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Low temperature solvothermal synthesis of nanosized NiSb as a Li-ion battery anode material
Journal of Alloys and Compounds 441 (2007) 231–235
Low temperature solvothermal synthesis of nanosized NiSb as a Li-ion battery anode material J. Xie ∗ , X.B. Zhao, H.M. Yu, H. Qi, G.S. Cao, J.P. Tu Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China Received 2 April 2006; received in revised form 11 September 2006; accepted 25 September 2006 Available online 24 October 2006
Abstract Nanosized intermetallic compound NiSb was successfully synthesized by a solvothermal route and studied as a promising anode material for secondary lithium-ion batteries. The as-prepared NiSb powder was characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and field emission scanning electron microscope (FESEM). The electrochemical performance of the nanosized NiSb electrode was investigated by constant current charge and discharge cycling and electrochemical impedance spectroscopy (EIS). It was found that the nanosized NiSb shows a higher initial capacity compared to microsized one prepared by a levitation-melting/ball-milling route due to larger specific surface area of the nanomaterial. The nanosized NiSb shows a rapid capacity fade due to the pulverization and exfoliation of active material caused by severe electrochemical grinding upon long-term cycling. © 2006 Elsevier B.V. All rights reserved. Keywords: Lithium-ion batteries; Nanosized NiSb; Anode material; Solvothermal route
1. Introduction One of the active topics in the research of lithium-ion batteries is the development for the high-capacity and high-cyclingbehavior anode materials to replace the conventional carbonbased materials. Since Fuji Film reported in 1997 [1] that some Sn-based oxides exhibited extremely larger Li-storage capacity than carbon-based materials, great interest has been paid to some Sn- or Sb-based intermetallic compounds [2–12]. The first irreversible capacity loss associated with the formation of Li2 O can be significantly reduced by using intermetallic compounds instead of oxides. However, these intermetallic compounds still exhibited rapid capacity fade due to the pulverization and exfoliation of active material caused by large volume changes upon cycling. Recently, many strategies are suggested to overcome this problem, one of which is to use intermetallic compound powder with small particle size. Yang et al. [13] reported that some Sb-based materials with reduced particle size showed good cycling stability. They attributed the improvement of cycling
∗
Corresponding author. Tel.: +86 571 85662587. E-mail address: [email protected] (J. Xie).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.087
stability to the reduced Li-ion diffusion path and small absolute volume changes by using powder with reduced particle size. More recently, Yin et al. [14,15] reported that nanostructured Ag–Fe–Sn systems, prepared by high-energy ball milling, exhibited excellent cycling behavior with a capacity of about 400 mA h g−1 retained up to 300 charge and discharge cycles. Besides ball-milling method, solution chemical route is often used to synthesize nanosized materials with uniform size distribution. In our previous studies [16–18], we have successfully synthesized some nanostructured Sb-based intermetallic compounds by a solvothermal (ST) route. These materials showed improved cycling stability compared to their microsized counterparts prepared by a levitation-melting/ball-milling (LB) route. In our present study, we will report the ST synthesis and electrochemical performance of another Sb-based intermetallic compound, nanosized NiSb (Nano-NiSb). For comparison, the electrochemical performance of microsized NiSb (Micro-NiSb) powder, prepared by LB route was also investigated. 2. Experimental Analytically pure NiCl2 ·6H2 O and SbCl3 were used as the precursors in the ST synthesis. The precursors with a Ni:Sb molar ratio of 1:1 were put into a Teflon-lined autoclave which was then filled with ethanol up to 85% of its 500 ml volume. After adding sufficient NaBH4 (A.R.) as the reductant, the autoclave
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J. Xie et al. / Journal of Alloys and Compounds 441 (2007) 231–235
