Designing Highly Reversible and Stable Zn Anodes for Next-Generation Aqueous Batteries

Abstract

The global imperative for sustainable energy has catalyzed the pursuit of next-generation energy storage technologies that are intrinsically safe, economically viable, and scalable. Aqueous zinc-ion batteries (AZIBs) present a promising solution to meet these demands. However, the metallic Zn anode, the heart of this technology, suffers from fundamental electrochemical instabilities—manifesting as dendrite growth and rampant parasitic reactions (e.g., corrosion and passivation)—that critically curtail battery lifespan and impede practical application. This review offers a comprehensive overview of the latest strategies designed to achieve a highly reversible and stable Zn anode. We meticulously categorize and analyze these innovations through the three integral components of the AZIBs: (i) intrinsic anode engineering, (ii) interfacial electrolyte chemistry regulation, and (iii) separator-induced transport modulation. By delving into the core scientific mechanisms and critically evaluating each approach, this work synthesizes a holistic understanding of the structure-property-performance relationships. We conclude by identifying the persistent challenges and, more importantly, proposing visionary perspectives on future research directions. This review aims to serve as a scientific guide for the rational design of highly reversible Zn anodes, paving the way for the next generation of high-performance, commercially viable aqueous batteries.

1. Introduction

The global transition to a sustainable energy economy is one of the most pressing challenges of the 21st century, demanding the development and large-scale deployment of energy storage systems that are intrinsically safe, sustainable, and economically viable. For decades, lithium-ion batteries (LIBs) have dominated the market due to their high energy density and long lifespan [1,2,3]. However, LIBs are hindered by their reliance on flammable organic electrolytes, which pose significant environmental pollution risks and safety hazards. Additionally, the disposal of spent batteries is complex and inefficient. Moreover, the limited natural abundance of lithium continues to drive high production costs. These drawbacks have led to increasing limitations in LIB development and have spurred an intensive global search for next-generation battery technologies based on principles of safety, sustainability, and earth-abundance [4,5]. Consequently, zinc-ion batteries (ZIBs) and sodium-ion batteries (SIBs) employing aqueous electrolytes have garnered significant attention. Figure 1a and Figure 1b, respectively, present the comprehensive parameters of Li/Zn/Na metal anodes and comparative advantages of the three battery systems. However, due to the instability and irreversibility of sodium in aqueous solutions, the practical development of SIBs is severely limited. These results clearly demonstrate the superior compatibility of zinc metal anodes with contemporary sustainable energy storage requirements. Within this landscape, aqueous zinc-ion batteries (AZIBs) (Figure 1c), first proposed in 2012, have attracted significant attention from the global academic community and are widely recognized as one of the most promising alternatives [6,7]. The compelling advantages of AZIBs stem from employing aqueous electrolytes, which make them non-flammable and environmentally friendly, coupled with a metallic Zn anode [8,9,10]. Notably, the zinc anode offers a high theoretical capacity of 820 mAh g⁻¹ (5833 mAh cm⁻³), a low redox potential (−0.76 V vs. SHE), and is earth-abundant [11,12]. These attributes not only enhance the safety and environmental profile of the battery but also ensure low cost and supply chain sustainability [13,14]. These characteristics substantially expand the application prospects of AZIBs in large-scale energy storage technologies.

As an indispensable component of AZIBs, the Zn anode is undoubtedly critical to the overall battery performance. However, despite the aforementioned inherent advantages of Zn anodes, there are also several intractable challenges, which may compromise the electrochemical stability of AZIBs. Similar to the case in LIBs, Zn²⁺ ions preferentially deposit at high-activity sites during charging, forming initial protrusions. The electric field intensity at these microscopic protrusions becomes significantly enhanced compared to planar regions. This tip-effect promotes further Zn²⁺ accumulation toward the protrusions, ultimately resulting in vertically aligned dendrite growth and negatively impacting the electrochemical activity at the anode/electrolyte interfaces [15]. If left uncontrolled, the continuous growth of dendrites will inevitably pierce the battery separator, leading to short circuits [8]. Compounding this issue is the persistent thermodynamic instability of Zn in aqueous media, which leads to severe corrosion. Specifically, the inherent reactivity disparity between Zn and hydrogen drives spontaneous hydrogen evolution reaction (HER) at the Zn anode interface. In addition, Zn²⁺ ions in aqueous electrolytes are surrounded by water molecules and anions through electrostatic interactions, forming stable solvation sheaths. The water molecules coordinated to Zn²⁺ ions exhibit high reactivity and readily undergo electrochemical reduction at the Zn electrode surface, generating insulating hydrogen gas bubbles and OH⁻ through electron transfer [16]. Therefore, the HER inevitably induces localized pH variations in the electrolyte. The generated OH⁻ ions attract corrosive anions and other reactive species in the electrolyte, leading to the precipitation of passivating byproducts. This process results in the formation of electrochemically inert layers on the Zn anode surface, such as Zn hydroxide sulfate (Figure 1d) [17,18]. These byproducts further hinder the Zn plating/stripping process. In short, the intertwined issues of dendrite formation, HER, corrosion and passivation starkly underscore the intrinsic electrochemical instability of Zn anodes in AZIBs. Overcoming these obstacles is paramount for the development of advanced AZIBs and forms the central focus of this review.

The design of highly reversible Zn anodes has emerged as a critical challenge and a primary focus in the research and development of advanced AZIBs. To this end, numerous advanced strategies have been developed to address this challenge. These strategies primarily revolve through the three integral components of the AZIBs: (i) intrinsic anode engineering (e.g., coating artificial protective layers or fabricating alloy anodes to enhance stability) [19,20], (ii) interfacial electrolyte chemistry regulation (e.g., introducing electrolyte additives or designing non-liquid electrolytes), and (iii) separator-induced transport modulation (e.g., optimizing conventional separators or designing novel separators) [21,22]. This review provides a comprehensive and critical overview of these frontier strategies. By dissecting the underlying mechanisms of each approach, we distill the fundamental scientific principles that govern reversible Zn electrochemistry. Furthermore, we identify their limitations and chart forward-looking perspectives on future research trajectories. Meanwhile, the potential for synergistic cross-application between these methodologies and integration with artificial intelligence (AI) algorithms are emphasized. Our work aims to provide innovative insights for developing next-generation stable Zn anodes with high reversibility, thereby accelerating the industrial-scale deployment of safe and sustainable aqueous batteries in next-generation energy storage systems.

2. Issues and Mechanisms Involving Zn Anode

2.1. Dendrite Growth

The formation of Zn dendrites is arguably the most pernicious and well-documented failure mode plaguing AZIBs, initiating a cascade of degradation processes. During cycling, the incessant growth of high-surface-area, structurally fragile dendrites is intrinsically linked to the formation of “dead Zn”—electrochemically inactive metallic fragments that become electronically isolated from the anode current collector. This continuous and irreversible loss of active material manifests as a rapid decay of coulombic efficiency (CE) and a pronounced fade in overall battery capacity. On a macroscopic level, the uncontrolled, anisotropic growth of these sharp structures can ultimately penetrate the separator. Such a phenomenon will cause internal short circuits and trigger catastrophic battery failure [23,24]. This deleterious failure mode is fundamentally rooted in the inability to maintain a uniform electrodeposition on the Zn anode surface. Even a seemingly pristine Zn anode surface inherently possesses nanoscale topographical asperities and crystallographic defects. During the plating process, these points of irregularity serve as nucleation hotspots where charge density intensifies, creating amplified local electric fields [25,26]. This non-uniform electric field, in turn, drives a highly localized ion flux, compelling Zn²⁺ to preferentially accumulate and deposit onto these tips rather than uniformly across the anode surface. This process establishes a destructive, self-amplifying feedback loop: initial deposition creates a larger protrusion, which further intensifies the electric field, attracting even more ions and accelerating vertical, runaway growth. Therefore, the dendrite issue stems from a convoluted interplay between inhomogeneous electric fields, non-uniform ion distribution, and the resulting unstable nucleation and growth kinetics [24,27].

Based on the above analysis, achieving homogeneous nucleation is a critical design principle for highly reversible Zn anodes, which can be achieved by engineering the interfacial environment. For instance, modifying the anode surface to create a zincophilic interface can provide an abundance of low-energy nucleation sites, effectively lowering the nucleation overpotential and encouraging uniform initial plating [28]. Concurrently, regulating the fast and even transport of ions to the anode surface is equally important. Optimizing electrolyte composition and separator design can induce faster ion transfer kinetics and shorten ion concentration gradients. These strategies demonstrate significant efficacy in suppressing Zn dendrites.

2.2. Hydrogen Evolution Reaction

Beyond the morphological instability of dendrites, the Zn anode confronts a fundamental thermodynamic predicament when interfaced with aqueous electrolytes. This conflict manifests primarily as the parasitic HER, which can be categorized into spontaneous HER and electrochemical HER [29]. The higher reactivity of Zn compared to hydrogen induces spontaneous oxidation of Zn anodes in weakly acidic electrolytes:

Zn + 2H⁺ → Zn²⁺ + H₂↑

(1)

Moreover, due to the significantly lower standard reduction potential of Zn²⁺/Zn (−0.76 V vs. SHE) compared to hydrogen evolution (0 V vs. SHE), thermodynamically favorable electrochemical HER inevitably occurs during each charging process [30,31]. The reaction equations are as follows:

2H₂O + 2e⁻ → 2OH⁻ + H₂↑ (neutral or alkaline electrolytes)

(2)

2H⁺ + 2e⁻ → H₂↑ (acidic electrolytes)

(3)

The irreversible reaction generates hydroxide ions, altering local pH conditions and compromising Zn utilization efficiency [32]. Simultaneously, accumulated hydrogen gas on the Zn anode surface not only physically masks active sites but also increases internal battery pressure [33]. This compromises cycle life and CE while posing risks of swelling or rupture [34,35]. Critically, HER and dendrite growth are not independent phenomena. The H₂ bubbles that blanket the anode disrupt the uniformity of the ion flux and electric field, creating localized “hot spots” that elevate the overpotential required for Zn deposition. This, in turn, exacerbates the very conditions that favor dendrite nucleation and growth. The newly formed, high-surface-area dendrites then provide an abundance of fresh, catalytically active sites for further hydrogen evolution, thus accelerating the entire degradation cascade [25].

