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Reaction mechanisms of MnMoO4 for high capacity anode material of Li secondary battery
Solid State Ionics 146 (2002) 249 – 256 www.elsevier.com/locate/ssi
Reaction mechanisms of MnMoO4 for high capacity anode material of Li secondary battery Sung-Soo Kim, Seiichiro Ogura, Hiromasa Ikuta, Yoshiharu Uchimoto, Masataka Wakihara * Department of Applied Chemistry, Tokyo Institute of Techonology, 2-12-1, Ookayama, Meguro, Tokyo 152-8552, Japan Received 10 May 2001; accepted 12 August 2001
Abstract Crystalline MnMoO4 was synthesized using a conventional solid reaction method and investigated for its physical and electrochemical properties as an anode material for Li secondary battery. The reversible amount of Li insertion/removal of MnMoO4 anode during the first cycle was about 800 mA h/g, accompanied by irreversible structural transformation into amorphous material. The amorphization during the first Li insertion was investigated by structural analysis using XRD of electrode. The charge compensation during Li insertion/removal was examined by measurement of X-ray Absorption Near Edge Structure (XANES) spectroscopy. Despite its irreversible structural transformation to amorphous during the first lithiation, subsequent cycles showed a reasonable cyclability. This paper presents the electrochemical properties of MnMoO4 and discusses the mechanism underlying the Li insertion/removal process. D 2002 Elsevier Science B.V. All rights reserved. Keywords: MnMoO4; Amorphization; XANES; Li secondary battery
1. Introduction The Li-ion rechargeable batteries are considered as the most suitable power sources for portable electronic devices due to their high capacity and energy density. Generally, Li-ion rechargeable batteries consist of intercalation compounds for both cathode and anode electrode materials. One is a lithiated transition metal oxide as the cathode and the other is graphite as the anode. However, the graphite anode material commonly used in Li-ion rechargeable batteries suffers from small capacity per unit weight (about 350 mA *
Corresponding author. Tel.: +81-3-5734-2145; fax: +81-35734-2146. E-mail address: [email protected] (M. Wakihara).
h/g) and/or per unit volume due to its low density in spite of its low redox potential and good cycle life. Furthermore, due to the diffusivity, the rate capability of graphite material also needs improvement. To overcome these disadvantages, considerable amounts of attempts have been made to find out alternative anode materials, including metal oxide (MO, M = Co, Ni, Fe) [1], tin-based material [2], vanadium-based oxide materials [3,4], in place of graphite anodes. Notably, vanadium-based oxides as active materials for Li secondary battery have been studied, since the vanadium oxides have interesting characteristics from a standpoint of variety of oxidation states. Recently, several researchers including our group have described the low potential Li-insertion behavior in vanadium-based oxides such as RVO4 (R = In, Cr, Fe,
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 1 ) 0 1 0 1 3 - X
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Al, Y) [3] and MnV2O6 [4]. Molybdenum oxides should be attractive as anode material because they also have various oxidation states like vanadium. Previously, the molybdenum oxide MoO2 as anode material of the lithium rocking chair battery was proposed by Auborn and Baberio [5] more than 10 years ago, but their study became limited by experimental conditions such as poor stability of the electrolyte at low potential. In this study, we synthesized molybdenum-based oxide MnMoO4 as a new anode material and described the lithium insertion/removal behavior at low potential. The XRD measurement and X-ray absorption study of O K-edge, Mn and Mo L-edge have provided insight on the structural transformation and electrode reaction mechanism.
ered with polyethylene film and subjected to ex-situ XRD measurements in order to understand the transformation of crystal structure. Mo L-edge X-ray Absorption Near Edge Structure (XANES) of the synthesized powder during the lithiation process was measured on BL7A at UVSOR (Okazaki, Japan) with a ring energy of 750 MeV and a stored current of 70– 220 mA in a mode of total electron yields. The KTP double crystal monochromator was used. The absolute energy scale was calibrated by using literature values of Mo L23-edge in MoO3. Mn L23edge XANES and oxygen K-edge XANES were measured on BL8B1 beam line at UVSOR with ring energy of 750 MeV in a mode of total electron yield at room temperature.