was sealed immediately and heated to 220 ◦ C and maintained at this temperature for 24 h for the synthesis reaction. Then the autoclave was cooled down to room temperature naturally. The obtained precipitate was filtered, washed several times with ethanol and distilled water, and dried under a vacuum of about 1 Pa at 110 ◦ C for 12 h. The phase purity of the ST powder was analyzed by X-ray diffraction (XRD) using a Rigaku-D/MAX-2550PC diffractometer equipped for Cu K␣ radia˚ in the range of 2θ = 10–80◦ . The powder morphology was tion (λ = 1.5406 A) observed on a JEM-2110 transmission electron microscope (TEM) and a FEI SIRION field emission scanning electron microscope (FESEM). The electrode slurry was prepared by mixing and stirring active powder, acetylene black, and polyvinylidene fluoride (PVDF) in the weight ratio 80:10:10 in NMP. The electrode slurry was coated onto Ni foam current collector. After vacuum-dried at 110 ◦ C for 12 h, the electrodes were assembled into coin-type half-cells in an Ar-filled glove box using metallic lithium foil as the counter electrode, a solution of 1 M LiPF6 dissolved in 50 wt.% ethylene carbonate (EC)–50 wt.% dimethyl carbonate (DMC) as the electrolyte, and polypropylene (PP) film (Celgard 2300) as the separator. The electrochemical performance of the NiSb electrodes was measured by constant current charge and discharge cycling between 0.05 and 1.5 V with a current density of 20 mA g−1 . The electrochemical impedance spectroscopy (EIS) measurements were performed using a frequency response analyzer Solartron FRA 1260 coupled with an electrochemical interface Solartron SI 1287. The impedance spectra were recorded by applying an ac voltage of 5 mV amplitude in a frequency range of 1 MHz–0.1 Hz after the cell has been undergone desired charge and discharge cycles and has been left at open circuit voltage (OCV) state for 24 h to achieve equilibrium.
3. Results and discussion Fig. 1 shows the XRD patterns of NiSb powder prepared by the ST. The diffraction peaks are consistent with the hexagonal NiSb phase with space group P63 /mmc (JCPDS File: 75-0604). The lattice constants calculated by diffraction data are a = 3.927 ˚ c = 5.127 (0.002) A, ˚ which are in good agreement (0.002) A, ˚ c = 5.09 A ˚ (JCPDS File: 75with the standard values of a = 3.9 A, 0604). This means that ST route is an ideal method to synthesize intermetallic compounds with relatively low temperature and simple route. According to the XRD results, a possible reaction mechanism is suggested as follows: NiCl2 + 2NaBH4 → Ni + H2 + 2BH3 + 2NaCl
(1)
Fig. 1. XRD patterns of NiSb prepared by the (a) ST and the (b) LB route.
Fig. 2. TEM images of Nano-NiSb prepared by the ST route.
SbCl3 + 3NaBH4 → Sb + 23 H2 + 3BH3 + 3NaCl
(2)
Ni + Sb → NiSb
(3)
Note that compared to the sample prepared by the LB route, the sample prepared by the ST route shows broader diffraction peaks, which means it is composed of particles with a smaller size. Fig. 2 shows the TEM images of NiSb powder prepared by the ST route. As seen in figure the NiSb powder is composed of small granules with grain size about 60 nm and large aggregates which are stacked from small grains. These granules and aggregates are connected each other and form a network structure. By contrast, the particle size of sample prepared by the LB route is in microsize as clearly shown in Fig. 3(b). Fig. 4 compares charge and discharge curves between NanoNiSb and Micro-NiSb. As shown in the figure, the first charge (Li-removal) and discharge (Li-insertion) capacities of NanoNiSb are 491 and 805 mA h g−1 , respectively. For both samples, its first discharge curve is composed of two quasi-plateaus. The first one at about 0.5 V can be attributed to the reduction decomposition and subsequent formation of solid electrolyte interface (SEI) layer [19], and the second one at about 0.3 V can be attributed to the irreversible decomposition of NiSb structure and the formation of Li3 Sb composition since the discharge plateaus in the subsequent cycles are increased to above 0.5 V. Compared to Micro-NiSb, Nano-NiSb exhibits a larger first charge capacity. In our opinion, it is the larger specific surface area of Nano-NiSb that contributes to its larger specific capacity. In addition, after first discharge process, both the charge and discharge plateaus of Nano-NiSb are almost overlapped indicating highly reversible electrochemical lithiation and delithiation reactions. Fig. 5 compares the cycling stability between Nano-NiSb and Micro-NiSb. Note that Nano-NiSb exhibits better cycling stability than Micro-NiSb in the initial several cycles. As suggested by Besenhard et al. [20], intermetallic anodes generally show rapid capacity fade in the initial cycles followed by relatively slow capacity fade. This is due to the fact that the intermetallic compounds undergo large absolute volume changes upon the alloying and dealloying of Li-ions with active center in the initial cycles especially during the first discharge process,
J. Xie et al. / Journal of Alloys and Compounds 441 (2007) 231–235
233
Fig. 3. SEM images of (a) Nano-NiSb and (b) Micro-NiSb.
