Understanding the Role of Zinc Hydroxide Sulfate and its Analogues in Mildly Acidic Aqueous Zinc Batteries: A Review

Mildly acidic aqueous zinc batteries (AZBs) have attracted tremendous attention for grid storage applications with the expectation to tackle the issues of Li‐ion batteries on high cost and poor safety. However, the performance, particularly energy density and cycle stability of AZBs are still unsatisfactory when compared with LIBs. To help the development of AZBs, a lot of effort have been made to understand the battery reaction mechanisms and precedent microscopic and spectroscopic analyses have shown flake‐like large particles of zinc hydroxide sulfate (ZHS) and its analogues formed on the surfaces of cathodes and anodes in sulfate and other electrolyte systems during cycling. However, because of the complexity of the thermodynamics and kinetics of aqueous reactions to understand different battery conditions, controversies still exist. This article will review the roles of ZHS discussed in recent representative references aiming to shine light on the fundamental mechanisms of AZBs and pave ways to further improve the battery performance.


Introduction
Concerns on environmental pollution, energy crisis, and safety have been crucial topics, motivating researchers to develop eco-friendly and highly efficient energy storage systems (ESS).[12] Second, electrochemical potential of Zn redox reaction is low (i.e., −0.76 V versus standard hydrogen electrode (SHE)) and the theoretical capacity of Zn is high DOI: 10.1002/smtd.20230096515][16] Third, the manufacture cost for fabricating AZBs is expected to be low ($25 kW h −1 ) because of the intrinsic nature of aqueous battery and high resistance of cell components to air atmosphere and natural moisture. [17,18]espite these attractive advantages, there are challenging obstacles in approaching the commercialization stage of mildly acidic AZBs.[21] In the case of cathode, active materials to simultaneously accomplish high reversible capacity and stability are scarce. [22,23]Efforts have been made intensively to address these issues.[41][42][43] These multimodal approaches more or less have pushed the cell performance of mildly acidic AZBs to higher levels.Many review articles have summarized the progress and systematically categorize these precedent strategies in relation to the performance of mildly acidic AZBs. [18,19,22,23,44,45]However, to the best of our knowledge, there is not many review papers focusing on ZHS, which commonly forms on the surfaces of electrodes in mildly acidic AZBs.
ZHS usually is referring to a category of zinc hydroxide sulfate salts with different amounts of hydroxyl and sulfate anions as well as various amounts of crystalline water.It generally forms on the cathode surface during discharge of the AZB in ZnSO 4 electrolyte and reversibly dissolved in the electrolyte in the subsequent charge process.ZHS also has been found on the surface of Zn anode in AZBs.Unlike on the cathode surface, ZHS formation on the Zn anode surface seems to be irreversible.Recently, the precipitation of ZHS-like analogues also has been observed in other mildly acidic electrolytes.Zn 5 (OH) 8 (NO 3 ) 2 •nH 2 O [46] in Zn(NO 3 ) 2 based electrolyte, and Zn 5 (OH) 8 Cl 2 •nH 2 O [47] in ZnCl 2 based electrolyte have been reported.ZHS and its analogues were previously considered byproducts which are not directly involved in the electrochemical reactions of cathodes and anodes, hence the role of ZHS and its analogues on the cell performance has  [19] Copyright 2021, American Chemical Society.
not attracted much attention over the past few years.However, recently, quite several studies have revealed that ZHS, similar to the interface engineering in other battery systems, [48] at the electrode and electrolyte interface plays important role to the battery performance.Understanding the evolution of ZHS, the common "side product", potentially provides insight into the reaction mechanism of aqueous batteries.In this regard, this review focuses on ZHS and its analogues aiming to understand the phenomenon and mechanisms of ZHS formation and dissolution reactions on the surface of electrode as well as the role of ZHS in affecting the electrochemistry of the anode and cathode in mildly acidic AZBs.In specific, we summarize the prevalent mechanisms of ZHS formation and the positive and negative effects of ZHS on the cell performance of AZBs according to the results reported in some recent work.While further in-depth studies are still needed to clearly illuminate some of the controversial topics, we envision that the discussion and outlook in this review reveal some truth on the reaction mechanisms and will inspire more research for the development of highly rechargeable AZBs.

Crystal Structures and Formation Mechanisms of ZHS and its Analogues
ZHS usually is referring to a category of zinc hydroxide sulfate salts formed in zinc sulfate electrolyte.It can have different amounts of hydroxyl and sulfate anions as well as various amount of crystalline water with a general crystalline formula of Zn x (OH) 2x-2y (SO 4 ) y •nH 2 O. [49,50] The precipitation in other mildly acidic electrolytes such as Zn 5 (OH) 8 (NO 3 ) 2 •nH 2 O [46] in Zn(NO 3 ) 2 based electrolyte, and Zn 5 (OH) 8 Cl 2 •nH 2 O [47] in ZnCl 2 based electrolyte are ZHS-like analogues.Figure 1 below show crystal structures of ZHS and some of its analogues.
Taking Zn x (OH) 2x-2y (SO 4 ) y •nH 2 O as the model ZHS material, the crystal structure of ZHS generally corresponds to layered double hydroxides which consist of brucite-like Zn(OH) 2 layers containing extra neutral H 2 O molecules and SO 4 2− anions positioned between the layered-structure (Figure 1). [19,51,52]The crystal structure of ZHS is dependent on the radius of anion inserted between Zn(OH) 2 layers. [53]The interlayer distance of Zn(OH) 2 layer (typically in the range of 7-11 Å) is dependent on the num-ber of H 2 O molecules (e.g., 3, [54] 4, [55] and 5 [56] ) which is highly related with the local concentration of H 2 O molecules at the interface of electrolyte and electrode. [57,58]It should be noted that this H 2 O molecule can be easily extracted from the crystal structure of ZHS by thermal energy, [59] hence ex-situ analyses after drying the electrode at high temperature may provide distorted information on ZHS structure.Therefore, in-situ analyses are more reliable/suitable for clearly understanding the structures and mechanisms of ZHS formation and dissolution in batteries.

