A Pullulan polysaccharide-based flame-retardant polyelectrolyte hydrogel for high-safety flexible zinc ion capacitors

Potential safety hazards such as leakage, flammability and thermal runaway of liquid electrolytes in conventional energy storage devices have seriously hindered their further development. In this work, a flame-retardant polyacrylamide/pullulan/phytic acid (PAM/PUL/PA) hydrogel electrolyte is prepared by using PA as flame-retardant additive, PAM as main polymer chain by one-step radical polymerization method. The PAM/PUL/PA hydrogel shows good flame-retardant properties with limiting oxygen index of up to 58 %, high mechanical performance with stretch up to 1535 % and 92 kPa tensile stress. Zinc ion capacitor (ZIC) assembled with the PAM/PUL/PA hydrogel as electrolyte and separator exhibits high ionic conductivity of 22.54 mS cm^-1, high specific capacity of 111.5 mAh g^-1 at 0.1 A g^-1 and energy density of 97.57 Wh kg^-1 at a power density of 49.99 W kg^-1, as well as good capacitance retention of 74.6 % even after 10,000 cycles. Furthermore, the ZIC has outstanding flexibility, the capacitance retention rate almost unchanged even after 1000 consecutive bending of deformations. These excellent properties are attributed to the hydrogen bonds easily formed by polar functional groups in hydrogels and the electrostatic interactions with metal ions. This work presented a simple and general pathway to prepare high-safety flexible energy storage devices with multifunctional properties.