All-Climate Energy-Dense Cascade Aqueous Zn-I₂ Batteries Enabled by a Polycationic Hydrogel Electrolyte

The practical development of aqueous zinc-iodine (Zn-I₂) batteries is greatly hindered by the low energy density resulting from conventional I⁰/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-I₂ 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 Br⁰/Br^-. Simultaneously, it activates the high voltage I⁺/I⁰ redox reaction through interhalogen formation. Consequently, sequential and highly reversible redox reactions involving I⁰/I^-, I⁺/I⁰, and Br⁰/Br^- are achieved with the assistance of -NR₃ ⁺ 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 I₂ 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.