Materials and structural design for preferable Zn deposition behavior toward stable Zn anodes

Benefiting from the high capacity of Zn metal anodes and intrinsic safety of aqueous electrolytes, rechargeable Zn ion batteries (ZIBs) show promising application in the post‐lithium‐ion period, exhibiting good safety, low cost, and high energy density. However, its commercialization still faces problems with low Coulombic efficiency and unsatisfied cycling performance due to the poor Zn/Zn2+ reversibility that occurred on the Zn anode. To improve the stability of the Zn anode, optimizing the Zn deposition behavior is an efficient way, which can enhance the subsequent striping efficiency and limit the dendrite growth. The Zn deposition is a controlled kinetics‐diffusion joint process that is affected by various factors, such as the interaction between Zn2+ ions and Zn anodes, ion concentration gradient, and current distribution. In this review, from an electrochemical perspective, we first overview the factors affecting the Zn deposition behavior and summarize the modification principles. Subsequently, strategies proposed for interfacial modification and 3D structural design as well as the corresponding mechanisms are summarized. Finally, the existing challenges, perspectives on further development direction, and outlook for practical applications of ZIBs are proposed.


| INTRODUCTION
2][3] Natural sources such as solar, wind, and tidal energy are widely used; however, direct use of these energies is difficult and is heavily impacted by the natural environment.Connecting energy harvesting and end-use systems for integration through electrical energy is a promising solution.5][6][7][8] Among all available ESSs, the lithium-ion battery (LIB), discovered in 1989, has grown rapidly due to its high operating voltage, excellent energy density, and good cycling stability. 9,10To date, commercial LIBs account for 63% of the global battery market.However, the safety issues brought by toxic and flammable organic electrolytes and the diminishing resources greatly limit their development.Therefore, researchers are prompted to explore more preferable battery systems with high safety, low cost, and high energy density.2][13][14][15] According to the hydrogen evolution reaction (HER) potential in aqueous solution (E H2 = −0.059× pH), certain requirements are then put forward for the metal anode of ARMBs.The higher redox potentials for Ca (−2.87 V vs. standard hydrogen electrode [SHE]), Mg (−2.37 V vs. SHE), and Al (−1.66 V vs. SHE) make them hardly used as rechargeable anodes directly, limited by competing HER and electrolyte corrosion. 16,17In comparison, Zn is advantageous in suitable redox potential (−0.762 V vs. SHE), resulting in good Zn/Zn 2+ reversibility during battery cycling. 18,19Besides, the ultralow cost (~US$2.4kg −1 greatly lower than Li ~US$19.2][22] Aqueous Zn-based batteries first originated in 1799 with the discovery of the voltaic cell. 23,245][26][27][28][29][30] Among them, alkaline Zn-based batteries dominated the early market based on their good capacity.2][33] Until 2012, rechargeable Zn-MnO 2 batteries were revisited, and mild Zn-ion batteries (ZIBs) ushered in rapid development. 346][37][38][39][40] Despite considerable efforts, the further development of ZIBs is severely limited by the cyclic stability of the Zn anode.Although compared to alkaline electrolytes, neutral/mild acidic electrolytes are effective in reducing corrosion problems in the Zn anode, the occurrence of side reactions and the dendrite growth still severely limit the service life of ZIBs. 41enerally, ZIBs use neutral/mild acidic ZnSO 4 as the electrolyte with Zn/Zn 2+ redox reaction occurs on the surface of the Zn anode (Zn(s)← →Zn 2+ (aq) + 2e − ). 42herefore, achieving desirable reversibility of Zn plating/stripping is a significant factor for long-cycle Zn anodes.][45][46] Essentially, the goal of these strategies is to tune the Zn deposition morphology to achieve high surface flatness and densification, as Zn deposition morphology greatly affects the subsequent striping efficiency and thus the electrode cycling stability.The Zn deposition is a controlled kinetics-diffusion joint process that is affected by the interaction between Zn 2+ ions and Zn anodes, ion concentration gradient, and current distribution, which begins with Zn nucleation and followed by Zn 2+ ions migrating to the interfacial nucleation sites driven by an electric field. 47,48The Zn deposition behavior is also affected by a combination of many aspects, including side reactions (HER and Zn corrosion), nucleation overpotential, crystal orientation, electric field strength, Zn 2+ flux, and deposition position (Figure 1). 42,49,50The inhomogeneous deposition behavior results in low Coulombic efficiency (CE) and the formation of "dead zinc," which eventually evolves into dendrites during the cycle.Due to the high mechanical strength of Zn (Young's modulus, 108 GPa), the formed sharp Zn dendrites can easily pierce the separator and cause the failure of the battery.Despite a variety of effective strategies has been proposed to optimize Zn deposition behavior, the latest explosive increase of research work on the Zn anode requires a detailed and systematic summary.More importantly, there is a lack of comprehensive reviews discussing key factors that guide Zn deposition behavior from a fundamental perspective.
In this review, focusing on the regulation of Zn deposition behavior, we dedicate to outlining and classifying the effects of materials and structural design on Zn deposition behavior from the perspective of electrochemical reactions.First, we explored the challenges affecting the Zn deposition behavior and analyzed the modification principles.Subsequently, strategies for interfacial modification and 3D structural design along with corresponding mechanisms to regulate the Zn deposition behavior are summarized.Finally, current challenges and future outlooks with theoretical and experimental considerations are discussed.We hope this review will provide valuable insights for researchers looking for long-cycling metal anodes and highperformance batteries.

| CHALLENGES AND PRINCIPLES FOR GOVERNING ZN DEPOSITION BEHAVIOR
During the cycling of the Zn anode, it is expected that the deposited Zn can be completely stripped.However, in the Zn deposition process, the microenvironment at the Zn/ electrolyte interface changes constantly and is influenced by Zn/Zn 2+ kinetics as well as Zn 2+ ion diffusion, making it difficult to keep homogeneous.Uneven Zn deposition is accompanied by competitive reactions, while concentrated Zn deposition makes stripping difficult, resulting in low CE and battery capacity degradation.Therefore, to achieve long-cycle Zn anodes, controllable and uniform Zn deposition behavior is desired.In this section, we discuss the key factors affecting the Zn deposition behavior from the perspective of electrochemical reactions, including side reactions, Zn nucleation, crystal orientation, electric field strength, Zn 2+ flux, and Zn deposition selectivity, and then principles for Zn deposition regulation are summarized.
Although the Zn metal achieves better cycling performance in neutral/mild acidic electrolytes, side reactions, including HER and Zn corrosion, still occur.According to the Nernst equation, the potential of HER in neutral/mild acidic electrolytes is about −0.29 to −0.41 V (pH 5-7); therefore, theoretically, the Zn metal cannot be used directly as an anode material in aqueous batteries. 51Nevertheless, the Zn/ Zn 2+ redox reaction is still ahead of the competition due to the high HER overpotential of the Zn metal.However, in neutral/mild acidic electrolytes, Zn 2+ ions are susceptible to water bonding and tend to combine with six water molecules to form the Zn 2+ hydration layer ([Zn(H 2 O) 6 ] 2+ ). 52,53In the deposition process, a pre-dehydrated process of [Zn(H 2 O) 6 ] 2+ is needed before the occurrence of the Zn/Zn 2+ redox reaction, which greatly increases the energy barrier of the Zn/Zn 2+ redox reaction and promotes the HER process.The H 2 gas generated by the HER will increase the internal pressure of the battery and destroy the electrode surface structure, thus damaging the uniformity of Zn deposition. 54,55More importantly, the decomposition of water molecules on the Zn metal surface will cause an increase in the OH − concentration, which will favor the formation of the byproduct Zn 4 SO 4 (OH) 6 .xH 2 O (ZSO). 56,57The formation of ZSO is shown in Equation ( 1), where SO 4 2− and Zn 2+ are provided by the electrolyte.

