Solid State Ionics 181 (2010) 1451–1455
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Solid State Ionics 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 / s s i
Synthesis and electrochemical performance of Li2FeSiO4/C as cathode material for lithium batteries Xiaobing Huang a,b,c, Xing Li d, Haiyan Wang e, Zhonglai Pan c, Meizhen Qu a, Zuolong Yu a,c,⁎ a
Chengdu Institute of Organic Chemistry, Chinese Academy of Science, Chengdu 610041, China Graduate University of the Chinese Academy of Sciences, Beijing 100049, China Zhongke Laifang Energy and Technology Co. Ltd., China d The Institute of Optics and Electronics the Chinese Academy of Sciences, China e School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China b c
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 20 May 2010 Received in revised form 2 August 2010 Accepted 3 August 2010 Keywords: Lithium ion battery Solid-state reaction Li2FeSiO4/C Pitch
Li2FeSiO4/C cathode material was synthesized by a traditional solid-state reaction method with Li2CO3, FeC2O4·2H2O, nano SiO2 as starting materials and pitch as the carbon source. For comparison, the Li2FeSiO4/C with glucose as the carbon source and the pristine Li2FeSiO4 were also prepared. The as-prepared materials were characterized by X-ray diffraction, scanning electron microscopy, elementary analyzer, BET speciﬁc surface area, galvanostatic charge-discharge test and AC impedance spectroscopy. The results demonstrated that the Li2FeSiO4/C composites showed better electrochemical properties compared with the pristine Li2FeSiO4. Surprisingly, the Li2FeSiO4/C with pitch as carbon source exhibited the best electrochemical properties among the three samples, it delivered a speciﬁc discharge capacity of 139 mAh g−1, 127 mAh g−1, 118 mAh g−1 and 103 mAh g−1 at 0.2 C, 0.5 C, 1 C and 2 C, respectively. After 100 cycles at the rate of 1 C, the discharge capacity remained 93.6% of its initial value. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, Li2FeSiO4 has been proposed as a promising cathode material for Li-ion batteries because of its low raw materials cost, environmental friendliness, high safety and electrochemical stability [1–6]. Furthermore, the presence of Si–O bonds should afford the same lattice stabilization effect as in LiFePO4. The lower electronegativity of Si vs. P would result in a lowering of the FeII ⇔ FeIII couple. Therefore Li2FeSiO4 often possesses a lower electronic band gap and a higher electronic conductivity in comparison with LiFePO4 . Its theoretical capacity is calculated as 166mAh g−1 based on the following reaction: þ
Li2 FeSiO4 ↔LiFeSiO4 + Li + e
Interestingly, a shift was observed in the potential plateau from 3.1 V to 2.8 V after the ﬁrst cycle, suggesting a phase transition to a more stable structure . Unfortunately, just like LiFePO4, Li2FeSiO4 also suffers from the problem of poor electronic conductivity and slow diffusion rate of lithium ion . As reported, synthesis of pure Li2FeSiO4 with small particle size and Li2FeSiO4/C composite material were considered as ⁎ Corresponding author. Chengdu Institute of Organic Chemistry, Chinese Academy of Science, Chengdu 610041, China. Tel.: + 86 28 85229790; fax: + 86 28 85242280. E-mail address: [email protected]
(Z. Yu). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.08.007
two kinds of methods to improve the electronic conductivity [9–11]. It is well known that carbon coating can not only provide conductive connections between the active particles which are favorable to the electron transfer, but also decrease effectively the particle size of the active materials during the heat treatment, shortening the diffusion path of lithium ion [2,12]. In an earlier study, Nien et al.  found that various carbon precursors directly affected the electronic conductivity of the carbon, which is proportional to the performance of the Li2FeSiO4/C composite. Therefore, selecting an appropriate carbon precursor is crucial for tailoring the ﬁnal properties of carboncoated composite powders. To date, Li2FeSiO4/C with various carbon sources such as sucrose, polyethylene-poly (ethylene glycol) , citric acid , glucose  and carbon black  have been reported successively. However, little research have been attempted to synthesize Li2FeSiO4/C with pitch as the carbon source. It's pointed out that the residual carbons obtained from pyrolysis of glucose, citric acid, and sucrose are usually non-graphitazable, however, functionalized aromatic pitch could form more highly graphitized carbons[18– 22]. The more the graphitized carbon, the better the electronic conductivity. For this reason, the Li2FeSiO4/C with pitch as the carbon source might exhibit better electrochemical performance than the Li2FeSiO4/C with other common carbon sources such as glucose. In the present work, the Li2FeSiO4/C composite with pitch as carbon source was synthesized, for comparison, the Li2FeSiO4/C composite with glucose as carbon source and the pristine Li2FeSiO4 were also investigated.
