Rescuing zinc anode–electrolyte interface: mechanisms, theoretical simulations and in situ characterizations

The research interest in aqueous zinc-ion batteries (AZIBs) has been surging due to the advantages of safety, abundance, and high electrochemical performance. However, some technique issues, such as dendrites, hydrogen evolution reaction, and corrosion, severely prohibit the development of AZIBs in practical utilizations. The underlying mechanisms regarding electrochemical performance deterioration and structure degradation are too complex to understand, especially when it comes to zinc metal anode–electrolyte interface. Recently, theoretical simulations and in situ characterizations have played a crucial role in AZIBs and are exploited to guide the research on electrolyte engineering and solid electrolyte interphase. Herein, we present a comprehensive review of the current state of the fundamental mechanisms involved in the zinc plating/stripping process and underscore the importance of theoretical simulations and in situ characterizations in mechanism research. Finally, we summarize the challenges and opportunities for AZIBs in practical applications, especially as a stationary energy storage and conversion device in a smart grid.


Introduction
Since Kang et al. rst proposed the concept of aqueous zinc-ion batteries (AZIBs), 1 AZIBs have received tremendous attention due to their high safety, abundance, and satisfactory performance.3][4] However, currently, AZIBs still encounter many challenges that hinder their further development as a practical alternative to commercial batteries (e.g., Li-ion batteries).Although researchers have made great efforts to improve the electrochemical performance, some tough issues remain, including the co-intercalation of proton and Zn 2+ , dissolution of cathode materials, sluggish kinetics during charge and discharge, and unstable Zn metal anode (ZMA). 5,6Among these challenges, the ZMA is the most serious one, resulting in the quick capacity decay of AZIBs in their long-term service.
In the aqueous electrolyte, ZMA will undergo a chemical conversion reaction between Zn atoms and Zn 2+ , i.e., the plating/stripping of ZMA, during the discharge and charge processes.It is generally accepted that the problems occurring on the ZMA-electrolyte interface are the main reasons for ZMA's instability.For example, the nucleation and growth of Zn 2+ on the ZMA surface during Zn plating is usually accompanied with the growth of irregular Zn dendrites, which will penetrate the separator membrane and eventually cause the short circuit in AZIBs.In addition, the side reactions taking place at the ZMAelectrolyte interface during the plating/stripping (e.g., hydrogen evolution reaction (HER) and corrosion) can cause low coulombic efficiency (CE) and low capacity, which considerably degrades the cycling stability of ZMA.In fact, the issues happening at the ZMA-electrolyte interface are interdependent and entangled with each other, as shown in Fig. 1, which would break the balance of the ZMA-electrolyte interface and nally lead to the failure of ZMA and AZIBs. 7,8en addressing the instability issues of ZMA, it requires a comprehensive understanding of the underlying mechanisms and inuencing factors that are related to the ZMA-electrolyte interface.For instance, during Zn plating, the zinc ions near the ZMA-electrolyte interface will undergo a series of reaction steps, such as migration, desolvation, diffusion, and reduction at the nucleation sites.Due to the different electric eld and Zn 2+ distribution, Zn atoms prefer to accumulate and continuously grow at some protuberances of the ZMA surface, which results in Zn dendrites aer repeated Zn plating.0][11] Moreover, Zn(H 2 O) 6 2+ , as the primary solvation structure in the aqueous electrolyte, is considered as a crucial factor that induces HER at the ZMAelectrolyte interface.The occurrence of HER would then lead to a localized pH elevation at the ZMA-electrolyte interface, thereby exacerbating the corrosion reaction at the surface of ZMA. 12 Based on the above understanding, it is apparent that the stabilization of the ZMA-electrolyte interface, determined by interface roughness, Zn 2+ solvation structure, and electrolyte pH, is the key for solving the problems of unstable ZMA in the aqueous electrolyte.Accordingly, the improvements of the ZMA-electrolyte interface should be conducive to enhancing the electrochemical performance of AZIBs in theory. 13,141][22][23] To further improve these strategies for better AZIBs, it is important to uncover the relationships between interface structure and battery performance. 24,25There are many characterization methods that have been applied to investigate the structure of the ZMA-electrolyte interface.However, these methods are mostly ex situ techniques, which fail to provide continuous information to clearly illustrate the dynamic evolution of the ZMA-electrolyte interface.Such a lack of information of the interface structure from ex situ characterization might be less effective in guiding future research on the ZMA-electrolyte interface.Thus, in situ characterization methods that can provide abundant information of the battery during any period of charge and discharge are indispensable for explaining the structure and performance relationship of the ZMA-electrolyte interface.Moreover, as a supplementary technique for in situ characterizations, theoretical simulations provide detailed information about the structure change of the ZMA-electrolyte interface at atomic and electronic levels under specied conditions.Apparently, the combination of in situ characterizations and theoretical simulations should be an ideal scheme for understanding how to improve the ZMAelectrolyte interface.However, only a few studies summarize the important roles of in situ characterizations and theoretical simulations in the research on the ZMA-electrolyte interface.
To ll the gap of current studies, we focus on discussing three objects in this review, i.e., ZMA-electrolyte interface

Electrolyte engineering-induced protective mechanisms
In recent years, there has been a signicant increase in research investigating various protection strategies that aim at enhancing the stability of the ZMA-electrolyte interface.Taking into account the simplicity and efficiency in controlling the electrolyte compositions, we primarily focus on summarizing the in situ electrolyte engineering strategies.Hence, the coverage of other protection strategies in this section is not exhaustive.More detailed information of other protection strategies can be obtained from ref. 26 and 27.The commonly used electrolyte engineering strategies can be classied into the following categories: electrolyte additives, [28][29][30] co-solvent engineering, [31][32][33][34] gelelectrolytes, 35,36 deep eutectic electrolytes, [37][38][39] and high concentration salts 22,[40][41][42][43] This section, as depicted in Fig. 2, presents a review of the mechanisms involved in electrolyte engineering, including the provision of more zincophilic sites, regulation of crystal orientation, modulation of solvation structure, weakening of the hydrogen bond network, maintenance of pH, and construction of the SEI layer.
Table 1 systematically summarizes the electrolyte composition and electrochemical properties of ZMA under different mechanisms.However, due to the lack of evaluation standard, the underlying mechanisms for each additive remain unclear.Currently, the main issues of additive strategies are the high additive dosage, low current density and shallow discharge depth, which fail to meet the application requirements.Moreover, the impacts of additive on the electrolyte stability and cathode compatibility are sometimes overlooked.Therefore, further studies are required to understand the ZMA-electrolyte interface, which mainly include the design and selection of electrolyte additive, the structure-function relationship between the additive/interface components and electrochemical properties, as well as the investigation of the dynamic performance and electrochemical behavior of ZMA.

