Recent progress of dendrite‐free stable zinc anodes for advanced zinc‐based rechargeable batteries: Fundamentals, challenges, and perspectives

Zinc‐based batteries are a very promising class of next‐generation electrochemical energy storage systems, with high safety, eco‐friendliness, abundant resources, and the absence of rigorous manufacturing conditions. However, practical applications of zinc‐based rechargeable batteries are impeded by the low Coulombic efficiency, inferior cyclability, and poor rate capability, due to the instability of zinc anode. Herein, effective strategies for dendrite‐free zinc anode are symmetrically reviewed, especially highlighting specific mechanisms, delicate design of electrode and current collectors, controlled electrode|electrolyte interface, ameliorative electrolytes, and advanced separators design. First, the particular mechanisms of dendrites formation and the associated fundamentals of the stable Zn metal anodes are presented elaborately. Then, recent key strategies for dendrites prevention and hydrogen evolution reaction suppression are categorized, discussed, and analyzed in detail in view of the electrodes, electrolytes, and separators. Finally, the challenging perspectives and major directions of stable zinc anodes are briefly discussed for further industrialization and commercialization of zinc‐based rechargeable batteries.


INTRODUCTION
In response to global climate changes and fossil fuels depletion, an exploration of low-carbon energy resources, such as the wind, solar, and geothermal energy, [1][2][3] has been an obvious tendency in recent years, However, seasonal characteristics and climatic conditions with uncontrolled features stimulate real-time constant energy supply to meet the demand of portable and smart electronics. 4 Therefore, electrical energy storage systems may help alleviate many of the inherent instabilities and deficiencies in the intermittent renewable sources, which not only decouples the electricity generation from terminal customers but also allows the energy storage opportunities for electrical microdevices. 5 Remarkably, lithium-based batteries, as a typical power source, dominate the present energy storage market and occupy the mainstream concern of researchers, owing to the lightweight, high energy density, and high working voltage. 6 However, the widespread application of lithium-based batteries may be hindered by the toxicity and flammability of organic electrolytes, low abundance of high-cost lithium, and highly reactive lithium metal. 7 Alternatively, zinc-based batteries (ZBs) are becoming a potential complementary technology, showing comparable energy density, satisfactory cost-effectiveness, and high safety. [8][9][10][11][12][13] To date, ZBs have attracted renewed and extensive attention and entered a new era with unparalleled advantages, such as low cost, easy manufacture, and unlimited conditions of strict instrument and atmosphere. 14 Remarkable advantages stimulate the additional enthusiasm for ZBs, ascribed to the superior theoretically electrical performance of high capacity of 820 mA h/g or 5855 mA h/cm 3 , comparatively low redox potential (−0.76 V vs. standard hydrogen electrode [SHE]) and excellent stability in air and humid atmosphere for available anode in the aqueous electrolytes. 10 Since 1799, metallic zinc anode has been applied as a conspicuous anode in the primary and secondary batteries attributed to high theoretical capacity and relatively low redox potential. At the beginning, the zinc anode appeared as an irreversible electrode due to the harmful by-products and limited rechargeability. To solve this intractable issue, Yamamoto et al., in 1986, 15 replaced the alkaline electrolyte in the primary batteries with the mild neutral zinc sulfate electrolyte. Particularly, Xu et al. 16 presented a novel mechanism based on the zinc anode, α-MnO 2 cathode, and mild aqueous electrolytes (ZnSO 4 or Zn(NO 3 ) 2 ) through two electrochemical processes of zinc stripping into Zn 2+ and thus Zn 2+ intercalation into tunnels of α-MnO 2 . Since then, ZBs with mild electrolytes come into their prosperity. At present, ample cathodes of ZBs have been exploited with high electrochemical performance, including vanadium-based materials, 17 manganese-based materials, 18,19 Prussian blue analogs-based materials, 20,21 and organic materials. 22 The superior performance of most cathode materials could contribute to the exciting gravimetric specific capacity, whereas the energy density of the whole device is limited by the greatly excessive weight of zinc metal anode, and the development of stable zinc anode is still in its primitive stage. It is explained that excessive zinc metal anode is assigned to the consumption of zinc anode, due to the zinc dendrites formation, "dead" zinc, and surface corrosion on the zinc metal surface. [23][24][25] Furthermore, the concomitant hydrogen evolution and severe zinc dendrites could lead to the low Coulombic efficiency, separator puncture, and possible short circuits. [26][27][28][29][30] In addition, zinc dendrites increase the specific surface area and further exacerbate the side reactions, contributing to the increased surface roughness and ample active sites accelerating the zinc dendrites formation to some extent. 31 Considering the above issues and drawbacks, there are serious main challenges to overcome for dendrite-free anodes and high energy density of the whole devices. It is delightful that vast efforts have been intrigued on zinc anode in the recent years, owing to the pursuit of high safety and long shelf-time. At present, the booming prosperity of related dendrite-free metal anode has brought new opportunities for zinc anode according to the referenced design strategies. 32 In this regard, methods for dendrite-free alkali metal anode, 33 such as artificial solidelectrolytes interface construction, 6 solvation design, and electrolytes addition, 34 display a notable electrochemical performance with long-term cyclability and splendid rate capability. Although some key strategies have been proven with superior effectiveness, there is still a lack of related in-depth study and clear mechanisms of zinc anode for practical industrialization, especially as anode component in the microdevices. To this end, it is urgently imperative to figure out the specific mechanism of zinc anode during the ion intercalation/extraction, in-depth analysis and optimization of zinc anode for further practical application.
With the above concerns in mind, some key challenges and fundamental mechanisms have been analyzed in detail, and current major strategies for dendrite-free zinc anode could be comprehensively summarized in this review ( Figure 1) as following aspects: (i) delicate design of electrode and current collectors for uniform zinc deposition sites, (ii) controlled electrode|electrolyte interface for homogenous zinc ion flow, (iii) ameliorative electrolytes through additives or novel construction of electrolytes solvation, and (iv) advanced separators design for orderly uniform zinc ion transportation. The relationship among these issues, surface and electrode|electrolyte interface engineering strategies and the electrochemical F I G U R E 1 Summarization of the key strategies for dendrite-free stable zinc anodes. [31][32][33][34][35][36][37][38] performance will be systematically discussed. Finally, we systematically conclude the new advances of dendrite-free anodes for ZBs and propose some possible prospects for further boosting electrochemical performance.

MECHANISM AND CHALLENGES OF ZINC ANODE
So far, zinc metal has been widely deployed as anode in the ZBs, despite of abundant merits of ZBs zinc anodes still suffer from the harmful bottlenecks, obstructing the scaleup industrialization of ZBs. In recent works of ZBs, the zinc anode reaction is usually recognized as the chemical convention of Zn stripping and Zn 2+ plating accompanied by discharging/charging operations: Zn = Zn 2+ +2 e − . The thermodynamic driving force could be depicted as ΔG = zF(φ − φ • ) = zFη, 35 where ΔG, z, F, φ, η, and φ • represent the Gibbs free energy change of the reaction, reactive electron number, Faraday constant, potential, overpotential, and equilibrium potential for the reaction, respectively. This electrochemical reaction is actually nonequilibrium state ascribed to the existence of overpotential. 36 Notably, the electrode|electrolyte interface at the surface of zinc anode displays steady evolution in the deposition/stripping reaction, resulting in the electrochemical performance changes of ZBs. 37,38 To obtain a comprehensive and thorough understanding of shortcomings of zinc anode and the synergistic effect of ZBs, numerous works have been carried out ground on experiments and theoretical simulation. Presently, it is acknowledged that the development of zinc anode is impeded by numerous issues, originating from dendrites formation, hydrogen evolution, corrosion and passivation, and their interactive influence. 39,40

