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Electrochemical cyclability of oxysulfide spinel Li1.03Al0.2Mn1.8O3.96S0.04 material for lithium secondary batteries
www.elsevier.nl/locate/elecom Electrochemistry Communications 2 (2000) 6–9
Electrochemical cyclability of oxysulfide spinel Li1.03Al0.2Mn1.8O3.96S0.04 material for lithium secondary batteries Yang-Kook Sun * Electrochemistry Laboratory, Samsung Advanced Institute of Technology, 103-6 Moonji-Dong, Yusong-Gu, Daejon 305-380, South Korea Received 21 September 1999; received in revised form 18 October 1999; accepted 18 October 1999
Abstract Oxysulfide spinel, Li1.03Al0.2Mn1.8O3.96S0.04 with well-developed octahedral structure was synthesized by a sol-gel method using glycolic acid as a chelating agent. The structural integrity of the oxysulfide spinel was characterized by charge–discharge cycling experiments and X-ray diffraction (XRD). The Li1.03Al0.2Mn1.8O3.96S0.04 electrode shows excellent cyclability. The oxysulfide spinel after cycling retains its original cubic spinel phase in all operating voltage regions (4.4–1.15 V). q2000 Elsevier Science S.A. All rights reserved. Keywords: Lithium secondary batteries; Sol-gel method; Oxysulfide spinel; Lithium manganese oxide; Jahn–Teller distortion
1. Introduction Spinel LixMn2O4, where xs0–2, has been extensively studied electrochemically due to its potential use as the most promising cathode material for lithium secondary batteries [1–4]. When Liq ions are intercalated–deintercalated in the range of 0-x-1 the open circuit voltage of the LiNLixMn2O4 cell is ;4 V, whereas when 1-x-2, the cell discharges at ;3 V. If such intercalation and deintercalation are reversible, tetragonal Li2Mn2O4 at a discharged state of ;3 V and lMnO2 at a charged state of ;4 V are formed, respectively. The cyclability of lithium secondary batteries is closely related with the structural integrity of the host materials in the process of charge and discharge [5]. Although the unit cell contracts and expands by 7.6% in the 4 V region, the lattice contraction and expansion proceed so gradually and isotropically that cubic symmetry of the spinel is maintained. However, a slow capacity loss during cycling has been observed even in many 4 V cells [6,7]. On the contrary, when the LixMn2O4 electrodes are discharged in the 3 V region (1-x-2), an average Mn valency -3.5 and the presence of the Jahn–Teller active Mn3q ion lead to a tetragonal distortion away from the cubic spinel structure (Jahn– Teller distortion). This structural distortion is too large for the spinel framework to withstand its structural integrity during cycling [1,8]. This explains why the cubic spinel * Tel.: q82-42-865-4074; fax: q82-42-865-4061; e-mail: yksun@sait. samsung.co.kr or [email protected]
LiMn2O4 has a more stable cyclability in the 4 V region than in the 3 V region and is limited to use only for 4 V electrodes. When LiNLixMn2O4 cells are overdischarged (x)2), the open circuit voltage of the cell occurs at 1.2 V. The additional lithium can be incorporated into Li2Mn2O4 to yield a layered structure Li2Mn2O4 [8,9]. The cubic symmetry of the pristine spinel phase is recovered upon subsequent removal of the lithium. However, the electrochemical cyclability of the spinel in this region is not reported. In order to improve the cyclability of the LiMn2O4 electrodes in the 4 and 3 V regions, many research groups have studied cation substitution for Li and Mn, and anion substitution (F) for O in LiMn2O4 [10–12]. Although there have been many reports to overcome Jahn–Teller distortion in the 4 and 3 V regions [13–15], no research groups have overcome Jahn–Teller distortion in the spinel LiMn2O4 phase. We report here the electrochemical cyclability of a new sulfur-doped spinel, Li1.03Al0.2Mn1.8O3.96S0.04, by a sol-gel method using glycolic acid as a chelating agent.
