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Manganese-Based Cathode Materials for Aqueous Zinc Ion Batteries

Shihao Zhou , Xianwen Wu , Yanhong Xiang , Ling Zhu , Zhixiong Liu , Caixian Zhao

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (4) : 649 -669.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (4) : 649-669 DOI: 10.7536/PC200512
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Manganese-Based Cathode Materials for Aqueous Zinc Ion Batteries
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Abstract

Aqueous zinc-ion batteries(AZIBs) have a broad application prospect in large-scale energy storage field with low cost, high safety and high environmental friendliness characteristics. At present, the cathode materials which have attracted much attention have become the research hotspot. Manganese-based compound is one of the most promising cathode materials in the market due to the advantages of abundant resources, environmental friendliness and low price. In this paper, the structural characteristics of different manganese-compounds and manganese-based AZIBs involved four kinds of energy storage mechanisms in the charging and discharging process are reviewed in detail, and the existing problems and optimization strategies of AZIBs manganese-based cathode materials are discussed. Finally, the possible research direction of AZIBs cathode materials is proposed, which is expected to play a certain role in the development of AZIBs.

Contents

1 Introduction

2 Manganese-based cathode materials for aqueous zinc ion batteries

2.1 MnO2

2.2 Mn2O3

2.3 Mn3O4

2.4 MnO

2.5 Other manganese compounds

3 Energy storage mechanism of manganese-based aqueous zinc ion batteries

3.1 Zn2+ insertion/extraction mechanism

3.2 Chemical conversion reaction mechanism

3.3 Zn2+ and H+ co-insertion/co-extraction

3.4 Dissolution/deposition mechanism

4 Problems and optimization strategies of manganese-based cathode materials

4.1 Doping

4.2 Surface modification

4.3 Introduction of defects

4.4 “Pillar” effect

4.5 Composite

4.6 Structural design

4.7 Electrolyte optimization

4.8 Other strategies

5 Conclusion and outlook

Graphical abstract

Keywords

aqueous zinc-ion batteries / cathode material / manganese compounds / optimization strategies / charge discharge mechanism

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Shihao Zhou, Xianwen Wu, Yanhong Xiang, Ling Zhu, Zhixiong Liu, Caixian Zhao. Manganese-Based Cathode Materials for Aqueous Zinc Ion Batteries. Progress in Chemistry, 2021, 33(4): 649-669 DOI:10.7536/PC200512

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1 Introduction

With the increase of global fossil energy consumption and greenhouse gas emissions, the problem of greenhouse effect is increasing day by day, and people put forward higher requirements for various green and sustainable energy storage technologies. At the same time, it also promotes the rapid development of advanced energy storage technology[1].
Among them, the manufacturing process of high energy density lithium-ion batteries (LIBs) is becoming more and more mature, and their market share has far exceeded that of lead-acid, nickel-cadmium and nickel-hydrogen secondary batteries[2]. However, the application of LIBs in large-scale energy storage has been greatly hindered by the increasingly prominent problems such as the high cost of assembly, the harmfulness of organic electrolytes and the uneven distribution of global lithium resources[3⇓~5]. The need for alternatives has also contributed to the development of sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). On the one hand, sodium and potassium are in the same main group as lithium, and they have similar chemical properties. On the other hand, the global reserves of sodium and potassium are abundant and the cost is low. Even so, the large-scale application of SIBs and PIBs still faces safety and environmental challenges, especially the large-scale use of toxic, flammable and volatile organic electrolytes[6⇓~8].
Compared with non-aqueous secondary batteries with higher cost, rechargeable secondary batteries with aqueous solution as electrolyte have the advantages of low cost, high safety, high ionic conductivity, easy processing and simple assembly process, which are very suitable for large-scale energy storage applications[9]. Among many aqueous batteries, aqueous zinc-ion batteries (AZIBs) with zinc metal or zinc-intercalated compounds as the negative electrode and Zn2+ in the electrolyte are increasingly favored by researchers[10⇓~12]. In the process of charging and discharging, in the aspect of the negative electrode, the Zn2+ is rapidly deposited and dissolved on the surface of the metal zinc or is reversibly deintercalated in the zinc intercalation compound; On the positive side, unlike the traditional rocking-chair LIBs, the zinc storage mechanism in AZIBs system is complex and controversial. At present, there are four main storage mechanisms of AZIBs: :Zn2+ intercalation/deintercalation mechanism, chemical transformation mechanism, H+/Zn2+ co-intercalation/deintercalation mechanism and dissolution/deposition mechanism[13,14]. The battery system also has the following characteristics: (1) the potential to zinc is higher, -0.763V(Zn2+/Zn)vs a standard hydrogen electrode, thereby ensuring the electrochemical stability in an aqueous solution; (2) that deposition and dissolution of zinc ion are reversible, unlike alkaline zinc batteries, near-neutral or slightly acidic electrolyte (E. G., pH = 3. 6 ~ 6) can largely avoid the formation of zinc dendrites and by-products such as ZnO, thus enabling long cycle life of the battery[15]; (3) Unlike monovalent metal ions, divalent zinc ions involve the transfer of two electrons during the charge-discharge process, which makes AZIBs have higher volumetric energy density[16]. Table 1 shows the comparison of the characteristics of AZIBs and LIBs. AZIBs has obvious competitiveness in industrial application[9,17⇓⇓~20]. First, the price advantage, LIBs is $300 per kilowatt-hour, AZIBs is only $65 per kilowatt-hour; Secondly, the safety and recovery of aqueous electrolyte are better than that of organic electrolyte. In addition, multivalent AZIBs, which allow multiple electron transfers in electrochemical reactions, make it possible to achieve high energy and high power densities for batteries.
However, there are few AZIBs cathode materials that can be commercialized at present, and their commercialization needs to meet the following conditions: insoluble in aqueous solution[21]; The cost is low and the raw materials are abundant; The conductivity and the stability are good, and the structural phase transition is not easy to occur in the charging and discharging process; Higher working voltage and wider voltage window; It has high capacity and high energy density, and can accommodate a large number of metal ions in the geometric structure. In recent years, the main types of AZIBs cathode materials reported are as follows: (1) Manganese-based compounds, especially MnO2, have high redox potential (~ 1.3 V), and the theoretical specific capacity of one electron is as high as 308 mA·h·g-1;(2). Vanadium-based compounds, especially the layered V2O5 and the double-layer structure containing water, have become a fast channel for ion migration[18,21⇓⇓⇓⇓⇓~27]. In addition, the diversity of ion coordination also makes the structure of vanadium materials rich. (3) Prussian blue analogs, which contain mixed-valence cations, most commonly iron ferricyanide. It usually has a face-centered cubic structure with a three-dimensional open skeleton and large ion insertion sites, which can realize the rapid transport of ions. (4) Chevreil-phase compounds (MxMoT8,M=metal,T=S,Se,Te), which have an open rigid structure allowing rapid and reversible intercalation of guest ions, and Mo6S8 are most commonly used in AZIBs; (5) polyanionic compounds, which are composed of polyanionic groups and transition metal elements (such as Na3V2(PO4)3, Na3V2(PO4)2F3, etc.), and because polyanions have adjustable inductivity to the redox potential of the material, higher potential can be obtained; (6) Organic compounds, mainly quinone compounds (Calix [4] quinone and p-chloroaniline) and conductive polymers (polyaniline, polypyrrole, PEDOT). Organic compounds mainly have the characteristics of light weight, high cost efficiency, multi-electron reaction and adjustable electrochemical window. (7) Other compounds, such as bismuth sulfide, cobaltosic oxide, etc. Among them, manganese-based compounds with low cost, non-toxicity and abundant resources are the preferred cathode materials for commercial applications of AZIBs due to their high capacity, high energy density and high working voltage.
Therefore, the structural characteristics of different manganese-based compounds and four energy storage mechanisms involved in the charge-discharge process of manganese-based AZIBs are reviewed in detail in this paper. In addition, the existing problems of manganese-based cathode materials (such as low conductivity, manganese dissolution, electrostatic repulsion during Zn2+ diffusion, by-products and oxygen evolution reaction during energy storage) and their optimization strategies were discussed. Finally, the research direction of AZIBs cathode materials was prospected and the paper was summarized.

2 Manganese-based cathode material for aqueous zinc ion battery

Manganese, as a transition metal element with abundant natural resources, has many variable valences, such as Mn2+, Mn3+, Mn4+ and Mn7+.Therefore, there are many stable manganese-based compounds (MnO, MnO2, Mn2O3, Mn3O4, ZnMn2O4, MnS, etc.)[28]. Due to the different crystal structure, valence state, morphology and specific surface area, different manganese-based compounds show different electrochemical behavior and electrochemical performance. In the past few years, manganese-based compounds have attracted more and more attention from researchers, and various manganese compounds have been used as cathode materials for AZIBs[14,29].

