- Email: [email protected]
Synthesis and electrode performance of carbon coated Na2FePO4F for rechargeable Na batteries
Electrochemistry Communications 13 (2011) 1225–1228
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Synthesis and electrode performance of carbon coated Na2FePO4F for rechargeable Na batteries Yoshiteru Kawabe a, b, Naoaki Yabuuchi a, Masataka Kajiyama a, Norihito Fukuhara b, Tokuo Inamasu b, Ryoichi Okuyama b, Izumi Nakai a, Shinichi Komaba a,⁎ a b
Department of Applied Chemistry, Tokyo University of Science, 1–3 Kagurazaka, Shinjuku, Tokyo 162–8601, Japan R&D Center, GS Yuasa International Ltd., Minami-ku, Kyoto 601–8520, Japan
a r t i c l e
i n f o
Article history: Received 4 July 2011 Received in revised form 21 August 2011 Accepted 24 August 2011 Available online 31 August 2011 Keywords: Na battery Sodium insertion
a b s t r a c t Carbon-coated Na2FePO4F is synthesized by a simple solid-state method with ascorbic acid as carbon source. Structural characterization of Na2FePO4F by synchrotron X-ray diffraction, scanning/transmission electron microscopy, and Raman spectroscopy reveals that ascorbic acid effectively suppresses the particle growth of Na2FePO4F, forming the nano-sized carbon coated materials. Electrode performance of Na2FePO4F for rechargeable sodium batteries is also examined. The carbon-coated Na2FePO4F sample (1.3 wt% carbon) delivers initial discharge capacity of 110 mAh g-1 at a rate of 1/20 C (6.2 mA g-1) with well-defined voltage plateaus at 3.06 and 2.91 V vs. Na metal. The sample also shows acceptable capacity retention and rate capability as the positive electrode materials for rechargeable Na batteries, which is operable at room temperature. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, the demand for the large scale batteries for the energy storage within an electrical grid system is rapidly growing to effectively utilize electricity from power plants and temporally store the electricity from solar cells and wind turbines. The materials abundance is the fundamental requirement to design the electrode materials for such large-scale applications. Iron and manganese-based electrode materials are the promising candidates among the compounds consisting of series of 3d-transtion metals. As the iron-based system, triphylite-type lithium iron phosphate, LiFePO4, is extensively studied for the practical lithium batteries [1]. LiFePO4 is one of the promising electrode materials for the large scale applications if the lithium resource is unlimited. Although lithium is widely distributed in the earth's crust, lithium is not regarded as abundant elements. The materials cost of lithium is increasing after the commercialization of lithium-ion batteries. In contrast to lithium, material resource of sodium is unlimited everywhere and sodium is the second-lightest and smallest alkali metal next to lithium. Rechargeable sodium-ion batteries [2], which consist of two different sodium-ion insertion materials, are the promising candidate for the large-scale applications. Recently, a layered fluorinated iron phosphate, Na2FePO4F, has been reported [3], and electrode performance has been examined as positive electrode materials for both sodium [4] and lithium [3] batteries. Among the sodium and iron-based insertion materials, such as NaFeO2
[5], NaFeF3 [6] and NaFePO4 [7], Na2FePO4F shows relatively high operating voltage (ca. 3.0 V vs. Na) and fair reversibility [4]. The strong ionicity of fluorine over oxygen (or oxyanion) results in the increase in operating voltage and thus increases energy density as the electrode materials. Electron localization relates to the strong iconicity of fluorine, however, reduces the intrinsic electrical conductivity. Indeed, micrometer-sized Na2FePO4F prepared by a solid-state method shows poor electrode performance compared with nanometer-sized sample prepared by an ionothermal method [4]. Reducing the particle size and shortening the migration distance within the particle are needed to optimize the electrode performance. In addition to reducing the particle size, carbon coating is an effective way to improve the electrode performance, which has been widely investigated for LiFePO4. When carbon sources such as sucrose [8] and ascorbic acid [9,10] are added to precursors of LiFePO4, the material surface is coated with carbon, which simultaneously suppresses the particle size growth [8]. In this paper, we report the synthesis and electrode performance of Na2FePO4F in a sodium cell. It is shown that ascorbic acid is effective carbon source to obtain the nanometer-sized Na2FePO4F by a simple solidstate method, and thus the electrode performance is drastically improved. From these results, we demonstrate the possibility of carbon coated Na2FePO4F as the positive electrode for rechargeable sodiumion batteries. 2. Experimental
