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3−xCox]O2 as a cathode material for lithium ion battery
Electrochimica Acta 56 (2011) 7088–7091
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical properties of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 as a cathode material for lithium ion battery Haibo Ren a , Xiang Li a , Zhenghe Peng b,∗ a b
Department of Chemistry and Chemical Engineering, Henan University of Urban Construction, Pingdingshan, 467036, PR China Department of Chemistry, Wuhan University, Wuhan, 430072, PR China
a r t i c l e
i n f o
Article history: Received 4 March 2011 Received in revised form 24 May 2011 Accepted 26 May 2011 Available online 6 June 2011 Keywords: Composite materials Sintering Crystal structure Electrochemical properties
a b s t r a c t Layered Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 (0 ≤ x ≤ 1/3) cathode materials are synthesized by a solvent evaporation method. Although XRD shows that Li[Ni1/3 Mn1/3 Al1/3 ]O2 has no obvious impurity phase, it has poor electrochemical properties. To improve its capability, part of Al in Li[Ni1/3 Mn1/3 Al1/3 ]O2 compound is replaced by Co in this study. The samples are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and charge–discharge test. The results indicate that the introduction of Co has a large influence on the morphology, structure and electrochemical performances of the samples, which become more excellent with an increase of Co content in compounds. Meanwhile, the high-temperature behavior of the samples is also investigated. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
The lithium-ion batteries (LIBs) with high energy density and power capability are extensively used as electrochemical power sources in a lot of modern equipments, such as mobile telephones, laptop computers, video-cameras and so on [1–4]. The most popular cathode material for LIBs is LiCoO2 due to its good recharge ability and easy preparation. However, the high price and toxicity of the cobalt have limited the farther application of LiCoO2 . Recently, many compounds as cathode materials for lithium ion batteries are of great interest and potential candidates to replace the commercial LiCoO2 , such as LiNiO2 , LiMn2 O4 , LiMnO2 , LiNi1/2 Mn1/2 O2 , as well as Li[Ni1/3 Co1/3 Mn1/3 ]O2 and their derivatives [5–12]. In addition, the solid solution of LiAlO2 with various lithiated transition-metal oxides, e.g., Lix Aly Co1−y O2 , Lix Aly Mn1−y O2 and Lix Aly Coz Mn1−y−z O2 [13–18] have been studied on account of their potentially increasing the intercalation voltage and cathode energy density. In this work, the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 (0 ≤ x ≤ 1/3) compounds are prepared and the effect of Co content on the morphology, structure and electrochemical properties of the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 will also be investigated.
Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 (x = 0, 1/12, 1/8, 1/4, 1/3) compounds were synthesized by a solvent evaporation method. A stoichiometric amount of LiC2 H3 O2 ·2H2 O, NiC2 H3 O2 ·4H2 O, CoC2 H3 O2 ·4H2 O, MnC2 H3 O2 ·4H2 O and Al(NO3 )3 ·9H2 O were used as the starting materials. These reagents were dissolved into a proper amount of distilled water and a mauve solution was obtained. The resulting solution was first heated at 100 ◦ C with magnetic stirring until viscous, and then dried at 120 ◦ C for 12 h to obtain a mixed precursor. The mixed precursor was pre-heated at 500 ◦ C for 6 h and finally calcined at 900 ◦ C for 20 h in air to yield final products. X-ray diffraction (XRD) patterns were evaluated using Xray diffraction (Shimadzu XRD-6000) with Cu K␣ radiation ˚ Lattice parameters and unit-cell volumes were cal( = 1.54056 A). culated by a least squares method with FullProf Suite program. Particle morphology of the powders after calcination was observed using a scanning electron microscope (SEM, QUANTA-200). Electrochemical charge–discharge experiments were performed using the CR2016 coin-type cell. Test cathode electrodes were prepared by mix 80:15:5 (mass ratio) of active material, acetylene black and PTFE binder, respectively, in isopropyl alcohol. The test cells were assembled with the electrode prepared above as cathode, lithium metal as anode, and Celgard 2300 film as separator in an argonfilled glove box. The electrolyte was 1 M LiPF6 dissolved in EC + DMC (1:1 volume ratio).
