Physical and Electrochemical Investigations of Various

PHYSICAL AND ELECTROCHEMICAL INVESTIGATIONS OF VARIOUS

DINITRILE PLASTICIZERS IN HIGHLY CONDUCTIVE POLYMER

ELECTROLYTE MEMBRANES FOR LITHIUM ION BATTERY APPLICATIONS

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Chenrun Feng

May, 2017

PHYSICAL AND ELECTROCHEMICAL INVESTIGATIONS OF VARIOUS

DINITRILE PLASTICIZERS IN HIGHLY CONDUCTIVE POLYMER

ELECTROLYTE MEMBRANES FOR LITHIUM ION BATTERY APPLICATIONS

Chenrun Feng

Thesis

Approved: Accepted:

Advisor: Department Chair Dr. Thein Kyu Dr. Sadhan C. Jana

Committee Member Dean of the College Dr. Xiong Gong Dr. Eric J. Amis

Committee Member Dean of the Graduate School Dr. Zhenmeng Peng Dr. Chand Midha

Date

ABSTRACT

To investigate physical and electrochemical properties of polymer electrolyte membranes (PEMs) containing various dinitriles such as succinonitrile (SCN), glutaronitrile (GLN) and adiponitrile (ADN), binary and ternary phase diagrams of poly(ethylene glycol) diacrylate (PEGDA), GLN and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) blends were firstly established in this thesis. The binary phase diagram of PEGDA/GLN system was self-consistently solved based on the combined free energies of Flory-Huggins theory for liquid-liquid demixing and phase field theory for crystal solidification. Computed liquidus and solidus lines were compared with crystal melting temperatures of the binary pairs, obtained by differential scanning calorimetry (DSC) measurement. The binary phase diagram of LiTFSI/GLN system was drawn according to crystal melting temperatures of the binary pairs determined by DSC measurement. Then coexistence regions of each binary phase diagram were verified by polarized optical microscopy.

Subsequently, the ternary phase diagram of PEGDA/GLN/LiTFSI at 25 oC were established. Guided by isotropic regions within ternary phase diagrams established in this thesis and previous studies, polymer electrolyte membranes (PEMs) plasticized by various dinitriles thus fabricated via photo-polymerization afforded transparent, homogeneous films. The ionic conductivity of these PEMs was determined by AC

iii impendence spectrometer, which showed high ionic conductivity up to 10-3 S/cm at room temperature. Of particular interest is that GLN-PEM reveals the highest ion conductivity among the three PEMs tested. To analyze the electrochemical performance of PEMs used in lithium-ion batteries, SCN-PEM, GLN-PEM, and

ADN-PEM were assembled into Li4Ti5O12/PEM/Li and LiFePO4/PEM/Li half-cells.

The half-cell containing GLN-PEM exhibits the best charge-discharge cycling performance, which is consistent with the highest ionic conductivity of the GLN plasticized PEM.

iv ACKNOWLEDGEMENTS

First of all, I would like to give my sincere appreciation to my supervisor, Dr.

Thein Kyu for his kind and patient indoctrination for my Master thesis. He not only teaches me as a master student, but also guides me how to think independently as a researcher. With his patient guidance, I have accomplished this thesis. I would also like to express my gratitude to my committee member: Dr. Gong Xiong and Dr.

Zhenmeng Peng for their suggestions and comments.

Secondly, I am very glad to study in the Department of Polymer Engineering at the University of Akron. Benefited from nice environment and facilities, I can focus on my researches and studies very well. My appreciation also goes to National

Science Foundation (NSF) for providing financial support of this project(NSF-DMR

1502543).

Thanks to all my group members, for their tolerance and understanding during my thesis. Without their helpful assistance, I cannot finish my thesis. Last but not least, I thank my parents in China for their love and support to me.

v TABLE OF CONTENTS

LIST OF TABLES ...... xx

LIST OF FIGURES ...... x

CHAPTER

I. INTRODUCTION ...... 1

II. BACKGROUND ...... 3

2.1. Introduction of Batteries ...... 3

2.1.1. Basic concepts of batteries ...... 4

2.1.2. Battery operations ...... 4

2.2. Introduction to Lithium Ion Batteries...... 8

2.2.1. Lithiumion battery operations ...... 9

2.2.2. Basic concepts of lithiumion battery performance ...... 12

2.2.3. The development of cathode in lithiumion batteries ...... 15

2.2.4. The development of anode in lithiumion batteries ...... 18

2.2.5. The development of electrolyte in lithium ion batteries ...... 19

2.3. Polymer Electrolyte for Lithium Ion Batteries ...... 21

2.3.1. Development of polymer electrolyte ...... 22

2.3.2.Polymer electrolyte based on dinitrile solvents for high voltage

applications ...... 23

vi III. MATERIALS AND EXPERIMENTS ...... 26

3.1. Materials ...... 26

3.1.1. Poly (ethylene glycol) diacrylate (PEGDA) ...... 26

3.1.2. Succinonitrile(SCN) ...... 26

3.1.3. Glutaronitrile (GLN) ...... 27

3.1.4. Adiponitrile (ADN) ...... 27

3.1.5. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) ...... 27

3.1.6. 2,2-Dimethoxy-2-phenylacetophenone ...... 27

3.1.7. Materials used in thermal analysis of PEMs ...... 29

3.1.8. Materials used in battery preparation and electrochemical tests ...... 29

3.2 Experiment Techniques ...... 30

3.2.1. Polymer electrolyte preparation and membrane fabrication ...... 30

3.2.2. Thermal analysis of polymer electrolyte membranes ...... 31

3.2.3.Electrochemical analysis of polymer electrolyte membranes ...... 32

IV. BINARY AND TERNARY PHASE DIAGRAMS OF VARIOUS DINITRILES

PLASTICIZED POLYMER ELECTROLYTE SYSTEMS ...... 34

4.1. Introduction ...... 34

4.2. Experimental Sections ...... 35

4.2.1. Solution Blending Method ...... 35

4.2.2. Differential Scanning Calorimetry Analysis ...... 36

vii 4.2.3. Polarized optical microscopic characterization ...... 37

4.3. Results and Discussion ...... 37

4.3.1. Binary phase diagram of PEGDA, GLN and LiTFSI blends...... 37

4.3.2. Ternary phase diagram ...... 42

4.4. Conclusions ...... 43

V. IONIC CONDUCTIVITY OF UV-CROSSLINKED POLYMER ELECTROLYTE

MEMBRANES CONTAINING VARIOUS PLASTICIZERS ...... 44

5.1. Introduction ...... 44

5.2. Experimental Sections ...... 45

5.2.1. Membrane fabrication and ionic conductivity measurement ...... 45

5.2.2. Thermal analysis for polymer electrolyte membranes (PEMs) ...... 46

5.3. Results and Discussion ...... 47

5.4. Conclusions ...... 54

VI. EVALUATION OF ELECTROCHEMICAL STABILITY FOR PEMS IN

LITHIUM ION BATTERY APPLICATIONS ...... 56

6.1. Introduction ...... 56

6.2. Experimental Sections ...... 57

6.3. Results and Discussion ...... 58

5.4 Conclusions ...... 68

VII. SUMMARY AND RECOMMENDATIONS ...... 70

viii 7.1. Summary ...... 70

7.2. Recommendations ...... 72

REFERENCE...... 82

ix LIST OF TABLES

Table Page

Table 1. Current density and position of redox peaks...... 60

x LIST OF FIGURES

Figure Page Figure 2.2. Conventional structure of a lithium-ion battery with cobalt oxide cathode and graphite anode bridged by an organic electrolyte, and a separator in between. (Reproduced from [15] with permission) ...... 9

Figure 2.3. a) Binary phase diagram of PEGDA/ADN blends around with POM pictures taken in each coexistence region. b)Binary phase diagram of LiTFSI/ADN blends combine with POM pictures taken in certain crystals coexistence regions. The overlapping area was enlarged and shown in inset...... 24

Figure 2.4. Ternary phase diagram of PEGDA/ADN/LiTFSI at 25 oC. ... 25

Figure 3.1. The chemical structure of PEGDA ...... 28

Figure 3.2. The chemical structure of SCN ...... 28

Figure 3.3. The chemical structure of GLN ...... 28

Figure 3.4. The chemical structure of ADN ...... 28

Figure 3.5. The chemical structure of LiTFSI ...... 28

Figure 3.6. The chemical structure of photo initiator Irgacure® 651 ...... 28

Figure 4.1. Binary phase diagram of PEGDA/LiTFSI blends and morphological POM pictures taken at certain coexistence regions. A wide isotropic region was noticed as denoted by isotropic.(reproduced from [107] with permission) ...... 35

Figure 4.2. DSC thermograms of PEGDA/GLN blends as a function of composition, exhibiting the declining trends of the melting transitions of both constituents...... 39

Figure 4.4. The DSC thermograms of LiTFSI/GLN blends, showing the variation of crystal melting endothermic peaks as a function of composition. DSC thermogram of LiTFSI/GLN 95/5 were zoomed in the enlarged inserted figure. 41

Figure 4.5. Binary phase diagram of LiTFSI/GLN blends combines with POM pictures taken in certain crystal coexistence regions. The overlapping area was enlarged and shown in the inset. A large isotropic area existed in the picture...... 42

Figure 4.6. Ternary phase diagram of PEGDA/GLN/LiTFSI at 25 oC...... 43

Figure 5.1. Geometry scheme of the cell, 10mm(length)×10mm(width)×1mm(depth), used for AC impedance measurement...... 46

Figure 5.2. Composition plots of SCN-PEM (blue), GLN-PEM (black), ADN-PEM (red) on the ternary phase diagram at 25 °C, The concentrations of PEGDA/plasticizer/LiTFSI are 6.7/80/13.3, 10/70/20, 13.3/60/26.7, 16.7/50/33.3, 20/40/40, 23.3/30/46.7...... 47

Figure 5.3. Ionic conductivity of SCN-PEM (blue), GLN-PEM (black), ADN-PEM (red), with increasing amount of plasticizers(from 30 wt% to 80 wt%)...... 48

Figure 5.4. Concentration plots of SCN-PEM (blue), GLN-PEM (black), ADN-PEM (red) on the ternary phase diagram at 25 °C...... 49

Figure 5.5. Ionic conductivity of SCN-PEM (blue), GLN-PEM (black), ADN-PEM (red) with weight fraction of PEGDA/Plasticizer/LiTFSI 20/70/10, 20/65/15, 20/60/20,20/55/25, 20/50/30, 20/45/35, 20/40/40, 20/35/45...... 50

Figure 5.7. TGA thermograms of 20/50/30 PEGDA/SCN/LiTFSI PEM (dash blue), 20/50/30 PEGDA/GLN/LiTFSI PEM (dash black), 20/50/30 PEGDA/ADN/LiTFSI PEM (dash red), neat SCN (blue), neat GLN (black) and neat ADN (red)...... 52

xii Figure 5.8. DSC thermograms of 20/50/30 PEGDA/SCN/LiTFSI PEM (blue), 20/50/30 PEGDA/GLN/LiTFSI PEM (black), and 20/50/30 PEGDA/ADN/LiTFSI PEM (red)...... 53

Figure 6.1. Cyclic voltammograms of half-cells a) LiFePO4/GLN-PEM/Li, b) LiFePO4/ADN-PEM/Li, c) Li4Ti5O12/GLN-PEM/Li, d) Li4Ti5O12/ADN-PEM/Li...... 59

Figure 6.2. Galvanostatic charge-discharge cycling performance of LiFePO4/PEM/Li half-cells. a1)discharge capacities of half-cell with SCN-PEM for 80 cycles at 0.25 C. a2)voltage against charge-discharge capacity curves of half-cell with SCN-PEM. b1)discharge capacities of half-cell with GLN-PEM for 80 cycles at 0.25 C and 0.1 C. b2)voltage against charge-discharge capacity curves of half-cell with GLN-PEM. c1)discharge capacities of half-cell with ADN-PEM for 80 cycles at 0.25 C and 0.1 C. c2)voltage against charge-discharge capacity curves of half-cell with ADN-PEM...... 62

Figure 6.3. Galvanostatic charge-discharge cycling performance of Li4Ti5O12/PEM/Li half-cells. a1)discharge capacities of half-cell with SCN-PEM for 80 cycles at 0.25 C. a2)voltage against charge-discharge capacity curves of half-cell with SCN-PEM. b1)discharge capacities of half-cell with GLN-PEM for 80 cycles at 0.25 C. b2)voltage against charge-discharge capacity curves of half-cell with GLN-PEM. c1)discharge capacities of half-cell with ADN-PEM for 80 cycles at 0.25 C. c2)voltage against charge-discharge capacity curves of half-cell with ADN-PEM...... 66

Figure 7.1. Specific discharge capacity as a function of cycle number for o LiFePO4/GLN-PEM/Li half-cells during cycling at 25 C (unfilled circle) and 60 oC (filled triangle)...... 74

Figure 7.2. Cyclization mechanism of GLN radical reaction...... 74

xiii CHAPTER I

INTRODUCTION

In recent decades, with the shortage of fossil fuel resource and global warming, electrical energy has become the major energy resource in manufacturing, transportation and human’s daily life. Meanwhile, people are seeking ways to develop energy storage devices with higher capacity and longer life. One of the most attractive research fields is the lithium ion batteries. Lithium ion batteries have high energy density and are light-weight. They are suitable for portable electronic devices and electric vehicles. Traditional lithium ion batteries use the liquid electrolyte, which contains organic solvents. Due to the high flammability of organic solvents, traditional Li ion batteries have a lot of safety issues. Hence, the solid state polymer electrolyte membranes (PEMs) have been proposed to improve thermal and mechanical stability of lithium ion batteries.

