Comprehensive review on zinc‐ion battery anode: Challenges and strategies

Abstract

Zinc‐ion batteries (ZIBs) have been extensively investigated and discussed as promising energy storage devices in recent years owing to their low cost, high energy density, inherent safety, and low environmental impact. Nevertheless, several challenges remain that need to be prioritized before realizing the widespread application of ZIBs. In particular, the development of zinc anodes has been hindered by many challenges, such as inevitable zinc dendrites, corrosion passivation, and the hydrogen evolution reaction (HER), which have severely limited the practical application of high‐performance ZIBs. This review starts with a systematic discussion of the origins of zinc dendrites, corrosion passivation, and the HER, as well as their effects on battery performance. Subsequently, we discuss solutions to the above problems to protect the zinc anode, including the improvement of zinc anode materials, modification of the anode–electrolyte interface, and optimization of the electrolyte. In particular, this review emphasizes design strategies to protect zinc anodes from an integrated perspective with broad interest rather than a view with limited focus. In the final section, comments and perspectives are provided for the future design of high‐performance zinc anodes.

INTRODUCTION

Increasing energy demand and aggravating environmental pollution have led to the development of renewable and clean energy. However, the rational and efficient storage and utilization of renewable energy remains challenging because of the instability of energy access and output.^1–3 Among the currently available energy storage technologies, batteries are capable of reversibly converting chemical energy into electrical energy and are regarded as the most promising and efficient energy storage technology owing to their high power and energy densities, low cost, simple manufacturing process, and long cycle life.^4–6

Among the various rechargeable batteries, lithium-ion batteries (LIBs) are currently the dominant type used in electric vehicles and mobile electronic devices owing to their long life, high energy density, and superior power density.^7–12 However, the progress and development of LIBs have been impeded by the scarcity and high price of lithium metal, as well as the toxicity and flammability of the electrolyte.^13,14 Compared with conventional LIBs, zinc-ion batteries (ZIBs) have received increasing attention owing to their distinct merits, such as nontoxicity, low price, abundance resources, low redox potential (−0.76 V vs. standard hydrogen electrode [SHE]), superior theoretical specific capacity (820 mAh g⁻¹), and low environmental impact.¹⁵

The structure of ZIBs, analogous to that of LIBs, consists of four components: the cathode, electrolyte, separator, and anode.^6,16–18 The cathode is composed of a binder, conductive agent, and active material, which is typically a metal oxide with a tunnel or layer structure. The electrolyte acts as reservoir for zinc ions shuttling between the cathode and anode, playing a crucial role in defining the electrochemically stable potential window and ionic conductivity. The separator mainly prevents short circuits caused by contact of the cathode and anode while leaving transport tunnels for zinc ions to pass through. The anode is generally a pure zinc plate or porous zinc powder electrode. Similar to LIBs, ZIBs are a “rocking chair battery”, where the charge storage process depends on the transport mobility of zinc ions between the cathode and anode (Figure 1A). There are four main energy storage mechanisms in ZIBs: (1) insertion–extraction^21–25; (2) displacement–intercalation reaction^26–28; (3) anion redox conversion^28–31; and (4) dissolution–deposition.^32–36 In the past few decades, the electrochemical performance of zinc cathode materials has been greatly improved through unremitting exploration; however, the practical application of anode materials remains in its infancy owing to insufficient understanding of the anodic reaction mechanism and issues arising from the zinc anode, which have become a bottleneck for the commercialization of ZIBs.^37–41 Charge storage in ZIB anodes can be summarized as a reversible zinc ion deposition/stripping mechanism.⁴² Normally, zinc ions are reduced and deposited on the zinc anode surface during the charging process of ZIBs. During the discharging process, the zinc ions on the anode surface are stripped and oxidized to soluble zinc ions, which migrate through the electrolyte toward the cathode owing to the electric field.^8,31 However, the deposition and stripping of zinc ions do not occur in a single region on the surface of the zinc anode, and zinc ions are more likely to be deposited on the threaded dislocations on the zinc anode surface owing to the effect of dislocations on the electric field distribution, which leads to the formation of zinc dendrites.^32,43 The growth of zinc dendrites not only aggravates side reactions (i.e., corrosion and the hydrogen evolution reaction [HER]) by enlarging the exposed zinc anode area, lowers the battery capacity, and increases the impedance, but also prompts the system into a vicious cycle where the expanded interface further stimulates by-product formation, incubates additional zinc dendrite nucleation sites⁴⁴ and ultimately leads to battery breakdown (Figure 1B).⁴⁵ Therefore, if these problems can be relieved or even eliminated through reasonable strategies, the performance of ZIBs will be greatly improved, and the industrialization of ZIBs will make a great step forward.^4,5,46–48

FIGURE 1. (A) Schematic of the structure and working principles of ZIBs. Reproduced with permission.19 Copyright 2019, Elsevier. (B) Diagram of the effects that degrade zinc anode performance. Reproduced with permission.20 Copyright 2020, Wiley-VCH

This article is divided into two major sections describing the main obstructions faced by ZIB anodes and their corresponding optimization strategies. The first part provides a comprehensive analysis of the causes of zinc dendrites, corrosion passivation, and the HER, along with their adverse effects on the performance of the batteries. In the second part, recently reported strategies to solve the above issues are categorized into three sections for detailed discussions: (1) investigation and design of zinc subject materials, (2) modification of the electrolyte–anode interface, and (3) optimization of the electrolyte system. In addition to a critical review of recent progress, we tentatively propose several pivotal perspectives for future breakthroughs in high-performance ZIB anodes at the end of this review.

ISSUES CONFRONTING ZINC ANODES Causes and effects of zinc dendrites

The metal dendrite problem is a notorious issue commonly encountered in the battery industry. Ever since the commercialization of LIBs, this phenomenon has remained a non-negligible concern, and it is an even larger problem for the relatively young ZIB technology. Fortunately, the academic community has established a well-developed understanding of the zinc dendrite growth mechanism, based on which precise resolution of the issue is now possible. It is commonly recognized that zinc dendrites are caused by inhomogeneous nucleation due to nonideal surface properties, such as a nonuniform electric field, nonuniform ion distribution, and unrestricted in-plane migration of surface-adsorbing zinc ions. The growth of zinc dendrites can be described as follows (Figure 2A–D): (A) during the initial stage of battery cycling, zinc ions are generally reduced at energetically favorable charge transfer sites, forming small zinc bumps on the anode surface; (B) zinc ions are inclined to aggregate on these bumps owing to the lower surface energy, forming the initial dendrites²⁰; (C) the “tip effect” of dendrites enhances the local electric field and further promotes dendrite growth at the surface. (D) zinc dendrites gradually grow, eventually puncturing the separator.^43,50 Without interference, zinc dendrites will continue growing to a large volume until “dead zinc” emerges as the dendrites eventually fracture and peel off from the electrode. Furthermore, because relatively high current densities are more likely to induce nonuniform electric field and ion distributions, zinc dendrites can be slightly suppressed by using low current densities, while they grow rapidly and cause short circuits in the battery at high current densities (Figure 2E–L).⁴⁹ Therefore, smooth electrodes, defect-free surfaces, and well-dispersed nucleation sites are key factors for uniform nucleation and are desirable for ZIB applications.

FIGURE 2. (A–D) The growth mechanism of zinc dendrites. Reproduced with permission.43 Copyright 2018, Wiley-VCH. (E–G) Atomic force microscopy (AFM) topography maps of the Zn electrodes cycled under constant capacity of 0.1 mAh cm−2 at 1, 5, and 10 mA cm−2 to investigate the nucleation behavior at the preliminary stage. (H–J) AFM maps of Zn the electrode cycled to a higher capacity of 0.5 mAh cm−2 to investigate the growing behavior of Zn dendrites under different current densities. (K,L) The evolution of the height distribution of the electrode surface in terms of current density. Reproduced with permission.49 Copyright 2019, Wiley-VCH

Because of the inhomogeneous deposition of zinc ions on the electrode surface, zinc dendrites are influenced by the electrode interface where zinc is deposited, electric field density distribution, ion concentration distribution, and nucleation sites.⁵¹ The loose structure and rough surface formed by zinc dendrites increase the specific surface area of the anode and further promote side reactions, such as corrosion and the HER. As these dendrites gradually accumulate and grow, they increase the thickness of the battery and risk puncturing the battery diaphragm, ultimately causing a short circuit.⁵¹ Worse still, “dead zinc” forms if these loose dendrites fracture and disconnect from the current collector, substantially reducing the coulombic efficiency and capacity that the zinc anode can deliver. In addition, the fractured dendrites become dispersed in and react with the electrolyte to generate by-products,⁵² increasing the impedance of the battery and further decreasing its coulombic efficiency.

