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

The practical development of aqueous zinc-iodine (Zn-I) batteries is greatly hindered by the low energy density resulting from conventional I/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 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/Br. Simultaneously, it activates the high voltage I/I redox reaction through interhalogen formation. Consequently, sequential and highly reversible redox reactions involving I/I, I/I, and Br/Br are achieved with the assistance of -NR units in BAVBr, effectively suppressing interhalogen hydrolysis in aqueous electrolyte. The cascade reactions lead to a high area capacity of 0.76 mAh cm at a low I loading of 1 mg cm or 760 mAh g 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.