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Influence of carbon coating porosity on the electrochemical performance of LiFePO4 cathode
Electrochemistry Communications 13 (2011) 726–729
Contents lists available at ScienceDirect
Electrochemistry Communications 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 / e l e c o m
Influence of carbon coating porosity on the electrochemical performance of LiFePO4 cathode Fangfang Pan, Xueyang Chen, Hongju Li, Xiaodong Xin, Qinqin Chang, Kangning Jiang, Wen-lou Wang ⁎ Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China
a r t i c l e
i n f o
Article history: Received 1 March 2011 Received in revised form 8 April 2011 Accepted 19 April 2011 Available online 22 April 2011 Keywords: Lithium iron phosphate Carbon coating Dual porosity
a b s t r a c t Electrochemical performance of LiFePO4/C should be strongly dependent on the carbon coating structure for core-shell nanocomposites. In this work two types of LiFePO4/C composites with the same carbon coating thickness were prepared using a sol–gel technique, enabling direct comparison of the influence of carbon coating porosity on the performance of LiFePO4/C cathodes. It was found that the material with a distribution of dual mesopores delivers better electrochemical performance than the one with a narrow pore size distribution. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Lithium iron phosphate, which was first studied by Padhi et al. [1], is considered as a promising cathode material for new generation lithium-ion batteries due to its high theoretical capacity, relative low cost, good thermal stability and low environmental impact [2–5]. However, as most of olivine LiMPO4 phosphates, intrinsic low ionic and electronic conductivities of LiFePO4 restrict its high electrochemical activities and prevent its large-scale application in electric vehicles (EVs) and hybrid electric vehicles (HEVs). One of the challenging issues to achieve high power density using LiFePO4 as electrode materials is to tackle its kinetic problems, sluggish mass and charge transport [6]. Porous LiFePO4/C composites have been proved to be potentially attractive materials for high-rate cathodes [13–17,19,21,22]. The work of Gaberscek et al. [13,15,19], Munichandraiah et al. [16] and Cao et al. [22] indicate that a porous structure created inside the LiFePO4/C composite offers improved kinetics and electrochemical performance of electrode material. Studies of Doherty et al. [17], Wu et al. [14] and He et al. [21] suggest that the incorporation of porous carbon matrix can bring considerable enhancement to electrode materials in terms of power and energy densities. Carbon coating which can yield composite cathodes with better performance has been widely used for improving the electronic conductivities of poor electronically conductive electrode materials [7–13,22,23]. However, as for the influence of carbon coating porosity on the electrochemical performance of LiFePO4/C, there are very few
⁎ Corresponding author. Tel.: + 86 551 3606143; fax: + 86 551 3601592. E-mail address: [email protected] (W. Wang). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.04.021
reports addressing this issue until now. Probably because perfect surface coatings are often difficult to achieve and carbon coating introduced into LiFePO4 is generally considered amorphous and porous. In the current work, a simple method was used to prepare LiFePO4/C composites with nanosized particles, high specific area and fine carbon coating. The distribution of pores in the carbon coating layer was controlled by appropriate selection of synthesis parameters. And the carbon coating porosity on rate performance of LiFePO4 composite cathodes was investigated. 2. Experimental As a starting precursor, iron (III) citrate was dissolved at 60 °C in water. Then citric acid, with the citric acid to iron molar ratio of 1.2:1 (sample A) and 2:1 (sample B), was added under constant stirring until the ferric citrate was completely dissolved. Separately, an equimolar water solution of LiH2PO4 was prepared from lithium hydroxide (LiOH, AR) and ammonium dihydric phosphate (NH4H2PO4, AR). Clear solutions were mixed together and magnetically stirred to get a transparent solution. The mixed solution was stirred until a pale green wet gel was formed. Wet gels of sample A and sample B were aged and dried separately with different method before sintered in an inert (pure nitrogen) atmosphere at 650 °C for 7 h to obtain LiFePO4/C composites. The specific area and porosity of LiFePO4/C samples were examined by adsorption-desorption of N2 using ASAP 2020 instrument. Carbon content in the samples was analyzed using Elementar (Vario ELIII). TEM and HRTEM were performed on a JEOL-2010 highresolution transmission electron microscope at 200 kV. The positive electrodes were prepared by pressing a mixture of the active materials, acetylene black, and binder (polyvinylidene fluoride,