Causes and effects of corrosion passivation

The corrosion of anodic zinc in electrolytes is mainly divided into chemical and electrochemical corrosion.⁵³ In alkaline media, chemical corrosion-induced passivation dominates on the zinc anode surface. At pH 14, the standard redox potential of Zn/ZnO (−1.26 V vs. SHE, Equation 1) is below the potential of the HER (−0.83 V vs. SHE, Equation 2):[Image Omitted. See PDF] [Image Omitted. See PDF]Therefore, chemical corrosion, namely, the reaction of Zn with water to produce ZnO and H₂, possesses a positive electromotive force and is thermodynamically favorable. Chemical corrosion roughens the anode surface, which further facilitates parasitic reactions, while the resulting ZnO by-product of this corrosion increases the internal resistance of ZIBs, passivates surface activity to a large extent, and eventually leads to capacity fading.

In contrast, although mild chemical corrosion is present in neutral or acidic electrolytes, electrochemical corrosion becomes the major mechanism for surface passivation. Taking the ZnSO₄ electrolyte as an example, during the charging and discharging processes of the battery, the main reactions occurring at the ZIB anode surface are as follows^54,55:

Zn deposition:[Image Omitted. See PDF]HER:[Image Omitted. See PDF]Side reaction:[Image Omitted. See PDF]Metallic Zn is oxidized to Zn²⁺ during discharge and attracts anions in the electrolyte (e.g., OH⁻ and SO₄²⁻); the two side reactions produce electrochemical corrosion products (Figure 3). Similar to its chemical counterpart, electrochemical corrosion of active zinc results in a severe deterioration of the charge/discharge efficiency and capacity of ZIBs. The formation of by-products (e.g., ZnSO₄Zn(OH)₂₃·xH₂O) inevitably consumes the active zinc ions and electrolyte, which leads to capacity fading to a certain extent. Moreover, these insoluble by-products adhere to the electrode surface, not only covering some active nucleation sites, but also forming rough bumps on the surface of the zinc anode; this induces a nonideal surface and promotes the formation of zinc dendrites. Furthermore, owing to their poor conductivity, these by-products increase the impedance of the entire battery, slowing the transport of electrons and ions at the anode, thus leading to passivation of the anode and further reduction in the coulombic efficiency of the battery.⁸ Hydrogen by-products can be generated by both chemical and electrochemical corrosion. However, gaseous hydrogen plays a very different role than that of the solid by-products mentioned above. Thus, it is discussed separately in Section 2.3.

FIGURE 3. (A) In-situ optical microscopy picture of electrolyte-zinc interface showing bubble formation due to parasitic HER in 0.5 M LiTFSI + 0.5 M Zn (TFSI)2 at 0.2 mA cm−2. Schematic diagrams illustrating (B) the corrosion reaction and HER at Zn anode surface in an aqueous electrolyte and (C) their unfavorable influences at electrolyte-zinc anode interface. Reproduced with permission.44 Copyright 2019, Elsevier

Causes and effects of the electrochemical HER

As shown in Section 2.2, the HER tends to occur in alkaline electrolytes before the deposition of zinc. In contrast, despite the thermodynamics in neutral and slightly acidic media (${E_{H^{+}}}^{\mathrm{\theta }}$(0 V) > ${E_{{\mathit{Zn}}^{2+}}}^{\mathrm{\theta }}$ (−0.76 V)), the reduction of Zn²⁺ generally occurs in advance of the HER owing to sluggish surface kinetics, the low activity of protons, and the high HER overpotential at the zinc surface. However, this does not mean that the HER is trivial under neutral conditions. Under certain circumstances, such as a large polarization during charging, high current density, and low potential, the HER still occurs within several minutes (Figure 3A).^31,44,56

The HER has the following effects on the performance of batteries: (1) the irreversible HER, which is competitive with reversible zinc deposition, may reduce battery reversibility and coulombic efficiency because the HER continuously consumes the zinc metal anode, reducing the coulombic efficiency and capacity of the battery; (2) flammable H₂ gas from the HER increases the internal pressure of the battery, which eventually leads to cell inflation and even explosion⁵⁷; (3) the HER depletes protons near the electrolyte–zinc interface, creating a local alkaline environment. This generates corrosive by-products and a passivation layer, eventually leading to battery performance degradation/breakdown.⁵⁸

RESEARCH AND DESIGN OF ZINC SUBJECT MATERIALS

In principle, the structure of the zinc anode defines its properties; therefore, modification of the main body of the zinc anode is a logical first step to improve anode performance.

Generally, the design principles of anode materials include the following requirements: (a) provide a stable framework to support the long-term plating/stripping cycles of zinc ions without collapse or critical shape change⁵⁹; (b) offer affluent interspace with interconnected tunnel pathways for fast transport and storage of ions and anions⁶⁰; (c) afford a large surface area with effective reactive sites to increase the contact area between the anode and electrolyte and induce uniform zinc deposition⁶¹; (d) possess excellent conductivity and hydrophilicity to promote infiltration of the electrolyte and complete plating/stripping cycles⁶²; and (e) remain physically and chemically stable in acidic or alkaline electrolytes with the capability to form a solid electrolyte interphase (SEI) on the surface⁶³. According to the above principles, we classify the structure design strategies of zinc anodes into three categories: (1) construction of three-dimensional (3D) anodes, (2) fabrication of zinc alloy anodes, and (3) addition of additives to the anode.

Construction of 3D anodes

Because of the limited specific surface area and insufficient contact with the electrolyte, planar zinc anodes are prone to a nonuniform current density and ion concentration, which lead to the formation of zinc dendrites and limit the further development of high-performance ZIBs. In contrast to planar anodes, 3D anodes afford highly stable skeleton structures, which prevent shape change or structural collapse of the anode during the plating and stripping cycles of zinc⁵⁹; provide broader pathways, which facilitate the transport of charges and ions,²⁰ and offer a large reactive surface area and abundant nucleation sites, which induce the uniform distribution of zinc ions.^20,64 As a result, 3D anodes increase the utilization efficiency of zinc anodes,⁶⁵ inhibit the growth of zinc dendrites,⁶⁶ lower the overpotential,⁶⁶ and improve the overall electrochemical performance of ZIBs. Hence, replacing a flat electrode with a 3D anode is a superior and straightforward method for increasing the area of the electrolyte-anode interface.

Use of zinc metal as the anodes subject

The essential goal of a 3D anode is to increase the exposed and available surface area of the active material such that more nucleation sites are accessible, and a lower local current density can be controlled to promote a uniform dendrite-free zinc plating process. Accordingly, Rolison et al.⁴⁷ developed a 3D sponge zinc anode by mixing zinc powder, carboxymethylcellulose, and sodium dodecyl sulfate, which delivered >90% of the theoretical depth of discharge in primary (single-use) cells.

In addition, Guo et al.⁶⁷ developed a 3D porous dual-channel pathway (DCP) zinc electrode with well-balanced ion/electron conduction for highly reversible dendrite-free zinc plating/stripping, which significantly suppressed the occurrence of undesirable reactions compared with that of conventional zinc foils (Figure 4A). These advantages were demonstrated by the reduced overpotential of symmetric batteries and substantial extension of the plating/stripping lifetime (1400 h at 0.1 mAh cm⁻² and 200 h at 10 mAh cm⁻²).

