Electrolyte/electrode interphase regulation with methylthiolation ionic liquids for high-voltage quasi–solid-state Li metal batteries

Quasi-solid-state lithium metal batteries (QSSLMBs) are hindered by interfacial instability at high-voltage cathodes and uncontrolled lithium dendrite growth at the anode, limiting their cycling life and energy density. To overcome these challenges, we have developed a piperidine-based ionic liquid (IL) additive featuring a methylthiolation cationic segment that stabilizes the interphase of layered oxide cathodes and lithium anodes under fast charging and high-voltage conditions. These ILs preferentially oxidize to form a robust cathode-electrolyte interphase with LiF and sulfide species, which effectively suppresses dissolution of transition metal and enhances interfacial ion transport. Simultaneously, the IL-derived diverse inorganic species composite interphase at the anode improves mechanical strength and inhibits lithium dendrite growth. The resulting quasi-solid-state electrolyte demonstrates stable operation at a high voltage of 4.5 V, high mass loading of 21.6 mg cm^-2, high-energy density of 407 Wh kg^-1, and broad temperature range from -20° to 60°C, advancing the practical application of QSSLMBs.