Recent developments in three-dimensional Zn metal anodes for battery applications

Aqueous zinc (Zn) ion batteries (AZIBs) are regarded as one of the promising candidates

YX030003; Natural Science Foundation of Jiangsu Province, Grant/Award Number: BK20210604; King Abdullah University of Science and Technology

| INTRODUCTION
Energy storage technology is an important component of most renewable energy systems.In particular, lithium (Li)-ion batteries (LIBs), as the main electrochemical energy storage system, have dominated the market of chemical energy storage applications in recent decades.Although LIBs are used almost everywhere in our daily life, various concerns about their safety, limited Li resources, high production costs, sustainability, and affordability have hindered some of their applications. 1These existing drawbacks make it essential that academia and industry develop new energy storage technologies that can complement LIBs in some applications.Therefore, to break through the bottleneck of LIBs, several novel battery systems have been developed in recent years, including sodium-ion batteries (SIBs), potassium ion batteries (PIBs), sulfur (S)-based batteries, and aqueous zinc (Zn) ion batteries (AZIBs). 2 Compared with LIBs, AZIBs offer several advantages, including low cost, environmental benignity, and high safety.The battery is intrinsically safe due to the chemical stability of Zn and the fact that the entire battery system operates in an incombustible aqueous electrolyte.In addition, metallic Zn, as the anode material of Zn-ion batteries, has received huge research attention because of its abundance, low cost, high theoretical capacity (e.g., gravimetric capacity of 820 mAh g À1 or volumetric capacity of 5855 mAh cm À3 ) and relative low electrochemical potential (À0.762 V vs. standard hydrogen electrode). 3However, Zn metal also suffers from several problems.When used as anode material of AZIBs, Zn metal anodes face challenges related to dendrites growth and side reactions (i.e., hydrogen evolution, corrosion, and side product generation).3b,4 These problems are serious impediments to the further expansion of AZIBs.Especially, the formation of Zn dendrites poses a significant risk as they can penetrate the separator, resulting in direct contact between the cathode and anode.This contact can lead to internal short circuits, ultimately causing the battery to fail. 5 Over the years, various attempts have been adopted to resolve the problems of Zn metal anodes, including optimizing electrolyte composition, 6 employing epitaxial growth of Zn, 7 building artificial interphase on the anode, 8 and introducing a three-dimensional (3D) structure to Zn. 9 Among them, the introduction of a 3D structure is considered to be one highly effective approach to tackle the problem of Zn dendrite formation, since it can provide evenly distributed surface current density, homogeneous ion flux, large inner space, and released Zn plating stress at the anode (Figure 1).However, while much research has used 3D Zn anode structure to suppress dendrites and achieved improved performance, an overall summary of applying this strategy in the Zn metal anode field is still lacking.Since AZIBs operate in a very different electrolyte system compared to alkali metal anodes (Li, sodium [Na], and potassium [K])-based batteries (i.e., aqueous vs. organic), a systematic review article presenting the comprehensive analysis on AZIBs is necessary and timely.Herein, we aim to provide such a review on 3D structural Zn metal anodes.This review provides an overview of the fundamentals of 3D Zn metal anode design and performance in aqueous electrolytes covering development history, working principles, and future opportunities and challenges.The recent advances in 3D structured Zn metal anode are summarized and discussed according to preparation methods, surface modification, gradient design, and side reaction inhibition.Future perspectives of 3D structured Zn metal anode are thoroughly discussed in this review.By discussing potential advancements and avenues for improvement, this review aims to offer fresh insights and valuable guidance for the design of 3D-structured Zn metal anodes, ultimately enhancing the performance of Zn-ion batteries.

| HISTORY OF 3D STRUCTURE DESIGN IN ZN METAL ANODE
A timeline describing recent progress in the development of 3D structural Zn anode in AZIBs is shown in Figure 2.
The research on AZIBs actually dates back to around the 19th century.Alessandro Volta invented the voltaic pile in 1799, marking the inception of the first aqueous Zn battery. 10In 1886, the primary Zn-C dry battery was designed, and its modified version has been practically used until now. 11After that, Thomas Edison developed the nickel-zinc (Ni-Zn) battery as the first rechargeable Zn battery in 1901. 12However, Ni-Zn batteries faced uncontrollable self-discharge phenomena and the uncontrollable growth of Zn metal dendrite.In 1986, to tackle these bottlenecks, Yamamoto et al. developed the first mild aqueous ZnSO 4 electrolyte for AZIBs, which greatly improved the electrochemical reversibility of the original Zn anode. 13After this, the commercialization of Li-ion batteries largely delayed the development of the aqueous Zn-ion batteries, but in recent years Zn batteries have received renewed interest.
It is known that planar-structured Zn foil can be directly used as anode material for AZIBs.However, the inhomogeneous dispersion of charge density and anion concentration always affects Zn plating and stripping behavior, resulting in dendrite growth and hindering the practical applications of AZIBs.Therefore, with the revival of AZIB research in recent years, mitigating the formation and growth of Zn metal dendrite by inducing 3D structures for Zn metal anodes has received tremendous attention as a means to enhance the electrochemical performance of AZIBs.
In 2017, Rolison et al. developed a 3D sponge Zn metal anode that achieved 90% theoretical depth of discharge (DOD) without dendrite growth. 14However, the sponge Zn anode obtained from assembled Zn powder still faced probable structure collapse during the long-term charge-discharge process.To further enhance the electrochemical performance of Zn metal anodes, researchers have proposed combining pure Zn metal with other materials, such as alloying materials and conductive hosts.Wang et al. utilized a graphite felt with high conductivity as the host to accommodate metallic Zn. 15 The as-prepared self-supporting composite anode provided a large electroactive area that can facilitate the transportation of electrons and ions and allowed homogeneous Zn deposition.Similar to this study, considerable efforts have been made to achieve a highly stable Zn composite anode by inducing carbon-based and metalbased hosts. 16esides carbon/metal-based materials as advanced hosts, replacing planar Zn metal anodes with highsurface-area Zn or Zn alloys anodes has been another strategy to achieve high reversibility of Zn during plating and stripping. 17Note that, high-surface-area Zn or Zn alloy anodes with significantly increased electroactive areas can greatly reduce the local charge accumulation, facilitate the homogeneous transportation of electrolyte ions, and release stress driven by Zn plating. 19Therefore, the advanced structured Zn or Zn alloy can promote F I G U R E 2 Timeline showing the history of 3D structured Zn anode in AZIBs.Pictures are reproduced with permission from: Copyright 2020, 10 American Chemical Society; Copyright 2019, 11 Elsevier; Copyright 2020, 12 Wiley-VCH; Copyright 2014, 18 and under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence: published by The Royal Society of Chemistry.Copyright 2016, 13b American Chemical Society; Copyright 2017, 14 The American Association for the Advancement of Science; Copyright 2017, 15  homogeneous Zn plating/stripping and mitigate dendrite formation.More recently, Guan and co-workers proposed a triple-gradient host that integrates multi-functional gradients, including pore, conductivity, and zincophilicity, for a dendrite-free Zn metal anode.9b The host with the triple-gradient design exhibits several advantages: (i) The porous host introduces higher Zn 2+ ion flux.(ii) The charge transport kinetics of the electrode is faster in the region of high conductivity, thus promoting the concentration distribution of Zn 2+ in the electrodes.(iii) The different zincophilicities of the electrode will induce the deposition and dissolution of Zn in specific areas.(vi) The synergistic effect of these triple gradients could realize desired bottom-up deposition and topdown dissolution behavior of Zn metals.

| Opportunities of 3D-structured Zn anode
In rechargeable aqueous Zn anode systems, the uneven distribution of electric field and Zn ion (Zn 2+ ) flux on flat Zn metal anodes can result in uncontrollable Zn metal plating/stripping, leading to the growth of dendrites.However, by transforming planar Zn foils into a 3D structure, the nucleation barrier for Zn 2+ can be reduced, facilitating the deposition and dissolution of Zn at the anode.This tailored 3D structure enables better control over the electrochemical processes, mitigating dendrite growth and improving the overall performance of the Zn metal anode system. 20Specifically, a 3D-structured Zn anode has the following features.

