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Electrochemical Energy Reviews

, Volume 2, Issue 3, pp 395–427 | Cite as

Electrode Materials for Rechargeable Zinc-Ion and Zinc-Air Batteries: Current Status and Future Perspectives

  • Dan Yang
  • Huiteng Tan
  • Xianhong RuiEmail author
  • Yan YuEmail author
Review article

Abstract

Advanced energy storage systems hold critical significance in satisfying the ever-increasing global demand for energy. And as a viable and effective alternative to lithium-ion batteries that dominate the current energy market, Zn-based batteries [i.e. Zn-ion batteries (ZIBs) and Zn-air batteries (ZABs)] have attracted extensive research efforts. Zn metal possesses many advantages because of its high theoretical capacity, its inexpensiveness and its good safety characteristic, and in recent years, tremendous efforts have been carried out to accelerate the development of ZIBs and ZABs with various electrode materials and electrocatalysts being proposed and investigated. In addition, with advances in characterization techniques, the underlying reaction mechanisms of these materials are also being elucidated. Therefore, this review will provide a comprehensive summary of the latest progress in various electrode materials adopted in the current ZIBs and ZABs along with corresponding mechanisms. Specifically, Mn- and V-containing cathode materials for ZIBs and associated reaction mechanisms will be thoroughly discussed, and emerging cathodes such as Prussian blue analogues, NASICON-type nanostructures and organic compounds will be presented. In terms of ZABs, this review will discuss three major types of electrocatalysts, including noble metals, heteroatom-doped carbons and transition metal oxides/sulphides/phosphides/nitrides. In addition, as a critical factor in the performance of Zn-based batteries, challenges encountered by the current Zn anodes and strategies developed to tackle these issues will be discussed as well. Finally, a short summary including the current progress and future perspectives of ZIBs and ZABs will be provided.

Graphical Abstract

Keywords

Zn-ion battery Zn-air battery Cathode material Electrocatalyst Zn anode 

1 Introduction

The demand for clean and renewable energy is increasing globally and currently; solar energy and wind energy are the most popular renewable energy resources in the world. Scientists have estimated that renewable energy will account for 70–85% of the total electricity used by 2050, and consequently, the development of massive energy storage systems is among the challenges associated with the growing demand for the large-scale production and conservation of these renewable energy sources. Thus far, lithium-ion technologies account for more than 95% of new energy storage deployments, and it is predicted that the demand for lithium will triple by 2025. However, although lithium-ion battery (LIB) consumption grew 73% from 2010 to 2014, production only increased 28% and over the next 15 years, the production of LIBs is predicted to slow down with issues related with the scarcity of lithium metal becoming severer. In addition, the insufficient energy density of LIBs (the theoretical value is ~ 400 Wh kg−1) also limits further application in high-power energy storage devices [1, 2, 3] and the use of highly active lithium metal and flammable organic electrolytes brings about serious safety concerns as well as increased manufacturing costs [4, 5].

Based on this, researchers have explored various alternative metal ions for rechargeable batteries based on important properties such as the ionic radius, atomic mass, standard potential and theoretical energy density (Table 1 and Fig. 1) and among these, considerable efforts have been made to develop similar alkali metal ion batteries (e.g. Na+, K+) due to their natural abundance [6, 7, 8]. However, given the intrinsic safety issues surrounding Na and K metals such as the flammability of alkali metals and organic electrolytes, the deployment of alkali metal-based battery systems, especially in wearable electronics that might be in contact with skin, is unrealistic. Alternatively, metal ions such as Mg2+ and Zn2+, possessing high natural abundance and stability in aqueous electrolytes, have gained increasing attention from researchers [9]. Here, although Mg2+ possesses potential in terms of its large theoretical energy density (2840 Wh kg−1), issues of low reduction potential which can cause rapid self-discharge arising from the hydrogen evolution reaction (HER) limit practical application. In addition, it is also thermodynamically unfavourable to build rechargeable Mg-ion batteries.
Table 1

Properties of typical metal ions being explored for rechargeable batteries

Element

Li+

Na+

K+

Mg2+

Zn2+

Ionic radius (Å)

0.76

1.02

1.38

0.72

0.74

Atomic mass (g mol−1)

6.9

23

39.1

24.3

65.4

Standard potential (V vs. SHE)

− 3.04

− 2.71

− 2.94

− 2.37

− 0.76

Theoretical energy density (Wh kg−1)

3458

1106

935

2840

1086

Fig. 1

Volumetric and specific capacities of typical metals

Alternatively, battery systems based on metal zinc (e.g. Zn-ion and Zn-air batteries) can provide comparable or even superior performances to LIBs [10, 11], and zinc possesses many obvious advantages over lithium [12, 13, 14, 15, 16, 17, 18]. This is because zinc is a readily available and inexpensive mineral with resources totalling 1.9 billion tons worldwide and costs about a third of what lithium costs. In addition, zinc weighs about half of what lithium weighs in comparable applications and demonstrates both high gravimetric (820 mAh g−1) and volumetric (5855 mAh L−1) theoretical capacities (Fig. 1) [18]. Furthermore, zinc is environmentally benign, transforming into zinc oxide (the main component of baby powder) after use in batteries and can be easily recycled. More importantly, zinc metal is compatible with aqueous electrolytes, which not only offers much higher ionic conductivities (~ 1 S cm−1 vs. 1–10 mS cm−1 for non-aqueous organic electrolytes) [11], but also provides safer and easier future scale-up applications as compared with organic lithium-ion cells. Based on these advantageous features, tremendous efforts have been carried out in the study of Zn-based battery systems and within the past 5 years from 2014 to 2018, the number of literature focused on Zn-ion and Zn-air batteries (ZIBs and ZABs) has increased almost eightfold (Fig. 2).
Fig. 2

Literatures published within the past 5 years (2014–2018) for ZIBs and ZABs. Data are collected from Web of Science searched with “Zn ion battery” and “Zn air battery” as keywords

Because of the large body of the literature involved and the few excellent reviews already summarizing the progress of Zn-based battery systems in earlier years [14, 15, 17, 19, 20, 21, 22, 23, 24, 25, 26], this review will only highlight the progress reported in recent years. In this review, challenges faced by the current electrode materials (i.e. cathodes for ZIBs, electrocatalysts for ZABs and Zn anodes) will be discussed and strategies developed to tackle these challenges will be presented. And at the end of each section, a table will be provided as an overview and straightforward comparison of representative electrode materials. Overall, the goal of this review is to provide a key perspective in this field in the hopes of advancing further breakthroughs in Zn-based battery technologies.

2 Principles of ZIBs and ZABs

2.1 Principle of ZIBs

Similar to LIBs, the reversible charge and discharge of ZIBs and ZABs are realized through the migration of Zn2+ ions [27], and typically, a ZIB is composed of a zinc metal anode, an aqueous electrolyte and a cathode for the accommodation of Zn2+ ions [28]. During discharge, anodic zinc is dissolved in the form of Zn2+ ions and rapidly diffuses and intercalates into the cathode with a layered/tunnel structure to generate an electron current flow in the electrical loop. The schematic principle of the operation of a typical ZIB is shown in Fig. 3a.
Fig. 3

Schematics of the configuration and mechanism of a a Zn-ion battery and a b Zn-air battery

2.2 Principle of ZABs

ZABs are composed of four main components, including an air electrode comprised of a catalyst-painted gas diffusion layer (GDL), an alkaline electrolyte, a separator and a zinc electrode (Fig. 3b) [29]. During the discharge process, the zinc anode oxidizes and reacts with OH to yield soluble zincate ions (Zn(OH) 4 2− ) [30]. These zincate ions subsequently precipitate spontaneously if supersaturated, forming insoluble zinc oxide. During this process, hydrogen evolution reactions (HERs) may occur concurrently and cause self-corrosion of the Zn anode and generate explosive hydrogen gas, which not only lowers active material utilization, but also increases safety concerns. On the cathode side, oxygen diffuses into the porous electrode driven by a concentration gradient and oxygen reduction reactions (ORRs) occur at the triple phase boundary of the solid working electrode, the liquid electrolyte and the gas phase. Afterwards, hydroxyl ions migrate in the electrolyte from the air cathode to the metallic anode and complete the battery reaction. During charging, these reaction pathways are reversed with the reduction of zincate ions back to zinc and the release of oxygen (Eqs. 15).

Anode:
$$ {\text{Zn }} + \, 4{\text{OH}}^{ - } \leftrightarrow {\text{Zn}}\left( {\text{OH}} \right)_{4}^{2 - } + \, 2{\text{e}}^{ - } $$
(1)
$$ {\text{Zn}}\left( {\text{OH}} \right)_{4}^{2 - } \leftrightarrow {\text{ZnO }} + {\text{ H}}_{2} {\text{O }} + 2{\text{OH}}^{ - } $$
(2)
Cathode:
$$ {\text{O}}_{2} + \, 2{\text{H}}_{2} {\text{O }} + \, 4{\text{e}}^{ - } \leftrightarrow 4{\text{OH}}^{ - } $$
(3)
Overall reaction:
$$ {\text{Zn }} + {\text{ O}}_{2} \to 2{\text{ZnO}} $$
(4)
Parasitic reaction:
$$ {\text{Zn }} + \, 2{\text{H}}_{2} {\text{O}} \to {\text{Zn}}\left( {\text{OH}} \right)_{2} + {\text{ H}}_{2} $$
(5)

3 Electrodes for ZIBs

3.1 Cathode Materials for ZIBs

Rechargeable ZIBs are promising energy storage devices due to the tremendous advantages of zinc metal over other metals. However, the development of ZIB technologies has lagged behind other multivalent metal-based batteries due to the absence of suitable cathode materials. To address this, extensive efforts have been devoted to the direct transfer of electrode materials that can accommodate Li+ ion insertion/extraction to aqueous ZIB systems. However, although the radius of Zn2+ ions (0.74 Å) is similar to that of Li+ ions (0.76 Å), the larger atomic mass and stronger positive polarity of Zn2+ ions result in slower transport kinetics and lower solid-state solubility in bulk electrodes, causing current ZIBs to often suffer from sluggish kinetics. In addition, zinc storage mechanisms in cathode materials have not been well established yet. Therefore, the following section will selectively highlight the latest progress in typical cathode materials developed for ZIBs and discuss the proposed mechanisms during charge and discharge.

3.1.1 Manganese-Based Cathodes

Applications of manganese (Mn) oxides in Zn battery systems can be traced back to 1865, and until now, it is still a hot research topic in which manganese oxides have been widely studied as cathodes for ZIBs due to their low costs, natural abundance, environmentally benignity and relatively high operating voltages. However, discussions of manganese-based oxides are complex due to many polymorphs (e.g. α, β, γ, δ and ε-MnO2) that have been investigated as cathodes for ZIBs [31, 32, 33, 34, 35]. Despite this, these polymorphs can be roughly categorized into tunnel and layered types based on their crystal structure.