2.1 MnO2

Due to the high working voltage and high theoretical capacity(308 mA·h·g-1for 0.5 mol/L Zn2+),MnO2in MnO2),AZIBs have become a common cathode material[30]。 In general,various arrangements of the unit danyuan octahedron through corner/edge sharing result in different crystal forms of the MnO2.These crystal structures are roughly divided into three categories:tunnel structures,layered structures,and 3D structures 。

2.1.1 Tunnel MnO2

Among the three types,the tunnel MnO2has a variety of crystal forms,and their tunnel sizes are also different.These include the[2×2]tunnel-typeα-MnO2,the[1×1]tunnel-typeβ-MnO2,the[1×1]and[1×2]tunnel-typeγ-MnO2,the[2×3]tunnel-type R-MnO2(fig.1D),and the[3×3]tunneling Todorokite-MnO2(perovskite type )[31][32][33][9][34]。 As shown in fig.1A,where theα-MnO2has a large[2×2]tunnel structure with four MnO6octahedral cells sharing a common edge along the c-axis.Due to the appropriate tunnel size,the diffusion ability and structural stability of Zn2+can be combined,which is not available in other crystalline MnO2,soα-MnO2has become the most widely studied cathode material for MnO2.As shown in fig.1b,theβ-MnO2is composed of octahedral MnO6single chains through shared vertices,forming a[1×1]tunnel structure along the c-axis,which is the most thermodynamically stable crystal structure of the MnO2.However,due to its narrow tunnel,it is not conducive to the diffusion of Zn2+.As shown in fig.1C,theγ-MnO2is actually a symbiont between theβ-MnO2and the R-MnO2,and generally has mixed tunnels of[1×1]and[1×2],and has better Zn2+diffusion capability than theβ-MnO2.As shown in fig.1e,the perovskite-type MnO2has a structure similar to that of aα-MnO2.The[3×3]tunnel structure can accommodate various cations and water molecules,provide structural support and other cation characteristics for the perovskite-type MnO2,not only make up for the defect that the structure is easy to collapse,but also improve the diffusion ability of the Zn2+ 。
Different tunnel structures lead to different electrochemical properties,and the MnO2with larger tunnel has faster Zn2+diffusion ability.Although the tunnel-type MnO2shows excellent Zn2+diffusion ability and structural stability,the tunnel structure limits the zinc storage ability of the MnO2and affects the specific capacity of the MnO2.According to recent reports,there is also a structural phase transition in the process of charging and discharging of tunnel MnO2[32,35,36]。 Therefore,the collapse of the intrinsic structure inevitably leads to capacity decay。

2.1.2 Layered MnO2

As shown in Figure 1f,the layered crystal form of MnO2(δ-MnO2)arranges the MnO6octahedron into a sheet structure by sharing edges,and the Birnessite layer spacing is generally around 7.7Å[37⇓~39]。 Due to the lack of manganese atoms in the octahedral center of some MnO6in the structure,or the replacement of Mn4+by some Mn3+to form octahedrons,there are a large number of negative charges in theδ-MnO2structure.Therefore,the interlayer between these sheets can contain water molecules or hydrated cations,which play a role in balancing the interlayer charge and preventing the collapse of the structure.Layered structures with different interlayer spacings can be obtained by intercalating different water molecules and hydrated cations.Among them,the Birnessite-type and Buserite-type structures(the layer spacing is generally 11 11Å)have one and two layers of water molecules in the layered polymorphic MnO2,respectively[40]。
Layeredδ-MnO2have relatively large interlayer spacing and faster Zn2+diffusion ability,which is conducive to the storage and transportation of Zn2+,and may be the most ideal cathode material for MnO2.However,the electrochemical reaction is prone to interlayer phase transition,which leads to the collapse of the structure and seriously affects the stability.Therefore,only by finding a suitable method to support the layered structure,can theδ-MnO2have a better development 。

2.1.3 3D MnO2

In addition to the above two types,there is also a 3D type(spinel structure)of MnO2,includingλ-MnO2andε-MnO2.The tetrahedron and octahedron in theλ-MnO2are occupied by Mn2+and Mn3+,respectively,and there are no tunnels or layers,which belong to close packing,as shown in Figure 1G[41]。 This also leads to the fact that the zinc storage capacity ofλ-MnO2may not be optimistic,the existing research is less,and the mechanism of zinc storage is rarely reported.ε-MnO2belongs to Aktenskite-MnO2(hexagonal pyrolusite),which is hexagonal crystal system,and its color is generally black and brown-black[42]。 Metastable phase consisting of coplanar MnO6and YO6octahedra(Y represents vacancy),exhibiting hexagonal symmetry,with Mn4+randomly occupying 50%of the octahedral sites,as shown in Fig.1H 。
Due to the close packing of the spinel MnO2,the capacity decay is not usually caused by the structural phase transition.However,H+/Zn+are difficult to embed in tightly packed structures,making them less capacitative.However,ε-MnO2is reported to have a dissolution/deposition mechanism,so it can show excellent specific capacity 。

2.2 Mn2O3

Mn2O3is a kind of manganese oxide with+3 valence of manganese element,which is an amphoteric oxide and generally black in color[43]。 There are two crystal forms of Mn2O3,α-Mn2O3andγ-Mn2O3,of whichα-Mn2O3is more common.γ-Mn2O3can not exist stably for a long time at room temperature,and will gradually change toα-Mn2O3.As shown in fig.2a,theα-Mn2O3belongs to the orthorhombic system,and its basic unit is the same as the MnO2,that is,the MnO6octahedron.Theα-Mn2O3are connected by one or two oxygen atoms MnO6the octahedral corners of the octahedron 。
With the insertion of Zn2+,the Mn2O3undergoes a structural phase transition from orthorhombic to layered phase,that is,theα-Mn2O3transforms into layered Zn-birnesite phase,which corresponds to the reduction of Mn3+to Mn2+,while the charging process is just the opposite[43]。 This makes the Mn2O3,which is at the disadvantage of energy storage structure,have better Zn2+diffusion rate and zinc storage capacity.However,the dissolution phenomenon caused by structural transformation is still inevitable 。

2.3 Mn3O4

Mn3O4is a mixed-valence oxide of manganese[44]。 The valence state of manganese in Mn3O4is both+3 and+2.It is generally believed that the oxide is a mixture of MnO and Mn2O3.As shown in fig.2b,the Mn3O4has a spinel structure,and the Mn3+and the Mn2+occupy octahedral and tetrahedral positions of the spinel structure,respectively 。
The Mn3O4is transformed into a layered structure during the first cycle of charge and discharge,and then is reversibly charged and discharged by the layered structure[44]。 The Mn3O4does not change the phase structure repeatedly in the circulation process,which is beneficial to the stability of the structure.The transition from the spinel type to the tunnel type may be the direct reason for the considerable capacity of the Mn3O4 。

2.4 MnO

MnO is the lowest valence oxide of manganese,also known as manganous oxide[45]。 The valence of manganese in MnO is+2,and its crystal structure is similar to that of NaCl,as shown in fig.2C.Common MnO is generally gray-green or grass-green,which can not exist stably in the air and is easily oxidized to other high-valence manganese oxides by O2in the air 。
Theoretically,the crystal structure of MnO is not suitable for the intercalation of H+/Zn2+,but MnO is beneficial to induce the formation of manganese defects during the first charge process,thus activating the electrochemical performance of MnO.The formation of manganese defects will provide a lower energy barrier for the migration of Zn2+,thus promoting the reversible intercalation and deintercalation of Zn2+from the MnO host,which is the main reason for the excellent electrochemical properties of MnO[46,47]。

2.5 Other manganese-based compound

2.5.1 ZnMn2O4

Inspired by the success of LiMn2O4,spinel MFe2O4(M=Zn(Ni or Cu),Co3O4,ZnCo2O4and ZnMn2O4are also used as electrode materials for batteries[48,49]。 The ZnMn2O4with a spinel structure can be used for the AZIBs,the structure is similar to a[1×1]tunnel type MnO2,the tunnel space is completely occupied by Zn,and Zn and O form a ZnO4unit,as shown in fig.2D 。
The zinc storage capacity of perfect spinel ZnMn2O4is not optimistic,but its performance has been greatly improved by introducing defects,doping and improving the structure 。

2.5.2 MnS

Transition metal sulfides are commonly used as electrode materials in Li+,Na+,or Mg2+cell systems.Inspired by this,Liu et al.First proposed the use of MnS in AZIBs[50]。 MnS has stableα-MnS and metastableβ-MnS andγ-MnS.Among them,α-MnS is easy to synthesize,and the crystal form belongs to NaCl,as shown in Figure 2e。
the ex situ XRD study of MnS cathode shows that MnS may suffer from complex electrochemical activation in the first few cycles,but the mechanism is still unclear.After the first few cycles,the MnS phase remains relatively stable,which means that MnS exhibits high stability during long cycles。

3 Energy Storage Mechanism of Manganese-based Aqueous Zinc-ion Batteries

Unlike the energy storage mechanism of lithium/sodium-ion batteries, the reaction mechanism of manganese-based AZIBs is complex and controversial[51]. Four energy storage mechanisms are involved in the charge-discharge process of AZIBs: :Zn2+ intercalation/deintercalation, chemical conversion, H+/Zn2+ co-intercalation/deintercalation, and dissolution/deposition[13,14].

