Fabrication and Characterizations of Lithium Aluminum Titanate Phosphate Solid Electrolytes for Li-Based Batteries

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Fabrication and characterizations of lithium aluminum titanate phosphate solid electrolytes for Li-based batteries

Anurag Yaddanapudi Wright State University

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This Thesis is brought to you for free and open access by the Theses and Dissertations at CORE Scholar. It has been accepted for inclusion in Browse all Theses and Dissertations by an authorized administrator of CORE Scholar. For more information, please contact [email protected]. FABRICATION AND CHARACTERIZATIONS OF LITHIUM ALUMINUM TITANATE PHOSPHATE SOLID ELECTROLYTES FOR LI-BASED BATTERIES

A Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Materials Science and Engineering

ANURAG YADDANAPUDI

B. Tech, Osmania University,2016

2018

Wright State University

WRIGHT STATE UNIVERSITY

GRADUATE SCHOOL

December 7, 2018

I HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY SUPERVISION BY Anurag Yaddanapudi ENTITLED Fabrication and characterizations of lithium aluminum titanate phosphate solid electrolytes for Li-based batteries. BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR DEGREE OF Master of Science in Materials Science and Engineering. ______Hong Huang, Ph.D. Thesis Director ______Joseph C. Slater, Ph.D., P.E. Chair, Department of Mechanical and Materials Engineering Committee on final examination: ______Hong Huang, Ph.D. ______Amir A. Farajian, Ph.D. ______Michael Rottmayer, Ph.D. ______Barry Milligan, Ph.D. Interim Dean of the Graduate School

ABSTRACT Yaddanapudi, Anurag. M.S.M.S.E Department of Mechanical and Materials Engineering, Wright State University, 2018. Fabrication and characterizations of lithium aluminum titanate phosphate solid electrolytes for Li-based batteries Demands for electric vehicles and flexible electronics have escalated research in developing high-performance lithium batteries based on solid-state chemistry. The present work is to develop highly-conductive and flexible solid electrolyte for such applications.

Lithium aluminum titanate phosphate (LATP or Li1.3Al0.3Ti1.7(PO4)3), both in ceramic pellets and free-standing composite membranes, have been fabricated. The crystal structure, surface morphology, and ionic conductivity are systematically studied.

LATP pellets are prepared using solid state reaction approach. The results indicate that calcine temperature has significant impacts on the phase impurity and sintering temperature and duration have more impacts on the grain size and porosity of LATP pellets.

At the optimal conditions, the highest bulk conductivity of LATP electrolyte reaches

1.5*10-3 S/cm at room temperature with an activation energy of 0.206 eV. The as-prepared

LATP has high conductivities comparable with liquid electrolytes, which is feasible for applications to all-solid-state lithium batteries.

Ceramic electrolyte can be composited with polymer electrolyte to enable flexible battery design. In this study, LATP-based electrolyte membranes are fabricated in composite with a lithiated polymer, i.e. polyvinylidene fluoride (PVDF) dissolved with lithium perchlorate (LiClO4), via the casting method. It is found curing temperature has influences on ionic conductivities of the composite membrane and high casting temperature

can cause the decomposition of PVDF. Appropriate LATP composition can increase the ionic conductivity, mechanical strength while maintaining the flexibility of the composite membrane. Raman spectroscopic analysis suggests there exists certain interactions among the three components in the composite membrane.

TABLE OF CONTENTS

Chapters

1. INTRODUCTION ...... 1

2. LITERATURE REVIEW...... 4

2.1 Overview ...... 4 2.2 Types of Electrode Materials ...... 4 2.3 Types of Electrolyte Materials...... 7 2.3.1 Liquid Electrolytes...... 7 2.3.2 Polymer Electrolytes...... 10 2.3.2.1 Solid Polymer Electrolytes...... 12 2.3.2.1 Gel Polymer Electrolytes...... 15 2.3.3 Solid State Electrolytes...... 17 2.3.4 Composite (ceramic - polymer) Electrolytes...... 27

3. SYNTHESIS AND CHARACTERIZATION OF LITHIUM ALUMINUM TITANATE

PHOSPHATE (LATP) ...... 29

3.1 Synthesis of Li1.3Al0.3TI1.7 (PO4)3 LATP Electrolyte...... 29 3.2 Structural and Morphological Characterizations of the As-prepared LATP...... 31 3.2.1 Structure and Phase ...... 31 3.2.2 Morphologies of LATP Specimens...... 32 3.2.2.1 Impacts of Calcination Temperature...... 33 3.2.2.2 Impacts of calcination duration on grain size of LATP...... 34 3.2.2.3 Impacts of sintering temperature and sintering time...... 36 3.2.3 Ionic Conduction Characterization of Li1.3Al0.3TI1.7 (PO4)3 ...... 37 3.2.3.1 Impacts of temperature on ionic conductivities...... 42 3.2.4 Conclusion...... 44 4. SYNTHESIS AND CHARACTERIZATION OF LITHIATED POLYMER COMPLEX

(LATP/PVDF) FOR LI ION BATTERIES...... 45

4.1 Fabrication of PVDF-LiClO4/LATP Composite Electrolyte ...... 46 4.2Characterization of PVDF-LiClO4/LATP Composite Electrolyte...... 47 4.2.1 Dimensional Stability...... 47 4.2.2 Ionic Conductivity...... 49

4.2.3 Raman Spectroscopy Studies...... 52 4.3 Conclusion...... 56

5. CONCLUSION AND FUTURE WORK ...... 58

REFERENCES ...... 60

vii

LIST OF FIGURES

Figure 1 Sketch of a lithium-ion battery made up of LiCoO2 cathode and graphite anode ...... 2

Figure 2 Ragone plot for various energy storage systems ...... 2

Figure 3 Schematic structures of some representative anode materials used in Li-ion batteries ...... 5

Figure 4 A plot showing the potentials and capacities of common anode materials used in Li-ion batteries-5 ...... 5

Figure 5 Sketch of the crystal structures of different cathode materials...... 6

Figure 6 A plot showing the potentials and capacities of common cathode materials ...... 6

Figure 7 Garnet and Perovskite crystal structure ...... 17

Figure 8 Crystal structure of Lithium Aluminum Titanate Phosphate(LATP) ...... 23

Figure 9 A schematic flow chart showing the fabrication of Li1.3Al0.3Ti1.7(PO4)3 powders and LATP pellets...... 31

Figure 10 XRD spectra of the as-prepared Li1.3Al0.3Ti1.7(PO4)3 samples obtained at different calcine and sintering conditions ...... 32

Figure 11 SEM images of LATP powders obtained at different calcination temperatures (a) 850oC-3hrs; (b) 950oC-3hrs; (c) EDS spectrum of LATP powders calcined 950oC-3hrs...... 33

Figure 12 SEM images of LATP samples calcined at different duration ...... 34

Figure 13 EDS spectra of LATP pellet calcined at 850oC for 10hrs and sintered 950oC for 6hrs ...... 35

Figure 14 SEM images of LATP pellets sintered at different duration ...... 37

Figure 15 A typical EIS spectrum obtained from LATP specimen (calcined 950oC- 6hrs-sintered 950oC-6hrs) at room temperature ...... 38

viii

Figure 16 Complied ionic conductivities of all LATP samples obtained as a function of calcination temperature, calcination time, sintering temperature, and sintering time ...... 39-40

Figure 17 (a) EIS spectra of LATP obtained at 30oC to 100oC for sample calcined 950oC-6hrs and sintered 950oC-6hrs (b) an Arrhenius plot for sample calcined 950oC-6hrs and sintered 950oC-6hrs ...... 43-44

Figure 18 Images of PVDF/LiClO4 membranes casted at different temperatures showing the changes in color and flexibility ...... 47

Figure 19 A Swagelok cell used for conductivity measurement of the electrolyte membrane...... 49

Figure 20 EIS spectra of the composite membranes with different LATP content prepared at curing temperature of 70oC-3hrs ...... 50

Figure 21 Conductivities as a function of LATP content of the membranes prepared at different curing temperatures...... 51

Figure 22 Conductivity vs. temperature in Arrhenius plot obtained from LATP-50 membrane casted 100oC for 4hrs and curing temperature 70oC 3hrs...... 52

Figure 23 Raman spectra of PVDF membrane prepared using two different solvents NMP and DMF ...... 53

Figure 24 Raman spectra of PVDF-LiClO4 membrane casted at different temperatures and compared with virgin PVDF casted at the same condition...... 55

Figure 25 Raman spectra of PVDF-LiClO4/LATP comparison with PVDF- LiClO4 and PVDF membranes casted at same conditions ...... 55

Figure 26 A hypothetic ion transfer mechanism in the PVDF-LiClO4-LATP system...... 56

LIST OF TABLES

Table 1 Structures and physical properties of solvents used in the liquid electrolytes...... 8

Table 2 Some lithium salts used in liquid electrolytes and their physical properties as well as typical ionic conductivities in PC or EC/DMC...... 9

Table 3 Properties of common ionic liquids employed in lithium-ion batteries...... 10

Table 4 Some SPEs and their conductivities reported in works of literature ...... 14

Table 5 Some GPEs and their conductivities reported in literature ...... 16

Table 6 Different Garnet Electrolyte Systems and their conductivities reported in literature...... 18

Table 7 Different Perovskite Electrolyte Systems and their conductivities reported in literature...... 19

Table 8 Different LISICON Electrolytes and their conductivities reported in literature...... 21

Table 9 Different LIPON Electrolytes and their conductivities reported in literature ...... 21

Table 10 NASICON based lithium ion conductors and reported conductivities in literature...... 25

Table 11 Various Composite electrolytes and reported conductivities in literature...... 27

Table 12 Stoichiometric Amounts of Precursors: for L1.3Al0.3Ti1.7(PO4)3 ...... 29

Table 13 Composition and the amounts of each component added to fabricate the series of PVDF-LiClO4/LATP composite ...... 46

Table 14 Dimensional stability of the composite electrolyte membranes fabricated whereas at different casting temperatures ...... 48

ACKNOWLEDGEMENTS

I want to take this opportunity to sincerely thank my advisor, Dr. Hong Huang, for all the supports and inspiration she provided throughout my graduate life. She has been a great advisor and teacher, whose energy and passion for research has inspired me to become an outstanding student and researcher. She has always been helpful and appreciative of all my new ideas, thus making my graduate life exciting.

I would also like to thank Dr. Amir A. Farajian and Dr. Michael Rottmayer for their being in my thesis committee and advising me with valuable suggestions.

I want to thank the Department of Materials Science and Engineering and the

Wright State University for giving me an opportunity to travel all the way from my home country, India, to complete my graduate studies.

