The 3D Flower–Like MnV12O31·10H2O as a High‐Capacity and Long‐Lifespan Cathode Material for Aqueous Zinc‐Ion Batteries

Selecting the right cathode material is a key component to achieving high‐energy and long‐lifespan aqueous zinc‐ion batteries (AZIBs); however, the development of cathode materials still faces serious challenges due to the high polarization of Zn2+. In this work, MnV12O31·10H2O (MnVO) synthesized via a one‐step hydrothermal method is proposed as a promising cathode material for AZIBs. Because the stable layered structure and hieratical morphology of MnVO provide a large layer space for rapid ion transports, this material exhibits high specific capacity (433 mAh g−1 at 0.1 A g−1), an outstanding long‐term cyclability (5000 cycles at a current density of 3 A g−1), and an excellent energy density (454.65 Wh kg−1). To illustrate the intercalation mechanism, ex situ X‐Ray diffraction, Fourier transform infrared spectroscopy, and X‐ray photoelectron spectroscopy are adopted, uncovering an H+/Zn2+ dual‐cation co‐intercalation processes. In addition, density‐functional theory calculation analysis shows that MnVO has a delocalized electron cloud and the diffusion energy barrier of Zn2+ in MnVO is low, which promotes the Zn2+ transport and consequently improves the reversibility of the battery upon deep cycling. The key and enlightening insights are provided in the results for designing high‐performance vanadium‐oxide‐based cathode materials for AZIBs.

crystal structures, e.g., cationic pillaring agents and vanadiumoxygen (V-O) skeleton. [27]These intercalated metal ions can act as "pillars" to facilitate greater Zn 2þ diffusivity along interlayer channels and easily facilitate Zn 2þ insertion/extraction, enabling long-cycle stability even at high currents for discharge/ charge, [26][27][28][29] such as Ba 1.2 V 6 O 16 ⋅3H 2 O, [30] Mg x V 2 O 5 ⋅nH 2 O, [31] Na 2 V 6 O 16 ⋅2.14H 2 O, [32] and Na 2 V 6 O 16 ⋅1.63H 2 O. [33] In addition, it has been illustrated that the cathode material becomes more structurally and thermodynamically stable by introducing Mn(II) ions to enlarge the crystal plane spacing of hydrated vanadate. [34]On the one hand, manganese vanadate exhibits excellent Zn ions intercalation due to its characterized by enhanced electrical conductivity and fast ion diffusion. [34]In contrast, the Mn 2þ form stable chemical bonds with O atoms in the VO x slabs with an Mn-O bond energy of 362 kJ mol À1 , which is higher than the bond energy of Na-O (270 kJ mol À1 ), Mg-O (358 kJ mol À1 ), K-O (277 kJ mol À1 ), and Zn-O (280 kJ mol À1 ). [35]Nevertheless, the current researches on manganese vanadate as cathode material for AZIBs are still in its infancy, with the limitation of low specific capacity and poor long-term cycle stability.Therefore, the constant development of novel cathode materials through material design for the AZIBs is highly desired.
Based on the previous description, considering the high bond energy of 362 kJ mol À1 formed among Mn-O bonds, the layered MnV 12 O 31 •10H 2 O (MnVO) linked by Mn-O bonds was synthesized by a one-step hydrothermal method, which exhibited excellent zinc-ion storage performance in terms of high specific capacity (433 mAh g À1 at a current density of 0.1 A g À1 ), sufficient energy density (454.65 Wh kg À1 at a current density of 0.1 A g À1 ), and prolonged cycling stability (5000 cycles at a current density of 3 A g À1 and the rate of capacity retention is 81.47%).The electrochemical reaction process of MnVO in AZIBs was investigated by a series of ex situ characterizations, which proved that the structure of MnVO during the intercalation/extraction of Zn 2þ is highly reversible.In addition, the presence of structured water stabilizes the structure of MnVO, which could be against repeated discharge/charge current shocks and accelerate the diffusion kinetics of Zn 2þ by acting as charge shielding.Theoretical calculations show that MnVO has a delocalized electron cloud, which helps to promote the chargetransfer kinetics at the electrode/electrolyte interface, and Zn 2þ can achieve fast migration in the MnVO structure.Therefore, the excellent electrochemical performance is mainly due to the structural stability of MnVO and its fast ion mobility.The research of this work has constructive significance for expanding the types of vanadium-based materials and exploring high-performance cathode materials for AZIBs.