∗ Corresponding author. Tel.: +86 27 8721 8504. E-mail address: [email protected] (Z. Peng). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.05.104
H. Ren et al. / Electrochimica Acta 56 (2011) 7088–7091
Fig. 1. XRD patterns of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 powders sintered at 900 ◦ C for 20 h in air. (a) x = 0, (b) x = 1/12, (c) x = 1/8, (d) x = 1/4 and (e) x = 1/3.
3. Results and discussion Fig. 1 shows typical XRD patterns of all the synthesized Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 (x = 0, 1/12, 1/8, 1/4, 1/3) materials at 900 ◦ C for 20 h. All peaks are sharp and well-defined, suggesting that compounds are generally well crystallized, which can be indexed on the basis of the hexagonal ␣-NaFeO2 structure (space ¯ The XRD patterns of the samples with different Co group: R3m). contents are quite similar. No extra diffraction peaks from related secondary phases or impurities are found, which is different from the finding of Hu et al. [19]. It is believed that all atoms are being incorporated within the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 structure and that the homogeneous solid solution of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 could be prepared successfully in the present synthesis conditions. The clear splitting of the lines assigned to Miller indices (0 0 6, 1 0 2) and (1 0 8, 1 1 0) for the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 compounds (x = 0, 1/12, 1/8, 1/4, 1/3) indicates good characteristic of layer struc-
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ture [20,21]. Hwang et al. [22] believed that the intensity ratio of the (0 0 3) and (1 0 4) peaks (I(0 0 3) /I(1 0 4) ), R, could be used to identify the cation mixing degree. The smaller the R value, the higher the disordering. As shown in Table 1, R values are significantly increased from 0.89 to 1.59, which shows that the cation mixing is reduced with the increase of Co content. In order to illustrate the relation between the unit cell and Co content, the lattice parameters of the samples are calculated by least square method using 10-diffraction lines. The determined parameters are also summarized in Table 1. It is clearly observed that a, c lattice parameters and the unit cell volume increase with more Co content, which is an inevitable result that Al3+ (rAl 3+ = 0.0535 nm) are replaced by Co3+ (rCo 3+ = 0.0545 nm). While c/a values that indicate hexagonal structure disorder [23] are virtually unchanged. Normally, the crystal structure and morphology are two main factors which affect the electrochemical performance of the materials. Fig. 2 shows the SEM micrographs of the samples with different Co contents. The powder of all the samples consists of quasispherical particle. The agglomeration of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 powders (x = 0 and 1/12) can be found from Fig. 2a and b, while there does not exist in Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 powders (x = 1/8, 1/4 and 1/3) from Fig. 2c–e. The particle size of the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 compounds with x = 0 and 1/12 is smaller than that with x = 1/8 and 1/4, and the one with x = 1/3 is the largest, indicating the particle size increases with the Co content increased. In addition, the particle size distribution of the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 powders becomes more uniform at higher Co content. It is also observed that the crystallinity of the synthesized powders becomes better with higher Co content. From above it can come to a conclusion that the particle size, distribution and crystallinity of the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 compounds are influenced by the introduction of Co. Figs. 3 and 4 show the initial charge–discharge curves and cycling performances of the samples, respectively. The cell is cycled between 3.0 and 4.3 V at room temperature at a current density of 20 mA g−1 (0.1 C), here the C-rate is calculated with 200 mA hg −1 as a theoretical capacity. As shown in Fig. 3, the first charge/discharge specific capacities are 141/116, 159/132, 178/150, 188/160 and 194/168 mA h g−1 with a coulombic efficiency about
Fig. 2. SEM micrographs of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 powders sintered at 900 ◦ C. (a) x = 0, (b) x = 1/12, (c) x = 1/8, (d) x = 1/4 and (e) x = 1/3.
