Present and Future Perspective on Electrode Materials for

http://pubs.acs.org/journal/aelccp

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*,†

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†

Department of Nano-Technology and Advanced Materials Engineering & Sejong Battery Institute, Sejong University, Seoul 143-747, South Korea ‡ School of Engineering, National Laboratory Astana, Nazarbayev University, 53 Kabanbay Batyr Avenue, Astana 010000, Kazakhstan § Department of Energy Engineering, Hanyang University, Seoul 04763, South Korea ABSTRACT: The zinc-ion battery (ZIB) is a 2 century-old technology but has recently attracted renewed interest owing to the possibility of switching from primary to rechargeable ZIBs. Nowadays, ZIBs employing a mild aqueous electrolyte are considered one of the most promising candidates for emerging energy storage systems (ESS) and portable electronics applications due to their environmental friendliness, safety, low cost, and acceptable energy density. However, there are many drawbacks associated with these batteries that have not yet been resolved. In this Review, we present the challenges and recent developments related to rechargeable ZIB research. Recent research trends and directions on electrode materials that can store Zn2+ and electrolytes that can improve the battery performance are comprehensively discussed.

A

high ionic conductivity of the aqueous electrolyte provides fast ion diffusion. In addition, the battery assembly process does not necessarily require a controlled atmosphere such as a dry room and glovebox, which could reduce the overall battery price. In the RALB system, it was demonstrated that Li ions can be intercalated/deintercalated in aqueous media. Since then, many studies have focused on enhancing the electrochemical performance of RALBs.5 However, because of the recent increase in the price of lithium due to the limited lithium resources, large-scale applications have not yet been realized.6 As an alternative to LIBs, mono- and multivalent metal ions are extensively under study; their characteristics are summarized in Table 1. For largescale applications, lead-acid batteries are still dominant because of their low cost and durability, although the lead compounds affect environmental issues and are harmful to human health even in low concentrations.7 From the above-mentioned points of view, rechargeable zincion batteries (ZIBs) are considered as the most promising candidates for grid energy storage and may provide an alternative to the currently used lead-acid batteries employing toxic lead compounds. The low price, nontoxicity, and abundance of zinc make it very attractive. In addition, Zn metal has a high theoretical gravimetric and volumetric capacity of 820 mAh g−1 and 5855 mAh cm−3, respectively. In 1986, Yamamoto et al.8

s the consumption of fossil fuels continues to increase, there is an attendant increase in the concentration of toxic gases like NOx and SOx in the atmosphere, where such gases are associated with global warming. Renewable energy sources such as wind and solar power are the most promising candidates for reducing the use of fossil fuels and offer the advantages of being free and unlimited. Despite being unlimited, these resources are intermittent, and they are not always available. To take advantage of these free and unlimited energy sources, there is a need to store the energy when it is available, where battery systems play an important role.1 Since their introduction by Sony in 1991, Li-ion batteries (LIBs) have become the leading energy storage systems (ESS) on the market compared to other rechargeable batteries.2 LIBs power small electronics like laptops, digital cameras, and mobile phones. Many electric vehicles (EVs) are now powered by LIBs because of their light weight and high-capacity features, which lead to high energy density. However, when it comes to largescale applications such as in stationary grid energy storage or EVs, the high cost and safety concerns related to LIBs become very important issues.3 Recently, there have been several accidents involving the explosion and ignition of LIBs, as in the case of Tesla cars and Samsung smart phones, where these LIBs employ flammable organic electrolytes. Rechargeable aqueous Li-ion batteries (RALBs) were introduced by Dahn et al. in 1994, where flammable organic electrolytes were replaced with water-based electrolytes.4 The advantage of this system over nonaqueous systems is that the © 2018 American Chemical Society

Received: August 22, 2018 Accepted: September 25, 2018 Published: September 25, 2018 2620

DOI: 10.1021/acsenergylett.8b01552 ACS Energy Lett. 2018, 3, 2620−2640

Review

Cite This: ACS Energy Lett. 2018, 3, 2620−2640

ACS Energy Letters

Review

materials that enable reversible intercalation of Zn2+, including descriptions of manganese oxides, vanadium oxides, molybdenum sulfides, Prussian blue (PB) analogues, and others (Figure 1c). Related cathode materials, their potential ranges, and energy densities are summarized in Figure 1d and Table 2. Moreover, we discuss the latest development of electrolyte media, including aqueous, nonaqueous, and quasi-solid-state (QSS) electrolytes for rechargeable ZIBs. Electrode Materials. Manganese-Based Composites. Manganese oxide materials have been widely used in primary and secondary battery applications. The utilization of manganese oxides in ZIBs is very attractive owing to their low cost, environmental benignity, high operation voltage, and high theoretical capacity (∼308 mAh g−1 for 0.5 mol of Zn in MnO2).8−53 These oxides exhibit very rich chemistry and can be synthesized in different crystalline forms such as MnO2,8−40,46,48,49,51−53 Mn2O3,45 Mn3O4,47,50 and MnO. Notably, MnO2, with outstanding structural flexibility, exists in a variety of crystallographic polymorphs such as α-, β-, γ-, δ-, λ-, R-, and ε-type (Figure 2).9 The fundamental MnO6 octahedral units, in which the Mn4+ is surrounded by six oxygen neighbors, linked via the edges and/or corners, form crystallographic structures that correspond to different polymorphs. Some polymorphs possess tunnel-like structures, such as α-, β-, γ-, and R-MnO2, with 2 × 2 (4.6 × 4.6 Å2), 1 × 1 (2.3 × 2.3 Å2), and randomly arranged 1 × 2 and 1 × 1 and 1 × 2 (2.3 × 4.6 Å2) tunnels, respectively. Moreover, a todorokitetype MnO2 has the largest 3 × 3 tunnels (7 × 7 Å2). A δ-type MnO2 possesses a layered-like structure with a large interlayer distance (∼7 Å). The unique δ-type two-dimensional structure

Table 1. Comparison of Several Charge Carrier Ions charge carrier

electrode potential vs SHE [V]

Shannon’s ionic radii [Å]

specific gravimetric capacity [mAh g−1]

specific volumetric capacity [mAh cm−3]

Li+ Na+ K+ Mg2+ Ca2+ Zn2+ Al3+

−3.04 −2.71 −2.93 −2.37 −2.87 −0.76 −1.66

0.76 1.02 1.38 0.72 1.00 0.74 0.535

3862 1166 685 2205 1337 820 2980

2066 1129 586 3832 2072 5855 8046

introduced the zinc aqueous battery system, replacing the alkaline electrolyte with zinc sulfate electrolyte. The advantage of this system is that there is no formation of byproducts (such as ZnO and Zn(OH)2) (Figure 1a), unlike the case in rechargeable Zn−air or Zn alkaline batteries, which means that Zn metal can be used as an anode. This is one of the advantages of Zn metal compared to other metals (Table 1); specifically, it can be dissolved and deposited more easily than Mg metal. Difficulties in settling cost and safety issues of LIBs have led us to revisit ZIBs with mild aqueous electrolytes. As seen from Figure 1a, the number of annual publications on ZIBs continues to increase rapidly (Figure 1b). Therefore, in this Review, we summarize the latest developments in ZIBs with respect to the battery components, such as the electrode materials and electrolyte media. We exclude the separator because, at present, absorbent glass mat (AGM) is used in almost all ZIBs, which is commercially available and well-known in lead-acid batteries. Particular attention is dedicated to the latest development of electrode

Figure 1. (a) Pourbaix diagram of Zn in aqueous solution. (b) Number of publications. (c) Schematic illustration of a ZIB, and (d) Ragone plot of active materials for Zn2+ storage. 2621

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α-MnO2 V-doped α-MnO2 α-MnO2 nanorod α-MnO2 nanorod todorokite [3 × 3] α-MnO2 nanorod α-MnO2@ PEDOT α-MnO2/C α-MnO2/CNT α-MnO2 nanorod α-MnO2 nanofibres α-MnO2 α-MnO2/C polypyrrole coated α-MnO2 α-MnO2 γ-MnO2 β-MnO2 nanorod β-MnO2 nanorod R-MnO2 δ-MnO2 nanoflakes δ-MnO2 δ-MnO2 polyaniline δ-MnO2 λ-MnO2/C λ-MnO2 ε-MnO2 ε-MnO2 Mn2O3 Mn3O4 CuHCF CuHCF CuHCF ZnHCF ZnHCF@MnO2 BL-V2O5 Zn0.25V2O5·nH2O Ca0.25V2O5·nH2O Na0.33V2O5 V2O5·nH2O V2O5 V2O5

cathode material 1−1.9 1−1.8 1−1.8 1−1.8 0.7−2 1−1.8 1−1.8 1−1.8 1−1.9 0.7−2 1−1.8 1−1.9 1−1.8 0.8−1.8 0.8−2.0 1−1.8 0.8−1.9 1−1.8 1−1.8 1−1.8 1−1.85 0.05−1.9 1−1.8 0.8−2 0.8−1.9 1−1.8 1−1.8 1−1.9 0.8−1.9 0.8−1.9 1.4−2.1 1.2−2.1 0.8−2.0 1.4−1.9 0.3−1.5 0.5−1.4 0.6−1.6 0.2−1.6 0.2−1.6 0.2−1.6 0.4−1.4

1 M ZnSO4 1 M ZnSO4 1 M ZnSO4 1 M ZnSO4 PVA/ZnCl2/MnSO4 PVA/ZnCl2/MnSO4 1 M ZnSO4 2 M ZnSO4 + 0.5 M MnSO4 1 M ZnSO4 2 M ZnSO4 + 0.1 M MnSO4 2 M ZnSO4 + 0.2 M MnSO4 2 M ZnSO4 + 0.1 M MnSO4 gelatin + borax polymer aqueous gelatin + PAM 1 M ZnSO4 3 M Zn(CF3SO3)2 + 0.1 M Mn(CF3SO3)2 1 M ZnSO4 1 M ZnSO4 + 0.05 M MnSO4 1 M ZnSO4 1 M ZnSO4 + 0.1 M MnSO4 0.5 M AN-Zn(TFSI)2 2 M ZnSO4 + 0.1 M MnSO4 3 M Zn(CF3SO3)2 1 M ZnSO4 + 0.05 M MnSO4 2 M ZnSO4 + 0.2 M MnSO4 2 M ZnTFS + 0.2 M MnCl2 + PVA 2 M ZnSO4 2 M ZnSO4 1 M ZnSO4 20 mM ZnSO4 1 M ZnSO4 1 M ZnSO4 0.5 M ZnSO4 0.5 M AN-Zn(TFSI)2 1 M ZnSO4 1 M ZnSO4 3 M Zn(CH3F3SO3)2 3 M Zn(CF3SO3)2 3 M Zn(CF3SO3)2 3 M ZnSO4

voltage [V]

1 M ZnSO4 or 1 M Zn(NO3)2

electrolyte

Table 2. Summary of Experimentally Evaluated Active Materials for an Aqueous ZIB

70/50 (66 mA g−1) 100/300 (1 A g−1) 70/30 (0.2C) 92/5000 (5C) 94/3000(3 A g−1) 50.3/130 (2C) 94/500 (2C) 97/1000 (2772 mA g−1) 64/45 (0.5 mA cm−2) 94/2000 (0.65 C) 75/200 (200 mA g−1) 95/100 (60 mA g−1) 44/100 (83 mA g−1) 100/400 (500 mA g−1) 60/100 (0.04C) 40/5000 (2000 mA g−1) 94/500 (500 mA g−1) 100/300 (100 mA g−1) 66/50 (0.3C) 85/100 (100 mA g−1) 87/2000 (100 mA g−1) 70/300 (500 mA g−1) 77/20 (20 mA g−1) 96.3/100 (60 mA g−1) 87/50 (1C) 76/100 (60 mA g−1) 77/1000 (500 mA g−1) 87/120 (0.1C) 80/1000 (8C) 78/5000 (80C) 93/1000 (1 A g−1) 71/900 (6 A g−1) 91.1/4000 (5 A g−1) 75/400 (2 A g−1)

272 (66 mA g−1) 190 (0.1 A g−1) 195 (0.05C) 285 362.2 (300 mA g−1) 175 (50 mA g−1) 143.2 (1C) 306.0 (2772 mA g−1) 285 (0.5 mA cm−2) 225 (0.65 C) 270 (100 mA g−1) 220 (60 mA g−1) 252 (83 mA g−1) 266 (0.33C) 123 (12.3 mA g−1) 280 (200 mA g−1) 150 (500 mA g−1) 106.5 (100 mA g−1) 290 (0.3C) 188 (100 mA g−1) 148 (100 mA g−1) 239.2 (100 mA g−1) 56 (20 mA g−1) 55 (60 mA g−1) 55 (1C) 65 (60 mA g−1) 118 (100 mA g−1) 196 (0.1C) 282 (1C) 340 (0.2C) 367.1 (0.1 A g−1) 381 (60 mA g−1) 450 (10th) (0.2 A g−1) 224 (100 mA g−1)

65/50 (83 mA g ) 44/75 (83 mA g−1) 95/50 (0.2C) 93.6/1000 (0.5 A g−1) 83.7/300 (1.11 A g−1)

(83 mA g ) (16 mA g−1) (0.5C) (0.5 A g−1)

−1

366.6 (0.74 A g−1)

233 323 108 353

−1

49/100 (66 mA g−1)

266 (66 mA g−1)

retention, %/cycle 77/100 (6C)

210 (0.5C)

specific capacity [mAh g−1] (first) ref

24 25 26 30 33 46 101 49 28 29 32 48 37 38 39 53 42 44 34 51 45 50 58 59 61 62 63 64 65 66 67 68 69 70

23

17 18 21 22

16

12

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electrolyte 1 M ZnSO4 3 M Zn(CF3SO3)2 3 M Zn(CF3SO3)2 1 M Zn(CF3SO3)2 1 M ZnSO4 1 M ZnSO4 1 M ZnSO4 3 M Zn(CF3SO3)2 1 M ZnSO4 2 M ZnSO4 0.3 M Zn(TFSI)2 3 M Zn(CF3SO3)2 3 M Zn(CH3F3SO3)2 3 M Zn(CF3SO3)2 1 M ZnSO4 1 M ZnSO4 0.5 M Zn(CH3COO)2 2 M Zn(CF3SO3)2 1 M ZnSO4 0.1 M ZnSO4 3 M Zn(CF3SO3)2 3 M Zn(CF3SO3)2 1 M zinc trifluoromethanesulfonate (Zn(OTf)2) 1 M (Zn(OAc)2, ([Ch]OAc) (70%) 0.5 M Zn(ClO4)2 in acetonitrile (AN) 1 M ZnSO4 2 M ZnSO4 + 0.2 M CoSO4 polyacrylamide hydrogel 0.3 M Zn(OTf)2 in MeCN

cathode material

LiV3O8 H2V3O8 H2V3O8 /graphene Na1.1V3O7.9@rGO α-Zn2V2O7 Zn3V2O7(OH)2·2H2O Na2V6O16·3H2O Na2V6O16·1.63H2O K2V6O16·2.7H2O K2V8O21 Fe5V15O39(OH)9 9H2O VO2 VO2(B) rGO/VO2 V0.91Al0.05O1.52(OH)0.77 VS2 Na3V2(PO4)3 Na3V2(PO4)2F3 Mo6S8 Mo6S8 poly(benzoquinonyl sulfide) Quinone tetrachloro-1,4-benzoquinone FeFe(CN)6 K0.86Ni[Fe(CN)6]0.954(H2O)0.766 V3O7·H2O/rGO Co3O4 Co3O4 ZnNi1/2Mn1/2CoO4

