An Active Halide Catholyte Boosts the Extra Capacity for All‐Solid‐State Batteries

Replacing flammable organic liquid electrolytes with nonflammable solid electrolytes (SEs) in lithium batteries is crucial for enhancing safety across various applications, including portable electronics, electric vehicles, and scalable energy storage. Since typical cathode materials do not possess superionic conductivity, Li-ion conduction in the cathode predominantly relies on incorporating a significant number of SEs as additives to form a composite cathode, which substantially compromises the energy density of solid-state lithium batteries. Here, a halide SE, Li₃VCl₆ is demonstrated, which not only exhibits a decent Li⁺ conductivity, but more importantly, delivers a highly reversible capacity of approximately 80 mAh g^-1 with an average voltage of 3 V versus Li⁺/Li. The ionic conductivity of Li₃VCl₆ experiences marginal fluctuations upon electrochemical lithiation/delithiation, as its prototypical solid-solution reaction results solely in a reduction of lithium vacancy. When combined with the traditional LiFePO₄ cathode, the active Li₃VCl₆ catholyte enables an impressive capacity of 217.1 mAh g^-1 _LFP and about 50% increase in energy density compared with inactive catholytes. Harnessing the integrated mass of the catholyte-which can serve as an active material-presents an opportunity to boost the extra capacity, rendering it feasible in applications.