Polyether(e.g. Polyethylene oxide(PEO)) with ether oxygen groups can form coordination bonds with lithium ions to transport lithium ions by polymer chain motions. Regarding to previous studies of PEMs in our group, poly(ethylene glycol) diacrylate (PEGDA) with ethenyl groups for photo-polymerization was used as polymer matrix and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was used as the lithium salt in the PEM system. The ionic conductivity of polymer/salt system can

1 be improved by adding plasticizer succinonitrile (SCN) to accelerate polymer chain motion and ion dissociation. By virtue of the fundamental studies on the effect of

SCN in PEMs, completely amorphous PEMs fabricated by UV crosslinking can achieve superionic conductivity (10-3 S/cm) for battery applications. In this thesis, various dinitrile plasticizers (SCN, adiponitrile (ADN) and glutaronitrile (GLN)) with increasing number of carbon atoms were used in PEM systems to investigate its physical and electrochemical properties.

Chapter II mainly reviewed lithium ion battery innovations in electrolytes and electrodes. The potential developments and challenges in lithium ion batteries were also claimed in this chapter.

In Chapter IV, a system of PEGDA, GLN and LiTFSI was investigated to compare with PEGDA/SCN/LiTFSI and PEGDA/ADN/ LiTFSI systems. The binary and ternary phase diagrams of PEGDA/GLN/LiTFSI system were established to guide the fabrication of PEMs.

In Chapter V, PEMs were made by UV crosslinking within amorphous regions of ternary phase diagrams. The ionic conductivities of PEMs containing various dinitrile plasticizers (SCN, GLN, ADN) were tested and compared. Then, the thermal stability and phase behavior of those three PEMs were investigated.

In Chapter VI, SCN-PEM, GLN-PEM and ADN-PEM were assembled into

Li4Ti5O12/PEM/Li and LiFePO4/PEM/Li half-cells to investigate its electrochemical stability. Cyclic voltammograms and galvanostatic cycling stability of half-cells with various dinitrile plasticizers were tested and analyzed.

2 CHAPTER II

BACKGROUND

2.1. Introduction of Batteries

Since the Second Industrial Revolution, electricity has been playing a key role in our modern life. Up to now, the generation of electrical energy mainly relies on combustion of fossil fuels, which has caused the energy crisis. One the other hand, burning fossil fuels can produce greenhouse gases and can also emit exhaust gases, causing environmental pollution. The world urgently needs alternative energy sources like wind, solar, thermal energy, which are renewable and environmentally friendly.

To store the electricity harvested from these new energy resources and popularize the usage of hybrid electric vehicles instead of fuel burning vehicles, the energy storage systems are introduced, which convert electrical energy into chemical energy, such as batteries, fuel cells and electrochemical capacitors. Fuel cells were designed to replace the combustion engines with high energy density and low environmental implication, but still under development due to the high costs, low power density and short life time. Electrochemical capacitors, which were considered as high power systems, also have been applied in memory protection of electronic devices and hybrid vehicles.

Compared to fuel cells and electrochemical capacitors, batteries, which have both high energy density and high power density, have been widely applied in human's daily life.

3 2.1.1. Basic concepts of batteries

Battery is an electrical energy storage device which can realize the conversion between electrical energy and chemical energy. A battery is composed by an electron conductive cathode and anode separated by an ion conductive electrolyte.

The anode: An electrode where active materials get oxidized and donate electrons to the external circuit. It serves as positive electrode in the charge process and negative electrode in the discharge process.

The cathode: An electrode where active materials get reducted and gain electrons from the external circuit. It serves as a negative electrode in the charge process and positive electrode in the discharge process.

Active materials: The chemical elements or compounds in the electrodes can undergo electrochemical reactions (oxidation or reduction) to store or deliver the electricity.

Electrolyte: Ionic conductive materials between anode and cathode transport ions and close the circuits. The electrolytes are mostly some aqueous or nonaqueous solution of salts, acids or alkalis. There are also polymeric or ceramic electrolytes.

2.1.2. Battery operations

A battery is operated by the driving force from the electrical potential difference between half-cell reactions happened in two electrodes. The positive electrode contains chemical compounds which undergo redox reactions at higher voltage range than active materials in the negative electrode. During the discharge process, the

4 negative electrode where oxidative half-cell reactions take place serves as the anode.

For example, the material in anode side denoted as M can generate n electrons to become Mn+ by oxidation. When the anode is connected to the external load, the electrons flow from the anode to the cathode, which is the positive electrode. The cathode gaining electrons from the anode occurs in the reductive half-cell reactions.

For instance, the positive electrode material is X. X gains n electrons becoming Xn-.

The chemical reactions were shown in Equation 2.1, 2.2 and 2.3.

At the negative electrode:

n  M  M  ne (2.1) At the positive electrode:

 n X  ne  X (2.2)

Overall reaction (Discharge):

n n M  X  M  X (2.3)

If the electrochemical reactions happening in discharge process are irreversible, the batteries are classified as the primary batteries (nonrechargeable batteries). If the reactions happened during the discharge process can be reversed and recharged, the batteries are called the secondary batteries (rechargeable batteries).

Rechargeable batteries are working as storage devices to convert electrical energy into electrochemical energy. This process is called charging. The positive pole of external power supply should be connected to the positive electrode of the battery, and the negative pole should be connected with the negative electrode.

5 During the charging process, half-cell reactions of two electrode were reversed, as shown in Equation 2.4, 2.5 and 2.6. The negative electrode undergoing reductive reactions is the cathode, and the positive electrode occurring oxidation is the anode.

At the negative electrode:

n  M  ne  M (2.4) At the positive electrode:

n  X  X  ne (2.5)

Overall reaction (charge):

n n M  X  M  X (2.6)

2.1.3. The Primary and Secondary Batteries

The earliest primary battery is Daniell cell, which is firstly constructed by a

British chemist John Frederic Daniell in 1836. Daniel cell is working with a zinc electrode immersed in zinc sulfate solution as the anode, a copper electrode immersed in copper(II) sulfate solution as the cathode, and a salt bridge in between.

Daniel cell can reach the voltage about 1.1 V. By the late 1860s, Daniel cell was supplanted by Leclanché cell (1.5V), which used a zinc anode, a cathode manganese dioxide mixed with a little carbon, and ammonium chloride solution as the electrolyte. It was also well known as “ Dry cell” after changing the aqueous electrolyte to NH4Cl paste and replacing the cell container with zinc anode. Today, the Leclanché cell is limited to use in low-drain devices due to the shortage of low capacity and polarization. The alkaline-manganese batteries have been commercially

6 manufactured and widely used in toys, tape recorders, radios handheld calculators and electric clocks since 1950s. Compared to the Leclanché cell, the shelf life and energy density are improved by changing the electrolyte into KOH and replace the zinc electrode with zinc power packed on the current collector. However, these aqueous electrolyte batteries are suffering with the low battery operating voltage. So, scientists begin to use organic electrolyte for high voltage battery applications[1,2].

Lithium can form a stable, solid electrolyte interface(SEI) to protect the electrodes during the electrochemical reaction with organic electrolyte. With the high specific capacity (3.86 Ah/g) and high reduction potential (-3.045 V) of lithium, the primary lithium batteries use organic electrolyte can reach higher energy densities than the former ones. Primary lithium batteries mostly use the lithium metal as the anode and materials such as CuS,I2,MnO2, CFx, V2O5 and SO2 as the cathode. Among those lithium primary batteries, the Li-CFx and Li-MnO2 battery are the most popular product in the market for electronics applications, such as cameras, watches and calculators.

The secondary batteries (rechargeable batteries) are designed for energy storage and long-term used electronic devices. Rechargeable batteries can be used for several times, keeping stable capacities due to the reversible electrochemical reactions. The lead-acid battery firstly appeared in 1850s. Due to low cost, good cyclic performance and wide temperature range, lead-acid batteries are widely used in automobile starting, emergency lighting and uninterruptible power supplies (UPS).

Up till 1990s, newer Ni-MH and lithium ion batteries were invented and rapidly

7 replaced the lead-acid batteries with higher cyclic capacity and energy density.

Ni-MH batteries are considered to apply in hybrid electric vehicles for high energy storage capacities. Lithium ion batteries taking advantage of light weight and high energy density became popular in 1990s. The following section will mainly talk about developments in lithium ion batteries.

2.2. Introduction to Lithium Ion Batteries.

In recent decades, with the shortage of fossil fuel resource and lack of energy storage devices, advanced batteries harvesting sustainable energy begin to play more and more important role. One of the most attractive research fields is the lithium ion batteries, with high energy density and light-weight suitable for portable electronic devices. In 1970s, the concept of Lithium-ion battery was first introduced by replacing the metallic lithium with the lithium compounds which can accept and release lithium ions[3-5]. The reversible intercalation of graphite anode was found and developed well during 1970s to 1980s[6-9]. At the meantime, the reversible intercalation of lithium metal oxide cathode was applied and developed[10-12]. In

1895, Akira Yoshino applied the carbonaceous as anode and lithium cobalt oxide

(LiCoO2) as cathode into the lithium batteries, representing the birth of current lithium ion batteries[13]. Sony company promoted the secondary lithium ion batteries to the commercial reality in the early 1990s, due to the application of lithium insertion compounds (LiCoO2) as the cathode materials and carbon as the

8 anode materials. These batteries can operate over 3.6V and high energy density of

150 Wh Kg-1[14]. Since then, rechargeable lithium-ion batteries, which are light, compact, present high open circuit voltage, and high energy density, are used predominantly in portable electronic devices such as mobile phone, laptops and digital cameras. The most commonly used lithium-ion batteries consist of graphite anode, e.g., mesocarbon microbeands, (MCMB), lithium metal oxide cathode LiMO2, e.g., LiCoO2, electrolyte containing lithium salt, e.g., LiPF6, mixed with organic solvent, e.g., EC-DMC, and a separator, shown in Figure 2.1[15].

Figure 2.1. Conventional structure of a lithium-ion battery with cobalt oxide cathode and graphite anode bridged by an organic electrolyte, and a separator in between. (Reproduced from [15] with permission)

2.2.1. Lithium ion battery operations The first step of lithium ion battery is discharging. During the discharge process, the overall potential in the battery drives electrons moving from the negative electrode to the positive electrode, generating current flow through the external

9 loading. The negative electrode as the anode happen the oxidative reactions to release lithium ions. The positive electrode as the cathode with reductive reaction to trap lithium ions. In lithium ion batteries, the anode and cathode side are determined by discharge process. The overall reaction and half cell reactions are listed in

Equation 2.7, 2.8 and 2.9.

