Chemistry and Electrochemistry of Low-Temperature Manganese Oxides as Lithium Intercalation Compounds

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1 3226 Journal of The Electrochemical Society, 147 (9) (2000) Chemistry and Electrochemistry of Low-Temperature Manganese Oxides as Lithium Intercalation Compounds S. Franger, a S. Bach, a,z J. P. Pereira-Ramos, a and N. Baffier b a Centre National de la Recherche Scientifique, UMR 7582, LECSO, Thiais, France b Centre National de la Recherche Scientifique, UMR 7574, LCAES, Paris, France The synthesis of lithiated manganese oxides, Li 0.45 MnO 2, and lithiated mixed manganese cobalt oxides, Li 0.45 Mn 0.85 Co 0.15 O 2, obtained via a sol-gel process combined with an ion-exchange reaction is reported. X-ray powder diffraction patterns reveal that these compounds exhibit a tetragonal symmetry. These tetragonal phases present attractive properties as rechargeable cathode materials for secondary lithium batteries. During cycling experiments performed at discharge-charge rates of C/20, a stabilization of the specific capacity around 160 mah g 1 (0.6 Li per mol) occurs after the 40th cycle for the best compound obtained with an optimal composition of 15% in cobalt ions, Li 0.45 Mn 0.85 Co 0.15 O The Electrochemical Society. S (99) All rights reserved. Manuscript submitted December 9, 1999; revised manuscript received June 5, This was Paper 152, presented at the Honolulu, Hawaii, Meeting of the Society, October 17-22, Lithiated transition metal oxides LiMO 2 (M Co, Ni, Mn) have been extensively studied as cathode materials for commercial rechargeable lithium batteries. 1-3 Among these, lithium manganese oxides are promising candidates as cathodes in these systems due to their low cost, abundance, and nontoxicity. Two kinds of synthesis are usually reported for preparing lithiated manganese oxides. One is a synthesis via solid-state reaction. The other one is the use of low-temperature techniques, consisting of ion-exchange reactions or sol-gel processes, which give the advantage of homogeneity. Some of these lithiated manganese oxides, prepared via solidstate reactions 4-8 as the spinel oxide, LiMn 2 O 4, exhibit electrochemical characteristics strongly dependent on the synthesis temperature. 6-8 As far as the Li 1 x Mn 2 O 4 (0 < x < 1) is concerned, previous studies 3,9-11 have shown that the Jahn-Teller distortion in the spinel manganese oxide provokes a large capacity fading with cycling performed at the 3 V range. This distortion is caused by Mn 3 ions inducing a crystal symmetry change from cubic to tetragonal during lithium insertion. Manganese dissolution according to the disproportionation reaction, 2Mn 3 o Mn 4 Mn 2, and an electrochemical reaction between electrolytes and cathode materials in the charged state, have been shown to take place. 3,12-14 This explains the poor cycling behavior of Li 1 x Mn 2 O 4 in the 3 V range. Hence, a reversible but limited capacity of only 110 mah g 1 can be recovered on cycling in the 4 V range with Li 1 x Mn 2 O 4. Recently, a layered monoclinic LiMnO 2 has been obtained from the use of ion-exchange reactions, but chemical studies have emphasized the difficulty of synthesizing a single-phase material LiMnO 2 by ion exchange from -NaMnO 2. The resulting compound consists in fact of a mixture of two very close structures, a monoclinic LiMnO 2 and a tetragonal phase Li 2 Mn 2 O Moreover, electrochemical data on this compound indicate a conversion of the cathode material into a spinel-type structure on cycling with a decrease of the capacity Doping this material with cobalt to form LiMn 1 y Co y O 2 has been suggested as an alternative to LiMnO 2 since the structural instability of the latter during cycling seems to be limited in the presence of cobalt. 