FIGURE 4. (A) Illustrative comparison of the repeated plating/peeling behavior of the original Zn foil and DCP-Zn. Reproduced with permission.67 Copyright 2020, Elsevier. (B–G) Scanning electron microscope picture of zinc anode obtained by different substrate deposition. Reproduced with permission.68 Copyright 2019, Wiley-VCH. Reproduced with permission.69 Copyright 2019, The American Chemical Society. Reproduced with permission.70 Copyright 2019, The American Chemical Society. Reproduced with permission.71 Copyright 2021, Wiley-VCH. Reproduced with permission.72 Copyright 2017, Wiley-VCH. Reproduced with permission.73 Copyright 2018, The Royal Society of Chemistry. Reproduced with permission.74 Copyright 2018, The Royal Society of Chemistry

Use of other collectors as the anode subject

In addition, 3D anodes can be realized by electrodeposition on different current collectors, including various metal materials such as porous copper,⁵⁹ copper foam,^62,69 3D porous Ti,⁷¹ steel mesh,³⁵ tin host,⁷⁵ metal–organic frameworks (MOFs) derived materials,^76,77 zinc foam,⁶⁸ Mxene,^70,78,79 graphene materials,^{71,78,80–84} carbon cloth (CC),⁸⁵ carbon nanotubes yarns,⁸⁶ graphite felt (GF),⁸⁷ carbon fiber (CF),⁷³ and carbon nanotubes (CNTs)⁷⁴ (Figure 4B–G). For example, Wang et al.⁸⁷ prepared a novel 3D zinc anode on highly conductive GF. The GF support material provided a larger active area for zinc deposition, reduced the voltage hysteresis and charge transfer resistance during cycling, and achieved a dendrite-free zinc anode with high cycling stability. In addition to the absence of dendrites, 3D zinc anodes plated on current collectors with excellent mechanical strength have broad prospects in flexible wearable devices.⁸⁵ According to previous studies, the substrates of these electrodes share some common traits. First, the substrate should be flexible, bendable, and have sufficient mechanical strength to support and maintain the 3D structure. Second, the substrate material should possess a zincophilic surface that exhibits favorable compatibility with zinc.^88,89 Finally, the substrate should be inert to reactions with the electrolyte and should have excellent stability in the electrolyte. It is notable that while the adoption of the above 3D-structured zinc anodes reduces the local current density to avoid dendrite growth, it inevitably introduces an unintended increase in the HER due to the increased contact surface with the aqueous electrolyte.

Fabrication of zinc alloy anode

As previously mentioned, corrosion is one of the major drawbacks limiting the application of zinc anodes. The alloying strategy incorporates an anti-corrosion metal with the vulnerable zinc metal to attain a stable structure against mildly corrosive electrolytes. Specifically, the fabrication of alloy anodes by adding an appropriate amount of metal or organic corrosion inhibitors to the Zn anode not only impedes the formation of unexpected byproducts, such as Zn₄SO₄(OH)₆·nH₂O,⁹⁰ and shelters the surface of the zinc metal,^91,92 but also induces a uniform distribution of zinc ions, enhances the charge storage capacity,^93,94 decreases the energy of zinc nuclei, and lowers the polarization overpotential.⁹⁵ Various alloy anodes have proven to be capable of boosting the electrochemical performance of ZIBs, such as CuZn,⁹³ AlZn,^94,96 PbSnZn,⁹⁵ and SnCuZn.⁹⁷ For example, Cai et al. introduced the corrosion-resistant metal Cu into Zn metal anodes to construct Cu/Zn alloy anodes with a compact structure, and the assembled CuZn/Zn electrodes exhibited excellent corrosion resistance (Figure 5A,B). At 1 mA cm⁻² with the capacity controlled to 0.5 mAh cm⁻², the obtained electrodes demonstrated a cycling stability of more than 1500 cycles with almost no change in overpotential after resting for 1 month, while the bare electrodes showed a higher overpotential (>400 mV) and larger voltage fluctuations under the same conditions, demonstrating the importance of corrosion resistance in enhancing the overall performance of the battery (Figure 5C,D).⁹⁸ Beyond the strengthened anti-corrosion properties, it is notable that alloying strategy can confine zinc deposition within the interspacing of nanopatterns formed by other stable metal components of the alloy. For instance, Wang et al.⁹⁶ proposed an effective strategy for alloying the eutectic components of zinc and aluminum to form lamella-nanostructured electrodes.

FIGURE 5. (A) Scheme for Cu/Zn alloy fabrication procedure. (B) Schematic representation of the function of CuZn alloy on CuZn/Zn electrode. (C) Voltage curves of Cu/Zn-30d||Cu/Zn-30d symmetric battery at 1 mA cm−2 and 0.5 mAh cm−2, inset illustrates the high-resolution voltage curves at different times. (D) Linear polarization curves of Cu/Zn and CuZn/Zn electrons within 3 M ZnSO4 electrolyte. Reproduced with permission.98 Copyright 2020, Elsevier. (E) Schematic diagram of eutectic strategy to suppress dendrites and cracks. Reproduced with permission.96 Copyright 2020, Springer Nature. (F) Cycle performance diagram of inorganic additive zinc anode batteries and commercial zinc foil batteries. Reproduced with permission.99 Copyright 2018, Wiley-VCH. (G) Cycling performance diagram of organic additive zinc anode batteries and commercial zinc foil batteries. Reproduced with permission.100 Copyright 2018, The American Chemical Society

The presence of aluminum lamellae in the layered composition of alternating zinc and aluminum nanoflakes can significantly inhibit the corrosion passivation of zinc anodes and reduce the generation of ZnO and Zn(OH)₂ at the anode interface. In addition, the nucleus/shell Al/Al₂O₃ interlayer nanopatterns generated in situ are self-passivated to prevent zinc deposition and thus limit the subsequent growth of zinc within their interspacing (Figure 5E), which enables dendrite-free zinc stripping/plating for more than 2000 h in the electrolyte and remarkable electrochemical performance.

Addition of additives to zinc anodes

In addition to the above two technical routes for the design of zinc anodes, the addition of suitable additives to the zinc anode is considered another vital approach. The additives can be added simply by mixing with powdered zinc or during synthesis (e.g., electrolyte additives for zinc electrodeposition), which inhibits the formation of unexpected products (such as Zn₄SO₄(OH)₆·nH₂O,⁹¹ improves the structural stability of the zinc anode, enhances the contact area between the electrolyte and electrode,¹⁰¹ and cements the electrode particles together to prevent dissolution by the electrolyte.⁴³ For example, the addition of conductive carbon additives not only increases the electrical conductivity of the anode but also allows more uniform deposition of zinc during cycling. Here, activated carbon (AC) is used as an example. AC is a specially treated carbon material, which is prepared by heating organic raw materials (fruit shells, coal, wood, etc.) under the condition of air isolation to reduce noncarbon components (this process is called carbonization). Then, after reacting with gas, the surface is eroded to produce a microporous structure (this process is called activation). Because the activation process is particularly microscopic (surface erosion of a large number of molecular carbons is called spot erosion), the AC surface contains numerous tiny pores. In brief, owing to its special treatment reaction process, AC possesses excellent electrical conductivity, a complex and well-developed pore structure, a large specific surface area, a high adsorption capacity, abundant surface chemical groups, and a relatively high specific adsorption capacity. When AC is added to the zinc anode, it provides abundant nucleation sites, and its well-developed porous structure accommodates the deposition of zinc dendrites and other passivation products generated during the charging and discharging of the zinc anode, instead of them being deposited on the surface of the zinc anode. This continually maintains a neat and active surface of the zinc particles, ensuring uniform deposition of zinc ions to a certain extent and considerably improving the reversibility of zinc dissolution–deposition. Xu et al.¹⁰² prepared a composite zinc anode by mixing zinc powder, AC, LA133 water-based binder, and carboxymethyl cellulose (CMC) with a mass ratio of 70:20:8:2 to form a slurry, which was uniformly coated on zinc foil as a battery anode. Li et al.⁹⁰ mixed polyvinylidene fluoride (PVDF), acetylene black (AB), and zinc particles with a mass proportion of 1:2:7 and added different amounts of AC to prepare composite zinc anodes. AC serves as a conductive carbon additive with a porous structure that provides ideal spaces to accommodate the deposition of zinc dendrites and other passivation products generated during the charging and discharging processes of the zinc cathode, which substantially enhances the performance of ZIBs.

