Challenges and Research Progress in Zinc Anode Interfacial Stability

Abstract

Aqueous zinc-ion batteries are regarded a promising energy storage system due to their high safety, low cost, high theoretical specific capacity (820 mAh g⁻¹), and low redox potential (−0.76 V). However, in practice, uneven Zn²⁺ deposition on the surface of the zinc anode can lead to the uncontrolled growth of zinc dendrites, which can puncture the separator and trigger a short-circuit in the cell. In addition, the inherent thermodynamic instability of weakly acidic electrolytes is prone to trigger side reactions like hydrogen evolution reaction and corrosion, further weakening the stability of the zinc anode. These problems not only affect the cycle life of the battery, but also lead to a significant decrease in electrochemical performance. Therefore, how to effectively inhibit the unwanted side reactions and guide the uniform deposition of Zn²⁺ to suppress the growth of dendrites becomes a key challenge in constructing a stable zinc anode/electrolyte interface. Therefore, this paper systematically combs through the main bottlenecks and root causes that hinder the interfacial stability of zinc anodes at present, and summarizes the existing solutions and the progress made. On this basis, this paper also analyzes the application potential of polymer materials in enhancing the interfacial stability of zinc anodes, which provides new ideas for the direction of subsequent research.

1. Introduction

The over-exploitation of traditional fossil energy sources has not only led to the gradual depletion of resources, but also triggered a range of serious environmental problems. In the face of this global challenge, the promotion of clean energy development and the strengthening of environmental protection have become an important mission shared by all mankind. Against this backdrop, China formally put forward the “dual carbon targets” of carbon peaking and carbon neutrality in 2020, with the aim of accelerating the transformation of the energy structure and vigorously promoting the development of clean energy. However, clean energy resources like wind, solar, and tidal energy are characterized by intermittency and instability, which makes it difficult for them to independently support large-scale, efficient electrochemical energy storage systems. To solve this problem, chemical batteries, as an efficient energy storage technology, can effectively store and release electrical energy, thus providing key support for the stable utilization of clean energy sources [1,2].

Lithium-ion batteries stand out among the many energy storage technologies by virtue of their significant advantages like high energy density and being without a memory effect, and have been extensively applied in many fields. Yet, lithium-ion batteries also face many challenges: scarce lithium resources, high prices, high production costs, and safety issues such as the toxicity and flammability of organic electrolytes [3,4]. In view of this, the development of a new, safe, efficient, and resourceful rechargeable battery has become an urgent need in the current energy field.

Aqueous zinc-ion batteries, as an emerging energy storage technology, have attracted much attention due to their unique properties and significant advantages. The zinc anode has a high theoretical capacity (820 mAh g⁻¹) and a low redox potential (−0.76 V vs. SHE), which enables it to release more energy during the discharge process, thus enhancing the energy conversion efficiency. Zinc is abundant in the Earth’s crust at an abundance of 79 ppm, which is 2.8-times that of lithium, and is abundant and low-cost [5,6]. In addition, the aqueous electrolyte is non-flammable and environmentally friendly, significantly reducing safety risks while minimizing potential environmental impacts. These combined advantages place aqueous zinc-ion batteries in a favorable position in the field of energy storage technology and are expected to provide significant support for the global energy transition and sustainable development.

From a historical perspective, the use of zinc metal as an anode material for batteries dates back to the Volta Pile in 1799. To this day, zinc metal remains the anode material of choice for many battery technologies, covering a wide range of areas such as zinc-ion batteries, zinc–air batteries, zinc–carbon dioxide batteries, zinc-based liquid flow batteries, and zinc-based flexible batteries [7,8,9,10]. Compared to lithium-based batteries, zinc-based batteries exhibit remarkable benefits regarding cost and safety. Compared with other alkali metals (Li, Na, Ca, Mg, K), Zn metal has a good volume specific capacity and a wide range of applications.

Aqueous zinc-ion batteries usually consist of a cathode, anode, separator, and electrolyte. The mechanism of operation of metallic zinc anodes in weakly acidic or neutral electrolytes relies on the reversible deposition and dissolution of Zn²⁺ (Zn ↔ Zn²⁺ + 2e⁻) [11,12,13]. During discharge, Zn²⁺ is released from the zinc anode and migrates through the electrolyte to the cathode, where it participates in an electrochemical reaction on the anode side. During charging, the opposite occurs and Zn²⁺ is reduced to Zn atoms deposited back onto the anode. The zinc anodes of zinc-based batteries are all based on the redox mechanism of Zn²⁺/Zn. However, differences in the electrolyte and cathode materials enable diverse electrochemical reaction mechanisms on the cathode side, which can be summarized into the following categories based on the reaction properties [14]: (1) insertion-type mechanisms, (2) conversion-type mechanisms, (3) coordination-type mechanisms, (4) catalytic-type mechanisms. In zinc-ion batteries with inorganic materials such as manganese-based oxides or vanadium-based oxides as cathodes, the insertion-type reaction mechanism is mainly observed; in zinc-based flow batteries, the conversion-type reaction mechanism is dominant; for zinc-ion batteries with organic materials as cathodes (zinc-organic batteries), the coordination-type reaction mechanism is the main feature; moreover, the catalytic-type reaction mechanism is mainly found in zinc–air and zinc–carbon dioxide batteries, in which the cathode promotes the redox reaction through a catalyst.

Although aqueous zinc-ion batteries have a wide range of application prospects, they still face interfacial side reactions like dendrite, HER, and corrosion, which seriously hinder the development of aqueous zinc-ion batteries. Therefore, this paper will discuss the current status of zinc anode development in detail by exploring the root causes of zinc anode interfacial side reactions and sorting out the current mainstream solutions (Figure 1a,b). It aims to provide a more comprehensive understanding and innovative experimental ideas for subsequent zinc anode research.

