Aqueous Zinc-ion Batteries

Xie Zhiying; Zheng Xinhua; Wang Mingming; Yu Haizhou; Qiu Xiaoyan; Chen Wei

doi:10.7536/PC230329

Progress in Chemistry >

2023 , Vol. 35 >Issue 11: 1701 - 1726

DOI: https://doi.org/10.7536/PC230329

Review

Aqueous Zinc-ion Batteries

Xie Zhiying ¹ ,
Zheng Xinhua ³ ,
Wang Mingming ³ ,
Yu Haizhou ² ,
Qiu Xiaoyan ^,¹^,^* ,
Chen Wei ^,³^,^*

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¹ Institute of Advanced Materials, School of Flexible Electronics (Future Technologies), Nanjing Tech University, Nanjing 211816, China
² Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816, China
³ School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, China

*Corresponding author e-mail: iamxyqiu@njtech.edu.cn(Xiaoyan Qiu),

weichen1@ustc.edu.cn (Wei Chen)

Received date: 2023-03-29

Revised date: 2023-05-08

Online published: 2023-08-07

Supported by

National Natural Science Foundation of China(22005141)

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Abstract

Aqueous zinc-ion batteries (AZIBs) have great advantages in terms of safety, low cost, high theoretical capacity and element abundance, which shows great potential in large-scale energy storage applications. The development of high-performance AZIBs has become a widely interesting topic recently. Although much progress has been made in AZIBs, the low energy density, poor ionic dynamics and short cycling life limit the commercialization of AZIBs. This review summarizes the challenges, recent progress and corresponding strategies for the development of cathodes, anodes, electrolytes, and energy storage mechanisms of AZIBs. It provides useful guidance for researchers in the battery area to design and develop high performance AZIBs.

Contents

1 Introduction

2 Dissolution of the cathode materials

2.1 Manganese-based materials

2.2 Vanadium-based materials

3 Electrostatic interaction

4 Oxygen/hydrogen evolution reaction

4.1 Oxygen evolution reaction

4.2 Hydrogen evolution reaction

5 Zinc dendrite and corrosion

5.1 Corrosion,passivation and zinc dendrite

5.2 Anode modification

6 Conclusion and outlook

6.1 Design of advanced cathode materials

6.2 Optimization of electrolyte

6.3 Surface modification of zinc anode and developing new anode materials

6.4 Design of high-performance separator

Key words： aqueous zinc-ion batteries; electrode material; electrolyte; capacity; cycling life; large-scale energy storage

Cite this article

Xie Zhiying , Zheng Xinhua , Wang Mingming , Yu Haizhou , Qiu Xiaoyan , Chen Wei . Aqueous Zinc-ion Batteries[J]. Progress in Chemistry, 2023 , 35(11) : 1701 -1726 . DOI: 10.7536/PC230329

1 Introduction

With the increasing energy consumption and the increasingly serious environmental problems, it is urgent for renewable energy and clean energy (such as wind, solar and tidal energy) to replace non-renewable energy such as oil and coal, which also promotes the rise of efficient and reliable power storage systems. Using large-scale energy storage to deal with the intermittency and randomness of renewable energy is the most important part of building the future smart grid. The basic criteria of an ideal large-scale energy storage system are low cost, high safety, environmental friendliness, long service life, energy efficiency and high energy density^[1]. In recent years, lithium-ion batteries have been widely used and developed in small storage devices, especially in communication and transportation industries, because of their high energy density, long cycle life and light weight^[2]. However, it still has problems in many aspects, such as cost, safety and environment. Lithium-ion batteries use toxic and flammable organic electrolytes, and reactions between the electrode and the electrolyte may pose additional risks. From the point of view of battery safety, as a new battery technology, aqueous zinc-ion batteries (AZIBs) have the characteristics of high ionic conductivity, low cost, safety and reliability, and easier assembly^[3].

Compared with other ion batteries, AZIBs have great advantages in terms of cost, safety, environmental compatibility and element reserves^[4]. It has attracted much attention because of its high volumetric capacity (5851 mAh·cm^-3) and theoretical specific capacity (820 mAh·g^-1), as well as its low redox potential (− 0.76 V versus standard hydrogen electrode). In addition, there are abundant zinc resources in nature, and zinc has the characteristics of environmental protection, non-toxicity, good ductility and easy processing. From Table 1, it is not difficult to find that aqueous zinc-ion batteries are superior to non-aqueous lithium/sodium/potassium-ion batteries in terms of ionic radius, price and theoretical volume capacity. In addition, the price of common materials for preparing the cathode of zinc-ion batteries, such as vanadium pentoxide and manganese ore, is lower than that of common raw materials (Li(NiMnCo)O₂ and LiCoO₂) for lithium-ion batteries^[5]. In the choice of electrolyte, organic electrolyte not only has potential safety hazards, but also costs several times higher than zinc sulfate^[6]. Therefore, AZIBs have good development potential in large-scale energy storage applications.

表1 水系锌离子电池与非水系锂/钠/钾离子电池的对比^[4]

Table 1 Aqueous zinc-ion battery vs. non-aqueous lithium/sodium/potassium-ion batteries^[4]

	Li	Na	K	Zn
Ionic radius [Å]	0.76	1.02	1.38	0.75
Cost (USD kg^-1)	19.2	3.1	13.1	2.2
Volumetric Capacity (mAh· cm^-3)	2061	1129	610	5855
Ionic conductivity (S·cm^-1)	10^-3~10^-2(organic electrolytes)			10^-1~6 (aqueous electrolytes)
Safety	Low		High

The development of aqueous zinc-ion batteries can be traced back to 1987, when Shoji et al. First used ZnSO₄ electrolyte instead of alkaline electrolyte and tested the performance of the battery^[7]. The use of neutral or weakly acidic ZnSO₄ solutions as rechargeable aqueous Zn/MnO₂ battery electrolytes opens the door for further research on water-based Zn/MnO₂ batteries. In aqueous zinc-ion batteries, the layered cathode material and the zinc anode are separated by a glass fiber separator in an aqueous electrolyte (Fig. 1), where the aqueous electrolyte is a salt solution containing Zn²⁺. During the charge/discharge process of the battery, zinc ions are reversibly intercalated/deintercalated at the positive electrode. During charging, the surface of the negative electrode is plated with zinc; Upon discharge, metallic zinc is stripped from the negative electrode to the positive electrode. In recent years, the research on zinc anode, electrolyte and cathode materials has once again triggered a research boom. Since 2018, thousands of articles related to AZIBs have been published. However, there are still many problems and challenges in the development of AZIBs electrode materials and even the whole battery system, such as the dissolution of the positive electrode, the electrostatic interaction between ions, the by-products of electrochemical reactions, the formation of zinc dendrites and the corrosion of the negative electrode^[8]^[9]^[10]^[11]. In this paper, the problems and challenges faced by aqueous zinc-ion batteries, as well as the latest solutions and progress, will be discussed and reviewed.

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图1 水系锌离子电池的构造示意图及存在的主要问题

Fig. 1 Schematic diagram of the construction of aqueous zinc ion battery and the main problems

2 Dissolution problem of cathode material

Cathode materials are critical to the capacity and specific energy of the battery. Traditional aqueous zinc-ion batteries use inorganic materials such as manganese (Mn), vanadium (V) -based compounds and Prussian blue analogues (PBAs) as cathode materials, and their electrochemical behavior mainly depends on the change of metal valence and the combination of charged metal ions with specific coordination ions to achieve redox reactions^[12]. The following zinc ion storage mechanisms are involved: intercalation/deintercalation of :Zn²⁺, co-insertion of Zn²⁺ and H⁺, dissolution-deposition mechanism, and chemical conversion reaction^[13]. H⁺ intercalation occurs in many oxide cathode materials, especially vanadium and manganese-based oxides, which are considered to be beneficial for zinc ion storage of most oxide cathode materials, which may be attributed to the preferential precipitation and intercalation of protons at the hydroxyl end or hydration interface^[14]. In addition to Zn²⁺/H⁺ co-intercalation, exclusive Zn²⁺ intercalation mechanisms have been identified in most non-oxide cathode materials, including Prussian blue analogs (PBAs), metal-organic frameworks, layered sulfides, and selenides^[15]. In aqueous zinc-ion batteries, the intercalation/deintercalation of Zn²⁺, compared with monovalent lithium ions, when intercalating into host materials, Zn²⁺ with larger atomic mass and higher charge density will produce larger electrostatic repulsive force and poorer transport kinetics, which will lead to the increase of interlayer spacing and the easy collapse of crystal structure^[16]. Therefore, the development of suitable cathode materials is a key factor for the application of aqueous zinc-ion batteries.

At present, the research of cathode materials for zinc-ion batteries is mainly focused on transition metal oxides. Among them, manganese-based and vanadium-based materials have been widely studied due to their various crystal structures, low price and high specific capacity^[17]. However, their poor conductivity, unstable crystal structure and dissolution of metal elements (V and Mn) in aqueous electrolyte will directly affect their cycle stability and capacity retention.

Aiming at the problems of poor conductivity and unstable structure,It can be solved by adding composite conductive agents (carbonaceous conductive agents and conductive polymers), using nanostructure technology, establishing structure engineering (metal ions, crystal water and organic compounds embedded in crystal structure) and defect engineering (oxygen vacancies and cation defects), which have been reported in many literatures^[15]^[16]^[18]. Most of the cathode research involving manganese-based and vanadium-based materials has the problem of active material dissolution. The dissolution of cathode materials will reduce the utilization of active materials and induce side reactions at the electrode interface, which will lead to structural degradation and performance degradation. This section will mainly summarize the dissolution problems of manganese-based and vanadium-based cathode materials and the corresponding improvement measures.

2.1 Manganese based material

Manganese-based oxides have been used as electrode materials for energy storage devices because of their high theoretical capacity, low cost, non-toxicity and easy preparation. The most typical one is manganese dioxide (MnO₂), which has a theoretical specific capacity of 308 mAh·g^-1 for single electron transfer reaction. The MnO₂ is an octahedral structure with the oxygen atom located at the vertex of the octahedron and the manganese atom in the middle of the octahedron^[19]. The edges of the octahedra are connected to each other to form single or double chains, and these chains and other chains aggregate to form polymorphs of various tunnel or layer structures. The tunnel-type MnO₂ crystal structure has excellent Zn²⁺ diffusion ability, and the layered MnO₂ crystal structure has a larger interlayer spacing, which is more conducive to the storage and transportation of Zn²⁺. There is also a spinel-like MnO₂ crystal structure, which shows better structural stability^[20]. Different crystal morphologies of MnO₂ have different effects on the reaction mechanism and electrochemical performance of AZIBs, but there is a common problem that the structural change of manganese-based materials will lead to the dissolution of manganese during battery cycling^[21]. This phenomenon will greatly reduce the utilization rate of the active material itself, reduce the conductivity of the material, and affect the dynamic performance of ion transmission.

The rechargeable aqueous Zn-MnO₂ battery, the first generation of zinc-manganese batteries to be investigated, has been intensively studied in order to understand the charge storage mechanism of manganese-based materials. For example, α-MnO₂, as a common cathode material, adopts a crystal structure with interconnected octahedra, forming a one-dimensional tunnel to facilitate reversible ion insertion/deinsertion. However, its working mechanism when coupled with zinc negative electrode in weak acidic electrolyte is still controversial. Although there are reports claiming that the tunnel phase of α-MnO₂ is completely transformed into a new phase state upon discharge and then restored upon charge, the highly reversible change of such a structure is questionable. In addition, most of the bulk characterization methods are carried out in the particle assembly stage, and their signals are easily disturbed by the presence of by-products or electrolyte residues.

Through atomic structure imaging experiments, researchers found that the possibility of Zn²⁺ intercalation into α-MnO₂ lattice is very low, but reversible H⁺ intercalation into α-H_xMnO₂ is dominant in the form of charge storage^[22]. By optimizing the proton intercalation kinetics to obtain better performance, the structure of cathode materials is easy to collapse with the change of lattice. The α-MnO₂ shows nanowire morphology, and the H⁺ is inserted into the MnO₂, maintaining the original tunnel phase and nanowire morphology. After several cycles of H⁺ intercalation/deintercalation, the MnO₂ host material will undergo repeated Mn valence changes accompanied by lattice distortion/recovery. A possible hypothesis is that the mass loss of the active material MnO₂ causes the capacity fading of the electrode material as the cycling proceeds. It can also be seen in the experiment that the MnO₂ nanowires are gradually decomposed during the cycling process, and after many cycles, the morphology of the nanowires can hardly be seen, and some needle-like nanoparticles appear, and the process is irreversible. Once they are formed and attached to the electrode, they cannot be electrochemically decomposed during the charge-discharge process, which is the main reason for the capacity fading.

2.1.1 Manganese ion addition strategy in electrolyte

To suppress the dissolution of Mn²⁺ ions in manganese-based cathode materials, additional addition of Mn²⁺ ions to the electrolyte has become the most commonly used optimization strategy according to the dissolution equilibrium theory. This method has also been applied to other Mn-based cathode materials, such as Mn₃O₄, Mn₂O₃, and β-MnO₂^[23]. The addition of MnSO₄ in the ZnSO₄ electrolyte can inhibit the dissolution of Mn in the α-MnO₂, and alleviate the problem of capacity fading caused by the dissolution of partial Mn²⁺ into the electrolyte due to redox reaction during cycling^[24]. It can be seen from Fig. 2a that the AZIBs of the electrolyte with and without the addition of MnSO₄ show relatively similar redox peaks, indicating that the addition of MnSO₄ has no effect on the redox reaction of the α-MnO₂. Meanwhile, it can be found from Figure 2B that the cycling performance of the battery is improved and its theoretical capacity is enhanced after the addition of MnSO₄ in the ZnSO₄ electrolyte. The reason for this phenomenon is that the added Mn²⁺ in the MnSO₄ may provide a balance between the dissolution of the Mn²⁺ in the electrode and the reoxidation in the electrolyte, thus maintaining the high stability of the electrode.