Table 2. continued 0.6−1.2 0.2−1.6 0.2−1.6 0.4−1.4 0.4−1.4 0.2−1.8 0.4−1.4 0.2−1.6 0.4−1.4 0.4−1.4 0.4−1.6 0.7−1.7 0.3−1.5 0.3−1.3 0.2−1.13 0.4−1.0 0.8−1.7 0.8−1.9 0.25−1.0 0.25−1.0 0.2−1.8 0.2−1.8 0.8−1.4 0.5−1.8 0.7−1.8 0.3−1.5 0.8−2.2 0.8−2.3 0.9−2.15

voltage [V]

retention, %/cycle 100/65 (133 mA g−1) 94.3/1000 (5.0 A g−1) 87/2000 (20C) 85/500 (1 A g−1) 85/1000 (4 A g−1) 68/300 (200 mA g−1) 80/1000 (14.44 A g−1) 90/6000 (5 A g−1) 82/500 (6 A g−1) 83/300 (6 A g−1) 80/300 (5 A g−1) 79/10000 (10 A g−1) 91.2/300 (5C) 99/1000 (4 A g−1) 67/50 (15 mA g−1) 98/200 (0.5 A g−1) 74/100 (0.5C) 97/4000 (1 A g−1) 95/150 (180 mA g−1) − 86/50 (0.2C) 87/1000 (0.5 A g−1) 70/200 (1C) 98/50 (mA cm−2) 90/35 (11.2 mA g−1) 79/1000 (5C) 92/5000 (4 A g−1) 94.6/2000 (2 A g−1) 83/200 (200)

specific capacity [mAh g−1] (first) 267 (15 mA g−1) 423.8 (0.1 A g−1) 400 (1/3C) 191 (50 mA g−1) 190.5 (300 mA g−1) 213 (50 mA g−1) 267 (1.8 A g−1) 352 (50 mA g−1) 217 (200 mA g−1) 247 (0.3 A g−1) 385 (0.1 A g−1) 265 (2nd) (100 mA g−1) 357 (0.25C) 276 (0.1 A g−1) 156 (15 mA g−1) 190.3 (0.05 A g−1) 97 (0.5C) 65 (0.08 A g−1) 92 (0.1C) 88 (0.05C) 203 (0.1C) 335 (20 mA g−1) 200 (0.2C) 50 (0.1 mA cm−2) 55 (11.2 mA g−1) 245 (1.5 A g−1) 158 (1 A g−1) 150 (0.5 A g−1) 180 (0.2C)

ref 71 72 73 74 75 76 77 78 79 80 81 82 83 84 86 87 88 89 95 96 97 98 99 103 106 120 121 121 122

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during cycling because of complex problems such as the low conductivity of the material, low structural stability, structural transformations, and manganese dissolution caused by the disproportionation reaction upon cycling. However, in recent years, some effective methods of mitigating these problems have been suggested. In the following section, we discuss the present literature on reversible Zn2+ intercalation into different manganese polymorphs and consider the reaction mechanism, phase stability upon Zn2+ insertion, and methods of improving the cycling performance. Tunnel-Type MnO2 Polymorphs (α-, β-, γ-, R-): The concept of a rechargeable ZIB employing a mild acidic electrolyte and MnO2 as a cathode was discussed in the late 1980s by Yamamoto et al.,8,10,11 where aqueous mild ZnSO4 solutions were exploited as a replacement for conventional alkaline electrolytes with promising results. The authors achieved good kinetics and reversibility (30 cycles), and the battery was operated at approximately 1.4 V vs Zn metal.8 After a 20 years hiatus from such research, in 2012, Kang and co-workers12−15 revived the topic of rechargeable aqueous ZIBs, demonstrating the possibility of using a mild aqueous electrolyte and α-MnO2 to reversibly host Zn2+. The intercalation of Zn2+ into the tunnels of α-MnO2 was characterized by electrochemical evaluation, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) measurements.12 From their reported XPS data (in the insertion (discharge) state, the ratio of Zn/Mn is 0.52, and in the extraction (charge) state, Zn/Mn equals 0.05), the authors claimed reversible insertion/extraction of Zn2+

Figure 2. Crystal structures of MnO2 polymorphs. The orange and blue octahedra surround spin-up and -down Mn atoms, respectively, while the red atoms represent O. Reprinted with permission from ref 9. Copyright 2018, American Chemical Society.

is characterized by the layer interspacing distance, which depends on the inserted cations and H2O. Among the various forms, λ-type MnO2 has a spinel structure. The properties of MnO2 depend strongly on the crystalline structure, morphology, and particle size. The various MnO2 polymorphs have been reported as host materials for Zn2+ insertion in mildly acidic or neutral aqueous electrolyte.8−40,46,48,49,51−53 Irrespective of the original crystal structure, manganese oxide electrodes suffer from capacity decay

Figure 3. (a) Schematic illustration of the reaction pathway of Zn insertion in the prepared γ-MnO2 cathode. (b) Three initial voltage profiles of the fabricated Zn/MnO2 cell cycled within the potential window of 0.8−1.8 V. (c) Cycling performance of the test cell (Reprinted with permission from ref 28. Copyright 2015, American Chemical Society). 2624

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into/from α-MnO2. The cathodic and anodic processes were described by the following equations:

Cathode:

Cathode: 2MnO2 + Zn 2 + + 2e− ↔ ZnMn2O4

(1)

Anode: Zn ↔ Zn 2 + + 2e−

(2)

Because MnO2 is a semiconductor with poor electronic conductivity, the use of a conducting carbon coating and doping with metals are effective strategies for enhancing the conductivity. Several coating and doping methods have been used to improve the performance of tunnel-type MnO2 in Zn cells.16,20−25,46 Kang et al.25 attempted to increase the electrical conductivity of α-MnO2 by using carbon nanotubes (CNTs). The rod-like α-MnO 2 /acid-treated CNT nanocomposites displayed both excellent storage properties with Zn2+ (∼400 mAh g−1 at 1 A g−1) and reversibility at various current rates. Thereafter, Oh et al.26,27 investigated the mechanism of Zn2+ insertion into α-MnO2 nanorods using the state-of-the-art analytical techniques. They proposed a reversible transition from tunneled α-MnO2 to layered δ-MnO2 (Zn-birnessite) by electrochemical reactions. The transition was initiated by the dissolution of manganese from α-MnO2 during the discharge process, which is caused by the disproportionation of Mn3+ into Mn4+ and Mn2+ and dissolution in water. However, the manganese ions were reversibly inserted into the layers of Zn-birnessite during charging to form the original tunnel structure, where the authors attributed these observations to intercalation rather than a conversion reaction.

H 2O ↔ H+ + OH−

(3)

MnO2 + H+ + e− ↔ MnOOH

(4)

1 2+ 1 x Zn + OH− + ZnSO4 + H 2O 2 6 6 1 ↔ ZnSO4 [Zn(OH)2 ]3 ·x H 2O 6

(5)

Anode: Zn ↔ Zn 2 + + 2e−

(6)

The only difference between the primary and secondary Zn− MnO2 batteries lies in the reversibility of the Zn anode reactions in mild and alkaline electrolytes, respectively. The poor cycling performance of the Zn anode in alkaline electrolyte was ascribed to the formation of an insulating ZnO powder layer onto the Zn anode. In contrast, in a mild 2 M ZnSO4 with 0.1 M MnSO4 aqueous electrolyte, the Zn anode exhibited excellent kinetics and stability toward Zn stripping/plating without any dendrite formation or insulating layer growth of ZnO.30 More details and an in-depth study of the reaction mechanism were introduced by Oh et al.31 through in situ monitoring of the structural changes in α-MnO2 and water chemistry alterations during reaction in aqueous zinc sulfate electrolyte. Contrary to the belief that zinc ions intercalate into the tunnels of α-MnO2, Oh et al. proposed that zinc hydroxide sulfate Zn4(OH)6(SO4)· 5H2O (Figure 4a) precipitates on the surface of α-MnO2. This precipitation is caused by the disproportionation of unstable trivalent manganese and dissolution in the electrolyte, resulting in a gradual increase in the pH of the electrolyte during discharging (Figure 4b). During the charge process, the pH of the electrolyte decreases due to recombination of manganese on the cathode, leading to redissolution of zinc hydroxide sulfate into the electrolyte. In situ XRD analysis confirmed this process (Figure 4c), and Mn K-edge X-ray absorption spectra showed no shift of the peaks of α-MnO2 during discharge/charge, where the two Mn K-edge absorption spectra were almost identical at fresh and discharged states. A solid−solution reaction and subsequent conversion reaction into β-MnO2 with [1 × 1] open tunnels involving Zn2+ was proposed by Kim et al.32 On the basis of in situ synchrotron, XANES, and EXAFS analyses, the Zn2+ were intercalated into the β-MnO2 framework, followed by the formation of Zn-inserted phases and precipitation of zinc hydroxide sulfate on the electrode surface. More recently, Mai et al.33 reported a two-step intercalation mechanism of Zn2+, where Zn2+ is first inserted into the layers and then into the tunnels of graphene scroll-coated α-MnO2. Recently, Sun et al.34 studied ε-MnO2 (akhtenskite structure without large open voids) and claimed that H+ and Zn2+ insertion/extraction occurred at the ε-MnO2 cathode. Proton insertion rather that hydronium ion (H3O+) insertion in the initial discharge process was confirmed by TGA analysis of the MnOOH product. This was the first report proposing a H+ and Zn2+ insertion reaction mechanism for aqueous ZIBs. Literature analysis of manganese oxides (MnO2) with tunnel structures and their mechanism of reaction in ZIBs with a mild aqueous electrolyte showed contradictory results. Several experiments demonstrated that the capacity of MnO2 is derived

A similar study was reported by Kim et al.,28 where mesoporous γ-MnO2 (with randomly arranged 1 × 2 and 1 × 1 tunnels) was used. The tunnel-type orthorhombic parent γ-MnO2 underwent a structural transformation to the spineltype ZnMn2O4 phase and two new intermediary phases, namely, tunnel-type γ-ZnxMnO2 and layered-type λ-ZnyMnO2. All of the phases with multioxidation states coexisted after completion of electrochemical Zn insertion (Figure 3a). Upon Zn2+ deinsertion, the majority of these phases with multioxidation states underwent reversion to the parent γ-MnO2 phase. The γ-MnO2 cathode delivered an initial discharge capacity of 285 mAh g−1 at 0.05 mA cm−2 with a defined plateau around at 1.25 V vs Zn2+/Zn (Figure 3b). The corresponding discharge capacity after 40 cycles was 158 mAh g−1, and the capacity fading was attributed to manganese dissolution rather than to structural distortion of MnO2 (Figure 3c). Chen et al.29 investigated Zn intercalation in β-MnO2 (1 × 1 tunnels) in aqueous 3 M Zn(CF3SO3)2 + 0.1 M Mn(CF3SO3)2 electrolyte. Phase transition from the tunneled to layered structure of Zn-buserite was observed during the first discharge, accompanied by reversible Zn2+ (de)intercalation in the Zn-buserite framework. Moreover, they elucidated the influence of the size of the tunnels on the electrode reaction in α-MnO2 (2 × 2), β-MnO2 (1 × 1), and γ-MnO2 (1 × 2 + 1 × 1) polymorphs. A main fact is that the phase transforms from tunnel to layer structure during the Zn2+ insertion (discharge) in all polymorphs, but the phase change did not depend on the size of the MnO2 tunnels. After further investigation for tunnel-type MnO2 in a mild aqueous electrolyte, Liu et al.30 proposed a reaction mechanism involving conversion between α-MnO2 and H+, similar to that of the primary alkaline MnO2 battery, as follows: 2625

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Figure 4. (a) Crystal structure of Zn4(OH)6SO4·5H2O. (b) pH variations during the first discharge−charge process (the pH level was measured on a separator that was soaked with electrolyte). (c) In situ XRD patterns of a cathode in a Zn/α-MnO2 cell with 1.0 M ZnSO4 aqueous electrolyte during the first discharge−charge cycle and the corresponding discharge−charge curve (Reproduced with permission from ref 31. Copyright 2016, WILEY-VCH).

capacities as high as 123 mAh g−1 and an operating voltage of 1.25 V vs Zn2+/Zn. They proved the intercalation of Zn2+ without proton participation in the organic acetonitrile-Zn(TFSI)2 electrolyte. Furthermore, a small amount of ZnO was observed on the surface of MnO2 in both the charged and discharged samples after the second cycle, but the byproduct did not propagate during further cycling. This may be proof that the ZnO is derived from the side reaction with water released from the structure of δ-MnO2. The overall electrochemical reaction was suggested as follows:

from the reversible Zn2+ insertion/extraction, whereas other experiments supported the reversible H+ insertion/extraction process based on the conversion reaction mechanism, whereas others affirmed the co-insertion/co-extraction of H+ and Zn2+. Oh et al.31 investigated not only the changes in the crystal structure of tunnel-type MnO2 with Zn2+ insertion but also electrolyte chemistry during electrochemical reactions. The result indicated that manganese dioxide actually undergoes a series of conversiontype reactions based on the reversible precipitation/dissolution of zinc hydroxide sulfate on the electrode surface, which is triggered by pH changes in the electrolyte. These findings highlighted the critical role of the electrolyte pH over the intercalation of Zn2+ into MnO2. The discrepancies in works show the importance of understanding the reaction mechanism governing the reversibility in tunnel-type MnO2 structures for further improvement of electrode materials for reversible Zn2+ storage. Layered-Type MnO2 Polymorph: Attempts have been made to reversibly intercalate Zn2+ into layered birnessite in both aqueous and organic electrolytes.35−39 Oh et al.35,36 presented a pioneering report on electrochemical cycling of Zn2+ in layered δ-MnO2, achieving an excellent performance of 300 mAh g−1 with the help of MnSO4 in aqueous ZnSO4 electrolyte.35 The authors claimed that the addition of MnSO4 improved the reversibility of the cathodic reaction and suppressed the formation of basic zinc sulfate on the electrode surface. Zn2+ intercalation into δ-MnO2 was further investigated by different groups. Kim et al.37 developed a nanostructured δ-MnO2 cathode that exhibited a discharge capacity of 250 mAh g−1 with a Coulombic efficiency (CE) of nearly 100% after 100 cycles. The authors claimed the formation of spinel-type ZnMn2O4 and layered-type δ-ZnxMnO2 at the end of discharge as confirmed by ex situ XRD and ICP analyses. In the work of Wang et al.,38 birnessite δ-MnO2 underwent H+ and Zn2+ insertion/extraction during charge/discharge processes, with an energy density of 364 Wh kg−1 and a capacity retention of 100% over 400 cycles at 500 mA g−1. Vaughey et al.39 investigated Zn2+ intercalation into δ-MnO2 in an organic acetonitrile-Zn(TFSI)2 electrolyte. The nonaqueous cell employing the K0.11MnO2·0.7H2O electrode demonstrated good reversibility and stability for 125 cycles, with

x Zn + K 0.11MnO2 ·0.7H 2O ↔ ZnxK 0.11 − yMnO2 + y K+ + 0.7H 2O

(7)

