Interfacial Redox Buffering Stabilizes Halide Solid Electrolytes Against Low-Potential Anodes

The practical adoption of solid-state batteries requires solid electrolytes that sustain stable interfaces with electrodes. Conventionally, electrolyte stability has been defined by its thermodynamic limits, assuming that operating beyond these boundaries causes irreversible decomposition. Such perspectives have deterred the integration of many solid electrolytes with low-potential anodes, leaving their behavior beyond the limit largely unexplored. Here, we demonstrate that the trivalent metal halide Li ₃ YCl ₆ (LYC) can be electrochemically driven below its calculated reduction limit while exhibiting a predominantly reversible redox response. Lowering the potential below the thermodynamic window activates a reversible lithiation process consistent with Y-centered redox, while the halide framework is retained. At the anode interface, this reversibility enables LYC to accommodate low-potential anodes by forming a self-limiting lithiation layer that kinetically suppresses continued reduction. Comparative anode screening with Li-In, Li-Si, and Li metal identifies practical operating window below the nominal reduction threshold, within which LYC maintains both phase retention and ion transport with manageable impedance growth. Utilizing the reversible regime, we realize coating-free LYC full cells with an NMC811 cathode and a Li-Si anode that cycle stably for over 500 cycles at high capacity (~ 140 mAh g ^-1 ) and extended cell voltages (> 4 V), expanding the practical anode selection beyond conventional thermodynamic-window constraints and enabling high-performance all-solid-state cells.