↔
(1) The resulting byproduct ZSO shows a porous structure that cannot block the electrolyte and terminate the corrosion reaction but instead creates more sites for the formation of H 2 . 58The side reactions exacerbate the inhomogeneity of the Zn electrode surface and the concentration polarization of Zn 2+ in the electrolyte, thus providing more nucleation for deposition and a stronger driving force for the formation of Zn dendrites.Recent studies have also shown that the formation of H 2 may not be caused by the decomposition of free water in the electrolyte but by the active water from hydrated Zn ([Zn(H 2 O) 6 ] 2+ ). 59,60During the dehydration process, the released highly reactive water molecules are more prone to decomposition to form H 2 and accompanying byproduct formation.Although the origin of the side reactions is unclear, the resulting H 2 gas and porous ZSO will greatly affect the homogeneity of the Zn deposition process and decrease CE.Limiting the occurrence of side reactions is, therefore, an important factor in regulating Zn deposition behavior toward longstable Zn anodes.
The Zn nucleation process is the initial stage of Zn deposition with new phase generation, which has an important influence on the subsequent deposition format and morphology.As previously reported, Zn atoms are first adsorbed onto the electrodes to form Zn nuclei.Then, additional Zn atoms form Zn clusters around these Zn nuclei.As further Zn is deposited, these clusters expand to form a mesoporous network. 61According to the principle of minimum Gibbs energy and classical nucleation theory, the Zn nucleation process is accompanied by a high energy barrier, which leads to the possibility of inhomogeneous Zn nucleation.Generally, Zn anodes show rough surfaces and large differences in surface free energy, which will result in differences in the nucleation overpotential and thus a nonuniform distribution of Zn nuclei.Given that Zn embryos usually act as reaction sites for Zn/Zn 2+ , inhomogeneous Zn nuclei in turn trigger the aggregated Zn deposition.The nucleation rate also has a great influence on the deposition compactness and cycling reversibility.When the nucleation rate is large, Zn nuclei with small size and large density are conducive to the formation of a flat surface.The relationship between nucleus density (N) and deposition time t is considered as 62 where N 0 is the available nucleation sites and A is the nucleation rate.In addition, it is worth noting that wafers grown from Zn nuclei with different sizes can experience lattice mismatches and dislocations during the joining process, which can also lead to the appearance of protrusions.Therefore, increasing the nucleation rate and density and reducing regional variations will facilitate the formation of thermodynamically stable and homogeneous Zn deposition.
The crystal orientation represents the growth pattern of the deposited Zn. 63,64 As mentioned before, the electrode substrate or predeposited Zn has an induced effect on the subsequent Zn deposition; therefore, the crystal heterogeneity of the electrode or predeposited Zn will guide the crystal orientation of the newly deposited Zn.Typically, Zn is a hexagonal compact-packed (hcp) structure, mainly present as Zn (002), Zn (100), and Zn (101) planes during the Zn deposition. 65,66When the Zn deposition favors the Zn (100) and Zn (101) planes, the deposited Zn is more inclined to grow at a large angle (70°-90°to the substrate) and form Zn nanosheets.However, since the Zn stripping process occurs preferentially from the bottom, the top Zn of the nanosheet will be easily evolved into "dead zinc."In contrast, when the deposition favors the Zn (002) plane, the deposited Zn is more inclined to grow at a small angle (0°-30°to the substrate) and induced horizontal deposition of Zn.In addition, density functional theory (DFT) calculations show that the low-index crystal planes of Zn (100) and Zn (101) possess low atomic coordination and high reactivity, which are more likely to trigger the occurrence of HER and Zn corrosion. 67The high-index facet Zn (002) has high atomic coordination and low reactivity, which will effectively limit the side reactions.Therefore, optimizing the intrinsic crystal orientation of the electrode or assisting with chemical bonding, to control the Zn deposition along the Zn (002) plane, is an effective way to improve the uniformity of Zn deposition.
The electric field distribution contributes to the Zn 2+ ion migration and determines the subsequent metal growth rate. 68,69During Zn deposition, the concentration of Zn 2+ ions on the electrode surface decreases, which creates a local space charge region and an electric field at the Zn/electrolyte interface.Nonuniform electric fields can lead to charge concentration and disturb the ion concentration gradient at the Zn anode surface, thus leading to aggregated deposition.From the perspective of the whole electrode, Zn anodes used in ZIBs commonly are commercial Zn foil or 2D composite Zn electrodes that possess relatively high electric field strengths and are prone to protrusion generation. 70In addition, as indicated by Gauss's law, when protrusions appear on the surface of the Zn anode, the local electric field intensity will increase instantaneously due to the high curvature, which can trigger rapid Zn deposition and ultimately the formation of dendrites (hot-spot effect).The effect of electric field strength on Zn deposition behavior can be depicted by Sand's time model, [71][72][73] which is as follows: where τ, D, C 0 , J, e, μ a , and μ c represent dendrite formation time, diffusion constant, initial electrolyte concentration, effective current density, electronic charge, and anion and cation mobility, respectively.It is can be found that the values of the electric field strength influence the formation of dendrites and the uniformity of Zn deposition.The electric field distribution is inhomogeneous at high current densities, whereas at low current densities, it is homogeneous and stronger where there are dendrites or sharp points.Therefore, a reduced and uniform local electric field is a key parameter to achieve homogeneous Zn deposition.Further, to achieve long cycling stability under high currents, it is necessary to reduce the average electric field intensity for the whole Zn anode.The Zn deposition process at the electrode/electrolyte interface is a dynamic equilibrium process with ion depletion and diffusion.Typically, the rate of the Zn/Zn 2+ redox reaction is significantly higher than the Zn 2+ diffusion, exhibiting a lower Zn 2+ ion concentration on the Zn anode surface. 74,75Therefore, Zn deposition can be considered a diffusion-limited process and the rate of Zn 2+ diffusion plays a key role in uniformity.When ion diffusion is restricted, concentration polarization occurs on the electrode surface, resulting in different deposition rates and benefits to the dendrite growth (concentration polarization is a phenomenon in which the electrochemical potential of a cell deviates from its equilibrium value due to the difference in ion concentration between the area near the electrode surface and the main body of the electrolyte). 19Therefore, homogenizing and enhancing Zn 2+ flux will be an efficient way to uniform Zn deposition processes.The Nernst-Planck equation is used to describe the mass transfer flux (J) at the electrode/electrolyte interface. 76


where D, V, T, K, z, v x , c, e, and x are the coefficient of diffusion, the electric potential, the thermodynamic temperature, the Boltzmann constant, the charge number, the convective velocity, the ion concentration, the unit charge, and the distance to the cathode, respectively.
To increase the Zn 2+ flux and reduce polarization, this can be achieved by increasing c and v x .Therefore, regulation of concentration and convection at the electrode/electrolyte interface have been used to mitigate the formation of dendrites.
The previous discussion mainly focused on how to achieve a uniform deposition morphology; however, Zn is thermodynamically unstable in a neutral/mild acidic solutions. 48,77Although various strategies have been proposed to reduce the uneven deposition of Zn, dendrite formation is still unavoidable, especially in harsh testing environments.The vertical growth of dendrites can puncture the separator and eventually cause battery failure.Therefore, it is an effective way to improve the stability of Zn anodes by controlling the Zn deposition process and preferentially depositing Zn in safe positions, away from the separator.To achieve selectivity in Zn deposition, various strategies have been proposed, such as electron shielding, gradient design, structure engineering, etc.
Based on the above discussion, the regulation of Zn deposition behavior needs to consider the Zn/Zn 2+ redox kinetics and Zn 2+ diffusion process on the Zn/electrolyte interface and can be carried out from the following aspects (Figure 2): (1) suppress the side reactions, (2) homogenize the Zn nucleation, (3) control the crystal orientation of Zn deposition, (4) reduce the local current density, (5) manipulate the Zn 2+ flux, and (6) achieve Zn deposition selectivity.Among them, the first three items are mainly related to kinetic optimization by interface modification, and the last three items mostly depend on the design of the 3D structure.Next, strategies proposed in recent years will be presented in detail.MODIFICATION Interface modifications are commonly used to improve the Zn/Zn 2+ reaction kinetics.Until now, various interfacial modification layers have been proposed to modulate the side reactions, Zn nucleation, and crystal orientation problems encountered during Zn deposition.In this section, related works are covered (Table 1).

| Suppressing side reactions
Zn is thermodynamically unstable in neutral/mild acidic solution.Side reactions, including HER and Zn corrosion, can be occurred during Zn deposition.Among them, HER is generated from the decomposition of free water in the electrolyte or active water in hydrated Zn ([Zn(H 2 O) 6 ] 2+ ).Zn corrosion refers to the formation of ZSO due to the increase of OH − concentration on the electrode surface, which is combined with SO 4 2− and Zn 2+ in the electrolyte.Therefore, the suppression of side reactions can be carried out by enhancing the HER overpotential, reducing the contact between the electrolyte (or water molecules) and the Zn anode, and limiting the diffusion of SO 4 . Until now, a variety of interfacial modification layers have been proposed, and here, we divide them into conductive and nonconductive categories based on their different deposition sites.Typically, the nonconductive interfacial modification layer is incapable of electron/ion exchange (i.e., the Zn/Zn 2+ redox reaction cannot take place); therefore, Zn is mainly deposited on the conductive substrate under it.In contrast, Zn can be directly deposited on the surface of the conductive interfacial modification layer.