Structure and Morphology of the Sample
The samples with different molar ratios of Mn and V were prepared by hydrothermal method.The phase type, crystallinity, and crystal structure of the sample were characterized by X-Ray diffraction (XRD), and the test results are shown in Figure 1a.Despite different molar ratios of Mn and V (Figure 1a and Figure S1, Supporting Information), all the samples show the characteristic peaks of MnV 12 O 31 •10H 2 O (PDF#47-0146) at 25.83°, 27.11°, 34.71°, 47.21°, 50.76°, and 60.36°, indicating that a layered MnVO has been obtained synthesized.As evaluated via energy-dispersive X-ray spectroscopy (EDX) (Figure 1b), the atomic ratio of Mn and V in the sample is identical to the theoretical stoichiometric proportion in MnV 12 O 31 •10H 2 O.The existence of Au is caused by the gold-spraying treatment on the surface of the sample.Furthermore, as shown in Table S1, Supporting Information, the inductively coupled plasma emission spectroscopy (ICP) test results showing that the atomic percentage of Mn and V elements is close to 1:12.Thermogravimetric analysis (TGA) (Figure 1c) is adopted to determine the content of structural water in the sample.The weight loss of the sample after 50 °C is 8.76%, which matches well with the mass percentage of water in MnVO.The results of EDX, ICP, and TGA confirm the successful synthesis of MnVO.To investigate the local structure of the as-prepared sample, Raman spectra were measured (Figure 1d).The bending vibration of the V=O bond (at 264 cm À1 ) originates from the terminal oxygen and the central vanadium. [36]The broad characteristic peak near 352 cm À1 corresponds to the free movement of water. [37]The signal at 512 cm À1 reflects the V 3 -O bond, which consists of triply coordinated O 3 by three V cations and the main connection VO 5 pyramid with three shared edges in the crystal structure. [38]The characteristic peaks at 674 and 712 cm À1 correspond to the tensile vibration of V 2 -O, indicating the presence of V-O-V connections in the crystal structure. [39]The highest peak occurs at 712 cm À1 , which indicates that the addition of Mn is beneficial to the orderly arrangement of V and O due to the synergistic action of the three ions.The peak at 891 cm À1 mainly corresponds to V-OH 2 . [40]For the spectra of the samples 600-800 cm À1 , the Lorentzian function was used for peak splitting.The two discernible peaks at 674 and 712 cm À1 are in good agreement with the stretching of V 2 -O (Figure 1d).The weak peak corresponding to 652 cm À1 in Figure 1e is mainly derived from the vibration of the Mn-O bond in the MnO 6 octahedron, indicating that the Mn ions are connected to the [VO n ] layer through chemical bonds. [41]o further determine the composition and elemental analysis of the product, the sample was characterized by XPS. Figure 1f shows the full XPS spectrum of MnVO, in which the characteristic peaks of O, V, Mn, and C can be observed without the presence of other impurity elements.The appearance of the C1s peak may be caused by the adsorption of CO 2 in the air on the sample surface during the detection process.The spectrum of O1s (Figure 1g) is divided into three peaks located at 530.05, 531.08, and 532.77eV, which are attributed to the bonding in different forms of O 2À in V-O, O-H, and H 2 O, respectively.In Figure 1h, the Mn 2p signal was detected in MnVO, indicating there are two valence states of Mn 2þ and Mn 3þ .Figure 1i shows the high-resolution XPS pattern of V 2p.The peaks of V2p 3/2 at 516.19 eV and V2p 1/2 at 524.32 eV correspond to the V 4þ oxidation state; the characteristic peaks of V2p 3/2 at 517.37 eV and V2p 1/2 at 525.25 eV correspond to the V 5þ oxidation state.Therefore, the vanadium elements in MnVO have two valence states, i.e., V 4þ and V 5þ .Different chemical states of V xþ (x = 3, 4, and 5) can be beneficial to the diffusion of Zn 2þ in the electrode material, which plays a good promoting role in improving the specific capacity of the battery. [42]It is speculated that MnVO can show excellent electrochemical properties with mixed valence states of V 4þ and V 5þ .Figure S2a-f, Supporting Information, is the high-resolution XPS spectra of O, Mn, and V elements at the molar ratio of Mn and V at 0.5:1 and 2:1.It can be seen from the figure that the forms of O and V elements are consistently compared with the sample when the molar ratio of Mn and V is 1:1 (Figure 1g-i).Since the content of V 4þ is similar and the effect of oxygen vacancies is close in the three samples, the difference in electrochemical performance originates from Mn 2þ and Mn 3þ .As shown in Table S2, Supporting Information, the relative contents of Mn 2þ and Mn 3þ were calculated from the XPS data of the three MnVO samples.Compared with the other two samples, the content of Mn 2þ in MnVO with the molar ratio of Mn and V at 1:1 is as high as 60.29%.Mn 2þ and V 4þ have electrons in 3D orbitals, which are beneficial to improve electrical conductivity and catalyze electrochemical reactions; when the Mn 2þ cations of the sample increase, the number of d orbitals located at the bottom of the conduction band and the top of the valence band increases. [34]Therefore, it is expected that MnVO with a molar ratio of Mn and V of 1:1 will have more excellent electrochemical performance.