3.1 Zn2+intercalation/deintercalation mechanism

During the charge-discharge process of AZIBs with manganese-based compounds as the positive electrode and zinc as the negative electrode,the redox reaction of manganese mainly occurs in the positive electrode,and the corresponding reversible intercalation and deintercalation mechanism of Zn2+is a mainstream view.Oh et al.First elucidated the intercalation/deintercalation mechanism of Zn2+inα-MnO2and the corresponding structural transformation[35]。 Upon discharge,Mn4+is partially reduced to Mn3+and subsequently disproportionated to Mn4+and Mn2+,with dissolution of manganese and formation of layered Zn-birnessite(birnessite).However,the Mn2+in the electrolyte will reversibly intercalate into the Zn-birnessite during the charging process to restore the original tunnel structure.Subsequently,they modified this argument,suggesting that the embedding of Zn2+into theα-MnO2would first lead to the formation of Zn-buserite(buserite),followed by the gradual detachment of embedded Zn2+and H2O from the interlayers of buserite,and finally to the formation of birnessite phase,as shown in Figure 3A[36]。 In addition,Kim et al.Observed the ZnMn2O4phase in the fully discharged electrode of theα-MnO2through in-situ XRD analysis,which supplemented the zinc generation phase in the Zn2+deintercalation mechanism[31]。 Zhang et al.Studied theβ-MnO2of AZIBs cathode materials,and confirmed that Zn2+also had structural transformation in the process of intercalation/deintercalation[32]。 Theβ-MnO2of the tunnel structure is transformed into the buserite of the layered structure during the intercalation of the Zn2+,and then the Zn2+is reversibly deintercalated in the layered phase,but the original tunnel structure is not restored.Due to the transition from the narrow tunnel to the layered structure,the Zn2+diffusion and zinc storage capacity are improved.Kim et al proposed that the reaction mechanism ofβ-MnO2nanorods in AZIBs was the combination of solid solution reaction and transformation reaction,and the essence was the embedding of Zn2+intoβ-MnO2[52]。 Not only the zinc intercalation phase is formed(as shown in fig.3B),but also a new phase ZnSO4·3Zn(OH)2·5H2O(ZHS)is precipitated on the surface.Subsequently,the intercalation/deintercalation mechanism of Zn2+in theγ-MnO2and the corresponding structural transition were also confirmed[33]。 As shown in fig.3C,at the beginning of the discharge,with the insertion of the Zn2+,part of theγ-MnO2is first transformed into a spinel ZnMn2O4.In the middle stage of discharge,with the insertion of Zn2+,the[1×2]tunnel of theγ-MnO2is occupied,forming a tunnel-typeγ-ZnxMnO2.At the end of the discharge,with the further embedding of the Zn2+,part of the phase will open the structural framework and finally form the L-ZnxMnO2phase.It is worth noting that intermediate phases such as ZnMn2O4andγ-ZnxMnO2will appear in this process,but they will not disappear completely.However,in the subsequent charged state,all phases will almost completely revert to theγ-MnO2configuration.Kim et al.proposed that during the initial cycle,the Zn2+intercalationδ-MnO2reversibly forms the L-ZnXMn2O4,the spinel ZnMn2O4,and the zinc intercalation phase,essentially maintaining a layered structure,as shown in fig.3D[37]。 However,after about 50 cycles,the conversion to irreversible spinel ZnMn2O4is complete,and manganese dissolution is also observed in the electrolyte after cycling.Subsequently,the capacity decreases greatly,which is mainly due to the formation of spinel phase and the dissolution of manganese 。
In addition to MnO2,many manganese-based cathode materials have also been observed to have Zn2+intercalation and deintercalation mechanisms.There is a structural transition of theα-Mn2O3during the charge-discharge process.During the discharge process,the originalα-Mn2O3transforms into layered Zn-birnesite phase with the intercalation of Zn2+,which corresponds to the reduction of Mn3+to Mn2+[43]。 Upon charging,the Zn-birnesite phase will be reduced to aα-Mn2O3,accompanied by oxidation of the Mn2+to a Mn3+,as shown in fig.3e.During the first charge and discharge of the Mn3O4,there is also a structural transformation,in which the Mn3+in the Mn3O4is oxidized to Mn4+,Mn2+and then to Mn5O8.The Mn5O8is then converted to birnessite by dissolving the Mn2+into the electrolyte and intercalating the H2O[44]。 Due to the incomplete electrochemical oxidation,the Mn5O8coexists with birnessite during the first charge.In the subsequent discharge process,the Zn2+is embedded into the birnessite interlayer space to form Zn-birnessite,and at the same time,a part of the Mn4+is reduced to Mn3+.Thereafter,the layered structure is reversibly charged and discharged,as shown in fig.3F.Zhou Jiang of Central South University and others used a pre-charging method to induce the formation of manganese defects in MnO,thus activating its electrochemical properties[45]。 The Mn vacancies in the Mn0.61□0.39O will provide a lower energy barrier for the migration of Zn2+,thereby facilitating the insertion/extraction of Zn2+from the MnO host[53]。 Wu Xianwen of Jishou University proposed that in the process of charging and discharging of ZnMn2O4,The Zn2+is extracted from the Zn-O tetrahedral site of the spinel ZnMn2O4in two parts(the intrinsic Zn2+of the ZnMn2O4and the Zn2+in the electrolyte )[54]。
The report of the intercalation/deintercalation mechanism of the Zn2+clearly illustrates for the first time that the influence of the intercalation/deintercalation of the Zn2+in the AZIBs on the structure of a host material helps us to comprehensively understand the change of a manganese ion valence state in the charge-discharge process of an MnO2,namely the problem of charge transfer and the fate of manganese ions,and also clarifies the problem of formation phases of Zn2+.This has made a significant contribution to the understanding of the redox reactions that occur during the charging and discharging of MnO2 。

3.2 Chemical Conversion Reaction Mechanism

Different from the reversible intercalation/deintercalation mechanism of Zn2+,Liu et al.Revealed the chemical conversion reaction mechanism in the process of studying the Zn/α-MnO2system,which occurs between theα-MnO2and MnOOH[55]。 The MnOOH phase appears in the XRD pattern in the discharge state(the reaction of theα-MnO2with the H+in solvent water).In order to achieve charge balance,the remaining OH-will react with ZnSO4and H2O in the electrolyte to form scaly basic zinc sulfate ZnSO4[Zn(OH)2]3·xH2O.In the STEM-EDS spectrum of theα-MnO2electrode under discharge,it is observed that the nanorods and nanoparticles contain O and Mn elements,while the Zn element is mainly distributed on the sheet compound.These experimental phenomena confirm the chemical conversion reaction mechanism,as shown in Figure 4A,B.The specific reaction process is as follows:
Anode:H2O↔H++OH-
α-MnO2+H+↔MnOOH
1 2 Zn2++OH-+ 1 6 ZnSO4+ x 6 H2O↔ 1 6 ZnSO4[(OH)2]3·xH2O
Negative electrode: 1 2 Zn 1 2 Zn2++e-
Liang et al.Synthesized a K0.8Mn8O16(KMO)containing K+and oxygen defects,which is also a chemical conversion reaction mechanism in the charge and discharge process of AZIBs[52]。 The XRD is shown in fig.4A,and the Zn4SO4(OH)6·5H2O(ZHS)and MnOOH phases appear during the discharge of the KMO electrode.SEM is shown in Figure 4 B.When the electrode is discharged to 1.0 V,a small amount of granular HxK0.8Mn8O16phase can still be observed except for the flaky ZHS,which may be due to the diffusion of H+into the KMO skeleton.Based on the above discussion,the energy storage process of KMO is shown in Figure 4C 。
The above chemical conversion mechanism means that the H+participates in the charge and discharge process of the MnO2,but the Zn2+will not be embedded into the MnO2,but will be chemically converted into an alkaline complex containing Zn by balancing the charge of the system.This also implies that the MnO2may use more than one way to store energy during the charging and discharging of AZIBs 。