| Evenly distributed surface current density
The electric field is the driving force of Zn 2+ migration that triggers the electrochemical reaction.The "space charge theory," initially proposed by Chazalviel, has been widely accepted as an explanation for the correlation between the formation of metal dendrites and the distribution of current density. 21Within this model, it is postulated that in a dilute electrolyte solution, metal ions undergo reduction.Consequently, a cation gradient is present between the anode and cathode surfaces in the battery system.During rapid discharge, the concentration of ions near the anode's surface diminishes, resulting in the formation of an electric field and a localized space-charge region at the interface between the electrolyte and anode.
At a critical current density value, denoted as J*, the local ionic concentration near the electrode surface rapidly drops to zero within a short time period, τ s , as described by Equations ( 1)-( 3).This critical condition marks the onset of dendrite formation.The variables J, μ a , and μ c represent the applied current density, anion mobility, and cation mobility, respectively.It is assumed that the variables μ, e, and D are concentrationindependent and represent mobility, electronic charge, and diffusion constant, respectively.C 0 represents the initial cation concentration, and L corresponds to the distance between the electrodes.
The formation of dendrites is closely linked to the concentration gradient of ions on the electrode surface.As described by Sand's time, this gradient becomes more pronounced on the surface of a planar electrode, resulting in a reduced ion concentration on the anode surface.This concentration gradient promotes the growth of dendrites. 22However, the transformation of a planar electrode into a three-dimensional porous electrode can be a game-changer.This modification significantly increases the specific surface area of the electrode, thereby reducing the concentration of local current density.As a result, the initial growth time of dendrites is prolonged, offering a potential solution to mitigate dendrite formation.This, in turn, increases τ s .In another study, Xie et al. studied the difference between bare Zn metal anodes and copper (Cu) nanowires coated Zn (Zn@CuNW) metal anodes.17h They found that, by coating the surface of Zn metal with a network of Cu NWs, a substantial increase in specific surface area is achieved.This, in turn, leads to a reduction in local current density and a more homogeneous distribution of Zn 2+ concentration (Figure 3A-C).These studies show that 3D-structured electrodes are an effective approach to controlling dendrite formation in Zn metal batteries.The reduction in local current density effectively extends the initial growth time of dendrites, resulting in a notable decrease in the risk of short circuits and potential battery failure.

| Homogeneous ion flux
The movement of Zn 2+ ions between the cathode and anode is a critical process that occurs during repeated charging and discharging cycles.The transportation of Zn 2+ ions plays a pivotal role in influencing the electrochemical deposition behavior and the resulting morphology of the Zn electrode.The traditional planar Zn foils with randomly rough surfaces can easily induce inhomogeneous charge accumulation in specific locations, leading to different current density distributions in different areas.
In addition, the concentration of the Zn 2+ ions at the interface of electrolyte/anode easily drops to zero at a high applied current density.Therefore, decreasing the local current densities and prolonging the τ s is worthwhile. 23ote that regulating ions' transportation behavior contains two principal strategies, namely, providing uniform Zn 2+ ions flux to facilitate flat Zn plating morphology and accelerating the Zn 2+ diffusion rate to improve electrolyte concentration polarization.According to the "space charge theory," the incorporation of a three-dimensional structure with a high specific surface area offers an increased electroactive reaction area and reduces the local current densities.As a result, it promotes the deposition of Zn metal in a more homogeneous manner.Moreover, the diffusion path of Zn 2+ in a 3D structure changes from 2D to 3D, which enables the electrolyte penetration in a 3D structured anode, thus narrowing the contact distance between Zn ions and electrodes, and mitigating the electrolyte concentration polarization. 24  Zn 2+ flux toward Zn anodes (Figure 4A,C). 25Finite element simulations illustrate that, with the uniform pore structures of the g-C 3 N 4 network, the Zn ion flux is more homogeneous, and a smaller concentration gradient can be achieved over time (Figure 4B,D).

| Large inner space
Almost all metal anode materials undergo volume expansion and contraction during the plating and stripping process.Regarding Zn metal anodes, the hostless feature of planar Zn metal leads to vigorous volume swelling and shrinking during repeated plating/stripping processes.As discussed above, the unevenly distributed surface current density and inhomogeneous ion flux easily cause inhomogeneous Zn metal deposition and even Zn dendrites growth.The uncontrolled growth of Zn metal dendrites poses risks such as separator penetration and direct contact with the cathode, which can cause short circuits in batteries.
Additionally, the chaotic growth of Zn metal dendrites can contribute to the formation of non-functional Zn regions, commonly known as "dead Zn," leading to a decline in battery performance and a decrease in Coulombic efficiency.Significantly, the molar volume plays a crucial role in determining the volume changes experienced by Zn metals with the same areal loading.Due to its larger molar volume in comparison to alkali metals such as Li, Na, and K, Zn metal undergoes more pronounced volume changes during repeated plating and stripping processes.Constructing a 3D structured Zn metal anode can provide ample inner space to effectively accommodate the volume expansion/contraction of Zn metal and suppress Zn metal dendrite growth.Wang and coworkers prepared a 3D porous Zn via organic acid etching and studied its Zn plating/stripping process versus plane Zn foils.9a It can be found that the Zn deposited on the plane Zn would form numerous dendritic Zn structures (Figure 5A-E).Copyright 2022, Elsevier.

| Released Zn plating stress
Over the past years, many researchers have revealed that stress can be generated on the planar substrate during the electrochemical deposition of metals, which strictly influences the deposition morphology of plated metal. 26or Zn metal anodes, internal stresses induced by Zn plating are the prevailing factor in the Zn dendrites formation on planar Zn surfaces.The non-equilibrium growth conditions of Zn metal atoms through the transportation process of grain boundaries determine the accumulation of internal stresses.The stresses cannot be effectively released in planar electrodes, driving the deposited metal to grow into disordered Zn metal, Zn dendrites, and even dead Zn.Constructing a 3D structured Zn metal anode will adequately mitigate the internal stress during the process of Zn metal plating.Tailoring planar and rigid Zn metal into the 3D structure or combined with the 3D porous host will bring enough buffer space to adapt the large volume changes in Zn metal plating/stripping and release the inner stress inside the 3D structured anodes, finally suppressing the Zn metal dendrites formation.Chen et al. studied the difference plating stresses of Zn on planar Zn (PL-Zn) and micro-groove-patterned Zn (P-Zn).17i The stress calculation reveals that the deposition of Zn would generate stress on the substrate.The stress cannot be released for the plane structure.In contrast, for the patterned structure, since Zn would preferentially deposit on these mesh-like channels, the stress induced by Zn deposition can be uniformly distributed in these channels and thus be relaxed (Figure 6A).Then, the PL-Zn and P-Zn are prepared on a soft polydimethylsiloxane (PDMS) substrate to study the Zn deposition behaviors.During Zn deposition, the PL-Zn electrode exhibited the formation of numerous wrinkles and cracks.In contrast, the P-Zn electrode maintained a smooth surface since the stress was relaxed (Figure 6B,C).

| Challenges for 3D-structured Zn metal anode
Zn metal anodes for AZIBs have garnered significant interest owing to their high specific capacity and costeffectiveness.In practical applications, the adoption of a 3D structured Zn metal anode has shown promising improvements in electrochemical performance compared to the planar Zn metal anode.However, the 3Dstructured Zn metal anodes still face several challenges as discussed below.

| Uniform Zn deposition
The 3D structure can inhibit the formation and growth of Zn dendrites based on the aspects discussed above.However, due to the mass transfer limitation of Zn 2+ ions, the deposition of Zn in the 3D framework may not be completely even.The top of the 3D framework could be more favorable for Zn deposition.This makes it difficult to achieve the desired dendrite-suppression effect of the 3D framework.It is known that Zn metals have a great advantage in terms of specific volumetric capacity.Introducing porosity into the Zn metal anode would inevitably reduce the volumetric energy density of the full cell.In addition, to achieve practical AZIBs, high Zn utilization and low N/P ratio should be fulfilled at the anode.While for 3D Zn metal anode, controlling the amount of Zn would be more challenging within the 3D framework.

| Side reactions
Introducing 3D or porous structures enhances the specific surface area of Zn metal anodes compared to the planar electrodes.As a consequence of the thermodynamic instability of metallic Zn in an aqueous solution, an increase in specific surface area results in a heightened susceptibility to side reactions, thereby deteriorating the performance of the anode.

| Mass production
Regarding fundamental research, various preparation methods have been developed for controlling pore structures and surfaces.However, appropriate techniques for the mass production of 3D Zn anodes still are needed to be optimized.Especially considering the different application scenarios of AZIB (e.g., grid-scale energy storage and wearable electronics), different preparation processes of 3D Zn anodes need to be developed accordingly.

| Storage of electrodes
Zn metal is easily oxidized in the air.Increasing the specific surface area of Zn metal will cause a larger area of the surface Zn oxide layer, which increases the interfacial impedance of the cell, resulting in a larger polarization of the cell.Moreover, oxidation would lead to the loss of Zn, which is also the active material at the anode.Thus, storage conditions for the fabricated 3D anodes become more challenging, especially for high-Zn-utilization anodes.In such cases, preserving the surface integrity of the 3D Zn anode becomes exceptionally crucial.
The aforementioned challenges present substantial obstacles to the performance and practical implementation of 3D Zn anodes.In order to tackle these issues and offer a better guarantee for the practical application of AZIBs, various attempts have been conducted to improve the electrochemical performance and cycling life of 3D Zn metal anode.In the following section, we will explore the advancements of these achievements regarding preparation method innovation, surface composition modification, gradient structure design, and side reaction inhibition.

| Innovation in preparation method
Conventional planar Zn foil and planar current collectors are unsuitable for regulating uniform Zn plating and stripping, which can lead to intensified preferential deposition and dendrite formation during cycling.These randomly distributed structures can also damage the continuity and electron transport pathways within the Zn anode.As a result, it is recommended to introduce 3D Zn anode structures that can suppress Zn dendrites with better cycling and safety performance.
Tailoring the conventional planar Zn electrodes into 3D structured Zn metal anodes is crucial for the mitigation of Zn dendrite formation.Generally, the main objective of incorporating a 3D structure in modified Zn is to enhance the surface area of the anode that can provide the balance of charge and mass transfer, resulting in homogeneous Zn metal deposition.Suppressing or mitigating the disordered Zn dendrite growth during repeated discharge/charge processes can be achieved by using 3D structured Zn anodes.According to the fabrication method, the 3D-structured Zn metal anode can be divided into the following categories (self-assembly, vacuum filtration, rolling process, 3D printing, electrochemical deposition, laser drilling, freeze drying, selective etching, slurring coating, high-temperature sintering, and multi-method synergy) as shown in Figure 7. Here, we will discuss the representative preparation methods in this section.

