Low-temperature resistant self-healing hydrogel electrolyte for dendrite-free flexible zinc-ion hybrid supercapacitors

Abstract

Flexible zinc-ion hybrid supercapacitors (ZHSC), have great potential as a novel energy storage device, and the appropriate electrolyte with stable low-temperature resistance and high conductivity is crucial for its widespread practical application. Herein, a poly (vinyl alcohol) (PVA)- carboxymethylcellulose sodium salt (CMC) hydrogel electrolyte is prepared by the repeated freezing method, with the advantages of low-temperature resistance, high conductivity, excellent mechanical properties (stretching, compression and torsion), and a certain degree of self-healing function to resist the damage of tearing. The PVA main chain in the gel forms a semi-interpenetrating network with CMC, which improves the mechanical strength of the gel, as well as enhances the binding ability with Zn2+ and promotes the uniform distribution of Zn2+. The incorporation of Li+ into the polymer network disrupts the crystalline domains, enhancing the viscosity and self-healing properties of the gel while improving its low-temperature resistance and electrical conductivity. The conductivity of the gel at 25/-20 °C is 5.17/2.37 mS cm−1. In addition, ethylene glycol (EG) is added to the gel as an antifreeze agent to help the gel not freeze easily at low temperatures. A low-temperature flexible ZHSC are prepared using PVA-CMC/Zn(CF3SO3)2/Li+/EG hydrogel electrolytes and carbonized ZIF-8 (HC) as the electrode. When at a current density of 2 mA cm−2, the device exhibits a specific capacitance of 282.7 mF cm−2 (141.4 F g−1) at room temperature, 270.4 mF cm−2 (135.2 F g−1) at −20 °C and even 61.7 mF cm−2 (135.2 F g-1) at −30 °C. Furthermore, the hydrogel electrolyte can efficiently inhibit the growth of zinc anode dendrites, thus guarantee the good capacitance retention of 81.7 % at 25 °C for 10,000 cycles and a coulombic efficiency of approximately 100 %. The device is also electrochemically stable under various bending conditions, and can power the timer at −30 °C, demonstrating new possibilities for the low-temperature flexible ZHSC.