F. Pan et al. / Electrochemistry Communications 13 (2011) 726–729
PVDF, dissolved in N-methyl-2-pyrrolidone, NMP) in a weight ratio of 80:15:5 on aluminum foil followed by a drying procedure in vacuum at 120 °C for 8 h. The Li metal was used as the counter and reference electrodes. 1 M LiPF6 in a 1:1 mixture of ethylene carbonate/diethyl carbonate was used as electrolyte. Charge and discharge performance was tested on a multi-channel battery tester (Shenzhen Neware, BTS, China) at different current densities. CV and EIS were measured on an electrochemical workstation (CHI660C) with a three-electrode system. 3. Results and discussions Morphology of the final products was characterized by transmission electron microscopy (TEM). Each primary crystallite is completely coated by a carbon layer to form core-shell LiFePO4/C structure as shown in TEM images in Fig. 1. In addition, we can also show directly in Fig. 1(a) and Fig. 1(d) that the carbon film is deposited on the surface of each particle. As disclosed in the HRTEM images shown in Fig. 1(b) and Fig. 1(e), the carbon films of both samples are about 3 nm thick. As pointed out by Zhou et al. [4], surface carbon shell can effectively restrict the crystallite growth of LiFePO4. Mean crystallite size of sample A and sample B estimated using Scherrer's formula is 56.4 and 27.7 nm, respectively. In order to further investigate the pore structure of the nanocomposite, a nitrogen isothermal-adsorption technique was used. As shown in Fig. 1(c) and Fig. 1(f), adsorption/desorption isotherms of sample A and sample B both exhibit a hysteresis which is typical of a mesoporous structure. According to BET analysis, total specific surface areas of sample A and sample B are 88.6 and 135.2 m2 g− 1, respectively. Shown in the inset of Fig. 1(c) and Fig. 1(f), the Barrett–Joyner–Halenda (BJH) pore-size distribution indicates that sample A has a dual pore-size distribution at 4 nm and 11 nm while sample B has a centered pore-size distribution at 4 nm. Since active particles in sample A and sample B are spherical and the average diameter of sample A is twice as much as that of sample B, mass fractions of carbon needed to cover sample B will be twice as many as
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that of sample A, agreeing with the elemental analysis results which show that carbon content of sample A and sample B is 5.56% and 11.83%, respectively. According to BET analysis, total specific surface areas of sample A and sample B are 88.6 and 135.2 m2 g− 1, respectively. If the specific surface area totally comes from the aggregated LiFePO4 particles, then the calculated value in terms of diameter for sample A and sample B should be 29.5 m2 g− 1 and 59.8 m2 g− 1, which are much smaller than the obtained results from BET analysis. So, the large specific surface area can be mostly attributed to the porous carbon phase in the composites. Additionally, Zhou et al. [4] reported the special BET surface of their synthesized core-shell LiFePO4/carbon composite with an average particle size of 36 nm to be 50 m2 g− 1. The difference may also be due to the contribution of porous carbon layer to the high specific area of LiFePO4/C composites prepared in this work. And thus it is reasonable for us to assume that the BJH distribution of pore sizes shown in the inset of Fig. 1(c) and Fig. 1(f) mainly represents the pore size distribution in the coating layer. Typical discharge curves of sample A and sample B are shown in Fig. 2(a) and Fig. 2(b). Excluding the mass of carbon in LiFePO4/C composite, sample A and sample B delivered a specific capacity of 169 mAhg− 1 and 161 mAhg− 1 at 1 C, respectively, which are both close to the theoretical capacity (170 mAhg− 1) of LiFePO4. Meanwhile, discharged at 10 C, sample A retained 74% of its full capacity when sample B retained 66% of its full capacity (based on the mass of LiFePO4 in the composite). Comparison of the rate performance of sample A and sample B indicates that the specific capacity and material utilization was improved by the introduction of larger mesopores which act as a transport system for electrolyte and increase the accessibility of smaller pores [18]. Cycling stability of asobtained samples with at different discharge current rates is presented in Fig. 2(c), from which we can see that both samples show good cycling stability at discharge rates of 1 C and 10 C. The ac impedance of the two types of cathodes was measured before cycling. The impedance spectra are shown in Fig. 3(a). In the high frequency region, the intercepts with the real axis of the sample
Fig. 1. Characterization of the as prepared LiFePO4/C composites: (a) and (b) selected TEM image and HRTEM image of sample A; (d) and (e) selected TEM image and HRTEM image of sample B; (c) and (f) nitrogen adsorption and desorption isotherms of sample A and sample B. Inset: BJH distribution of pores obtained from desorption branch.