2. Experimental Li1.03Al0.2Mn1.8O3.96S0.04 powders were prepared by a sol-gel method using glycolic acid as a chelating agent. Li(CH3COO)PH2O, Mn(CH3COO)2P4H2O, Al(NO3)3P 4H2O, and Li2S (cationic ratios of Li:Mn:Al:Li s 0.56:1.8:0.2:0.5) were dissolved in distilled water, and added dropwise to a continuously stirred aqueous solution of
1388-2481/00/$ - see front matter q2000 Elsevier Science S.A. All rights reserved. PII S 1 3 8 8 - 2 4 8 1 ( 9 9 ) 0 0 1 3 6 - 8
Thursday Dec 09 12:50 PM
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 146
Y.-K. Sun / Electrochemistry Communications 2 (2000) 6–9
glycolic acid. The molar ratio of glycolic acid to total metal ions was fixed at unity. The solution pH was adjusted to 9.5 using ammonium hydroxide. The resultant solution was evaporated at 70–808C until a transparent sol was obtained. As the evaporation of water proceeded, the sol turned into a viscous transparent gel. The resulting gel precursors were decomposed at 5008C for 10 h in air to eliminate the organic moiety. The decomposed powders were calcined at 8008C in air for 10 h and then in flowing oxygen for 15 h. The contents of lithium, aluminum, and manganese were measured using the inductively coupled plasma (ICP) method by dissolving the powders in dilute nitric acid. The sulfur was analyzed using a sulfur analyzer (LECO Co., CS 444) and the oxygen content was determined via a mass balance. The particle morphology was observed using a field emission scanning electron microscope (FE-SEM, Hitachi Co., S-4100). Powder X-ray diffraction (Rigaku, Rint-2000) with Cu Ka radiation was used to identify the crystalline phases of the materials and cycled electrodes. For the fabrication of the electrodes, the Li1.03Al0.2Mn1.8O3.96S0.04 powders were mixed with 12 wt.% carbon black and 8 wt.% polytetrafluoroethylene (PTFE), then pressed onto the aluminum Exmet. A lithium foil was used as anode. The electrolyte was a 1:1 mixture of ethylene carbonate (EC) and propylene carbonate (PC) containing 1 M LiClO4 by volume. The charge–discharge cycles were performed galvanostatically at a current density of 0.2 mA cmy2 with various voltage ranges.
7
Fig. 1. Scanning electron micrograph of the Li1.03Al0.2Mn1.8O3.96S0.04 powders.
3. Results and discussion The as-prepared powders were confirmed to have welldefined spinel phase structure with space group Fd3m, as shown in the X-ray diffraction (XRD) spectrum (see Fig. 3(a)). Fig. 1 shows a scanning electron micrograph of the powders. The particle morphology is analogous to that of singlecrystal-like gold with a cubic structure. The particles are shaped in a well-developed polyhedron which mainly consists of octahedral structure bounded by eight (111) planes. These are quite different in particle morphology from the spinel LiMn2O4 with a well-developed (100) plane. The average particle diameter was ;3 mm. The chemical analysis of the powders was proven to be Li1.03Al0.2Mn1.8O3.96S0.04. Fig. 2 shows charge–discharge curves for the Li1.03Al0.2Mn1.8O3.96S0.04 electrodes as a function of cycle number in the voltage ranges of (a) 4.4–3.4 V, (b) 2.4–3.5 V, and (c) 4.4–1.15 V. The Li1.03Al0.2Mn1.8O3.96S0.04 electrode in the 4 V region (Fig. 2(a)) initially delivered a discharge capacity of 85 mAh gy1. The charge–discharge curves have only one plateau. Although the material delivered a lower discharge capacity than the spinel LiMn2O4, it shows excellent cyclability without any capacity loss after the 50th cycle. We then examined the electrochemical cyclability of Li1.03Al0.2Mn1.8O3.96S0.04 in the 3 V region. The electrode
Thursday Dec 09 12:50 PM
Fig. 2. Cycling charge–discharge curves for the Li1.03Al0.2Mn1.8O3.96S0.04NLi cell in voltage ranges (a) 4.4–3.4, (b) 2.4–3.5, and (c) 4.4–1.15 V.
