- Email: [email protected]
Kinetics of synthesis olivine LiFePO4 by using a precipitated-sintering method
Journal of Alloys and Compounds 467 (2009) 390–396
Kinetics of synthesis olivine LiFePO4 by using a precipitated-sintering method Peixin Zhang a,b,∗ , Xinyang Li a , Zhongkuan Luo a , Xiaoqian Huang a , Jianhong Liu a , Qiming Xu b , Xiangzhong Ren a , Xun Liang a b
a School of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, PR China School of Materials Science and Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, PR China
Received 20 September 2007; received in revised form 28 November 2007; accepted 2 December 2007 Available online 15 January 2008
Abstract The LiFePO4 precursor was synthesized using a precipitation with raw materials LiOH·H2 O, (NH4 )2 HPO4 and FeSO4 ·7H2 O. The kinetics of synthesis olivine LiFePO4 was studied by using a differential thermal analysis (DTA) at different heating rates. The average activation energy of the reaction where the precursor form olivine LiFePO4 was 239.39 kJ mol−1 , calculated by Doyle–Ozawa and Kissinger methods. The reaction order, frequency factor, rate equation and kinetic equation of the reaction were determined by the Kissinger method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Fourier transform infrared spectrometer (FTIR). The sample synthesized by the precursor sintered for 6 h at 550 ◦ C shows a single-phase, regular morphology, well-distributed particle sizes and good electrochemical properties. © 2007 Elsevier B.V. All rights reserved. Keywords: Lithium-ion battery; LiFePO4 ; Kinetics; Activation energy; Precipitation
1. Introduction At present, rechargeable lithium-ion batteries as a sort of high-powered green battery are used as key components of telecommunication equipments, digital electronic equipments and portable computer systems. Because of their advantages such as low self-resistance, little self-discharge, high-specific capacity, no memorizing effect, rechargeable lithium-ion batteries are good candidates for electric vehicles (EVs) and hybrid electric vehicles (HEVs). Being lower-cost, simple in manufacturing and environment friendly, the lithium-ion battery is widely used. The lithium-ion battery is influenced by several factors in its commercial process, especially by the cathode material. The cathode material has an important effect on environmental problems, factors of electrochemical capabilities and feasibility of direct mass production. Also it affects the price of the lithium-ion battery. Transition metal oxides, such as LiCoO2 ∗ Corresponding author at: School of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, PR China. Tel.: +86 755 26538134; fax: +86 755 26538134. E-mail address: [email protected] (P. Zhang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.12.005
[1–3], LiNiO2 [4–6] and spinel LiMnO2 [7–9] have been studied as cathode materials. In 1997, Padhi et al. [10] reported firstly that olivine-structure LiFePO4 could reversibly extract/insert lithium-ion. The orthophosphates Li–M–PO4 (M = Fe, Co, Mn, Ni) are intensively studied as cathodes for lithium-ion batteries, especially LiFePO4 has attracted particular interest. LiFePO4 is inexpensive, non-toxic, environmentally benign, good thermal stability and has a theoretical specific capacity of 170 mAh g−1 . LiFePO4 is a good candidate for cathode materials used in lithium-ion batteries. Most of researches focus on improvements of synthesizing methods and electrochemical capabilities [11–15]. Up to now, the dynamic study of synthesizing olivine LiFePO4 has not been reported in detail. In this research, the LiFePO4 precursor has been synthesized using precipitation with the raw materials of LiOH·H2 O, (NH4 )2 HPO4 and FeSO4 ·7H2 O. The kinetics of synthesis olivine LiFePO4 was studied by means of a differential thermal analysis (DTA) analysis with different heating rates. The activation energy, reaction order, frequency factor and kinetic equation are provided. The theoretical gist for LiFePO4 preparation by sintering is also discussed.
P. Zhang et al. / Journal of Alloys and Compounds 467 (2009) 390–396
391
2. Experimental 2.1. Synthesizing procedures The molar ratio of Fe3 (PO4 )2 and Li3 PO4 were synthesized by using a precipitation with the aqueous solutions of LiOH·H2 O, (NH4 )2 HPO4 and FeSO4 ·7H2 O under pH control. The precipitate was filtered off, washed thoroughly with de-ionized water, then mixed the Fe3 (PO4 )2 and Li3 PO4 in the de-ionized water. After high-speed stirring several hours with blowing nitrogen gas into the system through a tube, the precursor was obtained, then washed with de-ionized water and filtered off. The precursor dried in a vacuum oven at 60 ◦ C for 8 h. After being dried, the precursor powder was mixed with 2 wt.% high-surface area carbon black and pressed into a disk-shaped pellet at 10 MPa pressure. The pellet was annealed at a certain temperature for a certain time in nitrogen gas flow, then the olivine LiFePO4 was obtained.
2.2. DTA experiments The 5–10 mg precursor was put into an Al2 O3 crucible with an empty Al2 O3 crucible as reference. In order to overcome the disadvantages, the DTA experiments were carried out with different heating rates of 5, 10, 15, 20 K min−1 , respectively, under nitrogen with a gas-flow rate of 30 ml min−1 on Netzsch4 STA 409 PC analyzer. According to the measured DTA data, dynamics parameters can be calculated by Doyle–Ozawa and Kissinger methods.
2.3. Structure and morphological characterization XRD profiles of the samples were measured on a Bruker D8 Advance diffractometer (Cu K␣ radiation). The scan rate was 0.04◦ s−1 and step was 0.02◦ . The samples were observed by using a JSM-5900 scanning electron microscope (SEM). The FTIR were recorded at room temperature on a Shimadzu FTIR-8300PCS spectrometer using the KBr pellet technique.
2.4. Electrochemical characterization A mixture composed of 5 wt.% of electrical conductive carbon powder, 85 wt.% of lithium iron phosphate powder and 15 wt.% PTFE latex were dried in a vacuum oven for 5 h at 60 ◦ C. After being dried, the mixture was rolled into flakes and stuck to the stainless steel grid plates. Each strip was mounted as positive electrode versus counter with lithium metal as reference electrodes in an experimental cell. 1 mol L−1 LiPF6 EC/DMC was used as electrolyte and the polypropylene no. 2400 separators were used. Assemblage of the cells was carried out in an argon box. The charge/discharge test used a LAND series battery test system (LAND, Wuhan, China) with a specific current of C/10 at room temperature.