Finally, I would like to offer my sincere thanks to my parents, sister and friends who have been behind me as moral support and provided encouragement throughout.

CHAPTER 1: INTRODUCTION

To meet the increasing demand for energy, vast resources of energy supply and storage become essential. Energy storage systems are gaining more development and attention with the increased power density and energy density. Furthermore, these systems can be light-weight, portable, eco-friendly, long cycle life, flexible, and fast rechargeable.

Representative electrochemical energy conversion and storage systems are batteries, capacitors, fuel cells.

A typical battery consists of cathode, anode, and electrolyte, as shown in Figure1.

The mechanism involved is a redox reaction occurring between the electrodes resulting in electricity generation, i.e., chemical energy is converted to electrical energy. Batteries are classified based on their abilities of the rechargeability: primary batteries and secondary batteries. Alkaline battery falls into the category of primary batteries which cannot be charged, whereas secondary batteries such as Li-ion batteries will reversibly convert energy upon charge/discharging cycle. Li-ion batteries have energy densities in the range of 10 Wh/kg -100 Wh/kg and power densities of 100 W/kg- 1000 W/kg [1].

Performances of the various electrochemical energy systems can be compared in the Ragone plot, see Figure 2 [1]. It is seen that the batteries possess high energy density with intermediate power density. Researchers are defining the new era of batteries which will have high energy density and power density through the development of new- generation electrode and electrolyte materials.

Figure 1 Sketch of a lithium-ion battery made up of LiCoO2 cathode and graphite anode

Figure 2 Ragone plot for various energy storage systems

The primary motivation behind this research is to reduce the dependence on fossil fuels and develop energy storage devices which are environmentally friendly. Our primary goal is to fabricate solid electrolyte material with excellent stability window and high conductivity for applications to all solid-state lithium batteries. Additionally, the problems associated with conventional liquid electrolytes, i.e., safety and dendrite growth can be suppressed. As a result of this, our present research on ceramic lithium ion conductor, i.e., lithium aluminum titanate phosphate (LATP) and its composite with lithiated polyvinylidene fluoride (PVDF) to achieve high conductivity, thin, and free-standing electrolyte membrane with excellent dimensional stability. The rest of this thesis is organized as the following:

Chapter 2 presents a literature review of materials including cathode and anode, with emphasized on different kinds of electrolytes utilized in lithium-based batteries.

Chapter 3 presents the results on synthesis and characterizations of

Li1.3Al0.3Ti1.7(PO4)3. The morphology, phase, and ionic conductivities of the as-prepared

LATPs are characterized by scanning electron microscopy (SEM), x-ray diffraction

(XRD), and electrochemical impedance spectroscopy (EIS). The impacts of the calcination and sintering conductions on the products are systematically studied.

Chapter 4 presents the results on fabricating LATP composited with lithiated polymer complex, PVDF-LiClO4. The conductivities of as prepared composite systems are determined as a function of LATP content. Potential interaction among the components in the composite electrolyte membranes is studied using Raman spectroscopy.

Chapter 5 summarizes the results from this study and briefly outlooks the potential future work in this direction.

CHAPTER 2: LITERATURE REVIEW

2.1 Overview

Researchers have been developing novel materials from electrode to electrolyte in the battery field. In the early 1970s the first non-rechargeable lithium battery was available in the market. Lithium is attractive because it is the lightest among all metals, has the lowest electrode potential, and hence the highest energy density. Efforts were made to move into the arena of rechargeable Li-ion batteries in the 1990s. Sony commercialized the first rechargeable Li-ion battery which was highly efficient and attracted the market all over the world. Today's market is highly dependent on Li-ion batteries in the applications from mobile phones, laptops, MEMS devices, to electric vehicles.

The basic principle involved in a Li-ion battery is that upon discharging the anode releases the electrons to the external circuit and lithium ions through the electrolyte membrane. Meanwhile, electrons and lithium ions are stored in the cathode. The reverse process occurs upon charging. The energy and power densities are determined by choice of pivotal materials for anode, cathode, and electrolyte in lithium-ion batteries.

2.2 Electrode Materials

The anode is one of the pivotal components in lithium-ion batteries. Using lithium metal anode led to dendrite growth and short circuit problems. The first-generation anode is carbon-based materials from graphite to various carbon cokes. Later, researchers have moved into secondary generation such as lithium titanate (LTO), tin(Sn), and silicon-based alloys. Presently, extensive studies are going on the above-mentioned materials and 4

problems in Li-ion battery industries. Figure 3 illustrates the structures of the different anode materials (graphite, lithium titanate oxide and lithiation of silicon) [2]. Figure 4 depicts the information about potentials and capacities of anode materials [3].

Figure 3 Schematic structures of some representative anode materials used in Li-ion batteries

Figure 4 A plot showing the potentials and capacities of common anode materials used in

Li-ion batteries

Cathode materials also play a prominent role in the design of batteries. These

materials act as host in chain network which can store/remove lithium ions reversibly.

Cathode materials are the entity of metal chalcogenides, transition metal oxides, and polyanion compounds. These materials exist in different crystal structural forms like layered, spinel, olivine, and tavorite which can be seen in Figure 5 [2]. The critical requirements for these materials are high volumetric capacity, high specific capacity, and high potential, volume expansion fraction. Figure 6 provide the information about potentials and capacities of different cathode materials [4].

Figure 5 Sketch of the crystal structures of different cathode materials.

Figure 6 A plot showing the potentials and capacities of common cathode materials

2.3 Electrolyte Materials

The electrolyte is crucial in ion transport between electrodes. Key requirements for an ideal electrolyte include: a) high ionic conductivity; b) electronic insulating; c) wide electrochemical stability window to be compatible with an electrode potential of anode and cathode; d) thermal stability in the operating temperature range; f) low toxicity and low- cost. Electrolytes used in the lithium-based batteries are classified as below: liquid electrolytes, polymer electrolytes, ceramic electrolytes, and composite electrolytes.

2.3.1 Liquid electrolytes

Liquid electrolytes developed in lithium-based batteries and commonly used in the present lithium ion batteries include i) non-aqueous electrolytes which consist of lithium salt soluble in organic or solvent mixture; and ii) ionic liquids.

The organic solvents should have low viscosities such that lithium ions movement is facile to ensure high ionic conductivity. The solvent should also have a high dielectric constant to dissociate lithium ions from the salt. Aprotic solvents consist of polar groups like carbonate, nitrile, ethers, and esters. Cyclic organic carbonates possess high dielectric constant and form low energy complex [5]. Some of the widely used solvents to dissolve the lithium salts are ethylene carbonate (EC) and propylene carbonate (PC). EC is solid at room temperature. Therefore, acyclic alkyl carbonates are usually added to reduce viscosity and melting point. Table 1 lists structures and some physical properties of the solvents used in the liquid electrolytes [6].

Table 1 Structures and Physical properties of solvents used in the liquid electrolytes.

In general, requirements for lithium salts are completely dissolvable in solvents with high dissociation constant. Moreover, the anion should be stable in the oxidation/reduction environment and relatively stable in the presence of trace water. It should be beneficial towards forming stable interfacial layers. The dissociation constant should be high to ensure more free lithium ions contributing to the conduction. Table 2 list some lithium salts and their physical properties as well as typical ionic conductivities in

PC or EC/DMC [6].

Table 2 Some lithium salts used in liquid electrolytes and their physical properties as well as typical ionic conductivities in PC or EC/DMC.

In consideration of the safety matters, ionic liquid (IL) electrolytes gain more attention in battery technology due to their advantageous features such as low vapor pressure, inflammable, low melting points, high electrochemical stability window(4-6V), and high resistance to oxidation. Ionic liquids are comprised of large cations (organic) and anions (both organic/inorganic) which make ionic forces weak leading to lower melting points. Cations of room temperature ionic liquids (RTILs) can be tetra alkyl ammonium

[R4N+] or cyclic amines like aromatic (pyridinium, imidazolium) and saturated

- - - - (piperidinium, pyrrolidinium). Anions vary from [BF4 ], [PF6 ], [AsF6 ], triflate [CF3SO3 ],

- - and imide [N(CF3SO2)2 ], [N(F2SO2)2 ]. The conductivity of RTILs is in range of 0.1-18 mS/cm [7]. Some of the considerations that need to be take into account while working

with ILs are high viscosity, slow diffusion process, solvation of Li ions, and double layer barrier at the electrode surface. Table 3 lists some properties of common ionic liquids employed in lithium-ion batteries [8].

Table 3 Properties of common ionic liquids employed in lithium-ion batteries

Cation Anion Tm (K) Tg (K) Conductivity(mS/cm) Viscosity(cP) Window(V)

EMI BF4 288.15 178 1.38 37.7 4.3

EMI TFSI 258.15 175.15 0.86 34 4.3

EMI BETI 272.15 188 0.34 61 4.1

BMI BF4 190.65 188.15 219 6.1

BMI TFSI 269.15 169.15 0.39 52 4.76

BMI PF6 283.15 193.15 0.1 450 5

2.3.2 Polymer electrolytes

Polymer electrolytes are in the trend to replace the liquid electrolytes due to their excellent interfacial stability, good chemical and thermal stability, good mechanical strength, high electrochemical stability window, enhanced ionic conductivity and capable of suppressing dendrite growth. First polymer electrolyte was reported by mixing polymer mix with alkali metal salts [9], which lead researchers to explore various polymer electrolytes for Li-ion batteries. Polymer matrix should comprise polar groups like -O; =O;

-N-; -P-; C=O; C=N to dissociate lithium salts resulting in lithiated polymer complexes.

The ion conductivity depends on the number of ions, electric charge, and ion mobility. The number of ions relies on dissociation coefficient of the salt in the polymer- salt complexes. Mobile ions transport with the help of polymer segmental motion, and

hence take place in non-crystalline phase regions [10,11]. The ion transport occurs from one site to another through these free volumes under the influence of the electric field. In some cases, ions travel through the cylindrical tunnel path formed in the polymer matrix

[12,13].

The insights of ion transfer mechanism lead to the relationship between conductivity and with temperature, which is presented by Vogel-Tammer-Fulcher (VTF) equation and Arrhenius equation [14,15,16].

The VTF equation is as the following,

-1/2 σ = σ0 T exp(-B / T-T0 ) (1)

Where σ0 is a pre-exponential factor; B is pseudo activation energy for σ; T0 is equilibrium glass transition temperature. Typically, T0 = Tg – 50K where Tg is the glass transition temperature. The σ vs. 1/ T plot is non-linear, which reflects the mechanism involved in segmental motion/breathing/relaxation of polymer chains.

The Arrhenius equation is shown below,

σ = σ0 exp(-Ea / kT ) (2)

Where σ0 is a pre-exponential factor, and Ea is activation energy of the conduction. The plot of σ vs. 1/ T is linear corresponding to the simple hopping mechanism within the polymer chain network.

