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Electrochemical properties of LiFePO4-multiwalled carbon nanotubes composite cathode materials for lithium polymer battery
Electrochemistry Communications 10 (2008) 1537–1540
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrochemical properties of LiFePO4-multiwalled carbon nanotubes composite cathode materials for lithium polymer battery Bo Jin a,b, En Mei Jin a, Kyung-Hee Park a, Hal-Bon Gu a,* a b
Department of Electrical Engineering, Chonnam National University, 300 Yongbong-dong, Gwangju 500-757, South Korea College of Materials Science and Engineering, Jilin University, Changchun 130025, China
a r t i c l e
i n f o
Article history: Received 7 June 2008 Received in revised form 19 July 2008 Accepted 2 August 2008 Available online 11 August 2008 Keywords: Olivine LiFePO4-MWCNTs composite Orthorhombic Hydrothermal method
a b s t r a c t LiFePO4-multiwalled carbon nanotubes (MWCNTs) composites were prepared by a hydrothermal method followed by ball-milling and heat treating. Cyclic voltammetry, ac impedance and galvanostatic charge/ discharge testing results indicate that LiFePO4-MWCNTs composite exhibits higher discharge capacity and rate capability than pure LiFePO4 at high-rate at room temperature. It is demonstrated that the added MWCNTs not only increase the electronic conductivity and lithium-ion diffusion coefficient but also decrease crystallite size and charge transfer resistance of LiFePO4-MWCNTs composite. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, lithium iron phosphate with an ordered olivine-type structure, LiFePO4, has attracted extensive attention due to low cost, safety and high compatibility with environment [1–9]. However, it is difficult to attain the full capacity because the electronic conductivity (10-9 S/cm) is very low, which leads to initial capacity loss and poor rate capability, and diffusion of Li+ ion across the LiFePO4/FePO4 boundary is slow due to its intrinsic character [1]. Many researchers have suggested solutions to this problem as follows: (i) coating with a conductive layer around the particles [10,11]; (ii) ionic substitution to enhance the electrochemical properties [12,13]; and (iii) synthesis of particles with well-defined morphology [14]. Li et al. [15] demonstrated that LiFePO4/MWCNTs composite cathode displayed the initial discharge capacity of 155 mAh/g at 0.1 C rate and the gradual decrease in discharge capacity upon cycling. Whittingham et al. [16] indicated that the added MWCNTs in pure LiFePO4 enhanced the electronic conductivity of the final product. Sakamoto et al. [17] suggested that V2O5/singlewalled carbon nanotubes composite electrode exhibited specific capacities in excess of 400 mAh/g at high discharge rates and retained this level of capacity on cycling. All the above papers used the liquid electrolyte. Zaghib et al. [18] reported the electrochemical performance of natural graphite-fibers/polyethylene oxide (PEO)based gel electrolyte/LiFePO4 batteries. Appetecchi et al. [19] suggested that Li/LiFePO4 polymer cells were capable of delivering * Corresponding author. Tel.: +82 62 530 0740; fax: +82 62 530 0077. E-mail address: [email protected] (H.-B. Gu). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.08.001
capacities exceeding 100 mAh/g at temperatures above 90 °C even at moderately high rates using PEO as a polymer matrix. However, up to now, there is no report on the electrochemical performance of LiFePO4-MWCNTs composite using polymer electrolyte especially at room temperature. In this study, MWCNTs were added to improve the electronic conductivity of pure LiFePO4. For the first time, we used 25PVDFLiClO4EC10PC10 as solid polymer electrolyte (SPE) to analyze the electrochemical properties of LiFePO4-MWCNTs composite by cyclic voltammetry (CV), ac impedance and galvanostatic charge/discharge tests at room temperature.
