Preparation of Electroactive Materials for High Performance Lithium-Sulfur Batteries

Item Type text; Electronic Dissertation

Authors Dirlam, Philip Thomas

Publisher The University of Arizona.

Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.

Download date 04/10/2021 08:48:47

Link to Item http://hdl.handle.net/10150/621564 PREPARATION OF ELECTROACTIVE

MATERIALS FOR HIGH PERFORMANCE

LITHIUM-SULFUR BATTERIES

Philip T. Dirlam

______

A Dissertation Submitted to the Faculty of the

DEPARTMENT OF CHEMISTRY AND BIOCHEMISTRY

In Partial Fulfilment of the Requirements

For the Degree of

DOCTOR OF PHILOSOPHY

WITH A MAJOR IN CHEMISTRY

In the Graduate College

THE UNIVERSITY OF ARIZONA

2016

THE UNIVERSITY OF ARIZONA

GRADUATE COLLEGE

As members of the Dissertation Committee, we certify that we have read the dissertation prepared by Philip Dirlam, titled Preparation of Electroactive Materials for High Performance Lithium-Sulfur Batteries and recommend that it be accepted as fulfilling the dissertation requirement for the Degree of Doctor of Philosophy.

______Date: 5/25/2016 Dong-Chul Pyun ______Date: 5/25/2016 Richard Glass ______Date: 5/25/2016 Neal Armstrong ______Date: 5/25/2016 Douglas Loy

Final approval and acceptance of this dissertation is contingent upon the candidate’s submission of the final copies of the dissertation to the Graduate College.

I hereby certify that I have read this dissertation prepared under my direction and recommend that it be accepted as fulfilling the dissertation requirement.

______Date: 5/25/2016 Dissertation Director: Dong-Chul Pyun

STATEMENT BY AUTHOR

This dissertation has been submitted in partial fulfilment of the requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this dissertation are allowable without special permission, provided that an accurate acknowledgement of the source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

SIGNED: Philip T. Dirlam

ACKNOWLEDGEMENTS

It is with great pleasure that I find myself writing this. The concept that the pursuit of this PhD would actually come to an end has truthfully always been arbitrary to me. It is surreal. The experience has introduced me to a degree of frustration that about drags one to the point of madness yet has also presented spells of true exhilaration.

My escapades through the numerous stages of chemical education that have culminated with this dissertation began with Dr. Kent Menzel who first sparked my intrigue with chemistry over a decade ago. I am grateful for the enthusiasm with which you taught that general chemistry course, it ultimately steered me into a field of study that

I have become earnestly passionate about.

I am indebted to my undergraduate advisor Dr. Philip Costanzo for taking me under his wing when I was just a kid who wandered into his office looking for some advice on what classes to take. The opportunities you presented to me to grow and learn as a chemist were the critical events that allowed me to find my way to graduate school and ultimately the successful completion of this degree. Even more, the mentorship and eventual friendship you provided pulled me back and corrected the tenuous course I was following. I never would have accomplished this without your support.

To my advisor, Dr. Jeff Pyun, I owe too much. You have shown me true dedication. Dedication not just to the discipline but most of all to the people who depend on you, your students and your family. You have taught me to be tenacious and unyielding. You have exemplified how with three parts hard work and a double shot of creativity great things can be achieved. Thank you for indulging my curiosity and allowing me to explore parts of the chemical space unknown to both of us. Thank you

for teaching me not just chemistry but also optimistically training me as an academic. I can’t hope to repay what I owe you but I will endeavour to pay it forward throughout my career.

I am grateful to Dr. Adam Simmonds and Dr. Clayton Shallcross for their mentorship. This dissertation would be of an entirely different flavor if it weren’t for your guidance. Adam, thank you for being an awesome and patient teacher. I am grateful for everything you taught me ranging from batteries and device fabrication to

EIS to MacGyvering any piece of equipment we could need. Clayton, thank you for showing me the ropes with my first electrochem experiments and all things conjugated polymers, it was a critical junction in my research and development as a chemist.

This would have been insurmountable without the support and friendship of my trusted confidants Eddie, Robb, Bereketab, Tristan, Jared, and Lawrence. In the chemistry realm, thank you for the countless hours of spit-balling ideas, trouble shooting stubborn experiments, studying and practicing talks. In the real world, thank you for all of the “good decisions” that were made and great times we had.

To my friends Anthony, X, and Ariel thank you for taking me in. You are like family and made Tucson and the U of A home.

To my Mom and Sister, thank you for always believing in me. Thank you for your never ending support even though I was a thousand miles away.

And to Sarah, thank you for loving me throughout all of this. Thank you for your patience and understanding with the countless long nights and too many weekends spent away from you toiling about in Old Chem. You were my anchor to reality and source of happiness.

In memory of my father,

Kenneth Dirlam, M.D.

CONTENTS

ABSTRACT……...……….……………………………………………………………11

1. THE USE OF POLYMERS IN LI-S BATTERY CATHODES ...... 16

1.1 INTRODUCTION ...... 16

1.2 APPLICATIONS OF POLYMERS IN LI-S CATHODES ...... 22

1.3 SUMMARY ...... 66

2. SINGLE CHAIN POLYMER NANOPARTICLES VIA SEQUENTIAL ATRP

AND OXIDATIVE POLYMERIZATION ...... 68

2.1 INTRODUCTION ...... 68

2.2 RESULTS AND DISCUSSION ...... 73

2.3 CONCLUSION ...... 86

3. IMPROVING THE CHARGE CONDUCTANCE OF ELEMENTAL SULFUR

VIA TANDEM INVERSE VULCANIZATION AND

ELECTROPOLYMERIZATION ...... 87

3.1 INTRODUCTION ...... 88

3.2 RESULTS AND DISCUSSION ...... 90

3.3 CONCLUSION ...... 97

4. INVERSE VULCANIZATION OF ELEMENTAL SULFUR WITH

1,4-DIPHENYLBUTADIYNE FOR CATHODE MATERIALS IN LI-S

BATTERIES ...... 98

4.1 INTRODUCTION ...... 99

4.2 RESULTS AND DISCUSSION ...... 101

4.3 CONCLUSION ...... 108

5. ELEMENTAL SULFUR AND MOLYBDENUM DISULFIDE COMPOSITES

FOR LI-S BATTERIES WITH LONG CYCLE LIFE AND HIGH-RATE

CAPABILITY ...... 109

5.1 INTRODUCTION ...... 110

5.2 RESULTS AND DISCUSSION ...... 114

5.3 CONCLUSION ...... 134

APPENDIX A. SUPPLEMENTARY INFORMATION FOR CHAPTER 2 ...... 135

A.1 MATERIALS ...... 136

A.2 INSTRUMENTATION AND METHODS ...... 136

A.3 EXPERIMENTAL PROCEDURES ...... 137

APPENDIX B. SUPPLEMENTARY INFORMATION FOR CHAPTER 3 ...... 146

B.1 MATERIALS ...... 147

B.2 INSTRUMENTATION AND METHODS ...... 147

B.3 EXPERIMENTAL PROCEDURES ...... 149

B.4 ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY (EIS) ...... 153

APPENDIX C. SUPPLEMENTARY INFORMATION FOR CHAPTER 4 ...... 156

C.1 MATERIALS ...... 157

C.2 INSTRUMENTATION AND METHODS ...... 157

C.3 EXPERIMENTAL PROCEDURES ...... 158

C.4 RESULTS AND DISCUSSION ...... 165

APPENDIX D. SUPPLEMENTARY INFORMATION FOR CHAPTER 5 ...... 176

D.1 MATERIALS ...... 177

D.2 INSTRUMENTATION AND METHODS ...... 177

D.3 EXPERIMENTAL PROCEDURES ...... 178

D.4 RESULTS AND DISCUSSION ...... 186

APPENDIX E. PERMISSIONS FOR REUSE OF COPYRIGHTED

MATERIALS ...... 202

REFERENCES...... 213

ABSTRACT

This dissertation is comprised of five chapters detailing advances in the synthesis and preparation of polymers and materials and the application of these materials in lithium-sulfur batteries for next-generation energy storage technology. The research described herein discusses progress towards overcoming three critical challenges presented for optimizing Li-S battery performance, specifically, addressing the highly electrically insulating nature of elemental sulfur, extending the cycling lifetime of Li-S batteries, and enhancing the charge discharge rate capability of Li-S cathodes.

The first chapter is a review highlighting the use of polymers in conventional lithium-sulfur battery cathodes. Li-S battery technology presents a grand opportunity to realize an electrochemical energy storage system with high enough capacity and energy density capable of addressing the needs presented by electrical vehicles and base load storage. Polymers are ubiquitous throughout conventional Li-S batteries and their use has been critical in overcoming the challenges presented for optimizing Li-S cathode performance towards practical implementation. The high electrical resistivity of elemental sulfur requires the incorporation of conductive additives in order to formulate it into a functional cathode. A polymer binder must be utilized to integrate the elemental sulfur as the active material with the conductive additives into an electrically conductive composite affixed to a current collector. The electrochemical action of the Li-S battery results in the electroactive sulfur species converting between high and low order lithium polysulfides as the battery is discharged and charged. These lithium polysulfides become soluble at various stages throughout this cycling process that lead to a host of complications including the loss of electroactive material and slow rate capabilities. The

use of polymer coatings applied to both the electroactive material and the cathode as a whole have been successful in mitigating the dissolution of lithium polysulfides by confining the redox reactions to the cathode. Elemental sulfur is largely intractable in conventional solvents and suffers from poor chemical compatibility limiting synthetic modification. By incorporating S-S bonds into copolymeric materials the electrochemical reactivity of elemental sulfur can be maintained and allow these polymers to function as the electroactive cathode materials while enabling improved processability and properties via the comonomeric inclusions. The use of inverse vulcanization, which is the direct copolymerization of elemental sulfur, is highlighted as a facile method to prepare polymeric materials with a high content of S-S bonds for use as active cathode materials.

The second chapter focuses on the synthesis and polymerization of a novel bifunctional monomer containing both a styrenic group to access free radical polymerization and a propylenedioxythiophene (ProDOT) to install conductive polymer pathways upon an orthogonal oxidative polymerization. The styrenic ProDOT monomer

(ProDOT-Sty) was successfully applied to a two-step sequential polymerization where the styrenic group was first leveraged in a controlled radical polymerization (CRP) to afford well defined linear homo- and block polymer precursors with pendant electropolymerizable ProDOT moieties. Subsequent treatment of the these linear polymer precursors with an oxidant in solution enabled the oxidative polymerization of the pendant ProDOT groups to install conductive polythiophene inclusions. Although the synthesis and CRP of ProDOT-Sty was novel, the key advance in this work was successful demonstration that sequential radical and oxidative polymerizations could be

carried out to install conductive polymer pathways through an otherwise nonconductive polymer matrix.

The third chapter expands upon the use of ProDOT-Sty to install conductive polymer pathways through a sulfur copolymer matrix. The highly electrically insulating nature of elemental sulfur precludes its direct use as a cathode in Li-S batteries and thus the use of ProDOT-Sty in the preparation of a high sulfur content copolymer with conductive inclusions was targeted to improve electrical properties. Inverse vulcanization of elemental sulfur with ProDOT-Sty and a minimal amount of 1,3- diisopropenylbenzene (DIB) was first completed to afford a sulfur rich copolymer with electropolymerizable side chains. Subsequently, the improved processability of the sulfur copolymer was exploited to prepare thin polymer films on electrode surfaces. The poly(ProDOT-Sty-co-DIB-co-sulfur) (ProDIBS) films were then subjected to oxidizing conditions via an electrochemical cell to invoke electropolymerization of the ProDOT inclusions and install conductive poly(ProDOT) pathways. Evaluation of the electrical properties with electrochemical impedance spectroscopy (EIS) revealed that the charge transfer resistance was reduced from 148 kΩ to 0.4 kΩ upon installation of the conductive poly(ProDOT) corresponding to an improvement in charge conductance of more than 95%. This also represented a key advance in expanding the scope of the inverse vulcanization methodology as the first example of utilizing a comonomer with a functional side chain.

The fourth chapter focuses on expanding the scope of the inverse vulcanization polymerization methodology to include aryl alkyne based comonomers and the application of new these new sulfur copolymers as active cathode materials in Li-S

batteries. The early work on developing inverse vulcanization relied heavily on the use of DIB as one of the few comonomers amenable to bulk copolymerization with elemental sulfur. One of the principal limitations in comonomer selection for inverse vulcanization is the solubility of the comonomer in molten sulfur. Generally it has been observed that aromatic compounds with minimal polarity are miscible and thus common classes of comonomers such as acrylates and methacrylates are immiscible and preclude their compatibility with inverse vulcanization. It was found that aryl alkynes are a unique class of compounds that are both miscible with molten sulfur and provide reactivity with sulfur centered radicals through the unsaturated carbon-carbon triple bonds.

Additionally, it was found that internal alkynes were best suited for inverse vulcanization to preclude abstraction of the somewhat acidic hydrogen from terminal alkynes. 1,4-

Diphenylbutadiyne (DiPhDY) was selected as a prototypical comonomer of this class of compounds for preparing high sulfur content copolymers via inverse vulcanization.

Poly(sulfur-co-DiPhDY) was prepared with various compositions of S:DiPhDY and these copolymers were formulated into cathodes for electrochemical testing in Li-S batteries.

The poly(S-co-DiPhDY) based cathodes exhibited the best performance reported at the time for a polymeric cathode material with the figure of merit of the first inverse vulcanizate to enable a cycle lifetime of up to 1000 cycles.

The fifth chapter details the preparation of composite materials composed of a sulfur or copolymeric sulfur matrix with molybdenum disulfide (MoS2) inclusions and the use of these materials for Li-S cathodes with rapid charge/discharge rate capabilities.

The higher order lithium polysulfide redox products (e.g., Li2S8 Li2S6) generated during

Li-S cycling are soluble in the electrolyte solution of the battery. The rate capability of

the Li-S battery is thus fundamentally limited by mass transfer as these electroactive species must diffuse back to the cathode surface in order to undergo further reduction

(discharge) or oxidation (charge). In order to limit the effective diffusion length of the soluble lithium polysulfides and therefore mitigate the diffusion limited rate, composite materials with fillers capable of binding the lithium sulfides were prepared. MoS2 was selected as the filler as simulations had indicated lithium polysulfide had a strong binding interaction with the surface of MoS2. Furthermore, it was demonstrated for the first time that metal chalcogenides such as MoS2 readily disperse in molten sulfur which enabled the facile preparation of the composite materials in situ. The composites were prepared by first dispersing MoS2 in liquid sulfur or a solution of liquid sulfur and DIB below the floor temperature of S8 (i.e. <160 °C). The dispersions were then heated above the floor temperature of S8 to induce ring opening polymerization of the sulfur phase and afford the composites. The composites were found to be potent active cathode materials in Li-S batteries enabling extended cycle lifetimes of up to 1000 cycles with excellent capacity retention. Furthermore, the composite materials were successful in enhancing the rate capability of the Li-S cathodes where reversible capacity of >500 mAh/g was achieved at the rapid rate of 5C (i.e. a 12 min. charge or discharge time).

15 The Use of Polymers in Li-S Battery Cathodes

1. THE USE OF POLYMERS IN LI-S BATTERY CATHODES

1.1 Introduction

1.1.1 Motivation for the Development of Li-S Technology

A shift in the current paradigm of procuring energy from fossil fuels to instead

securing it from cleaner, sustainable sources is a critical global enterprise for addressing

the world’s increasing energy needs while preserving the environment.1 Successfully

“decarbonizing” our energy consumption will rely on the ability to provide inexpensive

power from renewable sources such as solar, wind and tidal. Unfortunately these sources

are inherently erratic and relatively dispersed. Thus there is a need for efficient energy

storage technology built with abundant materials capable of supplying power when these

renewable sources are inactive.2 Furthermore, widespread implementation of a

transportation system based on electric vehicles requires the development of economical

energy storage systems with high energy density to allow for the production vehicles with

practical cost, size and driving range. Next-generation electrochemical energy storage

Philip T. Dirlam - 2016 16 The Use of Polymers in Li-S Battery Cathodes technology (i.e. batteries beyond Li-ion) provides a promising means to address these needs.3 Of the emerging battery systems the lithium-sulfur (Li-S) battery is exceptionally well suited to accommodate the demands posed by electric vehicles and could address stationary base power needs owing to notably high theoretical capacity of 1672 mAh/g and theoretical specific energy of 2500 Wh/kg.4-5

1.1.2 “Conventional” Cell setup for Li-S batteries and Complications Presented by the Use of Elemental Sulfur

A “conventional” Li-S cell setup is assembled with four integral components: the cathode, electrolyte, separator, and anode (Figure 1.1). The cathode is arguably the most complex of these components and is generally a composite of the electroactive material

(e.g. elemental sulfur), a conductive additive (e.g. carbon), and a polymer binder (Figure

1.1a). In this chapter, the extensive use of polymers in these subcomponents of the composite cathode will be discussed. The electrolyte in a Li-S battery is typically a solution of the lithium salt of bis(trifluoromethane)sulfonimide (LiTFSI) in a high dielectric ether based solvent mixture (e.g. 1,3-dioxolane, 1,2-dimethoxyethane).

However, there are also some examples of utilizing solid- and gel-polymer type electrolytes (Figure 1.1b). These polymer based electrolytes are less common could present a possible solution to a number of the problems inherent to the use of a liquid electrolyte.6-11 The separators most commonly employed are commercially available microporous polyolefin membranes (Figure 1.1c). These polyolefin membranes have been subjected to modification with a variety of functional polymers as a way to enhance

Li-S performance.12 Lastly, the anode is lithium metal which is generally included in the

Philip T. Dirlam - 2016 17 The Use of Polymers in Li-S Battery Cathodes

Li-S battery in an unmodified state, with only a few relevant reports detailing the use of polymers towards augmenting its function as the negative electrode.

Figure 1.1. General schematic of a Li-S cell constructed with a) a composite cathode comprised of the electroactive material (sulfur), conductive carbon, and polymer binder, b) electrolyte, and c) separator.

A number of the intrinsic properties of elemental sulfur present complications towards its successful use as a battery cathode. First, the innate electrical conductivity of elemental

-16 sulfur is extremely low. S8 is reported to have an electrical conductivity of only ~10

S/cm making it one of poorest conductors known and as such a difficult material to utilize as an electrode.13 This ultimately necessitates the inclusion of the two other constituents of the cathode, specifically the conductive additive and polymer binder, in order to yield a coherent electrode. The addition of these constituents in the cathode not only increases the complexity of this component but also decreases the effective energy

Philip T. Dirlam - 2016 18 The Use of Polymers in Li-S Battery Cathodes density of the cathode significantly. Furthermore, elemental sulfur is largely intractable in most solvents at ambient temperature. This makes chemical modification of sulfur via conventional synthetic means difficult and impedes ideal solution processing conditions for device fabrication.

1.1.3 Electrochemistry/Mechanism of the Li-S battery and the Resulting Challenges Towards Optimized Device Performance.

Elucidation of the complete electrochemical reaction mechanism between lithium and sulfur in a non-aqueous Li-S cell has been the subject of extensive experimental14-16 and computational studies.17 A comprehensive examination of the current understanding of the Li-S mechanism is beyond the scope of this paper and for a detailed assessment of the subject the reader is directed to a recent, thorough review by Offer et al.18 The requisite basis for a pertinent discussion on the Li-S battery can be provided with a

+ relatively simplified representation of the overall discharge mechanism of S8 + 16Li +

- 16e  8Li2S (Figure 1.2). The first phase in the discharge of a Li-S battery is the reduction of S8 to Li2S8. The Li2S8 is then, in part, reduced further to Li2S6 (which exists

•- in equilibrium as 2LiS3 ). These initial reduction reactions occur at a higher potential

(ca. 2.4 – 2.3 V vs Li/Li+) referred to as the high voltage plateau. The second phase in the discharge is attributed to the formation of Li2S4, conceivably via reductive dissociation of Li2S8 and is accompanied by a drop in the potential from ca. 2.3 to 2.1 V.

•- The third phase in the discharge is based around the equilibrium of Li2S6 with LiS3 and occurs at a lower potential (ca. 2.1 – 2.0 V) referred to as the low voltage plateau. The

•- reduction of LiS3 to Li2S3 is purported to be the dominant electrochemical reaction

Philip T. Dirlam - 2016 19 The Use of Polymers in Li-S Battery Cathodes

which upon subsequent association with of Li2S3 with Li2S4 affords Li2S6 (enabling

•- further discharge via LiS3 ) and Li2S as the ultimate discharge product.

The consequences of the fundamental electrochemical action of a Li-S cell presents considerable challenges towards optimized device performance. Throughout these various phases of the conversion reaction between S8 and Li2Sx, the solubility and volume of the Li/S redox products change immensely. The electroactive species undergo a repeated change from being intractable in the fully oxidized (charged) state as S8, to dissolving in the electrolyte solution as intermediate redox products (e.g. Li2S6) and again becoming intractable when extensively reduced (discharged) to Li2Sx (x≤2) (Figure 1.2).

This varied solubility of the Li/S redox products is systemic to many of the detriments of

Li-S battery performance including low Coulombic efficiency, active material loss, and poor charge/discharge rate capability. Additionally, the densities of the intractable electroactive species, S8 and Li2S, are 2.03 g/mL and 1.66 g/mL respectively. This considerable volume change of nearly 80% between the fully charged and discharged states induces notable mechanical stress to the carbon/binder framework of the cathode.

Over repeated deposition of the more voluminous Li2Sx discharge products, cracks and fissures begin to form throughout the cathode composite leading to electrically disconnected regions and eventually device failure.

Philip T. Dirlam - 2016 20 The Use of Polymers in Li-S Battery Cathodes

Figure 1.2. Schematic depicting the discharge process of a conventional Li-S cathode.

1.1.4 Application of Polymers throughout Li-S Cathodes

The application of polymeric materials is ubiquitous throughout the Li-S battery.

However, much of the work on developing functional Li-S cells has relied upon the use of polymeric materials that were developed and optimized for Li-ion technology. With the intense research in the Li-S field over the past years a much more detailed understanding of the mechanisms at work during Li-S cell operation has been elucidated.

Through these efforts it has become apparent that many of the polymers commonly employed in Li-S test cells are ill suited for accommodating the vastly different chemistries at work in a Li-S battery relative to Li-ion. This presents a great opportunity for the innovation of new polymeric materials specifically designed to address the challenges presented by the Li-S system. In this chapter, the Li-S battery cathode will be discussed from the perspective of the role polymeric materials play in each of the various components of a conventional composite cathode architecture. For each component of the cathode and specific applications therein the critical function of the polymeric

Philip T. Dirlam - 2016 21 The Use of Polymers in Li-S Battery Cathodes material will be presented. Additionally, for each application the material properties that are specifically desirable for overcoming obstacles in the Li-S system will identified and reports detailing notable advancements towards optimized device performance will be examined.

1.2 Applications of Polymers in Li-S Cathodes

1.2.1 Binders

The principal role of the polymer binder in the cathode formulation is to function as a matrix for integrating the electroactive sulfur and conductive carbon additive together into a composite. Therefore, the selection of the polymer binder should be guided by the principals of polymer matrix composite formation.19 Additionally the polymer binder serves to affix this composite on a current collector, such as aluminum foil, as a coherent, electrically conductive film. First, for a polymer to be effective in facilitating the formation of a conductive composite it must be able to promote cohesion of the carbon particles. This enables the percolation of electrically conductive pathways through the bulk of the composite to the current collector.20 Second, the polymer must also promote adhesion of the carbon filler with both sulfur and the current collector. This requires that there is a favorable interaction (i.e. wetting) of the surfaces of carbon, sulfur, and current collector with the polymer allowing for continuity between the carbon/sulfur constituents while attaching the composite to the current collector.20 Finally, it should be noted that these interactions are consequential both in the dispersion prior to coating and in the final composite.

Philip T. Dirlam - 2016 22 The Use of Polymers in Li-S Battery Cathodes

For the composite to function as a cathode upon assembly into a Li-S battery considerations of the interaction of the polymer binder with the electrolyte, which most commonly is a solution, should be taken. Foremost, the polymer must be insoluble in the electrolyte solvent in order to retain the composite structure. However, the polymer binder should be swellable by the electrolyte solution to allow for ion transport to the electroactive material during cell operation. The stability of the polymer binder both chemically and electrochemically should also be taken into consideration. The chemical environment during various stages of Li-S cell operation is aggressive owing to both the nucleophilicity of the polysulfide species and the notable alkalinity they impart. Thus the polymer binder must be relatively chemically inert in order to maintain integrity throughout long term battery operation. Fundamentally, the polymer binder must also be electrochemically stable within the potential window of interest for Li-S battery cycling

(i.e. 1.7 – 2.7 V vs. Li/Li+).

Fluoropolymers such as poly(tetrafluoroethylene) (PTFE, Figure 1.3a), and poly(vinylidene fluoride) (PVDF, Figure 1.3b) largely meet the compatibility and stability requirements outlined above and thus have been widely implemented in the research and development of composite cathodes. The first reports in the literature of prototype Li-S cells published by Brummer et al. and later by Yamin and Peled utilized

PTFE as the binder in high surface area carbon composite electrodes.14, 21 These seminal works focused on the utilization of electroactive sulfur introduced as a solution of lithium polysulfides rather than solid elemental sulfur and provided invaluable insights into the fundamental electrochemistry of nonaqueous Li-S cells. Since the revival of interest in

Li-S battery research approximately 20 years after these initial studies, much of the work

Philip T. Dirlam - 2016 23 The Use of Polymers in Li-S Battery Cathodes on Li-S cathodes has relied on the use of PVDF as a binder. This is presumably due to the fact that PVDF has been implemented in commercialized formulations for Li-ion technology and therefore it is widely available and optimized processing conditions have been determined. However, the use of PVDF is not ideal in the case of Li-S cathode formulation for a number of reasons. First, the solution processing of PVDF is principally limited to the use of N-methylpyrrolidone (NMP) as the solvent. NMP is not only is toxic but has a very low vapor pressure requiring conditions for its effective removal that are intensive enough to volatilize elemental sulfur via sublimation which complicates efficient cathode fabrication. Furthermore, PVDF is not ideal for managing the mechanical stress induced by the nearly 80% volume increase in the electroactive material upon discharge as it is semi-crystalline with a low elongation yield. The following section will review the application of polymers (other than PVDF) that have been investigated as binders in the formulation of composite cathodes towards optimizing

Li-S battery performance.

An early report by Kim and coworkers described the use of polyethylene oxide

(PEO, Figure 1.3c) as a polymer binder for Li-S cathodes.22 Additionally, they investigated the structural changes23 in addition to the rate capability and cycling characteristics24 of cathodes fabricated using poly(styrene-co-butadiene) (SBR, Figure

1.3d) as the polymer binder. With the use of PEO it was found that the agitation method employed when preparing the slurry of polymer, carbon, and sulfur in acetonitrile prior to coating had a notable influence on the structure of the coated cathode. Using mechanical stirring was shown to afford a more porous cathode structure compared to a more aggressive, ball milling method. It was also found that roll-pressing the as coated

Philip T. Dirlam - 2016 24 The Use of Polymers in Li-S Battery Cathodes

Figure 1.3. Structures of various polymers utilized as binders in the fabrication of sulfur/conductive carbon composite cathodes. cathodes was beneficial to device performance due to improved cohesion of the cathode materials and adhesion to the current collector leading to a stable reversible capacity of ca. 500 mAh/gsulfur for up to 100 charge/discharge cycles. A later report by Brandell et al. discussed this influence of polymer binder on cathode porosity in further detail.25

A study by Roh et al. compared a number of different binder systems utilizing

PTFE, carboxymethyl cellulose (CMC, Figure 1.3e), poly(vinyl alcohol) (PVA, Figure

1.3f), and poly(vinylpyrrolidone) (PVP, Figure 1.3g).26 The morphology of cathodes prepared with various mixtures of these binders was examined in terms of porosity and specific surface area and correlated to sulfur utilization. It was found that the pore

Philip T. Dirlam - 2016 25 The Use of Polymers in Li-S Battery Cathodes distribution was found to be largely independent of the polymer binder(s) employed while the specific surface area of the cathode was influenced greatly. The cathode prepared with a mixture of PTFE and CMC exhibited the highest surface area and consequently the greatest sulfur utilization of more than 70% (>1100 mAh/g).

The investigation of CMC as a beneficial component for a binder system continued, particularly in conjunction with SBR. Huang et al. showed that the SBR-

CMC binary binder system facilitated a considerably more coherent dispersion of sulfur and/or conductive carbon relative to PVDF (Figure 1.4) with the added benefit of being processable in water.27 A later report by Cairns et al. confirmed the improved Li-S battery performance of utilizing the SBR-CMC binder compared to PVDF and PEO with a carbon nanofiber-sulfur composite and ionic liquid electrolyte additive.28 The improved performance was attributed to the important fact that the SBR is elastomeric and thus more capable of accommodating the notable volume expansion of the Li-S discharge products. Further development of this approach by Cairns and coworkers led to the report of one of the highest performing Li-S cathodes which integrated the SBR-CMC binder with a graphene oxide/sulfur composite stabilized with a dynamic surfactant

(CTAB) coating (Figure 1.5a).29 This multifaceted approach towards enhancing battery performance afforded Li-S cells with a high sulfur loading that demonstrated both very high rate capabilities of up to 6C in addition to extremely long cycle lifetime of 1500 cycles with good capacity retention (Figures 5b,c).

Philip T. Dirlam - 2016 26 The Use of Polymers in Li-S Battery Cathodes

Figure 1.4. Confocal microscopy images showing the dispersion morphology of (a) Sulfur/Carbon Black/SBR–CMC, (b) Sulfur/Carbon Black/PVDF, (c) Carbon Black/SBR–CMC, (d) Carbon Black/PVDF, (e) Sulfur/SBR–CMC, and (f) Sulfur/PVDF. Adapted with permission from Ref. 27 © 2011 American Chemical Society.