Fundamentals of zinc dendrites formation
Generally, dendrites formation is a common phenomenon and a knotty issue during the deposition/stripping reaction, owing to the high electrochemical activity of zinc ions in thermodynamics. In the aqueous ZBs, the electrolytes could be divided into alkaline and mild electrolytes (pH ≈ 3-6), corresponding to the different mechanisms. 41,42 In the alkaline electrolytes (e.g., KOH), zinc anode could release two electrons and could be oxidized into Zn(OH) 4 2− on the metal surface according to the equation 43 44 which contributes to the high irreversibility of zinc metal in the alkaline electrolytes. Additionally, it is ineluctable corrosion of the zinc anode owing to the negative reduction potential even the existence of zinc precipitated solid zinc species, which occurs in accordance with the equation 45 Meantime, the reduction of products on the zinc anode could compete with the hydrogen evolution reaction (HER) (2H 2 O + 2e − = 2OH − + H 2 ). In the process of deposition on the preferable charging sites, small tips will conveniently occur after the zincates adsorption on the surface of zinc anode, promoting the accumulation of solvated zinc in accordance with the cusp effect and gradually varying into dendrites.
In the mild acidic electrolytes, the Zn 2+ cations usually are surrounded by six dipolar water molecules, appearing as [Zn(H 2 O) 6 ] 2+ because of the plentiful free water molecules, which is susceptible to the pH of the electrolytes. 46 The solvation sheath structure of [Zn(H 2 O) 6 ] 2+ ions possesses a high energy barrier due to the strong interaction between the water molecules and zinc ions for available deposit and strip at the zinc metal surface. 46 Despite the inadequate existence of OH − species to form zincates, Zn 2+ as a charge carrier displays the function as same as the [Zn(H 2 O) 6 ] 2+ in the alkaline electrolytes in the kinetic mechanism. 47 In spite of the profitless dendrites growth in the mild acid electrolytes, the overpotential could cause the dendrites formation and accelerate the dendrite growth electrochemically, especially at high current density. 48 The Zn 2+ gives a priority nucleation site and forms an initial protrusion in accordance F I G U R E 2 Mechanisms and pourbaix diagram for Zn 2+ . (A) Schematic illustration of zinc anode mechanisms. Pourbaix diagram for 10 −6 mol/L (B) and 10 −4 mol/L Zn 2+ (C). Source: (A) Reproduced with permission from Ref. [171]. Copyright 2020, American Chemical Society. (B and C) Reproduced with permission from Ref. [172]. Copyright 1991, Elsevier. with a two-dimensional diffusion, resulting in the formation of uneven electric field and stimulating the protrusion growth and dendrites formation. The uneven electric field could cause the inhomogeneous charge aggregation and ion concentration gradient ascribed to the "tip effect," promoting the dendrites formation and indirectly causing the short circuit ( Figure 2A). Furthermore, it is proposed that the thermodynamic mechanism of atomic cluster formation is correlative to the free energy nucleation of the new phase, 49 contributing to the different nucleation barriers on the surface and subsequently facilitating the zinc dendrites formation at low current density. Therefore, it is vital to control the zinc ion solvation structure, coordination environment, and the interface electric field for the prevention of zinc nucleation and dendrites hazard.

Mechanism of hydrogen evolution reaction
In the zinc metal anode systems, the HER indicates that the hydrogen generation occurs accompanied by selfcorrosion or electrochemical reaction in the mild/alkaline electrolytes. In theory, the potential of Zn 2+ /Zn is lower than that of HER in the whole pH region ( Figure 2B,C), suggestive of the low electronegativity of Zn. In addition, no overlap appears between the stable Zn region and electrochemically stable window of water, implying the little possible coexistence of Zn and H 2 O. There is a strong tendency to react with water, accompanied by zinc violent corrosion at the surface. 50 Along with the H 2 evolution, the local concentration of OH − species could be increased steadily on the surface, which indirectly promotes the insoluble precipitation of ZnO, Zn(OH) 2 , and other zinc composites on account of the coalescence with the Zn 2+ and electrolytes components. Furthermore, the H 2 evolution is the primary reason for battery failure, severe safety issues, and even for explosion toward practical applications, on account of the internal pressure increase during the charging/discharging process. 51,52 In general, the HER process displays different mechanisms in alkaline and mild acid electrolytes. (i) In the alkaline electrolytes, the standard reduction potential of ZnO/Zn is −1.26 V versus SHE, which is lower than the HER potential of −0.83 V versus SHE. Hence, the HER is thermodynamically uncontrolled and dominates the main reaction instead of the reduction of ZnO. The detrimentally irreversible reaction in the alkaline electrolytes impedes their practical applications in the actual scenarios. (ii) In the neutral or mild acid electrolytes, the standard reduction potential of Zn 2+ /Zn is −0.76 V versus SHE, which is higher than the HER potential in the mild acid electrolytes. Therefore, the HER resulting from self-corrosion could not appear spontaneously. Attributed to the superfluous H + in the electrolyte, it is convenient to obtain the electrons from electrolytes, resulting in the H 2 release and low Columbic efficiency. 44 Furthermore, the sites where the H 2 releases could be nonconductive because the H 2 bubbles absorbed on the zinc metal surface restrain the electrons transfers, resulting in the battery failure during the zinc deposition/stripping process. In general, the HER process in ZBs includes the Volmer step and the Heyrovsky step (or the Tafel step), which could be described as follows 8 : In these reactions, M and H ad represent the metal atom and the adsorbed H atom, respectively. Additionally, the Volmer step is speed controlling, and the high Tafel slope of 0.12 V and high Tafel intercept of 1.24 V for HER could contribute to the high overpotential. In view of this perspective, the HER and the surface corrosion of the metal anode are self-limited at the high current densities.

Corrosion and side reactions with by-products
Unfortunately, the corrosion and side reactions are inevitable and concomitant with the dendrites formation and HER, which could be divided into electrochemical corrosion and self-corrosion. According to the redox potential, zinc metal is more insensitive to water compared with the alkali metals. In fact, corrosion potential could not stand for the real state of zinc anode owing to the overpotential caused by polarization. Particularly, [Zn(H 2 O) 6 ] 2+ species desire desolvation energy to plate as Zn while approaching the zinc surface. In the alkaline electrolytes, it is impracticable to deposit/strip on the zinc metal surface, ascribed to the spontaneous formation of Zn(OH) 4 2− and ZnO 2 2− with the increased pH values in the discharging/charging process. 53 Even though the zinc deposition/stripping is possible and reasonable in the mild electrolytes, uneven corrosion still appears on account of the micro primary batteries, degrading the strength of zinc metal and making the surface violently rough with numerous pits. 54 In particular, the typical by-products of ZnSO 4 [Zn(OH) 2 ] 3 ⋅xH 2 O in the common ZnSO 4 electrolytes appear both on the cathode and anode simultaneously, which could be detected after several charging/discharging processes. 55 In detail, because of the part insertion of H + into the cathode materials, the local pHs of electrolytes steadily increase around the cathode. The concentration of OH − is not too high to generate the by-products in the first charging/discharging process. With the sustained increase of OH − concentration, the by-products of ZnSO 4 [Zn(OH) 2 ] 3 ⋅xH 2 O gradually form and adhere onto the cathode surface with Zn 2+ insertion. 56 In comparison, the by-product formation results from the electrostatic interaction and their limited solubility. Moreover, the Zn anode loses two electrons and varies from Zn 0 into Zn 2+ with a net positive charge in the anode surface. The Zn 2+ could attract the anions, such as SO 4 2− and OH − , which stimulates the fundamental reaction of ZnSO 4 [Zn(OH) 2 ] 3 ⋅xH 2 O formation. Therefore, the main corrosion reactions and side reactions in the ZnSO 4 electrolytes could be summarized as 57  O. These by-product formations consume the electrolytes, contributing to the decrease of the concentration and low Coulombic efficiency of zinc deposition/stripping. Furthermore, it is noxious for zinc anode that by-products show the poor ionic and electric conductivity and limited solubility plated onto the zinc anode surface. 58 In this case, the excess electrolytes are supposed to maintain stable cyclability, and the inferior conductivity of by-products demands an effective solution for the construction of stable zinc anode.

2.4
Interactive mechanisms for zinc anode Actually, the above factors display a synergistic influence on zinc anode and significant interactions with each other in the plating/stripping process. Initially, the HER could take place around the zinc anode surface, which not only consumes the H + in the electrolytes but also increases the OH − concentration. The increased pH value plays a significant role in accelerating the side reactions, dedicated to the inert corrosion, by-product formation, and the resulted uneven interfacial electrical field, especially for by-product generation and zinc dendrite growth. In turn, the zinc dendrites contribute to the porous structure and high specific surface area with numerous pits grown on the zinc anode, worsening the HER by supplying ample reactive sites. Besides, the increased specific surface area may decrease the current density, beneficial for HER and corrosion on the zinc anode due to the reduced overpotential, as well as the nonconductive by-products. As a result, the nonconductive by-product acceleration not only impedes the electron transfer but also hinders the zinc deposition to some extent. Furthermore, it is worth noting that the initial stripped Zn electrode (S-Zn) and plated Zn electrode (P-Zn) exhibited different electrochemical behaviors. Li et al. 59,60 reported that the dendrites grew in the pits of S-Zn, whereas dendrites grew on the protrusion of P-Zn, in which the dendrites from the pits are more heterogeneous than those on the surface. In addition, soft short circuit, which cannot be distinguished by the widely used galvanostatic cycling tests, prevails in a plethora of reported Zn anodes and leads to a super-stable illusion. Pertinent test protocols are proposed to be developed for the effective characterization of zinc anode. 61 Hence, the reaction on the zinc anode relies on the interactive effect among these factors, implying that once the HER issue is prevented or alleviated, the corrosion and zinc dendrite formation could be remitted at the same time.