Half-cell reaction in the positive electrode (cathode) LiMO  Li MO  xLi  xe 2 1x 2 (2.7) Half-cell reaction in the negative electrode (anode)

  yC  xLi  xe  Li C x y (2.8)

The overall reaction:

yC  LiMO2  LixCy  Li1xMO2 (2.9)

During the charging process, the external circuit gives an over-voltage to the battery, forcing the electrons in the external circuit to move from the positive electrode to the negative electrode. At the same time, lithium ions in the battery transport from the positive electrode to the negative electrode. The overall and half-cell reactions were reversed directions.

In current commercial lithium ion batteries, the cathode material provide lithium ion for the whole system. Actually, in cathode using LiCoO2, lithium ions cannot be completely released from the cathode materials(x<0.5). Only 0.5 Li/Co can be reversed without any capacity loss during cycling. So, the upper potential of

-1 delithiation is limited to 4.2 V and the capacity of LiCoO2 is only 140 mAh g , half

10 of the theoretical capacity[16]. The anode material, which is graphite here, can be reversibly intercalated by lithium ions around 0.1 - 0.2 V, forming LiC6 compounds[17]. The theoretical capacity is 372 mAh g-1, which is higher than

LiCoO2. But the low potential of lithiation causes the electrolyte reduction and forming solvent and salt reduction composites on the surface of graphite anode[18].

Composites on graphite surface are called solid electrolyte interface(SEI)[19]. The

SEI layer forms at the first discharge cycle and continue reducing the graphite capacity in next several cycles[20]. The thickness and components of SEI layer change during cycling, reducing the cycle life of batteries[21]. On the other hand, the formation of SEI layer can protect the graphite structure during lithiation and delithiation with 9% volume change in graphite anode[22]. To reduce the capacity decay and electrode failure, the SEI layer should be formed as stable, uniform, ion conductive and electron nonconductive structure on the anode surface. The structure of SEI composites is related to electrolyte components. Most commercial lithium ion batteries use EC-DMC as the solvent and LiPF6 as the salt, which can form a highly protective and passivated SEI layer on the surface of graphite anode[23]. The intercalation of Li ions into graphite can form up to idea stoichiometry LiC6 with nearly theoretical capacity. The reversible lithiation and delithiation can maintain sustainable capacity after hundred of cycles[24,25].

11 2.2.2. Basic concepts of lithium ion battery performance

Voltage(V): The theoretical cell voltage of lithium ion battery is called open circuit voltage, VOC. It is a thermodynamic value related to Gibbs free energy change in electrochemical reactions. The value of open circuit voltage depends on the redox reaction potential of active material in two electrodes, calculated in Equation 2.10.  Li   Li V  cath an cell F , F is the Faraday constant. (2.10)

Owning to the internal resistance R in the battery, the output voltage during discharge is lower than the open circuit voltage and the reaction voltage during the charge process is different from the equilibrium cell voltage. This phenomenon is polarization and the voltage difference is called overvoltage,  . The value of Vdis

and Vch can be calculated by Equation 2.11 and Equation 2.12. V V  V  I R dis oc dis oc dis (2.11)

V V  V  I R ch oc ch oc ch (2.12)

Capacity(Ah): The theoretical capacity of a battery is the total electricity generated or stored by electrochemical reactions of active materials in the battery.

The value of theoretical capacity depends on the quantity of active materials. In lithium ion batteries, charge/discharge capacity often evaluated by specific capacity, which means the total charge transferred by current in a charge-discharge cycle per

-1 mass of active materials (mact). The unit of specific capacity is Ah kg (gravimetric).

Equation 2.13 is used for calculating charge/discharge specific capacity for a given

12 current I. The cell specific charge/discharge capacity is related to the current I, representing the rate of electrochemical reactions happened in lithium ion batteries.

Many factors are distributed to the current rate in a cell, including the mass transfer of Li ions between electrolyte and electrode. At high charge/discharge current rate, the limited mass of Li ions intercalating and deintercalating the electrode cause a reversible loss of capacity. The current value also related to the charge transfer between electrode active particle and electrolyte, active particle and current collector.

Formation of SEI layer, electrode volume change, defects and decomposition cause irreversible loss of capacity during charge-discharge cycles. The irreversible capacity fade is called battery degradation.

t Idt 0 capacity  m act (2.13)

Energy (Wh), Specific Energy Density (Wh kg-1) and Volumetric Energy Density

(Wh L-1): The energy of a battery is the total energy delivered by the electrochemical system. Theoretical Energy is the product of theoretical capacity and voltage, depending on the type and amount of active material used in the battery. For lithium-ion batteries, specific Energy density is used for evaluation of the discharge process, which means the energy delivered by a battery per mass of active material.

The value of specific energy density is the product of specific discharge capacity and output voltage of the battery. The specific energy density can be measured by recording the discharge time of a battery with a constant Idis, in Equation 2.14. The energy density of current lithium-ion battery (~200 Wh kg-1) is still too low

13 compared with combustion of fossil fuels (~3000 Wh kg-1). Volumetric Energy

Density is used in some portable batteries for valuing the energy delivered by the battery per volume.

t IV(t)dt 0 specific energy density  m act (2.14)

Power density (W/h) and C rate: The output power of a battery system is the product of discharge current and voltage. Power density is the electric power delivered in the battery per mass of active materials. Power density is equal to the specific energy density divided by discharge time at a constant Idis. C rate is applied to represent the value of Idis , for example: 1 C rate means the current for discharging a battery in one hour and n C rate means the current for discharging a battery in 1/n hour.

Cycle life: The number of charge/discharge cycle before capacity fades to 80% of initial capacity is called cycle life of lithium ion batteries.

Shelf (calendar) life: The maximum time that a battery can maintain its original performance before usage.

Lithium ion batteries, which have high operating voltage, sufficient and sustainable capacity, enough energy and power density, long cycle and shelf life, safety and reliability, are in instant need of development for the shortage of efficient energy storage system and high-drain electric power source. The battery performance improved by cell engineering design and management has almost arrived its limits, but the new developments in battery chemistry and material design

14 still have large upside potential. Generally, there are three parts needed to be investigated:

1. The cathode materials with good structural stability, high rate of oxidation and reduction, high inherent capacity and operating voltage.

2. The anode with high specific capacity, low operating voltage, high electronic conductivity , quick ion transport, electrochemical and thermal stability.

3. The electrolyte which has high ionic conductivity, thermal stability, electrochemical stability and compatibility with electrodes.

In the whole battery systems, each part of the battery will affect another. Not only the individual part needs to be cared about, but also the interaction between each component also needs to be investigated.

2.2.3. The development of cathode in lithium ion batteries

The cathode of lithium-ion battery serves as positive electrode where active materials are typically transition metal oxides. Most commonly used transition metal oxides are LiCoO2, which can form layered structure for lithium ion intercalation and deintercalation. Although the performance of LiCoO2 is fine in most commercial batteries, the cost of cobalt is relatively high compared to other transition metals, like manganese, iron and nickel. Also, the degradation of LiCoO2 at high voltage limits energy density of lithium ion batteries[26].

1 4+ 3+ Layered LiMnO2 has a theoretical capacity of 285 mAh g with Mn /Mn redox couple. However, the pure LiMnO2 crystal is an orthorhombic structure with

15 only half of theoretical capacity[27-30]. The transformation to layered structure like

LiCoO2 with good lithium intercalation require adoption of other elements[31,32].

The addition of cobalt and nickel into LiMnO2 can form α-NaFeO2 layer type structure, e.g. LiNi1/3Co1/3Mn1/3O2 with high specific capacity around 150-200 mAh

-1 g [33-35]. The capacity of LiNixCoyMn1-x-yO2 is related to the operation voltage. For

-1 -1 instance, LiNi1/3Co1/3Mn1/3O2 generates capacity of 159 mAh g , 168 mAh g and

179 mAh g-1 at 2.8-4.2 V, 2.8-4.35 V and 2.8-4.5 V, reported by Lee et al[34]. But for higher operating voltage, the capacity decay by cycling increases a lot[36,37]. Small amounts of cobalt in LiNixCoyMn1-x-yO2 can improve the cycling performance, for the reason that cobalt improves conductivity and structure stability in the cathode[37-39]. Nickel has been reported to impede lithium transport, but it can stabilize the structure to improve cycling performance of LiNixCoyMn1-x-yO2 too[40].

Cathode Materials like LiNiO2, LiCoO2 and LiNixCoyMn1-x-yO2 all have the intrinsic advantage of high energy density for its compact lattice structure.

Another promising cathode material is lithium phosphates (LiMPO4, M=Mn, Ni,

Co, Fe) with olivine structure. Compare to transition metal oxides, O2- is replaced by

3- a polyanion (PO4 ), which occupies tetrahedral sites in olivine structure with transition metal(M) octahedral connected. So, lithium ions have free volumes to move along one-dimensional chains in [0 1 0] direction[41]. LiFePO4 is the most commonly used phosphate cathode material for nontoxicity, high power capacity, remarkable thermal stability and low cost. LiFePO4 has a theoretical capacity around

-1 170 mAh g with LiFePO4/FePO4 redox couple[42,43]. The oxidation process in

16 LiFePO4 cathode is shown in Equation 2.15. The oxidation occurs at low voltage

(3.4V) and oxidation products are stable ferric phosphates, so LiFePO4 cathodes are very safe and stable during cycling. However, polyanion materials suffer from poor inherent electronic conductivity[44]. This problem was solved by reducing particle size[45] and coating the surface of LiFePO4 with conductive carbon composites[46].

Synthetic LiFePO4 can reach up to 90% of its theoretical capacity after modification[47,48]. Besides, low operation voltage of LiFePO4 cathode limits the energy density of batteries. Other phosphates like LiMnPO4, LiCoPO4 and LiNiPO4 with high operating voltage (4.2 V, 4.8 V and 5 V, respectively) have been mixed with LiFePO4 in the cathode[49-54].

LiFePO  Li  FePO  e 4 4 (2.15)

Of course the surface property of active materials and transportation distance play an important role in electrode performance. Nanostructured electrodes with large surface area, short charge or mass transfer distance and freedom volume change attract more and more attention in recent decades. For example, layered

-1 MnO2 nanobelts have capacity of 230 mAh g up to 30 cycles[55] and platelet

-1 structured V2O5 films have capacity improved to 1240 mAh g [56]. However, the capacity retention of those nanostructured electrodes is still very low, compared with commercial electrodes.

17 2.2.4. The development of anode in lithium ion batteries

Lithium metal was used as anode material in primary lithium batteries. Lithium metal as an idea anode material for Lithium ion batteries has operating voltage of 0V vs. Li/Li+ and theoretical capacity up to 3860 mAh g-1[57]. Nevertheless, the growth of dendrites on lithium metal during cycling can cause short circuiting[58,59] and undergo thermal runaway reactions in batteries to catch fire[60,61]. So, lithium metal was replaced by carbon based materials for safety concerns in the mid 20th century. Graphite is one of the most popular carbon based intercalation compounds, widely used in commercial lithium ion batteries[62].

Graphite with layered planar structure has a theoretical capacity up to 372 mAhg-1 , 1 Li atom intercalating per 6 C atoms. Micronic graphite particles bonded with polymer binders can form a porous and oriented structure with nearly theoretical capacity[63,64]. Electrolyte solution permeates graphite electrode with enlarged reactive interface and relatively small volume expansion owing to the porous structure in graphite layers[65]. However, the electrolyte reduction products deposit on graphite electrode at low voltage range forming SEI[66]. SEI works as a passivation film on the surface of fresh carbons, consuming lithium ions and causing a rapid drop of capacity[67]. But an electron nonconductive and ion conductive SEI works as a protective film on the surface of graphite electrode to prevent graphite exfoliation by enhanced mechanical properties. SEI layers can also prevent further electrolyte reduction by blocking electron transport during cycling[68]. Various studies focused on SEI found that carbonate-based electrolyte can form stable SEI

18 structure on the surface of graphite electrode with sustainable battery performance during cycling[69-71]. Hence, the application of graphite anode in lithium ion batteries is restricted to a few types of electrolytes.

Lithium titanium oxide (Li4Ti5O12/LTO) is another anode material used for its safety lithium ion chemistries[72]. The unit crystal of LTO with spinal type structure can reversibly intercalate 3 lithium atoms to form rock salt type structure Li7Ti5O12, with a theoretical capacity of 175 mAh g-1[73-75]. The formula of this reaction is shown in Equation 2.16. With only 0.2% volume change from spinal to rock salt structure, LTO anode is well known as "zero strain" with long cycle life (up to ten thousand cycles)[76,77]. High equilibrium potential of the LTO anode (1.55V vs.