⁎ Corresponding author. Tel.: + 81 3 5228 8749; fax: + 81 3 5261 4631. E-mail address: [email protected] (S. Komaba). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.08.038
Na2FePO4F was prepared by a solid-state reaction from the stoichiometric amount of NaF (Nacalai tesque Inc., Kyoto, Japan, purity
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Y. Kawabe et al. / Electrochemistry Communications 13 (2011) 1225–1228
99.0%), NaHCO3 (Nacalai tesque Inc., N99.5%), FeC2O4·2H2O (Nacalai tesque Inc., 98%), and NH4H2PO4 (Nacalai tesque Inc., 99.0%) with or without ascorbic acid (Nacalai tesque Inc., 99.5%). The precursors were mixed using a ballmill. The mixture was heated at 300 °C for 2 h under the nitrogen stream. Thus obtained sample was again ground using a mortar and pestle, and then heated at 600 or 650 °C for 10 h under the nitrogen stream. The structural refinement of Na2FePO4F was carried out using the diffraction patterns obtained from synchrotron X-ray source (BL19B2, SPring-8, Japan). The X-ray wavelength was calibrated to 0.700 Å using a CeO2 standard. The morphological features of the samples were observed by using a scanning electron microscope (Carl Zeiss Inc., SUPRA40, Germany) and transmission electron microscope (EM-002BF, TOPCON TECHNOHOUSE Co. Ltd., Japan). Raman spectra of the samples were collected by using a Raman microscope (NRS2100, JASCO Co. Ltd., Japan). The amount of carbon in the samples was measured with a carbon combustion analyzer (EMIA-320 V, HORIBA Co. Ltd., Japan). Coin-type cells (2032 type) were assembled to evaluate the electrode performance of Na2FePO4F. Positive electrodes consisted of 80 wt% Na2FePO4F, 10 wt% acetylene black, and 10 wt% poly(vinylidene fluoride), which were mixed with NMP and pasted on Al foil, and then dried at 80 °C in vacuum. Metallic sodium is used as a negative electrode. Electrolyte solution used was 1.0 mol dm -3 NaClO4 dissolved in propylene carbonate (Kishida Chemical Co. Ltd., Japan) with fluorinated ethylene carbonate as an electrolyte additive [11]. A glass fiber filter (GB-100R, ADVANTEC Co. Ltd., Japan) was used as a separator.
a Iobs. Icalc. Iobs. - Icalc.
b [100]
c
5
15
Na Fe P
25
O F
35
2θ / deg. (λ = 0.70 Å)
b Carbon
PO43Intensity / a.u.
D-band
3. Results and Discussion
G-band
without ascorbic acid Fig. 1a shows a result of structural analysis of Na2FePO4F, which was prepared with 2.0 wt% ascorbic acid based on the weight of precursor, by the Rietveld method. Structural parameters refined are summarized in Table 1. A schematic illustration of the refined model is also shown in Fig. 1a. The data summarized in Table 1 is consistent with the previous reports [4,9]. Reliable factors obtained by the Rietveld analysis, such as Rwp and RB, are small enough, indicating that the layered fluorinated iron phosphate with the orthorhombic lattice (essentially isostructural with Na2FePO4OH [3]) is appropriate for the structural model. Although single phase Na2FePO4F without any impurity phases was successfully prepared regardless of the absence of ascorbic acid, the particle morphology is drastically influenced by the addition of the 2.0 wt% ascorbic acid as shown in Fig. 2. Fig. 2 shows the SEM image of Na2FePO4F samples synthesized using different precursors or temperatures. Figs. 2a and b compare the trend in crystal growth of Na2FePO4F without ascorbic acid. Relatively small primary particles (50 – 500 nm) are obtained at 600 °C. The crystal growth drastically accelerated at 650 °C, forming large micrometer size primary particles. When the ascorbic acid is added to the precursors, particle growth is effectively suppressed at 650 °C (Fig. 2c). Primary particle size of Na2FePO4F ranges from 30 to 200 nm. Although the uniform distribution of iron is observed by TEM/EDX analysis in Fig. 2f, the distribution of carbon was not clear (Fig. 2e). Therefore, the samples were further analyzed by Raman spectroscopy. Fig. 1b compares Raman spectra of the samples synthesized at 650 °C with or without ascorbic acid. In both samples, strong D and G-bands from elemental carbon on the materials are observed at 1345 cm -1 and 1595 cm -1, respectively. This is consistent with that a trace amount of carbon was detected by carbon combustion analysis in the sample without ascorbic acid, because the iron oxalate would be also another carbon source. In addition, existence of tetrahedral PO43- (point group symmetry of Td) with four internal modes is noted. The peaks at 940, 580, and 420 cm -1 are assigned to ν1 (A1), ν4 (F2), and ν2 (E) modes, respectively [12]. The asymmetric stretch ν3 (F2) mode is
2% ascorbic acid
300
800
1300
1800
Wavenumber / cm-1 Fig. 1. Structural analysis of Na2FePO4F samples; a) a result of Rietveld analysis on synchrotron XRD patterns of Na2FePO4F synthesized with 2 wt% ascorbic acid at 650 °C, and a schematic illustration of the crystal structure is shown as inset. (b) A comparison of Raman spectra of the samples prepared with or without ascorbic acid at 650 °C.