2. Issues and Challenges Facing Zinc Anodes

The capacity decay of aqueous zinc-ion batteries is significantly associated with the development of zinc anode dendrites, unwanted reactions on the zinc anode/electrolyte interface, and the collapse of the cell structure [17,18]. In particular, the uneven deposition of Zn²⁺ on the surface of the zinc anode during the charge/discharge cycle of aqueous zinc-ion batteries leads to the development of dendrites. If the development of dendrites is uncontrolled, in time it will permeate the separator and finally lead to an internal short-circuit [19,20]. In addition, direct interaction between the zinc anode and electrolyte hastens the corrosion of the electrode by water molecules, resulting interfacial side reactions like HER and passivation. The H₂ produced by HER leads to excessive pressure inside the cell, which directly affects the cycling performance [21,22,23]. Passivation produces by-products that adhere to the anode surface, and these electronically insulating by-products affect the electrodeposition/stripping of Zn²⁺ on the anode surface, leading to a reduction in the Coulombic efficiency [24,25]. It is worth noting that the growth of zinc dendrites and interfacial side reactions do not develop independently, but form a vicious cycle of “dendrite physical puncture–interfacial chemical corrosion–autocatalytic side reaction”, and this multi-mechanism coupling failure process leads to a continuous loss of the effective active material of the zinc anode [26]. Ultimately, the service life of the zinc anode is seriously shortened (as shown in Figure 2). The overall performance and practical applications of aqueous zinc-ion batteries are thus greatly limited [27,28]. In order to mitigate the capacity degradation of aqueous zinc-ion batteries and extend the cycle life of the batteries, researchers have focused on optimizing the zinc anode. An in-depth investigation of the root causes of these side reactions can provide a key basis for the development of targeted solution strategies, which in turn can significantly enhance the stability of the zinc anode interface.

2.1. Zinc Dendrite

The structure of the double electric layer is located on the zinc anode surface, which is the core region of the electrochemical reaction, and its structural evolution is significantly associated with the reliability of zinc anode interface. The bilayer is mainly composed of an inner Helmholtz layer, outer Helmholtz layer, and diffusion layer. In an ideal electrochemical reduction process, the solvated zinc ions first traverse the diffusion layer and reach the outer Helmholtz plane. Here, the zinc ions undergo desolventization and subsequently enter the inner Helmholtz plane where they gain electrons and are reduced to zinc atoms, which are ultimately deposited on the surface of the zinc anode. However, during actual cycling, the uneven diffusion and deposition of zinc ions tends to accumulate into tips and attract more zinc ions to be deposited, eventually forming dendrites. Especially in neutral or weakly acidic electrolytes, zinc dendrites tend to form randomly-oriented lamellar crystals instead of fractal structures. This dendrite formation mainly originates from the irregular distribution of electrons and Zn²⁺ at the zinc anode/electrolyte interface, leading to inhomogeneity in the electrodeposition process.

The formation of zinc dendrites can be categorized into two main cases. On the one hand, at fairly low current densities, the uneven deposition is largely attributed to irregular Zn²⁺ flux and horizontal diffusion of Zn²⁺ on the electrode surface. On the other hand, at increased current densities, the decrease and depletion of Zn²⁺ at the zinc anode/electrolyte interface is more rapid. When there is an insufficient supply of Zn²⁺ near the anode surface, cation depletion causes a localized space charge and intense electric field, which also leads to the formation of zinc dendrites, a phenomenon known as Sand behavior. Current density has a significant effect on the morphology of zinc dendrites. At low current densities, zinc deposition tends to be uniform, the surface is smooth, and only part of the area grows sparse and large zinc dendrites, which are spongy; at high current densities, the zinc dendrites grow into a dense and sharp hexagonal plate-like structure. This change is attributed to the current density which affects the reduction rate of zinc ions and the electric field distribution; high current density causes the electrode surface near the rapid reduction in zinc ion concentration to form a concentration gradient, prompting the growth of dendrites [29]. The zinc dendrite growth process is usually divided into two phases: the nucleation phase and the growth phase. In the nucleation stage, Zn²⁺ ions are trimmed to zinc atoms at remarkably active surface sites (e.g., Zn(100) planes); in the growth stage, zinc tends to be deposited at electron-rich sites, resulting in inhomogeneous deposition and zinc dendrite formation. In addition, other factors such as current density, electrolyte constituents, and the microstructure of zinc additionally impact the electrodeposition process [27,28].

The formation of zinc dendrites is affected by a variety of factors, including Zn²⁺ ion flux, electrode surface morphology, atomic structure, substrate composition, temperature, electrolyte concentration, inherent inhomogeneity at the solid electrolyte interface, and ion transfer kinetics [30,31]. These factors affect the growth of dendrites mainly by influencing the ion diffusion and mass transfer processes. Managing these factors is challenging but critical to minimizing dendrite development. Zinc dendrite growth may puncture the battery separator and result in internal short-circuits, while zinc dendrite fracture leads to rapid cell capacity degradation and reversible reductions, which severely impacts the cycling stability and cycle life of the zinc anode. In addition, the “tip effect” leads to zinc dendrites and at the same time generates “dead zinc”. The presence of “dead zinc” leads to a decrease in the actual capacity of the cell and a decrease in the Coulombic efficiency. Therefore, it is important to understand and control the formation of zinc dendrites to improve the performance and stability of zinc-based batteries.