Spinel-Type MnO2 Polymorph: Besides tunnel- and layeredmanganese oxides, the spinel-type oxide (λ-MnO2) has been proposed as a cathode for application in ZIBs.40−44 Successful utilization of LiMn2O4 as an electrode material for reversible intercalation of Li+ in aqueous media inspired the possibility of the spinel analogue for Zn2+ insertion. The first study on the chemical extraction of Zn from ZnMn2O4-based spinel indicated that the ideal spinel structure was unfit for Zn2+ insertion due to the high electrostatic repulsion among the Zn2+ cations within the lattice.41 Inspired by the knowledge that the introduction of cation vacancies in other spinel oxides opens up additional pathways for easier migration of divalent ions, cation-defective ZnMn2O4 spinel was proposed as a Zn insertion host material in the work of Chen and co-workers.42 The spinel/carbon hybrid material exhibited a reversible capacity of 150 mAh g−1 and a capacity retention of 94% over 500 cycles. The authors stated that the remarkable electrode performance resulted from facile charge transfer and Zn2+ insertion in the structurally robust spinel with abundant cation vacancies. The XRD results showed that the basic spinel lattice was maintained during charge and discharge, where the diffraction peaks gradually shifted to higher angle upon charge and returned to the original position upon discharge. The delivered capacity was attributed to Zn2+ insertion based on the absence of the vibrations of O−H bonds (from H+ insertion). Moreover, another proof of Zn2+ insertion was the fact that the electrode could deliver a reversible capacity of 2626

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∼90 mAh g−1 in a nonaqueous electrolyte comprising 0.1 M Zn(CF3SO3)2 in acetonitrile. Furthermore, 3 M Zn(CF3SO3)2based aqueous electrolyte enabled ∼100% Zn plating/stripping efficiency with long-term stability and suppressed Mn dissolution. More recently, Kim and co-workers43 also reported Mg2+ and Zn2+ co-intercalation into tetragonal MgMn2O4 spinel. The coin cell comprised of Zn metal, 1 mol L−1 (MgSO4 + ZnSO4) + 0.1 mol L−1 MnSO4 electrolyte, and a MgMn2O4 spinel electrode delivered an initial discharge capacity of 130 mAh g−1, which increased to 165 mAh g−1 within five cycles. The extraction/insertion of cations into the spinel was reversible, followed by the formation/dissolution of (MgMn)9Zn4(OH)22(SO4)2·8H2O salt.

relative to Zn2+ intercalation during the initial cycles, followed by Zn2+ insertion several cycles later. From the operando data set (Figure 5a), it can be seen that by the end of the first discharge plateau, the occupancy of Zn2+ at the 8c site decreased, while occupancy by Zn2+ at the 4a site increased. Thus, initially, the Zn2+ are intercalated into the tunnel site (first plateau) and subsequently occupy the Fe(CN)6 vacancy site (second plateau). Zhang and co-workers studied zinc hexacyanoferrates (Zn3[Fe(CN)6]2 (ZnHCFs), Figure 5b), demonstrating that the FeC6 octahedra are connected to the ZnN4 tetrahedra via CN ligands to form a porous 3D framework.62 The large open sites can accommodate alkaline metal cations A (A = Na+, K+, and Cs+) and water molecules. Electrochemical data showed reversible redox at around 0 V, which is associated with the dissolution/deposition of Zn metal. The other peaks at 1.6 and 1.9 V are attributed to Zn2+ insertion/extraction to/from the ZnHCF structure. The reactions that take place during cycling can be described as follows: Cathode:

Critical insight into the reaction mechanism in a Zn/MnO2 battery is the key remaining challenge to overcome. Zn2+ intercalation in manganese-based structures presents promising performance, displaying good reversibility and kinetics, and provides an energy density close to that of targets for ESS applications. The utilization of a manganese(II) salt (MnSO4 or Mn(CF3SO3)2) as an additive to the electrolyte prevents dissolution of Mn3+ and is preferred for obtaining long cycling stability. The reaction mechanism in these materials is not totally understood and needs to be further investigated both experimentally and theoretically. Prussian Blue and Its Analogues. PB and its analogues (PBAs) have a three-dimensional open framework with large interstitial sites, where these features are beneficial for fast charge and discharge.54,55 In addition to storing monovalent ions, PB and PBA can also store divalent (Mg2+, Ca2+, Sr2+, and Ba2+) ions.56,57 Wang et al.58 reported the use of copper hexacyanoferrate (CuHCF) nanocubes as a cathode for ZIBs. CuHCF acted as the PBA, where Cu was connected to N and Fe with C atoms, bridged by CN ligands. The cyclic voltammetry of CuHCF showed one cathodic and one anodic peak, associated with the insertion/extraction of Zn2+. Their XPS analysis suggested insertion of Zn2+ into the CuHCF structure via the reduction of Fe3+ to Fe2+. Further work on CuHCF was carried out by Trocoli and Mantia,59,60 where they improved the cyclability of the CuHCF electrode by employing a low concentration of Zn2+ in the electrolyte, which was beneficial for Zn metal dissolution and deposition during cycling. The related reactions can be described as below:

x Zn 2 + + 2x e− + Zn3[Fe(CN)6 ]2 ↔ Zn3 + x[Fe(CN)6 ]2 (10)

Anode: x Zn ↔ x Zn 2 + + 2x e−

Further electrochemical tests showed a capacity of approximately 65 mAh g−1 at 1C, with an average working potential of 1.7 V (Figure 5c), which is the highest operating voltage compared to that reported for other types of cathode materials. Zhang et al.63 improved the electrochemical performance of ZnHCF by wrapping it with manganese oxide (MnO2) nanosheets using an in situ coprecipitation method. The electrochemical performance of the composite was tested in a 0.5 M ZnSO4 aqueous solution. The combination of MnO2 nanosheets and ZnHCFs delivered a higher capacity than the bare electrodes due the synergistic effect from both compounds. The improved electrochemical performance of the MnO2wrapped ZnHFC electrode is contributed by the Fe3+/2+ redox of ZnHCFs and the outer MnO2 nanosheet layers that act as a buffer layer for facile ion diffusion into the ZnHCF structure (Figure 5d). In short summary, PB and PBAs displayed excellent cycling and rate performance. In addition, the operating voltage is one of the highest among other types of material to store Zn2+. However, its delivered capacity still needs to be further improved for making composite materials or making more vacancy sites in the crystal structure.

Cathode: x Zn 2 + + 2x e− + CuHCF ↔ ZnxCuHCF

(8)

Surface coating inhibits the dissolution of PB materials and improves structural stability.

Anode: x Zn ↔ x Zn 2 + + 2x e− 61

(0 ≤ x ≤ 0.5)

(11)

(9) 2+

Svensson et al. studied the insertion of Zn into the CuHCF structure in detail by using synchrotron X-ray diffraction. The voltage profile of CuHCF demonstrated that a capacity of 60 mAh g−1 was delivered at the single-step plateau, and after 50 cycles, the capacity retention was about 85%. However, with cycling, the operating voltage of the cell increased from 1.6 to 1.7 V, and two clear plateaus were observed. These changes can be explained by the dominance of the remaining K+

Vanadium-Based Composites. Johnson et al.64 developed a V2O5/Zn battery system with a nonaqueous (0.5 M Zn(TFSI)2) acetonitrile electrolyte. The cell delivered a discharge capacity of 196 mAh g−1 and charge capacity of 164 mAh g−1 in the first cycle. The delivered capacity indicates that approximately 0.59 mol of Zn2+ was intercalated into the V2O5 structure, activated by a V5+ /V4+ redox pair. A similar material 2627

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Figure 5. (a) Top-view elevation map (the intensity axis is perpendicular to the plane of the figure) displaying peak shifts of a CuHCF/Zn cell during the in operando XRD experiment, time evolution of the corresponding voltage profile (continuous line) of the first two cycles with the refined lattice parameter (circles) superimposed, and a plot of the time derivative of both the voltage (dE/dt) and the cell parameter (da/dt) and the occupancies of the two possible Zn2+ sites and their relation with the voltage profile during Zn2+ insertion followed by removal from the CuHCF framework (Reprinted from ref 61. Copyright 2017, with permission from Elsevier). (b) Crystal structure of rhombohedral ZnHCF. (c) Discharging curves of a Zn/ZnSO4/ZnHCF battery at a rate of 1C (60 mA g−1) (Reproduced with permission from ref 62. Copyright 2014, WILEY-VCH). (d) Graphical illustration of the ZnHCF@MnO2 electrode for Zn2+ storage (Reproduced with permission from ref 63. Copyright 2017, The Royal Society of Chemistry).

(Zn0.25V2O5·nH2O, ZVO) was studied in an aqueous (1 M ZnSO4) electrolyte.65 Zn2+ and water molecules in the structure acted as pillars to facilitate Zn2+ intercalation. The galvanostatic charge/discharge profiles of the ZVO electrode are shown in Figure 6a,b. The cell delivered a high capacity of 282 mAh g−1 at 1C, equivalent to the insertion of 1.1 mol of Zn2+. Moreover, the cell demonstrated excellent rate and cycling performance, even after 1000 cycles at 8C, with retention of more than 80% of its initial capacity. The excellent cyclability is attributed to water molecules that acted as pillars in the crystal structure. Recently, Alshareef et al.66 extended the work on ZVO by replacing the Zn2+ with Ca2+ (Ca0.25V2O5·nH2O, CVO) as a host for Zn2+. Because CVO has higher electrical conductivity than ZVO and due to the larger ionic radius of Ca2+ (1.00 Å) than Zn2+ (0.74 Å), the resulting interlayers between the V−O layers can be further expanded by the presence of large Ca2+ in the crystal structure. Ex situ XPS results proved that Ca ions do not move from the structure. As a result, CVO delivered a higher energy density than ZVO as Ca is much lighter than Zn. Furthermore, Mai et al.67 reported work on sodium-ion-stabilized vanadium oxide (Na0.33V2O5, NVO) nanowires, in which the Zn2+ were replaced with the larger Na+. The electrochemical performance of NVO was similar to that of the ZVO and CVO electrodes. Another interesting study was presented by Yang et al.,68 where they highlighted the important role of water in Zn2+ intercalation into the bilayer V2O5·nH2O structure. Even though there were no pillars of Zn2+, Ca2+, or Na+ between the layers of V−O sheets, water acted as a “lubricant” to facilitate movement of the Zn2+. When the water was removed from the V2O5 layer, the material

showed poor electrochemical performance, whereas watercontaining V2O5 showed outstanding and highly reversible Zn2+ (de)intercalation behavior. Lately, similar work was done by Cheng et al.;69 namely, H2O facilitated the electrostatic repulsion between the Zn2+ and host material. Also, Zhou and co-workers demonstrated that V2O5 can intercalate Zn2+ in highly concentrated ZnSO4 aqueous solution.70

The role of pillars in layered vanadium oxides is important to provide high mobility for Zn2+ and keep the structure stable. LiV3O8 shows interesting voltage profiles with plateaus that are observed during discharge, but only one during charging (Figure 6c).71 The cell delivered a capacity of over 250 mAh g−1 (16 mA g−1) with excellent rate performance. During the first plateau, the formation of ZnLiV3O8 was confirmed, and during the second plateau, addition of more Zn2+ led to formation of the ZnyLiV3O8 phase. However, during charging, there was direct transformation from ZnyLiV3O8 to LiV3O8. Another work by Mai et al.72 demonstrated that H2V3O8 (HVO) nanowires underwent Zn2+ (de)intercalation that occurred in two steps. The delivered capacity was approximately 400 mAh g−1 at 0.1 A g−1, which is the highest capacity reported in the literature for ZIBs. Furthermore, the system exhibited outstanding cycling performance at a high current of 5.0 A g−1 for 1000 cycles, with retention 2628

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Figure 6. (a) Galvanostatic discharge and charge profiles of the Zn0.25V2O5·nH2O freestanding cathode at a 1C rate. (b) Extended cycling performance at a 8C rate (2400 mA g−1) (Reprinted with permission from ref 65. Copyright 2016, Springer Nature). (c) Schematic of the Zn intercalation mechanism in the present LiV3O8 cathode (Reprinted with permission from ref 71. Copyright 2017, American Chemical Society).

performance. Park and co-workers85 predicted Zn2+ intercalation into the VO2(B) structure by first-principle calculations. The results showed that Zn2+ inserted/extracted by two phase reactions, as shown in Figure 7d−f. During the first step 0.125 mol and in the second step 0.375 mol of Zn2+ were inserted into VO2(B). In addition, the predicted operating voltage of 0.61 V was well matched with the experimental value, which is 0.7 V. Jo and co-workers86 introduced hollandite-type VO1.52(OH)0.77 with a [2 × 2] tunnel structure, where two distorted VO6 octahedra share an edge to form VO6 double chains (Figure 8a, top). The structural stability of VO1.52(OH)0.77 was improved by Al doping. The electrochemical tests (Figure 8a, bottom) showed that Zn2+ can be intercalated into the tunnels. Al-doped V1−xAlxO1.52(OH)1.77 (x = 0.05) demonstrated enhanced electrochemical performance, attributed to the high specific surface area by the introduction of Al into the structure. Layered vanadium sulfide (VS2) nanosheets were suggested as a Zn2+ intercalation electrode material by Mai et al.87 (Figure 8b, left). The intercalation of Zn2+ progressed via two steps (Figure 8b, right). According to their XRD and microscopic analyses, it was revealed that 0.09Zn2+ is first intercalated in the first plateau region, and the second plateau is associated with the insertion of another 0.14Zn2+ ions into the layered VS2 structure, forming Zn0.23VS2. The electrode displayed good cycling performance with a loss of only 2% of its initial capacity after 200 cycles. Huang et al.88 employed a Na superionic conductor (NASICON) structure comprising Na3V2(PO4)3 wrapped with graphene-like carbon. There are three sites where Na+ is located in the Na3V2(PO4)3 structure, two 18e sites and one 6b

of 94.3% of its initial capacity. This excellent electrochemical performance was achieved due to the weak hydrogen bonds between the layers of V−O, which were able to enhance the mobility of the Zn2+. Recently, Wang et al.73 reported improvement in the performance of H2V3O8 nanowires by wrapping with graphene sheets. Because of the high electrical conductivity of the graphene sheets, the electrode delivered enhanced electrochemical performance. Other varieties of families of vanadates showed that they can also intercalate Zn2+.74−81 VO2 is found to be a successful candidate for storage of Zn2+ in their structures.82 Yang et al.83 demonstrated VO2(B) nanofibers as a Zn2+ ion storage material. As shown in Figure 7a, VO2(B) has a unique tunnel structure with a tunnel size of 5.2 × 8.2 Å2 along the b- and c-axes, where the Zn2+ can intercalate. The cell delivered a high reversible capacity of 357 mAh g−1 at 0.25C, and the capacity remained at almost 100% after 50 cycles (Figure 7b). In addition, the VO2(B) nanofiber showed outstanding rate capability, a capacity of 171 mAh g−1 at 300C. Furthermore, XPS investigation confirmed that the VO2(B) is activated by the V4+/3+ redox pair. To increase the overall energy density of the VO2 electrode, Niu et al.84 engineered free-standing reduced graphene oxide (rGO)/VO2 composite films that were prepared by a combination of freeze-drying, high-temperature reduction, and mechanical compression. The free-standing rGO/VO2 composite film demonstrated better rate and cycling performance. More interestingly, because of their outstanding mechanical properties, the films were tested in a flexible ZIB system at different bending angles and showed stable electrochemical 2629

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Figure 7. (a) Schematic view of Zn2+ intercalation/deintercalation VO2(B) nanofibers projected along the direction of the b- and c-axes, respectively. (b) Cycling performance of VO2(B) nanofibers at 0.25C (inset: typical galvanostatic charge−discharge curve). (c) In situ XRD analysis of VO2(B) nanofibers. The corresponding high-resolution contour maps of (−601), (020), and (−404) peaks in the panel, respectively (Reproduced with permission from ref 83. Copyright 2018, WILEY-VCH). (d) Formation energy of ZnxVO2(B) (0 ≤ x ≤ 0.5), (e) calculated average redox potential and experimentally measured charge/discharge curve of ZnxVO2(B), and (f) expected mechanism of Zn intercalation into VO2(B) (Reprinted with permission from ref 85. Copyright 2018, American Chemical Society).