Polymer electrolytes are further classified into solid polymer electrolytes [SPE] and gel polymer electrolytes [GPE]. In SPEs polymer matrix directly dissolve lithium salt to maintain the mechanical properties of the system. GPEs contain some organic solvents as 11

plasticizers in addition to polymer matrix and lithium salts.

2.3.2.1 Solid polymer electrolytes

In SPEs extensive research was done on polyethylene oxide (PEO) - lithium salts derivatives. Polyether serves as a better matrix candidate due to its high conductivity when compared to other polymers. Most intriguing features of PEO are the flexible nature of the ethylene chain and the donor character of ether oxygen atoms. Researchers also reported working with low glass transition temperature polymers like polypropylene oxide (PPO), poly(bis(methoxy-ethoxy-ethoxy)-phosphazene (MEEP), and polysiloxanes (PSi), which consist of more amorphous phases at room temperatures. PPO has similar solvation ability to PEO. MEEP has stronger solvation ability to Li-ions due to the ether nature and nitrogen atoms in the polymer chain. PSi provides excellent mechanical strength due to [Si-O]n where each monomer unit has two preferential sites for crosslinking or functional side chains. PAN (polyacrylic nitrile) and its derivative copolymers are also attractive due to the presence of the nitrile group [17-20].

Lithium salts widely used in SPEs include LiClO4, LiPF6, LiBF4, LiAsF6,

LiCF3SO3, LiN(CF3SO2)2. Sequence of these salts with respect to mobility and dissociation constant are as the following:

Mobility: LiBF4> LiClO4 > LiPF6 >LiAsF6>LiCF3SO3>LiN (CF3SO2)2;

Dissociation: LiN (CF3SO2)2 > LiAsF6 > LiPF6 >LiClO4 >LiBF4 >LiCF3SO3.

The SPEs, in general, have low ionic conductivities in orders of 10-5 to 10-7 S/cm because ion transport occurs preferably in amorphous regions by segmental motions [21].

Research had been performed in order to increase ionic conductivity in these systems by structural modifications to reduce glass transition temperature and to accelerate the mobility of the ions in the polymer matrix. Attempts include copolymerization, the polymer in salt system, single Li-ion polymer electrolytes, and anion receptors. For instance, the

-4 conductivity of PAN in LiAlCl4 system is about 10 S/cm [18], and the polymer in salt system has very high lithium transference number. Zygadalo-Monikowaska et al., [22] reported work using binary salts LiBF4+LiDFOB (1:1) + acrylonitrile butyl acrylate copolymer which exhibited a conductivity of 10-5 S/cm at room temperature, higher than using pristine LiBF4 sample. Fanet et al.,[23] worked on ternary salt in the PEO system, i.e., LiClO4+LiNO3+LiOAc, where LiOAc is attributed to the increased conductivity up to

10-3S/cm in range of 20-130oC. Mishra et al. [24] reported the conductivity increased with the salt concentration due to the formation of percolation path in the polymer system.

Forsyth et al., [25,26] have studied the PAN-LiCF3SO3 system from which they submitted that the ionic conductivity depended on the salt concentration. They noticed that in the polymer-in-salt system glass transition temperature (Tg) is inversely proportional to salt content, and such a system is more glassy at room temperature to decouple from the structural relaxations.

The drawbacks in the highly conductive polymer-in-salt system have weak mechanical strength upon increasing the salt percentage. In single-ion polymer electrolytes, it was observed that the cations tend to stick to the polymer host wherein accumulation of the large anions increase polarization. Adding anion receptors and making anions to stick with polymer host may tackle such problem. Bushkova et al., [27] reported a single Li-ion system has high conductivity in the range of 10-3-10-4 S/cm. They attributed to the active

cation transport process in entire electrolyte system by the aid of the infinite clusters. Table

4 lists conductivity values reported in some SPE systems.

Table 4 Some SPEs and their conductivities reported in works of literature.

Solid Polymer Electrolyte Systems Conductivity (S/cm) Ref.

-7 o P(EO)20-LiBF4 6.32*10 @ 27 C 28

-7 o P(EO)20-LiClO4 2.78*10 @ 27 C 28

-6 o PEO-5wt% LiPF6 1.20*10 @ 25 C 29

-4 o PEO-11.1wt% LiAsF6 1.43*10 @ 25 C 30

-6 PEO-15wt%- LiCF3SO3 1*10 @ RT 31

-9 o P(EO)20- LiCF3SO3 1.88*10 @ 27 C 28

-4 o P(EO)24-LiN(CF3SO2)2 3.84*10 @ 50 C 32

-4 o PPO-LiX, X=ClO4, Br, CF3SO3, N(CF3SO2)2 >10 @ 25 C 33

-5 MEEP-10wt%- LiCF3SO3 1*10 34

-5 o MEEP-25wt%- LiCF3SO3 2.70*10 @ 30 C 35

-4 o P(Si)32- LiN(CF3SO2)2 4.50*10 @ 25 C 36

(PEO-PS)-LiTFSI block graft copolymer >10-4 @60oC 37

-4 (PEO-P(VDF-TrFE))-LiClO4 blend polymer 7*10 @ RT 38

-6 (PEO-PMMA)-LiClO4 blend polymer 10 @ RT 39

P(EO-PO)/LiTFSI copolymer 10-3 @ 80oC 40

P(EO-PO)/LiTFSI comb-type copolymer 10-5.5 @ 20oC 41

-4 o P(EO-TEC)/LiClO4 comb-type polymer >10 @ 30 C 42

(PEGMA-(MA-POSS)-EGDMA)-LiTFSI 1.60*10-4 @ 60oC 43 Organic/inorganic hybrid-branched copolymer

-5 o PVDF+LiClO4+PEO 2.62*10 @ 30 C 44

-5 PVDF+25wt% Li CF3SO3 >10 45

2.3.2.2 Gel polymer electrolytes

Gel polymer electrolytes are polymer electrolytes containing some aprotic solvents acting as plasticizer [46]. Different from SPEs, ions in GPEs ions move in the swollen polymer matrix. Some drawbacks reported on GPEs include lower mechanical strength, less compatibility with Li metal anode, and no better interfacial characteristics.

The primary function of the plasticizer is to increase the amount of the amorphous phase in the polymer system and to increase the dissociation capability. Three different kinds of plasticizers have been studies. Any organic solvents used in liquid electrolytes can act as plasticizers in the GPEs, such as EC (ethyl carbonate); PC (propylene carbonate);

DEC (diethyl carbonate); DMC (dimethyl carbonate); ɣ-BL (ɣ-butyrolactone). The low molar weight polymers can also serve as plasticizers, which include PEG (polyethylene glycol), MMPEG (monomethoxy polyethylene glycol), DMPEG (dimethyl polyethylene glycol), LiPEG (lithium polyethylene glycol), PEGDME (polyethylene glycol dimethyl ether), borate esters, Phthalates (e.g. DBP-dibutyl phthalate; DMP-dimethyl phthalate;

DOP-dioctyl phthalate); and SN(succinonitrile). Some RTILs(Room Temperature Ionic

Liquids) had been reported in GPEs like 1-ethyl-3methylmidazolium tricyanomethanide

(EMImTCM), N-N-dimethyl-N-propyl-N-butyl-Ammonium tricyanomethanide

(N1134TCM), Imidazolium-Tetra Alkylammounium dicationic PIC, 1-methyl-3-(2- acryloyloxthexyl) imidazolium tetrafluoroborate (MAHI-BF4). Table 5 lists conductivity values reported in some GPE systems with different plasticizers.

Table 5. Some GPEs and their conductivities reported in literature.

Gel Polymer Electrolyte Systems Conductivity (S/cm) Ref.

-4 o (PEO)20+LiCF3SO3 50wt% PEGDME. 1.24*10 @ 30 C 47

-3 o (PEO)15+LITFSI+10wt% PEGDME >10 @ 50 C 48

-4 o 6wt%PVC+24wt%PAN+8wt%LiClO4 +62wt%PC 0.701*10 @ 29 C 49

-5 PVC+PVDF+EC+PC+LiBF4 8.6*10 @ RT 50

-4 PMMA+EC/PC+LiClO4 5*10 51

-4 PMMA+LiN(CF3SO2)2+ 20wt% EC 1.28*10 52

-4 PMMA+LiN(CF3SO2)2+ EC+30wt% PC 2*10 52

-3 PVDF+ 1M LiPF6+EC/DMC/DEC(1:1:1) 1*10 @ RT 53 54 PVDF-HFP+ LiPF +EC/DMC/DEC(1:1:1) 0.07*10-3 @ RT 6

-3 PVDF-HFP+Urea+LiPF6+EC/DMC/DEC(1:1:1) 1.43*10 54 55 PVDF-HFP+ LiPF +EC/DEC 1*10-3 @ RT 6 PVDF-co-HFP+1M LiPF6+EC/DMC 2.3*10-3@ RT 56

-3 o PEO-PVDF-PGE-1MLiPF6+ EC/DEC(1:1) 4.5*10 @ 30 C 57

PEO-PVDF-PGE-1MLiTFSI+ EC/DEC(1:1) 4.9 *10-3 @ 30oC 57

-3 o PEO-PVDF-PGE-1MLiCF3SO3 + EC/DEC(1:1) 1.8*10 @ 35 C 57

-3 o PEO-PVDF-PGE-1MLiBF4+ EC/DEC(1:1) 2.8*10 @ 30 C 57

-3 o PEO-PVDF-PGE-1MLiClO4+ EC/DEC(1:1) 4.2 *10 @ 30 C 57

-3 o PVDF-PGE-1MLiPF6+ EC/DEC(1:1) 3.2*10 @ 30 C 57

-3 o 25wt%PVDF-HFP+EC/DEC(30/30 wt%) +15 wt% LiClO4 7.5*10 @ 30 C 58

-3 o 25wt%PVDF+EC/DEC(30/30 wt%) +15 wt% LiClO4 1.3*10 @ 30 C 58

-3 0.34*10 -LiBF4 @ RT 30wt%PVDF+60wt%EC/PC+10wt%LiX -3 0.47*10 - LiPF6 @ RT 59 (X=BF4;PF6;AsF6) -3 0.66*10 -LiAsF6 @ RT

-4 PVDF+ Li CF3SO3+ EC (30wt%) 4.3*10 60

-3 PVDF+EC/PC+LiN(SO2CF3)2) 2.2*10 @ RT 61

-3 PVDF(fibers)+1M LiPF6+ EC/DMC/DEC(1:1:1) 1*10 @ RT 62

-3 15 wt%PVDF+80wt%EC/PC+5wt%LiBF4 6.4*10 63

2.3.3 Solid state electrolytes

Recently, the research has moved into the arena of solid-state electrolyte systems whereby battery performances such as size, safety, energy density, thermal stability, mechanical properties, electrochemical stability window, etc. can be enhanced. The basic requirement for solid electrolytes is the high ionic conductivity greater than 10-4 S/cm with negligible electronic conductivity. The crystal structures of the various solid-state electrolytes which include LIPON, LISICON, Garnet, Perovskite, Argyrodite type, Anti- perovskite, etc. [64-67] Figure 7 presents the schematic structures of garnet-structured

Li5La3M2O12 (M=Ta, Nb)[68], perovskite-structured Li3xLa2/3-xTiO3, [68]. The typical conductivities of these different solid-state electrolytes are summarized in Tables 6-11.