2.2. Hydrogen Evolution Reaction

In the electrolyte system of conventional aqueous zinc-ion batteries, not only do a large number of free water molecules exist, but also water molecules interacting with Zn²⁺ exist, i.e., one Zn²⁺ is coordinated with six water molecules to form a typical [Zn(H₂O)₆]²⁺ solvation structure, in which the coordination structure of [Zn(H₂O)₆]²⁺ significantly affects the electrochemical deposition process. Guo et al. [32] revealed by in situ spectroscopic analysis that the hydrogen evolution reaction during Zn deposition mainly originates from solvated water rather than free water molecules, which is linked to the fact that the ligand action of Zn²⁺-H₂O weakens the O-H bonding energy and contributes to the preferential dissociation of solvated water (Figure 3). The H₂ and OH⁻ generated from this decomposition cannot form an effective passivation layer, resulting in a continuous exposure of the anode surface to the electrolyte, causing an irreversible loss of Coulombic efficiency. The reaction mechanism at the anode/electrolyte interface is critical in the HER process. Solvated water molecules interact more strongly with the anode surface. For example, at the anode surface, solvated water molecules can be adsorbed on the anode surface through interactions such as hydrogen bonding, and water molecules in this adsorbed state are more likely to undergo dissociative reactions to form reactive hydrogen species (e.g., hydrogen atoms or ions), whereas it is relatively difficult for free water molecules to approach the anode surface and react [33]. Although the use of a weakly acidic electrolyte can inhibit the growth of zinc dendrites and transform the zinc anode from an original electrode to a secondary auxiliary electrode, the hydrogen evolution reaction still cannot be completely circumvented [34,35]. Thermodynamic analysis further showed that the redox capacity of H⁺/H₂ was always lower than that of Zn²⁺/Zn irrespective of the electrolyte pH, which confirmed the necessity of the hydrogen evolution reaction at the intrinsic kinetic level [36]. This reaction not only causes energy loss, but also leads to localized electrolyte alkalization through the consumption of H⁺, which triggers the generation of Zn(OH)₂ and alkaline zinc sulfate by-products in the ZnSO₄ system to form an electrically insulating layer covering the anode, further aggravating the electrochemical performance degradation. In alkaline electrolytes, the movement of zinc salts is a “diffusion-controlled” process. Initially, the electric field strength is high at the uneven surface of the substrate, and the zinc salts preferentially reduce and accumulate at these sites, forming dendrites due to the tip effect. In neutral or weakly acidic electrolytes, on the other hand, the zinc nucleation behavior may be consistent with a transient nucleation mechanism, where the number of nucleation sites is smaller and activated at once, making it easier to form lamellar structures [37]. This multiple failure mechanism driven by solvated water decomposition constitutes a key scientific issue that needs to be broken through in Zn-based battery systems.

2.3. Surface Corrosion and Passivation

In the electrochemical reaction system of zinc-based batteries, the electrochemical dissociation behavior of water molecules on the surface of the anode has a noticeable effect on the stability of the zinc anode. During the charging process, OH⁻ ions generated from the dissociation of water molecules react with zinc salts in the electrolyte to generate by-products such as alkaline zinc sulfate (Zn₄SO₄(OH)₆·xH₂O), whose loose crystalline structure is unable to form an effective protective layer, but instead induces the sprouting and growth of zinc dendrites. Taking a typical ZnSO₄ electrolyte as an example, since the migration number of Zn²⁺ (0.2~0.5) is significantly lower than that of SO₄²⁻ ions, the rapidly migrating sulfate on the surface of the zinc anode preferentially induces heterogeneous Zn₄SO₄(OH)₆·xH₂O nucleation. The continuous accumulation of this by-product not only destroys the uniformity of zinc deposition, but also promotes the growth of dendrites through lattice stress, which becomes an important cause of battery failure.

Electrochemical corrosion dominates the failure mechanism of zinc anodes in mild (including slightly acidic) electrolyte environments. Zn²⁺ generated from the dissolution of metallic zinc combines with OH⁻ released from hydrogen evolution reactions to form insoluble deposits such as Zn(OH)₂ and ZnO. These by-products adhere to the electrode surface, obscuring the active nucleation sites and forcing the uneven deposition of zinc at the defective sites, forming rough bumps that promote the growth of zinc dendrites. Unlike the dense SEI film in lithium-ion batteries, the by-product layer on the zinc anode surface exhibits porous and lax characteristics, which cannot inhibit the continuation of the corrosion reaction [38]. The formation of a Zn(OH)₂ passivation layer is usually considered irreversible because it decreases the electrical conductivity of the zinc anode, increases the interfacial impedance, and reduces the active nucleation sites for zinc ions. However, this passivation effect can be reversed to some extent by specific surface treatments and electrolyte additives. For example, the addition of specific ligands to the electrolyte can restore some of the zinc anode activity by converting the Zn(OH)₂ passivation layer into a more ionically conductive composite layer during the charging process. In situ conversion of the native passivation layer of zinc into a composite interfacial layer with uniform Zn²⁺ transport and corrosion resistance is used to protect the zinc anode through a gel-supported release strategy. Localized alkalization causes surface passivation of zinc anodes by causing zinc corrosion, passivation, and hydrogen precipitation reactions that increase the generation and attachment of Zn(OH)₂. This localized alkaline environment changes the chemistry of the electrolyte and promotes the formation of additional by-products. For example, in a ZnSO₄ electrolyte, localized alkalization may lead to the formation of Zn₄SO₄(OH)_6−x H₂O (ZSH), whereas in a ZnCl₂ electrolyte, different by-products may be produced, but both lead to passivation of the zinc anode surface and degradation of performance [39,40]. This irreversible electrode/electrolyte depletion process leads to a continuous increase in the interfacial impedance, which becomes a source of cycling stability degradation.

The passivation of the zinc anode surface is significantly aggravated when the electrolyte exhibits alkaline characteristics. The high-pH environment promotes the rapid formation of passivation layers such as ZnO, which can temporarily inhibit the growth of dendrites, but its semiconducting properties severely hinder the interfacial ion/electron migration [41]. This lack of self-healing ability of the passivation layer leads to the gradual deactivation of the zinc anode during repeated charging and discharging processes, which ultimately results in a significant degradation of the discharge capacity and power density. This multi-mechanism coupled failure process highlights the complex regulation of electrode/electrolyte interface stability by water molecule-related reactions in the zinc-based battery system.

3. Zinc Anode Stabilization Strategies and Research Progress

In recent years, the research on zinc anode protection in aqueous zinc-ion batteries mainly focuses on zinc anode structure modification, electrolyte composition optimization, multifunctional separator design, and zinc anode interface modification. These programs effectively solve the key problems of zinc dendrite growth and interfacial parasitic reaction, thus significantly enhancing the cycling stability of zinc anodes.