was first discharged to the 2.4 V limit and then cycled between 3.5 and 2.4 V. It is worth noting that from the discharge– charge curves (Fig. 2(b)) the polarization (which means half the difference in voltage between the charge and discharge curves) decreased during cycling, indicative of an improved cyclability of the electrode. The electrode initially delivered 67 mAh gy1, increased rapidly up to the 25th cycle, and subsequently stabilized on further cycling. The discharge capacity after the 50th cycle reached 94 mAh gy1. The electrochemical cyclability of Li1.03Al0.2Mn1.8O3.96S0.04 showed
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 146
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Y.-K. Sun / Electrochemistry Communications 2 (2000) 6–9
quite different behavior compared with those of the spinel LiMn2O4 and its derivatives [13,16], in which most capacity loss occurs in this voltage region. This implies that the substitution of S for O is very effective in improving the cyclability in the lower voltage range. Fig. 2(c) shows the charge and discharge curves for a LiNLi1.03Al0.2Mn1.8O3.96S0.04 cell in the voltage range 4.4–1.15 V, encompassing 4, 3, and 1.2 V plateaux. The cell was first charged up to 4.4 V, and repeatedly cycled in the voltage range between 1.15 and 4.4 V. The discharge curves show tailing characteristics and follow no plateau at 1.2 V, which is a unique feature that is different from those of the spinel LiMn2O4 [8,9]. The characteristics of the voltage profile changed within the first few cycles, and stabilized on further cycling. The electrode initially delivered 260 mAh gy1, decreased slowly during cycling, and reached 218 mAh gy1 after the 20th cycle. Considering no capacity loss in the 4 and 3 V regions, the capacity loss of the Li1.03Al0.2Mn1.8O3.96S0.04 electrode in the voltage range 4.4–1.15 V could be attributed to the deterioration of electrical contact among the composite cathode interfaces (Li1.03Al0.2Mn1.8O3.96S0.04NPTFENcarbon black). The larger lattice change of the Li1.03Al0.2Mn1.8O3.96S0.04 particles due to a larger voltage drop (4.4–1.15 V) results in considerable strain on these interfaces and hence to a decrease in the capacity of the cathode during cycling [10,16]. In order to investigate the structural change of Li1.03Al0.2Mn1.8O3.96S0.04 during cycling, the electrode was characterized by means of XRD before and after cycling in various voltage regions. Fig. 3 shows the XRD patterns for the asprepared powders (a), and the electrodes cycled in the voltage ranges of 4.4–3.4 (b), 2.4–3.5 (c), and 4.4–1.15 V (d).
The LiNLi1.03Al0.2Mn1.8O3.96S0.04 cell was allowed to equilibrate for 5 h at fully discharged state. The cell was disassembled and then the electrode was dried for 1 day after removal from the cell. When comparing XRD patterns for the cycled electrodes in various voltage ranges (Fig. 3(b)–(d)) with that of as-prepared powders (Fig. 3(a)), we are unable to find any difference in the positions of the characteristic peaks for the typical spinel phase structure. Although there are some variations of the peak intensities, no tetragonal phases appeared. Considering the variation of the levels of preferential orientation in preparing the electrodes, it is reasonable to conclude that there is almost no difference in the XRD spectra of the cycled electrodes. This result indicates that the electrode structure retains its original cubic spinel phase even after cycling at the 1.2 V plateau. The preservation of the original cubic spinel phase after cycling in the 4.4–1.15 V region is quite different from the results reported by David et al. that the cubic spinel phase converts to the layered Li2MnO2 after the first discharge [8,9]. We report the excellent cyclability of LiMn2O3.98S0.02 in the 3 V region and LiAl0.24Mn1.76O4 in the 4 V region, which is attributed to substitution of Al for Mn in the 4 V region and S for O in the 3 V region [17]. The excellent cyclability of the Li1.03Al0.2Mn1.8O3.96S0.04 electrode in all the cycled voltage ranges is due to the combined effects of the substitution cations (Li, Al) for Mn and substitution anion (S) for O. The preservation of the cubic spinel phase of the cycled electrodes results in the enhancement of the electrochemical cyclability during cycling. This result encourages us to believe that the oxysulfide spinel Li1.03Al0.2Mn1.8O3.96S0.04 could overcome the Jahn–Teller distortion.
