Recent Advances on Challenges and Strategies of Manganese Dioxide Cathodes for Aqueous Zinc‐Ion Batteries

Aqueous zinc‐ion batteries (AZIBs) are regarded as promising electrochemical energy storage devices owing to its low cost, intrinsic safety, abundant zinc reserves, and ideal specific capacity. Compared with other cathode materials, manganese dioxide with high voltage, environmental protection, and high theoretical specific capacity receives considerable attention. However, the problems of structural instability, manganese dissolution, and poor electrical conductivity make the exploration of high‐performance manganese dioxide still a great challenge and impede its practical applications. Besides, zinc storage mechanisms involved are complex and somewhat controversial. To address these issues, tremendous efforts, such as surface engineering, heteroatoms doping, defect engineering, electrolyte modification, and some advanced characterization technologies, have been devoted to improving its electrochemical performance and illustrating zinc storage mechanism. In this review, we particularly focus on the classification of manganese dioxide based on crystal structures, zinc ions storage mechanisms, the existing challenges, and corresponding optimization strategies as well as structure–performance relationship. In the final section, the application perspectives of manganese oxide cathode materials in AZIBs are prospected.


Introduction
With the increasing social environmental problems and energy crisis, the goal of emission peak and carbon neutrality has gradually become a global consensus.Massive research and development efforts have been made around the world to develop advanced energy storage technology. [1]In the field of secondary batteries, rechargeable lithium-ion batteries (LIBs) have been widely studied due to its high energy density, wide operating voltage, and long-cycle performance, occupying a dominant position in the current energy storage market.4] Potassium-ion batteries (KIBs) and sodium-ion batteries (SIBs) as the choice of the next generation of electrical energy storage batteries, are feasible for massive applications due to their advantages of relatively rich resources, low cost, and high energy and power density.[19][20] So far, anode material for AZIBs is traditional Zn metal or modified novel Zn metal, such as growth 3D ZnOHF nanowire array on Zn foil, [21] nitrogen-doped Zn foil. [22]The latter can effectively inhibit the dendritic growth and the occurrence of side reactions on the Zn anode surface.[25][26] The comparison of the advantages and disadvantages of various cathode materials is shown in Figure 1.As a kind of potential cathode material, vanadium-based materials attract more attention due to its multiple valence states, superior rate capability, and a higher theoretical capacity.However, the low average operating voltage for vanadium-based materials should be further addressed.On the contrary, Prussian blue analogs could reach an output voltage of about 1.6 V, and 3D open-framework structures could also accelerate the rapid transport of Zn 2+ .Unfortunately, zinc storage capacity and rate capability should be further increased.Transition metal sulfides also suffer from low discharge voltage, inferior conductivity, and severe volume expansion, which hinder its further practical application.[47] As shown in Tables 1 and 2, all manganese oxide with different crystalline phases are composed of [MnO 6 ] octahedrons.The main difference is that the octahedron is connected in different ways, including sharing edge or/and angle, which leads to the formation of different crystal structures, such as chain structure, tunnel structure, and layered structure. [48]p to now, researchers have carried out numerous research works on MnO 2 cathode materials with different crystal structures in AZIBs.The results show that there are great differences in the electrochemical performance reflected by the different crystal structures.Meanwhile, the zinc storage capacity of MnO 2 with the same tunnel structure but different morphology is also significantly different.The main difference is that α-MnO 2 is constructed by a group of [MnO 6 ] octahedrons shared bilaterally with tunnel structure (2*2), in which the channel size is (4.6*4.6 A).Due to its relative stability and suitable channel, it is relatively easy to achieve rapid and reversible insertion and extraction for Zn 2+ ions. [49,50]Compared with α-MnO 2 , β-MnO 2 is with the narrowest (1*1) tunnel structure (2.3*2.3A), which is difficult to accommodate the intercalation/de-intercalation of Zn 2+ ions. [51]owever, β-MnO 2 has more stable thermodynamic properties than α-MnO 2 and even other crystalline forms MnO 2 . [47]γ-MnO 2 has a mixed tunnel structure of (1*1) and (1*2), which can be regarded as a symbiote of β-MnO 2 and R-MnO 2 .Besides, there are a lot of structure defects, such as stacking faults and vacancies.Therefore, it exhibits good electrochemical performance in AZIBs. [52]odorokite-MnO 2 has a (3*3) tunnel structure with the largest channel size, which can be stabilized by inserting water molecules or various cations into its tunnel.δ-MnO 2 has a two-dimensional layered structure and an interlayer spacing of about 0.7 nm, which is conducive for Zn 2+ ions insertion and extraction.[55] In addition, λ-MnO 2 and ε-MnO 2 of threedimensional spinel structure are not conducive to the diffusion of Zn 2+ ions due to the compact and limited 3D tunnel structure. [56]espite manganese dioxide materials having many advantages for AZIBs, there are three vital important problems that still inhibit its further development and application.Firstly, during the repeated insertion/extraction process of Zn 2+ , the structure of cathode materials will undergo irreversible phase transition, volume change, and structural collapse. [57,58][61] In addition, the poor electrical conductivity of MnO 2 materials affects the conduction of electrons, which is not conducive to the insertion and extraction of Zn 2+ ions. [62,63]The common problems seriously lead to the rapid capacity decay, unsatisfactory cycling stability, and rate capability of AZIBs in the process of cycling.In response to these challenges, researchers have devoted themselves to improving the electrochemical performance of AZIBs by using a variety of strategies such as surface modification, [64] nanostructure design, [65,66] defect engineering, [67,68] construction of composites, [69,70] doping, [56,71] and electrolyte modification. [72,73]Apart from that, numerous studies claim that MnO 2 cathode materials frequently suffer from complex structural transformation during electrochemical energy storage.Researchers have conducted in-depth research on the zinc storage mechanism of AZIBs, but it remains highly controversies.As a result, it is of great significance to review the advances and In this review, we first summarized the classification and structural features of MnO 2 cathode materials briefly.Then, we focus particularly on the main zinc storage mechanisms and current key challenges existing in MnO 2 as well as corresponding optimization strategies, including electrolyte modification, surface engineering, heteroatom doping, defect engineering, nanostructures, and so on.Finally, the practical consideration and future perspectives of MnO 2 cathode materials in developing high-performance AZIBs are prospected.α-MnO 2 @In 2 O 3 M ZnSO 4 + 0.1 M MnSO 4 1.0-1.8425 mAh g −1 at 0.1 A g −1 75 mAh g −1 after 3000 cycles at 3 A g −1 [ 194]   α-K 0.19 MnO 2 M Zn(CF 3 SO 3 ) 2 + 0.2 M Mn(CF 3 SO 3 ) 2 0.8-1.9211 mAh g −1 at 1 C 180 mAh g −1 after 400 cycles at 5 C [120]   α-MnO 2 /Super-P M ZnSO 4 1.0-1.8180 mAh g −1 at 1.0 A g −1 47% after 1000 cycles at 1.0 A g −1 Bi-doped α-MnO δ-MnO 2 NDs M ZnSO 4 + 0.1 M MnSO 4 1.0-1.9212.5 mAh g −1 at 0.1 A g −1 86.2% after 1000 cycles at 1 A g −1 [ 198]   ε-MnO 2 @N M ZnSO 4 + 0.5 M MnSO 4 1.0-1.8124 mAh g v1 at 5.0 A g −1 83% after 1000 cycles at 5 A g −1 Todorokite-MnO 2 M ZnSO 4 0.7-2.0108 mAh g −1 at 0.5 C 72% after 50 cycles at 0.5 C [199]   MnO 2 /rGO M ZnSO 4 + 0.1 M MnSO 4 1.0-1.9317 mAh g −1 at 0. insertion/extraction mechanism. [82]α-MnO 2 is one of the most concerned cathode materials for AZIBs.As early as 2012, Xu et al. [83] proposed that Zn 2+ could be easily stored in α-MnO 2 tunnel structure in Zn(NO 3 ) 2 stream electrolyte, and found that ZnMn 2 O 4 would be generated after Zn 2+ insert into α-MnO 2 , the reaction equations listed as follows: Wu and co-workers [45] designed a graphene scroll-coated α-MnO 2 cathode material for AZIBs.As illustrated in Figure 2a, it demonstrated that Zn 2+ was firstly inserted into the framework layer of α-MnO 2 to form Zn-busserite.When the continuously entered Zn 2+ cannot be fully accommodated, Zn 2+ will be inserted into (2*2) tunnel of α-MnO 2 by two-step intercalation mechanism, which was verified by in situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and galvanostatic intermittent titration technique (GITT) analysis.Kim et al. [84] studied the zinc storage performance of nanorod-type α-MnO 2 by synchronous XRD technology, and the results showed that the chemical process of Zn intercalation/extraction in α-MnO 2 nanorods was reversible, which was consistent with previous reports. [83]However, the crystal plane spacing of (110) plane in α-MnO 2 crystal structure changed with the process of Zn 2+ intercalation/extraction (Figure 2b), which provided buffer space for volume change, and thus made α-MnO 2 structure more stable.Khamsanga et al. [85] reported for the first time that the cathode of δ-MnO 2 with layered structure supported on graphite flake (noted as MNG) has the characteristics of rapid Zn 2+ insertion and extraction, as observed in Figure 2c.During the discharge process, the anode zinc metal will dissolve in the electrolyte containing Zn 2+ and insert into the structure of δ-MnO 2 in the form of Zn 2+ ions.The whole process is reversible, and forms ZnMn 2 O 4 with spinel structure and layered δ-Zn x MnO 2 . [54]Islam et al. [47] pointed out that zinc hydroxide sulfate hydrate (ZnSO 4 Á3Zn(OH) 2 Á5H 2 O, ZHS) new phase would be precipitated on the surface of the electrode when Zn 2+ was inserted into β-MnO 2 nanorods, and zinc insertion phase would be gradually formed with the precipitation of ZHS (Figure 2d).The intercalation process of Zn 2+ is carried out by both solution reaction and transformation reaction.Subsequently, Alfaruqi et al. [52] confirmed that Mn(II) phase would be generated during zinc intercalation of MnO 2 and showed that Mn(IV) would be reduced to Mn(III) and Mn(II) with the gradual insertion of Zn 2+ into the tunnel structure of γ-MnO 2 (Figure 2e).In other words, γ-MnO 2 gradually converts into ZnMn 2 O 4 with spinel structure, γ-Zn x MnO 2 with tunnel structure, and L-Zn y MnO 2 with layered structure in the progress of discharge.When fully charged, these phases can almost return to the original structure of γ-MnO 2 .
Researchers carried out a large number of studies on the mechanism of Zn 2+ insertion/extraction of MnO 2 , [86][87][88] and found that the mechanism also exists in cathode materials such as MnO and Mn 2 O 3 . [89,90]