2. Experimental The preparation of pure LiFePO4 was described in detail previously [20]. Five weight percentage of MWCNTs were added into the solution of LiFePO4 hydrothermally synthesized at 170 °C and N-methyl-2-pyrrolidone (NMP), the mixture was ball-milled for 10 h using a shaker type of ball mill (Planetary Mono Mill). After drying at 90 °C for 12 h, the powders were pelletized and further heated at 500 °C for 1 h in nitrogen atmosphere. After cooling to room temperature, the mixture of NMP and LiFePO4-MWCNTs composite was ball-milled again for 10 h. Finally, the mixture was dried at 90 °C for 12 h. For comparison, pure LiFePO4 without MWCNTs was synthesized by the same ball-milling and heating temperature. The composite electrodes were prepared by mixing pure LiFePO4 or LiFePO4-MWCNTs composite with carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 70:25:5 in NMP. The
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B. Jin et al. / Electrochemistry Communications 10 (2008) 1537–1540
synthesis of 25PVDFLiClO4EC10PC10 was described in detail previously [21]. Li/SPE/LiFePO4 and Li/SPE/LiFePO4-MWCNTs coin-type batteries (CR2032) were assembled with lithium metal as anode and 25PVDFLiClO4EC10PC10 as electrolyte. The automatic charge/ discharge equipment (WBCS3000, WonATech Co. Ltd.) was used to perform the galvanostatic charge/discharge tests and CV measurements at room temperature. The electronic conductivities of the samples were measured by a four-point probe method. The crystalline phases were identified with XRD (Dmax/1200, Rigaku) with Cu Ka radiation. Electrochemical impedance spectroscopy measurements were performed using an IM6 impedance system (Zahner Elektrik Co.). The spectrum was potentiostatically measured by applying an ac voltage of 20 mV over the frequency range from 2 MHz to 0.1 or 0.01 Hz. 3. Results and discussion
Fig. 2. The SEM image of LiFePO4-MWCNTs composite.
(022)
(321)/(212)
(102)/(221)/(401)
(112)/(202)
(311) (121) (410)
(301) (220)
(111)/(201)
(211)/(020)
(101) (210) (011)
Pure LiFePO4 (200)
Intensity (a. u.)
LiFePO4-MWCNTs
Standard
10
15
20
25
30
35
40
45
50
2 θ (degree) Fig. 1. The XRD patterns for pure LiFePO4 and LiFePO4-MWCNTs composite.
0.6 nd Pure LiFePO4 (2 ) th Pure LiFePO4 (4 )
0.4
th Pure LiFePO4 (6 ) nd LiFePO4-MWCNTs (2 )
Current (mA)
Fig. 1 shows the XRD patterns for pure LiFePO4 and LiFePO4MWCNTs composite. All the patterns can be indexed to a singlephase material having an orthorhombic olivine-type structure with a space group of Pnma, which is the same as the standard one. The crystallite size (D) was calculated by the Scherrer’s equation: D = 0.9k/b cos h, from the full-width-at-half-maximum b of four strong and well-resolved reflection peaks corresponding to [1 0 1], [1 1 1], [2 1 1] and [3 1 1] crystallographic directions and the mean value was calculated [22,23]. The crystallite sizes are 34 nm for pure LiFePO4 and 19 nm for LiFePO4-MWCNTs composite, respectively. It is believed that the small particle size is useful for the intercalation/de-intercalation process of lithium ions. There is no impurity in pure LiFePO4 and LiFePO4-MWCNTs composite. There is no obvious carbon diffraction peaks in LiFePO4-MWCNTs composite due to its low content and amorphous state. The added MWCNTs do not change the crystal structure of pure LiFePO4. Fig. 2 shows the SEM image of LiFePO4-MWCNTs composite. SEM observation shows that the MWCNTs intertwine with LiFePO4 particles together to form a three-dimensional network. The dispersed MWCNTs provide pathways for electron transference. Therefore, the electronic conductivity of LiFePO4-MWCNTs composite is improved from 5.86 109 S/cm for pure LiFePO4 to 1.08 101 S/cm (measured by a four-point probe method). Fig. 3 shows the cyclic voltammograms of pure LiFePO4 and LiFePO4-MWCNTs composite. For pure LiFePO4, oxidation and reduction peaks in the second cycle appear at around 3.65 and 3.17 V, respectively. The potential interval between two peaks is
0.2
LiFePO4-MWCNTs (4th) LiFePO4-MWCNTs (6th)
0.0 -0.2 -0.4 -0.6 1.5
2.0
2.5
3.0
3.5
4.0
4.5
Voltage (V) Fig. 3. The cyclic voltammograms of pure LiFePO4 and LiFePO4-MWCNTs composite at a scan rate of 0.1 mV/s.