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H. Ren et al. / Electrochimica Acta 56 (2011) 7088–7091
Table 1 Intensity ratio of I(0 0 3) /I(1 0 4) and lattice parameters for various samples. Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2
a (nm)
c (nm)
c/a
V (nm3 )
I(0 0 3) /I(1 0 4)
x=0 x = 1/12 x = 1/8 x = 1/4 x = 1/3
0.2852 0.2854 0.2856 0.2860 0.2863
1.4217 1.4219 1.4239 1.4255 1.4265
4.9842 4.9831 4.9858 4.9840 4.9833
0.1001 0.1003 0.1006 0.1010 0.1013
0.89 1.43 1.50 1.54 1.59
82.3%, 83.0%, 84.3%, 85.1% and 86.6% for Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 samples (x = 0, 1/12, 1/8, 1/4, 1/3), respectively. It is obvious that the initial discharge capacities and the coulombic efficiency of the first cycling have an increase with the increasing of Co content in Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 , which is mainly contributed from the reduction of the cell polarization with the addition of Co. In addition, from Fig. 3 charge curves shift upwards and discharge curves do downwards with an increase in the Co content, implying that increasing the Co content can reduce the resis-
tance for the diffusion of lithium ions. According to Fig. 4, at the end of the 20th cycle, the retained discharge capacities for Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 samples (x = 0, 1/12, 1/8, 1/4, 1/3) are 105, 124, 141, 153 and 163 mA h g−1 , respectively, which are 90.5%, 93.9%, 94.0%, 95.6% and 97.0% of initial discharge capacity. It is obvious that the long-term cyclic performance is improved with an increase of Co content and the Li[Ni1/3 Mn1/3 Co1/3 ]O2 compound shows the best reversible capacity and cycling performance among the synthesized Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 compounds. All the above results are consistent with those reported [24,25], with the change of Co content. The theoretical calculations [19] showed that the Al-substituted compounds should hold the advantages of better performance, high potential and high capacity as a cathode for lithium-ion batteries. However, there is some discrepancy between computational and our experimental works. This is mainly because the calculation does not take into account the effects of cation mixing, electronic conduction, lithium diffusion and charge transfer reaction (kinetics) on the performance of the materials. In order to further investigate the electrochemical performances of the samples, Fig. 5 also gives the cycling stability of these materials at 55 ◦ C between 3.0 and 4.3 V at a current density of 20 mA g−1 (0.1 C). It can be seen from Fig. 5 that the initial discharge capacities for all samples are improved compared with those at room temperature, which should be attributed to the increase in electronic conductivity and lower resistance at a higher temperature. However, at the end of the 20th cycle, the retained discharge capacities for Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 samples (x = 0, 1/12, 1/8, 1/4, 1/3) are 34.1%, 70.1%, 85.3%, 91.7% and 95.3% of initial discharge capacity, respectively. It is clear that the synthesized Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 samples with higher Co content hold better high-temperature cycling performance.
Fig. 4. Specific discharge capacity as a function of cycle number for Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 operated between 3.0 and 4.3 V at room temperature under a current density 20 mA g−1 . (a) x = 0, (b) x = 1/12, (c) x = 1/8, (d) x = 1/4 and (e) x = 1/3.
Fig. 5. Specific discharge capacity as a function of cycle number for Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 operated between 3.0 and 4.3 V at 55 ◦ C under a current density 20 mA g−1 . (a) x = 0, (b) x = 1/12, (c) x = 1/8, (d) x = 1/4 and (e) x = 1/3.
Fig. 3. Initial charge–discharge curves of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 cells operated between 3.0 and 4.3 V at room temperature under a current density 20 mA g−1 . (a) x = 0, (b) x = 1/12, (c) x = 1/8, (d) x = 1/4 and (e) x = 1/3.
