Present and Future Perspective on Electrode Materials for

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Present and Future Perspective on Electrode Materials for Rechargeable Zinc-Ion Batteries Aishuak Konarov, Natalia Voronina, Jae Hyeon Jo, Zhumabay Bakenov, Yang-Kook Sun, and Seung-Taek Myung ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01552 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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ACS Energy Letters

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Present and Future Perspective on Electrode

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Materials for Rechargeable Zinc-Ion Batteries

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Aishuak Konarov1, §, Natalia Voronina1, §, Jae Hyeon Jo1, Zhumabay Bakenov2, Yang-Kook

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Sun3,*, Seung-Taek Myung1, **

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1

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Institute, Sejong University, Seoul 143-747, South Korea

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2

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Batyr Av., Astana 010000, Kazakhstan

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3

Department of Energy Engineering, Hanyang University, Seoul 04763, South Korea

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§

These authors contributed equally to this work.

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AUTHOR INFORMATION

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Corresponding Author

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*Tel.: 82 2 2220 0524, fax: 82 2 2282 7329, e-mail: [email protected] (Y. Sun)

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**Tel.: 82 2 3408 3454, fax: 82 2 3408 4342, e-mail: [email protected] (S. Myung)

Department of Nano-Technology and Advanced Materials Engineering & Sejong Battery

School of Engineering, National Laboratory Astana, Nazarbayev University, 53 Kabanbay

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ACS Paragon Plus Environment

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Abstract

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The zinc-ion battery (ZIB) is a two century-old technology, but has recently attracted renewed

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interest owing to the possibility of switching from primary to rechargeable ZIBs. Nowadays,

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ZIBs employing a mild aqueous electrolyte are considered one of the most promising candidates

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for emerging energy storage systems (ESS) and portable electronics applications due to their

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environmental friendliness, safety, low cost, and acceptable energy density. However, there are

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many drawbacks associated with these batteries that have not yet been resolved. In this review,

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we present the challenges and recent developments related to rechargeable ZIB research. Recent

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research trends and directions on electrode materials that can store Zn2+ and electrolytes that can

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improve the battery performance are comprehensively discussed.

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TOC GRAPHICS

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As the consumption of fossil fuels continues to increase, there is an attendant increase in the

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concentration of toxic gases like NOx and SOx in the atmosphere, where such gases are

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associated with global warming. Renewable energy sources such as wind and solar power are the

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most promising candidates for reducing the usage of fossil fuels and offer the advantages of

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being free and unlimited. Despite unlimited, these resources are intermittent, and they are not

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always available. To take advantage of these free and unlimited energy sources, there is a need to

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store the energy when it is available, where battery systems play an important role.1

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Since their introduction by Sony in 1991, Li-ion batteries (LIBs) have become the leading

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energy storage systems on the market compared to other rechargeable batteries.2 LIBs power

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small electronics like laptops, digital cameras, and mobile phones. Many electric vehicles (EVs)

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are now powered by LIBs because of their lightweight and high-capacity features, which lead to

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high energy density. However, when it comes to large-scale applications such as in stationary

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grid energy storage or EVs, the high cost and safety concerns related to LIBs become very

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important issues.3 Recently, there have been several accidents involving the explosion and

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ignition of LIBs, as in the case of Tesla cars and Samsung smart phones, where these LIBs

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employ flammable organic electrolytes.

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Rechargeable aqueous Li-ion batteries (RALBs) were introduced by Dahn et al. in 1994,

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where flammable organic electrolytes are replaced with water-based electrolytes.4 The advantage

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of this system over non-aqueous systems is that the high ionic conductivity of the aqueous

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electrolyte provides fast ion diffusion. In addition, the battery assembly process does not

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necessarily require a controlled atmosphere such as a dry room and glovebox, which could

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reduce the overall battery price. In the RALB system, it was demonstrated that Li ions can be


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intercalated/deintercalated in aqueous media. Since then, many studies have focused on

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enhancing the electrochemical performance of RALBs.5 However, because of the recent increase

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in the price of lithium due to the limited lithium resources, large-scale applications have not yet

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been realized.6 As an alternative to LIBs, mono and multivalent metal ions are extensively under

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study, their characteristics are summarized in Table 1. For large-scale applications, lead-acid

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batteries are still dominant because of their low cost and durability, although the lead compounds

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affect environmental issue and harmful to human health even in low concentrations.7

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From the above-mentioned points of view, rechargeable ZIBs are considered as the most

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promising candidates for grid energy storage and may provide an alternative to the currently used

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lead-acid batteries employing toxic lead compounds. The low price, nontoxicity, and abundance

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of zinc make it very attractive. In addition, Zn metal has a high theoretical gravimetric and

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volumetric capacity of 820 mAh g−1 and 5855 mAh cm−3, respectively. In 1986, Yamamoto et

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al.8 introduced the zinc aqueous battery system, replacing the alkaline electrolyte with zinc

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sulfate electrolyte. The advantage of this system is that there is no formation of byproducts (such

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as ZnO and Zn(OH)2) (Figure 1a), unlike the case in rechargeable Zn-air or Zn alkaline batteries,

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which means that Zn metal can be used as an anode. This is one of the advantages of Zn metal

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compared to other metals (Table 1); specifically, it can be dissolved and deposited more easily

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unlike Mg metal. Difficulties in settling cost and safety issue of LIBs have led us to revisit ZIBs

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with mild aqueous electrolytes. As seen from Figure 1a, the number of annual publications on

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ZIBs continues to increase rapidly (Figure 1b). Therefore, in this review, we summarize the

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latest developments in ZIBs with respect to the battery components, such as the electrode

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materials and electrolyte media. We exclude the separator, because, at present, absorbent glass

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mat (AGM) is used in almost all ZIBs, which is commercially available and well-known in lead-


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acid batteries. Particular attention is dedicated to the latest development of electrode materials

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that enable reversible intercalation of Zn2+, including descriptions of manganese oxides,

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vanadium oxides, molybdenum sulfides, Prussian blue analogs, and others (Figure 1c). Related

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cathode materials, their potential ranges, and energy densities are summarized in Figure 1d and

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Table 2. Moreover, we discuss the latest development of electrolyte media, including aqueous,

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non-aqueous, and quasi-solid-state electrolytes for rechargeable ZIBs.

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Figure 1. (a) Pourbaix diagram of Zn in aqueous solution. (b) The number of publications. (c)

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schematic illustration of ZIB and (d) Ragone plot of active materials for Zn2+ storage.

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1. Electrode materials

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1.1. Manganese-based composites

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Manganese oxide materials have been widely used in primary and secondary battery

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applications. The utilization of manganese oxides in ZIBs is very attractive owing to their low

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cost, environmental benignity, high operation voltage, and high theoretical capacity (~308 mAh

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g−1 for 0.5 mol of Zn in MnO2).8-53 These oxides exhibit very rich chemistry and can be

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synthesized in different crystalline forms such as MnO2,8-40,46,48,49,51-53 Mn2O3,45 Mn3O4,47,50 and

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MnO. Notably, MnO2, with outstanding structural flexibility, exists in a variety of

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crystallographic polymorphs such as α-, β-, γ-, δ-, λ-, R-, and ε-type (Figure 2).9

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Figure 2. Crystal structures of MnO2 polymorphs. The orange and blue octahedra surround

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spin up and down Mn atoms, respectively, while the red atoms represent O. Reprinted with

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permission from ref. 9, Copyright 2018. American Chemical Society.

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The fundamental MnO6 octahedral units, in which the Mn4+ is surrounded by six oxygen

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neighbors, linked via the edges and/or corners, form crystallographic structures that correspond

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to different polymorphs. Some polymorphs possess tunnel-like structures, such as α-, β-, γ-, and

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R-MnO2, with 2 × 2 (4.6 × 4.6 Å2), 1 × 1 (2.3 × 2.3 Å2), randomly arranged 1 × 2 and 1 × 1 & 1

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× 2 (2.3 × 4.6 Å2) tunnels, respectively. Moreover, a todorokite-type MnO2 has the largest 3 × 3


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tunnels (7 × 7 Å2). A δ-Type MnO2 possesses a layered-like structure with a large interlayer

2

distance (~7 Å). The unique δ-type two-dimensional structure is characterized by the layer

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interspacing distance, which depends on the inserted cations and H2O. Among the various forms,

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λ-type MnO2 has a spinel structure. The properties of MnO2 depend strongly on the crystalline

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structure, morphology, and particle size.

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The various MnO2 polymorphs have been reported as host materials for Zn2+ insertion in

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mildly acidic or neutral aqueous electrolyte. 8-40,46,48,49,51-53 Irrespective of the original crystal

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structure, manganese oxide electrodes suffer from capacity decay during cycling because of

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complex problems such as the low conductivity of the material, low structural stability, structural

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transformations, and manganese dissolution caused by the disproportionation reaction upon

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cycling. However, in recent years, some effective methods of mitigating these problems have

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been suggested. In the following section, we discuss the present literature on reversible Zn2+

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intercalation into different manganese polymorphs and consider the reaction mechanism, phase

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stability upon Zn2+ insertion, and methods of improving the cycling performance.

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Tunnel-type MnO2 polymorphs (α-, β-, γ-, R-)

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The concept of a rechargeable ZIB employing a mild acidic electrolyte and MnO2 as a cathode

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was discussed in the late 1980s by Yamamoto et al.8,10,11, where aqueous mild ZnSO4 solutions

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were exploited as a replacement for conventional alkaline electrolytes with promising results.

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The authors achieved good kinetics and reversibility (30 cycles), and the battery was operated at

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approximately 1.4 V vs. Zn metal.8 After a 20 years hiatus from such research, in 2012, Kang

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and coworkers12-15 revived the topic of rechargeable aqueous ZIB, demonstrating the possibility

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of using a mild aqueous electrolyte and α-MnO2 to reversibly host Zn2+. The intercalation of Zn2+


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into the tunnels of α-MnO2 was characterized by electrochemical evaluation, X-ray

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photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) measurements.12 From their

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reported XPS data (in the insertion (discharge) state, the ratio of Zn/Mn is 0.52, and in the

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extraction (charge) state, Zn/Mn equals 0.05), the authors claimed reversible insertion/extraction

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of Zn2+ into/from α-MnO2. The cathodic and anodic processes were described by the following

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equations:

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Cathode: 2MnO2 + Zn2+ + 2e- ↔ ZnMn2O4

(1),

8

Anode: Zn ↔ Zn2+ + 2e-

(2).

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Because MnO2 is a semiconductor with poor electronic conductivity, the use of a conducting

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carbon coating and doping with metals are effective strategies for enhancing the conductivity.

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Several coating and doping methods have been used to improve the performance of tunnel-type

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MnO2 in Zn cells.16, 20-25, 46 Kang et al.25 attempted to increase the electrical conductivity of α-

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MnO2 by using carbon nanotubes (CNTs). The rod-like α-MnO2/acid-treated CNT

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nanocomposites displayed both excellent storage properties with Zn2+ (~400 mAh g−1 at 1 A g−1)

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and reversibility at various current rates. Thereafter, Oh et al.26,27 investigated the mechanism of

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Zn2+ insertion into α-MnO2 nanorods using the-state-of-the-art analytical techniques. They

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proposed a reversible transition of from tunneled α-MnO2 to layered δ-MnO2 (Zn-birnessite) by

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electrochemical reactions. The transition was initiated by the dissolution of manganese from α-

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MnO2 during the discharge process, which is caused by the disproportionation of Mn3+ into Mn4+

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and Mn2+ and dissolution in water. However, the manganese ions were reversibly inserted into

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the layers of Zn-birnessite during charging to form the original tunnel structure, where the

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authors attributed these observations to intercalation rather than a conversion reaction.


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Figure 3. (a) Schematic illustration of the reaction pathway of Zn-insertion in the prepared γ-

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MnO2 cathode. (b) The initial three voltage profiles of the fabricated Zn/MnO2 cell cycled within

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the potential window of 0.8−1.8 V (c) Cycling performance of the test cell (Reprinted with

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permission from ref 28. Copyright 2015, American Chemical Society).

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A similar study was reported by Kim et al.28, where mesoporous γ-MnO2 (with randomly

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arranged 1 × 2 and 1 × 1 tunnels) was used. The tunnel-type orthorhombic parent γ-MnO2

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underwent a structural transformation to the spinel-type ZnMn2O4 phase and two new

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intermediary phases, namely, tunnel-type γ-ZnxMnO2 and layered-type λ-ZnyMnO2. All the

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phases with multi-oxidation states coexisted after completion of electrochemical Zn-insertion


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(Figure 3a). Upon Zn2+ deinsertion, the majority of these phases with multi-oxidation states

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underwent reversion to the parent γ-MnO2 phase. The γ-MnO2 cathode delivered an initial

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discharge capacity of 285 mAh g−1 at 0.05 mA cm−2 with a defined plateau around 1.25 V vs.

