English

• 1. INTRODUCTION

  The aqueous rechargeable batteries are a pro-mising class of batteries due to their high opera-tional safety, low-cost, and environmental benignity. To date, large amount of researches on aqueous batteries based on alkali metal cations (e.g., Na⁺ and K⁺) and multivalent charge carriers (e.g., Mg²⁺, Al³⁺, and Zn²⁺) have been reported^{[1, 2]}. Among all aqueous batteries, rechargeable Zn/MnO₂ batteries with mild ZnSO₄ or Zn(CF₃SO₃)₂ aqueous electrolytes presented outstanding advantages, including low cost, high energy density (~820 mAh g^-1 for Zn anode, and ~308 mAh g^-1 for 1 e^- transfer of MnO₂ cathode), high operating potential (~1.35 V), as well as high safety and environmental friendliness^[3]. Although the alkaline Zn-MnO₂ batteries have become dominant in primary battery market, the rechar-geable mild Zn-MnO₂ batteries are still in the experimental stage, and are plagued by their capacity fading problem due to issues in both sides of MnO₂ cathode (Mn²⁺ dissolution, phase conversion, collapse of layered structure, and formation of inactive ZnMn₂O₄) and zinc anode (hydrogen evolution, zinc dendrite, and corrosion).

  In the past 10 years, researches focusing on improving battery performance of MnO₂ cathode in aqueous Zn batteries (ZIBs) have been extensively conducted. Various manganese oxide phases have been reported as host materials for H⁺/Zn²⁺ insertion in a mild aqueous electrolyte, including α-, β-, γ-, δ-, and λ-MnO₂, etc^[4]. This diverse phase structure of MnO₂ influences greatly on their electrochemical reactions during cycling. Despite the variation of electrochemical reactions, reasons for the capacity fading problems of manganese oxides during cycling are nearly the same, i. e., the formation of electrochemically inactive ZnMn₂O₄. Thus, in this perspective, we provided a brief review on the phase structures, charge storage mechanisms, as well as the capacity fading issues of MnO₂ in mild aqueous ZIBs. Besides, the strategies applied for improving battery performance of MnO₂ cathode are also demonstrated. Finally, some potential future research directions based on our personal perspec-tives and related research topics from other researchers are also demonstrated. This article provides a comprehensive overview focusing on recent progress and future perspectives of MnO₂ cathode for developing superior aqueous ZIBs.

  2. SOME ISSUES FOR MnO₂ELECTRODES

  For a MnO₂ cathode in ZIBs, some very striking features are listed as follows: diverse phase structure, phase transition during cycling, multiple charge storage mechanisms, as well as capacity fading issues. It is well known that MnO₂ exhibits a lot of crystal structures, typically including α-MnO₂ with [2×2] tunnels, β-MnO₂ with [1×1] tunnels, γ-MnO₂ with [1×2] tunnels, δ-MnO₂ with layer structure, λ-MnO₂ with 3D structure, and so on (Fig. 1a). The diverse crystallographic forms of MnO₂ are attri-buted to the different arrangement of [MnO₆] octahedra which are linked by sharing their edges or corners to form the tunnel, layer, or 3D structures. Some cations reside in the [2×2] tunnels of α-MnO₂ or interlayer space of δ-MnO₂, including Na⁺, K⁺, NH⁴⁺, Mg²⁺, Cs⁺, H₂O, etc., which play an important role in stabilizing the tunnel or layer structures, and greatly affect the battery performance of MnO₂ cathode^[4]. Besides, even though the manganese oxides present similar crystal structures, slight dif-ferences exist due to the different synthesis methods, hydrothermal temperature, annealing time, elec-trolyte addictive, as well as the choices of oxidant (KMnO₄, (NH₄)₂S₂O₈, etc.) and reductant (MnSO₄, carbon, ethanol, etc.)^{[5, 6]}. These slight differences are inflected in the following aspects: chemical valence of Mn, crystal orientation, amount of pre-inserted H₂O or other cations, micro-morphology, the BET surface area, and so forth, and present vital effect on tuning the electrochemical reactions of MnO₂ during cycling. Looking through the reported literatures, α- and δ-MnO₂ have been mostly investigated as cathode in ZIBs because of their large open tunnels and large interlayer spacing facilitating the H⁺/Zn²⁺ intercalation/extraction processes^[7-11]. However, for the industrialization development of mild aqueous Zn/MnO₂ batteries, γ-MnO₂ also displays high research value due to its possibility of mass produc-tion through electrolytic or chemical methods.

