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C by solid–liquid reaction milling method
Powder Technology 197 (2010) 309–313
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Short Communication
Synthesis of LiFePO4/C by solid–liquid reaction milling method Xu-heng Liu, Zhong-wei Zhao ⁎ School of Metallurgical Science and Engineering, Central South University, ChangSha, 410083, China Key Laboratory of Hunan Province for Metallurgy and Material Processing of Rare Metals, ChangSha, 410083, China
a r t i c l e
i n f o
Article history: Received 18 April 2009 Received in revised form 14 September 2009 Accepted 27 September 2009 Available online 3 October 2009 Keywords: Li-ion battery LiFePO4/C Solid–liquid reaction milling
a b s t r a c t LiFePO4/C was synthesized by the method of solid–liquid reaction milling, using FeCl3·6H2O, Li2CO3 and (NH4)2HPO4 and glucose, which was used as reductant (carbon source). The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), TG-DTA analysis, infrared absorption carbon– sulfur analysis and electrochemical performance test. The sample synthesized at 680 °C for 8 h showed, at initial discharge, a capacity of 155.8, 153.2, 148.5, 132.7 mAh g− 1 at 0.2 °C, 0.5 °C, 1 °C and 3 °C rate respectively. The sample also showed an excellent capacity retention as there was no significant capacity fade after 10 cycles. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In 1997, Padhi et al. reported that LiFePO4 could be used as a cathode material in lithium rechargeable batteries [1]. LiFePO4 has olivine-type phase and its theoretical capacity is 170 mAh g− 1. This material has many advantages compared with conventional cathode materials such as LiCoO2, LiNiO2 and LiMn2O4, namely, it is thermally stable in the fully charged state, environmentally benign and inexpensive [2–7]. It is considered as a promising cathode material for lithium ion battery, especially for large cell applications such as electric vehicles. However, LiFePO4 has a very low electronic conductivity. As a consequence, the rate capability and the cycle performance of this material would be lowered [8–12]. Therefore, many efforts have been devoted to increase and optimize the conductivity of LiFePO4. Currently, there are a lot of different methods, such as solid-state reaction [13], co-precipitation [14] and microwave heating [15], developed to synthesize LiFePO4. The mechanism of solid–liquid reaction milling is to generate product by mechanical forces [16]. During milling processing, the newly formed reaction product on the surface of the solid material peels off and breaks into fine particles during the continuous collisions among the balls and between the balls with the wall of the cylinder. The fresh surface when the solid peeled off helps to accelerate the solid–liquid reaction rate to form new reaction product layer. These cycles will not stop until the solid material is consumed. Finally, nanometer-sized particles can be prepared via the solid–liquid reaction milling. Ding Chen et al. [17] show that intermetallic powders of Fe–Sn could be obtained by solid–liquid reaction milling at 1033 K, with the particle size of 50–100 nm. ⁎ Corresponding author. Tel.: +86 731 8830476; fax: +86 731 8830477. E-mail address: [email protected] (Z. Zhao). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.09.019
It can be expected that the reaction between the solid material and the liquor may be induced during milling under the mechanochemical effect. In this research, LiFePO4/C was synthesized by solid–liquid reaction milling, which was different from the conventional solid– solid milling [18] to synthesize LiFePO4, for low-cost FeCl3·6H2O is used as iron source, and glucose is used as conductive additive and reducing agent in this paper. 2. Experimental The process of synthesizing LiFePO4/C by solid–liquid reaction milling was as follows: (1) FeCl3·6H2O (AR) was dissolved in ethanol, then Li2CO3 (AR), (NH4)2HPO4 (AR) and glucose (11 wt.% to FeCl3·6H2O), which are insoluble in ethanol, were added to the solution in a stoichiometric ratio of Li:Fe:P = 1.05:1:1; (2) The solid– liquid mixture was grinded for 30 min to form the precursor, then dried in oven at 70 °C in the open air; (3) the precursor was then heated in a tubular furnace at 600–720 °C under N2 atmosphere for several hours, and then cooled slowly to the ambient. Some toxic components, such as gaseous HCl in the exhaust gas, were absorbed by limes via the gas absorption system. The phases in the product were characterized by X-ray diffraction analysis (XRD, Rint-2000, Rigaku) with the Cu Kα radiation. The microstructure of the product was observed by scanning electron microscopy. Thermal analysis of the precursor was measured on SDT Q600 TG-DTA at the temperature between 10 and 900 °C with 10 °C/ min heating rate under argon flow. Carbon content left in the product was measured by infrared carbon–sulfur analyzer. Cl− content left in the sample was measured by inductive coupled plasma emission spectrometer (ICP). To make electrode, a mixture of 80 wt.% sample, 10 wt.% carbon black and 10 wt.% of polyvinylidene fluoride (PVDF) were mixed