Figure 7 a) Garnet crystal structure of Li5La3M2O12 (M=Ta, Nb). It consists of 3 tetrahedral, six octahedral and three trigonal sites where ions occupy; b) crystal structure of Li3xLa2/3-xTiO3 perovskite.

Garnet-type electrolyte was first developed in 1968 which comprised of

Li3Ln3M2O12 [69] where Ln= Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and M

= Te or W. Kasper reported that the TeO6 octahedron sites are more stable whereas structure distortion occurs in the presence of W. As the garnet-type electrolytes have low stability at ambient temperatures, more work was directed to increase the stability of garnets. The lattice constant and ionic radius are key parameters while designing the garnet structure system. Few electrolyte systems of this category are listed below in Table 6.

Table 6 Different garnet electrolytes and their conductivities reported in literatures.

Garnet-Structured Electrolytes Conductivity(S/cm) Ref.

-6 o Li5La3M2O12 M=Ta, Nb 10 @ 25 C; Ea(Nb)=0.43eV; Ea(Ta)=0.56eV 70

-5 o Li6BLa2M2O12 ;M=Ta, Nb ; B = Ba, Sr 4*10 @ 22 C; Ea=0.4eV for Ba doped. 71

-4 o Li7La3Zr2O12 (LLZO) 3*10 @ 25 C 72

-4 o Li6.75La3Zr1.75Nb0.25O12 8*10 @ 25 C 73

-4 o Bulk: 9.56*10 @ 25 C; Ea = 0.29eV Li7.6La3Y0.06Zr1.94O12 74 -4 o Total: 8.1*10 @ 25 C; Ea = 0.26eV

-3 Li7-xLa3Zr2-xTaxO12 (x=0.60) 1*10 @ RT; Ea = 0.35eV 75

-3 o -3 o Li6.55Ga0.15La3Zr2O12 1.3*10 @ 24 C; 2.2*10 @ 42 C 76

-3 o Li6.4Al0.2-x GaxLa3Zr2O12 0 ≤ x ≤ 0.2 1.2*10 @ 20 C 77

-6 o Li6.25La3Zr2Ga0.25O12 1.46*10 @ 25 C; Ea=0.25eV 78

-3 o Li6.55+yGa0.15La3Zr2-yScyO12 Y = 0.1 1.8*10 @ 27 C; Ea=0.29eV 79

-3 Li6.20Ga0.3La2.95Rb0.05Zr2O12 1.62*10 @ RT 80

Perovskite structure has a general chemical formulation in ABO3 wherein A is typically alkaline/rare earth metal ion occupying at corners of the cube; B is transition

metal ion occupying at the center of the lattice, and O is oxygen ions sitting at face-centered positions. Perovskite structure belongs to cubic lattice unit cells with a space group of

Pm3̅m symmetry. A representative perovskite-structured electrolyte is Li3xLa2/3-xTiO3

(LLTO), where B sites are filled by Li and La ions; A with Ti [68]. Li3xLa2/3-xTiO3 showed bulk conductivity 1*10-3 S/cm and total conductivity of 2*10-5 S/cm at room temperature with x = 0.34 [81]. One of the interesting features of this LLTO system is oxygen ions form bottlenecks which hinder Li ions to diffuse easily to neighboring sites. Varying lattice parameters of crystal system one can manipulate this bottleneck size. Up to date

Li3/8Sr7/16Zr3/4Ta1/4O3 (LSTZ) and Li3/8Sr7/16Ta3/4Hf1/4O3 (LSTH) are promising candidates for solid state Li ion batteries whereas further improvements are needed so one can enhance the ionic conductivity. Few electrolyte systems of this category are listed below in Table

Table 7. Different perovskite electrolytes and their conductivities reported in literatures.

Perovskite Electrolyte Conductivity(S/cm) Ref.

+2 -3 o Li1/2La1/2TiO3 (5mol% of Sr ) Bulk:1.5*10 @27 C 82

-5 o LiSr1.6Zr1.3Ta1.7O9 1.35*10 @30 C 83

-4 -4 o Li3/8Sr7/16Zr3/4Ta1/4O3 (LSTZ) Bulk:3.5*10 ; Total:2.7*10 @27 C 84

Li2x-ySr1-xTayZr1-yO3 ; Bulk:2.8*10-4; Total:2*10-4 @27oC @y=0.75 85 x=0.75y; y=0.6;0.7;0.75;0.77;0.8

-4 -4 -4 o Li3/8Sr7/16Ta3/4Zr1/4O3 (LSTZ) Bulk:2*10 ; Grain: 1.3*10 ; Total:10 @30 C 86

-4 o Li3/8Sr7/16Ta3/4Hf1/4O3 (LSTH) 3*10 @25 C; Ea=0.36eV 87

-5 o -4 o Li3/8Sr7/16Ta3/4Nb1/4O3 (LSNZ) 2*10 @30 C; 1.65*10 @100 C;Ea=0.26eV 88

-5 o Li Sr Ta Nb O + sintering 1.98*10 @30 C; Ea=0.34eV 3/8 7/16 3/4 1/4 3 89 aid(B2O3) +20wt% excess Li

LISICON structure belongs to the orthorhombic unit cell with space group Pnma, and all cations occupy tetrahedral sites [90]. Li3+x(P1-xSix)O4 represents LiSICON and can

+5 +4 +4 be obtained by aliovalent substitution of P by Si /Ge in ɣ-Li3PO4 which creates fast ion conduction. It was observed that the substitution generates excess Li ions which not only fully packed at tetrahedral sites but also occupy other interstitial sites leading to shorter diffusion path and hence high conductivity 3*10-6S/cm [91]. Another class of

LiSICON family is substituting O by S forming Thio-LiSICON. Li3+x(P1-xSix)S4 has higher conductivity than LiSICON by two orders of magnitude, i.e., 6*10-4 S/cm [91]. Although

- Hong et al. [92] developed electrolyte Li14Zn (GeO4)4 showing the conductivity of 1.25*10

1 S/cm at 300oC, Uaplen et al. [93] reported that at room temperature its conductivity was in the order of 10-6 S/cm. LISCON systems have conductivities in the range of 10-5 to 10-6

S/cm which are not suitable for solid-state battery application. However, LISICON electrolyte is highlights for its stability towards lithium. Research is further going on to improve the conductivity by employing different synthesis routes or by doping to tune the structure. A few LISICON electrolyte systems are described in Table 8.

LiPON electrolyte is abbreviated of lithium phosphorous oxynitride and dominant with the amorphous phase. Conductivity is in the orders of 10-6 to 10-5 S/cm and has excellent stability window up to 5.5 volts vs. lithium. The stability makes LiPON promising electrolytes in the field of thin film batteries. A few electrolyte systems of this category are listed in Table 9.

Table 8. Different LISICON electrolytes and their conductivities reported in literatures.

LISICON Electrolyte Conductivity(S/cm) Ref.

-5 o -2 o Li3.6Ge0.6V0.4O4 4*10 @18 C; 10 @190 C; Ea=0.44eV 94

-6 Li3.6Si0.6P0.4O4 10 @ RT 95

-5 o Li10.42Si1.5P1.5Cl0.08O11.92 For Si: 1.03*10 @27 C; Ea=0.44eV 96 -5 o Li10.42Ge1.5P1.5Cl0.08O11.92 For Ge: 3.75*10 @27 C; Ea=0.39eV

-4 Li4Al1/3Si1/6Ge1/6P1/3O4 9*10 97

-3 Li10GeP2S12 1.2*10 @ RT 98

-3 o -3 o Li10SnP2S12 Grain: 7*10 @27 C, Total: 4*10 @27 C 99

-3 Li10SiP2S12 2.3*10 100

-3 Li11Si2PS12 2*10 101

Table 9. Different LIPON electrolytes and their conductivities reported in literatures.

Electrolyte Conductivity (S/cm) Ref.

-6 o Li3.1PO3.8N0.16 -S1 S1:2*10 @25 C -6 o Li3.1PO3.8N0.22-S2 S2:2.4*10 @25 C -6 o Li3.1PO3.8N0.46-S3 S3:3.3*10 @25 C 102 -8 o Li2.7PO3.9+40%O2 & Ar-S4 S4*7*10 @25 C -8 o Li3.6Si0.19PO8.2O0.42+ 40%O2 and Ar- S5 S5:20*10 @25 C

-6 o Li3PO4+ Li2O (1:2), N/O ratio : 0.29/x 6.4*10 @25 C 103

-6 Li3PO4+ N2:Ar (1:8) 4.5*10 @RT 104

-6 o Li3.18PO1.69N1.39, N/O :0.82 4.9*10 @22 C; Ea=0.55eV 105

-6 LixPO2.1N0.6, N/O=0.29 2.85*10 @ RT; Ea=0.47eV 106

-6 Li3.2PO3N1, N/O=0.33 3.1*10 @ RT; Ea=0.57ev 107

t -6 -6 LiO Bu+TDMAP (in O2/NH3/Ar) 5.9*10 @190nm; 5.3*10 @95nm. 109 Li PO N (x<0.1)-S1 S1: 6.7*10-6@24oC; E =0.48eV. 3.8 2.8 x a 110 -6 o Li2.9PO2.9N0.5 -S2 S2: 1.4*10 @24 C; Ea=0.58eV.

NASICON electrolyte was first developed by Hong and Goodenough et al. [111] in 1976 as Na1+xZr2SixP3-xO12 which is Na ions conductor. General formula for such structural systems are represented as ABX(PO4)3 where A site consists of alkali ions/alkaline-earth ions; B&X sites comprised of mostly transition metal ions with different oxidation states ranging from II to V. In the past 30 years there was substantial research done on these electrolytes by replacing Na with Li ions. LTP (LiTi2(PO4)3) - lithium titanate phosphate, LGP ((LiGe2(PO4)3) - lithium germanium phosphate, LHP

((LiHf2(PO4)3) -lithium hafnium phosphate, and LZP ((LiZr2(PO4)3) - lithium zirconium phosphate, based on this parent structure, have been developed. LTP and LGP showed excellent stability window and high conductivity.

