- Email: [email protected]
C composite prepared by an in situ polymerization restriction method
Journal of Alloys and Compounds 563 (2013) 264–268
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Superior electrode performance of LiFePO4/C composite prepared by an in situ polymerization restriction method Jian Chen a, Yong-Cun Zou a, Feng Zhang b, Yuan-Chun Zhang a, Fei-Fan Guo a, Guo-Dong Li a,⇑ a b
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China Laboratory of Advanced Rechargeable Batteries, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
a r t i c l e
i n f o
Article history: Received 11 December 2012 Received in revised form 22 February 2013 Accepted 22 February 2013 Available online 4 March 2013 Keywords: Electrode materials Microstructure Coating materials Precipitation
a b s t r a c t The LiFePO4/C composite is prepared by heating the mixture of resorcinol–formaldehyde gel and FePO4, synthesized by an in situ polymerization restriction method, and lithium acetate dihydrate in the atmosphere of nitrogen. The physical and electrochemical properties of the LiFePO4/C composite are investigated by X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy and electrochemical measurements. The discharge capacity of LiFePO4 is as high as 155.6 mA h g1 in the first cycle at 0.5C, and it could remain 144.0 mA h g1 after 50 cycles. Even at the high rates of 10C, 20C and 50C, the initial discharge capacities of the electrodes exhibit 115.6 mA h g1, 84.5 mA h g1 and 67.8 mA h g1, and the electrodes deliver capacity retention of 89.5%, 90.9% and 85.7% after 1000 cycles, respectively. The outstanding electrochemical performance could be attributed to the small particle size and good electronic conductivity of the composite. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction During the past decade, LiMPO4 compounds (M = Fe, Mn, Co) have attracted extensive attention as cathode materials for lithium-ion batteries because of their promising electrochemical performances [1,2]. Among these materials, LiFePO4 is one of the most competitive cathode materials due to its low cost, environmental benignity and excellent thermal stability [1,3]. Furthermore, LiFePO4 has a flat discharge plateau at about 3.4 V versus Li/Li+, which is compatible with most commonly used organic electrolytes [4]. Unfortunately, its power performance is greatly limited by the poor electronic conductivity and low lithium ion diffusivity [1,5–8]. Thus far, tremendous effective approaches have been investigated to overcome these obstacles by minimizing the particle size [9], applying conductive surface coatings [10–12] and doping with metal ions [13–15]. It should be noted that carbon coating is a common strategy to enhance the electronic conductivity. On the other hand, smaller LiFePO4 particles can reduce the diffusion path of lithium ions and increase the effective active surface area [7]. Wang et al. [5] reported that the combination of the carbon coating and nano-technology was the most effective approach to improve the high-rate performance of LiFePO4. Up to now, various methods have been developed for the synthesis of LiFePO4, including solid-state process [16–18], spray ⇑ Corresponding author. Tel.: +86 431 85168318; fax: +86 431 85168624. E-mail address: [email protected] (G.-D. Li). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.02.131
technique [10], sol–gel method [19–21], mechanical activation [22–24], microwave synthesis [25], solvothermal synthesis [26– 28] and chemical vapor deposition [29]. The electrochemical performances of LiFePO4 materials prepared by the above approaches were improved remarkably. However, many reported methods require expensive raw material, long annealing times or several grinding steps. Furthermore, the particle size is inevitably increased due to the long annealing time and high temperature, which is not favorable to improve the electrochemical performance of LiFePO4. In addition, some LiFePO4/C composites were synthesized by mixed and ground the LiFePO4 powders with carbon sources rather than in situ generated carbon [8,22,23]. The latter approach should be more effective to restrict the crystallite growth of LiFePO4 particles and improve the electrochemical performance of LiFePO4 [30–34]. Therefore, it is still a challenge to prepare nano-sized LiFePO4/C composites at high temperature. Here, we developed an in situ polymerization restriction method for the synthesis of LiFePO4/C composite using inexpensive Fe3+ salt as raw material and resorcinol–formaldehyde (RF) gel as carbon source. Carbonaceous net, generated in situ, restricts the crystallite growth of LiFePO4 particles and provides a conducting web between the LiFePO4 particles, which can effectively improve the electrochemical performance of the LiFePO4 material [23,31]. A schematic representation of the synthesis was elaborated, and the electrochemical performance of the LiFePO4/C composite was measured systematically.