| Nonconductive interfacial modification layer
As previously mentioned, the electrolyte is involved in the occurrence of side reactions.Therefore, physically reducing the contact between the Zn anode and the electrolyte can limit the occurrence of side reactions.It can be inferred that materials with water insolubility, insulating properties, and good Zn 2+ ionic conductivity are beneficial to protect the Zn anode from side reactions.(1) Polymer materials are receiving a lot of attention.Polyvinylidene difluoride (PVDF) and hydrogels, which are often used as electrode binders and electrolyte carriers, can act as elastic waterblockers due to their good ionic conductivity and electrochemical stability.For example, Hieu et al. 78 demonstrated good side reaction resistance and CE by spin-coating β-PVDF on Zn anodes.Park et al. 79 achieved a high CE of 99.8% in over 1000 cycles by applying agarose hydrogels.(2) Metal compounds exhibit high HER overpotentials and can also be used as interfacial modification layers.ZnMoO 4 , 122 ZnMnO x , 123 ZnSe, 124 and ZnF 2 80 have been shown to be effective in reducing the occurrence of side reactions.As shown in the schematic diagram in Figure 3A, the passivation layer slows down the Zn deposition and creates a "leveling effect" to minimize uneven deposition.In addition, the dense passivation layer acts as a barrier between the electrolyte and the Zn surface, increasing its corrosion resistance.(3) Nonetheless, polymer materials usually have weak ionic conductivity, which may lead to a high energy barrier for Zn/Zn 2+ redox reaction, while metal compounds with poor mechanical flexibility are prone to rupture during cycling, thus polymer/metal compound composites may be a better choice.For example, Guo et al. 81 reported that the polydimethylsiloxane (PDMS)/ TiO 2-x coating can dynamically adapt to volume changes and inhibit dendrite growth (Figure 3B).Among them, the PDMS layer is used as an elastic H 2 O/O 2 barrier and the decorated TiO 2 nanoparticles contribute to the homogeneous Zn electrodeposition.Similarly, PVDF/TiO 2 82 and thermoplastic polyurethane/Zn-alginate (TPU/ZA) 83 have been also reported.Zn powders have a large specific surface area, and when used in Zn anodes, the side reaction is a key factor in the uniformity of Zn deposition.Zhang et al. 125 achieved good Zn anode stability by using ethylene-vinyl acetate copolymer-carbon nanotube (CNT-EVA) copolymers as an ion conductive and protective layer, which effectively reduced the contact between Zn powder and the electrolyte.
Although the above works mention that the modification layer can reduce the electrolyte-electrode contact and thus limit the occurrence of side reactions, it is worth noting that Zn 2+ ions in neutral/mild acidic solution mainly exist in the form of hydrated Zn.The dehydration process of hydrated Zn before the Zn/Zn 2+ reaction will increase its reaction energy barrier, thereby promoting the occurrence of HER.Besides, the active water produced during dehydration may be the source of H 2 gas.Therefore, accelerating the dehydration process of hydrated Zn through interface modification can effectively promote Zn/Zn 2+ reaction kinetics and suppress the side reactions.The mechanisms usually rely on hydrogen bonds or chemical bonds to inhibit the diffusion of water or destroy the structure of hydrated Zn.For example, Zhao et al. 84 coated a layer of the PA membrane (composed of polyamide (PA) and zinc trifluoromethane-sulfonate (Zn(TfO) 2 )) on the Zn foil to suppress side reactions.The polymer interface is water/O 2 -resistant and acts as a buffer layer to retard the chaotic bulk water electrolyte as well as dissolved O 2 .More importantly, the abundant hydrogen Zn@Nafion-Zn-X -97, 120, 0.
As shown in Equation ( 1), the formation of ZSO requires the participation of SO  89 used Nafion-Zn-X as a modification layer to limit the SO 4 2− ion concentration on the electrode surface.Due to the negatively charged framework and small pore size of Zn-X zeolite, Zn 2+ can hop along the organic-inorganic interface, while the SO 4 2− anion and free H 2 O will be blocked.The experimental results show that the SO 4 2− ion permeability value of Nafion-Zn-X can be as low as 0.61 × 10 −9 cm 2 /s.Different from the above methods, Chen et al. 90 reported a strategy of "anchoring + repelling" SO percent (%), cycle, current density (mA/cm 2 ), capacity (mA•h/cm 2 ) time (h), current density (mA/cm 2 ), capacity (mA•h/cm 2 ) Ref.
According to the Nernst equation, pH is a key parameter affecting the theoretical HER potential.A lower pH value is more likely to induce HER, while a higher pH value is more prone to the formation of irreversible Zn oxides.Therefore, a moderate and stable pH value is suitable for the long-cycle stability of Zn anodes.Qi et al. 91 constructed an enamel-like layer of nano-hydroxyapatite (Ca 5 (PO 4 ) 3 (OH), nano-HAP) on the Zn anode to enhance the cycling stability (Figure 3E).The nano-HAP as a slightly alkaline material can effectively stabilize the pH at the electrode/electrolyte interface.The pH-time curve shows that the pH value of the electrolyte finally stabilizes at 5.1 and possesses a higher pH buffering capacity.As a result, the E-nHAP@Zn electrode can exhibit a high CE of 99.8% over 1000 cycles.
For example, Cai et al. 92 introduced a Cu metal layer on the surface of the Zn anode through a displacement reaction (Figure 4A).The as-fabricated Cu/Zn anode showed a much-reduced corrosion rate and improved stability in comparison to the bare Zn electrode due to the good chemical stability and more positive redox potential of Cu.As shown in Figure 4B, the Cu/Zn electrode can show a low corrosion potential of −0.964 V better than the bare Zn electrode (−0.976V).Hong et al. 95 prepared a layer of the Sb metal on the surface of the Zn anode with SbCl 3 solution.DFT calculations show that the metallic Sb has a high hydrogen adsorption Gibbs free energy (ΔG H ) of 1.37 eV, which could suppress the HER.Similarly, Xiao et al. 128 prepared the In metal on the surface of the Zn anode (Figure 4C).The DFT calculation results show that the adsorption free energy of Zn (101) for hydrogen is about 0.181 eV, while that of In ( 101) is higher, i.e., 0.772 eV, indicating that it is very difficult for H 2 to precipitate from its surface.Scanning electron microscopy (SEM) images show that, after In modification, HER is inhibited, which avoids the formation of ZSO, so the deposited Zn could keep a fresh surface during Zn plating (Figure 4D,E).For metal alloys, Wang et al. 129 fabricated Zn-Sn alloys on 3D carbon felt (CF) substrates by electrodeposition (Figure 4F).DFT calculations show that the replacement of the Zn atom with the Sn atom could enlarge the ΔG H* of surrounding adsorption sites (top, bridge, and hollow sites) and inhibit the production of H 2 at those sites.Hydrogen production was quantitatively measured during repeated plating/stripping tests in symmetrical cells.After the plating/stripping reaction of 34 mA•h, the hydrogen evolution (0.20083 mmol/cm 2 ) of the Zn-Snbased battery is only half that of the pure Zn-based battery (0.39333 mmol/cm 2 ).Similarly, Li et al. 93 demonstrated that AgZn 3 alloys also contribute to the improvement of side reaction resistance.
Further, to better control the side reactions, Dong et al. 131 prepared a conductive/nonconductive composite modification layer by combining with inert metals (In, Sb, Sn) and polyacrylamide (PAM) (Figure 4G).They find that the inert metal can effectively suppress the H 2 generation and corrosion reactions, due to the large HER overpotential.The combination of the inert metal and PAM polymer coatings can synergistically enhance the reaction homogeneity and charge-transfer kinetics during the Zn plating/striping process.As a result, the ZnIn-PAM electrode can exhibit a stable cycle for 400 h at a high current density/capacity of 10 mA/cm 2 / 10 mA•h/cm 2 (Zn utilization rate: 57%).

| Homogenizing Zn nucleation
Zn nucleation is the initial stage of the Zn deposition process.The morphology and distribution of Zn nuclei have a significant influence on the subsequent Zn deposition.Commonly, the Zn nucleation is accompanied by a higher energy barrier due to the formation of new phases (minimum Gibbs energy principle and classical nucleation theory), which is the main factor in inhomogeneous Zn nucleation.Therefore, enhancing the interaction between Zn anodes and Zn 2+ ions (commonly referred to as zincophilicity) is an effective method to kinetically promote uniform Zn nucleation.Here, according to the working mechanism, we classify the various interfacial modification layers into conductive and nonconductive categories.The nonconductive interfacial modification layer is incapable of electron/ion transfer and acts as a "pump" during the nucleation process, pulling Zn 2+ ions from the electrolyte to the electrode surface and thus accelerating Zn nucleation.
The conductive interfacial modification layer acts as a deposition substrate and accelerates the conversion of Zn/Zn 2+ on its surface, through stronger interaction with Zn 2+ .Until now, research on nonconductive interfacial modification layers is mostly focused on metal compounds, gel systems, and carbon-based materials, which contain abundant electronegative groups.For conductive interfacial modification layers, MXene-based materials, metals, or alloys are commonly used.

| Nonconductive interfacial modification layer
The operation of the nonconductive interface modification layer relies on the strong interaction of electronegative groups with Zn 2+ , which enhances Zn 2+ concentration at the electrode/electrolyte interface and promotes its homogeneous distribution, thereby accelerating the kinetics of the Zn/Zn 2+ reaction.
ZnX compounds (X represents the electronegative group, including O, S, Se, P, F, etc.) have been shown to be effective in promoting Zn nucleation.For example, Hao et al. 96 prepared a ZnS interfacial layer on the Zn anode for the first time.The robust and homogeneous ZnS interphase can effectively protect the occurrence of side reactions of the Zn anode.More important, as shown by the DFT calculations (Figure 5A), the bonding interaction occurs between the S atoms and Zn atoms, which modifies the charge distribution and further leads to an unbalanced charge distribution at the interphase.The unbalanced charge distribution can accelerate the Zn 2+ diffusion at the ZnS@Zn interphase.As a result, the ZnS interfacial layermodified electrode can achieve a high CE of more than 99.2% after 10 cycles (Figure 5B).The ZnP has also been shown positive for the uniformity of Zn nucleation. 97DFT calculations revealed that ZnP has a stronger interaction with Zn 2+ than bare Zn, both close to (c-P) but also far from (f-P) the single substitutional P atom (Figure 5C).Moreover, the binding energy is further significantly enlarged when multiple P atoms (m-P) occupy the positions of Zn atoms, illustrating a good Zn affinity due to the additional P of the coating.Benefiting from the strong electronegativity of F − , ZnF 2 has also been shown to promote Zn nucleation. 80DFT results reveal that the Zn 2+ insertion barrier on Zn@ZnF 2 is greatly lower than that of bulk ZnF 2 and bare Zn, meaning that Zn 2+ is more easily inserted into Zn@ZnF 2 .Accordingly, the Zn@ZnF 2 preferentially provides electrostatic attraction toward Zn 2+ , accelerating the kinetics "through" and reducing the deposition barrier.Similarly, ZnSe 98 and ZnO 132 also have been shown to favor the Zn nucleation process, benefiting from the Se and O electronegative groups.Unlike the above-mentioned nonmetallic elemental groups, the carbonyl oxygen group is also highly electronegative and has been shown to enhance Zn nucleation homogeneity.Wang et al. 99 prepared ZnC 2 O 4 -2H 2 O (ZCO) layers on the Zn anode by the acid etching method.DFT calculations revealed that the highly electronegative oxygen possesses a strong binding energy with Zn 2+ and effectively shortens the migration path of Zn 2+ .
In addition to ZnX compounds, heteroatom-doped carbon-based materials or polymer materials can also be used to improve the Zn nucleation process.In particular, N-doped carbon materials have gained a lot of attention.Liu et al. 133 demonstrated that C 3 N 4 acts as an effective layer to promote the uniformity of Zn deposition by coating g-C 3 N 4 on the surface of the Zn anode through 3D printing technology.Theoretical calculations show that the binding energy of g-C 3 N 4 to Zn 2+ is greatly stronger than that of bare Zn.The N of g-C 3 N 4 has a pair of p-electrons and a high electronegativity of N (χ = 3.04).This kind of configuration is favorable for capturing the positively charged Zn 2+ during Zn-plating to form a relatively stable structure.Jiang et al. 134 further explored the effect of C 3 N 4 on Zn nucleation.Molecular dynamic (MD) simulation reveals that, with the g-C 3 N 4 layer, most of Zn 2+ can be formed in the complexing state in combination with g-C 3 N 4 molecular clusters to minimize the system energy, thus reducing the Zn nucleation barrier.Similarly, nitrogen-doped graphene oxide (N-GO) also has been shown to exhibit good zincophilic properties, based on its abundant nitrogen-doped groups. 135ZIF-8 with a desirable structure is widely used in the fields of catalysis and energy storage. 136In fact, abundant electronegative functional groups in ZIF-8 can effectively enhance the interaction between the electrode and Zn 2+ .Liu et al. 100 prepared ZIF-8 on the Zn anode by hydrothermal methods (Figure 5D).The obtained Zn@ZIF anode can deliver a high average CE of 99.4% at 5 mA/cm 2 and 5 mA•h/cm 2 for over 300 cycles, benefiting the enhanced uniformity of Zn deposition.
Electronegative groups such as SO 3− , OH − , and CN − can also accelerate the nucleation process of Zn.Quantum dots with small particle sizes (less than 10 nm) and tunable electronegative functional groups show superiority in promoting Zn nucleation.Zhang et al. 101 prepared multipolar functional carbon quantum dots on the Zn anode by a hydrothermal method, which contained abundant cyano and aldehyde groups.DFT  102 calculations show that the Zn 2+ ions on the aldehyde group and cyano-group are substantially lower than that of Zn 2+ on the Zn substrate, which is in favor of improving nucleation sites (Figure 5E).Similarly, Han et al. 137 achieved uniform Zn nucleation and long-cycle stability with graphene quantum dots, based on their abundant electronegative groups, including -NH 2 , -OH, and -COOH.Inorganic materials commonly show poor mechanical flexibility, which easily leads to rupture.To better withstand the volume change during Zn deposition and cycling, the introduction of electronegative groups into the gel system is a better choice.For example, Yang et al. 102 introduced a polyanionic hydrogel on the Zn anode (Figure 5F).The hydrogel framework with zincophilic SO 3 2− functional groups enables the uniform Zn nucleation and deposition process.DFT results showed that the sulfonated polyacrylamide (PAAm) unit generates much stronger binding energy toward Zn 2+ via Zn-O interaction than that of the unsulfonated one, suggesting a high zincophilicity of the polyanionic hydrogel.

