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Effect of precursor and synthesis temperature on the structural and electrochemical properties of Li(Ni0.5Co0.2Mn0.3)O2
Electrochimica Acta 75 (2012) 393–398
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Effect of precursor and synthesis temperature on the structural and electrochemical properties of Li(Ni0.5 Co0.2 Mn0.3 )O2 Kuichen Wu a , Fei Wang a , Lulu Gao a , Man-Rong Li a,b , Lingli Xiao a , Liutao Zhao a , Sujuan Hu a , Xiaojun Wang a , Zhongling Xu a,∗ , Qingguo Wu a,c,∗∗ a
Zhejiang WELLY Energy Corporation, 48 Xingyuan Rd, Cixi City, Zhejiang 315301, PR China Department of Chem. & Chem. Bio., Rutgers, The State University of New Jersey, 610 Taylor Rd., Piscataway, NJ, 08854, USA c College of Mater. Sci. & Chem. Eng., Ningbo University, 818 Fenghua Rd, Ningbo City, Zhejiang 315211, PR China b
a r t i c l e
i n f o
Article history: Received 7 January 2012 Received in revised form 8 May 2012 Accepted 8 May 2012 Available online 17 May 2012 Keywords: Lithium-ion batteries Cathode materials Li(Ni0.5 Co0.2 Mn0.3 )O2 Solid-state reaction
a b s t r a c t Li(Ni0.5 Co0.2 Mn0.3 )O2 layered materials were synthesized by solid-state reaction using Li2 CO3 and three transition-metal hydroxide precursors of composition (Ni0.5 Co0.2 Mn0.3 )(OH)2 (NMC Hydroxide) with different physical properties. Characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical testing, etc., the final Li(Ni0.5 Co0.2 Mn0.3 )O2 products showed different physical and electrochemical properties depending on their synthesis temperatures and the properties of transition-metal hydroxide precursors were got. Higher reaction temperature results in bigger primary particle size (PPS) and broader size distribution. Precursor with smaller PPS results in larger PPS when synthesized at the same condition. The electrochemical performance is related to the physical properties of Li(Ni0.5 Co0.2 Mn0.3 )O2 . Better crystallized and cation ordered layered material has higher initial capacity while smaller and uniform PPS results in higher capacity retention rate. The Li(Ni0.5 Co0.2 Mn0.3 )O2 synthesized at 880 ◦ C for 10 h in atmosphere using (Ni0.5 Co0.2 Mn0.3 )(OH)2 with smallest PPS size as the starting precursor showed the best overall electrochemical properties with a high discharge capacity over 171 mAh/g with a capacity retention >96% after 50 cycles at 1C rate in a half battery and tap density about 2.7 g/cm3 . © 2012 Elsevier Ltd. All rights reserved.
1. Introduction With traditional natural resources running low and increasing public concern on worsening environmental problems, alternative clean, sustainable and renewable energy sources are being sought all over the world. As a key ring in the chain of this energy revolution, lithium-ion batteries (LIBs) have been widely studied and developed in the last two decades. At present, LiCoO2 is still the dominant cathode material in commercial LIBs market, but with the increasing demands for large scale LIBs, its relatively high cost, toxic and safety problems plus its low practically reachable capacity against the theoretical capacity prohibit it from wider application, especially in automotive battery region [1]. Recently, a transition-metal oxide LiNix Mny Co1−x−y O2 with similar layered structure to that of LiCoO2 has attracted significant attention as a potential substitute for LiCoO2 for its lower cost, less toxicity,
∗ Corresponding author. Tel.: +86 137 3216 9649; fax: +86 574 6322 3715. ∗∗ Corresponding author at: Zhejiang WELLY Energy Corporation, 48 Xingyuan Rd, Cixi City, Zhejiang 315301, PR China. Tel.: +86 574 63226758; fax: +86 574 63226308. E-mail addresses: [email protected] (Z. Xu), [email protected] (Q. Wu). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.05.035
good rate capability, better thermal stability and cycling performance, and higher practical reversible capacity [2–6]. However, the performance of this complicated material is heavily dependent on its synthesis conditions, which makes it difficult to synthesis especially for large scale industrial preparation. Comparing to other synthesis methods like solid state synthesis, spray drying and sol–gel methods, co-precipitation method with proper precursors offers the best controllable results and favorable for industrial production [7–10]. Co-precipitation method can be classified into two different strategies, namely carbonate co-precipitation route and hydroxide co-precipitation route. The latter one is a more efficient technology and most often used in industry which can easily provide homogeneous precursor [Nix Mny Co1−x−y ](OH)2 to get ideal homogeneous and high performance LiNix Mny Co1−x−y O2 cathode material with controllable morphology, high tap-density and better processability. Generally, there are two crucial factors throughout the preparation of LiNix Mny Co1−x−y O2 by hydroxide co-precipitation route: (1), the properties of precursors made by hydroxide co-precipitation; and (2), the synthesis conditions at a given calcination temperature of the raw mixture of lithium salt and precursor. During the synthesis process, each step could influence the properties of final products on crystal structure, composition,
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morphology, and electrochemical performance. Thus, the optimizing and controlling of both factors during synthesis is of importance to form high performance well-ordered layered LiNix Mny Co1−x−y O2 cathode materials [8]. However, most studies are based on laboratory research which cannot be applied directly into industrial synthesis. So it is necessary to explore the optimized conditions for industrial large scale production of this material. In our present work, the layered Li(Ni0.5 Co0.2 Mn0.3 )O2 compounds were synthesized via calcination in different synthesis conditions (with synthesis temperature ranging from 850 to 910 ◦ C) from the mixtures of Li2 CO3 lithium source and three different commercial supplied spherical (Ni0.5 Co0.2 Mn0.3 )(OH)2 precursors prepared by co-precipitation method, in order to reveal the relationship between physical and electrochemical properties of Li(Ni0.5 Co0.2 Mn0.3 )O2 and the synthesis strategy. The effects of the precursors and synthesis conditions on the structure, morphology and electrochemical properties of the layered Li(Ni0.5 Co0.2 Mn0.3 )O2 were discussed. 