Polymers with properties of flexibility, abundant polar groups, and eco-friendly are prospected to be a promising agents to modify the surface of Zn anodes (Hong et al., 2022b). Adsorption sites induced by the functional groups of polymers play a crucial role in transferring Zn²⁺ along the polymer chains to the reaction interface, ensuring the uniform distribution of Zn²⁺ at the molecular scale through fast ion transport rates. For example, an ion-selective polymer glue was designed and coated on the Zn surface (Jiao et al., 2021). This polymer layer suppresses the contact of the Zn anode with the electrolyte, effectively blocking water diffusion while allowing rapid Zn²⁺ ion migration and thereby facilitating the uniform electrodeposition. A multifunctional polymeric interphase composed of polyamide (PA) and zinc trifluoromethane sulfonate (Zn (TfO)₂) was also developed (Zhao et al., 2019). The PA layer with abundant hydrogen bonding effectively hinders the direct contact of water molecule with Zn, suppressing the side reactions and dendrite growth. A semi-interpenetrating network (s-IPN) structure with hydrophilic and robust polymer network was generated for the protection of Zn anode (Ye et al., 2023). Benefiting from epoxide and amine groups in s-IPN, the symmetric batteries with modified anode showed high cycling stability of over 2,000 h.

Usually, the single unamended polymer protective layers are not sufficient to significantly improve the cycling performance and mechanical property of Zn anodes. Therefore, the modification of single polymers was proposed to enhance Zn anode property. For example, the self-adaptable poly (dimethylsiloxane) (PDMS)/TiO_2−x coated Zn anode (Figure 2A). (PDMS)/TiO_2−x was designed to dynamically adapt to volume changes and inhibited dendrite growth, consequently stabilizing the Zn surface (Guo et al., 2022). Rapid and uniform transfer of Zn²⁺ were induced by the synergistic effect of oxygenvacancy-rich TiO_2−x and PDMS.

Inorganic nonmetallic materials have been employed as non-conductive coatings on metallic Zn anode, which benefits from their rapid ion conductivity and chemical inertess (Hong et al., 2021), thus effectively stabilizing the anode interface and increasing anode performance (Figure 2B). Liu et al. (2021) proposed a Zn-metal oxide Ohmic contact interface model to improve the reversibility of Zn anodes. Zn anodes modified by the metal oxides with high work function, such as CeO₂, TiO₂, WO₃, and MoO₃, distinctly attract the cations to diffuse inward. The interface modified Zn anode with CeO₂ layers exhibited rapid Zn²⁺ diffusion kinetic, enhanced Coulombic efficiency, and dendrite-free Zn²⁺ deposition.

In recent years, inorganic salts have emerged as a viable option for interface protection of Zn metals. Amorphous zinc phosphate (AZP) nanofilm with flexible thickness was generated as a solid electrolyte interphase (SEI) for Zn anode (Li et al., 2023b). Consequently, numerous shortened pathways for rapid Zn²⁺ transportation were constructed by the amorphous layer. The nanofilm with hydrophobic propterty effectively prevents the direct contact of water with the Zn anode, thereby retarding the interfacial HER and suppressing associated corrosion and passivation.

A robust inorganic interface with strong adhesion on the Zn surface can also be fabricated by a simple acid etching method. Four types acids, including oxalic acid, phytic, citric, and phosphoric acid were used to regulate the interfaces of Zn anodes (Wang et al., 2021a). With more exposing (002) plane, high corrosion-resistance and suppressed dendritic deposition were achieved. The zinc phosphate@Zn (PPZ@Zn) electrode with high extent of texturing and compact coating endowed stable Zn stripping/plating interfaces.

Alloying with one or more metal elements provides an effective method for enhancing the protection of the Zn electrode. Mechanical strength, corrosion resistance, and electrical conductivity would be improved by these alloy anodes (Hong et al., 2022a). The involvement of the electrochemically inactive metal elements, such as Cu, Ag, and Mn in the alloys reduces the atomic defects in the Zn anodes. This reduction in defects not only hinders the formation of by-products and inhibits corrosion on the Zn surface but also significantly lowers the nucleation energy barrier.

