Ionic Liquid-based Gel Polymer Electrolytes For Lithium-ion Batteries

다국어 초록 (Multilingual Abstract)

In the last three decades, lithium-ion batteries (LIBs) have received much attention due to their properties such as light weight, high energy density, high ionic conductivity, and negligible memory effect. LIBs could be a promising candidate for energy storage in the face of green alternatives to fossil fuels. Their commercialized version consists of a graphite anode, liquid electrolyte, a cathode made up of metal oxides and a separator. These liquid electrolytes play a vital role in LIBs and ionic conductivity of 10-2 S cm-1 can be achieved.
Traditional liquid electrolytes (LEs) pose safety risks due to their flammability and volatility. Ionic liquids (ILs) offer a safer alternative, boasting inherent properties such as non-flammability, thermal stability, and high electrochemical stability, making them ideal for lithium-ion battery applications. The polymeric form of ILs, poly(ionic liquids) (PILs), is a new class of polymer electrolytes which combine the properties of ILs and polymer. However, differently from conventional solid polymer electrolytes (SPEs), in the case of PILs no additional salt needs to be added to the SPE since the pendant counter-ions are free to move and responsible for the ionic conductivity. PILs offers high thermal and electrochemical stability in addition to mechanical stability. Moreover, PILs with bulky anions like bis(trifluoromethanesulfonyl)imide (TFSI) show self-healing properties benefiting the development of flexible polymer electrolytes for wearable electronics. Despite the beneficial properties, use of PILs as battery electrolyte has been hampered due to their low ionic conductivities at room temperature. To address this issue various strategies have been implemented including co-polymerization, addition of plasticizers (like LE, inorganic nanoparticles, ILs, etc.) which has led to enhanced conductivities and exceptional cycling performance.
In the first chapter of this thesis, quantitative analysis of factors affecting the physicochemical behavior of ILs in electrolyte solution has been studied. Since solvents or ILs interact with dissociated ions, it is crucial to understand the factors that influence the solvent-solute relationship. Therefore, the ionic interactions are measured by nuclear magnetic resonance (NMR) spectroscopy and quantitively expressed in terms of donor number (DN). DN of ILs offering valuable insights for developing safer and more efficient electrolytes for lithium-ion batteries (LIBs) with enhanced charge-carrying capacity.
In the second chapter, crosslinked network polymer of poly(acrylonitrile-r-vinylidene diazide) with tethered tetrazolium rings (xPAN+) as highly stable and ion-selective gel polymer electrolytes (GPEs) for lithium metal batteries. The cationic rings play a crucial role in facilitating the movement of Li ions by interacting with both anions and solvent molecules, resulting in a remarkable transference number and ionic conductivity. Furthermore, the half-cell exhibited excellent capacity retention.
In the third chapter, polymer coated silica nanoparticles-based imine vitrimers (P@SNP-PHT) used as polymer electrolytes (PE). These electrolytes show self-healing ability at room temperature without any external stimuli. Moreover, the introduction of P@SNP in polymer matrix increases both stress and strain values of P@SNP-PHT films. P@SNP-PHT films shows high ionic conductivity and moderate lithium transference number with excellent electrochemical stability.
The research investigates the fundamental properties of ionic liquids and their interactions with Li salts, aiming to develop novel ionic-polymer electrolyte systems with enhanced lithium transference numbers and self-healing capabilities.

목차 (Table of Contents)

• Chapter 1. Introduction 3
• 1.1 Background and Motivation 3
• 1.2 Electrolytes 7
• 1.3 Types of Electrolytes 7
• 1.3.1 Liquid Electrolytes 7
• 1.3.1.1 Carbonate Electrolytes 7
• 1.3.1.2 Ionic Liquids 8
• 1.3.2 Polymer Electrolyte 9
• 1.3.2.1 Gel Polymer Electrolyte 11
• 1.3.2.2 Solid Polymer Electrolyte 11
• 1.3.2.3 Composite Polymer Electrolyte 12
• 1.4 Research Objective 15
• Chapter 2. Donor Numbers for Ionic liquids and carbonate solvents by using 7Li and 23Na as probe 18
• 2.1 Introduction 18
• 2.2 Experimental 22
• 2.3 Result and discussion 23
• 2.4 Conclusion 39
• Chapter 3. Tetrazolium-Crosslinked Polyacrylonitrile Gel Polymer Electrolyte with High Lithium Transference Number for Lithium Metal Batteries 40
• 3.1 Introduction 40
• 3.2 Experimental 42
• 3.2.1 Materials 42
• 3.2.2 Synthesis 43
• 3.2.3 Characterization 44
• 3.2.4 Electrochemical properties of xPAN+ 45
• 3.2.5 Battery test 46
• 3.3 Result and discussion 46
• 3.3.1 Fabrication of PAN-based GPE with tetrazolium (xPAN+) 46
• 3.3.2 Mechanical and swelling properties of xPAN+ GPEs 56
• 3.3.3 Electrochemical performance of xPAN+ 60
• 3.3.4 Battery performance of xPAN+ 69
• 3.4 Conclusion 73
• Chapter 4. Self-healing Solid Polymer Electrolytes composed of Polymers coated silica nanoparticles. 74
• 4.1 Introduction 74
• 4.2 Experimental section 75
• 4.2.1 Materials 75
• 4.2.2 Synthesis 76
• 4.2.3 Characterization 77
• 4.2.3 Electrochemical properties of P@SNP-PHT films 77
• 4.3 Result and discussion 78
• 4.3.1 Characterization and self-healing performance of P@SNP-PHT 78
• 4.3.2 Mechanical properties of P@SNP-PHT 83
• 4.3.3 Electrochemical performance of P@SNP-PHT 86
• 4.4 Conclusion 91
• Chapter 5. Concluding remarks and future work 92
• References 98