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F. Pan et al. / Electrochemistry Communications 13 (2011) 726–729
Fig. 2. (a and b) Discharge potential profiles of sample A and sample B at different rates (1 C, 2 C, 5 C and 10 C); (c) cyclic performance of samples A and B at 1 C and 10 C discharge rates.
A and sample B composite cathodes are at 260 Ω and 265 Ω (obtained from computer simulation), respectively. The values are believed to be the total resistance related to the charge transfer resistance [20] which for sample A and sample B are almost the same, indicating that sample A and sample B have the same interfacial property. Fig. 3(b) and Fig. 3(c) show cyclic voltammograms of LiFePO4/C composite samples. Under low scan rate (0.1 mV s− 1), sample A and sample B showed similar peak currents. However, when the scan rate increases to 1 mV s− 1, the peak current increase, which is a good indicator of electrode reaction kinetics, is much greater for sample A than for sample B, highlighting the improved high rate performance of sample A as discussed earlier. And this feature can be ascribed to the reduced transfer resistance of lithium ions in the coating layer with dual
Fig. 3. (a) AC impedance spectra for fresh cell assembled with sample A and sample B as cathode; (b) and (c) CV curves of sample A and sample B at scanning rate of 0.1 mV s− 1 and 1 mV s− 1.
porosity in sample A which provides quick pathways for ions to penetrate. The above electrochemical results demonstrate that kinetic performance of sample A is better than that of sample B. The reason could be that carbon coating porosity plays an important role in improving the rate performance of LiFePO4/C composites. The network of mesopores centered at 4 nm and relatively larger pores ranging from 5 nm to 20 nm appear to facilitate the electrolyte diffusion to the surface of the active material and provide favorable channels for lithium ions to penetrate. It is desirable electrochemical applications for electrode material to have an optimal combination of high porosity and surface area with good transport properties. A
F. Pan et al. / Electrochemistry Communications 13 (2011) 726–729
hierarchical pore system is considered to serve better as paths for fast ionic and electronic in a battery electrode [18]. Therefore it is more optimal to introduce a hierarchical pore system featuring dual porosity into the electrode materials to deliver both good capacity and achieve good capability. 4. Conclusions Pore size distribution in the carbon coating plays an important part in material utilization at different discharge rates. LiFePO4/C material with a distribution of well-defined dual mesopores delivers better electrochemical performance than the one with a narrow pore size distribution. Acknowledgements This work was supported by grants from Natural Science Foundation of Anhui province (Grant no. 090414178) and National Synchrotron Radiation Laboratory. References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. [2] R. Dominko, M. Bele, J.M. Goupil, M. Gaberscek, D. Hanzel, I. Arcon, J. Jamnik, Chem. Mater. 19 (2007) 2960.