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X. Liu, Z. Zhao / Powder Technology 197 (2010) 309–313
together in N-methyl-2 pyrolidone. The mixture, which is slurry, was then deposited uniformly onto a thin Al foil. After heating the filmed Al foil for 12 h at 120 °C under vacuum, the dried filmed Al foil, which is known as cathode, then was cut into disks with a diameter of 1.3 cm for assembling CR2025 coin-type cell. The electrolyte was 1 mol·L− 1 LiPF6 dissolved in ethyl carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) in a molar ratio of 1:1:1. The Li foil was used as an anode and the Celgard 2400 as the separator. CR2025 coin-type cell was then measured by Land-CT2001A (Land, Wuhan). The results showed that the charge–discharge limit was of 2.3–4.2 V. Electrochemical impedance spectroscopy (EIS) and cyclic voltammograms (CV) performance of the cell were measured and recorded via CHI660A (Chenghua, Shanghai). The amplitude of AC signal was 5 mV over the frequency range between 100 kHz and 0.1 Hz, and the scan rate of CV was 0.1 mVs− 1 in the voltage range of 2.5–4.5 V.
Fig. 2. TG-DTA profiles for precursor.
3. Results and discussion FeCl3·6H2O can be dissolved in ethanol, but Li2CO3, (NH4)2HPO4 and glucose are insoluble in ethanol. The direct result of solid–liquid milling is the reduction of solid particle size. Meantime, the mechanical forces may result in the deformation and fracture of the particles, such as translocation of lattice defects. The change in the structure and in the chemical reactivity of solid particles will happen due to deformation and fracture. The mechanochemical process increases the free energy of the system and enhances the reactivity and activity of the raw material, thereby, the reactions between Fe3+ and Cl− in the solution and solid materials of Li2CO3 and (NH4)2HPO4 would be induced under the mechanochemical effect over the period of solid–liquid reaction milling process. The formed product at the solid–liquid interface is peeled off during the continuous collisions of balls, and the reaction will go on until the raw materials run out. Meanwhile, homogenous precursor was also achieved during the milling. Fig. 1 shows the X-ray diffraction pattern of the precursor. Diffraction peaks of NH4H2PO4, NH4Cl and Li3PO4 can be observed in Fig. 1, but there is no obvious diffraction peak of ferric compound, which means the ferric compound is amorphous. The TG-DTA plots for precursor by solid–liquid reaction milling are shown in Fig. 2. In the range of 10–122.4 °C, an endothermic peak was observed, which result from the volatilization of moisture of the precursor. As the temperature increases to 250 °C, a strong endothermic peak and weight loss of 20% appears in the TG-DTA plots. It seems that NH4H2PO4 is decomposed. Another strong endothermic peak appears at 316.4 °C which indicated the decomposition of NH4Cl. Along with the rise of the temperature, the glucose was carbonized and Fe3+ was reduced to Fe2+. LiFePO4 was synthesized in this stage. Fig. 3 shows the X-ray diffraction pattern of LiFePO4/C after being heated in N2 at 680 °C for 8 h. XRD result demonstrates that all diffraction peaks can be attributed to the orthorhombic olivine phase LiFePO4 (space group: Pnma). Amorphous carbon was generated by the pyrolysis of glucose,
Fig. 1. XRD pattern of the precursor.
Fig. 3. XRD pattern of LiFePO4/C synthesized at 680 °C, 8 h.
as XRD shows no obvious diffraction peak of it. As it is known, Cl− is a contaminant for Li-ion batteries. The result measured by ICP shows that the content of Cl− in LiFePO4/C is no more than 2 wt ppm after the heating process. The calculated content of carbon is 3 wt.%, however, the leftover carbon content is about 2.86 wt.% under the measurement of infrared carbon–sulfur analyzer. This is because a few amount of carbon was lost during the heating process. Fig. 4 shows the X-ray diffraction patterns of LiFePO4/C synthesized at 600, 650, 680 and 720 °C for 8 h. From the figure, LiFePO4 was successfully synthesized at 600 °C, but some peaks of minor impurities can still be observed as the arrows show. XRD result of the sample synthesized at 650 °C demonstrates that all diffraction peaks are attributed to the orthorhombic olivine LiFePO4. The intensity of diffraction peaks gradually increases along with the increase
Fig. 4. XRD pattern of LiFePO4/C synthesized at different temperatures.
