Building better zinc-ion batteries: A materials perspective

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Building Better Zinc-ion Batteries: A Materials Perspective Pan He , Qiang Chen , Mengyu Yan , Xu Xu , Liang Zhou , Liqiang Mai , Ce-Wen Nan PII: DOI: Reference:

S2589-7780(19)30025-9 https://doi.org/10.1016/j.enchem.2019.100022 ENCHEM 100022

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EnergyChem

Received date: Revised date: Accepted date:

28 May 2019 11 October 2019 12 October 2019

Please cite this article as: Pan He , Qiang Chen , Mengyu Yan , Xu Xu , Liang Zhou , Liqiang Mai , Ce-Wen Nan , Building Better Zinc-ion Batteries: A Materials Perspective, EnergyChem (2019), doi: https://doi.org/10.1016/j.enchem.2019.100022

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Highlights 

Summarizes how to build better ZIBs base on the perspective of materials.



The system from the Zn anode to a series of important cathode materials.



Determine-effect of the chosen electrolyte and possible optimization strategies of ZIBs.



Challenges, future outlook and latest exciting developments on ZIBs research.

Building Better Zinc-ion Batteries: A Materials Perspective

Pan Hea,1, Qiang Chena,1, Mengyu Yana,b, Xu Xua, Liang Zhoua, Liqiang Maia,*, and Ce-Wen Nana,c

a

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,

International School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, P. R. China. b

Department of Materials Science and Engineering, University of Washington, Seattle,

Washington, 98195-2120, USA. c

Department of Materials Science and Engineering, University of Tsinghua, Beijing, 100000, P.

R. China. 1

These authors contributed equally.

*Correspondence: [email protected] (L. M.)

Abstract: Since they were first developed in 1899, Zn-ion batteries featuring an alkaline electrolyte became an important energy storage device, accounting for ~80% of the batteries manufactured in the U.S. in 2010. In recent years, however, the rechargeable Li-ion batteries (LIBs) have superseded older technologies. Now, the development of non-alkaline aqueous electrolyte-based rechargeable Zn-ion batteries have demonstrated potential for use the stationary energy storage devices (ESDs) and may eventually become an indispensable component of the smart electric grid of the future. In this review, we present the fundamentals, challenges, and the latest exciting

developments on Zn-ion battery research. The detailed discussion is organized around the individual parts of the system from the Zn anode to a series of important cathode materials. This review then addresses the determine-effect of the chosen electrolyte on battery performance and summarizes possible optimization strategies. Finally, several new avenues of research for Zn-ion batteries, including hybrid and flexible batteries, are reviewed.

Keywords: Zn-ion batteries, Zn anode, cathode materials, electrolyte

1. Introduction A range of renewable energy resources (solar, wind power, tidal, and others) are being widely explored in response to increasing global energy demands and the urgency of developing more environmentally responsible power sources to counter the damaging effects of harvesting and/or utilizing conventional fossil fuels.1, 2 While indisputably cleaner, these renewal energy sources are not without their challenges.3 In particular, in order to be able to reliably store and efficiently utilize these intermittent sources of power, scientists are working to develop sustainable and high-performance energy storage systems (ESSs).4-6 Wherein various electrical ESSs, are promising for both electric vehicles and portable electronic equipment because of their high energy density.7, 8 In prior decades, various battery technologies (Table 1), including of Ni-Cd, Ni-MH, Pb-acid, and LIBs have been intensively utilized in ESSs.9-12 However, currently available ESSs cannot satisfy the growing market demands in terms of longevity, size, and environmental practicality. For instance, while Ni-Cd, Ni-MH and Pb-acid batteries have been widely used in the past, their low-energy density, limited lifetime, and environmental risks make them increasingly inappropriate as ESSs.13 Additionally, even though LIBs are generally

regarded as the most prospective candidate for ESSs for their high energy density and longer energy lifetime, the escalating price of Li components and their persistent safety concerns make current LIBs too expensive to scale up for large-scale energy storage.2, 14

Table 1. Comparison of different batteries.

Compared with lithium, zinc presents a promising alternative given its relatively high abundance, low toxicity, higher safety parameters, and low potential-thus making it appropriate for use in aqueous systems.15, 16 Although the Zn2+ ion radius is relatively small compared to the alkali metal ions (0.75 Å, Table 2), it's never easy to easy to find applicable electrode materials due to the much stronger electrostatic interaction between the divalent zinc ion and the electrode material than that of lithium ion.17, 18 It should also be noted that electrodes based on zinc anodes (e.g., Zn/Air, Zn/AgO, Zn/NiOOH, Zn/MnO2 cells) have been favored for use in ESSs and other electronic devices since the invention of the battery more than 200 years ago (Scheme 1).19-32 Indeed, such zinc-based electrodes also remain attractive for secondary applications.

Scheme 1. Timeline of the development of Zn-based batteries.

Table 2. Recently, numbers of studies have focused on the use of metallic zinc as the anode in mild/neutral aqueous electrolytes in the quest for high-performance and long-life secondary zinc ion batteries (ZIBs). Compared with earlier state-of-the-art aqueous batteries (such as Ni-Cd, Pbacid and Ni-MH), the ZIBs are based on different working principles. The charge storage process of ZIBs mainly depends on the migration of Zn ions between anode and cathode. As a comparison, a ZIB demonstrates a similar working principle to that of a LIB with its rockingchair design, which was put forward by Kang and colleagues in 2012.27 The ZIBs compose of a zinc anode, a mild neutral aqueous electrolyte and a cathode material capable of storing zinc ions. In the aqueous electrolyte containing zinc ions, the cathode electrode is as host for the insertion/desertion of zinc ions, while the anode electrode relies on the reversible zinc stripping/plating (Fig. 1). Although enormous progress has been evidenced in studies pertaining to ZIBs since that 2012 report-notably the development of various cathode materials, the exploration of electrolytes, and the protection/modification of zinc anodes-there are still many scientific and technical challenges to be resolved before its large-scale application. In this review, we summarize how to build better ZIBs from the perspective of materials. This detailed discussion is organized around the respective parts of the system from the Zn anode to a series of cathode materials. Then, the determine-effect of an electrolyte on battery performance is emphasized, after which probable optimization strategies are summarized. Finally, some new branches of ZIBs, including hybrid and flexible batteries, are reviewed. We hope that this review can draw more attention on the materials for ZIBs and promote their practical applications.

Fig. 1.

2. Zinc Anode Zn anode demonstrates different behaviors depending on the pH of electrolyte and electrode‟s potential (Fig. 2a). According to the Pourbaix diagram, the balanced-line (dotted line) of hydrogen evolution is well below the equilibrium line of Zn dissolution/deposition in a pH range of 0 – 7, which indicates that the hydrogen evolution will dominate the entire reaction only in strongly acid solutions.37 At a pH level of > 4.0, the hydrogen evolution is limited by high overpotential. In mild acidic or neutral solutions, including zinc salt of Zn(CH3COO)2, ZnSO4, Zn(CF3SO3)2 and Zn(TFSI)2, the reaction of a Zn anode can be described as Zn ↔ Zn2+ + 2e-.38 When functioning in an alkaline electrolyte solution, it should be considered to be Zn + 4OH - ↔ [Zn(OH)4]2-, or Zn + 2OH- ↔ Zn(OH)2 + 2e-, which can be further irreversibly reduced in this following step: [Zn(OH)4]2- ↔ ZnO + H2O + 2OH-, or Zn(OH)2→ ZnO + H2O.39-41

2.1 Zinc corrosion behavior In acidic or neutral aqueous solutions in the absence of a complexation agent, the Zn dissolution product is Zn2+, which is rapidly hydrated by 10–12 H2O molecules per Zn2+, with a radius of 0.74–0.83 nm.42 The standard electrochemical potential of Zn/Zn2+ is subject to the concentration of Zn2+, which is written as: E0 = – 0.763 + 0.059/2 × lg[Zn2+] (V). The common compounds of the corrosion products are ZnSO4, ZnSO4·7H2O, Zn(OH)2, and ZnCO3 (at nearly neutral pH).42 According to the standard potential equilibrium equation, the corrosion potential will increase with increasing Zn2+ concentration, which results from Zn2+ acting as a cathodic inhibitor. Note that the corrosion behavior of Zn in an alkaline media shows much different

behavior with acidic/neutral solutions, with ZnO as the main corrosion product on the Zn surface when the pH > 5.8.40, 42

Fig. 2.

2.2 Zinc dendrite formation hypothesis The formation of zinc dendrite is largely dependent on local current density and the electrolyte‟s pH value. Although the Zn anode demonstrates higher solubility of Zn salts and higher working potential in an alkaline electrolyte, related dendrite growth and shape changes are more serious compared with using a mild electrolyte.43 Based on the diffusion-limited aggregation model, Chamoun et al. put forward an interpretation of zinc dendrite formation, as illustrated in Fig. 2b.44 Specifically, the dendrites are composed of primary dendrites, secondary dendrites, and even multi-dendrites. The primary dendrites are formed on the edge facets of zinc hexagonal crystals, while branched growth at the edge of primary dendrites form the secondary dendrites. The growth of these branches at the dendritic spines lead to the formation of threedimensional threadlike networks composed of nanoporous zinc. López et al. suggested that the motif crystals, formed by growth of dendrites, were interconnected by a matrix of dendritic backbones.45 Similar to the formation of lithium dendrites in LIBs, it is generally believed that the formation and corrosion of Zn dendrites are mainly due to uneven distribution of Zn2+ on the planar or two-dimensional zinc foil.46, 47 The primary tiny dendrite tip on the traditional zinc foil planar collector can be used as the charge center of the electric field, and the accumulation of charge will further amplify the dendrite growth. Although the Zn dendrite formation can be

greatly suppressed in mild/neutral electrolytes, it‟s still a threat to the battery safety, and more researches are expected.

2.3 Protection of the zinc anode In rechargeable ZIBs, the anode mainly utilizes zinc foil, porous zinc powder, zinc compound, organic electrode, and so on. During the charge/discharge process, the zinc ions are platting/stripping on the surface of zinc foils, thereby leading to dendritic deposits.48 These dendrites are normally needle-shaped and grow continuously on the Zn surface when a battery is functioning. Eventually, the dendrites will penetrate the membrane and reach the cathode of the battery, which can result in a sudden short circuit and a precipitous drop in capacity to near zero. In order to tackle the issue of dendrite-induced short circuit, researchers have made crucial improvements to rechargeable aqueous zinc ion batteries. Cui et al. reported a backside-plating configuration to avoid short circuits in Zn-based batteries, as illustrated in Fig. 3a (half-cells model).49 In a conventional configuration, the Zn electrode and the working electrode (copper) are facing each other and separated by a separator, which may cause battery failure result from the formation of dendrites from the Zn electrode side under repeated cycles. In the backside-plating configuration, an insulating layer (made of 30 mm polypropylene) is coated on the „front‟ side and the edges of copper foil to make Zn ions deposited on the open opposite surface of copper foil. Therefore, the Zn dendrite will not be formed on the counter electrode and further turned into reliable components, thereby ensuring the battery away from the short circuit. Gupta et al. reported a novel anode of hyper-dendritic zinc (HD Zn) to instead of conventional Zn foil.50 As shown in Fig. 3b, the over-potential is significantly reduced by using a HD Zn anode during the charge step (Zn ions deposited process).

