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High-performance cathode based on microporous Mo-V-Bi oxide for Li battery and the investigation by operando X-ray absorption fine structure Zhenxin Zhang, Satoshi Ishikawa, Masaki Kikuchi, Hirofumi Yoshikawa, Qi Lian, Heng Wang, Toshiaki Ina, Akihiro Yoshida, Masahiro Sadakane, Futoshi Matsumoto, and Wataru Ueda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07195 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 24, 2017

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ACS Applied Materials & Interfaces

High-performance cathode based on microporous Mo-V-Bi oxide for Li battery and the investigation by operando X-ray absorption fine structure Zhenxin Zhang, 1,2 Satoshi Ishikawa, 1 Masaki Kikuchi, 1 Hirofumi Yoshikawa,*3 Qi Lian, 3 Heng Wang, 4 Toshiaki Ina, 5 Akihiro Yoshida, 1 Masahiro Sadakane, 6 Futoshi Matsumoto, 1 Wataru Ueda*1 1

Faculty of Engineering, Kanagawa University, Rokkakubashi, Kanagawa-ku, Yokohama-shi, Kanagawa, 221-8686, Japan. 2

Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta-cho 4259, Midori-ku, Yokohamacity, Kanagawa, 226-8503, Japan.

3

School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan.

4

School of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, China.

5

Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo 679-5198, Japan.

6

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan. KEYWORDS. Transition metal oxide, redox property, electrochemical property, Li battery, operando XAFS.

ABSTRACT: The development of cathode-active material of Li battery is important for the current emerging energy transferring and saving problems. A stable crystalline microporous complex metal oxide based on Mo, V, and Bi is an active and suitable material for Li battery. High capacity (380 Ah/kg) and stable cycle performance are achieved. X-ray absorption near-edge structure analyses demonstrate that the original Mo6+ and V4+ ions are reduced to Mo4+ and V3+ in the discharging process, respectively, which results in a 70-electron reduction per formula. The reduced metal ions can be reoxidized reversibly in the next charging process. Furthermore, extended X-ray absorption fine structure analyses reveal that the Mo-O bonds in the material are lengthened in the discharging process probably due to interaction with Li+ without change of the basic structure.

Introduction Energy transformation and storage become an important issue, because of the increasing attention of the energy generation, utilization, and consumption issues in a sustainable society. Li batteries (LBs) is a potential solution for the issues due to the advantages of high energy density, large current, low cost, reusability, and low environmental burden. 1–3 A variety of materials have been used as cathode-active materials for LBs, including metal oxides, 4–6 metal-oxygen clusters (polyoxometalates, POMs), 7–10 organic small molecules, 11,12 and metal organic frameworks (MOFs). 13 Developing of new cathode-active materials for LBs to achieve high capacity and stable cycle performance is important. The cathode-active materials are expected to have redox activity, stable structure, high discharging voltage, high energy density, and Li migration under electrochemical condition. Crystalline microporous transition metal oxide mainly comprised of metal oxygen octahedra. The examples of

the materials are not many. To date, only manganese oxides, 14,15 crystalline Mo-V complex oxides (orth-MoVO), 16–18 ε-Keggin based microporous materials, 4,6,19–21 ring-like POM-based materials, 22 porous vanadoborates, 23,24, and porous phosphovanadates 25,26 have been synthesized and investigated. With combination of redox properties and microporosity, microporous transition metal oxide would have great application potential in LBs. Indeed, OrthMoVO shows activity for LBs, because of its multielectron transfer property and microporosity. 27 However, the low stability and poor reversibility of the material under the electrochemical condition leads to losing ca. 40% battery capacity within 10 cycles, which limits the application of the material in LBs. 27 The properties of transition metal oxides are able to be tuned by modifying composition of the material without changing the basic structure. The stability of orth-MoVO is expected to be enhanced by modification of the chemical composition of the material. Our strategy is incorpora-

