Open-structured vanadium dioxide as an intercalation host for Zn ions

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Open-structured vanadium dioxide as an intercalation host for Zn ions: investigation by first-principles calculation and experiments Jae-Sang Park, Jae Hyeon Jo, Yauhen Aniskevich, Aliaksei Bakavets, Genady Ragoisha, Eugene Streltsov, Jongsoon Kim, and Seung-Taek Myung Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02679 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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Chemistry of Materials

Open-structured vanadium dioxide as an intercalation host for Zn ions: investigation by first-principles calculation and experiments Jae-Sang Parka, ‡, Jae Hyeon Joa, ‡, Yauhen Aniskevichb, Aliaksei Bakavetsb, Genady Ragoishac, Eugene Streltsovb, Jongsoon Kima,*, Seung-Taek Myunga,* a

Department of Nano Technology and Advanced Materials Engineering, Sejong University, Gunja-dong, Gwangjin-gu, Seoul, 05006, South Korea b Belarusian State University, Nezalezhanastsi Av.4, Minsk 220030, Belarus c Research Institute for Physical Chemical Problems, Belarusian State University, Minsk 220030, Belarus ‡These authors equally contributed to this work.

ABSTRACT: Zinc-ion batteries are emerging as next-generation rechargeable batteries that can operate in aqueous electrolytes. We first find feasibility of open-structured VO2(B) as a Zn2+ intercalation host. Bond valence sum energy map predicts that four Zn2+ ion sites (ZnC, ZnA1, ZnA2, ZnC’) can exist in the structure. Using first-principles calculations, it is verified that 0.5 mol of Zn2+ ions can be reversibly (de)intercalated with an average voltage of ~0.61 V (vs. Zn2+/Zn), which is comparable with the experimental results. The specific capacity of VO2(B) at 50 mA g−1 is maintained up to ~365 mAh g−1 corresponding to the storage capacity of ~0.57 mol Zn2+ ions into the framework of VO2(B), and its redox reaction occurs at ~0.61 V. The high capacity is maintained for 200 cycles, with capacity retention of 80% (288 mAh g−1). Moreover, the capacity delivered by the VO2(B) electrode is stable even with cycling at a rate of 5C (1750 mA g−1) at approximately 110 mAh g−1. This high-power capability of VO2 is supported by the theoretical approach based on first-principle calculation, which shows the activation barrier for Zn2+ diffusion in the VO2(B) structure. These findings demonstrate the potential of open-structured VO2(B) as a new candidate material.

1. Introduction State-of-the-art lithium-ion batteries (LIBs) meet the criteria for energy storage applications because of the lightness of lithium as well as the high energy density and high-power capability of the battery systems.1 Therefore, LIBs are currently used in many portable to large-scale energy storage applications. However, the recent rapid growth of large-scale LIB applications may result in the explosive escalation of the price of LIBs. 2-3 The use of sodium, the sixth most abundant element and an element with essentially unlimited resources everywhere, could be an answer to these problems. However, the large ionic size of Na+ relative to Li+ leads to simultaneous phase transition during Na+ insertion into the host structure for cathodes. Optimization of non-aqueous electrolytic compositions and overcoming the issue of sensitivity to moisture are still in progress in sodium-ion battery systems. One strategy to address these issues is to explore another alternative battery system based on zinc, which is an abundant and low-cost element that is compatible with water, exhibits low toxicity, and is easy to handle in air. Its stability in water stems from the high overpotential for hydrogen evolution and results in a moderate voltage window (~2 V vs. Zn2+/Zn) for aqueous zinc-ion batteries (A-ZIBs) employing a metallic Zn electrode.4 The most important feature of these batteries is that zinc is recyclable.5 These advantages have opened a path not