2. Experiments Three different commercial supplied NMC hydroxides labeled NCM-1, NCM-2 and NCM-3, respectively, were introduced in our present study. Li(Ni0.5 Co0.2 Mn0.3 )O2 powders were synthesized using Li2 CO3 with the NMC Hydroxides precursors by the solidstate reaction. Li2 CO3 and the NMC Hydroxides (in mole ratio Li:(Ni0.5 Co0.2 Mn0.3 )(OH)2 = 1.05:1) were mixed completely by ball milling. Each mixture was pre-calcined at 480 ◦ C for 6 h and then calcined at different temperatures (850–910 ◦ C) for 12 h in a box furnace with a heating rate of 5 ◦ C min−1 with continuous air flow and then cooled down naturally. The as-prepared samples are named after the name of its precursor followed by synthesis temperature, e.g. the sample prepared with the precursor NCM-2 at 860 ◦ C is marked as NCM-2-860. Powder X-ray diffraction (XRD, XRD-6100, SHIMADZU, Japan) was performed with Cu K␣ to identify the structure and crystalline phase of the precursors and as-prepared compounds. The particle morphologies of the resulting compound were examined by a scanning electron microscope (SEM, InTouchScope JSM-6010LA, JEOL, Japan) with a W filament operating at 20 KeV. The electrochemical performances of the as-prepared compounds were tested in a battery testing system using CR2032 coin-type cells. Slurry was formed with active material mixed with acetylene black and polyvinyl difluoride (PVDF) in N-methyl-2pyrrolidone (NMP) with a weight ratio of 80:10:10. Such slurry was then coated onto an aluminum foil current collector evenly with a coating instrument and punched into 1.5 cm in diameter discs after drying. The electrolyte used was 1 M LiPF6 – ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1by weight, GuotaiHuarong New Chemical Materials Co., Ltd., China). The prepared cathodes were assembled in a glove box protected by argon with Li metal foil as the anode and a porous polypropylene film as the separator. The electrochemical characterization was performed by a Dynamic Li-ion/Polymer Li-ion Battery Automatic Testing Device (Chenwei Electronic Technology Co., Ltd., China) with charge and discharge current density 0.1–5C and a cut-off voltage of 3.0–4.8 V at room temperature. 3. Results and discussion 3.1. Characterization of (Ni0.5 Co0.2 Mn0.3 )(OH)2 precursors To reveal how the conditions of precursors leads to a change in physical and electrochemical property of Li(Ni0.5 Co0.2 Mn0.3 )O2
Fig. 1. XRD results of the precursors NCM-1, NCM-2 and NCM-3. Extra peaks marked by the asterisks are from Co(OH)2 secondary phase.
products, firstly we studied the three different commercial supplied precursors by XRD, as shown in Fig. 1. All the precursors show a layered structure with a P-3m1 space group and slightly different lattice parameters, as listed in Table 1. However, the peak broadening of the precursors are different from each other which is due to different PPS. The mean PPS of the precursors are estimated from XRD by Le Bail fitting based on Scherrer Equation, which are 52.9 nm, 59.9 nm and 62.3 nm for NCM-1, NCM-2 and NCM-3, respectively. The PPS of the precursor can influent that of the final products in the same synthesis condition. Secondary phase is observed in precursor NCM-1 and NCM-3, as marked by the asterisks in Fig. 1 which is attribute to the presence of Co(OH)2 and there is more in NCM-3. The unexpected presence of Co(OH)2 has no visible impact on the properties of final products, as will show latter. The precursors aggregate in spheroidal agglomerates with average secondary particle diameter of about 12 m, as shown by SEM in Fig. 2. The spheroidal agglomerates are a combination of nanosized primary particles flakes, which are assembled vertically to the agglomerate surface. The loose packed secondary particles of NCM-1 have irregular shape with uniform size and bumpy surface, which has the lowest tap density of 2.21 g/cm3 . Secondary particles of NCM-3 are most compactly packed in a proper size distribution with regular spheroidal morphology and smooth surface which make NCM-3 has a much higher tap density of 2.24 g/cm3 . Although the big secondary particles of NCM-2 are loose packed with cracks, it has a large amount of extra small agglomerates with irregular shape which can fill the space between big ones and offers a slightly higher tap density of 2.25 g/cm3 than that of NCM-3. 3.2. Effect of synthesize temperature on the properties of Li(Ni0.5 Co0.2 Mn0.3 )O2 The relationship between synthesis temperature and physical properties of LiNi0.5 Co0.2 Mn0.3 O2 was studied with the samples using the same precursor NCM-2 and different temperatures range from 850 to 910 ◦ C. Representative XRD patterns of samples named NCM-2-850, NCM-2-860, NCM-2-870, NCM-2-880 and NCM-2-910 are plotted in Fig. 3 to demonstrate the relationship. As previous samples, they all adopt an ␣-NaFeO2 -type layered structure with a R-3m space group. The high angle peaks are getting sharper and narrower, and the peak splitting of (0 0 6)/(1 0 2) and (1 0 8)/(1 1 0) which is an indicator of characteristic of layered structure [11,12] is getting evident with increasing synthesis temperature which
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Table 1 Tap densities and lattice parameters with mean particle sizes of the precursors estimated by XRD by Le Bail fitting. (Ni0.5 Co0.2 Mn0.3 )(OH)2
˚ a (A)
˚ c (A)
Mean particle size (nm)
Tap density (g/cm3 )
NCM-1 NCM-2 NCM-3
3.02941(1) 3.07586(1) 2.97957(1)
4.61415(1) 4.60465(1) 4.59250(1)
52.9 59.9 62.3
2.21 2.25 2.24
Table 2 Lattice parameters of LiNi0.5 Co0.2 Mn0.3 O2 synthesized at different temperatures and relative intensities between certain peaks. LiNi0.5 Co0.2 Mn0.3 O2
˚ a (A)
˚ c (A)
c/a
NCM-2-850 NCM-2-860 NCM-2-870 NCM-2-880 NCM-2-890 NCM-2-900 NCM-2-910
2.8695 2.8693 2.8681 2.86851 2.86956 2.86826 2.86983
14.2331 14.2341 14.2303 14.2404 14.2338 14.2290 14.2357
4.9601 4.9609 4.9616 4.9644 4.9603 4.9608 4.9605
I1 0 4 /I0 0 3 94.0% 95.7% 90.7% 88.4% 90.6% 92% 92.8%
(I0 0 6 + I1 0 2 )/I1 0 1 43.1% 41.1% 41.1% 38.1% 42.1% 39.7% 40.5%
Fig. 2. SEM morphologies of the precursors NCM-1 (a), NCM-2 (b) and NCM-3 (c).
