Structural Characterization of 2D Zirconomolybdate by Atomic Scale

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Article Cite This: Inorg. Chem. 2017, 56, 14306-14314

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Structural Characterization of 2D Zirconomolybdate by Atomic Scale HAADF-STEM and XANES and Its Highly Stable Electrochemical Properties as a Li Battery Cathode Qianqian Zhu,† Zhenxin Zhang,*,†,‡ Masahiro Sadakane,§ Futoshi Matsumoto,† Norihito Hiyoshi,∥ Akira Yamamoto,⊥,# Hisao Yoshida,⊥,# Akihiro Yoshida,† Michikazu Hara,‡ and Wataru Ueda*,† †

Faculty of Engineering, Kanagawa University, Rokkakubashi, Kanagawa-ku, Yokohama-shi, Kanagawa, 221-8686, Japan Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta-cho 4259, Midori-ku, Yokohama-city, Kanagawa, 226-8503, Japan § Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan ∥ Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino, Sendai 983-8551, Japan ⊥ Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan # Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Kyotodaigaku Katsura, Nishikyo, Kyoto 615-8520, Japan

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S Supporting Information *

ABSTRACT: The structural determination of nanomaterials and their application in energy storage and transfer are of great importance. Herein, a layered zirconomolybdate with a twodimensional structure was synthesized. Atomic resolution electron microscopy was utilized for direct visualization of the structure that was further confirmed by powder X-ray diffraction and X-ray absorption near-edge structure analyses. The structure of the molecular sheet was stable at a high temperature in an oxidative atmosphere. The electrochemical performance of the material was evaluated with a Li battery composed of the calcined material as a cathode. Li ions were reversibly inserted and extracted between the layers without collapse of the structure of the material. The electrochemical properties of the material were derived from the reversible redox activity of the Mo ions and Zr ions in the material as well as the flexibility of the molecular layer of the material.

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INTRODUCTION Transition metal oxides possess outstanding properties, including structure tunability, composition diversity, acidity, and multielectron transfer property, and are receiving increasing attention. Owing to their multielectron transfer property, transition metal oxides exhibit interesting redox ability and can be used as cathode-active materials for chemical batteries, such as Li batteries (LBs), and for current energy transfer and storage.1−5 However, as cathode-active materials, transition metal oxides have a disadvantage that the material structures easily decompose during electrochemical processes, leading to a decrease in battery capacity.6−10 The anisotropic assembly of metal oxygen units in a certain plane with a diameter on a molecular level forms a molecular sheet based on a metal oxide.11−14 These crystalline metal oxides possess 2-dimensional (2D) anisotropy, and thus, they © 2017 American Chemical Society

are regarded as unique fully inorganic 2D macromolecules. The unique 2D crystal structure facilitates easy tuning of the molecular nanosheet in the crystal. The single sheet can be fabricated by different chemical and physical methods, such as ion-exchange or ultrasonication, leading to alteration of the distance between each sheet and even to isolation of single nanosheets from the crystal. A variety of metal oxide molecular sheets, such as Ti oxides,15−17 Mn oxides,18,19 Nb/Ta oxides,20,21 W oxides,22 and Ru oxides,23 have been synthesized, investigated, and applied to many fields, including semiconductivity, ferroelectricity, superconductivity, photochromism, and electrochromism. Furthermore, the electrochemical properties of molecular sheets and their potential to solve Received: September 22, 2017 Published: November 3, 2017 14306

