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

Sep 12, 2018 - Zinc-ion batteries are emerging as next-generation rechargeable batteries that can operate in aqueous electrolytes. We first find feasi...
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Cite This: Chem. Mater. 2018, 30, 6777−6787

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*,† †

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Department of Nano Technology and Advanced Materials Engineering, Sejong University, Gunja-dong, Gwangjin-gu, Seoul 05006, South Korea ‡ Belarusian State University, Nezalezhanastsi Av. 4, Minsk 220030, Belarus § Research Institute for Physical Chemical Problems, Belarusian State University, Minsk 220030, Belarus S Supporting Information *

ABSTRACT: Zinc-ion batteries are emerging as next-generation rechargeable batteries that can operate in aqueous electrolytes. We first examine the feasibility of open-structured VO2(B) as a Zn2+ intercalation host. A bond-valence sum energy map predicts that four Zn2+-ion sites (ZnC, ZnA1, ZnA2, and ZnC′) can exist in the structure. Using first-principles calculations, we 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 of Zn2+ ions in 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-principles 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 and 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 insertion of Na+ into the host structure for cathodes. Optimization of non-aqueous electrolytic compositions and overcoming the issue of sensitivity to moisture are still being pursued in sodium-ion battery systems. One strategy for addressing 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 © 2018 American Chemical Society

aqueous zinc-ion batteries (A-ZIBs) employing a metallic Zn electrode.4 The most important feature of these batteries is the fact that zinc is recyclable.5 These advantages have opened a path not only to creating 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 abundance. Among MnO2 compounds such as α-MnO2, βMnO2, γ-MnO2, δ-MnO2, and ε-MnO2, α-MnO2 with a [2 × 2] tunnel structure has been investigated as a potential cathode material for A-ZIBs.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 Received: June 25, 2018 Revised: September 11, 2018 Published: September 12, 2018 6777

DOI: 10.1021/acs.chemmater.8b02679 Chem. Mater. 2018, 30, 6777−6787

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Chemistry of Materials

were examined using scanning electron microscopy (SEM) (JXA8100, JEOL) and transmission electron microscopy (TEM) (JEM3010, 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) 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. 2.3. Electrochemical Properties. The electrodes consisting of the active material [VO2(B) or VO2(B)/rGO composite] were mixed with a conducting agent (Ketjen black) and polytetrafluoroethylene dissolved in N-methyl-2-pyrrolidone at a weight ratio of 8:1:1 to form a homogeneous slurry. The slurry was coated onto a stainless steel 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 versus Zn2+/Zn at a current density of 50 mA g−1 at 25 °C. 2.4. Postcycled 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) in Pohang, South Korea. The K-edge XANES data were obtained in total electron yield mode, and the sample current was recorded.18 2.5. Computational Details. 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) parametrization 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 a Ueff value of 4.2 eV as reported in the study of other V-based materials.28,29 All the calculations were performed with an energy cutoff of 500 eV until the remaining force in the system converged to 98.5% for both electrodes (Figure 5b). The delivered capacity was higher than the

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 panels a−c of Figure 2, it is anticipated that Zn2+ ions can be intercalated into four sites, 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. On the basis of 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 shown in Figure 3c, 0.125 mol of Zn2+ ions is intercalated into the ZnA2 site of VO2(B), Zn0.25VO2(B), at ∼0.78 V, and additional insertion of 0.375 6780

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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 facile electron conduction ability, which increased the electrical conductivity of VO2(B) to 2.4 × 10−4 S cm−1. On the basis of the electrochemical performance described above, 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 contribution of the capacitive behavior to the charging process in Zn 2+ intercalation and deintercalation was evaluated using 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 spectrum (screenshot of the PDEIS spectrometer program31,32) 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 ref 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 the Supporting Information (Experimental and Figure S5). The potential dependences of doublelayer 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 dependence of the reciprocal Warburg coefficient, Aw−1, on the electrode potential shows two distinct maxima in the forward and backward scans (Figure 6e). Aw−1 maxima typically indicate characteristic potentials of diffusion-controlled interfacial charge transfer; the two maxima indicate the occurrence of two processes or a two-stage process related to Zn 2+ 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 (volts per second). 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

