Recyclable biomacromolecular hydrogel electrolytes for Zn-based aqueous batteries

Metallic Zn-based aqueous batteries (ZABs) have re-emerged as promising candidates for large-scale energy storage. However, their commercialization is still hindered by challenges such as uncontrolled zinc dendrite growth, hydrogen evolution and severe corrosion [1]. Conventional liquid electrolytes, although simple, are particularly susceptible to these issues [2]. To overcome these limitations, hydrogel polymer electrolytes (HPEs) have attracted growing interest. By suppressing free water activity, regulating Zn²⁺ flux and providing mechanical reinforcement against dendrites, HPEs can effectively mitigate these problems [3]. In addition, HPEs prevent electrolyte leakage and can simultaneously serve as separators, thereby simplifying cell configuration and imparting flexibility to ZABs [4].

However, the controllable fabrication of HPEs remains a significant challenge. Conventional hydrogels typically exhibit a trade-off between ionic conductivity and mechanical strength, excelling in one property at the expense of the other. Moreover, processing HPEs into thin films (<100 μm), which is essential for energy-dense cells, is particularly difficult. Sustainability further complicates the issue, as most petroleum-derived HPEs are neither recyclable nor environmentally friendly. Recently, Chen and co-workers reported in National Science Review a universal electrogelation strategy for the in situ construction of biomacromolecule-based hydrogel electrolytes. The resulting electrogelated chitosan hydrogel within 50 μm simultaneously achieves high mechanical strength (2.0–4.4 MPa), high ionic conductivity (10.1–19.5 mS cm⁻¹) and excellent recyclability, thereby enabling reversible Zn plating/stripping and enhancing environmental sustainability [5].

Hydrogels are typically formed via sol–gel transitions, which can be triggered under various conditions to regulate both the gelation process and the resulting hydrogel structure. The choice and chemical nature of gelation triggers exert a profound influence on the success of the intended application. Numerous mechanisms have been reported, including temperature variation, component mixing, pH adjustment, redox-state alteration, enzyme addition, electromagnetic irradiation and ultrasound [6]. Nevertheless, for battery-related applications, these conventional approaches generally fail to achieve precise control over hydrogel thickness and homogeneity. In contrast, the potential-mediated pH variation strategy enables accurate in situ gelation, allowing controllable fabrication of hydrogel thickness and composition directly on the zinc anode surface (Fig. 1a). Compared with traditional chemical cross-linking methods, electrogelation offers unique advantages such as fast gelation, precise thickness control, uniform micro-/nanoporous structures, excellent geometric adaptability and scalability. Looking ahead, this electrogelation strategy could be extended beyond Zn to other multivalent metal systems (e.g. Mg and Al). Furthermore, its universal compatibility with diverse biopolymers underscores broad potential in tailoring electrochemical interfaces.

Figure 1.

(a) Triggering conditions for hydrogel formation. (b) Main concerns in the development of HPEs.

It is worth noting that, unlike biomedical hydrogels, which have been extensively investigated and even commercialized [7], hydrogels for ZABs remain at a relatively early stage of development. For device-level applications, HPEs must simultaneously satisfy multiple requirements, including mechanical robustness, flexibility, thermal and chemical stability, as well as favorable electrochemical performance. Generally, HPEs can be categorized into non-cross-linked, physically cross-linked and chemically cross-linked networks. Non-cross-linked electrolytes typically suffer from poor mechanical integrity and often require the use of additional separators. By contrast, cross-linked hydrogels offer enhanced robustness and flexibility. For instance, a polyvinyl alcohol/Zn(CF₃SO₃)₂ hydrogel electrolyte with physical cross-linking can autonomously self-heal via hydrogen bonding without external stimuli [8], while a chemically cross-linked polyacrylamide electrolyte exhibits good tensile strength (273 kPa) and remarkable stretchability of ≤3000% strain [9]. These examples demonstrate that cross-linked networks offer more stable structures, providing both mechanical strength and flexibility. Nevertheless, conventional HPEs are not without limitations. Due to the presence of water molecules in polymer networks, they inevitably freeze and lose elasticity at subzero temperatures, severely impairing their electrochemical performance and restricting their practical use in cold environments [10]. Proper modification strategies are therefore required to overcome these challenges under specific conditions. Moreover, beyond achieving better mechanical properties and electrochemical stability, large-scale manufacturability and cost control remain critical issues for the practical deployment of HPEs. In this regard, scalable preparation methods should be as simple as possible, while the raw materials must be abundant and low-cost to ensure economic feasibility.

Although HPEs have undergone significant advancements in recent years, their translation into practical battery systems still faces several key challenges. Future research should emphasize the following aspects (Fig. 1b).

Optimization of fundamental electrolyte properties: The essential physicochemical parameters of HPEs require systematic improvement. Ionic conductivity, cation transference number, electrochemical stability window and thermal adaptability must be carefully optimized, as they directly determine overall battery performance. Striking a balance between high ionic conductivity and mechanical robustness is particularly important, while extending the electrochemical stability window to accommodate higher-voltage cathodes will further enhance energy density. In addition, achieving reliable performance across a broad temperature range from subzero to elevated conditions will be crucial for expanding the applicability of HPEs to diverse environments.

Engineering stable electrode–electrolyte interfaces: The interface between hydrogels and electrodes remains a major bottleneck. Problems such as insufficient contact, parasitic reactions, corrosion and dendrite formation compromise long-term stability. Addressing these challenges calls for the rational modification of hydrogel compositions and the incorporation of functional additives to stabilize interfacial chemistry. At the same time, advanced in situ and operando characterization techniques should be applied to monitor interfacial evolution in real time. These approaches will not only clarify the mechanisms behind interfacial degradation, but also

provide guidance for the rational design of robust electrode–electrolyte interfaces.

Practical considerations for device integration: For hydrogel electrolytes to be commercially viable, practical issues of raw material selection, salt chemistry, film thickness, mechanical integrity and scalable processing must be prioritized. Cost-effective and abundant raw materials will be essential to ensure economic feasibility. Minimizing hydrogel thickness is necessary to reduce internal resistance and increase energy density, highlighting the need for new fabrication strategies with precise thickness control. Furthermore, as HPEs usually serve dual roles as both electrolyte and separator, they must possess sufficient mechanical strength, elasticity and toughness to withstand operational stresses. Equally important is the scalability of fabrication methods; industrial roll-to-roll processing or other scalable techniques should be developed to meet the requirements of large-scale energy-storage applications.

In conclusion, the future development of HPEs will depend on an integrated approach that simultaneously tackles their intrinsic physicochemical properties, interfacial stability and practical manufacturability. By addressing these aspects in parallel, hydrogel electrolytes can move from laboratory-scale proof-of-concept systems to commercially deployable solutions, thereby enabling safer, more sustainable and higher-performance ZABs and beyond.

FUNDING

This work was supported by the National Key R&D Program of China (2024YFE0101100), the National Natural Science Foundation of China (U24A2060, 22088101 and 22279023), the Fundamental Research Funds for the Central Universities (20720250005), the Science and Technology Commission of Shanghai Municipality (25DZ3002901, 25PY2600100 and 2024ZDSYS02), and the Shanghai Pilot Program for Basic Research – Fudan University 21TQ1400100 (25TQ012). The authors thank the AI for Science Foundation of Fudan University (FudanX24A1035) and the National Research Foundation, Singapore, under its Singapore-China Joint Flagship Project (Clean Energy).

Conflict of interest statement. None declared.