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Electrochemical properties of orthorhombic LiMnO2 prepared by one-step middle-temperature solid-state reaction
Journal of Alloys and Compounds 346 (2002) 255–259
L
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Electrochemical properties of orthorhombic LiMnO 2 prepared by one-step middle-temperature solid-state reaction Z.P. Guo*, G.X. Wang, K. Konstantinov, H.K. Liu, S.X. Dou Institute for Superconducting and Electronic Materials, University of Wollongong, Wollogong, NSW 2522, Australia Received 8 March 2002; accepted 22 March 2002
Abstract Orthorhombic LiMnO 2 with small grain size was successfully synthesised using a one-step middle-temperature solid-state reaction (OSSS). Compared with the methods other researchers used, the OSSS method is much simpler. It is a one-step method without intermediate regrinding or other treatments. The o-LiMnO 2 obtained showed a high initial capacity 180 mAh / g when cycled at a current density of 0.5 mA / cm 2 at room temperature, and the rate capability was improved due to the smaller particle size. Therefore, the one-step middle temperature solid-state reaction could be a promising method for o-LiMnO 2 compound synthesis. 2002 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Electrode materials; Solid state reactions; Electrochemical reactions
1. Introduction The layered oxide materials LiMO 2 (M5Co, Ni, Mn . . . ) and the LiMn 2 O 4 spinel are the most widely studied cathode materials for lithium secondary batteries with high energy density [1–3]. The Mn-based materials have attracted wide attention as intercalation cathode materials because of their low cost and nontoxicity. However, the LiMn 2 O 4 presents a significant capacity loss when cycled in the 3 and 4 V region [4,5]. The onset of a collective Jahn–Teller distortion and the consequent structural instability that occurs when Li x Mn 2 O 4 is discharged to an average Mn valence ,3.5 (i.e. when x.1) is generally believed to be responsible for the rapid capacity loss when discharge reaches the 3 V plateau [6]. Orthorhombic LiMnO 2 (space group Pmnm, hereafter referred to as o-LiMnO 2 ) shows better cyclability than LiMn 2 O 4 when both the 4 and 3 V plateaus are utilised [7,8]. Orthorhombic LiMnO 2 of the ordered rock salt structure described by Johnston and Keikes [9] and Hoppe et al. [10] has been studied by many research groups [11–15]. It has a capacity in the range of 30–250 mAh / g depending on the synthetic method and sintering temperature [11–20]. From a review of previous studies, we have found the following problems. The first was the complexity of the *Corresponding author. Fax: 161-2-4221-5731. E-mail address: [email protected] (Z.P. Guo).
synthetic process. For low temperature synthesis, most groups used an excess amount of lithium salt of the lithium / sodium exchange reaction to form the homogeneous LiMnO 2 phase. It requires a long reaction time and other reaction steps. Even for high temperature synthesis, very sensitive synthetic conditions and some treatments to improve the reaction between the starting materials are needed. Second, o-LiMnO 2 , which was synthesised at high temperature, needed considerable time to reach the maximum discharge capacity at room temperature. Although it critically depends on current density and the cycle test conditions, this indication is not desirable so far as use of this cathode material for lithium secondary batteries is concerned. Finally, although the o-LiMnO 2 synthesised at low temperature has a high capacity, significant capacity fade is reported upon cycling over both the 4 and 3 V plateaus. On the other hand, Croguennec et al. have reported that active material having a small grain size ,1 mm and stacking faults induced a moderately enhanced discharge capacity up to 200 mAh / g [18]. According to their crystal structure analysis, the intercalation of the LiMnO 2 compound takes place primarily on the grain surface, and small grain size can induce rapid lithium diffusion between the bulk and its surface. Therefore, if an o-LiMnO 2 phase with small grain size and with moderate stacking faults is successfully synthesised, specific discharge capacity and cycle performance would probably be improved.