implies the better crystallized layered structure and bigger particle size. The patterns are almost identical to each other and did not show any significant shift of the reflections or any additional
Fig. 3. XRD patterns of samples synthesized with NCM-2 precursor at temperature 850, 860, 870, 880, 890, 900 and 910 ◦ C, respectively.
peak, which is indicative of single-phase materials without any byproduct in the range of synthesis temperature. Lattice parameters and relative peak intensities are estimated from XRD by Le Bail fitting, as listed in Table 3. The relative intensities of the certain peaks in XRD and the value of c/a demonstrate the crystallization and the level of antisite disordering between Ni2+ and Li+ . Generally, smaller I1 0 4 /I0 0 3 (Ih k l is the integral intensity of peak (h k l) hereafter), smaller (I0 0 6 + I1 0 2 )/I1 0 1 and larger c/a indicate crystallization, better hexagonal ordering and lower Ni2+ and Li+ antisite disordering, and results in better electrochemical performance [13,14]. With increasing temperature, the c/a ratio increased gradually to reach its maximum at 880 ◦ C and then decreased after that. This means the sample NCM-2-880 has the best stable layered structure and neither higher temperature nor lower temperature is helpful to get better layered structure. As a result of high Ni2+ content, the I1 0 4 /I0 0 3 values are quite high in these samples which indicate the high level of disordering between Ni2+ and Li+ . After comparing the value of c/a, I1 0 4 /I0 0 3 and (I0 0 6 + I1 0 2 )/I1 0 1 , we can see that NCM-2-880 has the best overall structural properties and should have the best electrochemical performance of the samples [15,16]. Thus, 880 ◦ C should be the best reaction temperature for high performance samples. To evaluate the effect of synthesis temperature on the PPS, samples synthesized at different temperatures are checked by SEM and
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Fig. 4. The mean primary particle size as a function of synthesis temperature.
PPS are measured which is plotted as a function of temperature, as shown in Fig. 4. The PPS is increasing almost linearly with increasing temperature. Sample NCM-2-880 has well defined particle shape and the most uniform particle size distribution. However, the particle size distributed in a broad range when the temperature is beyond 900 ◦ C, some particles have a dimension more than 1 m. 3.3. Effect of precursors on the properties of Li(Ni0.5 Co0.2 Mn0.3 )O2 The Li(Ni0.5 Co0.2 Mn0.3 )O2 compounds NCM-1-880, NCM-2-880 and NCM-3-880 synthesized in the same condition at 880 ◦ C with the three precursors are studied and compared with each other. Fig. 5 is the SEM images of the as-prepared samples. It can be seen from the images that the spheroidal morphology of the precursors are kept after calcinations, but the shapes of the primary particles are changed from nano-sheets to larger polyhedrons. The compactness of the spheroidal agglomerates is related to that of the precursors. NCM-1-880 has the biggest primary particles and agglomerates with the highest porosity, most bumpy and rough surface, results in lowest tap density of 2.11 g/cm3 . NCM-2-880 has smaller primary particles and smoother agglomerates with lower porosity and surface cracks, together with the large amount of extra small agglomerates, make it has a higher tap density of 2.40 g/cm3 .
Agglomerates of sample NCM-3-880 are made up of the smallest primary particles and have the highest compactness and smoothest surface with proper size distribution, offers the sample with the highest tap density 2.69 g/cm3 . As shown in Fig. 6, the PPS distributions of the samples are measured randomly from more than 200 particles for each sample by SEM and the mean particle sizes for NCM-1-880, NCM-2-880 and NCM-3-880 are 874(±292) nm, 648(±232) nm and 336(±98) nm, respectively. Compared with that of precursors (see in Table 1), it can be seen that smaller PPS of the precursor results in bigger PPS of the as-prepared samples. The relationship between PPS of precursors and corresponding products are shown in Fig. 7. NCM-3-880 has the most uniform and narrow particle size distribution, which is due to the uniform agglomerate size of its precursor. XRD were performed to check the structure and impurity of the samples, as shown in Fig. 8. All the samples have narrow and sharp peaks which indicate the formation of highly ordered structure which can be fitted by a layered structure with R-3m space group, similar to that of LiCoO2 . Different levels of peak splitting of (0 0 6)/(1 0 2) and (0 1 8)/(1 1 0) are due to the different PPS. Details of each unit cells after Le Bail fitting are listed in Table 3. No impurity peak was observed from the XRD patterns. This implies that the presence of Co(OH)2 observed in precursors NCM-1 and NCM-3 has no direct harm for the final products. As listed in Table 2, I1 0 4 /I0 0 3 and (I0 0 6 + I1 0 2 )/I1 0 1 are 89.9%, 88.4%, 87.4% and 40.1%, 38.1%, 43.4% for NCM-1-880, NCM-2-880, NCM-3-880, respectively. We can see that NCM-2-880 has smaller I1 0 4 /I0 0 3 , the smallest (I0 0 6 + I1 0 2 )/I1 0 1 ratios and the biggest c/a ratio (4.644) over the other two samples (4.9637 and 4.9635 for NCM-1-880 and NCM-3-880, respectively). Taking all these numbers together, we would expect that NCM-2-880 should have best layered structure and crystallization, lowest Ni2+ and Li+ antisite disordering than the other two samples, and thus should have the best initial electrochemical performance. The details of the electrochemical study are shown below. 3.4. Electrochemical performance study Fig. 9 is the 1st charge/discharge profiles of NCM-2-880 and NCM-3-880 vs lithium metal (0.1C, first row) and corresponding cycling performance (1C, second row) of NCM-2-880 and NCM3-880 cycled between 2.8 and 4.3 V. Both the charge/discharge curves are very smooth and consistent and show the typical potential plateaus at 3.75 V which is corresponding to the Ni2+ /Ni4+
Fig. 5. SEM morphologies of the as-prepared samples (a) NCM-1-880, (b) NCM-2-880 and (c) NCM-3-880, respectively.