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

reported values for other electrode materials such as α-MnO2 (230 mAh g−1),6 γ-MnO2 (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 bare VO2(B), and the retained capacity was approximately 131 mAh g−1 after 200 cycles (Figure 5a,b). 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 (Figure 5a,b). This excellent capacity may have been partially due to the presence of the electro-conducting rGO sheets, which enables the electrochemical reaction to be more reversible. 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 panels c and d of Figure 5. The composite electrode exhibited a discharge capacity of ∼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 bare VO2(B) and increased to a range of 10−8 to 10−11 S cm−1 for the VO2(B)/rGO electrode. The better diffusion is related to the high electrical conductivity of VO2(B)/rGO because the rGO sheets enable facile electron tansfer to lower the ohmic resistance during galvanostatic titration. Some earlier reports also showed a similar tendency when rGO sheets were 6781

DOI: 10.1021/acs.chemmater.8b02679 Chem. Mater. 2018, 30, 6777−6787

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

electrochemical reaction is an intercalation of Zn2+ into the VO2(B) framework and that the pseudocapacitance contribution is negligible. On the basis of the results for intercalation of Zn2+ into the VO2(B) framework presented above, 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 1 mol of Zn2+ ions could be inserted into the open framework of the 1 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 reaction within a continuous solid solution between two end members.19 Hence, ex situ XRD and XANES analyses were performed to understand the related reaction as a function of Zn2+ content in δ = ZnδVO2 (Figure 7b−d). The ex situ XRD patterns in Figure 7b reveal a gradual peak shift toward smaller 2θ angles 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 larger 2θ angles 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 and/or 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 results presented above, we find it is clear that the Zn2+ insertion/extraction reaction is dominant for VO2(B), namely, VO2(B) + 0.57Zn2+ + 1.14e−1 ↔ Zn0.57VO2(B). The capacity retention values for VO2(B) and the VO2(B)/ rGO composite were ∼40 and ∼80%, respectively, after 200 6782

DOI: 10.1021/acs.chemmater.8b02679 Chem. Mater. 2018, 30, 6777−6787

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Figure 6. (a) Potentiodynamic electrochemical impedance spectrum of zinc intercalation and deintercalation in the VO2(B)/rGO electrode (dE/dt = 0.61 mV s−1). The voltammogram on the transparent front plane of the parallelepiped and three-dimensional 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 the current observed in the cyclic voltammogram.

was pulverized into nanorod particles piece by piece or, more seriously, appeared on a platelike byproduct, 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, the electric contact could become more

cycles. The postcycled electrodes were examined using XRD and TEM to confirm the structural stability of VO2(B) that permitted long-term repetitive Zn2+ insertion and/or extraction (Figure 8a−c). The crystal structure of bare VO2(B) was not visible in the XRD pattern (Figure 8a). Surprisingly, the postcycled bare VO2(B) electrode was mainly composed of Zn4SO4(OH)6·0.5H2O (JCPDS Card No. 44-0674, marked by the check symbols). The original spherical nanorod assembly 6783

DOI: 10.1021/acs.chemmater.8b02679 Chem. Mater. 2018, 30, 6777−6787

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

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 the zinc metal anode before and after 200 cycles via SEM (Figure S7a,b). The surface state of Zn metal did not change even after the 200 cycles, indicating that Zn metal is stable against the acidic electrolyte for prolonged cycles. The series of electrochemical reactions for the VO2(B)/rGOs electrode are summarized in Figure 9.

isolated among active VO2(B) particles, thereby accelerating the 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 like 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

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 a plausible capacity retention for 200 cycles. Through combined firstprinciples calculations and experiments, it was verified that among four Zn2+-ion sites in VO2(B), named ZnC, ZnA1, ZnA2, and ZnC′, 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 6784

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Figure 8. (a) Ex situ XRD patterns of pristine VO2(B), 200-cycle VO2, and VO2/rGO composite electrodes in a Zn cell. TEM images of (b) bare VO2(B) and (c) the VO2(B)/rGO composite. Schemes of (d) the production of Zn4SO4(OH)6·0.5H2O and (e) electro-conduction change by particle pulverization and restacking on rGO.