The morphologies of the MnVO were characterized by scanning electron microscope (SEM).As shown in Figure 2a-c, MnVO exhibits a morphology of a 3D flower composed of nanosheets.Meanwhile, the thickness of the nanosheets is about 20 nm (Figure S3, Supporting Information).Figure S4, Supporting Information, shows the SEM of the molar ratio of Mn and V at 0.5:1 and 2:1, it can be seen from the figure that a small part of MnVO in the two molar ratios forms nanoflowers composed of nanosheets.Unfortunately, most of them exhibit blocky morphology, compared to the samples with a molar ratio of Mn and V of 1:1 (Figure 2a-c).The MnVO formed by these two molar ratios has fewer exposed active sites, and the contact area with the electrolyte is small under the same conditions, which is unfavorable to the performance of the batteries.TEM images in Figure 2d-e further indicate that MnVO with a 1:1 molar ratio of Mn and V is formed by the aggregation of the nanosheets.The lattice fringes in the transmission electron microscope (TEM) image (Figure 2f ) were measured to be 0.343 nm which is consistent with the diffraction peak at 25.83°in the XRD pattern (Figure 1a).Furthermore, element mapping (Figure 2g) of the MnVO in the SEM image shows that three elements (V, O, and Mn) uniformly distribute throughout the sample.The 3D hieratical morphology of the as-prepared MnVO is favorable for the intrusion of electrolyte and more reactive sites, thereby increasing the ability of charge transfer at the electrode/electrolyte interface.

The Electrochemical Performances
To study the electrochemical performance of MnVO as the cathode material in AZIBs, metal zinc was used as the negative electrode, 3M Zn(CF 3 SO 3 ) 2 aqueous solution was used as the electrolyte, and the glass fiber was used as the separator to assemble the batteries for electrochemical performances test.Figure 3a shows the initial 4 cyclic voltammetry (CV) curves at a scan rate of 0.1 mV s À1 and with a voltage window of 0.3-1.8V. Two pairs of redox peaks, located at 0.69/0.45and 1.1/0.93V, are observed, indicating multistep insertion/extraction processes.The consistency of the CV curves in these four cycles indicates excellent reversibility of the electrode material.Galvanostatic charge-discharge curves of the MnVO electrode in the range of 0.3-1.8V are shown in Figure 3b.The material delivers a reversible capacity of 433 mAh g À1 at a specific current of 0.1 A g À1 .In addition, the charge-discharge profiles further confirm the high reversibility and stability of the electrode material, which is identical to the CV curves (Figure 3a).The rate capability of the as-prepared MnVO was evaluated.The galvanostatic discharge-charge profiles of the MnVO obtained at different current densities are shown in Figure 3c, and the specific capacities upon the tests are displayed in Figure 3d.The discharge capacities at 0.1, 0.5, 1, 3, and 5 A g À1 are 433, 336, 266, 117, and 63 mAh g À1 , respectively.After the specific current is switched from 5 A g À1 back to 0.1 A g À1 , the discharge capacity recovers to 408 mAh g À1 , which corresponds to a recovery rate of 94.3%.It demonstrates that the high-rate operation has limited influence on the host structure of MnVO.Subsequently, to demonstrate the application potential of the MnVO cathode material, its energy density and the power density based on the mass of the MnVO active material were calculated from the rate performance (Figure 3e).The energy density of MnVO reaches 454.65 Wh kg À1 at 0.1 A g À1 and the power density is 5250 W kg À1 at 5 A g À1 .Figure 3e shows the Ragone diagram of MnVO developed in this work, and some other recently reported cathode materials, such as Zn 3 V 2 O 7 (OH) 2 ⋅2H 2 O, [43] Na 2 V 6 O 16 ⋅1.63H 2 O, [33] K 2 V 8 O 21 , [27] VS 2 , [44] V 3 O 7 ⋅H 2 O, [23] K 0.23 V 2 O 5 , [45] Mn 3 O 4 , [46] Cu 3 V 2 O 7 (OH) 2 ⋅2H 2 O, [47] Na 0.56 V 2 O 5 , [48] and H 2 V 3 O 8 . [49]Compared with those recently reported materials, MnVO developed in this work exhibits higher energy density in a wide power density range, which indicates the potential application of MnVO cathode material.