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图2 MnO₂电极在2 mol/L ZnSO₄水电解质中添加和不添加0.1 mol/L MnSO₄添加剂的(a)CV曲线和(b)分别在C/3和1 C下的循环性能图^[24]。(c)多孔ZnMn₂O₄正极材料第一圈之后在三种不同电解质中的充放电曲线及(d)相应的循环性能^[25]

Fig. 2 (a) CV curves and (b) cycling performance at C/3 and 1 C for MnO₂ electrode in 2 mol/L ZnSO₄ aqueous electrolyte with and without 0.1 mol/L MnSO₄ additive^[24]. Copyright 2016, Springer Nature (c) Charge and discharge curves of porous ZnMn₂O₄ cathode material in three different electrolytes after the first cycle and (d) corresponding cycling performance^[25]. Copyright 2020, Elsevier

Spinel ZnMn₂O₄(ZMO) as cathode materials show excellent electrochemical performance, but the capacity loss is up to about 30% after 300 cycles. To reduce the capacity loss of ZMO and enhance the cycling performance of the battery, Mn²⁺ can be added to the electrolyte. The results show that the contribution of Mn²⁺ to the specific capacity of manganese-based oxides (about 2% ~ 3%) is insignificant in the range of termination voltage (1. 85 V). However, the added Mn²⁺ itself helps to improve the specific capacity of the battery. In the presence of pure MnSO₄ electrolyte, the charge/discharge capacity of the ZMO cathode material is as high as 396/218 mAh·g^-1, respectively (Figure 2C). Figure 2D demonstrates the capacity retention of each electrolyte after cycling at a current density of 100 mA·g^-1 for ZMO. The initial discharge capacity when using ZnSO₄ electrolyte was 119 mAh·g^-1, and the capacity decreased to 43 mAh·g^-1 after 60 cycles. Although the initial discharge capacity is as high as 226 mAh·g^-1 when MnSO₄ electrolyte is used alone, its cycle stability is poor (only 81 mAh·g^-1 after 60 cycles), which may be due to the absence of Zn²⁺ in the electrolyte, resulting in the difficulty in maintaining the electrochemical equilibrium of the system^[25]. It is worth noting that when the system uses the ZnSO₄ electrolyte with the addition of Mn²⁺, the capacity increases with the number of cycles, and its maximum capacity is 285 mAh·g^-1.

2.1.2 Positive electrode protection layer strategy

In addition to the addition of additional manganese salts to the electrolyte, the dissolution of Mn can also be suppressed by coating the manganese-based material with a protective layer^[26]. The vacancy enrichment of β-MnO₂ and its surface coating can not only improve the intercalation kinetics, but also inhibit the dissolution of manganese. In 2021, Ding et al. Selected β-MnO₂ as the electrode material and synthesized β-MnO₂ with abundant oxygen vacancies (VO) and graphene oxide (GO) coating (Figure 3A), in which VO can enhance the ion kinetic performance, and the GO layer coated on the surface can inhibit the dissolution of Mn^[27]. The synergistic effect of defect engineering and interface optimization can improve the cycling stability of MnO₂ cathode through the interaction between them. In the first eight cycles, GO coating had little effect on the volumetric delivery of β-MnO₂ at low current density, and the electrode coated with GO had a small polarization and a high voltage efficiency. And regardless of the small or large current density, the cycle performance of the GO coated electrode and the β-MnO₂ electrode will have a process of first activation, and then continue to decrease in the subsequent cycles. At the same time, the discharge capacity of the coated GO electrode is significantly higher than that of the β-MnO₂, and it can be cycled for 2000 cycles at a current rate of 4 C. Therefore, the preparation of composite materials coated with GO and VO on the β-MnO₂ can not only improve the dissolution problem of Mn, but also improve the ion transport kinetic performance and the cycle stability of the electrode.

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图3 (a)具有氧空位β-MnO₂@Graphene氧化物正极材料示意图^[27]。(b)MnO₂/Mn₂O₃@PPy复合材料在水系锌离子电池中的应用^[28]。(c)超薄聚苯胺涂层的单晶纳米椭圆体电极材料^[29]。(d)针状微纳PDA@MnO₂@NMC复合材料^[30]

Fig. 3 (a) Schematic diagram of β-MnO₂@Graphene oxide cathode material with oxygen vacancies^[27]. Copyright 2021, Springer (b) MnO₂/Mn₂O₃@PPy composite in aqueous Zn-ion battery^[28].Copyright 2021, Elsevier. (c) Ultrathin polyaniline coated single crystal nano-ellipsoid electrode materials^[29]. Copyright 2022, American Chemical Society (d) Needle-like micro/nano PDA@MnO₂@NMC composites^[30]. Copyright 2022, American Chemical Society

It is still a great challenge to explore a simple, convenient, large-scale and low-cost method to produce advanced manganese-based cathode materials for practical application. In 2022, Huang et al. Proposed a facile and scalable strategy for in-situ growth of polypyrrole (PPy) thin layers on the surface of MnO₂/Mn₂O₃ nanocomposites, combining molten salt synthesis and self-initiated polymerization. The nanoscale structure provides a short diffusion path for electrons and electrolyte ions; Meanwhile, the polypyrrole coating not only enhances the overall conductivity of the nanocomposite, but also avoids the direct contact between MnO₂/Mn₂O₃ and electrolyte, and the dissolution of manganese is largely inhibited^[28]. Thanks to this, the cell obtains a high specific capacity (289.8 mAh·g^-1,0.2 A·g^-1). Moreover, when manganese ions were not added to the electrolyte in advance, the cycle performance remained excellent (96.7% after 1000 cycles under the condition of 1 A·g^-1). These properties are all superior to the MnO₂/Mn₂O₃ nanocomposite without PPy coating. This strategy (Figure 3B) opens a new way to achieve the synthesis of long-lived, high-rate electrode materials. It is worth noting that by adding extraneous reagents or constructing additional protective layers, it is likely to increase the cost or destroy the energy density of the whole device, inevitably increasing the weight and volume. In order to obtain high energy density, an ultra-thin polyaniline (PANI) layer was coated on the electrode surface by self-initiated polymerization (Figure 3C), which can not only inhibit the dissolution of manganese and protect the structural integrity of the electrode, but also provide a fast electron transport channel during the charge process^[29]. High energy density, excellent rate capability, and long cycle life can be achieved with Mn₂O₃/PANI as the electrode. In addition, in order to integrate the advantages of different coatings, the composite coating strategy has also attracted attention. For example, a composite coating of carbon material (NMC) and polydopamine (PDA) was constructed on the surface of the needle-like manganese dioxide electrode (Figure 3D). The NMC carbon substrate can provide abundant active sites for overgrown MnO₂ nanowires, which improves the conductivity and ensures the rapid transmission of ions. PDA has abundant hydrophilic groups and excellent adhesion performance, which can stabilize the integrity of the material, inhibit the dissolution of manganese, and show excellent electrochemical performance^[30].

In addition, the addition of conductive polymers such as polyethylene dioxythiophene (PEDOT) protective layer is generally used to improve the conductivity of manganese-based materials, and there is no detailed mechanism for the dissolution of manganese^[33]. However, it has been reported in detail in vanadium-based materials, so it will not be introduced too much here, but will be described in the following relevant sections.

2.1.3 Artificial construction strategy of electrolyte interface

The electrolyte is reduced on the negative electrode side or oxidized on the positive electrode to produce a passivation layer. The thin layer formed on the negative electrode side by electrolyte decomposition is called solid electrolyte interphase (SEI), and the passivation layer formed on the positive electrode side is called positive electrolyte interphase (CEI)^[34]. The CEI layer (CaSO₄·2H₂O) formed on the surface of the Ca₂MnO₄ cathode was observed by in-situ electrochemical charging process (Figure 4A), and its electronic insulation and ionic conductor properties were confirmed by density functional theory calculations, indicating that the CEI layer can effectively inhibit the dissolution of manganese^[31]. Compared with the α-MnO₂, the CMO electrode with the CEI layer exhibits superior electrochemical performance, and the mechanism is shown in fig. 4A. On the one hand, for α-MnO₂ cathode, tetravalent manganese is reduced to trivalent manganese during discharge, which easily leads to Jahn-Teller effect, and the disproportionation reaction caused by Jahn-Teller effect leads to the dissolution of divalent manganese ions. During the discharge process, the dissolved divalent manganese ions and the inserted zinc ions produce electrostatic repulsion, which reduces the diffusion rate of zinc ions and leads to a rapid decline in the capacity of the cathode. On the other hand, the dissolution of manganese ions and the Jahn-Teller effect should lead to more sluggish intercalation kinetics in α-MnO₂. For the CMO electrode, the CEI film can inhibit the dissolution of manganese ions and protect the positive electrode. Therefore, the idea of in-situ generation of CEI film to protect the cathode is helpful for the development of aqueous zinc-ion batteries.

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图4 (a)α-MnO₂和CMO的反应机理^[31]。(b)CEI/H₂O界面相互作用能的理论计算,充放电曲线及循环稳定性^[13]。(c)利用黏合剂原位生长CEI示意图^[32]

Fig. 4 (a) Reaction mechanism of α-MnO₂ and CMO^[31]. (Copyright 2017 American Chemical Society) (b) Theoretical calculation of CEI/H₂O interface interaction energy, charge/discharge curves and cycling stability^[13]. (Copyright 2022, American Chemical Society) (c) Schematic diagram of in situ growth of CEI using adhesives^[32]. Copyright 2021, Wiley

In view of this idea, researchers began to construct CEI membranes to improve the electrochemical performance of batteries. For example, the artificial CEI layer constructed by paraffin can well inhibit the dissolution of manganese ions to stabilize the MnO₂ cathode and prevent water from contacting with conductive graphite, thus effectively balancing the kinetics of zinc ion intercalation and the stability of cathode materials^[13]. The paraffin-based CEI can provide excellent capacity retention of the battery (more than 60% higher than that without CEI), and even with a positive electrode load up to 25.2 mg/cm², it still provides a capacity retention of more than 70% after 5000 cycles (Figure 4B). The theoretical results show that the interaction energy at the interface is higher than that without CEI, which proves the repulsive force at the interface. In addition, the processing cost is a problem that must be considered in the commercialization of aqueous zinc-ion batteries. The introduction of adhesives with high cost performance into CEI can improve the chemical properties and reduce the preparation cost. Polyvinylidene fluoride (PVDF) is often used as a binder because of its good thermal stability and adhesion ability. It was originally designed for batteries containing electromechanical electrolytes, but there is limited research on aqueous batteries. And N-methyl-2-pyrrolidone (NMP) is required as a solvent to form the PVDF slurry. The slurry is toxic, environmentally unfriendly and expensive. Natural or synthetic water-soluble polymer binders can enhance chemical/physical interactions and improve electrode stability, especially polysaccharides. Polysaccharide sodium alginate (SA) compounded with hydrophobic polytetrafluoroethylene (PTFE) can be used as CEI. The carboxyl group in SA acts as an anionic polyelectrolyte to adsorb zinc ions, while PTFE provides a hydrophobic component (Figure 4 C). Compared with PVDF/NMP, the cost of the hybrid CEI is reduced, the flux of zinc ions is improved, and the specific capacity of the battery is improved^[32].

2.1.4 Defect Engineering Strategy

Manganese-based cathode materials with high energy density are troubled by manganese dissolution and structural damage, which can be alleviated by adding manganese ions or coatings in the electrolyte. However, it is still a challenging problem to achieve the cycle durability of batteries by improving the inherent stability of manganese-based cathode itself. Therefore, it is crucial to develop cathode materials that enable rapid diffusion of guest ions. Inherent defects such as oxygen vacancies can not only promote the diffusion and adsorption of reactants in electrochemistry, but also improve the conductivity of metal oxides^[35]. K_0.8Mn₈O₁₆ is considered to be a cathode material with high energy density as well as long cycle performance due to its oxygen defect vacancy stabilized by potassium ions, which increases the density of donors, improves the conductivity and opens the MnO₆ polyhedral wall for ion diffusion. And the oxygen deficiency plays a key role in fast reaction kinetics and high capacity. The K_0.8Mn₈O₁₆ cathode material with stable structure and rich oxygen defects was constructed by doping α-MnO₂ with K⁺, and its electrode polarization was small^[36]. According to ICP-AES, the dissolved Mn²⁺ elements in the cycling process of K_0.8Mn₈O₁₆ and α-MnO₂ electrodes were analyzed.It is found that the dissolution of manganese element can indeed be effectively alleviated during the cycling process of K_0.8Mn₈O₁₆ with oxygen defects, so that it can obtain a higher discharge capacity at 100 mA·g^-1 than that of α-MnO₂ (Figure 5A).