H. Ren et al. / Electrochimica Acta 56 (2011) 7088–7091
4. Conclusions Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 (0 ≤ x ≤ 1/3) cathode materials have a hexagonal ␣-NaFeO2 structure and have no obvious impurityrelated peaks. As the Co content increased, the lattice parameters and the particle size of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 (0 ≤ x ≤ 1/3) tend to increase and the crystallinity gets more excellent. The electrochemical experiment indicates that the samples with higher Co content hold better electrochemical performance. The Li[Ni1/3 Mn1/3 Co1/3 ]O2 compound shows the best performance among the synthesized compounds. Its initial discharge capacity is 168 mA h g−1 (3–4.3 V, 20 mA g−1 ) and it holds better cycling performance. In addition, the introduction of Co also improve the high-temperature cycling property of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 . Acknowledgements We gratefully acknowledge the financial support from the National Nature Science Foundation of China (Nos. 29833090, 29771025 and 20573078). References [1] I.B. Weinstock, J. Power Sources 110 (11) (2002) 471. [2] K. Amine, C.H. Chen, J. Liu, M. Hammond, A. Jansen, D. Dees, I. Bloom, D. Vissers, G. Henriksen, J. Power Sources 97–98 (2001) 684.
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[3] A.M. Andersson, D.P. Abraham, R. Haasch, S. MacLaren, J. Liu, K. Amine, J. Electrochem. Soc. 149 (2002) A1358. [4] D. Ra, K. Hana, J. Power Sources 163 (2006) 284. [5] Y. Zhang, C. Wang, C. Wang, J. Electrochem. Soc. 156 (2009) A527. [6] Z. Su, Z.W. Lu, X.P. Gao, J. Power Sources 189 (2009) 411. [7] Q.S. Tong, Y. Yang, J.C. Shi, J. Electrochem. Soc. 154 (2007) A656. [8] Y. Koyama, Y. Makimura, I. Tanaka, H. Adachi, T. Ohzuku, J. Electrochem. Soc. 151 (2004) A1499. [9] K.M. Shaju, G.V. Subba Rao, B.V.R. Chowdari, Electrochim. Acta 48 (2002) 145. [10] B.J. Hwang, Y.W. Tsai, D. Cariler, G. Ceder, Chem. Mater. 15 (2003) 3676. [11] H. Ren, Y. Huanga, Y. Wang, Z. Li, P. Cai, Z. Peng, Y. Zhou, Mater. Chem. Phys. 117 (2009) 41. [12] Y. Sun, C. Ouyang, Z. Wang, X. Huang, L. Chen, J. Electrochem. Soc. 151 (2004) A504. [13] A. Van der Ven, M.K. Aydinol, G. Ceder, G. Kresse, J. Hafner, Phys. Rev. B 58 (1998) 2975. [14] Z. Lu, D.D. MacNeil, J.R. Dahn, Electrochem. Solid-State Lett. 4 (2001) A200. [15] G. Ceder, Y.M. Chiang, D.R. Sadoway, M.K. Aydinol, Y.-I. Jang, B. Huang, Nature 392 (1998) 694. [16] A.M. Kannan, A. Manthiram, J. Electrochem. Soc. 150 (2003) A349. [17] S.H. Park, K.S. Park, Y.K. Sun, K.S. Sun, K.S. Nahm, Y.S. Lee, M. Yoshio, Electrochim. Acta 46 (2001) 1215. [18] M. Guilmard, L. Croguennec, C. Delmas, Chem. Mater. 15 (2003) 4484. [19] S.K. Hu, T.C. Chou, B.J. Hwang, G. Ceder, J. Power Sources 160 (2006) 1287. [20] T.H. Cho, S.M. Park, M. Yoshio, T. Hirai, Y. Hideshima, J. Power Sources 142 (2005) 306. [21] A. Rougier, P. Gravereau, C. Delmas, J. Electrochem. Soc. 143 (4) (1996) 1168. [22] B.J. Hwang, R. Santhanam, C.H. Chen, J. Power Sources 114 (2003) 244. [23] J.M. Kim, H.T. Chung, Electrochim. Acta 49 (2004) 937. [24] F. Zhou, X. Zhao, Z. Lu, J. Jiang, J.R. Dahn, Electrochem. Commun. 10 (2008) 1168. [25] F. Zhou, X. Zhao, J.R. Dahn, J. Electrochem. Soc. 156 (4) (2009) A343.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical properties of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 as a cathode material for lithium ion battery Haibo Ren a , Xiang Li a , Zhenghe Peng b,∗ a b
Department of Chemistry and Chemical Engineering, Henan University of Urban Construction, Pingdingshan, 467036, PR China Department of Chemistry, Wuhan University, Wuhan, 430072, PR China
a r t i c l e
i n f o