only to realizing safe and environmentally benign energy storage devices but also to reducing their cost. Earlier works focused on the investigation of mainly cathode materials exhibiting electrochemical intercalation of Zn2+ ions into MnO2 with hollandite,6 birnessite,7 and todorokite8 structures, which are attractive because of their low toxicity and earth abundance. Among MnO2 compounds such as α-MnO2, β-MnO2, γ-MnO2, δ-MnO2, and ε-MnO2, α-MnO2 with a [2 × 2] tunnel structure has been investigated as a potential cathode material for AZIBs.9-13 Many previous works have reported on the electrochemical performance related to intercalation of Zn2+. 6-8 Recent attention has been diverted toward the exploration of high-capacity electrode materials for A-ZIB systems. Vanadium-based materials are particularly attractive because of the range of oxidation states of VxOy from +2 to +5 (VO, V2O3, VO2, and V2O5) depending on the oxygen coordination and structure motifs such as V2O5, V3O7, and V4O10. He et al.14 reported that VS2 delivered a discharge capacity of 190 mAh g−1 at 0.5C (50 mA g−1) because of the insertion of Zn2+ and capacitive behavior. In addition, Kundu et al.15 reported the delivery of a high capacity of 282 mAh g−1 from Zn0.25V2O5∙nH2O and of 375 mAh g−1 from layered V3O7∙nH2O via complex insertion of Zn2+.16 In addition to these materials, new zinc materials such as NASICON-type Na3V2(PO4)3,17 hollandite-type VO1.52(OH)0.77,18 LiV3O8,19, and the Chevrel phase Mo6S8 have been reported as cathode materials that are

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Chemistry of Materials active because of Zn2+ insertion. One of the interesting features of these materials is that they allow facile insertion of Zn2+ ions into their crystal structure.

Zn2+ intercalation. The reaction is activated by V4+/3+ redox accompanying the insertion of Zn2+ ions into the open structure, which is stable for prolonged cycles. We propose VO2(B), which is capable of zinc storage, as a cathode material for ZIBs. 2. Experimental Material preparation VO2(B) was synthesized via a solvothermal reaction of V2O5 (Samchun) with ethanol in an autoclave at 150 °C for 12 h under autogenous pressure. After the reaction, the products were filtered and then washed with ethanol several times. The precipitated products were dried overnight under vacuum at 80 °C. The products were dispersed onto reduced graphene oxide (rGO) sheets (IDT International), for which the ratio of active material to rGO was 99:1 by weight, in ethanol by sonication for 1 h, and the resulting product was heated at 150 °C for 12 h in an autoclave. The obtained mixture was washed and dried under vacuum overnight at 80 °C to evaporate the solvent. Material characterization Powder X-ray diffraction (XRD; X’Pert, PANalytical) using Cu Kα radiation was used to analyze the crystalline phase of the produced powders in the 2θ range from 10° to 60° with a step size of 0.03°. The FULLPROF Rietveld program was used to analyze the powder XRD patterns.22 The prepared

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Herein, we propose a new electrode material for Zn2+ insertion: VO2(B) with an open structure. As VO2(B) is known to form at relatively low temperature via hydrothermal, solvothermal, and other soft chemistry processes, we adopted a simple solvothermal method to synthesize VO2(B) that produces a sphere-shaped nanorod assembly. According to reports in the literature, these open structures tend to allow Li+ insertion into the crystal structures of TiO2(B)20 and VO2(B).21 Because of the similarity of the ionic radii of Li+ (0.76 Å ) and Zn2+ (0.74 Å ), it is anticipated that Zn2+ ions could be inserted into the crystal structure. Notably, two-electron transfer from Zn0 to Zn2+ enables the delivery of a capacity twice that achieved in Li and Na cells. Advantages related to not only the capacity but also the cost-effectiveness, low toxicity, ease of handling in air for all processes including cell fabrication and use of an aqueous solution drive investigation into electrode materials for A-ZIBs. For the first time, we unveil the electrode performance and reaction mechanism behind the reversible electrochemical behavior in Zn cells using a bondvalence sum (BVS) energy map and first-principles calculations. We predicted four sites for Zn2+ ion intercalation into the VO2(B) structure, which are designated as ZnC, ZnA1, ZnA2, and ZnC sites. Through combined studies using first-principles calculations and experiments, we suggest that VO2(B) exhibits an average operation voltage of approximately 0.7 V (vs. Zn2+/Zn) and that the Zn2+ ions are sited in the ZnA2 site for

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Figure 1. Rietveld refinement result of XRD data for (a) VO2(B)/rGO composite and (b) crystal structure of VO2(B).

Figure 2. Bond-valence sum energy map of VO2(B) for (a) ab plane, (b) bc plane, and (c) ac plane showing all possible Zn ion sites in the VO2(B) structure.


