Practical high-energy aqueous zinc-bromine static batteries enabled by synergistic exclusion-complexation chemistry

Zn-Br batteries with Br−/Br0/Br+ redox have faced challenges concerning the practical energy density and the dissolution/hydrolysis of high-valence polybromide ions (Br3− and BrCl2−). We here introduce a practical Zn-Br battery that harnesses the synergy effects of complexation chemistry in the electrode and the salting-out effect in the aqueous electrolyte. The kosmotropic ZnSO4 electrolyte, with its strong salting-out effect due to the intrinsic structured water formation ability, suppresses the dissolution/hydrolysis of the chaotropic pyridinium-polybromide complexes in the electrode. Such exclusion-complexation chemistry also creates a quasi-solid cathode with improved redox kinetics while effectively inhibiting the oxygen and chlorine evolution reactions. The highly reversible Br−/Br0/Br+ redox couples endow a close-to-theoretical specific capacity of 215 mAh g−1 with an average Coulombic efficiency (CE) of 99.8%. Pouch cells demonstrate a practical high energy density of 106 Wh kg−1, a remarkable energy efficiency of 87.8%, and a low bill of materials (∼$28 per kWh), showcasing its potential for scaling up applications.