Electrolyte and Additive Engineering for Zn Anode Interfacial Regulation in Aqueous Zinc Batteries

Aqueous Zn‐metal batteries (AZMBs) have gained great interest due to their low cost, eco‐friendliness, and inherent safety, which serve as a promising complement to the existing metal‐based batteries, e.g., lithium‐metal batteries and sodium‐metal batteries. Although the utilization of aqueous electrolytes and Zn metal anode in AZMBs ensures their improved safety over other metal batteries meanwhile guaranteeing their decent energy density at the cell level, plenty of challenges involved with metallic Zn anode still await to be addressed, including dendrite growth, hydrogen evolution reaction, and zinc corrosion and passivation. In the past years, several attempts have been adopted to address these problems, among which engineering the aqueous electrolytes and additives is regarded as a facile and promising approach. In this review, a comprehensive summary of aqueous electrolytes and electrolyte additives will be given based on the recent literature, aiming at providing a fundamental understanding of the challenges associated with the metallic Zn anode in aqueous electrolytes, meanwhile offering a guideline for the electrolytes and additives engineering strategies toward stable AZMBs in the future.


Introduction
Over the past few decades, the heavy consumption of fossil fuels for both daily use and industrial production has resulted in much environmental damages, while the increasing global population are still constantly raising the energy demands.It is DOI: 10.1002/smtd.202300268urgent to develop clean energy resources, such as wind power, [1] solar power, [2] tidal power, [3] as supplements or even replacements for the conventional energy resource from fossil fuels.However, the intermittent nature of these clean energy resources makes them difficult to be integrated into the power grid.Thereby, energy storage systems become the key technology to manage the power generation and electricity demands, which help to achieve low-carbon and sustainable development of human society.Secondary batteries are the most prominent in this regard and many secondary battery systems have emerged in the past decade, among which lithiumion batteries (LIBs), [4][5][6][7] sodium-ion batteries (SIBs), [8][9][10][11] and zinc-ion batteries (ZIBs) [12][13][14][15][16] are the most popular ones.Although the former two are studied more commonly, especially LIBs, regarding their relatively sophisticated technology and dominance in electric vehicle and consumer electronics markets, they still have a few drawbacks, e.g., the limited availability and expensive cost of lithium resources, the comparably low energy density of SIBs, and the toxicity of organic liquid electrolytes used in both LIBs and SIBs.27] Aqueous Zn-metal batteries (AZMBs), with aqueous electrolytes and metallic zinc anode, exhibit several inherent advantages over LIBs and SIBs, that the aqueous batteries manifest intrinsic safety, [16,[28][29][30][31][32][33] high ionic conductivity compared with organic electrolyte or gel electrolyte, [34] low cost, [35][36][37][38][39] facile manufacturing process, [28,31,[40][41][42][43] and non-toxicity. [16,32,44,45]Metallic Zn possess high volumetric and gravimetric theoretical capacities (5854 mAh cm −3 or 820 mAh g −1 ), [46,47] low redox potential (−0.76 V vs standard hydrogen electrode (SHE)), [48] and high overpotential for hydrogen evolution, [49,50] rendering it a preferable anode material superior to the majority of intercalation-type materials in aqueous electrolytes.Kang et al. first established the concept of zinc-ion or zinc-metal batteries in 2011, by revealing the reaction mechanism of zinc ions intercalating in MnO 2 in a mild aqueous electrolyte. [51]However, the practical application of AZMBs is still largely hampered by the interfacial issues involved with aqueous electrolytes and metal anode employed, including dendrite growth, [52] hydrogen evolution reaction (HER), [53] and zinc corrosion or passivation. [38,53]As in other metal-based batteries, the unceasing cycling of Zn plating and stripping results in unavoidable formation of dendrites in aqueous electrolytes.Although the Zn dendrites in AZMBs may not cause safety hazards of fire or explosion as those in LIBs or SIBs with organic electrolytes, their uncontrollable growth can still accelerate the capacity fading and shorten the battery life. [54]Unfortunately, the negative effects of these interfacial issues are mutually entangled and their synergetic effects may further exaggerate the problems. [55,56]hile the dendrites formed at the anode surface provide an expanded interface for HER, [55] HER tends to change the concentration of OH − near the zinc anode and facilitate their reaction, thus producing inert corrosion byproducts. [57]In return, the byproducts of corrosion and HER can induce nonuniform local electric field and concentration gradient on the coarse deposition surface, creating more zinc dendrites nucleation sites. [58]here have been a number of methods developed to address these interfacial issues involved in aqueous zinc metal batteries, such as coating a protection layer, [59][60][61][62] constructing a threedimensional (3D) [63][64][65] or 2D anode structure, [66,67] and introducing zinc alloy anode, [65] etc.However, the large-scale applications of these approaches are still limited despite their proved effectiveness in improving the interfacial stability between Zn anode and aqueous electrolytes and reversibility of metallic Zn.Instead, the engineering of aqueous electrolytes is a simple and affordable method to enhance the cycling stability of AZMBs.This strategy can be achieved by designing the electrolyte formulation and/or adding electrolyte additives, where the former is determined by the selection of zinc salts and solvent (water) concentration, while the latter depends on the modified interactions between solvents, anode surface, and Zn 2+ ions.Impressively, many cheap and environmental-friendly aqueous electrolytes and additives have been developed, further extending the intrinsic advantages of AZMBs.Herein, a comprehensive summary of the aqueous electrolyte engineering strategies will be given based on AZMBs, including the mechanisms behind electrolyte/anode interfacial issues, aqueous electrolyte formation, and electrolyte additives de- veloped so far.This article manages to provide a profound insight into the aqueous electrolyte engineering strategies and guide the future research directions of AZMBs.

Configuration and Working Principles of AZMBs
As other secondary batteries, an AZMB is composed of four essential components, namely electrodes, an electrolyte, a separator, and current collectors at each electrode of the battery that connects to the terminals of the cell, illustrated by Figure 1.The electrodes, including cathode and anode, host zinc ions and provide capacity for the battery cell, where Zn 2+ ions from Zn anode repeatedly intercalate/deintercalate in the cathode materials during the discharging/charging process.The typically used cathode materials contain a variety of compounds, including vanadium-based, [68][69][70] manganese-based, [71][72][73][74] prussianblue-based, [75] carbon-based, [76] and organic-based, [77][78][79] while the anode material in AZMBs is metallic Zn.The separator is usually a porous polymetric film that permits the pass-through of Zn 2+ but blocks the exchange of electrons in the internal circuit.The electrolyte in AZMBs acts as a conductive medium allowing Zn 2+ shuttling between electrodes.This component presents the most obvious difference between Zn-metal batteries and other metal-based batteries, that aqueous electrolytes are commonly chosen in AZMBs in contrast to the organic liquids used in LIBs or SIBs.
In 1986, it was the first time that Yamamoto's group introduced the mildly acidic ZnSO 4 aqueous electrolyte and the MnO 2based aqueous zinc battery exhibited decent cycle stability, [80] where the Zn 2+ ions from zinc anode combined with water molecules in aqueous electrolytes, thus forming Zn(H 2 O) 6

2+
to move within the ion conducting medium.The aqueous electrolytes in zinc-ion batteries can also be alkaline, that KOH aqueous electrolyte is the most traditional one. [81]Compared to neutral or weakly acidic aqueous electrolytes, alkaline aqueous electrolytes are subject to severe side reactions with MnO 2 cathode and irreversible consumption of Zn 2+ , leading to fast capacity decay and short lifespan of full-cell batteries.Therefore, recent research has shifted the primary focus on the development of neutral and mild aqueous electrolytes in AZMBs, while various salts have been studied, e.g., Zn(NO 3 ) 2 , [82] ZnCl 2 , [83] ZnSO 4 , [84] Zn(BF 4 ) 2 , [85] Zn(CH 3 COO) 2 , [86,87] Zn(ClO 4 ) 2 , [88,89] Zn(CF 3 SO 3 ) 2 , [90] and Zn(TFSI) 2 . [91]At the moment, the energy density of the 18650-type commercial LIBs has reached 250 Wh kg −1 or 670 Wh L −1 , [92] while that of SIBs usually lies in the range of merely 75-165 Wh kg −1 or 250-375 Wh L −1 . [93][96] Although AZMBs have not realized their commercial application up till now, they are still very tempting for future production and market due to their attractive advantages.