4. Conclusions A new oxysulfide spinel, Li1.03Al0.2Mn1.8O3.96S0.04, with a well-developed octahedral structure was synthesized by a sol-gel method using glycolic acid as a chelating agent. The electrochemical performance of the oxysulfide spinel Li1.03Al0.2Mn1.8O3.96S0.04 shows excellent cyclability without any capacity loss in the 4 V region and increases slowly during cycling in the 3 V region. The cycled electrode retains its original cubic spinel phase in all operating voltage regions, which overcomes Jahn–Teller distortion.
References
Fig. 3. XRD patterns for (a) Li1.03Al0.2Mn1.8O3.96S0.04 powders and the Li1.03Al0.2Mn1.8O3.96S0.04 electrodes cycled in the voltage ranges (b) 4.4– 3.4, (c) 2.4–3.5, and (d) 4.4–1.15 V.
Thursday Dec 09 12:50 PM
[1] M.M. Thackeray, P.G. David, P.G. Bruce, J.B. Goodenough, Mater. Res. Bull. 18 (1983) 461. [2] T. Ohzuku, M. Kitagawa, T. Hirai, J. Electrochem. Soc. 137 (1990) 769. [3] K. Amine, H. Tukamoto, H. Yasuda, Y. Fujita, J. Electrochem. Soc. 143 (1996) 1607. [4] Y. Xia, Y. Zhou, M. Yoshio, J. Electrochem. Soc. 144 (1997) 2593. [5] M.M. Thackeray, J. Electrochem. Soc. 142 (1995) 2558. [6] R.J. Gummow, A. de Kock, M.M. Thackeray, Solid State Ionics 69 (1994) 59.
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 146
Y.-K. Sun / Electrochemistry Communications 2 (2000) 6–9 [7] D.H. Jang, Y.J. Shin, S.M. Oh, J. Electrochem. Soc. 143 (1996) 2204. [8] M.M. Thackeray, Prog. Solid State Chem. 25 (1997) 1. [9] W.I.F. David, J.B. Goodenough, M.M. Thackeray, M.G.S.R. Thomas, Rev. Chim. Miner. 20 (1983) 636. [10] Y.-K. Sun, Solid State Ionics 100 (1997) 115. [11] A.D. Robertson, S.H. Lu, W.F. Averill, W.F. Howard Jr., J. Electrochem. Soc. 144 (1997) 3500. [12] G.G. Amatucci, J.M. Tarascon, US Patent No. 5 674 645 (1997).
Thursday Dec 09 12:50 PM
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[13] P. Barboux, J.M. Tarascon, F.K. Shokoohi, J. Solid State Chem. 94 (1991) 185. [14] J.M Tarascon, E. Wang, F.K. Shokoohi, W.R. McKinnon, S. Colson, J. Electrochem. Soc. 138 (1991) 2859. [15] H. Huang, P.G. Bruce, J. Power Sources 54 (1995) 52. [16] Z. Jiang, K.M. Abraham, J. Electrochem. Soc. 143 (1996) 1591. [17] Y.-K. Sun, Y.-S. Jeon, Electrochem. Commun., submitted for publication.