Li/Li+) avoids the growth of lithium dendrites, making battery more safety. Besides, the operating window of LTO is above 1V, preventing the formation of SEI layer[78,79]. So, LTO particles with a nano-structure can be used to improve rate capacity[80-82]. However, high operating voltage of LTO anode limits specific energy density of lithium ion batteries. Li Ti O  3Li  3e  Li Ti O 4 5 12 7 5 12 (2.16)

2.2.5. The development of electrolyte in lithium ion batteries

Besides the abovementioned electrodes, electrolyte is another key issue to improve the performance of lithium-ion battery. Electrolyte applied in batteries must be ion conductive and electron nonconductive. Highly conductive electrolyte benefits rate capacity and power density of lithium ion batteries. All the electrolyte components are electron insulated to prevent short circuit of batteries.

19 Nonaqueous liquid electrolytes comprising lithium salt and solvent have been applied in lithium ion batteries for its wider electrochemical windows (up to 5.5 V vs.

Li/Li+) than aqueous electrolyte[83,84]. The solvent used in electrolyte must contain polar groups such as carbonyl (C=O), sulfonyl (S=O), nitrile (C≡N) and ether (-O-) to dissolve enough lithium salts[3]. Organic ethers and esters are most commonly used solvent in nonaqueous electrolytes[85]. The basic function of electrolytes is to transport lithium ions between two electrodes. To improve the ionic conductivity of electrolyte, solvents with low viscosity and high dielectric constant are preferred for high dissociation and migration of lithium ions[86]. Generally, two or more solvents are used together in most electrolyte system to create a high fluidity and high dielectric constant electrolyte[87-90]. Compared to variety of solvent, the choice of lithium salt is limited because small radius lithium ions are hard to dissociate in most simple salts. The anions of lithium salt are commonly complex of a simple anion core with a Lewis acid agent, such as lithium hexafluorophosphate(LiPF6)[91], lithium perchlorate (LiClO4)[92], lithium hexafluoroarsenate (LiAsF6)[93] and lithium tetrafluoroborate (LiBF4)[94][95]. Another kind of Lithium salts is composed by conjugated bases such as lithium bis(trifluoromethanesulfonyl)imide

(LiTFSI)[96]. In most liquid electrolyte systems, only one kind of lithium salts is used with two or more organic solvent for the limitation of anions, such as

EC-DMC/LiPF6 system[15]. In this system, EC is a highly polar organic solvent which can dissolve lithium salt efficiently. A perfect SEI film can be formed to protect electrode when EC is applied with graphite anode in most commercial

20 batteries. Because of high melting temperature(38 oC), EC often used together with some linear carbonates such as EMC, DEC or DMC to decrease the melting point and fluidity of electrolyte. However, low flash point of these cosolvent will cause safety issues at high operating temperatures. Commercial lithium-ion batteries using liquid electrolyte have caused some safety issues, such as recent Samsung Note 7 cell phones catching fire by lithium-ion battery burning[97]. Besides, poor mechanic strength of liquid electrolyte cause liquid leakage under certain pressure. Hence, solid state electrolytes with high thermal stability and mechanic strength are preferred in lithium ion batteries.

2.3. Polymer Electrolyte for Lithium Ion Batteries

Safety concerns about lithium ion batteries in recent decades are mainly attributed to thermal runaway caused burning and explosion. Thermal runaway is caused by exothermic chemical reactions between electrode and electrolyte, increasing the internal temperature of batteries[98]. When temperature raised up to electrolyte evaporation temperature or flash point, flammable organic solvents used in liquid electrolyte can be converted into flammable gases, catching fire and explosion[99]. One way to solve the safety problem caused by fire hazard of liquid electrolyte in lithium ion batteries is to replace the flammable liquid electrolyte by fire retardant polymer electrolytes.

21 2.3.1. The development of polymer electrolytes

Typical polymer electrolytes were based on poly(ethylene oxide) (PEO) and lithium salt complex. PEO with polyether chains can dissociate and transport lithium ions by polymer chain motions. It was reported that Li cations can form ion-dipole complexation with ether oxygen along PEO chains, and thus transported by

"trans-gauche" transformation of PEO chains[100-102]. LiTFSI as a lithium salt with non-coordinating anions has extensive charge delocalization[103]. LiTFSI can be easily dissociated by PEO and form a eutectic composition[134]. However, the ionic conductivity of PEO/salt system were limited by crystallinity and glass transition

-9 -5 temperature(Tg), changing from 10 S/cm to 10 S/cm at room temperature[104].

Therefore, to improve the ionic conductivity of PEO-based polymer electrolyte, nonvolatile plasticizers were added into PEO/salt system to reduce Tg and remove crystals.

In the former thesis of our group, Echeverri et al.[105] added succinonitrile

(SCN) as a solid plasticizer with good thermal stability into PEO/LiTFSI system, improving ionic conductivity up to 10-4 S/cm at ambient temperature. In this system, solid plasticizer SCN not only improved PEO chain mobility, but also ionized lithium salts. Due to high polarity, SCN can dissolve LiTFSI with only composition of 5 wt%, reported by Long et al[106]. Later on, PEO was replaced with photo-curable poly(ethylene glycol) diacrylate (PEGDA) to improve the mechanical strength of polymer electrolyte[107]. Thus, PEMs composed by SCN as solid plasticizer, LiTFSI as high conductive salt, and PEGDA as polymer matrix was

22 subsequently fabricated, with superionic conductivity (~10-3 S/cm). More electrochemical stability issues were further investigated by He et at.[108,109] In this test, PEM's anodic stability was up to 4.8 V vs. Li/Li+, which is much higher than most carbonate, sulfone, and ionic liquid electrolyte systems. The wide electrochemical window of polymer electrolyte is closely related to the solid plasticizer SCN. It was found that SCN have used in high voltage lithium ion batteries with anodic stability up to 6 V vs. Li/Li+[110]. Besides, others also found that aprotic aliphatic dinitrile solvents with a chemical formula of NC-(CH2)n-CN

(n=3-8) have high anodic stability up to 7-8 V vs. Li/Li+[111,112].

2.3.2.Polymer electrolytes based on dinitrile solvents for high voltage applications

Dinitriles (NC-(CH2)n-CN) such as succinonitrile (SCN), glutaronitrile (GLN) and adiponitrile (ADN) with increasing number of methylene groups (n=2,3,4) exhibit high thermal and electrochemical stabilities. As reported, dinitrile solvents are commonly used with LiTFSI as safety electrolytes for high voltage batteries. The choose of lithium salt is limited by moderate dielectric constant of dinitriles.

Generally, the dielectric constant of dinitriles will decline with increasing number of methylene groups. But the boiling point and flash point of dinitrile solvents will increase with incremental methylene groups. The ionic conductivity of dinitrile/LiTFSI electrolyte is around 10-3 S/cm, lower than conventional liquid electrolyte but still higher than most polymer electrolytes[110,113-114]. In this thesis, dinitrile solvents (SCN,ADN,GLN) were used as plasticizers in polymer electrolyte

23 membranes mentioned before, to investigate their physical and electrochemical properties for lithium ion battery applications.

The ternary and binary phase diagrams to fabricate ADN plasticized PEMs has been established in the former thesis, shown in Figure 2.2 and Figure 2.3. The binary and ternary phase diagrams of PEGDA/SCN/LiTFSI were established by Echeverri et al.[105][107] So, binary and ternary phase diagrams of PEGDA/GLN/LiTFSI system will be studied in this thesis.

Figure 2.2. a) Binary phase diagram of PEGDA/ADN blends around with POM pictures taken in each coexistence region. b)Binary phase diagram of LiTFSI/ADN blends combine with POM pictures taken in certain crystals coexistence regions. The overlapping area was enlarged and shown in inset.

Figure 2.3. Ternary phase diagram of PEGDA/ADN/LiTFSI at 25 oC.

25 CHAPTER III

MATERIALS AND EXPERIMENTS

3.1. Materials

3.1.1. Poly (ethylene glycol) diacrylate (PEGDA)

PEGDA is a semi-crystalline polymer at room temperature depending on its molecular weight. The structure of PEGDA is shown in Figure 3.1. PEO segments can transport Li-ions and PEGDA networks provide mechanical support for PEMs. The number averaged molecular weight (Mn) of the PEGDA in this thesis is 700g/mol. It was purchased from Sigma-Aldrich and dried for 24 hours in the vacuum oven at room temperature before use.

3.1.2. Succinonitrile (SCN)

SCN is an organic plasticizer which has high thermal stability (boiling point of about 266 oC and high flash point of about 113 oC) along with high anodic stability. It serves as an effective ionizer to dissociate the Li+ ions in PEMs. The chemical structure of SCN is depicted in Figure 3.2, The SCN (99% purity) used in this research was purchased from Sigma-Aldrich.

26 3.1.3. Glutaronitrile (GLN)

GLN is another organic plasticizer used in PEMs. The GLN used in this thesis was purchased from Signa-Aldrich (99% purity). The boiling point of GLN is

285-287 oC and the flash point of GLN is 113 oC. The chemical structure of GLN is shown in Figure 3.3. GLN has one more methylene group than SCN.

3.1.4. Adiponitrile (ADN)

ADN was used in PEMs as an organic plasticizer with two more methylene groups than SCN. The boiling point of ADN is 295 oC and flash point is 163 oC. The

ADN used in this experiment was purchased from Sigma-Aldrich (99% purity) and chemical structure is shown in Figure 3.4.

3.1.5. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)

LiTFSI was used in PEMs as a salt which performs both good chemical and thermal stability. The structure of LiTFSI is shown in Figure 3.5. The LiTFSI has a melting temperature around 234 oC and decomposed at around 350 oC. High dissociation capability between Li+ cation and large TFSI- anion also results in high ionic conductivity of PEM. The LiTFSI (>99.95% purity) used in this thesis was purchased from Sigma-Aldrich and dried at 170 oC in a vacuum oven for 24 hours before use.

3.1.6. 2,2-Dimethoxy-2-phenylacetophenone

2,2-Dimethoxy-2-phenylacetophenone (Irgacure® 651) is the photo initiator used to crosslink PEGDA. Irgacure® 651 was bought from Aldrich. The structure of the Irgacure® 651 is shown in Figure 3.6.

Figure 3.1. The chemical structure of PEGDA

Figure 3.2. The chemical structure of SCN

Figure 3.3. The chemical structure of GLN

Figure 3.4. The chemical structure of ADN

Figure 3.5. The chemical structure of LiTFSI

Figure 3.6. The chemical structure of photo initiator Irgacure® 651

28 3.1.7. Materials used in thermal analysis of PEMs

Solvents: Methylene chloride (99.9% purity) and acetone (99.9% purity) were purchased from Fisher Scientific and Mallinckrodt, respectively. Above solvents were mixed at 40:1 weight ratios. They were used in this thesis to dissolve mixtures with high lithium salt concentration. The boiling point of methylene chloride and acetone is around 40 oC and 56 oC, separately.

3.1.8. Materials used in battery preparation and electrochemical tests

Lithium foil: Lithium foils working as reference electrode in half-cells were purchased from Alfa Aesar (99.9% purity). The thickness of the Li foil is 0.75mm.

Lithium foils were stored and used in a glovebox with argon gas circulation.

LiFePO4: LiFePO4 is one of the cathode materials for lithium-ion batteries. The

-1 theoretical capacity of LiFePO4 is 170 mAh g . LiFePO4 was purchased from MTI and dried 12 hours in a vacuum oven at 110 oC before use.

Li4Ti5O12: Li4Ti5O12 was purchased from MTI. It works as anode material and

-1 has a theoretical capacity of 175 mAh g . Li4Ti5O12 was dried 12 hours in a vacuum oven at 110 oC before use.

Carbon Black (SUPER P®): Carbon black served as the electron conductivity enhancer in both cathode and anode. Carbon black used in this thesis was purchased from MTI and dried in the vacuum oven at 110 °C for 12 hours before making slurry.