The molecular origin of this relentless interfacial attack is rooted in the solvation chemistry of Zn²⁺ in water. In a typical aqueous electrolyte, Zn²⁺ form octahedral [Zn(H₂O)₆]²⁺ solvation structures through electrostatic interactions with surrounding water molecules [36]. During plating, these solvation sheaths are transported to the anode surface. The water molecules within these sheaths, brought into direct contact with the anode, are highly susceptible to reduction. They become the primary fuel for HER. Therefore, the underlying mechanism driving HER is the electrochemically mediated interaction between the Zn anode and these reactive, solvating water molecules [29]. This insight reveals a critical strategic principle: mitigating HER requires more than simply blocking water; meanwhile, it demands a fundamental re-engineering of Zn²⁺ solvation environment to reduce the activity of interfacial water [37].

2.3. Corrosion and Passivation

The cascade of degradation does not end with HER. The localized alkalinization of the interface, caused by the accumulation of hydroxide ions from HER, creates a fertile chemical environment for further corrosion and passivation. These reactions, between the hydroxide ions and the electrolyte salt, precipitate a host of electrochemically inert inorganic species directly onto the anode surface which exhibit low solubility and porous structures [38]. This process forms a physical passivation layer that blankets the active surface and chokes the kinetics of Zn deposition and dissolution. It also dramatically increases the interfacial impedance [39], thereby severely compromising battery lifespan and performance. The specific chemical composition of this detrimental layer is dictated by the electrolyte’s anionic chemistry: for instance, Zn₄SO₄(OH)₆·4H₂O in ZnSO₄ electrolytes [38], ZnO/Zn(OH)₂ in ZnCl₂ electrolytes [40], and Zn₅(OH)₈(NO₃)₂·2H₂O in Zn(NO₃)₂ electrolytes [41]. Notably, this analysis reveals a crucial insight: dendrite growth, HER and passivation are not discrete failure modes. Rather, they are a destructive trinity, locked in a self-perpetuating cycle of degradation. The process unfolds as follows: (i) initial dendrite protrusions create additional active sites that accelerate HER; (ii) HER generates insulating H₂ bubbles that disrupt uniform ion flux, promoting further dendrite growth, and also produces hydroxide ions; (iii) the excess hydroxide ions triggers the formation of a passivating layer; (iv) this unevenly distributed passivation layer further exacerbates current density heterogeneity, creating new hotspots for dendrite nucleation. This vicious cycle is the fundamental reason for the poor reversibility and rapid failure of Zn anodes in AZIBs.

However, a deep understanding of this interconnected failure mechanism also illuminates the path forward. It suggests that a truly effective strategy will likely not target one issue in isolation but will instead disrupt the entire degradation cycle. Intervening at any single point—be it suppressing initial dendrite nucleation, inhibiting HER, or preventing byproduct precipitation—can weaken the entire destructive cascade. Indeed, the most advanced strategies often achieve synergistic effects by developing multifunctional mechanisms from diverse directions, as discussed below.

3. Intrinsic Anode Engineering

3.1. Inorganic Coatings

3.1.1. Oxides Coatings

The engineering of artificial solid-electrolyte interphases (SEIs) using chemically inert and mechanically robust inorganic oxides constitutes a foundational and highly effective strategy for passivating Zn anodes. These coatings serve as multifunctional shields that not only physically obstruct dendrite proliferation but also dielectrically modulate the interfacial electrochemical environment. Based on their primary mechanism of action, they can be broadly classified into two generations: conventional insulating oxides and functional transition metal oxides.

Conventional oxides have attracted significant attention due to their high natural abundance, non-polluting nature, and excellent insulating properties. Historically, Al₂O₃ is one of the first materials investigated. It was initially shown to raise the HER overpotential when applied via a simple chemical solution method, confirming its efficacy in mitigating corrosion [42]. The advent of atomic layer deposition (ALD) enabled the fabrication of ultrathin, conformal Al₂O₃ layers, as exemplified by the 100Al₂O₃@Zn anode (Figure 2a) [43]. Such coatings physically reinforce the anode structure, enhance electrolyte wettability, and homogenize Zn²⁺ flux via a spatial confinement effect. Thus, they effectively suppress both dendrite growth and corrosion [44]. Similarly, SiO₂ layers have been shown to enforce a planar deposition morphology by providing a high density of uniform Zn²⁺ transport channels [45]. Crucially, the SiO₂ coating offers a powerful secondary benefit: it prevents solvated water molecules from approaching the anode surface, thereby starving the parasitic HER and byproduct formation reactions [46]. It has been demonstrated that no dendrites or byproducts were observed on the surface of the SiO₂-protected Zn (SiO₂-Zn) anode after long-term cycling, whether at low or high current densities (Figure 2b) [47]. As a result, the SiO₂-Zn//SiO₂-Zn symmetrical cell operates over 1600 h.

Beyond simple physical barriers, certain transition metal oxides leverage more sophisticated electrochemical phenomena. Specifically, a significant disparity in dielectric constant and conductivity exists between the well-insulating transition metal oxide coating and the Zn anode. Under the influence of the electric field, the charge accumulates at the interface, thereby producing a macroscopic polarization phenomenon, which is designated as Maxwell-Wagner polarization [48]. Coatings such as Sc₂O₃ and Nb₂O₅ leverage this effect to promote the localized enrichment of Zn²⁺, thus influencing the internal electric field. This in turn induces a uniform flux and stabilizes the nucleation of Zn²⁺ [49,50]. The insulating Sc₂O₃ coating, for instance, provides a direct and fundamental blockade against HER by physically hindering electron transport from the anode to reactive water molecules at the interface [48]. In addition, the hydrophobic multi-channel coating structure accelerates Zn²⁺ transport and ensures uniform charge distribution. Similarly, as an n-type semiconductor, Nb₂O₅’s higher work function enables efficient electron transport through ohmic contact. It combines this unique property with a high dielectric constant to simultaneously smooth ion distribution via the Maxwell-Wagner effect and accelerate both electron transfer and Zn deposition kinetics. This results in a dense, flat Zn deposition morphology (Figure 2c) and remarkable cycling stability, with a low voltage hysteresis of ~112 mV for over 1000 h at 1.0 mA cm⁻²/0.5 mAh cm⁻² [51].

In summary, oxide coatings provide a potent strategy for stabilizing Zn anodes. By forming well-defined ion-conducting channels, they effectively suppress dendrite formation, while their hydrophobicity can mitigate parasitic side reactions. Nevertheless, this approach presents an inherent trade-off: the very denseness and chemical inertness that ensure stability concurrently lead to sluggish interfacial kinetics. Consequently, it compromises rate performance. Moreover, the mechanical rigidity of oxides coating is often inadequate to withstand the anode’s volumetric fluctuations during long-term cycling, leading to risks of fracture and delamination. Therefore, future research should focus on nanostructuring to enhance surface area and ion transport, developing composites with fast-ion conductors to decouple stability from conductivity. At the same time, there is an urgent need to pay attention to computationally guided discovery of amorphous, flexible oxides that achieve an optimal balance of all requisite properties.

Figure 2. (a) SEM images of bare Zn and 100Al₂O₃@Zn. Reproduced with permission from ref. [43]. Copyright 2020, Royal Society of Chemistry. (b) Schematic illustration of Zn²⁺ deposition on bare Zn and SiO₂-Zn anodes. Reproduced with permission from ref. [47]. Copyright 2024, Elsevier. (c) Schematic illustration for Zn²⁺ transport and electrodeposition behavior of Nb₂O₅-coated Zn anode in aqueous electrolyte. Reproduced with permission from ref. [51]. Copyright 2022, Elsevier. (d) Schematic illustration of morphology evolution for bare and nano-CaCO₃-coated Zn foils during Zn stripping/plating cycling. Reproduced with permission from ref. [52]. Copyright 2018, John Wiley and Sons. (e) Schematic illustration of the Zn deposition process on MMT@Zn and UMMT@Zn anodes. Reproduced with permission from ref. [53]. Copyright 2024, Elsevier. (f) Schematic diagrams of Zn deposition process on bare Zn and KL-coated Zn and a detailed schematic illustration of confined Zn²⁺ transmission in KL. Reproduced with permission from ref. [54]. Copyright 2020, John Wiley and Sons.

Figure 2. (a) SEM images of bare Zn and 100Al₂O₃@Zn. Reproduced with permission from ref. [43]. Copyright 2020, Royal Society of Chemistry. (b) Schematic illustration of Zn²⁺ deposition on bare Zn and SiO₂-Zn anodes. Reproduced with permission from ref. [47]. Copyright 2024, Elsevier. (c) Schematic illustration for Zn²⁺ transport and electrodeposition behavior of Nb₂O₅-coated Zn anode in aqueous electrolyte. Reproduced with permission from ref. [51]. Copyright 2022, Elsevier. (d) Schematic illustration of morphology evolution for bare and nano-CaCO₃-coated Zn foils during Zn stripping/plating cycling. Reproduced with permission from ref. [52]. Copyright 2018, John Wiley and Sons. (e) Schematic illustration of the Zn deposition process on MMT@Zn and UMMT@Zn anodes. Reproduced with permission from ref. [53]. Copyright 2024, Elsevier. (f) Schematic diagrams of Zn deposition process on bare Zn and KL-coated Zn and a detailed schematic illustration of confined Zn²⁺ transmission in KL. Reproduced with permission from ref. [54]. Copyright 2020, John Wiley and Sons.

3.1.2. Natural Minerals Coatings

Besides simple binary oxides, a compelling research avenue has emerged that harnesses the intricate structures and chemistries of natural minerals. This “geo-inspired” design paradigm typically utilizes materials like CaCO₃, montmorillonite (MMT), and kaolinite (KL). It is compelling not only due to their immense natural abundance and low cost but also in their inherently functional architectures—lamellar/porous structures and abundant surface active sites—that render them significant raw materials for the contemporary design of stable Zn anodes.

