In conventional aqueous liquid electrolytes, functional additives enable the construction of Zn²⁺ concentration gradients primarily through competitive coordination, interfacial adsorption, and modulation of the electric double layer. Additives containing electron-donating functional groups can partially replace water molecules in the primary solvation sheath of Zn²⁺, thereby reconstructing its solvation structure and altering desolvation kinetics^[69,70]. Owing to their preferential adsorption on the Zn surface, these additives locally increase the amount of coordinated Zn²⁺ ions at the electrode and electrolyte interface, generating spatial asymmetry in the chemical potential between the bulk electrolyte and the interfacial region. This gradient drives directional Zn²⁺ flux, alleviates ion depletion during plating, and homogenizes nucleation behavior, thereby suppressing dendrite growth and stabilizing interfacial reactions.

On the basis of this mechanism, Wang et al. introduced N, S-doped quantum dots (NSQDs) into the electrolyte to construct a surface Zn²⁺ concentration gradient and regulate deposition behavior^[71]. The mechanistic basis lies in the abundance of oxygen-containing functional groups located on the side chains of the NSQDs. Upon contact with the Zn foil, these groups undergo partial reduction and anchor onto the metal surface, forming Zn-O bonds^[72]. This interfacial chemical coupling creates a Zn²⁺-affinitive layer that is locally enriched with Zn²⁺ ions near the electrode, generating a directional ion flux from the bulk electrolyte toward the surface. As a result, numerous uniformly distributed nucleation sites are established, lowering the nucleation overpotential and homogenizing Zn electrodeposition^[73]. The formation of this functional interfacial layer was verified by energy dispersive X-ray spectroscopy (EDS) mapping, which revealed uniform distributions of O, N, and S, which were consistent with those of the NSQDs coating, confirming intimate interfacial integration [Figure 2A]. Owing to the regulated Zn²⁺ distribution, the Zn electrode exhibited a significantly smoother and denser morphology after 10 cycles in a symmetric cell at 10 mA cm^-2 with an areal capacity of 0.5 mAh cm^-2, as evidenced by scanning electron microscopy (SEM) [Figure 2B]. This interfacial gradient effect is further translated into remarkable full-cell performance. In a Zn-zinc vanadate (ZVO) configuration, the NSQDs-modified system maintained a high specific capacity of 208.4 mAh g^-1 even at an ultrahigh current density of 10 A g^-1, substantially outperforming its conventional electrolyte counterpart (44.4 mAh g^-1). When the current density was restored to 1 A g^-1, the cell recovered a high capacity of 278.9 mAh g^-1, demonstrating excellent rate capability and reversibility [Figure 2C]. These improvements originate from additive-induced interfacial Zn²⁺ enrichment and the resulting mitigation of localized ion depletion, which collectively suppress dendrite growth and stabilize the electrochemical reaction environment.

Additive hydrogen bond interactions offer an effective strategy to regulate the Zn electrode interface by restructuring the interfacial water network and tuning Zn²⁺ solvation chemistry. In aqueous electrolytes, the hydrogen-bonded network of water governs proton activity, Zn²⁺ coordination structure, and interfacial reaction kinetics^[74]. The introduction of hydrogen bond-active additives, such as molecules containing hydroxyl, amino, or carbonyl functional groups, enables competitive interactions with water molecules, thereby reducing free water activity and weakening proton transport pathways^[75,76]. This suppression of interfacial water reactivity mitigates the hydrogen evolution reaction and limits the formation of hydroxide byproducts. Simultaneously, the adsorption and interfacial enrichment of such additives reconstruct the electric double layer and modify the Zn²⁺ solvation sheath, lowering the desolvation barriers and homogenizing the ion flux. The resulting asymmetric hydrogen-bond environment establishes an ordered interfacial microstructure, which alleviates local ion depletion, stabilizes Zn nucleation, and improves deposition uniformity.

On the basis of this mechanism, Zong et al. introduced diethyl phosphoramidate (DP) as an electrolyte additive to simultaneously regulate hydrogen bonding interactions and Zn²⁺ solvation behavior^[77]. The polar P=O and N-H functional groups in DP strongly interact with water molecules, disrupting the intrinsic hydrogen bonding network of the aqueous electrolyte and thereby reducing free water activity^[78]. This suppression of water reactivity mitigates interfacial pH fluctuations and inhibits parasitic side reactions, particularly the hydrogen evolution reaction. Moreover, DP participates in the Zn²⁺ solvation sheath, partially replacing coordinated water molecules and reconstructing the local coordination environment^[79]. The modified solvation structure decreases the desolvation energy barrier and accelerates interfacial charge transfer kinetics while simultaneously decreasing the probability of water reduction at the electrode surface. Beyond solvation regulation, DP undergoes interfacial reactions that generate a gradient organic-inorganic hybrid solid electrolyte interphase (SEI) in situ [Figure 2D]. This interphase, enriched with DP-derived species and inorganic Zn compounds, provides abundant and energetically favourable nucleation sites, guiding Zn deposition preferentially along the thermodynamically stable (002) crystallographic plane. The resulting interfacial layer not only homogenizes the electric field distribution but also establishes a more uniform Zn²⁺ flux toward the electrode surface, thereby suppressing dendrite formation^[43]. In situ optical observations at 5 mA cm^-2 clearly reveal this regulatory effect. In the absence of DP, Zn deposition rapidly develops surface protrusions that evolve into dendritic structures, and this behavior indicates severe ion flux heterogeneity. In contrast, the DP-containing electrolyte enables dense and uniform Zn plating throughout the deposition process, and these results confirm that the reconstructed interfacial microenvironment promotes homogeneous nucleation and growth [Figure 2E]. The interfacial stabilization induced by DP translates directly into enhanced electrochemical performance at the full-cell level. Although cells with and without DP exhibit comparable initial capacities at low current density, the DP-modified system maintains a substantially higher capacity under high-rate conditions and demonstrates excellent reversibility upon returning to lower current densities [Figure 2F]. These improvements originate from the synergistic effects of hydrogen bond disruption, solvation-structure reconstruction, and gradient SEI formation, which collectively alleviate interfacial side reactions, homogenize Zn²⁺ distribution, and stabilize long-term Zn plating/stripping behavior.