Polyvinylidene Fluoride (PVDF): PVDF works as the binder material in both anode and cathode, connecting electrode materials with current collectors. PVDF used here was purchased from Sigma Aldrich (Mw=534,000). PVDF was dried in

29 the vacuum oven at 110 oC for 12 hours before making slurry with CB and active materials.

1-Methy-2-pyrrolidinone (NMP): NMP is a solvent used to dissolve the electrode materials forming a uniform slurry. NMP (anhydrous (99.5%)) was purchased from Sigma Aldrich.

Aluminum Foil: Al foil is the current collector on the cathode side. The Al foil purchased from MTI has a thickness of 15m and purity higher than 99.3%.

Cuprum Foil: Cu foil is the current collector on the anode side. Cu foil was purchased from MTI with 9 m thickness and >99.9% purity.

3.2 Experiment Techniques

Experimental methods for sample preparation, PEM fabrication, electrode preparation, half-cell assembly, and characterization techniques for electrolyte solutions, PEMs and half-cells are introduced in this section.

3.2.1. Polymer electrolyte preparation and membrane fabrication

Before PEM fabrication, binary and ternary phase diagrams of

PEGDA/plasticizer/LiTFSI system were established to guide processing. These binary and ternary solutions were prepared in an argon gas filled glovebox. Because the binary and ternary phase diagrams of PEGDA/SCN/LiTFSI system and

PEGDA/ADN/LiTFSI system have already been established, only

PEGDA/ADN/LiTFSI system was studied in this thesis. The composition of PEMs was chosen from ternary phase diagrams. For example, 20/50/30

30 PEGDA/GLN/LiTFSI represents the blends containing 20 wt% PEGDA, 50 wt%

GLN and 30 wt% LiTFSI. The mixtures were blended by mechanical stirring at

80 °C until completely removing the crystals, and then cured by photo-polymerization to fabricate PEMs.

For the binary mixtures of PEGDA/GLN and LiTFSI/GLN, samples with different concentrations were mixed in isotropic state for differential scanning calorimetry (DSC) characterization. The ternary mixtures were prepared in the same way to establish the ternary phase diagram. According to the ternary phase diagram at 25 °C, isotropic polymer electrolytes with different concentrations were prepared and sealed in amber vials before fabricating PEMs. PEMs were fabricated by photo-polymerization of the isotropic polymer electrolytes prepared above. 2 wt%

Irgacure® 651 was added into the liquid electrolyte blends and dissolved well before

UV crosslinking. Then, the solution was placed into a mold and covered by a cover glass on top. An isotropic polymer electrolyte membrane was formed by applying

UV light for 20 minutes.

3.2.2. Thermal analysis of polymer electrolyte membranes

Differential Scanning Calorimetry (DSC):

The phase transition temperatures of aforementioned mixtures and polymer electrolyte membranes (PEMs) were measured by DSC (TA Instruments, Model Q200) under constant nitrogen flow. Samples were weighted in recommended range between

5 mg and 10 mg and hermitically sealed into aluminum pans in the glovebox. Another sealed aluminum pan without sample was used as a reference in each measurement.

31 Thermal Gravimetric Analysis (TGA):

The thermal stability of polymer electrolyte membranes and neat components were determined by TGA (TA Instruments, Model Q50). Samples weighting among the preferred amount of 10-15 mg were placed in a platinum pan. Before each measurement, the empty weight of platinum pan was tiled at least three times.

3.2.3.Electrochemical analysis of PEMs

Electrodes preparation:

Electrode slurries were prepared by mixing active materials (LiFePO4 and

® Li4Ti5O12), PVDF and carbon black in NMP by sonication (Sonifier , Branson).

Li4Ti5O12/CB/PVDF slurry were mixed at the weight ratio of 80/15/5 and

LiFePO4/CB/PVDF slurry were mixed at the weight ratio of 80/10/10. The electrode slurry was coated on the current collector (Al foil and Cu foil). Coated Al foil and

Cu foil were placed on a hot stage to remove solvent NMP at 60 °C for 2 hours, and then dried in a vacuum oven for 12 h at 110 °C. The dried electrodes were stored in a glovebox before use.

Battery assembly:

Two types of 2032 coin cells (LiFePO4/PEM/Li and Li4Ti5O12/PEM/Li) were assembled in a glovebox filled with argon gas. LiFePO4/PEM/Li and

Li4Ti5O12/PEM/Li half-cells were assembled for galvanostatic charge-discharge cycling and cyclic voltammetry (CV) measurements.

32 AC Impedance Spectroscopy:

The ionic conductivity of various polymer electrolyte membranes was measured by impedance spectroscopy analyzer (HP 4192A LF). A hot chamber was used to control the temperature applied on samples during measurement. The complex impedance was measured with a voltage amplitude of 5 mV, sweeping from 13 MHz to 5 Hz.

Cyclic Voltammetry (CV):

Cyclic voltammetry tests of half-cells (LiFePO4/PEM/Li, Li4Ti5O12/PEM/Li) were conducted by CV (Model 1260 and 1287, Solartron Analytical Inc.).

Electrochemical stability of half-cells containing different polymer electrolyte membranes was determined by using Li4Ti5O12 or LiFePO4 as working electrode and

Li foil as reference electrode as well as counter electrode.

Galvanostatic charge-discharge cycling:

Charge-discharge cycling test of LiFePO4/PEM/Li and Li4Ti5O12/PEM/Li half cells were performed by using an 8-channel battery cycler (MIT Corp.). For

LiFePO4/PEM/Li half-cells, the operating voltage is 2.5 V-4.0 V for

LiFePO4/PEM/Li half-cells and 1.0 V-2.5 V for Li4Ti5O12/PEM/Li half-cells, with a constant current rate of 0.25 C and 1 C.

33 CHAPTER IV

BINARY AND TERNARY PHASE DIAGRAMS OF VARIOUS DINITRILES

PLASTICIZED POLYMER ELECTROLYTE SYSTEMS

4.1. Introduction

In order to fabricate completely amorphous polymer electrolyte membranes, the isotropic region of precursor mixtures should be determined by establishing phase diagrams. PEGDA/SCN/LiTFSI polymer electrolyte mixtures were previously studied by Echeverri et al[107]. The binary and ternary phase diagrams of

PEGDA/ADN/LiTFSI system has also been determined in the present author’s bachelor thesis. Hence, only the binary and ternary phase diagrams of

PEGDA/GLN/LiTFSI system will be studied in this section.

To establish the ternary phase diagram of PEGDA/GLN/LiTFSI blends, we should firstly study the binary phase behavior of three binary mixtures accordingly. The binary phase diagram of PEGDA/LiTFSI has already been obtained by Echeverri et al, shown in Figure 4.1[107]. The pure LiTFSI, PEGDA and the coexistence region of LiTFSI crystal with isotropic liquid as well as PEGDA crystal with isotropic liquid were observed by polarized optical microscope (Figure 4.1A, B, C, D, E, F). In this Chapter, binary phase diagram of PEGDA/GLN and LiTFSI/GLN blends and ternary phase diagram of PEGDA/GLN/LiTFSI were established and verified by microscope pictures.

4.2. Experimental Sections

4.2.1. Solution blending

To determine the binary phase diagrams of PEGDA/GLN, LiTFSI/GLN and ternary phase diagram of PEGDA/GLN/LiTFSI blends, various concentrations of

PEGDA, GLN and LiTFSI mixtures were blended in the isotropic melt state. For the

PEGDA/GLN binary system, various PEGDA and GLN mixtures were blended by mechanical stirring at 60 oC for 15 minutes to ensure completely mixing. Because

LiTFSI is sensitive to moisture, LiTFSI/GLN binary mixtures and

35 LiTFSI/GLN/PEGDA ternary mixtures were blended in a glove box under continuous argon gas flow. For high LiTFSI concentration blends, solvent mixture of methylene chloride/acetone at 40:1 weight ratio was used to ensure complete dissolution of the LiTFSI in these mixtures. Removing the solvent was conducted in a hot stage under constant nitrogen flow at 65 oC. Finally, all the samples were sealed in amber vials and kept in the glove box until use.

4.2.2. DSC Analysis

To determine the melting temperature of various binary and ternary mixtures,

DSC (TA Instruments, Model Q200) was used. Samples weighing 5-10 mg were sealed hermetically into Aluminum Hermetic pans, by using a crimping machine in a glove-box to prevent moisture absorption. An empty pan was used as the reference.

DSC scans were acquired under a constant nitrogen flow at 25 mL/min.

In each DSC scan of PEGDA/GLN blends, samples was first heated to 50 oC and cooled down to -80 oC at the rate of 5 oC/min, then isothermally kept at this temperature for 15 minutes. After that, the samples were heated up to 50 oC at the rate of 5 oC/min. In the case of LiTFSI/GLN mixtures, DSC scanning steps were the same as the former one, except for the third heating cycles. Samples with concentration of LiTFSI/GLN 95/5 and pure LiTFSI were heated up to 250 oC, others were heated to 80 oC. The ternary mixture of PEGDA/GLN/LiTFSI also conducted in the same way as the binary systems, but the ramping rate was 10 oC

/min and heating step was up to 80 oC. All the scans were performed twice, because the first scan was designed to remove the thermal history during sample preparation.

36 Only the data from the second run was used in the analysis. The melting transition points of various concentration mixtures were determined from the position of the endothermic peaks derived from DSC data, and further used in phase diagram simulations.

4.2.3. Polarized optical microscopic characterization

Polarized optical microscopic (POM, Olympus BX60) pictures were taken by a

35 mm digital camera (EOS 5D Mark II, Canon) to observe the phase behavior in different regions in phase diagrams. The mixed solution was placed on a glass slide with a cover slip after removing solvent, and then transferred to a sample heating/cooling chamber (TMS93, Linkam) under constant nitrogen flow to afford inert environment. The samples were firstly heated up to 50 oC and cooled down to

-50 oC, then heated to certain temperatures consistent with DSC analysis. Crystal structures were observed at different temperatures by cross-polarized optical microscope.

4.3. Results and Discussions

4.3.1. Binary phase diagram of PEGDA, GLN and LiTFSI blends

To draw the binary phase diagram of PEGDA/GLN blends, DSC thermograms of various compositions were vertically plotted at 10 wt% intervals in Figure 4.2.

The melting temperature of pure GLN is -28 oC. With the increasing of PEGDA concentration, melting transition temperature of GLN decreased. Similarly, the

37 o melting transition temperature of PEGDA (Tm=15 C) declined with increasing GLN plasticizer. No endothermic peak appeared at the intermediate concentrations

(PEGDA 20 wt%-40 wt%), which indicates GLN and PEGDA are miscible.

A melting point depression behavior of PEDGA/GLN binary phase diagram was observed by plotting the melting transition temperature points in Figure 4.3. The liquidus and solidus lines in this binary phase diagram were drawn by self-consistently solving the thermodynamic theory which combines the

Flory-Huggins free energy for liquid-liquid demixing and Landau free energy of crystallization[115-117]. Equation 4.1 was applied in the self-consistent calculation.

 1 f  , ,   f   1 f   ln  ln 1 x  x  2  2x    x  2  1 1 2 1 2 r r aa ac 1 cc 1 2 ac 2 1 2 (4.1)

The material and experiment parameters used are: the number of statistical

c 1 segments r1  25 , r2  5 . The heat of fusion H1  4600J mol ,

c 1 H2 11720J mol . The melting temperatures of each constituent are Tm1=15 °C,

Tm2=-28 °C. The subscripts 1 and 2 denote PEGDA and GLN, respectively. The adjustable parameters were set at Cw= 0.5 and a= -0.55.The details were written in our previous published papers[116,118].

Figure 4.2. DSC thermograms of PEGDA/GLN blends as a function of composition, exhibiting the declining trends of the melting transitions of both constituents.

The phase diagram thus drawn showed a eutectic point at -31 oC around 25 wt% of PEGDA concentration. The liquidus line drawn by our theory almost captured the experimental trend and the theory also figured out the solidus line, which cannot be obtained by experiment. Dash solidus lines were drawn to fit the theoretical trend and experiment data. In addition, the corresponding microscope pictures were shown in Figure 4.3A and Figure 4.3B to verify coexistent regions determined by theory model. A well-developed lamellar crystals of PEGDA were shown in Figure A.