3. Results and discussion 2. Experimental MnMoO4 powder sample was prepared by conventional solid reaction methods. Starting material used was MnCO3 (99.9% Soekawa chemicals) and MoO3 (99.9% Soekawa). These reagents were mixed in stoichiometric ratio in agate mortar and the mixture was heat-treated at 600 C for 24 h in air atmosphere. From here, we could get crystalline MnMoO4 powdered sample. The phase identification was carried out by powder X-ray diffractometry using Rigaku RINT2500V with CuKa radiation. The samples for the electrochemical measurement were prepared by mixing crystalline MnMoO4, acetylene black as conductive agent and polytetrafluoroethylene (PTFE) binder (55, 40 and 5 wt.%, respectively) in an agate mortar and were made in the form of film (100-mm thickness). The film was then cut into a disk form (5.4-mm diameter). Cells were fabricated by coupling this disc with lithium foil with the same area as counter-electrode using microporous polypropylene film (Celgard2400) as separator. One molar LiClO4 dissolved in ethylene carbonate (EC)/diethylene carbonate (DEC) (volume ratio = 1:1) was used as the electrolyte. The electrochemical measurement was carried out galvanostatically at various current densities at room temperature in a glove box under argon atmosphere. The cut-off voltage was set at 0.0 and 2.0 V vs. Li/Li + . The electrodes removed at different depths of lithiation during the first cycle were cov-
The crystal structure of the synthesized powder was examined by X-ray diffractometry analysis. The welldefined peak obtained confirms that the synthesized compound is MnMoO4 without any impurity phases and the JCPDS data (card number 27-1280) also provides the exact match. Fig. 1 shows the measured powder diffraction profile for MnMoO4. The schematic diagram of MnMoO4 structure is shown in Fig. 2. This structure has been known as alpha-MnMoO4, which is composed of octahedrally coordinated Mn and tetrahedrally coordinated Mo by oxygen [6]. Mn and Mo XANES measurements for the synthesized powder have been carried out to investigate the oxidation states of Mn and Mo and to verify the chemical formula. XANES results are shown in Fig. 3 with reference
Fig. 1. XRD pattern of synthesized MnMoO4.
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Fig. 2. Schematic figure of MnMoO4 unit cell.
materials that have various oxidation states. The Mn Ledge XANES shows two strong absorption features due to the spin-orbit splitting of the Mn 2p core hole. The absorption shown in Fig. 3 of about 640 –645 eV is 2p3/2 (L3) edge. In Fig. 3a, the spectra for the edge jumps for MnCO3 and MnMoO4 are very close to each other. Thus, we can assume that Mn exists as Mn2+ in MnMoO4. Mo L-edge XANES spectra of MnMoO4 and MoO3 presented in Fig. 3b illustrate that the valence of Mo in MnMoO4 should be + 6. The oxidation state derived from the XANES spectra and the well-defined XRD pattern obtained for the prepared sample confirm that the compound is stoichiometric MnMoO4. Furthermore, the spectra of Mo L-edge XANES of MnMoO4 and MoO3, which involve the transitions from the 2p core levels into the empty 4d orbital, show separation of two peaks clearly. Generally, in an octahedral coordinated metal, the d-orbitals are split into triply degenerate t2g and doubly degenerate eg orbitals. The electrons in eg orbitals are repelled more strongly by the negative charge since electrons are located along the bonding axes than those of t2g orbitals that point between the axes. Thus, the t2g orbitals lie lower in energy than the eg orbitals, whereas in tetrahedral coordination, triply degenerate t2 orbitals lie higher in energy than doubly degenerate e orbitals. The Mo atoms in MoO3 are in an octahedral environment and Fig. 3b indicates the typical 4d splitting in two sets of eg and t2g symmetry. On the other hand, the Mo atoms in MnMoO4 are in a tetrahedral environment and Fig. 3b shows ligand field
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splitting parameter (DT: energy difference of t2 and e) is about 1.6 eV. The difference in d-orbital splitting for tetrahedral and octahedral coordinations has been reported for several molybdates previously [7]. In order to obtain charge – discharge profile of MnMoO4, the cell was subjected to several cycles at constant current mode. The charge– discharge profiles obtained are depicted in Fig. 4. The initial charge capacity was about 1800 A h/kg and the reversible capacity in consequent discharge process was around 1000 A h/kg. During the first charge, the lithium intercalation process exhibited a plateau around 0.8 V vs. Li/Li+ , which is not observed in the following cycles. The X-ray diffraction patterns obtained for the fresh electrode as well as for the electrode charged up to different cut-off voltages are shown in Fig. 5. The principal peaks gradually decreased with lithiation and the peaks disappeared completely at 0.29 V vs. Li/Li+ . When the discharged electrodes were charged again, we could not observe the crystalline peaks, suggesting that an irreversible phase transformation into amorphous lithiated material occurred during the first lithium insertion. This phenomenon of amorphization during the first lithiation was reported previously in several vanadates and molybdates [1,3,4,8]. The difference between the first charge profile and the second charge curves is also indicative of the two different mechanisms operating in the lithium insertion process. Furthermore, a series of new Bragg peaks appeared between 0.5 and 0.25 V during the first lithiation and disappeared at fully lithiated electrode. The XRD patterns of electrode between 0.5 and 0.25 V together with the Bragg peak of NaCl-type compound VO are shown in Fig. 6. Although these peaks were too broad to be identified exactly, it was enough to assume that the amorphization process of MnMoO4 has an intermediate state of NaCl-type structure that has equal amounts of anion and cation by the appearance of new peaks of NaCl-type compound. The lattice constant calculated from the new Bragg peaks of NaCl-type ˚ and this compound observed at 0.29 V was 4.30 A value agrees fairly with the theoretical lattice constant ˚ ) which was calculated by of Li2MnMoO4 (4.27 A using the ionic radii of Shannon [9]. The similar behavior of vanadium redistribution with lithiation was observed in vanadate or molybdenum substituted vanadate, which was prepared electrochemically or by chemical methods [10].