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

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DOI: 10.1021/acs.chemmater.8b02679 Chem. Mater. 2018, 30, 6777−6787

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(4) Hou, Z.; Zhang, X.; Li, X.; Zhu, Y.; Liang, J.; Qian, Y. Surfactant widens the electrochemical window of an aqueous electrolyte for better rechargeable aqueous sodium/zinc battery. J. Mater. Chem. A 2017, 5, 730−738. (5) Freitas, M. B. J. G.; de Pietre, M. K. Electrochemical recycling of the zinc from spent Zn−MnO2 batteries. J. Power Sources 2004, 128, 343−349. (6) Alfaruqi, M. H.; Gim, J. H.; Kim, S. J.; Song, J. J.; Jo, J. G.; Kim, S. H.; Mathew, V.; Kim, J. K. Enhanced reversible divalent zinc storage in a structurally stable α-MnO2 nanorod electrode. J. Power Sources 2015, 288, 320−327. (7) Alfaruqi, M. H.; Mathew, V.; Gim, J. H.; Kim, S. J.; Song, J. J.; Baboo, J. P.; Choi, S. H.; Kim, J. K. Electrochemically Induced Structural Transformation in a γ-MnO2 Cathode of a High Capacity Zinc-Ion Battery System. Chem. Mater. 2015, 27, 3609−3620. (8) Alfaruqi, M. H.; Gim, J. H.; Kim, S. J.; Song, J. J.; Pham, D. T.; Jo, J. G.; Xiu, Z.; Mathew, V.; Kim, J. K. A layered δ-MnO2 nanoflake cathode with high zinc-storage capacities for eco-friendly battery applications. Electrochem. Commun. 2015, 60, 121−125. (9) Kumar, A.; Sanger, A.; Kumar, A.; Kumar, Y.; Chandra, R. An efficient α-MnO2 nanorods forests electrode for electrochemical capacitors with neutral aqueous electrolytes. Electrochim. Acta 2016, 220, 712−720. (10) Carvalho, B. B.; Pegoretti, V. C. B.; Celante, V. G.; Dixini, P. V. M.; Gastelois, P. L.; Macedo, W. A. A.; Freitas, M. B. J. G. Effect of temperature on the electrochemical synthesis of MnO2 recycled from spent Zn−MnO2 alkaline batteries and application of recycled MnO2 as electrochemical pseudocapacitors. Mater. Chem. Phys. 2017, 196, 126−136. (11) Hill, L. I.; Verbaere, A.; Guyomard, D. MnO2 (α-, β-, γ-) compounds prepared by hydrothermal-electrochemical synthesis: characterization, morphology, and lithium insertion behavior. J. Power Sources 2003, 119−121, 226−231. (12) Alfaruqi, M. H.; Gim, J. H.; Kim, S. J.; Song, J. J.; Pham, D. T.; Jo, J. G.; Xiu, Z.; Mathew, V.; Kim, J. K. A layered δ-MnO2 nanoflake cathode with high zinc-storage capacities for eco-friendly battery applications. Electrochem. Commun. 2015, 60, 121−125. (13) Han, S.-D.; Kim, S. J.; Li, D. G.; Petkov, V.; Yoo, H. D.; Phillips, P. J.; Wang, H.; Kim, J. J.; More, K. L.; Key, B.; Klie, R. F.; Cabana, J.; Stamenkovic, V. R.; Fister, T. T.; Markovic, N. M.; Burrell, A. K.; Tepavcevic, S.; Vaughey, J. T. Mechanism of Zn Insertion into Nanostructured δ-MnO2: A Nonaqueous Rechargeable Zn Metal Battery. Chem. Mater. 2017, 29, 4874−4884. (14) He, P.; Yan, M.; Zhang, G.; Sun, R.; Chen, L.; An, Q.; Mai, L. Layered VS2 Nanosheet-Based Aqueous Zn Ion Battery Cathode. Adv. Energy Mater. 2017, 7, 1601920−1601927. (15) Kundu, D.; Adams, B. D.; Duffort, V.; Vajargah, S. H.; Nazar, L. F. A high-capacity and long-life aqueous rechargeable zinc battery using a metal oxide intercalation cathode. Nat. Energy 2016, 1, 16119. (16) Kundu, D.; Hosseini Vajargah, S.; Wan, L.; Adams, B. D.; Prendergast, D.; Nazar, L. W. Aqueous vs. nonaqueous Zn-ion batteries: consequences of the desolvation penalty at the interface. Energy Environ. Sci. 2018, 11, 881−892. (17) Li, G.; Yang, Z.; Jiang, Y.; Zhang, W.; Huang, Y. Hybrid aqueous battery based on Na3V2(PO4)3/C cathode and zinc anode for potential large-scale energy storage. J. Power Sources 2016, 308, 52− 57. (18) Jo, J. H.; Sun, Y.-K.; Myung, S.-T. Hollandite-type Al-doped VO1.52(OH)0.77 as a zinc ion insertion host material. J. Mater. Chem. A 2017, 5, 8367−8375. (19) Alfaruqi, M. H.; Mathew, V.; Song, J. J.; Kim, S. J.; Islam, S.; Pham, D. T.; Jo, J. G.; Kim, S. H.; Baboo, J. P.; Xiu, Z.; Lee, K.-S.; Sun, Y.-K.; Kim, J. K. Electrochemical Zinc Intercalation in Lithium Vanadium Oxide: A High-Capacity Zinc-Ion Battery Cathode. Chem. Mater. 2017, 29, 1684−1694. (20) Kim, K.-T.; Yu, C.-Y.; Kim, S.-J.; Sun, Y.-K.; Myung, S.-T. Carbon-coated anatase titania as a high rate anode for lithium batteries. J. Power Sources 2015, 281, 362−369.

