Research Progress of Anode Materials for Zinc-Based Aqueous Battery in a Neutral or Weak Acid System

Xianwen Wu; Fengni Long; Yanhong Xiang; Jianbo Jiang; Jianhua Wu; Lizhi Xiong; Qiaobao Zhang

doi:10.7536/PC210453

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (11) : 1983-2001 DOI: 10.7536/PC210453

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Research Progress of Anode Materials for Zinc-Based Aqueous Battery in a Neutral or Weak Acid System

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Abstract

Zinc is an ideal electrode material for green rechargeable batteries because of its abundant raw materials, light weights, excellent electrical conductivity and ductility, as well as high theoretical specific capacity. The zinc-based aqueous battery with the neutral or weak acidic aqueous solution as electrolyte and zinc as the anode has the characteristics of high security, low cost and non-toxic battery materials, simple preparation process, and environmentally friendly. It has abroad application prospects in the field of storage devices for energy and power-driven battery. However, the problems such as zinc dendrite, hydrogen evolution, corrosion, and passivation during the process of charging and discharging limit its practical application. In this paper, the existing problems and current solutions of zinc anode for zinc-based aqueous batteries were reviewed, and the developmental trend of the anode has prospected.

Contents

1 Introduction

2 The challenges of zinc anode

2.1 Zinc dendrite

2.2 Corrosion and passivation

2.3 Hydrogen evolution

3 Optimizing strategy for zinc anode

3.1 Additives

3.2 Zinc alloy

3.3 Surface modification

3.4 Structural design

3.5 Intercalation-type anode

3.6 Electrolyte optimization

4 Summary and outlook

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Keywords

aqueous zinc ion battery / electrode material / zinc anode / dendrite / structure design

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Xianwen Wu, Fengni Long, Yanhong Xiang, Jianbo Jiang, Jianhua Wu, Lizhi Xiong, Qiaobao Zhang. Research Progress of Anode Materials for Zinc-Based Aqueous Battery in a Neutral or Weak Acid System. Progress in Chemistry, 2021, 33(11): 1983-2001 DOI:10.7536/PC210453

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1 Introduction

The continuous consumption of natural fossil energy has led to the drying up of resources, and the waste discharged by industrial development has also caused serious environmental pollution. Energy and environmental issues are the key issues to be solved for human survival and development. It will be the general trend to vigorously develop renewable energy such as solar energy, tidal energy and geothermal energy. However, most of the renewable resources are discontinuous in time and space, and it is necessary to develop advanced energy storage equipment to store these energy and output it intelligently and smoothly^{[1⇓⇓⇓⇓~6]}. Lithium-ion batteries (LIBs) have dominated the energy storage market since the 1990s due to their good cycle performance and high energy density^{[7⇓⇓⇓~11]}. However, the flammability and toxicity of organic electrolytes, the scarcity of lithium resources and the high cost of assembly limit the further development of LIBs in practical applications^{[12⇓⇓⇓~16]}. Potassium ion batteries (PIBs) and sodium ion batteries (SIBs) with abundant natural resources and cheap raw materials are considered to be better alternatives to LIBs because alkali metals such as sodium and potassium have similar chemical properties with lithium^{[17⇓⇓⇓⇓⇓~23]}. However, the inherent safety problems of organic electrolytes remain unsolved^[24⇓~26]. Compared with metal secondary batteries in organic systems, aqueous batteries have the advantages of low toxicity, low cost, free and simple assembly conditions, good battery stability and rate performance, and have considerable application prospects in portable electronic communication and energy storage devices^[27⇓~29]. Among them, zinc is one of the most desirable anode materials for aqueous batteries due to its high theoretical specific capacity (V vs SHE), high volume energy density (5855 mh·cm^-3), low redox potential (-0.76 V vs SHE), non-toxicity, good conductivity, small ionic radius and abundant natural reserves^{[30⇓⇓~33]}. In the 1970s, the reversible water-based Zn-Mn battery using alkaline electrolyte was successfully developed, which undoubtedly opened the door to a new world for researchers^[34]. However, with the deepening of the research on the battery system, it is found that the positive and negative surfaces of the battery will be seriously corroded and passivated in alkaline medium, which will lead to serious specific capacity attenuation and unstable cycle performance of the battery^[35]. In 1986, Takakazu et al. First developed the Zn/ZnSO₄/MnO₂ battery system. This kind of aqueous Zn-MnO2 battery (namely aqueous Zn-ion battery, AZIBs) with neutral or weak acidic aqueous solution as electrolyte has the advantages of high energy density, wide electrochemical window and high ionic conductivity, and can be used as the most promising alternative to alkaline Zn-MnO2 battery^[36]. In 2012, Professor Kang Feiyu's team of Tsinghua University once again studied AZIBs in depth^[37]. In the same year, Academician Chen Pu, University of Waterloo, Canada, first reported a mixed water battery (ReHAB) with zinc chloride + lithium chloride as the mixed electrolyte, zinc as the negative electrode and LiMn₂O₄ as the positive electrode^[38]. Since then, the research of zinc-based aqueous secondary batteries under neutral or weak acidic electrolyte conditions has stepped into the fast lane.

As mentioned above, zinc-based aqueous batteries under neutral or weak acid electrolyte conditions currently studied are mainly divided into two categories. One is ReHAB, which uses zinc as the negative electrode, lithium or sodium/zinc mixed salt as the electrolyte, and lithium or sodium-containing compounds as the positive electrode to construct battery systems such as Zn/LiMn₂O₄, Zn/LiV₂PO₄, Zn/Li₃V₂(PO₄)₃ and Zn/Na₃V₂(PO₄)₃^{[38⇓⇓⇓⇓~43]}. The other is AZIBs, which uses metal zinc as the negative electrode, zinc salt as the electrolyte, and zinc-intercalable compounds as the positive electrode to construct Zn/MnO₂, Zn/V₂O₅, Zn/KMn₈O₁₆ and other battery systems^{[37,44⇓~46]}. In recent years, with the increasingly in-depth research on the cathode materials of zinc-based aqueous batteries, anode materials have begun to attract much attention. However, in neutral or weak acidic electrolyte, the problems of hydrogen evolution, corrosion, passivation and dendrite growth of zinc anode during the charge-discharge process lead to poor reversibility and short cycle life of the battery, which greatly hinders the development and practical application of zinc-based aqueous batteries^[47,48].

Based on the above problems, this paper discussed the problems faced by the metal zinc anode of zinc-based aqueous batteries, summarized the current modification strategies of zinc anode, and briefly summarized the recent research progress of zinc-based aqueous batteries. Finally, some feasible research methods on the anode were proposed in this paper, aiming at promoting the development of zinc-based aqueous batteries.

2 Challenges faced by zinc anode

For the zinc-based aqueous battery, the initial research is in the alkaline electrolyte system, but the problems of corrosion, passivation and zinc dendrite caused by alkaline electrolyte will have a great impact on the performance of the battery^[35]. This is because the abundant OH^- will react with the negative electrode to form basic zincate. This by-product has low solubility and deposits on the surface of the zinc anode, resulting in passivation of the anode, reducing the active sites on the surface, making zinc ions unable to deposit uniformly, and promoting the growth of zinc dendrites, which will eventually lead to short circuit problems, a sharp decline in cycle stability, and damage to battery life. In the neutral or weak acid electrolyte system, these problems can be alleviated to some extent, but still can not be completely avoided.