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00498-X
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Based on our previous research [21], we successfully synthesised MT-o-LiMnO 2 with a small grain size (,1 mm) using a one-step middle-temperature solid-state reaction (OSSS). Compared with the method other researchers used, the OSSS method is much simpler. We emphasise that the LiMnO 2 in this study was synthesised by a one-step method without intermediate regrinding or other treatments. The characteristics and electrochemical performance of the samples obtained were investigated and were compared with that of o-LiMnO 2 prepared by high temperature solid-state reaction (In this method, Li 2 CO 3 and Mn 2 O 3 were used as starting materials, and the mixture were first calcined for 3 h at 650 8C, then fired at 1000 8C for a further 24 h. The samples were reground between firings).
2. Experimental The o-LiMnO 2 material was synthesised using Mn(CH 3 COO) 2 ?4H 2 O (Aldrich 99.5%), LiOH?H 2 O (Aldrich 99.9%) and citric acid (Aldrich 99%) as starting materials. The Li:Mn atomic ratio was 1.05:1, higher than the stoichiometric ratio of 1:1 to offset lithium loss by Li 2 O evaporation during o-LiMnO 2 synthesis. These powders were mixed and thoroughly ground in an agate mortar. The mixture was loaded into alumina crucibles and calcined in a tube furnace at 800 8C for 12 h under a flow of argon. Powder X-ray diffraction (1730 X-ray diffractometer) using CuKa radiation was employed to identify the crystalline phase of the synthesised materials. The lithium and manganese concentrations in the resulting materials were analysed using an inductively coupled plasma spectrometer (ICP). The particle morphologies of the resulting
compound was observed using a scanning electron microscope (SEM). The electrochemical characterisations were performed using PTFE cells. The cathode was prepared by mixing o-LiMnO 2 powder with 10 wt.% carbon black and 5 wt.% PVDF (polyvinylidene fluoride) solution. The o-LiMnO 2 and carbon black powders were first added to a solution of PVDF in N-methyl-2-pyrrolidinone (NMP) to make a slurry with appropriate viscosity. Al foil was then used to coat the mixture. After the electrode was dried at 140 8C for 2 h in vacuum, it was compressed at a rate of about 150 kg / cm 2 . PTFE test cells were assembled in an argon-filled glove box, where the counter electrode was Li metal and the electrolyte was 1 M LiPF 6 dissolved in a 50:50 (v / v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). These cells were cycled between 2.0 and 4.4 V at room temperature to measure the electrochemical response.
3. Results and discussion
3.1. Crystal structure The XRD patterns of the two types of o-LiMnO 2 compound are shown in Fig. 1. All XRD peaks except for the marked peaks (attributed to the small amount of Li 2 MnO 3 ), are indexed on the basis of the orthorhombic phase with the space group Pmnm. Compared with the o-LiMnO 2 prepared by high temperature solid-state reaction (HT-o-LiMnO 2 ), some XRD peaks of o-LiMnO 2 prepared by the one-step middle-temperature solid-state reaction (MT-o-LiMnO 2 ), broadened remarkably and / or were characterised by a significant asymmetry, while some lines remained unchanged. A full width at half-maximum
Fig. 1. X-ray diffraction patterns of o-LiMnO 2 materials. Above: HT-o-LiMnO 2 ; below: MT-o-LiMnO 2 .