18,20 Another way of chimie-douce synthesis such as the sol-gel process can be used for the synthesis of this kind of compounds. Sol-gel process chemistry provides homogeneous mixing of reactants on the molecular level and can also be used to control shape, morphology, and particle size in the resulting products. Previous results obtained in our group have shown the benefit of using the sol-gel method to get new and/or high-performance cathodic materials, especially in the case of V 2 O 5 based compounds and MnO 2 oxides z Bach@glvt-cnrs.fr The major problem in developing sol-gel processes for the synthesis of manganese oxides with near MnO 2 stoichiometry is the lack of suitable Mn(IV) molecular precursors in aqueous solution. An alternative consists of the use of redox reactions between fumaric acid and aqueous permanganate solutions 22 in order to get Mn(IV) and then the building of Mn O Mn bonds to form an oxide network through polycondensation reactions. In this paper, a new way of synthesis has been developed to prepare lithiated manganese oxides through an ion-exchange procedure from the sol-gel lamellar -Na 0.7 MnO 2. In a second step, these experiments have been performed with cobalt ions in order to modify and to improve both the structure and the electrochemical properties of the lithiated manganese oxide. We prove that these new Li- (Mn) 1 x (Co) x -oxides (x 0.15) are characterized by a high specific capacity and cycle life compared to Li 1 x Mn 2 O 4 in the potential range V. Experimental The average oxidation state, Z Mn, of manganese in the sample was determined by the following procedure. 24 The sample (ca mg) was dissolved in 50 cm 3 of concentrated H 2 SO 4, 50 cm 3 H 2 O, and in presence of an excess of ferrous(ii) ammonium sulfate until complete dissolution. After cooling to C, the excess of ferrous(ii) ammonium sulfate was potentiometrically titrated with potassium permanganate. At the same time a blank was run under identical conditions. The chemical composition of the compounds was made by elemental analysis (inductively coupled plasma-mass spectroscopy, ICP-MS). From both values, x, y, and z in Li x Mn (1 y) Co y O z were calculated. X-ray diffraction experiments (XRD) were performed with an Inel diffractometer using Cu K radiation ( Å). The electrolyte used was 1 mol L 1 LiClO 4, dried under vacuum at 180 C for 15 h, dissolved in twice-distilled propylene carbonate obtained from Fluka. The working electrode consisted of a stainless steel grid (7 mm diam, 0.2 mm thick) with a geometric area of 1 cm 2 on which the cathode material was pressed (5 t/cm 2 ). The cathode was made of a mixture of active material (80 wt %), graphite (7.5 wt %), acetylene black (7.5 wt %), and Teflon as binder agent (5%). The film is obtained by mixing the oxide powder, carbon, and Teflon for 10 min for 100 mg of preparation. Electrochemical studies were carried out in two-electrode cells (Swagelok type). This cell was prepared inside the dry box by placing a clean lithium metal disk (7 mm diam), a glassfiber separator soaked with the electrolytic solution, and the cathode pellet into a Teflon container with two stainless steel terminals. Electrochemical measurements were made with a MacPile apparatus Results and Discussion The synthesis procedure of lithiated manganese oxides is summarized in Fig. 1. Three main synthesis steps are required. Each step

2 Journal of The Electrochemical Society, 147 (9) (2000) 3227 corresponds to the synthesis of the following materials: (i) sol-gel -Na 0.7 MnO 2, (ii) Na 0.45 MnO 2 0.6H 2 O, and (iii) Li 0.45 Mn 1 y Me y O 2. Synthesis of the sol-gel -Na 0.7 MnO Stable manganese oxide can be easily obtained via the reduction of permanganate NaMnO 4 according to the following reactions HO 2 CCH CHCO 2 H 2[MnO 4 ] 8[H 3 O] o 2CO 2 HO 2 C-COH 2Mn 3 13H 2 O [1] (Glycoxylic acid) Under basic conditions (ph > 7), the carbon-carbon bond cannot be cleaved, but the aldehyde function could be oxidized, leading to oxalic acid