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A modified mechanical activation synthesis for carbon-coated LiFePO4 cathode in lithium batteries
Materials Letters 61 (2007) 3822 – 3825 www.elsevier.com/locate/matlet
A modified mechanical activation synthesis for carbon-coated LiFePO4 cathode in lithium batteries Jae-Kwang Kim a , Jae-Won Choi a , Gouri Cheruvally a , Jong-Uk Kim a , Jou-Hyeon Ahn a,⁎, Gyu-Bong Cho b , Ki-Won Kim b , Hyo-Jun Ahn b a
b
Department of Chemical and Biological Engineering, ITRC for Energy Storage and Conversion, Gyeongsang National University, 900, Gajwa-dong, Jinju 660-701, Republic of Korea School of Advanced Materials Science and Engineering, ITRC for Energy Storage and Conversion, Gyeongsang National University, 900, Gajwa-dong, Jinju 660-701, Republic of Korea Received 31 October 2006; accepted 19 December 2006 Available online 23 December 2006
Abstract An efficient synthesis based on mechanical activation (MA) was developed for carbon-coated lithium iron phosphate (LiFePO4/C). The conventional MA process was modified by introducing two initial steps of slurry phase blending of the ingredients and solvent removal by rotary evaporation, so as to get an intimate mixing and homogenous dispersion of conductive carbon in the sample. Phase-pure, nanometer-sized particles of the active material covered with a porous, nanometer-sized web of carbon were obtained. LiFePO4/C exhibited remarkably good electrochemical properties when evaluated as cathodes in room temperature lithium cells. An initial discharge capacity of 166 mAh/g (corresponding to 97.6% of theoretical capacity) was achieved at 0.1 C-rate. A very stable cycle performance was also realized; good capacity retention up to 100 cycles was achieved at different current densities. © 2006 Elsevier B.V. All rights reserved. Keywords: Electronic materials; Nanomaterials; Cathode materials; Lithium iron phosphate; Lithium batteries
1. Introduction Lithium iron phosphate (LiFePO4) is being actively investigated as a promising cathode material for lithium batteries. In addition to providing a reasonably high theoretical capacity of 170 mAh/g, LiFePO4 offers advantages such as a flat discharge voltage at 3.4 V vs lithium, a wide safety margin of usage for organic electrolytes, high thermal and chemical stability, low material cost, low toxicity and improved safety. However, LiFePO4 suffers from the limitations of poor electronic conductivity and slow lithium ion diffusion, and hence performs unsatisfactorily at lower temperatures and/or higher current densities [1,2]. An easy and successful approach to overcome the insulating nature has been to coat the active particles with conductive carbon, either by its incorporation as a powder initially [3,4] or by in-situ generation by pyrolysis of organic/ ⁎ Corresponding author. Tel.: +82 55 751 5388; fax: +82 55 753 1806. E-mail address: [email protected] (J.-H. Ahn). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.12.038
polymer compounds during the synthesis [2,5–13]. The attempts to overcome the limitation of lithium ion diffusion focus mainly on synthesizing small particles with high purity [14]. Different synthesis techniques including solid-state reactions [1,12,14] and solution methods [7,9,15] have been developed for preparing LiFePO4. The amorphous to crystalline phase change of LiFePO4 occurs at high temperatures (≥ 450 °C) [6,15]. Conventional solid-state reactions are adopted by many researchers since phase-pure material is obtained with a proper control of firing conditions. However, the lengthy and complex procedures requiring repeated grindings and calcinations usually lead to the formation of larger particles with lower performance. In mechanical activation (MA) method, a high energy ball milling step is introduced before thermal treatment which results in forming an intimate mixture of the reactants and effectively reduces the thermal treatment time and temperature, thus arresting the undesirable crystal growth. Some recent studies have highlighted the effectiveness of MA process for the synthesis of small and phase-pure particles of LiFePO4 [3,10,11].
