Influence of lithium salt anions on the interfacial properties of PEO-based solid-state electrolytes

The use of poly(ethylene oxide) (PEO)-based solid-state electrolytes have shown potential to improve both the energy density and safety performance of lithium-metal batteries. However, these electrolytes often form unstable interfaces with lithium metal anodes, compromising the durability and sustained performance of solid-state lithium-metal batteries. In this study, three lithium salts, lithium (fluorosulfonyl) (trifluoromethanesulfonyl)imide (LiFTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), were each mixed in equimolar ratios with tetraethylene glycol dimethyl ether (G4) and introduced as a conductivity enhancer in PEO-based solid-state electrolytes. The effects of the lithium salt anions on the electrolyte properties were systematically investigated. Among the three, the fluorosulfonyl group was found to enhance ionic conductivity, while the trifluoromethanesulfonyl group improved thermal stability. Notably, the synergistic interaction between these two groups in LiFTFSI led to the formation of a stable solid-electrolyte interphase (SEI), characterized by a higher content of inorganic species and reduced organic components. As a result, LiFTFSI/G4-based solid-state electrolytes enabled stable cycling for 200 cycles at a 0.5C rate in LiFePO₄-based solid-state lithium-metal batteries, achieving a capacity retention of 91 %. This study provides valuable insights into the optimization of high-efficiency solid-state lithium-metal batteries by elucidating the distinct roles of lithium salt anions.