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Fig. 6. Primary particle size distribution estimated from more than 200 particles for each sample by SEM of NCM-1-880, NCM-2-880 and NCM-3-880 from left to right, respectively.
redox couple [13]. NCM-3-880 delivered charge capacity around 198 mAh/g against the discharge capacity of 171 mAh/g corresponding to 86% CE during the first cycle with current density of 0.1C, which is better that that of NCM-2-880 with charge/discharge capacity around 198 mAh/g and 168 mAh/g, and CE 85%, respectively. NCM-2-880 and NCM-3-880 deliver an initial discharge capacity of approximately 168 and 155 mAh/g with 1C charge/discharge rate, respectively. After 50 cycles, the capacity of the two samples
reduced to 155 and 153 mAh/g, with capacity retention rates 92.7% and 97.1%, respectively. The difference between the two samples on their electrochemical performance is related to the difference in physical properties including PPS distribution and crystal microstructure. As we have already shown, NCM-2-880 has a lower level of Ni2+ and Li+ disordering (smallest I1 0 4 /I0 0 3 ratio), highest c/a ratio and best hexagonal ordering (lowest (I0 0 6 + I1 0 2 )/I1 0 1 ratio), and suggesting a better defined layered structure which are convenient for lithium to pass thus own a better initial capacity. After continuous cycling, the ordering of the compound was destroyed and has a faster capacity loss than that of NCM-3-880 which has the smallest and most uniform primary particle size and a nicer cycling performance with little change in capacity even after 50 cycles with 1C discharge rate.
Fig. 7. Primary particle size of products as a function of primary particle size of precursors.
Fig. 8. XRD results of the precursors NCM-1-880, NCM-2-880 and NCM-3-880.
Fig. 9. 1st charge/discharge curves with current density of 0.1C (top) and cycling performance with current density of 1C (bottom) of samples NCM-2-880 and NCM3-880, respectively.
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Table 3 Lattice parameters of LiNi0.5 Co0.2 Mn0.3 O2 calculated from XRD by Le Bail fitting and relative intensities between certain peaks. LiNi0.5 Co0.2 Mn0.3 O2
˚ a (A)
˚ c (A)
c/a
I1 0 4 /I0 0 3
(I0 0 6 + I1 0 2 )/I1 0 1
NCM-1-880 NCM-2-880 NCM-3-880
2.869 2.868 2.871
14.241 14.240 14.248
4.9637 4.9644 4.9635
89.9% 88.4% 87.4%
40.1% 38.1% 43.4%
4. Conclusion Using three commercial supplied spheroidal NMC hydroxides with different characters as precursors, layered structure phase-pure Li(Ni0.5 Co0.2 Mn0.3 )O2 compounds with spheroidal agglomerate morphology were synthesized at different calcination temperatures from 880 to 910 ◦ C with LiCO3 as lithium supplier which were evaluated by XRD, SEM, and ICP to ascertain the physical properties. It was found that the physical and electrochemical properties are determined by the physical properties of precursors and synthesis temperature. (1) The morphology of Li(Ni0.5 Co0.2 Mn0.3 )O2 is consistent with that of its precursor. (2) Smaller PPS results in bigger Li(Ni0.5 Co0.2 Mn0.3 )O2 primary particle size. The presence of Co(OH)2 impurity in precursors has no evident impact on phase purity of final products. (3) The irregular morphology of Li(Ni0.5 Co0.2 Mn0.3 )O2 agglomerate with loose, bumpy and rough surface results in low tap density, and vice versa. (4) Calcination to higher temperatures yield Li(Ni0.5 Co0.2 Mn0.3 )O2 compounds with bigger PPS, but the size distribution getting broader with temperature over 900 ◦ C. (5) The electrochemical performance of Li(Ni0.5 Co0.2 Mn0.3 )O2 is related to its primary particle size, crystallization and level of cation ordering. Small and uniform primary particles result in better cycling performance while better crystallization and cation ordering results in higher initial capacity. It is worth noting that above results can be applied to other NCM materials with different compositions to a certain extent according to our preliminary work on NMCs with Mn:Ni:Co = 4:4:2 and Mn:Ni:Co = 1:1:1 compositions which provide an instruction for optimizing other NMCs synthesized by solid-reaction with precursors.
It was found that the Li(Ni0.5 Co0.2 Mn0.3 )O2 samples synthesized at 880 ◦ C are of the most uniform regular primary particles and well defined layered structure, which should have a better electrochemical performance than the others. The sample NCM-3-880 with highest electrochemical performance and tap density (∼2.7 g/cm3 ) was synthesized with the precursor with smallest primary particle size and relatively high tap density (NCM-3) and was found to deliver discharge capacity of 171 mAh/g during the first cycle with charge efficiency of 85% and high cycling performance. References [1] Y. Chen, G.X. Wang, K. Konstantinov, H.K. Liu, S.X. Dou, Journal of Power Sources 119–121 (0) (2003) 184. [2] T. Ohzuku, Y. Makimura, Chemistry Letters (7) (2001) 642. [3] Y.K. Sun, S.T. Myung, B.C. Park, J. Prakash, I. Belharouak, K. Amine, Nature Materials 8 (4) (2009) 320. [4] S. Patoux, M.M. Doeff, Electrochemistry Communications 6 (8) (2004) 767. [5] Y. Koyama, I. Tanaka, H. Adachi, Y. Makimura, T. Ohzuku, Journal of Power Sources 119–121 (2003) 644. [6] I. Belharouak, Y.K. Sun, J. Liu, K. Amine, Journal of Power Sources 123 (2) (2003) 247. [7] S. Zhang, Electrochimica Acta 52 (25) (2007) 7337. [8] X.Y. Zhang, W.J. Jiang, A. Mauger, Qilu, F. Gendron, C.M. Julien, Journal of Power Sources 195 (5) (2010) 1292. [9] C. Deng, L. Liu, W. Zhou, K. Sun, D. Sun, Electrochimica Acta 53 (5) (2008) 2441. [10] X. Luo, X. Wang, L. Liao, S. Gamboa, P.J. Sebastian, Journal of Power Sources 158 (1) (2006) 654. [11] J.-M. Kim, H.-T. Chung, Electrochimica Acta 49 (6) (2004) 937. [12] J.H. Kim, C.S. Yoon, Y.K. Sun, Journal of the Electrochemical Society 150 (4) (2003) A538. [13] J.R. Dahn, U. von Sacken, C.A. Michal, Solid State Ionics 44 (1–2) (1990) 87. [14] J.N. Reimers, E. Rossen, C.D. Jones, J.R. Dahn, Solid State Ionics 61 (4) (1993) 335. [15] C.-Y. Hu, J. Guo, X.-Y. Wang, The Chinese Journal of Nonferrous Metals 18 (9) (2008) 1721. [16] H.-j. Guo, M. Zhang, X.-H. Li, X.-M. Zhang, Z.-X. Wang, W.-J. Peng, M. Hu, Transactions of Nonferrous Metals Society of China 15 (5) (2005) 1185.