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tion of the 3rd metal ion in Orth-MoVO, which would enhance the stability of the material and improve the cycle performance of corresponding LBs. Furthermore, analyzing the material under working condition to obtain the structure information using an operando characterization technique is critical to understand the electrochemical property of the material. 28,29 Herein, transition metal oxide based on Mo, V, and Bi was synthesized, denoted as MoVBiO. Structural characterization demonstrated that MoVBiO was an isostructural material of orth-MoVO. Elemental analysis and energy-dispersive X-ray spectroscopy (EDX)elemental mapping indicated that the Bi ions were uniformly incorporated in the structure of the material. A high discharging capacity of LBs with a stable cycle performance was achieved using the calcined material as a cathode. X-ray absorption near-edge structure (XANES) analyses demonstrated that metal ions (Mo and V) were reversibly oxidized/reduced during the charging/discharging process. Extended X-ray absorption fine structure analyses (EXAFS) indicated that Mo-O bond length changed reversibly during the electrochemical process without changing the structure. Experimental Ethylammonium isopolymolybdate (EATM) was prepared by evaporation method. 30 MoO3 (22.594 g, 0.150 mol) was added into the solution with 70% ethylamine solution and water, followed by the stirring for 30 min. After MoO3 was dissolved completely, the mixed solution was evaporated under a vacuum condition at 70 oC until all of the liquid was evaporated. Obtained powder was dried in air at 80 oC overnight. EATM (1.7996 g, 10 mmol based on Mo) was dissolved in 20 mL of water and stirred for 10 min. To this solution, 20 mL of VOSO4 (0.6580 g, 2.5 mmol) solution was added with stirring for 10 min. Then, BiOCl (0.3260 g, 1.3 mmol) was added to the mixture followed by adjustment of pH to 2 by H2SO4 (2 M). The mixture was introduced into a 50-mL Teflon-lined stainless-steel autoclave and degassed by N2 bubbling for 10 min. which was heated at 175 oC for 20 h with tumbling (ca. 1 rpm). After the autoclave was cooled to room temperature, the crude solid was recovered from the solution by filtration and dried at 80 oC overnight. The crude solid was dispersed in HCl solution (1.2 M), the amount of which was 25 times higher than the crude solid content. The mixture was stirred for 30 min. The solid was obtained by filtrated, which was washed with HCl solution several times, washed with 500 mL of water, and dried in oven over night. Then, 1.0026 g of the material was obtained. Calcd for C4N2H66Mo30.5V9.5Bi1.1O137, C, 0.80; N, 0.47; H, 1.10; Mo, 48.99; V, 8.10; Bi, 3.85, Found: C, 0.84; N, 0.50; H, 0.39; Mo, 48.68; V, 7.89; Bi, 3.70. The material was calcined at 400 oC for 2 h under air with a temperature increasing rate of 10 oC/min in an oven, denoted as MoVBiAC400. Calcd for H40Mo30.5V9.5 Bi1.1O132, C, 0.00; N, 0.00; H, 0.69; Mo, 50.53; V, 8.35; Bi,

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3.97, Found: C, 0.00; N, 0.00; H, 0.25; Mo, 50.66; V, 8.27; Bi, 3.79 Characterization Powder X-ray diffraction (XRD) patterns were obtained on a Ultima IV X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation (tube voltage: 40 kV, tube current: 40 mA). Fourier Transform Infrared (FTIR) spectroscopy was carried out using a JASCO FT/IR-4200 instrument (JASCO, Japan). Field emission scanning electron microscopy (FE-SEM) images and EDX analysis were obtained using a SU-8010 system (Hitachi, Japan). Temperature-programmed desorption mass spectrometry (TPD-MS) measurements were carried out from 40 oC to 600 oC at a heating rate of 10 oC/min under He flow (flow rate: 50 mL/min). Samples were set up between two layers of quartz wool. A TPD apparatus (BEL Japan, Inc.) equipped with a quadrupole mass spectrometer (M100QA; Anelva) was used to detect NH3 (m/z = 16) and H2O (m/z = 18). Thermogravimetric analysis-differential thermal analysis (TG-DTA) was carried out up to 600 oC at a heating rate of 10 oC/min under nitrogen flow (flow rate: 50 mL/min) with Thermo plus TG-8120 (Rigaku, Japan). N2 adsorption-desorption measurements were carried out on BEL sorp max (BEL, Japan). The adsorption was conducted at -197 oC. Before adsorption the material was pre-treated at 350 oC for 2 h under high vacuum. Elemental compositions were determined by an inductive coupling plasma (ICP-AES) method (ICPE-9000, Shimadzu). CHN elemental composition was determined at Material Analysis Suzukake-dai Center, Technical Department, Tokyo Institute of Technology. Rietveld refinement and powder diffraction pattern simulation The structures of MoVBiO was refined by powder XRD Rietveld refinement. 31 The initial structure was obtained from the previous paper. Pattern parameters and lattice parameters of the material were refined by the Pawley method. Then isotropic temperature factors were given for each atom. Rietveld analysis was started with the initial models of the material, and lattice parameters and pattern parameters were from Pawley refinement. Every atom position was refined. Occupancy of atoms in the framework was fixed without further refinement, and occupancies of atoms for water and cations were refined with consideration of results of elemental analysis. Finally, the pattern parameters were refined again for obtaining the lowest Rwp value. Crystallographic parameters and atom position of the materials are shown in Table S 1 and Table S 2. Theoretical calculation The structures of the material were optimized using the DMol3 program 32,33 in Materials Studio software package. We employed the Perdew-Burke-Ernzerhof (PBE) generalized gradient functional and DND basis set. Electrochemical measurements The cathode, which included 10wt% of MoVBiAC400, was prepared as follows: MoVBiAC400, carbon black, and