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Zn2+/Zn (Figure 3b). The corresponding discharge capacity after 40 cycles was 158 mAh g−1, and

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the capacity fading was attributed to manganese dissolution rather than to structural distortion of

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MnO2 (Figure 3c).

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Chen et al.29 investigated Zn intercalation in β-MnO2 (1 × 1 tunnels) in aqueous 3 M

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Zn(CF3SO3)2 + 0.1 M Mn(CF3SO3)2 electrolyte. Phase transition from the tunneled to layered

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structure of Zn-buserite was observed during the first discharge, accompanied by reversible Zn2+

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(de)intercalation in the Zn-buserite framework. Moreover, they elucidated the influence of the

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size of the tunnels on the electrode reaction in α-MnO2 (2 × 2), β-MnO2 (1 × 1), and γ-MnO2 (1 ×

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2 + 1 × 1) polymorphs. A main fact is that the phase transforms from tunnel to layer structure

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during the Zn2+ insertion (discharge) in all polymorphs, but the phase change did not depend on

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the size of the MnO2 tunnels. After further investigation for tunnel-type MnO2 in a mild aqueous

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electrolyte, Liu et al.30 proposed a reaction mechanism involving conversion between α-MnO2

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and H+, similar to that of the primary alkaline MnO2 battery, as follows:

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Cathode: H2O ↔ H+ + OHMnO2 + H+ + e- ↔ MnOOH

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ଵ

ଵ

௫

(4), ଵ

Zn2++ OH- + ଺ ZnSO4 + ଺H2O ↔ ଺ZnSO4[Zn(OH)2]3·xH2O (5), ଶ

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(3),

Anode:

Zn ↔ Zn2+ + 2e-

(6).

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The only difference between the primary and secondary Zn-MnO2 batteries lies in the

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reversibility of the Zn anode reactions in mild and alkaline electrolytes, respectively. The poor

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cycling performance of the Zn anode in alkaline electrolyte was ascribed to the formation of an


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insulating ZnO powder layer onto the Zn anode. In contrast, in a mild 2 M ZnSO4 with 0.1 M

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MnSO4 aqueous electrolyte, the Zn anode exhibited excellent kinetics and stability towards Zn

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stripping/plating without any dendrite formation or insulating layer growth of ZnO.30

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More details and an in-depth study of the reaction mechanism were introduced by Oh et al.31

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through in situ monitoring of the structural changes in α-MnO2 and water chemistry alterations

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during reaction in aqueous zinc sulfate electrolyte. Contrary to the belief that zinc ions

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intercalate into the tunnels of α-MnO2, Oh et al. proposed that zinc hydroxide sulfate

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Zn4(OH)6(SO4)·5H2O (Figure 4a) precipitates on the surface of α-MnO2. This precipitation is

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caused by the disproportionation of unstable trivalent manganese and dissolution in the

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electrolyte, resulting in a gradual increase in the pH of the electrolyte during discharging (Figure

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4b). During the charge process, the pH of the electrolyte decreases due to recombination of

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manganese on the cathode, leading to redissolution of zinc hydroxide sulfate into the electrolyte.

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In situ XRD analysis confirmed this process (Figure 4c), and Mn K-edge X-ray absorption

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spectra showed no shift of the peaks of α-MnO2 during discharge/charge, where the two Mn K-

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edge absorption spectra were almost identical at fresh and discharged states.

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Figure 4. (a) Crystal structure of Zn4(OH)6SO4·5H2O. (b) pH variations during the first

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discharge–charge process (the pH level was measured on separator which was soaked with

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electrolyte). (c) In situ XRD patterns of a cathode in a Zn/α-MnO2 cell with 1.0 M ZnSO4

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aqueous electrolyte during the first discharge–charge cycle and the corresponding discharge–

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charge curve. (Reproduced with permission from ref 31. Copyright 2016. WILEY-VCH).

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A solid-solution reaction and subsequent conversion reaction into β-MnO2 with [1 × 1] open

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tunnels involving Zn2+ was proposed by Kim et al.32. Based on in situ synchrotron, XANES, and

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EXAFS analyses, the Zn2+ were intercalated into the β-MnO2 framework, followed by the

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formation of Zn-inserted phases and precipitation of zinc hydroxide sulfate on the electrode

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surface. More recently, Mai et al.33 reported a two-step intercalation mechanism of Zn2+, where

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Zn2+ is first inserted into the layers and then into the tunnels of graphene scroll-coated α-MnO2.

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Recently, Sun et al.34 studied ε-MnO2 (akhtenskite structure without large open voids) and

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claimed that H+ and Zn2+ insertion/extraction occurred at the ε-MnO2 cathode. Proton insertion

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rather that hydronium ion (H3O+) insertion in the initial discharge process was confirmed by

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TGA analysis of the MnOOH product. This was the first report proposing a H+ and Zn2+

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insertion reaction mechanism for aqueous ZIBs.

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Literature analysis of manganese oxides (MnO2) with tunnel structures and their mechanism of

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reaction in ZIBs with a mild aqueous electrolyte showed contradictory results. Several

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experiments demonstrated that the capacity of MnO2 is derived from the reversible Zn2+

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insertion/extraction, whereas other experiments supported the reversible H+ insertion/extraction

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process based on the conversion reaction mechanism, whereas others affirmed the co-

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insertion/co-extraction of H+ and Zn2+. Oh et al.31 investigated not only the changes in the crystal

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structure of tunnel-type MnO2 with Zn2+ insertion, but also electrolyte chemistry during


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electrochemical reactions. The result indicated that manganese dioxide actually undergoes a

2

series of conversion-type reactions based on the reversible precipitation/dissolution of zinc

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hydroxide sulfate on the electrode surface, which is triggered by pH changes in the electrolyte.

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These findings highlighted the critical role of the electrolyte pH over the intercalation of Zn2+

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into MnO2. The discrepancies in works show the importance of understanding the reaction

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mechanism governing the reversibility in tunnel-type MnO2 structures for further improvement

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of electrode materials for reversible Zn2+ storage.

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Layered-type MnO2 polymorph

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Attempts have been made to reversibly intercalate Zn2+ into layered birnessite in both aqueous

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and organic electrolytes.35-39 Oh et al.35,36 presented a pioneering report on electrochemical

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cycling of Zn2+ in layered δ-MnO2, achieving an excellent performance of 300 mAh g−1 with the

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help of MnSO4 in aqueous ZnSO4 electrolyte.35 The authors claimed that the addition of MnSO4

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improved the reversibility of the cathodic reaction and suppressed the formation of basic zinc

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sulfate on the electrode surface. Zn2+ intercalation into δ-MnO2 was further investigated by

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different groups. Kim et al.37 developed a nanostructured δ-MnO2 cathode that exhibited a

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discharge capacity of 250 mAh g−1 with a Coulombic efficiency (CE) of nearly 100% after 100

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cycles. The authors claimed the formation of spinel-type ZnMn2O4 and layered-type δ-ZnxMnO2

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at the end of discharge as confirmed by ex situ XRD and ICP analyses. In the work of Wang et

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al.38, birnessite δ-MnO2 underwent H+ and Zn2+ insertion/extraction during charge/discharge

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processes, with an energy density of 364 Wh kg−1 and a capacity retention of 100% over 400

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cycles at 500 mA g−1. Vaughey et al.39 investigated Zn2+ intercalation into δ-MnO2 in an organic

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acetonitrile-Zn(TFSI)2 electrolyte. The non-aqueous cell employing the K0.11MnO2·0.7H2O


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electrode demonstrated good reversibility and stability for 125 cycles, with capacities as high as

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123 mAh g−1 and an operating voltage of 1.25 V vs. Zn2+/Zn. They proved the intercalation of

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Zn2+ without proton participation in the organic acetonitrile-Zn(TFSI)2 electrolyte. Furthermore,

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a small amount of ZnO was observed on the surface of MnO2 in both the charged and discharged

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samples after the second cycle, but the byproduct did not propagate during further cycling. This

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may be a proof that the ZnO is derived from the side reaction with water released from the

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structure of δ-MnO2. The overall electrochemical reaction was suggested as follows:

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xZn + K0.11MnO2·0.7H2O ↔ ZnxK0.11-yMnO2 + yK+ + 0.7H2O

(7).

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Spinel-type MnO2 polymorph

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Besides tunnel- and layered-manganese oxides, the spinel-type oxide (λ-MnO2) has been

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proposed as a cathode for application in ZIBs.40-44 Successful utilization of LiMn2O4 as an

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electrode material for reversible intercalation of Li+ in aqueous media inspired the possibility of

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the spinel analogue for Zn2+ insertion. The first study on the chemical extraction of Zn from

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ZnMn2O4-based spinel indicated that the ideal spinel structure was unfit for Zn2+ insertion due to

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the high electrostatic repulsion among the Zn2+ cations within the lattice.41 Inspired by the

17

knowledge that the introduction of cation vacancies in other spinel oxides opens up additional

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pathways for easier migration of divalent ions, cation-defective ZnMn2O4 spinel was proposed as

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a Zn-insertion host material in the work of Chen and coworkers.42 The spinel/carbon hybrid

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material exhibited a reversible capacity of 150 mAh g−1 and a capacity retention of 94% over 500

21

cycles. The authors stated that the remarkable electrode performance resulted from the facile

22

charge transfer and Zn2+ insertion in the structurally robust spinel with abundant cation

23

vacancies. The XRD results showed that the basic spinel lattice was maintained during charge


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and discharge, where the diffraction peaks gradually shifted to higher angle on charge and

2

returned to the original position on discharge. The delivered capacity was attributed to Zn2+

3

insertion based on the absence of the vibrations of O−H bonds (from H+ insertion). Moreover,

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another proof of Zn2+ insertion was the fact that the electrode could deliver a reversible capacity

5

of ∼90 mAh g−1 in a non-aqueous electrolyte comprising 0.1 M Zn(CF3SO3)2 in acetonitrile.

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Furthermore, 3M Zn(CF3SO3)2-based aqueous electrolyte enabled ∼100% Zn plating/stripping

7

efficiency with long-term stability and suppressed Mn dissolution.

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More recently, Kim and co-workers43 also reported Mg2+ and Zn2+ co-intercalation into

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tetragonal MgMn2O4 spinel. The coin cell comprised of Zn metal, 1 mol L−1 (MgSO4 + ZnSO4) +

10

0.1 mol L−1 MnSO4 electrolyte, and a MgMn2O4 spinel electrode delivered an initial discharge

11

capacity of 130 mAh g−1, which increased to 165 mAh g−1 within five cycles. The

12

extraction/insertion

13

formation/dissolution of (MgMn)9Zn4(OH)22(SO4)2·8H2O salt. (QUOTE 1)

of

cations

into

the

spinel

was

reversible,

followed

by

the

14

Zn2+ intercalation in manganese-based structures presents promising performance, displaying

15

good reversibility and kinetics, and provides an energy density close to that of targets for ESS

16

applications. The utilization of a manganese (II) salt (MnSO4 or Mn(CF3SO3)2) as an additive to

17

the electrolyte prevents dissolution of Mn3+ and is preferred for obtaining long cycling stability.

18

The reaction mechanism in these materials is not totally understood and needs to be further

19

investigated both experimentally and theoretically.

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1.2. Prussian Blue and its analogues

22

Prussian blue (PB) and its analogues (PBAs) materials have a three-dimensional open

23

framework with large interstitial sites, where these features are beneficial for fast charge and


15


Page 16 of 74

1

discharge.54,55 In addition to storing monovalent ions, PB and PBA can also store divalent (Mg2+,

2

Ca2+, Sr2+, and Ba2+) ions.56,57 Wang et al.58 reported the use of copper hexacyanoferrate

3

(CuHCF) nano-cubes as a cathode for ZIBs. CuHCF acted as the PBA, where Cu was connected

4

to N and Fe with C atoms, bridged by CN ligands. The cyclic voltammetry of CuHCF showed

5

one cathodic and one anodic peak, associated with the insertion/extraction of Zn2+. Their XPS

6

analysis suggested insertion of Zn2+ into the CuHCF structure via the reduction of Fe3+ to Fe2+.

7

Further work on CuHCF was carried out by Trocoli and Mantia,59,60 where they improved the

8

cyclability of the CuHCF electrode by employing a low concentration of Zn2+ in the electrolyte,

9

which was beneficial for Zn metal dissolution and deposition during cycling. The related

10

reactions can be described as below:

11

Cathode: xZn2+ + 2xe- + CuHCF ↔ ZnxCuHCF

(8),

12

Anode: xZn ↔ xZn2+ + 2xe–

(9).