Alkaline Electrolyte
The metallic zinc anode loses two electrons to Zn 2+ ions, which react with OH − in the alkaline electrolyte and form Zn (OH) 2 as a start.Zn(OH) 2 keeps reacting with OH − that ends up with Zn (OH) 4  2− , resulting in its increasing concentration until a critical value, then forming Zn(OH) 2 at the electrolyte/electrode interface [97] and eventually turning into ZnO covering the surface of anode.This process can lead to an irreversible loss of Zn anode by forming a thick layer of electrochemically inactive ZnO. [98]uch abovementioned reactions in alkaline electrolytes can be illustrated by the following equations: (1) Furthermore, the general reaction mechanism can be described as: [99] Discharging Overall : Zn

Charging
ZnO

Neutral and Mild Electrolytes
In neutral or mildly acidic aqueous electrolytes, Zn metal anode is in a different environment from the alkaline electrolytes, where metallic Zn loses an electron and become Zn 2+ to combine with H 2 O octahedral, thus forming [Zn (H 2 O) 6 ] 2+ .Due to the large size of [Zn(H 2 O) 6 ] 2+ , this complex has to remove the outer water layer before inserting into cathode or plating on Zn anode.These following equations explain the reactions that occurred at the anode in neutral or mildly acidic electrolytes: Furthermore, the general reaction mechanism could be simply represented by the following equations: Discharging Charging While the reaction mechanisms of zinc anode under different pH environment have been introduced, the following content in this review article will primarily concentrate on the remaining challenges in neutral and mild aqueous electrolytes, emphasizing the corresponding strategies to mitigate the Zn anode instability.

The Challenges of Zinc Anode in Aqueous Electrolytes
Due to the merits of high-capacity metallic Zn anode combined with non-flammable aqueous electrolytes featured by high conductivity, AZMBs exhibit both high energy density and improved battery safety.However, a few problems still need to be tackled, e.g., depletion of active zinc mass and unstable battery cycling, which are mainly ascribed to the undesirable issues at the interface of the Zn metal anode and aqueous electrolytes.These interfacial issues include (1) the formation of zinc dendrites or "dead Zn"; (2) HER; and (3) corrosion and passivation.Figure 2 is a schematic diagram illustrating each one of them, as well as their mutual relations.Although the specific mechanisms behind these interfacial issues may be slightly different in alkaline electrolytes or neutral and mild electrolytes, they are ubiquitous in AZMBs and significantly deteriorating the cell performance and lifespan.

Zn Dendrite Growth
Metal dendrites growth may be the most notorious problem in alkali-metal batteries, that the undesirable formation and drastic growth of Zn dendrites are responsible for the rapid decline of capacity and battery failure in AZMBs.To effectively design modification strategies to mitigate or even eliminate such an issue, thoroughly understanding the mechanisms behind the process of Zn dendrites formation and growth is of great importance.In general, the formation of zinc dendrites is evoked by the ununiform electrodeposition of Zn on the nonideal surface of Zn anode during the repeated stripping/plating process.At the initial stage, the deposition of Zn 2+ is decided by the transport kinetics of Zn 2+ in liquid phase mass transfer near the anode.During charging, Zn 2+ ions near the surface of the metallic Zn anode are drawn toward the anode under the drive of electric field force, then reduced to metallic Zn with electrons obtained from zinc anode and deposited on the nucleation sites if the nucleation overpotential is overcome. [100,101]The surface-adsorbing Zn atoms can freely migrate in plane and accumulate with other constantly generated ones at the energetically favorable charge transfer sites at the anode surface, forming an undulate surface morphology consisting of multiple small bumps of deposited Zn. [102][103][104] These small bumps are the so-called initial Zn core.Meanwhile, a concentration gradient is created between the electrolyte/anode interface and the bulk electrolyte because of the deficiency of Zn 2+ ions that have been extracted to the metallic Zn, which causes a shift in potential from the equilibrium potential of electrode.Such a deviation of potential can promote the reduction of Zn 2+ near the elevated area on the surface of anode or the previously deposited Zn where the surface energy is lower. [58,105]The uneven surface after the initial deposition and formation of Zn cores, induces the nonuniform electric field, especially at the tips of the Zn protrusions.The electric field intensity at those tips is much stronger than elsewhere due to the accumulated surface charge at the area with greater curvature, [106][107][108][109] that further facilitates the deposition of Zn 2+ at the tips and rapid growth of dendrites, known as "tip effect".In terms of high surface energy, the active nucleation sites for Zn 2+ ions also include dislocation, boundaries, and surface impurities, in addition to the tips of Zn cores. [110]Therefore, Zn dendrites formation and growth process are essentially determined by the ununiform ion concentration, uneven electric field, and surface morphology of metal anode, which are directly associated to the electrolyte formulation, current density, charging time, temperature, and other side reactions occurred at the anode interface. [111]s Zn dendrites continue to grow from the anode toward the cathode, they either end up penetrating the polymer sepa-rator and causing short circuit of the battery cell, or fracture into small pieces of zinc bulk that are insulated from the electrode mass, forming "dead Zn".These fractured dendrites dispersed in the electrolyte substantially increase the interfacial impedance and reduce the Coulombic efficiency (CE) and anode capacity, while aggravating its reaction with electrolyte and generation of byproducts. [112]Meanwhile, the rampant growth of Zn dendrites provides a breeding bed for other side reactions, e.g., corrosion and HER, because of the increased specific surface area of the loose structure and rough surface of anode. [113]

Hydrogen Evolution Reactions
HER is a unique and grievous problem in AZMBs that unceasingly occurs during the rest and operation of the battery cells.Hydrogen evolution exhibits distinct mechanisms in different electrolytes.In alkaline electrolytes, it tends to occur ahead of Zn deposition owing to the lower standard reduction potential of Zn/ZnO (−1.26 V versus SHE) than that of hydrogen evolution (−0.83 V vs SHE), which water preferably obtain electrons in oxidized state. [114]In other words, Zn and H 2 O that coexist in the AZMBs with alkaline electrolytes are thermodynamically unstable and prone to react and release hydrogen. [115]The related reactions are shown as Equations 16 and 17, Intuitively, the same condition should apply in neutral and mild electrolytes, considering the standard reduction potential of Zn/Zn 2+ (−0.76 V vs SHE) is still lower than that of H + /H 2 ((0 V vs SHE), with their reactions shown in Equations 18 and 19. [116] 2+ + 2e − ↔ Zn (−0.76 V) (18) However, the reduction Zn 2+ ions happens in advance of HER in neutral and mild electrolytes because of the high HER overpotential on Zn metal, [117] sluggish surface kinetics, [113] and low activity of H + .According to the Tafel equation:  = blogi + a ( equals to the HER overpotential, a is the constant value, b is called Tafel slope which is also a constant value and i stands for the current density).For different metals, the value of b almost remains unchanged (≈0.12 V), while the constant a for zinc metal in aqueous solutions is comparably high, leading to the high HER overpotential on Zn metal surface and suppression of hydrogen generation.Then again, although the rate of HER may be limited in some way in neutral and mild electrolytes, when the Zn deposition voltage surpasses the electrochemical stability window of water, the evolution of hydrogen is still unavoidable.In the commonly used electrolyte of 2 M ZnSO 4 aqueous solution, for instance, the Zn plating process takes place at a voltage lower than −0.15 V versus Zn/Zn 2+ , which is outside the electrochemical stability window of water (−0.05 to 1.7 V vs Zn/Zn 2+ ). [118]In practice, HER in neutral and mild electrolytes is also subject to kinetics factors, such as the polarization during charging, [102,119] hydrogen concentration, [120] operating temperature, [121][122][123] and current density, [124] etc.In other words, HER can also be significantly facilitated by the non-uniform Zn deposition and non-ideal anode surface.
HER deteriorates the battery cells in many ways: (1) the irreversible HER shares electrons with the reversible deposition of Zn 2+ that lowers the battery reversibility and current efficiency; (2) the unceasing generation of hydrogen consumes both zinc anode and aqueous electrolytes, resulting in reduced CE and capacity of the batteries; (3) the adsorption of evolved hydrogen gas on Zn anode surface impedes the nucleation of Zn and induces large overpotential [62] and non-uniform deposition; [125] (4) the gradual buildup of hydrogen gas raises the internal pressure of the cell, [29] causing the inflation of battery package and electrolyte leakage, [46] while the evolution of flammable gas carries the risk of fire and explosion; (5) A local alkaline environment can establish when HER depletes the protons in the vicinity of anode/electrolyte interface, that promote anode corrosion and formation of passivation layer. [120]