X. Liu, Z. Zhao / Powder Technology 197 (2010) 309–313
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Fig. 5. SEM photographs of LiFePO4/C synthesized at different temperatures A—600 °C; B—650 °C; C—680 °C; D—720 °C.
of temperature, which indicates that the temperature plays an important role in improving the crystallization. The contents of carbon in LiFePO4/C synthesized at 600, 650, 680 and 720 °C for 8 h are 2.91 wt.%, 2.88 wt.%, 2.86 wt.% and 2.82 wt.%, respectively. The results show that the loss of carbon slowly increases along with the increase of temperature. Fig. 5 shows the SEM micrographs of the LiFePO4/C synthesized at 600, 650, 680 and 720 °C for 8 h. The images show that the particle size of LiFePO4 prepared at 600, 650 and 680 °C is 2– 4 μm and the carbon coated on the particle of LiFePO4 is easily observed. However, the particle size synthesized at 720 °C significantly increases to around 10 μm, which is impeditive to the lithium ion diffusion. The charge/discharge profiles of the LiFePO4/C synthesized at different temperatures for 8 h between 2.3 and 4.2 V at 0.2°C rate are shown in
Fig. 6. Charge–discharge curves of LiFePO4/C synthesized at different temperatures at 0.2 °C rate.
Fig. 6. The discharge voltage plateau is between 3.3 and 3.4 V for samples synthesized at different temperatures. The cell of sample prepared at 600 °C delivers a discharge capacity of 136.1 mAh g− 1. Among all the LiFePO4/C samples, the one prepared at 680 °C exhibits the highest discharge capacity of 155.8 mAh g− 1, which is about 91.6% of the theoretical capacity of LiFePO4. The sample prepared at 720 °C shows discharge capacity of only 149.5 mAh g− 1. It is likely that the growth of particles restricts the intercalation/deintercalation of lithium ion. To compare the effect of temperature on the conductivity of LiFePO4/C, AC impedance analysis was introduced. In Fig. 7, the AC impedance spectra were a combination of a semicircle in high frequencies and a straight line in low frequencies. Fig. 7 shows that the shapes of the impedance spectra nearly overlap with each other, which indicate that the global
Fig. 7. AC impedance spectroscopies of LiFePO4/C synthesized at different temperatures.
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Fig. 10. The cyclic voltammograms curves of LiFePO4/C at different cycles. Fig. 8. Charge–discharge curves of LiFePO4/C synthesized under 680 °C at different rates.
4. Conclusion conductivities of LiFePO4/C synthesized at different temperatures are nearly equivalent. Fig. 8 shows the charge/discharge profiles, between 2.3 and 4.2 V, of LiFePO4/C synthesized at 680 °C for 8 h at different rates. It is clear that the voltage of discharge plateau increases significantly along with the increasing of the rate. With rate increasing from 0.5 °C to 1 °C and to 3 °C, the discharge capacities were of 153.2 mAh g− 1, 148.5 mAh g− 1, 132.7 mAh g− 1 respectively. This is because the utilization of the active material decreases when the rate increases. Fig. 9 shows the cyclic performance of LiFePO4/C synthesized at 680 °C for 8 h at different rates. We can see that the LiFePO4/C exhibits the discharge capacity of 154.7 mAh g− 1, 152.4 mAh g− 1, 147.6 mAh g− 1 and 132.7 mAh g− 1 under respectively 0.2 °C, 0.5 °C, 1 °C and 3 °C rate at the 10th cycle. This sample shows excellent capacity retention and was examined by cyclic voltammetry. The cell was tested between 2.5 V and 4.5 V at a scanning rate of 0.1 mVs− 1, and the result of the 1st and 10th cycle is shown in Fig. 10. The CV curves show that the anodic and cathodic peaks appear at 3.59 and 3.31 V, which correspond to the charge–discharge reaction of the Fe2+/Fe3+ redox couple. The CV curve of the 1st cycle is nearly superimposed to the curve of the 10th cycle, which demonstrates that the LiFePO4/C has an excellent cyclic performance.
Fig. 9. Cycle performance of LiFePO4/C synthesized under 680 °C at different rates.
