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Synthesis and electrochemical characterization of pillared layered Li1−2xCaxCoO2
Journal of Physics and Chemistry of Solids 67 (2006) 1343–1346 www.elsevier.com/locate/jpcs
Synthesis and electrochemical characterization of pillared layered Li1K2xCaxCoO2 W.S. Yang, X.M. Li, L. Yang, D.G. Evans, X. Duan * Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, P.O. Box 98, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China
Abstract The pillared layered Li1K2xCaxCoO2 has been obtained for the first time by a molten salt ion exchange reaction. The X-ray diffraction data of the materials show that they are single phase and retain the layered a-NaFeO2 type structure. The electrochemical testing of Li1K2xCaxCoO2 cells with an upper cutoff potential of 4.5 and 4.7 V (vs. LiC/Li) show that the specific capacity of the oxides have been increased, respectively, to 175 and 211 mAh gK1, and retain better charge–discharge cycling performance. The XRD data shows these improvements are attributed to the pillaring role of Ca2C which helps to suppress unwanted phase transition. q 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; C. X-ray diffraction; D. Electrochemical properties
1. Introduction Layered LiCoO2 is the preferred material in commercial lithium-ion batteries in view of its ease of synthesis and high reversibility. Although the theoretical capacity of LiCoO2 can achieve to 274 mAh gK1, in practice, when cycled with an upper cutoff voltage of about 4.2 V (vs. LiC/Li) (corresponding to repeated extraction/insertion of about 0.5 Li per LiCoO2), the specific capacity is only about 140 mAh gK1. To obtain a higher capacity, it must be charged above 4.2 V but this results in rapid capacity loss, thought to be caused by side reaction with the electrolyte and the structural instability of LixCoO2 with x!0.5 [1]. To counteract this problem, some groups have coated LiCoO2 with oxides in order to suppress the side reactions [2–4]. Partial substitution of Co with other metals has also been attempted with the aim of stabilizing the layered structure [5–8]. However, the capacity retention of coated or doped LiCoO2 is still unacceptable for industrial applications. layered LiCoO2 is isostructural with the rhombohedral R3m a-NaFeO2 type structure. It is an ordered rock salt structure with edge sharing CoO6 octahedral linked to form CoO2 sheets. They give an unobstructed two-dimensional pathway. It has been
* Corresponding author. Tel.: C86 10 6442 5395; fax: C86 10 6442 5385. E-mail address: [email protected] (X. Duan).
0022-3697/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2006.01.067
shown that the removal of the lithium ions during extraction results in a set of phase change, i.e. from O3/H1-3/O1 [1,9– 13], and the O3/H1-3 phase change near 4.5 V leads to rapid capacity loss in LixCoO2 [13]. We show here a novel way suppress this unwanted phase change by introducing Ca2C into the interlayer galleries to give pillared layered Li1K2xCaxCoO2 materials. 2. Experimental 2.1. Sample preparation LiCoO2 precursor was prepared by sol–gel method using citric acid (C6H10O7) as chelating agent. The gel was formed at 120 8C by mixing LiOH and Co(NO3)$6H2O (Li/CoZ1.05/1) with citric acid (C6 H10O7) solution. The gel was then dried at 120 8C in vacuum to yield organic polymer foam. After baked at 500 8C for 3 h, it was calcined in air at 850 8C for 6 h to obtain the LiCoO2 precursor. The high purity Ca(NO3)2$4H2O and LiCoO2 with a molar ratio of 5:1 were mixed and heated at 70 8C for a period of time (1, 2 and 4 h), respectively, then the products were washed by acetone. Finally, the products were dried at 70 8C in vacuum for 3 h and calcined at 500 8C for 10 h. 2.2. Characterization X-ray diffraction (XRD) was recorded on a Shimadzu XRD6000 diffractometer using a monochromatized Cu Ka1 source
1344
W.S. Yang et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1343–1346 Table 2 The oxidation and reduction peak potentials of LiCoO2 and Li0.95Ca0.025CoO2
003
700 600
Sample
100
d c b a
0 20
40
60
80
2θ/° Fig. 1. The XRD patterns of LiCoO2 and Li1K2xCaxO2 samples with different period of ion exchange reaction time (a) LiCoO2, (b) 1 h, (c) 2 h and (d) 4 h.