Fig. 4. Charge and discharge curves of (a) Nano-NiSb and (b) Micro-NiSb for the first three cycles.
during which, severe pulverization and exfoliation of the active materials occur. However, after initial cycles, the intermetallic anode becomes stable as reflected by the relatively slow capacity fade. The better cycle stability in the initial cycles for NanoNiSb means that Nano-NiSb exhibits smaller absolute volume changes compared to Micro-NiSb. As indicated in Fig. 3, the Nano-NiSb powder is composed of nanosized cotton-like particles, which are loosely stacked with nanosized pores between
Fig. 5. Comparison of cycling stability between (a) Nano-NiSb and (b) MicroNiSb.
them. The nanosized pores can act as buffer to mitigate the volume changes resulting in better cycle stability. In contrast, Micro-NiSb powder consists of microsized particles with irregular shapes and the aggregates of the particles. In addition, the sharp angles and edges produced during ball milling are easy to exfoliate upon severe volume changes, leading to a rapid capacity fade. Although Nano-NiSb exhibits better initial cycling stability, it also suffers from rapid capacity fade upon long-term charge and discharge cycles. As seen in Fig. 5, Nano-NiSb shows a capacity fade with a rate even faster than Micro-NiSb after 10 cycles, which indicates that great microstructure changes indeed occur within the Nano-NiSb electrode. To investigate the exact capacity fade mechanism, EIS technique was used. EIS is a powerful tool to study the electrochemistry-related phenomena especially the electrode resistance variation during the electrode reactions. Fig. 6(a) shows the Nyquist plots of Nano-NiSb electrode after desired cycles. The symbols and solid lines represent the experimental points and fitting results using the equivalent circuit (Fig. 6(b)), respectively. The typical impedance spectra show two partially overlapped semicircles in high and middlefrequency range, and a sloping line in low frequency range. According to the literature [21], the symbol Rs in the equivalent circuit represents the resistance of electrolyte solution and cell component, which corresponds to the intercept of highfrequency semicircle at Z -axis. The R1 and CPE1 elements,
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J. Xie et al. / Journal of Alloys and Compounds 441 (2007) 231–235
Table 1 Fitting results of Nyquist plots using the equivalent circuit Rs ()
R1 ()
CPE1 Y
After 10 cycles After 20 cycles
6.73 9.09
6.35 8.97
1.7 × 10−5 1.0 × 10−6
R2 ()
Rw ()
CPE2
n
Y
n
0.607 0.897
3.8 × 10−4
0.822 0.613
7.01 14.78
3.2 × 10−4
39.41 42.59
corresponding to the high-frequency semicircle, represent the SEI layer resistance and the capacitance due to the dielectric relaxation of the SEI layer, respectively. In the equivalent circuit, the CPE element instead of a pure capacitor (C) is used because of the non-homogeneity of the composite electrode system. CPE can be generally expressed as: nπ nπ YCPE = Yωn cos + jYωn sin (4) 2 2
particles during cycling. The large change of Rw indicates that Li-ion diffusion in bulk active material is also hindered due to the profound microstructure changes within the electrode. As a result, the EIS results are in good consistency with the cycling behavior shown in Fig. 5.