0.48 V. However, in the case of LiFePO4-MWCNTs composite, the oxidation and reduction peaks in the second cycle appear at around 3.64 and 3.2 V, respectively. The potential interval between two peaks is 0.44 V less than that of pure LiFePO4. The redox peak profile of LiFePO4-MWCNTs composite is more symmetric and spiculate than that of pure LiFePO4, demonstrating that the reversibility and reactivity of LiFePO4-MWCNTs composite are enhanced due to improvement of electronic conductivity and the reduced diffusion length resulting from a decrease in the crystallite size by MWCNTs. Fig. 4 shows (a) the cycling performance of pure LiFePO4, pure LiFePO4 with ball-milling and LiFePO4-MWCNTs composite and (b) the relationship between capacity and discharge rate of pure LiFePO4 and LiFePO4-MWCNTs composite. The discharge capacity of pure LiFePO4 after 1 cycle is 111 mAh/g, and decreases to 96 mAh/g after 30 cycles. For pure LiFePO4 with ball-milling, the discharge capacity after 1 cycle is 110 mAh/g, and decreases to 99 mAh/g after 7 cycles, and subsequently retains the stable discharge capacity. However, LiFePO4-MWCNTs composite displays more stable discharge capacity retention than pure LiFePO4 and pure LiFePO4 with ball-milling until 30 cycles and the discharge capacity is 115 mAh/g. In conclusion, the improvement of pure LiFePO4 is due to MWCNTs and not ball-milling. Fig. 4b demonstrates that high-rate capability of LiFePO4-MWCNTs composite
B. Jin et al. / Electrochemistry Communications 10 (2008) 1537–1540
centration of lithium-ion (7.69 103 mol/cm3), and r is the Warburg factor which is associated with Zre.
140
Discharge capacity (mAh/g)
(a)
120
Z re ¼ Rs þ Rct þ rx1=2
100 140 Pure LiFePO4 LiFePO4-MWCNTs
(b) 120
Capacity (mAh g-1)
80 60
100 80 60 40 20
40
0 0.01
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LiFePO4-MWCNTs
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Pure LiFePO4 with ball-milling
0 0
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10
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25
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Cycle number Fig. 4. (a) The cycling performance of pure LiFePO4, pure LiFePO4 with ball-milling and LiFePO4-MWCNTs composite at C/4 rate at room temperature. (b) The relationship between capacity and discharge rate of pure LiFePO4 and LiFePO4MWCNTs composite.