In conventional liquid electrolyte systems, ion concentration gradients can be deliberately constructed by introducing functional separators with selective ion transport properties between the anode and cathode. Unlike inert porous membranes that serve only as physical barriers, these engineered separators contain fixed charges, polar functional groups, or well-defined ion-conductive channels that regulate Zn²⁺ migration through electrostatic interactions, selective coordination, and Donnan exclusion effects^[80,81]. This selective transport behavior redistributes Zn²⁺ flux and creates spatial asymmetry in the ion concentration within the electrolyte region, thereby establishing a controlled chemical potential gradient toward the Zn electrode surface. Moreover, the tailored internal structure of the separator modulates the local electric field distribution and constrains ion diffusion pathways, which alleviates localized ion depletion and suppresses the field amplification that typically drives dendritic growth. In certain systems, solvent transport and water activity near the anode are also regulated, leading to reduced hydrogen evolution and surface passivation. Through the combined effects of ion sieving, electric field homogenization, and mass transport control, functional separators provide an effective platform for stabilizing Zn²⁺ flux and improving interfacial deposition behavior.

On the basis of this design principle, Liu et al. developed a bifunctional composite separator (BC-FP) using a nanoconfinement and gradient ion-guiding strategy to regulate Zn²⁺ transport and deposition^[82]. The separator integrates a bacterial cellulose layer with a filter paper substrate, enabling synergistic control over ion migration and interfacial kinetics. The bacterial cellulose component is rich in zincophilic hydroxyl groups that can coordinate with Zn²⁺ ions, thereby reconstructing local transport pathways and stabilizing the interfacial microenvironment by suppressing parasitic side reactions [Figure 3A]. Moreover, the hydrophilic filter paper layer facilitates directional Zn²⁺ transport through capillary-driven electrolyte uptake, promoting a continuous and guided ion flux toward the anode. The intrinsic gradient pore structure across the composite further modulates mass transport by enhancing ion diffusion while preventing localized ion depletion. This structural asymmetry establishes a spatially regulated Zn²⁺ concentration profile and homogenizes the desolvation process of hydrated Zn²⁺ ions at the electrode surface, leading to uniform nucleation behavior. After cycling a Zn symmetric cell at 1 mA cm^-2 for 30 min, the Zn electrode under BC-FP conditions exhibited a dense and smooth surface morphology, indicating that the Zn deposition stabilized compared with that of conventional separators [Figure 3B]. In full cells employing a Zn anode and a vanadium-based cathode, the BC-FP separator consistently delivers higher specific capacities across a wide range of current densities than glass fibre separators do. At high rates, the capacity advantage becomes particularly pronounced, and the cell maintains excellent reversibility when the current density is restored to lower values [Figure 3C].

In addition to directly regulating ion transport, ion concentration gradients can also be engineered by tailoring the interactions between separators and water molecules, thereby indirectly modulating Zn²⁺ solvation and interfacial mass transport. In aqueous electrolytes, water not only serves as the solvent but also governs proton activity, hydrogen bonding networks, and the solvation structure of Zn²⁺ ions. By introducing separators with strong water affinity, hydrogen bond acceptor or donor groups, or confined hydrophilic nanochannels, the local distribution and mobility of water molecules can be spatially regulated. Such separators can preferentially adsorb or immobilize water within their matrix, effectively reducing free water activity near the Zn electrode and reconstructing the hydrogen bonding network^[83,84]. This confinement alters the primary solvation sheath of Zn²⁺, often decreasing the number of coordinated water molecules and lowering desolvation barriers at the interface. Simultaneously, the asymmetric distribution of water across the separator establishes a gradient in solvent activity and chemical potential, which indirectly drives a more uniform Zn²⁺ flux toward the electrode surface. This regulated interfacial microenvironment mitigates pH fluctuations, suppresses hydrogen evolution, and prevents the accumulation of hydroxide byproducts.