| Conductive interfacial modification layer
The conductive interfacial modification layer can undergo electron/ion exchange and thus can be considered a deposited substrate during the Zn deposition process.Therefore, it is required to lower the nucleation energy barrier.Typically, enhancing the interaction of the conductive interfacial modification layer with Zn 2+ or applying the alloying effect is useful.At present, a wide range of zincophilic metals has been studied, including Au, Ag, Cu, Sb, Sn, Bi, and Ce.For example, Cui et al. 103 achieved uniform Zn deposition behavior by preparing Au nanoparticles on the Zn anode through magnetron sputtering.Deng et al. 104 demonstrated that Ce can be used to reduce the nucleation overpotential of Zn.Further, Xiong et al. 127 investigated the effect of Sn crystal textures on the Zn deposition behavior (Figure 6A).The results confirm that the metallic Sn can enhance the adsorption of Zn 2+ on the electrode and Sn (101) has a stronger binding energy to Zn 2+ than Sn (200).Hong et al. 95 prepared a layer of Sb on the Zn anode by the replacement method and confirmed that Sb (001) can be effective in homogeneous Zn nucleation.To better compare the zincophilicity of different metals, three metals, including Bi, Sb, and Sn, were prepared on the Zn anode by Xiong et al. 127 DFT and experimental results showed that Sn interacted more strongly with Zn 2+ than the other two and was therefore more conducive to promoting the nucleation process of Zn.
Although, zincophilic metals have strong interactions with Zn 2+ .However, some studies suggest that zincophilic metals will alloy with Zn during the initial Zn deposition process, thereby exhibiting a small nucleation overpotential, as the reversible alloying process can effectively lower the nucleation barrier.For example, Chen et al. 105 prepared Ag nanoparticles on the surface of carbon cloth (CC) using an inkjet method.Ag can react with Zn to form zincophilic AgZn 3 alloys during Zn deposition, which can function as the Zn resource to offset the irreversible loss of active Zn during cycling.Nevertheless, some researchers believe that the obtained alloy is not reversible, and the zincophilic alloy is the actual material that promotes the kinetics of subsequent Zn deposition.For example, Wang et al. 130 prepared a layer of Ag particles on the surface of the Zn metal by a displacement reaction.The pristine Ag nanoparticles attached to the Zn surface can be easily bound to the plated Zn atoms during the first plating process and transformed into a nonreversible AgZn 3 alloy phase, thereby reducing the nucleation overpotential of Zn.The strong zincophilicity of AgZn 3 facilitates the subsequent deposition process of Zn, which in turn achieves excellent cycling stability.The direct preparation of metal alloys on the Zn anode also promotes the kinetics of Zn nucleation.For example, Zhou et al. 106 prepared a layer of Cu-Zn alloys on the surface of the Zn metal by the electrodeposition method.DFT calculations results showed that the Zn nucleation barrier can be greatly reduced due to the strong zincophilicity of Cu in the alloy (Figure 6B).
Conductive carbon-based materials with heteroatom doping can also achieve homogeneous Zn nucleation, based on their electronegative groups.The mechanism of Zn deposition on N-doped carbon surfaces was investigated by Xie et al. 61 They found that Zn deposition followed a path from the absorption of Zn at single atomic sites to Zn clusters and mesoporous Zn networks.In the initial stage of Zn deposition, Zn 2+ ions combine with electronegative N to form Zn-N bonds.The binding of these to zincophilic sites induces a critically spacious distribution of the initial Zn nuclei.Therefore, N-doped carbon with uniform zincophilic sites exhibits homogeneous Zn deposition, as well as improved electrochemical properties.Xu et al. 107 prepared O,N-doped flowershaped carbon on the surface of the Zn anode (Figure 6C).DFT calculations reveal that, among oxygen/nitrogen dopants, ether (C-O), carboxylic acid (-O-C=O-), and pyrroline N groups show strong binding to Zn 2+ , making them favorable for the Zn nucleation point.MOF-derived carbon also has abundant N electronegativity groups.Wang et al. 140 demonstrated that ZIF-8-derived carbon can effectively induce Zn deposition near the Zn-N groups, resulting in uniform Zn nucleation.MXene has good electrical conductivity and contains a variety of anionic groups and can therefore be used to improve the Zn nucleation process.Zhang et al. 138 applied MXene to modify the Zn deposition behavior for the first time, by an in situ spontaneous reduction/assembly strategy (Figure 6D).Lower nucleation barriers and more uniform Zn nucleation were achieved due to the presence of negatively charged oxygen-containing groups on the MXene layer.Further, Tian et al. 141 introduced the zincophilic metal Sb on the MXene surface.The MXene@Sb-300 can exhibit a smooth surface after Zn plating/stripping for 200 cycles based on the zincophilic properties of MXene and Sb.
Conductive/nonconductive composite modification layers have also been studied.ZnF 2 -Ag nanoparticles were prepared on the surface of the Zn anode by Wang et al. 108 DFT results show that the electron donation of Zn to AgZn 3 @Zn surface is much higher than those of ZnF 2 and bare Zn, indicating the larger driving force for Zn 2+ adsorption (Figure 6E).In addition, the adsorption energy of H 2 O on ZnF 2 @Zn is much larger than that of bare Zn and AgZn 3 @Zn, indicating that ZnF 2 can accelerate the dehydration process of hydrated Zn.As a result, the symmetric cell assembled by ZnF 2 -Ag@Zn exhibits ultralow overpotential (60 mV at 14 mA/cm 2 ) and ultralong cycle life (2200 h at 2 mA•h/cm 2 ).
The Zn nucleation rate and uniformity are temperature sensitive.Controlling the Zn nucleation and growth uniformity in both high-and low-temperature environments plays an important role in ZIB application.The influence of temperature on Zn nucleation was investigated by Su et al. 139 They found that low-temperature environments have a higher barrier to nucleation and result in smaller and denser Zn nuclei.The hightemperature environment has a lower barrier to nucleation and leads to larger and less dense Zn nuclei (Figure 6F).Therefore, the high nucleation barrier at low temperatures and the uneven growth of nuclei at high temperatures are problems to be solved.To address the high energy barrier for Zn nucleation at low temperatures, Chen et al. 109 introduced single-atom Bi to modify the Zn anode.Bi-N 4 moieties served as Zn nucleation sites to increase Zn nucleation kinetics.The uniform and high-density distribution of Bi−N 4 moieties can effectively enhance Zn nucleation density and accelerate the Zn nucleation rate.At high temperatures, the uneven temperature is an important factor leading to inhomogeneous Zn deposition.Therefore, uniform heat distribution and the fast cooling rate of the electrodes are crucial.Chen et al. 105 introduced Ag nanoparticles into CC (AgNPs@CC).The introduction of Ag increased the thermal conductivity of CC from 1.4 to 5.4 W/(m•K).This apparent thermal conductivity enhancement ensures uniform temperature distribution and suppresses dendrite growth.