The cyclability of MnVO prepared with different Mn to V ratios, i.e., 0.5:1, 1:1, and 2:1, has been evaluated at 0.5 A g À1 .The specific capacity and Coulombic efficiency upon cycling are summarized in Figure 3f and S5, Supporting Information.The Coulombic efficiency of the three samples is around 100%, which indicates high reversibility.The sample with an Mn: V ratio of 2:1 exhibits the lowest capacity retention of 74.6% after 200 cycles.Under the same test protocol, a Mn: V ratio of 1:1 leads to the highest capacity retention of 91.2%.Furthermore, this MnVO was subjected to a high current rate of 3 A g À1 to evaluate its durability upon long-term cyclability, and the results are summarized in Figure 3g.MnVO exhibits a stable capacity upon 5000 cycles, finally reaching an impressive capacity of 144.2 mAh g À1 , which corresponds to a capacity retention rate of 81.5%.As shown in Table S3, Supporting Information, the cycling performance of MnVO is better than most materials compared with those reported at present.These electrochemical tests demonstrate the remarkable cyclability and rate capability of MnVO, which implies fast and stable charge transport at the electrode/electrolyte interface and ion diffusion in the bulk of MnVO, respectively.These two tightly relate to the 3D hieratical morphology and robust crystal structure.

Kinetic Performance
To understand the electrochemical process of MnVO in the cells, the kinetics of the MnVO cathode material has been investigated.Figure S6a, Supporting Information, shows the CV curves of the MnVO cathode material at different scan rates from 0.1 to 0.4 mV s À1 , and every curve has two pairs of clear redox peaks.It is well known that the peak current (i) is related to the scan rate (v) and their relationship can be expressed by the following equations [33] i ¼ av b (1) The slope b can be received from the log(i) to log(v) graphs and b values are usually between 0.5 and 1.When the capacity is fully provided by the ion-diffusion-controlled process, b is equal to 0.5; when the capacity is contributed fully by the capacitive process, b is equal to 1.The aforementioned equations were used for fitting and the b values of the four peaks were 0.617, 0.670, 0.985, and 0.913, respectively (Figure S6b, Supporting Information).Therefore, Peak 1 and Peak 2 are mainly controlled by ion diffusion, while Peak 3 and Peak 4 are mainly controlled by the capacitive process.So the whole electrochemical process is influenced by ion-diffusion and capacitive process.Furthermore, the corresponding capacitive contributions at different scan rates can be determined by the following equations [33] i where k 1 and k 2 represent the redox reaction coefficients of the capacitive process and diffusion process under the specific voltage, respectively; k 1 v and k 2 v 1/2 correspond to the contributions of the capacitive process and ion diffusion, respectively.As shown in Figure S6c, Supporting Information, the capacitive process contributions have been calculated as 39.12%, 45.18%, 45.45%, 52.05%, 52.84%, and 56.13% at scan rates of 0.1, 0.15, 0.2, 0.25, 0.3, and 0.4 mV s À1 , respectively, which demonstrates that the capacitive process gradually dominates the electrochemical reaction for increasing scan rates.As shown in the electrochemical impedance spectroscopy (EIS) (Figure S6d, Supporting Information), the charge-transfer resistance in the high-frequency region becomes smaller after cycling, which further indicates that the ion-diffusion rate of the electrode material is faster after activation in the later stage.In addition, As known, in the Nyquist plots, high frequency is mainly Ohm impedance, which reflects the impedance caused by the electrolyte, separator, and electrodes.Medium and high frequencies are semicircular arcs and related to charge-transfer impedance.Low frequency is Warburg impedance and shows a sloping line associated with the diffusion behavior of ions between electrodes and electrolytes ion diffusion. [50,51]In Figure S6d, Supporting Information, after the material is activated in the later stage, a new semicircle appears in the high-frequency region.This indicates that the electrolyte has made sufficient contact with the material, corresponding to the charge-transfer process at the electrode-electrolyte