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Jan 9, 2019 - Our work not only opens a new door to the practical application of VO2 in ZIB ... open cage-like structures that allow the rapid interca...
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An Ultrastable and High-Performance Zn/VO2 Battery Based on a Reversible Single-Phase Reaction Lineng Chen, Yushan Ruan, Guobin Zhang, Qiulong Wei, Yalong Jiang, Tengfei Xiong, Pan He, Wei Yang, Mengyu Yan, Qinyou An, and Liqiang Mai Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03409 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Chemistry of Materials

An Ultrastable and High-Performance Zn/VO2 Battery Based on a Reversible Single-Phase Reaction Lineng Chen,† Yushan Ruan,† Guobin Zhang,



Qiulong Wei,∏ Yalong Jiang,



Tengfei

Xiong,† Pan He, † Wei Yang, † Mengyu Yan, *,‡ Qinyou An,* ,† and Liqiang Mai*,†,§ †

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,

International School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China ‡

Materials Science and Engineering Department, University of Washington, Seattle,

Washington 98195-2120, United States §

Department of Chemistry, University of California, Berkeley, California 94720-3880,

United States ∏

Department of Materials Science and Engineering, University of California, Los Angeles,

California 90095-1595, United States

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ABSTRACT The aqueous zinc ion batteries (ZIBs) composed of inexpensive zinc anode and nontoxic aqueous electrolyte are attractive candidates for large-scale energy storage applications. However, their development are limited by cathode materials, which often deliver inferior rate capability and restricted cycle life. Herein, the VO2 nanorods show significantly electrochemical performance when used as intercalation cathode for aqueous ZIBs. Specifically, the VO2 nanorods display high initial capacity of 325.6 mAh g-1 at 0.05 A g-1, good rate capability and excellent cycling stability of 5000 cycles at 3.0 A g-1. Furthermore, the VO2 unit cell expands in a, b, and c directions sequentially during discharge process and contracts back reversibly during charge process, the zinc storage mechanism is revealed to be a highly reversible single-phase reaction by operando techniques and corresponding qualitative analyses. Our work not only opens a new door to the practical application of VO2 in ZIB systems, but also broadens the horizon in understanding the electrochemical behavior of rechargeable ZIBs.

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1. INTRODUCTION Given the incremental concerns of energy shortage and climate change, the application of sustainable energy resources such as wind, solar and water power has become the global trends.1,2 Battery technologies provide a superior option to store these renewable energy systems.3,4 Among various battery systems, lithium ion batteries (LIBs) is desirable candidates because of the high specific energy and technical maturity. However, the finite Li resource and poor safety limit their large-scale applications.5,6 In search of alternative batteries, people pay extensive attention to the systems based on Na, K, Mg, Zn, Al and Ca owing to their abundant resources and low cost.7-10 Among these options, rechargeable aqueous zinc ion batteries (ZIBs) have gradually received increasing worldwide attentions because of the following four advantages: (a) the excellent properties of zinc, such as cheap resources, high theoretical capacity (819 mAh g-1) and relatively low redox potential (-0.76 V vs. standard hydrogen electrode); (b) the nontoxic, noncorrosive and low cost aqueous electrolyte; (c) the ability to transfer more than one electron and thereby facilitate more energy storage than the univalent batteries; (d) facile assembly process, low manufacturing cost, high safety and environmental friendliness.10-12 Previous explorations of cathode materials for ZIBs mostly focused on manganese dioxide (MnO2) and Prussian blue analogue (PBA). Typically, ZIBs can utilize many forms of polymorphic MnO2, including α-MnO2, γ-MnO2, β-MnO2, ε-MnO2, δ-MnO2 and todorokite-type MnO2.11-22 Nevertheless, the aqueous Zn/MnO2 battery delivers a weak rate performance due to the slow reaction dynamics.13 PBA materials, such as CuHCF, ZnHCF and KZnHCF, have open cage-like structures that allow the rapid intercalation and deintercalation of Zn2+ without significant changes of the crystal structure.23-28 Whereas, PBA electrodes usually face the problems of delivering limited capacities and suffering from O2 evolution due to the high operating voltage.28,29