3.1. Separator Design

The separator functions in isolating the cathode and anode in the battery, and also offers a place for active ions to move. Hence, the separator is important for electrolyte diffusion and battery security. Liang et al. found [42] that Zn²⁺ was more likely to be deposited at locations on the zinc anode corresponding to separator pores. They recommended that the pore size affects the deposition of Zn and that micropores are more prone to direct the development of dendrites. Therefore, a more stable anode can be obtained by using glass fiber paper with regular pores as a separator compared with filter paper. Since the diffusion and deposition of Zn²⁺ in the electrolyte are impacted according to the concentration gradient, dendrites are easily produced when the concentration gradient is significantly high. Drawing from the aforementioned theory, Liu et al. [43] reported a cation exchange membrane based on crosslinked polyacrylonitrile (PAN) to decrease the ion concentration gradient (Figure 4a). The long cycle stability of Zn//Zn cells with and without PAN-S film was tested at a current density of 0.5 mA cm⁻². The symmetric cell without PAN-S film was deactivated after 11 turns of stable cycling, while the time–voltage curve exhibited a hysteresis voltage of ~280 mV. In contrast, the Zn//Zn symmetric cell containing PAN-S film can be stably cycled for more than 350 cycles with a hysteresis voltage less than 40 mV. This may be because the PAN-S film selectively transports cations and promotes homogeneous Zn²⁺ fluxes, which helps to inhibit Zn dendrites and obtain a homogeneous Zn deposition layer (Figure 4b). PAN-S membranes have a more uniform pore size distribution than conventional diaphragms. This uniformity helps to effectively inhibit the formation of dendrites and promotes a more uniform distribution of ion flux, resulting in improved battery performance and cycle stability. The PAN-S membrane effectively balances cation selectivity with Zn²⁺ diffusion rate. The sulfonic acid group (-SO³⁻) of the PAN-S membrane not only imparts a good ionic conductivity, but also increases the water content of the membrane through its hygroscopic properties, thus improving the ionic conductivity. This design achieves a uniform ionic flux distribution, promotes uniform nucleation of zinc, and effectively inhibits the formation of dendrites. In contrast, the irregular pore distribution in commercial diaphragms may lead to uneven local ion concentration and accelerate the growth of dendrites. Therefore, these properties of PAN-S membranes make them excellent in inhibiting dendrite growth and improving cell cycle stability. Inspired by wood architecture, Ma et al. [44]. prepared a wood anisotropic thin separator composed of nanofibrillated cellulose (NFC) and chitosan by biomimetic design. The V-NFC-CS separator not only effectively inhibited the vertical deposition and parasitic reaction of zinc, but also enhanced the zinc anode stripping/plating kinetics and provided a good reversibility (Figure 4c). Compared with conventional separators, Zn//Zn cells using the V-NFC-CS separator have a cycle life of 1000 h while significantly reducing overpotential (Figure 4d).

3.2. Anode Structure Optimization

A three-dimensional (3D) zinc anode framework can be successfully constructed by depositing zinc ions on a three-dimensional (3D) collector. This 3D zinc anode possesses greater electrochemical active sites, which can promote a homogeneous electric field distribution and thus effectively inhibit the growth of zinc dendrites [45,46]. Similarly, Zhang et al. [47] constructed a three-dimensional structural framework using CuO nanowires to modify the Cu network. By controlling the deposition and exfoliation of Zn²⁺ on this skeleton, the formation of Zn dendrites was thus suppressed (Figure 5a). Zn//Zn symmetric cells were prepared and the cycling life of zinc anodes before and after zinc foil modification was tested. As shown in Figure 5b, the CM@Zn//CM@Zn battery was only cycled stably for 60 h, and the polarization voltage enhanced abruptly after 60 h, which could be attributed to the generation of multiplied “dead zinc” on the zinc surface. On the contrary, CM@CuO@Zn//CM@CuO@Zn can be stably cycled for at least 340 h. This suggests that the Cuo nanowire modification can help to promote the uniform ionic distribution and electric field during zinc deposition, and effectively inhibit zinc dendrites. In addition, by growing 3D interconnected ZnF₂ substrates on the Zn foil’s surface to form a multifunctional protective layer, not only was the Zn²⁺ flux redistributed, but also the desolvation activation energy was significantly reduced to achieve stable and simple Zn deposition kinetics (Figure 5c) [48]. The long-time cycling stability of Zn//Zn symmetric cells and Zn-ZnF₂//Zn-ZnF₂ symmetric cells was tested under 0.5 mA cm⁻² test conditions. As shown, the Zn//Zn symmetric cell failed after about 50 h of cycling. However, the Zn-ZnF₂//Zn-ZnF₂ symmetric cell can be stably cycled for more than 700 h. This may be due to the fact that ZnF₂ can provide a uniform ion diffusion channel for Zn²⁺, which helps to promote the uniform deposition of Zn²⁺ and the formation of a smooth zinc deposition layer (Figure 5d). In a study by Guan et al. [49], an easily imprintable gradient zinc anode (PVDF-Sn@Zn) was reported, which was designed as a double-gradient structure with a hydrophobic insulating PVDF layer at the top and a hydrophilic conductive Sn layer at the bottom. This structure effectively prevents the corrosion of zinc metal in the electrolyte and inhibits side reactions. In addition, the gradient conductivity enables the optimization of the electric field distribution, Zn²⁺ ion flux, and local current density at the bottom of the microchannel, resulting in a bottom-up deposition behavior of the zinc metal, which ensures homogeneous deposition and prevents short-circuits caused by dendrite growth.

3.3. Surface Protection Layer

The immediate contact between the zinc anode and the electrolyte is a key factor to induce zinc dendrite formation and side reactions. Based on this, researchers have found that a functional protective layer on the zinc anode’s surface, can effectively boost the performance (Table 1) [50]. The main role of zinc anode surface protective layer in aqueous zinc ion battery is as follows: (1) The protective layer is situated between the zinc anode and the electrolyte, playing the role of a physical protective layer, preventing the water or oxygen in the electrolyte from directly contacting the surface of the zinc anode. It effectively reduces corrosion or side reaction and improves the stability of the zinc surface. (2) The conductive protective layer can change the local electrochemical environment at the zinc anode surface and form an even electric field, thus guiding the uniform nucleation of Zn²⁺. (3) The dense protective layer obtains a certain degree of mechanical strength, which can offset the volume change in the zinc anode during charging and discharging, and improve the utilization rate and cycling stability of the zinc anode [51]. Assorted artificial interfacial layers have been documented, like metal oxides and their compounds (Sc₂O₃ [52] and Na₂TiO₃ [53]), metals and their alloys (Zn-Al [54], AgZn₃ [55], and MoS₂ [56]), carbon-based protective layers (reduced graphene oxide (rGO) [57], graphitic carbon nitride (g-C₃N₄) [58], and hydrogen-substituted graphitic diynes (HsGDY) [59]), polymeric materials (polyimide [60], polypyrrole (PPy) [61], and polyamide (PA) [62]), and inorganic salts (kaolinite [63] and NaTi₂(PO₄)₃ [64]).