interface.Subsequently, the solid-phase-diffusion kinetics of Zn 2þ was investigated by the galvanostatic intermittent titration technique (GITT).Each step corresponds to a current flow of 50 mA g À1 for 10 min followed by 1 h, and the voltage range is between 0.3 and 1.8 V. Figure S6e, Supporting Information, presents the galvanostatic charge-discharge curves of the GITT test of the MnVO electrode.The Zn 2þ -diffusion coefficient (D GITT zn 2þ ) was calculated by the following equation [52] where τ is the constant current pulse time; L is the thickness of the electrode; ΔE s corresponds to the change in steady-state voltage; and ΔE t is the change in cell voltage at constant current minus the IR loss during each current step.According to Equation ( 5), the diffusion coefficient D GITT zn 2þ of Zn 2þ during the charge and discharge processes of the electrode material was calculated, the D GITT zn 2þ is between 1.22 Â e À12 and 6.55 Â e À9 (Figure S6f, Supporting Information) during the insertion/ extraction of Zn 2þ , indicating its excellent Zn 2þ -diffusion kinetics. [53,54]

Reaction Mechanism
In addition to the kinetics, the electrochemical energy-storage mechanism of the MnVO electrode materials was explored via ex situ XRD, FTIR, and XPS.The electrodes at the different states of charge upon the initial dis-/charge at 0.1 A g À1 were used for the ex situ characterizations.The evolution of the crystal structure of MnVO was characterized by ex situ XRD, as shown in Figure 4a-d.Three distinct diffraction peaks, located at 12.9°, 19.4°, and 32.9°, gradually appear during the discharge process (discharge voltage from 0.7 to 0.3 V), accompanied by increasing intensities.These peaks are assigned to the layered doublehydroxide Zn x (OTf ) y (OH) 2xÀy •nH 2 O, which has been proven to be an indicator of the H þ intercalation mechanism. [34,55,56]pecifically, due to the hydrolysis of Zn(OTf ) 2 , the mildly acid electrolyte contains free H þ ; upon the discharge process, H þ is intercalated into MnVO, which leads to locally increased pH and consequent formation of Zn x (OTf ) y (OH) 2xÀy •nH 2 O. Therefore, MnVO in the aqueous electrolyte involves not only the Zn 2þ de-/intercalation but also those of H þ .As illustrated in Figure 4c,d, the diffraction peaks of MnVO in the ranges of 22-28°and 44-52°shift to lower 2θ during the discharging process, which further indicates the insertion of Zn 2þ into the MnVO electrode material, because the de-/intercalation of H þ has little influence on the interlayer distance of the vanadium oxides. [57]Upon the subsequent charging process (charging voltage from 0.3 to 0.85 V), the diffraction peaks of MnVO recover to the nearly initial 2θ position and the diffraction peaks of Zn x (OTf ) y (OH) 2xÀy •nH 2 O disappear completely, demonstrating that Zn 2þ and H þ have been extracted from the electrode material.Overall, ex situ XRD demonstrates the high reversibility and structural stability of MnVO for H þ /Zn 2þ insertion/extraction, which is beneficial for the cyclability of this electrode material (Figure 3f,g).
Figure 4e shows the results of the ex situ FTIR spectroscopy tests.The characteristic peaks of hydroxyl groups (OH À ) located at 3497 and 1616 cm À1 gradually increase upon discharge and recover upon the subsequent charging process, implying the insertion/extraction of water molecules and the formation/ decomposition of Zn x (OTf ) y (OH) 2xÀy •nH 2 O.The characteristic peak of the Mn-O bond at 1386 cm À1 barely changes during the discharge and charge processes, [58] showing the stability of the Mn-O bonds.The peaks of S=O (at 1232 and 1172 cm À1 ) and C-F (at 1034 cm À1 ) are strengthened significantly upon the discharge process, proving the accumulation of Zn x (OTf ) y (OH) 2xÀy •nH 2 O. Upon charging, all the characteristic peaks nearly return to the same state as the original one, which indicates the disappearance of Zn x (OTf ) y (OH) 2xÀy •nH 2 O and is consistent with the results of ex situ XRD.In addition, the peak located at 994 cm À1 corresponds to the stretching vibration mode of V 5þ = O, and its intensity becomes lower upon the discharge, indicating a partial transition from V 5þ to V 4þ . [38,39]During the subsequent charging process, this characteristic peak recovers completely, demonstrating high reversibility.