Article history: Received 4 March 2011 Received in revised form 24 May 2011 Accepted 26 May 2011 Available online 6 June 2011 Keywords: Composite materials Sintering Crystal structure Electrochemical properties
a b s t r a c t Layered Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 (0 ≤ x ≤ 1/3) cathode materials are synthesized by a solvent evaporation method. Although XRD shows that Li[Ni1/3 Mn1/3 Al1/3 ]O2 has no obvious impurity phase, it has poor electrochemical properties. To improve its capability, part of Al in Li[Ni1/3 Mn1/3 Al1/3 ]O2 compound is replaced by Co in this study. The samples are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and charge–discharge test. The results indicate that the introduction of Co has a large influence on the morphology, structure and electrochemical performances of the samples, which become more excellent with an increase of Co content in compounds. Meanwhile, the high-temperature behavior of the samples is also investigated. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
The lithium-ion batteries (LIBs) with high energy density and power capability are extensively used as electrochemical power sources in a lot of modern equipments, such as mobile telephones, laptop computers, video-cameras and so on [1–4]. The most popular cathode material for LIBs is LiCoO2 due to its good recharge ability and easy preparation. However, the high price and toxicity of the cobalt have limited the farther application of LiCoO2 . Recently, many compounds as cathode materials for lithium ion batteries are of great interest and potential candidates to replace the commercial LiCoO2 , such as LiNiO2 , LiMn2 O4 , LiMnO2 , LiNi1/2 Mn1/2 O2 , as well as Li[Ni1/3 Co1/3 Mn1/3 ]O2 and their derivatives [5–12]. In addition, the solid solution of LiAlO2 with various lithiated transition-metal oxides, e.g., Lix Aly Co1−y O2 , Lix Aly Mn1−y O2 and Lix Aly Coz Mn1−y−z O2 [13–18] have been studied on account of their potentially increasing the intercalation voltage and cathode energy density. In this work, the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 (0 ≤ x ≤ 1/3) compounds are prepared and the effect of Co content on the morphology, structure and electrochemical properties of the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 will also be investigated.
Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 (x = 0, 1/12, 1/8, 1/4, 1/3) compounds were synthesized by a solvent evaporation method. A stoichiometric amount of LiC2 H3 O2 ·2H2 O, NiC2 H3 O2 ·4H2 O, CoC2 H3 O2 ·4H2 O, MnC2 H3 O2 ·4H2 O and Al(NO3 )3 ·9H2 O were used as the starting materials. These reagents were dissolved into a proper amount of distilled water and a mauve solution was obtained. The resulting solution was first heated at 100 ◦ C with magnetic stirring until viscous, and then dried at 120 ◦ C for 12 h to obtain a mixed precursor. The mixed precursor was pre-heated at 500 ◦ C for 6 h and finally calcined at 900 ◦ C for 20 h in air to yield final products. X-ray diffraction (XRD) patterns were evaluated using Xray diffraction (Shimadzu XRD-6000) with Cu K␣ radiation ˚ Lattice parameters and unit-cell volumes were cal( = 1.54056 A). culated by a least squares method with FullProf Suite program. Particle morphology of the powders after calcination was observed using a scanning electron microscope (SEM, QUANTA-200). Electrochemical charge–discharge experiments were performed using the CR2016 coin-type cell. Test cathode electrodes were prepared by mix 80:15:5 (mass ratio) of active material, acetylene black and PTFE binder, respectively, in isopropyl alcohol. The test cells were assembled with the electrode prepared above as cathode, lithium metal as anode, and Celgard 2300 film as separator in an argonfilled glove box. The electrolyte was 1 M LiPF6 dissolved in EC + DMC (1:1 volume ratio).