3.3.1. Carbon-Based Material Protection Layer

Carbon-based materials are recognized as ideal materials for the protective shielding layer of zinc anodes due to their combination of abundant resource reserves, excellent electrical conductivity, mechanical stability, and large specific surface area [65]. Currently, diverse carbon-based materials have been widely used as zinc anode protective layer materials, including carbon black [66], graphite [67], graphene [59], carbon nanotubes (CNTs) [68], graphene oxide (GO) [69], and its reduced state (rGO) [69]. The high electrical conductivity of carbon materials can effectively regulate the electric field distribution on the anode surface and significantly reduce the interfacial impedance, thus avoiding the local accumulation of charge during the cycling process and promoting the uniform transport of zinc ions and stabilizing the electroplating/stripping behavior; the large specific surface area characteristics of carbon materials offer ample active sites for zinc deposition, effectively reducing the local current density and stifling the development of zinc dendrites [70].

For example, Zhou et al. [71] prepared an ultra-thin (120 nm) protective layer of N-doped graphene oxide (NGO) on zinc foil, as shown in Figure 6a,f. The nitrogen-containing functional groups of graphene oxide effectively promote the directional deposition of Zn²⁺ on the (002) crystalline surface. As shown in the DFT calculations in Figure 6b, there is a high binding energy (−0.24, −0.46 eV) between C=O, Npr functional groups, and Zn²⁺, which suggests that the introduction of the NGO coating helps to promote the trapping of Zn²⁺ and facilitates the preferential deposition of Zn²⁺. Meanwhile, Bader charge analysis showed more electrons in the frontier orbitals of the Npr groups near the Fermi energy level, suggesting that Npr doping in graphene could provide more active sites for Zn deposition. Cycling stability tests of Zn//Zn symmetric cells and NGO@Zn//NGO@Zn symmetric cells at 1 mA cm⁻² current density were prepared. The comparison revealed that the Zn//Zn symmetric cell short-circuited after 190 h of cycling. In contrast, the NGO@Zn//NGO@Zn symmetric cell could cycle stably for at least 1200 h (Figure 6c). This may be due to the fact that the N-containing functional groups can effectively adsorb Zn²⁺, which can help to direct the Zn²⁺ to be uniformly deposited at the zinc foil surface and avoid the growth of dendrites. The XRD profiles of Zn foil surfaces with and without NGO protective layers after electroplating were tested, and distinct Zn₄(SO₄) (OH)₆·5H₂O characteristic peaks were found on the bare Zn surface, while there were no obvious heterogeneous peaks on the NGO@Zn surface (Figure 6d). This indicates that the mechanism of Zn²⁺ deposition on the surface of NGO@Zn is stable and reversible, which contributes to reducing the occurrence of interfacial side reactions. An average Coulombic efficiency of over 99.5% was found for NGO@Zn//Cu half-cells over 250 cycles (Figure 6e). In contrast, the Coulombic efficiency of the Zn/Cu cell declined rapidly due to the continuous generation of Zn dendrites. This indicates that the structure of the NGO@Zn electrode is stable, and the N-functional groups in the NGO film can uniformly guide zinc deposition and effectively inhibit the growth of dendrites.

3.3.2. Inorganic Non-Metallic Protective Layer

Inorganic non-metallic materials (e.g., TiO₂, Al₂O₃, ZrO₂, ZnO, etc.) have been used as non-conductive protective layers on zinc anodes [72,73,74,75,76], which are attributed to their excellent chemical inertness characteristics, and are able to form a physical barrier layer at the zinc anode/electrolyte interface, effectively isolating the electrolyte from direct contact with the zinc anode. Through this interfacial isolation mechanism, the occurrence of interfacial side reactions was significantly suppressed, which provided an important guarantee for the enhancement of the electrochemical stability of aqueous zinc-ion batteries. Notably, this protection strategy realizes the effective regulation of the interfacial reaction kinetics while maintaining the electrochemical activity of the zinc anode by modulating the interfacial ion transport properties.

For example, Li et al. [77] prepared a porous rutile TiO₂ interfacial layer consisting of uniform spherical nanoparticles with a diameter of 100 nm (Figure 7a). Cycling stability was tested for Zn//Zn and Zn@TiO₂//Zn@TiO₂ symmetric cells at 0.2 mA cm⁻² current density. The Zn//Zn symmetric cell fails after 112 h. On the contrary, the Zn@TiO₂//Zn@TiO₂ symmetric cell could operate stably for more than 160 h, while the polarization voltage decreased significantly (Figure 7b,c). This suggests that the TiO₂ protective layer helps to promote the rapid migration of Zn²⁺, and the porous configuration of the Zn@TiO₂ anode surface directs the Zn²⁺ flux to be evenly deposited on the anode surface, thus inhibiting the development of dendrites. Moreover, Zhou et al. [78] thermally decomposed alkaline zinc sulfate nanosheets into sieve-like zinc oxide nanosheets (Figure 7d) and successively stacked several layers of network-structured nanosheets as a protective layer at the zinc anode interface. As can be seen from Figure 7e, the Zn//Zn cell can be stably cycled for 50 h at 0.2 mA cm⁻² current density, compared to the Zn@ZnO//Zn@ZnO cell which can be stably cycled for at least 1000 h. This suggests that the different layers are interconnected by nanoparticles (200~500 nm in diameter) to form an overall framework of sieve-like nanosheets. This particular structure greatly boosts the specific surface area, and the pore size (1~5 nm) on the zinc oxide nanosheets provides a transport channel for the diffusion of Zn²⁺. Also, the hydrophobic ZnO layer reduces the continuous interface between the zinc anode and water molecules, thus controlling side reactions and the development of zinc dendrites.