729
[3] B.L. Ellis, W.R.M. Makahnouk, Y. Makimura, K. Toghill, L.F. Nazar, Nat. Mater. 6 (2007) 749. [4] Y. Wang, Y. Wang, E. Hosono, K. Wang, H. Zhou, Angew, Chem. Int. Ed. 47 (2008) 7461. [5] J.B. Goodenough, Y. Kim, Chem. Mater. 22 (2010) 587. [6] R. Amin, P. Balaya, J. Maier, Electrochem, Solid State Lett 10 (2007) A13. [7] H. Huang, S.-C. Yin, L.F. Nazar, Electrochem, Solid-State Lett. 4 (2001) A170. [8] F. Croce, A.D. Epifanio, J. Hassoun, A. Deptula, T. Olczac, B. Scrosati, Electrochem, Solid State Lett. 5 (2002) A47. [9] R. Dominko, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, S. Pejovnik, J. Jamnik, J. Electrochem. Soc. 152 (2005) A607. [10] C.-W. Ong, Y.-K. Lin, J.-S. Chen, J. Electrochem. Soc. 154 (2007) A527. [11] Y. Lin, H. Pan, M. Gao, Y. Liu, J. Electrochem. Soc. 154 (2007) A1124. [12] D. Choi, P.N. Kumta, J. Power Sources 163 (2007) 1064. [13] R. Dominko, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, J.M. Goupil, S. Pejovnik, J. Jamnik, J. Power Sources 153 (2006) 274–280. [14] X.-L. Wu, L.-Y. Jiang, F.-F. Cao, Y.-G. Guo, L.-J. Wan, Adv. Mater. 21 (2009) 2710. [15] R. Dominko, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, J.M. Goupil, S. Pejovnik, J. Jamnik, J. Power Sources 153 (2006) 274–280. [16] N.N. Sinha, C. Shivakumara, N. Munichandraiah, ACS Appl. Mater. Interfaces 2 (2010) 2031. [17] C.M. Doherty, R.A. Caruso, B.M. Smarsly, P. Adelhelm, C.J. Drummond, Chem. Mater. 21 (2009) 5300. [18] Y.S. Hu, P. Adelhelm, B.M. Smarsly, S. Hore, M. Antonietti, J. Maier, Adv. Funct. Mater. 17 (2007) 1873. [19] R. Dominko, J.M. Goupil, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, J. Jamnik, J. Electrochem. Soc. 152 (2005) A858. [20] X.L. Li, F.Y. Kang, X.D. Bai, W.C. Shen, Electrochem. Commun. 9 (2007) 663. [21] J.Q. Zhao, J.P. He, J.H. Zhou, Y.X. Guo, T. Wang, S.C. Wu, X.C. Ding, R.M. Huang, H.R. Xue, J. Phys. Chem. C 115 (2011) 2888. [22] J.F. Qian, M. Zhou, Y.L. Cao, X.P. Ai, H.X. Yang, J. Phys. Chem. C 114 (2010) 3477. [23] S.W. Oh, S. Myung, S.M. Oh, K.H. Oh, K. Amine, B. Scrosati, Y.K. Sun, Adv. Mater. 22 (2010) 4842.
Contents lists available at ScienceDirect
Electrochemistry Communications 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 / e l e c o m
Influence of carbon coating porosity on the electrochemical performance of LiFePO4 cathode Fangfang Pan, Xueyang Chen, Hongju Li, Xiaodong Xin, Qinqin Chang, Kangning Jiang, Wen-lou Wang ⁎ Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China
a r t i c l e
i n f o
Article history: Received 1 March 2011 Received in revised form 8 April 2011 Accepted 19 April 2011 Available online 22 April 2011 Keywords: Lithium iron phosphate Carbon coating Dual porosity
a b s t r a c t Electrochemical performance of LiFePO4/C should be strongly dependent on the carbon coating structure for core-shell nanocomposites. In this work two types of LiFePO4/C composites with the same carbon coating thickness were prepared using a sol–gel technique, enabling direct comparison of the influence of carbon coating porosity on the performance of LiFePO4/C cathodes. It was found that the material with a distribution of dual mesopores delivers better electrochemical performance than the one with a narrow pore size distribution. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Lithium iron phosphate, which was first studied by Padhi et al. [1], is considered as a promising cathode material for new generation lithium-ion batteries due to its high theoretical capacity, relative low cost, good thermal stability and low environmental impact [2–5]. However, as most of olivine LiMPO4 phosphates, intrinsic low ionic and electronic conductivities of LiFePO4 restrict its high electrochemical activities and prevent its large-scale application in electric vehicles (EVs) and hybrid electric vehicles (HEVs). One of the challenging issues to achieve high power density using LiFePO4 as electrode materials is to tackle its kinetic problems, sluggish mass and charge transport [6]. Porous LiFePO4/C composites have been proved to be potentially attractive materials for high-rate cathodes [13–17,19,21,22]. The work of Gaberscek et al. [13,15,19], Munichandraiah et al. [16] and Cao et al. [22] indicate that a porous structure created inside the LiFePO4/C composite offers improved kinetics and electrochemical performance of electrode material. Studies of Doherty et al. [17], Wu et al. [14] and He et al. [21] suggest that the incorporation of porous carbon matrix can bring considerable enhancement to electrode materials in terms of power and energy densities. Carbon coating which can yield composite cathodes with better performance has been widely used for improving the electronic conductivities of poor electronically conductive electrode materials [7–13,22,23]. However, as for the influence of carbon coating porosity on the electrochemical performance of LiFePO4/C, there are very few
