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High performance lithium cobalt oxides prepared in molten KCl for rechargeable lithium-ion batteries
Electrochemistry Communications 6 (2004) 505–509 www.elsevier.com/locate/elecom
High performance lithium cobalt oxides prepared in molten KCl for rechargeable lithium-ion batteries Hongying Liang a, Xinping Qiu a,*, Shicao Zhang b, Zhiqi He a, Wentao Zhu a, Liquan Chen a b
a Department of Chemistry, Tsinghua University, Beijing 100084, China Department of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China
Received 23 February 2004; accepted 4 March 2004 Published online: 10 April 2004
Abstract Lithium cobalt oxides (LiCoO2 ) powders have been easily prepared using molten KCl as high-temperature solvent. X-ray diffraction (XRD) and scanning electron microscope (SEM) measurements indicate that the obtained LiCoO2 powders are homogenous and the particle size of LiCoO2 powders can be controlled by flux content and heating time. For a LiCoO2 sample heating at 850 °C for 1 h, at molar ratio of KCl/CoO ¼ 4, the particle-size distribution is narrow at about 1 lm and most particles show spherical shape. Electrochemical studies indicate this sample has an excellent discharge capacity and cycling performance. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Lithium cobalt oxides; Lithium-ion batteries; Flux; Particle size
1. Introduction High-performance rechargeable lithium-ion battery is one of the most promising power sources for portable electronic devices. The layered transitional metal oxides LiCoO2 is the main commercially used cathode material for the rechargeable lithium-ion batteries, because of its high specific capacity, high operating cell voltage and excellent rechargeable capability. The commercial LiCoO2 is commonly prepared by solid-state reaction, which requires high-temperature heating for a long time and results in inhomogeneity, irregular morphology, and broad particle-size distribution of powder. To overcome these disadvantages, various solution-based techniques have been developed, such as sol–gel [1,2], co-precipitation [3,4], hydrothermal synthesis [5], starch-assisted combustion method [6], and so on. Most of these methods can produce materials *
Corresponding author. Fax: +86-10-62794234. E-mail addresses: [email protected], lianghy@mail. tsinghua.edu.cn (X. Qiu). 1388-2481/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2004.03.004
with better homogeneity and finer powder than the normal solid-state reaction method. However, these methods require difficult condition and a large quantity of solvent and organic materials like citric acid, ethylene glycol, polyvinyl alcohol, etc. [7]. A molten salt synthesis or flux growth synthesis method has been reported to be one of the simplest means to prepare multi-component oxides [8]. Molten salts as reaction media provide an alternative to aqueous chemistry by offering the possibility to change the solubility and the reactivity of the reactants. Recently, Tang et al. [9] obtained well-crystallized LiCoO2 using LiCl-flux method with flux content of LiCl/Co(OH)2 ¼ 19.6 (molar ratio). They also developed a ‘‘Li-containing flux method’’ to prepare lithium manganese oxides [10,11]. Han et al. [12] had prepared LiCoO2 using eutectic mixtures of LiCl– Li2 CO3 molten salts. They found that at molar ratio of total Li ion/Co ion ¼ 7, the LiCoO2 powders prepared at 800 and 900 °C for 24 h show good capacity retention and deliver, on average, approximately 100 and 120 mA h/g after 40 cycles. They also prepared
3.1. Structure analysis Excessive KCl was observed obviously around the wall of the crucible when KCl content reach N ¼ 4. So we have not tried any higher content KCl than N ¼ 4 in the present study. Fig. 1 shows the XRD patterns of the powder heating at 850 °C for 8 h using different content KCl. The diffraction patterns indicate that the crystal system is identical to that of single-phase hexagonal
108 110 113
009 107
104
N=1 20
30
2. Experimental
3. Results and discussion
N=4
N=2
10
CoO (Hainan Jinyi New Materials Co.), LiOH H2 O and KCl (AR, Beijing Chemical Reagent Co.) were mixed with a mortar and pestle. The particle size of CoO is on the order of nanometer as observed by scanning electron microscope (SEM). The flux content was defined as N by the molar ratio of KCl/CoO. The mixtures were heated at 850 °C for 8 or 1 h in air, and then cooled to ambient temperature. The products were immersed in deionized water, washed and filtered to remove the residual fluxes. Finally, the products were dried at 130 °C for 24 h. The phase composition of the powder was identified by X-ray diffraction (XRD) using a D/MAX-RB diffractometer with Cu Ka radiation. The morphology of the products was observed by SEM (LEO1530). The working electrodes were composed of LiCoO2 , carbon black and polyvinylidene fluoride at a weight ratio of 85:10:5. The mixed slurry was uniformly cast on aluminum foil, dried at 110 °C for 24 h and rollingpressed. The surface density of the active material is about 6–8 mg/cm2 . The electrochemical cells were assembled with pure lithium foil as counter electrode, Cellgard 2300 as the separator and 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (1:1 v/v) as the electrolyte. All cells were assembled and sealed in an argon-filled glove box and test on a Neware charge– discharge cycler.
