A new simple preparation method for water-in-salt-type cellulose hydrogels filled with ZnBr2 is reported. The ZnBr2 hydrogel electrolyte can be applied to zinc-ion hybrid supercapacitors and Zn-Br interfacial batteries. The water-in-salt ZnBr2 hydrogel serves as a stable electrolyte for good electrochemical performance in zinc-ion hybrid supercapacitors without redox reactions and can participate in the charging/discharging process of the Zn‒Br interfacial battery through the redox reaction of halogen anions. The assembled zinc-ion hybrid supercapacitor exhibits a specific capacitance of 241.8 F g-1 at 0.5 A g-1. Furthermore, after a 10000-cycle-long charging/discharging test at 5 A g-1, the coulombic efficiency is found at nearly 100%. The interfacial battery reaches the highest energy efficiency of 66% at a current density of 3 mA cm-2 at 0–1.8 V and displays a good cycle life with an 88% average coulombic efficiency at 3 mA cm-2. This work expands the application of hydrogels and provides new possibilities for optimizing the high performance of electrolytes.