Providing zincophilic sites
The operation of Zn nucleation involves the migration, desolvation, and diffusion of Zn 2+ , and the diffusion process precisely depends on the zincophilic sites.The dense zincophilic sites favor achieving a constant 3D diffusion process, thereby guiding the subsequent uniform plating of Zn.However, the rampant 2D diffusion process occurring at the ZMA-electrolyte interface would lead to the production of Zn dendrites.For instance, Zhang et al. 44 proposed that the polyacrylamide additive with abundant acyl groups serves as a guiding intermediary to provide more nucleation sites.A uniform electric eld and charge distribution are discovered, Fig. 2 Protective mechanism of electrolyte engineering for ZMA.
resulting in a smooth ZMA surface during cycling.Wang et al. 45 selected sericin molecules as electrolyte additives, which contain zincophilic functional groups.The proposed electrolyte additive adsorbs on the ZMA surface, resulting in denser ZMA aer Zn plating.Zhao et al. 46 took advantage of the hydrophobic internal cavity and hydrophilic external surface of cyclodextrins to manipulate and boost the kinetics and stability of ZMA.
Furthermore, the incorporation of high valence cations additives, such as La 3+ and Ce 3+ ions, preferentially occupy the electron-rich active sites.This leads to Zn nucleation and growth taking place at relatively inert areas, 47 weakening the repulsive force of electrical double layer (EDL) 48 and guiding Zn nucleation towards gradual nucleation rather than instantaneous nucleation.Recently, Dai et al. 49 introduced the concept of an "escort effect" of Ni 2+ ion additives, which can deposit in advance and result in subsequent Zn plating via underpotential deposition.Regarding the relevant mechanisms, Xie et al. 50roposed a mesoporous Zn plating method, as illustrated in Fig. 3a.Hollow carbon spheres were chosen as the anode materials.Zn 2+ was reduced to single-atom Zn on zincophilic sites in hollow carbon spheres, and then Zn clusters were formed by these single-atom Zn.These Zn clusters further expanded to form a mesoporous Zn network, leading to a uniform Zn plating layer.Apart from the electrolyte engineering, for more zincophilic site design, please consult ref. 51.

Regulating crystal orientation
Zn has a typical hexagonal close-packed (hcp) structure, as shown in Fig. 3b, 52 which is characterized by crystal planes with high relative texture coefficients (RTCs), predominantly including the (002), ( 100) and (101) crystal planes.Among all the Zn crystal planes, the (002) crystal plane has a low surface energy.4][55] Based on this characteristic, Zheng et al. 56 reported the epitaxial regulation concept of the Zn nucleation and growth, as illustrated in Fig. 3c.Furthermore, owing to its higher free energy of H adsorption and stripping energy of Zn, the (002) crystal plane could effectively inhibit HER and signicantly avoid the side reaction. 35,55Consequently, a number of studies have recently designed to optimize the growth of the (002) crystal plane.The common methods of the (002) crystal plane modulation, such as surface texture design, surface coating, separator modication and current density control, have been systematically reviewed in ref. 57-60.Apart from the abovementioned design and optimization, electrolyte additives also play an ingrained role during the plating process due to their specic adsorption effects.Sun et al. 61 conducted research on a series of organic additives and concluded that using different additives in the plating process  can modify the crystallographic properties and surface morphology of the ZMA.In this regard, the functions of these additives can be summarized as below.
Very recently, there have been divergent viewpoints.On the one hand, achieving a stable cycling performance with high capacity from (002)-textured zinc is challenging due to the signicant lattice distortion and uneven distribution of the electric eld. 71On the other hand, it is found that the (002) crystal plane is chemically unstable and prone to be corroded by water in aqueous electrolytes, leading to the formation of detrimental zinc hydroxide sulfate hydrate (ZHS). 72More importantly, it is unknown whether the above strategies are still effective when a higher depth of discharge is applied.

Modulating solvation structure
It is well known that the strong solvation of Zn 2+ with H 2 O molecules exacerbates the electrochemical polarization and charge transfer resistance of Zn 2+ . 73Moreover, the strong interactions will lead to the HER of solvated water. 74A systematic summary and comprehensive introduction on the evolution of the solvation structure affected by electrolyte additives is presented in ref. 13.Typically, Zn 2+ is solvated with six water molecules to form hydrated zinc ions, i.e., [Zn(H 2 O) 6 ] 2+ , 75 leading to a high energy barrier of 289.3 kcal mol −1 , and can be improved employing electrolyte additives. 61,76For instance, Xu et al. 77 introduced silk broin (SF) as an electrolyte additive, and the [Zn(H 2 O) 4 (SF)] 2+ solvation structure is formed.The desolvation process releases SF on the ZMA surface and in situ forms hydrostable and self-healable protective lm.Yao et al. found that the triethylmethyl ammonium (TMA) cation can participate in the constitution of [Zn(SO 4 ) 2 (TMA) 3 ] + structure in the electrolyte.The obtained solvated structure reduces the number of reactive H 2 O molecules and inhibits by-product formation. 78oreover, methanol, as an antisolvent, can modulate the coordination structure of Zn 2+ . 79,80Based on the above-mentioned improvement of ZMA, it is necessary to analyze how additive molecules or cations inherently reconstruct the typical solvation structure, which could effectively improve the electrochemical performance.Yang et al. proposed a stability constant (K), which represents the equilibrium constant of the complexation reaction. 81This method is served as a universal standard to understand the anchoring strength between the additive ligands and solvated Zn 2+ , and the dynamic evolution of Zn plating process is shown in Fig. 3d.In addition, Shao's work 82 reported that solvents with a higher donor number were favorable for stabilizing the ZMA-electrolyte interface as the donor number reects the solvation ability between solvent molecules and Zn 2+ .
Apart from organic electrolyte additives or solvents, there are other categories of effective additives.Zhang et al. 83 utilized a chloride salt with a bulky cation (1-ethyl-3-methylimidazolium chloride, EMImCl) to form an anion-type water-free solvation structure ZnCl 4 2− in the electrolyte.They also designed an ammonium halide additive. 84I − is acted as an electron donor and coordinates with Zn 2+ to form the solvation structure ZnI(H 2 O) 5 + .The stability of the solvation structure is effectively enhanced, making it much easier to inhibit HER.

