Understanding the Electrical Mechanisms in Aqueous Zinc Metal Batteries: From Electrostatic Interactions to Electric Field Regulation

Aqueous Zn metal batteries are considered as competitive candidates for next‐generation energy storage systems due to their excellent safety, low cost, and environmental friendliness. However, the inevitable dendrite growth, severe hydrogen evolution, surface passivation, and sluggish reaction kinetics of Zn metal anodes hinder the practical application of Zn metal batteries. Detailed summaries and prospects have been reported focusing on the research progress and challenges of Zn metal anodes, including electrolyte engineering, electrode structure design, and surface modification. However, the essential electrical mechanisms that significantly influence Zn2+ ions migration and deposition behaviors have not been reviewed yet. Herein, in this review, the regulation mechanisms of electrical‐related electrostatic repulsive/attractive interactions on Zn2+ ions migration, desolvation, and deposition behaviors are systematically discussed. Meanwhile, electric field regulation strategies to promote the Zn2+ ions diffusion and uniform Zn deposition are comprehensively reviewed, including enhancing and homogenizing electric field intensity inside the batteries and adding external magnetic/pressure/thermal field to couple with the electric field. Finally, future perspectives on the research directions of the electrical‐related strategies for building better Zn metal batteries in practical applications are offered.


Introduction
Electricity generated from renewable resources, such as wind, solar, and tide, reduces our dependence on the consumption of fossil fuels.However, the distribution and intermittent issues of these clean energy resources make them unreliable for large-scale power grids.Therefore, the development DOI: 10.1002/adma.202309726 of low-cost and reliable energy storage systems has become a key challenge to solving these problems.Lithium-ion (LIBs) and sodium-ion (SIBs) batteries are currently the most attractive candidates due to their high energy density and excellent cycling performance. [1]However, using flammable and toxic organic electrolytes increases the risk of their implementation in large-scale energy storage systems.Recently, Zn metal batteries (ZMBs) have become a promising alternative due to their overwhelming advantages in safety given the use of aqueous electrolytes.The abundance of Zn element in the earth's crust, the low redox potential of Zn/Zn 2+ (−0.76 V vs standard hydrogen), and the high theoretical specific capacity of Zn metal (820 mAh g −1 , 5854 mAh L −1 ) also increase the competitiveness of ZMBs in large-scale energy storage applications. [2]queous rechargeable ZMBs have a charge storage mechanism similar to Li metal batteries, which undergoes the shuttling of Zn 2+ ions between the anode and cathode during the charge and discharge processes.The cathode materials are usually transition metal oxides, such as manganese and vanadium-based compounds, functioning as hosts for inserting and extracting Zn 2+ ions.The anode is the Zn metal that undergoes repeated electrochemical plating/stripping of Zn 2+ ions during cycling. [3]Recent studies demonstrate that poor reversibility and severe side reactions of Zn anode are the major obstacles to achieving high-performance ZMBs.First, due to the uneven surface of the pristine Zn foil, the protrusions attract a large amount of Zn 2+ ions for deposition due to the "tip effect." [4]The uneven aggregation and nucleation phenomena induce severe dendrite growth, eventually penetrating the separators and causing a short circuit.Second, metallic Zn electrodes exhibit high thermodynamic reactivity toward aqueous electrolytes, leading to severe side reactions at the Zn/electrolyte interface.The side reactions, including Zn electrode corrosion and hydrogen evolution reaction (HER), continuously consume the limited electrolytes and induce the formation of insulating by-products (e.g., Zn 4 SO 4 (OH) 6 ) on Zn anodes. [5]Therefore, research on suppressing the dendrite formation of Zn anodes as well as inhibiting the side reactions will have a significant impact on the development of practical ZMBs.
][26][27][28][29][30][31][32] For instance, Yang et al. conducted a systematic discussion on the influencing factors of dendrite formation on Zn anodes and comprehensively summarized the research progress of Zn anode protection strategies. [24]Zhang et al. thoroughly summarized the development of aqueous electrolytes from the optimization strategies to the fundamental investigations of electrolyte/electrode interfaces (EEIs), especially the feasible modification approaches and advanced characterization technologies to EEIs. [25]Li et al. revealed the mystery of water molecules in the entire electrochemical energy storage process of ZMBs, including existing problems from the perspectives of the electrolytes, anodes, and cathodes. [26]In addition, Liu et al. reviewed the influence of temperature on the electrolyte behaviors in ZMBs under harsh conditions and provided perspectives on the advanced electrolytes toward industrial applications. [33]lthough the above effects from the perspectives of electrochemical activity, thermodynamic stability, and other physical features enhance the performance of ZMBs, we have to mention that the electrical mechanisms, which play a significant role in inhibiting dendrite growth, suppressing the hydrogen evolution reaction (HER), and accelerating the reaction kinetics of Zn metal anodes, have been neglected.In particular, the Zn deposition process is jointly controlled by reaction kinetics and diffusion limitations, which are affected by the Zn 2+ ion concentration gradient, current densities, nucleation overpotentials, crystal orientations of the Zn metal substrates, etc. [23,34] Besides, the electrostatic effects, such as electrostatic attraction and electrostatic repulsion, also significantly affect the Zn 2+ ion deposition behaviors.For instance, the electrostatic repulsions between the electric double layers (EDLs) of Zn deposits usually induce a loose nucleation morphology, resulting in dendritic growth.Therefore, developing effective approaches to weaken this repulsion force could achieve a dense Zn deposition layer. [35]On the other hand, the electrostatic attractions between unlike charges attract Zn 2+ ions to migrate and deposit at preferred positions, significantly improving Zn 2+ ions diffusion dynamics and reducing concentration polarization. [36]In addition, some polar additives can generate electrostatic attraction to Zn 2+ ions to replace some wa-ter molecules in the solvation structures, significantly inhibiting side reactions.Furthermore, considering the electric field is the only driving force for Zn 2+ diffusion during the operation of ZMBs, [2] the strength and uniformity of the electric field between the anode and cathode are essential to Zn 2+ ion diffusion, nucleation, and growth processes.Meanwhile, the coupling effect between an external magnetic/pressure/thermal field and an electric field also significantly influences Zn dendrite growth.
However, to the best of our knowledge, the electrical mechanisms behind the Zn 2+ diffusion, desolvation, and deposition behaviors for dendrite-free Zn metal anodes have only been scantily reviewed.In this review, we emphasize the effects of electricalrelated electrostatic interactions and electric field regulation on the migration and deposition behaviors of Zn 2+ ions (Figure 1).The electrical mechanisms of Zn 2+ ions deposition can be classified into two categories.The first category is to utilize the repulsive/attractive effects of electrostatic interactions to suppress Zn dendrite formation.The second category is to regulate the electric field, including enhancing and uniforming the intensity of the electric field, coupling the external magnetic/pressure/thermal field with the electric field, and establishing necessary potential differences in the protective layer on the Zn anode to promote fast and homogenous Zn 2+ diffusion.Finally, we provide unresolved challenges and future perspectives based on the electrical mechanisms in ZMBs that require further in-depth research.

Regulating Zn 2+ Ions Transport, Deposition Behaviors, and Coordination Environment by Electrostatic Interactions
Electrostatic interaction is the primary interaction between anions and cations inside the battery, and the sufficient use of electrostatic interactions can effectively regulate the migration and deposition of Zn 2+ ions.Electrostatic interactions include the repulsion between like charges and the attraction between unlike charges.In this section, we summarized the strategies of using electrostatic interaction to facilitate high-efficiency and dendritefree Zn deposition in aqueous electrolytes, including reducing electrostatic repulsion of Zn deposits for compact electrodeposition, forming electrostatic shielding to inhibit dendrite growth, constructing an ideal Ohmic contact interface to facilitate electron transfer, and applying protective layers with polar functional groups and negative charges to attract Zn 2+ ions diffusion.

Reducing Electrostatic Repulsion among Zn Deposits
In aqueous ZMBs, solvated Zn 2+ ions migrate to the Zn anode under the impact of the electric field induced by the voltage difference between the cathode and anode.Then Zn 2+ ions undergo an electron reduction reaction on the anode surface.The unreduced Zn 2+ ions and H 2 O molecules in the solvated structure and SO 4  2− ions remain on the anode surface, forming an electric double layer (EDL). [37]Based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, the interactions between Zn deposits in aqueous electrolytes are mainly related to the van der Waals (VDW) attractive force and the electrostatic EDL repulsion force. [38,39]As shown in Figure 2a, the interaction between two different ions is the result of the superposition between the VDW attractive force and the EDL repulsion force, with the strongest attraction and the densest deposition at the primary minimum (Wp) value.For electrochemical Zn deposits in ZnSO 4 electrolyte, the maximum energy barrier (ΔW) from the secondary minimum value (Ws) to the primary minimum value (Wp) is too high to overcome, resulting in the deposition of Zn in a loose form.The potential measurement result demonstrates that the Zn deposits are negatively charged in aqueous ZnSO 4 electrolyte.In this case, reducing the repulsive force between the EDLs of the Zn deposits could reduce the energy barrier and induce dense Zn deposition.
The morphology of the Zn deposits is usually hexagonal platelets, and the thickness of the platelets gradually increases  and La 3+ -ZnSO 4 electrolytes; the corresponding growth models of the Zn deposits. [35]Copyright 2022, Springer Nature.
with the increase of current density in aqueous ZnSO 4 electrolyte (Figure 2b). [35]This loose and separated distribution is caused by the strong repulsive force between negatively charged Zn deposits, which prevents the consolidation of platelets.In contrast, the introduction of La 3+ ions into the aqueous electrolyte, as high-valence competitive ions of Zn 2+ ions, could effectively weaken the electrostatic repulsion between Zn deposits and form a denser deposition layer on the anode (Figure 2b).Therefore, the amount of the net charge on Zn deposits has an important impact on the deposition morphology.According to the EDL theory, the potential of the Zn surface increases from  0 to  ς after absorbing Zn 2+ ions to neutralize electrons in the ZnSO 4 electrolyte.After introducing high-valent and inert La 3+ ions into the electrolyte, one La 3+ ion can neutralize three electrons, while one Zn 2+ ion can only neutralize two electrons.More electrons on the Zn anode are neutralized in EDL in La 3+ -ZnSO 4 electrolyte, resulting in the potential  ′  and fewer net charges (Figure 2c).The weakened EDL repulsive force between the deposits facilitates coherent Zn deposition along the 002 plane.Using high-valence competitive cations to neutralize negatively charged Zn deposits provides a facile method to form compact and dense Zn deposition layers.

