Unlocking High-Valent Chalcogen Redox with Halide-Rich Electrolytes for High-Energy Lithium Batteries

Elemental chalcogens (Ch) are promising positive materials for sustainable, high-energy lithium batteries, yet their chemistry is generally limited by the two-electron Ch⁰/Ch^2- conversion below 2.5 V. This leaves the high-valent redox regime largely unexplored due to the instability of oxidized chalcogen species. Here, we report a halide-rich electrolyte design that enables reversible high-valent chalcogen redox in lithium batteries. By using soluble organic halide salts with asymmetric cations, the electrolyte provides active chloride or bromide anions to promote high-valent redox and stabilize oxidized intermediates, thereby enabling a redox-amphoteric selenium (Se) conversion pathway. This transition from reduction-only chemistry (Se^2-/Se⁰) to three-electron conversion (Se^2-/Se⁰/Se⁺) is evidenced by a distinct plateau at ~2.6 V, corresponding to the Se⁰/Se⁺ process. Consequently, the Li | |Se cell achieves a reversible discharge capacity of 980 mAh g^-1 and a specific energy of 2003 Wh kg_Se^-1 with stable cycling performance over 200 cycles at 400 mA g^-1. This strategy is further extended to sulfur and selenium sulfide materials, activating high-valent conversion. These results establish a potentially general route to access high-valent, multi-electron chalcogen chemistry, broadening the energy density limits for next-generation batteries.