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Fig. 3. (a) Mn L-edge XANES spectrum of MnMoO4 and various Mn compounds as reference. (b) Mo L-edge XANES spectrum of MnMoO4 and MoO3.
The Mo L-edge XANES spectra variation during the first Li insertion and Li removal process is shown in Figs. 7 and 8, respectively. The lines represent the
energy positions of Mo6+ and Mo0, which were determined by the measurements of Mo L-edge XANES spectrum of MoO3 in this work and metallic Mo in
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Fig. 4. Charge – discharge profile of MnMoO4/Li test cell. The point represents the potential that measured the XRD of the electrode.
previous reports [11], respectively. Fig. 7 shows that peak shift of Mo6+ to reduced state with lithium insertion into MnMoO4 and Fig. 8 shows re-oxidation of Mo to 4+ at 2.0 V vs. Li/Li+ during the lithium removal process. In Fig. 7, we can observe that the valence of Mo is 5+ in the spectrum at 0.8 V and the separated peak shape is not changed, though it is not enough to conclude that Mo remained in tetrahedral coordination. Furthermore, this result consists of XRD patterns up to 0.7 V during lithium insertion, which does not show amorphization as mentioned above (in Fig. 5). Considering the disappearance of peak sepa-
Fig. 5. XRD patterns of the electrode at various potentials during the first cycle.
ration at 0.6 V during Li insertion, it implies crystal structure transformation to amorphous. In addition to the structural transformation, multiplets of electronic structure can also be assumed for the disappearance of peak separation. In the case of low oxidation number of Mo, 4d orbitals after electron promotion by X-ray absorption have more than two electrons. Therefore, the 4d orbitals should be multiplied by the electron – electron repulsion integrals. The valence change of Mo
Fig. 6. XRD patterns of the electrode between 0.5 and 0.25 V. The diffraction pattern of VO with NaCl-type structure was shown in the bottom.
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Fig. 7. Mo L-edge XANES spectrum of MnMoO4 electrode during the first lithium insertion.
in MnMoO4 with lithium insertion by the Mo L-edge XANES measurement is presented in Fig. 9. We assumed that Mo was reduced from 6+ to 1+ or 2+
at full lithiation and oxidized to 4+ by removal of lithium according to the results of Mo L-edge XANES Notice that the presented potentials which show meas-
Fig. 8. Mo L-edge XANES spectrum of MnMoO4 electrode during lithium removal.
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Fig. 9. Variation of valence of Mo in MnMoO4 during lithium insertion/removal estimated from Mo L-edge XANES.