2.1 Zinc dendrite

The growth of zinc dendrites is related to the nucleation process of zinc ions, and the nucleation process is affected by the electric field distribution, ion concentration and surface energy^[49]. Firstly, the uneven electric field and ion concentration distribution on the surface of the negative electrode make the nucleation of zinc ions uneven, which provides conditions for the formation of dendrites. Both of them are related to the unrestricted two-dimensional diffusion process of zinc ions adsorbed on the surface of the anode^[50,51]. At present, commercial zinc sheet is mainly used as the negative electrode in the zinc-based aqueous battery. Zinc is not only the current collector, but also provides the Zn²⁺ for the battery. The surface of the current collector can not be absolutely flat due to the limitation of process conditions. These "unevenness" will lead to uneven distribution of electric field on the surface of the negative electrode. Zinc ions will preferentially deposit on the sites with more charges, high current density and high energy on the surface of the negative electrode, and gradually form small protrusions. The convex tip will produce "point discharge" effect, that is, the convex tip has a larger curvature and a larger surface charge density, which further attracts zinc ions to deposit at the convex tip and intensifies the growth of dendrites^[52]. Secondly, the surface porosity of the two-dimensional planar metal zinc foil is low, the interface nucleation barrier is large, the active sites are few, and the zinc ions are not easy to nucleate and deposit. In the region of high ion concentration and fast ion transport, the nucleation barrier will decrease, and the Zn²⁺ will nucleate locally on the surface of zinc electrode^[49]. Finally, during the cycle of the battery, the metal zinc will be repeatedly deposited and dissolved. However, in order to reduce the exposed area of the negative electrode surface and reduce the surface energy, zinc ions tend to deposit on the small bumps that have been formed^[53]. As the cycle goes on, these bumps will continue to be accumulated by zinc ions to grow into zinc dendrites, and the sharp thorns of the dendrites can easily pierce the diaphragm to make the positive and negative electrodes directly contact, causing short circuit of the battery, resulting in significant attenuation of battery capacity, poor cycle performance and shortened cycle life.

To sum up, when zinc ions migrate to the vicinity of the negative electrode, they diffuse unevenly with the electrolyte interface of the electrode, nucleate and aggregate at the energy center with lower nucleation potential, and gradually form a bulge, which makes the distribution of the electric field more uneven and promotes the growth of dendrites^[51]. With the cycling, the current density at the tip of the dendrite increases, which will attract more zinc ions to deposit, and intensify the growth of the dendrite as well as the attenuation of the electrode capacity. Therefore, the key to solve the problem of zinc dendrite is to make the electric field distribution on the surface of metal zinc anode more uniform or to induce the uniform deposition of zinc ions during cycling.

2.2 Corrosion and passivation

Negative corrosion can be divided into self-corrosion and electrochemical corrosion. Self-corrosion often occurs in alkaline medium, because the reduction potential of H⁺ is higher than that of Zn²⁺, and the oxidation is stronger. The reaction formula is as follows:

(1)Zn²⁺+2e^-→Zn(E^θZn²⁺=-0.76V)

(2)2H⁺+2e^-→H₂↑(E^θH⁺=0V)

(3)Zn+2H⁺→H₂↑+Zn²⁺

(4)3Zn²⁺+6OH^-+ZnSO₄+nH₂O→ZnSO₄[Zn(OH)₂]₃·nH₂O

Formula (1) is a zinc deposition reaction, formula (2) is a hydrogen evolution reaction, formula (3) is an auto-corrosion reaction, and formula (4) is an electrochemical corrosion reaction. The self-corrosion reaction can also be regarded as the self-discharge reaction of the battery, which will make the efficiency and specific capacity of the zinc anode unable to reach the ideal state^[31,51,54]. Its microscopic nature can be explained that the potential of zinc electrode with non-uniform surface is different in different regions, so in the aqueous electrolyte environment, the surface of the negative electrode will produce many small devices that can convert chemical energy into electrical energy, and the synergistic effect of these small devices is the self-corrosion reaction of the negative electrode^[50,55]. In the weak acid or neutral electrolyte system without complexing agent, the self-corrosion phenomenon will be weakened a lot.

Electrochemical corrosion often occurs in neutral or weak acid electrolyte systems, that is, irreversible consumption of zinc anode occurs in the process of repeated charge-discharge cycles, such as dead zinc falling off in the electrolyte during the growth of zinc dendrites. Another example is the inert by-product generated in the electrochemical reaction of the zinc anode dissolution product Zn²⁺, whose reaction equation is listed in Formula (4), and the electrochemical corrosion will affect the cycle performance of the battery^[56]. Because these irreversible by-products have low solubility and poor conductivity, they are easy to saturate in the electrolyte and deposit on the surface of zinc anode to reduce its active sites and reaction surface area, which is called passivation of zinc anode^[57,58]. With the increase of cycle times, the by-products covering the surface of the negative electrode will affect the normal dissolution of zinc, reduce the active materials that can participate in the reaction, and reduce the specific surface area of the electrode. Essentially, the specific surface area of the electrode decreases, the ratio of area to volume increases accordingly, and the reaction system will polarize, which can alleviate corrosion to a certain extent, but will further deteriorate the performance of the battery. It can be seen that both self-corrosion and electrochemical corrosion have a serious negative impact on the cycle stability of zinc-based aqueous batteries. In the final analysis, the source of corrosion is the reaction between metal zinc anode and electrolyte solvent water, and trying to isolate the direct contact between anode and water is the fundamental solution to the problem of corrosion and even hydrogen evolution.

2.3 Hydrogen evolution

At the junction of the zinc cathode and the electrolyte, the polarization potential makes the zinc-based aqueous battery prone to hydrogen evolution reaction, as shown in formula (2). Even though

E θ H + > E θ Z n 2 + (E θ

is expressed as the standard electrode potential), the deposition of Zn²⁺ occurs before the hydrogen evolution reaction in the 2 mol·L^-1ZnSO₄ electrolyte environment, because H⁺ has a high hydrogen evolution overpotential, slow hydrogen evolution kinetics and low activity in neutral or weak acidic media^[49,59]. However, the hydrogen evolution reaction is still unavoidable: firstly, the uneven ion distribution of the negative electrode makes some active sites available for the hydrogen evolution reaction; Secondly, the larger polarization potential during charging will intensify the hydrogen evolution reaction process^[49,60,61]. Hydrogen evolution reaction usually occurs in the local high-energy area, in which water is consumed and OH^- is produced. If the concentration of local OH^- is too high, electrochemical corrosion will occur, which consumes zinc anode and water to produce by-products such as zinc hydroxide and basic zinc sulfate^[62,63]. The actual hydrogen evolution is also affected by thermodynamic and kinetic factors. The change from liquid H₂O to gaseous H₂ will increase the internal pressure of the battery. If the battery is sealed, the hydrogen evolution reaction will eventually damage the internal expansion of the battery, and the electrolyte will leak^[64]. Like the corrosion reaction, the hydrogen evolution reaction also originates from the direct contact and reaction between the negative electrode and water, and isolating the direct contact between the negative electrode and water is also the key to inhibiting the hydrogen evolution reaction.

The uneven electric field and ion distribution on the surface of the negative electrode promote the formation and growth of zinc dendrites, increase the specific surface area of the negative electrode, and increase the contact range between metal zinc and water molecules, so that hydrogen is released faster and more violently^[65]; Water decomposes into H⁺ and OH^- at room temperature, and the precipitation of hydrogen increases the amount of free OH^- in some areas, which increases the pH in this area and also accelerates the corrosion passivation at the interface^[66]. The irreversible inactive substances generated by corrosion passivation are attached to the zinc negative electrode, which reduces the active sites on the surface of the electrode, makes the negative electrode plane more uneven, enlarges the polarization phenomenon, and makes the electric field distribution, ion concentration distribution and surface energy distribution unbalanced. The zinc dendrite will grow faster and quickly pierce the diaphragm, resulting in the scrap of the energy storage device^[67]. To sum up, the formation of zinc dendrites will promote the hydrogen evolution reaction, and the hydrogen evolution reaction will help the corrosion reaction on the surface of the negative electrode, and a series of by-products brought by corrosion will aggravate the formation of dendrites. As shown in Figure 1, this is a vicious circle process, like the broken window effect, if the battery is not properly modified, it will be short-circuited in a short time, and once one of the links is successfully suppressed, other problems can also be alleviated to a certain extent^[51].