Figure 8. (a) (top) First discharge−charge curves of bare VO1.52(OH)0.77, V0.95Al0.05O1.52(OH)0.77, and V0.91Al0.09O1.52(OH)0.77; (bottom) Zn2+ intercalation/deintercalation into/out of the V1−xAlxO1.52(OH)0.77 (x = 0−0.09) tunnel model structure (Reproduced with permission from ref 86. Copyright 2017, The Royal Society of Chemistry). (b) (left) Schematic illustration of the operation mechanism of Zn/VS2 batteries. (right) Charge and discharge curves at a current from 0.05 to 2.0 A g−1 (Reproduced with permission from ref 87. Copyright 2017, WILEY-VCH).

first and second charge profiles, which is different from the work of Huang et al.88 The cell illustrated improved cycling performance, retaining 95% of its initial capacity after 4000 cycles at 1 A g−1 (Figure 9d). As a result, even though most of the vanadiumbased electrodes present low operation voltages, they deliver high capacity due to the various oxidation states of vanadium. Molybdenum-Based Composites. Chevrel phase Mo6S8 is an attractive electrode material for the storage of monovalent (Li+, Na+) ions as well as multivalent (Mg2+, Zn2+, Al3+) ions (Figure 10a) due to its unique open-crystal structure and rigid framework.90−92 The Chevrel phase Mo6S8 structure is built from six molybdenum atoms that form an octahedral Mo6 cluster and eight sulfur atoms that are face-capped on top of the eight trigonal faces of the octahedron. Six of the eight sulfur atoms are also bonded axially to a molybdenum atom of a neighboring cluster, resulting in the 3D network crystal structure (Figure 10b). Pioneering work on Zn2+ intercalation into the Mo6S8 structure was done by Schollhorn et al.,93,94 where they proved that Zn2+

site. It is well-known from the sodium-ion battery system that the deintercalation of Na+ from the 18e site is easier than that from the 6b site in the NASICON-type structure. As illustrated in the scheme in Figure 9a, during the initial charge, two Na+ ions can be extracted from the structure, and during discharge, Zn2+ can be intercalated into the NASICON structure. The resulting voltage profile of Na3V2(PO4)3 is shown in Figure 9b. Note that the first charge profile is due to the removal of two Na+ ions, but Zn2+ is intercalated into the structure at the first discharge. From the second cycle, shuttling of Zn2+ progressed, so that the resulting operation voltage for charge is lowered but there is no change in the discharge profile compared to the first discharge. To increase the operating voltage of Na3V2(PO4)3 in ZIBs, Jiang et al.89 introduced fluorine-doped Na3V2(PO4)2F3 as a cathode material. Because of the strong ionicity induced by the presence of fluorine in the crystal structure, the resulting operation was raised to 1.62 V versus Zn2+/Zn, which is shown in Figure 9c. However, there was no clear difference between the 2630

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Figure 9. (a) Schematic representation for phase transition of the Na3V2(PO4)3 cathode during cycling. (b) Galvanostatic charge−discharge curves (Reprinted from ref 88. Copyright 2016, with permission from Elsevier). (c) Three initial charge/discharge profiles of the CFF-Zn// N3VPF@C battery at 0.08 A g−1. (d) Cycling performance of the CFFZn//N3VPF@C battery at a current of 1 A g−1 (Reprinted from ref 89. Copyright 2018, with permission from Elsevier).

as an anode material (Figure 10c). The voltage profile displayed two plateaus associated with the intercalation of Zn2+ in two steps even at high rates, as mentioned above. Zn2+ can be intercalated into the Mo6S8 structure in both aqueous and nonaqueous electrolyte. The intercalation process undergoes via two steps: (Step 1) Zn 2 + + Mo6S8 + 2e− ↔ ZnMo6S8

(12)

(Step 2) Zn 2 + + ZnMo6S8 + 2e− ↔ Zn2Mo6S8

(13)

The formation of the final discharged product, Zn2Mo6S8, was confirmed by XRD. Moreover, the study was extended to full cell configuration using zinc polyiodide catholyte as the cathode. The full cell showed excellent electrochemical performance with 90% capacity retention after 350 cycles at 600 mA g−1. Hong et al.96 studied the structural and electrochemical properties of the ZnxMo6S8 Chevrel phase (x = 1, 2) that was prepared by electrochemically inserting Zn2+ into the Mo6S8 structure in aqueous electrolyte. One of the possible Zn1 sites is in this sixmembered ring. The first plateau (n = 2.2) corresponds to the formation of ZnMo6S8, and the second plateau (n = 4.16) corresponds to the additional insertion of Zn2+, resulting in Zn2Mo6S8. The unit cell volumes increased by 1.5% after the first plateau and 5.3% after the second plateau. The distance between the molybdenum centers was reduced due to the reduction of molybdenum by insertion of the Zn2+. More elaboration is required to increase the capacity and rate performances via surface modification of the active materials.

Figure 10. (a) Schematic illustration of Mo6S8 application in various rechargeable ion batteries. (Reproduced with permission from ref 90. Copyright 2017, WILEY-VCH). (b) Local structure around the Zn1 positions in ZnMo6S8 (Reprinted with permission from ref 96. Copyright 2016, American Chemical Society). (c) Rate capability of Mo6S8 for Zn2+ intercalation in 1.0 M ZnSO4 aqueous electrolytes (Reprinted with permission from ref 95. Copyright 2016, American Chemical Society).

can intercalate into the Chevrel phase. Liu et al.95 synthesized Mo6S8 nanocubes for use in aqueous (1 M ZnSO4 in water) and nonaqueous (1 M Zn(ClO4)2 in acetonitrile) rechargeable ZIBs 2631

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Other Zn Ion Intercalation Materials. Organic Electrodes. Recently, organic electrode materials have emerged as an attractive host for monovalent (Li+, Na+, K+) and divalent (Mg2+) ions due to their light weight and environmentally friendly features. 97 In addition, because of the weak intermolecular van der Waals forces, ion intercalation would be easier. On the basis of this, several studies have focused on the intercalation of Zn2+ into organic electrodes.98,99 Kundu and co-workers reported that tetrachloro-1,4-benzoquinone can store Zn2+ in its structure.99 The organic electrode was tested against Zn metal in aqueous (1 M Zn(OTf)2) electrolyte. The cell delivered a capacity of about 200 mAh g−1, with a smooth voltage profile. However, the water-based electrolyte promoted the phase evolution between p-chloranil and discharged Zn-pchloranil, which caused poor cycling performance. Electrolyte. As summarized in the previous sections, most of the active materials presented different electrochemical performance in the same operating window depending on the use of non/aqueous systems. It is clear that judicious choice of the electrolyte is as important as the choice of the electrode material for the operation of ZIBs; this is because the electrolyte plays key roles in forming protective layers at both the cathode and anode and influences the formation of byproducts such as ZnO or Zn(OH)2 and ZnSO4-based compounds. From this thought, ZIB electrolytes should exhibit the following requisite properties: (1) chemical stability, (2) electrochemical stability at high and low voltage, (3) the ability to isolate electrons and conduct ions, and (4) safety and low cost. To develop electrolytes with all of these characteristics, many research groups are investigating the characteristics of the salts and solvents. The electrolyte systems for ZIBs can be roughly classified into two types, i.e., aqueous and nonaqueous. In the context of the electrolytes studied herein, we discuss both aqueous and nonaqueous electrolytes. Mild Aqueous Electrolyte. First, aqueous electrolytes are discussed due to their advantages such as low cost, safety, and safe handling in open air. The electrolyte salts in aqueous electrolyte systems are clearly important components for use in ZIBs. Most of the reported electrochemical intercalation experiments have been performed in aqueous systems using mild aqueous electrolytes such as ZnSO4 and Zn(NO3)2 salts and Zn(CF3SO3)2. The significant influence of the electrolyte salts is clearly observed in manganese- and vanadium-based materials. Kang et al.12 investigated the electrochemical performance of an α-MnO2/Zn cell using two types of electrolyte salts, namely, 1 mol L−1 ZnSO4 and Zn(NO3)2 in DI water. The ZIBs employing the mild aqueous electrolyte presented good discharge capacity at various rates from 0.5C to 126C. The cycle life performance of the ZIB, in terms of the capacity retention and CE, was investigated in a continuous cycling test at a 6C charge/discharge rate. After 100 cycles, the discharge capacity of the ZIB remained near 100% of the original value, and the CE was about 100% for all cycles, indicating good electrochemical performance. The cell performance featured excellent capacity rechargeability in terms of the capacity retention and CE, even at 100% depth of discharge. It was reported that the zinc ions could rapidly dissolve electrochemically as Zn2+ and deposit reversibly, leading to a very high capacity in mild 1 mol L−1 ZnSO4 and Zn(NO3)2 aqueous solution. However, capacity fading was generally observed during the 30th cycle. The 1 mol L−1 ZnSO4 and Zn(NO3)2 systems are acidic electrolytes (pH = 4.2) that can cause

corrosion of metals and dissolution of the transition metal and current collectors, resulting in poor long-term reliability. To overcome these side effects, Chen et al.29 attempted the use of Zn(CF3SO3)2 having a bulky anion, which enhanced the reactivity and stability of the Zn metal and active materials due to the high ionic conductivity and electrochemical stability of the β-MnO2/Zn system. The cell employing the 3 mol L−1 Zn(CF3SO3)2 electrolyte delivered a much higher initial discharge capacity than that with 3 mol L−1 ZnSO4 (275 vs 120 mAh g−1) at 0.65C However, similar capacity fading was observed during cycling due to the loss of the active mass. Thus, Mn2+ salts were added to the Zn(CF3SO3)2 electrolyte to balance the equilibrium of dissolution of Mn2+ from the MnO2 electrode. To exclude the anion effect, Mn(CF3SO3)2 was selected as the electrolyte additive, with concentrations from dilute 0.01 M to saturated 0.1 M. The optimal electrolyte composition was 3 M Zn(CF3SO3)2 + 0.1 M Mn(CF3SO3)2, which resulted in a higher CE and ionic conductivity, as well as a high capacity of 225 mAh g−1, even after 100 cycles. Summarizing the salt effect, SO42−-based electrolytes such as ZnSO4 and Zn(NO3)2 presented an increase in the capacity within the first several cycles, which was attributed to the activation process. Furthermore, CF3SO3−-based electrolytes such as Zn(CF3SO3)2 delivered a much higher initial discharge capacity (275 mAh g−1 at 0.65C) and resulted in capacity stabilization from 10 cycles. The difference in the performance could be ascribed to the Zn(CF3SO3)2 solution that not only features higher ionic conductivity but also enables faster kinetics and higher stability of Zn plating/stripping as compared with sulfate and alkaline electrolytes. More importantly, the narrow electrochemical stability range of aqueous electrolytes limits the operating voltage of the host materials, where it is thought that protons or hydronium ions (H3O+) participate in the intercalation of zinc ions. The major issues of corrosion of the metal and current collector by the acidic electrolyte, which can lead to poor long-cycle reliability, must thus be resolved. Aqueous-Based Solid-State/Gel-Type Electrolytes. Recently, the development of safe, flexible, and wearable electronics has garnered significant attention for application to smart watches, pulse sensors, clothes, and cellular phones. In this regard, it is important to develop a rechargeable battery that can be worn very safely under various harsh conditions. Zhi et al.49 investigated a gelatin and grafting polyacrylamide (PAM)based hierarchical polymer electrolyte (HPE) using an aqueous solution of ZnSO4 and MnSO4 in an α-MnO2@CNT/Zn cell (Figure 11a). The HPE developed by Zhi et al. consisted of a highly porous 3D architecture that was filled with gelatin chains and exhibited a high level of wettability in the polymeric network. Thus, the HPE solid-state electrolyte presented a high ionic conductivity of 1.76 × 10−2 S cm−1 while maintaining great flexibility and favorable mechanical strength (7.76 MPa). Interestingly, ZIBs using the HPE solid-state electrolyte exhibited an excellent specific capacity of 306 mAh g−1 at 61 mA g−1 and superior capacity retention of 97% during 1000 cycles at 2772 mA g−1. The electrochemical performance of the HPE solid-state electrolyte in ZIBs under a variety of conditions was evaluated by subjecting it to cutting, bending, hammering, combustion, washing, weight loading, drilling, and sewing. Especially, thermal safety and fire-resistant performance are very important issues for practical wearable applications. As a result of Figure 11b, the unpackaged ZIBs using HPE solid-state electrolyte were exposed to fire using an alcohol burner and then 2632

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Figure 11. (a) Schematic of the SSE ZIB using HPE and (b) combustion tests (Reproduced with permission from ref 49. Copyright 2018, The Royal Society of Chemistry) and (c) schematic diagram of fabrication and encapsulation of the yarn-ZIB using the cross-linked PAM-based electrolyte (Reproduced with permission from ref 102. Copyright 2018, The Royal Society of Chemistry). (d) Schematic diagram showing the structure and the fabrication protocol of the shape memory wire battery (Reprinted with permission from ref 100. Copyright 2018, American Chemical Society).

did not catch fire even after being exposed to the fire for more than 5 min, demonstrating a surprising high-temperature durability. Besides, the ZIB presented a high capacity retention of 87.2%. The various tests demonstrated that this ZIB using the HPE solid-state electrolyte exhibited excellent electrochemical performance under a variety of extreme conditions.