∗ Corresponding author. Tel.: +86 27 8721 8504. E-mail address: [email protected] (Z. Peng). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.05.104
H. Ren et al. / Electrochimica Acta 56 (2011) 7088–7091
Fig. 1. XRD patterns of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 powders sintered at 900 ◦ C for 20 h in air. (a) x = 0, (b) x = 1/12, (c) x = 1/8, (d) x = 1/4 and (e) x = 1/3.
3. Results and discussion Fig. 1 shows typical XRD patterns of all the synthesized Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 (x = 0, 1/12, 1/8, 1/4, 1/3) materials at 900 ◦ C for 20 h. All peaks are sharp and well-defined, suggesting that compounds are generally well crystallized, which can be indexed on the basis of the hexagonal ␣-NaFeO2 structure (space ¯ The XRD patterns of the samples with different Co group: R3m). contents are quite similar. No extra diffraction peaks from related secondary phases or impurities are found, which is different from the finding of Hu et al. [19]. It is believed that all atoms are being incorporated within the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 structure and that the homogeneous solid solution of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 could be prepared successfully in the present synthesis conditions. The clear splitting of the lines assigned to Miller indices (0 0 6, 1 0 2) and (1 0 8, 1 1 0) for the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 compounds (x = 0, 1/12, 1/8, 1/4, 1/3) indicates good characteristic of layer struc-
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ture [20,21]. Hwang et al. [22] believed that the intensity ratio of the (0 0 3) and (1 0 4) peaks (I(0 0 3) /I(1 0 4) ), R, could be used to identify the cation mixing degree. The smaller the R value, the higher the disordering. As shown in Table 1, R values are significantly increased from 0.89 to 1.59, which shows that the cation mixing is reduced with the increase of Co content. In order to illustrate the relation between the unit cell and Co content, the lattice parameters of the samples are calculated by least square method using 10-diffraction lines. The determined parameters are also summarized in Table 1. It is clearly observed that a, c lattice parameters and the unit cell volume increase with more Co content, which is an inevitable result that Al3+ (rAl 3+ = 0.0535 nm) are replaced by Co3+ (rCo 3+ = 0.0545 nm). While c/a values that indicate hexagonal structure disorder [23] are virtually unchanged. Normally, the crystal structure and morphology are two main factors which affect the electrochemical performance of the materials. Fig. 2 shows the SEM micrographs of the samples with different Co contents. The powder of all the samples consists of quasispherical particle. The agglomeration of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 powders (x = 0 and 1/12) can be found from Fig. 2a and b, while there does not exist in Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 powders (x = 1/8, 1/4 and 1/3) from Fig. 2c–e. The particle size of the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 compounds with x = 0 and 1/12 is smaller than that with x = 1/8 and 1/4, and the one with x = 1/3 is the largest, indicating the particle size increases with the Co content increased. In addition, the particle size distribution of the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 powders becomes more uniform at higher Co content. It is also observed that the crystallinity of the synthesized powders becomes better with higher Co content. From above it can come to a conclusion that the particle size, distribution and crystallinity of the Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 compounds are influenced by the introduction of Co. Figs. 3 and 4 show the initial charge–discharge curves and cycling performances of the samples, respectively. The cell is cycled between 3.0 and 4.3 V at room temperature at a current density of 20 mA g−1 (0.1 C), here the C-rate is calculated with 200 mA hg −1 as a theoretical capacity. As shown in Fig. 3, the first charge/discharge specific capacities are 141/116, 159/132, 178/150, 188/160 and 194/168 mA h g−1 with a coulombic efficiency about
Fig. 2. SEM micrographs of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 powders sintered at 900 ◦ C. (a) x = 0, (b) x = 1/12, (c) x = 1/8, (d) x = 1/4 and (e) x = 1/3.