˚ ) with a step size of 0.028 and 4 s per scan. (lZ1.5406 A Elemental analysis for metal ions was tested using Inductively Coupled Plasma Emission Spectroscopy (Shimadzu ICPS7500). The electrochemical cycling performance of electrode materials was examined using a two-electrode test cell with lithium foil as the negative electrode. The positive electrode was a mixture of Li0.95Ca0.025CoO2 (or LiCoO2), acetylene black and polyvinylidene fluoride (PVDF) in the weight ratio of 85:10:5. Cell assembly was carried out in an argon filled glove box (H2O!1 ppm, O2!1 ppm) with an electrolyte of 1 mol LK1 LiPF6 in EC–DMC (1:1 ratio in volume) solution and Celgard 2400 as separators membrane. The electrochemical cycling performance was collected using a LAND CT2001A computer-controlled apparatus within the potential range of 3.0 to 4.5 or 4.7 V (vs. LiC/Li) at a constant current density of 0.2 mA cmK2. Cyclic voltammetry (CV) was recorded with a 15 mV/s sweep rate using a ZAHNER IM6e electrochemical workstation. 3. Results and discussion 3.1. Effects of molten salt ion exchange reaction time on the samples A serious of pillared layered Li1K2xCaxCoO2 was obtained by reaction of LiCoO2 precursor and Ca(NO3)2$4H2O for different period of ion exchange reaction time. The patterns of XRD and the results of element analysis are shown in Fig. 1 and Table 1, respectively. Table 1 The element analysis results and the chemical composition of the samples with different period of ion exchange reaction time Reaction time (h)
Li (wt%)
Ca (wt%)
Co (wt%)
Chemical formation
0 1 2 4
7.09 6.73 6.59 6.45
0.00 0.80 1.01 1.32
60.21 58.92 58.87 58.94
LiCoO2 Li0.97Ca0.019CoO2 Li0.95Ca0.025CoO2 Li0.93Ca0.033CoO2
0.025CoO2
Reduction peak potential V (vs. LiC/Li)
DV (V)
4.014 3.972
3.856 3.857
0.150 0.115
The XRD patterns indicate that the ion exchange reaction time does not affect the structure of the samples, and all the samples are single phase and retain the layered a-NaFeO2 type structure. The data of Table 2 shows that the content of LiC in samples decrease and the content of Ca2C increase in relationship of 2:1 with the reaction time prolonged. But if the Ca2C instead of too more LiC, the specific capacity of the sample will lose more, so we adopted 2 h as the ion exchange reaction time in this study. 3.2. Electrochemical performance The electrochemical cycling performance of Li0.95Ca0.025 CoO2 and LiCoO2 under higher cutoff potential (such as 4.5 and 4.7 V vs. LiC/Li) is shown in Fig. 2. It is evident that the Li0.95Ca0.025CoO2 cathode shows high specific capacity and excellent capacity retention than LiCoO2 under higher cutoff potential. This indicates we can access a specific capacity of more than 140 mAh gK1 in LiCoO2 and keep excellent cycling behavior by the pillaring method. The cyclic voltammetry (CV) curves of LiCoO2 and Li0.95Ca0.025CoO2 at a sweep rate of 15 mV sK1 which are carried out during the first cycle are illustrated in Fig. 3. The CVs clearly show three pair of current peaks, respectively, but the peak potentials are different as shown in Table 2. The oxidation peak potential of Li0.95Ca0.025CoO2 is lower, while its reduction peak potential is higher than that of LiCoO2, which imply that the reversibility of Li0.95Ca0.025CoO2 is better than that of LiCoO2 and the exist of Ca2C benefit to the diffusion of LiC.