split into two different signals at 1030 and 995 cm -1 because of the distortion of PO43-, which is consistent with the Rietvled analysis (PO distances ranges from 1.52 to 1.56 Å) and also the observation for Table 1 Summary of the structural parameters of Na2FePO4F. The sample was synthesized with 2.0 wt% ascorbic acid in the precursor at 650 °C. Chemical Formula Na2FePO4F S. G., Pbcn a = 5.2126(2) Å, b = 13.7971(5) Å, c = 11.7318(4) Å Rwp = 6.0 %, RB = 1.9 %
Na Na Fe P O O O O F F a
Wyckoff
g
x
y
z
B/Å2
8d 8d 8d 8d 8d 8d 8d 8d 4c 4c
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
0.264(1) 0.249(1) 0.2347(5) 0.2020(6) 0.261(1) 0.277(1) -0.089(1) 0.339(1) 0 0
0.2448(3) 0.1256(4) 0.0105(1) 0.3813(3) 0.3872(4) 0.2805(4) 0.3939(6) 0.4639(5) 0.1237(7) 0.6007(6)
0.3276(3) 0.0827(3) 0.3268(1) 0.0870(2) -0.0407(5) 0.1345(4) 0.1039(4) 0.1498(5) 0.25 0.25
0.88(6) 0.88(6) 0.44(4) 0.4a 0.7a 0.7a 0.7a 0.7a 0.7a 0.7a
not refined.
Y. Kawabe et al. / Electrochemistry Communications 13 (2011) 1225–1228
c
b
a
200 nm
d
1227
100 nm
1 µm
e
f
500 nm
500 nm
500 nm
Fig. 2. SEM images of Na2FePO4F samples synthesized at (a) 600 and (b) 650 °C without ascorbic acid, and (c) at 650 °C with 2 wt% ascorbic acid. TEM image of the sample synthesized at 650 °C with 2 wt% ascorbic acid, which is the same sample used in (c), is also shown in (d), and (e) carbon and (f) iron mapping by energy dispersive X-ray spectroscopy.