Although there is dendrite formation when starting with HD Zn, the initially dendritic structure can mitigate the volume expansion and densities over time, with a relatively less rough morphology. Also, the HD Zn electrodes show higher surface area compared with sheet Zn anodes, which can increase the kinetics of Zn2+ reduction, thus enhancing the ability of the Zn electrode to keep up with the rapid insertion kinetics of the CuHCF electrode. Electrode additives represent a common approach for enhancing the cycle performance of a Zn anode. Kang et al. demonstrated a significantly improved Zn anode‟s cycling life by adding activated carbon featuring a well-developed and rich pore structure.51 The Zn ions are preferential to deposit in the pores structure of activated carbon instead of depositing on the surface of metal zinc (Fig. 3c). Also, these pores structure of activated carbon can accommodate the insoluble anodic products and the deposition of Zn dendrites, which make the surface of Zn particles maintain highly active, leading to significant enhancements in cycle performance. In another work, Mantia et al. found that the electro-deposition efficiency was greatly improved (increase from 85% to 98%) by using the zinc electrode with the addition of layered double hydroxide.52 The layered double hydroxide can diminish the potential drop during Zn2+ deposition, resulted from a more compact zinc deposition compared with the dendritic forming on zinc particles. Long et al. designed a Zn anode with a 3D architecture that featured interconnected Zn domains and large void volume with a “sponge” shape (Fig. 3d).53 The threedimensional (3D) architecture demonstrated improved performance of the Zn anode (reaching 90% utilization) due to specific structural characteristics: (a) amplifying the electrified interface, thus making the current distribute uniformly throughout the anode structure; (b) the creation of interconnected pathways within the interior of the porous structure, thereby enhancing longrange electronic conductivity; and (c) maintaining the electrode shape during the Zn

dissolution/precipitation process, preventing it from forming macroscale dendrites (Fig. 3e). Sun et al. investigated the effects of different organic additives in the electroplating process of zinc anodes.54 The result showed that the morphology and crystallographic properties of the Zn surface exerted a profound effect on dendrite formation, which can be regulated through an electroplating process with different organic additives (thiourea, PEG-8000, sodium dodecyl sulfate, cetyltrimethylammonium bromide) in the plating solution (Fig. 3f). It is more likely to format dendrite of the anode when the Zn surfaces are dominated by (100) and (110) facets than (002) and (103) facets, which caused by the interaction between the tip interface and the diffusion field out of the crystal, and zinc metal with exposed (100) and (110) planes shows higher surface area than that exposed (002) and (103), promoting to format dendrite.55 Furthermore, more studies on the thermodynamics and dynamics condition for the Zn dendrite formation are expected, which can guide us solve the dendrite formation problem and protect the zinc anode. The electrochemical properties of zinc anode can be effectively improved by constructing the surface structure of zinc anode with protective layer. Kang et al. designed a uniform buffer layer on the surface of zinc metal by a porous nano-CaCO3 coating.56 This strategy can effectively prevent the formation of large protrusions/dendrites, which can easily penetrate the separator and lead to short circuit of the battery. The zinc anode coated with porous nano-CaCO3 layer shows better cycling performance. After 1000 cycles, the zinc ion battery still shows a capacity of 177 mA h g-1 at 1 A g-1. The construction of zinc compound and organic electrode is also one of the effective methods to inhibit zinc dendrites and improve the cell cycle performance. Li et al. proved that a trilayer 3D CC-ZnO@C-Zn anode exhibits excellent anti dendrite performance.57 The trilayer is obtained by deposing Zn on carbon cloth, which modified

by the ZnO@C core-shell. Furthermore, Wang et al. reported a highly reversible dendrite-free Zn anodes by using the host of metal-organic framework.58 In zinc-ion hybrid capacitors and zinciodine batteries, the Zn@ZIF-8-500 anode electrode exhibits excellent cycling performance. In summary, zinc anode suffers from side reactions (corrosion, H2 revolution, dendritic zinc, passivation), making ZIBs be risk for potential security problems. Although several useful methods (composite anode, surface modification, structural design and electrolytes addition) have been put forward to protect the Zn anode, any large-scale applications with high utilization rates that involve these batteries will need further study.

Fig. 3.

3. Cathode materials Cathode materials play an important part for ZIBs, which are required of properties as follows: (1) Due to the high polarization of Zn2+ and the relative sluggish Zn2+ diffusion, cathodes featured with special tunnel structure, layered structure, framework structure with large pores are capable of working as host for the extraction/insertion of Zn2+; (2) be stable in the potential range between hydrogen evolution reaction and oxygen evolution reaction; (3) high energy density with both high capacity and working voltage; (4) long-life during repeat charge/discharge process; (5) low-cost and eco-friendly. This considering, various cathode materials have been discussed in the following section.

3.1 Manganese Oxides

For centuries, manganese oxides featuring various tunneled and layered structures have attracted extensive attention and various applications. It is generally known that the fundamental unit within the crystalline structure of MnO2 consists of one manganese atom surrounded by six oxygen atoms. The MnO6 octahedral subunits form crystallographic structures by continuously linking to the adjoining subunits and sharing edges and vertices, which can form different crystal structure, such as tunneled α-MnO2, β-MnO2, δ-MnO2, ε-MnO2 and layered γ-MnO2 (Fig. 4). Generally, various crystallographic polymorphs of MnO2 have been used as the cathode material for ZIBs.

Fig. 4.

Among them, α-MnO2 is highly attractive as a cathode material for ZIBs because of its particular 2 × 2 tunnel crystal structure consisted of four edges sharing a MnO6 octahedral along the z-axis. In 2012, Xu et al. proposed a new battery composing of zinc anode, α-MnO2 cathode, and mild aqueous electrolyte (Fig. 5a).27 Unlike traditional commercial alkaline batteries, these batteries depend on a reversible insertion of zinc ions into the tunnel structures of α-MnO2. Accordingly, it can deliver a capacity of 210 mA h g−1 with an average discharge potential of 1.3 V at 0.5 C, resulting in an energy density of 225 Wh kg−1 according to the weight of cathode materials. Nonetheless, the battery shows limited capacity at high current rates and poor cycling stability. Some measures have been proposed for improving capacity and cycle stability, such as expanding the tunnel of MnO2 and designing MnO2/CNT nanocomposites.59, 60 Interestingly, the reaction mechanism of Zn-insertion/de-insertion in/from α-MnO2 polymorphs remains controversial and elusive. Lee et al. reported the Zn2+ intercalation

mechanism, which relates to a reversible phase transition between a tunneled-structure α-MnO2 and a layered-structure Zn-birnessite (Fig. 5b).61 Shortly thereafter, the researchers confirmed that Zn-birnessite was not a direct reaction product of Zn2+ insertion in an α-MnO2 structure, but rather the result of the loss of water molecules and intercalated zinc ions from the layered structure of buserite (Fig. 5c).62 Recently, Pan et al. reported a highly reversible conversion reaction in Zn/MnO2 systems, composing of α-MnO2 nanofibres cathode, zinc anode and 2 M ZnSO4 aqueous electrolyte with 0.1 M MnSO4 additive.29 A high capacity of 285 mA h g-1 (at C/3), high operating voltage of 1.44 V, and long cycling life of 5000 cycles (at 5 C) with the capacity retention of 92% are achieved. What‟s more, they revealed a different reaction mechanism based on the conversion between MnOOH and MnO2 rather than Zn2+ ion intercalation into MnO2 to form spinel ZnMnO4, or a tunnel or layered ZnxMnO2, which was proved by the HRTEM results of the discharge products (Fig. 5d). Very recently, Wang et al. proved that a consequent hydrogen ion and zinc ions insertion/extraction in/from the MnO2 cathode are happened in Zn/MnO2 battery.63 As illustrated in Fig. 5e, the GITT measurement showed that the total overvoltage in region II (0.6 V) was almost 10 times more than that in region I (0.08 V). The large overvoltage was due to both slow ion diffusion and large voltage jump in region II (Fig. 5e). Further, there are only gently capacity drops in region I compare to largely dropped in the region II, when the discharge current rates increased from 0.3 to 6.5 C, indicating that the reaction kinetics in region II was extremely slower than that in the region I. So, it was highly considered that the voltage plateau in region I was attributed to the hydrogen ion insertion, while the voltage plateau in region II was mainly because the Zn2+ insertion, which were also proved by the ex-situ XRD results (Fig. 5f).

Fig. 5.

In general, β-MnO2 is unable to incorporate Zn2+ due to its narrow tunnel (1×1) structure.64 While Chen et al. reported ZIBs based on β-MnO2 nanowire cathode.65 The cathode shows a capacity of 225 mA h g−1 at 0.65 C, and long-term cycle stability with 94% capacity retention over 2000 cycles (Fig. 6a-b). Additionally, the researchers confirmed the evolution of the cathode structure, showing that the tunneled structure of MnO2 was transformed to the layered structure of B-ZnxMnO2·nH2O during the first discharge process. Over subsequent cycles, a reversible Zn ion de/insert in the Zn-buserite framework was confirmed (Fig. 6c). Layered δ-MnO2 demonstrates relatively good electrochemical performance in a Zn/δMnO2 cell, owing to ~7.0 Å large interlayer distance that facilitates the guest-ion insertion/extraction. Alfaruqi et al. explored the electrochemical properties of a δ-MnO2 cathode in an aqueous electrolyte (1.0 M ZnSO4), showing that it can deliver a capacity of ∼240 mA h g−1 at 83 mA g−1 (Fig. 6d).66 However, further studies are needed to elucidate the formation of spinel-type ZnMn2O4 and layered-type δ-ZnxMnO2 on complete discharge cycling. Han et al. described a Zn/δ-MnO2 cell in non-aqueous acetonitrile-Zn(TFSI)2 electrolyte, which yielded capacities of 123 mA h g−1 at 12.3 mA g−1, with ≥ 99% Coulombic efficiency for 125 cycles (Fig. 6e).67 In contrast to prior studies using an aqueous electrolyte, there is only reversible Zn2+ insertion into the δ-MnO2 in the non-aqueous system, without proton participation. The γ-MnO2 phase, comprised of (1 × 2, size ∼ 2.3 × 4.6 Å, ramsdellite) + (1 × 1, size ∼ 2.3 × 2.3 Å, pyrolusite) tunnels blocks, is typically synthesized at low temperatures. Alfaruqi et al. reported the feasibility of Zn-insertion in γ-MnO2 cathode.68 As shown in Fig. 6f, during the Zninsertion process, the γ-MnO2 phase undergoes three structural transformations: the ZnMn2O4

phase (spinel-type), the γ-ZnxMnO2 phase (tunnel-type), and the L-ZnyMnO2 phase (layeredtype). In early stages of discharge, the Zn-insertion into γ-MnO2 results in the ZnMn2O4 phase. As the discharge proceeds, a portion of the spinel-type ZnMn2O4 phase undergoes a transformation into the γ-ZnxMnO2 phase with the intercalation of zinc. As the discharge proceeds further, the Zn-containing tunnels expand and transform, resulting in the L-ZnyMnO2 phase (layered-type). At the final stage on complete Zn-insertion, the three phases, viz., ZnMn2O4, γ-ZnxMnO2, and L-ZnxMnO2, are co-existing. However, these three phases are observed to revert back almost fully to the initial γ-MnO2 phase. As such, the Zn/δ-MnO2 battery shows a poor cycling stability, with only ~60% capacity retention over 50 cycles (Fig. 6g). The reaction mechanism of Zn/MnO2 battery is still controversial. Three different redox mechanisms (Zn2+ insertion, conversion reactions, and the hydrogen/zinc co-insertion) have been proposed in MnO2 cathode, and a summary table is as follows:

Fig. 6.

