Synthesis and electrochemical characterization of sulfur-doped Li3PO4 for all-solid-state battery applications

Isovalent and aliovalent cation-doping have been widely investigated to enhance ion transport in γ‑Li3PO4-type lithium superionic conductors (LISICONs). Anion doping, however, has been restricted to a full replacement of oxide ions by sulfide ions in some compounds (the so-called thio-LISICONs). The replacement of oxide ions by sulfide ions enhances both ion conductivity and deformability of the materials. Similar to other sulfide-type solid-electrolytes, thio-LISICONs, however, showed limited electrochemical stability against oxidation. In this report, a sonication-assisted liquid-phase synthesis approach was employed to achieve substantial sulfur‑oxygen mixing in γ-type Li3PO4. Sulfur-doped Li3PO4 possessed an ion conductivity four orders of magnitude higher than that of γ-Li3PO4, and a notably low activation energy of ion transport of 0.40(8) eV. Sulfur-oxygen mixing in sulfur-doped Li3PO4 enhanced electrochemical stability against oxidation (up to 4 V vs. Li+/Li) compared with sulfur-based Li3PS4. This study paves the way for developing mixed-anion phases in the γ-type Li3PO4 structure with enhanced electrochemical properties for all-solid-state battery applications.