13

Svensson et al.61 studied the insertion of Zn2+ into the CuHCF structure in detail by using

14

synchrotron X-ray diffraction. The voltage profile of CuHCF demonstrated that a capacity of 60

15

mAh g−1 was delivered at the single-step plateau, and after 50 cycles, the capacity retention was

16

about 85%. However, with cycling, the operating voltage of the cell increased from 1.6 to 1.7 V,

17

and two clear plateaus were observed. These changes can be explained by the dominance of the

18

remaining K+ relative to Zn2+ intercalation during the initial cycles, followed by Zn2+ insertion

19

several cycles later. From the operando dataset (Figure 5a), it can be seen that by the end of the

20

first discharge plateau, the occupancy of Zn2+ at the 8c site decreased, while occupancy by Zn2+

21

at the 4a site increased. Thus, initially, the Zn2+ are intercalated into the tunnel site (first plateau)

22

and subsequently occupy the Fe(CN)6 vacancy site (second plateau).

(0 ≤ x ≤ 0.5)


16

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ACS Energy Letters

1

Zhang and coworkers studied zinc hexacyanoferrates (Zn3[Fe(CN)6]2 (ZnHCFs), Figure 5b),

2

demonstrating that the FeC6 octahedra are connected to the ZnN4 tetrahedra via CN ligands to

3

form a porous 3D framework.62 The large open sites can accommodate alkaline metal cations A

4

(A = Na+, K+, and Cs+) and water molecules. Electrochemical data showed reversible redox at

5

around 0 V, which is associated with the dissolution/deposition of Zn metal. The other peaks at

6

1.6 V and 1.9 V are attributed to Zn2+ insertion/extraction to/from the ZnHCF structure. The

7

reactions that take place during cycling can be described as follows:

8

Cathode: xZn2+ + 2xe– + Zn3[Fe(CN)6]2 ↔ Zn3+x[Fe(CN)6]2

(10),

9

Anode: xZn ↔ xZn2+ + 2xe–

(11).

10

Further electrochemical tests showed a capacity approximately 65 mAh g−1 at 1C, with an

11

average working potential of 1.7 V (Figure 5c), which is the highest operating voltage compared

12

to that reported for other types of cathode materials. Zhang et al.63 improved the electrochemical

13

performance of ZnHCF by wrapping it with manganese oxide (MnO2) nanosheets using an in

14

situ co-precipitation method. The electrochemical performance of the composite was tested in

15

0.5 M ZnSO4 aqueous solution. The combination of MnO2 nanosheets and ZnHCFs delivered a

16

higher capacity than the bare electrodes due the synergistic effect from both compounds. The

17

improved electrochemical performance of the MnO2-wrapped ZnHFC electrode is contributed by

18

the Fe3+/2+ redox of ZnHCFs and the outer MnO2 nanosheet layers that act as a buffer layer for

19

facile ion diffusion into the ZnHCF structure (Figure 5d).

20

In short summary, PB and its analogues displayed excellent cycling and rate performance. In

21

addition, the operating voltage is the one of the highest among other types of material to store

22

Zn2+. However, its delivered capacity still needs to be further improved making composite

23

materials or making more vacancy site in the crystal structure. (QUOTE 2).


17


Page 18 of 74

1 2

Figure 5. (a) Top-view elevation map (intensity axis is perpendicular to the plane of the figure)

3

displaying peak shifts of a CuHCF/Zn cell during the in operando XRD experiment, time

4

evolution of the corresponding voltage profile (continuous line) of the first two cycles with the

5

refined lattice parameter (circles) superimposed and a plot of the time derivative of both the

6

voltage (dE/dt) and the cell parameter (da/dt) and the occupancies of the two possible Zn2+ sites

7

and their relation with the voltage profile during Zn2+ insertion followed by removal from the

8

CuHCF framework (Reprinted from ref 61, Copyright 2017, with permission from Elsevier). (b)

9

Crystal structure of rhombohedral ZnHCF. (c) Discharging curves of Zn/ZnSO4/ZnHCF battery

10

at a rate of 1 C (60 mA g-1) (Reproduced with permission from ref 62. Copyright 2014. WILEY-

11

VCH). (d) Graphical illustration of the ZnHCF@MnO2 electrode for Zn2+ storage (Reproduced

12

with permission from ref. 63, Copyright 2017. The Royal Society of Chemistry).

13 14

1.3. Vanadium-based composites


18

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ACS Energy Letters

1

Johnson et al.64 developed a V2O5/Zn battery system with non-aqueous (0.5 M Zn(TFSI)2)

2

acetonitrile electrolyte. The cell delivered a discharge capacity of 196 mAh g−1 and charge

3

capacity of 164 mAh g−1 in the first cycle. The delivered capacity indicates that approximately

4

0.59 mol of Zn2+ was intercalated into the V2O5 structure, activated by V5+/V4+ redox pair. A

5

similar material (Zn0.25V2O5·nH2O, ZVO) was studied in an aqueous (1 M ZnSO4) electrolyte.65

6

Zn2+ and water molecules in the structure acted as pillars to facilitate Zn2+ intercalation. The

7

galvanostatic charge/discharge profiles of the ZVO electrode are shown in Figures 6a−b. The cell

8

delivered a high capacity of 282 mAh g−1 at 1C, equivalent to the insertion of 1.1 mol of Zn2+.

9

Moreover, the cell demonstrated excellent rate and cycling performance, even after 1000 cycles

10

at 8C, with retention of more than 80% of its initial capacity. The excellent cyclability is

11

attributed to water molecules which acted as pillars in the crystal structure. Recently, Alshareef

12

et al.66 extended the work on ZVO by replacing the Zn2+ with Ca2+ (Ca0.25V2O5·nH2O, CVO) as a

13

host for Zn2+. Because CVO has higher electrical conductivity than ZVO, and due to the larger

14

ionic radius of Ca2+ (1.00 Å) than Zn2+ (0.74 Å), the resulting interlayers between the V–O layers

15

can be further expanded by the presence of large Ca2+ in the crystal structure. Ex situ XPS results

16

proved that Ca ions does not move from structure. As a result, CVO delivered a higher energy

17

density than ZVO as Ca is much lighter than Zn. Furthermore, Mai et al.67 reported work on

18

sodium ion-stabilized vanadium oxide (Na0.33V2O5, NVO) nanowires, in which the Zn2+ were

19

replaced with the larger Na+. The electrochemical performance of NVO was similar to that of the

20

ZVO and CVO electrodes. Another interesting study was presented by Yang et al.68, where they

21

highlighted the important role of water in Zn2+ intercalation into the bilayer V2O5·nH2O

22

structure. Even though there were no pillars of Zn2+, Ca2+, or Na+ between the layers of V-O

23

sheets, water acted as a “lubricant” to facilitate movement of the Zn2+. When the water was


19


Page 20 of 74

1

removed from the V2O5 layer, the material showed poor electrochemical performance, whereas

2

water-containing V2O5 showed outstanding and highly reversible Zn2+ (de)intercalation behavior.

3

Lately, similar work was done by Cheng et al.69; namely, H2O facilitated the electrostatic

4

repulsion between the Zn2+ and host material. Also, Zhou and coworkers demonstrated that V2O5

5

can intercalate Zn2+ in highly concentrated ZnSO4 aqueous solution.70 (QUOTE 3).

6

LiV3O8 shows interesting voltage profiles with plateaus that are observed during discharge, but

7

only one during charging (Figure 6c).71 The cell delivered a capacity of over 250 mAh g−1 (16

8

mA g−1) with excellent rate performance. During the first plateau, the formation of ZnLiV3O8

9

was confirmed, and during the second plateau, addition of more Zn2+ led to formation of the

10

ZnyLiV3O8 phase. However, during charging, there was direct transformation from ZnyLiV3O8 to

11

LiV3O8. Another work by Mai et al.72 demonstrated that H2V3O8 (HVO) nanowires underwent

12

Zn2+ (de)intercalation occurred in two steps. The delivered capacity was approximately 400

13

mAh g−1 at 0.1 A g−1, which is the highest capacity reported in literature for ZIBs. Furthermore,

14

the system exhibited outstanding cycling performance at a high current of 5.0 A g−1 for 1000

15

cycles, with retention of 94.3% of its initial capacity. This excellent electrochemical performance

16

was achieved due to the weak hydrogen bonds between the layers of V–O, which was able to

17

enhance the mobility of the Zn2+. Recently, Wang et al.73 reported improvement in the

18

performance of H2V3O8 nanowires by wrapping with graphene sheets. Because of the high

19

electrical conductivity of the graphene sheets, the electrode delivered enhanced electrochemical

20

performance. Other variety family of vanadates showed that they can also intercalate Zn2+.74-81


20

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ACS Energy Letters

1 2

Figure 6. (a) Galvanostatic discharge and charge profiles of the Zn0.25V2O5 nH2O freestanding

3

cathode at a 1C rate. (b) Extended cycling performance at a 8C rate (2,400mA g-1) (Reprinted

4

with permission from ref 65. Copyright 2016, Springer Nature). (c) Schematic of the Zn-

5

intercalation mechanism in the present LiV3O8 cathode (Reprinted with permission from ref 71.

6

Copyright 2017, American Chemical Society).

7 8

VO2 is found as a successful candidate for storage of Zn2+ into their structures.82 Yang et al.83

9

demonstrated VO2 (B) nanofibers as a Zn2+ ion storage material. As shown in Figure 7a, VO2 (B)

10

has a unique tunnel structure with a tunnel size of 5.2 × 8.2 Å2 along the b- and c-axes, where the

11

Zn2+ can intercalate. The cell delivered a high reversible capacity of 357 mAh g−1 at 0.25C, and


21


Page 22 of 74

1

the capacity remained at almost 100% after 50 cycles (Figure 7b). In addition, the VO2 (B)

2

nanofiber showed outstanding rate capability, a capacity of 171 mAh g−1 at 300C. Furthermore,

3

XPS investigation confirmed that the VO2 (B) is activated by V4+/3+ redox pair. To increase the

4

overall energy density of the VO2 electrode, Niu et al.84 engineered free-standing reduced

5

graphene oxide (rGO)/VO2 composite films that were prepared by combination of freeze-drying,

6

high temperature reduction, and mechanical compression. The free-standing rGO/VO2 composite

7

film demonstrated better rate and cycling performance. More interestingly, because of their

8

outstanding mechanical properties, the films were tested in a flexible ZIB system at different

9

bending angles and showed stable electrochemical performance. Park and coworkers85 predicted

10

Zn2+ intercalation into VO2 (B) structure by first principal calculations. The results showed that

11

Zn2+ inserted / extracted by two phase reactions as shown in Figure 7d-f. During the first step

12

0.125 mol and in the second step 0.375 mol of Zn2+ is inserted into VO2(B). In addition, the

13

predicted operating voltage of 0.61V is well matched with experimental value, which is 0.7V.

14 15

Figure 7 (a) Schematic view of Zn2+ intercalation/de-intercalation VO2 (B) nanofibers projected

16

along the direction of the b- and c-axes, respectively. (b) Cycling performance of VO2 (B)

17

nanofibers at 0.25C (inset: typical galvanostatic charge – discharge curve). (c) In situ XRD


22

Page 23 of 74 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

1

analysis of VO2 (B) nanofibers. The corresponding high-resolution contour maps of (−601),

2

(020), and (−404) peaks in panel, respectively (Reproduced with permission from ref 83.

3

Copyright 2018. WILEY-VCH). (d) Formation energy of ZnxVO2(B) (0 ≤ x ≤ 0.5), (e) calculated

4

average redox potential and experimentally measured charge/discharge curve of ZnxVO2(B), and

5

(f) expected mechanism of Zn intercalation into VO2(B) (Reprinted with permission from ref 85.

6

Copyright 2018. American Chemical Society) .

7

Jo and coworkers86 introduced hollandite-type VO1.52(OH)0.77 with a [2 × 2] tunnel structure,

8

where two distorted VO6 octahedra share an edge to form VO6 double chains (Figure 8a: top).

9

The structural stability of VO1.52(OH)0.77 was improved by Al doping. The electrochemical tests

10

(Figure 8a: bottom) showed that Zn2+ can be intercalated into the tunnels. Al-doped V1-

11

xAlxO1.52(OH)1.77

12

the high specific surface area by the introduction of Al into the structure.

(x = 0.05) demonstrated enhanced electrochemical performance, attributed to

13

Layered vanadium sulfide (VS2) nanosheets was suggested as a Zn2+ intercalation electrode

14

material by Mai et al.87 (Figure 8b: left). The intercalation of Zn2+ was progressed via two steps

15

(Figure 8b: right). According to their XRD and microscopic analyses, it was revealed that 0.09

16

Zn2+ are first intercalated in the first plateau region, and the second plateau is associated with the

17

insertion of another 0.14 Zn2+ ions into the layered VS2 structure, forming Zn0.23VS2. The

18

electrode displayed good cycling performance with a loss of only 2% of its initial capacity after

19

200 cycles.