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Effect of precursor and synthesis temperature on the structural and electrochemical properties of Li(Ni0.5 Co0.2 Mn0.3 )O2 Kuichen Wu a , Fei Wang a , Lulu Gao a , Man-Rong Li a,b , Lingli Xiao a , Liutao Zhao a , Sujuan Hu a , Xiaojun Wang a , Zhongling Xu a,∗ , Qingguo Wu a,c,∗∗ a
Zhejiang WELLY Energy Corporation, 48 Xingyuan Rd, Cixi City, Zhejiang 315301, PR China Department of Chem. & Chem. Bio., Rutgers, The State University of New Jersey, 610 Taylor Rd., Piscataway, NJ, 08854, USA c College of Mater. Sci. & Chem. Eng., Ningbo University, 818 Fenghua Rd, Ningbo City, Zhejiang 315211, PR China b
a r t i c l e
i n f o
Article history: Received 7 January 2012 Received in revised form 8 May 2012 Accepted 8 May 2012 Available online 17 May 2012 Keywords: Lithium-ion batteries Cathode materials Li(Ni0.5 Co0.2 Mn0.3 )O2 Solid-state reaction
a b s t r a c t Li(Ni0.5 Co0.2 Mn0.3 )O2 layered materials were synthesized by solid-state reaction using Li2 CO3 and three transition-metal hydroxide precursors of composition (Ni0.5 Co0.2 Mn0.3 )(OH)2 (NMC Hydroxide) with different physical properties. Characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical testing, etc., the final Li(Ni0.5 Co0.2 Mn0.3 )O2 products showed different physical and electrochemical properties depending on their synthesis temperatures and the properties of transition-metal hydroxide precursors were got. Higher reaction temperature results in bigger primary particle size (PPS) and broader size distribution. Precursor with smaller PPS results in larger PPS when synthesized at the same condition. The electrochemical performance is related to the physical properties of Li(Ni0.5 Co0.2 Mn0.3 )O2 . Better crystallized and cation ordered layered material has higher initial capacity while smaller and uniform PPS results in higher capacity retention rate. The Li(Ni0.5 Co0.2 Mn0.3 )O2 synthesized at 880 ◦ C for 10 h in atmosphere using (Ni0.5 Co0.2 Mn0.3 )(OH)2 with smallest PPS size as the starting precursor showed the best overall electrochemical properties with a high discharge capacity over 171 mAh/g with a capacity retention >96% after 50 cycles at 1C rate in a half battery and tap density about 2.7 g/cm3 . © 2012 Elsevier Ltd. All rights reserved.
1. Introduction With traditional natural resources running low and increasing public concern on worsening environmental problems, alternative clean, sustainable and renewable energy sources are being sought all over the world. As a key ring in the chain of this energy revolution, lithium-ion batteries (LIBs) have been widely studied and developed in the last two decades. At present, LiCoO2 is still the dominant cathode material in commercial LIBs market, but with the increasing demands for large scale LIBs, its relatively high cost, toxic and safety problems plus its low practically reachable capacity against the theoretical capacity prohibit it from wider application, especially in automotive battery region [1]. Recently, a transition-metal oxide LiNix Mny Co1−x−y O2 with similar layered structure to that of LiCoO2 has attracted significant attention as a potential substitute for LiCoO2 for its lower cost, less toxicity,
∗ Corresponding author. Tel.: +86 137 3216 9649; fax: +86 574 6322 3715. ∗∗ Corresponding author at: Zhejiang WELLY Energy Corporation, 48 Xingyuan Rd, Cixi City, Zhejiang 315301, PR China. Tel.: +86 574 63226758; fax: +86 574 63226308. E-mail addresses: [email protected] (Z. Xu), [email protected] (Q. Wu). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.05.035
good rate capability, better thermal stability and cycling performance, and higher practical reversible capacity [2–6]. However, the performance of this complicated material is heavily dependent on its synthesis conditions, which makes it difficult to synthesis especially for large scale industrial preparation. Comparing to other synthesis methods like solid state synthesis, spray drying and sol–gel methods, co-precipitation method with proper precursors offers the best controllable results and favorable for industrial production [7–10]. Co-precipitation method can be classified into two different strategies, namely carbonate co-precipitation route and hydroxide co-precipitation route. The latter one is a more efficient technology and most often used in industry which can easily provide homogeneous precursor [Nix Mny Co1−x−y ](OH)2 to get ideal homogeneous and high performance LiNix Mny Co1−x−y O2 cathode material with controllable morphology, high tap-density and better processability. Generally, there are two crucial factors throughout the preparation of LiNix Mny Co1−x−y O2 by hydroxide co-precipitation route: (1), the properties of precursors made by hydroxide co-precipitation; and (2), the synthesis conditions at a given calcination temperature of the raw mixture of lithium salt and precursor. During the synthesis process, each step could influence the properties of final products on crystal structure, composition,
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morphology, and electrochemical performance. Thus, the optimizing and controlling of both factors during synthesis is of importance to form high performance well-ordered layered LiNix Mny Co1−x−y O2 cathode materials [8]. However, most studies are based on laboratory research which cannot be applied directly into industrial synthesis. So it is necessary to explore the optimized conditions for industrial large scale production of this material. In our present work, the layered Li(Ni0.5 Co0.2 Mn0.3 )O2 compounds were synthesized via calcination in different synthesis conditions (with synthesis temperature ranging from 850 to 910 ◦ C) from the mixtures of Li2 CO3 lithium source and three different commercial supplied spherical (Ni0.5 Co0.2 Mn0.3 )(OH)2 precursors prepared by co-precipitation method, in order to reveal the relationship between physical and electrochemical properties of Li(Ni0.5 Co0.2 Mn0.3 )O2 and the synthesis strategy. The effects of the precursors and synthesis conditions on the structure, morphology and electrochemical properties of the layered Li(Ni0.5 Co0.2 Mn0.3 )O2 were discussed. 