Reaction Mechanism Optimization of Solid-State Li-S Batteries with a PEO-Based Electrolyte

摘要

The shuttle effect of long-chain polysulfides (Li2Sn, n = 4-8) from the multistep reactions reduces the cycling life of solid-state lithium-sulfur (Li-S) batteries with a poly(ethylene oxide) (PEO)-based solid polymer electrolyte (SPE). Moreover, the ambiguous reaction mechanism of polysulfides in an SPE also limits the development of high-performance solid-state Li-S batteries. Here, a solid-state Li-S cell with a much-improved cycling performance is reported by coating the sulfur cathode with a layer of polyvinylidene fluoride (PVDF), which not only suppresses the formation of soluble polysulfides, but also changes the reaction mechanism of the sulfur from a multistep "solid-liquid-solid" reaction to a single-step "solid-solid" reaction. These results show that long-chain polysulfides are insoluble and unstable in PVDF polymers with a low solvent property, which facilitates the direct transformation of elemental sulfur to solid Li2S2/Li2S without the formation of intermediary products. However, the strong PEO-Li2Sn (n = 4-8) attraction causes a dissolution of polysulfides in PEO. The introduction of a polymer with a low solvent property in the sulfur cathode would be promising for the development of solid-state Li-S batteries with a long cycling life.