STRATEGIES FOR DENDRITE-FREE ZINC ANODE
Recently, the design and optimization of zinc anode dominates the main concern, aiming at the well-designed assembly of ZBs with long cyclability and superior rate capability. Current strategies are summarized in this section as follows, including elaborate design of electrode structure, well-defined interface between the electrode and electrolyte, and ameliorative electrolytes preparation. At present, zinc powder possesses the remarkable advantages with low cost, tenability, and processability. However, zinc powder anode suffers from severe corrosion and hydrogen evolution due to the higher specific surface area and higher reactivity compared with the zinc foil anode. Therefore, some effective strategies have been exploited for the effective protection of zinc powder, such as semisolid zinc powder-based slurry, 62 the hybridization of commercial zinc powder and pristine graphene, 63 and spontaneous reaction of Zn powder and graphene oxide. 64

Elaborate design of electrode structure
To alleviate the dendrite formation of zinc anode and improve the zinc anode adaptation at high current density, it is a prerequisite that the zinc anode electrode structure should be well designed with high conductivity and the decreased HER reactivity. To be specific, numerous works about the design of electrode structure concentrate on the 3D structure design, 65 alloying anode, 66 and "rocking-chair" anode without zinc metal utilization. 67

3.1.1
Well-designed highly conductive 3D structures In fact, the polarization of Zn deposition exerts a harmful effect on the dendrite growth and battery failure, which could be categorized into concentration polarization and electrochemical polarization, ascribed to the slow Zn 2+ diffusion rate and sluggish Zn 2+ reaction and deposition. The intrinsic nucleation and the overpotential of zinc deposition vary with the different substrates and areal current densities, shedding light on the importance of the substrate for zinc deposition. Plentiful conductive 3D hosts have been developed for decreasing the polarization of the zinc deposition by providing affluent reactive sites for nucleation and deposition, benefiting from the native advantage with a high surface area of 3D hosts. Besides, porous skeleton could furnish enough volume buffer space for zinc anode during the zinc plating/stripping, which could moderate the zinc dendrites and improve the long cyclability. 68 Typically, porous current collectors display large specific surface area with ample reactive sites, uniform electrical field distribution, and effective electrolyte infiltration into the current collector skeleton, according to the sufficient protection of lithium anode. 65,69 At present, it is verified that porous current collectors are effective for dendrites prevention in zinc anode during the contiguous plating/stripping process. For instance, Cu foam 70,71 was developed as the carrier for zinc deposition to form Zn@Cu foam through an electrochemical deposition approach ( Figure 3A). The resulting zinc anode exhibited a low overpotential of 65.2 mV, high Coulombic efficiency of approximately 100%, and stable cyclability, contributing to the superior full cell with β-MnO 2 cathode. Moreover, it is worth noting that Cu foam shows the lowest overpotential among the different current collectors, such as steel mesh (214.07 mV), steel foil (128.76 mV), Ni foam (101.4 mV), carbon cloth (104.1 mV), and carbon paper (124.9 mV). Inspired by the 3D structure of current collectors, the designed self-existent 3D skeletons of zinc electrodes were exploited with abundant pores and cavities. 72,73 In detail, the 3D zinc anode was prepared through organic acid etching strategy by Wang et al., 72 representing hierarchical porous architecture with hydrophilic surface. This porous texture enabled the zinc anode with suppressed dendrite formation and long cyclability for 1500 h at 1 mA/cm 2 and 1 mA h/cm 2 , especially for the full cell with improved rate capability. Further, Bie et al. 73  zinc structure with zincophilic ZnSe overlayer, acquired through a one-step electrochemical scanning technique and sequentially electrodeposition. The obtained 3D zinc framework with ZnSe overlayer (3D-Zn@ZnSe, Figure 3B) contributed to the long stable cyclability of 2000 h at 0.5 mA/cm 2 and 0.5 mA h/cm 2 , as well as high Coulombic efficiency of 99.28%. Benefiting from the reduced local current density and superior adaption to the volume expansion, the zinc anode exhibited accelerated zinc kinetics and restrained zinc dendrite growth with low potential. The assembled full cell based on the 3D-Zn@ZnSe anode and V 2 O 5 cell displayed an extraordinary capacity retention of 90.63% after 8500 cycles, which shed great light on the combined impact on the polyporous structure and protective layer. Meantime, bi-continuous metallic zinc nanoporous structures were obtained with a stable electrochemical transition between the Zn and ZnO ( Figure 3C). 74 It is worth noting that the development of microscopy and spectroscopic measurements is beneficial to the valuable insights of the novel materials design, clearly probing the kinetics-controlled structural evolution of Zn and ZnO. In addition, the resulted porous Zn anode for Zn||NiOOH cell delivered an areal capacity of 30 mA h/cm 2 at 60% depth of discharging, while the constructed Zn-air batteries dispalyed stable 80 h operation.
Synchronously, some carbon substrates and MXenebased materials are certificated with superior conductivity for uniform zinc deposition and effective regulation of zinc ion flux. [75][76][77][78][79][80][81][82] For example, 3D fiber architectures assembled the zincophilic Sn nanoparticle with an N-doped hollow carbon sphere through a hard-template strategy, appearing large specific surface area ( Figure 3D), hierarchical hollow structure, and low nucleation barriers. The constructed Zn anode presented a prolonged cyclability over 370 h with high Coulombic efficiency, derived from inferior electrocatalytic property on HER and low nucleation overpotential. 83 Furthermore, the hierarchical hollow structure not only supplied numerous reactive sites but also provided enough buffer space for volume expansion during the plating/stripping process. Therefore, it is feasible to address the zinc dendrites through the combination of highly conductive carbon substrate and zincophilic species. Inspired by the ample reactive sites and functional groups on the MXene-based materials, the MXene coupled with graphene in a 3D scaffold was developed by an oriented freezing approach. 84 The resulted 3D architecture ( Figure 3E) denoted an in situ solid electrolyte interface of ZnF 2 on the electrode/electrolyte surface owing to the fluorine terminations in MXene, which prevented HER (3.8 mmol/h cm 2 ) and formation of some inactive by-products of Zn(OH) 4 2− notarized by in/ex situ characterization. As a result, the constructed symmetric cell exhibited admirable rate capability at 10 mA/cm 2 and long cyclability after 1000 cycles, ascribed to the affluent zincophilic sites and micropores in the 3D skeleton for homogenous zinc deposition and low overpotential.
So far, although there are plentiful works about 3D zinc anode for dendrite-free design to construct sandwiched ZBs, zinc anode design in the zinc ion microbatteries for micropower source is seriously neglected. Furthermore, it is stated that zinc foil will bring about vast issues functioning as current collectors and working electrodes at the meantime, which could give rise to severe problems facing scalable production. 85 It is hazard to the full cell that the morphologies of Cu and zinc powder changed with the aging process, on account of the hydrogen formation. Zinc powder/current collector was validated for more practical utilization, which was exploited with tin decorated on the Cu foil for ameliorated HER. The resulted full cell with a low negative/positive ratio of 10:7 revealed remarkable cyclability, which shed light on the calendar life of ZBs. Meanwhile, Duan et al. 86 fabricated Zn-Mn microbatteries with 3D nanocone array structures ( Figure 3F,G), benefiting to the electron-ion transport, excellent conductive framework, and shortening the pathways. However, the cyclability is far from satisfactory owing to the naked zinc anode with large specific surface area, which is favorable for the zinc dendrites formation and growth. Considering the available function of MXene for dendrites prevention during the fabrication of sandwiched ZBs, MXene was verified for high-performance zinc powder anode with low polarization and charge transfer resistance ( Figure 3H). 87 The MXene nanoflakes were adopted as the electrons and ions redistributor through electrostatic self-assembly method, exhibiting a low lattice mismatch (∼10%) for a coherent heterogeneous interface between deposited Zn(0 0 02) facet and MXene(0 00 2) facet ( Figure 3I). The low-barrier heterogeneous interface, highly conductive electron-ion network, and low lattice mismatch contributed to the high Coulombic efficiency of near 100%.