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图5 (a)H⁺扩散到具有完美结构和氧缺陷结构的KMO中的示意图^[36]。(b)氧空位缺陷产生示意图^[37]。(c)氧空位缺陷ZMO纳米管阵列合成示意图^[38]

Fig. 5 (a) Schematic representation of H⁺ diffusion into KMO with perfect structure and oxygen defect structure^[36] (Copyright 2019, Wiley). (b) Schematic diagram of oxygen vacancy defect generation^[37] (Copyright 2022, American Chemical Society). (c) Schematic diagram of the synthesis of oxygen vacancy-defective ZMO nanotube arrays^[38]. Copyright 2021, Elsevier

Doping metal ions in the lattice can also promote the diffusion of Zn²⁺ in the material and improve the stability of the cathode material. Defects can be created, for example, by doping with highly conductive silver, using electron transfer between silver and manganese to form Ag-O-Mn bridge bonds^[37]. The formation of Ag-O-Mn bonds can drive the generation of oxygen vacancy defects (Figure 5B), which plays an important role in increasing the diffusion kinetics of Zn²⁺.

Defect engineering can also improve the active sites and conductivity of the electrode. Manganese-based oxide materials with spinel structure have shown great potential in ZIB due to their rich oxidation States as well as reversible Zn²⁺ intercalation. However, its poor conductivity, insufficient active sites and large volume change lead to its insufficient reaction rate and cycle performance. Using N-doped coupled oxygen vacancy defect modulation ZMO nanotube arrays (Figure 5C), electrode materials with good conductivity, efficient ion transport, abundant active sites, and enhanced capacity can be obtained^[38].

The introduction of interlayer ions can effectively adjust the crystal structure of cathode materials, which is helpful to promote the diffusion of zinc ions during charge and discharge. In addition, the introduced ions can act as a pillar to prevent material collapse after zinc ion intercalation, thereby improving the stability of the cathode material during long-term cycling^[39]. In addition to introducing ions, molecules can also be introduced into cathode materials with layered structures, such as water molecules, which have been shown to work with oxide frameworks to weaken electrostatic interactions and promote reaction kinetics in electrochemical processes^[40]. In addition, organic molecules can also reduce the interaction between zinc ions and oxides and enhance the reversibility of zinc ion intercalation^[41].

In addition to single metal ions, researchers have also tried metal ion doping using bimetallic oxides. Zhang et al. Synthesized a Ni²⁺ doped Mn₂O₃(NM) derived from NiMn bimetallic hydroxide, which has a good inhibition effect on the dissolution of Mn^[42]. The element mapping shows that the Mn₂O₃ is uniformly distributed and dominant in the NM doped with a small amount of Ni ions (NM20), and the BET surface area can be increased to 35.8 m²·g^-1, which increases the active sites and is beneficial to faster ion diffusion and charge transfer. The doped Ni²⁺ can promote the internal charge rearrangement of Mn₂O₃, enhance its conductivity, and ultimately improve the mechanical and electrochemical properties of NM. Moreover, the doped Ni²⁺ can effectively stabilize the Mn-O bond of Mn₂O₃ by reducing the formation energy, thereby inhibiting manganese dissolution and finally achieving higher reversibility. At the same time, through the electrochemical performance test, it was found that the specific capacity after doping was 3 times higher than the original (Fig. 6).

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图6 (a)Mn₂O₃和NM20的电荷分布。(b)Mn₂O₃和NM20系列材料的循环性能^[42]

2.2 Vanadium-based material

Compared with MnO₂ and Prussian blue analogs, vanadium-based compounds show higher reversible capacity, better rate capability and longer cycle life despite their lower working voltage. Therefore, in recent years, vanadium oxides, metal vanadates, vanadium phosphates and so on have also attracted wide attention of researchers^[43]. A variety of vanadium-based compounds, such as layered or vanadates, as well as vanadium-based nanomaterials, have been successfully used as cathode materials for zinc-ion batteries because of their low cost, diverse crystal structures, multivalency of vanadium (V²⁺, V³⁺, V⁴⁺, V⁵⁺), and large ion channels, which provide a broad spatial size for Zn²⁺ intercalation/deintercalation^[44]. However, there are still some problems in the use of vanadium-based compounds as cathode materials in aqueous zinc-ion batteries, such as poor conductivity, irreversible phase transition caused by the destruction of the structure of zinc ions in the process of multiple intercalation/deintercalation, and slow ion diffusion. There have been many studies to improve their performance. However, the reported vanadium oxide cathode materials are still largely limited by their inherent electrochemical properties in storing divalent zinc ions^[45]. For example, the dissolution of vanadium in acidic/neutral electrolyte will lead to the degradation of electrode structure, electrolyte pollution and corrosion of zinc anode. From many studies of vanadium-based materials, it is found that there are only a few strategies to deal with the dissolution of vanadium in aqueous zinc-ion batteries.

2.2.1 Cation pre-intercalation strategy

By pre-intercalating multivalent cations (mainly Mg²⁺, Cu²⁺, Ba²⁺) into the positive electrode interlayer, an electrochemically robust positive electrode structure can be provided. These cations can form strong ionic bonds, thus preventing the dissolution of vanadium^[46]. NaV₃O₈ is a promising cathode material consisting of layered V₃O₈ and intercalated sodium ions^[47]. It is worth mentioning that the interlayer spacing of NaV₃O₈ (0.708 nm) is large enough to ensure the intercalation/deintercalation of Zn²⁺(0.074 nm), while H⁺ can also stably exist between the layered V₃O₈. Therefore, NaV₃O₈ with nanostructure is one of the ideal cathode materials for simultaneous H⁺ and Zn²⁺ co-intercalation/deintercalation processes in ZIB. For metal vanadates, the dissolution of vanadium is accompanied by the dissolution of other metal ions. It has been reported in the literature that 1 mol/L Na₂SO₄ was added to 1 mol/L ZnSO₄ electrolyte to study the zinc storage performance of NaV₃O₈·1.5H₂O(NVO) as a cathode material^[48]. The electrolyte with the addition of Na₂SO₄ is more reversible (Figure 7 a) and has better cycling performance. As the number of cycles increases, the performance of the NVO cathode material in pure ZnSO₄ electrolyte decreases sharply (Fig. 7 B), which is caused by the rapid dissolution of the NVO electrode in ZnSO₄ aqueous solution and the formation of dendrites. When the Na₂SO₄ is added, the Na⁺ can change the dissolution equilibrium of the NVO positive electrode to the Na⁺ and hinder the continuous dissolution of the NVO, thereby greatly improving the cycle stability of the NV electrode. In addition, the zinc dendrite/deposition during charge-discharge process can also be avoided by adding Na⁺ positive ions with lower reduction potential to the electrolyte according to the electrostatic screening mechanism (Fig. 7 C).

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图7 (a)NVO电极分别在1 mol/L ZnSO₄和1 mol/L ZnSO₄+1 mol/L Na₂SO₄电解质中的循环性能图,(b)NVO电极在ZnSO₄电解液中的循环性能。插图是不同时间下1 mol/L ZnSO₄和1 mol/L ZnSO₄+1 mol/L Na₂SO₄电解质中NVO电极的变化图像,(c)Na₂SO₄添加剂抑制NVO纳米带的溶解和锌枝晶的形成的示意图^[48]

Fig. 7 (a) Plots of cycling performance of NVO electrode in 1 mol/L ZnSO₄ and 1 mol/L ZnSO₄ + 1 mol/L Na₂SO₄ electrolyte, respectively, (b) cycling performance of NVO electrode in ZnSO₄ electrolyte. Insets are images of the changes of NVO electrodes in 1 mol/L ZnSO₄ and 1 mol/L ZnSO₄ + 1 mol/L Na₂SO₄electrolytes at different times, (c) schematic diagram of the inhibition of dissolution of NVO nanoribbons and formation of zinc dendrites by Na₂SO₄ additives^[48]. Copyright 2018, Springer Nature

The pre-intercalation of multivalent cations can provide an electrochemically robust cathode structure, but these ionic bonds may greatly shorten the interlayer distance of the cathode, resulting in slow Zn²⁺ migration and thus limiting the Zn²⁺ ion storage capacity, so further research is needed to improve the stability of cathode materials^[49]. In view of this, Wang et al. Prepared a barium vanadate stabilized by hydrated barium ion as a cathode material for aqueous zinc-ion batteries by hydrothermal synthesis^[50]. Ultrastable layered structures, namely Ba_xV₂O₅·nH₂O(BVO-1), Ba_1.2V₆O₁₆·3H₂O(BVO-2), and BaV₆O₁₆·3H₂O(BVO-3), were obtained by adjusting the amount of barium precursor. By comparison, the layered Ba_xV₃O₈ nanobelt (BVO-2) has the best rate capability and cycle stability, and its zinc ion kinetics is fast. At the same time, according to the ex situ FTIR spectrum (Fig. 8A), it can be seen that it shows the smallest by-product characteristic peak in the process of zinc ion intercalation, which can better and effectively inhibit the dissolution of the cathode. By observing the state of the cathode material in the electrolyte at different times (Figure 8 B), it can be found that the dissolution of vanadium is effectively controlled.

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图8 BVO-1、BVO-2、BVO-3分别(a)在完全放电状态下的FTIR光谱,(b)在2 mol/L ZnSO₄电解质中不同时间的光学图像^[50]。不同比例浓度插入阳离子的NZVO正极的电化学性能图:(c)0.1 mV·s^-1扫速下的CV曲线图,(d)不同倍率下速率性能,(e)在0.1 A·g^-1电流速率下,NZVO-4在400圈后容量保留率为99.6%^[51]

Fig. 8 (a) FTIR spectra of BVO-1, BVO-2 and BVO-3 in the fully discharged state, (b) optical images at different times in 2 mol/L ZnSO₄ electrolyte^[50]. Copyright 2020, American Chemical Society. Electrochemical performance of NZVO cathodes with different proportional concentrations of inserted cations. (c) CV plot at 0.1 mV·s^-1 sweep rate, (d) rate performance, (e) 99.6% capacity retention of NZVO-4 after 400 cycles at 0.1 A·g^-1 current rate^[51]. Copyright 2022, American Institute of Physics

A more stable vanadate cathode material can be obtained by pre-inserting dual cations. For example, Yu et al. Synthesized a new cathode material with a robust structure, Na_xZn_yV₃O₈·nH₂O(NZVO), by pre-inserting Zn²⁺ and Na⁺ into the intermediate layer of the V₃O₈-type vanadate crystal structure, which realized the rapid and stable extraction/insertion of Zn²⁺^[51]. Among them, the strong ionic bond formed between the pre-inserted cation and the V₃O₈ skeleton can effectively solve the problem of positive electrode dissolution, while inhibiting the generation of electrochemical by-products, thus improving the cycle performance of electrode materials. It can be seen from fig. 8 (C) that the cathodes prepared with different proportions of pre-inserted ions all show good reversibility. From Fig. 8 (d) and (e), it can be found that the charge screening effect caused by high crystal water content, as well as the fast migration kinetics of Zn²⁺, make the average capacity of NZVO-4 higher and the cycling stability better.

2.2.2 Electrolyte optimization strategy

At present, most of the electrolytes used in zinc-ion batteries are ZnSO₄ or Zn(OTF)₂ with weak acidity. Due to the strong polarity of water molecules, vanadium-based materials are easily dissolved under such electrolyte conditions, resulting in severe capacity fading during cycling. In addition, the zinc ion in aqueous solution exists as a hydrate coordinated by six water molecules ([Zn(H₂O)₆]²⁺). During the discharge process, the coordinated water molecules and zinc ions will be embedded in the interlayer or tunnel together, so excessive coordinated water molecules will accumulate in the interlayer, which accelerates the dissolution of vanadium. Moreover, the co-intercalated solvated water molecules can form hydrogen bonds with O^2- in the lattice, thus weakening the strength of V — O bonds and leading to the structural collapse of cathode materials. In the Zn(OTF)₂ electrolyte, the number of free water molecules can be reduced by introducing a certain amount of lithium triflate (LiOTF) with high solubility^[49]. LiOTF can not only reduce the number of water molecules, inhibit its activity and improve the stability of the interface between the electrode and the electrolyte, but also reduce the number of molecules between the layers and channels and enhance the stability of the crystal structure.

Aqueous electrolytes help to alleviate the safety problems caused by organic electrolytes, but are still limited by their narrow electrochemical window. In general, high-concentration electrolytes can effectively broaden the electrochemical stability window, optimize the transport process of zinc ions, and reduce the proportion of water molecules in the overall electrolyte, thus effectively improving the coulombic efficiency of the system. Because the electrolyte concentration has a great influence on the performance of ZIB, a lot of research has been done on high concentration electrolytes, and the concept of "water in salt" has been proposed in recent years. However, in order to realize the practical application of "water in salt" in aqueous zinc-ion batteries, it is necessary to optimize the ratio of salt to water, select highly water-soluble salts and reduce the cost of this "water in salt" electrolyte. Therefore, it is imperative to explore new electrolytes. Inorganic electrolytes are a potential candidate because of their stability and scalability. For example, Zhou Jiang et al. Of Central South University discovered an inorganic Zn²⁺ conductor electrolyte (ZHAP-Zn) with less water, and conceived a solid-liquid mixed Zn²⁺ ion transport channel for aqueous zinc-ion batteries^[52]. The surface of ZHAP particles is compact and flat, and the interstices between particles facilitate the storage of electrolyte, thus shortening the ion transport path and promoting ion conductivity. This electrolyte has a high ion transference number, which can reduce the dissolution of vanadium and inhibit the growth of zinc anode dendrite.