Co-Intercalation/Extraction Mechanism
Compared with the non-aqueous electrolyte, the aqueous electrolyte of ZIBs contains a lot of H + ions.[93] However, there are some differences between H + and Zn 2+ in ion radius, thermodynamics, and reaction kinetics, so H + will be inserted into the cathode materials before Zn 2+ . [94,95]Huang et al. [96] deeply researched and comprehensive analyzed the energy storage mechanism of Zn/MnO 2 batteries in a mixed aqueous solution of ZnSO 4 and MnSO 4 by Mn-Zn-O phase diagram, E-pH diagram, and so on.In the initial discharge state, Zn x Mn 2 O 4 , MnOOH, and Mn 2 O 3 formed with the insertion of H + and Zn 2+ .Zn x Mn 2 O 4 formed when Zn 2+ is inserted into α-MnO 2 at about 1.4 V, while ZHS formed due to the increase in pH.The reaction equations of α-MnO 2 /Zn battery were as follows: 2− all participated in a series of complex electrochemical reactions during the charging and discharging process, and the storage mechanism is relatively complex.This discovery also provides guidance and helps in further research.Gao et al. [97] confirmed the existence of H + /Zn 2+ co-Energy Environ.Mater.2023, 6, e12575 intercalation/extraction mechanism in α-MnO 2 in aqueous electrolyte, which is consistent with the reports of Liu and coworkers. [73,98]As displayed in Figure 3b, H + inserted into α-MnO 2 earlier than Zn 2+ in the discharge process, which resulted in the enrichment of OH − and the formation of basic zinc sulfate [Zn(OH) 2 ] 3 ZnSO 4 Á5H 2 O, while ZnMn 2 O 4 formed after Zn 2+ inserted.It is also found that the intercalation/extraction process of H + is more reversible than that of Zn 2+ .Jiang et al. [99] showed that the composite material of multiple manganese oxide and carbon exhibited considerable reversible specific capacity and excellent rate capability (Figure 3c), which is mainly because of the combined effect of internal charge transfer and H + /Zn 2+ coinsertion/extraction in the material.
As shown in Figure 3d, Jin et al. [100] demonstrated a storage mechanism in which both non-diffusion-controlled Zn 2+ insertion and H + conversion reaction work together, and pointed out the sequence relationship between the two reactions.In Zn(TFSI) 2 -based electrolyte, the non-diffusion-controlled Zn 2+ will first be inserted in δ-MnO 2 to dominate the initial discharge process.Then, H + reacts with δ-MnO 2 to form MnOOH and Zn(TFSI) 2 [Zn(OH) 2 ] 3 discharge compound at the same time.The charging process is highly reversible.Wang et al. [101] pre-intercalated Na + and H 2 O molecules into δ-MnO 2 and promoted  [45] Copyright 2018, Wiley-VCH.b) Schematic illustration for the zinc insertion into tunnel-structured α-MnO 2 , which causes the expansion of tunnel and hence increases the interplanar spacing of adjacent (110) planes.Reproduced with permission. [84]Copyright 2015, Elsevier.c) Chemistry schematics of the zinc-ion battery.Zn 2+ ions migrate between tunnels of the MNG cathode and Zn anode.The inset on the right shows Zn 2+ -ion insertion and interconnection between δ-MnO 2 and graphite.Reproduced with permission. [85]Copyright 2019, Springer Nature.d) In situ synchrotron XRD pattern of β-MnO 2 nanorod cathode recorded during electrochemical discharge/charge and a close-up view of the corresponding (101) plane reflection.Reproduced with permission. [47]Copyright 2017, The Royal Society of Chemistry.e) Schematic illustration of the reaction pathway of Zn insertion in the prepared γ-MnO 2 cathode.Reproduced with permission. [52]Copyright 2015, Springer Nature.b) XRD patterns of α-MnO 2 electrode and Zn electrode in fully discharged state.Reproduced with permission. [97]Copyright 2020, Wiley-VCH.c) Schematic illustration of Zn 2+ /H + insertion/de-insertion mechanism and good cycle performance of the MnO 2 /MnO@C cathode.Reproduced with permission. [99]the storage of Zn 2+ and also explored the energy storage mechanism of co-intercalation/extraction of H + and Zn 2+ in δ-MnO 2 .Figure 3e shows the constant-current intermittent titration curve of Na 0.44 Mn 2 O 4 Á1.5H 2 O (δ-NMOH) during discharge.Region I is the embedding process of H + and region II is the embedding process of Zn 2+ .It is obvious that the superpotential of region II is higher, about 10 times that of region I.The ion diffusion coefficient in region I is much larger than that of region II.This further confirms that the discharge process of the material is composed of H + and Zn 2+ co-insertion.Before this, Sun et al. also proposed the same conclusion. [102]Recently, ex situ XRD revealed that two obvious diffraction peaks of ZnMn 2 O 4 and MnOOH were detected when the cathode material was fully discharged to the D state (Figure 3f).It verified that the reaction mechanism involved the interaction of Zn 2+ and H + . [103]The same storage mechanism also exists in other manganese oxides, such as MnO.Wang et al. [104] firstly explained the zinc storage mechanism of the commercial MnO particle cathode material.Due to the effect of electrochemical activation, MnO 2 nanosheets will be formed on the surface of MnO particles during the first charge process.In the subsequent charge and discharge cycle, MnO 2 will go through two-generation stages of MnOOH and ZnMn 2 O 4 .ZHS phase also formed on the surface of cathode material due to proton insertion.Schematic illustration of phase transition mechanism is shown in Figure 3g. [105]n brief, H + /Zn 2+ co-insertion/extraction mechanism reveals H + preferentially inserts into the host material to generate MnOOH, and then ZnMn 2 O 4 is generated through the insertion of Zn 2+ during the discharge process.Due to the consumption of H + , [Zn(OH) 2 ] 3 Zn-SO 4 ÁxH 2 O is formed and makes the system charge neutral.However, basic zinc sulfate gradually dissolves and is highly reversible during the charging process.The mechanism of H + /Zn 2+ co-insertion/extraction is common in AZIBs.

Conversion Reaction Mechanism
Aside from the two energy storage mechanisms mentioned above, chemical conversion reaction mechanism has also been proposed, which is mainly a reversible electrochemical behavior between MnO 2 and MnOOH/ZHS. [106,107]Figure 4a shows the reaction path of β-MnO 2 in the cyclic process. [108]In the initial discharge process, β-MnO 2 will react with protons from water to generate MnOOH, which will convert into Mn 2+ subsequently.On the first charge, MnOOH and Mn 2+ transform and deposit to ε-MnO 2 .In subsequent cycles, deposited MnO 2 reacts with protons to form MnOOH, and partially dissolves to Mn 2+ during the discharge process, accompanied by the formation of ZHS.Mn 2+ deposited into MnO 2 and ZHS disappears during charging.The specific reaction process in cathode material can be expressed as follows: The participation of H + in electrode reaction process was further confirmed.As exhibited in Figure 4b, the pH value increases in the discharge process and decreases during the charge process.In order to further verify the chemical conversion reaction between MnO 2 and MnOOH as well as the formation mechanism of ZHS, Pan et al. [73] studied the changes in structure and morphology in α-MnO 2 electrode in the cycling process comprehensively.Highresolution transmission electron microscopy (HRTEM) images of the electrode at the first discharge to 1 V (Figure 4c,d