The high-resolution XPS spectra of Zn and C elements on the MnVO electrode at the initial, fully discharged, and fully charged states are shown in Figure 4f,g, respectively.In the Zn 2p XPS spectra of the electrode at the fully discharged state, the peaks at  [59] respectively.In the spectra of the fully discharged electrode, a new signal peak at 293.2 eV appears and is assigned to the C-F bond in Zn x (OTf ) y (OH) 2xÀy •nH 2 O.When the electrode is recharged to 1.8 V, the signal peak disappeared.The C-F bond at 290.66 eV may come from the residual electrolyte (Zn(CF 3 SO 3 ) 2 ) on the sample surface.Sustainable energy fuels exhibits XPS spectra of O, Mn, and V elements in the fully discharged and fully charged MnVO electrode.After the complete discharge, the signal peak (532.83eV) corresponding to water molecules in the XPS spectrum of O 1s is enhanced.Meanwhile, a new peak appears at 534.39 eV and can be attributed to water molecules coordinated with Zn 2þ . [56]oth these two signals return to the initial state (Figure 1g) when the electrode is charged to 1.8 V.These results confirm again the reversible decomposition of Zn x (OTf ) y (OH) 2xÀy ⋅nH 2 O.The XPS spectrum of Mn 2p is shown in Figure 4i.The peak positions of Mn 2p 3/2 and Mn 2p 1/2 do not shift and the ratio of Mn 2þ and Mn 3þ is similar at the full discharge/charge process.It implies that Mn is not a redox center of MnVO, and Mn with a stable local structure is very suitable to be used as a pillaring agent to stabilize the structure of MnVO for reversible insertion/extraction of Zn 2þ .The characteristic peaks of V2p 3/2 in the initial state (Figure 1i) can be divided into peaks located at 517.37 and 516.19 eV energy levels, corresponding to the two oxidation states of V 5þ and V 4þ , respectively.When the electrode is discharged to 0.3 V, the V2p 3/2 of V 5þ and V 4þ shifts to 517.85 and 516.73 eV, respectively, which may be caused by the V-bond rearrangement due to the insertion of Zn 2þ . [27]In addition, it is also observed that the intercalation of the guest cations, i.e., H þ or/and Zn 2þ , leads to more intense V 4þ peaks and weaker V 5þ .When the battery is fully charged to 1.8 V, despite the increased content of V 5þ , the content of V 4þ is not fully recovered, which means that Zn 2þ is not completely de-intercalated upon the first charge process.This is consistent with the previous phenomenon observed from the results of ex situ XRD and FTIR.The previous analysis about ex situ XPS further confirms the insertion/extraction of Zn 2þ and the involvement of vanadium-based redox centers in the electrochemical reactions.
In addition, the morphological evolution of MnVO electrode material during discharge/charge was also investigated.Figure S7a, Supporting Information, shows the SEM mappings of a fully discharged electrode.The sheet products can be observed in the secondary electron image (Figure S7a, Supporting Information) and the elemental mapping of these sheet products proves the uniform distribution of Zn, F, O, and S elements, which further illustrates the existence of Zn x (OTf ) y (OH) 2xÀy •nH 2 O.The 3D flower-like substance under the sheet product is MnVO.Figure S7b,c, Supporting Information, displays the SEM images and corresponding elemental mapping of MnVO in fully discharged and fully charged electrodes, respectively.The Zn element exhibits stronger signals in the fully discharged sample than the fully charged sample, which confirms again that Zn 2þ is intercalated into the material during the discharged process and there are also some residual Zn 2þ in the fully charged sample.The detected Mn element in both states implies its relatively stable presence in the crystal structure.The excellent electrochemical performance of MnVO electrode material comes from the fast transport of Zn 2þ , which not only relates to the transportation across the electrolyte/ electrode interface but also in the bulk of MnVO.The former has been demonstrated in Section 2.1.
To understand how Zn 2þ is transported in the bulk of the material, a density-functional theory calculation was conducted.Figure 5a shows the structure of Zn 2þ inserted in MnVO.