2. Experiments Three different commercial supplied NMC hydroxides labeled NCM-1, NCM-2 and NCM-3, respectively, were introduced in our present study. Li(Ni0.5 Co0.2 Mn0.3 )O2 powders were synthesized using Li2 CO3 with the NMC Hydroxides precursors by the solidstate reaction. Li2 CO3 and the NMC Hydroxides (in mole ratio Li:(Ni0.5 Co0.2 Mn0.3 )(OH)2 = 1.05:1) were mixed completely by ball milling. Each mixture was pre-calcined at 480 ◦ C for 6 h and then calcined at different temperatures (850–910 ◦ C) for 12 h in a box furnace with a heating rate of 5 ◦ C min−1 with continuous air flow and then cooled down naturally. The as-prepared samples are named after the name of its precursor followed by synthesis temperature, e.g. the sample prepared with the precursor NCM-2 at 860 ◦ C is marked as NCM-2-860. Powder X-ray diffraction (XRD, XRD-6100, SHIMADZU, Japan) was performed with Cu K␣ to identify the structure and crystalline phase of the precursors and as-prepared compounds. The particle morphologies of the resulting compound were examined by a scanning electron microscope (SEM, InTouchScope JSM-6010LA, JEOL, Japan) with a W filament operating at 20 KeV. The electrochemical performances of the as-prepared compounds were tested in a battery testing system using CR2032 coin-type cells. Slurry was formed with active material mixed with acetylene black and polyvinyl difluoride (PVDF) in N-methyl-2pyrrolidone (NMP) with a weight ratio of 80:10:10. Such slurry was then coated onto an aluminum foil current collector evenly with a coating instrument and punched into 1.5 cm in diameter discs after drying. The electrolyte used was 1 M LiPF6 – ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1by weight, GuotaiHuarong New Chemical Materials Co., Ltd., China). The prepared cathodes were assembled in a glove box protected by argon with Li metal foil as the anode and a porous polypropylene film as the separator. The electrochemical characterization was performed by a Dynamic Li-ion/Polymer Li-ion Battery Automatic Testing Device (Chenwei Electronic Technology Co., Ltd., China) with charge and discharge current density 0.1–5C and a cut-off voltage of 3.0–4.8 V at room temperature. 3. Results and discussion 3.1. Characterization of (Ni0.5 Co0.2 Mn0.3 )(OH)2 precursors To reveal how the conditions of precursors leads to a change in physical and electrochemical property of Li(Ni0.5 Co0.2 Mn0.3 )O2
Fig. 1. XRD results of the precursors NCM-1, NCM-2 and NCM-3. Extra peaks marked by the asterisks are from Co(OH)2 secondary phase.
products, firstly we studied the three different commercial supplied precursors by XRD, as shown in Fig. 1. All the precursors show a layered structure with a P-3m1 space group and slightly different lattice parameters, as listed in Table 1. However, the peak broadening of the precursors are different from each other which is due to different PPS. The mean PPS of the precursors are estimated from XRD by Le Bail fitting based on Scherrer Equation, which are 52.9 nm, 59.9 nm and 62.3 nm for NCM-1, NCM-2 and NCM-3, respectively. The PPS of the precursor can influent that of the final products in the same synthesis condition. Secondary phase is observed in precursor NCM-1 and NCM-3, as marked by the asterisks in Fig. 1 which is attribute to the presence of Co(OH)2 and there is more in NCM-3. The unexpected presence of Co(OH)2 has no visible impact on the properties of final products, as will show latter. The precursors aggregate in spheroidal agglomerates with average secondary particle diameter of about 12 m, as shown by SEM in Fig. 2. The spheroidal agglomerates are a combination of nanosized primary particles flakes, which are assembled vertically to the agglomerate surface. The loose packed secondary particles of NCM-1 have irregular shape with uniform size and bumpy surface, which has the lowest tap density of 2.21 g/cm3 . Secondary particles of NCM-3 are most compactly packed in a proper size distribution with regular spheroidal morphology and smooth surface which make NCM-3 has a much higher tap density of 2.24 g/cm3 . Although the big secondary particles of NCM-2 are loose packed with cracks, it has a large amount of extra small agglomerates with irregular shape which can fill the space between big ones and offers a slightly higher tap density of 2.25 g/cm3 than that of NCM-3. 3.2. Effect of synthesize temperature on the properties of Li(Ni0.5 Co0.2 Mn0.3 )O2 The relationship between synthesis temperature and physical properties of LiNi0.5 Co0.2 Mn0.3 O2 was studied with the samples using the same precursor NCM-2 and different temperatures range from 850 to 910 ◦ C. Representative XRD patterns of samples named NCM-2-850, NCM-2-860, NCM-2-870, NCM-2-880 and NCM-2-910 are plotted in Fig. 3 to demonstrate the relationship. As previous samples, they all adopt an ␣-NaFeO2 -type layered structure with a R-3m space group. The high angle peaks are getting sharper and narrower, and the peak splitting of (0 0 6)/(1 0 2) and (1 0 8)/(1 1 0) which is an indicator of characteristic of layered structure [11,12] is getting evident with increasing synthesis temperature which
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Table 1 Tap densities and lattice parameters with mean particle sizes of the precursors estimated by XRD by Le Bail fitting. (Ni0.5 Co0.2 Mn0.3 )(OH)2
˚ a (A)
˚ c (A)
Mean particle size (nm)
Tap density (g/cm3 )
NCM-1 NCM-2 NCM-3
3.02941(1) 3.07586(1) 2.97957(1)
4.61415(1) 4.60465(1) 4.59250(1)
52.9 59.9 62.3
2.21 2.25 2.24
Table 2 Lattice parameters of LiNi0.5 Co0.2 Mn0.3 O2 synthesized at different temperatures and relative intensities between certain peaks. LiNi0.5 Co0.2 Mn0.3 O2
˚ a (A)
˚ c (A)
c/a
NCM-2-850 NCM-2-860 NCM-2-870 NCM-2-880 NCM-2-890 NCM-2-900 NCM-2-910
2.8695 2.8693 2.8681 2.86851 2.86956 2.86826 2.86983
14.2331 14.2341 14.2303 14.2404 14.2338 14.2290 14.2357
4.9601 4.9609 4.9616 4.9644 4.9603 4.9608 4.9605
I1 0 4 /I0 0 3 94.0% 95.7% 90.7% 88.4% 90.6% 92% 92.8%
(I0 0 6 + I1 0 2 )/I1 0 1 43.1% 41.1% 41.1% 38.1% 42.1% 39.7% 40.5%
Fig. 2. SEM morphologies of the precursors NCM-1 (a), NCM-2 (b) and NCM-3 (c).