using a polymer electrolyte, the yarn-ZIBs were tested under a variety of conditions, such as from straight to 300% strain, bent, knotted, twisted, and soaked in DI water. The solid-state yarnZIB employing the polymer electrolyte exhibited excellent waterproof features, tailorability, knottability, and stretchability. Using this concept, Fan and co-workers developed QSS ZIBs using an aqueous ZnSO4-based gel-type electrolyte.101 This QSS ZIB exhibited ultrastable flexibility due to the gel-type electrolyte and Zn array anode. It presented a high first CE of 95% and delivered a high reversible capacity of 204 mAh g−1 at 0.5C. Interestingly, the QSS ZIBs exhibited high rate performance and delivered discharge capacities of 160 mAh g−1 at 10 C and 101 mAh g−1 even at 50 C. In addition, the QSS ZIB presented long-term cycle stability over 2000 cycles at 20C and an unprecedented energy density of 115 Wh kg−1. Similar to solidstate ZIBs, the QSS ZIBs could be bent without deterioration of their discharge performance, and there was no obvious capacity sacrifice (over 96% retention) even after 100 bending cycles. To improve the performance of flexible and wearable energy storage devices, Zhi et al.102 investigated a smart wire-shaped ZIB with shape memory function, derived from a temperaturetriggered shape memory effect, by using a gelatin−borax-based gel-type polymer electrolyte with aqueous ZnSO4/MnSO4 solution (Figure 11c). The gelatin−borax-based gel-type polymer electrolyte exhibited higher ionic conductivity (2.0 × 10−2 S cm−1) than the normal gel polymer electrolyte (1.39 × 10−2 S cm−1) due to the addition of borax, which also conferred enhanced water retention capability and electrochemical performance. As a result, smart wire-shaped ZIBs using the gelatin− borax-based gel-type polymer electrolyte delivered a discharge

Solid-state ZIBs offer extreme safety performance under various severe conditions, such as being cut, bent, hammered, punctured, or even set on fire. Another new concept for a solid-state electrolyte for ZIBs was also presented by Zhi et al.,100 who developed a cross-linked PAM-based polymer electrolyte as a matrix host for an aqueous solution of ZnSO4 and MnSO4 (Figure 11c). The solid-state polymer electrolyte showed very good compatibility between the metal salts and cross-linked PAM. Thus, it exhibited a high ionic conductivity of 16.5 × 10−3 S cm−1, indicating stable and fast ion transport due to the presence of the highly porous network structure in the polymer matrix and many hydrophilic groups, such as amide groups. The developed polymer electrolyte exhibited a high tensile strength of 273 kPa and excellent stretchability to 3000% strain in the constructed yarnZIBs. Interestingly, this yarn-ZIB presented a high discharge capacity of 302.1 mAh g−1 at 60 mA g−1 and superior cycling retention of 98.5% during 500 cycles, even at a current of 2 A g−1. To evaluate the electrochemical performance of the yarn-ZIBs 2633

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Figure 12. (a) Handmade flower using the xanthan gum electrolyte. (b) Schematic showing the structure of our gum Zn−MnO2 battery using xanthan gum film electrolyte (Reproduced with permission from ref 103. Copyright 2018, The Royal Society of Chemistry). (c) Schematic illustrations of morphology evolution for bare and nano-CaCO3-coated Zn foils during Zn stripping/plating cycling. (d,e) GCD curves and galvanostatic cycling performance of Zn/ZnSO4 + MnSO4/CNT/MnO2 (Reproduced with permission from ref 119. Copyright 2018, WILEY-VCH).

capacity of 174.2 mAh g−1 at 0.5C and retained a capacity of 60 mAh g−1 even at 4C. Moreover, the capacity retention was very well maintained in the gelatin−borax-based gel-type polymer electrolyte, presenting a discharge capacity of 135.2 mAh g−1 at 2C and a CE of over 99% over 1000 cycles. The electrochemical performance in the bent state was evaluated by fixing the battery. The flexible ZIBs employing the gelatin−borax-based gel-type polymer electrolyte displayed good battery performance even when bent to 90° and were functional even after 500 bending cycles. In summary, ZIBs employing solid-state/gel-type electrolytes do not seriously affect zinc dendrite, such that they exhibit a high level of wettability and ionic conductivity while maintaining great flexibility and favorable mechanical strength. Therefore, the concept of rechargeable ZIBs can be applied to reliable power sources, and solid-state/gel-type electrolytes offer the possibility for the development of flexible and wearable energy storage devices as the most suitable electrolyte for ZIBs. Aqueous-Based Bioelectrolytes. In addition to solid electrolytes, gel-type bioelectrolytes, which have a high ionic conductivity (similar to that of aqueous-based electrolytes such as ZnSO4/MnSO4 solution) are currently being studied for application to flexible and wearable devices. Li et al.103 investigated various stable gum-type sulfate-tolerant bioelectrolytes using a xanthan biopolymer with ZnSO4/MnSO4 solution for a MnO2@CNT/Zn cell (Figure 12a,b). This gum-type bioelectrolyte had a high ionic conductivity of 1.46 × 10−2 S cm−1 even at −8 °C (2.5 × 10−3 S cm−1). As advantages, the gum-type electrolyte permitted hydration, enhanced viscosity, and self-adjustment of the shape. ZIBs employing the gel-type bioelectrolyte exhibited a high discharge capacity of 260 mAh g−1 at 1C and good capacity retention (90%) during 330 cycles at 1C. Furthermore, the cell presented a capacity of 127 mAh g−1 over 1000 cycles, even at 5C. The ZIBs presented good electrochemical performance in the bent and twisted

states, implying that the gum-type electrolyte is less affected on the side effect by undesirable dendrite formation. Endres and co-workers104,105 investigated a PB/Zn cell using the new concept of a bioionic liquid-water electrolyte. The bioionic liquid-water electrolyte consisted of 1.0 mol L−1 Zn(OAc)2/([Ch]OAc + 30 wt % water). Interestingly, the ionic liquid, choline acetate, exhibits unique solubility in water. When [Ch]OAc was dissolved in 20 wt % of water, the zinc morphology was compact and dense. However, when [Ch]OAc was dissolved in over 50 wt % of water, the zinc deposited as discrete thin plates with perpendicular orientation. Moreover, the [Ch]OAc solution changed from an ionic liquid-like solution to an aqueous-like solution in 40 wt % water. On the basis of these results, [Ch]OAc in 30 wt % of water was employed as the electrolyte for the biodegradable ZIB. Thus, the electrolyte possessed the properties of an ionic liquid as well an aqueous medium. The corresponding ZIBs presented a reversible electrochemical reaction and a discharge capacity of 120 mAh g−1 at a current of 10 mA g−1 (0.1C). Although the rate capability decreased drastically at 60 mA g−1 with a discharge capacity of about 31 mAh g−1, the zinc metal exhibited good reversibility without the formation of Zn dendrites. In summary, bioionic liquids are good candidates for ZIB research due to the lack of research on electrolytes. Although bioionic liquids have good cyclability and the zinc metal exhibited good reversibility without the formation of Zn dendrites, rate capability is poorer than that of batteries employing mild aqueous electrolytes. Thus, one needs to improve the rate capability via new research such as using additive materials. Nonaqueous Systems. Organic Electrolyte: To exclude the issues related to mild aqueous electrolytes, some groups investigated organic electrolyte systems based on Zn(ClO4)2, Zn(TFSI)2, and Zn(CF3SO3)2 in acetonitrile. Hong et al.106 studied the electrochemical performance of a K1.51Ni[Fe(CN)6]0.954(H2O)0.77 (KNF-152)/Zn cell using an organic electrolyte, namely, 0.5 mol L−1 Zn(ClO4)2 in acetonitrile. The 2634

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cycling current of zinc in the organic electrolyte was compared with that in the mild aqueous ZnSO4 electrolyte to evaluate Zn metal plating; zinc presented excellent deposition/dissolution reversibility with 99.9% CE in 0.5 mol L−1 Zn(ClO4)2 in acetonitrile over 20 cycles. This indicates that the organic electrolyte is suitable for high-voltage and highly reliable ZIBs due to the high reversibility of the metal deposition/dissolution reaction. However, organic electrolytes have fatal disadvantages in terms of the rate capability. The initial discharge capacity of 55.6 mAh g−1 at 0.2C (11.2 mA g−1) declined drastically with increasing C-rates. This is related to the ionic conductivity of the electrolyte. The ionic conductivity of 0.5 mol L−1 Zn(ClO4)2 in acetonitrile and the aqueous electrolyte was 25.7 and 71.4 mS cm−1, respectively, at room temperature. Therefore, the rate capability in the organic electrolyte was significantly lower than that in the aqueous electrolyte due to the low ionic conductivity of organic electrolytes. A promising organic electrolyte comprising Zn(TFSI)2 in acetonitrile was also reported by Vaughey et al.,39 who demonstrated Zn insertion in a δ-MnO2/Zn cell using 0.5 mol L−1 Zn(TFSI)2 in acetonitrile. Although the cell presented a good CE of over 99% during cycling, the discharge capacity increased from 95 to 123 mAh g−1 over 20 cycles, and drastic capacity fading was observed from the 30th cycle. The authors claimed that improved electrode wetting enhanced the capacity by increasing the electronic contact of the cathode, and the capacity fading was attributed to decomposition of the organic electrolyte and structural changes. It was also suggested that the passivation layer formed at the cathode−organic electrolyte interface by decomposition of the organic electrolyte could contribute to capacity fading during cycling based on electrochemical impedance spectroscopy. When the δ-MnO2/Zn cell using 0.5 mol L−1 Zn(TFSI)2 was subjected to a C-rate test, the capacity decreased drastically from 100 to 30 mAh g−1 with a change in the current from 12.3 to 308 mA g−1. This indicates that the organic electrolyte showed poor rate capability, where the charge transfer resistance was increased. Therefore, the ZIB using 0.5 mol L−1 Zn(TFSI)2 was ineffective for overcoming the rate capability challenge. A significant difference between the aqueous and organic electrolyte was clearly observed with the V3O7·H2O electrode developed by Nazar et al.107 They investigated the effect of aqueous (1 mol L−1 ZnSO4 in DI water) and organic (0.25 mol L−1 Zn(CF3SO3)2 in acetonitrile) electrolytes on the electrochemical performance of ZIBs. Although the two electrolytes presented similar discharge/charge voltage profiles and inflection regions at the first cycle, the delivered capacity was significantly different. The cell employing aqueous 1 mol L−1 ZnSO4 showed a very high 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 vs Zn2+/Zn and good cyclability with 80% reversible capacity retention at a current of 8C (3000 mA g−1) during 200 cycles. The cell employing the organic 0.25 mol L−1 Zn(CF3SO3)2 in acetonitrile electrolyte exhibited an extremely poor capacity of 58 mAh g−1 at a current of 5 mA g−1, and there was an increase in the specific capacity from 58 to 175 mAh g−1 over 50 cycles. Interestingly, this phenomenon has been reported for organic electrolyte systems, related to the decrease in charge transfer at the negative electrode. However, the study revealed that the electrochemical behavior of the Zn2+ ions in the aqueous/organic electrolyte systems was overall similar. Ionic conductivities of electrolytes are summarized in Table 3. In summary, although organic electrolyte-based batteries exhibit excellent cycle capability without significant proton

Table 3. Ionic Conductivities of Electrolytes type aqueous type

ionic conductivities (S cm−1)

electrolyte

3 M ZnSO4 3 M Zn(CF3SO3)2 3 M Zn(CF3SO3)2 + 0.1 M Mn(CF3SO3)2 0.5 mol L−1 Zn(ClO4)2 aqueous-based solid- gelatin + PAM using an aqueous state/gel-type solution of ZnSO4 and MnSO4 cross-linked PAM using an aqueous solution of ZnSO4 and MnSO4 aqueous ZnSO4-based gel gelatin + borax polymer using an aqueous solution of ZnSO4 and MnSO4 aqueous-based xanthan biopolymer using an biotype aqueous solution of ZnSO4 and MnSO4 1.0 mol L−1 Zn(OAc)2/([Ch]OAc + 30 wt % water) nonaqueous type 0.5 mol L−1 Zn(ClO4)2 in acetonitrile 0.5 mol L−1 Zn(TFSI)2 in acetonitrile 0.25 mol L−1 Zn(CF3SO3)2 in acetonitrile

ref

∼1.5 ∼4.1 ∼6

29 29 29

0.714 × 10−2 1.76 × 10−2

105 49

16.5 × 10−3

99

8.1 × 10−2 2.0 × 10−2

100 101

1.46 × 10−2

102

unknown

103, 104

0.257 × 10−2 S cm−1 unknown

105

unknown

106

39

participation and higher reversibility (over 99.9%) of the metal deposition/dissolution process, their rate performance is poorer than that of batteries employing mild aqueous electrolytes. It seems likely that interfacial charge transfer processes and the ionic conductivity play a major role in determining the rate capability in these electrolyte systems. Anode Material. Zn Metal. Zinc is thought to be one of the best options as an anode material because it is a good reducing agent with a sufficiently negative potential (−0.76 V vs standard hydrogen electrode), has a high theoretical capacity, and is stable against corrosion in aqueous solutions, relatively cheap, and nontoxic. Zn anodes were used for the first time in 1799 in the Voltaic pile in the first electric battery that could continuously provide an electric current to a circuit.108 Later, in 1866, Leclanche invented a battery comprising a zinc anode and MnO2 cathode, which has remained the major primary battery to date, with an annual market value of 10 billion dollars. Since then, several zinc primary and secondary systems have been proposed and developed, including Zn/NH4Cl/MnO2, Zn/ZnCl2/MnO2, Zn/KOH/MnO2, Zn/NaOH/CuO, and Zn/ZnSO4/MnO2 systems. However, to date, the rechargeability of batteries based on zinc anodes remains a significant challenge because of zinc corrosion and the formation of electrochemically inactive products via reduction of the electrode in the alkaline electrolyte and the formation of metal dendrites and protrusions due to uneven Zn electrostripping/electroplating in neutral electrolytes.109,110 Because this Review is mainly devoted to ZIBs employing a mild acidic aqueous electrolyte, possible ways to increase the stability and reversibility of Zn metal anodes under these conditions are discussed. The documented knowledge and methods of solving the problems of dendrite formation by Zn in mild acidic aqueous electrolytes are limited. The most effective methods to suppress dendrite growth involve changing the architectures of the Zn electrode111,112,116 and introducing additives into the electrolyte113−115 or electrode.117−119 Porous Zn metal anodes have advantages over planar Zn because they possess a higher surface 2635