a
4
2
1
st
2nd - 5th 1
Capacity / mAh g-1
Voltage / V
3
150
2% ascorbic acid 100
50
without ascorbic acid 0 0
0
5
10
15
20
Cycle Number 0
40
120
80
Capacity / mAh g-1
b
4
1/20 C (6.2 mA g-1) 3
Voltage / V
Na3PO4 [13] and NaFePO4 [7]. In contrast, it is noted that Raman scattering by PO43- was not observed for the sample synthesized with ascorbic acid. It is believed that the particles would be effectively covered with carbon similarly to the LiFePO4 system [8]. Indeed, by the carbon combustion analysis, approximately 1.3 wt% of carbon was detected for the Na2FePO4F with ascorbic acid. From these results, we conclude that 2 wt% of ascorbic acid is the effective carbon source, and a layer of carbon coats the materials surface and suppresses the growth of the particles. Galvanostatic charge/discharge curves of a Na/Na2FePO4F cell are shown in Fig. 3a in voltage of 2.0 – 3.8 V. The Na cell was cycled at a rate of 6.2 mA g -1 at room temperature. The carbon coated Na2FePO4F sample can deliver approximately 110 mAh g -1 of reversible capacity, which corresponds to approximately 90% of theoretical capacity based on the 1 electron redox of iron. Two well-defined voltage plateaus are observed at 3.06 and 2.91 V with the small polarization. The electrode performance of the sample with 1.3% carbon in the Na cell is highly improved compared with previous report, in which a solid-state method or ionothermal method is used for the materials synthesis combined with subsequent carbon coating step (ca. 5 wt% carbon in the sample) [4]. Change in the reversible capacity is plotted in the inset of Fig. 3a. Although the electrode can retain ca. 75% of reversible capacity after the 20 cycle test, the cycleability of Na2FePO4F must be further improved to use it as the electrode materials for rechargeable sodium batteries in the future. The discharge capacity of Na2FePO4F synthesized at 600 °C without ascorbic acid is also shown in the Fig. 3a inset. The sample prepared without the ascorbic acid is almost electrochemically inactive under this experimental condition, presumably because of the poor electrical conduction. Rate capability of the Na/Na2FePO4F cell was also examined as shown in Fig. 3b. The Na cell was charged at a constant current of 6.2 mA g -1 (1/20 C rate) to 3.8 V followed by holding at 3.8 V, and then discharged to 2.0 V at different rates. The sample can deliver approximately 50% of reversible capacity at 124 mA g -1 (1 C rate) based on the capacity obtained at 6.2 mA g -1. The sample shows acceptable rate capability, which is comparable to the lithium system, even though the size of sodium ions is much larger than that of lithium. From these results, we conclude that the carbon coated Na2FePO4F is one of the candidate electrode materials for the rechargeable sodium-ion batteries operable at room temperature, by combining Na2FePO4F with the hard carbon negative electrodes [2].
2
8C
4C
1 C 1/2 C 1/5 C 1/10 C (124 mA g-1)
2C
1
0
0
40
80
120
Capacity / mAh g-1 Fig. 3. (a) Galvanostatic charge/discharge curves of the Na/Na2FePO4F cell cycled at a rate of 6.2 mA g-1. Capacity retention of the Na2FePO4F samples synthesized with or without ascorbic acid is shown in the inset (a). (b) The rate capability of the Na2FePO4F sample, which contains 1.3% coated carbon.
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Y. Kawabe et al. / Electrochemistry Communications 13 (2011) 1225–1228
Acknowledgements This study was partly supported by KAKENHI (No. 21750194) and NEXT program of JSPS. The authors thank Dr. Ni Jiang Feng (Suzhou University, China) for fruitful discussions. References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B.J. Goodenough, Electrochemical Society 144 (1997) 1188. [2] S. Komaba, W. Murata, T. Ishikawa, N. Yabuuchi, T. Ozeki, T. Nakayama, A. Ogata, K. Gotoh, K. Fujiwara, Advanced Functional Materials (2011). doi:10.1002/ adfm.201100854. [3] B.L. Ellis, W.R.M. Makahnouk, Y. Makimura, K. Toghill, L.F. Nazar, Nature Materials 6 (2007) 749. [4] N. Recham, J.N. Chotard, L. Dupont, K. Djellab, M. Armand, J.M. Tarascon, Journal of the Electrochemical Society 156 (2009) A993.