formation HO 2 C-COH 2Mn 3 3H 2 O o HO 2 C-CO 2 H 2Mn 2 2[H 3 O] [2] (Oxalic acid) Under these conditions, Mn 2 ions are not stable and could be oxidized into MnO 2, with an excess of Mn(VII). This is experimentally obtained with a molar ratio NaMnO 4 /C 4 O 4 H 4 3 3HO 2 CCH CHCO 2 H 10[MnO 4 ] 10[H 3 O] o 10MnO 2(gel) 18H 2 O 6CO 2 3HO 2 C-CO 2 H [3] and with the Na counterion to the overall reaction 3HO 2 CCH CHCO 2 H 10NaMnO 4 O 10MnO 2(gel) 4H 2 O 6CO 2 3NaO 2 C-CO 2 Na 4NaOH [4] This exothermic reaction produces a reddish sol which becomes quickly a brown gel. It could be suggested that the gel matrix consists of manganese oxide cross-linked by a network of partially oxidized fumaric acid fragments as oxalic acid. 18,19 The mean oxidation state of Mn is around 4. The Na counterion remains trapped inside the gel network. After air drying at room temperature of the gel, the amorphous MnO 2 xerogel is prepared. Calcination of the resulting brown xerogel produced a fine brown/ black powder. The formation of the crystalline layered sodium ternary oxide, -Na 0.7 MnO 2, is observed at 600 C (Fig. 2a). This compound consists of layers of edge-sharing MnO 6 octahedra, with Na atoms located between these MnO 6 layers. The distance between two consecutive MnO 6 layers is 5.5 Å. The important preferred orientation corresponds to the stacking of the layers parallel to the (a, b) plane (Fig. 2a). At 600 C, only 70% of sodium ions have reacted with the manganese oxide, the remaining sodium ions being adsorbed at the surface of the compound. Synthesis of Na 0.45 MnO 2 0.6H 2 O. 25 Figure 2b shows the XRD pattern changes during the water washing treatment of -Na 0.7 MnO 2. Rapidly, the diffraction line corresponding to -Na 0.7 MnO 2 (d 5.5 Å) decreases in its intensity with a constant increase in intensities of new diffraction lines, d 7.02, 3.53, 2.41, and 2.15 Å (Fig. 2b). After 1 h, the reaction is over and leads to the final product Na 0.45 MnO 2 nh 2 O characterized by this new system of diffraction lines. This new system (d 7.02, 3.53, 2.41, and 2.15 Å) corresponds to the signature features of synthetic birnessite materials. After filtration, the remaining solid was washed with water and dried at 60 C. The water content of the product is about 0.6H 2 O. Figure 1. Schematic synthesis procedure. Figure 2. (a) Schematic lamellar structure of -Na 0.7 MnO 2 ; (b) evolution of the XRD patterns (Cu K ) of the sol-gel -Na 0.7 MnO 2 as a function of the time of washing treatment; (c) schematic lamellar structure of Na 0.45 MnO 2 0.6H 2 O.

3 3228 Journal of The Electrochemical Society, 147 (9) (2000) The peak corresponding to a d value of 7 Å is particularly significant, since it indicates one single layer of water between octahedral sheets. The partial removing of Na ions from the interlayer spacing and the accommodation of water molecules between MnO 2 layers are responsible for the increase of the distance between two successive slabs (Fig. 2c). Synthesis of Li x Mn 1 y Me y O 2. A quantitative ion-exchange reaction was carried out by refluxing Na 0.45 MnO 2 0.6H 2 O (5 g) for 2 days in aqueous solution containing a large excess of lithium hydroxide (1 mol L 1 ) with or without a metallic cation (Me Co 2 from Co(CH 3 CO 2 ) 2 4H 2 O). After cooling and filtration, the remaining solid was washed with water and dried at 60 C. The same water content as that found in Na 0.45 MnO 2 0.6H 2 O is observed. The anhydrous final compound is obtained after a thermal treatment at 300 C for 5 h and corresponds to the following formula: Li 0.45 MnO 2. The XRD of the sample Li 0.45 MnO 2 0.6H 2 O obtained after heattreatment at 60 C (Fig. 3a) indicates a mixture of two phases, three diffraction lines, 002: d 7.02 Å, 100: d 4.84 Å (shoulder), 004: d 3.53 Å can be indexed on the basis of a hexagonal cell of birnessite with the parameters a 5.60 Å and c Å while the other