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by several groups. Three different reaction mechanisms have been proposed so far: capacity derived from the reversible Zn2+ insertion/extraction; the reversible H+ insertion/extraction, followed by precipitation/dissolution of zinc hydroxide sulfate on the electrode surface; the reversible co-insertion/co-extraction of Zn2+ and H+. The ambiguity arises from the absence of rigorous protocols ensuring data reproducibility and standardization of the measurements. Developing and applying different electrochemical methods, together with theoretical tools, which can provide definite proof of Zn2+ intercalation from multiple in situ characterization techniques, are a necessity for solving the above challenge. Second, ZIBs still suffer from the paucity of cathode materials that can store Zn2+ due to the relatively small ionic radius of divalent Zn2+ and consequently high charge density. The main difficulty associated with the intercalation of Zn2+ into a solid framework is to redistribute the multiple charges carried by each Zn2+ cation in the local structure of solid frameworks.123 For instance, charge transfer should be accompanied by a drastic transition in the electronic configurations or a sudden adjustment in the coordination environment or bond length, which can be thermodynamically unfavorable in many cases. Therefore, hard redistribution of the ionic charge is the main problem of Zn2+ mobility in an inorganic host. The low mobility of the Zn2+ remains a significant obstacle in the search for cathodes; however, many chemistries remain unexplored and need to be developed and tested. Third, in consideration of safety, conductivity, and cost, aqueous mild ZnSO4 electrolytes show the best prospective for ZIBs. However, batteries employing such electrolytes suffer from capacity decay because of electrode dissolution and formation of dendrites on the surface of the Zn anode. Recently, Chen demonstrated that the Zn(CF3SO3)2 aqueous electrolyte can effectively suppress the formation of dendrites, leading to long life of ZIBs.29 However, from an industrial point of view, the price of Zn(CF3SO3)2 is currently prohibitive. Therefore, the search for better electrolytes continues, and their discovery is essential to the eventual realization of the long-term cycling stability of ZIBs. Fourth, with the development of flexible electronic devices, the demand for safe, low-cost, and wearable batteries has intensified. Unfortunately, with current technology, flexible LIBs are vastly limited in terms of safety and cost. Therefore, wearable ZIBs with QSS aqueous electrolytes49,100−102 hold great potential for practical wearable applications. Efforts toward unearthing versatile solid-state electrolytes and improving the performance of ZIBs must be emphasized. In addition to the aforementioned challenges, self-discharge rate needs to be carefully examined in order to make ZIB technology a practical reality in ESS applications. This problem in fact is not unique to only ZIBs but is common to other batteries, including LIBs, which have the lowest self-discharge rate. Overall, the promising combination of safety, environmental benefit, and high energy density should allow ZIBs with mild aqueous electrolytes to become leading candidates for energy storage solutions. The mature primary ZIB technology can provide requisite solutions to the energy demand provided that adequate zinc and MnO2 electrodes can be designed, and the production and commercialization of rechargeable ZIBs with mild acidic aqueous electrolyte can potentially be scaled up in the near future.

area that is more accessible to the electrolyte. Recently, Wang et al. designed a novel Zn@graphite felt structure,116 which decreased the voltage hysteresis and increased the cycling stability of a full cell with a PB cathode in a mild acidic electrolyte. Kang et al.117 demonstrated that introducing activated carbon additives into the Zn anode can significantly improve the reversibility and electrochemical reaction kinetics of the Zn anode and suppress the formation of basic zinc sulfate, leading to enhancement of the cycle performance of the Zn/ZnSO4/MnO2 battery. The pores of the activated carbon accommodated the deposition of Zn dendrites and insoluble anodic products, leading to an increase in the cycling stability. Organic and inorganic additives have also been used to suppress dendrite formation and corrosion of the Zn anodes.118,119 However, whereas direct introduction of the additives into the electrolyte can constrain the formation of dendrites, it can, at the same time, influence the other components of the battery and in general can decrease the lifespan of the battery. Recently, Chen et al.118 proposed different organic additives such as cetyltrimethylammonium bromide (CTAB), poly(ethylene glycol) (PEG), sodium dodecyl sulfate (SDS), and thiourea for modification of an electroplated Zn anode via rendering the crystallographic surfaces to commercial Zn foil. Tafel fitting of the linear polarization data revealed that Zn anodes employing the organic additives exhibited 6−30 times lower corrosion currents. When Zn-SDS was used as the anode in a rechargeable hybrid aqueous battery, the float current decreased by as much as 2.5-fold. Among these electroplated anodes, Zn-SDS was the most suitable for aqueous batteries due to its low corrosion rate, low dendrite formation, low float current, and high capacity retention after 1000 cycles. Zhi et al.119 reported a porous nanoCaCO3 coating as a buffer layer to obtain uniform and positionspecific plating of Zn metal. The proposed schematic illustrations of Zn-plating on bare and nano-CaCO3-coated Zn foils are graphically depicted in Figure 12c. The strategy proposed by the authors effectively prevented the development of large protrusions/dendrites that may easily pierce the separators and cause cell shorting. The applicability of the nano-CaCO3 layer was demonstrated in a Zn/ZnSO4 + MnSO4/CNT/MnO2 rechargeable aqueous battery, which showed dramatically improved cycling stability of the device with a capacity of 177 mAh g−1 at 1 A g−1 after 1000 cycles (Figures 12d,e). Therefore, only fragmentary data on the influence of the different methods of mitigating the formation of Zn dendrites in aqueous mild acidic electrolyte and improving the battery performance in terms of cycle life, capacity, and CE are available. The strategies for improving the performance of zinc electrodes should be implemented directly to achieve an exact battery configuration given that many factors play crucial roles in affecting the battery performance. Summary and Perspective. ZIBs employing a mild acidic electrolyte are considered promising alternatives to conventional LIBs due to their cost-effectiveness, safety, and high energy density. Notably, the low cost of Zn and environmental safety of the aqueous electrolyte make these batteries promising candidates for ESS. A gravimetric energy density of around 400 Wh kg−1 is also very promising (Figure 1d). To date, considerable progress on aqueous ZIBs has been achieved, but there are still limitations that should be overcome. First, fundamental investigations on the reaction mechanism in ZIBs should be intensified. As summarized, the mechanism of Zn2+ intercalation into the MnO2 structure is still under debate 2636

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AUTHOR INFORMATION

Corresponding Authors

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ACKNOWLEDGMENTS

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REFERENCES

Review

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science, and Technology of Korea (NRF-2015M3D1A1069713, NRF2017R1A2A2A05069634, NRF-2017K1A3A1A30084795).

*E-mail: [email protected]. Tel.: 82 2 2220 0524. Fax: 82 2 2282 7329 (Y.-K.S.). *E-mail: [email protected]. Tel.: 82 2 3408 3454. Fax: 82 2 3408 4342. (S.-T.M.). ORCID

Yang-Kook Sun: 0000-0002-0117-0170 Seung-Taek Myung: 0000-0001-6888-5376

(1) Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928−935. (2) Yoshino, A. The Birth of the Lithium-Ion Battery. Angew. Chem., Int. Ed. 2012, 51, 5798−5800. (3) Luo, J. Y.; Xia, Y. Y. Aqueous Lithium-Ion Battery LiTi2(PO4)3/ LiMn2O4 with High Power and Energy Densities as well as Superior Cycling Stability. Adv. Funct. Mater. 2007, 17, 3877−3884. (4) Li, W.; Dahn, J. R.; Wainwright, D. S. Rechargeable Lithium Batteries with Aqueous Electrolytes. Science 1994, 264, 1115−1118. (5) Kim, H.; Hong, J.; Park, K. Y.; Kim, H.; Kim, S. W.; Kang, K. Aqueous Rechargeable Li and Na Ion Batteries. Chem. Rev. 2014, 114, 11788−11827. (6) Hwang, J.-Y.; Myung, S.-T.; Sun, Y.-K. Sodium-Ion Batteries: Present and Future. Chem. Soc. Rev. 2017, 46, 3529−3614. (7) Winter, M.; Brodd, R. J. What Are Batteries, Fuel Cells, and Supercapacitors? Chem. Rev. 2004, 104, 4245−4270. (8) Yamamoto, T.; Shoji, T. Rechargeable Zn|ZnSO4|MnO2-type Cells. Inorg. Chim. Acta 1986, 117, L27−L28. (9) Juran, T. R.; Young, J.; Smeu, M. Density Functional Theory Modeling of MnO2 Polymorphs as Cathodes for Multivalent Ion Batteries. J. Phys. Chem. C 2018, 122, 8788−8795. (10) Shoji, T.; Hishinuma, M.; Yamamoto, T. Zinc-Manganese Dioxide Galvanic Cell Using Zinc Sulphate as Electrolyte. Rechargeability of the Cell. J. Appl. Electrochem. 1988, 18, 521−526. (11) Shoji, T.; Yamamoto, T. Charging and Discharging Behavior of Zinc-Manganese Dioxide Galvanic Cells Using Zinc Sulfate as Electrolyte. J. Electroanal. Chem. 1993, 362, 153−157. (12) Xu, C.; Li, B.; Du, H.; Kang, F. Energetic Zinc Ion Chemistry: The Rechargeable Zinc Ion Battery. Angew. Chem., Int. Ed. 2012, 51, 933−935. (13) Wei, C.; Xu, C.; Li, B.; Du, H.; Kang, F. Preparation and Characterization of Manganese Dioxides with Nano-Sized Tunnel Structures for Zinc Ion Storage. J. Phys. Chem. Solids 2012, 73, 1487− 1491. (14) Xu, C.; Chiang, S. W.; Ma, J.; Kang, F. Investigation on Zinc Ion Storage in Alpha Manganese Dioxide for Zinc Ion Battery by Electrochemical Impedance Spectrum. J. Electrochem. Soc. 2013, 160, A93−A97. (15) Kang, F.; Xu, C.; Li, C.; Du, H. U.S. Patent US8 663844B2; 2014. (16) Alfaruqi, M. H.; Islam, S.; Mathew, V.; Song, J.; Kim, S.; Tung, D. P.; Jo, J.; Kim, S.; Baboo, J. P.; Xiu, Z.; Kim, J. Ambient Redox Synthesis of Vanadium-Doped Manganese Dioxide Nanoparticles and Their Enhanced Zinc Storage Properties. Appl. Surf. Sci. 2017, 404, 435−442. (17) Alfaruqi, M. H.; Gim, J.; Kim, S.; Song, J.; Jo, J.; Kim, S.; Mathew, V.; Kim, J. Enhanced Reversible Divalent Zinc Storage in a Structurally Stable α-MnO2 Nanorod Electrode. J. Power Sources 2015, 288, 320− 327. (18) Alfaruqi, M. H.; Islam, S.; Gim, J.; Song, J.; Kim, S.; Pham, D. T.; Jo, J.; Xiu, Z.; Mathew, V.; Kim, J. A High Surface Area Tunnel-Type αMnO2 Nanorod Cathode by a Simple Solvent-Free Synthesis for Rechargeable Aqueous Zinc-Ion Batteries. Chem. Phys. Lett. 2016, 650, 64−68. (19) Tafur, J. P.; Abad, J.; Roman, E.; Fernandez Romero, A. J. Charge Storage Mechanism of MnO2 Cathodes In Zn/MnO2 Batteries Using Ionic Liquid-Based Gel Polymer Electrolytes. Electrochem. Commun. 2015, 60, 190−194. (20) Yang, Z.; Wang, X.; Huang, Y. First-Principles Study on the Doping Effects of Al in α-MnO2. Current Applied Physic. 2015, 15, 1556−1561.

Author Contributions #

A.K. and N.V. contributed equally to this work.

Notes

The authors declare no competing financial interest. Biographies Aishuak Konarov received his Masters of Applied Science degree from the University of Waterloo in 2014. He is currently a Ph.D. candidate in the Department of Nano Engineering at Sejong University, South Korea, under the supervision of Professor Seung-Taek Myung. His research focuses on synthesis and characterization of active materials for lithium-ion, sodium-ion, and zinc-ion batteries, as well as lithium− sulfur batteries. Natalia Voronina received a Ph.D. degree from Moscow State University in 2009. After having worked at Samsung SDI from 2011 to 2014, she decided to continue her research career aiming at battery technology. She has currently been working for another Ph.D. with Professor Seung-Taek Myung in the Department of Nano Engineering at Sejong University, South Korea. Her current research interests have focused on the development of new materials for sodium-ion and zincion batteries. Jae Hyeon Jo is presently a Ph.D. candidate in the Department of Nano Engineering at Sejong University, South Korea, under the supervision of Professor Seung-Taek Myung. His research focuses on materials development in the fields of energy conversion and storage, such as cathode and anode materials for sodium-ion batteries and zinc-ion batteries. Zhumabay Bakenov is a Professor of Chemical Engineering at Nazarbayev University, Director of the Center for Energy and Advanced Materials Science of National Laboratory Astana, Kazakhstan. He received his Doctor of Engineering (Chemical Engineering) degree from Tokyo Institute of Technology, Japan and Ph.D. degree from the Kazakh National Academy of Science, Kazakhstan. His research interests are advanced energy storage and conversion systems; synthesis of functional nanomaterials, including biological application; environmental engineering remediation of soil and water; and modelling and analysis of electrochemical and thermal processes in lithium-ion batteries. https://nu.edu.kz/faculty/zhumabay-bakenov Yang-Kook Sun received his Ph.D. degree from the Seoul National University, Korea. He was group leader at the Samsung Advanced Institute of Technology and contributed to commercialization of the lithium polymer battery. He has worked at Hanyang University in Korea as a professor since 2000. His research interests are the synthesis of new electrode materials for lithium-ion batteries, Na-ion batteries, Li−S batteries, and Li−air batteries. http://escml.hanyang.ac.kr/new/ professor.html Seung-Taek Myung is a Professor of Nano Engineering at Sejong University, South Korea. He received his Ph.D. degree in Chemical Engineering from Iwate University, Japan, in 2003. His research interests embrace development of electroactive materials and corrosion of current collectors of rechargeable lithium and sodium batteries. http://dasan.sejong.ac.kr/~smyung/member.htm 2637

