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/Ch2- 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 (Se2-/Se0) to three-electron conversion (Se2-/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 kgSe-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.