Forming Electrostatic Shielding
When Zn 2+ ions deposit on the anode, due to the uneven surface of the bare Zn anode, a "tip effect" will occur at the protrusions to gather a large number of electrons, thus attracting Zn 2+ ions to accumulate and deposit at the tips of the protrusions and promoting the rapid growth of dendrites.Many studies have discovered that the "electrostatic shielding" formed by electrostatic attraction can be used to shield the protruding area of the Zn anode, preventing the deposition of Zn 2+ ions and driving the migration of Zn 2+ ions toward flat areas. [40,41]The strategies for forming "electrostatic shielding" include applying electrolyte additives and optimizing the electrode structure.
To shield the protrusions of the Zn anode, diethyl ether (Et 2 O) was used as an electrolyte additive to facilitate homogenous Zn deposition. [42]When an appropriate amount of Et 2 O was added to the electrolyte, the organic molecules preferentially adsorbed on the protrusions and formed electrostatic shielding (Figure 3a).The adsorption of Et 2 O weakened the "tip effect" and hindered the deposition of Zn 2+ ions on the protrusions, inducing Zn 2+ ions to migrate and deposit in the around flat areas without Et 2 O adsorption, leading to uniform Zn deposition.The concentrations of Et 2 O in the electrolyte showed a significant impact on the deposition behavior of Zn 2+ ions.At high concentrations (>3 vol%), the excess Et 2 O molecules adsorbed on the flat areas of the Zn anode, which reduced the selectivity of Zn 2+ deposition.While, at low concentrations (<1 vol%), the incomplete coverage of the protrusions by Et 2 O molecules still led to severe dendrite growth.
The effect of brominated complexing agents as electrolyte additives on the deposition behavior of Zn 2+ ions was also studied. [43]When 1-ethyl-1-methyl pyrrolidinium bromide (MEP•Br) was added to the electrolyte at different concentrations, it was found that there were significant differences in the morphologies of deposited Zn.Furrow ripples with thinly crumpled morphology were observed in the electrolyte with 0.3 m MEP•Br.With the increasing concentrations of the MEP•Br additive, the thickness of the ripples also increased, and the gaps between the corrugated deposits gradually disappeared.The changes in morphology after the addition of MEP•Br can be explained by the electrostatic shielding mechanism.During the Zn deposition process, a large number of electrons on the protrusions attracted the aggregation of MEP cations, forming a positive electrostatic shielding on the protrusions.MEP cations showed a repulsive effect on Zn 2+ ions near the protrusions, thus inducing the deposition of Zn 2+ ions in the flat area near the protrusions.The electrostatic shielding effect of MEP cations can be maintained until the completion of Zn deposition, thus significantly inhibiting the growth of dendrites.
The electrostatic shielding formed by electrolyte additives in the above studies only regulate Zn 2+ ions migration, which prevents the aggregation and nucleation of Zn 2+ ions on the protrusions.However, this strategy cannot prevent the solvated H 2 O molecules from poisoning the Zn anodes, which causes severe side reactions (e.g., hydrogen evolution reactions) during the Zn deposition process.In addition, the desolvation process is the rate-determining step in the charge transfer into active material. [6]Therefore, exploring multi-functional electrolyte additives that can guide the uniform Zn deposition and reducing side reactions could further increase the cycling stability of Zn metal anodes.Zhang et al. used the chloride salt with bulky cation (1-ethyl-3-methylimidazolium chloride, EMImCl) as an electrolyte additive to block the Zn 2+ -water interaction, release the bound water in the cation-type solvation structure of Zn(H 2 O) 6 2+ , and form ZnCl 4 2− as an anion-type solvation structure in the electrolyte. [44]During the Zn deposition process, strong negatively charged electric fields were generated at the tips, which attracted bulky cation (EMIm + ) to form "electrostatic shielding" and repelled ZnCl 4 2− anions.Therefore, ZnCl 4 2− anions preferred to deposit on the flat surface area of the Zn anode with a weak electric field, facilitating uniform Zn deposition.Positively charged chlorinated graphene quantum dot (Cl-GQD) additives were also introduced into the aqueous electrolyte to suppress dendrite formation. [45]During the charging pro-cess, the positively charged Cl-GQDs were adsorbed onto the Zn surface, acting as an electrostatic shielding layer.Furthermore, the relative hydrophobic properties of the chlorinated groups also provided a hydrophobic protective interface for the Zn anode.
Proteins from natural resources were used as multi-functional electrolyte additives to form electrostatic shielding and suppress side reactions.A small amount of silk peptide was used as an electrolyte additive to improve the performance of the Zn anode. [46]ompared with silk sericin and silk fibroin, the silk peptide has abundant polar groups (─COOH and ─NH 2 ).Due to the strong interactions between the polar groups (─COOH and ─NH 2 ) of the silk peptide and Zn 2+ ions, [47] silk peptide tends to replace the solvated H 2 O molecules and weaken the association between Zn 2+ and SO 4 2− , resulting in a decrease of H 2 O and SO 4 2− in the modified solvation structure.Meanwhile, the silk peptide molecules anchoring on the protrusions of the Zn anode produce an electrostatic shielding effect, which prevents Zn 2+ ions 2D diffusion and accumulation at the protrusion, thus inhibiting the growth of Zn dendrites.Compared with pure ZnSO 4 electrolyte, the Zn nucleation overpotential in ZnSO 4 electrolyte containing silk peptide increased by 57 mV, resulting in reduced Zn nuclei sizes and increased nuclei population density. [48]Consequently, the excellent cycle life (3000 h) and high Coulombic efficiency (99.7%) of Zn anodes were achieved in 2 m ZnSO 4 electrolyte with only 5 mg mL −1 of silk peptide (≈0.49USD L −1 ), demonstrating potential practical applications of using silk peptide in ZMBs.
Zwitterionic ionic liquids (ZIL) contain covalently bound cationic and anionic moieties, which show potential electrochemical applications as multi-functional electrolyte additives to form electrostatic shielding.The addition of the ZIL additive changes the solvate structures of Zn 2+ ions in the electrolyte and the EDLs on both cathodes and anodes (Figure 3b,c). [49]In the ZnSO 4 electrolyte, positively charged Zn 2+ ions are tightly adsorbed on the negatively charged Zn anode surface.The H 2 O molecules and SO 4 2− anions were also dispersed nearby, forming a traditional EDL (Figure 3b), which leads to hydrogen evolution reactions and generates by-products.After adding ZIL to the electrolyte, ZIL autonomously directs its cations toward the surface of the Zn anode under the impact of an electric field, forming a selfadaptive EDL (Figure 3c).The ZIL molecules occupy the active sites of the electrode and expel the H 2 O molecules out of the EDL, which significantly suppresses the side reactions.In addition, due to the positively charged imidazole ring at one end of ImS (3-(1-methylimidazole) propanesulfonate), the ImS molecules accumulate at electron-rich protrusions under the impact of an electric field, forming a dynamic electrostatic shield to suppress dendrite growth.The other end of the ImS molecule is ─SO 3 − , which has a strong interaction with the Zn 2+ ions, ensuring the 3D diffusion of Zn 2+ ions (Figure 3d). [50]This could increase the ion migration rate and guide Zn 2+ ions deposition to flat areas.Both the Zn||Zn symmetric cell and the Zn||NaV 3 O 8 full cell exhibited excellent rate performance and extended cycling stability.
The electrode structure design also brings electrostatic shielding effects.A eutectic Zn/Al alloy anode with alternating Zn and Al lamellar nanostructures were designed to eliminate Zn dendrite formation (Figure 3e). [51]The redox potential of Al 3+ /Al  [42] Copyright 2019, Elsevier.b) Schematic illustration of traditional EDL.c) Schematic illustration of self-adaptive EDL.d) Schematic illustration of the Zn plating/stripping processes in ImS/ZnSO 4 electrolyte. [49]Copyright 2022, The Royal Society of Chemistry.e) Eutectic Zn/Al alloys with a lamellar structure composed of alternative Zn and Al nanolamellas in situ produce core/shell interlayer patterns during the Zn stripping to guide the subsequent Zn plating. [51]Copyright 2020, Springer Nature.f) Schematic illustration of Zn deposition on different modified Zn electrodes. [54]Copyright 2022, American Chemical Society.
(−1.66 V vs SHE) [52] is lower than that of Zn 2+ /Zn (−0.76 V vs SHE). [53]During the Zn deposition process, the exposed surfaces of the Al lamellas were preferentially oxidized to insulating Al 2 O 3 shells, which blocked the direct electronic contact between Zn 2+ ions and the Al lamellas.This prevented the reduction of Zn 2+ ions and formed positive electrostatic shields around the Al/Al 2 O 3 lamellas.Ultimately, Zn 2+ ions could only undergo reduction deposition on Zn lamellas between two Al lamellas.Therefore, the Zn lamellas are the Zn 2+ ions charge carriers, and the Al lamellas serve as the hosts to accommodate Zn deposition, thus significantly improving the reversibility and stability of Zn anodes.A similar strategy was also explored by sputtering the Albased protective coatings on Zn anodes to establish electrostatic shielding effects.In order to verify the optimal ratio of Al/Zn in the protective film, three different anodes were prepared (Al@Zn, Zn 0.34 Al 0.66 @Zn, and Zn 0.73 Al 0.27 @Zn). [54]As shown in Figure 3f, there are significant differences in the diffusion and deposition behaviors of Zn 2+ ions on the alloy protective layers with different Al contents.For the Al@Zn anode, due to the low redox potential of Al/Al 3+ , a dense insulating Al 2 O 3 film was quickly formed outside the Al particles, which prevented Zn deposition and generated electrostatic shields on the Al protective layer.Therefore, Zn 2+ ions could only pass through the gaps between Al particles and be deposited on the Zn foil.In order to reduce the nucleation energy, subsequent Zn 2+ ions were deposited preferentially on the previously formed Zn nuclei, leading to the growth of dendrites between Al particle boundaries.In contrast, a proper amount of Al in the protective layer (Zn 0.73 Al 0.27 ) produced moderate electrostatic shielding effects, forcing Zn 2+ ions to diffuse evenly and deposit homogeneously at the exposed Zn sites in the alloy particles.The Zn 0.73 Al 0.27 @Zn anode could realize a long lifespan of up to 3000 h at a practical operating condition of 1.0 mA cm −2 and 1.0 mAh cm −2 .

Establishing Ohmic Contact Interfaces
[57][58] Based on the type and concentration of carriers in semiconductors, semiconductors can be classified into p-type semiconductors and n-type semiconductors. [59]In p-type semiconductors, holes are the majority carriers, while in n-type semiconductors, electrons are the majority carriers.When in contact with the metals, the holes in p-type semiconductors and the electrons in n-type semiconductors migrate to the interface. [60,61]As for the Zn anode surface modification, we need to build a charge enriched region composed of electrons at the interface to attract Zn 2+ ions and promote ion transport.Therefore, some n-type semiconductors can be used as interface materials for Zn anode modification.The formation of the Ohmic contact interface is closely related to the work function (W, eV), [62] as shown in Figure 4a.The energy level of the electrons inside a solid is usually lower than that of the electrons on the surface of the solid.Therefore, electrons inside a solid are equivalent to being trapped in a "potential well."A certain amount of energy is required for the electrons to escape from the "potential well" and transfer from the solid to the outside.This part of the energy is defined as the work function.The higher the value of the work function, the harder it is for electrons to escape from the solid.When metals contact semiconductors, there are two different types of Ohmic contact interfaces.As shown in Figure 4b, if the work function value of the metal is higher than that of the semiconductor, electrons from the semiconductor side flow into the metal side and form a negative charge center near the interface.Meanwhile, electron holes are generated on the semiconductor side to create a positive charge center due to the loss of electrons. [63]An electric field from the semiconductor to the metal is formed at the interface (Figure 4d), which gradually increases with the flow of electrons until reaching a balancing status to prevent further electron diffusion.Thus, the electrons at the metal interface need to overcome a high potential energy barrier to reach the semiconductor (Figure 4e).In this case, the interface contact impedance cannot be ignored during the metal deposition process. [64]ontrarily, if the value of the work function of a metal is smaller than that of a semiconductor, electrons from the metal flow toward the semiconductor, forming a very thin and highly conductive region denoted as the negative charge center on the semiconductor side (Figure 4c).Due to the deficit of electrons, a positive charge center is formed on the metal side.In this case, there is no potential energy barrier to prevent electron diffusion and almost no interface contact impedance (Figure 4f).This is the ideal Ohmic contact model for metal deposition. [63]stablishing an ideal Ohmic contact interface requires the metals to have a lower work function than semiconductor materials.It is worth noting that the work function of Zn metal (3.6-3.8 eV) is lower than that of many metal oxides, thus establishing an ideal Ohmic contact interface on Zn anode.Liu et al. proposed the construction of a CeO 2 -based Ohmic contact interface on the Zn anode, which significantly improved the reversibility of Zn plating/stripping. [55]In Figure 5a, (E F ) Zn and (E F ) CO represent the Fermi energy levels of the Zn metal and the semiconductor oxide (CeO 2 ), respectively.E 0 is the vacuum energy level; W Zn and W CO represent the work functions of Zn metal and CeO 2 , respectively.Since the work function of Zn metal is lower than that of CeO 2 , electrons prefer to move from Zn metal to CeO 2 until they reach equilibrium status, forming an ideal Ohmic contact interface (Figure 5b).Due to the movement of electrons, an electron enrichment region is established at the interface on the CeO 2 side.The electrons in this region attract Zn 2+ ions through electrostatic interactions and significantly affect repelling anions, thereby improving the diffusion kinetics of Zn 2+ ions and reducing the nucleation barrier of Zn (Figure 5c).In addition, due to the presence of a CeO 2 protective layer, free water molecules and solvated water molecules cannot participate in the side reactions to corrode the Zn anode.
In another study, niobium pentoxide (Nb 2 O 5 ) with a high dielectric constant () was used to construct a protective layer on the Zn anode. [56]As described in Figure 5d, under the electric field inside the battery during charging, the Nb 2 O 5 layer undergoes polarization, with the positive charges converging on the side near the anode and the negative charges converging on the other side toward the cathode.Therefore, under the locally formed polarization electric field, the Nb 2 O 5 layer acts as a Zn 2+   [55] Copyright 2021, Wiley-VCH.d) Schematic illustration for Zn 2+ ions transport and electrodeposition behaviors induced by Nb 2 O 5 protective layer.e,f) The band structure of Zn metal and n-type semiconductor Nb 2 O 5 before (e) and after contact (f). [56]Copyright 2022, Elsevier.
ions transport channel to evenly distribute ions flux.At the same time, the Lewis acid active sites at the Nb atoms show strong interactions with the H 2 O molecules.Thus, the H 2 O molecules in the solvated structure were restricted outside the protective layer, effectively inhibiting the hydrogen evolution reaction. [65]ore importantly, as an n-type semiconductor, the work function of Nb 2 O 5 is appreciably higher than that of Zn metal, [66] which induces facile electron transfer through the ideal Ohmic contact interface (Figure 5e,f).The electrons in the Zn anode move toward Nb 2 O 5 , forming an Ohmic contact interface and constructing an electron enrichment region in Nb 2 O 5 , which accelerates electron transfer and induces Zn 2+ to transport rapidly and uniformly.The Nb 2 O 5 -protected Zn anode achieved significantly improved Zn plating/stripping reversibility with stable cycling performance up to 1000 h (5 mA cm −2 and 5 mAh cm −2 ).In addition, Xiong et al. reported a charge-transfer complex electrolyte additive, 7,7,8,8-tetracyanoquinodimethane (TCNQ), which was tightly adsorbed on the surface of the Zn anode and constructed an ideal Zn-Zn(TCNQ) 2 -based Ohmic contact interface. [67]This interface significantly guided rapid ions/electrons transport and suppressed direct contact between H 2 O and Zn anodes.