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polyvinylidene ﬂuoride (PVDF) in the weight ratio of 1:7:2, were mixed homogeneously. Then, ca. 0.5 mL of 1-methyl2-pyrrolidinone was added. The mixture was pressed into a plate of a thickness of ca. 0.3 mm. The plate was cut into small discs that were dried under vacuum for 2 h.

background subtraction, and normalization using the atomic absorption coefficients. The k ranges in the Fourier transforms were approximately 2-12 Å-1 for Mo K-edge and 2-11 Å-1 for V K-edge.

Li metal was used as the anode material. The anode was isolated from the cathode by a polyolefin film separator. The cathode and anode were set in a coin cell with an electrolyte (LiPF6) in a mixed solution of diethyl carbonate (DEC) and ethylene carbonate (EC) (DEC: EC = 1: 1) in an inert atmosphere. The battery was denoted as MoVBiAC400-LBs. The charging/discharging measurements were performed at a constant current density of 1 mA cm-2 in the voltage range of 1.5~4.0 V.

Results and discussion

Herein, the theoretical capacity was calculated using the following equation:

MoVBiO was prepared under a hydrothermal condition using organoammonium cation (EMTA) as a Mo source, VOSO4 as a V source, and BiOCl as a Bi source. After hydrothermal synthesis for 20 h, deep color solid materials were recovered. The un-reacted Bi compound was washed by excess HCl solution. Compared with orth-MoVO, 17 the characteristic diffraction peaks and IR bands of the materials were the same, indicating that these two materials were iso-structural materials and the main bonding state of the materials was almost the same (Figure 1).

Q = (nF)/(3.6Mw), where Q is theoretical capacity, n is the numbers of reduced electrons, F is Faraday constant, and Mw is molar mass of an unit cell. Battery analyses of MoVBiAC400-LBs for XAFS For operando X-ray absorption fine structure (XAFS) measurements, the batteries were fabricated in a similar way using a special battery cell with a Kapton film as an X-ray window. The cathode containing MoVBiAC400, carbon black, and PVDF was prepared in the same way as the coin cell. For Mo and V K-edge XAFS measurements, the weight ratio of MoVBiAC400, carbon black, and PVDF was 30:60:10, while for Bi L3-edge XAFS measurement, the weight ratio was 50:40:10 to obtain an appropriate absorption intensity. The charging/discharging tests of the fabricated MoVBiAC400-LBs were carried out in the voltage range of 1.5-4.0V at a constant current of 0.5 mA, 1.0 mA, and 1.0 mA for Mo K-edge, V K-edge, and Bi L3-edge, respectively.

Figure 1. A) XRD patterns and B) FTIR spectra of a) MoVBiO and b) MoVBiAC400.

Operando XAFS measurements Operando XAFS measurements were performed in the transmission mode at room temperature in the BL01B1 at the SPring-8 (8.0 GeV, 100mA). The operando battery cell was placed between two transmission ion chambers. The intensities of the incident (I0) and transmitted (It) X-rays were detected by ion chambers. To avoid chemical decomposition of the cathode by X-ray, a quick XAFS (QXAFS) method was adapted by which one spectrum was obtained every 3 min (Mo) and 5 min (V and Bi) during battery reaction. We also measured QXAFS of the standard materials (a) MoO2, MoO3, TBA3[PMo12O40], (b) V2O3, VO2, V2O5, and (c) Bi2O5, Bi2O3, Bi powder to analyze the relationship between the absorption energy of Mo K-edge, V K-edge, and Bi L3-edge and the averaged valence of Mo, V, and Bi. Analysis of K-edge XAFS XANES spectra were obtained by pre-edge background subtraction and subsequent normalization using the software Athena. EXAFS spectra were obtained by the standard procedures in Feffit (Artemis). The EXAFS oscillation functions k3x(k) were obtained by pre-edge base line subtraction, edge-energy determination, post-edge

