Polymer Hydrogel Electrolytes for Flexible and Multifunctional Zinc-Ion Batteries and Capacitors

Flexibility and multifunctionality are now becoming inevitable worldwide tendencies for electronic devices to meet modern life’s convenience, efficiency, and quality demand. To that end, developing flexible and wearable energy storage devices is a must. Recently, aqueous zinc-ion batteries (ZIBs) and zinc-ion capacitors (ZICs) stand out as two of the most potent candidates for wearable electronics due to their excellent electrochemical performance, intrinsic safety, low cost, and functional controllability. Simultaneously, polymer electrolytes’ introduction and rational design, especially various hydrogels, have endowed conventional ZIBs and ZICs with colorful functions, which has been regarded as a perfect answer for energy suppliers integrated into those advanced wearable electronic devices. This review focuses on the functional hydrogel electrolytes (HEs) and their application for ZIBs and ZICs. Previously reported HEs for ZIBs and ZICs were classified and analyzed, from the flexibility to mechanical endurance, temperature adaptability, electrochemical stability, and finally cell-level ZIBs and ZICs based on multifunctional HEs. Besides introducing the diverse and exciting functions of HEs, working principles were also analyzed. Ultimately, all the details of these examples were summarized, and the related challenges, constructive solutions, and futural prospects of functional ZIBs and ZICs were also dedicatedly evaluated.