Research progress on the design of electrolyte additives and their functions for zinc-ion batteries

In recent years, zinc-ion batteries (ZIBs) have been considered one of the most promising candidates for next-generation electrochemical energy storage systems due to their advantages of high safety, high specific capacity and high economic efficiency. As an indispensable component, the electrolyte has the function of connecting the cathode and the anode, and plays a key role in the performance of the battery. Different types of electrolytes have different effects on the performance of ZIBs, and the use of additives has further developed the research on modified electrolytes, thus effectively solving many serious problems faced by ZIBs. Therefore, to further explore the improvement of ZIBs by electrolyte engineering, it is necessary to summarize the current status of the design of various electrolyte additives, as well as their functions and mechanism in ZIBs. This paper analyzes the challenges faced by different electrolytes, reviews the different solutions of additives to solve battery problems in liquid electrolytes and solid electrolytes, and finally makes suggestions for the development of modified ZIB electrolytes. It is hoped that the review and strategies proposed in this paper will facilitate development of new electrolyte additives for ZIBs.


Introduction
Lithium-based battery technology has advanced significantly during the past 30 years, both commercially and academically 3 Co-first authors.* Authors to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.[1,2].Due to their high energy density and extended cycle life, rechargeable lithium-ion batteries (LIBs) have dominated the energy market, as they are being used in everything from electric vehicles and mobile devices to energy storage systems and smart grid storage.However, growing concerns about potential safety issues, high cost, limited lithium resources and environmental impact have prompted the search for effective alternative battery systems [3,4].Fortunately, zinc-ion batteries (ZIBs) may offer a promising solution to these problems, due to their high theoretical capacity (820 mAh g −1 , 5855 mAh cm −3 ), low redox potential (−0.76 V versus SHE), Research advances in various kinds of electrolytes and additives in Zn-ion batteries.This includes liquid electrolytes (divided into aqueous systems, organic systems and water-organic hybrid systems), solid electrolytes (divided into hydrogel solid systems, polymeric gel systems and all-solid systems).Reprinted from [11], Copyright (2022), with permission from Elsevier.[13]  [© 2021 Wiley-VCH GmbH].Reprinted from [22], Copyright (2022), with permission from Elsevier.Reprinted from [23], Copyright (2022), with permission from Elsevier.[24] John Wiley & Sons.[© 2021 Wiley-VCH GmbH].
high abundance and superior safety compared to lithium and other metals, which support their development as an electrochemical system for large-scale energy storage applications [5][6][7].The migration of Zn 2+ ions between Zn metal anode and cathode materials is the charge storage mechanism in ZIBs, and the electrolyte, as the main component of ZIBs, plays an important role in Zn 2+ transport between the anode and cathode, determining the electrochemically stable potential window, and affecting the efficiency and storage behavior of the Zn 2+ repository [6,8].According to the current research on ZIB electrolytes, electrolytes can be divided into liquidstate electrolytes and solid-state electrolytes.Liquid electrolytes include aqueous systems [9], organic systems [10] and water-organic hybrid systems [11], while solid electrolytes mainly include hydrogel solid systems [12], polymeric gel systems [13] and all-solid systems [14].
The modification of the electrolyte also affects the performance of the battery, such as electrochemical stability, plating/stripping reversibility, cycle life and even the reaction mechanism [15,16].To date, the electrolytes of ZIBs have mainly consisted of aqueous solutions, which are safer, more environmentally friendly and more ionically conductive than organic electrolytes [17].However, challenges of aqueous ZIBs include the limited electrochemical stability window (1.23 V), the growth of Zn dendrites, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).Organic electrolytes can effectively inhibit these side reactions and prolong cycle life.Solid-state electrolytes have sufficient mechanical strength to inhibit dendrite growth [18][19][20].Many excellent studies in the past have shown that the use of electrolyte additives for electrolyte modification is an effective method, and different kinds of additives can effectively solve the above problems and also play different roles in different types of electrolytes.The influence and related mechanisms of various electrolytes and additives on batteries in recent years are presented in figure 1.For example, in aqueous electrolytes, Zhang et al [21] introduced tetrasodium ethylenediaminetetraacetate salt (Na 4 EDTA) into ZnSO 4 electrolyte to inhibit dendritic Zn deposition and H 2 precipitation.Miao et al [22] designed a three-functional co-solvent, tetraglyme (G4), and demonstrated that G4 containing five O-ethers has the ability to inhibit water-induced parasitic reactions in the test ethers.Therefore, the use of additives in aqueous electrolytes can inhibit side reactions and dendrite growth.In organic electrolytes, Ma et al [23], by coupling hydrated Zn(BF 4 ) 2 salts with trimethyl phosphate (TMP) solvents, not only formed a protective ZnF 2 -Zn 3 (PO 4 ) 2 interface in situ on the surface of the Zn electrode, but were also able to significantly inhibit cathode dissolution.In a solid hydrogel electrolyte, Liu et al [24] developed a polyacrylamide chitin nanofiber (PAM-ChNF) hydrogel electrolyte in which the as-prepared ZIBs can be self-recharged by VO 2 cathode exposure to air for different times, and after adding a small amount of acetic acid to the hydrogel electrolyte, the cycle life of self-charging ZIBs can be extended.
With the deepening of research on electrolyte additives for ZIBs and the summary of previous studies, it can be seen that although great progress has been made, there are still many obstacles, and the main problems can be summarized as follows: (1) aqueous electrolytes are facing the adverse effects of positive electrode dissolution, growth of Zn dendrites, HER, electrostatic interaction, etc [15,25]; (2) polymer/hydrogel electrolytes face problems, such as poor mechanical strength, low ionic conductivity and electrolyte aging effect, which hinder their use in practical applications [19,26]; (3) the solubility of Zn 2+ in non-aqueous solutions is poor; most organic solvents are flammable, resulting in the low safety of ZIBs; organic cathode materials are easily soluble in organic solvents, which can easily lead to battery capacity attenuation and short cycle life [10,27].To our knowledge, although there are reviews on aqueous ZIB electrolytes, a comprehensive discussion of various types of electrolytes and their additives is not available.Therefore, this paper focuses on the effect of additives on different types of electrolytes, describes in detail the mechanism of action of modified electrolytes on ZIBs, and compares the advantages and disadvantages of different types of electrolytes in practical applications, hoping that this review can provide auxiliary help for researchers in this field and promote the further development of ZIBs.

Additives in liquid-state electrolytes of ZIBs
To date, the study of liquid electrolytes in ZIBs has mainly included aqueous electrolytes [28], organic electrolytes [27] and water-organic mixed electrolytes [11].

Aqueous electrolyte
Aqueous electrolytes have been extensively studied due to their non-flammability and high ionic conductivity.However, these challenges, such as severe cathode dissolution, Zn dendrite growth and HER, will reduce the reversibility of the Zn metal anode, resulting in a terrible electrochemical performance and hindering the practical application of ZIBs [3,5].Compared with electrode modification strategies, modifying electrolytes by adding inexpensive additives seems to be a promising strategy to solve these problems [29].As shown in figure 2, according to the different effects of additives in the electrolyte, they can be divided into on the Zn anode and on the cathode according to the battery composition [3].

Additive for interface regulation of the anode.
Since the formation of Zn dendrites and passivization byproducts (such as ZnO and Zn(OH) 2 ) on Zn anodes is greatly reduced in the weakly acidic electrolyte compared with the traditional alkaline electrolyte, this causes the reversibility and cycle life of the Zn anode in the weakly acidic electrolyte to have greater room for improvement.Compared with the traditional alkaline electrolyte, the formation of Zn dendrites and passivization byproducts (such as ZnO and Zn(OH) 2 ) on the surface of the Zn anode is largely suppressed, which means that the reversibility and cycle life of the Zn anode in weakly acidic electrolytes has more room for improvement.During the cycles of plating/stripping in weakly acidic electrolytes, the Zn anode is primarily affected by two factors: inhomogeneous Zn deposition and irreversible side reactions, which leads to the growth of Zn dendrites, corrosion reaction and HER [5,30,31].Figure 3 describes the main challenges and mechanism of damage to the Zn anode.The growth of Zn dendrites is described in figure 3(a).Under the influence of an electric field, it is easy for Zn 2+ to migrate to an energy location that is conducive to charge transfer.As a result, it is a simple process for the aggregation of Zn 2+ to occur and finally form the nucleation point of Zn dendrites.After nucleation, Zn 2+ is reduced and deposited at the nucleation sites during subsequent cycles.The deposition of Zn 2+ on the surface of the anodes is uneven due to the influence of both the electric field and concentration gradient [32,33].Under acidic conditions, two irreversible reactions of corrosion and hydrogen evolution occur (equations ( 1) and ( 2)) and are depicted in figures 3(b) and (c): The corrosion and hydrogen evolution chemical mechanisms of Zn metal were presented in the equations, in which Zn atoms on the surface of Zn metal enter the electrolyte solution as Zn 2+ and electrons migrate to the cathode reaction site.The formation of a porous and loose corrosion layer in the electrolyte solution means that the electrolyte can penetrate the innermost layer of the bulk Zn metal and thus lead to persistent corrosion and the formation of H 2 [31].