J.-K. Kim et al. / Materials Letters 61 (2007) 3822–3825
In this paper, we report the preparation of cathode active carbon-coated LiFePO4 (LiFePO4/C) by a modified MA process. The electrochemical properties of LiFePO4/C in lithium batteries would be decided considerably by the nature as well as the effectiveness of the conductive carbon coating around the particles. For attaining an effective coating, it would be beneficial if a uniform dispersion of carbon powder is obtained in the precursor sample before subjecting to ball milling. With this in view, the conventional MA process was modified and the material so synthesized had uniform carbon coating and good electrochemical properties. 2. Experimental LiFePO4/C was synthesized from Li2CO3, FeC2O4·2H2O, and NH4H2PO4 (all chemicals of 99% purity from Aldrich) taken in stoichiometric quantities along with 7.8 wt.% of acetylene black powder (Alfa). The process steps were: (1) initial mixing by magnetic stirring of all ingredients together as a slurry in 60 wt.% of triply distilled water at room temperature for 7 h, (2) rotary evaporator drying at 70 °C for 2 h at 60 rpm to yield a solid powder, (3) high energy ball milling in a hardened steel vial (zirconia balls, ball-to-powder weight ratio 10:3, SPEX mill, 1000 rpm, room temperature, 15 h, argon atmosphere), (4) mechanical pressing to make pellets, and (5) thermal treatment at 600 °C for 10 h in nitrogen atmosphere. The ball milling and thermal treatment conditions were adopted based on our earlier study on the optimization of MA processing parameters for the synthesis of LiFePO4 [16]. The crystallographic structure was determined by X-ray diffraction (XRD: D8 Advance, Bruker AXS) using CuKα radiation. Scanning electron microscopy (SEM) imaging and energy dispersive X-ray (EDX) mapping were done using SEM–EDX (Philips XL30 S FEG). The nature and thickness of the carbon coating was observed with high resolution transmission electron microscopy (HR-TEM) (JEM-3010, JEOL). The specific surface area was computed from N2 sorption data (ASAP 2010 Analyzer) using the Brunauer– Emmett–Teller (BET) method. The chemical composition was determined by inductively coupled plasma (ICP) analysis (Atomscan 25, Optima 4300DV) and carbon content by elemental analyzer (CHNS-932, LECO). The cathode was prepared from the LiFePO4/C powder, carbon black and poly(vinylidene fluoride) (Aldrich) binder taken in the ratio 70:20:10 by weight. The components were mixed as viscous slurry in N-methyl pyrrolidone solvent and cast uniformly on aluminum foil and dried at 95 °C under vacuum for 12 h. The film so obtained was cut into circular discs of the area 0.95 cm2 and mass ∼ 3.0 mg for use as the cathode. Electrochemical coin cells were assembled with lithium metal as anode, LiFePO4/C as cathode and Celgard@ 2200 separator with 1 M LiPF6 in EC:DMC (1:1 vol) as electrolyte. Cyclic voltammetry (CV) was done at a scan rate of 0.1 mV/s between 2.0 and 4.5 V. Electrochemical tests were carried out using an automatic galvanostatic charge–discharge unit, WBCS3000 battery cycler, between 2.0 and 4.6 V at room temperature.
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3. Results and discussion The molar ratio of Li:Fe:P in the sample of LiFePO4/C agreed with the theoretical ratio of 1:1:1. The carbon content was analyzed to be 14.0 wt.%, which corresponded well with the theoretical value of 14.1% based on the added carbon. These analytical results indicate that the molar composition was preserved during processing and no loss of carbon by way of burning out occurred during the firing step. Fig. 1 shows the XRD pattern of the sample, identified to be orthorhombic, ordered olivine structure belonging to Pnma space group (all peaks are indexed; standard spectrum also shown). Peaks corresponding to significant amounts of impurities such as Fe3+ compounds are absent in the spectrum. As has been observed by other researchers also, carbon in the sample provides an efficient reductive atmosphere and effectively converts traces of Fe3+ (that might be formed during firing) back to Fe2+, and this results in the formation of a single phase material [10,13]. Fig. 2(a) presents the SEM image of the material showing nearspherical particles with a uniform morphology. The particle size range is 65–90 nm, with an average particle size of 80 nm and specific surface area of 18.3 m2/g. Adopting the conventional MA process, LiFePO4/C particles of the size 0.1–2 μm and specific surface area of 2.1–7.6 m2/g has been reported by Kwon et al. [3]. Thus, it is seen that the modifications introduced in this study result in attaining much smaller particles with a narrow size range and higher surface area. This indicates that compared to the conventional process, the modified process leads to (1) an intimate blending of the ingredients resulting in a more uniform particle size, and (2) a more homogenous distribution of carbon black throughout the sample that limits the particle growth effectively. SEM also reveals that the particles have a rough surface without sharp edges and/or clean cleavage surfaces; an added indication of the efficient carbon coating achieved. The homogeneity of chemical composition as determined by SEM–EDX mapping is presented in Fig. 2(b–e). The elemental mapping images obtained for C, Fe and P match with that of the corresponding SEM image. Since carbon also follows the same uniform distribution and image contrast as that of the other two elements, it is concluded that LiFePO4/C contains carbon in a uniformly distributed manner. The nature of surface coating of carbon, as analyzed by TEM, is shown in Fig. 3. Knowing the specific surface area of the particles (18.3 m2/g), carbon content (14.0 wt.%), and the true density of carbon (1.8–2.1 g/cm3), the approximate thickness of the carbon coating was calculated theoretically to be 4 nm [8]. TEM analysis shows a nano-
Fig. 1. XRD spectra of (a) standard Pnma orthorhombic LiFePO4. (b) LiFePO4/ C prepared by the modified MA process.