On the basis of this mechanism, Dong et al. developed an 18 μm thick cellulose separator with a negatively charged surface and gradient hydrophobicity to regulate the solvent distribution and Zn²⁺ transport^[85]. The negatively charged functional groups within the separator create an electrostatic shielding effect that repels Cl^- anions while promoting more uniform Zn²⁺ flux through selective ion migration [Figure 3D]. Moreover, the gradient hydrophobic architecture induces asymmetric wettability across the separator thickness, as evidenced by the distinct water contact angles on the two surfaces, which confirms the establishment of a spatially differentiated solvent environment [Figure 3E]. This asymmetric solvent distribution results in a stepwise desolvation pathway for hydrated Zn²⁺ ions, gradually reducing the number of coordinated water molecules as the ions approach the anode. The resulting localized high-concentration electrolyte environment near the electrode effectively suppresses hydrogen evolution and mitigates interfacial side reactions. Through the combined effects of electrostatic regulation, solvent confinement, and progressive desolvation, the separator homogenizes interfacial Zn²⁺ transport and stabilizes deposition behavior. Consequently, compared with control systems lacking gradient structural design, full cells employing this separator exhibit superior rate capability and consistently higher specific capacities across a wide range of current densities, demonstrating that solvent-mediated gradient engineering is an effective strategy for enhancing the interfacial stability and electrochemical performance of AZBs [Figure 3F].

Notably, the formation of ion concentration gradients is not merely a secondary consequence of interfacial modification but also serves as a fundamental driving factor governing Zn deposition behavior. While interfacial phenomena such as solid electrolyte interphase formation, surface adsorption, and solvation structure regulation can influence local electrochemical environments, their primary role in many systems is to induce or stabilize spatially nonuniform Zn²⁺ distributions. These gradients establish chemical potential differences that directly regulate the ion flux, electric field distribution, and nucleation kinetics at the Zn electrode surface. In this sense, ion concentration gradients represent the underlying physical origin of homogenized Zn²⁺ transport and uniform deposition.

Moreover, it is important to recognize that gradient formation is often intrinsically coupled with other interfacial effects. For example, additive adsorption or SEI formation can simultaneously modify the electric double layer and promote localized Zn²⁺ enrichment, while confined polymer networks in hydrogel systems regulate both ion transport and solvent activity^[86]. These processes act cooperatively to reinforce gradient stability and amplify its regulatory effect on interfacial reactions. Therefore, rather than being independent or competing mechanisms, ion concentration gradients and interfacial modifications should be understood as synergistically coupled, where gradients provide the primary driving force for Zn²⁺ flux regulation, and interfacial effects serve as essential enablers that facilitate and stabilize the gradient structure.

By designing distinct network structures in different regions of hydrogel electrolytes, ion concentration gradients can be established through spatially differentiated ion transport and solvent confinement behavior. In hydrogel systems, the polymer network governs ion mobility, water distribution, and Zn²⁺ solvation structure by controlling pore size, crosslinking density, and the distribution of functional groups. Regions with high crosslinking density or reduced pore size impose stronger confinement effects, lowering ion diffusivity and partially immobilizing water molecules, whereas loosely crosslinked domains provide faster ion migration pathways and higher ionic conductivity. This structural heterogeneity generates differences in Zn²⁺ transport kinetics and local chemical potential across the hydrogel, thereby creating a controllable ion concentration gradient^[89,90]. Simultaneously, functional groups embedded within the polymer matrix can selectively coordinate with Zn²⁺ or participate in hydrogen bonding with water, further modulating the solvation structure and desolvation dynamics near the electrode interface. As hydrated Zn²⁺ ions migrate from bulk-like regions toward more confined domains adjacent to the anode, a progressive desolvation process occurs, leading to localized Zn²⁺ enrichment and reduced water activity at the interface. This asymmetric microenvironment homogenizes the ion flux, suppresses hydrogen evolution, and stabilizes Zn nucleation behavior.

On the basis of this principle, Wang et al. developed a gradient network hydrogel electrolyte composed of polyvinyl alcohol (PVA), cellulose nanofibres (CNF), and graphene oxide (GO) to regulate Zn²⁺ transport and interfacial kinetics in AZBs^[91]. The design integrates a low-density PVA and CNF (PC) layer on the cathode side with a high-density PVA, CNF, and GO (PCG) layer adjacent to the Zn anode, thereby establishing spatially differentiated transport properties within a single electrolyte. The cathode-facing layer contains wider transport channels and a higher water content, which facilitates rapid ion migration and maintains high ionic conductivity [Figure 4A]. In contrast, the dense interfacial network enriched with carboxyl and hydroxyl groups enhances Zn²⁺ coordination and promotes progressive desolvation as ions approach the anode. This confined microenvironment reduces local water activity, suppresses parasitic reactions, and homogenizes Zn²⁺ flux at the electrode surface. Simulated Zn²⁺ concentration fields confirm that compared with homogeneous hydrogels, the gradient structure results in a more uniform interfacial ion distribution, which is consistent with the proposed mechanism of concentration gradient regulation. The electrochemical benefits are reflected in the rate performance, where cells employing the gradient hydrogel deliver average discharge capacities of 250.6, 236.2, 205.7, 159.3, 125.0, 103.6, 89.7, and 236.5 mAh g^-1 at current densities of 0.15, 0.3, 0.6, 1.2, 1.8, 2.4, 3, and 0.15 A g^-1, respectively [Figure 4B]. These values consistently exceed those obtained with hydrogels with homogeneous structures, demonstrating that gradient network engineering effectively regulates Zn²⁺ transport, stabilizes interfacial deposition, and enhances overall electrochemical performance.