Tunnel-type cathodes including α (2 × 2 tunnels, size ~ 4.6 Å), β (1 × 1 tunnel, 2.3 Å × 2.3 Å along its c-axis) and γ (composed of randomly arranged 1 × 1, size ~ 2.3 Å × 2.3 Å and 1 × 2, size ~ 2.3 Å × 4.6 Å tunnel blocks)-MnO2 and α-MnO2 have been widely reported to be able to facilitate fast and reversible Zn-ion storage. This is because α-MnO2 can undergo a transition from the tunnel to layer type in its crystal structure to enable fast Zn-ion intercalation. Here, layer structured intermediates possess much larger spacings; however, this is also accompanied by increased stress within the structure and active dissolution of Mn in the electrolyte, which can increase structural collapse and result in rapid capacity fading and poor cycling performance. Researchers have also reported that the addition of small amounts of MnSO4 in neutral electrolytes is an efficient strategy to prevent Mn2+ ion dissolution [36] and that the tunnel structure can be stabilized through the addition of small numbers of potassium ions (K+) [37]. However, the intercalation of K+ can hinder Zn2+ ion diffusion, and the large steric and electrostatic interactions within the narrow α-MnO2 tunnel walls (~ 0.7 nm) can compromise performance. For example, Wu et al. [38] recently used a graphene scroll to coat the surface of α-MnO2 and reported that this coating effectively prevented the dissolution of Mn2+ ions into the electrolyte and optimized the performance of the corresponding ZIBs in which a high energy density of 406.6 Wh kg−1 (382.2 mAh g−1) with excellent long-term cycling stability (94% capacity retention after 3000 cycles at 3 A g−1) was achieved. In addition to α-MnO2, other tunnel-type MnO2 has also been explored. For example, Lee et al. [37] used todorokite-type MnO2 with a one-dimensional 3 × 3 tunnel structure (size ~ 7.0 Å) through the b-axis as the cathode for a ZIB and reported that the larger tunnel size in combination with the crystal water inside the structure can effectively shield the electrostatic interactions between Zn2+ ions and tunnel walls, allowing for enhanced rate performances superior to that of α-MnO2 in which capacity retention over 40% even at a high current density of 10 C can be achieved.

The performance of Mn-based materials is also closely related to particle size, morphology, defects and orientation in the crystal structure [39, 40]. For example, rutile β-MnO2 possesses an arrow tunnel spacing of 2.3 Å but a highly stable crystal structure and in earlier attempts by Kim et al. [41] to use bulk β-MnO2 as a cathode, only poor performances were achieved. However, the researchers subsequently fabricated β-MnO2 into nanorods with the (101) planes exposed and reported that the resulting β-MnO2 cathode delivered a high discharge capacity of 270 mAh g−1 at 100 mA g−1, a ~ 75% capacity retention and 100% Coulombic efficiency at 200 mA g−1 for 200 cycles (Fig. 4). Recently, Chen et al. [42] also investigated β-MnO2 cathode performances by using a concentrated Zn(CF3SO3)2 electrolyte with a Mn(CF3SO3)2 additive, allowing for the suppression of Mn2+ dissolution and the formation of a uniform porous MnOx nanosheet layer on the cathode surface, which is beneficial to maintain electrode integrity. As a result, the β-MnO2 cathode in this study exhibited high reversible capacity, high rate capability and stable cyclability in which a high reversible capacity of 225 mAh g−1 and long-term cyclability with 94% capacity retention over 2000 cycles were achieved.
Fig. 4

a SEM and b TEM images of the β-MnO2 nanorod sample (enlarged area of b: illustration of the crystallographic structure and 1 × 1 open tunnels in the β-MnO2 nanorod). c Cyclability of the β-MnO2 nanorod cathode in the ZnSO4 electrolyte with the addition of MnSO4. Reprinted with permission from Ref. [41], copyright 2017, Royal Society of Chemistry

In theory, layered structures with large spacings are more favourable for Zn2+ ion diffusion and storage and can contribute to superior electrochemical performances. However, the performance of reported layered-type MnO2 has been moderate [33] in which researchers report that during the cycling process, the electrode structure can undergo a layered-to-spinel transition, which leads to much smaller tunnel sizes [43]. This structural transformation mainly originates from the intrinsically high mobility and solubility of Mn2+ ions, and efforts to suppress the migration of Mn include the substitution of Mn with fixed valence cations and more electronegative and low-mobility transition metal ions. In addition, researchers also report that the intercalation of hydrated cations can effectively shield the electrostatic interactions between Zn2+ ions and the host material and facilitate Zn2+ ion diffusion. However, the presence of excessive hydrated cations can increase structural stress in layered structures and can result in poor cycling performances. For example, Xia et al. [44] demonstrated in a recent study that polymers such as polyaniline (PANI) can be used as an efficient intercalating material (Fig. 5) because it can help to create enlarged spacings and act as a “glue” to strengthen the crystal structure during cycling, thereby enhancing performances significantly. And in their study, the novel hierarchical structure retained a high capacity of 280 mAh g−1 after 200 cycles (90% utilization of the theoretical capacity of MnO2), and even at a high current density of 2000 mA g−1, the resulting PANI-intercalated MnO2 nanolayer composite exhibited a stable discharge capacity of ~ 125 mAh g−1 (up to 40% utilization) for over 5000 cycles.
Fig. 5

a Schematic of the expanded intercalated structure of PANI-MnO2 nanolayers. b Cycling performance (red) and corresponding Coulombic efficiency (blue) at 200 mA g−1.Reprinted with permission from Ref [44], copyright 2018, Nature Publishing Group

Interestingly, the layered-to-spinel transition has always been regarded as a spontaneous process until an alternative process was revealed and investigated recently in which Nam et al. [45] found that spinel Mn3O4 can transform to layered birnessite AxMnO2·yH2O (A = monovalent or divalent cations) if electrochemically cycled in aqueous electrolyte solutions. Furthermore, Kim et al. [46] conducted a thorough investigation into this behaviour and identified that this alternative phase transition is mediated by the formation of a transition phase at the phase boundary that can trap crystal water in a stepwise manner in which the phase transition progresses through the layer-by-layer propagation of the transient phase towards the original spinel. In addition, recent findings reported by Kang et al. [47] further confirmed the spinel to layer transition behaviour of Mn3O4 and reported that this can produce decent rate and cycling performances if applied as a cathode for ZIBs (Fig. 6).
Fig. 6

Schematic of the transition of Mn3O4 into layered-type birnessite during the first cycle. Reprinted with permission from Ref [47], copyright 2018, Elsevier

Along with electrochemical performance, researchers are also trying to unveil the underlying reaction mechanisms of Mn-based electrodes to obtain deep insights and guide future design. However, the storage mechanisms of Zn2+ ions in Mn-based electrodes remain elusive and at times controversial. Based on previous understandings, the storage of Zn2+ ions mostly relies on the reversible insertion/extraction of Zn2+ within electrode materials. However, in recent years, with extensive research and advances in in situ and ex situ characterization techniques (e.g. transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and Raman spectroscopy), three plausible mechanisms have been proposed [42, 48, 49, 50]. The first mechanism involves the reversible insertion/extraction of Zn2+ ion in which Wu et al. [38] proposed a two-step intercalation mechanism with Zn2+ cations intercalating first into layers and subsequently into tunnels of α-MnO2 nanowires. The second mechanism involves the co-insertion/extraction of H+ and Zn2+ ions in which Huang et al. [44] proposed the sequential intercalation of H+ and Zn2+ into layered birnessite MnO2 due to the appearance of zinc hydroxide sulphate hydrate (Zn(OH)2)3(ZnSO4)(H2O)5) in XRD results (Fig. 7a) and the presence of a reversible Zn–O band in Raman spectra (Fig. 7a, b). This co-insertion mechanism was also supported by Sun et al. [48] based on the consecutive appearance of MnOOH and ZnMn2O4 after the cycling of ε-MnO2 cathodes in their study. And finally, the third mechanism involves the conversion reaction in which Pan et al. [49] in their study reported that they were able to only detect the MnOOH phase (nanorod and nanoparticle aggregates in Fig. 7c) in the discharged products and that combined with elemental analyses (Fig. 7d), the researchers concluded that the discharge products were generated from the conversion reaction between MnO2 and protons from water (MnO2 + H+ + e↔ MnOOH). In addition, no ZnxMnO2 products from the intercalation of Zn2+ as previously recognized were observed. These three proposed mechanisms have been discussed in several recent reviews; however, arguments remain over the actual zinc storage mechanism, which may stem from the difficulty in accurately identifying intermediates formed by Mn-based electrodes during the electrochemical reaction process. For example, Oh et al. [34, 51] conducted thorough investigations into the Zn2+ ion storage mechanism of α-MnO2, and initially, the phase at the end of the discharge process was identified to be Zn-birnessite with a similar but slightly distorted structure as ZnMn3O7·3H2O, possessing an interlayer distance around 7 Å [51]. However, further investigations revealed that due to the active dissolution of Mn into the electrolyte, the actual intermediates possessed layered structures with a much larger interlayer distance (11 Å) to enable the fast diffusion of Zn2+ ions [34]. To add to the complexity, the structural transformation behaviours and intermediates varied among different MnO2 polymorphs. For example, Kim et al. [52] reported that the structure of γ-MnO2 electrodes can transform into the spinel-type Mn(III) phase (ZnMn2O4), but was found to be able to also transform into two new intermediary Mn(II) phases including tunnel-type γ-ZnxMnO2 and layered-type L-ZnyMnO2 and that these phases with multi-oxidation states can coexist after the complete electrochemical Zn insertion.
Fig. 7

a Selected ex situ XRD patterns of PANI-intercalated MnO2 nanolayers during the charge-discharge process. b Schematic of the sequential insertion of H+ and Zn2+. Reprinted with permission from Ref. [44], copyright 2018, Nature Publishing Group. c TEM and d STEM-HAADF images of the MnO2 electrode at the discharged state. Reprinted with permission from Ref. [49], copyright 2016, Springer Nature

Driven by ever-growing global demand, flexible and wearable energy storage devices are being developed to power next-generation electronics [53, 54, 55, 56, 57]. And with compatibility to aqueous electrolytes, solid-state ZIBs possess better safety characteristics as compared with LIBs which require organic electrolytes. However, solid-state rechargeable ZIBs have rarely been reported due to the lack of electrolytes with high Zn2+ ion conductivity as well as the unavoidable issue of Mn2+ ion dissolution in electrolytes, which can result in poor stability and cyclability. Despite this, Zhi et al. [58] recently reported that they successfully integrated MnO2 with a surface polypyrrole coating into a wire-shaped ZIB and obtained impressive performances. In addition, issues related to rapid degradation and poor mechanical strength have also been alleviated in which Li et al. [59] in their study developed a gelatin and polyacrylamide (PAM)-based polymer electrolyte combined with MnO2/CNTs electrodes. Here, the as-fabricated device demonstrated remarkable electrochemical performances even under harsh conditions. And overall, these studies provide guidelines for the integration of ZIBs into flexible and wearable energy storage technologies.

3.1.2 Vanadium-Based Cathodes

Vanadium-based materials have been extensively applied as cathodes for aqueous ZIBs [60, 61], and rich V–O diagrams can provide diverse polymorphs with many specific advantages. For example, the open framework of the mother V2O5 can provide channels for ion diffusion and charge transfer can be simultaneously realized through electron hopping with valence exchange between V4+/V3+ and V5+. In addition, the existence of this mixed valence state brings about oxygen vacancies and enhances electrical conductivity. However, due to its unstable structure, weak conductivity and a low ion diffusion coefficient, bulk V2O5 electrodes often undergo rapid capacity decay upon cycling and demonstrate unsatisfactory stability. Alternatively, in contrast to layer-structured V2O5, vanadium dioxide VO2(B) possesses a shear-type structure that can provide larger resistances to lattice shearing against ion insertion/extraction and therefore is expected to provide better stability. In addition, the lattice spacing of VO2(B) is large enough to allow Zn2+ to insert/extract conveniently. Typical VO2 (B) materials are built from distorted VO6 octahedra, which share both corners and edges, forming tunnel-like frameworks (0.82 nm2 along the b-axis, 0.34 nm2 along the a-axis and 0.5 nm2 along the c-axis) [62], and the application of VO2 (B) has been extensively demonstrated in lithium-, sodium- and magnesium-ion batteries [63, 64, 65, 66]. Furthermore, many studies have investigated the performance of VO2(B) for ZIBs. For example, Ding et al. [67] recently investigated the behaviour of VO2 (B) nanofibers in ZIBs and reported that due to the unique tunnel-type framework and robust crystal structure, the VO2(B) nanofibers displayed stable intercalation pseudocapacitance behaviours with ultrafast Zn2+ ion diffusion kinetics in which a high reversible capacity of 357 mAh g−1, an excellent rate capability of 171 mAh g−1 at 300 C and high energy and power densities were achieved. Furthermore, researchers report that electrodes based on VO2(B) nanoparticles can further be optimized through composition with conductive materials such as graphene and carbons. For example, Niu et al. [68] fabricated 3D freestanding reduced graphene oxide/VO2 (rGO/VO2) composite films using a freezing-drying process and reported that the use of conducting additives and binders was effectively avoided in which the resulting freestanding 3D architecture endowed the electrode with a high energy density of 65 Wh kg−1 even at a high-power density of 7.8 kW kg−1.

