Advanced High-Voltage\nElectrolyte Design Using Poly(ethylene\nOxide) and High-Concentration Ionic Liquids for All-Solid-State Lithium-Metal\nBatteries

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes\n(SPEs)\nare among the most promising materials for solid-state lithium metal\nbatteries (LMBs) due to their inherent safety advantages; however,\nthey suffer from insufficient room-temperature ionic conductivity\n(up to 10<sup>–6</sup> S cm<sup>–1</sup>) and limited\noxidation stability (<4 V). In this study, a novel “polymer-in-high-concentrated\nionic liquid (IL)” (PiHCIL) electrolyte composed of PEO, <i>N</i>-propyl-<i>N</i>-methylpyrrolidinium bis(fluorosulfonyl)\nimide (C<sub>3</sub>mpyrFSI) IL, and LiFSI is designed. The EO/[Li/IL]\nratio has been widely varied, and physical and electrochemical properties\nhave been explored. The Li-coordination and solvation structure has\nbeen explored through Fourier-transform infrared spectroscopy and\nsolid-state magic-angle spinning nuclear magnetic resonance. The newly\ndesigned electrolyte provides a promisingly high oxidative stability\nof 5.1 V and offers high ambient temperature ionic conductivity of\n5.6 × 10<sup>–4</sup> S cm<sup>–1</sup> at 30 °C.\nLi|Li symmetric cell cycling shows very stable and reversible cycling\nof Li metal over 100 cycles and a smooth dendrite-free deposition\nmorphology. All-solid-state cells using a composite lithium iron phosphate\ncathode exhibit promising cycling with 99.2% capacity retention at\na C/5 rate over 100 cycles. Therefore, the novel approach of PiHCIL\nenables a new pathway to design high-performing SPEs for high-energy-density\nall-solid-state LMBs.