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(21) Lee, J.; Ju, J. B.; Cho, W. I.; Cho, B. W.; Oh, S. H. TodorokiteType MnO2 as a Zinc-Ion Intercalating Material. Electrochim. Acta 2013, 112, 138−143. (22) Qiu, W.; Li, Y.; You, A.; Zhang, Z.; Li, G.; Lu, X.; Tong, Y. HighPerformance Flexible Quasi-Solid-State Zn-MnO2 Battery Based on MnO2 Nanorod Arrays Coated 3D Porous Nitrogen-Doped Carbon Cloth. J. Mater. Chem. A 2017, 5, 14838−14846. (23) Zeng, Y.; Zhang, X.; Meng, Y.; Yu, M.; Yi, J.; Wu, Y.; Lu, X.; Tong, Y. Achieving Ultrahigh Energy Density and Long Durability in a Flexible Rechargeable Quasi-Solid-State Zn−MnO2 Battery. Adv. Mater. 2017, 29, 1700274. (24) Islam, S.; Alfaruqi, M. H.; Song, J.; Kim, S.; Pham, D. T.; Jo, J.; Kim, S.; Mathew, V.; Baboo, J. P.; Xiu, Z.; et al. Carbon-Coated Manganese Dioxide Nanoparticles and Their Enhanced Electrochemical Properties for Zinc-Ion Battery Applications. J. Energy Chem. 2017, 26, 815−819. (25) Xu, D.; Li, B.; Wei, C.; He, Y. B.; Du, H.; Chu, X.; Qin, X.; Yang, Q. H.; Kang, F. Preparation and Characterization of MnO2/acidtreated CNT Nano Composites for Energy Storage with Zinc Ions. Electrochim. Acta 2014, 133, 254−261. (26) Lee, B.; Lee, H. R.; Kim, H.; Chung, K. Y.; Cho, B. W.; Oh, S. H. Elucidating the Intercalation Mechanism Of Zinc Ions into α-MnO2 for Rechargeable Zinc Batteries. Chem. Commun. 2015, 51, 9265−9268. (27) Lee, B.; Yoon, C. S.; Lee, H. R.; Chung, K. Y.; Cho, B. W.; Oh, S. H. Electrochemically-Induced Reversible Transition from the Tunneled to Layered Polymorphs of Manganese Dioxide. Sci. Rep. 2015, 4, 6066. (28) Alfaruqi, M. H.; Mathew, V.; Gim, J.; Kim, S.; Song, J.; Baboo, J. P.; Choi, S. H.; Kim, J. Electrochemically Induced Structural Transformation in a γ MnO2 Cathode of a High Capacity Zinc-Ion Battery System. Chem. Mater. 2015, 27, 3609−3620. (29) Zhang, N.; Cheng, F.; Liu, J.; Wang, L.; Long, X.; Liu, X.; Li, F.; Chen, J. Rechargeable Aqueous Zinc-Manganese Dioxide Batteries with High Energy and Power Densities. Nat. Commun. 2017, 8, 405. (30) Pan, H.; Shao, Y.; Yan, P.; Cheng, Y.; Han, K. S.; Nie, Z.; Wang, C.; Yang, J.; Li, X.; Bhattacharya, P.; et al. Reversible Aqueous Zinc/ Manganese Oxide Energy Storage from Conversion Reactions. Nat. Energy 2016, 1, 1−7. (31) Lee, B.; Seo, H. R.; Lee, H. R.; Yoon, C. S.; Kim, J. H.; Chung, K. Y.; Cho, B. W.; Oh, S. H. Critical Role of pH Evolution of Electrolyte in the Reaction Mechanism for Rechargeable Zinc Batteries. ChemSusChem 2016, 9, 2948−2956. (32) Islam, S.; Alfaruqi, M. H.; Mathew, V.; Song, J.; Kim, S.; Kim, S.; Jo, J.; Baboo, J. P.; Pham, D. T.; Putro, D. Y.; et al. Facile Synthesis and The Exploration of Zinc Storage Mechanism of β-MnO2 Nanorod with Exposed (101) Planes as a Novel Cathode Material for High Performance Eco-Friendly Zinc-Ion Battery. J. Mater. Chem. A 2017, 5, 23299−23309. (33) Wu, B.; Zhang, G.; Yan, M.; Xiong, T.; He, P.; He, L.; Xu, X.; Mai, L. Graphene Scroll-Coated α-MnO2 Nanowires as High-Performance Cathode Materials for Aqueous Zn-Ion Battery. Small 2018, 14, 1703850. (34) Sun, W.; Wang, F.; Hou, S.; Yang, C.; Fan, X.; Ma, Z.; Gao, T.; Han, F.; Hu, R.; Zhu, M.; et al. Zn/MnO2 Battery Chemistry With H+ and Zn2+ Coinsertion. J. Am. Chem. Soc. 2017, 139, 9775−9778. (35) Kim, S. H.; Oh, S. M. Degradation Mechanism of Layered MnO2 Cathodes in Zn/ZnSO4 /MnO2 Rechargeable Cells. J. Power Sources 1998, 72, 150−158. (36) Kim, S. H.; Oh, S. M. Aqueous Zinc Sulfate (II) Rechargeable Cell Containing Manganese (II) Salt and Carbon Powder. U.S. Patent US6 187475B1; 2001. (37) Alfaruqi, M. H.; Gim, J.; Kim, S.; Song, J.; Pham, D. T.; Jo, J.; Xiu, Z.; Mathew, V.; Kim, J. A Layered δ-MnO2 Nanoflake Cathode with High Zinc-Storage Capacities for Eco-Friendly Battery Applications. Electrochem. Commun. 2015, 60, 121−125. (38) Qiu, N.; Chen, H.; Yang, Z.; Sun, S.; Wang, Y. Low-Cost Birnessite as a Promising Cathode for High-Performance Aqueous Rechargeable Batteries. Electrochim. Acta 2018, 272, 154−160.

(39) Han, S. D.; Kim, S.; Li, D.; Petkov, V.; Yoo, H. D.; Phillips, P. J.; Wang, H.; Kim, J. J.; More, K. L.; Key, B.; et al. Mechanism of Zn Insertion into Nanostructured δ-MnO2: A Nonaqueous Rechargeable Zn Metal Battery. Chem. Mater. 2017, 29, 4874−4884. (40) Yuan, C.; Zhang, Y.; Pan, Y.; Liu, X.; Wang, G.; Cao, D. Investigation of the Intercalation of Polyvalent Cations (Mg2+, Zn2+) into λ-MnO2 for Rechargeable Aqueous Battery. Electrochim. Acta 2014, 116, 404−412. (41) Knight, J. C.; Therese, S.; Manthiram, A. Chemical Extraction of Zn from ZnMn2O4-Based Spinels. J. Mater. Chem. A 2015, 3, 21077− 21082. (42) Zhang, N.; Cheng, F.; Liu, Y.; Zhao, Q.; Lei, K.; Chen, C.; Liu, X.; Chen, J. Cation- Deficient Spinel ZnMn2O4 Cathode in Zn(CF3SO3)2 Electrolyte for Rechargeable Aqueous Zn-Ion Battery. J. Am. Chem. Soc. 2016, 138, 12894−12901. (43) Soundharrajan, V.; Sambandam, B.; Kim, S.; Mathew, V.; Jo, J.; Kim, S.; Lee, J.; Islam, S.; Kim, K.; Sun, Y.-K.; et al. Aqueous Magnesium Zinc Hybrid Battery: An Advanced High Voltage and High Energy MgMn2O4 Cathode. ACS Energy Lett. 2018, 3, 1998−2004. (44) Wu, X.; Xiang, Y.; Peng, Q.; Wu, X.; Li, Y.; Tang, F.; Song, R.; Liu, Z.; He, Z.; Wu, X. Green-Low-Cost Rechargeable Aqueous ZincIon Batteries Using Hollow Porous Spinel ZnMn2O4 as the Cathode Material. J. Mater. Chem. A 2017, 5, 17990−17997. (45) Jiang, B.; Xu, C.; Wu, C.; Dong, L.; Li, J.; Kang, F. Manganese Sesquioxide as Cathode Material for Multivalent Zinc Ion Battery with High Capacity and Long Cycle Life. Electrochim. Acta 2017, 229, 422− 428. (46) Li, B.; Chai, J.; Ge, X.; An, T.; Lim, P. C.; Liu, Z.; Zong, Y. Sheeton-Sheet Hierarchical Nanostructured C@MnO2 for Zn-Air and ZnMnO2 Batteries. ChemNanoMater. 2017, 3, 401−405. (47) Zhu, C.; Fang, G.; Zhou, J.; Guo, J.; Wang, Z.; Wang, C.; Li, J.; Tang, Y.; Liang, S. Binder-Free Stainless Steel@Mn3O4 Nanoflower Composite: A High-Activity Aqueous Zinc-Ion Battery Cathode with High-Capacity and Long Cycle-Life. J. Mater. Chem. A 2018, 6, 9677− 9683. (48) Chamoun, M.; Brant, W. R.; Tai, C. W.; Karlsson, G.; Noréus, D. Rechargeability of Aqueous Sulfate Zn/MnO2 Batteries Enhanced by Accessible Mn2+ Ions. Energy Storage Mater. 2018, 15, 351−360. (49) Li, H.; Han, C.; Huang, Y.; Huang, Y.; Zhu, M.; Pei, Z.; Xue, Q.; Wang, Z.; Liu, Z.; Tang, Z.; et al. An Extremely Safe and Wearable Solid-State Zinc Ion Battery Based on a Hierarchical Structured Polymer Electrolyte. Energy Environ. Sci. 2018, 11, 941−951. (50) Hao, J.; Mou, J.; Zhang, J.; Dong, L.; Liu, W.; Xu, C.; Kang, F. Electrochemically Induced Spinel-Layered Phase Transition of Mn3O4 in High Performance Neutral Aqueous Rechargeable Zinc Battery. Electrochim. Acta 2018, 259, 170−178. (51) Wang, K.; Zhang, X.; Han, J.; Zhang, X.; Sun, X.; Li, C.; Liu, W.; Li, Q.; Ma, Y. High-Performance Cable-Type Flexible Rechargeable Zn Battery Based on MnO2@CNT Fiber Microelectrode. ACS Appl. Mater. Interfaces 2018, 10, 24573−24582. (52) Alfaruqi, M. H.; Islam, S.; Putro, D. Y.; Mathew, V.; Kim, S.; Jo, J.; Kim, S.; Sun, Y.-K.; Kim, K.; Kim, J. Structural Transformation and Electrochemical Study of Layered MnO2 in Rechargeable Aqueous Zinc-Ion Battery. Electrochim. Acta 2018, 276, 1−11. (53) Huang, J.; Wang, Z.; Hou, M.; Dong, X.; Liu, Y.; Wang, Y.; Xia, Y. Polyaniline-Intercalated Manganese Dioxide Nanolayers as a HighPerformance Cathode Material for an Aqueous Zinc-Ion Battery. Nat. Commun. 2018, 9, 2906. (54) Xu, Y.; Zheng, S.; Tang, H.; Guo, X.; Xue, H.; Pang, H. Prussian Blue and Its Derivatives as Electrode Materials for Electrochemical Energy Storage. Energy Storage Mater. 2017, 9, 11−30. (55) Wessells, C. D.; Huggins, R. A.; Cui, Y. Copper Hexacyanoferrate Battery Electrodes with Long Cycle Life and High Power. Nat. Commun. 2011, 2, 1−5. (56) Wang, R. Y.; Shyam, B.; Stone, K. H.; Weker, J. N.; Pasta, M.; Lee, H. W.; Toney, M. F.; Cui, Y. Reversible Multivalent (Monovalent, Divalent, Trivalent) Ion Insertion in Open Framework Materials. Adv. Energy Mater. 2015, 5, 1401869. 2638

DOI: 10.1021/acsenergylett.8b01552 ACS Energy Lett. 2018, 3, 2620−2640

ACS Energy Letters

Review

(57) Wang, R. Y.; Wessells, C. D.; Huggins, R. A.; Cui, Y. Highly Reversible Open Framework Nanoscale Electrodes for Divalent Ion Batteries. Nano Lett. 2013, 13, 5748−5752. (58) Jia, Z.; Wang, B.; Wang, Y. Copper Hexacyanoferrate with a WellDefined Open Framework as a Positive Electrode for Aqueous Zinc Ion Batteries. Mater. Chem. Phys. 2015, 149−150, 601−606. (59) Trocoli, R.; La Mantia, F. An Aqueous Zinc-Ion Battery Based on Copper Hexacyanoferrate. ChemSusChem 2015, 8, 481−485. (60) Kasiri, G.; Trocoli, R.; Bani Hashemi, A.; La Mantia, F. An Electrochemical Investigation of the Aging of Copper Hexacyanoferrate During the Operation in Zinc-Ion Batteries. Electrochim. Acta 2016, 222, 74−83. (61) Renman, V.; Ojwang, D. O.; Valvo, M.; Gomez, C. P.; Gustafsson, T.; Svensson, G. Structural-Electrochemical Relations in the Aqueous Copper Hexacyanoferrate-Zinc System Examined by Synchrotron X-Ray Diffraction. J. Power Sources 2017, 369, 146−153. (62) Zhang, L.; Chen, L.; Zhou, X.; Liu, Z. Towards High-Voltage Aqueous Metal-Ion Batteries Beyond 1.5 V: The Zinc/Zinc Hexacyanoferrate System. Adv. Energy Mater. 2015, 5, 1400930. (63) Lu, K.; Song, B.; Zhang, Y.; Ma, H.; Zhang, J. Encapsulation of Zinc Hexacyanoferrate Nanocubes with Manganese Oxide Nanosheets for High-Performance Rechargeable Zinc Ion Batteries. J. Mater. Chem. A 2017, 5, 23628−23633. (64) Senguttuvan, P.; Han, S. D.; Kim, S.; Lipson, A. L.; Tepavcevic, S.; Fister, T. T.; Bloom, I. D.; Burrell, A. K.; Johnson, C. S. A High Power Rechargeable Nonaqueous Multivalent Zn/V2O5 Battery. Adv. Energy Mater. 2016, 6, 1600826. (65) Kundu, D.; Adams, B. D.; Duffort, V.; Vajargah, S. H.; Nazar, L. F. A High-Capacity and Long-Life Aqueous Rechargeable Zinc Battery Using a Metal Oxide Intercalation Cathode. Nat. Energy 2016, 1, 16119. (66) Xia, C.; Guo, J.; Li, P.; Zhang, X.; Alshareef, H. N. Highly Stable Aqueous Zinc-Ion Storage Using a Layered Calcium Vanadium Oxide Bronze Cathode. Angew. Chem., Int. Ed. 2018, 57, 3943−3948. (67) He, P.; Zhang, G.; Liao, X.; Yan, M.; Xu, X.; An, Q.; Liu, J.; Mai, L. Sodium Ion Stabilized Vanadium Oxide Nanowire Cathode for High-Performance Zinc-Ion Batteries. Adv. Energy Mater. 2018, 8, 1702463. (68) Yan, M.; He, P.; Chen, Y.; Wang, S.; Wei, Q.; Zhao, K.; Xu, X.; An, Q.; Shuang, Y.; Shao, Y.; et al. Water-Lubricated Intercalation in V2O5·nH2O for High-Capacity and High-Rate Aqueous Rechargeable Zinc Batteries. Adv. Mater. 2018, 30, 1703725. (69) Zhang, N.; Dong, Y.; Jia, M.; Bian, X.; Wang, Y.; Qiu, M.; Xu, J.; Liu, Y.; Jiao, L.; Cheng, F. Rechargeable Aqueous Zn−V2O5 Battery with High Energy Density and Long Cycle Life. ACS Energy Lett. 2018, 3, 1366−1372. (70) Zhou, J.; Shan, L.; Wu, Z.; Guo, X.; Fang, G.; Liang, S. Investigation of V2O5 as a Low-Cost Rechargeable Aqueous Zinc Ion Battery Cathode. Chem. Commun. 2018, 54, 4457−4460. (71) Alfaruqi, M. H.; Mathew, V.; Song, J.; Kim, S.; Islam, S.; Pham, D. T.; Jo, J.; Kim, S.; Baboo, J. P.; Xiu, Z.; et al. Electrochemical Zinc Intercalation in Lithium Vanadium Oxide: A High-Capacity Zinc-Ion Battery Cathode. Chem. Mater. 2017, 29, 1684−1694. (72) He, P.; Quan, Y.; Xu, X.; Yan, M.; Yang, W.; An, Q.; He, L.; Mai, L. High-Performance Aqueous Zinc−Ion Battery Based on Layered H2V3O8 Nanowire Cathode. Small 2017, 13, 1702551. (73) Pang, Q.; Sun, C.; Yu, Y.; Zhao, K.; Zhang, Z.; Voyles, P. M.; Chen, G.; Wei, Y.; Wang, X. H2V3O8 Nanowire/Graphene Electrodes for Aqueous Rechargeable Zinc Ion Batteries with High Rate Capability and Large Capacity. Adv. Energy Mater. 2018, 8, 1800144. (74) Cai, Y.; Liu, F.; Luo, Z.; Fang, G.; Zhou, J.; Pan, A.; Liang, S. Pilotaxitic Na1.1V3O7.9 Nanoribbons/Graphene as High-Performance Sodium Ion Battery and Aqueous Zinc Ion Battery Cathode. Energy Storage Mater. 2018, 13, 168−174. (75) Sambandam, B.; Soundharrajan, V.; Kim, S.; Alfaruqi, M. H.; Jo, J.; Kim, S.; Mathew, V.; Sun, Y.-K.; Kim, J. Aqueous Rechargeable Zn2+ Ion Battery: An Imperishable and High Energy Zn2V2O7 Nanowire Cathode through Intercalation Regulation. J. Mater. Chem. A 2018, 6, 3850−3856.