3.3 H+/Zn2+co-intercalation/deintercalation mechanism

Due to the different thermodynamic and kinetic characteristics of the intercalation of the H+and the Zn2+,the H+and the Zn2+can be simultaneously intercalated by a suitable host material.Wang et al first proposed the reaction mechanism of co-intercalation/deintercalation of H+and Zn2+in the study of Zn/ε-MnO2system[56]。 The galvanostatic intermittent titration(GITT)curve of the positive electrode of theε-MnO2during discharge is shown in Figure 5A.The overpotential of the second discharge plateau is 0.6 V,which is almost 10 times the overpotential of the first discharge plateau(0.08 V).The reason for the gradual change of voltage after discharge in GITT is mainly due to ion diffusion,and the significant difference between region I and region II may be attributed to different ion intercalation[30]。 Region I is due to the insertion of the H+,due to the large size of the Zn2+and the strong electrostatic interaction between the divalent Zn2+and the host lattice,while region II may be due to slow insertion of the Zn2+[28]。 This hypothesis was further confirmed by comparing the discharge curves of the electrodes in the MnSO4electrolyte with and without ZnSO4.In the pure MnSO4electrolyte,there is no discharge plateau at 1.3 V,as shown in Fig.5B.As shown in fig.5C,in the electrolyte containing Zn2+,the MnOOH phase was observed after the electrode was discharged to 1.3 V due to the intercalation of H+.However,the ZnMn2O4phase can only be observed after a further discharge to 1.0 V,which proves that the main reaction in region II is caused by the intercalation of Zn2+ 。
Wang et al.First used commercial MnO particles as the cathode material of ZIBs and expounded its energy storage mechanism in detail[57]。 During the first charging process,the MnO particles are electrochemically oxidized into porous layered MnO2nanosheets.During the subsequent cycle,the newly formed MnO2undergoes a typical H+/Zn2+intercalation/deintercalation process.In addition,the intercalation of the H+results in an increase in the pH value of the electrolyte and the formation of(Zn(OH)2)3ZnSO4·3H2O nanosheets on the surface of the positive electrode,as shown in fig.5d 。
Pan et al.Found that there are H+conversion reactions and non-diffusional intercalation of Zn2+in Zn(TFSI)2based electrolytes forδ-MnO2[58]。 The non-diffusion-controlled Zn2+storage mechanism controls the first charge storage step of theδ-MnO2without a significant phase transition,while the diffusion-controlled H+conversion reaction controls the subsequent reaction,as shown in fig.5e.Unfortunately,theδ-MnO2has undergone a serious structural phase transition.Since then,Zhu et al.Have described a co-intercalation mechanism forδ-MnO2without structural phase transition[39]。 At the beginning of the discharge,the H+is embedded in the middle layer of theδ-MnO2nanosheet.The Zn2+then begins to embed theδ-MnO2,H+and continues to embed.With the continuous insertion of H+,the pH value of the electrolyte increases,and a Zn4(SO4)(OH)6·nH2O(ZHS)is formed on the surface of the electrode.During the charging process,the Zn2+and the H+in theδ-MnO2are released,and the ZHS is gradually dissolved with the release of the H+ 。
The H+/Zn2+co-intercalation/deintercalation mechanism is a supplement to the single Zn2+intercalation/deintercalation mechanism.This point of view can also logically explain the situation that“H+first intercalates into MnO2to form MnOOH,and then Zn2+intercalates,and MnOOH is converted into ZnMn2O4”.Although this mechanism reveals what kind of ions are inserted into the MnO2in the solution,it does not focus on the structural changes of the MnO2.There is no transformation of Mn2+in this mechanism,so we can infer that most MnO2do not have serious dissolution phenomenon and do not have a large structural transformation 。

3.4 Dissolution/deposition reaction mechanism

The dissolution/deposition reaction mechanism was first proposed by Oh et al.,which breaks away from the view that the host MnO2material should maintain a stable crystal structure[59]。 Fig.6a shows an in situ XRD pattern of theα-MnO2electrode during discharge/charge.As the discharge progresses,a new phase of the Zn4(OH)3SO4·5H2O(ZHS),represented by the red squares,gradually appears.At the end of the discharge,the characteristic peak of ZSH dominates.At the end of the subsequent charge,only the characteristic peak of theα-MnO2was observed,indicating that the formation of ZHS was reversible.EDX analysis of the discharge electrode in Fig.6d shows that the zinc content in the nanorods(region I)is negligible,whereas the agglomerates on the nanorods(region II)are mainly composed of zinc oxide(Fig.6C).This indicates that the Zn2+is mainly deposited on the surface of the positive electrode as ZHS,rather than embedded in the tunnel of theα-MnO2.After full charge,the electrode presents a single MnO2nanorod(Fig.6B),indicating good reversibility of energy storage 。
The detailed energy storage process is as follows:In the discharge process,the MnO2is electrochemically reduced to Mn3+and then disproportionated to Mn2+dissolved in the electrolyte,and the pH value of the electrolyte gradually increases.A ZHS precipitate is for on that surface of the electrode.During the charging process,Mn2+is deposited on the electrode to form a MnO2,resulting in the redissolution of ZHS into the electrolyte.In addition,similar phenomena were also found in the Todorokite-MnO2,indicating that the change of electrolyte pH value plays a key role in the reaction mechanism of AZIBs 。
the appearance of dissolution/deposition mechanism completely breaks the inherent understanding of researchers that AZIBs cathode needs intercalation ions to store energy,and also provides the possibility of energy storage for more cathode materials with unsatisfactory structure(not conducive to intercalation/deintercalation)。
With the development of research,the interaction of multiple energy storage mechanisms in the electrode has been reported.In 2020,Zhou Jiang et al.Proposed a new viewpoint:the dissolution/deposition reaction mechanism is dominant in the energy storage process of Zn/MnO2aqueous zinc-ion batteries,accompanied by the cation(H+,Zn2+)intercalation/deintercalation reaction mechanism[60]。 They prepared two cathode materials with different crystal forms(MnO2(α-MnO2andδ-MnO2).The two MnO2were studied by in situ XRD.The results show that a large number of new Zn4(OH)6SO4·4H2O(ZHS)phases are formed during the discharge process of the battery,but they gradually disappear during the subsequent charge process,which indicates that ZHS has an obvious reversible phase transition.Most of theα-MnO2disappears after the first discharge,and only a small amount is produced in the subsequent charge and discharge process.The shift of the peak position of theδ-MnO2indicates that the crystal structure of theδ-MnO2has changed and transformed into a birnesite-MnO2,which is also confirmed by the SEM image.As shown in Figure 7A,the morphology of the originalα-MnO2powder is nanorods.After discharging to 0.8 V,the surface of the positive electrode appears flaky ZHS,with a small amount ofδ-MnO2.After charging to 1.8 V,a large number of interconnected porous Birnesite-MnO2materials appeared everywhere on the surface of the positive electrode.As shown in fig.7B,the morphology of the originalδ-MnO2powder is nanometer-sized,and theδ-MnO2positive electrode and theα-MnO2undergo the same surface morphology transformation.As shown in fig.7c,the energy storage process can be divided into two parts:(1)During the first discharge,the host materialα-MnO2orδ-MnO2reacts with water to form Mn2+and OH- 。
The surrounding ZnSO4reacts immediately with the generated OH-to convert to ZHS,which is highly reversible and will also effectively consume a large amount of H2O around the MnO2.The lack of active water inhibits the continued dissolution of the MnO2,resulting in a decrease in the Mn2+content and pH of the electrolyte.During the first charge,ZHS reacts with the Mn2+to form the birnesite-MnO2.A similar dissolution/deposition mechanism occurs again in subsequent cycles,with the birnesite-MnO2as the host material instead of the original MnO2.This deposition/dissolution mechanism dominates the entire energy storage process and contributes most of the specific capacity.(2)In the whole process of energy storage,the process of intercalation and deintercalation is insignificant,that is,the H+/Zn2+can only be intercalated into the residual undissolved MnO2material in a limited way.In addition,Kim et al.Found that there are two energy storage mechanisms for ZnMn2O4in ZnSO4+MnSO4electrolyte[61]。 As shown in fig.7d,the Mn2+exhibits a deposition/dissolution mechanism by electrochemically depositing MnOxonto the electrode surface during charging.At the same time,the Zn2+will be intercalated into the ZnMn2O4and the MnOxrespectively,showing the mechanism of Zn2+intercalation/deintercalation.At the same time,basic zinc sulfate also precipitates on the surface of the electrode 。
the combined effect of multiple energy storage mechanisms breaks The understanding that there is only one energy storage mechanism for a cathode material of AZIBs,and inspires researchers to explore a more complete energy storage mechanism.The changeable energy storage mode enables manganese-based cathode materials to store energy in a variety of environments,and also stimulates the energy storage potential of cathode materials。