Table 3.

3.2 Vanadium-based Materials Vanadium-based candidates for cathode materials in complementary alkali-ion batteries have captured much attention, owing to their high theoretical capacity, their layered structure that is suitable for ion-intercalation, and low cost.69 Most recent studies examining vanadiumbased candidates have focused on their potential applications beyond use in alkali-ion batteries, and instead are directed toward their use in multivalent ion battery technologies, such as

magnesium ion batteries,70, 71 aluminum ion batteries,72, 73 and ZIBs.74-76 It is worth noting that the larger radius and multi-charge features of Mg or Zn ions may lead to novel reaction mechanisms and exciting electrochemical properties. Also, these systems are facing a series of new challenges associated with the specific performance of the ion-transmission species and the resulting effects on the vanadium-based materials. In this section, the electrochemical performance, reaction mechanisms, developmental advances, and challenges of vanadium-based cathode materials for ZIBs will be discussed in detail. The catalyst orthorhombic vanadium pentoxide (V2O5), typically synthesized by combining a hydrothermal reaction with solid-state annealing, suffers from rapid capacity fading in ZIBs owing to its poor structural stability. A useful structural optimization strategy (phase control) to enhance the performance has been employed for vanadium-based electrodes of ZIBs. Senguttuvan et al. reported a bi-layered V2O5 structure, synthesized by an electrochemical deposition with extended intralayer spacing, which accommodates the insertion of Zn ions.30 The BL-V2O5@CFS electrode is synthesized by directly grow BL-V2O5 on carbon foam substrate (CFS), which is efficient for electron transport during battery operation (Fig. 7a). Two pairs of reduction/oxidation peaks are observed at 0.77/0.88 and 0.90/0.99 V during the first cycle. A new peak at 0.71 V appears with the diminishing of the peak at 0.99 Vas the cycle number increases, which is possibly due to the exchange of solvent molecules with the electrolyte. Because no characteristic band stretch from acetonitrile solvent molecules could be detected by Raman spectroscopy after the first discharge and charge, future characterizations are needed to confirm the solvent intercalation feasibility. The Zn/BL-V2O5 battery shows the capacity of 170 mA h g−1 at C/10 over 120 cycles, and the energy density is ≈144 Wh kg−1 based on the cathode electrode masses (Fig. 7b-c).

Fig. 7.

Another general method for providing structural stability during cycling is to use interlayer metal ions to stabilize the interlayer structure.69 Interlayer metal ions (MxV2O5, M = Na+, Zn2+, Mg2+, Ca2+) and/or structural water in the layered open framework crystal structure act as pillars, leading to expanded layer spacing, ensuring fast and reversible Zn2+ (de)insertion, thereby providing structural stability during cycling. He et al. investigated Na ion intercalation V 2O5 compounds (Na0.33V2O5 or NVO) nanowire cathode for ZIBs.77 The intercalation of Na+ between the [V4O12]n layers could work as “pillars” to improve the stability of the 3D tunnel structure upon ion intercalation/de-intercalation (inset in Fig. 7d). Also, the Na0.33V2O5 demonstrated improved electronic conductivity in comparison to bulk V2O5, owing to the intercalation of sodium ions. A remarkable capacity retention ratio of 93% was verified over more than 1000 cycles at high rates of 1.0 A g−1, with the Coulombic efficiency approaching 100% in all cycles. In contrast, the capacity of bulk V2O5 electrodes fade rapidly upon cycling, with only 52.9 mA h g−1 over 680 cycles (Fig. 7e). The outstanding performance of NVO can be attributed to their high conductivity and stable layered structure, improved and stabilized by the “Na+ pillars” of NVO. Compared with the monovalent “Na+ pillar” in a MxV2O5 structure, the divalent metal ions (Zn2+, Mg2+, Ca2+) are bonded with oxygen atoms, leading to stronger ionic bonds that aid in supporting the layers and effectively preventing the collapse of the structure.78 Nazar and coworkers reported layered and hydrated V2O5 derivative (Zn0.25V2O5·nH2O or ZVO, n ≈ 0.85) cathodes for ZIBs.79 The ZVO evidenced an open two-dimensional (2D) framework structure

with ZnO6 octahedra acting as pillars (Fig. 8a). The battery cycled in potential range of 0.5–1.4 V, a very high capacity of 282 mA h g−1 was initially obtained at 1 C rate (1 C = 300 mA g−1), of which 278 mA h g−1 was recovered upon charging with an average working voltage of ≈ 0.81 V. From 1 C to 8 C, significantly higher discharge capacities of ≈ 260 mA h g−1 could be delivered, with only 7% capacity decreased against the 1 C capacity (Fig. 8b). Extended cycling performance at an 8 C rate showed an impressive capacity retention of 81% of the highest achievable capacity (260 mA h g−1) after 1,000 cycles; moreover, it approached 100% of the corresponding Coulombic efficiency (Fig. 8c). Structural evolutions were investigated via operando XRD characterization (Fig. 8d). Results showed that the interlayer distance increased from 10.8 Å to 12.9 Å upon ZVO immersion in the electrolyte, which was attributed to water intercalation. The ZVO structure underwent a phase transition at the Zn2+ content ≈ 0.55, as well as two solid-solution regimes during Zn2+ intercalation (Fig. 8e). Interestingly, the interlayer distance sharply decreased from 12.3 Å to 11.0 Å at zinc concentrations between 0.55 and 0.6, which was related to dislodge of water molecules. Although a more detailed analysis is needed to elucidate this phenomenon, water molecules certainly play a significant role in the kinetic behavior of Zn2+ intercalation. Compared to the ZnO6 octahedra in ZVO, the double-layer Ca0.24V2O5·nH2O (CVO, n ≈ 0.83) shows a more open crystal structure due to the larger CaO7 polyhedra between the (V4O10)n layers. Alshareef and colleagues investigated CVO cathodes for ZIBs.80 The CVO cathodes delivered an initial capacity of 340 mA h g−1 at 60 mA g−1, and a long cycling life at 80 C (with 96% capacity retention after 3000 cycles) (Fig. 8h). Compared to ZVO, the improved performance of CVO cathode is attributed to their lower molecular weight and density, the larger increase in the interlayer space (Fig. 8f), and higher electrical conductivity (Fig. 8g).

Fig. 8.

The layered-type structure of AxV3O8 (A = H, Li, Na; x = 1 or 2) indicates the operational feasibility of ZIBs owing to the wide valence couples (V5+/V4+/V3+) of vanadium and large interlayer spacing.81-83 The monoclinic AVO (A = H, Li, Na) system is comprised of V3O8 layers linked by “A” ions in the tetrahedral and interstitial octahedral sites that make up the V3O8 layers. Specifically, the V3O8 layers are composed of VO5 trigonal bi-pyramids and VO6 octahedra, linked by corner-shared O atoms (Fig. 9a). He et al. designed high performance ZIBs using H2V3O8 nanowire cathode in 3 M Zn(CF3SO3)2 aqueous electrolyte.84 The resulting Zn/H2V3O8 battery exhibits an extremely high capacity of 423.8 mA h g−1 at 0.1 A g−1, a stable specific capacity of ~250 mA h g−1, and an excellent cycling stability (94.3% over 1000 cycles) (Fig. 9bc). This prominent performance is attributed to the open-layered structure that is suitable for the intercalation/de-intercalation of Zn2+, and higher electrical conductivity owing to the mixture valence of vanadium. In addition, the one-dimensional nanowire morphology enabled a short Zn2+ diffusion path and large electrode/electrolyte interface, which exerts a significant performance impact. Kim et al. investigated zinc insertion in a flake-type LiV3O8 (LVO) cathode for use in ZIBs.85 An average discharge capacity of 256 mA h g−1 was achieved at 16 mA g−1, 172 mA h g−1 at 133 mA g−1 with 75% retention after 65 cycles, and Coulombic efficiencies approaching 100% in all cycles (Fig. 9d-e). Unlike the electrochemical lithiation in LVO (coexisting two-phase domain), the Zn intercalation mechanism is a progressive single-phase transition between the ZnLiV3O8 phase and solid-solution ZnyLiV3O8 (y > 1) phase (Fig. 9f). Although the LiV3O8 structural framework undergoes a slight change during the electrochemical

reaction, the layered structure can be well maintained, indicating a reversible reaction. Liang et al. described a Na1.1V3O7.9 (NVO) nanoribbon/graphene cathode for ZIBs, which showed a capacity of ~220 mA h g-1 at 300 mA g−1, and a reversible capacity of 171 mA h g−1 over 100 cycles.86

Fig. 9.

Since Nazar et al. reported the effect of structural water for ZIBs, Yan et al. reported a detailed study of the effect of structural water molecules in V2O5·nH2O (n ≈ 1.29) for the use in ZIBs.87 The plate-like morphology V2O5·nH2O (VOG) with a highly interconnected nanowire framework supported by reduced graphene oxide, shows high conductivity (Fig. 10a). Interestingly, the bilayer distance shows increasement from 10.4 to 13.5 Å (Fig. 10b) during Zn2+ intercalation in VOG, which is opposite to the result of Zn2+ intercalation in ZVO with the interlayer distance decreased from 12.3 Å to 11.0 Å (Fig. 8e). The reason is that the structural water can serves as a charge screening media between the V2O5 bilayers and Zn2+, which resulted in an increasing distance between the neighbouring oxygen ions and Zn2+. During the charge process, the interlayer distance reduced from 12.6 to 10.4 Å, which related to the formation of hydrogen bonds among the intercalation of H2O molecules, CF3SO3−, Zn2+, and lattice oxygen which pulls VOG bilayers closer. Benefited from this “lubricating” effect, the VOG cathodes delivered a high capacity of 381 mA h g−1 at 0.06 A g−1, 372 mA h g−1 at 0.3 A g−1, and ≈ 86% capacity retention (319 mA h g−1) were confirmed when the current density increased by a factor of 50 times (15 A g−1). Even at an extremely high rate of 30 A g−1, a reversible discharge capacity of 248 mA h g−1 can still be achieved (Fig. 10c).

Fig. 10.