uring points of XANES in Fig. 9 are not open circuit voltage (OCV). Therefore, there is a difference in the valence of Mo between Li insertion and removal, since these potentials ignore the overpotential. The valence change in Mo is not enough to explain the capacity of MnMoO4. Furthermore, the state of Mn more reduced than Mn2+ is difficult to imagine. Thus, it is conceivable that there is the contribution of oxygen besides the reduction of molybdenum during lithium insertion. Previously, the role of anions in the lithium insertion/removal process was suggested in anode materials such as nitride [13] or vanadium oxide material [3]. The N K-edge EELS study, whose spectra are essentially the same as XANES, on the new anode material Li2.6Co0.4N have showed that the lithium extraction decreases the occupancy of the nitrogen 2p orbitals [14]. This means that nitrogen orbitals besides those of Co play an important role in keeping the charge balance. Accordingly, it can be expected that there are similar anion contributions to charge compensation for our material. To investigate the role of oxygen, O K-edge XANES was measured (Fig. 10). We could observe two peaks around 530 eV which can be attributed to transition to the unoccupied band derived from the mixing of the Mo 4d with oxygen 2p states [12] and a broad peak around 535 –545 eV which can be assigned to that which originated from the mixing of the Mo 5sp or Mn 4sp with O 2p orbital, respectively. The peak with a high energy side is broad, so that the peak is difficult to be divided into
two characters (Mo 5sp and Mn 4sp). The peak splitting at 530 eV can be explained by the hybridization of tetrahedral-coordinated Mo e orbital and t2 orbital with O 2p orbital and the gap of these peaks was 1.5 eV, which shows fair agreement with the result of the splitting gap (1.6 eV) of Mo e and Mo t2 orbital in Mo L-edge XANES shown in Fig. 3b. Rodriguez et al. [12] have also reported consistent results on NiMoO4 that has the same structure as alpha-MnMoO4. This indicates that the Mo 4d band strongly hybridized with the oxygen 2p band, i.e., Mo)O bond has strong covalent character. This result implies that there is contribution of oxygen, as previously suggested by Denis [3] and
Fig. 10. O K-edge XANES spectrum of MnMoO4.
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Poizot [15] through the hybridization of Mo 4d and O 2p orbitals during lithium insertion. Consequently, it can be thought that the reduction also occurs in oxygen 2p orbital and that the oxidation by lithium removal partly takes place using oxygen 2p orbital as well as Mo 4d orbital.
4. Conclusions We reported the electrochemical properties of MnMoO4 as a new type of anode material for lithium secondary battery. This high capacity anode, realized during the first lithiation ( 1800 A h/kg) and in the subsequent lithiation ( 1000 A h/kg), could be attributed to the oxygen contribution to lithium insertion by the accommodation of electrons in the hybridization orbital. During the first lithiation, amorphization, which has an intermediate state of NaCl structure, was observed in XRD measurement of the electrode. The valence of Mo and Mn in MnMoO4 measured by XANES was 6+ and 2+, respectively. The valence of Mo during lithium insertion/removal investigated by XANES is not enough to explain the Li amount that is inserted and removed into this material. Through the O K-edge XANES measurement, we can observe the role of the anion to charge compensation in the electrochemical Li insertion/removal. Although this anode material delivers high capacity, it exhibits large irreversibility and amorphization at the first cycle. It needs further intensive investigation to un-
derstand the lithium insertion mechanisms underlying it, so as to improve cycle life to make it a commercially available material. References [1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496. [2] Y. Idota, T. Kubota, A. Mastufuji, Y. Maekawa, T. Miyasaka, Science 276 (1997) 1395. [3] S. Denis, E. Baudrin, M. Touboul, J.-M. Tarascon, J. Electrochem. Soc. 144 (1997) 3886. [4] S.-S. Kim, H. Ikuta, M. Wakihara, Solid State Ionics 139 (2001) 57. [5] J.J. Auborn, Y.L. Baberio, J. Electrochem. Soc. 134 (1987) 638. [6] A. Clearfiled, A. Moini, P.R. Rudolf, Inorg. Chem. 24 (1985) 4606. [7] J.A. Rodriguez, J.C. Hanson, S. Chaturvedi, A. Maiti, J.L. Brito, J. Phys. Chem. B 104 (2000) 8145. [8] F. Leroux, G.R. Goward, W.P. Power, L.F. Nazar, Electrochem. Solid-State Lett. 1 (1998) 255. [9] R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751. [10] C. Delmas, Industrial chemistry library, in: G. Pistoia (Ed.), Lithium Batteries, vol. 5, Chap. 12, 1994, p. 475. [11] F.F. Ferreira, T.G.S. Cruz, M.C.A. Fantiti, M.H. Tabacniks, S.C. Castro, J. Morais, A. Siervo, R. Landers, A. Gorenstein, Solid State Ionics 136 – 137 (2000) 357. [12] J.A. Rodriguez, J.C. Hanson, S. Chaturvedi, A. Maiti, J.L. Brito, J. Chem. Phys. 112 (2000) 935. [13] T. Shodai, Y. Sakurai, T. Suzuki, Solid State Ionics 122 (1999) 85. [14] S. Suzuki, T. Shodai, J. Yamaki, J. Phys. Chem. Solids 59 (1998) 331. [15] P. Poizot, E. Baudrin, S. Laruelle, L. Dupont, M. Touboul, J.-M. Tarascon, Solid State Ionics 138 (2000) 31.