CaCO₃ serves as a prime example of using particle-based coatings to engineer a physically confining buffer layer [55]. The particle size is a critical design parameter; while both micro- and nano-scale CaCO₃ particles have been used, nano-particulate coatings offer superior performance [52]. This structural characteristic facilitates (i) a more uniform electrolyte concentration gradient and (ii) accelerated nucleation kinetics on the Zn surface. The abundant nanopores effectively confine small-sized Zn nuclei within the pores, physically preventing their vertical growth and achieving a reduction in electrode polarization. The lower polarization suggests a reduced nucleation energy barrier for Zn²⁺, thereby ensuring a uniform plating process (Figure 2d). Furthermore, layered silicate minerals such as MMT and KL offer a more sophisticated, dual-mode of action, combining physical guidance with potent chemical regulation. Zn-based MMT coatings, prepared by the ion-exchange method, provide ordered ion-transport channels. These channels can be used as highways for Zn²⁺ transport [56]. A portion of the Al³⁺ in this layered crystalline structure is substituted by Mg²⁺, resulting in the negatively charged interface. This intrinsic negative charge acts as an electrostatic rudder, steering and concentrating Zn²⁺ towards the anode, further ensuring a high concentration of surface Zn²⁺. The coating guarantees the stability of the battery’s operation at high current densities [57]. Building on this foundation, a study employed urea to intercalate MMT (UMMT). Its strongly Zn²⁺-adsorbing –C=O and –NH₂ groups not only accelerated Zn²⁺ diffusion but also prevented water molecules access. This mechanism amplifies the desolventizing effect of the coating and suppresses side reactions with greater efficacy (Figure 2e). The symmetric cell exhibited a significantly longer lifespan of over 1300 h at 6 mA cm⁻² with a capacity of 3 mAh cm⁻² [53]. Similarly, KL coatings function as a nanoscale sieve (selective transport of Zn²⁺), with uniform pores that restrict disordered ion movement [54], while its surface, rich in zincophilic O–H and Si–O bonds provides abundant adsorption sites that guide homogeneous electrochemical deposition trajectories of Zn²⁺ (Figure 2f).

In summary, coatings derived from natural minerals offer a compelling combination of low cost, eco-friendliness, and inherent functionality, underscore their considerable research potential in the field of inorganic metal compound materials for the stabilization of Zn anodes. The current well-established industrial system of mineral mining and processing has laid a solid foundation for the commercialization of this strategy, and is expected to provide sufficient convenience for the market entry of AZIBs with mineral coatings in the future. Nevertheless, their application is not without challenges. A critical drawback is the inherent compositional heterogeneity and batch-to-batch inconsistency of mined minerals, which can hinder reproducible performance. Furthermore, existing studies show insufficient exploration into the secondary processing of mineral materials. The true future of this field may lie not in simply applying natural minerals as coatings, but in leveraging our understanding of natural mineral architectures to create “geomimetic” mineral-derived coatings, ultimately yielding performance that surpasses that of their natural counterparts.

3.2. Macromolecule Organic Coatings

In sharp contrast to the rigid, inorganic shields discussed previously, macromolecular organic coatings provide soft and dynamic interfaces for Zn anode stabilization. Composed of lightweight elements like C, H, and O, organic compounds possess distinct advantages such as biomass-derived synthesis feasibility and tunable molecular architectures [58]. Their architectures can be precisely programmed with specific functional groups. This allows for the creation of coatings that are not merely passive barriers but active participants in regulating the complex interfacial environment.

3.2.1. Synthetic Organic Coatings

The vast design space of synthetic polymers has been leveraged to create protective coatings with targeted functionalities, such as the ultrathin covalent organic frameworks (COFs) [59], polyacrylonitrile (PAN) [60] and poly(vinyl butyral) (PVB) [61], which were first extensively explored.

As a kind of crystalline nanoporous materials [62], COFs are highly valued for their high chemical stability and high porosity. A variety of COF-based electrodes have been reported to date. For example, COFs formed from the aldehyde precursor, 1,3,5-trichloroglucitol, and aromatic amine linkers can be uniformly coated onto Zn anode surfaces via a simple immersion method and exhibit excellent water stability. The processed Zn anode possesses a significantly lower nucleation overpotential (58 mV vs. 146 mV for bare Zn) and nearly eliminates the energy barrier between initial nucleation and subsequent growth (10 mV vs. 48 mV for bare Zn) [59]. This indicates that the COFs coating promotes homogeneous nucleation and Zn²⁺ flux (Figure 3a). Additionally, the strong adhesive contact between COFs and the Zn anode interface physically suppresses lateral Zn²⁺ transport, reflecting their high interfacial compatibility. This plays a critical role in suppressing Zn dendrites. Meanwhile, the shielding effect of COFs against anions mitigates the side reactions, significantly enhancing anode stability. The coated Zn anode symmetric cell maintains stability over 420 h at a current density of 1 mA cm⁻².

Harnessing the intrinsic chemistry of flexible polymer chains, materials like PAN and PVB offer an equally potent and stabilization strategy. The zincophilic nitrogen-containing groups of PAN can act as atomic-level anchors to reduce the nucleation overpotential. Simple thermal treatment can cyclize PAN into a delocalized π-conjugation structure [63,64], redistribute the current density and homogenize the electric field distribution on the Zn anode surface, thereby suppressing dendrite growth. Building on this, Yang et al. designed a cyclized PAN (cPAN) coating layer on the Zn anode via a combined spin-coating and thermal treatment approach [60]. By further incorporating Zn(OTf)₂ into the cPAN matrix to form a composite coating layer (denoted as cPANZ), the ion transport efficiency is significantly enhanced (Figure 3b). The cPANZ-Zn symmetric cell stably operates for 600 h and the cPANZ-Zn//MnO₂ full cell retains a high capacity of 180 mAh g⁻¹ after 300 cycles, with no noticeable side products observed. Similarly, PVB can also be uniformly deposited onto the Zn anode surface via a simple spin-coating process, preventing direct contact between the anode and active water [61]. Its effectiveness stems from the highly polar oxygen-containing functional groups (hydroxyl and acetal moieties). These groups serve as powerful zincophilic sites, creating a robust adhesive bond with the anode while directing uniform electrolyte distribution. Additionally, their exceptional electrolyte wettability reduces the interfacial free energy, facilitating even plating (Figure 3c). The reported symmetric cell can operate for 2200 h.

According to the aforementioned studies, the types and abundance of functional groups in organic protective layers directly correlate with their effectiveness in safeguarding the Zn anodes. Following this principle, a high-yield carbon dots (CDs) [65], as a functional artificial interface layer, was rationally designed. The abundant polar functional groups (–CHO and –C≡N) contribute to the repulsion effect, shielding active water molecules and anions from approaching the Zn anode surface. Therefore, they mitigate side reactions and enhance Zn²⁺ reaction kinetics (Figure 3d). The electrochemically stable CDs coating enabled the Zn anode to achieve an extended cycling stability of 3000 h at 1 mA cm⁻² [65]. Additionally, a recent study integrated a Zn²⁺ conductive polymer matrix with counter-anion trapping agents to construct a near-single Zn²⁺ conducting (NSIC) protective coating that decouples cation and anion transport [66]. This sophisticated architecture synergistically integrates two components: (i) a polymer matrix decorated with sulfonic acid groups, serving as a high-mobility channel for Zn²⁺, and (ii) amine-functionalized metal–organic frameworks that act as precisely tuned “molecular cages” to selectively trap and immobilize counter-anions (Figure 3e). The synergistic integration achieves nearly single Zn²⁺ migration—quantified by a high Zn²⁺ transport number (t_Zn²⁺) of 0.91. The symmetric cell with Zn-NSIC demonstrates stable operation over 1000 h at a high current density of 10 mA cm⁻² (Figure 3f) [66]. However, these organic protective layers often fail to simultaneously achieve high ionic and electronic conductivity. To address this, a mixed-conductive polythiophene coating named pgBTTT was recently designed [67]. Unlike other polymer coatings, pgBTTT exhibits dual ion/electron conductivity, which synergistically enhances both the electrodeposition kinetics and stability of Zn anodes. Coupled with its zincophilic ethylene glycol side chains, pgBTTT significantly accelerates Zn²⁺ transport and demonstrates exceptional interfacial compatibility with the anode.

In summary, although notable progress has been achieved in engineering organic-coated Zn anodes, the intrinsic electrochemical and mechanical instability of these soft interfacial layers under long-term cycling remains a critical bottleneck. Future research is anticipated to converge on the rational design of “intelligent” macromolecular interfaces endowed with adaptive or autonomous self-healing functionalities, as well as the construction of structurally robust organic-inorganic hybrid interphases to ensure interfacial integrity and long-term performance.

Figure 3. (a) Scheme illustrating differences in the plating/stripping behaviors of bare versus COF-coated electrodes. Reproduced with permission from ref. [59]. Copyright 2021, John Wiley and Sons. (b) Schematic illustration of pure Zn and cPANZ-Zn anodes during electrochemical process. Reproduced with permission from ref. [60]. Copyright 2024, John Wiley and Sons. (c) Schematic illustration of morphology evolution for both the bare Zn and PVB-coated Zn anodes during repeated cycles of stripping/plating. Reproduced with permission from ref. [61]. Copyright 2020, John Wiley and Sons. (d) Schematic depicting the effect of CDs stabling Zn metal electrode. Reproduced with permission from ref. [65]. Copyright 2022, John Wiley and Sons. (e) Schematic illustration of Zn²⁺ transference on Zn-NSIC anode; (f) galvanostatic cycling performances of symmetrical cells with Zn-NSIC and bare Zn anodes at 10 mA cm⁻²/5 mAh cm⁻². Reproduced with permission from ref. [66]. Copyright 2025, John Wiley and Sons.

Figure 3. (a) Scheme illustrating differences in the plating/stripping behaviors of bare versus COF-coated electrodes. Reproduced with permission from ref. [59]. Copyright 2021, John Wiley and Sons. (b) Schematic illustration of pure Zn and cPANZ-Zn anodes during electrochemical process. Reproduced with permission from ref. [60]. Copyright 2024, John Wiley and Sons. (c) Schematic illustration of morphology evolution for both the bare Zn and PVB-coated Zn anodes during repeated cycles of stripping/plating. Reproduced with permission from ref. [61]. Copyright 2020, John Wiley and Sons. (d) Schematic depicting the effect of CDs stabling Zn metal electrode. Reproduced with permission from ref. [65]. Copyright 2022, John Wiley and Sons. (e) Schematic illustration of Zn²⁺ transference on Zn-NSIC anode; (f) galvanostatic cycling performances of symmetrical cells with Zn-NSIC and bare Zn anodes at 10 mA cm⁻²/5 mAh cm⁻². Reproduced with permission from ref. [66]. Copyright 2025, John Wiley and Sons.

3.2.2. Natural Organic Coatings

In addition to artificially engineered organic protective coatings, recent studies have explored the application potential of natural bio-membranes as protective coatings for Zn anodes [68,69]. According to their conclusions, natural bio-membranes are not merely a “green” alternative but can deliver protective efficacy equivalent to sophisticated synthetic counterparts.