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Fig. 2. (a) SEM of LiFePO4/C. (b)–(e) SEM–EDX mapping: (b) SEM image, and (c–e) corresponding EDX mapping of C, Fe and P respectively.
sized web of amorphous carbon, approximately 5 nm in thickness, surrounding the LiFePO4 particles. The coating is seen to be highly porous and uniform, existing across the entire surface of particles, enveloping and connecting them, thus minimizing inter-particle agglomeration. By adopting a liquid-based powder preparation method, Chung et al. synthesized LiFePO4/C with a nano-web coating of carbon, which resulted in improved electrochemical properties for the material [7]. In the present study also, the efficient carbon coating formed could lead to improved charge transfer kinetics of the material and result in higher reversible capacity, as detailed below. Charge–discharge performance of lithium cell with LiFePO4/C cathode, tested at 0.1 C-rate is shown in Fig. 4(a). The charge and
Fig. 3. TEM image of LiFePO4/C.
Fig. 4. (a) Initial charge–discharge curves at 0.1 C. Inset: CV at a scan rate of 0.1 mV/s, 2.0–4.5 V. (b) Cycle performance at different C-rates.
discharge current of a battery is measured in C-rate. Thus, for the cell with LiFePO4 as the cathode active material, having a theoretical specific capacity of 170 mAh/g, the charge–discharge at 1 C-rate corresponds to a current density of 170 mA over a period of 1 h for 1 g of active material. The cell reaction voltage at ∼3.4 V, agrees with the observations in CV (oxidation and reduction peaks at 3.6 and 3.3 V respectively), shown in the inset of Fig. 4(a). The characteristic of extremely flat cell reaction voltage of LiFePO4 is observed here also [11,12]. The difference between the charging and discharging voltage is very low at 0.1 C (0.06 V), indicating good kinetics of redox reaction at comparatively low discharge currents. A discharge capacity of 166 mAh/g (97.6% of theoretical capacity) is obtained at 0.1 C, demonstrating that almost a full capacity can be achieved in the initial cycle. The performance during the first cycle is retained well during the subsequent cycles also. The cycle performance of the cell was studied up to 100 cycles at different C-rates and the results are shown in Fig. 4(b). LiFePO4/C shows excellent long-term cycling property. The cell retains 92% of its initial discharge capacity after 100 cycles at 0.1 Crate. With an increase in C-rate, polarization is enhanced due to slow lithium ion diffusion at the solid, two-phase FePO4/LiFePO4 interface and the resulting limitation of the material to cope up with the fast reaction kinetics at high C-rates [1]. The initial discharge capacities at 1, 2 and 3 C-rates are 142, 132 and 113 mAh/g, respectively. Thus, even at the high current densities of 2 C and 3 C, the cell performs respectively at 77.6% and 66.5% of theoretical capacity. The discharge capacities are higher than those obtained using LiFePO4/C samples prepared by the conventional MA process in (1) our laboratory (152 and 129 mAh/g at 0.1 and 1.0 C respectively) and (2) those reported by
J.-K. Kim et al. / Materials Letters 61 (2007) 3822–3825
Kwon et al. (156 and 110 mAh/g at 0.05 and 1 C respectively) [3]. The charge–discharge efficiencies are also high (N 96%) at all C-rates. The enhanced performance of the material processed by the modified MA process is attributed to the combined effect of achieving small, phasepure particles and an efficient carbon coating.
