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LiFePO4@C cathode materials synthesized from FePO4@PAn composites
Materials Letters 81 (2012) 115–118
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
LiFePO4@C cathode materials synthesized from FePO4@PAn composites Ningyu Gu ⁎, Xinghua He, Yang Li Department of Chemistry, Nanchang University, Nanchang 330031, China
a r t i c l e
i n f o
Article history: Received 6 March 2012 Accepted 1 May 2012 Available online 9 May 2012 Keywords: Lithium ion batteries Cathode material Lithium iron phosphate In situ polymerization
a b s t r a c t Carbon coated LiFePO4 cathode materials were successfully synthesized by heating the precursors of FePO4@PAn (polyaniline) and equimolar LiOH·H2O under Ar flow. We demonstrated a method that could precisely control the carbon shell in a single reaction step. The precursors of FePO4@PAn were prepared by in situ polymerization of aniline and precipitation of FePO4 in one pot in the presence of hydrogen peroxide as oxidizer. In the obtained core–shell cathode composite, the LiFePO4 core phase was crystalline while the carbon shell had the thickness of several nanometers. The optimized cathode material exhibited satisfactory rate and cycle performance, which virtues are quite suitable for power lithium ion batteries. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As one of the most promising cathode materials for power lithium ion batteries, lithium iron phosphate (LiFePO4) with the olivine structure has attracted much attention due to its theoretical capacity of 170 mAh g− 1, certain safety, good cycling stability, environmental benignity, and low cost of the raw materials since its discovery in 1997 [1]. However, the poor intrinsic electric conductivity and slow diffusion of Li+ make it difficult to obtain good performance at high power C rates for applications in electric vehicles (EVs) and hybrid electric vehicles (HEVs) [2]. To enhance the rate performance of LiFePO4, many efforts have been made, including carbon coating [3–5], metal-ion doping [6–8], and reducing the particle size [9–11], etc. Carbon coating is a quite effective technique, for carbon deposited on the surface improves the electronic conductivity of LiFePO4 particles and also hinders the particle growth during the sintering in an inert atmosphere. In common routes, the carbon-coating process was carried out after the preparation of LiFePO4 core. By big contrast, some researchers took a new method to prepare LiFePO4@C composite cathode materials from FePO4@PAn (polyaniline) composite [12,13]. In their work, Fe3+ acted as the oxidant for aniline and as a raw material, but the process required additional carbon, which would result in the difficulty to control the amount of carbon in LiFePO4@C composites. We propose a route that avoids this issue and can precisely control the resulting carbon content. Adequate hydrogen peroxide was used as oxidant instead of Fe3+ for the complete polymerization of aniline in this work. Afterwards PAn played a significant role as the only carbon source in the preparation of LiFePO4@C composites. After determining the addition of aniline, well-controlled amount of carbon could be gotten in LiFePO4@C
⁎ Corresponding author. Tel./fax: + 86 791 83969247. E-mail address: [email protected] (N. Gu). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.003
composites. Since PAn is conductive for Li+ transport, the FePO4 core phase can be adequately lithiated before PAn shell is carbonized. In addition, the choice of PAn as the precursor of carbon-shell is also a key factor influencing the quality of the composite cathode material since the benzene ring moiety in PAn has trends in giving rise to the sp2 coordinated carbon coating [14], which is more conductive for electron and more favorable for lithium ion diffusion compared with the sp3 coordinated carbons [15]. Here we will show the electrochemical performance of this type of cathode materials. 