implies the better crystallized layered structure and bigger particle size. The patterns are almost identical to each other and did not show any significant shift of the reflections or any additional
Fig. 3. XRD patterns of samples synthesized with NCM-2 precursor at temperature 850, 860, 870, 880, 890, 900 and 910 ◦ C, respectively.
peak, which is indicative of single-phase materials without any byproduct in the range of synthesis temperature. Lattice parameters and relative peak intensities are estimated from XRD by Le Bail fitting, as listed in Table 3. The relative intensities of the certain peaks in XRD and the value of c/a demonstrate the crystallization and the level of antisite disordering between Ni2+ and Li+ . Generally, smaller I1 0 4 /I0 0 3 (Ih k l is the integral intensity of peak (h k l) hereafter), smaller (I0 0 6 + I1 0 2 )/I1 0 1 and larger c/a indicate crystallization, better hexagonal ordering and lower Ni2+ and Li+ antisite disordering, and results in better electrochemical performance [13,14]. With increasing temperature, the c/a ratio increased gradually to reach its maximum at 880 ◦ C and then decreased after that. This means the sample NCM-2-880 has the best stable layered structure and neither higher temperature nor lower temperature is helpful to get better layered structure. As a result of high Ni2+ content, the I1 0 4 /I0 0 3 values are quite high in these samples which indicate the high level of disordering between Ni2+ and Li+ . After comparing the value of c/a, I1 0 4 /I0 0 3 and (I0 0 6 + I1 0 2 )/I1 0 1 , we can see that NCM-2-880 has the best overall structural properties and should have the best electrochemical performance of the samples [15,16]. Thus, 880 ◦ C should be the best reaction temperature for high performance samples. To evaluate the effect of synthesis temperature on the PPS, samples synthesized at different temperatures are checked by SEM and
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Fig. 4. The mean primary particle size as a function of synthesis temperature.
PPS are measured which is plotted as a function of temperature, as shown in Fig. 4. The PPS is increasing almost linearly with increasing temperature. Sample NCM-2-880 has well defined particle shape and the most uniform particle size distribution. However, the particle size distributed in a broad range when the temperature is beyond 900 ◦ C, some particles have a dimension more than 1 m. 3.3. Effect of precursors on the properties of Li(Ni0.5 Co0.2 Mn0.3 )O2 The Li(Ni0.5 Co0.2 Mn0.3 )O2 compounds NCM-1-880, NCM-2-880 and NCM-3-880 synthesized in the same condition at 880 ◦ C with the three precursors are studied and compared with each other. Fig. 5 is the SEM images of the as-prepared samples. It can be seen from the images that the spheroidal morphology of the precursors are kept after calcinations, but the shapes of the primary particles are changed from nano-sheets to larger polyhedrons. The compactness of the spheroidal agglomerates is related to that of the precursors. NCM-1-880 has the biggest primary particles and agglomerates with the highest porosity, most bumpy and rough surface, results in lowest tap density of 2.11 g/cm3 . NCM-2-880 has smaller primary particles and smoother agglomerates with lower porosity and surface cracks, together with the large amount of extra small agglomerates, make it has a higher tap density of 2.40 g/cm3 .
Agglomerates of sample NCM-3-880 are made up of the smallest primary particles and have the highest compactness and smoothest surface with proper size distribution, offers the sample with the highest tap density 2.69 g/cm3 . As shown in Fig. 6, the PPS distributions of the samples are measured randomly from more than 200 particles for each sample by SEM and the mean particle sizes for NCM-1-880, NCM-2-880 and NCM-3-880 are 874(±292) nm, 648(±232) nm and 336(±98) nm, respectively. Compared with that of precursors (see in Table 1), it can be seen that smaller PPS of the precursor results in bigger PPS of the as-prepared samples. The relationship between PPS of precursors and corresponding products are shown in Fig. 7. NCM-3-880 has the most uniform and narrow particle size distribution, which is due to the uniform agglomerate size of its precursor. XRD were performed to check the structure and impurity of the samples, as shown in Fig. 8. All the samples have narrow and sharp peaks which indicate the formation of highly ordered structure which can be fitted by a layered structure with R-3m space group, similar to that of LiCoO2 . Different levels of peak splitting of (0 0 6)/(1 0 2) and (0 1 8)/(1 1 0) are due to the different PPS. Details of each unit cells after Le Bail fitting are listed in Table 3. No impurity peak was observed from the XRD patterns. This implies that the presence of Co(OH)2 observed in precursors NCM-1 and NCM-3 has no direct harm for the final products. As listed in Table 2, I1 0 4 /I0 0 3 and (I0 0 6 + I1 0 2 )/I1 0 1 are 89.9%, 88.4%, 87.4% and 40.1%, 38.1%, 43.4% for NCM-1-880, NCM-2-880, NCM-3-880, respectively. We can see that NCM-2-880 has smaller I1 0 4 /I0 0 3 , the smallest (I0 0 6 + I1 0 2 )/I1 0 1 ratios and the biggest c/a ratio (4.644) over the other two samples (4.9637 and 4.9635 for NCM-1-880 and NCM-3-880, respectively). Taking all these numbers together, we would expect that NCM-2-880 should have best layered structure and crystallization, lowest Ni2+ and Li+ antisite disordering than the other two samples, and thus should have the best initial electrochemical performance. The details of the electrochemical study are shown below. 3.4. Electrochemical performance study Fig. 9 is the 1st charge/discharge profiles of NCM-2-880 and NCM-3-880 vs lithium metal (0.1C, first row) and corresponding cycling performance (1C, second row) of NCM-2-880 and NCM3-880 cycled between 2.8 and 4.3 V. Both the charge/discharge curves are very smooth and consistent and show the typical potential plateaus at 3.75 V which is corresponding to the Ni2+ /Ni4+
Fig. 5. SEM morphologies of the as-prepared samples (a) NCM-1-880, (b) NCM-2-880 and (c) NCM-3-880, respectively.