Corrosion and Passivation
Both chemical corrosion and electrochemical corrosion can happen to the metallic Zn anode in aqueous electrolytes.The chemical corrosion, or referred as self-corrosion, is prevalent in alkaline electrolytes owing to the more positive redox potential of HER (−0.83 V vs SHE, Equation 17) than that of Zn/ZnO (−1.26 V vs SHE, Equation 16), that the reaction between the metallic Zn with water is thermodynamically spontaneous, producing H 2 and ZnO.It should be noted that the ZnO-contained byproducts tend to adhere to and roughen the anode surface, being unstable during the large volume change of Zn anode and subject to constant growth, which significantly raises the electronic impedance at the interface.In neutral and mild electrolytes, the electrochemical corrosion mechanism is dominant during the repeated cycling of charging and discharging, which irreversibly consumes the Zn metal anode, causing the loss of its active mass and inert byproducts on the surface. [55]The irreversible loss of active mass of anode mainly attributes to the Zn dendrites growth as well as emergence of "dead Zn", as illustrated in Section 3.1.Meanwhile, during discharge, the oxidation of metallic Zn induces concentrated Zn 2+ ions near the Zn surface and attracts anions from electrolyte, triggering the parasitic reactions and formation of corrosion products upon the establishment of loose and porou [Zn(OH) 2 ] 3 •ΧH 2 O(ZSH) layer.For example, in the most widely used ZnSO 4 -contained aqueous electrolyte, the following equations describe the formation of ZSH on metallic Zn. [54] Zn ↔ Zn 2+ + 2e − (20) While the electrochemical corrosion reactions directly consume the Zn anode and electrolytes causing capacity fading, the formation of insoluble byproducts, e.g., ZSH, can reduce the amount of active nucleation sites and lead to an uneven anode surface, which results in unregulated deposition of Zn and dendrites growth.Moreover, the diffusion of ions or electrons at the anode surface is significantly restricted due to the poor conductivity of the byproducts, further raising the energy barrier for Zn deposition and lowering the CE of the battery. [126,127]orse still, in contrast to the dense and homogeneous passivation layers usually formed by corrosion in strong alkaline electrolytes, [128] byproduct layers formed in neutral and mild electrolyte cannot act as solid electrolyte interphase (SEI) layers because they formed the loose structure which showed hexagonal layered stacking.Therefore, their loose structure couldn't effectively impede the direct contact between electrolyte and zinc anode. [54,125]And a series of side reactions are still occurring at the Zn anode/electrolyte interface.So, it is important to find effective approaches to address these interfacial issues in AZMBs, not only to mitigate the severe damages they caused, but also to suppress their mutual effects which significantly accelerate the battery failures.Researchers have proposed numerous solutions to those issues in recent years.The modification approaches typically follow three types of technique routes, that are surface regulation, structure construction, and electrolyte engineering, while the former two strategies are based on the design of Zn anode.For example, Renjie Chen's group.created nitrogen (N)doped graphene oxide (NGO) to obtain a parallel and ultrathin interface modification layer (≈120 nm) on Zn foil, that the N-NGO@Zn||LiMn 2 O 4 pouch cells maintained a high energy density at depth of discharge of 36%, i.e.,164 Wh kg −1 after 178 cycles. [129]Besides, Chengjun Xu's group introduced the layerby-layer structure zinc anode (Sn/Cu/Zn) to achieve high performance zinc-ions battery, [130] where the Sn/Cu/Zn composite anode showed improved cycle performance (over 250 h at 0.5 mA cm − 2 ) than the bare zinc anode.However, both the surface regulation and structure construction strategies have their limits, for example, the complex process, resulting in the increase process time and energy consumption.Moreover, structure collapse of Zn anode may still happen after long cycling time, leading to poor cycling stability.Therefore, it is difficult for them to realize large industrial applications.On the other hand, the electrolyte formulation could be an ideal approach to make up for those disadvantages that talked above mentioned.They are easy to formulate, consuming less time and energy, leading to low cost, and exploring new and green electrolyte formulation is also comparably reasonable for companies, electrolyte formulation has great potential for large industrial application.

Aqueous Electrolytes
Currently, zinc sulfate (ZnSO 4 ), zinc trifluoromethanesulfonate (Zn(CF 3 SO 3 ) 2 ), and zinc bis (trifluoromethylsulfonyl)imide (Zn(N(CF 3 SO 2 ) 2 ) 2 ) (known as Zn(TFSI) 2 ), are most commonly used in mild electrolytes because of their stability during plating/stripping process. [131]However, water existed in the solvents tend to decompose at increased operating voltage and raises a series of problems.Therefore, the activity of water should be restrained concerning the inhabitation of parasitic reactions.[139] On the other hand, deep eutectic solvents (DES) are offered as another choice, [122,[140][141][142] that change the solvation structure of Zn 2+ and desolvation energy, largely improved compared to the highly active [Zn(H 2 O) 6 ] 2+ in normal aqueous electrolytes.The summary of aqueous electrolyte engineering will be divided into three parts in this review, i.e., the dilute/concentrated electrolyte, the WiS electrolyte, the DES electrolytes (Figure 3).They are essential for manipulating reactions occurring at the electrolyte/anode interface and have huge impacts on the performance of AZMBs.

Dilute/Concentrated Electrolytes
With extensive choices of zinc salts including ZnSO 4 , [143] ZnCl 2 , [83] and Zn (CH 3 COO) 2 , [86] etc., the concentration of solutions acts a crucial role in electrochemical properties of the aqueous electrolytes, that dilution or concentrated electrolytes are defined by different water content.Each Zn 2+ ion in dilute electrolytes is coupled to 4 to 6 water molecules, and this high concentration of water enables quick ion conduction in the solution.A low-concentration perchlorate-based salt called 3 M Zn (ClO 4 ) 2 electrolyte was investigated by Hang Zhou and colleagues.This electrolyte displayed a high conductivity of 4.23 mS cm at extremely low temperatures (−50 °C) (Figure 4a), [89] indicating its superior anti-freezing ability and stable operation.However, the overly abundant water also results in various problems, for example, the limited electrochemical stability window aroused from water decomposition.As a proof, cathode dissolution was observed in conventional dilute electrolyte (1 Mm ZnSO 4 or 1 Mm Zn(TFSI) 2 ) due to the insufficient electrochemical stability window. [75]On the contrary, the electrolyte with low water content (1 m Zn(TFSI) 2 + 21 m LiTFSI) highly suppresses the activity of water owing to the extensive coordination, that results in stable open framework of Zn storage and high reversible capacity of 60.2 mAh g −1 over 1600 cycles (Figure 4b). [75]Strong interactions between the water molecule around Zn 2+ and Zn 2+ result in a high energy barrier for the deposition of Zn 2 on the anode.Meanwhile, the excess water molecules in electrolyte would compete with Zn 2+ for electrons, thereby worsening hydrogen evolution and deteriorating the CE of the batteries.By reducing the amount of water, Bing Joe Hwang's group prepared the concentrated aqueous electrolyte   [89] Copyright 2022, Wiley.b) 1 m Zn (TFSI) 2 + 21 m LiTFSI concentrated aqueous electrolyte's cycling efficiency of Zn||NiHCF batteries.Reproduced with permission. [75]Copyright 2021, Wiley.c) Diagram showing the formation of passivation layer Zn 4 (OH) 6 SO 4 •xH 2 O and the solution structure of Zn 2+ during electrodeposition in dilute electrolyte (left) and corresponding concentrated electrolyte (right).d) SEM photography of the surface morphology following 100 iterations of plating and stripping process.e) Using concentrated aqueous electrolyte, the ability of Zn on Cu foil to be stripped off the metal and the coulombic efficacy of Zn||Cu cell.Reproduced with permission. [134]Copyright 2020, Amer Chemical Society.f) Zn||stainless steel cell linear sweep voltammetry at a scan rate of 10 mV s −1 .Reproduced with permission. [146]Copyright 2023, Amer Chemical Society.efficiency of 99.21% over 1000 h under 0.2 mA cm 2 (Figure 4e), [134] while the corresponding dilute electrolyte ) only provided an average columbic efficiency of 97.54% and relatively short cycle life, indicating the alleviated interfacial stability and improved cyclability using concentrated electrolyte, as illustrated by Figure 4c.In addition to extending the lifespan of AZMBs, some of the researches have broadened the electrochemical windows of the electrolytes.A multi-hydroxyl polymer (polyethylene glycol, PEG) cosolvent was introduced by Jin zhou's group. 2 m of zinc trifluoromethanesulfonate (Zn(OTf) 2 ) in PEG/H 2 O (50 vol% PEG + 50 vol% H 2 O) was investigated and the zinc metal batteries could be operated over wide temperatures between −20 and 80 °C. [144]when the temperature is low, PEG molecules tend to absorb on the zinc metal, suppress the zinc dendrite growth, and when the temperature is high, PEG molecules help suppress the parasitic reactions.Organohydrogen electrolytes (OHEs) are prepared by swelling freeze-dried hydrogels of poly (2-acrylamide-2-methylpropane sulfonic acid)/polyacrylamide in a binary solvent electrolyte of ethylene glycol and water (EG/H 2 O, water content 10% v/v) containing ZnCl 2 /NH 4 Cl, it endowed zinc-metal batteries wide operating temperature (−30 to 80 °C) with excellent capacity retentions of 88.8% after 1500 cycles at −30 °C and 44.8% after 1000 cycles at 80 °C. [145]Hybrid electrolytes of water and a polar aprotic N, N-dimethylformamide is utilized and forms Zn 5 (CO 3 ) 2 (OH) 6 solid electrolyte interphase (SEI) on the Zn surface to achieve good performance of zinc-metal batteries over a wide temperature range.Bing Joe Hwang's group utilized a highly concentrated salt electrolyte (HCE) with dual salts (1 m Zn(OTf) 2 + 20 m LiTFSI), [146] that the electrochemical stability window was expanded up to 2.856 V, as shown in Figure 4f.Another example is the concentrated nitrate electrolyte (2.5 m Zn(NO 3 ) 2 + 13 m LiNO 3 aqueous solution) with DMA diluted, [147] and the widest electrochemical window is determined to be 3.1 V. Based on these studies, it appears feasible to raise the salt concentration of aqueous electrolytes to broaden their electrochemical stability window and improve interfacial stability.