Apart from conductive carbon-based additives¹⁰¹ (such as AC, CNTs, and AB), other materials are also added into the anodes. Sun et al.⁹⁹ used an electrodeposition technique to construct composite zinc anodes containing different inorganic additives (indium sulfate, boric acid, and tin oxide). When indium sulfate and tin oxide were incorporated into the zinc anode, zinc dendrite growth and corrosion were strongly suppressed, whereas the boric-acid-doped zinc anode inhibited the generation of by-products during charging and discharging. Compared with batteries using pure zinc anodes, batteries using the new composite zinc anode materials exhibited superior cycle stability and higher capacity retention (Figure 5F). Sun et al.¹⁰⁰ synthesized zinc composite anode materials by adding the organic additives cetyltrimethylammonium bromide (CTAB), polyethylene glycol (PEG-8000), sodium dodecyl sulfate (SDS), and thiourea (TU) to the plating solution. The experimental data showed that the zinc anode materials prepared with various organic additives not only reduced the corrosion current by 6–30 times compared with that of the pure zinc anode, but also possessed a higher capacity retention (Figure 5G). Owing to the presence of organic ligands and surfactants, the crystallographic orientation of the zinc anode was successfully controlled to preferentially expose various planes. Based on this comparative study, zinc dendrites were found to preferentially grow on (100) and (110) planes, whereas they are unlikely to develop when a zinc substrate dominated by (002) and (103) planes, namely, a lattice-matched substrate, is used for zinc plating.¹⁰³

For the above two experimental additives, the crystal orientation induction method is essential to effectively inhibit the growth of zinc dendrites and thus achieve uniform zinc deposition. Crystal orientation induction influences the surface morphology and dendrite growth conditions by impacting the crystal growth direction. For example, Sawada et al.¹⁰⁴ found that the (100) crystal plane orientation was the most prone to zinc dendrites because the crystal growth direction was 70°–90° from the substrate, which was extremely favorable for dendrite growth. Li et al.¹⁰⁵ summarized the relationship between crystal orientation and dendrite growth: when the angle between the substrate and the crystal growth direction is 0°–30° (e.g., [002], [103], and [105] crystal planes), the growth of zinc dendrites is effectively suppressed, and the deposition is more uniform; when the crystal growth direction is 70°–90° from the substrate (e.g., [100] and [110] crystal planes), the growth of zinc dendrites is favored. In both studies, Sun synthesized new zinc anodes by adding additives. In addition to a higher corrosion resistance, the synthesized zinc electrodes had a crystal orientation of (002) or (103), which exhibited a relatively high resistance to dendrite growth, effectively suppressing the generation of dendrites and by-products and allowing for a more uniform deposition of zinc metal. Consequently, the batteries assembled from these anodes exhibited better cycling stability and a higher capacity retention.¹⁰³

MODIFICATION OF THE ELECTROLYTE–ANODE INTERFACE

In addition to the modification of the anode itself, various complex reactions occur at the electrolyte/electrode interface (EEI),^19,39,106 therefore, to better protect the zinc anode, we should concentrate on the optimization of the electrolyte–anode interface.^40,48 EEI optimization is primarily a matter of protective barrier design and separator design. Both suitable barriers and separators provide protection for zinc anodes and improvement of battery performance. However, regarding the practical application, if barrier and separator are simultaneously prepared in the batteries, the assembly feasibility and energy density of the batteries will be decreased. Therefore, it is better to adopt one of them according to the actual situation of the battery.

Protective barrier design

According to the working principle and structure of ZIBs, the zinc anode directly contacts the electrolyte, resulting in corrosion of the zinc anode. Hence, inserting a protective barrier, namely, protective layers, between the electrolyte and zinc anode can effectively avoid the direct contact of zinc with the corrosive electrolyte (Figure 6).¹⁰⁷ On the one hand, reasonable design of the functional surface barrier can homogenize the electric field and ion concentration distribution at the anode surface, guide the uniform deposition/dissolution of zinc, and thus inhibit the generation of zinc dendrites, corrosion passivation, and the HER to a certain extent. On the other hand, this barricade can influence the growth of zinc and reduce the deformation of the zinc anode.^109,110 Herein, we summarize the design principles of barriers for anodes as follows: (a) excellent physical and electrochemical stability with corrosion resistance to the electrolyte¹¹¹; (b) good electrical conductivity, high dielectric constant, and inferior zincophilicity⁴⁰; (c) uniform distribution of polar groups and suitable quantity of pores³¹; (d) good wetting effect with zinc and low charge transfer resistance with abundant active sites.^65,108 The compositions of effective protective layers are diverse, and the corresponding working mechanisms vary. Based on the working principles and the interaction of the protective layer with zinc, surface coating protection can be classified into (a) artificially coated barriers, (b) adsorption barriers, and (c) redox-assisted self-assembled barriers.

FIGURE 6. Schematic diagram of stabilized zinc anode with TiO2 coating. (A) Repeated plating/stripping cycles contribute to zinc corrosion and HER. The decomposition of the solvent in aqueous electrolytes expedites the hydrolysis of Zn2+, resulting in the formation of less conductive Zn(OH)2. Reproduced with permission.107 Copyright 2020, Wiley-VCH. (B) The thin layer TiO2 coating leading to a stabilized cyclic deposition/stripping procedure eliminates severe gas release and the formation of low conductivity Zn(OH)2. Reproduced with permission.57 Copyright 2018, Wiley-VCH. (C) Comparison of zinc ions behaviors during the plating and stripping process with/without CNG separator and consolidation mode of artificial CNG layer. Reproduced with permission.105 Copyright 2021, The Royal Society of Chemistry. (D) Schematic Mechanisms illustration of undecorated and decorated by MOF-PVDF. Reproduced with permission.108 Copyright 2019, The American Chemical Society. (E) Schematic illustration of zinc ions behavior during the deposition process. Reproduced with permission.31 Copyright 2019, The Royal Society of Chemistry. (F) Comparison of stripping process of zinc ions. Reproduced with permission.109 Copyright 2018, Wiley-VCH

According to previous studies, artificially coated barriers offer abundant preferential pathways for zinc ions and an appropriate contact area by inhibiting the unexpected generation of by-products (Figure 7).^{31,105,107–109} For example, Zhao et al.⁵⁷ demonstrated for the first time that atomic barrier deposition of a titanium dioxide coating can be used as a protective barrier for zinc anodes in ZIBs. The barrier effectively suppressed the corrosion and passivation of the zinc anode, thereby reducing the generation of corrosion by-products and gases and significantly improving the coulombic efficiency. This artificially coated titanium dioxide barrier boosted the cycling performance of a zinc–manganese dioxide battery to 1000 cycles with an 85% capacity retention at 3 mA cm⁻², dramatically enhancing the performance of the zinc-based battery (Figure 8). Yan et al.¹¹² introduced a zinc-based montmorillonite (MMT) interlayer as a protective layer for zinc anodes, which provided ample transport pathways for zinc ions with a high transference number, resulting in suppressed corrosion, passivation, and zinc dendrites. The assembled symmetric zinc batteries delivered a highly stable and small overpotential of 50 mV and a long life span of 1000 cycles at 1 mA cm⁻²/0.25 mAh cm⁻² and an overpotential of 100 mV and a long life span of over 1000 h (77% depth of discharge) at an ultrahigh current and capacity of 10 mA cm⁻²/45 mAh cm⁻². Deng et al.¹⁰⁷ employed kaolin to protect the zinc anode because it possesses a considerable number of pathways to modulate the transport of zinc ions and abundant sites to absorb zinc ions, which enhance the anti-corrosion capacity of the zinc anode and induce the uniform deposition of zinc ions. Hence, the morphology of the kaolin-protected zinc anode (Figure 7A–D) remained highly stable after 600 cycles at 0.5 A g⁻¹ in a Zn//MnO₂ battery, and the assembled symmetric battery using the kaolin-protected zinc anode cycled for 800 h at 1.1 mAh cm⁻² with the side reactions suppressed (Figure 8A). Another function of artificially coated barriers is to provide a porous structure that can guide uniform electrolyte flux and regulate the Zn deposition rate. Similarly, Kang et al.¹⁰⁹ reported that porous nano-CaCO₃ layers artificially coated on zinc anodes could guide uniform plating/stripping of zinc, and batteries with nano-CaCO₃-coated zinc anodes exhibited a higher discharge capacity of 42.7% after 1000 cycles (177 vs. 124 mAh g⁻¹ at 1 A g⁻¹) compared to that of batteries using bare zinc anodes (Figure 7I–L). Liu et al.¹⁰⁸ reported a thin zincophilic layer composed of PVDF and MOF nanoparticles, which offered uniform electrolyte flux and lower charge transfer resistance. The Zn//MnO₂ battery using this coating layer exhibited stable performance with inhibited zinc dendrites for over 500 cycles and a markedly suppressed overpotential (Figure 8C).