200
Discharge capacity (mAh g–1)
104
108 110 113
200
107
300
LiCoO2 Li0.95Ca 105
400 101 102 006
Intensity
500
Oxidation peak potential V (vs. LiC/Li)
150
100 Li0.95Ca0.025CoO2 (4.5V) LiCoO2
50
(4.5V)
Li0.95Ca0.025CoO2 (4.7V) LiCoO2
(4.7V)
0 0
1
2
3
4
5
6
7
8
9
10
11
Cycle number Fig. 2. Electrochemical cycling performance of LiCoO2 and Li0.95Ca0.025CoO2 materials with different upper cutoff potential.
W.S. Yang et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1343–1346
5 cycles during the cutoff potential of 3.0–4.5 V (vs. LiC/Li) at a current density of 0.2 mA cmK2, finally the cells were charged to 4.5 V and then disassembled. The Li0.95Ca0.025 CoO2 cathode films were washed with DMC and the content of Ca and Co in the material was analyzed, and the results are shown in Table 3. According to Table 3, we find that the atomic ratio of Ca/Co in the material is not changed after cycling. The result indicates that the Ca2C cannot be extracted from the interlayer space when charged. The XRD patterns of LiCoO2 and Li0.95Ca0.025CoO2 materials charged, respectively, to 4.5, 4.6 and 4.7 V (vs. LiC/Li) are shown in Fig. 4. When charged to 4.5 V, the O3(003) peaks of these two samples moved to lower angle, it means the interlayer spacing expanded. However, the expanded extent of Li0.95Ca0.025CoO2 is smaller than that of LiCoO2. When charged to 4.6 V, a new peak which is known as (006) peak of H1-3 phase appeared obviously [13]. However, Li0.95Ca0.025CoO2 almost does not change to H1-3 from O3 phase at the same potential. This indicates that Ca2C pillars can suppress the unwanted phase transition.
800 600
(b) (a)
0 –200 –400 –600 3.6
3.8
4.0
4.2
4.4
E ( V vs.Li+/Li) Fig. 3. Cyclic voltammetry curves of (a) LiCoO2 and (b) Li0.95Ca0.025CoO2 materials at a sweep rate of 15 mV/s. Table 3 Results of the element analysis of Li0.95Ca0.025CoO2 before and after cycling
Uncharged Cycled
Ca (wt%)
Co (wt%)
Ca/Co atomic ratio
1.01 0.892
58.87 52.75
0.025 0.025
4. Conclusion Pillared layered Li1K2xCaxCoO2 has been synthesized by a molten salt ion exchange method. All the samples are single phase and have the layered a-NaFeO2 type structure. The electrochemical cycling performance shows that pillared layered Li0.95Ca0.025CoO2 has higher specific capacity and retain better charge–discharge cycling performance than LiCoO2. The XRD patterns show that Li0.95Ca0.025CoO2 almost does not change from the O3 to the H1-3 phase even at 4.6 V (vs. LiC/Li) while LiCoO2 exists mainly as the H1-3 phase at the
3.3. Analysis of the role of Ca2C pillars In order to gain insight into the effect of Ca2C on the electrochemical behavior of electrode material, the composition and structure of Li0.95Ca0.025CoO2 after charging are investigated further subsequently. After assembling test cells with Li0.95Ca0.025CoO2 as the cathode material, the test cells were charged and discharged for O3(003) 2θ=18.86
O3(003) 2θ=18.46
x in LixCoO2
O3(003) 2θ=18.34
OCV
1.00 0.32
O3(003) 2θ=18.85
x in LixCa0.025CoO2
I (mA cm–2)
400 200
1345
H1-3(006) 2θ=18.83
O3(003) 2θ=18.38
0.95
4.5V O3(003) 2θ=18.41
0.26
H1-3(006) 2θ=18.81
4.6V
0.19
H1-3(006) 2θ=18.85 4.7V
0.12
H1-3(006) 2θ=18.84
O3(003) 2θ=18.47
0.05
0.12 18
19
18
19
20
2θ/° Fig. 4. The XRD patterns of LiCoO2 (left) and Li0.95Ca0.025CoO2 (right) charged to different upper cutoff potential.
