Electrode Properties of Li2MnO3 (C2/m) for a Lithium-Battery Cathode in Several Charge-Discharge Potential Ranges

Yasuhiro Nakao1, Kiyoshi Ozawa2*, Hiroki Fujii2, Takashi Mochiku2, Hideo Iwai2, Yoshinori Tsuchiya2 and Naoki Igawa3

1 Honda Engineering Co., Ltd., 6-1 Hagadai, Haga-Machi, Haga-Gun, Tochigi 321-3395, Japan 2 National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

3 Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1195, Japan * Corresponding author: Fax: 81-29-859-2301 e-mail: OZAWA.Kiyoshi@nims.go.jp

Li2MnO3 powder with a monoclinic cell related to the space group of C2/m was synthesized by a simplified coprecipitation method, followed by a combustion technique. Its electrode properties as a lithium-battery cathode were investigated in the charge-discharge potential ranges of 2.0 – 4.6, 2.0 – 4.8, and 2.0 – 5.0 V. In all the potential ranges, the electrochemical capacit ies gradually increased during cycling, and the increased capacities significantly depended on the potential ranges. Compared to the discharge capacities for the potential ranges of 2.0 – 4.6 and 2.0 – 5.0 V, the former merely increased from 12.5 to 22.5 mAhg-1 between the 1st and 99th cycles, whereas the latter increased from 20.0 to 110.2 mAhg-1 during the same cycling. The structural variation in Li2MnO3, which is induced by the cycling, also shows a fluctuation depending on the potential range, and conversion to the rhombohedral phase is detected during cycling in the potential range of 2.0 – 5.0 V. Key words: Li2MnO3, Lithium-battery cathode, Charge-discharge potential range

1. INTRODUCTION

Layered manganese-based lithium oxides, such as LiMnO2 (C2/m or Pmnm), Li2MnO3 (C2/m), and the solid solutions of Li2MnO3-LiMO2 (M = Ni and Co: R 3 m or C2/m), are an attractive material as a lithium-battery cathode because they are safer, cheaper, and less toxic than LiCoO2 or LiNiO2 and exhibit a higher operating voltage (3.5 V vs. Li/Li+ in average). However, in general, they show a poor cycle life due to the breakup of particles that occurs with the phase transition during cycling, and thus, the need for improving their capacity retention is recognized when they are used in practical applications.

It is commonly accepted that the electrochemical Li+ deinsertion in manganese-based cathode materials occurs in conjunction with the oxidation of Mn up to a maximum oxidation state of 4+. However, since the charge state of Mn in Li2MnO3 or the solid solutions of Li2MnO3-LiMO2 is already 4+, the electrochemical activity is considered to result from the enhanced Li+ deinsertion and insertion accompanying the activated manganese redox reaction (Mn3+/Mn4+) following the extraction of oxygen molecules induced during cycling.1-3 Except for such a mechanism, the proton exchange with Li+ between the active material3 and the redox reaction (O2-/O) at the electrode surface1 were also

proposed. The extraction of oxygen molecules occurs at the

electrode surface, as a result of which it is recognized that nanoparticles with a large surface area, prepared at a lower combustion temperature of 400 – 600 C, achieve enhanced electrochemical activities in the Li2MnO3 and solid solutions of Li2MnO3-LiMO2 systems.2,4 In this paper, we report the electrode properties and structural variation of Li2MnO3 as a lithium-battery cathode during cycling in the charge-discharge potential ranges of 2.0 – 4.6, 2.0 – 4.8, and 2.0 – 5.0 V. 2. EXPERIMENTAL

Li2MnO3 powder was synthesized based on a modification of a previously reported method.5 Manganese sulfate (MnSO4·5H2O) was dissolved in distilled water, added to an aqueous solution of sodium carbonate (Na2CO3) containing an equimolar amount of Na2CO3 with manganese sulfate, and stirred for 24 h. The produced precipitate in the solution was filtered with suction through a 0.2-m pore size polytetrafluoroethylene (PTFE) membrane, washed with distilled water, and dried at 110 C. The resulting precipitate was mixed with lithium hydroxide (LiOHH2O) after calcination at 500 C in air and then heated at 900 C for 12 h under flowing oxygen to obtain the Li2MnO3 powder.