FIGURE 7. Scanning electron microscope photography of: (A–D) undecorated Zn and KL-Zn anode after different cycles. Reproduced with permission.107 Copyright 2020, Wiley-VCH. (E–H) bare Zn and CNG/Zn anode after the deposition of Zn with different times. Reproduced with permission.105 Copyright 2021, The Royal Society of Chemistry. (I–L) Comparison of bare Zn and nano-CaCO3-coated Zn anode after cycled. Reproduced with permission.109 Copyright 2018, Wiley-VCH. (M–P) the interface after the deposition of zinc ions on (M) bare Ti, (N, O) PA-coated Ti and (P) PA-coated Ti after tearing off the layer. Reproduced with permission.31 Copyright 2019, The Royal Society of Chemistry. Bare Zn anode (Q) before and (R) after cycling, MOF-coated Zn anode (S) before and (T) after cycling. Reproduced with permission.108 Copyright 2019, The American Chemical Society

FIGURE 8. Electrochemical properties of: (A) symmetrical cells with bare Zn and KL-Zn anodes at different current densities. Reproduced with permission.107 Copyright 2020, Wiley-VCH. (B) PA-coated Zn foil and bare Zn foil at the current density of 0.5 mA cm−2. Reproduced with permission.31 Copyright 2019, The Royal Society of Chemistry. (C) MOF-PVDF-coated Zn foil and bare Zn foil at the current density of 1 and 3 mA cm−2. Reproduced with permission.108 Copyright 2019, The American Chemical Society. (D) Bare Zn and CNG/Zn anodes. Reproduced with permission.105 Copyright 2021, The Royal Society of Chemistry

In addition, a well-designed protective layer can induce oriented zinc deposition with growth orientations that match the existing crystallographic directions of the zinc anode substrate. This makes the subsequent zinc deposition more uniform and denser, resulting in improved electrochemical stability for ZIBs. Shin et al.¹¹³ proved this theory by proposing an artificially cross-linked SEI layer on the surface of a zinc anode, which was fabricated by employing gelatin as the coating material. The active surface of the zinc anode was protected from corrosive byproducts, and the electrochemical stability was enhanced. Similarly, Wang et al. developed an effective layer using a ferroelectric polymer material (poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))), which formed a static electric field between the zinc anode and coating layer that regulated zinc deposition and inhibited zinc dendrites. Symmetric zinc batteries employing this layer demonstrated a long life span of 2000 h at 0.2 mAh cm⁻² and an outstanding rate performance with a current density up to 15 mA cm⁻².¹¹⁴ The interaction between the zinc anode and protective barrier plays an important role in defining the adherence force between them. Automatically coated or self-assembled protective layers generally possess better stability and strength against electrode volume changes. For instance, Deyab et al.⁹² used polyoxyethylene (40) nonylphenyl ether as a barrier molecule to effectively isolate zinc metal from the electrolyte and inhibit corrosion. This protection was realized by molecule adsorption at the anode surface through the interaction between functional groups and the surface zinc atoms. It was found that the side reaction suppression capability was positively correlated with the concentration of ethylene naphthalene dicarboxylate surfactant. In a similar self-assembly manner but via a redox reaction mechanism, Xia et al.¹¹⁵ developed a convenient and facile method involving the spontaneous reduction of graphene oxide (GO) by metallic zinc, which allows the reduced GO to assemble at the surface of zinc anode with a controllable thickness. This self-assembled soft layer possesses a large specific surface area and excellent electrical conductivity, which favors uniform zinc deposition, restrains the growth of zinc dendrites, and ensures an outstanding full-battery cycle life and lower overpotential (~20 mV at 1 mA cm⁻²) compared with the control sample. In addition to those described in this section, the anti-corrosion layers reported thus far include MOF,⁷⁶ solid electrolyte,¹⁰⁸ and kaolin barriers.¹⁰⁷

Separator modification

The operation of ZIBs relies heavily on the effective and rapid shuttling of zinc ions between two electrodes, which is affected dramatically by not only the properties of the electrolyte, but also the elastic modulus, absorption rate, permeability, pore shape, and separator homogeneity. Therefore, it is important to functionalize or modify the separators following the above principles, not only to strengthen the abilities of the separator, but also to protect the anodes.^116–119 Conventional separators in ZIBs, such as filter paper and glass fiber, are unable to simultaneously satisfy all these requirements; therefore, new diaphragm candidates are urgently needed.¹²⁰ As illustrated in Figures 9A,B, a cellulose-based Sn-layer-modified separator with outstanding zincophilicity and superior conductivity was adept in inducing the uniform distribution of electronics and ions and modulating the horizontal deposition of zinc. As a result, the stabler Zn anode without dendrite was finally earned, which was the same as the result from the modified glass fiber separator depicted in Figure 9C,D. After summarizing the design principle of separators with excellent performance, we discuss in detail the influence of various characteristics on the performance of separators. The necessary characteristics possessed by high-performance separators are as follows: (a) excellent compatibility with the electrodes and electrolyte and capability of resisting the corrosion of the electrolyte¹²³; (b) thermal and electrochemical stability without shape change at large current densities and at high or low temperatures¹²⁴; (c) uniform distribution of ion pores with an appropriate diameter and porosity¹²⁵; (d) mechanical stability with low ion transport resistance, high elastic Young's modulus, and homogeneity^126,127; (e) abundant functional groups with high wettability^128,129; (f) ability to induce oriented zinc deposition on the (002) zinc plane¹³⁰; (g) priority transport of cations and uniform ion flux distribution.^{120,131–133}

FIGURE 9. Schematic representatives of: the modulated distribution of electric field of Zn anodes with (A) the pristine and (B) modified separator. Reproduced with permission.121 Copyright 2021, Springer Nature. (C) the preparation route of g-C3N4/GF separator. Reproduced with permission.122 Copyright 2021, The Royal Society of Chemistry. (D) the effect of the Janus separator and glass fiber separator on the behaviors of zinc ions during the plating/stripping cycles. Reproduced with permission.119 Copyright 2020, WILEY-VCH

Numerous studies have enhanced the overall performance of separators by investigating the effects of the elastic modulus, absorption rate, permeability, pore shape, and homogeneity. For example, Wang et al. demonstrated a novel separator comprising polyvinyl alcohol (PVA) and Lyocell dual-layer (PLD) that exhibited remarkable electrolyte retention and submicrometer-sized pores with an adjustable size and large surface area, which enhanced the electrochemical performance.¹³⁴

Lee et al. proposed a cross-linked separator based on polyacrylonitrile (PAN) with preferential cation exchange, uniform ion flux, good mechanical strength, and an ultrahigh thickness of 30 μm, which delivered a long life span of over 350 cycles in Zn//n symmetric cells and inhibited polarization and zinc dendrites.¹²⁰ Likewise, Cao et al. proposed a cellulose nanofiber-ZrO₂ composite (ZC) separator that possessed an outstanding zinc ion transference number, high ionic conductivity, and a high dielectric constant, which modulated the uniform distribution of zinc ions with preferential and rapid transportation kinetics and stabilized the anode by inhibiting side and passivation reactions. Consequently, the battery assembled using this separator delivered a high coulombic efficiency of 99.5% and exceptional cyclability of 2000 h at 0.5 mA cm⁻². In addition, the assembled Zn||ZnSO₄||MnO₂/graphite coin and pouch cells demonstrated a high rate capability and long-term cyclability with excellent flexibility and integration capability.¹²⁷

In addition, a high zinc ion transference number with a low activation energy is one of the most vital characteristics of effective separators; therefore, many studies have focused on this property to improve the performance of separators. Ghosh et al.¹²⁸ proposed for the first time a Zn²⁺-integrated Nafion ionomer separator that could effectively curb irregular and inhomogeneous zinc plating on zinc anodes. The nonporous cation-selective Nafion films guarantee the preferred mass transfer of Zn²⁺ with a uniform ion flux perpendicular to the anode surface, such that zinc ions can be deposited evenly and densely on the anode surface. The ZIB system assembled from this separator retained 88% of its initial capacity after a long period of 1800 cycles at a current density of 10 A g⁻¹, which is quite impressive. This case shows that, in addition to the electrolyte–anode interface, the separator has a decisive impact on the deposition of zinc on the anode surface. Unfortunately, discussions on the effects of different separators on the anode performance of ZIBs currently receive too little attention, and the research progress on separators in terms of zinc anode protection has been slow.