2. Experimental The LiFePO4@C cathode materials were synthesized from FePO4@PAn composites. Firstly, FePO4@PAn precursors were prepared by in situ polymerization of PAn and precipitation of FePO4 in one pot. Typically, 100 mL of 0.3 mol L − 1 Fe(NO3)3·9H2O aqueous solution was slowly added to a flask containing 100 mL of 0.3 mol L − 1 NH4H2PO4 aqueous solution and some aniline while stirring it mechanically. 50 mL of aqueous solution containing 5 mL 30 wt.% H2O2 was added into this mixture to complete the oxidization polymerization of aniline. The pH of the solution was adjusted to 1.78 by dropping ammonia [16]. The reaction lasted for over 24 h at room temperature and the resultant precipitant was filtered and washed several times with distilled water and acetone, and dried at 50 °C in vacuum. Secondly, the obtained FePO4@PAn composite was mixed with an equimolar amount of LiOH·H2O and the mixture was ground. At last, each precursor mixture was pre-heated at 400 °C for 5 h and finally calcined at 700 °C for 12 h under argon to obtain the LiFePO4@C composites, marked as sample LFP@C-1, LFP@C-2, and LFP@C-3, corresponding to the sample prepared with 0.25, 0.50, and 1.00 mL aniline, respectively. Fourier transform infrared (FTIR) spectrum was recorded with a Nicolet 60XR-IR spectrophotometer (America). X-ray diffraction (XRD) measurement was carried out on a Bruker D8 Focus X-ray diffractometer
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(Germany) using Cu Kα radiation (λ =1.5406 Å) filtered by graphite monochromator. The particle microstructure and surface morphology were observed by scanning electron microscope (SEM, FEI XL30 ESEMFEG) and transmission electron microscope (TEM, FEI TECNAI F20). The amount of carbon in the final sample was determined by the HCS-140 elemental analyzer (China). The electrochemical properties of the cathode electrodes were characterized in 2025 coin-type cells with lithium metal foil as the counter electrode. The cathode electrodes were fabricated with the active material (AM), super P carbon black (SP) and polyvinylidene fluoride (PVDF) in the weight ratio of 85:10:5. Charge and discharge performances were carried out on a battery test system (LAND CT2001A, China). Electrochemical impedance spectra (EIS) were tested on electrochemical workstation (Zahner Zennium, Germany).
3. Results and discussion Fig. 1a gives the FTIR spectrum of the FePO4@PAn composite. The peak at 1059 cm − 1 corresponds to PO43− bond. The peaks at 1428, 1512, and 1625 cm − 1 correspond to benzene rings stretch in PAn. Fig. 1b shows powder XRD patterns of the obtained LiFePO4@C composites. All main diffraction peaks were indexed in orthorhombic olivine LiFePO4 (JCPDS Card No. 40–1499). This indicates that a crystallized pure LiFePO4 can be obtained by heating FePO4@PAn in Ar since PAn-derived carbon and hydrogen can reduce Fe(III) in the
duration of calcinations. However, the Fe2P2O7 impurity was observed in the XRD pattern of sample LFP@C-3, which may be ascribed to excessive thickness of PAn preventing the facile lithiation of FePO4. The finally residual carbon content of sample LFP@C-1, LFP@C-2, and LFP@C-3 is of about 0.02%, 3.86%, and 10.13% in weight, respectively. The SEM images of the LiFePO4@C composites are illustrated in a, b, and c of Fig. 2. From the SEM images it can be clearly seen that the average diameter of sample LFP@C-1 primary particles is larger than the others. The reason is probably that almost all the generated carbon of FePO4@PAn composite prepared with the least aniline only reduces Fe(III), but cannot hinder the particle growth. Sample LFP@C2 has the primary particle size of 50–100 nm from the TEM image in Fig. 2d. When zoomed in further as shown in Fig. 2e to observe the surface morphology, it shows that the highly crystalline LiFePO4 particle is surrounded by a less-disordered ~5 nm thick surface carbon layer. Fig. 3a shows typical charge/discharge cures of the LiFePO4@C composites at 0.1 C rate at room temperature, where 1 C corresponds to 170 mA g − 1. The results show that sample LFP@C-1, LFP@C-2, and LFP@C-3 delivered a discharge