A prime example is the employing of a natural reed membrane, which possesses hierarchically organized, multi-scale ion channels (Figure 4a) [70]. This intrinsic architecture, combined with abundant hydroxyl groups along the channel walls, orchestrates a uniform and rapid flux of Zn²⁺. More profoundly, the reed membrane exhibits a unique ability to form in situ electrochemical Zn–O bonds with the underlying anode during cycling. This dynamic chemical anchoring grafts the protective layer directly onto the metal, creating an exceptionally stable and low-resistance interface that dramatically enhances the anode’s reversibility (Figure 4b).

Furthermore, a protective membrane made from chitosan (CS) and sodium alginate (SA), two abundant polysaccharides derived from marine life, has been reported [71]. The functional groups on these biopolymers aggressively compete with water for coordination sites within the Zn²⁺ solvation sheath, significantly reducing the desolvation energy of Zn²⁺. This active water displacement, coupled with the homogeneous transport pathways provided by the membrane’s hydroxyl network, simultaneously guides even Zn deposition and suppresses parasitic side reactions (Figure 4c). It has been confirmed that a symmetric cell protected by the CS/SA film can undergo stable cycling for over 6500 h (Figure 4d).

Due to their direct derivation from biomass, the acquisition cost of these materials may be lower than that of synthetic coatings upon large-scale market deployment. At the same time, their inherent biodegradability reduces the environmental burden from spent batteries, aligning more closely with contemporary green development principles in commercialization. Notably, their unique fusion of elite electrochemical performance with inherent biocompatibility offers substantial potential to address the most stringent safety demands of on-skin and in-body electronics, significantly strengthening the viability of AZIBs for the future of bio-integrated devices. However, this strategy still faces critical commercialization challenges that demand immediate attention. The physicochemical properties of bio-membranes are highly sensitive to cultivation conditions, making it difficult to ensure consistent quality during mass production. Additionally, their relatively slow natural growth cycles may not align with the rapid pace required for industrial manufacturing. Therefore, establishing standardized extraction protocols and developing high-throughput production technologies are indispensable for bringing natural bio-membrane-based AZIBs to market.

3.2.3. Organic Coatings with Halogens

A particularly powerful strategy for optimizing organic coatings involves the incorporation of halogens, leveraging their extreme electronegativity for atomic-level control over the interfacial environment. Fluorine, as the most electronegative element, has been demonstrated to strongly interact with Zn and reduce surface charge transfer resistance, making it a persistent research focus [62,72]. This principle was powerfully demonstrated through a highly oriented, crystalline 2D porous fluorinated covalent organic framework (denoted as TpBD-2F) synthesized in situ on the Zn anode [73]. The incorporation of fluorine into the zinc-friendly hydrophobic coating results in the formation of the complex one-dimensional fluorinated nanocanal network. This network aggressively repels solvated water molecules, erecting a formidable kinetic barrier that effectively suppresses the HER (Figure 5a). Meanwhile, the strong C–F dipoles create a defined electrostatic landscape, lowering the energy barrier for Zn²⁺ to shed their hydration shells. Once desolvated, these ions are guided by the channels into a highly ordered, planar deposition pattern, thereby ensuring the longevity of the Zn anode. The TpBD-2F protective layer allows symmetric Zn cells to operate stably for over 1200 h at 2 mA cm⁻². Similarly, Zhao et al. developed an additional ultrathin fluorinated organic thin-film protective coating through a more complex process. With the synergistic effect of fluorine and organic frameworks, the symmetric cell operated stably for over 1700 h at 5 mA cm⁻² [74].

In summary, protective films derived from organic halogen compounds offer advantages of zincophilicity, regulation of rapid and uniform Zn²⁺ transport, shielding of active water molecules, and high safety, markedly suppressing Zn dendrites while mitigating HER and passivation. Although most current research focuses on synthetic organic interface materials, natural biogenic membranes have proven equally meritorious and warrant significant attention. Furthermore, while fluorine has proven to be a potent tool, a vast design space remains unexplored in the systematic incorporation of other elements (e.g., B, P, S, or other halogens) to introduce new functionalities.

3.3. Alloying Anodes

The design of alloy anodes represents an important shift from extrinsic surface protection to intrinsic bulk modification. By incorporating secondary metallic elements into the Zn matrix, this approach fundamentally modulates the thermodynamic and kinetic behavior of the anode. In contrast to surface coatings, which are prone to mechanical degradation during extended cycling, the inherent structural integration of alloy anodes offers superior long-term stability and operational reliability, with its core role primarily reflected in lattice matching and altering surface electrochemical behavior.

Cu metal exhibits low cost, corrosion resistance, and a significant electrochemical potential difference with Zn, which provides the overpotential necessary for Zn dissolution [80]. These characteristics make Cu a commonly utilized anode alloying material in AZIBs. Previous studies have utilized an electrochemical-assisted annealing method to synthesize 3D nanoporous (3D NP) Zn-Cu alloys [75]. This alloy features a vast, electrochemically active surface area, which enhances Zn utilization efficiency (Figure 5b). The interfacial properties of the Zn-Cu alloy exhibit intrinsic excellent lattice compatibility with pure Zn. This near-perfect lattice matching provides a seamless, low-energy, and epitaxially guided pathway for subsequent Zn deposition. Consequently, the 3D NP Zn-Cu alloy electrode effectively mitigates Zn dendrite formation and passivation while demonstrating superior cycling performance with 5000 cycles. However, this synthesis process remains complicated and time-consuming. Subsequently, a simple displacement reaction strategy for fabricating 3D leaf-like Cu@Zn composite electrodes was reported [76]. This network can be readily prepared by immersing Zn foil in CuSO₄ aqueous solution (Figure 5c). This distinctive structure acts as a 3D conductive scaffold, providing a vast surface area that distributes current. It also offers abundant, zincophilic sites for uniform initial nucleation. The true ingenuity of this design, however, is revealed during electrochemical cycling. A highly stable intermetallic CuZn₅ phase in situ forms on the Cu surface. Benefiting from high lattice compatibility, it exhibits lower Zn binding energy, which dramatically accelerates interfacial charge transfer, allowing Zn to deposit and dissolve with minimal resistance (Figure 5d). This dynamic, self-optimizing interface enabled a symmetrical cell to achieve stable cycling for 1300 h.

In addition to Cu, alloying Zn with other common industrial metals, such as Al [77], Ti [78], and Sn [79], also regulates its electrochemical behavior and passivate its reactive surface. For instance, leveraging the structural compatibility between Al and Zn, Wang et al. fabricated a eutectic Zn₈₈Al₁₂ (at%) alloy anode with alternating nanolamellas [77]. During Zn stripping, the more electrochemically active Al in situ generates an Al/Al₂O₃ core/shell structure. This configuration deeply regulates the original electrochemical reactions on the surface of the Zn anode. Specifically, Al core protects the Zn surface from the formation of byproducts such as ZnO or Zn(OH)₂, while the insulating Al₂O₃ shell guides the uniform electro-reduction of Zn²⁺ at the Zn sites, fundamentally suppressing dendrite formation and growth (Figure 5e). The Zn₈₈Al₁₂//Zn₈₈Al₁₂ symmetric cell exhibits negligible voltage hysteresis even after operating for over 2000 h. Based on similar principles, a Zn-Ti alloy anode has recently been reported [78]. In this system, precisely controlled alloying induces the formation of the intermetallic compound TiZn₁₆ at grain boundaries. These TiZn₁₆ phases exhibiting high zincophilicity and thermodynamic stability, act as preferential nucleation sites. This significantly lowers the nucleation overpotential and mitigates the propensity for intergranular electrochemical corrosion (Figure 5f). Similarly, ZnSn alloys fabricated via ion sputtering technology can also provide uniform nucleation sites, leading to a more homogeneous electric field distribution and Zn²⁺ electroreduction deposition [79]. Moreover, during charge/discharge processes, this alloy reduces the HER overpotential and corrosion current. These combined attributes—guided nucleation and suppressed parasitic reactions (Figure 5g)—enable stable galvanostatic cycling for 1000 h in ZnSn//ZnSn symmetric cells.

Designing alloy anodes represents a shift from extrinsic surface protection to intrinsic bulk stabilization, making it one of the most promising strategies for achieving long-term reversibility. However, the critical challenge lies in fabrication. Current fabrication methods lack the scalability and precision required to create uniform, optimized alloy structures. The central challenge for the field, therefore, is to develop scalable, bottom-up fabrication routes that offer atomic-level control over composition and microstructure. Mastering such techniques will be the key to unlocking the full performance benefits of alloying Zn anodes.

To better demonstrate the specific performance of the modified anodes mentioned above,

Table 1. Main parameters and cycle lives of cells with modified Zn anodes.

Modification Strategy	Electrode	Electrolyte	Current Density/Areal Capacity (mA cm−2/mAh cm−2)	Cycle Life	CE (%)	Ref.
3.1.1. Oxides Coatings	100Al2O3@Zn	3 M Zn(SO3CF3)2	1.0/1.0	500 h	≈100	[43]
	SiO2-Zn	2 M ZnSO4	0.1/0.1	1600 h	99.8	[47]
			4.0/2.0	1200 h
	Sc2O3-coated Zn	2 M ZnSO4 + 0.1 M MnSO4	2.0/2.0	240 h	99.85	[48]
	Nb2O5@Zn	2 M ZnSO4	10.0/2.5	490 h	97	[49]
			20.0/5.0	120 h
	Nb2O5@Zn	2 M ZnSO4	1.0/0.5	1000 h	98.43	[51]
3.1.2. Natural Minerals Coatings	Nano-CaCO3-coated Zn	3 M ZnSO4 + 0.1 M MnSO4	0.25/0.05	836 h	84.7	[52]
	MMT-Zn	2 M ZnSO4	1.0/0.25	1000 cycles	96.3	[56]
			10.0/45.0	1000 h
	MMT@Zn	2 M ZnSO4	0.5/0.125	5600 h	≈100	[57]
			5.0/1.25	1800 h
	UMMT@Zn	2 M ZnSO4	1.0/0.5	2000 h	99.7	[53]
			6.0/3.0	1300 h
	KL-Zn	2 M ZnSO4 + 0.1 M MnSO4	4.4/1.1	800 h	≈100	[54]
3.2.1. Synthetic Organic Coatings	COF@Zn	2 M ZnSO4	1.0/1.0	420 h	99.9	[59]
	cPANZ-Zn	2 M ZnSO4	0.5/-	600 h	≈100	[60]
	PVB@Zn	1 M ZnSO4	0.5/0.5	2200 h	99.4	[61]
	Zn@CDs	2 M ZnSO4	1.0/1.0	3000 h	99.6	[65]
			4.0/1.0	850 h
	Zn@NSIC	2 M ZnSO4	10.0/5.0	1000 h	99.9	[66]
			20.0/1.0	3000 h
	Zn@pgBTTT	2 M ZnSO4	2.0/1.0	1700 h	99.75	[67]
			5.0/1.0	500 h
3.2.2. Natural Organic Coatings	Reed interlayer-coated Zn	2 M ZnSO4	3.0/1.5	1450 h	99.77	[70]
			5.0/5.0	260 h
	(CS/SA)4-Zn	2 M ZnSO4	1.0/1.0	6500 h	≈100	[71]
			5.0/2.5	1750 h
3.2.3. Organic Coatings with Halogens	ZnBOF/Zn	2 M ZnSO4 + 0.05 M ZnF2	2.0/1.0	5000 h	99.6	[72]
			10.0/1.0	2500 h
	TpBD-2F@Zn	2 M ZnSO4	2.0/2.0	1200 h	99.5	[73]
	FCOF@Zn	2 M ZnSO4	5.0./1.0	1700 h	98.4	[74]
			40.0/1.0	700 h
3.3. Alloying Anodes	3D NP Zn-Cu	2 M KOH + 0.02 M Zn(CH3COO)2	2.0/-	300 h	≈100	[75]
	3D leaf-like Cu@Zn	2 M ZnSO4	0.5/0.5	1300 h	81.6	[76]
	Zn88Al12	2 M ZnSO4	0.5/	2000 h	≈100	[77]
	Zn-Ti	3 M ZnSO4	2.0/2.0	1100 h	99.85	[78]
	ZnSn	2 M ZnSO4	0.5/0.25	1000 h	≈100	[79]