Figure 2. a) SEM image of MoVBiO, b) elemental mapping of Mo, c) V, d) Bi, and e) O. The morphology of MoVBiO was observed by SEM, showing that MoVBiO was a rod-shaped material with ca. 100 nm in diameter and ca. 2 μm in length (Figure 2a). The elemental composition and distribution were characterized by EDX and elemental mapping. The material comprised of Mo, V, and Bi, and the ratio estimated by EDX was Mo: V: Bi = 33.5: 11.6: 1 (Figure 2b-e). Mo, V, and Bi uniformly distributed in the material, indicating that the Bi ions were incorporated in the structure of MoVBiO, although the content was low. The chemical composition was further determined by elemental analysis, showing that C: N: Mo: V: Bi = 4: 2: 30.5: 9.5: 1.1, the



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chemical formula of which was summarized to be (C2H5N)2[Bi1.1Mo30.5V9.5O112]·25H2O. Thermal analysis (TG-DTA and TPD-MS) indicated that the guest molecules were removed from MoVBiO during heat treatment (Figure S 1). The weight loss was recorded by TG; the desorbed molecules were monitored TPD-MS. There were two weight losses during heating. The one at 120 oC was ascribed to water desorption, which was corresponded to the weakly bound water. The weight loss above 300 oC was corresponded to the decomposition of organoammonium species, because the desorption of water (m/z = 18), NH3 (m/z = 16), and CO2 (m/z = 44) was observed at the same time (Figure S 1b). MoVBiO was calcined at 400 oC in air, and the resulting material was denoted as MoVBiAC400. Compared with MoVBiO, the calcined material exhibited almost the same characteristic XRD patterns and FTIR spectra (Figure 1). The structure of the material did not change after calcination. Chemical composition of MoVBiAC400 was determined by elemental analysis, showing that the ratio of Mo: V: Bi did not change after calcination. The chemical formula of MoVBiAC400 was [Bi1.1Mo30.5V9.5O112]·20H2O.

Figure 3. a) Comparison of the experimental XRD pattern with the simulated XRD pattern of MoVBiO by the Rietveld refinement, b) enlarged XRD pattern (5-16 degree) c) enlarged XRD pattern (24-48 degree), and d) refined structure by the Rietveld refinement, pentagonal units are highlighted with yellow, Mo (or V): blue, O: red.

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a-b plane along the c direction with a short distance from Mo1-O1 to Mo11-O11 was denoted as Mo-OS bond, and the average length was 1.681 Å (Figure 4c, black), which was similar to the Mo=O double bonds. 25,35 The 2nd group of the Mo-O bonds was the bonds in the a-b plane of the material, which connected pentagonal units with other metal-oxygen octahedra in the a-b plane. The average length of the Mo-O bond was 1.929 Å, which was assigned as Mo-Oab (Figure 4b, yellow). The last group of the Mo-O bonds, from Mo1-O1’ to Mo11-O11’, was ascribed to the bond for connection of each a-b plane along the c direction with a long distance, denoted as Mo-OL (Figure 4c, blue). Theoretical calculation showed the same trend of bond length compared with the data of single crystal analysis (Table 1).

Figure 4. Structure representations of a) unit cell, the yellow color indicating the simplest structural fragment of the material, b) the atom type in the structure fragment in a-b plane, and c) in the c direction, Mo (or V): blue, O: red. Table 1. Bond length of orthorhombic phase using different methods. Single crystal analysis (Å)

DFT calculation (Å)

EXAFS (Å)

Range

Average

Range

Average

The material structure was refined by powder XRD Rietveld analysis (Figure 3). Rietveld refinement demonstrated that the simulated XRD pattern was similar to the experimental one, indicating that the proposed structure was correct. The structure of the material constructed by connection of pentagonal units with metal oxygen octahedra (Figure 3d). There were six-member metal-oxygen octahedra ring and seven-member metal-oxygen octahedra ring, which acted as micropores that were accessible to small molecules or ions. 17 N2 adsorption-desorption measurement for MoVBiAC400 showed that the material was a microporous material (Figure S 2). The micropore of the material was not blocked after incorporation of Bi. The Bi ions were located in the six-member metal-oxygen octahedra ring. 30