ð2Þ
Herein, Rs is the resistance of SPE, and Rct is the charge transfer resistance. The diameter of the semicircle on the real axis is approximate equal to Rct. The Rct values are 1000 X for pure LiFePO4 and 280 X for LiFePO4-MWCNTs composite, respectively. Fig. 5b shows the relationship plot between Zre and reciprocal square root of the angular frequency (x1/2) at low-frequency region. The lithiumion diffusion coefficients of pure LiFePO4 and LiFePO4-MWCNTs composite were calculated, and are 2.41 1015 and 1.50 1014 cm2/s, respectively, which are lower than the reported values (9.98 1014 cm2/s for virginal LiFePO4 and 1.58 1013 cm2/s for doped LiFePO4 at room temperature [12], and 1.70 1012 cm2/s for LiFePO4/C at 20 °C [5], but the liquid electrolytes were used in their papers. It is obvious that the Rct drastically decreases and lithium-ion diffusion coefficient increases when adding MWCNTs, and therefore enhances the charge/discharge performance of LiFePO4-MWCNTs composite, and this is consistent with the results in Fig. 3 and Fig. 4. 4. Conclusions
is obviously ameliorated, and this is due to an increase in the electronic conductivity and the reduced diffusion length resulting from a decrease in the crystallite size by MWCNTs. It is consistent with the CV results in Fig. 3. Fig. 5a shows the impedance spectra of pure LiFePO4 and LiFePO4-MWCNTs composite, which were measured in the fully discharged state after cycling. The depressed semicircles in the medium frequency are related to the charge transfer process. The inclined lines in the lower frequency are attributed to the Warburg impedance, which is associated with lithium-ion diffusion in LiFePO4 electrode. The lithium-ion diffusion coefficient could be calculated using the following equation [10]:
D ¼ R2 T 2 =2A2 n4 F 4 c2 r2
LiFePO4-MWCNTs composites have been synthesized successfully by a hydrothermal method followed by ball-milling and heat treating. The electronic conductivity of LiFePO4-MWCNTs composite is 1.08 101 S/cm, which is eight orders of magnitude higher than that of pure LiFePO4. The added MWCNTs not only increase the electronic conductivity and the lithium-ion diffusion coefficient but also decrease crystallite size and the charge transfer resistance, and thus, obviously improve the discharge capacity and high-rate capability of LiFePO4-MWCNTs composite especially at 1 °C and 3 °C rates at room temperature. These results indicate that LiFePO4-MWCNTs composite could be used as a cathode material for litsium polymer batteries.
ð1Þ
Herein, 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 oxidization, F is the Faraday constant, C is the con-
Acknowledgment This work was supported by the Korea Research Foundation Grant funded by the Korea Government (MOEHRD) (KRF-2007521-D00184). References
1400 (b)
1200 Pure LiFePO4 1000 Pure LiFePO4 LiFePO4-MWCNTs
800
800 600
-Z"im (Ω)
Z're (Ω)
1000
(a)
600
400
200
0
400
0
200
400
600
800 1000 1200 1400
Z're (Ω)
LiFePO4-MWCNTs
200 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
ω−1/2 Fig. 5. (a) The impedance spectra of pure LiFePO4 and LiFePO4-MWCNTs composite. (b) The relationship plot between Zre and x1/2 at low-frequency region.
[1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. [2] H.-C. Kang, D.-K. Jun, B. Jin, E.M. Jin, K.-H. Park, H.-B. Gu, K.-W. Kim, J. Power Sources 179 (2008) 340. [3] S. Yang, Y. Song, P.Y. Zavalij, M.S. Whittingham, Electrochem. Commun. 4 (2002) 239. [4] M.S. Islam, D.J. Driscoll, C.A.J. Fisher, P.R. Slater, Chem. Mater. 17 (2005) 5085. [5] X.Z. Liao, Z.F. Ma, Q. Gong, Y.S. He, L. Pei, L.J. Zeng, Electrochem. Commun. 10 (2008) 691. [6] K. Dokko, K. Shiraishi, K. Kanamura, J. Electrochem. Soc. 152 (2005) A2199. [7] S. Yang, P.Y. Zavalij, M.S. Whittingham, Electrochem. Commun. 3 (2001) 505. [8] H.-B. Gu, D.-K. Jun, G.-C. Park, B. Jin, E.M. Jin, J. Nanosci. Nanotechnol. 7 (2007) 3980. [9] H. Liu, G.X. Wang, D. Wexler, J.Z. Wang, H.K. Liu, Electrochem. Commun. 10 (2008) 165. [10] A.J. Bard, L.R. Faulkner, Electrochemical Methods, second ed., Wiley, 2001, p. 231. [11] E.M. Jin, B. Jin, D.-K. Jun, K.-H. Park, H.-B. Gu, K.-W. Kim, J. Power Sources 178 (2008) 801. [12] H. Liu, Q. Cao, L.J. Fu, C. Li, Y.P. Wu, H.Q. Wu, Electrochem. Commun. 8 (2006) 1553. [13] S.-Y. Chung, J.T. Bloking, Y.-M. Chiang, Nat. Mater. 1 (2002) 123. [14] A. Yamada, S.C. Chung, K. Hinikuma, J. Electrochem. Soc. 148 (2001) A224. [15] X.L. Li, F.Y. Kang, X.D. Bai, W.C. Shen, Electrochem. Commun. 9 (2007) 663. [16] J.J. Chen, M.S. Whittingham, Electrochem. Commun. 8 (2006) 855.