Dissolution/Deposition Mechanism
As early as 2020, Guo et al. [109] further supplemented the zinc storage mechanism of Zn//MnO 2 batteries and proposed dissolution/deposition reaction mechanism, which found that H + and Zn 2+ insertion/ extraction occurred simultaneously in the reaction process.As shown in Figure 5a, the host material reacts with H 2 O to generate Mn 2+ and OH − in the initial discharge process, then OH − reacts with the surrounding ZnSO 4 and generates a new ZHS phase, which consumes a large amount of H 2 O around MnO 2 effectively.However, the consumption of active H 2 O inhibits the subsequent dissolution of MnO 2 , which leads to the increase in Mn 2+ content and pH in the electrolyte.ZHS and Mn 2+ transfer to birnessite-MnO 2 in the first charging process.This mechanism still occurs in the subsequent cycles, but the host material changes from original MnO 2 to birnessite-MnO 2 .In the whole process of energy storage, dissolution/deposition reaction mechanism plays a dominant role and contributes most of the specific capacity, while the accompanying intercalation/de-intercalation reaction process is not particularly critical.Wang et al. [110] Li's team [111] proposed a MnO 2 /Mn 2+ redox chemical reaction with interfacial regulation.As shown in Figure 5f, MnO 2 dissolves due to the H + conversion during the first discharge, leading to increased pH value and Mn 2+ concentration in electrolyte.ZHS formed at the end of the discharge, and complex dissolution/ deposition reactions dominate the subsequent process.During the charging process, the co-deposition of Zn 2+ and Mn 2+ began to form Zn-rich birnessite Zn x Mn 3 O 7 , accompanied by the dissolution of ZHS.Subsequently, Zn x Mn 3 O 7 was converted to Mn 7 O 13 with the Zn 2+ extraction and Mn 2+ deposition.With the co-insertion of Zn 2+ and H + , the phase transition goes in the opposite direction, and further insertion of H + leads to the dissolution of Zn x Mn 3 O 7 as well as the formation of ZHS.In situ XRD patterns of α-MnO 2 / Zn battery confirmed the formation of new-phase ZHS in the charging and discharging process (Figure 5g). [112]Recently, the zinc storage mechanism of D-MnO 2 ultrafine nanowire with abundant crystal defects (oxygen vacancies and holes) involves the H + and Zn 2+ co-insertion/extraction process, as well as the deposition/dissolution mechanism of ZHS nanosheets. [113]The result is consistent with Liang and Zhong's research. [114,115]he zinc storage mechanisms are also widely presented in other manganese oxides, such as Mn 2 O 3 , Mn 3 O 4 , and MnO.Among them, Mn 2 O 3 undergoes a reversible phase transition and forms layered-type MnO 2 (Zn-birnessite) in the process of Zn 2+ insertion/extraction. [31] Spinel Mn 3 O 4 shows electrochemical inactivity in Zn/MnO 2 batteries because Mn 3 O 4 is transformed to intermediate Mn 5 O 8 and finally, to Zn-birnessite during Zn 2+ ions insertion into the interlamination of birnessite. [34]Besides, due to the low activity of the Mn(II)-based material and difficult ion insertion, it is impossible to deliver any discharge capacity.In the subsequent charging process, MnO is transformed into MnO 2 .In conclusion, Mn 2 O 3 , Mn 3 O 4 , and MnO can also display high reversible capacity.This attributes to the Zn-birnessite generated due to the phase transition of these manganese oxides formed during the initial charge/discharge processes.Then, the subsequent charge and discharge processes are similar to that of the MnO 2 materials.At present, zinc storage mechanism needs to be further studied to improve the electrochemical performance of AZIBs.

Optimized Strategies of Enhancing Zinc Storage
MnO 2 has the advantages of good safety, high output voltage, and excellent capacity.It has become one of the most promising cathode materials for AZIBs.Unfortunately, there are three major challenges in MnO 2 , which seriously hampers its practical application and development: a) Structure instability: it is mainly volume change and structural damage of MnO 2 electrode due to repeated Zn 2+ ions insertion/extraction during the charge/discharge processes.b) Manganese dissolution: the active substance is dissolved in electrolyte by disproportionation reaction during the cycles, which directly leads to rapid capacity attenuation.c) Poor electrical conductivity: It generates sluggish electrons' transport kinetics and limits the electrons' transfer rate.These problems seriously impede the further development of MnO 2 cathode materials. [116]Figure 6 shows the currently reported modification strategies to solve the problems mentioned above and achieve performance-enhanced AZIBs.

Enhancement of Structural Stability
In the long-term cycle process, with the repeated insertion and extraction of Zn 2+ /H + , the initial structure will have irreversible phase change or volume expansion, and the structure of the electrode material will change greatly, resulting in structural collapse.It seriously affects the cycle stability and rate capability of AZIBs.To make the material  [109] Copyright 2020, Elsevier.b) Ex situ XRD patterns at different depths of charge/discharge.SEM images of MnO 2 /rGO electrode at c) A state, d) B state, and e) C state.Reproduced with permission. [110]Copyright 2020, Wiley-VCH.f) Schematic diagram of the proposed charge storage mechanism of β-MnO 2 cathode.Reproduced with permission. [111]Copyright 2022, Elsevier.g) In situ XRD patterns of a cathode in a Zn/α-MnO 2 cell with 1.0 M ZnSO 4 electrolyte during the first dischargecharge cycle at C/20 and the corresponding discharge-charge curve.Reproduced with permission. [112]Copyright 2016, Wiley-VCH.
Energy Environ.Mater.2023, 6, e12575 structure more stable, the solutions proposed by researchers include pillar engineering, heteroatom doping, defect engineering, bond/interface engineering, and controllable nanostructures.

Pillar Engineering
119] α-K 0.19 MnO 2 cathode material was synthesized by pre-intercalating high content K + into α-MnO 2 nanotubes with tunnel structure, and contrast sample α-K 0.07 MnO 2 nanotubes was obtained by processing concentrated HNO 3 (Figure 7a). [120]It is observed that the insertion potential of α-K 0.19 MnO 2 active electrode (1.41/1.17V) is higher than α-K 0.07 MnO 2 electrode (1.39/1.13V) (Figure 7b), indicating that the presence of K + helps promote the insertion/de-insertion process of H + and Zn 2+ .In addition, part of the tunnel = structured MnO 2 will be transformed into a layered structure (Zn-busserite) with H + and Zn 2+ inserted into MnO 2 .The pre-intercalated K + ions as pillars can stabilize the layered structure of MnO 2 , thus promoting Zn 2+ diffusion and improving the electrons' conductivity.Zhang's group [49] also reached a similar conclusion on the intercalation of La 3+ .Aside from metal ions, crystal water can also stabilize the crystal structure of electrode materials and increase the layer spacing. [121,122]Nam et al. [123] synthesized layered MnO 2 with ~10 wt% content of crystal water by electrochemical conversion of Mn 3 O 4 in aqueous solution.Density functional theory (DFT) calculations and extended X-ray absorption fine structure analyses show that Zn 2+ will eventually form a Zn-Mn dumbbell structure with the intercalation of Zn 2+ coordinated with water molecules (Figure 7c), and this stable structure plays a key role in the electrochemical performance of the layered MnO 2 .Interlayer crystal water can not only increase the layer spacing of MnO 2 and make it have a highly stable lattice skeleton but also effectively screen the electrostatic interaction between Zn 2+ and the host framework, further promoting the diffusion of Zn 2+ , so that the sustainable cycle during the process of charge and discharge shows excellent rate capability.To stabilize the layered structure and achieve better performance of δ-MnO 2 , preintercalated Na + and water molecules into δ-MnO 2 is an effective strategy. [101]When Na + and H 2 O are used as the pillars, the spacing between the intermediate layers is 0.72 nm.It can reach a specific capacity of 278 mAh g −1 at 1 C (106 mAh g −1 at 20 C) and capacity retention of 98% after 10 000 cycles.The properties of cathode materials are far superior to those reported by other teams (Figure 7d).The same conclusion was obtained for Na 0.55 Mn 2 O 4 Á 0.57 H 2 O prepared by Zhai et al. [124] However, phase transitions and structural instability of MnO 2 caused by water intercalation during the charging and discharging process, the capacity attenuates rapidly and the cycle stability is unsatisfied.Therefore, it is a good method to select polyaniline as the object of intercalation material and insert it into MnO 2 nanolayer through simple interfacial reaction (Figure 7e). [95]Intercalation of polyaniline enhances and expands the layered structure of MnO 2 .It effectively avoids phase transformation and structure collapse caused by repeated insertion/ extraction of hydrated H + /Zn 2+ , which is beneficial to charge storage.Various guest materials as pillar in the host frame not only expand the interlayer distance of the cathode materials for accelerating the ions diffusion rate and make Zn 2+ /H + more easily insert/extract in the process of charge and discharge but also effectively stabilize the crystal structure.Pillar engineering is a vital method to realize high zinc ions storage and long cycle life.

Heteroatom Doping
Doping metal cations in manganese dioxide compounds is an effective strategy to improve the cyclic stability of electrode materials. [125,126]hang et al. [49] designed a cathode material of intercalating La 3+ in δ-MnO 2 nanoflowers (LMO).LMO nanoflowers are composed of thin slices intersecting each other, with a total thickness of 5-10 layers.The measured interlaminar spacing is larger than pure δ-MnO 2 (MO), indicating that La 3+ intercalation can increase the interlaminar spacing of MO.It is also confirmed by XRD results (Figure 8a).La 3+ intercalation enables LMO to obtain a larger interlaminar spacing and specific surface area (SSA).Moreover, the existence of La 3+ can be used as the structure support and effectively prevent the structure collapse of the cathode material in the process of charge and discharge, leading to outstanding cycle stability and rate capability.Wang et al. [127] synthesized Ce-doped MnO 2 nanorod-like cathode material.Ce doping leads to the transformation of phase structure from the initial β-MnO 2 structure to the α phase.The capacity retention of Ce doping is about twice higher than that of β-MnO 2 after 100 cycles.The results show that Ce doping increases the size of MnO 2 tunnel structure (Figure 8b), which is conducive to the rapid and reversible diffusion of Zn 2+ and alleviates the dissolution of manganese, resulting in good electrochemical stability and reversibility.Besides, doping of Ni 2+ can also improve the structural stability, regulate the storage behavior of H + , and promote the proton migration dynamics (Figure 8c). [128]The doping of nonmetallic elements can also improve the cycle stability of AZIBs.Ndoped ε-MnO 2 (MnO 2 @N) was realized by means of electrochemical deposition and heat treatment under nitrogen atmosphere. [56]It was found that the oxygen vacancy generated after nitrogen doping would improve the conductivity of MnO 2 @N, and the formed Mn-N bond could stabilize the structure of the cathode material and prevent rapid capacity decay during discharge.