Metallic Cu, characterized by excellent chemical/physical stability and cost-effectiveness, is considered an effective element for the generation of alloy anodes. Zhou et al. (2022) demonstrated a strategy to address issues on Zn surface by electrochemically depositing Zn onto the Cu surface. Xu et al. (2021) generated a robust protective layer by immersing Zn plate into an aqueous solution of CuSO₄. Owing to the strong zincophilicity and inert nature of Cu, alleviated interface corrosion and inhibited dendrite growth were realized. Simultaneously, the structural features of Zn-Cu could tune uniform nucleation and deposition of Zn²⁺ to suppress dendrite growth, thereby endowing alloying anode with lowered nucleation overpotential and polarization, prolonged cycling stability and increased CE. The reduction potential of Sn is −0.14 V, higher than that of Zn (−0.76 V vs. SHE). Sn was selected to generate a protective layer on the Zn foil (Li et al., 2021a). The alloy anode fabricated increases the hydrogen precipitation energy barrier and lowers the deposition energy barrier, promoting the Zn²⁺ deposition kinetics.

Recently, Zhao et al. (2023a) developed a conventional metallurgical process for the construction of Zn-Ti alloy anode. The grain boundary of metallic Zn was modified by the construction of the Zn-Ti dual-phase alloy (Figure 2C). In this alloy, Ti-containing solid solutions and intermetallic compounds was formed and thermodynamically stabilized at grain boundaries. Moreover, the Ti-containing alloy anode with high interface zincophilicity substantially inhibited intergranular corrosion and HER during cycling, diminished nucleation energy barrier, and promoted favorable nucleation mode.

Apart from the direct interface engineering of the Zn anodes, the incorporation of additives into the electrolyte is regarded as another effective strategy for the in-situ construction of stable interface. For instance, polyacrylamide (PAM), a low-cost polymer containing ketone units, was employed as an effective electrolyte additive (Zhang et al., 2019). The overpotential of zinc nucleation was significantly reduced and the stability of zinc deposition as enhanced. Liu et al. (2023) found that cetyltrimethyl ammonium bromide (CTAB) not only adsorbed onto the zinc anode surface, thereby regulating Zn²⁺ deposition orientation and inhibiting dendrite formation, but also modified the solvation structure of Zn²⁺ to reduce HER. Feng et al. (2023) designed a polymer additive (PMCNA) engineered by copolymerizing 2-methacryloyloxyethyl phosphorylcholine (MPC) and N-acryloyl glycinamide (NAGA) (Figure 2D). PMCNA adsorbed onto the Zn anode surface, forming a interface protective layer in situ to resist side reactions and guided Zn nucleation depositing along the (002) plane.

In addition to organic molecules, metal ions have been used as additives added into liquid electrolytes to enhance the anode stability. For instance, Hu et al. (2022) reported that CeCl₃, when used as an additive, could effectively inhibit the growth of Zn dendrites. They revealed that Ce³⁺can be preferentially adsorb on the surface of the Zn anode, forming H₂O-poor electrical double layer that inhibits side reaction through the electrostatic shielding effect. Specifically, the Ce³⁺ ions accumulated around the Zn protrusions, forming an interfacial “electrostatic shield” with a regional positive electric field. The electric field prevented the further accumulation of Zn²⁺ ions through the electrostatic force, impeding the deposition of Zn²⁺ ions and rendering the even Zn plating upon cycling.

Anode interface engineering aims to impede the direct interaction between zinc and the electrolyte, thereby effectively mitigating dendrite formation, corrosion, and side reactions (Wu et al., 2024). Polymer and Inorganic nonmetallic materials as coatings exhibit sufficient conductivity and compatibility with Zn metal. Nonetheless, thinner coatings may be prone to cracking and shedding, compromising anode stability, while excessively thick coatings could elevate impedance and hinder ionic conductivity.

For the Zn alloy anodes, it is significant to optimize the deposition time, solution concentration, and voltage to dictate alloy property (Du et al., 2023). The researchers predominantly focus on binary alloys rather than ternary alloys. Therefore, further exploration of binary alloys is warranted.

Electrolyte additives is regarded as an effective strategy for the in-situ construction of stable interface (Zhao et al., 2023b). However, the presence of additives in the electrolyte may impact Zn ion migration, leading to decreased ionic conductivity and complicating the interface of the anode. Therefore, optimizing the concentration of additives poses a challenge for this strategy.

