SiO 2 Nanoparticles‐Induced Antifreezing Hydrogel Electrolyte Enables Zn–I 2 Batteries with Complete and Reversible Four‐Electron‐Transfer Mechanisms at Low Temperatures

Abstract 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‐SiO 2 ) is developed through the spontaneous gelation of a high‐concentration electrolyte (1 m Zn(OAc) 2 + 21 m LiCl, HC) with SiO 2 nanoparticles, enabling low‐temperature operation of quasi‐solid‐state Zn–I 2 batteries with complete and reversible four‐electron‐transfer processes. Abundant interactions between dispersed SiO 2 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 ZnCl 2 ‐based concentrated electrolytes, thereby enhancing reaction kinetics of the I 2 /I + conversion. Benefiting from synergistic manipulation of ionic interaction, water activity, and Cl − activity, the HC‐SiO 2 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–I 2 batteries.