| Construction of 3D structured Zn metal anodes
As previously mentioned, dendrite growth can occur due to the formation of non-uniform electric fields and ion flux on planar substrates.To address these issues, reconstructing planar Zn into a 3D porous structure can induce a homogeneous distribution of ion flux and electric field, thereby promoting homogeneous nucleation and deposition of Zn.Directly using Zn powder has been put forward as a substitute for planar Zn electrodes to delay the formation of Zn dendrite.The self-assembly of Zn powders into a 3D Zn powder anode offers a notable advantage by substantially enhancing the specific surface area of the anode.The increased surface area provides a greater number of nucleation sites, facilitating dendritefree growth and promoting homogeneous deposition of Zn metal over a certain duration. 27However, neither the surface state of the 3D Zn powder metal anode nor the grain boundaries between the Zn powders can maintain a stable state for a long time, and the increased specific surface area of the Zn electrode leads to the aggregation of random Zn microstructures, leading to vigorous interfacial reactions that exacerbate the electrochemical performance of the cell.Despite having excellent processability and structural compatibility, applying 3D Zn powder metal anodes brings about potential concerns regarding corrosion and failure mechanisms.In order to address the challenges associated with volume changes during repetitive Zn stripping/plating, a novel approach involves the development of a semi-solid Zn anode.This innovative anode consists of Zn powder dispersed within a conductive elastic rheological network and incorporates Sn additives. 28The rheological properties of the semi-liquid Zn powder anode (SLA) release the stresses induced by galvanization effectively, promoting 3D homogeneous Zn 2+ flux and enabling homogeneous Zn plating on the anode.Additionally, to create a practical and high-performing Zn powder anode, a 3D porous MXene@Zn composite was developed. 29In the 3D porous MXene@Zn composite anode, a highly lattice-matched MXene sheet layer is employed as a medium for electron and ion redistribution, as well as a support for the Zn micro-powder.The MXene lamellae, carrying a negative charge, facilitate the connection of monodisperse particles through electrostatic forces, resulting in a macroscopic conductive Zn powder anode.To prevent significant lattice stresses, a uniform deposition process is employed to ensure that the exposed (0002) crystalline surface of the MXene lamellae maintains the same crystal structure as the deposited Zn.The lattice mismatch between the (0002) crystalline surfaces of MXene and Zn is kept below 10%.This approach helps to minimize any potential strain and promotes a more stable and efficient performance of the composite anode.
Similar to the self-assembly method, vacuum filtration allows the assembly of 1D nanowires into membrane electrodes with a network structure.Xie et al. presented a vacuum filtration method that is environmentally friendly for the fabrication of 3D zincophilic Cu nanowire networks as the host to stabilize Zn metal anodes.17h The as-prepared 3D zincophilic Cu nanowire networks exhibited several advantages: (1) the porous nature of 3D nanowire structure favors uniform Zn 2+ flux, (2) the nanoscale size of the nanowires network endows the 3D Zn anode with a large surface area, reducing the local charge accumulation and homogenizing Zn 2+ concentration field, (3) the abundant facets and edge sites of Cu nanowires facilitate uniform nucleation and homogeneous Zn metal deposition, enhancing the electrochemical performance of Zn metal anodes.
Apart from fabricating a conductive network, patterning 3D Zn metal anode through template-assisted mechanical calendering has also effectively lowered the local current density by constructing a large surface area.17g,26c,30 In comparison to the pristine bare Zn foil, the patterned Zn metal anodes reconstructed the surface morphology from 2D to 3D, inducing homogeneous Zn deposition and dissolution without dendrite growth.Cao and co-workers developed an imprinted Zn metal anode that integrates conductive and hydrophilic gradients for long-term Zn metal anodes without dendrite formation.17j The 3D dual-gradient Zn metal anode synergistically modified the distribution of local charge accumulation (or current density) and Zn 2+ flux, inducing preferentially metallic Zn deposition with the "bottom-up" model in the reconstructed Zn microchannels and suppressing Zn dendrite growth.In addition, Chen et al. developed a metal-mesh-assisted rolling method to construct a groove-patterned Zn metal anode (Figure 8A).17i Specifically, the groove-patterned Zn metal anode has the following merits: (1) The utilization of 3D patterned Zn metal with a large surface area effectively reduces the local current density, facilitating a uniform Zn 2+ flux for consistent Zn nucleation and deposition.(2) The micro-grooves of 3D Zn anode effectively released the stress generated from Zn plating process and thus suppressed the growth of Zn dendrite.(3) The metal-mesh-assisted rolling method can adapt the large-production fabrication of 3D patterned Zn metal anodes.
In recent years, 3D printing technology has been widely adopted, offering a means of producing highly customized and porous conductive metal electrodes through rapid prototyping.9c,17g,26c This technique is distinct from other traditional 3D printing methods, requiring a high-power laser to sinter meltable powder materials into solid structures.In a recent study, a novel approach utilizing direct 3D printing was developed by Zeng et al. for fabricating a hierarchical porous Zn metal anode (Figure 8B).26c The as-prepared 3D Zn metal anode (3DP-ZA) demonstrated remarkable characteristics such as mechanical flexibility, effective stress dissipation, and structural stability.As a result, the 3DP-ZA anode achieved a low overpotential of 35 mV for 330 h in a symmetrical cell under an applied current density of 1 mA cm À2 along with a cycling capacity of 1 mAh cm À2 .Besides constructing 3D Zn metal anodes by directly printing Zn powders as the building block, 3D printing is also suitable for constructing conductive hosts to regulate the plating and stripping process of Zn.For instance, Zhang and co-workers introduced a noteworthy advancement by employing 3D printing techniques to fabricate a 3D Ni-based host featuring multi-channel lattice structures.
Laser techniques have gained significant popularity in the fabrication of 3D Zn metal anodes for energy storage devices.The 3D Zn anode construction involves the deposition of Zn onto a conductive substrate in  32 Copyright 2022, Wiley-VCH.(F) A schematic diagram illustrates the procedure of fabrication of the 3D-ZGC host by freeze-drying method.Reproduced with permission. 33Copyright 2022, Wiley-VCH.
a controlled manner to create a porous architecture.The porosity and surface morphology of the anode is critical.The use of lasers enables precise control over the deposition process, resulting in a highly porous structure that enhances the anode's electrochemical performance.Huang and co-workers used laser lithography to obtain a Zn metal anode with a periodic concave-convex structure (Figure 8C). 31The as-prepared 3D Zn electrode with ordered patterned shapes generated a large surface area, which shows several advantages: (1) The periodic surface geometry creates the periodic electric field and related current density distribution.(2) The laser-induced surface oxides provide abundant zincophilic sites that lead to a low nucleation overpotential for homogeneous Zn plating behavior.(3) The cooperative interaction between periodic surface geometry and zincophilic sites facilitates the Zn metal plating and stripping kinetics, ultimately enhancing the electrochemical performance of Zn metal anodes.
The presence of a humid atmosphere can induce thermodynamic instability in Zn, leading to the formation of passivation layers and by-products on its surface, which seriously hinders the charge transfer and the homogeneous distribution of electric fields.Therefore, designing and constructing clean Zn metal electrodes without passivation layers significantly enhances the cycling life of Zn metal anodes.This surface etching strategy can remove the native passivation layer on pristine Zn foil and reconstruct the surface into the 3D structure.Through the selective etching of the grain boundaries and weak crystallographic planes of the planar Zn metal, Wang et al. were able to fabricate a 3D ridge-like Zn metal electrode on its surface. 34This method shows several advantages: (i) the selected dilute hydrochloric acid solution can efficiently clean out the oxide layer and impurities from the raw surface of the Zn foil electrode, leading to a uniform contact interlayer for electrolyte; (ii) the presence of the Zn metal electrode with a uniform surface provides advantages for achieving homogeneous Zn deposition and dissolution, (iii) the high-surface-area ridge-like Zn anode increases the electroactive area, reduces the charge accumulation, lowers local current density, and promote uniform Zn 2+ flux, which suppresses the formation of Zn metal dendrites.As a result, the 3D ridge-like Zn metal anode can stably run for 200 h with a low voltage hysteresis of $20 mV under the current density of 1 mA cm À2 and a cycling capacity of 0.5 mAh cm À2 .Compared to inorganic acids, using organic acids for etching Zn electrodes is a novel field that enables the regulation of highly reactive Zn etching rates, leading to the construction of 3D Zn anodes with high porosity.Wang et al. proposed a mixed organic acid solution with trifluoromethanesulfonic acid (TFA) and acetonitrile (AN) (Figure 8D).9a Compared to inorganic acids (e.g., hydrochloric acid), employing TFA-AN as an etchant avoids drastic reactions on the surface of Zn metal electrode.A mild etching process facilitates the generation of abundant pore architecture from nanometer to micrometer scale on the surface of Zn metal electrodes, leading to homogeneous Zn nucleation and deposition.
Bulk or planar Zn electrode always suffers from inhomogeneous nucleation and deposition, restricting their commercial applications.The incorporation of a 3D structure effectively enhances the surface area, leading to reduced local current density and inhibition of Zn dendrite growth.However, the design of 3D structured Zn metal anodes still faces a crucial problem: suppressing the Zn dendrites formation and simultaneously increasing the energy density of the batteries.Therefore, it is essential to produce a lightweight Zn anode or host with exceptional mechanical properties, high porosity, and low weight for practical applications with high-energy density.Zhou et al. developed a foldable 3D porous MXene-based aerosponge via freezing treatment and freeze-drying route (Figure 8E). 32Using the freeze-drying technique, the as-prepared 3D host exhibited a uniform directional pore structure with abundant zincophilic sites, which realized uniform Zn nucleation and deposition without forming dendrite and inactive by-products.As a result, the flexible pouch full cell with Zn@3D aerosponge anode and LiMn 2 O 4 cathode shows a high gravimetric capacity of 97 mAh g À1 over 60 cycles, exhibiting excellent mechanical flexibility and high electrochemical performance under different fold treatments.Similarly, Xue and co-workers employed the freezedrying method to construct a porous host (3D-ZGC) with abundant zincophilic ZnO sites and a stable carbon scaffold (Figure 8F). 33The as-prepared 3D-ZGC can effectively mitigate the huge volume expansion of Zn during continuous Zn plating, enabling high structure reversibility of composite Zn anode.
3D structured Zn anode is a new material for aqueous Zn ion battery anode, which possesses excellent electrochemical properties, good electrical conductivity, and stable cycling performance, thus attracting extensive interest.Overall, the 3D Zn anode obtained from the abovementioned method has advantages and disadvantages and needs to be selected according to the actual needs.Among them, self-assembled Zn nanofibers or Zn powder can obtain a 3D Zn anode with regular morphology and well-formed pore structure.However, the contact resistance between wire-wire or particleparticle can easily cause large contact resistance and increases cell polarization.An effective and straightforward approach involves the incorporation of 3D conductive hosts to accommodate the deposition and dissolution of Zn metal by vacuum filtration or freeze-drying.However, the nucleation and growth rates need to be precisely controlled to regulate the morphology of deposited Zn.In addition, electrolyte and interface composition are also key factors that influence the electrochemical performance of Zn-ion batteries when utilizing a 3D Zn metal anode.The composition and concentration of the electrolyte can regulate the morphology and structure of the 3D Zn anode, and it also affects the electrochemical performance of the electrode.By controlling the composition of the interface between the electrode and electrolyte, it is possible to manipulate the morphology and structure of the deposited Zn, thereby influencing its electrochemical properties.Therefore, these key issues need to be carefully controlled while preparing 3D Zn anodes (Table 1).