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Recently, layered and tunnel-type vanadium-based compounds (VS2,30 V2O5,31 H2V3O7,32 LiV3O8,33 Na2V6O16·3H2O,34 Na3V2(PO4)3,35 Ca0.25V2O5·nH2O,36 Zn0.25V2O5·nH2O,37 Zn3V2O7(OH)2·2H2O38, Mg0.34V2O539 and ZnxMo2.5+yVO9+z40) have been used as cathode materials for rechargeable aqueous ZIBs. The reported electrochemical performances were mainly attributed to the open-framework crystal structure and the multiple oxidation states of vanadium. Among various vanadium oxides, B phase vanadium dioxide (VO2) stands out because of its unique layered structure formed by edge-sharing VO6 octahedra, which support the intercalation and deintercalation of, magnesium ions (0.66 Å), lithium ions (0.69 Å)and even sodium ions (1.02 Å).41-43 It is expected that zinc ions (0.74 Å) can insert into the layered structure of VO2 reversibly. Herein, we demonstrate a highly reversible intercalation reaction in the aqueous Zn/VO2 battery by using the 1 M zinc sulfate (ZnSO4) solution as the electrolyte and VO2 nanorods as the cathode materials. The VO2 nanorods exhibit a high capacity of 325.6 mAh g-1 at a 0.05 A g-1 and deliver excellent long-term cyclic stability with a capacity retention of 86% after 5000 cycles at 3.0 A g-1. Furthermore, the zinc storage mechanism of VO2 electrode for ZIBs is revealed by various operando techniques and quantitative analyses.

2. EXPERIMENTAL SECTION 2.1 Materials synthesis We have adopted a rapid and simple hydrothermal method for synthesis of highly homogeneous VO2 nanorods. 4 mmol V2O5 (Xiya Reagent, GR, 99.5%), 9.6 mmol H2C2O4·2H2O (Sinopharm Chemical Reagent limited corporation, 98%), and 3.4 mmol PEG-4000 (Sinopharm Chemical Reagent limited corporation, CP, 3500-4500) were added into 66 mL H2O. After continuous magnetic stirring at 40 ºC in a water bath for 24 h, the homogeneous liquid was obtained. The preparation of VO2 microspheres : 4 mmol V2O5 (Xiya Reagent, GR, 99.5%), 12 4

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Chemistry of Materials

mmol H2C2O4·2H2O (Sinopharm Chemical Reagent limited corporation, 98%), were added into 30 mL H2O. After continuous magnetic stirring at 40 ºC in a water bath for 24 h, the homogeneous liquid was formed. Then 30 mL ethyl alcohol was added into the homogeneous liquid and stirred for 20 minutes. Subsequently, two samples were transferred to a 100 mL Teflon-lined stainless steel autoclave to be maintained at 180 °C for 24 h. Afterward, the systems were cooled down to room temperature naturally. The prepared products were successfully collected by washing with deionized water and pure alcohol for several times and finally drying at 70 °C in air for 24 h. 2.2 Material characterizations The crystallographic characterization of the product was measured by a Bruker D8 Advance X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were collected with a JEM-2100F STEM/EDS microscope. Energy dispersive X-ray spectra (EDS) was recorded using an Oxford EDS IE250 system. Field emission scanning electron microcopy (FESEM) images were obtained with a JEOL-7100F microscope. Raman spectrum measurement was achieved using a Renishaw RM-1000 laser Raman microscopy system. X-ray photoelectron spectroscopy (XPS) measurements were performed using a VG Multi Lab 2 000 instrument. 2.3 Electrochemical measurements VO2 electrode slice was prepared by mixing VO2 nanorods, acetylene black, and poly tetrafluoroethylene (PTFE) at a weight ratio of 6:3:1, the solvent is isopropyl alcohol which is volatile, then the slurry was evenly grinded, tableted and cut into Φ6 mm electrodes. 1 mol L1

zinc sulfate (ZnSO4) solution was used as the electrolyte, and glass fiber membrane and

Zinc foil were used as the separator and anode, respectively. CR2016-type coin cells were 5

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assembled in the air atmosphere to evaluate the electrochemical performances with a LAND battery testing system (CT2001A). Cyclic voltammograms (CV) were tested on an electrochemical workstation (CHI600E). All of the tests were measured at room temperature.