diffraction peaks can be ascribed to a tetragonal phase with a Å and c Å. After removal of an interlayer of water at 300 C, the 00l diffraction lines (002: d 7.02 Å and 004: d 3.53 Å) disappear, and only the 100 line (d 4.84 Å) typical of a disordered birnessite remains. At the same time, all the other peaks due to the tetragonal lattice are unchanged except their intensities, which increase with temperature (Fig. 3b and c). In fact, the diffraction lines corresponding to a d value of 4.84 Å can belong to two different systems, the birnessite characterized by a hexagonal unit cell and the monoclinic LiMnO 2. However, the two additional lines at d 7.02 and 3.53 Å belonging unambiguously to the birnessite system tend to show the presence of birnessite in samples heat-treated at 60 and 300 C. At 600 C (Fig. 3c), the XRD pattern can be indexed on the basis of a tetragonal unit cell (I4 1 /amd) with the lattice parameters a Å and c Å (Fig. 3c). The XRD pattern of Li 0.45 Mn 0.85 Co 0.15 O 2 is found to be close to that of Li 0.45 MnO 2 and can also be indexed on a tetragonal unit cell with the parameters a Å and c Å (Fig. 3d). It can then be suggested that Mn 3 ions are probably substituted by Co 3 ions in MnO 2 layers during the ion-exchange procedure. The solids henceforth are referred to as samples (A) Li 0.45 MnO 2 prepared at 300 C, (B) Li 0.45 MnO 2 prepared at 600 C, and (C) Li 0.45 Mn 0.85 Co 0.15 O 2 prepared at 300 C. Electrochemical studies. The electrochemical behavior of the compounds A and B was examined in a 1 mol L 1 LiClO 4 in propylene carbonate (Fig. 4a and b). Chronopotentiometric curves for the reduction-oxidation processes at low current density (C/20 rate) clearly demonstrate that the total faradaic balance (x 0.5 Li per mol) corresponds to the reduction of most of the Mn 4 (ca. 90%, Z Mn 3.55) available in the initial tetragonal structure. For sample A, only one Li insertion process appears at around 2.9 V (Fig. 4a). With a cutoff voltage of 4.2 V for the charge process, only 0.5 Li ions can be extracted from the host lattice. The discharge curve of sample B exhibits two steps (Fig. 4b). A flat voltage plateau at 2.7 V, then a voltage decrease from 2.8 to 2.3 V followed by a second insertion step near 2.2 V with the additional accommodation of ca. 0.2 Li ions/mno 2. This latter region has been also reported by other groups 19,20 for nonstoichiometric spinel oxides Li 2 O ymno 2 (y 2.5). This step never appears during the oxidation process. During the charge, a larger faradaic balance of 0.65 Li per mol is recovered. This means that about 0.15 additional Mn 3 are oxidized at around 4 V with the further extraction of about 0.15 Li ions from Li 0.45 MnO 2 in a potential range where Li extraction (0 < x < 1) from 8a sites of the spinel Li 1 x Mn 2 O 4 occurs. 3,6-8 Regarding the second discharge curve, the electrochemical behavior of sample A and B is found to be slightly improved in terms of specific capacity corresponding to a Li uptake of 0.48 and 0.50 Li ions for the samples A and B, respectively (Fig 4a and b). Figure 3. Evolution of the XRD patterns (Cu K ) of lithium manganese oxide: (a) Li 0.45 MnO 2 0.6H 2 O at 60 C, (b) Li 0.45 MnO 2 at 300 C/5 h, (c) Li 0.45 MnO 2 at 600 C/5 h, and (d) Li 0.45 Mn 0.85 Co 0.15 O 2 at 300 C/5 h. Figure 4. (a) Chronopotentiometric curves for the two first reduction/oxidation cycles at low current density (C/20 rate) of Li 0.45 MnO 2 heat-treated at 300 C (sample A) in a 1 mol L 1 LiClO 4 /propylene carbonate. (b) Chronopotentiometric curves for the two first reduction/oxidation cycles at low current density (C/20 rate) of Li 0.45 MnO 2 heat-treated at 600 C (sample B) in a 1 mol L 1 LiClO 4 /propylene carbonate. (c) Evolution of the specific capacity and the faradaic yield as a function of the number of cycles of Li 0.45 MnO 2 heat-treated at ( ) 300 and ( ) 600 C. (cycling limits V, C/20.)