23


Page 24 of 74

1 2

Figure 8 (a) (top) First discharge–charge curves of bare VO1.52(OH)0.77, V0.95Al0.05O1.52(OH)0.77,

3

and V0.91Al0.09O1.52(OH)0.77; (bottom) Zn2+ intercalation/de-intercalation into/out of the V1-

4

xAlxO1.52(OH)0.77

5

Copyright 2017. The Royal Society of Chemistry). (b) (left) Schematic illustration of the

6

operation mechanism of Zn/VS2 batteries. (right) Charge and discharge curves at the current

7

from 0.05 to 2.0 A g−1 (Reproduced with permission from ref 87. Copyright 2017. WILEY-

8

VCH).

(x = 0–0.09) tunnel model structure (Reproduced with permission from ref. 86,

9

Huang et al.88 employed a Na superionic conductor (NASICON) structure comprising

10

Na3V2(PO4)3 wrapped with graphene-like carbon. There are three sites where Na+ are located in

11

the Na3V2(PO4)3 structure, two 18e sites and one 6b site. It is well known from the sodium ion

12

battery system that the deintercalation of Na+ from the 18e site is easier than from the 6b site in

13

the NASICON-type structure. As illustrated in the scheme in Figure 9a, during the initial charge,

14

two Na+ can be extracted from the structure, and during discharge, Zn2+ can be intercalated into

15

the NASICON structure. The resulting voltage profile of Na3V2(PO4)3 is shown in Figure 9b.

16

Note that the first charge profile is due to the removal of two Na+ but Zn2+ is intercalated into the

17

structure at the first discharge. From the second cycle, shuttling of Zn2+ are progressed, so that

18

the resulting operation voltage for charge is lowered but there is no change in the discharge


24

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ACS Energy Letters

1

profile compared to the first discharge. To increase the operating voltage of Na3V2(PO4)3 in

2

ZIBs, Jiang et al.89 introduced fluorine-doped Na3V2(PO4)2F3 as a cathode material. Because of

3

the strong ionicity induced by the presence fluorine in the crystal structure, the resulting

4

operation was raised to 1.62 V versus Zn2+/Zn which shown in Figure 9c. However, there was no

5

clear difference between the first and second charge profile, which is different from the work

6

Huang et al.88 The cell illustrated improved cycling performance, retaining 95% of its initial

7

capacity after 4000 cycles at 1 A g−1 (Figure 9d). As a result, even though most of the vanadium-

8

based electrodes present low operation voltages, they deliver high capacity due to the various

9

oxidation state of vanadium.

10 11

Figure 9. (a) Schematic representation for phase transition of Na3V2(PO4)3 cathode during

12

cycling. (b) galvanostatic charge–discharge curves (Reprinted from ref 88, Copyright 2016, with

13

permission from Elsevier). (c) Initial three charge/discharge profiles of CFF-Zn//N3VPF@C


25


Page 26 of 74

1

battery at 0.08 A g−1. (d) Cycling performance of CFFZn// N3VPF@C battery at a current of 1 A

2

g−1 (Reprinted from ref 89, Copyright 2018, with permission from Elsevier).

3 4

1.4. Molybdenum-based composites

5

Chevrel phase Mo6S8 is an attractive electrode material for the storage of monovalent (Li+,

6

Na+) ions as well as multivalent (Mg2+, Zn2+, Al3+) ions (Figure 10a) due to its unique open

7

crystal structure and rigid framework.90-92 The Chevrel phase Mo6S8 structure is built from six

8

molybdenum atoms that form an octahedral Mo6 cluster and eight sulfur atoms that are face-

9

capped on top of the eight trigonal faces of the octahedron. Six of the eight sulfur atoms are also

10

bonded axially to a molybdenum atom of a neighboring cluster, resulting in the 3D network

11

crystal structure (Figure 10b). Pioneering work on Zn2+ intercalation into the Mo6S8 structure

12

was done by Schollhorn et al.93,94, where they proved that Zn2+ can intercalate into the Chevrel

13

phase. Liu et al.95 synthesized Mo6S8 nanocubes for use in aqueous (1 M ZnSO4 in water) and

14

non-aqueous (1 M Zn(ClO4)2 in acetonitrile) rechargeable ZIBs as an anode material (Figure

15

10c). The voltage profile displayed two plateaus associated with the intercalation of Zn2+ in two

16

steps even at high rates, as mentioned above. Zn2+ can be intercalated into the Mo6S8 structure in

17

both aqueous and non-aqueous electrolyte. The intercalation process undergoes via two steps:

18

(Step 1) Zn2+ + Mo6S8 + 2e– ↔ ZnMo6S8

(12),

19

(Step 2) Zn2+ + ZnMo6S8 + 2e– ↔ Zn2Mo6S8

(13).

20

The formation of the final discharged product, Zn2Mo6S8, was confirmed by XRD. Moreover,

21

the study was extended to full cell configuration using zinc polyiodide catholyte as the cathode.

22

The full cell showed excellent electrochemical performance with 90% capacity retention after

23

350 cycles at 600 mA g−1. Hong et al.96 studied the structural and electrochemical properties of


26

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ACS Energy Letters

1

the ZnxMo6S8 Chevrel phase (x = 1, 2) that was prepared by electrochemically inserting Zn2+ into

2

the Mo6S8 structure in aqueous electrolyte. One of the possible Zn1 sites is in this six-membered

3

ring. The first plateau (n = 2.2) corresponds to the formation of ZnMo6S8, and the second plateau

4

(n = 4.16) corresponds to the additional insertion of Zn2+, resulting in Zn2Mo6S8. The unit cell

5

volumes increased by 1.5% after the first plateau and 5.3% after the second plateau. The distance

6

between the molybdenum centers was reduced due to the reduction of molybdenum by insertion

7

of the Zn2+. More elaboration is required to increase the capacity and rate performances via

8

surface modification of the active materials.

9 10

Figure 10. (a) Schematic illustration of Mo6S8 application in various rechargeable ion batteries.

11

(Reproduced with permission from ref 90. Copyright 2017. WILEY-VCH). (b) Local structure

12

around the Zn1 positions in ZnMo6S8 (Reprinted with permission from ref 96. Copyright 2016,

13

American Chemical Society). (c) Rate capability of Mo6S8 for Zn2+ intercalation in 1.0 M ZnSO4


27


Page 28 of 74

1

aqueous electrolytes (Reprinted with permission from ref 95. Copyright 2016, American

2

Chemical Society).

3 4

2. Other Zn ion intercalation materials

5

2.1. Organic electrodes

6

Recently, organic electrode materials have emerged as an attractive host for monovalent (Li+,

7

Na+, K+) and divalent (Mg2+) ions due to their lightweight and environmentally friendly

8

features.97 In addition, because of the weak intermolecular van der Waals forces, ion

9

intercalation would be easier. Based on this, several studies have focused on the intercalation of

10

Zn2+ into organic electrodes.98,99 Kundu and coworkers reported that tetrachloro-1,4-

11

benzoquinone can store Zn2+ in its structure.99 The organic electrode was tested against Zn metal

12

in aqueous (1 M Zn(OTf)2) electrolyte. The cell delivered a capacity of about 200 mAh g−1, with

13

a smooth voltage profile. However, the water-based electrolyte promoted the phase evolution

14

between p-chloranil and discharged Zn-p-chloranil, which caused poor cycling performance.

15 16

3. Electrolyte

17

As summarized in the previous sections, most of the active materials presented different

18

electrochemical performance in the same operating window depending on the use of non-

19

/aqueous systems. It is clear that judicious choice of the electrolyte is as important as the choice

20

of the electrode material for the operation of ZIBs; this is because the electrolyte plays key roles

21

in forming protective layers at both the cathode and anode and influences the formation of by-

22

products such as ZnO or Zn(OH)2 and ZnSO4-based compounds. From this thought, ZIB

23

electrolytes should exhibit the following requisite properties: 1) chemical stability, 2)


28

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ACS Energy Letters

1

electrochemical stability at high and low voltage, 3) the ability to isolate electrons and conduct

2

ions, and 4) safety and low-cost. To develop electrolytes with all these characteristics, many

3

research groups are investigating the characteristics of the salts and solvents. The electrolyte

4

systems for ZIBs can be roughly classified into two types, i.e., aqueous and non-aqueous. In the

5

context of the electrolytes studied herein, we discuss both aqueous and non-aqueous electrolytes.

6

3.1. Mild aqueous electrolyte

7

First, aqueous electrolytes are discussed due to their advantages such as low-cost, safety, and

8

safe handling in open air. The electrolyte salts in aqueous electrolyte systems are clearly

9

important components for use in ZIBs. Most of the reported electrochemical intercalation

10

experiments have been performed in aqueous systems using mild aqueous electrolytes such as

11

ZnSO4 and Zn(NO3)2 salts and Zn(CF3SO3)2. The significant influence of the electrolyte salts is

12

clearly observed in manganese- and vanadium-based materials.

13

Kang et al.12 investigated the electrochemical performance of an α-MnO2/Zn cell using two

14

types of electrolyte salts, namely, 1 mol L−1 ZnSO4 and Zn(NO3)2 in D.I. water. The ZIBs

15

employing the mild aqueous electrolyte presented good discharge capacity at various rates from

16

0.5 C to 126C. The cycle life performance of the ZIB, in terms of the capacity retention and

17

coulombic efficiency, was investigated in a continuous cycling test at 6C charge/discharge rate.

18

After 100 cycles, the discharge capacity of the ZIB remained near 100% of the original value,

19

and the coulombic efficiency was about 100% for all cycles, indicating good electrochemical

20

performance. The cell performance featured excellent capacity rechargeability in terms of the

21

capacity retention and coulombic efficiency, even at 100% depth of discharge. It was reported

22

that the zinc ions could rapidly dissolve electrochemically as Zn2+ and deposit reversibly, leading

23

to a very high capacity in mild 1 mol L−1 ZnSO4 and Zn(NO3)2 aqueous solution. However,


29


Page 30 of 74

1

capacity fading was generally observed during the 30th cycle. The 1 mol L−1 ZnSO4 and

2

Zn(NO3)2 systems are acidic electrolytes (pH = 4.2) that can cause corrosion of metals and

3

dissolution of the transition metal and current collectors, resulting in poor long-term reliability.

4

To overcome these side effects, Chen et al.29 attempted the use of Zn(CF3SO3)2 having a bulky

5

anion, which enhanced the reactivity and stability of the Zn metal and active materials due to the

6

high ionic conductivity and electrochemical stability of the β-MnO2/Zn system. The cell

7

employing the 3 mol L−1 Zn(CF3SO3)2 electrolyte delivered a much higher initial discharge

8

capacity than that with 3 mol L−1 ZnSO4 (275 vs. 120 mAh g−1) at 0.65 C However, similar

9

capacity fading was observed during cycling due to the loss of the active mass. Thus, Mn2+ salts

10

were added to the Zn(CF3SO3)2 electrolyte to balance the equilibrium of dissolution of Mn2+

11

from the MnO2 electrode. To exclude the anion effect, Mn(CF3SO3)2 was selected as the

12

electrolyte additive, with concentrations from dilute 0.01 M to saturated 0.1 M. The optimal

13

electrolyte composition was 3 M Zn(CF3SO3)2 + 0.1 M Mn(CF3SO3)2, which resulted in a higher

14

coulombic efficiency (CE) and ionic conductivity, as well as a high capacity of 225 mAh g−1,

15

even after 100 cycles. Summarizing the salt effect, SO42−-based electrolytes such as ZnSO4 and

16

Zn(NO3)2 presented an increase in the capacity within the first several cycles, which was

17

attributed to the activation process. Furthermore, CF3SO3−-based electrolytes such as

18

Zn(CF3SO3)2 delivered a much higher initial discharge capacity (275 mAh g−1 at 0.65C) and

19

resulted in capacity stabilization from 10 cycles. The difference in the performance could be

20

ascribed to the Zn(CF3SO3)2 solution that not only features higher ionic conductivity, but also

21

enables faster kinetics and higher stability of Zn plating/stripping as compared with sulfate and

22

alkaline electrolytes.


30

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ACS Energy Letters

1

More importantly, the narrow electrochemical stability range of aqueous electrolytes limits the

2

operating voltage of the host materials, where it is thought that protons or hydronium ions

3

(H3O+) participate in the intercalation of zinc ions. The major issues of corrosion of the metal

4

and current collector by the acidic electrolyte, which can lead to poor long-cycle reliability, must

5

thus be resolved.

