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carbon composites as cathode materials for lithium-ion batteries
Advanced Powder Technology 24 (2013) 593–598
Contents lists available at SciVerse ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Effect of carbon content and calcination temperature on the electrochemical performance of lithium iron phosphate/carbon composites as cathode materials for lithium-ion batteries Xiaodong Wang ⇑, Ke Cheng, Jingwei Zhang, Laigui Yu, Jianjun Yang ⇑ Key Laboratory of Ministry of Education for Special Functional Materials, Henan University, Kaifeng 475004, China
a r t i c l e
i n f o
Article history: Received 17 March 2012 Received in revised form 23 July 2012 Accepted 2 November 2012 Available online 23 January 2013 Keywords: Lithium iron phosphate Water-soluble phenol-formaldehyde resin Lithium-ion batteries Cathode material
a b s t r a c t Lithium iron phosphate/carbon (LiFePO4/C) composites were prepared by a convenient method with water-soluble phenol-formaldehyde resin as the carbon precursor. The morphology, crystalline structure, thermal stability, and composition of as-prepared LiFePO4/C composites were investigated by scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, and Raman spectrometry. Their electrochemical performance was examined based on cyclic voltammogram with a LAND battery testing system while the effect of carbon content and calcination temperature was highlighted. Results show that carbon content and calcination temperature dramatically influence the discharge capacities and rate performance of LiFePO4/C composites. The optimal calcination temperature is 700 °C, and the optimal carbon content (mass fraction) is 8.7%. The LiFePO4/C composite prepared under the optimal conditions exhibits an initial room temperature discharge capacity of 150.2 mA h g 1 at a 0.2 C rate and a constant discharge capacity of about 105.7 mA h g 1 at a 20.0 C rate after 50 cycles, showing promising potential as a novel cathode material for lithium ion batteries. Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Since its discovery in 1997 [1,2], LiFePO4 has been emerging as the most potential cathode material for lithium ion batteries, in particular for high power application, owing to its high theoretical capacity (170 mA h g 1), low cost, environmental friendliness, and high thermal stability. However, its poor electric conductivity (less than 10 13 S cm 1) results in dramatic decrease in power at a high current density, which limits its battery performance and hinders its commercial use. To overcome this drawback, many researchers have made great efforts to prepare nanocrystalline LiFePO4 [3–7] and encapsulate LiFePO4 with carbon layer [8–11] or dope LiFePO4 with metal or nonmetal elements [12–15]. Among various methods to increase the electric conductivity of LiFePO4, nanocrystallization and encapsulation with carbon layer are the most effective ones, because nanocrystalline particles can shorten the trips of Li+ ions intercalation and extraction thereby improving the migration rate of ions [16,17], while carbon layer can inhibit the growth of crystalline particles and improve electric conductivity [18,19]. Recently, we reported the electrochemical performance of carbon-coated LiFePO4 cathode materials for lithium-ion batteries with resorcinol–formaldehyde polymer, soluble starch, sucrose, ⇑ Corresponding authors. Tel./fax: +86 378 3881358. E-mail address: [email protected] (X. Wang).
and citric acid monohydrate as the carbon precursors. To our disappointment, although carbon-coated LiFePO4 prepared with resorcinol–formaldehyde polymer as the carbon precursor exhibits a high initial discharge capacity [20], its rate performance still needs to be improved. Therefore, in the present research we select water-soluble phenol-formaldehyde (WSPF) resin as a carbon precursor to prepare LiFePO4/C with a convenient method, hoping to improve the rate performance of LiFePO4/C as a cathode material for lithium-ion batteries. This article reports the preparation, characterization, and high-rate performance of as-prepared LiFePO4/C while the effect of carbon content and calcination temperature on the electrochemical performance of LiFePO4/C composites is highlighted.
2. Experimental 2.1. Preparation of amorphous FePO4 The procedure for preparing amorphous nano-FePO4 is similar to that reported elsewhere [20]. Briefly, in a proper amount of distilled water were dissolved some polyethylene glycol-400 (PEG400) as the surfactant and Fe(NO3)39H2O as the iron source. Into resultant mixed solution was added an equimolar amount of H3PO4 under vigorous stirring, followed by heating to 60 °C (with
0921-8831/$ - see front matter Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2012.11.001
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X. Wang et al. / Advanced Powder Technology 24 (2013) 593–598
a water bath) and stirring for 60 min. Then a proper amount of ammonia water (NH3H2O) was slowly added to the mixed solution under vigorous stirring to allow immediate formation of a milkwhite precipitate and to adjust the solution pH to be about 2. After 4 h of continuous stirring, the precipitate was filtered and washed with distilled water for several times until pH = 6. As-washed precipitate was dried at 100 °C for 12 h in a vacuum oven and then ground into yellowish-white powder of amorphous FePO4. 2.2. Preparation of WSPF resin WSPF resin was synthesized by the method modified with reference to literature [21]. In brief, 1.84 g of LiOH powder was added in 18.82 g of melted phenol and allowed to react at 50 °C for 30 min under vigorous stirring yielding a non-transparent viscous brown solution. Into the viscous brown solution was slowly added 40 mL of formaldehyde solution (37%) allowing a gradual change of solution color to transparent brown in association with a slow rise of temperature to 80 °C. Transparent claret-red WSPF resin solution was obtained after continuous stirring at 80 °C for 30 min. 2.3. Preparation of LiFePO4/C composites Different amounts (0.15, 0.2, 0.25, and 0.30 mL) of WSPF resin were respectively mixed with 1.31 g of amorphous FePO4, 0.27 g of Li2CO3 and a little amount of water. Resultant mixtures were ground for 30 min, dried at 80 °C for 12 h, heat-treated at 350 °C for 2 h in flowing argon atmosphere, calcinated at 600–750 °C for 12 h in flowing argon atmosphere, cooled to room temperature, and ground again to yield LiFePO4/C composites. 2.4. Characterization and electrochemical test Powder X-ray diffraction (XRD, X’ Pert Pro MPD, Philips; Cu Ka radiation) analysis was employed to identify the crystalline phase of as-prepared LiFePO4/C composites. The surface morphology and particle size of the composites were investigated using a JSM5600LV scanning electron microscope (SEM). Thermogravimetric analysis (TGA) was conducted with an EXSTAR 6000 thermal analysis system at a heating rate of 10 °C min 1. Room