2.2.3 Solid Electrolyte Interface Construction Strategy

The dissolution of vanadium in aqueous electrolyte, as well as the formation of by-products during charge and discharge, can lead to severe capacity fading and reduced cycle life. As mentioned earlier, the dissolution of manganese can be inhibited by constructing a solid electrolyte interface, which is also suitable for vanadium-based cathode materials. Atomic layer deposition (ALD) is an effective method for coating uniform and conformal layers on complex and high specific surface area structures. The ALD passivation layer can act as an artificial solid electrolyte interface to inhibit the side reactions occurring at the electrode and electrolyte interface, thereby improving the electrochemical performance of the battery. Cui Yi and Alshareef et al. Designed an ultrathin HfO₂ film with an artificial solid-electrolyte interface (Fig. 9 B)^[53]. The film was uniformly and densely deposited by atomic layer deposition (ALD) method (Figure 9 a), which not only reduced the formation of by-products (Zn₄SO₄(OH)₆·xH₂O) on the surface of Zn₃V₂O₇(OH)₂·2H₂O(ZVO), but also inhibited the dissolution of ZVO cathode in the electrolyte.

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图9 (a)原始ZVO正极和通过原子层沉积涂覆HfO₂的ZVO的制造过程的示意图,(b)HfO₂涂层在ZVO上的高角环形暗场扫描透射图^[53]。(c)原位CEI层策略设计的示意图^[54]。(d)制备V₂O₅@PEDOT/CC 的示意图^[55]

Fig. 9 (a) Schematic of the fabrication process of pristine ZVO cathode and ZVO coated with HfO₂ by atomic layer deposition, (b) high angle annular dark field scanning transmission electron microscopy of HfO₂ coating on ZVO^[53]. Copyright 2019, American Chemical Society (c) Schematic of in situ CEI layer strategy design^[54]. Copyright 2021, Wiley (d) Schematic of the preparation of V₂O₅@PEDOT/CC^[55]. Copyright 2019, Wiley

The spontaneous dissolution of vanadium in aqueous solution is the main problem, and metal ions can leach from the vanadium oxide interlayer during the electrochemical process, resulting in structural collapse and capacity fading. However, inspired by this phenomenon, the researchers believe that if the leached metal ions are directly precipitated on the vanadium-based materials, they may form a CEI protective layer on the surface in situ. In response to this idea, Miao Ling et al. Of Huazhong University of Science and Technology precipitated metal ions on the surface immediately during the electrochemical process and transformed them into SrCO₃CEI layers^[54]. This CEI layer (Fig. 9 C) can effectively avoid the contact between vanadium-based materials and polar water molecules in the electrolyte, which can not only inhibit the dissolution of vanadium, but also reduce the self-discharge effect of the open circuit voltage at rest, thus obtaining better cycle performance.

In addition, it was found that poly (3,4-ethylenedioxythiophene) (PEDOT), which has high conductivity and stability, can not only effectively improve the electrode conductivity, but also be used as a protective layer to inhibit the collapse of nanostructures. However, the application of PEDOT in vanadium pentoxide-based electrodes was not reported until 2019. Hu Xianluo et al. Of Huazhong University of Science and Technology deposited PEDOT with a thickness of 5 nm on the surface of vanadium pentoxide nanosheets as a protective layer^[55]. The synergistic effect between the V₂O₅ nanosheet array, which provides sufficient zinc storage active sites (Figure 9D), and the PEDOT coating shell, which increases the zinc ion/electron transport kinetics and further acts as a protective layer to suppress the structural collapse during cycling, contributes to the high rate performance and long cycling performance of the battery.

2.3 Sum up

In general, ZIBs have made rapid progress and made significant research progress in recent years. However, there are still some difficulties in realizing its industrialization and commercialization in a real sense. For example, as far as cathode materials are concerned, although both manganese-based and vanadium-based materials have shown excellent performance, the dissolution problem of vanadium and manganese in aqueous electrolyte has not been completely solved. Among them, the dissolution of manganese-based oxides as ZIBs cathode materials has two major problems, namely, the rapid capacity fading during cycling and the poor conductivity. In order to improve the electrochemical performance of manganese dioxide, researchers have proposed a series of research strategies, including nanostructure design, defect engineering, composite with conductive carbon-based materials, and surface coating^[56]. Moreover, the structural characteristics of vanadium-based compounds also have an important impact on their electrochemical properties. In addition to its own structural characteristics, some other design strategies have been developed to improve its electrochemical performance, including the insertion of metal cations, the surface coating of artificial SEI film, and the appropriate addition of additives in aqueous electrolyte^[57]^[58]^[59]. The design and development of cathode materials with large storage capacity, high discharge voltage, stability and easy intercalation/deintercalation path is a great challenge for the research and development of high-performance ZIB. Although the above methods can improve the performance of cathode materials to some extent, it is difficult to meet all the performance requirements of cathode materials through one strategy or one material in practice. On this road, researchers still need to make unremitting efforts and actively explore.

3 Electrostatic interaction between ions

In the multivalent aqueous zinc-ion battery, the strong electrostatic interaction between the host material and the ion caused by the high charge density limits the solid diffusion rate of the Zn²⁺ and slows down the reaction kinetics^[60]. At the same time, it leads to the increase of interlayer spacing and the acceleration of skeleton bending vibration, which eventually leads to the collapse of the structure. Since the solid diffusion rate of Zn²⁺ is limited, the slow diffusion rate can lead to the accumulation of Zn²⁺ in the crystal structure, and once the amount of accumulated Zn²⁺ reaches the maximum amount that the material can support, an irreversible phase transformation will occur. Electrostatic interaction is the essence of the formation of chemical bonds and ionic bonds, including electrostatic attraction and repulsion. Ionic bond is a chemical bond formed by electrostatic interaction between anions and cations after an atom gains or loses electrons^[61]. Different anions and cations have different radii and electrical properties, and the crystal space lattice formed is also different. In ZIB, the interlayer spacing between vanadium oxide layers is enhanced during the intercalation/deintercalation of Zn²⁺ during charge/discharge, which is related to the electrostatic interaction force^[62].

Wang et al. Prepared nanobelt Mg_0.26V₂O₅·0.73H₂O(MVO) cathode materials by hydrothermal method, matched with metal zinc anode, and realized aqueous zinc-ion batteries with high area-specific capacity under high load.Based on a series of test data and density functional theory (DFT) calculations, the energy storage mechanism of aqueous zinc-ion batteries was clarified, and it was proved that the Zn²⁺ was intercalated/deintercalated in the form of hydrated zinc ions during the charge-discharge process^[63]^[64]. The deintercalation of hydrated zinc ions can play the role of charge shielding, effectively reduce the Coulomb repulsion between the electrode and electrolyte interface, thereby reducing the activation energy of ion transfer at the positive electrode interface, and reduce the desolvation loss of hydrated zinc ions in the intercalation/deintercalation process, so as to achieve rapid zinc storage capacity^[65]. In addition, the excellent structural stability and large interlayer spacing of the cathode material also ensure the high reversibility of the intercalation/deintercalation of hydrated zinc ions, thus achieving the excellent electrochemical performance of MVO cathode material. As shown in Fig. 10 (a) and (B), the initial discharge capacity of MVO was 1.77 mAh·cm^-2, and the capacity retention was over 93% after 100 cycles. Although the VO electrode showed a higher initial capacity of 1.79 mAh·cm^-2, the capacity fading was severe. The capacity of MVO is higher than that of VO at different current rates, and when the current returns to the 0.1 A·g^-1, MVO has almost no significant decay. Fig. 10 (C) demonstrates the intercalation mechanism of dissolved Zn²⁺ during charge-discharge process, and MVO is able to combine the Zn²⁺ cation with six water molecules as a solvation shell due to the large interlayer spacing. This indicates that Zn²⁺ desolvation is not required during intercalation/deintercalation, effectively reducing the ion transfer activation energy.

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图10 (a)MVO和VO电极在0.1 A·g^-1电流下的循环性能图,(b)不同电流密度下的MVO和VO电极的倍率性能图,(c)溶解Zn²⁺的嵌入/脱出机理示意图^[63]。(d)NaV₃O₈晶体结构,(e)β-Na_0.33V₂O₅晶体结构^[66]

Fig. 10 (a) Cycling performance plots of MVO and VO electrodes at 0.1 A·g^-1 current, (b) multiplicative performance plots of MVO and VO electrodes at different current densities, (c) schematic representation of the embedding/deembedding mechanism of dissolved Zn^2+[63]. Copyright 2020, Wiley (d) NaV₃O₈ crystal structure, (e) β-Na_0.33V₂O₅ crystal structure^[66]. Copyright 2019, Wiley

In addition, the high charge density of Zn²⁺ and the strong electrostatic interaction with them make them easier to combine with oxygen atoms in vanadate^[67]. Therefore, the intercalated ions in the vanadate structure may squeeze out the rest of the lattice, dissolve in the electrolyte, and thus lead to rapid capacity decay^[68].

Generally speaking, the metal ion used for intercalation is related to the connection mode of oxygen atoms. Single chain tends to show stronger electrostatic interaction force and better stability than multiple chains^[64]. Liang Shuquan et al. Constructed sodium vanadate with layered structure NaV₃O₈ (Na₅V₁₂O₃₂ and HNaV₆O₁₆·4H₂O) and tunnel structure β-Na_0.33V₂O₅ (Na_0.76V₆O₁₅).The storage/release behavior of Zn²⁺ ions in these two typical structures was also studied in depth^[66]. From Fig. 10 (d), it can be found that the structure of the NaV₃O₈-type compound consists of V₃O₈ polyhedral layers, and the intercalated sodium ions are mainly located between the layers of octahedral sites, showing a typical layered structure. Among them, the oxygen atoms at the surface of the layer are singly and triply connected. β-Na_0.33V₂O₅ is a tunnel structure formed by the combination of VO₆ octahedra and VO₅ square pyramids along the B axis. The inserted sodium ion will connect with a single oxygen atom between the tunnels (Fig. 10e), and it is proved that the cycle performance of a single oxygen atom connection is better. The problem of electrostatic interaction between ions is another form of positive electrode dissolution, and optimizing the dissolution equilibrium is an effective strategy, such as adding stable metal ions to the electrolyte.

In the study of electrolyte additives, it has been found that some inorganic and organic additives have an effect on the morphology of zinc deposition. For example, metal salts containing Pb²⁺ and Bi³⁺ have high overpotential for hydrogen evolution, which can effectively inhibit the hydrogen evolution reaction. However, heavy metals will aggravate environmental pollution and increase costs. Rare earth metal salts can refine the grain size and adsorb on the active sites of Zn crystal, thus inhibiting the dendrite growth. Li Li et al. Of Beijing Institute of Technology first used cerium chloride (CeCl₃) as an effective, low-cost and green electrolyte additive to promote the formation of a dynamic electrostatic shielding layer around the zinc protrusions and induce the uniform deposition of zinc^[69]. The schematic diagram shows that Ce³⁺ can be preferentially and selectively adsorbed on the surface of zinc anode to form an electric double layer (Fig. 11), inhibiting the subsequent dendrite growth through the electrostatic shielding effect.

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图11 CeCl₃添加剂对锌沉积过程的影响示意图^[69]

Considering that electron additives nucleate on the surface, leading to dendrites and by-products, while ionic additives polarize and affect the rate performance of batteries, both methods directly affect the cycling of batteries. Fu Lijun et al., Nanjing University of Technology, introduced an artificial hybrid electron-ion conductive coating (Alg-Zn + AB @ Zn) composed of zinc alginate gel (Alg-Zn) and acidic conductive carbon black (AB) on zinc surface^[70]. This coating not only provides more Zn nucleation sites, but also effectively reduces the Zn nucleation overpotential. In addition, the conductive carbon black provides a stable conductive channel on the zinc surface, which can coordinate the electric field, guide the uniform deposition of Zn, and also alleviate the polarization of the ion-regulated coating (Fig. 12).

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图12 镀锌示意图和不同负极对称电池的性能比较^[70]

4 Oxygen/hydrogen evolution reaction

Due to the high conductivity and low voltage window of aqueous electrolyte, the electrochemical behavior of electrolyte/electrode interface in aqueous zinc-ion battery is more complex than that in organic electrolyte, which leads to many side reactions and makes the cycle stability of aqueous battery worse^[71]. The electrochemical behavior and evolution of electrode/electrolyte interface (EEI) are the key factors affecting the energy density, cycle performance and power density of batteries, and have been a research difficulty^[72]. For example, the hydrogen evolution reaction (HER) at the anode/electrolyte interface (AEI) and the oxygen evolution reaction (OER) at the cathode/electrolyte interface (CEI)^[72]^[73]. In addition to the storage process of zinc, many ions are involved in the electrochemical reaction on EEI, such as the formation of by-product Zn₄(SO₄)(OH)₆·nH₂O(ZHS) and its analogues, which generally occur in cathode materials (manganese-based and vanadium-based)^[74]^[75]. When ZHS dissolves on the electrolyte/electrode surface, the pH value increases, and it reversibly dissolves into the electrolyte as the pH value decays during charging. Due to the low conductivity of ZHS, the reversible precipitation/dissolution of ZHS by-products during charge/discharge does not significantly hinder the insertion of zinc ions in subsequent cycles^[76]. However, the gradual accumulation of Zn²⁺ after continuous circulation will inevitably lead to the consumption of water electrolyte. This problem can be alleviated by constructing an electrode with a porous morphology to accommodate the precipitation of by-products, while also providing more reaction sites for Zn²⁺ and reducing the impedance^[77]. In addition, the increase of ZHS material will lead to the volume expansion of the electrode, which will aggravate the mechanical stress and reduce the cycling performance^[78].

In order to maximize the capacity of the electrode material, the electrochemical window of the aqueous zinc-ion battery should be less than the decomposition voltage of the electrolyte^[77]. The electrochemical stability window of water is only 1. 23 V, in which the electrode material can not exert its maximum capacity, and the decomposition of water will inevitably occur, which will lead to the occurrence of hydrogen/oxygen evolution reaction, the change of pH value near the electrode, and the deterioration of the stability of the electrode material^[79]. Reducing the generation of by-products is essential for improving the performance of aqueous zinc-ion batteries, even the whole aqueous battery system. In the following chapters, this problem will be analyzed and summarized.