[5] S. Okada, Y. Takahashi, T. Kiyabu, T. Doi, J.-I. Yamaki, T. Nishida, ECS Meeting Abstracts 602 (2006) 201. [6] Y. Yamada, T. Doi, I. Tanaka, S. Okada, J. Yamaki, Journal of Power Sources 196 (2011) 4837. [7] K. Zaghib, J. Trottier, P. Hovington, F. Brochu, A. Guerfi, A. Mauger, C.M. Julien, Journal of Power Sources 196 (2011) 9612. [8] Z.H. Chen, J.R.J. Dahn, Electrochemical Society 149 (2002) A1184. [9] B. Ellis, W.H. Kan, W.R.M. Makahnouk, L.F. Nazar, Journal of Materials Chemistry 17 (2007) 3248. [10] J. Ni, M. Morishita, Y. Kawabe, M. Watada, N. Takeichi, T. Sakai, Journal of Power Sources 195 (2010) 2877. [11] T. Ishikawa, W. Murata, M. Yuta, N. Yabuuchi, S. Komaba, Y. Osawa, A. Ito, In The 78th Annual Meeting of the Electrochemical Society of Japan, Abs. 1B09 Yokomaha, Japan, 2011. [12] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Sixth ed., John Wiley & Sons, Inc, New Jersey, 2009. [13] A. Ghule, N. Baskaran, R. Murugan, H. Chang, Solid State Ionics 161 (2003) 291.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Synthesis and electrode performance of carbon coated Na2FePO4F for rechargeable Na batteries Yoshiteru Kawabe a, b, Naoaki Yabuuchi a, Masataka Kajiyama a, Norihito Fukuhara b, Tokuo Inamasu b, Ryoichi Okuyama b, Izumi Nakai a, Shinichi Komaba a,⁎ a b
Department of Applied Chemistry, Tokyo University of Science, 1–3 Kagurazaka, Shinjuku, Tokyo 162–8601, Japan R&D Center, GS Yuasa International Ltd., Minami-ku, Kyoto 601–8520, Japan
a r t i c l e
i n f o
Article history: Received 4 July 2011 Received in revised form 21 August 2011 Accepted 24 August 2011 Available online 31 August 2011 Keywords: Na battery Sodium insertion
a b s t r a c t Carbon-coated Na2FePO4F is synthesized by a simple solid-state method with ascorbic acid as carbon source. Structural characterization of Na2FePO4F by synchrotron X-ray diffraction, scanning/transmission electron microscopy, and Raman spectroscopy reveals that ascorbic acid effectively suppresses the particle growth of Na2FePO4F, forming the nano-sized carbon coated materials. Electrode performance of Na2FePO4F for rechargeable sodium batteries is also examined. The carbon-coated Na2FePO4F sample (1.3 wt% carbon) delivers initial discharge capacity of 110 mAh g-1 at a rate of 1/20 C (6.2 mA g-1) with well-defined voltage plateaus at 3.06 and 2.91 V vs. Na metal. The sample also shows acceptable capacity retention and rate capability as the positive electrode materials for rechargeable Na batteries, which is operable at room temperature. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, the demand for the large scale batteries for the energy storage within an electrical grid system is rapidly growing to effectively utilize electricity from power plants and temporally store the electricity from solar cells and wind turbines. The materials abundance is the fundamental requirement to design the electrode materials for such large-scale applications. Iron and manganese-based electrode materials are the promising candidates among the compounds consisting of series of 3d-transtion metals. As the iron-based system, triphylite-type lithium iron phosphate, LiFePO4, is extensively studied for the practical lithium batteries [1]. LiFePO4 is one of the promising electrode materials for the large scale applications if the lithium resource is unlimited. Although lithium is widely distributed in the earth's crust, lithium is not regarded as abundant elements. The materials cost of lithium is increasing after the commercialization of lithium-ion batteries. In contrast to lithium, material resource of sodium is unlimited everywhere and sodium is the second-lightest and smallest alkali metal next to lithium. Rechargeable sodium-ion batteries [2], which consist of two different sodium-ion insertion materials, are the promising candidate for the large-scale applications. Recently, a layered fluorinated iron phosphate, Na2FePO4F, has been reported [3], and electrode performance has been examined as positive electrode materials for both sodium [4] and lithium [3] batteries. Among the sodium and iron-based insertion materials, such as NaFeO2
[5], NaFeF3 [6] and NaFePO4 [7], Na2FePO4F shows relatively high operating voltage (ca. 3.0 V vs. Na) and fair reversibility [4]. The strong ionicity of fluorine over oxygen (or oxyanion) results in the increase in operating voltage and thus increases energy density as the electrode materials. Electron localization relates to the strong iconicity of fluorine, however, reduces the intrinsic electrical conductivity. Indeed, micrometer-sized Na2FePO4F prepared by a solid-state method shows poor electrode performance compared with nanometer-sized sample prepared by an ionothermal method [4]. Reducing the particle size and shortening the migration distance within the particle are needed to optimize the electrode performance. In addition to reducing the particle size, carbon coating is an effective way to improve the electrode performance, which has been widely investigated for LiFePO4. When carbon sources such as sucrose [8] and ascorbic acid [9,10] are added to precursors of LiFePO4, the material surface is coated with carbon, which simultaneously suppresses the particle size growth [8]. In this paper, we report the synthesis and electrode performance of Na2FePO4F in a sodium cell. It is shown that ascorbic acid is effective carbon source to obtain the nanometer-sized Na2FePO4F by a simple solidstate method, and thus the electrode performance is drastically improved. From these results, we demonstrate the possibility of carbon coated Na2FePO4F as the positive electrode for rechargeable sodiumion batteries. 2. Experimental