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Neutron diffraction data were collected by monochronized neutron radiation at a wavelength of 1.8244 Å at room temperature using a high-resolution powder diffractometer (HRPD) at the research reactor (JRR-3M) of the Japan Atomic Energy Agency. The synthesized Li2MnO3 powder (about 5 g) was contained in a cylindrical vanadium cell (5-mm radius, 55-mm height). Based on the neutron diffraction data, the structural parameters of the Li2MnO3 were refined using the Rietveld refinement program RIETAN-FP.6 X-ray photoelectron (XPS) measurement was carried out using the PHI Quantera SXM spectrometer (ULVAC-PHI). Transmission electron microscope (TEM) measurements were conducted using a JEOL JEM4000EX TEM with an accelerating voltage of 400 kV.

The electrochemical measurements were carried out using a 2032-type coin cell. The electrolyte was a 1 M solution of LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) mixed at the volume ratio of 1: 1 (Ube Industries, Ltd., Japan). The anode was a lithium metal foil disk (11-mm diameter, 0.5-mm thickness). A porous

polypropylene film was used as the separator. The cathode consisted of an aluminum grid on which the ground Li2MnO3 powder, mixed with 15 wt% carbon black and 10 wt% PTFE powder, was pressed; these cathodes were dried at 140 C for 4 h under vacuum before placing them in the cells. The charge-discharge data were collected at current densities of 20 mAg-1 and 23 C. It must be noted that every cycling started from the charge process in this study.

3. RESULTS AND DISCUSSION

The results of the Rietveld refinement showed that the synthesized Li2MnO3 is quite crystalline and the structure is assigned to a monoclinic symmetry corresponding to the space group C2/m (z = 4).7 As shown in Fig. 1 and Table I, almost all the Bragg reflections can be indexed to the monoclinic cell, and both low R factors and good fitting between the observed and calculated patterns are acquired. It is noted that the material is characterized by a mutual substitution between the lithium and manganese ions on

Table I. Summary of Rietveld refinement for Li2MnO3 with neutron diffraction data measured at room temperature. Atom Site g x y z B/nm2 Li(1) 2b 0.73(4) 0 1/2 0 0.0049(6) Mn(1) 2b 0.27(4) 0 1/2 0 = BLi(1) Li(2) 2c 1 0 0 1/2 0.011(3) Li(3) 4h 1 0 0.659(1) 1/2 0.007(2) Mn(2) 4g 0.87(2) 0 0.1692(7) 0 = BLi(1) Li(4) 4g 0.13(2) 0 = yMn(2) 0 = BLi(1) O(1) 4i 1 0.2197(8) 0 0.2251(9) 0.0067(7) O(2) 8j 1 0.2524(6) 0.3235(3) 0.2234(5) 0.0056(4) Rwp = 11.68%, Rp = 8.71%, RR = 16.59%, RB = 3.65%, RF = 4.11%, Re = 9.43%, S = 1.24 a = 0.49329(3) nm, b = 0.85351(4) nm, c = 0.50260(3) nm, = 109.314(3), V = 0.1997(2) nm3

Figure 1 Rietveld refinement pattern of the neutron-diffraction data measured for the synthesized material. The plus marks (+) are the raw diffraction data, and the overlapped continuous line is the calculated pattern. The short vertical lines below the pattern indicate the positions of the allowed Bragg reflections. The difference between the observed and calculated intensities is shown at the bottom on the same scale.