Figure B was taken in the coexistence region of crystal PEGDA and isotropic liquid of PEGDA/GLN blends, from which small changes of PEGDA lamellar crystals can be observed. All the pictures were taken under cross polarization.

Figure 4.3. The Binary phase diagram of PEGDA/GLN blends around with POM pictures taken in each coexistence region. A eutectic phase behavior was shown and the dash line was drawn for guidance. For the LiTFSI/GLN system, thermograms of various LiTFSI/GLN concentrations were drawn in Figure 4.4. The melting transition temperature of pure

GLN is -28 oC. No crystal and melting peaks appeared within the temperature range by adding up to 90 wt% of LiTFSI. The blends were completely amorphous, which proves the high ion dissociation occurs when LiTFSI interacts with GLN. This is consistent with ADN/LiTFSI system and SCN/LiTFSI system[119].The melting transition temperature of pure LiTFSI located at 230 oC and 150 oC, which predicted a solid-solid phase transformation mentioned by Hendeson et al[120]. Similar phase transition appeared at 95 wt% of LiTFSI concentration with two melting peaks at

201 oC and 140 oC.

The melting transition temperatures from DSC thermograms were plotted against the LiTFSI concentration with interval of 10 wt% in Figure 4.5. Dash lines were drawn to connect the experimental data points. Each coexistent region was determined by theory and experiment, then verified by microscope pictures obtained under polarized conditions. The pure LiTFSI crystals were shown in Figure 4.5C and Figure 4.5D.

Figure 4.5A and Figure 4.5B were taken in the high LiTFSI concentration region, which show LiTFSI crystals containing a minor amount of the GLN component. A wide isotropic region appeared in binary phase diagram of LiTFSI/GLN, on account of the high affinity of the nitrile groups with lithium ions. As reported in

+ − + − succinonitrile[121] and acetonitrile[122], Li(CNCH2CH2CN)2 X and Li(CH3CN)4 X complexes were formed by four cyano groups coordinating one Li cation. Hence, with high polarity of GLN, nearly 95 wt% LiTFSI were dissolved in only 5 wt% GLN.

4.3.2. Ternary phase diagram

With the information provided by binary phase diagrams of each two components obtained above, the ternary phase diagram of PEGAD/GLN/LiTFSI blends at 25 oC was established by DSC, in Figure 4.6. The ternary diagram exhibits a wide isotropic region, which serves a processing composition window for fabrication of completely amorphous polymer electrolyte membranes (PEMs). At 25 oC, only a small range of the triangular plane was covered by LiTFSI crystal regions, shown by microscope picture in Figure 4.6.

Figure 4.6. Ternary phase diagram of PEGDA/GLN/LiTFSI at 25 oC.

4.4. Conclusions

In this chapter, binary and ternary phase diagrams of PEGDA/GLN/LiTFSI blends were established to guide the processing window for achieving completely amorphous PEMs. In the binary system of PEGDA/GLN, GLN as a plasticizer with high miscibility in PEGDA promoted polymer chain mobility and declined the melting transition temperature. In the binary system of LiTFSI/GLN, nearly 95 wt%

LiTFSI were dissolved by only 5 wt% GLN solvent, showing high ion dissociation ability of GLN. According to binary phase diagrams, ternary phase diagram of

PEGDA/GLN/LiTFSI was determined with only a small crystal region shown in rich lithium salt concentrations. Compared with PEGDA/SCN/LiTFSI system and

PEGDA/ADN/LiTFSI system, PEGDA/GLN/LiTFSI system had lower melting transition temperatures and higher ion dissociation.

43 CHAPTER V

IONIC CONDUCTIVITY OF UV-CROSSLINKED POLYMER ELECTROLYTE

MEMBRANES CONTAINING VARIOUS PLASTICIZERS

5.1. Introduction

In previous chapter, wide isotropic regions in ternary phase diagrams were determined, where polymer electrolytes may have high ion transport. Within the isotropic concentrations for PEGDA/SCN/LiTFSI, PEGDA/GLN/LiTFSI and

PEGDA/ADN/LiTFSI systems, transparent and amorphous polymer electrolyte membranes (SCN-PEM, GLN-PEM, ADN-PEM) were fabricated by UV crosslinking in this chapter.

According to previous investigation of ionic conductivity in SCN plasticized

PEMs by He. et al[123], the ionic conductivity of PEMs was enhanced by increasing amount of dissociated salts and accelerating polymer chain mobility. The room temperature ionic conductivities of SCN-PEMs, GLN-PEMs and ADN-PEMs were measured by changing the ratio of PEGDA/Plasticizer/LiTFSI. To compare the effect of plasticizers in ionic conductivity of three different systems, the concentration of plasticizers was changed from 30 wt% to 80 wt% by keeping the ratio of

PEGDA/LiTFSI at 1:2. For the application of PEMs in lithium-ion batteries, PEMs with suitable mechanical strength were made by keeping the concentration of PEGDA

44 at 20 wt%. The ratio of Plasticizer/LiTFSI was changed to enhance the ionic conductivity of PEMs for lithium ion battery applications. Besides, the phase transition and thermal stability of PEMs were investigated in this chapter.

5.2. Experimental Sections

5.2.1. Membrane fabrication and ionic conductivity measurement

Various ratios of PEGDA/SCN/LiTFSI, PEGDA/GLN/LiTFSI and

PEGDA/ADN/LiTFSI ternary blends were determined by the isotropic region in the ternary phase diagram to fabricate the conductive polymer electrolyte membrane via photo-polymerization. Subject to the PEGDA weight, 2 wt% of photo initiator

Irgacure® 651 were added in the mixtures. Then, the mixtures with initiator were placed in a special cell, which is designed for AC impedance measurement, shown in

Figure 5.1. The conductive membrane was formed by cross-linking the matrix polymer under uniform UV-light illumination using a UV cure lamp (Bondwand 350 nm) operated at an intensity of 5 mW/cm2 for 15 min. All the steps above were conducted in a glovebox under an argon gas circulation. A rubber ring was added between two electrodes of this special cell equipment for better sealing and taken outside for ionic conductivity measurement.

Ionic conductivities of PEMs were measured by an AC impendence spectrometer (HP4192A, Hewlett-Packard). The applied voltage was 5 mV and the frequency was varied from 13 MHz to 5 Hz during the measurement. In order to determine the conductivity variation with the temperature, temperature control was

45 employed by using a hot chamber with the temperature controller. All samples were kept in the hot chamber for at least 20 minutes to ensure the PEMs were evenly heated. Each ionic conductivity measurement was performed three times and averaged the results.

Figure 5.1. Geometry scheme of the cell, 10mm(length)×10mm(width)×1mm(depth), used for AC impedance measurement.

5.2.2. Thermal analysis for polymer electrolyte membranes (PEMs)

Thermal degradation of plasticizers (SCN, GLN and ADN) and their PEMs were measured by thermal gravimetric analysis (TGA, TA Instruments, Model Q50).

Samples were weighted between 10 mg and 15 mg and tiled before loading on the instrument. Samples of neat plasticizers were tested by raising the temperature up to

500 oC at the rate of 10 oC/min. Samples of PEMs were measured up to 800 oC at the same rate of 10 oC/min. All the measurements were conducted under constant nitrogen flow.

46 Differential Scanning Calorimetry (DSC, TA Instruments, Model Q200) was applied to study the phase transition phenomena of PEMs containing various plasticizers. Samples were weighted around 5-10 mg and sealed into Aluminum

Hermetic pans. An empty pan was sealed as reference too. All the sample preparation steps were performed in a glovebox filled with argon gas to avoid moisture. All the samples were heated up to 50 oC first, and cooled down to -80 oC, then heated up to

40 oC again at the rate of 5 oC/min under constant nitrogen flow. The first heating step was applied to remove the thermal history of samples. The second heating process was used for analysis.

5.3. Results and Discussions

The ionic conductivity of SCN-PEMs, GLN-PEMs and ADN-PEMs were measured at the compositions marked in Figure 5.2. The composition points in three phase diagrams were chosen by keeping the ratio of PEGDA/LiTFSI at 1:2.

Figure 5.3. Ionic conductivity of SCN-PEM (blue), GLN-PEM (black), ADN-PEM (red), with increasing amount of plasticizers(from 30 wt% to 80 wt%). The ionic conductivities of three kinds of PEMs were enhanced by increasing amount of plasticizers, shown in Figure 5.3. All the PEMs at high plasticizer concentrations manifested superionic room temperature ionic conductivity (10-3

S/cm), indicating that plasticizers play an important role in enhancing the ionic conductivity of PEMs. It is well known that the ionic conductivity is inversely related to the activation energy for the ion transport. The way to enhance the ionic conductivity is reducing the activation energy for ion transport, which might be realized by promoting polymer chain motion and ion dissociation. However, the

SCN plasticized PEMs crystallized at high SCN concentration regions, which limited the ionic conductivity of PEMs with SCN concentrations higher than 60 wt%.

GLN plasticized PEMs showed the highest ionic conductivity (up to 2.3×10-3 S/cm)

48 among three kinds of PEMs, which can be explained by its high LiTFSI dissolving ability and high miscibility with PEGDA improved in binary phase behaviors in

Chapter III. It is obvious to find that the uptrend of the conductivity of

GLN-PEMs and ADN-PEMs at high plasticizer concentrations(60 wt%-80 wt%) became slower than at low plasticizer concentrations. It should be noted that the high plasticizer concentration also means low salt and polymer concentration. Despite the ion transport was improved by increasing plasticizer, the reducing amount of dissociated ions affected the efficiency of plasticizers.

Although PEMs with higher plasticizer concentrations exhibit higher ionic conductivity, the concentration of polymer matrix should be kept at certain values in order to afford sufficient mechanical support. Therefore, in the following study, the concentration of PEGDA was fixed at 20 wt%.

The ionic conductivity variation will be studied upon changing the ratio of

LiTFSI/ plasticizer. The chosen concentrations are plotted in Figure 5.4.

Figure 5.4. Concentration plots of SCN-PEM (blue), GLN-PEM (black), ADN-PEM (red) on the ternary phase diagram at 25 °C.

From Figure 5.5, ionic conductivities of PEMs were enhanced by increase the concentration of LiTFSI from 10 wt% to 30 wt%. However, further addition of

LiTFSI caused a decline of ionic conductivity. The ionic conductivity of PEMs is influenced by polymer chain motions and the amount of dissociated ions. With the initial increase of salt concentrations, dissociated ions increased within the dissociating capability of polymer and plasticizer. Then, the ionic conductivity of

PEMs turned down by further addition of lithium salts. On the one hand, the limited amount of ether oxygen in PEGDA caused incomplete dissociation of lithium salt and formed coordinate bonds with lithium ions, restricting polymer chain motions.

50 On the other hand, increasing amount of salts decreased amount of plasticizers, and thus obstructed ion dissociation and chain mobility. Therefore, ionic conductivity of

PEMs arrived maximum values around 10-3 S/cm at 25%-30% amount of LiTFSI.

To keep both mechanical strength and superionic conductivity, PEMs with the ratio of PEGDA/ Plasticizer/LiTFSI at 20/50/30 were chosen to for application in the lithium-ion batteries.

Figure 5.6. Plots of ionic conductivity versus reciprocal absolute temperature for 20/50/30 PEGDA/SCN/LiTFSI PEM (blue), 20/50/30 PEGDA/GLN/LiTFSI PEM (black), and 20/50/30 PEGDA/ADN/LiTFSI PEM (red). To study the conductivity performance of PEMs plasticized by different dinitriles at high temperatures, the ionic conductivity of PEMs with 20/50/30

PEGDA/Plasticizer/LiTFSI concentrations were measured as a function of reciprocal temperature in Figure 5.6. The ionic conductivity of PEMs was increased from 10-3

S/cm at 25 oC to 10-2 S/cm at 150 oC, indicating superb high temperature

51 conductivity performance of PEMs plasticized by dinitriles. A linear increase of conductivity showed from 25 oC to 100 oC, suggesting a typical Arrhenius behavior[109]. At very high temperatures (from 100 oC to 150 oC), the slower uptrend of ionic conductivity could be explained by the Vogel-Tammann-Fulcher

(VTF) equation[3,124]. PEMs plasticized by SCN,GLN and ADN exhibited excellent ionic conductivity even at very high temperature( up to 150 oC).

