Design Principles for Aqueous Stability of Lithium Halide and Oxyhalide Solid Electrolytes from First-Principles Pourbaix Analysis

Abstract Halide solid electrolytes (SEs) are promising candidates for all-solid-state batteries but remain susceptible to moisture-induced degradation that is not captured by conventional electrochemical stability windows. Here, we present a systematic aqueous stability landscape for lithium halide SEs (LiaMXb; M = Ta, Zr, Sc, Y, In, Er; X = F, Cl, Br) using a first-principles Pourbaix diagram framework. We demonstrate that fluorides consistently exhibit superior stability relative to chlorides and bromides, a trend arising from intrinsic M–X bonding covalency. Our analysis reveals that degradation in heavier halides is primarily driven by the thermodynamic formation of high-valency oxyanions under oxidizing conditions, rather than simple hydrolysis. Furthermore, rational oxygen substitution (e.g., Li2.5ZrCl5.5O0.5) suppresses this oxidative decomposition, effectively enhancing stability over the pristine Li2ZrCl6 framework. These findings establish thermodynamic design principles for moisture-tolerant SEs and provide a predictive map for water-mediated synthesis routes.