230 Electrode Properties of Li2MnO3 (C2/m) for a Lithium-Battery Cathode in Several Charge-Discharge Potential Ranges

the ab plane with z = 0 (the LiMn2O6 layer). Figure 2 shows the cycling data measured for the Li/Li+/Li2MnO3 cell in the potential ranges of 2.0 – 4.6, 2.0 – 4.8, and 2.0 – 5.0 V, in which the charge and discharge capacities are plotted as a function of the number of cycles. It is observed that the electrochemical capacities gradually increased over all the cycles in all the charge-discharge potential ranges and the increases significantly depended on the potential range. A remarkable increase in the capacities was especially evident in the potential range of 2.0 – 5.0 V. For example, compared with the discharge capacities in the potential ranges of 2.0 – 4.6 and 2.0 – 5.0 V, the former merely increased from 12.5 to 22.5 mAhg-1 between the 1st and 99th cycles, whereas the latter increases from 20.0 to 110.2 mAhg-1 during the same cycling regime.

As described later in the TEM section, we assume that the synthesized Li2MnO3 is mostly comprised of rod-like particles with ~120-nm diameter and ~500-nm length. Such a particle size is considerably larger than the values of 70 – 90 nm reported for Li2MnO3, which shows enhanced electrochemical activities.2 Thus, we concluded that the charge-discharge potential range also plays a major role in the electrode properties of Li2MnO3. This characteristic seems to be common knowledge in the field of battery studies. However, it is noteworthy that the systematic increases in the capacities related to the charge-discharge potential ranges may be first reported in this study. In order to investigate the extraction of oxygen molecules and manganese redox reaction (Mn3+/Mn4+) during the cycling, XPS measurements were carried out for the as-prepared Li2MnO3 and cathode samples after 99 cycles in the potential ranges of 2.0 – 4.8 and 2.0 – 5.0 V; it is noteworthy that all the samples were washed with acetone and then dried at 120 C for 1 h under vacuum before the measurements. The XPS core spectra

of the Mn and O are shown in Fig. 3. For the Mn spectra (Fig. 3a), the binding energies of Mn 2p1/2 and Mn 2p3/2 in the as-prepared material are estimated to be 654.1 and 642.4 eV, respectively. These values are in close agreement with those reported for MnO2,8 suggesting the 4+ charge state of Mn in the as-prepared material. The spectral positions of both Mn 2p1/2 and Mn 2p3/2 in the cycled samples shift to lower binding energies when compared to those in the as-prepared material, and the positions for the potential range of 2.0 – 5.0 are also shown to be slightly lower than those for the potential range of 2.0 – 4.8 V. In addition, the peaks of Mn 2p1/2

and Mn 2p3/2 in the cycled samples tend to become asymmetrical due to swelling toward the lower binding energies. These observations indicated the production of the 3+ charge state of Mn during the cycling and suggest that such a production is increasingly accelerated as the maximum voltage for the charge process increases. In contrast, as shown in Fig. 3b, the spectral positions of O 1s for the cycled samples shift to a higher binding energy than that for the as-prepared material; the top position of the as-prepared material is 529.5 eV, whereas the positions of the cycled samples in the potential ranges of 2.0 – 4.8 and 2.0 – 5.0 V are 531.1 and 531.6 eV, respectively. It was reported that the spectral position of O 1s, depending on the chemical species and

Figure 2 Cycling data measured for the Li/Li+/Li2MnO3 cell in the potential ranges of 2.0 – 4.6, 2.0 – 4.8, and 2.0 – 5.0 V at the current densities of 20 mAg-1.

Figure 3 XPS core spectra of Mn (a) and O (b) for the as-prepared material (A), cycled cathode sample in the potential range of 2.0 – 4.8 V (B), and cycled cathode sample in the potential range of 2.0 – 5.0 V (C). The cycled cathode samples were obtained after 99 cycles in the respective potential ranges.