J. Chen et al. / Journal of Alloys and Compounds 563 (2013) 264–268 2. Experimental 2.1. Powder preparation and treatment The RF/FePO4 composite was prepared by a mild hydrothermal method. Firstly, stoichiometric amounts of ferric nitrate nonahydrate (Fe(NO3)39H2O) and phosphoric acid were dissolved into 30 mL of de-ionized water, and the solution was stirred for 30 min to obtain a mixture with homogeneity. The concentration of Fe3+ was 0.025 mol L1. Then, the desired amounts of resorcinol (C6H6O2) and hexamethylenetetramine ((CH2)6N4) were added into the solution, and the resulting solution was placed into a 50 mL Teflon-lined stainless steel autoclave and maintained at 85 °C for 48 h. After the autoclave was cooled to room temperature, the resulting RF/FePO4 composite was washed several times with de-ionized water and dried at 80 °C for 12 h. The LiFePO4/C composite was synthesized by milling the obtained RF/FePO4 composite with CH3COOLi2H2O and heating at 750 °C for 6 h under flowing N2. 2.2. Structural characterization The as-synthesized product was examined by X-ray diffraction using Cu K radiation (k = 1.5406 Å) with 2h ranging from 10° to 60°. The Raman spectrum was recorded on a Renishaw Model 1000 model confocal microscopic Raman spectrometer with an excitation wavelength of 514.5 nm. The morphology and particle size were observed through a field emission scanning electron microscope (JEOL JSM-6700F) and a transmission electron microscope (Hitachi H-8100). The carbon content of the LiFePO4/C composite was determined by thermogravimetric analysis (TGA) on a Netzsch STA 449C instrument from room temperature to 800 °C at a heating rate of 10 °C min1 under oxygen atmosphere. 2.2.1. Electrochemical measurements The working electrode was prepared by mixing 80 wt.% LiFePO4/C composite, 10 wt.% PVDF and 10 wt.% acetylene black with NMP (1-methyl-2-pyrrolidone) to form the slurry, which was then spread onto an aluminum foil and dried at 120 °C for 24 h in a vacuum oven. The batteries were assembled in an argon-filled gloved-box, in which oxygen and moisture level less than 1 ppm, and the electrolyte was 1 M LiPF6 in a mixture of EC (ethylene carbonate), DMC (dimethyl carbonate) and EMC (ethylmethyl carbonate) (1:1:1 by weight). Typically, a working electrode of 1.5 cm2 was prepared with the active material mass loading of 3 mg per cm2. The coin cell was fabricated using lithium metal as counter electrode. Cyclic voltammetry (CV) measurements were performed on a CHI 660A electrochemical workstation. CVs were conducted in the cut-off voltage range of 2.0–4.2 V versus Li/Li+ at a scan rate of 0.2 mV s1. Galvanostatic charge/discharge tests were carried out between 2.0 and 4.2 V using a LAND CT2001A cell testing apparatus. All of the electrochemical tests were performed at room temperature. The C-rates and storage capacities were calculated on the mass of LiFePO4 with the amount of carbon being subtracted (1C = 170 mA g1).
3. Results and discussion Hexamethylenetetramine was usually used as a potential precipitant of homogeneous precipitation. It is known that hexamethylenetetramine is subject to hydrolysis into formaldehyde and ammonium ions under acidic conditions [35], which is the most crucial step for the preparation of RF/FePO4. On the one hand, the pH value of the solution increases and facilitates the formation of FePO4 precipitation; on the other hand, formaldehyde, formed in situ, as one reactant for the raw material of the resorcinol–formaldehyde gel. The process for the preparation of RF/FePO4 composite is assumed as follow:
ðCH2 Þ6 N4 þ 4Hþ þ 6H2 O ! 6CH2 O þ 4NHþ4
ð1Þ
Fe3þ þ PO3 4 ! FePO4 #
ð2Þ
CH2 O þ C6 H6 O2 ! resorcinol-formaldehydeðRFÞgel
ð3Þ
It is known that a polymer with a cross-linked three dimensional network could be obtained via the reaction between resorcinol and formaldehyde [36]. The RF gel, formed during the synthesis process, could inhibit the growth and agglomeration of the FePO4 particles, which further restricts the in situ crystallite growth of LiFePO4. The residual carbon from the pyrolysis of RF
265
gel can effectively increase the electron conduction between the particles and decrease the charge-transfer resistance of the cells [37]. XRD pattern of the LiFePO4/C composite is shown in Fig. 1. All of the diffraction peaks can be indexed to standard pattern of olivine structured LiFePO4 with orthorhombic crystal structure (JCPDS No.40-1499, space group: Pnma). There are no diffraction peaks related to Fe2P and Fe3P, which are usually present in LiFePO4 materials [20]. Furthermore, there are no diffraction peaks of carbon due to its amorphous form [11]. The electrochemical performance of active material is greatly affected by the morphology and particle size [11]. Fig. 2a shows the SEM image of RF/FePO4 composite. It is clear that most of the RF/FePO4 particles are bulk, and the particle size distribution is very wide. And the LiFePO4/C composite was obtained via the reaction between RF/FePO4 and CH3COOLi2H2O during calcination in the presence of N2. In this process, the RF gel worked as the reductant. It should be noted that a reducing gas could be produced during the carbonization of the RF gel such as CO [38]. The SEM and TEM images suggest that the LiFePO4 nano-particles are densely embedded in the carbon framework, as shown in Fig. 2b–e. Fig. 2f shows the high resolution transmission electron microscopy (HRTEM) image of LiFePO4/C composite. The lattice fringes with a width of 0.35 nm correspond to the (1 1 1) plane of LiFePO4 [34], and the thickness of the amorphous carbon, coated on LiFePO4, is around 3 nm. The nano-sized LiFePO4 particles can shorten the diffusion path of lithium ions and increase the effective reaction areas between the LiFePO4 particles and the surrounding electrolyte. The carbon generated by the carbonization of RF gel in inert atmosphere can effectively improve the electronic conductivity of LiFePO4. Raman experiments are extremely sensitive to the surface components due to the limited penetration depth of the incident laser into the sample [39]. To detect the surface components of LiFePO4/ C composite, the Raman spectrum of the LiFePO4/C composite is measured, as shown in Fig. 3a. It can be seen that the bands in the range of 1100–1460 cm1 and 1500–1700 cm1 are attributed to the D-band (disorder-induced phonon mode) and G-band (graphite band) of carbon, respectively [40]. In addition, the symmetric PO4 stretching vibration of LiFePO4 is not observed in the spectrum of the sample, which could be ascribed to the screening effect of carbon [41]. This result indicates a uniform carbon coating on the surface of LiFePO4. The amount of carbon in LiFePO4/C composite is determined by thermogravimetric analysis. Fig. 3b shows the TGA curve of the LiFePO4/C composite tested in oxygen. It should be mentioned that the pure LiFePO4 can be oxidized to Li3Fe2(PO4)3 and Fe2O3 in air from 200 °C to 500 °C with a weight gain of 5.07 wt.% [42]. The
Fig. 1. X-ray diffraction pattern of LiFePO4/C composite.
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Fig. 2. SEM images of (a) RF/FePO4 and (b and c) LiFePO4/C composite, (d and e) TEM images of LiFePO4/C composite and (f) The HRTEM image of the square area in (e).
composite is oxidized to Li3Fe2(PO4)3 and Fe2O3 at about 250 °C with the mass of the sample increasing. Further increasing the temperature, the weight-loss should be attributed to the oxide of the carbon in the sample. Therefore, the content of carbon in the LiFePO4/C composite is around 10.0 wt.% by taking into account the theoretical weight gain (5.07%) of the pure LiFePO4 [43]. Fig. 4 shows the CV curves of LiFePO4/C composite at a scan rate of 0.2 mV s1 from 2.0 to 4.2 V. Each of the CV curves includes an oxidation peak and a reduction peak, which corresponds to the charge/discharge reaction of the Fe2+/Fe3+ redox couple [16,32,37]. The good overlap of CV curves indicates the high reversibility of lithium insertion/deinsertion reactions in the LiFePO4/C composite. The electrochemical performance of LiFePO4/C composite is investigated with lithium insertion/deinsertion process. Fig. 5 shows the charge/discharge curves of the LiFePO4/C electrode at 0.5C rate with a voltage window of 2.0–4.2 V. The electrode delivers a high discharge capacity of 155.6 mA h g1 in the first cycle, which accounts for 91.5% of the theoretical capacity of LiFePO4 (170 mA h g1), and it remains 144.0 mA h g1 after 50 cycles. The flat voltage plateaus at about 3.4 V and 3.5 V represent the insertion and extraction of lithium ion, implying the two-phase redox reaction of LiFePO4 [13]. However, there is another voltage plateau around 2.5 V, which can be attributed to the presence of trace
Fe2O3 [44,45]. In addition, the voltage difference between charge and discharge plateaus remains the same up cycling, showing that the LiFePO4/C composite has both excellent reaction reversibility and high structure stability. Based on the results discussed above, it can be concluded that the LiFePO4/C composite has superior capacity and cycling performance at a low charge/discharge rate of 0.5C. However, high-rate properties of the sample are characterized for potential applications in high power lithium-ion batteries. The rate capability of the LiFePO4/C electrode at different C-rates from 0.5C to 50C is presented in Fig. 6. Average discharge capacities are obtained as 154.5, 143.9, 136.2, 119.4, 107.5, 84.1 and 65.8 mA h g1, respectively, for discharge rates of 0.5C, 1C, 2C, 5C, 10C, 20C and 50C. With increased current rates, the discharge capacities decrease regularly. In addition, the discharge capacities recover to the original value when the current density returns to 0.5C rate, indicating that the LiFePO4/C composite has good electrochemical reversibility and structural stability even after fast-charging and high-current discharging. The typical discharge profiles of LiFePO4/C electrode at different current rates from 0.5C to 50C between 2.0 and 4.2 V are shown in Fig. 7. At the rate of 0.5C, the electrode delivers a discharge capacity of 155.6 mA h g1, and a flat voltage platform is observed, corresponding to the Fe2+/Fe3+ redox reaction. It should be noted that
J. Chen et al. / Journal of Alloys and Compounds 563 (2013) 264–268
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Fig. 5. Charge–discharge curves of the LiFePO4/C composite at 0.5C rate.