Weakening hydrogen bond network
The high activity of H 2 O molecules poses a threat at the ZMAelectrolyte interface. 85In addition, the original hydrogen bond network among H 2 O molecules, which is an important indicator of HER, 86 can be easily destroyed by applying co-solvents or deep eutectic electrolyte. 87,88Moreover, a new hydrogen bonding network containing multifunctional groups can be constructed, which in turn inhibits the activity of the H 2 O molecule. 89For instance, xylitol additive inhibits HER, accelerates cation migration by expelling active H 2 O molecules, and facilitates the reconstruction of the hydrogen bond network. 90ecently, a study demonstrates that (aminomethyl)phosphonic acid (AMPA) promotes the formation of an ordered hydrogen bond network at the interfacial H 2 O, resulting in the inhibition of water-induced H 2 production (Fig. 3e). 913][94] To validate the fundamental role of additives in the broken hydrogen bond network, a series of Lewis basic organic molecules with lone pair electrons were proposed. 95Due to the unique strong electronegativity of the lone pair of electrons, the tetrahedral structure of H 2 O molecules and their original hydrogen bond network were broken.

Maintaining proton concentration (pH)
According to the Pourbaix diagram, it is indicated that HER is a pH-dependent reaction and an inevitable thermodynamic process.The sources of HER on ZMA can be attributed to the following three reasons: the high reactivity of solvated H 2 O, the purity of ZMA and the irregular surface morphology. 96The occurrence of HER results in the release of more OH − , leading to an elevation in pH value and exacerbating the corrosion reactions on ZMA.For a thorough understanding, please consult the pH evolution law of the ZMA-electrolyte interface and its impact mechanism on the formation of Zn dendrites, as rstly revealed by Yang et al. 97 Therefore, the pH value of the ZAM-electrolyte interface is crucial, and the additives will disrupt the pH value of the pristine electrolyte.As the amount of additive is increased, the pH value is either increased or decreased: the lower suitable pH of the electrolyte can diminish the formation of ZHS, 98,99 and an appropriate increase in the electrolyte pH benets the suppression of HER; 100,101 moreover, an excessively high pH value (>5.47) would exacerbate corrosion. 102For instance, a specic quantity of Na 4 EDTA will trigger the formation of the ZHS precipitate in the electrolyte. 103Therefore, a pH buffer electrolyte additive is required to keep an appropriate pH value, such as acetic acid/acetate (HOAc/OAc − ), 104,105 pyridine/pyridinium, 106 and imidazole/imidazolium. 107,108These electrolyte additives act as pH regulators to help maintain an appropriate acid-base balance during the charge/discharge process, ultimately mitigating the interfacial HER and corrosion reactions (as depicted in Fig. 3f). 104Interestingly, Zhang et al. presented a molecular switch strategy that utilizes the reversible structural changes of g-butyrolactone (GBL) and g-hydroxybutyrate (GHB) under varying pH levels. 109[112]

Constructing solid electrolyte interphase (SEI) layer
The in situ construction of an Zn 2+ conductive SEI layer that can be achieved through the reduction and decomposition of electrolyte, exhibiting enhanced durability and stability in providing protective effects.The presence of this in situ layer effectively impedes corrosion and promotes a homogeneous distribution of cations, attributable to its chemical bonding with the anode surface.Nevertheless, the establishment of a compact and enduring SEI layer in aqueous electrolytes faces a considerable obstacle due to the concurrent production of gas and inert byproducts during the decomposition of these electrolytes. 113Additional insights into the formation and composition of SEI can be ascertained in ref. 114, and Fig. 3g illustrates the mechanical characteristics of different types of SEI layer. 25Therefore, the introduction of an additional concept, referred to as the EDL, becomes imperative as it considerably impacts the chemical properties and morphology of the SEI through the arrangement of molecular and ionic assemblies in close proximity to the anode. 115However, numerous additives merely adsorb onto the ZMA surface to generate a EDL without undergoing subsequent reduction to form an SEI.In response to this issue, Huang et al. conducted a comprehensive evaluation of 15 organic additives and iden-tied the pivotal determinant of these additives' protective effectiveness as their capacity to generate an SEI layer. 116nlike the traditional organic electrolyte, ZMA suffer from severe surface corrosion and the formation of non-uniform ZHS and ZnO compounds.By introducing a small quantity of Zn(OH) 2 additive into the ZnSO 4 electrolyte 117 or Na 2 SO 4 additives into zinc triuoromethanesulfonate (Zn(OTF) 2 ) electrolytes, 118 the conversion of the non-uniform ZHS into a homogeneous and durable SEI layer is achieved.Li et al. designed a low-concentration aqueous Zn(OTF) 2 -Zn(NO 3 ) 2 electrolyte.An insulating Zn 5 (OH) 8 (NO 3 ) 2 $2H 2 O passivation layer was rst formed, which then transformed into a more stable Zn-ion conductive Zn 5 (CO 3 ) 2 (OH) 6 layer and ZnF 2 -rich SEI layer in sequence. 119From the perspective of composition, prior investigations demonstrate that the SEI layers enriched in ZnF 2 , [120][121][122] ZnS 123,124 and zinc phosphate (Zn 3 (PO 4 ) 2 ) are propitious to enhance the stability of the ZMA-electrolyte interface. 125,126These constituents of the SEI layer are either derived from the decomposition products of additives or the reduction of anions promoted by additives.
Apart from inorganic additives, organic additives play an equally important protective role.For instance, the ZnCl 2 -H 2 O-DMSO electrolyte forms an SEI layer based on Zn 12 (SO 4 ) 3 -Cl 3 (OH) 15 through the decomposition of solvated DMSO. 127On the one hand, organic additives can construct an anode-molecular interface. 128,129On the other hand, they can decompose directly to produce organic-inorganic SEI layers. 130,131In summary, additives containing nitrogen or sulfur functional groups can provide signicant protection.However, the precise mechanism by which these active groups participate in the formation process of the SEI layer, especially when it involves the capture of certain intermediates, is poorly understood.Therefore, the advancement of theoretical simulations and in situ characterizations will contribute to uncovering the underlying mechanisms.

Theoretical simulations
Theoretical simulations have been widely conducted in literatures to investigate the mechanisms of reactions happening at the ZMA-electrolyte interface.Using theoretical simulations can considerably improve the understanding of the fundamental mechanisms. 132,133This section will specically summarize the functions of various theoretical simulation methods in investigating the ZMA-electrolyte interface.