Fig. 1. The TG–DTA curves of LiFePO4 precursor at 10 K min−1 heating rate.
appeared in the system and the nucleation happened within this temperature range. The exothermic peak was emerging within temperature range of 550–700 ◦ C in the DTA curve with a steady mass according to the TG curve. The temperature range to the exothermic peak is the process of crystal phase transformation from amorphous precursor to olivine LiFePO4 . After crystal nucleus formed, with the rise of temperature, the crystal nucleus grew up, and then a large quantity of olivine LiFePO4 is thus synthesized with a great deal of heat dissipation. The synthesizing process of the olivine LiFePO4 is a simplex crystal phase transformation process after crystal water decomposition. The experiments were carried out under nitrogen with a gasflow rate of 30 ml min−1 and different heating rates of 5, 10, 15 and 20 K min−1 . The peak temperature, enthalpy variation and peak areas of endothermic peak and exothermic peak could be explored under different heating rates. The DTA curves of lithium iron phosphate precursor were given in Fig. 2. According to the measurement zero line, it could be seen from Fig. 2 that there was an exothermic peak in each DTA curve within the temperature range of 550–700 ◦ C which was the range of crystal
3. Results and discussion 3.1. Non-isothermal kinetics The thermogravimetric and differential thermal analyses (TG–DTA) curves of lithium iron phosphate precursor at the heating rate of 10 K min−1 were given in Fig. 1. According to the measurement zero line of the DTA curve, there were two peaks including an endothermic peak and an exothermic peak. The endothermic peak appeared within the range of 80–350 ◦ C in the DTA curve with a mass decrease dramatically according to the TG curve. This mass decreasing trend was getting slower and slower when temperature went up. The temperature range of the endothermic peak is caused by the decomposition of the crystal water gradually. Within the temperature range of 450–550 ◦ C, there is a slight heat dissipation with a slow rate. As the temperature rose gradually, the very small crystallize grain
Fig. 2. DTA curves of LiFePO4 precursor at different heating rates: (a) 5 K min−1 ; (b) 10 K min−1 ; (c) 15 K min−1 ; (d) 20 K min−1 .
392
P. Zhang et al. / Journal of Alloys and Compounds 467 (2009) 390–396
phase transformation. With the heating rate rising up gradually, the exothermic peak was moving to a high temperature and the heat dissipation of nucleation became unsteady. When the heating rate was getting higher, the interior temperature gradient increased which made the deviate of thermal effect range and procedure temperature increased. Lower heating velocity makes the nucleation carry on slowly, and the system can immediately attain temperature balance within a very short time. So it can make crystal nucleus very small and grow slow, then the synthesized crystal particle size distribution becomes very narrow and the modality becomes easy to control. 3.1.1. Dynamics parameters In this paper, from the viewpoint of chemistry reaction dynamics, the variations of chemical reaction velocity respect to time and temperature were studied, and the dynamic equations were induced. Doyle–Ozawa and Kissinger methods were adopted to calculate the activation energy of the reaction of the precursor become olivine LiFePO4 . The three formulas listed below were basic principles: dα the mass action law : = k(1 − α)n dt E the Arrhenius formula : k = A exp − RT the heating rate formula :
β=
dT dt
(1) (2) (3)
Doyle–Ozawa method applied formula (4) that was deduced from above formulas to calculate the activation energy: log β=log
AE E −2.315 − 0.4567 , f (α) = (1 − α)n Rf (α) RT
(4)
When the fractional conversion α is certain, regardless reaction order, the function f(α) concerning fractional conversion α is a constant. For a certain reaction, the frequency factor A can be determined, and log[AE/Rf(α)] is also a constant. The plot of log β ∼ T−1 is a straight line and the slope is (−0.4567ER−1 ) [16–18]. With different heating rates β and the same fractional conversion α, we can plot the relation curve of the log β ∼ T−1 , the activation energy E of the precursor becoming olivine LiFePO4 can be calculated from the slope of the straight line. Using the Doyle–Ozawa method, the diagrams of log β ∼ T−1 for the exothermic peak was showed in Fig. 3. As shown in Fig. 3, every line expressed percent conversion of 10–100% from the right to the left. The slopes, correlation coefficients and activation energy of every line were showed in Table 1. The reaction activation energy was various for different percent conversion. In addition, the process could be divided into two stages: 70.36–159.63 kJ mol−1 and 193.11–211.01 kJ mol−1 . In the first stage, the activation energy obviously increases with the increment of fractional conversion, but on a lower level all the time and the reaction carried on more easily and rapidly. In the second stage, the activation energy leveled off, on the average of about 190 kJ mol−1 . The reaction carried on stably and slowly. In the whole process the average apparent activation energy was 160.07 kJ mol−1 .
Fig. 3. Curves of log β vs. T−1 of activation energy for the reaction using Doyle–Ozawa method (every line regressed by calculating point of symbol 1–10 expresses reaction degree of 10–100% from right to left in the figure).
In terms of the Kissinger principle, assuming that the reaction velocity was maximum at peak temperature and the reaction followed dynamics equations of Kissinger [19], according to mass action law (1), the Arrhenius formula (2) and heating rate formula (3), formula (5) can be deducted. dα A E (5) = exp − (1 − α)n dT β RT For T = Tmax (where Tmax is the maximum temperature of the peak), then d dα =0 (6) dT dt When formula (6) was substituted into formula (5), we get E An E n−1 (7) = ) exp − (1 − α max 2 RTmax β RTmax If n = 1, then formula (7) can be rewritten as E E A exp − = 2 RTmax β RTmax
(8)
Table 1 Activation energy and linear correlation coefficients of different reaction degrees at the exothermic peak by Doyle–Ozawa method α (%)
Slope
Correlation coefficient
E (kJ mol−1 )
10 20 30 40 50 60 70 80 90 100
−3.86468 −4.85644 −6.49505 −8.76892 −10.60804 −10.75755 −10.64314 −10.3428 −10.00133 −11.59081
−0.97181 −0.95321 −0.96500 −0.98577 −0.99726 −0.99916 −0.99749 −0.99053 −0.97153 −0.96017
70.36 88.41 118.24 159.63 193.11 195.84 193.75 188.29 182.07 211.01
Average E (kJ mol−1 )
160.07
P. Zhang et al. / Journal of Alloys and Compounds 467 (2009) 390–396
393
Generally, the activation energy of chemical reaction is 60–250 kJ mol−1 . If the activation energy is lower than 40 kJ mol−1 , the reaction velocity is very fast and difficult to measure. When this happens, the reaction then becomes difficult to control. If the activation energy is higher than 400 kJ mol−1 , the reaction velocity is very slow, then rigorous condition or activators are indispensable to expedite the reaction. The average activation energy of the reaction process where the precipitated precursor becomes olivine LiFePO4 was 239.39 kJ mol−1 , as suggested by Doyle–Ozawa and Kissinger methods. The precursor acquiring a lower energy can overcome energy barrier and become olivine LiFePO4 . This is the basic reason why the precursor synthesized by precipitation method can produce olivine LiFePO4 at a lower temperature for a shorter sintering time. −2 ) vs. T −1 of the peak at different heating rates by Fig. 4. Curves of ln(βTmax max Kissinger method.