Constructing Protective Layers with Polar Functional Groups or Negative Charges
Constructing a protective layer with polar functional groups or negative charges on the surface of Zn anode can induce uniform transport of Zn 2+ ions through electrostatic attraction and improve the Zn 2+ ions migration rate. [7][70] A 3D nanoporous ZnO coating (Zn@ZnO-3D) layer was applied to modify the Zn anode. [71]The geometrically optimized O element significantly induced extra charge concentrations at the surface.Moreover, the lower energy barrier of Zn 2+ inserted into Zn@ZnO-3D was validated by the decrease in Gibbs free energy, which was almost eight times lower than that of bare Zn (Figure 6a).As a result, this new structure accelerated the migration and deposition kinetics of Zn 2+ ions by prioritizing electrostatic attraction to Zn 2+ ions rather than solvated Zn 2+ ions in the EDLs.Subsequently, the strong solvation sheath of free H 2 O molecules near the Zn anode is reduced, effectively preventing the generation of H 2 .As mentioned above, the Zn@ZnO-3D anode can accelerate the kinetics of Zn deposition by reducing the deposition barrier and the electrostatic attraction of Zn 2+ ions in the dense layer of EDLs, as well as reducing the desolvation energy (Figure 6b).Therefore, an average zinc utilization rate of 99.55% and long-term stability of 1000 cycles were achieved, and the Zn@ZnO-3D||MnO 2 full cell showed no capacity fading after 500 cycles at 0.5 A g −1 with a specific capacity of 212.9 mA h g −1 .
A cellulose nanocellulowhisker-graphene (CNG) membrane, which was assembled by graphene (GN) and cellulose nanocellulowhisker (CNW) to regulate the deposition of Zn. [72] GN with a high electrical conductivity is in the inner layer, while CNW with a low electrical conductivity is the outer layer.CNW has a large number of oxygen-containing functional groups (─O─H, ─C─O─C─, and ─C─O).Due to the existence of these polar functional groups, the outer surface is negatively charged, with an average potential of −24.5 mV.As depicted in Figure 6c, this negatively charged CNG membrane effectively shields the anions to suppress the side reactions and accelerates the rapid transport and uniform distribution of Zn 2+ ions to achieve a dendritefree Zn anode.Moreover, GN is hydrophobic, while CNW is hydrophilic.This "dual effect" for H 2 O includes the attraction by CNWs and the repulsion by GN, which effectively isolates Zn foils from bulk electrolyte.Thus, the CNG membrane effectively helps Zn 2+ ions shed their solvation sheath and significantly inhibits Zn anode corrosion.
In addition, a "zincophilic" polyanionic (─SO 3 − ) hydrogel was developed as the anode protective layer, which significantly inhibited the side reactions, increased the flux of Zn 2+ ions and promoted their uniform migration by using the electrostatic effect. [73]The hydrogel is rich in negative charges, and ─SO 3 − plays a crucial role in ions transport.Zn 2+ ions can be strongly trapped and smoothly transported toward Zn metal in the entire hydrogel layer.Meanwhile, the negatively charged ─SO 3 − groups can repel SO 4 2− ions.In addition, in order to firmly adhere the hydrogel to the anode surface, a silane coupling agent is also used, and a Zn─O bond is formed at the hydrogel/Zn interface.The Zn─O interaction can induce additional charge concentration at the O atom, which further generates electrostatic attraction for Zn 2+ ions, thus effectively reducing the activation energy barrier of desolvation. [71]he above-mentioned protective layers can be divided into inorganic and organic layers based on their composition.Inorganic layers densely cover the surface of the anode, preventing direct contact between the electrolyte and the anode.However, achieving high ionic conductivity at room temperature is a huge challenge.In addition, the poor mechanical flexibility makes the inorganic protective layer collapse easily due to the significant volume changes during the Zn plating/stripping processes.On the other hand, organic coatings are flexible with abundant polar functional groups, which promote the migration of Zn 2+ ions.However, their adhesion to the Zn anode is weak and they are prone to detachment during cycling.Therefore, an organic-inorganic hybrid protective layer containing Nafion and Zn-X zeolite nanoparticles was synthesized as a resilient hybrid protective layer for Zn anodes. [74]As shown in Figure 6d, Nafion contains abundant ─SO 3 − (sulfonic acid group), which strongly interacts with Zn 2+ ions, resulting in the formation of a bridge bond structure (∼SO 3 − ─Zn 2+ ∼) between zeolite nanoparticles and Nafion.The Nafion layer is composed of hydrophilic and hydrophobic regions, and only the hydrophilic channel surrounded by the hydrophilic group ─SO 3 − allows H 2 O molecules to pass through.Therefore, the limited contact between the electrolyte and the Zn anode can diminish the hydrogen evolution reaction.However, it should also be noted that due to the presence of hydrophilic channels, the pure Nafion layer can only partially block anions (SO 4 2− ) and H 2 O. [75] Based on this condition, hydrophilic Zn-X zeolite is introduced into the hydrophilic region of the Nafion layer, reducing the size of the hydrophilic cavity and further inhibiting the Zn dendrites and side reactions.Most importantly, due to the negative charge and small pore size of the framework of Zn-X molecular sieve, Zn 2+ ions can hop along the organicinorganic interface under electrostatic action, while other and its corresponding energy barrier compared with bare Zn. [71] Copyright 2020, The Royal Society of Chemistry.c) Schematic illustration of Zn plating behavior with CNG membrane and combination mode of artificial CNG layer. [72]Copyright 2021, The Royal Society of Chemistry.d) Ions transport mechanisms in Nafion-Zn-X protective layers. [74]Copyright 2020, Wiley-VCH.e) Schematic illustration of protective effect of SR layer. [76]Copyright 2022, Wiley-VCH.f) Schematic illustration of the negatively charged NSQD layer. [36]Copyright 2023, Elsevier.receptor (denoted SO 4 2− receptors as SR) membrane on Zn anode. [76]The SR membrane has abundant NH functional groups and possesses strong hydrogen bonding donor capability.Therefore, it is easy to form hydrogen bonds with SO 4 2− in the electrolyte, trapping SO 4 2− outside the membrane, thus forming a negatively charged protective layer.Due to the negative charge feature of the membrane, free SO 4 2− ions will be blocked by electrostatic repulsion, which inhibits the occurrence of side reactions (Figure 6e).The migration rate of Zn 2+ ions on the SR-coated Zn anode was significantly improved by the electrostatic attraction effect, with t Zn 2+ increasing to 0.76, effectively alleviating the reaction kinetic hysteresis caused by concentration polarization.Thanks to this versatility, the SR-protected Zn anode can achieve an average Coulombic efficiency of over 99% and excellent cycling performance of 10 000 cycles at 10 mA cm −2 .
A similar strategy was applied by Wang et al. to construct a protective layer with the negative charge on the surface of Zn anode by introducing N, S-doped carbon quantum dots (NSQDs) as additives to reduce the interface concentration gradient and achieve uniform Zn deposition. [36]The oxygen-containing groups of NSQD will be reduced by Zn foil and interact with oxidized Zn 2+ to form Zn-O bonds on the Zn surface, thereby forming the NSQD layer with abundant nucleation sites (i.e., O, N, and S-containing functional groups).The negatively charged NSQD layer significantly reduces the ions concentration gradient on the surface of the Zn anode, and homogenizes the electric field at the interface, promoting the Zn 2+ transfer at a high rate and inhibiting dendrite growth and HER (Figure 6f).The use of anionic polymers as protective layers can also regulate ion transport through electrostatic interactions.Anionic polymers generally have highly acidic substituents, such as carboxylic acids, sulfonic acids, and phosphoric acids at the side chains, and these acids undergo ion dissociation to form polymers with negative charges in the side chains. [77]Therefore, they can undergo electrostatic attractions with cations or other positively charged substances.Carboxymethylcellulose (CMC) was used on Zn-graphite composite anodes as an artificial solid electrolyte interface (ASEI), which significantly improved the lifespan of the battery. [78]Due to the presence of carboxyl groups (─COOH) in CMC, the ASEI tends to coordinate with Zn 2+ ions, providing abundant channels for rapid ion transport and reducing desolvation energies.This electrostatic attraction effect can promote the uniform distribution of Zn 2+ ions near the Zn electrode, exhibiting a rapid kinetics of Zn 2+ ions transfer.Similarly, Zhu et al. reported a method of using polyimide as a Zn anode protective layer, which can prevent Zn metal corrosion and reduce capacity loss. [69]The carbonyl oxygen in polyimide coordinated with Zn 2+ ions minimized the concentration gradient near the electrode/electrolyte interface, thereby achieving rapid kinetics and reducing the overpotentials of Zn plating/stripping.
[83][84] Importantly, the porous properties of MOFs and COFs can be utilized to couple with electrostatic interactions to improve the reversibility, cycling stability, and lifespan of Zn anodes.Wang et al. prepared a porphyrinbased 2D MOF nanoarray (denoted as Zn-TCPP). [12]The deprotonated carboxyl group and N-containing sites in Zn-TCPP increased the adsorption of Zn 2+ ions through electrostatic interactions, promoting the pre-seeding of Zn 2+ on Zn-TCPP nanoarrays layers.Subsequently, Zn 2+ ions laterally deposited on these MOF nanoarrays and further bottom-up grew from the Zn anode surface, achieving a dendrite-free anode.Besides, Zhao et al. prepared a zincophilic covalent organic framework (COF; TpPa-SO 3 H) film through interfacial reaction to stabilize Zn anode (TpPa-SO 3 H@Zn). [85]The sulfonic acid groups of the TpPa-SO 3 H film have electrostatic attraction to Zn 2+ ions, promoting uniform deposition of Zn 2+ ions.In addition, the TpPa-SO 3 H can release abundant protons (H + ) and achieve a dynamic equilibrium with OH − , thereby inhibiting the formation of byproducts such as Zn 4 SO 4 (OH) 6 •xH 2 O.Despite recent progress in Zn metal anode protection, MOFs, COFs, and their derivatives still face several challenges that require further investigations to achieve high-performance Zn anodes (i.e., increasing the stability of aqueous electrolytes). [79,82]he strategies of using electrostatic interactions, including electrostatic repulsion and electrostatic attraction, have been realized via adding electrolyte additives, designing appropriate Ohmic contact interfaces and constructing negatively charged protective layers.Reducing the electrostatic repulsion among Zn deposits suppresses the dendrite growth, which is conducive to the formation of uniform Zn deposition.This strategy has the advantages of low cost and low energy consumption, but some of the electrolyte additives are not reusable, resulting in poor sustainability.In addition, electrolyte additives can form electrostatic shields at the protrusions, significantly inhibiting dendrite growth.However, these additives have the drawback of being flammable and not environmentally friendly.Therefore, it is necessary to focus on discovering environmentally friendly and recyclable electrolyte additives.The use of electrostatic attraction promotes the transport of Zn 2+ ions, thus facilitating uniform Zn deposition.In addition, constructing multifunctional protective layers with polar functional groups or negative charges on Zn anodes improves the transport rate of Zn 2+ ions, repels SO 4 2− anions and restricts H 2 O molecules at the same time, significantly inhibiting side reactions.The uniformity, thickness, ionic conductivity, and mechanical properties of the protective layers require further optimization.