3.3.3. Protective Layer of Polymer

In aqueous electrolytes, zinc ions are usually present in the form of [Zn(H₂O)₆]²⁺ clusters, which are more inclined to be deposited at the tips of high charge densities, leading to the growth of dendrites [22,79,80]. Organic polymers are often used as interfacial layers for the coordination with zinc ions due to their excellent mechanical properties and chemical stability [81,82]. These polymers are characterized by their polar groups, exhibiting strong coordination with Zn²⁺. Specific functional groups can be precisely designed and formed into stable SEIs by chemical synthesis. These functional groups optimize Zn²⁺ transport, enhance charge transfer kinetics, reduce nucleation overpotential, and inhibit dendrite growth, thus effectively inhibiting side reactions and corrosion [83].

Jiao et al. [84] used an in situ method to prepare an interfacial layer of ionic sieve (IS) consisting of bacterial cellulose on the surface of the zinc anode (Figure 8a), which effectively restricted the formation of zinc dendrites. As presented in Figure 8b, the Zn@IS//Zn@IS symmetric cell can be operated stably for 3000 h at 0.5 mA cm⁻² current density, which is much higher than the cycling lifespan of the Zn//Zn symmetric cell (300 h). This shows that, the 3D porous interfacial layer constructed from bacterial cellulose provided channels for the conduction of zinc ions and promoted a regular flux of zinc ions in the electrolyte, which facilitated the uniform deposition of zinc ions on the surface of the zinc anode. Also, when the groups in the ion–ligand layer are present in succession, these groups can give rise to ion-transfer channels upon coordination with Zn²⁺. Meanwhile, the Figure 8c LSV curve shows that the Zn@IS hydrogen precipitation potential is significantly positively shifted compared with that of bare Zn. Meanwhile, the Tafel slope value decreases from 110.3 mV dec⁻¹ to 47.8 mV dec⁻¹. This is primarily because the IS protective layer excludes the ligand water molecules and effectively isolates the direct interaction between the water molecules and the zinc anode, thus alleviating side reactions such as HER and corrosion. For example, Lee et al. [85] used polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene-grafted-methylenebutenedioic acid (SEBS-MA) as a zinc anode interfacial layer and obtained a significant performance (Figure 8d). A Zn//Zn symmetric cell was prepared, and Zn@SEBS-MA//Zn@SEBS-MA could maintain stable cycling for 3200 h at 3 mA cm⁻², which was much higher than the cycle life of the Zn//Zn symmetric cell (200 h), while the polarization voltage was reduced (Figure 8e). This may be due to the fact that the maleic anhydride group in the interfacial layer includes the polar functional group carboxyl group, which is capable of ion selection and effectively inhibits the generation of by-products on the anode surface by selectively shielding anions (e.g., SO₄²⁻) and water molecules.

An organic–inorganic composite interface engineered with enhanced interfacial cycling stability through isoleucine coating (Ile) was theoretically proposed by Peng et al. [86]. The Ile coating supported the three-dimensional diffusion of Zn²⁺ and improved ion transport kinetics. Oxygen vacancies formed during electrochemical oxidation increased conductivity and provided additional active sites for Zn²⁺ storage. The specific capacity of the Ile-α-MnO₂ electrode reached 334.8 mAh g⁻¹ at 0.1 A g⁻¹.

Table 1. Comparison of long cycle stability of symmetric batteries with different protective layer materials reported in the literature.

Protective Layer	Electrolyte	Current Density	Cyclic Life	Reference
NGO	2M Zn(SO4)2	1 mA cm−2	1200 h	[71]
TiO2	2M Zn(SO4)2	0.2 mA cm−2	160 h	[77]
ZnO	2M Zn(SO4)2	0.2 mA cm−2	1000 h	[78]
IS	2M Zn(SO4)2	0.5 mA cm−2	3000 h	[84]
SEBS-MA	2M Zn(SO4)2	3 mA cm−2	3200 h	[85]
Zn@Nafion-Zn-X	2M Zn(SO4)2	3 mA cm−2	10,000 h	[87]

Table 1. Comparison of long cycle stability of symmetric batteries with different protective layer materials reported in the literature.

Protective Layer	Electrolyte	Current Density	Cyclic Life	Reference
NGO	2M Zn(SO4)2	1 mA cm−2	1200 h	[71]
TiO2	2M Zn(SO4)2	0.2 mA cm−2	160 h	[77]
ZnO	2M Zn(SO4)2	0.2 mA cm−2	1000 h	[78]
IS	2M Zn(SO4)2	0.5 mA cm−2	3000 h	[84]
SEBS-MA	2M Zn(SO4)2	3 mA cm−2	3200 h	[85]
Zn@Nafion-Zn-X	2M Zn(SO4)2	3 mA cm−2	10,000 h	[87]

3.4. Optimization of Electrolyte Composition

Electrolyte engineering, as a simple and practical method, significantly improves the stability of zinc anodes by precisely modulating the solvation structure and the interfacial properties between the electrolyte and the anode. Specifically, the optimization of electrolyte composition can effectively inhibit the corrosion phenomenon of zinc anodes and the growth of zinc dendrites; this is not only effective, but also simple and easy to implement. According to the different formation principles and working mechanisms, electrolyte engineering can be categorized into “salt-in-water” electrolytes, aqueous low-eutectic electrolytes, and electrolyte additives.

3.4.1. “Salt-in-Water” Electrolytes

The “salt-in-water” electrolyte is based on the “salt-water” effect, and is developing rapidly, although it is relatively late in the research. The “salt-in-water” electrolytes used in aqueous zinc-ion batteries can be broadly categorized into multi-component “salt-in-water” electrolytes and single-component “salt-in-water” electrolytes. They not only help to expand the electrochemical stabilization window of the aqueous electrolyte and inhibit the dissolution of active materials in the anode, but also mitigate the development of zinc dendrites and stabilize the zinc anode by forming a stable solid electrolyte interface. The basic components of conventional aqueous electrolytes are mainly water and salt. A very small amount of salt is dissolved in water to form a dilute solution, commonly referred to as a “salt-in-water” electrolyte. When the concentration of salt steadily increases to the point where it exceeds water in both weight and volume, a “salt-in-water” electrolyte structure is formed. For example, a 30 m ZnCl₂ electrolyte can be operated efficiently down to −70 °C for 450 h of stable operation [88]. On the other hand, the application of salt-in-water electrolytes also facilitates the operation of batteries or supercapacitors at very high temperatures (more than 100 °C), because salt-in-water electrolytes effectively prevent the electrolyte from evaporating at high temperatures [89].

