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Recent Advances in Aqueous Zinc-ion Batteries Guozhao Fang, Jiang Zhou, Anqiang Pan, and Shuquan Liang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01426 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 7, 2018
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ACS Energy Letters
Recent Advances in Aqueous Zinc-ion Batteries Guozhao Fang a, Jiang Zhou a,b*, Anqiang Pan a,b and Shuquan Liang a,b* a
School of Materials Science and Engineering, Central South University, Changsha 410083, P. R. China
b
Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Central South
University, Changsha 410083, P. R. China
Abstract: Although current high-energy-density lithium-ion batteries (LIBs) have taken over the commercial rechargeable battery market, the increasing concerns about limited lithium resources, highcost, and insecurity of organic electrolyte scale up limit their further development. Rechargeable aqueous zinc-ion batteries (ZIBs), the alternative battery chemistry, have paved the way not only for realizing environmentally benign and safe energy storage devices, but also reducing the manufacturing costs of next-generation batteries. This review underscores recent advances in aqueous ZIBs; these include the design of high reversible Zn anode, optimization of electrolyte, and a wide range of cathode materials and their energy storage mechanisms. We also present recent advanced techniques that aim at overcoming the current issues in aqueous ZIBs system. This review on the future perspectives and research directions will provide a guide for future aqueous ZIBs study.
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1. Introduction Electrochemical energy storage devices, such as rechargeable batteries, are critical for overcoming the worldwide energy challenge.1 Due to their high energy density and long-cycle-life, the nonaqueous lithium-ion batteries (LIBs) have dominated the portable electronics and emerging electric/hybrid vehicles, and now are considered as the possible choice for future electric vehicles and grid-scale energy storage systems.2 However, the increasing concerns about limited lithium resources, high-cost, and safety issue strongly limit their further development for large-scale applications.3 Sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) are plausible alternatives to LIBs since these devices are based on relatively abundant and cheap sodium (potassium) elements as well as their similar chemical properties to lithium, but they suffer from the low energy density, highly toxic and flammable electrolyte, high operating cost and security issue.4-6 The drawbacks of these organic-based systems motivate us to explore the alternative battery chemistry with low-cost, high safety, and long-cycle-life. Aqueous rechargeable batteries are a promising class of batteries for grid-scale electrochemical energy storage due to their low-cost, high operational safety and environmental benignity.7 Furthermore, the aqueous electrolytes offer two orders of magnitude higher ionic conductivities (~1 S cm-1) than that of nonaqueous electrolytes (~1–10 mS cm-1).8 So far, a variety of aqueous batteries based on naturally abundant alkali metal cations (e.g. Na+ and K+),9, 10 and multivalent charge carriers (e.g. Mg2+, Al3+ and Zn2+) are under development.11 Obviously, the systems employing multivalent ions can, in principle, achieve higher specific capacity and energy density due to the multiple electrons involved in redox reactions.12,
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The development of aqueous magnesium-ion batteries (MIBs) is restricted since the
available cathode materials for this system have been limited to very few compounds owing to the sluggish Mg2+ diffusion in host lattices.14 In addition, the passivation of the Mg anode greatly prevents further transport of Mg2+ ions, which needs to find a usable anode material to build more powerful 2
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aqueous MIBs.11, 15, 16 Aqueous aluminum-ion batteries (AIBs) have also been proposed owing to the high volumetric capacity (ca. 8040 mA h cm−3) and good gravimetric capacity (ca. 2980 mA h g−1) of aluminum anode.17-19 Nonetheless, the formation of protective oxide (Al2O3) film on anode in aqueous electrolytes (4 < pH < 8) accompanied by a decrease in cell efficiency and electrode potential, as well as tendency of aluminum to corrode unevenly,19-21 make aqueous AIBs still in the experimental stage and are limited to the development of primary high-energy systems. In this regard, aqueous zinc-ion batteries (ZIBs) with mild neutral pH (or slightly acidic) electrolyte hold particular promise for grid-scale energy storage.22 Since Volta et al. employed metallic zinc (Zn) in the first battery in 1799, Zn anode has been regarded as an ideal negative electrode in various primary and secondary Zn-based batteries such as Zn-Mn, Ni-Zn, and Zn-air batteries due to its high theoretic capacity, relatively low redox potential, low cost, and high safety.23-25 Among them, alkaline Zn/MnO2 batteries show great potential and have become dominant in primary battery chemistry.8, 26 Later efforts were paid to develop rechargeable alkaline Zn-MnO2 batteries, unfortunately, they were plagued by the poor cycle life and inferior discharge performance due to the formation of zinc dendrite and irreversible discharged species.27-29 In the year of 1988, Shoji et al. firstly reported a rechargeable aqueous Zn-MnO2 batteries using mild neutral or slightly acidic electrolyte (ZnSO4 aqueous electrolyte),30 which opens the door of aqueous zinc-ion batteries (ZIBs). So far, many significant works of aqueous ZIBs have been reported in the literatures including metallic zinc (Zn) anode,31, 32 and cathode materials (e.g. manganese-based oxides,22, 33-35 prussian blue analogs,36, 37 vanadium-based oxides,38-40 polyanion compounds,41, 42 chevrel phase compounds,43, 44 sustainable quinone analogs,45, 46 etc.). Recently, Fan et al. have summarized part of cathode material and their reaction mechanisms as well as electrolytes.47 In this review, the energy storage mechanisms in these systems are systematically analyzed and summarized. Furthermore, the merits and issues as 3
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well as the optimization strategies related to various cathodes, Zn anode and aqueous electrolytes are discussed. Finally, some potential future research directions from our personal perspectives or other related research topics are also presented. This review provides a comprehensive overview focusing on the recent progress, challenges, and future perspectives in aqueous ZIBs.