4. Conclusion Nanometer-sized, phase-pure particles of LiFePO4/C cathode active material were synthesized by a modified MA process. The intimate mixing of ingredients together with the conductive carbon carried out in the modified process resulted in achieving particles of uniform morphology with an efficient nano-web of carbon covering the particles. The material had remarkably good electrochemical properties when used as cathode in lithium cells, exhibiting high discharge capacities and stable cycle performance. The results obtained in this study emphasize the importance of achieving an intimate mixing of the components including the conductive carbon powder prior to the mechanical mixing and thermal treatment of the material. Acknowledgements This research was supported by the MIC (Ministry of Information and Communication), Korea, under the ITRC (Information Technology Research Center) support program. Gouri Cheruvally is thankful to the KOFST for the award of Brain Pool Fellowships, and J.-K. Kim and J.-W. Choi acknowledge the partial support by the Post Brain Korea 21 Project in 2006.
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References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. [2] T. Nakamura, Y. Miwa, M. Tabuchi, Y. Yamada, J. Electrochem. Soc. 153 (2006) A1108. [3] S.J. Kwon, C.W. Kim, W.T. Jeong, K.S. Lee, J. Power Sources 137 (2004) 93. [4] H.S. Kim, B.W. Cho, W.I. Cho, J. Power Sources 132 (2004) 235. [5] H. Huang, S.C. Yin, L.F. Nazar, Electrochem. Solid-State Lett. 4 (2001) A170. [6] S. Franger, F.L. Cras, C. Bourbon, H. Rouault, J. Power Sources 119–121 (2003) 252. [7] H.T. Chung, S.K. Jang, H.W. Ryu, K.B. Shim, Solid State Commun. 131 (2004) 549. [8] R. Dominko, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, S. Pejovnik, J. Jamnik, J. Electrochem. Soc. 152 (2005) A607. [9] Y. Hu, M.M. Doeff, R. Kostecki, R. Finones, J. Electrochem. Soc. 151 (2004) A1279. [10] X.Z. Liao, Z.F. Ma, L. Wang, X.M. Zhang, Y. Jiang, Y.S. He, Electrochem. Solid-State Lett. 7 (2004) A522. [11] S. Franger, C. Bourbon, F.L. Cras, J. Electrochem. Soc. 151 (2004) A1024. [12] C.H. Mi, X.B. Zhao, G.S. Cao, J.P. Tu, J. Electrochem. Soc. 152 (2005) A483. [13] A. Ait-Salah, K. Zaghib, A. Mauger, F. Gendron, C.M. Julien, Phys. Status Solidi, A Appl. Res. 203 (1) (2006) R1. [14] A. Yamada, S.C. Chung, K. Hinokuma, J. Electrochem. Soc. 148 (2001) A224. [15] S. Scaccia, M. Carewska, P. Wisniewski, P.P. Prosini, Mater. Res. Bull. 38 (2003) 1155. [16] J.K. Kim, J.W. Choi, G. Cheruvally, J.U. Kim, J.H. Ahn, G.B. Cho, K.W. Kim, H.J. Ahn, J. Power Sources (in press).