Electric Field Regulation Strategies between Anodes and Cathodes
There are two major strategies for regulating the electric field inside the ZMBs, including enhancing the intensity of the local electric field and even electric field distribution.Dielectric materials (ZrO 2 , SiO 2 , TiO 2 , BaTiO 3 , etc.) can respond to external electric fields and then polarize to generate a built-in directional electrical field, which significantly enlarges the local electric field.When the electric field strength increases, the migration rate of Zn 2+ ions also significantly increases.This effectively solves the problem of slow Zn 2+ ions migration caused by the solvated structure, accelerating the transfer kinetics and reducing the Zn 2+ ions concentration gradient.In addition, we need to consider the impact of Zn 2+ ions migration rate on battery performance and the important role of Zn 2+ ions migration direction and deposition position.Coating materials with high electronic conductivity on the Zn anode surface to drive the redistribution of surface electrons and applying a designed 3D current collector to lower the anode-localized current density can uniformize the electric field, promoting the uniform migration of Zn 2+ and avoiding the aggregation of Zn 2+ , thereby inhibiting Zn dendrite growth and improving long cycle life.

Enhance Localized Electric Field Intensity
The Zn deposition process at the anode/electrolyte interface is a dynamic equilibrium process with ions depletion and diffusion.Generally, Zn 2+ ions are coordinated by six dipolar water molecules in mild aqueous solutions, in which Zn ions mainly exist in the form of solvated (Zn(OH 2 ) 6 ) 2+ .This phenomenon significantly leads to high charge transfer resistance and sluggish kinetics during the Zn deposition. [2]Therefore, the kinetics of the Zn/Zn 2+ redox reaction are significantly higher than those of Zn 2+ ions diffusion, leading to a lower concentration of Zn 2+ ions on the surface of the Zn anodes.This limited diffusion process seriously affects Zn 2+ ions distribution.In addition, due to the difference in Zn 2+ ions concentration between the anode surface and the electrolyte, the electrochemical potential of the battery may deviate from its equilibrium status, which is known as the concentration polarization phenomenon.The Nernst−Planck equation used to describe the ions migration flux at the electrode/electrolyte interface is expressed as follows [86] In the equation, J is the ion diffusion flux, q is the unit charge, C is the concentration, D is the diffusion coefficient, K is Boltzmann constant, T is the temperature, V is the electric potential, x is the distance, and v x refer to, convective velocity.The derivative of V to x in the equation represents the electric field intensity.Therefore, enhancing the electric field intensity can increase the flux of Zn 2+ ions and reduce the concentration polarization.

Enhancing Localized Electric Field Strength by Directional Polarized Electric Field
Regulating the dielectric property of the electrode/electrolyte interface can greatly affect the Zn 2+ deposition behavior. [87][89][90][91] Using dielectric materials to construct protective layers on Zn anodes and synthesize/modify separators has been reported to enhance the localized electric field strength between cathodes and anodes. [92]The dielectric materials respond to the external electric field by means of induction, resulting in the position shift of the negative charge and positive charge centers inside the dielectric materials.The noncoincidence of positive and negative centers in the crystal structure will generate electric dipole moments.The changes in the electric dipole moment follow the direction of the electric field between the cathode and anode. [93,94]The direction polarized electric field induced by the electric dipole can promote the Zn 2+ ions flux. [95]olarization Caused by the Internal Electric Field: Protective layers made of dielectric materials can be used to generate directional polarization electric fields on Zn anodes.The mechanism of such protective layers could be explained by the Maxwell-Wagner polarization theory (called "space charge polarization" or "interfacial polarization").The Zn anode protected by the ZrO 2 layer showed excellent cycling performance and flat Zn deposition morphology. [88]The ZrO 2 coating layer can provide more controllable nucleation sites for Zn 2+ ions deposition by enhancing the localized electric field through polarization, thus guaranteeing a uniform Zn stripping/plating (Figure 7a).The ZrO 2coated Zn anode possesses a lower nucleation overpotential of 55.2 mV compared to the pristine Zn anode (75.1 mV).A similar strategy was also adopted by Wu et al. [95] They use BaTiO 3 (BTO) as the protective layer for Zn metal anodes.BTO is a typical perovskite-type material with an ABO 3 structure that exhibits polarization under an applied electric field.This is due to the deviation of Ti 4+ cations from the center of the [TiO 6 ] octahedra (Figure 7b). [96]The direction of the electric dipole moment can be switched by the external electric field, as shown in Figure 7c.Therefore, the BTO layer can provide a uniform Zn 2+ transfer channel and directional electric dipoles to guide the homogeneous Zn 2+ ions migration in stripping and plating processes.The full cell with the BTO@Zn anode exhibited a significantly improved capacity retention of 67% after 300 cycles, compared with the cell with the pristine Zn anode (only 16% capacity retention after the 66th cycle).The improved cycling performance indicates that BTO@Zn can endure extensive stripping/plating.However, it was found that there was no significant difference in corrosion potential and current values between the bare Zn anode and the BTO@Zn anode.The contribution of the BTO protective layer in preventing side reactions was insignificant.
A composite separator using cellulose nanofibers and ZrO 2 ceramic nanoparticles was prepared for ZMBs. [97]The developed ZrO 2 /cellulose separator can promote the nucleation process of Zn 2+ ions and accelerate the Zn 2+ ions diffusion kinetics due to the high dielectric constant of ZrO 2 .Under the effect of an external electric field, the positive charge center in ZrO 2 shifts toward one direction, and the negative charge center shifts to the opposite direction, forming an electric dipole, which induces a directional polarization electric field at the anode interface.Meanwhile, the electric dipole rotates under the action of torque in the external electric field, making its electric dipole moment turn in the direction of the external electric field.As shown in Figure 7d, during the stripping process, the internal electric field direction points to the cathode, and the negative charge terminal of the polarized ZrO 2 points to the Zn anode.By contrast, during the plating process, the internal electric field points to the Zn anode.The electric dipole turns and the negative charge terminal of ZrO 2 points to the cathode.Therefore, in both stripping and plating processes, the directional polarization electric field from the ZrO 2 /cellulose separator can enhance the migration of Zn 2+ ions.In addition, the polarized ZrO 2 nanoparticles can effectively inhibit other side reactions by electrostatic repulsion of SO 4 2− , and the ZrO 2 /cellulose separator can also act as a sluggish physical barrier to prevent dendrite growth.Consequently, the symmetric cells with the ZrO 2 /cellulose separator achieved superior cyclic stability (over 2000 h under 0.5 mA cm −2 ) and excellent Coulombic efficiency (99.5% at 8 mA cm −2 , 4 mAh cm −2 ).
Zhao et al. constructed a multi-functional semi-immobilized ionic liquid interface (SIP) on the surface of the Zn anode. [89]his interface layer can induce a polarized electric field to accelerate Zn 2+ transport and effectively block the water molecules and anions from the Zn anode, significantly inhibiting hydrogen evolution reactions.As illustrated in Figure 8a, SIP is composed of polyacrylonitrile (PAN) skeleton and semi-immobilized  [88] Copyright 2020, Wiley-VCH.b) Schematic diagram of the Ti ion migration in the [TiO 6 ] octahedral interstitial sites under the external electric field.c) Schematic of the mechanism of Zn 2+ ions transport at the BTO@Zn/electrolyte interface during Zn plating process. [95]Copyright 2021, Springer Nature.d) Schematic illustration of possible migration process of Zn 2+ ions when passing through the ZrO 2 /cellulose separators. [97]Copyright 2021, Elsevier.
IL-functionalized SiO 2 nanoparticles (SiO 2 @IL).The semiimmobilized SiO 2 @IL is coupled with the PAN to provide solid-liquid ion-transport channels, where the PAN polymer skeleton transports ions via the coordination of ─CN groups and Zn 2+ ions.Meanwhile, the semi-immobilized IL with free anion provides a continuous liquid transport environment.Since the fixed SiO 2 nanoparticles and H 2 O can form H-O bonds, H 2 O molecules are captured outside the protective layer and cannot contact the Zn anode.More importantly, the insulating SiO 2 nanoparticles establish a directional polarization electric field under the influence of the external electric field.In Figure 8b, it can be observed that due to the rough surface of the bare Zn anode, the electric field is severely uneven.After applying the SIP protective layer, the induced electric field becomes uniform significantly (Figure 8c).The electric field intensity near the SiO 2 particles is significantly higher than other areas in the electrolyte, allowing Zn 2+ ions to rapidly move through the interface layer in an orderly way.Therefore, the SIP protective layer reduces the Zn 2+ ions concentration gradient (Figure 8d,e).For the cell with the pristine Zn anode, the transference number of Zn 2+ ions was only 0.414.In contrast, the transference number of Zn 2+ ions was increased to 0.85 for the cell with Zn@SIP anode.The symmetrical cell with Zn@SIP anodes achieved excellent cycling stability up to 75 h at 20 mA cm −2 and 10 mAh cm −2 .However, inorganic protective layers, such as SiO 2 , BaTiO 3 , etc., always show low mechanical flexibility and are easily damaged by the huge volume changes under high charge/discharge capacity cycling.Usually, we use the doctor blade coating method to graft  d,e) concentration field of the bare Zn anode (d) and the Zn@SIP anode (e) after 300 s. [89] Copyright 2022, Wiley-VCH.f) Schematic setup for corona poling treatment.g) Schematic of electric dipoles movement during poling.h) Electric field simulations on BTO-coated Zn (h1) and poled BTO-coated Zn electrode (h2).i) Concentration field simulations for BTO-coated Zn (i1) and poled BTO-coated Zn electrode (i2) after a constant diffusion duration of 1 s (equilibrium state). [100]Copyright 2021, Wiley-VCH.
the inorganic interface layer onto the Zn electrode, [98,99] which is simple and cost-effective, but it relies heavily on the choice of binders.In addition, it is necessary to reduce the thickness of the interface layer to increase the volumetric energy density of ZMBs. [24]olarization Obtained via Pretreatment: Due to the low voltages of the aqueous batteries (1-3 V), the polarization generated by the dielectric materials under the electric field in the battery is relatively limited, which restricts its contribution to promoting the rapid migration of Zn2+ ions.Zou et al. proposed a high-voltage corona-poling (HVCP) method to pretreat the coating layer on the Zn anode to enhance the polarization of the dielectric materials significantly. [100]As shown in Figure 8f, the corona discharge is initiated by applying a high voltage, and then the electric charges from the corona needle are sprayed onto the surface of the polymer-BTO coating layer, forming a bottom-up electric field in the polymer-BTO coating layer.Under the strong electric field, the Ti ions deviate from their symmetric position within the [TiO 6 ] octahedra, inducing the separation of the positive and negative charge centers and forming electric dipoles in the polymer-BTO coating layer that lined up in the direction of the applied electric field (Figure 8g).Due to the high degree of polarization after HVCP treatment, the electric field intensity inside the pretreated polymer-BTO layers is about 400 V mm −1 .Therefore, the orientation of the electric dipole does not switch with the electric fields inside the batteries during the charge/discharge processes.In addition, the electric polarization in the polymer-BTO layer cannot be neutralized by the cations and anions in the electrolyte.Copyright 2021, Wiley-VCH.f) Preparation of Au nanoparticle decorated Zn foils (NA-Zn anodes).g) Schematic illustration of Zn plating on B-Zn and NA-Zn. [108]Copyright 2019, American Chemical Society.
The electric field and ions concentration simulation results show significant differences before and after poled BTO coating.The poled BTO coating can significantly reduce the relative electric field intensity at the protuberances (Figure 8h).Meanwhile, the electric field intensity between the two adjacent BTO particles after HVCP treatment has been significantly enhanced, which is consistent with the expected rapid Zn 2+ ions transfer behaviors.In addition, the poled BTO-coated Zn electrode demonstrates a much more uniform and strengthened Zn 2+ ions flux along the entire Zn surface, and the local Zn 2+ ions gathering around the protrusions is mitigated (Figure 8i).Therefore, under the synergetic impact of the "homogeneous channeling (homochanneling) effect" and "ion-pumping effect," the surface of the Zn anode coated with BTO polarized by HVCP treatment possesses a uniform and high concentration Zn 2+ ions flux.The phenomenon of Zn 2+ ions accumulation around the projection is also reduced.Benefiting from the multifunctional protective coating layer, the cells with BTO-coated Zn electrodes achieved high reversibility even at a high capacity of 10 mAh cm −2 and a high rate of 40 mA cm −2 .
A dielectric polymer, poly(vinylidene fluoridetrifluoroethylene) (P(VDF-TrFE)), was selected by Wang et al., to form a thin protective layer on the surface of Zn anodes.(Figure 9a). [101]Since the dipole orientation of the polymer molecule can be changed by the direct-current polarization process, a negative dipole center of the dielectric polymer can be formed on the surface in contact with the electrolyte. [93]This protective layer provides an internal electrostatic field between the polymer coating and the Zn anode, inducing Zn 2+ ions to migrate toward the corner between the raised area and the polymer surface, allowing zinc to grow horizontally along the coating surface.As shown in Figure 9b, a 32 V voltage was directly applied to the polymer coating layer, and the direction of the polarization could be changed by changing the current direction.Calvin Probe microscope technology was used to measure the potentials of the dielectric polymer surfaces in different polarization directions.The surface potentials of the samples with upward polarization, unpolarized and downward polarization are about 350, 75, and −200 mV, respectively.From the perspective of the electric field and ion concentration distribution on the electrode surface (Figure 9c), a new local electrostatic field can be generated between the protrusion and the coating layer after polarization.The favorable electric field distribution on the surface of the Zn anode is the key to inhibiting dendrite growth. [102]The coating layer with upward polarization treatment has a higher ions concentration at the protrusion, which makes it easy to promote the growth of dendrites.However, for the downward polarized coating layer, there is a lower electrochemical potential between the protrusion and the coating corner (Figure 9d), and Zn 2+ ions converge at the corner between the protrusion and the coating (Figure 9e), which drives Zn 2+ ions to nucleate horizontally and effectively inhibits dendrite growth.][105][106][107] Instead, it drives Zn 2+ ions to deposit and grow horizontally by "guiding Zn growth" under the impact of the local electric field.Based on the above experimental data and theoretical analysis, the coated Zn anode with optimized downward polarization treatment showed stable cycling performance for 2000 h in the symmetric coin cells at 0.2 mA cm −2 and 0.2 mAh cm −2 .

