Preparation of LiCoO2 nanofibers by electrospinning technique

Preparation of LiCoO2 nanofibers by electrospinning technique

ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 67 (2006) 1423–1426 www.elsevier.com/locate/jpcs Preparation of LiCoO2 nanofibers by elec...

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ARTICLE IN PRESS

Journal of Physics and Chemistry of Solids 67 (2006) 1423–1426 www.elsevier.com/locate/jpcs

Preparation of LiCoO2 nanofibers by electrospinning technique Changlu Shaoa,, Na Yua, Yichun Liua, Rixiang Mub a

Center for Advanced Optoelectronic Functional Materials Research, Northeast Normal University, Changchun 130024, China b Nanoscale Materials and Sensors, Department of Physics, Fisk University, Nashville, TN, USA Received 28 September 2005; received in revised form 1 December 2005; accepted 11 January 2006

Abstract Using a sol–gel processing and electrospinning technique, extrathin fibers of PVA (polyvinyl alcohol)/lithium chloride/cobalt acetate composite were prepared. After calcinations of the above precursor fibers at 6001C, LiCoO2 nanofibers with a diameter of 100–150 nm, were successfully obtained. Measurements of TG/DTA, IR, XRD, Raman, SEM, EDS, respectively, were performed to characterize the properties of the as-prepared materials. We observed a strong correlation between crystalline phase and morphology of the fibers and calcinations temperature. r 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; C. X-ray diffraction

1. Introduction Lithium-transition-metal-oxides, such as LiCoO2, LiNiO2, LiMn2O4, LiMnO2, and LiV3O8, are all highperformance cathode materials for lithium–ion batteries [1]. As a result of the intense search for high-specific energy cathode materials for use in lithium–ion rechargeable battery technology, LiCoO2 has become the first, and one of the most promising for commercial application [2], due to its good capacity, high specific energy, good power rates, low self-discharge, and excellent cycle life [3,4]. The traditional method for synthesizing LiCoO2 is a solid-state reaction at high temperature [5,6], which may result in inhomogeneity, abnormal grain growth, and poor control of stoichiometry. Many advanced chemical processes, such as the sol–gel process, spray decomposition, and precipitation and freeze-drying rotary evaporation, supercritical drying, have been developed to prepare high-active materials of high purity and crystallinity [7–10]. Among them, sol–gel synthesis is considered as an important method to prepare these kinds of materials, in which all the components can be homogenously distributed to atomic scale, thus allowing a reduction of heating temperature and Corresponding author.

E-mail address: [email protected] (C. Shao). 0022-3697/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2006.01.104

sintering time. And, pure phase products with good crystallizability, homogeneity and uniform particles morphology could be obtained by sol–gel method [11–14]. Recently, a novel method for making inorganic nanofibers by combining the sol–gel processing with electrospinning technique together was found [15–18]. One of the attractive features associated with this method is that the obtained nanofiber mats possess high surface areas and small pore sizes [19]. In this paper, we describe the preparation of the electrospun nanofibers of LiCoO2 with the potential for improved electrode properties as a result of high surface areas. 2. Experimental For the preparation of nanofibers of LiCoO2, 1.50 g of lithium chloride (LiCl  1H2O) and 0.36 g of cobalt acetate (Co(CH3COO)2  4H2O) in the required molar ratio for the formation of LiCoO2, were dissolved in 10.0 g of deionized water. Then 40.0 g of aqueous PVA (poly(vinyl alcohol)) solution of 10 wt% was dropped slowly into the above solution with thorough stirring. Thus, a viscous solution of PVA/lithium chloride and cobalt acetate composites was obtained. Then, it was contained in a plastic capillary. A copper pin connected to a high-voltage generator was placed in the solution, and the solution was kept in the