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powders were examined using scanning electron microscopy (SEM; JXA-8100, JEOL) and transmission electron microscopy (TEM; JEM-3010, JEOL). The obtained VO2(B) and VO2(B)/rGO composite were further characterized using Raman spectroscopy (inVia, Renishaw). The DC electric conductivity was measured using the direct volt–ampere method (CMT-SR1000, AIT), in which disc samples were contacted with a four-point probe. X-ray photoelectron spectroscopy (XPS, PHI5600, PerkinElmer, USA) measurements were performed in macro mode (3 mm × 3 mm) to obtain information about the chemical states of the vanadium elements. The samples were first transferred into a hermetically sealed transfer chamber in a glovebox and then transferred into the vacuum chamber of the XPS machine, preventing exposure to air or water for the XPS measurements.

at a current density of 50 mA g−1 at 25 °C.

Electrochemical properties The electrodes consisting of the active material (VO2(B) or VO2(B)/rGO composite) were mixed with conducting agent (Ketjen black) and polytetrafluoroethylene dissolved in Nmethyl-2-pyrrolidone at a weight ratio of 8:1:1 to form a homogeneous slurry. The slurry was coated onto a stainlesssteel foil substrate, pressed, and dried at 80 °C overnight in a vacuum oven. The electrolyte was a 1 M ZnSO4 solution (pH = 4.2),23 and the Zn metal counter electrode and working electrode were separated by a glass filter in R2032-type coin cells, which were tested in the range of 0.3–1.1 V vs. Zn2+/Zn

Density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP).24 We used projector-augmented wave (PAW) pseudopotentials25 with a plane-wave basis set as implemented in VASP. Perdew–Burke–Ernzerhof (PBE) parameterization of the generalized gradient approximation (GGA)26 was used for the exchange-correlation functional. The GGA+U method27 was adopted to address the localization of the d-orbital in V ions, with the Ueff value of 4.2 eV as reported in the study on other V-based material.28-29 All the calculations were performed with an energy cutoff of 500 eV until the remaining

Post-cycled electrodes Ex situ XRD and X-ray absorption near-edge structure (XANES) analyses were used to monitor the structural evolution during the electrochemical tests. To prepare the ex situ XRD samples, the electrodes were recovered from the cycled coin cells, washed with deionized water, and heated in a vacuum oven overnight at 80 °C. The XANES measurements were performed at the 8C Nano XAFS beamline at Pohang Accelerator Laboratory (PAL), Pohang, South Korea. The K-edge XANES data were obtained in total electron yield mode, and the sample current was recorded.18 Computational details

Figure 3. (a) Formation energy of ZnxVO2(B) (0 ≤ x ≤ 0.5), (b) calculated average redox potential and experimentally measured charge/discharge curve of ZnxVO2(B), and (c) expected mechanism of Zn intercalation into VO2(B).


Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. (a) ZnA2–ZnA2 diffusion pathways and (b) calculated Zn diffusion activation barriers in VO2(B).

force in the system converged to less than 0.05 eV Å −1 per unit cell. The phase stability of ZnxVO2 (0 ≤ x ≤ 0.5) was investigated by generating all the configurations of Zn for specific Zn contents and calculating the formation energies. CASM software30 was used to generate all the Zn/vacancy configurations at each composition, followed by full DFT calculations on a maximum of 30 configurations with the lowest electrostatic energy at each composition used to obtain the convex hull of ZnxVO2 (0 ≤ x ≤ 0.5). Nudged elastic band (NEB) calculations22 were conducted to determine the activation barrier of Zn2+ ion diffusion in the VO2(B) structure. A unit cell consisting of four formula units of VO2(B) was used, and one Zn vacancy was generated to model the Zn-ion diffusion. We considered six intermediate states between the first and final images of a single Zn diffusion event. During the NEB calculation, all the structures were allowed to relax within the fixed lattice parameters. Potentiodynamic electrochemical impedance spectroscopy Potentiodynamic electrochemical impedance spectroscopy (PDEIS)31-34 was applied for investigation of the complex multi-frequency ac–dc response of the VO2(B)/rGO electrodes in zinc intercalation and deintercalation processes. Analysis of the impedance spectra variation with the potential in terms of equivalent circuits with variable parameters was performed using the PDEIS analysis software.35 The VO2(B)/rGO electrodes for the PDEIS measurements were prepared by contacting the end-plane face of 0.5-mm Pt wire with a suspension of VO2(B)/rGO, PVDF and acetylene black in NMP with subsequent drying under air at 80 °C for 12

