Electrochemistry The Electrochemical Society of Japan Communication
Received: January 9, 2017 Accepted: May 17, 2017 Published: October 5, 2017 http://dx.doi.org/10.5796/electrochemistry.85.630
Electrochemistry, 85(10), 630–633 (2017)
Synthesis and Electrochemical Properties of Fe3C-carbon Composite as an Anode Material for Lithium-ion Batteries Ayuko KITAJOU, Shinji KUDO, Jun-ichiro HAYASHI, and Shigeto OKADA* Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-koen, Kasuga 816-8580, Japan * Corresponding author:
[email protected] ABSTRACT Novel Fe3C carbon composite was proposed as new low cost and low environmental impact anode for Li-ion battery. Although the delithiated capacity of Fe3C without AB were 100 mAh g−1 in the first cycle at a rate of 7 mA g−1, that of Fe3C with AB was improved to 230 mAh g−1 at a rate of 50 mA g−1. Moreover, nanoparticle-Fe3C carbon composite including a relatively high-purity Fe3C could be obtained from α-Fe2O3 and ion-exchange resin. The best initial delithiated capacity more than 320 mAh g−1 at a rate of 50 mA g−1 and improved anode performance were obtained in the Fe3C-carbon composite sintered at 650°C. © The Electrochemical Society of Japan, All rights reserved.
Keywords : Iron Carbide, Carbon Composite, Anode Properties, Li-ion Batteries 1. Introduction Li-ion batteries have the highest energy density among rechargeable batteries on the market and therefore have been widely adopted for advanced portable electronic devices and hybrid vehicles. Commercially available Li-ion batteries generally consist of layered rocksalt-type LiCoO2 as the cathode material, graphite carbon as the anode material, and non-aqueous organic solvents containing Li salts as the electrolyte solution. Although graphite offers high electrical conductivity and good cyclability, its low theoretical specific capacity (372 mAh g¹1) and poor rate capability preclude its application to high-performance batteries. Moreover, additional issues such as cost and safety must be improved especially for large-scale Li-ion batteries. For a long time, carbides have scarcely been considered as anode materials for Li-ion batteries due to their low theoretical capacity and poor electronic conductivity.1 However, 2 dimensional transition metal carbides such as Ti3C2 called MXenes have attracted attention as anodes for large-scale Li-ion batteries.2–4 Moreover, recently, nanomaterials based on Fe3C, the same carbide as MXene, have been also tried to use as anode. For example, Su et al. prepared core-shell Fe@Fe3C/C nanocomposites, which showed a large delithiated capacity of 500 mAh g¹1.5 However, the reported Fe3Ccarbon composite anode contained a large amount of metallic iron as an impurity, and had a carbon content as high as 40% or more.6–9 In addition, the reported Fe3C-carbon composite anode has a large irreversible capacity of over 500 mAh g¹1. In this work, the anode properties of Fe3C without and with carbon were investigated in order to clarify the Li storage capacity of Fe3C. Moreover, the nanoparticle Fe3C-carbon composite having a high-purity Fe3C was prepared from A-Fe2O3 particulate and ionexchange resin, and its anode properties were evaluated in half cell with Li metal counter electrode. 2. Experimental A-Fe2O3 particulates were synthesized by a precipitation method. 36 g of Fe(NO3)3·9H2O was dissolved in deionized 500 mL water and then precipitated as hydroxide by the adjustment of pH at 8.2 with Na2CO3. After aging at 60°C for 1.5 h, the slurry was filtered, and the precipitate was washed with distilled water until the filtrate showed neutral pH. The precursor thus obtained dried at 80°C for 630
12 h under vacuum and finally calcined at 200°C for 2 h to obtain AFe2O3 particulates. Fe3C-carbon composites were prepared from the obtained A-Fe2O3 particulate and ion-exchange resin. The mixtures of 0.7 g of A-Fe2O3 particulate and 2.5 g of ion-exchange resin (Diaion WK11, Mitsubishi chemical) were sintered at 600°C, 650°C, 700°C, and 750°C for 1 h in N2 atmosphere. The obtained powders were characterized using powder X-ray diffraction (XRD, 50 kV and 300 mA, CuKA, Rigaku TTRIII). The particle size, particle morphology and EDS (Energy Dispersive X-ray Spectrometer) mapping were observed by using a transmission electron microscope (TEM; JEOL JEM 2100F). The particle size of the commercially available Fe3C was measured using Laser scattering particle size distribution analyzer (LA-950, HORIBA). The carbon content in the samples was calculated from the amount of residue in its combustion on a thermogravimetric