Layered vanadates (MxVnOm, M = metal ion, e.g. Zn2+ [69, 70, 71], Ca2+ [72], Na+ [73], Li+ [74], K+ [75] and Ag+ [76]) with or without crystal water have also drawn extensive attention in recent years, and the performance of Zn0.25V2O5·nH2O [72], Na2V6O16·3H2O [77], Na0.33V2O5 [78], NaV3O8 [79], Na1.1V3O7.9 [73], NaV6O16·1.63H2O [80], K2V2O8 [75], Ag0.4V2O5 [76], FeV15O39(OH)9·9H2O [81], LixV2O5·nH2O [74], H2V3O8 [82, 83] and MgxV2O5·nH2O [84] has been systematically investigated. For example, as compared with other layered vanadium oxides such as V2O5 and VO2 that are held together by weaker van der Waals interactions, Zhou et al. [85] reported that the hydrogen bonds in H2V3O8 can provide better structural integrity and stability during charge/discharge processes and that the high average valence (+ 4.67) can contribute to more active redox sites and larger specific capacities. In addition, Wang et al. [83] reported that due to the synergistic effects arising from the structural merits of H2V3O8 nanowires (NW) and the high conductivity of graphene networks, H2V3O8 NW/graphene composites can exhibit superior zinc-ion storage performances including a high capacity of 394 mAh g−1 at 1/3 C, a high rate capability of 270 mAh g−1 at 20 C and excellent cycling stability up to 2000 cycles with capacity retention of 87%. As for sodium vanadates (NVO), these materials possess typical layered structures with structural flexibility and can serve as guest species to store Li+ and Na+ ions with good electrochemical properties. And considering that the ionic radii of Li+ (0.74 Å) and Zn2+ (0.76 Å) are almost equal, NVO electrodes possess excellent potential in ZIBs. For example, Mai et al. [78] reported that in Na0.33V2O5, intercalating Na+ between [V4O12]n layers can act as “pillars” to increase the 3D stability of the tunnel structure upon ion insertion/extraction (Fig. 8a) and that reversible charge and discharge can be realized due to the formation of ZnxNa0.33V2O5 in which a high specific capacitance of 367.1 mAh g−1 at 0.1 A g−1 and long-term cyclic stability with capacity retention over 93% for 1000 cycles can be obtained. Furthermore, Guo et al. [79] thoroughly compared the performance and energy storage mechanisms of three typical structures of sodium vanadates, including NaV3O8·xH2O and HNaV6O16·4H2O with a layered structure and β-Na0.33V2O5 with a tunnel structure (Fig. 8b). Here, the researcher reported that the structure and configuration of the crystal structure play a key role in determining the electrochemical performance in which layered structures generally show much higher capacities as compared with the tunnel structure (Fig. 8c). However, these researchers also reported that the long-term stability of the layered structured sodium vanadates can be compromised by the collapse of the layered structure and the formation of secondary phases upon cycling in which only 71% and 42% capacity retention after 2000 cycles was observed for layer-structured Na5V12O32 and HNaV6O16·4H2O, respectively (Fig. 8d). Alternatively, tunnel-structured Na0.33V2O5 displayed relatively low capacitances, but much better stability (Fig. 8c). A similar conclusion was reached by Liang et al. [75] who systematically investigated the electrochemical performance of potassium-doped vanadates with different crystal structures including tunnel-type K2V8O21 and K0.25V2O5 and layered-type K2V6O16·1.57H2O and KV3O8. In this study, tunnel-type electrodes proved to be stabler than layered ones and specifically, the K2V8O21 electrode with a tunnel-type structure demonstrated a high capacity of 247 mAh g−1 at 0.3 A g−1as well as excellent cyclic stability up to 300 cycles.
Fig. 8

a Schematic of the zinc-storage mechanism in NVO electrodes. Reprinted with permission from Ref [78], copyright 2018, John Wiley and Sons. b The crystal structures of NaV3O8 and β-Na0.33V2O5. c Cycling performances of NVO electrodes at 500 mA g−1.d Cycling performance of Na5V12O32at 4 A g−1.Reprinted with permission from Ref [79], copyright 2018, John Wiley and Sons

For cation-doped vanadates, electrochemical performance is determined by many parameters, including composition, doped cation valence states and crystal structure. For example, as compared with monovalent alkaline metal cations (e.g. Li+, Na+ and K+), divalent metal ions bonded with oxygen atoms can result in stronger ionic bonds. And typically, if applied as a cathode in ZIBs, Zn0.25V2O5·nH2O can deliver much higher capacities (> 300 mAh g−1) and long-term cycling stability (> 1000 cycles) as well as impressive rate capabilities [11]. In a further study, Xia et al. [72] discovered that the replacement of Zn2+ by Ca2+ can effectively enlarge the cavity between V4O10 layers (Fig. 9a–c) and that due to the lower molecular weight and better electrical conductivity (Fig. 9d), Ca0.25V2O5·nH2O can provide improved performances as compared with Zn0.25V2O5·nH2O, in which a high capacity of 340 mAh g−1 can be achieved at 0.2 C (Fig. 9e,f).
Fig. 9

a Crystal structure of Ca0.25V2O5·nH2O. b, c Typical XRD patterns and d electricity conductivities of Ca0.25V2O5·nH2O and Zn0.25V2O5·nH2O. e Rate performance and f cycle stability of Ca0.25V2O5·nH2O. Reprinted with permission from Ref [72], copyright 2018, John Wiley and Sons

In vanadates containing crystal water, H2O molecules always play an important role in the performance of electrodes. Many reports have attempted to interpret the fundamental mechanisms of the promotion effect arising from H2O molecules, and a recognized agreement is that solvating water can work as a charge shield for metal ions and can reduce their effective charge and their interaction with host framework functions, thereby promoting Zn2+ ion diffusion [79, 86]. In addition, researchers also suggest that the intercalation of H2O molecules can open up diffusion tunnels. For example, Wang et al. [87] used a simple hydrothermal process to synthesize V10O24·12H2O dendrites with a large interlayer spacing of 1.4 nm and reported a specific capacitance of 164.5 mAh g−1 and stable cycling performances with 80.1% retention after 3000 cycles. Alternatively, the interlayer spacing in some layered electrodes can decrease during discharge due to the attraction between inserted Zn2+ ions and water molecules. For example, Xia et al. [72] reported that the interspacing of Ca0.25V2O5·nH2O electrodes can change from 10.6 Å to 10.0 Å during discharge. Furthermore, Wang et al. [88] investigated the effects of H2O in electrolytes and in electrode lattices on the thermodynamics and kinetics of reversible multivalent-ion intercalation chemistry based on a model platform of layered VOPO4. Here, the researchers reported that the intercalation of H2O molecules into the interlayer of VOPO4·2H2O can significantly expand the basal spacing from 4.2 Å to 7.4 Å as compared with VOPO4 (Fig. 10a, b) and that the migration of “free” H2O molecules can effectively promote the diffusion of Zn2+ ions at the electrolyte/electrode interface. Furthermore, the researchers reported that crystalline H2O molecules in the lattice can shift Zn2+ ion insertion potentials to a higher range (Fig. 10c, d).
Fig. 10

a XRD patterns of pristine VOPO4,VOPO4·2H2O and heated VOPO4·2H2O. b Schematic of the transformation between VOPO4 and VOPO4·2H2O. c, d Schematic of VOPO4 and VOPO4·2H2O in 0.1 M Zn(OTf)2-AN without water and with 1% H2O, respectively. Reprinted with permission from Ref [88], copyright 2018, John Wiley and Sons

The Zn2+ ion storage mechanism in vanadium-based electrodes has also been widely studied, and most reported electrodes follow a traditional intercalation/deintercalation mechanism. However, Liang et al. [76] recently reported a mechanism involving a combination of displacement and intercalation by using Ag0.4V2O5 as a cathode for a ZIB (Fig. 11). In this mechanism, the substitution of Ag+ for Zn2+ can construct more available sites for Zn2+ ion storage and the reduction of Ag+ to Ag0 can increase the electrical conductivity of the electrode. And as a result, the electrode in this study delivered a high capacitance of 144 mAh g−1 even after cycling at a high current density of 20A g−1 for 4000 cycles. Furthermore, Wan et al. [89] proposed a mechanism based on the co-insertion of H+ and Zn2+ into NaV3O8·1.5H2O nanobelt positive electrodes in which the simultaneous intercalation of the dual carriers resulted in a high and reversible capacity of 380 mAh g−1 and capacity retention of 82% after cycling for over 1000 cycles.
Fig. 11

Schematic of the combination of the displacement and intercalation mechanisms in Ag0.4V2O5 during the discharging/charging process. Reprinted with permission from Ref [76], copyright 2018, Elsevier

3.1.3 Prussian Blue (PB) Analogues

Prussian Blue (PB) is a well-known electrode material dating back to 1936 and currently, PB, and its derivatives, PBAs (AxMy[B(CN)6]z·mH2O (x, y, z, m = stoichiometric numbers, A, B = alkaline metal, M = transition metal), are still being actively investigated as electrode materials for different batteries [90]. This is because the crystal structure of PB with its open framework can endow excellent rate capabilities that can realize fast charge and discharge. However, the main issues associated with PB and PBA electrodes are their rapid capacity fading, poor cycling performance and the fact that the voids and H2O present in the crystal are not conductive. And thus far, extensive efforts have been carried out to investigate the storage behaviour of PBA for monovalent alkaline metal ions such as Li+, Na+ and K+ [91, 92, 93, 94]. However, in terms of divalent Zn2+ ion storage, research efforts are relatively less and reported performances have been moderate. For example, in contrast to extensively explored cubic metal hexacyanoferrates (MeHCFs), Zhang et al. [95] studied rhombohedral zinc hexacyanoferrate (ZnHCF) with large open sites in a porous 3D framework and reported a high voltage of 1.7 V with an exceptionally high energy density of 100 Wh kg−1 in application as a cathode for ZIBs. Furthermore, Hong et al. [96] investigated the performance of potassium nickel hexacyanoferrate in organic electrolytes and reported that this cell can achieve a reversible discharge capacity of 55.6 mAh g−1 at a rate of 0.2 C with a discharge voltage at 1.19 V (vs. Zn2+/Zn). In another study, La Mantia et al. [97] applied copper hexacyanoferrate (CuHCF) as cathodes for ZIBs using a 20 mM aqueous solution of ZnSO4 as the electrolyte and reported that their cell was able to provide a 90% theoretical capacity with capacity retention of 96.3% after 100 cycles and an average discharge potential of 1.73 V, demonstrating good rate capabilities. However, the researchers in this study also reported the difficulty of achieving stable electrodes that can last for 1000 cycles or more.