| Modification of surface composition
The pore structure design of the Zn electrode has a significant impact on the plating/stripping behavior and overall electrochemical performance of the Zn metal anode.In 3D structured Zn metal anodes, both charge transfer and ion transport processes play crucial roles in battery performance.At low current densities, the charge transfer process governs the electrochemical kinetics, while ion diffusion becomes dominant as the current densities increase.To promote homogeneous Zn nucleation and deposition, a high Zn-ion concentration is desirable as it extends the dendrite growth threshold time.However, the reduction of Zn ions at the separator/electrode surface interface can lead to topgrowth Zn metal deposition and even the formation of Zn dendrites (Figure 9A).To address this issue, it is crucial to rationally design the 3D porous structure and incorporate surface modifications that regulate both ion diffusion and charge transfer (Figure 9B). 35In the following section, we will discuss the surface modification design of 3D Zn that regulate the homogeneous Zn deposition and dissolution behavior.According to the materials constitution of the substrate host, the surface modifications are divided into 3D metal-based hosts and 3D carbon-based hosts.
The metal micro/nanoporous skeleton electrode possesses exceptional electrical conductivity and a large specific surface area, enabling various pathways to facilitate uniform deposition of Zn metal.These porous structures can be realized using metal substrates such as Cu nanowires, Cu mesh, Cu foam, Ag aerogel, porous Ti, and Ni nanotube.
For single-crystal metals with specific facets and defects, each crystallographic facet and defect site has different adsorption energy for Zn atoms. 36A rational design of the conductive substrate can induce preferential nucleation of metallic Zn in specific regions and thus regulate homogeneous Zn plating behavior.One approach involves attaching zincophilic compounds to the metal substrate, creating active nucleation sites that facilitate the uniform Zn 2+ flux and deposition.In this section, we will explore the utilization of a 3D metal substrate for the 3D Zn anode.
In comparison with pristine Zn foil, the 3D metallic hosts exhibit excellent structural stability and good conductivity, facilitating the accommodation of Zn deposition inside the framework. 37A previous study discussed the nucleation behavior of Zn metal on various metal material substrates and found that the metallic Cu shows a lower nucleation barrier than other metallic substrates (such as Sn, Ti, and Ag). 38Therefore, 3D Cu is an ideal substrate for metallic Zn anodes due to its structure stability, abundant pores, and high electrical conductivity.The morphology evolution of Zn metal is significantly influenced by the pore structure of 3D architectures.The pore structure plays a crucial role in affecting the distribution of surface charge and ion concentration, which in turn affects the morphology of Zn metal.However, it can be challenging to isolate the specific impact of the pore structure factors on the initial nucleation process.22b Moreover, as a result of the short ion diffusion pathways between the cathode and anode, coupled with the concentrated electric field on the top portion of the electrode, lead to preferential deposition of Zn metal in that region, where Zn 2+ ions receive electrons during the initial nucleation.This is why it is crucial to promote uniform nucleation and deposition of Zn.Gaining a comprehensive understanding of the nucleation process of Zn on 3D metal substrates is highly significant for the rational design of 3D metal structures that can effectively regulate the homogeneous plating and stripping behavior of Zn.Introducing zincophilic sites can facilitate this goal as it promotes homogeneous Zn nucleation and deposition.For instance, Xie et al. reported a novel strategy by employing zincophilic 3D Cu nanowires as the anode host to regulate the Zn deposition/ dissolution behavior (Figure 10A).The as-prepared 3D Cu nanowires networks significantly improve the Zn metal anodes from several aspects, including reducing local current density, homogenizing Zn 2+ concentration, suppressing the side reactions and inducing the uniform Zn plating/stripping behavior.Importantly, with the help of finite element analysis (FEA) simulation and density functional theory (DFT) calculations, the series simulation results revealed that different facets and edge sites of the Cu nanowires exhibit different zincophilicity to induce Zn nucleation/deposition.This study provides new research thoughts to design specific crystal metal materials with unique facets and edge sites to enhance the electrochemical performance of Zn metal anodes.17h Unlike the 3D Cu nanowires network obtained by vacuum filtration, the commercial Cu foam could be directly used as the Zn host.Zhou et al. reported a thermal infusion method to create a stable Zn composite anode by using 3D Cu foam. 39In this study, 3D foam Cu with high mechanical strength and structural stability can adsorb metallic Zn inside the framework through a melt-wet-cooling process (Figure 10B).The comprehensive analysis of in/ex-situ tests characterization revealed three merits of using 3D foam Cu as a stable host for Zn deposition and dissolution, reduces the nucleation barrier and effectively inhibits the hydrogen evolution process and by-products during cycling.As a result, the excellent performance of the Zn@3D foam Cu composite electrode was demonstrated through 1000 cycles without dendrite formation, exhibiting an overpotential of only 44 mV, even at a high current density of 10 mA cm À2 .Additionally, when integrated into full cells with LMO cathodes, the Zn@3D foam Cu anode-based cell exhibited superior rate performance, delivering a capacity of 73 mAh g À1 , significantly outperforming bare Zn-based batteries, which achieved a capacity of 54 mAh g À1 .In addition to Cu nanowire networks and foam-like hosts, porous metal aerogel demonstrates an attractive structure system for Zn metal anodes.For example, Ling et al. developed a lightweight silver nanowire aerogel (3D AgNWA) as the Zn host by a facile self-assembly strategy (Figure 10C).With the simulation calculations and electrochemical test results, the 3D AgNWA exhibits several advantages, such as a 3D cross-linked network that enables a uniformly distributed electric field, abundant zincophilic sites that illustrating the synthesis of the Zn@3D foam Cu composite anode; optical images of the Zn@3D foam Cu composite anode; XRD patterns illustrating the composite materials with 30 μm (red pattern) and 50 μm (blue pattern) Zn foil; XPS profiles of Zn 2p peak obtained from the Zn@3D foam Cu composite anode.Reproduced with permission. 39Copyright 2022, Wiley-VCH.(C) Schematic diagram illustrating the electric field distribution during the electrochemical deposition of Zn metal on different structures (planar and 3D); schematic illustration of the vertical self-assembly process for fabricating 3D AgNWA; top-view SEM images displaying the morphology of 3D AgNWA; TEM and high-resolution TEM images revealing the microstructure of AgNWA; XRD pattern of AgNWA.Reproduced with permission. 40Copyright 2022, Elsevier.(D) Schematic illustration clarifying the fabrication of NOCA@CF product; SEM images demonstrating the pristine NOCA@CF (top), Zn plating at 100th (middle), Zn plating at 200th (bottom) at a current density of 1 mA cm À2 .Reproduced with permission. 30Copyright 2022, Wiley-VCH.(E) Schematic diagram illustrating the synthesis process of CM@CuO host and CM@CuO@Zn composite anodes.Reproduced with permission. 42Copyright 2020, Wiley-VCH.(f) Schematic depiction of the dealloying process for fabrication of 3D TiÀTiO 2 .Reproduced with permission.17f Copyright 2021, American Chemical Society.
support uniform Zn plating and stripping behavior, and high porosity that accommodates a high Zn metal loading. 40ompared to free-standing metal electrodes, enhancing the electrochemical performance of Zn metal anodes can be achieved by incorporating zincophilic species onto the surface of porous metal substrates.An and coworkers developed a 3D flexible composite metal host via loading N and O co-doped porous carbon nanostructures on commercial foam Cu current collector (NOCA@CF) through a green vacuum distillation route. 41Figure 10D shows that Zn dendrites appear on the bare Cu foam surface, while the surface of NOCA@CF remains smooth even after 200 cycles.This is due to the superior conductivity and large surface area of NOCA@CF.The modified Cu foam with abundant N/O dual-doping sites improves hydrophilicity and zincophilicity, which promotes uniform deposition of Zn 2+ and inhibits Zn dendritic formation.During the charge/discharge process, the stable NOCA layer effectively reduces the nucleation overpotential of Zn, leading to improved electrochemical performance.Additionally, the Zn@NOCA@CF/ LiMn 2 O 4 full battery demonstrates exceptional performance.
In addition to the strategy of modified heteroatoms on the surface of metal substrates, loading zincophilic nanoparticles on the surface of porous metal substrates is an effective method for tuning the behavior of homogeneous Zn deposition.Zhang et al. decorated CuO nanoparticles on the Cu mesh (CM) via electrochemical reduction and thermal treatment (Figure 10E). 42Specifically, to achieve a dendrite-free anode (CM@CuO@Zn), Zn was deposited onto the CM@CuO substrate.Density functional theory (DFT) calculations indicate that CuO plays a crucial role in facilitating the uniform distribution of Zn 2+ ions and promoting the deposition of Zn.The CM@CuO@Zn symmetric cell exhibits remarkable cycling stability and low voltage hysteresis (20 mV) when operated at a current density of 1 mA cm À2 with a cycling capacity of 1 mAh cm À2 .During Zn deposition on CM@CuO, an apparent incipient voltage drop was observed due to the significantly low polarization voltage associated with both Zn nucleation and CuO conversion reactions.The reduction of CuO to Cu facilitated the formation of conductive networks, enabling the rapid transfer of electrons.This phenomenon allowed for efficient electron transport, leading to enhanced electrochemical performance.
Similarly, An et al. utilized a template-free electrodeposition associated with a vapor-dealloying technique to fabricate a porous 3D Ti skeleton embedded with zincophilic TiO 2 nanoparticles (3D Ti-TiO 2 ).17f This unique combination of materials and techniques resulted in a 3D structure that not only acts as a Zn deposit vessel, but also reduces local current density, thereby enhancing homogeneous Zn nucleation and deposition.The zincophilic TiO 2 layer serves a crucial role in enhancing the interaction between Zn and Ti, as well as improving the wettability between the 3D Ti-TiO 2 electrode and the electrolyte.This, in turn, facilitates the homogeneous nucleation of Zn during electrodeposition.In addition, the stable cycling of the resulting full batteries with Zn@3D Ti-TiO 2 as anode and S-MXene@MnO 2 cathode was stably run over 500 cycles even at a high current density of 5 A g À1 (Figure 10F).This impressive performance is a testament to the effectiveness of the 3D Ti-TiO 2 structure and zincophilic TiO 2 layer in facilitating homogeneous Zn deposition.