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h. The thus obtained composition was used as the working electrode in a three-electrode cell with Zn counter and reference electrodes and a 1 M ZnSO4 aqueous electrolyte solution. The potential dependent frequency response was probed at 28 frequencies from 439 to 9 Hz with 2-mV steps of the potential staircase applied at 0.61 mV s−1 net scan rate. 3. Results and Discussion The crystal structures of the products were determined by Rietveld refinement by assuming a monoclinic structure with C2/m space group (Figure-S1a and Figure 1a). The observed pattern matched with the calculated one of VO2(B) (JCPDS No. 81-2392) without the presence of any impurities. Similarly, the reduced graphene oxide (rGO)-treated VO2(B) was identified as a single phase with the same monoclinic structure. The broadened XRD pattern is likely to associate with reduction of V4+O2(B) from V5+2O5, accompanied by oxygen evolution during the solvothermal reaction.Thus, it is observed in Figure 1a that overall XRD peak of VO2(B) are broad. According to Rietveld refinement of the XRD data, the addition of rGO did not affect the crystal structure. In addition, the results presented in Table S1 further confirm that no significant differences were observed between the lattice parameters of the bare VO2(B) and VO2(B)/rGO composite. Schematic illustrations of the crystal structure of monoclinic VO2(B) are presented in Figure 1b using the structural coordinates obtained from the Rietveld refinement. The framework of VO2(B) is described as stacks of (011)-oriented sheets of double layers of edge-sharing VO6 octahedra, which are connected through the corners of the facing octahedra. This structure generates tunnels that are 3.725 Å in width and 4.083 Å in height; these dimensions are sufficient to accommodate ionic moieties along the z-direction. The assynthesized VO2(B) and VO2(B)/rGO composite were examined using transmission electron microscopy (TEM) (Figures S1b and 1c). Microscopic results suggest that the VO2(B) powders were well dispersed onto the rGO sheets (Figure S2). The spherical particles, appearing a tone of black in the images (Figures S1d and e), were composed of numerous nanorod primary particles ranging from 200 to 300 nm in length (Figures S1b and c). The compositization process did not alter the particle morphology. Selected-area diffraction (SAD) patterns also confirmed that no change occurred in the crystal structure after the treatment for the adhesion onto rGO (Insets of Figure S1b and c), which agrees with the X-ray diffraction (XRD) data. The appearance of the G band provides evidence of the presence of electro-conducting rGO sheets with VO2(B) (Figure S3). The resulting electrical conductivity increased to 2.4 × 10−4 S cm−1 for the VO2(B)/rGO composite from 2 × 10−7 S cm−1 for VO2(B). The several observed bands are typical features of VO2(B), and the observed bands were identical for both the bare VO2(B) and VO2(B)/rGO composite. Using the BVS energy map method based on the structural information of VO2(B), we attempted to locate the possible Zn2+ ion diffusion pathways in the crystal lattice. Because the BVS energy map method is based on the calculation of the bonding interaction among atoms such as Zn, V, and O in the crystal structure, we expected to find the stable atomic positions and diffusion path for Zn2+ ions with the lowest free energy determined by attraction or repulsion of each atom in the crystal structure. As shown in Figures 2a–c, it is anticipated that Zn2+ ions can be intercalated into four sites,


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Figure 5. (a) Continuous cycles of bare VO2 and VO2/rGO composite and (b) resulting cyclability with Coulombic efficiency; (c) rate capabilities of bare VO2 and VO2/rGO composite and (d) resulting rate capability.

named ZnC, ZnA1, ZnA2, and ZnC’, which is consistent with the possible Li sites at VO2(B) and TiO2(B) (Table S2). 36-39 Assuming insertion of Zn2+ into the VO2(B) structure, the Zn2+ ions are three-dimensionally connected to each other (Table S2), implying the possibility of facile diffusion of Zn2+ ions in the VO2(B) structure. Based on the structural information concerning the possible Zn atomic sites at VO2(B) obtained from the BVS map, we verified the phase reaction and theoretical redox potential of VO2(B) (vs. Zn2+/Zn) using first-principles calculations. Figure 3a shows the formation energies and corresponding convex hull of ZnxVO2(B) with Zn contents of x (0 ≤ x ≤ 0.5). These results suggest two phenomena: i) ZnxVO2(B) undergoes two kinds of two-phase reactions with an intermediate phase at x ≈ 0.12 during (de)intercalation of Zn2+ in ZnxVO2(B) (0 ≤ x ≤ 0.5) and ii) the predicted redox potential of ZnxVO2(B) is ~0.61 V (vs. Zn2+/Zn). As observed in Figure 3b, these data coincide with our preliminary electrochemical results for VO2(B) tested in a Zn cell. These results demonstrate that the ZnA2 site is considered the most preferred Zn atomic site when Zn2+ ions are intercalated into the VO2(B) structure. As observed in Figure 3c, 0.125 mol of Zn2+ ions are intercalated into ZnA2 site of the VO2(B), Zn0.25VO2(B), at ~0.78 V, and additional insertion of 0.375 mol of Zn2+ ions follows to form