Inhibiting the growth of Zn dendrites.
According to the previous explanation of the genesis of Zn dendrites, Zn 2+ is more likely to accumulate in an energy-advantageous position, and as the cycle continues, it will lead to uneven deposition of Zn.Therefore, the growth of Zn dendrites can be inhibited by optimizing the electric field distribution on the electrode surface and reducing the concentration of Zn 2+ between the electrode surface and the electrolyte.For example, Cao et al [34] innovatively added dimethyl sulfoxide (DMSO) to ZnCl 2 -H 2 O solution to prepare ZnCl 2 /H 2 O-DMSO electrolyte.It is shown in figure 4(a) that the decomposition of the solvated H 2 O is inhibited due to the preferential solvation of DMSO with Zn 2+ and the strong interaction of H 2 O-DMSO.In addition, the product formed by the decomposition of the solvated DMSO, which forms a dense solid electrolyte interface (SEI) layer on the surface of the Zn anode rich in ZnS, preventing the growth of Zn dendrites and further inhibiting water decomposition (figure 4(b)).However, instead of using highly toxic DMSO, Shang et al [35] used a common nontoxic food and drug additive, propylene glycol (PG), as an electrolyte additive for aqueous Zn-ion batteries and achieved a stable dendrite-free cycling.PG can adsorb the protrusions and reaction crystal junction points, which removes hydrogen bubbles from the Zn surface and adjusts the Zn deposition rate due to hydrophobic properties (-CH 3 tail and hydrocarbon backbone) (figure 4(c)).In contrast, in PG-free electrolytes, several hydrogen bubbles and porous Zn deposition can be observed (figure 4(d)).As a result, the PG can not only adjust Zn deposition morphology effectively, but also perform long-term stable cycling in aqueous Zn-ion electrolytes under harsh cycling conditions (figure 4(e)).Hu et al [36] first applied a rare earth metal-type addition to Zn batteries.Cerium chloride (CeCl 3 ) is an effective, low-cost, and green electrolyte additive that can facilitate the formation of a dynamic electrostatic shielding layer around the Zn protuberance to guide the uniform deposition of Zn.As displayed in (figure 4(f)), as the cycle continues, Zn 2+ prefers to deposit around the cusps rather than the smooth regions of the Zn anode because Zn 2+ and the electric field tend to concentrate around the sharp edges with a lower radius of curvature ('tip effect').After introducing CeCl 3 additives, Ce 3+ aggregated at the tip of the semi-ellipse Zn protuberance and formed a strong electrostatic shield to promote Zn 2+ deposition on the smooth region of anodes without the formation of a Zn dendrite (figure 4(g)).Compared with single ion additives, anionic and cationic synergies can not only achieve uniform Zn deposition and inhibit the growth of Zn dendrites, but also improve the ion transport efficiency and cycle life of ZIB.For example, Guo et al [37] reported a synergistic metal salt electrolyte additive (LiCl) to improve the cycle ability of Zn.According to the results of XPS (figure 4(h)), the introduction of lithium ions (Li + ) results in the growth of Li 2 O or Li 2 CO 3 on the Zn surface, which forms a non-conductive shielding layer to effectively prevent the growth of Zn dendrites.Similarly, the presence of Cl − in the electrolyte contributes to the formation of mixtures of ZnCl 4 2− and ZnCl 3 and reduces the pH of the electrolyte, finally suppressing the formation of ZHS and ZnO.Thus, under the synergistic action of an anion (Cl − ) and a cation (Li + ), a stable cycle without dendrites is realized, the side reactions are reduced and the total capacity is improved.In addition to forming a hydrophobic layer and an electrostatic shielding layer on the surface of the Zn anode to inhibit the growth of Zn dendrites, Wu et al [38] introduced 2-methyl imidazole (Hmim) as an electrolyte additive, which built a solid inorganic-organic Zn-rich (Zn 4 SO 4 (OH) 6 /Zn(Hmim)) SEI film in situ on the surface of the Zn anode.As shown in figure 4(i), a smooth plating layer composed of ultrafine nanoparticles is clearly observed on a Zn foil after plating for 30 min at a current density of 1 mA cm −2 with an Hmim-added electrolyte.Therefore, the Hmim electrolyte additive can act as a refining agent to facilitate uniform Zn deposition.More importantly, the in situ Zn 4 SO 4 (OH) 6 •3H 2 O/Zn(Hmim) interface could substantially increase the corrosion resistance of the Zn anode, reduce the occurrence of side reactions, and restrict the growth of Zn dendrite due to its excellent structural stability and great mechanical strength.We conclude that different kinds of additives can inhibit the growth of Zn dendrites by forming an electrostatic shielding layer on the surface of the Zn anode or by forming an interface protective film, in order to achieve uniform deposition of Zn and improve cycle stability.The presence of some additives also reduces the occurrence of side reactions, allowing aqueous Znion batteries to obtain a longer cycle life.

Inhibiting side reactions.
Previous research has demonstrated that the use of additives can effectively prevent the occurrence of side reactions.For example, Deng et al [39] improved the reversibility of a Zn anode by adjusting the electrolyte solvation structure with the high donor number solvent additive N,N-dimethylacetamide (DMA).It is shown in figure 5(a) that DMA as an organic solvent additive with a high donor number and one carbonyl functional group as a hydrogen bond acceptor can realize reconstruction of a Zn 2+ solvation sheath and regulate the quantity of hydrogen bonds in a 3D network, This helps inhibit the growth of dendrites and suppresses the occurrence of side reactions on the anode during cycling.In addition, the high adsorption energy between Zn (002) plane and DMA additive, which homogenizes the fine-grained deposition manner and texture of Zn (002) plane to mitigate corrosion occurrence.In addition to DMA, there are organic solvents that, due to their hydrophobicity, can theoretically produce strong reciprocal repellence with water, which makes it possible to break the hydrogen bond network of water.Furthermore, the solvation interaction between H 2 O and Zn 2+ can be favorably reduced by coordinating the hydrophobic solvent into the Zn 2+ solvation sheath.According to the rule of 'solubility-like', the hydrophobicity of the molecule is inversely proportional to its polarity, indicating that the low-polarity molecule exhibits high hydrophobicity.Therefore, Miao et al [40]  Compared to H 2 O-hydrophilic organic hybrid electrolyte, the hydrophobic solvent-based hybrid system is more conducive to destroying H-bond networks in water due to the hydrophobic interaction.Therefore, DEC, as the most hydrophobic additive, has the strongest ability to disrupt the H-bond network of water.Figure 5(c) illustrates the interface chemistry of Zn anodes in aqueous electrolytes with and without hydrophobic DEC, respectively.In blank electrolyte (BE), HER inevitably occurs on the Zn metal surface due to the high activity of water, resulting in an increased local concentration of OH − .This would accelerate the Zn corrosion, byproduct formation and dendrite growth.However, the H 2 O + DEC hybrid electrolyte significantly stabilizes the Zn electrolyte interface due to its numerous advantages: (1) The hydrophobic DEC cosolvent by favorably breaking the H-bond network of water and reducing the activity of water.(2) The Zn 2+ solvation sheath with hydrophobic DEC coordination can inhibit the water decomposition by reducing the solvating H 2 O number.
(3) DEC preferentially adsorbing onto Zn can create an H 2 Opoor electrical double layer near Zn and guide a homogeneous Zn 2+ deposition.
Compared with the additive-modified Zn 2+ solvated shell, the preparation of an artificial interface layer on the surface of the Zn anode can also achieve uniform Zn deposition and inhibit the HER phenomenon [41].However, the complex manufacturing process and the volume changes caused by the plating/stripping process are the main shortcomings in commercial applications and cycle testing.Consequently, SEI with self-healing properties constructed with additives is more conducive to achieving high-performance aqueous Zn-ion batteries.For example, Huang et al [42] introduced 2 mM SeO 2 into 2 M ZnSO 4 electrolyte (Sel/ZnSO 4 ) to construct an in situ inorganic SEI on the surface of the Zn anode.It is shown in figure 5(d) that byproducts, such as Zn 4 SO 4 (OH) 6 •xH2O, were hardly observed on the surface of the Zn anode due to the presence of Se, which demonstrated the corrosion resistance of the Sel/ZnSO 4 electrolyte.This is due to the hydrolysis reaction of the SeO 2 additive to form SeO 3 2− in ZnSO 4 solution, and the initial reduction process of SeO 3 2− leads to the formation of a Se film on the surface of the Zn anode, which is able to inhibit the parasitic reaction between the Zn anode and the electrolyte.Furthermore, this addition strategy has unique self-healing properties that recover cracks caused by volume changes, ensuring the durability of the ZnSe layer.It is shown in figure 5(e) that the Zn plating process in different electrolytes was explored by in situ light microscopy.In the ZnSO 4 electrolyte, Zn protrusions were detected on the exposed area.In contrast, no such dendrites were observed on the cracked areas generated in the Sel/ZnSO 4 electrolyte due to the recovery of the ZnSe-SEI layer by pre-reduction of SeO 3 2− .Therefore, the strategy of using an SEI layer with self-healing characteristics is beneficial to improving the cycle stability of the Zn anode and then the cycle stability of the full cells.

Multifunctional additives.
According to the summary of previous studies, it is not difficult to see that the ideal electrolyte additive should meet the following three requirements at the same time: (1) the binding capacity of the additive to Zn 2+ should be higher than that of H 2 O, which adjusts the electrolyte solvation structure by replacing H 2 O in the Zn 2+ solvated sheath, thereby minimizing H 2 O activity [43][44][45][46]; (2) additives should be able to adsorb more easily on the surface of Zn anode and prevent parasitic reactions, especially corrosion of the Zn metal anodes [47][48][49]; (3) finally, a solid SEI layer can be formed in situ on the anode surface, which facilitates ion transport, uniform Zn deposition, and further inhibits parasitic reactions [41,42,50].Therefore, more and more research is beginning to focus on the realization of multifunctional additives [51,52].
For example, Yang et al [53] proposed the introduction of a high donor number solvent tetramethylurea (TMU) into the electrolyte, which satisfies all three requirements at the same time.First, it is shown in figure 6(a) that TMU will preferentially adsorb on the surface of Zn, inhibiting corrosion and parasitic reactions.Second, it is shown in figure 6(b) that TMU will replace H 2 O in the Zn 2+ solvated sheath, significantly weakening the activity of water.Finally, TMU contributes to the formation of an inorganic-organic bilayer SEI for uniform and fast Zn 2+ transport.For example, Zhao et al [54] used PFOA with a strong electronegative perfluoroalkyl chain as an additive to improve the cycling ability of a Zn anode.Since the free water molecules near the PFOA adsorption layer were limited by the hydrophobicity of the long perfluorocarbon chain in the PFOA additive (figure 6(c)), both the waterrelated side reactions and the migration of sulfate anions to the Zn anode were inhibited, causing the electrochemical window to be expanded to 2.1 V (figure 6(d)).Most electrolyte additives act on the anode to suppress the growth of dendrites and side reactions, resulting in uniform Zn deposition and high reversibility [55][56][57].However, in practice, the challenges faced by aqueous Zn-ion batteries are not only applicable to the anode.The effect of the cathode also needs to be treated with caution.

Additive for cathode interface regulation.
The previous section reviewed the effect of electrolyte additives on the anodes, and this section will discuss in detail the effect of additives on the cathode of aqueous Zn-ion batteries.To date, significant progress has been achieved in the study of aqueous Zn-ion battery cathode materials.However, some issues remain to be addressed (figure 2).The challenges faced and the corresponding reasons can be granularly categorized into (1) sluggish cathode-electrolyte interface dynamics caused by poor electron conductivity; (2) increased impedance and loss of active material caused by cracking of the cathode material; (3) the dissolution of the cathode material leads to rapid capacity decay; (4) uncontrollable byproducts lead to reduced reversibility of the battery; (5) insufficient energy density limits the theoretical specific capacity [58][59][60].These problems are limited not only by the cathode material itself, but also by the type of electrolyte and additives.To overcome these problems, many studies have been devoted to improving reaction kinetics, suppressing cathode solubility and inhibiting byproducts.Electrolyte additives have attracted much attention as one of the most convenient and most efficient methods.