where ω is the angular frequency, j = (−1)1/2 , and n are constants. The value of n denotes the degree of distortion of the capacitance–resistance arc and when n = 1, CPE becomes an ideal capacitor [22]. The R2 and CPE2 elements, corresponding to the middle-frequency semicircle, represent the charger transfer resistance between the electrolyte and the active material and the associated double-layer capacitance, respectively, and Rw is referred to the semi-infinite Warburg impedance, which is related to the solid phase diffusion of Li-ions within the bulk electrode. The fitting results are given in Table 1. Note that Rs , R1 , R2 and Rw are all on the increase during 10 charge and discharge cycles, especially for R2 , whose value is doubled. The significant rise of R2 can be ascribed to the growth of the SEI film on the newly formed interfaces between the active material and the electrolyte resulting from the pulverization of the
The nanosized NiSb was successfully synthesized by the ST route and it exhibits a higher charge capacity compared to the microsized one prepared by LB in the initial cycles due to the large specific surface area of the nanomaterial. The rapid capacity fade of this material during the long-term cycling is caused by pulverization (leading to the growth of SEI film) and exfoliation of the active material caused by the severe electrochemical grinding. Further work should be undertaken to optimize microstructure of Nano-NiSb for the practical use of this material in the secondary Li-ion batteries.
4. Conclusions
Acknowledgements The work is supported by National Natural Science Foundation of China (No. 50201014), PFDP of the Education Ministry of China (No. 20010335045), and China Postdoctoral Science Foundation (No. 2005038278). References
Fig. 6. EIS results of Nano-NiSb electrode: (a) Nyquist plots and the fitting results and (b) the equivalent circuit.
[1] Y. Idolta, T. Kubata, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science 276 (1997) 1395. [2] O. Mao, R.A. Dunlap, J.R. Dahn, J. Electrochem. Soc. 146 (1999) 405. [3] O. Mao, J.R. Dahn, J. Electrochem. Soc. 146 (1999) 414. [4] O. Mao, J.R. Dahn, J. Electrochem. Soc. 146 (1999) 423. [5] S.F. Yang, P.Y. Zavalij, M.S. Whittingham, Electrochem. Commun. 5 (2003) 587. [6] H. Kim, Y.J. Kim, D.G. Kim, H.J. Sohn, T. Kang, Solid State Ionics 144 (2001) 41. [7] R. Alc´antara, F.J. Fern´andez-Madrigal, P. Lavela, J.L. Tirado, J.C. Jumas, J. Olivier-Fourcade, J. Mater. Chem. 9 (1999) 2517. [8] F.J. Fern´andez-Madrigal, P. Lavela, C. P´erez Vicente, J.L. Tirado, J. Electroanal. Chem. 501 (2001) 205. [9] D. Larcher, L.Y. Beaulieu, O. Mao, A.E. George, J.R. Dahn, J. Electrochem. Soc. 147 (2000) 1703. [10] X.B. Zhao, G.X. Cao, C.P. Lv, L.J. Zhang, S.H. Hu, T.J. Zhu, B.C. Zhou, J. Alloys Compd. 315 (2001) 265. [11] L.M.M. Fransson, J.T. Vaughey, R. Benedek, K. Edstr¨om, J.O. Thomas, M.M. Thackeray, Electrochem. Commun. 3 (2001) 317. [12] L.M.M. Fransson, J.T. Vaughey, K. Edstr¨om, M.M. Thackeray, J. Electrochem. Soc. 150 (2003) 86. [13] J. Yang, Y. Takada, N. Imanishi, O. Yamamoto, J. Electrochem. Soc. 146 (1999) 4009.