analyzer (TG, Hitachi, STA7200), considering the composition of iron species, which was analyzed by XRD. Mixtures of the 450 mg of obtained Fe3C-carbon composite and 25 mg of acetylene black (AB, Denki Kagaku) were put in an Arfilled atmosphere control container with T3-ZrO2 balls (ca. 20 g). The mixtures were ball-milled using a planetary mill (Fritsch, Pulverisette7) at 600 rpm under Ar for 30 min. For comparison, the commercially available 450 mg of Fe3C (Rare Metallic Co.) and 25 mg of AB were also ball-milled at 600 rpm under Ar for 30 min. To assess electrochemical performance, anodes were prepared by mixing the Fe3C-carbon composite/AB or Fe3C/AB with a 5 wt% PAA binder (polyacrylic acid, MW: 250,000, Wako) in Nmethylpyrrolidione. The slurry was coated on copper foil and dried at 100°C until the solvent had evaporated completely. The component ratio of active material (Fe3C-Fe carbon composite or the commercially available Fe3C):AB:PAA binder was 90:5:5 wt%. The electrochemical performance of each obtained product was evaluated with a 2032 coin-type cell using a non-aqueous electrolyte (1 M LiPF6/EC:DMC = 1:1 in volume, Tomiyama Pure Chemical Industries) and a polypropylene separator (Celgard 3501, Celgard) against lithium metal (Honjo Metal Co.). All the cells were assembled in an Ar-filled glove box. 3. Results and Discussion Figure 1(a) shows the lithiation/delithiation profiles of the commercially available Fe3C anode with and without AB between 0.01 and 3.0 V. The commercially Fe3C without AB showed a
Electrochemistry, 85(10), 630–633 (2017)
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Figure 1. (a) Lithiation/delithiated profiles of Fe3C with and without AB at a rate of 7 mA g¹1 (without AB) and 50 mA g¹1 (with AB). (b) Cyclablilty and (c) rate capability of Fe3C with AB between 0.01 and 3.0 V. lithiated capacity of 300 mAh g¹1 and a delithiated capacity of 100 mAh g¹1 in first cycle. Although this lithated capacity almost corresponds to the theoretical capacity based on the 2Li+ reaction (300 mAh g¹1), the delithiated capacity was unsatisfactory at a rate of 7 mA g¹1 (0.2 mA cm¹2). On the other hand, Fe3C with AB showed similar lithiated capacity of 300 mAh g¹1 and the delithiated capacity of 230 mAh g¹1 even at a rate of 50 mA g¹1. These lithiated/ delithiated capacity was larger than that of previous report about Fe3C which was found to store only 1/6 Li per unit (26 mAh g¹1).6 Figure 1(b) shows the cyclability of Fe3C with AB anode in the potential window between 0.01 and 3.0 V. The rechargeable capacity was maintained at 250 mAh g¹1 even after 20 cycles, and the capacity retention of Fe3C with the AB anode was 109%. The delithiated rate capability of Fe3C with the AB anode between 0.01 and 3.0 V (Fig. 1(c)) was studied at different current densities. The delithiated capacity at 200 mA g¹1 was 160 mAh g¹1, namely this delithiated capacity corresponds to 78% of that at 50 mA g¹1. These results suggested that the Fe3C can function sufficiently as an anode for Li-ion batteries. However, because of the high hardness of Fe3C, the particle size was wiely distributed from 200 nm to 400 µm even after ball milling and making it difficult to obtain uniform nanoparticles. We therefore tried to synthesize Fe3C particulate from A-Fe2O3 particulate and ion-exchange resin. The Fe3C-carbon composites were obtained by carbonization of the ion-exchange resin mixed with A-Fe2O3. The large weight loss of the mixture around 400°C in Fig. 2(a) is caused mainly by pyrolysis of the resin. The other two smaller weight losses around 200°C and 600°C indicate the reduction of iron oxides. Figure 2(b) shows the XRD profiles of the Fe3C-carbon composites sintered at various temperatures. All of the samples could be indexed as Fe3C, metallic iron, and carbon. Moreover, the proportion of each component in the obtained Fe3C-carbon composite was determined from the XRD peak intensity (Fe3C and iron) and TG analysis (carbon), as shown Table 1. Here the compositions of Fe3C and iron were estimated by using the PDXL program (Rigaku). The ratio of carbon contained in each sample was 40%, and the Fe3C content was highest in the Fe3C-carbon composite sintered at 650°C. The amount of carbon contained in the obtained samples was comparable to that of Su et al.5 Figure 2(c) shows the TEM image and EDS mapping of the obtained Fe3C-carbon composite at sintered various temperature. The particle size of Fe3C in the obtained composite was ca. 10– 200 nm, and Fe3C was dispersed uniformly in the carbon. These
Table 1. DTA.