In the context of AZIBs, by employing a porous metal substrate characterized by excellent electrical conductivity and a significant specific surface area, it becomes possible to achieve a more uniform distribution of ions and an electric field.To further enhance the stability of the Zn metal anode, zincophilic materials can be added to the 3D metal framework.However, it is important to note that the utilization of bulky or thick metal substrates may impose limitations on the gravimetric/volumetric energy densities of the associated AZIBs.Additionally, the hydrogen evolution reaction (HER) can pose a challenge when metal is used as a substrate.By carefully selecting the appropriate metal substrate and incorporating zincophilic materials, it may be possible to overcome these challenges and enable the practical application of high-performance AZIBs.
Carbon materials have become increasingly popular for various applications owing to their abundant availability, low density, versatile structural properties, and ease of manufacturing. 43Carbon-based materials offer a conductive and flexible matrix suitable for Zn plating, making them an excellent choice for host materials.
Among the various carbon-based substrates, the outstanding electrical conductivity, porosity, high specific surface area, and mechanical stability of carbon nanotubes have made them extensively employed in the realm of energy storage. 44These properties make carbon nanotubes rendering them an excellent choice as a substrate material for accommodation of Zn plating.Due to their unique properties, the 3D flexible carbon nanotube (CNT) framework as the host for Zn deposition/dissolution has gained significant attention.Zeng et al. were the first to utilize this approach to construct a dendrite-free Zn/CNT anode, as illustrated in Figure 11A.16h The process involves growing carbon nanotubes on carbon cloth (CC) and subsequently synthesizing a Zn/CNT anode through electrochemical deposition.The resulting structure features a conductive network of 3D interconnected carbon nanotubes uniformly covering each CC.The incorporation of this structure offers an enlarged surface area and enhances the homogenization of the electrical properties of Zn deposition.After 10 min of Zn electrodeposition, nanosheets evenly form on the surface of CNT.Compared to pristine carbon cloth, CNTs can suppress dendrite growth due to their porous surface structure and high specific surface area.Additionally, the Zn/CNT symmetrical cell demonstrates significant improvement, with a lower nucleation overpotential than conventional cells.Furthermore, by utilizing CNT-MnO x @PEDOT (PEDOT: poly(3,-4-ethylenedioxythiophene)) as the cathode and Zn/CNT as the anode, the resulting Zn//MnO 2 full battery demonstrates excellent mechanical flexibility and high capacity retention even after 1000 cycles.
Carbon cloths (CC) are extensively employed in energy storage applications owing to their exceptional conductivity, good corrosion resistance and flexibility, making them ideal for constructing stable hosts in 3D Zn anode constructions.CC can load various active nanomaterials to construct a 3D porous and highly conductive network, providing a larger surface area and enhancing Zn nucleation and deposition homogenization.For instance, Li et al. developed an aminefunctionalized CC as the host to construct the orderly The schematic representations depicting the deposition behavior of Zn on carbon cloth and carbon nanotube (CNT) electrodes.Reproduced with permission.16h Copyright 2019, Wiley-VCH.(B) Schematic illustration showcasing the Zn deposition process on amine-functionalized carbon cloth (CC).Reproduced with permission. 45Copyright 2021, American Chemical Society.(C) Schematic representations depicting the synthesis of N-doped vertical graphene (N-VG) on CC and the subsequent deposition of Zn on N-VG@CC electrode.Scanning electron microscopy (SEM) images displaying N-VG@CC at various magnifications, along with an SEM image of Zndeposited N-VG@CC.Reproduced with permission. 46Copyright 2021, Wiley-VCH.(D) Schematic depiction elucidating the preparation of 3D printed N-doped carbon (3DP-NC) host and the resulting 3DP-NC@Zn anode.Reproduced with permission. 30Copyright 2022, Wiley-VCH.(E) Schematic illustration illustrating the synthetic process of Cu nanoboxes (Cu NBs) on N-doped carbon fibers (NCFs); corresponding SEM images showing Cu 2 O nanocrystals (NCs) (left), Cu 2 O@CuS NCs (middle), and CuS NBs (right).Reproduced with permission. 48Copyright 2022, Wiley-VCH.(F) Schematic illustration demonstrating the fabrication process of Sn-embedded porous carbon fiber; SEM and transmission electron microscopy (TEM) images of Sn-PCF.Reproduced with permission. 49Copyright 2022, Elsevier.
growth of the 3D Zn anode (Figure 11B). 45DFT calculations show that the introduction of amine groups enhances the binding energy between Zn atoms and the CC substrate while inhibiting the decrease of conductivity induced by Zn atom adsorption.The resulting dendrite-suppressed anode exhibits an extended cycling life of over 250 h at a current density of 0.5 mA cm À2 in a symmetrical cell.Furthermore, the use of the dendritefree anodes significantly enhances the electrochemical performance of Zn/MnO 2 full batteries.Specifically, when employing amine-functionalized CC anodes, the full-cell exhibits an impressive capacity retention of 94.0% and a Coulombic efficiency (CE) of 93.6% after 6800 cycles, demonstrating excellent long-term stability.
In a recent study by Cao et al., a novel method was introduced to prepare a dendrite-free Zn@N-VG@CC anode.The approach involved the in situ growth of nitrogen-doped vertical graphene nanosheets on carbon cloth (CC), followed by Zn plating (Figure 11C). 46These nitrogen-doped vertical graphene nanosheets exhibit excellent electrical conductivity and high surface area allows efficient electrochemical reactions.The resulting N-VG@CC plated skeleton exhibits excellent zincophilicity and high conductivity, enabling uniform Zn deposition and obtaining a high reversible anode with no dendrite growth.DFT calculations have revealed that the presence of N atoms lowers the nucleation barrier of Zn, facilitating homogeneous nucleation and enhancing battery performance.Furthermore, the 3D nanosheet array structure helps reduce the electric field intensity and prevents dendrite formation.Overall, the remarkable performance of button cells and flexible pouch batteries utilizing MnO 2 @N-VG@CC cathode signifies a promising direction for the future development of energy storage devices.In addition, the 3D printing technique can also be used to construct a 3D carbon-based anode with hierarchical porous structures.17g, 30,47 Recently, Zeng et al. reported a 3D printed reservoir-integrated nitrogen-doped carbon substrate with enhanced functionality (3DP-NC, Figure 11D). 30The incorporation of a customized 3D printed structure with ordered pore structure brings three advantages.First, the presence of micro-sized holes leads to a reduction in local current density and promotes a uniform distribution of the electric field.Second, these holes serve as reservoirs, ensuring unimpeded ion diffusion and facilitating a steady supply of ions.Third, the N-doped interfacial modified method promotes the zincophilic nature of the surface, lowering the nucleation barrier and enabling homogeneous Zn metal deposition.Consequently, the Zn-deposited 3D printed N-doped carbon electrode (3DP-NC@Zn) exhibits a dendrite-free morphology and demonstrates highly reversible Zn plating and stripping, with an impressively low overpotential of 15.3 mV at 10 mA cm À2 .These exceptional characteristics contribute to the outstanding lifespan of the 3DP-NC@Zn electrode, surpassing 380 h under a cycling capacity of 1 mAh cm À2 and a fixed current density of 1 mA cm À2 .The remarkable achievements of the 3DP-NC@Zn electrode, characterized by its dendrite-free nature and exceptional durability, establish a new paradigm for high-performance metal batteries, showcasing the vast potential of the distinctive 3D printing approach.
The electrospinning method has been a promising technology that has gained wide attention in various fields.In recent years, researchers have also explored its potential application in the field of batteries to construct 3D flexible electrodes.A significant breakthrough was conducted by Zeng et al. who utilized a straightforward electrospinning technique to fabricate a versatile N-doped 3D carbon fiber (NCF) material modified with Cu nanoboxes, known as Cu NBs@NCFs (Figure 11E). 48The unique structure of the NCFs allows them to reduce the local current density and accommodate the disordered volume change of Zn.In addition, the N-doped carbon fibers exhibit a significantly lower nucleation overpotential for Zn compared to conventional carbon fibers (CFs).During the charge/discharge process, the formation of a zincophilic Cu-Zn alloy and a solid-solution interface plays a crucial role in reducing the nucleation barriers for Zn and facilitating uniform Zn deposition.Remarkably, the Cu NBs@NCFs-Zn anode demonstrated an impressive Coulombic efficiency of 98.8% at a high current density of 5.0 mA cm À2 and a cycling capacity of 1 mAh cm À2 even after 1000 cycles.Furthermore, in a symmetric cell configuration with Cu NBs@NCFs-Zn anodes, stable cycling is achieved for over 450 h at a current density of 2 mA cm À2 , which is four times longer than that observed with pure planar Zn metal anode.These results suggest that the electrospinning method can be an effective approach to constructing flexible 3D scaffolds for batteries and can potentially lead to the development of more efficient and long-lasting batteries in the future.Similar to this study, Yang and colleagues developed a unique 3D micro-scaffold based on 3D porous carbon fibers (PCF) modified with Sn (Sn-PCF) using the electrospinning method (Figure 11F). 49The fabrication process involved multiple steps, including hardtemplate synthesis, SiO 2 redispersion on polyacrylonitrile (PAN), carbonization, etching, and thermal reduction.The resulting interconnected 3D conductive carbon fiber network, in conjunction with abundant zincophilic Sn sites, forms a highly effective and efficient system to eliminate the "tip effect" by standardizing the charge accumulation and Zn 2+ flux.Notably, the Sn coating plays a crucial role in this process by creating physical confinement for dendrites by mitigating the "tip effect," this technology can help improve the overall performance of batteries.By incorporating metallic Sn, which exhibits strong Zn 2+ T A B L E 2 Parameters of surface modified and gradient structured Zn metal anodes for AZIBs.

