An inexpensive electrolyte with double-site hydrogen bonding and a regulated Zn²⁺ solvation structure for aqueous Zn-ion batteries capable of high-rate and ultra-long low-temperature operation

Aqueous Zn-ion batteries (AZIBs) present tremendous promise for large-scale energy storage owing to their intrinsically high safety, low cost and environmental friendliness. However, a huge challenge is the freezing of aqueous electrolytes at low temperatures. Herein, we report the introduction of formamide (FA), a low-cost, safe co-solvent with a high dielectric constant, as both a hydrogen bonding acceptor and donor, with an inexpensive zinc salt (zinc acetate), and the hybrid electrolyte exhibits a low freezing point lower than −40 °C and high ionic conductivity in a frozen environment. Experiments together with theoretical calculations demonstrate that FA can effectively anchor water molecules in a double-site hydrogen bonding interaction configuration and interrupt the original hydrogen bonding network, as well as confine free water activity, which significantly decreases the freezing point of the electrolyte and reduces the occurrence of hydrogen evolution reactions. In addition, the stronger coordination interaction between FA and Zn²⁺ can regulate the solvation structure of Zn²⁺, replace the water in the Zn²⁺ solvation sheath, realize uniform Zn deposition, avoid the formation of dendrites and inhibit the corrosion on the Zn surface. Consequently, an ultralong cycling life of 9500 h at 0.5 mA cm⁻² in the Zn||Zn symmetric cell at −30 °C, a record cycling stability and cumulative capacity among the reported electrolytes for low-temperature AZIBs, and a high Coulombic efficiency of 99.91% in the Zn||Cu asymmetric cell are achieved. Zn||PANI batteries with the designed electrolyte possess ultra-high stable reversible capacity (107 mA h g⁻¹ at 1 A g⁻¹ for 22 000 cycles and 121 mA h g⁻¹ at 5 A g⁻¹ for 5500 cycles) and excellent rate capability (124.8 mA h g⁻¹ at 10 A g⁻¹) at -30 °C. Even at −40 °C, Zn||PANI cells can cycle 6000 times with a capacity of 80 mA h g⁻¹ at 5 A g⁻¹. This work provides a facile and feasible strategy to design inexpensive, stable and high-performance aqueous zinc ion batteries for applications at low temperatures.