capacity of 63.4, 161.1, and 121.6 mAh g − 1, respectively. Due to its least residual carbon leading to the lowest electron conductivity, sample LFP@C-1 had the worst capacity performance. That sample LFP@C-3 delivered lower discharge capacity than sample LFP@C-2 is likely because: a) excess PAn hindered the solid reaction so that the Fe2P2O7 impurity was generated and lowered the proportion of active material LiFePO4, and b) overabundant carbon also reduced the LiFePO4 proportion in the LiFePO4@C composite. To investigate rate performances and cycling stabilities, sample LFP@C-2 was tested for up to hundreds of cycles at different rates shown in Fig. 3b and c. The results from Fig. 3b show that at rates of 1, 2, 5, and 10 C, sample LFP@C-2 delivered a discharge capacity of 152.1, 147.5, 128.7, and 109.6 mAh g − 1, respectively. Obviously, the discharge capacity decreased and polarization increased with the C rate increasing. However, it still exhibited high capacities and had flat charge/discharge voltage plateaus even at high rates of 5 and 10 C. From Fig. 3c, the discharge capacity retention of sample LFP@C-2 after 300 cycles was 98.5%, 97.4%, 98%, and 95.5%, at rates of 1, 2, 5, and 10 C, respectively, implying that it possessed excellent cycling stability. Especially, the capacity retentions after 1000 cycles reach 91.6% (5 C) and 87.1% (10 C). To study the electrode reaction kinetic, EIS spectra of sample LFP@C-1, LFP@C-2, and LFP@C-3 were tested under fully discharge state. Each Nyquist plot includes a compressed semicircle in the high-middle frequency region and an inclined line in the low frequency region, as is represented in Fig. 3d. The ohmic resistance (Rs), namely the intercept with the real axis (Zre) in the high frequency region, represents the resistance of the electrolyte and electrode. The intercept with Zre in the middle frequency region is approximately the charge transfer resistance (Rct), related to Li+ interfacial transfer between the surface film and LiFePO4 particle interface. The line in low frequency region is assigned to the Warburg impedance (Zw), associated with Li+ diffusion within the solid-state bulk of LiFePO4. The Rct value of sample LFP@C-1, LFP@C-2, and LFP@C-3 is about 384, 30, and 156 Ohm, respectively, indicating that less final carbon would result in heavier polarization in sample LFP@C-1 and more residual carbon coating would block the Li+ transport between inner LiFePO4 and electrolyte [17] in sample LFP@C-3. 4. Conclusions
Fig. 1. (a) FTIR spectrum of the FePO4@PAn composite prepared with 0.50 mL aniline; (b) powder XRD patterns of the LiFePO4@C composites using Cu Kα radiation.
In sum, the LiFePO4@C composites were synthesized by heating the precursors of FePO4@PAn with LiOH·H2O under Ar flow in this work. The results show that the optimized LiFePO4@C composite with 3.86 wt.% carbon content was a crystalline pure phase and possessed 50–100 nm primary particles and about 5 nm thick surface carbon coating layer. At rates of 0.1, 1, 2, 5, and 10 C, it delivered a discharge capacity of 161.1, 152.1, 147.5, 128.7, and 109.6 mAh g − 1,
N. Gu et al. / Materials Letters 81 (2012) 115–118
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Fig. 2. SEM images of sample LFP@C-1 (a), LFP@C-2 (b), and LFP@C-3 (c); TEM images (d, e) of sample LFP@C-2.
respectively, with flat voltage plateaus and excellent cycling stability for charge/discharge behavior. These demonstrate that the as-prepared LiFePO4@C composite is a superior cathode material for power lithium ion batteries.
Acknowledgements This work was financially supported by Jiangxi Provincial Department of Science and Technology (grant no. 2010BGB01001), and
Fig. 3. (a) Typical charge/discharge curves of the LiFePO4/C composites at 0.1 C rate between 2.0 V and 4.2 V vs. Li/Li+; (b) Typical charge/discharge curves and (c) cyclic performance of sample LFP@C-2 at 1, 2, 5, and 10 C rates; (d) EIS of the LiFePO4/C composites with the frequency range of 10− 1–105 Hz and the insert equivalent circuit of impedance spectra.
