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• 1. PROSPECTS FOR MnO₂-BASED AQUEOUS Zn-ION BATTERY

  Aqueous rechargeable zinc-ion batteries (AZIBs) have been considered as promising alternatives to lithium-ion batteries for electrochemical energy storage. There are two typical overwhelming advantages that make AZIBs stand out among the various electrochemical energy storage systems (organic alkaline ion batteries, e.g., Na⁺ and K⁺, and aqueous batteries, e.g., Na⁺, K⁺, Mg²⁺, and Al³⁺). Firstly, some advantages come from the utilization of aqueous electrolyte. The aqueous electrolytes could deliver 2 orders of magnitude higher ionic conductivities (~ 1 S cm^-1) than the non-aqueous electrolytes (~1-10 mS cm^-1)^[1]. Moreover, aqueous electrolyte reduces the fabrication cost comparing with the usage of organic solvents. In addition, it is an important point that the high operational safety and environmental friendliness derived from aqueous electrolyte endows AZIBs with strong competitiveness. Secondly, zinc metal can directly be used as the anode in the AZIBs, which makes the AZIBs more promising than other aqueous batteries (Al³⁺, Mg²⁺, Na⁺, K⁺). Zn anode features the low oxidation-reduction potential of Zn²⁺/Zn (-0.76 V vs. standard 2H⁺/H₂ potential), which can deliver higher energy density^[2]. Moreover, the low-cost Zn metal can decrease the manufacturing cost.

  To date, various electrode materials have been developed for AZIBs, such as Prussian blue analogues^[3], metal oxides (MnO₂, Co₃O₄)^{[4, 5]}, vanadium oxides (V₂O₅·H₂O) and their derivatives^{[6, 7]}, polyanion compounds^[8], organic materials (polyaniline)^[9], etc. The energy storage mechanisms in AZIBs are complicated and controversial. So far, there are two main redox reactions in AZIBs: Zn²⁺ insertion/extraction and the chemical conversion reaction. According to the storage mechanism, different electrolytes are chosen for the AZIBs. The conversion-type cathodes need an alkaline electrolyte, while the insertion-type ones use neutral (or slightly acidic, ZnSO₄) electrolytes. In comparison, the insertion-type cathodes that use the neutral electrolytes are more environmentally friendly. In the case of the insertion-type mechanism, many com-pounds with tunnel-type and layered-type structure, such as Prussian blue analogues, and manganese oxide-based and vanadium oxide-based materials, enable the insertion/extraction of Zn²⁺ ions into/from their hosts due to the small ionic radius of Zn²⁺ (0.74 Å). Among these insertion-type cathode candidates, manganese oxides are more ideal candidates for Zn-ion storage owing to their intrinsic properties of low cost and environmental friendliness. MnO₂ has a variety of complicated crystal structures, however, giving rise to enormous effects on the electrochemical performance of AZIBs in terms of Zn-ion storage.

  2. ENORMOUS EFFECTS OF THE MnO₂ STRUCTURE ON THE ELECTROCHEMICAL PERFORMANCE

  MnO₂ has various crystal structures, including λ-MnO₂, δ-MnO₂, β-MnO₂, ɑ-MnO₂, γ-MnO₂, and R-type and todorokite MnO₂, as shown in Fig. 1. The basic crystal structural unit is the MO₆ octahedron, which is composed of six oxygen atoms and one manganese atom. These MnO₂ structures are classified into three main types: tunnel-structured (β-MnO₂, ɑ-MnO₂, γ-MnO₂, and todorokite MnO₂), layered structured (δ-MnO₂), and spinel structured (λ-MnO₂). The tunnel size depends on the number of interconnected MnO₆ octahedra. β-MnO₂ presents [1×1] tunnels along the c-axis, as shown in Fig. 1(a); ɑ-MnO₂ shows a large [2×2] tunnel size with four interconnected MnO₆ octahedra (Fig. 1(b)); γ-MnO₂ grows along the b-axis to form randomly arranged [1×1] and [1×2] hybrid tunnels (Fig. 1(c)); todorokite MnO₂ presents larger [3×3] tunnels, as shown in Figure 1(d). δ-MnO₂ presents a layered structure with interlayer spacing of ~ 7 Å (Fig. 1(e)).

