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.

Lithium metal batteries (LMBs) are attracting attention for their potential to enhance energy density while offering safety over conventional Li-ion batteries (LIBs) with flammable liquid organic electrolytes. However, realizing LMBs presents a formidable challenge, and developing compatible and effective solid-state electrolytes (SSEs) has been proposed as one effective strategy to address the challenge. 

SSEs are typically classified into inorganic solid electrolytes (ISEs), solid polymer electrolytes (SPEs), and solid composite electrolytes (SCEs), while each type has inherent limitations that prevent them from forming an ideal SSE. The SSEs using polymer are promising in further development owing to the capabilities of the polymer in facilitating Li+ transport and enabling the operation of LMBs with high-voltage cathodes. However, the current polymers in LMBs suffer from poor high-voltage stability, making it challenging to achieve long cycle life. Poly(ionic liquid)s (polyILs), a new type of polymer that incorporates the properties of ionic liquids (ILs), including wide electrochemical stability window (ESW) and high ionic conductivity, into polymer frameworks, offer a promising alternative to the traditional polymers in SSEs.

This thesis aims to develop polyIL-based SSEs with enhanced ionic conductivity, a wide ESW, a high lithium transference number (tLi+), reduced electrodes/electrolyte interface resistance, and suppression of lithium dendrites growth, ultimately enabling LMBs with extended cycle life. These objectives are achieved by tuning the constituents of the polyIL-based SSEs. The specific achievements of this thesis are as follows:

1. The application of ILs in SSEs and their effects on LMB performance were reviewed and summarized. The analysis highlighted that ILs can improve ionic conductivity, broaden the ESW of electrolytes, and enhance interface contact between the electrode and electrolyte. Considering the overall performance of ILs, including high ionic conductivity, a wide ESW, and cost-effectiveness, EMIMTFSI was selected for subsequent experiments.

2. Three F-based Li-salts were selected to prepare SSEs using poly(ethylene oxide) host and polyimide substrate. The investigation focused on the impact of F content and chemical structures (F-connecting bonds) of these Li-salts on the cell performance and uncovering the formation process of LiF in the solid electrolyte interphase (SEI). The results revealed that the F-connecting bond plays a more significant role than the F element content, resulting in slightly better cell performance using LiPFSI than LiTFSI and substantially better performance than LiFSI. The preferential breakage of bonds in LiPFSI was found to be related to its position to the Li anode. Consequently, the LiPFSI reduction mechanism was proposed.

3. Using the template method, a polyIL-based SCE was synthesized with boron nitride (BN) nanosheets as inert inorganic fillers. BN was chosen due to its high specific surface area and porous structure. An optimal BN content of 1.6 wt% was found to increase the amorphous regions of the polyIL, facilitating Li+ migration, and enhancing both tLi+ and ionic conductivity. The Li//LiFePO4 cell assembled with the optimized SCE delivered a stable capacity of up to 152 mA h g−1 after 300 cycles.

4. A concentration gradient poly(ionic liquid) (polyIL)-based SCE (GSCE) was synthesized via natural sedimentation and photopolymerization to simultaneously meet the distinct requirements of both the cathode and the lithium metal anode. The concentration of active inorganic filler was optimized, with 5 wt% as the optimal content. Compared to the uniform SCE, the GSCE demonstrated a higher tLi+ and improved ionic conductivity. As a result, the Li/GCSE-5/LMFP cell operated at a cut-off voltage of 4.3 V and exhibited a long cycle life.

5. A polyIL-based SSE was developed by combining a polyIL material host with a modified cellulose acetate (CA)-polyIL substrate to enrich diverse functional groups. This design effectively mitigated the non-uniform filler distribution within the polymer host while maintaining high mechanical strength and facilitating the Li+ migration. Additionally, the use of the same polyIL-based material as a cathode binder significantly improved their interfacial compatibility. As a result, the developed LMB demonstrated stable operation at a high cut-off voltage of 4.8 V and an extended cycle life.