Recent advances in stabilization strategies for zinc anodes in aqueous zinc-ion batteries

Abstract

Rechargeable aqueous metal-ion batteries are promising alternative energy storage devices in the post-lithium-ion era due to their inherent safety and environmental compatibility. Among them, aqueous zinc ion batteries (AZIBs) stand out as next-generation energy storage systems, offering low cost, high safety, and eco-friendliness. Nevertheless, the instability of Zn metal anodes, manifested as Zn dendrite growth, interfacial side reactions, and hydrogen (H₂) evolution, remains a major obstacle to commercialization. To address these challenges, extensive research has been conducted to understand and mitigate these issues. This review comprehensively summarizes recent advances in Zn anode stabilization strategies, including artificial solid electrolyte interphase (SEI) layers, structural optimization, electrolyte modification, and bioinspired designs. These approaches collectively aim to achieve uniform Zn deposition, suppress parasitic reactions, and enhance cycling stability. Furthermore, it critically evaluates the advantages and feasibility of different strategies, discuss potential synergistic effects of multi-strategy integration, and provide perspectives for future research directions.

Graphical abstract

Keywords

aqueous zinc-ion batteries (AZIBs) / Zn anode / stability / artificial solid electrolyte interphase (SEI) layers / electrolyte modification / bioinspired

1 Introduction

With the rapid growth of the global economy, energy demand continues to rise steadily, intensifying the strain on energy supply-demand balance. Concurrently, extensive combustion of fossil fuels has led to environmental issues such as greenhouse effects and air pollution, profoundly impacting human living conditions [1]. Addressing energy and environmental challenges has become imperative; China has elevated “carbon peaking” and “carbon neutrality” to national policy levels, accelerating the transition to solar, wind, and other clean energy sources. In this context, developing low-cost, high-efficiency energy storage systems (ESSs) is critical for energy efficiency and clean energy advancement [2].

Among electrochemical energy storage technologies, lithium-ion batteries (LIBs), which are widely used in digital consumer products and electric vehicles, dominate the secondary battery market [3]. However, LIB face inherent safety risks because their operating voltage exceeds the thermodynamic stability window of water (1.23 V), necessitating the use of flammable, toxic organic electrolyte [4].

The development of novel of ESS is therefore an urgent problem in the energy storage industry [5]. This problem must be solved with the utmost urgency. To be suitable for large-scale energy storage applications, the batteries should meet the following criteria: high safety, low cost, excellent performance (including multiplication performance as well as cycling performance), and environmental friendliness. Aqueous zinc-ion batteries (AZIBs), as an ESS, possess the characteristics of substitutability, safety, and environmental protection [6]. The aqueous electrolyte not only exhibits high ionic conductivity, but is also non-flammable, thereby fundamentally overcoming the issues of flammability and explosion associated with traditional organic electrolytes [7].

With the advantages of high theoretical capacity (820 mAh/g), low redox potential (−0.76 V versus standard hydrogen electrode), and high natural zinc (Zn) abundance (about 200 times that of lithium resources), AZIBs are widely regarded as the most promising next-generation ESS [8]. AZIBs signify a substantial advancement in the domain of renewable energy storage technology [9]. These batteries are distinguished by their environmental friendliness, safety, cost-effectiveness, and non-combustible properties, thereby classifying them as high-security electrochemical energy storage devices.

Despite the long history of zinc-ion battery development, dating back to the invention of the voltaic pile in the early 18th century and the pioneering work of Daniels in 1836, the energy density remains relatively low, limiting their application to low-power electronic devices. Nevertheless, AZIBs have garnered attention due to their inherent safety advantages [10]. MnO₂, with its abundant resources, low toxicity, high discharge potential, and large theoretical capacity, is considered one of the most promising cathode materials for AZIBs [11].

As illustrated in Fig.1, Zn-based battery technology has evolved significantly since the introduction of the Zn-copper battery in 1883 [12], marking one of the earliest developments in this field. Subsequent innovations, including Zn-air [13], Zn-bromine hybrid [14], and zinc-iodine batteries [18], have substantially advanced the technology. The incorporation of iodine as a positive active material enables reversible redox reactions at the Zn anode, offering high theoretical energy density and potential applications in distributed energy storage systems. Most recently, the 2023 development of self-decoupled mild-acidic Zn-air battery [19] has further expanded the potential of Zn-based battery technologies.

Zn-manganese dry batteries, alkaline batteries, and other metal Zn-based batteries, with their long history in electrochemical energy storage, remain important due to their stable discharge performance. In AZIBs, the anode material critically determines redox efficiency [20], which directly influence first-cycle efficiency and cycling stability. The rationale behind the selection of Zn metal as the anode material for AZIBs is multifaceted. First, Zn is an abundant and widely available mineral resource. Next, it is easily processed into electrode materials. Finally, Zn-manganese dry batteries, alkaline batteries and other Zn-based batteries have been commercially available for many years. The Zn-based anode industry has been relatively mature for some time, and it is therefore logical to conclude that Zn metal is the optimal choice for AZIBs [21]. Rechargeable AZIBs, with their inherent safety, environmental compatibility, and cost-effectiveness, offer a compelling solution for next-generation energy storage [22].

Recent advances in zinc battery technology and high-performance electrode materials have strengthened the foundation for large-scale energy storage and renewable energy integration. AZIBs are typically composed of a layered metal oxide cathode, Zn metal anode, aqueous electrolyte, and separator. Notably, Zn metal’s air stability allows battery assembly in the air inert atmosphere protection. The working mechanism of AZIBs relies on the loss of electrons from Zn metal during the charging and discharging processes, which results in the production of Zn²⁺. The gain of electrons from Zn ions during charging enables the reduction of these ions to Zn metal. The reaction of the Zn anode in a near-neutral aqueous system can be expressed as

Discharge (anode):

(1)\mathrm{Z}\mathrm{n}\to {\mathrm{Z}\mathrm{n}}^{2+}+2{\mathrm{e}}^{-}

Charge (anode):

(2){\mathrm{Z}\mathrm{n}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{Z}\mathrm{n}

It is imperative to note that Zn anodes necessitate a substantial quantity of Zn²⁺ during the charging process. Consequently, the electrolyte of AZIBs must comprise one or more soluble Zn salts, thereby ensuring the provision of a copious amount of Zn²⁺ for the anode reaction. The common Zn salts worthy of note are: ZnCl_2, Zn(NO₃)₂, ZnSO₄, and Zn trifluoromethanesulfonate (Zn(OTF)₂) (Zn(CF₃SO₃)₂). Among these, Zn(OTF)₂ or ZnSO₄ electrolyte is notable for its neutral or weakly acidic nature, which confers enhanced stability and corrosion resistance on the battery case, making them the most widely adopted in practice [23].

2 Challenges for Zn anodes

Despite their cost competitiveness and growing market adoption, ZIBs still face challenges such as dendrite formation, lower energy density and limited cycle life. A primary bottleneck lies in the interface between Zn anode and aqueous electrolytes, which severely compromises the electrochemical performance of commercially available Zn metal anode in practical applications [24].

In general, the main issues associated with Zn anodes in alkaline electrolytes can be classified into three main categories: corrosion passivation, Zn dendrite growth, and hydrogen precipitation corrosion. This section systematically examines these failure modes and overviews current strategies for Zn anode modification [25], providing a roadmap for overcoming these barriers to commercialization.

The dynamic coupling mechanism between the electrochemical behavior of the Zn anode interface and the side reactions is revealed in Fig.2. The reorganization of the solvent layer and the desolvation process of Zn²⁺ are visualized by the dynamic changes of the ligand water molecules: the stable ligand structure of the [Zn(H₂O)₆]²⁺ at the initial stage is progressively lost by the electric field to form a desolvated transition state, releasing active water molecules to participate in the hydrogen precipitation reaction through proton transfer. The hydrogen precipitation reaction is facilitated by the participation of the chemical transition state and the active water molecules that are released. This process involves a proton transfer mechanism.

