A novel redox bromide-ion additive hydrogel electrolyte for flexible Zn-ion hybrid supercapacitors with boosted energy density and controllable zinc deposition

With the rapid development of flexible wearable electronic devices and the growing energy demands of modern society, flexible energy storage equipment is attracting increasing attention. Recently, flexible Zn-ion hybrid supercapacitors (ZHSs), as a new type of flexible energy storage device, have been reported. However, the limited energy density of the currently reported flexible ZHSs should be further improved to realize their large-scale applications. Herein, we designed a novel redox bromide-ion additive hydrogel electrolyte (SA-Zn-Br) for flexible Zn-ion hybrid supercapacitors (BH-ZHSs) via the introduction of extra faradaic contributions (3Br-/Br3-) into the hydrogel electrolyte to improve their energy density. Additionally, the assembled flexible BH-ZHS displays a maximum energy density of 605 W h kg-1 at a power density of 1848 W kg-1 at an amazing voltage of 2.6 V, which is better than that of most reported flexible ZHSs. After a 5000 cycle charge/discharge cycling test, capacity retention of 87.7% is retained. Interestingly, the strong interactions between the charged groups and Zn2+ ion in the SA-Zn-Br hydrogel electrolyte can harmonize Zn2+ migration with uniform nucleation on a Zn foil surface, leading to layered zinc deposition. Additionally, the SA-Zn-Br hydrogel electrolyte can also serve as an inhibitor of water/oxygen, resulting in the mitigation of corrosion and highly reversible zinc stripping/depositing. The strategy described in this study should provide a new insight for exploring flexible ZHSs with boosted energy density and controllable zinc deposition.