6

3.2. Aqueous-based solid-state/gel-type electrolytes

7

Recently, the development of safe, flexible, and wearable electronics has garnered significant

8

attention for application to smart watches, pulse sensors, clothes, and cellular phones. In this

9

regard, it is important to develop a rechargeable battery that can be worn very safely under

10

various harsh conditions. Zhi et al.49 investigated a gelatin and grafting polyacrylamide (PAM)-

11

based hierarchical polymer electrolyte (HPE) using an aqueous solution of ZnSO4 and MnSO4 in

12

an α-MnO2@CNT/Zn cell (Figure 11a). The HPE developed by Zhi et al. consisted of a highly

13

porous 3D architecture that was filled as gelatin chains and exhibited a high level of wettability

14

in the polymeric network. Thus, the HPE solid-state electrolyte presented a high ionic

15

conductivity of 1.76 × 10−2 S cm−1 while maintaining great flexibility and favorable mechanical

16

strength (7.76 MPa). Interestingly, ZIBs using the HPE solid-state electrolyte exhibited an

17

excellent specific capacity of 306 mAh g−1 at 61 mA g−1 and superior capacity retention of 97%

18

during 1000 cycles at 2772 mA g−1. The electrochemical performance of the HPE solid-state

19

electrolyte in ZIBs under a variety of conditions was evaluated by subjecting it to cutting,

20

bending, hammering, combustion, washing, weight loading, drilling, and sewing. Especially,

21

thermal safety and fire-resistant performance are of very importance issue for practical wearable

22

applications. As a result of Figure 11b, the unpackaged ZIBs using HPE solid-state electrolyte

23

was exposed to fire using an alcohol burner, and then did not catch fire even after being exposed


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1

to the fire for more than 5 min, demonstrating a surprising high-temperature durability. Besides,

2

the ZIB presented and a high capacity retention of 87.2%. The various tests demonstrated that

3

this ZIB using the HPE solid-state electrolyte exhibited excellent electrochemical performance

4

under a variety of extreme conditions. (QUOTE 4).

5

Another new concept for a solid-state electrolyte for ZIBs was also presented by Zhi et al.100,

6

who developed a cross-linked polyacrylamide (PAM)-based polymer electrolyte as a matrix host

7

for an aqueous solution of ZnSO4 and MnSO4 (Figure 11c). The solid-state polymer electrolyte

8

showed very good compatibility between the metal salts and cross-linked polyacrylamide

9

(PAM). Thus, it exhibited a high ionic conductivity of 16.5 × 10−3 S cm−1, indicating stable and

10

fast ion transport due to the presence of the highly porous network structure in the polymer

11

matrix and many hydrophilic groups, such as amide groups. The developed polymer electrolyte

12

exhibited a high tensile strength of 273 kPa and excellent stretchability to 3000% strain in the

13

constructed yarn-ZIBs. Interestingly, this yarn-ZIB presented a high discharge capacity of 302.1

14

mAh g−1 at 60 mA g−1 and superior cycling retention of 98.5% during 500 cycles, even at a

15

current of 2 A g−1. To evaluate the electrochemical performance of the yarn-ZIBs using a

16

polymer electrolyte, the yarn-ZIBs were tested under a variety of conditions, such as from

17

straight to 300% strain, bent, knotted, twisted, and soaked in D.I. water. The solid-state yarn ZIB

18

employing the polymer electrolyte exhibited excellent waterproof features, tailor-ability,

19

knottability, and stretchability.

20

Using this concept, Fan and coworkers developed quasi-solid-state (QSS) ZIBs using an

21

aqueous ZnSO4-based gel-type electrolyte.101 This QSS-ZIB exhibited ultrastable flexibility due

22

to the gel-type electrolyte and Zn array anode. It presented a high first coulombic efficiency of

23

95% and delivered a high reversible capacity of 204 mAh g−1 at 0.5C. Interestingly, the QSS-


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ZIBs exhibited high rate performance and delivered discharge capacities of 160 mAh g−1 at 10 C

2

and 101 mAh g−1 even at 50 C. In addition, the QSS-ZIB presented long-term cycle stability over

3

2000 cycles at 20 C and an unprecedented energy density of 115 Wh kg−1. Similar to solid-state

4

ZIBs, the QSS-ZIBs could be bent without deterioration of their discharge performance, and

5

there was no obvious capacity sacrifice (over 96% retention) even after 100 bending cycles.

6 7

Figure 11. (a) Schematic of the SSE-ZIB using HPE and (b) Combustion test ((Reproduced with

8

permission from ref. 49, Copyright 2018. The Royal Society of Chemistry) and (c) Schematic

9

diagram of fabrication and encapsulation of the yarn ZIB using the cross-linked PAM-based

10

electrolyte (Reproduced with permission from ref. 102, Copyright 2018. The Royal Society of

11

Chemistry). (d) Schematic diagram showing the structure and the fabrication protocol of the


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1

shape memory wire battery (Reprinted with permission from ref 100. Copyright 2018, American

2

Chemical Society).

3 4

To improve the performance of flexible and wearable energy storage devices, Zhi et al.102

5

investigated a smart wire-shaped ZIB with shape memory function, derived from a temperature-

6

triggered shape memory effect, by using a gelatin−borax-based gel-type polymer electrolyte with

7

aqueous ZnSO4/MnSO4 solution (Figure 11c). The gelatin−borax-based gel-type polymer

8

electrolyte exhibited higher ionic conductivity (2.0 × 10−2 S cm−1) than the normal gel polymer

9

electrolyte (1.39 × 10−2 S cm−1) due to the addition of borax, which also conferred enhanced

10

water retention capability and electrochemical performance. As a result, smart wire-shaped ZIBs

11

using the gelatin−borax-based gel-type polymer electrolyte delivered a discharge capacity of

12

174.2 mAh g−1 at 0.5C and retained a capacity of 60 mAh g−1 even at 4C. Moreover, the capacity

13

retention was very well maintained in the gelatin−borax-based gel-type polymer electrolyte,

14

presenting a discharge capacity of 135.2 mAh g−1 at 2C and a coulombic efficiency of over 99%

15

over 1000 cycles. The electrochemical performance in the bent state was evaluated by fixing the

16

battery. The flexible ZIBs employing the gelatin−borax-based gel-type polymer electrolyte

17

displayed good battery performance even when bent to 90° and were functional even after 500

18

bending cycles.

19

In summary, ZIBs employing solid-state/gel-type electrolytes do not seriously affect zinc

20

dendrite, such that they exhibit a high level of wettability and ionic conductivity while

21

maintaining great flexibility and favorable mechanical strength. Therefore, the concept of

22

rechargeable ZIBs can be applied to reliable power sources and solid-state/gel-type electrolytes


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1

offer the possibility for the development of flexible and wearable energy storage devices as the

2

most suitable electrolyte for zinc ion batteries.

3 4

3.3. Aqueous-based bio-electrolytes

5

In addition to solid electrolytes, gel-type bio-electrolytes, which have a high ionic conductivity

6

(similar to that of aqueous-based electrolytes such as ZnSO4/MnSO4 solution) are currently being

7

studied for application to flexible and wearable devices.

8

Li et al.

103

investigated various stable gum-type sulfate-tolerant bio-electrolytes using a

9

xanthan bio-polymer with ZnSO4/MnSO4 solution for a MnO2@CNT/Zn cell (Figures 12a and

10

b). This gum-type bio-electrolyte had a high ionic conductivity of 1.46 × 10−2 S cm−1 even at −8

11

°C (2.5 × 10−3 S cm−1). As advantages, the gum-type electrolyte permitted hydration, enhanced

12

viscosity, and self-adjustment of the shape. ZIBs employing the gel-type bio-electrolyte

13

exhibited a high discharge capacity of 260 mAh g−1 at 1C and good capacity retention (90%)

14

during 330 cycles at 1C. Furthermore, the cell presented a capacity of 127 mAh g−1 over 1000

15

cycles, even at 5C. The ZIBs presented good electrochemical performance in the bent and

16

twisted states, implying that the gum-type electrolyte is less affected on the side effect by

17

undesirable dendrite formed.


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1 2

Figure 12. (a) A hand-made flower using the xanthan gum electrolyte. (b) Schematic showing

3

the structure of our gum Zn–MnO2 battery using xanthan gum film electrolyte (Reproduced with

4

permission from ref. 103, Copyright 2018. The Royal Society of Chemistry). (c) Schematic

5

illustrations of morphology evolution for bare and nano-CaCO3-coated Zn foils during Zn

6

stripping/plating cycling. (d,e) GCD curves and galvanostatic cycling performance of

7

Zn/ZnSO4+MnSO4/CNT/MnO2 (Reproduced with permission from ref 119. Copyright 2018.

8

WILEY-VCH).

9 10

Endres and coworkers104,105 investigated a Prussian blue/Zn cell using the new concept of a

11

bio-ionic liquid-water electrolyte. The bio-ionic liquid-water electrolyte consisted of 1.0 mol L−1

12

Zn(OAc)2/([Ch]OAc + 30 wt% water). Interestingly, the ionic liquid, choline acetate, exhibits

13

unique solubility in water. When [Ch]OAc was dissolved in 20 wt% of water, the zinc

14

morphology was compact and dense. However, when [Ch]OAc was dissolved in over 50 wt% of

15

water, the zinc deposited as discrete thin plates with perpendicular orientation. Moreover, the


36

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[Ch]OAc solution changed from an ionic liquid-like solution to an aqueous-like solution in 40

2

wt% water. Based on these results, [Ch]OAc in 30 wt% of water was employed as the electrolyte

3

for the biodegradable zinc ion battery. Thus, the electrolyte possessed the properties of an ionic

4

liquid as well an aqueous medium. The corresponding ZIBs presented a reversible

5

electrochemical reaction and a discharge capacity of 120 mAh g−1 at a current of 10 mA g−1 (0.1

6

C). Although the rate capability decreased drastically at 60 mA g−1 with a discharge capacity of

7

about 31 mAh g−1, the zinc metal exhibited good reversibility without the formation of Zn

8

dendrites.

9

In summary, bio-ionic-liquids are good trend and attempt for Zn-ion battery research due to the

10

lack of research on electrolytes. Although bio-ionic-liquids have good cyclability and the zinc

11

metal exhibited good reversibility without the formation of Zn dendrites, rate capability is poorer

12

than that of batteries employing mild aqueous electrolytes. Thus, it needs to improve the rate

13

capability via new research such as using the additive materials.

14 15

3.2. Non-aqueous systems

16

3.2.1. Organic electrolyte

17

To exclude the issues related to mild aqueous electrolytes, some groups investigated organic

18

electrolyte systems based on Zn(ClO4)2, Zn(TFSI)2, and Zn(CF3SO3)2 in acetonitrile. Hong et

19

al.106 studied the electrochemical performance of a K1.51Ni[Fe(CN)6]0.954(H2O)0.77 (KNF-152)/Zn

20

cell using an organic electrolyte, namely, 0.5 mol L−1 Zn(ClO4)2 in acetonitrile. The cycling

21

current of zinc in the organic electrolyte was compared with that in the mild aqueous ZnSO4

22

electrolyte to evaluate Zn metal plating; zinc presented excellent deposition/dissolution

23

reversibility with 99.9% coulombic efficiency in 0.5 mol L−1 Zn(ClO4)2 in acetonitrile over 20


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1

cycles. This indicates that the organic electrolyte is suitable for high-voltage and highly reliable

2

ZIBs due to the high reversibility of the metal deposition/dissolution reaction. However, organic

3

electrolytes have fatal disadvantages in terms of the rate capability. The initial discharge capacity

4

of 55.6 mAh g−1 at 0.2C (11.2 mA g−1) declined drastically with increasing C-rates. This is

5

related to the ionic conductivity of the electrolyte. The ionic conductivity of 0.5 mol L−1

6

Zn(ClO4)2 in acetonitrile and the aqueous electrolyte was 25.7 mS cm−1 and 71.4 mS cm−1,

7

respectively, at room temperature. Therefore, the rate capability in the organic electrolyte was

8

significantly lower than that in the aqueous electrolyte due to the low ionic conductivity of

9

organic electrolytes.

10

A promising organic electrolyte comprising Zn(TFSI)2 in acetonitrile was also reported by

11

Vaughey et al.39, who demonstrated Zn insertion in a δ-MnO2/Zn cell using 0.5 mol L−1

12

Zn(TFSI)2 in acetonitrile. Although the cell presented a good coulombic efficiency of over 99%

13

during cycling, the discharge capacity increased from 95 mAh g−1 to 123 mAh g−1 over 20

14

cycles, and drastic capacity fading was observed from the 30th cycle. The authors claimed that

15

improved electrode wetting enhanced the capacity by increasing the electronic contact of the

16

cathode, and the capacity fading was attributed to decomposition of the organic electrolyte and

17

structural changes. It was also suggested that the passivation layer formed at the cathode−organic

18

electrolyte interface by decomposition of the organic electrolyte could contribute to the capacity

19

fading during cycling based on electrochemical impedance spectroscopy. When the δ-MnO2/Zn

20

cell using 0.5 mol L−1 Zn(TFSI)2 was subjected to a C-rate test, the capacity decreased

21

drastically from 100 mAh g−1 to 30 mAh g−1 with a change in the current from 12.3 mA g−1 to

22

308 mA g−1. This indicates that the organic electrolyte showed poor rate capability, where the


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1

charge transfer resistance was increased. Therefore, the zinc ion battery using 0.5 mol L−1

2

Zn(TFSI)2 was ineffective for overcoming the rate capability challenge.