| Controlling crystal orientation
The crystal orientation represents the growth pattern of the deposited Zn, and the horizontal growth pattern is beneficial to the uniform Zn deposition and long-cycle stability.Generally, the growth patterns with Zn (100) and Zn (101) planes are more inclined to generate nanosheets, while the Zn (002) plane promotes the horizontal growth of Zn.Therefore, various methods have been proposed to induce Zn growth along the Zn (002) plane, including lattice matching and deposition induction.In addition, controlling the crystallization orientation of byproducts to grow along the horizontal orientation is also an effective way for uniform Zn deposition processes.
The pattern of Zn deposition matches the crystal orientation of the substrate.The interfacial modification layers with a similar lattice structure to Zn (002) will induce Zn deposition along the Zn (002) plane.For the first time, Archer and colleagues reported in 2019 on an epitaxial growth route for Zn crystals that exploits the low lattice mismatch between Zn (002) and the prepared graphene (Figure 7A). 63The Zn atoms prefer to be deposited in an orientation locked to the graphene substrate, resulting in a well-organized sheet morphology parallel to the Zn (002) orientation.Inspired by this remarkable work, Li et al. 110 synthesized a series of MXene (-Cl 2 , -Br 2 , -I 2 ) with halogen functional groups by the molten salt method, in which the most widely exposed (001) plane of MXene belongs to the same hcp structure as Zn (002) (Figure 7B).They also found that the outermost halogens regulate the tiling rather than the stacking of Zn 2+ ions on the MXene substrate, thereby promoting Zn deposition along the Zn (002) plane.Yan et al. 111 systematically explored the mechanism of Zn deposition along the Zn (002) plane induced by the Cu (100) from an atomic structure perspective.Theoretical simulations show that the initial deposition of Zn atoms on the Cu (100) electrode is underpotential deposition (UPD).The atomic arrangement of the deposited layer resembles more closely the face-centered cubic structure of Cu (100).As this crystal structure is similar to the hcp structure of Zn (002), the subsequent deposition of Zn on the initially deposited layer is transformed into an overpotential deposition (OPD) with an hcp structure.Figure 7C shows a schematic diagram of the crystal orientation of the Zn UPD and OPD layers on Cu (100).Similarly, Lu et al. 112 achieved the deposition of AgZn 3 on the Zn anode by ion sputtering, which shows abundant exposure of AgZn 3 (002) (Figure 7D).The vertical crystal plane matching between AgZn 3 (002) and Zn (002) can drive the Zn deposition along with the Zn (002) plane.
Controlling Zn growth along the Zn (002) crystal plane through electrode polarization or Zn 2+ diffusion modulation is also an effective way.Wang et al. 142 prepared a polarized ferroelectric polymer material (polyvinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) as the interfacial modification layer on the Zn anode.This layer could provide an inner electrostatic field between the polymer coating and Zn metal, inducing the locally concentrated Zn ions at the polymer coating surface, thus enabling the Zn to grow horizontally along the Zn (002) plane.Luo et al. 113 prepared the tetracyanoquinonedimethylethane anion (TCNQ 2− )-modified Zn anode (TCNQ@Zn) through in situ etching of Zn foil by TCNQ.Benefiting from the abundant -CN groups of TCNQ 2− , TCNQ 2− can be used as an ion pump to continuously pump Zn 2+ in the electrolyte to the Zn anode surface.Since the Zn 2+ migration barrier on TCNQ 2− -modified Zn ( 101) is significantly higher than that on Zn (002), the 2D diffusion on Zn (101) can be effectively suppressed and the 3D diffusion on Zn (002) can be induced to form an ordered deposition morphology.Chen et al. 114 also achieved the Zn (002) plane deposition pattern by effectively tuning the interfacial ion diffusion and deposition energy barriers through PVA coating.
When metallic Zn is directly used as the Zn anode, exposing more Zn (002) crystal planes on the electrode surface by physical or chemical treatment is an effective method to control the Zn deposition behavior.Currently, two methods are commonly used, including acid etching and rolling.Liu et al. 56 achieved more exposure of the Zn (002) crystal faces by acid etching methods.Due to the high reactivity, molybdate and phytate can selectively etch the Zn metal on the Zn (100) and Zn (101) crystal faces.Zhou et al. 67 first proposed the rolling method to modulate the crystal orientation of the Zn metal, based on the basal plane slip mechanism.Further, Chen et al. 143 modified the crystal texture of the Zn metal by a cold-rolling method (Figure 8A,B).The effect of the grain size of Zn on the formation of Zn (002) crystal planes is explored.They found that the initial grain size plays a key role in influencing the grain boundary deformation ability and the resistance to lattice dislocation, and the best grain size for Zn (002) formation is around 30 μm.The grain size of Zn can be modulated by optimizing the annealing temperature.Pu et al. 144 optimized the crystal texture of deposited Zn using single-crystal Zn-metal anodes.They found that the polycrystalline Zn substrate has a certain lattice mismatch that results in a nonperfect fit when the two Zn islands meet (Figure 8C).Defect regions form between these islands (i.e., grains), making them susceptible to other side reactions and nonplanar Zn growth.For single-crystal Zn (002), the uniform orientation of the substrate allows for perfect stitching when the individual plating islands are combined together to form a larger single crystal (Figure 8D).As mentioned above, the occurrence of side reactions is inevitable, and therefore, controlling the morphology and crystal orientation of the byproducts is also an effective way to improve the uniformity of Zn deposition.Zheng et al. 115 modified Zn anodes with magnetronsputtered TiN and then investigated the effect of TiN crystal orientation on the uniformity of Zn deposition.DFT and experimental results reveal that the binding energy of TiN (200) with the pyramidal structure (vertical growth) of ZSO is much higher than that with the ringlike structure (lateral growth) of ZSO, suggesting the latter is more thermodynamically favorable and the ZSO nanosheets tend to grow parallel to the surface.

| Multiple-functional modification
Although the above three methods can effectively improve the kinetics of Zn deposition, the cycling stability of Zn anodes still faces challenges in extreme environments.Multiple-functional modification may be more favorable to encountering various problems in the Zn deposition process.Here, we classify the multiple-functional modification layers into conductive and nonconductive categories.

| Nonconductive interfacial modification layer
Heteroatom-doped carbon-based materials, as mentioned above, contain abundant electronegative groups, which can effectively reduce the Zn nucleation overpotential of the Zn anode.Actually, it also has a positive effect on suppressing side reactions and guiding the crystal orientation of the deposited Zn.Zhou et al. 135 synthesized a nitrogen (N)-doped graphene oxide (NGO) film on Zn foil by the Langmuir-Blodgett method (Figure 9A).They indicate that the modification layer with high-ionic conductivity and electronic insulation can effectively reduce the contact between the electrolyte and the electrode, thereby reducing the occurrence of side reactions.The electronegative N and O groups effectively enhance the affinity between Zn 2+ and the electrode, thus can reduce the Zn nucleation barrier and promoting the uniform distribution of Zn nuclei.More importantly, the parallel graphene layer and beneficial zincophilic traits of the N-doped groups lead to a desirable Zn (002) deposition behavior.As a result, the pouch cells assembled based on the NGO@Zn anode can maintain 80% initial capacity after 178 cycles at a high DOD of 36%.Similarly, Wang et al. 116 prepared a fluorine-doped amorphous carbon (CF) on the Cu substrate.They point out that the Zn (002) plane shows more preferential adsorption on CF than Zn (100) and Zn (101) planes.Zhang et al. 117 proposed a cellulose nanowhisker-graphene (CNG) film for regulated Zn deposition.The CNG film can act as a dehydration layer and benefit to the dehydration process of hydrated Zn through ion repulsion, thereby limiting the waterinduced corrosion reaction.Furthermore, this CNG layer with negative surface charges can simultaneously generate a deanionization shock by spreading cations but screening anions to obtain redirected Zn deposition parallel to the Zn (002) plane.It was also mentioned before that ZnSe can effectively improve the uniformity of Zn nucleation.Further, Yang et al. 98 demonstrated F I G U R E 8 (A) Schematic Zn foil preparation by mechanical calendering.(B) Schematic crystal plane evolution process during repeated cold-rolling.Reproduced with permission: Copyright 2022, American Chemical Society. 143Comparison of Zn electroplating sequences on (C) semimatched substrate and (D) perfect-matched substrate.Reproduced with permission: Copyright 2022, Wiley-VCH. 144hat ZnSe can exhibit a similar lattice structure to the Zn (002) plane, thus promoting the growth of Zn along the (002) plane.Combined with the good HER and Zn corrosion resistance, the ZnSe@Zn anode exhibits uniform Zn deposition behavior and excellent cycling stability.As a result, the designed ZnSe@Zn anode shows a high Zn utilization rate of 99.2% and an extended cycle life of 1530 h during repetitive Zn plating/stripping.
Zhao et al. 118 achieved improved Zn deposition behavior using FCOF as a multifunctional layer (Figure 9B).The electronegative F groups in FCOF can effectively enhance the kinetics of Zn nucleation.The strong hydrophobic properties of the nanopores repel the passage of H 2 O molecules when the hydrated Zn pass through, thereby accelerating the dehydration process of the hydrated Zn.Theoretical calculations show that F groups have higher binding energies with the Zn (200) plane than Zn (100), and the Zn (100) planes are more easily detached during striping, thus promoting the deposited Zn growth along the Zn (002) plane.Zhao et al. 119 applied ionic liquids to modulate Zn deposition behavior.They demonstrated that ionic liquids can reduce the dehydration potential of hydrated Zn and limit the diffusion of free water on the electrode surface, thus effectively suppressing side reactions.Furthermore, it can induce homogeneous nucleation and horizontal deposition along the Zn (002) plane under a selfpolarizing electric field.As a result, the Zn@SIP electrode can be applied in a wide temperature range and the assembled symmetrical cells can achieve long cycles of 2100, 550, and 350 h at −10 °C, −35 °C, and 60 °C, respectively.

| Conductive interfacial modification layer
Wang et al. 120 improved the electrochemical properties of Zn anodes by using the ZnTe semiconductor as the multiple-functional layer (Figure 9C).The good electronic conductivity and zincophilic properties of ZnTe ensure good Zn nucleation kinetics and preferential Zn growth along the (002) crystal plane.Theoretical calculations show that the Te atom has a strong binding interaction with the Zn atom and a higher binding energy with the Zn (002) than with the Zn (100).In combination with good HER and Zn corrosion resistance, the ZnTe@Zn electrode achieves good Zn plating/ striping reversibility and cycling stability.
As previously mentioned, Zn deposition is a process involving nucleation, growth, and connection; therefore, reducing the disorder of the crystal island linkage is also a key factor in homogeneous Zn deposition.Li et al. 121 prepared multicomponent Cu-Zn alloys on the Zn anode by magnetron sputtering.These Cu-Zn alloys show high redox potential, low HER activity, good corrosion resistance, and zincophilic properties and thus can effectively suppress side reactions and homogenize the Zn nucleation process.More importantly, this multicomponent Cu-Zn alloy layer can effectively alleviate the lattice distortion of Zn during the crystal island linkage process and avoid the formation of dendrites (Figure 9D).As a result, the symmetric cells assembled by Cu-Zn@Zn electrodes can maintain a stable cycle of 5496 h at 1 mA/cm 2 for 1 mA•h/cm 2 .

| 3D STRUCTURAL DESIGN
3D structural design has an important impact on Zn 2+ ion diffusion, based on the fact that they can affect the electric field strength and Zn 2+ flux and also can regulate the deposition sites of Zn. 3D structures with a high surface area can effectively reduce the local electric field strength and regular units allow for a homogeneous Zn 2+ flux distribution.In addition, 3D structures can buffer the volume variation during the Zn deposition process and special structural designs can also achieve selectivity in Zn deposition, thus prolonging the cycling performance of the Zn anode.Multiphysics field simulations have an important role in the design of 3D structures.It can contribute to a better understanding of the Zn deposition process and summarize the influencing factors, thus providing future design directions for 3D Zn anodes.In this section, related works are covered (Table 2).