4 Problems and Solutions of Manganese-based Cathode Material

In recent years, manganese-based cathode materials have shown different electrochemical behaviors and electrochemical properties. However, manganese-based materials with different valences may have very different electrochemical properties. However, they all have the following common problems: (1) The conductivity of manganese-based materials is poor, which seriously hinders the conduction of electrons and leads to poor charge-discharge rate performance. (2) In the process of charging and discharging, the dismutation reaction of positive active material and its dissolution in water directly lead to the rapid attenuation of capacity. (3) In the process of Zn2+ intercalation/deintercalation, the strong electrostatic interaction between divalent Zn2+ and host ions leads to the low diffusion coefficient of Zn2+. (4) In the process of Zn2+ diffusion, the crystal structure of many manganese-based cathode materials is unstable, and even structural collapse and irreversible phase transition occur, resulting in poor cycle stability. (5) In the process of charge transport at the anode, because the theoretical decomposition voltage of oxygen is 1.23 V vs H+/H2, the energy storage process will be affected by the competition of oxygen evolution reaction (OER) when the voltage is too high. This not only reduces the coulombic efficiency of AZIBs, but also accelerates the evolution of hydrogen due to the increase of H+, which eventually leads to the loss of electrolyte. (6) Batteries often produce some unexpected by-products during long-term cycling. ZHS is formed in most of the energy storage mechanisms, but the reason for its formation seems to be the same, that is, the pH value of the electrolyte increases with the consumption of H+, and then ZHS is formed on the surface of the electrode. ZHS is reversibly dissolved in the electrolyte with decreasing pH[55,56,59]. The formation of ZHS material will lead to the volume expansion of the electrode surface, increase the surface mechanical stress and reduce the stability of the material[62].

4.1 Doping

the poor intrinsic conductivity of manganese-based compounds is a common factor affecting The rate capability of batteries[63,64]。 At present,there are two ways to dope manganese-based compounds,one is bulk doping,the other is doping the cladding layer with foreign elements(such as N,P,S,F)[65]。
Doping metal cations in manganese-based compounds is a common means to improve the conductivity and cycle stability of materials[66,67]。 It is reported that some researchers have prepared Ce-dopedα-MnO2by a simple hydrothermal method,and confirmed that Ce is doped into the host lattice of the MnO2by XRD(as shown in Figure 8A),Rietveld structure refinement and XPS,so that the MnO2is transformed fromβphase toαphase[68]。 The surface average oxidation state(AOS)of the three kinds of manganese increases in the order ofα-MnO2(3.53)<Ce-MnO2(3.75)<β-MnO2(3.98).Compared with theβ-MnO2without Ce doping,the AOS of the Ce-doped manganese is lower,which is due to the phase transition of the MnO2.However,at the same phase(αphase),the AOS of the MnO2is significantly improved by doping Ce.It is well known that the electrochemical reactions involved in the intercalation/deintercalation of Zn2+are stimulated by manganese redox.Therefore,the high oxidation state of manganese is beneficial to improve the specific capacity.In addition,GITT was used to calculate the diffusion rate of Zn2+.Compared with theβ-MnO2electrode,doping Ce in the MnO2structure can effectively improve the diffusion coefficient of Zn2+,which is conducive to the rapid and reversible migration of Zn2+,so it has higher rate capability.Hu et al.Demonstrated that V doping significantly improves the electronic conductivity of MnO2by using first-principles calculations and four-probe method[69]。 Subsequently,a simple redox method to prepare V-doped MnO2for AZIBs was reported[70]。 As shown in fig.8B,the formation of the MnO2was confirmed by XRD studies,accompanied by an anisotropic extension of the MnO2lattice,indicating the incorporation of V into the MnO2framework.V doping improves the conductivity of the MnO2,and the formed porous morphology increases the specific surface area,which is more conducive to the penetration of electrolyte.These advantages help to promote the chemical kinetics of the guest ions and ultimately achieve greater Zn2+intercalation.Zhao et al.Formed oxygen vacancy(VO)Ti-MnO2nanowires by surface gradient Ti doping[71]。 Ti doping results in the decrease of the valence state of Mn and the formation of VO.The doping of Ti and VOopens the complete octahedral wall of the MnO6,which leads to the imbalance of charge distribution and local electric field in the crystal structure and accelerates the transfer of ions/electrons.The diffusion coefficients of the Zn2+and the H+in the Ti-MnO2are improved,which is also proved by the alternating current impedance diagram.In addition,inspired by the NiMn2O4,Long et al.Synthesized carbon-coated porous spinel nanoparticles(NixMn3-xO4@C)by sol-gel method combined with freeze-drying technology[72]。
Carbon coating can improve the conductivity of manganese oxides(such as MnO2@porous-C,β-MnO2@C,λ-MnO2@C,etc.),while doping N in carbon materials can introduce some structural defects,which can further improve the reactivity and conductivity of carbon materials[73⇓~75]。 The researchers synthesized a composite material(ZnMn2O4/NG)of N-doped graphene(NG)coated with ultra-fine nanoparticle ZnMn2O4by one-step hydrothermal method[76]。 NG provides a more efficient electron transport pathway for ZnMn2O4nanoparticles and also stabilizes the material structure,allowing it to withstand greater volume expansion during cycling.In addition,due to the synergistic effect and excellent electron conductivity,its cycle life is very long,and its capacity retention rate is 97.4%.Earlier,the metal-organic framework template strategy has been used to prepare a porous framework and N-doped manganese-based cathode material(MnOx@N-C )[53]。 As shown in Figure 8d,its unique porous structure,onion-like N-doped carbon and amorphous carbon shell are conducive to improving the cycle stability and rate capability of the material.This approach has also been shown to be effective in other manganese compounds(e.g.,Mn3O4@NC,MnO@NC )[47,77]。 In summary,N-doping is a common,simple and feasible method to improve the electrochemical performance of AZIBs cathode materials。

4.2 Surface modification

As an effective way to protect the positive electrode,surface modification(i.e.surface coating or surface coating)can alleviate the problems of dissolution of positive active material and structural collapse[78,79]。 Graphene and carbon nanotubes(CNTs)are two of the most common highly conductive carbon materials.In 2018,researchers reported a kind of AZIBs with graphene coatedα-MnO2as the anode[80]。 As shown in fig.9a,the graphene is uniformly coated on the MnO2nanowires with an average width of 5 nm,which improves the conductivity of the MnO2nanowires and alleviates the dissolution of the cathode material during cycling.At that time,it was the cathode material with the highest specific energy in the field of AZIBs(as shown in Figure 9 B),and had long-term cycle stability,with a capacity retention rate of 94%.Subsequently,α-MnO2@CNT has also been studied and reported[81]。
the application of conductive polymers in surface modification can also avoid The dissolution of active substances[82]。 Taking advantage of the high conductivity of conductive polymers,researchers have demonstrated superior performance of AZIBs in composite gels by coating MnO2and rGO(MnO2@rGO@PANI)with polyaniline(PANI )[83]。 The coating of rGO and PANI on the surface of the MnO2can inhibit the dissolution of manganese(as shown in Figure 9c,d),and improve the rate capability and cycle stability of the AZIBs electrode material.In addition,some researchers have successfully electrodeposited conductive PEDOT on the surface of ZnMn2O4[84]。 The coating of oxides on cathode materials is often neglected.As we all know,indium oxide(In2O3)is widely used in solar cells,supercapacitors,lithium ion batteries and other fields because of its high conductivity[85,86]。 Gou et al.Successfully prepared In2O3coated TiO2 nanotubes under simple and mild hydrothermal conditions.The results of electrochemical kinetics studies showed that the cycle stability and rate capability of AZIBs were greatly improved due to the improvement of conductivity[87]。