⁎ Corresponding author. Tel.: + 86 551 3606143; fax: + 86 551 3601592. E-mail address: [email protected] (W. Wang). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.04.021
reports addressing this issue until now. Probably because perfect surface coatings are often difficult to achieve and carbon coating introduced into LiFePO4 is generally considered amorphous and porous. In the current work, a simple method was used to prepare LiFePO4/C composites with nanosized particles, high specific area and fine carbon coating. The distribution of pores in the carbon coating layer was controlled by appropriate selection of synthesis parameters. And the carbon coating porosity on rate performance of LiFePO4 composite cathodes was investigated. 2. Experimental As a starting precursor, iron (III) citrate was dissolved at 60 °C in water. Then citric acid, with the citric acid to iron molar ratio of 1.2:1 (sample A) and 2:1 (sample B), was added under constant stirring until the ferric citrate was completely dissolved. Separately, an equimolar water solution of LiH2PO4 was prepared from lithium hydroxide (LiOH, AR) and ammonium dihydric phosphate (NH4H2PO4, AR). Clear solutions were mixed together and magnetically stirred to get a transparent solution. The mixed solution was stirred until a pale green wet gel was formed. Wet gels of sample A and sample B were aged and dried separately with different method before sintered in an inert (pure nitrogen) atmosphere at 650 °C for 7 h to obtain LiFePO4/C composites. The specific area and porosity of LiFePO4/C samples were examined by adsorption-desorption of N2 using ASAP 2020 instrument. Carbon content in the samples was analyzed using Elementar (Vario ELIII). TEM and HRTEM were performed on a JEOL-2010 highresolution transmission electron microscope at 200 kV. The positive electrodes were prepared by pressing a mixture of the active materials, acetylene black, and binder (polyvinylidene fluoride,
F. Pan et al. / Electrochemistry Communications 13 (2011) 726–729
PVDF, dissolved in N-methyl-2-pyrrolidone, NMP) in a weight ratio of 80:15:5 on aluminum foil followed by a drying procedure in vacuum at 120 °C for 8 h. The Li metal was used as the counter and reference electrodes. 1 M LiPF6 in a 1:1 mixture of ethylene carbonate/diethyl carbonate was used as electrolyte. Charge and discharge performance was tested on a multi-channel battery tester (Shenzhen Neware, BTS, China) at different current densities. CV and EIS were measured on an electrochemical workstation (CHI660C) with a three-electrode system. 3. Results and discussions Morphology of the final products was characterized by transmission electron microscopy (TEM). Each primary crystallite is completely coated by a carbon layer to form core-shell LiFePO4/C structure as shown in TEM images in Fig. 1. In addition, we can also show directly in Fig. 1(a) and Fig. 1(d) that the carbon film is deposited on the surface of each particle. As disclosed in the HRTEM images shown in Fig. 1(b) and Fig. 1(e), the carbon films of both samples are about 3 nm thick. As pointed out by Zhou et al. [4], surface carbon shell can effectively restrict the crystallite growth of LiFePO4. Mean crystallite size of sample A and sample B estimated using Scherrer's formula is 56.4 and 27.7 nm, respectively. In order to further investigate the pore structure of the nanocomposite, a nitrogen isothermal-adsorption technique was used. As shown in Fig. 1(c) and Fig. 1(f), adsorption/desorption isotherms of sample A and sample B both exhibit a hysteresis which is typical of a mesoporous structure. According to BET analysis, total specific surface areas of sample A and sample B are 88.6 and 135.2 m2 g− 1, respectively. Shown in the inset of Fig. 1(c) and Fig. 1(f), the Barrett–Joyner–Halenda (BJH) pore-size distribution indicates that sample A has a dual pore-size distribution at 4 nm and 11 nm while sample B has a centered pore-size distribution at 4 nm. Since active particles in sample A and sample B are spherical and the average diameter of sample A is twice as much as that of sample B, mass fractions of carbon needed to cover sample B will be twice as many as