ZHS on the Surface of Cathode
Lee et al. investigated the formation and dissolution phenomenon of ZHS on the surface of a cathode by in-situ x-ray diffraction (XRD) analysis with a Zn-MnO 2 cell (Figure 2a). [59]uring the initial discharge, representative crystalline peaks corresponding to ZHS appears and continues to increase in intensity until the end of discharge.These ZHS peaks gradually decrease in intensity during the subsequent charge process and completely disappear at full charge.It indicates that formation and dissolution of ZHS is highly reversible during the discharge and charge process.The reversible formation of ZHS also was observed by Huang et al. using scanning electron microscopy (SEM) analyses (Figure 2b,c). [60]The SEM image of MnO 2 cathode after initial discharge shows that ZHS formed has a morphology of micrometer-sized two-dimensional flakes (Figure 2b).After charge, ZHS flakes almost completely disappeared under SEM (Figure 2c), consistent with in-situ XRD analysis.Despite the high reversibility of ZHS formation and dissolution, we need to pay attention that most of cathode surface can be partially covered by large particles of ZHS which affects the overall electrochemical performance of AZBs in some way.
The formation of ZHS during discharge process is commonly observed at the cathode, regardless of the type of cathode materials (e.g., inorganic, [61,62] and organic [63] cathodes).A generally acknowledged mechanism ascribes the ZHS formation to the electrochemical reaction involving the insertion and extraction of H + to/from the cathodes. [59,64]As shown in Figure 2d and   [59] Copyright 2016, Wiley.b) SEM images of the MnO 2 cathode at discharged and c) charged state.Scale bar is 10 μm.Reproduced with permission. [60]Copyright 2018, Springer Nature.d) Schematics of ZHS formation and dissolution mechanisms at the cathode.2d).With the increase of the depth of discharge (DOD), the local pH increases.When it reaches a critical point, the reaction of these emerging OH − species with Zn 2+ , SO 4 2− , and H 2 O results in the formation of ZHS in the vicinity of the cathode surface (Table 1a).During the charge process, H + ions are extracted from the cathode and local pH decreases with the increase of the state of charge (SOC), causing the dissolution of ZHS if no additional energy is needed.In this mechanism, reversible electrochemical insertion/extraction of H + to/from cathode active materials are triggered to induce the change of local OH − concentration on the surface of cathodes, linking the reversible precipitation/dissolution of ZHS to the battery specific capacity and cycle life.The reversibility of the ZHS formation in long-term cycling and under practical conditions of high loading and deep charge/discharge conditions are to be further verified.
Another perspective on the ZHS formation mechanism at the cathode surface involves O 2 reduction reactions. [65] 1b).This new mechanism is very effective in explaining the existing experimental results.While systematic, high precision experiments are needed for validation of the O 2 reduction and zero intercalation of H + .It is going to be a very complicated task considering the overall potential of ORR reactions on different catalysts surface and the solubility of O 2 in electrolytes.

ZHS on the Surface of Zn Metal Anode
The formation of ZHS occurs at the cathode as well as at the Zn anode.
Step-by-step reactions are as follows: i) H + in mildly acidic electrolyte receives the electrons from Zn metal and converts to hydrogen (H 2 ) (i.e., 2H + + 2e − → H 2 ) [19] and ii) this HER on the surface of Zn anode resulted in the concentration increase for OH − anions near the Zn anode surface (Figure 3a).The increase of local pH induces the chemical reaction of OH − anions with Zn 2+ , SO 4 2− anions, and H 2 O molecules and subsequent ZHS precipitation.Reaction c in Table 1 shows the total reaction formula for HER and ZHS formation reaction while Figure 3a shows the schematic of the ZHS formation.Guo research group observed the formation of ZHS flake-like large particles on the surface of Zn anodes after soaking the Zn anode in the 1 M ZnSO 4 electrolyte for one week (Figure 3b), [20] consistent with thermodynamic fundamentals of Zn Pourbaix diagram. [66]Ex-ternal potential applied to the anode side under the conditions in which actual Zn aqueous cell operates accelerates the kinetics of HER reactions.Vigorous precipitation of ZHS is usually observed after cycling.Unlike the mechanism of ZHS formation on the cathode surface, there is only one chemical reaction mechanism for the ZHS precipitation at the anode side currently.The ZHS formation at the anode side most likely is irreversible because it is mainly derived from the irreversible side reaction of electrolyte decomposition.Figure 3c shows the ex-situ XRD pattern of the pristine Zn anode and the anode after 1 and 50 cycles. [20]Significant increase of the ZHS crystalline peak was observed as cycle number increases from 1 to 50 (Figure 3c), corroborating the poor reversibility of ZHS formation and dissolution reaction on the Zn anode surface.
From the above discussion, ZHS formation on either anode or cathode surface involves two steps: (i) change of OH − concentration and (ii) precipitation of ZHS by chemical reactions of OH − with electrolyte species (Table 1).Considering the mildly acidic electrolyte with the pH of ∼4, no matter what the mechanism is, H 2 O involves in the reactions.In order to improve the battery reversibility, it is essential to improve the reaction reversibility involving H 2 O and ZHS or use other ions as charge carriers for the redox reactions while mitigating the reactions involving H 2 O.

