Gel Polymer Electrolytes Based on Poly(vinylidene fluoride-<i>co</i>-hexafluoropropylene) and Salt-Concentrated Electrolytes for High-Voltage Lithium Metal Batteries

Although high-voltage lithium (Li) metal batteries are promising next-generation energy storage devices, their practical use is hindered by their poor cycling stability owing to low electrolyte compatibility with both Li metal anodes and 5 V-class cathodes. In this study, we report that the gelation of salt-concentrated electrolytes with weakly coordinating poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) effectively improves the cycling stability of high-voltage Li metal batteries. The PVDF-HFP-based gel polymer electrolyte with a salt-concentrated electrolyte comprising lithium bis(fluorosulfonyl)amide (LiFSA) and sulfolane (SL) achieves a high Coulombic efficiency and dense deposition morphology of Li metal anodes, along with sufficient oxidation stability against 5 V-class cathodes. Experimental and computational analyses show that the solvation structures of SL–Li+–FSA–, similar to those in the original concentrated electrolyte, are maintained in the PVDF-HFP matrix, which leads to the formation of a low-resistance solid electrolyte interphase (SEI) rich in lithium fluoride and sulfur compounds. These findings indicate that the low-resistance SEI in the gel polymer electrolyte promotes dense Li deposits, which suppresses electrolyte decomposition and inactive Li formation, improving the Coulombic efficiency of Li metal anodes. We demonstrate that the stable cycling of a Li metal battery with a 5 V-class LiNi0.5Mn1.5O4 cathode is enabled by the gel electrolyte, which inhibits the deposition of transition metals dissolved from the cathode onto the anode. This electrolyte and interface design is an effective strategy for developing 5 V-class Li metal batteries and can be applied to other high-energy-density metal batteries with high-voltage cathodes.