| Reducing the local current density
As previously mentioned, Zn growth is accompanied by the migration of Zn 2+ ions to the interfacial nucleation sites driven by an electric field; therefore, the magnitude and uniformity of the electric field strength are important factors in the uniformity of Zn deposition.Zn anodes with a planar structure show low surface area and high electric field strength, tending to cause agglomerative deposition and resulting in protrusions and tips.Protrusions and tips further increase the local electric field strength and accelerate the deposition rate at this site, ultimately leading to dendrite formation ("tip effect") after long cycling.In contrast, 3D structures with a high surface area can effectively uniform and reduce the local electric field strength. 177,178To achieve a high specific surface area, the design of 3D structures focuses on preparing porous structures, applying 3D structural hosts, and preparing 3D modification layers.
Porous structures are usually prepared by etching or anodizing methods.Wang et al. 145 fabricated hierarchical porous structures on the surface of the Zn anode by using organic acid etching (TFA-AN@Zn) (Figure 10A).The porous structure can improve the interfacial wettability and carrier path of the Zn anode, increase the number of nucleation sites, and reduce the local current density.As a result, the TFA-AN@Zn electrode achieves a low nucleation overpotential (11 mV vs. 164 mV for Zn foil) and long stability (930 h at 2 mA•h/cm 2 ) (Figure 10B,C).Guo et al. 146 achieved a porous Zn anode by an anodized process (denoted as DCP-Zn).DCP-Zn can provide highspeed paths for ions/electrons and increase the effective solid-electrolyte interfacial area, thereby effectively reducing the local current density and buffering the volume change during the Zn deposition process (Figure 10D).In addition, the porous structure provides additional storage space for the deposited Zn, which is beneficial for high-capacity cycling.As a result, the symmetrical cells can stably cycle over 200 h at 10 mA•h/ cm 2 .Further, Bie et al. 147 coated a zincophilic ZnSe layer on an anodized Zn anode.As shown in Figure 10E, the inner cavity of the 3D-Zn formed by electro-oxidation can effectively increase the surface area and Zn/Zn 2+ reaction sites, thus reducing the local current density on the electrode surface.Besides, the ZnSe overlayer can restrain the side reactions and promote efficient dehydration, resulting in the acceleration of the Zn deposition kinetics.As a result, the symmetric cell based on 3D-Zn@ZnSe exhibits a long cycling lifespan (2000 h at 0.5 mA•h/cm 2 ).Chen et al. 148 built a porous Zn anode with a patterned microgroove structure by the calendaring method (Figure 10F).The specific structure can not only release the stress across the grid-patterned grooves but also accommodate the volume change during the Zn plating/stripping process.Notably, the groove-patterned Zn anodes coupling with Nafion film coating (P-Zn) can effectively depress the side reactions.
In addition to building a porous structure on the Zn anode, the construction of the 3D host can optimize the electric field distribution.For example, Fan et al. 149 coated a unique In@Zn@In trilayer on 3D porous Cu as a Zn anode host through electrodepositing In, Zn, and In percent (%), cycle, current density (mA/ cm 2 ), capacity (mA•h/cm 2 ) time (h), current density (mA/ cm 2 ), capacity (mA•h/cm 2 ) Ref.
The design of 3D nanostructures on the surface of the Zn anode can also effectively reduce the local current density.For example, Tao et al. 157 prepared Cu-dispersed foliate zinc-coordinated zeolite imidazole framework (ZIF-L) nanosheets on the Ti mesh (CuZIF-L@TM) to achieve a dendrite-free Zn anode (Figure 11D).The 3D skeleton of the Ti mesh and nanonetwork formed by the interconnected ZIF-L nanoflakes provided a large surface area for Zn deposition and greatly reduced the local current density.As a result, at a high depth of discharge (DOD) of 50% and a high current density of 6 mA/cm 2 , the CuZIF-L@TM Zn-based symmetrical cell keeps a small hysteresis of about 80 mV for 650 h.Similarly, Zeng et al. 68 prepared a 3D CNT framework on CC (Figure 11E).The porous structure effectively accomplished homogeneous  148 electric field distribution and achieved good cycling stability at a high DOD of 28%.Further, Cao et al. 158 grew 3D nitrogen-doped vertical graphene (N-VG@CC) on CC as a dendrite-free Zn anode.The obtained electrode not only achieves a low local current density with an enlarged surface area, but the zincophilic N group can benefit the Zn nucleation process, thus achieving a uniform Zn deposition process.Similarly, ZIF-8, 159 Zn array, 179 CuO nanowires, 69 etc. are also used to enlarge the surface area and uniformize the electric field distribution of the Zn anode.

| Manipulating Zn 2+ flux
The Zn 2+ flux determines the speed of charge transfer and influences the deposition location.The nonuniform Zn 2+ flux resulting from inhomogeneous electric field distribution and concentration polarization is the main reason for the formation of dendritic Zn.In general, the 2D planar structure is difficult to provide a good channel for ion transport, which not only affects deposition kinetics but also causes ions to gather in places with high local current density and then form dendrites.In comparison, the 3D structure can distribute the Zn 2+ flux and increase the Zn 2+ ion concentration on the electrode surface, thus facilitating the Zn deposition uniformity. 180esigning the 3D anode host with an array structure is an effective method to regulate Zn 2+ flux as the regular units can order the Zn 2+ ion distribution and enhance the Zn 2+ ion diffusion. 181For example, Zhou et al. 160 built a 3D hierarchical Zn host via a facile and scalable method with balsa wood veneer as the raw material (Figure 12A).It is found that the obtained electrode can considerably facilitate the dehydration process, improve Zn deposition kinetics, and inhibit Zn metal corrosion, thereby significantly stabilizing Zn deposition behavior.More importantly, the electrode restricted the 2D diffusion of Zn 2+ ions and achieved uniform Zn 2+ flux in a 3D pattern.As confirmed by AFM images, the wood@Ni@Zn surface only becomes slightly rougher after cycling (Figure 12B).3D structured metal mesh with a regular structure can be effectively used to regulate Zn 2+ flux when coated with zincophilic materials.For example, Yu et al. 161 grew Ag-Zn alloys on the surface of stainless-steel mesh (AZ-SSM) and achieved a desirable Zn anode host with fast ion diffusion (Figure 12C).With a high surface area and porous structure, the 3D AZ-SSM exhibits a homogeneous electric field and fast electrochemical kinetics, significantly suppressing the growth of zinc dendrites.The fast ion diffusion alloy layer accelerates the Zn 2+ migration in an orderly manner to homogenize Zn 2+ flux.Similarly, a nanoporous Sn layer was coated on Cu mesh by Jian et al., 162 which also confirms that the special 3D structure can homogenize electric field and Zn 2+ flux distribution.A 3D carbon skeleton with high electronic conductivity is also a good option.A 3D porous graphene-carbon nanotube scaffold decorated with metal-organic framework-derived ZnO/C nanoparticles (3D-ZGC) is fabricated as the host for dendrite-free Zn-metal composite anodes. 163The mechanically robust 3D scaffold can homogenize Zn 2+ ion flux during the long-term plating/stripping process and uniformize the electric field distribution on the surface of the 3D-ZGC host (Figure 12D).
The nanoarrays also facilitate the regulation of Zn 2+ flux.Guo et al. 164 integrated a thin interfacial layer consisting of an array of ZnSiO 3 (ZSO) nanosheets on the surface of a Zn foil by a simple wet chemical method.As illustrated in Figure 13A, the evenly distributed micro/ mesopores on the ZSO nanosheets and the hydrophilic layer promote the charge/mass transfer processes, thus reducing the nucleation barrier and polarization.Moreover, Zn 2+ flux is equalized by the uniform nanosheet array structure, increasing more active nucleation sites, reducing F I G U R E 12 (A) Graphic illustration of Zn deposition onto the surface of wood@Ni@Zn.(B) AFM images of wood@Ni@Zn before and after 50 cycles at 2 mA/cm 2 and 2 mA•h/cm 2 .Reproduced with permission: Copyright 2022, Elsevier. 160(C) Schematic illustration of 3D metal mesh with a fast zinc-ion diffusion alloy layer.Reproduced with permission: Copyright 2022, Elsevier. 161(D) Schematic illustration of Zn plating on the 3D-ZGC host in different states.Reproduced with permission: Copyright 2022, Wiley-VCH. 163ocal current density, and finally eliminating the "tip effect."Similarly, the 3D interconnected ZnF 2 matrix was electrodeposited onto a Zn foil substrate in an aqueous solution containing 1.0 mol/L NH 4 F (Figure 13B). 74Ion flux simulation results reveal that the inhomogeneous distribution of the Zn 2+ flux on the surface of Zn foil generates "hot spots," resulting in the formation of Zn dendrites.After building a 3D ZnF 2 matrix, the magnitude of Zn 2+ flux in the whole electrode architecture is simultaneously enhanced and homogenized.As a result, the assembled Zn@ZnF 2 ||Zn@ZnF 2 cell can display stable voltage profiles with a small hysteresis beyond 500 h at 10 mA/cm 2 (Figure 13C).Further, a 3D ZnOHF nanoarray was fabricated on the Zn anode by a simple hydrothermal method. 70They found that the array units can order the current density and Zn 2+ flux, thus facilitating the uniformity of Zn deposition (Figure 13D).Wang et al. 165 reported a 3D Zn-phosphorus (ZnP) solid solution alloy as the artificial protective layer on the Zn anode (Zn@RP-NC).The 3D ZnP solid solution alloy is formed by the deposited Zn and red P during the plating process.The concentration gradient of the electrolyte on the electrode surface can be redistributed by this protective layer, thereby achieving a uniform Zn 2+ flux (Figure 13E).The ZnO-3D interfaces are also fabricated to decrease the local current density and uniform the Zn 2+ flux. 166The Zn@ZnO-3D electrode with low activation energy and deposition overpotential achieves fast transfer and deposition kinetics.Similarly, Kim et al. 167 reported a Zn anode comprising a Zn hexagonal pyramid array (HPA) coated with a functionalized ZnO layer.The obtained Zn anode achieves good cycling stability.After the 10th cycle, it can maintain its array morphology (Figure 13F).

| Achieving Zn deposition selectivity
The 3D porous frameworks can regulate the local current density and Zn 2+ ion flux, thus offering the possibility of selective Zn deposition.Preferentially depositing Zn in a safe place, away from the separator, can effectively limit the risk of short circuits caused by dendrites piercing the separator.Moreover, the 3D structure with abundant void space can achieve a large Zn deposition capacity, which is beneficial for high-capacity cycling.In this section, reports about space-induced deposition and space confinement strategies will be covered.