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Education Department of Jiangxi Province (grant nos. GJJ11278 and GJJ12042). References [1] Padhi AK, Nanjundaswamy KS, Goodenough JB. J Electrochem Soc 1997;144: 1188–94. [2] Broussely M. Electric and Hybrid Vehicles. Amsterdam: Elsevier; 2010. p. 305–45. [3] Dominko R, Gaberscek M, Drofenik J, Bele M, Pejovnik S, Jamnik J. J Power Sources 2003;119–121:770–3. [4] Doeff M, Wilcox J, Yu R, Aumentado A, Marcinek M, Kostecki R. J Solid State Electrochem 2008;12:995–1001. [5] Cheng F, Wan W, Tan Z, Huang Y, Zhou H, Chen J, et al. Electrochim Acta 2011;56: 2999–3005. [6] Wang D, Li H, Shi S, Huang X, Chen L. Electrochim Acta 2005;50:2955–8.
[7] Wang GX, Needham S, Yao J, Wang JZ, Liu RS, Liu HK. J Power Sources 2006;159: 282–6. [8] Hua N, Wang C, Kang X, Wumair T, Han Y. J Alloys Compd 2010;503:204–8. [9] Saravanan K, Reddy MV, Balaya P, Gong H, Chowdari BVR. Vittal JJ. J Mater Chem 2009;19:605. [10] Wang B, Qiu Y, Ni S. Solid State Ionics 2007;178:843–7. [11] Lim J, Mathew V, Kim K, Moon J, Kim J. J Electrochem Soc 2011;158:A736–40. [12] Wang Y, Wang Y, Hosono E, Wang K, Zhou H. Angew Chem Int Ed 2008;47: 7461–5. [13] Yin X, Huang K, Liu S, Wang H, Wang H. J Power Sources 2010;195:4308–12. [14] Nien Y-H, Carey JR, Chen J-S. J Power Sources 2009;193:822–7. [15] Zhang W-J. J Power Sources 2011;196:2962–70. [16] Kandori K, Kuwae T, Ishikawa T. J Colloid Interface Sci 2006;300:225–31. [17] Dominko R, Bele M, Gaberscek M, Remskar M, Hanzel D, Pejovnik S, et al. J Electrochem Soc 2005;152:A607–10.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
LiFePO4@C cathode materials synthesized from FePO4@PAn composites Ningyu Gu ⁎, Xinghua He, Yang Li Department of Chemistry, Nanchang University, Nanchang 330031, China
a r t i c l e
i n f o
Article history: Received 6 March 2012 Accepted 1 May 2012 Available online 9 May 2012 Keywords: Lithium ion batteries Cathode material Lithium iron phosphate In situ polymerization
a b s t r a c t Carbon coated LiFePO4 cathode materials were successfully synthesized by heating the precursors of FePO4@PAn (polyaniline) and equimolar LiOH·H2O under Ar flow. We demonstrated a method that could precisely control the carbon shell in a single reaction step. The precursors of FePO4@PAn were prepared by in situ polymerization of aniline and precipitation of FePO4 in one pot in the presence of hydrogen peroxide as oxidizer. In the obtained core–shell cathode composite, the LiFePO4 core phase was crystalline while the carbon shell had the thickness of several nanometers. The optimized cathode material exhibited satisfactory rate and cycle performance, which virtues are quite suitable for power lithium ion batteries. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As one of the most promising cathode materials for power lithium ion batteries, lithium iron phosphate (LiFePO4) with the olivine structure has attracted much attention due to its theoretical capacity of 170 mAh g− 1, certain safety, good cycling stability, environmental benignity, and low cost of the raw materials since its discovery in 1997 [1]. However, the poor intrinsic electric conductivity and slow diffusion of Li+ make it difficult to obtain good performance at high power C rates for applications in electric vehicles (EVs) and hybrid electric vehicles (HEVs) [2]. To enhance the rate performance of LiFePO4, many efforts have been made, including carbon coating [3–5], metal-ion doping [6–8], and reducing the particle size [9–11], etc. Carbon coating is a quite effective technique, for carbon deposited on the surface improves the electronic conductivity of LiFePO4 particles and also hinders the particle growth during the sintering in an inert atmosphere. In common routes, the carbon-coating process was carried out after the preparation of LiFePO4 core. By big contrast, some