Due to the advantages of cost-effectiveness, abundant availability, relatively low toxicity and multivalent states (Mn²⁺, Mn³⁺, Mn⁴⁺, Mn⁷⁺), manganese-based oxides cathodes have been extensively investigated as the cathode materials for ZIBs over the past decade (Fang et al., 2018; Liu et al., 2021; Hong et al., 2023). MnO₂, as one of the most important manganese-based oxides materials, has a variety of crystal structures, including α-MnO₂, β-MnO₂, δ-MnO₂ and so on. These structures consist of [MnO₆] octahedra as the basic structural units, interconnected by shared corners or edges into chain-, tunnel-, and layered-type configurations.

As the most extensively studied of MnO₂, α-MnO₂ is composed of octahedral MnO₆ in a double chain style, possessing a 2 × 2 tunnel in its structure. Due to the substantial tunnels in its structural framework, α-MnO₂ could serve as a host material for foreign cations, including Zn²⁺ ions. For instance, Pan et al. (Pan et al., 2016) demonstrated a reversible Zn/MnO₂ system where aqueous ZnSO₄ based solution was used as the electrolyte, and α-MnO₂ nanofibers were used as the cathode materials (Figure 3B). For the α-MnO₂ nanofiber cathode, it exhibited an operating voltage of 1.44 V, a specific capacity of 210 mA h g⁻¹ at 0.2C and a capacity retention of 92% over 5,000 cycles. Sun et al. (2017) proposed an in-situ electrodeposition method to prepare a binder-free nanocrystal α-MnO₂ cathode, donated as MnO₂@CFP. Stable cycling performance up to 10,000 cycles at 6.5C with a low capacity decay rate of 0.007% per cycle was achieved. Meanwhile, the consequent H⁺ and Zn²⁺ insertion mechanism in Zn/MnO₂ battery chemistry was revealed.

β-MnO₂ is characterized by the most stable 1 × 1 tunnel-type structure constructed by MnO₆ octahedra single chains through the corner sharing. Recent studies have confirmed the excellent zinc storage performance of β-MnO₂. Liu et al. (2019) synthesized β-MnO₂ nanorods through a microwave hydrothermal method. A high-capacity delivery of 288 mA h g⁻¹ at 0.05C and cycling stability with capacity retentions of 84.3% after 1000 cycles at 2C were achieved. The capacity fading of the β-MnO₂ cathode was primarily attributed to the formation of inactive ZnMn₂O₄ after long-term charge/discharge cycling. Zhang et al. (2017) reported a high-performance rechargeable aqueous ZIBs system with β-MnO₂ cathode, which delivered a high capacity of 225 mA h g⁻¹ at 0.65C. The tunnel-structured manganese dioxide polymorphs underwent a phase transition to layered zinc-buserite on first discharging, enabling subsequent intercalation of zinc cations in the latter structure.

δ-MnO₂ is constructed by co-angled [MnO₆] octahedrons, forming a two-dimensional infinite layer. Recently, layered δ-MnO₂ was also received consierable interest as cathode materials for ZIBs. Remarkably high battery performances was demonstrated and attributed to the large interlayer distance (approximately 7.0 Å) of δ-MnO₂. Yao et al. (2023) reported a two-step strategy to prepare pre-intercalation of ammonium ions in layered δ-MnO₂ nanosheets, denoted as AMO, as the cathode material for ZIBs (Figure 3C). The AMO delivered a reversible Zn storage capacity of 128.7 mA h g⁻¹ over 10000 cycles at 4 A g⁻¹ without significant capacity decay. Furthermore, the intercalated ammonium ions serve as the interlayer pillars to expand the lattice spacing of AMO and form hydrogen-bond networks, thus relieving the Jahn–Teller effect and enabling fast Zn²⁺ ion diffusion kinetics.

Vanadium-based oxides are widely investigated as cathode materials for ZIBs due to the manifold vanadium oxidation states (V^x+ (x = 2, 3, 4, 5)) and diverse redox properties. The fundamental V–O coordination polyhedra assemble into different frameworks of vanadium oxides, with V coordination polyhedra transformed from tetrahedron through square pyramid, trigonal bipyramid and regular octahedra. The changes in both complex polyhedra and V oxidation states can be systematically tuned to accommodate Zn²⁺ ions, resulting in outstanding electrochemical performance (Wang et al., 2023; Wang et al., 2023).