A modified mechanical activation synthesis for carbon-coated LiFePO4 cathode in lithium batteries Jae-Kwang Kim a , Jae-Won Choi a , Gouri Cheruvally a , Jong-Uk Kim a , Jou-Hyeon Ahn a,⁎, Gyu-Bong Cho b , Ki-Won Kim b , Hyo-Jun Ahn b a
b
Department of Chemical and Biological Engineering, ITRC for Energy Storage and Conversion, Gyeongsang National University, 900, Gajwa-dong, Jinju 660-701, Republic of Korea School of Advanced Materials Science and Engineering, ITRC for Energy Storage and Conversion, Gyeongsang National University, 900, Gajwa-dong, Jinju 660-701, Republic of Korea Received 31 October 2006; accepted 19 December 2006 Available online 23 December 2006
Abstract An efficient synthesis based on mechanical activation (MA) was developed for carbon-coated lithium iron phosphate (LiFePO4/C). The conventional MA process was modified by introducing two initial steps of slurry phase blending of the ingredients and solvent removal by rotary evaporation, so as to get an intimate mixing and homogenous dispersion of conductive carbon in the sample. Phase-pure, nanometer-sized particles of the active material covered with a porous, nanometer-sized web of carbon were obtained. LiFePO4/C exhibited remarkably good electrochemical properties when evaluated as cathodes in room temperature lithium cells. An initial discharge capacity of 166 mAh/g (corresponding to 97.6% of theoretical capacity) was achieved at 0.1 C-rate. A very stable cycle performance was also realized; good capacity retention up to 100 cycles was achieved at different current densities. © 2006 Elsevier B.V. All rights reserved. Keywords: Electronic materials; Nanomaterials; Cathode materials; Lithium iron phosphate; Lithium batteries
1. Introduction Lithium iron phosphate (LiFePO4) is being actively investigated as a promising cathode material for lithium batteries. In addition to providing a reasonably high theoretical capacity of 170 mAh/g, LiFePO4 offers advantages such as a flat discharge voltage at 3.4 V vs lithium, a wide safety margin of usage for organic electrolytes, high thermal and chemical stability, low material cost, low toxicity and improved safety. However, LiFePO4 suffers from the limitations of poor electronic conductivity and slow lithium ion diffusion, and hence performs unsatisfactorily at lower temperatures and/or higher current densities [1,2]. An easy and successful approach to overcome the insulating nature has been to coat the active particles with conductive carbon, either by its incorporation as a powder initially [3,4] or by in-situ generation by pyrolysis of organic/ ⁎ Corresponding author. Tel.: +82 55 751 5388; fax: +82 55 753 1806. E-mail address: [email protected] (J.-H. Ahn). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.12.038
polymer compounds during the synthesis [2,5–13]. The attempts to overcome the limitation of lithium ion diffusion focus mainly on synthesizing small particles with high purity [14]. Different synthesis techniques including solid-state reactions [1,12,14] and solution methods [7,9,15] have been developed for preparing LiFePO4. The amorphous to crystalline phase change of LiFePO4 occurs at high temperatures (≥ 450 °C) [6,15]. Conventional solid-state reactions are adopted by many researchers since phase-pure material is obtained with a proper control of firing conditions. However, the lengthy and complex procedures requiring repeated grindings and calcinations usually lead to the formation of larger particles with lower performance. In mechanical activation (MA) method, a high energy ball milling step is introduced before thermal treatment which results in forming an intimate mixture of the reactants and effectively reduces the thermal treatment time and temperature, thus arresting the undesirable crystal growth. Some recent studies have highlighted the effectiveness of MA process for the synthesis of small and phase-pure particles of LiFePO4 [3,10,11].
J.-K. Kim et al. / Materials Letters 61 (2007) 3822–3825
In this paper, we report the preparation of cathode active carbon-coated LiFePO4 (LiFePO4/C) by a modified MA process. The electrochemical properties of LiFePO4/C in lithium batteries would be decided considerably by the nature as well as the effectiveness of the conductive carbon coating around the particles. For attaining an effective coating, it would be beneficial if a uniform dispersion of carbon powder is obtained in the precursor sample before subjecting to ball milling. With this in view, the conventional MA process was modified and the material so synthesized had uniform carbon coating and good electrochemical properties. 2. Experimental LiFePO4/C was synthesized from Li2CO3, FeC2O4·2H2O, and NH4H2PO4 (all chemicals of 99% purity from Aldrich) taken in stoichiometric quantities along with 7.8 wt.% of acetylene black powder (Alfa). The process steps were: (1) initial mixing by magnetic stirring of all ingredients together as a slurry in 60 wt.