3 Optimization Strategy of Zinc Anode

With regard to the difficulties faced by the negative electrode of zinc-based water-based batteries, researchers have mainly used six measures to achieve the purpose of improvement: (1) additives; (2) Zinc alloy; (3) surface modification; (4) Structural design; (5) an embedded negative electrode; (6) Electrolyte optimization.

3.1 Additive

It is a common research strategy to improve the performance of zinc-based aqueous battery system by adding additives. The additives include electrode structure additives and electrolyte additives, which will be described in detail in the following electrolyte optimization section.

Electrode structure additives are usually divided into non-metallic additives and metal additives. Non-metallic additives include conductive materials such as graphite, acetylene black, carbon nanotubes and activated carbon, which do not directly intervene in the deposition and dissolution of the negative electrode, but can improve the properties of the negative electrode itself, improve the cycle period and capacity retention rate of the battery.It is often used to improve the performance of zinc powder electrode. For example, Kang et al added different mass ratios of activated carbon (AC) to zinc powder-based negative electrode (ZnAB) prepared by zinc powder: acetylene carbon black (ACET): polyvinylidene fluoride (PVDF) in the ratio of 7:2:1^[58,68]^[69]. As shown in Figure 2a and Table 1, the galvanostatic charge-discharge cycle experiments show that the capacity retention rates of the batteries are 62.5%, 77.9% and 85.6% after 80 cycles when 5 wt%, 8 wt% and 12 wt% AC are added to the zinc anode, respectively.The capacity retention was 56. 7% better than that of the battery using unmodified zinc anode, which indicated that the electrochemical reaction kinetics and reversibility of zinc anode modified by activated carbon were improved, and the cycle performance of zinc anode in neutral electrolyte was improved. As shown in Figure 2b, the AC with porous structure can not only improve the conductivity of the zinc powder-based anode, but also build a three-dimensional support structure due to its porous characteristics, so that the zinc dendrites and other substances generated in the redox process can be contained in the structural pores, so as to prevent them from attaching to the surface of the structure and causing damage to the battery. The AC can also inhibit the formation of inactive alkaline zinc sulfate (Zn₄SO₄(OH)₆·nH₂O), so that the zinc powder-based composite anode can maintain a uniform, flat and active reaction interface, and is also very conducive to inhibiting other potential side reactions. However, the dosage of electrode additives should be moderate, as shown in Table 1 and Figure 2a. The electrochemical performance of ZnAB + 15 wt% AC was further tested, and it was found that its capacity retention rate was significantly lower than that of ZnAB + 12 wt% AC, which indicated that there was a parabolic curve between the dosage of electrode additives and their modification effect on zinc anode, and there was a peak in the dosage of additives.

Layered double hydroxides (LDH) are widely used as metal additives. Adding metal additives in the synthesis process of zinc anode can improve the electrodeposition efficiency of zinc anode, inhibit the occurrence of side reactions such as hydrogen evolution, prolong the battery life cycle and stabilize its electrochemical performance^[70]. Gonz González et Al. doped Zn-Al-LDH powder and zinc powder at four ratios of 0:1, 1:1, 2:1 and 4:1 to prepare a zinc powder-based composite anode. The deposition and dissolution efficiency of the modified electrode was effectively improved, and the doping ratio had a certain impact on the performance^[71]. Because the electrodeposition reaction of zinc on the zinc anode is closely related to the performance and composition of the electrode itself, the addition of LDH into the anode can eliminate the potential drop at the initial reduction of Zn²⁺ and avoid the hydrogen evolution reaction at the initial discharge stage of the battery. As shown in fig. 2C, the layered LDH can also provide more sites for zinc deposition. Compared with the uneven surface of the pure zinc anode, the zinc anode with LDH has a more flat and dense surface, which effectively inhibits the dendrite and corrosion of the anode. Subsequently, the team prepared Cu-doped Zn-Al-CO₃ layered double hydroxides by constant pH co-precipitation method, and mixed them as additives with zinc powder in four ratios of 0:1, 1:1, 1:2 and 2:1 to prepare zinc negative electrodes^[72]. For example, when the ratio of Zn to LDH is 1 ∶ 1 and 1 ∶ 2, the deposition and dissolution efficiency of the two are almost the same, and the former can even stabilize faster. After assembled with Prussian blue analogue (CuHCF), the performance of the battery was not good, and the capacity did not exceed the 60 m Ah g^-1 in the first 50 cycles, but compared with the combination without additives, the Cu-doped Zn-Al-CO₃-LDH still achieved the purpose of modification. This is not only attributed to the layered structure of LDH (Figure 2D), but also because the Cu doping enhances the electrode conductivity.

3.2 Zinc alloy

Mixing zinc with metal or non-metal elements with strong corrosion resistance, high hydrogen evolution overpotential and excellent chemical stability to prepare high-performance zinc alloys for zinc anode is also an effective means to inhibit side reactions such as zinc dendrite and corrosion and improve the stability of zinc anode. Wang et al. Made zinc-aluminum eutectic alloy Zn₈₈Al₁₂(at%) by low temperature calcination method. This alloy with layered structure will form zinc-aluminum alternating nano-film on the surface of the anode, which is an effective strategy to solve the problems of low coulombic efficiency of zinc metal anode and irreversible growth of zinc dendrite^[73]. The layered nanostructure not only promotes the dissolution of zinc in the precursor eutectic Zn₈₈Al₁₂(at%) alloy, but also forms Al₂O₃ oxide protective layer on the surface of aluminum, which can guide the subsequent deposition and growth of zinc ions, so that zinc can be deposited/dissolved without dendrite in the oxygen-free aqueous electrolyte for more than 2000 H. These outstanding electrochemical performances are confirmed in aqueous zinc-ion batteries. As shown in Fig. 3A, the battery with a negative electrode of Zn₈₈Al₁₂ alloy and a positive electrode of K_xMnO₂ can provide high energy density (230 Wh·kg^-1) at high power (550 kW·kg^-1), and it still maintains 100% capacity after 200 H of charge-discharge cycling. The excellent electrochemical performance of Zn₈₈Al₁₂ alloy /K_xMnO₂ is related to the electrochemical behavior of Zn and Al.

Although the standard equilibrium potential of V vs SHE (− 1.66 V vs SHE) is much lower than that of Zn²⁺/Zn, the formation of an insulating Al₂O₃ layer on the Al surface prevents the dissolution of Al. This makes the Zn and Al layers play different roles in the charge-discharge process, as shown in Fig. 3B: the Zn layer provides Zn²⁺ charge carriers, while the Al layer acts as a 2D carrier skeleton for the negative electrode to accommodate the deposition of Zn²⁺. And since the insulating Al₂O₃ layer substantially prevents electrons from transferring from Al to the Zn²⁺, without reducing the Zn²⁺,A positive electrostatic shield is formed around the Al/Al₂O₃, so that the Al/Al₂O₃ nano-template guides the Zn²⁺ to be uniformly deposited along the interlayer spacing of the center of the Zn precursor. This can significantly inhibit the passivation of the Zn anode, prevent the generation of dendrites, and reduce the generation of ZnO or Zn(OH)₂ at the anode interface.