015
101 006 102
LiCo0:8 M0:2 O2 (M ¼ Al, Ni) using the same method [13]. In these studies, Li-containing salts as molten salt were utilized as both solvent and reacting species. The reaction usually needs a large amount of Li-containing molten salt and the content of molten salts is constant. The flux content had no effect on particle size of the powders. In this paper, LiCoO2 powders were easily prepared using a relatively low content of molten KCl as hightemperature solvent. The particle size of powders can be controlled by KCl content. The obtained LiCoO2 powders are homogeneous and have good electrochemical performance.
003
H. Liang et al. / Electrochemistry Communications 6 (2004) 505–509
Intensity (a.u.)
506
40 50 2θ (Degree)
60
70
80
Fig. 1. XRD patterns of the LiCoO2 powders heating at 850 °C for 8 h using different content KCl.
LiCoO2 for N ¼ 1, 2 and 4. No impurity peaks are detected. The lattice constants a, c values and c=a ratio of the powders are listed in Table 1. These data are identical to those obtained from a single-crystal XRD study within exper[14] (a ¼ 2:8161(5) and c ¼ 14:0536(5) A) imental errors. The well-defined doublets (006, 102) and (108, 110) indicate an ordered distribution of lithium and cobalt ions exists in the structure [15]. The intensity ratios of XRD lines (003)/(104) and (003)/(102) increase with the increasing KCl content, which means the sample has a better-developed layer structure. The presence of molten salt phase probably facilitates the formation of the LiCoO2 phase. Since the diffusion rates of the components in molten salts are much higher than those in the solid-state reaction. It probably takes less time to carry out the reaction. So the heating time was shortened to 1 h. The XRD patterns of the powder are shown in Fig. 2. We did not find peaks referring to impurities. The diffraction patterns are identical to those of the powders heating for 8 h that indicate the formation of single-phase hexagonal LiCoO2 . The intensity ratios of XRD lines (003)/ (104) and (003)/(102) also increase with the increasing KCl content. The lattice constants a, c values and c=a ratio of the powders are given in Table 1 too. These data are identical to those obtained from a single-crystal XRD study within experimental errors [14]. Table 1 The lattice constants a, c and c=a ratio of LiCoO2 powders heating at 850 °C for 1 and 8 h with different content KCl Sample 8 8 8 1 1 1
h, h, h, h, h, h,
N N N N N N
¼1 ¼2 ¼4 ¼1 ¼2 ¼4
a (A)
c (A)
c=a
2.8155(6) 2.8167(6) 2.8156(8) 2.8168(8) 2.8164(4) 2.8169(1)
14.0479(6) 14.0529(4) 14.0497(1) 14.0589(9) 14.0528(8) 14.0561(9)
4.9894 4.9890 4.9898 4.9910 4.9896 4.9899
H. Liang et al. / Electrochemistry Communications 6 (2004) 505–509
507
3.2. Morphology, particle-size and distribution of the particles
Intensity (a.u.)
N=4
N=2
N=1 10
20
30
40
50
60
70
80
2θ (Degree) Fig. 2. XRD patterns of the LiCoO2 powders heating at 850 °C for 1 h using different content KCl.