4.3 Introduction of defects

Defect engineering can endow new materials with new functions,such as optical,magnetic,and electronic properties[88⇓~90]。 Among the various types of defects,VOplay a particularly important role in modifying the surface chemistry and geometry of oxides[91⇓~93]。 For example,the VOis generated in the MnO2lattice,and the Gibbs free energy of Zn2+adsorption near the VOhas a thermal equilibrium value.This allows the Zn2+insertion/extraction process to be highly reversible and requires fewer electrons to form the Zn2+chemical bond.This means that more electrons can be contributed to the non-localized electron cloud of the material,thereby increasing the specific capacity.However,for pure MnO2,once the Zn2+is adsorbed on the surface of the MnO2,the enhanced chemical bond between Zn and O will hinder the subsequent desorption of the Zn2+.These unintercalated Zn2+ultimately act as a physical barrier,reducing the effective electrochemically active surface area of the MnO2.It is reported that the VOrichδ-MnO2(Od-MnO2)as the cathode material of AZIBs shows promising electrochemical performance[94]。 Fig.10 a is a schematic diagram showing the insertion/extraction of the Zn2+of theδ-MnO2and the Od-MnO2.In addition,the oxidation state of Mn was studied by X-ray near-edge absorption structure spectroscopy(XANES).The results of XANES confirm(fig.10 B)that the average oxidation state of manganese in the obtained Od-MnO2is between+3 and+4,and the crystal structure of the prepared Od-MnO2sample is considered to be anoxic.It is worth noting that the Od-MnO2after 4 months and the fresh Od-MnO2show similar K-edge X-ray absorption near edge spectra(see fig.10 C),indicating that the prepared Od-MnO2is stable for a long time,which provides a possibility for commercial application of the electrode.VOrichβ-MnO2have also been reported to have excellent electrochemical properties[95]。
Earlier reported studies on the chemical extraction of zinc from spinel ZnMn2O4have shown that the ideal spinel structure is not suitable for the insertion/extraction of Zn2+[96]。 This is because in an ideal spinel,when the Zn2+migrates from one tetrahedral site(4A)to another tetrahedral site(4A),it passes through an unoccupied octahedral site(8C),thus generating a large electrostatic repulsion with the manganese in the adjacent octahedral sites(8d),which strongly hinders the diffusion of the Zn2+,as shown in Fig.10d.However,the introduction of cation vacancies from spinel oxides(Co0.37Mn2.15O4)provides an additional path for the migration of divalent ions(Mg2+).Inspired by this,Chen et al.First introduced cation vacancies into spinel ZnMn2O4and prepared spinel ZnMn1.86Y0.14O4(Y:(manganese vacancies)with abundant manganese vacancies[97,98][99]。
In contrast,the spinel rich in manganese vacancies allows the Zn2+to diffuse more easily without too many electrostatic barriers,resulting in a higher migration rate of the Zn2+,thus speeding up the electrode kinetic process.Subsequently,the oxygen-rich vacancy ZnMn2O4(OD-ZMO)was also reported[84]。 According to the vacancy formation energy(Evac)of Zn2+to evaluate the insertion/extraction process of Zn2+,DFT calculation shows that the Evacvalue of(101)surface of OD-ZMO(2.36 eV)is significantly lower than that of pure ZnMn2O4(3.87 eV)(Fig.11A).The decrease in Evacmeans that vacancies are more likely to be generated at the Zn site of the Od-ZMO,which facilitates the insertion/extraction of Zn2+.In addition,the capacity and rate capability of ZnMn2O4are closely related to their Zn2+mobility.As shown in fig.11B,C,the mobility of zinc atoms near the VOexhibits a low barrier of 0.24 eV,while the mobility barrier of zinc atoms in the cell without oxygen defects is 0.39 eV 。
Therefore,we can speculate that cation vacancies may be more inclined to help Zn2+move from one site to another,while VOmay be more inclined to help Zn2+escape from the site.The two kinds of vacancies can increase the diffusion coefficient of the Zn2+,thereby improving the electrochemical performance of the ZnMn2O4 。

4.4 "Pillar"effect

Due to the poor conductivity and structural stability of manganese oxides,the capacity decay is fast in the process of Zn2+intercalation/deintercalation.It is reported that the crystal water not only increases the interlayer spacing,but also effectively shields the electrostatic interaction between the guest ions and the host skeleton[100]。 At the same time,the diffusion of guest ions in the host material is accelerated,which is helpful to stabilize the host structure during the cycle.A birnessite with a high amount of water of crystallization(10 wt%)in the interlayer was synthesized by electrochemical conversion of Mn3O4in aqueous solution[101]。 By DFT calculation and EXAF analysis,the Zn2+is more likely to coordinate to the octahedral triangular shared site(TCS),where the Zn2+is coordinated to three water molecules by forming a tridentate bond with the MnO2layer(fig.12A,state 1 )[102,103]。 At the same time,due to the electrostatic repulsion of the Zn2+,the manganese atoms in the MnO2layer facing the Zn2+are pushed upward,leaving a vacancy in the MnO2layer(fig.12A,state 2).Next,the moving manganese atom forms an overhanging octahedron by coordinating to three water molecules,because the octahedral coordination of Mn has 0.88 eV less energy than the tetrahedral coordination(see fig.12A,state 3 vs 2).Finally,the so-called Zn-Mn"dumbbell"structure is formed[104]。 The presence of water molecules alleviates the dissolution of the Mn2+.Proper interlayer spacing and high water content help to mitigate manganese dissolution and stabilize the host skeleton during cycling,resulting in sustainable cycling 。
In addition to crystal water,metal ions(K+,Na+,La3+,etc.)Embedded in manganese oxides can also improve their stability,conductivity and ion diffusion rate.A KMO containing a K+and a VOwas mentioned earlier[52]。 The results show that the embedded K+can support the tunnel structure of theα-MnO2and inhibit the dissolution of manganese.The defective octahedron(Fig.12B),which promotes ion diffusion,plays a key role in the rapid reaction and capacity improvement of KMO.It is also reported that KxMn8-xO16(x=0.2~1)nanocrystals prepared by K+intercalation and structural properties are used in AZIBs anode.Nanocrystalline electrodes have the advantages of good stability,high energy density and high ion diffusion coefficient,and have potential competitiveness in the field of energy storage(Fig.12c )[105][106]。 Wang et al stabilized the layered structure by embedding the Na+and the H2O into theδ-MnO2[107]。 As shown in fig.12d,the interlayer has a gap of 0.72 nm with the Na+and the H2O as pillars,activating the intrinsic high performance of theδ-MnO2.The results showed that the Na0.46Mn2O4·1.4H2O battery had high rate capability,and the capacity retention rate was as high as 98%after 10 000 cycles due to the smooth diffusion of Zn2+in the intercalation structure.Zhang et al.Synthesized La3+intercalatedδ-MnO2(LMO(nanoflowers)by precipitation method.La3+can be used as a support to prevent structural collapse during cycling.The intercalation ofδ-MnO2can increase the interlayer spacing and reduce the intercalation impedance of Zn2+[108]。 In addition,there are also studies on the insertion of non-metallic ions into the interlayer of MnO2,and the insertion of oxygen defects and phosphorus ions is achieved simultaneously by phosphorylation[109]。 The former can improve the conductivity of the MnO2,while the latter can expand the interlayer spacing of the MnO2and accelerate the ion transfer 。
Conductive polymers can be used not only for surface modification,but also for supporting interlayer spacing and stabilizing structure[110]。 The intercalation of polyaniline(PANI)into the interlayer ofδ-MnO2(as shown in Figure 12e)can effectively eliminate the phase transition and subsequent structural collapse induced by the intercalation of hydrated H+/Zn2+,which is of great significance for obtaining high charge storage capacity and long cycle life[111]。