In addition to being mixed with aluminum to prepare a eutectic alloy, the introduction of metals with high hydrogen evolution overpotential such as copper, gallium, titanium, bismuth and the like to prepare a high-performance zinc alloy can also effectively inhibit the corrosion of the negative electrode. Cai et al. introduced corrosion-resistant metal copper on the negative electrode of zinc metal by low temperature calcination method, and constructed a compact and smooth copper/zinc composite electrode. With the occurrence of redox reaction, the copper/zinc electrode will eventually degenerate into a copper-zinc nano-alloy/zinc mixture^[74]. As shown in fig. 3C, under the cycling condition of 1 mA·cm^-2, the symmetrical battery assembled with ordinary zinc sheets has a severe polarization phenomenon after standing for a short period of time, and the overpotential changes greatly. However, the overpotential of the symmetrical battery assembled by Cu-Zn nano-alloy/Zn electrode was very stable after 720 hours of shelving, and at least 1500 cycles of stable reaction could be ensured after shelving. As shown in fig. 3D, this is because the zinc metal anode is prone to uneven oxidation reaction in aqueous solution at room temperature, that is, electrochemical corrosion, accompanied by the generation of basic zinc sulfate and the occurrence of hydrogen evolution. The chemical corrosion of zinc metal anode will lead to the consumption of water molecules and active metal zinc in the electrolyte, and the increase of electrode impedance due to the coverage of inert by-products, which will eventually deteriorate the performance of the battery and lead to the rapid scrapping of the battery. Cu-Zn nano-alloy/Zn anode with compact structure, high hydrogen evolution overpotential and good corrosion resistance can effectively inhibit the chemical corrosion and dendrite of the anode, and achieve the purpose of improving the performance of the anode of aqueous batteries.

Alloy technology is frequently used in the research of anticorrosive materials. The design scheme of alloy modification of zinc electrode provides the direction of thinking and research for the preparation of zinc anode materials with long cycle life, no dendrite and corrosion resistance, which is conducive to the development of better and more stable secondary zinc-based aqueous batteries and the promotion of modification for other aqueous batteries^[75,76].

3.3 Surface modification

Surface modification of the anode is also an effective strategy. A stable and mild electrode/electrolyte interface can be formed by introducing an interfacial protective layer, which can effectively inhibit the growth of zinc dendrites in the anode, thereby reducing the occurrence of side reactions such as corrosion and hydrogen evolution^[77,78]. The method is usually to form a protective layer on the zinc sheet by direct coating or electrodeposition of inorganic or organic substances. The reported functional protective layers include inorganic mineral coatings (such as nanoporous CaCO₃), carbon-based coatings (such as graphene), and metal alloy coatings (e.g. Liquid Ga-In alloy coatings)^[79]^[80,81]^[82]. The modification principle is not completely consistent due to the different types of coatings, but it usually plays the role of ion absorption and interface protection. On the one hand, the protective layer can effectively improve the deposition mode of zinc ions and enhance its kinetic performance, and the coating with higher porosity can induce relatively uniform electrolyte flux and deposition dissolution rate to achieve the effect of dendrite-free deposition. The principle is that the porous structure of the interface guides and controls the ion absorption rate and deposition sites. On the other hand, the interface protective layer prevents the negative electrode from being in direct contact with the electrolyte, reduces the polarization potential of the material, effectively inhibits the occurrence of a series of interface side reactions, reduces the loss of the negative electrode, protects the interface reaction of the zinc-based aqueous battery, and prolongs the service life of the battery.

Nanoporous materials with layered structure are widely used in surface modification. For example, Liang et al. Synthesized ZrO₂ coating on zinc anode by sol-gel method, and prepared a new anode Zn@ZrO₂, which can promote the uniform deposition and dissolution of zinc anode surface^[83]. The anode has the advantages of low polarizability (24 mV at 0.25 mA·cm^-2), high Coulombic efficiency (99. 36% at 20 mA·cm^-2) and long cycle life (more than 3800 H at 0.25 mA·cm^-2). As shown in Table 2, under the current density condition of 1 A·g^-1, the V₂O₅//Zn@ZrO₂ battery still has a reversible capacity of 204.2 mAh·g^-1 after 1145 cycles, and the discharge capacity is almost equal to the charge capacity; However, the specific capacity of the V₂O₅//Zn cell decays to 30.4 mAh·g^-1, and the coulombic efficiency is only 15.8%. Fig. 4A shows the action mechanism of the ZrO₂ coating. A series of known electrode side reactions occur at the interface of the uncoated pure zinc anode, and the electrode surface becomes uneven; However, the zinc anode coated with ZrO₂ is smooth and flat. This is because the zinc anode coated with ZrO₂ has lower polarization potential, reduces the nucleation potential energy at the interface, and increases the nucleation centers to guide the uniform deposition and dissolution of Zn²⁺ in the continuous cycling process. As an interfacial protective layer, ZrO₂ is an effective "courtyard wall" to prevent the negative electrode from contacting with water in a large area, at the same time, its high hydrogen evolution overpotential can also inhibit the occurrence of hydrogen evolution reaction, effectively inhibit or eliminate several side reactions, and the coating can naturally improve the reversibility and stability of the battery. Deng et al. Proposed a kaolin (KL) coating with electron insulation, ion conduction (selective through Zn²⁺) and uniform gap distribution (3. 0 nm) to alleviate the corrosion and dendrite of zinc anode^[84]. Fig. 4 C compares the deposition phenomena of the unmodified negative electrode and the coated kaolin negative electrode: the unmodified negative electrode rapidly generates dendrite, corrosion, passivation and hydrogen evolution phenomena at the interface due to the non-directional migration of Zn²⁺; Kaolinite-coated anode can effectively control the migration process of Zn²⁺ and make the anode enter a stable three-dimensional diffusion process, because the chemical formula of kaolinite is Al₂Si₂O₅(OH)₄, which has a typical 1:1 layered silicate structure and abundant ion adsorption sites, such as O — H bonds and Si — O bonds, which can reduce the amount of free Zn²⁺, thus controlling the deposition step of stable Zn²⁺ and inhibiting the formation of zinc dendrites. In addition, the kaolin coating also has electrical insulation and ionic conductivity, which can prevent the solution from contacting the Zn negative electrode in a large area and effectively avoid the generation of corrosion by-products such as zinc oxide and basic zinc sulfate. As shown in Table 3, the electrochemical performance of the Zn//MnO₂ battery and the KL-Zn//MnO₂ battery after 600 cycles at the current density of 0.5 A·g^-1 was compared, and the KL-Zn//MnO₂ battery had significantly higher specific capacity and capacity retention. Zhao et al. Deposited a protective layer of amorphous TiO₂ on the surface of pure zinc sheet by atomic layer deposition (ALD) technology, and prepared a new type of zinc anode Zn@TiO₂^[85]. As shown in fig. 4B, the amorphous TiO₂ protective layer is deposited on the surface of the negative electrode as a passivation protective layer with good conductivity and stable structure to isolate a wide range of reactions between the metal zinc negative electrode and the solvent, thus preventing the generation of hydrogen evolution reaction and corrosion by-products and hindering the continuous growth of dendrites.

The organic electrolyte used as the coating has better protection effect on the negative electrode than the porous material. Cao et al. Used a hydrophobic organic compound Zn(TFSI)₂-TFEP thin layer on the zinc surface, and fixed it on the surface of the negative electrode by using the organophilic metal-organic framework, which can directly prevent the intimate contact between the surface of the zinc negative electrode and the aqueous electrolyte, fundamentally prevent the germination of dendrite and other side reactions, and make the battery reaction extremely stable^[86]. Fig. 4D shows the change of the surface of the zinc electrode before and after coating the Zn(TFSI)₂-TFEP layer, which is obviously changed from silver to cyan. Fig. 4E shows a surface structure and reaction schematic diagram of a zinc anode coated with a thin Zn(TFSI)₂-TFEP layer, the Zn(TFSI)₂-TFEP coated with an organic compound has hydrophobicity, and when zinc ions in the aqueous electrolyte enter the hydrophobic organic electrolyte, the exchange of a solvent layer will occur, thereby preventing water molecules from contacting the surface of metal zinc; During the reaction, the hydrophobic organic compound can also be converted into a solid electrolyte interface film with healing ability and high ion transmission rate to further prevent the side reaction between water molecules and metal zinc. The double-layer SEI/Zn(TFSI)₂-TFEPstructure can effectively improve the deposition and dissolution efficiency of zinc, inhibit the growth of zinc dendrite, and improve the cycling stability of metal zinc.