COMSOL simulations
As mentioned earlier, the stability of the ZMA-electrolyte interface can be inuenced by interface roughness.An uneven interface can induce the "tip effect", which depends on the intensity of the electric eld and the Zn 2+ concentration near the ZMA-electrolyte interface. 134As shown in Fig. 4a and b, COMSOL simulation indicated that electrons and ions tend to accumulate at the sharp tips on bare ZMA.Additionally, micronscale concavities or pits on ZMA can also enhance the intensity of the electric eld. 135,136The former worsens dendrite growth, while the latter is more conducive to enhancing the stability of the ZMA-electrolyte interface because it leads to a uniform electric eld and Zn 2+ distribution.][141] Following this theory simulation, researchers discovered that an exaggerated electric eld heterogeneity occurs when the bending radius is less than 5 mm. 1424][145] Furthermore, COMSOL simulations were roughly estimated to predict the surface morphology and the hydrogen diffusion ux. 122,146

Density functional theory (DFT) calculations
It is generally accepted that DFT calculations are indispensable tools in numerous scientic and engineering disciplines, providing valuable guidance and explanations for experiments.In this section, the application of DFT calculations for the research of the ZMA-electrolyte interface is described as follows.
(i) Tuning the correlations, such as the adsorption energy of additive on ZMA-electrolyte interface, binding energy of additive with Zn 2+ , free energy of H adsorption 147,148 and energy of the stripping process. 55,1491][152][153] Moreover, the charge density distribution and surface electrostatic potential intuitively reect the strong interaction of Zn 2+ and precise active site for Zn 2+ nucleation, 49,154,155 respectively.As shown in Fig. 4c, DFT calculations show that the surface conguration 156 or special Zn deposition sites 55 can impact the adsorption energy of Zn atoms.Aside from zincophilic sites, lower diffusion energy barriers support the formation of a 2D deposition mode, leading to uniform and dendrite-free Zn deposition. 157,158Then, a unique zincophobic repulsion mechanism was proposed in the presence of an additive adsorption layer. 159,160n addition, the binding energy between different additives and Zn 2+ is closely related to the solvation structure.In traditional electrolyte, Zn 2+ solvated with six water molecules forms a hydrated zinc ions, resulting in a formation energy of −5.58 eV. 161By comparing the binding energy or desolvation  100) and (002) crystal planes. 55Copyright 2021, Wiley.(d) Comparison of the energy barrier required for normal and novel solvation structures to de-solvate through DFT calculation. 166Copyright 2023, Elsevier.(e) Energy profiles of Zn cluster at different deposition states. 169Copyright 2023, American Chemical Society.(f and g) Snapshot of the electrolyte containing xylitol additive (f) and its corresponding RDFs and CN (g) collected from MD simulations. 30Copyright 2023, Wiley.
3][164][165] As shown in Fig. 4d, DFT calculations indicated that the desolvation process for additive molecule detachment can be considered as a rate-determining step.In this case, additive molecules tend to replace the active H 2 O molecules in the solvation structure, thereby driving the active H 2 O molecules away from the ZMA-electrolyte interface during the desolvation process.Thus, the HER caused by active H 2 O molecules is alleviated. 166Furthermore, Zhou et al. rstly investigated the effects of the molecular electrostatic polarity on the desolvation of hydrated Zn 2+ , which is used as a key factor to select a suitable additive to regulate reversible Zn plating/ stripping chemistry. 167ii) Evaluating the kinetics and stability of the Zn 2+ deposition and diffusion on ZMA surface. 168Conventionally, it was believed that a low adsorption energy and rapid Zn 2+ diffusion coefficient contribute to the fast and uniform nucleation and deposition of Zn hexagonal crystal. 143For instance, by comparing the 3D charge density variances and Gibbs free energy of the deposited Zn 2+ , as well as the energy of the Zn cluster on different matrixes (Fig. 4e), an accelerated reaction kinetics and uniform Zn deposition were implemented on Ag surfaces. 169Furthermore, to thoroughly investigate the mechanism of Zn dendrite formation induced by residual stress and lattice defects, the Gibbs free energy of Zn crystals under different stress and defect states was evaluated using DFT calculations. 170

Molecular dynamics (MD) simulations
MD simulation can provide an in-depth electrolyte analysis, such as anion solvation structure, 171 ionic conductivity, 172 and spontaneous chemical reactivity. 173From the perspective of molecular dynamics, the evolution of the electrode-electrolyte interfaces can be simulated (Fig. 4f and g). 30The applications of MD simulations in the research of AZIBs to date can be summarized as follows.8][179] (3) Estimating the hydrogen bond network and activity of H 2 O. 80,92,180 (4) Revealing the effects of additive molecules on the separator or ZMA. 129,181herefore, MD simulations can help understand the changes in the electrolyte during a proposed system in a visualized way.For instance, an electrochemically and thermally stable Zn 5 (-OH) 6 (CO 3 ) 2 -containing SEI layer was successfully achieved by the decomposition of N,N-dimethylformamide (DMF). 182The corresponding MD simulation indicates that most of the Zn 2+ migration occur in the Zn 5 (OH) 6 (CO 3 ) 2 phase along the [010] lattice plane.Moreover, the concentration-dependent effects in an electrolyte-containing propylene glycol (PG) additive were studied by the MD simulation, indicating that a self-assembled mediated lm formation occurs at a low concentration. 183ecently, Yang et al. proposed a bulk-phase reconstructed ZMA with abundant zincophilic sites, which considerably improved the resistance to dendrite growth and side reactions even aer deep stripping.In the case, MD simulations were employed to analyze the nitrogen bombardment process at different nitrogen species energies, substrate temperature, and substrate vacancy concentrations to guide the acquisition of a bulk-phase reconstructed ZMA. 184

In situ characterization
Theoretical simulations provide valuable insights into the protective mechanisms of electrolyte engineering strategies and guide their further development.Furthermore, experimental evidence, obtained through in situ characterizations, is also indispensable for a direct understanding of the mechanisms.In this section, we summarize the applications of various novel in situ characterizations that have been employed to investigate the ZMA-electrolyte interface, such as in situ visualizing characterizations, in situ spectroscopy/mass spectrometry characterizations, in situ physicochemical characterizations and other advanced characterizations.For a schematic representation of the relevant in situ characterizations and more detailed information about the in situ battery structure, please consult the references of 185-187.