727
that of sample A, agreeing with the elemental analysis results which show that carbon content of sample A and sample B is 5.56% and 11.83%, respectively. According to BET analysis, total specific surface areas of sample A and sample B are 88.6 and 135.2 m2 g− 1, respectively. If the specific surface area totally comes from the aggregated LiFePO4 particles, then the calculated value in terms of diameter for sample A and sample B should be 29.5 m2 g− 1 and 59.8 m2 g− 1, which are much smaller than the obtained results from BET analysis. So, the large specific surface area can be mostly attributed to the porous carbon phase in the composites. Additionally, Zhou et al. [4] reported the special BET surface of their synthesized core-shell LiFePO4/carbon composite with an average particle size of 36 nm to be 50 m2 g− 1. The difference may also be due to the contribution of porous carbon layer to the high specific area of LiFePO4/C composites prepared in this work. And thus it is reasonable for us to assume that the BJH distribution of pore sizes shown in the inset of Fig. 1(c) and Fig. 1(f) mainly represents the pore size distribution in the coating layer. Typical discharge curves of sample A and sample B are shown in Fig. 2(a) and Fig. 2(b). Excluding the mass of carbon in LiFePO4/C composite, sample A and sample B delivered a specific capacity of 169 mAhg− 1 and 161 mAhg− 1 at 1 C, respectively, which are both close to the theoretical capacity (170 mAhg− 1) of LiFePO4. Meanwhile, discharged at 10 C, sample A retained 74% of its full capacity when sample B retained 66% of its full capacity (based on the mass of LiFePO4 in the composite). Comparison of the rate performance of sample A and sample B indicates that the specific capacity and material utilization was improved by the introduction of larger mesopores which act as a transport system for electrolyte and increase the accessibility of smaller pores [18]. Cycling stability of asobtained samples with at different discharge current rates is presented in Fig. 2(c), from which we can see that both samples show good cycling stability at discharge rates of 1 C and 10 C. The ac impedance of the two types of cathodes was measured before cycling. The impedance spectra are shown in Fig. 3(a). In the high frequency region, the intercepts with the real axis of the sample
Fig. 1. Characterization of the as prepared LiFePO4/C composites: (a) and (b) selected TEM image and HRTEM image of sample A; (d) and (e) selected TEM image and HRTEM image of sample B; (c) and (f) nitrogen adsorption and desorption isotherms of sample A and sample B. Inset: BJH distribution of pores obtained from desorption branch.
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F. Pan et al. / Electrochemistry Communications 13 (2011) 726–729
Fig. 2. (a and b) Discharge potential profiles of sample A and sample B at different rates (1 C, 2 C, 5 C and 10 C); (c) cyclic performance of samples A and B at 1 C and 10 C discharge rates.
A and sample B composite cathodes are at 260 Ω and 265 Ω (obtained from computer simulation), respectively. The values are believed to be the total resistance related to the charge transfer resistance [20] which for sample A and sample B are almost the same, indicating that sample A and sample B have the same interfacial property. Fig. 3(b) and Fig. 3(c) show cyclic voltammograms of LiFePO4/C composite samples. Under low scan rate (0.1 mV s− 1), sample A and sample B showed similar peak currents. However, when the scan rate increases to 1 mV s− 1, the peak current increase, which is a good indicator of electrode reaction kinetics, is much greater for sample A than for sample B, highlighting the improved high rate performance of sample A as discussed earlier. And this feature can be ascribed to the reduced transfer resistance of lithium ions in the coating layer with dual
Fig. 3. (a) AC impedance spectra for fresh cell assembled with sample A and sample B as cathode; (b) and (c) CV curves of sample A and sample B at scanning rate of 0.1 mV s− 1 and 1 mV s− 1.