Defect Engineering
Defect engineering technology enables materials to have some novel properties.To a large extent, defect engineering can improve the electrochemically active surface area and significantly improve the electrochemical performance.The studies on oxygen defects and cationic defects are very extensive. [129,130]Xiong et al. [131] prepared MnO 2 rich in oxygen vacancy (Od-MnO 2 ) in the lattice.Gibbs free energy of Zn 2+ adsorption near oxygen vacancy (V O ) is close to the thermal neutral value (0.05 eV), which is remarkably greater than the original MnO 2 (Figure 8d).Therefore, the adsorption/desorption process of Zn 2+ on Od-MnO 2 is more reversible than the original MnO 2 (Figure 8e, f), resulting in a higher electrochemical active surface area.Researchers also found that the generation of oxygen defects resulted in fewer electrons required to form Zn-O bond, and thus more electrons delocalized into the electrode, significantly increasing the specific capacity of the electrode.MnO 2 can also generate oxygen vacancies through doping of non-metallic elements. [132]N-MnO 2-X @TiC/ C core/branch arrays were synthesized via anchoring N-doped MnO 2 nanosheets (N-MnO 2-X ) with oxygen vacancy on highconductivity TiC/C nanorods, in which manganese oxide exists in the form of δ-MnO 2 (Figure 8g).The introduction of N doping and oxygen vacancy not only further reduces the band gap but also effectively reduces the charge density of MnO 2 (Figure 8h), and significantly improves the conductivity of MnO 2 .In addition, oxygen vacancy can enhance the surface capacitance contribution.Due to the synergistic effect of N doping and Mn 2+ preaddition in electrolyte, the N-MnO 2-X @TiC/C electrode displays faster reaction kinetics, higher discharge capacity, and better cycling performance.It is important to note that the introduction of Mn defects or other cation defects can greatly promote the structural stability of cathode materials. [68]Recently, Guo et al. [133] prepared a Mn 3 O 4 nanoparticle rich in Mn defects, which not only effectively regulates the surface electronic properties but also provides more active sites for electrochemical reactions, and further promotes the reaction kinetics of electrodes.At the same time, the cathode material maintained a stable structure during the cycle and showed outstanding electrochemical performance.
Defect engineering strategy solves the key problems of poor conductivity and structural stability in the charging and discharging process of AZIBs, and provides very meaningful insights into the design and development of high-performance AZIBs.

Bond/Interface Engineering
The strategy of combining manganese oxide with other organic materials or inorganic materials has been proved to have good improvement effect.This can be attributed to the formation of heterogeneous interfaces or chemical bond.A novel organic/inorganic hybrid MnO 2 cathode half-wrapped by aromatic polymers was prepared by a facile twostep electrodeposition method. [134]In the hybrid cathode, the organic chains with C=N groups provide additional zinc storage sites, and the newly formed Mn-N bonding effectively promotes ion diffusion and mitigates the dissolution of Mn atoms.Furthermore, the Mn-O-C  [120] Copyright 2019, The Royal Society of Chemistry.c) The formation of Zn-Mn dumbbell structure (numbers are interatomic distances).Reproduced with permission. [123]Copyright 2019, The Royal Society of Chemistry.d) Comparison in cycle life and capacity retention for several typical ZIBs.Reproduced with permission. [101]Copyright 2019, American Chemical Society.e) Schematic illustration of PANI-intercalated MnO 2 .Reproduced with permission. [95]Copyright 2018, Springer Nature.
Energy Environ.Mater.2023, 6, e12575 bonds between heterogeneous interfaces significantly improve the conductivity and integrity of the hybrid cathodes.MnO x @N-C Mn-based composite with porous framework and N-doping was prepared by the metal-organic framework template strategy. [89]The conductive network formed by onion N-doped carbon and amorphous carbon shell, which significantly improves the conductivity of active material, promotes the diffusion process of Zn 2+ and keeps good mechanical stability of the structure, as displayed in Figure 9a.Qiu et al. [135] selected the nitrogen-doped carbon cloth (N-CC) with 3D porous surface as the flexible support to uniformly deposit MnO 2 nanorods arrays and weeny Zn nanoparticles, respectively.The obtained N-CC@MnO 2 and N-CC@Zn are used as the cathode and anode of AZIBs, and can provide a high capacity of 353 mAh g −1 at 0.5 A g −1 , showing good cycle stability, rate capability, and good flexibility.Growing β-MnO 2 nanolayer on carbon cloth (MnO 2 @CC) exhibits excellent properties. [136]This is mainly due to the unique structure of MnO 2 which makes its outer surface fully in contact with electrolyte, and the inner surface is closely bonded with the carbon fabric with high conductivity.In particular, the outstanding performance of MnO 2 @NC is attributed to the hierarchical core-shell structure formed by incorporating MnO 2 with Ndoped carbon nanowires. [137]In addition, N atoms can elongate the Mn-O bond and reduce the valence state of Mn 4+ ions by delocalizing the electron cloud (Figure 9b,c), thus weakening the electrostatic repulsion of ion insertion and improving the capacity and rate performance of the electrode.Combined manganese oxide with other forms of carbon, such as N-doped carbon, [138] onion-like carbon, [139] N-doped hollow carbon spheres, [140] and activated eggplant carbon, [141] has been reported successively.
Shang et al. [142] coated a layer of V 2 O 5 on the surface of α-MnO 2 microspheres.Average discharge voltage of MnO 2 @V 2 O 5 composite materials was 1.3 and 1.4 V higher than that of MnO 2 (Figure 9d), and it has high specific capacity and small polarization.The V 2 O 5 coating greatly enhanced the kinetics of ion insertion/extraction and inhibited the dissolution of electrode materials during the electrochemical Figure 8. a) XRD patterns of the MO and LMO samples.Reproduced with permission. [49]Copyright 2019, The Royal Society of Chemistry.b) Crystallographic structure of β-MnO 2 and 0.1 mmol Ce-doped MnO 2 .Reproduced with permission. [127]Copyright 2019, American Chemical Society.c) GITT of the first charge/discharge cycle and corresponding diffusion coefficients.Reproduced with permission. [128]Copyright 2022, Wiley-VCH.d) Calculated adsorption energies for Zn 2+ on the surfaces of perfect σ-MnO 2 and σ-MnO 2 with oxygen vacancies.Schematic illustration of Zn 2+ adsorption/desorption for e) perfect MnO 2 and f) MnO 2 with oxygen vacancies.Reproduced with permission. [131]Copyright 2019, Wiley-VCH.g) XRD patterns of MnO 2 @TiC/C and N-MnO 2-x @TiC/C.h) State density of MnO 2 and MnO 2-x .Reproduced with permission. [132]Copyright 2019, Wiley-VCH.
Energy Environ.Mater.2023, 6, e12575 reaction process.Moreover, the presence of V 2 O 5 can alleviate the volume expansion of MnO 2 in the cycle process and enhance the structural stability of electrode materials.Intriguingly, in situ-formed Bi 3+ could alleviate the dissolution of Mn 3+ and stabilize the structure of MnO 2 by forming Bi-O interactions (Figure 9e), and Bi 2 Mn 4 O 10 formed by competition can inhibit irreversible ZnMn 2 O 4 phase generated, thus obtaining better Zn 2+ transport kinetics during repeated charging and discharging. [143]In addition, it has been reported that the formation of Ag-O-Mn bond will lead to the increase in oxygen vacancy content, which has a considerable impact on the properties of Reproduced with permission. [89]Copyright 2018, Wiley-VCH.b) Mn 2p XPS spectra and c) Raman spectra of MnO 2 @NC and MnO 2 @CC.Reproduced with permission. [137]Copyright 2022, Wiley-VCH.d) Galvanostatic charge/discharge (GCD) curves of MnO 2 and MnO 2 @V 2 O 5 electrodes at 0.1 A g −1 .Reproduced with permission. [142]Copyright 2021, Elsevier.e) Schematic diagram of the incorporation of Bi 3+ ions stabilizing the Mn polyhedrons.Reproduced with permission. [143]Copyright 2021, American Chemical Society.f) The structure of Mn 3 O 4 -GDY.Reproduced with permission. [146]Copyright 2022, Elsevier.g) N 2 adsorption/desorption isotherm of the as-prepared MnO 2 material.Reproduced with permission. [151]Copyright 2019, The Royal Society of Chemistry.h) Schematics of Zn 2+ ions diffusion in δ-MnO 2 (left) and disordered MnO 2 (right).Reproduced with permission. [152]Copyright 2021, Springer Nature.i) Nyquist plots of KMOH@C, neat KMOH, and δ-MnO 2 cathodes before and after cycles at 0.5 A g −1 .j) Schematic illustration of the Zn 2+ /electron transport in KMOH@C with oxygen vacancies.Reproduced with permission. [153]Copyright 2021, Elsevier.k) Optimized geometrical structures of α-(Mn 2 O 3 -MnO 2 ), Mn in purple and O in red.l) The spin-resolved density of states (DOS) of α-(Mn 2 O 3 -MnO 2 ).Reproduced with permission. [156]Copyright 2020, American Chemical Society.
Energy Environ.Mater.2023, 6, e12575 the material. [144]These composite materials show excellent electrochemical performance when used as cathode materials of AZIBs.
β-MnO 2 /PPy three-dimensional mesoporous microsphere was synthesized by chemical oxidation polymerization method, in which PPy nanowire and β-MnO 2 nanorods are well connected and show a large SSA. [145]it significantly enhanced the zinc storage performance and structural stability of β-MnO 2 .In order to improve the diffusion and storage efficiency of zinc ions in Mn 3 O 4 nanoparticles, Mn 3 O 4graphdiyne (Mn 3 O 4 -GDY) composite material was prepared by in situ chemical method (Figure 9f). [146]The coating of GDY with the unique electronic structure can limit Mn 3 O 4 nanoparticles to a small size, which is conducive to electron and mass transfer, and can reduce the large fluctuation of Mn 3 O 4 volume in the charging and discharging process, and improve the stability of AZIBs.Importantly, Mn 3 O 4 -GDY electrode can obtain a specific capacity of 490.3 mAh g −1 at 50 mA g −1 , which is about twice the theoretical capacity of MnO 2 .Of course, the composites formed between manganese oxide and polymers have also been reported. [147,148]