4.1 Oxygen evolution reaction

Electrochemical water splitting includes both HER and OER reactions, and each half-reaction follows a different reaction path depending on the electrochemical and electronic properties of the electrode surface^[80]. The activity, reaction kinetics, and stability vary with the intrinsic properties of the electrode surface and the actual electrochemical conditions. This problem can be solved by constructing a stable electrode electrolyte interface or suppressing the activity of water^[81]^[82]. The OER process involves the transfer of multiple electrons, which can lead to slow kinetics and affect the overpotential^[83]. OER tends to occur at low concentrations of electrolytes above 1.8 V, so the use of high concentrations of electrolytes is the preferred option regardless of cost. Nevertheless, the operating voltage of AZIBs is still limited and the energy density is insufficient, which is much lower than that of non-aqueous batteries.

The specific water splitting voltage, which affects the battery performance, depends on the pH value and the composition of the electrolyte, which are the two main aspects of the current research to expand the stable voltage range^[84]^[85]. However, simple pH adjustment does not always enlarge the electrolyte voltage window because the potential difference of OER and HER does not increase accordingly. In addition, the change of pH value of electrolyte may lead to the change of redox potential of electrode materials. Although no intercalation electrode materials for ZIB have been reported to exhibit this feature, lessons can be learned from other electrodes. Similarly, the construction of a storage system with a mixture of electrolytes of different pH may result in a wide voltage window, which may be a solution for the development of low-cost and high-capacity aqueous batteries.

Studies have shown that high concentration of electrolyte has a great influence on the electrochemical stability window and the potential of electrode materials. The enhancement of electrolyte concentration can increase the potential difference between OER and HER. Some researchers have proposed a high-concentration "water in salt" electrolyte technology in aqueous supercapacitors to broaden the electrolyte voltage window^[86]. As shown in Fig. 13 (a ~ C), the window of this highly concentrated aqueous electrolyte (37 mol/L KFSI) expands to about 2.8 V with the formation of the electrode/electrolyte interface. Fig. 13 (d) shows that the aqueous supercapacitor using the electrolyte can be cycled 50,000 times at a high voltage of 2.3 V, and the coulombic efficiency is close to 100% at both low (1 C) and high (5 C) charge-discharge rates, respectively. Lithium salt can form a more effective protective interface on the negative electrode, while further inhibiting the water activity on the surface of the negative electrode and the positive electrode^[87]. They calculated the relative lithium activity coefficient in the electrolyte based on the peak potential shift measured by CV using different concentrations of LiTFSI electrolyte. Among them, the increase of activity coefficient at high concentration, reflects the scarcity of water as free solvent and the enhancement of cation/anion interaction^[88]. The "water-in-salt" electrolyte and the electrolyte with appropriate concentration can effectively expand the electrochemical stability window of the aqueous electrolyte^[89].

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图13 基于KFSI电解液的物理化学性能比较。(a)不同KFSI电解液浓度下的电化学稳定窗口。正极(b)和负极(c)附近区域的放大图。(d)电池在2.3 V电压及1 A/g电流密度时的容量保持率和库仑效率^[86]

Fig. 13 Comparison of physicochemical properties of KFSI-based electrolytes. (a) ESWs of KFSI electrolyte with different concentrations. Magnified view of the regions outlined near (b) cathodic scan and (c) anodic scan. (d) Capacitance retention and Coulombic efficiency at an operation voltage of 2.3 V at a current density of 1 A/g^[86]. Copyright 2021, Elsevier

In the "water-in-salt" electrolyte, the mass ratio of salt to water is only 1 ∶ 2.6, and the electrochemical window of the electrolyte is increased to 3. 0 V. Subsequently, "water in bisalt" (WIBS) electrolyte was also reported. The mass ratio of salt to water in the electrolyte is 1 ∶ 2, and the stable electrochemical window is further broadened to more than 3. 1 V. At present, this kind of electrolyte provides a novel idea for electrochemical energy storage technology in aqueous system, and its potential needs to be further explored. In addition, new systems related to this type of electrolyte are constantly being proposed, but at the same time, there are a series of basic science and application problems to be explored. For example, its ion transport mechanism is not yet clear, its cost needs to be reduced, and its safety and performance need to be evaluated.

4.2 Hydrogen evolution reaction

Compared with OER, solving the problem of HER faces more serious challenges, because HER has good reaction kinetics (HER is a two-electron reaction process, while OER is a four-electron reaction process), which is more likely to occur during battery cycling^[90]. Even trace amounts of hydrogen can severely deteriorate the electrode structure during cycling. In acid electrolyte, there are a lot of hydrides. The covalent bond of the hydride is weak, which is beneficial to each step of the reaction, and the reaction kinetics is fast^[91]. In addition, the formation of zinc dendrites will promote the hydrogen evolution reaction, which will accelerate the corrosion of the negative electrode surface, and a series of by-products produced by corrosion will also aggravate the formation of dendrites^[92].

The process of hydrogen evolution not only breaks the chemical bonds in water molecules, but also breaks the hydrogen bonds between water molecules. Thus, HER and hydrogen bond formation are in a competitive relationship^[93]. The conventional way to inhibit HER is to increase the pH value of the electrolyte and reduce the potential of HER, but the strong alkaline solution will affect the stability of most electrode materials. Although highly concentrated electrolyte solutions have been widely studied, their effects on HER inhibition are not satisfactory. In addition, a highly concentrated electrolyte solution may significantly increase the cost of the battery. In view of this, there is an urgent need for a simple and universal strategy to reduce the inherent activity of H₂O in HER. Chen Jun of Nankai University and Ren Xiaodi of University of Science and Technology of China cooperated to control the hydrogen bond between water molecules by introducing dimethyl sulfoxide (DMSO) from the thermodynamic point of view, so as to achieve the purpose of inhibiting water electrolyte HER^[94]. As a "hydrogen bond acceptor", DMSO can replace the weak hydrogen bond interaction in H₂O-H₂O with the strong hydrogen bond interaction between DMSO-H₂O, which makes the deprotonation process of water molecules difficult, thus inhibiting hydrogen evolution^[95]. During the charge and discharge process, water molecules migrate to the electrode surface together with cations, and then undergo a deprotonation process to produce hydrogen. This step not only changes the chemical bonds in the water molecules, but also breaks the intermolecular hydrogen bonds. Therefore, the activity of water can be reduced by adjusting the hydrogen bond structure of water, thereby inhibiting HER in AZIBs. The infrared imaging technique shows that the mixture of DMSO and H₂O has an obvious exothermic phenomenon, and the heat released is caused by the formation of hydrogen bonds between DMSO and H₂O^[96]. The current of 0.1 mA·cm^-2 was chosen as the onset potential to define the hydrogen evolution potential. When the concentration of LiTFSI in the electrolyte decreased from 1 mol/L to 0. 5 mol/L, the voltage decreased by 0. 45 V, and it was found that the hydrogen evolution potential was inhibited. The above proves that it is feasible to reduce the activity of free water and reduce the hydrogen evolution reaction from the thermodynamic point of view.

In recent years, there have been more and more studies on electrolytes, and the effect of adding organic molecules to the electrolyte to stabilize zinc anode is remarkable. However, the mechanism of this effect has not been explored deeply enough. Zhi Chunyi of City University of Hong Kong analyzed the mechanism by studying the principle of electrochemical stability window, and discussed the process of zinc ion intercalation/deintercalation, the causes of side reactions and the formation of dendrites^[97]. Hydrogen evolution and oxygen evolution usually require a high potential, which leads to the hydrogen evolution reaction of the negative electrode during zinc deposition. Zinc is thermodynamically unstable in aqueous solution and prone to corrosion, depending mainly on the pH of the solution. Many studies have shown that neutral or weak acidic water electrolyte is the first choice for aqueous zinc-ion batteries. Organic molecules have strong interaction with zinc, which can be adsorbed on the surface of zinc, thus improving the compatibility of zinc anode with electrolyte.

Continuous hydrogen evolution will cause some pH changes, resulting in the formation of by-products, which will increase the curvature and irregularity of the electrode/electrolyte interface, increase the contact area, and accelerate the hydrogen evolution reaction. The key to improve the performance of aqueous zinc-ion battery is to study the amount of hydrogen produced on the electrode during zinc deposition. Ma et al. Synthesized a dense and uniform zinc fluoride protective layer with high Zn²⁺ conductivity^[98]. By in situ ion metathesis, it was found that the Zn/ZnF₂ interface was bonded tightly by covalent bonds, which achieved a long time of dendrite-free growth, hindered the contact between Zn and water molecules in the aqueous electrolyte, and avoided the occurrence of side reactions such as hydrogen evolution and zinc corrosion.

In a word, in aqueous zinc-ion batteries, the hydrogen evolution reaction at the electrode/electrolyte interface changes the liquid phase into the gas phase, which increases the internal pressure of the battery and causes the battery to swell and damage. Moreover, the hydrogen evolution reaction will affect the deposition of zinc, reduce the coulombic efficiency, lead to the change of pH in the system, and accelerate the production of by-products on the surface of the anode. In alkaline electrolyte, the standard potential of zinc/zinc oxide (− 1.26 V vs. SHE) is lower than the hydrogen evolution potential (− 0.83 V vs. SHE), and from the thermodynamic point of view, the hydrogen evolution reaction will occur preferentially compared with the reduction reaction of zinc oxide, affecting the cycle life of the battery. However, in a weak acidic environment, the standard reduction potential of V vs. SHE (− 0.76 V vs. SHE) is lower than the hydrogen evolution potential (0 V vs. SHE), and the hydrogen evolution reaction tends to occur. Considering the limited voltage window of the electrolyte and the existing cathode materials, the above strategy to inhibit the hydrogen evolution reaction can be used as an alternative to optimize the aqueous zinc-ion battery module. At the same time, people have done a lot of research work in the exploration and modification of metal anode of aqueous zinc-ion battery.

5 Side Reaction Problems of Zinc Anode

Zinc is one of the best negative electrodes for aqueous ion batteries because of its availability in nature, low price, good chemical stability, low redox potential and easy processing. However, due to the uneven distribution of electric field, zinc anode has side reactions such as zinc dendrite growth, corrosion and passivation, which is one of the problems hindering the further development of AZIBs^[99]^[100]^[101].

5.1 Corrosion, passivation and zinc dendrite

Due to the active interface between the electrode and the electrolyte, some uncontrollable side reactions may occur during the storage of ions or the operation of the battery. In the commonly used 1 mol/L ZnSO₄ electrolyte with pH value of about 5, besides hydrogen evolution reaction, the surface of zinc anode will also be corroded. Modification of the inherent characteristics of weak acid electrolytes, such as ZnSO₄ solutions, which have inherently limited solubility and coulombic efficiency for zinc stripping/plating, can effectively avoid anode corrosion and maintain continuous anode protection^[102]. There are two main ways to prevent side reactions by modifying the electrolyte: reducing the activity of water to inhibit its stripping and increasing the reaction overpotential to delay corrosion^[103]^[104].

Negative passivation is another side reaction in AZIBs. Taking the MnO₂ cathode as an example, the H⁺/Zn²⁺ co-insertion reaction is the energy storage mechanism in weak acidic electrolyte. Due to faster H⁺ diffusion and weaker electrostatic interaction with the host lattice, H⁺ is preferentially inserted into MnO₂, and the resulting electrolyte pH increases, leading to the formation of a basic zinc sulfate passivation layer. When the distribution of electric field and concentration gradient on the zinc deposition interface is not uniform, it is easy to lead to the growth of zinc dendrite^[105]. The process of zinc nucleation usually takes place in a region where the initial concentration of zinc ions is high, and then the zinc ions will begin to deposit where the crystals are already present to reduce the surface free energy. Finally, coarse dendrites are gradually generated, and the process is the same as zinc deposition in alkaline electrolyte. Usually, the brittle dendrite is needle-like, and its tip acts as a charge center in the subsequent reaction and triggers the tip effect, resulting in continuous charge accumulation, which promotes the growth of zinc dendrite, resulting in capacity fading and short circuit caused by piercing the diaphragm. Yufit et al. Studied the failure mechanism of zinc batteries with and without commercial microporous separators by multi-scale and tomography techniques, and found that the dendrite growth, dissolution and regeneration in these two cases eventually led to the formation of dendrite layers with different morphologies on the electrode and separator^[106].

First, zinc dendrites were deposited on a cone-shaped zinc anode. The growth, dissolution, and regrowth of zinc dendrites in the absence of a diaphragm were observed in situ by radiography. The secondary dendrite grows on the backbone of the primary dendrite rather than directly on the electrode tip. However, the morphology of the primary and secondary dendrites does not change with the change of the trunk and leaf-like branches, indicating that the growth of dendrites strongly depends on the local crystallography and the current along the secondary dendrite direction. At the same time, the growth rate of dendrites at different positions of the tip was estimated by radiographic data, and it was found that the growth rate at these positions was not uniform, which was contrary to the reported linear growth rate. This phenomenon indicates the change of local current during dendrite growth, which is likely to be related to the increase of active surface area caused by secondary and tertiary dendrite growth, as well as the density and size of surrounding primary dendrites. Moreover, they studied the effects of low current density and high current density on zinc dendrite growth, and found that the higher the working current density, the shorter the dendrite initiation time and the larger the dendrite height.