Recently, Alshareef et al. reported the cathode of zinc pyrovanadate (Zn3V2O7(OH)2·2H2O) for ZIBs.88 As shown in Fig. 11a, the Zn3V2O7(OH)2·2H2O showed an open-framework structure, composed of zinc oxide layers and V2O74- pillars, with structural water randomly filling in vacancies, yielding an interlayer spacing of 0.719 nm. The Zn/Zn3V2O7(OH)2·2H2O battery delivered a capacity of 213 mA h g−1 at 50 mA g−1, and a capacity of 101 mA h g−1 was achieved after 300 cycles, with 68% capacity retention (Fig. 11b-c). Very recently, Kim et al. described a α-Zn2V2O7 nanowire cathode for ZIBs.89 Compared with the Zn3V2O7(OH)2·2H2O cathode, the α-Zn2V2O7 evidenced improved cycling stability owing to its thortveitite structure. The discharge curve of a Zn2V2O7 electrode showed two continue discharge plateau from 0.81 V to 0.4 V (Fig. 11d). Impressively, a high reversible capacity of 138 mA h g−1 at high rate of 4000 mA g−1 was achieved after 1000 cycles with a capacity retention of 86% (Fig. 11e). A summary comparison of the storage mechanism in different type of vanadium-based materials is as follows:

Fig. 11.

Table 4.

3.3 Chalcogenides Layered transition-metal dichalcogenides (TMDs) are being scrutinized for use in ESDs owing to their graphene-like structure with a large intercalation layer and high electrical conductivity.90, 91 He et al. designed a ZIB using VS2 nanosheet as the cathode and Zn metal as

the anode in 3 M ZnSO4 aqueous electrolyte (Fig. 12a).31 The VS2 cathode shows a capacity of 190.3 mA h g−1 at 50 mA g−1, and 115.5 mA g−1 at a high rate of 2.0 A g−1; moreover, a discharge capacity of 110.9 mA h g−1 can be achieved after 200 cycles (Fig. 12b-c). According to an electrochemical kinetics analysis of multi-rate cyclic voltammetry measurements, the capacitive contribution plays a dominant role in the total capacity. Kang et al. investigated the performance of different types of transition metal compounds, including borides (TiB2 and ZrB2), oxides (TiO2, Fe3O4, and MoO3) and sulfides (MnS, MoS2, and WS2).92 Their preliminary results indicated that MnS displayed a capacity of 70 mA h g−1 after 100 cycles at 500 mA g−1 (Fig. 12d), while the other cathodes showed poor performance. Given these preliminary findings, further systematical studies are needed to confirm these performance parameters. Liu et al. investigated the Mo6S8 electrode with reversible intercalation of Zn2+, showing that the Mo6S8 compound evidenced a reversible discharge capacity of 79 mA h g−1 at 45 mA g−1; additionally, ∼60 mA h g−1 was obtained after 150 cycles (Fig. 12e-f).93

Fig. 12.

3.4 Prussian blue analogues Prussian blue analogues (PBAs) represent another interesting class of cathode material that could host a range of alkaline earth metal ions owing to their open-framework structure.94 Moreover, they also show the possibility as hosts for Zn2+ intercalation. Among them, zinc hexacyanoferrates (ZnHCFs) display a different structure compared to cubic MeHCFs. The ZnHCFs consist of the ZnN4 tetrahedra andFeC6 octahedra, which further form threedimensional framework by CN ligands (Fig. 13a). Liu et al. investigated ZnHCFs with

rhombohedral structure, as intercalation hosts for Zn2+.32 The CVs at a scan rate of 2 mV s−1 showed a reversible reduction peak at 1.61 V and an oxidation level of 1.94 V (Fig. 13b). The Zn/ZnSO4/ZnHCFs battery shows a capacity of 65.4 mA h g−1 at 1 C (60 mA g−1), 52.5 mA h g−1 at 5 C, 32.3 mA h g−1 at a 20 C rate, and an expected energy density of 60 Wh kg−1 (Fig. 13c). In addition, Liu et al. reported well-defined FeFe(CN)6 as the cathode for ZIBs, which delivered excellent performance of 122 mA h g−1 at 10 mA g−1 (Fig. 13d).94 On the downside, however, the discharge capability decreased rapidly with increasing rate (about 30 mA h g−1 at 60 mA g−1), which was attributed to slow Zn2+ diffusion limited by the high viscosity of the IL-based electrolyte. A CuHCF electrode displayed a pair of oxide peaks at ~0.6 V/~0.8 V, which was ascribed to the insertion/extraction of Zn2+ (Fig. 13e).95 Specific energy density levels of 45.7 Wh kg−1 at 60 mAg−1, and 33.8 Wh kg−1 at 600 mAg−1 have been obtained (Fig. 13f).96

Fig. 13.

3.5 NASICONs Another interesting candidate for incorporation in ZIBs is the NASICON-type material. Huang et al. incorporated Na3V2(PO4)3 as a host material for ZIBs.97 The Na3V2(PO4)3/Zn battery showed a pair of peaks at 1.05 and 1.28 V, which was attributed to the insertion/extraction of Zn2+ from/into NaV2(PO4)3. Note that an oxidation peak at ~ 1.46 V was observed in the first cycle of the CV curves, which was caused by the Na+ de-intercalation from the Na3V2(PO4)3 structure (Fig. 14a). The Na3V2(PO4)3 cathode delivered a reversible capacity of 97 mA hg−1 at 0.5C, with a capacity retention of 74% over 100 cycles (Fig. 14b). As indicated in the reaction mechanism displayed in Fig. 14c, the Na3V2(PO4)3 phase was formed during the first

step, followed by a transform between Na3V2(PO4)3 and ZnxNaV2(PO4)3 in subsequent cycles. Another similar NASICON-typed material, Na3V2(PO4)2F3, showed 0.5 V higher potential than Na3V2(PO4)3, and a more stable structure owing to the strong affinity of adjacent F atoms. Recently, Jiang et al. investigated the performance of a Na3V2(PO4)2F3 cathode for use in ZIBs.98 The Na3V2(PO4)3/Zn battery exhibited two pairs of redox peaks (1.25/1.35 and 1.6/1.7 V), yielding a high average voltage of 1.62 V (Fig. 14d). A reversible capacity of 60 mA h g−1 at 0.2 A g−1, and 33 mA h g−1 at high rate of 3 A g−1 were achieved (Fig. 14e). Impressively, the battery showed stable cycling performance (95% capacity retention), which was maintained over 4000 cycles at 1 A g−1 (Fig. 14f).

Fig. 14.

3.6 Spinels and post-spinels In addition to the aforementioned cathode materials, ZnMn2O4 nanostructures have shown great potential for application in ZIBs owing to the analogous spinel structure of LiMn 2O4.99, 100 The ZnMn2O4 consists of a MnO6 unit, with Mn ions occupying 1/2 of the octahedral sites, and Zn ions residing in 1/8 of the tetrahedral sites (Fig. 15a). Chen et al. reported nanostructures of ZnMn2O4 as a promising cathode material for ZIBs.101 The CV curves at 0.2 mV s−1 showed two pairs of redox peaks at 1.1/1.55 and 1.35/1.6 V, corresponding to the stepwise de/intercalation of Zn2+ from/into the spinel framework (Fig. 15b). Note that the initial cycle displayed slightly different behavior in comparison to subsequent cycles, which claimed to be associated with the gradual activation of the cathode. The ZnMn2O4 electrode delivered stable capacities at around 150 mA h g−1 at 50 mA g−1; and a remarkably high capacity retention of 94% was obtained over

500 cycles. In addition, the introduction of Mn vacancies in the ZnMn 2O4 spinel opened up additional pathways for easier Zn-ion migration without any significant electrostatic barrier, leading to faster electrode kinetics compared to the stoichiometric ZnMn2O4. Inspired by the work detailed above, the Wu group designed hollow porous ZnMn2O4 microspheres as a host material for ZIBs (Fig. 15c); the resulting battery displayed a high rate (3200 mA g−1) capacity of 70.2 mA h g−1.102 Note that cycling curves showed a parabolic shape when using the electrolyte with the addition of the MnSO4. The increased capacity at the onset was attributed to the oxidation of Mn2+ of the electrolyte; however, the decreased capacity observed in subsequent cycles requires further investigation (Fig. 15d).

Fig. 15.

In summary, Manganese oxide and vanadium-based materials are the most studied materials among all candidate cathodes for zinc ion batteries. Among various kinds of cathodes, manganese oxides and vanadium-based materials show outstanding performance. Although MnO2 cathodes shows high working potential (~1.3 V), the limited capacities (especially at high rates) are still needed to be improved. Vanadium -based materials show higher rate performances and capacities, but the low working potential (< 1.0 V) prevents its farther application. As shown in Fig. 16, Mn-based materials show higher energy density, while V-based materials have higher power density. Therefore, these two materials have the most promising application prospects. It should not be ignored that the dissolution of cathode materials in aqueous electrolytes also threat their electrochemical stability.75, 97 In addition, the slow diffusion of Zn2+ caused by the strong electrostatic interaction between divalent Zn2+ and cathode materials (especially for MnO2 based cathodes)also lead to slow reaction kinetics of the system, which seriously restricts the high-rate

performance of ZIBs.103 Furthermore, charge transfer may be accompanied by a drastic change of electron configuration, a change of coordination environment, or a sudden adjustment of bond length, which is also unfavorable to thermodynamics.104 Therefore, the performance of cathode materials is still needed to be improved. Cathode materials featured with high porosity,105 large surface area,79 large lattice spacing,31, 106 and structural water,81, 87 can promote the diffusion of Zn2+, thereby improve the performance of ZIBs.

Fig. 16.

4. Electrolytes The deposition/dissolution efficiency of a Zn anode and the electrochemical stability of cathode materials are related to the selection of the electrolyte for incorporation in a ZIB. For example, a Zn anode shows different behaviors in aqueous electrolytes at various pH levels, which is discussed earlier in this report. Generally, aqueous electrolytes display orders of level higher ionic conductivity than non-aqueous electrolytes at a given concentration, leading to better performance at high power. In contrast, for ZIBs, non-aqueous electrolytes (such as organic electrolyte) show little advantages over aqueous electrolytes due to their high toxicity, inflammability, high cost and potential safety problems. And ZIBs in non-aqueous electrolytes always show lower energy density than in aqueous electrolytes, which make it uncompetitive. 67, 77

Additionally, gel electrolytes demonstrate improved potential in applications for flexible Zn

batteries.

4.1 Aqueous electrolyte

The most common aqueous electrolytes for ZIBs are mild solutions of ZnSO4 and Zn(CF3SO3)2, since a Zn electrode is not stable in an aqueous electrolyte containing Cl- and NO3due to their strong causticity. Chen et al. systematically investigated ZnSO4 and Zn(CF3SO3)2 electrolytes, showing that both solutions evidenced a wide electrochemical window (above 2.3 V), indicating that the O2 evolution reactions are significantly suppressed (Fig. 17a-b).101 Compared to ZnSO4, the Zn(CF3SO3)2 exhibits faster kinetics and better reversibility of Zn deposition/dissolution, which can be attributed to quicker Zn2+ transportation and charge transfer resulting from the decreasing water molecules surrounding Zn2+ caused by bulky CF3SO3- anions. The salt concentration will also show a significant effect on the properties of electrolytes. As shown in Fig. 17c, the ionic conductivity of electrolyte decreases with increasing solution viscosity when the concentration of Zn(CF3SO3)2 salt increases from 1 to 4 M. The increased viscosity of the electrolyte will enhance solvation stability, thereby improving transport kinetics for cations/anions. Similar to aqueous LIBs described in previous studies, the water-activity induced side reactions can be reduced by increasing the salt concentration.