Mo-OS

1.6271.726

1.681

1.7511.777

1.762

1.64

Mo-Oab

1.7562.077

1.929

1.8052.080

1.941

1.96

Mo-OL

2.2722.361

2.310

2.2292.256

2.243

2.48

Figure 4 shows the minimum structure fragment of the material. Table 1 summarized the bond length of the material from single crystal analysis 34 and DFT calculation. According to the bond length and the atomic position, all of the Mo-O bonds were classified into three groups, from short to long generally. The bonds for connection of each

Figure 5. a) The charging/discharging curves MoVBiAC400-LBs and b) cycle performance.

of

Electrochemical property of the material MoVBiAC400 was used as a cathode-active material for LBs. The material, carbon black, and binder were mixed


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homogeneously to obtain the cathode, which was used to assemble coin cells for LBs performance tests. The charging/discharging experiments were carried out. The voltage window was set as 1.5-4.0 V. The first discharging capacity was ca. 380 Ah/kg (Figure 5a). The cycle performance of MoVBiAC400-LBs was good. After 20 times’ cycles, the battery only lost ca. 8% of the initial capacity (Figure 5b). The rate capability experiment was carried out. As the current increased, the capacity decreased correspondingly. The capacities obtained at 1.0 mA, 5.0 mA, and 10.0 mA were 370, 270, and 170 Ah/kg, respectively. When the current returned to 1.o mA, the capacity recovered, indicating that the material was stable (Figure S 3). The result demonstrated that MoVBiAC400 was a stable and active material for LBs.

2C)/discharging (1D and 2D) processes, respectively. This structure was well-known as a formally dipole forbidden excitation from 1s to 4d antibonding orbitals directed principally along the Mo=O (Mo-OS in MoVBiAC400) bond (Figure 4). Therefore, the appearance and disappearance of this shoulder indicated the presence and absence of Mo-OS. It demonstrated that the short Mo-O bonds (Mo-OS) in O, 1C and 2C changed to longer bonds in the discharging state (1D and 2D), denoted as Mo-OS,d.

Operando XANES analyses of MoVBiAC400-LBs The investigation of the cathode-active material under an operando condition during the battery reaction is interesting and important to obtain real time material information for further understanding the property. MoVBiAC400-LBs was studied using the operando XAFS analysis. Figure 6 shows the charging/discharging curves of MoVBiAC400-LBs measured with in situ battery cells for operando XAFS studies. A discharging capacity of ca. 360 Ah/kg in the first two cycles was obtained, which was close to the results obtained from the coin cell of MoVBiAC400-LBs (Figure 5a).

Figure 6. Charging/discharging curves of MoVBiAC400LBs during XAFS measurements for the 1st charging (black line), the 1st discharging (green line), the 2nd charging (red line), and the 2nd discharging (blue line). The valence change of the metal elements was monitored by XANES. Figure 7a and Figure S 4 show the normalized Mo K-edge XANES spectra at the specific voltages, and the original sample, the sample after the 1st charging, the sample after the 1st discharging, the sample after the 2nd charging, and the sample after the 2nd discharging were denoted as O, 1C, 1D, 2C, and 2D, respectively. There was a low-energy shift of the absorption curves in discharging (1D), while the curve returned to the original position after the next charging process (2C). The spectra of O, 1C and 2C were almost overlapped, implying that the initial oxidation state of Mo was fully charged. A shoulder peak around 20010 eV in the pre-edge region, which appeared/disappeared in the charging (O, 1C and

Figure 7. Normalized a) Mo K-edge XANES spectra for O, 1C, 1D, 2C, and 2D, b) V K-edge XANES spectra for O, 1C, 1D, 2C, and 2D, and c) Bi L3-edge XANES for O, 1C, and 1D. The absorption edge energy of Mo K-edge XANES was utilized to determine the valence of the absorbing Mo atom. Figure S 5 shows a linear relationship between the Mo oxidation state and the X-ray absorption edge energy for the reference compounds, Mo0, Mo4+O2, and TBA3[PMo6+12O40] (TBA = tetrabutyl ammonium). The edge energy was defined, when the intensity reached 60% of the absorption energy. By using the linear relation, the average oxidation state of the Mo ions in MoVBiAC400 was plotted as a function of the battery voltage in Figure 8a. The initial Mo valence at open circuit voltage was ca. 6.0, which illustrated that the Mo ions were Mo6+ in the original MoVBiAC400. During the 1st charging, this value kept constant, which implied that the as-prepared battery was in a charged state. In the 1st discharging, the valence did not change in 4.0-3.0V, then this value quickly decreased from 6.0 to 3.8 (3.0-1.5 V). The 2nd charging in-