For n = / 0 and n = / 1, due to (n − 1)(2RTmax /E) 1, then 2RTmax n−1 ≈ 1 + (n − 1) ≈1 (9) n(1 − αmax ) E When formula (9) was substituted into formula (7) and the same formula (8) could be inferred. In this paper, we use formula (8) to calculate the corresponding dynamic constants. When formula (8) was natural logarithmic transformed, we get β RA E ln = ln (10) − 2 Tmax E RTmax For a certain reaction, the frequency factor A is constant, and 2 ) ∼ (1/T ln(RA/E) is also a constant. The plot of ln(β/Tmax max ) −1 is a line and the slope is (−ER ). Using the Kissinger method, 2 ) ∼ (1/T the diagrams of ln(β/Tmax max ) for the exothermic peak was shown in Fig. 4. The slope is equal to (−ER−1 ) and the activation energy can be calculated out. The frequency factor A was obtained according to formula (8). In terms of the Kissinger principle, the shape index S is defined as the absolute value of the ratio of the slops of tangents to the curve at the inflexion points. Then we can use formula (11) to calculate the reaction order n [19]. n = 1.26S 0.5
(11)
The calculated corresponding dynamic constants were listed in Table 2.
3.1.2. Rate equation The reaction mechanism was assured after obtaining dynamics parameters in terms of unequal isothermal process. Within infinite time intervals, it could be considered that an unequal isotherm process was as an isothermal process. The reaction velocity could replace the general formula during the isotherm process. In accordance with formula (5), with the calculated corresponding dynamic constants, the rate equation of exothermic peak was 2.3939 × 105 dα 18 = 3.164 × 10 exp − (1 − α)0.9133 (12) dt RT 3.2. Structure and morphological characterization Fig. 5 shows the XRD patterns of LiFePO4 precursor and a series of samples synthesized by sintering at various synthesizing temperatures for 6 h in nitrogen gas. The precursor is amorphous. The profiles of the reflection peaks are quite low, wide and out-of-order according to Fig. 5(a). The precursor was sintered at different synthesizing temperatures for 6 h then the olivine LiFePO4 was synthesized. All diffraction lines can be attributed to the olivine type phase LiFePO4 and to a minor impurity phases in (c) and (d). Additionally, in the pattern (b), the profiles of the reflection peaks are quite narrow, symmetric and without impurity phases. Therefore, a single-phase and well-crystallized product is indicated by the precursor sintered at 550 ◦ C for 6 h. The reason why the impurity phases appeared in (c) and (d) samples is that a higher sintering temperature sped the crystal phase transformation process, so that the reaction velocity had a striking dissimilarity in the internal part of the system, and
Table 2 Kinetic parameters for different heating rates by Kissinger method β (K/min)
Tmax (K) S n A (×10−18 )
5
10
15
20
871.35 0.6859 1.0435 3.225
886.15 0.5344 0.9211 2.991
894.65 0.4416 0.8373 2.918
897.05 0.4565 0.8513 3.451
Average
Slope
Correlation coefficient
E (kJ/mol)
0.5294 0.9133 3.164
−38.33368
−0.99122
318.71
394
P. Zhang et al. / Journal of Alloys and Compounds 467 (2009) 390–396
Fig. 5. XRD patterns of LiFePO4 precursor and olivine LiFePO4 : (a)LiFePO4 precursor; (b) 550 ◦ C sintered 6 h (c) 600 ◦ C sintered 6 h; (d) 650 ◦ C sintered 6 h.
the crystal phase transformation reaction became miscellaneous. Shown in Fig. 6 are the SEM images of a series of samples obtained at different synthesizing temperatures for 6 h in nitrogen gas. The particle size of the LiFePO4 synthesized by precipitation is very small as shown in Fig. 6. Additionally, the sample obtained after sintering 6 h at 550 ◦ C consist of well-distributed, near-spherical shaped particles whose average particle size is about 200 nm, which is the smallest without agglomeration. With the exaltation of sintering temperature, the
distribution of particle sizes became more and more wide, and the agglomeration phenomenon was more and more obvious. The agglomeration and the wide distribution of particle size attributed to the higher sintering temperature which increased the speed of the crystal phase transformation. The higher transformation speed makes the crystal nucleus grow rapidly and the distribution of the particle size become wider due to the agglomeration happens. The smaller the particle size, the higher specific surface area is. A high-specific surface area is advantageous to internal lithium-ion diffusion, and the good electrochemical properties material can be obtained. From the view of chemistry reaction dynamics, because the activation energy of crystal phase transformation is a little larger than 200 kJ mol−1 , the precursor can become olivine LiFePO4 at 550 ◦ C in a short time. 550 ◦ C is the starting temperature of the crystal phase transformation. At this temperature, a low energy that the crystal phase synthesizing process dissipated could overcome the energy barrier and the precursor became olivine LiFePO4 . When the sintering temperature is low, the growth of crystal is slow. The regular surface texturing and welldistributed olivine LiFePO4 sample can be synthesized. The degree of crystallinity and the average particle diameter of LiFePO4 which was synthesized at 550 ◦ C for different sintering time are given in Fig. 7. When the sintering time is less than 6 h, the energy which the samples obtained could not overcome the energy barrier and could not produce the olivine LiFePO4 . Therefore the degree of crystallinity of the samples is lower. When the sintering time is more than 6 h, the agglomeration happened. Then the particle size is larger and the minor impurity phases were synthesized because of asymmetrical dis-
Fig. 6. SEM patterns of LiFePO4 synthesized at different sintering temperatures: (a) 550 ◦ C; (b) 600 ◦ C; (c) 650 ◦ C; (d) 700 ◦ C.