4.2 Concentrated electrolyte Water-in-salt electrolytes provide the extraordinary opportunity to expand the electrochemical stability window of Zn based batteries.107 Wang et al. demonstrated that using the extremely concentrated Zn-ion electrolyte (1 M Zn(TFSI)2 + 20 M LiTFSI) is an effective method for promoting dendrite-free plating/stripping of Zn with nearly 100% CE.108 As illustrated in Fig. 17d, Zn2+ is expected to surround by six water molecules in the dilute electrolyte (5 M Li+ + 1 M Zn2+), with little contribution from the TFSI-. The anions begin to occupy the Zn2+-solvation sheath in the intermediate concentrated electrolyte (10 M Li+ + 1 M

Zn2+). In the highest concentrated electrolyte (20 M LiTFSI+1 M Zn(TFSI)2), vicinity of Zn2+ is coordinated with TFSI- to form close ion pairs (Zn-TFSI)+instead of Zn-(H2O)6)2+, which can effectively prevent the H2 evolution, leading to dendrite-free Zn plating/stripping with nearly 100% CE.

4.3 Gel electrolyte Compared to the aqueous electrolytes, the gel electrolytes feature a solid-like dimensional stability, which are promising for flexible ZIBs. The reported gel electrolytes typical composing of Zn-containing salts (ZnCl2,109,

110

ZnSO4,111,

112

Zn(CF3SO3)2),98,

105

additive (LiCl,110

MnSO4,113, 114 Na2SO4),83 polymeric frameworks (poly(vinyl alcohol, PVA;105, 115, 116 gelatin;83, 113, 117

gum;109 polyacrylamide, PAM)118-120 and a certain amount of water. The polymeric

frameworks can not only provide high mechanical integrity for the devices, but also enable good contact for electrodes/electrolyte interfaces. In addition, gel electrolytes also play important role in reducing system side effects (dissolution of electrode materials, the growth of zinc dendrites, water decomposition) due to its limited free water. However, the relatively low ionic conductivity and poor mechanical strength still limits its further applications.121

4.4 The additives of electrolytes Electrolytes additives represent another common approach for enhancing the cycle performance of battery systems. On one hand, the use of electrolyte additives in an alkaline electrolyte (e.g., inorganic acids, zincate, and LiOH) have a positive impact on the complex deposition/dissolution dynamics of the Zn/Zn2+ reaction process, which can hinder dendrite formation.47, 122 On the other hand, the dissolution of the electrode in neutral/mild electrolytes

can be efficiently suppressed by selecting suitable electrolyte additives.83 The electrolyte additives also can stable the structure of electrode. For instance, Kang et al. introduced a 0.5 M MnSO4 additive into 2 M ZnSO4 electrolytes, which greatly improved the capacity and cycle lifetime.60 A similar phenomenon was described by Liu et al. (Fig. 17e).103 Specifically, in nonadditive electrolytes, the Zn/MnO2 battery displayed distinctive capacity fading during the initial fifteen cycles, which is ascribed to the dissolution of Mn2+ resulting from Mn3+ disproportionation in the electrolyte during cycling. However, the equilibrium of Mn2+ between the dissolution MnO2 electrodes and the electrolyte can be balanced, when MnSO4 is incorporated into the electrolyte, thereby stabilizing the electrodes. What‟s more, it‟s found that the additives Mn2+ in the electrolyte will be oxidized to form solid MnO2 on the cathode, which further works as host materials for Zn2+ storage in recent studies.123, 124 Recently, Chen et al. reported that the addition of Na2SO4 into a ZnSO4 electrolyte will not only prevent the dissolution of a NaV3O8·1.5H2O (NVO) cathode, but will also suppress Zn dendrite formation (Fig. 17f).83 The reason is that dendrite formation during the charge process will be avoided when Na+ with a lower reduction potential is introduced into the electrolyte, based on the electrostatic shield mechanism. Consequence, the cycle life of the Zn/NVO system can be greatly improved (Fig. 17g).

4.5 Solid electrolyte interface (SEI) In nonaqueous LIBs, the formation of solid electrolyte interface (SEI) at the anode is caused by the electrolyte decomposition in initial step of a newly assembled battery, which stabilizes cycling.125 The formation of SEI layers are heavy relied on the salt concentration of the aqueous electrolyte.126, 127 Unfortunately, in a diluted aqueous electrolyte system (Common electrolyte; 1-

4 M ZnSO4/Zn(CF3SO3)2), the direct degradation products of water are either soluble ions or volatile gases, which make it hard to form stable SEI.128 In a highly concentrated (21 M “waterin-salt”) LiTFSI-based electrolyte, SEI layer (LiF, Li2CO3 and Li2O) is observed in the initial cycles, which help suppress hydrogen and oxygen evolution.126 The formation of SEI layer in concentrated aqueous electrolytes is quite similar to SEIs in nonaqueous electrolytes, which are contributed by two possible pathways: (1) The formation of LiF owing to the reduction of clusters or anion complexes; and (2) The formation of Li2CO3 and Li2O caused by the reduction of CO2 and O2 dissolved in the electrolyte. It's worth noting that SEI would change (corrosion, cracking, dissolution and reforming) over repeated cycling, and the concentrated electrolyte are responsible for repairing and maintaining such an aqueous SEI. In summary, ZIBs working in aqueous electrolytes show advantages than in non-aqueous electrolytes in term of cost, safety and performances. Among them ZnSO4 (1-4 M) and Zn(CF3SO3)2 (1-3 M) aqueous solutions are widely used in recent studies since their outstanding performance. Compared to ZnSO4, Zn(CF3SO3)2 exhibits faster kinetics and better reversibility of Zn deposition/dissolution, while its price is too expensive (>18 times than ZnSO4). It should be noted that the slightly acidic properties of both ZnSO4 and Zn(CF3SO3)2 (pH: 3 - 4) can promote the corrosion of zinc anode, which will kill the performance of ZIBs and lead to potential safety issues caused by the producing of H2 during corrosion. More studies related to these corrosions are expected in future since they are ignored in most reported works caused by far excess electrolyte and zinc content. Although there are some improvements of performances for ZIBs or hybrid ions batteries by using high-concentrated electrolytes, we need to consider it carefully whether they are promising for aqueous batteries or not, for the following reasons: (1) high-concentrated salt lead to high density of the electrolyte. For example, it needs add > 6 g mL-

1

LTFSI salt in 21 M LTFSI-based electrolyte, which reduces the energy density of the system

and increases the costs for aqueous batteries; (2) high-concentrated salt resulted in high viscosity, which reduces ions diffusion in electrolyte. As for gel electrolytes, they are currently preferable for developing flexible ZIBs, but the relatively poor rate performance, caused by low ionic conductivity, should not be ignored when large-scale applications. To develop practical and environment-friendly aqueous electrolyte, more studies are expected on the additives of electrolytes, since they have shown some amazing effect on ZIBs.

Fig. 17.

5. Hybrid batteries (Cathode||Electrolyte (A+Zn2+)|| Zn) As discussed above, it is still a great challenge for the reversible storage of Zn2+ in the cathodes resulted from the relative sluggish Zn2+ diffusion (caused by divalent) in host materials, which hinders the development of cathode materials for aqueous batteries. As for the traditional aqueous Li/Na batteries, which are facing challenges of serious capacity fading and complicated full-cell designing since Li/Na metals cannot directly be served as the anode due to their highly active properties.129, 130 The introduction of hybrid ions (Li+/Na+ + Zn2+) into aqueous battery system can combine the advantages of zinc anode and the traditional aqueous Li/Na batteries (fast ion diffusion and high working voltage). As a result, the hybrid ions batteries not only feature the properties of higher working voltage and improved life, but also broaden the designing of aqueous system.131 However, compared to divalent Zn2+ insertion/extraction, the monovalent Li+/Na+ transfer may cause lower capacities with the same amounts of ions embedding. Rechargeable hybrid aqueous batteries based on mixed electrolyte (Li+/Na+ + Zn2+),

lithium/sodium intercalation compounds, and the Zn anode have been designed.132 As illustrated in Fig. 18a-b, this system is composed of a Li/Na host cathode which is prefer for Li+/Na+ insertion but difficult for Zn2+ insertion, a hybrid electrolyte and a Zn anode. During the charge/discharge process, the metal Li+/Na+ inserted or extracted from the cathode material, along with the Zn dissolution/deposition at the anode in the hybrid electrolyte. This novel design combines the advantages of the respective electrodes, thereby enhancing the electrochemical performance of the device.128

Fig. 18.

5.1 Hybrid Li/Zn batteries Chen et al. reported the development of an aqueous Zn/LiMn2O4 battery made up of a 2 M Li2SO4 + 1 M ZnSO4 aqueous electrolyte, which evidenced two distinct discharge plateaus with average voltages located at 1.89 V and 1.75 V, thus indicating a two-step Li ion extraction/insertion behavior (Fig. 19a).133 The binder-free LiMn2O4/CNT electrode delivered a capacity of 116 mA h g-1 at 480 mA g−1, with a capacity retention of 79.3% after 300 cycles (Fig. 19b). In another work, Chen et al. discussed the electrochemical mechanisms associated with a Li3V2(PO4)3/Zn hybrid battery in a 1M Li2SO4 + 2M ZnSO4aqueous electrolyte.134 Their results revealed the appearance of three pairs of redox peaks in the range of 0.7-2.1 V, suggesting a stepwise de/intercalation of Li+ into during the charge-discharge process (Fig. 19c). The LVP/Zn battery delivered an initial capacity of 113.5 mA h g−1 at 24 mA g−1, and 85.4% capacity retention ratio after 200 cycles (Fig. 19d).

Fig. 19.

Inspired by the knowledge that the stability window can be expanded to ~3.0 volts using a highly concentrated aqueous electrolyte (Water-in-salt electrolytes),108 Cui et al. developed a high-voltage battery by using a 21 M LiTFSI + 0.5 M ZnSO4 hybrid electrolyte with LiMn0.8Fe0.2PO4 cathode.135 This “water-in-salt” electrolyte evidenced an expanded stability window, which supported the electrochemical process of LiMn0.8Fe0.2PO4 and Zn electrodes (Fig. 20a). The LiMn0.8Fe0.2PO4 showed a working voltage exceeding 1.8 V, delivered a reversible capacity of ~140 mA hg−1 at 0.1 C (1 C = 170 mA g−1), and yielded an energy density of 183 Wh kg−1. Recently, Mai et al. reported a Zn/V2O5 hybrid-ion battery using a “water-in-salt” electrolyte (1 M Zn(CF3SO3)2 + 21 M LiTFSI), which demonstrated improved electrochemical performance.136 The CV curves of the Zn/V2O5 with 21 M LiTFSI + 1 M Zn(CF3SO3)2 electrolyte showed improved discharge plateaus (0.90 and 1.10 V), as well as improved cycling performance in a Zn(CF3SO3)2-LiTFSI electrolyte (95% capacity retention after 160 cycles) compared with the performance of a pure 1 M Zn(CF3SO3)2 electrolyte (25% capacity retention over 90 cycles) (Fig. 20b-c). The authors claim that the increased working voltage and improved performance are attributed to the intercalation/deintercalation of Li+ instead of the Zn2+ insertion/extraction in this high-concentration Zn(CF3SO3)2-LiTFSI electrolyte. However, recently studies show that this high-concentration electrolyte also promote dendrite-free plating/stripping of Zn anode, resulted in an improved performance of Zn anode.107 This considering, the improved performance in “water-in-salt” electrolyte for hybrid ions batteries are related to the improvements of both cathode and anode materials.