1346
W.S. Yang et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1343–1346
same potential. This indicates that Ca2C pillars can suppress the unwanted phase transition. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 20301002) and Beijing Nova Fund (Grant No. H013610350112). References [1] Z. Chen, Z. Lu, J.R. Dahn, J. Electrochem. Soc. 149 (2002) A1604. [2] Z. Chen, J.R. Dahn, Electrochem. Solid-State Lett. 5 (2002) A213.
[3] Y.J. Kim, H. Kim, B. Kim, D. Ahn, J.-G. Lee, T.-J. Kim, D. Son, J. Cho, Y.-W. Kim, B. Park, Chem. Mater. 15 (2003) 1505. [4] J. Cho, J.-G. Lee, B. Kim, B. Park, Chem. Mater. 15 (2003) 3190. [5] C. Julien, Solid State Ionics 157 (2003) 57. [6] S. Gopukumar, Y. Jeong, K.B. Kim, Solid State Ionics 159 (2003) 223. [7] I. Yanase, T. Ohtaki, M. Watanabe, Solid State Ionics 151 (2002) 189. [8] C.-H. Han, Y.-S. Hong, K. Kim, Solid State Ionics 159 (2003) 241. [9] J.N. Reimers, J.R. Dahn, J. Electrochem. Soc. 139 (1992) 2091. [10] T. Ohzuku, A. Ueda, J. Electrochem. Soc. 141 (1994) 2972. [11] G.G. Amatucci, J.M. Tarascon, L.C. Klein, J. Electrochem. Soc. 143 (1996) 1114. [12] A.V. der Ven, M.K. Aydinol, G. Ceder, J. Electrochem. Soc. 145 (1998) 2149. [13] Z. Chen, J.R. Dahn, Electrochim. Acta 49 (2004) 1079.
Synthesis and electrochemical characterization of pillared layered Li1K2xCaxCoO2 W.S. Yang, X.M. Li, L. Yang, D.G. Evans, X. Duan * Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, P.O. Box 98, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China
Abstract The pillared layered Li1K2xCaxCoO2 has been obtained for the first time by a molten salt ion exchange reaction. The X-ray diffraction data of the materials show that they are single phase and retain the layered a-NaFeO2 type structure. The electrochemical testing of Li1K2xCaxCoO2 cells with an upper cutoff potential of 4.5 and 4.7 V (vs. LiC/Li) show that the specific capacity of the oxides have been increased, respectively, to 175 and 211 mAh gK1, and retain better charge–discharge cycling performance. The XRD data shows these improvements are attributed to the pillaring role of Ca2C which helps to suppress unwanted phase transition. q 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; C. X-ray diffraction; D. Electrochemical properties
1. Introduction Layered LiCoO2 is the preferred material in commercial lithium-ion batteries in view of its ease of synthesis and high reversibility. Although the theoretical capacity of LiCoO2 can achieve to 274 mAh gK1, in practice, when cycled with an upper cutoff voltage of about 4.2 V (vs. LiC/Li) (corresponding to repeated extraction/insertion of about 0.5 Li per LiCoO2), the specific capacity is only about 140 mAh gK1. To obtain a higher capacity, it must be charged above 4.2 V but this results in rapid capacity loss, thought to be caused by side reaction with the electrolyte and the structural instability of LixCoO2 with x!0.5 [1]. To counteract this problem, some groups have coated LiCoO2 with oxides in order to suppress the side reactions [2–4]. Partial substitution of Co with other metals has also been attempted with the aim of stabilizing the layered structure [5–8]. However, the capacity retention of coated or doped LiCoO2 is still unacceptable for industrial applications. layered LiCoO2 is isostructural with the rhombohedral R3m a-NaFeO2 type structure. It is an ordered rock salt structure with edge sharing CoO6 octahedral linked to form CoO2 sheets. They give an unobstructed two-dimensional pathway. It has been
* Corresponding author. Tel.: C86 10 6442 5395; fax: C86 10 6442 5385. E-mail address: [email protected] (X. Duan).
