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

PEO-based solid polymer electrolytes (SPEs) are regarded as excellent candidates for solid-state lithium metal batteries (LMBs) 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 newly designed crosslinked polymer-in-concentrated ionic liquid (PCIL) SPE consisting of PEO, C3mpyrFSI and LiFSI 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 crosslinked SPEs exhibited a promisingly high oxidative stability of 4.9V vs. Li+/Li and high ambient temperature ionic conductivity of 4 x10-4 S cm-1. Stable and reversible lithium plating/stripping is demonstrated in symmetrical Li||Li cells over hundreds of hours. High loading solid-state LFP||Li cells showed favourable cycling with over 90% capacity retention at 0.1C over 100 cycles at 50°C. High voltage solid-state LMO||Li cells exhibited promising cycling with 93% capacity retention at 0.2 C rate over 50 cycles at 50°C. Both cathodes utilized a high areal capacity of 1 mAh cm-2. The synergistic combination of crosslinking and concentrated ionic liquid electrolyte to a robust SPE enables a new pathway for designing high-performing PEO-based SPEs for high energy density solid-state LMBs.