⁎ Corresponding author. Tel.: + 81 3 5228 8749; fax: + 81 3 5261 4631. E-mail address: [email protected] (S. Komaba). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.08.038
Na2FePO4F was prepared by a solid-state reaction from the stoichiometric amount of NaF (Nacalai tesque Inc., Kyoto, Japan, purity
1226
Y. Kawabe et al. / Electrochemistry Communications 13 (2011) 1225–1228
99.0%), NaHCO3 (Nacalai tesque Inc., N99.5%), FeC2O4·2H2O (Nacalai tesque Inc., 98%), and NH4H2PO4 (Nacalai tesque Inc., 99.0%) with or without ascorbic acid (Nacalai tesque Inc., 99.5%). The precursors were mixed using a ballmill. The mixture was heated at 300 °C for 2 h under the nitrogen stream. Thus obtained sample was again ground using a mortar and pestle, and then heated at 600 or 650 °C for 10 h under the nitrogen stream. The structural refinement of Na2FePO4F was carried out using the diffraction patterns obtained from synchrotron X-ray source (BL19B2, SPring-8, Japan). The X-ray wavelength was calibrated to 0.700 Å using a CeO2 standard. The morphological features of the samples were observed by using a scanning electron microscope (Carl Zeiss Inc., SUPRA40, Germany) and transmission electron microscope (EM-002BF, TOPCON TECHNOHOUSE Co. Ltd., Japan). Raman spectra of the samples were collected by using a Raman microscope (NRS2100, JASCO Co. Ltd., Japan). The amount of carbon in the samples was measured with a carbon combustion analyzer (EMIA-320 V, HORIBA Co. Ltd., Japan). Coin-type cells (2032 type) were assembled to evaluate the electrode performance of Na2FePO4F. Positive electrodes consisted of 80 wt% Na2FePO4F, 10 wt% acetylene black, and 10 wt% poly(vinylidene fluoride), which were mixed with NMP and pasted on Al foil, and then dried at 80 °C in vacuum. Metallic sodium is used as a negative electrode. Electrolyte solution used was 1.0 mol dm -3 NaClO4 dissolved in propylene carbonate (Kishida Chemical Co. Ltd., Japan) with fluorinated ethylene carbonate as an electrolyte additive [11]. A glass fiber filter (GB-100R, ADVANTEC Co. Ltd., Japan) was used as a separator.
a Iobs. Icalc. Iobs. - Icalc.
b [100]
c
5
15
Na Fe P
25
O F
35
2θ / deg. (λ = 0.70 Å)
b Carbon
PO43Intensity / a.u.
D-band
3. Results and Discussion
G-band
without ascorbic acid Fig. 1a shows a result of structural analysis of Na2FePO4F, which was prepared with 2.0 wt% ascorbic acid based on the weight of precursor, by the Rietveld method. Structural parameters refined are summarized in Table 1. A schematic illustration of the refined model is also shown in Fig. 1a. The data summarized in Table 1 is consistent with the previous reports [4,9]. Reliable factors obtained by the Rietveld analysis, such as Rwp and RB, are small enough, indicating that the layered fluorinated iron phosphate with the orthorhombic lattice (essentially isostructural with Na2FePO4OH [3]) is appropriate for the structural model. Although single phase Na2FePO4F without any impurity phases was successfully prepared regardless of the absence of ascorbic acid, the particle morphology is drastically influenced by the addition of the 2.0 wt% ascorbic acid as shown in Fig. 2. Fig. 2 shows the SEM image of Na2FePO4F samples synthesized using different precursors or temperatures. Figs. 2a and b compare the trend in crystal growth of Na2FePO4F without ascorbic acid. Relatively small primary particles (50 – 500 nm) are obtained at 600 °C. The crystal growth drastically accelerated at 650 °C, forming large micrometer size primary particles. When the ascorbic acid is added to the precursors, particle growth is effectively suppressed at 650 °C (Fig. 2c). Primary particle size of Na2FePO4F ranges from 30 to 200 nm. Although the uniform distribution of iron is observed by TEM/EDX analysis in Fig. 2f, the distribution of carbon was not clear (Fig. 2e). Therefore, the samples were further analyzed by Raman spectroscopy. Fig. 1b compares Raman spectra of the samples synthesized at 650 °C with or without ascorbic acid. In both samples, strong D and G-bands from elemental carbon on the materials are observed at 1345 cm -1 and 1595 cm -1, respectively. This is consistent with that a trace amount of carbon was detected by carbon combustion analysis in the sample without ascorbic acid, because the iron oxalate would be also another carbon source. In addition, existence of tetrahedral PO43- (point group symmetry of Td) with four internal modes is noted. The peaks at 940, 580, and 420 cm -1 are assigned to ν1 (A1), ν4 (F2), and ν2 (E) modes, respectively [12]. The asymmetric stretch ν3 (F2) mode is
2% ascorbic acid
300
800
1300
1800
Wavenumber / cm-1 Fig. 1. Structural analysis of Na2FePO4F samples; a) a result of Rietveld analysis on synchrotron XRD patterns of Na2FePO4F synthesized with 2 wt% ascorbic acid at 650 °C, and a schematic illustration of the crystal structure is shown as inset. (b) A comparison of Raman spectra of the samples prepared with or without ascorbic acid at 650 °C.