Reaction mechanisms of MnMoO4 for high capacity anode material of Li secondary battery Sung-Soo Kim, Seiichiro Ogura, Hiromasa Ikuta, Yoshiharu Uchimoto, Masataka Wakihara * Department of Applied Chemistry, Tokyo Institute of Techonology, 2-12-1, Ookayama, Meguro, Tokyo 152-8552, Japan Received 10 May 2001; accepted 12 August 2001
Abstract Crystalline MnMoO4 was synthesized using a conventional solid reaction method and investigated for its physical and electrochemical properties as an anode material for Li secondary battery. The reversible amount of Li insertion/removal of MnMoO4 anode during the first cycle was about 800 mA h/g, accompanied by irreversible structural transformation into amorphous material. The amorphization during the first Li insertion was investigated by structural analysis using XRD of electrode. The charge compensation during Li insertion/removal was examined by measurement of X-ray Absorption Near Edge Structure (XANES) spectroscopy. Despite its irreversible structural transformation to amorphous during the first lithiation, subsequent cycles showed a reasonable cyclability. This paper presents the electrochemical properties of MnMoO4 and discusses the mechanism underlying the Li insertion/removal process. D 2002 Elsevier Science B.V. All rights reserved. Keywords: MnMoO4; Amorphization; XANES; Li secondary battery
1. Introduction The Li-ion rechargeable batteries are considered as the most suitable power sources for portable electronic devices due to their high capacity and energy density. Generally, Li-ion rechargeable batteries consist of intercalation compounds for both cathode and anode electrode materials. One is a lithiated transition metal oxide as the cathode and the other is graphite as the anode. However, the graphite anode material commonly used in Li-ion rechargeable batteries suffers from small capacity per unit weight (about 350 mA *
Corresponding author. Tel.: +81-3-5734-2145; fax: +81-35734-2146. E-mail address: [email protected] (M. Wakihara).
h/g) and/or per unit volume due to its low density in spite of its low redox potential and good cycle life. Furthermore, due to the diffusivity, the rate capability of graphite material also needs improvement. To overcome these disadvantages, considerable amounts of attempts have been made to find out alternative anode materials, including metal oxide (MO, M = Co, Ni, Fe) [1], tin-based material [2], vanadium-based oxide materials [3,4], in place of graphite anodes. Notably, vanadium-based oxides as active materials for Li secondary battery have been studied, since the vanadium oxides have interesting characteristics from a standpoint of variety of oxidation states. Recently, several researchers including our group have described the low potential Li-insertion behavior in vanadium-based oxides such as RVO4 (R = In, Cr, Fe,
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 1 ) 0 1 0 1 3 - X
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Al, Y) [3] and MnV2O6 [4]. Molybdenum oxides should be attractive as anode material because they also have various oxidation states like vanadium. Previously, the molybdenum oxide MoO2 as anode material of the lithium rocking chair battery was proposed by Auborn and Baberio [5] more than 10 years ago, but their study became limited by experimental conditions such as poor stability of the electrolyte at low potential. In this study, we synthesized molybdenum-based oxide MnMoO4 as a new anode material and described the lithium insertion/removal behavior at low potential. The XRD measurement and X-ray absorption study of O K-edge, Mn and Mo L-edge have provided insight on the structural transformation and electrode reaction mechanism.
ered with polyethylene film and subjected to ex-situ XRD measurements in order to understand the transformation of crystal structure. Mo L-edge X-ray Absorption Near Edge Structure (XANES) of the synthesized powder during the lithiation process was measured on BL7A at UVSOR (Okazaki, Japan) with a ring energy of 750 MeV and a stored current of 70– 220 mA in a mode of total electron yields. The KTP double crystal monochromator was used. The absolute energy scale was calibrated by using literature values of Mo L23-edge in MoO3. Mn L23edge XANES and oxygen K-edge XANES were measured on BL8B1 beam line at UVSOR with ring energy of 750 MeV in a mode of total electron yield at room temperature.