The purpose of surface modification is to prevent the excessive reaction of the electrode and electrolyte interface, and the common coating is the solid-liquid interface. Recently, Liu et al. Successfully introduced a new battery strategy of liquid-liquid interface coordination by coating an electrochemically inert liquid Ga-In alloy layer on the zinc anode. This new liquid-liquid interface is more effective than the solid-liquid interface for the side reactions at the anode interface of aqueous batteries^[82]. The reason is that the liquid Ga-In alloy layer has high affinity for Zn and high conductivity for electrons, and the liquid alloy-electrolyte interface also has fast electron/mass transport characteristics. At the same time, the newly generated Zn atoms on the surface of liquid Ga-In will rapidly diffuse into the alloy interlayer through the binary alloy phase, because Zn atoms prefer to be embedded In the alloy rather than combine with themselves, as shown in Figure 4F. When the ratio of Zn exceeds the limit of Ga-In-Zn alloy, the extra Zn will spontaneously grow into the liquid alloy instead of depositing on the surface through diffusion process, which avoids the growth of zinc dendrites, and the hydrogen evolution overpotential of Ga-In @ Zn anode increases, which enhances the corrosion resistance. The corresponding symmetric battery of this material can last for more than 2100 H at the lowest polarization voltage of 0.25 mA·cm^-2 (24 mV), and the Ga-In @ Zn anode was assembled into a Ga-In@Zn//MnO₂ battery, and the whole battery can provide a stable life of more than 1000 cycles at 1000 mA·g^-1. Although surface modification can avoid most of the dendrite and corrosion problems, it will also hinder the transmission speed of Zn²⁺, which is not yet mature for practical application, and finding better and more suitable coatings is still the focus of research.

3.4 Structure Design

One of the main reasons for the formation of zinc dendrites is that the uneven surface of the negative electrode leads to high ion nucleation potential energy and few nucleation sites. To reduce the surface energy, the Zn²⁺ tends to deposit on the nucleated sites. Therefore, reducing the zinc nucleation overpotential and increasing the nucleation sites are the key to inhibit the dendrite formation. Usually, the zinc electrode or current collector is designed by three-dimensional (3D) structure. The modified 3D zinc anode often presents a porous structure and has a wider reaction area, which increases the deposition sites of zinc in the electrochemical reaction process and reduces the nucleation overpotential of zinc, so that the Zn²⁺ can achieve the purpose of shunting in the deposition process and grow into dendrites naturally and unconditionally^[51,87,88]. The reported 3D zinc anode current collectors can be divided into carbon-based current collectors (such as carbon cloth, graphite felt, carbon fiber, carbon nanotube, etc.), metal current collectors (such as copper foam, copper foil, nickel foam), metal-organic frameworks (MOFs) materials, etc^[89]^[90]^[91]^[92]^[93⇓~95]^[96]. Guo Yuguo's team prepared self-supporting Zn @ GF materials by electrodeposition method using graphite felt (GF) with good conductivity as working electrode and zinc foil as counter electrode and reference electrode^[90]. The porous and highly conductive Zn @ GF anode material can accommodate zinc ions in the framework, so that the zinc ions can be transported and deposited faster and more compactly, the difference between the charge-discharge voltage curves is reduced, and the cycle stability of the battery is improved. As shown in fig. 5A, the self-supporting Zn @ GF anode was combined with high-quality Prussian Blue (HQ-NaFe) cathode material to prepare a mixed aqueous zinc-based battery with good rate performance. Subsequently, the team used electrochemical deposition to in-situ generate composite 3D Zn @ CFs anode on carbon fibers (CFs)^[91]. Similarly, this unique composite has more nucleation sites and reaction area, as well as a highly conductive framework that can withstand changes in internal stress during zinc deposition/dissolution. As shown in Fig. 5 B, the battery based on this 3D Zn @ CFs anode and α-MnO₂ nanofiber anode has a high rate capacity and good cycle stability. Although the performance of zinc-plated electrode based on carbon-based current collector is quite good, which provides a good research direction for the realization of dendrite-free zinc anode, the cost of this kind of carbon substrate is high, which is not conducive to the practical application of batteries, and it is still necessary to find a suitable substitute.

Therefore, in addition to conductive carbon materials, metal-based current collectors are also in the public eye. As mentioned above, the lower zinc nucleation overpotential means that the energy barrier of zinc deposition on the surface of the current collector is lower, which is conducive to the uniform deposition of zinc ions, can prevent the formation of zinc dendrites, can also alleviate other side reactions to a certain extent, can properly improve the utilization rate of metal zinc, and make the battery performance close to the ideal value. Therefore, for the first time, Shi et al. systematically studied and compared the zinc nucleation overpotential of different metal current collectors to determine the most suitable substrate for highly reversible deposition/dissolution behavior^[97]. The copper foam has the lowest zinc nucleation overpotential (only 65.2 mV) and also exhibits an ideal discharge efficiency (close to 100%). Zinc @ copper foam anode was prepared by electrodeposition of zinc on copper foam. The zinc @ copper foam anode had a low polarization voltage and a stable voltage hysteresis curve. The zinc @ copper foam cathode was assembled with a β-MnO₂ anode to form a full cell. As shown in Fig. 5C and Table 4, the zinc @ copper foam //β-MnO₂ full cell achieved a stable cycle of more than 600 cycles and a low capacity fading of only 0.0218% per cycle on average. The good performance of the cell is mainly attributed to the role of copper foam, which has the characteristics of low zinc nucleation overpotential, high electronic conductivity, many nucleation sites in porous structure, and fast zinc ion transport and deposition dissolution reaction. These advantages can ensure the highly reversible deposition/dissolution behavior of aqueous zinc ion batteries, solve the problems of "queuing" and "traffic jam" in the process of zinc ion deposition, and naturally prevent the excessive growth of zinc dendrites.

In addition to the above two types of materials, metal-organic framework (MOF) materials are also an effective way to solve the dendrite problem of anode. Wang et al. Prepared a new zinc anode (ZIF-8-500) of MOF-ZIF-8 by annealing at 500 ℃^[96]. Because of its porous structure, high hydrogen evolution overpotential and the presence of trace zinc in the metal-organic framework, ZIF-8-500 greatly improves the overall performance of the battery when it is used as the anode of secondary zinc-based aqueous battery. Yuksel et al. Also used a convenient and low-cost wet chemical method to prepare MOF-based zinc anode. The MOF-based zinc anode has hydrophilic and porous surface, which is conducive to the diffusion of Zn²⁺ and uniform charge distribution^[98]. As shown in fig. 5d, the modified zinc anode has better stability than the unmodified zinc anode, and its porous structure can prevent the growth of zinc dendrites, improve the reversibility of the electrode, reduce the metal loss rate, and inhibit the capacity fading of the anode. These results indicate that MOF-based anodes have potential applications in future dendrite-free Zn-based aqueous batteries. Although the increase of the surface area of 3D zinc cathode can effectively eliminate the zinc dendrite, with the increase of the specific surface area, the contact area with water expands, which inevitably promotes the hydrogen evolution reaction and even corrosion. Although the rapid zinc reaction rate and the wide spread of zinc anode make hydrogen evolution and corrosion less harmful to the battery, it may be a necessary development trend in the future to consider the combination of various modification methods to improve the performance of anode.

3.5 Embedded negative electrode

At present, the anode materials of zinc-based aqueous batteries are mainly zinc and its modified products, and the modification methods are mainly focused on structural design, surface coating and so on. The reduction potential of zinc is much higher than that of lithium (-3. 04 V vs SHE), and the potential range of zinc ion intercalation anode materials can be explored is narrow. The reported intercalation anode materials for zinc-based aqueous batteries include organic anthraquinone compounds, Chevrel phase compounds (V vs SHE, Mo₆S₈), titanium-based compounds (TiS₂) and so on^[51]^[99]^[100]^[101]^[102].