3

A significant difference between the aqueous and organic electrolyte was clearly observed with

4

the V3O7·H2O electrode developed by Nazar et al.107 They investigated the effect of aqueous (1

5

mol L−1 ZnSO4 in D.I. water) and organic (0.25 mol L−1 Zn(CF3SO3)2 in acetonitrile) electrolytes

6

on the electrochemical performance of ZIBs. Although the two electrolytes presented similar

7

discharge/charge voltage profiles and inflection regions at the first cycle, the delivered capacity

8

was significantly different. The cell employing aqueous 1 mol L−1 ZnSO4 showed a very high

9

capacity of over 400 mAh g−1 at a current of 1C (375 mA g−1) in the voltage range of 0.4−1.1 V

10

vs. Zn2+/Zn and good cyclability with 80% reversible capacity retention at a current of 8C (3000

11

mA g−1) during 200 cycles. The cell employing the organic 0.25 mol L−1 Zn(CF3SO3)2 in

12

acetonitrile electrolyte exhibited an extremely poor capacity of 58 mAh g−1 at a current of 5 mA

13

g−1, and there was an increase in the specific capacity from 58 to 175 mAh g−1 over 50 cycles.

14

Interestingly, this phenomenon has been reported for organic electrolyte systems, related to the

15

decrease in charge transfer at the negative electrode. However, the study revealed that the

16

electrochemical behavior of the Zn2+ ions in the aqueous/organic electrolyte systems was overall

17

similar. Ionic conductivities of electrolytes are summarized in Table 3.

18

In summary, although organic electrolyte-based batteries exhibit excellent cycle capability

19

without significant proton participation and higher reversibility (over 99.9%) of the metal

20

deposition/dissolution process, their rate performance is poorer than that of batteries employing

21

mild aqueous electrolytes. It seems likely that interfacial charge transfer processes and the ionic

22

conductivity play a major role in determining the rate capability in these electrolyte systems.

23


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4. Anode material

2

4.1. Zn metal

3

Zinc is thought to be one of the best options as an anode material because it is a good reducing

4

agent with a sufficiently negative potential (−0.76 V vs. standard hydrogen electrode), has a high

5

theoretical capacity, and is stable against corrosion in aqueous solutions, relatively cheap, and

6

non-toxic. Zn anodes were used for the first-time in 1799 in the Voltaic pile in the first electric

7

battery that could continuously provide an electric current to a circuit.108 Later, in 1866,

8

Leclanche invented a battery comprising a zinc anode and MnO2 cathode, which has remained

9

the major primary battery to date, with an annual market value of 10 billion dollars. Since then,

10

several zinc primary and secondary systems have been proposed and developed, including

11

Zn/NH4Cl/MnO2, Zn/ZnCl2/MnO2, Zn/KOH/MnO2, Zn/NaOH/CuO, and Zn/ZnSO4/MnO2

12

systems. However, to date, the rechargeability of batteries based on zinc anodes remains a

13

significant challenge because of zinc corrosion and the formation of electrochemically inactive

14

products via reduction of the electrode in the alkaline electrolyte and the formation of metal

15

dendrites and protrusions due to uneven Zn electrostripping/electroplating in neutral

16

electrolytes.109,110 Because this review is mainly devoted to ZIBs employing a mild acidic

17

aqueous electrolyte, possible ways to increase the stability and reversibility of Zn metal anodes

18

under these conditions are discussed.

19

The documented knowledge and methods of solving the problems of dendrite formation by Zn

20

in mild acidic aqueous electrolytes are limited. The most effective methods to suppress dendrite

21

growth involve changing the architectures of the Zn-electrode111,112,116 and introducing additives

22

into the electrolyte113-115 or electrode.117-119

23

planar Zn because they possess a higher surface area that is more accessible to the electrolyte.

Porous Zn metal anodes have advantages over


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Recently, Wang et al. designed a novel Zn@graphite felt structure,116 which decreased the

2

voltage hysteresis and increased the cycling stability of a full cell with a PB cathode in a mild

3

acidic electrolyte. Kang et al.117 demonstrated that introducing activated carbon additives into the

4

Zn anode can significantly improve the reversibility and electrochemical reaction kinetics of the

5

Zn anode and suppress the formation of basic zinc sulfate, leading to enhancement of the cycle

6

performance of the Zn/ZnSO4/MnO2 battery. The pores of the activated carbon accommodated

7

the deposition of Zn dendrites and insoluble anodic products, leading to an increase in the

8

cycling stability.

9

Organic and inorganic additives have also been used to suppress dendrite formation and

10

corrosion of the Zn anodes.118,119 However, whereas direct introduction of the additives into the

11

electrolyte can constrain the formation of dendrites, it can, at the same time, influence the other

12

components of the battery and in general can decrease the lifespan of the battery. Recently, Chen

13

et al.118 proposed different organic additives such as cetyltrimethylammonium bromide (CTAB),

14

polyethylene-glycol (PEG), sodium dodecyl sulfate (SDS), and thiourea for modification of an

15

electroplated Zn anode via rendering the crystallographic surfaces to commercial Zn foil. Tafel

16

fitting of the linear polarization data revealed that Zn anodes employing the organic additives

17

exhibited 6−30 times lower corrosion currents. When Zn-SDS was used as the anode in a

18

rechargeable hybrid aqueous battery, the float current decreased by as much as 2.5-fold. Among

19

these electroplated anodes, Zn-SDS was the most suitable for aqueous batteries due to its low

20

corrosion rate, low dendrite formation, low float current, and high capacity retention after 1000

21

cycles. Zhi et al.119 reported a porous nano-CaCO3 coating as a buffer layer to obtain uniform

22

and position-specific plating of Zn metal. The proposed schematic illustrations of Zn-plating on

23

bare and nano-CaCO3-coated Zn foils are graphically depicted in Figure 12c. The strategy


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1

proposed by the authors effectively prevented the development of large protrusions/dendrites that

2

may easily pierce the separators and cause cell shorting. The applicability of the nano-CaCO3

3

layer was demonstrated in a Zn/ZnSO4 + MnSO4/CNT/MnO2 rechargeable aqueous battery,

4

which showed dramatically improved cycling stability of the device with a capacity of 177 mAh

5

g−1 at 1 A g−1 after 1000 cycles (Figures 12d and e).

6

Therefore, only fragmentary data on the influence of the different methods of mitigating the

7

formation of Zn dendrites in aqueous mild acidic electrolyte and improving the battery

8

performance in terms of cycle life, capacity, and CE are available. The strategies for improving

9

the performance of zinc electrodes should be implemented directly to achieve an exact battery

10

configuration given that many factors play crucial roles in affecting the battery performance.

11 12

5. Summary and perspective

13

ZIBs employing a mild acidic electrolyte are considered promising alternatives to conventional

14

LIBs due to their cost-effectiveness, safety, and high energy density. Notably, the low cost of Zn

15

and environmental safety of the aqueous electrolyte make these batteries promising candidates

16

for ESS. The gravimetric energy density of around 400 Wh kg−1 is also very promising (Figure

17

1d). To date, considerable progress on aqueous ZIBs has been achieved, but there are still

18

limitations that should be overcome.

19

Firstly, fundamental investigations on the reaction mechanism in ZIBs should be intensified.

20

As summarized, the mechanism of Zn2+ intercalation into the MnO2 structure is still under debate

21

by several groups. Three different reaction mechanisms have been proposed so far: capacity

22

derived from the reversible Zn2+ insertion/extraction; the reversible H+ insertion/extraction,

23

followed by precipitation/dissolution of zinc hydroxide sulfate on the electrode surface; the


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1

reversible co-insertion/co-extraction of Zn2+ and H+. The ambiguity arises from the absence of

2

rigorous protocols ensuring data reproducibility and standardization of the measurements.

3

Developing and applying different electrochemical methods, together with theoretical tools,

4

which can provide definite proof of Zn2+ intercalation from multiple in situ characterization

5

techniques, are a necessity for solving the above challenge.

6

Second, ZIBs still suffer from the paucity of cathode materials that can store Zn2+ due to the

7

relatively small ionic radius of divalent Zn2+ and consequently high charge density. The main

8

difficulty associated with the intercalation of Zn2+ into a solid framework is to redistribute the

9

multiple charges carried by each Zn2+ cation in the local structure of solid frameworks.123 For

10

instance, charge transfer should be accompanied by a drastic transition in the electronic

11

configurations or a sudden adjustment in the coordination environment or bond length, which

12

can be thermodynamically unfavorable in many cases. Therefore, hard redistribution of the ionic

13

charge is the main problem of Zn2+ mobility in inorganic host. The low mobility of the Zn2+

14

remains a significant obstacle in the search for cathodes; however, many chemistries remain

15

unexplored and need to be developed and tested.

16

Third, in consideration of safety, conductivity, and cost, aqueous mild ZnSO4 electrolytes

17

show the best prospective for ZIBs. However, batteries employing such electrolytes suffer from

18

capacity decay because of electrode dissolution and formation of dendrites on the surface of the

19

Zn anode. Recently, Chen demonstrated that the Zn(CF3SO3)2 aqueous electrolyte can effectively

20

suppress the formation of dendrites, leading to long life of ZIBs29. However, from the industrial

21

point of view, the price of Zn(CF3SO3)2 is currently prohibitive. Therefore, the search for better

22

electrolytes continues, and their discovery is essential to the eventual realization of the long-term

23

cycling stability of ZIB.


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1

Fourth, with the development of flexible electronic devices, the demand for safe, low cost, and

2

wearable batteries has intensified. Unfortunately, with current technology, flexible LIBs are

3

vastly limited in terms of safety and cost. Therefore, wearable ZIBs with quasi-solid-state

4

aqueous electrolytes49,100-102 hold great potential for practical wearable applications. Efforts

5

towards unearthing versatile solid-state electrolytes and improving the performance of ZIBs must

6

be emphasized.

7

In addition to the aforementioned challenges, self-discharge rate needs to be carefully

8

examined in order to make ZIB technology a practical reality in ESS applications. This problem

9

in fact is not unique only to ZIBs, but is common to other batteries, including LIBs, which have

10

the lowest self-discharge rate.

11

Overall, the promising combination of safety, environmental benefit, and high energy density

12

should allow ZIBs with mild aqueous electrolytes to become leading candidates for energy

13

storage solutions. The mature primary ZIB technology can provide the requisite solutions to the

14

energy demand provided that adequate zinc and MnO2 electrodes can be designed, and the

15

production and commercialization of rechargeable ZIBs with mild acidic aqueous electrolyte can

16

potentially be scaled up in the near future.

17 18

AUTHOR INFORMATION

19

Corresponding Author

20

*Tel.: 82 2 2220 0524, fax: 82 2 2282 7329, e-mail: [email protected] (Y. Sun)

21

**Tel.: 82 2 3408 3454, fax: 82 2 3408 4342, e-mail: [email protected] (S. Myung)

22

Notes


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ACS Energy Letters

1

The authors declare no competing financial interest.

2

BIOGRAPHIES

3

Aishuak Konarov received his Master of Applied Science degree from University of

4

Waterloo in 2014. He is currently a Ph.D. candidate in the Department of Nano Engineering at

5

Sejong University, South Korea, under the supervision of Professor Seung-Taek Myung. His

6

research focuses on synthesis and characterization of active materials for lithium-ion, sodium-ion

7

and zinc-ion batteries, as well as lithium-sulfur batteries.

8

Natalia Voronina received a Ph.D. degree from Moscow State University in 2009. After

9

having worked in Samsung SDI from 2011 to 2014 she decided to continue her research career

10

aiming at battery technology. She has currently been working for another Ph.D. with Professor

11

Seung-Taek Myung in the Department of Nano Engineering at Sejong University, South Korea.

12

Her current research interests have focused on the development of new materials for sodium-ion

13

and zinc-ion batteries.

14

Jae Hyeon Jo is presently a Ph.D. candidate in the Department of Nano Engineering at Sejong

15

University, South Korea, under the supervision of Professor Seung-Taek Myung. His research

16

focuses on materials development in the fields of energy conversion and storage, such as cathode

17

and anode materials for sodium-ion batteries and zinc-ion batteries.

18

Zhumabay Bakenov is a Professor of Chemical Engineering at Nazarbayev University,

19

Director of Center for Energy and Advanced Materials Science of National Laboratory Astana,

20

Kazakhstan. He received his Doctor of Engineering (Chemical Engineering) degree from Tokyo

21

Institute of Technology, Japan and PhD degree from the Kazakh National Academy of Science,

22

Kazakhstan. His research interests are advanced energy storage and conversion systems,

23

synthesis of functional nanomaterials, including biological application; environmental


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1

engineering – remediation of soil and water; modelling and analysis of electrochemical and

2

thermal processes in lithium-ion batteries. http://batterykazakhstan.com/index.php/en/zhumabay-

3

bakenov-en/

4

Yang-Kook Sun received his Ph.D. degree from the Seoul National University, Korea. He was

5

group leader at the Samsung Advanced Institute of Technology and contributed to

6

commercialization of the lithium polymer battery. He has worked at Hanyang University in

7

Korea as a professor since 2000. His research interests are the synthesis of new electrode

8

materials for lithium-ion batteries, Na-ion batteries, Li−S batteries, and Li−air batteries.

