The demand for electric vehicles is increasing rapidly as the world is preparing for a fossil fuel-free future in the automotive field. Lithium battery technologies are the most effective options to replace fossil fuels due to their higher energy densities. However, safety remains a major concern in using lithium as the anode, and the development of non-volatile, non-flammable, high conductivity electrolytes is of great importance. In this dissertation, a gel polymer electrolyte (GPE) consisting of ionic liquid, lithium salt, and a polymer has been developed for their application in lithium batteries. A comparative study between GPE and ionic liquid electrolyte (ILE) containing batteries shows a superior cyclic performance up to 5C rate and a better rate capability for 40 cycles for cells with GPE at room temperature. The improvement is attributed to GPE’s improved stability voltage window against lithium as well as higher lithium transference number. The performance of the GPE in lithium-sulfur battery system using sulfur-CNT cathodes shows superior rate capability for the GPE versus ILE for up to 1C rates. Also, GPE containing batteries had higher capacity retention versus ILE when cycled for 500 cycles vii at C/2 rate. Electrochemical impedance spectroscopy (EIS) studies reveal interfacial impedances for ILE containing batteries grew faster than in GPE batteries. The accumulation of insoluble Li2S2/Li2S on the electrodes decreases the active material thus contributes to capacity fading. SEM imaging of cycled cathodes reveals cracks on the surface of cathode recovered from ILE batteries. On the other hand, the improved electrochemical performance of GPE batteries indicates better and more stable passivation layer formation on the surface of the electrodes. Composite GPE (cGPE) containing micro glass fillers were studied to determine their electrochemical performance in Li batteries. GPE with 1 wt% micro fillers show superior rate capability for up to 7C and also cyclic stability for 300 cycles at C/2 rate. In situ, EIS also reveals a rapid increase in charge transfer resistance in GPE batteries, responsible for lowering the capacity during cycling. Improved ion transport properties due to ion-complex formations in the presence of the micro fillers, is evidenced by improved lithium transference number, ionic conduction, and ion-pair dissociation detected using Raman spectroscopy.

Energy storage is crucial for intermittent renewable energy sources, electric vehicles, and portable devices. The continuously increasing energy consumption in these industries necessitates the enhancement of commercial lithium-ion batteries (LIB), especially regarding their safety and energy density. Historically, aqueous electrolytes were the norm in the battery industry. Prior to the development of lithium batteries, most commercially significant batteries used water as the solvent. In the past decade, "highly concentrated" electrolytes resurrected the notion of an aqueous lithium-ion battery (ALIB). Significant efforts have been made since then to comprehend the interfacial stability of these high-concentration electrolytes, and make them suitable for use in batteries especially high voltage ones. Another candidate for future batteries is All-Solid-State Batteries (ASSB) as they have the potential to double, or even triple, the energy density figures we currently achieve in LIBs mainly due to their ability to utilize lithium metal anode which has the highest specific capacity among anodes (3860 mAh g⁻¹), lowest reduction potential (-3.04 V vs SHE), and low density (0.53 g cm⁻³). This thesis first proposes a phenomenological model to describe the microstructure of aqueous electrolyte and the relation between their phase diagrams with ionic conductivity; highlighting a common correlation between the eutectic composition and peak ionic conductivity in conductivity isotherms. we then propose an empirical model correlating ionic conductivity with both molar concentration and temperature. The aim of this portion of the thesis is to provide an in depth understanding of aqueous electrolytes' physical properties in a way that can help researchers optimize the energy density and the cost of ALIBs. Moving further, the thesis presents two novel composite solid electrolytes (CSE) that were developed and fully characterized. Both of which were composed of the following four components; polyethylene oxide (PEO), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, lithium lanthanum titanate (LLTO) perovskite inorganic ceramic and the polymer plasticizer succinonitrile (SN). The careful formulation of these CSEs was based on the trade-off between film forming ability and ionic conductivity. The optimized polymer rich CSE proved to have better characteristics when compared to its ceramic rich alternative. ASSBs employing both CSEs were successfully charged and discharged when coupled with lithium metal anode and in-lab prepared composite cathode. The developed thin and flexible CSEs could be utilized in small applications (Wh-KWh) such as in consumer electronics and flexible biomedical devices (e.g., pacemakers) or larger applications (kWh-MWh) such as in EVs and large format storage for the electrical grid.