Inhibiting cathode dissolution.
Cathode dissolution is an unavoidable problem in almost all aqueous batteries.Currently, cathode materials commonly used in aqueous Zn-ion batteries include manganese-based compounds, vanadium-based oxides and vanadates, Prussian blue analogues, organic compounds and other materials [61,62].The above materials are usually dissolved in aqueous solutions, mainly including the dissolution of V and Mn.When the divalent Zn 2+ is repeatedly inserted/deintercalated in Mnbased compound materials, the dissolution of Mn is affected by Jahn-Teller deformation, resulting in poor cycle stability [63].Practical applications of V-based materials face the challenges of cathode dissolution and rapid capacity fading, especially at low current densities due to the side reactions between V-based materials and H 2 O [64].Cathode dissolution reduces the utilization of the active material, which can also lead to structural degradation and performance degradation of electrodes.Therefore, to inhibit cathode dissolution, many effective solutions have been proposed.For example, Liu et al [65] selected a strong solvating triethyl phosphate (TEP) solvent with a high Gutmann donor number of 26 kcal mol −1 as an electrolyte additive to suppress cathode dissolution in ZIBs by adjusting the solvation structure of the electrolyte.The optimized 0.5 M-Zn(OTf) 2 /TEP:H 2 O electrolyte (OTf is trifluoromethanesulfonate with a 1:1 TEP:H 2 O volume ratio) exhibits a TEP-dominated major solvated sheath structure (figure 7(a)).The strong coordination of TEP with Zn 2+ and H 2 O molecules leads to a TEP-dominated solvated sheath around Zn 2+ , which can significantly reduce the activity of water and inhibit cathode solubilization.In addition, Mei et al [66] prepared a Zn(OTf) 2 -TEP/H 2 O-80% mixed electrolyte by adjusting the TEP addition content.The presence of MXene nanosheets and crystal water in the heterostructured cathode greatly promote electron transfer and Zn 2+ intercalation kinetics by testing the long cycle performance of a 2D hydrated V oxide/Ti 3 C 2 -MXene heterogeneous cathode.Importantly, the aqueous electrolyte containing TEP avoids the inactivity of V cathodes in organic electrolytes (Zn(OTf) 2 -TEP) and solves the problem of V dissolution in the aqueous electrolyte (Zn(OTf) 2 -H 2 O).It is shown in figure 7(b) that the effect of electrolytes with and without TEP on the solubility of V-based cathodes can be observed.In the Zn(OTf) 2 -H 2 O electrolyte, the solution turns yellow, indicating a serious process of V dissolution.Therefore, Zn(OTf) 2 -TEP/H 2 O-80% hybrid electrolyte largely inhibits the dissolution of V in the water electrolyte.This work provides a new perspective on the capacity fading mechanism of V-based cathodes, which facilitates the construction of long-term ZIBs.In addition, similar to the anode, the construction of a cathode-electrolyte interface (CEI) layer on the surface of the cathode can also effectively inhibit cathode dissolution [67].However, these methods often have the disadvantages of relatively slow ion diffusion kinetics, high cost and complex synthesis steps.Therefore, in order to maintain the high safety and low cost advantages of aqueous Zn-ion batteries, electrolyte additives with simple operation and low concentration should be preferred.For example, Wang et al [68] introduced N-methylpyrrolidone (NMP) molecules with a donor number of 27.3 into the ZnSO 4 electrolyte at a low concentration of 5%, since NMP exhibits a higher HOMO energy level than water, the LUMO energy trends of water, NMP, and Zn 2+ -NMP complexes are 2.408, 0.785 and −1.808 eV (figure 7(c)), respectively, indicating an increase in decomposition during Zn reduction.This is because NMP is more likely to adsorb on the upper surface of the cathode to form CEI than water.Thus, the dissolution of V is effectively inhibited, and the capacity retention during the cycle is enhanced.

Decrease in the production of byproducts.
The accumulation of byproducts on the cathode surface will cause the electrode volume to expand, which increases mechanical stress and reduces the long-term cycling of the battery.Therefore, exploring additives that can be used to reduce or inhibit byproducts is of great significance for the further development of aqueous Zn-ion batteries.For example, Wang et al [69] developed a multifunctional ZIB electrolyte additive 1-phenylethylamine hydrochloride (PEA).In a full-battery test with polyaniline (PANI) as the cathode, the Cl − from PEA enters the PANI chain during charging.Since the mean coordination number of water to Cl − is less than that of SO 4 2− , solvated Cl − releases fewer water molecules around the charged PANI, thereby inhibiting harmful side reactions on the cathode (figure 8(a)).Chen et al [70] introduced hydrophilic PEG to regulate the solvation environment of the electrolyte and to improve the stability of the electrode.Proton shielding can be achieved by locking most free water molecules through the PEG network, eliminating the formation of non-electrochemically active byproducts.Therefore, the storage behavior of the electrode in this modified electrolyte is dominated by Zn 2+ insertion/extraction without the formation of byproducts.Figures 8(b) and (c) display FESEM images of the electrode after 100 cycles in pure H 2 O and H 2 O-50% PEG electrolytes, respectively.It is clear that a large number of flaky byproducts were grown on the electrode surface in pure H 2 O. From the point of view of electrochemical storage kinetics, this byproduct interface layer will prevent ion transport, especially reversible ion insertion/extraction.In sharp contrast, no significant byproducts were observed on the electrode surface in the H 2 O-50% PEG electrolyte.In addition, an excellent preserved structure of ZVO formed after an in situ phase transition.Based on the above results, it is confirmed that by regulating the voltage and solvation environment, stable cycling can be achieved and harmful effects caused by byproduct generation can be eliminated.Furthermore, sodium lauryl sulfate acts as an anionic surfactant and is hydrophilic.Guo et al [71] used it as an electrolyte additive to inhibit cathode byproducts.It is shown in figure 8(d) that OH − readily reacts with ZnSO 4 and H 2 O in the additive-free electrolyte, forming numerous ZHS byproducts on the Na 0.44 MnO 2 (NMO) surface.However, in the hybrid electrolyte, SDS forms a protective layer on the surface of the cathode through electrostatic adsorption, which in turn inhibits the formation of ZHS byproducts (figure 8(e)).In addition, some ionic additives play a significant role in inhibiting byproduct formation.For example, Zhou et al [72] proposed a new hybrid electrolyte with Al 3+ as an additive to construct aqueous Zn-ion batteries composed of a metal Zn anode and a K 0.51 V 2 O 5 (KVO) nanoplate cathode.The XRD results of figure 8(f) show that after 100 cycles in a fully discharged state using an electrolyte without Al 3+ , a new peak representing basic Zn salts appears at 2θ of 12.23 • .After the introduction of Al 3+ , the formation of alkaline Zn salts was effectively inhibited.

Other issues.
For the cathode, electrolyte additives can not only inhibit the dissolution of the cathode, but also reduce the production of byproducts.More beneficial effects can be discovered, such as the effect on electrochemical kinetics.Chang et al [73] introduced thiourea (TU) into Zn-S batteries to improve the reaction kinetics for ZnS conversion to S. Figure 9(a) shows the mechanism of TU effect on the S@KB cathode.With the addition of TU, TU − can be formed, the lone pair of electrons of nitrogen atoms and negatively charged S atoms, and can interact with the Zn atoms in ZnS, reducing the bonding energy of Zn-S bonds.In addition, Met + is formed during charging, and its very center can interact with the S atoms of ZnS, reducing the bond energy of Zn-S bonds, and then accelerating its fracture to form Zn 2+ .In the end, satisfactory results were obtained, and the aqueous Zn-S battery with TU additive showed excellent cycling performance and high capacity.In addition, Li et al [74] proposed an important but rarely mentioned issue of anode-cathode interaction, which is a significant factor that triggers the degradation of aqueous Zn-ion batteries.To fundamentally resolve the capacity decay of V-based aqueous Zn-ion batteries, they used Al 2 (SO 4 ) 3 (expressed as ASO) as a multi-purpose electrolyte additive to regulate the OH −mediated cross-communication between the Zn anode and sodium vanadate (NaV 3 O 8 •1.5H 2 O, expressed as NVO) cathode.It is shown in figure 9(b) that from the XRD test, in the electrolyte of 2.5 M Zn(OTf) 2 + 0.2 M ASO, the diffraction peak of Zn absolutely dominates even after 400 cycles.These results show that ASO additive can inhibit the interface water decomposition and prevent the continuous accumulation of OH − on the surface of the anode.Second, the hydrolysis of Al 3+ consumes OH − to create a more acidic environment.This electrolyte acidification is expected to convert dissolved NVO from VO 2 (OH) 2− to VO 2+ , preventing undesirable phase transitions from NVO to Zn pyrocyanate (ZVO).Finally, Al 3+ can be firmly anchored in the middle layer of NVO by partially substituting Na + , acting as a strong pillar to maintain the structural integrity of the cathode during repeated charge/discharge cycles.It is shown in figure 9(c) that the Zn/NVO battery assembled using an ASO-containing electrolyte verifies the effectiveness of the proposed crosscommunication operation strategy.Similarly, Gong et al [75] proposed the introduction of diethylenetriamine (DETA) into a 2 m ZnSO 4 electrolyte, which plays different roles in the cathode and anode (figure 9(d)).In the anode, DETA preferentially adsorbs on the surface of the Zn metal, forming an organic SEI layer composed of DETA in situ and inhibiting dendrite formation on the surface of the Zn anode.In solution, DETA will participate in adjusting the solvation structure of Zn 2+ due to the higher binding energy of EDTA molecules and Zn 2+ than that of H 2 O. Importantly, the substances in DETA, the cathode and electrolyte will form an in situ CEI layer on the cathode surface, which not only inhibits the dissolution of V 2 O 5 , but also guides the transformation of V 2 O 5 into a more stable structure.It is shown in figure 9(e) that in a DETA-40 electrolyte system, the crystal type of the V 2 O 5 cathode has changed after cycling, which is advantageous for the intercalation/extraction of Zn 2+ and increases the Zn storage capacity of the V 2 O 5 material.

Non-aqueous liquid electrolytes
The electrolyte solvent is an important medium for dissolving and dissociating conductive salts and its properties are crucial for rechargeable batteries, which is the most important factor in determining the stability of Zn anodes [76][77][78].In general, the cycling ability of Zn-based aqueous batteries is determined by the number of Zn anodes.According to the discussion in the previous section, some challenges including Zn dendrite, corrosion and hydrogen evolution will lead to poor reversibility of Zn anodes, which is usually associated with the thermodynamic instability of Zn in aqueous electrolytes.This causes major problems with Zn anodes, including inhibitable Zn dendrite growth [78][79][80][81].Conversely, a thermodynamically stable Zn anode can be provided in the organic electrolyte, which is beneficial to reduce the corrosion process.Unfortunately, despite a great deal of effort investigating non-aqueous (organic) electrolytes, attempts to apply organic electrolytes in Zn batteries are still relatively rare.The reasons are as follows: (1) compared to aqueous electrolytes, organic electrolytes have low ionic conductivity, which limits the diffusion of Zn 2+ ions into the cathode lattice; (2) the flammability of organic electrolytes; (3) Zn 2+ the poor solubility of Zn salt in most non-aqueous solutions; (4) organic cathode materials are usually soluble in organic solvents, resulting in capacity fading and a short cycle life.Furthermore, the problem is even worse at high temperatures [10,27,78,82].

