Toward flexible zinc-ion hybrid capacitors with superhigh energy density and ultralong cycling life: the pivotal role of ZnCl2 salt-based electrolytes.

Zinc (Zn) ion hybrid capacitors (ZIHCs) are promising energy storage devices for emerging flexible electronics, but they still suffer from trade-off in energy density and cycling life. Herein, we prove that such a dilemma can be well-addressed by deploying ZnCl 2 based electrolytes. Combining experimental studies and density functional theory (DFT) calculations, for the first time, we demonstrate an intriguing chloride ion (Cl - ) facilitated desolvation mechanism in hydrated [ZnCl] + (H 2 O) n-1 (with n=1-6) clusters. Based on this mechanism, a "water-in-salt" type hydrogel electrolyte filled with ZnCl 2 was developed to concurrently improve the energy storage capacity of porous carbon materials and the reversibility of Zn metal electrode. The resulting ZIHCs deliver a battery-level energy density up to 217 Wh kg -1 at a power density of 450 W kg -1 , an unprecedented cycling life of 100,000 cycles, together with excellent low-temperature adaptability and mechanical flexibility.