The Role of ZHS on Cathode Electrochemistry
Despite a few controversial points on the formation mechanisms of ZHS, it is plain fact that ZHS is heavily formed on the cathode surface during the discharge process and remains until some point in the charge process before the complete dissolution.Considering the physical/chemical states at the electrode-electrolyte interface are crucial factors to determine the overall battery performance, ZHS affects electrochemistry of cathodes in various ways.

The Effect of ZHS on the Transfer of Electrons and Ions
It can be intuitively expected that precipitation of large particles on the surface of cathode materials greatly affects the transfer behaviors of electrons and ions throughout the cathode. [67]In this regard, Cheng's group carried out ex-situ XRD and in-situ impedance analyses to understand the ZHS effect on the charge transfer in the organic cathode cycled with ZnSO 4 electrolyte. [63]s shown in Figure 4a, ZHS starts to form at 1.4 V during discharge according to the ex-situ XRD patterns of the cathode.The intensity of ZHS crystalline peaks gradually increases until the potential reaches 0.1 V (the last point of discharge), indicating the continuous growth of ZHS on the cathode surface.Correspondingly, the in-situ electrochemical impedance spectroscopy (EIS) results show that the semicircle size of Nyquist plot also continues to increase during discharge from 1.6 V to 0.1 V (Figure 4a).This well-matched proportional relationship between the intensity of ZHS crystalline peaks and the semicircle size of Nyquist plot implies that ZHS formation on cathode surface causes the increase of the charge transfer resistance.During the subsequent charge process from 0.1 V to 1.6 V, the semicircle size of Nyquist plot gradually decreases, as the intensity of ZHS crystalline peaks  [20] Copyright 2020, Wiley.c) XRD patterns of Zn anode before cycling and after cycling.Reproduced with permission. [20]Copyright 2022, Wiley.b) Discharge and charge profiles and Nyquist plots of the Zn-MnO 2 cell with different state of discharge process.Reproduced with permission. [71]Copyright 2019, Royal Society of Chemistry.c) Schematics of effect of ZHS on transfer behavior of electrons and ions on the surface of MnO 2 cathodes.
decrease.It is noteworthy that the semicircle size of Nyquist plot at the 1.6 V after 1 cycle is almost similar with that of Nyquist plot at the 1.6 V of at the beginning of the discharge.This indicates the charge transfer properties of cathode active material itself did not change significantly after 1 cycle.The reversible precipitation and dissolution of the majority of ZHS during the discharge and charge process is responsible for the charge transfer resistance change.[70] For example, Chi et al. also characterized the charge transfer resistance of two different MnO 2 cathodes by in-situ EIS analysis. [68]They specially designed MnO 2 cathode (ZMOC) which shows smaller and more evenly distributed ZHS precipitation than pristine MnO 2 cathode by regulating the electric field.Interestingly, ZMOC cathode exhibits much smaller change of the semicircle size of Nyquist plot than the pristine MnO 2 cathode, of which the semicircle size of Nyquist plot drastically increases and decreases during the discharge and charge process, respectively.It indicates that the formation of large and uncontrolled ZHS particles on cathode surface is not favorable for facile charge transfer, which increases the internal resistance of AZBs.
In another effort, Wang et al. investigated the Warburg coefficient of MnO 2 cathode by in-situ EIS analysis. [71]The cathode at different discharged states were collected (i.e., point H and J which were discharged to 1.4 and 1.3 V, respectively) (Figure 4b).At these different discharged states, ZHS formation behaviors during discharge are expected to be significantly dif-ferent.The cathode stopped at point J (corresponding to the 2nd plateau) shows a larger amount of ZHS precipitation than the cathode stopped at point H (corresponding to the 1st plateau).In Nyquist plots of these two cathodes, the sloping line of the cathode stopped at point H is steeper than that of the cathode stopped at point J (Figure 4b).Considering that this sloping line observed at low frequency region has proportional relationship with the Warburg diffusion coefficient of charge carrying ions in the electrode, [72][73][74] the ion transfer behavior clearly is disrupted by large ZHS particles precipitated on the surface of cathode.Consequently, the ZHS formation during the discharge process can act as a physical barrier which deteriorates the facile transfer of electrons and ions (e.g., H + and Zn 2+ ) at the interface of electrode and electrolyte (Figure 4c). [69]everaging this understanding, strategies to mitigate the ZHS formation phenomenon on the surface of cathodes are rational to facilitate the transfer behaviors of electrons and ions.Guo et al. coated ultrathin hafnium oxide (HfO 2 ) film with the thickness of 5 nm on the surface of Zn 3 V 2 O 7 (OH) 2 •2H 2 O (ZVO) cathode using atomic layer deposition method. [70]This artificial HfO 2 film decreases the accessibility of electrolyte components (i.e., reactants of the chemical reactions of ZHS formation) to the surface of ZVO cathode.As a combined result of the surface coating and primarily Zn 2+ cation insertion, large flake-like ZHS particles are not observed in the ex-situ SEM image of HfO 2 coated ZVO after cycling, whereas the SEM image of pristine ZVO without artificial layer after cycling shows the huge amount of ZHS particle on the surface (Figure 5a).Furthermore, EIS analyses were performed to estimate the charge transfer resistance of ZVO and  [70] Copyright 2019, American Chemical Society.b) The estimated charge transfer resistance of ZVO and HfO 2 coated ZVO cathodes during the discharge and charge process.Reproduced with permission. [70]Copyright 2019, American Chemical Society.c) Schematics of the effect of pH buffer additives to stabilize the pH value.Reproduced with permission. [75]Copyright 2023, Wiley.d) XRD patterns of carbon paper, pristine MnO 2 @carbon paper, cycled MnO 2 @carbon paper cathode with/without NHP additives.Reproduced with permission. [75]Copyright 2023, Wiley.
HfO 2 coated ZVO cathodes (Figure 5b).In the case of ZVO cathodes, the charge transfer resistance increases drastically during the discharge because of the formation of insulating ZHS on ZVO surface.During the subsequent charge, charge transfer resistance decreases reversibly with ZHS dissolution.Meanwhile, HfO 2 coated ZVO cathodes show almost same charge transfer resistance to that of the ZVO at the beginning of discharge process, indicating that artificial HfO 2 layer does not affect charge transfer behavior of ZVO cathode before the formation of ZHS.It is noteworthy that the HfO 2 coated ZVO cathode shows highly stable charge transfer resistance without any significant change in the entire discharge and charge cycle.It implies that the suppression of ZHS formation by coating the HfO 2 artificial layer on the surface of ZVO cathode can stabilize the charge transfer behavior at the surface of cathode by inhibiting the formation of ZHS.
In addition to cathode surface modification, electrolyte engineering strategy also has been studied to suppress the ZHS formation on cathode surface.Zhang et al. introduced pH buffer additive (i.e., ammonium dihydrogen phosphate (NHP)) in ZnSO 4 based electrolyte. [75]They suggested that (NH 4 ) + and (H 2 PO 4 ) − of NHP additives can react with OH − anions to form NH 3 •H 2 O Figure 6.a) Cycle performance of Zn/ZHS cell in 2.0 M ZnSO 4 electrolyte.Reproduced with permission. [76]Copyright 2020, Elsevier.b) Schematics of the role of ZHS in the discharge process.Reproduced with permission. [76]Copyright 2020, Elsevier.c) Optical images of MnO@C cathode with/without ZHS layer in the ZnSO 4 electrolyte.Reproduced with permission. [80]Copyright 2022, Elsevier.d) Comparison of Mn 2+ dissolution of MnO@C cathode with and without ZHS layer in the ZnSO 4 electrolyte.Reproduced with permission. [80]Copyright 2022, Elsevier.and (HPO 4 ) 2− + H 2 O, respectively, when the local concentration of OH − increases (Figure 5c).(H 2 PO 4 ) − of NHP also can react with H + ions to form H 3 PO 4 when the local concentration of H + increases.These chemical reactions of NHP additives with H + and OH − efficiently stabilize the pH fluctuation at the electrode and electrolyte interface.As a result, ZHS formation is suppressed.The XRD patterns of cycled MnO 2 cathode with NHP additive containing electrolyte in Figure 5d does not exhibit any crystalline peaks corresponding to ZHS compared to the one without NHP.The effect of electrolyte engineering on mitigated ZHS formation happens at both the cathode and anode.The synergistic effect of suppressed ZHS formation on both cathode and anode surface by NHP additive delivers improved performance over the one without NHP.