By using the porous diaphragm and then carrying out the same experimental operation as above, it can be observed that the dendrite grows gradually after the zinc deposition occurs (fig. 14). The main effect of the insulating porous separator is to change the local potential distribution in the pores of the separator and to limit the mass of zincate and other ions during their transport through the separator, thus affecting the growth of internal dendrites. As the deposition proceeds, a large number of dendrites penetrate the diaphragm at various locations and continue to grow in different directions on the diaphragm surface. However, the detached dendrites can accumulate in the diaphragm to form a pathway leading to a short circuit. Part of the current flowing through the zinc anode is divided by the short-circuit current, so the dissolution rate is significantly reduced, and the growth of other new dendrites is not found. Until the dendrite does not increase after regrowth, the inside of the battery has been completely short-circuited. In general, zinc dendrites may not be a problem at low current densities or loads, but at high current densities or loads, dendrites may quickly damage the cell.

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图14 使用多孔隔膜对锌枝晶生长、溶解和再生长的研究(A)200 s、(B)300 s、(C)430 s、(D)590 s和(E)890 s时锌枝晶的生长,(F)120 s、(G)240 s、(H)680 s时的锌枝晶溶解,(I)304 s和(J)656 s时的锌枝晶再生^[106]

Fig. 14 Study of zinc dendrite growth, dissolution and regrowth using porous separator. Zinc dendrite growth at (A) 200 s, (B) 300 s, (C) 430 s, (D) 590 s and (E) 890 s, (F) 120 s, (G) 240 s, (H) 680 s for each time of zinc dendrite dissolution, (I) 304 s and (J) 656 s for each time of zinc dendrite regrowth^[106]. Copyright 2019, Cell Press

Based on the interface structure and modification methods of zinc anode, many materials, such as metal particles, composite particles, polymers and so on, have optimized the interface modification through different mechanisms, which can achieve the effect of inhibiting side reactions^[107]. In the construction of interface engineering, effective confinement effect, uniform interface electric field, increasing nucleation sites, electrostatic shielding, crystal orientation induction and other aspects have obvious effects on inhibiting dendrite growth^[108].

Considering that zinc anode has a great influence on the interfacial behavior, it is necessary to change its interfacial composition and structure to change the electron distribution, which can directly affect the diffusion of H⁺ and Zn²⁺, and is expected to solve the problems of zinc dendrite and hydrogen evolution. Therefore, Zhou et al. Designed a stable Zn | Sn alloy anode with uniform second phase and local electron effect, which can limit the reduction, diffusion and aggregation of interfacial hydrogen ions^[109]. The DFT calculation shows that the electronegativity of Sn (1.96) is stronger than that of Zn (1.65), so the electrons in the alloy anode will be preferentially concentrated near Sn, forming a local electronic effect. It was found by in situ optical microscopy that the surface of the alloy anode was flat and no bubbles were produced within 1 H, which showed a significant effect on inhibiting hydrogen evolution.

An "epitaxial electrodeposition" strategy to suppress zinc dendrites at the source, published in Science in 2019^[110]. Zheng et al. First deposited a graphene layer on the surface of stainless steel to make it epitaxial to the basal surface of zinc metal, thus minimizing the lattice strain. Graphene has a low lattice mismatch, which can effectively drive zinc deposition with a specific crystal orientation relationship. After that, when metal zinc is electrodeposited on the graphene epitaxial substrate, the crystalline orientation of zinc is preferentially parallel to the electrode, forming a plate-like stacking structure rather than a dendrite, and the resulting epitaxial zinc anode achieves excellent reversibility at a medium and high rate in thousands of cycles. Zinc exhibits a tendency to deposit in a planar form, implying a lower thermodynamic free energy associated with the nearest packing plane exposed in hexagonal close-packed metals. Therefore, candidate substrates for zinc epitaxial electrodeposition should show a similar atomic arrangement to the zinc planes. A suspension composed of graphene flakes in NMP is scraped on a substrate, and the zinc deposition direction formed on the graphene is consistent and arranged in a specific orientation relationship. Epitaxial deposition of zinc was found to be significantly improved in terms of reversibility by coulombic efficiency data. A battery assembled by utilizing epitaxial zinc deposition as a negative electrode and a conventional α-MnO₂ as a positive electrode, the N: P ratio is 2:1, and the battery keeps good capacity in 1000 cycles under the condition of 8 mA·cm^-2. It is proved that this method can effectively change the direction of zinc deposition and inhibit the dendrite growth.

The corrosion process of zinc is mainly due to the dissolution of Zn²⁺ into the liquid phase formed by the spalling of zinc foil during discharge, resulting in the increase of local concentration. At the same time, by-products such as zinc hydroxide and zincate are formed due to the reaction of Zn²⁺ and OH^-, which will passivate the original zinc. When the aqueous electrolyte is slowly reduced in the reaction, this corrosion and passivation are often irreversible, resulting in a sharp decline in cycle performance. In addition, corrosion usually leads to self-discharge, while passivation increases the impedance and prevents further reaction of the zinc negative electrode, resulting in poor coulombic efficiency of plating/stripping. The formation of zinc dendrite mainly exists in the process of electroplating, and is affected by the "tip effect". The "tip effect" can be eliminated by using the "electrostatic shielding" effect in the electroplating process, for example, a layer of magnetron sputtering aluminum-based alloy protective layer is plated on the zinc surface to adjust the electrostatic shielding effect^[111]. Aluminum can form a surface insulating alumina layer, and the Zn²⁺ will be adsorbed on the alumina shell to form an electrostatic shielding layer to prevent further deposition of the Zn²⁺. By controlling the Al content in the Zn-Al alloy film, the strength of the electrostatic shielding can be controlled, and the "tip effect" can be effectively eliminated. It can be clearly seen from the laser scanning confocal microscope image that the "tip effect" is well controlled.

Deposition occurs during zinc plating, which may penetrate the battery separator and cause internal short circuit. At present, there are few reports on diaphragm research in the solution strategy of zinc dendrite. In 2021, Zhou Jiang and Liang Shuquan of Central South University invented an ultrathin three-dimensional acrylonitrile (PAN) nanofiber separator^[112]. The cyano groups on the surface of the membrane guide the ordered transport of zinc ions through N-Zn bonds, and regulate the nucleation and deposition of zinc ions. The symmetrical battery prepared by PAN separator has good stability, and the dendrite-free deposition layer is a major advantage. Compared with the common GF separator, the PAN separator is thinner, which can effectively reduce the pH gradient of the electrolyte and promote the uniform deposition of zinc. At the same time, the generation of by-products and dendrite growth on the cathode are inhibited, which provides a new idea for the development of ZIBs.

In a word, the formation of zinc dendrites, the corrosion and passivation of the negative electrode do not exist independently, but have a great influence on each other. The formation of zinc dendrite will increase the surface area of the negative electrode and promote the hydrogen evolution reaction. Hydrogen evolution reaction can cause local pH change, form electrochemical inert corrosion by-products deposited on the surface of the negative electrode, and intensify the electrode polarization, which in turn promotes the formation of dendrites. Therefore, while researchers are working to solve one problem, they may also indirectly alleviate one or more other problems. But in some cases, the strategy we use to solve one of these problems may also worsen the other. Therefore, when solving practical problems, researchers should make a concrete analysis of specific problems and consider them comprehensively.

5.2 Cathode modification

The formation, corrosion and passivation of zinc dendrites will accelerate the formation of side reactions inside the battery, resulting in the rapid consumption of active zinc. In the cycle process, it has a greater impact on the coulombic efficiency, which seriously threatens the life of the battery, and has attracted the attention of many researchers. Therefore, the modification of the negative electrode is also one of the key factors to improve the performance of aqueous zinc-ion batteries.

5.2.1 Surface modification

In the previous part of cathode material modification, it was mentioned that surface modification and coating can improve the electrochemical performance. Surface modification refers to endowing the surface with new properties, such as hydrophilicity, biocompatibility, antistatic properties, etc., on the premise of maintaining the original properties of materials or products. In view of the fact that the formation of zinc dendrites is mainly caused by the uneven contact between the electrode and the zinc anode surface, Kang et al. Proposed a strategy of constructing a porous protective layer on the zinc surface to provide uniform penetration of the electrolyte to eliminate the harmful dendrites^[113]. The nano CaCO₃ layer has uniform pore channels to guide uniform and position the selected zinc ion stripping/plating on the zinc foil interface, which can ensure the uniform penetration of electrolyte, followed by the nucleation of uniformly distributed zinc, and finally the formation of small-sized zinc crystals (Figure 15 A). After 100 charge-discharge cycles, the surface of the nano CaCO₃ layer still maintains a flat porous structure, and has better cycle stability, higher specific capacity and better capacity retention rate than the bare zinc anode.

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图15 (a)裸锌负极和纳米CaCO₃涂层负极的形貌演化图^[113]。(b)超薄氮(N)掺杂氧化石墨烯(NGO)层的复合锌金属负极示意图^[114]

Fig. 15 (a) Morphological evolution of bare zinc anode and nano-CaCO₃ coated anode^[113]. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (b) Schematic diagram of a composite zinc-metal anode with an ultrathin nitrogen (N)-doped graphene oxide (NGO) layer^[114]. Copyright 2021, Wiley-VCH GmbH

Recently, impressive ultrathin interfaces have been obtained using techniques such as atomic layer deposition and chemical vapor deposition to modify zinc anodes. These technical approaches, which facilitate the further synthesis of artificial interfacial layers, with some of the electrodes exhibiting excellent cycling performance, provide new insights into the construction of high-performance zinc metal anodes via ultrathin interfacial modification. However, the development of aqueous zinc metal anode is still in its infancy, and further research is still needed on the simple, low-cost and environmentally friendly ultra-thin interface process, the synthesis and preparation of thin films with uniform thickness, stable structure and easy processability, the in-situ characterization of interface stability and the corrosion mechanism of anode. Zhou et al. Designed a composite zinc metal anode with an ultrathin nitrogen (N) -doped graphene oxide (NGO) layer by using a one-step Langmuir-Blodgett method, which can optimize the growth of zinc deposition and inhibit the formation of dendrites, and obtain a stacked and ordered nanocrystal deposition layer instead of dendritic zinc dendrites (Fig. 15b)^[114]. At the same time, the flat and uniform deposition layer morphology reduces the contact between the electrolyte and the negative electrode, which can effectively inhibit the HER reaction and the formation of by-products. Moreover, they explored the mechanism through in situ and ex situ characterization techniques and simulation data. It is found that the N functional groups in the coating can lead to zinc deposition on the (002) crystal face, and the ultra-thin layer can inhibit the occurrence of HER side reaction and the formation of Zn₄(OH)₆SO₄ in weak acid electrolyte. A specific capacity of 112 mAh·g^-1 was finally obtained, as well as a capacity retention of 94% after 300 cycles.

It is well known that the formation of zinc dendrites originates from the non-uniform distribution and nucleation of Zn on the surface of the negative electrode caused by the "tip effect". The zinc nucleation can be optimized and the formation of zinc dendrites can be prevented by improving the uniform local current (or electric field distribution) from the point of view of hydrophilicity, zincphilicity and HER inhibition of the protective layer. To endow the coating with versatility, Zhu et al. Proposed the strategy of constructing a Ti₃C₂T_x-MXene(MX-TMA) coating with a low zinc nucleation barrier on the zinc foil surface to regulate the deposition of dendrite-free zinc metal^[115]. This strategy provides abundant zincphilic sites and better hydrophilicity for the interfacial protective layer, facilitates the transport of Zn²⁺, achieves uniform electrodeposition of zinc, and suppresses interfacial side reactions (Fig. 16)^[59]. DFT theory shows that the strong adsorption between doped naphthoquinone (Nq) and zinc ions is helpful to guide the rapid deposition of zinc ions. Hydrophilicity between the electrode and electrolyte is a key factor affecting impedance, polarization, and kinetics. Compared with bare zinc, MX-TMA @ Zn has better hydrophilicity, which makes the electrolyte better wet the electrode sheet. Chronoamperometry analysis shows that the current density of bare zinc increases with time, and there is a "tip effect" process of zinc deposition, which evolves into the growth of dendrites. However, MX-TMA @ Zn only shows lateral diffusion at the beginning, and then diffuses to each zincphilic site in a three-dimensional form, finally forming a dense zinc layer. At the current density of 2 mA·cm^-2, MX-TMA @ Zn achieves more excellent cycling performance, and the coulombic efficiency can be maintained at 99.7% after 1000 cycles.

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图16 (a)MX-TMA@Zn的制备示意图。(b)锌离子与不同基底的结合能理论计算结果。(c)电解液和MX-TMA@Zn以及裸锌之间的接触角。(d)裸锌和MX-TMA@Zn的循环性能图^[115]

Fig. 16 (a) Schematic diagram of the preparation of MX-TMA@Zn. (b) Theoretical calculation of binding energy of Zn ions with different substrates. (c) Contact angle between electrolyte and MX-TMA@Zn and bare zinc. (d) Cycle performance diagram of bare zinc and MX-TMA@Zn^[115]. Copyright 2022, Elsevier

The creation of a protective layer can be used as one of the strategies for anode modification, and the exploration of new protective layer materials can provide new opportunities to solve the problems existing in the anode. metal-organic framework materials (metal organic frameworks, MOF) have been used in lithium-sulfur batteries due to their high porosity, low density, large specific surface area, and tunable pore size. It has been found that conductive MOF-based materials have high ionic conductivity, large amount of ion transfer, and strong ability in Zn²⁺storage, which has also been proposed to be used in aqueous zinc-ion batteries. Pu et al. Synthesized a series of MOF materials and found that Mn (BTC) (BTC refers to 1,3,5-benzenecarboxylic acid) has excellent ability to store Zn²⁺^[116]. With Mn (BTC) as cathode material and ZIF-8 @ Zn as anode, 92% capacity retention can be obtained after 900 cycles.