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 146
Electrochemical cyclability of oxysulfide spinel Li1.03Al0.2Mn1.8O3.96S0.04 material for lithium secondary batteries Yang-Kook Sun * Electrochemistry Laboratory, Samsung Advanced Institute of Technology, 103-6 Moonji-Dong, Yusong-Gu, Daejon 305-380, South Korea Received 21 September 1999; received in revised form 18 October 1999; accepted 18 October 1999
Abstract Oxysulfide spinel, Li1.03Al0.2Mn1.8O3.96S0.04 with well-developed octahedral structure was synthesized by a sol-gel method using glycolic acid as a chelating agent. The structural integrity of the oxysulfide spinel was characterized by charge–discharge cycling experiments and X-ray diffraction (XRD). The Li1.03Al0.2Mn1.8O3.96S0.04 electrode shows excellent cyclability. The oxysulfide spinel after cycling retains its original cubic spinel phase in all operating voltage regions (4.4–1.15 V). q2000 Elsevier Science S.A. All rights reserved. Keywords: Lithium secondary batteries; Sol-gel method; Oxysulfide spinel; Lithium manganese oxide; Jahn–Teller distortion
1. Introduction Spinel LixMn2O4, where xs0–2, has been extensively studied electrochemically due to its potential use as the most promising cathode material for lithium secondary batteries [1–4]. When Liq ions are intercalated–deintercalated in the range of 0-x-1 the open circuit voltage of the LiNLixMn2O4 cell is ;4 V, whereas when 1-x-2, the cell discharges at ;3 V. If such intercalation and deintercalation are reversible, tetragonal Li2Mn2O4 at a discharged state of ;3 V and lMnO2 at a charged state of ;4 V are formed, respectively. The cyclability of lithium secondary batteries is closely related with the structural integrity of the host materials in the process of charge and discharge [5]. Although the unit cell contracts and expands by 7.6% in the 4 V region, the lattice contraction and expansion proceed so gradually and isotropically that cubic symmetry of the spinel is maintained. However, a slow capacity loss during cycling has been observed even in many 4 V cells [6,7]. On the contrary, when the LixMn2O4 electrodes are discharged in the 3 V region (1-x-2), an average Mn valency -3.5 and the presence of the Jahn–Teller active Mn3q ion lead to a tetragonal distortion away from the cubic spinel structure (Jahn– Teller distortion). This structural distortion is too large for the spinel framework to withstand its structural integrity during cycling [1,8]. This explains why the cubic spinel * Tel.: q82-42-865-4074; fax: q82-42-865-4061; e-mail: yksun@sait. samsung.co.kr or [email protected]
LiMn2O4 has a more stable cyclability in the 4 V region than in the 3 V region and is limited to use only for 4 V electrodes. When LiNLixMn2O4 cells are overdischarged (x)2), the open circuit voltage of the cell occurs at 1.2 V. The additional lithium can be incorporated into Li2Mn2O4 to yield a layered structure Li2Mn2O4 [8,9]. The cubic symmetry of the pristine spinel phase is recovered upon subsequent removal of the lithium. However, the electrochemical cyclability of the spinel in this region is not reported. In order to improve the cyclability of the LiMn2O4 electrodes in the 4 and 3 V regions, many research groups have studied cation substitution for Li and Mn, and anion substitution (F) for O in LiMn2O4 [10–12]. Although there have been many reports to overcome Jahn–Teller distortion in the 4 and 3 V regions [13–15], no research groups have overcome Jahn–Teller distortion in the spinel LiMn2O4 phase. We report here the electrochemical cyclability of a new sulfur-doped spinel, Li1.03Al0.2Mn1.8O3.96S0.04, by a sol-gel method using glycolic acid as a chelating agent.