The Effect of ZHS on Discharge Capacity and Cyclability
The above preceding studies elucidated the formation of large ZHS particles on cathode surface is not favorable to facilitate the charge transfer behavior.Thus, the inhibition of the ZHS formation is helpful to improve the cell performance of AZBs.Besides the charge transfer behaviors, discharge capacity and cycling stability also are crucial to the performance of AZB and can be affected by the formation of ZHS.The ZHS has been considered electrochemically inactive.The Zn/ZHS cell (Zn and ZHS for anode and cathode, respectively) with ZnSO 4 electrolyte exhibits the negligible capacity (1.7 mA g −1 ) at the 1st cycle and the capacity remains low even after 50 cycles (Figure 6a). [76]This corroborates that capacity contribution of ZHS itself is negligible for AZBs.ZHS formed during the discharge covers the active surface of MnO 2 cathodes (Figure 6b) inhibits the subsequent access of electrolyte component.It causes the decrease of the amount of active H 2 O on the surface of cathode materials. [76]In the case of MnO 2 cathode in mildly acidic electrolyte (i.e., pH range of 4-5, H + concentration of ≈10 −4 −10 −5 mol L −1 ), the H + ions generated from the dissociation of active H 2 O is a main source for the electrochemical reaction with MnO 2 cathode materials. [53]Thus, poor accessibility of active H 2 O to the MnO 2 cathode surface because of the formation of ZHS leads to the decrease of discharge capacity.
ZHS covering the cathode surface also can block the Mn 2+ from depositing back onto the cathode during the subsequent charge process (Figure 6b).Several attempts have been tried to demonstrate this negative effect of ZHS on discharge capacity. [77]n these experiments, increased capacity has been observed for the cells after first discharge and removal of the ZHS deposition by mild acid solution washing.While the results are convincing, control experiments with more systematical design that considers all kinds of variable introduced to the system is needed to have an unambiguous conclusion.Furthermore, the suppressed accessibility of electrolyte components on the surface of MnO 2 cathode by the formation of ZHS may be a double-edged sword.The dissolution of Mn 2+ into electrolyte also is considered one of the major issues that cause the poor cycle stability of MnO 2 cathode. [24,78,79]The poor accessibility of H 2 O molecules which are good solvent for Mn 2+ to the cathode can help inhibit the Mn 2+ dissolution issues and improve the cycling stability.Du and coworkers investigated that the effect of ZHS nanoflake layer on the dissolution behaviors of Mn 2+ . [80]It is found that the ZHS nanoflake coated MnO@C cathode exhibits different Mn 2+ dissolution behaviors compared to the uncoated MnO@C cathode.When the uncoated MnO@C cathode at discharged states was immersed in ZnSO 4 electrolyte, the color of solution changed slightly after 14 days, implying the dissolution of cathode species into the electrolyte at some extent (Figure 6c).Whereas the electrolyte shows no notable color change after the ZHS nanoflake coated MnO@C cathode at discharged state soaked for 14 days.The inductively coupled plasma optical emission spectrometer (ICP-OES) analysis result of electrolyte for uncoated MnO@C cathode shows drastic increase of concentration ratio of Mn 2+ to Zn 2+ , consistent with results of optical images (Figure 6d).The ICP-OES does not show significant change of concentration ratio of Mn 2+ to Zn 2+ for the electrolyte that soaks ZHS nanoflake coated MnO@C cathode.Consequently, the formation of ZHS on the surface of MnO 2 cathode that has dissolution issue of active materials into the electrolyte can mitigate the loss of active species.
It must be mentioned that even though Du and coworkers demonstrated the inhibition of Mn 2+ dissolution phenomenon by artificial ZHS layer, apple-to-apple comparison of the cell performance of mildly acidic AZB between MnO 2 cathode with and without ZHS layer was not performed.Strictly speaking, it is still unclear whether ZHS layer can improve the cycle stability of the mildly acidic AZBs under operated conditions.Additional electrochemical testing and ex-situ or in-situ analyses are required to correlate the cycle stability of real AZBs with the effect of ZHS layer on suppressing the Mn 2+ dissolution.