Organic electrolytes.
With the advances in organic electrolyte systems research, some strategies of organic electrolyte modification were proposed to solve the existing challenges of organic electrolytes with respect to battery performance.For example, the flammability of organic electrolytes can be solved by using non-flammable organic solvents [27,83,84].Naveed et al [85] proposed a new high-safety Zn battery non-aqueous electrolyte based on TMP, N-methyl formamide (NMF) and Zn(OTf) 2 .TMP was used as the main solvent to ensure the non-flammability of the modified electrolyte, and the introduction of the NMF additive enabled the symmetrical battery to achieve stable cycling at higher current density, and the current capability of the Zn anode was enhanced by NMF bonding with the Zn anode and Zn 2+ , guiding Zn nucleation and realizing no dendritic cycle (figure 10(a)).In addition, Ma et al [86] used hydrated Zn(BF 4 ) 2 salt as an additive by changing the type of additive to achieve the dendrite-free and corrosion-free Zn electrodeposition.Similarly, Ma et al used hydrated Zn(ClO 4 ) 2 •6H 2 O as an additive to prepare a high-performance electrolyte [23].This modified organic electrolyte not only has a H 2 O-poor Zn 2+ -solvation sheath and low water activity, but can also significantly enhance the reversibility of Zn and widen the electrochemical window.In contrast, Chen et al [87] chose a carbonate Zn electrolyte (1.5 M Zn(TFSI) 2 /EMC) as a research platform to construct a micro-heterogeneous anionic solvation structure by introducing TMP.TMP preferentially binds with Zn 2+ and TFSI − in the form of TMP-solvated ion pairs and aggregates.Thus, a micro-heterogeneous anion solvation structure is formed where TFSI − anions are confined in TMP-rich domains (figure 10(b)), converting TFSI -complexed EMC into stable uncoordinated EMC.Notably, this is the first successful attempt to modulate the anionic coordination environment to restore the electrochemical tolerance of carbonates.This method is different to the literature reports on high-voltage carbonate electrolyte using highly concentrated Li salts, which is difficult to achieve in the case of Znbased carbonate due to the low solubility of Zn salts.In addition, Zn/graphite batteries based on TMP-EMC solvents have high-power densities up to 5.68 kW kg −1 , due to the high ionic conductivity of the electrolyte and the main pseudo-capacitive behavior of TFSI − embedded graphite.
In addition to the flammability of organic solvents, the cathodes of ZIBs in organic electrolytes also face challenges, such as high solubility of cathode material, poor ionic conductivity and low-rate performance [88,89].Ma et al [90] prepared a tetraethylene glycol dimethyl ether-based organic electrolyte consisting of LiClO 4 •3H 2 O and Zn(ClO 4 ) 2 •6H 2 O for the construction of Zn-MnO 2 batteries.The monovalent Li + ion effectively reduces the electrostatic interaction between the insertion ion and the crystal structure, which improves the structural stability of the cathode material.In addition, the aqueous tetrameric electrolyte inhibits the activity of H 2 O molecules, avoiding the formation of Zn 4 ClO 4 (OH) 7 byproducts.At the same time, as shown in figure 10(c), a highly stable stripping/plating (more than 1500 h) is achieved in the modified electrolyte.In addition, the battery can work normally, after storage for more than 1 year, indicating an ultra-long storage life.Wang et al [89] prepared a nonaqueous electrolyte (Cu 2+ -DMF) using N,N-dimethyl formamide (DMF) as a substrate and copper salt (Cu(OAc) 2 ) as an additive.It is shown in figure 10(d) that the combination of the high thermodynamic stability of Zn anodes in DMF electrolytes and the Cu-Zn alloy interface in vitro can surpass the properties of aqueous and non-aqueous electrolytes.Furthermore, the battery with Cu 2+ -DMF electrolytes shows high-rate capabilities.At a current density of 20.0 mA cm −2 with a capacity of 220.0 mAh cm −2 , the symmetrical cell shows a low over-voltage hysteresis (about 50 mV).In organic electrolytes, in addition to solving the flammability problem, it is also particularly important to address the effect of organic solvents on the cathode.Dong et al [91] proposed a nonaqueous phosphate-based electrolyte with 0.5 M Zn 2+ and 1.0 M Na + bications in TMP solvent.Not only does it have flame retardant capability, but it also exhibits great versatility for rechargeable Zn batteries with other polyanionic-type cathodes (e.g.Na 3 V 2 (PO 4 ) 2 F 3 (NVPF) and Na 3 V 2 (PO 4 ) 3 (NVP).The working mechanism of rechargeable Zn/NVPOF batteries in dual cationic electrolytes is reversibly extracting/inserting Na + from the NVPOF cathode and electrochemically depositing/dissolving Zn at the anode.The relevant reactions are as follows: Overall: The common reaction mechanism of NVPOF, NVPF and NVP cathodes is the reversible insertion of Na + into the polyanion crystal structure without the co-intercalation layer of Zn 2+ in non-aqueous bicationic electrolytes.This organic electrolyte exhibits high anodic stability as well as good compatibility with polyanionic cathodes.The use of this organic system electrolyte broadens the choice of cathode and provides an effective idea for the development of Zn batteries with high reversibility, high energy and high safety.

Water-organic mixed electrolytes.
In aqueous electrolytes, the strong solvation of H 2 O molecules can lead to the growth of Zn dendrites.In the organic electrolyte, the charge transfer resistance is increased due to the high desolvation energy [92,93].By the addition of organic solvents to aqueous electrolytes (or the introduction of aqueous additives into organic electrolytes) [65,94], these hybrid electrolytes combine the advantages of aqueous electrolytes and organic electrolytes so that high-performance ZIBs have long-term stability, excellent rate capability, slow self-discharge and can achieve synergistic effects on cost, safety and performance through the simple operation of the electrolyte system [90,92,99].For example, Shang et al [95] used TEGDME as a cosolvent to prepare an 'ether-in-water' electrolyte.The anion and cation pairs can be separated effectively due to the excellent interfacial stability, low viscosity and large dielectric constant of electrolyte.The designed solvated sheath structure of Li 4 (TEGDME)(H 2 O) 7 not only overcomes the narrow electrochemical window of H 2 O, but also facilitates the formation of interfacial chemistry.It is shown in figure 11(a) that the high-quality SEI generated by the reduction of Li + 2 (TFSI − ) and Li + 4 (TEGDME) effectively inhibits hydrogen precipitation and electrode dissolution.In addition, in a hybrid electrolyte of 1 M Zn(OTf) 2 water/acetonitrile (ACN), the ACN solvent can effectively inhibit the formation of Zn dendrites and increase the rate capacity of the device.The hybrid electrolyte promotes sulfur redox by inhibiting the activity of H 2 O around the surface of the electrode, increasing the specific capacity [96].This view is also confirmed by Meng et al [11].Through the study of the acetonitrile-water co-solvent (AWCS) electrolyte, its influence on the electrochemical performance and energy storage mechanism of the whole battery was deeply explored.It is confirmed that acetonitrile-aqueous co-solvent electrolytes can solve the key problems of the anode and cathode at the same time, which not only enhances the stable cycle of waterborne Zn batteries, but also improves the practicality.It is shown in figure 11(b) that ACN molecules break the hydrogen bond network of H 2 O and regulate the solvent structure, inhibiting the formation of byproducts and the decomposition of water.In addition, the ACN can form an adaptive layer at the interface due to the strong Zn affinity, which not only facilitates the diffusion of Zn 2+ , but also shields the charge at the tip to achieve uniform deposition of Zn 2+ .On this basis, the introduction of organic co-solvents effectively inhibits the dissolution of cathode materials and the generation of byproducts.The appropriate ratio of acetonitrile to water not only maintains an adequate redox reaction, but also improves the cycling stability, which is the key reason for keeping the battery stable and with a reversible cycle.Hybrid electrolytes not only have beneficial effects on cathodes and anodes, but are also useful in a wide range of operating temperatures.The electrochemical performance of aqueous electrolyte at low temperatures is usually significantly reduced due to the phase change, viscosity and solvation state of the aqueous electrolyte.Chang et al [97] used ethylene glycol (EG) with a high boiling point and a relatively low freezing point as an additive to expand the applicable temperature range of the battery.Due to the unique solvation interaction between EG and Zn 2+ , the HB between EG and H 2 O is enhanced, which effectively breaks the hydrogen bond between H 2 O and H 2 O, resulting in a hybrid electrolyte with low freezing point and high ionic conductivity even at −40 • C.Moreover, the introduction of EG reduces the solvation interaction of Zn 2+ with H 2 O, which is beneficial to reduce side reactions, such as HER on the Zn anode.Encouraged by these advantages, the versatility of hybrid electrolytes was verified by using ZIBs (intercalation/de-intercalation reactions), which exhibit high-capacity density, high-power density, and high cycle stability over a wide temperature range (figures 11(c) and (d)).In addition, anode-free Zn metal batteries have been extensively studied due to their high-energy density, and Ming et al [98] designed a hybrid electrolyte based on the salinization effect by adding Zn trifluoromethanesulfonate (Zn(OTf) 2 ) to a mixture of PC and water.Mixing electrolytes can efficiently modulate the Zn 2+ solvated structure (figures 11(e) and (f)), which is essential for forming a waterproof solid-electrolyte interface.The hydrophobic interface of the anode and the significantly reduced amount of free H 2 O can inhibit the occurrence of parasitic reactions and improve the cycle stability of the Zn-based battery (figure 11(g)).

Additives in solid-state ZIB electrolytes
There are still many problems with liquid electrolytes in ZIBs including the relatively narrow electrochemical window, the growth of Zn dendrites, the HER and the OER [100][101][102].Fortunately, solid electrolytes for ZIBs can well overcome the challenges faced by liquid electrolytes for ZIBs [103].Therefore, solid electrolytes for ZIBs have received increasing attention in recent years.
Compared with liquid electrolyte, solid electrolyte has the following advantages [104,105].(1) The solid electrolyte not only serves as a mechanical barrier in the battery to withstand extreme external stresses, but also inhibits the formation of dendrites, further extending the life of the battery [106][107][108].(2) The solid electrolyte does not suffer from liquid leakage, which makes it much safer than the liquid electrolyte, making it possible to make some safe and wearable flexible electronic devices that are in direct contact with the human body [109,110].(3) The solid electrolyte can alleviate the problem of side reactions, reduce the dissolution of active substances on the electrode and inhibit the corrosion of the electrode, improving the electrochemical performance of ZIBs [18].(4) Solid electrolytes have excellent mechanical and easy-to-assemble properties, which provide more significant advantages in the preparation of flexible cells [111,112].Due to these advantages, solid-state electrolytes have become the current trend in the development of ZIBs.
Solid-state electrolytes can be roughly divided into two major categories, one being all-solid electrolytes and the other quasi-solid electrolytes.Quasi-solid electrolytes are composed of solvents and polymer chains, which can be classified into three categories based on the internal solvent: water, organic solvents and ILs.
However, there are still some challenges in the practical application of solid-state electrolytes [113].(1) The evaporation of H 2 O in quasi-solid electrolyte at room temperature will result in water loss from hydrogel polymer electrolyte (GPE) and degradation of electrochemical performance.Similarly, the poor storage performance of quasi-solid electrolytes at extreme temperature environments is a fundamental drawback.Freezing occurs at low temperatures, and water loss occurs at high temperatures in quasi-solid electrolytes.These problems will lead to a rapid decline in the electrochemical performance of ZIBs [114,115].(2) The mechanical properties of the quasi-solid electrolyte are not very good at lasting bending.In addition, after repeated pressing, bending, folding, etc, the battery will be changed to a great extent.For example, the electrical properties will be drastically reduced, which still makes the ZIBs unsatisfactory for making human wearable electronic devices [116].
In addition to the problems listed above, there are remaining problems with quasi-solid-state electrolytes for ZIBs, which seriously hinder the large-scale development of solid-state electrolytes for ZIBs.According to previous studies, introducing additives into electrolyte to improve the electrolyte performance is one of the practical, simple and fast methods to effectively solve the above-mentioned problems, and mixing different types of additives can also improve the performance of electrolyte.It is shown in figure 12 that the effects and the related mechanisms of different types of additives on the solid electrolyte of the battery have been introduced in recent years.For example, in the polymer system, Rong et al [117] prepared an organic hydrogel by introducing EG into the hydrogel, demonstrating that EG can improve the low-temperature tolerance performance of the hydrogel electrolyte, thus resisting the effects of extreme cold temperatures.Mo et al [115] designed an EG-waPUA/PAM freeze-resistant hydrogel electrolyte for ZIB cells by combining EG with polyurethane acrylate (PUA) through covalent bonding, providing a new idea for hydrogel electrolytes for ZIBs to face extreme lowtemperature environments.In addition, Liu et al [116] introduced DMSO into the ZnSO 4 hydrogel electrolyte to suppress the water side reactions and the growth of Zn dendrites.In the all-solid-state system, Hiralal et al [118] introduced inorganic particle-titanium dioxide into the Zn-polyethylene oxide (PEO) system, which greatly improved the ionic conductivity and mechanical stability of this electrolyte through the interaction of titanium dioxide with the all-solid electrolyte.
Therefore, it is important to find suitable additives in solid electrolytes of ZIBs and explore the mechanism of adding suitable additives to improve the above-mentioned problems.In this section, we will focus on the research progress of adding suitable additives in solid electrolytes of ZIBs to overcome their problems.