split into two different signals at 1030 and 995 cm -1 because of the distortion of PO43-, which is consistent with the Rietvled analysis (PO distances ranges from 1.52 to 1.56 Å) and also the observation for Table 1 Summary of the structural parameters of Na2FePO4F. The sample was synthesized with 2.0 wt% ascorbic acid in the precursor at 650 °C. Chemical Formula Na2FePO4F S. G., Pbcn a = 5.2126(2) Å, b = 13.7971(5) Å, c = 11.7318(4) Å Rwp = 6.0 %, RB = 1.9 %
Na Na Fe P O O O O F F a
Wyckoff
g
x
y
z
B/Å2
8d 8d 8d 8d 8d 8d 8d 8d 4c 4c
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
0.264(1) 0.249(1) 0.2347(5) 0.2020(6) 0.261(1) 0.277(1) -0.089(1) 0.339(1) 0 0
0.2448(3) 0.1256(4) 0.0105(1) 0.3813(3) 0.3872(4) 0.2805(4) 0.3939(6) 0.4639(5) 0.1237(7) 0.6007(6)
0.3276(3) 0.0827(3) 0.3268(1) 0.0870(2) -0.0407(5) 0.1345(4) 0.1039(4) 0.1498(5) 0.25 0.25
0.88(6) 0.88(6) 0.44(4) 0.4a 0.7a 0.7a 0.7a 0.7a 0.7a 0.7a
not refined.
Y. Kawabe et al. / Electrochemistry Communications 13 (2011) 1225–1228
c
b
a
200 nm
d
1227
100 nm
1 µm
e
f
500 nm
500 nm
500 nm
Fig. 2. SEM images of Na2FePO4F samples synthesized at (a) 600 and (b) 650 °C without ascorbic acid, and (c) at 650 °C with 2 wt% ascorbic acid. TEM image of the sample synthesized at 650 °C with 2 wt% ascorbic acid, which is the same sample used in (c), is also shown in (d), and (e) carbon and (f) iron mapping by energy dispersive X-ray spectroscopy.
a
4
2
1
st
2nd - 5th 1
Capacity / mAh g-1
Voltage / V
3
150
2% ascorbic acid 100
50
without ascorbic acid 0 0
0
5
10
15
20
Cycle Number 0
40
120
80
Capacity / mAh g-1
b
4
1/20 C (6.2 mA g-1) 3
Voltage / V
Na3PO4 [13] and NaFePO4 [7]. In contrast, it is noted that Raman scattering by PO43- was not observed for the sample synthesized with ascorbic acid. It is believed that the particles would be effectively covered with carbon similarly to the LiFePO4 system [8]. Indeed, by the carbon combustion analysis, approximately 1.3 wt% of carbon was detected for the Na2FePO4F with ascorbic acid. From these results, we conclude that 2 wt% of ascorbic acid is the effective carbon source, and a layer of carbon coats the materials surface and suppresses the growth of the particles. Galvanostatic charge/discharge curves of a Na/Na2FePO4F cell are shown in Fig. 3a in voltage of 2.0 – 3.8 V. The Na cell was cycled at a rate of 6.2 mA g -1 at room temperature. The carbon coated Na2FePO4F sample can deliver approximately 110 mAh g -1 of reversible capacity, which corresponds to approximately 90% of theoretical capacity based on the 1 electron redox of iron. Two well-defined voltage plateaus are observed at 3.06 and 2.91 V with the small polarization. The electrode performance of the sample with 1.3% carbon in the Na cell is highly improved compared with previous report, in which a solid-state method or ionothermal method is used for the materials synthesis combined with subsequent carbon coating step (ca. 5 wt% carbon in the sample) [4]. Change in the reversible capacity is plotted in the inset of Fig. 3a. Although the electrode can retain ca. 75% of reversible capacity after the 20 cycle test, the cycleability of Na2FePO4F must be further improved to use it as the electrode materials for rechargeable sodium batteries