Fig. 3. (a) Raman spectrum of the as-prepared LiFePO4/C composite and (b) TGA curve of the LiFePO4/C composite.
Fig. 6. Rate performance of the LiFePO4/C composite at different rates.
Fig. 4. CV curves of the LiFePO4/C composite in the voltage range of 2.0–4.2 V at a scan rate of 0.2 mV s1.
not only the plateau capacity but also the sluggish slope capacity contributed to the total discharge capacities at the current rates smaller than 10C. But the mechanism of the sluggish slop capacity is still not clear [27,40]. As the discharge rates reach 20C and 50C, the total discharge capacities are mainly contributed by the sluggish slop capacity. The discharge platform voltage decreases with the current rate increasing, which attributes to the increase in polarization resistance [46]. In addition, the discharge voltage declines rapidly as the discharge rate increases from 10C to 20C. It may be attributed to the significant change of lithium storage mechanism. For 20C to 50C, the discharge voltage changes a little maybe due to the same lithium storage mechanism of 20C and 50C. Although the discharge capacity gradually decreases with the current rate increasing, a discharge capacity of 120.0 mA h g1
Fig. 7. The typical discharge curves of LiFePO4/C composite at various current rates.
has been achieved at 5C. Even when the current rate increases to 50C, the discharge capacity can reach 63.9 mA h g1. The superior capacities at high rates could be attributed to the small particle size and good electronic conductivity of the composite. For HEVs or PHEVs applications, batteries not only need to charge and discharge at high current rates but also have a longterm cycling [4]. The cycling stability of the LiFePO4/C composite at high rates is shown in Fig. 8. The sample electrodes deliver the discharge capacities of 103.5 mA h g1, 76.8 mA h g1 and 58.1 mA h g1 after 1000 cycles at 10C, 20C and 50C, and the electrodes deliver capacity retention of 89.5%, 90.9% and 85.7%, respec-
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Fig. 8. Cycle performance for prepared LiFePO4/C composite at high rates.
tively. The superior capacity and outstanding cycling performance indicate that the structure of the LiFePO4/C nano-particles is very stable, and the insertion and extraction of lithium ion process is quite reversible even at high charge/discharge rates. 4. Conclusions In summary, the LiFePO4/C composite with good electrode performance has been prepared by an in situ polymerization restriction method. The formed RF gel can effectively inhibit the growth of LiFePO4 particles, and the carbon from the pyrolysis of RF gel obviously increases the electron conduction between the particles and reduces electrode polarization. As the cathode material for lithium-ion batteries, the LiFePO4/C composite showed high discharge capacities and superior cycling performances at various charge/discharge rates. The enhanced electrochemical performance was attributed to the small particle size and good electronic conductivity of the composite. All in all, the synthesized composite is a good candidate for cathode materials of high power lithiumion batteries. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (21071060). References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188–1194. [2] E. Markevich, R. Sharabi, O. Haik, V. Borgel, G. Salitra, D. Aurbach, G. Semrau, M.A. Schmidt, N. Schall, C. Stinner, J. Power Sources 196 (2011) 6433–6439. [3] Y. Yin, M. Gao, J. Ding, Y. Liu, L. Shen, H. Pan, J. Alloys Comp. 509 (2011) 10161– 10166. [4] G. Wang, H. Liu, J. Liu, S. Qiao, G. Max Lu, P. Munroe, H. Ahn, Adv. Mater. 22 (2010) 4944–4948. [5] Q. Wang, S.X. Deng, H. Wang, M. Xie, J.B. Liu, H. Yan, J. Alloys Comp. 553 (2013) 69–74.