3.1.2
Alloying anode To date, there are plenty of related works about Zn-alloying anodes to resolve the dendritic metal deposition, such as Zn-In, 88,89 Zn-Al, 90 Zn-Ag, 91 Zn-Au, 92 Zn-Cu, [93][94][95] and Zn-Mn 96 alloys. Specifically, it is revealed by Zhang et al. 91 that the intrinsic reasons for the effective impedance of dendrite growth could be attributed to the zero potential barrier of some Zn-soluble materials and spontaneous alloying reaction during the reduction of Zn 2+ . As shown in Figure 4A, Ag islands as the seeding materials on the carbon paper induced the zinc deposition on the Ag islands rather than the carbon substrate, accompanied by the Zn x Ag 1−x alloying formation on the islands uniformly. 91 During the plating/stripping process, the formation of Zn x Ag 1−x alloying occurred preferentially derived from the negative Gibbs free energy of formation. In consequence, the fabricated zinc anode achieved impressive long-term cyclic stability and excellent anticorrosion property. Furthermore, liquid gallium-indium alloy coating on the zinc anode ( Figure 4B) preferred inward deposition and depicted a higher HER overpotential than the reference zinc anode. 97 The liquid interface effectively controlled the zinc nucleation behaviors through the alloying-diffusion process, confirmed by the in situ characterizations. The built zinc anode with liquid alloy layer displayed a small polarization of 24 mV at 0.25 mA/cm 2 and facilitated a prolonged cyclability of 2400 h, providing a universal idea for future development of metal batteries. Besides, Zn-Cu alloying reaction was in situ performed at the zinc anode surface to form a Zn-rich alloy interlayer that is zincophilic and isotypic to the zinc. 93 With the densely packed micrometer-sized Zn particles on the Cu foils, steady zinc anode was fabricated with a spontaneous Zn-Cu alloying reaction promoted by deep eutectic solvent (DES)-based electrolyte ( Figure 4C). This alloy interlayer enabled the zinc anode with compact deposition, suppressed corrosion, and swelling, leading to the high utilization of more than 25% in the full cell and stable cyclability after 7000 cycles. It is remarkable that the high depth of discharge of 40% in the symmetric cell indicated great potential application for urgently required gravimetric/volumetric energy densities with a thin and compact electrode design. In another work, Zn-In alloy anode was synthesized through a simple and typical smelting-rolling method ( Figure 4D), with trace indium in the bulk anode. 88 The adsorption of the uniformly dispersed indium in the bulk anode was devoted to the instantaneous nucleation, whereas the exclusion part for nucleation was prevented simultaneously. The as-designed Zn-In alloy anode displayed stable stripping/plating for 2500 h at 4.4 mA/cm 2 , contributing to the satisfied cyclability of the full cell with a high capacity retention of 96.44% after 1000 cycles. This method offered a practical and reliable strategy for the solvation of the severe zinc dendrites formation and by-product generation. Moreover, a lamella-nanostructured eutectic Zn-Al alloy anode was constructed to tackle the issues of dendrites growth and low Coulombic efficiency. 66 Core/shell Al/Al 2 O 3 interlamellar nanopatterns ( Figure 4E,F) helped to guide the uniform deposition of zinc and promote the stripping from the eutectic Zn 88 Al 12 (at%) alloys. 66 Specifically, some by-product formations of ZnO and Zn(OH) 2 and the electroreduction of Zn 2+ on the electrode surface were inhibited by the in situ formation of the interlamellar nanopatterns with an Al/Al 2 O 3 core/shell counterparts. The resulted zinc anode discovered an extraordinary stripping/plating process and low overpotential for 2000 h in the O 2 -absence ZnSO 4 electrolyte. The splendid electrochemical performance of the eutectic Zn-Al alloy resulted in the high energy density of the full cell with 142 W h/kg.

"Rocking-chair" anode
Owing to low discharge depth, inferior electrode utilization, severe zinc dendrites, low Coulombic efficiency of stripping/plating process and intricate side reactions, the shelf life and electrochemical performance of zinc metalbased full cells were rigorously deteriorated by the zinc metal anode. 67,98,99 Notably, the unsatisfactory gravimetric and volumetric energy densities were caused by the excessive zinc metal compared to the cathode, which contributed to the impacts of zinc dendrites and "dead" zinc generations. In analogy with lithium-ion batteries, in which lithium metal was replaced with the intercalated carbon or oxide, "rocking-chair" ZBs were exploited to solve the above issues, in the principle of the ions intercalation/extraction between the anode and cathode. Therefore, abundant "rocking-chair" anodes were testified to fabricate the metal-free ZBs, based on the anode current collector, intercalative zinc metal-free anode, separator, electrolytes, zinc-rich cathode, and cathode current collector ( Figure 4G). It could be described that the mechanism in the "rocking-chair" zinc-ion batteries is the reversible shuttle of the Zn 2+ between the intercalative anode and cathode through the electrolyte. 100,101 The Zn 2+ will extract from the cathode and intercalate into the anode after the diffusion in the electrolyte when the batteries are in the charging process, whereas the Zn 2+ will reversibly intercalate into the cathode from the extraction in the anode during the discharging process. For instance, graphene aerogel decorated with mixed ZnCO 3 MnCO 3 (ZMG) assembled with a nanotubular sulfidated Ni-Co-Fe-layered double hydroxide (LDHS) contributed to the successful establishment for "rockingchair" ZBs based on the alkaline electrolyte ( Figure 4H). 102 As a consequence, the constructed ZBs not only delivered a comparable specific capacity of 89 mA h/g at 12 A/g, a rate capability of 27 mA h/g at 300 A/g, and an extremely high working potential of 1.8 V but also showed an unprecedented capacity retention of 96% after 17 000 cycles with a high discharge depth of 100%, owing to the electrochemical pulse-driven regenerative mechanism. In KOH electrolyte, this superbattery exhibited a working voltage window of 1.1 V (Figure 4I), ascribed to the both capacitive and battery-like energy storage processes. This outstanding electrochemical performance could be attributed to (i) the bimetallic Zn-Mn anode contribution for the preven-tion of zinc dendrites and homogeneously zinc deposition, and (ii) the effective suppression of HER and high depth of discharging. In addition, (NH 4 ) 2 V 10 O 25 ⋅8H 2 O@Ti 3 C 2 T x (NHVO@Ti 3 C 2 T x ) film 103 was proposed as anode for constructing the "rocking-chair" ZBs, due to the high specific capacity and low redox potential of vanadium oxide, as well as the high conductivity of Ti 3 C 2 T x . The as-prepared NHVO@Ti 3 C 2 T x film disclosed an ultrathin framework with 14 nm (Figure 4J,K). The synergistic impact of highly conductive Ti 3 C 2 T x with the plentiful active sites and the vanadium oxide with high specific surface area could build a rapid electron-ion network, equipped the ZBs with an admirable specific capacity of 131.7 mA h/g and remarkable energy density of 97.1 W h/kg based on the whole mass of anode and cathode. The observed zinc ion transfer could be realized in the cathode and anode ( Figure 4L), after subtle match between the anode and cathode based on the gravimetric specific capacity ( Figure 4M). The full cell based on the well-matched anode and cathode delivered a highcapacity retention of 92% after 6000 cycles at the current density of 2 A/g. Therefore, "rocking-chair" zinc metalfree anode offers a positive insight for practical full cells with high energy density and power density. The "rockingchair" zinc anode could improve actual gravimetric and volumetric capacities through the well-matched capacity and supply a large working potential and high safety; however, the rocking-chair ZBs still suffer from the limited energy density, which should be overcome for practical application.