Philip T. Dirlam - 2016 27 The Use of Polymers in Li-S Battery Cathodes

Figure 1.5. (a) Schematic of the S–GO nanocomposite structure with typical morphology of the S–GO composites examined by scanning electron microscopy. (b) Cycling performance of CTAB-modified S–GO composite cathodes at different rates. (c) Long- term cycling test results of the Li/S cell with CTAB-modified S–GO composite cathodes with an extremely low capacity decay rate (0.039% per cycle). The S–GO composite contained 80% S, and elastomeric SBR/CMC binder was used. 1 M LiTFSI in

The use of a variety of other polar/ionic polymers have also received attention as binders in efforts to mitigate diffusion of lithium polysulfides away from the cathode via dipolar interactions. Novák and coworkers investigated the efficacy of utilizing Nafion as the polymer binder. They demonstrated that Nafion was plausible as a binder materials, however, even after application of an additional Nafion layer coated on top of the cathode

Philip T. Dirlam - 2016 28 The Use of Polymers in Li-S Battery Cathodes the dissolution of polysulfides was still prevalent.30 Li et al. later reported that Nafion which had been neutralized with LiOH to afford a lithiated Nafion analogue (Li+-Nafion) was more effective than the commercially available form of the polymer which is acidic due to sulfonic acid side chains.31 It was found the Li+-Nafion blended with PVP as a multicompent binder system was even more effective than either polymer utilized alone and furthermore that the introduction of SiO2 inclusions to this blend further enhanced device performance as evidenced by stable, reversible capacity of >1100 mAh/g after 50 cycles (Figure 1.6). A similar combination of Nafion/PVP was later described by Zhang et al. where the two polymers were used in the fabrication of cathodes via layer-by-layer assembly.32

Figure 1.6. Cycling performance of the cell using PVDF, PVP, Li+-Nafion, polymer blend of Li+-Nafion with PVP, and SiO2-impregnated polymer blend as the binder at 25 °C. Reproduced with permission from Ref. 31 © 2015 Elsevier.

Insight into the nature of the enhanced device performance realized with the use of PVP and other polymer binders with polar side chains was elucidated by Cui et al.33

They first provided a theoretical basis for the observed improvement in Li-S performance

Philip T. Dirlam - 2016 29 The Use of Polymers in Li-S Battery Cathodes

where ab initio simulations were used to determine the interaction of Li2S with a variety of polar functional groups (Figures 1.7a-d,f-h). Based on these simulations, it was determined that the interaction of lithium sulfides with halogenated groups such as the C-

F functionality present on PVDF is considerably weaker than carbonyl containing functional groups. This determination led to a detailed experimental investigation of

PVP as a polymer binder in Li-S batteries utilizing Li2S as the electroactive sulfur source.

It was first demonstrated that the stronger binding affinity of PVP with Li2S compared to

PVDF facilitated the formation of a more uniform dispersion of Li2S and conductive carbon (Figures 1.7e,i). During evaluation of the electrochemical performance the cathodes fabricated with PVP binder compared to PVDF it was shown that the PVP was able to reduce the concentration of sulfur that diffused into the electrolyte by more than

50% (Figure 1.7j). By mitigating the loss of electroactive species the PVP based cathodes enabled devices with longer cycle lifetime and reduced capacity fade delivering nearly 1000 mAh/gsulfur after more than 200 cycles (Figure 1.7k). Following the rational design and successful implementation of PVP by Cui et al. further reports on the use of polymers with polar side chains have been disclosed including PVP/ PEO blends 34-35 and

PVA36 in addition to a recent report on the unique use of polyamidoamine (PAMAM) dendimers rich with ester and lactam functionality as binders.37

Philip T. Dirlam - 2016 30 The Use of Polymers in Li-S Battery Cathodes

Figure 1.7. (a) Table showing the calculated binding energy of Li2S with various functional groups (R) based on the framework of vinyl polymers –(CH2–CHR)n–. Ab initio simulations showing the most stable configuration and calculated binding energy of

Li2S with (b) ester, (c) ketone, (d) lactam/PVP, (f) amide, and (h) alkyl fluoride/PVDF. Optical microscopy and digital camera images (inset) showing the electrode slurry of (e)

Li2S/carbon black/PVP binder and (i) Li2S/carbon black/PVDF binder in N-methyl-2- pyrrolidinone (60 : 35 : 5 by weight in both cases). (j) Percentage of sulfur in the electrolyte relative to the total sulfur mass on the electrode after cycling at 0.2 C using

PVP binder in comparison with PVDF binder (k) Specific capacity of Li2S cathodes using PVP binder cycled over 200 cycles at 0.2 C, in comparison with PVDF binder. Adapted with permission from Ref. 33 © 2013 Royal Society of Chemistry.

The utility of water soluble binder systems have been investigated due to the attractive feature of an environmentally friendly solvent system.38 Many of these binders are based on biopolymers, including gelatin,39-45 alginate,46-47 starch,48 chitosan,49 and

Philip T. Dirlam - 2016 31 The Use of Polymers in Li-S Battery Cathodes gum Arabic (GA). The latter, a natural polymeric material composed of a mixture of polysaccharides and glycoproteins derived from the Acacia senegal tree, was reported by

Zhang and coworkers as a multifunctional binder.50 It was found that the electrochemical performance of Li-S cathodes fabricated with gum Arabic far exceeded cathodes fabricated with either gelatin or PVDF in terms of cyclability. The GA based cathode delivered a reversible capacity of 1090 mAh/gsulfur after 50 cycles compared to 450 mAh/g and 310 mAh/g with gelatin and PVDF respectively (Figure 1.8a) in addition to enabling a very high rate capability of up to 10C (Figure 1.8b) and long term cyclability delivering a capacity of >800 mAh/g after 500 cycles (Figure 1.8c). The enhanced performance of the GA containing cathodes was correlated to improved mechanical properties elucidated via scanning probe microscopy. Scratches were induced at 1 mN over a 10 μm track where a smooth scratch with limited cracking was observed for the cathode fabricated with gum Arabic (Figure 1.8d). The friction coefficient was determined over the track of the scratches imparted on GA, PVDF and gelatin containing composite cathodes where the GA based coating was found to maintain the highest friction coefficient along the entire track indicating the GA facilitated best adhesion of the three binders (Figure 1.8e). Furthermore, indentation tests were conducted to investigate the reduced modulus (Figure 1.8f) and hardness (Figure 1.8g) of the composite cathodes employing different binders. The cathodes fabricated with GA again showed more attractive mechanical properties than the PVDF and gelatin based cathodes where a lower overall modulus and hardness were measured suggesting a better flexibility more suitable for accommodating the stress induced from the expansion of the electroactive material upon discharge.

Philip T. Dirlam - 2016 32 The Use of Polymers in Li-S Battery Cathodes

Figure 1.8. (a) Electrochemical cycling performance of sulfur cathodes with gum Arabic (S@GA), PVDF (S@PVDF), and gelatin (S@Gelatin) binders at a rate of C/5. (b) The discharge capacities of the S@GA electrode at various C rates. (c) Long term cycling performance of the sulfur cathode with GA binder. (d) In situ 3D nanoscratch image of the sulfur@GA cathode via scanning probe microscopy with (e) friction coefficeints determined along the track of 10 μm long scratches induced composite cathodes with the different binders. (f) Average reduced modulus and (g) hardness of the S@GA, S@PVDF, and S@Gelatin cathodes obtained via nanoindentation testing. Adapted with permission from Ref. 50 © 2015 John Wiley and Sons.

Another unique approach has been the use of conjugated polymers as the polymer binder. As most of the polymers which are utilized as binders are electrically insulating this strategy is of interest for maximizing the conductivity of the carbon/binder composite

Philip T. Dirlam - 2016 33 The Use of Polymers in Li-S Battery Cathodes framework. G. Liu et al. first demonstrated the efficacy of utilizing poly(ethylenedioxythiophene) (PEDOT) as a polymer binder in combination with micrometric sulfur in enhancing Li-S performance compared to cathodes fabricated with

PVDF binder.51 They extended the scope of the conductive polymer binders in a subsequent study to include a unique copolymer of 9,9-dioctylfluorene, fluorenone, and methylbenzoic ester (PFM, Figure 1.9a). The structure of the PFM copolymer was designed to provide electrical conductivity via the polyfluorene backbone with the octyl side chains enhancing processability and electrolyte uptake. The fluorenone comonomer was chosen to tune the electronic structure and should have a binding affinity for lithium sulfides and the methylbenzoate was selected to improve the mechanical properties in addition to providing additional polysulfide binding capability.52 Comparison of the electrochemical performance of sulfur cathodes with PFM, PEDOT, PVP, and PVDF revealed that the unique PFM copolymer enabled the highest, stable capacity over 150 cycles (Figure 1.9b).53 The composite cathode integrity was qualitatively evaluated as shown with photographs of the cathodes taken after cell disassembly and solvent washing

(Figure 1.9b inset, top row). Additionally the degree of polysulfide loss from the cathode was qualitatively assessed upon visual inspection of the separator as indicated by the degree of coloration (Figure 1.9b inset, bottom row). The cathodes fabricated with the

PFM and PVP clearly show greater integrity with the retention of the majority of the cathode coating intact relative to the PEDOT and PVDF. Furthermore, the degree of polysulfide diffusion into the separator was notably reduced in the PFM and PVP cells and was attributed to the binding affinity of lithium sulfides with the carbonyl containing side chains. The combination of increased conductivity and cathode integrity coupled

Philip T. Dirlam - 2016 34 The Use of Polymers in Li-S Battery Cathodes

Figure 1.9. (a) Structure of the conductive polymer binders PFM (poly(9,9- dioctylfluorene-co-fluorenone-co-methylbenzolate)) and PEDOT (poly(ethylenedioxythiophene)). (b) Cycling performance of sulfur cathodes fabricated with various binders (inset) showing photographs after cycling and cell dissambly of (top) cathodes after rinsing with DOL/DME and (bottom) separators. Adapted with permission from Ref. 53 © 2015 Elsevier. with reduction of polysulfide diffusion in the PFM based cathode was correlated with the overall improved cycling performance observed.

Polymers have also been introduced to composite cathodes as a supplement to the formal polymer binder to modify the interface between the carbon and sulfur constituents

Philip T. Dirlam - 2016 35 The Use of Polymers in Li-S Battery Cathodes and/or act as polysulfide hosts during battery operation.54-63 Conjugated polymers have been the focus of many examples implementing this strategy.64-76 An initial report by Yu et al. demonstrated the use of poly(pyrrole) (PPy) nanowires as sulfur hosts. The PPy was prepared in a biphasic system using CTAB as a surfactant and ammonium persulfate to initiate oxidative polymerization of pyrrole. This afforded PPy in a wormlike morphology that was then mixed with liquid sulfur yielding a PPy/S composite which was used as the electroactive material. Cathodes fabricated with the PPy/S composite, carbon black and a poly(acrylonitrile) type binder were shown to exhibit better cycling performance over a short period of 20 cycles with the PPy/S cathode delivering ca. 700 mAh/gsulfur compared to cathodes prepared with just elemental sulfur which delivered 500 mAh/g.64 Tour and coworkers reported on the use of a composite of poly(aniline) decorated graphene nanoribbons (PANI-GNRs) and elemental sulfur for the electroactive cathode material. The PANI-GNRs were prepared by oxidatively polymerizing aniline in the presence of GNRs and sulfur subsequently introduced by evaporating a solution of S8 in carbon disulfide (Figure 1.10a). The sulfur-PANI-GNRs (Figures 10b,c) were formulated into a composite cathode along with carbon black and PVDF binder and shown to enable Li-S batteries with high capacity and notably extended cycle lifetimes delivering >500 mAh/gsulfur after 400 cycles at 0.4C (Figure 1.10d).

Philip T. Dirlam - 2016 36 The Use of Polymers in Li-S Battery Cathodes

Figure 1.10. (a) Scheme showing the preparation of sulfur/poly(aniline)/graphene nanoribbons (SPGs). (b and c) SEM of SPGs at different magnifications. (d) Li-S cycling performance of cathodes fabricated with SPGs as the electroactive material. Adapted with permission from Ref. 69 © 2014 American Chemical Society.

1.2.2 Coatings

Polymers have been widely implemented as coatings to confine the Li/S redox reactions to the cathode and limit the effective diffusion length of the soluble lithium polysulfides. Polymer coatings have been used to both encapsulate the electroactive material in the form of colloids, particulates, and composites in addition to being applied to the cathode as a whole. The polymers utilized for these coatings would ideally have electrochemical properties which facilitate charge and/or ionic conduction to the electroactive material. Further considerations for the polymer coatings are similar to

Philip T. Dirlam - 2016 37 The Use of Polymers in Li-S Battery Cathodes those outlined for polymer binders. Specifically, it would be ideal for the polymer coatings to have mechanical properties which can accommodate the substantial volume change of the Li/S redox products. Also, the coatings should be electrochemically stable within the 1.7 V to 2.7 V potential window in addition to being chemically stable to the alkaline environment and nucleophilic polysulfides generated during Li-S cell operation.

Practical concerns in terms of the ease of preparation and amenability to scale up are relevant to consider for the ultimate goal of commercializing Li-S technology. Also towards the goal of commercialization, the mass and volume of the polymer coatings should be minimal. This is a key design consideration in order ensure that the total electroactive sulfur loading maximized and therefore the energy density of the cathode is not compromised.

1.2.2.1 Polymer Coating of Colloidal/Particulate Sulfur

Encapsulation of colloidal or particulate elemental sulfur and Li2S with conjugated polymers including poly(pyrrole) (PPy),77-84 poly(thiophene) (PTh),85-87 poly(aniline) (PANI),88-92 and poly(3,4-ethylenedioxythiophene) (PEDOT)93-94 has been investigated extensively. The seminal example of coating sulfur with a (conjugated) polymer was completed by H. K. Liu and coworkers.77 In their initial report PPy was deposited on particulate sulfur via oxidative polymerization of pyrrole utilizing FeCl3 as the oxidant and toluenesulfonate as the dopant. The use of S-PPy as the electroactive material in Li-S cathodes enabled higher capacity of ~600 mAh/g after 20 cycles compared to ~350 mAh/g for bare sulfur particles. However, it was noted that PPy is also electrochemically active in the potential window used for cycling the Li-S cell and was contributing approximately 16 mAh/gPPy of reversible capacity. This also points to

Philip T. Dirlam - 2016 38 The Use of Polymers in Li-S Battery Cathodes the PPy undergoing doping/dedoping (oxidation/reduction) at different stages of charge and discharge which would affect the electrical conductivity of the coating as conjugated polymers are poor conductors in their neutral states.

Wu et al. completed a similar study where the use poly(thiophene) (PTh) was investigated as a conjugated polymer coating on sulfur powder affording a composite like material.85 They later presented a more extensive study on the strategy of using PTh coating of particulate S in order to prepare core@shell particles of sulfur@polythiophene with varied S:PTh loading.86 In this system the S@PTh particles were prepared by first treating the sulfur with a solution of the oxidant (i.e. FeCl3) followed by a slow addition of thiophene monomer (Figure 1.11a). Upon introduction of the thiophene to the

FeCl3/sulfur mixture oxidative polymerization was induced to afford sulfur particles encapsulated with PTh (Figure 1.11b). Electrochemical evaluation of the S@PTh materials revealed a notably reduced charge transfer resistance (RCT) as indicated by electrochemical impedance spectroscopy (EIS) with the S@PTh materials compared to bare sulfur (Figure 1.11c). The extent of RCT reduction was observed to correlate with the PTh content of the materials where the S@PTh with the highest PTh content of 38 wt% (S-PTh A) had the lowest RCT of 93 Ω compared to 28.1 wt% PTh (S-PTh B) and 17 wt% PTh (S-PTh C) with RCT of 122 Ω and 220 Ω respectively. The S@PTh were implemented as electroactive cathode materials and found to enable considerably enhanced Li-S performance compared with sulfur when cycled over the relatively broad

1.0 V to 3.0 V vs Li/Li+ potential window (Figure 1.11d) where the S-PTh B cathode delivered specific capacity of 830 mAh/gsulfur after 80 cycles compared to the pristine sulfur cathode with a capacity of only 282 mAh/gsulfur. It was noted that there was a

Philip T. Dirlam - 2016 39 The Use of Polymers in Li-S Battery Cathodes

Figure 1.11. (a) Synthetic scheme for the preparation of core@shell sulfur@polythiophene (S@PTh) materials with (b) corresponding SEM image of S@PTh. (c) Nyquist plots from electrochemical impedance spectroscopy (EIS) analysis of S@PTh with various S:PTh amounts (S-PTh A = 38 wt% PTh, S-PTh B = 28.1 wt% PTh, and S-PTh C = 17 wt% PTh) with fitting to a Randells circuit. (d) Plot of specific capacity (based on sulfur loading) versus cycle number for cathodes fabricated with S- PTh B compared with pristine sulfur cycled between 1.0 V and 3.0 V vs. Li/Li+ (e) Plot of specific capacity versus cycle number for poly(thiophene) cycled between 1.0 V and 3.0 V vs. Li/Li+. Adapted with permission from Ref. 86 © 2011 American Chemical Society. minimal contribution to the capacity from the poly(thiophene) coating of ca. 10 mAh/g

(Figure 1.11e). Finally, it was found that the PTh encapsulation was able to suppress the polysulfide shuttle where a coulombic efficiency >90% was observed for the S@PTh

Philip T. Dirlam - 2016 40 The Use of Polymers in Li-S Battery Cathodes

based cathodes in Li-S cells tested without LiNO3 added to the electrolyte. As a lower coulombic efficiency is principally results from dissolved lithium polysulfides diffusing to the Li anode surface where they are chemically reduced and thus require additional electrochemical oxidation (i.e. overcharging) the higher coulombic efficiency was indicative of reduced polysulfide dissolution/diffusion.

The nature of the enhanced Li-S performance observed via conductive polymer coating was elucidated by Cui et al. where they prepared hollow colloidal S particles encapsulated in PEDOT, PPy, or PANI (Figures 12a,b) and presented theoretical simulations describing the interaction between lithium sulfides and these conductive polymers (Figures 1.12c,d).95 With an optimized conductive polymer coating thickness of 20 nm the encapsulated sulfur nanoparticles enabled notable electrochemical performance as Li-S cathode materials. The best performance was exhibited by the

PEDOT coated sulfur with a high capacity of >800 mAh/gsulfur was exhibited over 500 cycles (Figure 1.12e). The enhanced performance of the PEDOT system was attributed in part to the high binding affinity with the EDOT groups of > 1.0 eV which was nearly double the binding interactions calculated for Py and ANI. Furthermore, the design of the nanoparticles with a hollow interior allows for accommodation of the volume expansion of the electroactive material upon discharge.96 The conductive polymer coated sulfur nanoparticle based cathodes were also found to exhibit high rate capabilities with the

PEDOT system enabling reversible capacity >600 mAh/gsulfur at the very fast rate of 4C.

The high rate capability was correlated to faster electrode kinetics as observed via EIS analysis of the conductive polymer coated sulfur nanoparticles in addition to confinement of the electroactive sulfur by the polymer coatings.

Philip T. Dirlam - 2016 41 The Use of Polymers in Li-S Battery Cathodes

Figure 1.12. (a) Scheme showing the preparation of conductive polymer coated sulfur nanospheres. (b) Structures of the conductive polymers utilized for encapsulation with m and n indicating the doped and neutral forms of the polymers respectively. (c, d) Ab initio simulations showing the most stable configurations with calculated binding energies shown in parentheses of (c) Li2S and (d) Li–S· species with the heteroatoms (oxygen, sulfur, or nitrogen) in PEDOT, PPy, and PANI. (e) Cycling performance of Li-S cells fabricated with hollow sulfur nanospheres encapsulated in 20 nm thick coatings of PANI, PPy, and PEDOT over 500 cycles at a rate of C/2 and (f) rate capability of the

Philip T. Dirlam - 2016 42 The Use of Polymers in Li-S Battery Cathodes

An alternative strategy to the use of conventional conjugated polymer coatings has recently been investigated with the use of poly(dopamine) (PD) as a unique polymer encapsulating agent.97 The seminal example of utilizing PD was reported by Jin et al. where PD-coated S nanosheets (Figure 1.13a) were utilized along with a poly(acrylic acid) binder and carboxylic acid functionalized multi-walled carbon nanotubes

(MWCNTs) as the conductive additive in a multifaceted approach to improving Li-S performance.98 This study emphasized a design strategy aimed at improving the mechanical properties and stability of the cathode as a whole by integrating all three components of the cathode into a crosslinked network. To accomplish this Jin and coworkers chose the polydopamine coating to afford an electroactive material with reactive sites (e.g. amines, alcohols) for covalent bond formation with the carboxylic acid functional binder and MWCNTs (Figure 1.13b). The crosslinked cathode delivered a high capacity of ca. 500 mAh/gtotal cathode for up to 500 cycles (Figure 1.13c) and improved the capacity retention by nearly 20% per cycle compared to non-crosslinked cathodes.

Another example of utilizing PD coatings was reported by Chen et al. where sulfur colloids were first prepared by decomposition of sodium thiosulfate with PVP as a surfactant which were subsequently coated with PD via base promoted polymerization of dopamine (Figure 1.14a).99 Li-S batteries tested with these PD coated sulfur nanospheres as the electroactive material exhibited long cycle lifetimes with excellent capacity retention delivering a specific capacity of >600 mAh/gsulfur after 900 cycles (Figure

1.14b). The enhanced performance was attributed to binding interactions of the lithium polysulfides with the PD coating as supported with DFT calculations and XPS analysis.

Philip T. Dirlam - 2016 43 The Use of Polymers in Li-S Battery Cathodes

Figure 1.13. (a) Schematic of the preparation of poly(dopamine) (PD) coated few- layered sulfur nanosheets (FLSNSs). (b) Schematic of the interfaces formed with amide bond formation between the PD coating with poly(acrylic acid) (PAA) binder and carboxylic acid functional multi-walled carbon nanotubes (MWCNTs). (c) Li-S cycling performance of the crosslinked cathode over 500 cycles at a rate of 1 A/g. Note that capacity and rate are based on total cathode mass. Adapted with permission from Ref. 98 © 2013 American Chemical Society.

Philip T. Dirlam - 2016 44 The Use of Polymers in Li-S Battery Cathodes

Figure 1.14. (a) Synthetic scheme for the preparation of poly(dopamine) coated sulfur nanoparticles (S@PD NPs) before and after discharge showing interaction between lithium polysulfide with PD. (b) Cycling performance of Li-S cells fabricated with

1.2.2.2 Polymer Coating of Sulfur/Carbon Composite Electroactive Materials

The use of polymer coatings has also been applied to sulfur/carbon composite electroactive materials. The most prevalent example of this strategy was presented in the seminal work of Nazar et al. where PEG coatings were applied to mesoporous

Philip T. Dirlam - 2016 45 The Use of Polymers in Li-S Battery Cathodes carbon/sulfur (CMK-3/S) composite materials.100 This work was innovative as the first example of using mesoporous carbon as a sulfur host in addition to the demonstration that polymer coating could further enhance the electrochemical performance of the composite materials. Comparison of the cycling performance of CMK-3/S with PEO coated composites (CMK-3/S-PEG) revealed a higher capacity of >1000 mAh/gsulfur after 20 cycles for the PEG coated composite materials relative to ~800 mAh/gsulfur for the uncoated composites (Figure 1.15a). This improved capacity was attributed to the PEO coating inhibiting the dissolution of lithium polysulfides from the cathode as indicated by a lower concentration of sulfur present in the electrolyte solution (Figure 1.15b). Later reports further investigated the utility of PEG encapsulation of C/S composites101-102 where a unique study by Huang et al. extended the strategy of using PEG as a coating to a more complicated coaxial nanocable construct.103 The C/S composites were prepared by coating MWCNTs in PPy which was subsequently carbonized to afford a nitrogen doped porous carbon (NPC) layer. The porous carbon was then infiltrated with sulfur and finally coated with PEG (MWCNT@S/NPC@PEG, Figure 1.16a). The multilayered

MWCNT@S/NPC@PEG coaxial composite materials were found to exhibit higher capacity relative to composites of only S/ NPC with the PEG coated composites exhibiting better capacity retention over 100 cycles (Figure 1.16b). However, The PEG encapsulation was not observed to influence the rate capability to a significant degree with both the coated and non-coated composites exhibiting high rate capabilities (Figure

1.16c) indicating the enhanced rate capability was due to the MWCNT/NPS core.

Philip T. Dirlam - 2016 46 The Use of Polymers in Li-S Battery Cathodes

Figure 1.15. (a) Cycling performance of mesoporous carbon/sulfur composites (CMK- 3/S) and PEG coated carbon/sulfur composites (CMK-3/S-PEG) with inset showing a schematic of the mesoporous carbon infiltrated with sulfur. (b) Plot of sulfur content in the electrolyte solution versus cycle number for CMK-3/S-PEG, CMK-3/S, and a conventional carbon black based sulfur cathode as determined via elemental analysis of the electrolyte solution. Adapted with permission from Ref. 100 © 2009 Nature Publishing Group.

Philip T. Dirlam - 2016 47 The Use of Polymers in Li-S Battery Cathodes

Figure 1.16. (a) Schematic of the preparation of coaxial sulfur/N-doped porous carbon/multi-walled carbon nanotubes encapsulated in poly(ethylene glycol) (S/NPC@MWCNT-PEG). (b) Li-S cycling performance of composites before (S/NPC@MWCNT) and after PEG encapsulation (S/NPC@MWCNT-PEG) compared to composites of only S and NPC. Adapted with permission from Ref. 103 © 2013 Royal Society of Chemistry

Philip T. Dirlam - 2016 48 The Use of Polymers in Li-S Battery Cathodes

Similar to the colloidal/particulate sulfur based systems described above, conjugated polymers including PEDOT,104-106 PANI,107-114 PPy,115-121 and polyphenylene122 have also been extensively studied for the coating of carbon/sulfur composite electroactive materials. A seminal report by Cui et al. expanded on the use of

PEG coatings with CMK-3/S composites discussed previously to using poly(3,4- ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS) in order to better confine lithium polysulfides (Figure 1.17a) and improve ionic and electrical conductivity.104

Comparison of the Li-S cycling performance of bare CMK-3/S with PEDOT:PSS coated

CMK-3/S composites revealed that the conductive polymer coating extended the cycle lifetime from 120 cycles to 175 cycles and reduced the extent of the capacity fade (Figure

1.17b). The efficacy of the PEDOT:PSS coating in mitigating the dissolution of polysulfides is apparent upon comparison of coulombic efficiency (i.e. ratio between discharge and charge capacities) between the bare and coated composites (Figure 1.17c).

The coulombic efficiency of the bare CMK-3/S cathode was 92–94% while the

PEDOT:PSS coating enabled a higher efficiency of 96–98%. This result provided good indication that the polymer coating was effective in reducing the degree of polysulfide dissolution/diffusion as the extent of the overcharging required to complete the cycle was lower than in the uncoated composite.

Philip T. Dirlam - 2016 49 The Use of Polymers in Li-S Battery Cathodes

Figure 1.17. (a) Schematic depicting the mitigation of polysulfide dissolution/diffusion from CMK-3/S composites via conductive polymer coating. (b) Comparison of Li-S cycling performance of bare and PEDOT:PSS coated CMK-3/S composite electroactive materials with (c) coulombic efficiency over first 100 cycles. Adapted with permission from Ref. 104 © 2011 American Chemical Society.

Philip T. Dirlam - 2016 50 The Use of Polymers in Li-S Battery Cathodes

A number of polymers with polar/ionic side chains such as PVP,63 and Nafion,123 in addition to bio-inspired polymers amylopectin124 and poly(dopamine)125-127 have also been explored as a coatings for carbon/sulfur composite electroactive materials. W. Li and coworkers reported an exemplary demonstration of the effectiveness of PD as a coating for both carbon nanotube/sulfur composites in addition to a coating for the separator. The PD coated composites were prepared by first functionalizing the surface of carbon nanotubes via a hydrothermal route with sodium hydroxide in an autoclave to afford hydroxylated carbon nanotubes (HCNTs). Colloidal sulfur was then deposited on the HCNTs via acidic decomposition of sodium thiosulfate affording S-HCNTs.

Subsequently, the base promoted polymerization of dopamine was carried out in the presence of S-HCNTs to install a with a poly(dopamine) membrane coating affording the

PD coated S-HCNTs (PD-S-HCNTs) (Figure 1.18a). This process was demonstrated to be relatively scalable where gram quantities of the PD-S-HCNTs were prepared (Figure

1.18b). Li-S batteries fabricated with the PD-S-HCNTs as the active material and PD coated separators exhibited exceptional cycling performance. The PD coatings enabled ultra-long cycle lifetimes of >3000 cycles with low capacity decay of only 0.018% per cycle at a high rate of 2C (Figure 1.18c). In addition to long-term cyclability the PD based system had a very high rate capability, delivering reversible capacity >700 mAh/gsulfur at a current density of 8.0 A/gsulfur (i.e. rate of 5C) (Figure 1.18d). The notable improvement in device performance was attributed to the PD membrane on the sulfur/carbon composite facilitating ion transport to the electroactive sulfur while being flexible enough to accommodate the volume expansion upon discharge to low order polysulfides. Furthermore, simulations showed a strong binding of sulfide species with

Philip T. Dirlam - 2016 51 The Use of Polymers in Li-S Battery Cathodes the poly(dopamine) present as the active material and separator coating which was correlated to both the reduced capacity fade and improved rate capability observed during

Li-S testing due to the reduction of polysulfide diffusion.

Figure 1.18. (a) Scheme for the preparation of poly(dopamine) coated sulfur/hydroxy functionalized carbon nanotubes (PD-S-HCNTs) with expansion showing schematic representation of the composite material. (b) Photograph showing the scaled up synthesis

Philip T. Dirlam - 2016 52 The Use of Polymers in Li-S Battery Cathodes of PD-S-HCNTs in multigram quantity. Performance of Li-S batteries fabricated with PD-S-HCNTs as the active material and PD coated polyethylene separator showing (c) capacity versus cycle number over 3162 cycles at 2C and (d) capacity at various current densities over 100 cycle periods ranging from 3.2 A/gsulfur to 8.0 A/gsulfur (note: rate of 1C

1.2.2.3 Polymer Coating of the Entire Cathode (Incorporation of a Polymer Interlayer)

The strategy of incorporating of a polymer “interlayer” has also been investigated with the application of a polymer coating to the cathode as a whole. The seminal example utilizing this approach was reported by Wei and coworkers where a PEG coating was applied to a cathode comprised of an array of sulfur-coated carbon nanotubes (Figure

1.19a).95 The cathode fabrication was completed by first synthesizing CNTs via carbonization of xylene with a ferrocene catalyst affording CNT arrays with a height of ca. 500 μm. Sulfur was then incorporated into the CNT array via diffusion of liquid sulfur. Finally the PEG coating was applied via spincoating to afford a ca. 50 μm thick

PEG layer (Figure 1.19b). Comparison of the electrochemical performance of the S-CNT cathodes with and without the PEG barrier revealed that the PEG coated cathode exhibited a higher capacity and nominally better capacity retention over 100 cycles compared to the uncoated S-CNT cathode (Figure 1.19c). The improved capacity was attributed to the PEG coating reducing extent of irreversible electroactive material loss to dissolution. Evaluation of the rate capability of the S-CNT cathodes with and without

PEG coating again showed that the PEG coating was beneficial where notably higher capacities were delivered at rates ≤2C (Figure 1.19d). However at the high rate of 5C the capacity of the PEG coated cathode was reduced dramatically to ca. 150 mAh/gsulfur compared to the uncoated cathode which delivered a capacity of ca. 450 mAh/gsulfur. This

Philip T. Dirlam - 2016 53 The Use of Polymers in Li-S Battery Cathodes indicated that although the PEG coating was effective in mitigating the irreversible loss of soluble polysulfides it impeded the effective diffusion of electroactive material to the conductive framework. EIS analysis of the S-CNT cathodes corroborated this interpretation of the rate study data where the charge transfer resistance of the PEG coated S-CNT cathode was more than twice that of the pristine S-CNT cathode (i.e. 18 Ω and 7.6 Ω respectively).95

The use of conductive polymers as an interlayer has also received attention with examples of electrically conductive polymers such of PANI,96 PPy,128-133 and PEDOT134 recently demonstrated in addition to ionically conductive Nafion based coatings.135-137

Kim et al. reported on the utilization of Nafion coated cathodes with a particular emphasis on the importance of complete cathode coverage for maximizing improved electrochemical performance. Conventional cathodes comprised of sulfur, carbon black, and PVP binder were fabricated on an Al foil current collector. The Nafion coating was installed via a spray coating method utilizing conditions that delivered the Nafion to the cathode surface in a gel state. This unique spray coating approach afforded cathodes completely enveloped in Nafion without penetration of the perfluoroionomer into the porous cathode structure (Figures 1.20a – d). Furthermore the spray coating technique enabled facile tuning of the Nafion loading where the coatings were varied from 0.1 mg/cm2 to 1.6 mg/cm2. Evaluation of the electrochemical performance of the Nafion enveloped cathodes revealed that an intermediate coating density of 0.8 mg/cm2 was ideal for longer term cycling delivering the highest capacity of >800 mAh/gsulfur after 100

2 cycles (NL 0.8, Figure 1.20e). The cathode coated with 1.6 mg/cm exhibited a similarly high capacity however the capacity fade was more pronounced over 100 cycles

Philip T. Dirlam - 2016 54 The Use of Polymers in Li-S Battery Cathodes

Figure 1.19. (a) Schematic and (b) SEM image of the sulfur/CNT array with PEG interlayer coating. (c) Plot of capacity versus cycle number comparing the cycling performance of S-CNT cathode with and without PEG coating at a rate of 0.1C. (d) Rate capability of S-CNT cathode with and without PEG coating at various C-rates ranging from 0.2C to 5C. Adapted with permission from Ref. 95 © 2013 Elsevier Ltd.