LiFePO4/C was synthesized by the method of solid–liquid reaction milling, using FeCl3·6H2O, Li2CO3 and (NH4)2HPO4 and glucose. The XRD results indicated that pure LiFePO4/C can be synthesized above 650 °C. SEM revealed that the LiFePO4/C prepared at 680 °C was with particles of the size of around 2–4 μm and exhibited good electrochemical performance. The initial discharge capacities of the sample were around 155.8, 153.2, 148.5, 132.7 mAh g− 1 at 0.2 °C, 0.5 °C, 1 °C and 3 °C rate, respectively. It also showed an excellent capacity retention of the material, as no obvious capacity fade after 10 cycles is observed. Meanwhile, all the raw materials are inexpensive and with good operability. Therefore, it is feasible to achieve industrial production of LiFePO4/C for Li-ion batteries. Acknowledgement The work was supported by the National Key Basic Research Program of China (973 Program, 2007CB613603) and the New-Century Training Programme Foundation for the Talents by the State Education Commission (NCET-05-0692). References [1] A.K Padhi, K.S Nanjundaswamy, J.B. Goodenough, Phospho-olivines as positiveelectrode materials for rechargeable lithium batteries, Journal of the Electrochemical Society 144 (1997) 1188–1194. [2] C. Masquelier, A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, New cathode materials for rechargeable lithium batteries: the 3-D framework structures Li3Fe2 (XO4)3 (X = P, As), Journal of Solid State Chemistry 135 (1998) 228–234. [3] J. Sugiyama, T. Noritake, T. Hioki, et al., A new variety of LiMnO2: high-pressure synthesis and magnetic properties of tetragonal and cubic phases of LixMn1 − xO (x ~0.5), Materials Science and Engineering B84 (2001) 224–232. [4] S. Scaccia, M. Carewska, P. Wisniewski, et al., Morphological investigation of submicron FePO4 and LiFePO4 particles for rechargeable lithium batteries, Materials Research Bulletin 38 (2003) 1155–1156. [5] K.S. Park, J.T. Son, H.T. Chung, S.J. Kim, C.H. Lee, K.T. Kang, H.G. Kim, Surface modification by silver coating for improving electrochemical properties of LiFePO4, Solid State Communications 129 (2004) 311–314. [6] Kaoru Dokko, Shohei Koizumi, Keisuke Sharaishi, Kiyoshi Kanamura, Electrochemical properties of LiFePO4 prepared via hydrothermal route, Journal of Power Sources 165 (2007) 656–659. [7] S.F. Yang, Y.N. Song, P.Y. Zavalij, M.S. Whittingham, Hydrothermal synthesis of lithium iron phosphate cathodes, Electrochemistry Communications 4 (2001) 505–508. [8] J.F. Ni, H.H. Zhou, J.T. Chen, X.X. Zhang, LiFePO4 doped with ions prepared by coprecipitation method, Materials Letters 59 (2005) 2361. [9] G.X. Wang, L. Yang, Y. Chen, J.Z. Wang, Bewlay Steve, H.K. Liu, An investigation of polypyrrole-LiFePO4 composite cathode materials for lithium-ion batteries, Electrochimica Acta 50 (2005) 4649–4654. [10] Yike Chen, Progress in research of cathode material LiFePO4 in Li-ion batteries, Chinese Journal of Power Sources 27 (2003) 487–490. [11] G. Arnold, J. Garche, R. Hemmer, S. Ströbele, C. Vogler, M. Wohlfahrt-Mehrens, Fineparticle lithium iron phosphate LiFePO4 synthesized by a new low-cost aqueous precipitation technique, Journal of Power Sources 119–121 (2003) 247–251.
X. Liu, Z. Zhao / Powder Technology 197 (2010) 309–313 [12] Jun-biao Lu, Zhong-tai Zhang, Zi-long Tang, Zi-shan Zheng, A novel anode material for lithium ion batteries—LiFePO4, Rare Metal Materials and Engineering 33 (2004) 679–683. [13] Sylvain Franger, Frederic Le Cras, Carole Bourbon, Helene Rouault, Comparison between different LiFePO4 synthesis routes and their influence on its physicochemical properties, Journal of Power Sources 119–121 (2003) 252–257. [14] Xiang-qian Shen, Yun Zhan, Jian-xin Zhou, Mao-xiang Jing, Preparation of LiFePO4 by coprecipitation–calcination process, Journal of Functional Materials 8 (2006) 1198–1203. [15] Lei Wang, Yudai Huang, Rongrong Jiang, Dianzeng Jia, Preparation and characterization of nano-sized LiFePO4 by low heating solid-state coordination method and microwave heating, Electrochimica Acta 24 (2007) 6778–6783.