Using Uniform "High Curvature Tips" to Enhance Localized Electric Field Intensity
A dense layer made of conductive nanoparticles can be constructed on the surface of the Zn anode to form uniformly distributed "high curvature tips."Compared with the pristine Zn anode, the charge density at the tip becomes higher, and the electric field intensity near the tip becomes stronger.Therefore, the use of "high curvature tips" can enhance the localized electric field on the Zn anode surface, strengthen the 2D diffusion of Zn 2+ ions and have the effect of inhibiting dendrites.As shown in Figure 9f, Cui et al. synthesized an Au-modified Zn anode (NA-Zn-60) by a magnetron sputtering method. [108]The highly conductive Au seed crystals with high curvature can form a strong local electric field around them, which has strong attractions for Zn 2+ ions, thus promoting the rapid migration and uniform deposition of Zn 2+ ions (Figure 9g).The repeated Zn plating/stripping tests demonstrate this uniform coating shows a positive effect on long-term cycling.After 100 cycles, the Zn deposit is still dense with a smooth surface.The symmetric cell with NA-Zn-60 anodes showed lower overpotential and better cycling stability than the cell with the pristine Zn anodes.It should be noted that, using this strategy, we need to make sure the tips are uniformly and compactly distributed on the surface of the Zn anode. [92]onuniform dispersed tips on the Zn anodes will lead to dendrite growth.

Uniform Electric Field Intensity
The electric field inside the battery is the driving force induces the shuttling of Zn 2+ ions between the two electrodes, which plays a crucial role in the diffusion and deposition of Zn 2+ ions.Uneven electric fields can lead to charge accumulation and accelerate the ions concentration gradient at the Zn anode surface, thus leading to aggregated Zn deposition.The "tip effect" also increases the electric field intensity at the protrusions of Zn anodes and weakens the electric field strength in other areas. [4]This uneven electric field distribution intensifies the aggregation and deposition of Zn 2+ ions in the raised area to form dendrites, as shown in Figure 10a.In addition, the following electrochemical stripping process occurs at both the dendrite sites and the Zn metal substrate.Then, the subsequent Zn 2+ ions plating step preferentially occurs at the protrusions with large curvature, and this repeated stripping/plating process leads to significant accumulation of Zn dendrites, known as the "accumulation effect." [24]This accumulation effect induces the dendrites to grow orthogonally to the Zn anodes, eventually penetrating the separators and leading to short circuits in the batteries.Therefore, the uniform distribution of the electric field inside the battery avoids uneven deposition and inhibits dendrite growth, thus significantly improving the cycle life of ZMBs.

Optimizing Electrode Structure to Uniform Electric Field Intensity
Conductive Interlayers: The redistribution of electrons on the surface of the Zn anode can even the electric field.[111][112] Xia et al. used a onestep facile strategy to obtain a spontaneously rGO-coated Zn foil (Zn/rGO) as the composite anode.The Zn 2+ ions were reduced on the surface of rGO interlayer and the inner space between each rGO nanosheets to promote the uniform Zn 2+ ions electrodeposition and improve the cycling stability of ZMBs. [111]This structure shows several advantages.First, layered rGO can provide stable scaffolds with excellent hydrophilicity and a large surface area for Zn stripping/plating, significantly reducing the concentration polarization and suppressing uneven deposition.Second, the rGO nanosheets provide rich nucleation sites for Zn deposition.The increase in nucleation sites can reduce the average size of the nucleus, forming uniform and fine deposits. [48]In addition, the highly conductive materials redistribute electrons and uniform the electric field.Compared with bare Zn, the Zn/rGO anode exhibits lower overpotential (20 mV at 1 mAcm − 2 ) and significantly extended cycle life.[115] Zhang et al. explored a charge-enriched strategy toward dendrite-free Zn anode through introducing MXene-based polypyrrole (MXene-mPPy) interlayers, which uniformed interfacial electric fields and ions fluxes (Figure 10b). [114]In addition, due to the abundant mesoporous structure of MXene-mPPy and the abundant functional groups in MXene, the wettability of the interlayer toward aqueous electrolyte has been improved, which facilitates the diffusion rate of Zn 2+ ions and reduces the ions concentration gradient on the anode surface.Therefore, a dendrite-free Zn anode with an ultra-long cycle life of up to 2500 h and excellent rate capability has been achieved.
Porous Conductive Skeletons: The strategy of using highly conductive interlayer to even the distribution of the electric field shows some limitations. [2]After the conductive layer is completely covered by the Zn deposit, the "tip effect" may occur again, resulting in dendrite growth.In addition, due to the direct contact between the conductive layer and the electrolyte during cycling, electron transfer occurs along with the side reactions, which significantly reduces the interface stability of the Zn anodes. [116]herefore, alternative strategies for designing 3D porous conductive skeletons have been developed.
Porous skeleton-based anodes, especially copper-based materials are widely used in ZMBs, such as copper mesh, copper foam, and porous copper, due to their excellent zincophilicity Figure 10.a) Schematic illustration describing the detailed accumulation effect of Zn dendrites. [24]Copyright 2020, Wiley-VCH.b) Schematic illustrations of the Zn plating behaviors on MXene-mPPy/Zn. [114]Copyright 2022, Wiley-VCH.c) The corresponding schematic illustration of Zn deposition on the 3D Ni. d) Simulations of the relative intensity distributions of localized electric field for 3D Ni-Zn electrode. [120]Copyright 2021, Wiley-VCH.e) Schematic illustration on the underlying dendrite growth mechanism for Bulk-NC@Zn and 3DP-NC@Zn electrodes. [121]Copyright 2022, Wiley-VCH.f,g) The schematic diagrams of 3DP-HG (f) and 3DP-BU (g) scaffolds illustrate the Zn-deposition habits with different electron/ion fluxes. [128]Copyright 2023, Wiley-VCH.
[119] Shi et al. reported that using Cu foam skeleton as the zinc anode host enabled the uniform distribution of electrons. [117]During electroplating, due to the uniform distribution of electric field and sufficient contact area with electrolyte, the gradient concentration of Zn 2+ ions near the anode is significantly reduced, thus significantly inhibiting the dendrite growth.
In addition, Zhang et al. fabricated a 3D Ni-Zn anode with the multi-channel lattice structures using 3D printing technology.During the Zn plating process, the electrode surface remained smooth and flat (Figure 10c). [120]Compared with the planar Zn anode, 3D Ni-Zn anode shows a larger specific surface area and uniform electric field distribution.Moreover, due to the higher current density inside the 3D multichannel than that on the upper surface of the skeleton (Figure 10d), Zn 2+ ions preferentially deposit uniformly into the 3D microchannels.By matching the 3D Ni-Zn anode with a polyaniline-intercalated vanadium oxide cathode, the assembled ZMBs achieved excellent long-term cycling stability and high specific capacity, with a capacity reten-tion rate of 80% after 1000 cycles at a current density of 10 Ag −1 .Similarly, Zeng et al. used the 3D printing technology to synthesize a nitrogen-doped carbon host to render the electric field uniform. [121]The high content of N doping improves the electronic conductivity of the host.To demonstrate that the porous framework is the determining factor to influence the electric field distribution and smooth Zn deposition, a non-printed bulk nitrogen-doped carbon host with similar dimension was fabricated and used for Zn deposition.In Figure 10e, the 3D-printed structure provides a uniform electric field and sufficient nucleation sites to induce uniform deposition of Zn 2+ ions.In addition, the reservoir-integrated structure with hierarchical multichannels contributes to a homogeneous ionic field within the host to ensure a uniform Zn deposition that directs Zn growth along with the reservoir holes built by the adjacent printed strips.In contrast, the Zn deposition behavior on the bulk host with low surface area and blocked pore channels is in a completely different situation.Due to the inhomogeneous distribution of the electrons on the anode surface, the electric field is uneven, resulting in the accumulation of Zn 2+ ions in electron-intensive areas and serious dendrite growth after only a few cycles.Therefore, the importance of the 3D-layered porous structure is clearly evidenced.On the macro level, the anode is presented as a square hole with orderly strips printed layer by layer.The high conductivity of carbon materials and unique structure ensure the redistribution of the electrons.At the micro level, the interwoven carbon nanotubes generate abundant micro-scale pores, which can be used to contain electrolyte.Therefore, the electrodes show excellent wettability toward an aqueous electrolyte.
Gradient Designed Skeletons: Due to the diffusion limitation of the bulk electrolyte, an increase in the thickness of the 3D anode inevitably leads to an increased ion flux on the top of the 3D anode, causing the nucleation and growth of Zn 2+ ions on the top surface. [122,123]This phenomenon becomes more severe at high current density.Therefore, it is essential to study the ionic/electronic field modulation in 3D-structured anodes during Zn deposition.Rational design of key battery components with varying microstructure along the charge-transport direction to realize optimal local charge-transport dynamics can compensate for reaction polarization, which accelerates electrochemical reaction kinetics. [124][127] He et al. used a 3D-printed Zn anode with layer-by-layer bottom-up attenuation of Ag nanoparticles (3DP-BU@Zn), which causes a gradually decreased electron distribution from the bottom to the top. [128]his electron distribution also affects the concentration distribution of Zn 2+ ions, establishing dual-gradient electron/ion fluxes, which dominate the bottom preferential zinc deposition behavior.In contrast, the 3D scaffold with a homogeneous distribution of Ag nanoparticles (3DP-HG) tends to deposit Zn 2+ ions on the top surface, causing a "top-growth" behavior, as schemed in Figure 10f.Therefore, the dual-gradient electron/ion fluxes endowed by the 3DP-BU scaffold ensure a stable bottom-aggregated Zn-deposition manner, which is beneficial for suppressing dendrite growth and improving the cycling life of ZMBs (Figure 10g).

