Improving Cycling Stability of Vanadium Sulfide (VS₄) as a Li Battery Cathode Material Using a Localized High-Concentration Carbonate-Based Electrolyte

Vanadium sulfide (VS4) is a promising cathode material for rechargeable Li batteries because it exhibits high capacity and can address the problematic dissolution of lithium polysulfides and the low electrical conductivity of sulfur cathodes. However, VS4 cathodes suffer from low Coulombic efficiency and fast capacity fading during battery cycling. Here, we report a localized high-concentration electrolyte comprising lithium bis(fluorosulfonyl)imide (LiFSI), ethylene carbonate (EC), propylene carbonate (PC), and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE), which not only stabilizes Li metal anodes but also improves the Coulombic efficiency and cycling stability of the cathode. Further, we propose that the dissolution of V is one of the reasons for the poor cycling stability of VS4. The localized high-concentration electrolyte has a local coordination structure similar to the high-concentration LiFSI/EC:PC electrolyte, which suppresses the dissolution of V from VS4 during discharge/charge cycling. Spectroscopic analysis revealed that a LiF-rich layer is formed on the surface of VS4 in the electrolyte, which also should prevent the dissolution of V from the cathode and the decomposition of the solvents. Our findings suggest that the limited dissolution of V from VS4 and the LiF-rich cathode–electrolyte interphase layer inhibit the degradation of the cathode, resulting in the improved cycling stability in the electrolyte. This concept of electrolyte design will pave the way for developing high-energy-density Li|transition metal sulfide batteries with stable performance.