LTP material has rhombohedral symmetry with space group (R3c). LTP structure consists of octahedrons (TiO6) and tetrahedrons (PO4) sites which form a 3Dimensional chain of the network by sharing oxygen atoms at corners of the structure. This chain of networks is lithium ion diffusion pathway. LTP structure is highly rigid and stable due to the lattice geometry. The structure gets rigidity from the tetrahedrons (PO4) which shares a common oxygen atom between them. The composition can be flexible upon doping different species whereby its structure maintains [112]. The structure has two cationic vacant sites termed as M1 and M2 sites M1 sites are gaps between two adjacent facing octahedral sites (2MO6) along c-axis. M2 sites are gaps between two parallel chains of PO4.

In pristine LTP phase lithium completely occupies the M1 sites. Conductivity of LTP was

3.88*10-6 S/cm (298K) [117]. Miguel Paris et al. [118] reported NMR studies on Li diffusion mechanism in LTP as a function of temperature. Below 230K Li ions occupy completely M1 sites; 230-330K Li ions occupation in M2 sites with an increase of

temperature possessing activation energy of 0.47eV; higher than 330K activation energy drops whereby increases Li ions motion in both sites M1, M2 sites.

In 1980’s Aono et al. [114] substituted trivalent cations in the LTP lattice by

III IV replacing some tetravalent cations resulting in the formation of Li1+xMx M2-x (PO4)3. In order to dope the trivalent cations into LTP, the ionic radii should be comparable. The doped NASICON electrolytes showed enhancement of conductivity. It was also seen that new sites for Li ions are generated in the lattice, occurring in the M2 cavity which is known

-3 as M3 site [115,116]. Li1+xAlxTi2-x(PO4)3 conductivity reached 10 S/cm at room temperature which all other structured solid electrolytes, and comparable to conventional liquid electrolyte system. Figure 8 illustrates a crystal structure of LATP [113].

Figure 8 a) Crystal structure of lithium aluminum titanate phosphate (LATP), wherein

Green (M1) and Orange (M2) corresponds to cationic sites where Li ions occupy, the blue color is occupied Ti or Al ions, purple by Phosphate ions. b) Arrows pointing out in the figure shows Li ions diffusion from M1 to M2 sites.

Studies have reported by varying the dopants, dopant concentration, synthesis procedure, precursor materials in Li1+xMxTi2-x(PO4)3 (M=Al, Sc, Ga, In, Eu). By doping aluminum into Li1+xAlxTi2-x(PO4)3 the highest conductivities can be obtained at x = 0.3.

When x > 0.3 the formation of AlPO4 impurity was observed [125]. Furthermore, the lattice parameters of the LATP crystal decreased by increasing the doping concentration. The decrease in lattice parameters is due to large differences of ionic radii between Al and Ti.

Using scandium(Sc+3) as dopant lattice parameters increased and also led to structure distortion [125]. Therefore, the amount of the cations incorporated in the structure are in order of Ga+3

Investigations were carried out on the different synthesis procedures such as mechanochemical, magnetron sputtering, co-precipitation, sol-gel, hydrothermal, solid- state method, microwave processing, spray drying, spark plasma sintering, etc. Comparing all these procedures co-precipitation, sol-gel and solid-state methods showed high conductivities. In order to achieve high conductivities, the densification of the pellet should be very high whereby the grain growth would be homogeneous with less porosity. It was reported that conductivity could be enhanced by lowering the microcracks across the boundaries and reduction in secondary phases and maintaining critical grain size of LATP less than 1.6 µm [131]. Employing different sources of aluminum and phosphate has been studied, wherein due to solubility differences secondary phase (AlPO4) formation may occur [136,137]. These sources also impact on crystal structure and morphologies at different sintering conditions. In phosphate, sources pH was a key parameter, and at basic conditions there was no impurity (AlPO4) was observed [138]. Influence of the impurity

phase (AlPO4) increased the amount of the microcracks at grain boundaries which in turn act as a passive layer for lithium diffusion path. The volatilization of the lithium precursor at higher temperatures will result in the formation of other phases (TiO2, Li4P2O7) and low ionic conductivity [140,144]. The studies using effects of dispersants had been reported.

Replacing ethylene glycol with glucose has increased the ionic conductivity, which is due to homogeneity in the system created by the hydroxyl groups which prevents the metal complex reactions [159].

The research was later focused on by replacing Ti+4 with Ge+4 to form LAGP

-3 -5 (Li1+xAlxGe2-x (PO4)3) which showed conductivity in orders of 10 to 10 S/cm. It was also reported that GeO2 as secondary phase formed in LAGP with an increase in dopant concentration. The cell volume is lower due to comparable ionic radius of Al+3. The structure of LAGP is more superior than LATP for Li-ion migration [147].

Table 10 summarizes NASICON based lithium ion conductors and reported conductivities in literature.

Table 10 NASICON based lithium ion conductors and reported conductivities in literatures.

NASICON Electrolyte Conductivity(S/cm) Ref.

6 o x=0.1-1.6*10 @25 C; Ea=0.46eV -6 o x=0.05-1.4*10 @25 C; Ea=0.50eV -6 o Li1+xTi2-xAlx(PO4)3 ; 0

-4 o Li1.3M0.3Ti1.7(PO4)3 M=(Al; Sc), solid state 7*10 @25 C Ea=0.35ev. 126

-3 Li1.3Al0.3Ti1.7(PO4)3-Sol-gel 1.32*10 @ Room Temperature; Ea=0.326ev. 127 25

+3 -6 o Al : 2.5*10 ;Ea=0.28ev @25 C +3 -7 o Li1+xTi2-xRx(PO4)3; X=0.2; Ga :6.6*10 ; Ea=0.33ev @25 C +3 +3 +3 +3 +3 -6 o R= Al ;Ga ;Sc ;In Sc :1.4*10 ; Ea=0.33ev @25 C 127 +3 -6 o In :1.5*10 ; Ea=0.32ev @25 C

-4 o Li1+xAlxTi2-x(PO4)3, 2.9*10 @25 C; Ea=0.31eV x=0,0.3,0.35,0.4,0.45,0.5,0.6 128

-5 Li1.3Eu0.3Ti1.7(PO4)3, solid state reaction 6*10 ; Ea=0.32eV 129

-3 -4 Li1.4Al0.4Ti1.6(PO4)3, co-precipitation Bulk: 2.19*10 ; Total: 1.83*10 ; Ea=0.35eV. 130

Li Al Ti (PO ) 1.3 0.3 1.7 4 3 S1: 0.2*10-3 @25 oC S1:Moderate Purity Fine grains(MPFG) S2:0.49*10-3 @25oC S2:High Purity Coarse Grain(HPCG) 131 S3:0.67*10-3@25 oC S3:High Purity Fine Grains(HPFG)

-4 o o Li1+xAlxTi2-x(PO4)3 0

-4 o Li1.3Al0.3Ti1.7(PO4)3 , molten flux 7.02*10 @25 C; Ea=0.29eV 134

-3 Li1+xAlxM2-x(PO4)3;0

-3 S1: bulk: 1.2*10 ; Ea=0.19eV Li1.5Al0.5Ti1.5(PO4)3, sol-gel -5 Total: 7.1*10 ; Ea=0.35eV. Al(NO3)3-water soluble-S1 -3 S2: Bulk: 3.1*10 ; Ea=0.31eV 136 Al(C3H7O)3-water insoluble-S2 -4 Total: 4.5*10 ; Ea=0.28eV Li Al Ti (PO ) sol-gel 1.5 0.5 1.5 4 3, S1: Bulk:1.10*10-3; Total:9.95*10-5 H PO -S1 3 4 S2: Bulk:1.46*10-3; Total:5.22*10-4 NH H PO -S2 137 4 2 4 S3: Bulk:1.19*10-3; Total:2.16*10-4 (NH4)2HPO4-S3

-3 -4 Li1.4Al0.4Ti1.6(PO4)3, citric acid sol gel Bulk: 2.09*10 ;Total: 6.13*10 ; Ea=0.29eV 138

-3 -3 Li1.3Al0.3Ti1.7(PO4)3 + 85% H3PO4 solution Bulk: 2.48*10 ; Total: 1.21*10 ; Ea=0.26eV 139

-4 Li1.3Al0.3Ti1.7(PO4)3, co-precipitation 1.6*10 @RT 140

-5 Li-Al-Ti-P-O (target) + N2 atmosphere. 1.22*10 @RT; Ea=0.44eV magnetron sputtering @500oC 142

-4 o Li1.5Al0.5Ti1.5(PO4)3, sol-gel (large-scale) 6.9*10 @25 C 143

-4 o Li1.3Al0.3Ti1.7(PO4)3 , sol-gel S1:1*10 @30 C; S1: No dispersant S2:1.6*10-4@30oC o S2: Ethylene Glycol S3:6*10-4@30 C; Ea=0.31ev 144 S3: Glucose

-4 Li1.3Al0.3Ti1.7(PO4)3, spray drying method 6.22*10 @RT 146

2.3.4 Composite (Ceramic-Polymer) electrolytes

Current research on the composite electrolyte system includes use of different ceramic fillers, varying filler concentration, and different sizes of fillers. Significant improvements had been reported regarding interfacial stability [150,151], mechanical properties [152], ionic conductivity [153] and Li-ion transference number.

Fillers are classified into two classes inert and active fillers. Inert fillers the particles indirectly involve in the conduction process, via increasing free volume, the mobility of ions and segmental motion of polymer chains [154,155]. Active fillers directly participate in the conduction process by adding Li ions concentrations and extra conducting path in the polymer matrix. Extensive research was carried out using different ceramic fillers such as Al2O3, AlO[OH]n, TiO2, SiO2, BaTiO3, super acid oxides, clays, LiAlO2,

Li2O, LATP, LLTO, and LAGP. Table 11 summarizes conductivities of some composite electrolytes with different ceramic fillers and polymer complexes.

Table 11 Various composite electrolytes and reported conductivities in literatures.

Composite Electrolyte Conductivity( S/cm) Ref.