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capillary by adjusting the angle between capillary and the fixing bar. A grounded iron drum, covered with an aluminum foil, served as the counter electrode. A voltage of 12 kV was applied to the solution and a dense web of fibers was collected on the aluminum foil. The obtained fiber mats were dried 5 h at 701C under vacuum, and then calcined at 400–7001C at a rising rate of 41 min1 and remained 3 h at the required temperature. TG and DTA were performed on a NETZSCH STA 449C thermoanalyzer in an air atmosphere. Infrared spectra were obtained on Magna 560 FT-IR spectrometer with a resolution of 1 cm1, KBr wafers were used, and the weight percentage of nanofibers in KBr was about 0.5%. XRD patterns of the samples were recorded by a Siemens D5005 Diffractometer, scans were made from 41 to 701 (2y) at the speed of 21 min1, Ni-filtered CuKa was used. Raman spectra were performed on a HR 800 spectrometer from 100 cm1 to 2000 cm1. For SEM investigation, a Hitach-600 was used. EDS spectra was given by a X L 30E SEM FEG Scanning electron microscope. 3. Results and discussion 3.1. TG/DTA Fig. 1 showed the thermal behavior of the precursor fibers of PVA/lithium chloride/cobalt acetate composites, which showed that most of the organic components belonged to PVA, the–CH3COO group of cobalt acetate, the–Cl group of lithium chloride, and other volatiles (H2O, COx, etc.), were removed at the temperature below 6001C. The weight loss of the precursor fibers terminated at 5001C and three discrete regions of weight loss occurred at about 200, 370, 4001C, respectively. As observed in the DTA curve, the exothermic peak at about 2001C in the DTA curve was thought to be due to the decomposition of cobalt acetate into Co3O4 [20], the peak at about 3701C was assigned to the reaction of freshly formed Co3O4 with

lithium chloride to form LiCoO2 [20] and the degradation of PVA by a dehydration on the polymer side chain [21], the peak at about 4001C was associated with the decomposition of PVA main chain [22] and the continuous combination of lithium chloride with cobalt acetate. Up to a temperature of 6001C, there was no change in weight loss, indicating the formation of pure inorganic oxide. 3.2. IR spectra Fig. 2 gave the FT-IR spectra for the PVA/lithium chloride/cobalt acetate composite fibers and for those calcined at different temperature. As observed in Fig. 2a and b, due to the reaction of LiCl  H2O with Co(CH3COO)2 4H2O and the decomposition of PVA, the peaks at about 3413 cm1(s), 2941 cm1(s), 1636 cm1(w), 1 1 1 1570 cm (w), 1450 cm (s), 1336 cm (w), 1095 cm1(s), 921 cm1(w), 847 cm1(s), corresponding to nC–H, nC–C, nC–O, nO–H, nLi–Cl, respectively [23], weakened or disappeared after calcinations at 4001C, and two new strong peaks around 667 cm1 and 576 cm1 assigned to nCo–O, nLi–O of LiCoO2 appeared [24]. When calcinations at 6001C (Fig. 2c), all the peaks corresponding to the organic groups of PVA and lithium chloride and cobalt acetate disappeared, meanwhile the intensity of the peaks at about 667 cm1 and 576 cm1 enhanced, indicating the formation of pure LiCoO2 phase at this temperature. Notably, there always existed 1450 and 3415 cm1 peaks in Fig. 2a–c, which should be assigned to H2O absorbed by the fibers samples or the KBr wafers. These results illustrated that the organic molecules could be removed completely from PVA/lithium chloride/cobalt acetate composite fibers when the calcinations temperature was above 6001C. And the LiCoO2 species could be obtained above this temperature.

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Fig. 1. TG–DTA curves of precursor fibers of PVA/lithium chloride/ cobalt acetate composites.

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Fig. 2. FT-IR spectra of various fibers samples: (a) PVA/lithium chloride/ cobalt acetate composites fibers; (b) calcinations at 400 1C; (c) calcinations at 600 1C.