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H. Ren et al. / Electrochimica Acta 56 (2011) 7088–7091
Table 1 Intensity ratio of I(0 0 3) /I(1 0 4) and lattice parameters for various samples. Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2
a (nm)
c (nm)
c/a
V (nm3 )
I(0 0 3) /I(1 0 4)
x=0 x = 1/12 x = 1/8 x = 1/4 x = 1/3
0.2852 0.2854 0.2856 0.2860 0.2863
1.4217 1.4219 1.4239 1.4255 1.4265
4.9842 4.9831 4.9858 4.9840 4.9833
0.1001 0.1003 0.1006 0.1010 0.1013
0.89 1.43 1.50 1.54 1.59
82.3%, 83.0%, 84.3%, 85.1% and 86.6% for Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 samples (x = 0, 1/12, 1/8, 1/4, 1/3), respectively. It is obvious that the initial discharge capacities and the coulombic efficiency of the first cycling have an increase with the increasing of Co content in Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 , which is mainly contributed from the reduction of the cell polarization with the addition of Co. In addition, from Fig. 3 charge curves shift upwards and discharge curves do downwards with an increase in the Co content, implying that increasing the Co content can reduce the resis-
tance for the diffusion of lithium ions. According to Fig. 4, at the end of the 20th cycle, the retained discharge capacities for Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 samples (x = 0, 1/12, 1/8, 1/4, 1/3) are 105, 124, 141, 153 and 163 mA h g−1 , respectively, which are 90.5%, 93.9%, 94.0%, 95.6% and 97.0% of initial discharge capacity. It is obvious that the long-term cyclic performance is improved with an increase of Co content and the Li[Ni1/3 Mn1/3 Co1/3 ]O2 compound shows the best reversible capacity and cycling performance among the synthesized Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 compounds. All the above results are consistent with those reported [24,25], with the change of Co content. The theoretical calculations [19] showed that the Al-substituted compounds should hold the advantages of better performance, high potential and high capacity as a cathode for lithium-ion batteries. However, there is some discrepancy between computational and our experimental works. This is mainly because the calculation does not take into account the effects of cation mixing, electronic conduction, lithium diffusion and charge transfer reaction (kinetics) on the performance of the materials. In order to further investigate the electrochemical performances of the samples, Fig. 5 also gives the cycling stability of these materials at 55 ◦ C between 3.0 and 4.3 V at a current density of 20 mA g−1 (0.1 C). It can be seen from Fig. 5 that the initial discharge capacities for all samples are improved compared with those at room temperature, which should be attributed to the increase in electronic conductivity and lower resistance at a higher temperature. However, at the end of the 20th cycle, the retained discharge capacities for Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 samples (x = 0, 1/12, 1/8, 1/4, 1/3) are 34.1%, 70.1%, 85.3%, 91.7% and 95.3% of initial discharge capacity, respectively. It is clear that the synthesized Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 samples with higher Co content hold better high-temperature cycling performance.
Fig. 4. Specific discharge capacity as a function of cycle number for Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 operated between 3.0 and 4.3 V at room temperature under a current density 20 mA g−1 . (a) x = 0, (b) x = 1/12, (c) x = 1/8, (d) x = 1/4 and (e) x = 1/3.
Fig. 5. Specific discharge capacity as a function of cycle number for Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 operated between 3.0 and 4.3 V at 55 ◦ C under a current density 20 mA g−1 . (a) x = 0, (b) x = 1/12, (c) x = 1/8, (d) x = 1/4 and (e) x = 1/3.
Fig. 3. Initial charge–discharge curves of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 cells operated between 3.0 and 4.3 V at room temperature under a current density 20 mA g−1 . (a) x = 0, (b) x = 1/12, (c) x = 1/8, (d) x = 1/4 and (e) x = 1/3.