Z.P. Guo et al. / Journal of Alloys and Compounds 346 (2002) 255–259
(FWHM) in the (011) peak for HT-o-LiMnO 2 was 0.18, indicating that the HT-o-LiMnO 2 material is in a wellordered orthorhombic structure, while MT-o-LiMnO 2 showed an increased FWHM to 0.1768, indicating that the density of stacking faults has been increased, i.e. structural disorder [18]. This behaviour can also be observed in the increased broadening of the (120), (122) peaks. Croguennec et al. proposed that the smaller the crystallites / crystals, the higher the number of stacking faults [18]. Therefore, we can assume that the grain size of MT-o-LiMnO 2 is smaller than that of HT-o-LiMnO 2 . To confirm this assumption, the particle morphologies (Fig. 2) of the compounds were observed using a SEM. A SEM micrograph of the HT-o-LiMnO 2 compound is presented in Fig. 2a and compared with the MT-o-LiMnO 2 compound (Fig. 2b). The HT-o-LiMnO 2 compound has flake-shaped particles with an average size of 2–15 mm, whereas the MT-o-LiMnO 2 compound has similar spherical-shape
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crystals. The particle size of MT-o-LiMnO 2 (0.3 mm) is much smaller than that of HT-o-LiMnO 2 . Other phenomena observed from XRD patterns are a decrease in the Li 2 MnO 3 impurity phase when using the middle-temperature solid-state reaction. This is believed to be due to the existence of organic groups (citric acid and CH 3 COO 2) in the reactant. These organic groups could react during sintering with free oxygen (possibly present in small quantities due to inadequate furnace sealing) and yield CO 2 , thus decreasing the density of oxygen and thus reducing the impurity phase Li 2 MnO 3 in the MT-oLiMnO 2 . Tang et al. reported that the transformation of o-LiMnO 2 in a mixture of LiMn 2 O 4 and Li 2 MnO 3 was accompanied by the absorption of oxygen gas during the calcination process (3LiMnO 2 11 / 2O 2 →Li 2 MnO 3 1 LiMn 2 O 4 ) [19]. The firing of citric acid could also lead to the temperature increasing very quickly in some areas, thereby increasing the lithium evaporation and loss. An ICP measurement verified this. The Li:Mn ratios in MT-oLiMnO 2 and HT-o-LiMnO 2 , are 96% and 99%, respectively. The lower Li:Mn ratio may be another reason for the smaller particle size in MT-o-LiMnO 2, as Croguennec et al. believed that an excess of LiOH can act as a flux agent favouring crystal growth. The preparation methods also affect the overall constants of o-LiMnO 2 . The lattice constants a, b and c for HT-o-LiMnO 2 were calculated to be 2.806, 5.758 and ˚ respectively, while those of the MT-o-LiMnO 2 4.574 A, ˚ respectively. were 2.800, 5.766 and 4.582 A,
3.2. Electrochemical characteristics
Fig. 2. (a) SEM image of HT-o-LiMnO 2 material. (b) SEM image of MT-o-LiMnO 2 material.
PTFE cells containing o-LiMnO 2 materials were cycled at room temperature. The cells were cycled in a voltage window of 2.0–4.4 V at a constant current density of 0.5 mA / cm 2 . The first charge curves are shown in Fig. 3. The two types of o-LiMnO 2 exhibited a slightly different charging behaviour in the first charge. As shown in Fig. 3, the HT-o-LiMnO 2 electrode was quickly charged from OCV to 3.7 V and then followed an ascending plateau between 3.7 and 4.4 V. For the MT-o-LiMnO 2 electrode, charging the voltage from OCV to 3.7 V was relatively slower, and the ascending plateau between 3.7 and 4.4 V was lower. The first charging process consisted in removing Li from o-LiMnO 2 . The plateau recorded is characteristic of a phase transition, as expected from the change in the Mn electronic configuration (d3 instead of d4 in the pristine phase) and from the removal of the Li ions from the structure. It is obvious that the Li in MT-o-LiMnO 2 is easier to remove because the charging plateau is lower. This confirmed the theory of Crogunnec et al., i.e. the smaller the sample crystal / crystallites, the more Li can be removed [18]. Fig. 4 shows the evolution of the discharge specific capacity vs. the number of cycles. Following the phase transition occurring at the first charge, several cycles
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Fig. 3. Galvanostatic mode first charge of o-LiMnO 2 samples. (1) HT-o-LiMnO 2 ; (2) MT-o-LiMnO 2 .