It is well known that heat generated during battery operations can volatilize or even ignite solvent in liquid electrolytes, so thermal stability of plasticizers in PEMs is critical for battery safety. Thermal stability of neat SCN, GLN, ADN and PEMs

52 with the ratio of PEGDA/plasticizer/LiTFSI at 20/50/30 were tested for application in batteries, shown in Figure 5.7. The thermal stability of each components were represented by the thermal degradation onset temperature[125] at a weight loss of

95 %, shown by the intersection points on temperature axis. The thermal degradation temperatures of pure SCN, GLN and ADN were around 106 oC, 117 oC and 144 oC, respectively. PEMs plasticized by SCN, GLN and ADN were thermally stable until

115 oC, 118 oC and 135 oC, respectively. The thermal stability of PEMs was limited by the degradation of dinitriles, but still higher than most liquid electrolytes.

Figure 5.8. DSC thermograms of 20/50/30 PEGDA/SCN/LiTFSI PEM (blue), 20/50/30 PEGDA/GLN/LiTFSI PEM (black), and 20/50/30 PEGDA/ADN/LiTFSI PEM (red).

53 Phase transition phenomena of three obtained PEMs were studied by DSC, shown in Figure 5.8. A sharp melting peak appeared around -40 oC in SCN-PEM, representing plastic crystal melting transition of SCN. Another crystallization peak showed up around 5 oC corresponding to polymerization induced crystallization in

SCN-PEM. For GLN-PEM, no phase transition behavior was detected by DSC at temperature range from -80 oC to 50 oC, suggesting its potential battery applications at extremely low temperature. However, melting peaks emerged around -20 oC to

-10 oC in ADN-PEM, limiting the temperature window of batteries using

ADN-PEMs.

5.4. Conclusions

In this chapter, highly conductive, completely amorphous PEMs using different dinitrile plasticizers were successfully fabricated by UV crosslinking within the wide isotropic window of the ternary phase diagrams, especially at high plasticizer concentrations. In this highly conductive membrane, PEGDA provides the mechanical strength and transport lithium ions, whereas dinitrile plasticizers improve polymer chain mobility and dissociate the Li cations from the TFSI complex anions, thereby enhancing the ionic conductivity. As a result, the ionic conductivity of PEMs was measured up to a superionic conductivity of 10-3 S/cm. Of particular interest is that GLN-PEM reveals the highest ion conductivity among the three PEMs tested, due to its high ion dissociating ability and high miscibility with the precursor PEM system. For further application of PEMs in lithium ion batteries, not only superionic

54 conductivity is required, but also mechanical strength is preferred to prevent lithium dendrite growth. PEMs with ratio of PEGDA/Plasticizer/LiTFSI at 20/50/30 were chosen to investigate its ionic conductivity at high temperatures. All the PEMs showed excellent high temperature conductivity performance. For safety concerns of plasticizers used in batteries, Thermal stability of different plasticizers in PEMs was tested. The generally thermal stability was up to 120 oC. For wide temperature usage of PEMs in lithium-ion batteries, the phase transition behavior of PEMs form -80 oC to 50 oC were studied by DSC. As a result, GLN-PEM remained amorphous until -80 oC, showing potential applications in extremely low temperature.

55 CHAPTER VI

EVALUATION OF ELECTROCHEMICAL STABILITY FOR PEMS IN LITHIUM

ION BATTERY APPLICATIONS

6.1. Introduction

In the previous chapter, ionic conductivity of PEMs containing three different plasticizers (SCN,GLN,SCN) was affected by concentration of polymer, plasticizer and lithium salt. To investigate the effect of plasticizers with increasing chain length in PEM’s electrochemical properties, the concentration of PEGDA, plasticizer and

LiTFSI should be controlled in the same ratio. PEMs with the same composition of

20/50/30 PEGDA/ plasticizer/ LiTFSI having a relatively high ionic conductivity were chosen to apply in galvanostatic charge-discharge cycling, for the reason that high ion conductive electrolyte can decrease internal resistance of cells to improve charge-discharge performance.

In this chapter, half-cells of Li4Ti5O12/PEM/Li and LiFePO4/PEM/Li were assembled to test galvanostatic charge-discharge cycling performance of PEMs containing various plasticizers for further application in Li-ion batteries with the same

-1 half-cell configurations. Li4Ti5O12 with a theoretical capacity of 175 mAh g was applied to test anode half-cell performance of PEMs for its high rate capacity, ultra-long cycle life, high thermal stability and safety[72,76,77]. LiFePO4 with a

56 theoretical capacity of 170 mAh g-1 was used to study cathode half-cell performance of PEMs for its high rate capacity, thermal stability and low cost[41,42,46,47].

Redox reaction peaks of half-cells containing SCN-PEMs, GLN-PEMs and

ADN-PEMs were examined by cyclic voltammetry. Galvanostatic cycling stability of half-cells were tested within the voltage range of cathode and anode materials.

The cycling performance of three kinds of PEMs were analyzed and compared with each other to figure out the effect of plasticizers in PEMs for battery applications.

6.2. Experimental Sections

To fabricate SCN-PEMs, GLN-PEMs and ADN-PEMs, 20/50/30 PEGDA/ plasticizer/ LiTFSI mixtures were prepared as described in Chapter III. 0.2 wt% of photo initiator Irgacure® 651 were added in mixtures before photo-polymerization.

The mixtures were placed onto a glass slide with the depth of 200 m cubes spaced by four layers of scotch tape. The surface of mixtures was smoothed and uniformed by cover glasses. Then, the mixtures were cured into polymer electrolyte membranes under UV-light for 20 minutes at room temperature.

To assemble half-cells, PEMs were cut into circles fitting the size of 2032 coin cells and covering electrodes. In half-cells of Li4Ti5O12/PEM/Li and

LiFePO4/PEM/Li using LiFePO4 or Li4Ti5O12 as working electrodes and Li foil as the reference and counter electrode, PEMs were sandwiched between two electrodes.

57 For the CV measurement, the half-cell redox reaction peaks were measured at a

+ scan rate of 0.1 mV/s within the voltage range of 1.0 V-2.5 V vs. Li/Li for Li4Ti5O12

+ and 2.5 V-4.2 V vs. Li/Li for LiFePO4.

According to the analysis of cyclic voltammetry, galvanostatic charge-discharge cycling performance of half-cells were tested under current rate of 1 C and 0.25C within the voltage limits determined by CV. An 8-channel battery cycler (MIT Corp) were applied to measure galvanostatic cycling performance of half-cells.

6.3. Results and Discussions

To determine the electrochemical performance of PEMs used with cathode and anode materials, cyclic voltammograms of Li4Ti5O12/PEM/Li and LiFePO4/PEM/Li half cells containing GLN-PEM and ADN-PEM in Figure 6.1 were tested at scan rate of 0.1 mV/s for first three cycles. The red, blue and purple line in Figure 6.1 represents the first, second and third cycle, respectively. The electrochemical behaviors of SCN-PEM using in the same half-cells were examined by He. et al[108].

Figure 6.1. Cyclic voltammograms of half-cells a) LiFePO4/GLN-PEM/Li, b) LiFePO4/ADN-PEM/Li, c) Li4Ti5O12/GLN-PEM/Li, d) Li4Ti5O12/ADN-PEM/Li.

Cyclic voltammograms of LiFePO4/PEM/Li half-cells containing GLN-PEM and ADN-PEM were shown in Figure 6.1a and Figure 6.1b, respectively. In both cases, the cyclic voltammograms showed a single oxidation and a single reduction peak around 3.7 V and 3.2 V vs. Li/Li+. The oxidation peak in anodic scan corresponds to lithium ion insertion to LiFePO4, and reduction peak in cathodic scan corresponds to lithium ion extraction from LiFePO4. No additional peaks showed up during cycling implying no changes to electrochemical reaction process of

LiFePO4.The results of peak positions and current densities for three charge-discharge cycles were shown in Table 1. Form Table 1.The peak current

59 densities in Figure 6.1a were slightly higher than in Figure 6.1b, and the potential difference between anodic and cathodic peaks in Figure 6.1a were smaller than in

Figure 6.1b. Both suggested that half-cells containing GLN-PEMs had less lithiation/delithiation polarization than half-cells with ADN-PEMs. The first cycle in figure 6.1a was slightly different from the second and third cycle, manifesting a few irreversible reactions happened in half-cells containing GLN-PEM. But in Figure

6.1b, a pronounced difference between the first cycle and rest cycles indicated that some irreversible changes were taken place in half-cells containing ADN-PEMs. The shape and size of the second and third cycles were consistent in both systems, indicating stable cycling performance after the first cycle.

Current density of peaks (mA cm-2) Position of peaks (V) Difference between anodic and Sample Cycle Number Anodic Cathodic Anodic Cathodic cathodic peak potentials: Δox-red C1 0.118 -0.094 3.644 3.232 0.412(V) a C2 0.110 -0.093 3.658 3.223 0.435 C3 0.108 -0.093 3.660 3.219 0.441 C1 0.092 -0.085 3.672 3.196 0.476 b C2 0.111 -0.088 3.685 3.183 0.502 C3 0.108 -0.087 3.709 3.177 0.532 C1 0.345 -0.251 1.730 1.433 0.297 c C2 0.344 -0.270 1.728 1.448 0.280 C3 0.338 -0.259 1.733 1.443 0.290 C1 0.326 -0.249 1.686 1.483 0.203 d C2 0.324 -0.292 1.687 1.490 0.197 C3 0.319 -0.288 1.688 1.491 0.197 Table 1.Current density and position of redox peaks. Similarly, Figure 6.1c and Figure 6.1d showed cyclic voltammograms of

Li4Ti5O12/PEM/Li half-cells containing GLN-PEM and ADN-PEM. A pair of sharp redox peaks appeared around 1.7 V and 1.4 V vs. Li/Li+ in both systems, corresponding to lithium ion insertion/extraction to and from Li4Ti5O12. The

60 cathodic peak of the first cycle was different from others in both system, indicating significant irreversible reactions happened in the first discharge process. It was related to electrolyte-electrode interface reactions caused by the amount of carbon black in LTO electrode, reported by other literatures too[126,127]. The electrolyte-electrode interface reaction products deposited on the surface of LTO electrode forming SEI layers during the first discharge process in both systems. The results of peak positions and current densities were shown in Table 1. From peak positions listed in Table 1, the larger potential difference between oxidation and reduction peaks in half-cell containing GLN-PEMs than in half-cells with

ADN-PEMs revealed more lithiation/delithiation polarizations in GLN-PEM system.

However, the peak current densities of GLN-PEM system were much higher than the one of ADN-PEM system, indicating better ion transport in GLN-PEM. The consistence of shape and size in the second and third cycles indicated reversibility of half-cell reactions after the first cycle in both systems.

62 According to the stable cycling voltage range of each half-cell analyzed from cyclic voltammograms, galvanostatic charge-discharge cycling of half-cells containing SCN-PEM, GLN-PEM and ADN-PEM were measured within determined voltage ranges. Figure 6.2 exhibited 1)discharge capacities of LFP half-cells with different PEMs for 80 cycles at different current rate, and 2)voltage against charge-discharge capacity curves.

Figure 6.2a1 showed specific discharge capacity of LiFePO4/SCN-PEM/Li half-cell measured at the rate of 0.25 C. The initial discharge capacity was around

135 mAh g-1. After 50 cycles, the capacity retention was about 90%. For 80 cycles,

82% of initial capacity was retained. To analyze the detailed information of each charge-discharge cycle, the voltage vs. capacity curves at 10th, 30th and 50th cycle were plotted in Figure 6.2a2. The potential difference between charge and discharge plateau was around 0.3 V at 10th cycle and increased to 0.5 V at 50th cycle, indicating the internal resistance of half-cell containing SCN-PEM increased gradually during charge-discharge cycling. That can be the reason of the capacity reduction.