0022-3697/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2006.01.067
shown that the removal of the lithium ions during extraction results in a set of phase change, i.e. from O3/H1-3/O1 [1,9– 13], and the O3/H1-3 phase change near 4.5 V leads to rapid capacity loss in LixCoO2 [13]. We show here a novel way suppress this unwanted phase change by introducing Ca2C into the interlayer galleries to give pillared layered Li1K2xCaxCoO2 materials. 2. Experimental 2.1. Sample preparation LiCoO2 precursor was prepared by sol–gel method using citric acid (C6H10O7) as chelating agent. The gel was formed at 120 8C by mixing LiOH and Co(NO3)$6H2O (Li/CoZ1.05/1) with citric acid (C6 H10O7) solution. The gel was then dried at 120 8C in vacuum to yield organic polymer foam. After baked at 500 8C for 3 h, it was calcined in air at 850 8C for 6 h to obtain the LiCoO2 precursor. The high purity Ca(NO3)2$4H2O and LiCoO2 with a molar ratio of 5:1 were mixed and heated at 70 8C for a period of time (1, 2 and 4 h), respectively, then the products were washed by acetone. Finally, the products were dried at 70 8C in vacuum for 3 h and calcined at 500 8C for 10 h. 2.2. Characterization X-ray diffraction (XRD) was recorded on a Shimadzu XRD6000 diffractometer using a monochromatized Cu Ka1 source
1344
W.S. Yang et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1343–1346 Table 2 The oxidation and reduction peak potentials of LiCoO2 and Li0.95Ca0.025CoO2
003
700 600
Sample
100
d c b a
0 20
40
60
80
2θ/° Fig. 1. The XRD patterns of LiCoO2 and Li1K2xCaxO2 samples with different period of ion exchange reaction time (a) LiCoO2, (b) 1 h, (c) 2 h and (d) 4 h.
˚ ) with a step size of 0.028 and 4 s per scan. (lZ1.5406 A Elemental analysis for metal ions was tested using Inductively Coupled Plasma Emission Spectroscopy (Shimadzu ICPS7500). The electrochemical cycling performance of electrode materials was examined using a two-electrode test cell with lithium foil as the negative electrode. The positive electrode was a mixture of Li0.95Ca0.025CoO2 (or LiCoO2), acetylene black and polyvinylidene fluoride (PVDF) in the weight ratio of 85:10:5. Cell assembly was carried out in an argon filled glove box (H2O!1 ppm, O2!1 ppm) with an electrolyte of 1 mol LK1 LiPF6 in EC–DMC (1:1 ratio in volume) solution and Celgard 2400 as separators membrane. The electrochemical cycling performance was collected using a LAND CT2001A computer-controlled apparatus within the potential range of 3.0 to 4.5 or 4.7 V (vs. LiC/Li) at a constant current density of 0.2 mA cmK2. Cyclic voltammetry (CV) was recorded with a 15 mV/s sweep rate using a ZAHNER IM6e electrochemical workstation. 3. Results and discussion 3.1. Effects of molten salt ion exchange reaction time on the samples A serious of pillared layered Li1K2xCaxCoO2 was obtained by reaction of LiCoO2 precursor and Ca(NO3)2$4H2O for different period of ion exchange reaction time. The patterns of XRD and the results of element analysis are shown in Fig. 1 and Table 1, respectively. Table 1 The element analysis results and the chemical composition of the samples with different period of ion exchange reaction time Reaction time (h)
Li (wt%)
Ca (wt%)
Co (wt%)
Chemical formation
0 1 2 4
7.09 6.73 6.59 6.45
0.00 0.80 1.01 1.32
60.21 58.92 58.87 58.94