Fig. 3 shows the SEM photographs of the LiCoO2 powders heating at 850 °C for 8 or 1 h with different content KCl. All the samples are composed of wellcrystallized polygonal particles without agglomeration. With the increasing flux content, the particles size decreases and the distribution becomes narrow. At the same flux content, the particle size becomes larger with the increasing heating time. For a sample heating for 8 h, N ¼ 1, the sample shows a broader particle-size distribution in the 2–10 lm range. When the flux content increases to N ¼ 4, the particle-size distribution becomes narrow and most particles is averaged at about 2 lm. As the heating time shorten to 1 h, N ¼ 4, the particles average size decreases to 1 lm. The particle-size distribution is narrow and most particles show spherical shape. All these indicate that the presence of KCl flux hinders the growth up of LiCoO2 powder and the
Fig. 3. SEM photographs of the LiCoO2 powders heating at 850 °C for 8 or 1 h with different content KCl: (a) 8 h, N ¼ 1; (b) 1 h, N ¼ 1; (c) 8 h, N ¼ 2; (d) 1 h, N ¼ 2; (e) 8 h, N ¼ 4; (f) 1 h, N ¼ 4.
508
H. Liang et al. / Electrochemistry Communications 6 (2004) 505–509
particle-size of LiCoO2 powder can be controlled by the flux content and heating time.
as seen from SEM photograph. The charge and discharge profiles of the cell with this sample are given in Fig. 5. The discharge capacity is 127 mA h/g in the first cycle and increases gradually in the beginning six cycles. In the subsequent cycles, the discharge capacity keeps stable (changes from 137 mA h/g in the 6th cycle to 133 mA h/g in the 30th cycle) and shows excellent cycling behavior. These results suggest that a
3.3. Electrochemical properties
Discharge capacity (mAh/g)
The electrochemical performance of a cell is mainly governed by Liþ diffusion length and diffusion rate. Diffusion length depends on particle microstructure and particle size. The formation of large crystallites tends to lower the kinetics of diffusion. As a result, polarization is increased during charge and discharge, and thus reduces workability of the cathode. By contrast, smaller particles promote shorter pathways for solid-state diffusion of Li ions, results in better cycleability and charge retention. Therefore, particle-size has a direct influence on the electrochemical performance of the cell [16]. Fig. 4 shows the voltage vs. specific discharge capacity of the cells with LiCoO2 powders heating at 850 °C for 8 h or 1 h with different content KCl. The cells were cycled between 4.25 and 3 V with a constant current density of 0.6 mA/cm2 at 20 °C. At the end of the 20th cycle, for the sample a, b, c, d, e and f in Fig. 4, the capacity is 128, 119, 128, 113, 128 and 139 mAh/g, respectively. It is apparent that the cycleability of the cell composed of sample heating for 1h, N ¼ 4 is better than those with the other samples due to its smaller particle size and better homogeneity
4.4 4.2
Potential (V)
4.0 3.8 3.6 3.4 10
3.2 15 1
3.0
5 20 3025
2.8 0
20
40
60
80
100
140
Fig. 5. Charge and discharge profiles of the sample heating at 850 °C for 1 h, N ¼ 4. The current density is 0.6 mA/cm2 . The cutoff voltage is from 3 to 4.25 V.
140
140
120
120
100
100
(a)
(b)
80
80
140
140
120
120
100
100
(c)
(d)
80
80
140
140
120
120
100
100
(d) 80
120
Specific capacity (mAh/g)
(f) 80
0
5
10
15
20
25
30 0
5
10
15
20
25
30
Cycle number Fig. 4. Evolution of discharge capacity with the number of cycles of the cells with sample: (a) 8 h, N ¼ 1; (b) 1 h, N ¼ 1; (c) 8 h, N ¼ 2; (d) 1 h, N ¼ 2; (e) 8 h, N ¼ 4; (f) 1 h, N ¼ 4. The cells cycled at a constant current density of 0.6 mA/cm2 between 3 and 4.25 V.
H. Liang et al. / Electrochemistry Communications 6 (2004) 505–509
smaller size particle with better homogeneity and a well-developed layered structure has an excellent discharge capacity and cycling performance. 4. Conclusion In this paper, a KCl flux synthesis method has been used to prepare LiCoO2 for lithium ion batteries. LiCoO2 powders were easily obtained with homogeneity, regular morphology, and narrow particle-size distribution using molten KCl as a high-temperature solvent. The particles size decreases and the distribution becomes narrow with the increasing flux content. The LiCoO2 sample heating at 850 °C for 1 h, N ¼ 4, has smaller particle size and better homogeneity than the other samples. Most particles are averaged at 1 lm and show spherical shape. This sample has an excellent discharge capacity and cycling performance. The discharge capacity is 127 mA h/g in the first cycle and 133 mA h/g in the 30th cycle. This sample is an excellent cathode material for lithium rechargeable batteries. Acknowledgements The authors appreciate the financial support of the State Key Basic Research Program of PRC (2002CB211803).