Concurrently, the competitive decomposition of {\mathrm{S}\mathrm{O}}_4^{2-} with Zn²⁺ instigates the corrosion reaction, which engenders corrosion products that nucleate preferentially at defects. This, in turn, results in the anomalous growth of dendrites in regions of current density concentration. The problem encountered in the Zn anode can be summarized as follows: ① the increased desolvation energy barrier exacerbates the local ion concentration gradient and promotes dendrite tip growth; ② the hydrogen bubbles generated by the hydrogen evolution reaction (HER) hinder uniform zinc deposition and accelerate porous by-product accumulation; ③ the insulating properties of corrosion products increase interfacial impedance, creating a vicious cycle.

2.1 Zn dendrite growth

As with lithium metal batteries, AZIBs are confronted with the serious problem of dendrite growth. The formation of dendrites on the Zn anode surface may puncture the separator and establish direct contact with the cathode, potentially causing short-circuiting and battery failure. A stable Zn anode necessitates uniform Zn²⁺ deposition and dissolution. However, similar to Li⁺/Na⁺ and other metal ions, Zn²⁺ at the Zn anode surface exhibits uneven ionic flux distribution, particularly evident in electric bilayer formation. The spontaneous 2D diffusion of Zn²⁺ results electrolyte Zn²⁺ aggregation on the anode surface, forming low-barrier nucleation sites. This leads to the accumulation of Zn²⁺ in the electrolyte on the surface of the Zn anode, ultimately causing dendritic and inhomogeneous deposition morphologies of Zn²⁺ [26,27].

In particular, Zn²⁺ tend to deposit at energetically favorable charge transfer sites on the anode, forming initial minor protuberances. To reduce surface energy, subsequent Zn²⁺ preferentially deposits on these minor protuberances, causing them to grow into dendrites.

Scanning electron microscopy reveals remarkable multi-scale morphological evolution of the Zn anode surface (Fig.3(a)) [28]. During initial nucleation, irregular lamellar dendrites form, with their smooth surfaces and sharp edges indicating a laminar growth mechanism dominated by ion diffusion during the self-assembly process. The hydration environment (Fig.3(b)) [29] promotes radial alignment of rod-like crystals (~5 μm) along the radial direction, showing clear anisotropic growth characteristics attributed to the intermediate phase-regulated interfacial energies formed through the participation of water molecules.

Fig.3(c) [30] demonstrates that dendritic dendrite formation is characterized by a higher curvature of the protruding dendrites in comparison to the surrounding smooth regions. This phenomenon can be attributed to the “tip effect,” which engenders a comparatively substantial electric field within the elevated dendrites. This electric field exerts an attraction on Zn²⁺, resulting in their accumulation and subsequent deposition within the branch crystals. Subsequent observation reveals that the plume-like branching structure (Fig.3(d)) [30] manifests a stepped texture at the 3 μm scale, consistent with the growth pattern of helical dislocations caused by surface energy anisotropy. These features reflect the dynamic coupling between ion concentration gradients, electric field distribution, and mechanical stress fields during Zn deposition.

As illustrated in Fig.4(a) [31], Zn exhibits characteristic polyhedral crystal aggregates at the 10 μm scale after electroplating. The dendrites manifest a pronounced dendritic structure, characterized by interspersed branches that exhibit a lack of orderliness. The radius of curvature at the tip is less than 1 μm. Cross-sectional analysis reveals vertically oriented dendritic growth from the substrate, accompanied by localized porosity, indicating non-uniform Zn deposition during plating. This dendritic morphology strongly corelates with the “tip effect,” where high-curvature regions enhance local electric fields, promoting preferential Zn²⁺ deposition and forming a dynamic “growth-dissolution” cycle. Such non-uniform deposition leads to dendrite detachment from the substrate, forming electrochemically inactive “dead zinc,” that reduces Coulombic efficiency (CE) by over 30%.

Fig.4(b) [32] demonstrates that the initial charging phase involves layered dendrites growth, which subsequently evolves into three-dimensional expansion. The accelerated growth rate at dendrite tips indicates a selective expansion of dendrites along grain boundary regions. As shown in Fig.4(c) [33], the In-containing alloy electrode develops a lamellar crystal structure after plating, where jagged interlamellar interfaces between the lamellae effectively suppress lateral dendrite propagation. Alloying modifies Zn crystal growth kinetics by reducing surface energy anisotropy and inhibiting dislocation-dominated dendrite growth. Furthermore, alloying decreases the Zn²⁺diffusion coefficient by two orders of magnitude, forcing uniform ion deposition along interlamellar gaps. This results in lamellar interstitials for uniform deposition.

In Fig.4(d) [34], the Zn anode surface exhibits a nanospike structure, without forming a continuous dendrite network. Surface remodeling generates a passivation layer that reduces surface reactivity and modifies crystal structure, which, in turn, induces the uniform deposition of Zn ions along the axial direction of the spikes.

2.2 Zn self-corrosion and hydrogen evolution

The corrosion of Zn metal anodes has the potential to reduce the utilization rate of the anodes and affect the electrochemical performance of the batteries, which represents a significant challenge in the field of anode research. The thermodynamic activity of Zn metal in an aqueous electrolyte renders the Zn/electrolyte interface susceptible to a range of side reactions, the most significant of which are Zn anode self-corrosion and hydrogen evolution. The self-corrosion of the Zn anode is primarily attributable to the weak acidity of aqueous electrolytes, such as ZnSO₄. During the battery shelving stage, the H⁺ ion reacts chemically with the Zn anode, resulting in the consumption of anode active substance and a consequent reduction in battery capacity.

Hydrogen evolution usually occurs during high-rate charging, which is one of the key indicators of battery performance. Under high rate charging and discharging conditions, the anodes is subjected to high current densities. For example, when a current density of 24 mA/cm² is applied to a bare Zn plate, the Zn²⁺ on the cathode surface is rapidly depleted due to the high current density. However, the limited diffusion rate of Zn²⁺ from the electrolyte to the anode surface results in a localized Zn²⁺ concentration that is too low, which produces an overpotential and contributes to a preferential hydrogen evolution reaction. Although the Zn anode has a high overpotential for hydrogen evolution and can reduce the rate of the reaction, continuous hydrogen precipitation at a low rate still leads to an increase in the internal pressure of the cell and a decrease in the CE. In addition, hydrogen evolution also occurs during the cell shelving phase, affecting cell performance.

During the discharge process, the Zn metal anode undergoes a transition from metallic Zn (Zn⁰) to Zn ions (Zn²⁺), resulting in a relatively large concentration of Zn ions in the electrolyte near the anode, which is locally net positively charged. This attracts hydroxyl ions (\mathrm{O}{\mathrm{H}}^{-}) and anions (e.g., {\mathrm{S}\mathrm{O}}_4^{2-}, \mathrm{C}{\mathrm{l}}^{-}, etc.) in the electrolyte to undergo complexation reactions, resulting in the production of electrochemical corrosion products such as Zn hydroxysulfate, Zn hydroxide, and Zn oxide. These corrosion by-products typically exhibit low solubility and poor electrical conductivity within the electrolyte. When they reach a state of supersaturation and are deposited on the anode surface, they cover the reactive sites, impeding electron and ion transport and ultimately leading to the passivation of the anode. Furthermore, this irreversible corrosion and passivation also result in electrolyte depletion, which has an additional detrimental impact on battery performance.

Specifically, when the Zn electrode contacts with the electrolyte, the Zn anode will form several miniature protocells with solid impurities in the electrolyte or anode. This primary cell can react during the shelving phase of the cell, leading to H₂ precipitation and an increase in OH^- concentration, which can also lead to the production of passivation layers such as {\mathrm{Z}\mathrm{n}}_4{\mathrm{S}\mathrm{O}}_4{\left(\mathrm{O}\mathrm{H}\right)}^{6-}x{\mathrm{H}}_2\mathrm{O} [35]. The uninterrupted side reactions not only continue to damage the anode interface, leading to an increase in the overpotential voltage, but also continue to consume the active mass of the Zn anode and the electrolyte. This further leads to a decrease in battery capacity. Concurrently, the \mathrm{Z}{\mathrm{n}}_4\mathrm{S}{\mathrm{O}}_4{\left(\mathrm{O}\mathrm{H}\right)}^{6-}x{\mathrm{H}}_2\mathrm{O} passivation layer formation and H₂ precipitation will also result in gradual Zn²⁺ diffusion from the electrolyte to the Zn anode surface, leading to an increase in Zn anode volume [36].

