Substitution of Li ₃ BS ₃ : Revealing New Superionic Conductor Phases and the Significance of Crystallinity

Understanding ion transport in solid Li-ion conductors is critical for developing solid-state batteries with improved safety and energy density. Initial studies of lithium thioborates have demonstrated superionic conductivity in Li₃BS₃ through substitution and amorphization. However, the mechanisms underlying these improvements remain unclear. Contrary to the hypothesis that conductivity is enhanced by an increase in mobile-carrier concentration, we show that aliovalent substitutions of Cl for S, Al for Li, and Si for B primarily decrease activation energy and increase ionic conductivity by forming noncrystalline phases. Microstructural changes can also independently modify the conductivity by at least an order of magnitude. Our findings highlight the importance of understanding local structure, crystallinity, and microstructure to decouple multiple contributions to ion mobility in solid-state electrolytes. We also report a new crystalline phase with a nominal stoichiometry of Si_0.1-Li_2.9B_0.9S₃, previously observed as an unidentified precipitate from a glass, which has an ionic conductivity of 1.56 × 10^-3 S cm^-1 at 25 °C.