IMPROVED DESIGN OF THE ELECTROLYTES SYSTEM

As an integrated component of ZIBs, the electrolyte plays an essential role in establishing the electrochemical stability window and transporting zinc ions between the anode and cathode during the charging and discharging processes.^{34,79,135,136} For zinc anodes, a suitable electrolyte not only attenuates zinc dendrites, corrosion passivation, and the HER, but also improves the coulombic efficiency and reversibility of the battery.^{10,38,137,138} Therefore, selecting a well-matched electrolyte is also an effective method to restrict zinc anode corrosion. For this reason, in-depth studies on the development of zinc ion electrolytes have been conducted.^139,140 After a broad review of a large number of references, the design principles of electrolytes are summarized as follows: (a) possesses outstanding electrical and ionic conductivity with a wide electrochemical stable potential window¹⁴¹; (b) remains stable without decomposition at high voltages and temperatures and does not freeze at low temperatures⁶³; (c) can form an SEI with a low solvation effect¹⁴²; (d) is low-cost, nontoxic, and inert to zinc metal with high energy efficiency.¹²⁶ Moreover, we introduce the optimization strategy of ZIB electrolytes from the following three aspects based on the abovementioned principles: (1) design optimization of the electrolyte system, (2) application of electrolyte additives, and (3) optimization of the electrolyte concentration and dosage.

Design optimization of the electrolyte system

The major zinc-based battery electrolytes can be broadly classified into five categories: (1) aqueous electrolytes, (2) ionic liquid electrolytes, (3) gel polymer electrolytes, (4) organic electrolytes, and (5) all-solid electrolytes. Because the characteristics of each electrolyte determine its inherent merits and deficiencies, it is possible to design the main body of the electrolyte based on the structure–activity relationship to intentionally obtain a desirable electrolyte that protects the anode in batteries.

Aqueous electrolytes

Aqueous electrolytes have emerged as the most prevalent zinc ion reservoirs for zinc-based batteries owing to their low environmental impact, intrinsic safety, and low cost.^48,143–145 Researchers have spared no efforts to develop a number of high-performance electrolytes, as summarized in Table 1, including alkaline electrolytes such as KOH¹⁴⁶ and NaOH,¹⁴⁷ zinc salts electrolytes such as ZnSO_4,^148,152 Zn(NO₃)₂,²⁴ Zn(ClO₄)₂,¹⁴⁹ ZnF₂,¹⁵⁰ ZnCl₂,¹⁵¹ and Zn(CH₃COO)₂.⁹ Among them, primitively developed alkaline electrolytes (e.g., NaOH and KOH) frequently suffer from severe zinc anode dissolution, zinc dendrites, and corrosion passivation during battery cycling,¹⁵³ whereas excessively acidic electrolytes also corrode zinc anodes. Therefore, neutral and mildly acidic electrolytes (e.g., ZnSO₄) have become the most widely used in aqueous ZIBs. Apart from ZnSO₄,^154,155 Zn(CF₃SO₃)₂ is a popular aqueous electrolyte. Zhang et al.²⁴ discovered that a battery using a Zn(CF₃SO₃)₂ electrolyte possesses faster plating/stripping kinetics and superior reversible cycling stability compared with that using ZnSO₄ because the large CF₃SO₃⁻ anion can reduce the number of H₂O molecules hydrated with zinc ions, decrease the effect of solubilization, and accelerate the migration and charge transfer of zinc ions (Figure 10A–D). An analysis of current aqueous ZIB electrolytes revealed that the most widely used aqueous electrolytes are ZnSO₄ and Zn(CF₃SO₃)₂, which are extensively accepted for their excellent compatibility with the anode and intrinsic safety. However, the inherent defects of aqueous-based electrolytes include rapid battery capacity decay, the inevitable HER, and a narrow thermodynamic stable potential window (1.23 V), which should be treated with sufficient caution when employing aqueous-based electrolytes.

TABLE 1 The electrochemical performance comparison of batteries using various aqueous electrolytes

Electrolytes	Concentration	Battery electrochemical performance	References
KOH	6 M	With six stripping/plating cycles at 0.2 mA cm−2	[146]
NaOH	5 M	With a CE of 99.97%, an EE of 88.07%, and a VE of 88.11% at 80 mA cm−2	[147]
ZnSO4	1 M	With ~80% of the capacity retention after 200 cycles at 8 C	[148]
Zn(NO3)2	1 M	Electrolyte decomposition voltage <1.5 V (vs. Zn2+/Zn)	[24]
Zn(ClO4)2	3 M	With a high ionic conductivity of 4.23 mS cm−1 at −50°C	[149]
ZnF2	20 mM	A reversible specific charge retention of 95% after 200 cycles	[150]
ZnCl2	7 M	With capacity retention of 89.4% after 9000 cycles at 5 A g−1	[151]
Zn(CF3SO3)2	3 M	With ~100% Zn plating/stripping efficiency for 800 h	[24]
Zn(CH3COO)2	0.5 M	With 74% capacity after 100 cycles at 0.5 C	[9]

FIGURE 10. (A) Constant current cycling for Zn/Zn batteries using 3 M ZnSO4 and 3 M Zn (CF3SO3)2 electrolytes at current density of 0.1 mA cm−2. The inset expands the voltage distribution at the 1st and 25th cycles. (B) Rate performance and long-term cycling properties of Zn/Zn batteries using 3 M ZnSO and 3 M Zn (CF3SO3)2 electrolytes. (C,D) Cyclic voltammograms for zinc anodes in (C) 1 M Zn (CF3SO3)2 and (D) 1 M ZnSO4 in aqueous electrolytes with 0.5 mV s−1 and a range from −0.2 to 2.0 V. Reproduced with permission.24 Copyright 2016, The American Chemical Society. Cycling (charge/discharge) behaviors for zinc air batteries using molten Li0.87Na0.63K0.50CO3 with eutectic electrolyte at 550°C together with 3 M NaOH and 1 M ZnO: (E) Cycling characterization. (F) Coulomb efficiency and voltage efficiency. (G) Average voltage for charging, open circuit, and discharging. Reproduced with permission.156 Copyright 2016, Elsevier

Ionic liquid electrolytes

At high temperatures, mass transfer and electrode reaction kinetics are drastically accelerated; however, conventional aqueous electrolytes are prone to volatilization at operating temperatures over 100°C. In stark contrast, ionic liquid electrolytes are characterized by high thermal stability, low volatility, and a wide electrochemical window, thus avoiding the effects of electrolyte evaporation and allowing the development of ZIBs that can operate at high temperatures.^{106,157–159} For instance, a promising new rechargeable zinc–air battery with a molten Li_0.87Na_0.63K_0.50CO₃ eutectic electrolyte mixed with NaOH and ZnO was first demonstrated by Liu et al.¹⁵⁶ The battery achieved nearly perfect reversible deposition/stripping of zinc with a coulombic efficiency of 96.9% over 110 cycles at a high temperature of 550°C and average charging and discharging voltages of 1.43 and 1.04 V, respectively (Figure 10E–G). Overall, ionic liquid electrolytes are typically beneficial for zinc plating/stripping; however, their further development is limited by their slow ionic conductivity and high viscosity.

Gel polymer electrolytes

The presence of active water molecules in aqueous electrolytes inevitably leads to uncontrolled side reactions (anode dissolution, corrosion passivation, and the HER) in ZIBs, whereas gel electrolytes effectively alleviate these issues because of their low active water content and high ionic conductivity.^{140,160–166} Furthermore, as a result of their good physical flexibility and processability and the urgent demand for flexible wearable energy storage devices, gel polymer electrolytes have emerged as an active area of research, and it is vital to explore functional and applicable gel electrolytes for ZIB anodes.^167–169 A well-defined 3D zinc alginate-based (Alg-Zn) gel electrolyte was prepared by Tang et al.¹⁷⁰ based on the ionic cross-linking method. The gel electrolyte suppressed the generation of irreversible by-products and the formation of dendrites through strong interactions between the deprotonated carboxyl groups (COO⁻) on the polymer chain and the zinc anode surface, thus accomplishing high reversibility and dendrite-free growth of zinc metal anodes (Figure 11A–H).