H. Ren et al. / Electrochimica Acta 56 (2011) 7088–7091
4. Conclusions Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 (0 ≤ x ≤ 1/3) cathode materials have a hexagonal ␣-NaFeO2 structure and have no obvious impurityrelated peaks. As the Co content increased, the lattice parameters and the particle size of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 (0 ≤ x ≤ 1/3) tend to increase and the crystallinity gets more excellent. The electrochemical experiment indicates that the samples with higher Co content hold better electrochemical performance. The Li[Ni1/3 Mn1/3 Co1/3 ]O2 compound shows the best performance among the synthesized compounds. Its initial discharge capacity is 168 mA h g−1 (3–4.3 V, 20 mA g−1 ) and it holds better cycling performance. In addition, the introduction of Co also improve the high-temperature cycling property of Li[Ni1/3 Mn1/3 Al1/3−x Cox ]O2 . Acknowledgements We gratefully acknowledge the financial support from the National Nature Science Foundation of China (Nos. 29833090, 29771025 and 20573078). References [1] I.B. Weinstock, J. Power Sources 110 (11) (2002) 471. [2] K. Amine, C.H. Chen, J. Liu, M. Hammond, A. Jansen, D. Dees, I. Bloom, D. Vissers, G. Henriksen, J. Power Sources 97–98 (2001) 684.
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[3] A.M. Andersson, D.P. Abraham, R. Haasch, S. MacLaren, J. Liu, K. Amine, J. Electrochem. Soc. 149 (2002) A1358. [4] D. Ra, K. Hana, J. Power Sources 163 (2006) 284. [5] Y. Zhang, C. Wang, C. Wang, J. Electrochem. Soc. 156 (2009) A527. [6] Z. Su, Z.W. Lu, X.P. Gao, J. Power Sources 189 (2009) 411. [7] Q.S. Tong, Y. Yang, J.C. Shi, J. Electrochem. Soc. 154 (2007) A656. [8] Y. Koyama, Y. Makimura, I. Tanaka, H. Adachi, T. Ohzuku, J. Electrochem. Soc. 151 (2004) A1499. [9] K.M. Shaju, G.V. Subba Rao, B.V.R. Chowdari, Electrochim. Acta 48 (2002) 145. [10] B.J. Hwang, Y.W. Tsai, D. Cariler, G. Ceder, Chem. Mater. 15 (2003) 3676. [11] H. Ren, Y. Huanga, Y. Wang, Z. Li, P. Cai, Z. Peng, Y. Zhou, Mater. Chem. Phys. 117 (2009) 41. [12] Y. Sun, C. Ouyang, Z. Wang, X. Huang, L. Chen, J. Electrochem. Soc. 151 (2004) A504. [13] A. Van der Ven, M.K. Aydinol, G. Ceder, G. Kresse, J. Hafner, Phys. Rev. B 58 (1998) 2975. [14] Z. Lu, D.D. MacNeil, J.R. Dahn, Electrochem. Solid-State Lett. 4 (2001) A200. [15] G. Ceder, Y.M. Chiang, D.R. Sadoway, M.K. Aydinol, Y.-I. Jang, B. Huang, Nature 392 (1998) 694. [16] A.M. Kannan, A. Manthiram, J. Electrochem. Soc. 150 (2003) A349. [17] S.H. Park, K.S. Park, Y.K. Sun, K.S. Sun, K.S. Nahm, Y.S. Lee, M. Yoshio, Electrochim. Acta 46 (2001) 1215. [18] M. Guilmard, L. Croguennec, C. Delmas, Chem. Mater. 15 (2003) 4484. [19] S.K. Hu, T.C. Chou, B.J. Hwang, G. Ceder, J. Power Sources 160 (2006) 1287. [20] T.H. Cho, S.M. Park, M. Yoshio, T. Hirai, Y. Hideshima, J. Power Sources 142 (2005) 306. [21] A. Rougier, P. Gravereau, C. Delmas, J. Electrochem. Soc. 143 (4) (1996) 1168. [22] B.J. Hwang, R. Santhanam, C.H. Chen, J. Power Sources 114 (2003) 244. [23] J.M. Kim, H.T. Chung, Electrochim. Acta 49 (2004) 937. [24] F. Zhou, X. Zhao, Z. Lu, J. Jiang, J.R. Dahn, Electrochem. Commun. 10 (2008) 1168. [25] F. Zhou, X. Zhao, J.R. Dahn, J. Electrochem. Soc. 156 (4) (2009) A343.