Methods for anodes
S-MXene@MnO 2 , 0.5 A g À1 , 100 S-MXene@MnO 2 , 5 A g adsorption and a high Zn 2+ surface diffusion barrier, regulated nucleation and uniform Zn deposition can be achieved, reducing HER tendency and promoting highly reversible Zn plating/stripping.When paired with a NaVO cathode, the Sn-PCF@Zn anodes-based full cells exhibit high capacity retention, indicating potential for long-term and high-performance batteries.This innovative approach provides a new avenue for high-performance 3D Zn anode design with high modification of the surface of 3D structured anodes.This represents a significant step forward in achieving dendrite-free and durable metal batteries (Table 2).

| Design of gradient structure
To develop a 3D Zn metal anode toward higher Zn utilization, higher energy density, and more uniform Zn deposition, designing a gradient structure in the 3D Zn metal anode is a good strategy.In conventional 3D Zn metal anodes, the porosity or composition is uniform throughout the framework, in which the 3D structure can inhibit the dendrite growth, but the effect would be affected under certain conditions, for example, high-capacity Zn deposition.Even a zincophilic layer is pre-decorated on the 3D structure, once the deposited Zn covers the substrate, the following Zn deposition would occur on Zn, and dendrite could be formed (Figure 12A).Introducing a gradient structure to the 3D Zn anode, for example, an electrochemically inert layer on the top and a zincophilic layer on the bottom, can ensure the Zn deposited in the coating from the bottom to the top, thus achieving a better dendrite inhibition effect (Figure 12B). 50ao and co-workers reported a triple-gradient electrode for the 3D Zn metal anodes, where the porosity, zincophilicity, and conductivity are all designed in a gradient distribution along the electrode.9b In this triplegradient electrode, the porosity gradient is provided via using three nickel foams with different mesh sizes, the conductivity gradient is provided via pre-oxidizing the surface of the Ni foam into NiO in the top layer, and the zincophilicity gradient is provided via loading Ag in the Ni foam in the bottom layer (Figure 13A).Such gradient design greatly enhanced the Zn plating/stripping stability at the electrode, where a bottom-up plating behavior of Zn can be achieved.The symmetric Zn cell utilizing the triple-gradient anode demonstrates excellent cycling stability, maintaining stable performance at a high current density of 10 mA cm À2 and a cycling capacity of 1 mAh cm À2 for over 250 h, much better than other electrodes, including non-gradient, one-gradient, and two-gradient (Figure 13B).The symmetric cell using such anodes also shows stable voltage profiles in the rate test from 0.5 to 10 mA cm À2 , and only a slight increase in the deposition overpotential is observed (Figure 13C).
Moreover, generating a gradient of the zincophilic material in the 3D anode can also achieve good Zn deposition regulation.He et al. prepared 3D printing inks with different weight ratios of Ag nanoparticles and printed them in a different sequence to fabricate a 3D anode with gradient-distributed Ag (Figure 13D).9c Since Ag is a highly conductive and zincophilic material, generating the Ag gradient with decreasing concentration from the bottom to the top in the 3D anode (denoted as 3DP-BU) can achieve the best electric field and Zn ion flux distribution compared to 3DP-HG (homogeneous Ag concentration from the bottom to the top) and 3DP-UB (decreasing concentration from top to bottom), as indicated in Figure 13E.By observing the 3DP-BU@Zn anode, it can be seen that the Ag and Zn signals show an increasing trend from the bottom to the top, while the C signal shows a homogeneous distribution (Figure 13F).This gradient design allows for better utilization of the specific surface area of the 3D framework, as evidenced by the lowest Zn deposition overpotential 3DP-BU electrode compared to other electrodes (Figure 13G).Since the Zn deposition is more uniform in the 3DP-BU electrode, the 3DP-BU@Zn anode exhibits higher stability in the Zn-Cu asymmetric cells than other anodes (Figure 13H).The approach involves incorporating different materials or modifying the host structure to create a gradient architecture.By carefully engineering the composition, porosity, or morphology of the anode, various beneficial effects can be achieved.First, a gradient-structured host can promote higher Zn utilization by providing a larger active surface area and optimizing the electrodeelectrolyte interface, which facilitates faster reaction kinetics.Second, the gradient structure enables homogeneous Zn deposition and dissolution.By tailoring the distribution of Zn 2+ ion flux or controlling the conductivity, it is possible to induce directional nucleation and deposition of metallic Zn at specific regions within the anode.Lastly, the gradient structure, especially when using metal materials, provides mechanical robustness and structural stability to the anode.This minimizes the potential for electrode deformation or degradation during cycling, ensuring the long-term durability and reliable performance of the 3D Zn metal anode.Overall, the gradient architecture in the anode offers a promising approach for achieving higher Zn utilization, promoting uniform Zn deposition, and enhancing the mechanical integrity of the electrode.