To understand the degradation mechanisms of ZIBs and improve cycle life spans, Kasiri et al. [98] studied the effects of electrolytes (nature and concentration) and current rates on the degradation of cathodes during battery operation. Here, the researchers reported that anion concentrations can play an important role in the performance of batteries in which lower Zn2+ ion concentrations in the electrolyte can yield better stability, suggesting that cathode degradation is not caused by the dissolution of active materials but due to phase transformations during the charge-discharge process in which upon cycling, Zn2+ ions from the electrolyte can occupy positions in a CuCHF structure and form a ZnCHF phase that can affect subsequent Zn2+ ion intercalation. In a further study, Svensson et al. [99] investigated the storage mechanism of Zn2+ in a CuCHF framework structure using operando synchrotron X-ray diffraction and revealed that there was no apparent change in unit-cell contraction even with the high cation insertion content. Alternatively, the researchers in this study also reported that Fe(CN)6 vacancies within the CuHCF framework can play an important role in the structural-electrochemical behaviour of this system in which the hopping of Zn2+ from cavity sites to vacant ferricyanide may be responsible for the observed structural changes during the electrochemical cycling process. In the past 2 years, the performances of PBA-based electrodes have also been improved by fabricating them into hybrid battery systems. For example, Hou et al. [100] developed a hybrid Na/Zn aqueous rechargeable battery based on sodium manganese hexacyanoferrate Na2MnFe(CN)6 and zinc metal and increased the stability window to 2.5 V through the addition of sodium dodecyl sulphate (SDS) as a surfactant. Furthermore, Wang et al. [101] proposed a stable hybrid Na-Zn aqueous battery (Fig. 12a, b) with large discharge capacities and good rate performances (Fig. 12c, d) in which the hybrid full cell delivered excellent cycling stability with ~ 80% capacity retention after 1000 cycles at 300 mA g−1 (Fig. 12e).
Fig. 12

a Schematic for the NaFe-PB//Zn aqueous hybrid-ion battery. b The crystal structure of [Fe2(CN)6]. c, d and e Galvanostatic discharge/charge voltage profiles, rates and long cycling performances of the full cell. Reprinted with permission from Ref [101], copyright 2017, Elsevier

Due to its open-framework crystal structure, PB and its analogues can be used as electrodes for Zn2+ ion storage, providing remarkable rate performances and high voltage windows at around 1.7 V. However, capacities are relatively low (< 60 mAh g−1) and fade quickly during long-time cycling, resulting in the poor stability of electrodes. To reveal the degradation mechanisms, the interactions of Zn2+ with vacant sites and H2O in the crystal structure and anions in the electrolytes have thus far been investigated. However, key factors remain unclear, and more advanced characterization techniques and systematic control over impacting parameters are needed to further clarify the degradation mechanisms and facilitate the improvement of such electrodes.

3.1.4 Other Cathodes

NASICON (sodium superionic conductor)-structured materials are also appealing electrodes for ZIBs, and the facile insertion of Li+ and Na+ has been extensively demonstrated in NASICON-type electrodes [102, 103, 104]. Recently, NASICON-type Na3V2(PO4)3 was also tested as a cathode for ZIBs in which Huang et al. [105] reported that a 1.1 V voltage and 97.5 mAh g−1 capacities can be achieved for a Na3V2PO4 cathode. However, the researchers in this study also reported that the electrode suffered from inadequate cyclability (74% capacity retention after 100 cycles). Researchers also report that the incorporation of Na3V2(PO4)3 electrodes into rechargeable hybrid aqueous batteries (ReHABs) can significantly improve rate capabilities and cyclabilities. For example, Islam et al. [106] incorporated a Na3V2(PO4)3/C electrode into a ReHAB system and reported a high rate capability of 66 mAh g−1 at 32 C and excellent cyclability over 1000 cycles (72% retention of the initial capacity). In addition, the adoption of other NASICON-type electrodes with better stability is also an alternative solution. For example, Li et al. [27] investigated Na3V2(PO4)2F3 as an electrode, which possesses 0.5 V higher voltage and better structural stability than Na3V2(PO4)3 due to the strong affinity of the F atoms towards the surroundings, and reported that the electrode exhibited ultrastable cycling performances with 95% capacity retention over 4000 cycles at a current density of 1 A g−1 if applied as a cathode for ZIBs.

In the exploration and design of suitable cathodes for ZIBs, researchers have also used clues from the study of Zn2+ ion storage behaviours in traditional electrode materials for LIBs and SIBs. For example, Liu et al. [107] systematically investigated the performance of various transition metal oxides, sulphides and borides as cathodes for ZIBs and found that MnS can be used as a promising cathode for ZIBs, exhibiting a higher capacity of 221 mAh g−1 as compared with other electrode materials. In addition, Wang et al. [108] reported that Mo6S8 can enable the reversible storage of Zn2+ in both aqueous and non-aqueous electrolytes with a specific capacity of ~ 90 mAh g−1 and aqueous Zn/VS2 batteries were reported by Mai et al. [109] in which the researchers reported that the large interlayer spacing (5.76 Å) of VS2 enabled the facile diffusion of Zn2+ ions, allowing the corresponding battery to deliver a high capacity of 190.3 mAh g−1 at a current density of 0.05 A g−1. In a further study, Ma et al. [110] studied a Zn/Co3O4 battery based on a Co(III) rich-Co3O4 nanorod electrode in a mild electrolyte and revealed a novel conversion mechanism between CoO and Co3O4 in which the battery delivered a high voltage of 2.2 V and a capacity of 205 mAh g−1 with extreme cycling stability of 92% capacity retention even after 5000 cycles. NiCo2O4 is another popular cathode material that has been widely adopted. However, its main challenges are the inferior conductivity and insufficient concentration of active sites. Here, Zeng et al. [111] recently tried to introduce oxygen vacancies along with the modification of phosphate ions (P-NiCo2O4−x) on the surface of NiCo2O4 electrodes in which the successful modification of the phosphate ions on the NiCo2O4 was confirmed through electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) (Fig. 13a–d). And as a result, the researchers reported that the resulting electrode possessed improved surface reactivities and surface kinetics, achieving an admirable capacity of 309 mAh g−1 at a high current density of 6 A g−1 (Fig. 13e).
Fig. 13

a EPR spectra of NiCo2O4 and P-NiCo2O4−x. bd XPS survey scans showing the O 1 s b Co 2p c and P 2p d spectra for NiCo2O4 and P-NiCo2O4−x.e Cycling performances of the NiCo2O4 and P-NiCo2O4−x electrodes over 3000 cycles. Reprinted with permission from Ref [111], copyright 2018, John Wiley and Sons

Along with significant breakthroughs in inorganic materials, organic compound-based cathodes have also been investigated for ZIBs. Due to their soft and flexible crystal structures, organic compounds possess distinguished advantages over inorganic cathode materials for ZIBs. For example, organic compound-based cathodes are held together by weak van der Waals forces and only show modest Coulomb repulsions to ion diffusion. In addition, the malleable and soft lattice of organic compound-based cathodes can allow for the reorientation of inserted molecules and enable the reversible intercalation of hard divalent cations [112, 113, 114, 115]. Therefore, researchers are trying to use organic compound-based cathodes for aqueous ZIBs, which results in intriguing properties with improved stability and large capacities. For example, various quinone compounds with carbonyls in para-position [i.e. 1,4-naphthoquinone (1,4-NQ), 9,10-anthraquinone (9,10-AQ) and C4Q] and ortho-position [1,2-naphthoquinone (1,2-NQ) and 9,10 phenanthrenequinone (9,10-PQ)] were investigated as cathodes in aqueous ZIBs by Zhao et al. [114] in which all selected quinone electrodes demonstrated feasibility for highly reversible Zn2+ ion uptake and quinone (C4Q), possessing an open bowl structure and eight carbonyls, exhibiting a high capacity of 335mAh g−1 with an energy efficiency of 93% at 20 mA g−1 and a long life span of 1000 cycles with capacity retention of 87% at 500 mA g−1. In another example, Kundu et al. [112] tested tetrachloro-1,4-benzoquinone as a cathode for ZIBs in mild acidic aqueous electrolytes and achieved a high capacity above 200 mAh g−1 with a voltage of 1.1 V (Fig. 14a) in which further investigations revealed that the corresponding charge-discharge process was realized through a phase transition between p-chloranil and Zn-p-chloranil following a water-assisted phase transfer mechanism (Fig. 14b, c). However, the researchers in this study suggested that although the common dissolution associated with organic compounds was absent, the growth of large discharge products can result in capacity fading upon cycling. Based on this, the researchers further developed an efficient nanoconfinement strategy to confine the organic molecules into CMK-3 carbon channels and further reported significantly enhanced rechargeability and cyclability of the resulting electrode (Fig. 14d, e).
Fig. 14

a Cyclic voltammograms (CV) of the p-chloranil cathode. b, c Schematic and DFT structural model of the H2O assisted phase transfer mechanism. Cyclability and Coulombic efficiency of the p-chloranil-infused CMK-3 electrode at a 0.2 d and 1C rate e (1C: 217 mA g−1). Reprinted with permission from Ref. [112], copyright 2018, American Chemical Society

The electrochemical performances of cathode materials for ZIBs are summarized in Table 2.
Table 2

Electrochemical performances of various cathode materials for ZIBs

Cathode

Electrolyte

Capacity (mAh g−1)/current density (A g−1)

Stability (capacity (mAh g−1) or capacity retention (%)/cycle number/current density (A g−1)

Reference

Graphene-coated α-MnO2

2 M ZnSO4 + 0.2 M MnSO4

382/0.3, 166/3

362.6/100/0.3

[38]

β-MnO2

1 M ZnSO4

270/0.1, 86/1

75%/200/0.2

[41]

β-MnO2

3 M Zn(CF3SO3)2 + 0.1 M Mn(CF3SO3)2

225/0.2, 100/10

94%/2000/6.5

[42]

Layered MnO2

1 M ZnSO4

222/0.1, 61/1

97/50/0.1

[43]

PANI-intercalated MnO2

2 M ZnSO4 + 0.1 M MnSO4

260/0.05, 110/3

280/200/0.2

[44]

Mn3O4

2 M ZnSO4

239.2/0.1, 124/2

106.1/300/0.5

[47]

α-MnO2

2 M ZnSO4 + 0.2 M MnSO4

290/0.09

150/20/1.3 C

[48]

MnO2

1 M ZnSO4 + 0.1 M MnSO4

143.2/1 C, 86.8/5 C

74.2%/850/5 C

[58]

MnO2

polymer

306/0.06, 150/1.8,

97%/1000/2.7

[59]

Mn2O3

2 M ZnSO4

148/0.1, 38/2

117.2/30/0.1

[116]

MnO2

Polyacrylamide (PAM)

302.1/0.06,17.7/1.5

98.5%/500/2

[117]

ZnMn2O4

1 M ZnSO4 + 0.05 M MnSO4

70.2/3.2

106.5/300/0.1

[118]

Na3V2(PO4)2F3

2 M Zn(CF3SO3)2

60/0.2, 33/3

46/4000/1

[27]

V2O5

3 M Zn(CH3F3SO3)2

470/0.2386/10

91.1%/4000/5

[60]

V2O5

3 M ZnSO4

224/0.1

113/400/2

[61]

VO2(B)

3 MZn(CF3SO3)2

357/0.1, 171/51.2

274.1/300/2

[67]

Graphene/VO2

3 MZn(CF3SO3)2

276/0.1, 120/35

276/1000/4

[68]

Zn2(OH)VO4

Fumed silica/ZnSO4

204/0.5 C, 101/50 C

125/2000/20 C

[69]

Zn2V2O7

1 M ZnSO4

203.4/0.3, 170/4.4

197.4/200/0.3, 138/1000/4

[70]

Zn3V2O7(OH)2·2H2O

1 M ZnSO4

213/0.05, 54/3

101/300/0.2

[71]

Ca0.25V2O5·nH2O

1 M ZnSO4

340/0.2 C, 72/80 C

96%/3000/80 C

[72]

Na1.1V3O7.9/graphene

1 M Zn(CF3SO3)2

238/0.05, 184/0.3

171/100/0.3

[73]