2. Experimental Li1.03Al0.2Mn1.8O3.96S0.04 powders were prepared by a sol-gel method using glycolic acid as a chelating agent. Li(CH3COO)PH2O, Mn(CH3COO)2P4H2O, Al(NO3)3P 4H2O, and Li2S (cationic ratios of Li:Mn:Al:Li s 0.56:1.8:0.2:0.5) were dissolved in distilled water, and added dropwise to a continuously stirred aqueous solution of
1388-2481/00/$ - see front matter q2000 Elsevier Science S.A. All rights reserved. PII S 1 3 8 8 - 2 4 8 1 ( 9 9 ) 0 0 1 3 6 - 8
Thursday Dec 09 12:50 PM
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 146
Y.-K. Sun / Electrochemistry Communications 2 (2000) 6–9
glycolic acid. The molar ratio of glycolic acid to total metal ions was fixed at unity. The solution pH was adjusted to 9.5 using ammonium hydroxide. The resultant solution was evaporated at 70–808C until a transparent sol was obtained. As the evaporation of water proceeded, the sol turned into a viscous transparent gel. The resulting gel precursors were decomposed at 5008C for 10 h in air to eliminate the organic moiety. The decomposed powders were calcined at 8008C in air for 10 h and then in flowing oxygen for 15 h. The contents of lithium, aluminum, and manganese were measured using the inductively coupled plasma (ICP) method by dissolving the powders in dilute nitric acid. The sulfur was analyzed using a sulfur analyzer (LECO Co., CS 444) and the oxygen content was determined via a mass balance. The particle morphology was observed using a field emission scanning electron microscope (FE-SEM, Hitachi Co., S-4100). Powder X-ray diffraction (Rigaku, Rint-2000) with Cu Ka radiation was used to identify the crystalline phases of the materials and cycled electrodes. For the fabrication of the electrodes, the Li1.03Al0.2Mn1.8O3.96S0.04 powders were mixed with 12 wt.% carbon black and 8 wt.% polytetrafluoroethylene (PTFE), then pressed onto the aluminum Exmet. A lithium foil was used as anode. The electrolyte was a 1:1 mixture of ethylene carbonate (EC) and propylene carbonate (PC) containing 1 M LiClO4 by volume. The charge–discharge cycles were performed galvanostatically at a current density of 0.2 mA cmy2 with various voltage ranges.
7
Fig. 1. Scanning electron micrograph of the Li1.03Al0.2Mn1.8O3.96S0.04 powders.
3. Results and discussion The as-prepared powders were confirmed to have welldefined spinel phase structure with space group Fd3m, as shown in the X-ray diffraction (XRD) spectrum (see Fig. 3(a)). Fig. 1 shows a scanning electron micrograph of the powders. The particle morphology is analogous to that of singlecrystal-like gold with a cubic structure. The particles are shaped in a well-developed polyhedron which mainly consists of octahedral structure bounded by eight (111) planes. These are quite different in particle morphology from the spinel LiMn2O4 with a well-developed (100) plane. The average particle diameter was ;3 mm. The chemical analysis of the powders was proven to be Li1.03Al0.2Mn1.8O3.96S0.04. Fig. 2 shows charge–discharge curves for the Li1.03Al0.2Mn1.8O3.96S0.04 electrodes as a function of cycle number in the voltage ranges of (a) 4.4–3.4 V, (b) 2.4–3.5 V, and (c) 4.4–1.15 V. The Li1.03Al0.2Mn1.8O3.96S0.04 electrode in the 4 V region (Fig. 2(a)) initially delivered a discharge capacity of 85 mAh gy1. The charge–discharge curves have only one plateau. Although the material delivered a lower discharge capacity than the spinel LiMn2O4, it shows excellent cyclability without any capacity loss after the 50th cycle. We then examined the electrochemical cyclability of Li1.03Al0.2Mn1.8O3.96S0.04 in the 3 V region. The electrode
Thursday Dec 09 12:50 PM
Fig. 2. Cycling charge–discharge curves for the Li1.03Al0.2Mn1.8O3.96S0.04NLi cell in voltage ranges (a) 4.4–3.4, (b) 2.4–3.5, and (c) 4.4–1.15 V.