Water-in-Salts (WiS) Electrolytes
Compared to either dilute or concentrated electrolytes, i.e., saltsin-water electrolytes, where the amount of water still greatly outnumbers that of zinc salts in solution, water-in-salts electrolytes contain very little water content, that its volume and mass are significantly lower than those of zinc salts. [148]By minimizing the free water molecules contained, WiS electrolytes manifest widened electrochemical stability windows. [149]In 2015, Suo et al. designed an electrolyte solution with incredibly high salt content, where the cation solvation shell changed because of insufficient water to neutralize the cation charge, thereby achieving ESW of 3 V and successfully suppressing hydrogen evolution and electrode oxidation. [150]Jiri Cervenka's team prepared a WiS electrolyte based on Zn(ClO 4 ) 2 for Zn g-1raphite dual-ion batteries, which has wide voltage window of 2.80 V. [151] The chaotropic ClO 4 − anions in the water-in-salts electrolyte inhibit the ability of water reacting with other compounds, meanwhile reshaping the solvation shell of Zn 2+ due to the interaction between ClO 4 − anions and H 2 O.In addition, the high concertation of Zn 2+ ions prevent Zn anode corrosion, enabling a low overpotential of Zn 2+ plating/stripping (Figure 5a) and high cut-off voltage (2.5 V Zn/Zn 2+ ) (Figure 5b) in aqueous Zn-graphite dual-ion battery.And they also applied ClO 4 -based WiS electrolyte Al(ClO 4 ) 3 as the suitable electrolyte with cathode material graphite, resulting in the wide electrochemical window of 4.0 V. [153] Unfortunately, the impact of various concentrations of ClO 4 − anions on solvation shell of H 2 O was not fully explained.Chunsheng Wang's group introduced aqueous electrolytes based on 1 M Zn(TFSI) 2 with various concentrations of LiTFSI. [152]Using Fourier transform infrared (FTIR) (Figure 5c) and nuclear magnetic resonance (NMR) spectroscopies (Figure 5d), they examined the relationship between various aqueous electrolyte concentrations and the interaction between Zn 2+ ions, H 2 O, and TFSI − .The water molecules may become severely confined within the Li + solvation structures at higher LiTFSI concentrations, which significantly reduces the amount of water near Zn 2+ ions.In order to theoretically study the Zn 2+ solvation structure in aqueous electrolytes with 1M Zn (TFSI) 2 and three concentrations of LiTFSI ((5 m, 10 m, and 20 m), Molecular Dynamics (MD) stimulations were also performed through the polarizable APPLE&P force field (Figure 5e).When the concentration of LiTFSI increased to 10 m, the water surrounded by Zn 2+ ions was replaced by anions simultaneously.
With CE close to 100%, this WIS electrolyte (1 m Zn (TFSI) 2 + 20 m LiTFSI) demonstrated the highly reversible and dendritefree Zn plating/stripping process.The hybrid Zn-LiMn 2 O 4 battery using such a WIS electrolyte exhibited 85% capacity retained after 400 cycles, while the Zn/O 2 battery manifested a high energy density of 300 W h kg −1 for 200 cycles.Recently, another highly concentrated salt electrolyte (HCE) with dual salts (1 m Zn(OTf) 2 + 20 m LiTFSI) showed outstanding capacity retention at 92% after 300 cycles and an average CE of 99.62% in Zn-LiMn 2 O 4 cells. [152]In sharp contrast to the average CE of 96.91%, the battery using low-concentration electrolyte (LCE) deteriorated quickly and short-circuited after 66 cycles. [146]

Deep Eutectic Solvents (DESs)
Although WiS electrolytes have demonstrated its broadened electrochemical stability window and enhanced interfacial stability with metallic Zn, their application is still hindered by the high cost arising from the increased amount of LiTFSI used.Deep eutectic solvents (DESs) become a promising candidate considering their similar benefits as WiS electrolytes but much lower cost.In addition, DESs can be easily formulated and tend to maintain stability in a wider range of temperatures above and below room temperature. [154]DES is composed of two or more components, which is a eutectic mixture of Lewis or Brønsted acids and bases containing a variety of anionic and/or cationic species, [155] or considered as a free-flowing solution consisting of two or more solid organic materials, that has a melting temperature lower than that of an ideal liquid mixture. [156]DESs are facile to prepare, that the molar ratio between components or water content can be adjusted to meet different requirements.The solvation shell structure of Zn 2+ ions can be altered thereby, when a specific quantity of water is introduced into DES, which results in an increased Zn 2+ ion conductivity.Mai's group developed a novel eutectic electrolyte with special solvation structure, containing ethylene glycol (EG) and ZnCl 2 , that achieved the dendritefree ZIBs with long cycle life. [157]With the optimized molar ratio of ZnCl 2 /EG (1: 4), the ionic conductivity of DES was maximized to 1.15 mS cm −1 .The EG within the DES interacted with Zn 2+ and changed their solvation structure, resulting in the creation of [ZnCl(EG)] + and [ZnCl(EG) 2 ] + complex cations.Their decomposition induced a Cl-rich organic-inorganic hybrid solid electrolyte interphase film formed on the metallic zinc anode surface (Figure 6a), that assists the reversible plating and stripping of Zn.Therefore, the Zn symmetric cell exhibited highly reversible Zn plating/stripping and long-term stability of 3200 h at 1 mA cm −2 and 1 mAh cm −2 (Figure 6b), while the polyaniline||Zn cell manifested good cycling stability of 10 000 cycles with 78% of capacity retained.Guanglei Cui's group designed a new electrolyte named "water-in-DES", that contains approximately 30 mol% H 2 O in a eutectic mixture of urea, LiTFSI, Zn(TFSI) 2 . [122]In such a "waterin-DES" electrolyte, the nature of deep eutectic solvents was inherited despite of the addition of water content (Figure 6c), that effectively suppressed the water activity due to the interaction between DES and water, while the advantages of aqueous electrolytes were also maintained benefiting the ionic conductivity and viscosity.The high-voltage Zn/LMO batteries using "waterin-DES" electrolyte demonstrated the stable cycling of 600 cycles ) Graph illustrating the connection between cut-off potentials, released capacities, and coulombic efficiencies.Reproduced with permission. [151]Copyright 2022, The Royal Society of Chemistry.c) The evolution of the FTIR spectrum between 3800 and 3100 cm −1 with rising salt concentration.d) The modification of 17 O nuclei's chemical shifts in solvent as a result of variations in salt content.e) Molecular dynamics analyses of the Zn 2+ -solvation structure.f) The cycling stablity and CE of Zn/LiMn 2 O 4 full cell in HCZE at 4 C. Reproduced with permission. [152]Copyright 2018, Springer Nature.g) The cycle performance of Zn/LMO full cell in HCE and LCE at charge rate of 0.1 C and discharge rate of 0.2 C. Reproduced with permission. [146]Copyright 2021, American Chemical Society.at 0.5 C with over 80% of capacity retained, or with over 90% retained after 300 cycles at 0.1 C (Figure 6d).Likewise, Zhanliang Tao et al. prepared a similar "water-in-DES" electrolyte based on ZnCl 2 , acetamide, and water, that had benefits of both ionic liquid and concentrated electrolytes. [141]This "water-in-DES" electrolyte changed the solvation structure of Zn 2+ and reduced its desolvation energy barrier, thereby lowering the nucleation overpotential and favoring uniform Zn nucleation (Figure 6f).The formation mechanism of ZnCl 2 -acetamide-H 2 O was investigated through spectra analysis and density functional theory (DFT) calculations, that [ZnCl(acetamide) 3 ] + was found to be tetrahedral structure (Figure 6e).The superior cycling stability of this "water-in-DES" electrolyte was proved by the Zn/Ti cell, which worked for 1000 cycles with an average CE of 98%.The corresponding Zn||PNZ full-cell battery exhibited a stable cycling for 10 000 cycles, with capacity retention of 85.7% to the initial 72.3 mA h g −1 .However, despite of the effectiveness of DES electrolytes in suppression of side reactions, most of them can only work under low current densities, typically from 0.05 to 0.5 mA cm −2 , which are much lower than that of dilute electrolytes.
Except for the abovementioned three categories of aqueous electrolytes, there are some other special aqueous electrolyte .Reproduced with permission. [157]Copyright 2022, Wiley-VCH.c) Results from Raman analysis for the LZ-DES/nH 2 O and pure LZ-DES (the Li/urea ratio of 1:3.8).d) Comparison of the Zn/LMO cell cycling ability in LZ-DES/2H 2 O at different rates to that in 0.5 m LiTFSI + 0.5 m Zn (TFSI) 2 .Reproduced with permission. [122]Copyright 2019, Elsevier.e) DFT calculations of the optimized coordination structures of [ZnCl(acetamide) 3 ] + and [ZnCl(acetamide) 2 (H 2 O)] + ,respectively.f) Schematic representations of the unaltered, electrostatic shielding mechanism and enhanced solvation shell-based Zn nucleation and growth process, respectively.Reproduced with permission. [141]Copyright 2021, Wiley-VCH.
formulations.For example, a hybrid electrolyte containing an equal amount of water and glycerol was prepared by Zhang et al., [158] that enabled the flat and smooth Zn metal surface after the repeated stripping and deposition of Zn 2+ , without dendrites formed.The added glycerol has a high affinity to the Zn metal and strong binding interactions with Zn 2+ that adjusts its solvation-shell structure in hybrid aqueous electrolyte to form (Zn (OH 2 ) 2 (C 3 H 8 O 3 ) 3 ) 2+ ions with less coordinated water.Thereby, it is effective to inhibit the side reactions at the electrolyte/anode interface.With homogeneous deposition and nucleation of Zn on metal surface, the Zn||Zn symmetric cell manifested a stable cycling of more than 1500 h at 1 mA cm −2 via addition of glycerol, while the Zn‖CaV 6 O 16 •3H 2 O (CaVO) full cells provided a high-capacity retention of 86.6% with a reversible capacity of 136 mAh g −1 after 400 cycles.