3. Results and discussion 2. Experimental MnMoO4 powder sample was prepared by conventional solid reaction methods. Starting material used was MnCO3 (99.9% Soekawa chemicals) and MoO3 (99.9% Soekawa). These reagents were mixed in stoichiometric ratio in agate mortar and the mixture was heat-treated at 600 C for 24 h in air atmosphere. From here, we could get crystalline MnMoO4 powdered sample. The phase identification was carried out by powder X-ray diffractometry using Rigaku RINT2500V with CuKa radiation. The samples for the electrochemical measurement were prepared by mixing crystalline MnMoO4, acetylene black as conductive agent and polytetrafluoroethylene (PTFE) binder (55, 40 and 5 wt.%, respectively) in an agate mortar and were made in the form of film (100-mm thickness). The film was then cut into a disk form (5.4-mm diameter). Cells were fabricated by coupling this disc with lithium foil with the same area as counter-electrode using microporous polypropylene film (Celgard2400) as separator. One molar LiClO4 dissolved in ethylene carbonate (EC)/diethylene carbonate (DEC) (volume ratio = 1:1) was used as the electrolyte. The electrochemical measurement was carried out galvanostatically at various current densities at room temperature in a glove box under argon atmosphere. The cut-off voltage was set at 0.0 and 2.0 V vs. Li/Li + . The electrodes removed at different depths of lithiation during the first cycle were cov-
The crystal structure of the synthesized powder was examined by X-ray diffractometry analysis. The welldefined peak obtained confirms that the synthesized compound is MnMoO4 without any impurity phases and the JCPDS data (card number 27-1280) also provides the exact match. Fig. 1 shows the measured powder diffraction profile for MnMoO4. The schematic diagram of MnMoO4 structure is shown in Fig. 2. This structure has been known as alpha-MnMoO4, which is composed of octahedrally coordinated Mn and tetrahedrally coordinated Mo by oxygen [6]. Mn and Mo XANES measurements for the synthesized powder have been carried out to investigate the oxidation states of Mn and Mo and to verify the chemical formula. XANES results are shown in Fig. 3 with reference
Fig. 1. XRD pattern of synthesized MnMoO4.
S.-S. Kim et al. / Solid State Ionics 146 (2002) 249 – 256
Fig. 2. Schematic figure of MnMoO4 unit cell.
materials that have various oxidation states. The Mn Ledge XANES shows two strong absorption features due to the spin-orbit splitting of the Mn 2p core hole. The absorption shown in Fig. 3 of about 640 –645 eV is 2p3/2 (L3) edge. In Fig. 3a, the spectra for the edge jumps for MnCO3 and MnMoO4 are very close to each other. Thus, we can assume that Mn exists as Mn2+ in MnMoO4. Mo L-edge XANES spectra of MnMoO4 and MoO3 presented in Fig. 3b illustrate that the valence of Mo in MnMoO4 should be + 6. The oxidation state derived from the XANES spectra and the well-defined XRD pattern obtained for the prepared sample confirm that the compound is stoichiometric MnMoO4. Furthermore, the spectra of Mo L-edge XANES of MnMoO4 and MoO3, which involve the transitions from the 2p core levels into the empty 4d orbital, show separation of two peaks clearly. Generally, in an octahedral coordinated metal, the d-orbitals are split into triply degenerate t2g and doubly degenerate eg orbitals. The electrons in eg orbitals are repelled more strongly by the negative charge since electrons are located along the bonding axes than those of t2g orbitals that point between the axes. Thus, the t2g orbitals lie lower in energy than the eg orbitals, whereas in tetrahedral coordination, triply degenerate t2 orbitals lie higher in energy than doubly degenerate e orbitals. The Mo atoms in MoO3 are in an octahedral environment and Fig. 3b indicates the typical 4d splitting in two sets of eg and t2g symmetry. On the other hand, the Mo atoms in MnMoO4 are in a tetrahedral environment and Fig. 3b shows ligand field
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splitting parameter (DT: energy difference of t2 and e) is about 1.6 eV. The difference in d-orbital splitting for tetrahedral and octahedral coordinations has been reported for several molybdates previously [7]. In order to obtain charge – discharge profile of MnMoO4, the cell was subjected to several cycles at constant current mode. The charge– discharge profiles obtained are depicted in Fig. 4. The initial charge capacity was about 1800 A h/kg and the reversible capacity in consequent discharge process was around 1000 A h/kg. During the first charge, the lithium intercalation process exhibited a plateau around 0.8 V vs. Li/Li+ , which is not observed in the following cycles. The X-ray diffraction patterns obtained for the fresh electrode as well as for the electrode charged up to different cut-off voltages are shown in Fig. 5. The principal peaks gradually decreased with lithiation and the peaks disappeared completely at 0.29 V vs. Li/Li+ . When the discharged electrodes were charged again, we could not observe the crystalline peaks, suggesting that an irreversible phase transformation into amorphous lithiated material occurred during the first lithium insertion. This phenomenon of amorphization during the first lithiation was reported previously in several vanadates and molybdates [1,3,4,8]. The difference between the first charge profile and the second charge curves is also indicative of the two different mechanisms operating in the lithium insertion process. Furthermore, a series of new Bragg peaks appeared between 0.5 and 0.25 V during the first lithiation and disappeared at fully lithiated electrode. The XRD patterns of electrode between 0.5 and 0.25 V together with the Bragg peak of NaCl-type compound VO are shown in Fig. 6. Although these peaks were too broad to be identified exactly, it was enough to assume that the amorphization process of MnMoO4 has an intermediate state of NaCl-type structure that has equal amounts of anion and cation by the appearance of new peaks of NaCl-type compound. The lattice constant calculated from the new Bragg peaks of NaCl-type ˚ and this compound observed at 0.29 V was 4.30 A value agrees fairly with the theoretical lattice constant ˚ ) which was calculated by of Li2MnMoO4 (4.27 A using the ionic radii of Shannon [9]. The similar behavior of vanadium redistribution with lithiation was observed in vanadate or molybdenum substituted vanadate, which was prepared electrochemically or by chemical methods [10].