  Figure 1

  

  Figure 1.  Examples illustrating characteristics of MnO₂ cathode. (a) Multiple phase structures of MnO₂; (b) Schematic illustrating the phase transition mechanism of α-MnO₂ during Zn²⁺ intercalation process. From ref.^[8], copyright of nature; (c) TEM morphology and EDS mapping of Mn, O and Zn for β-MnO₂ nanofiber after long-term cycle, and the schematic of ZnMn₂O₄ structure. From ref.^[14], copyright of elsevier; (d) H⁺/Zn²⁺ synthetic intercalation mechanism for a manganese oxide nanosheet. From ref.^[22], copyright of Wiley

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  For MnO₂ with tunnel structures, such as α-MnO₂, β-MnO₂ and γ-MnO₂, some researchers have repor-ted the existence of phase transitions between tunnel structures and layer or/and spinel structures during cycling (Fig. 1b)^{[9, 12,13]}. This phase conversion upon cycling is very harmful to the cycling stability of MnO₂ cathode, and the reasons may be as follows: a) the structure collapse of MnO₂ due to the internal microscopic stresses generated from repeated phase transitions^[13], b) the inevitable Mn²⁺ dissolution from tunnel walls of MnO₂ during discharging^[9], and c) the formation of spinel-type ZnMn₂O₄ with weak electrochemical activity (Fig. 1c)^[14]. Based on this standpoint, researchers wish to find a cathode ma-terial with well maintained crystal structure during cycling, for example, the layer-type δ-MnO₂^[15], spinel-type Mn₃O₄ and ZnMn₂O₄^[16], etc. However, when applied as cathode in ZIBs, the δ-MnO₂ suffers from poor rate properties, and Mn₃O₄ or ZnMn₂O₄ presents low capacity delivery. To resolve the above issues, strategies promoting battery performance of δ-MnO₂ and ZnMn₂O₄ have been proposed in recent years, which will be described in the next section.

  Except phase transition issues during cycling, the charge storage mechanism is also important for achieving high-performance of MnO₂ cathode in ZIBs. Single Zn²⁺ insertion^[17] and single H⁺ insertion^[18] for α-MnO₂ cathodes have been pro-posed in 2015 and 2017, respectively. Nowadays, it has been widely accepted that H⁺ and Zn²⁺ are co-inserted into MnO₂ cathode during discharge in a mild aqueous media (Fig. 1d)^[19]. However, con-troversies remain on the insertion sequence of H⁺ and Zn²⁺. For example, Wang’s group reported a subsequent H⁺ and Zn²⁺ insertion mechanism for an electrodeposited MnO₂^[20]; Liu’s group presented a non-diffusion controlled Zn²⁺ intercalation and subsequent H⁺ conversion reaction for δ-MnO₂^[21]; and our group proposed a H⁺/Zn²⁺ synthetic intercalation mechanism for a novel phase of man-ganese oxide^[22]. Despite these differences, we find that all manganese oxides with H⁺/Zn²⁺ co-insertion mechanisms exhibit superior rate and capacity properties. Especially, the high rate performance of MnO₂ is mainly attributed to the capacity con-tribution of H⁺ insertion. However, when applying H⁺ insertion, the electrode integrity of MnO₂ cathode can be destroyed by the repeated generation and diminish of the by-products (Zn₄(OH)₆(SO₄)·5H₂O) during cycling^[23], which is bad for cycling stability of MnO₂ cathode. By tuning the H⁺/Zn²⁺ insertion ratio, a balance among rate performance, capacity property, and cycling stability can be obtained to achieve a high-performance MnO₂ cathode, which is a very valuable research direction for ZIBs in the future.

  3. STRATEGIES TO PROMOTE ELECTROCHEMISTY OF MnO₂ ELECTRODES

  Recently, some strategies for improving the cycling stability and capacity of MnO₂ cathode have been reported, and a new concept based on depo-sition-dissolution mechanism was also proposed for the future energy storage. To enhance cycling sta-bility of MnO₂ cathodes, two effective methods are reported, including 1) tuning discharge/charge potential ranges, and 2) stabilizing the layer-structure of MnO₂ through pre-insertion of large cations or molecules. A pioneering discovery was conducted by Liu’s group^[24], in which they present that the rate-limiting and irreversible conversion reactions occur at cell voltage lower than 1.26 V. Thus, by limiting the charge/discharge reactions in a potential range (1.8~1.3 V) higher than 1.26 V, ultra-long life of Zn/MnO₂ cells can be achieved (Fig. 2a and b). Here, we consider that the “rate-limiting and irreversible conversion reactions” refer to the sluggish Zn²⁺ intercalation process and the formation of inactive ZnMn₂O₄.