Electrolyte Additives
In addition to the electrolyte formulation, aqueous electrolyte additives can also alleviate the side reactions at the anode interface, [158] broaden the electrochemical stability window, [159] suppress the dendrite growth, [115,160] and limit the diffusion mechanism, [161] thereby improving the interfacial stability. [162]lthough additive is not necessarily the constituent part of an aqueous electrolyte, additive-contained electrolytes exhibit much longer cycle life compared to those without additives.Electrolyte additives provide a low cost, eco-friendly, and easy-operating solution to the interfacial stability issues, which favors the large-scale industrialization of AZMB.These additives can be broadly categorized into ionic additives, [163,164] organic additives, [103,[165][166][167] inorganic additives, [168,169] and metal additives. [112]Ionic additives have the most versatile functions yet simple preparation, that they can not only modify the interfacial stability, but also enhance the ionic conductivity of the aqueous electrolytes.][178] While inorganic and metal additives are less studied compared to the other two due to their limited solubility, they are proved to largely improve the performance of aqueous electrolytes in metallic zinc anode even at a very little concentration. [168,179]onsidering the complicated classification by composition, the electrolyte additives will be distinguished and introduced based on the mechanisms, mainly divided into five types: inducing nucleation on anode surface, [180][181][182] creating electrostatic shield, [134,183,184] adjusting solvation structure, [185][186][187] regulating deposition orientation, [160,[188][189][190] in situ building SEI layer [191][192][193][194]  (Figure 7).97][198]

Inducing Nucleation
In most cases, the accumulation of deposited Zn 2+ ions ascribe to the uneven electric field and restricted nucleation sites on the metallic Zn anode surface.Therefore, the key to suppress formation and growth of Zn dendrite and occurrence of interfacial side reactions, lies in the establishment of uniform surface electric field and the creation of evenly spread of nucleation sites with refined nucleation size.
Jiaqian Qin et al. added a small amount of graphene oxide (GO) as an additive into the ZnSO 4 electrolyte, [199] that successfully eliminated the Zn dendrites and achieved highly stable Zn anode during cycling.Due to the preferable electrostatic interactions between GO particles and metallic Zn, as well as its firm adherence on the surface, the distribution of the electric field is homogenized.Meanwhile, the deposition of Zn 2+ is guided by the oxygen-containing polar groups on the surface of GO parti-cles, that quickly conduct Zn 2+ to the anode surface to form a uniform Zn deposition layer.As a matter of fact, both the nucleation overpotential and charge transfer resistance on the Zn anode surface were largely reduced by the GO electrolyte additives, while additional nucleation sites are induced and evenly distributed on the anode surface, as illustrated by Figure 8a.Compared to the uncontrolled electrodeposition of Zn in the pristine electrolyte, which usually results in uneven anode surface and formation of dendrites, a smooth and flat surface of Zn anode can be maintained during the battery cycling, owing to the improved nucleation process and enhanced reaction kinetics by adding GO additives.As a result, the cycle life of Zn symmetric cell with GO added was improved to five times longer than that with pristine electrolyte, while the Zn||Ti battery with GO achieved a high CE of 99.16% over 100 cycles at 1 mA cm −2 and 1 mAh cm −2 , with polarization of only 116 mV.Han et al. reported the graphene quantum dots (F-GQDs) as an electrolyte additive. [200]Under the action of highly electronegative polar groups (-OH, -COOH, -NH 2 , and -SCN), F-GQDs preferentially adsorb on the Zn anode surface to render a highly hydrophilic surface with low nucleation energy, and homogeneous distribution of electric field (Figure 8b).F-GQDs are acting as active nucleation sites for Zn 2+ , that regulate the uniform Zn deposition at a nano scale and prevent the .Reproduced with permission. [200]Copyright 2021, American Chemical Society.b) Schematic of in situ formation of F-GQDs on anode surface under the action of polar groups with abundant nucleation sites provided.c) Cycling performance of symmetric Zn cells in electrolytes with or without F-GQDs added, at 0.5 mA cm −2 & 0.25 mAh cm −2 , and 10 mA cm −2 & 5 mAh cm −2 , respectively.Reproduced with permission. [201]opyright 2023, Elsevier.d-f) SEM imaging of cross-section morphology of Zn anodes in d) pristine electrolytes after 100 h cycling, and e) EDTAadditive electrolytes after 100 h cycling, and f) EDTA-additive electrolytes after 500 h cycling.g-i) SEM imaging of surface morphology of Zn anodes in g) pristine electrolytes after 100 h cycling, and h) EDTA-additive electrolytes after 100 h cycling, and i) EDTA-additive electrolytes after 500 h cycling.j) Cycling performance of the symmetric Zn cells at 2 mA cm −2 and 2 mAh cm −2 with the corresponding.k) CE of Zn plating/stripping process.l) Cycling performance of Zn||V 2 O 5 battery at 2 A g −1 .Reproduced with permission. [202]Copyright 2022, American Chemical Society.
particle aggregation and formation of Zn dendrites.The Zn||Zn symmetric cell with F-GQDs cycled more than 1800 h at a current density of 0.5 mA cm -2 , or 450 h at 10 mA cm −2 (Figure 8c), while the Zn||Ti battery exhibited a high average CE of 99.6%, demonstrating the improved stability of Zn anode with F-GQDs additive.The Zn|MnO 2 full-cell battery using F-GQDs-added electrolytes delivered an initial specific capacity of 254.6 mAh g −1 at 1 A g −1 , with 200.1 mAh g −1 retained after 500 cycles and a capacity retention of 78.6 %.Xie et al. reported ethylenediaminetetraacetic acid (EDTA) as an electrolyte additive by creating active nucleation sites for Zn deposition. [201]Due to the Zn affinity of EDTA, it forms an adsorption layer on the Zn anode surface, that exhibited enhanced driving force for Zn nucleation and abundant nucleation spots under the action of strong chelation between EDTA and Zn 2+ .Zn 2+ ions tend to deposit in a large area of the anode surface, with reduced critical grain size from the very beginning of the Zn plating, guiding the formation of a dense and smooth deposition layer on Zn, as shown in (Figure 8d-i).As a result, the Zn||Zn symmetric cell with 0.04 m EDTA in ZnSO 4 electrolyte cycled stably for 3000 h at 2 mA cm −2 and 2 mAh cm −2 (Figure 8j), with CE of 99.68% (Figure 8k), while the Zn||V 2 O 5 full cell exhibited outstanding rate performance of 258 mAh g −1 at 2 A g −1 and improved cyclability of 199.7 mAh g −1 after 500 cycles at 2 A g −1 (Figure 8l).Besides, other than alleviating the dendrite growth, additives may also have the ability of widening the operation electrochemical window.For example, dimethyl sulfoxide(DMSO) additive was added into a type of multi-component crosslinked hydrogel electrolyte proposed by Bingang Xu's group, the full cell equipped with this type of electrolyte could be operated under −40 °C to 60 °C.

