SiO₂ Nanoparticles-Induced Antifreezing Hydrogel Electrolyte Enables Zn-I₂ Batteries with Complete and Reversible...

Four-electron-transfer aqueous zinc-iodine batteries hold significant promise for large-scale energy storage due to their high specific capacities. However, achieving four-electron-transfer mechanisms under subzero temperatures remains challenging due to freezing point limitations of conventional aqueous electrolytes and sluggish reaction kinetics. Herein, an antifreezing hydrogel electrolyte (HC-SiO₂) is developed through the spontaneous gelation of a high-concentration electrolyte (1 m Zn(OAc)₂ + 21 m LiCl, HC) with SiO₂ nanoparticles, enabling low-temperature operation of quasi-solid-state Zn-I₂ batteries with complete and reversible four-electron-transfer processes. Abundant interactions between dispersed SiO₂ nanoparticles and cations enlarge ion-pair distances, reducing close ion-pair formation and lowering the freezing temperature (-60.7 °C). Furthermore, the quasi-solid-state hydrogel electrolyte combines advantages of reduced water activity and disrupted hydrogen-bond networks, effectively suppressing I⁺ hydrolysis while inhibiting ice nucleation. Additionally, the utilization of low-concentration Zn(OAc)₂ combined with high-concentration LiCl increases availability of free Cl^- by mitigating strong ionic interaction in conventional ZnCl₂-based concentrated electrolytes, thereby enhancing reaction kinetics of the I₂/I⁺ conversion. Benefiting from synergistic manipulation of ionic interaction, water activity, and Cl^- activity, the HC-SiO₂ hydrogel achieves a high capacity of 490.9 mAh g^-1 and durable lifespan exceeding 11,000 cycles at -20 °C. These findings offer valuable insights for advancing practical low-temperature Zn-I₂ batteries.