3.4.2. Aqueous Low-Eutectic Electrolyte

By adjusting the types and ratios of zinc salts and organic solvents, aqueous low-eutectic electrolytes with different physical and chemical properties can be prepared. According to the way water is involved in their preparation, aqueous low-eutectic electrolytes can be roughly divided into two categories: aqueous co-solvent low-eutectic electrolytes and hydrated low-eutectic electrolytes. The aqueous co-solvent low-eutectic electrolyte has a high ionic conductivity and low viscosity and is prepared by adding an appropriate amount of water to the deep eutectic electrolyte, whereas the hydrated low-eutectic electrolyte consists of zinc salts containing crystalline water directly with organic solvents without the need for additional water as a co-solvent, and this electrolyte can provide such advantages as stabilized zinc plating/peeling, inhibition of zinc dendritic crystal growth, and a wider electrochemical stabilization window [90]. Dimethyl sulfoxide (DMSO) has been selected as a co-solvent for aqueous low-eutectic electrolytes because of its room temperature melting point, high dielectric constant, and high Gutmann donor number. For example, a Zn//Zn battery using DMSO could operate stably for more than 1200 h at −20 °C without observing the formation of dendrites or by-products, in contrast to a common electrolyte that freezes at low temperatures and fails to operate. DMSO was able to preferentially dissolve Zn²⁺ and promote the removal of water molecules from the solvated sheath of Zn²⁺. In addition, the hydrogen bond formed between DMSO and H₂O significantly reduces the freezing point of the electrolyte, thereby substantially improving the low-temperature cycling stability of aqueous zinc-ion batteries [91].

3.4.3. Electrolyte Additives

The introduction of additives into the electrolyte is considered to be the most direct and effective method, eliminating the need for a complex electrode design process and the excessive use of inert substances for protective layer protection. The current views on the mechanism of action of additives can be divided into the following two types: first, coordination additives regulate the solventized structure of Zn²⁺ by replacing part of the solvated water molecules, inhibit the activity of H₂O, and attenuate the interaction between Zn²⁺ and H₂O, thus inhibiting side reactions [31,92]; second, adsorbent additives enhance the electrode interface performance, and they are usually preferentially and uniformly adsorbed on the surface of the Zn anode to form a water-repellent layer that induces the uniform deposition of Zn²⁺, while isolating the Zn anode from direct contact with the free water and inhibiting the hydrogen precipitation and corrosion that occurs at the anode interface [93,94].

Yang et al. [95] introduced triethylammonium ion (TMA) as an additive into the ZnCl₂ electrolyte (Figure 9a). As shown in Figure 9b, DFT calculations indicate that the binding energy of [ZnCl₄(TMA)₃]⁺ is higher (−22.1632 eV) compared to Zn(H₂O)₆²⁺ (−15.3272 eV). Thus, Zn²⁺ is more inclined to form ZnCl₄(TMA)₃⁺. The Zn//Zn symmetric cell with TMA electrolyte additive could be stably cycled for over 2000 h at 1 mA cm⁻² current density, whereas the Zn//Zn battery without the TMA electrolyte additive short-circuited after only 35h (Figure 9c). This suggests that TMA is preferentially adsorbed onto the zinc anode surface and effectively enhances Zn anode stability through the formation of ZnCl₄(TMA)₂ complexes. This complex was able to exclude solvated water molecules, thus inhibiting the hydrogen evolution reaction. The XRD map of the circulating zinc foil (Figure 9d) showed that the peak of the by-product Zn₅(OH)₈Cl₂·₂O (ZHC) was significantly weakened by the introduction of the additive, which was attributed to the fact that the repulsive effect of the TMA cation on the water molecules could effectively inhibit the generation of by-products. Therefore, the TMA cation as an electrolyte additive can effectively enhance the reversibility of zinc plating/stripping at the zinc anode. Mao et al. [96] introduced g-butyrolactone (GBL) into a ZnSO₄ electrolyte, in which the carbonyl group in GBL coordinates with Zn²⁺, chelating into the solvated sheath and increasing ionic stability (Figure 9e). The Zn//Zn battery using the 2MG1 electrolyte had a stable cycle life of 1170 h under 10 mA cm⁻² test conditions. In comparison, the Zn//Zn battery using a blank electrolyte short-circuited after 70 h of cycling (Figure 9f). This is due to the fact that GBL preferentially adsorbs to the surface of the Zn anode, effectively inhibiting 2D diffusion at the Zn anode interface and promoting the uniform deposition of Zn ions.

Highly polar organic molecules can effectively inhibit the growth of Zn dendrites due to their “electrostatic shielding” effect. As shown in Figure 9g, the use of a small amount (2 vol%) of diethyl ether (Et₂O) as an electrolyte additive has a significant enhancement effect on Zn²⁺ deposition [97]. Highly polarized Et₂O molecules can preferentially adsorb to the initial Zn tip due to the high local electric field generated by the continuous irregular Zn²⁺ deposition on the initially rough Zn surface. The electrostatic shielding layer effectively inhibits the inhomogeneous deposition of Zn²⁺ on the zinc surface, enabling the zinc anode to maintain a flat surface during continuous cycling. Notably, the ether molecules are relatively stable and do not participate in the electron transfer process during zinc deposition. As shown in the figure at 0.2 mA cm⁻² current density, the Zn//Zn symmetric cell containing Et₂O additive can be stably cycled for 250 h without obvious polarization voltage fluctuation, indicating that Et₂O molecules can effectively enhance the cycling stability and reversibility of zinc anodes. On the contrary, the Zn//Zn battery containing blank electrolyte had obvious voltage fluctuations during cycling and short-circuited at 150 h (Figure 9h).