appear necessary for the materials to reach their optimal capacity, up to 40 cycles at a current density of 0.5 mA / cm 2 in the case of the HT-o-LiMnO 2 compounds. In contrast, the full capacity is obtained after only 3–4 cycles for the MT-o-LiMnO 2 materials. The initial capacity is 180 mAh / g at room temperature. As can be seen, the need for fewer cycles to reach the sample highest capacity is correlated with the sample crystal / crystallite size. This is likely to be correlated with kinetic reasons. Since the crystal size of HT-o-LiMnO 2 is large, the inner area of the crystal may not be charged, although the surface area has already reacted. This means that the HT-o-LiMnO 2 samples may contain a substantial amount of unreacted oLiMnO 2 phase, probably in the crystal core, i.e. covered by a spinel-type phase present at the crystal surface. Experiments were then carried out at different rates (C /n with n . 10) in order to compare the reversible capacity
changes with n (Fig. 5). (Fresh cells were cycled until the capacity reached a steady-state value first, then cells were cycled at different rates.) In agreement with a kinetic limitation, the capacity gain is more important for the HT-o-LiMnO 2 material than for MT-o-LiMnO 2 at low rates. For MT-o-LiMnO 2 , the specific capacity is about 81% of the best at C / 10, whereas it is only 48% for HT-o-LiMnO 2 . A slow discharge rate allows for the formation of the material in fewer cycles. All these results prove that the rate capability is better for the smaller crystal materials (MT-o-LiMnO 2 ). Therefore, we succeeded in synthesising a new type of o-LiMnO 2 material by a one-step middle temperature solid-state reaction with an excellent cycle performance as well as a high initial discharge capacity at room temperature. Further work is now in progress to test the high temperature performance and more clearly reveal the
Fig. 4. Discharge specific capacity vs. cycle number of o-LiMnO 2 samples cycled at 0.5 mA / cm 2 in the 4.4–2.0 V range.
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Fig. 5. Changes of normalised discharge capacity (C /Cmax ) for cells with active electrodes of o-LiMnO 2 at room temperature with a varying charge–discharge rate.
mechanism of the reaction. These results will be reported elsewhere.
4. Conclusion Orthorhombic LiMnO 2 was synthesised using Mn(CH 3 COO) 2 ?4H 2 O, LiOH?H 2 O and citric acid as starting materials at 800 8C in an argon atmosphere. Compared with the HT-o-LiMnO 2 , some of the XRD peaks of MT-o-LiMnO 2 broadened remarkably and / or were characterised by a significant asymmetry, indicating that the density of stacking faults had been increased, i.e. structural disorder increased. MT-o-LiMnO 2 shows slightly larger b and c values compared with HT-o-LiMnO 2 . MT-o-LiMnO 2 also showed a high initial capacity—180 mAh / g when cycled at a current density of 0.5 mA / cm 2 at room temperature, and the rate capability was improved due to the smaller particle size. Therefore, the one-step middle temperature solid-state reaction could be a promising method for o-LiMnO 2 compound synthesis.
References [1] K. Mizushima, P.C. Jones, P.J. Wiseman, J.B. Goodenough, Mater. Res. Bull. 15 (1980) 783.
