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The pursuit of reliable and high-performance batteries has fueled extensive research into new battery chemistries and materials, aiming to enhance the current lithium-ion battery technologies in terms of energy density and safety. Among the potential advancements, solid-state batteries (SSBs) have captured significant attention as the next-generation energy storage technology. One key factor contributing to their appeal is the utilization of solid-state electrolytes (SSEs) with a wide electrochemical stability window (ESW), making SSBs compatible with high-voltage cathodes. The energy density of SSBs can be further improved by employing the “holy-grail” anode, Li-metal, which boasts the lowest working voltage (-3.04 V vs. Li+/Li) and an ultrahigh theoretical capacity (3860 mAh g−1). Consequently, these batteries are referred to as lithium metal batteries (LMBs). However, realizing the full potential of LMBs presents formidable challenge, including the low ionic conductivity of current SSEs, large interfacial resistance between SSE and electrodes, uncontrollable interfacial reactions, and the growth of Li dendrites.  Typically, SSEs can be categorized into three types. Among these, solid composite electrolytes (SCEs) are considered the most promising choice for solid-state LMBs due to their combination of high ionic conductivity and excellent mechanical strength from inorganic solid electrolytes (ISEs) and the flexibility and good interface compatibility provided by solid polymer electrolytes (SPEs). Polymeric ionic liquids (PolyILs), which contain both ionic liquid-like moieties and polymer frameworks, have emerged as highly attractive alternatives to traditional polymers in SCEs.  The overall objective of this thesis was to develop PolyIL-based SCEs with enhanced ionic conductivity, wide ESW, high Li+ transference number, and reduced electrodes/electrolyte interface resistance. The main progress achieved in this thesis is as follows: 1. We selected three F-based Li-salts to prepare SPEs using poly(ethylene oxide) and polyimide. The investigation focused on assessing the impact of molecular size, F content, and chemical structures (F-connecting bonds) of these Li-salts. Additionally, we aimed to uncover the formation process of LiF in the solid electrolyte interphase (SEI). The result revealed that the F-connecting bond plays a more significant role than the molecular size and F element content, resulting in slightly better cell performance using LiPFSI compared to LiTFSI and substantially better performance compared to LiFSI. The preferential breakage of bonds in LiPFSI was found to be related to its position to Li anode. Consequently, we proposed the LiPFSI reduction mechanism based on these findings. 2. Using the template method, we synthesized a monolayer SCE with enhanced Li+ transference number and high ionic conductivity. In this study, boron nitride (BN) nanosheets with a high specific surface area and richly porous structure were employed as inert inorganic filler. These BN nanosheets played a crucial role in homogenizing the Li+ flux and facilitating the Li+ transmission to suppress Li dendrite growth. When integrated into a LiFePO4//Li cell with the optimized SCE, the assembled battery demonstrated remarkable cycling performance.  3. A monolayer GSCE with multifunctionality was synthesized via a natural sedimentation and subsequent UV-curing polymerization technique. This innovative method capitalizes on intrinsic gravity, allowing for the integration of multiple functions within a single layer, thereby eliminating the additional interlayer resistance. The developed GSCE provides an optimum Li+ transportation path and enhanced Li+ transference number, leading to an enhanced ionic conductivity and a long cycle life of Li//Li cells and SSLMBs. Compared with the monolayer uniform SCEs, the gradient structure also alleviates the uncoordinated thermal expansion between fillers and PolyIL, avoiding increased stress during the cycle and battery capacity fade.

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