Reducing the Average Current Density to Even the Electric Field Intensity
The current density near the anode surface is a key parameter to control the Zn deposition behavior of a high-performance Zn metal anode. [129]The "space charge theory" has been used to explain the relationship between dendrite formation and current density distribution. [130]Specifically, metal ions are reduced in a dilute electrolyte solution, and the cation gradient exists between the surfaces of the anode and cathode.Once a critical current density J* (Equations ( 2)) is reached, the current can only be sustained for a certain period called Sand's time  (Equation (3)), after which cations become depleted in the electrolyte, breaking the electrical neutrality at the plated electrode surface, which generates an electric field and local space-charge region on the interface between the electrolyte and the Zn anode. [131] where , D, C 0 , J, e, L,  a , and  c represent dendrite formation time, diffusion constant, initial electrolyte concentration initial concentration of cation, effective current density, electronic charge, distance between the electrodes, and anion and cation mobility, respectively.According to the description of Sand's time, reducing the current density near the Zn anode decreases the ions concentration gradient and postpones the initial dendrite formation.Therefore, transforming the planar electrode into a porous electrode can effectively increase the specific surface area of the electrode and reduce the local current density distribution, which delays the initial growth time of the dendrite. [132]hus, under the same current condition, the current density is greatly reduced, and the uneven distribution of the electric field in dendrites or protrusions will be relatively weakened.
A variety of porous current collectors with high specific surface areas were developed to reduce the local current densities of the Zn anodes.Zeng et al. designed a flexible 3D carbon nanotube (CNT) network as a highly conductive skeleton for Zn deposition. [133]As a comparison, the original carbon cloth (CC) was used as the substrate for Zn electrodeposition.The electric field distribution on the surface of the Zn/CC anode is severely uneven, and the electric field intensity of the protrusions is significantly higher than that of the other regions.In contrast, the Zn/CNT anode exhibits a uniform electric field distribution due to its 3D structure, considerably reducing the local current density.Consequently, a highly flexible symmetric cell based on the Zn/CNT electrodes exhibits appreciably low voltage hysteresis (27 mV) and excellent cycling stability without dendrite formation.In another study, Li et al. demonstrated a hierarchical confinement effect toward Zn deposits by constructing Coembedded carbon cage (denoted as CoCC) fibers with zincophilic properties (Figure 11a). [134]The Zn deposit is compactly encapsulated within the interspace among the CoCC fibers and inside the hollow carbon cages.The Zn deposition behavior can be effectively regulated due to the following reasons: First, the zincophilic Co site effectively reduces the nucleation barrier of initial Zn deposition.Second, the carbon cages with a high surface area reduce the local current density and spatially regulate the uniform Zn growth within the cages.This porous integrated network shows a more uniform current density distribution than the pristine Zn foil (Figure 11b,c).The ZMB with CoCC anode exhibits excellent rate performance and stable cycling life (over 800 cycles).A dual-channel porous Zn (DCP-Zn) anode was proposed by Guo et al. for dendritic-free Zn plating/stripping. [135]he porous Zn skeleton is composed of continuously interconnected Zn nanoparticles, and the DCP-Zn anode with a continuous conductive structure has a relatively uniform electric field distribution and abundant Zn nucleation sites.Therefore, Zn deposition is limited within the cavities of the skeleton rather than preferentially depositing on the tips of Zn protrusions.Even after multiple cycles, it can still prevent the upward growth of Zn dendrites from penetrating the separator.Another benefit of DCP-Zn anode is its promotion of battery kinetics.The DCP-Zn provides high-speed transfer channels for ions and electrons, increases the interface area between the Zn anode and electrolyte, reduces local current density and buffers volume changes, thus inhibiting the formation of "dead zinc" during repeated cycles.
At present, most strategies show a significant inhibitory effect on dendrite growth under low areal current densities  [134] Copyright 2022, American Chemical Society.d) Illustration of the electric field distribution toward the proposed electrochemical deposition processes of Zn metal on 3D structure.e) 3D models of the electric field distributions for AgNWA. [136]Copyright 2022, Elsevier.f) A Janus separator harnessing one-side directly grown VG carpet to help lower local current density and homogenizing ion distribution.g) Current distribution in the 3D scaffold.h) Electric field distribution of 3D scaffold structure (Janus separator case).i) Electric field distribution of 2D planar structure (pristine separator case). [137]Copyright 2020, Wiley-VCH.
(0.5-5 mA cm −2 ) and areal capacities (0.5-2 mAh cm −2 ), but the stability of ZMBs at high current densities still faces significant challenges.Ling et al. proposed a strategy that can effectively improve the cycle life of ZMBs in extreme conditions.They prepared 3D-light silver nanowire aerogels (AgNWA) by a vertical self-assembly method. [136]As an excellent zincophilic material, the silver substrate has the most suitable binding energy toward Zn atoms.Although Cu and Ti substrates have higher binding energies with Zn atoms, the strong binding energy between the substrate and Zn atoms increases the resistance during the Zn stripping process, which eventually reduces the reversibility of the Zn anode during repeated Zn plating/stripping.In addition, the AgNWA shows a porous morphology with the cross-linked networks, which increases the specific area and significantly reduces the current density on the electrode surface.As for the pristine Zn anode, due to its rough surface, an extremely uneven electric field distribution is formed at the protrusions at high current densities.By contrast, owing to the advantages of the 3D structure, the electric field on the surface of the AgNWA electrode is more uniform (Figure 11d,e).In addition, the interlaced Ag nanowires also contribute to forming a fast electronic conductive network at the interface, which further promotes uniform Zn deposition/stripping during cycling.Therefore, 3D AgNWA maintained a super stable Coulombic efficiency of 99.8% under a high current density of 40 mA cm −2 and a high capacity limitation of 10 mAh cm −2 .
Although the construction of 3D current collectors can significantly achieve uniform electric field distribution, this strategy usually introduces an extra host, which inevitably increases the volume and weight of the battery.This increases the manufacturing cost and reduces the overall energy density of ZMBs.Meanwhile, the structural uniformity of the 3D current collectors could be challenging to control. [2]To avoid the drawbacks of increasing the anode volumes, Li et al. modified the separators to effectively render the electric field more uniform and regulate the transport of Zn 2+ ions inside the battery. [137]he glass fiber separator in ZMBs always experiences dendrite growth, which ultimately pierces the separator and causes an internal short circuit.By in-situ growing a layer of vertical graphene (VG) carpet on one side of the glass fiber separator, the Zn dendrite growth can be effectively suppressed, and the cycling life of ZMBs can be significantly increased (Figure 11f).The VG carpet perfectly guides the uniform deposition of Zn 2+ ions onto the separator.In this regard, the 3D VG framework can serve as a continuation of the anode for hosting the Zn deposition.Introducing a 3D conductive VG skeleton significantly increases the surface area of the anode (Figure 11g), effectively reducing the current density (from 1.0 to 0.38 mA cm −2 ).At the same time, such a 3D VG skeleton provides a uniformly distributed electric field (Figure 11h).In contrast, the cell with a commercial glass fiber separator shows an unevenly distributed electric field near the surface of the Zn anode (Figure 11i), which will lead to rapid dendrite formation.Therefore, the unique feature of the modified separator brings excellent reversibility and stability to the Zn metal anodes.The V 2 O 5 ||Zn full cell with the VG-modified separator provides an energy density of 182 Wh kg −1 , while maintaining 75% capacity over 1000 cycles.

Using External Magnetic/Pressure/Thermal Field to Regulate the Electric Field
When the electrochemical process is carried out under the action of a magnetic field, electromagnetic interactions cause convection of the liquid electrolyte, resulting in the magnetohydrodynamic (MHD) effect. [138]Specifically, the MHD effect is produced by the interaction of the magnetic field (B) and electric field (E).The charged particles that are cutting magnetic lines of force can be subjected to the Lorentz force (F = qvB), in which F is the Lorentz force, q is the quantity of electric charge, v is the velocity of the charged particles, and B is the magnetic flux intensity. [139]142] Zn ions migration and deposition behaviors can be significantly influenced by the action of a certain electric field force.Due to the presence of protrusions on the Zn anode surface, the electric field direction that was originally parallel to the magnetic field direction is deflected and then intersects with the magnetic field.Only the Zn ions around the protrusions will be affected by the Lorentz force (When B and v are parallel, the Lorentz force is zero, and when B and v are orthogonal, the Lorentz force is the highest). [143]Therefore, the Zn 2+ ions diffusion near the protrusion is jointly affected by the electric field force and Lorentz force, which are dispersed through MHD "stirring" the electrolyte.This results in the expansion of the deposition area and a more uniform ions distribution, ultimately forming a flat deposition morphology (Figure 12a,b). [144]n addition, the pressure-electric field coupling effect formed inside the battery also has a significant influence on suppress-ing dendrite growth.[147] Applying certain pressure on the pouch cells has been demonstrated to be beneficial in forming a smooth and dense Li deposition layer (Figure 12c), which weakened the uneven distribution of the electric field caused by the "tip effect." [148]At the same time, squeezing the pouch cells reduces the distance (d) between the anodes and the cathodes, which enhances the internal electrical strength (E = U/d).External pressure not only has a significant impact on traditional pouch cells with liquid electrolyte but also has an impact on allsolid-state batteries.The limitation of the "solid-solid" contact between the electrode and the solid electrolytes (SEs) severely hinders the interfacial charge transport.[151] When the external force increases, it improves interface contact and produces high bonding strength, which significantly reduces the interface impedance (Figure 12d). [152]ecent research has discovered the deposition and nucleation behavior of metal ions on metal anodes are also significantly dependent on temperature.Temperature is a vital thermodynamic factor that directly affects the ions diffusion coefficient and charge transfer coefficient. [153]However, studies focused on the early nucleation and growth behavior of Zn metal at various temperatures are still absent.The denser initial deposition of Zn 2+ ions could be regulated by an external temperature field, which will provide favorable electric field distribution on the Zn anode surface for the subsequent deposition process.This thermal-electric field coupling effect requires further investigation.Yan et al. explored the temperature-dependent nucleation and growth behavior of Li dendrites and constructed a dendritefree Li metal anode by increasing the temperature from 20 to 60 °C. [154]They found that an increase in temperature leads to an increase in Li nucleation size, a decrease in nucleation potential barrier, and a more dense growth of Li.Su et al. conducted a study on the effect of cyclic temperature on the size and surface density of Zn crystal nuclei and found that low temperature induced smaller and denser Zn crystal nuclei.Eventually, they developed a cooling-treatment-based self-healing strategy to in situ eliminate dendrites. [155]Additionally, the thermal distribution affects the uniformity of ions and the reaction kinetics, [156] thus influencing Zn nucleation and deposition.Liu et al. were inspired by natural phenomenon and designed a topology-optimized biomimetic honeycomb Zn anode, which achieved a smooth distribution of current-stress-thermal field, achieving multiple field regulation effects, and improved cycling stability. [157]The reaction mechanisms for the coupling effect between thermal field and electric field inside ZMBs need to be further understood.