summarizes their key parameters in terms of cycling performance.

4. Interfacial Electrolyte Regulation

In addition to direct anode modification, electrolyte optimization also represents a promising strategy. As the ionic conduction medium between cathode and anode, electrolyte’s dynamic chemical environment dictates the behavior of Zn²⁺, the stability of the electrode-electrolyte interface, and the ultimate performance of the entire cell. Precisely tuning the electrolyte’s components can fundamentally alter the Zn²⁺ solvation structure, charge transfer kinetics, and the formation of stable SEIs. These interventions effectively achieve uniform and reversible Zn deposition while suppressing corrosion and passivation by tailored interfacial chemistry. The recent pertinent research is advancing along four major fronts, including adjustments of the bulk solution and the complete redesign of the electrolyte: (i) regulating Zn salt concentration, (ii) developing functional electrolyte additives, (iii) designing structured hydrogel electrolytes, and (iv) pioneering all-solid-state electrolytes.

4.1. Zn Salt Concentration

The concentration of Zn salt serves as a critical parameter in governing the solvation structure, interfacial side reactions, and deposition morphology of Zn²⁺. The fundamental electrochemical behavior of Zn electrodes can be systematically optimized by modulating the coordination ratio between Zn²⁺, anions and solvent molecules in the electrolyte. Theoretically, increased electrolyte concentration should result in a reduction in concentration polarization at the Zn anode surface. This phenomenon also decreases the number of water molecules in the solvated sheath of Zn²⁺ and free water molecules contained in the electrolyte itself. These alterations promote the uniform deposition of Zn and reduce the probability of hydrogen evolution through water decomposition.

Conventional aqueous electrolytes, typically containing 1–3 M Zn salt, are known as salt-in-water [81]. In this regime, an abundance of free and loosely coordinated water molecules provides ample fuel for parasitic reactions, offering only limited protection for the anode. Upon increasing the salt concentration, water-in-salt (WIS) electrolytes exhibit substantial configuration alterations in solvation structure, particularly in the near-saturation regime, which has attracted considerable research attention. For instance, a 20 M LiTFSI/1 M Zn(TFSI)₂ electrolyte restricts the movement of active water molecules, confining them predominantly within the Li solvation sheath layers (Figure 6a) [82]. In contrast, the solvation sheath of Zn²⁺ exhibits minimal water molecule content. This distinctive solvation configuration significantly impedes HER and passivation. Other WIS electrolytes, including 8 M NaClO₄/0.4 M Zn(CF₃SO₃)₂ [83] and 1 M Zn(OAc)₂/31 M KOAc [84], are designed based on the similar principle to ensure adequate stabilization of the Zn anodes, which renders them suitable for long-life battery cycling. However, the generally low molar ratio of Zn in such electrolytes limits the number of nucleation sites on the Zn anodes surface. To resolve this, a recent study explored the impact of Zn molar ratio on the properties of highly concentrated salts [85]. The comparative analysis revealed that interconnected, extensive, and dense Zn nuclei were formed on the Zn foil as the Zn molar ratio increased. This finding indicates that an increase in Zn²⁺ concentration brings about more nucleation sites, consequently leading to a decrease in nucleation overpotential. This study identified an optimal Zn molar ratio of 0.5, which balanced the need for reduced water activity with the requirement for abundant nucleation sites to achieve superior rate capability. While all the aforementioned studies employed multi-cation design strategies, Cai et al. reported a ZnCl₂/ZnBr₂/Zn(OAc)₂ aqueous electrolyte that precluded the introduction of cations other than Zn²⁺. They also achieved a record supersolubility up to 75 M, breaking physical solubility limits by formation of acetate-capped water-salt oligomers [86]. Without doubt, this work provides a novel insight for designing high-concentration Zn salt electrolytes.

However, it is important to note that an underlying fact is often overlooked: pure deionized water is electrochemically inert and highly resistant to electrolysis. This implies that the primary culprit for the HER is not free water, but rather the water molecules that become polarized and activated once bound within the Zn²⁺ hydration shell. This insight leads to a fascinating and plausible hypothesis: decreasing the concentration of Zn salts would instead inhibit the side reactions. Based on this counterintuitive theory, an ultra-low concentration of Zn salt (0.3 M ZnSO₄) was reported [87]. The reduced content of hydrated Zn²⁺ is equally effective in suppressing the HER and widening the electrochemical window (1.4~2.3 V of Zn-Fe₄[Fe(CN)₆]₃ full cell and 0.5~1.5 V of Zn-polyaniline (PANI) full cell as shown in Figure 6b). Further advancing this field, a groundbreaking evaluation metric—the anionic polarity index (API)—was recently reported [90]. This index provides deeper mechanistic insights into how anions in low-concentration zinc salts suppress water dissociation and modulate solvation sheaths. These paradigm-shifting discoveries collectively underscore the significance of fundamental research in this field.

4.2. Electrolyte Additives

The deployment of electrolyte additives is arguably the most cost-effective and scalable strategy for enhancing Zn anode stability [91]. Notably, such additives typically deliver multifaceted benefits: (i) enhanced Zn²⁺ transport kinetics, (ii) preferential planar Zn deposition, and (iii) effective suppression of interfacial corrosion. To date, reported electrolyte additives can be broadly classified into inorganic and organic categories.

4.2.1. Inorganic Additives

Certain inorganic oxides have been demonstrated to effectively reconfigure the Zn²⁺ solvation sheaths when applied as anode coatings, suggesting their promising application as functional electrolyte additives. Among these, Al₂O₃ nanoparticles have recently attracted significant attention for their dual functionalities in AZIBs [92]. On one hand, Al₂O₃ modifies the solvation structure of Zn²⁺, reducing desolvation energy and improving ion diffusion coefficients and reaction kinetics. On the other hand, it suppresses active water molecules by disrupting the hydrogen-bonding network, effectively guiding Zn deposition and mitigating side reactions. Additionally, SiO₂ molecules inherently possess abundant silanol bonds and hydroxyl groups on their surfaces, which are beneficial for adsorbing active water molecules and weakening hydrogen-bonding interactions [93,94]. When combined with Al₂O₃ in electrolytes, a “soggy-sand” electrolyte can be formulated (Figure 6c), where the SiO₂ acts as a stationary “sand” that immobilizes free water, while the Zn²⁺, facilitated by Al₂O₃, flow through the resulting water-depleted channels [88]. This cooperative design enabled the symmetric cell to exhibit a prolonged stability of 2500 h.

Beyond insoluble oxide particles, a wide range of soluble ionic additives, such as Na⁺ [95], Mn²⁺ [96], ${\mathrm{I}}_3^{-}$ [97], and Br⁻ [98], have been employed in recent years to enhance the stability of AZIBs. These additives not only contribute to the stabilization of the Zn anode but also, in many cases, improve the electrochemical performance of the corresponding cathode. Among them, Mn²⁺ has been extensively utilized in Zn-MnO₂ battery systems where it mitigates cathode dissolution. But its utility does not readily transfer to other battery types, reflecting an inherent limitation shared by most ionic additives. Therefore, there is an urgent need to explore more universally applicable inorganic additives that can robustly and consistently protect the Zn anodes across a variety of AZIBs chemistries. In addition, to explore novel mechanisms for suppressing Zn dendrites, the Ce³⁺ trivalent cations were introduced into AZIBs [89]. Morphological studies revealed that the Ce³⁺ preferentially migrates to and adsorbs at the very tips of nascent dendrites on the anode surface. This accumulation of positive charge homogenizes the electric field creating a dynamic, self-healing electrostatic shielding effect that locally repels the incoming flux of Zn²⁺. The effect causes a transition from instantaneous nucleation to flattened progressive nucleation of Zn²⁺ (Figure 6d). With 0.01 M Ce₂(SO₄)₃ additive, symmetric cells can cycle stably for over 700 h at 5 A g⁻¹. Similarly, the introduction of CeCl₃ that creates analogous dynamic shielding over Zn protrusions, effectively suppressing dendrites, with symmetric cells remaining stable for 2600 h at 2 mA cm⁻² [99].

In short, inorganic additives have gradually gained widespread attention due to their low-cost, simplicity, and eco-friendly nature. However, as reported above, current research has predominantly focused on the impact of single additives, resulting a lack of in-depth exploration into the synergistic effects between different additives. While the successful pairing of SiO₂ and Al₂O₃ particles offers potential for cumulative benefits among oxides, a significant knowledge gap remains. The true frontier lies in understanding and designing systems that bridge different additive classes—for instance, how a stationary nanoparticle might interact with a dynamic, mobile ionic shield. Deconvoluting these complex, multi-component interactions is the critical next step toward the rational design of advanced.

4.2.2. Organic Additives

Relative to inorganic additives, organic additives display unique advantages owing to their superior structural diversity and molecular tunability. Through precise molecular design strategies, including functional group modification and molecular chain length control, these materials enable stable chemical modulation, establishing them as a cutting-edge approach for enhancing Zn anode stability. These strategies primarily operate through two distinct mechanisms: forming protective surface films or actively remodeling the Zn²⁺ solvation sheath.