% of triply distilled water at room temperature for 7 h, (2) rotary evaporator drying at 70 °C for 2 h at 60 rpm to yield a solid powder, (3) high energy ball milling in a hardened steel vial (zirconia balls, ball-to-powder weight ratio 10:3, SPEX mill, 1000 rpm, room temperature, 15 h, argon atmosphere), (4) mechanical pressing to make pellets, and (5) thermal treatment at 600 °C for 10 h in nitrogen atmosphere. The ball milling and thermal treatment conditions were adopted based on our earlier study on the optimization of MA processing parameters for the synthesis of LiFePO4 [16]. The crystallographic structure was determined by X-ray diffraction (XRD: D8 Advance, Bruker AXS) using CuKα radiation. Scanning electron microscopy (SEM) imaging and energy dispersive X-ray (EDX) mapping were done using SEM–EDX (Philips XL30 S FEG). The nature and thickness of the carbon coating was observed with high resolution transmission electron microscopy (HR-TEM) (JEM-3010, JEOL). The specific surface area was computed from N2 sorption data (ASAP 2010 Analyzer) using the Brunauer– Emmett–Teller (BET) method. The chemical composition was determined by inductively coupled plasma (ICP) analysis (Atomscan 25, Optima 4300DV) and carbon content by elemental analyzer (CHNS-932, LECO). The cathode was prepared from the LiFePO4/C powder, carbon black and poly(vinylidene fluoride) (Aldrich) binder taken in the ratio 70:20:10 by weight. The components were mixed as viscous slurry in N-methyl pyrrolidone solvent and cast uniformly on aluminum foil and dried at 95 °C under vacuum for 12 h. The film so obtained was cut into circular discs of the area 0.95 cm2 and mass ∼ 3.0 mg for use as the cathode. Electrochemical coin cells were assembled with lithium metal as anode, LiFePO4/C as cathode and Celgard@ 2200 separator with 1 M LiPF6 in EC:DMC (1:1 vol) as electrolyte. Cyclic voltammetry (CV) was done at a scan rate of 0.1 mV/s between 2.0 and 4.5 V. Electrochemical tests were carried out using an automatic galvanostatic charge–discharge unit, WBCS3000 battery cycler, between 2.0 and 4.6 V at room temperature.
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3. Results and discussion The molar ratio of Li:Fe:P in the sample of LiFePO4/C agreed with the theoretical ratio of 1:1:1. The carbon content was analyzed to be 14.0 wt.%, which corresponded well with the theoretical value of 14.1% based on the added carbon. These analytical results indicate that the molar composition was preserved during processing and no loss of carbon by way of burning out occurred during the firing step. Fig. 1 shows the XRD pattern of the sample, identified to be orthorhombic, ordered olivine structure belonging to Pnma space group (all peaks are indexed; standard spectrum also shown). Peaks corresponding to significant amounts of impurities such as Fe3+ compounds are absent in the spectrum. As has been observed by other researchers also, carbon in the sample provides an efficient reductive atmosphere and effectively converts traces of Fe3+ (that might be formed during firing) back to Fe2+, and this results in the formation of a single phase material [10,13]. Fig. 2(a) presents the SEM image of the material showing nearspherical particles with a uniform morphology. The particle size range is 65–90 nm, with an average particle size of 80 nm and specific surface area of 18.3 m2/g. Adopting the conventional MA process, LiFePO4/C particles of the size 0.1–2 μm and specific surface area of 2.1–7.6 m2/g has been reported by Kwon et al. [3]. Thus, it is seen that the modifications introduced in this study result in attaining much smaller particles with a narrow size range and higher surface area. This indicates that compared to the conventional process, the modified process leads to (1) an intimate blending of the ingredients resulting in a more uniform particle size, and (2) a more homogenous distribution of carbon black throughout the sample that limits the particle growth effectively. SEM also reveals that the particles have a rough surface without sharp edges and/or clean cleavage surfaces; an added indication of the efficient carbon coating achieved. The homogeneity of chemical composition as determined by SEM–EDX mapping is presented in Fig. 2(b–e). The elemental mapping images obtained for C, Fe and P match with that of the corresponding SEM image. Since carbon also follows the same uniform distribution and image contrast as that of the other two elements, it is concluded that LiFePO4/C contains carbon in a uniformly distributed manner. The nature of surface coating of carbon, as analyzed by TEM, is shown in Fig. 3. Knowing the specific surface area of the particles (18.3 m2/g), carbon content (14.0 wt.%), and the true density of carbon (1.8–2.1 g/cm3), the approximate thickness of the carbon coating was calculated theoretically to be 4 nm [8]. TEM analysis shows a nano-
Fig. 1. XRD spectra of (a) standard Pnma orthorhombic LiFePO4. (b) LiFePO4/ C prepared by the modified MA process.