4. Structural Characteristics of Polymer-Based Materials and Their Application in Aqueous Zinc Electrodes

Polymeric materials can be broadly categorized into two main groups: natural polymers and synthetic polymers. Natural polymers, such as sodium alginate [98], carrageenan [99], polyaspartic acid [100], chitosan [101], etc., are known for their excellent hydrophilicity and degradability. Synthetic polymers exhibit excellent performance in terms of mechanical properties, electrical conductivity, ionic transport capacity, and stability due to their precisely designed molecular structures, such as polyacrylonitrile [102], polyacrylamide [103], polyaniline [104], polyvinyl alcohol [105], polyacrylic acid [106], and polyvinylidene fluoride [107].

The introduction of polymer additives into aqueous electrolytes is an effective strategy to enhance the reversibility and cycling stability of zinc anodes. The mechanism of action of polymer additives can be divided into the following aspects: First, the polymer additives in the polymer chain of the functional group Zn²⁺ interact, replacing part of the coordination water, reducing the number of active water molecules in the Zn²⁺ solvated structure. Second, the polymer has a strong interaction with water molecules in the electrolyte, which can destroy the hydrogen bonding network between water molecules and thus inhibit the water activity. In addition, the polymer additives are preferentially adsorbed on the zinc anode at the zinc anode/electrolyte interface to form a functional protective layer (e.g., electrostatic shielding layer or localized hydrophobic layer), which effectively inhibits the formation of ZnO zinc dendrites and the occurrence of side reactions [108]. For example, polystyrene sulfonate (PSS) was introduced as an electrolyte additive to enhance the electrochemical performance of Zn anodes [109]. Symmetric cells of different systems were prepared to test the long cycle stability of Zn//Zn batteries at 1 mA cm⁻². The 2% PSS electrolyte could be stably cycled for 3000 h without significant voltage fluctuation. The battery without PSS electrolyte was short-circuited after 400 h of cycling. This shows that PSS enhances the diffusion kinetics of zinc ions by generating a high-density water shell with strong hydrogen bonding and a more ordered structure in the electrolyte. In addition, the adsorption of PSS chains on the Zn anode shields zinc from direct contact with water molecules and inhibits side reactions caused by water decomposition. When the polymer additive has a higher molecular weight, it has a longer molecular chain. Additive calculations were performed for both short- and long-chain PSS polymer electrolytes. Notably, the long-chain PSS electrolytes exhibited significantly better Zn²⁺ diffusion kinetics, with diffusion coefficients as high as 1.4:1 over the short-chain electrolytes.

Constructing a polymer protective layer on zinc anodes physically blocks direct contact between the zinc anode and the electrolyte, which is an effective way to inhibit interfacial side reactions and zinc dendrites. Polymer protective layers often have significant advantages over other interfacial engineering strategies. The research group of Guo [110] modified Zn anodes with insulating polymers (PVB). Thanks to the abundant polar functional groups on the polymer chain, the PVB protective layer has a good hydrophilicity and ionic conductivity, which effectively inhibits the side reactions and the formation of Zn dendrites. The cycle life of the PVB@Zn//PVB@Zn battery is up to 2200 h under 0.5 mA cm⁻² test conditions, which is nearly 10 times higher than the 260 h cycle life of the Zn//Zn battery. Wu’s group utilized cell membrane-derived phosphorylcholine amphiphiles with carboxymethyl chitosan on a zinc anode to design a phosphorylcholine amphiphile protective layer (PZIL) [111]. The PZIL@Zn//PZIL@Zn symmetric cell was cycled stably at a current density of 40 mA cm⁻² for 1000 h. However, the Zn//Zn symmetric cell shorted out after the first few cycles. This may be due to the preferential adsorption of choline groups on PZIL onto the zinc anode, which facilitates the removal of H₂O from the zinc anode surface. The charged phosphate groups lower the desolvation energy barrier of Zn²⁺ by chelating with Zn²⁺ and form faster ion migration channels.

5. Conclusions

Zinc anodes, as the main component of aqueous zinc-ion batteries, directly affect the cycling durability and lifespan of the battery. However, irregular zinc dendrites and interfacial side reactions like HER and corrosion seriously affect the interfacial stability of zinc anodes, which in turn hinders the development of aqueous zinc-ion batteries. In this paper, the main dilemmas faced by zinc anodes and their root causes are systematically sorted out, and the current mainstream solutions and their working principles are discussed in depth, with a special focus on the employment of polymer materials in enhancing the interfacial stability of zinc anodes. This review aims to provide in-depth insights and innovative ideas for the development of efficient and stable zinc anodes, contributing to research breakthroughs in related fields. In addition, this paper proposes that the design of zinc anodes can be further optimized in the following aspects.

(1)

Optimization of the solvation structure and additive species of electrolytes to develop electrolytes that maintain stability across a broad temperature spectrum. Extreme high temperatures accelerate the reaction kinetics and exacerbate the uncontrollable side reactions at the zinc anode interface, while low temperatures slow down the kinetics, increase the cell polarization, and impede the desolvation mechanism of zinc ions. Therefore, the development of electrolytes adapted to a wide temperature range is of considerable importance in promoting the commercialization of aqueous zinc-ion batteries.

(2)

Organic–inorganic composite protective layer materials were designed to enhance the zinc anode interface stability. Although the pure polymer protective layer can provide some protection, it suffers from low ionic conductivity [112], insufficient mechanical strength and stability, and is prone to be pierced and detached by zinc dendritic crystals; whereas the inorganic non-metallic protective layer, despite its high hardness, is brittle, has difficulty adapting to the volume change in the zinc anode in the charging and discharging process, and is prone to brittle cracking. By preparing organic–inorganic composites, the synergistic optimization of protective layer performance can be achieved to overcome the limitations of single materials.

(3)

Adoption of advanced characterization and monitoring techniques. Current zinc anode interfacial stability studies rely on non-in situ methods, and more sophisticated and sensitive characterization and real-time detection techniques need to be developed in order to deeply explore their working principles. The integration of electrochemistry, spectroscopy, microscopy, and structural analysis for comprehensive characterization of aqueous zinc-ion batteries is essential to advance their development and performance optimization.