-2 S1: PVDF+ 25wt%SiO2 + 1MLiPF6+ EC/PC(1:1) S1:3.5*10 @RT -3 156-159 S2: PVDF+ 15wt% SiO2 +1MLiPF6+EC/DMC(1:1) S2:1.4*10 @RT

-3 S1:PVDF-HFP+SiO2+ 1MLiClO4+ EC/PC(1:1). S1: 4.3*10 @RT 160-161 -2 S2:PVDF-HFP+4wt%SiO2+ LiClO4+ DEC/PC(1:1). S2:1.04*10 @RT

-3 S1:PVDF-HFP+ TiO2+DMBITFSI+LiPF6 S1:1.03*10 @RT -3 S2: PVDF-HFP+ TiO2 +EC/PC(1:1)+LiClO4 S2: 2.1*10 @RT -3 162-165 S3: PVDF-HFP+6.5wt%TiO2 +LiPF6+EC/DEC(1:1) S3: 1.66*10 @RT -3 o S4:PVDF-HFP+TiO2 +EC/DMC(1:1:1)+LiPF6 S4:0.98*10 @20 C

-2 PVDF-HFP+ LiN(CF3SO2)2 +AlO[OH]n (Nano) 1.1*10 @RT 166

-3 o PVDF-HFP+BaTiO3(7.5wt%)+LiBETI+ EC/PC 1.26*10 @30 C 167

-3 o PVDF-TrFE+BaTiO3(4wt%)+1M LiClO4.3H2O in PC 1.04*10 @25 C;Ea=0.11eV 168

-3 PVDF-HFP+CaCO3 (1:2)+1M LiPF6+EC/DMC 1.38*10 @ RT, Ea=0.07eV 169

-2 PVDF-HFP+ZrO2 +LiClO4 +EC/DEC 1.1*10 @ RT 170

-5 o PEO+LAGP(70wt%)+LiClO4 1*10 @25 C 171

PEO+LAGP(20wt%)+LiTFSI 6.76*10-4 @60oC 172

-4 o PEO+LATP(10wt%,Nano)+LiClO4 1.7*10 @20 C 173

PEO+LGPS(1wt%)+LiTFSI 1.8*10-5 @25oC 174

-3 PVDF-HFP+LiAlO2(Nano)+LiClO4+EC/DEC 8.1*10 175

-3 PAN+LATP(15wt%)+1M LiPF6+ EC/EMC 3.6*10 @RT 176

-3 PAN+LLTO (15wt%)+1M LiPF6+EC/EMC 1.95*10 @RT 177

-3 PVDF+LATP+1M LiPF6+EC/EMC/DMC 0.96*10 @ RT, Ea=0.52eV 178

-5 o PEO+LATP(15wt%) +LiClO4 1.387*10 @25 C 179

LATP+PVDF-HFP(Fibers) 10-5 to 10-4 180

PVDF+LATP (5wt%)+1MLiTFSI+DOL/DME(1:v/v) 3.31*10-4 181 1wt%LiN3

CHAPTER 3: SYNTHESIS AND CHARACTERIZATIONS OF

LITHIUM ALUMINUM TITANATE PHOSPHATE (LATP)

This chapter will present the synthesis and characterization results of lithium aluminum titanate phosphate, i.e. Li1.3Al0.3Ti1.7 (PO4)3 (LATP), obtained in this study.

3.1 Synthesis of Li1.3Al0.3Ti1.7(PO4)3 Electrolytes

Table 12 Stoichiometric amounts of precursors used for synthesis of L1.3Al0.3Ti1.7(PO4)3

Precursor Weight or (Volume)

C6H5Li3O7.4H2O + 15 mol% excess lithium 4.7381 grams

Concentrated Nitric Acid (HNO3) 1.902 ml

Aluminum Hydroxide (Al(OH)3) 0.78 grams

Ammonium Dihydrogen Phosphate(NH4H2PO4) 11.738 grams

Titanium Isopropoxide(TiC8H24O4) 17.298 ml

Citric Acid (CA) 3.881 grams

Ethylene Glycol (EG) 4.426 ml

Distilled Water 35.106 ml

The Li1.3Al0.3Ti1.7(PO4)3 powders were prepared by using the following precursors trilithium tetrahydrate citrate (C6H5Li3O7.4H2O), concentrated nitric acid (HNO3), distilled water, aluminum hydroxide (Al(OH)3), ammonium dihydrogen phosphate (NH4H2PO4), citric acid (CA), ethylene glycol (EG), titanium isopropoxide (TiC12H28O4). Table 12 lists

the stoichiometric amounts of the precursors used to synthesize LATP.

Figure 9 illustrates the flow chart of synthesizing LATP powders and pellets.

Firstly, the 70% concentrated nitric acid was diluted with distilled water to 5%. Then

TiC12H28O4 and diluted nitric acid were mixed together under continuous stirring.

Afterwards, the appropriate amounts of lithium and aluminum precursors, i.e.,

C6H5Li3O7.4H2O and Al(OH)3, were added to the mixture until homogeneity was achieved.

Lithium precursor tends to evaporate due to the high vapor pressure of lithium oxide at high temperatures resulting in the actual deficiency of lithium in the product [141,182].

Therefore, excess lithium of 15 mol% was added to compensate for this loss. Aluminum content was selected at x = 0.3 for Li1+xAlxTi2-x(PO4)3, because at this composition the conductivity was reported as the highest. Later, citric acid was added as complexation agent to create a transparent solution, and ammonia was added until pH value reaches 5. At last,

NH4H2PO4 and EG were added to the entire solution. The molar ratio of Al: CA: EG was maintained as 1:2:8. Subsequently, the solution was heated at 170oC on the hot plate under continuous stirring. During this process, excessive solvent and water gradually evaporated and meanwhile polymerization and esterification occurred to generate sol. The sol was transferred to furnace and kept at 180oC until black gel formed.

The as-prepared gel was grounded and calcined in crucibles at the preset temperatures in the range of 850-1000oC for the preset duration varying from 3hrs, 6hrs, to 10hrs to obtain crystalline LATP powders. To break the large agglomerates, ball milling was performed for 5hrs at high speeds with the help of ethanol and zirconia balls.

The powders are then pressed into pellets using the isostatic press at 20MPa for 5 minutes. These LATP pellets are further sintered in the temperature range of 850-1000oC

for 3hrs, 6hrs, or 10hrs to increase the crystallization and density of the pellet.

Figure 9 A schematic flow chart showing the fabrication of L1.3Al0.3Ti1.7(PO4)3 powders and LATP pellets.

3.2 Structural and Morphological Characterizations of the As-prepared LATP

3.2.1. Structure and phase

Brucker D8 XRD was employed to determine the phase of LATP material. The

XRD spectra were obtained using Cu Kα radiation, 2θ range from 10o to 90o with a step size 0.01o and step scan 1 sec.

Figure 10 presents the XRD spectra of the Li1.3Al0.3Ti1.7 (PO4)3. The spectra match well with crystalline LATP reported in literatures and indexed with rhombohedral symmetry(R3c). It is noteworthy that LATP specimens calcined at 950oC for shorter time like 3hrs have some very small amounts of TiO2 impurities manifested by the presence of diffraction peak at 2θ = 27.1o and 36.4o. Further, calcination at 850oC for 10 hrs in

fabricating leads to the formation of AlPO4 impurity phase with the presence of diffraction peak at 2θ = 25.4o. At higher temperatures loss lithium material occurs which results formation of TiO2 impurity. No other impurities were found in LATP products.

Figure 10 XRD spectra of the as-prepared L1.3Al0.3Ti1.7(PO4)3 samples obtained at different calcine and sintering conditions.

3.2.2. Morphologies of LATP specimens

Scanning electron microscopy (SEM, FEI Lyra SEM/FIB system) was employed to visualize the grain size and porosity of the LATP specimens fabricated at different heat treatment process. ImageJ software was used to quantify the grain size and porosity of the

LATP materials.

3.2.2.1. Impacts of calcination temperature

The main function of calcination was to eliminate the carbon contents in LATP and to increase the crystallinity. In this study, the LATP samples were obtained at different calcination temperatures at 850oC, 900 oC, 950 oC, and 1000oC.

(c)

Figure 11 SEM images of LATP powders obtained at different calcination temperatures

(a) 850oC-3hrs; (b) 950oC-3hrs; (c) EDS spectrum of LATP powders calcined 950oC-

3hrs.

Figure 11 (a) and (b) shows the typical SEM images of LATP powders obtained at calcination temperatures of 850oC and 950oC, respectively and calcined time is 33

maintained as 3hrs. High calcination temperature is beneficial to the crystallization of

LATP. Figure 11 (c) shows EDS (energy dispersive spectroscopy) spectrum of the LATP powders calcined at 950oC-3hrs. Clearly, all elements except lithium are detected. Lithium cannot be detected due to its low characteristic radiation. The molar ratio among Al, P, and

O agrees with the nominal value in LATP.

3.2.2.2. Impacts of calcination duration on grain size of LATP

Figure 12 shows SEM images of LATP pellets calcined at 950oC for different durations (3hrs, 6hrs) and compared with the sample calcined 850oC 10hrs. For consistency, all sintering parameters of these three samples are the same. Comparing

Figure 12 (a) and (b), it is seen that increase in the calcination duration improved the crystallization and grain growth progression in LATP samples. The average particle size of the for “cal-3hrs” sample is 0.52 µm, while the particle size of the “cal-6hrs” sample increases to 1.51 µm. For longer duration of 10hrs, i.e. “cal-10hrs”, there was no significant increase in particle size (1.7 µm) in LATP. However, longer calcine duration can lead to the formation of AlPO4.

(a) (b) (c)

Figure 12 SEM images of LATP samples calcines for (a) 3hrs calcined at 950oC; (b) 6hrs, calcined at 950oC, (c) 10hrs calcined at 850oC. All samples are sintered at 950oC for 6hrs. 34

Spectrum 7 Spectrum 6

Figure 13 EDS spectra of LATP pellet calcined at 850oC for 10hrs and sintered

950 for 6hrs (a) obtained at point 7; (b) obtained at point 6.

Actually, longer calcine duration can lead to the formation of AlPO4 even at lower calcine temperature of 850oC. Seen in Figure 12 (c) and Figure 13 (a), the dark large amorphous region corresponds to the AlPO4. Figure 13 (b) and (c) shows EDS spectra of the sample obtained at two different areas. The one is on the cubical particle aligns well with LATP. In contrast, the one on the thin amorphous flat-sheet covering many cubical

particles corresponds to the AlPO4 impurity. Formation of AlPO4 occurs mainly in acidic conditions, it also observed this phase decreases the total ionic conductivity but enhances the densification of the pellet.

Hence, the optimized working condition for our LATP samples is 6hrs which exhibited highest bulk conductivity.

3.2.2.3. Impacts of sintering temperature and sintering time

Sintering helps to improve grain growth, densify the pellet, reduce porosity in

LATP. However, sintering at higher temperatures and/or for longer duration may lead to local Li2O evaporation and hence transformation into AlPO4. All these microstructural factors will affect the ionic conductivities. On the other hand, sintering at high temperature is kinetically beneficial to grain growth. The large grain size means reduced the grain boundaries. It is known that lithium ions traveling in grain are fasters with low activation barrier than traveling across grain boundaries. In consideration of the above aspects, the sintering temperature of 950oC is selected for the highest conductivities which will be discussed in the following section.

