Synergistic Combination of Cross-Linked Polymer and Concentrated Ionic Liquid for Electrolytes with High Stability in Solid-State Lithium Metal Batteries

Poly(ethylene oxide)-(PEO-based solid polymer electrolytes (SPEs) are regarded as excellent candidates for solid-state lithium metal batteries (SSLMBs) due to their inherent safety advantages, processability, low cost, and excellent Li+ ion solvation. However, they suffer from limited oxidation stability (up to 4 V vs Li⁺/Li). In this study, a crosslinked polymer-in-concentrated ionic liquid (PCIL) SPE consisting of PEO, <i>N</i>-propyl-<i>N</i>-methylpyrrolidinium bis(fluorosulfonyl)imide (C₃mpyrFSI) ionic liquid (IL), and lithium bis(fluorosulfonyl)imide (LiFSI) salt is developed. The adopted UV-crosslinking strategy synergistically reduces PEO crystallinity while increasing the amount of encompassed lithium salt and IL and improves PEO oxidative stability, therefore leading to enhanced electrochemical performance. The physical and electrochemical properties of both linear and crosslinked SPEs are explored and compared. The designed cross-linked SPEs exhibited a promisingly high oxidative stability of 4.9 V vs Li⁺/Li and high ambient temperature ionic conductivity of 4 × 10^-4 S cm^-1. Stable and reversible lithium plating/stripping is demonstrated in symmetrical Li||Li cells over hundreds of hours. High-loading solid-state lithium iron phosphate (LFP)||Li cells show favorable cycling with over 90% capacity retention at 0.1C over 100 cycles at 50 °C. High voltage solid-state lithium manganese oxide (LMO)||Li cells exhibit promising cycling with a 93% capacity retention at a 0.2 C rate over 50 cycles at 50 °C. Thus, the combination of concentrated ionic liquid electrolytes in a crosslinked PEO-based matrix enables a pathway for designing high-performing SPEs for high energy density solid-state LMBs.