Hydrogel electrolytes
The hydrogel electrolyte can effectively prevent the leakage of liquid due to the low active water content and high elastic modulus, which can greatly improve the safety performance of ZIBs [25].In addition, the hydrogel electrolyte has good mechanical properties, high ductility and processability, and is also an ideal electrolyte with good practical application prospects [119].Unfortunately, there are still many problems with the hydrogel electrolyte of ZIBs in practical applications [120,121].For example, the hydrogel electrolyte cannot fully adapt to an extreme temperature environment [122] and shows very poor mechanical strength [123] as well as low ionic conductivity [124] in an extreme environment.To alleviate these problems, the experimentalists have proposed many ways to solve the problems.However, since most of the modification strategies are complicated or expensive, it is best to solve these problems by introducing cheap additives into the hydrogel polymer electrolyte of ZIBs.The challenges of the hydrogel electrolyte of ZIBs are presented in figure 13.Conventional hydrogel electrolytes will inevitably face the serious challenge of long-term use at extreme low temperatures.For example, conventional hydrogel electrolytes inevitably freeze and lose conductivity at subzero temperature, which limits their practical applications at low temperatures [117].At the same time, the frozen hydrogel electrolyte will lose many of its original high-quality properties, including good mechanical and electrochemical properties [127].
Recently, the experimenter proposed that there are three different forms of water in the hydrogel electrolyte, namely, unbound water, weakly bound water and strongly bound water [128].Most of the water molecules in the hydrogel electrolyte are in unbound water form.At low temperatures, the free water molecules connected by hydrogen bonds are tightly arranged to form ice [129].On the other hand, weakly bound water and strongly bound water can interact strongly with some functional groups in the network inside the electrolyte, respectively, which facilitates the lowering of the freezing point of the hydrogel.This problem can be solved by breaking the hydrogen bonds between free water molecules in the hydrogel electrolyte.For example, the presence of free water can be suppressed by introducing polyols as additives or enhancing the interaction between water molecules and the hydrogel network [130].For example, hydrophilic functional groups, such as -COOH or -CONH 2 , are introduced into the hydrogel electrolyte [131,132].It is worth noting that the main reason for the high ionic conductivity of hydrogel electrolyte is that the hydrogel electrolyte internal network has enough free water molecules, which can make the electrolyte inside the ions accelerate their movement, reaching high ionic conductivity of the electrolyte.Limiting the free movement of water molecules in the electrolyte can facilitate the application of hydrogel electrolytes at extremely cold temperatures.Encouragingly, after extensive research, experimenters have found how to balance the low-temperature resistance and ionic conductivity of the hydrogel electrolyte, and even improve both properties within the hydrogel electrolyte.
According to previous studies, the addition of suitable organic solvents to the hydrogel electrolyte can promote its efficient work at extremely low temperatures.These organic solvents include glycerol (GL) [133] and EG [134].Polyols can bind to freely moving water molecules in the hydrogel electrolyte through strong hydrogen bonding, which can destroy the force between the original water molecules.This can inhibit the formation of ice crystals in solution and reduce the freezing temperature of the hydrogel electrolyte so that the hydrogel electrolyte can remain a gel at low temperatures and maintain a better chemical property [129,135].
In order to investigate the influence of organic solvent fraction of hydrogel electrolytes at low temperature, Wang et al [136] prepared a group of hydrogel electrolytes with different ratios of water to EG and used them to assemble ZIBs.The experimental results show that the hydrogel electrolyte with an H 2 O/EG ratio of 2:1 can not only have a good ionic conductivity at low temperature, but also have high cell capacity.The reason for these results may be due to the ability of alcohol substances to produce some very stable molecular clusters with the water molecules in the hydrogel electrolyte, which can decrease the freezing point of the hydrogel electrolyte [137].However, with the increase in organic solvent content, the content of free water molecules in the hydrogel electrolyte freely decreases, which inhibits the conductivity of the electrolyte [132].
With the continuous development of the battery industry, the addition of only one type of polyol to the hydrogel electrolyte has been unable to meet the market requirements for high conductivity with ZIBs.Meanwhile, we also found that the addition of only one polyol may reduce the ionic conductivity and chemical reaction rate of the hydrogel electrolyte, which severely limits the large-scale use of ZIBs [138].In order to better solve the various problems associated with freezing of the hydrogel electrolyte at low temperatures, researchers have proposed a variety of schemes.For example, Wei et al [139] proposed to add different kinds of organic substances to the hydrogel electrolyte to solve the problems through the synergistic effect of multiple substances.The hydrogel electrolyte was composed of PAM, ZnSO 4 (ZS), GL and acetonitrile (AN), in which GL and AN were combined as functional additives for the system.The experimental results show that the ZIBs prepared by the hydrogel electrolyte can still maintain good ionic conductivity at −20 • C, with good mechanical properties, stable cycling ability and wider working temperature window.In addition, the mechanism of interactions within the hydrogel electrolyte network can be illustrated by theoretical calculations.The results show that the formation energy between GL-H 2 O and PAM-H 2 O is much lower than that of H 2 O-H 2 O (figure 14(a)).GL and PAM are likely to form hydrogen bonds, which can break the hydrogen bond between the original water molecules, reducing the activity of water.Thus, the freezing point of the hydrogel electrolyte shows a drop phenomenon (figure 14(b)).In addition, using large amounts of organic solvents to replace the water molecules in the traditional hydrogel electrolytes is also a good method to inhibit the freezing of the hydrogel electrolytes at low temperatures.The principle of this approach is very simple as a controlled method to convert a mixture of water and organic solvents.For instance, Wang et al [12] prepared a composite hydrogel electrolyte with high ionic conductivity (figure 14(c)).The hydrogel electrolyte was obtained by immersing the mixture of guar gum and sodium alginate (GG/SA) in ZnSO 4 /MnSO 4 /EG.In this hydrogel electrolyte, EG can disrupt large amounts of hydrogen bonds between water molecules, inhibiting the formation of crystal lattices of ice.Therefore, the ZIBs with this hydrogel electrolyte display a high ionic conductivity (6.19 mS cm −1 ) and an excellent electrochemical performance (175.5 mAh g −1 at 0.1 A g −1 ) at −20 • C.This experiment provides a new idea for the development of antifreeze hydrogel electrolyte.
Furthermore, in addition to using common antifreeze EG and GL in experiments, developing new antifreeze additives is also a good method to promote the development of hydrogel electrolytes for ZIBs at extreme temperatures.Previously, DMSO [140], methanol [141] and sorbitol [124] were proposed as effective antifreeze additives to broaden the working temperature window and electrochemical properties of hydrogel electrolytes.For instance, Lu et al [142] prepared a hydrogel electrolyte of PAAm/DMSO/Zn(CF 3 SO 3 ) 2 multiple components (figure 14(d)), where the DMSO can be combined with the free water in the hydrogel electrolyte, thus destroying the original hydrogen bonds between water molecules and water molecules, further suppressing the freezing point of the hydrogel electrolyte.Therefore, the electrolyte can be completely converted into a crystalline state even at an extreme temperature of −40 • C (figure 14(e)), and the ZIBs with the hydrogel electrolyte can be stabilized for more than 75 000 runs at −40 • C at a current density of 2 A g −1 .In addition, Quan et al [143] prepared a hydrogel electrolyte from crop straw and added sorbitol to the electrolyte as a functional additive (figure 14(f)).Sorbitol has abundant hydroxyl group, which can destroy the interaction between water molecules by forming hydrogen bonds with water molecules, inhibiting the formation of ice crystals and reducing the freezing temperature of hydrogel electrolyte.However, the introduction of organic additives inevitably reduces the ionic conductivity of the hydrogel electrolytes [144].Moreover, the organic solvent does not strongly interact with the polymer network, making the stability of the hydrogel electrolyte very poor [145].Adding organic additives to the hydrogel electrolytes is not a perfect solution.Therefore, Ou et al [145] prepared a cyclic carbonate based on a carbon dioxide (CO 2 ) source as a crosslinker agent, polymerized with potassium polyacrylateacrylamide to form a hydrogel electrolyte that can maintain good performance at a low temperature, and it can be applied to Zn-ion batteries.The experimental results show that the Znion battery with this hydrogel electrolyte has a wide working temperature window, good cycling stability and capacity retention when the temperature is reduced to −20 • C.This is due to the strong interaction between the polar oxygen groups and water molecules, which breaks the hydrogen bond between water and water, and reduces the freezing point of the hydrogel electrolyte.