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Fig. 6. Primary particle size distribution estimated from more than 200 particles for each sample by SEM of NCM-1-880, NCM-2-880 and NCM-3-880 from left to right, respectively.
redox couple [13]. NCM-3-880 delivered charge capacity around 198 mAh/g against the discharge capacity of 171 mAh/g corresponding to 86% CE during the first cycle with current density of 0.1C, which is better that that of NCM-2-880 with charge/discharge capacity around 198 mAh/g and 168 mAh/g, and CE 85%, respectively. NCM-2-880 and NCM-3-880 deliver an initial discharge capacity of approximately 168 and 155 mAh/g with 1C charge/discharge rate, respectively. After 50 cycles, the capacity of the two samples
reduced to 155 and 153 mAh/g, with capacity retention rates 92.7% and 97.1%, respectively. The difference between the two samples on their electrochemical performance is related to the difference in physical properties including PPS distribution and crystal microstructure. As we have already shown, NCM-2-880 has a lower level of Ni2+ and Li+ disordering (smallest I1 0 4 /I0 0 3 ratio), highest c/a ratio and best hexagonal ordering (lowest (I0 0 6 + I1 0 2 )/I1 0 1 ratio), and suggesting a better defined layered structure which are convenient for lithium to pass thus own a better initial capacity. After continuous cycling, the ordering of the compound was destroyed and has a faster capacity loss than that of NCM-3-880 which has the smallest and most uniform primary particle size and a nicer cycling performance with little change in capacity even after 50 cycles with 1C discharge rate.
Fig. 7. Primary particle size of products as a function of primary particle size of precursors.
Fig. 8. XRD results of the precursors NCM-1-880, NCM-2-880 and NCM-3-880.
Fig. 9. 1st charge/discharge curves with current density of 0.1C (top) and cycling performance with current density of 1C (bottom) of samples NCM-2-880 and NCM3-880, respectively.
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Table 3 Lattice parameters of LiNi0.5 Co0.2 Mn0.3 O2 calculated from XRD by Le Bail fitting and relative intensities between certain peaks. LiNi0.5 Co0.2 Mn0.3 O2
˚ a (A)
˚ c (A)
c/a
I1 0 4 /I0 0 3
(I0 0 6 + I1 0 2 )/I1 0 1
NCM-1-880 NCM-2-880 NCM-3-880
2.869 2.868 2.871
14.241 14.240 14.248
4.9637 4.9644 4.9635
89.9% 88.4% 87.4%
40.1% 38.1% 43.4%
4. Conclusion Using three commercial supplied spheroidal NMC hydroxides with different characters as precursors, layered structure phase-pure Li(Ni0.5 Co0.2 Mn0.3 )O2 compounds with spheroidal agglomerate morphology were synthesized at different calcination temperatures from 880 to 910 ◦ C with LiCO3 as lithium supplier which were evaluated by XRD, SEM, and ICP to ascertain the physical properties. It was found that the physical and electrochemical properties are determined by the physical properties of precursors and synthesis temperature. (1) The morphology of Li(Ni0.5 Co0.2 Mn0.3 )O2 is consistent with that of its precursor. (2) Smaller PPS results in bigger Li(Ni0.5 Co0.2 Mn0.3 )O2 primary particle size. The presence of Co(OH)2 impurity in precursors has no evident impact on phase purity of final products. (3) The irregular morphology of Li(Ni0.5 Co0.2 Mn0.3 )O2 agglomerate with loose, bumpy and rough surface results in low tap density, and vice versa. (4) Calcination to higher temperatures yield Li(Ni0.5 Co0.2 Mn0.3 )O2 compounds with bigger PPS, but the size distribution getting broader with temperature over 900 ◦ C. (5) The electrochemical performance of Li(Ni0.5 Co0.2 Mn0.3 )O2 is related to its primary particle size, crystallization and level of cation ordering. Small and uniform primary particles result in better cycling performance while better crystallization and cation ordering results in higher initial capacity. It is worth noting that above results can be applied to other NCM materials with different compositions to a certain extent according to our preliminary work on NMCs with Mn:Ni:Co = 4:4:2 and Mn:Ni:Co = 1:1:1 compositions which provide an instruction for optimizing other NMCs synthesized by solid-reaction with precursors.
It was found that the Li(Ni0.5 Co0.2 Mn0.3 )O2 samples synthesized at 880 ◦ C are of the most uniform regular primary particles and well defined layered structure, which should have a better electrochemical performance than the others. The sample NCM-3-880 with highest electrochemical performance and tap density (∼2.7 g/cm3 ) was synthesized with the precursor with smallest primary particle size and relatively high tap density (NCM-3) and was found to deliver discharge capacity of 171 mAh/g during the first cycle with charge efficiency of 85% and high cycling performance. References [1] Y. Chen, G.X. Wang, K. Konstantinov, H.K. Liu, S.X. Dou, Journal of Power Sources 119–121 (0) (2003) 184. [2] T. Ohzuku, Y. Makimura, Chemistry Letters (7) (2001) 642. [3] Y.K. Sun, S.T. Myung, B.C. Park, J. Prakash, I. Belharouak, K. Amine, Nature Materials 8 (4) (2009) 320. [4] S. Patoux, M.M. Doeff, Electrochemistry Communications 6 (8) (2004) 767. [5] Y. Koyama, I. Tanaka, H. Adachi, Y. Makimura, T. Ohzuku, Journal of Power Sources 119–121 (2003) 644. [6] I. Belharouak, Y.K. Sun, J. Liu, K. Amine, Journal of Power Sources 123 (2) (2003) 247. [7] S. Zhang, Electrochimica Acta 52 (25) (2007) 7337. [8] X.Y. Zhang, W.J. Jiang, A. Mauger, Qilu, F. Gendron, C.M. Julien, Journal of Power Sources 195 (5) (2010) 1292. [9] C. Deng, L. Liu, W. Zhou, K. Sun, D. Sun, Electrochimica Acta 53 (5) (2008) 2441. [10] X. Luo, X. Wang, L. Liao, S. Gamboa, P.J. Sebastian, Journal of Power Sources 158 (1) (2006) 654. [11] J.-M. Kim, H.-T. Chung, Electrochimica Acta 49 (6) (2004) 937. [12] J.H. Kim, C.S. Yoon, Y.K. Sun, Journal of the Electrochemical Society 150 (4) (2003) A538. [13] J.R. Dahn, U. von Sacken, C.A. Michal, Solid State Ionics 44 (1–2) (1990) 87. [14] J.N. Reimers, E. Rossen, C.D. Jones, J.R. Dahn, Solid State Ionics 61 (4) (1993) 335. [15] C.-Y. Hu, J. Guo, X.-Y. Wang, The Chinese Journal of Nonferrous Metals 18 (9) (2008) 1721. [16] H.-j. Guo, M. Zhang, X.-H. Li, X.-M. Zhang, Z.-X. Wang, W.-J. Peng, M. Hu, Transactions of Nonferrous Metals Society of China 15 (5) (2005) 1185.