[2] J.R. Dahn, U. von Sacken, C.A. Michel, Solid State Ionics 44 (1990) 87. [3] D. Guyomard, J.M. Tarascon, Solid State Ionics 69 (1994) 222. [4] J.M. Tarascon, E. Wang, F.K. Sehokoohi, J. Electrochem. Soc. 138 (1991) 2859. [5] J. Baker, R. Koksbang, M.Y. Saidi, Solid State Ionics 82 (1995) 143. [6] M.M. Thackeray, P.G. David, P.G. Bruce, J.B. Goodenough, Mater. Res. Bull. 18 (1983) 461. [7] R.J. Gummow, D.C. Liles, M.M. Thackeray, Mater. Res. Bull. 28 (1993) 1249. [8] A.R. Amstrong, P.G. Bruce, Nature 381 (1996) 499. [9] W.D. Johnston, R.R. Keikes, J. Am. Chem. Soc. 78 (1956) 3255. [10] R. Hoppe, G. Brachtel, M. Jansen, Z. Anorg. Allg. Chem. 417 (1975) 1. [11] T. Ohzuku, A. Ueda, T. Hirai, Chem. Express 7 (1992) 193. [12] J.N. Reimers, E.W. Fuller, E. Rossen, J.R. Dahn, J. Electrochem. Soc. 140 (1993) 396. [13] S.T. Myung, S. Komaba, N. Kumagai, Chem. Lett. 1 (2001) 80. [14] P. Strobel, J.P. Levy, J. Cryst. Growth 66 (1984) 257. [15] R.J. Gummow, M.M. Thackeray, J. Electrochem. Soc. 141 (1994) 1178. [16] L. Croguennec, P. Deniard, R. Brec, A. lecerf, J. Mater. Chem. 5 (1995) 1919. [17] L. Croguennec, P. Deniard, R. Brec, P. Biensan, M. Broussely, Solid State Ionics 89 (1996) 197. [18] L. Croguennec, P. Deniard, R. Brec, J. Electrochem. Soc. 144 (1997) 3323. [19] W. Tang, H. Kanoh, K. Ooi, J. Solid State Chem. 142 (1999) 19. [20] I.J. Davidson, R.S. McMillan, J.J. Murray, J.E. Greedan, J. Power Sources 54 (1995) 232. [21] Xia Xi, Guo Zaiping, Nuli Yanna, J. New Mat. Electrochem. Systems, 3 (2000) 327.
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Electrochemical properties of orthorhombic LiMnO 2 prepared by one-step middle-temperature solid-state reaction Z.P. Guo*, G.X. Wang, K. Konstantinov, H.K. Liu, S.X. Dou Institute for Superconducting and Electronic Materials, University of Wollongong, Wollogong, NSW 2522, Australia Received 8 March 2002; accepted 22 March 2002
Abstract Orthorhombic LiMnO 2 with small grain size was successfully synthesised using a one-step middle-temperature solid-state reaction (OSSS). Compared with the methods other researchers used, the OSSS method is much simpler. It is a one-step method without intermediate regrinding or other treatments. The o-LiMnO 2 obtained showed a high initial capacity 180 mAh / g when cycled at a current density of 0.5 mA / cm 2 at room temperature, and the rate capability was improved due to the smaller particle size. Therefore, the one-step middle temperature solid-state reaction could be a promising method for o-LiMnO 2 compound synthesis. 2002 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Electrode materials; Solid state reactions; Electrochemical reactions
1. Introduction The layered oxide materials LiMO 2 (M5Co, Ni, Mn . . . ) and the LiMn 2 O 4 spinel are the most widely studied cathode materials for lithium secondary batteries with high energy density [1–3]. The Mn-based materials have attracted wide attention as intercalation cathode materials because of their low cost and nontoxicity. However, the LiMn 2 O 4 presents a significant capacity loss when cycled in the 3 and 4 V region [4,5]. The onset of a collective Jahn–Teller distortion and the consequent structural instability that occurs when Li x Mn 2 O 4 is discharged to an average Mn valence ,3.5 (i.e. when x.1) is generally believed to be responsible for the rapid capacity loss when discharge reaches the 3 V plateau [6]. Orthorhombic LiMnO 2 (space group Pmnm, hereafter referred to as o-LiMnO 2 ) shows better cyclability than LiMn 2 O 4 when both the 4 and 3 V plateaus are utilised [7,8]. Orthorhombic LiMnO 2 of the ordered rock salt structure described by Johnston and Keikes [9] and Hoppe et al. [10] has been studied by many research groups [11–15]. It has a capacity in the range of 30–250 mAh / g depending on the synthetic method and sintering temperature [11–20]. From a review of previous studies, we have found the following problems. The first was the complexity of the *Corresponding author. Fax: 161-2-4221-5731. E-mail address: [email protected] (Z.P. Guo).