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Superior electrode performance of LiFePO4/C composite prepared by an in situ polymerization restriction method Jian Chen a, Yong-Cun Zou a, Feng Zhang b, Yuan-Chun Zhang a, Fei-Fan Guo a, Guo-Dong Li a,⇑ a b
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China Laboratory of Advanced Rechargeable Batteries, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
a r t i c l e
i n f o
Article history: Received 11 December 2012 Received in revised form 22 February 2013 Accepted 22 February 2013 Available online 4 March 2013 Keywords: Electrode materials Microstructure Coating materials Precipitation
a b s t r a c t The LiFePO4/C composite is prepared by heating the mixture of resorcinol–formaldehyde gel and FePO4, synthesized by an in situ polymerization restriction method, and lithium acetate dihydrate in the atmosphere of nitrogen. The physical and electrochemical properties of the LiFePO4/C composite are investigated by X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy and electrochemical measurements. The discharge capacity of LiFePO4 is as high as 155.6 mA h g1 in the first cycle at 0.5C, and it could remain 144.0 mA h g1 after 50 cycles. Even at the high rates of 10C, 20C and 50C, the initial discharge capacities of the electrodes exhibit 115.6 mA h g1, 84.5 mA h g1 and 67.8 mA h g1, and the electrodes deliver capacity retention of 89.5%, 90.9% and 85.7% after 1000 cycles, respectively. The outstanding electrochemical performance could be attributed to the small particle size and good electronic conductivity of the composite. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction During the past decade, LiMPO4 compounds (M = Fe, Mn, Co) have attracted extensive attention as cathode materials for lithium-ion batteries because of their promising electrochemical performances [1,2]. Among these materials, LiFePO4 is one of the most competitive cathode materials due to its low cost, environmental benignity and excellent thermal stability [1,3]. Furthermore, LiFePO4 has a flat discharge plateau at about 3.4 V versus Li/Li+, which is compatible with most commonly used organic electrolytes [4]. Unfortunately, its power performance is greatly limited by the poor electronic conductivity and low lithium ion diffusivity [1,5–8]. Thus far, tremendous effective approaches have been investigated to overcome these obstacles by minimizing the particle size [9], applying conductive surface coatings [10–12] and doping with metal ions [13–15]. It should be noted that carbon coating is a common strategy to enhance the electronic conductivity. On the other hand, smaller LiFePO4 particles can reduce the diffusion path of lithium ions and increase the effective active surface area [7]. Wang et al. [5] reported that the combination of the carbon coating and nano-technology was the most effective approach to improve the high-rate performance of LiFePO4. Up to now, various methods have been developed for the synthesis of LiFePO4, including solid-state process [16–18], spray ⇑ Corresponding author. Tel.: +86 431 85168318; fax: +86 431 85168624. E-mail address: [email protected] (G.-D. Li). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.02.131
technique [10], sol–gel method [19–21], mechanical activation [22–24], microwave synthesis [25], solvothermal synthesis [26– 28] and chemical vapor deposition [29]. The electrochemical performances of LiFePO4 materials prepared by the above approaches were improved remarkably. However, many reported methods require expensive raw material, long annealing times or several grinding steps. Furthermore, the particle size is inevitably increased due to the long annealing time and high temperature, which is not favorable to improve the electrochemical performance of LiFePO4. In addition, some LiFePO4/C composites were synthesized by mixed and ground the LiFePO4 powders with carbon sources rather than in situ generated carbon [8,22,23]. The latter approach should be more effective to restrict the crystallite growth of LiFePO4 particles and improve the electrochemical performance of LiFePO4 [30–34]. Therefore, it is still a challenge to prepare nano-sized LiFePO4/C composites at high temperature. Here, we developed an in situ polymerization restriction method for the synthesis of LiFePO4/C composite using inexpensive Fe3+ salt as raw material and resorcinol–formaldehyde (RF) gel as carbon source. Carbonaceous net, generated in situ, restricts the crystallite growth of LiFePO4 particles and provides a conducting web between the LiFePO4 particles, which can effectively improve the electrochemical performance of the LiFePO4 material [23,31]. A schematic representation of the synthesis was elaborated, and the electrochemical performance of the LiFePO4/C composite was measured systematically.
