High‐Performance Zinc Halogen Aqueous Battery Exploiting [BrCl ₂ ] ⁻ Storage in Ketjenblack by Reconstructing Electrolyte Structure

Halogen redox couples offer distinct merits due to high redox potential and low cost, but they usually suffer from poor reversibility. Here, we report highly reversible [Br₂Cl]^-/Br^- and [BrCl₂]^-/Cl^- redox couples via harnessing interhalogen chemistry within nanoporous Ketjenblack (KB) carbon as a host for zinc-based aqueous batteries. The tunable halogen reactions are facilitated by controllable coordinating chemistry of [ZnCl_4-xBr_x]^2- in a hydrated deep eutectic solvent electrolyte comprising ZnCl₂, ZnBr₂, and choline chloride (C₅H₁₄ClNO). In an optimized electrolyte, the KB electrode delivers a high discharge capacity of 535 mAh g_KB ^-1 or 214 mAh g^-1 when the stored [BrCl₂]^- is counted at 100 mA g^-1, with an average discharge voltage of ~1.6 V, and a record high energy density of 788 Wh kg_KB ^-1 or 315 Wh kg^-1, when considering the mass of [BrCl₂]^- as well. Benefiting from the interhalogen coordination chemistry and KB pore host, the pouch cell delivers an areal capacity of 2.3 mAh cm^-2 at 0.5 mA cm^-2, storing an energy density of ~80 Wh kg^-1 based on the mass of both electrodes. This work offers a new strategy to enhance the reversibility of static aqueous halogen batteries for energy storage applications.