synthetic process. For low temperature synthesis, most groups used an excess amount of lithium salt of the lithium / sodium exchange reaction to form the homogeneous LiMnO 2 phase. It requires a long reaction time and other reaction steps. Even for high temperature synthesis, very sensitive synthetic conditions and some treatments to improve the reaction between the starting materials are needed. Second, o-LiMnO 2 , which was synthesised at high temperature, needed considerable time to reach the maximum discharge capacity at room temperature. Although it critically depends on current density and the cycle test conditions, this indication is not desirable so far as use of this cathode material for lithium secondary batteries is concerned. Finally, although the o-LiMnO 2 synthesised at low temperature has a high capacity, significant capacity fade is reported upon cycling over both the 4 and 3 V plateaus. On the other hand, Croguennec et al. have reported that active material having a small grain size ,1 mm and stacking faults induced a moderately enhanced discharge capacity up to 200 mAh / g [18]. According to their crystal structure analysis, the intercalation of the LiMnO 2 compound takes place primarily on the grain surface, and small grain size can induce rapid lithium diffusion between the bulk and its surface. Therefore, if an o-LiMnO 2 phase with small grain size and with moderate stacking faults is successfully synthesised, specific discharge capacity and cycle performance would probably be improved.
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00498-X
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Z.P. Guo et al. / Journal of Alloys and Compounds 346 (2002) 255–259
Based on our previous research [21], we successfully synthesised MT-o-LiMnO 2 with a small grain size (,1 mm) using a one-step middle-temperature solid-state reaction (OSSS). Compared with the method other researchers used, the OSSS method is much simpler. We emphasise that the LiMnO 2 in this study was synthesised by a one-step method without intermediate regrinding or other treatments. The characteristics and electrochemical performance of the samples obtained were investigated and were compared with that of o-LiMnO 2 prepared by high temperature solid-state reaction (In this method, Li 2 CO 3 and Mn 2 O 3 were used as starting materials, and the mixture were first calcined for 3 h at 650 8C, then fired at 1000 8C for a further 24 h. The samples were reground between firings).
2. Experimental The o-LiMnO 2 material was synthesised using Mn(CH 3 COO) 2 ?4H 2 O (Aldrich 99.5%), LiOH?H 2 O (Aldrich 99.9%) and citric acid (Aldrich 99%) as starting materials. The Li:Mn atomic ratio was 1.05:1, higher than the stoichiometric ratio of 1:1 to offset lithium loss by Li 2 O evaporation during o-LiMnO 2 synthesis. These powders were mixed and thoroughly ground in an agate mortar. The mixture was loaded into alumina crucibles and calcined in a tube furnace at 800 8C for 12 h under a flow of argon. Powder X-ray diffraction (1730 X-ray diffractometer) using CuKa radiation was employed to identify the crystalline phase of the synthesised materials. The lithium and manganese concentrations in the resulting materials were analysed using an inductively coupled plasma spectrometer (ICP). The particle morphologies of the resulting
compound was observed using a scanning electron microscope (SEM). The electrochemical characterisations were performed using PTFE cells. The cathode was prepared by mixing o-LiMnO 2 powder with 10 wt.% carbon black and 5 wt.% PVDF (polyvinylidene fluoride) solution. The o-LiMnO 2 and carbon black powders were first added to a solution of PVDF in N-methyl-2-pyrrolidinone (NMP) to make a slurry with appropriate viscosity. Al foil was then used to coat the mixture. After the electrode was dried at 140 8C for 2 h in vacuum, it was compressed at a rate of about 150 kg / cm 2 . PTFE test cells were assembled in an argon-filled glove box, where the counter electrode was Li metal and the electrolyte was 1 M LiPF 6 dissolved in a 50:50 (v / v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). These cells were cycled between 2.0 and 4.4 V at room temperature to measure the electrochemical response.
3. Results and discussion
3.1. Crystal structure The XRD patterns of the two types of o-LiMnO 2 compound are shown in Fig. 1. All XRD peaks except for the marked peaks (attributed to the small amount of Li 2 MnO 3 ), are indexed on the basis of the orthorhombic phase with the space group Pmnm. Compared with the o-LiMnO 2 prepared by high temperature solid-state reaction (HT-o-LiMnO 2 ), some XRD peaks of o-LiMnO 2 prepared by the one-step middle-temperature solid-state reaction (MT-o-LiMnO 2 ), broadened remarkably and / or were characterised by a significant asymmetry, while some lines remained unchanged. A full width at half-maximum
Fig. 1. X-ray diffraction patterns of o-LiMnO 2 materials. Above: HT-o-LiMnO 2 ; below: MT-o-LiMnO 2 .