Zn0.5VO2(B) at ~0.55 V. The intrinsic Zn2+ ionic diffusion into the ZnA2 site of the VO2(B) structure was also investigated using first-principles calculations using the nudged elastic band (NEB) method. As shown in Figures 4a–b, when Zn2+ ion diffusion occurred between each ZnA2 site, the activation barrier was ~586 meV, of which the value is similar to activation barrier for other divalent ion insertion. 36-38 This finding indicates that facile Zn2+ ion migration into the crystal structure of VO2(B) is possible. Modifications such as nanosizing, nanostructuring, or compositization with electroconducting materials would be beneficial for increasing the rate of diffusion. Therefore, it is anticipated that the openstructured VO2(B) would exhibit good electrode performance. The detailed structural information of ZnC, ZnA1, ZnA2, and ZnC’ sites at VO2(B) were tabulated in Table S2. To confirm the expected results based on the first-principles calculations, the synthesized VO2(B) and VO2(B)/rGO composite were electrochemically tested in the range of 0.3– 1.1 V (vs. Zn2+/Zn) by applying a constant current density of 50 mA g−1 at 25 °C (Figure 5a). Both electrodes had similar voltage profiles for the first cycle with an average operating voltage of ~0.7 V (vs. Zn2+/Zn), which agrees with the calculated value, with discharge capacities of 340 mAh g−1 for VO2(B) and 365 mAh g−1 for the VO2(B)/rGO composite. The



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E (vs. Zn /Zn) / V Figure 6. (a) Potentiodynamic electrochemical impedance spectrum of zinc intercalation and deintercalation in VO2(B)/rGO electrode, dE/dt = 0.61 mV s-1. The voltammogram on the transparent front plane of the parallelepiped and 3D impedance spectrum result from the same potential scan and were extracted from the complex ac–dc response of the object using PDEIS software. (b) Equivalent electric circuit derived from PDEIS data using the PDEIS software. Potentiodynamic profiles of circuit parameters derived from PDEIS for VO2(B)/rGO electrodes: (c-1) double-layer capacitance, Cdl, and (c-2) pseudocapacitance, Cp; inverses of charge-transfer resistances, (d-1) Rd–1 and (d-2) Rr–1; (e) inverse Warburg constant, A−1; and (f) I(capacitive) compared with current observed in cyclic voltammogram.

capacities were reversible for charge: 338 mAh g−1 for VO2(B) and 360 mAh g−1 for VO2(B)/rGO, with first Coulombic efficiencies (CE) over 98.5% for both electrodes (Figure 5b). The delivered capacity was higher than the reported values for other electrode materials such as α-MnO2 (230 mAh g−1),6 γMnO2 (285 mAh g−1),7 δ-MnO2 (250 mAh g−1),8 Na3V2(PO4)3 (92 mAh g−1),17 and LiV3O8 (256 mAh g−1).19 An abrupt

capacity drop was evident during the initial 50 cycles for the bare VO2(B), and the retained capacity was approximately 131 mAh g−1 after 200 cycles (Figures 5a and 5b). In addition, the VO2(B)/rGO composite electrode maintained a higher discharge capacity of 288 mAh g−1 after 200 cycles, retaining 80% of the initial capacity (Figures 5a and 5b). This excellent capacity may have been partially due to the presence of the


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Figure 7. (a) Initial discharge–charge profile of VO2/rGO composite electrodes tested in Zn cell. The circles indicate the points at which the ex situ XRD measurements were performed. (b) Ex situ XRD patterns of VO2/rGO composite in Zn cells. (c) Lattice parameter variation during first cycles of VO2/rGO composite. (d) K-edge XANES spectra obtained during discharging and charging.

electro-conducting rGO sheets, which electrochemical reaction more reversible.