X. Huang et al. / Solid State Ionics 181 (2010) 1451–1455
impedance spectrum was measured by using a Solatron 1260 Impedance Ananlyzer in the frequency range 10−1 Hz to 106 Hz.
3. Results and discussion
Fig. 1. XRD proﬁles of as-prepared Li2FeSiO4 samples: (a) pristine Li2FeSiO4; (b) Li2FeSiO4/C composite with glucose; (c) Li2FeSiO4/C composite with pitch.
2. Experimental All the samples were prepared by a solid-state reaction in the present paper. Li2FeSiO4 was synthesized from Li2CO3 (99.5%), FeC2O4·2H2O (99.7%) and nano SiO2 (99.5%, Anhui China). Firstly, the raw materials in a stoichiometric ratio were dispersed into acetone with thoroughly ball-milling to form a slurry mixture. As follow, the obtained slurry was dried at 80 °C in a vacuum circumstance for 2 h. The dried powder was pressed into pellets and heated in a horizontal quartz tube oven under ﬂowing argon gas, ﬁrst at 350 °C for 5 h and then at 700 °C for 10 h to obtain the ﬁnal Li2FeSiO4. The synthesis methods of Li2FeSiO4/C composite with pitch and glucose as carbon sources respectively were similar to that of Li2FeSiO4, with pitch (high soften point coal pitch, Shenzheng China) and glucose, respectively mixed with raw materials in acetone. Pitch and glucose were also heated in a horizontal quartz tube oven under ﬂowing argon gas, ﬁrst at 350 °C for 5 h and then at 700 °C for 10 h to obtain the pyrolized carbon. The structure of the as-prepared Li2FeSiO4 samples was characterized by X-ray powder diffraction analysis (XRD) measurement using the Philips X'Pert Pro MPD DY1219 with a Cu Kα radiation source and the morphology was observed by scanning electron microscope (SEM FEI INSPECT-F). The speciﬁc surface area was measured by nitrogen adsorption/desorption at −196 °C using a Builder SSA-4200 apparatus. The actual amount of carbon in the ﬁnal Li2FeSiO4/C composites was determined by elemental analysis instrument (EA). The electrochemical properties of the samples were evaluated using CR2032 coin-type cell. The cell consisted of a cathode and a lithium metal anode separated by a Celgard 2400 separator. The cathode was prepared by mixing 80% active material with 10% SuperP carbon and 10% LA-132 binder. The mixture was made into slurry by a mortar and pestle using water as the solvent. The slurry was then coated on an aluminum foil and dried at 100 °C for 10 h in a vacuum oven. The electrolyte was 1 M LiPF6/EC:DEC:DMC (1:1:1 in volume). The coin cells were assembled in an argon-ﬁlled glove box. Galvanostatic charge-discharge measurements were performed in a potential range of 1.5–4.8 V at room temperature (25 °C). The AC
The X-ray diffraction patterns of the as-prepared Li2FeSiO4 samples are shown in Fig. 1. The main diffraction peaks of all the obtained samples are in good agreement with the previous report by Nyten et al. . Then we identiﬁed it as Li2FeSiO4 phase with good crystallization ﬁtted using Pmn21 space group. A little Fe3O4 impurity is detected in all samples. The lattice parameters of the Li2FeSiO4 samples are calculated and listed in Table 1. Clearly, the lattice parameters (a, b and c) of three samples are similar to those of Nyten et al. , which indicates that the addition of the carbon has no interfere with the main reaction. Furthermore, the pristine Li2FeSiO4 shows a relatively sharp peak with the higher intensity.While the Li2FeSiO4/C composites indicate the decreasing intensity of the peaks and broadening peaks. The results above suggest that the Li2FeSiO4/C composites probably have smaller crystallite sizes. It might be due to the presence of carbon hindering the growth of Li2FeSiO4 particles during the calcination process, which results in the smaller crystallite sizes of the Li2FeSiO4/C composites. Fig. 2 shows XRD patterns of carbons produced by pyrolysis of pitch and glucose. As observed, the graphite peak at 2θ = 26° of pyrolysis carbon from pitch is much higher than that of pyrolysis carbon from glucose, which indicates that pitch is favorable to form more highly graphitized carbons during the heat treatment process. Fig. 3 shows the SEM images of the Li2FeSiO4 samples. Nanoparticles can be observed in all images. The formation of nanoparticles could be attributed to the extensive ball milling process. A tendency toward agglomeration of individual particles can be also observed in all samples. Besides, as observed, image A reveals that the pristine Li2FeSiO4 forms larger particle sizes, whereas image B and image C show that the Li2FeSiO4/C composites are much smaller in size than the pristine Li2FeSiO4. Interestingly, the Li2FeSiO4/C composite with pitch has the smallest particle sizes among three samples. The smaller particle sizes of the Li2FeSiO4/C composites could be attributed to the following reasons: as well known, the addition of carbon source such as glucose and pitch into the precursor hinders particle growth of Li2FeSiO4 particles during the calcination process, resulting in smaller particle sizes. The smallest particle sizes of the Li2FeSiO4/C composite with pitch might be ascribed to the following reasons: the highly active carbon pyrolysis of pitch homogeneously coated on the surface of a fresh crystallized Li2FeSiO4 particle, which results in hindering the particle growth efﬁciently and thus the formation of the Li2FeSiO4/C composite with the smallest particle sizes.
Table 1 The lattice parameters of pristine Li2FeSiO4 and Li2FeSiO4/C composites.
Pristine Li2FeSiO4 Li2FeSiO4/C composite with glucose Li2FeSiO4/C Composite with pitch
6.2567 6.2563 6.2561
5.3080 5.3236 5.3387
5.0066 5.0058 5.0054
Fig. 2. XRD proﬁles of carbon: (a) pyrolysis of glucose; (b) pyrolysis of pitch.