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creased the valence from 3.8 to 6.0 in the range of 1.5-3.0 V, which became a constant in 3.0-4.0 V, demonstrating that the Mo4+ ions fully returned to Mo6+ and the reduced material was able to be re-oxidized reversibly. The average valence change of the Mo ions was ca. 2.0 during the electrochemical process, and this indicated that an approximately 60-electron reduction per MoVBiAC400 formula occurred, which contributed a battery capacity of ca. 300 Ah/kg.

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Mo ions in the discharging process, since the reduction potential of Mo was lower than that of V. In the case of Bi L3-edge XANES (Figure 7c), the lowerenergy spectra shifted, indicating that the local environment around the Bi ions was changed. The oxidation state of Bi3+ in the fresh MoVBiAC400 was reduced below 2.5 V in the discharging process, although the exact valence change could not be determined precisely. Since the experimental capacity of MoVBiAC400-LBs (350 Ah/kg) was slightly larger than that estimated from the redox of the Mo and V ions (346 Ah/kg), reduction of Bi3+ contributed a little to the whole capacity due to the low Bi amount in MoVBiAC400. According to the above analyses, a more than 70electron reduction per MoVBiAC400 formula occurred in the discharging process, which resulted in the high capacity of the MoVBiAC400-LBs. The valence change of Mo and V was reversible, which led to the high stability of cycle performance during the charging/discharging experiment. Bi occupied the micropores, which might stabilize the structure under electrochemical condition and improve the cycle performance compared with the previous research. 27

Figure 8. Average valence of a) Mo and b) V in MoVBiAC400-LBs as a function of the cell voltage. V K-edge XANES spectra in Figure 7b and Figure S 6 exhibited a pre-edge absorption feature around 5455 eV. The spectra showed an increase in intensity of pre-edge peak after discharging. The increase in the pre-edge peak intensity was ascribed to a decrease in the V d orbital occupancy after Li insertion. This suggested that the symmetry of V ions in VO6 decreased after discharging. The average oxidation state of the V ions in the MoVBiAC400 (Figure 8b) was calculated using a linear relationship between the V oxidation state and the X-ray absorption edge energy of the reference materials, V3+2O3, V4+O2, and V5+2O5 (Figure S 7). The value of V valence was ca. 4.0 in the fresh material. In the 1st discharging from 4.0 V to 2.5 V, the V valence rapidly decreased to 3.4, then further decreased to 3.35. The 2nd charging increased the V valence from 3.35 to 3.93. Similar to the 1st discharging, the V valence firstly decreased from 3.93 to 3.26 in the voltage of 4.0 V to 2.5 V, then it further decreased to 3.25 in the 2nd discharging. The change in the average V valence was ca. 0.8 in the discharging process, which indicated that most of the V4+ ions in the MoVBiAC400 were reduced to V3+ reversibly. The V valence change corresponded to a capacity of ca. 46 Ah/kg. It was reasonable that reduction of the V ions was observed before the reductions of the

Figure 9. a) Fourier transforms of the Mo K-edge EXAFS spectra, b) local structure change of the material based on EXAFS, Mo6+: blue, Mo4+: green, O: red, and c) ex situ XRD patterns for O, 1C, 1D, 2C, and 2D.