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The rate capability is shown for the VO2(B)/rGO composite tested from 0.5C (170 mA g−1) to 10C (3.4 A g−1) in Figures 5c and 5d. The composite electrode exhibited a discharge capacity of approximately 285 mAh g−1 at a rate of 0.5C and was still active even at 10C (3.38 A g−1), delivering a discharge capacity of 63 mAh g−1, which coincides with the results of the first-principles calculations that predicted a reasonable activation barrier energy of ~474 meV despite the divalent Zn2+ ion diffusion. In addition, the diffusion coefficients for Zn2+, measured during discharge, were calculated using the galvanostatic intermittent titration technique (GITT) for the bare and rGO composite material (Figure S4). The diffusion coefficients ranged from 10−9 to 10−12 S cm−1 for the bare VO2(B) and increased to a range of 10−8 to 10−11 S cm−1 for the rGO/VO2(B) electrode. The better diffusion is related to the high electrical conductivity of rGO/VO2(B) because the rGO sheets enable facile electron tansfer so as to lower the ohmic resistance during galvanostatic titration. Some earlier reports also show similar

tendency when rGO shees were used. 40-42 The aforementioned findings indicate that the compositization of VO2(B) using the rGO sheets clearly improved the electrode performance, presumably because of their ability for facile electron conduction, which increased the electrical conductivity of VO2(B) to 2.4 × 10−4 S cm−1. Based on the above electrochemical performance, the VO2(B)/rGO composite exhibited superior electrode performance in terms of both capacity retention and rate capability. To further investigate the electrochemical kinetics of the VO2(B)/rGO electrode, the capacitive behavior contribution to the charging process in Zn2+ intercalation and deintercalation was evaluated using potentiodynamic electrochemical impedance spectroscopy (PDEIS). The PDEIS technique measures the frequency response of the electrochemical system in a cyclic potential scan with simultaneous acquisition of the cyclic voltammogram (CV); hence, the charging current of the intrinsic capacitive elements derived from the potentiodynamic impedance spectra can be compared with the total current that passes through the electrochemical interface. Figure 6a presents the PDEIS


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Figure 8. (a) Ex situ XRD patterns of pristine VO2(B), 200th cycled VO2, and VO2/rGO composite electrodes in Zn cell. TEM image of (b) bare VO2(B) and (c) VO2(B)/rGO composite. Scheme of (d) Zn4SO4(OH)6∙0.5H2O produced process and (e) electro-conduction change by particle pulverization and restacking on rGO.

spectrum (screenshot of the PDEIS spectrometer program 31-32) of the VO2(B)/rGO electrode in a cyclic scan in the potential range of 0.3–1.1 V (vs. Zn2+/Zn). The potentiodynamic profile of the impedance spectra was further analyzed in terms of equivalent electric circuits using the method and software routines described in 35. Figure 6b presents the equivalent electric circuit of the VO2(B)/rGO electrode shown in Figure 6a. The details of the equivalent circuit are explained in supporting information (Experimental and Figure S5). The potential dependences of the double-layer capacitance Cdl (Figure 6c-1) and pseudocapacitance Cp (Figure 6c-2) show two distinct features, which correspond to the two-stage process observed in the CV in Figure 6a. The concerted variations of both capacitances may be explained by the surface reaction effect on the double-layer structure, in accordance with the assumption of the surface origin of the charge-transfer resistance (Rr); namely, the Rr–Cp branch of the equivalent circuit (Figure 6b). It should be noted that frequency response analysis in impedance spectroscopy gives true electric capacitances, such that both Cdl and Cp, despite their different origins, comply with the electric capacitance. Figure 6d presents the potentiodynamic profiles of the reciprocal charge-transfer resistances, Rd−1 (Figure 6d-1) and Rr−1 (Figure 6d-2), of the diffusion-controlled and surface limited reactions, respectively. Rd−1 dominates over Rr−1 in the lower potential region, whereas the difference between these variables becomes less significant above 0.8 V. The reciprocal Warburg coefficient, Aw−1, dependence on the electrode potential shows two distinct maxima in the forward and backward scans (Figure 6e). Maxima in Aw−1 typically indicate