porosity in sample A which provides quick pathways for ions to penetrate. The above electrochemical results demonstrate that kinetic performance of sample A is better than that of sample B. The reason could be that carbon coating porosity plays an important role in improving the rate performance of LiFePO4/C composites. The network of mesopores centered at 4 nm and relatively larger pores ranging from 5 nm to 20 nm appear to facilitate the electrolyte diffusion to the surface of the active material and provide favorable channels for lithium ions to penetrate. It is desirable electrochemical applications for electrode material to have an optimal combination of high porosity and surface area with good transport properties. A
F. Pan et al. / Electrochemistry Communications 13 (2011) 726–729
hierarchical pore system is considered to serve better as paths for fast ionic and electronic in a battery electrode [18]. Therefore it is more optimal to introduce a hierarchical pore system featuring dual porosity into the electrode materials to deliver both good capacity and achieve good capability. 4. Conclusions Pore size distribution in the carbon coating plays an important part in material utilization at different discharge rates. LiFePO4/C material with a distribution of well-defined dual mesopores delivers better electrochemical performance than the one with a narrow pore size distribution. Acknowledgements This work was supported by grants from Natural Science Foundation of Anhui province (Grant no. 090414178) and National Synchrotron Radiation Laboratory. References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. [2] R. Dominko, M. Bele, J.M. Goupil, M. Gaberscek, D. Hanzel, I. Arcon, J. Jamnik, Chem. Mater. 19 (2007) 2960.
729
[3] B.L. Ellis, W.R.M. Makahnouk, Y. Makimura, K. Toghill, L.F. Nazar, Nat. Mater. 6 (2007) 749. [4] Y. Wang, Y. Wang, E. Hosono, K. Wang, H. Zhou, Angew, Chem. Int. Ed. 47 (2008) 7461. [5] J.B. Goodenough, Y. Kim, Chem. Mater. 22 (2010) 587. [6] R. Amin, P. Balaya, J. Maier, Electrochem, Solid State Lett 10 (2007) A13. [7] H. Huang, S.-C. Yin, L.F. Nazar, Electrochem, Solid-State Lett. 4 (2001) A170. [8] F. Croce, A.D. Epifanio, J. Hassoun, A. Deptula, T. Olczac, B. Scrosati, Electrochem, Solid State Lett. 5 (2002) A47. [9] R. Dominko, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, S. Pejovnik, J. Jamnik, J. Electrochem. Soc. 152 (2005) A607. [10] C.-W. Ong, Y.-K. Lin, J.-S. Chen, J. Electrochem. Soc. 154 (2007) A527. [11] Y. Lin, H. Pan, M. Gao, Y. Liu, J. Electrochem. Soc. 154 (2007) A1124. [12] D. Choi, P.N. Kumta, J. Power Sources 163 (2007) 1064. [13] R. Dominko, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, J.M. Goupil, S. Pejovnik, J. Jamnik, J. Power Sources 153 (2006) 274–280. [14] X.-L. Wu, L.-Y. Jiang, F.-F. Cao, Y.-G. Guo, L.-J. Wan, Adv. Mater. 21 (2009) 2710. [15] R. Dominko, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, J.M. Goupil, S. Pejovnik, J. Jamnik, J. Power Sources 153 (2006) 274–280. [16] N.N. Sinha, C. Shivakumara, N. Munichandraiah, ACS Appl. Mater. Interfaces 2 (2010) 2031. [17] C.M. Doherty, R.A. Caruso, B.M. Smarsly, P. Adelhelm, C.J. Drummond, Chem. Mater. 21 (2009) 5300. [18] Y.S. Hu, P. Adelhelm, B.M. Smarsly, S. Hore, M. Antonietti, J. Maier, Adv. Funct. Mater. 17 (2007) 1873. [19] R. Dominko, J.M. Goupil, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, J. Jamnik, J. Electrochem. Soc. 152 (2005) A858. [20] X.L. Li, F.Y. Kang, X.D. Bai, W.C. Shen, Electrochem. Commun. 9 (2007) 663. [21] J.Q. Zhao, J.P. He, J.H. Zhou, Y.X. Guo, T. Wang, S.C. Wu, X.C. Ding, R.M. Huang, H.R. Xue, J. Phys. Chem. C 115 (2011) 2888. [22] J.F. Qian, M. Zhou, Y.L. Cao, X.P. Ai, H.X. Yang, J. Phys. Chem. C 114 (2010) 3477. [23] S.W. Oh, S. Myung, S.M. Oh, K.H. Oh, K. Amine, B. Scrosati, Y.K. Sun, Adv. Mater. 22 (2010) 4842.