High‐Performance Zinc Halogen Aqueous Battery Exploiting [BrCl2]− 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 [ZnCl4‐xBrx]2− in an aqueous deep eutectic solvent electrolyte comprising ZnCl2, ZnBr2, and choline chloride. In an optimized electrolyte, the KB electrode delivers a high discharge capacity of ~535 mAh gKB–1 or 215 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 kgKB–1 or 311 Wh kg–1, when considering the mass of [BrCl2]– as well. Benefiting from the interhalogen coordination chemistry and KB pore host, the pouch cell prototype 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.