DOI: 10.1021/acs.inorgchem.7b02420 Inorg. Chem. 2017, 56, 14306−14314

Article

Inorganic Chemistry

Temperature-programmed desorption mass spectrometry (TPDMS) measurements were carried out from 40 to 620 °C at a heating rate of 10 °C/min in He flow (flow rate: 50 mL/min). Samples were set up between two layers of quartz wool. A TPD apparatus (BEL, Japan) equipped with a quadrupole mass spectrometer (M-100QA. Anelva) was used to detect NH3 (m/z = 16) and H2O (m/z = 18). Thermogravimetric-differential thermal analysis (TG-DTA) was carried out up to 600 °C at a heating rate of 10 °C/min in N2 flow (flow rate: 50 mL/min) on a Thermo plus TG-8120 (Rigaku, Japan). Field emission scanning electron microscopy (FE-SEM) images and energy-dispersive X-ray spectroscopy (EDX) were obtained using an SU-8010 system (Hitachi, Japan). Atomic force microscopy (AFM) images were obtained on an Agilent 5500 scanning probe microscope (Agilent Technologies, USA) in air by a silicon cantilever 10 nm in radius. HAADF-STEM images were obtained with an ARM-200F electron microscope (JEOL, Japan) operated at 200 kV with a CEOS probe aberration corrector. The probe convergence semi-angle was 14 mrad, and the collection angle of the HAADF detector was 54−175 mrad. Obtained images were treated for noise removal with the Local 2D Wiener Filter in the HREM-Filters Pro software (HREM Research Inc., Japan). STEM samples were prepared through deposition on holey carbon-coated copper grids by attaching powder directly on the grids and blowing away excess powder. Metal compositions were determined using an inductively coupled plasma atomic emission spectroscopy (ICP-AES) method, and the CHN elemental compositions were determined in the Material Analysis Suzukake-dai Center, Technical Department, Tokyo Institute of Technology. Computer-Based Simulations. The structure modeling, theoretical calculations, and Rietveld refinement were carried out on the Materials Studio software package (Accelrys). The models of MoZrO and MoZrAC400 were optimized by density functional theory (DFT) using the DMol3 program.25,26 The Perdew−Burke−Ernzerhof (PBE) generalized gradient function and a DND basis set were applied for each calculation. Rietveld refinement was carried out on the Reflex program in the same software package. Electrochemical Measurements. A cathode composed of ca. 10 wt % MoZrAC400, 70 wt % acetylene black (AB, DENKA BLACK, DENKI KAGAKU KOGYO, Japan), and 20 wt % binder of polyvinylidene fluoride (PVdF, KF9130, Kureha, Japan) was prepared. Briefly, MoZrAC400 was homogeneously mixed with acetylene black for 15 min in a mortar. Then, a solution of PVdF (13 wt %) in Nmethyl-2-pyrrolidone was added to the mixture. The slurry was further mixed in a mortar and coated on aluminum foil with a thickness of ca. 0.1 mm. The coated aluminum foil was dried under high vacuum at 100 °C for ca. 1 h. The coated aluminum foil was cut into cycle plates (Φ 16 mm), which were further dried in a vacuum oven at 80 °C overnight. The mass loading of the active material was 0.12 mg/cm2. The electrodes were transferred to a glovebox for the assembly of LBs. The assembly of the coin cell (CR2032) used for testing the LBs performance is shown in Figure S 1. Li metal was used as an anode. The anode was isolated from the cathode by a porous polypropylene film separator (Celgard3401). The cathode and anode were set in a solution containing 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:2 by volume, Ube Chemicals, Japan) in an inert atmosphere glovebox. The battery was denoted MoZrAC400-LBs The electrochemical properties of MoZrAC400 were tested by cyclic voltammetry (CV) and charging/discharging cycle tests (Bio Logic, VMP3). CV measurements were conducted under the following conditions: scan rate: 1 mV/s; Einitial = open circuit potential; and cycle voltage range: 4.0−1.0, 4.0−1.5, and 4.0−2.0 V. The charging/ discharging cycle experiments were conducted over the voltage range of 4.0−1.5 V or 4.0−2.0 V versus Li/Li+ at different constant current densities. Herein, the relationship between capacity and transferred electrons was described using the following equation

energy transfer and storage issues in LBs are areas of great interest. The determination of a material’s structure is one of the most important steps in material science because many properties and applications are highly related to structure. Structure determination using X-ray diffraction technology for layered materials is difficult, because the crystal habit of the layered material makes the size of a diameter too small. In recent research, an interesting layered molybdenum-zirconium oxide was reported.24 However, the detailed structure was not confirmed, and the electrochemical properties of the material are yet to be understood. Herein, the detailed structure characterization of layered molybdenum-zirconium oxide, denoted as MoZrO, was conducted using atomic-resolution high-angle annular darkfield scanning transmission electron microscopy (HAADFSTEM), X-ray absorption near-edge structure (XANES), powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), elemental analysis, and theoretical calculations, which demonstrated that the material was composed of [H0.1Zr1.05Mo1.95O8]n nanosheets assembled with H2O and NH3. The guest molecules were removed by calcination, shrinking the space between the layers and causing parallel movement of the single layers. The calcined material was used as a cathode-active material for LBs, and stable cycling performance was obtained. Li ions reversibly migrated between the layers during the electrochemical process, and Mo and Zr ions were spontaneously reduced or oxidized, respectively, without changing the layered structure of the material.