This process is a competitive reaction with ZnO reduction during Zn deposition in an alkaline electrolyte. The hydrogen evolution reaction can be expressed as

(3)\mathrm{Z}\mathrm{n}(\mathrm{s})+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2\uparrow \uparrow +{\mathrm{Z}\mathrm{n}(\mathrm{O}\mathrm{H})}_2

In summary, the self-corrosion and hydrogen evolution reaction occur in the shelving stage and the charging/discharging stage, which seriously affects the charging performance and the electrochemical performance of AZIBs. Therefore, the problem of Zn self-corrosion and hydrogen evolution must be solved in order to achieve the practical application and commercial development of high-performance AZIBs.

3 Zn anode protection strategy

As mentioned above, Zn dendrite growth, Zn self-corrosion, and hydrogen evolution are the main factors affecting the performance of AZIBs. So far, researchers have made great efforts to develop various strategies to mitigate the above challenging problems. From the point of view of Zn anode protection, the general strategies can be divided into main categories: construction of artificial solid electrolyte interphase (SEI) layer, electrolyte additives, bioinspired strategies, and structural optimization strategies, as described below.

3.1 Construction of an artificial SEI layer

The construction of an artificial SEI layer is a common method of creating a physical protection layer [37]. The artificial SEI layers can effectively isolate the Zn anode from direct contact with the electrolyte, thus inhibiting the parasitic reactions. In addition, the artificial SEI layers usually have abundant Zn²⁺ binding sites, which facilitate the uniform deposition of Zn²⁺ and suppress dendrite formation. By stabilizing the dense SEI film, it can inhibit Zn anode self-corrosion and hydrogen evolution.

In contrast to LIBs, which are capable of self-passivation through corrosion [38], a dense SEI film cannot be spontaneously generated on the Zn anode surface in typical aqueous electrolytes. Although \mathrm{Z}{\mathrm{n}}_4\mathrm{S}{\mathrm{O}}_4{\left(\mathrm{O}\mathrm{H}\right)}^{6-}x{\mathrm{H}}_2\mathrm{O} can be formed at the Zn/electrolyte (ZnSO₄ electrolyte system) interface, it cannot prevent the electrolyte from contacting the anode surface to inhibit side reactions through a dense SEI layer as in the case of the graphite anode of LIBs [39]. Therefore, the Zn metal will continue reacting with the electrolyte until the Zn metal or electrolyte is completely consumed.

In this case, artificially constructing a dense and stable SEI film is an effective strategy to enhance the electrochemical performance of the Zn anode, and the performance of the SEI film itself also determines the performance of the Zn anode. Based on the composition, these artificial SEI films can be classified into three main categories: inorganic, organic, and inorganic/organic composite protective layers.

Inorganic SEI membranes are mainly composed of inorganic materials such as carbonates and metal oxides. Liu et al. [40] developed a hydrophobic, dense, and fast Zn²⁺ conductor-type inorganic SEI membrane, which solved Zn dendrite growth and inhibits water erosion simultaneously, thus significantly enhancing the reversibility and stability of Zn anodes.

The inorganic SEI film consisted of Zn ferrocyanide (ZnHCF), whose formation process was based on the “etching-nucleation-growth” mechanism shown in Fig.5. In a weakly acidic aqueous solution of 1 mol/L K₃[Fe(CN)₆] (pH~3.0), Zn²⁺ precipitated from the metallic Zn surface and instantly complexed with \mathrm{F}\mathrm{e}{\left(\mathrm{C}\mathrm{N}\right)}_6^{3-} anion ligand. The Zn²⁺ nucleated on the Zn surface and gradually grew into a dense and homogeneous ZnHCF film within 10 min.

The inorganic SEI membrane prepared by this method has a thickness of only 1 μm and exhibits high hydrophobicity. Theoretical computational simulations and electrochemical experimental characterization demonstrated that the hydrophobic and dense inorganic SEI membrane (HB-ZnHCF) can inhibit the passage of water molecules and prevent the accumulation of free water molecules, which can otherwise lead to hydrogen evolution problems.

Furthermore, the crystal structure of HB-ZnHCF contained a multitude of ion channels (with a radii up to 5 Å) and the ‒CN group was highly Znophilic, which enhanced Zn²⁺ diffusion kinetics (Zn²⁺ migration number up to 0.86). Additionally, HB-ZnHCF could homogenize the electric field distribution of the Zn anode and reduce interfacial concentration polarization, thereby facilitating dendrite-free Zn deposition.

In addition to the HB-ZnHCF protective layer, numerous inorganic SEI protective layers have been widely employed, including TiO₂ [41,42], ZnO [43], ZnS [44], BaTiO₃ [45], ZrO₂ [46], among others. In 2018, an inorganic protective layer composed of nanoporous CaCO₃ was reported. This was achieved by uniformly coating Zn foil with CaCO₃ using a scraper method, forming a CaCO₃ SEI film that facilitated relatively uniform electrolyte flux and galvanic deposition rate. The nano-CaCO₃ protective layer, characterized by high porosity, allowed aqueous electrolyte penetration while its electrically insulating properties directed Zn²⁺ deposition between the CaCO₃ layer and Zn surface. This uniform deposition effectively prevented Zn dendrite formation and growth, thereby extending the cycle life.

The artificially constructed SEI layer’s density and stability prevent direct electrolyte-Zn anode contact, effectively suppressing Zn self-corrosion and hydrogen evolution reactions. Consequently, the Zn anode achieves high CE and prolonged cycle life. However, inorganic materials possess inherent limitations, such as high brittleness and susceptibility to fracture during bending or under significant Zn anode volume changes.

Nanoyang’s group at Tianjin University [47] proposed a dynamic adaptive hydrophobic interface controlled by electric field-triggered ionic valves as shown in Fig.6(a), addressing the divergent interfacial water requirements during Zn deposition/stripping. The system employs amphiphilic quaternary ammonium surfactants as ionic valves, whose arrangement and density change according to different electric field directions, enabling spontaneous formation and dissolution of hydrophobic interfaces to regulate water channel permeability. After optimizing interfacial hydrophobicity and Zn²⁺ transport balance, octyltrimethylammonium bromide (C₈TAB) was selected as the ionic valve to stabilize the Zn anode/electrolyte interface to achieve highly stable and reversible cycling, a novel approach for aqueous AZIB development.

The CnTA⁺ ionic valve undergoes electrostatic adsorption under anode bias and reversible desorption under positive bias, creating hydrophobic interfaces during Zn deposition that disappear during stripping. Adjusting the CnTA⁺ hydrophobic carbon chain length balances interfacial hydrophobicity and [Zn(H₂O)_n]²⁺ transport, simultaneously suppressing side reaction and inducing uniform Zn deposition. The C₈TA⁺ system significantly improves Zn anode reversibility, enabling stable cycling for more than 2500 h with a CE of 99.8%, while full cells maintain 85% capacity retention over 1000 cycles.

Data-driven material design provides efficient SEI optimization pathways. For example, Fitz et al. [12] employed machine learning to screen 168000 MOF structures to develop a cerium-ferric bimetallic MOF (Ce‒Fe MOF), whose ion screening channels and zinc affinity sites (‒CN groups) synergistically reduced Zn²⁺ desolvation energy barriers to 0.32 eV, enabling Zn symmetric batteries with more than 4300 h of cycle life. This “high-throughput computing + experimental validation” model significantly accelerates the discovery of highly stable SEI materials, providing an efficient approach for precise design of complex interfacial layers.