(NL 1.6, Figure 1.20e). In general it was observed that any amount of Nafion coating led to better Li-S performance relative to a bare cathode. The coulombic efficiency of the

Nafion coated cathodes was found to improve with increased coating density where the

NL 1.6 cathode had an efficiency >90% over 100 cycles compared with the uncoated cathode which exhibited coulombic efficiency ≤80% (Figure 1.20f).

Philip T. Dirlam - 2016 55 The Use of Polymers in Li-S Battery Cathodes

Figure 1.20. Schematic comparing (a) Nafion coated S/C composite active materials with (b) a Nafion enveloped cathode prepared via spray coating. (c) SEM image of Nafion enveloped cathode with (d) cross sectional image. (e) Li-S cycling performance of Nafion enveloped cathodes over 100 cycles with 0.1 to 1.6 mg Nafion/cm2 loadings and (d) corresponding coulombic efficency. Adapted with permission from Ref. 136 © 2014 John Wiley and Sons.

A unique approach with polymer coating of the cathode was reported by

Goodenough and coworkers where they demonstrated utilizing thiol-ene chemistry to install crosslinked polymer coatings on sulfur/carbon paper cathodes.138 A multifunctional thiol, specifically pentaerythritol tetrakis(3-mercaptopropionate) (PETT), was used in conjuction with a variety of dienes with fluoroalkane (FC), ethylene oxide

(EO), esters, or sulfone linkers (Figure 1.21a). A mixture of the thiol, diene of interest, and photoinitiator was drop cast onto sulfur loaded carbon paper and subsequently irradiated with UV light to promote the thiol-ene reaction affording crosslinked polymer coatings with various structures (Figures 1.21b – e). The sulfur/carbon paper with photocured polymer coatings were utilized directly as the cathode in Li-S test cells. The

PETT-EO and PETT-Ester coatings were found to facilitate the best cycling performance with comparable capacites

Philip T. Dirlam - 2016 56 The Use of Polymers in Li-S Battery Cathodes

Figure 1.21. (a) Monomers utilized in the synthesis of polymer coatings via thiol ene chemistry and (b – e) the resulting crosslinked polymers. (f) Li-S cycling performance of sulfur/carbon paper cathodes with the various crosslinked polymer coatings indicated in (b – c) cycled between 1.5 V and 3.0 V vs. Li/Li+ at a rate of 0.5C. Adapted with permission from Ref. 138 © 2015 The Royal Society of Chemistry.

>800 mAh/gsulfur over the first 100 cycles (Figure 1.21f). After 100 cycles the PETT-EO based cathode had better capacity retention relative to the PETT-Ester coated cathode which exhibited ca. 700 mAh/gsulfur and 600 mAh/gsulfur respectively after 200 cycles.

The PETT-FC cathode delivered considerably lower capacity than the EO and Ester based systems which was similar to uncoated sulfur/carbon paper however with a longer cycle lifetime. The PETT-Sulfone coated cathode exhibited the poorest performance with a very low intial capacity of ca. 300 mAh/gsulfur and a shorter lifetime than the other coated cathodes of 160 cycles. The variable performance of the different polymer coatings was correlated in part with the degree of swelling with electrolyte solution

Philip T. Dirlam - 2016 57 The Use of Polymers in Li-S Battery Cathodes measured for each polymer which was related to the different glass transition temperatures of the polymers. A moderate swelling of 50% by mass was found to be optimal as with the PETT-EO and PETT-Ester based polymers compared to the high degree of swelling with the PETT-FC polymer of nearly 80% and low swelling with the

PETT-Sulfone polymer of 20%.138

1.2.3 Polymeric Electroactive Materials

Polymers have been investigated as electroactive cathode materials for Li-S batteries. The principal function of these polymers is to serve as the source of S-S bonds for electrochemical reduction (discharge) and oxidation (charge). It is desirable for the polymeric active materials to have a high content of S-S bonds which exhibit analogous electrochemical activity to S8 with respect to Li. The high S-S content polymers must also have the capability to participate in repetitive reduction and oxidation in order to facilitate long cycle lifetimes. The role of the non S-S containing component of the

(co)polymers should be considered for imparting useful properties to the polymeric active material towards improving cathode fabrication and performance. It is ideal for the polymeric active materials to be prepared via facile methods amenable to large scales and that the structure contain a majority of S-S bonds in order to maximize the energy density when fabricated into a cathode.

The seminal demonstrations of utilizing polymers with S-S bonds accessible for redox reactions with lithium as polymeric cathode materials for secondary Li batteries was initially described by de Jonghe and coworkers. They presented this concept where a variety of polymers with backbones containing disulfide linkages were prepared via oxidation of the respective di- and trithiol monomers affording materials which they

Philip T. Dirlam - 2016 58 The Use of Polymers in Li-S Battery Cathodes termed solid redox polymerization electrodes (SRPEs) (Figure 1.22).139-141 The SRPEs were successfully demonstrated as functional electroactive materials in cathodes fabricated with carbon black and PEO binder which were assembled into batteries with a solid polymer electrolyte composed of PEO/LiTf and Li metal anode.139-141 Evaluation of the solid-state cells employing the various polydisulfides as electroactive materials revealed the polymer synthesized from 2,5-dimercapto-1,3,4-thiadiazole (DMcT, Figure

1.22b) to exhibit the most promising electrochemical performance. The poly(DMcT) cathode exhibited a cycle life of up to 83 cycles at 75 % utilization of the S-S bond content, however for this performance to be achieved the battery needed to be cycled at elevated temperatures due to slow redox kinetics of the poly(DMcT) and solid-state cell configuration. These promising initial studies inspired further investigation of polydisulfides, especially poly(DMcT), as an electroactive cathode materials for rechargeable Li batteries.142-148 A notable example was described by Sotomura and coworkers where poly(DMcT) was combined with PANI into a composite material that was employed as the active cathode material. It was reported that the incorporation of the

PANI accelerated the redox kinetics of the poly(DMcT) in addition to acting as an additional electroactive material when a broader potential window (4.50 to 2.25 V vs.

Li/Li+) was used during cycling. The DMcT-PANI cathodes were found to be operational at room temperature due to the improved kinetics and exhibited cycle lifetimes of up to 100 cycles with an overall capacity retention of 80 – 90%.142 Polymers with poly- amide,149 anthracene,150-151 aniline,152-153 pyrrole,154 and phenylene155 based main-chains where disulfides were instead positioned in side chains or crosslinks have

Philip T. Dirlam - 2016 59 The Use of Polymers in Li-S Battery Cathodes

Figure 1.22. (a) General scheme for the preparation of polydisulfide electroactive materials from oxidation of thiol monomers with iodine. (b) Chemical structures of the monomers utilized for preparing SRPEs from. Adapted from Refs. 139-141. also been investigated cathode materials in Li batteries, however these materials generally exhibited poor electrochemical performance relative to the polydisulfides.

To prepare polymeric materials with electroactive S-S bonds of higher order

(e.g., trisulfides, tetrasulfides, etc.) the strategy of post-polymerization sulfurization has been investigated. This was first reported by Trofimov and coworkers where a number of polymers including poly(styrene),156-157 poly(vinylpyridine),157 poly(ethylene),157 poly(acetylene),158 and poly(2-thienylthio-2-thienyl-2-methylene) were subjected to deep sulfurization by reaction with elemental sulfur at elevated temperatures. The sulfurization was reported to yield complex mixtures of polyheteroaromatic compounds with polysulfide linkages and sulfur contents of up to 80 wt%. Cyclic voltammetry revealed that the redox behavior of the sulfurized polymers against lithium was largely similar to elemental sulfur, indicating that the materials were suitable as active cathode materials for Li-S batteries. The sulfurized polyacetylene exhibited the highest capacity

Philip T. Dirlam - 2016 60 The Use of Polymers in Li-S Battery Cathodes of the various sulfurized polymers tested as cathode materials with a capacity of >400 mAh/g delivered over 25 cycles.158

Yang et al. later extended this sulfurization strategy to a polyyne analog generated via treatment of poly(vinylidene chloride) with base and subsequent heating with elemental sulfur (Figure 1.23a). The resulting sulfurized polymeric material, termed carbyne polysulfide, was found to contain 54 wt% of S and was tested as a Li-S cathode material with two different electrolytes (Figure 1.23b). The carbyne polysulfide exhibited good electrochemical performance with the standard LiTFSI in DOL/DME electrolyte, with a capacity of >800 mAh/gsulfur delivered over 200 cycles. However, the observed performance with LiPF6 in a carbonate solvent system was better where the carbyne polysulfide cell delivered a reversible capacity of nearly 1000 mAh/gsulfur over

200 cycle. The latter result was interesting as carbonate based electrolytes are typically unstable to the lithium polysulfides generated during Li-S cycling. This indicates the carbyne polysulfide active material does not generate soluble lithium polysulfide redox products like a conventional sulfur based cell. This was also indicated in the discharge curves where the high voltage plateau at ca. 2.4 V, which is attributed to the formation of soluble lithium polysulfides, was not observed.159

Philip T. Dirlam - 2016 61 The Use of Polymers in Li-S Battery Cathodes

Figure 1.23. (a) Synthetic scheme for the preparation of carbyne polysulfide. (b) Cycling performance of carbyne polysulfide with 1M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 v/v) and 1M LiTFSI in dioxolane/dimethoxy ethane (DOL/DME, 1:1 v/v) at a rate of 0.1C between 1.0 – 3.0 V vs. Li/Li+. Adapted with permission from Ref. 159 © 2013 The Royal Society of Chemistry.

The post-polymerization sulfurization of poly(acrylonitrile) (PAN) has received significant attention as a route to a relatively high performing active cathode material.160-

176 The seminal work on this system was completed by Wang and coworkers where they simply heated sulfur and PAN at 300 °C under Ar to afford the S-PAN material.160-161

Evaluation of the electrochemical performance of the S-PAN showed it was effective as an active cathode material where cathodes fabricated with S-PAN exhibited reversible capacity >600 mAh/g over 50 cycles. In these initial reports it was proposed that S-PAN was simply a composite of elemental sulfur and conductive polymer formed from

Philip T. Dirlam - 2016 62 The Use of Polymers in Li-S Battery Cathodes cyclization of PAN and subsequent dehydrogenation of the cyclized structure by sulfur.

This was a reasonable assumption as it is well known that the nitrile side chains on linear

PAN undergo cyclization upon thermal treatment at T > 200 °C to afford a ladder-type polymer which upon dehydrogenation yields a conjugated system of fused nitrogen containing heterocycles.177 However, upon further investigation by Buchmeiser and coworkers a more complex structure was elucidated and it was proposed S-PAN contains thioamide and poly(sulfide) linkages in addition to the PAN derived backbone (Figure

1.24).163

One of the highest performance applications of S-PAN was as an active material was reported by Wang et al. where graphene oxide nanosheets (GNS) were integrated with S-PAN nanoparticles to afford microparticulate composites. The composite active material was prepared by spray drying a dispersion of graphene oxide and colloidal PAN yielding a PAN@GNS precursor that was subsequently pyrolyzed in the presence of elemental sulfur (pPAN-S@GNS, Figure 1.25a). Evaluation of the electrochemical performance of the pPAN-S@GNS showed the composed enabled high capacity of

Philip T. Dirlam - 2016 63 The Use of Polymers in Li-S Battery Cathodes

Figure 1.25. (a) Schematic with corresponding SEM images showing the preparation of pPAN-S@GNS via spray drying a dispersion of graphene oxide (GO) nanosheets (GNS) and PAN nanoparticles followed by pyrolysis in the presence of S8. (b) Cycling performance of pPAN-S@GNS based cells cycled at a rate of 0.2C between 1.0 – 3.0 V vs. Li/Li+. Adapted with permission from Ref. 174 © 2014 John Wiley and Sons.

>1200 mAh/gsulfur with minimal loss over 300 cycles (Figure 1.25b). In addition, the pPAN-S@GNS facilitated notably high rate capability delivering reversible capacity of more than 600 mAh/g up to the very rapid rate of 10C.174

High sulfur content copolymers prepared via inverse vulcanization have been of particular interest as electroactive cathode materials in Li-S batteries178-184 and will be the focus of a number of the studies presented in subsequent chapters. Inverse vulcanization is facile method for preparing polymeric materials with a high content of S-S bonds where molten elemental sulfur is directly utilized as a monomer in a bulk

Philip T. Dirlam - 2016 64 The Use of Polymers in Li-S Battery Cathodes copolymerization.185 By introducing a high degree of branching and enabling the formation of polysulfide loops and rings via connectivity installed with the vinylic comonomer units, the depolymerization that is prevalent in homopolymeric sulfur is suppressed. This allowed for the first time the preparation of a polymeric form of sulfur that is stable at room temperature and amenable to conventional polymer processing.186-

188 Furthermore, the ability to introduce new functionality into these polymeric materials which are majority sulfur via the incorporation of functional comonomers presents a unique opportunity for overcoming some of the detrimental properties of S8 such as its extremely high resistivity.189

The initial report on high sulfur content copolymers prepared via inverse vulcanization focused on the use of 1,3-diisopropenylbenzene (DIB) as a comonomer affording poly(sulfur-r-1,3-diisopropenylbenzene) (poly(S-r-DIB, Figure 1.26).185 The

S-S bonds comprising the Poly(S-r-DIB) copolymer structure were shown to have analogous electrochemical reactivity compared to elemental sulfur enabling the successful demonstration of the copolymer as an active material in Li-S cathodes.185

Further investigation of poly(sulfur-r-DIB) as an electroactive material in Li-S cells revealed that copolymers prepared with a high sulfur content of 90 wt% were optimal for delivering high capacity (>1000 mAh/gsulfur, Figure 1.27a) and long cycle lifetimes of up to 500 cycles (Figure 1.27b). It is proposed that the extended cycle lifetime and enhanced capacity retention is due to the comomomer units forming organosulfides upon discharge which act as an adventitious impurity that co-deposit with lower order polysulfides that effectively “plasticize” the intractable Li2S chalks and suppress irreversible deposition.190 In addition, the inverse vulcanization process has the

Philip T. Dirlam - 2016 65 The Use of Polymers in Li-S Battery Cathodes advantage of being amenable to large scale production where poly(S-r-DIB) electroactive materials were successfully prepared on kilogram scale.191

Figure 1.26. Scheme showing the preparation poly(sulfur-r-1,3-diisopropenylbenzene) (poly(S-r-DIB) via inverse vulcanization of elemental sulfur.

1.3 Summary The critical role polymeric materials have played towards optimizing lithium- sulfur battery cathode performance has been discussed. First, an overview of the basic components and operating principals of a conventional Li-S battery constructed with a composite cathode comprised of electroactive material, conductive carbon, and polymer was described. The key considerations for the polymer binder were then discussed where the specific need for an alternative binder to PVDF capable of accommodating the notable volume change of the Li-S redox products was emphasized. The use of polymers as coatings for both the electroactive material and the cathode as a whole was then discussed, noting the benefits provided by confining the Li/S redox reactions to the cathode in reducing capacity loss and extending device lifetimes by mitigating

Philip T. Dirlam - 2016 66 The Use of Polymers in Li-S Battery Cathodes

Figure 1.27. (a) Electrochemical performance of poly(S-r-DIB) prepared with various sulfur:comonomer compositions. (b) Long term cycling and of Li-S battery with poly(S- r-DIB) prepared with 90 wt% sulfur and 10 wt% DIB at a rate of 0.1C with inset showing rate study of capacity at various current densties. All cycling performed between 1.7 V and 2.6 V vs. Li/Li+. Reproduced with permission from Ref. 192 © 2014 American Chemical Society. polysulfide dissolution. Finally the use of polymers as the electroactive material was presented and the particular utility of high sulfur content copolymers prepared via inverse vulcanization was highlighted as this will be the focus of a number of the following chapters.

Philip T. Dirlam - 2016 67 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization

2. SINGLE CHAIN POLYMER NANOPARTICLES VIA SEQUENTIAL ATRP AND OXIDATIVE POLYMERIZATION

With contributions from Kyle J. Arrington (synthesis of ProDOT-Sty monomer), Hyo Ju

Kim (synthetic support), Woo Jin Chung (synthetic support), Rabindra Sahoo (design of

ProDOT-OH synthesis), Lawrence J. Hill (electron microscopy), Kookeon Char (support

of Hyo Ju Kim), and Jeffrey Pyun (co-author and PI).

Reproduced in part from Ref. 193 by permission of The Royal Society of Chemistry

2.1 Introduction The creation of polymeric nanoparticles (PNPs) 194-197 has attracted significant

attention over the past decade as a route to well-defined, functional nanomaterials.

Synthesis and processing advances of these materials have enabled precise control of

composition, size and functionality to impart intriguing new properties for their potential

Philip T. Dirlam - 2016 68 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization use in drug delivery,198-202 polymer blends, 203-206 biological imaging,207-210 chemosensors,211 optoelectronics,212 and biomimetic sytems.202, 213 In general, a number of different “top-down” and “bottom-up” techniques have been developed for the synthesis of polymeric colloids, depending on the size, morphology and functionality of targeted materials. The vast majority of reports on the preparation of polymeric colloids utilized dispersed media methods (i.e., emulsion, suspension, and dispersion polymerizations).214 The application of lithographic approaches has recently been reported for the preparation of micro- and nanoscopic polymeric colloids based on the

PRINT methodology.215-217 For the preparation of well-defined polymeric nanoparticles, lyotropic self-assembly of amphiphilic block copolymers has widely been used, most notably in the formation of shell-crosslinked NPs (SCKs) as a route to sub 100 nm-sized colloids of precise size and functionality.218-225

An alternative approach to the synthesis of PNPs with sub-20 nm size dimensions has been accomplished via the intramolecular crosslinking of dendritic and linear polymer precursors. 210, 226-231 While dendrimers are inherently pure compounds with precise placement of functionality, the use of dendritic precursors for the preparation of

PNPs is complicated by the multi-step synthesis required to prepare these materials.

Hence, the preparation of PNPs has been explored via the preparation of functional, linear (co)polymer precursors, followed by intramolecular crosslinking, or non-covalent intramolecular interactions. The marriage of controlled radical polymerization (CRP) with these intramolecular crosslinking reactions was elegantly pioneered by Hawker et al.232 to prepare a variety of well-defined PNPs. This approach has utilized CRP (e.g.,

ATRP,233 NMP,234-235 RAFT236) to synthesize well-defined linear polymers containing

Philip T. Dirlam - 2016 69 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization functional comonomer units carrying orthogonal, and reactive moieties to promote single chain collapse. Recent advances in macromolecular engineering have enabled similar advances in the preparation of “folded” single chain polymers via the installation of intramolecular chemical junctions.197, 237

The concept of intramolecularly crosslinked polymer chains was initially described by Martin and Eichinger238-239 and has since been elaborated with improved synthetic methods. The seminal work of Hawker et al., employed linear polystyrenic copolymers synthesized by NMP which were subsequently thermally treated to trigger dimerization of benzocyclobutene (BCB) units.232 This general method has been utilized for the preparation of a variety of PNPs.203, 226, 240 Consequently, advances in the synthesis of PNPs via the intramolecular chain collapse methodology have proceeded to create polymeric colloids with more well-defined translation of the primary copolymer sequence into intramolecularly collapsed PNPs with controllable 3-D architectures.

Numerous synthetic strategies have been developed to prepare the linear copolymer precursors from a variety of living and/or controlled polymerization methods, along with the development of novel intramolecular crosslinking or association chemistry. Linear copolymer precursors have been synthesized by controlled radical polymerization techniques using ATRP,241-245 RAFT,206, 209, 223, 246-255 and NMP,208, 218, 222, 226, 240, 256-258 as well as ring-opening metathesis polymerization (ROMP),218, 222, 259-264 acyclic diene metathesis polymerization (ADMET),262-263 and single electron transfer-living radical polymerization (SET-LRP).265 Intramolecular crosslinking chemistries have been developed based on thermally triggered crosslinking of vinylbenzylsulfones,208, 256 benzoxazines,252 and retro Diels-Alder,257 as well as a BCB-amine grafted onto

Philip T. Dirlam - 2016 70 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization poly(acrylic acid) that dimerized at lower temperatures.255 Other crosslinking chemistry that has been employed to prepare intramolecularly collapsed polymers includes Grubbs’ cross metathesis,266 free radical polymerization,246, 258 azide-alkyne206, 209, 237, 247, 254 and thiol-ene201, 262, 264 click chemistries. More recently, the concept of intramolecular collapse of single polymer chains has been conducted, exploiting supramolecular associations via hydrogen bonding to form PNPs. 243-244, 248, 259-260 Furthermore, the preparation of these types of single chain polymer nanoparticles has spurred the developement of new crosslinking or self-association chemistry based on N-alkylation of

PVP,267 urea linkages generated via isocyanates,223 Bergman cyclization of ene-diynes,212,

242, 265 coupling via the Curtius rearrangement,249 C-alkylation via a nitrene intermediate,251 Glaser-Hay coupling,253 guest-host interactions,245 reversible disulfide bridging,261 vulcanization,218 carbodiimide coupling,222 amide bond formation,264 and photocrosslinking of methacrylates,241 cinnamoyl groups268 and coumarins.250

In all of these existing single chain polymer nanoparticle systems, the crosslinking moieties primarily serve to permanently “lock-in” the 3-D nanoparticle architecture formed in dilute polymer solutions. Hence, there is a clear opportunity to develop intramolecular crosslinking chemistry that concurrently promotes NP formation and incorporates new functionality into the polymeric nanoparticle. The incorporation of

(opto)electroactive species via electropolymerization of side chains groups (e.g., thiophenes) is an attractive example of this approach to create conjugated polymeric inclusions within the crosslinked colloid. A related approach by Advincula and coworkers demonstrated the preparation of conjugated shell-crosslinked nanoparticles by the synthesis and subsequent electropolymerization of Fréchet-type benzyl ether

Philip T. Dirlam - 2016 71 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization dendrimers carrying peripheral carbazole moieties.229, 269 However, this approach has not been extensively investigated for single chain polymer nanoparticles prepared via intramolecular crosslinking, or association methods.

The ability to prepare single chain polymeric nanoparticles with conjugated polymer inclusions is facilitated by access to a multifunctional monomer that can be polymerized via CRP and oxidative polymerization in a sequential fashion. Monomers and conjugated polymers have been explored based on acrylic functional thiophenes,270-

274 alkylenedioxythiophenes,275-277 carbazoles,273 and fluorenes,273 norbornyl functional alkylenedioxythiophenes,278-280 and styrenic functional ethylenedioxythiophene.281 These resulting copolymers have been utilized for photolithography,270-271 the creation of conductive ultra-thin films,272, 274 tuning the absorbance of optoelectronic materials,278 fabricating transparent conductive thin films,275 optical diffraction gratings,271 and large- area electrochromic films.276

Herein, we report on the synthesis, ATRP and intramolecular crosslinking of a novel styrenic functional propylenedioxythiophene (ProDOT-Sty) to prepare single chain polymer nanoparticles. ProDOT-based monomers have widely been investigated as a route to electroactive polythiophenes that can be readily modified.282-287 To our knowledge, this is the first example of this type of methodology using a styrenic-ProDOT monomer in conjunction with CRP and oxidative polymerization. Exploitation of

ProDOT side chains via electrochemical processes resulted in intramolecular crosslinking of pendant thiophene side chain groups to afford single chain polymer nanoparticles, where interconnected ProDOT moieties both chemically crosslinked the polymer and created electroactive pathways within newly formed colloids.

Philip T. Dirlam - 2016 72 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization

2.2 Results and Discussion

2.2.1 Synthesis and ATRP of ProDOT-Sty

Figure 2.1. Synthetic schemes for preparation of (a) ProDOT-Sty monomer and (b) homopolymers of poly(ProDOT-Sty) via ATRP.

In the first step of the total synthesis, ProDOT-Sty (2, Figure 2.1) was synthesized via the transetherification of 3,4-dimethoxythiophene with 1,1,1- tris(hydroxymethyl)methane to form ProDOT-OH (1, Figure 3.1a), which was further esterified with 4-vinylbenzoic acid by DCC coupling to yield ProDOT-Sty as a white solid in reasonable overall yield (64%, Figure 2.1a). This synthetic route was attractive since all of the starting materials were commercially available and the methodology involved in the synthesis took advantage of standard synthetic transformations to yield

ProDOT-Sty in large quantities for further use in polymerizations. Moreover, the

ProDOT-Sty monomer exhibited significant handling and storing advantages, in comparison with (meth)acrylate derivatives of ProDOT-OH, which autopolymerized within a few days, despite storage at low temperature (< 10 °C).

Philip T. Dirlam - 2016 73 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization

With the ProDOT-Sty monomer in hand, investigation into the ATRP of this compound was conducted to ascertain conditions to prepare well-defined poly(ProDOT-

Sty) homopolymers. While a number of catalytic systems were investigated, the use of a copper(I) halide salt, with either N,N,N’,N’,N’-pentamethyldiethylenetriamine

(PMDETA) or 4,4’-dinonyl-2,2’-bipyridine (dNbpy) ligands were observed to be the most efficient. Initial comparisons of the polymerization kinetics of ProDOT-Sty by

ATRP revealed significantly higher reactivity relative to conventional styrenic monomers, requiring lower reaction temperature and the use of reduced catalyst loading.

The optimal catalytic system for the ATRP of ProDOT-Sty was determined to be

Cu(I)Br/PMDETA, primarily due to the reduced cost of PMDETA vs. dNbpy. The

ATRP of ProDOT-Sty required the monomer to be dissolved in anisole (1.22 M) as the monomer was a white crystalline solid and ethyl α-bromoisobutyrate (1.22 x 10-2 M) was used as the initiator with the CuIBr/PMDETA catalyst system (1.22 x 10-2 M) at T = 80

I °C ([M]0:[I]0:[Cu ]:[PMDETA] = 100:1:1:1, see Figure 2.1).

The kinetics of homopolymerization of ProDOT-Sty by ATRP was tracked using both 1H NMR spectroscopy and SEC, where the semilogarithmic plot of monomer consumption and percent conversion versus time predominantly revealed a linear correlation of ln([M]0/[M]) versus time indicative of a constant number of active chains throughout the polymerization. Under these reaction conditions, high monomer conversions (74 %) were achieved within 9 hrs (540 min) (Figure 2.2a). The evolution of apparent molar mass from SEC versus monomer conversion (Figure 2.2b) was also observed to exhibit a linear correlation, confirming that the controlled polymerization

Philip T. Dirlam - 2016 74 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization was achieved under these conditions. Deviation from theoretical molecular weight was observed and

Figure 2.2. a) First-order plot of monomer conversion vs. time and b) plot of number average molecular weight and polydispersity vs monomer conversion ([M]o : [I]o : [L] : [CuI] = 100 : 1 : 1 : 1, 80°C).

Philip T. Dirlam - 2016 75 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization

Figure 2.3. GPC traces during the ATRP of ProDOT-Sty at increasing reaction time I ([M]o : [I]o : [L] : [Cu ] = 100 : 1 : 1 : 1, 80°C). attributed to the deviation of hydrodynamic volume of poly(ProDOT-Sty) relative to PS standards. SEC traces of kinetic samples from the ATRP of ProDOT-Sty (Figure 2.3) progressively shifted toward shorter elution times, ultimately yielding uniform molecular weight distributions. From SEC, the apparent molar masses of poly(ProDOT-Sty) homopolymers prepared via ATRP could be readily controlled (Mn = 2,000-12,500 g/mol) while maintaining low polydispersity (Mw/Mn < 1.15; Figures 2.2b, 2.3).

2.2.2 Preparation of SCPNs from poly(ProDOT-Sty) precursors

For the preparation of polymeric nanoparticles, homopolymers of poly(ProDOT-

Sty) (Mn apparent = 11,400 g/mol; Mw/Mn = 1.11) were polymerized in solution to promote intramolecular crosslinking of poly- and oligo(ProDOT) units. In these series of experiments, the pendant ProDOT side groups of linear poly(ProDOT-Sty) precursors

Philip T. Dirlam - 2016 76 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization were oxidatively coupled (Figure 2.4a) primarily in an intramolecular fashion to afford crosslinked polymeric nanoparticles with interconnected ProDOT units bound to a polystyrenic backbone. Solution electropolymerizations of linear poly(ProDOT-Sty) precursors were conducted at fairly dilute concentration (c = 5 mg/mL of homopolymer) to suppress extensive intermolecular coupling. These oxidative polymerizations were

Figure 2.4. Synthetic route and graphical representation of (a) the oxidative polymerization of pendant ProDOT groups of poly(ProDOT-Sty) yielding conjugated polymeric nanoparticles and (b) their subsequent reduction to poly(vinylbenzyl alcohol) and poly(ProDOT-OH).