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Table 2 Speciﬁc surface area of pristine Li2FeSiO4 and Li2FeSiO4/C composites. Speciﬁc surface area (m2 g−1) Pristine Li2FeSiO4 Li2FeSiO4/C composite with glucose Li2FeSiO4/C composite with pitch
15.6 64.2 88.2
The ﬁnal quantities of residual carbon obtained from pyrolysis of various carbon sources in the Li2FeSiO4/C composites were measured by an elemental analyzer (EA). It is 12.8%, 13.3% for the Li2FeSiO4/C composite with glucose and the Li2FeSiO4/C composite with pitch, respectively. The data from the BET measurement shown in Table 2, give a speciﬁc surface area of 15.6 m2 g−1, 64.2 m2 g−1, 88.2 m2 g−1 for the pristine Li2FeSiO4, the Li2FeSiO4/C composite with glucose and the Li2FeSiO4/C composite with pitch, respectively. The Li2FeSiO4/C composite with pitch shows the largest speciﬁc surface area, which is in good agreement with the smallest particle size observed by SEM. Fig. 4 depicts the charge–discharge voltage proﬁles for the Li2FeSiO4 samples at the rate of 0.2 C in a potential range of 1.5–4.8 V. The capacity is determined by the mass of Li2FeSiO4 not including the carbon. It can be observed that the pristine Li2FeSiO4 has a low discharge capacity of 13 mAh g−1, while the Li2FeSiO4/C composite with glucose and the Li2FeSiO4/C composite with pitch deliver a relatively high discharge capacity of 122 mAh g−1 and 131 mAh g−1, respectively. The much higher discharge capacity of the Li2FeSiO4/C composites in comparsion with the pristine Li2FeSiO4 could be attributed to the following reasons: carbon coating can provide conductive connections between the active particles which are favourable to the electron transfer, and the smaller particle size of the active materials (see in Fig. 3) can shorten the diffusion path of lithium ion . Furthermore, the difference between the charge and discharge plateau potentials of the Li2FeSiO4/C composites is obviously smaller than that of the pristine Li2FeSiO4, and the Li2FeSiO4/C composite with pitch has the smallest difference. It was reported that the difference between the charge and discharge plateau potentials is related to the polarization of the cell system, and the smaller the difference, the less the polarization [23,24]. Therefore, the Li2FeSiO4/C composite with pitch has the smallest polarization. Cyclic performance of the Li2FeSiO4/C samples with glucose as carbon source and pitch as carbon source at 0.2 C, 0.5 C, 1 C to 2 C are shown in Fig. 5. As seen clearly, the discharge capacities decrease with increasing the rate for both samples. However, the Li2FeSiO4/C composite with pitch delivers a relatively higher capacities and better rate capacities in comparison with the Li2FeSiO4/C composite with glucose. The maximum discharge capacity measured for the Li2FeSiO4/C composite with pitch at
