Anion-cation synergy enables reversible seven-electron redox chemistry for energetic aqueous zinc-iodine batteries

Aqueous zinc-iodine batteries have drawn intensive attention from battery community due to the high theoretical capacity and low cost. However, the traditional two-electron-transfer and four-electron-transfer mechanisms suffer from low capacity and inferior stability due to the sluggish kinetics and low I- utilization. Herein, we design a seven-electron-transfer Zn||I-2 battery via the anion-cation synergistic electrode reaction in layered BiI3. The three-electron-transfer of Bi-0/Bi3+, together with the four-electron-transfer of I-/I+, endows Zn||BiI3 battery with high capacity (similar to 370 mAh g(-1) at 1 A g(-1)) and high energy density. The chemical confinement of I- by Bi3+ and the effective conversion of I- in BiI3 eradicate side reactions and enable I- with high utilization rate. Zn||BiI3 battery delivers a good cycling stability (10,000 cycles) and a high coulombic efficiency. This work presents an anion-cation synergy method for stabilizing Zn||I-2 batteries and other conversion-type batteries such as Zn||Br-2, Li||I-2, and Li||Br-2.