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[16] Ding Chen, Zhen-hua Chen, Gang Chen, Pei-yun Huang, Fabrication of TiAl, NiAl & FeAl intermetallic compounds powder by solid–liquid reaction milling, Journal of Hunan University 2 (2004) 20–24. [17] Ding Chen, Zhen-hua Chen, Gang Chen, Pei-yun Huang, Fabrication of Fe–Sn intermetallic compound powders by solid–liquid reaction milling, The Chinese Journal of Nonferrous Metals 3 (2003) 579–583. [18] Cheol Woo Kim, Moon Hee Lee, Woon Tae Jeong, Kyung Sub Lee, Synthesis of olivine LiFePO4 cathode materials by mechanical alloying using iron(III) raw material, Journal of Power Sources 146 (2005) 534–538.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Short Communication
Synthesis of LiFePO4/C by solid–liquid reaction milling method Xu-heng Liu, Zhong-wei Zhao ⁎ School of Metallurgical Science and Engineering, Central South University, ChangSha, 410083, China Key Laboratory of Hunan Province for Metallurgy and Material Processing of Rare Metals, ChangSha, 410083, China
a r t i c l e
i n f o
Article history: Received 18 April 2009 Received in revised form 14 September 2009 Accepted 27 September 2009 Available online 3 October 2009 Keywords: Li-ion battery LiFePO4/C Solid–liquid reaction milling
a b s t r a c t LiFePO4/C was synthesized by the method of solid–liquid reaction milling, using FeCl3·6H2O, Li2CO3 and (NH4)2HPO4 and glucose, which was used as reductant (carbon source). The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), TG-DTA analysis, infrared absorption carbon– sulfur analysis and electrochemical performance test. The sample synthesized at 680 °C for 8 h showed, at initial discharge, a capacity of 155.8, 153.2, 148.5, 132.7 mAh g− 1 at 0.2 °C, 0.5 °C, 1 °C and 3 °C rate respectively. The sample also showed an excellent capacity retention as there was no significant capacity fade after 10 cycles. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In 1997, Padhi et al. reported that LiFePO4 could be used as a cathode material in lithium rechargeable batteries [1]. LiFePO4 has olivine-type phase and its theoretical capacity is 170 mAh g− 1. This material has many advantages compared with conventional cathode materials such as LiCoO2, LiNiO2 and LiMn2O4, namely, it is thermally stable in the fully charged state, environmentally benign and inexpensive [2–7]. It is considered as a promising cathode material for lithium ion battery, especially for large cell applications such as electric vehicles. However, LiFePO4 has a very low electronic conductivity. As a consequence, the rate capability and the cycle performance of this material would be lowered [8–12]. Therefore, many efforts have been devoted to increase and optimize the conductivity of LiFePO4. Currently, there are a lot of different methods, such as solid-state reaction [13], co-precipitation [14] and microwave heating [15], developed to synthesize LiFePO4. The mechanism of solid–liquid reaction milling is to generate product by mechanical forces [16]. During milling processing, the newly formed reaction product on the surface of the solid material peels off and breaks into fine particles during the continuous collisions among the balls and between the balls with the wall of the cylinder. The fresh surface when the solid peeled off helps to accelerate the solid–liquid reaction rate to form new reaction product layer. These cycles will not stop until the solid material is consumed. Finally, nanometer-sized particles can be prepared via the solid–liquid reaction milling. Ding Chen et al. [17] show that intermetallic powders of Fe–Sn could be obtained by solid–liquid reaction milling at 1033 K, with the particle size of 50–100 nm. ⁎ Corresponding author. Tel.: +86 731 8830476; fax: +86 731 8830477. E-mail address: [email protected] (Z. Zhao). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.09.019