temperature Raman spectrum was recorded with a Renishaw RM-1000 microscopic Raman spectrometer (457.5 nm excitation; 10 mW power). As-prepared LiFePO4/C powder was mixed with carbon black and polyvinylidene fluoride (PVDF) in N-methylpyrrolidinon at mass fractions of 80%, 12%, and 8% generating a mixed slurry. The slurry was spread onto Al foil and dried in vacuum at 120 °C for 12 h giving electrodes. Cells with a LiFePO4/C loading of 2 mg cm 2 were assembled in an argon-atmospherefilled glove box. The solution of 1 mol L 1 LiPF6 in the mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 was used as the electrolyte. A LAND battery testing system was performed at room temperature to galvanostatically charge and discharge the cells at a voltage of 2.5–4.2 V. 3. Results and discussion Previous studies indicate that calcination temperature plays a key role in the preparation of LiFePO4/C cathode materials for lithium-ion batteries, and the optimal calcination temperature is generally regarded as 700 °C or 750 °C at which LiFePO4 crystal is well harvested and prevented from growing into large particles [22–25]. In the present research, the calcination temperature is discretionarily selected as 700 °C. Fig. 1 shows the XRD patterns of LiFePO4/C composites prepared with different amounts of WSPF resin as the carbon precursors and calcinated at 700 °C. All the diffraction peaks can be indexed to orthorhombic olivine-type structure of LiFePO4 with Pnma space group (JCPDS no. 83-2092). The lattice parameters of various as-synthesized LiFePO4/C composites are listed in Table 1. No diffraction peaks of carbon were detected, possibly due to its poor crystallinity (as shown in Fig. 6). The carbon contents (mass fractions) of LiFePO4/C composites prepared with different amounts (0.15, 0.2, 0.25, and 0.3 mL) of WSPF resin precursor, determined from TGA data with the method reported by Zhao et al. [7], are about 4.3%, 6.5%, 8.7%, and 10.2%, respectively. Fig. 2 shows the initial charge and discharge curves of LiFePO4/C composites with different carbon contents (calcination temperature: 700 °C; discharge rate: from 0.2 C to 2 C). At a discharge rate of 0.2 C, as-prepared LiFePO4/C composites with carbon contents of 4.3%, 6.5%, 8.7%, and 10.2% have specific capacities of 97.8,
Fig. 1. XRD patterns of LiFePO4/C composites with different carbon contents prepared at a calcination temperature of 700 °C.
X. Wang et al. / Advanced Powder Technology 24 (2013) 593–598 Table 1 Refined lattice parameters and cell volume of various LiFePO4/C composites with different carbon contents (deduced from relevant XRD data). Name of LiFePO4/C compositesa
Composite Composite Composite Composite
sample sample sample sample
1 2 3 4
Lattice parameters and cell volume a (nm)
b (nm)
c (nm)
v (nm3)
1.0327 1.0334 1.0332 1.0311
0.5994 0.6006 0.6012 0.5997
0.4694 0.4693 0.4697 0.4689
0.2906 0.2913 0.2918 0.2900
a Carbon contents (mass fractions) of composite samples 1–4 are 4.3%, 6.5%, 8.7%, and 10.2%.
125.4, 151.2, and 135.8 mA h g 1, respectively. With increasing discharge rate, the capacities of the composite samples with lower carbon contents drop dramatically. Namely, at a discharge rate of 2 C, the capacities of composite samples 1 and 2 with carbon contents of 4.3% and 6.5% are 65.4 mA h g 1 and 95.8 mA h g 1, and their capacity ratios of 2.0 C/0.2 C are 66.9% and 76.4%, respectively. Obviously, composite samples 3 and 4 with higher carbon contents of 8.7% and 10.2% have larger capacities (141.1 mA h g 1 and 124.1 mA h g 1) and larger capacity ratios of 2.0 C/0.2 C (93.5% and 91.4%) than composite samples 1 and 2 with lower carbon contents. This means that as-prepared LiFePO4/C composites with higher carbon contents are superior in capacity and rate performance.
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Fig. 3 shows the SEM images of LiFePO4/C composites with different carbon contents prepared at a calcination temperature of 700 °C. It is noticeable that LiFePO4/C composites with lower carbon contents (4.3% and 6.5%) have irregular shapes and emerge as large particles, and the composite samples with higher carbon contents (8.7% and 10.2%) appear as small spherical particles with a diameter of about 200 nm. Therefore, we deduce that the differences in the capacity and rate performance of LiFePO4/C composites with different carbon contents are mainly attributed to their different carbon contents and size of LiFePO4/C particles. On one hand, doping a higher content of carbon contributes to increase conductivity [19] thereby enhancing rate performance and suppressing the growth of LiFePO4 crystallites [26]. On the other hand, the small particle size of electrode materials readily facilitates the intercalation/extraction of lithium ions into/from the electrode thereby improving the capacity and rate performance [16,17]. Besides, LiFePO4/C composite containing 8.7% C possesses the maximum specific capacity, but the specific capacity of the LiFePO4/C composite containing 10.2% C is reduced to some extent as compared with that of the composite containing 8.7% C. This is possibly because an excessive amount of carbon leads to reduced ratio of active substance (i.e., LiFePO4) to carbon and decreased tap density of the composite sample. Fig. 4 shows the cycling performance of LiFePO4/C composites with different carbon contents (calcination temperature: 700 °C;
Fig. 2. Initial charge and discharge curves of LiFePO4/C composites with carbon contents (mass fractions) of (a) 4.3%, (b) 6.5%, (c) 8.7%, and (d) 10.2% (calcination temperature: 700 °C; discharge rate: from 0.2 C to 2 C).