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EXPERIMENTAL SECTION

Synthesis of MoZrO. (NH4)6Mo7O21·4H2O (AHM) (1.471 g, 8.33 mmol based on Mo) was dissolved in 20 mL of water, and ZrOCl2·8H2O (0.269 g, 0.833 mmol) was dissolved in 20 mL of water. The ZrOCl2·8H2O solution was added to the solution of AHM. The resulting solution was stirred for 10 min, followed by transfer of the mixture to a 50 mL Teflon-lined stainless-steel autoclave that was set in an oven and heated to 175 °C for 1 week. After the expected reaction time and the autoclave cooled down, the mixture was recovered by filtration. The resulting solids were washed with water three times and dried at 50 °C overnight. Then, 0.338 g of the material was obtained (yield of 16% based on Mo). Elemental Analysis: Calcd for N0.6Mo1.95Zr1.05O12H9.9: N, 1.70; Mo, 37.94; Zr, 19.42; H, 2.01, Found: N, 1.60; Mo, 37.54; Zr, 19.59; H, 1.28. MoZrO was calcined in air at different temperatures (100, 200, 300, 400, 500, and 600 °C) for 2 h with an increasing temperature ramp rate of 10 °C/min. The calcined materials are denoted MoZrAC100, MoZrAC200, MoZrAC300, MoZrAC400, MoZrAC500, and MoZrAC600. Elemental Analysis (MoZrAC400): Calcd for N0.1Mo1.95Zr1.05O9.5H3.4: N, 0.32; Mo, 42.56; Zr, 21.78; H, 0.77, Found: N, 0.37; Mo, 42.66; Zr, 21.93; H, 0.07. Characterization. XRD patterns were obtained on an RINT2200 (Rigaku, Japan) with Cu Kα radiation (tube voltage: 40 kV, tube current: 40 mA). FTIR spectroscopy was carried out on a JASCO-FT/ IR-6100 (JASCO, Japan). XPS was performed on a JPS-9010MC (JEOL, Japan). Spectrometer energies were calibrated using the C1s peak at 284.7 eV. For XPS measurements of the used cathodes, the cells were disassembled in an Ar atomsphere and the cathodes were transferred to XPS without exposure to air. Diffuse reflectance ultraviolet−visible (DR-UV−vis) spectra were obtained using a JASCO V-550 UV−vis-spectrophotometer equipped with an ISN470 reflectance spectroscopy accessory (JASCO, Japan). Mo-K and ZrK edge X-ray Absorption Fine Structure (XAFS) measurements were carried out in transmission mode using a Si(311) double-crystal monochromator at the BL01B1 beamline of SPring-8, Japan.

Capacity (mAh/g) = (neF )/(3.6M ) (mAh/g) 14307

DOI: 10.1021/acs.inorgchem.7b02420 Inorg. Chem. 2017, 56, 14306−14314

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Inorganic Chemistry where F is the Faraday constant (96 485 C/mol), ne is the number of transferred electrons, and M is the molecular weight of the material based on its chemical formula.

