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.