509
References [1] P. Barboux, J.M. Tarascon, F.K. Shokoohi, J. Solid State Chem. 94 (1991) 185. [2] B. Garcia, J. Farcy, J.P. Pereira-Ramos, J. Electrochem. Soc. 144 (1997) 1179. [3] X. Qiu, X. Sun, W. Shen, N. Chen, Solid State Ionics 93 (1997) 335. [4] H. Chen, X. Qiu, W. Zhu, P. Hagenmuller, Electrochem. Comm. 4 (2002) 488. [5] D. Larcher, M.R. Palacin, G.G. Amatucci, J.M. Tarascon, J. Electrochem. Soc. 144 (1997) 408. [6] P. Kalyani, N. Kalaiselvi, N. Muniyandi, J. Power Sources 111 (2002) 232. [7] E. Zhecheva, R. Stoyanova, M. Gorova, R. Alcantara, J. Mopales, J.L. Tirado, Chem. Mater. 8 (1996) 1429. [8] P. Afanasiev, C. Geantet, Coord. Chem. Rev. 178–180 (1998) 1725. [9] W. Tang, H. Kanoh, K. Ooi, Electrochem. Solid-State Lett. 1 (1998) 145. [10] X. Yang, W. Tang, H. Kanoh, K. Ooi, J. Mater. Chem. 9 (1999) 2683. [11] W. Tang, X. Yang, Z. Liu, S. Kasaishi, K. Ooi, J. Mater. Chem. 12 (2002) 2991. [12] C. Han, Y. Hong, C.M. Park, K. Kim, J. Power Sources 92 (2001) 95. [13] C. Han, Y. Hong, K. Kim, Solid State Ionics 159 (2003) 241. [14] J. Akimoto, Y. Gotoh, Y. Oosawa, J. Solid State Chem. 141 (1998) 298. [15] D. Guyomard, J.M. Tarascon, J. Electrochem. Soc. 139 (1992) 937. [16] O.A. Shlyakhtin, Y.S. Yoon, Y. Oh, J. Eur. Ceramic Soc. 23 (2003) 1893.
High performance lithium cobalt oxides prepared in molten KCl for rechargeable lithium-ion batteries Hongying Liang a, Xinping Qiu a,*, Shicao Zhang b, Zhiqi He a, Wentao Zhu a, Liquan Chen a b
a Department of Chemistry, Tsinghua University, Beijing 100084, China Department of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China
Received 23 February 2004; accepted 4 March 2004 Published online: 10 April 2004
Abstract Lithium cobalt oxides (LiCoO2 ) powders have been easily prepared using molten KCl as high-temperature solvent. X-ray diffraction (XRD) and scanning electron microscope (SEM) measurements indicate that the obtained LiCoO2 powders are homogenous and the particle size of LiCoO2 powders can be controlled by flux content and heating time. For a LiCoO2 sample heating at 850 °C for 1 h, at molar ratio of KCl/CoO ¼ 4, the particle-size distribution is narrow at about 1 lm and most particles show spherical shape. Electrochemical studies indicate this sample has an excellent discharge capacity and cycling performance. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Lithium cobalt oxides; Lithium-ion batteries; Flux; Particle size
1. Introduction High-performance rechargeable lithium-ion battery is one of the most promising power sources for portable electronic devices. The layered transitional metal oxides LiCoO2 is the main commercially used cathode material for the rechargeable lithium-ion batteries, because of its high specific capacity, high operating cell voltage and excellent rechargeable capability. The commercial LiCoO2 is commonly prepared by solid-state reaction, which requires high-temperature heating for a long time and results in inhomogeneity, irregular morphology, and broad particle-size distribution of powder. To overcome these disadvantages, various solution-based techniques have been developed, such as sol–gel [1,2], co-precipitation [3,4], hydrothermal synthesis [5], starch-assisted combustion method [6], and so on. Most of these methods can produce materials *
Corresponding author. Fax: +86-10-62794234. E-mail addresses: [email protected], lianghy@mail. tsinghua.edu.cn (X. Qiu). 1388-2481/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2004.03.004