038-1466) (Figure 1g). An IR band shift was observed (Figure 1B), indicating that the bonding state of the material changed slightly after calcination. Both of the peaks for H2O (1620 cm−1) and NH3 (1400 cm−1) decreased remarkably after calcination in air, demonstrating that the calcination removed H2O and NH3 from the material. The FTIR peak at 985 cm−1 shifted to a higher wavenumber, indicating that the desorption of H2O and NH3 from the material caused the shortening of the MoO terminal bond. The FTIR spectrum of MoZrO is shown in Figure 1. The IR bands at 1620 and 1400 cm−1 were ascribed to H2O and NH3,28 respectively. The peaks ca. 985 and 800−600 cm−1 were derived from the terminal MoO bond and the bridge Mo−O−Mo(Zr) bond, respectively. MoZrO and MoZrAC400 were observed by SEM, showing that the materials had a platelike morphology (Figure S 6a,b), and thin nanoplates were observed in the image with a plate diameter of ca. 1 μm. After calcination at 400 °C, the morphology and size of MoZrAC400 did not change significantly. AFM was carried out for MoZrAC400. The thickness of the nanoplate was analyzed by a line-profile analysis, exhibiting that the thickness was ca. 30 nm (Figure S 6c). Structural Characterization. The powder XRD pattern of MoZrAC400 was indexed with a monoclinic cell (a = 16.3810 Å, b = 4.9706 Å, c = 7.1208 Å, β = 107.2293°) and a space group of C2/C (Table S 1). The initial structure was solved using the charge flipping algorithm, showing a distribution of heavy metal ions in the structure (Table S 2). The metal ion distribution was directly observed by HAADF-STEM, which confirmed the initial structure from the charge-flipping algorithm. HAADF-STEM images (Figure 2) demonstrated that the calcined material was a layered-type material. The cross-sectional image of the layers in the b−c plane exhibited the arrangement of Mo and Zr in the molecular layer with a layer distance of 7.84 Å, and a lattice parameter of b = 4.97 Å. On the basis of the HAADF-STEM observation, the atomic distribution of the heavy metal ions proposed a molecular sheet composed of three sublayers of transition metal ions with two types of sites (Figure 2a), two side sites and one central site. The side sites were ascribed to the Mo ions, and the central sites were ascribed to the Zr ions. The structure obtained by the charge-flipping method also fitted the other views of MoZrAC400 in corresponding planes in HAADF-STEM (Figure 2b,c). The detailed coordination state of oxygen for MoZrAC400 was confirmed by XANES (Figure 3).29−31 To obtain the detailed coordination of Mo and Zr, the Mo-K edge and the ZrK edge XANES measurements were carried out with the reference samples including Mo foil, Na2MoO4·2H2O, MoO3, AHM, and Ba2NiMoO6, for Mo and ZrSiO4, t-ZrO2, m-ZrO2, Zr(SO4)2·4H2O, and SrZrO3 for Zr. The adsorption edge position of MoZrAC400 was similar to the MoVI- and ZrIVbased reference samples, indicating that the valences of Mo and Zr in the material were 6+ and 4+, respectively. This result was also supported by our XPS results. The shape of the pre-edge depends on the coordination state. For the Mo-K edge, Ba2NiMoO6 was composed of only the regular Mo-O octahedra; AHM and MoO3 were composed of distorted MoO6 octahedra; and Na2MoO4·2H2O was composed only of regular MoO4 tetrahedra. The pre-edge peak of MoZrAC400 was similar to those of AHM and MoO3, indicating that the material was probably composed of distorted MoO6 octahedra (Figure 3A). For Zr, the XANES spectrum of

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RESULTS AND DISCUSSION Synthesis and Characterization of MoZrO and MoZrAC400. MoZrO was prepared using a hydrothermal method with the precursor solution containing AHM and ZrOCl2 at 175 °C for 1 week, and a white powdery solid was obtained. The material was synthesized with different amounts of Mo and Zr precursors. The XRD patterns showed that the mole ratio of Mo:Zr = 10:1 in the precursor solution was the best (Figure S 2). Elemental analysis for MoZrO exhibited that the ratio of N:Mo:Zr = 0.6:1.95:1.05. XPS measurements (Figure S 3) revealed that the valence of the Mo ion was MoVI and that the Zr ion was ZrIV in the material. UV−vis spectra (Figure S 4) also confirmed that the Mo ion was in the highest oxidation state (MoVI), because there were no signals observed in the 500−800 nm range.27 The chemical formula of MoZrO was estimated to be (NH3)0.6[H0.1ZrIV1.05MoVI1.95O8]·4H2O. Thermal analysis (TG-DTA and TPD-MS) showed that calcination removed the existing guest molecules, H2O and NH3, from MoZrO. The weight of the material decreased gradually with increasing treatment temperature from 40 to 600 °C (Figure S 5a). The rate of the weight decrease was fast at a low temperature and gradually slowed down with an increase in temperature. The desorption of H2O (m/z = 18) and NH3 (m/ z = 16) was monitored by TPD-MS (Figure S 5b). NH3 desorbed from the material at 250 °C. H2O desorbed from MoZrO exhibited two desorption maxima. The first desorption peak below 150 °C corresponded to H2O that was weakly bound to the layers, and the second desorption peak appeared at a higher temperature, which might correspond to H2O that formed by structural decomposition. MoZrO was calcined at different temperatures. XRD patterns (Figure 1A) showed that the diffraction peaks for the (0 0 1)