Electrode|electrolyte interface reconstruction
In general, the zinc electrodeposition is related to the two interfaces on the anode, containing the zinc host materials and zinc/electrolytes interface that influenced the electron and ion transfer during the stripping/plating process. 23,104 From this point of view, it is vital to control and optimize the interface of zinc host materials and the electrode|electrolyte interface to regulate the interfacial electric field and ion flux for uniform zinc transfer and nucleation. In spite of multiple practical strategies based on the zinc host electrode design, it is also critical to figure out the interface mechanism and further exploitation of interface engineering for dendrite-free zinc anode. Specifically, solid-electrolyte interphase (SEI) could be established as intermediary to constructively reconcile electrode|electrolyte interface, accompanied by hydrogen and oxygen generations from the water near the zinc anode surface. In this aspect, the interface engineering is one key to reconstruct the interface between the anode and electrolytes. To date, current works mainly concentrated on the two aspects of artificial coating and in situ pretreatment construction. 44,105

3.2.1
Artificial construction of SEI Interfacial layers usually performed as the counterparts for inducing the homogenous zinc deposition and suppressing the dendrites generation. It is acknowledged that the artificial SEI should satisfy these following desires: (i) chemical and electrochemical inertness in the aqueous electrolytes, (ii) excellent mechanical strength withstanding the volume change and dendrites formation, and (iii) favorable wettability to electrolyte or unblocked ion transfer channel. Therefore, some inorganic materials, including CaCO 3 , 106 TiO 2 , 107 and ZrO 2 , 108,109 were suitable to be functionalized as the interface layers without the electrical or ionic contact with the zinc anode. Notably, faceted TiO 2 with relatively low zinc affinity was discovered as a protective layer, denoted to the low potential hysteresis and flat voltage plateau in the symmetric cells. 110 It was deduced from the density functional theory (DFT) calculation that the (0 0 1) and (1 0 1) facets of TiO 2 showed relatively low Zn affinity, whereas the (1 0 0) facet presented a high Zn affinity leading to the prior accumulation during the zinc deposition ( Figure 5A,B). The faced TiO 2 sheet was synthesized with a large crystal size parallel to the (0 0 1) facet, compared to the commercial TiO 2 bulk ( Figure 5C). The highly exposed (0 0 1) facet was discovered with a long cyclability of 460 h at 1 mA/cm 2 for 1 mA h/cm 2 and smooth layer during the plating/stripping process through regulating the orientation of the TiO 2 facets with low Zn affinity. It is indicated that the adjustment and control of the crystal facet are rewarding to the even nucleation and deposition and furnish with an insightful understanding of the native mechanism of metal affinity. In analogy with the orientation control of the artificial SEI interface, a surface-preferred (0 0 2) crystal plane was detected for high-performance zinc anode with dendrite-free, no-byproducts, and neglectable HER compared to the (1 0 0) plane. 111 The Zn anode featured with (0 0 2) crystal plane with homogeneous interfacial electrical field and strong adsorption energy, denoted to the long cyclic life of 500 h and high Coulombic efficiency of 97.71%, facilitating with the full cells for long-term cyclability of 2000 cycles. The methods of regulating the preferred orientation crystal of zinc anode provided a novel direction for highly reversible and safe zinc anode. Remarkably, single-crystal (0 0 2) Zn was demonstrated for defect-suppression constructing outstanding performance. 112 The lack of grain boundaries, the perfect homoepitaxial lattice, and the sole expression of (0 0 2) facet supply regular zinc deposition with singlenucleation sites parallel to the zinc surface. Uniform flat deposition at 200 mA/cm 2 could be obtained, whereas a flat anode surface was well sustained after 1200 cycles at 4 mA h/cm 2 , along with a high Coulombic efficiency of 99.94%. Inspired by the orientation deposition, ferroelectric polymer-inorganic composites decorated on the zinc anode with tunnel-rich and nanoporous characterization were proved that ferroelectricity or the piezoelectricity of commercial BaTiO 3 and polyvinylidene fluoride could induce the even and controllable electric polarization and guide ordered ion transfer. 113 However, the ion flux inhomogeneity and fast-charging stability are inferior due to the caused poor polarization of the coating layer and limited polarized domains following the direction of the self-built electric field (10 V/mm corresponding to the battery voltage of 2 V and the thickness of 200 μm via the equation of E = V/d, where E, V, and d refer to the electric field, battery voltage, and the distance between the anode and cathode, respectively). It is certificated that adopting the pre-poled ferroelectric coating layers on the zinc anode is an effective strategy to immobilize and maximize the piezoelectric capability of the coating layer along its polarization direction ( Figure 5D). Through the electric field and concentration field stimulations ( Figure 5E), it is concluded that the poled BaTiO 3 enabled the zinc anode with enhanced zinc flux owing to the cation attraction on the bottom surface and accelerated zinc ion migration. Through chronoamperometry with an overpotential of −150 mV, it could be inferred that the confined random 2D zinc ion diffusion and homogenous 3D zinc ion diffusion were supplied by the poled BaTiO 3 -coated zinc anode attributed to the initiation time of less than 3 min ( Figure 5F). Furthermore, the facets and edge sites of Cu nanowires ( Figure 5G) with high zincophilic were demonstrated to induce a uniform zinc deposition/nucleation with homogeneous electric field and concentration field, leading to ultralong cyclability of 2800 h of symmetric cells and high rate capability for full cells. 114 The zinc anode decorated with Cu nanowires as SEI delivered inhibition of zinc dendrites, HER, and by-product suppression, which triggered a novel way to long shelf life and high rate capability of zinc-ion batteries. Typically, the reversible epitaxial electrodeposition on the metal anode could be obtained through textured conducting electrode coatings equipped with low lattice mismatch with the metal anode. 115,116 This low lattice mismatch with anode opens research avenues to establish highly reversible ZBs in the concept of epitaxy regulation. A fluid-based route was developed for creating aligned graphene coatings, which could produce long-lived orientation parallelly following the shear plane. As a result, graphene decorated on the zinc anode could drive a locked crystallographic orientation of zinc deposition, contributing to the elegant reversibility of zinc anode. Meanwhile, nitrogen-doped graphene oxide was prepared through a Langmuir-Blodgett approach with a parallel and controllable interface layer of 120 nm on the zinc anode ( Figure 5H). 117 Devoted to the prior directional deposition of zinc anode along with the (0 0 2) facet and zincophilic N-doped functional groups, the resulted zinc anode displayed effective suppression of HER and passivation. Furthermore, it is evidenced through in situ Raman profiles ( Figure 5I) that Zn(OH) 4 − species could be formed in the high-voltage region along with the violent HER and severe side reactions. The passivation and Zn(OH) 4 − species suppression were achieved by the N-doped graphene protection of zinc anode ( Figure 5J), ascribed to the directional plating morphology preventing the close contact between the electrode and electrolytes.

In situ SEI formation
Different from the artificial SEI with manual construction, in situ SEI formation is ancillary and obtained at the interface during the repeated plating/stripping owing to the unique substrate and special electrolyte additives. The in situ SEI layer is formed at the interfaces during the cyclic process, leading to the unmodified thickness of the SEI. In addition, not only the transfer kinetics and the electron transfer of Zn 2+ can be affected by the changed thickness along with the stripping/plating process, but also the ionic conductivity cannot be effectively evaluated with multiple constituents. However, the in situ SEI formation could be self-repaired because the new ingredient will repair the protective SEI layer once the cracks occur on the interlayer. 118,119 In this aspect, the in situ SEI will always be refreshed covering the zinc anode during the deposition, which can solve knotty problems of artificial SEI with cracks and degradation after continued plating/stripping. Attributed to the valid impact of the in situ SEI at the interface and no complex manufacturing process of doctor-blading or spin coating techniques, the in situ SEI still possesses a promising potential for dendrite-free zinc anode with special ability to prevent the by-product formation and undesired HER. At present, ample strategies for constructing in situ SEI have been reported, mainly including the liquid-gas reactions and solid-liquid reactions on the zinc surface. Typically, an in situ electrochemical polymerization of dopamine was carried out to build dendrite-free surface with strong adhesion of polydopamine ( Figure 6A). 120 This in situ polymeric SEI is facilitated with ample functional groups and remarkable hydrophilicity for uniform zinc ion flux, zinc nucleation, and high conductivity. As a consequence, the zinc anode delivered high Coulombic efficiency of 99.5%, high areal capacity at high current density (30 mA h/cm 2 at 30 mA/cm 2 , Figure 6B), and impressive cyclability. Moreover, the electronic resistivity of the SEI layer was detected by the voltage-current response of the protected zinc anode ( Figure 6C), revealing an electronic resistivity of 7.46 × 10 6 Ω cm (σ = 1.34 × 10 −7 S/cm) for the necessary potential gradient for zinc-ion diffusion. This work sheds new light on the in situ polymerization strategy for stable and effective SEI formation, stimulating the further development of in situ strategies. Meanwhile, some inorganic composites were put forward with meliorative substrate or other element doping. It is reported that a facile replacement reaction was triggered at the zinc anode surface between the zinc metal and the indium chloride solution (Figure 6D), after which the SEI layer was fabricated through electrochemical activation. 121 The SEI layer with 3D interdigitated zinc anode/SEI architecture was characterized with amorphous indium hydroxide sulfate in the interphase, contributing to the high ionic conductivity, large zinc ion transference number, and extraordinary electronic resistivity. Impressively, no direct contact between the zinc anode and mild electrolytes and the stable host for the accommodation of volume change led to the outstanding performance with an extremely small overpotential of 10 mV even at the high current density of 20 mA/cm 2 and high areal capacity of 20 mA h/cm 2 . This outstanding electrochemical performance stimulated the related development for high rate capability and superior energy density. Moreover, an in situ grown ZnSe layer 54 decorated on the one side of the commercial zinc anode was exploited through chemical vapor deposition ( Figure 6E), revealing an optimized electrode|electrolyte interface with promising and available scalability. This derived ZnSe layer induced the oriented deposition of zinc anode following the (0 0 2) plane at the initial plating/stripping process, accompanied by long cyclability of 1530 h at 1 mA/cm 2 and 1 mA h/cm 2 . The constructed full cells based on the V 2 O 5 and zinc anode with in situ SEI exhibited splendid capacity retention of 84% after 1000 cycles. The zincophilic in situ grown ZnSe cultivator on the zinc anode may promote the practical application featured with versatility and simplicity.
In addition to the impact of zinc anode well design for the in situ SEI formation, electrolyte additives were proved with benefits to the in situ SEI formation, due to the novel ingredient appearance resulted from the zincophilic element in the electrolytes. For instance, a zinc fluoriderich organic/inorganic hybrid SEI was observed in an acetamide (Ace)-Zn(TFSI) 2 eutectic electrolyte (ZES). 122 Attributed to the low binding energy with metal ions and prior decomposition sources, the dissociated TFSI − was available, compared to the conventional ions of BF 4 − and PF 6 − . With Ace as the hydrogen bond donor, the homogeneous and transparent electrolytes were obtained with satisfactory thermal adaptability in the operational temperature region. As a result, the symmetric cell presented a gradually decreased overpotential from 55 to 39 mV in the ZES electrolytes with long stability after 1000 h ( Figure 6F). Compared to the uneven surface of the zinc anode in the 1 M Zn(TFSI) 2 , the surface in the ZES electrolytes indicated a homogenous deposition of zinc ions. It is verified from the DFT geometry that [ZnTFSI(Ace) 2 ] + species with bidentate coordination exhibited a uniform molecular electrostatic potential energy surface distribution with a comparatively low binding energy ( Figure 6G,H). Furthermore, time-of-flight secondary-ion mass spectrometry ( Figure 6I) was applied to figure out the surface components information, which pointed out that ZnF 2 increased gradually with sulfides and nitrides decreased. The results implied that the inner SEI mainly contained ZnF 2 and the outer SEI mainly contained S/N-based composites. Therefore, these in situ SEI strategies provide a novel solution to tackle a variety of dilemmas that appeared in the multivalent metal ion batteries.