researchers took a new method to prepare LiFePO4@C composite cathode materials from FePO4@PAn (polyaniline) composite [12,13]. In their work, Fe3+ acted as the oxidant for aniline and as a raw material, but the process required additional carbon, which would result in the difficulty to control the amount of carbon in LiFePO4@C composites. We propose a route that avoids this issue and can precisely control the resulting carbon content. Adequate hydrogen peroxide was used as oxidant instead of Fe3+ for the complete polymerization of aniline in this work. Afterwards PAn played a significant role as the only carbon source in the preparation of LiFePO4@C composites. After determining the addition of aniline, well-controlled amount of carbon could be gotten in LiFePO4@C
⁎ Corresponding author. Tel./fax: + 86 791 83969247. E-mail address: [email protected] (N. Gu). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.003
composites. Since PAn is conductive for Li+ transport, the FePO4 core phase can be adequately lithiated before PAn shell is carbonized. In addition, the choice of PAn as the precursor of carbon-shell is also a key factor influencing the quality of the composite cathode material since the benzene ring moiety in PAn has trends in giving rise to the sp2 coordinated carbon coating [14], which is more conductive for electron and more favorable for lithium ion diffusion compared with the sp3 coordinated carbons [15]. Here we will show the electrochemical performance of this type of cathode materials. 2. Experimental The LiFePO4@C cathode materials were synthesized from FePO4@PAn composites. Firstly, FePO4@PAn precursors were prepared by in situ polymerization of PAn and precipitation of FePO4 in one pot. Typically, 100 mL of 0.3 mol L − 1 Fe(NO3)3·9H2O aqueous solution was slowly added to a flask containing 100 mL of 0.3 mol L − 1 NH4H2PO4 aqueous solution and some aniline while stirring it mechanically. 50 mL of aqueous solution containing 5 mL 30 wt.% H2O2 was added into this mixture to complete the oxidization polymerization of aniline. The pH of the solution was adjusted to 1.78 by dropping ammonia [16]. The reaction lasted for over 24 h at room temperature and the resultant precipitant was filtered and washed several times with distilled water and acetone, and dried at 50 °C in vacuum. Secondly, the obtained FePO4@PAn composite was mixed with an equimolar amount of LiOH·H2O and the mixture was ground. At last, each precursor mixture was pre-heated at 400 °C for 5 h and finally calcined at 700 °C for 12 h under argon to obtain the LiFePO4@C composites, marked as sample LFP@C-1, LFP@C-2, and LFP@C-3, corresponding to the sample prepared with 0.25, 0.50, and 1.00 mL aniline, respectively. Fourier transform infrared (FTIR) spectrum was recorded with a Nicolet 60XR-IR spectrophotometer (America). X-ray diffraction (XRD) measurement was carried out on a Bruker D8 Focus X-ray diffractometer
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(Germany) using Cu Kα radiation (λ =1.5406 Å) filtered by graphite monochromator. The particle microstructure and surface morphology were observed by scanning electron microscope (SEM, FEI XL30 ESEMFEG) and transmission electron microscope (TEM, FEI TECNAI F20). The amount of carbon in the final sample was determined by the HCS-140 elemental analyzer (China). The electrochemical properties of the cathode electrodes were characterized in 2025 coin-type cells with lithium metal foil as the counter electrode. The cathode electrodes were fabricated with the active material (AM), super P carbon black (SP) and polyvinylidene fluoride (PVDF) in the weight ratio of 85:10:5. Charge and discharge performances were carried out on a battery test system (LAND CT2001A, China). Electrochemical impedance spectra (EIS) were tested on electrochemical workstation (Zahner Zennium, Germany).
