Disorder in the Antifluorite Sulfide Cathode: Li_1.7Cu_0.15S₂

Cation and anion substitutions in Li₂S have improved their performance as high-capacity cathodes, but electron and ion transport in solid solutions of Li₂S have not been fully understood due to intricate site disorder within the antifluorite structure. Herein, neutron diffraction identified a unique disordered structure of Cu-substituted Li₂S (Li_1.7Cu_0.15S) within an antifluorite framework where Cu occupies an octahedral site in addition to the canonical tetrahedral one. This disorder leads to partial sulfur oxidation and facilitates Li⁺transport, as revealed by X-ray absorption, impedance, and magnetic measurements. Density functional theory calculations support the entropically stabilized nature of this disordered structure and shed insight on the influence of Cu substitution on electronic structure, vacancy formation, and ion transport. The proposed dual-site occupation suggests a mechanism for the improved performance of Li₂S-based cathodes with cation substitutions.