Improving water retention capacity.
Traditional hydrogel electrolytes are very vulnerable to the influence of external environmental conditions.Even in a room temperature environment, it will inevitably make the freely moving water molecules inside the hydrogel electrolyte suffer from evaporation, which seriously affects the normal use of hydrogel electrolyte [125] (figure 15(a)).To alleviate the evaporation of hydrogel electrolyte in the daily environment, experimenters have proposed many methods.For example, ILs can be used to replace water molecules in hydrogel electrolytes to fundamentally solve the problem of water evaporation by preparing non-aqueous gel electrolytes [146].However, this method not only complicates the fabrication process, but also increases the cost of the device.
According to previous studies, adding organic solvents to the hydrogel electrolyte can reduce the chemical potential of the electrolyte.In general, there is a reverse trend between the chemical potential of the electrolyte and the anti-dehydration potential of the electrolyte, that is, when the chemical potential of the electrolyte is lower, the anti-dehydration performance of the electrolyte will be better [124].At the same time, the water molecules in the hydrogel electrolyte are firmly locked in because the organic solutes can combine with the freely moving water molecules in the hydrogel electrolyte, thus avoiding the evaporation of water (figure 15(b)).Based on the antidehydration mechanism of hydrogel electrolyte mentioned above, a lot of research has been conducted on adding organic solvent to hydrogel electrolyte to improve its water retention ability [132].For example, Wang et al [147] prepared a hydrogel electrolyte based on PAM and GL (figure 15(c)).The strong hydrogen-bonding interactions between PAM and GL and water not only avoid the freely moving water molecules in the hydrogel electrolyte, but also restrict its evaporation at room temperature or even high temperature.Meanwhile, dehydration tests of PAM-H 2 O-GL electrolytes with different content of GL were carried out.The results show that when the addition of GL reaches 40 vol%, the hydrogel electrolyte exhibits the best anti-drying performance (figure 15(d)).Using a similar experimental mechanism, Mo et al [125] prepared an organic hydrogel electrolyte made of EG/alginate/PAM (figure 15(e)).EG can be closely connected to the freely moving water molecules in the hydrogel electrolyte through hydrogen bonds, well locking the water tightly in it to prevent the water in the hydrogel electrolyte from volatilization into the external environment and avoid affecting the normal operation of the hydrogel electrolyte of the ZIBs for a long time.The two substances mentioned above are very common in ZIB hydrogel additives used to solve the problem of dehydration caused by temperature.
In addition, DMSO [142], sorbitol and various alcohols can also promote the ZIB hydrogel electrolyte to achieve the same anti-dehydration effect.However, according to numerous reports, the addition of organic substances will decrease the ionic conductivity of hydrogel electrolyte.Therefore, it is also a good solution to disperse the inorganic particles in the hydrogel to form a nanocomposite material to improve the water retention ability of the hydrogel electrolyte [132].For example, Fan et al [148] synthesized a porous-structured poly(vinyl alcohol) (PVA)-based nanocomposite GPE with the optimum addition of silica (SiO 2 ).Due to the combination of the hydroxyl groups on the surface of SiO 2 and the freely moving water molecules in the electrolyte, the water retention performance of the hydrogel electrolyte is improved.Therefore, the same mechanism can also be used in the future development of ZIB hydrogel electrolyte by adding inorganic particles to hydrogel electrolytes to improve its water retention performance.

Improving the mechanical properties.
Hydrogel electrolytes are often used to prepare flexible batteries due to their excellent mechanical properties.However, most of these reported hydrogel electrolytes do not contain Zn 2+ ions.Therefore, these hydrogel electrolytes cannot be directly used to prepare wearable ZIBs.To solve this problem, the hydrogel electrolyte has to be immersed in the electrolyte containing Zn 2+ for a long time to ensure that the battery has enough Zn 2+ ions.Unfortunately, the degree of expansion of the hydrogel electrolyte will inevitably increase during the long-term immersion process, resulting in a decline in its mechanical properties [149].In addition, flexible batteries in practical applications will inevitably often bend and stretch during daily operation, which creates challenges.These challenges can also make the mechanical properties of hydrogel electrolytes not meet the standard for daily use, so that the ZIB hydrogel electrolyte cannot be used in the flexible battery [150].In order to better achieve good mechanical properties of hydrogel electrolytes after repeated stretching, bending and other operations, the experimenter proposed a variety of operational methods.For example, the addition of Zn salt during the process of preparing hydrogel electrolyte, which can avoid the absence of Zn 2+ in the hydrogel electrolyte and the need for long-term immersion [151].However, this practice is not suitable for the improvement of all ZIB hydrogel electrolytes with poor mechanical properties.Therefore, adding additives to improve the mechanical properties of hydrogel electrolytes becomes a difficult task.
According to previous studies, the addition of polyols to hydrogel polymer electrolytes can improve the mechanical properties of the hydrogel electrolyte [132].For example, Quan et al [143] added sorbitol to the cellulose hydrogel electrolyte and achieved the effect of 'water in salt' (WIS) by using concentrated ZnCl 2 in the hydrogel electrolyte.The experimental results show that sorbitol is rich in hydroxyl groups, which enables it to have more abundant hydrogen bonds with the cellulose backbone so that the mechanical properties of the electrolyte can be greater than those of the pure cellulose electrolyte.In order to further highlight the excellent enhancement effect of sorbitol on the mechanical properties of the hydrogel electrolyte, three commonly used organic additives including EG, GL and DMSO were added into the cellulose electrolyte with the same amount of sorbitol.Compared to three other organic additives, the hydrogel electrolytes with sorbitol additives exhibit the best mechanical properties.
In order to further explore the specific mechanism of organic additives to improve the mechanical properties of the hydrogel polymer electrolyte, Chen et al [152] used different kinds of organic compounds containing hydroxyl groups as additives to the hydrogel electrolyte, including EG, GL and sorbitol.The results show that the hydrogel electrolyte constructed with sorbitol has the highest mechanical capacity due to the compact intertwined structure between sorbitol and hydrogel electrolyte.In addition, the same effect can be achieved by adding inorganic particles to the ZIB hydrogel polymer electrolyte [153][154][155].For example, Abbasi et al [156] prepared a novel nanocomposite hydrogel electrolyte consisting of PAM as the gel-forming polymer and graphene oxide (PGO) as the additive (figure 16(a)).Moreover, the acid function of GO can promote its dispersion and improve the compatibility between the additive and the hydrogel electrolyte.The experimental results show that PGO can improve the electrochemical and mechanical properties of hydrogel electrolyte.
In addition, the mechanical properties of hydrogel electrolyte can be improved by accelerating its synthesis based on the special mechanism and adding inorganic particles in the preparation process.For example, Ji et al [157] prepared a rigid and hydrophilic Na-montmorillonite lamella and ZnSO 4 salt plasticized PAM-based (MMT-PAM) hybrid hydrogel electrolyte by a controlled accelerated polymerization mechanism (figure 16(b)).Compared with the traditional preparation of hydrogel electrolyte, this method can avoid the problem of poor mechanical properties caused by the hydrogel electrolyte in the solution (figure 16(c)).Second, the mechanical properties of the hydrogel electrolyte can be further improved due to the addition of Na-montmorillonite.At the same time, the experimental results show that ZIBs prepared from the hydrogel electrolyte have excellent electrochemical performance and stable cycling ability, good compression resistance and self-healing properties.However, in general, the addition of small numbers of inorganic particles to the hydrogel polymer electrolyte is indeed a very effective method to improve the mechanical properties of hydrogel electrolytes.Unfortunately, the excessive doping of inorganic particles may cause some damage to the internal structure of the hydrogel electrolyte [132].In order to avoid excessive addition of inorganic particles or organic solvent problems, researchers can add inorganic particles and organic solvent to the hydrogel electrolyte.Since a number of experiments have shown that this method is also a good choice, there is a new idea that adding two or a variety of different types of additives can improve the mechanical properties of hydrogel electrolyte using a synergistic method.For example, Wang et al [136] added EG and borax, two different types of additives, to PVA containing lithium chloride and Zn chloride, and prepared a transparent hydrogel polymer electrolyte by cooling and freezing (figure 16(d)).The EG in the electrolyte can make the hydrogel electrolyte also have good electrochemical and mechanical properties at extreme low temperature.Second, due to the addition of borax, the hydrogel polymer electrolyte can not only show good self-healing ability, but also good mechanical properties.
3.1.4.Reducing side reactions.Although hydrogel electrolytes largely prevented the occurrence of some side reactions triggered by water, small amounts of water in the hydrogel electrolyte can still have a significant impact on battery life [158].In order to prevent the occurrence of side reactions caused by water molecules, many common solutions have been proposed.First, constructing an artificial SEI layer in the electrolyte [159].Unfortunately, a series of problems associated with artificial SEI hinder the long-term cycle of the battery, such as damage, poor contact with the interface and poor electrochemical performance.Second, preparing the electrolytes with a high concentration of salt [160] or adding additives to electrolytes [161].However, most of the methods will not only lead to an increase in production costs, but also cannot effectively solve the problem.Therefore, it is very important to choose the right method to reduce the water-induced side effects.
In the traditional water electrolyte, due to water molecules producing many side effects, the prepared battery will show poor electrochemical performance, greatly restricting the development of the Zn-ion battery.In most experiments, the preparation of a WIS electrolyte is chosen to improve this problem, where the amount of salt exceeds the solvent in weight and volume.Pan et al [162] prepared a 'waterin-gel' electrolyte (WIG), in which the weight of salt and polymer exceeds the weight of water.The results show that the WIG electrolyte can effectively inhibit the side effects of water and improve the cycling ability of the battery.In addition, Wang et al [163] fabricated a double cross-linked hydrogel electrolyte by immersing the poly(2acrylamido-2-methylpropane-sulfonic acid)/polyacrylamide (PAMPS/PAAM) hydrogel in an EG/H 2 O solution containing ZnCl 2 (figure 17(a)).The presence of EG in the hydrogel electrolyte allows the free water molecules to be tightly locked between the hydrogel network, effectively limiting the dendrite growth and the occurrence of side reactions.In addition, He et al [158] fabricated a novel hydrogel with high water retention, which was constructed by copolymerizing sulfobetaine and acrylamide in Zn(ClO 4 ) 2 solution.A hydrogel frame with hydrophilic and charged groups is prepared (figure 17(b)).The hydrogel electrolyte can not only build more ion migration channels, but also accelerate the migration speed of ions in the hydrogel electrolyte.Meanwhile, interfaces can also be generated to reduce water-related side reactions, leading to a uniform deposition of Zn.

Gel polymer electrolytes
Gel polymer electrolyte is usually composed of the polymer as a skeleton with a uniformly dispersed liquid solution [164].Consequently, the GPE has all the advantages of both the liquid electrolyte and the solid electrolyte [165,166].The liquid in the GPE can make a great contribution to the ionic conductivity and interface stability, and at the same time, the polymer part can provide relatively strong mechanical properties and a certain flexibility to the GPE [167].The solvents in the GPE can be water, organic solvent and IL.In section 3.1, we have described the current improvement measures of the hydrogel electrolyte, which are not extensively covered in this chapter.It is worth noting that both the hydrogel electrolyte and the water system electrolyte of ZIBs experience the same problem caused by water molecules, which inhibits ZIBs during the long-term stable cycle [18].Therefore, the experimenter believes that the study of a non-water GPE is currently the best method to develop ZIBs that combine high performance and safety [168].This is due to the fact that the non-water GPE can avoid some problems caused by water so that ZIBs can have better long-term stable circulation.Unfortunately, there are still some significant problems in the GPE, the most prominent of which is the low conductivity of non-drainage polymer electrolytes [169,170].Therefore, the experimenter considers the problem of how to make gel polymer more efficient in the first place, in order to prepare GPEs with better electrochemical performance, stronger mechanical performance and long-term use as the first priority [171].