The Effect of ZHS on the MnO 2 Deposition
As discussed in Section 3.1 and 3.2, several pioneering researchers suggest that the formation of ZHS is not favorable for facilitating the transfer behaviors of charge carriers nor for increasing the discharge capacity of MnO 2 cathode.Thus, strategies to suppress the ZHS formation reaction is believed to be required to achieve high-performance mildly acidic AZBs.On the other hand, some other researchers propose that ZHS is helpful to enhance the performance of mildly acidic AZBs by lowering the overpotential of conversion reaction of Mn 2+ to MnO 2 and hence accelerating the deposition of MnO 2 during charge.
Lee et al. designed a cyclic voltammetry (CV) study and observed that MnO 2 deposition onto ZHS cathode (synthesized by simple chemical reaction at alkaline condition) has lower redox potential than onto bare carbon paper cathode. [81]The CV analysis was performed on ZHS coated stainless steel and carbon paper cathode with an electrolyte that consists of 1.0 M ZnSO 4 + 0.2 M MnSO 4 (Figure 7a).Interestingly, the initial anodic peak corresponding to electrochemical oxidation reaction of Mn 2+ to Mn 4+ (i.e., MnO 2 ) sharply appears at 1.55 V in the CV result of ZHS cathode, whereas CV of carbon paper cathode exhibits the relatively broad anodic peak at above 1.68 V.The difference of the anodic peak potential and shape between ZHS and carbon paper cathodes indicates that ZHS is helpful to enhance the electrochemical oxidation reaction of Mn 2+ , hence, facilitates the deposition of MnO 2 during the charge process.
Zhou research group also recently reported the positive effect of ZHS on oxidation electrochemistry of Mn 2+ during charge process. [82]In their experiment, they designed a beaker cell with three different electrodes (i.e., Ti, Zn, and MnO 2 electrodes) and ZnSO 4 solution electrolyte (Figure 7b) for electrochemical testing and used SEM to track the electrode surface evolution.For the 1st discharge, Zn and MnO 2 electrodes were used as reference/counter and working electrode, respectively.The conversion reaction from MnO 2 to Mn 2+ occurred during the discharge, so Mn 2+ species were partially dissolved into the electrolyte.For the subsequent charge process and following cycles, Zn and Ti electrodes were used as reference/counter and working electrode, respectively.In the 1st charge, Mn 2+ dissolved in electrolyte gradually converts to Mn 4+ , hence, MnO 2 is deposited on the surface of Ti electrode during the charge process.This charge voltage profile shows the MnO 2 deposition on Ti surface occurs at 1.73 V (versus Zn/Zn 2+ ) (Figure 7c).The inset SEM images of Ti electrode shows the formation of MnO 2 .The inset SEM image of Ti electrode after the 2nd discharge process shows the formation of ZHS.The voltage profile of the 2nd charge shows the MnO 2 deposition reaction occurs at 1.52 V (versus Zn/Zn 2+ ), lower than MnO 2 deposition potential in the 1st charge.It indicates that MnO 2 deposition on the surface of ZHS-containing cathode is kinetically more favorable than on the surface of cathode without ZHS.
Very recently, Bao's research group proposed a ZHS-assisted deposition-dissolution model for Zn-Mn batteries based on their Zn-ZHS experimental design. [55]It was believed that ZHS directly participates in electrochemical reduction and oxidation reactions and the reversible conversion between ZHS and layered zinc vernadite (Zn x MnO(OH) 2 ) nanosheet through the transfer of Mn 2+ and H + during discharge and charge process is key to high-performance Zn-MnO 2 batteries (Figure 8a).Almost linear relationship between the ZHS amount and charge capacity of Mn 2+ oxidization (Figure 8b) was observed when ZHS cathode was cycled against Zn metal in 2 M ZnSO 4 + 0.5 M MnSO 4 electrolyte.Fluorine-doped tin oxide (FTO) substrate with half surface coated with ZHS clearly showed the deposition and dissolution reaction whereas no reaction was observed at the half surface without ZHS coating (Figures 8c-e).These experiments together with systematic characterization clearly demonstrated the critical role of ZHS to Zn-MnO 2 batteries and beyond.However, because of the complicated aqueous reactions and materials structures, extensive and thorough examination is needed to unambiguously validate whether ZHS is "active material" or "by-product" that has a catalytic function and intimately associates to the redox reaction.As aforementioned, the formation of ZHS is helpful to improve the kinetics of charge reactions and can possibly result in the preferential deposition if charged within an appropriate voltage window.it would not be too surprised to have a linear relationship between the charge capacity and ZHS amount if there is excess amount of electrolyte and enough surface area for material deposition.Reproduced with permission. [81]opyright 2021, Royal Society of Chemistry.b) Schematic diagram of the three electrodes test.Reproduced with permission. [82]Copyright 2022, Elsevier.c) The SEM images of Ti foam and corresponding charge-discharge curves using Ti foam as cathode.Reproduced with permission. [82]Copyright 2022, Elsevier.