Figure 1. (A) XRD patterns and (B) FTIR spectra of (a) MoZrO, (b) MoZrAC100, (c) MoZrAC200, (d) MoZrAC300, (e) MoZrAC400, (f) MoZrAC500, and (g) MoZrAC600.

plane started to shift from ca. 8° to ca. 11.5° at 200 °C, and the corresponding d-spacing between each layer changed from ca. 10.7 to ca. 7.9 Å during heating. Shortening the distance between the molecular layers was due to desorption of H2O and NH 3 , as indicated in the thermal analysis. This phenomenon widely existed in layered-type materials.22 The structure of the layer was stable at 500 °C in air. Further increasing in the heat-treatment temperature to 600 °C generated the hexagonal Zr(MoO4)2 (database number: 0014308

DOI: 10.1021/acs.inorgchem.7b02420 Inorg. Chem. 2017, 56, 14306−14314

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Inorganic Chemistry

Figure 3. Comparison of the XANES spectra of MoZrO and MoZrAC400 with the reference samples (A) Mo, (a) MoZrO, (b) MoZrAC400, (c) AHM, (d) MoO3, (e) Na2MoO4·2H2O, (f) Ba2NiMoO6, and (g) Mo foil; (B) Zr, (a) MoZrO, (b) MoZrAC400, (c) Zr(SO4)2·4H2O, (d) m-ZrO2, (e) t-ZrO2, (f) SrZrO3, and (g) ZrSiO4.

structures of MoZrO and MoZrAC400 were proposed for further Rietveld refinement. The structures of MoZrO and MoZrAC400 were refined by Rietveld analysis under the assumption that Mo had six oxygen atoms with distorted octahedral coordination, and Zr had 8 oxygen atoms initially (Tables S 4 and S 5). The simulated patterns were similar to the experimental ones in both cases (Figure 4Aa,Ba), indicating that the proposed structures were reasonable. For the structure of a single layer, while the heavy metal distribution of the materials before and after calcination was the same, the oxygen position was different. For a single layer in the b−c plane, the arrangement of each linear ZrO8 and MoO6 along the b-axis was in the same direction when it arranged along the c-axis in the case of MoZrO (Figure 4Ab,d), whereas, for MoZrAC400, the arrangement direction was reversed (Figure 4Bb,d). As a result, the lattice parameter c of MoZrAC400 almost doubled compared to that of MoZrO. Furthermore, the manner in which single layers accumulated to form the crystal was also different. After calcination, the space between each molecular sheet decreased due to the removal of guest molecules, and the packing of the molecular sheet shifted along the c-axis with ca. 0.2 nm (Figure 4Ac,Bc). The different oxygen coordination caused the bonding state to change, which resulted in a slight difference in the FTIR spectra of the material before and after calcination (Figure 1B). The structures of MoZrO and MoZrAC400 were further optimized by DFT calculation. The basic structure of the material did not change dramatically compared to those determined by XRD, XANES, and HAADF-STEM, which demonstrated that the models were also theoretically correct. Electrochemical Properties of the Material. MoZrAC400 was used as a cathode-active material for LBs. Before the electrochemical experiments, the cathodes composed of ca. 10 wt % MoZrAC400, 70 wt % AB, and 20 wt % PVdF were prepared. The XRD patterns of the cathode showed a characteristic peak at ca. 11° (Figure 5 and Figure 1A). The broad peak between 10° and 30° was ascribed to AB. The

Figure 2. HAADF-STEM images (left) and the corresponding proposed distribution of heavy metal ions (right) of MoZrAC400 in (a) the (−1 0 2) plane, (b) the (−1 2 2) plane, and (c) the (−11 8 16) plane. The dashed line indicates the unit cell of the material. Mo (blue), and Zr (orange).

