Toward Flexible Zinc‐Ion Hybrid Capacitors with Superhigh Energy Density and Ultralong Cycling Life: The Pivotal Role of ZnCl2 Salt‐Based Electrolytes

Zinc 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 show that such a dilemma can be well‐addressed by deploying ZnCl2 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]+(H2O)n−1 (with n=1–6) clusters. Based on this mechanism, a water‐in‐salt type hydrogel electrolyte filled with ZnCl2 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. A chloride ion facilitated desolvation effect in aqueous ZnCl2 solutions increases the energy storage capacity of porous carbon electrodes. By utilizing this effect, a flexible zinc ion hybrid capacitor assembled from a ZnCl2 based hydrogel electrolyte and matching porous carbon materials delivers a battery‐level energy density and excellent cycling life.