Organic artificial SEI layers represent another research focus for Zn anode protection due to their ability to precisely regulate interfacial electrochemical behavior. A hydrophobic protective layer constructed via in situ complexation reactions [48] utilized the long carbon chain of octadecylphosphate (OPA) to block water molecules while guiding the uniform deposition through the strong coordination of the phosphate group with Zn²⁺. This interfacial layer enabled symmetric batteries to cycle for more than 4000 h at 2 mA/cm² and 2 mAh/cm², with full cells retaining 85% capacity after 1000 cycles at 5 A/g. The core mechanism is to inhibit the hydrogen precipitation reaction while maintaining the rapid migration of Zn²⁺ by regulating the balance between interfacial hydrophobicity and ionic conductivity, which provides a paradigm for the dynamic response design of organic SEIs.

Simultaneously, an artificial protective layer based on organic polyamide (PA) was constructed on Zn metal Fig.6(b) [49]. The unique hydrogen bonding network and strong coordination with Zn²⁺ within the PA layer not only accelerated the Zn²⁺ migration kinetics and guided homogeneous nucleation of Zn²⁺, but also exploited the hydrophobicity of PA to effectively prevent aqueous electrolyte-Zn anode contact. These advantages enabled the first demonstration of dendrite-free, highly reversible Zn deposition at a high area capacity of 10 mAh/cm².

Polyvinyl butyral (PVB) resin represents another viable artificial SEI material, particularly suitable for applications requiring strong bonding, optical transparency, multi-surface adhesion, durability, and flexibility [52]. The long carbon chains and oxygen-containing functional groups in PVB facilitate Zn²⁺ diffusion through the SEI film. Moreover, PVB exhibits excellent adhesion and mechanical flexibility when interfaced with Zn-metal substrates, effectively suppressing side reactions and dendritic crystal growth. Remarkably, the Zn metal surface maintains its smooth and uniform morphology even after 400 cycles.

Various other organic materials have been explored for AZIB anode protection, including polyimide [53], β-phase poly(vinylidene difluoride) [54], polyacrylonitrile [55], and commercial cyanoacrylates [56]. In summary, organic SEI protective layers can modulate Zn²⁺ diffusion kinetics through their abundant functional groups, thereby inhibiting Zn dendrite formation. Additionally, these organic SEI layers provide complete coverage of the entire Zn metal surface, effectively preventing direct electrolyte-anode contact. However, organic materials often suffer from limited mechanical strength, necessitating careful evaluation of mechanical properties when selecting potential organic SEI components.

In this study Fig.6(c) [50], a 20-minute material surface modification (AS) by was explored to significantly improve properties relevant to artificial SEI construction in AZIBs. As demonstrated in Fig.6, the initial AS surface structure underwent substantial modification after 20 min of treatment, exhibiting increased homogeneity and density. This transformation primarily resulted from the introduction and diffusion of hydrogen ion (H⁺).

For AZIBs, the construction of an artificial SEI layer is of paramount importance for enhancing the performance and stability of the battery. Conventional AZIBs experience Zn²⁺ deposition and stripping during the charging and discharging process, often resulting in structural damage and side reactions of anode surface. In contrast, brief surface treatment can create a uniform, dense artificial SEI layer on the anode surface by promoting closer atomic packing in the surface structure, thereby reducing defects and voids, and enhancing the overall material strength and stability. The introduction of hydrogen ions (H⁺⁾ not only promote the rearrangement of surface atoms but also to interact with Zn ions to form a stable SEI layer. This artificial SEI layer effectively suppresses side reactions while enhancing the transport efficiency of Zn ions, thereby improving the cycling stability and capacity retention of the battery.

To further enhance the performance and stability of the battery, various UiO-67 material sizes (B-10, B-15, and B-20) was evaluated (Fig.6(d)) [51]. Comparative analysis revealed significant differences in battery performance. UiO-67, as an advanced material, forms a stable SEI layer that improves the embedding and detachment process of Zn ions, enhancing cycle life and energy density of the battery. Precise optimization of UiO-67 size and structure holds great potential for enhancing the performance of AZIBs, thereby facilitating their widespread adoption in practical applications.

Combining the advantages of inorganic and organic components, the organic‒inorganic composite protective layer has emerged as a major research hotspot, Chen et al. [57] developed a PVA@SR-ZnMoO₄ coating with a SEI-like structure (SR representing {\mathrm{S}\mathrm{O}}_4^{2-} receptor), which effectively stabilizes the Zn anode (Fig.7). The flexible PVA@SR outer layer enhances the flexibility of the inorganic component and improves the stability of coating during high-capacity cycling. The ZnMoO₄ of the inorganic component effectively suppresses dendrite growth and side reactions, while its interaction with PVA creates a fast migration pathway and promotes Zn²⁺ desolvation.

In addition to these, robust inorganic-organic ZnF₂-Zn₅(CO₃)₂(OH)₆ bilayer SEIs [58] and Nafion-Zn-X [59] are also widely used for constructing artificial SEI layers. In these organic‒inorganic composite artificial SEIs, the organic component enhances flexibility, prevents cracking during volume changes, and facilitates Zn²⁺ migration, while the inorganic component removes solvated water, inhibits water decomposition, and prevents direct Zn-water contact while allowing Zn²⁺ transport.

In their quest for sustainable SEI materials, Wang et al. [60] developed a low-cost artificial SEI layer from industrial by-product phosphogypsum (PG). The CaSO₄·2H₂O in PG forms a porous layer on the zinc surface through an “etch-nucleation-growth” mechanism. This layered structure provides fast Zn²⁺ transport channel and releases Ca²⁺ that preferentially adsorbs at dendrite tips, forcing Zn²⁺ to nucleate uniformly in the inert regions. This design enables the anode to reach a CE of 99.5% after 500 cycles at 1 mA/cm² and over 1000 h of stable cycling, offering a new path for the high-value utilization of industrial solid waste.

Therefore, the composite SEI layer combines the advantages of organic and inorganic materials to enhance Zn anode performance. Based on these artificial SEI studies, an optimal SEI layer should meet these requirements:

1) Chemical stability: The composition of the artificial SEI film must be chemically stable in the aqueous electrolyte, which means the artificial SEI film cannot dissolve in or react with the electrolyte.

2) Complete coverage: The artificial SEI film must completely cover the surface of the Zn anode to prevent electrolyte contact and subsequent side reactions (self-corrosion and hydrogen evolution).

3) Selective conductivity: The artificial SEI membrane must have high Zn²⁺ and low electronic conductivities. Once the protective layer covers the Zn metal surface, Zn²⁺ and electron transfer pathways should be considered. High ionic conductivity ensures fast migration kinetics of Zn²⁺ between the protective layers and fast conduction of Zn²⁺ thus reducing polarization; low electronic conductivity ensures deposition of Zn²⁺ underneath the protective layer.

4) Strong adhesion: The artificial SEI film must maintain tight adhesion to the Zn anode. Under the stripping/plating condition of 1 mAh/cm², the average thickness variation of the Zn anode is about 1.7 µm, which poses a great challenge to the stability of the SEI layer. Therefore, a tight adhesion of the SEI layer to the Zn anodes during stripping/plating is highly desired.

5) Mechanical flexibility: The SEI layer must have high mechanical flexibility to ensure its integrity under repeated volume changes during long-term cycling.

Fig.8(a) illustrates a ZIF-8 of metal organic framework (MOF) artificial SEI layer, demonstrating a molecular structure containing Zn, C, N, and H elements with structural features that can provide a basis for the construction of stable and dense SEI membranes [31]. The abundant microporous structure in MOFs can achieve solvent removal of hydrated Zn²⁺ and uniform deposition of Zn²⁺. In addition, ionic covalent organic frameworks (iCOFs) materials also show promise for constructing SEI.