Figure 14 show SEM images of LATP pellets calcined at the optimal conditions

(950oC for 6hrs) and sintered at 950oC for different times (3hrs, 6hrs, and 10hrs). Increase in sintering duration increases more symmetrical grain growth in the system up to 6hrs.

Further increase led to uneven growth of grains which creates bridges for lithium ions transportation and more loss of lithium material in the system. Grain sizes for 3hrs, 6hrs,

10hrs were 2.8µm, 4.8 µm, 4.79 µm.

Figure 14 SEM images of LATP pellets sintered at different times while keeping sintering temperature at 950oC and calcination at optimal conditions, i.e., 950oC for 6hrs.

3.2.3. Ionic conduction characteristics of Li1.3Al0.3Ti1.7(PO4)3

Ionic conductivities of LATP electrolytes were obtained using electrochemical impedance spectroscopy approach.

The LATP pellets were coated with silver paint (TED PELLA INC) on both sides which act as working and counter electrodes. A copper wire was attached to coat sample on each side. Electrochemical impedance spectra studies were done with the help of Gamry

Potentiostat Reference 3000. The spectra were collected at AC amplitude of 200mV in the frequency range of 1MHz to 1Hz with 50 points per decade. From EIS studies, the impedance of the LATP pellet was determined. Based on the resistance and LATP dimension, ionic conductivities were calculated.

Figure 15 A typical EIS spectrum obtained from LATP specimen (calcined 950-6hrs- sintered 950-6hrs) at room temperature

Figure 15 shows a typical EIS spectrum obtained from a LATP pellet at room temperature. The spectrum has an intercept with x-axis of 83.06 ohm. At high frequency regime, a semicircle with a diameter of 1.2 kohm. At low frequencies a linear tail is dominant. In an electrochemical system, the low-frequency impedances are correlated with electrode reaction/process, while the high-frequency regime in EIS reflects ionic conduction characteristics in electrolyte. For solid ionic electrolytes, bulk and grain boundary conductions correspond to high-frequency and intermediate-frequency impedances, respectively [182,183,184].

In this research, we focused on study the relationship between bulk conductivities

and synthesis conditions, e.g. calcination temperature, duration and sintering temperature, duration. From figure 15, the bulk conducting impedance is 83 ohm. Accordingly, bulk conductivity of the sample (calcined 950oC-6hrs, sintered 950oC-6hrs) is calculated

1.5mS/cm. This value is the highest among all the LATP samples in this research.

3.2.3.1 Impacts of synthesis conditions on ionic conductivities

Figure-16 Complied ionic conductivities of all LATP samples obtained as a function of calcination temperature, calcination time, sintering temperature, and sintering time.

Figure 16 presents the complied ionic conductivities of all LATP samples obtained as a function of calcination temperature, calcine time, sintering temperature and sintering time. The average trend suggests the highest conductivities occur at calcination temperature 950oC, calcination time 6hrs, sintering temperature 950oC, sintering time 6-10 hrs. Among these four parameters, calcination temperature appears to has the most significant impacts. This is consistent with our highest conductivity obtained at all these optimal conductions.

The maximum conductivity occurring at calcination temperature of 950oC and 6 hrs can be attributed to the following aspects. Firstly, calcination processing at low temperatures and short duration leads to the presence of titanium oxide (TiO2) impurity.

Secondly, at higher temperatures or long duration lithium volatilization is significant resulting the formation of AlPO4. The presence of AlPO4 and TiO2 impurities could lead to fractures (microcracks) and blocking lithium ions pathway resulting in low conductivities (bulk and grain). All these impurities were observed by XRD at 850oC,

1000oC. Thirdly, with elimination of impurity, higher-temperature and longer calcination can facilitate better crystallization of LATP. Hence, 950oC and 6hrs are favorable compared with 900oC and 3hrs. Consequently, the optimized conditions with the highest bulk conductivity are around calcination at 950oC for the 6hrs duration.

The impact of crystallization is also reflected on sintering conditions. One can see that at lower sintering duration LATP particles are not fully crystallized with different shapes and more like unsymmetrical growth. The LATP grains sintered for 6hrs are well- developed in symmetric shapes with clearly distinguishable the grain boundaries. The further increase in sintering temperature and duration (1000oC,10hrs) results in Li loss and

formation of AlPO4 which will dominate at the grain boundaries and suppress Li mobility.

Ionic conductivity depends on microstructure of pellet (density, porosity and particle size).

Therefore, a dense pellet with very high particle size would reduce the resistance at grain boundaries even when impurity phase is high [185]. Furthermore, Thokchom’s [186] revealed that AlPO4 forms complex with Li ions which could be source for space charge mediation or could block the lithium diffusion pathway. Therefore, the optimized conditions for sintering is 950oC for 6hrs.

3.2.3.2 Impacts of temperature on ionic conductivities

Bulk ionic conductivity (σ) of LATP has shown an increasing trend with increase of temperature (T) in the range of 30oC-100oC obeying Arrhenius relationship. Figure

17(a) shows impedance spectra obtained at different temperature and Figure 17 (b) shows a plot of log (σ) vs 1/T obtained from the LATP fabricated at the optimal conditions obtained, i.e. calcined at 950oC – 6hrs and sintered 950oC – 6hrs. The activation energy of conduction is 0.206 eV which matches with literature [187].

Figure 17 (a) EIS spectra of LATP obtained at 30oC to 100oC for sample calcined 950oC-

6hrs and sintered 950oC-6hrs.

Figure 17 (b) an Arrhenius plot showing Log (σ) vs 1000/T for sample calcined 950oC-

6hrs and sintered 950oC-6hrs.

3.3 Conclusion

We have successfully fabricated highly conductive in a more economical way which can be used for all solid-state lithium ion batteries. Bulk conductivities are found to be affected by the presence of impurity and microstructure (grain size, crystallinity, microcracks etc.) of sintered pellets, which are caused during the synthesis. It is found that in the present system, the optimal synthesis conditions are calcination at 950oC for 6hrs followed by sintering at 950oC for 6hrs, which will result in well crystalline micro-size dense LATP pellets and eliminate the formation of common impurity phases like TiO2

AlPO4. The highest bulk conductivity of 1.5mS/cm and low activation energy of 0.206 eV are obtained on the LATP synthesized at the optimal conditions.

CHAPTER 4: FABRICATION AND CHARACTERIZATION OF

LATP-BASED COMPOSITE MEMBRANE

This chapter will present results on fabricating and characterizing free-standing composite electrolyte membranes. The composite system is polyvinylidene fluoride

(PVDF) lithiated with lithium perchlorate (LiClO4) and filled with active ceramic filler lithium aluminum titanate phosphate (LATP), i.e., PVDF-LiClO4/LATP.

As discussed in Chapter 2, PVDF is a favorable polymer electrolyte candidate because it has a strong electron accepting group -F-, excellent electrochemical stability, thermal stability, and excellent mechanical strength. PVDF-based SPEs possess the conductivities in the range of 10-4 to 10-5 S/cm whereas GPEs in range of 10-3 S/cm.

Addition of active ceramic fillers like LATP can increase high ionic conductivities, thermal stability, and mechanical strength.

Choosing lithium perchlorate (LiClO4) as salt is for its properties like high mobility, high dissociation constant, stable against hydrolysis, and excellent electrochemical stability (5V vs. Li) [5,188]. Additionally, the anion nature being strong oxidizer readily reacts with polymer and quickly dissolves in the system. Furthermore, it is most stable against moisture among various lithium salts, make it relatively easy to fabricate in the less-stringent environment. Although LiClO4 is excluded from lithium-battery products due to safety concern, the potential explosion hazard can be significantly suppressed in the solid composite electrolytes.

4.1 Fabrication of PVDF-LiClO4/LATP Composite Electrolyte

The polyvinylidene fluoride used in this study has an average molecular weight of

534,000. N-Methyl-2-Pyrrolidone (NMP) is used as the solvent to dissolve PVDF. NMP has been stability and solubility compared with acetone, dimethylformamide (DMF), dimethyl sulfoxide (DMSO) [189]. NMP and LiClO4 were purchased from ACS reagent-

Sigma Aldrich. LATP powders were synthesized as described in the previous chapter.

After several trials, the content of PVDF in NMP was fixed to 1mg: 10ml for the approperiate viscosity, which is essential for memebrane casting. The molar ratio of VDF to Li was kept 6.2. The content of LATP over the sum of LATP and PVDF was varied from 0 to 60 wt%. Table 13 lists the compositional details of the series composite membranes, i.e. PVDF, PVDF-LiX, LATP-20, LATP-30, LATP-40, LATP-50 to LATP-

60.

Table 13 Composition and the amounts of each component added to fabricate the series of

PVDF-LiClO4/LATP composites

Moles of [VDF]: Sample LATP(g) PVDF(g) LiClO4(g) Moles of Li PVDF [Li]

-3 PVDF-LiClO4 0 0.75 0.20 1.88*10 0.0117 6.2

LATP-20 0.15 0.60 0.160 1.504*10-3 9.37*10-3 6.2

LATP-30 0.225 0.525 0.140 1.316*10-3 8.20*10-3 6.2

LATP-40 0.3 0.45 0.120 1.12*10-3 7.03*10-3 6.2

LATP-50 0.375 0.375 0.100 9.4*10-4 5.85*10-3 6.2

LATP-60 0.5625 0.375 0.100 9.4*10-4 5.85*10-3 6.2

Firstly, PVDF was dissolved into NMP solvent and lithium perchlorate was added under continuous stirring until a homogeneous phase was achieved. Meanwhile, LATP powders (see in Chapter 3) was ball-milled for 24 hrs in the presence of NMP to reduce the particle size. The ball-milled LATP powders were then ultra-sonicated for 24 hrs to reduce the agglomerates. After LATP was added into the PVDF-LiClO4-NMP solution, the mixture was stirred continuously at preset curing temperatures (70oC, 80oC, 100oC) for

3 hrs to achieve a homogenous solution. The volume of the total solution is kept around

15ml. This composite solution was then casted in small stainless steel can (15mm in diameter) at different temperatures (80oC, 90oC, 100oC, 120oC, 140oC) for 4 hrs. The film thickness was then measured by a micrometer. It was observed that the most film prepared in this approach has approximately 43µm in thickness.

4.2 Characterizations of the PVDF-LiClO4/LATP Composite Membranes

4.2.1. Dimensional stability

Figure 18. Images of PVDF/LiClO4 membranes casted at different temperatures showing the changes in color and flexibility.

