Solvation-engineering-enabled quasi-solid positive electrode for four-electron aqueous Zn||Br batteries

Diffusion, crossover, and hydrolysis of bromine species in aqueous electrolytes impede the application of Zn||Br batteries with four-electron 2Br+/Br2/2Br- conversion. Here, we study a ligand-assisted sparingly solvating electrolyte using ZnSO4·7H2O and urea. The high solubility of urea suppresses water activity, while its stronger affinity toward bromine enables preferential coordination over water molecules. This synergistic effect creates a quasi-solvent-free microenvironment that limits bromine solvation and diffusion, resulting in a robust quasi-solid cathode. Moreover, the compact solvation structure favors the formation and stability of Br+ species by strengthening nucleophilic urea coordination and inhibiting hydrolysis. Consequently, the fabricated Zn||Br battery exhibits reversible 4e bromine chemistry with a capacity of 417.5 mAh g-1 at 1 A g-1 (based on KBr). A zinc battery coupling the 4e iodine positive electrode and 4e bromine (from the electrolyte) is constructed, yielding a total 8e transfer with a capacity of 897.8 mAh g-1 and a specific energy of 1,344 Wh kg-1 at 2 A g-1 (based on KI).