P. Zhang et al. / Journal of Alloys and Compounds 467 (2009) 390–396
395
Fig. 7. The degree of crystallinity and average particle diameter of LiFePO4 synthesized at different sintered time. Fig. 9. Initial charge/discharge profile of olivine LiFePO4 .
tribution of Li+ , Fe2+ and PO4 3− . And the degree of crystallinity of the sample is lower too. The sample synthesized at 550 ◦ C for 6 h has the highest degree of crystallinity and the smallest particle size. The energy that this sample obtained can overcome the energy barrier exactly. The Fourier transform infrared spectroscopy (FTIR) is known to be a good means for investigating of the structure at a local scale. The FTIR images of precursor and olivine LiFePO4 samples were shown in Fig. 8. Fig. 8(a) is Fourier transforms infrared spectroscopy of the precursor, the absorption of FTIR spectra originates from the intramolecular vibrations of the (PO4 )3− unit, which involve the displacement of oxygen atoms at frequencies closely related to those of the free molecule within 940–1120 cm−1 and 540–650 cm−1 ranges [20]. From Fig. 8, the FTIR spectra (b), (c) and (d) of olivine LiFePO4 samples are finer and narrower, compared with Fig. 8(a), which is within ranges of 940–1120 cm−1 and 540–650 cm−1 . In curve (d), compared with curves (b) and (c), there is an absorption gap extending from 650 to 950 cm−1 where located vibra-
Fig. 8. FTIR patterns of LiFePO4 precursor and olivine LiFePO4 : (a) LiFePO4 precursor; (b) 700 ◦ C sintered 6 h; (c) 550 ◦ C sintered 10 h; (d) 550 ◦ C sintered 6 h.
tions associated with other phosphate anions such as (P2 O7 )4− and (P3 O10 )5− [21]. The absence of any structure in this gap indicates that such complexes are not present in LiFePO4 sample obtained at 550 ◦ C sintered for 6 h. The samples such as shown in Fig. 8(b), which were synthesized at a temperature higher than 550 ◦ C for 6 h, contained impurity phases to a certain extent. The same is true of sample Fig. 8(c) which was synthesized at 550 ◦ C for more than 6 h. In Fig. 8(d), the absorption at 1138 cm−1 originates from the symmetric and anti-symmetric stretching vibrations of O–P–O. Because the discrepancy of the two vibrations is extremely small, there is only one peak at 1138 cm−1 . The absorption at 1061 and 1096 cm−1 originate from anti-symmetric stretching vibration of P–O. The absorption at 976 and 637 cm−1 originate from originate stretching vibration of P–O. The absorption at 579 cm−1 originate from originate anti-symmetric bending vibration of O–P–O. The absorption at 552 and 471 cm−1 originate from originate symmetric bending vibration of O–P–O. The absorption at 505 cm−1 originate from originate perturbation bending vibration of O–P–O [22].
Fig. 10. Cycling performance of olivine LiFePO4 sample.
396
P. Zhang et al. / Journal of Alloys and Compounds 467 (2009) 390–396
3.3. Electrochemical results
References
The single-phase olivine LiFePO4 was synthesized at 550 ◦ C by sintering the precursor for 6 h. We use this sample to assemble experimental cell according to the method discussed in Section 2.4. Fig. 9 depicts the initial voltage profiles of this sample at the rate of C/10. The initial capacity of this sample is 119.5 mAh g−1 at room temperature with the charge plateau at 3.5 V and the discharge plateau at 3.3 V. Fig. 10 shows the cycling performance of this olivine LiFePO4 sample. At the C/10 rate, the specific discharge capacity fading of this sample is lower than 2% after 8 cycles at room temperature.
[1] K. Mizushima, P.C. Jones, P.J. Wiseman, et al., Mater. Res. Bull. 15 (1980) 783–789. [2] M. Zou, M. Yoshio, S. Gopukumar, et al., Chem. Mater. 15 (2003) 4699–4702. [3] A. Clemencon, A.T. Appapillai, S. Kumar, et al., Electrochim. Acta 52 (2007) 4572–4580. [4] M.G.S.R. Thomas, W.I.F. David, J.B. Goodenough, et al., Mater. Res. Bull. 20 (1985) 1137–1146. [5] N. Kalaiselvi, A.V. Raajaraajan, B. Sivagaminathan, et al., Ionics 9 (2003) 382–387. [6] H. Gu, Y. Zhai, Y. Tian, et al., Chin. J. Mater. Res. 21 (2007) 97– 101. [7] T. Ohzuku, M. Kitagawa, T. Hirai, J. Electrochem. Soc. 137 (1990) 769–775. [8] S. Yao, S. Zhang, N. Xu, et al., Chin. J. Power Sources 27 (2003) 554– 557. [9] P. Suresh, A.K. Shukla, N. Munichandraiah, J. Power Sources 161 (2006) 1307–1313. [10] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188–1194. [11] F. Croce, A.D. Epifanio, J. Hassoun, et al., Electrochem. Solid-State Lett. 5 (2002) A47–A50. [12] K.S. Park, K.T. Kang, S.B. Lee, et al., Mater. Res. Bull. 39 (2004) 1803–1810. [13] P.P. Prosini, M. Carewska, S. Scaccia, et al., Electrochim. Acta 48 (2003) 4205–4211. [14] S. Franger, F. Le Cras, C. Bourbon, et al., J. Power Sources 119–121 (2003) 252–257. [15] Z. Xu, L. Xu, Q. Lai, et al., Mater. Chem. Phys. 105 (2007) 80–85. [16] M.I. Bopu, M.D. Youde, S. Wang, et al., Differential Thermal Analysis, Beijing Normal University Press, Beijing, 1981. [17] X. Shen, Differential Thermal Gravity Analysis and Non-isothermal Solid Reaction Dynamics, Metallurgical Industry Publishing House, Beijing, 1995. [18] Z. Cai, Thermal Analysis, Higher Education Press, Beijing, 1990. [19] H.E. Kissinger, Anal. Chem. 29 (1957) 1702–1706. [20] R.A. Nyquist, R.O. Kagel, Infrared Spectra of Inorganic Compounds (3800–45 cm−1 )], Academic Press, New York, 1997. [21] A. Ait Salah, A. Mauger, C.M. Julien, et al., Mater. Sci. Eng. B: Solid-State Mater. Adv. Technol. 129 (2006) 232–244. [22] Y. Bai, F. Wu, C. Wu, Chin. J. Light Scatter. 15 (2005) 231–236.
4. Conclusion By using Doyle–Ozawa and Kissinger methods, the average activation energy of the reaction process where the precipitated precursors produced olivine type LiFePO4 was suggested 239.39 kJ mol−1 . The reaction orders, the frequency factors and the kinetic equation of the reaction were determined by the Kissinger method. The kinetic equation was dα 2.3939 × 105 18 = 3.164 × 10 exp − (1 − α)0.9133 dt RT By sintering the precursor for 6 h at 550 ◦ C, the sample was synthesized. It has more single-phase, more regular morphology, well-distributed particles and better electrochemical properties. At the rate of C/10, the initial specific discharge capacity of this sample reached 119.5 mAh g−1 with lower than 2% capacity fading after 8 cycles at room temperature. Acknowledgments This work was financially supported by National Natural Science Foundation of China (grant no. 50474092) and Shenzhen Government’s Plan of Science and Technology (grant no. 200505).