LiCoO2 Li0.97Ca0.019CoO2 Li0.95Ca0.025CoO2 Li0.93Ca0.033CoO2
0.025CoO2
Reduction peak potential V (vs. LiC/Li)
DV (V)
4.014 3.972
3.856 3.857
0.150 0.115
The XRD patterns indicate that the ion exchange reaction time does not affect the structure of the samples, and all the samples are single phase and retain the layered a-NaFeO2 type structure. The data of Table 2 shows that the content of LiC in samples decrease and the content of Ca2C increase in relationship of 2:1 with the reaction time prolonged. But if the Ca2C instead of too more LiC, the specific capacity of the sample will lose more, so we adopted 2 h as the ion exchange reaction time in this study. 3.2. Electrochemical performance The electrochemical cycling performance of Li0.95Ca0.025 CoO2 and LiCoO2 under higher cutoff potential (such as 4.5 and 4.7 V vs. LiC/Li) is shown in Fig. 2. It is evident that the Li0.95Ca0.025CoO2 cathode shows high specific capacity and excellent capacity retention than LiCoO2 under higher cutoff potential. This indicates we can access a specific capacity of more than 140 mAh gK1 in LiCoO2 and keep excellent cycling behavior by the pillaring method. The cyclic voltammetry (CV) curves of LiCoO2 and Li0.95Ca0.025CoO2 at a sweep rate of 15 mV sK1 which are carried out during the first cycle are illustrated in Fig. 3. The CVs clearly show three pair of current peaks, respectively, but the peak potentials are different as shown in Table 2. The oxidation peak potential of Li0.95Ca0.025CoO2 is lower, while its reduction peak potential is higher than that of LiCoO2, which imply that the reversibility of Li0.95Ca0.025CoO2 is better than that of LiCoO2 and the exist of Ca2C benefit to the diffusion of LiC.
200
Discharge capacity (mAh g–1)
104
108 110 113
200
107
300
LiCoO2 Li0.95Ca 105
400 101 102 006
Intensity
500
Oxidation peak potential V (vs. LiC/Li)
150
100 Li0.95Ca0.025CoO2 (4.5V) LiCoO2
50
(4.5V)
Li0.95Ca0.025CoO2 (4.7V) LiCoO2
(4.7V)
0 0
1
2
3
4
5
6
7
8
9
10
11
Cycle number Fig. 2. Electrochemical cycling performance of LiCoO2 and Li0.95Ca0.025CoO2 materials with different upper cutoff potential.
W.S. Yang et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1343–1346
5 cycles during the cutoff potential of 3.0–4.5 V (vs. LiC/Li) at a current density of 0.2 mA cmK2, finally the cells were charged to 4.5 V and then disassembled. The Li0.95Ca0.025 CoO2 cathode films were washed with DMC and the content of Ca and Co in the material was analyzed, and the results are shown in Table 3. According to Table 3, we find that the atomic ratio of Ca/Co in the material is not changed after cycling. The result indicates that the Ca2C cannot be extracted from the interlayer space when charged. The XRD patterns of LiCoO2 and Li0.95Ca0.025CoO2 materials charged, respectively, to 4.5, 4.6 and 4.7 V (vs. LiC/Li) are shown in Fig. 4. When charged to 4.5 V, the O3(003) peaks of these two samples moved to lower angle, it means the interlayer spacing expanded. However, the expanded extent of Li0.95Ca0.025CoO2 is smaller than that of LiCoO2. When charged to 4.6 V, a new peak which is known as (006) peak of H1-3 phase appeared obviously [13]. However, Li0.95Ca0.025CoO2 almost does not change to H1-3 from O3 phase at the same potential. This indicates that Ca2C pillars can suppress the unwanted phase transition.