This approach utilizes organic molecules that spontaneously adsorb onto the Zn surface, creating a robust physical and electrochemical barrier. For example, the polar groups on fucoidan (FCD), a natural polysaccharide, enable it to chemisorb onto the anode. When incorporated into the original electrolyte as an organic corrosion inhibitor, FCD effectively mitigates the impact of corrosive elements in aqueous environments through the strong interaction between its electronegative oxygen atoms and Zn atoms (Figure 7a) [100]. Similarly, as a molecular additive, (aminomethyl)phosphonic acid (AMPA) not only exhibits specific zincophilic affinity in the electrolyte but also restructures active water molecules into a highly ordered hydrogen-bonding network (Figure 7b) [101]. This engineered interphase homogenizes the Zn²⁺ flux and raises the energy barrier for parasitic reactions, enabling lifespans exceeding 900 h at a demanding 5 mA cm⁻². In addition, Peng et al. recently used nonionic amphoteric polysorbate (PS) as a functional additive, which can produce preferential chemisorption on the Zn surface and form the directional arrangement at the Zn/electrolyte interface [102]. These PS molecules can also assist in the construction of organic-inorganic hybrid interphase, which effectively homogenizes electric fields and suppresses side reactions. Remarkably, the PS-containing electrolyte enables an ultra-long cycle life of 8060 h in Zn//Zn symmetric cell.

A more advanced strategy employs organic molecules that directly intervene in the Zn²⁺ coordination environment to facilitate its desolvation, such as cetyltrimethyl ammonium bromide (CTAB) [103], dimethyl sulfoxide (DMSO) [104], β-cyclodextrin (β-CD) [105], glycine/valine molecules [106], and N,N-di-(2-picolyl)ethylenediamine (NDPA) [107]. Specifically, the Br⁻ counter-ion from CTAB and the DMSO exhibit similar effects: both can interact with water molecules and displace them from the primary Zn²⁺ coordination sphere, forming more stable solvation sheaths with fewer bound water molecules. The difference lies in DMSO’s additional ability to confer temperature-resistant properties to aqueous batteries, a feature analogous to the previously reported antisolvent methanol [108]. The inner diameter of β-CD with cavity feature is larger than that of [Zn(H₂O)₆]²⁺, thus contributing to the entry and desolvation of [Zn(H₂O)₆]²⁺. The carboxyl and amino functional groups in the small molecule glycine/valine are able to chelate with metal ions and displace solvated water molecules in a strongly alkaline environment. Notably, The NDPA with four solvation sites can regulate Zn²⁺ diffusion with efficiency. The introduced –NH₂ functional groups form hydrogen bonds with surrounding water molecules, creating significant steric hindrance (Figure 7c) and reducing the dehydration energy (Figure 7d). Benefiting from these features, fast and even Zn deposition is achieved on the anode (Figure 7e). In a similar manner, butane-2,3-dione (BD) employs the steric site-blocking effect to repel solvated Zn²⁺ clusters and increase the charge transfer impedance. It homogenizes the Zn²⁺ deposition process by kinetically regulating the electroreduction process [109]. Moreover, as a novel additive, β-alanyl-L-histidine (AH) exhibits dual functionalities [110]. It not only reconstructs the solvation structure through its diverse functional groups but also adsorbs on the Zn surface to form an interphase that can homogenize the Zn²⁺ flux. Benefiting from these dual characteristics, the incorporation of AH enables stable operation of symmetric cells for 6000 h. The effectiveness of these additives in suppressing Zn dendrite formation, parasitic reactions, and interfacial passivation has been experimentally validated, providing novel insights for the design of highly stable Zn anodes.

Figure 7. (a) Schematic illustration of the anode interfacial mechanism in 1 M ZnSO₄ (ZSO) aqueous solution electrolytes and ZSO with 25 mM FCD (FCD-ZSO) electrolytes. Reproduced with permission from ref. [100]. Copyright 2024, John Wiley and Sons. (b) Comparative schematic of interfacial water structures in benchmark ZnSO₄ electrolyte (BE) and ZnSO₄ electrolyte containing the AMPA additive (BE/AMPA). Reproduced with permission from ref. [101]. Copyright 2023, John Wiley and Sons. (c) Schematic representation of the molecule design principles for the super-zincophilic NDPA; (d) desolvation process and the corresponding desolvation energy values of the solvated Zn²⁺ in DPA- and NDPA- containing electrolytes; (e) schematic illustration of Zn plating process in ZSO and ZSO + NDPA electrolytes. Reproduced with permission from ref. [107]. Copyright 2025, John Wiley and Sons.

Figure 7. (a) Schematic illustration of the anode interfacial mechanism in 1 M ZnSO₄ (ZSO) aqueous solution electrolytes and ZSO with 25 mM FCD (FCD-ZSO) electrolytes. Reproduced with permission from ref. [100]. Copyright 2024, John Wiley and Sons. (b) Comparative schematic of interfacial water structures in benchmark ZnSO₄ electrolyte (BE) and ZnSO₄ electrolyte containing the AMPA additive (BE/AMPA). Reproduced with permission from ref. [101]. Copyright 2023, John Wiley and Sons. (c) Schematic representation of the molecule design principles for the super-zincophilic NDPA; (d) desolvation process and the corresponding desolvation energy values of the solvated Zn²⁺ in DPA- and NDPA- containing electrolytes; (e) schematic illustration of Zn plating process in ZSO and ZSO + NDPA electrolytes. Reproduced with permission from ref. [107]. Copyright 2025, John Wiley and Sons.

In summary, the extensive chemical tunability of organic molecules provides a versatile platform for Zn anode interfacial engineering. Although the performance varies with structural features, their underlying mechanisms converge toward a common goal: controlling Zn²⁺ solvation, transport, and deposition. Importantly, additive design must also ensure cathode compatibility to achieve synergistic electrolyte-electrode optimization at the full-cell level. Looking ahead, AI and machine learning are expected to accelerate the discovery and rational design of organic additives, enabling data-driven electrolyte formulation strategies.

4.3. Hydrogel Electrolytes

Despite extensive research on liquid electrolytes, their inherent limitations—such as leakage risks and rigid form-factor constraints—present significant challenges for practical applications. As a promising alternative, quasi-solid-state hydrogel electrolytes, constructed from densely crosslinked polymeric networks, have garnered considerable attention. Their excellent electrochemical properties and robust mechanical strength not only protect Zn anodes effectively but also enable the fabrication of shape-conformable and flexible batteries, meeting the energy storage demands of wearable devices (Figure 8a) [111].

Polymer backbones such as polyvinyl alcohol (PVA) and polyacrylamide (PAM) provide the fundamental mechanical integrity. While simple crosslinking suffices for basic function, next-generation hydrogels require exceptional fatigue resistance to withstand the stresses of repeated Zn plating/stripping. A key strategy involves creating densely interpenetrating networks. For instance, a chitosan (CS) and PAM dual-network hydrogel (C-PAMCS) [111], formed by pre-mixing the Zn salt into the precursor solution before crosslinking, exhibits dramatically superior mechanical robustness compared to its single-network counterpart (Figure 8b). This enhanced resilience, stemming from efficient stress dissipation within the interpenetrating network, is critical for achieving ultra-long cycle life under high-rate and high-capacity conditions. Accordingly, it enables stable operation for over 1500 h at a demanding 10 mA cm⁻²/10 mAh cm⁻² (Figure 8c). This represents significant improvements over traditional aqueous electrolytes (AE).

Beyond mechanical support, the hydrogel can host molecules or additives that regulate interfacial chemistry. For example, incorporating zincophilic betaine (BT) molecules and ${\mathrm{BF}}_4^{-}$ anions into a gelatin/PAM hydrogel creates a synergistic system [112]. The BT molecules preferentially adsorb on the Zn surface to facilitate desolvation, while the ${\mathrm{BF}}_4^{-}$ anions react in situ to form a ZnF₂-rich transport layer. This engineered interface within the gel matrix simultaneously guides uniform Zn²⁺ flux and suppresses dendrite growth (Figure 8d), leading to stable cycling for 2400 h.

Furthermore, the intrinsic water content of hydrogels renders them susceptible to freezing at sub-zero temperatures, leading to ion transport failure and device malfunction. To overcome this limitation, Wang et al. incorporated ethylene glycol (EG) into the guar gum (GG)/ammonium alginate (SA) system to develop an anti-freezing GG/SA/EG hydrogel electrolyte (Figure 8e) [113]. The physicochemical crosslinking of the natural polymers GG and SA increased the ionic conductivity to 25.37 mS cm⁻¹. The antifreeze mechanism involves the formation of stable molecular clusters between EG and H₂O molecules. These clusters compete with hydrogen bonds in water, thereby disrupting the formation of ice crystal lattice and lowering the freezing point. Consequently, the GG/SA/EG hydrogel electrolyte maintained an ionic conductivity of 6.19 mS cm⁻¹ even at −20 °C. Additionally, the cosolvent strategy is also applicable to anti-freezing hydrogel design. For instance, introducing N,N-dimethylformamide (DMF) to construct a binary solvent (H₂O-DMF), followed by mixing with PAM and ZnSO₄, yielded a hydrogel electrolyte (denoted as PZD) [114]. The synergistic effect of DMF and PAM not only guided uniform Zn²⁺ flux (Figure 8f), but also disrupted the hydrogen-bonding network among water molecules, achieving broad-temperature stability for PZD. At −20 °C, symmetric cells using PZD operated stably for over 300 h at 0.1 mA cm⁻²/0.1 mAh cm⁻². Nevertheless, these anti-freezing hydrogels generally exhibit relatively low ionic conductivity, an inherent disadvantage relative to conventional liquid electrolytes.

The quasi-solid-state nature of hydrogel electrolytes is advantageous for suppressing both side reactions and corrosion. By establishing intimate interfacial contact with the Zn anode, they suppress lateral ion diffusion and promote uniform two-dimensional Zn deposition [115]. This characteristic, combined with their excellent mechanical robustness, makes them ideal candidates for flexible energy storage systems. Despite these merits, their practical implementation is severely hampered by low ionic conductivity. Therefore, future design strategies can focus on constructing straight-oriented ion channels and high-porosity multi-network structures within hydrogel electrolytes to achieve high ionic conductivity.