Broadening the electrochemical window.
Most of the solutions in non-hydro GPEs are organic solvents, such as PC and vinyl carbonate (EC), to ensure that the GPE can maintain a relatively good electrochemical window [18].ZIBs prepared by GPE can be better used in the market.However, due to the low solubility of Zn salt in PC or PC/EC and its combustible properties, these will hinder ZIB GPE in the preparation process [172].
Therefore, in order to make the GPE with a wide electrochemical window, researchers have proposed the use of ILs instead of organic solvents.The unique properties of ILs, such as non-flammability, high ionic conductivity and wide electrochemical window [173], facilitate the dissolution of Zn [174].Liu et al [175] prepared a GPE consisting of poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate(EMITf) and Zn(OTf) 2 .The experimental results show that when the IL is added to the polymer electrolyte, it greatly improves the electrochemical performance of the Znion battery.The experiment also compares all-solid electrolytes and GPEs introduced into different concentrations of IL (figure 18(a)).Finally, it was determined that ILPE-Zn-4 (the mass ratio of EMITf:Zn(OTf) 2 :PVDF-HFP is 0.4:0.4:1)was the optimal concentration.ZIBs prepared at this concentration not only showed high ionic conductivity, but also had a wide electrochemical stability window at room temperature.Kumar et al [176] prepared a GPE consisting of poly(vinylidene fluoride-co-hexafluoropropylene) and 1ethyl-3-methylimidazolium tetrafluoroborate and Zn(OTf) 2 .The experimental results show that the polymer electrolyte can solve many problems, such as dendrite formation, cathode dissolution and side reaction of the anode.
Moreover, in addition to replacing organic solvents with ILs, Murali and Samuel [172] proposed a new idea to alleviate this problem of GPE.The nanocomposite GPE is made of 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI) IL and inorganic matter silica added to poly(vinyl chloride) (PVC)/poly(ethyl methacrylate) (PEMA) mixture together with a [Zn(OTf) 2 ] salt system.The experimental results show that the Zn-ion cells prepared from the GPE have an electrochemical stability window of up to 5.07 V.The enhanced electrochemical stability is due to the interaction between silica and the −CF 3 SO 3 ions in the electrolyte, which hinders the decomposition of the Zn salt anion, and the ZIBs prepared from this electrolyte can have a wide electrochemical window.However, it is also found that excessive silica agglomerates in the electrolyte, thus limiting the electrochemical properties of ZIBs prepared from the electrolyte.In addition, due to the combined action of IL and inorganic particles, the battery maintains a very high ionic conductivity at room temperature (figure 18(b)).Furthermore, Prasanna and Austin Suthanthiraraj prepared a new type of nanocomposite GPE (NCGPEs) [177] by adding tin oxide (SnO 2 ) nanofiller, 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide IL (EMIMTFSI) PVC/poly(ethyl methacrylate) (PEMA) mixed into Zn(OTf) 2 salt solution.The experimental results show that the Zn cell prepared using this electrolyte has a high ionic conductivity of 4.92 × 10 −4 S cm −1 at room temperature, and the electrochemical stability window can reach 4.37 V.

Improving the ionic conductivity.
In the non-water GPE, improving the ionic conductivity is also very important.The experimenter proposed that this characteristic could be improved by adding additives to the GPE so that ZIB GPE could better meet the market demand.
A lot of research shows that one only needs to add the inorganic particles to the GPE so that the inorganic particles can greatly improve the electrochemical properties of GPE.The principle is that the inorganic particles can reduce the polymer crystallinity of the electrolyte, and at the same time make the polymer amorphous phase content increase proportionally [178].For example, Sai Prasanna and Austin Suthanthiraraj [179] prepared a new composite nanogel polymer electrolyte, and inorganic particle-zirconia was added to the electrolyte (the electrolyte is denoted as NCGPE).The experimental results show that adding zirconia to the GPE can indeed greatly improve the conductivity.Moreover, most inorganic particles will produce the same results as the zirconia-added polymer gel electrolyte mentioned above [180].However, it is worth noting that if there is an excess of inorganic particles in the GPE, the electrochemical performance will decrease.
According to some reports, it is a better choice to add ILs to polymer gel electrolytes than traditional inorganic particles [181,182].This is because IL is a nonvolatile liquid, which can not only avoid the volatilization of solvents, but also be very easily incorporated into the polymer, to better transport Zn ions in the GPE, and further improve the ionic conductivity of the polymer gel electrolyte [183].For example, Tafur and Fernández Romero [184] added 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide (EMIM TFSI) and Zn(CF 3 SO 3 ) 2 to poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) to produce an ionic GPE that can show very good ionic conductivity at room temperature.In addition, Rathika and Austin Suthanthiraraj [185] prepared a new GPE by adding room-temperature IL 1-ethyl-3-methylimidazolium hydrogen sulfate (EMIHSO 4 ) to a PEO/PVdF blend containing Zn triflate (Zn(CF 3 SO 3 ) 2 ) salt.The experimental results show that with the increase in the concentration of IL, the electrochemical performance of the electrolyte is significantly improved.This is due to the addition of an IL polymer gel electrolyte that can make the salt in the electrolyte highly dissociative.At the same time, the cation in IL and inorganic salts can interact with each other to release more Zn ions in the electrolyte.

All-solid electrolytes
As known, in recent years, with the development of the battery industry, ZIBs are usually considered to be one of the very safe and reliable flexible batteries with a very broad research value in the battery industry [186].Therefore, ZIBs are widely used in various large-scale industries, but it is worth noting that when there is any liquid in the battery, including water harmless to the human body, that is encapsulated in a possible deformation of the battery, it can lead to some safety problems [187].At the same time, when the whole solid electrolyte material and liquid electrolyte material are of the same type, the two are compared and it is found that all-solid electrolytes have greater advantages.For example, all-solid electrolytes are refractory to combustion.There will be no possibility of liquid leakage and it can prevent the generation of Zn dendrite, further hindering the battery short-circuit problem [188].Therefore, the practicability and safety of all-solid electrolytes will be much better than that of liquid electrolytes and quasi-solid electrolytes in ZIBs.In addition, the all-solid electrolyte also has the advantages of higher adaptability to the working temperature, excellent mechanical strength, a simple preparation process and is inexpensive [189].Therefore, at present, the development of all-solid electrolytes for ZIBs is one of the more advantageous methods for preparing safer ZIBs.Unfortunately, all-solid electrolytes in practical applications have many serious shortcomings [190], and are unable to meet the requirements of practical applications.

3.3.1.
Improving the ionic conductivity.The full-solid electrolyte is an electrolyte prepared by adding the required metal salt to the polymer matrix, wherein the transport of metal ions in the full-solid electrolyte occurs through the segmented movement of the polymer chain in the amorphous state above the glass transition temperature (T g ) [191].That is to say, when the all-solid electrolyte is highly crystallized, its conductivity is very poor.Generally speaking, when the T g in the all-solid-state electrolyte is low, the segmented motion of the polymer chain can effectively improve, which is conducive to the transport of metal ions in the electrolyte, thus improving the ion conductivity of the all-solid electrolyte [192].Unfortunately, in the all-solid electrolyte without any additives, its conductivity is unable to meet the current market demand for the all-solid electrolyte of ZIBs, which seriously limits the development of ZIBs [8].
As known, the research direction of all-solid electrolytes for almost all batteries is basically started from the research of PEO, which is due to the many excellent properties of PEO, such as good mechanical properties, good flexibility, etc [193].Unfortunately, PEO itself has a semi-crystalline property, which makes its ionic conductivity not very ideal, and it has a very low ionic conductivity (≈10 −6 S cm −1 ) even at room temperature.At present, ensuring that the polymer in the full-solid electrolyte can maintain the amorphous state structure is the key to the development of high ionic conductivity.Therefore, the problem of inhibiting the crystallization of polymers at room temperature is an important task [104].
According to previous studies, researchers have found many very effective methods to improve the ion conductivity of all-solid electrolytes for ZIBs, such as changing the preparation method of all-solid electrolytes [194], adding inorganic substances to all-solid-state electrolytes [195] or choosing appropriate Zn salts to add to all-solid-state electrolytes [196].The method of adding Zn salt to the all-solid electrolyte to improve its ionic conductivity plays an irreplaceable role in the development of the all-solid electrolyte of ZIBs.The principle of this method is that the crystallization degree of the polymer can be reduced through the complexation/dissolution of Zn ions in the polymer network, which enables the all-solid electrolyte to show better stability and excellent ionic conductivity [196].In addition, different Zn salt concentrations also have different effects on the ionic conductivity of all-solid electrolytes.For example, Karan et al [197] added different concentrations of Zn(CF 3 SO 3 ) 2 to PEO to study how much the ratio of Zn salt to PEO content can optimize the ionic conductivity of all-solid electrolysis.
In addition, adding inorganic particles to all-solid electrolytes is a relatively simple and quick method.The difference is that different inorganic particles are added to all-solid electrolytes.This method can destroy the crystallization of all-solid electrolytes, improve the ion conductivity of ZIBs, and make the all-solid electrolytes of ZIBs, as far as possible, meet the large-scale use of ZIBs in the market [198].There are many inorganic fillers that are often added to the all-solid electrolyte as additives to improve the ionic conductivity, such as alumina, silicon dioxide and titanium dioxide [199].Its principle is that inorganic particles can use their own shape and characteristics to interact with the polymer matrix site to form an ion transport channel in the all-solid electrolyte, thus greatly improving the ion transport rate to further improve the conductivity and electrochemical stability of the polymer [200].For instance, Nancy and Suthanthiraraj [201] prepared an all-solid electrolyte consisting of PEO and polypropylene glycol complexed with Zn(CF 3 SO 3 ) 2 .However, the experimental results show that this method does not reach the best state as expected.We further improve the ionic conductivity of the all-solid electrolyte while adding inorganic particle-alumina to the all-solid electrolyte.The experimental results show that the addition of alumina can indeed improve the ionic conductivity, but with the increase in the alumina content, the ionic conductivity shows a downward trend.

3.3.2.
Widening the operating temperature.In addition to its poor ionic conductivity at room temperature, the allsolid electrolyte of ZIBs performs even more unsatisfactorily at low or high temperatures.For example, it has a short storage time and poor long-term stable cycle ability at extreme temperatures [100].
To ameliorate this phenomenon, the experimenters added different substances to the all-solid-state electrolyte.For example, Chen et al [100] for the first time solved the dendrite and serious side reactions and passivation of the Zn metal anode by filling poly(methyl acrylate) grafted MXenes (PVHF/MXene-g-PMA) into poly(vinylidene fluoride-cohexafluoropropylene)-based solid polymer electrolyte (SPE) (figure 19(a)).At the same time, the article also pointed out that due to the strong electrostatic bonding of Zn 2+ , the conductivity of the all-solid electrolyte of ZIBs always presents a bad state.However, since the addition of the well-dispersed MXenes in this experiment, the migration number of Zn 2+ in it is higher, thus increasing the ionic conductivity of this electrolyte.Another advantage is that the developed all-solid ZIBs can also be applied in a very wide range of temperatures.This can be performed at a temperature of −35 • C-100 • C (figure 19(b)) and the ZIBs exhibit a significant improvement in both cycle and shelf life (up to 90 d).The solventfree MXene, which has significant advantages in specific surface area and surface functional groups, can achieve high safety, significant durability and excellent long shelf life in the preparation of all-solid electrolyte ZIBs, eliminating many troublesome side reactions.In order to alleviate the agglomeration problem caused by the addition of inorganic particles in all-solid electrolytes, researchers have changed the direction of ZIB all-solid-state electrolyte additives from inorganic particles to IL.This is due to the unique quality characteristics of IL, which enables good electrochemical battery performance.This method has been widely used in LIBs.The mechanism of action is by destroying the polymer chain of some substances to reduce the crystallinity of the polymer to further improve the ionic conductivity of all-solid electrolytes [202].Therefore, the experimenter added IL to the solid electrolyte of lithium ions to the all-solid electrolyte with a similar mechanism to improve its conductivity [18].For example, Ma et al [14]