It can be expected that the reaction between the solid material and the liquor may be induced during milling under the mechanochemical effect. In this research, LiFePO4/C was synthesized by solid–liquid reaction milling, which was different from the conventional solid– solid milling [18] to synthesize LiFePO4, for low-cost FeCl3·6H2O is used as iron source, and glucose is used as conductive additive and reducing agent in this paper. 2. Experimental The process of synthesizing LiFePO4/C by solid–liquid reaction milling was as follows: (1) FeCl3·6H2O (AR) was dissolved in ethanol, then Li2CO3 (AR), (NH4)2HPO4 (AR) and glucose (11 wt.% to FeCl3·6H2O), which are insoluble in ethanol, were added to the solution in a stoichiometric ratio of Li:Fe:P = 1.05:1:1; (2) The solid– liquid mixture was grinded for 30 min to form the precursor, then dried in oven at 70 °C in the open air; (3) the precursor was then heated in a tubular furnace at 600–720 °C under N2 atmosphere for several hours, and then cooled slowly to the ambient. Some toxic components, such as gaseous HCl in the exhaust gas, were absorbed by limes via the gas absorption system. The phases in the product were characterized by X-ray diffraction analysis (XRD, Rint-2000, Rigaku) with the Cu Kα radiation. The microstructure of the product was observed by scanning electron microscopy. Thermal analysis of the precursor was measured on SDT Q600 TG-DTA at the temperature between 10 and 900 °C with 10 °C/ min heating rate under argon flow. Carbon content left in the product was measured by infrared carbon–sulfur analyzer. Cl− content left in the sample was measured by inductive coupled plasma emission spectrometer (ICP). To make electrode, a mixture of 80 wt.% sample, 10 wt.% carbon black and 10 wt.% of polyvinylidene fluoride (PVDF) were mixed
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X. Liu, Z. Zhao / Powder Technology 197 (2010) 309–313
together in N-methyl-2 pyrolidone. The mixture, which is slurry, was then deposited uniformly onto a thin Al foil. After heating the filmed Al foil for 12 h at 120 °C under vacuum, the dried filmed Al foil, which is known as cathode, then was cut into disks with a diameter of 1.3 cm for assembling CR2025 coin-type cell. The electrolyte was 1 mol·L− 1 LiPF6 dissolved in ethyl carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) in a molar ratio of 1:1:1. The Li foil was used as an anode and the Celgard 2400 as the separator. CR2025 coin-type cell was then measured by Land-CT2001A (Land, Wuhan). The results showed that the charge–discharge limit was of 2.3–4.2 V. Electrochemical impedance spectroscopy (EIS) and cyclic voltammograms (CV) performance of the cell were measured and recorded via CHI660A (Chenghua, Shanghai). The amplitude of AC signal was 5 mV over the frequency range between 100 kHz and 0.1 Hz, and the scan rate of CV was 0.1 mVs− 1 in the voltage range of 2.5–4.5 V.
Fig. 2. TG-DTA profiles for precursor.
3. Results and discussion FeCl3·6H2O can be dissolved in ethanol, but Li2CO3, (NH4)2HPO4 and glucose are insoluble in ethanol. The direct result of solid–liquid milling is the reduction of solid particle size. Meantime, the mechanical forces may result in the deformation and fracture of the particles, such as translocation of lattice defects. The change in the structure and in the chemical reactivity of solid particles will happen due to deformation and fracture. The mechanochemical process increases the free energy of the system and enhances the reactivity and activity of the raw material, thereby, the reactions between Fe3+ and Cl− in the solution and solid materials of Li2CO3 and (NH4)2HPO4 would be induced under the mechanochemical effect over the period of solid–liquid reaction milling process. The formed product at the solid–liquid interface is peeled off during the continuous collisions of balls, and the reaction will go on until the raw materials run out. Meanwhile, homogenous precursor was also achieved during the milling. Fig. 1 shows the X-ray diffraction pattern of the precursor. Diffraction peaks of NH4H2PO4, NH4Cl and Li3PO4 can be observed in Fig. 1, but there is no obvious diffraction peak of ferric compound, which means the ferric compound is amorphous. The TG-DTA plots for precursor by solid–liquid reaction milling are shown in Fig. 2. In the range of 10–122.4 °C, an endothermic peak was observed, which result from the volatilization of moisture of the precursor. As the temperature increases to 250 °C, a strong endothermic peak and weight loss of 20% appears in the TG-DTA plots. It seems that NH4H2PO4 is decomposed. Another strong endothermic peak appears at 316.4 °C which indicated the decomposition of NH4Cl. Along with the rise of the temperature, the glucose was carbonized and Fe3+ was reduced to Fe2+. LiFePO4 was synthesized in this stage. Fig. 3 shows the X-ray diffraction pattern of LiFePO4/C after being heated in N2 at 680 °C for 8 h. XRD result demonstrates that all diffraction peaks can be attributed to the orthorhombic olivine phase LiFePO4 (space group: Pnma). Amorphous carbon was generated by the pyrolysis of glucose,
Fig. 1. XRD pattern of the precursor.