was first discharged to the 2.4 V limit and then cycled between 3.5 and 2.4 V. It is worth noting that from the discharge– charge curves (Fig. 2(b)) the polarization (which means half the difference in voltage between the charge and discharge curves) decreased during cycling, indicative of an improved cyclability of the electrode. The electrode initially delivered 67 mAh gy1, increased rapidly up to the 25th cycle, and subsequently stabilized on further cycling. The discharge capacity after the 50th cycle reached 94 mAh gy1. The electrochemical cyclability of Li1.03Al0.2Mn1.8O3.96S0.04 showed
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 146
8
Y.-K. Sun / Electrochemistry Communications 2 (2000) 6–9
quite different behavior compared with those of the spinel LiMn2O4 and its derivatives [13,16], in which most capacity loss occurs in this voltage region. This implies that the substitution of S for O is very effective in improving the cyclability in the lower voltage range. Fig. 2(c) shows the charge and discharge curves for a LiNLi1.03Al0.2Mn1.8O3.96S0.04 cell in the voltage range 4.4–1.15 V, encompassing 4, 3, and 1.2 V plateaux. The cell was first charged up to 4.4 V, and repeatedly cycled in the voltage range between 1.15 and 4.4 V. The discharge curves show tailing characteristics and follow no plateau at 1.2 V, which is a unique feature that is different from those of the spinel LiMn2O4 [8,9]. The characteristics of the voltage profile changed within the first few cycles, and stabilized on further cycling. The electrode initially delivered 260 mAh gy1, decreased slowly during cycling, and reached 218 mAh gy1 after the 20th cycle. Considering no capacity loss in the 4 and 3 V regions, the capacity loss of the Li1.03Al0.2Mn1.8O3.96S0.04 electrode in the voltage range 4.4–1.15 V could be attributed to the deterioration of electrical contact among the composite cathode interfaces (Li1.03Al0.2Mn1.8O3.96S0.04NPTFENcarbon black). The larger lattice change of the Li1.03Al0.2Mn1.8O3.96S0.04 particles due to a larger voltage drop (4.4–1.15 V) results in considerable strain on these interfaces and hence to a decrease in the capacity of the cathode during cycling [10,16]. In order to investigate the structural change of Li1.03Al0.2Mn1.8O3.96S0.04 during cycling, the electrode was characterized by means of XRD before and after cycling in various voltage regions. Fig. 3 shows the XRD patterns for the asprepared powders (a), and the electrodes cycled in the voltage ranges of 4.4–3.4 (b), 2.4–3.5 (c), and 4.4–1.15 V (d).
The LiNLi1.03Al0.2Mn1.8O3.96S0.04 cell was allowed to equilibrate for 5 h at fully discharged state. The cell was disassembled and then the electrode was dried for 1 day after removal from the cell. When comparing XRD patterns for the cycled electrodes in various voltage ranges (Fig. 3(b)–(d)) with that of as-prepared powders (Fig. 3(a)), we are unable to find any difference in the positions of the characteristic peaks for the typical spinel phase structure. Although there are some variations of the peak intensities, no tetragonal phases appeared. Considering the variation of the levels of preferential orientation in preparing the electrodes, it is reasonable to conclude that there is almost no difference in the XRD spectra of the cycled electrodes. This result indicates that the electrode structure retains its original cubic spinel phase even after cycling at the 1.2 V plateau. The preservation of the original cubic spinel phase after cycling in the 4.4–1.15 V region is quite different from the results reported by David et al. that the cubic spinel phase converts to the layered Li2MnO2 after the first discharge [8,9]. We report the excellent cyclability of LiMn2O3.98S0.02 in the 3 V region and LiAl0.24Mn1.76O4 in the 4 V region, which is attributed to substitution of Al for Mn in the 4 V region and S for O in the 3 V region [17]. The excellent cyclability of the Li1.03Al0.2Mn1.8O3.96S0.04 electrode in all the cycled voltage ranges is due to the combined effects of the substitution cations (Li, Al) for Mn and substitution anion (S) for O. The preservation of the cubic spinel phase of the cycled electrodes results in the enhancement of the electrochemical cyclability during cycling. This result encourages us to believe that the oxysulfide spinel Li1.03Al0.2Mn1.8O3.96S0.04 could overcome the Jahn–Teller distortion.