Electrostatic Shielding Effect
Some of the electrolyte additives can establish an electrostatic shield in the vicinity of protuberance on the uneven anode surface, that adsorbs on the high local current density sites prior to the Zn 2+ , thus engendering the electrostatic repulsion to approaching Zn 2+ and shifting the deposition to the adjacent flat regions.
A cationic surfactant-type electrolyte additive, tetrabutylammonium sulfate (TBA 2 SO 4 ), was first proposed by Changbao Zhu et al. [178] During the plating process of Zn, the nonredox TBA + ions preferentially adsorb near the humps on surface of Cu substrate, forming a zincphobic shielding film that regulates the initial Zn nucleation and suppresses the formation of dendritic morphology due to the electrostatic force against hydrated Zn 2+ in the electrolytes.In this way, both lateral diffusion of Zn 2+ and "tip effect" are largely restricted (Figure 9a), that a uniform Zn deposition with flat surface can be observed on the Cu foil, in sharp contrast to the highly dendritic and mossy Zn protrusions grown without TBA 2 SO 4 added.The electrostatic shielding effect of the zincphobic TBA + cations was also verified by DFT calculations, that the depositing Zn 2+ ions have to overcome an energy barrier of ≈0.55 eV to pass through the TBA + shielding layer (Figure 9b).Likewise, tetramethylammonium sulfate (TMA 2 SO 4 ) electrolyte additive was reported by Dunmin Lin et al., [184] which also effectively inhibited the stacking of deposited Zn on the Zn anode surface, as well as side reactions of corrosion and HER, by shifting the Zn 2+ deposition from the protrusion tips to other regions.The surface morphology of Zn foil was investigated via SEM imaging, that a flat surface with well-ordered Zn deposition layers was observed after cycling in TMA 2 SO 4 -added ZnSO 4 aqueous electrolyte for 100 h, presenting the best result compared to others containing different aqueous electrolytes additives (Figure 9c-h).The long-term cycling stability of symmetric Zn||Zn cell was demonstrated after adding a small amount of TMA 2 SO 4 (0.25 mm L −1 ) into the ZnSO 4 electrolyte, exhibiting a cycle life of 1800 h at 0.5 mA cm −2 with 0.5 mAh cm −2 (Figure 9i).The Zn||TMA 2 SO 4 @ZnSO 4 || MnO 2 full cell delivered a high initial capacity of 181.3 mAh g −1 at 0.2 A g −1 , with 98.72% of capacity retained after 200 cycles, as shown in Figure 9j.Reported by Hu et al., [203] the inorganic rare earth metal type electrolyte additive, cerium chloride (CeCl 3 ) was applied in AZMBs to alleviate the dendrite growth based on the electrostatic-shielding mechanism.By adding 2 g L −1 CeCl 3 in the 2 m ZnSO 4 aqueous electrolyte, a dynamic electrostatic shield was established on the anode surface, where the Ce 3+ acted as competitive cation against Zn 2+ , regulating the Zn 2+ deposition to the flat region for an even surface of Zn metal.This phenomenon was proved via finite element modeling (FEM) simulation, that Zn 2+ are moved to adjacent flat regions because of the electrostatic repulsion of concentrated Ce 3+ near tips (Figure 9k).The Zn symmetric cell with ZnSO 4 +CeCl 3 electrolyte exhibited an enhanced cycling stability of 2600 h at 2 mA cm −2 , with CE of 99.7%, while the LiFePO 4 ||Zn full cell delivered an adequate discharge capacity of 84 mAh g −1 at 2 C with 80% of its initial capacity retained after 400 cycles, (Figure 9l).

Adjusting Solvation Structure
Instead of shuttling between the electrodes in a single-ion form, the Zn 2+ ions tend to coordinate with six free H 2 O molecules in aqueous solutions and exist in a configuration of [Zn (H 2 O) 6 ] 2+ cluster.The active water molecules released from this solvation structure of hydrated complex, in the electrodeposition of Zn 2+ , can easily trigger various side reactions at the electrolyte/anode interface and deteriorate the interfacial stability. [204]Thereby, modifying the solvation structure of hydrated Zn 2+ is essentially effective to alleviate the side reactions and dendrite-free Zn anode.
By adding 0.2 wt.% PAM in 1 m ZnSO 4 , the Zn||Cu cell delivered a high CE over 99.65% under 2 mA cm −2 , 2 mAh cm −2 , as well as stable cycle life over 1300 h (Figure 10c).Wang et al. introduced a small amount of silk peptide as electrolyte additives to mitigate the parasitic reactions at the electrolyte/anode interface. [196]Verified by DFT calculations shown in Figure 10d, the highly soluble silk peptide contains abundant polar groups, such as -COOH and -NH 2 , that strongly interact with Zn 2+ and reduced the coordinated active H 2 O and SO 4 2− (Figure 10e).In this way, the side reactions of Zn anode are essentially suppressed, and a uniform and stable Zn deposition process is ensured as observed in Figure 10g.Benefiting from the electrostatic shielding effect of the silk peptide anchored on the anode surface, as well as the isolation between Zn and aqueous electrolyte (Figure 10f), the symmetric Zn||Zn exhibited stable cycling of 3000 h at 1 mA cm −2 and 1 mAh cm −2 with CE of 99.7% (Figure 10h).

Regulating Electrodeposition Orientation
Because the direction of crystal growth largely determines the surface morphology and dendrites formation, it is critical to control the Zn deposition orientation regarding its hexagonal close-packed (hcp) structure with high anisotropy platelets. [219]pecifically, as the typical Zn (101) and Zn (110) crystal planes are mostly vertically positioned against the Zn surface which benefits the dendrite growth, the (002) plane benefits the even Zn deposition due to the flat atom arrangement and even charge distribution. [220]By regulating the crystallographic orientation of electrodeposited Zn, the stability of Zn anode can be largely improved during the battery cycling. [221,222]ecently, Yingjin Wei's group reported a unique colloidal aqueous zinc electrolyte prepared by adding the oleic acid (OA) into a 2 m ZnSO 4 solution. [223]Instead of directly interacting with Zn 2+ and H 2 O or changing the solvation structure of Zn 2+ , such a ) n ] 2+ penetrating the TBA + cationic shielding on the surface of Zn metal anode with potential energy change with varying distance.Reproduced with permission. [178]Copyright 2020, American Chemical Society.c-h) SEM imaging of Zn anodes after cycling for c) 0 h, d) 100 h in pristine electrolytes without additives, e) 100 h with TMA 2 SO 4 , f) 100 h with TMAAc, g) 100 h with TMACl, and h) 100 h with TMANO 3 .i-j) Cycling performance of Zn symmetric cells at 0.5 mA cm −2 with 0.5 mAh cm −2 and Zn||TMA 2 SO 4 @ZnSO 4 || MnO 2 full cells at 0.2 A g −1 , respectively.Reproduced with permission. [184]Copyright 2022, Academic Press Inc., Elsevier Science.k) finite element modeling (FEM) simulation of Zn deposition after 2 min in Ce 3+ added electrolytes.l) Cycling performance of LiFePO 4 ||Zn full cell in ZnSO 4 +CeCl 3 electrolyte at 2 C. Reproduced with permission. [203]Copyright 2022, Wiley-VCH. 2− -polymer" bonding network and adjustment of space charge region of Zn anode with different functional polymer additives.c) Cycling performance of Zn anodes in 1 M ZnSO 4 aqueous electrolytes with and without 0.2 wt.% PAM additive at 2 mA cm −2 , 2 mAh cm −2 ; Reproduced with permission. [162]Copyright 2021, American Chemical Society.d) DFT calculations of bonding energies between Zn 2+ and different polar groups on silk peptide chains.e) Solvation structure of hydrated Zn 2+ with or without silk peptide.f) DFT calculation of adsorption energy of H 2 O and silk peptide on Zn (002) surface.g) In situ optical imaging of cross-section morphology of Zn deposition through time at 10 mA cm −2 .h) Cycling performance of Zn symmetric cells at 1 mA cm −2 and 1 mAh cm −2 .Reproduced with permission. [196]Copyright 2022, Wiley-VCH.
"temporary electrolyte additive" induces a stable OA spread on the metallic Zn anode surface due to the strong polar effect of -COOH group, that effectively guides the layer-by-layer deposition of Zn(002) horizontally on the anode surface, as shown in Figure 11a.While Zn platelets usually fail to deposit horizontally along the preferred (002) crystal plane because of the lat-tice mismatch between Zn (002) and polycrystalline Zn foil, the OA molecules can cling to the (002) surface of deposited Zn (Figure 11b), resulting in a flat and smooth anode surface.Meanwhile, the firm OA layer with hydrophobic alkyl chain protect the Zn metal anode from immediate contact with water in an aqueous electrolyte, further mitigating the side reactions at the Reproduced with permission. [223]Copyright 2023, Elsevier.e) Schematic of Zn deposition in ZnSO 4 (left) and ZnX 2 -PEG300 (right; X = Cl, Br, or I) electrolytes.f) SEM imaging of the large hexagonal Zn platelet formed in ZnI 2 -5%PEG300 electrolytes.g) XRD pattern of Zn deposits after growing for 24 h at 4 mA cm −2 .h) Cycling performance of Zn||Zn symmetric cells with different electrolytes at 25 mA cm −2 , 3.2 mAh cm −2 .Reproduced with permission. [224]Copyright 2022, Wiley-VCH.
anode/electrolyte interface.As a result, the Zn||Cu asymmetric cell using OA-added ZnSO 4 electrolyte manifested an ultra-long stable cycling over 3340 cycles and high CE of 99.63% (Figure 11c), while the Zn‖MnO 2 full cell provided a high discharge capacity of 215.1 mAh g −1 at 1A g −1 with capacity retention of 98.9% after 1100 cycles (Figure 11d).A short chain PEG300 was used as an additive in ZnI 2 electrolytes, which effectively regulated the crystal growth structure and deposition orientation of Zn on the metal anode surface, illustrated by Figure 11e, owing to the affinity of instantly formed PEG-Zn 2+ -aI − (a = 1,2,3) complexes to the (002) Zn crystal facets. [221,224]Compared to the sphere-like crystal structure of Zn grown in ZnSO 4 electrolytes, Zn deposits in ZnI 2 -PEG300 electrolytes are prone to form in a large hexagonal platelet shape (Figure 11f) and grow along the direction of 2D substrate with (002) surface exposed (Figure 11g).Thereby, the reversibility of Zn anode in ZnI 2 -PEG300 is largely improved compared to that in ZnSO 4 electrolytes, that the Zn symmetric cell (Figure 11h) with PEG300 added showed a stable cycling for over 4000 cycles and 1200 h at 25 mA cm −2 and 3.2 mAh cm −2 .