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[17] J.S. Sakamoto, B. Dunn, J. Electrochem. Soc. 149 (2002) A26. [18] K. Zaghib, P. Charest, A. Guerfi, J. Shim, M. Perrier, K. Striebel, J. Power Sources 146 (2005) 380. [19] G.B. Appetecchi, J. Hassoun, B. Scrosati, F. Croce, F. Cassel, M. Salmon, J. Power Sources 124 (2003) 246. [20] B. Jin, H.-B. Gu, Solid State Ionics 178 (2008) 1907.
[21] J.-U. Kim, Y.-J. Jo, G.-C. Park, W.-J. Jeong, H.-B. Gu, J. Power Sources 97–98 (2001) 450. [22] M.A.E. Sanchez, G.E.S. Brito, M.C.A. Fantini, G.F. Goya, J.R. Matos, Solid State Ionics 177 (2006) 497. [23] G. Arnold, J. Garche, R. Hemmer, S. Strobele, C. Volger, M. Wohlfahrt-Mehrens, J. Power Sources 119–121 (2003) 247.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrochemical properties of LiFePO4-multiwalled carbon nanotubes composite cathode materials for lithium polymer battery Bo Jin a,b, En Mei Jin a, Kyung-Hee Park a, Hal-Bon Gu a,* a b
Department of Electrical Engineering, Chonnam National University, 300 Yongbong-dong, Gwangju 500-757, South Korea College of Materials Science and Engineering, Jilin University, Changchun 130025, China
a r t i c l e
i n f o
Article history: Received 7 June 2008 Received in revised form 19 July 2008 Accepted 2 August 2008 Available online 11 August 2008 Keywords: Olivine LiFePO4-MWCNTs composite Orthorhombic Hydrothermal method
a b s t r a c t LiFePO4-multiwalled carbon nanotubes (MWCNTs) composites were prepared by a hydrothermal method followed by ball-milling and heat treating. Cyclic voltammetry, ac impedance and galvanostatic charge/ discharge testing results indicate that LiFePO4-MWCNTs composite exhibits higher discharge capacity and rate capability than pure LiFePO4 at high-rate at room temperature. It is demonstrated that the added MWCNTs not only increase the electronic conductivity and lithium-ion diffusion coefficient but also decrease crystallite size and charge transfer resistance of LiFePO4-MWCNTs composite. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, lithium iron phosphate with an ordered olivine-type structure, LiFePO4, has attracted extensive attention due to low cost, safety and high compatibility with environment [1–9]. However, it is difficult to attain the full capacity because the electronic conductivity (10-9 S/cm) is very low, which leads to initial capacity loss and poor rate capability, and diffusion of Li+ ion across the LiFePO4/FePO4 boundary is slow due to its intrinsic character [1]. Many researchers have suggested solutions to this problem as follows: (i) coating with a conductive layer around the particles [10,11]; (ii) ionic substitution to enhance the electrochemical properties [12,13]; and (iii) synthesis of particles with well-defined morphology [14]. Li et al. [15] demonstrated that LiFePO4/MWCNTs composite cathode displayed the initial discharge capacity of 155 mAh/g at 0.1 C rate and the gradual decrease in discharge capacity upon cycling. Whittingham et al. [16] indicated that the added MWCNTs in pure LiFePO4 enhanced the electronic conductivity of the final product. Sakamoto et al. [17] suggested that V2O5/singlewalled carbon nanotubes composite electrode exhibited specific capacities in excess of 400 mAh/g at high discharge rates and retained this level of capacity on cycling. All the above papers used the liquid electrolyte. Zaghib et al. [18] reported the electrochemical performance of natural graphite-fibers/polyethylene oxide (PEO)based gel electrolyte/LiFePO4 batteries. Appetecchi et al. [19] suggested that Li/LiFePO4 polymer cells were capable of delivering * Corresponding author. Tel.: +82 62 530 0740; fax: +82 62 530 0077. E-mail address: [email protected] (H.-B. Gu). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.08.001
