Oxygen-tuned aluminum-based halide solid electrolytes enabling low-voltage anode compatibility in all-solid-state batteries

Abstract

Developing solid electrolytes with a wide electrochemical window, high ionic conductivity, and facile processability is essential for realizing high-energy-density all-solid-state batteries. In this work, we report a new family of aluminum-based oxychloride solid electrolytes with tunable oxygen/chlorine ratios, designed to overcome the critical reduction instability that limits the widespread adoption of halide-based electrolytes. Our study on the series of oxychlorides elucidates a complex tradeoff between oxygen content and electrolyte performance, particularly reduction onset potential and ionic conductivity. While increased oxygen content in the electrolyte delays the onset of reduction, it also induces strong propensity in Al–O bond formation, which simultaneously promotes the segregation of chlorine-rich impurities such as LiCl. Notably, we find that this residual LiCl phase initiates reductive decomposition, prematurely triggering electrolyte breakdown. Guided by this insight, we identify an optimized composition, Li_1.1AlO_1.1Cl₃, that balances reduction stability and ionic conductivity. This new electrolyte enables stable cycling with a conventional 0.6 V-class alloy anode without requiring a secondary anolyte, delivering 188.8 mAh g⁻¹ (LiNi_0.8Co_0.1Mn_0.1O₂ cathode) with 80% capacity retention over 250 cycles. More strikingly, it also supports stable operation with a low-voltage 0.3 V-class anode in the same solid-state configuration, achieving ∼91.5% capacity retention after 100 cycles, representing one of the most stable cycle performances reported for halide-based solid electrolytes paired with low-voltage anodes. These findings redefine the anode compatibility of halide solid electrolytes and point toward new design principles for next-generation solid-state battery systems.