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Fig. 3. (a) Mn L-edge XANES spectrum of MnMoO4 and various Mn compounds as reference. (b) Mo L-edge XANES spectrum of MnMoO4 and MoO3.
The Mo L-edge XANES spectra variation during the first Li insertion and Li removal process is shown in Figs. 7 and 8, respectively. The lines represent the
energy positions of Mo6+ and Mo0, which were determined by the measurements of Mo L-edge XANES spectrum of MoO3 in this work and metallic Mo in
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Fig. 4. Charge – discharge profile of MnMoO4/Li test cell. The point represents the potential that measured the XRD of the electrode.
previous reports [11], respectively. Fig. 7 shows that peak shift of Mo6+ to reduced state with lithium insertion into MnMoO4 and Fig. 8 shows re-oxidation of Mo to 4+ at 2.0 V vs. Li/Li+ during the lithium removal process. In Fig. 7, we can observe that the valence of Mo is 5+ in the spectrum at 0.8 V and the separated peak shape is not changed, though it is not enough to conclude that Mo remained in tetrahedral coordination. Furthermore, this result consists of XRD patterns up to 0.7 V during lithium insertion, which does not show amorphization as mentioned above (in Fig. 5). Considering the disappearance of peak sepa-
Fig. 5. XRD patterns of the electrode at various potentials during the first cycle.
ration at 0.6 V during Li insertion, it implies crystal structure transformation to amorphous. In addition to the structural transformation, multiplets of electronic structure can also be assumed for the disappearance of peak separation. In the case of low oxidation number of Mo, 4d orbitals after electron promotion by X-ray absorption have more than two electrons. Therefore, the 4d orbitals should be multiplied by the electron – electron repulsion integrals. The valence change of Mo
Fig. 6. XRD patterns of the electrode between 0.5 and 0.25 V. The diffraction pattern of VO with NaCl-type structure was shown in the bottom.
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Fig. 7. Mo L-edge XANES spectrum of MnMoO4 electrode during the first lithium insertion.
in MnMoO4 with lithium insertion by the Mo L-edge XANES measurement is presented in Fig. 9. We assumed that Mo was reduced from 6+ to 1+ or 2+
at full lithiation and oxidized to 4+ by removal of lithium according to the results of Mo L-edge XANES Notice that the presented potentials which show meas-
Fig. 8. Mo L-edge XANES spectrum of MnMoO4 electrode during lithium removal.
S.-S. Kim et al. / Solid State Ionics 146 (2002) 249 – 256
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Fig. 9. Variation of valence of Mo in MnMoO4 during lithium insertion/removal estimated from Mo L-edge XANES.