3. Results and discussion Fig. 1a gives the FTIR spectrum of the FePO4@PAn composite. The peak at 1059 cm − 1 corresponds to PO43− bond. The peaks at 1428, 1512, and 1625 cm − 1 correspond to benzene rings stretch in PAn. Fig. 1b shows powder XRD patterns of the obtained LiFePO4@C composites. All main diffraction peaks were indexed in orthorhombic olivine LiFePO4 (JCPDS Card No. 40–1499). This indicates that a crystallized pure LiFePO4 can be obtained by heating FePO4@PAn in Ar since PAn-derived carbon and hydrogen can reduce Fe(III) in the
duration of calcinations. However, the Fe2P2O7 impurity was observed in the XRD pattern of sample LFP@C-3, which may be ascribed to excessive thickness of PAn preventing the facile lithiation of FePO4. The finally residual carbon content of sample LFP@C-1, LFP@C-2, and LFP@C-3 is of about 0.02%, 3.86%, and 10.13% in weight, respectively. The SEM images of the LiFePO4@C composites are illustrated in a, b, and c of Fig. 2. From the SEM images it can be clearly seen that the average diameter of sample LFP@C-1 primary particles is larger than the others. The reason is probably that almost all the generated carbon of FePO4@PAn composite prepared with the least aniline only reduces Fe(III), but cannot hinder the particle growth. Sample LFP@C2 has the primary particle size of 50–100 nm from the TEM image in Fig. 2d. When zoomed in further as shown in Fig. 2e to observe the surface morphology, it shows that the highly crystalline LiFePO4 particle is surrounded by a less-disordered ~5 nm thick surface carbon layer. Fig. 3a shows typical charge/discharge cures of the LiFePO4@C composites at 0.1 C rate at room temperature, where 1 C corresponds to 170 mA g − 1. The results show that sample LFP@C-1, LFP@C-2, and LFP@C-3 delivered a discharge capacity of 63.4, 161.1, and 121.6 mAh g − 1, respectively. Due to its least residual carbon leading to the lowest electron conductivity, sample LFP@C-1 had the worst capacity performance. That sample LFP@C-3 delivered lower discharge capacity than sample LFP@C-2 is likely because: a) excess PAn hindered the solid reaction so that the Fe2P2O7 impurity was generated and lowered the proportion of active material LiFePO4, and b) overabundant carbon also reduced the LiFePO4 proportion in the LiFePO4@C composite. To investigate rate performances and cycling stabilities, sample LFP@C-2 was tested for up to hundreds of cycles at different rates shown in Fig. 3b and c. The results from Fig. 3b show that at rates of 1, 2, 5, and 10 C, sample LFP@C-2 delivered a discharge capacity of 152.1, 147.5, 128.7, and 109.6 mAh g − 1, respectively. Obviously, the discharge capacity decreased and polarization increased with the C rate increasing. However, it still exhibited high capacities and had flat charge/discharge voltage plateaus even at high rates of 5 and 10 C. From Fig. 3c, the discharge capacity retention of sample LFP@C-2 after 300 cycles was 98.5%, 97.4%, 98%, and 95.5%, at rates of 1, 2, 5, and 10 C, respectively, implying that it possessed excellent cycling stability. Especially, the capacity retentions after 1000 cycles reach 91.6% (5 C) and 87.1% (10 C). To study the electrode reaction kinetic, EIS spectra of sample LFP@C-1, LFP@C-2, and LFP@C-3 were tested under fully discharge state. Each Nyquist plot includes a compressed semicircle in the high-middle frequency region and an inclined line in the low frequency region, as is represented in Fig. 3d. The ohmic resistance (Rs), namely the intercept with the real axis (Zre) in the high frequency region, represents the resistance of the electrolyte and electrode. The intercept with Zre in the middle frequency region is approximately the charge transfer resistance (Rct), related to Li+ interfacial transfer between the surface film and LiFePO4 particle interface. The line in low frequency region is assigned to the Warburg impedance (Zw), associated with Li+ diffusion within the solid-state bulk of LiFePO4. The Rct value of sample LFP@C-1, LFP@C-2, and LFP@C-3 is about 384, 30, and 156 Ohm, respectively, indicating that less final carbon would result in heavier polarization in sample LFP@C-1 and more residual carbon coating would block the Li+ transport between inner LiFePO4 and electrolyte [17] in sample LFP@C-3. 4. Conclusions
Fig. 1. (a) FTIR spectrum of the FePO4@PAn composite prepared with 0.50 mL aniline; (b) powder XRD patterns of the LiFePO4@C composites using Cu Kα radiation.