Fig. 20.

5.2 Na-Zn hybrid batteries Similar to aqueous Li-Zn hybrid batteries, Na-Zn hybrid batteries are also of significant interest to researchers. For example, the Wu et al. developed a Zn/Na0.95MnO2 rechargeable hybrid battery, which showed a capacity of 60 mA h g−1 at 2 C in “0.5 M CH3COONa +0.5 M Zn(CH3COO)2” aqueous electrolyte, with an average discharge voltage of 1.4 V.137 The battery also evidenced outstanding capacity retention of 92% after 1000 full cycles at 4 C (Fig. 21a), while the discharge capacity decreased rapidly at high rate, with only 12 mA h g−1 at 15 C (Fig. 21b).

Fig. 21.

Huang et al. recently reported a Na-Zn hybrid battery comprised of Na3V2(PO4)3 (NVP) cathode, zinc anode, and “0.5 M Zn(CH3COO)2 + 0.5 M CH3COONa” aqueous electrolyte.138 Subsequent analysis results revealed two flat charge/discharge plateaus at 1.42 and 1.47 V when cycled at 0.5 C, corresponding to the V4+/V3+ redox couples. The NVP electrode exhibited discharge capacities of 91, 86, 69, and 60 mA h g−1 at 0.5, 1, 10, and 20 C, respectively (Fig. 22a). Kim et al. reported a high-voltage insertion Na3V2O2x(PO4)2F3-2x and multi-walled carbon nanotube composite cathode for Na-Zn hybrid batteries.139 The Zn/Na3V2O2x(PO4)2F32x/MWCNT

battery showed an average voltage exceeding 1.65 V, delivered an initial capacity of

54 mA h g−1 at 1 C, and demonstrated an 85% capacity level after 400 cycles (Fig. 22b).

Fig. 22.

It should also be noted that Prussian blue analogues are another cathode for Na-Zn hybrid batteries. This class of material typically shows excellent cyclic stability while with limited specific capacities owing to their open framework structure. The Qian group developed Na2MnFe(CN)6 cathode for Na-Zn hybrid batteries, which showed excellent Na+ storage behavior in an aqueous battery system.140 Specifically, they reported three flat discharge voltage plateaus at 1.8, 1.65, and 1.4 V. And as shown in Fig. 23a, their Na-Zn battery displayed a capacity of 137 mA h g−1 at 80 mA g−1, while also exhibiting about 75% capacity retention after 2000 cycles at 0.8 A g−1 (Fig. 23b). Concurrently, Ma et al. described the development of a nickel hexacyanoferrate (NiHCF) cathode.141 Their battery was cycled in a 0.5 M Na2SO4 + 50 mM ZnSO4 electrolyte within the potential range of 0.9 − 1.9 V; it delivered the capacity of 76.2 mA h g−1 at 100 mA g−1, with around 81% capacity retention over 1000 cycles (Fig. 22c-d). The loss of capacity was attributed to the slow dissolution of the Na2MnFe(CN)6 cathode and limited reversibility.

Fig. 23.

6. Flexible Zn-ion batteries Also of increasing interest to researchers are the flexible quasi/all-solid-state Zn-based batteries principally due to a range of attractive features such as high flexibility, cost effectiveness, lightweight properties, good safety, and eco-friendliness.142-145 Steingart et al. developed a flexible mesh-embedded Zn-MnO2 alkaline battery architecture via the 3D printing method.146

Fig. 24a illustrates the fabrication steps involved in producing a mesh-embedded flexible electrode. A slurry of MnO2/Zn was stencil-printed on 50-mesh size nylon-mesh substrates, followed by curing in an oven atmosphere to remove residual solvent, after which a piece of MnO2/Zn electrode was obtained (Fig. 24b). The flexible Zn-MnO2 battery displayed a capacity of 5.6 mA h cm−2 at 0.5 mA with an open circuit potential of 1.52 V. The flexible electrochemical performance of this battery was tested by bending the device around cylinders of varying diameters (0.95, 1.27, 2.54 and 3.81 cm) (Fig. 24c). Findings revealed improved discharge performance after bending owing to compression, which led to closer interfacial contact between the electrodes and the PGE electrolyte. In another work, the Steingart group reported a stretchable MnO2/Zn cell architecture with active materials embedded in stretchable fabric (Fig. 24d).147 The battery showed a capacity of 3.875 mA h cm−2 with an open circuit potential (OCV) of 1.5 V. Moreover, the capacities were maintained when the fabric tested under 50% and 100% strain (Fig. 24e-f). In addition, the fabric current collector maintained sufficient contact with the electrodes even after repeatedly stretched by 100%, which resulted from the reduced shear force on the current collector-electrode interfaces by the “wave” configuration. Although the flexible Zn-MnO2 battery based on a silver-based current collector showed high flexibility (bending, stressing) and conductivity, its scale-up potential is limited by the high cost of the raw current collector materials. As a possible alternative, carbon fiber has been employed as a current collector due to its attractive features namely, low cost, high mechanical strength, low weight, and high flexibility. For instance, Zou et al. proposed a new-type fiber-type zinc-carbon battery with MnO2 on the positive electrode.148 A plastic tube was chosen as the host for the electrodes with an electrolyte filling in the voids (Fig. 24g). The MnO2 paste and carbon fiber were composed of a core-shell structure, with the diameter of the MnO2/carbon fiber

electrode in the range of 300–500 μm (Fig. 24h). The zinc–carbon battery exhibited a discharge capacity of 158 mA h g−1 at 70 mA g−1 with an open circuit voltage of 1.5 V. Interestingly, this device demonstrated almost no capacity loss when the bending radius was 3.0, 1.5, or 0.7 cm (Fig. 24i).

Fig. 24.

In addition to the mentioned printing and coating methods for preparing flexible electrodes, electrodepositing also represents an effective method for preparing flexible devices. Lu et al. reported a flexible quasi-solid-state (QSS) Zn-MnO2 battery made up of a Zn nanosheet anode, a MnO2@PEDOT cathode, with a modified PVA gel electrolyte (Fig. 25a).116 Both the MnO2@PEDOT cathode and Zn nanosheet anode were deposited on a carbon cloth (CC) by electrodeposition method. The Zn nanosheets were grown on a carbon fiber, forming a freestanding structure of ≈50 nm in thickness (Fig. 25b). Encouragingly, a superior capacity of 282.4 mA h g−1 was obtained at 0.37 A g−1 for this flexible QSS Zn/MnO2@PEDOT battery (Fig. 25c). More interestingly, the flexible device showed little capacity deterioration upon bending or twisting; it also performed well under temperatures ranging from 5 °C to 40 °C. Moreover, this battery device showed excellent flexibility, which enhances its utility for being incorporated into clothing or powering the LED lights of a wristwatch when connected in serial (Fig. 25d). Recently, Li et al. designed a novel 3D architecture with active materials directly grown on a nitrogen-doped CC (N-CC), which demonstrated higher surface area and improved conductivity.110 The stable flexible Zn/MnO2 battery was composed of tiny Zn nanoparticles and MnO2 nanorod arrays with a PVA/ZnCl2/MnSO4 gel electrolyte (Fig. 25e). The battery

evidenced capacity levels ranging from 328 to 201 mA h g−1 as the current density was increased from 0.5 A g−1 to 6 A g−1, yielding an energy density of 440 W h kg−1 at the power density of 7.9 kW kg−1 (Fig. 25f). The electrochemical performance of these tandem devices (two/three devices connected in series) exhibited an enhanced voltage window and was capable of illuminating light-emitting diode (LED) indicators (Fig. 25g). The development of flexible and wearable ZIBs offers a promising prospect for portable and flexible electronic devices. Applications of gel-containing or solid-state electrolytes to flexible water systems show great commercial potential in recent years. However, the ionic conductivity of the gel electrolytes and mechanical strength of reported flexible ZIBs systems are expected to be improved. The development of flexible and wearable ZIBs offers a promising prospect for portable and flexible electronic devices. Table 5 summarizes the recently reported electrochemical properties and device structure of flexible ZIBs.

Fig. 25.

Table 5.

7. Summary and Perspectives The past decade has witnessed enormous progress in the development of ZIBs, including the protection/modification of the zinc anode, a detailed analysis of related electrolytes, and the design of various cathode materials. Despite these advances, there are still many scientific and technical challenges to be resolved before ZIBs can enter the consumer market in any significant way. Firstly, zinc anodes remain predisposed to dendrite formation, making ZIBs at risk for

potential security problems. Although researchers have suggested methods for protecting the Zn anode, any large-scale applications with high utilization rates that involve these batteries will need further study. Secondly, side effects that include H2/O2 evolution, the corrosion of current collectors, and proton intercalation in aqueous electrolytes must be resolved. Thirdly, the narrow voltage window and relative low working potential still limit the energy density of ZIBs. To address this issue, some effective measures should be considered: (1) Designing suitable cathode materials with high capacity (> 300 mA h g−1) and high working potential (> 1.2 V) are extremely urgent. Among various kinds of cathodes, manganese oxides and vanadium-based materials show outstanding performance. Although MnO2 cathodes shows high working potential (~1.3 V), the limited capacities (especially at high rates) are still needed to be improved. Vanadium -based materials show higher rate performances and capacities, but the low working potential (< 1.0 V) prevents its farther application. (2) The voltage window can be increased by inhibiting the decomposition of water. By using highly concentrated aqueous electrolyte, the stability window can be expanded to ~3.0 V, which makes high-voltage materials (LiNi0.5Mn1.5O4, LiMPO4, Li-rich et al.) possible to be used as the cathode for ZIBs or hybrid batteries. (3) It should be noted that Zn anode in alkaline electrolyte (~1.26 V) shows lower working potential than in non-alkaline (~ 0.76 V) electrolytes, which could improve the system‟s voltage if the dendrite and other problems are addressed. Although MnO2 and vanadium-based cathodes show outstanding performance, it must be noted that both of these cathode materials, have been shown to dissolve in an aqueous electrolyte, to some degree, which is harmful to the system. To address this issue, a number of effective measures have been developed for improving performance: (a) using electrolyte additives to inhibit the dissolution of electrodes, (b) taking advantage of the “ion pillars” to stabilize the

structure, and (c) making use of structural water to support the layer and lubricate the framework of the electrodes. Finally, the issue of energy-density efficiency (discharge/charge energy density) must be further investigated if ZIBs are to be widely commercialized. As a typical example, the Zn/MnO2 system shows operating voltages of ~1.3/1.6 V during the discharge/charge process; as a result, only ~81.2% energy density efficiency is achieved. In a Zn/Zn0.25V2O5 battery (i.e., a typical vanadium-based system), operating voltages are in the range of ~0.7/0.9 V during the discharge/charge process, with a corresponding energy density efficiency of ~77.8%. The energy density efficiencies are even worse at high current rates due to serious polarization for ZIBs. For stationary energy-storage devices-ones requiring an all-vanadium redox flow battery, a rechargeable Zn-ion battery, a Na-ion battery, or a lead-acid cell and other parameters must also be taken into consideration. First, future studies must address cycling aging costs, which include the cost of the cell itself, installation fees, and maintenance costs. Indeed, researchers have yet to prove that Zn-ion batteries show the ability to replace the redox flow battery, which is almost maintenance-free during operation. To their credit, however, ZIBs can be fabricated in air and use aqueous electrolyte; such features could be advantageous in reducing the cost of the cell and in minimizing installation fees. Another parameter of significant environmental importance is the ability to recycle these cells and Zn-ion batteries have a decided edge over organic electrolytebased Na-ion batteries and lead-acid cell. Nonetheless, more experimental data are needed to evaluate the recyclability of Zn-ion batteries and all-vanadium redox flow batteries. The third key factor is the temperature-dependent performance of these cells. It is currently not economically feasible to construct temperature-control systems for these batteries. Specifically, for commercial applications, Zn-ion batteries need to tolerate operating temperatures ranging from 0 – 40 oC, which is currently not the case. Should these various challenges be overcome,

ZIBs could represent a highly viable source of battery power for personal and consumer electronics.