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the binding energies of the oxygen ion (O2-) in the lattice and free oxygen ions based on Li2O or Li2CO3, are 529 and 532 eV, respectively.1,9,10 Thus, it was suggested that oxygen molecules are extracted from the active material during cycling, and the extracted oxygen molecules exist at the electrode surface. In addition, such an extraction is progressively activated during the cycling with the increasing maximum voltage for the charge process, in a manner similar for the production of Mn3+. In general, it is not easy to precisely estimate the structural variation in the manganese-based cathode materials with cycling based on the powder X-ray diffraction (XRD) data alone. This is due to the decrease in the crystallinity upon cycling and the similar XRD profile between the anticipated materials. In this study, TEM diffraction measurements were carried out for the

individual particles at several electrochemical stages. Figure 4 shows the bright-field TEM images of the as-prepared Li2MnO3 and 99-cycled samples in the potential ranges of 2.0 – 4.8 and 2.0 – 5.0 V, together with the corresponding selected-area electron-diffraction patterns taken for one of the respective particles. For the as-prepared material (Fig. 4a), a striped pattern, lying perpendicular to the c axis, was observed in the particles, suggesting the existence of a lattice defect due to a stacking fault.11,12 The main spots in the diffraction pattern can be indexed to a monoclinic cell with the space group of C2/m. For the cycled sample in the potential range of 2.0 – 4.8 V (Fig. 4b), the representative particle was found to have a rod-like morphology with ~120 nm diameter and ~500 nm length. In the diffraction patterns, there are two types of patterns due to a cubic spinel (Fd3m) and a monoclinic phase (C2/m), and both phases occasionally coexist in one particle, as shown in the image. Here, diffraction spots, indicated by the white circle in the Fd3m diffraction pattern, are probably due to multi-reflections related to a localized defect in the spinel.13 For the cycled sample in the potential range of 2.0 – 5.0 V (Fig. 4c), besides the phases of Fd3m and C2/m, the diffraction pattern due to a rhombohedral structure related to -NaFeO2 (R3m) was detected.

It is known that layered lithium manganese oxides, including Li2MnO3 (C2/m), convert to a spinel phase during cycling.14-16 The TEM data showed that the transformation to the rhombohedral LiMnO2 (R3m) is also induced by the cycling when the maximum voltage for the charge process exceeds ~4.8 V. However, it was also reported that the LiMnO2 with a high symmetry R 3 m is unstable at room temperature due to a cooperative monoclinic distortion based on the Jahn-Teller activity of Mn3+.14 We consider that the formation of the rhombohedral LiMnO2 (R3m) would be related to the oxygen vacancies induced by the cycling. For the LiMnO2 with oxygen vacancies, the average crystal field in the cycled sample, formed by oxygen molecules around Mn3+ ions, must show a lower symmetry than that in the stoichiometric LiMnO2. The crystal field of the lower symmetry causes an energy splitting of two degenerated 3d orbits, and the energy of the spin system is thus lowered. In the non-degenerated state, the energy increase caused by lattice distortion cannot be compensated. Consequently, the Jahn-Teller distortion is impossible or suppressed in the cycled sample.

However, a full discussion of the mechanism proposed in this study must await further detailed structural observations, including the quantitative analysis of oxygen contents. For this purpose, we are planning to carry out in situ neutron-diffraction measurements in the Li2MnO3 system during cycling in

Figure 4 TEM images of the as-prepared material (a), 99-cycled cathode sample in the potential range of 2.0 – 4.8 V (b), and 99-cycled cathode sample in the potential range of 2.0 – 5.0 V (c). The corresponding selected-area electron-diffraction patterns are also shown. The estimated d values are as follows: d = 4.71 and 1.44 Å for 111 and 444 planes of Fd3m (b), respectively, and d = 1.44 and 4.68 Å for 110 and 003 planes of R3m (c), respectively.

232 Electrode Properties of Li2MnO3 (C2/m) for a Lithium-Battery Cathode in Several Charge-Discharge Potential Ranges

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(Received March 12, 2013; Accepted April 18, 2013)