Controllable Nanostructures
Adjusting the nanostructure of electrode material allows it to have larger SSA and more internal pores, thus providing more active sites and shorter ion/electron transport paths for the electrochemical process, further accelerating the reaction kinetics.In addition, the nanostructure is beneficial to maintain the structural integrity of electrode, so it has better cyclic stability.Mesoporous structure and high SSA of electrode materials are conducive to rapid diffusion and transport of electrolyte ions, providing enough active sites. [149,150]Based on this, Wang et al. [151] designed Zn 2+ -stabilized mesoporous, MnO 2 flower-like nanosphere, which is combined with ultra-thin nanosheets and forms a layered porous structure.Both mesoporous structure and high SSA of MnO 2 are conducive to rapid diffusion and transport of electrolyte ions.N 2 adsorption/desorption isotherm of MnO 2 materials confirms the presence of a unique mesoporous structure (Figure 9g).Cyclic voltammetry (CV) curves remain in a good shape after many cycles, indicating the high reversibility and stability of the redox reaction.Using δ-MnO 2 as the precursor, disordered MnO 2 material with the coexistence of crystalline and amorphous structure was prepared by one-step water bath synthesis method. [152]It has an ultra-high specific capacity of 636 mAh g −1 at 0.1 A g −1 , and still remains 216 mAh g −1 after 400 cycles at 1.0 A g −1 .The main reason is the disordered hybrid structure of MnO 2 with more active site and irregular Mn-O keys, which alleviate the problems of structure collapse and manganese dissolution, and improve the stability of electrode material during the process of repeated charging and discharging (Figure 9h).Zhai et al. [153] reported a yolk-shell structured K-birnessite@mesoporous carbon nanospheres (KMOH@C) with K + pre-intercalation and rich oxygen vacancies.Figure 9j shows the schematic diagram of Zn 2+ /electron transport in KMOH@C.The electrochemical impedance spectroscope (EIS) illustrates the charge transfer resistance of KMOH@C cathode is the lowest under different cyclic states (Figure 9i).It is because the synergy of pre-intercalated K + , abundant oxygen vacancy, and special mesoporous carbon shell make KMOH@C cathode have better conductivity and greatly improved Zn 2+ storage performance.The construction of heterostructures can promote the transfer of Zn 2+ ions in the process of charge and discharge, and provide remarkable properties at their interface. [154,155]α-(Mn 2 O 3 -MnO 2 )-500 heterostructures were constructed by in situ phase transformation at 500 °C. [156]It has better nanorods morphology and perfect heterostructure.The unique structural advantage can provide much more active site, and effectively promote the intercalation and extraction of Zn 2+ .Figure 9k shows the optimized geometrical structures of α-(Mn 2 O 3 -MnO 2 ).DFT was used to compare the state density of the sample, and it was confirmed that the heterostructure at the boundary and interface improved the electron conductivity and ion/electron transfer (Figure 9l).
Nanostructure design can significantly improve the electrochemical properties of manganese-based compounds.Materials that combine small size, hollow structure, and porous structure can promote full contact at the interface of electrode/electrolyte and provide more active sites. [157]Most importantly, nanostructures greatly alleviate the problem of structural collapse and improve the cyclic stability of cathode materials.It is because of these significant advantages that nanostructure engineering has been widely reported.

Inhibition of Manganese Dissolution
At present, although MnO 2 cathode material has been widely studied due to its environment friendliness, low cost, multiple crystal structures, suitable voltage window, and other advantages, manganese dissolution makes the specific capacity of the electrode decay rapidly and further affects the cycle stability of the batteries.To alleviate Mn dissolution during the cycle, the addition of Mn 2+ in ZnSO 4 electrolyte or replacing ZnSO 4 electrolyte with Zn(CF 3 SO 3 ) 2 or Zn(CH 3 COO) 2 or other new electrolytes is an effective strategy.In addition, surface coating is also an effective way to prevent manganese dissolution.

Optimization of Electrolyte
Electrolyte regulation strategy provides a good idea for the development of performance-advanced Zn/MnO 2 batteries in the future.Pan et al. [73] found that the specific capacity of Zn/MnO 2 battery was 210 and 255 mAh g −1 at C/5 in the initial two cycles in mild ZnSO 4 electrolyte, but the capacity of the battery decreased rapidly after two cycles.Inductively coupled plasma (ICP) technique confirmed that it was caused by the disproportionation of Mn 3+ into Mn 2+ dissolved into electrolyte during the cycle.In order to change the dissolution balance of Mn 2+ in the MnO 2 electrode, MnSO 4 was added to the electrolyte on the basis of the previous experiment.As illustrated in Figure 10a,b, CV curves of the battery after pre-adding Mn 2+ showed similar redox behavior, indicating MnSO 4 additive did not change the redox process of the MnO 2 cathode.However, it is obvious that electrode exhibits a higher specific capacity, which can reach 285 and 260 mAh g −1 at C/ 3 and 1 C after adding Mn 2+ .Moreover, the capacity can still maintain 92% after 5000 cycles at 5 C, with better cycle stability and rate performance.Soon after, Qiu et al. [158] also studied the role of Mn 2+ in electrolyte, revealing that pre-adding Mn 2+ into electrolyte has a nonnegligible impact on batteries, and nearly 18.9% of the capacity is attributed to the contribution of Mn 2+ in electrolyte.Zeng et al. [72] studied the electrochemical properties of γ-MnO 2 in 1.0 M Zn(CH 3 COO) 2 and 0.4 M Mn(CH 3 COO) 2 mixture electrolyte.The specific capacity of the battery is 1.0 mAh cm −2 at 5.0 mA cm −2 , and it can still deliver reversible discharge capacity of 0.98 mAh cm −2 after 1000 cycles.Acetate anion not only changes the surface properties of MnO 2 cathode but also creates a highly compatible surrounding for Zn anode.As Energy Environ.Mater.2023, 6, e12575 shown in Figure 10d, potentiodynamic behaviors of different electrolytes were compared. [36]Zn(CF 3 SO 3 ) 2 electrolyte showed a decreased overpotential upon cycling, while ZnSO 4 showed a large overpotential.Moreover, the separation of charge-discharge voltage indicates Zn (CF 3 SO 3 ) 2 has higher energy efficiency.When 3 M Zn(CF 3 SO 3 ) 2 was selected as the electrolyte, the cathode showed excellent electrochemical performance.It may be due to the fact that compared with SO 4 2− , the larger volume of CF 3 SO 3 − can reduce the water molecules and solvation effect around Zn 2+ , and promote the transport of Zn 2+ and charge transfer as well as effectively reduce the dissolution loss of Mn 2+ in spinel.As electrolyte additives, Mn(CF 3 SO 3 ) 2 was added into the electrolyte of Zn(CF 3 SO 3 ) 2 for rechargeable Zn//MnO 2 batteries. [159]The introduction of Mn 2+ can form a uniform porous MnO x nanosheet on the cathode surface, which protects the integrity of the cathode Reproduced with permission. [73]Copyright 2016, Springer Nature.c) Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) of MnO 2 @graphene scroll.Reproduced with permission. [45]Copyright 2018, Wiley-VCH.d) Galvanostatic cycling of Zn/Zn symmetrical cells at 0.1 mA cm −2 in 3 M ZnSO 4 and 3 M Zn(CF 3 SO 3 ) 2 electrolytes.Insets enlarge the voltage profiles of the 1st and 25th cycles.Reproduced with permission. [36]Copyright 2016, American Chemical Society.e) Cycling performances of MGS at 0.3 and 1.0 A g −1 after activation at 0.1 A g −1 .Reproduced with permission. [45]Copyright 2018, Wiley-VCH.f) SEM images of MnO 2 /rGO/PANI.Reproduced with permission. [168]Copyright 2019, Springer Nature.g) Galvanostatic discharge comparison curve in 100 mA g −1 of α-MnO 2 and CMO.Reproduced with permission. [169]Copyright 2019, American Chemical Society.
structure and mitigates Mn 2+ dissolution.Lastly, Qin et al. [160] added Al 3+ ions into the mixture electrolyte of ZnSO 4 and MnSO 4 , in which Al 3+ with Brønsted weak acid not only serves as the proton-donor reservoir to maintain the electrolyte acidity near the electrode surface and prevents the formation of ZHS during discharging but also introduces oxygen vacancies by doping Al 3+ during charging, which weakens the Mn-O bond and facilitates the dissolution reaction during discharging.The above results indicate that adding Mn 2+ or corresponding salt into the electrolyte and finding a more suitable electrolyte are the keys to solving the problem of Mn dissolution in the charging and discharging process of AZIBs. [118,159,161]n addition, the use of solid or gel electrolytes mitigates the growth of zinc dendrites, and effectively relieves the dissolution of manganese originating from their limited water medium and great adsorption affinity, [162] thus conducing to achieve long-term cyclability.For example, Chen et al. [163] demonstrated that as the gel electrolyte, boraxcrosslinked polyvinyl alcohol/glycerol shows excellent cycling durability with high capacity retention of about 90% after 2000 cycles for a flexible Zn-MnO 2 battery.Recently, based on polyacrylamide hybridized and layered double hydroxides, a designed solid electrolyte was utilized to inhibit the formation of Zn 2 MnO 4 with electrochemical inactivity during the charge/discharge progresses. [164]