Fig.8(b) compares bare Zn and Zn@iCOF-ED deposition surfaces during the ion deposition process, suggesting that Zn@iCOF-ED may form an artificial SEI layer with the role of modulating the ion deposition, and that this SEI layer better stabilizes the surface of the Zn anode and reduces the undesirable reaction with the electrolyte [28]. Fig.8(c) depicts ion-trapping structures with tentacles, including processes such as rapid desolvation, directed ion channeling, and controlled deposition, demonstrating the mechanism of action of an artificial SEI layer with a special function on Cu electrodes, which may help to achieve a more homogeneous and stable ionic transport to enhance the performance and stability of Zn anodes. This mechanism may help realize a more uniform and stable ion transport, thus improving the performance and stability of Zn anode [29].

These different types of artificial SEI layer structures and mechanisms of action provide diverse ideas and methods for constructing efficient Zn anode protection, which can help solve the problems faced by Zn anodes in aqueous electrolytes and improve the overall performance of Zn batteries.

Constructing artificial SEI layers using inorganic, organic, or composite materials is an effective strategy to modulate Zn²⁺ deposition kinetics. Frontiers include entropy-driven interfacial modulation, such as porous structures of MOFs/iCOFs to induce uniform nucleation, and machine learning-assisted design to screen MOF structures for optimized SEI conductivity. Reconstruction of the Zn²⁺ solvation shell layer via hydrogen bonding, electrostatic interactions, and reactive additives, combined with high-concentration “water-in-salt” electrolytes or pH buffer system to inhibit side reactions. Emerging directions include dynamic electrolyte flow field design to mimic biofluidic transport and enhance ionic homogeneity, alongside high-entropy electrolytes for multi-component ion synergy to suppress dendrites. Some biomimetic strategies mentioned above include mimicking bio-adhesion, ion chelation, and low-temperature anti-freezing mechanisms. Meanwhile, biomimetic nanochannels and bio-based composite SEIs represent further frontier explorations. Structural optimization through developing 3D host frameworks, gradient pore structures, and composite anodes is promising. However, the emerging approaches like self-healing metal-organic frameworks embedded in 3D structures and dynamic covalent bonding for post-dendrite self-healing after dendrimer penetration prove more effective. Strategies are evolving from single modification to “structure-electrolyte-SEI” synergy. Future research will focus on entropy-driven interface design and AI-driven material discovery integration to promote the breakthrough of Zn batteries toward high energy density, wide temperature tolerance, and long cycle life.

In the performance optimization of symmetric batteries (Zn||Zn), multiple Zn anode modulation strategies precisely control deposition behavior across different dimensions to drive performance improvements. As observed from the time, current density, and areal capacity data in Fig.9, the 3D porous structure constructs interconnected ion transfer channels and provides uniform deposition sites for Zn²⁺, addressing dendrites and swelling issues through spatial design. The interfacial modification layer, physically or chemically controlled, limits the dendrite length to 5 μm [41], alleviating deposition non-uniformity. Alloying modification and single-atom site design optimize nucleation at the atomic scale and improve deposition kinetics. The gradient structure design modulates Zn²⁺ migration gradient and suppress vertical dendrite growth.

These strategies synergize across spatial structure, interface chemistry, atomic modulation, electrolyte design, and crystal orientation to enhance CE, extend cycle life, inhibit dendrite growth, and control volume expansion, enabling stable symmetric battery operation. The core logic involves regulating the dynamics and thermodynamics of zinc deposition, breaking the dendrite-expansion-hydrogen removal cycle, and achieving uniform deposition, stable cycling, and efficient utilization. This systematic approach provides a systematic technological path for Zn anode design. The data in Fig.9, including time, current density, and areal capacity for each strategy, clearly illustrate performance variations under different working conditions.

In the performance optimization of full batteries, the synergistic effect of various zinc anode strategies with cathode materials and electrolyte demonstrates significant enhancement in the dimensions of capacity, cycle stability, multiplicity, and environmental adaptability. As observed from the 3D data (capacity retention, test condition, cycle number) in Fig.10, through the fine regulation of anode structure, combined with the appropriate electrolyte optimization, the full battery achieves performance breakthroughs in multiple scenarios:

At the capacity and cycle level, it achieves an optimal balance between high capacity and long lifespan while demonstrating exceptional cycle stability. The FeHCF-related strategy [40] enables the I₂ full battery to cycle 10000 times at high rates, which verifies excellent fast charging and discharging capability. Meanwhile, the hydrogel electrolyte empowers the VO₂ full battery to work across a wide temperature range from −10 to 40 °C, breaking through the temperature limitations of traditional aqueous batteries [45].

The core logic of these optimizations lies in improving the deposition uniformity and stability of the zinc anode and reducing the interfacial impedance through the anode structure design.

Both high-capacity anode and high-rate systems have achieved system-adapted performance enhancement through systematic synergies, providing multi-dimensional technical support for AZIB commercialization, demonstrating a systematic breakthrough from material innovation to system integration.

In the performance optimization of AZIBs, different Zn anode strategies not only significantly improve the stability and CE of symmetric batteries through multi-dimensional synergy, achieving thousands to tens of thousands of cycles, but also form highly efficient synergies with selected cathode materials and electrolytes in the whole battery system to enable comprehensive performance breakthroughs. These advances provide systematic solutions from material innovation to system integration for the practical application of AZIBs in energy storage, low temperature operation, and other challenging scenarios.

3.2 Electrolyte modification

As an additional crucial component of the interfacial phase, the additive interacts with Zn²⁺ and actively participates in the desolvation of hydrated Zn²⁺. This interaction reduces the concentration of active water during deposition, thereby effectively suppressing side reactions. Electrolyte additives are defined as small amounts of substances added to the electrolyte with the purpose of improving the electrochemical properties of the electrolyte and enhancing the quality of cathode deposition.

According to the mechanism of action of the additives, electrolyte additives can be categorized into hydrogen bonding additives, electrostatic additives, and reactive additives. These types of electrolyte additives are considered as the most convenient and economical way to improve interfacial Zn reversibility.

Hydrogen bonding type additives enhance the stability of Zn ions by forming hydrogen bonds with them, optimizing the deposition location and reducing dendrite growth. For example, compounds containing amino or carboxyl groups improve the uniformity of the deposited layer by forming hydrogen bonds with Zn ions [62]. Nonionic surfactants such as Tween-80 provide a new direction for electrolyte modification. Its hydroxyl and ether groups can replace water molecules in the solvated sheath of Zn²⁺, reconfiguring the hydrogen bonding network and inhibiting the participation of active water in the hydrogen precipitation reaction. Meanwhile, the hydrophobicity of the long carbon chains forms a physical barrier on Zn surface, enabling the symmetric cell to cycle stably for over 4000 h at 1 mA/cm², and the capacity retention rate of the full cell after 1000 cycles is more than 80%. This dual mechanism of “solvation modulation + interface isolation” demonstrates the synergistic effect of surfactants in inhibiting dendrite and corrosion [66].

Electrostatic additives, on the other hand, inhibit Zn dendrite growth by forming a positively charged electrostatic shielding layer on Zn anode surface and utilizing the electrostatic shielding effect. For example, the addition of Na⁺ ions from the ZnSO₄ electrolyte effectively inhibited Zn dendrite generation through the electrostatic shielding effect [63].

Reactive additives react chemically with Zn ions to form a stable SEI layer, thus inhibiting dendrite growth and side reactions. For example, some polymer or inorganic salt-based additives reduced side reactions and dendrite growth by reacting with Zn ions to produce insoluble products that cover Zn anode surface [64–66]. Although various electrolyte additives have been proposed to solve the issues of Zn anodes and improve battery performance, common additives only regulate deposited Zn morphology but do not effectively protect it from contacting with the aqueous electrolyte. Therefore, to optimize the battery performance to meet the market demand, different strategies need to be combined to effectively mitigate all the drawbacks of metallic Zn anodes.