Figure 5b shows the differential charge density distribution of inserted Zn 2þ in MnVO.The yellow cloud is the accumulation area of electrons; the blue cloud is the dissipation area of electrons (the isosurface value is 0.005 eV Å À3 ).The results indicate that MnVO possesses a delocalized electron cloud, which helps to facilitate the charge-transfer kinetics, thereby improving the reversibility of the battery during deep cycling.The diffusion path of Zn 2þ after entering the material lattice is simulated according to the structural characteristics of the MnVO.The migration of Zn 2þ in the interlayer of MnVO along the a-axis (path I) and b-axis (path II) is shown in Figure 5c,d, respectively.Subsequently, the diffusion barriers of Zn 2þ in MnVO along these two paths have been further calculated and are shown in Figure 5e.The diffusion barrier along the a-axis is 0.395 eV (path I), and the diffusion barrier along the b-axis is 0.297 eV (path II).These values are much lower than the various reported vanadium-based compounds, such as MgV 2 O 6 ⋅1.7H 2 O (0.896 eV), [59] Mg 0.55 V 2 O 5 ⋅0.8H 2 O (1.16 eV), [59] V 2 O 5 ⋅1.75H 2 O (0.81 eV), [60] VO 2 (0.78 eV), [61] NaCa 0.6 V 6 O 16 (1.08eV), [62] K 2 V 8 O 21 (1.34 eV), [63] and V 6 O 13 (0.87 eV), [64] further demonstrating the remarkable Zn-ions-diffusion kinetics in the MnVO cathode material.
Based on the results, a schematic diagram of the Zn 2þ /H þ hybrid intercalation mechanism of MnVO in the AZIBs is shown in Figure 6.First, Zn 2þ is inserted into the MnVO cathode material in the discharge voltage range of 1.8-0.7 V, resulting in lattice changes and peak shifts in XRD patterns.When the discharge voltage is 0.7-0.3V, the decomposition of water molecules in the electrolyte generates a large amount of OH À , which reacts with the electrolyte to obtain Zn x (OTf ) y (OH) 2xÀy •nH 2 O, and the remaining same amount of H þ also enters into the positive electrode active material lattice together with Zn 2þ .During the first charging process (0.3-0.85 V), Zn 2þ and H þ are extracted from the MnVO cathode material and accompanied by the dissolution of Zn x (OTf ) y (OH) 2xÀy •nH 2 O, and H þ was completely extracted and regenerated with OH À to form water molecules.When fully charged to 1.8 V, the release of Zn 2þ reaches the maximum, but it is still not completely released.

Conclusions
MnVO has been successfully synthesized via a simple hydrothermal method and investigated as a cathode material for AZIBs.The large layered space and 3D hieratical morphology of MnVO provide a large space for the accommodation of the cationic charge carriers and fascinate their rapid transport.As a result, MnVO outputs a high discharge specific capacity of up to 433 mAh g À1 (at a specific current of 0.1 A g À1 ), excellent cycling stability (capacity retention rate of 81.47% after 5000 cycles at a specific current of 3 A g À1 ).Due to the remarkable capacity and rate capability, it exhibits an energy density of 454.65 Wh kg À1 and a power density of 5250 W kg À1 .In addition, ex situ XRD, FTIR, and XPS tests have demonstrated a Zn 2þ /H þ co-insertion mechanism of MnVO in the AZIBs.The high energy density and long cycle life endow this material with promising potential applications in the low-cost and high-safety AZIBs.

Experimental Section
Sample Preparation: V 2 O 5 of 0.182 g was dissolved in 35 mL of deionized water with constant stirring, which was followed by dropwise addition of 5 mL of 30% H 2 O 2 .After being stirred for 20 min, the mixture was added with•MnCl 2 •4H 2 O in different molar ratios (Mn: V = 0.5:1, 1:1, 2:1, respectively) and stirred at room temperature for another 2 h to form a homogeneous solution.The aforementioned solution was transferred to a reaction kettle which was kept at 180 °C for 24 h.The solid product was washed with ethanol and deionized water three times when cooling down to room temperature, and then freeze-dried to obtain a dark red MnVO powder.In this work, the performance of MnVO synthesized when the molar ratio of Mn and V was 1:1 was discussed in the previous stage, so this work mainly studies various properties of Mn and V molar ratio of 1:1.The XRD and long-cycle stability of the other two molar ratios are in Supporting Information.
