Polymerized-ionic-liquid-based solid polymer electrolyte for ultra-stable lithium metal batteries enabled by structural design of monomer and crosslinked 3D network

Abstract

Solid polymer electrolytes (SPEs) have attracted much attention for their safety, ease of packaging, cost-effectiveness, excellent flexibility and stability. Poly-dioxolane (PDOL) is one of the most promising matrix materials of SPEs due to its remarkable compatibility with lithium metal anodes (LMAs) and suitability for in-situ polymerization. However, poor thermal stability, insufficient ionic conductivity and narrow electrochemical stability window (ESW) hinder its further application in lithium metal batteries (LMBs). To ameliorate these problems, we have successfully synthesized a polymerized-ionic-liquid (PIL) monomer named DIMTFSI by modifying DOL with imidazolium cation coupled with TFSI⁻ anion, which simultaneously inherits the lipophilicity of DOL, high ionic conductivity of imidazole, and excellent stability of PILs. Then the tridentate crosslinker trimethylolpropane tris[3-(2-methyl-1-aziridine)propionate] (TTMAP) was introduced to regulate the excessive Li⁺-O coordination and prepare a flame-retardant SPE (DT-SPE) with prominent thermal stability, wide ESW, high ionic conductivity and abundant Li⁺ transference numbers (t_Li+). As a result, the LiFePO₄|DT-SPE|Li cell exhibits a high initial discharge specific capacity of 149.60 mAh g⁻¹ at 0.2C and 30 °C with a capacity retention rate of 98.68% after 500 cycles. This work provides new insights into the structural design of PIL-based electrolytes for long-cycling LMBs with high safety and stability.