Gradient concentration fields can also be constructed through the formation of chemical bonds between the hydrogel electrolytes and the Zn electrode surface, which creates an interfacially anchored and chemically coupled transport layer. When functional groups within the hydrogel network, such as hydroxyl, carboxyl, amino, or sulfonate groups, form coordination or covalent interactions with surface Zn atoms, a tightly bound interphase is generated that differs structurally and chemically from the bulk gel^[57,92]. Compared with the interior hydrogel matrix, this bonded interfacial region exhibits modified ion mobility, solvent distribution, and coordination environments. Chemical anchoring enhances Zn²⁺ affinity at the interface and promotes localized ion enrichment, thereby establishing a spatial gradient in Zn²⁺ concentration from the bulk electrolyte toward the electrode surface. Simultaneously, the constrained polymer chains and coordinated functional groups facilitate partial desolvation of hydrated Zn²⁺ ions and reduce free water activity in the interfacial zone, which suppresses hydrogen evolution and mitigates surface passivation. Interfacial bonding also stabilizes the electric field distribution and minimizes interfacial impedance fluctuations during repeated plating and stripping. Through the synergistic effects of chemical anchoring, solvation reconstruction, and mass transport modulation, hydrogel-Zn interfacial bonding provides a robust strategy for generating controlled concentration gradients and enhancing Zn deposition uniformity and cycling stability in AZBs.

On the basis of this concept, Cui et al. constructed gradient hydrogel networks via an epitaxial polymerization strategy that enables covalent anchoring of the polymer matrix onto the Zn surface^[90]. The resulting networks exhibit an asymmetric distribution of negatively charged groups along the thickness direction, which promotes selective Zn²⁺ transport and increases the Zn²⁺ transference number to 0.65, markedly higher than that of homogeneous hydrogel networks with identical polymer contents. Confocal laser scanning microscopy (CLSM) revealed that the network density gradually decreased from the Zn side interface toward the bulk region, confirming the well-defined gradient architecture, whereas homogeneous hydrogels displayed uniform density along the Z axis [Figure 4C]. This structural asymmetry establishes differentiated ion mobility and solvent confinement, where the dense interfacial layer restricts lateral diffusion and regulates Zn²⁺ desolvation, while the upper region maintains sufficient ionic conductivity. Chronoamperometric measurements further elucidate the transport mechanism. In homogeneous hydrogels, the continuously increasing current within 400 s indicates two-dimensional diffusion and nonuniform deposition [Figure 4D]. In contrast, the gradient hydrogel exhibited a stable current plateau after 143 s, characteristic of three-dimensional diffusion, demonstrating that the high-density interfacial network effectively homogenized the Zn²⁺ flux and stabilized the nucleation behavior^[82]. Notably, the term “stable” used here does not imply a strictly constant current response but rather refers to the relatively reduced fluctuation amplitude and smoother evolution of the current over time. These interfacial transport advantages translate to improved electrochemical performance, with full cells delivering capacities of 67, 60, 53, 48, and 44 mAh g^-1 at current densities of 0.1, 0.2, 0.5, 1, and 2 A g^-1, respectively, compared with 57, 50, 43, 36, and 30 mAh g^-1 for homogeneous hydrogel systems [Figure 4E]. The superior rate capability confirms that covalently anchored gradient networks effectively regulate interfacial ion transport and reaction kinetics at the Zn electrode.

Hydrogel electrolytes possess sufficient mechanical robustness and structural flexibility to enable the lamination of multiple hydrogel layers with distinct physicochemical properties, thereby providing a platform for constructing spatially controlled ion concentration gradients. When hydrogels with different crosslinking densities, pore structures, functional group distributions, or water contents are stacked together, each layer exhibits unique ion transport kinetics, solvent activity, and Zn²⁺ coordination behavior. Regions with loose networks and high water content typically offer high ionic conductivity and rapid Zn²⁺ migration, whereas densely crosslinked or functionally enriched layers impose stronger confinement effects, reduce water mobility, and enhance Zn²⁺ coordination interactions^[93,94]. This structural heterogeneity generates differences in diffusion coefficients and chemical potentials across the electrolyte thickness, resulting in a directional Zn²⁺ flux from high-mobility domains toward more confined interfacial regions. As hydrated Zn²⁺ ions traverse these layers, progressive desolvation and solvent reorganization occur, leading to localized Zn²⁺ enrichment and reduced free water activity near the electrode surface. The resulting asymmetric transport environment homogenizes the interfacial ion distribution, suppresses hydrogen evolution, and stabilizes Zn nucleation and growth. Through the integration of mass transport modulation, solvent confinement, and solvation structure tuning, laminated hydrogel electrolytes enable rational gradient engineering to enhance the interfacial stability and electrochemical performance of AZBs.

