Advanced High-Voltage Electrolyte Design Using Poly(ethylene oxide) and High-Concentration Ionic Liquids for All-Solid-State Lithium Metal Batteries

Poly(ethylene oxide) (PEO) based solid polymer electrolytes (SPEs) are one of the most promising materials for solid-state lithium metal batteries (LMBs) due to their inherent safety advantages, however, they suffer from insufficient room temperature ionic conductivity (up to 10−6 S cm−1) and limited oxidation stability (< 4V). In this study, a novel “polymer-in-high concentrated ionic liquid” (PiHCIL) electrolyte composed of PEO, N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl) imide (C3mpyrFSI) ionic liquid (IL) and LiFSI is designed. The EO: [Li/IL] ratio has been widely varied and their physical and electrochemical properties have been explored. The Li-coordination and solvation structure has been explored through FTIR spectroscopy and solid-state magic angle spinning (MAS) NMR. The designed electrolyte provides promisingly high oxidative stability of 5.1V and offers high ambient temperature ionic conductivity of 5.6 x10-4 S cm-1. Li|Li symmetric cell cycling showed very stable and reversible cycling of Li metal over 100 cycles and smooth, dendrite-free deposition morphology. All-solid-state cells using composite LFP cathode exhibited promising cycling with 99.2% capacity retention at C/5 rate over 100 cycles. Therefore, the novel approach of PiHCIL enables a new pathway to design high-performing SPEs for high-energy density all-solid-state LMBs.