Fig. 3. SEM images of the Li2FeSiO4 samples: (A) pristine Li2FeSiO4; (B) Li2FeSiO4/C composite with glucose; (C) Li2FeSiO4/C composite with pitch. Fig. 4. The charge and discharge curves for pristine Li2FeSiO4, Li2FeSiO4/C composite with glucose and Li2FeSiO4/C composite with pitch.
X. Huang et al. / Solid State Ionics 181 (2010) 1451–1455
Fig. 5. Cyclic performance of Li2FeSiO4/C composites with glucose and pitch as carbon sources, respectively at difference rates.
Fig. 7. Nyquist plots of pristine Li2FeSiO4, Li2FeSiO4/C composite with glucose and Li2FeSiO4/C composite with pitch.
a rate of 0.2 C, 0.5 C, 1 C and 2 C is 139 mAh g−1, 127 mAh g−1, 118 mAh g−1 and 103 mAh g−1, respectively. While that of the Li2FeSiO4/C with glucose is 130 mAh g−1, 118mAh g−1, 105 mAh g−1 and 87 mAh g−1 at a rate of 0.2 C, 0.5 C, 1 C and 2 C, respectively. The results above could be attributed to two main reasons: (1) the Li2FeSiO4/C composite with pitch has the smaller particle size (see in Fig. 3), which results in shortening the diffusion path of lithium ion, further expediting the ion transport. Therefore, it shows high reversible capacities. (2) Pitch with aromatic functional groups favors the formation of more highly graphitized carbons during pyrolysis, and consequently, improves electronic conductivity of the coated carbon. Improved electronic properties of the residual carbon can assure good contact between active particles, which can predetermine their electrochemical performance . Cyclic performance of the Li2FeSiO4/C composite with pitch at the rate of 1 C is shown in Fig. 6. The charge–discharge process of the cell was performed after the ﬁrst 40 cycles at various rates from 0.2 C, 0.5 C, 1 C to 2 C. It can be observed that the initial discharge capacity is 115 mAh g−1, and 93.6% of the initial discharge capacity after 100 cycles is remained. Meanwhile, the charge and discharge efﬁciency remains about at 99%. The results above indicate that the Li2FeSiO4/C composite with pitch shows better capacity retention ability. Fig. 7 shows the electrochemical impedance spectra of the Li2FeSiO4 samples, which were measured in the 50% discharge state after 10 cycles of charge–discharge at a rate of 0.2 C. It can be observed that each plot has a semicircle and a straight line. According to the
literature , the intercept at the Z′ axis in the high frequency corresponds to the ohmic resistance (Re), which represents the resistance of the electrolyte. The semicircle in the middle frequency range indicates the charge transfer resistance (Rct). The straight line in the low frequency is associated with lithium ion diffusion in Li2FeSiO4. The lithium ion diffusion coefﬁcient could be calculated from the low frequency plots according to the following equation [2,26,27]:
Fig. 6. Cyclic performance of the Li2FeSiO4/C composite with pitch at 1 C.