Yan et al. Used 9,10-anthraquinone instead of metal zinc as a negative electrode material, combined it with ZnMn₂O₄ positive electrode material as shown in fig. 6a, and assembled a full battery using a mixed solution of zinc sulfate and manganese sulfate as an electrolyte^[99]. When the reaction takes place, zinc ions are intercalated and deintercalated in the positive electrode, and also in the negative electrode. As shown in fig. 6B, the 9,10-anthraquinone //ZnMn₂O₄ battery has high discharge capacity and long cycle stability, and its capacity loss rate is only 5.6% after 500 cycles, and it also shows ideal coulombic efficiency. Because 9,10-anthraquinone has a high theoretical specific capacity (257 mAh·g^-1) and a high reduction potential (0.45 V vs Zn²⁺/Zn), zinc ions are not deposited, dendrites are not derived, there is no negative electrode interface with uneven surface, and there is no trouble of side reactions such as hydrogen evolution and corrosion.

3.6 Electrolyte optimization

Aqueous electrolyte system is the most potential alternative to organic electrolyte because of its low cost, safety, non-toxic, simple battery assembly conditions and high ionic conductivity. However, due to the low electrode capacity, much lower energy density and narrow electrochemical stability window of aqueous electrolytes, zinc-based aqueous batteries can not be successfully applied in practical applications^[104]. Therefore, it is not only necessary to modify the metal zinc anode itself, but also to optimize the electrolyte, which is a common method to improve the electrochemical performance of secondary zinc-based aqueous batteries. Electrolyte is the channel connecting the positive and negative electrodes of the battery and the medium of ion transmission, and it is an indispensable part of the battery structure. At present, the main electrolytes used in zinc-based aqueous batteries include aqueous electrolytes, hydrogel electrolytes and organic electrolytes^[105⇓~107]. Among them, aqueous electrolyte has the advantages of low cost, good safety, high ionic conductivity and simple battery assembly conditions, and has become the most widely used secondary zinc-based aqueous battery electrolyte system. Hydrogel electrolyte is the best choice for flexible wearable devices^[108]. However, there are a large number of active water molecules in aqueous electrolyte, which will lead to the dissolution of cathode materials and a series of side reactions between anode materials and water inevitably, and the voltage window of aqueous electrolyte is narrow (about 1.23 V).The range of cathode materials to be selected for batteries becomes smaller, and materials with better theoretical specific capacity and stability may not be used for zinc storage, so the modification of electrolytes is crucial^[87].

In this paper, the method of electrolyte optimization is simply divided into the following four parts: (1) the type and concentration of electrolyte; (2) electrolyte additive; (3) a hydrogel electrolyte; (4) Organic electrolyte.

3.6.1 Type and concentration of electrolytes

Optimizing the electrolyte of secondary zinc-based aqueous battery is the best way to control the electrode/electrolyte interface reaction and improve the surface performance of metal zinc anode material. At present, a series of soluble zinc salts commonly used in aqueous battery electrolytes include ZnSO₄, Zn(CF₃SO₃)₂, Zn(CH₃COO)₂, Zn(NO₃)₂, Zn(TFSI)₂ and so on^[109]. Among them, ZnSO₄ is the most widely used because of its safety, stability, low cost, easy compatibility with electrode materials and non-toxic and pollution-free^[110⇓~112]. Zn(CF₃SO₃)₂ has been widely concerned because of its faster reaction kinetics and higher reversibility in the deposition and dissolution processes of metal Zn^[113⇓~115]. Large volume of CF₃SO₃^- can reduce the solvation effect of Zn²⁺ and promote the rapid transmission of ions and electrons, but the high cost of Zn(CF₃SO₃)₂ also limits its further commercial application^[115].

In addition to the type of zinc salts, the concentration of soluble zinc salts also has a profound impact on the physical and chemical properties of batteries. The same salt at different concentrations produces different battery performance, and the electrolyte with higher concentration will make the aqueous battery have better electrochemical performance. Because in aqueous media, metal cations do not exist in the form of free ions, such as water molecules will wrap around zinc ions and combine with them to form solvent ionic groups such as [Zn(H₂O)₆]²⁺structures^[116]. Due to the existence of solvent ionic groups, zinc ions need to overcome a higher nucleation barrier, and the nucleation centers are correspondingly reduced. It is only a matter of time before the dendrite grows to the extent that the battery is short-circuited, and too many water molecules will promote the occurrence of side reactions such as hydrogen evolution and corrosion. The rapid performance decline and ultra-short service life of the battery are foreseeable^[109]. The increase of electrolyte solute concentration will reduce the number of water molecules in the electrolyte, so as to achieve the purpose of reducing nucleation potential energy and inhibiting side reactions.

On this basis, Wang et al. reported a zinc-based mixed aqueous battery with a high concentration of electrolyte (1 mol·L^-1Zn(TFSI)₂+20 mol·L^-1LITFSI^[116]. Among them, the bis (trifluoromethylsulfonyl) imide ion (TFSI^-) has a neutral pH after dissolution, which can retain the water in the electrolyte in an open environment, so that the metal zinc anode can maintain nearly 100% coulombic efficiency during the deposition and dissolution cycle, and no zinc dendrite is produced during the cycle. As shown in fig. 7A, the zinc-based mixed aqueous battery assembled by the LiMn₂O₄ of the electrolyte and the cathode material has very good reversibility. The aqueous battery with the high-concentration mixed electrolyte has an ultra-long service life and excellent stability, and the capacity loss rate is only 20% after 4000 cycles. The ionic groups formed by the combination of the zinc ions and solute anions contribute to the good result, and the TFSI^- concentration of the electrolyte used in the battery is larger than that of the (ZnTFSI)⁺ groups formed by the 20 mol·L^-1,TFSI^- around the zinc ions, thereby remarkably preventing the combination of [Zn(H₂O)₆]²⁺ and ensuring the structural stability of the zinc negative electrode in the cycle process. This high concentration electrolyte system is called "water-in-salt" electrolyte. Although this electrolyte has unique long-term stability, its cost is too high, and the solution is too viscous to hinder ion transport, and its manufacture and assembly are time-consuming and laborious, which breaks the inherent characteristics of zinc-based water-based batteries, such as low cost, fast and convenient assembly.

In order to reduce the cost, Chen et al. Used relatively cheap acetate to study the electrochemical performance of the battery with 1 M Zn(OAc)₂+31 M KOAc as the electrolyte, α-MnO₂-TiN/TiO₂ as the positive electrode and metal Zn as the negative electrode. The electrolyte has a wider electrochemical window (~ 3.4 V), which will improve the stability and energy density of the battery^[117]. As shown in Fig. 7 B, after the cycle test, almost no dendrite was formed on the zinc anode of the battery, the battery could provide 304.6 mAh·g^-1 discharge capacity and 368.5 Wh·kg^-1 energy density, and the coulombic efficiency could also be stabilized at about 99%. This is due to the higher concentration of the acetate electrolyte salt, which, like the TFSI^- salt, prevents the formation of [Zn(H₂O)₆]²⁺ by the incorporation of (Zn-Ac)⁺ groups. The water-in-salt electrolyte is favorable for improving the cycle performance and the service life of the aqueous zinc-ion battery.

In addition to the "water-in-salt" electrolyte, the molten salt electrolyte also has a similar effect, because the solute presents a very concentrated state, eliminating the interference of active water on the zinc negative electrode. Chen et al. Realized more than 4000 dendrite-free stable cycles of zinc anode using ZnCl₂·2.33H₂O molten hydrate as electrolyte^[118]. Although they are both in the state of high concentration and little solute, the "water-in-salt" electrolyte is essentially different from the molten electrolyte. They both aim to reduce the interference of active water on the negative electrode, but the "water-in-salt" electrolyte still contains free water molecules that can not be ignored.