K2V8O21

2 M ZnSO4

247/0.3, 92/4

185/100/1

[75]

Ag0.4V2O5

3 M ZnSO4

237/0.5, 216/5

144/4000/20

[76]

Na2V6O16·3H2O

1 M ZnSO4

268/5 C, 152/40 C

228/300/15 C

[77]

Na0.33V2O5

3 M Zn(CH3F3SO3)2

276.6/0.2,96.4/2

93%/1000/1

[78]

Na5V12O32

2 M ZnSO4

281/0.5

71%/2000/4

[79]

Fe5V15O39(OH)9·9H2O

0.3 M Zn(TFSI)2

385/0.1, 80/7

80%/300/5

[81]

H2V3O8

3 M Zn(CF3SO3)2

423.8/0.1, 113.9/5

94.3%/1000/5

[82]

H2V3O8/graphene

3 M Zn(CF3SO3)2

336/0.1, 215/3

240/2000/6

[83]

V2O5·nH2O

3 M Zn(CH3F3SO3)2

381/0.06,248/30

71%/900/6

[86]

V10O24·12H2O

3 M Zn(CF3SO3)2

159/0.2, 80/10

83.1%/200/0.5

[87]

NaV3O8·1.5 H2O

1 M ZnSO4 + 1 M Na2SO4

380/0.1221/1

82%/1000/4

[89]

V3O7·H2O

1 MZnSO4

375/0.375, 275/3

80%/200/3

[119]

LiV3O8

1 M ZnSO4

250/16, 29/1666

172/65/0.133

[120]

MnS

2 M ZnSO4

221/0.1, 110/0.5

70/100/0.5

[107]

VS2

1 M ZnSO4

190.3/0.05, 115.5/2

110.9/200/0.5

[109]

Co3O4

2 M ZnSO4 + 0.2 M CoSO4

200/0.5, 62/8

110/5000/4

[110]

P-NiCo2O4−x

1 M KOH + 0.05 M Zn(CH3COO)2

309.2/6, 172.1/18.1

96.4/5000/25.3

[111]

CMK-3-p-chloranil

1 M Zn(OTf)2

170/0.04, 118/0.21

90/100/0.04, 83/200/0.21

[112]

Calix [4]quinone

3 M Zn(CF3SO3)2

335/0.02, 172/1

87%/1000/0.5

[114]

4 Electrocatalysts for ZAB Air Electrodes

Currently, the development of practical and viable rechargeable ZABs remains challenging. In typical ZABs, oxygen reduces to hydroxyl ions that combine with zinc ions at the anode to form zincate ions (Zn(OH) 4 2− ), which can subsequently decompose to produce ZnO. However, the electrochemical reaction kinetics of oxygen in practice is generally slow in the absence of catalysts in which the difficulty stems from the exceptionally strong O=O bond (498 kJ mol−1), which is extremely difficult to break electrochemically. Here, an oxygen electrocatalyst can assist in bond activation and cleavage and can therefore effectively accelerate the ORR or OER process, and in recent years, many electrocatalysts have been developed. In this review section, recent representative studies will be highlighted and electrocatalysts for ZAB air electrodes will be discussed by dividing them into three major categories, including: (1) noble metal-based alloys and oxides; (2) carbon or heteroatom-doped carbon materials; and (3) metal oxides/sulphides/nitrides and their composites that are coupled with carbon.

4.1 Noble Metal-Based Alloys and Oxides

Noble metal-based electrocatalysts (e.g. Pt/C, Ir/C and RuO2) have been shown to possess excellent catalytic activities towards ORR or OER. However, costs are high, and none of these commercial catalysts can adequately catalyse both ORR and OER reactions. Therefore, recent research has been devoted to the reduction of costs as well as the enrichment of functionality of noble metal-based electrocatalysts. In addition, researchers report that the coupling of PtM alloys with transitional metal oxides or complexes to fabricate hybrid materials is a viable strategy and demonstrates great promise to simultaneously catalyse ORR and OER. For example, a variety of Pt-M (M = Fe [121, 122], Co [123, 124], Ni [125, 126] and other transition metals) alloys with different structures and morphology have demonstrated superior catalytic activities towards ORR as compared with Pt. In one study, Mukherjee et al. [127] embedded PtCo nanoalloys in CoOx matrixes and reported bifunctional catalytic activities towards ORR and OER in which a combined overpotential of 756 mV vs. RHE was achieved. In another study, Cui et al. [128] fabricated robust Fe3Mo3C-supported IrMn clusters as a bifunctional air electrode and reported long-term cycling performances over 200 h with high efficiency. In a later study, Cui et al. [129] further developed a Ni3FeN-supported Fe3Pt nanoalloy as a bifunctional catalyst for ZABs and obtained a long-term cycling performance of over 480 h at 10 mA cm−2 (Fig. 15). Overall, the design of these carbon-free catalysts is based on the fact that the adoption of Ni3FeN as a support can effectively tackle carbon corrosion issues encountered by most carbon dispersed catalysts and the fact that despite Pt showing remarkable ORR properties, its activity towards OER is insufficient and the alloying of Pt with Fe to form ordered intermetallic phases can significantly enhance catalytic performances and simultaneously reduce the consumption of expensive Pt. As an example, Wang et al. [130] integrate CoPt nanoparticles with SiO2 through the reduction of Pt4+ and Co2+ ions co-adsorbed into diatomite (DTM) and reported that the presence of diatomite can contribute to enhanced catalytic activities and long-term stability of the catalyst in which the specific and mass activity of the resulting CoPt-1/DTM-C for ORR was found to be 2.5 and 3 times higher as compared with CoPt-c/C catalysts (0.74 mA cm−2 and 286 mA mg−1 at 0.9 V vs. RHE, respectively). In addition, the researchers also reported that if applied in ZABs, CoPt-9/DTM-C can produce a power density of 140 mW cm−2 and a specific capacity of 616 mAh g−1, exceeding the performance of the sample without DTM. In another example, You et al. [131] investigated binary Ru-Sn oxide catalysts for ZABs and reported optimized performances in which the introduction of Sn not only lowered costs, but also optimized the relatively sluggish ORR reactivity of the RuO2 catalyst. Here, a Zn-air cell made of the above catalyst produced a peak power density of 120 mW cm−2 at a current density of 235 mA cm−2 with outstanding charge-discharge stability of over 80 h.
Fig. 15

a SEM and b TEM images of Fe3Pt/Ni3FeN. c ORR and d OER polarization curves. e Bar plot of the kinetic current densities at 0.9 V in 0.1 M KOH before and after accelerated durability tests (ADT). f Current densities at 1.6 V before and after 1000 cycles. Reprinted with permission from Ref. [129], copyright 2017, John Wiley and Sons

4.2 Heteroatom-Doped Carbon Materials

Carbon materials (graphene, carbon nanotubes, graphite, porous carbon, etc.) have been extensively studied as efficient electrocatalysts because of their large specific surface areas, good conductivities and low costs [132, 133, 134]. However, the performances achieved from pure carbon materials are usually unsatisfactory, and in recent years, numerous studies have shown that the doping with heteroatoms (i.e. N, P, S, F and B) can enhance the electrochemical activity of carbon materials. This is because the unequal size and electronegativity of heteroatoms between carbon atoms can allow the introduction of heteroatoms into carbon matrixes to modulate charge distributions and electronic properties and induce defects in electrodes, facilitating OER and ORR in ZABs [135]. For example, Liu et al. [136] demonstrated that the chemisorption of oxygen molecules and oxygen-containing intermediates on carbon materials can be enhanced during ZAB operations through heteroatom doping. In another example, Lei et al. [29] fabricated 2DP-doped carbon nanosheets (2D-PPCNs) with tuneable porosity by adopting P2O5 as the carbonizing agent and reported that as a result of the high concentration of P dopants and exposed active sites in combination with the optimized porosity in the resulting electrode, the catalyst delivered excellent activities towards both ORR and OER in which a lower overpotential (365 mV) as compared with Ir/C catalysts (381 mV) was achieved. In addition, the researchers reported that if integrated into an air-breathing cathode for rechargeable ZABs, the corresponding ZAB can demonstrate better cell performances than noble metal benchmark catalysts and a higher durability with over 1000 charge-discharge cycles. Furthermore, N, F and B ternary-doped carbon fibres were prepared by Wang et al. [137] through a simple electrospinning process, and the researchers reported that the ternary doping induced higher catalytic activities towards ORR through an efficient 4e transfer mechanism as compared with single N-doped carbon fibres. Moreover, Hang et al. [138] fabricated a defect-enriched and pyridinic-N (PN) dominated bifunctional electrocatalyst with a novel core-shell architecture (DN-CP@G) through the in situ exfoliation of graphene from carbon paper followed by a high-temperature ammonia treatment and reported that the resulting catalyst provided a high discharge performance and outstanding long-term cycle stability with at least 250 cycles, outperforming mixed Pt/C and Ir/C electrodes.

Despite these remarkable improvements, it is still challenging to achieve both satisfactory OER and ORR performance for most metal-free anion-doped carbon materials. Here, researchers report that the coordination between metal and heteroatoms in carbon matrixes (especially in defective sites) and the introduction of transition metals can effectively modify local electronic structures and therefore optimize intermediate adsorption, leading to superior activities comparable to precious metal catalysts. Based on this, many promising electrocatalysts with single or bimetal M-Nx centres (M=Fe, Co, FeCo, etc.) in single carbon catalysts have been developed and investigated in recent years [139, 140, 141, 142, 143, 144, 145]. For example, Tang et al. [141] developed an effective defect engineering method to achieve targeted active sites in Co/N/O tri-doped graphene catalysts. Here, the researchers integrated the resulting catalyst into a rechargeable and flexible solid ZAB and achieved a high open-circuit voltage of 1.44 V, a stable discharge voltage of 1.19 V and a high energy efficiency of 63% at 1.0 mA cm−2 even under bending conditions. Furthermore, Li et al. [144] used active salt and silica nanoparticles as templates to synthesize meso/microporous FeCo-Nx-CN nanosheets with exceptionally high surface areas and reversible oxygen electrocatalytic performances for both ORR and OER (Fig. 16).
Fig. 16

a Schematic of the process of active salt/silica-templated 2D meso/micro-FeCo-Nx-CN. b Corresponding TEM image. A comparison of the surface area c and Egap difference d between samples with/without 30% silica NPs. Reprinted with permission from Ref [144], copyright 2018, John Wiley and Sons

To fabricate multi-component doped carbon, high-temperature annealing is always necessary, which can induce the collapse of carbon sheets during carbonization and lower accessible surface areas, compromising performance. To tackle this problem, Zhu et al. [146] synthesized FeNx/C catalysts through the pyrolysis of thiourea and agarose containing α-Fe2O3 nanoplates and reported that the α-Fe2O3 nanoplates can prevent the aggregation of carbon sheets to effectively improve the specific surface area during carbonization. Furthermore, due to high surface areas and rich porosity, metal-organic frameworks (MOFs) are another important type of electrocatalysts with active M-Nx-C centres [147, 148, 149, 150, 151]. For example, Mu et al. [152] investigated the synthesis of Fe/N/S-doped CNTs from hydrazine hydrate and ferrous sulphate-treated ZIF-8 and reported that the treatment with hydrazine hydrate can reduce Fe ions and prevent their quick aggregation during pyrolysis, inducing synergetic doping effects from Fe, N and S atoms. And as a result, the catalyst exhibited excellent ORR activities in both alkaline and acid electrolytes.