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Fig. 3. SEM images of LiFePO4/C composites with different carbon contents prepared at a calcination temperature of 700 °C.
Fig. 4. The cycling performance of LiFePO4/C composites with different carbon contents (calcination temperature: 700 °C; discharge rate: from 0.2 C to 2 C). Inset shows the high rate (5 C, 10 C and 20 C) cycling performance of LiFePO4/C composite sample with 8.7% C.
discharge rate: from 0.2 C to 2 C), where the inset shows the high rate (5 C, 10 C and 20 C) cycling performance of LiFePO4/C composite with 8.7% C. It can be seen that all as-prepared LiFePO4/C composites with different carbon contents exhibit excellent cycling performance. Particularly, LiFePO4/C composite with 8.7% C possesses the highest discharge capacity, and it retains a discharge capacity of 105.7 mA h g 1 even after 50 cycles (see inset in Fig. 4). Thus we can infer that WSPF resin can well soak fine FePO4 particles and prevent them from agglomerating into too large particles during calcination, resulting in improved cycling performance of LiFePO4/C composites. In the meantime, carbon derived
from WSPF resin can encapsulate and intertwine LiFePO4 particles thereby restricting their volume change in charge/discharge processes and facilitating the migration of electrons between different LiFePO4 particles, also leading to improved cycling performance of the composites. To investigate the effect of calcination temperature on the electrochemical performance of LiFePO4/C composites, we also examined the discharge capacity and rate performance of LiFePO4/C composites (carbon content: 8.7%) calcinated at 600–750 °C. As seen in Fig. 5a, the LiFePO4/C composite prepared at a calcination temperature of 700 °C shows the highest discharge capacity.
X. Wang et al. / Advanced Powder Technology 24 (2013) 593–598
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Fig. 5. Cycling performance at various discharge rates (a) and XRD patterns (b) of LiFePO4/C composites calcinated at 600–750 °C (carbon content 8.7%).
corresponds to disordered structured carbon (D band), and that at 1590 cm 1 mainly corresponds to graphitized structured carbon (G band) [27,28]. The integrated intensity ratio of D band to G band (ID/IG) in the Raman spectrum represents the degree of graphitization; and a decrease in the ID/IG ratio means a higher degree of graphitization. In this article, the ratio of ID/IG ratio is 0.806, which indicates that during the preparation of LiFePO4/C composite, WSPF resin is pyrolyzed to form graphitized carbons with a lower degree of disorder thereby favoring to increase the conductivity [29]. 4. Conclusions
Fig. 6. Raman spectrum of LiFePO4/C composite calcinated at 750 °C (carbon content: 8.7%).
Considering relevant XRD data shown in Fig. 5b, we can infer that the electrochemical performance of LiFePO4/C composites prepared at different calcination temperatures is highly dependent on their crystalline structure. Namely, the composite samples prepared at 600 °C and 650 °C both contain a small amount of impurities, corresponding to their lower discharge capacities. The LiFePO4/C composite obtained at a calcination temperature of 700 °C shows very weak XRD peaks of impurities, corresponding to its highest discharge capacity. Interestingly, although the LiFePO4/C composite prepared at 750 °C shows no XRD peaks of impurities (all of its XRD peaks can be indexed to single phase ordered olivine structure of the orthorhombic space group Pnmb (JCPDS card no. 83-2092)), its discharge capacity is much lower than that of the composite calcinated at 700 °C. This is possibly because LiFePO4 particles tend to excessively grow at a too high calcination temperature like 750 °C forming larger particles which hinder the intercalation/extraction of lithium ions into/from the electrode. Therefore, we suggest that LiFePO4/C composites should be calcinated at 700 °C so as to acquire the best discharge capacity and rate performance, even though a small amount of impurities is generated thereat. Fig. 6 shows the Raman spectrum of LiFePO4/C composite calcinated at 750 °C (carbon content: 8.7%). The peak at 1350 cm 1
LiFePO4/C composites have been prepared by a convenient method using water-soluble phenol-formaldehyde resin as the carbon precursor. The electrochemical performance of as-prepared LiFePO4/C composites as the cathode materials for lithium-ion batteries has been investigated while the effect of carbon content and calcination temperature on the electrochemical performance is highlighted. It has been found that the carbon content and calcination temperature dramatically influence the discharge capacities and rate performance of LiFePO4/C composites. The optimal calcination temperature and carbon content are suggested as 700 °C and 8.7%, respectively. Resultant LiFePO4/C composite prepared under the optimal conditions exhibits an initial room temperature discharge capacity of 150.2 mA h g 1 at a 0.2 C rate, and its constant discharge capacity after 50 cycles at a 20.0 C rate is as much as 105.7 mA h g 1, showing great potentials as a novel cathode material for lithium-ion batteries. Acknowledgments This research is supported by the National Natural Science Foundation of China (Grant Nos. 20973054, 20971037, and 21103042), the Key Laboratory Foundation of Henan Province (Grant No. 122300413205) and the Postdoctoral Scientific Research Foundation of Henan University (Grant No. BH2011054). References [1] A.K. Padhi, K.S. Nanjundaswamy, C. Masquelier, J.B. Goodenough, Phopshoolivines as positve-electrode materials for rechargeable lithium batteries, Journal of Electrochemical Society 144 (1997) 1188–1194. [2] A.K. Padhi, K.S. Nanjundaswamy, C. Masquelier, J.B. Goodenough, Eeffet of structure on the Fe3+/Fe2+ redox couple in iron phosphate, Journal of Electrochemical Society 144 (1997) 1609–1613.