Future perspectives
In recent years, ZIBs have become a hot research topic due to their numerous advantages.However, challenges, such as the growth of Zn dendrites, HER and the dissolution of cathode materials have hindered their practical application.To circumvent these issues, the modification strategy of using electrolyte additives has attracted extensive attention.First, the liquid electrolyte additives are summarized reviewed from the perspective of aqueous and liquid non-aqueous systems, and the working principles involved are discussed.Second, the additives of solid electrolytes are summarized, analyzed and discussed from the perspective of all-solid electrolyte and quasi-solid electrolyte, respectively.Although the modification of the electrolyte by the addition of additives can improve the performance of ZIBs and satisfactory results have been achieved, more effort can be made in the following areas to facilitate further development.
(1) Explore new multi-functional electrolyte additives.The electrolyte acts as a bridge between the cathode and anode and makes contact with both at the same time.Therefore, it is feasible to develop an electrolyte additive that can act on both the cathode and the anode.Due to the different needs of the cathode and anode, for example, the cathode side needs to inhibit cathodic dissolution and byproducts, etc, while the anode side has to inhibit Zn dendrite growth and reduce side reactions, etc.This multi-functional electrolyte additive is the development trend of future ZIBs.However, due to the lack of comprehe nsive research on electrolyte additives, the mechanism of different types of electrolyte additives is not clear enough.Therefore, it is an important goal to study the effect and mechanisms of multi-functional electrolyte additives.(2) Explore the synergy of composite additives.Due to the limited range of a single additive, two or more kinds of composite additives may be more effective.However, it is not only necessary to avoid the consumption of multiple additives by mutual reaction, but also to ensure that the effect of 1 + 1 > 2 can be obtained after its application, which inevitably requires synergistic effects between additives.Therefore, the development of complex additives with synergistic effects is a good strategy.(3) Exploring the microstructure of the electrolyte at nano and molecular level.The structure of electrolytes is an important factor for the long-term stable use of batteries.Further research can further promote the development of electrolyte modification with additives.Therefore, some advanced characterization methods, especially in situ technique, can be used to deeply study the microstructure of the electrolyte at nano and molecular level, in order to facilitate the design of ZIB additives and the electrolytes.(4) Improve the study of all-solid-state electrolytes.All-solid electrolytes can fundamentally solve some of the problems caused by water.However, due to the low ion conductivity of all-solid electrolytes, these cannot be applied to ZIBs.In this respect, the research is not comprehensive enough.Therefore, it is necessary to explore the good solid electrolytes that can improve the comprehensive performance of the ZIBs.Some successful experience in solidstate LIBs can be used for reference.
In summary, the research progress of ZIBs requires further systematic research and scientific exploration.It is important to continuously improve the electrolyte and electrolyte additive strategy, which may play a key role in the commercialization of ZIBs in the future.51922099), and Fundamental Research Funds for the Central Universities (Grant No. buctrc202104).

Figure 2 .
Figure 2. Existing challenges for cathodes and anodes in aqueous Zn-ion batteries, and the positive impact additives can play.

Figure 3 .
Figure 3. (a) Growth process of Zn dendrites.(b) and (c) are the chemical mechanisms of the corrosion reaction and the hydrogen evolution reaction, respectively.

Figure 4 .
Figure 4. (a) Scheme of Zn 2+ solvation structure and Zn surface passivization in H 2 O (left) and H 2 O-DMSO (right) solvents.(b) SEM images of the Zn electrode in Zn/Zn symmetrical cells after 50 plating/stripping cycles at 0.5 mA cm −2 and 0.5 mAh cm −2 in ZnCl 2 -H 2 O-DMSO electrolytes.White lines indicate the etching depth in the cross-section view.Reprinted with permission from [34].Copyright (2020) American Chemical Society.(c) Due to to PG's hydrophobic nature (-CH 3 tail and hydrocarbon backbone), it can remove the hydrogen bubbles from the Zn surface that, in the PG-free electrolyte, induce porous and flaky Zn deposit formation.(d) In situ optical microscopy investigation of Zn deposition in a transparent Zn/Zn cell with PG-free and 10% PG electrolytes.(e) Cycling performance of AC/Zn cells in the three electrolytes; [35] John Wiley & Sons.[© 2022 Wiley-VCH GmbH].(f), (g) Potential and current distribution of Zn deposition at a simulation time of 2 min in ZSO electrolyte (f) and ZSO/Ce electrolyte (g); gray lines with the arrows and the black lines at the bottom represent the current and the initial surface of the Zn anode.[36] John Wiley & Sons.[© 2022 Wiley-VCH GmbH].(h) O 1s of Zn surface after charge-discharge process with 2 M LiCl as the additive.Reprinted with permission from [37].Copyright (2021) American Chemical Society.(i) SEM images of Zn anodes plated in electrolyte with Hmim for 30 min at a current density of 1 mA cm −2 .Reprinted from [38], Copyright (2023), with permission from Elsevier.
use four typical carbonate-based solvents with different hydrophobicity (i.e.ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC) and diethyl carbonate (DEC)) as additives to protect a Zn anode.The hydrophobicity of the four solvents gradually increases from EC to DEC (figure 5(b)).

Figure 8 .
Figure 8.(a) Schematic diagram of the battery storage mechanism of raw electrolyte and PEA-modified electrolyte.[69] John Wiley & Sons.[© 2023 Wiley-VCH GmbH].FESEM images of the VNxOy/C electrode after 100 cycles at the current density of 0.5 A g −1 operated in the electrolyte of (b) pure H 2 O and (c) the H 2 O-50% PEG system.Reprinted from [70], Copyright (2023), with permission from Elsevier.Schematic illustration of the hybrid aqueous battery storage mechanism with (d) pristine electrolyte and (e) SDS-modified electrolyte.Reprinted from [71], Copyright (2022), with permission from Elsevier.(f) XRD patterns of the fully discharged KVO cathode after 100 cycles in three different electrolytes.Reproduced from [72], with permission from Springer Nature.

Figure 10 .
Figure 10.(a) SEM images of Zn deposits at the electrode surface cycled in TMPNMF electrolyte after 800 cycles of Zn deposition/stripping. Reprinted from [85], Copyright (2023), with permission from Elsevier.(b) Design strategy of an EMC-based electrolyte with a microheterogeneous anion solvation network for high-voltage Zn/graphite cells.[87] John Wiley & Sons.[© 2020 Wiley-VCH GmbH].(c) Comparison of the cycling performance (capacity retention versus cycle number) of Zn-MnO 2 batteries using the three electrolytes.Illustration is the optical image of two CR2032-type coin cells that are assembled using aqueous and hydrous organic electrolytes and stored for different storage times.Reprinted from [90], Copyright (2023), with permission from Elsevier.(d) Charge/discharge profiles of Zn/Zn symmetric cells in various electrolytes: DMF (black), AN (red), TMP (blue), H 2 O (pink), TEP (green).Reprinted from[89], Copyright (2022), with permission from Elsevier.

Figure 11 .
Figure 11.(a) Schematic illustration of the anode SEI formation in EIWE.[95] John Wiley & Sons.[© 2020 Wiley-VCH GmbH].(b) Schemes illustrating different reaction processes of Zn 2+ solvation structure and interfacial interaction between the Zn anode surface and electrolyte in AWCS electrolytes.Reprinted from [11], Copyright (2022), with permission from Elsevier.(c) Cycling stability at 20 • C with a current density of 5 A g −1 .(d) Cycling stability of EG 40 at −20 • C with a current density of 0.2 A g −1 .Reproduced from[97] with permission from the Royal Society of Chemistry.Snapshot of the MD simulation cell and corresponding RDF plots for (e) 1 M Zn(OTf) 2 in water, (f) 50% PC-sat.Insets in panels are the corresponding representative solvation structure within a 0.3 nm scale.(g) Anode-free Cu-ZnMn 2 O 4 batteries in different electrolytes with a current density of 0.5 mA cm −2 (∼350 mA g −1 ).Reprinted with permission from[98].Copyright (2022) American Chemical Society.

Figure 12 .
Figure 12.Influence of various electrolyte additives on the solid electrolyte of the battery and related mechanisms.(a) Structure of the battery studied in nanomaterial-enhanced all-solid flexible Zn-carbon batteries.Reprinted with permission from [118].Copyright (2010) American Chemical Society.(b) Schematic illustration of the preparation and structural characterization of the anti-freezing conductive organohydrogels.[117] John Wiley & Sons.[© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim].(c) Schematic illustration of the strong hydrogen bonds between EG-waPUA, water and PAM in the AF-gel.Reproduced from [115] with permission from the Royal Society of Chemistry.(d) Schematic diagram of the synthesis of PAMPS/PAAM dual-network hydrogel.Reprinted from [116], Copyright (2022), with permission from Elsevier.

Figure 13 .
Figure 13.Challenges facing the current hydrogel polymer electrolyte of ZIBs: (a) elastic stability of polyacrylamide (PAM)-hydrogel electrolytes after a day's storage at various temperatures.Reproduced from [125].CC BY 4.0.(b) Performance deterioration due to freezing of aqueous electrolyte at low temperatures.Reprinted with permission from [126].Copyright (2022) American Chemical Society.(c) Low ion conductivity of electrolyte in low-temperature hydrogel solid electrolytes.(d) Photograph of PAMPS/PAAM hydrogel in a completely unloaded state and 50-cycle distortion.Reprinted from [116], Copyright (2022), with permission from Elsevier.

Figure 14 .
Figure 14.(a) Formation energy from DFT calculations (PAM is viewed as one unit and the long chains are replaced by -CH 3 ).(b) Photos of the hydrogels at different temperatures.Reprinted from [139], Copyright (2022), with permission from Elsevier.(c) Schematic illustration of the fabrication of GG/SA and GG/SA/EG hydrogel electrolytes.Reprinted from [12], Copyright (2021), with permission from Elsevier.(d) Schematic diagram of the synthesis process of the PDZ-H electrolyte.(e) Digital photos of PDZ-H electrolyte at different temperatures from 20 to −40 • C; [142] John Wiley & Sons.[© 2022 Wiley-VCH GmbH].(f) Schematic illustration of the synthesis process at different states.Reprinted from [143], Copyright (2022), with permission from Elsevier.

Figure 16 .
Figure 16.(a) Schematic view of GO phosphonation reaction and PAM-PGO structure showing hydrogen bonds.Reprinted from [156], Copyright (2022), with permission from Elsevier.(b) Schematic illustration of the synthesis route of the MMT-PAM hydrogel through a controllable accelerated polymerization mechanism.(c) Schematic illustration of the synthesis route of the MMT-PAM hydrogel through a traditional route; (b), (c) Reprinted from [157], Copyright (2023), with permission from Elsevier.(d) Illustration of the spatial structure of PVA-based hydrogel electrolyte in flexible solid-state Zn-polymer batteries with practical functions.Reprinted from [136], Copyright (2021), with permission from Elsevier.

Figure 17 .
Figure 17.(a) Schematic illustration of the swelling method for the preparation of OHE, and structure merits of OHE.Reprinted from [163], Copyright (2022), with permission from Elsevier.(b) Schematic illustration of the construction of an ion migration channel under the applied electric field.Reproduced from [158].CC BY 4.0.