The road ahead for hydrogel electrolytes in aqueous zinc metal batteries

Regulating the water network and solvation structure

In AZMB systems, free water activity, hydrogen-bond (HB) network connectivity, and the number of water molecules in the primary Zn²⁺ solvation sheath jointly govern the kinetics of hydrogen evolution, corrosion, and byproduct formation. HEs leverage polymer networks and functional components to tune water speciation and the local coordination environment of Zn²⁺, thereby mitigating water-driven side reactions^[14].

A widely adopted strategy is to reconstruct the HB network through multipoint interactions, decreasing the fraction of free water and lowering bulk water activity. Polyhydroxy molecules and other multidentate hydrogen-bond donors can simultaneously engage water molecules and polymer chains. For instance, the introduction of sucrose into polyacrylamide-based hydrogels simultaneously regulates the HB network and the Zn²⁺ solvation structure, enabling Zn||Zn symmetric cells to operate stably for over 3,970 h at 0.5 mA cm^-2 and -10 °C. The corresponding Zn||VO₂ full cell also exhibits excellent durability, sustaining 10,000 cycles at 1 A g^-1 even at 0 °C^[15]. However, it is important to note that excessive immobilization of water or an overly compact polymer network can reduce the concentration of mobile charge carriers, increase viscous resistance, and generate locally starved Zn²⁺ microenvironments^[16]. Moreover, spatially nonuniform microenvironments can induce local flux heterogeneity, exacerbating polarization under high-rate conditions^[17].

A second approach targets the Zn²⁺ solvation sheath directly by introducing functional groups or molecules that compete with water in a controlled manner, partially replacing water molecules within the first solvation shell^[18]. Cosolvents and urea-based molecules can coordinate with Zn²⁺ while simultaneously tuning gel swelling through hydrogen bonding. For instance, tetramethylurea incorporated into polyvinyl alcohol or polyacrylamide gels can lower water activity and adjust solvation composition, contributing to improved corrosion resistance and cycling stability at elevated temperatures^[9,19]. In zwitterionic polyelectrolyte gels, increasing salt concentration can reshape solvation through changes in ionic strength and preferential ion–polymer interactions^[20].

Overall, microenvironment engineering aims to lower water activity while optimizing Zn²⁺ solvation. In our view, its effectiveness should be evaluated not only by its ability to suppress water-driven side reactions, but also by whether it preserves sufficient and spatially uniform Zn²⁺ supply. Otherwise, reducing water activity alone may simply shift the bottleneck from parasitic chemistry to transport limitations under high-rate or high-loading conditions^[21]. We argue that microenvironment engineering remains limited in practice by the lack of standardized protocols for quantifying water activity and solvation structure under operating, rather than equilibrium, conditions.

Selective bulk ion transport and decoupling

In HEs, bulk ion transport is not fully captured by ionic conductivity alone. It is also governed by $$ t_{Z n^{2+}} $$, the ion association state, and the connectivity and tortuosity of transport pathways^[22-27]. Low $$ t_{Z n^{2+}} $$ (or dominant anion transport) can result in the rapid formation of concentration gradients under an applied current, leading to severe concentration polarization^[28,29]. This can cause interfacial Zn²⁺ depletion, higher local overpotential, and nonuniform nucleation, which may in turn promote dendritic growth and accelerated side reactions^[22,30]. Transport regulation, therefore, targets a higher effective Zn²⁺ flux, reduced anion contribution, and lower tortuosity, so that ion supply remains more uniform.

Hydrogels with polyanionic backbones or anion binding sites immobilize anions through electrostatic interactions, increasing $$ t_{Z n^{2+}} $$ and alleviating concentration polarization. A sulfonate-based polyanionic hydrogel ($$ t_{Z n^{2+}} $$ = 0.79) reshapes the Zn²⁺ solvation shell and regulates the interfacial potential distribution, enabling stable Zn||Zn cycling for over 1,100 h at 5 mA cm^-2[22]. An anion-philic design combining weak cation-dipole coordination with strong anion adsorption ($$ t_{Z n^{2+}} $$ = 0.71) further supports pouch cell operation at 4.9 mAh cm^-2 over 270 cycles^[31]. Regulating the microenvironment to weaken Zn²⁺-counterion pairing can thus enhance the mobile Zn²⁺ fraction and flux responsiveness. Meanwhile, the incorporation of ordered porous fillers or oriented channels can lower tortuosity and promote uniformity, collectively contributing to a more stable ion supply at high areal capacities and current densities^[23,32,33].

Enhanced bulk transport, however, does not necessarily translate into interfacial stability^[34]. Interfacial reactions and morphology evolution are highly localized and can be amplified by electrode roughness and reaction heterogeneity^[35]. Even with an enhanced ion supply, desolvation, nucleation, and growth can still become the dominant lifetime limiting steps. This motivates direct regulation of interfacial processes. In our view, meaningful progress requires a standardized framework that reports $$ t_{Z n^{2+}} $$, ionic conductivity, and concentration polarization together under identical operating conditions, enabling identification of the true rate-limiting transport parameter.

Interfacial desolvation and deposition regulation

After Zn²⁺ reaches the electrode surface, it undergoes desolvation, nucleation, and growth. Even when bulk transport is improved, a high desolvation barrier, elevated local water activity, or a nonuniform interfacial ion supply can still lead to heterogeneous nucleation and unstable deposition. This is often accompanied by the thickening of byproduct layers and rising polarization^[36].

Interfacial regulation generally follows two complementary routes. One is to build structure-guided interfaces that promote uniform nucleation and controlled growth. For example, introducing silk fibroin into hydrogels can yield an ordered, β-sheet-rich network via solvent-induced assembly. Zincophilic polar groups in this network can tune Zn²⁺ solvation and promote preferential growth along the (002) plane, thereby reducing dendrite formation and side reactions^[6].

The second route is to construct dynamic, adaptive interfaces that preserve interfacial contact stability during cycling. For instance, a zinc alginate hydrogel interlayer uses zincophilic carboxylate groups and a negatively charged network to guide Zn²⁺ transport, repel SO₄^2-, and facilitate interfacial desolvation. Meanwhile, its dynamic flexibility accommodates volume changes and repairs microcracks during cycling, sustaining stable Zn deposition for over 2,200 h without dendrite or byproduct formation^[37]. These examples highlight that bulk ion supply and interfacial desolvation kinetics must be co-designed, yet we note that desolvation barriers and their dependence on local water activity remain poorly quantified relative to the field’s focus on dendrite morphology.