MoZrAC400 was similar to that of Zr(SO4)2·4H2O,32 suggesting that the Zr species was in an 8 coordination state (Figure 3B). In the case of MoZrO, the XRD pattern was indexed and exhibited a monoclinic cell with lattice parameters (a = 21.5484 Å, b = 4.9868 Å, c = 3.5749 Å, β = 89.7310°) and space group (C2), which are listed in Table S 1. There were also two sites for heavy metal ions: the central site and the side site (Table S 3). The heavy metal distribution of MoZrO in a single layer was the same as that of MoZrAC400, which demonstrated that the basic structure of the single molecular layer did not change after calcination in air. For oxygen coordination, the XANES spectra of MoZrO and MoZrAC400 almost overlapped, indicating that the coordination state of oxygen in both materials was similar, and the valences of Mo and Zr in the asprepared oxides were 6+ and 4+, respectively. On the basis of HAADF-STEM observation and XANES analysis, the initial 14309

DOI: 10.1021/acs.inorgchem.7b02420 Inorg. Chem. 2017, 56, 14306−14314

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Inorganic Chemistry

Figure 4. Rietveld refinement and structure models of (A) MoZrO and (B) MoZrAC400. (a) Comparison of the simulated XRD pattern from the Rietveld refinement with the experimental one; inset: enlarged patterns for high-angle data, structural models of (b) the a−b plane, (c) the a−c plane, and (d) the b−c plane of the materials. Mo (blue), Zr (orange), and O (blue).

particle sizes of AB and MoZrAC400 were 200 and 500 nm, respectively (Figure S 6). MoZrAC400 exhibited electrochemical redox properties over the large voltage windows of 4.0−2.0, 4.0−1.5, and 4.0−1.0 V versus Li/Li+ (Figure 7). The experiment started from the open circuit voltage, followed by scanning to a high potential (4.0 V), and then scanning back to a low potential of 2.0, 1.5, or 1.0 V. When the voltage window of 4.0−2.0 V was applied (Figure 7a), the redox process was reversible, and the CV curves of the 1st, 2nd, and 10th cycles almost overlapped, indicating that the material was stable under these conditions. When the voltage window was extended to 4.0−1.5 V, the peak current of the material’s redox peaks decreased, and the position of the peaks shifted with the cycles, indicating that the structure of the material might change gradually (Figure 7b). When the lower potential limit was further changed to 1.0 V (Figure 7c), the redox process of the material became completely irreversible, and the material completely collapsed. The charging/discharging experiments were conducted for MoZrAC400-LBs. The current density affected the capacity of MoZrAC400-LBs. When a low current was applied, a high capacity was obtained (Figure 7d). The increase in the current density decreased the capacity of the battery which probably derived from an insufficient redox reaction. For MoZrAC400LBs, the 1st discharging voltage gradually decreased to 2.0 and 1.5 V, and battery capacities of 210 and 117 mAh/g were obtained, respectively (Figure 7e). The charging/discharging experiments were repeated to obtain the cycling performance of

structures of MoZrAC400 did not change after being made into cathodes.

Figure 5. (a) XRD patterns and (b) enlarged XRD patterns of the MoZrAC400-based cathode during the charging/discharging experiment under different voltage ranges.

The as-prepared cathode was further investigated using SEM (Figure 6a), indicating that MoZrAC400 was well dispersed in AB. The EDX elemental mapping images clearly exhibited that the distribution of Mo, Zr, and O was concentrated in the particle in the SEM image, and the ratio of Mo:Zr was 2:1, which demonstrated that the particle was MoZrAC400. The 14310

DOI: 10.1021/acs.inorgchem.7b02420 Inorg. Chem. 2017, 56, 14306−14314

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Figure 6. SEM images and EDX elemental mapping of (a) the cathode of MoZrAC400-LBs before the electrochemical test, (b) after the first discharging to 2.0 V, (c) after the second charging to 4.0 V, and (d) over 50 times’ charging/discharging. Mo (red), Zr (green), and O (blue).