Acknowledgements This work was supported by the National Natural Science Fund for Distinguished Young Scholars (51425204), the National Natural Science Foundation of China (51521001, 51702247), the National Key R&D Program of China (2016YFA0202603), the Program of Introducing Talents of Discipline to Universities (B17034), the Yellow Crane Talent (Science & Technology) Program of Wuhan City and the Fundamental Research Funds for the Central Universities (WUT: 2018IVB034, 2018IVA088, 2018III025).

Author Contributions L. Q. Mai and P. He proposes the topic, P. He and Q. Chen finish the main parts on Zn anode, electrolyte and cathode materials, P. He, M. Y. Yan and X. Xu on summary and perspectives, all authors contributed to the preparation of the manuscript.

Conflicts of interest There are no conflicts to declare.

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Figures and tables

Fig. 1. Schematic illustration of ZIBs.

Fig. 2. (a) Pourbaix diagram of the system Zn/H2O, 10−4 M Zn2+ considering ZnO as solid substance. Reprinted with permission.37 Copyright 1990, Elsevier Ltd. (b) Scheme of zinc dendrite formation based on the diffusion-limited aggregation model. Reprinted with permission.44 Copyright 2015, Springer Nature.

Fig. 3. (a) Scheme of backside-plating configuration. Reproduced with permission.49 Copyright 2016, Springer Nature. (b) The charge and discharge profiles of HD Zn and Zn sheet anode. Reproduced with permission.50 Copyright 2015, Elsevier. (c) The schematic illustration for the deposition of anodic products in activated carbon. Reprinted with permission.51 Copyright 2015, ECS. (d) Photograph and SEM images of a 3D Zn sponge. (e) Cross-sectional schematic of the dissolution–precipitation conventional ad hoc powder-bed electrodes and the 3D-wired Zn sponge electrode. Reprinted with permission.53 Copyright 2014, Royal Society of Chemistry. (f) SEM images of synthesized anode with/without different additives. Reproduced with permission.54 Copyright 2017, American Chemical Society.

Fig. 4. Schematic diagram of various crystal structures MnO2: (a) α-MnO2, β-MnO2, (c) γ-MnO2, (d) ε-MnO2, and (e) δ-MnO2, respectively.

Fig. 5. (a) Schematics of Zn2+ storage between α-MnO2 cathode and Zn anode. Reprinted with permission.27 Copyright 2012, Wiley-VCH (b) Schematic Zn2+ intercalation into α-MnO2 and Zn-birnessite. Reprinted with permission.61 Copyright 2014, Nature Publishing Group (c) In situ

XRD patterns of α-MnO2 cathode. Reprinted with permission.62 Copyright 2015, Royal Society of Chemistry. (d) SEM of MnO2 electrodes discharged to 1.0 V (yellow) and then charged back to 1.8 V in the first cycle (blue). Reproduced with permission.29 2016 Macmillan Publishers Limited. (e) GITT profiles of the Zn/MnO2@CFP cell. (f) Ex-situ XRD patterns of the MnO2@CFP cathode at depth of discharge. Reproduced with permission.63 Copyright 2017, American Chemical Society.

Fig. 6. The performance of Zn/β-MnO2 cell, (a) discharge/charge profiles at varying C rates, and (b) long-cycle performance at rate of 6.5 C. (c) The schematic illustrating of the mechanism. Reproduced with permission.65 Copyright 2017, Springer Nature. The performance of Zn/βMnO2 cell, (d) in 1 M ZnSO4 aqueous electrolyte, Reproduced with permission.66 Copyright 2015, Elsevier. (e) In 0.5 M AN-Zn(TFSI)2 electrolyte. Reproduced with permission.67 Copyright 2017, American Chemical Society. (f) Schematic illustration of the reaction pathway of Zninsertion in the prepared γ-MnO2 cathode, and (g) Cyclic performance at 0.5 mA cm−2. Reproduced with permission.68 Copyright 2015, American Chemical Society.

Fig. 7. (a) SEM image of BL-V2O5. (b) Galvanostatic charge/discharge curves, and (c) cycling performance of the Zn/BL-V2O5 cell. Reproduced with permission.30 Copyright 2016, WileyVCH. (d) FESEM image and (inset) the crystal structure of Na0.33V2O5. (e) Cycling performance of Na0.33V2O5 at 1.0 A g−1. Reproduced with permission.77 Copyright 2018, Wiley-VCH.

Fig. 8. (a) Crystal structure of Zn0.25V2O5·nH2O viewed along the [110] direction. (b) Rate capability, and (c) extended cycling performance at 8 C rate (2,400 mA g−1). (d) Operando XRD measurement during the second electrochemical cycle. (e) Scheme showing reversible water intercalation into Zn0.25V2O5·nH2O immersed in electrolyte/H2O, and the water deintercalation accompanying Zn2+ intercalation upon electrochemical discharge. Reproduced with permission.79 Copyright 2016, Macmillan Publishers Limited. (f) Typical XRD pattern of Ca0.25V2O5·xH2O. (g) Electricity conductivities of Zn0.25V2O5·xH2O and Ca0.25V2O5·xH2O. (h) Cycle performance of Ca0.25V2O5·xH2O at 80 C. Reproduced with permission.80 Copyright 2018, Wiley-VCH.

Fig. 9. (a) Crystal structure of AxV3O8 (A = H, Li, Na; x = 1 or 2). (b) Charge/discharge profiles at 0.1 A g−1 of the initial three cycles. (c) Cycle performance at 5.0 A g−1 for 1000 cycles of H2V3O8 cathode. Reproduced with permission.84 Copyright 2017, Wiley-VCH. (d) Initial five voltage profiles and (e) cycle performance of LiV3O8 electrode. (f) Schematic of the Znintercalation mechanism in the present LiV3O8 cathode. Reproduced with permission.85 Copyright 2017, American Chemical Society.

Fig. 10. (a) The bright-field TEM image with selected area electron diffraction pattern of asprepared V2O5·nH2O (VOG). (b) The proposed crystal structures of pristine VOG, after charging to 1.3 V, and discharging to 0.2 V. (c) Rate capability of VOG and VOG-350. Reproduced with permission.87 Copyright 2017, Wiley-VCH.

Fig. 11. (a) Structural characterizations of as-synthesized Zn3V2O7(OH)2·2H2O nanowires. (b) Galvanostatic charge–discharge profiles. (c) Cycle performance at 200 mA g−1 of Zn/ZVO battery. Reproduced with permission.88 Copyright 2018, Wiley-VCH. (d) Galvanostatic discharge/charge profiles at 300 mA g−1. (e) Prolonged cycle performance of Zn/Zn2V2O7 battery. Reproduced with permission.89 Copyright 2018, Royal Society of Chemistry.

Fig. 12. (a) Schematic illustration of the operation mechanism of Zn/VS2 batteries. (b) Charge and discharge curves, and (c) cyclic properties at 0.5 A g−1. Reproduced with permission.31 Copyright 2017, Wiley-VCH. (d) Cycle performance of MnS at 500 mA g−1. Reproduced with permission.92 Copyright 2017, Royal Society of Chemistry. (e-f) Charge and discharge curve, and cyclic properties of Mo6S8 at 45 mA g−1. Reproduced with permission.93 Copyright 2016, American Chemical Society.

Fig. 13. (a) Crystal structure of rhombohedral zinc hexacyanoferrate. (b) CVs at a scan rate of 2 mV s−1, (c) rate capability of the Zn/ZnHCFs battery (1 C = 60 mA g−1). Reproduced with permission.32 Copyright 2015, Wiley-VCH. (d) Rate capability of the Zn/FeFe(CN)6 battery. Reproduced with permission.94 Copyright2016, American Chemical Society. (e) Typical cyclic voltammograms at 1 mV s−1 of Zn/CuHCF battery. Reproduced with permission.95 Copyright2014, Elsevier. (f) Rate capability of the Zn/CuHCF battery. Reproduced with permission.96 Copyright 2015, Wiley-VCH.

Fig. 14. (a) CV behavior at a scan rate of 0.1 mV s−1. (b) Cycle performance at a charge/discharge rate of 0.5 C of Na3V2(PO4)3/C sample. (c) Schematic representation for phase transition of Na3V2(PO4)3 cathode during cycling of Zn/Na3V2(PO4)3 battery. Reproduced with permission.97 Copyright 2016, Elsevier. (d) CV curves at 0.2 mVs−1. (e) Rate capacity and (f) cycle performance at 1.0 A g−1 of CFF-Zn/NVPF@C battery. Reproduced with permission.98 Copyright 2018, Elsevier.

Fig. 15. (a) Proposed Zn2+ diffusion pathway in ZnMn2O4 (ZMO) spinel without and with Mn vacancies. (b) CVs of ZMO/C electrode scanning at 0.2 mV s−1. Reproduced with permission.101 Copyright 2016, American Chemical Society. (c) TEM image and (inset) high-resolution TEM of ZnMn2O4. (d) Cycle performance at 100 mA g−1 of ZnMn2O4/Zn battery. Reproduced with permission.102 Copyright 2018, Royal Society of Chemistry.

Fig. 16. Ragone plots of representative cathode materials in ZIBs.

Fig. 17. (a) Cyclic voltammograms of a Zn electrode in an aqueous electrolyte of (a) 1 M Zn(CF3SO3)2 and (b) 1 M ZnSO4 at the scan rate of 0.5 mV s−1 between -0.2 and 2.0 V. (c) Characterization of viscosity and ionic conductivity in an aqueous Zn(CF3SO3)2 electrolyte with different concentrations (1−4 M). Reproduced with permission.101 Copyright 2016, American Chemical Society. (d) Schematic for MD studies of the Zn2+-solvation structure, representative Zn2+-solvation structures in the electrolytes with 1 M Zn(TFSI)2 and three concentrations of LiTFSI (5 M, 10 M and 20 M). Reproduced with permission.106 Copyright 2018, Springer Nature. (e) Improved electrochemical performance of Zn/MnO2 batteries in optimal aqueous electrolyte with and without 0.1 M MnSO4 additive in a 2 M ZnSO4 aqueous electrolyte at C/3 and 1 C, respectively. Reproduced with permission.103 Copyright 2016, Springer Nature. (f) Schematic diagram: Na2SO4 additive suppresses the dissolution of NVO nanobelts and the formation of Zn dendrites and (g) cycle performance. Reproduced with permission.83 Copyright 2016, Springer Nature.