2. Energy Storage Mechanisms in Aqueous ZIBs Systems Unlike the well-established lithium/sodium-ion based energy storage chemistries (e.g. insertion/conversion-/and alloying reaction mechanism) to storage monovalent alkali metal cations,48 the reaction mechanisms in aqueous ZIBs system are complicated and controversial. So far, the reaction mechanisms involved remain a topic of discussion and are under developed. According to the previous literatures, the redox reactions in aqueous ZIBs system mainly involve three mechanisms: Zn2+ insertion/extraction,22, 38, 49 chemical conversion reaction,50 and H+/Zn2+ insertion/extraction,51, 52 which are summarized and discussed in this section.
2.1. Zn2+ Insertion/Extraction Mechanism Many compounds with tunnel-type and layered-type structure enable the insertion/extraction of Zn2+ ions into/from their hosts owing to the small ionic radii of Zn2+ (0.74 Å). For example, Kang et al. conducted an aqueous ZIB, which is composed of an α-MnO2 cathode, a zinc anode, and a mild ZnSO4 or Zn(NO3)2 aqueous electrolyte (Figure 1A),22 demonstrating that the charge storage mechanism in aqueous Zn/α-MnO2 system is based on the migration of Zn2+ ions between tunnels of α-MnO2 cathode and Zn anode, namely a typical Zn2+ insertion/extraction reaction mechanism as follow: Cathodic process: Zn2+ +2e- +2α-MnO2 ↔ZnMn2 O4 (1) Anodic process: 4
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Zn↔Zn2+ +2e-
(2)
Soon afterwards, Kang’s group further revealed the Zn2+ insertion into the host of α-, β-, γ-, and δtype MnO2 with different tunnel structures as their main charge storage mechanism.53 It should be noted that some studies also focused on Zn2+ insertion into the α-MnO2 host, but they demonstrated a phase transition from tunneled structure (α-MnO2) to layered Zn-buserite or birnessite,54, 55 not a spinel ZnMn2O4 phase mentioned above. Kim et al. reported an aqueous Zn/γ-MnO2 cell, in which γ-MnO2 undergoes a complex multiphase transformation on successive insertion of Zn2+.49 As shown in Figure 1B, in the early stages, partial γ-MnO2 is transferred into spinel-type ZnMn2O4. The further inserted Zn2+ ions occupy the 1×2 tunnels of γ-MnO2 to form a tunnel-type γ-ZnxMnO2 phase. As more Zn2+ ions are inserted, Zn-containing tunnels in a portion of γ-ZnxMnO2 tend to expand and open-up the structural framework to form a layered-type L-ZnyMnO2 phase. For the β-MnO2, it seems to hardly incorporate Zn2+ ions due to its narrow 1×1 tunnels,53 however, Chen et al. elucidated that a layer-type B-ZnxMnO2·nH2O phase was generated during the initial discharge, followed by reversible insertion/extraction of Zn2+ ions in this layered structure (Figure 1C).56 In some of them, the insertion/extraction of Zn2+ is homogeneous (e.g. α-, and β-MnO2), while in others, it is heterogeneous (e.g. γ-MnO2). The different inserted behaviors of Zn2+ ions in these MnO2 may be attributed to the difference in crystallographic polymorphs that change the ion insertion thermodynamics and kinetics.57 Except for MnO2, other manganese-based oxides are also investigated. For instance, Chen et al. reported a cation-deficient spinel ZnMn2O4 cathode with highly reversible insertion/extraction of Zn2+.33 The ex-situ XRD patterns were conducted at different charge/discharge states to elucidate the evolution of ZnMn2O4 cathode. As shown in Figure 1D, the diffraction peaks gradually shift to higher angle when charged from 0.8 to 1.9 V, demonstrating the shrink of lattice spacing after the extraction of Zn2+. As expected, the diffraction peaks are recovered upon full discharging back to 0.8 V, suggesting good reversibility of the ZnMn2O4 cathode. 5
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Figure 1 Typical Examples of Manganese-Based Electrode Materials Based on Zn2+ Insertion/Extraction Mechanism. (A) Schematics of the chemistry of the aqueous ZIBs, in which the migration of Zn2+ ions are between tunnels of α-MnO2 cathode and Zn anode. Reprinted with permission.22 Copyright 2012, WileyVCH. (B) Schematic illustration of Zn2+ insertion in γ-MnO2 cathode. Reprinted from reference
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. (C)
Schematic illustration of reactions in aqueous Zn/β-MnO2 cell. Reprinted with permission.56 Copyright 2017, Macmillan Publishers. (D) Ex-situ XRD patterns of ZnMn2O4 cathode. Reprinted from reference 33.
In addition to the progress of manganese-based oxides, other materials with Zn2+ insertion reaction mechanism are also being intensively studied. Nazar et al. opened a new chapter in Zn/vanadium-based oxides battery, which is constructed by a Zn0.25V2O5·nH2O cathode with a Zn2+ (de)intercalation mechanism.38 At the initial state, the structure of pristine Zn0.25V2O5·nH2O was modified via water intercalation upon immersion in the electrolyte, which results in an increase in the interlayer distance, thus expands the galleries for facile Zn2+ intercalation. During the discharge/charge reaction proceeds, the insertion/extraction of 1.1 Zn2+ ions into/from Zn0.25V2O5·nH2O is highly reversible, corresponding to the following equation: 6
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Zn0.25 V2 O5 ·nH2 O+1.1Zn2+ +2.2e- ↔Zn1.35 V2 O5 ·nH2 O (3) However, this process is complex as elucidated from an operando XRD investigation (Figure 2A). In detail, two solid-solution regimes (0.25
