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.