It is noticed that the membranes gradually changed into yellowish color upon increasing the temperature from 100oC to 140oC. As the casting temperature was set at

170oC, the membrane changed into black color. Figure 18 shows typical images of the obtained free-standing PVDF membranes casted at different temperatures. Later Raman spectroscopy suggests PVDF decompose at high casting temperatures beyond 100oC. High electronic conductivities were noticed due to the decomposition of PVDF.

Table 14 list the dimensional stability of the composite films. With the addition of

LATP beyond 60wt% and increasing the casting temperatures above 100oC, some membranes became too brittle to be usable.

Table 14 Dimensional stability of the composite electrolyte membranes fabricated at different casting temperatures

Electrolyte 80oC-4hrs 90oC-4hrs 100oC-4hrs 120oC-4hrs 140oC-4hrs

PVDF-LiClO4 good good good good good

LATP-20 good good good good good

LATP-30 good good good good poor

LATP-40 good good good good poor

LATP-50 ok ok ok poor poor

LATP-60 ok ok ok poor poor

In consideration of applications to flexible lithium-batteries, the electrolyte membrane should be free-standing, fast processing, and have negligible electronic conductivity, the casting condition of the series composite membranes was fixed at 100oC for 4 hours.

4.2.2. Ionic conductivities of the membranes

Conductivity measurements were carried out using Swagelok cell setup and schematic as shown in Figure 19. The Swagelok cell consists of a closing cap, spacer,

Teflon cylinder, and bottom membrane. All the components were heated at 100oC for 3-

4hrs to remove the adsorbed moisture, before they were transfer into an argon –filled glove box with controlled the oxygen and moisture less than 5 ppm. The composite membrane is sandwiched between two lithium spacers and assembled in the bottom cap — further, its sealed by assembling Teflon cylinder, spacer, and closing cap. All the assembling was performed in the Ar-filled glove box.

Figure 19 A Swagelok cell used for conductivity measurement of the electrolyte membrane.

The ionic conductivities of the composite membranes were determined using electrochemical impedance spectroscopy. The EIS spectra were obtained using Gamry

Potentiostat Reference 3000 with an AC voltage of 100 mV in the frequency range of

1MHz to 1Hz with 50 points per decade. 49

It is interesting to note that conductivities of the membranes prepared at different curing temperature are slightly different, wherein the curing temperature at 70 oC leads to the highest conductivity values. The curing temperature may result in different amount of crystallinity of PVDF which in turn affect the mobility of Li ions.

Figure 20 shows the EIS spectra at different curing temperature and different amounts of LATP. From EIS spectra, resistances corresponding to ionic conduction in the membrane, which is usually occurred at high frequencies, are determined, from which conductivities are calculated based on the membrane thickness and area.

Figure 20 EIS spectra of the composite membranes with different LATP content prepared at curing temperature of 70oC-3hrs.Circular dot points are used to estimate conductivity

Figure 21 shows the conductivity as a function of LATP content. It is seen that the conductivity increases with the increase of LATP filler content and reaches the maximum at 50 wt%. The highest conductivity at room temperature is 4.02*10-4 S/cm obtained from the LATP-50 membrane cured at 70oC-3hrs and casted at 100oC-4hrs.

Figure 21 Conductivities as a function of LATP content of the membranes prepared at different curing temperatures.

Figure 22 Conductivity vs. temperature in Arrhenius plot obtained from LATP-50 membrane casted 100oC for 4hrs and curing temperature 70oC 3hrs.

Figure 22 shows the conductivities of LATP-50 increase with temperature, which appears to follow Arrhenius relationship. This increase was due to the faster movement of polymer chains facilitating rapid lithium ion transport in the composite matrix. The activation energy of the conduction in LATP-50 membrane is 0.23 eV with conductivity at

95oC of 2.01*10-3S/cm.

4.2.3. Preliminary Raman spectroscopic studies

All Raman spectra are measured in region 0 to 2000 cm-1 region on Rainahsaw system using 514cm-1 wavelength.

Figure 23 Raman spectra of PVDF membrane prepared using two different solvents NMP and DMF. Note: PVDF-DMF was casted at 80oC while PVDF-NMP was casted at 100oC.

Figure 23 shows the Raman spectra of PVDF membrane prepared using two different solvents NMP and DMF. Both samples were cured at 70oC, but their casting temperature was varied. These samples are designated as PVDF-DMF-80oC-6h, PVDF-

NMP-100oC-6h. Seen in Figure 23, the membranes casted using DMF solvent have a dominant band at 839 cm-1 and strong band (shoulder) at 795 cm-1. In comparison, the

PVDF-NMP membrane has a higher contribution from 809 cm-1 band. Other major peaks like 839 cm-1,1433 cm-1,1526 cm-1 originating from C-H stretching are all observed in both case. PVDF polymer consists of three different crystalline phases, i.e., α, β, ɣ phases.

Basing on these, it has three distinct molecular conformations, i.e., form-I-where polymer 53

chains with all trans structure, form-II with trans-Gauche Sequences, form-III with three trans bonds separated by gauche bond [190]. indicating the presence of α-phase, β, ɣ-phases and characteristic of C-H stretching mode [191]. Therefore, the bands corresponding to

839 cm-1,795 cm-1 consists of form-I, form-II wherein overall DMF casted membrane is comprised trans-zigzag conformations. Furthermore, NMP casted membrane dominated with 809 cm-1 band has lower number of trans gauche sequences. The Raman spectra suggested the PVDF in the PVDF-DMF-80oC-6h membrane is mainly the alpha phase, while the PVDF phases in the PVDF-NMP-100oC-6h membrane are dominant by beta and gamma phases.

Figure 24 shows the Raman spectra of PVDF-LiClO4 membranes casted at different temperature and compared with PVDF membrane. The characteristic peak of

- -1 perchlorate anion (ClO4 ) is observed at 933 cm [192] corresponding to free perchlorate

-1 anions. The peak corresponding to 1526 cm band is missing in PVDF-LiClO4 system was due to deprotonation of the polymer.

Figure 25 shows the Raman spectra of PVDF-LiClO4/LATP membranes. LATP bands can be seen in the region from 300-500 cm-1 which are due to symmetrical bending

-1 vibrations of PO4 group, whereas peaks in 900-1100 cm correspond to vibrations of PO3

-1 group, and those between 1100-1300 cm are vibrations of PO2 group [193].The disappearance of the peaks corresponding to PVDF and lithium salt might suggest the reduced crystallinity of the PVDF and some interactions among the three components.

Figure 24 Raman spectra of PVDF-LiClO4 membrane casted at different temperatures and compared with virgin PVDF casted at the same condition.

Figure 25 Raman spectra of PVDF-LiClO4/LATP comparison with PVDF- LiClO4 and

PVDF membranes casted at same conditions (100oC and 6 hrs with NMP). 55

It is hypothesized, basing on the Raman Spectra, that there exist the interactions among

PVDF, Li salt, LATP in the composite membrane. As a consequence, lithium ions are dissociated from LiClO4 and attached to PVDF. The lithium ions can transfer upon PVDF segmental movement or transfer through LATP conducting channels, resulting in the enhancement in the conductivity. However, excess LATP in the composite membrane, e.g.

60 wt% or beyond, can reduce the volume of PVDF which may lead to reduction of ionic conductivity. Figure 26 depicts the complex ion transfer mechanism of PVDF-LiClO4-

LATP system.

Figure 26. A hypothetic ion transfer mechanism in the PVDF-LiClO4-LATP system.

4.3 Conclusion

Free-standing flexible PVDF-LiClO4 solid composite membranes with LATP content from 0 to 60 wt% have been successfully fabricated. The key parameters influencing the electrolyte preparation include curing temperature, casting temperature,

and LATP filler concentration. Casting temperatures exceeding 140oC led the decomposition of PVDF polymer resulting in the fragile films. Ionic conductivity cured at

70oC for 3hrs showed best results. Ionic conductivities of the composites with different filler concentrations in the range of 0 to 60 wt% were determined. The membrane with

50wt% LATP showed the highest conductivity 4.02*10-4 S/cm at room temperature with an activation energy of 0.23eV. Preliminary Raman spectroscopic studies indicate there exist certain interactions among the three components. The interactions result in the reduced crystallinity of PVDF, more dissociated lithium ions from lithium salt, and extra rapid transport paths for lithium ions between PDVF and LATP. All these can contribute to the enhanced ionic conductivities.

CHAPTER 5: CONCLUSION AND FUTURE WORK

Solid and flexible electrolyte materials with excellent stability window and high conductivity are demanded in all solid-state lithium batteries. This research is focused on fabricating and characterizing ceramic lithium ion conductor, i.e., lithium aluminum titanate phosphate (LATP) and its composite with lithiated polyvinylidene fluoride

(PVDF), to achieve high conductivity, thin, and free-standing electrolyte membrane with excellent dimensional stability.

We have successfully fabricated lithium aluminum titanate phosphate (LATP) pellets using a sol-gel route which is economical and easy to scale up. The optimized conditions in LATP fabrication are found around calcination temperature of 950oC for 6hrs and densification of the pellet at a sintering temperature of 950oC for 6hrs. Short calcination duration led to TiO2 impurity and longer calcination times to AlPO4. The highest bulk conductivity of 1.5 mS/cm is obtained at room temperature and the activation energy of conduction is 0.206 eV. These results are comparable with those reported in literatures.

Further, free-standing and flexible composite solid polymer electrolyte membranes consisting of PVDF-LiClO4/LATP are fabricated. Both curing temperature and casting temperature can influence the electrolyte preparation and dimension stability. Ionic conductivity cured at 70oC for 3hrs showed best results. The ionic conductivities of composite electrolytes increase with LATP content and reach maximum at 50 wt %. The

ionic conductivity at room temperature is 4.02*10-4S/cm for LATP-50 with activation energy of 0.23eV. Raman spectroscopic studies indicate there exist certain interactions among the three components, which may reduce crystallinity of PVDF, facilitate lithium ion dissociation from lithium salt, and provide extra transport path for lithium ions. All these will be beneficial to the enhancement of ionic conduction.

In future, replacing the lithium salt with LiTFSI(Lithium bis(trifluoromethane sulfonyl)imide ) may lead to the enhancement of ionic conductivity owing to the plasticizer effect of TFSI anion. Differential Scanning Calorimetry (DSC) studies should be performed to determine the ceramic filler addition on the thermal stabilities. DSC and XRD characterizations can also provide quantitative information of PVDF crystallinity. Raman spectroscopic analysis together with other characterization techniques may provide insight on interactions among the components in the composite membrane and their correlation with ionic conductivities at different temperatures.

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