4.1.
In situ visualizing characterization 4.1.1.In situ optical microscopy (OM).Similar to Li-ion batteries, 188,189 it is essential to monitor the morphology and structure evolution of the ZMA in real-time.ZMA protection strategies have been well studied using in situ OM to investigate the differences of dendrite growth under different conditions.Without any protection strategies, irregular dendrites and gas bubbles form on the ZMA surface during the plating process, which is due to the inhomogeneous Zn nucleation and severe HER in traditional electrolyte (Fig. 5a). 42Moreover, during the stripping process, the partial dissolution of Zn could be observed (Fig. 5b), 190 which leads to the formation of "dead Zn" and consequently low coulombic efficiency (CE). 191,1924][195] Through the in situ OM observation, pits are formed aer the rst stripping process.These pits exacerbate the subsequent growth of dendrites.In contrast, pre-deposited ZMA exhibits a more uniform morphology in subsequent cycles. 196xcept for the above cross-section observation, the surfacesection perspective is also extensively utilized to observe the interfacial atness and nucleation sites. 192For instance, a stress-generation-release phenomena on the coating layers during the Zn plating process was directly observed, as shown in Fig. 5c.In detail, a large number of Zn grains emerge on the ZMA surface and soon vanish in a ash.As the plating time increases, signicant uctuations and wrinkles appear due to the huge stress exceeding the threshold coating layer strain.Immediately aerwards, the ZMA surface gradually recovers to a at state due to the groove deposition model. 197n situ OM is also applied to reveal the effects of current density on Zn plating.Generally, higher current density is supposed to result in more Zn dendrite formation on the ZMA surface, 198 which is considered as a guidance to control the Chemical Science Review plating behaviors and crystal orientation of Zn deposition by tuning the current densities. 199However, Yu et al. clearly demonstrated that the in situ OM technique used for observing ZMA is not a sufficiently reliable characterization for describing the HER and dendrite growth process due to the signicant randomness of various deposition morphologies in different regions. 27.1.2.In situ atomic force microscopy (AFM).1][202] For AZIBs, the plating and stripping processes of Zn could be monitored by in situ AFM. 203ig. 5d illustrates the surface morphology evolution on ZMA within a single Zn plating/stripping process.During the plating process, two nascent nuclei emerge and then gradually turn into micron-sized particles through radial expansion.During the stripping process, Zn dendrites are homogeneously dissolved identied by a color change from dark yellow to light yellow. 204esides, AFM images are used to monitor Zn dendrite formation at different current densities and depth of discharge.At a very early stage, the size of the dendrite increases while a higher current density is applied.Besides, the formation of Zn dendrites is accelerated with a higher depth of discharge. 135n addition, AFM is also used to support the effects of surface modication.For example, Chen et al. polished the Zn foil using sand papers with a proper grit size, which demonstrate a stable cycling performance.AFM images showed that Zn deposition mainly happens in the concaves generated by sand papers and proves that smaller grit size contributes to atter anode surface, i.e., dendrite free surface. 192Zhang et al. performed in situ AFM to reveal the plating patterns in different electrolytes.As a result, a dot-distributed Zn nucleation mechanism was observed in traditional ZnSO 4 electrolyte.However, an anion-type water-free Zn 2+ solvation structure electrolyte leads to plane-distributed Zn nucleation. 83Keist et al. investigated the evolution of Zn deposition morphology using in situ AFM and concluded that uniformly distributed Zn nucleus gradually become larger and thicker in a layer-by-layer growth mode, leading to the smooth surface and improved cycling performance in an imidazolium-based ionic liquid electrolyte. 155,205Moreover, Wang et al. presented that a smoother hexagonal surface and pyramid-like structures with sharp edges were observed in a concentrated electrolyte through in situ AFM images, whereas a rough and undulating topography was observed in a dilute electrolyte. 206Despite the use of in situ AFM for the direct observation of the surface topology of ZMA during the rst plating stage, using in situ AFM to monitor the formation of SEI layers or EDL remains underexplored.
4.1.3.In situ X-ray computed tomography (X-CT).X-ray computed tomography (X-CT) 207,208 has been employed to achieve the 4D observation of the Zn plating/stripping process.As shown in Fig. 5e and f, aer a statistical quantication of the mean curvature, the 3D rendering image for the entire domain can be drawn, and different colors represent different compositions. 190,208Therefore, X-CT is suitable for studying the 3D structure of ZMA, such as the surface morphology of the cycled Zn@3DCu 3 anode 209 and the spatial distribution of ZnO in porous ZMA. 210Meanwhile, Pu et al. employed the in situ X-CT technique to demonstrate plated Zn and Zn substrates without disassembling the battery.The obtained results further indicate that the epitaxial Zn does not possess interior defects aer the plating. 211They also demonstrated that the gas bubbles generated on the Zn surface can physically deactivate Zn in the following plating/stripping processes, which is monitored by the in situ X-CT technique (Fig. 5g). 212In addition, the X-CT technique can verify the detailed and homogeneous Zn plating behavior aer constructing a protective SEI layer or employing additives. 213For instance, Zhang et al. demonstrated that the homogeneous Zn plating behavior can be achieved on the hydrophobic-zincophilic SEI layer because a relatively at surface has been detected by in situ X-CT, 214 while extensive "dead Zn" was shown in in situ X-CT images when using additive-free electrolyte. 215Although the above mentioned examples demonstrate the potentials of the X-CT technique in promoting a deep understanding of the protective mechanism, several challenges still need to be overcome, including expanding the scanning range and improving the accuracy and resolution of the detected area. 216,217.1.4.In situ electron microscopy.To monitor the local chemical reactions on the interface with higher resolution, electron microscopy, such as in situ scanning electrochemical microscopy (SECM) and in situ transmission electron microscopy (TEM), is employed.As shown in Fig. 6a, vertical oriented patterns are observed on the polished ZMA through in situ SECM.In contrast, an SEI layer can guide the horizontal accumulation of thin Zn sheets at the early stage of Zn deposition to suppress the growth of dendrites.218 As indicated by in situ TEM, particles and needle-like precipitates are formed on the Pt electrode during the plating process (Fig. 6b), occurring before 4 s and aer 7 s, respectively.The dendritic branch angle is approximately 60°.During the stripping process (Fig. 6c), the particles near the Pt surface is more likely to be stripped away than the dendrite tips and the detached Zn dendrite is a potential risk for "dead Zn".219 Very recently, an electrostatic shielding effect mechanism has been further conrmed via in situ liquid-cell TEM, which is close to the practical AZIBs batteries.220 In this case, as shown in Fig. 6d, LiCl will induce preferential secondary nucleation on the previously deposited Zn akes along the (002) plane, resulting in the formation of stacked zinc akes.