On the basis of this concept, Lu et al. proposed a directional electrolyte gradient strategy that establishes a Zn²⁺ concentration gradient from the separator toward the anode to simultaneously optimize wettability and interfacial deposition behavior^[95]. In this configuration, a high-concentration electrolyte integrated with a sodium carboxymethyl cellulose binder is coated directly onto the Zn surface, while a low-concentration electrolyte fully infiltrates the glass fibre separator [Figure 5A]. This spatially differentiated design creates distinct physicochemical environments across the cell. The low-concentration region ensures sufficient wettability and facilitates long-range Zn²⁺ migration within the separator, whereas the high-concentration region adjacent to the Zn anode promotes dense and uniform Zn deposition by reducing water activity, suppressing parasitic reactions, and stabilizing the interfacial electric field. Optical observations after 10 h of evaporation reveal a gradual transition in light transmittance from top to bottom, which is consistent with an increasing concentration of crystalline hydrated zinc sulfate, confirming the formation of the intended concentration gradient [Figure 5B]. The distribution of carboxymethyl cellulose particles further supports this gradient structure, with spherical and chain-like aggregates concentrated near the bottom region and flake-like structures observed near the top, reflecting morphology-dependent migration behavior during solvent redistribution. The electrochemical advantages of this gradient design are evident in full cells assembled with Zn anodes and V₂O₅ cathodes. Under gradient electrolyte conditions, the cell retains a specific capacity of 92 mAh g^-1 after 800 cycles, whereas a cell using a conventional liquid electrolyte decreases to 37 mAh g^-1 after only 200 cycles. The gradient system also exhibits more stable and narrowly distributed Coulombic efficiency, indicating suppressed side reactions and stabilized interfacial kinetics [Figure 5C]. These results demonstrate that a spatially controlled electrolyte concentration effectively regulates Zn²⁺ flux and interfacial reactions, leading to significantly enhanced cycling stability.

Moreover, this gradient electrolyte design strategy can be extended to Zn-I₂ battery systems, where it simultaneously regulates Zn anode stability and iodine redox kinetics through spatially differentiated ionic environments. In Zn-I₂ batteries, performance degradation typically arises from two coupled issues: dendritic Zn deposition at the anode and the dissolution and shuttling of polyiodide species at the cathode^[96]. By constructing a Zn²⁺ concentration gradient from the separator toward the Zn surface, a high-concentration electrolyte region adjacent to the anode reduces water activity, suppresses hydrogen evolution, and homogenizes Zn²⁺ flux, thereby stabilizing Zn plating and stripping. Moreover, maintaining a relatively low concentration environment near the separator and cathode preserves sufficient ionic conductivity and facilitates efficient redox conversion of iodine species^[97,98]. Concentration asymmetry also influences the distribution and diffusion behavior of polyiodide intermediates, as modified solvent activity and ion-pairing interactions can restrict their outwards migration and mitigate shuttle effects. Consequently, the gradient electrolyte establishes a coordinated regulatory mechanism that balances Zn²⁺ transport, solvent reactivity, and iodine conversion dynamics.

On the basis of this principle, Liu et al. constructed an asymmetric hydrogel electrolyte to achieve spatially differentiated interfacial regulation in Zn-I₂ batteries^[96]. On the anode side, a Zn²⁺-crosslinked sodium alginate and carrageenan double-network hydrogel was employed to guide Zn²⁺ transport and stabilize the deposition behavior [Figure 5D]. The abundant coordination sites within this dense network promote uniform Zn²⁺ distribution, facilitate controlled desolvation, and suppress dendritic growth, thereby enabling dendrite-free Zn plating and long-term interfacial stability. On the cathode side, a highly conductive PVA-reinforced poly(3,4-ethylenedioxythiophene) polystyrene hydrogel (PVA-PEDOT) was introduced to immobilize polyiodide intermediates and accelerate electron transfer, effectively mitigating the shuttle effect while enhancing I₂ to I^- redox kinetics. This asymmetric configuration integrates ion transport regulation at the anode with redox confinement at the cathode, forming a coordinated gradient microenvironment across the cell. After operation at 5 mA cm^-2 for 100 h, the Zn surface remained flat and dense, with a height variation of only 24 μm, confirming highly uniform deposition [Figure 5E]. The electrochemical advantages are reflected in the rate performance, where the full cell delivers capacities of 210 and 150 mAh g^-1 at 0.1 and 10 A g^-1, respectively, and nearly fully recovers its initial capacity when the current density returns to 0.1 A g^-1 [Figure 5F]. Compared with cells using conventional liquid electrolytes, cells using asymmetric hydrogel electrolytes demonstrate markedly improved stability and reversibility, highlighting the effectiveness of spatially engineered hydrogel systems in regulating interfacial reactions and enhancing battery performance.