  Figure 1

  

  Figure 1.  Schematic illustration of the crystal structures of MnO₂: (a) β-MnO₂; (b) ɑ-MnO₂; (c) λ-MnO₂; (d) todorokite MnO₂; (e) δ-MnO₂; (f) γ-MnO₂; (g) R-MnO₂^[10]

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  The crystal structure of MnO₂ has an enormous effect on the Zn²⁺ storage performance. Among these MnO₂ polymorphs, λ-MnO₂, β-MnO₂ and todorokite MnO₂ are not ideal hosts for Zn²⁺ insertion owing to their limited three-dimensional (3D) tunnel structures. Although todorokite MnO₂ has large tunnels, the tunnels are already occupied by a number of Mn²⁺ ions, which affects the average oxidation of manganese, resulting in its low theoretical capacity of 99 mAh·g^-1[11]. β-MnO₂ is intrinsically electrochemically inactive for Zn-ion storage. The electrochemical performances of these structures for Zn²⁺ storage, however, could be improved by modulating them through the following approaches: 1) exposing the preferred crystal orientation; 2) introducing oxygen defects into the structure; and 3) doping exotic atoms or molecules (K⁺, polyaniline) into the MnO₂ framework to widen the interlayer spacing. Kim et al. were the first to explore the proposal that tunnel-type β-MnO₂ nanorods with exposed (101) planes could present excellent Zn²⁺ storage performance. In contrast to its bulk counterpart which showed no electrochemical reactivity, the β-MnO₂ nanorod electrode exhibited a high discharge capacity of 270 mAh·g^-1 at the 100 mA·g^-1 rate and long cycling stability (75% capacity retention) over 200 cycles (Fig. 2(a, b))^[12] because the exposure of these specific planes of the crystal structure promoted facile ionic transport during the electrochemical reaction. Generally, Zn²⁺ is difficult to intercalate into a MnO₂ host due to the strong electrostatic interaction between Zn²⁺ and the MnO₂ lattice. Once Zn²⁺ is adsorbed onto the MnO₂ surface, the strong chemical bonds between Zn and O would hinder the subsequent Zn²⁺ desorption process and these un-desorbed Zn²⁺ ions would eventually act as a physical barrier, reducing the available electroche-mically active surface area of MnO₂. As a result, the rate capability of MnO₂ is not ideal. It was reported that the introduction of oxygen vacancies into MnO₂ would be a feasible approach for developing high-performance cathodes for zinc-ion batteries^[13]. According to the first principles calculations, the Gibbs free energies of Zn²⁺ adsorption are close to thermoneutral values (~0.05 eV) for oxygen-deficient MnO₂. In comparison, the Gibbs free energy of Zn²⁺ adsorption onto MnO₂ without oxygen vacancies is significantly reduced to about –3.31 eV (Fig. 2(c)). This indicates that Zn²⁺ adsorption/desorption onto/from oxygen-deficient MnO₂ would be more reversible than for MnO₂ without oxygen vacancies, enhancing the electrochemical performance of MnO₂ cathode. Manganese dissolution, which gives rise to sharp capacity decay, is still a major issue for MnO₂ as cathode for aqueous zinc-ion batteries. Liang's group demonstrated that introducing K⁺ ions into the structure could stabilize the Mn-based cathode^[16]. In order to characterize the dissolution of Mn element, inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis was conducted to test the Mn concentration in 2 M ZnSO₄ electrolyte. The results showed that the α-MnO₂ exhibited fast dissolution of Mn element during cycling and that the level of dissolved Mn in the electrolyte was much higher than that for K_0.1MO₂ (Fig. 2(f)), suggesting that Mn dissolution is suppressed as the K⁺ ions are intercalated into the structure and bonded with the MnO₆ polyhedra (Fig. 2(e)). Usually, MnO₂ hosts suffer from serious structural transformation during cycling processes, leading to significant capacity fading. Huang et al. designed polyaniline-intercalated layered manganese dioxide (PANI-MnO₂), which could efficiently eliminate the phase transformation induced by Zn²⁺-insertion. As a result, it showed excellent cycling performance with high capacity of 280 mAh·g^-1 (Fig. 2(i))^[16].