The Role of ZHS on Anode Electrochemistry
While the role of ZHS in the cathode part is still controversial, the formation of ZHS on the surface of Zn anode is usually considered detrimental to the overall cell performance of mildly acidic AZBs.The ZHS formation on the Zn anode surface is mainly derived from irreversible HERs.ZHS formation is not very reversible and its amount on the metal anode gradually accumulates and covers the anode surface as cycle number increases. [20]imilar to cathode part, ZHS formation on the surface of Zn anode can disrupt the transfer behaviors of charge carriers.Considering that ZHS precipitation is a chemical reaction with consumption of ZnSO 4 salt and active H 2 O molecules, the accumulation of solid-phase ZHS also indicates the continuous loss of salt and solvent molecules.It slowly deteriorates the ionic conductivity of the electrolyte. [83,84]The irreversible consumption of H 2 O solvent molecules also leads electrolyte to reach the sat-uration state and partial crystallization of salt after long-term cycling. [85,86]n this context, strategies to suppress the ZHS formation reaction on the Zn anode surface have been widely studied.Zhang et  al. proposed that the use of carbon dots interlayer with abundant electronegative functional groups on the Zn anode (Zn@CDs) can suppress the precipitation reaction of ZHS (Figure 9a). [87]his carbon dots layer can act as a physical barrier to partially suppress the direct contact of Zn anode with excess electrolyte, which inhibits the HER side reactions at the Zn anode.Furthermore, the electronegative functional groups on carbon dots can electrostatically repulse the SO 4 2− anions, leading to the lack of reactant to precipitate the ZHS on the Zn surface.As a result, crystalline peak corresponding to ZHS is not observed in the XRD pattern of cycled Zn@CDs, whereas XRD pattern of uncoated Zn anode exhibits the sharp ZHS crystalline peaks (Figure 9b).When this specially designed Zn@CDs anode is assembled with Reproduced with permission. [55]Copyright 2022, Wiley.b) Charge areal capacity with different active mass of ZHS.Reproduced with permission. [55]Copyright 2022, Wiley.c-e) Digital photos of the transparent Zn-ZSH/FTO battery at different charge and discharge states at 200 mA g −1 .Reproduced with permission. [55]Copyright 2022, Wiley.
Na-doped V 2 O 5 (NVO) cathode, Zn@CDs/NVO full cell shows highly improved cycle stability over the full cell with bare Zn anode.Other protective layers on the surface of Zn anode such as fullerene and covalent organic frameworks also have been widely studied to prevent the ZHS formation reaction and improve the cycle stability of mildly acidic AZBs. [88,89]Strategies including the design of highly concentrated electrolyte [90] and hydrogel-type electrolyte [91] also have been considered efficient owing to the drastic decrease of activity of H 2 O.
Very recently, several researchers suggested a new perspective to achieve high-performance mildly acidic AZBs by turning the ZHS from "obsolete wastes" to "useful valuables". [92,93]Sung and coworkers prepared the nano-sized uniform porous ZHS layer on the surface of Zn anode (Zn/PZL) and demonstrated significantly improved performance. [92]They electrodeposited a layer of Mg(OH) 2 on Zn anode and followed by its conversion to porous ZHS layer in aqueous ZnSO 4 based electrolyte.The porous ZHS layer prepared by the unique method is not composed of the irregular large particles, but uniformly distributed 200 nm-sized small flake-like particles (Figure 9c).EIS analysis results indicate that charge transfer resistance of Zn/PZL is lower than that of bare Zn (Figure 9d), which does not match with general cognition that ZHS products increase the charge transfer resistance of electrode.Xin et al. also reported interesting experimental results, corroborating Sung and coworkers' discovery. [93]hey deposited a layer of uniformly controlled ZHS on the Zn anode surface by using Zn(OH) 2 as an additive.Interestingly, the charge transfer resistance of Zn anode with uniform ZHS layer (i.e., using 0.02 M Zn(OH) 2 ) is lower than bare Zn anode with the deposition of irregular ZHS particle (i.e., using bare 2 M ZnSO 4 without additive) (Figure 9e).The cycle stability of Zn-organic cathode full cell with uniform ZHS deposition also improved significantly (Figure 9f).These works provided an alternative aspect for future research that it may not be reasonable to consider ZHS simply as a bad byproduct which can deteriorate the cell performance of AZBs.ZHS has intrinsic 2D layer structure providing good ion diffusion channels. [92,93]The ZHS of bulk dimension/morphology and high crystallinity, however, can lead to the increase of the ion diffusion length, poor accessibility of electrolyte, and decrease of electronic conductivity at the electrode-electrolyte interface.Furthermore, the structure and morphology of ZHS can also affect the reversibility of  [87] Copyright 2022, Wiley.b) XRD patterns of each electrode after cycling tests at 2 mA cm −2 with a deposited amount of 1 mA h cm −2 .Reproduced with permission. [87]Copyright 2022, Wiley.c) Surface SEM image of Zn/PZL electrode.Reproduced with permission. [92]Copyright 2023, Elsevier.d) Nyquist plots of bare Zn and Zn/PZL electrodes at the different cycle numbers.Reproduced with permission. [92]Copyright 2023, Elsevier.e) Nyquist plots of Zn/Zn symmetric cells with and without Zn(OH) 2 additive.Reproduced with permission. [93]Copyright 2021, American Chemical Society.f) Cycle performance of Zn/organic cathode cells with and without Zn(OH) 2 additive at 0.5 A g −1 .Reproduced with permission. [93]Copyright 2021, American Chemical Society.
the deposition/dissolution or the conversation/catalysis process of ZHS during battery cycling.Therefore, it is reasonable to propose that the negative effect of the conventional ZHS formation phenomenon (e.g., increase of charge transfer resistance) on cell performance may be derived from the irregular and uncontrolled large particle properties, not necessarily from the intrinsic properties of ZHS (e.g., high Zn ion diffusivity).Modulation of physical properties of ZHS can be one of future ways to maximize the facile charge transfer behaviors and subsequently improve the cell performance of mildly acidic AZBs.That includes but not limited to modulating the particle size at nanoscale level, controlling the uniform distribution/coverage on the electrode surface, as well as construction of well-controlled porous architectures.