characteristic potentials of diffusion-controlled interfacial charge transfer; the two maxima either indicate the occurrence of two processes or a two-stage process related to Zn2+ intercalation/deintercalation. The contribution of the capacitive branches of the equivalent circuit to the current observed in the CV is limited by the capacity of the capacitors to acquire electric charge, which is quantitatively determined by their capacitances: I (capacitive) = (Cdl+Cp) v, where v is the scan rate, V s−1. A comparison of the capacitive limiting current, I(capacitive), with the current of the VO2(B)/rGO electrode observed in the CV is presented in Figure 6f. The capacitive contribution appears to be insignificant, negligible for the left part of the potential cycle up to 0.6 V and very small above 0.6 V. This finding indicates that the related electrochemical reaction is a Zn2+ intercalation process into the VO2(B) framework and that the pseudocapacitance contribution is negligible. Based on the above results on Zn2+ intercalation into the VO2(B) framework, we further investigated the structural changes during the electrochemical reaction resulting from intercalation of Zn2+ ions. As observed in Figure 7a, the VO2(B)/rGO composite had a discharge capacity of approximately 365 mAh g−1, which can be expressed as Zn0.57VO2 assuming a theoretical capacity of VO2(B) of 645 mAh g−1. This theoretical capacity was calculated by assuming that one mol of Zn2+ ions could be inserted into the open framework of the one mol of VO2(B) structure. VO2(B) exhibited a smooth S-shaped discharge curve upon discharge (Figure 7a). Note that the reaction is a typical insertion


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Figure 9. Schematic illustration of repetitive electrochemical reaction for VO2(B)/rGO composite.

reaction within a continuous solid solution between two end members.19 Hence, ex situ XRD and X-ray absorption nearedge structure (XANES) analyses were performed to understand the related reaction as a function of Zn2+ content in δ = ZnδVO2 (Figures 7b–d). The ex situ XRD patterns in Figure 7b reveal a gradual peak shift toward lower 2θ angle upon discharge (reduction) as the electrochemical reaction progressed. Such a progressive shift is generally associated with the insertion of ionic species into a host structure.18 The shift (specifically of the main peaks at 2θ = 25.2° and 45.1°) was attributed to the insertion of Zn2+ into the tunnels of the VO2(B) framework. Upon charging (oxidation), the VO2(B) peaks progressively shifted again toward higher 2θ angle with the extraction of Zn2+ from the host structure. Notably, the expansion and contraction of the crystal structure occurred reversibly, with a reversible linear increase and decrease in the lattice parameters as a function of Zn2+ content in δ = ZnδVO2 (Figure 7c). The formation of a solid solution is indicative of a single-phase reaction accompanied by insertion/extraction of ionic species. This variation in the lattice parameters is related to the change in the oxidation state of vanadium. As observed in the XANES spectra (Figure 7d), the tetravalent vanadium was evidently reduced to +3 upon discharge and oxidized to +4 upon charge. In addition, the variation of the spectra in the pre-edge region also supports the occurrence of the V4+/3+ redox reaction of VO2(B). Considering the ionic radii of V3+ (0.64 Å ) and V4+ (0.58 Å ), the lattice expansion with the insertion of Zn2+ ions into the VO2(B) structure during discharge was expected because the oxidation state of vanadium approaches +3. Upon charge, the oxidation state of vanadium increased to +4, accompanied by contraction of the crystal structure and extraction of Zn2+ from the host structure. Summarizing the above results, it is clear that the Zn2+

insertion/extraction reaction is dominant for VO2(B), namely, VO2(B) + 0.57 Zn2+ + 1.14 e−1 ↔ Zn0.57VO2(B). The capacity retention for VO2(B) and the VO2(B)/rGO composite were approximately 40% and 80%, respectively, after 200 cycles. The post-cycled electrodes were examined using XRD and TEM to confirm the structural stability of VO2(B) that permitted long-term repetitive Zn2+ insertion/extraction (Figures 8a–c). The crystal structure of the bare VO2(B) was not visible in the XRD pattern (Figure 8a). Surprisingly, the post-cycled bare VO2(B) electrode was mainly composed of Zn4SO4(OH)6∙0.5H2O (PDF #44-0674, marked by the check symbols). The original spherical nanorod assembly was pulverized into nanorod particles piece by piece or, more seriously, appeared on a plate-like by-product, presumably Zn4SO4(OH)6∙0.5H2O (Figure 8b). Even though the active material was pulverized into nanorod particles during prolonged cycles, the particles could be active if not isolated by the Zn4SO4(OH)6∙0.5H2O plates, which are electric insulators. This finding indicates that as more Zn4SO4(OH)6∙0.5H2O is produced, more isolation of the electric contact could be generated amongst active VO2(B) particles, thereby accelerating capacity fade during cycling. A schematic illustration of the production of Zn4SO4(OH)6∙0.5H2O is presented in Figure 8d. In contrast, the original VO2(B) structure was preserved for the VO2(B)/rGO composite even after 200 cycles (Figure 8a). The TEM results also confirm the presence of spherical VO2(B) particles as the majority component (Figure 8c); some particles were deformed into nanorods but were still attached on the rGO sheets. The main difference between the cycled electrodes was the location of the pulverized nanorods, namely, on the sheet of inactive Zn4SO4(OH)6∙0.5H2O or on the electro-conducting rGO sheet. It is possible that the