Philip T. Dirlam - 2016 77 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization

Figure 2.5. (a) Semi-logarithmic plot of monomer consumption with time of the oxidative polymerization of poly(ProDOT-Sty) into polymeric nanoparticles as monitored by 1H NMR spectroscopy (b) Plot of apparent number average molecular weight vs. conversion and polydispersity of poly(ProDOT-Sty) during oxidative polymerization.

Philip T. Dirlam - 2016 78 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization

conducted with an excess of iron(III) chloride (FeCl3) as the oxidizing agent in a chloroform solution of the homopolymer (where [ProDOT] = 0.3 M) at T = 50 °C for 5 hrs. This poly(ProDOT-Sty) precursor solution was initially clear and colorless, but yielded a dark blue solution after treatment with FeCl3, which qualitatively confirmed chemical connectivity of ProDOT side chain moieties into conjugated poly- and oligo(ProDOT) units. Since the poly(ProDOT-Sty) homopolymer was initially soluble in CHCl3 and CDCl3, the kinetics of electrochemical polymerization of ProDOT side chains could be monitored (Figure 2.5) by both 1H NMR spectroscopy (as noted by the consumption of aromatic protons on thiophene rings at δ  6.1 ppm, see Appendix A for spectra) and by the changes in the apparent molar mass using SEC (Figure 2.6).

The kinetics of the oxidative polymerization in the semilogarithmic plot of

ProDOT side chain group electropolymerization (Figure 2.5a) revealed a nonlinear correlation of ln([M]0/[M]) versus time and a ProDOT conversion of almost 30% after 5 hrs of reaction time. At the early stages of the oxidative polymerization, the reaction mixture was still homogeneous, enabling solution characterization by NMR spectroscopy and SEC. However, soon after a reaction time of 5 hrs, the formation of a blue-colored precipitate was observed, which was indicative of intermolecular coupling of thiophene moieties from poly(ProDOT-Sty) linear precursors. SEC of aliquots taken from soluble samples from these oxidative polymerizations over various reaction times confirmed the formation of intramolecularly crosslinked polymer nanoparticles, as noted by the contraction of apparent molecular mass with the increase of thiophene conversion

(Figures 2.5b, 2.6). The initial increase in polydispersity after the first hour of the

Philip T. Dirlam - 2016 79 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization

Figure 2.6. GPC traces during oxidative polymerization of pendant ProDOT groups on poly(ProDOT-Sty) at increasing reaction time.

Figure 2.7. Optical absorption spectra of poly(ProDOT-Sty) before and after oxidative polymerization of pendant ProDOT groups affording SCPNs.

Philip T. Dirlam - 2016 80 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization

Figure 2.8. GPC traces of poly(ProDOT-Sty) SCPNs and poly(ProDOT-OH) isolated after reductive degradation of the SCPNs with LiBH4 utilizing a UV detector set at 350 nm. oxidative polymerization was attributed to some extent of intermolecular coupling, after which the gradual focusing of PDI occurred as expected with the formation of PNPs

(Figure 2.6). SEC revealed the emergence of a higher molecular weight shoulder after longer reaction times, indicative of intermolecular coupling (of different polymer nanoparticles) during the oxidative polymerization.

In addition to NMR spectroscopy and SEC characterization, optical absorbance spectroscopy was conducted to confirm the formation of poly-, or oligo(ProDOT) inclusions within polymeric nanoparticles (Figure 2.7). The solution spectrum of electropolymerized polymer nanoparticles revealed two broad absorbance peaks centered around 475 nm and 825 nm, which was consistent with the formation of neutral and

Philip T. Dirlam - 2016 81 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization doped polythiophenes based on ProDOT units. Doping of polythiophene inclusions was attributed to the presence of excess FeCl3 present in the oxidative polymerization.

Further chemical evidence for the formation of electrochemically polymerized

ProDOT side chain groups from linear poly(ProDOT-Sty) precursors was sought out by reductive degradation experiments to cleave the polystyrenic backbone from electrochemically fused ProDOT units (Figure 2.4b). In these experiments, the ester linkage within electrochemically polymerized polymeric nanoparticles was cleaved with lithium borohydride (LiBH4) to afford poly(vinylbenzyl alcohol) and poly- or oligo(ProDOT-OH). The complete reduction of ester linkages in electropolymerized polymer nanoparticles was confirmed by 1H NMR spectroscopy (see Appendix A) and the resulting mixture of poly(vinylbenzyl alcohol) and poly(ProDOT) was subject to GPC analysis using UV detection set at 350 nm as to observe only the bathochromically shifted conjugated species (Figure 2.8). The SEC of electrochemically polymerized polymeric nanoparticle before LiBH4 treatment exhibited a retention time around 19 min

(Mn apparent = 8,900 g/mol; Mw/Mn = 1.22), which drastically increased to longer retention time and lower apparent molecular weight (Mn apparent = 500; Mw/Mn = 1.65) after reductive degradation.

A direct comparison of poly(ProDOT-Sty) nanoparticles and linear poly(ProDOT-

Sty) precursors was conducted of solution cast samples on carbon coated TEM grids to ascertain the effect of oxidative coupling of ProDOT units on the shape persistence of these materials (Figure 2.10). For this particular study, linear poly(ProDOT-Sty) precursors of slightly higher molar mass (Mn = 36,000; Mw/Mn = 1.6) were utilized.

Philip T. Dirlam - 2016 82 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization

Figure 2.9. Synthetic route and graphical representation of the oxidative polymerization of pendant ProDOT groups on poly(ProDOT-Sty)-b-poly(tert-butyl acrylate) yielding a hybrid linear-nanoparticle polymer architecture.

Figure 2.10. TEM images of (a) poly(ProDOT-Sty) nanoparticles and (b) pristine poly(ProDOT-Sty) drop cast from 0.05 mg/mL solutions in chloroform and stained with

RuO4 (scale bars = 100 nm).

Philip T. Dirlam - 2016 83 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization

TEM imaging of the linear poly(ProDOT-Sty) precursors showed contiguous, featureless macromolecular assemblies, consistent with polymers of a linear architecture.

Conversely, after oxidative coupling of ProDOT units, TEM imaging confirmed the presence of globular-like features with nominal sizes of 16-18 nm. While some coalescence of the nanoparticle features was observed, there was a significantly higher fraction of discrete globular features observed relative to the interdigitated, featureless, thin films imaged from the solution cast linear precursor. Similar trends in the comparative morphology of linear vs. single chain nanoparticles have been reported for

BCB functional block copolymers.226, 240

2.2.3 Synthesis and SCPN formation of poly(tert-butyl acrylate)-b- poly(ProDOT-Sty)

Finally, to suppress the intermolecular coupling during electrochemical polymerization, the preparation of poly(tert-butyl acrylate)-b-poly(ProDOT-Sty) (ptBA- b-poly(ProDOT-Sty)) block copolymers was completed where the ptBA block was introduced to sterically impede intermolecular crosslinking events. Linear copolymers of ptBA-b-poly(ProDOT-Sty) were prepared by the sequential polymerization of tert-butyl acrylate, followed by chain extension with ProDOT-Sty via ATRP. In these experiments, a ptBA macroinitiator (Mn SEC = 8,400 g/mol; Mw/Mn = 1.08) was first prepared by similar conditions employed for the hompolymerization of ProDOT-Sty by ATRP.

Comparative SEC traces (Figure 2.11) elucidated a shift to higher molecular weight after the chain extension to form a linear copolymer of poly(tert-butyl acrylate)-b- poly(ProDOT-Sty) (Mn = 19,100 g/mol; Mw/Mn = 1.15) with retention of low polydispersity. Copolymer composition and molecular weight determined by 1H NMR

Philip T. Dirlam - 2016 84 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization

Figure 2.11. GPC traces of the poly(t-butyl acrylate) macroinitiator ( ) (Mn = 8400 g/mol, PDI = 1.08) and poly(tert-butyl acrylate)-b-poly(ProDOT-Sty) before ( ) (Mn

= 19,100, PDI = 1.15) and after ( ) (Mn apparent = 17,200 g/mol, PDI = 1.19) oxidative polymerization.

spectroscopy were in reasonable agreement with SEC values (Mn NMR = 21,300 g/mol).

With the block copolymer in hand, the ProDOT side chain groups of linear ptBA- b-poly(ProDOT-Sty) precursors were then oxidatively polymerized by treatment with

FeCl3 to form particle-coil like block copolymers with a hybrid linear-nanoparticle architecture (Figure 2.9). The comparative SEC traces of poly(tert-butyl acrylate)-b- poly(ProDOT-Sty) after the oxidative polymerization of pendant ProDOT groups showed a shift to a smaller apparent molecular weight (Mn SEC =19,100 g/mol to Mn SEC = 17,200 g/mol, Figure 2.11). The reduction in hydrodynamic volume relative to the linear block copolymer was again attributed to the intramolecular oxidative coupling and chain

Philip T. Dirlam - 2016 85 Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative Polymerization collapse. The SEC trace corresponding to the post-oxidative polymerization shows a small shoulder at higher molecular weight compared with the peak molecular weight.

This could be attributed to some degree of intermolecular coupling during oxidative polymerization, or a small degree of thermal self-initiation during the ATRP of ProDOT-

Sty.226 Nevertheless, both the low polydispersity and prolonged colloidal stability of the oxidatively polymerized hybrid linear-nanoparticle copolymers confirmed the viability of this approach using ptBA as steric stabilizing segments to suppress the intermolecular crosslinking of ProDOT side chain groups. The hybrid linear-nanoparticle copolymers were found to remain in solution even at long reaction time in the presence of FeCl3, confirming that that intermolecular coupling was successfully suppressed. Optical absorbance spectroscopy of these electrochemically polymerized copolymer nanoparticles also confirmed the formation of chemically connected conjugated ProDOT units (see Appendix A).

2.3 Conclusion We report on a new synthetic approach to prepare single chain polymer nanoparticles

(SCPNs) containing conjugated ProDOT units via the combination of ATRP and electrochemical polymerization. The synthesis of a novel ProDOT functional styrenic monomer enabled the preparation of well-defined polystrenic linear precursors via ATRP that were subsequently oxidatively polymerized to promote tandem intramolecular crosslinking of ProDOT side chains and installation of electroactive moieties. Future work with these materials will examine the electrochemical properties of these systems, along with the investigation of charge transfer processes through single chain polymer nanoparticles with poly(ProDOT) inclusions.

Philip T. Dirlam - 2016 86 3. IMPROVING THE CHARGE CONDUCTANCE OF ELEMENTAL SULFUR VIA TANDEM INVERSE VULCANIZATION AND ELECTROPOLYMERIZATION

With contributions from Adam G. Simmonds (assistance with electrochemical

characterization), R. Clayton Shallcross (assistance with electropolymerization), Kyle J.

Arrington (synthesis of ProDOT-Sty monomer), Woo Jin Chung (synthetic support),

Jared J. Griebel (assistance with polymer processing and characterization), Lawrence J.

Hill (electron microscopy), Richard S. Glass (concept development and experimental

design), and Jeffrey Pyun (co-author and PI).

Reproduced in part from Ref. 189 by permission of The American Chemical Society.

Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse Vulcanization and Electropolymerization

3.1 Introduction The direct use of elemental sulfur as a feedstock for the preparation of high sulfur content copolymers is an attractive route to a unique class of materials exhibiting intriguing electrochemical and optical properties for lithium-sulfur (Li-S) batteries,288-292 infrared optics,186 and nanomaterials.293-295 We recently developed a polymerization methodology termed inverse vulcanization where molten elemental sulfur is employed as both a comonomer and the solvent in a facile route to polymeric materials with a high content of sulfur.288 Our initial reports on inverse vulcanization have demonstrated that the intrinsic chemical instability of polymeric sulfur,296 along with its poor processing

297 characteristics can be circumvented by copolymerization of S8 with 1,3- diisopropenylbenzene (DIB) yielding poly(sulfur-random-1,3-diisopropenylbenzene)

(poly(S-DIB)) to form chemically stable and processable materials. Synthetic accessibility to these very high sulfur content copolymers enabled the application of these materials in high refractive index IR optics186 and as active cathode materials in Li-S batteries.289-290

Despite the advantages afforded by inverse vulcanization, numerous synthetic challenges remain which preclude optimal performance of sulfur and sulfur based copolymers in target applications such as Li-S batteries. The extreme resistivity of sulfur

15 13 (ca. 10 Ω·m) is a principal limitation in fully exploiting its exceptional capacity versus lithium in electrochemical devices. The highly resistive nature of S8 is commonly addressed by blending with conductive carbons, or fabrication of nanocomposite materials with conductive inclusions.298 We aim to overcome this limitation by expanding the scope of inverse vulcanization to include functional comonomers designed to enhance the electrical properties of the resulting sulfur copolymer. These new functional sulfur

Philip T. Dirlam - 2016 88 Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse Vulcanization and Electropolymerization copolymers with both improved electrical and electrochemical properties are anticipated to also produce improved polymeric cathode materials for Li-S batteries.299

Synthetic accessibility to functional 3,4-propylenedioxythiophene (ProDOT) monomers300 provided an attractive route to prepare electroactive comonomer for inverse vulcanization with sulfur. Such ProDOT derivatives have enabled direct wiring of conductive polymer pathways through nanocomposite thin films301 and been shown to be readily processable.302 Furthermore, the efficacy of polymers bearing pendant electropolymerizable moieties as precursors to materials with conjugated polymer inclusions has been aptly demonstrated with conventional polymer systems by a number of groups,273, 303 notably Sotzing et al.279, 304-307 and Advincula et al.274, 308-309

Herein, we report the incorporation of functional styrenic comonomers, specifically with pendant 3,4-alkylenedioxythiophene groups (i.e., ProDOT-Sty, Figure

3.1) into high sulfur content copolymers via inverse vulcanization. This key synthetic advance allowed for the installation of electroactive side chain groups that were post- electropolymerized. We previously established the utility of ProDOT-Sty for synthesizing polymeric materials with electroactive polymer components via sequential controlled radical and oxidative polymerization.193 Furthermore, we found that ProDOT-Sty was miscible with liquid sulfur enabling direct copolymerization via free radical processes.

Using this approach, soluble, and chemically stable sulfur copolymer precursors carrying

ProDOT side chain groups were solution processed into thin films onto supporting electrode substrates. Post-electropolymerization then enabled preparation of high sulfur content copolymers with conjugated poly(ProDOT) segments. The electrical properties of this novel sulfur copolymer were interrogated with electrochemical impedance

Philip T. Dirlam - 2016 89 Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse Vulcanization and Electropolymerization spectroscopy (EIS) which revealed a greater than 95% decrease in the charge transfer resistance (RCT) upon installation of the conductive polymer pathways.

3.2 Results and Discussion

3.2.1 Preparation of Sulfur Rich Copolymer with Electropolymerizable Side Chains

To prepare a sulfur rich copolymer with electropolymerizable side chains via inverse vulcanization a solution of ProDOT-Sty in liquid elemental sulfur. The polymerization was thermally initiated using homolytic ring-opening polymerization

(ROP) of S8 to form copolymers of ProDOT-Sty and sulfur which were successively reacted with DIB to yield poly(ProDOT-Sty-random-DIB-random-sulfur) (ProDIBS).

ProDIBS was then solution processed into thin films on indium tin oxide (ITO) and subjected to electropolymerization conditions to form interconnected poly(ProDOT) segments in the sulfur copolymer matrix (Figure 3.1).

ProDOT-Sty was chosen as the multifunctional monomer to enable post- electropolymerization for its chemical compatibility with sulfur. The styrenic component provided reactivity amenable to radical copolymerization with sulfur and the relatively nonpolar aromatic construct favored miscibility with liquid sulfur. The use of ProDOT-

Sty was also attractive due to its straightforward synthesis from

Philip T. Dirlam - 2016 90 Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse Vulcanization and Electropolymerization

Figure 3.1. Synthetic scheme for the preparation of sulfur and thiophene based copolymers using tandem inverse vulcanization and electropolymerization of elemental sulfur and ProDOT-Sty. commercially available starting materials where briefly, transetherification of 3,4- dimethoxythiophene with tris(hydroxymethyl)ethane followed by a Stieglich esterification with 4-vinylbenzoic acid yielded the styrenic functional thiophene in large quantities and good yield. Successful incorporation of ProDOT-Sty into a chemically stable high sulfur content copolymer necessitated a two-step terpolymerization sequence.

Philip T. Dirlam - 2016 91 Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse Vulcanization and Electropolymerization

Initially ProDOT-Sty was reacted with liquid sulfur diradicals to afford linear copolymeric species. However, linear copolymers of sulfur and vinylic comonomers have also been found to suffer a similar propensity for depolymerization at RT as in the case of polymeric sulfur. Hence, a modest amount of DIB was added after the initial poly(ProDOT-Sty-r-sulfur) copolymerization, which afforded chemically stable, glassy copolymers. The effective suppression of depolymerization afforded by DIB also introduced branching and presumably intramolecular “loops” formed by termination of sulfur radicals.288

1H NMR spectroscopy of the ProDIBS terpolymer confirmed the successful copolymerization of elemental sulfur, ProDOT-Sty and DIB (comonomer feed ratios of

50:40:10 wt%, respectively) with complete conversion of the vinylic groups and retention of the electropolymerizable ProDOT functionality in the inverse vulcanization step (see

Appendix B). Size exclusion chromatography (SEC) in tetrahydrofuran (THF) indicated that terpolymers possessed low molar mass and relatively high polydispersity (Mn apparent =

2000 g/mol; Mw/Mn = 1.75) consistent with earlier results for branched poly(S-DIB) copolymers.

3.2.2 Processing of ProDIBS Thin Films on ITO Electrodes with Subsequent Electropolymerization

Thin ProDIBS films on ITO electrodes were fabricated by solution processing for performing electrochemical experiments. Films were spun cast from a copolymer solution in CHCl3/toluene onto freshly cleaned ITO coated glass affording homogeneous films with thickness of 88 nm as determined by scanning electron

Philip T. Dirlam - 2016 92 Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse Vulcanization and Electropolymerization

Figure 3.2. Schematic of solution processing and electrochemical oxidation of ProDIBS films with representative cross sectional SEM images (a) before and (b) after electropolymerization to install conjugated poly(ProDOT) inclusions.

Philip T. Dirlam - 2016 93 Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse Vulcanization and Electropolymerization

Figure 3.3. Cyclic voltammogram of potentiodynamic oxidative polymerization scans 5, 10, 15, 20, and 30 of ProDIBS thin film on ITO. microscopy (SEM) imaging of a film cross-section (Figure 3.2a).

To enable oxidative polymerization of the ProDOT side chains a three-electrode electrochemical cell was employed with the ProDIBS coated ITO as the working electrode and a platinum mesh counter electrode referenced to Ag/AgNO3 (all potentials reported herein are referenced to Ag/Ag+). The effective area of the ProDIBS film on

ITO (1 cm2) was immersed in supporting electrolyte (100 mM TBAFP/acetonitrile) and electrochemical polymerization of pendant ProDOT moieties was carried out with potentiodynamic scanning between -0.5 V and 1.2 V (Figure 3.3). An irreversible oxidation was observed upon scanning positive of 1.0 V corresponding to ProDOT oxidation and subsequent electropolymerization.

Following the electropolymerization, the appearance of a redox couple at E1/2 ≈

0.3 V was observed and shown to increase in peak current upon successive scans (Figure

3.3). This behavior is characteristic of oxidative doping and reductive dedoping of a

Philip T. Dirlam - 2016 94 Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse Vulcanization and Electropolymerization conjugated polymer with the increasing peak current indicative of conjugated polymer growth.310 After installing conductive polymer pathways in the ProDIBS film cross sectional SEM confirmed integrity of the film was retained during electrochemical treatment which was accompanied by a slight reduction in film thickness (~4.5%) consistent with crosslinking of a polymer film.

3.2.3 Characterization of Electrical Properties via Electrochemical Impedance Spectroscopy

To interrogate the effect of introducing conjugated polymer moieties on the electrical properties of ProDIBS thin films, electrochemical impedance spectroscopy

(EIS) was conducted at varying stages of the electropolymerization (Figure 3.4).

Impedance spectra of (p)ProDIBS films on ITO were measured between 150 kHz and 0.1

Hz after increasing numbers of oxidative polymerization scans in the three electrode cell described previously. As conjugated polymers are most conductive in a non-neutral oxidation state, measurements were taken at 0.8 V.

Evaluation of EIS data plotted in the complex plane (Figure 3.4) and interpreted with a modified Randles circuit (see Appendix B) revealed an apparent charge transfer resistance (graphically conveyed as the diameter of the semicircular feature) of 148 kΩ for pristine ProDIBS. Subsequent to installation of poly(ProDOT) pathways, the impedance response of pProDIBS showed a decrease in RCT to 2.6 kΩ after 5 polymerization scans.. Results indicated an even further decrease in RCT to a minimum of

0.4 kΩ after 20 or more polymerization scans which is within an order of magnitude of the RCT reported for poly(dibenzyl-ProDOT) thin films on

Philip T. Dirlam - 2016 95 Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse Vulcanization and Electropolymerization

Figure 3.4. Nyquist plots of impedance spectra of (p)ProDIBS at increasing number of potentiodynamic oxidative polymerization scans with expansions of lower impedance regions (top to bottom). Frequency ranged from 150 kHz to 0.1 Hz.

Philip T. Dirlam - 2016 96 Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse Vulcanization and Electropolymerization

ITO (96 Ω)311 and corresponds to over 95% decrease in charge transfer resistance in comparison to the pristine ProDIBS. This observed decrease in RCT of pProDIBS fundamentally indicates an increase in charge conductance (G) as it is simply the inverse of resistance (G = R-1). These dramatic effects in the electrochemical charge conductance of pProDIBS films clearly confirmed the presence of conjugated polythiophene segments in the otherwise highly electrically resistive sulfur copolymer matrix.

3.3 Conclusion We report the first example of preparing high sulfur content polymeric materials with functional side chains utilizing the inverse vulcanization methodology, where styrenic ProDOT and polythiophene moieties were successfully incorporated into elemental sulfur based copolymers. To our knowledge this system is the first report of improving the conductivity of sulfur, or a very high sulfur content polymeric material using a copolymerization approach. This system points to a new direction in polymer chemistry for the preparation of functional sulfur copolymers and for sulfur materials with enhanced conductivity with potential applications to improve Li-S batteries.

Philip T. Dirlam - 2016 97 Inverse Vulcanization of Elemental Sulfur with 1,4-Diphenylbutadiyne for Cathode Materials in Li-S Batteries

4. INVERSE VULCANIZATION OF ELEMENTAL SULFUR WITH 1,4- DIPHENYLBUTADIYNE FOR CATHODE MATERIALS IN LI-S BATTERIES

With contributions from Adam G. Simmonds (development of Li-S battery fabrication), R.

Tristan S. Kleine (diyne monomer synthesis), Ngoc A. Nguyen (thermal analysis), Laura

E. Anderson (synthetic support), Adam O. Klever (synthetic support and TGA), Philip J.

Costanzo (support of Tristan S. Kleine), Michael E. Mackay (support of Ngoc A.

Nguyen), Richard S. Glass (experimental design and data interpretation), and Jeffrey

Pyun (co-author and PI).

Reproduced in part from Ref. 243 by permission of The Royal Society of Chemistry

Philip T. Dirlam - 2016 98 Inverse Vulcanization of Elemental Sulfur with 1,4-Diphenylbutadiyne for Cathode Materials in Li-S Batteries

4.1 Introduction Sustainable exploitation of inherently sporadic renewable energy sources such as solar, wind, and tidal necessitates the development of electrical energy storage systems constructed with earth abundant materials.2 State-of-the-art electrochemical storage based upon Li-ion technology typically relies on materials with compromising abundance such as cobalt-based oxides for the electroactive cathode material and even when the full potential of Li-ion technology is realized it is unlikely to enable comprehensive electrified transportation or satisfy stationary base load requirements.312 Thus the challenge remains to develop electroactive components for battery electrodes which utilize both sustainable and earth abundant materials capable of meeting these demands.

Next generation battery technologies which utilize electroactive polymers as the electrode materials are a promising route to developing sustainable electrochemical energy storage devices capable of facilitating the use of intermittent renewable energy sources.299, 313-320

High sulfur content copolymers have recently been of particular interest as a class electroactive polymeric cathode materials for secondary lithium batteries due to their high specific capacity and exceptional cycle life.288-289, 291-292, 321 The analogous electrochemical reactivity of the high S-S bond content of these copolymers compared with elemental sulfur (S8) enables their use as the active material in lithium-sulfur (Li-S) batteries.288 Preparation of these sulfur-rich polymers have been achieved via the polymerization methodology termed, inverse vulcanization, where molten elemental sulfur is directly utilized as a comonomer in a free radical copolymerization.

Furthermore, sulfur is an economical, earth abundant material making its direct use with inverse vulcanization attractive for development of sustainable next generation electrochemical storage technology. Inverse vulcanization of S8 has thus far been

Philip T. Dirlam - 2016 99 Inverse Vulcanization of Elemental Sulfur with 1,4-Diphenylbutadiyne for Cathode Materials in Li-S Batteries demonstrated with a few comonomers such as 1,3-diisopropenylbenzene (DIB),186-187, 288-

289, 292, 295, 322-323 1,3-diethynylbenzene (DEB),291 oleylamine,294 and a styrenic propylenedioxythiophene.189 In an effort to further expand the scope of inverse vulcanization and produce a copolymer with additional functionality capable of enhancing performance as the active cathode material for Li-S batteries we have investigated 1,4-diphenylbutadiyne (DiPhDY) as a new comonomer.

The choice of using a butadiyne comonomer was motivated by previous reports of preparing small molecule multiple-sulfur containing heterocycles e.g. dithiolodithioles and particularly thiophenes such as the thieno-3,4-pentathiepin reported by Swager et. al. upon treating diarylbutadiynes with sulfur in solution.324-325 By applying the inverse vulcanization methodology to the reaction of S8 and DiPhDY we aimed to prepare a copolymer with a high content of S-S bonds in place of the intramolecular polysulfide rings (i.e. pentathiepin) reported for small molecule analogs. Incorporation of heterocycles such as thiophene into the sulfur-rich copolymer was desirable as Li-S cathodes formulated with polar and aromatic heterocycle containing materials have been shown to increase device performance by promoting adsorption of the soluble lithium polysulfides generated during battery cycling and thus mitigating loss of active material.33, 95, 326

Herein we report the inverse vulcanization of elemental sulfur with 1,4- diphenylbutadiyne for the first time to yield poly(sulfur-co-1,4-diphenylbutadiyne)

(poly(S-co-DiPhDY)) and establish its utility as an active cathode material in Li-S batteries. We demonstrate that the sulfur content of poly(S-co-DiPhDY) could be easily modified by simply varying the feed ratio of S8 and DiPhDY comonomers. It was found

Philip T. Dirlam - 2016 100 Inverse Vulcanization of Elemental Sulfur with 1,4-Diphenylbutadiyne for Cathode Materials in Li-S Batteries that copolymers synthesized with ≥ 50 wt% DiPhDY exhibited solubility in common polar organic solvents (e.g. CHCl3, THF, CH2Cl2) allowing for conventional solution based characterization such as NMR spectroscopy, size exclusion chromatography

(SEC), and cyclic voltammetry (CV) to be completed. Additionally, the effect of poly(S- co-DiPhDY) composition on thermal properties were investigated where copolymers prepared with increasing DiPhDY loadings were found to have higher glass transition temperatures (Tg) respectively. Finally, very high sulfur content copolymers (90 wt% sulfur) were prepared for formulating Li-S battery cathodes where a maximum sulfur content is desirable. Utilizing these electroactive polymers with a high content of S-S bonds, Li-S batteries were fabricated that demonstrated exceptional capacity (800 mAh/g at 300 cycles, C/5 rate) and the longest cycle lifetime (850 cycles) reported to date of a

Li-S battery employing an electroactive polymer.

4.2 Results and Discussion

4.2.1 Synthesis of Poly(Sulfur-co-1,4-Diphenylbutadiyne) (poly(S-co- DiPhDY)) via Inverse Vulcanization

To prepare poly(S-co-DiPhDY) via inverse vulcanization a mixture of elemental sulfur and 1,4-diphenylbutadiyne was heated above the melting point of S8 to yield a homogeneous solution of the two comonomers and facilitate bulk free radical copolymerization. We propose that the copolymerization proceeds with the

Philip T. Dirlam - 2016 101 Inverse Vulcanization of Elemental Sulfur with 1,4-Diphenylbutadiyne for Cathode Materials in Li-S Batteries

Figure 4.1. Scheme for the inverse vulcanization of elemental sulfur with 1,4- diphenylbutadiyne (DiPhDY) yielding poly(sulfur-co-1,4-diphenylbutadiyne) initial formation of a dithiolodithiole (1, Figure 4.1) that subsequently rearranges to form a 2,5-diphenylthiophene with bridging S-S connectivity at the 3 and 4 positions (Figure

4.1). Elucidating the structure of the resulting copolymer and gaining insight into the course of the copolymerization was challenging due to the broad range of possible products and isomers that have been alluded to in the literature upon sulfurization of

(bis)alkynes,327-331 along with broad resonances observed in both 1H and 13C NMR spectroscopies. Initial NMR spectroscopy experiments revealed the presence of diphenyldithiolodithiole as a component of the reaction mixture generated upon inverse vulcanization of S8 with low loadings of DiPhDY (i.e. 5 wt%). To interrogate the role of the dithiolodithiole species in the copolymerization compound 1 was synthesized and isolated according to the method of Blum et al.324 and subjected to a series of control experiments examining homopolymerization and copolymerization of 1 with S8 and other

Philip T. Dirlam - 2016 102 Inverse Vulcanization of Elemental Sulfur with 1,4-Diphenylbutadiyne for Cathode Materials in Li-S Batteries intermediates. It was found that the dithiolodithiole 1 did not homopolymerize in the melt

(T = 175 ºC) nor was it reactive with additional S8 as determined by size exclusion chromatography (see Appendix C). In both cases however a second small molecule was observed via NMR with signals characteristic of a 2,5-diphenylthiophene presumably formed via rearrangement of the dithiolodithiole 1 to trithiolethiophene 2. Such a rearrangement would be in agreement with earlier related reports on multi-sulfur containing heterocycles such as dithiins which form thiophenes by the extrusion of a sulphur atom upon thermal treatment.332-336 In other control experiments we observed that diphenyldithiolodithole 1 afforded low molar mass copolymer when reacted with another equivalent of DiPhDY as observed from SEC (see Appendix C).