FIGURE 11. (A–D) scanning electron microscope (SEM) images of copper foil after 1 h (left column) and 5 h (right column) of galvanization in 2 M zinc sulfate 0.2 M manganese sulfate aqueous electrolyte. (E–H) SEM images of copper foil after 1 h (left column) and 5 h (right column) of galvanization in Alg-Zn electrolyte. (I) Self-discharge testing after 10 cycles at 0.2A g−1 and left to stand for 60 h at full charge. (J) Cycling properties after 60 h of resting. (K) Galvanization/peel performances of symmetrical batteries at 1.77 mA cm−2 after being left at room temperature for 60 h. Reproduced with permission.170 Copyright 2020, Elsevier. (L) Long-term constant current cycling of zinc/zinc symmetric batteries at 1 mA cm−2 in Zn (OTf)2-TMP-DMC/DMF electrolyte. (M) Coulombic efficiency of zinc/stainless steel batteries in Zn(OTf)2-TMP-DMC/DMF electrolyte. Reproduced with permission.171 Copyright 2019, Wiley-VCH

Furthermore, the Alg-Zn gel electrolyte maintained a low overpotential and a steady voltage hysteresis curve without high voltage fluctuations. ZIBs based on this gel electrolyte exhibited a high rate performance, high cycling performance, excellent shelf life, and self-recovery capability (Figure 11I–K). Although gel electrolytes have several advantages, such as effective mitigation of anode side reactions, excellent flexibility, and excellent galvanic performance, their inferior mechanical strength and chemical instability remain bottlenecks in the practical application of these electrolytes.⁷⁴

Organic electrolytes

Organic electrolytes have a wide electrochemical stability window and favorable zinc anode compatibility. More importantly, most of the previously mentioned side reactions (e.g., passivation) are related to water molecules; therefore, nonaqueous organic electrolytes largely relieve the possible surface side reactions of the zinc anode.^{7,59,172–176} Naveed et al.¹⁷¹ performed considerable research in the field of organic electrolytes for ZIBs. They constructed stabilized zinc anodes using an intrinsically safe trimethyl phosphate-based electrolyte to obtain highly stable and dendrite-free zinc anodes. As the ZIB was repeatedly charged and discharged, the original zinc foil gradually transformed into graphene-like layered deposits with the assistance of trimethyl phosphate, a surfactant, and zinc phosphate molecular templates. This self-evolved morphology of the zinc anode contributed to an improved performance at 5 mA cm⁻², an extremely high coulombic efficiency of up to 99.68%, and a long-term high reversibility for electroplating/stripping, lasting for more than 5000 h without zinc dendrite formation (Figure 11L,M). This is the first report on the use of triethyl phosphate as a solvent to attain a dendrite-free zinc anode with superior performance.¹⁷¹ Stable plating/stripping over 3000 h and a prominent coulombic efficiency of 99.68% were obtained for ZIBs using this electrolyte. The electrolyte exhibited favorable compatibility with zinc anodes and potassium copper hexacyanoferrate (KCuHCf) cathodes, retaining a discharge capacity of 74% after 1000 cycles at a rate of 1 C, and the corresponding full ZIBs exhibited long-cycle stability and a relatively high rate capability. However, most organic electrolytes are flammable, which counters the original purpose of ZIB development to emphasize safety and reduce the environmental impact.

All-solid electrolytes

As long as there are active water molecules in the electrolyte, the HER and zinc dendrites inevitably occur at the electrolyte–anode interface. Even though gel electrolytes suppress the above-mentioned side reactions to a certain degree, a small quantity of water molecules remains; thus, these reactions cannot be completely eliminated.^177,178 However, an all-solid electrolyte possesses no free water molecules, making it a satisfactory candidate for tackling issues such as the HER, liquid leakage, and evaporation.^179–184 The majority of currently available all-solid electrolytes are composed of a mixture of polymer substrates and zinc salts. Ma et al.¹⁸⁵ constructed an all-solid-state ZIB filled with a poly(ethylene oxide)/ionic-liquid-based zinc salt consisting of a poly(vinylidene fluoride-hexafluoropropylene) thin-film solid polymer electrolyte. This solid-state electrolyte provided a dendrite-free, side-reaction-free, and highly reversible zinc anode, which exhibited an excellent lifetime of 30 000 cycles at 2 A g⁻¹ at room temperature, a relatively high ionic conductivity of 1.69 × 10⁻² S cm⁻¹, and a wide operating temperature window from −20 to 70°C. However, the diffusion rate of Zn²⁺ ions is generally sluggish in solid electrolytes compared to that in their liquid counterparts, and the incompatible contact between the solid electrolyte and anode results in high interfacial impedance, which significantly hampers their large-scale application.

Application of electrolyte additives

The addition of additives to electrolytes is another efficient way to inhibit the formation of zinc dendrites because they optimize the zinc deposition sites and promote homogenous deposition of zinc ions on the surface of the zinc anode. According to their components, electrolyte additives are mainly categorized into organic and metal-ion additives. The most common additives are organic additives, including, but not restricted to, organic acids, aldehydes, polymers, and surfactants. They not only play an essential role in modifying the zinc deposit grain size and the postdeposition surface roughness of the zinc anode,¹⁸⁶ but also increase the cathodic overpotential and inhibit the deposition kinetics of zinc ions.^187–189 Hashemi et al.¹⁹⁰ proposed branched polyethyleneimine (BPEI) as an electrolyte additive to improve the zinc deposition kinetics and postzinc-deposition morphology. BPEI changed the surface topography of the deposited zinc layer from layered hexagonal microcrystal agglomerates to a dense layer (Figure 12A–F). More surprisingly, BPEI dramatically suppressed the zinc deposition kinetics and reduced the growth rate of zinc grains adhered to the electrode surface, efficiently alleviating the abrupt local depletion of zinc ions to ensure flat and compact zinc deposition. Xu et al.¹⁹¹ discovered that the addition of a modest amount of diethyl ether could reduce the plating/stripping overpotential over a long period of up to 250 h (Figure 12G). In this case, the preferential adsorption of ether molecules at zinc dendrites acted as an electrostatic shield to repel positively charged zinc ions and thus effectively suppresses the further growth of zinc dendrites (Figure 12H). Polyacrylamide, a low-cost polymer containing acyl groups, can also suppress the growth of zinc dendrites as an electrolyte additive in ZIBs because of the strong selective adsorption of divalent zinc ions on acyl groups. As zinc ions are transferred along the polymer chains, polyacrylamide, as a medium for zinc deposition, can promote the uniform distribution of zinc on the anode surface (Figure 12I,J).¹⁸⁰ It is also worth noting that the co-deposition of overdosed organic additives on the electrode surface may occur as a side reaction, extending the battery life span and performance. For example, in a study on the adverse effects of additives on the performance of zinc anodes, Chen et al found that excess polyethylene glycol 200 (PEG200) addition (2 vol.% compared to the proper amount of 1 vol.%), caused a further negative shift in the corrosion potential and increased the corrosion current density (Figure 12K). This indicates that when utilizing electrolyte additives, the concentration of additives must be controlled with extra caution. Quaternary ammonium (triethylmethyl ammonium, TMA) can effectively realize long-term cyclability and electrochemical stability by providing a charge shielding effect and participating in the composition of contact ion pairs. Consequently, the uniform deposition of zinc ions can be achieved, and unwanted by-products can be suppressed, as reported by Yao et al.,¹⁹³ who added TMA to diluted electrolytes based on ZnCl₂ and ZnSO₄ solutions. Further analysis of the assembled Zn||Zn symmetric cell batteries using these electrolytes revealed excellent cyclabilities of over 2145 h at 1 mA cm⁻² with a low overpotential of 20 mV and of over 500 h at 5 mA cm⁻². Additionally, the Zn||Ti half-cells stably operated for 750 cycles with a retained coulombic efficiency of over 97.7%.

FIGURE 12. Scanning electron microscope pictures for electrodeposited zinc layers in 0.5 M branched polyethyleneimine (BPEI)-free zinc sulfate (A–C) and 300 ppm BPEI zinc sulfate (D–F) at different applied current densities: (A,D) 13.5 mA cm−2, (B,E) 60 mA cm−2, and (C,F) 110 mA cm−2. Reproduced with permission.190 Copyright 2017, Elsevier. (G) Cyclic plating/stripping profiles for Zn–Zn symmetric batteries at 0.2 mA cm−2 with (red curve) and without (blue curve) Et2O. (H) Schematic diagram of the morphological evolution of the zinc anode during the zinc stripping/plating cycling in electrolytes with and without Et2O additives. Reproduced with permission.191 Copyright 2019, Elsevier. (I) Schematic diagram of dendrite-free plating on copper mesh 3D zinc anode with electrolyte containing PAM electrolyte additive. (J) Computational model illustrating the zinc adsorption on PAM. Reproduced with permission.180 Copyright 2019, Wiley-VCH. (K) Summary of the effect of different amounts of PEG200 on the transient evolution of corrosion potential and current. Reproduced with permission.192 Copyright 2018, Wiley-VCH