In addition to additives, the electrolyte system design optimizes the interfacial behavior at the molecule level by adjusting the solvent composition and ionic concentration, mainly including:

1) Hydrogel-based electrolyte: The structure of hydrogel-based electrolyte is characterized with a polymer network (e.g., polyacrylamide PAM, sodium alginate) that immobilizes water molecules to form a quasi-solid system, combining ionic conductivity and mechanical flexibility. In terms of physical barrier, the gradient pore structure of Janus hydrogel inhibits both Zn dendrites (penetration depth < 5 μm) and anodic dissolution (90% reduction in MnO₂ dissolution) [67]. It can also be chemically adapted, such as the double crosslinked cellulose hydrogel (DCZ-gel) developed by Wuhan University, which releases ligand water through hydroxyl complexation with Zn²⁺ to enable the symmetric cell to cycle for 2000 h at 0.5 mA/cm², with Zn surface roughness only 1/5 that of liquid electrolytes [68].

2) High concentration of electrolyte: The presence of elevated salt concentrations gives rise to the formation of a “water-in-salt” system, wherein the solvated sheath layer of Zn²⁺ is predominantly characterized by OTF^‒ and free water. This results in the binding of water, thereby impeding both HER and zinc corrosion. X-ray absorption spectroscopy (XAS) reveals that the coordination bond length of OTF with Zn²⁺ exhibits a shortening, accompanied by enhanced interfacial stability [69]. A high concentration of electrolyte used in conjunction with hydrogen bonding additives (e.g., silk peptides) can further reduce water activity, resulting in a reduction of Zn deposition overpotential from 200 to 80 mV and a 3-fold increase in cycle life [70].

3) pH buffered electrolyte: Organic buffers such as pyridine and imidazole are introduced to maintain the pH of the electrolyte at 4−6 and inhibit the direct reaction between Zn and H⁺. For example, pyridine equilibrates the pH fluctuation (ΔpH < 0.3) in charging and discharging by proton trapping, which increases the hydrogen overpotential by 200 mV and reduces the corrosion rate of zinc significantly [71]. The interfacial adsorption effect is evident in that the N-containing functional groups are preferentially adsorbed on Zn surface through electrostatic interaction, which induces the preferential orientation of Zn (002) crystal surface and reduces the surface energy required for the formation of dendritic crystals [12].

3.3 Bioinspired strategy for stabilizing Zn anodes

A variety of modification strategies for AZIBs have been mentioned above, all of which significantly improve the cycle life of the Zn anode, but some of them are not favorable for the large-scale application of AZIBs. One of the core advantages of AZIBs over LIBs in large-scale energy storage systems is their environmental friendliness [72], which ensures that there is no risk of environmental pollution after large-scale deployment.

High-magnification and stable zinc-ion cells operating at low temperatures are ideal for practical applications, but are challenged by slow kinetics and severe corrosion. Cedarwood inhibits cytosol solidification and maintains rapid nutrient transport at low temperatures. Inspired by frost-resistant plants, Bu et al. [73] reported trace amounts of hydroxyl-rich electrolyte additives (Fig.12(a)), which achieve a dual remodeling effect on high-performance low-temperature Zn ion batteries. This additive with high Zn adsorption capacity not only remodels the main solvent shell of Zn²⁺ through alternating H₂O molecules, but also forms a shielding layer and thus remodels Zn surface, effectively enhancing the kinetics of the rapid desolvation reaction of Zn²⁺ and preventing Zn anode corrosion.

Wang et al. [74] proposed a hydrolysis strategy of filipin protein to promote its structural reorganization and the emergence of more ‒NH₂, ‒COOH as shown in Fig.12(b). The results show that filipin, as a hydrolysate of filipin protein, is a highly efficient, low-cost, and environmentally friendly electrolyte additive, which can significantly extend the cycle life of AZIBs. Compared with filaggrin and filipin, silk peptide molecules have higher solubility in Zn sulfate electrolyte and contain more polar groups on the peptide chains, which play a key role in mitigating Zn anode side reactions.

Seaweeds (macroalgae) are marine plants that absorb and accumulate essential metal cations such as Fe³⁺, Cu²⁺, and Zn²⁺ from seawater. Its negatively charged polysaccharides can chelate with metal cations to form stable complexes. Inspired by the accumulation of cations and anti-aging properties of macroalgae, a surface modification strategy was proposed to assist the formation of a robust in situ solid electrolyte interface (SEI) through a bionic anionic layer as shown in Fig.12(c) [75]. Sodium alginate (SA), a polysaccharide extracted from macroalgae, contains free carboxyl and hydroxyl groups that chelate with divalent cations such as Zn²⁺, Ca²⁺, Ba²⁺, and Al³⁺, to form a conductive “egg carton” hydrogel structure. After coating the Zn anode surface with SA, the plating/stripping process initiates in situ SEI formation to build a uniform Zn²⁺ diffusion layer. This process forms a protective hydrogel layer on Zn surface and enhances its stability. The SA layer acts as an anionic layer to modulate the Zn²⁺ desolvation structure and promotes the preferential deposition of Zn(002) facets to form a homogeneous and dense deposition layer. As shown in Fig.12(d) [75], the affinity of Zn²⁺ ions for alginate induced a well-aligned acceleration channel for uniform Zn deposition due to the anionic group of COO^‒. Even under high depth of discharge (DOD) conditions, the Zn anode coated with an in situ SEI layer maintained a stable Zn stripping/deposition behavior with a low overpotential (0.114 V). Meanwhile, the in situ SEI layer also enhances the mechanical strength of the Zn anode, which can effectively prevent Zn dendrites from piercing the separator, thus realizing the improvement of the electrochemical performance of AZIBs.

In exploring strategies for Zn anode protection, bionic thinking offers an innovative solution. Organisms in nature, such as mussels, demonstrate superior adhesion capabilities by secreting adhesion proteins to firmly adhere to various substrates in a moist environment. This adhesion mechanism inspired the design of a novel artificial solid electrolyte interface layer (SEI layer) that mimics the structure and function of mussel adhesion proteins [47] to enhance the stability between the Zn anode and the electrolyte. By constructing a polydopamine network similar to mussel adhesion proteins on the surface of the Zn anode, Zn dendrite growth and side reactions can be inhibited, and the CE and cycling stability of the Zn anode can be significantly improved. In addition, this bionic SEI layer not only improves the battery performance but also reduces the potential impact on the environment, which align with the development of green energy storage technology.

Zn, as an essential trace element, exists widely in all living organisms, and the colorful plants and animals in nature absorb and utilize Zn in different ways. The development of Zn anode protection strategies based on the prototype of cellular absorption of Zn ions, therefore, is also one of the research hot spots.

He et al. [76] used biomass materials to form a high-performance SEI layer on the Zn anode in situ. The SEI layer was formed in situ on the surface of the Zn anode by chelating sodium alginate with Zn ions, and the in situ SEI layer was able to reduce the interfacial activation energy and stabilize Zn(002) deposition. The in situ SEI layer acts as an anionic layer that controls the Zn²⁺ desolvation structure, which helps prioritize plating on the Zn(002) surface, resulting in a uniform and dense deposition layer. Among these, the hydrogen bonding-type mechanism of action of the bionic SEI layer plays a crucial role in the Zn anode protection of AZIBs. It is mainly manifested in the hydrogen bonds formed between the dopamine groups and the surface of the Zn anode. This process enhances the stability of the Zn anode, while also effectively reducing the growth of Zn dendrites. The presence of hydrogen bonds promotes the uniform deposition of Zn ions on the surface of the Zn anode, thus inhibiting the growth of dendrites. In addition, the hydrogen-bonded SEI layer reduces the occurrence of side reactions and improves CE by reducing the direct contact between the Zn anode and the electrolyte. This biomimetic design significantly improves the cycling stability of the Zn anode and extends the service life of the battery.

