Synergistic Combination\nof Cross-Linked Polymer and\nConcentrated Ionic Liquid for Electrolytes with High Stability in\nSolid-State Lithium Metal Batteries

Poly(ethylene oxide)(PEO-based solid polymer\nelectrolytes\n(SPEs) are regarded as excellent candidates for solid-state lithium\nmetal batteries (SSLMBs) due to their inherent safety advantages,\nprocessability, low cost, and excellent Li+ ion solvation. However,\nthey suffer from limited oxidation stability (up to 4 V vs Li<sup>+</sup>/Li). In this study, a crosslinked polymer-in-concentrated\nionic liquid (PCIL) SPE consisting of PEO, <i>N</i>-propyl-<i>N</i>-methylpyrrolidinium bis(fluorosulfonyl)imide (C<sub>3</sub>mpyrFSI) ionic liquid (IL), and lithium bis(fluorosulfonyl)imide\n(LiFSI) salt is developed. The adopted UV-crosslinking strategy synergistically\nreduces PEO crystallinity while increasing the amount of encompassed\nlithium salt and IL and improves PEO oxidative stability, therefore\nleading to enhanced electrochemical performance. The physical and\nelectrochemical properties of both linear and crosslinked SPEs are\nexplored and compared. The designed cross-linked SPEs exhibited a\npromisingly high oxidative stability of 4.9 V vs Li<sup>+</sup>/Li\nand high ambient temperature ionic conductivity of 4 × 10<sup>–4</sup> S cm<sup>–1</sup>. Stable and reversible lithium\nplating/stripping is demonstrated in symmetrical Li||Li cells over\nhundreds of hours. High-loading solid-state lithium iron phosphate\n(LFP)||Li cells show favorable cycling with over 90% capacity retention\nat 0.1C over 100 cycles at 50 °C. High voltage solid-state lithium\nmanganese oxide (LMO)||Li cells exhibit promising cycling with a 93%\ncapacity retention at a 0.2 C rate over 50 cycles at 50 °C. Thus,\nthe combination of concentrated ionic liquid electrolytes in a crosslinked\nPEO-based matrix enables a pathway for designing high-performing SPEs\nfor high energy density solid-state LMBs.