Electrochemical Performance and Stability of Li ₇ Si ₂ S ₇ I Solid Electrolytes in Solid-State Battery Cathode Composites

The development of solid electrolytes (SEs) for high-performance solid-state batteries (SSBs) requires not only favorable electrochemical stability but also interfacial compatibility with diverse cathode chemistries. In this study, we systematically benchmark recently discovered Li7Si2S7I against the well-established argyrodite Li5.5PS4.5Cl1.5 as an SE in composite cathodes. Despite exhibiting comparable oxidative stability, Li7Si2S7I demonstrates markedly different behaviors depending on the cathode chemistry. In composite cathodes with uncoated LiNi0.83Co0.11Mn0.06O2 as the cathode active material and Li7Si2S7I as the electrolyte, rapid degradation occurs, with capacity retention dropping to 5% after 30 cycles, driven by fast degradation kinetics and interfacial instability toward the formation of SiOx species as a thermodynamic sink. In contrast, sulfur–carbon–Li7Si2S7I composite cathodes show good performance in half-cells, comparable to that of the argyrodite benchmark. The reversible oxidative redox processes of Li7Si2S7I in sulfur-based systems highlight its promise for Li–S and other oxygen-free battery chemistries. Overall, this work emphasizes the importance of a holistic approach to SE evaluation, integrating chemical and electrochemical stability with degradation kinetics, to inform the rational design of next-generation SSB materials.