Kinetics of synthesis olivine LiFePO4 by using a precipitated-sintering method Peixin Zhang a,b,∗ , Xinyang Li a , Zhongkuan Luo a , Xiaoqian Huang a , Jianhong Liu a , Qiming Xu b , Xiangzhong Ren a , Xun Liang a b
a School of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, PR China School of Materials Science and Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, PR China
Received 20 September 2007; received in revised form 28 November 2007; accepted 2 December 2007 Available online 15 January 2008
Abstract The LiFePO4 precursor was synthesized using a precipitation with raw materials LiOH·H2 O, (NH4 )2 HPO4 and FeSO4 ·7H2 O. The kinetics of synthesis olivine LiFePO4 was studied by using a differential thermal analysis (DTA) at different heating rates. The average activation energy of the reaction where the precursor form olivine LiFePO4 was 239.39 kJ mol−1 , calculated by Doyle–Ozawa and Kissinger methods. The reaction order, frequency factor, rate equation and kinetic equation of the reaction were determined by the Kissinger method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Fourier transform infrared spectrometer (FTIR). The sample synthesized by the precursor sintered for 6 h at 550 ◦ C shows a single-phase, regular morphology, well-distributed particle sizes and good electrochemical properties. © 2007 Elsevier B.V. All rights reserved. Keywords: Lithium-ion battery; LiFePO4 ; Kinetics; Activation energy; Precipitation
1. Introduction At present, rechargeable lithium-ion batteries as a sort of high-powered green battery are used as key components of telecommunication equipments, digital electronic equipments and portable computer systems. Because of their advantages such as low self-resistance, little self-discharge, high-specific capacity, no memorizing effect, rechargeable lithium-ion batteries are good candidates for electric vehicles (EVs) and hybrid electric vehicles (HEVs). Being lower-cost, simple in manufacturing and environment friendly, the lithium-ion battery is widely used. The lithium-ion battery is influenced by several factors in its commercial process, especially by the cathode material. The cathode material has an important effect on environmental problems, factors of electrochemical capabilities and feasibility of direct mass production. Also it affects the price of the lithium-ion battery. Transition metal oxides, such as LiCoO2 ∗ Corresponding author at: School of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, PR China. Tel.: +86 755 26538134; fax: +86 755 26538134. E-mail address: [email protected] (P. Zhang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.12.005
[1–3], LiNiO2 [4–6] and spinel LiMnO2 [7–9] have been studied as cathode materials. In 1997, Padhi et al. [10] reported firstly that olivine-structure LiFePO4 could reversibly extract/insert lithium-ion. The orthophosphates Li–M–PO4 (M = Fe, Co, Mn, Ni) are intensively studied as cathodes for lithium-ion batteries, especially LiFePO4 has attracted particular interest. LiFePO4 is inexpensive, non-toxic, environmentally benign, good thermal stability and has a theoretical specific capacity of 170 mAh g−1 . LiFePO4 is a good candidate for cathode materials used in lithium-ion batteries. Most of researches focus on improvements of synthesizing methods and electrochemical capabilities [11–15]. Up to now, the dynamic study of synthesizing olivine LiFePO4 has not been reported in detail. In this research, the LiFePO4 precursor has been synthesized using precipitation with the raw materials of LiOH·H2 O, (NH4 )2 HPO4 and FeSO4 ·7H2 O. The kinetics of synthesis olivine LiFePO4 was studied by means of a differential thermal analysis (DTA) analysis with different heating rates. The activation energy, reaction order, frequency factor and kinetic equation are provided. The theoretical gist for LiFePO4 preparation by sintering is also discussed.
P. Zhang et al. / Journal of Alloys and Compounds 467 (2009) 390–396
391
2. Experimental 2.1. Synthesizing procedures The molar ratio of Fe3 (PO4 )2 and Li3 PO4 were synthesized by using a precipitation with the aqueous solutions of LiOH·H2 O, (NH4 )2 HPO4 and FeSO4 ·7H2 O under pH control. The precipitate was filtered off, washed thoroughly with de-ionized water, then mixed the Fe3 (PO4 )2 and Li3 PO4 in the de-ionized water. After high-speed stirring several hours with blowing nitrogen gas into the system through a tube, the precursor was obtained, then washed with de-ionized water and filtered off. The precursor dried in a vacuum oven at 60 ◦ C for 8 h. After being dried, the precursor powder was mixed with 2 wt.% high-surface area carbon black and pressed into a disk-shaped pellet at 10 MPa pressure. The pellet was annealed at a certain temperature for a certain time in nitrogen gas flow, then the olivine LiFePO4 was obtained.
2.2. DTA experiments The 5–10 mg precursor was put into an Al2 O3 crucible with an empty Al2 O3 crucible as reference. In order to overcome the disadvantages, the DTA experiments were carried out with different heating rates of 5, 10, 15, 20 K min−1 , respectively, under nitrogen with a gas-flow rate of 30 ml min−1 on Netzsch4 STA 409 PC analyzer. According to the measured DTA data, dynamics parameters can be calculated by Doyle–Ozawa and Kissinger methods.
2.3. Structure and morphological characterization XRD profiles of the samples were measured on a Bruker D8 Advance diffractometer (Cu K␣ radiation). The scan rate was 0.04◦ s−1 and step was 0.02◦ . The samples were observed by using a JSM-5900 scanning electron microscope (SEM). The FTIR were recorded at room temperature on a Shimadzu FTIR-8300PCS spectrometer using the KBr pellet technique.
2.4. Electrochemical characterization A mixture composed of 5 wt.% of electrical conductive carbon powder, 85 wt.% of lithium iron phosphate powder and 15 wt.% PTFE latex were dried in a vacuum oven for 5 h at 60 ◦ C. After being dried, the mixture was rolled into flakes and stuck to the stainless steel grid plates. Each strip was mounted as positive electrode versus counter with lithium metal as reference electrodes in an experimental cell. 1 mol L−1 LiPF6 EC/DMC was used as electrolyte and the polypropylene no. 2400 separators were used. Assemblage of the cells was carried out in an argon box. The charge/discharge test used a LAND series battery test system (LAND, Wuhan, China) with a specific current of C/10 at room temperature.