capacities exceeding 100 mAh/g at temperatures above 90 °C even at moderately high rates using PEO as a polymer matrix. However, up to now, there is no report on the electrochemical performance of LiFePO4-MWCNTs composite using polymer electrolyte especially at room temperature. In this study, MWCNTs were added to improve the electronic conductivity of pure LiFePO4. For the first time, we used 25PVDFLiClO4EC10PC10 as solid polymer electrolyte (SPE) to analyze the electrochemical properties of LiFePO4-MWCNTs composite by cyclic voltammetry (CV), ac impedance and galvanostatic charge/discharge tests at room temperature.
2. Experimental The preparation of pure LiFePO4 was described in detail previously [20]. Five weight percentage of MWCNTs were added into the solution of LiFePO4 hydrothermally synthesized at 170 °C and N-methyl-2-pyrrolidone (NMP), the mixture was ball-milled for 10 h using a shaker type of ball mill (Planetary Mono Mill). After drying at 90 °C for 12 h, the powders were pelletized and further heated at 500 °C for 1 h in nitrogen atmosphere. After cooling to room temperature, the mixture of NMP and LiFePO4-MWCNTs composite was ball-milled again for 10 h. Finally, the mixture was dried at 90 °C for 12 h. For comparison, pure LiFePO4 without MWCNTs was synthesized by the same ball-milling and heating temperature. The composite electrodes were prepared by mixing pure LiFePO4 or LiFePO4-MWCNTs composite with carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 70:25:5 in NMP. The
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B. Jin et al. / Electrochemistry Communications 10 (2008) 1537–1540
synthesis of 25PVDFLiClO4EC10PC10 was described in detail previously [21]. Li/SPE/LiFePO4 and Li/SPE/LiFePO4-MWCNTs coin-type batteries (CR2032) were assembled with lithium metal as anode and 25PVDFLiClO4EC10PC10 as electrolyte. The automatic charge/ discharge equipment (WBCS3000, WonATech Co. Ltd.) was used to perform the galvanostatic charge/discharge tests and CV measurements at room temperature. The electronic conductivities of the samples were measured by a four-point probe method. The crystalline phases were identified with XRD (Dmax/1200, Rigaku) with Cu Ka radiation. Electrochemical impedance spectroscopy measurements were performed using an IM6 impedance system (Zahner Elektrik Co.). The spectrum was potentiostatically measured by applying an ac voltage of 20 mV over the frequency range from 2 MHz to 0.1 or 0.01 Hz. 3. Results and discussion
Fig. 2. The SEM image of LiFePO4-MWCNTs composite.
(022)
(321)/(212)
(102)/(221)/(401)
(112)/(202)
(311) (121) (410)
(301) (220)
(111)/(201)
(211)/(020)
(101) (210) (011)
Pure LiFePO4 (200)
Intensity (a. u.)
LiFePO4-MWCNTs
Standard
10
15
20
25
30
35
40
45
50
2 θ (degree) Fig. 1. The XRD patterns for pure LiFePO4 and LiFePO4-MWCNTs composite.