Establishing Potential Differences in the Protective Layers
The high transfer rate of Zn 2+ through the anode protective layers also plays an important role in inhibiting dendrite growth and reducing concentration polarization. [158]Therefore, it is critical to  [144] Copyright 2019, Wiley-VCH.c) Schematic illustration of the morphology of Li dendrites without and with external pressure. [148]Copyright 2021, Wiley-VCH.d) Schematic illustration of the Li-SE interfacial creep process with the external pressure. [152]opyright 2022, Elsevier.establish the necessary potential gradient within the protective layers of Zn anodes to promote the migration of Zn 2+ ions. [159]he insulating materials are applied to the surface of the Zn anode, and the high resistance of the insulating materials will form a high potential difference (ΔE) between the anode surface and the coating layer surface (Figure 13a).The lowest potential region is near the interface between the coating layer and the Zn anodes.The highest potential will be at the interface between the coating layer and the electrolyte.Therefore, it will provide a driving force for Zn 2+ ions to migrate to the Zn anode, thus imparting excellent conductivity.In addition, due to the high resistivity characteristics of the protective layer, it also avoids the reduction of Zn 2+ ions on the protective layer toward the electrolyte.
A dense ZnS protective layer was prepared via an in situ vaporsolid method for Zn anodes. [160]ZnS has poor electron conductivity but high ionic conductivity.Therefore, introducing the insulating ZnS brings a high resistance layer on the surface of the Zn anode, which establishes the necessary potential gradient to drive the diffusion of Zn 2+ ions.The transference number of Zn 2+ ions obtained from the symmetric cell with pristine Zn anodes is only 0.33.A large concentration gradient is easily formed on the pristine Zn anode surface during Zn deposition, significantly limiting the rate performance of ZMBs. [158]In contrast, the Zn 2+ ions transference number obtained from the symmetric cell with the modified ZnS@Zn is significantly increased, which indicates that the protective layer has an obvious blocking effect on SO 4 2− (t Zn 2+ + t SO4 2− = 1).Similarly, a poly(vinyl butyral) (PVB) polymer protective layer was coated on the surface of the Zn anode by a spin-coating coating method (Figure 13b).The polyvinyl alcohol group in PVB promoted ions transfer, and the dense PVB layer prevented the solvated water and anions from contacting with the anode, thus inhibiting the hydrogen evolution reactions. [161]Importantly, PVB is an electronic insulator with a measured resistivity of 2.4 × 10 5 Ω cm.Due to its high resistance, a high potential gradient is established within the membranes to drive the diffusion of Zn 2+ ions toward the Zn anode.Thus, the PVB-protected Zn (PVB@Zn) electrode provided an extended cycle life of 2200 h at 0.5 mA cm −2 in the symmetric cell.
Wang et al. proposed the Nafion/Zn 3 (PO 4 ) 2 (NFZP) protective layer (Figure 13c) to stabilize Zn anodes. [75]The Nafion layer is composed of hydrophilic and hydrophobic regions (Figure 13d), and only the hydrophilic channel surrounded by the hydrophilic group ─SO 3 − allows H 2 O to pass through.However, considering some H 2 O and anions still pass through the Nafion layer, an inorganic layer of zinc phosphate is introduced under the Nafion layer to further inhibit side reactions.Meanwhile, the measured resistivity of NFZP composite protective layer is ≈2.5 × 10 4 Ω cm.Due to the poor electronic conductivity of NFZP, a high potential gradient is formed between the protective layer, which promotes abundant Zn 2+ transfer through the protective layer.The ionic Zn cell and the PVB@Zn-PVB@Zn cell during repeated cycles of stripping/plating. [161]Copyright 2020, Wiley-VCH.c) Schematic illustration of the fabrication process of the NFZP@Zn electrode.d) Zinc ion transmission mechanism on NFZP composite layer.e) Zn ion conductivity of NFZP, Nafion, and Zn 3 (PO 4 ) 2 layer. [75]Copyright 2022, Elsevier.f) Schematic diagram of the reaction between CuF 2 solution and Zn foil. [162]Copyright 2022, The Royal Society of Chemistry.g) Schematic depiction of Zn deposition behavior on pure Zn and Zn@IS anodes. [165]Copyright 2021, American Chemical Society.
conductivity of the inorganic/organic composite protective layer is measured to be as high as ≈8 × 10 −4 S cm −1 , which is 1 to 3 orders of magnitude higher than that of the corresponding inorganic protective layer or organic protective layer (Figure 13e).
Zhi et al. [162] and Liang et al. [159] had similar research ideas, using insulating materials mixed with alloy particles to form protective coating layers on the metal anodes.Zhi et al. pointed out the drawbacks of two classic coating strategies.The first strategy is the use of porous conductive coatings, such as carbon nanotubes, [133] MXene, [21] etc., which can increase the surface area of the anode, reduce the current density, and suppress the dendrite formation.However, it cannot avoid electron transfer between the conductive substrate and the electrolyte, which causes electrolyte decomposition and hydrogen evolution.The second strategy is to use insulating materials for Zn anode coatings, such as ZnF 2 , [106] ZrO 2 , [88] etc., to regulate Zn 2+ ions transfer and suppress side reactions.However, it increases the internal resistance and retards the ion-transfer kinetics of the battery, especially under high current density, leading to high voltage polarization.Based on these considerations, Zhi et al. immersed Zn foil into CuF 2 solution and formed a coating layer of insulating ZnF 2 with CuZn alloy, as described in Figure 13f.This strategy endows with the advantages of the porous conduc-tive structure and the insulating coating.CuZn alloy increases the surface area and provides more nucleation sites for Zn deposition.Meanwhile, the insulating fluorinated component covering the CuZn alloy leads to high resistivity (2.1 × 10 3 Ω cm).The high resistivity of the protective layer prevents the direct reduction of Zn 2+ ions outside the coating, inhibiting the decomposition of the electrolyte.The second function of the insulating layer is to form a potential difference between coatings to drive the migration of Zn 2+ ions toward the Zn anode.
Elastic materials are also used to modify Zn anodes.Their unique mechanical strength can buffer the volume fluctuation during repeated Zn plating/stripping processes.As a protective layer, it serves as an excellent physical barrier to prevent water molecules from poisoning the Zn anode.However, elastic materials are usually hydrophobic with poor ionic conductivities. [163]o solve this problem, Liu et al. designed a combination of a thermoplastic material and an ionic conductive polymer material. [164] hybrid interface is formed by interpenetrating Zn-alginate (ZA) into a porous thermoplastic polyurethane (TPU) network to achieve dendrite-free Zn metal anode at high current densities and high capacity limitations.Due to the presence of the TPU fiber matrix, the protective layer has good mechanical stability, adapting to severe volume fluctuation and allowing for high-capacity cycling.At the same time, the protective layer serves as the anti-corrosion interface of the Zn anode, inhibiting side reactions.Most importantly, the formed protective layer has a low electronic conductivity (1.5 × 10 −10 S), forming a potential gradient inside the protective layer and driving the migration of Zn 2+ ions toward the Zn anode.
Although insulating materials can establish potential gradients between layers to drive ions diffusion, they also increase the internal resistance of the battery and cause voltage polarization at high current density.Therefore, to solve this problem, researchers tend to composite these materials with 3D conductive materials.Jiao et al. conducted a 3D structural design of insulation materials by using environmentally friendly and lowcost bacterial cellulose (BC) nanofibers to design a 3D porosity ion sieve (IS) on the surface of Zn anode (Zn@IS). [165]As shown in Figure 13g, due to the presence of a large number of polar ─OH groups on IS, IS has a higher binding energy with Zn than solvated water molecules, which contributes to the desolvation and inhibits hydrogen evolution reaction.In addition, due to the steric hindrance effect, the uniform Zn 2+ ions deposition can be easily guided.Moreover, the insulating IS coating establishes a potential difference along the vertical direction of the Zn anode, thus forming a gradually decreased potential gradient from the IS surface to the Zn anode surface, where the potential at the Zn anode surface is the lowest, which provides enough energy to induce the reaction and deposition of Zn 2+ ions.As expected, the Zn@IS || Zn@IS symmetric cell stably cycled for 3000 h with the capacity limitation of 0.25 mA h cm −2 , and the overpotential slightly increased from 73.5 to 78.1 mV.In contrast, the pure Zn || Zn symmetric cell only cycled for 300 h with a rapidly increased overpotential from 127.2 to 376.8 mV.
The strategies of electric field regulation, including enhancing the electric field and uniforming the electric field have been realized via coating protective layers, designing current collectors, and coupling the electric fields with external magnetic/pressure/thermal fields.Dielectric materials modified on the Zn anode surface can respond to external electric fields and then polarize to generate a built-in directional electrical field, which significantly enlarges the local electric field.This strategy significantly improves the cycling life of the Zn metal battery.However, inorganic protective layers constructed with dielectric materials typically lack mechanical flexibility.In addition, by coating the surface of the anode with a conductive coating layer or building 3D current collectors, the redistribution of electrons on the anode can be achieved to regulate the surface electric field.However, due to the direct contact between the conductive layer/current collectors and the electrolyte during the cycle, electrons will be gained and lost at the electrolyte/electrode interface, which is detrimental to the electrochemical performance of Zn metal batteries.At the same time, attention should be paid to the issue of hydrogen evolution reaction.Besides, when the magnetic field is coupled with the internal electric field of the battery, a magnetohydrodynamic (MHD) effect occurs, which can form a uniform deposition morphology at the protrusion.However, this strategy typically requires the use of a magnetic field generator or magnets, making it challenging to apply in practice.

