Beneficial redox activity of halide solid electrolytes empowering high-performance anodes in all-solid-state batteries

All-solid-state batteries receive ample attention due to their potential to outperform Li-ion batteries in safety characteristics and energy density. The latter holds true if they are compatible with next-generation high-capacity anodes. However, most highly ion-conductive solid electrolytes decompose at the low operating potentials of next-generation anodes, leading to irreversible lithium loss and increased cell resistances. Here we introduce the concept of the dynamic stability of solid electrolytes, and demonstrate how this phenomenon is utilized to improve all-solid-state battery performance. Halide electrolytes such as Li3YCl3Br3 and Li2ZrCl6, considered unstable at low working potentials, are shown to exhibit a structurally reversible redox activity beyond their electrochemical stability windows. Low potentials result in reversible lithiation of these halide solid electrolytes, introducing three advantages to the benefit of all-solid-state battery performance. First, the dynamic stability window is wider than their electrochemical stability window, thereby increasing their compatibility with anodes. Second, the lithiation of these halides increases their ionic conductivity rather than compromising it. Finally, the solid electrolyte contributes to the reversible capacity of the all-solid-state battery. The benefit of this dynamic stability window is demonstrated through halide-based cost-effective red phosphorus anodes that fall within this window, resulting in high reversible capacities (2308 mAh g-1), high rate capacity retention (1024 mAh g-1 at 7.75 mA cm-2) and an extended cycle life (61% retention after 1780 cycles). Furthermore, high areal capacity (7.65 mAh cm-2) and stability (70% retention after 1000 cycles) are achieved for exclusive halide-based full cells with uncoated high-voltage cathodes in combination with red phosphorous anodes. The beneficial redox activity of halide electrolytes that is unveiled, opens up novel application scenarios and suggests new solid electrolyte and solid-state battery design principles to enhance performance.