All‐Climate Energy‐Dense Cascade Aqueous Zn‐I 2 Batteries Enabled by a Polycationic Hydrogel Electrolyte

Abstract The practical development of aqueous zinc‐iodine (Zn‐I 2 ) batteries is greatly hindered by the low energy density resulting from conventional I 0 /I − conversion and the limited temperature tolerance. Here, a temperature‐insensitive polycationic hydrogel electrolyte borax‐bacterial cellulose / p (AM‐ co ‐VBIMBr) (denoted as BAVBr) for achieving an energy‐dense cascade aqueous Zn‐I 2 battery over a wide temperature range from −50 to 50 °C is designed. A comprehensive investigation, combining advanced spectroscopic investigation and DFT calculations, has revealed that the presence of Br species in the gel electrolyte facilitates the conversion reaction of Br 0 /Br − . Simultaneously, it activates the high voltage I + /I 0 redox reaction through interhalogen formation. Consequently, sequential and highly reversible redox reactions involving I 0 /I − , I + /I 0 , and Br 0 /Br − are achieved with the assistance of −NR 3 + units in BAVBr, effectively suppressing interhalogen hydrolysis in aqueous electrolyte. The cascade reactions lead to a high area capacity of 0.76 mAh cm −2 at a low I 2 loading of 1 mg cm −2 or 760 mAh g −1 based on the mass of iodine, demonstrating exceptional long‐term cycling stability over a wide temperature range from −50 to 50 °C. This study offers valuable insights into the rational design of electrolytes for high‐energy aqueous batteries, specifically tailored for wide‐temperature operation.