In addition, amino acids as protein components have been widely used in the field of anode protection for aqueous AZIBs. Wen et al. [77] proposed an interfacial engineering strategy by introducing a Zn-L-cysteine functional layer (Cys-Zn) with unique sulfhydryl groups on the surface of the Zn anode. The Cys-Zn layer has been demonstrated to enhance the hydrophobicity of the Zn anode, thereby preventing direct contact between the electrolyte and the Zn anode, thus reducing the corrosion of the Zn anode. Moreover, the Cys-Zn layer facilitates the uniform deposition of Zn. Furthermore, L-cysteine has the capacity to etch the Zn anode in situ during the construction of the Cys-Zn layer. This results in the preferential exposure of the (002) Zn planes, facilitating the uniform deposition of Zn, and removing the natural oxide layer on the Zn foil. Consequently, the electrochemical surface area is increased whereas the interfacial resistance is decreased. Huang et al. [78] selected aspartic acid (Asp) and its Zn salt (Zn-Asp) as electrolyte additives for AZIBs. These additives regulated the Zn²⁺ solvation structure by preferential adsorption of Asp on the (002) crystalline surface of the Zn anode. Additionally, Asp preferentially coordinated to Zn²⁺ in the electrolyte, thereby inhibiting the Zn anode self-corrosion and hydrogen evolution side reactions. This enabled the directional deposition of Zn²⁺ on the (002) crystalline surface.

Fig.12 illustrates the subtleties of the bionic structure in several dimensions. The molecular structures in the bionic electrolyte environment also clearly present the spatial arrangement and interaction patterns of the molecules in the electrolyte environment. The orderly combination and synergistic effect of these molecular structures are the key elements for the construction of an efficient biomimetic electrolyte system, which is of great significance for optimizing the electrochemical performance of AZIBs.

As demonstrated in Fig.13(a) [79], a clear distinction emerges between static and flowing electrolytes in electrochemical deposition. The left Fig.13(a) illustrates the static electrolyte, while the right Fig.13(a) shows the flowing electrolyte. In static electrolyte, the transport of Zn²⁺ is predominantly contingent on the concentration gradient, a factor that can give rise to inhomogeneous deposition of Zn²⁺ on the anode surface and the subsequent formation of irregular deposits.

In contrast, flow electrolytes, in which the introduction of flow velocity is implemented, have been shown to significantly improve the efficiency of Zn²⁺ transport. The flow electrolyte mitigates the impact of the concentration gradient through continuous fluid motion, enabling Zn ions to be more uniformly distributed on the anode surface and thereby promoting uniform deposition. The presence of hydrogen bonds within the flowing electrolyte serves to augment the transport efficiency of Zn²⁺ within the SEI layer. The presence of hydrogen bonding facilitates the movement of Zn ions through the SEI layer, thereby reducing transport resistance and enhancing interfacial dynamics. This enhancement of uniformity, in turn, leads to an improvement in the performance and stability of the electrochemical cell. Consequently, the flow electrolyte has been shown to enhance the transport efficiency and deposition uniformity of Zn ions by combining the flow rate and hydrogen bonding, thus providing a superior solution for the electrochemical deposition process. The simulation mechanism of the Zn-air battery and the interactions of its core components are depicted in Fig.13(b) [80], in which the air anode is connected to the Zn anode through the electrolyte to form a complete electrochemical reaction path to complete the discharge process. Fig.13(b) demonstrates a substantial change in the interfacial electrode reaction (IEA) modulus and strength through the introduction of varying alloying compositions. This modulus and strength exhibit a slight decrease in comparison to the pure Zn anode. However, the IEA strength undergoes significant enhancement due to the enhancement of interfacial stability resulting from alloying. This phenomenon corresponds to the “hydrogen-bonding regulation” mechanism identified in the study of biomimetic SEI layers. Both mechanisms address the dendrite problem of the Zn anode through interfacial engineering, impeding the growth of dendrites, and reducing side reactions, thereby enhancing the cycle life of the battery. As illustrated in Fig.13, this phenomenon exhibits a superior response to the formation mechanism of hydrogen bonding and the stability of the SEI layer. This, in turn, promotes the optimization of uniform deposition and interfacial dynamics, reduces the side reactions, and improves CE.

Emulating the hydrogen bonding network characteristic of mussel adhesion proteins achieves the self-healing and dynamic adaptability of the SEI layer, providing a novel concept for biomimetic interface design. The experimental data further validate the practical efficacy of the hydrogen bonding mechanism, substantiating the fundamental premise of the “bionic strategy” for battery lifespan enhancement.

As can be seen from the above discussion, the protection strategy for the Zn anode revolves around constructing an artificial SEI layer, modifying the electrolyte, structural optimization, and biomimetic simulation. The synergistic effect of these strategies gradually becomes more pronounced. The artificial SEI layer is created using an inorganic/organic composite (e.g., ZnHCF/PVA@ZnMoO₄ [55]) forming a physical barrier with ionic conductivity and mechanical toughness that can block electrolyte penetration while leading to homogeneous Zn²⁺ deposition. Electrolyte modification reconfigures the solvated structure of Zn²⁺ at a molecular level through hydrogen bonding, electrostatic interactions, and reactive additives, thereby inhibiting the thermodynamic driving force of hydrogen precipitation and dendrite growth. Bionic strategies further mimic the dynamic adaptability of biological interfaces, such as self-healing adhesive layers of polydopamine and ion-chelating templates of alginate. Optimization of Zn anode structures suppresses dendrite growth by regulating electrode morphology and interface dynamics, with typical strategies including three-dimensional fluidization and pore engineering. These structures can cycle 3000 times at high current densities with a CE of 99.67% [81].

Based on this, Al-Abbasi et al. [82] further emphasized that future development must integrate advantages of multiple strategies to develop biomimetic dynamic interfaces exhibiting both high ionic conductivity and mechanical stress-resistant stability. They also suggested that combining these approaches with high-concentration electrolytes could optimize AZIBs’ migration kinetics, thereby overcoming interfacial failure limitations at high discharge depths while promoting the development of AZIBs with long life and high security.

The commercial feasibility assessment of the four Zn anode stabilization strategies evaluates cost, scalability, synthesis complexity, and manufacturability. First, the electrolyte modification excels in scalability and manufacturability, is cost-effective, and is well-suited for large-scale industrial applications. Then, the artificial SEI layer achieves a balance between cost and synthesis complexity, offering a notable low-cost advantage. Furthermore, the structural optimization is manufacturable but has complex synthesis, with weaker cost control and scalability. Finally, the bio-inspired strategies have complex synthesis and high costs, but show potential in manufacturability. Fig.14 provides an intuitive comparison of the industrial applicability of each strategy. Among them, electrolyte modification is more suitable for industrialization due to its dual advantages of cost and scalability. Structural optimization has advantages in material preparation and process implementation, while artificial SEI layers and bio-inspired strategies requiring overcoming cost and synthesis bottlenecks to enhance their competitiveness. This radar chart provides a multidimensional decision-making reference for the design of Zn²⁺ battery anodes, clarifies improvement directions for each strategy, and combines literature and application scenarios to enhance industrial guidance value, ultimately promoting the industrial adoption of Zn anode stabilization strategies.

3.4 Structural optimization strategy

The structural optimization of Zn anodes provides a huge specific surface area, significantly reduces the local current density, facilitates the uniform electric field distribution, and promotes the uniform nucleation and growth of Zn. Typical strategies include 3D fluidization [81], pore engineering [84] and composite Zn anodes [85]. In principle, the 3D structure uniformizes electric field distribution by virtue of high porosity, enabling the 3D-RFGC@Zn to cycle 3000 times at 120 mA/cm² with a CE of 99.67% [81], compared with bare Zn. Meanwhile pore engineering guides the homogeneous deposition of Zn²⁺ through physically confined domains, e.g., when the Cu mesh electrode is coupled with a strongly coordinated anionic electrolyte with Zn²⁺ strong coordination anion electrolyte, the CE increased from 85% to 92% [81]. In addition, the composite anode promotes the formation of Zn-friendly sites via surface chemical modulation, resulting in a twofold increase in Zn nucleation density. One-step thermal chemical vapor deposition enables the synthesis of 3D layered structures, and general techniques such as laser lithography and template methods are summarized by inducing substrate surface reduction through electrolyte reimpregnation.