Considering the XPS results, the reduction peak at ca. 2.5 V in the CV profiles was assigned to the reduction of MoVI to MoV, and the peak at 1.5 V could be assigned to the reduction of ZrIV to ZrIII and of MoV to MoIV. The number of transferred electrons estimated from XPS was close to that estimated from battery performance. This indicated that the capacity of the battery was derived from the redox reaction of the Mo and Zr ions with Li insertion (Table 1). The Li signal was observed by using XPS after discharging (Figure S 7), indicating the insertion of the Li ions into the structure of MoZrAC400, while the valence of MoVI was reduced (Figure 8). After charging, the Li peak disappeared, indicating that the Li ions were extracted from the structure and that the Mo ions were reoxidized (Figure 8). The structure of MoZrAC400 after the charging/discharging experiments was analyzed by powder XRD (Figure 5). The peak at ca. 11.2° corresponding to the space of the molecular layer shifted to a lower angle after discharging, indicating that the layer of the material swelled after discharging, due to the insertion of Li ions between layers of the material. Furthermore, the XRD pattern of the cathode after charging (4.0 V) showed that the peak of MoZrAC400 shifted back to its original position, indicating that the extraction of the Li ions during the charging process caused shrinkage of the layer of the material layer (Figure 5). The Li site in the discharged material was studied by DFT calculation. Compared with the model of MoZrAC400, the lattice parameter of the discharged MoZrAC400 increased in the a direction, which is in agreement with the XRD pattern (Figure 5 and Table S 6). Li was inserted in the layer of the material, and Li closely interacted with two terminal MoO bonds and a Mo−O−Zr bridge bond with a length of 1.924− 2.174 Å (Figure S 8).

MoZrAC400-LBs. For the battery that was discharged to 1.5 V, ca. 82% of the initial capacity remained after 20 cycles. When the battery was discharged to 2.0 V, ca. 94% of the capacity remained after 20 cycles. The battery lost only 9% of its initial capacity after 50 cycles. After the cycling performance tests, the cathodes were characterized by SEM and EDX elemental mapping (Figure 6b,c). The elements Mo, Zr, and O were distributed homogeneously within the particle in the SEM image, and EDX demonstrated that Mo:Zr = 2:1, indicating that the particle was MoZrAC400. Therefore, the chemical composition as well as the morphology of the material did not change after discharging/charging. XPS was conducted for the MoZrAC400-based cathodes to investigate the oxidation states of the metal ions in the material after the charging/discharging experiments (Figure 8). The fresh cathode exhibited MoVI and ZrIV, which did not change compared to MoZrO (Figure 8a and Figure S 3). The metal ion was reduced after discharging. The oxidation state decreased with the decrease in potential. On discharging to 2.0 V, the MoVI ions were mainly reduced to MoV ions with a slight reduction of ZrIV to ZrIII, and the molar ratio of MoVI/ MoV = ca. 0.23 (Figure 8c,d). When this battery was charged back to 4.0 V, the oxidation states of the Mo and Zr ions were reoxidized to their original states of MoVI and ZrIV (Figure 8e,f), respectively. For discharging to 1.5 V, the MoVI ions in the materials were further reduced to MoV and MoIV (Figure 8g). The ratio of MoV/MoIV was ca. 1.05. The ZrIV ions were also reduced to ZrIII, with the ratio of ZrIV/ZrIII = 1.23. The oxidation states of Mo and Zr ions in the material were reoxidized to MoVI and ZrIV, respectively, after the battery was charged to 4.0 V (Figure 8i,j). 14311

DOI: 10.1021/acs.inorgchem.7b02420 Inorg. Chem. 2017, 56, 14306−14314

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Figure 7. CV curves of MoZrAC400-LBs at a scan rate of 1 mV/s with a low potential of (a) 2.0 V, (b) 1.5 V, and (c) 1.0 V. (d) The discharging curves of MoZrAC400-LBs with different current densities; inserted picture: capacity vs current. (e) The cycle performance of MoZrAC400-LBs with the current of 0.1 mA; inset: the corresponding charging/discharging curves from 4.0 to 2.0 V.