Fig. 1. The TG–DTA curves of LiFePO4 precursor at 10 K min−1 heating rate.
appeared in the system and the nucleation happened within this temperature range. The exothermic peak was emerging within temperature range of 550–700 ◦ C in the DTA curve with a steady mass according to the TG curve. The temperature range to the exothermic peak is the process of crystal phase transformation from amorphous precursor to olivine LiFePO4 . After crystal nucleus formed, with the rise of temperature, the crystal nucleus grew up, and then a large quantity of olivine LiFePO4 is thus synthesized with a great deal of heat dissipation. The synthesizing process of the olivine LiFePO4 is a simplex crystal phase transformation process after crystal water decomposition. The experiments were carried out under nitrogen with a gasflow rate of 30 ml min−1 and different heating rates of 5, 10, 15 and 20 K min−1 . The peak temperature, enthalpy variation and peak areas of endothermic peak and exothermic peak could be explored under different heating rates. The DTA curves of lithium iron phosphate precursor were given in Fig. 2. According to the measurement zero line, it could be seen from Fig. 2 that there was an exothermic peak in each DTA curve within the temperature range of 550–700 ◦ C which was the range of crystal
3. Results and discussion 3.1. Non-isothermal kinetics The thermogravimetric and differential thermal analyses (TG–DTA) curves of lithium iron phosphate precursor at the heating rate of 10 K min−1 were given in Fig. 1. According to the measurement zero line of the DTA curve, there were two peaks including an endothermic peak and an exothermic peak. The endothermic peak appeared within the range of 80–350 ◦ C in the DTA curve with a mass decrease dramatically according to the TG curve. This mass decreasing trend was getting slower and slower when temperature went up. The temperature range of the endothermic peak is caused by the decomposition of the crystal water gradually. Within the temperature range of 450–550 ◦ C, there is a slight heat dissipation with a slow rate. As the temperature rose gradually, the very small crystallize grain
Fig. 2. DTA curves of LiFePO4 precursor at different heating rates: (a) 5 K min−1 ; (b) 10 K min−1 ; (c) 15 K min−1 ; (d) 20 K min−1 .
392
P. Zhang et al. / Journal of Alloys and Compounds 467 (2009) 390–396
phase transformation. With the heating rate rising up gradually, the exothermic peak was moving to a high temperature and the heat dissipation of nucleation became unsteady. When the heating rate was getting higher, the interior temperature gradient increased which made the deviate of thermal effect range and procedure temperature increased. Lower heating velocity makes the nucleation carry on slowly, and the system can immediately attain temperature balance within a very short time. So it can make crystal nucleus very small and grow slow, then the synthesized crystal particle size distribution becomes very narrow and the modality becomes easy to control. 3.1.1. Dynamics parameters In this paper, from the viewpoint of chemistry reaction dynamics, the variations of chemical reaction velocity respect to time and temperature were studied, and the dynamic equations were induced. Doyle–Ozawa and Kissinger methods were adopted to calculate the activation energy of the reaction of the precursor become olivine LiFePO4 . The three formulas listed below were basic principles: dα the mass action law : = k(1 − α)n dt E the Arrhenius formula : k = A exp − RT the heating rate formula :
β=
dT dt
(1) (2) (3)
Doyle–Ozawa method applied formula (4) that was deduced from above formulas to calculate the activation energy: log β=log
AE E −2.315 − 0.4567 , f (α) = (1 − α)n Rf (α) RT
(4)
When the fractional conversion α is certain, regardless reaction order, the function f(α) concerning fractional conversion α is a constant. For a certain reaction, the frequency factor A can be determined, and log[AE/Rf(α)] is also a constant. The plot of log β ∼ T−1 is a straight line and the slope is (−0.4567ER−1 ) [16–18]. With different heating rates β and the same fractional conversion α, we can plot the relation curve of the log β ∼ T−1 , the activation energy E of the precursor becoming olivine LiFePO4 can be calculated from the slope of the straight line. Using the Doyle–Ozawa method, the diagrams of log β ∼ T−1 for the exothermic peak was showed in Fig. 3. As shown in Fig. 3, every line expressed percent conversion of 10–100% from the right to the left. The slopes, correlation coefficients and activation energy of every line were showed in Table 1. The reaction activation energy was various for different percent conversion. In addition, the process could be divided into two stages: 70.36–159.63 kJ mol−1 and 193.11–211.01 kJ mol−1 . In the first stage, the activation energy obviously increases with the increment of fractional conversion, but on a lower level all the time and the reaction carried on more easily and rapidly. In the second stage, the activation energy leveled off, on the average of about 190 kJ mol−1 . The reaction carried on stably and slowly. In the whole process the average apparent activation energy was 160.07 kJ mol−1 .
Fig. 3. Curves of log β vs. T−1 of activation energy for the reaction using Doyle–Ozawa method (every line regressed by calculating point of symbol 1–10 expresses reaction degree of 10–100% from right to left in the figure).
In terms of the Kissinger principle, assuming that the reaction velocity was maximum at peak temperature and the reaction followed dynamics equations of Kissinger [19], according to mass action law (1), the Arrhenius formula (2) and heating rate formula (3), formula (5) can be deducted. dα A E (5) = exp − (1 − α)n dT β RT For T = Tmax (where Tmax is the maximum temperature of the peak), then d dα =0 (6) dT dt When formula (6) was substituted into formula (5), we get E An E n−1 (7) = ) exp − (1 − α max 2 RTmax β RTmax If n = 1, then formula (7) can be rewritten as E E A exp − = 2 RTmax β RTmax
(8)
Table 1 Activation energy and linear correlation coefficients of different reaction degrees at the exothermic peak by Doyle–Ozawa method α (%)
Slope
Correlation coefficient
E (kJ mol−1 )
10 20 30 40 50 60 70 80 90 100
−3.86468 −4.85644 −6.49505 −8.76892 −10.60804 −10.75755 −10.64314 −10.3428 −10.00133 −11.59081
−0.97181 −0.95321 −0.96500 −0.98577 −0.99726 −0.99916 −0.99749 −0.99053 −0.97153 −0.96017
70.36 88.41 118.24 159.63 193.11 195.84 193.75 188.29 182.07 211.01
Average E (kJ mol−1 )
160.07
P. Zhang et al. / Journal of Alloys and Compounds 467 (2009) 390–396
393
Generally, the activation energy of chemical reaction is 60–250 kJ mol−1 . If the activation energy is lower than 40 kJ mol−1 , the reaction velocity is very fast and difficult to measure. When this happens, the reaction then becomes difficult to control. If the activation energy is higher than 400 kJ mol−1 , the reaction velocity is very slow, then rigorous condition or activators are indispensable to expedite the reaction. The average activation energy of the reaction process where the precipitated precursor becomes olivine LiFePO4 was 239.39 kJ mol−1 , as suggested by Doyle–Ozawa and Kissinger methods. The precursor acquiring a lower energy can overcome energy barrier and become olivine LiFePO4 . This is the basic reason why the precursor synthesized by precipitation method can produce olivine LiFePO4 at a lower temperature for a shorter sintering time. −2 ) vs. T −1 of the peak at different heating rates by Fig. 4. Curves of ln(βTmax max Kissinger method.
