Tannic acid-modified polyvinyl alcohol hydrogel electrolyte enabling dendrite-free and flexible aqueous zinc-ion batteries

Abstract

Aqueous zinc-ion batteries (AZIBs) hold great promise for large-scale and flexible energy storage owing to their high safety and low cost. However, conventional polyvinyl alcohol (PVA)-based hydrogel electrolytes suffer from poor mechanical robustness, low ionic conductivity, and uncontrolled Zn dendrite growth. Herein, a sustainable composite hydrogel electrolyte is developed by introducing plant-derived tannic acid (TA) into the PVA matrix through a freeze-thaw process. The abundant phenolic hydroxyl groups in TA form strong hydrogen bonds with PVA chains and coordinates with Zn2+ ions, constructing a highly interconnected and ionically conductive network. The optimized PVA–TA hydrogel exhibits a markedly improved tensile strength of 10.3 MPa, excellent flexibility (190% elongation), and a high ionic conductivity of 8.92 mS cm-1 along with an ultrahigh Zn2+ transference number of 0.66, all of which are substantially superior to the values of the pure PVA hydrogel (4.9 MPa, 5.9 mS cm-1, and 0.40, respectively). Such synergistic effects enable uniform Zn deposition and stable plating/stripping over 1200 h with minimal polarization (~ 100 mV). Zn||MnO2 full cell deliver a high reversible capacity 151 mAh g-1 at 0.5 C and retain 148 mAh g-1 after 100 cycles. Moreover, flexible pouch cell maintain stable operation even under severe bending (120°) and partial cutting, demonstrating outstanding mechanical resilience and interfacial stability. This work provides a green and scalable strategy for constructing hydrogen-bonded, polyphenol-functionalized hydrogels, offering new insights for safe and flexible zinc-ion energy storage systems.