In addition to organic additives, metal ions are commonly used as additives. Metal-ion additives can be classified into two general categories based on their mechanism: co-deposition and electrostatic additives. The former type, such as Bi³⁺, can be deposited on the anode zinc before the reduction of zinc ions, increasing the anode conductivity while promoting uniform zinc deposition; however, the displacement reaction between zinc metal and metal-ion additives can vastly affect the battery performance.^180,191,192 The latter type, such as Cr³⁺, does not involve co-deposition but helps to establish a positive self-healing electrostatic shield surrounding the zinc seed crystal to repel approaching zinc ions from the seed to adjacent deposition sites, eventually enabling uniform zinc plating.¹⁹⁴ Cao et al. developed a highly stable inorganic colloidal electrolyte by introducing lithium magnesium silicate into zinc sulfate, which regulated the solvation structure of zinc ions, promoted the uniform distribution of zinc ions with rapid transportation dynamics, and suppressed the growth of zinc dendrites. The electrolyte also inhibited corrosion, side reactions, and byproducts owing to its excellent ability to absorb free H₂O molecules, transform the free H₂O molecules into absorbed ones, lower the electrostatic interactions of anions and cations in the electrolyte, and reduce the energy barrier during the desolvation and solvation processes. Not surprisingly, the assembled Zn||Zn symmetric batteries showed ultra-stable cyclabilities of 1000 h at 0.5 mA cm⁻² and 400 h at 2.5 mA cm⁻², which are superior to those of other zinc sulfate-based electrolytes based on different strategies.¹⁹⁵

Optimization of electrolyte concentration and dosage

In addition to optimizing the electrolyte system and the application of additives, adjusting the concentration and dosage of the electrolyte is a practical strategy for optimizing the electrolyte and protecting the zinc anode.^{44,146,196–202} The representative method of regulating electrolyte concentration is the “water-in-salt” idea,^19,39,106 which has drawn considerable attention in recent years. These high-concentration electrolytes have enormous potential to inhibit corrosion passivation and the HER because there are insufficient water molecules available for side reactions (Figure 13). In early research on LIBs, a high-concentration 21 M LiN(CF₃SO₂)₂ electrolyte was shown to inhibit the decomposition of water because of the strong coulombic bonding between the oxygen atoms of the water molecules and Li⁺.

FIGURE 13. (A) Schematic illustrations of solvation structures of zinc ions and the homologous reactions on the interfaces in 0.5 M (ZnClO4)2 and 0.5 M Zn(ClO4)2 + 18 M NaClO4. Reproduced with permission.197 Copyright 2021, The Royal Society of Chemistry. (B) Schematic illustrations for the formation and solution structure of the passivation layer (Zn4(OH)6SO4·xH2O) during the deposition of zinc on the Cu substrate using LZSAE and CZSAE. Reproduced with permission.199 Copyright 2020, The American Chemical Society. (C) Binding energies of the Zn(H2O)x(NH3)6−x2+ complexes and the corresponding molecular models. Reproduced with permission.198 Copyright 2021, Elsevier. (D) Study on the molecular dynamics of the Zn2+-solvation structure with different three concentrations of LiTFSI. Reproduced with permission.146 Copyright 2018, Springer Nature

Corrosion passivation, which can be accelerated by the pH change induced by the HER, is also attenuated, demonstrating an improved corrosion resistance with a negative shift in the corrosion potential.^203,204 Similarly in ZIBs, the “water-in-salt” category of electrolytes has exceptional performance in protecting zinc anodes, but they are too expensive to be used on a large scale. Beyond the electrolyte concentration, electrolyte dosage is often a pertinent issue in ZIBs. An insufficient electrolyte volume hinders the transport of zinc ions, whereas surplus electrolyte favors anode corrosion. However, there are only limited studies pertaining to electrolyte dosage, and the understanding of electrolyte dosage and its effect on the electrochemical performance of ZIBs is still in its infancy.¹⁹⁹ As an example, Wang et al.¹⁴⁶ reported a high-concentration electrolyte comprising 1 M Zn(TFSI)₂ and 20 M LiTFSI for ZIBs, which optimized the solvation structure of zinc ions and inhibited zinc dendrites and the HER by providing abundant TFSI⁻ to surround the zinc ions instead of water molecules (Figure 13D). Additionally, Zhu et al.¹⁹⁷ revealed that a high-concentration Na solute could reduce the quantity of free H₂O molecules and uniformly distribute the electronic state of the electrolyte, resulting in a reversible and uniform distribution of zinc deposition. Their work proved that a high-concentration electrolyte is an effective strategy to induce the uniform distribution of zinc ions and suppress the formation of byproducts during the plating and stripping processes.

SUMMARY AND PERSPECTIVES

ZIBs have demonstrated tremendous potential for application in large-scale energy storage devices; therefore, a summary of the challenges and solutions to issues confronting ZIB anodes is warranted. This review begins by introducing the main problems encountered by zinc anodes, including zinc dendrites, corrosion passivation, and the HER. The formation of zinc dendrites is a result of the inhomogeneous distribution of the electric field and ion concentration, and corrosion passivation can cause dramatic decay of battery capacity and coulombic efficiency during cycling; the HER is a wide-ranging issue that is associated with the acidity of the electrolyte, the content of water molecules in the electrolyte, the reduction potential, and the specific surface area. In addition to the above understanding, solutions to the existing research on the problems of zinc anodes are summarized in three classes: (1) investigation and design of zinc materials, (2) modification of the electrolyte–anode interface, and (3) optimization of the electrolyte system. The aforementioned anode problems are generally inextricably coupled with each other; that is, they aggravate and interact with each other, running the entire battery in a self-accelerated vicious cycle (dendrite → side reaction → dendrite). Large polarization abruptly depletes zinc ions and induces diffusion-controlled mass transport with a nonuniform ion concentration distribution, whereas inhomogeneous surface properties (such as defects) distort the electric field intensity and create a nonuniform crystal seed distribution. These nonideal conditions incubate the initial growth of zinc dendrites in the form of small bumps with enlarged surface areas, thus aggravating corrosion passivation and the HER. Worse still, the by-products of the two processes exacerbate nonuniform zinc ion transport/reduction and produce additional zinc dendrites. Eventually, the continuously deteriorating coulombic efficiency and battery capacity lead to battery failure.⁴⁵

Similar to the interconnectedness of the problems on zinc anodes, the strategies for solving the problems of zinc anodes are not isolated. For example, 3D anodes can effectively regulate zinc deposition, but the increased specific surface area exacerbates corrosion passivation and the HER. The “water-in-salt” strategy can inhibit the HER and corrosion passivation to some extent but has a high cost. Therefore, to address all the issues of zinc anodes to obtain a superior anode material, a synergistic and comprehensive perspective is required to optimize the entire system. Despite the promising advances in zinc anode protection and performance, many challenges remain that need to be addressed, and there is considerable room for progress. Therefore, we offer the following recommendations and perspectives:

• For challenges other than dendrites, corrosion passivation, and the HER, zinc anodes should possess excellent performance merits, including high reversibility of zinc ion plating/stripping, high discharge capacity retention, long cycle stability, and reliability in universal environments. More importantly, the anode and its manufacturing must have a low environmental impact and cost to realize large-scale applications.
• When considering zinc anode protection strategies, the focus should extend beyond the exploration of the zinc anode itself, and the importance of the anode–electrolyte interface and electrolyte optimization should be considered. The electrolyte has a tremendous impact on the anode; however, current research on electrolytes is in the initial stage, which includes the in situ state of the electrolyte, the components of the electrolyte, the proper match between the electrolyte and the electrode, and the practical considerations of the electrolyte. Therefore, more emphasis should be placed on the exploration of the electrolyte because it is an unavoidable aspect that must be considered for high-performance ZIBs.
• At the mechanism level, the failure of the zinc anode has not been thoroughly recognized. It is expected that the reaction and transport mechanism at the surface of the zinc anode can be further clarified through in situ characterizations and theoretical simulations, and the combination of these investigations may provide insight and design principles for more targeted solutions.

ACKNOWLEDGMENTS

This work was financially supported by the Projects of International Cooperation and Exchanges (BZ2018010), the Postdoctoral Science Foundation of China (Grant no. 2021M690980), the National Natural Science Foundation of China (Grant no. 51772093, 52103053), the National Key R&D Program of the Ministry of Science and Technology of China (Grant no. 2021YFB2400403), the Distinguished Youth Foundation of Hunan Province (Grant no. 2019JJ20010), and the “1515” Talent Cultivation Plan of Hunan Agricultural University.

CONFLICT OF INTEREST

The authors declare no conflict of interest.