4 Journal of The Electrochemical Society, 147 (9) (2000) 3229 Figure 6. (a) Chronopotentiometric curves at low current density (C/20 rate) of Li 0.45 Mn 0.85 Co 0.15 O 2 heat-treated at 300 C in a 1 mol L 1 LiClO 4 /propylene carbonate. (b) Evolution of the specific capacity and the faradaic yield as a function of the number of cycles of Li 0.45 MnO 2 heat-treated at 300 C [cycling limits V, ( ) C/20] and Li 0.55 Mn 0.85 Co 0.15 O 2 heat-treated at 300 C [cycling limits V, ( ) C/20 and ( ) C/5]. Figure 5. (a) Evolution of the specific capacity and the faradaic yield as a function of the number of cycles of Li 0.45 MnO 2 heat-treated at ( ) 300 and ( ) 600 C. (Cycling limits V, C/20). (b) Voltage profiles of the first and fortieth cycles (cycling limits V, C/20) of Li 0.45 MnO 2 heat-treated at 600 C. (c) Voltage profiles of the first and fortieth cycles (cycling limits V, C/20) of Li 0.45 MnO 2 heat-treated at 300 C. The cycling behavior of both compounds in the potential window V is summarized Fig. 4c. A fast and continuous decrease of the specific capacity appears. The best results are obtained for compound A with 65% of the initial capacity, i.e., 100 mah g 1 recovered after the 40th cycle. When the low cutoff voltage is adjusted to 2.5 instead of 2 V, both compounds show a better capacity retention and deliver, respectively, 120 Ah kg 1 for sample B and around 130 Ah kg 1 for the other compound (Fig. 5a) after 40 cycles. This stable specific capacity contrasts with the results reported in the voltage range V. This can be explained by the detrimental influence of a larger amount of Mn 3 ions (Jahn-Teller ions) introduced when a cutoff voltage of 2 V is used. The change observed in the voltage profiles between the first and fortieth cycles suggests a partial and progressive transformation from tetragonal in cubic spinel symmetry (Fig. 5b and c) with two reduction steps at 4 and around 3 V at the 40th cycle. This trend also appears in the evolution of the discharge profiles of sample B (Fig. 5b). This phenomenon is less marked for sample A (Fig. 5c). However, in all cases, Fig. 5a shows that a stable capacity is achieved with a capacity value of around 120 mah g 1 in the voltage range V, including the potential window used for the spinel compounds Li 1 x Mn 2 O 4 and Li 1 x Mn 2 O 4. Figure 6 presents the best results obtained for the optimal composition of 15% for cobalt ions in Li 0.45 Mn 0.85 Co 0.15 O 2 (sample C) in the potential window V. 26 The material is first charged to 4.2 V, allowing the extraction of 0.15 Li ions at around 4 V (Fig. 6a). A first and small insertion step near 3.7 V occurs before an S-shaped reduction curve near 2.75 V. The overall Li uptake is around 0.6 Li per mole of oxide. After the first cycle, Li insertion takes place in Li 0.3 Mn 0.85 Co 0.15 O 2 with an initial capacity of 170 mah g 1 at C/20 rate. The capacity retention is satisfactory with a remarkable improvement by around 25% compared with the undoped oxide of the specific capacity with a stable value of 160 mah g 1 after 40 cycles (Fig. 6b). Similar tests performed at a higher discharge-charge rate, C/5 instead of C/20, lead to very close behavior (Fig. 6b), showing the attractive properties of sample C both in terms of cycle life and rate capability. These results make the Li 0.45 Mn 0.85 Co 0.15 O 2 material one of the best rechargeable lithiated manganese oxide cathode materials with the compound LiMn 0.9 Co 0.1 O 2 of P. G. Bruce et al., 18 also characterized by a high specific capacity but using a larger potential window (4.8/2.6 V). A deeper investigation of structural, morphological, and electrochemical properties is required in order to understand fully the details of the electrochemical process. Acknowledgment Financial support by the CEA/CEREM is gratefully acknowledged. Centre National de la Recherche Scientifique assisted in meeting the publication costs of this article. References 1. T. Ohzuku, A. Ueda, M. Nagayama, Y. Iwakoshi, and H. Komori, Electrochim. Acta, 38, 1159 (1993). 2. M. Broussely, J. P. Planchat, G. Rigobert, D. Virey, and G. Sarre, J. Power Sources, 68, 8 (1997). 3. R. J. Gummow, A de Kock, and M. M. Thackeray, Solid State Ionics, 69, 59 (1994). 4. J. N. Reimers, E. W. Fuller, E. Rossen, and J. R. Dahn, J. Electrochem. Soc., 140, 3396 (1993). 5. I. J. Davidson, R. S. McMillian, J. J. Murray, and J. E. Greedan, J. Power Sources, 54, 232 (1995). 6. J. M. Tarascon, W. R. McKinnon, F. Coowar, T. N. Bowmer, G. Amatucci, and D. Guyomard, J. Electrochem. Soc., 141, 1421 (1994).

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