Like conventional liquid systems, ion concentration gradients in hydrogel electrolytes can be established through the incorporation of functional additives that synergistically interact with both the polymer network and solvated Zn²⁺ species. In hydrogel matrices, ion transport is inherently influenced by network confinement, fixed charges, and solvent distribution. When additives containing coordination groups or hydrogen bond-active moieties are introduced, they can selectively bind Zn²⁺ ions or restructure the local water environment, thereby modifying solvation configurations and ion mobility within specific regions of the gel. Because diffusion and segmental motion in hydrogels are spatially heterogeneous, compared with those in additive-free regions, Zn²⁺ transport kinetics and chemical potentials in hydrogels in additive-enriched regions often differ^[58,99]. This spatial variation generates a controllable Zn²⁺ concentration gradient across the electrolyte thickness. Near the anode interface, additive-induced coordination and solvent confinement promote progressive desolvation, reduce free water activity, and homogenize Zn²⁺ flux, whereas the bulk region maintains sufficient ionic conductivity for efficient charge transport. In some systems, additive participation in interfacial reactions further stabilizes the electrode surface by forming a reinforced interphase integrated with the polymer network. Through the combined effects of solvation tuning, selective coordination, and network-dependent mass transport modulation, additive functionalization in hydrogel electrolytes provides a versatile strategy for constructing ion concentration gradients and enhancing interfacial stability in AZBs.

On the basis of this strategy, Liu et al. introduced a zincophilic betaine (BT) additive into a functional hydrogel electrolyte to regulate interfacial chemistry and construct a favourable solid electrolyte interphase^[100]. Owing to the robust interfacial contact established by the in situ sol-gel transition, BT molecules preferentially adsorb onto the Zn surface through their zinc-affinitive -COO groups, whereas the hydrophobic -CH₃ groups orient toward the electrolyte, thereby reconstructing the electric double layer and inducing the formation of a hybrid SEI with an organic-rich outer layer [Figure 6A]. This adsorption configuration displaces water molecules from the inner Helmholtz plane and simultaneously suppresses their replenishment from the outer Helmholtz plane, establishing a water-deficient interface that mitigates hydrogen evolution and other parasitic reactions. Compared with its ex situ-assembled counterpart, the in situ-constructed Zn/gel-BT/Zn symmetric cell exhibits markedly lower interfacial resistance, confirming improved interfacial contact and accelerated Zn²⁺ transport [Figure 6B]. Owing to the regulated SEI formation and homogenized ion flux, the symmetric cell achieves stable cycling for up to 450 h at 5 mA cm^-2 and 10 mAh cm^-2 [Figure 6C]. In Zn-Cu asymmetric cells, the gel BT electrolyte maintains exceptionally high Coulombic efficiency as the current density increases from 1 to 10 mA cm^-2 and preserves ultrahigh reversibility when the current density is switched back to 1 mA cm^-2, whereas cells without BT display pronounced efficiency fluctuations under elevated current densities [Figure 6D]. These results collectively demonstrate that BT-enabled interfacial reconstruction in hydrogel electrolytes effectively suppresses water-induced side reactions and uncontrolled dendrite growth, thereby enhancing the stability and reversibility of Zn plating and stripping.

From a mechanistic perspective, the solvation and desolvation behavior of Zn²⁺ ions plays a critical role in governing interfacial reaction kinetics and deposition morphology. In aqueous electrolytes, Zn²⁺ typically exists in a strongly hydrated coordination environment, most commonly forming octahedral [Zn(H₂O)₆]²⁺ complexes stabilized by hydrogen bonding networks. The desolvation process, which involves partial or complete removal of coordinated water molecules prior to electron transfer, constitutes a key energy barrier that directly influences the nucleation overpotential and charge transfer kinetics at the Zn electrode. This process is highly sensitive to the local coordinated environment and water activity.

The introduction of ion concentration gradients can indirectly modulate Zn²⁺ solvation structures by altering the spatial distribution of ions and solvent molecules near the interface. In regions with elevated Zn²⁺ concentrations, the relative availability of free water molecules decreases, leading to reduced water activity and a higher probability of contact ion pairs or solvent-shared ion configurations^[101]. This shift in the coordination environment facilitates partial desolvation and decreases the energy barrier for Zn²⁺ reduction. Conversely, in bulk-like regions with higher water activity, fully hydrated Zn²⁺ species dominate, supporting efficient long-range ion transport. Such spatial variation establishes a progressive desolvation pathway as Zn²⁺ ions migrate toward the electrode, coupling ion transport with solvation dynamics.

Furthermore, water activity not only governs solvation structure but also influences interfacial side reactions, particularly the hydrogen evolution reaction and hydroxide formation. Reduced water activity near the Zn surface suppresses proton reduction and stabilizes the interfacial pH, thereby mitigating parasitic reactions and surface passivation^[102]. Therefore, effective gradient engineering should consider not only the Zn²⁺ concentration distribution but also the coupled regulation of coordination chemistry and water activity. This integrated perspective provides a more comprehensive understanding of how gradient structures influence interfacial reaction kinetics and highlights the importance of solvation transport coupling in the design of advanced electrolyte systems for AZBs.

