A new selection criterion for voltage windows of aqueous zinc ion hybrid capacitors: achieving a balance between energy density and cycle stability.

Aqueous zinc-ion hybrid supercapacitors (ZHSs) are promising energy storage devices owing to their high energy and power density. However, the selection of the voltage window to achieve the balance between energy density and cycling stability is still difficult. Herein, the protective effect of by-products deposited on the surface structure of the cathode material was studied based on the differences in the cycling stability of ultrathin carbon materials (UTCs) at different voltage windows. Through the calculation of the Gibbs free energy of the hydrogen evolution reaction (HER) process, the association of cathode by-products with voltage windows was described and confirmed for the first time: whether the by-product layer on the surface of the cathode material is broken or not can be considered a criterion for selecting the voltage window. Based on this, a flat pouch cell was fabricated in a wide voltage window of 0–1.9 V and exhibited an extremely high energy density of 267.9 W h kg⁻¹ coupled with good flexibility and long cycle life with a capacity retention of 90.0% after 3000 cycles. Overall, this work introduces the relationship between cathode by-products and voltage windows for the first time, useful for providing more specific criteria for the selection of voltage windows. [ABSTRACT FROM AUTHOR]