In Situ Forming SEI
There is another type of electrolyte additives decorating the metal anode surface through the in situ establishment of robust SEI layer, which not only isolates the Zn anode with water molecules in aqueous electrolytes, but also modifies the surface of anode to achieve the smooth deposition of Zn 2+ .A nonionic surfactant electrolyte additive, polyethylene glycol tert-octylphenyl ether (PEGTE), [179] was proved valid in in situ forming a H-ZnO passivation layer in honeycomb structure on the Zn anode surface, as shown in Figure 12a, protecting it from interfacial side reactions and regulating the distribution of surface electric field, as well as uniform Zn nucleation.Thereby, a uniform and dense layer of Zn deposition is ensured below the thin H-ZnO film (Figure 12b), as well as reduced contact to O 2 in aqueous electrolytes due the PEGTE-induced micelle particles.As a result, by additive 5wt.%PEGTE in aqueous electrolyte, the Zn metal achieved the improved cycling stability and reversibility for exceeding 2400 h or 1300 h at 5 mAh cm −2 (Figure 12c) or 10 mAh cm −2 , respectively.While an average CE of 99.2% was accomplished at 3 mAh cm −2 , Figure 12. a) Schematic of the Zn plating process in both pristine electrolyte (upper) and electrolyte with PEGTE-5 added (bottom).b) In situ optical microscopy tracking the Zn plating process in pristine electrolyte (upper) and in electrolyte with PEGTE-5 added (bottom).c) Cycling performance of symmetric Zn cells at 5 mA cm −2 and 5 mAh cm −2 .Reproduced with permission. 179Copyright 2022, American Chemical Society.d) In situ FEC-induced ZnF 2 -riched inorganic/organic hybrid SEI layer on metallic zinc anode surface.e-f) Cycling performance of e) Zn symmetric cells 4 mA cm −2 and 1 mAh cm −2 , and f) Zn||O d -NH 4 V 4 O 10 full cell at 5 A g −1 .Reproduced with permission. [192]Copyright 2023, Wiley-VCH.g) Schematic of TS-Ns-induced SEI film regulating Zn 2+ deposition.h) CE of Zn plating/stripping process in pristine electrolyte and TS-Ns-added electrolyte at 20 mA cm 2 and 5 mAh cm 2 .Reproduced with permission. [225]Copyright 2022, Elsevier.
the full cell Zn metal batteries using V 2 O 5 cathode provided an exceptional reversible capacity of 142 mAh g −1 at a low negative/positive capacity ratio (N/P ≈ 3), with stable cycling of 600 cycles.
Wu's group reported an electrolyte additive, [226] 2-methyl imidazole (Hmim), that is capable of in situ forming an inorganicorganic zinc-rich (Zn 4 SO 4 (OH) 6 /Zn(Hmim)) SEI film on the Zn anode surface.Both the side reactions and Zn dendrite formation are significantly restricted regarding the outstanding mechanical, chemical, and thermal stability of the SEI layer.Moreover, such an SEI layer exhibits chelation effect with Zn 2+ , that regulates the Zn deposition to plenty of active sites in smaller nuclei sizes.The improved Zn anode stability and reversibility were demonstrated by the ultra-stable cycling of symmetric Zn||Zn cell (2000 h at 2 mA cm −2 with CE of nearly 100%) and excel-lent performance of Zn-V 2 O 5 full cell, with high reversible capacity of 174.5 mAh g −1 after 400 cycles at 2 A g −1 , as well as CE of 99.63 %.Likewise, an inorganic/organic hybrid SEI (ZHS) layer rich in ZnF 2 was successfully built on the Zn anode surface to suppress the dendrite growth and adverse reactions, [192] via the addition of Fluoroethylene carbonate (FEC) in the ZnSO 4 aqueous electrolytes (Figure 12d).The HF component released from FEC, which is considered thermodynamically unstable in an aqueous environment, removes the zinc hydroxycarbonate passivation layer readily formed on the Zn anode surface and produce ZnF 2 .At the same time, the organic components from FEC can be further polymerized under the catalytic effect of the exposed Zn, that construct the ZHS layer on the anode surface.This ZHS layer serves as a multi-functional SEI film, which not only isolate the free H 2 O molecules and Zn bulk to inhibit the anode corrosion and HER, but also reduces the de-solvation active energy of Zn 2+ and facilitate Zn deposition kinetics.Consequentially, the SEI-coated Zn symmetric cell operated steadily for 1000 h at 4 mA cm −2 and 1 mAh cm −2 (Figure 12e), while the Zn||O d -NH 4 V 4 O 10 full cell remained a reversible capacity of 208 mAh −1 after 500 cycles at 5 A g −1 (Figure 12f).Wu Chao et al. introduced 2D ultrathin anionic tin sulfide nanosheets (TS-Ns) as an electrolyte additive, [225] that co-deposits with Zn 2+ to the Zn anode surface at the initial plating process and constructs an SEI layer in the following cycling to protect the Zn anode surface and guide uniform deposition of Zn during cycling (Figure 12g).To demonstrate the effectiveness of the in situ formed interfacial protection layer, a symmetric cell was assembled with an artificial interfacial layer consisting of TN-Ns coated on the Zn foil, which exhibited a cycle life almost ten times longer than that of PVDF-coated Zn cell.Correspondingly, the SS||Zn asymmetric cell delivered a high average CE of 99.6% over 500 cycles at 20 mA cm −2 and 5 mAh cm −2 (Figure 12h), while the Zn||Zn symmetric cell provided stable cycling for more than 3700 h at 0.2 mA cm −2 .