J. Chen et al. / Journal of Alloys and Compounds 563 (2013) 264–268 2. Experimental 2.1. Powder preparation and treatment The RF/FePO4 composite was prepared by a mild hydrothermal method. Firstly, stoichiometric amounts of ferric nitrate nonahydrate (Fe(NO3)39H2O) and phosphoric acid were dissolved into 30 mL of de-ionized water, and the solution was stirred for 30 min to obtain a mixture with homogeneity. The concentration of Fe3+ was 0.025 mol L1. Then, the desired amounts of resorcinol (C6H6O2) and hexamethylenetetramine ((CH2)6N4) were added into the solution, and the resulting solution was placed into a 50 mL Teflon-lined stainless steel autoclave and maintained at 85 °C for 48 h. After the autoclave was cooled to room temperature, the resulting RF/FePO4 composite was washed several times with de-ionized water and dried at 80 °C for 12 h. The LiFePO4/C composite was synthesized by milling the obtained RF/FePO4 composite with CH3COOLi2H2O and heating at 750 °C for 6 h under flowing N2. 2.2. Structural characterization The as-synthesized product was examined by X-ray diffraction using Cu K radiation (k = 1.5406 Å) with 2h ranging from 10° to 60°. The Raman spectrum was recorded on a Renishaw Model 1000 model confocal microscopic Raman spectrometer with an excitation wavelength of 514.5 nm. The morphology and particle size were observed through a field emission scanning electron microscope (JEOL JSM-6700F) and a transmission electron microscope (Hitachi H-8100). The carbon content of the LiFePO4/C composite was determined by thermogravimetric analysis (TGA) on a Netzsch STA 449C instrument from room temperature to 800 °C at a heating rate of 10 °C min1 under oxygen atmosphere. 2.2.1. Electrochemical measurements The working electrode was prepared by mixing 80 wt.% LiFePO4/C composite, 10 wt.% PVDF and 10 wt.% acetylene black with NMP (1-methyl-2-pyrrolidone) to form the slurry, which was then spread onto an aluminum foil and dried at 120 °C for 24 h in a vacuum oven. The batteries were assembled in an argon-filled gloved-box, in which oxygen and moisture level less than 1 ppm, and the electrolyte was 1 M LiPF6 in a mixture of EC (ethylene carbonate), DMC (dimethyl carbonate) and EMC (ethylmethyl carbonate) (1:1:1 by weight). Typically, a working electrode of 1.5 cm2 was prepared with the active material mass loading of 3 mg per cm2. The coin cell was fabricated using lithium metal as counter electrode. Cyclic voltammetry (CV) measurements were performed on a CHI 660A electrochemical workstation. CVs were conducted in the cut-off voltage range of 2.0–4.2 V versus Li/Li+ at a scan rate of 0.2 mV s1. Galvanostatic charge/discharge tests were carried out between 2.0 and 4.2 V using a LAND CT2001A cell testing apparatus. All of the electrochemical tests were performed at room temperature. The C-rates and storage capacities were calculated on the mass of LiFePO4 with the amount of carbon being subtracted (1C = 170 mA g1).
3. Results and discussion Hexamethylenetetramine was usually used as a potential precipitant of homogeneous precipitation. It is known that hexamethylenetetramine is subject to hydrolysis into formaldehyde and ammonium ions under acidic conditions [35], which is the most crucial step for the preparation of RF/FePO4. On the one hand, the pH value of the solution increases and facilitates the formation of FePO4 precipitation; on the other hand, formaldehyde, formed in situ, as one reactant for the raw material of the resorcinol–formaldehyde gel. The process for the preparation of RF/FePO4 composite is assumed as follow:
ðCH2 Þ6 N4 þ 4Hþ þ 6H2 O ! 6CH2 O þ 4NHþ4
ð1Þ
Fe3þ þ PO3 4 ! FePO4 #
ð2Þ
CH2 O þ C6 H6 O2 ! resorcinol-formaldehydeðRFÞgel
ð3Þ
It is known that a polymer with a cross-linked three dimensional network could be obtained via the reaction between resorcinol and formaldehyde [36]. The RF gel, formed during the synthesis process, could inhibit the growth and agglomeration of the FePO4 particles, which further restricts the in situ crystallite growth of LiFePO4. The residual carbon from the pyrolysis of RF
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gel can effectively increase the electron conduction between the particles and decrease the charge-transfer resistance of the cells [37]. XRD pattern of the LiFePO4/C composite is shown in Fig. 1. All of the diffraction peaks can be indexed to standard pattern of olivine structured LiFePO4 with orthorhombic crystal structure (JCPDS No.40-1499, space group: Pnma). There are no diffraction peaks related to Fe2P and Fe3P, which are usually present in LiFePO4 materials [20]. Furthermore, there are no diffraction peaks of carbon due to its amorphous form [11]. The electrochemical performance of active material is greatly affected by the morphology and particle size [11]. Fig. 2a shows the SEM image of RF/FePO4 composite. It is clear that most of the RF/FePO4 particles are bulk, and the particle size distribution is very wide. And the LiFePO4/C composite was obtained via the reaction between RF/FePO4 and CH3COOLi2H2O during calcination in the presence of N2. In this process, the RF gel worked as the reductant. It should be noted that a reducing gas could be