4.2.
In situ spectroscopy/mass spectrometry characterizations 4.2.1.In situ Raman spectroscopy.In contrast to the visualization research discussed in the above section, in situ Raman spectroscopy can provide a deeper understanding of the vibration information of molecules.So far, in situ Raman spectroscopy has been widely used to analyze the corrosion behavior of ZMA. 221,222For instance, the gradually emerging O-H stretching vibration of ZHS at 3633 cm −1 indicates the existence of irreversible corrosion reaction. 223The spectrum intensity of the SO 4 2− peak at 980 cm −1 becomes weakened during the plating process (Fig. 7a), suggesting that SO 4 2− was consumed due to ZHS formation. 224Moreover, the side reaction HER generated more OH − , which further accelerates the formation of Zn(OH) 4 2− species, hindering the subsequent transport of Zn 2+ .
Therefore, a Raman peak located at 465 cm −1 in Fig. 7b, assigned to the Zn(OH) 4 2− species, could be observed in the traditional electrolyte. 225The corrosion behavior can be suppressed by modifying ZMA via an oriented freezing process or thermal infusion strategy 226,227 because the Zn(OH) 4 2− peak at 465 cm −1 has disappeared, as shown in Fig. 7c.
Referring to the complexity of the EDL or the SEI layer on the ZMA, in situ Raman is an effective tool to monitor the in situ formation of the EDL and SEI layer. 228For instance, with the increase in the deposition time, the vibration peaks of the 3-(1methylimidazole)propanesulfonate (ImS) additive gradually increased and would not change upon cycling; it is supposed that ImS can adsorb on ZMA to form EDL without undergoing chemical decomposition. 229In addition, the gradual polymerization of the monomers to form the corresponding polymer is recorded by in situ Raman during cycling. 230As shown in Fig. 7d, the intermediates of in situ polymerization are detected, which would lead to the successful construction of a robust polydopamine layer on the ZMA eventually.This in situ polymeric SEI possesses abundant functional groups and outstanding hydrophilicity for regulating Zn nucleation to achieve dendrite-free Zn plating. 231n the case of articially-constructed ZMAs, in situ Raman spectroscopy also plays a signicant role in exploring the mechanism of Zn 2+ storage.For instance, the vibration mode of a 2D polyarylimide covalent organic framework (PI-COF) indicates its reversible conversion during cycling (Fig. 7e). 232urthermore, in situ Raman can be used to identify the preferential Zn 2+ nucleation sites, such as the diacetylene bonds in the graphdiyne nanowalls 233 and the O and OH terminal on the Ti 3 C 2 T x MXene. 234.2.2.In situ Fourier transform infrared spectroscopy (FTIR).In situ FTIR techniques can also support the study of ZMA by identifying the formation of new species and monitoring the decomposition of the electrolyte.Generally, the attenuated total reection (ATR) conguration is used in in situ FTIR experiments, especially in battery systems.Amaral et al. have summarized a plenty of in situ FTIR congurations and their corresponding signicant advancements. 235As for the ZMA-electrolyte interface, in situ FTIR has been widely employed to monitor molecular dynamics during plating/ stripping, particularly the migration of Zn 2+ and the desolvation process. 91,236As shown in Fig. 7f, during the plating process, the peak of the O-H band from the H 2 O molecule exhibits a blueshi, and the peak intensity of SO 4 2− becomes weakened.These could be explained as a repulsion between H 2 O molecules and SO 4 2− anions resulting from the desolvation 2− species on (b) Cu@Zn and (c) MGA@Zn anode. 227Copyright 2022, Wiley.(d) In situ Raman spectra of DA additive and its representative polymerization procedure for PDA. 231Copyright 2021, Royal Society of Chemistry.(e) In situ Raman spectra of the PI-COF electrode during the charging process. 232Copyright 2020, American Chemical Society.(f) In situ ATR-FTIR spectra of the O-H band and SO 4 2− anion vibration. 237Copyright 2023, Springer Nature.(g) Illustration of piezo-controlled macro-ATR monitoring of the interfacial area of the electrode. 238Copyright 2023, Wiley.(h) In situ ATR-FTIR spectra of conformation changes between GBL and GHB. 109Copyright 2023, Wiley.

Chemical Science Review
of Zn 2+ . 237Liu et al. employed in situ synchrotron FTIR characterization to monitor the effects of electrolyte additives on the hydrogen bond network, and the corresponding congurations are illustrated in Fig. 7g. 238With the introduction of the additive, a redshi of O-H is monitored together with increased peak intensity, which means a new hydrogen bond network between H 2 O and additives is established and the general water derived hydrogen bond network is destroyed. 39,239Moreover, in situ FTIR enables the direct observation of the reduction behavior of additive molecules, such as dimethyl methylphosphonate (DMMP) and N,N-dimethylformamidium tri-uoromethanesulfonate (DOTf). 238,239Zhang et al. reported gbutyrolactone (GBL) as the electrolyte additive to stabilize the pH value at the ZMA-electrolyte interface. 109ATR-FTIR spectroscopy was used to monitor the transformation between GBL and g-hydroxybutyrate (GHB) to unveil the mechanism for the adjustable pH in the battery system.Specically, the reversible C]O vibration during Zn plating/stripping has been identied, as shown in Fig. 7h.4.2.3.In situ X-ray diffraction (XRD).In situ XRD is mainly used to investigate the phase transformation of active materials. 240,241More details of the advanced in situ XRD characterizations about the cathodes for AZIBs are discussed in ref. 242-245.As for ZMA, in situ XRD technique can identify byproducts formation and measure crystal plane changes during the plating/stripping process.For instance, ZHS is a main byproduct while using ZnSO 4 based electrolyte and can be easily identied in the XRD data.Moreover, the characteristic peaks of ZHS are irreversible (Fig. 8a). 223,246Secondly, the oriented deposition behavior of Zn is also a hot topic, which could be monitored by collecting the corresponding relative texture coefficients (RTCs) values of all the crystal planes by in situ XRD.For example, the preferred oriented Zn deposition on ZMA could be identied by the XRD result as the (002) peak gradually becomes stronger.Generally, the preferred Zn(002) deposition means a horizontal stacked Zn plate on ZMA, ensuring a smooth ZMA during cycling (Fig. 8b). 234,247,248urthermore, in situ XRD can be used to study the evolution of ZMA structure and its reversibility, contributing to the analysis of "dead Zn" formation. 227,249.2.4.In situ differential electrochemical mass spectrometry (DEMS) and gas chromatography (GC).Generally, the amount of H 2 produced by the modied or protected ZMA is much less than that produced by pristine ZMA, known as HER inhibition.Bubble formation is hardly seen in the modied system, but it does not mean that the HER is completely eliminated.Therefore, in situ electrochemical gas chromatography (EC-GC) or Differential Electrochemical Mass Spectrometry (DEMS) are employed.For EC-GC, the amount of gas production can be quantied over a specic time period (typically 15 minutes), as shown in Fig. 8c.74,103,250 Based on DEMS, continuous cumulative gas production can be detected.160,251 Furthermore, the combination of linear sweep voltammetry and DEMS can accurately distinguish the potentials for Zn plating and HER (Fig. 8d).109 Fig. 8e demonstrates the H 2 evolution during the open-circuit voltage (OCV) state.[252][253][254] Also, the presence of plenty uctuation peaks indicates the HER and released bubbles.255 Briey, these in situ EC-GC or DEMS methods are primarily utilized for evaluating the H 2 evolution rate and electrolyte stability.256