Conclusion and Perspective
Aqueous Zn-metal batteries (AZMBs) have become a promising technology for energy storage due to their intrinsic safety, abundant resources, as well as adequate energy density.However, the metallic zinc anode in aqueous systems still suffers from a series of interfacial issues, which include Zn dendrites and "dead Zn" formation, hydrogen evolution reactions, and zinc anode corrosion and passivation.These problems may mutually reinforce each other and cause huge deterioration in the performance of AZMBs.So far, several strategies of aqueous electrolytes engineering have emerged to stabilize the interface between electrolytes and Zn metal anode, as well as enhancing the reversibility of metal anode, which are based on two aspects: the manipulation of aqueous electrolytes and the regulation of electrolytes additives.The design of aqueous electrolytes mostly relies on the tunning of salts and concentration, that dilute and concentrated electrolytes, water-in-salts (WiS) electrolytes, and deep eutectic solvent (DES) electrolytes have been extensively studied and proved to mitigate the side reactions at the electrolyte/anode interface.On the other hand, various electrolyte additives are found effective to establish stable interfaces and achieve highly reversible Zn anode during the repeated plating and stripping process.The major mechanisms of electrolyte additive optimizing interface include: (1) inducing uniform Zn nucleation on Zn anode surface by providing abundant nucleation sites and homogeneous electric field distribution, (2) electrostatically shielding the protuberance on anode surface to avoid overlapped Zn deposition, (3) adjusting the solvation structure of Zn 2+ with water or solvent anions replaced, (4) regulating the orientation of Zn electrodeposition to establish flat anode surface, (5) in situ forming a robust SEI layer to protect the bulk Zn anode.Excitingly, these modification approaches have achieved substantial results addressing the interfacial challenges concerning metallic Zn anode in aqueous electrolytes.
Nevertheless, based on the research results to date, none of the work in electrolyte engineering has come up with an ultimate solution to completely eliminate the interfacial issues in AZMBs.In fact, there are still plenty of room for future research of AZMBs. 1) Unveiling the formation mechanism of "dead Zn".Although it has been widely acceptedthat "dead Zn" comes from the fractured Zn dendrites, where the large chunks of Zn lose contact to the Zn bulk and fail to participate in the following stripping process, the actual formation mechanism may be rather complex than this.Meanwhile, the composition of "dead Zn" has not been quantitively determined.It is important to thoroughly understand the specific origin of "dead Zn" to reduce the possibility of low CE and significant decay of capacity.This implies the necessity of utilizing advanced characterization techniques in the future research, which can help to better understand the mechanisms of dendrite formation and identify effective strategies for suppression.This could include using in situ/operando techniques, such as microscopy and spectroscopy, to directly observe the growth and evolution of dendrites during battery operation.2) Precisely evaluating the effect of modification strategies based on a unified standard of testing conditions.The difference in testing conditions causes troubles in comparing the electrochemical performance of batteries at the same scale, which may result in misleading assessment.3) Designing non-toxic and safe electrolyte additives.While aqueous zinc metal batteries are generally considered safe and environmentally friendly, most of the electrolyte additives for AZMBs are still toxic or eco-unfriendly for now.Considering the need to further improve the safety and environmental impact of aqueous zinc metal batteries, more non-toxic additive materials should be developed in the future to further enhance the sustainability and safety of the technology.4) Scaling up the production of aqueous electrolytes and electrolyte additives.Despite of the great advance that has been achieved in the laboratory, there are not any commercialized AZMBs.The scalability of aqueous zinc metal batteries is another important perspective.Scaling up the production and deployment of aqueous zinc metal batteries to meet the growing demand for energy storage requires the development of cost-effective manufacturing processes and the optimization of the battery design to achieve high performance and stability.This should be considered in the future studies.

Figure 1 .
Figure 1.Schematic of the configuration of AZMBs including the major challenges involved with Zn metal anode.

Figure 4 .
Figure 4. a) Comparison of the 3 m Zn (ClO 4) 2 aqueous electrolyte conductivity-temperature relationship with others.Reproduced with permission.[89]Copyright 2022, Wiley.b) 1 m Zn (TFSI) 2 + 21 m LiTFSI concentrated aqueous electrolyte's cycling efficiency of Zn||NiHCF batteries.Reproduced with permission.[75]Copyright 2021, Wiley.c) Diagram showing the formation of passivation layer Zn 4 (OH) 6 SO 4 •xH 2 O and the solution structure of Zn 2+ during electrodeposition in dilute electrolyte (left) and corresponding concentrated electrolyte (right).d) SEM photography of the surface morphology following 100 iterations of plating and stripping process.e) Using concentrated aqueous electrolyte, the ability of Zn on Cu foil to be stripped off the metal and the coulombic efficacy of Zn||Cu cell.Reproduced with permission.[134]Copyright 2020, Amer Chemical Society.f) Zn||stainless steel cell linear sweep voltammetry at a scan rate of 10 mV s −1 .Reproduced with permission.[146]Copyright 2023, Amer Chemical Society.

Figure 5 .
Figure 5. a) Profiles of stripping-plating displaying overpotential and matching capacity.b) Graph illustrating the connection between cut-off potentials, released capacities, and coulombic efficiencies.Reproduced with permission.[151]Copyright 2022, The Royal Society of Chemistry.c) The evolution of the FTIR spectrum between 3800 and 3100 cm −1 with rising salt concentration.d) The modification of17 O nuclei's chemical shifts in solvent as a result of variations in salt content.e) Molecular dynamics analyses of the Zn 2+ -solvation structure.f) The cycling stablity and CE of Zn/LiMn 2 O 4 full cell in HCZE at 4 C. Reproduced with permission.[152]Copyright 2018, Springer Nature.g) The cycle performance of Zn/LMO full cell in HCE and LCE at charge rate of 0.1 C and discharge rate of 0.2 C. Reproduced with permission.[146]Copyright 2021, American Chemical Society.

Figure 7 .
Figure 7. Strategies of aqueous electrolyte additives engineering for stable metallic Zn anode.

Figure 8 .
Figure 8. a) Schematic of Zn deposition process on anode surface under different electric field distribution in pristine electrolytes (upper) and GO-added electrolytes (bottom).Reproduced with permission.[200]Copyright 2021, American Chemical Society.b) Schematic of in situ formation of F-GQDs on anode surface under the action of polar groups with abundant nucleation sites provided.c) Cycling performance of symmetric Zn cells in electrolytes with or without F-GQDs added, at 0.5 mA cm −2 & 0.25 mAh cm −2 , and 10 mA cm −2 & 5 mAh cm −2 , respectively.Reproduced with permission.[201]Copyright 2023, Elsevier.d-f) SEM imaging of cross-section morphology of Zn anodes in d) pristine electrolytes after 100 h cycling, and e) EDTAadditive electrolytes after 100 h cycling, and f) EDTA-additive electrolytes after 500 h cycling.g-i) SEM imaging of surface morphology of Zn anodes in g) pristine electrolytes after 100 h cycling, and h) EDTA-additive electrolytes after 100 h cycling, and i) EDTA-additive electrolytes after 500 h cycling.j) Cycling performance of the symmetric Zn cells at 2 mA cm −2 and 2 mAh cm −2 with the corresponding.k) CE of Zn plating/stripping process.l) Cycling performance of Zn||V 2 O 5 battery at 2 A g −1 .Reproduced with permission.[202]Copyright 2022, American Chemical Society.

Figure 10 .
Figure 10.a) Schematic of solvation structures of Zn 2+ in aqueous electrolyte with different polymer additives.b) Rearrangement of the "Zn 2+ -H 2 O-SO 42− -polymer" bonding network and adjustment of space charge region of Zn anode with different functional polymer additives.c) Cycling performance of Zn anodes in 1 M ZnSO 4 aqueous electrolytes with and without 0.2 wt.% PAM additive at 2 mA cm −2 , 2 mAh cm −2 ; Reproduced with permission.[162]Copyright 2021, American Chemical Society.d) DFT calculations of bonding energies between Zn 2+ and different polar groups on silk peptide chains.e) Solvation structure of hydrated Zn 2+ with or without silk peptide.f) DFT calculation of adsorption energy of H 2 O and silk peptide on Zn (002) surface.g) In situ optical imaging of cross-section morphology of Zn deposition through time at 10 mA cm −2 .h) Cycling performance of Zn symmetric cells at 1 mA cm −2 and 1 mAh cm −2 .Reproduced with permission.[196]Copyright 2022, Wiley-VCH.

Figure 11 .
Figure 11.a) The irregular and well-oriented Zn growth on metal anode surface in ZnSO 4 (upper) and ZnSO 4 -OA (bottom) electrolytes, respectively.b) The adsorption energy of OA molecules on different crystal planes of Zn. c) Comparison between Zn‖Cu cells with or without OA additives added at 1 mA cm −2 and 0.5 mAh cm −2 .d) Cycling performance of Zn‖MnO 2 full cells with different electrolytes at 1 A g −1 .Reproduced with permission.[223]Copyright 2023, Elsevier.e) Schematic of Zn deposition in ZnSO 4 (left) and ZnX 2 -PEG300 (right; X = Cl, Br, or I) electrolytes.f) SEM imaging of the large hexagonal Zn platelet formed in ZnI 2 -5%PEG300 electrolytes.g) XRD pattern of Zn deposits after growing for 24 h at 4 mA cm −2 .h) Cycling performance of Zn||Zn symmetric cells with different electrolytes at 25 mA cm −2 , 3.2 mAh cm −2 .Reproduced with permission.[224]Copyright 2022, Wiley-VCH.