0.6 nd Pure LiFePO4 (2 ) th Pure LiFePO4 (4 )
0.4
th Pure LiFePO4 (6 ) nd LiFePO4-MWCNTs (2 )
Current (mA)
Fig. 1 shows the XRD patterns for pure LiFePO4 and LiFePO4MWCNTs composite. All the patterns can be indexed to a singlephase material having an orthorhombic olivine-type structure with a space group of Pnma, which is the same as the standard one. The crystallite size (D) was calculated by the Scherrer’s equation: D = 0.9k/b cos h, from the full-width-at-half-maximum b of four strong and well-resolved reflection peaks corresponding to [1 0 1], [1 1 1], [2 1 1] and [3 1 1] crystallographic directions and the mean value was calculated [22,23]. The crystallite sizes are 34 nm for pure LiFePO4 and 19 nm for LiFePO4-MWCNTs composite, respectively. It is believed that the small particle size is useful for the intercalation/de-intercalation process of lithium ions. There is no impurity in pure LiFePO4 and LiFePO4-MWCNTs composite. There is no obvious carbon diffraction peaks in LiFePO4-MWCNTs composite due to its low content and amorphous state. The added MWCNTs do not change the crystal structure of pure LiFePO4. Fig. 2 shows the SEM image of LiFePO4-MWCNTs composite. SEM observation shows that the MWCNTs intertwine with LiFePO4 particles together to form a three-dimensional network. The dispersed MWCNTs provide pathways for electron transference. Therefore, the electronic conductivity of LiFePO4-MWCNTs composite is improved from 5.86 109 S/cm for pure LiFePO4 to 1.08 101 S/cm (measured by a four-point probe method). Fig. 3 shows the cyclic voltammograms of pure LiFePO4 and LiFePO4-MWCNTs composite. For pure LiFePO4, oxidation and reduction peaks in the second cycle appear at around 3.65 and 3.17 V, respectively. The potential interval between two peaks is
0.2
LiFePO4-MWCNTs (4th) LiFePO4-MWCNTs (6th)
0.0 -0.2 -0.4 -0.6 1.5
2.0
2.5
3.0
3.5
4.0
4.5
Voltage (V) Fig. 3. The cyclic voltammograms of pure LiFePO4 and LiFePO4-MWCNTs composite at a scan rate of 0.1 mV/s.
0.48 V. However, in the case of LiFePO4-MWCNTs composite, the oxidation and reduction peaks in the second cycle appear at around 3.64 and 3.2 V, respectively. The potential interval between two peaks is 0.44 V less than that of pure LiFePO4. The redox peak profile of LiFePO4-MWCNTs composite is more symmetric and spiculate than that of pure LiFePO4, demonstrating that the reversibility and reactivity of LiFePO4-MWCNTs composite are enhanced due to improvement of electronic conductivity and the reduced diffusion length resulting from a decrease in the crystallite size by MWCNTs. Fig. 4 shows (a) the cycling performance of pure LiFePO4, pure LiFePO4 with ball-milling and LiFePO4-MWCNTs composite and (b) the relationship between capacity and discharge rate of pure LiFePO4 and LiFePO4-MWCNTs composite. The discharge capacity of pure LiFePO4 after 1 cycle is 111 mAh/g, and decreases to 96 mAh/g after 30 cycles. For pure LiFePO4 with ball-milling, the discharge capacity after 1 cycle is 110 mAh/g, and decreases to 99 mAh/g after 7 cycles, and subsequently retains the stable discharge capacity. However, LiFePO4-MWCNTs composite displays more stable discharge capacity retention than pure LiFePO4 and pure LiFePO4 with ball-milling until 30 cycles and the discharge capacity is 115 mAh/g. In conclusion, the improvement of pure LiFePO4 is due to MWCNTs and not ball-milling. Fig. 4b demonstrates that high-rate capability of LiFePO4-MWCNTs composite
B. Jin et al. / Electrochemistry Communications 10 (2008) 1537–1540
centration of lithium-ion (7.69 103 mol/cm3), and r is the Warburg factor which is associated with Zre.