In situ physicochemical characterizations
Intrinsic properties, such as impedance, mass, and electrolyte conditions, are also crucial factors for AZIBs.These intrinsic properties affect the electrochemical performance, interface reactions, and electrolyte stability.Investigating the changes in intrinsic properties holds great signicance in improving the protection strategies, i.e., more stable ZMA-electrolyte interface, less side reaction, and higher safety.4.3.1.In situ electrochemical impedance spectroscopy (EIS).Electrochemical impedance spectroscopy (EIS) is an essential method to study reaction kinetics at the ZMAelectrolyte interface and the evolution of the SEI layer. 257,258As shown in Fig. 9a, the in situ EIS results demonstrate that the interface resistance of the pristine electrolyte gradually increases.On the other hand, the interface impedance in taurine electrolyte is more stable even aer a drastic reduction. 14,2591][262] In detail, during the plating process, the interface impedance decreases slightly, indicating that the newly deposited Zn nanoarrays expose more active sites. 263For a more comprehensive analysis, Dai et al. recorded the in situ EIS spectra and the performed distribution of relaxation time (DRT) analysis (Fig. 9b). 49The interfacial impedance (P3-P5) in the ZnSO 4 electrolyte increases signicantly aer the rst cycle of plating/stripping, while that in the Ni 2+ -ZnSO 4 electrolyte remains relatively constant, corresponding to a signicant decrease in the ZHS.In addition, in situ EIS can provide information about the surface roughness and "so shorts" of ZMA during cycling. 264,265.3.2.In situ electrochemical quartz crystal microbalance (EQCM).The electrochemical quartz crystal microbalance (EQCM), as a gravimetric analysis tool, establishes a relationship between the mass change and charge transfer of the active materials.Consequently, it can analyze the specic adsorption of additive, in-depth plating/stripping mechanisms, and formation of SEI. 266,267Zhang et al. carried out EQCM to assess the absorption behavior of N-methylimidazole (NMI) additive on ZMA (Fig. 9c and d).There is a signicant mass change in the electrolyte containing NMI additive when the potential changes, which is attributed to the adsorption of NMIH + on the interface at low potential and the desorption of NMIH + at high potential. 107In addition, EQCM records a negligible mass change resulting from the adsorption and desorption of H atoms and H 2 O in the pristine electrolyte. 268he plating/stripping process of Zn 2+ is generally believed as a two-electron reaction.However, Agrisuelas et al. conducted EQCM studies and characterized the presence of the Zn + intermediate, suggesting that the plating/stripping of Zn 2+ undergoes two consecutive single-electron transfer steps. 269In addition, EQCM is used to identify the side reactions such as the formation of byproduct.The mass change of ZMA is detected by EQCM during immersion in electrolyte with/ without dissolved oxygen.Much faster mass increment is recorded due to the growth of byproducts induced by dissolved oxygen during the rst resting state. 270Another example is the detection of SEI formation.Signicant unbalanced data of coulombic loss and gravimetric mass change is observed in the non-aqueous Zn(TFSI) 2 electrolyte due to the SEI formation process in the rst three cycles.However, in the aqueous electrolyte, no signicant loss of charge occurs, resulting in nearly 100% CE. 271 4.3.3.In situ proton concentration (pH).Moreover, proton concentration in the electrolyte plays an important role in AZIBs, showing up as the pH value at the ZMA-electrolyte interface.Generally, because Zn 2+ plating/stripping and HER process share an overlap potential range, which means that within the battery operation voltage range, HER is inevitably triggered. 272,273Symmetric batteries with conventional electrolyte suffer from a gradually increased pH environment, indicating the continuously occurring HER reaction. 101,106,112,274herefore, researchers applied an electrolyte additive to maintain the stability of the pH value, which has been described in Section 2.5. 14,101,275Recently, Zhang et al. 107 used a ternary pH ultra-microelectrode (UME, Fig. 9e) to detect the pH value by controlling the distance between the probe and ZMA to distinguish the pH evolution between the bulk and diffusion layers.
The results indicate that the OH − species produced by HER will not diffuse into the bulk electrolyte because of the strong interactions between OH − and Zn 2+ at the interface.However, there is no generally accepted suitable pH values for AZIBs because both low pH (2.8) 102 and high pH (5.14) 104 have been reported to favor ZMA stability.

Minority and advanced characterizations
Apart from the commonly used in situ characterizations mentioned above, some relatively rare but innovative strategies and characterizations have been used in AZIBs.As shown in Fig. 10a, the generation of bubble footprints can be observed on the surface of cycled ZMA, while in situ GC collects the bubbles and separates the hydrogen gas for analyzing whether the HER reaction is inhibited. 276The results indicate that uneven zinc deposition is more likely to occur around the bubbles, leading to the easier formation of dendrites.Moreover, a novel ultrasonic-guided wave technique was introduced (Fig. 10b) by Zhang et al.With the beginning of Zn plating, the velocity and attenuation of ultrasound change.These changes are related to the formation of dendritic structures on the ZMA-electrolyte interface.As dendrites grow, the attenuation of ultrasound increases, and the velocity decreases.The reason for its attenuation is that Zn dendrites increase the scattering and absorption of energy.Conversely, during the stripping process, the wave velocity increases. 277n addition, signicant advancements and applications have been achieved in the integrated characterizations of full-battery.For instance, Zhang et al. reported a exible pressure sensor composed of a solid-state ZIB, which can be used for accurate and continuous human pulse and limb movement detection.This ZIB-type pressure sensor can be used as both an energy storage device and an excellent pressure sensor, which effectively convert mechanical signals into electrical signals and output a stable high/low frequency response signal depending on the various interface resistance and isolation layer resistance. 278Similarly, a exible and electrochromic AZIBs has been reported by Wang et al.Once a short circuit occurs in the battery, the voltage rapidly drops to 0 V, and the polypyrrole (PPy, as cathode) layer quickly transitions from the oxidized state to the reduced state, causing the battery's color to change from black (normal operation state) to bright yellow (short circuit state). 279Sun et al. proposed a smartphone computing multispectral imaging reconstruction (MSIRC) strategy to unlock the ion storage behaviors in cathode materials.As indicated in Fig. 10d, by leveraging the relative reectance values at specic wavelengths, a "layer to layer" Zn 2+ storage mechanism and excessive Zn 2+ insertion was identied. 280esearchers used a smartphone to perform the in situ analysis of aqueous batteries through a transparent monitoring window.The authors extracted the optical characteristics of the materials using computational reconstruction methods, thereby enabling the monitoring of the chemical ion storage and realtime capacity changes of the materials.Most recently, articial intelligence has become more and more popular.Further investigations may take the advantage of this powerful tool to guide the research design, help understand complex mechanisms and kinetics, and accelerate the acquisition of experimental and computational data.