In the design of Zn²⁺ concentration gradients, advanced characterization techniques combined with quantitative simulation are indispensable for verifying gradient formation and clarifying how spatial Zn²⁺ distribution regulates interfacial reactions. Electrochemical measurements such as electrochemical impedance spectroscopy, galvanostatic intermittent titration, chronoamperometry, and transference number tests provide key kinetic parameters, including ionic conductivity, charge transfer resistance, Zn²⁺ diffusion coefficients, and Zn²⁺ transference numbers. These experimentally derived parameters can be incorporated into continuum-scale models based on the Nernst-Planck and Poisson equations or finite element simulations to quantitatively reconstruct Zn²⁺ concentration fields and electric potential distributions across the electrolyte and near the electrode interface^[103,104]. It should be clarified that the above two references support the fundamental governing equations and the general modelling framework for metal electrodeposition rather than a specific electrochemical system. Although these studies focus on Li metal systems, the underlying theoretical descriptions, including ion transport, electric field distribution, and interface evolution, are based on general electrochemical principles that are also applicable to Zn deposition. By correlating simulated ion flux profiles with operando or ex situ characterization methods such as in situ optical microscopy, CLSM, Raman mapping, X-ray photoelectron spectroscopy depth profiling, and electron microscopy of deposited Zn, directly linking gradient-regulated ion transport behavior with nucleation thermodynamics and growth morphology becomes possible. This integrated strategy, in which electrochemical data serve as input parameters for ion transport modelling and are cross-validated by spatially resolved structural and compositional analyses, enables a mechanistic understanding of how engineered Zn²⁺ concentration gradients suppress side reactions, homogenize interfacial current distribution, and stabilize Zn plating and stripping processes.

On the basis of the above strategy, Cui et al. designed a viscosity gradient electrolyte configuration by inserting filter paper to spatially divide the electrolyte into a high-viscosity Zn²⁺ ion buffer layer and a low-viscosity bulk transport region, thereby constructing an asymmetric Zn²⁺ concentration regulation pathway^[105]. In this architecture, the high viscosity region adjacent to the Zn anode retards Zn²⁺ migration and alleviates local ion depletion, which homogenizes interfacial Zn²⁺ flux and promotes uniform nucleation, whereas the low viscosity region maintains rapid ion transport and low internal resistance. Simulations of Zn²⁺ concentration profiles after charging at 5 mA cm^-2 for 720 s reveal that the decrease in Zn²⁺ concentration at the cathode surface is nearly identical in both the electrolyte with a Zn²⁺ ion buffer layer and the PVA gel electrolyte, indicating that the thin buffer layer can effectively regulate Zn electrodeposition and induce an ordered Zn²⁺ flux comparable to that of a full gel system [Figure 7A]. Moreover, the Zn²⁺ concentration near the anode in the buffer layer system remains close to that in the liquid electrolyte, confirming its spatially selective regulation behavior, and the opposite side preserves Zn²⁺ conductivity comparable to that of the liquid electrolyte. This asymmetric modulation enables uniform Zn deposition at lower overpotentials and suppresses parasitic reactions. When charged to an areal capacity of 2 mAh cm^-2 at 5 mA cm^-2, Zn deposited in the liquid electrolyte exhibits severe nonuniformity, and pronounced hydrogen bubble accumulation is observed in the gel electrolyte, whereas the viscosity gradient configuration yields dense and uniform Zn without obvious gas evolution, reflecting a stabilized interfacial environment [Figure 7B]. Quantitative analysis of deposition kinetics reveals that the initial deposition area coefficient k_a and the single-step deposition area increase coefficient k_s are lower under viscosity gradient conditions, indicating denser and more reversible Zn growth, whereas the initial Zn²⁺ accumulation coefficient k_m and the hydrogen evolution onset time coefficient p_h are greater, suggesting enhanced Zn²⁺ retention and reduced water activity at the interface [Figure 7C]. The smoother evolution of these parameters further confirms that the constructed Zn²⁺ concentration gradient effectively regulates interfacial reaction dynamics and improves cycling stability.

In addition to reconstructing Zn²⁺ concentration fields through electrochemically derived kinetic parameters and numerical simulations, fluorescence-based visualization techniques offer a direct and spatially resolved approach to probe ion concentration gradients in situ. This strategy relies on fluorescent probes whose emission intensity, wavelength shift, or lifetime are sensitive to the local Zn²⁺ concentration or coordination environment. Upon complexation with Zn²⁺, the electronic structure of the fluorophore is altered, leading to quantifiable optical responses that can be calibrated to the Zn²⁺ concentration. By integrating such probes into electrolytes or hydrogel matrices and monitoring fluorescence signals via CLSM or operando optical imaging, two-dimensional or three-dimensional maps of Zn²⁺ distribution can be constructed across the electrolyte and near the electrode interface^[106,107]. Time-resolved fluorescence imaging further enables the tracking of dynamic concentration evolution during plating and stripping processes, revealing ion depletion, accumulation, and flux heterogeneity under different current densities. When correlated with electrochemical data and modelled ion transport profiles, fluorescence visualization provides complementary validation of gradient formation and clarifies how localized Zn²⁺ enrichment or depletion governs nucleation behavior, interfacial stability, and dendrite growth. This combined optical and electrochemical methodology, therefore, establishes a powerful platform for elucidating the mechanistic role of engineered Zn²⁺ concentration gradients in AZBs.