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[3] K.F. Hsu, S.Y. Tsay, B.J. Hwang, Synthesis and characterization of nano-sized LiFePO4 cathode materials prepared by a citric acid-based sol–gel route, Journal of Materials Chemistry 14 (2004) 2690–2695. [4] Y. Wang, G. Cao, Developments in nanostructured cathode materials for high performance lithium-ion batteries, Advanced Materials 20 (2008) 2251–2269. [5] I. Bilecka, A. Hintennach, I. Djerdj, P. Novák, M. Niederger, Efficient microwaveassisted synthesis of LiFePO4 mesocrystals with high cycling stability, Journal of Materials Chemistry 19 (2009) 5125–5128. [6] M.H. Lee, J.Y. Kim, H.K. Song, A hollow sphere secondary structure of LiFePO4 nanoparticles, Chemical Communications 46 (2010) 6795–6797. [7] J.Q. Zhao, J.P. He, J.H. Zhou, Y.X. Guo, T. Wang, S.C. Wu, X.H. Ding, R.M. Huang, H.R. Xue, Facile synthesis for LiFePO4 nanospheres in tridimensional porous carbon framework for lithium ion batteries, Journal of Chemical Physics C 115 (2011) 2888–2894. [8] L.N. Wang, Z.G. Zhang, K.L. Zhang, A simple, cheap soft synthesis routine for LiFePO4 using iron (III) raw material, Journal of Power Source 167 (2007) 200– 205. [9] K. Zaghib, A. Mauger, F. Gendron, C.M. Julien, Surface effects on the physical and electrochemical properties of thin LiFePO4 particles, Chemistry of Materials 20 (2008) 462–469. [10] S.W. Oh, S.T. Myung, S.M. Oh, K.H. Oh, K. Amine, B. Scrosati, Y.K. Sun, Double carbon coating of LiFePO4 as high rate electrode for rechargeable lithium batteries, Advanced Materials 22 (2010) 4842–4845. [11] M.S. Pan, Z.T. Zhou, Carbon rich surface of LiFePO4 grain enhancing its rate capability, Materials Letters 65 (2011) 1131–1133. [12] D. Wang, H. Li, S. Shi, X. Huang, L. Chen, Improving the rate performance of LiFePO4 by Fe-site doping, Electrochimica Acta 50 (2005) 2955–2958. [13] H.C. Shin, S.B. Park, H. Jang, K.Y. Chung, W.I. Cho, C.S. Kim, B.W. Cho, Rate performance and structural change of Cr-doped LiFePO4/C during cycling, Electrochimica Acta 53 (2008) 7946–7951. [14] C.S. Sun, Y. Zhang, X.J. Zhang, Z. Zhou, Structural and electrochemical properties of Cl-doped LiFePO4/C, Journal of Power Source 195 (2010) 3680– 3683. [15] L. Yang, L.F. Jiao, Y.L. Miao, H.T. Yuan, Synthesis and characterization of LiFe0.99Mn0.01(PO4)2.99/3F0.01/C as a cathode material for lithium-ion battery, Journal of Solid State Electrochemistry 14 (2010) 1001–1005. [16] H. Gabrisch, J.D. Wilcox, M.M. Doeff, Carbon surface layers on a high-rate LiFePO4, Electrochemical and Solid-State Letters 9 (2006) A360–A363.
[17] D. Choi, P.N. Kumta, Surfactant based sol–gel approach to nanostructured LiFePO4 for high rate Li-ion batteries, Journal of Power Source 163 (2007) 1064–1069. [18] Y.L. Cao, L.H. Yu, T. Li, X.P. Ai, H.X. Yang, Synthesis and electrochemical characterization of carbon-coated nanocrystalline LiFePO4 prepared by polyacrylates–pyrolysis route, Journal of Power Source 172 (2007) 913–918. [19] Y.D. Cho, G.T.K. Feya, H.M. Kao, The effect of carbon coating thickness on the capacity of LiFePO4/C composite cathodes, Journal of Power Source 189 (2009) 256–262. [20] Y.C. Lan, X.D. Wang, J.W. Zhang, J.W. Zhang, Z.S. Wu, Z.J. Zhang, Preparation and characterization of carbon-coated LiFePO4 cathode materials for lithiumion batteries with resorcinol–formaldehyde polymer as carbon precursor, Powder Technology 212 (2011) 327–331. [21] L.M. Sun, M.Y. Li, B. Peng, M.Q. Lin, J.X. Guo, Synthesis and characterization of novel water soluble phenol-formaldehyde resin, Acta Petrolei Sinica (Petroleum Processing Section) 24 (2008) 63–68. [22] X.L. Wu, L.Y. Jiang, F.F. Cao, Y.G. Guo, L.J. Wang, LiFePO4 nanoparticles embedded in a nanoporous carbon matrix: superior cathode material for electrochemical energy-storage devices, Advanced Materials 21 (2009) 2710– 2714. [23] N. Meethong, Y.H. Kao, S.A. Speakman, Y.M. Chiang, Aliovalent substitutions in olivine lithium iron phosphate and impact on structure and properties, Advanced Functional Materials 19 (2009) 1060–1070. [24] S.Y. Chung, S.Y. Choi, T. Yamamoto, Y. Ikuhara, Orientation-dependent arrangement of antisite defects in lithium iron(II) phosphate crystals, Angewandte Chemie-International Edition 48 (2009) 543–546. [25] T.H. Cho, H.T. Chung, Synthesis of olivine-type LiFePO4 by emulsion-drying method, Journal of Power Source 133 (2004) 272–276. [26] K.F. Hsu, S.Y. Tsay, B.J. Hwang, Synthesis and characterization of nano-sized LiFePO4 cathode materials prepared by a citric acid-based sol–gel route, Journal of Materials Chemistry 14 (2004) 2690–2695. [27] M.M. Doeff, J.D. Wilcox, R. Kostecki, G. Lau, Optimization of carbon coatings on LiFePO4, Journal of Power Sources 163 (2006) 180–184. [28] G.L. Yang, A.F. Jalbout, Y. Xu, H.Y. Yu, X.G. He, H.M. Xie, R.S. Wang, Effect of polyacenic semiconductors on the performance of olivine LiFePO4, Electrochemical and Solid-State Letters 11 (2008) A125–A128. [29] T. Nakamura, Y. Miwa, M. Tabuchi, Y. Yamada, Structural and surface modifications of LiFePO4 olivine particles and their electrochemical properties, Journal of the Electrochemical Society 153 (2006) A1108–A1114.