4.4. All-Solid-State Electrolytes

Building upon the success of all-solid-state electrolytes (SSEs) in LIBs, recent efforts have introduced SSEs into AZIBs [116]. By replacing the liquid component with a solid ion conductor, dendrite penetration and leakage are fundamentally eliminated. However, unlike Li⁺ ions, Zn²⁺ ions with a higher valence state exhibit stronger electrostatic interactions with polymer chains, which restricts the transport kinetics of Zn²⁺ and results in lower ionic conductivity and higher interfacial impedance [117]. Since ion migration primarily occurs in amorphous regions, reducing the crystallinity of the electrolyte is considered a promising strategy to enhance ionic conductivity. The currently reported solid-state electrolytes are mainly based on low-crystallinity polymers such as poly(ethylene oxide) (PEO), PAN, and PAM. For instance, copolymerizing PEO with low-molecular-weight polypropylene glycol (PPG) can improve the overall ionic conductivity of the electrolyte to 0.075 mS cm⁻¹. To further optimize performance, Nancy and Suthanthiraraj incorporated zinc triflate at varying mass percentages, achieving a high ionic conductivity of 0.69 mS cm⁻¹ at room temperature [118]. Additionally, the introduction of nano-fillers like ZnO into PEO-based electrolytes has been shown to improve mechanical properties such as flexibility and Young’s modulus [119].

Based on current research progress, although the incorporation of appropriate Zn salts and inorganic nano-fillers has improved the overall performance of all-solid-state electrolytes, these advancements still fall short of meeting practical application requirements. This situation underscores the urgent need for novel material designs that can overcome the challenge of efficient divalent-ion transport in a solid-state host.

To more intuitively demonstrate the protective effect of the aforementioned electrolytes on Zn anodes,

Table 2. Main parameters and cycle lives of cells utilizing various modified electrolytes.

Modification Strategy	Electrolyte	Additives	Current Density/Areal Capacity (mA cm−2/mAh cm−2)	Cycle Life	CE (%)	Ref.
4.2.1. Inorganic Additives	1 M Zn(OAc)2 + 4 M LiOAc + NH3·H2O	Al2O3 nanoparticle	1.0/0.5	1000 h	99.2	[92]
			2.0/1.0	500 h
	2 M ZnSO4	Single-mesopore hollow SiO2 (smSiO2)	2.0/2.0	2000 h	99.93	[93]
			4.0/2.0	1600 h
			20.0/1.0	1200 h
			10.0/5.5	300 h
	2 M ZnSO4	Fumed silica (FS)	1.0/1.0	3000 h	99.58	[94]
	2 M ZnSO4	Al2O3 nanoparticle and SiO2	1.0/0.5	2500 h	99.52	[88]
			1.0/5.0	1100 h
			10.0/5.0	1000 h
	2 M ZnSO4	${\mathrm{I}}_3^{-}$	1.75/0.585	1430 h	98.83	[97]
	2 M ZnSO4	Br−	5.0/1.0	150 h	99.96	[98]
	1 M ZnSO4	Ce2(SO4)3	5.0/1.0	700 h	≈100	[89]
	2 M ZnSO4	CeCl3	2.0/1.0	2600 h	99.8	[99]
			40.0/10.0	170 h
4.2.2. Organic Additives	1 M ZnSO4	Fucoidan (FCD)	1.0/1.0	2700 h	99.5	[100]
			10.0/10.0	400 h
	2 M ZnSO4	(Aminomethyl)phosphonic acid (AMPA)	5.0/2.5	900 h	99.8	[101]
	1 M Zn(OTf)2	Polysorbate (PS)	1.0/1.0	8060 h	99.2	[102]
			5.0/5.0	4554 h
			10.0/10.0	1930 h
	1 M ZnSO4	Cetyltrimethyl ammonium bromide (CTAB)	1.0/0.5	2500 h	99.7	[103]
			2.0/1.0	2000 h
			4.0/4.0	400 h
	2 M ZnSO4	Dimethyl sulfoxide (DMSO)	1.0/1.0 (20 °C)	2100 h	99.73	[104]
			0.5/1.0 (−20 °C)	1200 h
	2 M ZnSO4	β-cyclodextrin (β-CD)	4.0/2.0	1700 h	99.56	[105]
			20.0/20.0	120 h
			40.0/20.0	77 h
	6 M KOH + 0.2 M Zn(OAc)2	Glycine	5.0/-	174 h	-	[106]
		Valine	5.0/-	204 h
	2 M ZnSO4	N,N-di-(2-picolyl)ethylenediamine (NDPA)	2.0/2.0	2000 h	99.61	[107]
			20.0/20.0	550 h
			60.0/1.0	300 h
	2 M ZnSO4	Methanol	-	-	99.7	[108]
	2 M ZnSO4	Butane-2,3-dione (BD)	1.0/1.0	2827 h	99.7	[109]
			5.0/2.5	1276 h
			50.0/50.0	423 h
	2 M ZnSO4	β-Alanyl-L-histidine (AH)	1.0/1.0	6000 h	99.28	[110]
			10.0/1.0	2000 h
4.3. Hydrogel Electrolytes	Zn(ClO4)2-PAM/CS hydrogel electrolyte (C-PAMCS)	-	5.0/5.0	2700 h	99.9	[111]
			10.0/10.0	1500 h
	Betaine-containing gelatin/PAM/Zn(BF4)2 (gel-BT)	-	2.0/0.5	2400 h	98.2	[112]
			5.0/10.0	450 h
	GG/SA/EG/ZnSO4/MnSO4 hydrogel electrolyte	-	0.2/- (0 °C)	200 h	≈100	[113]
			0.2/- (−20 °C)	200 h
	PAM/ZnSO4/N,N-dimethylformamide (PZD)	-	1.0/1.0	1600 h	≈100	[114]
			0.5/0.5	5600 h
4.4. All-Solid-State Electrolytes	PEO/1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([Emim]OTF)/Zn(OTF)2	ZnO	0.1/0.1	1800 h	≈95	[119]

summarizes the key cycling performance parameters of these Zn anodes from the reported works.

5. Separator-Induced Transport Modulation

The separator plays a crucial role in AZIBs by isolating the two electrodes to prevent short circuits while facilitating ion transport and blocking electron transfer. An ideal separator should not only allow efficient ion movement but also exhibit robust mechanical strength and strong compatibility with both the cathode and anode. Since separators are typically in direct contact with the Zn anode, their physicochemical properties not only reflect their protective effect on the Zn electrode but also influence the battery’s overall electrochemical performance and safety. Current separator design strategies can be broadly categorized into two categories: modification of conventional separators and construction of novel separators.

5.1. Modification of Conventional Separators

Commercial glass fiber (GF) separators are widely used in AZIBs due to their excellent chemical stability, high porosity, and superior water permeability. However, conventional GF separators suffer from non-uniform pore distribution and poor mechanical strength, which promote dendrite growth on the Zn surface and render them prone to puncture. As a result, they fail to protect the Zn anode effectively. A recent study comparing four common GF separators revealed that pore size and thickness have profound impacts on Zn deposition [120]. Zn deposits tend to fill pores exceeding micron-scale dimensions, leading to the formation of “dead” Zn and eventual battery short-circuit failure. Therefore, specific modifications to conventional GF separators are necessary to address these issues.

One prominent strategy is to apply functional coatings onto the GF scaffold. For instance, Wang et al. proposed a phthalocyanine (Pc)-modified GF (Pc@GF) separator [121]. During Zn deposition, Pc molecules coordinate with Zn²⁺ to form a macrocyclic conjugated ring structure, which subsequently generates an electrostatic field layer on the Zn surface, inducing uniform Zn²⁺ diffusion (Figure 9a). Additionally, the Pc@GF separator accelerates ion transport kinetics, enabling symmetric cells to stably operate for 700 h at 2.0 mA cm⁻²/2.0 mAh cm⁻². Coating GF with hydrated titanic acid (HTO) has also been proven effective [122]. The highly electronegative oxygen in HTO facilitates electron loss from Zn metal. At the same time, its strong water adsorption capability accelerates Zn²⁺ desolvation. Experimental results demonstrate that the modified separator ensures stable Zn anode cycling for over 1400 h. Moreover, CS suitable for organic coating can also be applied to coat GF separators [123]. Its hydroxyl, amino, and ether groups enable preferential adsorption of protons, Zn²⁺, and sulfate ions. The reduced proton activity effectively suppresses the HER, while the interaction between Zn²⁺/sulfate and CS increases the t_Zn²⁺, extending the operational lifespan of the Zn anode to 2900 h at 0.5 mA cm⁻²/0.5 mAh cm⁻² (Figure 9b). Similarly, coating GF separators with polyimide enriches them with –CO–N–CO– groups strongly interacting with Zn²⁺, thereby altering the Zn²⁺ solvation structure [124]. This modified separator enables the Zn anode to achieve stable cycling for 6000 h at 1 mA cm⁻².

In addition to coating strategies, architecting pore structure with heterolayer designs is also an effective strategy. This approach focuses on physically re-engineering the geometry of the ion pathway. A dual-layer separator (GF/BP) composed of a cellulose-based butter paper and a glass fiber membrane was designed [125], delivering stable symmetric cell operation for 5000 h at 0.5 mA cm⁻² (Figure 9c). This seemingly simple stacked architecture also achieves a highly reversible Zn anode, offering a novel design insight for separators modification.

5.2. Construction of Novel Separators

Although modifications can improve the compatibility between conventional GF separators and Zn anodes, them fail to address a critical limitation of the separator’s thickness. The development of thin and uniform separators is crucial, as it not only shortens ion migration pathways but also reduces concentration polarization, which plays a critical role in suppressing Zn dendrites. Therefore, increasing numbers of novel ultra-thin separators have been reported in recent studies.