Surface Engineering
A protective layer covering the surface of the electrode can effectively alleviate the dissolution of active substances. [165,166]Islam et al. [167] reported α-MnO 2 @C nanocomposite, which consists of α-MnO 2 nanoparticles and the coated carbon network.It had better discharge capacity and cycling than the original α-MnO 2 .Moreover, the conductivity and electrochemical activity of the α-MnO 2 @C cathode material are improved effectively.As observed in Figure 10c, graphene has also been used as a coating layer to modify the MnO 2 . [45]It was found that graphene played the same role as carbon coating, not only significantly increasing the specific capacity of cathode material but also showing good cycle stability and rate capability (Figure 10e).α-MnO 2 /graphene scroll (MGS) cathode exhibits a maximum capacity of 382.2 mAh g −1 after 10 cycles of activation at 0.3 A g −1 .It still has a high capacity of 87.4 mAh g −1 after 800 cycles at 7 A g −1 .
In addition to these different carbon-based materials, highly conductive polymers can also be selected as the coating of nanomaterials to prevent the dissolution of active substances and improve the electrical conductivity of materials. [168]Mao's team prepared the MnO 2 /rGO composite aerogel in a simple way and then coated polyaniline on the surface of MnO 2 /rGO to obtain MnO 2 /rGO/PANI composite with unique micro-/nanostructure.It can be seen that the compact and dense microstructure of MnO 2 /rGO is well protected after coating with a uniform layer of polyaniline, it promotes the interaction between the components (Figure 10f).Furthermore, the presence of N element in the energy-dispersive X-ray spectroscopy (EDX) element mapping indicates that polyaniline coating has been successfully realized.The specific capacity of MnO 2 /rGO/PANI composites can be increased to 241.1 mAh g −1 at 0.1 A g −1 .This value is much higher than the MnO 2 /rGO and the original sample MnO 2 .After 600 cycles, the capacity of MnO 2 /rGO/PANI composites still maintained at 82.7%.The above results indicate that the strategy of polyaniline coating not only enhances the conductivity of the electrode material but also inhibits the Mn dissolution and increases the discharge capacity of the batteries.Different from previous reports, Guo et al. [169] synthesized Ca 2 M-nO 4 (CMO) by the traditional sol-gel method, and found that a solid electrolyte interface (SEI) film of CaSO 4 Á2H 2 O would be generated on CMO surface through in situ characterization technique.This interface layer is beneficial to reduce the impedance and improve the interface, meanwhile, reducing the activation energy of battery.In addition, it is beneficial to the insertion/extraction of Zn 2+ and effectively inhibits the dissolution of manganese.Even at 1 A g −1 for 1000 cycles, it maintained superior stability without significant fluctuations in the specific capacity of the battery.GCD curves show the CMO still maintains good stability at the end of the discharge stage (Figure 10g).[172] This method provides more ideas and guidance for the development of AZIBs with excellent stability.The surface modification includes surface coating and the formation of SEI film.It can buffer volume change and large stress resulting from ion insertion/extraction, and also effectively inhibit the dissolution of active substances.It is an influential design strategy and has been widely used in AZIBs.

Improvement in Electrical Conductivity
The poor conductivity of manganese dioxide cathode material (10 −- 5 ~10−6 S cm −1 ) is not conducive to the diffusion of divalent ions, hinders the insertion and extraction of Zn 2+ ions, and reduces the overall performance of electrode.To solve this problem, the current reports mainly improve electrochemical performance by constructing composite/hybrid structures, introducing defects sites, heteroatom doping, and so on.

Heteroatom Doping
Doping non-metal elements is an effective method to enhance the electrochemical property of transition metal oxides. [173,174]CV curves of MnO 2 and sulfur-doped MnO 2 nanosheets (S-MnO 2 ) electrodes at a scan rate of 0.8 mV s −1 are similar in shape (Figure 11a). [175]However, the peak current of the cathode and anode in S-MnO 2 is higher than that of MnO 2 electrode, while the voltage gap of S-MnO 2 is smaller than that of MnO 2 .These results indicate that the S-MnO 2 has higher electrochemical activity and faster reaction kinetics.In addition, S-MnO 2 electrode exhibits lower charge transfer resistance and Warburg diffusion resistance, and the average diffusion coefficient of Zn 2+ ions is about an order of magnitude higher than that of MnO 2 electrode (Figure 11b).Besides, combined with theoretical calculations, it confirmed that sulfur doping in the structure of MnO 2 would reduce its band gap, thus improving the conductivity of MnO 2 .Moreover, the sulfur-doping process introduced abundant oxygen defects, improved ion/electron transfer in the material structure, and significantly improved the zinc ions storage performance.The spinel structure of ε-MnO 2 is unstable and will undergo serious changes when the temperature exceeds 300 °C.Zhang et al. [56] conducted nitrogen annealing on ε-MnO 2 at 200 °C, and obtained N-doped MnO 2 (MnO 2 @N) cathode material.The Mn-N bond formed in the process of doping can stabilize the structure and slow down the capacity decay of MnO 2 material.In addition, the oxygen defects in MnO 2 @N can improve the conductivity and accelerate the charge transfer and diffusion process of ions.
Energy Environ.Mater.2023, 6, e12575 In addition to anionic doping, [176] metal cationic doping is also widely used in the research of cathode materials for AZIBs. [32,177,178]oping V into two-dimensional MnO 2 gives it a more stable structure and a higher SSA. [71]The first-principles calculation and four-point probe method were used to prove that V doping would significantly improve the conductivity of MnO 2 (Figure 11c,d).Recently, it was verified that V doping and induced oxygen vacancies regulated the electronic structure of MnO 2 , thus improving the conductivity and reducing the Zn 2+ diffusion energy barrier. [179]Moreover, Bi-doping strategy can stabilize the tunnel structure of α-MnO 2 material through Bi-O bond, ameliorate the conductivity, weak the strength of chemical bond between Zn 2+ and O, and obtain excellent cycle stability. [75]oping heteroions with different valence states is also a good strategy.It can not only improve the conductivity of electrode materials but also introduce additional defects to improve the electrochemical performance. [180,181]Recently, Shao et al. [182] realized K and Fe doubledoped manganese-based compound (K, Fe-ZMO) based on the previous theory, in which K occupies the partial Zn sites and Fe occupies the partial Mn sites.The existence of oxygen defects in K, Fe-ZMO was verified by Raman spectra and photoluminescence spectra.Oxygen defects can change the local electronic structure of the materials and enhance their electrical conductivity.EIS shows that the doping of K and Fe results in lower charge transfer resistance and promotes charge transfer (Figure 11e).The change in state density of K, Fe-ZMO is depicted in Figure 11f.The band gaps of K, Fe-ZMO and ZMO are 0.7912 and 1.4718 eV, respectively.K, Fe-ZMO with a smaller band gap shows better conductivity.In addition, the double doping and oxygen defects can reduce the formation energy, and effectively stabilize  [175] Copyright 2022, Elsevier.c) States density and d) structures of pristine MnO 2 and interstitial V-doped MnO 2 .Reproduced with permission. [71]opyright 2015, The Royal Society of Chemistry.e) Nyquist plots of K, Fe-ZMO and ZMO at 50th charge-discharge at 0.1 A g −1 .f) TDOS of the K, Fe-ZMO and ZMO bulk phase.Reproduced with permission. [182]Copyright 2022, Elsevier.g) Comparison of Ragone plot of the MnO with other AZIBs cathode materials.Reproduced with permission. [90]Copyright 2020, Elsevier.h) The calculated tunnel structure of pristine and Ti-doped α-MnO 2 .Reproduced with permission. [184]Copyright 2019, Elsevier.
Energy Environ.Mater.2023, 6, e12575 the Fe-O and Mn-O bonds in K, Fe-ZMO, enhance the structure stability, and alleviate the manganese dissolution.