Fig. 3. XRD pattern of LiFePO4/C synthesized at 680 °C, 8 h.
as XRD shows no obvious diffraction peak of it. As it is known, Cl− is a contaminant for Li-ion batteries. The result measured by ICP shows that the content of Cl− in LiFePO4/C is no more than 2 wt ppm after the heating process. The calculated content of carbon is 3 wt.%, however, the leftover carbon content is about 2.86 wt.% under the measurement of infrared carbon–sulfur analyzer. This is because a few amount of carbon was lost during the heating process. Fig. 4 shows the X-ray diffraction patterns of LiFePO4/C synthesized at 600, 650, 680 and 720 °C for 8 h. From the figure, LiFePO4 was successfully synthesized at 600 °C, but some peaks of minor impurities can still be observed as the arrows show. XRD result of the sample synthesized at 650 °C demonstrates that all diffraction peaks are attributed to the orthorhombic olivine LiFePO4. The intensity of diffraction peaks gradually increases along with the increase
Fig. 4. XRD pattern of LiFePO4/C synthesized at different temperatures.
X. Liu, Z. Zhao / Powder Technology 197 (2010) 309–313
311
Fig. 5. SEM photographs of LiFePO4/C synthesized at different temperatures A—600 °C; B—650 °C; C—680 °C; D—720 °C.
of temperature, which indicates that the temperature plays an important role in improving the crystallization. The contents of carbon in LiFePO4/C synthesized at 600, 650, 680 and 720 °C for 8 h are 2.91 wt.%, 2.88 wt.%, 2.86 wt.% and 2.82 wt.%, respectively. The results show that the loss of carbon slowly increases along with the increase of temperature. Fig. 5 shows the SEM micrographs of the LiFePO4/C synthesized at 600, 650, 680 and 720 °C for 8 h. The images show that the particle size of LiFePO4 prepared at 600, 650 and 680 °C is 2– 4 μm and the carbon coated on the particle of LiFePO4 is easily observed. However, the particle size synthesized at 720 °C significantly increases to around 10 μm, which is impeditive to the lithium ion diffusion. The charge/discharge profiles of the LiFePO4/C synthesized at different temperatures for 8 h between 2.3 and 4.2 V at 0.2°C rate are shown in
Fig. 6. Charge–discharge curves of LiFePO4/C synthesized at different temperatures at 0.2 °C rate.
Fig. 6. The discharge voltage plateau is between 3.3 and 3.4 V for samples synthesized at different temperatures. The cell of sample prepared at 600 °C delivers a discharge capacity of 136.1 mAh g− 1. Among all the LiFePO4/C samples, the one prepared at 680 °C exhibits the highest discharge capacity of 155.8 mAh g− 1, which is about 91.6% of the theoretical capacity of LiFePO4. The sample prepared at 720 °C shows discharge capacity of only 149.5 mAh g− 1. It is likely that the growth of particles restricts the intercalation/deintercalation of lithium ion. To compare the effect of temperature on the conductivity of LiFePO4/C, AC impedance analysis was introduced. In Fig. 7, the AC impedance spectra were a combination of a semicircle in high frequencies and a straight line in low frequencies. Fig. 7 shows that the shapes of the impedance spectra nearly overlap with each other, which indicate that the global
Fig. 7. AC impedance spectroscopies of LiFePO4/C synthesized at different temperatures.
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X. Liu, Z. Zhao / Powder Technology 197 (2010) 309–313
Fig. 10. The cyclic voltammograms curves of LiFePO4/C at different cycles. Fig. 8. Charge–discharge curves of LiFePO4/C synthesized under 680 °C at different rates.
4. Conclusion conductivities of LiFePO4/C synthesized at different temperatures are nearly equivalent. Fig. 8 shows the charge/discharge profiles, between 2.3 and 4.2 V, of LiFePO4/C synthesized at 680 °C for 8 h at different rates. It is clear that the voltage of discharge plateau increases significantly along with the increasing of the rate. With rate increasing from 0.5 °C to 1 °C and to 3 °C, the discharge capacities were of 153.2 mAh g− 1, 148.5 mAh g− 1, 132.7 mAh g− 1 respectively. This is because the utilization of the active material decreases when the rate increases. Fig. 9 shows the cyclic performance of LiFePO4/C synthesized at 680 °C for 8 h at different rates. We can see that the LiFePO4/C exhibits the discharge capacity of 154.7 mAh g− 1, 152.4 mAh g− 1, 147.6 mAh g− 1 and 132.7 mAh g− 1 under respectively 0.2 °C, 0.5 °C, 1 °C and 3 °C rate at the 10th cycle. This sample shows excellent capacity retention and was examined by cyclic voltammetry. The cell was tested between 2.5 V and 4.5 V at a scanning rate of 0.1 mVs− 1, and the result of the 1st and 10th cycle is shown in Fig. 10. The CV curves show that the anodic and cathodic peaks appear at 3.59 and 3.31 V, which correspond to the charge–discharge reaction of the Fe2+/Fe3+ redox couple. The CV curve of the 1st cycle is nearly superimposed to the curve of the 10th cycle, which demonstrates that the LiFePO4/C has an excellent cyclic performance.