Both "water-in-salt" electrolyte and molten salt electrolyte can make aqueous zinc-ion batteries have excellent service life and good stability, but their high cost and complex preparation process will make them face great challenges in practical application. The presence of an excess of non-reactive ions in the electrolyte may also reduce the energy efficiency of the cell, and the long-term stability of zinc metal in most electrolytes remains to be improved.

3.6.2 Lectrolyte additive

In recent years, electrolyte additives have also been frequently used to improve the positive and negative electrodes of zinc-based aqueous batteries. It is worth noting that the choice of additives mainly depends on the type of battery cathode materials and solute salts^[109,119]. Electrolyte additives can be divided into organic additives, inorganic additives and metal additives^[120⇓~122].

The addition of the additive to the plating solution can change the crystal face of the negative electrode and lead the zinc ions to be deposited uniformly^[122]. Some additives will adhere to the negative electrode, reduce the polarization of the electrode, make the negative electrode not easy to produce dendrites, and reduce the probability of hydrogen evolution reaction. Wu Xianwen's team added gelatin (GL) and thiourea (TU) to the plating solution of zinc foil. The negative electrode containing additives in the plating solution had better charge-discharge performance than the ordinary negative electrode without modification, because the corrosion rate and side reactions of the battery were slowed down^[123]. Academician Chen Pu's team added more different kinds of organic substances [cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), polyethylene glycol (PEG-8000) and thiourea (TU) in turn] to the zinc anode plating solution to prepare anodes with different properties^[124]. As shown in fig. 8A, most of the batteries using the new anode have better capacity retention compared to the commercial zinc anode. Because the crystal morphology on the surface of the zinc electrode can be changed, the zinc negative electrode forms a crystal orientation which is not conducive to the growth of zinc dendrites and a more uniform and flat surface of the zinc negative electrode, thereby preventing the appearance of zinc dendrites and reducing the corrosion of zinc, but some additives,For example, the Zn-CTAB electrode shows strong (100) and (110) crystal faces (Fig. 8B), which are conducive to the growth of dendrites, which will make the surface of the negative electrode more uneven, aggravate the corrosion of zinc, and eventually lead to battery capacity fading and short circuit. Therefore, attention should be paid to the concentration and properties of additives when using additives to modify anode materials.

Referring TO the guiding function of organic matter on the surface crystal orientation of zinc anode, Academician Chen Pu's team further studied the galvanized anode materials with different inorganic additives [indium sulfate (In), tin oxide (TO), boric acid (BA)]^[125]. The mixed water battery with Zn-In and Zn-TO as the zinc anode can well inhibit the growth and corrosion of zinc dendrite, while the battery with Zn-BA as the zinc anode can inhibit the occurrence of side reactions. As shown in fig. 8C, the battery with the self-made zinc-plated anode has a significant improvement in cycle stability compared to the battery with the commercial zinc foil anode. After 1000 cycles, the capacities of Zn-In, Zn-TO, and Zn-BA batteries remained at 75%, 82%, and 78%, respectively, compared with 67% for commercial zinc foils. Consistent with the modification principle of organic additives, inorganic additives make the surface of zinc anode more inclined to grow to the crystal plane of (002) or (103), and inhibit the formation of zinc dendrites (Fig. 8d). Surface modification methods such as surface coating, coating and SEI film can effectively inhibit the dendrite growth. In addition to the usual complex means, the use of appropriate electrolyte additives can also produce interfacial film. Zeng et al. Introduced Zn(H₂PO₄)₂ salt additive into the electrolyte and proved that it could form a dense, stable and zinc-conductive interfacial film (hopeite layer) in aqueous zinc-ion batteries^[126]. The hopeite SEI (thickness 140 nm) enables dendrite-free zinc deposition and dissolution, facilitates uniform and rapid transport of zinc ions, and inhibits the occurrence of side reactions by isolating water in the electrolyte from the active zinc. The Zn/V₂O₅ full cell with the Zn(H₂PO₄)₂ salt additive retained 94.4% of its original capacity after 500 cycles.

In addition to the above two categories, pre-addition of appropriate metal ions in the electrolyte can usually be used to inhibit the dissolution of cathode active materials, ensure the structural stability of cathode materials, and enhance the overall stability of the battery^[127,128]. For example, in the zinc-based aqueous battery system with manganese-based compounds as the cathode, it is normal to use a series of manganese salts such as manganese trifluoromethanesulfonate and manganese sulfate as electrolyte additives^[111,129]. Of course, the choice of additives depends on the type of positive and negative electrodes. Suitable metal ions can not only inhibit the dissolution of positive active material, but also improve the dendrite growth and corrosion of zinc. The formation of zinc dendrites is caused by the uneven distribution of the electric field on the surface of the negative electrode and the local current around the tip. Therefore, metal ion additives with lower reduction potential than Zn²⁺ can accumulate at the dendrite tips and inhibit the further growth of zinc dendrites during charging through electrostatic shielding effect, which is equivalent to fundamentally preventing the accumulation of zinc dendrites.

For example, Zhang et al chose MgSO₄ as an additive to solve the capacity fading problem of Mg_xV₂O₅·nH₂O//ZnSO₄//Zn aqueous zinc-ion batteries^[130]. In this study, the electrochemical performance of the battery with different concentrations of ZnSO₄ and MgSO₄ electrolytes was compared, and the action mechanism of Mg²⁺ in the electrolyte was shown in Figure 9a. On the one hand, adding Mg²⁺ into the electrolyte will change the dissolution equilibrium between Mg²⁺ and MgVO cathode, hinder the continuous dissolution of cathode materials and structural collapse, and ensure the stability of the battery. On the other hand, the Mg²⁺ acts as a co-intercalated cation together with the Zn²⁺ during the discharge process, and then the removed Mg²⁺ is dissolved in the electrolyte and deposited on the negative electrode together with the Zn²⁺ during the charge process, and the addition of the Mg²⁺ during the charge and discharge process can improve the specific capacity of the battery. As shown in Fig. 9 B, the battery showed the optimal performance (0.1A·g^-1,374 mAh·g^-1) in the 1 M ZnSO₄+1 M MgSO₄ hybrid electrolyte and simultaneously possessed excellent rate capability (5 A·g^-1,175 mAh·g^-1).

3.6.3 Hydrogel electrolyte

Gel electrolyte is the development hotspot of zinc-based aqueous battery in the past two years, which is often used as the electrolyte of portable wearable equipment. It has high mechanical strength, not easy to damage, strong deformation recovery ability, low water content of electrolyte, no dendrite, hydrogen evolution and corrosion and a series of side reactions, so it is a potential research direction^[131,132]. There are many types of gel electrolytes, and only a few of them are briefly introduced in this paper^[95,133,134]. For example, Pan et al. Designed and constructed a ——NaCl+ZnSO₄+ of sodium alginate with the structure of "water-in-salt" electrolyte^[135]. The "water-in-gel" electrolyte was used to prepare a mixed zinc-based aqueous battery to overcome the problems of low gram capacity and low working voltage of the battery. Under this condition, the mixed aqueous battery could not only meet the voltage of 2. 1 V, but also obtain a 260 mAh·g^-1 specific discharge capacity.