  Figure 2

  

  Figure 2.  (a) Schematic illustration of (101) lattice planes in the β-MnO₂ structure^[12]. (b) Cycling performance of the β-MnO₂ with (101) planes as cathode in Zn-ion batteries^[12]. (c) Calculated adsorption energies for Zn²⁺ on the surface of perfect σ-MnO₂ and that with oxygen vacancies. V_O represents oxygen vacancy^[13]. (d) Ragone plots of σ-MnO₂ with and without V_O in Zn-ion batteries^[13]. (e) Incorporated K⁺ ions stabilized Mn polyhedra^[14]. (f) Element analysis of dissolved Mn²⁺ in 2 M ZnSO₄ aqueous electrolyte during cycling of K_0.1MnO₂ and ɑ-MnO₂^[14]. (g) Expanded structure of polyaniline (PANI)-intercalated MnO₂ layers^[15]. (h) High resolution transmission electron microscope (TEM) image of PANI-intercalated MnO₂ layers^[15], and (i) cycling performance of PANI-intercalated MnO₂^[15]

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  In summary, MnO₂-based AZIBs have some issues that need to be addressed, including Mn dissolution, phase transformation, and strong electrostatic interaction of Zn²⁺ with the host lattice, which are restricting the electrochemical performance of MnO₂. Their performance is determined by the crystal structure of their materials. Thus, there are some approaches that operate through structural modulation to improve the electrochemical performance of MnO₂-based AZIBs: 1) exposing the preferred crystal orientation; 2) introducing oxygen defects into the structure; and 3) doping exotic atoms or molecules (K⁺, polyaniline) into the MnO₂ framework to widen the interlayer spacing. As a result, properties including Zn-ion transport, phase transformation, and electrostatic interaction with Zn²⁺ could be effectively improved, giving rise to enhanced Zn²⁺ storage performance (high rate capability, cycling stability, etc.).

  
  
• 1. [1]

     Fang, G.; Zhou, J.; Pan, A.; Liang, S. Recent advances in aqueous zinc-ion batteries. ACS Energy Lett. 2018, 3, 2480–2501. doi: 10.1021/acsenergylett.8b01426

  2. [2]

     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. doi: 10.1002/adma.201800762

  3. [3]

     Yang, Q.; Mo, F.; Liu, Z.; Ma, L.; Li, X.; Fang, D.; Chen, S.; Zhang, S.; Zhi, C. Activating C-coordinated iron of iron hexacyanoferrate for Zn hybrid-ion batteries with 10000-cycle lifespan and superior rate capability. Adv. Mater. 2019, 31, 1901521.

  4. [4]

     Mo, F.; Liang, G.; Meng, Q.; Liu, Z.; Li, H.; Fan, J.; Zhi, C. A flexible rechargeable aqueous zinc manganese-dioxide battery working at –20 ℃. Energy Environ. Sci. 2019, 12, 706–715. doi: 10.1039/C8EE02892C

  5. [5]

     Wang, X.; Wang, F.; Wang, L.; Li, M.; Wang, Y.; Chen, B.; Zhu, Y.; Fu, L.; Zha, L.; Zhang, L.; Wu, Y.; Huang, W. An aqueous rechargeable Zn//Co₃O₄ battery with high energy density and good cycling behavior. Adv. Mater. 2016, 28, 4904–4911. doi: 10.1002/adma.201505370

  6. [6]

     Yan, M.; He, P.; Chen, Y.; Wang, S.; Wei, Q.; Zhao, K.; Xu, X.; An, Q.; Shuang, Y.; Shao, Y.; Mueller, K. T.; Mai, L.; Liu, J.; Yang, J. Water-lubricated intercalation in V₂O₅·nH₂O for high-capacity and high-rate aqueous rechargeable zinc batteries. Adv. Mater. 2018, 30, 1703725. doi: 10.1002/adma.201703725

  7. [7]

     Yang, Y.; Tang, Y.; Fang, G.; Shan, L.; Guo, J.; Zhang, W.; Wang, C.; Wang, L.; Zhou, J.; Liang, S. Li⁺ intercalated V₂O₅·nH₂O with enlarged layer spacing and fast ion diffusion as an aqueous zinc-ion battery cathode. Energy Environ. Sci. 2018, 11, 3157–3162. doi: 10.1039/C8EE01651H

  8. [8]

     Wan, F.; Zhang, Y.; Zhang, L.; Liu, D.; Wang, C.; Song, L.; Niu, Z.; Chen, J. Reversible oxygen redox chemistry in aqueous zinc-ion batteries. Angew. Chem. Int. Ed. 2019, 58, 7062–7067. doi: 10.1002/anie.201902679