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The electrochemical process of the MoZrAC400-based cathode was proposed. When the battery was discharged, Li inserted into the layer of the material, the Mo ions were reduced, and the distance between the layers expanded. When the charging process was conducted, Li ions were extracted from the material, the reduced Mo ions in the material were oxidized, and the layer distance also decreased. The layered structure of MoZrAC400 was important for the reversible electrochemical Li insertion and extraction from the structure without changing the basic structure. After MoZrO was calcined at 600 °C (MoZrAC600), the material transferred to the hexagonal Zr(MoO4)2. The CV curves exhibited an irreversible reduction peak ca. 2.1 V, which could not be reoxidized (Figure S 9).

CONCLUSIONS

In summary, we investigated the crystal structure of a layered complex metal oxide material based on Mo and Zr before and after calcination using HAADF-STEM images combined with XRD, FTIR, XPS, TPD-MS, TG-DTA, and elemental analyses. The electrochemical properties of the layered-type zirconomolybdate were studied using Li batteries. The material reversibly stored and released Li ions in the material during the charging/ discharging process. The cathode was characterized by XRD and XPS after the electrochemical process. Li ions were inserted into the structure. Meanwhile, Mo ions were reduced during discharging. Li ions were extracted from the structure, and Mo was oxidized to MoVI during the charging process. MoZrAC400-LBs was reused 50 times with only losing 9% loss 14312

DOI: 10.1021/acs.inorgchem.7b02420 Inorg. Chem. 2017, 56, 14306−14314

Inorganic Chemistry

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02420. Structure determination by PXRD, Rietveld refinement, assembly of the Li battery using MoZrAC400, XRD patterns, XPS profiles, UV−vis spectra, TG-DTA and TPD-MS profiles, SEM images, structure model of MoZrAC400, CV curves, crystallographic information and Rietveld refinement parameters, results for MoZrAC400 and MoZrO from the charge flipping method and peak assignments, structural information for MoZrO and MoZrAC400 from Rietveld analysis, and comparison of MoZrAC400 before and after discharging (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.Z.). *E-mail: [email protected] (W.U.). ORCID

Zhenxin Zhang: 0000-0002-9609-4691 Masahiro Sadakane: 0000-0001-7308-563X Futoshi Matsumoto: 0000-0001-6808-6531 Akira Yamamoto: 0000-0002-1329-0807 Hisao Yoshida: 0000-0002-2540-0225 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Novel Cheap and Abundant Materials for Catalytic Biomass Conversion (NOVACAM, FP7-NMP-2013-EU-Japan-604319) program of the Japan Science and Technology Agency (JST) and a Grantsin-Aid for Scientific Research (A) from the Ministry of Education, Culture, Sport, Science, and Technology of Japan (MEXT) (grant number: 15H02318). The authors also thank the Material Analysis Suzukake-dai Center, Technical Department, Tokyo Institute of Technology, for the elemental analyses. The XAFS measurements were performed at the BL01B1 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2017A1477).

Figure 8. XPS profiles of (a) Mo and (b) Zr in MoZrAC400-LBs, (c) Mo and (d) Zr in MoZrAC400-LBs after discharging to 2.0 V, (e) Mo and (f) Zr in MoZrAC400-LBs after discharging to 2.0 V, then charging to 4.0 V, (g) Mo and (h) Zr in MoZrAC400-LBs after discharging to 1.5 V, (i) Mo and (j) Zr in MoZrAC400-LBs after discharging to 1.5 V, then charging to 4.0 V.

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REFERENCES

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of initial capacity, demonstrating that MoZrAC400 is stable during the process.

Table 1. Comparison of Electron Transfer According to the Discharging Experiments and the XPS Measurements discharged material MoZrAC400-LBs, 2 V MoZrAC400-LBs, 1.5 V

method

electron transfer

battery XPS battery XPS

1.92 1.72 3.45 3.37 14313

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DOI: 10.1021/acs.inorgchem.7b02420 Inorg. Chem. 2017, 56, 14306−14314