An Anti‐Freezing Hydrogel Electrolyte for Flexible Zinc‐Ion Batteries Operating at −70 °C

Abstract

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···F to replace the OH···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 °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 °C, such as high capacity retention and remarkable cyclic stability. Furthermore, they also maintain stable electrochemical performance under different bending states even at −70 °C. This work broadens the practical application of flexible aqueous ZIBs at ultralow temperature.

Flexible Polymer Hydrogels for Wearable Energy Storage Applications

“…[284] At low temperatures, the arrangement of water molecules into an ordered lattice formation via intermolecular hydrogen bonding leads to the formation of ice crystals. [285] Consequently, to reduce the freezing point of hydrogel electrolytes, it is necessary to break the hydrogen bonding between the water molecules. [285] Water-miscible antifreezing solvents interact with water molecules to inhibit the formation of hydrogen bonding between water molecules.…”

“…[285] Consequently, to reduce the freezing point of hydrogel electrolytes, it is necessary to break the hydrogen bonding between the water molecules. [285] Water-miscible antifreezing solvents interact with water molecules to inhibit the formation of hydrogen bonding between water molecules. [285] Glycerol, ethylene glycol, and dimethyl sulfoxide (DMSO) are among the most common water-miscible solvents to improve the antifreezing properties of organohydrogels.…”

“…[285] Water-miscible antifreezing solvents interact with water molecules to inhibit the formation of hydrogen bonding between water molecules. [285] Glycerol, ethylene glycol, and dimethyl sulfoxide (DMSO) are among the most common water-miscible solvents to improve the antifreezing properties of organohydrogels. [286] Compared with other low-temperature protective agents, DMSO has a higher dielectric constant.…”

“…In addition, prepared flexible antifreezing Zn-ion batteries presented a suitable performance as wearable energy storage systems in a simulated practical application, where three antifreezing batteries were connected in series to a mechanical arm to power a light at room temperature and −70 °C (Figure 18e). [285] Continuous accumulation of heat generated through highspeed charge and ion transfer is another important challenge in the field of wearable energy storage systems. [290] Solid state sol-gel transition electrolytes based on the low-critical-solutiontemperature (LCST)-responsive polymer hydrogels, such as poly(isopropylacrylamide), poly(ethylene oxide)-poly(propylene oxide) copolymers, and methylcellulose have been considered as promising electrolytes to overcome this issue in wearable energy storage systems.…”

“…Antifreezing flexible Zn-ion batteries with polymer-hydrogel-based electrodes and electrolyte: a) the schematic of the assembled Zn-ion flexible battery; b) charge/discharge curves under different bending conditions; c) capacity and capacity retention of the flexible battery after bending at −70 °C; d) capacity stability and coulombic efficiency under repeated charge/discharge cycles in various bending conditions with the current density of 50 mA g −1 ; e) the digital images of three antifreezing flexible Zn-ion batteries integrated in series on a mechanical arm model (1, 2) providing power for a LED lamp (3.00 V) under different bending states at −70 °C (3-5). Reproduced with permission [285]. Copyright 2023, Wiley-VCH Gmb.…”