Defect Engineering
Zhu et al. [90] firstly reported an in situ electrochemical approach to activate MnO by inducing Mn defect and convert MnO with poor electrochemical activities into active cathode with high electrochemical activities.Zn-Mn battery composed of this cathode material can deliver an energy density of 383.88 Wh kg −1 at 135.6 W kg −1 , which is far better than the other cathodes of AZIBs (Figure 11g).The formation of Mn defects not only enhanced the conductivity of MnO but also provided a large channel and available active sites for insertion/extraction of Zn 2+ , without obvious structural collapse phenomenon.In addition, ion insertion kinetics of β-MnO 2 can be enhanced by introducing oxygen defect into β-MnO 2 (D-β-MnO 2 ). [183]DFT calculations forecast the main structure of β-MnO 2 may be H + insertion instead of Zn 2+ , and the Gibbs free energy of H + insertion β-MnO 2 may be reduced by the introduction of oxygen defect, which can enhance the conductivity of the electrode.Furthermore, the presence of oxygen deficiency increases ion absorption sites and opens additional ion intercalation channels, resulting in higher reactivity and reversible capacity.Ti-doped α-MnO 2 nanowires (Ti-MnO 2 ) were obtained by surface-gradient-doping Ti in tunnel-structured α-MnO 2 and generate oxygen vacancies simultaneously. [184]The surface gradient Ti doping not only causes the tunnel shrinkage of MnO 2 but also causes the reduced Mn valence state, resulting in oxygen vacancy (Figure 11h).Both the Ti doping and oxygen vacancy open [MnO 6 ] octahedral walls, leading to imbalanced charge distribution and local electric field in crystal structure, thus accelerating Reproduced with permission. [189]Copyright 2020, Wiley-VCH.c) SEM and d) corresponding particle size distribution image of ZnMn 2 O 4 /NG.Reproduced with permission. [190]Copyright 2019, Elsevier.e) EIS of ZNCMO@N-rGO and ZMO@N-rGO after 10 cycles.Reproduced with permission. [191]Copyright 2020, Elsevier.f) XRD patterns of the α-MnO 2 @In 2 O 3 nanotubes.g) Cycling performances of α-MnO 2 and α-MnO 2 @In 2 O 3 electrodes at 0.1 A g −1 .Reproduced with permission. [194]Copyright 2019, IOP Publishing.h) SEM images of Mn 2 O 3 @PPy microbox.Reproduced with permission. [197]Copyright 2019, American Chemical Society.
Energy Environ.Mater.2023, 6, e12575 the migration rates of ions/electrons.Because of this unique gradient structure, Ti-MnO 2 cathode achieves excellent high rate capability and long circular period.Oxygen vacancy has gradually become an effective strategy to improve materials' conductivity. [118]At present, defect engineering has proved to be a realistic strategy.By introducing defects, including cation vacancy and anion vacancy, the electronic conductivity and ion transport capacity of materials can be greatly enhanced, and the electrochemical property of cathode materials is significantly enhanced. [133,185]

Introduction of Nanocomposite
The development of manganese-based oxides has been inhibited by their rapid capacity decay and unsatisfactory rate capability.[188] Wang et al. [165] combined γ-MnO 2 nanorods with graphene, which improves the electrical conductivity of MnO 2 and also endows MnO 2 with the ability to adapt the structural damage and dissolution in the process of charging and discharging.It significantly solves the problem of low rate performance and cycling life of MnO 2 .As exhibited in Figure 12a, yolk-shell structure of MnO 2 @C cathode material consists of MnO 2 nanowire space confined in mesoporous carbon nanotube. [189]It exhibits high specific capacity and good cyclic stability because of the good electrical conductivity and mechanical stability of the carbon shell in MnO 2 @C (Figure 12b).Porous structure can enhance the diffusion kinetics of Zn 2+ and improve the utilization rate of active materials.This strategy is widely used.Zhai's group reported a similar yolk-shell structure cathode material. [153]N-doped graphene (NG)-coated ZnMn 2 O 4 nanoparticles composite material (ZnMn 2 O 4 /NG) was synthesized by one-step hydrothermal method. [190]SEM images show that ZnMn 2 O 4 /NG has a sandwich structure (Figure 12c), and corresponding particle size distribution images further confirm the diameter of ZnMn 2 O 4 ultrafine nanoparticles is only 21 nm (Figure 12d).The fine nanoparticles greatly enhance the pseudocapacitance behavior and shorten the diffusion route of Zn 2+ and electrons.Moreover, NG as a medium with high conductivity can promote the rapid transmission of electrons, stabilize the overall structure of the material, and alleviate the volume change upon cycling.In contrast, the composite of N-doped graphene with ZnMn 2 O 4 nanoparticles co-substituted by nickel and cobalt was prepared. [191]As presented in Figure 12e, the composite has a smaller charge transfer resistance.Large amounts of Mn ions are replaced by Ni and Co in the lattice, resulting in increased lattice spacing, which promotes the intercalation/de-intercalation process of Zn 2+ , stabilizes the spinel structure, and inhibits Jahn-Teller distortion of Mn 3+ .
In addition to the common conductive carbon materials, some metal oxides can also improve the materials' conductivity.It is reported that In 2 O 3 with high conductivity is widely used in LIBs, photocatalysis, and other fields, showing excellent performance. [192,193]As shown in Figure 12f, Gou et al. [194] successfully coated In 2 O 3 uniformly on the surface of α-MnO 2 nanotubes.Cyclic stability and rate capability of α-MnO 2 @In 2 O 3 cathode have been greatly improved because of the improved conductivity of In 2 O 3 coating.The maximum specific capacity of α-MnO 2 @In 2 O 3 electrode can reach 425 mAh g −1 at 0.1 A g −1 after 100 cycles, which is far superior to that of the original α-MnO 2 (Figure 12g).Besides, conductive polymers are also an effective strategy for improving electrical conductivity. [195]Polypyrrole (PPy)-coated α-MnO 2 core-shell nanorod composites show improved electrochemical performance. [196]It alleviated the continuous Mn 2+ dissolution and also significantly improved α-MnO 2 materials' conductivity.As shown in Figure 12h, Liu's team [197] also applied PPy to Mn 2 O 3 .The composite material is a microbox composed of a unique porous structure and the PPy protective layer, in which a large number of holes expedite ion transport and provide much more active interfaces for Zn 2+ storage, while PPy protective layer can inhibit the dissolution of the active material and also ensure a good conductive network and structural integrity.
In conclusion, unexpected electrical conductivity and excellent electrochemical properties can be obtained by combining cathode materials with other materials with unique advantages and exploiting synergistic effects between different components.This strategy also provides more ideas for researchers to modify manganese-based materials.

Summary and Perspectives
In this review, we briefly introduce the crystal structures and characteristics of different MnO 2 .Then, four energy storage mechanisms involved in MnO 2 cathode materials are comprehensively summarized.More importantly, the main challenges that existed in MnO 2 and corresponding optimization strategies are proposed and analyzed in depth.Simultaneously, the structure-performance relationship of MnO 2 cathode materials was summarized and discussed, as shown in Table 2. Despite MnO 2 cathode materials having been explored for a long time and great achievements having been made, there are still some key problems that need further research in order to realize commercial application as an ideal substitute for AZIBs.Based on our analysis, the future research directions of AZIBs are prospected in terms of the following aspects: Firstly, zinc storage mechanism of MnO 2 cathode is more complex than that of other materials, and further discussion and research are needed to reach a consensus.Some in situ techniques, such as electrochemical quartz crystal microbalance (EQCM), are further required for providing more comprehensive and reasonable complement for the electrochemical reaction mechanism.EQCM can examine realtime ion transport and provide precise information on mass changes during the charge/discharge processes.This method provides direct evidence for the mass transfer process, which can evaluate the contribution of mass transfer to the entire electrode process, and distinguish the mass difference in the H + /Zn 2+ ions intercalation and conversion reactions.In situ X-ray absorption spectroscopy (XAS) is also a powerful technique to elucidate the fundamental atomic/electronic structures of energy storage and conversion materials and monitor the material change with the charge state under different operating conditions.In addition, in situ Raman and in situ XRD can be utilized to further determine the structural evolution of electrode materials.In addition, much more special attention should be paid to the effect of new products in the cycle process on the interface between electrode and electrolyte, such as [Zn(OH) 2 ] 3 ZnSO 4 ÁxH 2 O, ZnMn 2 O 4 , and MnOOH.
Secondly, with the continuous expansion of the flexible energy storage field and the gradual application of flexible/wearable electronic devices, flexible zinc-ion batteries with low cost, high bending, and strong stretching have been rapidly developing.In the progress of battery deformation, the mechanical strength and water content of gel and closeness of contact at the interface of electrolyte/electrode affect the Energy Environ.Mater.2023, 6, e12575 performance of the batteries.Hence, it is important to design multifunctional gel electrolyte with reasonable structure because unique functional groups effectively mitigate the external stress.Adjusting the structure and constituent of the gel by selecting different polymer matrixes also shows excellent mechanical properties and functionalization, including ions and electrons conductivity.In addition, developing a multi-effect electrolyte additive may restrain the formation of Zn dendrites and occurrence of side reactions with enhanced cyclic stability of the cathodes.Therefore, the optimization of gel electrolytes has been a potential strategy to improve the battery performance of MnO 2 cathode materials.The interaction between mechanical behavior and electrode material, as well as solid/gel electrolytes with good mechanical strength and high ionic conductivity, should be considered.
Thirdly, in the modification of MnO 2 cathode material, many optimization strategies have been generalized.However, a single strategy cannot solve many problems at the same time.The doping strategy of MnO 2 is mainly focused on cathode ions doping or anion ions doping.However, the codoping of metal/non-metallic elements and precise control of doping sites in MnO 2 are less studied.Anionic doping is an alternative strategy to generate lattice vacancies in MnO 2 cathodes, in which non-metallic elements (such as S, N, P, and B) with low electronegative replace high electronegative O atoms while maintaining its original phase structure.Doping metal cations into host materials can enlarge the lattice spacing, decrease the crystal stress, and facilitate the insertion of Zn 2+ , as well as create oxygen vacancies and change the electronic properties of MnO 2 cathodes.Especially, transition metal elements (such as V, Cu, Co, Ni, Zn, and Fe) and rare-earth metal elements (La and Ce) with d orbital electron structure show better doping effect owing to the valence d-p orbital coupling.Therefore, dual doping of metallic cations and non-metallic anions may generate excellent synergistic effects.Moreover, the understanding of dual-doping metal/ non-metallic elements of MnO 2 cathodes in terms of structural changes and electrode dynamics has not been illustrated well, which may be an interesting direction in the near future.