For any electrocatalyst, the creation of more single-site dispersed active sites can enhance the number of electrocatalytic sites, which can boost oxygen electrode performances but also pose great challenges to preparation and synthetic strategies. Recently, Ma et al. [153] reported a catalyst with single-site active Fe-Nx species distributed on highly graphitic 2D porous nitrogen-doped carbon (PNC) that exhibited superior catalytic activities towards ORR/OER with a half-wave potential (E1/2) of 0.86 V for ORR and an overpotential of 390 mV at 10 mA cm−2(η10) for OER in an alkaline medium. In addition, Yang et al. [145] synthesized a single-atom cobalt electrocatalyst with CoN4 moieties dispersed on nitrogen-doped graphitic nanosheets (CoN4/NG) using a surfactant-assisted approach. Furthermore, Yang et al. [154] adopted the concept of an apically dominant mechanism in the improvement of the catalytic performance of nitrogen-doped carbon nanotubes (NCNTs) by precisely encapsulating CoNi nanoparticles (NPs) within the apical domain of NCNTs on Ni foam (CoNi@NCNT/NF) (Fig. 17a–d). Here, a ZAB coin cell using CoNi@NCNT/NF as an air electrode produced a peak power density of 127 mW cm−2 with an energy density of 845 Wh kg−1 and recharge ability over 90 h, which outperformed platinum group metal (PGM) catalysts (Fig. 17e–h).
Fig. 17

a, b SEM and c, dTEM images of the CoNi@NCNT/NF (the yellow circle in b shows the encapsulated CoNi NPs within the top of the CNTs). The scale bars in ad are: 5 μm, 200 nm, 50 nm and 5 nm, respectively. e Polarization curves and power density curves, f discharge curves, g discharge and charge polarization curves and h galvanostatic charge-discharge cycling of a ZAB coin cell using CoNi@NCNT/NF and Pt/C/NF as its cathode. Reprinted with permission from Ref [154], copyright 2018, John Wiley and Sons

4.3 Metal Oxides/Sulphides/Nitrides

In recent years, oxides or chalcogenides (e.g. sulphides, phosphides and nitrides) of earth-abundant elements have been extensively investigated as alternatives to precious metal-based electrocatalysts and showed promising OER and ORR activities and the electrochemical performances of perovskites [155], La/Ca oxides [156], MnS [157], CoSx [158, 159], Co9S8 [160], Co3O4 [161, 162, 163, 164, 165, 166], CoO [167], CoP [168, 169, 170, 171], CaMnO3 [172], NiCo2O4 [173, 174, 175] and NiCo2S4 [176, 177] have been systematically investigated very recently. Co3O4 is among the most extensively studied electrocatalyst materials due to its excellent electrochemical performance. For example, the thin atomic layer of Co3O4 (~ 1.6 nm) has been applied in flexible ZABs based on 1D yarns with wearable and knittable features and resulted in high rate capabilities and high cycling stability (Fig. 18) [163]. Lu et al. [161] reported that ultrathin 2D Co3O4 nanosheets with several atomic layers in thickness possess multiple advantages such as high surface areas and abundant coordinated unsaturated atoms, which can enable rapid charge transport. In addition, the ultrathin structure is more favourable for flexible ZABs as it shows excellent mechanical strength so that the battery can better withstand bending or twisting. However, despite decent OER activities, the ORR activity of Co3O4 is still unsatisfactory. Here, researchers suggest that the formation of composites with ORR-active Ag nanoparticles or the addition of dopants such as N is an efficient strategy to enhance the OER activity of Co3O4 electrodes [165]. For example, Yang et al. [178] reported that N doping can contribute to enhanced electronic conductivities, increased O2 adsorption strengths and improved reaction kinetics in which if assembled into ZABs, N-doped Co3O4 electrodes can exhibit a high volumetric capacity of 98.1 mAh cm−3 with outstanding flexibility. The downsizing of catalysts with increased contact areas and active facets can also enhance catalytic activity. However, the small size and high energy planes of the resulting catalyst can also cause severe aggregation of the nanoparticles and lower performance. As an efficient solution to this problem, Wang et al. [164] embedded hollow Co3O4 nanospheres within N-doped carbon nanowall arrays derived from a cobalt-based metal organic framework and reported that the unique hierarchical structure enabled the catalyst to demonstrate efficient catalytic activities towards both ORR and OER, with the corresponding battery achieving high open-circuit potentials (Eocv: 1.44 V), high capacities (387.2 mAh g−1) and excellent cycling stability, surpassing Pt- and Ir-based ZABs.
Fig. 18

a Schematic of yarn-based ZABs. b TEM and c FM images of the ultrathin Co3O4 nanosheets and d a corresponding thickness of ~ 1.6 nm. e Discharge and charge polarization curves, f ORR tests at 1600 rpm and g OER tests. Reprinted with permission from Ref [163], copyright 2018, John Wiley and Sons

Notably, recent studies suggest that bimetallic oxides such as CoNi [173], FeNi [179] or FeCo [175] oxides possess better activities as compared with single-element counterparts because of the multiple valences and abundant crystallographic structures [174]. The catalytic activity of bimetal oxides is influenced by elemental composition and crystallinity, and amorphous oxides are of particular interest due to increased active sites arising from defects and improved ionic conductivity due to abundant oxygen vacancies [180]. For example, Sun et al. [175] synthesized amorphous bimetal Fe/Co hydroxide/oxide nanoparticles (10–20 nm) inlaid on multi-walled N-doped carbon nanotubes and reported that the multi-component can contribute to synergetic benefits in which the Fe/Co nanoparticles can enable maximum contact areas on the carbon nanotubes, facilitating rapid electron transport and preventing aggregation of nanoparticles. And as a result, the as-prepared catalyst achieved a half-wave potential (E1/2) of 0.86 V for ORR and a low operating potential of 1.55 V at 10 mA cm−2(Ej=10) for OER in 1.0 m KOH. In another example, an efficient strategy was developed by Wei et al. [181] to synthesize amorphous Fe0.5Co0.5Ox, Ni0.4Fe0.6Oxand Ni0.33Co0.67Ox on N-doped reduced graphene oxide (NrGO), allowing for the simultaneous control of elemental compositions, sizes and crystallinities. And as a result, the catalyst demonstrated excellent OER activities with a Tafel slope of 30.1 mV dec−1 and an overpotential of 257 mV for 10 mA cm−2 as well as superior ORR activities with a large limiting current density of − 5.25 mA cm−2 at 0.6 V.

In addition to metal oxides, other chalcogenides such as metal sulphides and phosphides have also attracted considerable research interests. For example, Zhang et al. [160] used efficient interface engineering to implant Co9S8 nanoparticles into MOF generated carbon matrixes and reported excellent performances due to high-speed electron transfer at the interface. And in contrast to the direct loading of metal oxides on conductive carbon, Liu et al. [177] synthesized a unique architecture by wrapping urchin-like NiCo2S4 in S-doped graphene nanosheets and reported that because of the synergetic effects arising from both NiCo2S4 and S-doped graphene, the resulting catalyst displayed excellent electrocatalytic activities and long-term durability for both ORR and OER. In addition, the corresponding ZAB constructed by using this catalyst achieved high power densities, small charge/discharge gaps and excellent cycling stability. As for transition metal phosphides, these have shown decent activity towards OER but inferior performances in terms of ORR activity. Therefore, many studies have explored metal phosphides as efficient bifunctional catalysts with enhanced ORR activities, and Fe-doped CoP nanosheet arrays on Ni foam have demonstrated improved performances as compared with CoP/NF structures [170]. And as reported by Li et al. [169], CoP with a highly exposed (211) crystal plane and abundant surface phosphide atoms can show remarkable activity towards both OER and ORR (Fig. 19). Here, the researchers conducted density functional theory calculations and revealed that the desorption of OH* intermediates from cobalt sites is the rate-limiting step for both CoP and Co2P in ORR and that a high content of phosphide is essential to lower reaction barriers. In addition, the researchers also revealed that Co2P possesses higher OER activities due to the easier formation of Co2P@COOH heterojunctions, which are regarded as real active sites for OER on metal phosphides. Another advantage of adopting CoP supported on carbon-based substrates is that CoP can be oxidized prior to carbon materials in OER, which can prevent anodic currents from being applied to carbon and ensure the stability of carbon materials, which maintains their catalytic properties [171].
Fig. 19

a The crystal structures of CoP (above) and Co2P (below). b Chronoamperometric response of CoP, Co2P and Pt/C at a potential of 0.7 V versus RHE (900 rpm). Polarization curves for c HER and d OER of CoP, Co2P and Pt/C (or RuO2/C) in 1.0 m KOH. Reprinted with permission from Ref [169], copyright 2018, John Wiley and Sons

The performance of electrocatalysts reported in recent years for ZABs is summarized in Table 3.
Table 3

Performances of recently reported electrocatalysts for ZABs

Catalyst

ORR

OER

\( \Delta E \) (V) (Ej=10E1/2)

ZAB performance

Reference

Eonset(V)/E1/2 (V)/Tafel slope (mV dec−1)

Eonset (V)/Tafel slope (mV dec−1), η10 (V) or Ej=10(V)

Eocv (V)/over potential (V)/peak power density (mW cm−2)

2D-PPCN

0.92/0.85/–

η10: 0.365

0.74

1.40/0.87/–

[29]

Fe3Pt/Ni3FeN

–/0.93/–

η10: 0.365

[129]

RuSn(37-C|73-Ti)

1.42/0.75/120

[131]

AgNW/GA

  

–/–/331

[132]

CoNC@GF

0.99/0.87/75.7

η10: 0.43

1.51/–/154.4 (liquid)

1.4/1.04/85.6 (solid)

[136]

TD-CFs

1.46/–/–

[137]

DN-CP@G

–/0.801/–

Ej=10: 1.788

0.987

1.43/0.95/135

[138]

CoFe/N-C

1.03/0.821/86

1.525/249

–/–/128.6

[139]

N-GCNT/FeCo

1.03/–/66.8

–/99.5

Ej=10: 1.73

0.81

1.48/0.26/89.3

[140]

NGM-Co

–/–/58

0.95

–/1.12/152

[141]

Co-NHCs

0.99/0.81/–

–/96

1.556/0.84/239.8

[142]

NC-Co/CoNx

0.93/0.87/–

η10: 0.289

0.65 V

1.4/–/41.5

[143]

CoN4/NG

0.98/0.87/70

0.27/81

0.74

1.51/1.06/115

[145]

FeNx/C-700-20

1.1/0.9/93

–/219

1.6/27/–

[146]

MNG-CoFe

0.98/0.7/–

Ej=10: 1.34

0.64

–/0.61/97.7

[147]

Co-MOF/GF

0.78/0.7/–

–/72

η10: 0.22

0.75

1.332/86.2/–

[148]

Fe-MOF@CNTs-G

0.97/0.873/99.8

η10: 0.416

1.414/1.01/95.3

[151]

Fe/N/S-CNTs

0.987/0.887/73

1.49/–/111

[152]

FeNx-PNC

0.997/0.86/–

–/80

η10: 0.395

0.775

1.55/0.78/278

[153]

CoNi@NCNT/NF

0.97/0.87/–

Ej=10: 1.54

0.67

1.4/0.52/127

[154]

PrBa0.5Sr0.5Co2−xFexO5+δ

0.73/–/98

1.53/81

η10: 0.3

–/–/127

[155]

LCMO

0.92/0.83/–

1.51/–

1.37/–/203

[156]

Co/CoxSy@SNCF-800

0.83/0.74/–

–/68

Ej10: 1.536

0.7

1.37/–/230

[158]

CoSx@PCN/rGO

0.89/–/–

–/44

Ej=10: 1.57

0.79

1.38/–/–

[159]

Co9S8@NSCM

0.97/0.81/75

–/155

Ej=10: 1.6

0.79

1.45/–/179

[160]

Co3O4

1.33/1.92/–

[161]

Co3O4-NrGO

0.9/0.79/54

–/101

Ej=10: 1.72

0.93

1.31/–/–

[162]