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 identifying appropriate electrolytes is required to further improve the cell performances of rechargeable Zn cells.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02679. Rietveld refinement results for XRD data for bare VO2(B), TEM image and SAED pattern of bare VO2(B) and the VO2(B)/rGO composite, color of bare VO2(B) and the VO2(B)/rGO composite, SEM images of rGO sheets and the VO2(B)/rGO composite, Raman spectra of bare VO2(B) and the VO2(B)/rGO composite, GITT results for VO2(B) and the VO2(B)/rGO composite, equivalent electrical circuit derived from PDEIS data shown in Figure 6a, XPS spectra of the postcycled VO2(B)/rGO composite, wide scan and scans of V 2p, Zn 2p, and O 1s, SEM images of zinc metal and its magnified images before cycling and after 200 cycles, a table of Rietveld refinement results of XRD data for bare VO2(B) and the VO2(B)/rGO composite, and detailed structural information about ZnC, ZnA1, ZnA2, and ZnC′ sites at VO2(B) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jongsoon Kim: 0000-0002-4122-4874 Seung-Taek Myung: 0000-0001-6888-5376 Author Contributions

J.-S.P. and J.H.J. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 (NRF-2017R1A2A2A05069634 and NRF-2015M3D1A1069713), and the 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 (NRF-2017K1A3A1A30084795).

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DOI: 10.1021/acs.chemmater.8b02679 Chem. Mater. 2018, 30, 6777−6787