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[18] J. Xie, X.B. Zhao, G.S. Cao, M.J. Zhao, S.F. Su, J. Alloys Compd. 393 (2005) 283. [19] D. Aurbach, A. Zaban, A. Schechter, Y. Ein-Eli, E. Zinigrad, B. Markovsky, J. Electrochem. Soc. 142 (1995) 2873. [20] J.O. Besenhard, J. Yang, M. White, J. Power Sources 68 (1997) 87. [21] Y.C. Chang, J.H. Jong, G.T.K. Fey, J. Electrochem. Soc. 147 (2000) 2033. [22] K.M. Shaju, G.V. Subba Rao, B.V.R. Chowdari, Electrochim. Acta 48 (2003) 2691.
Low temperature solvothermal synthesis of nanosized NiSb as a Li-ion battery anode material J. Xie ∗ , X.B. Zhao, H.M. Yu, H. Qi, G.S. Cao, J.P. Tu Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China Received 2 April 2006; received in revised form 11 September 2006; accepted 25 September 2006 Available online 24 October 2006
Abstract Nanosized intermetallic compound NiSb was successfully synthesized by a solvothermal route and studied as a promising anode material for secondary lithium-ion batteries. The as-prepared NiSb powder was characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and field emission scanning electron microscope (FESEM). The electrochemical performance of the nanosized NiSb electrode was investigated by constant current charge and discharge cycling and electrochemical impedance spectroscopy (EIS). It was found that the nanosized NiSb shows a higher initial capacity compared to microsized one prepared by a levitation-melting/ball-milling route due to larger specific surface area of the nanomaterial. The nanosized NiSb shows a rapid capacity fade due to the pulverization and exfoliation of active material caused by severe electrochemical grinding upon long-term cycling. © 2006 Elsevier B.V. All rights reserved. Keywords: Lithium-ion batteries; Nanosized NiSb; Anode material; Solvothermal route
1. Introduction One of the active topics in the research of lithium-ion batteries is the development for the high-capacity and high-cyclingbehavior anode materials to replace the conventional carbonbased materials. Since Fuji Film reported in 1997 [1] that some Sn-based oxides exhibited extremely larger Li-storage capacity than carbon-based materials, great interest has been paid to some Sn- or Sb-based intermetallic compounds [2–12]. The first irreversible capacity loss associated with the formation of Li2 O can be significantly reduced by using intermetallic compounds instead of oxides. However, these intermetallic compounds still exhibited rapid capacity fade due to the pulverization and exfoliation of active material caused by large volume changes upon cycling. Recently, many strategies are suggested to overcome this problem, one of which is to use intermetallic compound powder with small particle size. Yang et al. [13] reported that some Sb-based materials with reduced particle size showed good cycling stability. They attributed the improvement of cycling
∗
Corresponding author. Tel.: +86 571 85662587. E-mail address: [email protected] (J. Xie).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.087
stability to the reduced Li-ion diffusion path and small absolute volume changes by using powder with reduced particle size. More recently, Yin et al. [14,15] reported that nanostructured Ag–Fe–Sn systems, prepared by high-energy ball milling, exhibited excellent cycling behavior with a capacity of about 400 mA h g−1 retained up to 300 charge and discharge cycles. Besides ball-milling method, solution chemical route is often used to synthesize nanosized materials with uniform size distribution. In our previous studies [16–18], we have successfully synthesized some nanostructured Sb-based intermetallic compounds by a solvothermal (ST) route. These materials showed improved cycling stability compared to their microsized counterparts prepared by a levitation-melting/ball-milling (LB) route. In our present study, we will report the ST synthesis and electrochemical performance of another Sb-based intermetallic compound, nanosized NiSb (Nano-NiSb). For comparison, the electrochemical performance of microsized NiSb (Micro-NiSb) powder, prepared by LB route was also investigated. 2. Experimental Analytically pure NiCl2 ·6H2 O and SbCl3 were used as the precursors in the ST synthesis. The precursors with a Ni:Sb molar ratio of 1:1 were put into a Teflon-lined autoclave which was then filled with ethanol up to 85% of its 500 ml volume. After adding sufficient NaBH4 (A.R.) as the reductant, the autoclave