Co3O4

1.3/–/7.3 Wh L−1

[163]

NC-Co3O4/CC

0.91/0.87/-

η10: 0.352

1.44/–/82 (mW cm−3)

[164]

Co3O4/Ag@NrGO

–/0.735/–

η10: 0.473

0.93

1.3/1.08/109

[165]

Co3O4/MnO2/PQ-7

0.95/–/–

1.6/–

1.4/–/257

[166]

CoP@CC

0.8/0.67/–

1.35/124

η10: 0.3

0.6/–/30

[168]

CoP

0.92/72.1

–/–

1.34/–/61

[169]

Fe0.33-CoP/NF

0.9/0.8/120

–/122

η10: 0.313

1.32/0.74/63

[170]

NiCo2O4@NiMn LDH

0.216/62.4

1.4/–/160.8

[174]

NiCo2S4/N-CNT

0.93/0.8/–

0.8

1.49/0.63/147

[176]

NiCo2S4

–/0.88/–

1.56/65

η10: 0.33

0.69

–/0.8/216.3

[177]

FeCo-DHO/NCNTs

0.91/0.8/59

–/–

0.93

–/1.003/326 (liquid)

–/1.085/60 (all-solid)

[175]

Fe0.5Co0.5Ox

–/–/62.2

–/30.1

0.74

1.43/–/86

[181]

Ni3FeN/NRGO

0.9/–/–

1.38/–

0.69

–/0.77/–

[182]

PFe-Pc

1.033/0.94/–

1.6/–/192

[183]

Co-N-SWCNTN

0.97/0.87/54

1.512/0.21/248

[184]

Fe,N-CNS

0.96/0.88/60.2

–/155.5

Ej=10: 1.84

0.96

1.38/–/128.7

[185]

am-Fe-Bi/NF

–/53

1.481/–/61.8

[186]

Fe3C/Co(Fe)Ox@NCNT

–/0.86/52.1

1.5/139

Ej=10: 1.58

0.72

1.48/0.88/231

[187]

5 Zinc Anodes

The energy storage capacity of Zn-based batteries is highly dependent on the Zn anode, and an ideal Zn anode should possess high utilization efficiencies to ensure large capacities and reversible charging and discharging. However, utilizations are below 1% for the most commonly adopted Zn metal foil anodes, and the performance of Zn anodes is limited by three phenomena including dendrite growth and shape change, passivation and hydrogen evolution reactions, and corrosion.

Because oxygen shows minimum overpotential in alkaline electrolytes and because of its high ionic conductivity, high concentration KOH is commonly adopted in ZABs. And in aqueous alkaline electrolytes, charge and discharge are realized through two consecutive reactions involving complexation (dissolution) and electroreduction in charging and electrooxidation and precipitation in discharging. These can be represented as [188]:
$$ {\text{Complexation/precipitation: ZnO }}\left( {\text{s}} \right) \, + {\text{ H}}_{2} {\text{O }}\left( {\text{l}} \right) \, + \, 2{\text{OH}}^{ - } \left( {\text{aq}} \right) \leftrightarrow {\text{Zn}}\left( {\text{OH}} \right)_{4}^{2 - } \left( {\text{aq}} \right) $$
(6)
$$ {\text{Electro - reduction/oxidation: Zn}}\left( {\text{OH}} \right)_{4}^{2 - } \left( {\text{aq}} \right) \, + \, 2{\text{e}}^{ - } \leftrightarrow {\text{Zn }}\left( {\text{s}} \right) \, + \, 4{\text{OH}}^{ - } \left( {\text{aq}} \right) $$
(7)
Here, the sluggish dynamics of the solid-solute-solid transformation on Zn anode surfaces can cause a series of issues that lead to poor cycling performances. One issue is that the dissolution of metal Zn can result in the saturation or over-saturation of Zn(OH) 4 2− in KOH solutions in which during charging, Zn(OH) 4 2− starts to deposit onto the surface of Zn anodes and a concentration gradient forms between the bulk solution and the near surface region. (Zn(OH) 4 2− will preferably precipitate in the area with higher concentration.) And after several cycles of charging and discharging, parts of the anode will become covered with dense Zn dendrites, which can further protrude into the electrolyte with even higher Zn(OH) 4 2− concentrations. This rapid growth of Zn dendrites and protrusions can result in anode shape changes and poor cycling performances. In addition, these dendrites/protrusions can pierce through separators and short-circuit the battery. Another issue is passivation, which is caused by the formation of ZnO on anode surfaces (Fig. 20) [189]. Because ZnO is insulating, the electric conductivity of electrodes can be greatly affected by the deposition of ZnO layers. More importantly, this ZnO deposition can block ion transport pathways during charge and discharge, resulting in high charging voltages and lower discharging potentials, compromising the capacity of the whole battery.
Fig. 20

Schematic of the passivation layer formation on Zn metal anodes. Reprinted with permission from Ref [189], copyright 2018, John Wiley and Sons

HER and Zn anode corrosion can also compromise Zn-based battery performances. Because HER is more thermodynamically favoured in high concentration alkaline electrolytes and possesses a more positive redox potential (− 0.83 V vs. SHE) than Zn/ZnO (− 1.26 V vs. SHE) in KOH with a pH of 14, the occurrence of HER in ZABs will not only cause self-discharge but also decrease the Coulombic efficiency of rechargeable batteries. And although neutral and mild acid electrolytes have been developed for ZABs to resolve issues encountered with alkaline electrolytes, side reactions and Zn anode corrosion are still inevitable. In the case of acidic electrolytes, Zn2+ is formed through Zn \( \leftrightarrow \) Zn2+ + 2e [190] and upon deposition of Zn2+ and formation of a highly active Zn surface, side reactions such as corrosion and HER can occur. In addition, the continuous production of hydrogen caused by HER depletes electrolytes and shortens battery life and the hydration effects of Zn2+ in water because solvent loss is further accelerated, allowing for the easy formation of zinc hydroxides. The consumption of Zn anodes by these active side reactions will also result in low Zn anode utilization efficiencies. In recent years, different strategies and concepts have been introduced to tackle these issues and improve the stability and cycling performance of Zn-based battery systems, and the optimization of electrode structures is undoubtedly the most viable option. The downsizing of Zn anodes has proven to be efficient in alleviating the passivation of ZnO on anode surfaces [191] in which, as emphasized in several studies, the optimal size of Zn or ZnO nanoparticles is in the range of 100 nm–2 μm [189, 192, 193]. For example, Chen et al. [193] proved that materials smaller than 2 μm can ensure full utilization of anode materials, and Liu et al. [189] reported that the merits of this nanostructuring towards the reversibility of electrodes disappear and the dissolution of anodes is accelerated if electrode sizes drop below 100 nm. Furthermore, Steingart et al. [194] used hyper-dendritic (HD) Zn to replace Zn sheets as an anode and reported that due to the formation of a more compact structure upon cycling, the formation of Zn dendrites on HD Zn can be effectively suppressed and the fast insertion kinetics of Zn2+ can be enhanced, leading to remarkably improved electrode rate performances and stability as compared with Zn sheet anodes.

Despite these results, the tuning of Zn anode morphology alone cannot adequately achieve ideal battery performances and simultaneously suppress passivation, dissolution and corrosion of Zn anodes, and Zn anode structures also need to be further enriched. For example, Parker et al. [12, 195] designed a monolithic, porous and aperiodic architecture that allowed an inner core of electron-conductive metallic Zn to undergo deep levels of discharge. Here, the 3D sponge Zn anode design enabled corresponding cells to be cycled hundreds to thousands of times without passivation or dendrite formation. The coating of Zn nanoparticles with protective/buffer layers is also an effective method to prevent the rapid dissolution of Zn anodes, and until now, various coating materials such as active carbon, metal oxide glass and polymers have been explored [196, 197]. For example, Jiang et al. [197] reported that a porous carbon layer can serve as nucleation sites and reservoirs to capture zincate ions from the electrolyte (8 M NaClO4 + 0.4 M Zn(CF3SO3)2) and alter the prioritization of Zn-ion deposition, resulting in an anode that can be cycled in the electrolyte for hundreds to thousands of cycles without dendrite formation. In another example, Kang et al. [198] proposed the addition of active carbon into Zn nanoparticles because the presence of carbon can provide spacing to accommodate dendrite formation so that Zn anodes can remain clean and active to sustain further reactions. Here, the researchers reported that with increased carbon additives, the surface of the Zn anode becomes smoother and that the capacity retention of their anode after several cycles with and without active carbon was 85.6% and 56.7%, respectively. Furthermore, Mai et al. [190] reported that the atomic layer deposition of TiO2 as a protective layer can allow Zn anodes to withstand harsh battery operation conditions and efficiently suppress surface dendrite formation. In another study, Kang et al. [199] conducted an uniform and position-selected plating of Zn metal with a porous nano-CaCO3 coating (Fig. 21a, b) and reported that if galvanostatically cycled at 1 A g−1, the corresponding battery with the nano-CaCO3-coated Zn anode can exhibit much better cycling stability in which a specific capacity as high as 236 mAh g−1 can be achieved in the first 500 cycles and remain at 177 mAh g−1 at the 1000th cycle (capacity retention = 86%). In this study, the battery using a bare Zn anode only achieved a smaller initial discharge capacity of 188 mAh g−1 and remained at 124 mAh g−1 after 1000 cycles, which is a significant difference. Coatings of functional oxides such as Bi2O3-ZnO-CaO [200] and Bi2O3-Li2O-ZnO [201] have also been demonstrated and have achieved improved rechargeability and cyclic stability as compared with uncoated zinc powder in which the functional oxide glasses were able to swell in aqueous KOH and lead to the formation of gels, allowing for the immobilization of discharge products near the anode surface.
Fig. 21

a Morphological evolution for bare and nano-CaCO3-coated Zn foils during Zn stripping/plating. b Typical charge-discharge profiles of Zn|ZnSO4 + MnSO4|Zn symmetric cells with bare (black) and nano-CaCO3-coated (red) Zn electrodes. Reprinted with permission from Ref [199], copyright 2018, John Wiley and Sons. c Schematic of the ion-sieving nanoshell design. d Specific capacities and Coulombic efficiencies of bare ZnO, ZnO@C and bulk Zn foil anodes. Reprinted with permission from Ref. [189], copyright 2018, John Wiley and Sons

Alternatively, Li et al. [202] adopted a strategy to load Zn nanoparticles onto nanotube arrays of Li-RTiO2 and reported that such architectures can exhibit enhanced electrochemical performances towards Zn deposition and dissolution during charge and discharge and can also exhibit robust long-term stability and excellent rate performances. As a result, a maximum volumetric energy density of 0.034 Wh cm−3 and a power density of 17.5 W cm−3 with 95% capacity retention after 20000 cycles were achieved. In another study, Schröder et al. [203] coated an anion-exchange ionomer (AEI), which is a ionomeric hydroxide-conducting polymer (IHCP) possessing a fraction of cationic head groups covalently bonded to the carbon backbone, onto a Zn anode, and achieved the selective permeation of hydroxide ions and zincate ions (Zn(OH) 4 2− ). Here, the researchers reported that this coating suppressed dendrite growth and obtained higher cycling stability in the resulting slurry-based Zn anode.

