KCS

Na₃PS₄ is an archetype in room-temperature (RT) Na⁺-conducting solid-state electrolytes. Various compositional modifications of this compound via iso/aliovalent substitution are known to have the high ionic conductivity (σ_ion) comparable to liquid electrolytes. In this work, we implement the Bayesian optimization (BO) algorithm to discover a new composition with high σ_ion in a multi-dimensional search space. The systematically designed search space used for our BO implementation consists of compositional variables (A = Ca²⁺, Y³⁺, La³⁺; M₁ = P⁵⁺, Sb⁵⁺; M₂ = Si⁴⁺, Ge⁴⁺, Sn⁴⁺; Q = S^2-, Se^2-, Te^2-; X = Cl^-, Br^-, I^- in (Na, A)_3±δ(M₁, M₂, W)(Q, X)₄) and processing variables (synthetic temperature and time). Na_2.81(W_0.22Si_0.10Sb_0.68)S_3.93Br_0.07 selected through BO is further refined via logical reasoning, and finally Br^-free Na_2.88(W_0.22Si_0.10Sb_0.68)S₄ is newly discovered. This heavily co-doped Na₃SbS₄ by Si⁴⁺ and W⁶⁺ shows high σ_ion of 20.2 and 7.4 mS·cm^-1 at RT and -20^oC, respectively, which is ascribed to the notably low activation barrier (0.14 eV) in Na₃SbS₄ families. Low electronic conductivity (2 x 10-10 S·cm^-1) and stable Na plating/stripping behaviors are also disclosed, implying that Na_2.88(W_0.22Si_0.10Sb_0.68)S₄ could be a promising SSE in Na₃SbS₄ families.