| Space-induced deposition
As mentioned before, the 3D structure can induce electric field concentration in the microchannels and enhance the Zn 2+ flux; therefore, Zn will preferentially deposit in the 3D microchannels.Inspired by this, Liu et al. 168 constructed a flexible ultrathin and ultralight Zn micromesh with regularly aligned microholes for the Zn anode by combining photolithography with electrochemical machining (Figure 14A).In situ microscopic observation demonstrates the inductive effect of the Zn micromesh on the ion concentration distribution and current distribution, where Zn 2+ ions are preferentially nucleated and deposited on the inner walls of the micropores.Zhang et al. 169 reported a 3D Ni-Zn framework with multichannel lattice structures as the Zn anode through 3D printing technology.They found that the multichannel structure with a large surface area not only can reduce the local current density and homogeneous electric field but more importantly can realize redistribution of current flow and lead Zn 2+ ions to flow into the channels.
Further Zeng et al. 182 prepared a zincophilic 3D N-doped carbon host by 3D printing technology.The 3D lattice structure can induce the migration of Zn 2+ to the microchannels, realizing the Zn deposition selectivity.The zincophilic N group can accelerate the Zn nucleation kinetics, thereby promoting the cycling stability of the Zn anode.Although the above two methods can realize the space-induced deposition of Zn, the complicated preparation process makes it hard for large-scale preparation. 170Huang et al. 171 proposed an interfacial engineering strategy for fabricating channeled Zn anodes via a one-step lasermicromachining technology (Figure 14B).The as-prepared Zn anode shows a periodic grid concave convex architecture with a rough surface.This unique structure enhances the interfacial wettability and optimizes the mass transfer kinetics of Zn plating/stripping.The concave convex patterned surface also generates a regular electrical field and associated current density fluctuations that suppress tip growth.As a result, these attributes ensure uniform Zn electrodeposition/electrostripping through a wide range of current densities.Further, to increase the accuracy of structures and reduce the cost of using laser technology, Cao et al. 172 reported a zincophilic Zn anode with 3D micropatterns by combining the imprinting strategy and femtosecond laser technology (Figure 14C).The preparation of the Zn anode is imprinted with a customized mold fabricated by a femtosecond laser, ensuring structural controllability and low costs.The obtained imprinted zincophilic Zn electrode expresses a unique microchannelinduced spatial selection deposition behavior, which not only optimized the Zn deposition process but also prevented the short circuit from vertical dendrite growth.In addition, this imprinting strategy can be easily scaled up for large-area pouch cells, and it is also proved to be efficient for other zincophilic metal alloys and metal compounds.

| Space confinement
Although the space-induced deposition can achieve preferential deposition of Zn in the microchannels, its deposition on the top surface still carries the risk of dendrites piercing the separator.Designing a special structure to realize the confinement deposition of Zn can effectively prolong the cyclic stability of the Zn anode.
Electrodes with gradient structures can achieve selectivity of Zn deposition through differences in conductivity, zincophilicity, or porosity. 183Shen et al. 173 prepared a 3D gradient Zn anode with Cu foam at the bottom, Ni foam at the middle, and NiO coating on the top, which created a gradual increase in Zn/Zn 2+ reaction resistance from the bottom to the top (Figure 15A).The as-prepared stratified deposition framework (SDF) electrode can achieve a stratified deposition of Zn metals from the bottom layer to the top layer due to the different overpotentials and binding energy of Zn deposition.Benefiting from the desirable deposition behavior, the Zn//MnO 2 cell assembled with the SDF electrode can stably run for 500 cycles with a high specific capacity of 141.9 mA•h/g and a capacity retention of 91.4% at the current density of 5 C (Figure 15B).Further, Gao et al. 174 achieved a triple gradient Zn anode by a simple imprinting process (Figure 15C).The triple-gradient design synergistically introduces higher Zn 2+ ion flux and optimizes local charge-transport dynamics at the bottom of the electrode, thus promoting the migration of Zn 2+ ions from the top to the bottom and realizing desired bottom-up deposition behavior for Zn metals.As a result, not only controllable and uniform Zn deposition is achieved, but also the possible short circuit from top dendrite growth is  47 prevented.Different from the construction of the gradient host, Liang et al. 184 prepared a zincophilic gradient on the surface of the Zn anode to realize the confinement deposition behavior.The coating consists of spatially graded fluorinated alloy (GFA) nanoparticles with ZnF 2 as the outermost layer and CuZn alloys as the inner layer.It can accommodate the dendrite-free morphology by inducing lateral growth within the GFA coating: by forming a CuZn alloy inside the CuZn particles to store the Zn and fill the voids between the GFA particles.
Electron shielding enables selective deposition of Zn via electrostatic effects.Wang et al. 47 proposed that Zn/Al alloys with alternating Zn and Al layered nanostructures can be used as reversible and dendritefree anode materials (Figure 15D).The unique lamellar structure promotes the reversibility of stripping/plating of Zn by making use of symbiotic less-noble Al lamellas, which in situ form interlamellar nanopatterns with an Al/Al 2 O 3 core/shell structure.Therein, the Al protects against the irreversible byproduct of ZnO or Zn(OH) 2 , while the insulating Al 2 O 3 shell prevents the electroreduction of Zn 2+ ions on the Al/Al 2 O 3 patterns and thus guides their electrodeposition on the precursor Zn sites, substantially eliminating the formation and growth of Zn dendrites.
Different from the bottom-up deposition behavior, Li et al. 185 achieved a hierarchical confinement deposition behavior by using porous Co-embedded carbon cages as a Zn anode host.The Zn deposition characteristics can be effectively regulated by the following: (i) Zincophilic Co sites effectively reduce the nucleation barrier, endowing direct Zn nucleation with high efficiency (within 0.5 mA•h/cm 2 ).(ii) The carbon cages with a high surface area can spatially constrain the uniform growth of metallic Zn within the cages (within 5.0 mA•h/cm 2 ).(iii) The porous all-in-one network would homogenize the electric field distribution and redistribute the Zn 2+ flux on a large scale (within 12 mA•h/cm 2 ).

| Multiphysical field simulation
The Zn deposition process is complex and the mechanism varies depending on the electrode system.With the development of computer science, theoretical simulation is gradually employed to investigate physical field changes.The simulation of electric field intensity, ion flux distribution, and current density is commonly used and combined with experiments to explore the details of reactions occurring on the Zn anode surface. 70,74,169,172he local current density is solved according to the Butler-Volmer equation (Equation ( 5)): where i loc represents the current density of the electrode; i 0 is the exchange current density; α a is the charge transfer coefficient in the anode direction; F represents the Faraday constant; η represents the activation overpotential; R represents the ideal gas constant; T represents the temperature in Kelvin; and α c represents the charge transfer coefficient in the cathode direction.
Ion migration is studied according to the diffusion and electric field migration equation (Equation ( 6)): where N Zn is the Zn 2+ diffusion flux; D Zn is the Zn 2+ diffusion coefficient; and c Zn ∇ is the Zn 2+ concentration gradient; μ m,Zn represents the mobility of Zn 2+ ; Z Zn represents the Zn 2+ band charge; and ϕ l represents the electric potential in solution.
Electric field simulations can explore the effect of concave and convex surfaces on Zn deposition behavior, in which strong electric fields mean fast deposition rates.As shown in Figure 16A, the electric field distribution on the bare CC surface is inhomogeneous.During cycling, Zn 2+ prefers to continuously deposit on these protuberances sites, consequently resulting in Zn dendrite growth. 68In contrast, the electric field streamlines become more uniform with the introduction of the 3D CNT scaffold.The simulation of the electric field well explains the 3D architecture-minimized Zn nuclei size and optimized deposition behavior.
Zn ion concentration distribution allows monitoring of Zn deposition behavior.The Zn micromesh with regularly aligned microholes is simulated. 168As shown in Figure 16B, it can be observed that the Zn micromesh electrode shows a stronger affinity of Zn 2+ ions and guides Zn 2+ ions to migrate into the internal wall of the microholes preferentially.Moreover, the Zn micromesh with higher ion concentrations both along the X and Y directions implies a compressed concentration polarization owing to the Zn-ion affinity of the microhole structure, which is beneficial for the Zn ion transport and a uniform deposition.
The magnitude of the current represents the electron transfer rate occurring at the electrode, which helps to determine the Zn deposition site.For example, Wu et al. 175 fabricated two types of 3D printing graphene arrays (3DGs), in which both tube arrays and pilar arrays can improve the reversibility of the Zn anode.To predict the Zn plating rate among 3DG substrates, the current density distribution simulation was conducted.It is shown in Figure 16C that the current density on the top surface that is in direct contact with the separator is much lower than that in the void space of tubes and interspace of the arrays, which indicates that the Zn plating rate on the top surface of 3DGs was much lower than the inner tubes and interspace of the arrays.These results confirm that the tube arrays have a confinement effect on the distribution of Zn 2+ ions, which can effectively avoid the formation of Zn dendrites on the top surface of the arrays.
The specific deposition path on the Zn anode surface can also be explored by simulating the electric field and deposition position.The 3D crosslinking structure of the silver nanowire aerogel (AgNWA) renders a uniformly distributed electric field for reversible Zn deposition/ stripping. 156To better show the advantages of the 3D structure in AgNWA, the electric field simulation at the anode interface was carried out by COMSOL calculation (Figure 16D,E).Benefiting from the 3D cross-linked fiber structure, the AgNWA displays a uniform electric field distribution, while the local electric field is inclined to concentrate near the protuberance of the planar anode surface observed from the 3D models.
Multiphysical field simulation is beneficial to guide a homogeneous Zn deposition on the anode surface and inhibit dendrite growth.Using a self-template method, Wang et al. 176  nanoarray (Figure 16F) on the Zn anode.The numerical simulations are conducted to reveal the Zn 2+ ion distribution on the Zn-TCPP/Zn electrode.At the initial stage, a higher Zn 2+ concentration can be observed on the surface of Zn-TCPP flakes due to the polarized negative charge, demonstrating the formation of the preseeding Zn 2+ layer, which can facilitate the Zn deposition on the Zn-TCPP flakes.When the deposition potential is applied, the current is directed to the Zn-TCPP flakes, thus promoting the Zn deposition on the MOF flake.Considering the nonconductive properties of the Zn-TCPP, the Zn deposition proceeds with a bottom-up approach from the Zn substrate, which leads to the spatially controlled U-shaped deposition without dendrite formation.
Multiphysical field simulation also can assist in the design of 3D structures for selective Zn deposition.The bottom-up deposition behavior in triple gradient electrodes can be explained through ion and electron transport. 174As explained by the electric field simulation in Figure 16G (the downward arrow indicates the direction of ion transfer, the upward arrow indicates the path of electron transfer, and the gray part indicates the deposited Zn metal), the electric field in the electrolyte channel accelerates the transfer of Zn ions from the top to the bottom.From the point of view of the electron transfer path, the bottom Ag layer has a lower charge transfer resistance and promotes the deposition of Zn on the bottom, rather than on the top NiO surface.