3. RESULTS AND DISCUSSION

Figure 1. Characterization of VO2 nanorods: a) Rietveld refinement result from the XRD data , b) Raman spectrum, c-e) FESEM and SEM-EDS images, f,g) TEM images, and h) SAED patttern. Figure 1a presents the high-resolution X-ray diffraction (XRD) patterns of the powder that was prepared through a hydrothermal reaction. The patterns were refined using the 6

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Chemistry of Materials

Rietveld method and identified as B phase VO2 (C2/m space group) with lattice parameters of a=12.06 Å, b=3.68 Å, c=6.42 Å and β=107.0° (Supplementary Note 1). In addition, the monoclinic structure of VO2 was drawn by VESTA software (inset in Figure 1a). The double layer V4O10 in VO2 share corners to form a shear-type structure which having onedimensional tunnels, so the as-synthesized VO2 nanorods may provide feasible access for zinc ion guest to available sites. The Raman spectrum of the VO2 nanorods is illustrated in Figure 1b, five peaks located at 140, 282, 408, 691 and 991 cm-1 correspond to the fundamental mode of VO2 monoclinic crystalline. Two peaks at 140 and 187 cm-1 are strongly consistent with the layered structure. The two peaks at 282 and 408 cm-1 identifies with the V=O bending vibration bonds. The peak at 691 cm-1 is which is in line with the doubly coordinated oxygen (V–O-V) stretching mode results from corner-shared oxygens of two pyramids. And the high-shift peak located at 996.5 cm-1 is in accord with the terminal oxygen (V=O) stretching vibration.44,45 Thr field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to explore the morphology and microstructure of VO2 nanorods. The VO2 sample is composed by uniform nanorods, with a length of 1-2 μm and a diameter of 100-150 nm (Figure 1f and S1). Energy-dispersive X-ray spectra (EDS) mappings of the synthesized VO2 demonstrate that V and O elements are distributed homogeneously in the sample (Figure 1c-e). The TEM image (Figure 1g) clearly shows two different lattice fringes. The lattice spacings are 6.15 and 3.56 Å, which in line with the (001) and (110) lattice planes, respectively. Besides, the selected area electron diffraction (SAED) pattern (Figure 1h) displays two distinct diffraction rings, which are in accordance with the (110) and (003) planes of monoclinic VO2 nanorods.

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Figure 2. Electrochemical performances of the Zn/VO2 batteries cycling in the voltage range of 0.2-1.2 V: a) CV curves of VO2 electrode at 0.1 mV s-1, b) Galvanostatic charge/discharge profiles at 0.05 A g-1, c) Cycling performance at 0.05 A g-1, d) Rate performance at several current densities from 0.1 to 5.0 A g-1 and e) Long cycling performance at 3.0 A g-1. The slurry of the cathode material was prepared by compounding VO2 nanorods, acetylene black, and polytetrafluoroethylene with a weight ratio of 6:3:1. Glass fiber and zinc metal foil were used as the separator and anode, respectively, and 1 M ZnSO4 was used as the electrolyte. The cyclic voltammetry (CV) curves of VO2 electrode in the voltage range of 0.01-2.0 V show a obviously irreversible peak located at about 1.70 V (Figure S2), and the XRD patterns of VO2 cathode materials in different states (Figure S3) show the VO2 electrodes undergoes the irreversible phase transition after charged to 1.7V. Therefore, the electrochemical measurements were performed in the range of 0.2-1.2 V. In Figure 2a, the initial three cycles of the CV curves at 0.1 mV s-1 are almost overlapping, which indicate the Zn/VO2 battery is highly reversible. The three pairs of reduction/oxidation peaks located at 1.05/0.74, 0.70/0.51 and 0.63/0.42 V, which are in keeping with the initial three charge/discharge curves at 0.05 A g-1 (Figure 2b). It's worth noting that high discharge capacities of 325.6, 315.9 and 310.6 mAh g-1 are received at this low current density. The 8

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EIS tests were carried out on the full-discharged Zn/VO2 cell at different cycles to grasp more insights about the electrochemical behavior.[46] In Figure S4, the initial charge transfer resistance is 798 Ω, and it decreases to 320 Ω after the first cycle. After 10 cycles, the charge transfer resistance even reached a value about 230 Ω. The dissolution and depositionan of the Zn metal anode could produce passive film, which leaded to the decrease of charge transfer resistance.20,32 Meanwhile, the good rate capability of the Zn/VO2 battery is demonstrated in Figure 2d. The average discharge capacities at different current densities of 0.1, 0.2, 0.5, 1.0, 2.0, 3.0 and 5.0 A g-1 are 283, 261, 215, 170, 124, 97 and 72 mAh g-1, respectively. Impressively, when the current density was adjusted stepwise from 5.0 to 0.1 A g-1, the capacity can be nearly recovered under various current densities. These results demonstrate the high reversibility of the redox reactions and the good stability of the crystal structure. The corresponding charge and discharge curves at various current densities are shown in Figure S5. In particular, at a low current density of 0.05 A g-1, the specific capacity can be retained 85% after 100 cycles (Figure 2c). Figure S6 shows the cyclability and the charge/discharge curves of VO2 nanorods at 0.2 A g-1. And the cycling performance at 1.0 A g-1 is further presented in Figure S7. Extraordinarily, when the current density is even 3 A g-1, the capacity still maintains 86% of the maximum capacity after 5000 cycles, and the coulombic efficiency is almost 100% for all cycles (Figure 2e). Compared with the reported cathode materials