In sum, the LiFePO4@C composites were synthesized by heating the precursors of FePO4@PAn with LiOH·H2O under Ar flow in this work. The results show that the optimized LiFePO4@C composite with 3.86 wt.% carbon content was a crystalline pure phase and possessed 50–100 nm primary particles and about 5 nm thick surface carbon coating layer. At rates of 0.1, 1, 2, 5, and 10 C, it delivered a discharge capacity of 161.1, 152.1, 147.5, 128.7, and 109.6 mAh g − 1,
N. Gu et al. / Materials Letters 81 (2012) 115–118
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Fig. 2. SEM images of sample LFP@C-1 (a), LFP@C-2 (b), and LFP@C-3 (c); TEM images (d, e) of sample LFP@C-2.
respectively, with flat voltage plateaus and excellent cycling stability for charge/discharge behavior. These demonstrate that the as-prepared LiFePO4@C composite is a superior cathode material for power lithium ion batteries.
Acknowledgements This work was financially supported by Jiangxi Provincial Department of Science and Technology (grant no. 2010BGB01001), and
Fig. 3. (a) Typical charge/discharge curves of the LiFePO4/C composites at 0.1 C rate between 2.0 V and 4.2 V vs. Li/Li+; (b) Typical charge/discharge curves and (c) cyclic performance of sample LFP@C-2 at 1, 2, 5, and 10 C rates; (d) EIS of the LiFePO4/C composites with the frequency range of 10− 1–105 Hz and the insert equivalent circuit of impedance spectra.
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Education Department of Jiangxi Province (grant nos. GJJ11278 and GJJ12042). References [1] Padhi AK, Nanjundaswamy KS, Goodenough JB. J Electrochem Soc 1997;144: 1188–94. [2] Broussely M. Electric and Hybrid Vehicles. Amsterdam: Elsevier; 2010. p. 305–45. [3] Dominko R, Gaberscek M, Drofenik J, Bele M, Pejovnik S, Jamnik J. J Power Sources 2003;119–121:770–3. [4] Doeff M, Wilcox J, Yu R, Aumentado A, Marcinek M, Kostecki R. J Solid State Electrochem 2008;12:995–1001. [5] Cheng F, Wan W, Tan Z, Huang Y, Zhou H, Chen J, et al. Electrochim Acta 2011;56: 2999–3005. [6] Wang D, Li H, Shi S, Huang X, Chen L. Electrochim Acta 2005;50:2955–8.
[7] Wang GX, Needham S, Yao J, Wang JZ, Liu RS, Liu HK. J Power Sources 2006;159: 282–6. [8] Hua N, Wang C, Kang X, Wumair T, Han Y. J Alloys Compd 2010;503:204–8. [9] Saravanan K, Reddy MV, Balaya P, Gong H, Chowdari BVR. Vittal JJ. J Mater Chem 2009;19:605. [10] Wang B, Qiu Y, Ni S. Solid State Ionics 2007;178:843–7. [11] Lim J, Mathew V, Kim K, Moon J, Kim J. J Electrochem Soc 2011;158:A736–40. [12] Wang Y, Wang Y, Hosono E, Wang K, Zhou H. Angew Chem Int Ed 2008;47: 7461–5. [13] Yin X, Huang K, Liu S, Wang H, Wang H. J Power Sources 2010;195:4308–12. [14] Nien Y-H, Carey JR, Chen J-S. J Power Sources 2009;193:822–7. [15] Zhang W-J. J Power Sources 2011;196:2962–70. [16] Kandori K, Kuwae T, Ishikawa T. J Colloid Interface Sci 2006;300:225–31. [17] Dominko R, Bele M, Gaberscek M, Remskar M, Hanzel D, Pejovnik S, et al. J Electrochem Soc 2005;152:A607–10.