Summary and outlook
In conclusion, AZIBs possess the advantages of high safety, low cost, and eco-friendliness and are one of the most promising energy storage and conversion devices for the applications in smart grid.Zn is considered as the most ideal anode material but encounters challenges such as Zn dendrites, HER, and corrosion.This review summarizes the theoretical simulations and multiscale in situ characterization for understanding the protective mechanism of the ZMA-electrolyte interface.
Nonetheless, researches on AZIBs are still at its early stages, and there are many critical scientic issues and underlying mechanisms waiting to be disclosed, as detailed below.
(i) The stripping mechanisms receive less attention, despite the fact that the stripping process is the rst step of ZMA in a full battery. 184,281As shown in Fig. 11a, the uneven stripping process leads to the formation of voids and/or cracks on ZMA, exposing highly active surface.The volume changes of ZMA aer repeated cycles lead to the formation of interspace/crevices between the modied layer and ZMA.The modied layer is thus easier to separate from the ZMA and loses its protective effect.
(ii) Inhibiting the bubble formation is challenging since it is difficult to fully control the HER.As shown in Fig. 11b, the gradual aggregation of bubbles physically isolates some Zn with the electrolyte, leading to the capacity attenuation of ZMA.However, this issue is normally neglected since excessive Zn is applied.
(iii) Similar to the preferred oriented Zn plating, is it possible to achieve selective stripping processes?For instance, selectively stripping Zn(101) and exposing more favorable Zn(002) (Fig. 11c) would in turn promote the subsequent plating process along the (002) crystal surface.
(iv) Is the EDL completely covered with adsorption-type additives?If not, is there still a signicant amount of solvated H 2 O that can directly contact ZMA?Therefore, it is crucial to distinguish the state of local environment from the near interface region to the bulk electrolyte.
(v) It is challenging to fabricate a functional SEI with high performance.For instance, when constructing the SEI layer, it is necessary to consider its thickness, mechanical strength, and self-healing ability.As shown in Fig. 11d, excellent self-healing ability is benecial for extending the battery life.Moreover, it is important to investigate and understand how the composition and microstructure of SEI inuence the desolvation and diffusion coefficient of Zn 2+ .
These issues severely affect the development of AZIBs in practical applications.It is important for researchers to understand the relationships among crystal structure, material composition, reaction mechanism, reaction kinetics, and electrochemical performance through advanced tools such as Fig. 11 Scientific issues and prospects for the plating/stripping process of ZMAs.

Chemical Science
Review theoretical simulations and in situ characterizations mentioned in this review.Besides, when it comes to in situ characterizations, researchers need to pay more attention to the reliability and validity of the required data and quality for practical applications.
(i) Developing and integrating a variety of in situ characterizations that can operate closer to actual operating conditions are essential for a more accurate understanding of the dynamic changes at the ZMA-electrolyte interface.
(ii) Improving the temporal and spatial resolution of in situ characterizations is crucial to rapidly capture the electrochemical processes occurring and subtle structural changes, which are essential for understanding the operation mechanisms and optimizing the performance of AZIBs.

Fig. 1
Fig.1The issues, influencing factors and modification strategies of the ZMA-electrolyte interface.

Fig. 3
Fig. 3 Illustration of electrolyte engineering for protecting ZMA.(a) Schematic illustration of the Zn deposition process. 50Copyright 2021, Wiley.(b) The illustration of the hexagonal close packed structure of Zn. 52 Copyright 2023, Royal Society of Chemistry.(c) Scheme illustrating the design principle of epitaxial metal electrodeposition. 56Copyright 2019, AAAS.(d) Dynamic evolution of an EDTA-Zn molecule during Zn plating. 81Copyright 2022, Wiley.(e) Comparative schematic of interfacial water structures in different electrolytes. 91Copyright 2023, Wiley.(f) Schematic illustration of the pH evolution in different electrolytes and the interfacial pH buffer mechanism enabled by the OAc − anion. 104Copyright 2022, Wiley.(g) Mechanical characteristics of different types of SEI layer. 25Copyright 2023, Wiley.

Fig. 4
Fig. 4 Illustration of theoretical simulations.(a) COMSOL simulation of Zn 2+ diffusion and distribution with different conditions. 135Copyright 2019, Wiley.(b) Electric field distribution simulation results of ZMAs with different scratches. 136Copyright 2023, Wiley.(c) The adsorption energy of Zn at different sites on the (100) and (002) crystal planes.55Copyright 2021, Wiley.(d) Comparison of the energy barrier required for normal and novel solvation structures to de-solvate through DFT calculation.166Copyright 2023, Elsevier.(e) Energy profiles of Zn cluster at different deposition states.169Copyright 2023, American Chemical Society.(f and g) Snapshot of the electrolyte containing xylitol additive (f) and its corresponding RDFs and CN (g) collected from MD simulations.30Copyright 2023, Wiley.

Fig. 6 Fig. 7
Fig. 6 Other in situ electron microscopy characterizations of ZMA.(a) In situ SECM feedback imaging of Zn electrodeposition process on freshly polished Zn substrates. 218Copyright 2023, Wiley.(b and c) In situ TEM images of the zinc plating/stripping process on Pt electrode. 219Copyright 2021, Elsevier.(d) In situ electrochemical LC-TEM imaging of Zn plating/stripping with LiCl additive. 220Copyright 2024, Wiley.

Fig. 9
Fig. 9 In situ physicochemical characterizations of ZMA.(a) In situ EIS of symmetrical batteries without/with taurine additive electrolyte. 14Copyright 2023, Wiley.(b) The distribution of relaxation times (DRT) plot of the in situ EIS data in ZS and Ni 2+ -ZS electrolyte. 49Copyright 2023, Wiley.(c and d) Cyclic voltammograms (CV) curves (c) and corresponding mass change (d) obtained from EQCM test. 107Copyright 2023, Wiley.(e) Diagram of UME detecting pH changes (inset is a microscopy image of pH UME). 107Copyright 2023, Wiley.

Fig. 10
Fig.10Minority and advanced characterizations of ZMA.(a) Schematic of the configuration used for observing H 2 evolution in situ.276Copyright 2024, Springer Nature.(b) Schematic of a transducer-waveguide assembly and ultrasonic signals on a waveguide before (blue) and after (red) the cycling.277Copyright 2022, Elsevier.(c) Schematics of a chromatic battery with a short circuit warning function.279Copyright 2018, Royal Society of Chemistry.(d) Scheme illustrating the process of smartphone multispectral imaging reconstruction and usage for in situ energy storage monitoring.280Copyright 2023, Wiley.

Table 1
Comparison of performance reported for different protective mechanisms via electrolyte engineering