140
Discharge capacity (mAh/g)
(a)
120
Z re ¼ Rs þ Rct þ rx1=2
100 140 Pure LiFePO4 LiFePO4-MWCNTs
(b) 120
Capacity (mAh g-1)
80 60
100 80 60 40 20
40
0 0.01
0.1
1
Pure LiFePO4
10
100
C-rate
LiFePO4-MWCNTs
20
Pure LiFePO4 with ball-milling
0 0
5
10
15
20
25
1539
30
Cycle number Fig. 4. (a) The cycling performance of pure LiFePO4, pure LiFePO4 with ball-milling and LiFePO4-MWCNTs composite at C/4 rate at room temperature. (b) The relationship between capacity and discharge rate of pure LiFePO4 and LiFePO4MWCNTs composite.
ð2Þ
Herein, Rs is the resistance of SPE, and Rct is the charge transfer resistance. The diameter of the semicircle on the real axis is approximate equal to Rct. The Rct values are 1000 X for pure LiFePO4 and 280 X for LiFePO4-MWCNTs composite, respectively. Fig. 5b shows the relationship plot between Zre and reciprocal square root of the angular frequency (x1/2) at low-frequency region. The lithiumion diffusion coefficients of pure LiFePO4 and LiFePO4-MWCNTs composite were calculated, and are 2.41 1015 and 1.50 1014 cm2/s, respectively, which are lower than the reported values (9.98 1014 cm2/s for virginal LiFePO4 and 1.58 1013 cm2/s for doped LiFePO4 at room temperature [12], and 1.70 1012 cm2/s for LiFePO4/C at 20 °C [5], but the liquid electrolytes were used in their papers. It is obvious that the Rct drastically decreases and lithium-ion diffusion coefficient increases when adding MWCNTs, and therefore enhances the charge/discharge performance of LiFePO4-MWCNTs composite, and this is consistent with the results in Fig. 3 and Fig. 4. 4. Conclusions
is obviously ameliorated, and this is due to an increase in the electronic conductivity and the reduced diffusion length resulting from a decrease in the crystallite size by MWCNTs. It is consistent with the CV results in Fig. 3. Fig. 5a shows the impedance spectra of pure LiFePO4 and LiFePO4-MWCNTs composite, which were measured in the fully discharged state after cycling. The depressed semicircles in the medium frequency are related to the charge transfer process. The inclined lines in the lower frequency are attributed to the Warburg impedance, which is associated with lithium-ion diffusion in LiFePO4 electrode. The lithium-ion diffusion coefficient could be calculated using the following equation [10]:
D ¼ R2 T 2 =2A2 n4 F 4 c2 r2
LiFePO4-MWCNTs composites have been synthesized successfully by a hydrothermal method followed by ball-milling and heat treating. The electronic conductivity of LiFePO4-MWCNTs composite is 1.08 101 S/cm, which is eight orders of magnitude higher than that of pure LiFePO4. The added MWCNTs not only increase the electronic conductivity and the lithium-ion diffusion coefficient but also decrease crystallite size and the charge transfer resistance, and thus, obviously improve the discharge capacity and high-rate capability of LiFePO4-MWCNTs composite especially at 1 °C and 3 °C rates at room temperature. These results indicate that LiFePO4-MWCNTs composite could be used as a cathode material for litsium polymer batteries.
ð1Þ
Herein, 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 oxidization, F is the Faraday constant, C is the con-
Acknowledgment This work was supported by the Korea Research Foundation Grant funded by the Korea Government (MOEHRD) (KRF-2007521-D00184). References
1400 (b)
1200 Pure LiFePO4 1000 Pure LiFePO4 LiFePO4-MWCNTs
800
800 600
-Z"im (Ω)
Z're (Ω)
1000
(a)
600
400
200
0
400
0
200
400
600
800 1000 1200 1400
Z're (Ω)
LiFePO4-MWCNTs
200 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
ω−1/2 Fig. 5. (a) The impedance spectra of pure LiFePO4 and LiFePO4-MWCNTs composite. (b) The relationship plot between Zre and x1/2 at low-frequency region.
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