  Figure 2

  

  Figure 2.  Strategies improving the battery performance of MnO₂ cathode. (a) Gibbs free energy vs. reaction coordinate showing the thermodynamic and kinetic properties of the redox reactions in Zn/MnO₂ cells with different rates; (b) Cycling performance of the cells at different voltage ranges (1.0～1.8 and 1.3～1.8 V); From ref.^[24], of ACS; (c) Structural illustration of the Na⁺ and H₂O intercalated layered δ-MnO₂; From ref.^[25], copyright of ACS; (d) Expanded intercalated structure of polyaniline-intercalated δ-MnO₂ nanolayers; ref.^[19], copyright of Nature; (e) Schematic illustration of the subsequent H⁺ and Zn²⁺ intercalation in α-K_0.19MnO₂ nanotubes; From ref.^[26], copyright of RSC; (f) Calculated adsorption energies for Zn²⁺ on the surfaces of perfect σ-MnO₂ and σ-MnO₂ with oxygen vacancies; From ref.^[27], copyright of Wiley; (g) Reversible charge/discharge processes based on deposition-dissolution mechanism. From ref.^[29], copyright of Wiley

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  Although the cycling stability of MnO₂ is greatly improved by tuning potential ranges, the capacity delivery is seriously reduced. Thus, it is not an ideal optimization solution for MnO₂ cathodes. However, this work gives an inspiration to us, that is, the key to improve the cycling stability of MnO₂ cathode is inhibiting the formation of inactive ZnMn₂O₄. Based on this principle, the stabilizing effects of cations (K⁺, Na⁺, other molecules, etc.) on the layer-structure of MnO₂ have been adapted to prevent the generation of inactive ZnMn₂O₄. For example, Zhi’s group reported a Na⁺ stabilized δ-MnO₂ (Fig. 2c)^[25], Xia’s group presented a polyaniline-intercalated δ-MnO₂ (Fig. 2d)^[19], and Zhang’s group proposed a K⁺ pre-intercalated α-MnO₂ (Fig. 2e)^[26] as high-per-formance cathode materials in ZIBs. Here, for K⁺ pre-intercalated α-MnO₂, K⁺ presents impressing stabilizing effect on the layer structure of manganese oxide which transformed from the original tunnel structure during discharge. It should be noticed that the stabilizing K⁺ not only comes from the pre-inserted K⁺ in α-MnO₂, but also from the K⁺ containing salts pre-added in electrolyte.

  Some researches exploring higher capacity of MnO₂ have also been reported in 2019. MnO₂ ca-thode present a theoretical capacity of ~308 mAh g^-1 for 1 e^- transfer (Mn⁴⁺/Mn³⁺) in ZIBs. Generally, the charge/discharge cycle is controlled in a potential range from 1.8 to 1.0 V vs. Zn/Zn²⁺, and most of capacity deliveries of MnO₂ cathode are lower than ~308 mAh g^-1. Defect engineering in the MnO₂ lattice has been proved as an effective way enhancing the attainable capacity. Xue’s group^[27] propose that the Zn²⁺ adsorption/desorption process on oxygen-deficient MnO₂ is more reversible as compared to pristine MnO₂ due to a thermos-neutral value of Gibbs free energy of Zn²⁺ adsorption in the vicinity of oxygen vacancies (Fig. 2f). Thus, the oxygen-deficient MnO₂ presented one of the highest capacities of 345 mAh g⁻¹ for a birnessite MnO₂ system. Similar results are also reported by Mai’s group^[28]. They produce defects in α-MnO₂ lattice through Ti substitution, which facilitates both the H⁺ and Zn²⁺ intercalation processes, and achieves a high capacity delivery.