Contents lists available at SciVerse ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Effect of carbon content and calcination temperature on the electrochemical performance of lithium iron phosphate/carbon composites as cathode materials for lithium-ion batteries Xiaodong Wang ⇑, Ke Cheng, Jingwei Zhang, Laigui Yu, Jianjun Yang ⇑ Key Laboratory of Ministry of Education for Special Functional Materials, Henan University, Kaifeng 475004, China
a r t i c l e
i n f o
Article history: Received 17 March 2012 Received in revised form 23 July 2012 Accepted 2 November 2012 Available online 23 January 2013 Keywords: Lithium iron phosphate Water-soluble phenol-formaldehyde resin Lithium-ion batteries Cathode material
a b s t r a c t Lithium iron phosphate/carbon (LiFePO4/C) composites were prepared by a convenient method with water-soluble phenol-formaldehyde resin as the carbon precursor. The morphology, crystalline structure, thermal stability, and composition of as-prepared LiFePO4/C composites were investigated by scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, and Raman spectrometry. Their electrochemical performance was examined based on cyclic voltammogram with a LAND battery testing system while the effect of carbon content and calcination temperature was highlighted. Results show that carbon content and calcination temperature dramatically influence the discharge capacities and rate performance of LiFePO4/C composites. The optimal calcination temperature is 700 °C, and the optimal carbon content (mass fraction) is 8.7%. The LiFePO4/C composite prepared under the optimal conditions exhibits an initial room temperature discharge capacity of 150.2 mA h g 1 at a 0.2 C rate and a constant discharge capacity of about 105.7 mA h g 1 at a 20.0 C rate after 50 cycles, showing promising potential as a novel cathode material for lithium ion batteries. Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Since its discovery in 1997 [1,2], LiFePO4 has been emerging as the most potential cathode material for lithium ion batteries, in particular for high power application, owing to its high theoretical capacity (170 mA h g 1), low cost, environmental friendliness, and high thermal stability. However, its poor electric conductivity (less than 10 13 S cm 1) results in dramatic decrease in power at a high current density, which limits its battery performance and hinders its commercial use. To overcome this drawback, many researchers have made great efforts to prepare nanocrystalline LiFePO4 [3–7] and encapsulate LiFePO4 with carbon layer [8–11] or dope LiFePO4 with metal or nonmetal elements [12–15]. Among various methods to increase the electric conductivity of LiFePO4, nanocrystallization and encapsulation with carbon layer are the most effective ones, because nanocrystalline particles can shorten the trips of Li+ ions intercalation and extraction thereby improving the migration rate of ions [16,17], while carbon layer can inhibit the growth of crystalline particles and improve electric conductivity [18,19]. Recently, we reported the electrochemical performance of carbon-coated LiFePO4 cathode materials for lithium-ion batteries with resorcinol–formaldehyde polymer, soluble starch, sucrose, ⇑ Corresponding authors. Tel./fax: +86 378 3881358. E-mail address: [email protected] (X. Wang).
and citric acid monohydrate as the carbon precursors. To our disappointment, although carbon-coated LiFePO4 prepared with resorcinol–formaldehyde polymer as the carbon precursor exhibits a high initial discharge capacity [20], its rate performance still needs to be improved. Therefore, in the present research we select water-soluble phenol-formaldehyde (WSPF) resin as a carbon precursor to prepare LiFePO4/C with a convenient method, hoping to improve the rate performance of LiFePO4/C as a cathode material for lithium-ion batteries. This article reports the preparation, characterization, and high-rate performance of as-prepared LiFePO4/C while the effect of carbon content and calcination temperature on the electrochemical performance of LiFePO4/C composites is highlighted.
2. Experimental 2.1. Preparation of amorphous FePO4 The procedure for preparing amorphous nano-FePO4 is similar to that reported elsewhere [20]. Briefly, in a proper amount of distilled water were dissolved some polyethylene glycol-400 (PEG400) as the surfactant and Fe(NO3)39H2O as the iron source. Into resultant mixed solution was added an equimolar amount of H3PO4 under vigorous stirring, followed by heating to 60 °C (with
0921-8831/$ - see front matter Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2012.11.001
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X. Wang et al. / Advanced Powder Technology 24 (2013) 593–598
a water bath) and stirring for 60 min. Then a proper amount of ammonia water (NH3H2O) was slowly added to the mixed solution under vigorous stirring to allow immediate formation of a milkwhite precipitate and to adjust the solution pH to be about 2. After 4 h of continuous stirring, the precipitate was filtered and washed with distilled water for several times until pH = 6. As-washed precipitate was dried at 100 °C for 12 h in a vacuum oven and then ground into yellowish-white powder of amorphous FePO4. 2.2. Preparation of WSPF resin WSPF resin was synthesized by the method modified with reference to literature [21]. In brief, 1.84 g of LiOH powder was added in 18.82 g of melted phenol and allowed to react at 50 °C for 30 min under vigorous stirring yielding a non-transparent viscous brown solution. Into the viscous brown solution was slowly added 40 mL of formaldehyde solution (37%) allowing a gradual change of solution color to transparent brown in association with a slow rise of temperature to 80 °C. Transparent claret-red WSPF resin solution was obtained after continuous stirring at 80 °C for 30 min. 2.3. Preparation of LiFePO4/C composites Different amounts (0.15, 0.2, 0.25, and 0.30 mL) of WSPF resin were respectively mixed with 1.31 g of amorphous FePO4, 0.27 g of Li2CO3 and a little amount of water. Resultant mixtures were ground for 30 min, dried at 80 °C for 12 h, heat-treated at 350 °C for 2 h in flowing argon atmosphere, calcinated at 600–750 °C for 12 h in flowing argon atmosphere, cooled to room temperature, and ground again to yield LiFePO4/C composites. 2.4. Characterization and electrochemical test Powder X-ray diffraction (XRD, X’ Pert Pro MPD, Philips; Cu Ka radiation) analysis was employed to identify the crystalline phase of as-prepared LiFePO4/C composites. The surface morphology and particle size of the composites were investigated using a JSM5600LV scanning electron microscope (SEM). Thermogravimetric analysis (TGA) was conducted with an EXSTAR 6000 thermal analysis system at a heating rate of 10 °C min 1. Room