800 600
(b) (a)
0 –200 –400 –600 3.6
3.8
4.0
4.2
4.4
E ( V vs.Li+/Li) Fig. 3. Cyclic voltammetry curves of (a) LiCoO2 and (b) Li0.95Ca0.025CoO2 materials at a sweep rate of 15 mV/s. Table 3 Results of the element analysis of Li0.95Ca0.025CoO2 before and after cycling
Uncharged Cycled
Ca (wt%)
Co (wt%)
Ca/Co atomic ratio
1.01 0.892
58.87 52.75
0.025 0.025
4. Conclusion Pillared layered Li1K2xCaxCoO2 has been synthesized by a molten salt ion exchange method. All the samples are single phase and have the layered a-NaFeO2 type structure. The electrochemical cycling performance shows that pillared layered Li0.95Ca0.025CoO2 has higher specific capacity and retain better charge–discharge cycling performance than LiCoO2. The XRD patterns show that Li0.95Ca0.025CoO2 almost does not change from the O3 to the H1-3 phase even at 4.6 V (vs. LiC/Li) while LiCoO2 exists mainly as the H1-3 phase at the
3.3. Analysis of the role of Ca2C pillars In order to gain insight into the effect of Ca2C on the electrochemical behavior of electrode material, the composition and structure of Li0.95Ca0.025CoO2 after charging are investigated further subsequently. After assembling test cells with Li0.95Ca0.025CoO2 as the cathode material, the test cells were charged and discharged for O3(003) 2θ=18.86
O3(003) 2θ=18.46
x in LixCoO2
O3(003) 2θ=18.34
OCV
1.00 0.32
O3(003) 2θ=18.85
x in LixCa0.025CoO2
I (mA cm–2)
400 200
1345
H1-3(006) 2θ=18.83
O3(003) 2θ=18.38
0.95
4.5V O3(003) 2θ=18.41
0.26
H1-3(006) 2θ=18.81
4.6V
0.19
H1-3(006) 2θ=18.85 4.7V
0.12
H1-3(006) 2θ=18.84
O3(003) 2θ=18.47
0.05
0.12 18
19
18
19
20
2θ/° Fig. 4. The XRD patterns of LiCoO2 (left) and Li0.95Ca0.025CoO2 (right) charged to different upper cutoff potential.
1346
W.S. Yang et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1343–1346
same potential. This indicates that Ca2C pillars can suppress the unwanted phase transition. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 20301002) and Beijing Nova Fund (Grant No. H013610350112). References [1] Z. Chen, Z. Lu, J.R. Dahn, J. Electrochem. Soc. 149 (2002) A1604. [2] Z. Chen, J.R. Dahn, Electrochem. Solid-State Lett. 5 (2002) A213.
[3] Y.J. Kim, H. Kim, B. Kim, D. Ahn, J.-G. Lee, T.-J. Kim, D. Son, J. Cho, Y.-W. Kim, B. Park, Chem. Mater. 15 (2003) 1505. [4] J. Cho, J.-G. Lee, B. Kim, B. Park, Chem. Mater. 15 (2003) 3190. [5] C. Julien, Solid State Ionics 157 (2003) 57. [6] S. Gopukumar, Y. Jeong, K.B. Kim, Solid State Ionics 159 (2003) 223. [7] I. Yanase, T. Ohtaki, M. Watanabe, Solid State Ionics 151 (2002) 189. [8] C.-H. Han, Y.-S. Hong, K. Kim, Solid State Ionics 159 (2003) 241. [9] J.N. Reimers, J.R. Dahn, J. Electrochem. Soc. 139 (1992) 2091. [10] T. Ohzuku, A. Ueda, J. Electrochem. Soc. 141 (1994) 2972. [11] G.G. Amatucci, J.M. Tarascon, L.C. Klein, J. Electrochem. Soc. 143 (1996) 1114. [12] A.V. der Ven, M.K. Aydinol, G. Ceder, J. Electrochem. Soc. 145 (1998) 2149. [13] Z. Chen, J.R. Dahn, Electrochim. Acta 49 (2004) 1079.