produced during the carbonization of the RF gel such as CO [38]. The SEM and TEM images suggest that the LiFePO4 nano-particles are densely embedded in the carbon framework, as shown in Fig. 2b–e. Fig. 2f shows the high resolution transmission electron microscopy (HRTEM) image of LiFePO4/C composite. The lattice fringes with a width of 0.35 nm correspond to the (1 1 1) plane of LiFePO4 [34], and the thickness of the amorphous carbon, coated on LiFePO4, is around 3 nm. The nano-sized LiFePO4 particles can shorten the diffusion path of lithium ions and increase the effective reaction areas between the LiFePO4 particles and the surrounding electrolyte. The carbon generated by the carbonization of RF gel in inert atmosphere can effectively improve the electronic conductivity of LiFePO4. Raman experiments are extremely sensitive to the surface components due to the limited penetration depth of the incident laser into the sample [39]. To detect the surface components of LiFePO4/ C composite, the Raman spectrum of the LiFePO4/C composite is measured, as shown in Fig. 3a. It can be seen that the bands in the range of 1100–1460 cm1 and 1500–1700 cm1 are attributed to the D-band (disorder-induced phonon mode) and G-band (graphite band) of carbon, respectively [40]. In addition, the symmetric PO4 stretching vibration of LiFePO4 is not observed in the spectrum of the sample, which could be ascribed to the screening effect of carbon [41]. This result indicates a uniform carbon coating on the surface of LiFePO4. The amount of carbon in LiFePO4/C composite is determined by thermogravimetric analysis. Fig. 3b shows the TGA curve of the LiFePO4/C composite tested in oxygen. It should be mentioned that the pure LiFePO4 can be oxidized to Li3Fe2(PO4)3 and Fe2O3 in air from 200 °C to 500 °C with a weight gain of 5.07 wt.% [42]. The
Fig. 1. X-ray diffraction pattern of LiFePO4/C composite.
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Fig. 2. SEM images of (a) RF/FePO4 and (b and c) LiFePO4/C composite, (d and e) TEM images of LiFePO4/C composite and (f) The HRTEM image of the square area in (e).
composite is oxidized to Li3Fe2(PO4)3 and Fe2O3 at about 250 °C with the mass of the sample increasing. Further increasing the temperature, the weight-loss should be attributed to the oxide of the carbon in the sample. Therefore, the content of carbon in the LiFePO4/C composite is around 10.0 wt.% by taking into account the theoretical weight gain (5.07%) of the pure LiFePO4 [43]. Fig. 4 shows the CV curves of LiFePO4/C composite at a scan rate of 0.2 mV s1 from 2.0 to 4.2 V. Each of the CV curves includes an oxidation peak and a reduction peak, which corresponds to the charge/discharge reaction of the Fe2+/Fe3+ redox couple [16,32,37]. The good overlap of CV curves indicates the high reversibility of lithium insertion/deinsertion reactions in the LiFePO4/C composite. The electrochemical performance of LiFePO4/C composite is investigated with lithium insertion/deinsertion process. Fig. 5 shows the charge/discharge curves of the LiFePO4/C electrode at 0.5C rate with a voltage window of 2.0–4.2 V. The electrode delivers a high discharge capacity of 155.6 mA h g1 in the first cycle, which accounts for 91.5% of the theoretical capacity of LiFePO4 (170 mA h g1), and it remains 144.0 mA h g1 after 50 cycles. The flat voltage plateaus at about 3.4 V and 3.5 V represent the insertion and extraction of lithium ion, implying the two-phase redox reaction of LiFePO4 [13]. However, there is another voltage plateau around 2.5 V, which can be attributed to the presence of trace
Fe2O3 [44,45]. In addition, the voltage difference between charge and discharge plateaus remains the same up cycling, showing that the LiFePO4/C composite has both excellent reaction reversibility and high structure stability. Based on the results discussed above, it can be concluded that the LiFePO4/C composite has superior capacity and cycling performance at a low charge/discharge rate of 0.5C. However, high-rate properties of the sample are characterized for potential applications in high power lithium-ion batteries. The rate capability of the LiFePO4/C electrode at different C-rates from 0.5C to 50C is presented in Fig. 6. Average discharge capacities are obtained as 154.5, 143.9, 136.2, 119.4, 107.5, 84.1 and 65.8 mA h g1, respectively, for discharge rates of 0.5C, 1C, 2C, 5C, 10C, 20C and 50C. With increased current rates, the discharge capacities decrease regularly. In addition, the discharge capacities recover to the original value when the current density returns to 0.5C rate, indicating that the LiFePO4/C composite has good electrochemical reversibility and structural stability even after fast-charging and high-current discharging. The typical discharge profiles of LiFePO4/C electrode at different current rates from 0.5C to 50C between 2.0 and 4.2 V are shown in Fig. 7. At the rate of 0.5C, the electrode delivers a discharge capacity of 155.6 mA h g1, and a flat voltage platform is observed, corresponding to the Fe2+/Fe3+ redox reaction. It should be noted that
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Fig. 5. Charge–discharge curves of the LiFePO4/C composite at 0.5C rate.