| Strategies of further enhancing the performances of 3D Zn metal anodes
The electrolyte in the presence of the Zn anode is prone to undergo a series of side reactions, leading to the decomposition of water molecules on the anode surface (e.g., hydrogen evolution, corrosion, and side product formation). 51For the substantially larger surface active area of the 3D Zn metal anode, the side effects on battery performance will be even more dramatic.Enhancing the stability of aqueous electrolytes to inhibit the decomposition of the water molecules is a very effective way of addressing these side reactions.The strategies to achieve this goal include constructing artificial solid electrolyte interphase, optimizing electrolyte composition, and employing hydrogel electrolytes.Prior to battery operation, a protective layer called the artificial solid electrolyte interphase (SEI) is formed on the surface of the Zn metal anode. 52Generally, by creating a thin layer in AZIB, this interphase effectively prevents direct contact between the Zn metal anode and the aqueous electrolyte, thereby inhibiting corrosion or unwanted reactions between the Zn and the electrolyte. 53eanwhile, the artificial SEI layer also allows the transfer of Zn ions, which would not affect the normal operation of the Zn anode. 54Constructing an artificial SEI layer can significantly improve the anti-corrosion ability of the 3D Zn metal anode.Chen et al. used a Nafion layer as the ASEI coating to prevent the 3D patterned Zn metal anode from corrosion.17i The side-view SEM images clearly show that the patterned 3D Zn is coated by the Nafion layer (denoted as N@P-Zn) (Figure 14A).Tafel plots show the decreased corrosion current after the Nation coating, indicating the suppressed side reactions with the Nafion coating (Figure 14B).The suppressed corrosion endowed by the Nafion coating is also indicated by a 7-day soaking test, where the N@P-Zn showed a more intact appearance (Figure 14C,D), suppressed side product formation (Figure 14E), and more stable impedance (Figure 14F,G).In addition, the DFT calculation demonstrated that Nafion exhibits enhanced water adsorption, alleviating the hydrogen evolution, and meanwhile the normal Zn deposition would not be  would lead to increased side reactions, while the CTO coating layer endowed the 3D electrode with excellent anti-corrosion ability, as indicated by the HER onset potential (Figure 14L), corrosion current (Figure 14M), and XRD spectra of side products (Figure 14N).More recently, Cao et al. reported a PVDF-Sn@Zn gradient anode, where the PVDF coating provides good anticorrosion ability to the composite gradient 3D anode. 55ptimizing the electrolyte composition is also important in stabilizing the aqueous electrolyte and suppressing the Zn metal anode without side reactions. 57Modulation of the electrolyte composition, for example, using some cosolvent and/or additives that are able to interact strongly with water molecules or adsorb on the surface of Zn metal anode, 58 can effectively enhance the electrochemical stability of 3D Zn anodes.Zhang et al. used polyacrylamide (PAM) as the additive in the electrolyte to increase the stability of the 3D Zn anode (Figure 15A). 59Tafel tests indicate that adding the PAM to the electrolyte can result in decreased corrosion current density (Figure 15B,C).Under low current density, the symmetrical cell shows significantly better cycling performance with PAM electrolyte additives (Figure 15D,E).Later on, Bayaguud et al. employed TBA 2 SO 4 , a cationic surfactant, as the electrolyte additive to regulate the deposition of Zn on Cu foam (Figure 15F). 60Since the TBA cations can be electrostatically adsorbed and aggregate on the surface of Zn metal anode, the deposited metallic Zn can be regulated more evenly, and a reduced corrosion rate is achieved.The alleviated corrosion is visualized in the Tafel plots, where a decreased corrosion current is achieved after adding TBA 2 SO 4 in the electrolyte (Figure 15G).With such an additive, the symmetrical Zn cell showed an obviously prolonged lifespan (Figure 15H).Furthermore, by regulating the pH value of the electrolyte, the occurrence of side reactions on the 3D Zn metal anode can be minimized, as these reactions tend to be accelerated in more acidic conditions. .Reproduced with permission. 60Copyright 2020, American Chemical Society.
AZIBs have great promise due to their safety and low cost but are hindered by some problems, including Zn dendrite formation at the anode.In this review article, we have reviewed the application of 3D structural design as a strategy to improve Zn metal anodes.The history, opportunities, and challenges have been carefully discussed.The progress in 3D structure engineering of Zn metal anode has been systematically summarized, including preparation method, surface composition modification, gradient structure design, and side reaction inhibition.In recent years, considerable efforts have been focused on this field and brought rapid development in 3D Zn metal anodes, but there is still a long way to go before their implementation in practical AZIBs.More efforts should be made in the following directions.1. Systematic pore structure studies: Pore structures are the key to dendrite suppression in 3D Zn anodes, which can provide uniform surface current density, ion flux distribution, stress distribution, and more internal space for Zn deposition.However, at this stage, although many new types of porous structures and advanced pore-making methods have been devised, systematic studies of pore parameters for understanding the electrochemical behavior of different pores are still lacking.For example, to achieve non-dendritic deposition and high Zn utilization at the anode, studying and optimizing the pore diameter, pore spacing, and porosity of the 3D Zn metal anode are still lacking and are urgently needed.To fulfill this, more experiments should be conducted coupled with simulations such as finite element calculations.
In addition, artificial intelligence can be applied herein in order to study the pore structure of 3D Zn metal anode and its impact on battery performance.2. Gradient structure design: Although a 3D structure can inhibit the formation of dendrites, the simple introduction of a three-dimensional structure, for example, mesh or foam, will hardly meet the future demand for high-performance AZIBs.In order to achieve higher Zn utilization and more uniform Zn deposition, gradient structure design of 3D Zn anode should be considered.On the one hand, hierarchical pore distribution should be introduced to the 3D Zn anode, where some pores are used to provide good electrolyte saturation, some are used to provide rapid Zn flux transfer, some are used for high-utilization Zn deposition, and these hierarchical pores cooperate to realize a highperformance 3D Zn anode.On the other hand, modifying the surface of the 3D Zn anode with different functional materials in gradient distribution to achieve desired Zn deposition is useful.For example, using zincophilic material to modify the bottom and electrochemically inert materials to modify the top of the 3D Zn can allow for more uniform deposition of Zn inside the 3D framework.In the future, the integration of hierarchical pore structure and gradient functionalization should be investigated for highperformance 3D Zn metal anodes.3. Large-scale and cost-effective fabrication: The scalability and commercialization of AZIBs require the development of cost-effective and scalable fabrication methods for 3D structured Zn metal anodes.Strategies that are compatible with roll-to-roll processing need to be strongly developed.For example, paste coating and mechanical calendering are promising methods for achieving large-scale and cost-effective fabrication of 3D metal anodes.In addition, using a roll-to-roll process to fabricate non-Zn (e.g., Cu or carbon) 3D frameworks and then infuse Zn via melt-diffusion is also worth investigating.Furthermore, considering the recent vigorous development of flexible wearable devices (an important application scenario of AZIBs), printing techniques such as screen printing and 3D printing are also important for high-throughput fabricating 3D Zn metal anodes.The printing processing should be investigated, such as printing a 3D conductive framework on Zn foil or directly printing Zn powder-based 3D structure.For the latter one, the composition of the ink also needs to be studied, for example, using appropriate binders and conductive agents to improve the performance and using appropriate pore-forming agents to enhance the porosity are both important.4. Electrolyte composition optimization: As discussed above, although the three-dimensional structure inhibits the formation of dendrites, its increased active surface area also provides a potential benefit regarding side reactions (i.e., hydrogen evolution, corrosion, and side product generation) at the Zn anode.Considering these side reactions are mainly contributed by the decomposition of water molecules in the electrolyte, decreasing the activity of the water molecules can effectively alleviate the side reactions.In this regard, optimizing the electrolyte composition for 3D Zn anode-based AZIBs can fulfill this goal.For example, it is helpful to develop novel Zn salt anions, functionalized additives or co-solvents with strong interactions with water molecules and thus decrease their activities.Also, using highly concentrated aqueous (also known as "water-in-salt") electrolytes can alleviate the side reactions that occur at the 3D Zn metal anode.In addition, the use of additives that contribute to the generation of stable SEI on the anode can effectively improve the stability of 3D Zn metal anodes.
Beyond these aqueous electrolytes, developing advanced hydrogel electrolytes is also useful in improving the stability of the 3D Zn anode.Especially by infiltrating the monomer precursor into the 3D framework of the Zn anode, followed by in situ polymerization (e.g., heatinitiated polymerization), good interfacial contact between the 3D Zn anode and the gel electrolyte can be achieved, which can lead to enhanced electrochemical performance.5. High Zn utilization investigation: Future AZIBs need to be developed with higher energy densities.For now, cathode materials are relatively more well investigated, including type of cathode material (e.g., manganese oxides, vanadium-based oxides, Prussian blue analogues, organic compounds, and polysulfides), and charge storage mechanism (e.g., Zn 2+ insertion/extraction, H + -assisted insertion/extraction, H + /Zn 2+ ion co-insertion/extraction, and Zn-involved conversion reactions).These studies have greatly helped increase the energy density of the cathode side.However, the overall energy density of the full cell and the Zn utilization at the anode is still a concern.Most AZIB studies have used excessive amounts of Zn at the anode side, which lowers battery gravimetric energy density.Therefore, work is still needed to determine the optimal amount of Zn that can be accommodated in the 3D Zn anode, to realize the stable operation of the 3D Zn anode at low N/P ratios, and to optimize the anode thickness to achieve efficient use of the 3D framework.These are all important research directions along the way to achieve high Zn utilization at the 3D anode.Moreover, as inspired by the emerging anode-free AZIBs, developing a Zn-free 3D anode that couples prezincified cathode can further enhance the energy density of AZIBs.
In addition to the challenges mentioned above, several key factors need to be addressed to enhance the commercialization of aqueous Zn metal batteries: (i) Development of low-cost and scalable manufacturing processes: It is crucial to establish manufacturing processes that are not only cost-effective but also scalable for large-scale production.This applies specifically to essential components like electrolytes, artificial solid electrolyte interphase, and separators.(ii) Optimization of test parameters and characterization methods: To fully exploit the combined effect of each battery component, it is important to optimize the test parameters and characterization methods.It will allow researchers to understand how each component interacts and impacts overall battery performance.By fine-tuning these parameters, battery efficiency can be improved.(iii) Understanding the interaction of 3D structured components: The integration of 3D structured components, such as 3D Zn metal anodes, electrolytes, and separators, plays a critical role.It is essential to gain a comprehensive understanding of how these components interact with each other and how they influence overall battery performance.This knowledge can guide further improvements in the design and functionality of the battery.
By focusing on the development of low-cost and scalable manufacturing processes, integrating 3D structured components, and optimizing test parameters and characterization methods, significant progress can be made in improving the commercialization of aqueous Zn metal batteries.These efforts will contribute to enhanced performance, scalability, and cost-effectiveness, making these batteries more viable for widespread adoption.We believe that practical 3D Zn metal anodes can be achieved, which will undoubtedly improve the potential of AZIB for commercialization.