uring points of XANES in Fig. 9 are not open circuit voltage (OCV). Therefore, there is a difference in the valence of Mo between Li insertion and removal, since these potentials ignore the overpotential. The valence change in Mo is not enough to explain the capacity of MnMoO4. Furthermore, the state of Mn more reduced than Mn2+ is difficult to imagine. Thus, it is conceivable that there is the contribution of oxygen besides the reduction of molybdenum during lithium insertion. Previously, the role of anions in the lithium insertion/removal process was suggested in anode materials such as nitride [13] or vanadium oxide material [3]. The N K-edge EELS study, whose spectra are essentially the same as XANES, on the new anode material Li2.6Co0.4N have showed that the lithium extraction decreases the occupancy of the nitrogen 2p orbitals [14]. This means that nitrogen orbitals besides those of Co play an important role in keeping the charge balance. Accordingly, it can be expected that there are similar anion contributions to charge compensation for our material. To investigate the role of oxygen, O K-edge XANES was measured (Fig. 10). We could observe two peaks around 530 eV which can be attributed to transition to the unoccupied band derived from the mixing of the Mo 4d with oxygen 2p states [12] and a broad peak around 535 –545 eV which can be assigned to that which originated from the mixing of the Mo 5sp or Mn 4sp with O 2p orbital, respectively. The peak with a high energy side is broad, so that the peak is difficult to be divided into
two characters (Mo 5sp and Mn 4sp). The peak splitting at 530 eV can be explained by the hybridization of tetrahedral-coordinated Mo e orbital and t2 orbital with O 2p orbital and the gap of these peaks was 1.5 eV, which shows fair agreement with the result of the splitting gap (1.6 eV) of Mo e and Mo t2 orbital in Mo L-edge XANES shown in Fig. 3b. Rodriguez et al. [12] have also reported consistent results on NiMoO4 that has the same structure as alpha-MnMoO4. This indicates that the Mo 4d band strongly hybridized with the oxygen 2p band, i.e., Mo)O bond has strong covalent character. This result implies that there is contribution of oxygen, as previously suggested by Denis [3] and
Fig. 10. O K-edge XANES spectrum of MnMoO4.
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Poizot [15] through the hybridization of Mo 4d and O 2p orbitals during lithium insertion. Consequently, it can be thought that the reduction also occurs in oxygen 2p orbital and that the oxidation by lithium removal partly takes place using oxygen 2p orbital as well as Mo 4d orbital.
4. Conclusions We reported the electrochemical properties of MnMoO4 as a new type of anode material for lithium secondary battery. This high capacity anode, realized during the first lithiation ( 1800 A h/kg) and in the subsequent lithiation ( 1000 A h/kg), could be attributed to the oxygen contribution to lithium insertion by the accommodation of electrons in the hybridization orbital. During the first lithiation, amorphization, which has an intermediate state of NaCl structure, was observed in XRD measurement of the electrode. The valence of Mo and Mn in MnMoO4 measured by XANES was 6+ and 2+, respectively. The valence of Mo during lithium insertion/removal investigated by XANES is not enough to explain the Li amount that is inserted and removed into this material. Through the O K-edge XANES measurement, we can observe the role of the anion to charge compensation in the electrochemical Li insertion/removal. Although this anode material delivers high capacity, it exhibits large irreversibility and amorphization at the first cycle. It needs further intensive investigation to un-
derstand the lithium insertion mechanisms underlying it, so as to improve cycle life to make it a commercially available material. References [1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496. [2] Y. Idota, T. Kubota, A. Mastufuji, Y. Maekawa, T. Miyasaka, Science 276 (1997) 1395. [3] S. Denis, E. Baudrin, M. Touboul, J.-M. Tarascon, J. Electrochem. Soc. 144 (1997) 3886. [4] S.-S. Kim, H. Ikuta, M. Wakihara, Solid State Ionics 139 (2001) 57. [5] J.J. Auborn, Y.L. Baberio, J. Electrochem. Soc. 134 (1987) 638. [6] A. Clearfiled, A. Moini, P.R. Rudolf, Inorg. Chem. 24 (1985) 4606. [7] J.A. Rodriguez, J.C. Hanson, S. Chaturvedi, A. Maiti, J.L. Brito, J. Phys. Chem. B 104 (2000) 8145. [8] F. Leroux, G.R. Goward, W.P. Power, L.F. Nazar, Electrochem. Solid-State Lett. 1 (1998) 255. [9] R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751. [10] C. Delmas, Industrial chemistry library, in: G. Pistoia (Ed.), Lithium Batteries, vol. 5, Chap. 12, 1994, p. 475. [11] F.F. Ferreira, T.G.S. Cruz, M.C.A. Fantiti, M.H. Tabacniks, S.C. Castro, J. Morais, A. Siervo, R. Landers, A. Gorenstein, Solid State Ionics 136 – 137 (2000) 357. [12] J.A. Rodriguez, J.C. Hanson, S. Chaturvedi, A. Maiti, J.L. Brito, J. Chem. Phys. 112 (2000) 935. [13] T. Shodai, Y. Sakurai, T. Suzuki, Solid State Ionics 122 (1999) 85. [14] S. Suzuki, T. Shodai, J. Yamaki, J. Phys. Chem. Solids 59 (1998) 331. [15] P. Poizot, E. Baudrin, S. Laruelle, L. Dupont, M. Touboul, J.-M. Tarascon, Solid State Ionics 138 (2000) 31.