with better homogeneity and finer powder than the normal solid-state reaction method. However, these methods require difficult condition and a large quantity of solvent and organic materials like citric acid, ethylene glycol, polyvinyl alcohol, etc. [7]. A molten salt synthesis or flux growth synthesis method has been reported to be one of the simplest means to prepare multi-component oxides [8]. Molten salts as reaction media provide an alternative to aqueous chemistry by offering the possibility to change the solubility and the reactivity of the reactants. Recently, Tang et al. [9] obtained well-crystallized LiCoO2 using LiCl-flux method with flux content of LiCl/Co(OH)2 ¼ 19.6 (molar ratio). They also developed a ‘‘Li-containing flux method’’ to prepare lithium manganese oxides [10,11]. Han et al. [12] had prepared LiCoO2 using eutectic mixtures of LiCl– Li2 CO3 molten salts. They found that at molar ratio of total Li ion/Co ion ¼ 7, the LiCoO2 powders prepared at 800 and 900 °C for 24 h show good capacity retention and deliver, on average, approximately 100 and 120 mA h/g after 40 cycles. They also prepared
3.1. Structure analysis Excessive KCl was observed obviously around the wall of the crucible when KCl content reach N ¼ 4. So we have not tried any higher content KCl than N ¼ 4 in the present study. Fig. 1 shows the XRD patterns of the powder heating at 850 °C for 8 h using different content KCl. The diffraction patterns indicate that the crystal system is identical to that of single-phase hexagonal
108 110 113
009 107
104
N=1 20
30
2. Experimental
3. Results and discussion
N=4
N=2
10
CoO (Hainan Jinyi New Materials Co.), LiOH H2 O and KCl (AR, Beijing Chemical Reagent Co.) were mixed with a mortar and pestle. The particle size of CoO is on the order of nanometer as observed by scanning electron microscope (SEM). The flux content was defined as N by the molar ratio of KCl/CoO. The mixtures were heated at 850 °C for 8 or 1 h in air, and then cooled to ambient temperature. The products were immersed in deionized water, washed and filtered to remove the residual fluxes. Finally, the products were dried at 130 °C for 24 h. The phase composition of the powder was identified by X-ray diffraction (XRD) using a D/MAX-RB diffractometer with Cu Ka radiation. The morphology of the products was observed by SEM (LEO1530). The working electrodes were composed of LiCoO2 , carbon black and polyvinylidene fluoride at a weight ratio of 85:10:5. The mixed slurry was uniformly cast on aluminum foil, dried at 110 °C for 24 h and rollingpressed. The surface density of the active material is about 6–8 mg/cm2 . The electrochemical cells were assembled with pure lithium foil as counter electrode, Cellgard 2300 as the separator and 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (1:1 v/v) as the electrolyte. All cells were assembled and sealed in an argon-filled glove box and test on a Neware charge– discharge cycler.
015
101 006 102
LiCo0:8 M0:2 O2 (M ¼ Al, Ni) using the same method [13]. In these studies, Li-containing salts as molten salt were utilized as both solvent and reacting species. The reaction usually needs a large amount of Li-containing molten salt and the content of molten salts is constant. The flux content had no effect on particle size of the powders. In this paper, LiCoO2 powders were easily prepared using a relatively low content of molten KCl as hightemperature solvent. The particle size of powders can be controlled by KCl content. The obtained LiCoO2 powders are homogeneous and have good electrochemical performance.
003
H. Liang et al. / Electrochemistry Communications 6 (2004) 505–509
Intensity (a.u.)
506
40 50 2θ (Degree)
60
70
80
Fig. 1. XRD patterns of the LiCoO2 powders heating at 850 °C for 8 h using different content KCl.