temperature Raman spectrum was recorded with a Renishaw RM-1000 microscopic Raman spectrometer (457.5 nm excitation; 10 mW power). As-prepared LiFePO4/C powder was mixed with carbon black and polyvinylidene fluoride (PVDF) in N-methylpyrrolidinon at mass fractions of 80%, 12%, and 8% generating a mixed slurry. The slurry was spread onto Al foil and dried in vacuum at 120 °C for 12 h giving electrodes. Cells with a LiFePO4/C loading of 2 mg cm 2 were assembled in an argon-atmospherefilled glove box. The solution of 1 mol L 1 LiPF6 in the mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 was used as the electrolyte. A LAND battery testing system was performed at room temperature to galvanostatically charge and discharge the cells at a voltage of 2.5–4.2 V. 3. Results and discussion Previous studies indicate that calcination temperature plays a key role in the preparation of LiFePO4/C cathode materials for lithium-ion batteries, and the optimal calcination temperature is generally regarded as 700 °C or 750 °C at which LiFePO4 crystal is well harvested and prevented from growing into large particles [22–25]. In the present research, the calcination temperature is discretionarily selected as 700 °C. Fig. 1 shows the XRD patterns of LiFePO4/C composites prepared with different amounts of WSPF resin as the carbon precursors and calcinated at 700 °C. All the diffraction peaks can be indexed to orthorhombic olivine-type structure of LiFePO4 with Pnma space group (JCPDS no. 83-2092). The lattice parameters of various as-synthesized LiFePO4/C composites are listed in Table 1. No diffraction peaks of carbon were detected, possibly due to its poor crystallinity (as shown in Fig. 6). The carbon contents (mass fractions) of LiFePO4/C composites prepared with different amounts (0.15, 0.2, 0.25, and 0.3 mL) of WSPF resin precursor, determined from TGA data with the method reported by Zhao et al. [7], are about 4.3%, 6.5%, 8.7%, and 10.2%, respectively. Fig. 2 shows the initial charge and discharge curves of LiFePO4/C composites with different carbon contents (calcination temperature: 700 °C; discharge rate: from 0.2 C to 2 C). At a discharge rate of 0.2 C, as-prepared LiFePO4/C composites with carbon contents of 4.3%, 6.5%, 8.7%, and 10.2% have specific capacities of 97.8,
Fig. 1. XRD patterns of LiFePO4/C composites with different carbon contents prepared at a calcination temperature of 700 °C.
X. Wang et al. / Advanced Powder Technology 24 (2013) 593–598 Table 1 Refined lattice parameters and cell volume of various LiFePO4/C composites with different carbon contents (deduced from relevant XRD data). Name of LiFePO4/C compositesa
Composite Composite Composite Composite
sample sample sample sample
1 2 3 4
Lattice parameters and cell volume a (nm)
b (nm)
c (nm)
v (nm3)
1.0327 1.0334 1.0332 1.0311
0.5994 0.6006 0.6012 0.5997
0.4694 0.4693 0.4697 0.4689
0.2906 0.2913 0.2918 0.2900
a Carbon contents (mass fractions) of composite samples 1–4 are 4.3%, 6.5%, 8.7%, and 10.2%.
125.4, 151.2, and 135.8 mA h g 1, respectively. With increasing discharge rate, the capacities of the composite samples with lower carbon contents drop dramatically. Namely, at a discharge rate of 2 C, the capacities of composite samples 1 and 2 with carbon contents of 4.3% and 6.5% are 65.4 mA h g 1 and 95.8 mA h g 1, and their capacity ratios of 2.0 C/0.2 C are 66.9% and 76.4%, respectively. Obviously, composite samples 3 and 4 with higher carbon contents of 8.7% and 10.2% have larger capacities (141.1 mA h g 1 and 124.1 mA h g 1) and larger capacity ratios of 2.0 C/0.2 C (93.5% and 91.4%) than composite samples 1 and 2 with lower carbon contents. This means that as-prepared LiFePO4/C composites with higher carbon contents are superior in capacity and rate performance.
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Fig. 3 shows the SEM images of LiFePO4/C composites with different carbon contents prepared at a calcination temperature of 700 °C. It is noticeable that LiFePO4/C composites with lower carbon contents (4.3% and 6.5%) have irregular shapes and emerge as large particles, and the composite samples with higher carbon contents (8.7% and 10.2%) appear as small spherical particles with a diameter of about 200 nm. Therefore, we deduce that the differences in the capacity and rate performance of LiFePO4/C composites with different carbon contents are mainly attributed to their different carbon contents and size of LiFePO4/C particles. On one hand, doping a higher content of carbon contributes to increase conductivity [19] thereby enhancing rate performance and suppressing the growth of LiFePO4 crystallites [26]. On the other hand, the small particle size of electrode materials readily facilitates the intercalation/extraction of lithium ions into/from the electrode thereby improving the capacity and rate performance [16,17]. Besides, LiFePO4/C composite containing 8.7% C possesses the maximum specific capacity, but the specific capacity of the LiFePO4/C composite containing 10.2% C is reduced to some extent as compared with that of the composite containing 8.7% C. This is possibly because an excessive amount of carbon leads to reduced ratio of active substance (i.e., LiFePO4) to carbon and decreased tap density of the composite sample. Fig. 4 shows the cycling performance of LiFePO4/C composites with different carbon contents (calcination temperature: 700 °C;
Fig. 2. Initial charge and discharge curves of LiFePO4/C composites with carbon contents (mass fractions) of (a) 4.3%, (b) 6.5%, (c) 8.7%, and (d) 10.2% (calcination temperature: 700 °C; discharge rate: from 0.2 C to 2 C).