  9. [9]

     Shi, H. Y.; Ye, Y. J.; Liu, K.; Song, Y.; Sun, X. A long cycle-life self-doped polyaniline cathode for rechargeable aqueous zinc batteries. Angew. Chem. Int. Ed. 2018, 57, 16359–16363. doi: 10.1002/anie.201808886

  10. [10]

      Tang, B.; Shan, L.; Liang, S.; Zhou, J. Issues and opportunities facing aqueous zinc-ion batteries. Energy Environ. Sci. 2019, 12, 3288–3304. doi: 10.1039/C9EE02526J

  11. [11]

      Lee, J.; Ju, J. B.; Cho, W. I.; Cho, B. W.; Oh, S. H. Todorokite-type MnO₂ as a zinc-ion intercalating material. Electrochim. Acta 2013, 112, 138–143. doi: 10.1016/j.electacta.2013.08.136

  12. [12]

      Islam, S.; Alfaruqi, M. H.; Mathew, V.; Song, J.; Kim, S.; Kim, S.; Jo, J.; Baboo, J. P.; Pham, D. T.; Putro, D. Y.; Sun, Y. K.; Kim, J. Facile synthesis and the exploration of the zinc storage mechanism of β-MnO₂ nanorods with exposed (101) planes as a novel cathode material for high performance eco-friendly zinc-ion batteries. J. Mater. Chem. A 2017, 5, 23299–23309. doi: 10.1039/C7TA07170A

  13. [13]

      Xiong, T.; Yu, Z. G.; Wu, H.; Du, Y.; Xie, Q.; Chen, J.; Zhang, Y. W.; Pennycook, S. J.; Lee, W. S. V.; Xue, J. Defect engineering of oxygen-deficient manganese oxide to achieve high-performing aqueous zinc ion battery. Adv. Energy Mater. 2019, 9, 1803815. doi: 10.1002/aenm.201803815

  14. [14]

      Trócoli, R.; La Mantia, F. An aqueous zinc-ion battery based on copper hexacyanoferrate. ChemSusChem. 2015, 8, 481–485. doi: 10.1002/cssc.201403143

  15. [15]

      Huang, J.; Wang, Z.; Hou, M.; Dong, X.; Liu, Y.; Wang, Y.; Xia, Y. Polyaniline-intercalated manganese dioxide nanolayers as a high-performance cathode material for an aqueous zinc-ion battery. Nat. Commun. 2018, 9, 2906. doi: 10.1038/s41467-018-04949-4

  16. [16]

      Fang, G.; Zhu, C.; Chen, M.; Zhou, J.; Tang, B.; Cao, X.; Zheng, X.; Pan, A.; Liang, S. Suppressing manganese dissolution in potassium manganate with rich oxygen defects engaged high-energy-density and durable aqueous zinc-ion battery. Adv. Funct. Mater. 2019, 29, 1808375. doi: 10.1002/adfm.201808375

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  Figure 1  Schematic illustration of the crystal structures of MnO₂: (a) β-MnO₂; (b) ɑ-MnO₂; (c) λ-MnO₂; (d) todorokite MnO₂; (e) δ-MnO₂; (f) γ-MnO₂; (g) R-MnO₂^[10]

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  Figure 2  (a) Schematic illustration of (101) lattice planes in the β-MnO₂ structure^[12]. (b) Cycling performance of the β-MnO₂ with (101) planes as cathode in Zn-ion batteries^[12]. (c) Calculated adsorption energies for Zn²⁺ on the surface of perfect σ-MnO₂ and that with oxygen vacancies. V_O represents oxygen vacancy^[13]. (d) Ragone plots of σ-MnO₂ with and without V_O in Zn-ion batteries^[13]. (e) Incorporated K⁺ ions stabilized Mn polyhedra^[14]. (f) Element analysis of dissolved Mn²⁺ in 2 M ZnSO₄ aqueous electrolyte during cycling of K_0.1MnO₂ and ɑ-MnO₂^[14]. (g) Expanded structure of polyaniline (PANI)-intercalated MnO₂ layers^[15]. (h) High resolution transmission electron microscope (TEM) image of PANI-intercalated MnO₂ layers^[15], and (i) cycling performance of PANI-intercalated MnO₂^[15]

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