Ameliorative electrolytes
Although there are a variety of strategies proposed to homogenize the zinc ion flux and electric field at the zinc surface, including the above states about optimization of zinc electrode architecture, novel zincophilic 3D structures, and artificial SEI construction, the possible crack of artificial SEI, difficult formation of in situ SEI, and limited zincophilic substrate still confined these strategies for zinc anode with high performance. Therefore, apart from the electrode design and the artificial SEI construction for dendrite-free anodes, multitudinous efforts were concentrated on the development of advanced electrolytes for dendrite-free zinc anode. To solve the challenges resulted from the aqueous electrolytes with serious HER, side reactions, and low Coulombic efficiency in the zinc anode, strategies based on the electrolytes were put forward and resultful for dendrite-free zinc anode. 41,42,123,124 In this section, the modifications of electrolytes for further actual application were symmetrically summarized in the aspects of aqueous electrolytes and nonaqueous electrolytes.

Aqueous electrolytes
It is well known that present ZBs are usually fabricated with neutral or mild acidic electrolytes, avoiding the irreversible reaction of ZnO formation and the resulted low Coulombic efficiency in alkaline electrolytes on the zinc anode. In this regard, various inorganic salt additives and concentration controlling have been verified for the effective suppression of dendrites formation and HER with pernicious by-products. Specifically, continuous efforts on aqueous electrolytes mainly contain the three aspects of inorganic salt additives, high-concentration electrolytes (water-in-salts) or localized high-concentration electrolytes, and gel or solid-state electrolytes. 125,126 In detail, the inorganic salt additives and concentration controlling of the electrolytes could adjust the ion flux and Zn 2+ solvation, accompanied by the possible decomposition forming the SEI layer on the surface. 127 Moreover, the gel electrolytes enable the ZBs with superior flexibility, increasing the Coulombic efficiency and regulating the formation of dendrites. In comparison, solid-state electrolytes could mitigate the side reactions occurring at the electrode|electrolyte interface but arising the issues of low ionic conductivity and interfacial compatibility. Great attentions to the aqueous electrolytes amelioration have been paid at present based on the above aspects. So far, ZnSO 4 , ZnClO 4 , ZnCl 2 , ZnF 2 , Zn(NO 3 ) 2 , and other zinc-based salts have been developed as main active electrolytes salts for ZBs. 41 Among them, ZnSO 4 is the most common electrolytes in ZBs, compared to the NO 3 − with strong oxidizability, 128 ClO 4 − with high overpotential, ZnF 2 with low solubility, 129 and ZnCl 2 with a narrowly stable potential window avoiding the Cl 2 production at the high potential. 130,131 At present, ZnSO 4 becomes popular in the ZBs, 132,133 due to the comparatively wide potential window and anodic stability. To further improve the electrochemical performance of ZnSO 4 electrolytes for dendrite-free zinc anode, the addition of inorganic salts in aqueous electrolytes is an effective strategy to address these issues in aqueous electrolytes. For instance, Zn(H 2 PO 4 ) 2 134 salt was introduced into the aqueous electrolytes without the side reactions and dendrites formation, leading to a dense, stable, and highly zinc-conductive in situ SEI layer on the zinc anode surface ( Figure 7A). With a thickness of 140 nm for hopeite layer on the zinc electrode, this zinc anode exhibited uniform and rapid zinc ion transfer kinetics without severe HER, passivation, and corrosion. It is testified by linear polarization measurements ( Figure 7B) that the corrosion potential of the decorated zinc anode increased from −0.949 to −0.945 V compared with the bare zinc anode, implying less possibility of corrosion reactions. Furthermore, the corrosion current was reduced by 372.63 μA/cm 2 , agreeing well with the tendency of corrosion potential for a low corrosion rate. This work strengthens the importance of the industrial phosphating coating technique through hopeite for stable metal anode. Typically, the lanthanum nitrate La(NO 3 ) 3 as supporting salt for ZnSO 4 aqueous electrolytes was prepared for circumventing the short life and inferior performance raised by zinc anode. 135 Through the chronoamperogram test in a three-electrode cell ( Figure 7C), the concentration change of electroactive species could be characterized. It is proved that the steady current density in ZnSO 4 electrolyte with a fast response to the overpotential of −200 mV (−26 mA/cm 2 ), is higher than that in La 3+ -ZnSO 4 electrolyte (−22 mA/cm 2 ) with a prolonged activation time. This difference could be attributed to the La 3+ adsorption on the electrode surface, leading to the decreased active nucleation sites and slow nuclei formation. With the addition of La 3+ addition, the competitive adsorption of the inert La 3+ on the surface of the Zn electrodes could avoid the electric double layer repulsion between the Zn deposits, contributing to the coherent electrodeposition of the Zn deposition of (0 0 2) facet ( Figure 7D).
In addition to these inorganic salts as support salts in the electrolytes, the electrolyte concentration or local concentration controlling also violently affects the electrochemical performance of zinc anode. Typically, a concept of anion concentration gradient layer was proposed by Wen et al. 136 through the in situ reaction between the zinc anode and the sulfonic acid polymer. A promoted ionic conductivity was realized by concentration gradient and high bulky sulfonate concentration, contributing to the fast and even zinc ion flux. Besides, numerous covalent organic frameworks 137 and metal-organic framework 104,138 were proven with the effective suppression of dendrites to maintain a supersaturated electrolyte layer on the surface, which enable the different solvation structure with that in bulk electrolytes for even zinc deposition. More importantly, high-concentration electrolyte development was carried out through the supporting Na salt (18 M NaClO 4 ) addition in the aqueous electrolytes. 139 The supporting salt of NaClO 4 not only exhibited an exothermic peak at −68.5 • C ( Figure 7F) for a low freezing point with crystal formation of a salt-water eutectic mixture but also showed the scarcity of free water molecules, low viscosity, and high ionic conductivity for the prevention of dendrites. According to the radial distribution functions (Figure 7G), the water molecules dominate the first and second solvation shells of zinc ions with two sharp peaks at 2.05 and 4.25 Å, whereas they appeared at 2.05 and 3.04 Å for highly concentrated electrolytes, indicating that the ClO 4 − replaced partial water molecules in the second hydration shell. Therefore, the changed solvation structure could result in the reversible and uniform zinc stripping/plating.