Ion-conducting polymer electrolytes (PES) are also considered to be effective in inhibiting the growth of zinc dendrites and the dissolution of active materials due to their limited water content. For example, Park et al. Proposed a novel single-ion conductor gel electrolyte based on zinc sulfonated covalent organic framework (TpPa-SO₃Zn_0.5)^[136]. The synthesized TpPa-SO₃Zn_0.5 material was mixed with polytetrafluoroethylene to prepare a flexible TpPa-SO₃Zn_0.5 membrane, as shown in fig. 10a and fig. 10B. The TpPa-SO₃Zn_0.5 membrane showed a single Zn²⁺ conductive behavior due to the presence of a delocalized sulfonate covalently linked to the oriented void, and the β-ketoenamine chain of the material could also maintain structural stability. Driven by this structure and physicochemical characteristics, the TpPa-SO₃Zn_0.5 film improves the redox reliability of the Zn metal anode, stabilizes the structure of the cathode material, and realizes rapid ion transport, as shown in Figure 10c. Finally, the modified Zn-MnO₂ full battery still has a specific capacity of 144 mAh·g^-1 after 800 cycles at a current density of 0.6 A·g^-1, with a capacity retention rate of 73%.

Although gel electrolyte has many advantages, when the traditional sandwich laminated structure is seriously damaged, such as trampling, shearing, beating and so on, the corresponding active material structure may fall off, the main components of the battery may be damaged and lost, and the flexible battery will be scrapped immediately. To this end, Yao et al. designed a method for the combination of flexible battery components that can be popularized and applied. As shown in Fig. 10d, after the positive and negative electrodes are prepared by the one-piece coating method, the rolling technology is used to apply mechanical pressure on the two electrode materials to make them closely combined, thus making a thin and integrated flexible battery^[137]. The flexible zinc ion battery prepared by the method can prevent the electrode components from being dislocated and falling off, and ensures that the battery has stable ion and electron conduction functions even if the battery is cut into window grilles. The flexible battery with the integrated structure has strong ability to resist external risks and has development prospects for the application of special dangerous situations, and the method also provides a new idea for the development of gel electrolyte.

3.6.4 Organic electrolyte

Although the aqueous electrolyte system has inherent safety, environmental friendliness and low cost, the problems of zinc dendrite, corrosion and hydrogen evolution in the aqueous electrolyte system can not be ignored^[138⇓~140]. The organic system shows thermal stability when participating in the battery reaction, the organic solvent does not wrap zinc ions like water molecules to form zinc-containing by-products, the loss of zinc in the electrolyte is reduced, the metal utilization rate is increased, and other interfacial side reactions are avoided^[109]. However, in the organic electrolyte system, the electrode has low specific capacity, poor conductivity, and the inherent shortcomings of the organic electrolyte: harsh operating environment, high cost, low safety, etc. These are new challenges, but it is also a good research direction to find an organic electrolyte suitable for zinc-based batteries^[141⇓~143]. Using intrinsically safe and non-flammable organic materials as electrolytes is a good solution. For example, Wang Jiulin's team first constructed a stable zinc-ion battery by using the flame retardant trimethyl phosphate (TMP) solvent with high safety^[141]. The capacity of the battery did not fade at high current density (5 mA·cm^-2, 10 mAh·cm^-2, and the charge-discharge capacity ratio was 99. 57%), and the dendrite did not form even if the reversible deposition and dissolution lasted for more than 5 000 H. As shown in figs. 11A and B, the zinc anode can be gradually deposited into a highly regular porous morphology during this cycle, and this in-situ formed porous structure can effectively reduce the real current density and accelerate the transport of zinc ions^[142].

Dong et al. Dissolved the mixture of Zn(CF₃SO₃)₂ and NaClO₄ in trimethyl phosphate to prepare an organic electrolyte (abbreviated as 0.5 mol·L^-1Zn+1.0 mol·L^-1Na in TMP) with high reaction potential, solvent stability and good compatibility with positive and negative electrode materials, which can be applied to secondary zinc-based battery systems^[143]. As shown in fig. 11c, there is a strong interaction between the TMP solvent and the zinc salt and sodium salt in the electrolyte, so that the free solvent in the electrolyte is reduced, the Zn²⁺/Na⁺ salt concentration is increased, and the stability of the positive and negative electrodes of the battery in the electrolyte is greatly improved. Compared with the non-aqueous organic environment, the zinc-based battery showed excellent performance in the 0.5 mol·L^-1Zn+1.0 mol·L^-1Na in TMP (the charge-discharge capacity ratio was 99. 8% after cycling for more than 5 000 H). Through the analysis of the surface morphology of the negative electrode, it can be found that the porous structure formed by multiple cycles of the negative electrode can better accommodate the organic electrolyte, so that the ion diffusion speed and electron transfer speed are faster, and the overall performance of the battery is improved. Organic electrolyte system has its inherent advantages, which can be seen from its application in lithium-ion batteries. However, one of the purposes of developing aqueous batteries is to overcome the inevitable toxicity and flammability of organic systems, while reducing costs and optimizing operability. Therefore, although organic electrolytes are good, they are used in aqueous batteries to put the cart before the horse.

4 Summary and prospect

Zinc-based aqueous battery is the most potential alternative electrochemical energy storage device for lithium-ion batteries. Zinc anode has the advantages of high theoretical capacity, low redox potential, green and non-toxic, strong safety, convenient assembly in air atmosphere, low operation cost and abundant natural reserves. Although the application prospect of zinc-based aqueous battery is good, the problems of low coulombic efficiency and poor cycle performance caused by zinc dendrite, corrosion, passivation, hydrogen evolution and other negative electrode interfacial reactions hinder the practical application and development of zinc-based aqueous battery. In this paper, the problems faced by zinc-based aqueous battery anode were briefly discussed, and the interaction between these defects was analyzed. In addition, the existing improvement strategies for the performance of metal zinc anode were summarized.The main methods include adding additives, designing three-dimensional zinc electrode current collector, coating functional protective layer on the anode, developing embedded zinc anode and optimizing electrolyte. On this basis, the following four ideas are put forward for the development direction of zinc-based aqueous battery anode in the future.

(1) a variety of zinc anode modification methods are combined to improve the electrochemical performance of the battery, such as designing a 3D current collector zinc anode, covering the surface of the anode with a functional protective film or using electrolyte additives, and combining a variety of modification methods to further alleviate the interfacial side reactions of the anode materials and improve the coulombic efficiency and cycle life of the battery.

And (2) designing and researching a novel separator material to enable the separator to have the capability of a surface protective film to replace a functional protective layer to inhibit zinc dendrite and promote the transport of ions, so that the deposition/dissolution process of a zinc negative electrode can be uniform and stable, and the service life of the battery is prolonged.

(3) Explore the development of solid electrolyte, gel electrolyte, organic electrolyte and other electrolytes with low water content and low corrosivity, and find ways to enhance the conductivity of such electrolytes, reduce viscosity and cost, effectively improve material stability while improving material capacity and reducing costs.

(4) To explore the inhibition mechanism of electrolyte additives on zinc dendrite in the process of zinc anode intercalation and deintercalation, to find low-cost and high-performance electrolyte additives suitable for commercial models, to avoid side reactions such as anode dissolution and zinc dendrite affecting the performance of batteries, and to realize the commercialization of zinc-based aqueous batteries.

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Funding

National Natural Science Foundation of China(52064013)

National Natural Science Foundation of China(52072323)

National Natural Science Foundation of China(51762017)

National Natural Science Foundation of China(52064014)

National Natural Science Foundation of China(51862008)

Key Program of Hunan Provincial Education Department(18A285)

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Cite this article

1 Introduction

2 Challenges faced by zinc anode

2.1 Zinc dendrite

2.2 Corrosion and passivation

2.3 Hydrogen evolution

3 Optimization Strategy of Zinc Anode

3.1 Additive

3.2 Zinc alloy

3.3 Surface modification

3.4 Structure Design

3.5 Embedded negative electrode

3.6 Electrolyte optimization

3.6.1 Type and concentration of electrolytes

3.6.2 Lectrolyte additive

3.6.3 Hydrogel electrolyte

3.6.4 Organic electrolyte

4 Summary and prospect

References

Funding

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