Fig. 9. Cycle performance of LiFePO4/C synthesized under 680 °C at different rates.
LiFePO4/C was synthesized by the method of solid–liquid reaction milling, using FeCl3·6H2O, Li2CO3 and (NH4)2HPO4 and glucose. The XRD results indicated that pure LiFePO4/C can be synthesized above 650 °C. SEM revealed that the LiFePO4/C prepared at 680 °C was with particles of the size of around 2–4 μm and exhibited good electrochemical performance. The initial discharge capacities of the sample were around 155.8, 153.2, 148.5, 132.7 mAh g− 1 at 0.2 °C, 0.5 °C, 1 °C and 3 °C rate, respectively. It also showed an excellent capacity retention of the material, as no obvious capacity fade after 10 cycles is observed. Meanwhile, all the raw materials are inexpensive and with good operability. Therefore, it is feasible to achieve industrial production of LiFePO4/C for Li-ion batteries. Acknowledgement The work was supported by the National Key Basic Research Program of China (973 Program, 2007CB613603) and the New-Century Training Programme Foundation for the Talents by the State Education Commission (NCET-05-0692). References [1] A.K Padhi, K.S Nanjundaswamy, J.B. Goodenough, Phospho-olivines as positiveelectrode materials for rechargeable lithium batteries, Journal of the Electrochemical Society 144 (1997) 1188–1194. [2] C. Masquelier, A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, New cathode materials for rechargeable lithium batteries: the 3-D framework structures Li3Fe2 (XO4)3 (X = P, As), Journal of Solid State Chemistry 135 (1998) 228–234. [3] J. Sugiyama, T. Noritake, T. Hioki, et al., A new variety of LiMnO2: high-pressure synthesis and magnetic properties of tetragonal and cubic phases of LixMn1 − xO (x ~0.5), Materials Science and Engineering B84 (2001) 224–232. [4] S. Scaccia, M. Carewska, P. Wisniewski, et al., Morphological investigation of submicron FePO4 and LiFePO4 particles for rechargeable lithium batteries, Materials Research Bulletin 38 (2003) 1155–1156. [5] K.S. Park, J.T. Son, H.T. Chung, S.J. Kim, C.H. Lee, K.T. Kang, H.G. Kim, Surface modification by silver coating for improving electrochemical properties of LiFePO4, Solid State Communications 129 (2004) 311–314. [6] Kaoru Dokko, Shohei Koizumi, Keisuke Sharaishi, Kiyoshi Kanamura, Electrochemical properties of LiFePO4 prepared via hydrothermal route, Journal of Power Sources 165 (2007) 656–659. [7] S.F. Yang, Y.N. Song, P.Y. Zavalij, M.S. Whittingham, Hydrothermal synthesis of lithium iron phosphate cathodes, Electrochemistry Communications 4 (2001) 505–508. [8] J.F. Ni, H.H. Zhou, J.T. Chen, X.X. Zhang, LiFePO4 doped with ions prepared by coprecipitation method, Materials Letters 59 (2005) 2361. [9] G.X. Wang, L. Yang, Y. Chen, J.Z. Wang, Bewlay Steve, H.K. Liu, An investigation of polypyrrole-LiFePO4 composite cathode materials for lithium-ion batteries, Electrochimica Acta 50 (2005) 4649–4654. [10] Yike Chen, Progress in research of cathode material LiFePO4 in Li-ion batteries, Chinese Journal of Power Sources 27 (2003) 487–490. [11] G. Arnold, J. Garche, R. Hemmer, S. Ströbele, C. Vogler, M. Wohlfahrt-Mehrens, Fineparticle lithium iron phosphate LiFePO4 synthesized by a new low-cost aqueous precipitation technique, Journal of Power Sources 119–121 (2003) 247–251.
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