An Anti-Freezing Hydrogel Electrolyte for Flexible Zinc-Ion Batteries Operating at-70 degrees C

Aqueous zinc-ion batteries (ZIBs) are considered as the promising candidate of flexible energy storage devices due to their high safety, eco-friendliness, and low-cost. However, conventional flexible aqueous ZIBs undergo severe capacity loss at bending and subzero temperature states. Herein, an anti-freezing hydrogel electrolyte based on zinc tetrafluoroborate (Zn(BF4)(2)) and polyacrylamide (PAM) is developed. Owing to the strong electronegativity of F atoms, BF4- anions interact with water molecules by O-H center dot center dot center dot F to replace the O-H center dot center dot center dot O between water molecules in Zn(BF4)(2)-PAM hydrogel electrolyte, inhibiting the formation of ice crystal lattice in hydrogel electrolyte at low-temperature. Therefore, the Zn(BF4)(2)-PAM hydrogel remains unfrozen state, good flexibility, and high ionic conductivity even at -70 degrees C, which ensures that they can act as the electrolytes of anti-freezing flexible ZIBs. As a proof of concept, anti-freezing flexible ZIBs with all-in-one integrated configuration are assembled based on Zn(BF4)(2)-PAM hydrogel. They display enhanced electrochemical performance at -70 degrees C, such as high capacity retention and remarkable cyclic stability. Furthermore, they also maintain stable electrochemical performance under different bending states even at -70 degrees C. This work broadens the practical application of flexible aqueous ZIBs at ultralow temperature.