LiCoO2 for N ¼ 1, 2 and 4. No impurity peaks are detected. The lattice constants a, c values and c=a ratio of the powders are listed in Table 1. These data are identical to those obtained from a single-crystal XRD study within exper[14] (a ¼ 2:8161(5) and c ¼ 14:0536(5) A) imental errors. The well-defined doublets (006, 102) and (108, 110) indicate an ordered distribution of lithium and cobalt ions exists in the structure [15]. The intensity ratios of XRD lines (003)/(104) and (003)/(102) increase with the increasing KCl content, which means the sample has a better-developed layer structure. The presence of molten salt phase probably facilitates the formation of the LiCoO2 phase. Since the diffusion rates of the components in molten salts are much higher than those in the solid-state reaction. It probably takes less time to carry out the reaction. So the heating time was shortened to 1 h. The XRD patterns of the powder are shown in Fig. 2. We did not find peaks referring to impurities. The diffraction patterns are identical to those of the powders heating for 8 h that indicate the formation of single-phase hexagonal LiCoO2 . The intensity ratios of XRD lines (003)/ (104) and (003)/(102) also increase with the increasing KCl content. The lattice constants a, c values and c=a ratio of the powders are given in Table 1 too. These data are identical to those obtained from a single-crystal XRD study within experimental errors [14]. Table 1 The lattice constants a, c and c=a ratio of LiCoO2 powders heating at 850 °C for 1 and 8 h with different content KCl Sample 8 8 8 1 1 1
h, h, h, h, h, h,
N N N N N N
¼1 ¼2 ¼4 ¼1 ¼2 ¼4
a (A)
c (A)
c=a
2.8155(6) 2.8167(6) 2.8156(8) 2.8168(8) 2.8164(4) 2.8169(1)
14.0479(6) 14.0529(4) 14.0497(1) 14.0589(9) 14.0528(8) 14.0561(9)
4.9894 4.9890 4.9898 4.9910 4.9896 4.9899
H. Liang et al. / Electrochemistry Communications 6 (2004) 505–509
507
3.2. Morphology, particle-size and distribution of the particles
Intensity (a.u.)
N=4
N=2
N=1 10
20
30
40
50
60
70
80
2θ (Degree) Fig. 2. XRD patterns of the LiCoO2 powders heating at 850 °C for 1 h using different content KCl.
Fig. 3 shows the SEM photographs of the LiCoO2 powders heating at 850 °C for 8 or 1 h with different content KCl. All the samples are composed of wellcrystallized polygonal particles without agglomeration. With the increasing flux content, the particles size decreases and the distribution becomes narrow. At the same flux content, the particle size becomes larger with the increasing heating time. For a sample heating for 8 h, N ¼ 1, the sample shows a broader particle-size distribution in the 2–10 lm range. When the flux content increases to N ¼ 4, the particle-size distribution becomes narrow and most particles is averaged at about 2 lm. As the heating time shorten to 1 h, N ¼ 4, the particles average size decreases to 1 lm. The particle-size distribution is narrow and most particles show spherical shape. All these indicate that the presence of KCl flux hinders the growth up of LiCoO2 powder and the
Fig. 3. SEM photographs of the LiCoO2 powders heating at 850 °C for 8 or 1 h with different content KCl: (a) 8 h, N ¼ 1; (b) 1 h, N ¼ 1; (c) 8 h, N ¼ 2; (d) 1 h, N ¼ 2; (e) 8 h, N ¼ 4; (f) 1 h, N ¼ 4.
508
H. Liang et al. / Electrochemistry Communications 6 (2004) 505–509
particle-size of LiCoO2 powder can be controlled by the flux content and heating time.
as seen from SEM photograph. The charge and discharge profiles of the cell with this sample are given in Fig. 5. The discharge capacity is 127 mA h/g in the first cycle and increases gradually in the beginning six cycles. In the subsequent cycles, the discharge capacity keeps stable (changes from 137 mA h/g in the 6th cycle to 133 mA h/g in the 30th cycle) and shows excellent cycling behavior. These results suggest that a
3.3. Electrochemical properties
Discharge capacity (mAh/g)
The electrochemical performance of a cell is mainly governed by Liþ diffusion length and diffusion rate. Diffusion length depends on particle microstructure and particle size. The formation of large crystallites tends to lower the kinetics of diffusion. As a result, polarization is increased during charge and discharge, and thus reduces workability of the cathode. By contrast, smaller particles promote shorter pathways for solid-state diffusion of Li ions, results in better cycleability and charge retention. Therefore, particle-size has a direct influence on the electrochemical performance of the cell [16]. Fig. 4 shows the voltage vs. specific discharge capacity of the cells with LiCoO2 powders heating at 850 °C for 8 h or 1 h with different content KCl. The cells were cycled between 4.25 and 3 V with a constant current density of 0.6 mA/cm2 at 20 °C. At the end of the 20th cycle, for the sample a, b, c, d, e and f in Fig. 4, the capacity is 128, 119, 128, 113, 128 and 139 mAh/g, respectively. It is apparent that the cycleability of the cell composed of sample heating for 1h, N ¼ 4 is better than those with the other samples due to its smaller particle size and better homogeneity
4.4 4.2
Potential (V)
4.0 3.8 3.6 3.4 10
3.2 15 1
3.0
5 20 3025
2.8 0
20
40
60
80
100
140
Fig. 5. Charge and discharge profiles of the sample heating at 850 °C for 1 h, N ¼ 4. The current density is 0.6 mA/cm2 . The cutoff voltage is from 3 to 4.25 V.