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Fig. 3. SEM images of LiFePO4/C composites with different carbon contents prepared at a calcination temperature of 700 °C.
Fig. 4. The cycling performance of LiFePO4/C composites with different carbon contents (calcination temperature: 700 °C; discharge rate: from 0.2 C to 2 C). Inset shows the high rate (5 C, 10 C and 20 C) cycling performance of LiFePO4/C composite sample with 8.7% C.
discharge rate: from 0.2 C to 2 C), where the inset shows the high rate (5 C, 10 C and 20 C) cycling performance of LiFePO4/C composite with 8.7% C. It can be seen that all as-prepared LiFePO4/C composites with different carbon contents exhibit excellent cycling performance. Particularly, LiFePO4/C composite with 8.7% C possesses the highest discharge capacity, and it retains a discharge capacity of 105.7 mA h g 1 even after 50 cycles (see inset in Fig. 4). Thus we can infer that WSPF resin can well soak fine FePO4 particles and prevent them from agglomerating into too large particles during calcination, resulting in improved cycling performance of LiFePO4/C composites. In the meantime, carbon derived
from WSPF resin can encapsulate and intertwine LiFePO4 particles thereby restricting their volume change in charge/discharge processes and facilitating the migration of electrons between different LiFePO4 particles, also leading to improved cycling performance of the composites. To investigate the effect of calcination temperature on the electrochemical performance of LiFePO4/C composites, we also examined the discharge capacity and rate performance of LiFePO4/C composites (carbon content: 8.7%) calcinated at 600–750 °C. As seen in Fig. 5a, the LiFePO4/C composite prepared at a calcination temperature of 700 °C shows the highest discharge capacity.
X. Wang et al. / Advanced Powder Technology 24 (2013) 593–598
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Fig. 5. Cycling performance at various discharge rates (a) and XRD patterns (b) of LiFePO4/C composites calcinated at 600–750 °C (carbon content 8.7%).
corresponds to disordered structured carbon (D band), and that at 1590 cm 1 mainly corresponds to graphitized structured carbon (G band) [27,28]. The integrated intensity ratio of D band to G band (ID/IG) in the Raman spectrum represents the degree of graphitization; and a decrease in the ID/IG ratio means a higher degree of graphitization. In this article, the ratio of ID/IG ratio is 0.806, which indicates that during the preparation of LiFePO4/C composite, WSPF resin is pyrolyzed to form graphitized carbons with a lower degree of disorder thereby favoring to increase the conductivity [29]. 4. Conclusions
Fig. 6. Raman spectrum of LiFePO4/C composite calcinated at 750 °C (carbon content: 8.7%).
Considering relevant XRD data shown in Fig. 5b, we can infer that the electrochemical performance of LiFePO4/C composites prepared at different calcination temperatures is highly dependent on their crystalline structure. Namely, the composite samples prepared at 600 °C and 650 °C both contain a small amount of impurities, corresponding to their lower discharge capacities. The LiFePO4/C composite obtained at a calcination temperature of 700 °C shows very weak XRD peaks of impurities, corresponding to its highest discharge capacity. Interestingly, although the LiFePO4/C composite prepared at 750 °C shows no XRD peaks of impurities (all of its XRD peaks can be indexed to single phase ordered olivine structure of the orthorhombic space group Pnmb (JCPDS card no. 83-2092)), its discharge capacity is much lower than that of the composite calcinated at 700 °C. This is possibly because LiFePO4 particles tend to excessively grow at a too high calcination temperature like 750 °C forming larger particles which hinder the intercalation/extraction of lithium ions into/from the electrode. Therefore, we suggest that LiFePO4/C composites should be calcinated at 700 °C so as to acquire the best discharge capacity and rate performance, even though a small amount of impurities is generated thereat. Fig. 6 shows the Raman spectrum of LiFePO4/C composite calcinated at 750 °C (carbon content: 8.7%). The peak at 1350 cm 1
LiFePO4/C composites have been prepared by a convenient method using water-soluble phenol-formaldehyde resin as the carbon precursor. The electrochemical performance of as-prepared LiFePO4/C composites as the cathode materials for lithium-ion batteries has been investigated while the effect of carbon content and calcination temperature on the electrochemical performance is highlighted. It has been found that the carbon content and calcination temperature dramatically influence the discharge capacities and rate performance of LiFePO4/C composites. The optimal calcination temperature and carbon content are suggested as 700 °C and 8.7%, respectively. Resultant LiFePO4/C composite prepared under the optimal conditions exhibits an initial room temperature discharge capacity of 150.2 mA h g 1 at a 0.2 C rate, and its constant discharge capacity after 50 cycles at a 20.0 C rate is as much as 105.7 mA h g 1, showing great potentials as a novel cathode material for lithium-ion batteries. Acknowledgments This research is supported by the National Natural Science Foundation of China (Grant Nos. 20973054, 20971037, and 21103042), the Key Laboratory Foundation of Henan Province (Grant No. 122300413205) and the Postdoctoral Scientific Research Foundation of Henan University (Grant No. BH2011054). References [1] A.K. Padhi, K.S. Nanjundaswamy, C. Masquelier, J.B. Goodenough, Phopshoolivines as positve-electrode materials for rechargeable lithium batteries, Journal of Electrochemical Society 144 (1997) 1188–1194. [2] A.K. Padhi, K.S. Nanjundaswamy, C. Masquelier, J.B. Goodenough, Eeffet of structure on the Fe3+/Fe2+ redox couple in iron phosphate, Journal of Electrochemical Society 144 (1997) 1609–1613.