4. Conclusions A new oxysulfide spinel, Li1.03Al0.2Mn1.8O3.96S0.04, with a well-developed octahedral structure was synthesized by a sol-gel method using glycolic acid as a chelating agent. The electrochemical performance of the oxysulfide spinel Li1.03Al0.2Mn1.8O3.96S0.04 shows excellent cyclability without any capacity loss in the 4 V region and increases slowly during cycling in the 3 V region. The cycled electrode retains its original cubic spinel phase in all operating voltage regions, which overcomes Jahn–Teller distortion.
References
Fig. 3. XRD patterns for (a) Li1.03Al0.2Mn1.8O3.96S0.04 powders and the Li1.03Al0.2Mn1.8O3.96S0.04 electrodes cycled in the voltage ranges (b) 4.4– 3.4, (c) 2.4–3.5, and (d) 4.4–1.15 V.
Thursday Dec 09 12:50 PM
[1] M.M. Thackeray, P.G. David, P.G. Bruce, J.B. Goodenough, Mater. Res. Bull. 18 (1983) 461. [2] T. Ohzuku, M. Kitagawa, T. Hirai, J. Electrochem. Soc. 137 (1990) 769. [3] K. Amine, H. Tukamoto, H. Yasuda, Y. Fujita, J. Electrochem. Soc. 143 (1996) 1607. [4] Y. Xia, Y. Zhou, M. Yoshio, J. Electrochem. Soc. 144 (1997) 2593. [5] M.M. Thackeray, J. Electrochem. Soc. 142 (1995) 2558. [6] R.J. Gummow, A. de Kock, M.M. Thackeray, Solid State Ionics 69 (1994) 59.
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 146
Y.-K. Sun / Electrochemistry Communications 2 (2000) 6–9 [7] D.H. Jang, Y.J. Shin, S.M. Oh, J. Electrochem. Soc. 143 (1996) 2204. [8] M.M. Thackeray, Prog. Solid State Chem. 25 (1997) 1. [9] W.I.F. David, J.B. Goodenough, M.M. Thackeray, M.G.S.R. Thomas, Rev. Chim. Miner. 20 (1983) 636. [10] Y.-K. Sun, Solid State Ionics 100 (1997) 115. [11] A.D. Robertson, S.H. Lu, W.F. Averill, W.F. Howard Jr., J. Electrochem. Soc. 144 (1997) 3500. [12] G.G. Amatucci, J.M. Tarascon, US Patent No. 5 674 645 (1997).
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[13] P. Barboux, J.M. Tarascon, F.K. Shokoohi, J. Solid State Chem. 94 (1991) 185. [14] J.M Tarascon, E. Wang, F.K. Shokoohi, W.R. McKinnon, S. Colson, J. Electrochem. Soc. 138 (1991) 2859. [15] H. Huang, P.G. Bruce, J. Power Sources 54 (1995) 52. [16] Z. Jiang, K.M. Abraham, J. Electrochem. Soc. 143 (1996) 1591. [17] Y.-K. Sun, Y.-S. Jeon, Electrochem. Commun., submitted for publication.
StyleTag -- Journal: ELECOM (Electrochemistry Communications)
Article: 146