Although these strategies have significant advantages in terms of high current density stability and volume expansion suppression, their limitations should not be overlooked. Critical challenges remain in balancing the porosity and conductivity of 3D structures, managing the complexity of compositional modulation of alloying, and ensuring the long-term stability of the surface modification layers—all of which require breakthrough solutions. Structural optimization must synergize with SEI layer engineering and electrolyte modification to overcome existing bottlenecks. For example, 3D-RFGC@Zn combined with FA-DX [84] hybrid electrolyte reduced the diffusion distance of Zn²⁺ through the “physically confined domains” of the radial carbon channels, and the coordination of carbonyl group and Zn²⁺ in FA-DX. Benefit from the above mentioned, the desolvation energy decreases by 20% and the nucleation site density increases to 7.34 × 10¹⁰ cm⁻², resulting in 3000 h of stable cycling at −35 °C with nearly 100% CE [84]. On the other hand, the surface reduction layer of the prepreg Cu substrate, in concert with the F-containing electrolyte, not only promotes the uniform adsorption of Zn²⁺, but also induces the generation of an ultrathin ZnF₂-containing SEI layer, which inhibits the interfacial corrosion and at the same time reduces the deposition overpotential from 120 to 50 mV [85].

In addition, the chemical complementarity between the interface-modified ZnF₂-Ag@Zn layer and the SEI layer increases the CE to 99.7%, and its strong Zn²⁺ adsorption ability synergized with the high ionic conductivity SEI layer to suppress side reactions. Integration of the gradient structure design with dynamic electrolyte management reduces local concentration polarization and improves Zn²⁺ deposition uniformity by 50%, with no dendrite protrusions after 2600 h of cycling at high loads. This multi-dimensional synergy of “structure-electrolyte-SEI layer” not only takes advantage of the physically confined domain of the 3D structure, but also optimizes the interfacial dynamics through electrolyte modulation, which provides a key support for the application of Zn²⁺ batteries in high-energy-density scenarios.

Among the four major strategies for stabilizing Zn anode, the SEI layer forms a dense physical barrier at the electrode-electrolyte interface, isolating Zn from the electrolyte to suppress side reactions. It combines ionic conductivity with electronic insulation, guiding the uniform deposition of Zn²⁺ by regulating the ionic transport path while blocking electrolyte penetration to stabilize the electrode interface. The hydrophobic, highly conductive barrier formed by the inorganic layer enables symmetric cell cycles to exceed 2500 h with a CE of 99.8% [68]. Inorganic materials cannot withstand great volume changes and are prone to cracking, while organic materials lack sufficient mechanical strength, which limits their large-scale application. Organic‒inorganic composite materials are expected to achieve high-performance cycling [54].

As shown in Fig.15, the electrolyte modification strategy offers advantages in terms of ease of operation and low cost. Hydrogen-bonding additives enable symmetric cells to cycle for over 4000 h, while high-concentration electrolytes can maintain cycling for 2000 h [66]. However, high-concentration systems are costly and have high viscosity, and some additives may adversely affect ion conductivity. The data in Fig.15 related to electrolyte modification show distinct cycle time performances at different current densities.

The biomimetic strategy achieves dynamic interface adaptation by mimicking biological structures. The chelation network formed by sodium alginate promotes oriented deposition on specific crystal faces, while silk peptide additives [74] maintain high-capacity retention at low temperatures. However, biological materials are costly, and large-scale preparation processes require optimization. The purification costs of natural materials also pose limitations.

Structural optimization strategies utilize 3D porous frameworks to distribute current density. Porous carbon-Zn composite materials demonstrate stable performance over 3000 cycles at high current densities, with a CE of 99.67% [84]. Additionally, Zn alloy anodes maintain a capacity decay rate of less than 0.1% per cycle after 200 cycles [80], indicating excellent cycling stability. However, achieving an optimal balance between conductivity and porosity in 3D structures remains a significant challenge. The complexity involved in alloying regulation further presents an application bottleneck, as the uniformity of conductivity still necessitates improvement.

These strategies exhibit complementary properties across different application scenarios, clearly demonstrated by the data in Fig.15. In the future, through the synergistic design of these four strategies, it is anticipated that key technical bottlenecks in Zn dendrite suppression and cycling stability will be overcome.

4 Conclusions and perspectives

In summary, the emergence of various strategies for stabilizing Zn anodes offers significant opportunities for improving the electrochemical performance of AZIBs. Numerous materials design and electrolytes modification techniques have been implemented to address key challenges including Zn dendrite formation and interfacial side reactions. Despite substantial progress achieved in recent years, Zn anodes in AZIBs remain in the infant stage. For future applications and advancement, the following critical challenges must be addressed.

4.1 Conclusions

Further studies are required to understand the interfacial side reactions and electrodeposition mechanism of Zn²⁺ near the Zn anode surface at the molecular level, as the chemical environment of Zn²⁺ near the anode surface differs from that in the bulk electrolyte. For instance, Zn²⁺ must undergo desolvation before deposition, with slower kinetic processes and higher local concentrations near the anode, while factors like pH, salt types, and interfacial water/oxygen content influence deposition mechanisms and side reactions. Despite extensive research on Zn²⁺ chemistry in electrolytes, analyzing the electrochemical processes at the anode surface remains challenging.

Meanwhile, the practical adoption of AZIBs is constrained by Zn anode reversibility and stability issues under high discharge depths: most studies focus on low areal capacities (< 1 mAh/cm²) and shallow discharge depths, whereas real-world full battery systems require low negative/positive capacity ratios (N:P), high cathode mass loading, and thin Zn foils. These requirements necessitate improved Zn utilization and operation at high discharge depths. Under such conditions, anode volume changes, uneven Zn deposition, and fresh Zn corrosion destabilize plating/stripping processes and degrade performance.

Inspired by biological structures like aquaporins, where hydrophobic groups in protein channels disrupt water hydrogen bonds to lower energy barriers for ion transport, bionic designs of SEI layers with hydrophilic-hydrophobic structures hold promise for accelerating hydrated Zn²⁺ desolvation, enhancing mass transfer kinetics during plating/stripping, and enabling bio-based antifreeze electrolytes to expand AZIBs’ temperature range.

However, the uncontrollable growth of zinc dendrites remains a key issue, as these needle-like structures can penetrate the diaphragm and lead to internal short circuits, which significantly shorten the cycle life. Additionally, the HER and corrosion reactions remain major obstacles, leading to poor CE and gradual anode degradation. These side reactions are exacerbated by dynamic interactions between water molecules and zinc ions in the electrolyte, promoting the formation of Zn(OH)₂ and other by-products that passivate the electrode surface.

4.2 Perspectives

In the future, it is necessary to build an integrated system of “artificial SEI-electrolyte -electrode,” combining machine learning-driven material design, the sustainability advantages of bio-based interfacial layers, and the dynamic regulation of flowing electrolyte, to break through the bottleneck of interfacial failure at high discharge depths. This multi-dimensional synergistic strategy will drive AZIBs toward long life, low cost, and environmental friendliness [86].

In short, a set of related structures and functional materials will be designed to enhance the Zn plating/stripping behavior and thus the cycling stability and rates efficiency of the battery. Future research should develop SEI layer materials with high ionic conductivity and mechanical flexibility. This is to ensure the effective management of volume changes and the mitigation of dendrite growth in Zn anodes during cycling. Concurrently, the development of novel bionic electrolytes and multifunctional additives is imperative to ensure efficient Zn ion transportation and uniform deposition, while concomitantly reducing the incidence of undesirable side reactions.

Furthermore, the improvement of Zn-based batteries’ performance necessitates a synergistic optimization of key components, including cathode materials, electrolytes, and separators. Future research should thus focus on the compatibility and synergistic effects among components to achieve an enhancement of battery performance. Through these efforts, AZIB technology is expected to advance toward enhanced electrochemical efficiency, improved operational safety, and superior environmental sustainability.

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