Fig. 18. Schematics of two hybrid aqueous batteries. (a) Li-Zn hybrid battery, (b) Na-Zn hybrid battery.

Fig. 19. (a) CV curves at 0.1 mV s−1. (b) Cycling stability at a rate of 4 C of binder-free LiMn2O4/CNT network electrode. Reproduced with permission.133 Copyright 2016, Springer Nature. (c) Stable CV at 0.1 mV s−1. (d) The cycle performance with pH of 2.5, 4.0, 4.5 and 5.0 at 0.2 C of LVP/Zn battery. Reproduced with permission.134 Copyright 2016, Elsevier.

Fig. 20. (a) CV curves of Zn at a scan rate of 0.2 mV s-1 and LiMn0.8Fe0.2PO4 at 0.1 mV s−1 in aqueous electrolyte containing LiTFSI (21 m) and ZnSO4 (0.5 m). Reproduced with permission.135 Copyright 2016, Elsevier. (b) The 15th cycle charge/discharge curves at 100 mA g−1. (c) Cycling stability at 100 mA g−1 of Zn/V2O5 batteries. Reproduced with permission.136 Copyright 2017, American Chemical Society.

Fig. 21. (a) Charge and discharge curves of the rod-like Na0.95MnO2 at different rates using Zn metal as the counter electrode, and (b) cycle performance of the Zn/Na0.95MnO2 at 4 C rates. Reproduced with permission.137 Copyright 2014, Royal Society of Chemistry.

Fig. 22. (a) Charge-discharge curves for the Zn/NVP battery at different C rates. Reproduced with permission.138 Copyright 2016, Elsevier. (b) Cycle performance and coulombic efficiencies of the Zn/Na3V2O2x(PO4)2F3-2x/MWCNT full-cell, and the inset shows the corresponding chargedischarge voltage profiles of the given full-cell. Reproduced with permission.139 Copyright 2015, Royal Society of Chemistry.

Fig. 23. (a) Charge/discharge profiles at a rate of 0.5C in the electrolyte with added SDS. (b) Long cycle life testing at a rate of 5 C (1 C = 160 mA g−1) for the Zn/Na2MnFe(CN)6 battery. Reproduced with permission.140 Copyright 2017, Royal Society of Chemistry. (c) Charge and discharge curves at different current densities. (d) Cycle life and Coulombic efficiency at 500 mA g−1 for the NiHCF/Zn battery. Reproduced with permission.141 Copyright 2016, Elsevier.

Fig. 24. (a) Flow diagram for making the printed MnO2 cathode and Zn anode with a printed silver current collector; (b) Optical image of the MnO2 electrode; (c) Discharge profile at 1 mA of the flexible battery when flexed to different radii of curvature while discharging. Reproduced with permission.146 Copyright 2011, Wiley-VCH. (d) Top-view of the MnO2-Zn stretchable battery. (e) Optical image of the battery under 0% strain and 100% strain; (f) discharge curve of the battery at 0, 50, and 100% strain at discharge current of 0.35 mA. Reproduced with permission.141 Copyright 2012, Wiley-VCH. (g) Schematic illustration of the fiber battery based on Zn wire and MnO2/carbon fiber. The inset at the left shows the cross section of the fiber battery and the discharge process. The inset at the right is an actual picture of the fiber battery; (h) Cross section of the MnO2/carbon fiber. (i) Discharge curves of the fiber battery under different bending radii at the same discharge current, the inset is a schematic illustration of the bending test. Reproduced with permission.148 Copyright 2013, Elsevier.

Fig. 25. (a) Schematic illustrations of flexible quasi-solid-state Zn–MnO2@PEDOT battery; (b) SEM images; (c) Galvanostatic charge/discharge curves; (d) Photograph of a watch with LED lights powered by three devices in series. Reproduced with permission.116 Copyright 2017, Wiley-VCH. (e) Schematic illustration of the flexible N-CC@MnO2/N-CC@Zn battery and the electrode preparation strategy. (f) Galvanostatic charge/discharge curves at different current densities. (g) Galvanostatic charge/discharge curves collected at 2.0 A g−1 for a single quasisolid-state NCC@MnO2/N-CC@Zn battery device. Reproduced with permission.110 Copyright 2017, Royal Society of Chemistry.

Table 1. Comparison of different batteries. Batteries

Capacity (Whkg−1)

Capacity (Wh L−1)

Life (cycles)

Voltage (V)

Ni-Cd

45–80

50–150

300–2000

1.2

Ni-MH

60–120

140–300

180–2000

1.25

Lead-acid

33–42

60–110

200–300

2.1

Li-ion

160–300

250–693

400–1200

3.2–3.85

Zn-ion

80–120

>500

>1000

0.6–1.75

Table 2. A comparison of several monovalent/multivalent metals. Element

Crust abundance (%)33

Electrode potential [V] vs SHE34

Ionic radius [Å]35

Hydrated ionic radius [Å]36

Gravimetric capacity (mA h g-1)

Volumetric capacity (mA h g-1)

Li

20ppm

-3.04

0.76

3.40-3.82

3862

2066

Na

2.3

-271

1.02

2.76-3.60

1166

1129

K

2.10

-2.93

1.38

2.01-3.31

685

620

Mg

2.30

-2.36

0.72

3.00-4.70

2206

3834

Ca

4.1

-2.87

1.00

4.12-4.20

1337

2072

Zn

75ppm

-0.76

0.75

4.04-4.30

820

5855

Al

8.2

-1.67

0.54

4.80

2980

8046

Table 3. The comparison of the storage mechanism in different type of MnO2 compounds. Performance Crystal

α-MnO2

Crystal structures

Tunnel structure 2*2 cross section

Mechanism and

Refs.

Current

Capacity

Retent

mA g-1

mA h g-1

ion %

Zn2+ insertion

~1800

~135

74

100

27

conversion reactions

1540

161

92

5000

29

2002

~70

~71

10000

63

2000

151

94

2000

65

250

~60

40

68

208

~54

100

66

structural evolution

+

2+

H / Zn co-insertion

cycles

tunneled structure→Znβ-MnO2

Tunnel structure 1*1 cross section

buserite 2+

Zn insertion in Znbuserite

γ-MnO2

Tunnel structure

γ-MnO2→spinel-type

1*1 and 1*2 cross

ZnMn2O4→layered-

sections

type L-ZnyMnO2

0.5 mA cm-2

δ-MnO2→spinel-type δ-MnO2

Layered Structure

ZnMn2O4→layeredtype δ-ZnyMnO2

83

Table 4. The comparison of the storage mechanism in different type of vanadium-based materials. Performance Mechanism and structural Crystal

Crystal structures

layered VO2

structure, VO6 octahedra

Capacity

Retent

mA g-1

mA h g-1

ion %

2000

274.1

92.6

300

75

3000

~100

86

5000

76

14.4

170

87

120

30

6000

~300

71

900

87

Zn2+ insertion

1000

218.4

93

1000

77

replacement/intercalation

5000

81

97

2000

78

2400

~210

80

1000

79

24000

72

96

3000

80

evolution

Zn2+ insertion Single-phase reaction Zn2+ insertion

BL-V2O5

in nonaqueous bilayer hydrated V2O5·nH2O NaxV2O5 MgxV2O5 ZnxV2O5

Zn2+ insertion CF3SO3-

2D and 3D layered structures

Refs. Current

Zn2+ insertion

CaxV2O5

cycles

H2V3O8

Layered

Zn2+ insertion

5000

140

94.3

1000

84

LiV3O8

Structure,VO6

Zn2+ insertion

133

172

75

65

85

H+/ Zn2+ co-insertion

4000

165

82

1000

83

200

101

68

300

88

4000

138

85

1000

89

octahedra and NaV3O8·1.5 H2O

VO5 square pyramids

Zn3V2O7(O H)2·2H2O Zn2V2O7

Layered Structure V2O74pillars

2+

Zn insertion

Table 5. Summary of selected recently reported flexible ZIBs. Cathode material

Electrolyte

Anode material

Capacity (mA h g-1)

Retention%/cycle

Refs.

MnO2

ZnSO4/MnSO4 gum

Zn plate

127 (5 C)

45/1000

109

MnO2 nanorod arrays

PVA/ZnCl2/MnSO4

Zn nanoparticles

353 (0.5 A g-1)

93.6/1000

110

ZnHCF@MnO2

ZnSO4/PVA

Zn plate

118 (0.1 Ag-1)

77/500

111

zinc orthovanadate

Fumed silica/ZnSO4

zinc array

204 (0.5 C)

89/2000

112

Na3V2(PO4)2F3@C

Zn(CF3SO3)2

Zn plate

62.1 (0.2 A g-1)

80.6/600

98

Polyaniline@carbon felts

Zn(CF3SO3)2/PVA

Zn wires

109 (0.5 A g-1)

91.7/200

105

MnO2@PPy@SS

ZnSO4/MnSO4/gelatin

Zinc on NT wires

135.2 (1C)

87/500

113

MnO2@CNT yarn

ZnSO4/MnSO4/ PAM

Zn CNT yarn

302.1 (60 mA g-1)

98.5/500

114

ZnV3O8·1.5H2O@the steel

gelation/ZnSO4

Zn film

160 (0.5 A g-1)

77/100

83

MnO2/CNT composite

gelatin/ PAM/PAN

Zn on CNT paper

~130 (2.8 A g-1)

97/1000

120

MnO2 nanorods

gelatin-based gel

Zn on CC

133 (4C)

77/1000

117

Expanded MoS2@CC

ZnSO4/MnSO4/PAM

Zn on CNT paper

202.6 (0.1 A g-1)

98.6/600

119

Biographical sketches Liqiang Mai: Liqiang Mai is Chair Professor of Materials Science and Engineering at Wuhan University of Technology (WUT). He is the winner of the National Natural Science Fund for Distinguished Young Scholars and Fellow of the Royal Society of Chemistry. He received his Ph.D. from WUT in 2004 and carried out his postdoctoral research with Prof. Zhong Lin Wang at Georgia Institute of Technology in 2006-2007. He worked as an advanced research scholar with Prof. Charles M. Lieber at Harvard University in 2008-2011 and Prof. Peidong Yang at University of California, Berkeley in 2017. His current research interests focus on new nanomaterials for electrochemical energy storage and micro/nano energy devices.

Pan He: Pan He received his B.S. degree in Department of Material and Chemical Engineering from China Three Gorges University in 2015. He is currently working toward the Ph.D. degree in Department of Materials Science of Engineering from Wuhan University of Technology and as Joint training doctoral student at Northwestern University since 2018. His current research focuses on new type energy storage devices, carbon materials and liquid phase process.

Qiang Chen: Qiang Chen obtained his B.S. degree (2014) from the Jingdezhen Ceramic Institute and master‟s degree (2017) from the Guilin University of Technology. He is currently working toward his Ph.D. degree in Materials Science at Wuhan University of Technology. His current research involves nanomaterials and devices for energy storage.