For n = / 0 and n = / 1, due to (n − 1)(2RTmax /E) 1, then 2RTmax n−1 ≈ 1 + (n − 1) ≈1 (9) n(1 − αmax ) E When formula (9) was substituted into formula (7) and the same formula (8) could be inferred. In this paper, we use formula (8) to calculate the corresponding dynamic constants. When formula (8) was natural logarithmic transformed, we get β RA E ln = ln (10) − 2 Tmax E RTmax For a certain reaction, the frequency factor A is constant, and 2 ) ∼ (1/T ln(RA/E) is also a constant. The plot of ln(β/Tmax max ) −1 is a line and the slope is (−ER ). Using the Kissinger method, 2 ) ∼ (1/T the diagrams of ln(β/Tmax max ) for the exothermic peak was shown in Fig. 4. The slope is equal to (−ER−1 ) and the activation energy can be calculated out. The frequency factor A was obtained according to formula (8). In terms of the Kissinger principle, the shape index S is defined as the absolute value of the ratio of the slops of tangents to the curve at the inflexion points. Then we can use formula (11) to calculate the reaction order n [19]. n = 1.26S 0.5
(11)
The calculated corresponding dynamic constants were listed in Table 2.
3.1.2. Rate equation The reaction mechanism was assured after obtaining dynamics parameters in terms of unequal isothermal process. Within infinite time intervals, it could be considered that an unequal isotherm process was as an isothermal process. The reaction velocity could replace the general formula during the isotherm process. In accordance with formula (5), with the calculated corresponding dynamic constants, the rate equation of exothermic peak was 2.3939 × 105 dα 18 = 3.164 × 10 exp − (1 − α)0.9133 (12) dt RT 3.2. Structure and morphological characterization Fig. 5 shows the XRD patterns of LiFePO4 precursor and a series of samples synthesized by sintering at various synthesizing temperatures for 6 h in nitrogen gas. The precursor is amorphous. The profiles of the reflection peaks are quite low, wide and out-of-order according to Fig. 5(a). The precursor was sintered at different synthesizing temperatures for 6 h then the olivine LiFePO4 was synthesized. All diffraction lines can be attributed to the olivine type phase LiFePO4 and to a minor impurity phases in (c) and (d). Additionally, in the pattern (b), the profiles of the reflection peaks are quite narrow, symmetric and without impurity phases. Therefore, a single-phase and well-crystallized product is indicated by the precursor sintered at 550 ◦ C for 6 h. The reason why the impurity phases appeared in (c) and (d) samples is that a higher sintering temperature sped the crystal phase transformation process, so that the reaction velocity had a striking dissimilarity in the internal part of the system, and
Table 2 Kinetic parameters for different heating rates by Kissinger method β (K/min)
Tmax (K) S n A (×10−18 )
5
10
15
20
871.35 0.6859 1.0435 3.225
886.15 0.5344 0.9211 2.991
894.65 0.4416 0.8373 2.918
897.05 0.4565 0.8513 3.451
Average
Slope
Correlation coefficient
E (kJ/mol)
0.5294 0.9133 3.164
−38.33368
−0.99122
318.71
394
P. Zhang et al. / Journal of Alloys and Compounds 467 (2009) 390–396
Fig. 5. XRD patterns of LiFePO4 precursor and olivine LiFePO4 : (a)LiFePO4 precursor; (b) 550 ◦ C sintered 6 h (c) 600 ◦ C sintered 6 h; (d) 650 ◦ C sintered 6 h.
the crystal phase transformation reaction became miscellaneous. Shown in Fig. 6 are the SEM images of a series of samples obtained at different synthesizing temperatures for 6 h in nitrogen gas. The particle size of the LiFePO4 synthesized by precipitation is very small as shown in Fig. 6. Additionally, the sample obtained after sintering 6 h at 550 ◦ C consist of well-distributed, near-spherical shaped particles whose average particle size is about 200 nm, which is the smallest without agglomeration. With the exaltation of sintering temperature, the
distribution of particle sizes became more and more wide, and the agglomeration phenomenon was more and more obvious. The agglomeration and the wide distribution of particle size attributed to the higher sintering temperature which increased the speed of the crystal phase transformation. The higher transformation speed makes the crystal nucleus grow rapidly and the distribution of the particle size become wider due to the agglomeration happens. The smaller the particle size, the higher specific surface area is. A high-specific surface area is advantageous to internal lithium-ion diffusion, and the good electrochemical properties material can be obtained. From the view of chemistry reaction dynamics, because the activation energy of crystal phase transformation is a little larger than 200 kJ mol−1 , the precursor can become olivine LiFePO4 at 550 ◦ C in a short time. 550 ◦ C is the starting temperature of the crystal phase transformation. At this temperature, a low energy that the crystal phase synthesizing process dissipated could overcome the energy barrier and the precursor became olivine LiFePO4 . When the sintering temperature is low, the growth of crystal is slow. The regular surface texturing and welldistributed olivine LiFePO4 sample can be synthesized. The degree of crystallinity and the average particle diameter of LiFePO4 which was synthesized at 550 ◦ C for different sintering time are given in Fig. 7. When the sintering time is less than 6 h, the energy which the samples obtained could not overcome the energy barrier and could not produce the olivine LiFePO4 . Therefore the degree of crystallinity of the samples is lower. When the sintering time is more than 6 h, the agglomeration happened. Then the particle size is larger and the minor impurity phases were synthesized because of asymmetrical dis-
Fig. 6. SEM patterns of LiFePO4 synthesized at different sintering temperatures: (a) 550 ◦ C; (b) 600 ◦ C; (c) 650 ◦ C; (d) 700 ◦ C.