2 4 4 2
D = R T = 2A n F C σ
Where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, n is the number of electrons per molecule during oxidation, F is the Faraday constant, C is the concentration of lithium ion, σ is the Warburg factor which is relative with Zre [2,26,27]. −1 = 2
Zre = RD + RL + σω
Where ω is frequency. The relationship between Zre and the reciprocal square root of frequency in the low frequency are demonstrated in Fig. 8. All the parameters obtained and calculated from EIS are shown in Table 3. It can be observed the Rct values of the Li2FeSiO4/C composites are smaller than that the pristine Li2FeSiO4, and the Li2FeSiO4/C composite with pitch has the smallest Rct value. Furthermore, the lithium ion diffusion coefﬁcient of the Li2FeSiO4/C composites are higher than that of the pristine Li2FeSiO4 and the
Fig. 8. The relationship curve between Zre and ω−1/2 in the low frequency: (a) pristine Li2FeSiO4; (b) Li2FeSiO4/C composite with glucose; (c) Li2FeSiO4/C composite with pitch.
X. Huang et al. / Solid State Ionics 181 (2010) 1451–1455 Table 3 Electrochemical impedance parameters of pristine Li2FeSiO4, Li2FeSiO4/C composite with glucose and Li2FeSiO4/C composite with pitch. Re (Ω) Rct (Ω) σ (Ω cm2 s−1/2) D (cm2 s−1) 2.1 Pristine Li2FeSiO4 Li2FeSiO4/C composite with 1.9 glucose Li2FeSiO4/C composite with pitch 1.8
Li2FeSiO4/C composite with pitch has the highest lithium ion diffusion coefﬁcient. This means that the kinetics of Li+ and electron transfer into the electrodes are the fastest for the Li2FeSiO4/C composite with pitch, which coincides well with the cyclic performance in Figs. 5 and 6 and the charge/discharge proﬁles in Fig. 4. 4. Conclusion Pristine Li2FeSiO4, and Li2FeSiO4/C composites with glucose and with pitch as carbon sources, respectively were prepared by a traditional solid-state reaction method. The results illustrate that Li2FeSiO4/C composites show better electrochemical performance than pristine Li2FeSiO4. However, Li2FeSiO4/C composite with pitch as carbon source has the best electrochemical performance among the three samples. The main reasons for the good electrochemical performance could be attributed to that Li2FeSiO4/C composite with pitch as carbon source has the smallest particle size, which results in shortening the diffusion path of lithium ion, further expediting the ion transport. Furthermore, pitch with functionalized aromatic form more highly graphitized carbons, which would lead to a better electronic conductivity. References  C. Deng, S. Zhang, B.L. Fu, S.Y. Yang, L. Ma, Mater. Chem. Phys. 120 (2010) 14.  S. Zhang, C. Deng, B.L. Fu, S.Y. Yang, L. Ma, J. Electroanal. Chem. 644 (2010) 150.  Z.L. Gong, Y.X. Li, G.N. He, J. Li, Y. Yang, Electrochem. Solid-State Lett. 11 (2008) A60.
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