of

aqueous

ZIBs,11-28,30-40

the

VO2

electrode

demonstrates

excellent

electrochemical performance in terms of cycling stability (Table S1). Additionally, TEM images and SAED pattern show that the VO2 electrode still maintains nanorod morphology after 1000 cycles at 1.0 A g-1 (Figure S8).

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Figure 3. a) CV curves at various scan rates from 0.1 to 1.0 mV s-1, b) The surfacecontrolled and diffusion-controlled contributions to capacity, c) Galvanostatic intermittent titration technique (GITT) curves at a current density of 20 mA g-1 and d) Zn2+ diffusivity vs. the state of discharge. Cyclic voltammetry (CV) measurements at multiple scan rates were performed to further investigate the electrochemical kinetics of the aqueous Zn/VO2 battery. The CV curves of the VO2 cathode measured at various scan rates from 0.1 to 1.0 mV s-1 is showed in the Figure 3a . The curves maintain similar shapes in line with the good rate capability of VO2 cathode, and the characteristic peaks become broader gradually. Based on the measured current at various scan rates, the surface-induced capacitive and diffusion-controlled processes are quantitatively separated.47 The general relationship between the measured peak currents ( i ) and scan rates ( v ) can be described with the following equation: i  avb

(1)

Where a and b are adjustable parameters. b  0.5 indicates the totally diffusioncontrolled process, while b  1 reveals surface-controlled process.47-49 The b values of the 10

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four peaks were calculated to be 0.80, 0.64, 0.68 and 0.76 (Figure S9), indicating that the corresponding redox reactions are limited by surface-controlled kinetics.46,50 Furthermore, the capacity can be divided into surface-controlled ( k1v ) and diffusion-controlled ( k2 v1/2 ) parts by the following equation:48,49 i (V)  k1v  k2 v

1

2

(2)

Figure 3b displays the surface-controlled and diffusion-controlled contributions to capacity ranging from 0.1 to 1.0 mV s-1. With the incremental scan rate, the contribution ratio of surface-controlled capacity gradually increases from 72.0% to 85.5%, which demonstrating that surface-controlled contribution occupies the absolute proportion.[30-33, 40] Meanwhile, we synthesized VO2 with microsphere morphology. Figure S10 shows that the VO2 sample is composed by inhomogenous microspheres with a diameter of 0.5-2 μm. Figure S11 shows the CV measurements of the ZIBS with VO2 microspheres at multiple scan rates. The contribution of surface-controlled capacity of VO2(B) nanorods is more than that of VO2 microspheres due to the higher-surface area of nanorod crystallites. Additionally, the Zn2+ diffusion coefficient of the VO2 cathode was investigated via the galvanostatic intermittent titration technique (GITT, Figure S12 and Supplementary Note 2). 51,52

A specific capacity of 334.5 mAh g-1 was measured in the GITT test (Figure 3c), which

in line with the discharge product of ZnxVO2 (x = 0.54) calculated by the discharge curve. Moreover, Figure 3d shows that the Zn2+ diffusion coefficient decreases from about 10-5.6 to 10-7.5 cm2 s-1 when the Zn2+ content increases in the cathode. The average Zn2+ diffusivity of the VO2 nanorods during the whole discharge process is 10-6.5 cm2 s-1, indicating the fast Zn2+ transport in the VO2 cathode.