After MOF materials, an important three-dimensional ordered material, covalent organic frameworks (COF), has emerged. This is a kind of organic porous crystalline materials connected by light elements (C, H, N, B, etc.) Through covalent bonds, which has the advantages of low density, high specific surface area and easy modification, and shows great potential in energy storage. As an anode protective layer of aqueous zinc-ion battery, COF can simultaneously control the flux of Zn²⁺ and the evolution of H₂. Lan Yagan and Chen Yifa of South China Normal University have prepared a series of zincphilic COFs, which can inhibit the dendrite growth and hydrogen evolution reaction of aqueous zinc-ion batteries^[117]. It is found that Zn-AAn-COF not only has strong adsorption for zinc, but also can effectively remove the free water in the electrolyte. Fourier transform infrared spectroscopy data showed C = C, C — N stretching at 1542 cm^-1 and 1270 cm^-1 and XRD pattern confirmed the presence of AAn-COF structure. After 6000 cycles, the retention rate was still 73. 2%, which achieved the effect of alleviating the hydrogen evolution reaction and inhibiting the zinc dendrite.

The interfacial coating usually consists of an organic protective layer and an inorganic protective layer. The organic interfacial layer has good flexibility and strong mechanical properties, which can adapt to the volume change of zinc anode during cycling and inhibit the growth of zinc dendrite. In addition, organic coatings usually contain zincphilic polar groups, which can induce uniform deposition of zinc ions. Inorganic materials have high stability and corrosion resistance in aqueous electrolyte, and are not easy to dissolve in water. Moreover, the inorganic coating can provide a Zn²⁺ channel, improve the mobility of the Zn²⁺, and promote the uniform nucleation, thereby inhibiting the dendrite growth. To integrate the advantages of organic and inorganic coatings, 2022 Wang et al. Studied an organic-inorganic synergistic protective layer (Nafion/Zn₃(PO₄)₂,NFZP), in which the organic layer (Nafion) is on the top and the inorganic layer (Zn₃(PO₄)₂) is on the bottom^[118]. The special structure and interface effect of this double protective layer can improve the ionic conductivity, stabilize the zinc coating, and inhibit the growth of dendrites and the occurrence of side reactions (Fig. 17).

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图17 裸锌以及NFZP@Zn复合层(a、c)初始和(b、f)50圈后的SEM图,(c、g)XRD,(d、h)EIS,(i)裸锌和NFZP@Zn复合层的镀锌行为示意图^[118]

Fig. 17 SEM images of bare zinc as well as NFZP@Zn composite layer (a, c) initially and (b, f) after 50 turns, (c, g) XRD, (d, h) EIS, (i) Schematic diagram of galvanic behavior of bare zinc and NFZP@Zn composite layer^[118]. Copyright 2022, Elsevier

5.2.2 Structural optimization

At present, there have been a large number of studies on the construction of zinc anode protective layer structure. The three-dimensional structure established on the surface of zinc anode includes fibrous, porous, ridged, etc^[119]. These three-dimensional structures can provide high specific surface area, more active ionic sites, and also enhance the wettability between the electrode and the electrolyte. Compared with 2D planar Zn, 3D Zn anode can alleviate the dendrite growth and improve the electrochemical performance. Self-supporting anode materials can be prepared by electrodeposition of zinc at constant voltage using highly conductive carbon fiber graphite felt (GF) as current collector^[120]. Graphite felt can provide a larger electroactive area, transport electrons faster, and promote zinc to deposit on the substrate faster, so that zinc can be pre-plated in several specific directions to achieve dendrite-free cycling of zinc-ion batteries, thus improving the cycling performance of batteries. The deposited zinc is contained within the graphite felt and grows compactly along the fibers. The thickness of the graphite felt after zinc deposition becomes smaller, which indicates that the problem of dendrite growth on the zinc anode is effectively solved.

It is worth noting that in order to keep up with the increasing pursuit of portable and wearable electronics, the development of flexible and wearable energy storage systems requires good mechanical strength. The ingenious introduction of carbon nanotube (CNT) three-dimensional framework structure on the surface of carbon cloth substrate (CC), as a zinc deposition/dissolution scaffold, can effectively inhibit the generation of zinc dendrites and other by-products, and produce flexible and stable aqueous zinc-ion batteries^[121]. The results show that the construction of three-dimensional CNT framework makes the electric field distribution on the electrode surface more uniform, which is more conducive to the rapid migration of zinc ions at the interface and uniform nucleation. The electrode experienced a rapid voltage drop at the beginning of zinc plating, corresponding to the nucleation process of metallic zinc on the heterogeneous electrode surface.

The CNT-framed electrode exhibited a smoother voltage drop compared to the pristine CC, evidencing a reduced nucleation overpotential and enhanced affinity with zinc. Even at higher current densities, the nucleation potential can still be found to be much higher, which may reduce the initial zinc nucleus size and further inhibit the formation of zinc dendrites. This indicates that the absence of zinc dendrites on the surface of the 3D CNT frame electrode is mainly due to its more uniform electric field distribution. The introduction of CNT three-dimensional framework interlayer can effectively improve the electrochemical cycle stability of metal zinc anode. The performance of the cell assembled with Zn/CNT was tested, and the cycle stability could still reach 110 H at a current density of 5 mA·cm^-2, and the excellent coulombic efficiency could be maintained. It retains 88.7% capacity after 1000 cycles and has significant mechanical flexibility.

5.2.3 Electrolyte optimization

In order to solve the problems of side reactions such as dendrite and zinc deposition on the zinc anode, in addition to adding coatings on the surface of the zinc anode or constructing other structures for optimization, the coulombic efficiency of the zinc electrode can also be enhanced from the aspect of electrolyte^[122]. The feasibility of introducing additives into the electrolyte to inhibit dendrite growth has been confirmed by many studies^[59]. These additives include inorganic additives and organic additives, which can be adsorbed on the surface of zinc to adjust the ion flux, so that zinc ions can be deposited uniformly. In addition to inhibiting the dissolution of the positive electrode, it also has a high overpotential for hydrogen evolution and prevents zinc corrosion and dendrite growth caused by zinc deposition. Most of the inorganic additives are metal ion salts, MXenes, graphene oxide, etc. Organic additives, such as polyethylene oxide (PEO), polyacrylamide (PAM), sodium dodecyl benzene sulfonate (SDBS), cetyltrimethylammonium bromide (CTAB), tetrabutylammonium sulfate (TBA₂SO₄), etc., are usually used to change the deposition morphology of zinc^[123]^[124]^[125]^[126]^[127]. The organic additives adsorbed on the zinc electrode can regulate the formation of zinc nuclei by screening the hydrated Zn²⁺ ions in the electrolyte, reducing the diffusion kinetics and the reduction rate of Zn²⁺, and promoting the uniform deposition of zinc and even dendrite-free growth, thus improving the electrochemical stability of zinc negative electrode.

The exploration of electrolyte additives can reduce the cost of zinc plating, improve the efficiency and make the zinc plating more uniform. In 2022, Wu et al. Used cyclohexane dodecanol (CHD), an organic additive, to construct [Zn(H₂O)₅(CHD)]²⁺ complex ions in ZnSO₄ electrolyte to promote the rapid shedding and nucleation of zinc during electroplating^[128]. At the same time, CHD is easy to adhere to the zinc anode to form a protective layer, which can not only effectively prevent the growth of dendrites, but also prevent the occurrence of hydrogen evolution reaction. According to the DFT simulation, the solvation shell of Zn²⁺ in the electrolyte and the surface coverage of zinc electrode are reconstructed simultaneously after the addition of small molecular organic additive CHD. The strong hydrogen bonding effect of the [Zn(H₂O)₅(CHD)]²⁺ adsorbed on the zinc anode on the water molecules effectively reduces the free water near the zinc anode, thus stabilizing the zinc anode.

Recently, some researchers have proposed the concept of a gel electrolyte, which has less free water than the general aqueous electrolyte. Composed of polymer chains, zinc salts, and plasticizers, such as antifreeze hydrogels, colloidal bioelectrolytes, and polyacrylamide gel electrolytes^[130]. This type of gel electrolyte helps to mitigate hydrogen evolution and zinc corrosion, while it also controls the Zn²⁺ flux, making zinc deposit uniformly and inhibiting dendrite growth to some extent. In 2022, Zhang et al. Prepared tannic acid (TA) modified sodium alginate (SA) composite gel electrolyte membrane by ionic crosslinking method^[129]. The phenolic hydroxyl group in tannic acid (TA) chelates with zinc ions (Figure 18 a), while the carboxyl group in sodium alginate (SA) coordinates with it, which can effectively reduce the activity of bound water molecules around the solvation shell of Zn²⁺. Through the enhanced ion confinement effect, the occurrence of side reactions and the formation of zinc dendrites are inhibited, and the uniform deposition of zinc ions is realized. The surface of TA-SA hydrogel (Fig. 18 B and C) is uniform, smooth and regularly arranged in cross section. The negative electrodes TA-SA and SA after the first cycle have relatively smooth surfaces due to the uniform stripping of zinc, while the pits in the liquid electrolyte may be caused by corrosion, uneven and disordered zinc dissolution. In addition, the gel electrolyte has good flexibility, which is a breakthrough for the application demand of wearability and broadens the application fields of aqueous zinc-ion batteries.

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图18 (a)TA-SA水凝胶电解质工作示意图。TA-SA水凝胶电解质的SEM表面图(b)、截面图(c),Zn/NH₄V₄O₁₀电池在(d)2 A·g^-1,(e)0.5 A·g^-1及零度时的循环性能^[129]

Fig. 18 (a) Schematic diagram of the operation of TA-SA hydrogel electrolyte. SEM surface view (b), cross-sectional view (c) of TA-SA hydrogel electrolyte, cycling performance of Zn/NH₄V₄O₁₀ cell at (d) 2 A·g^-1, and (e) 0.5 A·g^-1 under 0 ℃^[129]. Copyright 2022, Elsevier

There is an interaction between the aqueous electrolyte and the zinc anode, and the free water in it will have a serious impact on dendrite and hydrogen evolution, resulting in a rapid decline in battery performance. One of the effective solutions is to avoid the use of water molecules or to reduce the reaction in the electrolyte. Considering the safety and sustainability of batteries, Han et al. Developed a low-cost, non-flammable hydrated organic electrolyte Zn(BF₄)₂/EG, which promotes the formation of ZnF₂ passivation layer, avoids the growth of dendrites, and can operate at -30 ~ 40 ℃ and maintain good performance^[131]. Zinc sheets immersed in hydrated Zn(BF₄)₂ acid solution will corrode or even perforate, while when ethylene glycol is used as a solvent, because of its high boiling point and low freezing point, once the glass fiber diaphragm is saturated with ethylene glycol, it will immediately catch fire. In the comparative experiments, it was found that the 4 mol/L Zn(BF₄)₂/EG solution was non-flammable, the voltage hysteresis of the symmetric cell using 4 mol/L Zn(BF₄)₂/EG electrolyte was larger, meanwhile, the number of Zn²⁺ transfer was larger, and the corresponding symmetric cell showed better stability.

Correlating the properties of ligands with electrochemical performance allows one to explore the general principles of ion solvation chemistry for electrolyte design by varying the ligands. In 2022, Yang Quanhong et al. Of Tianjin University proposed the stability constant (K) as a general criterion for selecting ligands in electrolytes to adjust the solvation of zinc ions and improve the stability of zinc anode, and confirmed the effect of K on corrosion current density and nucleation overpotential theoretically and experimentally^[132]. The EDTA-based electrolyte with a large K value was selected, and the zinc anode was stably cycled at a high current density of 5 mA·cm^-2 for 3000 H, which indirectly indicated that the growth of zinc dendrite was inhibited. in order to integrate the strategies of structural design, interface modification and electrolyte optimization, Li et al. Developed a new three-dimensional electrode system, "all in one" (AIO), which integrates electrolyte and zinc anode^[133]. The electrode can effectively reduce free water, inhibit the growth of zinc dendrite, increase the contact area to promote the transfer of ions, accelerate the redox rate, and enhance the stability of zinc plating/stripping, thereby greatly improving the zinc anode material. Cu foam @ Zn was obtained by electroplating using copper foam sheet and zinc sheet as working electrode and contrast electrode. XRD characterization confirmed that there was no by-product formation. Then, palygorskite powder was added to the sodium alginate to obtain a mixed suspension, and Cu foam @ Zn and zinc sheets were placed again at the positions of the working electrode and the reference electrode, and the ions were crosslinked (fig. 19). Cu foam @ Zn can be tightly combined with gel, and this three-dimensional electrode (AIO) directly integrates the negative electrode, electrolyte and separator, which promotes the development of zinc-based batteries.

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图19 (a)制备AIO三维电极系统的操作步骤,(b)AIO电极的横断面照片,(c)扫描电镜图像,(d)利用AIO三维电极系统制作的扣电^[133]

Fig. 19 (a) Operating steps for preparing the AIO 3D electrode system, (b) cross-sectional photograph of the AIO electrode, (c) scanning electron microscope image, (d) buckling electrode fabricated using the AIO 3D electrode system^[133]. Copyright 2021, Oxford Univ Press

6 Conclusion and prospect

In a word, aqueous zinc-ion batteries have great potential for development because of their advantages of safety, environmental protection and low cost. As an efficient energy storage system, water-based zinc-ion battery has achieved many breakthroughs and is expected to be better developed in the near future. However, the practical application and commercialization of aqueous zinc-ion batteries are hindered by a series of derived problems in the charge-discharge process of the cathode, electrolyte and anode. In this review, the factors that hinder the development of aqueous zinc-ion batteries are introduced in detail, including the dissolution of the positive electrode, the electrostatic interaction between ions, the side reaction of electrochemical reaction, the zinc dendrite, and the passivation and corrosion of the negative electrode. The optimization strategies for the components of the battery system are discussed in depth, and the prospects are put forward from the following aspects.