Z.P. Guo et al. / Journal of Alloys and Compounds 346 (2002) 255–259
(FWHM) in the (011) peak for HT-o-LiMnO 2 was 0.18, indicating that the HT-o-LiMnO 2 material is in a wellordered orthorhombic structure, while MT-o-LiMnO 2 showed an increased FWHM to 0.1768, indicating that the density of stacking faults has been increased, i.e. structural disorder [18]. This behaviour can also be observed in the increased broadening of the (120), (122) peaks. Croguennec et al. proposed that the smaller the crystallites / crystals, the higher the number of stacking faults [18]. Therefore, we can assume that the grain size of MT-o-LiMnO 2 is smaller than that of HT-o-LiMnO 2 . To confirm this assumption, the particle morphologies (Fig. 2) of the compounds were observed using a SEM. A SEM micrograph of the HT-o-LiMnO 2 compound is presented in Fig. 2a and compared with the MT-o-LiMnO 2 compound (Fig. 2b). The HT-o-LiMnO 2 compound has flake-shaped particles with an average size of 2–15 mm, whereas the MT-o-LiMnO 2 compound has similar spherical-shape
257
crystals. The particle size of MT-o-LiMnO 2 (0.3 mm) is much smaller than that of HT-o-LiMnO 2 . Other phenomena observed from XRD patterns are a decrease in the Li 2 MnO 3 impurity phase when using the middle-temperature solid-state reaction. This is believed to be due to the existence of organic groups (citric acid and CH 3 COO 2) in the reactant. These organic groups could react during sintering with free oxygen (possibly present in small quantities due to inadequate furnace sealing) and yield CO 2 , thus decreasing the density of oxygen and thus reducing the impurity phase Li 2 MnO 3 in the MT-oLiMnO 2 . Tang et al. reported that the transformation of o-LiMnO 2 in a mixture of LiMn 2 O 4 and Li 2 MnO 3 was accompanied by the absorption of oxygen gas during the calcination process (3LiMnO 2 11 / 2O 2 →Li 2 MnO 3 1 LiMn 2 O 4 ) [19]. The firing of citric acid could also lead to the temperature increasing very quickly in some areas, thereby increasing the lithium evaporation and loss. An ICP measurement verified this. The Li:Mn ratios in MT-oLiMnO 2 and HT-o-LiMnO 2 , are 96% and 99%, respectively. The lower Li:Mn ratio may be another reason for the smaller particle size in MT-o-LiMnO 2, as Croguennec et al. believed that an excess of LiOH can act as a flux agent favouring crystal growth. The preparation methods also affect the overall constants of o-LiMnO 2 . The lattice constants a, b and c for HT-o-LiMnO 2 were calculated to be 2.806, 5.758 and ˚ respectively, while those of the MT-o-LiMnO 2 4.574 A, ˚ respectively. were 2.800, 5.766 and 4.582 A,
3.2. Electrochemical characteristics
Fig. 2. (a) SEM image of HT-o-LiMnO 2 material. (b) SEM image of MT-o-LiMnO 2 material.