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Figure 4. XPS peaks of Zn 2p (a) and V 2p (b) of VO2 electrodes in the original, extraction (charged to 1.2 V), and insertion states (discharged to 0.2 V). TEM-EDS element mapping images (c), HRTEM and SAED pattern (d) of the insertion state electrode. TEM-EDS element mapping images (e), HRTEM and SAED pattern (f) of the extraction state electrode. The Zn-storage mechanism of the Zn/VO2 battery had been investigated through ex-situ XPS, ex-situ TEM and ex-situ SAED measurements. Figure S13 confirmed the cathodes consisted of V and O elements at the original and insertion states. The presence of zinc element at the extracion states precisely proved the intercalation of Zn2+ guest into VO2 host. In Figure 4a, the intensity of Zn 2p peak in the insertion state is larger than those in the original and extraction states, which obviously demonstrated the insertion and extraction of 12

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Chemistry of Materials

Zn2+ in VO2 nanorods. Furthermore, the V valence states of the as-prepared VO2 and ZnxVO2 after three cycles of the discharge-charge process were examined in Figure 4b. In the original powder, only V4+ ions exist. After the discharge process, V3+ can be detected, indicating that Zn2+ guest inserted into the VO2 host. In the extraction state, the amount ratio of V3+/V4+ is demonstrated by the peak areas, which decreases from 2.68 to 0.49 owing to the almost complete deintercalation of Zn2+ from the electrode material. Ex-situ TEM images and SAED patterns at discharge and change states of VO2 electrode after three cycles were collected in Figure 4c-f. When discharged to 0.2 V, the spacing of (110) lattice plane extends to 3.79 Å, which increases by 5.8 % relative to the original state (Figure 4d). The expansion of lattice plane confirms the insertion of Zn2+ and fits well with the TEMEDS element mapping images (Figure 4c). After the full charge process companied by extraction of Zn2+ from the VO2 materials, the spacing of (110) plane decreases to 3.58 Å (Figure 4f), which nearly returns to the original state. Accordingly, negligible zinc element is detected in extraction states of VO2 electrodes (Figure 4e).

Figure 5. In-situ XRD test: a) Two-dimensional in-situ XRD patterns collected in the first cycle at 0.1 A g-1, divided into three stages. D1 and C1 (green), D2 and C2 (blue), D3 and C3 13

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(orange), b) Evolution of lattice parameter during the discharge (left) and charge (right) process , c) Most significant changes of lattice parameter in each stage and d) DFT calculation model of zinc ion intercalated sites into VO2 crystal.

To provide further insight into the structural change of the VO2 cathode during the insertion and extraction of zinc ions, in-situ XRD was performed. Figure 5a and Figure S15 show the in-situ XRD patterns collected in the first cycle at 0.1 A g-1, which reveals the reaction mechanism of VO2 cathode for aqueous ZIBs to be a reversible single-phase reaction. Peaks of lattice planes (002), (-401), (310), (003), (-601), (020) and (113) are clearly identified (Figure S11). The continuous shift indicates Zn2+ insertion/extraction in VO2 host. According to the relationship of a, b, c, h, k, l and β in the monoclinic system, the lattice parameter (a, b, c and cell volume) can be calculated (Figure 5b and Table S2). After a full discharge process, the change ratios of a, b and c are 2.72%, 2.53% and 1.31%, respectively, and the unit cell volume expands by 6.69% compared with the original state (Figure S16). The minor structural change of VO2 electrode during guestresults in the excellent long cycle performance and high capacity retention of the Zn/VO2 battery. In addition, the change of lattice parameter suggests the reversible single-phase reaction involving at least three stages, namely, stage D1 and C1 (green), stage D2 and C2 (blue), stage D3 and C3 (orange). Based on the results presented above, the overall electrochemical reaction of the Zn/VO2 battery can be described as follows: Anode: Zn  Zn 2+ + 2e-

(3)

Cathode: xZn 2+ +VO 2 + 2xe-  Zn x VO 2

 0  x  0.54 

(4)

Figure 5b and 5c intuitively present the relatively significant changes in lattice parameter of three stages, D1 (0 ≤ x < 0.07), D2 (0.07 ≤ x < 0.29) and D3 (0.29 ≤ x ≤ 0.54) in the discharge process, C3 (0.54 > x ≥ 0.25), C2 (0.25 > x ≥ 0.09) and C1 (0.09 > x ≥ 0.04) in the charge process. After analyzing the changes ratios of the lattice parameter of each stage 14