F I G U R E 3
Evenly distributed surface current density using 3D structure at the Zn anode.(A) A schematic illustration demonstrates the contrasting Zn plating processes on a bare Zn metal anode and Zn@CuNW (Zn coated with copper nanowires) metal anodes.Finite element simulations showcase the distribution of electric field and Zn 2+ concentration on both the (B) bare Zn anode and (C) Zn@CuNW anodes.Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/ by/4.0).17h Copyright 2021, The Author, Published by Springer Nature.

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Homogeneous ion flux using 3D structure at the Zn anode.Schematic illustration showing the Zn anode (A) without and (C) with a 3D g-C 3 N 4 network above.Finite element simulations showing the Zn 2+ concentration change as time proceeds (B) without and (D) with a 3D g-C 3 N 4 network.Reproduced with permission.

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Large inner space contributed by 3D structure at Zn anode.SEM images reveal the process of Zn plating and stripping on (A-E) planar Zn anode and (F-J) 3D porous Zn anode at 4 mA cm À2 .(K) Schematic illustration showing the plating and stripping behavior of Zn on 3D porous Zn anode.Reproduced with permission.9a Copyright 2022, Wiley-VCH.

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Released Zn plating stress contributed by 3D structure at Zn anode.(A) The figure presents schematic illustrations of planar Zn (PL-Zn) and micro-groove-patterned Zn (P-Zn) electrodes, accompanied by finite element calculations demonstrating the distribution of von Mises stress on the electrodes following zinc deposition.(B) Optical microscopy images and (C) scanning electron microscope (SEM) images showcase the process of Zn metal plating on PL-Zn and P-Zn electrodes.Reproduced with permission.Copyright 2022, 17i Elsevier.

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Overview of preparation methods for 3D Zn metal anode.

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I G U R E 8 Preparation methods of 3D-structured Zn metal anode.(A) The schematic illustration of the fabrication process for largescale patterned Zn metal electrode.Reproduced with permission.17i Copyright 2022, Elsevier.(B) A schematic diagram illustrates the 3D printing process for creating a 3D Zn metal anode.Reproduced with permission.26c Copyright 2023, Elsevier.(C) Schematic illustration of the formation of periodic concave-convex patterned Zn anode using nanosecond laser lithography.Reproduced with permission. 31Copyright 2022, Elsevier.(D) A schematic diagram depicts the acid etching process employed to fabricate a 3D porous Zn electrode.Reproduced with permission.9a Copyright 2022, Wiley-VCH.(E) Schematic illustration of the fabrication of a foldable 3D MXene-based aerogel through freezing treatment and freeze-drying strategy Reproduced with permission.

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I G U R E 9 Schematic illustration showing the possible routes of continuous Zn metal deposition in (A) homogeneous 3D metal Zn anode and (B) surface-modified 3D Zn metal anode.F I G U R E 1 0 Modification of surface composition of 3D metal-based host.(A) Schematic depiction of the fabrication process for a 3D host of Cu nanowires as the substrate for Zn metal anodes; Density Functional Theory (DFT) calculations investigating the absorption of Zn atoms on various facets and edges.Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).17h Copyright 2021, The Authors, Published by Springer Nature.(B) Schematic diagram

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I G U R E 1 2 Schematic illustration showing the Zn deposition (A) on zincophilic substrate and (B) in a gradient-designed 3D Zn anode.

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I G U R E 1 3 (A) Schematic representations showing the preparation process of the triple-gradient 3D host for AZIB, where the porosity, zincophilicity, and conductivity show an increasing gradient from top to bottom.(B) The cycling performance of symmetric cells employing various electrodes was evaluated at a current density of 10 mA cm À2 and a cycling capacity of 1 mAh cm À2 .(C) Rate performance of the symmetric cell using the triple-gradient 3D electrodes.Reproduced with permission.9b Copyright 2022, Wiley-VCH.(D) The preparation process of the 3D-printed electrode with gradient Ag concentration.(E) Schematic illustration showing the Zn plating behaviors on 3DP-HG@Zn, 3DP-BU@Zn, and 3DP-UB@Zn.(F) Element mapping images of the 3DP-BU@Zn anode.(G) Zn deposition profiles of different electrodes at 10 mA cm À2 .(H) The Coulombic efficiencies of asymmetric Zn-Cu cells utilizing different 3D Zn anodes were measured during cycling at a current density of 10 mA cm À2 and a cycling capacity of 1 mAh cm À2 .Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).9c Copyright 2023, The Author, Published by Wiley-VCH.affected due to its weak adsorption to the Zn atom (Figure 14H,I).Zhang et al. imitated the bee hair to prepare a copper-based super-branched structure for stabilizing the Zn anode, and a layer of CaTiO 3 (CTO) is coated on it to enhance its corrosion-resistant ability (Figure 14J,K).Introducing the 3D porous structure

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Inhibition of side reactions at 3D structured Zn metal anode via artificial solid electrolyte interphase (SEI) construction.(A) SEM images showing the side view of bare Zn electrode, patterned Zn electrode, and N@P-Zn electrode.(B) Tafel plots of different samples.Digital images of bare Zn electrode and N@P-Zn electrode (C) before and (D) after a 7-day electrolyte soaking test.(E) XRD spectra of these samples before and after the soaking test.Electrochemical impedance spectra of (F) N@P-Zn electrode and (G) bare Zn electrode before and after the soaking test.DFT calculation showing the adsorption energy of (H) water molecules and (I) Zn atom with the existence of Nafion molecules.Reproduced with permission.17i Copyright 2022, Elsevier.(J) The schematic illustration showing the design principle of the 3D biomimetic scaffold designed for Zn metal anodes.(K) SEM image of the 3D biomimetic scaffold.(L) LSV curves of different electrodes scanned in 1 M Na 2 SO 4 aqueous solution.(M) Tafel plots of different electrodes scanned in 2 M ZnSO 4 aqueous solution.(N) XRD spectra of different electrodes cycled at a current density of 1 mA cm À2 with a cycling capacity of 1 mAh cm À2 after the first Zn plating/stripping.Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). 56Copyright 2022, The Author, Published by Wiley-VCH.

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Inhibition of side reactions at 3D Zn metal anodes via electrolyte composition optimization.(A) Schematic illustration showing the Zn deposition on Cu mesh with and without the addition of PAM in the electrolyte.(B) Tafel plots of the electrolytes with different amounts of PAM addition and (C) the corresponding fitted corrosion current density.(D) Cycling performance of symmetrical Zn cells using different electrolytes and (E) the enlarged voltage profiles at 0.2 mA cm À2 , 1 mAh cm À2 .Reproduced with permission. 59Copyright 2019, Wiley-VCH.(F) Schematic illustration showing the Zn deposition process with and without TBA cation addition.(G) Tafel plots of Zn plates in different electrolytes.(H) Symmetrical Zn cells using different electrolytes and different anodes at 2 mA cm À2 , 2 mAh cm À2 T A B L E 1 Parameters of 3D structured Zn metal anodes for AZIBs.