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Operando EXAFS Analyses of MoVBiAC400-LBs Operando EXAFS analyses were performed for MoVBiAC400-LBs to obtain the real time local structural information of Mo, which was a main metal element in MoVBiAC400. Figure 9a shows the Fourier transform spectra for Mo K-edge EXAFS spectra for MoVBiAC400LBs during the charging/discharging experiments (Figure 6). The as-prepared sample (O) exhibited three main peaks at 1.2, 1.7, and 2.2 Å, which were assigned to the bond length values of 1.64, 1.96, and 2.48 Å. Comparing the bond length values from EXAFS with these from the crystallographic data and theoretical calculation (Table 1), three EXAFS peaks at 1.2, 1.7, and 2.2 Å were ascribed to the Mo-Os bond, the Mo-Oab bond, and the Mo-OL bond, respectively (Figure 4b,c and Figure 9b). The values estimated by EXAFS was close to these from the crystallographic data and theoretical calculation (Table 1). The EXAFS spectra of 1C was nearly the same as that of O, indicating that the structure did not change during this process. After the 1st discharging, 1D exhibited a different spectrum. The peak at 1.2 Å disappeared. A new peak at 1.55 Å appeared, which was corresponding to an average length of 1.88 Å. The disappearance of the peak at 1.2 Å and the appearance of the peak at 1.55 Å were resulted from lengthening the Mo-OS bonds to form longer bonds (Mo-OS,d) during discharging, as the XANES analysis for the discharging process indicated (Figure 7a). The EXAFS peak of Mo-OS,d might overlap with the signal of Mo-Oab. The change of the Mo-O bonds was attributed to the intercalation/deintercalation of Li ions with the framework of the material in battery reactions. The peak at 2.2 Å did not change, which indicated that the bond length of Mo-OL kept constant after discharging. This peak was significantly enhanced, which was probably caused by a shortening of the Mo-Mo distance due to the reduction of Mo6+ to Mo4+ during the discharging process, which widely existed in the reduced Mo based oxides. 36 The EXAFS analysis demonstrated that after discharging the OS-Mo-OL bond along c-axis increased (Figure 9b).

Figure 10. Structure representations of a) unit cell, b) heptagonal ring and enlarged four-member oxygen unit with a central Li in the wall of the heptagonal ring, and c) hexagonal ring, and d) pentagonal unit as indicated in a), Mo (or V): blue, Li: purple, O: red. Crystal structure change during charging/discharging was characterized by ex situ XRD (Figure 9c). The XRD

pattern of O was identical to MoVBiAC400. After discharging (1D), the peak for the (001) plane shifted to a lower angle, indicating that the lattice parameter c increased from 4.00 Å to 4.17 Å after discharging, and the distance of OS-Mo-OL along the c direction increased correspondingly. The increase in lattice parameter c (0.17 Å) was close to increase in the Mo-OS bond (0.24 Å) based on EXAFS analysis. Other characteristic diffraction peaks did not change during charging/discharging, indicating that the basic material structure was stable. The EXAFS profiles and XRD patterns of O, 1C, and 2C were almost the same, and 2D also showed the same profile and XRD pattern with 1D, which indicated that the local structure change of the material in charging/discharging was reversible. The possible Li sites were proposed by theoretical calculation. There were three types of sites for locating Li ions, heptagonal ring, hexagonal ring, and pentagonal unit, which were shown in Figure 10a-d. The micropores of the material were expected to facilitate the migration of Li in the bulk of the material during the charging/discharging processes. In the heptagonal ring and pentagonal unit, Li was imbedded into the wall of the framework and was almost in the center of the fourmember oxygen ring (Figure 10b,d). In the case of hexagonal ring, the Li ions had two sites. One was the in the center of the four-member oxygen ring (Li10 and Li11), and the other one was in the a-b plane (Li8 and Li9) (Figure 10c). The calculated relative energy of each state was close indicating that all of the sites were possible for Li ions. The length of the Mo-O bonds in the material after Li insertion was estimated using DFT calculation. We checked bond length of Mo-O in model with different Li sites (Figure 10) after discharging. As shown in Table S 3, it was found that the length of the Mo-OS bond increased dramatically after incorporating Li in the structure with only a slight change of other adjacent Mo-O bonds. The Mo-OS bond (1.627-1.726 Å based on single crystal analysis) increased to form Mo-OS,d (1.762-1.982 Å based on DFT calculation) (Table S 3), and lengthening Mo-OS after discharging was basically consistent with the result of the EXAFS and XRD analyses. After incorporation of Bi in the hexagonal channel, it might avoid the over discharge and further deformation of the structure of the material, which might cause the irreversible battery reaction and decomposition of the material. For V K-edge EXAFS (Figure S 8), sample O showed two main peaks at around 1.55 and 2.2 Å, which were attributed to two kinds of V-O bonds (1.89 Å V-O and 2.54 Å VO). The curve of 1C, 1D, 2C and 2D was nearly the same as that of O. We assumed that the local structure around V atoms did not change during the charging/discharging processes. Among the above analyses, the structure change of the material in charging/discharging was described as follow. Li inserted in the bulk of the material, causing the Mo-OS bond was lengthened. The Mo6+ ions were reduced to



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Mo4+, and the V4+ ions were reduced to V3+ simultaneously in the discharging process. The metal ions in material was re-oxidized in the next charging process, and the material returned to the original structure.