Fang et al. designed a 3D long-range ordered PAN nanofiber separator via electrospinning and applied it to Zn//ammonium vanadate (NVO) batteries. The N-Zn bonds on its surface effectively guided uniform Zn²⁺ transport (Figure 9d) [126]. This PAN separator, with a thickness of 69 µm, is 8 times thinner than conventional GF separators, which increases the t_Zn²⁺ to 0.85 and reduces the electrolyte concentration gradient. Another recent study reported a commercial hydrophilic polytetrafluoroethylene (PTFE) membrane as a viable alternative to GF separators, marking significant advancement [127]. The average pore size of approximately 200 nm on the PTFE membrane is much smaller than that of the GF separator (1.6 µm). Its uniformly distributed microporous structure prevents Zn particle deposition within pores, achieving rapid and homogeneous ion diffusion kinetics (Figure 9e). With a thickness of 50 µm, the PTFE membrane also achieved a high t_Zn²⁺ of 0.81. COFs, widely utilized for anode coatings and electrode optimization, can also be strategically engineered into functional separators. A representative case is the 25-μm-thick Zn²⁺-substituted sulfonate COF (COF-Zn) membrane, whose quasi-single-ion conductivity (t_Zn²⁺ = 0.87) and electronegative properties improve Zn anode’s stability by lowering water activity [129]. Zn//Zn batteries utilizing the COF-Zn film demonstrated a lifespan exceeding 2900 h at 0.2 mA cm⁻². Moreover, an optimized anisotropic separator (V-NFC-CS), consisting of nanofibrillated cellulose (NFC) and CS, can also overcome the challenge of achieving both a reduced thickness (23 μm) and effective dendrite suppression [130]. Benefiting from the biomimetic design, the mechanical properties can be considerably improved and Zn²⁺ transport (0.67) can be significantly facilitated within the vertically arranged channels. Therefore, the V-NFC-CS separator simultaneously manifests substantial Young’s modulus of 7.3 GPa and large ionic conductivity reaching 20.5 mS cm⁻¹. In addition, to further push the limits of separator thinness, Yang et al. developed an ultra-thin aramid nanofiber (ANF) separator with an astonishing thickness of only 5 µm through a simple chemical synthesis method (Figure 9f) [128]. The rich polar functional groups modified the Zn²⁺ solvation structure, reducing the desolvation energy from 1.12 eV to 0.81 eV. The crosslinked nanoporous structure, low desolvation energy, and excellent mechanical strength synergistically stabilize Zn plating morphology (Figure 9g). This design allows the Zn anode to stably operate for over 850 h at 5 mA cm⁻²/2.5 mAh cm⁻².

The aforementioned findings underscore a critical evolution in the understanding of separators: they are no longer viewed as simple physical dividers but as functional components essential for achieving highly reversible Zn anodes. The ultimate goal of separator engineering is to create an ideal anisotropic architecture, featuring an array of straight, vertically aligned channels. This would provide a perfectly uniform current distribution and the shortest possible ion transport path, simultaneously eliminating dendrites and minimizing cell impedance. While modifying conventional GF separators can improve their electrochemical performance, they fail to address the issue of excessive thickness. Conversely, although existing novel separator designs are versatile and tunable, they introduce higher costs. Therefore, there is an urgent need to explore cost-effective separator design technique that can effectively protect Zn anodes while maintaining performance.

To more clearly elucidate the protective effects of the aforementioned separators on Zn anodes,

Table 3. Main parameters and cycle lives of cells utilizing various separators.

Modification Strategy	Electrolyte	Separator	Current Density/Areal Capacity (mA cm−2/mAh cm−2)	Cycle Life	CE (%)	Ref.
5.1. Modification of Conventional Separators	2 M ZnSO4	Phthalocyanine-modified glass fiber (Pc-GF)	2.0/2.0	700 h	≈100	[121]
			5.0/5.0	160 h
			20.0/1.0	500 h
	2 M ZnSO4	Hydrated titanic acid-modified GF (HTO@GF)	1.0/1.0	1400 h	99.78	[122]
			2.95/2.95	300 h
	1 M ZnSO4	Chitosan-coated filter paper (CS-filter paper)	0.5/0.5	2900 h	99.6	[123]
			5.0/1.0	450 h
	0.5 M Zn(CF3SO3)2 in TMP (Trimethyl phosphate)	Tailored polyimide-coated GF (PI@GF)	1.0/1.0	6000 h	99.7	[124]
			2.0/1.0	3000 h
			6.0/1.0	1200 h
	2 M ZnSO4	Butter paper and GF membrane (GF/BP)	0.5/0.5	5000 h	99.0	[125]
			2.0/2.0	1600 h
5.2. Construction of Novel Separators	2 M ZnSO4	Polyacrylonitrile (PAN) nanofiber film	0.283/-	800 h	99.3	[126]
	2 M ZnSO4	Polytetrafluoroethylene (PTFE) membrane	0.5/0.25	3000 h	99.5	[127]
			1.0/1.0	1300 h
			10.0/1.0	900 h
	2 M ZnSO4	Quasi-single-ion conducting COF-based separator (COF-Zn)	1.0/1.0	350 h	98.3	[129]
			0.2/0.2	2900 h
	2 M ZnSO4	Anisotropic separator (V-NFC-CS)	5.0/25.0	300 h	99.0	[130]
			10.0/2.0	1000 h
	3 M Zn(CF3SO3)2 + 0.1 M MnSO4	Ultrathin (5 µm) aramid nanofiber (ANF)	2.0/1.0	1470 h	99.22	[128]
			5.0/2.5	850 h
			10.0/5.0	375 h

lists the key cycling performance parameters of these Zn anodes from the reported works.

6. Outlooks and Perspectives

AZIBs have emerged as promising alternatives to LIBs due to their inherent advantages of high specific capacity, low cost, and intrinsic safety. However, the poor reversibility of the Zn anode remains a critical bottleneck for which no sufficiently effective solution has yet been established. This review systematically analyzes three critical issues that degrade Zn anode lifespan: Zn dendrite formation, HER, surface corrosion and passivation. These interrelated phenomena present a formidable barrier to the practical application and commercialization of AZIBs. Based on rigorous theoretical frameworks, we comprehensively summarize current mitigation strategies, including Zn anode modification, electrolyte engineering, and functional separators design, which target component-level optimization across battery architectures to achieve highly reversible Zn plating/stripping. While these strategies have demonstrated remarkable achievements in advancing AZIBs technologies, substantial challenges remain in practical implementation and long-term stability.

Firstly, the fundamental principles governing Zn anode behavior are not yet fully understood. Although multiple theoretical frameworks, such as ion concentration gradient, surface electric field distribution, and solvation structure, have been proposed, quantitative correlations between these mechanisms and Zn deposition dynamics are still lacking. For instance, the extent to which varying electric field intensities differentially affect Zn plating/stripping remains unquantified. Surface coatings, electrolyte additives, and functional separators are known to modify Zn²⁺ solvation structures by replacing reactive water molecules in the solvation sheath. However, critical questions persist: what constitutes the optimal solvation sheath configuration for suppressing parasitic reactions and passivation? how do altered solvation structures influence electrochemical cycling processes at the atomic scale? To bridge this knowledge gap, a more profound understanding of these intrinsic mechanisms, ideally supported by direct visualization (e.g., through advanced techniques like in situ SEM and AFM), is imperative. Such deeper insights would enable rational design of targeted modification strategies, guiding researchers to achieve maximal performance improvements through minimal technical interventions while minimizing trial-and-error approaches and resource waste.

Secondly, there is a lack of standardized evaluation criteria. For critical parameters such as Zn anode reversibility, no universally accepted evaluation protocols currently exist. While numerous studies claim that they achieved highly stable Zn anodes and validated through prolonged cycling tests of symmetric cells, these conclusions often lack cross-study comparability due to the divergence in testing conditions. Experimental results across different works are typically based on heterogeneous parameters, including environmental temperature, current density, areal capacity, and electrolyte volume, all of which can significantly influence Zn anode longevity in distinct systems. Furthermore, the lack of unified testing protocols obscures the competitive advantages of novel research breakthrough, creating substantial barriers for advancing superior solutions. Therefore, the establishment of standardized evaluation metrics for critical parameters is imperative to facilitate rigorous comparative analysis and guide the researchers toward truly transformative solutions.

Thirdly, the synergistic application of multiple strategies remains underexplored. Most current studies focus on isolated research perspectives, often overlooking the potential for combined effects between different strategies and thus creating inherent limitations. Although Zn anode modification, electrolyte engineering, and separators design target anode protection from distinct directions, their underlying principles exhibit fundamental consistency and compatibility. For instance, CS not only serves as an organic coating material for Zn anodes but also demonstrates promising applications in hydrogel electrolyte preparation and separator coating modification. Such multifunctional material utilization could generate synergistic effects. Additionally, alloy anodes for guiding uniform Zn deposition, electrolyte additives suppressing passivation, and modified separators altering solvation structures all possess potential for concurrent application. As is well known, the ultimate industrialization of AZIBs necessitates stable energy supply under high loading and high current density. Although current AZIBs still fall short of these key indicators, the multi-strategy coupling approach will holistically enhance performance across multiple dimensions (e.g., long cycle life, high current output and high capacity). This integrated optimization is projected to bridge the gap between laboratory research and commercial applications, addressing diverse power requirements from portable electronics to industrial-scale energy storage systems. Thus, the integrated application of multiple strategies could potentially offer more comprehensive improvements in addressing interfacial challenges of Zn anodes and promoting the commercialization of AZIBs.

Finally, the systematic integration with advanced computational technologies remains insufficiently developed. As large language models and deep learning architectures undergo exponential advancement in materials science, there is an urgent need to establish an intelligent research framework tailored to Zn anode challenges. Currently, Ganti et al. have utilized machine learning algorithms to develop a data fusion machine learning-coupled meta learning model [131]. This model predicts battery properties, voltage and the specific capacity for diverse combinations of organic anode and cathode materials, enabling the identification of high-performance polymer electrodes. Furthermore, the integration of machine learning, AI, and response surface methodology has successfully optimized the fabrication and the enhancement of the properties of polymeric nanocomposites [132]. These advances provide critical insights for Zn anode optimization. Undoubtedly, harnessing high-performance computing-driven AI and machine learning algorithms can offer a powerful toolkit for deconvoluting complex mechanisms (e.g., the kinetics of dendritic growth) through sophisticated multi-scale modeling. A thorough elucidation of failure mechanisms constitutes an essential prerequisite for formulating effective optimization strategies. Notably, recent advancements in AZIBs research have generated a growing body of experimental data on Zn anode protection strategies, which constitutes an invaluable resource for training and validating these AI algorithmic models. However, as mentioned above, the currently available datasets suffer from inadequate classification, resulting in substantial disarray that significantly impedes model training processes. Moreover, the extant literature demonstrates a pronounced paucity of algorithmic implementations specifically dedicated to battery optimization, presenting formidable obstacles in establishing an intelligent computational framework for Zn anode enhancement. These challenges demand immediate scholarly attention, requiring prioritized foundational work to establish a bottom-up framework for robust, functionally viable AI-driven optimization models. The fusion of perfect intelligent models with conventional research methodologies could accelerate the targeted screening and optimization of highly reversible electrode interfaces. Therefore, the strategic integration of high-performance computational technologies represents an imperative pathway to resolve Zn anode stability issues and advance the commercialization of AZIBs.