Nonaqueous electrolytes
Restricted with limited solubility, low Coulombic efficiency, and narrow electrochemical stable window, the development of traditional aqueous electrolytes is hindered by these bottlenecks for practical constructions of ZBs. Therefore, nonaqueous electrolytes for zinc anode with high reversibility, no side reactions, superior recyclability, and excellent Coulombic efficiency were exploited through different additions of organic salt additives or novel organic solvents. 29,53,140,141 To be specific, Zn(CH 3 COO) 2 , Zn(TFSI) 2 , and Zn(CF 3 SO 3 ) 2 are the common organic salts for zinc-based electrolytes, which generally present complex by-products around the elec-trode|electrolyte interface but could broaden the working voltage window and improve the Coulombic efficiency with satisfactory kinetics of zinc stripping/plating behaviors to some extent. Notably, hydrous organic electrolytes design not only could promote the in situ formation of a favorably effective passivation layer to suppress the zinc dendrites formation and side reactions but also could embrace excellent nonflammability. 142 Additionally, hydrous organic electrolytes could devote to the decreased water reactivity and interfacial side reaction suppression, which could sustain stable thermodynamic performance. 143 Furthermore, the organic salts could act as support salts in aqueous electrolytes, presenting a changed solvation of zinc ions. For instance, Qiao et al. 144 reported that ethylene diamine tetraacetic acid tetrasodium salt (Na 4 EDTA) added into the ZnSO 4 electrolytes could prevent the dendrites formation and HER around the zinc anode, dominating the active sites for HER and water electrolysis. As shown in Figure 8A, the EDTA molecules could strongly adsorb on the electrode surface, replacing H 2 O adsorption on the surface due to the higher adsorption ability and active sites covering compared with the H 2 O molecules. The EDTA molecules contributed to the desolvation of Zn 2+ by dominating the solvation sheath of Zn 2+ because of the strong interactions between the EDTA and zinc ions. This unique solvation structure could ameliorate the interruption of H 2 O during the stripping/plating process, promoting uniform nucleation on the zinc anode surface. Obviously, the HER was suppressed by the addition of Na 4 EDTA, in which the current response shifted negatively to −1.12 V (Figure 8B), compared with the baseline electrolytes (1 M H 2 SO 4 ). Noticeably, a low-cost additive of glucose was proposed to regulate the Zn 2+ solvation structure for highly reversible plating/stripping ( Figure 8C). 145 It is demonstrated that the glucose could regulate the solvation of zinc ions and electrode-electrolyte interface, with replacing one H 2 O molecules from the Zn(H 2 O) 6 2+ structure. The decreased number of active H 2 O molecules, restrained side reactions, and active water decomposition are devoted to the comparable rate capability and remarkable cyclability of 2000 h at 1 mA/cm 2 and 1 mA h/cm 2 . In addition, Qiu et al. 146 revealed a solid zinc ion conductor based on the tailored eutectic liquids formed by bipolar ligands and organic zinc salts. Nano-TiO 2 performed as nucleation seeds resulted in the Zn(TFSI) 2 -based DES to form the crystal nuclei along the surface of the nanoparticles at ultralow undercooling ( Figure 8D), which weakened ionic associations and produced a high zinc transference number of 0.64. The resultant zinc anode displayed reversible Zn stripping/plating for over 4000 cycles. Inspired by the above strategies, the coupling functions of metal-organic framework (MOF) and organic salt additives were discovered for high-performance zinc anode ( Figure 8E). 147,148 In detail, a thin MOF layer decorated on the zinc anode and the hydrophobic Zn(TFSI) 2 -tris(2,2,2-trifluoroethyl)phosphate (TFEP) organic electrolyte were utilized to achieve long lifespan and high energy density. As a consequence, the obtained ZnF 2 -Zn 3 (PO 4 ) 2 SEI layer was formed to protect zinc anode and separate the electrode from the aqueous electrolytes, ascribed to the synergistic influence of MOF encapsulated Zn(TFSI) 2 -TFEP.
To eliminate the influence of interfacial effect between the electrode and electrolyte, the ionic liquid, quasisolid-state, and all-solid-state electrolytes were developed to circumvent this issue. 149 In terms of gel electrolytes, a zwitterionic gel poly(3-(1-vinyl-3-imidazolio) propanesulfonate) 150 was reported with multifunctional charged groups of sulfonate and imidazole, which could even the zinc nucleation and induce the deposition plane to (0 0 2). Besides, the zinc ion solvation structure could be manipulated by the charged groups to prevent the by-product formation and HER, along with a dendrite-free anode. Further, a solid polymer electrolyte of a poly(vinylidene fluoride-co-hexafluoropropylene) film filled with poly(ethylene oxide)/ionic-liquid-based Zn salt 48 ensured a stable electrochemical stability window and avoided HER occurrence ( Figure 8F). It is certified that the effective electrolytes design could contribute to the hydrogen-free, dendrite-free zinc anode with long cyclability over 1500 h at 2 mA/cm 2 and high Coulombic efficiency of almost 100%. However, this ionic liquid electrolyte application may affect the practical industrialization, ignoring the advantages of ZBs with low cost and simple manufacture. There is still a long way to construct the practical ZBs with high electrochemical performance, high safety, and competitive cost.

Decorated separator for uniform zinc ion flux
Non-electrode components for ZBs, such as the abovementioned SEI, electrolytes, current collectors, and separators, play an underestimated important role in the construction of ZBs with high electrochemical performance. Specially, the separators function as physical separations between the anode and cathode, which are supposed to possess the stability in the specific electrolytes, chemical stability, and mechanical strengths toward different pH values. [151][152][153] Presently, there are several common separators that appeared in the recent works on ZBs, contain-ing glass fiber, filter papers, and nonwoven fabrics. 151,154 Among the numerous separators, the glass fiber separators with highly porous architecture and beneficial wettability devote to the high ionic conductivity after electrolytes uptaking 360% but still suffer from the high cost and fragile properties. 155 In contrast, though the filter papers show low cost and comparable mechanical strengths, 156 their practical applications are still stuck by limited performance with uneven pores. Furthermore, nonwoven separators were usually constructed through electrospinning and melting spraying with a directional or random network structure. 157,158 Nevertheless, their high porosity (60%-80%), large pore sizes (20-50 μm), and hydrophobic property impede their scalable application for ZBs.
In this part, advanced separators were discussed, owing to their enormous impact on the ionic conductivity and complex interfacial chemistry near the electrode and electrolyte interfaces. Typically, it is reported that a Janus separator is equipped with parallelly grown graphene sheets with the modification of sulfonic cellulose on one side of glass fiber separator. 159 In Figure 9A,B, it is illustrated that the preparations of sulfonic cellulosegrafted/graphene-coated Janus separators were successfully achieved through spin coating in different configurations. In addition, the sulfonic acid groups (-SO 3 H) contributed to the preparation of sulfonic cellulose with an impressive adsorption impact of metal ions. Moreover, the hydrogen evolution polarization curves ( Figure 9C) proved a lower hydrogen evolution current density of zinc electrode with Janus separators, compared to the bare zinc electrode with glass fiber, indicating an effective strategy for suppression of corrosion reactions. As a result, the well-designed zinc anode is attributed to the high capacity and long cyclability in contrast with the related works on Zn//MnO 2 batteries ( Figure 9D). Besides, a Janus separator was fabricated through chemical vapor deposition with vertical graphene grown on one side of commercial glass fiber separators ( Figure 9E). 160 The constructed separators could help lower local current density and homogenize ion distributions for the spatial expansion of zinc deposition. As shown in Figure 9F, a direct plasma-enhanced chemical vapor deposition was applied to in situ grow vertical graphene nanosheets on glass fiber with CH 4 as carbon sources. Subsequently, air plasma treatment was dedicated to surface oxygen and nitrogen functional groups formation on the vertical graphene. Meanwhile, a simplified 3D model was performed to represent that such 3D vertical graphene scaffold could homogenize the electron and ion distributions ( Figure 9G). Additionally, zeolites (ZSM-5) could present an effective impact on zinc anode with zinc-ion uniform transportation and dendrite-free zinc deposition. Zhu et al. 157 reported that this ZnSO 4 -ZSM-5 mixed electrolyte with 3D channel system could lead to the F I G U R E 9 Advanced separators for uniform zinc ion flux: (A) schematic of the preparation of a Janus separator by spin coating for different devices; (B) synthesis of sulfonic cellulose by grafting sulfonic acid groups on the cellulose backbone; (C) hydrogen evolution polarization curves for zinc electrode with glass fiber and Janus separators; (D) comparison of the battery performance of zinc-based batteries (ZBs) with Janus separators and other related Zn//MnO 2 batteries; (E) schematic of the designed Janus separators for lowering local current density and homogenizing ion distributions; (F) schematic representation of synthetic process of Janus separator; (G) electric field distribution of 3D scaffold structure/Janus separators; (H) schematic of typical reaction process of bare zinc anode in ZnSO 4 electrolyte and regulating water activity using a ZSM-5 membrane; (I) 3D channel structure of ZSM-5; (J) Tafel plots of zinc anode with glass fiber or ZnSO 4 -ZSM-5 mixed electrolyte without separator. Source: Reproduced with permission from Refs. [157,159,160]. Copyright 2020 2022, Wiley-VCH. uniform zinc ion flux and transport through the regulation of the solvent structure and zinc-ion transport ( Figure 9H), in which the ZSM-5 molecules sieve was composed of two types of vertical cross channels and "Z"-shaped channels with 5.4 Å ( Figure 9I). The linear polarization measurements ( Figure 9J) revealed a reduced corrosion current density (i corr ) for strong corrosion resistance and low corrosion reaction rate. However, these strategies for protecting zinc anode based on the separators still lack systematic and deep investigation for dendrite-free mechanism.