4.2.2 Investigation of the Repeat Unit Structure of Poly(S-co-DiPhDY)

To further support the proposed structure of poly(S-co-DiPhDY), fully soluble copolymer samples were prepared with high DiPhDY loadings (60 wt%) and the higher molecular weight components of the mixture were isolated via a series of precipitations and flash chromatography. Cyclic voltammetry of the isolated higher molecular weight poly(S-co-DiPhDY) (Figure 4.2) revealed a largely irreversible oxidation with an onset at

0.5 V and peak current at a potential of 1.3 V corresponding to oxidation of the diphenylthiophene units which exhibited similar voltammetry to those of 2,5- arylthiophene derivatives as reported previously by

Philip T. Dirlam - 2016 103 Inverse Vulcanization of Elemental Sulfur with 1,4-Diphenylbutadiyne for Cathode Materials in Li-S Batteries

Figure 4.2. Cyclic voltammogram (2nd scan) of poly(S-co-DiPhDY) (2 mg/mL in 100 mM TBAFP/CH2Cl2, scan rate: 50 mV/s).

325 Swager et al. The minor redox couple at E1/2 = -0.2 V was attributed to residual small molecule impurity in the copolymer sample. The presence of cantenated S-S connectivity was supported by laser desorption mass spectrometry (see Appendix C) where mass distributions were observed with periodic variances equivalent to the mass of sulfur. These collective findings pointed to the formation of diphenylthiophene and S-S containing moieties as proposed in Figure 4.1.

Thermal analysis was also conducted on a series of poly(S-co-DiPhDY) prepared with varying feed ratios of S8 and DiPhDY to confirm the formation of a copolymer (as noted by the presence of distinct and tunable glass transition temperatures) and examine the effect of composition on the thermal properties of the material. Differential scanning calorimetry (DSC) thermograms (Figure 4.3a) of

Philip T. Dirlam - 2016 104 Inverse Vulcanization of Elemental Sulfur with 1,4-Diphenylbutadiyne for Cathode Materials in Li-S Batteries

Figure 4.3. a) DSC thermograms highlighting glass transition temperatures (Tg) (curves staggered for clarity) and b) TGA thermograms of S8 and poly(S-co-DiPhDY) prepared with varied sulfur : 1,4-diphenylbutadiyne (DiPhDY) feed ratios.

Philip T. Dirlam - 2016 105 Inverse Vulcanization of Elemental Sulfur with 1,4-Diphenylbutadiyne for Cathode Materials in Li-S Batteries poly(S-co-DiPhDY) prepared with DiPhDY feed ratios ranging from 10 to 60 wt% revealed that the Tg of poly(S-co-DiPhDY) increased respectively with DiPhDY content.

The lowest DiPhDY content copolymer (i.e. 10 wt%) exhibited a Tg of -26 °C Poly(S-co-

DiPhDY) and increases in Tg with increasing DiPhDY loadings to a maximum of 47 °C for 60 wt% DiPhDY were observed. Thermogravimetric analysis (TGA) was also conducted on the series of poly(S-co-DiPhDY) (Figure 4.3b). The TGA thermograms show an initial mass loss with an onset at 200 °C attributed to the volatilization of sulfur with a subsequent plateau in mass loss beginning at 300 °C corresponding to the overall sulfur content. A clear correlation between this mass loss with the S8 feed ratio utilized for inverse vulcanization was observed confirming that the sulfur content of the S8 :

DiPhDY ratio was retained in the final copolymer product.

4.2.3 Electrochemical Testing of poly(S-co-DiPhDY) in Li-S Batteries

To evaluate the electrochemical properties of these sulfur copolymers (and to structurally confirm the presence of S-S bonds) poly(S-co-DiPhDY) was utilized as the active cathode material in Li-S batteries. Cycling performance of coin cells fabricated with poly(S-co-DiPhDY) of various S : DiPhDY compositions revealed higher capacity was exhibited with increasing sulfur content of the copolymer (see Appendix C). Poly(S- co-DiPhDY) prepared with 90 wt% sulfur and 10 wt% DiPhDY was therefore selected for long term Li-S cycling studies. The high sulfur content copolymer exhibited a notable initial capacity of 1050 mAh/g and low capacity loss was observed as indicated by capacities of ≥ 800 mAh/g for the first 300 charge / discharge cycles (Figure 4.4a).

Additionally the batteries were found to have exceptional lifetimes with over 850 total

Philip T. Dirlam - 2016 106 Inverse Vulcanization of Elemental Sulfur with 1,4-Diphenylbutadiyne for Cathode Materials in Li-S Batteries

Figure 4.4. a) Cycling performance at C/5 of Li-S battery fabricated with poly(S-co- DiPhDY) prepared with a 10 wt% DiPhDY, 90 wt% sulfur feed ratio. b) Plot of potential versus charge/discharge capacity for the Li-S cell shown in (a) at 100 cycle intervals. c) Charge / discharge rate performance of Li-S battery with poly(S-co-DiPhDY) (10 wt% DiPhDY) at various current densities (1C = 1672 mAh). All capacities are specific capacity based on sulfur loading. cycles at a rate of C/5 while retaining high Coulombic efficiency (> 97%) throughout long term testing.

Plots of cell potential versus capacity at 100 cycle intervals (Figure 4.4b) show electrochemical behaviour consistent with more conventional Li-S cathode materials337-

338 suggesting that the polysulfide S-S connectivity between the organo-sulfur components of poly(S-co-DiPhDY) participates in analogous reactions to those of S8 during charge / discharge. No appreciable polarization was observed with extended

Philip T. Dirlam - 2016 107 Inverse Vulcanization of Elemental Sulfur with 1,4-Diphenylbutadiyne for Cathode Materials in Li-S Batteries cycling as the high voltage plateau during discharge at ca. 2.3 V corresponding to the lithiation of higher order polysulfides (e.g. S6-8 to Li2S6-8) followed by further reduction of these lithiated polysulfides lower order lithium polysulfides (e.g. Li2S2-5) from 2.25 V to the plateau at ca. 2.05 V was consistently observed during extended battery cycling.

These processes were found to be highly reversible as indicated by the charging profile with oxidation of the lower order lithiated polysulfides observed at ca. 2.25 V and their subsequent conversion to higher order polysulfides at potentials ≥ 2.3 V.

A rate study conducted to evaluate performance of the polymeric cathode material at various current densities (Figure 4.4c) found that poly(S-co-DiPhDY) was able to facilitate high capacities of over 800 mAh/g even at a faster, practical rate of 1C (1C =

1672 mAh). When cycled at a more rapid rate of 2C significantly reduced capacity (ca.

400 mAh/g) was observed which was attributed to the limited rate of diffusion of electroactive species, however high capacity was recovered upon returning to a less aggressive charge/discharge rate of C/5.

4.3 Conclusion We report the first example of inverse vulcanization of elemental sulfur with 1,4- diphenylbutadiyne. The facile bulk copolymerization was shown to yield poly(sulfur-co-

1,4-diphenylbutadiyne) as a high sulfur content copolymeric material with easily tunable sulfur content ranging from 90 wt% to 40 wt% sulfur. Copolymers prepared with up to

90 wt% sulfur were utilized as the active cathode material in Li-S batteries which demonstrated high capacities of > 800 mAh/g at a rate of C/5 for 300 cycles and the longest lifetime of a Li-S battery employing a sulfur copolymer prepared with inverse vulcanization yet reported.

Philip T. Dirlam - 2016 108 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

5. ELEMENTAL SULFUR AND MOLYBDENUM DISULFIDE COMPOSITES FOR LI-S BATTERIES WITH LONG CYCLE LIFE AND HIGH-RATE CAPABILITY

With contributions from Jungjin Park (Materials Characterization), Adam G. Simmonds

(battery characterization and electrochemical data analysis), Clay B. Arrington

(composite preparation scale-up), Tristan S. Kleine (synthetic support), Kenneth

Domanik (electron microprobe analysis), Jennifer L. Schaefer (dielectric spectroscopy),

Vladmir P. Oleshko (electron microscopy), Nicola Pinna (data interpretation), Yung-Eun

Sung (Support of Jungjin Park), and Jeffrey Pyun (co-author and PI).

Reproduced in part from DOI: 10.1021/acsami.6b03200, by permission of The American

Chemical Society .

Philip T. Dirlam - 2016 109 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

5.1 Introduction The development of advanced electrical energy storage technology from earth abundant materials remains a critical challenge toward alternative and renewable energy technologies.2 In this regard, lithium-sulfur (Li-S) batteries are of principal interest as an electrochemical energy storage platform theoretically capable of meeting many of the requirements for electrification of transportation.312 Furthermore, sulfur is a naturally abundant substance that is generated in enormous quantities in its elemental form as a byproduct from petroleum refining.339 This has prompted a tremendous research effort towards realizing the figures of merit for Li-S batteries which boast a high theoretical specific energy of 2600 Wh/kg and theoretical specific capacity 1672 mAh/g.340-343

A number of constraints inherent to the fundamental electrochemistry of operational Li-S cells need to be addressed in order to achieve enhanced device performance. The first complication is that elemental sulfur (S8) has an extremely high

15 13 electrical resistivity (ca. 10 Ω·m) which precludes direct utilization of S8 as an electrode. This issue has traditionally been addressed by blending the electrochemically active sulfur source with conductive carbon additives (e.g. carbon black, carbon fiber, etc.) and a polymer binder to afford a carbon/sulfur composite electrode with adequate electrical conductivity.343 The fundamental issue that complicates optimal cycling of Li-S batteries is the vastly different solubility of Li/S redox products in the electrolyte solution. Repeated electrochemical conversion between intractable species in the charged

(S8) or discharged (Li2S2/Li2S) states and soluble intermediate products (i.e. Li2Sx, 8 ≤ x ≤

3) ultimately leads to low Coulombic efficiency, pronounced capacity fade, short device lifetimes and poor rate capability.

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The poor Coulombic efficiency arises from the dissolved lithium polysulfides diffusing to the anode which are then directly reduced upon interaction with the lithium surface. As a result of this deleterious chemical reduction, additional electrochemical oxidation (i.e. charging) beyond the extent corresponding to the depth of discharge becomes necessary.337 This parasitic “polysulfide shuttle” effect has been most successfully suppressed with the incorporation of lithium nitrate as an electrolyte additive which substantially passivates the Li metal anode to direct reaction with polysulfides.344-

345 An alternate strategy to shut down the polysulfide shuttle effect by blocking the diffusion of lithium polysulfides to the anode surface has recently demonstrated by installing a polymer coating on the cathode via a layer-by-layer process.346

The capacity fade can be attributed in part to an irreversible loss of electroactive material through dissolution of polysulfides. Initial success towards addressing this challenge was demonstrated with the use of high surface area meso- and nanostructured carbons as polysulfide hosts.100, 347-349 This strategy was expanded to a more comprehensive physical confinement of the electroactive material via encapsulation of active sulfur species in carbon,350-354 or conductive polymers.104, 355 It has been shown that by increasing the extent of confinement to a degree where the electroactive sulfur species are sterically inhibited from forming higher order sulfides leads to further enhancement of device performance.356-357 Another approach to improve sulfur utilization, has been incorporating nanostructured sulfur into three-dimensional carbon networks.358 The recent demonstration of utilizing a 3-D graphene sponge highlights the ability of this approach to achieve notable Li-S battery performance.359 It has also been shown that the introduction of a properly designed redox mediator enables markedly

Philip T. Dirlam - 2016 111 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability increases sulfur utilization.360 Furthermore, a variety of distinctive cell designs have been reported with the introduction of a functional interlayer enabling extended cycling life time. Efforts towards grid-level storage have also been reported with the first demonstrations of hybrid redox flow systems.361-362

The short device lifetime and further contribution to capacity loss can be attributed to the formation of cracks and fissures throughout the conductive carbon architecture resulting in mechanical and electrical disconnection from the charge collector. This damage is induced by the repetitive deposition of insoluble discharge

338, 363 products which are nearly 80% more voluminous than S8 if fully lithiated to Li2S.

A number of demonstrations addressing this mechanical destruction mechanism of failure

364 have been reported including using a TiO2 yolk-shell nanostructured cathode and employing both an elastomeric binder along with a dynamic, surfactant based sulfur sequestration.29 Furthermore, redox active polymers have shown great utility as functional electrode materials299, 365 and we have demonstrated that high sulfur content copolymers prepared via inverse vulcanization are effective as active Li-S cathode

192, 288, 323, 366-367 materials capable of mitigating the detrimental Li2Sx precipitation.

Moreover, the inverse vulcanization process has gained attention as a robust method for affording processable polymeric materials186, 188-189 and nanocomposites294, 368-369 with a high content of S-S bonds including a number of additional demonstrations highlighting the use of these sulfur-rich polymers as Li-S cathode materials.181, 183, 291, 370-371

The rate capability of conventional Li-S cells is ultimately limited by mass transport as the electroactive species are in solution at various stages of charge/discharge.

In particular, the diffusion of soluble polysulfide products (e.g. Li2S8 to Li2S6) from the

Philip T. Dirlam - 2016 112 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability electrolyte back to the cathode surface fundamentally limits C-rate capability. Upon charging/discharging at high current densities the limited rate of diffusion of electroactive species results in a low effective concentration of active polysulfide species at the electrode and therefore induces considerable polarization.372 In order to successfully improve the rate capability there is a need for cathode materials capable of substantially decreasing the effective diffusion length of the soluble polysulfides.

Recently, the strategy of employing materials capable of anchoring the polysulfides to their surface via binding interactions373-374 has been implemented to limit the diffusion of soluble polysulfides from the cathode and enhance Li-S device performance. These efforts have focused on utilizing nanostructured materials to bind

375-376 polysulfide redox products which include materials based on Magnéli-phase Ti4O7,

377-378 379 titanium disulfide, nanoscopic molybdenum disulfide (MoS2), manganese dioxide,380 and titanium carbide.381 While the efficacy of these elegant systems is clear as evidenced by improved device performance, the preparation of such nanostructured materials often necessitates complex and energy intensive synthetic procedures. Thus the challenge of developing a facile, scalable method to prepare cathode materials capable of employing the polysulfide anchoring strategy from economical, commercially available starting materials was apparent.

Herein, we report on the facile preparation and application of sulfur/MoS2 and sulfur copolymer/MoS2 composites for use as inexpensive active cathode materials for improved Li-S batteries. We demonstrate for the first time that commercially available

MoS2, which consists of 2-D sheets of MoS2 in micron sized aggregate particles, directly disperses in molten elemental sulfur. This enabled the in-situ formation of composites

Philip T. Dirlam - 2016 113 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability with MoS2 inclusions in a sulfur matrix in scalable, one-pot process. MoS2 was selected as an additive for Li-S cathode materials to introduce polysulfide anchoring sites via the

MoS2 particulate fillers and enhance Li-S cathode performance as demonstrated by Cui et

373, 379 al. with nanosized MoS2 materials. Moreover, MoS2 was attractive owing to its relatively low cost and stability to ambient conditions (obviating the need for handling under inert atmosphere). These sulfur-rich composites, formulated into cathodes as the electroactive component, afforded Li-S batteries with augmented capacity retention

(average capacity loss of ca. 0.07 % per cycle) and lifetimes of up to 1000 cycles.

Furthermore, these composite materials yielded Li-S batteries with exceptional rate capability, delivering reversible capacity of up to 500 mAh/g at 5C. To our knowledge, this is the first example of utilizing composites of MoS2 and elemental sulfur or sulfur copolymers for enhanced Li-S cathode materials prepared with the important advantages of a scalable, one-pot synthesis directly from inexpensive, commercially available starting materials.

5.2 Results and Discussion

5.2.1 Preparation of Composites with a High Sulfur Content Matrix and MoS2 Inclusions

To prepare composites of sulfur and molybdenum disulfide, a mixture of MoS2

(325 mesh powder, Figure 5.1a) and elemental sulfur was heated above the melting point

(Tm) of S8 to yield a uniform dispersion of MoS2 in liquid sulfur (Figure 5.1b). These dispersions were observed to be macroscopically homogeneous, viscous inks when maintained at temperatures above the Tm of S8. For the preparation of composites with a matrix consisting wholly of sulfur, the dispersion was simply heated further (T > 160 °C)

Philip T. Dirlam - 2016 114 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

Figure 5.1. Scheme for the in-situ preparation of composites directly from commercially available elemental sulfur and molybdenum disulfide via the dispersion of MoS2 in liquid sulfur and subsequent (co)polymerization of the sulfur phase with a) SEM image of pristine MoS2 (scale bar is 100 μm), b) photograph of MoS2 dispersion in liquid sulfur, schematic representations and photographs of 100 g batches of c,d) sulfur/MoS2 and e,f) poly(S-r-DIB)/MoS2 composites.

to induce thermal ring-opening polymerization (ROP) of S8 affording a vitrified composite of micro-particulate MoS2 fillers in a polymeric sulfur matrix (Figure 5.1c,

MolyS). As homopolymeric sulfur is subject to depolymerization at room temperature296 we observed that the majority of the homopolymeric sulfur generated from this process reverted back to S8 as noted by X-ray diffraction (XRD, vide infra). Hence, we also

Philip T. Dirlam - 2016 115 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability explored the chemical stabilization of these polymeric sulfur matrices via inverse vulcanization of the sulfur phase with 1,3-diisopropenylbenzene (DIB). Upon heating a mixture of S8, MoS2 and DIB mixture above 160 °C, bulk copolymerization of S8 and

DIB was triggered, which afforded MoS2 composites with a high sulfur content copolymer matrix of poly(sulfur-random-1,3-diisopropenylbenzene) (poly(S-r-DIB))

(Figure 5.1e, MolyS-DIB). By simply modifying the feed ratios of MoS2 : S8 : DIB the relative loading of the different constituents in the composites could be readily tailored with retention of the initial feed ratio in the final materials confirmed by elemental analysis (See Appendix D). For the current study, we explored the effect of MoS2 loading relative to S8 where sulfur/MoS2 composites containing 10-50 wt% MoS2 were prepared (a composite with a sulfur-only matrix and 20 wt% MoS2 filler is termed

MolyS20). However, to afford composites with higher loadings of the electrochemically active sulfur phase, we also investigated poly(S-r-DIB)/MoS2 composites prepared with lower MoS2 loading (10-wt%), and a minimal amount of DIB (10-wt%) to maximize the amount of S8 (80-wt%).

To demonstrate the scalability and straightforward nature of the composite preparation, MolyS50 and MolyS10-DIB10 were prepared in 100 g scale batches (Figure

5.1d and Figure 5.1f respectively). The large scale preparations were conducted in an analogous manner as described above where S8 and MoS2 were heated above the melting temperature of sulfur with vigorous stirring to afford a suspension of MoS2 in liquid sulfur. A slightly elevated temperature of 145 °C was found to be more ideal in the dispersion formation step at larger scales. Additionally, longer times (ca. 30 min) were required for the dispersion to become uniform. Once the MoS2 was uniformly

Philip T. Dirlam - 2016 116 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability suspended, the mixture was then heated above the floor temperature of S8 to induce ROP and afford the final composite. For the preparation of the composite with the poly(S-r-

DIB) matrix, it was found that the process could be streamlined by simply loading all three constituents (i.e. S8, MoS2, and DIB) to the reaction vessel prior to the initial dispersion formation. Similar to the MolyS50 process, the larger scale MolyS10-DIB10 preparation required a longer time at elevated temperature (T = 145 °C) to allow for the sulfur and DIB to become homogenous and adequately disperse the MoS2.

5.2.2 Composite Morphology Characterization

Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) was utilized to interrogate the morphology of the composite materials used in this study. Backscattered electron (BSE) SEM imaging of the composite prepared with 50 wt% MoS2 and 50 wt% sulfur (MolyS50) showed two distinct phases with a regular distribution of a higher contrast, polydisperse particulate phase with dimensions of ca. 10-100 μm throughout a lower contrast matrix (Figure 5.2a, see Figures

5.1a, D.1 for SEM of pristine MoS2). EDS mapping revealed that the higher contrast phase shown in the BSE SEM imaging was Mo rich while the lower contrast matrix was principally devoid of Mo (Figures 5.2c,e). This result provides clear indication that the material is a composite with MoS2 inclusions in a sulfur matrix. The composite prepared with 10 wt% MoS2, 10 wt% DIB and 80 wt% sulfur (MolyS10-DIB10) was shown by

SEM and EDS to have a similar regular distribution of polydisperse MoS2 particles throughout the sulfur copolymer matrix although with a lower density of particles as

Philip T. Dirlam - 2016 117 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

Figure 5.2. SEM and EDS of as-prepared composites with backscattered electron (BSE) images of a) MolyS50 and b) MolyS10-DIB10 composites with respective c,d) Mo K EDS maps (blue indicating Mo rich and white indicating Mo poor regions), and e,f) BSE/Mo K overlay (blue indicating Mo rich and yellow indicating Mo poor regions). All scale bars are 100 μm.

Philip T. Dirlam - 2016 118 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability expected from the lower relative loading of MoS2 (Figure 5.2b). It is noted that there is a considerable overlap of the S K and Mo L signals detected via EDS, thus the fully resolved Mo K signals were utilized for EDS based elemental mapping to show the Mo rich phase attributed to MoS2 inclusions. Transmission electron microscopy (TEM) with

EDS and electron diffraction (ED) was also performed which provided additional support for the intimate mixing of MoS2 and sulfur in the composites (See Appendix D, Figure

D.3).

Electron microprobe analysis (EMPA) coupled with wavelength dispersive spectroscopy (WDS) WDS was also performed as the resolution of X-ray microanalysis via WDS is sufficient to resolve S K and Mo L signals. BSE imaging of MolyS50 and

MolyS10-DIB10 via EMPA again showed the materials were composed of two distinct phases with a higher contrast particulate phase regularly distributed throughout a lower contrast matrix (Figures 5.3a,b). WDS analysis of the brighter (i.e. higher contrast) phase revealed the particulate inclusions in both MolyS50 and MolyS10-DIB10 contained both

Mo and S (Figure 5.3c) while the lower contrast matrix was composed primarily of sulfur. These results demonstrated that SEM with EDS was a reliable technique for characterization of these composite materials, in particular for characterization of the Li-

S cathodes investigated in this study, vide infra. Furthermore, the EMPA with WDS provided definitive confirmation that the composites are composed of MoS2 inclusions in a sulfur or sulfur copolymer matrix.

Philip T. Dirlam - 2016 119 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

Figure 5.3. Backscattered electron images via electron microprobe analysis of a) MolyS50 and b) MolyS10-DIB10 with c) respective wavelength dispersive X-ray spectra obtained from the bright, high contrast inclusions and dark, lower contrast matrix with S K and Mo L lines for reference. Scale Bars are 100 μm.

Philip T. Dirlam - 2016 120 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

Raman spectroscopy was conducted to interrogate if the composite formation via dispersion of particulate MoS2 in molten sulfur was inducing any perturbation of the

MoS2 filler. The Raman spectrum of MolyS50 showed a shift in the A1g band peak of

-1 MoS2 to lower wavenumber by ~ 1.5 cm relative to pristine MoS2. This points to fewer average number of layers per MoS2 particle indicating that some exfoliation was possibly occurring (see Appendix D for a more detailed discussion, Figure D.4).382-383 Dielectric spectroscopy was also conducted on the MolyS50 composite to investigate if the MoS2 filler was able to enhance the electrical conductivity. While the conductivity of S8 was challenging using dielectric spectroscopy as a comparison, for MolyS50 composites

-9 -7 electrical conductivities (σ) ranging from 10 -10 S/cm were observed from RT to 90 °C

(See Appendix D, Figure D.5). With a general understanding of the bulk morphology of the MolyS composites, these materials were then formulated as the electroactive material along with conductive carbon and polymer binder as cathodes for Li-S batteries.

The electroactive cathode coatings with a thickness of ca. 15 μm on C-coated Al current collectors were studied before cycling via X-ray diffraction (XRD) and after cycling via SEM with EDS along with evaluation of the Li-S device performance in

2032-type coin cells. Furthermore, control experiments were conducted on cathodes fabricated with pristine S8 and MoS2 that were either directly formulated into cathode coatings, or subjected to a solid state premixing step via ballmilling for 5 min (ballmilled

S8/MoS2).

A comparative X-ray diffraction (XRD) study of uncycled cathodes formulated with as-received MoS2 and S8 powder samples versus those of cathode coatings cast from ballmilled S8/MoS2 and the MolyS50 composite was conducted (Figure 5.4).

Philip T. Dirlam - 2016 121 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

Figure 5.4. XRD patterns of MoS2, Elemental Sulfur (S8), Ballmilled S8/MoS2 (50 wt%

S8, 50 wt% MoS2) and MolyS50 composite formulated into cathodes on Al foil with indexes according to PDF-2 No. 00-037-1492 (MoS2), No. 01-078-1889 (S8), and No. 03- 065-2869 (Al).

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Comparison of the XRD patterns from cathodes formulated using as-received MoS2 and

S8 with the MolyS50 cathode confirmed the incorporation of MoS2 into the electroactive coating. Additionally the MolyS50 cathode was found to contain a notable crystalline sulfur component indicating that the sulfur matrix of the composite had depolymerized to

S8. Further investigation of the morphology of the electroactive materials in the cathode coatings was carried out with analysis of crystallite size via the Scherrer equation. It was found that the crystallite sizes of MoS2 and S8 were largely unchanged in the physically mixed (ballmilled) S8/MoS2 cathode while a reduction the crystallite size for both MoS2 and S8 was found in the MolyS50 cathode (See Appendix D for a more detailed discussion).

5.2.3 Electrochemical Evaulation of the S/ MoS2 Composites

To interrogate the fundamental electrochemistry of these electroactive materials, cyclic voltammetry was initially conducted to investigate the redox processes of

MolyS50 and MolyS10-DIB10 composites in the potential window of interest for Li-S battery cycling, i.e. 1.7 V to 2.7 V vs Li/Li+ (Figure 5.5a). The principal electrochemical transformation observed was reversible reduction and oxidation of S-S bonds indicated by reduction events at ~2.35 V and 2.05 V during the cathodic sweep and the oxidation occurring from 2.20 V to 2.65 V during the anodic sweep. The observed voltammetry was found to be analogous to that of elemental sulfur under the same conditions indicating that the composite materials would be suitable for further study as the active sulfur material in Li-S cathodes. Conversely, there was an observable discrepancy between the CV profiles of MolyS50 and S8, most notably in terms of the appearance of a new feature during the initial stage of oxidation at ca. 2.3 V. This discrepancy might at

Philip T. Dirlam - 2016 123 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

Figure 5.5. a) Cyclic voltammograms of MolyS50 (MoS2 : S, 50 wt% : 50 wt%),

MolyS10-DIB10 (MoS2 : DIB : S, 10 wt% : 10 wt% : 80 wt%) and S8 recorded at a scan rate of 25 µV/s in 2032 coin cells with Li serving as the counter and quasi-reference electrode. Current density is normalized to S loading. Inset shows the cyclic voltammogram recorded for pristine MoS2 under the same conditions. b) Potential vs. specific capacity plots of MolyS50, MolyS10-DIB10, Ballmilled S8/MoS2, and MolyS20 at 50 cycles c) Plot of specific capacity vs. cycle number for MolyS50, MolyS10-DIB10,

Ballmilled S8/MoS2, and MolyS20 recorded at a rate of 0.2C. Coulombic efficiency vs. cycle number was observed to be >95 % throughout cycling for all Li-S cells, plots omitted for clarity, see Figure D.7. All reported capacities are specific capacity based on electroactive sulfur loading (S mass from MoS2 is excluded). first be intuitively ascribed to a new redox state of the electroactive material. However, further analysis of the nature of this potential shift indicated that it was due to an increase

Philip T. Dirlam - 2016 124 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability in current, as observed with slow scan rate CV, rather than a change in the redox potential of the electroactive material (see Appendix D for a detailed discussion).

To address the possibility of MoS2 contributing to the capacity of Li-S batteries via electrochemical reaction of MoS2 with Li, CV was also performed on a cathode formulated with only pristine MoS2. It was found that MoS2 alone exhibited negligible electrochemical activity in the potential range relevant to Li-S battery cycling (Figure

5.5a inset). This result precluded any concern of a direct electrochemical reaction with

+ the MoS2 constituent of the composites (e.g. intercalation of Li ). Furthermore, this absence of electrochemical activity ensured the innate sulfur mass introduced via MoS2 would not contribute to the specific capacity measured during Li-S battery testing. It is also important to note that earlier reports using MoS2 as active cathode materials in various Li-batteries typically employed a large potential window for charge-discharge cycling (3 – 0 V vs. Li/Li+), with reversible Li+ intercalation occurring at ca. 1.1 V during discharge followed by decomposition upon further lithiation generating Li2S and Mo(0) in situ at ca. 0.6 V.384-386

To investigate the influence of the MoS2 inclusions in regard to Li-S battery performance a composition study of different MolyS composites with varying MoS2 loading from 10 – 50 wt% was conducted, where improved device performance was observed with higher MoS2 loading. Coin cells assembled utilizing MolyS10, MolyS20 and MolyS50 composites as the active cathode material were cycled from 2.6 V to 1.7 V at a rate of 0.2C (Figures 5.5b and 5.5c, see Figure D.6 for MolyS10 data). The MolyS active materials were found to exhibit high initial reversible capacity (>1000 mAh/gsulfur) with potential vs. specific capacity curves comparable to elemental sulfur. The

Philip T. Dirlam - 2016 125 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability discharge curves all showed a higher voltage plateau at ca. 2.35 V corresponding to the reductive formation of high order lithium polysulfides (e.g. Li2S6-8) followed by a sharp drop in potential to ca 2.05 V where a more extensive lower voltage plateau was observed which is attributed to successive reductions of high order lithium polysulfides to lower order species (e.g. Li2S6 to Li2S2). Long-term cycling of the MolyS Li-S cells revealed the

MoS2 inclusions enabled extended device lifetimes and reduced the extent of capacity fading (Figure 5.5c). The best performance was observed from the MolyS50 cathodes which exhibited a long cycle life of up to 1000 cycles with a low ~0.07 % capacity loss per cycle. Li-S batteries fabricated with MolyS10 and MolyS20 composites also exhibited extended lifetimes (600 cycles) and a moderate enhancement of capacity retention but overall, noticeably underperformed in comparison to batteries employing

MolyS50 formulated cathodes. To elucidate if the composite forming process with MoS2 in liquid sulfur was needed for enhancing cathode performance, a control experiment was conducted where the ballmilled S8/MoS2 blend (1:1 by mass) described previously was utilized as the active cathode material for testing in Li-S cells. It was found that the cathode with the ballmilled S8/MoS2 blend exhibited more pronounced capacity loss and shorter cycle lifetime relative to the MolyS50 composite prepared in situ via molten elemental sulfur (Figure 5.5c). This could be attributed to the uniform distribution of

MoS2 particles and reduced crystallite size afforded by the in situ premixing of MoS2 fillers in liquid sulfur during composite formation. This intimately mixed state of sulfur and MoS2 would be anticipated to facilitate more optimal utilization of both electrochemically active sulfur and access to the polysulfide anchoring sites on the surface of the MoS2.