On the basis of the above mechanism, Chen et al. introduced CLSM to realize operando visualization of ion transport by coupling fluorescence imaging with electrochemical processes in a neutral Zn air battery model system^[108]. A pH-sensitive fluorescent probe was employed to convert local ion transport-induced fluctuations in pH into measurable optical signals, thereby enabling indirect mapping of interfacial ion dynamics [Figure 7D]. After immersion in the ZnSO₄ electrolyte, pronounced fluctuations in the pH with pulsation behavior were observed near the Zn electrode, reflecting severe hydrogen evolution and periodic variations in reaction intensity, whereas a galvanized Zn electrode exhibited much smaller fluctuations in the pH, indicating improved corrosion resistance and suppressed parasitic reactions. At the air electrode, distinct spatial heterogeneity in ion transport was detected during both discharge and charge, and with increasing current density, the dominant transport mode gradually shifted from diffusion-controlled to convection diffusion-coupled behavior, highlighting the increasing contribution of convection to internal ion migration. To establish a quantitative relationship between the fluorescence response and pH, the emission spectra of an electrolyte containing 1 M ZnSO₄ and 500 μM 8-hydroxypyrene 1,3,6-trisulfonic acid trisodium salt were recorded under 458 nm excitation with emission collected from 480 to 580 nm [Figure 7E]. The fluorescence intensity increased monotonically with increasing pH and reached a maximum at approximately 510 nm, providing a calibration basis for confocal imaging. On the basis of this correlation, in situ imaging during charging revealed that the fluorescence intensity near the electrode decreased within the first 50 to 100 s, indicating a transient local pH decrease caused by the diffusion of alkaline species into the bulk electrolyte [Figure 7F]. These results demonstrate that fluorescence-based operando microscopy can be used to quantitatively visualize dynamic ion concentration gradients and elucidate interfacial transport mechanisms in Zn-based electrochemical systems.

Building upon the insights provided by advanced characterization techniques and simulation methods, it becomes possible to move beyond individual case analyses toward a more unified understanding of ion-concentration gradient engineering. By quantitatively resolving Zn²⁺ transport behavior, interfacial reaction kinetics, and spatial concentration distributions, these approaches enable direct comparisons across different gradient design strategies^[109]. Despite variations in material systems and structural configurations, the underlying regulatory mechanisms can be broadly categorized into ion transport modulation, solvation structure tuning, and interfacial electric field homogenization. For example, additive-based systems primarily influence Zn²⁺ coordination environments and electric double layer structures, whereas separator and hydrogel-based systems impose spatial confinement and directional ion pathways, leading to distinct gradient formation modes ranging from interfacial Zn²⁺ enrichment to continuous concentration profiles across the electrolyte^[110].

Importantly, the effectiveness of these strategies can be quantitatively evaluated through key electrochemical and transport parameters extracted from combined experimental and modelling analyses. Metrics such as the Zn²⁺ transference number, nucleation overpotential, interfacial resistance, and chronoamperometric response provide direct insight into the ion flux uniformity and interfacial stability^[111]. Moreover, performance indicators, including critical current density, Coulombic efficiency, and long-term cycling stability, reflect the practical impact of gradient regulation on suppressing dendrite growth and parasitic reactions. These quantitative descriptors establish a direct link between the gradient structure, ion transport behavior, and electrochemical performance. On the basis of these correlations, effective gradient engineering should simultaneously regulate interfacial Zn²⁺ flux while preserving sufficient bulk ionic conductivity, thereby balancing interfacial stability with overall transport efficiency. This integrated understanding, enabled by advanced characterization and simulation, provides general design principles for the rational development of high-performance aqueous Zn-based batteries.

Despite the significant progress achieved with advanced characterization techniques such as fluorescence imaging and CLSM, several limitations remain that should be carefully considered when experimental results are interpreted. First, the spatial resolution of these techniques is fundamentally constrained by optical diffraction limits, which may hinder accurate probing of the nanoscale ion distribution and interfacial structures. Second, the quantitative reliability of fluorescence-based methods depends strongly on probe calibration, local environmental sensitivity, and potential interference from factors such as pH variation, ionic strength, and probe aggregation, which can introduce uncertainties in correlating signal intensity with actual Zn²⁺ concentration. In addition, these techniques typically rely on exogenous probes that may perturb the native electrolyte environment or alter ion transport behavior.

Furthermore, capturing rapid and transient ion transport processes under practical operating conditions remains challenging because of limitations in temporal resolution and signal acquisition speed. As a result, experimentally observed concentration profiles may represent time-averaged or partially evolved states rather than true instantaneous distributions^[90]. To address these challenges, future efforts should focus on integrating high-resolution operando techniques with quantitative calibration strategies and complementary methods such as electrochemical modelling and spectroscopy. Such multimodal approaches are essential for achieving reliable, spatially resolved, and quantitatively accurate characterization of Zn²⁺ concentration gradients and interfacial processes in AZBs.