Summary and Outlook
Rechargeable AZBs in mildly acidic conditions with intrinsic safety and low cost have been considered promising for gridscale energy storage.However, the performance of the state-ofthe-art AZBs is still far from meeting the requirement of grid applications, particularly on energy density and cycle stability.Disruptive innovation is needed for future development of AZBs if we don't want to repeat the >100 years of history.Thorough understanding of the system is prerequisite for the next breakthrough.First, to understand the electrochemical and chemical reactions, charge transfers and structure evolutions in the bulk and at the interfaces and interphases in AZBs, we need to leverage the tremendous progress on the development of the varieties of advanced in-situ and ex-situ characterizations of high sensitivity, high spatial and temporal resolution and cryo capabilities (Figure 10).In this regard, the science in highly "local" areas, extremely "short" timescales, exceptionally "low" concentrations, and their effects across all the scales on the reaction thermodynamics/kinetics, the material structure evolution, and the electrochemical performance can be accurately captured.For example, the ZHS formation and dissolution is still mainly investigated by common XRD, x-ray photoelectron spectroscopy (XPS), SEM, transmission electron microscopy (TEM) or even in-situ pH probe.The source of OH − anions and the evolution of Zn 2+ ions with or without Mn 2+ ions are controversial.The dynamics of OH − concentration change (ΔC OH-) including the spatial and temporal factors on the behaviors of ZHS formation and distribution remain to be further ascertained.Developing and harnessing the advanced characterization capabilities will lead to understanding some of the information on water involvement and ZHS evolution, which we believe can pave the way to regulate the chemical and electrochemical reactions for high-performance AZBs.
Second, we need to gradually move from fundamental study to technology development by transferring the knowledge learned from model systems to AZBs that are relevant to practical conditions (Figure 10).For example, many of precedent research studied ZHS formation and dissolution reaction by analyzing the thin electrode performance for only a few cycles.The reversibility of the ZHS formation and dissolution reaction after long-term cycling is not certain.Also, the fundamental study may reveal ZHS has pros or cons to the battery performance at the same time, while the reversibility and dominant effect may vary when the battery has high loading electrode, large material utilization rate, lean electrolyte, or different testing rates/voltage windows.If the formation of ZHS at close to practical conditions involves the consumption of electrolyte components over long term cycling, it probably would mean irreversible loss of salt and solvent molecules, which will result into the failure of AZBs.Investigating AZBs such as the ZHS formation and dissolution behaviors at operating conditions close to practical applications is necessary.