Fig. 3. (a) Raman spectrum of the as-prepared LiFePO4/C composite and (b) TGA curve of the LiFePO4/C composite.
Fig. 6. Rate performance of the LiFePO4/C composite at different rates.
Fig. 4. CV curves of the LiFePO4/C composite in the voltage range of 2.0–4.2 V at a scan rate of 0.2 mV s1.
not only the plateau capacity but also the sluggish slope capacity contributed to the total discharge capacities at the current rates smaller than 10C. But the mechanism of the sluggish slop capacity is still not clear [27,40]. As the discharge rates reach 20C and 50C, the total discharge capacities are mainly contributed by the sluggish slop capacity. The discharge platform voltage decreases with the current rate increasing, which attributes to the increase in polarization resistance [46]. In addition, the discharge voltage declines rapidly as the discharge rate increases from 10C to 20C. It may be attributed to the significant change of lithium storage mechanism. For 20C to 50C, the discharge voltage changes a little maybe due to the same lithium storage mechanism of 20C and 50C. Although the discharge capacity gradually decreases with the current rate increasing, a discharge capacity of 120.0 mA h g1
Fig. 7. The typical discharge curves of LiFePO4/C composite at various current rates.
has been achieved at 5C. Even when the current rate increases to 50C, the discharge capacity can reach 63.9 mA h g1. The superior capacities at high rates could be attributed to the small particle size and good electronic conductivity of the composite. For HEVs or PHEVs applications, batteries not only need to charge and discharge at high current rates but also have a longterm cycling [4]. The cycling stability of the LiFePO4/C composite at high rates is shown in Fig. 8. The sample electrodes deliver the discharge capacities of 103.5 mA h g1, 76.8 mA h g1 and 58.1 mA h g1 after 1000 cycles at 10C, 20C and 50C, and the electrodes deliver capacity retention of 89.5%, 90.9% and 85.7%, respec-
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Fig. 8. Cycle performance for prepared LiFePO4/C composite at high rates.
tively. The superior capacity and outstanding cycling performance indicate that the structure of the LiFePO4/C nano-particles is very stable, and the insertion and extraction of lithium ion process is quite reversible even at high charge/discharge rates. 4. Conclusions In summary, the LiFePO4/C composite with good electrode performance has been prepared by an in situ polymerization restriction method. The formed RF gel can effectively inhibit the growth of LiFePO4 particles, and the carbon from the pyrolysis of RF gel obviously increases the electron conduction between the particles and reduces electrode polarization. As the cathode material for lithium-ion batteries, the LiFePO4/C composite showed high discharge capacities and superior cycling performances at various charge/discharge rates. The enhanced electrochemical performance was attributed to the small particle size and good electronic conductivity of the composite. All in all, the synthesized composite is a good candidate for cathode materials of high power lithiumion batteries. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (21071060). References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188–1194. [2] E. Markevich, R. Sharabi, O. Haik, V. Borgel, G. Salitra, D. Aurbach, G. Semrau, M.A. Schmidt, N. Schall, C. Stinner, J. Power Sources 196 (2011) 6433–6439. [3] Y. Yin, M. Gao, J. Ding, Y. Liu, L. Shen, H. Pan, J. Alloys Comp. 509 (2011) 10161– 10166. [4] G. Wang, H. Liu, J. Liu, S. Qiao, G. Max Lu, P. Munroe, H. Ahn, Adv. Mater. 22 (2010) 4944–4948. [5] Q. Wang, S.X. Deng, H. Wang, M. Xie, J.B. Liu, H. Yan, J. Alloys Comp. 553 (2013) 69–74.
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