V₂O₅ exhibits a layered structure bound together by weak van der Waals forces, showing a square-pyramidal coordination of V⁵⁺ linked to five oxygen atoms. The layered structure of V₂O₅ could facilitate the intercalation of Zn²⁺ and thus has been extensively investigated as an intercalation host for ZIBs. Ye et al. (2023) fabricated oxygen vacancy-enriched V₂O₅ structures, denoted as O_v-ZVO, by hydrothermal method followed by further annealing. It was revealed that the presence of oxygen vacancies reduced Zn²⁺ diffusion barriers and weakened the interaction between Zn and O atoms in Ov-ZVO (Figure 3D), thus leading to the excellent electrochemical performance. A remarkably high specific capacity of 402 mA h g⁻¹ at 0.1 A g⁻¹ and impressive energy output of 193 W h kg⁻¹ at 2673 W kg⁻¹ were demonstrated. In another investigation by Qi et al. (2023), a conductive polypyrrole (PPy) was incorporated with V₂O₅, resulting in PPy-intercalated V₂O₅ (PPy-V₂O₅). The intercalated PPy served to prevent structure collapse, contributing to the structure and cycling stability. A high reversible capacity of 450.6 mA h g⁻¹ at 0.5 A g⁻¹ and excellent capacity retention of 90.0% at 10 A g⁻¹ after 8500 cycles were achieved.

V₂O₃ is recognized as a promising electrochemical energy storage material with high capacity. V₂O₃ has a 3D structure-like tunnels formed by the edge-sharing of the adjacent octahedron with two common VO₆ octahedra. These tunnels would facilitate the intercalation of cations. The V-V chain allows for the transfer of vanadium 3 days electrons, inducing high electronic conductivity. The high electronic conductivity is an important property for application as a cathode material in ZIBs. Zhu et al. (2021) designed a vanadium defective V₂O₃ (V_d–V₂O₃) by a hydrothermal method and an ensuing annealing process. The vanadium vacancies created permanent sites for the preoccupation of a small amount of Zn²⁺, enhancing the stability of lattice and mitigating deterioration during the process of Zn²⁺ insertion/extraction. The V_d–V₂O₃ exhibited a capacity of 196 mA h g⁻¹ at 0.1 A g⁻¹ and achieved 30,000 stable cycles with a capacity retention rate of 81%.

Prussian blue analogs with features of substantial open structures, versatile ion-storage capabilities and abundant redox-active sites have garnered attention for application in ZIBs (Hurlbutt et al., 2018; Li et al., 2022). The general formula of PBAs is A _x M′ _y [M″(CN)₆] _z ∙nH₂O, where M′ and M″ represent transition-metal ions in the divalent or trivalent state, and A can be Li⁺, Na⁺, or Zn²⁺ ions. In the PBAs structure, M′-coordinated and M″-coordinated transition metal ions are interconnected by C≡N, forming a rigid open framework with a large interlayer for hosting A ions or water molecules, as well as 3D diffusion tunnels for A ions.

In a study conducted by Zeng et al. (2022), Cu-substituted Mn-PBAs double-shelled nanoboxes (CuMn-PBA DSNBs) were prepared by using efficient tannic acid etching and cation exchange methods (Figure 3E). The distinctive hollow structures, Cu substitution and Mn vacancies in the CuMn-PBA DSNBs electrode contributed to the improved Zn ion storage properties. A notable rate capability (116.8 mA h g⁻¹ at 0.1 A g⁻¹) and superior cycling stability (96.8% retention after 2000 cycles) were achieved. Yang et al. (2023) proposed the epitaxial growth of Fe on Mn-hexacyanoferrate to construct a core–shell double-atom-redox PBA (Mn@FeHCF). The in-situ surface reorganization effectively suppressed Jahn–Teller distortion and prevented Mn dissolution into the electrolyte in the core Mn-PBA. Mn@FeHCF delivered a high discharge capacity of 166 and 117 mA h g^–1 and a capacity retention of 72.4% and 83.9% over 400 and 4800 cycles at 0.1 and 2 A g^–1, respectively.