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3.3. X-ray diffraction spectra

3.5. Scanning electron microscopy

Fig. 3 gave the XRD patterns for various fibers samples. As shown in Fig. 3a, there existed a broad peak around 2y ¼ 201, corresponding to the (1 0 1) plane of PVA semicrystalline [25] in PVA/lithium chloride/cobalt acetate composite fibers. This result indicated that the crystallinity of PVA was largely influenced by the presence of lithium chloride and cobalt acetate in the PVA/lithium chloride/ cobalt acetate composite fibers, saying that there might be some interaction between PVA and lithium chloride and cobalt acetate molecules. Notably, after the PVA/lithium chloride/cobalt acetate composites fibers were calcined at 4001C (Fig. 3b), it was observed that crystalline peak of PVA disappeared and LiCoO2 [12,26,27] began to form along with some unreacted Co3O4 from the decomposition of cobalt acetate [20,28]. When increasing calcinations temperature to 6001C (Fig. 3c), the diffraction peaks conformed that LiCoO2 became the predominant phase. Notably, in a typical solid-state reaction, the LiCoO2 phase was formed at above 8001C [5,6]. Therefore, the electrospun fibers of PVA/lithium chloride/cobalt acetate composite derived from sol–gel and electrospinning processing could produce the LiCoO2 phase at a lower temperature.

A series of electron micrographs (Fig. 5) revealed the morphological changes that occurred during the calcinations of the precursor fibers. As shown in Fig. 5a, the precursor fibers appeared smooth and featureless. When calcined to 4001C (Fig. 5b), owing to the decomposition of PVA and the reaction between lithium chloride and cobalt

3.4. Raman spectra

Fig. 4. Raman spectra of PVA/lithium chloride/cobalt acetate composites fibers calcinations at 600 1C.

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Fig. 4 showed the spectra of the Raman measurements carried out on the LiCoO2 nanofibers calcined at 600 1C. Three broad bands were observed at about 472, 585, 665 cm1, corresponding to the A1g, Eg, F2g Raman active mode typical of LiCoO2 spinel structure [29]. This result showed that LiCoO2 nanofibers obtained at 600 1C was spinel phase.

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Fig. 3. XRD results for various fibers samples: (a) PVA/lithium chloride/ cobalt acetate composites fibers; (b) calcinations at 400 1C; (c) calcinations at 600 1C.

Fig. 5. SEM photographs of various fibers samples: (a) PVA/lithium chloride/cobalt acetate composites fibers; (b) calcinations at 400 1C; (c) calcinations at 600 1C; (d) calcinations at 700 1C.

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acetate, the diameters of fibers became smaller, and the surface became rougher than that were not calcined. When calcined at 6001C (Fig. 5c), nanofibers of LiCoO2, with regular morphology and diameters of 100–150 nm were prepared, and, EDS result showed that there was no Cl residue for the fibers calcined at 6001C. Notably, after increasing calcinations temperature to 7001C (Fig. 5d), the fibers morphology could not be stained, owing to the instability of the LiCoO2 under high temperature calcinations. 4. Conclusion For the first time, nanofibers of LiCoO2, with diameters of 100–150 nm, were prepared by using the electrospun fibers of PVA/lithium chloride/cobalt acetate composite as precursor and through calcinations treatment. These kinds of materials are expected to have improving electrode properties due to its high surface areas and 1D nanostructural properties. This method may also be used to synthesize other cathode materials for lithium–ion batteries and has good prospects for commercialization. Acknowledgements The present work is supported financially by the National Natural Science Foundation of China (no. 50572014), and the Key Project of Chinese Ministry of Education (no. 104071). References [1] R. Koksbang, J. Barker, H. Shi, M.Y. Saidi, Solid State Ionics 84 (1996) 1. [2] T. Nagaura, K. Tazawa, Prog. Batteries Solar Cells 9 (1990) 20. [3] E. Plichta, S. Slane, M. Uchiyama, M. Saloman, D. Chua, W. Ebner, H. Lin, J. Electrochem. Soc. 136 (1989) 1865. [4] H.F. Gibbard, J. Power Sources 26 (1989) 81.

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