continuous volume changes during repetitive charge and discharge could cause cracking and eventually exfoliation (self-pulverization) of the active materials. Those particles may be stacked onto the graphene sheets. Then, the restacked particles can again become active via the electro-conducting rGO sheets, which act similarly to current collectors for the restacked nanorod particles, as shown in Figure 8e. This phenomenon may enable high capacity retention for long-term cycling of the VO2(B)/rGO composite by repetitive insertion of Zn2+ into the VO2(B) structure. For this reason, the VO2(B)/rGOs composite electrode could exhibit excellent cycling performances for 200 cycles with high capacity, retaining 80% of the initial capacity (288 mAh g−1). We additionally observed the surface of zinc metal anode before and after 200 cycles through SEM (Figure S7a and b). The surface state of Zn metal did not alter even after the 200 cycles, indicating that Zn metal is stable against acidic electrolyte for prolonged cycles. The series of electrochemical reactions for VO2(B)/rGOs electrode are summarized in Figure 9. 4. Conclusion We first demonstrate the electrochemical activity of openstructured monoclinic VO2(B) in Zn cells. By forming a composite with rGO, VO2(B) was able to deliver a large capacity of approximately 365 mAh g−1 with plausible capacity retention for 200 cycles. Through combined firstprinciples calculations and experiments, it was verified that among four Zn2+ ion sites in the VO2(B), named ZnC, ZnA1, ZnA2, and ZnC sites, the preferred Zn ionic site was the ZnA2 site and reversible Zn2+ (de)intercalation from the VO2(B) structure occurred at an average operation voltage of ~0.7 V (vs. Zn2+/Zn). These results were experimentally confirmed by ex situ XRD and XANES analyses, which provided evidence of the insertion of Zn2+ followed by the V4+/3+ redox reaction, VO2(B) + 0.57Zn2+ + 1.14e−1 ↔ Zn0.57VO2(B). Additional work aimed at finding appropriate electrolytes is required to further improve the cell performances of rechargeable Zn cells.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Rietveld refinement result of XRD data for bare VO2(B). TEM image and SAED pattern of bare VO2(B) and VO2(B)/rGO composite. Color of bare VO2(B) and VO2(B)/rGO composite. SEM images of rGO sheets and VO2(B)/rGO composite, Raman spectra of bare VO2(B) and VO2(B)/rGO composite, GITT results for VO2(B) and rGO/VO2(B), Equivalent electrical circuit derived from PDEIS data shown in Figure 6a, XPS spectra of post-cycled VO2(B)/rGO composite: wide scan and scans of V2p, Zn2p, and O1s, SEM images of zinc metal and its magnified images of (a) before cycle and (b) after 200 cycles., Table of rietveld refinement results of XRD data for bare VO2(B) and VO2(B)/rGO composite., The detailed structural information of ZnC, ZnA1, ZnA2, and ZnC’ sites at VO2(B).

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank S.-J. Song of the National Center for Interuniversity Research Facilities for assistance with the TEM experiments. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science, and Technology of Korea (NRF2017R1A2A2A05069634) and (NRF-2015M3D1A1069713) and State Committee on Science and Technology of the Republic of Belarus. Also, this research was supported by the International Research & Development Program of the National Research Foundation of Korea(NRF) funded by the Ministry of Science and ICT of Korea (NRF2017K1A3A1A30084795).

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Table of Contents (TOC)

“ Intercalation reaction ” VO2(B) + 0.57 Zn2+ + 1 e-1 e- e-

↔ Zn 0.57VO2

Zn2+

e- e e- e

ZnA1 ZnA2

ZnA2 ZnA2

Znc Znc’ VO6

ZnA2 ZnA2

ZnA2

12 ACS Paragon Plus Environment

Open-structured vanadium dioxide as an intercalation host for Zn ions

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