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Fig. 2. (a) SEM of LiFePO4/C. (b)–(e) SEM–EDX mapping: (b) SEM image, and (c–e) corresponding EDX mapping of C, Fe and P respectively.
sized web of amorphous carbon, approximately 5 nm in thickness, surrounding the LiFePO4 particles. The coating is seen to be highly porous and uniform, existing across the entire surface of particles, enveloping and connecting them, thus minimizing inter-particle agglomeration. By adopting a liquid-based powder preparation method, Chung et al. synthesized LiFePO4/C with a nano-web coating of carbon, which resulted in improved electrochemical properties for the material [7]. In the present study also, the efficient carbon coating formed could lead to improved charge transfer kinetics of the material and result in higher reversible capacity, as detailed below. Charge–discharge performance of lithium cell with LiFePO4/C cathode, tested at 0.1 C-rate is shown in Fig. 4(a). The charge and
Fig. 3. TEM image of LiFePO4/C.
Fig. 4. (a) Initial charge–discharge curves at 0.1 C. Inset: CV at a scan rate of 0.1 mV/s, 2.0–4.5 V. (b) Cycle performance at different C-rates.
discharge current of a battery is measured in C-rate. Thus, for the cell with LiFePO4 as the cathode active material, having a theoretical specific capacity of 170 mAh/g, the charge–discharge at 1 C-rate corresponds to a current density of 170 mA over a period of 1 h for 1 g of active material. The cell reaction voltage at ∼3.4 V, agrees with the observations in CV (oxidation and reduction peaks at 3.6 and 3.3 V respectively), shown in the inset of Fig. 4(a). The characteristic of extremely flat cell reaction voltage of LiFePO4 is observed here also [11,12]. The difference between the charging and discharging voltage is very low at 0.1 C (0.06 V), indicating good kinetics of redox reaction at comparatively low discharge currents. A discharge capacity of 166 mAh/g (97.6% of theoretical capacity) is obtained at 0.1 C, demonstrating that almost a full capacity can be achieved in the initial cycle. The performance during the first cycle is retained well during the subsequent cycles also. The cycle performance of the cell was studied up to 100 cycles at different C-rates and the results are shown in Fig. 4(b). LiFePO4/C shows excellent long-term cycling property. The cell retains 92% of its initial discharge capacity after 100 cycles at 0.1 Crate. With an increase in C-rate, polarization is enhanced due to slow lithium ion diffusion at the solid, two-phase FePO4/LiFePO4 interface and the resulting limitation of the material to cope up with the fast reaction kinetics at high C-rates [1]. The initial discharge capacities at 1, 2 and 3 C-rates are 142, 132 and 113 mAh/g, respectively. Thus, even at the high current densities of 2 C and 3 C, the cell performs respectively at 77.6% and 66.5% of theoretical capacity. The discharge capacities are higher than those obtained using LiFePO4/C samples prepared by the conventional MA process in (1) our laboratory (152 and 129 mAh/g at 0.1 and 1.0 C respectively) and (2) those reported by
J.-K. Kim et al. / Materials Letters 61 (2007) 3822–3825
Kwon et al. (156 and 110 mAh/g at 0.05 and 1 C respectively) [3]. The charge–discharge efficiencies are also high (N 96%) at all C-rates. The enhanced performance of the material processed by the modified MA process is attributed to the combined effect of achieving small, phasepure particles and an efficient carbon coating.
4. Conclusion Nanometer-sized, phase-pure particles of LiFePO4/C cathode active material were synthesized by a modified MA process. The intimate mixing of ingredients together with the conductive carbon carried out in the modified process resulted in achieving particles of uniform morphology with an efficient nano-web of carbon covering the particles. The material had remarkably good electrochemical properties when used as cathode in lithium cells, exhibiting high discharge capacities and stable cycle performance. The results obtained in this study emphasize the importance of achieving an intimate mixing of the components including the conductive carbon powder prior to the mechanical mixing and thermal treatment of the material. Acknowledgements This research was supported by the MIC (Ministry of Information and Communication), Korea, under the ITRC (Information Technology Research Center) support program. Gouri Cheruvally is thankful to the KOFST for the award of Brain Pool Fellowships, and J.-K. Kim and J.-W. Choi acknowledge the partial support by the Post Brain Korea 21 Project in 2006.
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