Zincophilic zwitterionic hydrogel electrolyte towards dendrite-free zinc ion hybrid supercapacitors with anti-self-discharge ability

Hydrogel-based zinc ion hybrid supercapacitors (ZHSCs) with high flexibility and appealing energy/power densities have attracted increasing attention in energy storage fields. However, the Zn dendrite growth, water-related side reactions, and fast self-charge issues lead to severe performance degeneration of hydrogel-based ZHSCs. Herein, a zwitterion L-carnitine (CN) was integrated into hydrogel electrolyte to construct a dendrite-free and anti-corrosive hydrogel-based ZHSC with a slow self-discharge rate. CN can enter the Zn 2+ inner solvation sheath and isolate active water molecules from Zn anodes to induce homogeneous Zn 2+ deposition and inhibit undesired side reactions. The CN ionic complexes serving as ion transport channels enable the Zn 2+ to move along the migration channels, endowing the hydrogel electrolytes with a high Zn 2+ transference number of 0.847. Besides, CN with zwitterionic groups forms electrostatic interactions with cations and anions, which prevents the diffusion of moveable ions from the electrode surfaces to hydrogel electrolytes, thus effectively prolonging the self-discharge time of ZHSCs. Accordingly, the hydrogel-based ZHSCs exhibit dendrite-free and corrosive-free behaviors on Zn electrodes after long-time operation, presenting a capacitance retention of 75 % after 2000 charge and discharge cycles at 1 A/g. The hydrogel-based ZHSC also display a long self-discharge time of 3h, which is more than 9 times that of the one without CN. This work provides guidance on the design of dendrite-free and anti-self-discharge hydrogel-based zinc ion energy storage devices.