4.5 Compound

LIBs and SIBs have demonstrated that the construction of two-phase composites is important for improving electrochemical performance[112]。 First of all,the synergistic effect of two-phase composites is beneficial to inherit the advantages of the corresponding single-phase materials[113]。 Second,the introduction of a multiphase compound will increase the interfacial area,thereby providing additional ion storage sites and improving electronic and ionic conductivity[114,115]。 Finally,the two phases with stepwise redox reaction can effectively reduce the deep solid-state diffusion,thereby relieving the stress in the ion insertion/extraction process[116]。
Wu Xianwen et al.First prepared ZnMn2O4/Mn2O3composites with microsphere structure by microemulsion-assisted solvothermal method[117]。 As shown in Figure 13A,the diameter of the primary particles is about 30~40 nm,and the ultrafine particles have a large specific surface area,which greatly increases the contact area between the material and the electrolyte,facilitates the diffusion of the Zn2+,shortens the diffusion distance in the solid phase,and improves the cycle performance of the composite material at high rate.As shown in fig.13(B),the initial charge and discharge capacity of constant current charge and discharge at 0.5 A·g-1is only 15.7 mA·h·g-1,which is mainly attributed to the high electrostatic repulsion force on the Zn2+in the ZnMn2O4during the initial charge and discharge[99]。 However,the subsequent discharge capacity reaches a 82.6 mA·h·g-1,because the Zn2+may be intercalated into the host material of theα-Mn2O3with the reduction of the Mn3+to the Mn2+during the initial discharge process.Therefore,there is a huge increase in capacity,showing a good synergistic effect of the composite material.At the same time,the ZnMn2O4will undergo a transition to a layered Zn-birnesite phase,which shows excellent cycle stability in the subsequent cycling process[43]。 In addition,the diffusion kinetics at different scan rates were studied,as shown in figs.13C and d.The results show that the Zn2+in the composite has the behavior of ultrafast ion diffusion[99]。 This may be due to the fact that the two phases in the composite significantly enrich the redox reaction to buffer the stress during the insertion of the Zn2+[116]。
Subsequently,MnOx@N-C(MnO and MnO2composites),MnOx/PPy(MnO2and Mn3O4composites)were also reported,and these composites showed excellent rate capability and cycle stability.Recently,manganese-based oxides with other metal oxides have also been reported.Gou et al.Synthesized Mn2O3/Al2O3composites with microbeam structure by using MOFs as precursors[118]。 The results show that the Al2O3can effectively inhibit the dissolution of Mn2+in the disproportionation reaction of Mn3+,and the obtained material still has 118.0 mA·h·g-1after 1100 cycles at 1500 mA·g-1.While the pure Mn2O3has only 20 mA·h·g-1,the composite material not only shows higher capacity,but also has more excellent cycle stability 。

4.6 Structure Design

Zero-dimensional,one-dimensional and two-dimensional materials have their own characteristics.Using them as units to build nano-materials with various structures can not only retain the advantages of the original structure,but also obtain other structural characteristics at different scales.Uch as large size,high porosity,significant specific surface area,and excellent permeability,facilitate charge transfer and shorten ion diffusion paths,and facilitate storage/release of guest ions in the electrode material structure.Among them,the construction of hollow and porous structures is the most common.Wu Xianwen et al.Prepared hollow and porous ZnMn2O4by carbon microsphere template method[54]。 As shown in fig.14B,the specific surface area of the:ZnMn2O4calculated by BET was 31.9 m2·g-1,and the pore size distribution was 1.3 to 300 nm.The average pore size is about 31.2 nm,and there is abundant mesoporous structure in the sample,which is consistent with the hollow core-shell structure observed by TEM(Figure 14 a).This cathode material was used in AZIBs and achieved a specific capacity of 70.2 mA·h·g-1at a current density of 3.2 A·g-1,indicating a good rate capability of the material(Figure 14C).After 300 cycles,the capacity retention rate can reach 100%,indicating that the material can show excellent cycle stability at different rates(as shown in Figure 14 d).Manganese-based materials with a porous structure are more common,α-MnO2/CNT-HMs(nanofiber/carbon nanotube assembly microspheres),NixMn3-xO4@C,MnOx@N-C,Cube-like Mn3O4@C,porous MnO2membranes,and ZnMn2O4@PCPs(porous carbon polyhedra)all have a large number of porous structures[72][53][119][120][121]。
Adhesive-free free-standing composite electrodes are usually fabricated on Three-dimensional substrates by In-situ growth/deposition.three-dimensional substrates(such as carbon fiber paper(CFP),carbon fiber cloth(CFC),carbon fiber felt(CFF)and stainless steel welded mesh(SSWM))can not only improve the conductivity of the electrode,but also provide support for the active material to form a stable porous structure,and some substrates can also be used in flexible batteries.in addition,the absence of a binder greatly reduces the resistance of the material。
Tang et al.Prepared a nanoflower-like SSWM@Mn3O4composite by in-situ growth of Mn3O4on stainless steel mesh by hydrothermal method(Fig.15A )[122]。 As shown in fig.15B by SEM,the material is a multi-layer nanoflower structure composed of ultra-thin nanosheets,which is conducive to the migration and diffusion of Zn2+.The EIS spectra(Fig.15C)show that the SSWM@Mn3O4electrode has a lower electrochemical reaction impedance and a smaller Warburg impedance than the bare Mn3O4electrode,which is attributed to the fact that the binderless and 3D substrate effectively enhance the electron transfer kinetics in the electrode.Cao et al.Used zeolite-based imidazolium salt framework-67(ZIF-67)as a self-sacrificial template to grow MnO2hollow polyhedra in situ on CFC by hydrothermal method,which had a large specific surface area and was conducive to electrolyte penetration and inhibition of volume expansion,while CFC was conducive to electron transport[123]。 More importantly,thanks to the unique structural advantages,the flexible AZIBs assembled by MnO2/CFC can successfully light LED bulbs at different bending angles,showing excellent flexibility 。
Wang et al.Prepared the positive electrode of MnO2@CFP by in-situ electrodeposition of MnO2on carbon fiber paper[56]。 The electrodeposition process is shown in Figure 15d.As the Mn2+of the MnSO4solution is oxidized,a uniform MnO2is deposited on the surface of the carbon fiber paper(CFP)(see inset of Figure 15d).In addition,the SEM images further show that the MnO2is composed of interconnected nanosheets,forming a highly porous structure(fig.15e).The porous nature of the MnO2provides a rich electrode/electrolyte contact interface,which reduces ion diffusion paths and enables rapid electrochemical kinetics.The EIS spectrum(as shown in Fig.15f)shows that the positive electrode of the MnO2@CFP has a very low electrode reaction resistance,which remains almost unchanged after 100 cycles,indicating that the electrode has good cycle stability.Subsequently,electrodeposited MnO2@CFC and MnO2@CFF were also reported 。

4.7 Electrolyte optimization

The influence of electrolyte on the cycle stability of manganese-based cathode materials is sometimes even greater than the material itself.At present,ZnSO4aqueous solution is regarded as a common electrolyte for AZIBs because of its low price and excellent electrochemical properties[124]。 However,due to the Jahn-Teller effect,manganese-based cathode materials tend to dissolve in the Mn2+.Some researchers have added MnSO4to the electrolyte to greatly improve the utilization of MnO2active materials,providing higher capacity than materials without MnSO4[55]。 The results show that the electrode has higher cycle stability and electrochemical capacity when the 0.1 mol/L Mn2+is added to the electrolyte.However,due to the strong solvation of Zn2+in aqueous solution of ZnSO4,the diffusion rate of Zn2+is slow.Therefore,the cycle stability of the cathode material is not ideal,and a long activation process is needed to reach the maximum specific capacity[32]。 The excellent electrochemical performance of the Zn(CF3SO3)2can be attributed to the fact that the bulky CF3S O 3 - reduces the number of water molecules around the Zn2+,reduces the solvent effect,and promotes the migration and charge transfer of the Zn2+[99]。 In addition,the precipitation of the H2is effectively inhibited,and the zinc deposition/dissolution efficiency is also improved.Similar to the Zn(CF3SO3)2electrolyte,the Zn(TFSI)2+LiTFSI electrolyte reported by Wang et al.Can also reduce the solvation effect[125]。 Zn2+and a large number of TFSI+will form a tight(Zn-TFSI+),which will form a solvated sheath structure and significantly inhibit the generation of(Zn-(H2O)6)2+.However,the price of Zn(CF3SO3)2and Zn(TFSI)2electrolyte is very expensive,even more than ten times the price of ZnSO4.To sum up,the electrolyte of adding 0.1 mol/L MnSO4to the aqueous solution of ZnSO4has become the most common aqueous electrolyte for AZIBs 。
Ionic conducting polymer electrolytes(PEs)have attracted much attention because they contain less aqueous solution,can alleviate the dissolution of active materials caused by aqueous solution,and can effectively inhibit the growth of zinc dendrites[126]。 PEs are mainly divided into three categories:solid polymer electrolytes(SPEs),gel polymer electrolytes(GPEs)and hybrid polymer electrolytes(HPEs).They are all composed of polymer chains and salt media.SPEs have good mechanical properties,but have little effect on ionic conductivity.GPEs can improve ionic conductivity,but reduce mechanical strength.HPEs are made of two or more polymers through crosslinking,self-assembly or copolymerization,which have the advantages of both.PEs are widely used in the field of flexible batteries because of their portability.However,as the cycle goes on,the aqueous solution in the PEs will continue to be consumed,which will lead to a sharp decline in the diffusion capacity of ions,resulting in a rapid decline in their capacity。