Content of each element calculated from XRD and TG-
Sintered temperature
Fe3C
Fe
C
600°C 650°C 700°C 750°C
41 wt% 52 wt% 47 wt% 47 wt%
18 wt% 8 wt% 14 wt% 13 wt%
41 wt% 40 wt% 40 wt% 39 wt%
results suggested that the obtained Fe3C in the carbon composite consisted of nanoparticles. In addition, the particle size tended to grow as the sintering temperature increases. Figure 3(a) shows the lithiation/delithiation profiles of the Fe3C-carbon composites sintered at various temperatures. These electrochemical measurements were carried out using potential windows of 0.01–3.0 V at a rate of 50 mA g¹1. The initial delithiated capacities were 273 mAh g¹1 (600°C), 326 mAh g¹1 (650°C), 220 mAh g¹1 (700°C), and 226 mA g¹1 (750°C), respectively. The delithiated capacity of the composite sintered at 650°C was the largest among them. For comparison, the delithiation capacity of AB was 204 mAh g¹1 in Fig. 3(a). However, all samples had large irreversible capacities of ca. 400 mAh g¹1 in the first cycle and the discharge plateau around 1 V could hardly be confirmed in the second cycle. Therefore, we guess that the SEI film formation proceeds around 1 V in the first lithiated process, and that this is a cause of the irreversible capacity for AB and Fe3C-carbon composite anodes. Figure 3(b) is summarized the cyclability of the obtained Fe3C-carbon composite at a rate of 50 mA g¹1. The delithiated capacities were maintained 202 mAh g¹1 (600°C), 305 mAh g¹1 (650°C), 197 mAh g¹1 (700°C) and 202 mAh g¹1 (750°C) even after 20 cycles. Their capacity retentions were 78%, 94%, 92% and 90%, respectively. These result suggested that the composite sintered at 650°C has a better anode property among them. In addition, Fe3C-carbon composite sintered at various temperature was evaluated the rate capability as shown in Fig. 3(c). The delithiated capacities at 200 mA g¹1 were 197 mAh g¹1 (600°C), 245 mAh g¹1 (650°C) and 149 mAh g¹1 (700°C), namely this delithiated capacity corresponds to 75%, 79% and 71% of that at 50 mA g¹1, respectively. The high rate capability was achieved by preparation of the uniform Fe3C nanoparticle. Although the 631
Electrochemistry, 85(10), 630–633 (2017)
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Figure 2. (Color online) (a) TG curves of the mixture of Fe2O3 and ion-exchange resin and (b) XRD profiles of the obtained Fe3C-carbon composite sintered at various temperatures. (c) TEM image and EDS mapping of the obtained Fe3C-carbon composite.
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Figure 3. (a) Lithiation/delithiated profiles of Fe3C-carbon composite at a rate of 50 mA g¹1 between 0.01 V and 3.0 V. (b) Cyclability and (c) rate capability of the obtained Fe3C-carbon composite sintered at various temperature. 632
Electrochemistry, 85(10), 630–633 (2017) overpotentials of Fe3C and Fe3C-carbon composite were larger than that of graphite, which is used as an anode for general Li-ion batteries, the operating voltage of Fe3C was higher than that of graphite. Therefore, we can expect the suppression of the Li dendrite formation on the surface to Fe3C-carbon composite anode, because there is room to raise the cutoff potential for the Fe3C-carbon composite anode from the graphite anode. 4. Conclusion The sintering of A-Fe2O3 and ion-exchange resin successfully gave a nanoparticle Fe3C-carbon composite with a particle size of 10–200 nm. The obtained Fe3C-carbon composite was a mixture of Fe3C, metallic iron, and carbon. It had excellent cyclability and rate capability. On the other hand, it was clarified that commercially available Fe3C without carbon can not have sufficient reversibility as an anode for Li-ion batteries. Through the optimization process of the sintering temperature of A-Fe2O3 and ion-exchange resin, Fe3C and carbon composite sintered at 650°C showed the best initial delithiated capacity (326 mAh g¹1) and improved anode performance for Li-ion batteries.
Acknowledgment This work was financially supported by the Murata Science Foundation and the Cooperative Research Program of “Network Joint Research Center for Materials and Devices,” MEXT, Japan. References 1. K. Ichikawa and M. Achikita, Mater. Trans., 34, 718 (1993). 2. M. Naguib, J. Come, B. Dyatkin, V. Presser, P. L. Taberna, P. Simon, M. W. Barsoum, and Y. Gogotsi, Electrochem. Commun., 16, 61 (2012). 3. Q. Tang, Z. Zhou, and P. Shen, J. Am. Chem. Soc., 134, 16909 (2012). 4. D. Er, J. Li, M. Naguib, Y. Gogotsi, and V. B. Shenoy, ACS Appl. Mater. Interfaces, 6, 11173 (2014). 5. L. Su, Z. Zhou, and P. Shen, Electrochim. Acta, 87, 180 (2013). 6. D.-H. Liu, Y. Guo, L.-H. Zhang, W.-C. Li, T. Sun, and A.-H. Lu, Small, 9, 3852 (2013). 7. L.-Z. Bai, D.-L. Zhao, T.-M. Zhang, W.-G. Xie, J.-M. Zhang, and Z.-M. Shen, Electrochim. Acta, 107, 555 (2013). 8. S. Chen, J. Wu, R. Zhou, L. Zuo, P. Li, Y. Song, and L. Wang, Electrochim. Acta, 180, 78 (2015). 9. J.-Q. Huang, B. Zhang, Z.-L. Xu, S. Abouali, M. A. Garakani, J. Huang, and J.-K. Kim, J. Power Sources, 285, 43 (2015).
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