Conclusions and Perspective
Aqueous Zn-based batteries, with the advantages of low cost, excellent safety, and high power density have been considered as one of the most promising energy storage systems for the renewable energy industry.However, some critical challenges of Zn metal anodes, including dendrites formation, chemical corrosion, hydrogen evolution, and sluggish reaction kinetics, have seriously impeded the commercial application of Zn-based batteries.Various effects have been made from the perspectives of electrode materials and electrolyte optimization, electrode interface modification, and separator design.Comprehensive fundamental investigations to reveal the mechanisms of Zn nucleation and growth behaviors are essential to achieving high-performance ZMBs.Especially, the electrical mechanisms behind these improved strategies need to be fully understood.First, the electrostatic interaction on the anode surface can facilitate modulating the zinc coordination environment, the Zn 2+ transport rate, and migration direction.Meanwhile, the strength and uniformity of the electric field between the anode and cathode are essential to Zn 2+ ions diffusion, nucleation, and growth kinetics.
The main achievement roadmap for the application of the electrical mechanisms in ZMBs is summarized in Figure 14a.Strategy comparisons have been summarized based on the economy, electrochemical stability, sustainability, applicability, and energy efficiency (Figure 14b,c).For the strategies based on electrostatic interaction, electrolyte additives are usually used to reduce electrostatic repulsion between the electric double layers of Zn deposits or form electrostatic shielding at protrusions on the surface of the Zn anode.Finding appropriate electrolyte additives with high solubilities is essential.Although using electrolyte additives to change electrostatic interactions has lower costs and energy consumption, it causes difficulties in recycling.Especially, some additives are harmful to the environment.Forming a protective layer with polarity groups on the anode surface can significantly improve the cycle life of the Zn anode, but the additional coating process during electrode fabrication increases the production cost.For electric field regulation, the strategy of uniform electric field distribution shows good applicability.Due to the fact that the multi-field coupling method does not require additional substances to the battery, it is low-cost, reusable, and environmentally friendly.Detailed information on performance characteristics of ZMBs based on different electrical mechanisms has been summarized in Table 1.Although the latest developments from the perspective of electrical mechanisms have been reviewed, there is still some space for further improvement, as proposed below: i.The strategy of regulating deposition behavior by changing the electrostatic repulsion force between electric double layers (EDLs) is rarely mentioned.We believe that this approach desires more attention.Electrolyte additives with low cost and good stability need to be developed as well.Other approaches such as in situ forming a protective layer on Zn deposits to reduce the electrostatic repulsion effect could be considered.ii.There is limited research on improving ions transfer rate by enhancing the intensity of the electric field.Compared to monovalent Li + ions or Na + ions, the reaction kinetic of Zn 2+ ions deposition is slow due to the formation of strong solvation sheaths. [166]The low Zn 2+ ions diffusion rate significantly affects the rate performance of ZMBs.The corresponding strategies to solve this challenge include modifying the solvation structure of Zn 2+ in the electrolytes, constructing a surface modification layer to attract Zn 2+ ions through electrostatic interactions, and enhancing the electric field strength between the anode and cathode.Research on enhancing the localized electric field intensity is still relatively understudied.The polarization electric field induced by dielectric materials provides controllable nucleation sites for Zn 2+ ions and promotes rapid ions kinetics.At present, only a few dielectric materials have been used to form protective layers on Zn anodes.Other dielectric materials with good wettability to the aqueous electrolytes and excellent zincophilic properties should be explored.This strategy can also be used to improve Li/Na metal anodes, which could significantly inhibit dendrite growth and improve reaction kinetics.iii.The 3D structure design of Zn anode can reduce local charge accumulation, which is beneficial for uniform electric field distribution on the electrode surface and improves the Coulombic efficiency of Zn deposition/stripping.However, some excessively narrow channels of the 3D structure may hinder the electrolyte penetration, thereby restricting the deposition of Zn in those pores.In addition, the 3D structure is prone to a large amount of Zn deposit on the top of the porous substrates, leading to dendrite growth and internal short circuits.Therefore, further optimization of the local porous structure and the electronic conductivities of 3D substrates is essential for 3D porous Zn anodes.Constructing gradient electrodes with dense electrons at the bottom and sparse electrons at the top has been demonstrated to effectively induce bottom-up Zn deposition, which should be further optimized to maximize the inner space of the 3D substrates.iv.The application of multi-field regulation strategies for highperformance Zn anodes has not received much attention.Previous studies focused on using magnetic field generators or magnets to apply a magnetic field outside the battery, resulting in the magnetohydrodynamic (MHD) effect when the magnetic field couples with the internal electric field of the battery.However, this strategy might be difficult to apply in practical applications without an innovative design to apply the magnetic field.Other strategies employing pressure or  thermal field should be intensively studied for Zn anodes as well.v.Other key factors that affect the performance of Zn metal anode include their thermodynamic stability, electrochemical activity, and other physical properties.The thermodynamic instability and high electrochemical activity of Zn metal anodes in mild aqueous electrolytes lead to dendrite formation and severe side reactions such as hydrogen evolution and surface corrosion, which may cause short circuits and rapid capacity fading of the Zn batteries with poor cycling performance.Continuous HER also changes the local pH value of the electrolyte, resulting in the formation of byproducts (Zn 4 SO 4 (OH) 6 ⋅H 2 O, Zn(OH) 4 2− ) and triggering a series of interrelated adverse phenomena.Various effects have been reported to solve these problems, such as introducing protective layers, constructing 3D porous current collectors, forming Zn-based alloy anodes, exploring multifunctional electrolyte additives, and modifying separators.On the other hand, introducing electrical regulation can also help to solve these problems.For example, electrostatic repulsion strategies can prevent the contact between SO 4 2− and Zn anode, thus suppressing the generation of byproducts.Electrostatic shielding and uniform electric field strategies are also conductive to suppress dendrite growth, etc.Therefore, we believe that understanding the electrical mechanism is a necessary supplementary to the conventional improved strategies.It would be better to design the electrolytes, separators, or the Zn metal anodes for high-performance ZMBs by simultaneously considering the electrical regulation.
For future practical application of ZMBs, Zn metal anodes with high areal capacity, high-rate capability and minimized side reactions must be fully developed.The reaction mechanisms of the Zn plating/stripping require intensive investigations.Advanced characterization techniques with excellent temporal and spatial resolution will provide great opportunities to reveal the electrolyte solvation environment, interface composition, nucleation and growth behaviors, and dendrite growth conditions.Besides, various computational calculations and artificial intelligence techniques can also be applied to screen material composition, predict material properties, and explore structure-property relationships.This review focuses on electrical mechanisms in aqueous ZMBs, but it also has reference value to guide the development of other rechargeable battery systems that share similar fundamentals.

Figure 1 .
Figure 1.Summary of the developed strategies for dendrite-free Zn anodes from an electrical perspective.

Figure 2 .
Figure 2. a) The schematic of the interaction energy-distance curves described by DLVO theory.b) SEM images of the Zn electrodes for Zn||Zn cells in ZnSO 4 and La3+  -ZnSO 4 electrolytes.c) The comparison of the EDL of the Zn deposits in ZnSO 4 and La 3+ -ZnSO 4 electrolytes; the corresponding growth models of the Zn deposits.[35]Copyright 2022, Springer Nature.

Figure 3 .
Figure 3. a) Schematics of the morphology evolution during Zn plating in mild aqueous electrolyte with the Et 2 O additive.[42]Copyright 2019, Elsevier.b) Schematic illustration of traditional EDL.c) Schematic illustration of self-adaptive EDL.d) Schematic illustration of the Zn plating/stripping processes in ImS/ZnSO 4 electrolyte.[49]Copyright 2022, The Royal Society of Chemistry.e) Eutectic Zn/Al alloys with a lamellar structure composed of alternative Zn and Al nanolamellas in situ produce core/shell interlayer patterns during the Zn stripping to guide the subsequent Zn plating.[51]Copyright 2020, Springer Nature.f) Schematic illustration of Zn deposition on different modified Zn electrodes.[54]Copyright 2022, American Chemical Society.

Figure 4 .
Figure 4. a) Schematic diagram of the work function concept.b) Schematic diagram of electron movement if the work function of a metal is greater than that of a semiconductor.c) Schematic diagram of electron movement if the work function of a metal is smaller than that of a semiconductor.d) Schematic diagram of the electric field established due to electron transfer.e) Schematic diagram of the situation with potential energy barrier.f) Schematic diagram of the situation without potential energy barrier.

Figure 5 .
Figure 5. a) Schematic of electrons flowing from metallic Zn metal to CeO 2 semiconductor; W Zn and W CO represent the work functions of Zn metal and CeO 2 , respectively.(E F ) Zn and (E F ) CO represent the Fermi levels of Zn metal and CeO 2 , and E 0 is the vacuum level.b) Schematic illustration of the formation of an ideal Ohmic contact interface between Zn and metal oxides.c) The corresponding uniform Zn plating process upon cycling.[55]Copyright 2021, Wiley-VCH.d) Schematic illustration for Zn 2+ ions transport and electrodeposition behaviors induced by Nb 2 O 5 protective layer.e,f) The band structure of Zn metal and n-type semiconductor Nb 2 O 5 before (e) and after contact (f).[56]Copyright 2022, Elsevier.

Figure 6 .
Figure6.a) The calculation of Zn insertion energy barriers for Zn@ZnO-3D and bare Zn. b) Electric double layer structure in the vicinity of Zn@ZnO-3D and its corresponding energy barrier compared with bare Zn.[71]Copyright 2020, The Royal Society of Chemistry.c) Schematic illustration of Zn plating behavior with CNG membrane and combination mode of artificial CNG layer.[72]Copyright 2021, The Royal Society of Chemistry.d) Ions transport mechanisms in Nafion-Zn-X protective layers.[74]Copyright 2020, Wiley-VCH.e) Schematic illustration of protective effect of SR layer.[76]Copyright 2022, Wiley-VCH.f) Schematic illustration of the negatively charged NSQD layer.[36]Copyright 2023, Elsevier.
anions and free H 2 O molecules are blocked outside the protective layer.Chen et al. synthesized tris-(squaramide) to construct SO 4 2−

Figure 7 .
Figure7.a) Schematics of the stripping/plating processes on the ZrO 2 -coated Zn anode.[88]Copyright 2020, Wiley-VCH.b) Schematic diagram of the Ti ion migration in the [TiO 6 ] octahedral interstitial sites under the external electric field.c) Schematic of the mechanism of Zn 2+ ions transport at the BTO@Zn/electrolyte interface during Zn plating process.[95]Copyright 2021, Springer Nature.d) Schematic illustration of possible migration process of Zn 2+ ions when passing through the ZrO 2 /cellulose separators.[97]Copyright 2021, Elsevier.

Figure 9 .
Figure 9. a) Schematic illustration showing the Zn-deposition behaviors on selectively poled P(VDF-TrFE)-coated Zn (bottom).b) Schematics showing the polarization process of P(VDF-TrFE) coating.c) Electrostatic field distribution simulations of the P(VDF-TrFE)-coated Zn foil with different polarized directions.d) Enlarged calculation profiles at the Zn protrusion region of the distributions of electrochemical potential.e) Enlarged calculation profiles at the Zn protrusion region of the distributions of Zn ion concentration.[101]Copyright 2021, Wiley-VCH.f) Preparation of Au nanoparticle decorated Zn foils (NA-Zn anodes).g) Schematic illustration of Zn plating on B-Zn and NA-Zn.[108]Copyright 2019, American Chemical Society.

Figure 11 .
Figure 11.a) Schematic illustrations of Zn deposition behaviors on a Co-embedded carbon cage (CoCC) network.b) Simulation of current density distribution in the CoCC host.c) Simulation of electric field distribution of 3D CoCC host and planar bare Zn foil.[134]Copyright 2022, American Chemical Society.d) Illustration of the electric field distribution toward the proposed electrochemical deposition processes of Zn metal on 3D structure.e) 3D models of the electric field distributions for AgNWA.[136]Copyright 2022, Elsevier.f) A Janus separator harnessing one-side directly grown VG carpet to help lower local current density and homogenizing ion distribution.g) Current distribution in the 3D scaffold.h) Electric field distribution of 3D scaffold structure (Janus separator case).i) Electric field distribution of 2D planar structure (pristine separator case).[137]Copyright 2020, Wiley-VCH.

Figure 12 .
Figure 12. a,b) Schematic diagram of the electric field distribution and dendrite growth around the protrusions on the Zn anode without a magnetic field (a) and in the presence of a magnetic field (b).[144]Copyright 2019, Wiley-VCH.c) Schematic illustration of the morphology of Li dendrites without and with external pressure.[148]Copyright 2021, Wiley-VCH.d) Schematic illustration of the Li-SE interfacial creep process with the external pressure.[152]Copyright 2022, Elsevier.

Figure 13 .
Figure 13.a) Schematic depicting the function of the alloy-protected Zn foil.b) Schematic illustration of morphology evolution for both the bare Zn-Zn cell and the PVB@Zn-PVB@Zn cell during repeated cycles of stripping/plating.[161]Copyright 2020, Wiley-VCH.c) Schematic illustration of the fabrication process of the NFZP@Zn electrode.d) Zinc ion transmission mechanism on NFZP composite layer.e) Zn ion conductivity of NFZP, Nafion, and Zn 3 (PO 4 ) 2 layer.[75]Copyright 2022, Elsevier.f) Schematic diagram of the reaction between CuF 2 solution and Zn foil.[162]Copyright 2022, The Royal Society of Chemistry.g) Schematic depiction of Zn deposition behavior on pure Zn and Zn@IS anodes.[165]Copyright 2021, American Chemical Society.

Figure 14 .
Figure 14.a) Roadmap of major achievements in the application of electrical mechanisms.b) Comparison of electrostatic interaction manipulation strategies in the radar map.c) Comparison of electric field regulation strategies in the radar map. 4

Table 1 .
Summary of the electrical mechanisms and performance characteristics applied in ZMBs.