140
140
120
120
100
100
(a)
(b)
80
80
140
140
120
120
100
100
(c)
(d)
80
80
140
140
120
120
100
100
(d) 80
120
Specific capacity (mAh/g)
(f) 80
0
5
10
15
20
25
30 0
5
10
15
20
25
30
Cycle number Fig. 4. Evolution of discharge capacity with the number of cycles of the cells with sample: (a) 8 h, N ¼ 1; (b) 1 h, N ¼ 1; (c) 8 h, N ¼ 2; (d) 1 h, N ¼ 2; (e) 8 h, N ¼ 4; (f) 1 h, N ¼ 4. The cells cycled at a constant current density of 0.6 mA/cm2 between 3 and 4.25 V.
H. Liang et al. / Electrochemistry Communications 6 (2004) 505–509
smaller size particle with better homogeneity and a well-developed layered structure has an excellent discharge capacity and cycling performance. 4. Conclusion In this paper, a KCl flux synthesis method has been used to prepare LiCoO2 for lithium ion batteries. LiCoO2 powders were easily obtained with homogeneity, regular morphology, and narrow particle-size distribution using molten KCl as a high-temperature solvent. The particles size decreases and the distribution becomes narrow with the increasing flux content. The LiCoO2 sample heating at 850 °C for 1 h, N ¼ 4, has smaller particle size and better homogeneity than the other samples. Most particles are averaged at 1 lm and show spherical shape. This sample has an excellent discharge capacity and cycling performance. The discharge capacity is 127 mA h/g in the first cycle and 133 mA h/g in the 30th cycle. This sample is an excellent cathode material for lithium rechargeable batteries. Acknowledgements The authors appreciate the financial support of the State Key Basic Research Program of PRC (2002CB211803).
509
References [1] P. Barboux, J.M. Tarascon, F.K. Shokoohi, J. Solid State Chem. 94 (1991) 185. [2] B. Garcia, J. Farcy, J.P. Pereira-Ramos, J. Electrochem. Soc. 144 (1997) 1179. [3] X. Qiu, X. Sun, W. Shen, N. Chen, Solid State Ionics 93 (1997) 335. [4] H. Chen, X. Qiu, W. Zhu, P. Hagenmuller, Electrochem. Comm. 4 (2002) 488. [5] D. Larcher, M.R. Palacin, G.G. Amatucci, J.M. Tarascon, J. Electrochem. Soc. 144 (1997) 408. [6] P. Kalyani, N. Kalaiselvi, N. Muniyandi, J. Power Sources 111 (2002) 232. [7] E. Zhecheva, R. Stoyanova, M. Gorova, R. Alcantara, J. Mopales, J.L. Tirado, Chem. Mater. 8 (1996) 1429. [8] P. Afanasiev, C. Geantet, Coord. Chem. Rev. 178–180 (1998) 1725. [9] W. Tang, H. Kanoh, K. Ooi, Electrochem. Solid-State Lett. 1 (1998) 145. [10] X. Yang, W. Tang, H. Kanoh, K. Ooi, J. Mater. Chem. 9 (1999) 2683. [11] W. Tang, X. Yang, Z. Liu, S. Kasaishi, K. Ooi, J. Mater. Chem. 12 (2002) 2991. [12] C. Han, Y. Hong, C.M. Park, K. Kim, J. Power Sources 92 (2001) 95. [13] C. Han, Y. Hong, K. Kim, Solid State Ionics 159 (2003) 241. [14] J. Akimoto, Y. Gotoh, Y. Oosawa, J. Solid State Chem. 141 (1998) 298. [15] D. Guyomard, J.M. Tarascon, J. Electrochem. Soc. 139 (1992) 937. [16] O.A. Shlyakhtin, Y.S. Yoon, Y. Oh, J. Eur. Ceramic Soc. 23 (2003) 1893.