Electrochemically Formed Protective Interphase on Selenium Sulfide/Carbon Nanocomposite Cathodes for Stable Lithium Batteries

Selenium sulfide (SeS2) is an attractive next-generation cathode material because it combines the high gravimetric capacity of sulfur (S) with the high electronic conductivity of selenium (Se). However, like its parent S and Se cathodes, SeS2 undergoes severe polychalcogenide dissolution during electrochemical cycling, leading to fast capacity fade. While protective interphase layers can stabilize SeS2 cathodes, conventional coating methods require complex synthesis, limiting practical implementation. Here, we report a facile electrochemical approach to engineer protective interphases on SeS2/carbon composite cathodes by simply using a fluoroethylene carbonate (FEC)-containing electrolyte. During the initial formation cycle, FEC is preferentially reduced, in situ forming cathode electrolyte interphase (CEI) and solid electrolyte interphase (SEI) layers on the SeS2/C cathode and Li anode, respectively. These dual protective interphase layers effectively suppress polychalcogenide dissolution, enable rapid Li-ion conduction, and provide mechanical and chemical stability. Consequently, the FEC-assisted SeS2/C cathode retains over 98.5% of its initial capacity after 400 cycles at C/5, significantly outperforming the unprotected cathode, which suffers rapid capacity decay within 100 cycles. This work shows that electrolyte engineering via in situ electrochemical protective-layer formation offers a practical, effective strategy for stabilizing conversion-type cathodes.