CONCLUSIONS AND PERSPECTIVES
Environmental awareness and fossil resources exhaustion have stimulated the pursuit for green and sustainable energy sources. Up to now, ZBs are going into their booming prosperity, featured with high safety, low cost, and simple manufacture without severe environmental conditions, especially for the humidity and oxygen content. However, the direct utilization of zinc metal as anode has arose multiple challenging issues hindering their further practical application, due to the dendrite formation, severe HER, and complex side reactions. Therefore, various engineering strategies for dendrite-free zinc anode have been proposed to address the challenges, which have been emphasized in this review for four aspects, including the electrode structure design, different SEI layers on the electrode|electrolyte interface, various electrolytes, and decorated separators. For zinc electrode architecture design, 3D network, alloying and nonmetal anode based on the "rocking-chair" mechanism have been certified effectively for dendrite-free zinc anodes. In terms of SEI exploitation, novel artificial protective layers and in situ formation of SEI were verified for effectively separating the electrode from the electrolytes and preventing the dendrites formation. In addition, electrolyte optimization has been proved with effective additives, novel solvent, and other solid-state electrolytes for zinc-ion solvation controlling and suppression of HER, whereas decorated separators could even the zinc ion flux and homogenize the electric and ionic field. It is worth noting that only if addressing the challenging issues of zinc anode, low cost, high energy density, and power density, the following industrial scalability could be feasible. In our opinion, there are some challenging perspectives on the zinc anode proposed to accelerate the practical applications of ZBs ( Figure 10) as follows: (i) Efficient regulation of interfacial ionic transfer and electric field. The electric field and ionic transfer kinetics are devoted to the electrochemical reactions at the surface of zinc anode, because the electric field is the only driving force for zinc nucleation during the stripping/plating process. Although there are plenty of 3D networks and ionic conductive layers developed for zinc anode, it is still unclear when it comes to the industrial scalability, owing to the lack of rational design of electrode architecture with explicit mechanisms, simple manufacture, and convenient availability. On the one hand, the uniformity of 3D networks and the optimal spatial network size are neglected in the recent reports, the extra mass of inactive materials of which was detrimental to the energy density of the full cells to some extent. It is worth noting that hot-dip galvanization, an effective approach inducing the zinc surface coating through the immersion in molten zinc, could tightly bond the coating and substrate. On the other hand, the interfacial transfer resistances caused by the interfacial induced layers are increased, contributing to the battery failure at a high rate with a weak strength and uneven zinc stripping/plating in the case with a large capacity. Impressively, metal-free zinc anode 161 could be a promising candidate for eliminating the detrimental impact of zinc metal anode.
Hence, host materials are supposed to be characterized with zincophilic nature, beneficial to the zinc-ion transfer and stable mechanical strength during zinc stripping/plating. (ii) Precise controlling of the Zn 2+ solvation structure. Generally, zinc ions with comparatively high charge state, perform as solvated Zn 2+ surrounded by six water molecules, which will shuttle between the cathode and anode electrodes in aqueous electrolytes. It should be pointed out that the Zn 2+ solvation structure could be changed with the highconcentration electrolytes, [162][163][164][165] accompanied by the increased viscosity of the electrolyte. The decreased ionic conductivity attributes to battery break-off at high rates, leading to the inferior energy density and power density. Remarkably, DESs and ionic liquid-based electrolytes possess a promising potential for high-performance ZBs with controllable water molecules, the adjusted ratio of each component for appropriate density, conductivity, solubility, and viscosity. Furthermore, advanced gel and solid-state electrolytes with highly porous or hierarchical structures could improve the ionic conductivity and zincion migration kinetics to some extent. Importantly, the electrolyte additives with comprehensive functions could benefit to tailor the solvation structure, de-solvation promotion, and other realizable performances, which are regarded as an important direction for future mainstream research study in laboratory and practical industrialization. Considering the ionic conductivity, cation transfer, electrochemical stability window, interface stability, mechanical property, and scalability in practice, the coupling of above strategies with the minimization of water-related side reactions would promote the synergistic effects to fulfill low-cost, eco-friendly, and high electrochemical performance of ZBs. (iii) Utilization of interactive effect of the different components. It is well known that once one of the zinc anode problems is solved, other issues will be relieved to some extent. The challenges for dendrite-free zinc anode are not independent, which could be addressed from a holistic and comprehensive perspective. In the view of synergy, dendrite growth could accelerate the side reactions, including corrosion, passivation, and hydrogen evolution, along with the increased electrode polarization. In this aspect, advanced electrode design collaborating with optimized electrolytes would help achieve the double-effect inhibition of dendrite growth and severe HER. 166,167 Further, an "all-in-one" strategy combining the structural design, interface modification, and electrolyte optimization will help alleviate the zinc nucleation and dendrite formation. (iv) Deep and comprehensive understanding of dendrite-free stable zinc anode mechanisms based on the in situ characterizations and theoretical calculation. To be specific, the performance and kinetics of zinc anodes could be characterized through electrochemical testing, electron microscopy, and spectrum characterizations. The essential parameters, such as open-circuit voltage, operating voltage, energy efficiency, specific capacity, energy density, power density, and cycling stability, could be evaluated by the galvanostatic charging and discharging methods. [168][169][170] Further, the individual resistance, ionic transport number, and electrode polarization values could be characterized by electrochemical impedance spectroscopy and three-electrode testing through the Tafel curves, chronoamperogram characterization, or other test characterization technologies for verification of zinc deposition behavior. In addition, the in situ and ex situ techniques of electron microscopy and spectrum characterizations could devote to the in-depth understanding of visual and structural representations of the zinc anode. Although significant progress has been realized on dendrite-free zinc anode, the inner fundamental mechanisms are still vague without reasonable explanation. In consideration of the differences for SEI formation and interfacial zinc deposition chemistry between the alkaline and mild acid or neutral electrolytes, rational experiments, simulations, and theoretical calculations are supposed to be taken for fundamental principles of zinc nucleation and dendrites growth. Especially, real-time characterizations, such as in situ X-ray diffraction, transmission electron microscope, and Xray absorption spectroscopy, could observe an instant electrode change in the morphology, mechanical, or chemical characterizations during the stripping/plating process. Moreover, the relationship among the internal stresses, dendrite growth, and the influence to the surrounding zinc deposition remains elusive. Accompanied by the study on DFT, kinetics calculations on LUMO levels, and zinc ion diffusion energy barriers, the in-depth mechanism could be comprehensively figured out. More importantly, through the in situ characterizations combined with simulation calculation, the SEI formation mechanism and their changes in zinc deposition will be accurate, which can induce a superior construction of SEI and dendrite-free zinc anode.
In summary, great significance is put on the electrode architecture, electrolyte/electrode interface reconstruction, novel electrolytes for suppression of dendrite and HER in alkaline and mild acid electrolytes, preventions of corrosion and side reactions with by-products, and their interactive effect. Further, the feasible key directions and challenging perspectives in the regulation of interfacial ionic transfer and electric field, precise controlling of the Zn 2+ solvation structure, and utilization of interactive effect are proposed to boost the electrochemical performance of zinc anodes for rechargeable ZBs. Therefore, we believe that this review will shed new light on the direction of full-spectrum research progress of stable dendrite-free zinc anode and also present the reliable ways to accelerate future practical industrialization of high-performance ZBs.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.