(76) Xia, C.; Guo, J.; Lei, Y.; Liang, H.; Zhao, C.; Alshareef, H. N. Rechargeable Aqueous Zinc-Ion Battery Based on Porous Framework Zinc Pyrovanadate Intercalation Cathode. Adv. Mater. 2018, 30, 1705580. (77) Soundharrajan, V.; Sambandam, B.; Kim, S.; Alfaruqi, M. H.; Putro, D. Y.; Jo, J.; Kim, S.; Mathew, V.; Sun, Y.-K.; Kim, J. Na2V6O16· 3H2O Barnesite Nanorod: An Open Door to Display a Stable and High Energy for Aqueous Rechargeable Zn-Ion Batteries as Cathodes. Nano Lett. 2018, 18, 2402−2410. (78) Hu, P.; Zhu, T.; Wang, X.; Wei, X.; Yan, M.; Li, J.; Luo, W.; Yang, W.; Zhang, W.; Zhou, L.; et al. Highly Durable Na2V6O16·1.63H2O Nanowire Cathode for Aqueous Zinc-Ion Battery. Nano Lett. 2018, 18, 1758−1763. (79) Sambandam, B.; Soundharrajan, V.; Kim, S.; Alfaruqi, M. H.; Jo, J.; Kim, S.; Mathew, V.; Sun, Y.-K.; Kim, J. K2V6O16·2.7H2O Nanorod Cathode: A Boosting Intercalation Hub for High Energy Aqueous Rechargeable Zn-Ion Batteries. J. Mater. Chem. A 2018, 6, 15530− 15539. (80) Tang, B.; Fang, G.; Zhou, J.; Wang, L.; Lei, Y.; Wang, C.; Lin, T.; Tang, Y.; Liang, S. Potassium Vanadates with Stable Structure and Fast Ion Diffusion Channel as Cathode for Rechargeable Aqueous Zinc-Ion Batteries. Nano Energy 2018, 51, 579−587. (81) Peng, Z.; Wei, Q.; Tan, S.; He, P.; Luo, W.; An, Q.; Mai, L. Novel Layered Iron Vanadate Cathode for High-Capacity Aqueous Rechargeable Zinc Batteries. Chem. Commun. 2018, 54, 4041−4044. (82) Wei, T.; Li, Q.; Yang, G.; Wang, C. An Electrochemically Induced Bilayered Structure Facilitates Long-Life Zinc Storage of Vanadium Dioxide. J. Mater. Chem. A 2018, 6, 8006−8012. (83) Ding, J.; Du, Z.; Gu, L.; Li, B.; Wang, L.; Wang, S.; Gong, Y.; Yang, S. Ultrafast Zn2+ Intercalation and Deintercalation in Vanadium Dioxide. Adv. Mater. 2018, 30, 1800762. (84) Dai, X.; Wan, F.; Zhang, L.; Cao, H.; Niu, Z. Freestanding Graphene/VO2 Composite Films for Highly Stable Aqueous Zn-Ion Batteries with Superior Rate Performance. Energy Storage Mater. 2018, DOI: 10.1016/j.ensm.2018.07.022. (85) Park, J. S.; Jo, J. H.; Aniskevich, Y. M.; Bakavets, A. S.; Ragoisha, G. A.; Streltsov, E. A.; Kim, J. S.; Myung, S. T. Open-Structured Vanadium Dioxide as an Intercalation Host for Zn Ions: Investigation by First-Principles Calculation and Experiments. Chem. Mater. 2018, DOI: 10.1021/acs.chemmater.8b02679. (86) Jo, J. H.; Sun, Y.-K.; Myung, S.-T. Hollandite-Type Al-Doped VO1.52(OH)0.77 as a Zinc Ion Insertion Host Material. J. Mater. Chem. A 2017, 5, 8367−8375. (87) He, P.; Yan, M.; Zhang, G.; Sun, R.; Chen, L.; An, Q.; Mai, L. Layered VS2 Nanosheet-Based Aqueous Zn Ion Battery Cathode. Adv. Energy Mater. 2017, 7, 1601920. (88) Li, G.; Yang, Z.; Jiang, Y.; Jin, C.; Huang, W.; Ding, X.; Huang, Y. Towards Polyvalent Ion Batteries: A Zinc-Ion Battery Based NASICON Structured Na3V2(PO4)3. Nano Energy 2016, 25, 211−217. (89) Li, W.; Wang, K.; Cheng, S.; Jiang, K. A Long-Life Aqueous ZnIon Battery Based on Na3V2(PO4)2F3 Cathode. Energy Storage Mater. 2018, 15, 14−21. (90) Mei, L.; Xu, J.; Wei, Z.; Liu, H.; Li, Y.; Ma, J.; Dou, S. Chevrel Phase Mo6T8 (T = S, Se) as Electrodes for Advanced Energy Storage. Small 2017, 13, 1701441. (91) Ling, C.; Suto, K. Thermodynamic Origin of Irreversible Magnesium Trapping in Chevrel Phase Mo6S8: Importance of Magnesium and Vacancy Ordering. Chem. Mater. 2017, 29, 3731− 3739. (92) Levi, E.; Gershinsky, G.; Aurbach, D.; Isnard, O. Crystallography of Chevrel Phases, MMo6T8 (M= Cd, Na, Mn, and Zn, T= S, Se) and Their Cation Mobility. Inorg. Chem. 2009, 48, 8751−8758. (93) Schollhorn, R.; Kumpers, M.; Besenhard, B. O. Topotactic Redox Reactions of the Channel Type Chalcogenides Mo3S4 and Mo3Se4. Mater. Res. Bull. 1977, 12, 781−788. (94) Gocke, E.; Schramm, W.; Dolscheid, P.; Schollhorn, R. Molybdenum Cluster Chalcogenides Mo6X8: Electrochemical Intercalation of Closed Shell Ions Zn2+, Cd2+, and Na+. J. Solid State Chem. 1987, 70, 71−81. 2639

DOI: 10.1021/acsenergylett.8b01552 ACS Energy Lett. 2018, 3, 2620−2640

ACS Energy Letters

Review

(95) Cheng, Y.; Luo, L.; Zhong, L.; Chen, J.; Li, B.; Wang, W.; Mao, S. X.; Wang, C.; Sprenkle, V. L.; Li, G.; et al. Highly Reversible Zinc-Ion Intercalation into Chevrel Phase Mo6S8 Nanocubes and Applications for Advanced Zinc-Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8, 13673−13677. (96) Chae, M. S.; Heo, J. W.; Lim, S. C.; Hong, S. T. Electrochemical Zinc-Ion Intercalation Properties and Crystal Structures of ZnMo6S8 and Zn2Mo6S8 Chevrel Phases in Aqueous Electrolytes. Inorg. Chem. 2016, 55, 3294−3301. (97) Liang, Y.; Jing, Y.; Gheytani, S.; Lee, K.-Y.; Liu, P.; Facchetti, A.; Yao, Y. Universal Quinone Electrodes for Long Cycle Life Aqueous Rechargeable Batteries. Nat. Mater. 2017, 16, 841−848. (98) Zhao, Q.; Huang, W.; Luo, Z.; Liu, L.; Lu, Y.; Li, Y.; Li, L.; Hu, J.; Ma, H.; Chen, J. High-Capacity Aqueous Zinc Batteries Using Sustainable Quinone Electrodes. Sci. Adv. 2018, 4, eaao1761. (99) Kundu, D.; Oberholzer, P.; Glaros, C.; Bouzid, A.; Tervoort, E.; Pasquarello, A.; Niederberger, M. Organic Cathode for Aqueous ZnIon Batteries: Taming a Unique Phase Evolution Toward Stable Electrochemical Cycling. Chem. Mater. 2018, 30, 3874−3881. (100) Li, H.; Liu, Z.; Liang, G.; Huang, Y.; Huang, Y.; Zhu, M.; Pei, Z.; Xue, Q.; Tang, Z.; Wang, Y.; et al. Waterproof and Tailorable Elastic Rechargeable Yarn Zinc Ion Batteries by a Cross-Linked Polyacrylamide Electrolyte. ACS Nano 2018, 12, 3140−3148. (101) Chao, D.; Zhu, C.; Song, M.; Liang, P.; Zhang, X.; Tiep, N. H.; Zhao, H.; Wang, J.; Wang, R.; Zhang, H.; et al. A High-Rate and Stable Quasi-Solid-State Zinc-Ion Battery with Novel 2D Layered Zinc Orthovanadate Array. Adv. Mater. 2018, 30, 1803181. (102) Wang, Z.; Ruan, Z.; Liu, Z.; Wang, Y.; Tang, Z.; Li, H.; Zhu, M.; Hung, T. F.; Liu, J.; Shi, Z.; et al. A Flexible Rechargeable Zinc-Ion Wire-Shaped Battery with Shape Memory Function. J. Mater. Chem. A 2018, 6, 8549−8557. (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. (104) Liu, Z.; Pulletikurthi, G.; Endres, F. A Prussian Blue/Zinc Secondary Battery with a Bio-Ionic Liquid− Water Mixture as Electrolyte. ACS Appl. Mater. Interfaces 2016, 8, 12158−12164. (105) Liu, Z.; Bertram, P.; Endres, F. Bio-Degradable Zinc-Ion Battery Based on a Prussian Blue Analogue Cathode and a Bio-Ionic LiquidBased Electrolyte. J. Solid State Electrochem. 2017, 21, 2021−2027. (106) Chae, M. S.; Heo, J. W.; Kwak, H. H.; Lee, H.; Hong, S. T. Organic Electrolyte-Based Rechargeable Zinc-Ion Batteries Using Potassium Nickel Hexacyanoferrate as a Cathode Material. J. Power Sources 2017, 337, 204−211. (107) Kundu, D.; Hosseini Vajargah, S.; Wan, L.; Adams, B.; Prendergast, D.; Nazar, L. F. Aqueous vs. Nonaqueous Zn-Ion Batteries: Consequences of the Desolvation Penalty at the Interface. Energy Environ. Sci. 2018, 11, 881−892. (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. (109) Baugh, L. M. Corrosion and Polarization Characteristics of Zinc in NeutralAcid Media  II. Effect of NH+4 Ions and the Role of Battery Zinc Alloying Constituents. Electrochim. Acta 1979, 24 (6), 669−677. (110) Baugh, L. M. Corrosion and Polarization Characteristics of Zinc in NeutralAcid Media  I. Pure Zinc in Solutions of Various Sodium Salts. Electrochim. Acta 1979, 24 (6), 657−667. (111) Chamoun, M.; Hertzberg, B. J.; Gupta, T.; Davies, D.; Bhadra, S.; Van Tassell, B.; Erdonmez, C.; Steingart, D. A. Hyper-Dendritic Nanoporous Zinc Foam Anodes. NPG Asia Mater. 2015, 7, e178. (112) Parker, J. F.; Chervin, C. N.; Pala, I. R.; Machler, M.; Burz, M. F.; Long, J. W.; Rolison, D. R. Rechargeable Nickel−3D Zinc Batteries: An Energy-Dense, Safer Alternative to Lithium-Ion. Science 2017, 356, 415−418. (113) Gonzalez, M. A.; Trocoli, R.; Pavlovic, I.; Barriga, C.; La Mantia, F. Layered Double Hydroxides as a Suitable Substrate to Improve the Efficiency of Zn Anode in Neutral pH Zn-Ion Batteries. Electrochem. Commun. 2016, 68, 1−4.

(114) Bani Hashemi, A.; Kasiri, G.; La Mantia, F. The Effect of Polyethyleneimine as an Electrolyte Additive on Zinc Electrodeposition Mechanism in Aqueous Zinc-Ion Batteries. Electrochim. Acta 2017, 258, 703−708. (115) Wang, F.; Borodin, O.; Gao, T.; Fan, X.; Sun, W.; Han, F.; Faraone, A.; Dura, J. A.; Xu, K.; Wang, C. Highly Reversible Zinc Metal Anode for Aqueous Batteries. Nat. Mater. 2018, 17, 543−549. (116) Wang, L.-P.; Li, N.-W.; Wang, T.-S.; Yin, Y.-X.; Guo, Y.-G.; Wang, C.-R. Conductive Graphite Fiber as a Stable Host for Zinc Metal Anodes. Electrochim. Acta 2017, 244, 172−177. (117) Li, H.; Xu, C.; Han, C.; Chen, Y.; Wei, C.; Li, B.; Kang, F. Enhancement on Cycle Performance of Zn Anodes by Activated Carbon Modification for Neutral Rechargeable Zinc Ion Batteries. J. Electrochem. Soc. 2015, 162 (8), A1439−A1444. (118) Sun, K. E. K.; Hoang, T. K. A.; Doan, T. N. L.; Yu, Y.; Zhu, X.; Tian, Ye.; Chen, P. Suppression of Dendrite Formation and Corrosion on Zinc Anode of Secondary Aqueous Batteries. ACS Appl. Mater. Interfaces 2017, 9, 9681−9687. (119) Kang, L.; Cui, M.; Jiang, F.; Gao, Y.; Luo, H.; Liu, J.; Liang, W.; Zhi, C. Nanoporous CaCO3 Coatings Enabled Uniform Zn Stripping/ Plating for Long-Life Zinc Rechargeable Aqueous Batteries. Adv. Energy Mater. 2018, 8, 1801090. (120) Shen, C.; Li, X.; Li, N.; Xie, K.; Wang, J.; Liu, X.; Wei, B. Graphene-Boosted, High-Performance Aqueous Zn-Ion Battery. ACS Appl. Mater. Interfaces 2018, 10, 25446−25453. (121) Ma, L.; Chen, S.; Li, H.; Ruan, Z.; Tang, Z.; Liu, Z.; Wang, Z.; Huang, Y.; Pei, Z.; Zapien, J. A.; et al. Initiating a Mild Aqueous Electrolyte Co3O4/Zn Battery with 2.2 V-High Voltage and 5000-Cycle Lifespan by a Co(III) Rich-Electrode. Energy Environ. Sci. 2018, 11, 2521. (122) Pan, C.; Zhang, R.; Nuzzo, R. G.; Gewirth, A. A. ZnNixMnxCo2−2xO4 Spinel as a High-Voltage and High-Capacity Cathode Material for Nonaqueous Zn-Ion Batteries. Adv. Energy Mater. 2018, 8, 1800589. (123) Levi, E.; Levi, M. D.; Chasid, O.; Aurbach, D. A Review on the Problems of the Solid State Ions Diffusion in Cathodes for Rechargeable Mg Batteries. J. Electroceram. 2009, 22, 13−19.

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DOI: 10.1021/acsenergylett.8b01552 ACS Energy Lett. 2018, 3, 2620−2640