The critical role of oxygen-evolution kinetics in the electrochemical stability of oxide superionic conductors

Abstract

Fast ion-conducting solid electrolytes could potentially overcome two key limitations of liquid electrolytes used in today's battery systems, namely, their flammability and limited electrochemical stability. In addition to high ionic conductivity, achieving a wide electrochemical window (EW) to suppress electronic transport (self-discharge and short circuiting) is particularly challenging. The superionic conductor Li₇La₃Zr₂O₁₂ (LLZO) exhibits a wide EW (>5.0 V) while maintaining a good ionic conductivity of ∼10⁻⁴ mS cm⁻¹, and serves as a model for promising superionic conductors. However, the physical origin of its electrochemical stability is not fully understood. Here, density functional theory (DFT) calculations demonstrated that a major contribution to the wide EW in LLZO originated from the high-barrier kinetics of electrochemical oxygen evolution. The high reaction barrier is attributed to electron holes occurring above the Fermi level, which is consistent with the high-voltage anionic redox potential. Based on the electron–hole relaxation capacity and polyhedral rigidity, we establish a combined charge and bond distance parameter q/r³ which serves as the design principle for regulating electrochemical stability. Based on this principle, LLZO compounds with Nd³⁺, In³⁺, Sb³⁺, and Y³⁺ substituted in the Zr⁴⁺ site are predicted to have a wider electrochemical window of >5.5 V or even higher. The established anionic electrochemical mechanism for improving the electrochemical stability provides new insight into rationally designing and optimizing electrochemically stable superionic conductors.