in the future. The discharge capacity of Na2FePO4F synthesized at 600 °C without ascorbic acid is also shown in the Fig. 3a inset. The sample prepared without the ascorbic acid is almost electrochemically inactive under this experimental condition, presumably because of the poor electrical conduction. Rate capability of the Na/Na2FePO4F cell was also examined as shown in Fig. 3b. The Na cell was charged at a constant current of 6.2 mA g -1 (1/20 C rate) to 3.8 V followed by holding at 3.8 V, and then discharged to 2.0 V at different rates. The sample can deliver approximately 50% of reversible capacity at 124 mA g -1 (1 C rate) based on the capacity obtained at 6.2 mA g -1. The sample shows acceptable rate capability, which is comparable to the lithium system, even though the size of sodium ions is much larger than that of lithium. From these results, we conclude that the carbon coated Na2FePO4F is one of the candidate electrode materials for the rechargeable sodium-ion batteries operable at room temperature, by combining Na2FePO4F with the hard carbon negative electrodes [2].
2
8C
4C
1 C 1/2 C 1/5 C 1/10 C (124 mA g-1)
2C
1
0
0
40
80
120
Capacity / mAh g-1 Fig. 3. (a) Galvanostatic charge/discharge curves of the Na/Na2FePO4F cell cycled at a rate of 6.2 mA g-1. Capacity retention of the Na2FePO4F samples synthesized with or without ascorbic acid is shown in the inset (a). (b) The rate capability of the Na2FePO4F sample, which contains 1.3% coated carbon.
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Acknowledgements This study was partly supported by KAKENHI (No. 21750194) and NEXT program of JSPS. The authors thank Dr. Ni Jiang Feng (Suzhou University, China) for fruitful discussions. References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B.J. Goodenough, Electrochemical Society 144 (1997) 1188. [2] S. Komaba, W. Murata, T. Ishikawa, N. Yabuuchi, T. Ozeki, T. Nakayama, A. Ogata, K. Gotoh, K. Fujiwara, Advanced Functional Materials (2011). doi:10.1002/ adfm.201100854. [3] B.L. Ellis, W.R.M. Makahnouk, Y. Makimura, K. Toghill, L.F. Nazar, Nature Materials 6 (2007) 749. [4] N. Recham, J.N. Chotard, L. Dupont, K. Djellab, M. Armand, J.M. Tarascon, Journal of the Electrochemical Society 156 (2009) A993.
[5] S. Okada, Y. Takahashi, T. Kiyabu, T. Doi, J.-I. Yamaki, T. Nishida, ECS Meeting Abstracts 602 (2006) 201. [6] Y. Yamada, T. Doi, I. Tanaka, S. Okada, J. Yamaki, Journal of Power Sources 196 (2011) 4837. [7] K. Zaghib, J. Trottier, P. Hovington, F. Brochu, A. Guerfi, A. Mauger, C.M. Julien, Journal of Power Sources 196 (2011) 9612. [8] Z.H. Chen, J.R.J. Dahn, Electrochemical Society 149 (2002) A1184. [9] B. Ellis, W.H. Kan, W.R.M. Makahnouk, L.F. Nazar, Journal of Materials Chemistry 17 (2007) 3248. [10] J. Ni, M. Morishita, Y. Kawabe, M. Watada, N. Takeichi, T. Sakai, Journal of Power Sources 195 (2010) 2877. [11] T. Ishikawa, W. Murata, M. Yuta, N. Yabuuchi, S. Komaba, Y. Osawa, A. Ito, In The 78th Annual Meeting of the Electrochemical Society of Japan, Abs. 1B09 Yokomaha, Japan, 2011. [12] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Sixth ed., John Wiley & Sons, Inc, New Jersey, 2009. [13] A. Ghule, N. Baskaran, R. Murugan, H. Chang, Solid State Ionics 161 (2003) 291.