PTFE cells containing o-LiMnO 2 materials were cycled at room temperature. The cells were cycled in a voltage window of 2.0–4.4 V at a constant current density of 0.5 mA / cm 2 . The first charge curves are shown in Fig. 3. The two types of o-LiMnO 2 exhibited a slightly different charging behaviour in the first charge. As shown in Fig. 3, the HT-o-LiMnO 2 electrode was quickly charged from OCV to 3.7 V and then followed an ascending plateau between 3.7 and 4.4 V. For the MT-o-LiMnO 2 electrode, charging the voltage from OCV to 3.7 V was relatively slower, and the ascending plateau between 3.7 and 4.4 V was lower. The first charging process consisted in removing Li from o-LiMnO 2 . The plateau recorded is characteristic of a phase transition, as expected from the change in the Mn electronic configuration (d3 instead of d4 in the pristine phase) and from the removal of the Li ions from the structure. It is obvious that the Li in MT-o-LiMnO 2 is easier to remove because the charging plateau is lower. This confirmed the theory of Crogunnec et al., i.e. the smaller the sample crystal / crystallites, the more Li can be removed [18]. Fig. 4 shows the evolution of the discharge specific capacity vs. the number of cycles. Following the phase transition occurring at the first charge, several cycles
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Fig. 3. Galvanostatic mode first charge of o-LiMnO 2 samples. (1) HT-o-LiMnO 2 ; (2) MT-o-LiMnO 2 .
appear necessary for the materials to reach their optimal capacity, up to 40 cycles at a current density of 0.5 mA / cm 2 in the case of the HT-o-LiMnO 2 compounds. In contrast, the full capacity is obtained after only 3–4 cycles for the MT-o-LiMnO 2 materials. The initial capacity is 180 mAh / g at room temperature. As can be seen, the need for fewer cycles to reach the sample highest capacity is correlated with the sample crystal / crystallite size. This is likely to be correlated with kinetic reasons. Since the crystal size of HT-o-LiMnO 2 is large, the inner area of the crystal may not be charged, although the surface area has already reacted. This means that the HT-o-LiMnO 2 samples may contain a substantial amount of unreacted oLiMnO 2 phase, probably in the crystal core, i.e. covered by a spinel-type phase present at the crystal surface. Experiments were then carried out at different rates (C /n with n . 10) in order to compare the reversible capacity
changes with n (Fig. 5). (Fresh cells were cycled until the capacity reached a steady-state value first, then cells were cycled at different rates.) In agreement with a kinetic limitation, the capacity gain is more important for the HT-o-LiMnO 2 material than for MT-o-LiMnO 2 at low rates. For MT-o-LiMnO 2 , the specific capacity is about 81% of the best at C / 10, whereas it is only 48% for HT-o-LiMnO 2 . A slow discharge rate allows for the formation of the material in fewer cycles. All these results prove that the rate capability is better for the smaller crystal materials (MT-o-LiMnO 2 ). Therefore, we succeeded in synthesising a new type of o-LiMnO 2 material by a one-step middle temperature solid-state reaction with an excellent cycle performance as well as a high initial discharge capacity at room temperature. Further work is now in progress to test the high temperature performance and more clearly reveal the
Fig. 4. Discharge specific capacity vs. cycle number of o-LiMnO 2 samples cycled at 0.5 mA / cm 2 in the 4.4–2.0 V range.
Z.P. Guo et al. / Journal of Alloys and Compounds 346 (2002) 255–259
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Fig. 5. Changes of normalised discharge capacity (C /Cmax ) for cells with active electrodes of o-LiMnO 2 at room temperature with a varying charge–discharge rate.
mechanism of the reaction. These results will be reported elsewhere.
4. Conclusion Orthorhombic LiMnO 2 was synthesised using Mn(CH 3 COO) 2 ?4H 2 O, LiOH?H 2 O and citric acid as starting materials at 800 8C in an argon atmosphere. Compared with the HT-o-LiMnO 2 , some of the XRD peaks of MT-o-LiMnO 2 broadened remarkably and / or were characterised by a significant asymmetry, indicating that the density of stacking faults had been increased, i.e. structural disorder increased. MT-o-LiMnO 2 shows slightly larger b and c values compared with HT-o-LiMnO 2 . MT-o-LiMnO 2 also showed a high initial capacity—180 mAh / g when cycled at a current density of 0.5 mA / cm 2 at room temperature, and the rate capability was improved due to the smaller particle size. Therefore, the one-step middle temperature solid-state reaction could be a promising method for o-LiMnO 2 compound synthesis.
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