Figure 2 .
Figure 2. a) In-situ XRD patterns and voltage profiles of a cathode in a Zn-MnO 2 cell with 1.0 M ZnSO 4 electrolyte during the initial discharge and charge at 0.05 C rate.Reproduced with permission.[59]Copyright 2016, Wiley.b) SEM images of the MnO 2 cathode at discharged and c) charged state.Scale bar is 10 μm.Reproduced with permission.[60]Copyright 2018, Springer Nature.d) Schematics of ZHS formation and dissolution mechanisms at the cathode.
For V 3 O 7 •H 2 O cathodes tested in 1 M ZnSO 4 aqueous electrolyte, the capacity are believed to be dominantly contributed by the reversible insertion and extraction of Zn 2+ ions, whereas the involvement of H + ions in the electrochemical reaction of cathodes is negligible.As a result, the H 2 O decomposition is triggered by other reactions to form OH − to react with Zn 2+ , SO 4 2− forming ZHS, which has been observed in the operando XRD patterns during the discharge of V 3 O 7 •H 2 O. Since no pH change has been detected by the in-situ pH meter during the discharge, the hypothesis is that of the oxygen (O 2 ) dissolved in the electrolyte went through oxygen reduction reaction (ORR) to form OH − , which react with Zn 2+ and SO 4 2− and H 2 O to precipitate in the form of ZHS (Table

Figure 3 .
Figure 3. a) Schematics of ZHS formation mechanism at the Zn anode surface.b) SEM images of Zn anode before and after soaking in electrolyte for one week.Reproduced with permission.[20]Copyright 2020, Wiley.c) XRD patterns of Zn anode before cycling and after cycling.Reproduced with permission.[20]Copyright 2020, Wiley.

Figure 4 .
Figure 4. a) Ex-situ XRD patterns and in-situ EIS profiles of organic cathode with different charge/discharge states in 2.0 M ZnSO 4 electrolyte.Reproduced with permission.[63]Copyright 2022, Wiley.b) Discharge and charge profiles and Nyquist plots of the Zn-MnO 2 cell with different state of discharge process.Reproduced with permission.[71]Copyright 2019, Royal Society of Chemistry.c) Schematics of effect of ZHS on transfer behavior of electrons and ions on the surface of MnO 2 cathodes.

Figure 5 .
Figure 5. a) SEM images of cycled ZVO and HfO 2 coated ZVO cathodes.Reproduced with permission.[70]Copyright 2019, American Chemical Society.b) The estimated charge transfer resistance of ZVO and HfO 2 coated ZVO cathodes during the discharge and charge process.Reproduced with permission.[70]Copyright 2019, American Chemical Society.c) Schematics of the effect of pH buffer additives to stabilize the pH value.Reproduced with permission.[75]Copyright 2023, Wiley.d) XRD patterns of carbon paper, pristine MnO 2 @carbon paper, cycled MnO 2 @carbon paper cathode with/without NHP additives.Reproduced with permission.[75]Copyright 2023, Wiley.

Figure 7 .
Figure 7. a) CV curves of ZHS and carbon paper electrodes for the first cycles with potential scan rate of 0.03 mV s −1 .Reproduced with permission.[81]Copyright 2021, Royal Society of Chemistry.b) Schematic diagram of the three electrodes test.Reproduced with permission.[82]Copyright 2022, Elsevier.c) The SEM images of Ti foam and corresponding charge-discharge curves using Ti foam as cathode.Reproduced with permission.[82]Copyright 2022, Elsevier.

Figure 8 .
Figure 8. a) Schematics of the reversible conversion reaction between ZHS and Zn x MnO(OH) 2 .Reproduced with permission.[55]Copyright 2022, Wiley.b) Charge areal capacity with different active mass of ZHS.Reproduced with permission.[55]Copyright 2022, Wiley.c-e) Digital photos of the transparent Zn-ZSH/FTO battery at different charge and discharge states at 200 mA g −1 .Reproduced with permission.[55]Copyright 2022, Wiley.

Figure 9 .
Figure9.a) Schematics of the effect of carbon dots protective layer on the Zn anode surface.Reproduced with permission.[87]Copyright 2022, Wiley.b) XRD patterns of each electrode after cycling tests at 2 mA cm −2 with a deposited amount of 1 mA h cm −2 .Reproduced with permission.[87]Copyright 2022, Wiley.c) Surface SEM image of Zn/PZL electrode.Reproduced with permission.[92]Copyright 2023, Elsevier.d) Nyquist plots of bare Zn and Zn/PZL electrodes at the different cycle numbers.Reproduced with permission.[92]Copyright 2023, Elsevier.e) Nyquist plots of Zn/Zn symmetric cells with and without Zn(OH) 2 additive.Reproduced with permission.[93]Copyright 2021, American Chemical Society.f) Cycle performance of Zn/organic cathode cells with and without Zn(OH) 2 additive at 0.5 A g −1 .Reproduced with permission.[93]Copyright 2021, American Chemical Society.

Figure 10 .
Figure 10.The schematics of investigating mildly acidic AZBs from the fundamental study and technology development to the realization of practical cells.

Table 1 ,
H + inserts into the cathode material during the discharge

Table 1 .
Summary of electrochemical and chemical reactions for ZHS formation on the surface of different types of electrodes.Small Methods 2024, 8, 2300965 process, H 2 O molecules dissociates because the electrolyte pH is ≈4 to 6, corresponding to a low H + concentration of ≈10 −4 to 10 −6 M. The dissociation of H 2 O and the insertion of H + into cathode materials leads to the increase of the hydroxide ion (OH − ) concentration (i.e., increase of local pH) (Figure