Similarly, Figure 6.2b1 showed specific discharge capacities of

LiFePO4/GLN-PEM/Li half-cells at 0.25 C and 1 C. Figure 6.2b2 plotted the voltage against capacity curves at the rate of 0.25 C. For the current rate of 0.25 C in Figure

6.2b1, the initial discharge capacity was around 130 mAh g-1. After 50 cycles, the capacity retention was about 94%. After 80 cycles, 92% of initial capacity was retained. From Figure 6.2b2, the potential difference between two plateaus was less

63 than 0.3 V at 10th cycle, indicating smaller internal resistance than SCN-PEM system. Besides, the charge/discharge plateaus at 30th and 50th cycle were overlapped, revealing that the resistance in half-cell became stable after 30 cycles.

This can explain the better discharge capacity retention in Figure b1. For the discharge capacity at the rate of 1C in Figure 6.2b1, the initial capacity was about

117 mAh g-1 and 99% of initial capacity was retained after 80 cycles, manifesting high rate capacity and long cycle life of half-cells with GLN-PEM.

Figure 6.2c1 and Figure 6.2c2 exhibited the charge-discharge cycling performance of LiFePO4/ADN-PEM/Li half-cells. For the cycling performance of half-cell at the rate of 0.25 C in Figure c1, the initial discharge capacity of half-cell was about 130 mAh g-1. After 50 cycles, the capacity retention was about 90%. After

80 cycles, 76% of initial capacity was retained. The capacity decay of half-cell containing ADN-PEM was more than others, corresponding to lower ionic conductivity of ADN-PEMs discussed in Chapter V. This behavior was consistent with the large potential difference between charge and discharge plateaus shown in

Figure 6.2c2. The same trend of discharge decay was measured at the rate of 1 C in

Figure 6.2c1, the initial capacity was about 113 mAh g-1, and 82% of initial capacity was retained after 80 cycles.

The high capacity retention of each case in Figure 6.2 indicates that PEMs are compatible with LFP half-cells. The capacity fading is less than commonly observed in batteries with polymer electrolytes (70%-85%)[128-132]. The incremental internal resistance of LFP half-cells indicates that the capacity decay happened in LFP

64 half-cells can be related to LiTFSI caused corrosion of Al current collector at potential higher than 3.7 V vs. Li/Li+[133]. Another reason for capacity decay of

LFP half-cell can be the irreversible redox reactions happened between electrodes and electrolyte at high voltage range[113,114]. Therefore, the enhanced electrochemical performance of PEMs in LFP half-cells can be related to high anodic stability of dinitrile plasticizers that can protect electrolytes from uncontrolled decomposition. Remarkable capacity retention combined with reduced internal resistance were appeared in half-cell containing GLN-PEM. This can be related to high ionic conductivity of GLN-PEM.

66 The galvanostatic charge-discharge cycling performances of LTO half-cells containing various PEMs at 0.25C were described in Figure 6.3. Figure 6.3a1, Figure

6.3b1 and Figure 6.3c1 plotted the discharge capacities vs. cycle number in half-cells containing SCN-PEM, GLN-PEM and ADN-PEM, respectively. The capacity decay of first ten cycles was much quicker in LTO half-cells than in LFP half-cells. This can be explained by the electrolyte-electrode interfacial reactions in LTO half-cells, discussed in the analysis of CV curves previously. The dropped capacities were consumed to form SEI layer on the surface of LTO electrode. The SEI layers as a passive layer suppressed further interfacial reactions, preventing capacity decay after ten cycles. Figure 6.3a2, Figure 6.3b2 and Figure 6.3c2 were the voltage against capacity curves of half-cells containing SCN-PEM, GLN-PEM and ADN-PEM at

10th, 30th and 50th cycles. The polarization in LTO half-cells was smaller than LFP half-cells, for the reason of a “zero strain” intercalation mechanism of LTO structure[75,76].

In Figure a1, the initial discharge capacity of half-cell with SCN-PEM was about 166 mAh g-1. After 10 cycles, the capacity dropped to 88% of initial capacity.

After 50 cycles, the capacity retention was 80%. After 80 cycles, 76% of initial capacity was retained. The capacity reduction is more pronounced in first ten cycles and then the capacity became stable with the cycling. This trend appeared in Figure

6.3b1 and Figure 6.3c1 too. In Figure 6.3b1, the initial discharge capacity of half-cells containing GLN-PEM was around 171 mAh g-1. After ten cycles, the capacity dropped to 88% of initial capacity. After 50 cycles, the capacity retention

67 was 83%. After 80 cycles, 81% of initial capacity was retained. In figure c1, the initial discharge capacity of half-cells with ADN-PEM was around 170 mAh g-1.

After ten cycles, the capacity dropped to 83% of initial capacity. After 50 cycles, the capacity retention was 73%. After 60 cycles, 71% of initial capacity was retained.

All the LTO half-cells presented remarkable capacity retention after 10 cycles, manifesting that the formation of SEI layers stabilized further capacity decay. Hence,

LTO electrode is compatible with dinitrile plasticized PEMs. Compared with the other two half-cells, Half-cell containing GLN-PEM had better capacity retention.

After initial consuming of capacity, the discharge capacity became very stable from

10th cycle to 80th cycle, with only 7% capacity decay. Therefore, SEI layers formed in LTO half-cells using GLN-PEM have better passivation for LTO electrode.

For the voltage vs. capacity curves in Figure 6.3a2, Figure 6.3b2 and Figure

6.3c2, A bigger potential difference between charge and discharge plateaus appeared in Figure 6.3b2, indicating bigger internal resistance of LTO half-cell containing

GLN-PEM. It also improved that better passive SEI layer formed in half-cell containing GLN. The superior SEI layer formation can be related to the higher ionic conductivity in GLN-PEMs than other PEMs.

6.4. Conclusions

The electrochemical behavior of Li4Ti5O12/PEM/Li and LiFePO4/PEM/Li half cells containing various PEMs (SCN-PEMs, GLN-PEMs, ADN-PEMs) were demonstrated in this chapter. From the cyclic voltammograms of half-cells, adding

68 PEMs with different plasticizers doesn’t change the electrochemical reactions of

LFP and LTO electrode. But the oxidation and reduction peak positions and current density are affected by the variety of PEMs. According to the electrochemical stability window of PEMs in half-cells tested by CV, galvanostatic charge-discharge cycling measurement were applied. Either LFP or LTO half-cells of GLN-PEM possess the best battery performance, with 50 cycles capacity retention of 94% and

83%, respectively. batteries with polymer electrolyte generally have capacity retention from 70% to 85%, or even worse after 50 cycles. The pronounced performance of GLN plasticized PEM can be related to high ionic conductivity measured in Chapter V. Besides, the SCN-PEM and ADN-PEM containing half-cells still have relatively good cycling performance, with 50 cycles capacity retention from 73% to 90%. The enhanced electrochemical stability of all PEM containing half-cells proved that dinitrile plasticizers can improve the electrochemical performance of polymer electrolyte in LPF and LTO half-cells.

69 CHAPTER VII

SUMMARY AND RECOMMENDATIONS

7.1. Summary

In this thesis, PEMs plasticized by three different dinitriles (SCN, GLN and ADN) were fabricated via photo-polymerization afforded transparent, homogeneous films.

Ionic conductivity behaviors of three different dinitrile plasticized PEMs were studied by the guidance of ternary phase diagrams. Superionic conductive PEMs were fabricated at rich dinitrile concentrations for lithium-ion battery applications. Thermal and electrochemical stability of various dinitrile plasticized PEMs were investigated later on.

In Chapter IV, phase transition behaviors of precursor systems were investigated for dinitrile plasticized PEMs. Binary system of PEGDA/GLN and LiTFSI/GLN were firstly studied by DSC thermal analysis. Then, combined with free energies of

Flory-Huggins theory for liquid-liquid demixing and phase field theory for crystal solidification, binary phase diagrams of PEGDA/GLN and LiTFSI/GLN systems were self-consistently solved. GLN as a plasticizer had high miscibility with PEGDA, promoted polymer chain mobility and declined the melting transition temperature in

PEGDA/GLN system. Besides, GLN as a highly polarized solvent to LiTFSI generated wide isotropic region in phase diagram of GLN/LiTFSI. Furthermore,

70 ternary phase diagrams of PEGDA/GLN/LiTFSI system were established together with PEGDA/SCN/LiTFSI system and PEGDA/ADN/LiTFSI system to guide the processing window for achieving completely amorphous PEMs. Compared with

PEGDA/SCN/LiTFSI system and PEGDA/ADN/LiTFSI system,

PEGDA/GLN/LiTFSI system had lower melting transition temperatures and higher ion dissociation, affecting final properties of its PEM.

In Chapter V, The ionic conductivity studies of various dinitrile plasticized

PEMs were measured by AC impedance spectrometer at ambient temperature and high temperature. the room temperature ionic conductivity were increased up to superionic conductivity of 10-3 S/cm by adding more plasticizers into PEMs. Of particular interest is that GLN-PEM reveals the highest ion conductivity among the three PEMs tested, due to its high ion dissociating ability and high miscibility with

PEM precursors. For further application of PEMs in lithium-ion batteries, PEMs with ratio of PEGDA/Plasticizer/LiTFSI at 20/50/30 were chosen with superionic conductivity and certain mechanical strength to investigate its ionic conductivity at high temperatures. All the PEMs showed excellent conductivity performance, up to

10-2 S/cm at 150 oC. Fire retardant PEMs designed for battery safety were stable up to 120 oC, according to TGA test. Later on, the temperature window of PEMs used in lithium-ion batteries were tested by DSC. GLN-PEM remained amorphous until

-80 oC, which can be in extremely low temperature.

In Chapter VI, the electrochemical behavior of various dinitrile plasticizers in

Li4Ti5O12/PEM/Li and LiFePO4/PEM/Li half cells were demonstrated. From the

71 cyclic voltammograms of half-cells, stable electrochemical reactions of PEMs with

LFP and LTO electrode were determined after the first charge-discharge cycle. Only oxidation and reduction peak positions and current density are affected by the variety of PEMs. The results were confirmed by galvanostatic charge-discharge cyclic performance of these half-cells. Either LFP or LTO half-cells containing GLN-PEM possess the best battery performance, with 50 cycles capacity retention of 94% and

83%, respectively. Besides, the SCN-PEM and ADN-PEM containing half-cells still have better cycling performance than commonly reported in other cells using polymer electrolytes, with 50 cycles capacity retention from 73% to 90%. Results manifested that the electrochemical stability of half-cells containing polymer electrolyte can be improved by using dinitrile plasticizers.

7.2. Recommendations

In this thesis, plenty of studies focused on the physical and electrochemical properties of various dinitrile plasticized PEMs. The ionic conductivity of PEMs was improved by addition of dinitrile plasticizers. Besides, the electrochemical performance of polymer electrolytes in half-cells was enhanced by plasticizers. It could be related to the improvement of ionic conductivity or other reasons. The electrochemical performance can be affected by any part of batteries. Also, PEMs with reduce crystallinity and wider operating windows were achieved by the addition of dinitrile solvents. For the superiority of dinitrile plasticized PEMs, further studies

72 can be focused on high voltage and high/low temperature applications in lithium ion batteries.

Of particular interest, GLN plasticized PEMs showed the best ionic conductivity among others. The electrochemical performance of LFP and LTO half-cells containing GLN was the best too. Hence, thermal stability of GLN-PEM was studied in LFP half-cells, shown in Figure 7.1. The filled triangles represent discharge capacities of LFP half-cell for 50 cycles at 60 oC. The discharge capacities at 20 oC were plotted by unfilled circles for comparisons. The capacity retention was marked in the figure. As we all known, promising electrochemical reactions and ion transport at high temperature should enhance the performance of lithium ion batteries. However, the electrochemical stability at high temperature decreased, with

50 cycles 70% capacity retention. It can be explained by side reactions, increased at elevated temperature. Compared with high temperature performance of LEP half-cells containing SCN-PEM, the capacity retention of GLN-PEM is much higher.

One reason can be the cycliztion of GLN plasticizer, shown in Figure 7.2. GLN can easily form a six member ring structure by free radicals generated in batteries at high temperatures[135][136]. The conjugated double bonds in cyclic structure can increase the thermal stability of polymer electrolytes. However, this hypothesis still need to be improved by further studies.

Figure 7.1. Specific discharge capacity as a function of cycle number for o o LiFePO4/GLN-PEM/Li half-cells during cycling at 25 C (unfilled circle) and 60 C (filled triangle).

Figure 7.2. Cyclization mechanism of GLN radical reaction.

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