Transition metal dichalcogenides (TMDs) are typically 2D layered structures constructed by weak van der Waals forces. Transition metal dichalcogenides are denoted as MX₂, where M represents transition metals (e.g., V, Mo, Ti and W) and X denotes chalcogen atoms (S, Se and Te). Generally, TMDs can be classified into three crystal phases of 1T, 2H, and 3R with the different arrangements of the TMDs monolayers. The different arrange of the monolayers would lead to different properties (Lee et al., 2021; Yang et al., 2022; Shuai et al., 2023). Due to their sufficient spacing layer, large theoretical capacity and fast ion transfer kinetics, TMDs have been investigated as ZIB cathode. For example, He et al. (2017) demonstrated that nanosheets of VS₂ can accommodate 0.23 Zn²⁺ per formula units, giving a specific capacity of 190.3 mA h g⁻¹ at 0.05 A g⁻¹. Tan et al. (2022) employed a one-step hydrothermal method to fabricate 1T-VS₂ nanospheres assembled from nanosheets. The distinctive hierarchitectures of 1T-VS₂ delivered a high reversible capacities of 212.9 and 102.1 mA h g⁻¹ at 0.1 and 5 A g⁻¹, respectively. An ultralong cycling life with 86.7% capacity retention over 2000 cycles at 2 A g⁻¹was also achieved.

In addition to VS₂, MoS₂ has also been explored as a cathode material for ZIBs. Li et al. (2021b) reported a molecular engineering strategy (Figure 3F) which involves structure defects manufacturing and O-doping to enlarge interlayers and unlock basal planes of MoS₂ (donated as D-MoS₂-O). It was demonstrated that the incorporation of defects and O-doping in MoS₂ reduced Zn²⁺ migration barriers, leading the improved electrochemical performance. Superior rate performance (261 mA h g⁻¹ at 0.1 A g⁻¹ with 102.4 mA h g⁻¹ at 10 A g⁻¹) and cycling stability (90.5% capacity retention after 1000 cycles) were achieved.

The cathode materials for ZIBs are plagued by active materials dissolution during cycling, resulting in significant capacity decay. Consequently, preventing cathode dissolution is a critical step in enhancing the electrochemical performance of ZIBs. For instance, the dissolution of Mn arises from disproportionation and irreversible phase changes for the Mn-Based oxides, resulting in the structural collapse and the rapid decay of battery capacity. To address the issue of Mn dissolution, various strategies have been investigated, including material doping, structural modifications, and coating (Figure 4A) (Nolis et al., 2018; Venkatkarthick et al., 2020). As for vanadium compound, soluble intermediate and the intimate contact between crystal water in V-based oxides and electrolyte contribute to vanadium dissolution (Figure 4B). This challenge can be alleviated by incorporating V₂O₅ sol into the electrolyte. Moreover, approaches such as ion pre-intercalation and ion-exchange induced phase transitions have demonstrated efficacy in further fortifying the crystal structure of the material, thereby enhancing its stability in aqueous solutions.

The intercalation of H⁺ ions during discharge initiates the creation of basic zinc salts (BZS) on the surface of the cathode material, which are a type of layered double hydroxide (LDH). The precise composition and characteristics of the resulting BZS precipitate are contingent upon the electrolytes employed in battery operation. The emergence of by-products during battery cycling can give rise to several issues (Jia et al., 2020; Wang et al., 2021b). For instance, the by-products formed on the positive electrode surface hinder charge transfer at the electrolyte/cathode interface. Besides, irreversible charging and discharging processes resulting from the detachment and incomplete reversibility of ZSH formed by H⁺ intercalation. Therefore, building an artificial solid electrolyte interphase on the surface of the cathode materials can be effectively reduce the generation of by-products (Figure 4C).

As a type of multivalent battery, ZIBs faces challenges stemming from the strong electrostatic interaction between the cathode and Zn²⁺ ions, despite its inherent high specific capacity attributed to the involvement of multiple electrons in redox reactions. Compared with monovalent ions, Zn²⁺ ions own higher charge density, leading to electrostatic attraction with cathode materials, resulting in lower diffusion kinetics (Yong et al., 2020). Moreover, during deep charge/discharge cycles, a portion of Zn²⁺ ions tend to become trapped by electronegative atoms within the host lattice, inducing structural instability and reduced capacity in subsequent cycles. Introducing metal ions or water molecules into the cathode material layer offers a potential solution to alleviate the electrostatic interaction between multivalent Zn²⁺ ions and the host framework (Figure 4D). The method serves to decrease the high charge density of Zn²⁺ ions and reduce the activation energy required for charge transfer at the electrode interface. Furthermore, defect engineering can effectively decrease the electron density surrounding Zn²⁺ ions, thereby facilitating both ion diffusion and electron transfer processes.