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quantitatively (Figure 5c, S17 and Table S3), three features are presented as follows: (1) The overall discharge and charge processes are highly reversible. In the discharge process, the reaction proceeds via VO2-Zn0.07VO2-Zn0.29VO2-Zn0.54VO2, and correspondently, the evolution Zn0.54VO2-Zn0.25VO2-Zn0.09VO2-Zn0.04VO2 occurs during the charge process. In particular, the change of crystal parameter in D1 is exactly opposite to C1, just like D2 and C2, D3 and C3. (2) In each stage, the change ratios of a and c keep the same rule roughly. In detail, D2 expands by 2.3% in a and 1.6% in c, D3 expands by 0.1% in a and 0.5% in c. (3) The most noticeable changes of crystal structure appear in D2 and C2 stages. The volume expansion of D2 stage is 4.5%, and the volume contraction of C2 stage is 3.6%. Simultaneously, the expansion and shrinkage occur along a and c direction, while the b barely changes in both D2 and C2 stages. Therefore it can be deduced that the diffusion direction of Zn2+ in VO2 electrode is along the [010] crystal orientation, and the edgesharing VO6 octahedra expand along the a and c direction Accoring the changes of a, b, and c parameters during discharge and charge process, we used advanced computational techiques based on density functional theory (DFT, Supplementary Note 3) to investigate, at the atomic level, the key issues of zinc insertion sites and diffusion pathways in the VO2 crystal.53 The corresponding total energies, zinc sites and volume changes are listed in Table S4. The DFT calculation model of Zn2+ ion insertion into VO2 at different sites is showed in the Figure 5d. In the 2×2×2 k-points VO2 crtstal, the energy is -205.87 eV after the intercalation of four zinc ions. The small changes of the total energy and volume indicate the crystal structure is stable when 0.5 Zn2+ intercalate into the structure of VO2. For the first time, the lattice parameters of host materials can be dynamically analyzed by in-situ monitoring and corresponding quantitative calculation during the insertion/extraction of zinc ion guest in ZIB systems.

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4. CONCLUSIONS An ultrastable and high-performance aqueous Zn/VO2 battery is designed in this work. Satisfactorily, the VO2 nanorods deliver high specific capacity of 325.6 mAh g-1 at 0.05 A g1,

good rate performance and excellent capacity retention of 86% after 5000 cycles at 3.0 A

g-1, which is an outstanding long-term cycle performance among the reported cathode materials of the aqueous ZIBs. Furthermore, the zinc storage mechanism of the VO2 cathode materials has been proposed to be a highly reversible single-phase reaction. During the discharge and charge process, the VO2 cathode expands/shrinks periodically and features three stages with different lattice parameter. For the first time, the lattice parameters of host materials can be dynamically analyzed during the insertion/extraction of zinc ion guest. The Zn/VO2 battery with high cycling stability, low cost and high safety is believed to be a promising candidate for large-scale storage applications, and its reaction process will give a better understanding of the mechanism involved in ZIB systems.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. SEM image and XRD refinement of VO2, electrochemical performance of Zn/VO2 cells, CV curves and GITT analysis, capacity grading of the VO2 microsphere, in-situ XRD studies, change ratios of the lattice parameter, DFT low energy structures and atomic coordinates, tables of lattice parameter and comparison with the reported cathode materials of ZIBs.

AUTHOR INFORMATION 16

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Chemistry of Materials

Corresponding Authors * E-mail: [email protected] * E-mail: [email protected] * E-mail: [email protected] ORCID Liqiang Mai: 0000-0003-4259-7725 Qinyou An: 0000-0003-0605-4942 Mengyu Yan: 0000-0003-1028-0627 Author Contributions L. N. Chen and Y. S. Ruan contributed equally to this work. L. Q. Mai, L. N. Chen and Y. S. Ruan designed the experiments. L. N. Chen, M. Y. Yan and Y. S. Ruan performed the experiments. L. N. Chen, Y. S. Ruan, Q. Y. An, M. Y. Yan, G. Z. Zhang, Q. L. Wei, T. F. Xiong, P. He and W. Yang discussed the interpretation of results and co-wrote the paper. Y. L. Jiang calculated the lattice parameter. Thanks for the strong support in operando testing device from Mohamad Khajebashi. All authors have given approval of the final version of the manuscript. Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by the National Key Research and Development Program of 17

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China (2016YFA0202601, 2016YFA0202603), the Programme of Introducing Talents of Discipline to Universities (B17034), the National Natural Science Foundation of China (51602239, 51521001), the National Science Fund for Distinguished Young Scholars (51425204), and the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (WUT: 2018-KF-7, 2018-ZD-1). We thank Prof. Xudong Wang of the University of Wisconsin-Madison for strong support and stimulating discussions.

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