P. Zhang et al. / Journal of Alloys and Compounds 467 (2009) 390–396
395
Fig. 7. The degree of crystallinity and average particle diameter of LiFePO4 synthesized at different sintered time. Fig. 9. Initial charge/discharge profile of olivine LiFePO4 .
tribution of Li+ , Fe2+ and PO4 3− . And the degree of crystallinity of the sample is lower too. The sample synthesized at 550 ◦ C for 6 h has the highest degree of crystallinity and the smallest particle size. The energy that this sample obtained can overcome the energy barrier exactly. The Fourier transform infrared spectroscopy (FTIR) is known to be a good means for investigating of the structure at a local scale. The FTIR images of precursor and olivine LiFePO4 samples were shown in Fig. 8. Fig. 8(a) is Fourier transforms infrared spectroscopy of the precursor, the absorption of FTIR spectra originates from the intramolecular vibrations of the (PO4 )3− unit, which involve the displacement of oxygen atoms at frequencies closely related to those of the free molecule within 940–1120 cm−1 and 540–650 cm−1 ranges [20]. From Fig. 8, the FTIR spectra (b), (c) and (d) of olivine LiFePO4 samples are finer and narrower, compared with Fig. 8(a), which is within ranges of 940–1120 cm−1 and 540–650 cm−1 . In curve (d), compared with curves (b) and (c), there is an absorption gap extending from 650 to 950 cm−1 where located vibra-
Fig. 8. FTIR patterns of LiFePO4 precursor and olivine LiFePO4 : (a) LiFePO4 precursor; (b) 700 ◦ C sintered 6 h; (c) 550 ◦ C sintered 10 h; (d) 550 ◦ C sintered 6 h.
tions associated with other phosphate anions such as (P2 O7 )4− and (P3 O10 )5− [21]. The absence of any structure in this gap indicates that such complexes are not present in LiFePO4 sample obtained at 550 ◦ C sintered for 6 h. The samples such as shown in Fig. 8(b), which were synthesized at a temperature higher than 550 ◦ C for 6 h, contained impurity phases to a certain extent. The same is true of sample Fig. 8(c) which was synthesized at 550 ◦ C for more than 6 h. In Fig. 8(d), the absorption at 1138 cm−1 originates from the symmetric and anti-symmetric stretching vibrations of O–P–O. Because the discrepancy of the two vibrations is extremely small, there is only one peak at 1138 cm−1 . The absorption at 1061 and 1096 cm−1 originate from anti-symmetric stretching vibration of P–O. The absorption at 976 and 637 cm−1 originate from originate stretching vibration of P–O. The absorption at 579 cm−1 originate from originate anti-symmetric bending vibration of O–P–O. The absorption at 552 and 471 cm−1 originate from originate symmetric bending vibration of O–P–O. The absorption at 505 cm−1 originate from originate perturbation bending vibration of O–P–O [22].
Fig. 10. Cycling performance of olivine LiFePO4 sample.
396
P. Zhang et al. / Journal of Alloys and Compounds 467 (2009) 390–396
3.3. Electrochemical results
References
The single-phase olivine LiFePO4 was synthesized at 550 ◦ C by sintering the precursor for 6 h. We use this sample to assemble experimental cell according to the method discussed in Section 2.4. Fig. 9 depicts the initial voltage profiles of this sample at the rate of C/10. The initial capacity of this sample is 119.5 mAh g−1 at room temperature with the charge plateau at 3.5 V and the discharge plateau at 3.3 V. Fig. 10 shows the cycling performance of this olivine LiFePO4 sample. At the C/10 rate, the specific discharge capacity fading of this sample is lower than 2% after 8 cycles at room temperature.
[1] K. Mizushima, P.C. Jones, P.J. Wiseman, et al., Mater. Res. Bull. 15 (1980) 783–789. [2] M. Zou, M. Yoshio, S. Gopukumar, et al., Chem. Mater. 15 (2003) 4699–4702. [3] A. Clemencon, A.T. Appapillai, S. Kumar, et al., Electrochim. Acta 52 (2007) 4572–4580. [4] M.G.S.R. Thomas, W.I.F. David, J.B. Goodenough, et al., Mater. Res. Bull. 20 (1985) 1137–1146. [5] N. Kalaiselvi, A.V. Raajaraajan, B. Sivagaminathan, et al., Ionics 9 (2003) 382–387. [6] H. Gu, Y. Zhai, Y. Tian, et al., Chin. J. Mater. Res. 21 (2007) 97– 101. [7] T. Ohzuku, M. Kitagawa, T. Hirai, J. Electrochem. Soc. 137 (1990) 769–775. [8] S. Yao, S. Zhang, N. Xu, et al., Chin. J. Power Sources 27 (2003) 554– 557. [9] P. Suresh, A.K. Shukla, N. Munichandraiah, J. Power Sources 161 (2006) 1307–1313. [10] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188–1194. [11] F. Croce, A.D. Epifanio, J. Hassoun, et al., Electrochem. Solid-State Lett. 5 (2002) A47–A50. [12] K.S. Park, K.T. Kang, S.B. Lee, et al., Mater. Res. Bull. 39 (2004) 1803–1810. [13] P.P. Prosini, M. Carewska, S. Scaccia, et al., Electrochim. Acta 48 (2003) 4205–4211. [14] S. Franger, F. Le Cras, C. Bourbon, et al., J. Power Sources 119–121 (2003) 252–257. [15] Z. Xu, L. Xu, Q. Lai, et al., Mater. Chem. Phys. 105 (2007) 80–85. [16] M.I. Bopu, M.D. Youde, S. Wang, et al., Differential Thermal Analysis, Beijing Normal University Press, Beijing, 1981. [17] X. Shen, Differential Thermal Gravity Analysis and Non-isothermal Solid Reaction Dynamics, Metallurgical Industry Publishing House, Beijing, 1995. [18] Z. Cai, Thermal Analysis, Higher Education Press, Beijing, 1990. [19] H.E. Kissinger, Anal. Chem. 29 (1957) 1702–1706. [20] R.A. Nyquist, R.O. Kagel, Infrared Spectra of Inorganic Compounds (3800–45 cm−1 )], Academic Press, New York, 1997. [21] A. Ait Salah, A. Mauger, C.M. Julien, et al., Mater. Sci. Eng. B: Solid-State Mater. Adv. Technol. 129 (2006) 232–244. [22] Y. Bai, F. Wu, C. Wu, Chin. J. Light Scatter. 15 (2005) 231–236.
4. Conclusion By using Doyle–Ozawa and Kissinger methods, the average activation energy of the reaction process where the precipitated precursors produced olivine type LiFePO4 was suggested 239.39 kJ mol−1 . The reaction orders, the frequency factors and the kinetic equation of the reaction were determined by the Kissinger method. The kinetic equation was dα 2.3939 × 105 18 = 3.164 × 10 exp − (1 − α)0.9133 dt RT By sintering the precursor for 6 h at 550 ◦ C, the sample was synthesized. It has more single-phase, more regular morphology, well-distributed particles and better electrochemical properties. At the rate of C/10, the initial specific discharge capacity of this sample reached 119.5 mAh g−1 with lower than 2% capacity fading after 8 cycles at room temperature. Acknowledgments This work was financially supported by National Natural Science Foundation of China (grant no. 50474092) and Shenzhen Government’s Plan of Science and Technology (grant no. 200505).