Philip T. Dirlam - 2016 126 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

As alluded to previously, a MolyS10-DIB10 composite was also prepared as a route to increase the overall loading of electroactive sulfur in the coating, while retaining the benefits of MoS2 inclusions in the active material. Hence, the addition of 10-wt% DIB in the mixing step of the composite was conducted to form a copolymer matrix of poly(S-r-

DIB) containing MoS2 inclusions, with the aim of combining the advantages of the MoS2 filler with the “plasticization” qualities of poly(S-r-DIB) cathodes.192 Li-S batteries formulated from these copolymer composites exhibited analogous cycling behavior and high capacity retention relative to the MolyS50 based Li-S cells (Figure 5.5b,c). The

MolyS10-DIB10 cathode exhibited a relatively shorter cycle lifetime (595 cycles) but overall clearly offered comparable device performance to the MolyS50 based Li-S batteries with the added advantage of higher electroactive material loading.

To investigate the charge/discharge rate capability afforded by the high sulfur content composites, Li-S coin cells with MolyS50 and MolyS10-DIB10 based cathodes were cycled at increasing current densities corresponding to rates ranging from 0.10C to

5C (Figure 5.6a). The composite based active materials were found to enable significantly enhanced rate capabilities relative to elemental sulfur. The MolyS50 cathode demonstrated fidelity at the rapid rate of 5C where reversible capacity of up to 500 mAh/g was delivered when cycling between 2.6 to 1.7 V, which is one of a limited number of Li-S systems to date with this level of capacity retention at these very fast C- rates.29, 181, 356, 358-359, 370, 387 Rate capability enhancement was also observed with the

MolyS10-DIB10 based cathode which delivered reversible capacity of up to 500 mAh/g at 3C. The elemental sulfur based cathode was only able to deliver reversible capacity

≥500 mAh/g at rates slower than 1C. Comparison of these results provides clear

Philip T. Dirlam - 2016 127 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

Figure 5.6. a) Plot of discharge capacity vs. cycle number for MolyS50, MolyS10-

DIB10 and S8 based cathodes at various charge/discharge rates ranging from 0.10C to 5C. All reported capacities are specific capacity based on electroactive sulfur loading. Potential vs. capacity plot of b) elemental sulfur and c) MolyS50 based cathode discharge curves at increasing charge/discharge rates. Coulombic efficiency vs. cycle number was observed to be >95 % throughout cycling for all Li-S cells, plots omitted for clarity, see Figure D.8.

indication that the incorporation of MoS2 augments the rate capability of Li-S cathodes.

We propose that the enhanced rate capability is due to the binding of the soluble higher

373, 379 order polysulfides to the MoS2 inclusions, as indicated by the reports of Cui et al., which effectually encumbers the diffusion of these species away from the cathode. This anchoring effect is clearly manifested in the discharge curves of the MolyS50 cathode at increasing rates. The extent of the shift in apparent reduction potentials to lower voltages

Philip T. Dirlam - 2016 128 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability concomitant with concentration polarization388 (i.e. a low relative concentration of electroactive species available at the electrode) ultimately leading to hindered performance at faster rates is diminished compared to the elemental sulfur cathode

(Figures 5.6b and 5.6c).

5.2.4 Investigation of the Li Polysulfide Anchoring Capability of MoS2

To obtain additional evidence for the ability of MoS2 to bind lithium polysulfides

(Li2Sx) a set of experiments was conducted to provide qualitative confirmation via visualization of Li2Sx sequestration by MoS2. The first assay was completed by first preparing a solution of lithium polysulfides in 1,3-dioxolane/1,2-dimethoxyethane

(DOL/DME, 1:1 by vol.) via the direct reaction of S8 and Li metal. The dissolved polysulfides afforded a highly colored, dark yellow solution (Figures 5.7a,c, ca. 10 mM as Li2S8) which allowed for correlation of the Li2Sx concentration with the extent of coloration of the solution. A portion of the Li2Sx solution was then treated with either conductive carbon (Super C65) (Figure 5.7b) or MoS2 (Figure 5.7d) and digital images were subsequently recorded after 2 hours. It was found that the Li2Sx solution exposed to conductive C retained a notable dark yellow color over the course of 2 hours indicating a considerable amount of polysulfides remained in solution. However, it was observed that the Li2Sx solution treated with MoS2 became virtually colorless after 2 hours of exposure indicating that a minimal concentration of polysulfides remained dissolved. This result pointed to the affinity of the as received commerically available MoS2 to bind lithium polysulfides.

The second experiment for providing visual evidence of the anchoring of Li2Sx to

MoS2 was completed by observing the discharge of Li-S cathodes in a conventional

Philip T. Dirlam - 2016 129 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

Figure 5.7. Visual confirmation of lithium polysulfide anchoring capability of MoS2. a,c) Photographs of Li2Sx solutions (ca. 10 mM as Li2S8) in DOL/DME (1:1 v/v) as initially prepared. b) Li2Sx solution shown in 7a after 2 hours of exposure to conductive carbon (Super C65). d) Li2Sx solution shown in 7c 2 hours of exposure to MoS2. Photographs of a Li-S electrochemical cell with an e) elemental sulfur or f) MolyS50 based cathode throughout galvanostatic discharge at an effective rate of C/20 at 30 min, 10 hours and 20 hours.

Philip T. Dirlam - 2016 130 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability solution-based electrochemical cell setup. Cathodes were coated on Al charge collectors, as previously described for coin cell testing, with either S8 or MolyS50 as the active material. Rectangular electrodes were then cut taking care to ensure an equivalent loading of electroactive sulfur per cathode. An electrochemical cell with the cathode of interest and a metallic Li anode was assembled and sealed in an Ar-filled glovebox with subsequent addition of supporting electrolyte (LiTFSI and LiNO3 in DOL/DME). The cathodes were then galvanostatically discharged at a current density equivalent to C/20 and images of the electrochemical cell were recorded after 30 min, 10 hours, and 20 hours of discharge. The electrolyte solution during discharge of the S8 based cathode was observed to take on a faint yellow/green color after 30 min of discharge attributed to a small amount of dissolved lithium polysulfides (Figure 5.7e). The intensity of the yellow/green color from dissolved Li2Sx increased substantially after 10 hours of discharge and was largely retained even when the S8 cathode had discharged for 20 hours. This indicated that a considerable amount of polysulfides remained in solution and illustrates poor active material utilization as the color would be expected to dissipate upon deposition of the intractable low order polysulfides at the end of discharge. The electrochemical cell with the MolyS50 cathode exhibited a similar faint yellow/green color imparted to the electrolyte solution during the initial 30 min of discharge (Figure

5.7e). However, the extent of coloration of the electrolyte solution at 10 hours of discharge with the MolyS50 cathode was notably reduced compared to the S8 cell and was nearly colorless after 20 hours of discharge. The comparatively lower amount of dissolved lithium polysulfides observed during discharge in the cell employing the

Philip T. Dirlam - 2016 131 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

MolyS50 cathode further corroborates that the MoS2 inclusions are able to mitigate Li2Sx dissolution during Li-S cycling leading to enhanced performance.

To further investigate if the MoS2 was acting as a polysulfide anchor and directing the deposition of low order Li polysulfide discharge products, SEM with EDS was performed on MolyS50 cathodes which had been cycled 100 times in 2032 type coin cells and compared to cycled S8 based cathodes (Figure 5.8). Cathodes were extracted from disassembled coin cells in both the charged and discharged states and copiously washed with DOL:DME (50:50 v/v) to affect the removal of electrolyte salts and soluble

Li/S redox products. The SEM with EDS mapping of the charged elemental sulfur based cathode (Figure 5.8a) showed localized sulfur rich deposits characteristic of intractable low order lithium sulfide “slate” (e.g. Li2S2) which indicated that the washing protocol was effective in removing all higher order Li/S redox products. The elemental sulfur cathode in the discharge state (Figure 5.8b) was found to have similar sulfur-rich deposits with a higher incidence, as would be expected for a Li-S cell in the discharged state, in addition to notable cracking throughout the cathode structure. The SEM BSE imaging coupled with elemental mapping of the MolyS50 cathodes revealed a drastically different morphology and composition as compared to the S8 based cathodes. The MolyS50 cathodes imaged after the solvent rinsing protocol were found to exhibit a consistent distribution of S and Mo throughout the electrodes from both charged (Figure 5.8c) and discharged (Figure 5.8d) Li-S cells. Comparison of the Mo K mapping with the mapping corresponding to the overlapped S K and Mo L signals allows for deduction of regions containing a greater occurrence of sulfur rich material. It was observed that the Mo K maps were chiefly coincident with the S K/Mo L mapping providing further evidence that

Philip T. Dirlam - 2016 132 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

Li/S redox products were principally localized to the MoS2 inclusions and corroborating the function of MoS2 as a polysulfide anchor.

Figure 5.8. SEM BSE image with corresponding S K EDS mapping of cycled elemental sulfur cathodes after 70 cycles in the a) charged and b) discharged states. SEM with corresponding Mo K and S K/Mo L EDS mapping images of MolyS50 cathodes after 100 cycles in the c) charged and d) discharged states. All scale bars are 50 μm.

Philip T. Dirlam - 2016 133 Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High- Rate Capability

5.3 Conclusion We report a robust and scalable method for preparing sulfur/molybdenum disulfide composites by dispersing MoS2 directly in molten elemental sulfur with subsequent (co)polymerization of the sulfur phase. Our approach affords composite materials with both a high content of electroactive S-S bonds and polysulfide anchoring sites directly from economical, commercially available reagents in one-pot. We demonstrate that these composites are potent active cathode materials for Li-S batteries enabling extended device lifetimes of up to 1000 cycles with excellent capacity retention and rapid charge/discharge rate capabilities of up to 5C. Future work will focus on the effects of varying MoS2 particle size and morphology in order to reduce the MoS2 loading and maximize sulfur content while retaining the benefits of maximal polysulfide anchoring. We anticipate this straightforward methodology for the bulk preparation of composites with a sulfur rich matrix will encourage further research into utilizing new materials (e.g. other metal- chalcogenides, carbides etc.) as the dispersed constituent for implementing the polysulfide anchoring strategy in Li-S cathodes.

Philip T. Dirlam - 2016 134 Appendix A. Supplementary Information for Chapter 2

APPENDIX A. SUPPLEMENTARY INFORMATION FOR CHAPTER 2

This Appendix is included to provide experimental details and supporting information for

Chapter 2: Single Chain Polymer Nanoparticles via Sequential ATRP and Oxidative

Polymerization.

Reproduced in part from Ref. 193 by permission of The Royal Society of Chemistry

Philip T. Dirlam - 2016 135 Appendix A. Supplementary Information for Chapter 2

A.1 Materials 3,4-Dimethoxythiophene (97 %, Aldrich), 1,1,1-Tris(hydroxymethyl)ethane (99 %,

Aldrich), p-Toluenesulfonic acid monohydrate (98 %, Aldrich), N,N’-

Dicyclohexylcarbodiimide (DCC, 99 %, Aldrich), 4-(Dimethylamino)pyridine (DMAP,

99 %, Aldrich), 4-vinylbenzoic acid (96 %, TCI), N,N,N’,N”,N”-

Pentamethyldiethylenetriamine (PMDETA, 99 %, Aldrich), Ethyl α-bromoisobutyrate

(98 %, Aldrich), tert-Butyl acrylate (98 %, Aldrich), Methyl 2-bromopropionate (98 %,

Aldrich), Dowex 50WX8-200 ion exchange resin (Aldrich), Toluene (ACS grade, EMD),

Ethyl acetate (ACS grade, Macron), Hexanes (ACS grade, EMD), Dichloromethane

(ACS grade, EMD), Anisole (99 %, Alfa Aesar), Lithium borohydride (2.0 M in THF,

Aldrich), Sodium periodate (SPI), Ruthenium(IV) oxide hydrate (99.5%, SPI) were commercially available and used as received. Copper(I) bromide (98 %) was purchased from Aldrich and purified by stirring in glacial acetic acid, filtering, washing with ethanol and ethyl ether, and drying under vacuum before use.

A.2 Instrumentation and Methods 1H and 13C nuclear magnetic resonance (NMR) spectra were obtained using a Bruker

DRX 500 MHz or a BrukerAvance III 400 MHz spectrometer. Chemical shifts are

1 referenced to Me4Si (δ 0.00 ppm) for H NMR and residual CHCl3 (δ 77.0 ppm) in

13 CDCl3 for C NMR. Size exclusion chromatography (SEC) was performed in a tetrahydrofuran (THF) mobile phase with a Waters 1515 isocratic pump running three 5-

µm PLgel columns (Polymer Labs, pore size 104, 103 and 102 Å) at a flow rate of 1 mL/min with a Waters 2414 differential refractometer and a Waters 2487 dual- wavelength UV-vis spectrometer. Molar masses were calculated using the Empower

Philip T. Dirlam - 2016 136 Appendix A. Supplementary Information for Chapter 2 software (Waters), calibrated against low polydispersity linear polystyrene standards.

TEM images were obtained on a Phillips CM12 transmission electron microscope

(CM12) at 80 kV, using prepared carbon coated grids (Electron Microscopy Sciences, Cu, square, 300 mesh). Image analysis was performed using ImageJ software (Rasband, W.S.,

National Institutes of Health, http://rsb.info.nih.gov/ij/, 1997-2007)

A.3 Experimental Procedures

A.3.1 Synthesis of ProDOT-OH, (3-methyl-3,4-dihydro-2H- thieno[3,4-b][1,4]dioxepin-3-yl)methanol A 1 L 3-necked round bottom flask equipped with a magnetic stir bar and condenser was loaded with a solution of 3,4-dimethoxythiophene (5.0 g, 34.7 mmol) in toluene (850 mL). 1,1,1,-Tris(hydroxyl-methyl)ethane (5.4 g, 45.0 mml) and p- toluenesulfonic acid monohydrate (0.7 g, 3.5 mmol) were added sequentially. The resulting mixture was heated at T = 100 °C and stirred at this temperature for 3 days.

After cooling to room temperature, the solvent was removed by evaporation and the viscous black residue was dissolved in ethyl acetate (800 mL). The organic layer was washed with deionized water (400 mL) three times, dried over anhydrous MgSO4, filtered and concentrated under vacuum. The residue was purified by silica gel column chromatography using a gradient elution from 10 % ethyl acetate to 20 % ethyl acetate in

1 hexanes to afford a white solid (5.6 g, 80 % isolated yield). H NMR (500 MHz, CDCl3)

δ 6.47 (s, 2H), 4.07 (d, J = 11.95 Hz, 2H), 3.75-3.72 (m, 4H), 1.69 (t, J = 5.08 Hz, 1H),

13 0.95 (s, 3H); C NMR (125 MHz, CDCl3) δ 149.6, 105.6, 76.6, 65.8, 43.8, 17.0.

Philip T. Dirlam - 2016 137 Appendix A. Supplementary Information for Chapter 2

A.3.2 Synthesis of (ProDOT-Styrene), (3-methyl-3,4-dihydro-2H- thieno[3,4-b][1,4]dioxepin-3-yl)methyl 4-vinylbenzoate A 1 L 2- necked round bottom flask equipped with a magnetic stir bar was loaded with a solution of 4-vinylbenzoic acid (16.5 g, 111.3 mmol) in dichloromethane (400 mL) and the resulting solution was cooled under an ice bath to T = 0 °C. 4-

(Dimethylamino)pyridine (1.4 g, 11.5 mmol) and N,N’-dicyclohexylcarbodiimide (23.0 g,

111.3 mmol) were added sequentially to the reaction mixture and the resulting mixture was stirred at 0 °C for 15 minutes. A solution of ProDOT-OH (17.5 g, 87.4 mmol) in dichloromethane (450 mL) was added to the reaction mixture over 15 minutes at 0 °C.

The reaction mixture was stirred at 0 °C for 1 hour and warmed to room temperature slowly while stirring overnight. The reaction mixture was filtered to remove the urea precipitate and the filtrate was concentrated under vacuum. The residue was purified by silica gel column chromatography using 10 % ethyl acetate in hexanes as an eluent to afford a white solid (24.6 g, 86 % isolated yield)

1 H NMR (400 MHz, CDCl3) δ 7.99 (dt, J = 8.4, 2.0 Hz, 2H), 7.47 (dt, J = 8.4, 2.0 Hz,

2H), 6.76 (dd, J = 17.6, 10.9 Hz, 1H), 6.50 (s, 1H), 5.87 (dd, J = 17.6, 0.7 Hz, 1H), 5.40

(dd, J = 10.9, 0.7 Hz, 1H), 4.43 (s, 1H), 4.14 (d, J = 12.0 Hz, 1H), 3.83 (d, J = 12.0 Hz,

13 1H), 1.08 (s, 3H). C NMR (100 MHz, CDCl3) δ 166.1, 149.6, 142.2, 136.0, 129.9,

129.0, 126.2 116.7, 105.8, 76.3, 66.8, 42.9, 17.4. HRMS (ESI) m/z calculated for

+ C18H19O4S (M+H) 331.0999, observed 331.1000.

I A.3.3 ATRP of ProDOT-Sty ([M]o : [I]o : [L] : [Cu ] = 100 : 1 : 1 : 1) A typical polymerization was conducted as follows: A 10 mL Schlenk flask equipped with a magnetic stir bar was loaded with ProDOT-Sty (2.0 g, 6.1 mmol) and

Philip T. Dirlam - 2016 138 Appendix A. Supplementary Information for Chapter 2

Cu(I)Br (8.7 mg, 6.1 x 10-2 mmol). The flask was fitted with a rubber septum, evacuated and back-filled with argon five times. Deoxygenated anisole (5 mL) was added via purged syringe and the resulting mixture was stirred at room temperature until a homogeneous, light yellow solution was formed. PMDETA (12.6 L, 6.1 x 10-2 mmol) was added via purged syringe and the resulting mixture was stirred for 10 minutes to allow for the formation of the copper catalyst complex, yielding a light green solution.

Ethyl α-bromoisobutyrate (8.9 L, 6.1 x 10-2 mmol) was added via purged syringe to the reaction mixture. The resulting mixture was stirred at 80 °C until desired conversion was achieved after which the reaction was quenched by exposure to air. The copper catalyst was removed from the reaction mixture by stirring over Dowex 50WX8-200 ion exchange resin, followed by filtering the solution through a short plug of activated neutral alumina. The polymer was isolated via precipitation into a 10 fold excess of rapidly stirred methanol, followed by filtration to yield a white powder (0.58 g yield).

A.3.4 Synthesis of poly(tert-butyl acrylate) Macroinitiator A 25 mL Schlenk flask equipped with a magnetic stir bar was loaded with

Cu(I)Br (97.5 mg, 6.8 x 10-1 mmol). The flask was fitted with a rubber septum, evacuated and backfilled with argon three times. Deoxygenated tert-buty acrylate (20 mL, 136.0 mmol), deoxygenated acetone (5 mL) and deoxygenated PMDETA (0.14 mL,

6.8 x 10-1 mmol) were added sequentially via purged syringe. The resulting mixture was stirred at room temperature to form the copper catalyst complex, yielding a light green solution. Methyl 2-bromopropionate (0.15 mL, 1.36 mmol) was added via purged syringe to the reaction mixture and the resulting solution was heated at 60 °C for 6.5 hours after which the reaction was quenched by exposure to air. The copper catalyst was

Philip T. Dirlam - 2016 139 Appendix A. Supplementary Information for Chapter 2 removed from the reaction mixture by stirring over Dowex 50WX8-200 ion exchange resin, followed by filtering the solution through a short plug of activated neutral alumina.

The polymer was isolated via precipitation into a 10 fold excess of rapidly stirred a mixture of MeOH and cold H2O (1:1 = v:v), followed by filtration to give poly(tert-butyl acrylate) macroinitiator (3.8 g yield). Mn SEC = 9200 g/mol, Mw/Mn = 1.07.

A.3.5 Synthesis of poly(tert-butyl acrylate)-b-poly(ProDOT-Sty) A 10 mL Schlenk flask equipped with a magnetic stir bar was loaded with poly(tert-butyl acrylate) macroinitiator (570 mg,.1 x 10-2 mmol) and Cu(I)Br (8.7 mg, 6.1 x 10-2 mmol). The flask was fitted with a rubber septum, evacuated and back-filled with argon five times. Deoxygenated anisole (5 mL) was added via purged syringe and the resulting mixture was stirred at room temperature until a homogeneous solution was formed. PMDETA (12.6 L, 6.1 x 10-2 mmol) was then added via purged syringe and the resulting mixture was stirred for 10 minutes to allow for the formation of the copper catalyst complex, yielding a light green solution. The mixture was then degassed by three freeze-pump-thaw cycles. The resulting mixture was stirred at 80 °C overnight after which the reaction was quenched by exposure to air. The copper catalyst was removed from the reaction mixture by stirring over Dowex 50WX8-200 ion exchange resin, followed by filtering the solution through a short plug of activated neutral alumina. The polymer was isolated via precipitation into a 10 fold excess of rapidly stirred methanol at

0 °C, followed by filtration to yield a white crystalline powder (1.36 g yield). Mn SEC =

19,100 g/mol, Mw/Mn = 1.15.

Philip T. Dirlam - 2016 140 Appendix A. Supplementary Information for Chapter 2

A.3.6 Electrochemical oxidative polymerization of pendant ProDOT groups A typical polymerization was conducted as follows: A 100 mL round bottom flask with a magnetic stirbar was loaded with poly(tert-butyl acrylate)-b-poly(ProDOT-

Sty) (100.7 mg, 5 µmol). Chloroform (25 mL) was added and the mixture was stirred until a homogeneous solution resulted. A solution of FeCl3 (104 mg, 0.64 mmol) in chloroform (10 mL) was added and the solution was stirred at 50 °C for 5 hours. The resulting dark blue solution was stirred over Dowex 50WX8-200 ion exchange resin for 1 hour and then passed through a short plug of activated basic alumina to remove excess oxidant. The solvent was then removed under vacuum to yield a dark bluish black powder with 85% isolated yield. The product was analyzed by UV-Vis absorption spectroscopy as a solution in CHCl3 (Figure A.1).

Philip T. Dirlam - 2016 141 Appendix A. Supplementary Information for Chapter 2

Figure A.1. Optical absorption spectra of poly(tert-butyl acrylate)-b-poly(ProDOT-Sty) (dashed) and electrochemically polymerized polymer nanoparticles (solid).

Philip T. Dirlam - 2016 142 Appendix A. Supplementary Information for Chapter 2

Figure A.2. 1H NMR spectra of poly(ProDOT-Sty) over 5 hours of electrochemical oxidative polymerization.

A.3.7 TEM Sample Staining with RuO4

A solution of RuO4 was prepared as follows: Sodium periodate (1.00 g, 4.68 mmol) was dissolved in H2O (25 mL). The solution was cooled in an ice bath and ruthenium(IV) oxide hydrate (150 mg) was added. The mixture was allowed to come to room temperature while stirring for 3 hr.

Philip T. Dirlam - 2016 143 Appendix A. Supplementary Information for Chapter 2

Samples were then prepared by drop casting a chloroform solution of the material to be imaged onto C coated Cu TEM grids which were then exposed to RuO4 vapors for 1 hr at room temperature by placing the TEM grid on a PTFE coated stage in a secondary sealed container along with the RuO4 solution.

A.3.8 Reduction of Oxidized poly(ProDOT)-Sty to poly(ProDOT- OH) and poly(vinylbenzyl alcohol) A 10 mL round bottom flask with a magnetic stirbar was loaded with oxidized

III poly(ProDOT-Sty) that had been subjected to electrochemical oxidation with Fe Cl3 as previously described for 4 hours (45 mg, 4 μmol). The flask was then evacuated and backfilled with argon three times and dry THF (1 mL) was added yielding a dark blue solution after stirring. LiBH4 in THF (2.0 M, 1.0 mL) and MeOH (81 μL, 2.0 mmol) were then added sequentially which rapidly turned the solution a vibrant reddish orange color.

The reaction was allowed to proceed with stirring overnight at room temperature. The solution was then added to an excess of H2O (~50 mL) and carefully acidified with 1 N

HCl until ~pH 7.0. The resulting reddish purple precipitate was recovered via centrifugation at 10,000 rpm for 5 min (35.7 mg yield). The mixture of products was analyzed via 1H NMR spectroscopy (Figure A.3) and GPC employing a UV detector set at 350 nm.

Philip T. Dirlam - 2016 144 Appendix A. Supplementary Information for Chapter 2

Figure A.3. 1H NMR spectrum of poly(ProDOT-OH) and poly(vinylbenzyl alcohol) yielded from the LiBH4 reduction of oxidized poly(ProDOT-Sty).

Philip T. Dirlam - 2016 145 Appendix B. Supplementary Information for Chapter 3

APPENDIX B. SUPPLEMENTARY INFORMATION FOR CHAPTER 3

This Appendix is included to provide experimental details and supporting information for

Chapter 3: Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse

Vulcanization and Electropolymerization

Reproduced in part from Ref. 189 by permission of The American Chemical Society

Philip T. Dirlam - 2016 146 Appendix B. Supplementary Information for Chapter 3

B.1 Materials

Sulfur (S8, refined, 99.5%, Acros), 3,4-Dimethoxythiophene (97 %, Aldrich), 1,1,1-

Tris(hydroxymethyl)ethane (99 %, Aldrich), p-Toluenesulfonic acid monohydrate (98 %,

Aldrich), N,N’-Dicyclohexylcarbodiimide (DCC, 99 %, Aldrich), 4-

(Dimethylamino)pyridine (DMAP, 99 %, Aldrich), 4-vinylbenzoic acid (96 %, TCI),

Toluene (ACS grade, EMD), Ethyl acetate (ACS grade, Macron), Hexanes (ACS grade,

EMD), Dichloromethane (ACS grade, EMD), Chloroform-d (CDCl3, 0.01 % v/v TMS

Cambridge Isotope Laboratories), Tetrahydrofuran (THF, 99.8%, Fischer), Acetonitrile

(anhydrous, 99.8%, EMD) were commercially available and used as received. 1,3-

Diisopropenylbenzene (DIB, 97 %, TCI), was passed through a short plug of activated alumina to remove inhibitor prior to use, Tetrabutylammonium hexafluorophosphate

(TBAFP, 99 %, Fluka) was recrystallized from absolute ethanol and dried in vacuo before use. Indium Tin Oxide coated glass (ITO, Colorado Concepts) was cut into squares (A = 2 cm2) and cleaned by first scrubbing with an aqueous Triton-X 100 solution using a microfiber cloth followed by successive sonication in deionized water and absolute ethanol for 20 min each changing the solvent after 10 min. Prior to use a fresh ITO surface was produced by etching in a FeCl3 (2M) solution in conc. HCl for 10 s followed by rinsing with excess deionized water three times, and absolute ethanol two times.

B.2 Instrumentation and Methods 1H and 13C nuclear magnetic resonance (NMR) spectra were obtained using a Bruker

DRX 500 MHz or a BrukerAvance III 400 MHz spectrometer. Chemical shifts are

1 referenced to Me4Si (δ 0.00 ppm) for H NMR and residual CHCl3 (δ 77.0 ppm) in

Philip T. Dirlam - 2016 147 Appendix B. Supplementary Information for Chapter 3

13 CDCl3 for C NMR. Size exclusion chromatography (SEC) was performed in a tetrahydrofuran (THF) mobile phase with a Waters 1515 isocratic pump running three 5-

4800 FE-SEM and analyzed with ImageJ software (Rasband, W.S., National Institutes of

Health, http://rsb.info.nih.gov/ij). Electrochemical measurements (cyclic voltammetry, impedance spectroscopy) were performed with a Zahner Zennium Electrochemical

Workstation utilizing a three electrode electrochemical cell with ITO as the working electrode, a platinum mesh counter electrode, and a Ag/AgNO3 reference. Impedance spectroscopy data were fit with ZView software (ver. 2, Scribner Associates).

Philip T. Dirlam - 2016 148 Appendix B. Supplementary Information for Chapter 3

B.3. Experimental Procedures

B.3.1 Preparation of ProDOT-Sty was completed in two steps (Scheme S1) according to previously reported methods.193

Scheme B1. Synthesis of ProDOT-Sty from 3,4-dimethoxythiophene

B.3.1.1 Synthesis of ProDOT-OH ((3-methyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepin- 3-yl)methanol).

To a 1 L 3-necked round bottom flask equipped with a magnetic stir bar and reflux condenser was added 3,4-dimethoxythiophene (5.0 g, 34.7 mmol) and toluene (850 mL). 1,1,1,-Tris(hydroxyl-methyl)ethane (5.4 g, 45.0 mml) and p-toluenesulfonic acid monohydrate (0.7 g, 3.5 mmol) were then added sequentially. The resulting mixture was heated to 100 °C and stirred at this temperature for 3 days. After cooling to room temperature, the solvent was removed in vacuo. The resulting viscous black residue was dissolved in ethyl acetate (800 mL), washed with deionized water (3 x 400 mL), dried over anhydrous MgSO4, filtered and concentrated in vacuo. The residue was purified by silica gel column chromatography using a gradient elution from 10 % ethyl acetate to

20 % ethyl acetate in hexanes affording a white solid (5.6 g, 80 % isolated yield). 1H

NMR (500 MHz, CDCl3) δ 6.47 (s, 2H), 4.07 (d, J = 11.95 Hz, 2H), 3.75-3.72 (m, 4H),

13 1.69 (t, J = 5.08 Hz, 1H), 0.95 (s, 3H); C NMR (125 MHz, CDCl3) δ 149.6, 105.6, 76.6,

65.8, 43.8, 17.0.

Philip T. Dirlam - 2016 149 Appendix B. Supplementary Information for Chapter 3

B.3.1.2 Synthesis of ProDOT-Sty ((3-methyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepin- 3-yl)methyl 4-vinylbenzoate)

To a 1 L 2-necked round bottom flask equipped with a magnetic stir bar was added 4-vinylbenzoic acid (16.5 g, 111.3 mmol) and dichloromethane (400 mL) and the resulting mixture was stirred until a clear colorless solution resulted and then cooled to

0 °C in an ice bath. 4-(Dimethylamino)pyridine (1.4 g, 11.5 mmol) and N,N’- dicyclohexylcarbodiimide (23.0 g, 111.3 mmol) were added sequentially and the resulting mixture was stirred at 0 °C for 15 minutes. A solution of ProDOT-OH (17.5 g,

87.4 mmol) in dichloromethane (450 mL) was added to the reaction mixture over 15 minutes at 0 °C. The reaction mixture was stirred at 0 °C for 1 hour and then warmed to room temperature while stirring overnight. The reaction mixture was filtered to remove the urea precipitate and the filtrate was concentrated in vacuo. The residue was purified by silica gel column chromatography using 10 % ethyl acetate in hexanes as an eluent to

1 afford a white solid (24.6 g, 86 % isolated yield). H NMR (400 MHz, CDCl3) δ 7.99 (dt,

J = 8.4, 2.0 Hz, 2H), 7.47 (dt, J = 8.4, 2.0 Hz, 2H), 6.76 (dd, J = 17.6, 10.9 Hz, 1H), 6.50

(s, 1H), 5.87 (dd, J = 17.6, 0.7 Hz, 1H), 5.40 (dd, J = 10.9, 0.7 Hz, 1H), 4.43 (s, 1H), 4.14

(d, J = 12.0 Hz, 1H), 3.83 (d, J = 12.0 Hz, 1H), 1.08 (s, 3H). 13C NMR (100 MHz,

CDCl3) δ 166.1, 149.6, 142.2, 136.0, 129.9, 129.0, 126.2 116.7, 105.8, 76.3, 66.8, 42.9,

17.4.

