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

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 (Li a MX b; 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., Li 2.5 ZrCl 5.5 O 0.5 ) suppresses this oxidative decomposition, effectively enhancing stability over the pristine Li 2 ZrCl 6 framework. These findings establish thermodynamic design principles for moisture-tolerant SEs and provide a predictive map for water-mediated synthesis routes.