| CONCLUSIONS AND OUTLOOK
ZIBs possess multiple advantages such as greenness, safety, and high specific energy density, thus exhibiting great application prospects in the next generation of high-energy rechargeable batteries.However, they still suffer from low CE, rapid capacity degradation, and short-circuit problems caused by unstable Zn anodes.The reversibility of Zn plating/stripping is a key factor affecting the stability of Zn anodes.Among them, the Zn deposition behavior can greatly affect the subsequent stripping efficiency, and inhomogeneous Zn deposition is one of the main causes of low CE and dendrite growth.Therefore, it is important to optimize the uniformity and flatness of the Zn deposition process.The Zn deposition process is controlled by kinetics and Zn 2+ diffusion and is influenced by many aspects.In this review, starting from the influencing factors of the Zn deposition process, we analyze the basic principles of the modification direction and carry out a mechanistic analysis.Subsequently, based on those principles, a literature review is conducted and various strategies and approaches reported are summarized.Most importantly, we outline their potential regulatory mechanisms for optimizing Zn anodes, including nucleation modes, subsequent Zn growth, Zn 2+ diffusion behavior, etc.Specifically, as shown in Figure 17, from the design principle, the modification of the Zn anode can be divided into the kinetic modification and 3D structural design, of course, both of which can work synergistically.From the kinetic point of view, it can mainly be carried out from the ways suppressing the side reactions, homogenizing the Zn nucleation, and controlling the crystal orientation of deposited Zn.From the structural point of view, it can be mainly considered reducing the local current density, manipulating Zn 2+ flux, and achieving Zn deposition selectivity.
Although various methods have been proposed for optimizing the Zn deposition behavior and the cycling performance of Zn anodes has been improved, the commercialization of Zn anodes still remains a great challenge.Hence, we evaluate and discuss the challenges F I G U R E 17 Summary of optimization strategies for Zn deposition behavior.
simulation system should be according to the actual conditions.(8) Improving the practicality of ZIBs.To facilitate the commercialization of ZIBs, the cost of electrode materials and manufacturing processes should be considered to ensure competitiveness.Expensive materials and complex processes, such as precious metals, ALD, MLD, etc., should be avoided.To improve the utilization of the Zn anode, the negative/positive capacity (N/P) ratio should be highlighted when assembling full cells.The lower N/P ratio not only reduces the cost of ZIBs but also allows for a high specific energy density.In addition, the preparation of large-size soft pouch batteries with high performance is of good practical application.

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I G U R E 1 Schematic diagram of the categories, challenges, and optimization strategies of Zn deposition behavior.
). Due to the high affinity of NH groups for SO 4 2− ions, SR can efficiently combine with SO 4 2− ions in the bulk electrolyte to generate SR-SO 4 2− coating.As shown in Figure 3D, the corresponding electrostatic effect induced by the obtained negatively charged coating can repel the remaining free SO 4 2− ions, thereby reducing the SO 4 2− ion concentration on the electrode surface.Besides, SR and SR-SO 4 2− also show high HER overpotentials.Similarly, Park et al. 126 introduced (3-aminopropyl)triethoxysilane (APTES) on the surface of Zn anodes to modulate the Zn deposition process.The results reveal that the positively charged amine (-NH 2 ) groups of the artificial T A B L E 1 (Continued)

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I G U R E 3 (A) Schematic illustration of the passivation layer slowing the electrodeposition of Zn and protecting the electrode from corrosion by the electrolyte.Reproduced with permission: Copyright 2022, Wiley-VCH. 123(B) Schematic diagram of PDMS/TiO 2 interface coating and protective mechanism.Reproduced with permission: Copyright 2022, Wiley-VCH. 81(C) DFT of the binding energies of Zn adsorbed on PZIL with different groups, and schematic diagram of the Zn coating behavior on bare Zn and PZIL-Zn.Reproduced with permission: Copyright 2022, American Chemical Society. 87(D) Diagram of the role of the SR layer.Reproduced with permission: Copyright 2022, Wiley-VCH.

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I G U R E 4 (A) Schematic illustration of the chemical corrosion of the Zn metal and the protective effect of Cu-Zn alloys.(B) Linear polarization curve of the Cu/Zn electrode in a 3 mol/L ZnSO 4 electrolyte, suggesting that the corrosion was suppressed for the Cu/Zn electrode in comparison to the bare Zn electrode.Reproduced with permission: Copyright 2020, Elsevier. 92(C) Schematic illustration of the evolution of Zn deposition morphology on the bare Zn anode and Zn@In anode.Cross-sectional SEM images of (D) bare Zn and (E) Zn@In after Zn deposition.Reproduced with permission: Copyright 2022, The Royal Society of Chemistry. 128(F) Preparation of Zn-Sn alloys.Reproduced with permission: Copyright 2021, Wiley-VCH. 129(G) Schematic diagrams of the challenges in the pure Zn electrode and the protective mechanism of the ZnIn-PAM electrode.Reproduced with permission: Copyright 2022, Elsevier. 131SEM, scanning electron microscopy.

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I G U R E 5 (A) Schematic representation of the ZnS@Zn interphase of the ZnS@Zn-350 electrode.(B) CEs of Zn plating/stripping in bare Cu-Zn and ZnS@Cu-Zn cells with a capacity of 1 mA•h/cm 2 .Reproduced with permission: Copyright 2020, Wiley-VCH. 96(C) Zn adsorbed on bare Zn and ZnP coating at different adsorption sites.Reproduced with permission: Copyright 2021, Wiley-VCH. 97(D) Schematic diagrams for the Zn@ZIF anode during cycling.Reproduced with permission: Copyright 2020, Wiley-VCH. 100(E) Calculation models of Zn absorbed on the cyano-group and aldehyde group and corresponding binding energy.Reproduced with permission: Copyright 2022, Wiley-VCH. 101(F) Optimized configuration of the Zn 2+ adsorption on the -SO 3− group and Zn 2+ transport in the polyanionic hydrogel SEI layer.Reproduced with permission: Copyright 2022, Wiley-VCH.

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I G U R E 7 (A) Scheme illustrating the design principle of epitaxial metal electrodeposition.Reproduced with permission: Copyright 2019, American Association for the Advancement of Science. 63(B) Schematic illustration of Zn dendrite growth and MXene crystal structure with matching Zn deposition on MXene.Reproduced with permission: Copyright 2021, American Chemical Society. 110(C) Schematic illustration of crystallographic orientation of the Zn UPD and OPD layers on Cu (100).Reproduced with permission: Copyright 2022, American Chemical Society. 111(D) Schematic of dendrite-free Zn deposition based on vertical crystal plane matching.Reproduced with permission: Copyright 2022, Wiley-VCH.112

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I G U R E 9 (A) Schematic illustration of Zn plating on bare Zn foil (upper) and the NGO@Zn electrode (lower).Reproduced with permission: Copyright 2021, Wiley-VCH. 135(B) Mechanism comparison of the deposition processes for FCOF@Zn and bare Zn surfaces.Reproduced with permission.Copyright 2021, Springer Nature. 118(C) Schematic illustration showing Zn deposition behavior of the ZnTe@ Zn electrode.Reproduced with permission: Copyright 2022, Wiley-VCH. 120(D) Schematic illustration of Zn deposition on bare Zn and Cu-Zn@Zn.Reproduced with permission: Copyright 2022, Wiley-VCH.121

T A B L E 2
Electrochemical performance of different Zn anodes with 3D structural design.

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I G U R E 10 (A) Schematic diagram showing the fabrication of 3D porous Zn foil in nonaqueous organic TFA-AN acid.(B) Voltage profiles of TFA-AN@Zn and pristine Zn at the 1st plating under the current density of 4.0 mA/cm 2 with the capacity of 2.0 mA•h/cm 2 .(C) Long-term plating/stripping performance of TFA-AN@Zn and pristine Zn (insets are their magnified voltage profiles).Reproduced with permission: Copyright 2022, Wiley-VCH. 145(D) Schematic illustration of repeated Zn tripping/plating behaviors of DCP-Zn.Reproduced with permission: Copyright 2020, Elsevier. 146(E) Schematic illustration of the preparation of the 3D-Zn@ZnSe anode.Reproduced with permission: Copyright 2022, Wiley-VCH. 147(F) Schematic illustration of Zn plating/stripping behavior on P-Zn.Reproduced with permission: Copyright 2022, Elsevier.

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I G U R E 15 (A) Oriented deposition of Zn particles on SDF from the Cu layer to the Ni layer and eventually to the NiO layer.(B) Cycling performance comparison of the button Zn//MnO 2 full cells using SDF and Zn foil anodes at 5 C. Reproduced with permission: Copyright 2021, Wiley-VCH. 173(C) Schematic diagram for the triple-gradient electrode fabrication via the mechanical rolling method.Reproduced with permission: Copyright 2022, Wiley-VCH. 174(D) Al/Al 2 O 3 interlayer patterns associated with the insulative Al 2 O 3 shield facilitate the uniform deposition of Zn.Reproduced with permission: Copyright 2020, Springer Nature.
produced a porphyrin-based 2D MOF F I G U R E 16 (A) Models of the electric field distributions for a Zn/CC electrode and a Zn/CNT electrode after Zn nuclei formation.Reproduced with permission: Copyright 2019, Wiley-VCH. 68(B) In-electrolyte concentration of Zn ions with isoconcentration lines during the deposition process for the Zn micromesh electrode (left) and Zn film electrode (middle) and the cross-sectional 1D concentration profile along normal the X direction (right).Reproduced with permission: Copyright 2021, Wiley-VCH. 168(C) Current density distributions predicted by multiphysics models for the 3DGT (left) and 3DGP (right).Reproduced with permission: Copyright 2022, Elsevier. 175(D) 3D models of the electric field distributions for pure zinc foil (left) and AgNWA (right).(E) 3D models of the Zn atom deposition distributions for pure zinc foil (left) and AgNWA (right).Reproduced with permission: Copyright 2022, Elsevier. 156(F) Numerical simulation of the Zn 2+ concentration distribution within the Zn-TCPP/Zn electrode (left) and simulated potential and current distribution for Zn-TCPP/Zn and bare Zn in a 2 mol/L ZnSO 4 electrolyte (right).Reproduced with permission: Copyright 2022, Elsevier. 176(G) Schematic of the simulation of ion and electron transport paths within the triple-gradient electrode.Reproduced with permission: Copyright 2022, Wiley-VCH.174 Electrochemical performance parameters of different Zn anodes with interfacial modification.
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