A hydrogen-bonded zwitterionic hydrogel electrolyte constructs freeze-resistant and flame-retardant flexible zinc-ion capacitors

Abstract

Multifunctional hydrogel electrolytes are promising for safe and flexible Zn-based energy storage, yet simultaneously achieving mechanical robustness, wide temperature adaptability, and intrinsic flame retardancy remains a formidable challenge. Herein, a multifunctional PAM/CS/Pro hydrogel electrolyte is fabricated via free-radical polymerization by integrating chitosan (CS) and zwitterionic D-proline (Pro) into a polyacrylamide (PAM) network, utilizing levulinic acid (LA) to enable homogeneous CS incorporation and reinforce secondary interactions. Mechanistically, the Pro-derived carboxylate groups serve as zincophilic sites that coordinate Zn²⁺, homogenize interfacial ion flux, and promote uniform Zn deposition to suppress dendrite growth. Concurrently, the synergy between zwitterionic moieties and CS hydroxyls constructs a dense hydrogen-bond network; this effectively increases the bound water fraction to inhibit ice crystallization at subzero temperatures and mitigate dehydration at elevated temperatures. Consequently, the hydrogel exhibits high stretchability (1180% elongation), robust adhesion, and intrinsic flame retardancy (LOI = 29%, peak HRR = 118.16 kW m⁻²). Zn//Zn symmetric cells demonstrate stable plating/stripping for over 240 h, and flexible zinc ion capacitors (ZICs) based on this electrolyte operate reliably at −20 °C, achieving an energy density of 60.33 Wh kg⁻¹ at a power density of 149.99 W kg⁻¹. This work presents an integrated ion–water–thermal management strategy, offering a blueprint for designing intrinsically safe and environmentally tolerant electrolytes for next-generation wearable energy storage.