9

http://escml.hanyang.ac.kr/new/professor.html

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Seung-Taek Myung is a Professor of Nano Engineering at Sejong University, South Korea.

11

He received his Ph.D. degree in Chemical Engineering from Iwate University, Japan, in 2003.

12

His research interests embrace development of electroactive materials and corrosion of current

13

collectors of rechargeable lithium and sodium batteries.

14

http://dasan.sejong.ac.kr/~smyung/member.htm

15

QUOTES

16

1. Critical insight into the reaction mechanism in Zn/MnO2 battery is the key remaining

17 18 19 20 21 22 23

challenge to overcome. 2. Surface coating inhibits the dissolution of PB materials and improves structural stability. 3. Role of pillars in layered vanadium oxides is important to provide high mobility for Zn2+ and keep the structure stable. 4. Solid-state ZIB offers an extreme safety performance under various severe conditions, such as being cut, bent, hammered, punctured, or even put on fire.


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ACS Energy Letters

1 2

ACKNOWLEDGMENT

3

This research was supported by the Basic Science Research Program through the National

4

Research Foundation of Korea (NRF), funded by the Ministry of Education, Science, and

5

Technology of Korea (NRF 2015M3D1A1069713, NRF- 2017R1A2A2A05069634, NRF-

6

2017K1A3A1A30084795).

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Cathode for High-Capacity Aqueous Rechargeable Zinc Batteries. Chem. Commun. 2018, 54,

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4041-4044.

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Facilitates Long-Life Zinc Storage of Vanadium Dioxide. J. Mater. Chem. A. 2018, 6, 8006-

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8012.

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(84) Dai, X.; Wan, F.; Zhang, L.; Cao, H.; Niu, Z. Freestanding Graphene/VO2 Composite

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Films for Highly Stable Aqueous Zn-Ion Batteries with Superior Rate Performance. Energy

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Storage Mater. 2018, https://doi.org/10.1016/j.ensm.2018.07.022.

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(85) Park, J.S.; Jo, J.H.; Aniskevich, Y.M.; Bakavets, A.S.; Ragoisha, G. A.; Streltsov, E.A.;

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Kim, J.S.; Myung, S.T. Open-Structured Vanadium Dioxide as an Intercalation Host for Zn Ions:

6

Investigation by First-Principles Calculation and Experiments. Chem. Mat. 2018, DOI:

7

10.1021/acs.chemmater.8b02679.

8 9 10 11

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12

(88) Li, G.; Yang, Z.; Jiang, Y.; Jin, C.; Huang, W.; Ding, X.; Huang, Y. Towards Polyvalent

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Ion Batteries: A Zinc-Ion Battery Based NASICON Structured Na3V2(PO4)3. Nano Energy.

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2016, 25, 211-217.

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19

(91) Ling, C.; Suto, K. Thermodynamic Origin of Irreversible Magnesium Trapping in Chevrel

20

Phase Mo6S8: Importance of Magnesium and Vacancy Ordering. Chem. Mater. 2017, 29,

21

3731−3739


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(94) Gocke, E.; Schramm, W.; Dolscheid, P.; Schollhorn, R. Molybdenum Cluster

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Chalcogenides Mo6X8: Electrochemical Intercalation of Closed Shell Ions Zn2+, Cd2+, and Na+.

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J. Solid state Chem. 1987, 70, 71-81.

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(95) Cheng, Y.; Luo, L.; Zhong, L.; Chen, J.; Li, B.; Wang, W.; Mao, S.X.; Wang, C.;

10

Sprenkle, V.L.; Li, G. et al. Highly Reversible Zinc-Ion Intercalation into Chevrel Phase Mo6S8

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Nanocubes and Applications for Advanced Zinc-Ion Batteries. ACS Appl. Mater. Interfaces.

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2016, 8, 13673−13677.

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(96) Chae, M.S.; Heo, J.W.; Lim, S.C.; Hong, S.T. Electrochemical Zinc-Ion Intercalation

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Properties and Crystal Structures of ZnMo6S8 and Zn2Mo6S8 Chevrel Phases in Aqueous

15

Electrolytes. Inorg. Chem. 2016, 55, 3294−3301.

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(97) Liang, Y.; Jing, Y.; Gheytani, S.; Lee, K.-Y.; Liu, P.; Facchetti, A. Yao, Y. Universal

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Quinone Electrodes for Long Cycle Life Aqueous Rechargeable Batteries. Nat. Mater. 2017, 16,

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(98) Zhao, Q.; Huang, W.; Luo, Z.; Liu, L.; Lu, Y.; Li, Y.; Li, L.; Hu, J.; Ma, H.; Chen, J.

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High-Capacity Aqueous Zinc Batteries Using Sustainable Quinone Electrodes. Sci. Adv. 2018, 4,

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eaao1761.


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(99) Kundu, D.; Oberholzer, P.; Glaros, C.; Bouzid, A.; Tervoort, E.; Pasquarello, A.;

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Niederberger, M. Organic Cathode for Aqueous Zn-Ion Batteries: Taming a Unique Phase

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Evolution Toward Stable Electrochemical Cycling. Chem. Mater. 2018, 30, 3874−3881.

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(100) Li, H.; Liu, Z.; Liang, G.; Huang, Y.; Huang, Y.; Zhu, M.; Pei, Z.; Xue, Q.; Tang, Z.;

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Wang, Y. et al. Waterproof and Tailorable Elastic Rechargeable Yarn Zinc Ion Batteries by a

6

Cross-Linked Polyacrylamide Electrolyte. ACS Nano. 2018, 12, 3140−3148.

7

(101) Chao, D.; Zhu, C.; Song, M.; Liang, P.; Zhang, X.; Tiep, N.H.; Zhao, H.; Wang, J.;

8

Wang, R.; Zhang, H. et al. A High-Rate and Stable Quasi-Solid-State Zinc-Ion Battery with

9

Novel 2D Layered Zinc Orthovanadate Array. Adv. Mater. 2018, 30,1803181.

10

(102) Wang, Z.; Ruan, Z.; Liu, Z.; Wang, Y.; Tang, Z.; Li, H.; Zhu, M.; Hung, T.F.; Liu, J.;

11

Shi, Z. et al. A Flexible Rechargeable Zinc-Ion Wire-Shaped Battery with Shape Memory

12

Function. J. Mater. Chem. A. 2018, 6, 8549-8557.

13 14

(103) Zhang, S.; Yu, N.; Zeng, S.; Zhou, S.; Chen, M.; Di, J.; Li, Q. An Adaptive and Stable Bio-Electrolyte for Rechargeable Zn-Ion Batteries. J. Mater. Chem. A. 2018, 6, 12237-12243.

15

(104) Liu, Z.; Pulletikurthi, G.; Endres, F. A Prussian Blue/Zinc Secondary Battery with a Bio-

16

Ionic Liquid− Water Mixture as Electrolyte. ACS Appl. Mater. Interfaces. 2016, 8,

17

12158−12164.

18

(105) Liu, Z.; Bertram, P.; Endres, F. Bio-Degradable Zinc-Ion Battery Based on a Prussian

19

Blue Analogue Cathode and a Bio-Ionic Liquid-Based Electrolyte. J. Solid State Electrochem.

20

2017, 21, 2021-2027.


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(106) Chae, M.S.; Heo, J.W.; Kwak, H.H.; Lee, H,; Hong, S.T. Organic Electrolyte-Based

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Rechargeable Zinc-Ion Batteries Using Potassium Nickel Hexacyanoferrate as a Cathode

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Material. J. Power Sources. 2017, 337, 204-211.

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(107) Kundu, D.; Vajargah, S.H.; Wan, L.; Adams, B.; Prendergast D.; Nazar, L.F. Aqueous

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vs. Nonaqueous Zn-Ion Batteries: Consequences of the Desolvation Penalty at the Interface.

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Energy Environ. Sci. 2018, 11, 881-892.

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(108) Volta, A. On the Electricity Excited by the Mere Contact of Conducting Substances of Different Kinds. Philosophical Transactions of the Royal Society of London, 1800, 90, 403–431.

9

(109) Baugh, L.M. Corrosion and Polarization Characteristics of Zinc in Neutral—Acid Media

10

— II. Effect of NH+4 Ions and the Role of Battery Zinc Alloying Constituents. Electrochimica

11

Acta, 1979, 24(6), 669-677.

12

(110) Baugh, L.M. Corrosion and Polarization Characteristics of Zinc in Neutral—Acid Media

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— I. Pure Zinc in Solutions of Various Sodium Salts. Electrochimica Acta, 1979, 24(6), 657-

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15

(111) Chamoun, M.; Hertzberg, B.J.; Gupta, T.; Davies; D.; Bhadra, S.; Tassell, B.V.;

16

Erdonmez, C.; Steingart, D.A. Hyper-Dendritic Nanoporous Zinc Foam Anodes. NPG Asia

17

Mater. 2015, 7, e178-185.

18

(112) Parker, J. F.; Chervin, C.N.; Pala, I.R.; Machler, M.; Burz, M.F.; Long, J.W.; Rolison,

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D.R. Rechargeable Nickel–3D Zinc Batteries: An Energy-Dense, Safer Alternative to Lithium-

20

Ion. Science, 2017, 356, 415-418.


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ACS Energy Letters

1

(113) Gonzalez, M.A.; Trocoli, R.; Pavlovic, I.; Barriga, C.; Mantia, F.L. Layered Double

2

Hydroxides as a Suitable Substrate to Improve the Efficiency of Zn Anode in Neutral pH Zn-Ion

3

Batteries. Electrochim. Commun. 2016, 68, 1-4.

4

(114) Hashemi, A.B.; Kasiri, G.; Mantia, F.L. The Effect of Polyethyleneimine as an

5

Electrolyte Additive on Zinc Electrodeposition Mechanism in Aqueous Zinc-Ion Batteries.

6

Electrochim. Acta. 2017, 258, 703-708.

7

(115) Wang, F.; Borodin, O.; Gao, T.; Fan, X.; Sun, W.; Han, F.; Faraone, A.; Dura, J.A.; Xu,

8

K.; Wang, C. Highly Reversible Zinc Metal Anode for Aqueous Batteries. Nat. Energy. 2018,

9

17, 543-549.

10

(116) Wang, L.-P.; Li, N.-W.; Wang, T.-S.; Yin, Y.-X.; Guo, Y.-G.; Wang, C.-R. Conductive

11

Graphite Fiber as a Stable Host for Zinc Metal Anodes. Electrochimica Acta, 2017, 244, 172–

12

177

13

(117) Li, H.; Xu, C.; Han, C.; Chen, Y.; Wei, C.; Li, B.; Kang, F. Enhancement on Cycle

14

Performance of Zn Anodes by Activated Carbon Modification for Neutral Rechargeable Zinc Ion

15

Batteries, J. Electrochem. Soc., 2015, 162 (8), A1439-A1444

16

(118) Sun, K. E. K.; Hoang, T. K. A.; Doan, T. N. L.; Yu, Y.; Zhu, X.; Tian, Ye.; Chen P.

17

Suppression of Dendrite Formation and Corrosion on Zinc Anode of Secondary Aqueous

18

Batteries, ACS Appl. Mater. Interfaces, 2017, 9, 9681−9687

19

(119) Kang, L.; Cui, M.; Jiang, F.; Gao, Y.; Luo, H.; Liu, J.; Liang, W.; Zhi, C. Nanoporous

20

CaCO3 Coatings Enabled Uniform Zn Stripping/Plating for Long-Life Zinc Rechargeable

21

Aqueous Batteries, Adv. Energy Mater. 2018, 1801090.


63


1 2

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3

(121) Ma, L.; Chen, S.; Li, H.; Ruan, Z.; Tang, Z.; Liu, Z.; Wang, Z.; Huang, Y.; Pei, Z.;

4

Zapien, J.A. et al. Initiating a Mild Aqueous Electrolyte Co3O4/Zn Battery with 2.2 V-High

5

Voltage and 5000-Cycle Lifespan by a Co(III) Rich-Electrode. Energy Environ. Sci. 2018,

6

10.1039/C8EE01415A.

7

(122) Pan, C.; Zhang, R.; Nuzzo, R.G.; Gewirth, A.A. ZnNixMnxCo2–2xO4 Spinel as a High-

8

Voltage and High-Capacity Cathode Material for Nonaqueous Zn-Ion Batteries. Adv. Energy

9

Mater. 2018, 8, 1800589.