The use of ZnO instead of Zn is another popular concept in the synthesis of Zn anodes because the tailoring of ZnO nanostructures is more feasible and the formation of cracks caused by active material expansion during cycling can be avoided. For example, Liu et al. [204] grew ZnO nanorods onto carbon fibre paper and subsequently coated a strong and conductive TiNxOy layer onto the nanorod surface to seal the ZnO inside and reported that the resulting anode achieved a high specific capacity of 508 mAh g−1 and long-term cycling performances (7500 cycles when cycled under start-stop conditions). In another example, Schröder et al. [203] deposited a ZnO layer onto carbon mesh and subsequently protected this layer with the aionomeric hydroxide-conducting polymer (IHCP) layer and reported that the resulting anode delivered a specific discharge capacity of 351 mAh g−1 with a high depth of discharge (DoD) of 53.6%. Furthermore, Mai et al. [205] reported that a triple-layer anode in which ZnO@C core-shell nanorods were grown on carbon cloth followed by Zn deposition (CC-ZnO@C-Zn) achieved excellent anti-dendrite performances due to the 3D skeleton matrix. Liu et al. [193] also reported that the use of a ZnO pomegranate-like material in which ZnO nanoparticles were encapsulated and held in clusters by a carbon shell diaphragm was viable to suppress the passivation and dissolution of anodes and the corresponding battery demonstrated remarkable capacity retention and cycling stability under extremely harsh conditions. Finally, Wu et al. [189] studied an ion-sieving carbon nanoshell-coated ZnO nanoparticle anode (Fig. 21c) and reported that due to the suppressed passivation and dissolution effects arising from the nanosized ZnO and the microporous carbon shell, the anode achieved significantly improved performances as compared with Zn foil and bare ZnO nanoparticles even under extremely harsh testing conditions (closed cells, lean electrolytes and no ZnO saturation) (Fig. 21d).

In general, dendrites/protrusions on Zn anodes undergo a process of nucleation, plating and stripping and once a nucleus forms, enhanced electrical fields are established due to high curvatures, leading to the fast growth of dendrites or protrusions through the attraction of cations. Here, the adsorption of inert cations on the surface of dendrites or protrusion can suppress further growth through the repelling of incoming plating cations [206]. And therefore, the addition of MnSO4, Zn(CF3SO3)2 and Zn(CF3SO3)2/Mn(CF3SO3)2 into aqueous electrolytes can effectively suppress the formation of detrimental dendrites [36, 42]. For example, Wang et al. [207] recently demonstrated that the use of a high concentration electrolyte based on Zn(TFSI)2 and LiTFSI can effectively suppress dendrite growth during plating/stripping and that the corresponding electrode remained stable after > 200 cycles. However, researchers also report that the simple addition of MnSO4 into electrolytes cannot sufficiently suppress dendrite growth and that the prices of Zn(CF3SO3)2 and Mn(CF3SO3)2 are relatively high [199]. Therefore, it is desirable to further explore more efficient and inexpensive electrolyte additives. Alternatively, solid-state electrolytes (SSEs) can also prevent dendrite formation, and a crystalline single-ion Zn2+ SSE based on metal-organic frameworks with a fixed anionic microporous host and mobile Zn2+ ions was reported recently by Pan et al. [208] that demonstrated good compatibility with Zn metal anodes and allowed for dendrite-free, smooth and compact Zn deposition due to its solid microporous structure with a nanowetted interface between Zn and SSEs. And as a result, a specific capacity of 125 mAh g−1 was achieved over 250 cycles at 0.2 A g−1, 40% of which was retained even at 2 A g−1.

The design of membrane separators is another parameter that can be tuned to suppress dendrite formation because ion transport can be regulated by separators in which an ion exchange membrane may be beneficial. Here, selective ion conductive membranes can mitigate ion concentration gradients, which are the primary source of dendrite growth. In addition, the intermolecular channels of membranes can greatly enhance the uniformity of current distribution, resulting in uniform Zn deposition. For example, Liu et al. [209] fabricated a PAN-based ionic conducting membrane (a PAN-S membrane) and reported stable cycling with a low voltage hysteresis of < 40 mV through 350 cycles due to the effective suppression of dendrite formation.

The performances of recently reported Zn anode materials for ZIBs and ZABs are summarized in Table 4.
Table 4

Performances of Zn anodes for ZIBs and ZABs

Anode

Electrolyte

Battery performance (Specific capacity (mAh g−1) or capacity retention/cycle number)

Solved problems

Reference

ZnO/C

4.0 M KOH + 2.0 M KF + 2.0 M K2CO3

100%/500

Passivation and dissolution

[189]

ZnO/GO

4.0 M KOH + 2.0 M KF + 2.0 M K2CO3

86%/150

Passivation and dissolution

[192]

Zn-pome

4 M KOH

84%/40

Dissolution

[193]

Zn@C

8 M NaClO4 + 0.4 M Zn(CF3SO3)2

100%/1000

Dendrite formation

[197]

CaCO3/Zn

3 M ZnSO4 + 0.1 M MnSO4

177/1000

Dendrite formation

[199]

Zn/Bi2O3-Li2O-ZnO

6 M KOH

52/20

Passivation

[201]

Li-RTiO2

2 M KOH + 0.08 M ZnSO4∙7H2O

95%/20000

Dendrite growth, dissolution

[202]

ZnO/C/IHCP

4 M KOH

177/30

Corrosion, dissolution

[203]

ZnO@TiNxOy

4 M KOH + 2 M KF + 2 M K2CO3

508/7500

Passivation and dissolution

[204]

3D CC-ZnO@C-Zn

6 M KOH with 1.5 M ZnO (liquid); 6 M KOH-PVA (solid)

71.1%/5000 (liquid); 82%/1600 cycles (solid)

Dendrite formation

[205]

Zn foil

Solid-state electrolyte

125/250

Dendrite growth

[208]

6 Conclusion and Future Perspectives

Because of the rapid consumption and increasing cost of materials (e.g. metal Li and Co) used in current LIBs, ZIBs and ZABs have emerged as promising and competitive alternatives due to their low costs, large theoretical capacities and environmental benignity. However, the development of ZIBs and ZABs is still in nascent stages, and several major limitations need to be tackled before commercialization. In this review, the progress of ZIBs and ZABs is summarized with an emphasis on the design and optimization of electrode materials, including cathode materials for ZIBs and functional electrocatalysts for ZABs. Currently, the lack of suitable cathode materials is a major bottleneck of ZIBs, and manganese-based oxides have been extensively investigated because of their excellent electrochemical properties and natural abundance. However, although manganese oxides with rich polymorphs offer a wide range of possibilities, there is also the challenge of understanding the complex underlying mechanisms of their Zn-ion storage behaviours in which three distinct mechanisms have been proposed. The three mechanisms, which include insertion of Zn2+, co-insertion of H+ and Zn2+ and conversion reaction, are confusing and sometimes controversial. The controversy mainly originates from the existence of rich polymorphs and their facile transformation between each other during the charge and discharge process. Vanadium-based oxides are also potential candidates as cathodes for ZIBs, and the rich V–O diagram can provide diverse polymorphs with many specific advantages. Here, the open framework and facile valence exchange capabilities of mother V2O5 can enable fast ion diffusion and charge transfer. In addition, the existence of rich oxygen vacancies due to mixed valence states in the V-O structure can enhance electrical conductivity. However, the main issue associated with vanadium-based materials is the rapid capacity fading caused by the collapse of the crystal structure, and the search for vanadium oxides with more robust structures or the addition of heteroatoms such as H+, Zn2+ and Ca2+ as pillars to support the crystal structure is major strategies being adopted to optimize the performance of vanadium-based cathodes. Furthermore, PBA and NASICON-type materials have also been explored as possible cathodes for ZIBs because of their increased stability and cyclability. Moreover, transition metal oxides and sulphides have also demonstrated great potential as high voltage or high capacity electrodes, and because of the accessibility of aqueous electrolytes, organic cathodes are also being considered for ZIBs in which the dissolution of organic compounds can be suppressed by adopting aqueous electrolytes. Here, structural transformation is the main factor that needs to be considered in the further performance optimization of organic cathode materials.

As for ZABs, the sluggish kinetics of oxygen evolution and reduction reactions (OER and ORR) is major bottlenecks, and thus far, the development of ZABs has been hindered by the lack of electrocatalysts with sufficiently high activities to promote both ORR and OER. Here, precious metal-based electrocatalysts possess high activities but suffer from high raw material costs. As an alternative, carbon-based materials have shown great promise as catalysts. And although pure carbon materials possess poor performances, the doping with heteroatoms (N, B, F, etc.) or the creation of active M-Nx centres in carbon materials can remarkably enhance activities towards both ORR and OER. In addition, transition metal oxides/sulphides/nitrides/phosphides have been extensively studied in recent years and tremendous efforts are been devoted to the optimization of their performance based on the manipulation of composition, defect status, morphology and crystal structure.

To fully realize the potential of cathodes, it is also critical to develop comparable Zn anodes for ZIBs or ZABs and many strategies have been developed to suppress dendrite growth, passivation and HER corrosion in traditional Zn anodes, including the downsizing and structural modification of Zn or ZnO anodes, the coating or hybridization of Zn anodes with protective layers, the addition of efficient electrolyte additives and the tailoring of separator membrane designs.

In general, rechargeable ZIBs and ZABs technologies are still in primary stages and performances are not yet up to par with LIBs. Therefore, further improvements are essential to realize commercialization. Here, feasible options to achieve high-performance cathode materials for ZIBs rely on more precise engineering of electrode structures and several promising strategies have been proposed to obtain cathode materials with ideal architecture characteristics such as high surface areas, rich porosity and strengthened crystal structures with enlarged interplanar crystal spacing to enable stable and fast diffusion of Zn2+. In addition, structure defects such as oxygen vacancies, ion doping and highly active facets are intriguing topics worth exploring to further boost the performance of current ZIBs. Moreover, flexible and wearable batteries are vital to the development of commercial state-of-the-art wearable devices, and ZIBs are promising in this application because organic electrolytes can be avoided. Therefore, the development of electrode materials that are compatible with electrolytes used in current wearable devices is crucial to reduce interface barriers of ionic transport and achieve performances meeting commercial standards.

Lastly, the development of low-cost, highly efficient bifunctional catalysts that can simultaneously promote ORR and OER should be the focus of future research for ZABs. For example, despite improvements through synergetic doping in carbon materials, carbon-based electrodes are still susceptible to rapid corrosion at high oxidizing potentials during charging (OER) at the air electrode of ZABs and to carbon promoted electrolyte decomposition during discharge and charge. Therefore, the search for carbon alternatives or the development of novel strategies to achieve highly efficient catalysts that can effectively decrease overpotentials is vital. In addition, novel synthetic strategies that can allow for the precise generation and control of active sites in catalysts are also needed in which deeper understandings of the working mechanisms of these catalysts and the nature of their active sites can allow for the development of effective strategies to fabricate stable and reversible oxygen electrodes.

Overall, a combination of theoretical and computational studies with in situ/in-operando analyses that can quantitatively determine the number of active sites during battery operation is vital to achieve further breakthroughs in ZIBs and ZABs.

Notes

Acknowledgements

The authors gratefully acknowledge the National Key R&D Program of China (Grant No. 2018YFB0905400), the National Natural Science Foundation of China (Grant No. 51622210, 51872277, 21606003 and 51802044), the DNL Cooperation Fund, CAS (DNL180310), the Fundamental Research Funds for Central Universities (WK3430000004) and the Opening Project of CAS Key Laboratory of Materials for Energy Conversion.

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© Shanghai University and Periodicals Agency of Shanghai University 2019

Authors and Affiliations

  1. 1.Hefei National Laboratory for Physical Sciences at the Microscale, Department of Materials Science and Engineering, Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences (CAS)University of Science and Technology of ChinaHefeiChina
  2. 2.Guangzhou Key Laboratory of Low-Dimensional Materials and Energy Storage Devices, Collaborative Innovation Center of Advanced Energy Materials, School of Materials and EnergyGuangdong University of TechnologyGuangzhouChina
  3. 3.Dalian National Laboratory for Clean Energy (DNL)Chinese Academy of SciencesDalianChina
  4. 4.State Key Laboratory of Fire ScienceUniversity of Science and Technology of ChinaHefeiChina

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