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Page 8 of 11

Whittingham, M. S. Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104, 4271–4302.

(2)

Conclusion

Tarascon, J.-M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359–367.

In summary, the Bi incorporated material, MoVBiO, was synthesized. The characterization showed that the Bi ions was incorporated in the hexagonal channel of the material. The material after calcination in air was used as a stable cathode-active material for LBs, and a large capacity was obtained with a stable cycle performance. Operando XAFS analysis was used for investigation of the valence and structure change of MoVBiAC400-LBs during charging/discharging. XANES analysis demonstrated that Mo was reversibly oxidized (Mo6+) and reduced (Mo4+), and the valence of V was also oxidized and reduced from 4 to 3. The EXAFS, XRD, and theoretical calculation demonstrated that inserting Li in the discharging process lengthened the Mo-O bond along c-axis, which returned in the next charging process.

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Armand, M.; Tarascon, J.-M. Building Better Batteries. Nature 2008, 451, 652–657.

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Kawasaki, N.; Wang, H.; Nakanishi, R.; Hamanaka, S.; Kitaura, R.; Shinohara, H.; Yokoyama, T.; Yoshikawa, H.; Awaga, K. Nanohybridization of Polyoxometalate Clusters and Single-Wall Carbon Nanotubes: Applications in Molecular Cluster Batteries. Angew. Chem. Int. Ed. 2011, 50, 3471–3474.

(5)

Genovese,

M.;

Lian,

K.

Polyoxometalate

Modified

Inorganic–organic Nanocomposite Materials for Energy

ASSOCIATED CONTENT

Storage Applications: A Review. Curr. Opin. Solid State

Supporting information Other characterizations of TG-DTA, TPD-MS, N2 adsorption, rate capability, Mo K-edge XANES, V-K-edge XANES, V Kedge EXAFS, and crystallographic data are in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

Mater. Sci. 2015, 19, 126–137. (6)

Yang, H.; Song, T.; Liu, L.; Devadoss, A.; Xia, F.; Han, H.; Park, H.; Sigmund, W.; Kwon, K.; Paik, U. Polyaniline / Polyoxometalate Hybrid Nano Fi Bers as Cathode for Lithium Ion Batteries with Improved Lithium Storage

AUTHOR INFORMATION

Capacity. J. Phys. Chem. C 2013, 117, 17376–17381.

Corresponding Author [email protected] (H. Yoshikawa), [email protected] (W. Ueda)

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Sun, C.; Rajasekhara, S.; Goodenough, J. B.; Zhou, F. Monodisperse Porous LiFePO4 Microspheres for a High Power Li-Ion Battery Cathode. J. Am. Chem. Soc. 2011, 133,

Present Addresses

2132–2135.

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

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Ma, H.; Zhang, S.; Ji, W.; Tao, Z.; Chen, J. α-CuV2O6 Nanowires : Hydrothermal Synthesis and Primary Lithium

Funding Sources

Battery Application. J. Am. Chem. Soc. 2008, 130, 5361–5367.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported in part by a Grants-in-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 analysis. The synchrotron radiation experiments were performed at the BL01B1 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2016A1259).

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Hy, S.; Felix, F.; Rick, J.; Su, W.; Hwang, B. J. Direct In Situ Observation of Li2O Evolution on Li-Rich High-Capacity Cathode Material, Li[NixLi(1–2x)/3Mn(2–x)/3]O2 (0 ≤ x ≤0.5). J. Am. Chem. Soc. 2014, 136, 999–1007.

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Lu, K.; Hu, Z.; Xiang, Z.; Ma, J.; Song, B.; Zhang, J.; Ma, H. Cation Intercalation in Manganese Oxide Nanosheets: Effects on Lithium and Sodium Storage. Angew. Chem. Int. Ed. 2016, 55, 10448–10452.

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Zhao, Q.; Wang, J.; Lu, Y.; Li, Y.; Liang, G.; Chen, J.

ABBREVIATIONS

Oxocarbon Salts for Fast Rechargeable Batteries. Angew.

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