Characterization: In this work, XRD and ex situ XRD patterns of samples were obtained by using the D/max-3B X-Ray diffractometer produced by Rigaku Company.For ex situ XRD tests, the electrode samples unmounted from the cells at certain discharge/charge states were washed with deionized water and vacuum dried.The SEM images of the samples were taken using Quanta 200 instrument produced by FEI Company, and the accelerated voltage tested was 20-30 kV.The samples were tested with a JEM-2010 TEM under the accelerated voltage of 200 kV, and the TEM images were obtained.X-Ray photoelectron spectrometer (ESCALAB 250Xi) of Thermo Fisher Scientific was used for XPS spectrum characterization of the samples.ICP was conducted using a PlasmaQuant PQ9000 ICP.TGA was performed by MicroMELER/1600 H thermogravimetric analyzer.Infrared spectra of the samples were recorded using a Nicolet model 10 FTIR spectrometer for studying surface functional group information.
Electrochemical Measurements: The electrodes were fabricated via casting slurries with active substance (70 wt%), carbon black (20 wt%), and polyvinylidene fluoride (10 wt%) binder on titanium foil (current collector).N-methyl-2-pyrrolidone was used as the solvent.After being dried under vacuum at 80 °C for 8 h and punched to discs with diameters of 14 mm, the MnVO cathodes were obtained and the average MnVO loading was 1.62-1.95mg cm À2 .In addition, 3 M Zn(CF 3 SO 3 ) 2 (Zn(OTf ) 2 ) solution (the dosage about 50 μL) and metal zinc foil (the thickness was 0.02 mm) were used as the electrolyte and anode electrode, respectively.Finally, the materials were assembled into a standard CR 2032 buckle battery for testing.CV and EIS were tested with Chenhua electrochemical workstation.The rate and cycling performance were tested by a LAND CT2001A battery test system.

Figure 3 .
Figure 3. Electrochemical behaviors and kinetics of the Zn//MnVO battery: a) cyclic voltammetry (CV) curve at the scanning speed is 0.1 mV s À1 ; b) The charge-discharge curve of five cycles at a current density of 0.1 A g À1 ; c) galvanostatic discharge-charge profiles of the MnVO obtained at different current densities; d) rate performance; e) the Ragone plots in comparison with other reported materials for aqueous zinc-ion batteries (AZIBs); f,g) The long cycle performance and Coulombic efficiency of the MnVO.
1022.57 and 1045.68 eV are Zn 2p 3/2 and Zn 2p 1/2 , which originate from Zn x (OTf ) y (OH) 2xÀy •nH 2 O and Zn 2þ -intercalated into MnVO.For the electrode at a fully charged state, these peaks are still observed but exhibit weaker intensities.Since ex-XRD has demonstrated that Zn x (OTf ) y (OH) 2xÀy •nH 2 O completely disappeared upon the charge, the signal of Zn 2p at a fully charged state must result from the residual Zn 2þ in MnVO.The XPS spectra of C 1s (Figure 4g) further confirm the reversible formation of Zn x (OTf ) y (OH) 2xÀy •nH 2 O.In the C 1s XPS spectrum of the initial electrode, the peaks at 284.6, 285.6, and 286.8 eV are assigned to C=C, C-C, and C-O bonds,

Figure 4 .
Figure 4. a) The discharge/charge curve at the selected of the first cycle at 0.1 A g À1 of the MnVO electrode; b) ex situ XRD patterns; c,d) the magnified XRD patterns from (b); e) ex situ Fourier transform infrared spectroscopy (FTIR) spectra; f,g) ex situ high-resolution XPS spectra at pristine, fully discharged and charged states, for f ) Zn 2p, g) C 1s; and h-j) ex situ high-resolution XPS spectra of O 1s, Mn 2p, and V 2p in the fully discharged and fully charged states.

Figure 5 .
Figure 5. a) Structural diagram of inserted Zn in the MnVO structure; b) charge density difference of the inserted Zn 2þ in the MnVO electrode; c,d) simulated Zn 2þ -diffusion paths in the MnVO along the a and b axes; and e) diffusion energy barriers of Zn 2þ in MnVO along two paths (along a and b axes).

Figure 6 .
Figure 6.Schematic illustration of the reaction mechanism of the MnVO electrode.