6.1 Develop more advantageous cathode materials

In order to solve the problems of positive electrode dissolution and structural collapse, metal ions and water molecules are added to enhance the structural stability, so that the interlayer spacing is more firm, and the zinc ions can be removed/embedded more quickly. The surface of the positive electrode can also be coated with a conductive material enhanced by a carbon material, or the surface of the positive electrode can be coated with a conductive material to improve the conductivity of the surface of the negative electrode. The addition of conducting polymer can either improve the dissolution or enhance its conductivity, which is beneficial to the transport of Zn²⁺. In addition, defect engineering is mainly the generation of defects, which is beneficial to promote the diffusion and adsorption of reactants, and these defects can be used as hosts to improve the conductivity, so the introduction of defects can optimize the electrode. The voltage of the cathode materials used at present is not high. To improve their practical application, it is very important to increase the voltage, such as cobalt (Co), nickel (Ni), etc. To improve the voltage and capacity at the same time, it can be considered to introduce a structure combining two metals at the same time to achieve the integration of advantages.

6.2 Optimized electrolyte

The common problem of aqueous electrolyte is that it contains free water, because a large amount of free water will cause some irreversible reactions in the positive or negative electrode, which will directly affect the performance of the battery. The production of gel electrolyte can alleviate such problems, but the process of gel electrolyte is too complex and costly, and its mechanical and ductile properties as a flexible device still need to be enhanced^[134]. The development of metal ion gel electrolytes with conductive properties can not only take advantage of its framework function, but also improve the conductivity of Zn²⁺, thus reducing the ion transport gap and the formation of dendrites at high current densities. The utilization of zinc can be improved by constructing novel solvation structures in the electrolyte, which provides a new strategy for enhancing the energy density of aqueous zinc batteries^[135]. In addition, the electrochemical window can also be effectively widened by using the composite electrolyte^[136,137]. These measures are of great significance to the development of aqueous zinc-ion batteries.

6.3 Improving the surface of zinc anode and developing new anode materials

The stability problem of zinc anode in aqueous zinc-ion battery has not been well solved, which has become an obstacle to its commercialization and large-scale application. Problems such as dendrite growth due to limited diffusion of Zn²⁺, corrosion due to hydrogen evolution and water splitting also need to be solved. The nucleation and growth of zinc can be controlled by constructing a heterostructure interface, and the deposition and dissolution of zinc without dendrite growth can be realized at a higher capacity^[138]. At present, most of the strategies are aimed at the behavior of zinc deposition in electrolyte, but the interfacial reaction between electrolyte and anode has not been studied so far, so it is necessary to discuss the interfacial layer. By constructing a composite negative electrode of alloyed zinc, the alloyed zinc and the electrolyte act synergistically to enhance the regulation of zinc deposition. We can also start from the electrolyte to explore an electrolyte that can act on both the positive and negative electrodes, which can not only regulate the deposition behavior of zinc ions on the surface of the zinc negative electrode, but also act on the positive electrode interface to improve the cycle stability of the positive electrode material. In addition, the electrolyte can effectively control the free water and significantly inhibit the side reaction caused by the free water.

6.4 Research and development of high performance diaphragm materials

At present, there is little research on the separator of aqueous zinc-ion battery, and the separator material is an important part of the battery. Selecting the separator material with low resistance and high ion transmission capacity is conducive to separating the dissolved active material of the positive electrode and the growth of dendrites, so as to reduce the energy loss of the battery during high current discharge^[139,140]. At present, the glass fiber separator is the most widely used, but a small amount of metal ions in the glass fiber will increase the self-discharge of the battery, and the separator is thick and easy to adhere to the electrode, which is difficult to remove, and has a certain impact on the subsequent characterization test. From the aspects of cost saving and process optimization, the performance of the battery can be further improved by modifying polypropylene fiber, polyamide fiber and composite fiber to develop new separators more suitable for aqueous zinc-ion batteries.

In a word, there is still a long way to go to realize the large-scale commercial application of aqueous zinc-ion batteries. It is believed that the AZIBs technology with excellent performance will eventually be widely used and sustainably developed in large-scale energy storage and other fields through the design and continuous exploration of electrode materials, electrolytes, separators, etc^[141,142].

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Outlines

模态框（Modal）标题

Abstract

Cite this article

1 Introduction

表1 水系锌离子电池与非水系锂/钠/钾离子电池的对比[4]

图1 水系锌离子电池的构造示意图及存在的主要问题

2 Dissolution problem of cathode material

2.1 Manganese based material

2.1.1 Manganese ion addition strategy in electrolyte

图2 MnO2电极在2 mol/L ZnSO4水电解质中添加和不添加0.1 mol/L MnSO4添加剂的(a)CV曲线和(b)分别在C/3和1 C下的循环性能图[24]。(c)多孔ZnMn2O4正极材料第一圈之后在三种不同电解质中的充放电曲线及(d)相应的循环性能[25]

2.1.2 Positive electrode protection layer strategy

图3 (a)具有氧空位β-MnO2@Graphene氧化物正极材料示意图[27]。(b)MnO2/Mn2O3@PPy复合材料在水系锌离子电池中的应用[28]。(c)超薄聚苯胺涂层的单晶纳米椭圆体电极材料[29]。(d)针状微纳PDA@MnO2@NMC复合材料[30]

2.1.3 Artificial construction strategy of electrolyte interface

图4 (a)α-MnO2和CMO的反应机理[31]。(b)CEI/H2O界面相互作用能的理论计算,充放电曲线及循环稳定性[13]。(c)利用黏合剂原位生长CEI示意图[32]

2.1.4 Defect Engineering Strategy

图5 (a)H+扩散到具有完美结构和氧缺陷结构的KMO中的示意图[36]。(b)氧空位缺陷产生示意图[37]。(c)氧空位缺陷ZMO纳米管阵列合成示意图[38]

图6 (a)Mn2O3和NM20的电荷分布。(b)Mn2O3和NM20系列材料的循环性能[42]

2.2 Vanadium-based material

2.2.1 Cation pre-intercalation strategy

2.2.2 Electrolyte optimization strategy

2.2.3 Solid Electrolyte Interface Construction Strategy

图9 (a)原始ZVO正极和通过原子层沉积涂覆HfO2的ZVO的制造过程的示意图,(b)HfO2涂层在ZVO上的高角环形暗场扫描透射图[53]。(c)原位CEI层策略设计的示意图[54]。(d)制备V2O5@PEDOT/CC 的示意图[55]

2.3 Sum up

3 Electrostatic interaction between ions

图10 (a)MVO和VO电极在0.1 A·g-1电流下的循环性能图,(b)不同电流密度下的MVO和VO电极的倍率性能图,(c)溶解Zn2+的嵌入/脱出机理示意图[63]。(d)NaV3O8晶体结构,(e)β-Na0.33V2O5晶体结构[66]

图11 CeCl3添加剂对锌沉积过程的影响示意图[69]

图12 镀锌示意图和不同负极对称电池的性能比较[70]

4 Oxygen/hydrogen evolution reaction

4.1 Oxygen evolution reaction

图13 基于KFSI电解液的物理化学性能比较。(a)不同KFSI电解液浓度下的电化学稳定窗口。正极(b)和负极(c)附近区域的放大图。(d)电池在2.3 V电压及1 A/g电流密度时的容量保持率和库仑效率[86]

4.2 Hydrogen evolution reaction

5 Side Reaction Problems of Zinc Anode

5.1 Corrosion, passivation and zinc dendrite

图14 使用多孔隔膜对锌枝晶生长、溶解和再生长的研究(A)200 s、(B)300 s、(C)430 s、(D)590 s和(E)890 s时锌枝晶的生长,(F)120 s、(G)240 s、(H)680 s时的锌枝晶溶解,(I)304 s和(J)656 s时的锌枝晶再生[106]

5.2 Cathode modification

5.2.1 Surface modification

图15 (a)裸锌负极和纳米CaCO3涂层负极的形貌演化图[113]。(b)超薄氮(N)掺杂氧化石墨烯(NGO)层的复合锌金属负极示意图[114]

图16 (a)MX-TMA@Zn的制备示意图。(b)锌离子与不同基底的结合能理论计算结果。(c)电解液和MX-TMA@Zn以及裸锌之间的接触角。(d)裸锌和MX-TMA@Zn的循环性能图[115]

图17 裸锌以及NFZP@Zn复合层(a、c)初始和(b、f)50圈后的SEM图,(c、g)XRD,(d、h)EIS,(i)裸锌和NFZP@Zn复合层的镀锌行为示意图[118]

5.2.2 Structural optimization

5.2.3 Electrolyte optimization

图18 (a)TA-SA水凝胶电解质工作示意图。TA-SA水凝胶电解质的SEM表面图(b)、截面图(c),Zn/NH4V4O10电池在(d)2 A·g-1,(e)0.5 A·g-1及零度时的循环性能[129]

图19 (a)制备AIO三维电极系统的操作步骤,(b)AIO电极的横断面照片,(c)扫描电镜图像,(d)利用AIO三维电极系统制作的扣电[133]

6 Conclusion and prospect

6.1 Develop more advantageous cathode materials

6.2 Optimized electrolyte

6.3 Improving the surface of zinc anode and developing new anode materials

6.4 Research and development of high performance diaphragm materials

References

表1 水系锌离子电池与非水系锂/钠/钾离子电池的对比^[4]

图2 MnO₂电极在2 mol/L ZnSO₄水电解质中添加和不添加0.1 mol/L MnSO₄添加剂的(a)CV曲线和(b)分别在C/3和1 C下的循环性能图^[24]。(c)多孔ZnMn₂O₄正极材料第一圈之后在三种不同电解质中的充放电曲线及(d)相应的循环性能^[25]

图3 (a)具有氧空位β-MnO₂@Graphene氧化物正极材料示意图^[27]。(b)MnO₂/Mn₂O₃@PPy复合材料在水系锌离子电池中的应用^[28]。(c)超薄聚苯胺涂层的单晶纳米椭圆体电极材料^[29]。(d)针状微纳PDA@MnO₂@NMC复合材料^[30]

图4 (a)α-MnO₂和CMO的反应机理^[31]。(b)CEI/H₂O界面相互作用能的理论计算,充放电曲线及循环稳定性^[13]。(c)利用黏合剂原位生长CEI示意图^[32]

图5 (a)H⁺扩散到具有完美结构和氧缺陷结构的KMO中的示意图^[36]。(b)氧空位缺陷产生示意图^[37]。(c)氧空位缺陷ZMO纳米管阵列合成示意图^[38]

图6 (a)Mn₂O₃和NM20的电荷分布。(b)Mn₂O₃和NM20系列材料的循环性能^[42]

图9 (a)原始ZVO正极和通过原子层沉积涂覆HfO₂的ZVO的制造过程的示意图,(b)HfO₂涂层在ZVO上的高角环形暗场扫描透射图^[53]。(c)原位CEI层策略设计的示意图^[54]。(d)制备V₂O₅@PEDOT/CC 的示意图^[55]

图10 (a)MVO和VO电极在0.1 A·g^-1电流下的循环性能图,(b)不同电流密度下的MVO和VO电极的倍率性能图,(c)溶解Zn²⁺的嵌入/脱出机理示意图^[63]。(d)NaV₃O₈晶体结构,(e)β-Na_0.33V₂O₅晶体结构^[66]

图11 CeCl₃添加剂对锌沉积过程的影响示意图^[69]

图12 镀锌示意图和不同负极对称电池的性能比较^[70]

图13 基于KFSI电解液的物理化学性能比较。(a)不同KFSI电解液浓度下的电化学稳定窗口。正极(b)和负极(c)附近区域的放大图。(d)电池在2.3 V电压及1 A/g电流密度时的容量保持率和库仑效率^[86]

图14 使用多孔隔膜对锌枝晶生长、溶解和再生长的研究(A)200 s、(B)300 s、(C)430 s、(D)590 s和(E)890 s时锌枝晶的生长,(F)120 s、(G)240 s、(H)680 s时的锌枝晶溶解,(I)304 s和(J)656 s时的锌枝晶再生^[106]

图15 (a)裸锌负极和纳米CaCO₃涂层负极的形貌演化图^[113]。(b)超薄氮(N)掺杂氧化石墨烯(NGO)层的复合锌金属负极示意图^[114]

图16 (a)MX-TMA@Zn的制备示意图。(b)锌离子与不同基底的结合能理论计算结果。(c)电解液和MX-TMA@Zn以及裸锌之间的接触角。(d)裸锌和MX-TMA@Zn的循环性能图^[115]

图17 裸锌以及NFZP@Zn复合层(a、c)初始和(b、f)50圈后的SEM图,(c、g)XRD,(d、h)EIS,(i)裸锌和NFZP@Zn复合层的镀锌行为示意图^[118]

图18 (a)TA-SA水凝胶电解质工作示意图。TA-SA水凝胶电解质的SEM表面图(b)、截面图(c),Zn/NH₄V₄O₁₀电池在(d)2 A·g^-1,(e)0.5 A·g^-1及零度时的循环性能^[129]

图19 (a)制备AIO三维电极系统的操作步骤,(b)AIO电极的横断面照片,(c)扫描电镜图像,(d)利用AIO三维电极系统制作的扣电^[133]