Yuhui
Xu received his B.S. degree from the School of Materials Science and Engineering at the Xi'an University of Technology in 2021.He is currently a master's student at the Xi'an University of Technology.His research interests focus on the design, synthesis, and modification of manganesebased cathodes for aqueous zinc-ion batteries.Gaini Zhang is a lecturer in School of Materials Science and Engineering at the Xi'an University of Technology.She received her M.S. and Ph.D. degrees from the School of Shaanxi Normal University in 2012 and 2016, respectively.Her current research interests focus on energy storage materials and devices.Xifei Li is currently a full professor at the Xi'an University of Technology.He was awarded as 2018-2022 Highly Cited Researchers of Clarivate Analytics.He is an executive editor-in-chief of Electrochemical Energy Reviews, a vice president of the International Academy of Electrochemical Energy Science, and a fellow of the Royal Society of Chemistry.Dr. Li 0 s research group is currently working on optimized interfaces of the anodes and the cathodes with various structures for highperformance rechargeable batteries.Dr. Li has authored/coauthored 350 peer-reviewed articles with 19 800 citations and an H index of 73.Energy Environ.Mater.2023, 6, e12575 2 of 24 perspectives of MnO 2 cathode materials from the perspective of mechanism and optimization strategies for AZIBs.

λ-MnO 2 [ 79 , 80 ]
of double or triple chains of the [MnO 6 ] octahedra, resulting in 2D tunnels within the lattice. of [MnO 6 ] octahedra sharing opposite edges; each chain is corner-linked with four similar chains.related to rutile except the edge-sharing [MnO 6 ] octahedra single chain are replaced by double chains.structure, containing infinite two-dimensional sheets of edge-shared [MnO 6 ] octahedra.Defect spinel structure, Fd3m a = b = c = 8.09 N (m*n) c 42-1169Tetrahedral and octrahedral sites are occupied by the Mn 2+ and Mn 3+ .close packing of anions, with Mn 4+ statistically distributed over half the available octahedral interstices.

Figure 2 .
Figure2.a) Schematic illustration of the two-step intercalation mechanism of α-MnO 2 /graphene scrolls cathode.Reproduced with permission.[45]Copyright 2018, Wiley-VCH.b) Schematic illustration for the zinc insertion into tunnel-structured α-MnO 2 , which causes the expansion of tunnel and hence increases the interplanar spacing of adjacent (110) planes.Reproduced with permission.[84]Copyright 2015, Elsevier.c) Chemistry schematics of the zinc-ion battery.Zn 2+ ions migrate between tunnels of the MNG cathode and Zn anode.The inset on the right shows Zn 2+ -ion insertion and interconnection between δ-MnO 2 and graphite.Reproduced with permission.[85]Copyright 2019, Springer Nature.d) In situ synchrotron XRD pattern of β-MnO 2 nanorod cathode recorded during electrochemical discharge/charge and a close-up view of the corresponding (101) plane reflection.Reproduced with permission.[47]Copyright 2017, The Royal Society of Chemistry.e) Schematic illustration of the reaction pathway of Zn insertion in the prepared γ-MnO 2 cathode.Reproduced with permission.[52]Copyright 2015, American Chemical Society.

Figure 3 .
Figure3.a) The results of the cathode which is constant current discharged to 1.4 V and then constant voltage discharged at 1.4 V in 2 M ZnSO 4 + 0.5 M MnSO 4 electrolyte for 2 h: phase evolution of cathode during the first discharge process.Reproduced with permission.[96]Copyright 2015, Springer Nature.b) XRD patterns of α-MnO 2 electrode and Zn electrode in fully discharged state.Reproduced with permission.[97]Copyright 2020, Wiley-VCH.c) Schematic illustration of Zn 2+ /H + insertion/de-insertion mechanism and good cycle performance of the MnO 2 /MnO@C cathode.Reproduced with permission.[99]Copyright 2020, Wiley-VCH.d) Joint non-diffusion-controlled Zn 2+ intercalation and H + conversion reaction mechanism in δ-MnO 2 .Reproduced with permission.[100]Copyright 2019, Wiley-VCH.e) The discharge GITT profiles of the Zn-δ-NMOH cell.Reproduced with permission.[101]Copyright 2019, American Chemical Society.f) The second discharge/charge curve at 0.2 A g −1 , and the ex situ XRD patterns at different potentials of PMC-8 cathode.Reproduced with permission.[103]Copyright 2022, Wiley-VCH.g) Schematic illustration of phase transition mechanism.Reproduced with permission.[105]Copyright 2022, Elsevier.

Figure 5 .
Figure5.a) Schematic illustration of zinc storage mechanism in Zn/MnO 2 battery.Reproduced with permission.[109]Copyright 2020, Elsevier.b) Ex situ XRD patterns at different depths of charge/discharge.SEM images of MnO 2 /rGO electrode at c) A state, d) B state, and e) C state.Reproduced with permission.[110]Copyright 2020, Wiley-VCH.f) Schematic diagram of the proposed charge storage mechanism of β-MnO 2 cathode.Reproduced with permission.[111]Copyright 2022, Elsevier.g) In situ XRD patterns of a cathode in a Zn/α-MnO 2 cell with 1.0 M ZnSO 4 electrolyte during the first dischargecharge cycle at C/20 and the corresponding discharge-charge curve.Reproduced with permission.[112]Copyright 2016, Wiley-VCH.

Figure 6 .
Figure 6.Problems and optimized strategies of manganese dioxide cathode materials in AZIBs.

Table 1 .
Type and crystal structure of MnO 2.

Table 2 .
Electrochemical performance of various Mn-based cathode materials for AZIBs.
2e À Structural evolution of the cathode during the first discharge is shown in Figure 3a.In the initial charging state, Zn x Mn 2 O 4 , MnOOH, and Mn 2 O 3 are oxidized to α-MnO 2 with the release of Zn 2+ and H + , and ZHS reacts with Mn 2+ to generate ZnMn 3 O 7 Á3H 2 O.In the subsequent charge-discharge cycle, ZnMn 2 O 4 and ZnMn 3 O 8 are further generated on the surface of MnO 2 and serve as the hosts for Zn 2+ insertion.The results show that H + , Zn 2+ , Mn 2+ , and SO 4 ) show the lattice spacing of short nanorods and nanoparticles is 0.33 and 0.26 nm, which are consistent with the d-spaces of (210) and (020) planes in monoclinic MnOOH discharge products and indicate the formation of MnOOH phase.As shown in Figure4e, the elemental mapping images clearly display that Mn and O elements are well-distributed, while Zn elements are mainly distributed on the flake-like solid, which is ZnSO 4 [Zn(OH) 2 ] 3 ÁxH 2 O generated in the process of discharge.The nanorods and nanoparticles retain their morphology during the charging process, but the lattice spacing and crystallinity are restored to the original α-MnO 2 electrode.These phenomena fully indicate the reversible chemical conversion reaction between MnO 2 and MnOOH.Different from the previous two types of ions insertion energy storage mechanisms, chemical conversion reactions involve the formation of new materials.During the charging and discharging process, only H + is inserted/extracted into the cathode materials while Zn 2+ does not participate in this process and is mainly converted into basic zinc sulfate.Relatively few studies have been reported on this energy storage mechanism, which needs to be further studies.
designed a hybrid MnO 2 /rGO nanofilm with good electrochemical performance and high flexibility, which was constructed by 1D ultra-long MnO 2 nanowires and 2D reduced graphene oxide (rGO) nanosheets.As presented in Figure5b, the MnO 2 /rGO cathode exhibits a typical two-stage discharge process.When α-MnO 2 is discharged at the first step to point A, a new peak of MnOOH appears at about 26°, which corresponds to the process of proton insertion (formula 1).On continuous discharge to point B, a new set of peaks will generate.It is related to ZHS and ZnMn 2 O 4 new phase (formula 2), which corresponds to the insertion process of Zn 2+. Scanning electron microscopy (SEM) images show a large number of ZHS flake phases that appear on the surface of the electrode as the discharge process goes on (Figure5c-e).The generation of ZHS nanosheets is mainly caused by protons consumption (formula 3).When the electrode is charged to point C, α-MnO 2 nanowires return to their initial shape, and ZHS Energy Environ.Mater.2023, 6, e12575 nanosheets dissolve and release protons.The energy storage mechanism of continuous H + /Zn 2+ intercalation/extraction and ZHS flakes deposition/dissolution during the cycle was confirmed.The chemical reaction equations are as follows: