Iodine/Chlorine Multi-Electron Conversion Realizes High Energy Density Zinc-Iodine Batteries

Aqueous zinc-iodine (Zn-I₂) batteries are promising energy storage devices; however, the conventional single-electron reaction potential and energy density of iodine cathode are inadequate for practical applications. Activation of high-valence iodine cathode reactions has evoked a compelling direction to developing high-voltage zinc-iodine batteries. Herein, ethylene glycol (EG) is proposed as a co-solvent in a water-in-deep eutectic solvent (WiDES) electrolyte, enabling significant utilization of two-electron-transfer I⁺/I⁰/I^- reactions and facilitating an additional reversibility of Cl⁰/Cl^- redox reaction. Spectroscopic characterizations and calculations analyses reveal that EG integrates into the Zn²⁺ solvation structure as a hydrogen-bond donor, competitively binding O atoms in H₂O, which triggers a transition from water-rich to water-poor clusters of Zn²⁺, effectively disrupting the H₂O hydrogen-bond network. Consequently, the aqueous Zn-I₂ cell achieves an exceptional capacity of 987 mAh g_I2 ^-1 with an energy density of 1278 Wh kg_I2 ^-1, marking an enhancement of ≈300 mAh g^-1 compared to electrolyte devoid of EG, and enhancing the Coulombic efficiency (CE) from 68.2% to 98.7%. Moreover, the pouch cell exhibits 3.72 mAh cm^-2 capacity with an energy density of 4.52 mWh cm^-2, exhibiting robust cycling stability. Overall, this work contributes to the further development of high-valence and high-capacity aqueous Zn-I₂ batteries.