10

(123) Levi, E.; Levi, M.D.; Chasid, O.; Aurbach, D. A Review on the Problems of the Solid

11

State Ions Diffusion in Cathodes for Rechargeable Mg Batteries. J. Electroceram. 2009, 22, 13-

12

19.

13 14 15


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1

ACS Energy Letters

Table 1. Comparison of several charge carrier ions. Charge

Electrode potential

Shannon’s

Specific gravimetric

Specific volumetric

carrier

vs SHE [V]

ionic radii [Å]

capacity [mAh g-1]

capacity [mAh cm-3]

Li+

-3.04

0.76

3862

2066

Na+

-2.71

1.02

1166

1129

K+

-2.93

1.38

685

586

Mg2+

-2.37

0.72

2205

3832

Ca2+

-2.87

1.00

1337

2072

Zn2+

-0.76

0.74

820

5855

Al3+

-1.66

0.535

2980

8046

2


65

ACS Energy Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 66 of 74

Table 2. Summary of experimentally evaluated active materials for aqueous Zn-ion battery Cathode material Electrolyte Voltage Specific capacity [V]

α-MnO2

Retention, %

[mAh g-1] (1st)

/ cycle

Ref.

1 M ZnSO4 or 1 M Zn(NO3)2

1-1.9

210 (0.5C)

77/100 (6C)

12

1 M ZnSO4

1-1.8

266 (66 mA g-1)

49/100 (66 mA g-1)

16

α-MnO2 nanorod

1 M ZnSO4

1-1.8

233 (83 mA g-1)

65/50 (83 mA g-1)

17

α-MnO2 nanorod

1 M ZnSO4

1-1.8

323 (16 mA g-1)

44/75 (83 mA g-1)

18

Todorokite [3x3]

1 M ZnSO4

0.7-2

108 (0.5C)

95/50 (0.2C)

21

α-MnO2 nanorod

PVA/ZnCl2/MnSO4,

1-1.8

353 (0.5 A g-1)

93.6/1000 (0.5 A g-1)

22

PVA/ZnCl2/MnSO4,

1-1.8

366.6 (0.74 A g-1)

83.7/300 (1.11A g-1)

23

V-doped α-MnO2

α-MnO2@ PEDOT


66

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ACS Energy Letters

α-MnO2/C

1 M ZnSO4

1-1.8

272 (66 mA g-1)

70/50 (66 mA g-1)

24

α-MnO2/CNT

2 M ZnSO4 + 0.5 M MnSO4

1-1.9

190 (0.1 A g-1)

100/300 (1 A g-1)

25

α-MnO2 nanorod

1 M ZnSO4

0.7-2

195 (0.05C)

70/30 (0.2C)

26

α-MnO2 nanofibres


1-1.8

285

92/5000 (5C)

30

α-MnO2


1-1.9

362.2 (300 mA g-1)

94/3000(3A g-1)

33

α-MnO2/C


1-1.8

175 (50 mA g-1)

50.3/130 (2C)

46

Gelatin+Borax polymer aqueous

0.8-1.8

143.2 (1C)

94/500 (2C)

101

Polypyrrole coated αMnO2 97/1000 (2772 mA gα-MnO2

Gelatin+PAM

0.8-2.0

-1

306.0 (2772 mA g )

49 1

)

γ-MnO2

1 M ZnSO4

1-1.8

285 (0.5mA/cm2)

64/45 (0.5 mA cm-2)

28

β-MnO2 nanorod

3 M Zn(CF3SO3)2+0.1 M Mn(CF3SO3)2

0.8-1.9

225 (0.65 C)

94/2000 (0.65 C)

29


67


Page 68 of 74

β-MnO2 nanorod

1 M ZnSO4

1-1.8

270 (100 mA g-1)

75 / 200 (200 mA g-1)

32

R- MnO2


1-1.8

220 (60 mA g-1)

95/100 (60 mA g-1)

48

δ-MnO2 nanoflakes

1 M ZnSO4

1-1.8

252 (83 mA g-1)

44/100 (83 mA g-1)

37

δ-MnO2


1-1.85

266 (0.33C)

100/400 (500 mA g-1)

38

δ-MnO2

0.5M AN-Zn(TFSI)2

0.05-1.9

123 (12.3 mA g-1)

60/100 (0.04C)

39

40/5000 (2000 mA gPolyaniline δ-MnO2


1-1.8

280 (200 mA g-1)

53 1

)

λ-MnO2/C

3 M Zn(CF3SO3)2

0.8-2

150 (500 mA g-1)

94/500 (500 mA g-1)

42

λ-MnO2


0.8-1.9

106.5 (100 mA g-1)

100/300 (100mA g-1)

44

ε-MnO2


1-1.8

290 (0.3C)

66/50 (0.3C)

34

ε-MnO2

2 M ZnTFS+0.2 M MnCl2+PVA

1-1.8

188 (100 mA g-1)

85/100 (100 mA g-1)

51

Mn2O3

2 M ZnSO4

1-1.9

148 (100 mA g-1)

87/2000 (100 mA g-1)

45


68

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ACS Energy Letters

Mn3O4

2 M ZnSO4

0.8-1.9

239.2 (100 mA g-1)

70/300 (500mA g-1)

50

CuHCF

1 M ZnSO4

0.8-1.9

56 (20 mA g-1)

77/20 (20 mA g-1)

58

CuHCF

20 mM ZnSO4

1.4-2.1

55 (60 mA g-1)

96.3/100 (60 mA g-1)

59

CuHCF

1 M ZnSO4

1.2-2.1

55 (1C)

87/50 (1C)

61

ZnHCF

1 M ZnSO4

0.8-2.0

65 (60 mA g-1)

76/100 (60 mA g-1)

62

ZnHCF@MnO2

0.5 M ZnSO4

1.4-1.9

118 (100 mA g-1)

77/1000 (500 mA g-1)

63

BL-V2O5

0.5M AN-Zn(TFSI)2

0.3-1.5

196 (0.1C)

87/120 (0.1C)

64

Zn0.25V2O5 nH2O

1M ZnSO4

0.5-1.4

282 (1C)

80/1000 (8C)

65

Ca0.25V2O5·nH2O

1 M ZnSO4

0.6-1.6

340 (0.2C)

78/5000 (80C)

66

Na0.33V2O5

3 M Zn(CH3F3SO3)2

0.2-1.6

367.1 (0.1 A g-1)

93/1000 (1 A g-1)

67

V2O5·nH2O

3 M Zn(CF3SO3)2

0.2-1.6

381 (60 mA g-1)

71/900 (6 A g-1)

68


69


Page 70 of 74

V2O5

3 M Zn(CF3SO3)2

0.2-1.6

450 (10th) (0.2 A g-1)

91.1/4000 (5 A g-1)

69

V2O5

3M ZnSO4

0.4-1.4

224 (100 mA g-1)

75/400 (2 A g-1)

70

LiV3O8

1 M ZnSO4

0.6-1.2

267 (15 mA g-1)

100/65 (133 mA g-1)

71

H2V3O8

3 M Zn(CF3SO3)2

0.2-1.6

423.8 (0.1 A g-1)

94.3/1000 (5.0 A g-1)

72

H2V3O8 /Graphene

3M Zn(CF3SO3)2

0.2-1.6

400 (1/3C)

87/2000 (20C)

73

Na1.1V3O7.9@rGO

1M Zn(CF3SO3)2

0.4-1.4

191 (50 mA g-1)

85/500 (1 A g-1)

74

α-Zn2V2O7

1 M ZnSO4

0.4-1.4

190.5 (300 mA g-1)

85/1000 (4 A g-1)

75

Zn3V2O7(OH)2·2H2O

1M ZnSO4

0.2-1.8

213 (50 mA g--1)

68/300 (200 mA g-1)

76

Na2V6O16·3H2O

1M ZnSO4

0.4-1.4

267 (1.8 A g-1)

80/1000 (14.44 A g-1)

77

Na2V6O16·1.63H2O

3M Zn(CF3SO3)2

0.2-1.6

352 (50 mA g-1)

90/6000 (5 A g-1)

78

K2V6O16·2.7H2O

1M ZnSO4

0.4-1.4

217 (200 mA g-1)

82/500 (6 A g-1)

79


70

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ACS Energy Letters

K2V8O21

2 M ZnSO4

0.4-1.4

247 (0.3 A g-1)

83/300 (6 A g-1)

80

0.3 M Zn(TFSI)2

0.4-1.6

385 (0.1 A g-1)

80/300 (5 A g-1)

81

79/10000 (10 A g-1)

82

Fe5V15O39(OH)9 9H2O

265 (2nd) (100 mA gVO2

3M Zn(CF3SO3)2

0.7-1.7 1

)

VO2 (B)

3 M Zn(CH3F3SO3)2

0.3-1.5

357 (0.25C)

91.2/300 (5C)

83

rGO/VO2

3M Zn(CF3SO3)2

0.3-1.3

276 (0.1 A g-1)

99/1000 (4 A g-1)

84

1M ZnSO4

0.2-1.13

156 (15 mA g-1)

67/50 (15 mA g-1)

86

VS2

1 M ZnSO4

0.4-1.0

190.3 (0.05 A g-1)

98/200 (0.5 A g-1)

87

Na3V2(PO4)3

0.5M Zn(CH3COO)2

0.8-1.7

97 (0.5C)

74/100 (0.5C)

88

Na3V2(PO4)2F3

2M Zn(CF3SO3)2

0.8-1.9

65 (0.08 A g-1)

97/4000 (1 A g-1)

89

V0.91Al0.05O1.52(OH)0. 77


71


Page 72 of 74

Mo6S8

1 M ZnSO4

0.25-1.0

92 (0.1C)

95/150 (180 mA g-1)

95

Mo6S8

0.1 M ZnSO4

0.25-1.0

88 (0.05C)

-

96

3M Zn(CF3SO3)2

0.2-1.8

203 (0.1C)

86/50 (0.2C)

97

Quinone

3M Zn(CF3SO3)2

0.2-1.8

335 (20 mA g-1)

87/1000 (0.5 A g-1)

98

tetrachloro-1,4-

1 M zinc trifluoromethanesulfonate 0.8-1.4

200 (0.2C)

70/200 (1C)

99

benzoquinone

(Zn(OTf)2)

FeFe(CN)6

1 M (Zn(OAc)2, ([Ch]OAc) (70%)

0.5-1.8

50 (0.1 mA cm-2)

98/50 (mA cm-2)

103

0.5 M Zn(ClO4)2 in acetonitrile (AN)

0.7-1.8

55 (11.2 mA g-1)

90/35 (11.2 mA g-1)

106

V3O7·H2O/rGO

1M ZnSO4

0.3-1.5

245 (1.5 A g-1)

79/1000 (5C)

120

Co3O4

2 M ZnSO4 + 0.2 M CoSO4

0.8-2.2

158 (1 A g-1)

92/5000 (4 A g-1)

121

poly(benzoquinonyl sulfide)

K0.86Ni[Fe(CN)6]0.954 (H2O)0.766


72

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ACS Energy Letters

Co3O4

Polyacrylamide hydrogel

0.8–2.3

150 (0.5 A g-1)

94.6/2000 (2 A g-1)

121

ZnNi1/2Mn1/2CoO4

0.3 M Zn(OTf)2 in MeCN

0.9-2.15

180 (0.2C)

83/200 (200)

122


73


Page 74 of 74

Table 3. Ionic conductivities of electrolytes. Type

Electrolyte

Ionic conductivities

Ref

3M ZnSO4

~ 1.5 S cm-1

29

3M Zn(CF3SO3)2

~ 4.1 S cm-1

29

3M Zn(CF3SO3)2 + 0.1M Mn(CF3SO3)2

~ 6 S cm-1

29

0.5 mol L−1 Zn(ClO4)2

0.714 x 10-2 S cm−1

105

Gelatin+PAM using an aqueous solution of ZnSO4 and MnSO4

1.76 × 10−2 S cm−1

49

A cross-linked PAM using an aqueous solution of ZnSO4 and MnSO4

16.5 × 10−3 S cm−1

99

Aqueous ZnSO4-based gel

8.1 × 10−2 S cm–1

100

Gelatin+Borax polymer using an aqueous solution of ZnSO4 and MnSO4

2.0 × 10−2 S cm−1

101

xanthan bio-polymer using an aqueous solution of ZnSO4 and MnSO4

1.46 × 10−2 S cm−1

102

1.0 mol L−1 Zn(OAc)2/([Ch]OAc + 30 wt% water)

Unknown

103, 104

0.5 mol L−1 Zn(ClO4)2 in acetonitrile

0.257 x 10-2 S cm−1

105

0.5 mol L−1 Zn(TFSI)2 in acetonitrile

Unknown

39

0.25 mol L−1 Zn(CF3SO3)2 in acetonitrile

Unknown

106

Aqueous type

Aqueous-based Solid-state/geltype

Aqueous-based bio-type

Non-aqueous type


74

Present and Future Perspective on Electrode Materials for

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