  Based on the above discussions, the existing stra-tegies enhancing battery performance of MnO₂ cathode are all based on a conventional charge storage mechanism: promoting the H⁺/Zn²⁺ insertion process in a stable structure of MnO₂. However, it has got a bottleneck on further improving energy densities of Zn/MnO₂ battery systems. Thus, a new novel deposition-dissolution mechanism was proposed to provide the maximized electrolysis process (Fig. 2g). Qiao’s group^[29] proposed a high-voltage electrolytic Zn-MnO₂ battery, with a theoretical voltage of ~2 V and energy density of ~700 Wh kg⁻¹. In their study, Mn²⁺ ions in electrolyte can be oxidized to form solid MnO₂ on the carbon fiber during charging, and then reduced to Mn²⁺ ions dissolving back to electrolyte during discharging. This unique two-electron redox elec-trolysis reaction of Mn⁴⁺/Mn²⁺ was produced via a reversible proton and electron dynamics, and exhibi-ted high capacity delivery of MnO₂. Zhi’s group^[30] also reported a similar deposition-dissolution mechanism in Zn/MnO₂, Cu/MnO₂ and Bi/MnO₂ systems, respectively. This new kind of simple battery electrochemistry presents excellent capacity and rate performances, and is a prospective direction for further researches of ZIBs. Although this new mechanism is impressing, the practical use of Zn/MnO₂ battery encounters unprecedented challen-ges in the Zn anode side due to the very serious hydrogen evolution issue during charging in a strong acidic electrolyte.

  4. SUMMARY & PERSPECTIVE

  In summary, we have provided a brief introduction on charge storage mechanisms and some strategies to improve electrochemistry of MnO₂ cathode in ZIBs. Some solid conclusions and perspectives based on existing reports, including experiments and theore-tical calculations, are as follows: a) A clear acknowledgement on the relationship between the diverse phase structures and electrochemical reac-tions of MnO₂ is vital for designing high-perfor-mance MnO₂ cathode materials; b) Tuning H⁺/Zn²⁺ intercalation processes in a stable structure of MnO₂ phase is a promising research direction for the development of superior ZIBs; c) The key to improve the cycling stability of MnO₂ cathode is to prevent the formation of ZnMn₂O₄ during cycling; d) The method of pre-insertion of large-size cations or other molecules inside the layer-structure of MnO₂ is an effective way to prevent the generation of inactive ZnMn₂O₄, and hence improving the cycling property of MnO₂ electrode; e) Defect engineering will be an effective method for improving both capacity and rate properties of MnO₂ cathodes; f) The dissolu-tion/deposition mechanism of MnO₂ cathode is impressive, but the very serious hydrogen evolution issue in Zn anode side restricts its practical applica-tion in future. We believe the rechargeable aqueous Zn/MnO₂ batteries will play an important role in the next-generation energy storage devices, and to achieve this goal, further researches need to be done on electrochemical reactions and correlated strategies improving battery performance of MnO₂ cathode. This article combining reviews and perspectives of manganese oxides may aid in the future development of advanced cathodes for aqueous Zn ion batteries.

  
  
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• 

  Figure 1  Examples illustrating characteristics of MnO₂ cathode. (a) Multiple phase structures of MnO₂; (b) Schematic illustrating the phase transition mechanism of α-MnO₂ during Zn²⁺ intercalation process. From ref.^[8], copyright of nature; (c) TEM morphology and EDS mapping of Mn, O and Zn for β-MnO₂ nanofiber after long-term cycle, and the schematic of ZnMn₂O₄ structure. From ref.^[14], copyright of elsevier; (d) H⁺/Zn²⁺ synthetic intercalation mechanism for a manganese oxide nanosheet. From ref.^[22], copyright of Wiley

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  Figure 2  Strategies improving the battery performance of MnO₂ cathode. (a) Gibbs free energy vs. reaction coordinate showing the thermodynamic and kinetic properties of the redox reactions in Zn/MnO₂ cells with different rates; (b) Cycling performance of the cells at different voltage ranges (1.0～1.8 and 1.3～1.8 V); From ref.^[24], of ACS; (c) Structural illustration of the Na⁺ and H₂O intercalated layered δ-MnO₂; From ref.^[25], copyright of ACS; (d) Expanded intercalated structure of polyaniline-intercalated δ-MnO₂ nanolayers; ref.^[19], copyright of Nature; (e) Schematic illustration of the subsequent H⁺ and Zn²⁺ intercalation in α-K_0.19MnO₂ nanotubes; From ref.^[26], copyright of RSC; (f) Calculated adsorption energies for Zn²⁺ on the surfaces of perfect σ-MnO₂ and σ-MnO₂ with oxygen vacancies; From ref.^[27], copyright of Wiley; (g) Reversible charge/discharge processes based on deposition-dissolution mechanism. From ref.^[29], copyright of Wiley

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