B.3.2 Preparation of ProDIBS (poly(ProDOT-Sty-r-1,3- diisopropenylbenzene-r-sulfur)) via Inverse Vulcanization of ProDOT-Sty, DIB, and Sulfur (40 : 50 : 10 wt% respectively)

A typical polymerization was conducted as follows: To a 5 mL glass vial equipped with a magnetic stir bar was added elemental sulfur (500 mg, 1.95 mmol as S8)

Philip T. Dirlam - 2016 150 Appendix B. Supplementary Information for Chapter 3 and ProDOT-Sty (400 mg, 1.21 mmol). The mixture was heated to 180 °C with rapid stirring (500 rpm) for 5 min to facilitate the oligomerization of sulfur and ProDOT-Sty yielding a viscous orange liquid. 1,3-Diisopropenyl benzene (100 mg, 108 μL, 0.63 mmol) was then added and the reaction was allowed to stir at 180 °C until vitrification occurred (ca. 5 additional minutes). Following vitrification the product was maintained at 180 °C for 30 s before being rapidly cooled in a dry ice/acetone bath for 2 min. The vitreous reaction product was extracted by breaking the glass vial yielding a dark red glass (946 mg, 95% isolated yield).

1 Figure B.1. Annotated H NMR (400 MHz, CDCl3) spectrum of ProDIBS

Philip T. Dirlam - 2016 151 Appendix B. Supplementary Information for Chapter 3

Figure B.2. GPC Trace of ProDIBS indicating a Mn apparent = 2000 g/mol and Mw/Mn = 1.75.

Figure B.3. Thermogravimetric curve of ProDIBS indicating a mass loss of ca. 50% with an onset at 225 °C corresponding to sulfur content.

Philip T. Dirlam - 2016 152 Appendix B. Supplementary Information for Chapter 3

B.3.3 Fabrication of ProDIBS thin films on ITO via Spin Coating

A thin film of ProDIBS (ca. 90 nm thickness) was deposited onto a freshly cleaned ITO coated glass substrate (2 cm x 2 cm) by spin-coating from a solution of

ProDIBS (25 mg/mL) in 1:1 by volume toluene:CH2Cl2 with a two-step spin-coating protocol (step 1: ramp at 400 rpm/s to 1500 rpm for a total of 15 s, step 2: ramp at 665 rpm/s from 1500 to 3500 rpm for an additional 15 s).

B.3.4. Electrochemical Oxidative Polymerization of ProDOT Side Chains

An electrochemical cell was assembled with ProDIBS coated ITO as the working

2 electrode (A = 1 cm ), a Pt mesh counter electrode, and a Ag/AgNO3 reference electrode.

A 100 mM tetrabutylammonium hexafluorophosphate (TBAFP) solution in acetonitrile was bubbled with Ar for 30 min and then added to the cell as the supporting electrolyte.

The system was then allowed to rest for 5-10 min to allow for swelling of the polymer with the supporting electrolyte solution. Oxidative polymerization of the ProDOT moieties was then carried out by cycling the potential from 0.5 V to 1.2 V vs. Ag/Ag+ at a rate of 100 mV/s.

B.4 Electrochemical Impedance Spectroscopy (EIS)

B.4.1 Impedance Measurement Procedure

The same three-electrode electrochemical cell with TBAFP/MeCN supporting electrolyte previously described for electropolymerization experiments was utilized for

EIS measurements. Impedance spectra were recorded between 150 kHz and 0.1 Hz at E

= 0.8 V. Spectra were taken of the ProDIBS film on ITO in its pristine state and after various numbers of potentiodynamic polymerization scans in order to interrogate the

Philip T. Dirlam - 2016 153 Appendix B. Supplementary Information for Chapter 3 effect of increasing conversion of ProDOT side chains to poly(ProDOT) inclusions. The impedance spectra were then imported into ZView 2.0 software and the data fit to a modified Randles equivalent circuit (Figure B.5).

B.4.2 EIS Spectra Interpretation

Electrochemical impedance spectroscopy has proven an excellent method for interrogating the electronic properties of conducting polymer films as aptly introduced in a tutorial review by Rubinson and Kayinamura.389 An in-depth discussion of the theory of

AC impedance is beyond the scope of this work but a brief explanation of interpreting

EIS data represented in a Nyquist plot (Figure B.4) modeled with a Randles circuit390 modified with a constant phase element (Figure B.5) is presented. Modeling EIS data with an equivalent circuit allows for interpretation of the complex electrochemical processes in terms of common circuit elements (i.e. resistors, capacitors) and therefore extraction of practical values. The Nyquist plot of the real (Z) and imaginary (Z’) components of the total impedance as a function of frequency is a useful way to graphically represent complex impedance.

Impedance is measured from the surface of the working electrode to the reference electrode and the various circuit elements account for the different components of the system (electrolyte solution, and polymer film).391 The impedance of the supporting electrolyte solution is interpreted in the Randles circuit by a resistor (Rs). The resistance of the electrolyte solution is indicated on a Nyquist plot by the intercept of the high frequency response with the real impedance axis. The polymer film is modeled as a constant phase element (CPE) in parallel with a resistor (RCT) and Warburg element in series. The CPE is frequency dependent element associated with a non-ideal capacitance

Philip T. Dirlam - 2016 154 Appendix B. Supplementary Information for Chapter 3 attributed to film inhomogeneity and is represented in the Nyquist plot by the depression of the semi-circular feature. The Warburg element accounts for the impedance of charge carrier diffusion and is manifested in the Nyquist plot as the positively sloped line at low frequencies. The resistor (RCT) accounts for the impedance of charge transfer in the polymer film. It is graphically represented in the Nyquist plot as the diameter of the semi-circular feature at high and mid frequencies.

Figure B.4. Theoretical Nyquist plot annotated with select parameters relevant to interpretation of the complex impedance in terms of a Randles equivalent circuit.

Figure B.5. Randles circuit with constant phase element (CPE) in place of an ideal capacitor.

Philip T. Dirlam - 2016 155 Appendix C. Supplementary Information for Chapter 4

APPENDIX C. SUPPLEMENTARY INFORMATION FOR CHAPTER 4

This Appendix is included to provide experimental details and supporting information for

Chapter 4: Inverse Vulcanization of Elemental Sulfur with 1,4-Diphenylbutadiyne for

Cathode Materials in Li-S Batteries

Reproduced in part from Ref. 366 by permission of The Royal Society of Chemistry

Philip T. Dirlam - 2016 156 Appendix C. Supplementary Information for Chapter 4

C.1 Materials

Sulfur (S8, refined, 99.5%, Acros), 1,4-Diphenylbutadiyne (DiPhDY, 99%, Alfa

Aesar), 1,2-Dichlorobenzene (99%, Acros), Phenylacetylene (98%, Pfaltz & Bauer),

N,N,N’,N”,N”-Pentamethyldiethylenetriamine (PMDETA, 99 %, Aldrich), Copper (II) chloride (anhydrous, 98%, Alfa Aesar), (ethylenedinitrilo)tetraacetic acid disodium salt, dihydrate (EDTA, ACS grade, EMD), Chloroform (anhydrous, Aldrich),

Dichloromethane (ACS grade, Fischer Scientific), Copper(I) iodide (Aldrich, 98%), N,N-

Dimethylformamide (DMF, ACS grade, EMD), Sodium sulfide nonahydrate (Acros,

98%), 1,10-Phenanthroline (Sigma, ACS grade), Chloroform-d (CDCl3, 0.01 % v/v TMS

Cambridge Isotope Laboratories), Polyethylene (Aldrich, Mw ~4000 g/mol), Conductive carbon (Super C65, Timcal), Lithium bis(trifluoromethane)sulfonimide (Aldrich),

Lithium nitrate (Aldrich), Polypropylene separator (Celgard), Lithium foil (FMC), 1,3-

Dioxolane (Novolyte) and 1,2-Dimethoxyethane (Novolyte) were commercially available and used as received without further purification. Tetrabutylammonium hexafluorophosphate (TBAFP, 99 %, Fluka) was recrystallized from absolute ethanol and dried in vacuo before use.

C.2 Instrumentation and Methods 1H and 13C nuclear magnetic resonance (NMR) spectra were obtained using a

Bruker DRX 500 MHz or a BrukerAvance III 400 MHz spectrometer. Chemical shifts are

1 referenced to Me4Si (δ 0.00 ppm) for H NMR and residual CHCl3 (δ 77.0 ppm) in

13 CDCl3 for C NMR. Size exclusion chromatography (SEC) was performed in a tetrahydrofuran (THF) mobile phase with a Waters 1515 isocratic pump running three 5-

µm PLgel columns (Polymer Labs, pore size 104, 103 and 102 Å) at a flow rate of 1

Philip T. Dirlam - 2016 157 Appendix C. Supplementary Information for Chapter 4 mL/min with a Waters 2414 differential refractometer and a Waters 2487 dual- wavelength UV-vis spectrometer. Molar masses were calculated using the Empower software (Waters), calibrated against low polydispersity linear polystyrene standards.

Thermal gravimetric analysis (TGA) was performed with a Q50 TGA (TA instruments) from 25 °C to 600 °C at a rate of 20 °C/min under nitrogen atmosphere and data was analyzed with TA Universal Analysis software. XPS data were collected with monochromatic Al(Kα) radiation using a KRATOS 165 Ultra photoelectron spectrometer. Differential scanning calorimetry (DSC) characterization was carried out using a 2920 Modulated DSC (TA Instruments) running Thermal Solution 1.4E (TA

Instruments) software, or a Discovery DSC (TA instruments) under nitrogen atmosphere.

Cyclic voltammetry (CV) was performed with a Zahner Zennium Electrochemical

Workstation utilizing a three electrode electrochemical cell with a platinum button

2 working electrode (1 mm ), a platinum mesh counter electrode, and a Ag/AgNO3 reference. Battery cycling was performed at ambient temperature (Taverage = 23 °C) with an Arbin BT2000 battery tester controlled with MITS Pro 2.0 software.

C.3 Experimental Procedures

C.3.1 Preparation of Poly(Sulfur-co-1,4-Diphenylbutadiyne) (poly(S-co- DiPhDY))

C.3.1.1 General Synthetic Procedure (Inverse Vulcanization)

A general procedure for the preparation of poly(S-co-DiPhDY) is presented below and followed by specific conditions for different copolymer compostions. It was found that the inverse vulcanization of sulfur with DiPhDY at feed ratios of DiPhDY ≥ 20 wt%

Philip T. Dirlam - 2016 158 Appendix C. Supplementary Information for Chapter 4 resulted in a very vigorous, exothermic reaction and necessitated lower reaction temperatures accompanied by longer reaction times.

A typical polymerization was conducted as follows: To a 5 mL glass vial equipped with a magnetic stir bar was added elemental sulfur and 1,4-diphenylbutadiyne.

The mixture was heated with rapid stirring (500 rpm) until vitrification of the reaction mixture occurred or for a specified time. The resulting copolymer was then rapidly cooled in a dry ice/acetone bath for 2 min. The reaction product was extracted by breaking the glass vial.

C.3.1.1.1 Preparation of Poly(S-co-DiPhDY) with 10 wt% DiPhDY: The copolymerization was conducted according to the general procedure vide supra with S8 (900 mg, 3.51 mmol) and DiPhDY (100 mg, 0.49 mmol) at 175 °C for 10 min yielding a dark reddish purple glass. (0.91 g yield)

C.3.1.1.2 Preparation of Poly(S-co-DiPhDY) with 20 wt% DiPhDY The copolymerization was conducted according to the general procedure vide supra with S8 (800 mg, 3.12 mmol) and DiPhDY (200 mg, 0.99 mmol) at 175 °C for 10 min yielding a purple glass. (0.93 g yield)

C.3.1.1.3 Preparation of Poly(S-co-DiPhDY) with 40 wt% DiPhDY The copolymerization was conducted according to the general procedure vide supra with S8 (600 mg, 2.34 mmol) and DiPhDY (400 mg, 1.98 mmol) at 125 °C for 1 hr yielding a dark purple glass. (0.95 g yield)

Philip T. Dirlam - 2016 159 Appendix C. Supplementary Information for Chapter 4

C.3.1.1.4 Preparation of Poly(S-co-DiPhDY) with 60 wt% DiPhDY: The copolymerization was conducted according to the general procedure vide supra with S8 (400 mg, 1.56 mmol) and DiPhDY (600 mg, 2.97 mmol) at 125 °C for 1 hr yielding a dark purple glass. (0.87 g yield)

C.3.1.2 Isolation of Higher Molecular Weight Fraction

Poly(S-co-DiPhDY) (60 wt% DiPhDY) (1.0 g) was dissolved in dichloromethane

(20 ml) and added dropwise to an excess of rapidly stirred methanol (ca. 200 mL). The resulting precipitate was recovered via vacuum filtration. This was repeated for a total of three precipitations. The remaining small molecule contaminants were removed via flash silica gel chromatography with 5 % v/v CH2Cl2 in hexanes as the eluent. After elution of the small molecule component the remaining polymer was eluted with 100 % CH2Cl2.

The solvent was removed in vacuo affording poly(S-co-DiPhDY) as a glassy purple

1 13 solid. (456 mg yield) H NMR (400 MHz, CDCl3) δ 7.7 – 6.7 (br). C NMR (100 MHz,

CDCl3) δ 131 – 126 (br). Found: C, 65.08; H, 3.95; S, 31.77

C.3.2 Synthesis of 1,4-Diphenylbutadiyne (DiPhDY)

Commercially available 1,4-diphenylbutadiyne was principally used for the synthesis of poly(sulfur-co-1,4-diphenylbutadiyne) but preliminary experiments were conducted with DiPhDY synthesized according to a procedure adapted from Nielsen et al.392

Philip T. Dirlam - 2016 160 Appendix C. Supplementary Information for Chapter 4

Scheme C.1. Synthesis of 1,4-diphenylbutadiyne via Glaser-Hay Coupling

To a 25 mL round bottomed flask equipped with a magnetic stir bar was added phenylacetylene (0.204 g, 2.00 mmol), copper(II) chloride (0.538 g, 4.00 mmol) and methanol (4 mL). N, N, N’, N’’, N’’- Pentamethyldiethylenetriamine (0.930 g, 5.4 mmol) was then added to the stirred mixture which was subsequently bubbled with oxygen for 5 minutes. The reaction was stirred at 40°C overnight and then cooled to room temperature. The resulting solution was diluted with dichloromethane and washed with saturated EDTA (aq) (3x20 mL) followed by 1N HCl(aq) (2x20 mL). The organic layer was dried over anhydrous MgSO4, filtered and concentrated in vacuo. The product was isolated by passing a solution of the crude mixture in dichloromethane through a short plug of silica and removing the solvent in vacuo yielding a white solid (0.1313 g, 65%).

1 13 H NMR (400 MHz, CDCl3) δ 7.52 – 7.55 (m, 4H), 7.32 – 7.40 (m, 6H). C NMR (100

MHz, CDCl3) δ 132.52, 129.22, 128.46, 121.82, 81.57, 73.93.

C.3.3 Synthesis of Diphenyldithiolodithiole

3,6-Diphenyl-[1,2]dithiolo[4,3-c][1,2]dithiole (diphenyldithiolodithiole) was prepared using a procedure adapted from Blum et al.324

Philip T. Dirlam - 2016 161 Appendix C. Supplementary Information for Chapter 4

Scheme C.2. Synthesis of diphenyldithiolodithiole from 1,4-diphenylbutadiyne and elemental sulfur.

To a 50 mL round bottom flask equipped with a magnetic stirbar was added S8

(800 mg, 3.13 mmol), 1,4-diphenylbutadiyne (1.01 g, 5 mmol), and 1,2-dichlorobenzene

(20 mL). The mixture was heated to 145 °C with stirring for 52 hrs. The product was isolated via silica gel chromatography with hexanes as the eluent affording a dark red

1 crystalline solid (297 mg, 18%). H NMR (400 MHz, CDCl3) δ 7.65 – 7.53 (m), 7.53 –

13 7.43 (m). C NMR (100 MHz, CDCl3) δ 148.40, 137.92, 132.62, 130.09, 129.20, 128.60.

C.3.4 Synthesis of 2,5-Diphenylthiophene

2,5-Diphenylthiophene was prepared using a procedure adapted from Fu et al.393

Scheme C.3. Synthesis of 2,5-diphenylthiophene

To a 100 mL round bottom flask equipped with a magnetic stirbar was added 1,4- diphenylbutadiyne (505 mg, 2.5 mmol), copper(I) iodide (71 mg, 0.375 mmol), 1,10- phenanthroline (90 mg, 0.50 mmol), sodium sulfide nonahydrate (6.0 g, 25 mmol), and

DMF (25 mL). The mixture was stirred at 70 °C for 6 hrs. After cooling to room

Philip T. Dirlam - 2016 162 Appendix C. Supplementary Information for Chapter 4 temperature the mixture was diluted with water (75 ml) and extracted with ethyl acetate

(3 x 25 mL). The organic layers were combined and washed with saturated EDTA(aq) (2 x 50 mL), brine (50 mL), dried over anhydrous sodium sulfate and the solvent was removed in vacuo to afford the product as a light yellow solid that was used without

1 further purification (342 mg, 58%). H NMR (500 MHz, CDCl3) δ 7.67 (m, 4H), 7.43 (m,

13 4H), 7.33 (s, 2H), 7.32 (tt, 2H). C NMR (125 MHz, CDCl3) δ 143.64, 134.34, 128.94,

127.53, 125.66, 124.01.

C.3.5 Procedures for Diphenyldithiolodithiole Reactivity Control Experiment

C.3.5.1 Reactivity of Dithiolodithiole (Neat) at Inverse Vulcanization Temperature

Dithiolodithiole (6.0 mg, 18 μmol) was added to an NMR tube. The sample was heated to 175 °C for 30 min affording a clear red liquid and then allowed to cool to room temperature. CDCl3 (0.6 mL) was added and the resulting red solution was analyzed via

NMR spectroscopy. The solvent was then removed in vacuo and the residue was dissolved in THF for SEC analysis.

C.3.5.2 Reactivity of Dithiolodithiole with S8 at Inverse Vulcanization Temperature

Dithiolodithiole (5.4 mg, 16 μmol) and S8 (5.4 mg, 21 μmol) were briefly ground in a mortar and pestle to ensure adequate mixing. The mixture was transferred to an

NMR tube and heated to 175 °C for 30 min affording a clear orange solution and then allowed to cool to room temperature. CDCl3 (0.6 mL) was added and the resulting red solution was analyzed via NMR spectroscopy. The solvent was then removed in vacuo and the residue was dissolved in THF for SEC analysis.

Philip T. Dirlam - 2016 163 Appendix C. Supplementary Information for Chapter 4

C.3.5.3 Reactivity of Dithiolodithiole with DiPhDY at Inverse Vulcanization Temperature

Dithiolodithiole (5.7 mg, 17 μmol) and 1,4-diphenylbutadiyne (3.4 mg, 17 μmol) were briefly ground in a mortar and pestle to ensure adequate mixing. The mixture was transferred to an NMR tube and heated to 175 °C for 30 min affording a clear dark red solution and then allowed to cool to room temperature. CDCl3 (0.6 mL) was added and the resulting red solution was analyzed via NMR spectroscopy. The solvent was then removed in vacuo and the residue was dissolved in THF for SEC analysis

C.3.6 Fabrication and Testing of Lithium-Sulfur Coin Cell Batteries

A mixture of poly(S-co-DiPhDY) (10 wt% DiPhDY), conductive carbon, and poly(ethylene) in a 75:20:5 mass ratio respectively were ballmilled with chloroform to afford a slurry. The slurry was blade cast onto conductive carbon coated aluminum foil and allowed to dry under ambient conditions yielding an active layer with sulfur loading of ca. 0.75 mg/cm2. Circular cathodes (1 cm2) were punched and assembled with a poly(propylene) separator, lithium foil anode, and electrolyte (0.38 M lithium bis(trifluoromethane)sulfonimide, 0.38 M lithium nitrate in 1:1 v/v 1,3-dioxolane : 1,2- dimethoxyethane) into a CR2032 coin cell. Batteries were then cycled from 1.7 V to 2.6

V vs. Li/Li+. Note: All capacity measurements reported are specific capacity based on sulfur loading.

Philip T. Dirlam - 2016 164 Appendix C. Supplementary Information for Chapter 4

C.4 Results and Discussion

C.4.1 Characterization of Poly(S-co-DiPhDY) and Investigation of the Course of Polymerization

C.4.1.1 Isolation of Higher MW Poly(S-co-DiPhDY) Fraction

The isolation of a higher molecular weight fraction of poly(S-co-DiPhDY) from any oligomeric and small molecule components of the inverse vulcanizate was completed in order to interrogate the repeating unit structure of poly(S-co-DiPhDY). The majority of lower molecular weight species present in a sample of poly(S-co-DiPhDY) prepared with 60 wt% DiPhDY were able to be removed with a series of precipitations into hexanes. Residual small molecule contaminants were then easily separated by flash chromatography affording a higher molecular weight fraction of poly(S-co-DiPhDY) which was highly soluble in polar aprotic solvents allowing for characterization by SEC

(Figure C.1) and CV (Figure 1). SEC indicated that the polymer had a low apparent molecular weight and fairly broad polydispersity (Mn apparent = 2300 g/mol, Mw/Mn = 1.8) consistent with previous high sulfur content copolymers prepared via inverse vulcanization.

Philip T. Dirlam - 2016 165 Appendix C. Supplementary Information for Chapter 4

Figure C.1. SEC Trace of higher molecular weight poly(S-co-DiPhDY) (Mn apparent =

2300 g/mol, Mw/Mn = 1.8)

C.4.1.2 Diphenyldithiolodithiole Reactivity Control Experiments

Diphenyldithiolodithiole (1, Scheme 1) was identified as a soluble component of the crude copolymerization mixture resulting from the inverse vulcanization of sulfur with low DiPhDY (5 wt%) feed ratios. To investigate the role of diphenyldithiolodithiole in the copolymerization it was synthesized and isolated according to a previously reported method.324 The interaction of diphenyldithiolodithiole with the various components of the inverse vulcanization reaction was interrogated applying inverse vulcanization conditions (i.e. thermal treatment in the melt) to diphenyldithiolodithiole in the bulk, with additional sulfur, and with another equivalent of DiPhDY. SEC revealed that no significant homopolymerization of 1 occurred upon heating to 175 °C nor was

Philip T. Dirlam - 2016 166 Appendix C. Supplementary Information for Chapter 4 copolymerization with additional sulfur observed (Figure C.2). However, analysis of the thermally treated samples via NMR (Figures S3 and S4) revealed the appearance of an second small molecule component with resonances characteristic of a 2,5- diphenylthiophene derivatives reported in the literature.325 2,5-Diphenylthiophene was also synthesized to serve as a model compound for further comparison of the NMR spectra (Figure C.5). The splitting patterns of the 1H NMR signals and their relative chemical shifts were found to be similar to the small molecule that was formed upon thermal treatment of diphenyldithiolodithiole serving to further corroborate the proposed copolymerization pathway.

Figure C.2. SEC traces of 1,4-diphenylbutadiyne, diphenyldithiolodithiole, reaction mixture of thermally treated (T = 175 °C) diphenyldithiolodithiole neat, with DiPhDY, and with elemental sulfur (bottom to top respectively).

Philip T. Dirlam - 2016 167 Appendix C. Supplementary Information for Chapter 4

13 Figure C.3. Annotated C NMR (100 MHz, CDCl3) spectra of diphenyldithiolodithiole 1 (A, bottom) and reaction mixtures resulting from the thermal treatment (175 °C) of 1 neat (B, middle) and with S8 (C, top).

Philip T. Dirlam - 2016 168 Appendix C. Supplementary Information for Chapter 4

1 Figure C.4. Annotated H NMR (400 MHz, CDCl3) spectra of diphenyldithiolodithiole 1 (A, bottom) and reaction mixtures resulting from the thermal treatment (175 °C) of 1 neat

(B, middle) and with S8 (C, top).

Philip T. Dirlam - 2016 169 Appendix C. Supplementary Information for Chapter 4

C.4.1.3 2,5-Diphenylthiophene as a Model Compound for NMR Comparison

2,5-Diphenylthiophene was prepared to serve as a model compound for comparison of NMR analysis of the sulfur / DiPhDY inverse vulcanization reaction mixture. 1H NMR (Figure C.5) revealed similar signals, notably the triplet of triplets like signal at 7.32 ppm and second order multiplet at 7.43 ppm, to the new signals observed upon thermal treatment of diphenyldithiolodithiole at 7.34 and 7.43 ppm respectively

(Figure C.4). The signal observed further downfield at 7.68 ppm could not be directly compared due to overlap of the diphenyldithiolodithiol signals.

1 Figure C.5. Annotated H NMR (400 MHz, CDCl3) spectrum of 2,5-diphenylthiophene.

Philip T. Dirlam - 2016 170 Appendix C. Supplementary Information for Chapter 4

C.4.1.4 Laser Desorption Ionization Mass Spectrometry (LDI-MS)

LDI-MS was performed to confirm the presenence of polysulfane connectivity in the poly(S-co-DiPhDY) inverse vulcanizate. The LDI-MS spectrum (Figure SS6) showed the presence of a complex mixture of polymeric species with the notable feature of distrubutions spaced by 32 m/z units corresponding to cantentated S-S connectivity in poly(S-co-DiPhDY).

Figure C.6. LDI Mass Spectrum of poly(S-co-DiPhDY) (20 wt% DiPhDY)

Philip T. Dirlam - 2016 171 A

Appendix C. Supplementary Information for Chapter 4 B

C.4.1.5 X-Ray Photoelectron Spectroscopy (XPS)

X-Ray photoelectron spectroscopy revealed three discernable C species present in relative abundances that corroborate the proposed structure of the organosulfur component of poly(S-co-DiPhDY) (Figure C.7). XPS also served to corroborate the electrochemical evidence that the oxidation state of the S contained within poly(S-co-

DiPhDY) is similar to elemental sulfur (Figure C.8) with the S 2p region observed to be

394 analogous with the XPS spectra of S8.

Figure C.7. XPS Spectra of poly(S-co-DiPhDY) (20 wt% DiPhDY) highlighting C 1s

Philip T. Dirlam - 2016 172 Appendix C. Supplementary Information for Chapter 4

Figure C.8. XPS Spectra of poly(S-co-DiPhDY) (20 wt% DiPhDY) highlighting S 2p

C.4.2 Thermogravimetric Analysis of 1,4-Diphenylbutadiyne

TGA of DiPhDY (Figure C.9) was performed to provide a reference thermogram for the weight loss processes observed with poly(S-co-DiPhDY) prepared with various

S:DiPhDY feed ratios.

Philip T. Dirlam - 2016 173 Appendix C. Supplementary Information for Chapter 4

Figure C.9. TGA thermogram of 1,4-diphenylbutadiyne

C.4.3 Li-S Battery Performance of Poly(S-co-DiPhDY) with Various S:DiPhDY Compositions

Lithium-sulfur coin cells (2032 type) were fabricated with poly(S-co-DiPhDY) of various S:DiPhDY compositions as the active cathode material and cycled at a rate of C/5 from 1.7 V to 2.6 V vs. Li/Li+. Comparison of the cycling performance revealed improved performance (i.e. higher capacity) with copolymer compositions prepared with higher sulfur content (lower DiPhDY loading). Excellent capacity retention was observed with copolymers prepared with 10, 20, and 40 wt% DiPhDY while the isolated higher molecular weight poly(S-co-DiPhDY) suffered from more pronounced capacity fading.

Philip T. Dirlam - 2016 174 Appendix C. Supplementary Information for Chapter 4

Figure C.10. Cycling performance at C/5 of Li-S batteries fabricated with poly(S-co- DiPhDY) of various compositions. All capacities based on sulfur content.

Philip T. Dirlam - 2016 175 Appendix D. Supplementary Information for Chapter 5

APPENDIX D. SUPPLEMENTARY INFORMATION FOR CHAPTER 5

This Appendix is included to provide experimental details and supporting information for

Chapter 5: Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High-Rate Capability

Reproduced in part from DOI: 10.1021/acsami.6b03200 by permission of The American

Chemical Society

Philip T. Dirlam - 2016 176 Appendix D. Supplementary Information for Chapter 5

D.1 Materials

Sulfur (S8, refined, 99.5%, Acros), Molybdenum(IV) Sulfide (MoS2, 325 mesh powder, 99%, Alfa Aesar), Chloroform (anhydrous, Aldrich), Polyethylene (Aldrich, Mw

~4000 g/mol), Conductive carbon (Super C65, Timcal), Lithium bis(trifluoromethane)sulfonimide (LiTFSI, Aldrich), Lithium nitrate (Aldrich),

Polypropylene separator (Celgard), Lithium foil (FMC), 1,3-Dioxolane (Novolyte), 1,2-

Dimethoxy ethane (Novolyte), and CR2032 coin cell components (Stainless steel, MTI) were commercially available and used as received. 1,3-Diisopropenylbenzene (DIB,

97 %, TCI), was passed through a short plug of activated alumina to remove inhibitor prior to use.

D.2 Instrumentation and Methods Scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy

(EDS) was conducted with a Hitachi S3400 operated in reduced pressure mode for the as- prepared composites, a Hitachi S4700II CFEG-SEM for pristine MoS2, a SUPRA 55VP

FE-SEM (Carl Zeiss) for elemental sulfur and S8/MoS2 blend, and an FEI Inspec-S for analysis of cycled cathodes. SEM EDS mapping was processed with Thermo Noran

System Six (NSS, v3.2). Electron microprobe analysis (EMPA) was conducted with a

CAMECA SX50 electron microprobe to obtain BSE images and WDS spectra of the phases in the sample. WDS spectra were obtained using a standard PET crystal at 15 kV accelerating voltage, 20 nA beam current, and a 300 ms dwell time. Transmission electron microscopy (TEM) was performed on JEOL JEM-2100F (JEOL, Japan).

Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) were carried out with a Tecnai F20 (FEI) equipped with an EDAX

Philip T. Dirlam - 2016 177 Appendix D. Supplementary Information for Chapter 5

Tecnai 136-5 detector. TEM samples were prepared by briefly sonicating the sample in

THF and casting the afforded dispersion on a carbon coated TEM grid. X-ray diffraction

(XRD) was conducted using a high power X-ray diffractometer (Rigaku, Smartlab) equipped with a rotating disk anode and a Cu Kα radiation source (λavg = 1.5418 Å) at 45 kV and 200 mA. Raman spectroscopy was performed with a LabRam HR (High resolution, Horiba Hobin-Yvon) with excitation wavelength of 633 nm and peak fitting was completed with OrginPro 8.5. Dielectric spectroscopy was conducted with a

Novocontrol Broadband Dielectric Spectrometer outfitted with a cryostat and Quatro temperature control unit. Elemental analysis for C, H, and S was completed with a Perkin

Elmer PE2400-Series II, CHNS/O analyzer. Mo content was determined by inductively coupled plasma optical emission spectrometry (ICP-OES), samples were digested with

HNO3, HCl, and HF. Cyclic voltammetry (CV) was performed with a Zahner Zennium

Electrochemical Workstation at 25 μV/s from 2.7 to 1.7 V vs. Li/Li+. Coin cell testing was performed at ambient temperature with an Arbin BT2000 battery tester controlled with MITS Pro 2.0 software.

D.3 Experimental Procedures

D.3.1 Preparation of Composites with MoS2 Inclusions and a Sulfur Matrix (MolyS)

D.3.1.1 General Procedure for Preparation of MolyS Composites

A general procedure for the preparation of MolySx composites (where x indicates the wt% of MoS2) is presented below and followed by specific conditions for composites prepared with various S8:MoS2 loadings and various reaction scales.

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MolyS composites were prepared in general as follows: To a glass reaction vessel equipped with a magnetic stir bar was added molybdenum disulfide and elemental sulfur.

The mixture was heated at 135 °C or 145 °C with rapid stirring (750 rpm) until a uniform, black dispersion was afforded. The rapidly stirred dispersion was then heated further to

T > 160 °C until vitrification of the reaction mixture occurred. The resulting composite was then rapidly cooled in a dry ice/acetone bath. The vitreous reaction product was recovered from the reaction vessel yielding a grey/black solid that was utilized without further purification.

D.3.1.1.1 Preparation of MolyS10: 2 g Scale The composite was prepared according to the general procedure vide supra utilizing a 5 mL glass vial as the reaction vessel. S8 (1.80 g, 7.02 mmol) and MoS2 (200 mg, 1.25 mmol) were initially mixed at 135 °C for 5 min to uniformly disperse the MoS2 in liquid sulfur and subsequently heated at 175 °C with continued rapid stirring until vitrification (ca. 5 min) yielding a grey solid. (1.94 g recovered yield)

D.3.1.1.2 Preparation of MolyS20: 2 g Scale The composite was prepared according to the general procedure vide supra utilizing a 5 mL glass vial as the reaction vessel. S8 (1.60 g, 6.24 mmol) and MoS2 (400 mg, 2.50 mmol) were initially mixed at 135 °C for 5 min to uniformly disperse the MoS2 in liquid sulfur and subsequently heated at 175 °C with continued rapid stirring until vitrification (ca. 5 min) yielding a grey solid. (1.93 g recovered yield)

D.3.1.1.3 Preparation of MolyS50: 2 g Scale The composite was prepared according to the general procedure vide supra utilizing a 5 mL glass vial as the reaction vessel. S8 (1.00 g, 3.90 mmol) and MoS2 (1.00

Philip T. Dirlam - 2016 179 Appendix D. Supplementary Information for Chapter 5

g, 6.24 mmol) were initially mixed at 135 °C for 5 min to uniformly disperse the MoS2 in liquid sulfur and subsequently heated at 175 °C with continued rapid stirring until vitrification (ca. 5 min) yielding a greyish black solid. (1.90 g recovered yield)

D.3.1.1.4 Preparation of MolyS50: 10 g Scale The composite was prepared according to the general procedure vide supra utilizing a 20 mL glass vial as the reaction vessel. S8 (5.00 g, 19.5 mmol) and MoS2

(5.00 g, 31.2 mmol) were initially mixed at 145 °C for 23 min to uniformly disperse the

MoS2 in liquid sulfur and subsequently heated at 180 °C with continued rapid stirring until vitrification (ca. 5 min) yielding a greyish black solid. (9.5 g recovered yield)

D.3.1.1.5 Preparation of MolyS50: 20 g Scale The composite was prepared according to the general procedure vide supra utilizing a 60 mL glass culture tube as the reaction vessel. S8 (10.0 g, 39.0 mmol) and

MoS2 (10.0 g, 62.4 mmol) were initially mixed at 145 °C for 25 min to uniformly disperse the MoS2 in liquid sulfur and subsequently heated at 180 °C with continued rapid stirring until vitrification (ca. 5 min) yielding a greyish black solid. (19.2 g recovered yield)

D.3.1.1.6 Preparation of MolyS50: 100 g Scale The composite was prepared according to the general procedure vide supra utilizing a 250 mL beaker as the reaction vessel. S8 (50.0 g, 195 mmol) and MoS2 (50.0 g,

312 mmol) were initially mixed at 145 °C for 26 min to uniformly disperse the MoS2 in liquid sulfur and subsequently heated at 180 °C with continued rapid stirring until vitrification (ca. 6 min) yielding a greyish black solid. (98.0 g recovered yield)

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D.3.2 Preparation of MolyS-DIB Copolymer Composites (Poly(Sulfur- random-1,3-Diisopropenylbenzene) (poly(S-r-DIB) Copolymer Matrix with MoS2 Inclusions )

D.3.2.1 General Procedures for the Preparation of MolyS-DIB Composites

Two general procedures for the preparation of MolySx-DIBy copolymer composites (where x indicates the wt% of MoS2 and y indicates the wt% of DIB) are presented below and followed by specific conditions for composites prepared at various reaction scales.

Procedure A - MolyS-DIB composites were prepared in general as follows: To a glass reaction vessel equipped with a magnetic stir bar was added molybdenum disulfide and elemental sulfur. The mixture was heated at 135 °C with rapid stirring (750 rpm) until a uniform, black dispersion was afforded. 1,3-Diisopropenylbenzene was then added with continuous stirring until the comonomer and liquid sulfur were adequately mixed and a uniform dispersion was afforded. The rapidly stirred dispersion was then heated further to T > 160 °C until vitrification of the reaction mixture occurred. The resulting composite was then rapidly cooled in a dry ice/acetone bath. The vitreous reaction product was recovered from the reaction vessel yielding a glassy, reddish-black solid that was utilized without further purification.

Procedure B - The following simplified general procedure was also utilized: To a glass reaction vessel equipped with a magnetic stir bar was added S8, MoS2, and DIB.

The mixture was heated at 145 °C with rapid stirring (750 rpm) until a uniform, black dispersion was afforded. The rapidly stirred dispersion was then heated further to T >

160 °C until vitrification of the reaction mixture occurred. The resulting composite was then rapidly cooled in a dry ice/acetone bath. The vitreous reaction product was

Philip T. Dirlam - 2016 181 Appendix D. Supplementary Information for Chapter 5 recovered from the reaction vessel yielding a glassy, reddish-black solid that was utilized without further purification. Note: This simplified procedure was found to significantly expedite the preparation of MolyS-DIB composites in larger batches and is described below for the preparation of MolyS10-DIB10 at a 100 g scale.

D.3.2.1.1 Preparation of MolyS10-DIB10: 2 g Scale via Procedure A The composite was prepared according to the general procedure, vide supra, utilizing a 5 mL glass vial as the reaction vessel. S8 (1.60 g, 6.24 mmol) and MoS2 (200 mg, 1.25 mmol) were initially mixed at 135 °C for 3 min to disperse the MoS2 in liquid sulfur. 1,3-Diisopropenylbenzene (200 mg, 0.22 mL, 1.26 mmol) was then added and the mixture was stirred at 135 °C until the liquid phase was homogenous and a uniform dispersion was afforded (ca. 2 min). The dispersion of MoS2 in liquid S8/1,3- diisopropenylbenzene was subsequently heated at 175 °C with continued rapid stirring until vitrification (ca. 6 min) yielding a glassy reddish-black solid. (1.9 g recovered yield)

D.3.2.1.2 Preparation of MolyS10-DIB10: 10 g Scale via Procedure A The composite was prepared according to the general procedure, vide supra, utilizing a 20 mL glass vial as the reaction vessel. S8 (8.00 g, 31.2 mmol) and MoS2

(1.00 g, 6.25 mmol) were initially mixed at 135 °C for 31 min to disperse the MoS2 in liquid sulfur. 1,3-Diisopropenylbenzene (1.00 g, 1.08 mL, 6.32 mmol) was then added and the mixture was stirred at 145 °C until the liquid phase was homogenous and a uniform dispersion was afforded (ca. 5 min). The dispersion of MoS2 in liquid S8/1,3- diisopropenylbenzene was subsequently heated at 180 °C with continued rapid stirring until vitrification (ca. 4 min) yielding a glassy reddish-black solid. (9.7 g recovered yield).

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D.3.2.1.3 Preparation of MolyS10-DIB10: 20 g Scale via Procedure A The composite was prepared according to the general procedure, vide supra, utilizing a 60 mL glass culture tube as the reaction vessel. S8 (16.0 g, 62.4 mmol) and

MoS2 (2.00 g, 12.5 mmol) were initially mixed at 145 °C for 18 min to disperse the MoS2 in liquid sulfur. 1,3-Diisopropenylbenzene (2.00 g, 2.16 mL, 12.64 mmol) was then added and the mixture was stirred at 145 °C until the liquid phase was homogenous and a uniform dispersion was afforded (ca. 5 min). The dispersion of MoS2 in liquid S8/1,3- diisopropenylbenzene was subsequently heated at 180 °C with continued rapid stirring until vitrification (ca. 5 min) yielding a glassy reddish-black solid. (19.7 g recovered yield).

D.3.2.1.4 Preparation of MolyS10-DIB10: 100 g Scale via Procedure B The composite was prepared according to the general procedure B, vide supra, utilizing a 250 mL beaker as the reaction vessel. S8 (80.00 g, 311.9 mmol), MoS2 (10.00 g, 62.47 mmol) and 1,3-diisopropenylbenzene (10.00 g, 10.81 mL, 63.20 mmol) were stirred at 145 °C for 25 min to allow for the formation of a homogenous liquid S8/1,3- diisopropenylbenzene solution and facilitate thorough suspension of the MoS2. The resulting black dispersion of MoS2 in liquid S8/1,3-diisopropenylbenzene was subsequently heated at 180 °C with continued rapid stirring until vitrification (ca. 7 min) yielding a glassy reddish-black solid. (92.18 g recovered yield)

D.3.3 Fabrication and Testing of Lithium-Sulfur Coin Cell Batteries

A mixture of electroactive material (e.g. MolyS or MolyS-DIB composite), conductive carbon, and poly(ethylene) in a 75:20:5 mass ratio respectively were

Philip T. Dirlam - 2016 183 Appendix D. Supplementary Information for Chapter 5 ballmilled with chloroform to afford a slurry. The slurry was blade cast onto conductive carbon coated aluminum foil and allowed to dry under ambient conditions yielding an active cathode coating with an average thickness of 20 μm (as determined via SEM) with an active material loading of ca. 1.5 mg/cm2. Circular cathodes (1 cm2) were punched and assembled with a poly(propylene) separator, lithium foil anode, and electrolyte (0.38

M lithium bis(trifluoromethane)sulfonamide (LiTFSI), 0.38 M lithium nitrate in 1:1 v/v

1,3-dioxolane : 1,2-dimethoxyethane) into a CR2032 coin cell in an Ar-filled glove box.

Batteries were then cycled at various current densities corresponding to charge/discharge

+ rates of 0.1C to 5C (1C = 1.672 A/gsulfur) from 1.7 V to 2.6 V vs. Li/Li .

D.3.4 Li Polysulfide Anchoring Capability of MoS2 via Visual Discrimination

D.3.4.1 Preparation of Li Polysulfide (Li2Sx) Solution and Treatment with MoS2 and Super C65

A stock solution of lithium polysulfides was prepared as follows: To a 50 mL thick walled reagent bottle was added elemental sulfur (2.25 g, 8.79 mmol) and 1,3- dioxolane/1,2-dimethoxyethane (DOL/DME) (1:1 v/v, 30 mL) followed by Lithium foil

(0.14 g, 20 mmol) cut into ca. 0.25 cm2 pieces. The reagent bottle was tightly sealed and the reaction mixture was then sonicated in a bath sonicator for 1 hour at room temperature followed by a rest period (ca. 3 hrs) to allow the reaction mixture and sonication bath to cool. This sonication/rest cycle was repeated until there was no longer any solid elemental sulfur visible affording Li2Sx in DOL/DME (ca. 3.9 M assuming x =

8) as a deeply colored, dark red solution. The Li2Sx stock solution was passed through a

0.2 µm PTFE syringe filter prior to use. Note: All procedures were carried out in an Ar- filled glove box except the sonication step.

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To two separate 20 mL glass vials was added DOL/DME (1:1 v/v, 8.55 mL ea.) followed by an aliquot of the Li2Sx stock solution (0.15 mL) affording a dark reddish yellow solution (ca. 10 mM as Li2S8). MoS2 (100 mg, mmol) was then added to one solution and conductive carbon (Super C65) (100 mg) was added to the other. The mixtures were briefly mixed to disperse the particulate MoS2 or C and then allowed to rest. Digital images of the two mixtures were taken at 1 hour intervals to qualitatively assess the amount of Li2Sx remaining in solution by visual discrimination of the extent of coloration of the solution.

D.3.4.2 Observation of Galvanostatic Discharge of MolyS50 and S8 based Li-S Cathodes

A 2-electrode electrochemical cell was constructed from a Schlenk tube with a

29/42 opening and a rubber septum fitted with two leads consisting of coated 22 gauge wire with small alligator clips. Cathodes with either elemental sulfur or MolyS50 active material (fabricated as described above in section III-C) were cut into rectangles (~1 cm x

3 cm) with a portion of the electrode left uncoated for attachment of via alligator clip. It is noted that care was taken to ensure that the electroactive sulfur content was the same for each electrode with the various cathodes tested containing 0.93 mg of electroactive sulfur. In an Ar-filled glove box the cathode of interest was attached to one of the leads and a Li foil anode (~1 cm x 3 cm) was attached to the other. The electrodes were then lowered into the Schlenk tube and the septum was fitted tightly sealing the electrodes under Ar. The electrochemical reaction vessel was then removed from the glove box and supporting electrolyte (10 ml, 0.38 M LiTFSI with 0.32 M LiNO3 in DOL/DME 1:1 v/v) was added via standard air-free techniques. The cell was allowed to equilibrate at room temperature for 3 hours and was then galvanostatically discharged at an effective rate of

Philip T. Dirlam - 2016 185 Appendix D. Supplementary Information for Chapter 5

C/20. Digital images of were taken at intervals of increasing depth of discharge to qualitatively asses the amount of Li2Sx in solution by visual discrimination of the extent of coloration of the solution.

D.4 Results and Discussion

D.4.1 Elemental Analysis of MolyS50, MolyS20 and MolyS10-DIB10 composites

Elemental analysis was completed to assess if the composition expected from the feed ratio of starting materials was retained in the final composite materials. It was found that the measured elemental composition of the final composites was in good agreement with the expected composition assumed from the initial feed ratio of elemental sulfur, molybdenum disulfide, and 1,3-diisopropenylbenzene (Table D1).

C (wt %) H (wt %) S (wt %) Mo (wt %)

Sample Calc. Found Calc. Found Calc. Found Calc. Found MolyS10-DIB10 9.11 8.06 0.89 0.68 84.01 80.23 5.99 5.52 MolyS20 - - - - 88.01 89.47 11.99 10.92 MolyS50 - - - - 70.03 66.12 29.97 27.06

Table D1. Elemental Analysis Results for Composite Materials

D.4.2 FE-SEM of pristine starting materials (MoS2 and S8) and ballmilled S8/MoS2 blend.

SEM was utilized to investigate the morphology of the starting materials utilized in the formation of MolyS and MolyS-DIB composites. SEM imaging at lower magnification (x 1000, Figure D.1a) of the as-received MoS2 revealed a particulate material with plate-like morphology and polydisperse dimensions ranging from ca. 5 –

100 μm. Images of the MoS2 at higher magnification (x 50k, x 100k, Figures S1c, S1d) revealed the graphitic structure of MoS2 with many layers composing the larger aggregate

Philip T. Dirlam - 2016 186 Appendix D. Supplementary Information for Chapter 5 structures shown at lower magnification. SEM imaging of the as-received elemental sulfur showed particulate material with irregular morphology and polydisperse dimensions of ca. 10 – 100 μm (Figure D.2a). SEM of the blended S8 and MoS2 prepared by ball milling the two powdered reagents at 1:1 S8:MoS2 by mass for 5 minutes revealed a simple mixture of the two types of primary particles with no significant alteration of their respective morphologies (Figure D.2b).

Figure D.1. SEM images of as received MoS2 (325 mesh powder) with increasing magnification of a) 1k, b) 10k, c) 50k, and d) 100k times magnification.

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Figure D.2. SEM images of a) elemental sulfur and b) elemental sulfur/molybdenum disulfide blend prepared with 1:1 by mass elemental sulfur:molybdenum disulfide

(ballmilled S8/MoS2)

D.4.3 TEM with EDS and electron diffraction (ED) data for MolyS50

Transmission electron microscopy (TEM) was utilized to gain additional insight into the morphology of the composite samples suspended in a solvent and drop cast onto a carbon coated Cu grid. TEM coupled with EDS and ED of the MolyS50 composite confirmed the material is an intimate mixture of elemental sulfur and MoS2. Imaging revealed smaller (ca. 1 μm) sulfur rich regions intimately mixed with thin sheets of MoS2

(Figures S3a-f). This finding indicates that the composite formation led to a reduction in sulfur particle size and some degree of MoS2 exfoliation which is in agreement with

Raman spectroscopy and XRD analysis (vide infra). However, it is noted that the overlapping S K and Mo L EDS signals could be simply indicating the higher contrast regions shown in the S K/Mo L maps are of greater thickness. The corresponding electron diffraction (ED, Figure D.3g) data showed characteristic ED patterns from both sulfur and molybdenum disulfide (Figure D.3e).

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Figure D.3. a,d) TEM images of MolyS50 with respective b,e) S K / Mo L, and c,f) Mo K EDS maps and g) corresponding electron diffraction pattern with h) highlighted region shown with indexing to sulfur and MoS2 shown in cyan and green respectively.

D.4.4 Raman spectra of S8, MoS2, ballmilled S8/MoS2 and MolyS50

Raman spectroscopy was utilized to investigate if the composite formation was inducing exfoliation (reduction in the number of layers per aggregate particle) in the

1 MoS2 filler (Figure D.4). The characteristic peaks for E 2g and A1g Raman modes for

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Figure D.4. a) Raman spectra of MolyS50, Ballmilled S8/MoS2 (1:1 by mass), pristine 1 MoS2 and elemental sulfur (S8). b) Expansion of region containing MoS2 E 2g and A1g -1 peaks with fitting showing a ca. 1.5 cm shift to lower wavenumber of the A1g peak for the MoS2 constituent of the MolyS50 composite relative to pristine MoS2.

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-1 -1 bulk MoS2 at ca. 379 cm and 406 cm respectively have been shown to exhibit slight

1 shifts when the number of MoS2 layers is reduced. Comparison of the MoS2 E 2g and A1g

1 peaks with fitting showed a slight shift in the E 2g peak to lower wavenumber and a ca.

-1 1.5 cm shift to lower wavenumber of the A1g peak which is in agreement with the

382-383 literature for the Raman response transitioning from bulk to few layer MoS2.

However, it is noted that the observed shift in the A1g peak of MoS2 to lower wavenumber in the MolyS50 composite relative to pristine MoS2 could result from some degree of an interaction of the sulfur matrix with the MoS2 as there is precedence for a similar shift in the Raman response resulting from a charge transfer to MoS2 (i.e. n- doping).395-397

D.4.5 Dielectric spectroscopy of MolyS50

Dielectric spectroscopy was utilized to investigate the effect of the MoS2 inclusions on the electrical properties of the composite materials. MolyS50 was prepared for analysis by crushing the composite into a powder which was then transferred into the center of a donut shaped Teflon ring and sandwiched between 2 brass electrodes. The sample was heated to 120 °C and held at this temperature for 30 minutes to allow for the sulfur matrix to melt and afforded the composite sample as a pellet upon cooling. The electrical response was measured from 100 mHz to 10 MHz at 120 °C to 0 °C in 10 °C intervals where the low frequency plateau of the real part of the complex conductivity

(σ’) indicates the direct current (DC) conductivity, (σ) (Figure D.5). The observed electrical response gives clear indication that the sample is a true composite. The sharp drop in conductivity at ca. 50 – 60 °C and appearance of the inflection at mid-range frequencies is due to domain formation within the sample such that the conductivity

Philip T. Dirlam - 2016 191 Appendix D. Supplementary Information for Chapter 5 within some large, non-percolating domains is higher than that of the percolative, through-plane conductivity due to the sample thickness. The results show that the sample has conductivity as would be expected from the literature with the maximum conductivity

-4 398 below that of pure MoS2 (i.e. 10 S/cm). While this conductivity is much improved over that of bulk sulfur (i.e. ~10-17 S/cm) it is still drastically lower than that of carbon

(i.e. ~103 S/cm).13 Hence, it is unlikely that this improvement in conductivity is the cause for the improved electrochemical behavior of these materials as Li-S cathodes.

Figure D.5. Plot of real part of the complex conductivity (σ’) versus frequency for the MolyS50 composite at temperatures ranging from 0 °C to 120 °C as determined from dielectric spectroscopy measurements. The DC conductivity (σ) is indicated by the plateau value of σ’ at low frequency.

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D.4.6 Crystallite size determination of sulfur and MoS2 in electroactive cathode coatings from XRD with analysis via the Scherrer equation

The XRD diffractograms (Figure 4) of the pristine MoS2 and S8 coatings indicated initial crystallite sizes of 40.9 ± 4 Å and 563 ± 24 Å respectively. Analysis of the diffraction pattern of the ballmilled S8/MoS2 (1:1 by mass) coating showed very similar

MoS2 and sulfur crystallite sizes of 52 ± 3 Å and 533 ± 30 Å respectively and established that the processing of these materials during cathode fabrication did not dramatically reduce grain sizes of the respective components. However, XRD of the

MolyS50composite coating (where the MoS2 is pre-dispersed in liquid sulfur prior to cathode coating) revealed a notable reduction in MoS2 crystallite size to 34 ± 5 Å and a drastic reduction in crystallite size for the crystalline sulfur constituent to 98 ± 3 Å. This reduction of apparent crystallite sizes was attributed to the composite formation process of pre-dispersing the MoS2 filler in liquid sulfur.

D.4.7 Li-S testing data for MolyS10 based cathodes

The electrochemical performance of MolyS10 (composite prepared with a feed of

10 wt% MoS2, 90 wt% S8) as the active material for Li-S batteries was evaluated as part of the composition study to investigate effect of MoS2 loading. Cathodes were fabricated by coating a slurry of MolyS10 (electroactive component), Super C65 (conductive C), and polyethylene (binder) onto C-coated aluminum foil charge collectors. The MolyS10 based cathodes were assembled into CR2032 coin cells with a Li metal anode and electrolyte and cycled at a rate of 0.2C between 2.6 V and 1.7 V vs. Li/Li+ (Figure D.6).

The MolyS10 cell exhibited a high initial capacity of 1280 mAh/g and a high reversible

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Figure D.6. Li-S cell testing with MolyS10 composite as the electroactive cathode material. a) Plot of specific capacity and Coulombic efficiency vs. cycle number for galvanostatic charge/discharge at a rate of 0.2C between 2.6 to 1.7 V vs. Li/Li+. b) Potential vs. specific capacity plot at various cycles throughout the long term testing shown in (a). All reported capacities are specific capacity based on electroactive sulfur loading.

capacity of >950 mAh/g over the first ca. 60 cycles. After the initial 60 cycles the

MolyS10 cathode was found to experience considerable capacity fade however exhibited high Coloumbic efficiency near unity throughout long term testing and demonstrated a relatively long cycle lifetime of 650 cycles without suffering from catastrophic failure.

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D.4.8 Coulombic efficiency vs. cycle number plots for all Li-S cycling experiments

Coulombic efficiency (the ratio of discharge:charge capacities) was monitored for all Li-S cycling experiments to evaluate the extent of the parasitic self-discharge reactions that occur with the “polysulfide shuttle”. In both long term cycling experiments conducted at relatively slower rates of 0.2C (Figure D.7) and throughout rate studies

(Figure D.8) the Coulombic efficiency was found to be >95% which is attributed to the passivation of the Li anode surface via the LiNO3 electrolyte additive utilized in battery fabrication.

Figure D.7. Coulombic efficiency vs. cycle number plots for long term cycling tests of Li-S cells with cathodes employing a) MolyS50, b) MolyS10-DIB10, c) Ballmilled

S8/MoS2 (1:1 by wt.) blend and d) MolyS20 active materials corresponding to the cycle testing shown in Figure 5.5c.

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Figure D.8. Coloumbic efficiency vs. cycle number plots for rate studies of Li-S cells with cathodes employing a) MolyS50, b) MolyS10-DIB10 and c) elemental sulfur active materials corresponding to the capacity vs. cycle number data shown in Figure 5.6.

D.4.9 Comparison of EDS of MolyS50 cathodes after 100 cycles

Energy dispersive X-ray spectroscopy (EDS) was utilized to map the elemental composition of MolyS50 based cathodes after 100 cycles in the charged and discharged state (Figure 8); however, the Mo L signal overlaps with the S K signal in the 2-3 keV region (Figure D.9). Thus to allow for deduction of regions showing the presence of disparate sulfur rich material, the Mo K lines in the 17-20 keV region were utilized as they are completely resolved and presented for comparison to the Mo L/S K maps.

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Figure D.9. EDS spectra for MolyS50 cathodes after 100 cycles in the a) charged and b) discharged states.

D.4.10 Discussion of Observed Discrepancy between MolyS50 and Elemental Sulfur CV

The nature of the differences observed between the voltammetry measured for

MolyS50 and elemental sulfur based cathodes (Figure 5a) was addressed in terms of several factors including the nature of peak position in slow scan CV, the current-voltage characteristics prior to the peak, the increase in rate capability, and cycling voltage profiles.

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D.4.10.1 Shifts in Peak Position

Most electrochemical discussions on peak position in cyclic voltammetry are in reference to intermediate sweep rates (10-100 mV/s). At these rates, the peaks are formed when the reactant concentration at the electrode is depleted and diffusion takes over as the limiting rate. The reactant concentration in the bulk solution is assumed to remain unchanged during the course of the experiment. The peaks formed at these rates show a distinct voltage dependence as they typically lie within a certain voltage range from the standard redox potential (Eo).388

At much slower sweep rates (25μV/s), the peaks are also formed when the reactant concentration at the electrode is depleted, but in this case, the reactant concentration in the bulk solution is also depleted. Here, the peaks are formed as a result of the bulk solution being depleted throughout the cell, instead of just electrode localized diffusion as seen with faster sweep rates. Peaks in slow scan CV are not determined by a voltage dependence, but rather how much time does it take to deplete the cell. So, if a cell flows a higher current at a given voltage, the peak will shift towards the current onset potential simply because the reactant runs out sooner. The peak shifts in slow scan CV are predominantly determined by the current leading up to it and not the characteristic voltage dependence observed at faster scan rates.

D.4.10.2 Shifts in Current-Voltage

The onset potential leading up to the new oxidation peak appears to shift to a lower voltage with the MolyS50 based cathode. This shift, like the peak position, is also due to an increase in current rather than a change in sulfur’s redox potential. The current- voltage characteristics in this onset region are fundamentally exponential and the fractal

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Figure D.10. Model showing effect of a) current increase and b) voltage shift on the current/potential response in slow scan rate CV with insets showing expansion of the higher potential region of MolyS50 and elemental sulfur CV from Figure 5.5a. nature of this exponential dependence can manifest with current and voltage shifts appearing notably similar. This is illustrated in the model below (Figure D.10) where the anodic current in the lower half is identical for both the case of a current increase and as well as a voltage shift. In order to differentiate a voltage shift from a current increase, we must also look at the cathodic (upper) current. The insets of Figure D.10 (an expansion of the higher potential region of Figure 5.5a) shows that when considering both anodic and cathodic currents, the trends support a current increase rather than a voltage shift.

Note that the cathodic peak also shifts in the same manner as the anodic peak (higher current depletes reactants faster).

D.4.10.3 Increased Rate Capability and Cycling Voltage Profiles

Additional evidence supporting the assertion that the potential shift observed with slow scan CV is due to an alteration in the current rather than a change in the redox species is presented with evaluation of the different C-rate capabilities between MolyS50 and S8 based cathodes. It is clear that the addition of MoS2 inclusions enables a

Philip T. Dirlam - 2016 199 Appendix D. Supplementary Information for Chapter 5 significant improvement in the rate capability over elemental sulfur. While this result does not explicitly determine whether or not a change in sulfur’s redox potential has occurred, it does directly correlate with an increased (catalyzed) current.

Furthermore, the voltages observed in galvanic cycling are determined by both the redox potential of the active material and sources of polarization (i.e. impedance from series resistance and diffusion). These polarization factors only become prevalent at higher rates as shown in Figure 6. With slower cycling (C/5), polarization is minimized and the observed voltage plateaus are almost exclusively dependent on the redox potential. Figure 5.5b shows that even with 50%wt MoS2, the voltage does not significantly deviate from the typical voltage profile observed for elemental sulfur, indicating that sulfur’s redox potential remains unchanged in the presence of MoS2.

By comparison, the battery with 10% DIB and 10% MoS2 does have a characteristic voltage shift in the discharge profile. This is in agreement with our previous studies of S-DIB copolymers which exhibit a voltammetric response in accordance with a change in redox potential, presumably due to the introduction of C-S connectivity altering the redox potential, and becomes more prevalent with increased DIB comonomer loading. No such trend is observed with the MolyS composites at various

MoS2 loadings.

D.4.10.4 Conclusion of Observed Discrepancy between MolyS50 and Elemental Sulfur CV

The appearance of the new peak suggests that there is a notable interaction of the electroactive sulfur species with MoS2. We propose that this peak is not from a different chemical form of electroactive sulfur, but from the purported binding interaction of the electroactive sulfur with MoS2 at the electrode surface enabling the electroactive sulfur

Philip T. Dirlam - 2016 200 Appendix D. Supplementary Information for Chapter 5

which is participating in the interaction with MoS2 to achieve higher currents. We have several pieces of evidence supporting the presence of a current increase, we also have several pieces of evidence that suggest sulfur’s redox potential has not been changed.

From this we can concluded that the variations in electrochemical behavior observed with

MoS2 are a result of sulfur adsorbing to the MoS2 surface (current increase) rather than forming new chemical bonds (redox shift).

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