4.8 Other strategies

Due to the multivalency of Mn(+2,+3,+4),it is feasible to use a two-electron reaction instead of the common one-electron redox reaction(e.g.Mn4+/Mn2+compared to Mn4+/Mn3+),which not only increases the capacity,but also broadens the voltage window[127]。
Inspired by this,Qiao et al.Prepared a new type of high-voltage zinc-manganese battery(EZMB)and a typical dissolution/deposition reaction mechanism[17]。 The schematic diagram of EZMB is shown in Figure 16A.During charging,the Zn2+is deposited on the graphite foam to form the zinc foam negative electrode,and the Mn2+is deposited on the carbon fiber cloth to form the MnO2positive electrode.During discharge,Zn and MnO2are dissolved in the cathode and anode,respectively,and the whole process is completely reversible.As shown in fig.16(B),linear sweep voltammetry was performed on the zinc negative electrode and the MnO2positive electrode,respectively,and the HER and OER properties were obtained.Cathodic scanning showed zinc deposition and hydrogen evolution on the negative electrode,and anodic scanning showed MnO2deposition and oxygen evolution on the positive electrode.The results show that the minimum deposition voltage for simultaneous deposition of Zn and MnO2is~2.00 V,and the maximum deposition voltage is~2.41 V in the range of deposition potentials of H2and O2.Therefore,a constant voltage charge of 2.2 V was chosen to allow for both the success of the deposition and the avoidance of water decomposition.In the actual discharge process,the MnO2deposition/dissolution reaction makes the EZMB battery have a very high discharge plateau at 1.95 V and a 409 Wh·kg-1energy density.At a theoretical cell voltage of 1.991 V,the electrochemical reaction equation in EZMB is as follows:
Negative:Zn2++2e-↔Zn
E0=-0.763 V vs SHE
Anode:Mn2++2H2O↔MnO2+4H++2e-
E0=1.228 V vs SHE
Whole reaction:Zn2++Mn2++2H2O↔Zn+MnO2+4H+
E0=1.991 V vs Zn/Zn2+
Subsequently,some researchers also used electrodeposition to prepare the positive electrode of MnO2,and proposed a strategy to separate the electrolyte,which skillfully maximized the amount of charge transfer in the redox reaction[128]。 The zinc manganese battery(DZMB)uses neutral electrolyte to separate the two chambers containing alkaline and acidic electrolytes to avoid their neutralization,and the neutral electrolyte chambers are wrapped by anion and cation exchange membranes(with ion selectivity),respectively.By adopting the design,the working conditions of the positive electrode of the MnO2and the negative electrode of the Zn are separated,so that both the positive electrode and the negative electrode have redox reaction of double electron transfer,and the specific reaction is as follows:
Negative:Zn2++2OH-↔Zn(OH ) 4 2 - +2e-
E0=-1.199 V vs SHE
Anode:Mn2++2H2O↔MnO2+4H++2e-
E0=1.224 V vs Zn/Zn2+
As a result,the DZMB has an extremely high open-circuit voltage of 2.83 V and an energy density of 1621.7 W·h·kg-1.To better illustrate the reversibility of MnO2deposition/dissolution,MnO2electrodes obtained at different depths of discharge(DOD)and depths of charge(DOC)were analyzed by SEM.With the increase of DOD,the MnO2dissolved gradually.On the contrary,with the increase of DOC,the MnO2deposited gradually 。

5 Conclusion and prospect

In this paper, the latest research progress of AZIBs manganese-based cathode materials is reviewed, and the existing problems and optimization strategies of manganese-based cathode materials are discussed. In addition, the structure and electrochemical properties of various manganese-based materials in AZIBs are summarized (Table 2). However, in practical applications, it is still a challenge to find cathode materials with long cycle life and stable performance, and AZIBs still have the following bottlenecks in large-scale energy storage: (1) Low discharge voltage, the discharge voltage platform is generally 0.6 ~ 1.8 V.LIBs is as high as 3.2 ~ 5.0 V. (2) The specific energy is not high, and the 70~140 W·h·kg-1 is still significantly lower than that of the LIBs(180~230 W·h·kg-1);(3)[17,129]. (4) Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) occur, resulting in the loss of aqueous solution; (5) The working temperature range is narrow, generally 0 ~ 30 ℃, and it is difficult to be competent for energy storage in harsh environments.
In this regard, in-depth study of valuable research directions can play a guiding role in AZIBs with manganese-based materials as cathode, as follows: (1) The dissolution/deposition mechanism needs to be further studied, because the two-electron reaction can obtain a higher discharge platform, extend the voltage window, and further improve the specific energy of the battery[59,129]; (2) In the mechanism of H+/Zn2+ intercalation, due to the small ionic radius and valence state of H+, the electrode obtains very good kinetic properties, with significant capacity and excellent performance. Therefore, the specific capacity and the coulombic efficiency of the battery are expected to be improved by increasing the intercalation contribution of the H+. (3) In terms of electrolyte, the best way to solve the problem of loss of aqueous solution is to make solid/gel polymer electrolyte, and to solve the defect of narrow working temperature range; (4) In the aspect of negative electrode, zinc alloy, surface modification, structure design and embedded negative electrode are all good ways to solve the problems of zinc dendrite, corrosion, passivation and corrosion of negative electrode; (5) Manganese-based cathode materials need to be further optimized. Next, we put forward some prospects and predictions for the future research direction of manganese cathode.
In the existing optimization strategies of manganese-based materials, there are still many ideas that have not been tried. Although the doping of foreign elements (such as N, P, S, F) on carbon-based materials has been widely known, in the field of AZIBs, only carbon-doped N has been reported, while carbon-doped P, S, F and other non-metallic elements have not been tried, but there have been many reports in LIBs batteries.Doping elements with different electronegativities (such as N: 3.04, P: 2.19, S: 2.58, F: 3.98) can destroy the charge neutrality of carbon materials (such as graphene) and generate charge sites, thus improving the charge-discharge capacity of the electrode[124,130,131][124]. In recent years, metal cation doping and "pillar" effect modification of manganese-based compounds have been widely reported, but the types of cations reported are still very limited. Other metal ions, such as Al3+, Ca2+ and other metal elements, have smaller or similar atomic radii and similar valences than manganese ions, which may allow them to be doped into the lattice or interlayer[132,133]. At present, the surface modification of polymers and oxides is only a preliminary exploration. Both PEDOT and PAIN are electron-conducting polymers, not ion-conducting polymers, but Zn2+ and electron transport in the active species are equally important. Therefore, it is possible to achieve more excellent performance by using the bi-conductive polymer as a modification material in the positive electrode. Among them, although polydopamine does not belong to the above two categories, it may be another good choice [134~136] because of its simple coating process, good stability, strong hydrophilicity and easy-to-modify surface. In addition, the SnO2 has good stability and ultra-high conductivity, which may be a direction of oxide coatings[137]. In terms of structural design, nanofiber AZIBs cathode materials are rarely reported. Electrospinning technology is widely used in the synthesis of nanofiber materials because of its simple equipment and easy control of the synthesis process. It can not only synthesize nanofiber materials with high aspect ratio on a large scale, but also easily realize the design of morphology (such as hollow, porous structure), and even realize surface modification. In addition to the existing optimization strategies of manganese-based compounds, there are many research directions to try. For example, because of the high electronegativity of oxygen, the intercalation/deintercalation of Zn2+ is difficult, and the phase transition of manganese-based oxides is easy to occur during charge and discharge, resulting in inevitable capacity fading. The introduction of oxygen vacancies is one solution strategy, but in addition, structures with less or no oxygen (e.g., manganese-based carbides, manganese-based sulfides, manganese nitrides, manganese phosphides, manganese selenides) can reduce this effect, thereby reducing the phase transition of the structure.
To sum up, AZIBs need to be further studied systematically, especially for new manganese-based cathode materials. With the pursuit of convenience, the preparation of flexible and corrosion-resistant manganese-based cathode materials will be the future development trend in the field of energy storage.

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Funding

the National Natural Science Foundation of China(51704124)

the National Natural Science Foundation of China(51762017)

the National Natural Science Foundation of China(51262008)

the Research Foundation of Education Bureau of Hunan Province, China(18A285)

and the Huxiang Youth Talent Support Program(2018RS3098)

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