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[3] K.F. Hsu, S.Y. Tsay, B.J. Hwang, Synthesis and characterization of nano-sized LiFePO4 cathode materials prepared by a citric acid-based sol–gel route, Journal of Materials Chemistry 14 (2004) 2690–2695. [4] Y. Wang, G. Cao, Developments in nanostructured cathode materials for high performance lithium-ion batteries, Advanced Materials 20 (2008) 2251–2269. [5] I. Bilecka, A. Hintennach, I. Djerdj, P. Novák, M. Niederger, Efficient microwaveassisted synthesis of LiFePO4 mesocrystals with high cycling stability, Journal of Materials Chemistry 19 (2009) 5125–5128. [6] M.H. Lee, J.Y. Kim, H.K. Song, A hollow sphere secondary structure of LiFePO4 nanoparticles, Chemical Communications 46 (2010) 6795–6797. [7] J.Q. Zhao, J.P. He, J.H. Zhou, Y.X. Guo, T. Wang, S.C. Wu, X.H. Ding, R.M. Huang, H.R. Xue, Facile synthesis for LiFePO4 nanospheres in tridimensional porous carbon framework for lithium ion batteries, Journal of Chemical Physics C 115 (2011) 2888–2894. [8] L.N. Wang, Z.G. Zhang, K.L. Zhang, A simple, cheap soft synthesis routine for LiFePO4 using iron (III) raw material, Journal of Power Source 167 (2007) 200– 205. [9] K. Zaghib, A. Mauger, F. Gendron, C.M. Julien, Surface effects on the physical and electrochemical properties of thin LiFePO4 particles, Chemistry of Materials 20 (2008) 462–469. [10] S.W. Oh, S.T. Myung, S.M. Oh, K.H. Oh, K. Amine, B. Scrosati, Y.K. Sun, Double carbon coating of LiFePO4 as high rate electrode for rechargeable lithium batteries, Advanced Materials 22 (2010) 4842–4845. [11] M.S. Pan, Z.T. Zhou, Carbon rich surface of LiFePO4 grain enhancing its rate capability, Materials Letters 65 (2011) 1131–1133. [12] D. Wang, H. Li, S. Shi, X. Huang, L. Chen, Improving the rate performance of LiFePO4 by Fe-site doping, Electrochimica Acta 50 (2005) 2955–2958. [13] H.C. Shin, S.B. Park, H. Jang, K.Y. Chung, W.I. Cho, C.S. Kim, B.W. Cho, Rate performance and structural change of Cr-doped LiFePO4/C during cycling, Electrochimica Acta 53 (2008) 7946–7951. [14] C.S. Sun, Y. Zhang, X.J. Zhang, Z. Zhou, Structural and electrochemical properties of Cl-doped LiFePO4/C, Journal of Power Source 195 (2010) 3680– 3683. [15] L. Yang, L.F. Jiao, Y.L. Miao, H.T. Yuan, Synthesis and characterization of LiFe0.99Mn0.01(PO4)2.99/3F0.01/C as a cathode material for lithium-ion battery, Journal of Solid State Electrochemistry 14 (2010) 1001–1005. [16] H. Gabrisch, J.D. Wilcox, M.M. Doeff, Carbon surface layers on a high-rate LiFePO4, Electrochemical and Solid-State Letters 9 (2006) A360–A363.
[17] D. Choi, P.N. Kumta, Surfactant based sol–gel approach to nanostructured LiFePO4 for high rate Li-ion batteries, Journal of Power Source 163 (2007) 1064–1069. [18] Y.L. Cao, L.H. Yu, T. Li, X.P. Ai, H.X. Yang, Synthesis and electrochemical characterization of carbon-coated nanocrystalline LiFePO4 prepared by polyacrylates–pyrolysis route, Journal of Power Source 172 (2007) 913–918. [19] Y.D. Cho, G.T.K. Feya, H.M. Kao, The effect of carbon coating thickness on the capacity of LiFePO4/C composite cathodes, Journal of Power Source 189 (2009) 256–262. [20] Y.C. Lan, X.D. Wang, J.W. Zhang, J.W. Zhang, Z.S. Wu, Z.J. Zhang, Preparation and characterization of carbon-coated LiFePO4 cathode materials for lithiumion batteries with resorcinol–formaldehyde polymer as carbon precursor, Powder Technology 212 (2011) 327–331. [21] L.M. Sun, M.Y. Li, B. Peng, M.Q. Lin, J.X. Guo, Synthesis and characterization of novel water soluble phenol-formaldehyde resin, Acta Petrolei Sinica (Petroleum Processing Section) 24 (2008) 63–68. [22] X.L. Wu, L.Y. Jiang, F.F. Cao, Y.G. Guo, L.J. Wang, LiFePO4 nanoparticles embedded in a nanoporous carbon matrix: superior cathode material for electrochemical energy-storage devices, Advanced Materials 21 (2009) 2710– 2714. [23] N. Meethong, Y.H. Kao, S.A. Speakman, Y.M. Chiang, Aliovalent substitutions in olivine lithium iron phosphate and impact on structure and properties, Advanced Functional Materials 19 (2009) 1060–1070. [24] S.Y. Chung, S.Y. Choi, T. Yamamoto, Y. Ikuhara, Orientation-dependent arrangement of antisite defects in lithium iron(II) phosphate crystals, Angewandte Chemie-International Edition 48 (2009) 543–546. [25] T.H. Cho, H.T. Chung, Synthesis of olivine-type LiFePO4 by emulsion-drying method, Journal of Power Source 133 (2004) 272–276. [26] K.F. Hsu, S.Y. Tsay, B.J. Hwang, Synthesis and characterization of nano-sized LiFePO4 cathode materials prepared by a citric acid-based sol–gel route, Journal of Materials Chemistry 14 (2004) 2690–2695. [27] M.M. Doeff, J.D. Wilcox, R. Kostecki, G. Lau, Optimization of carbon coatings on LiFePO4, Journal of Power Sources 163 (2006) 180–184. [28] G.L. Yang, A.F. Jalbout, Y. Xu, H.Y. Yu, X.G. He, H.M. Xie, R.S. Wang, Effect of polyacenic semiconductors on the performance of olivine LiFePO4, Electrochemical and Solid-State Letters 11 (2008) A125–A128. [29] T. Nakamura, Y. Miwa, M. Tabuchi, Y. Yamada, Structural and surface modifications of LiFePO4 olivine particles and their electrochemical properties, Journal of the Electrochemical Society 153 (2006) A1108–A1114.