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J. Xie et al. / Journal of Alloys and Compounds 441 (2007) 231–235
was sealed immediately and heated to 220 ◦ C and maintained at this temperature for 24 h for the synthesis reaction. Then the autoclave was cooled down to room temperature naturally. The obtained precipitate was filtered, washed several times with ethanol and distilled water, and dried under a vacuum of about 1 Pa at 110 ◦ C for 12 h. The phase purity of the ST powder was analyzed by X-ray diffraction (XRD) using a Rigaku-D/MAX-2550PC diffractometer equipped for Cu K␣ radia˚ in the range of 2θ = 10–80◦ . The powder morphology was tion (λ = 1.5406 A) observed on a JEM-2110 transmission electron microscope (TEM) and a FEI SIRION field emission scanning electron microscope (FESEM). The electrode slurry was prepared by mixing and stirring active powder, acetylene black, and polyvinylidene fluoride (PVDF) in the weight ratio 80:10:10 in NMP. The electrode slurry was coated onto Ni foam current collector. After vacuum-dried at 110 ◦ C for 12 h, the electrodes were assembled into coin-type half-cells in an Ar-filled glove box using metallic lithium foil as the counter electrode, a solution of 1 M LiPF6 dissolved in 50 wt.% ethylene carbonate (EC)–50 wt.% dimethyl carbonate (DMC) as the electrolyte, and polypropylene (PP) film (Celgard 2300) as the separator. The electrochemical performance of the NiSb electrodes was measured by constant current charge and discharge cycling between 0.05 and 1.5 V with a current density of 20 mA g−1 . The electrochemical impedance spectroscopy (EIS) measurements were performed using a frequency response analyzer Solartron FRA 1260 coupled with an electrochemical interface Solartron SI 1287. The impedance spectra were recorded by applying an ac voltage of 5 mV amplitude in a frequency range of 1 MHz–0.1 Hz after the cell has been undergone desired charge and discharge cycles and has been left at open circuit voltage (OCV) state for 24 h to achieve equilibrium.
3. Results and discussion Fig. 1 shows the XRD patterns of NiSb powder prepared by the ST. The diffraction peaks are consistent with the hexagonal NiSb phase with space group P63 /mmc (JCPDS File: 75-0604). The lattice constants calculated by diffraction data are a = 3.927 ˚ c = 5.127 (0.002) A, ˚ which are in good agreement (0.002) A, ˚ c = 5.09 A ˚ (JCPDS File: 75with the standard values of a = 3.9 A, 0604). This means that ST route is an ideal method to synthesize intermetallic compounds with relatively low temperature and simple route. According to the XRD results, a possible reaction mechanism is suggested as follows: NiCl2 + 2NaBH4 → Ni + H2 + 2BH3 + 2NaCl
(1)
Fig. 1. XRD patterns of NiSb prepared by the (a) ST and the (b) LB route.
Fig. 2. TEM images of Nano-NiSb prepared by the ST route.
SbCl3 + 3NaBH4 → Sb + 23 H2 + 3BH3 + 3NaCl
(2)
Ni + Sb → NiSb
(3)
Note that compared to the sample prepared by the LB route, the sample prepared by the ST route shows broader diffraction peaks, which means it is composed of particles with a smaller size. Fig. 2 shows the TEM images of NiSb powder prepared by the ST route. As seen in figure the NiSb powder is composed of small granules with grain size about 60 nm and large aggregates which are stacked from small grains. These granules and aggregates are connected each other and form a network structure. By contrast, the particle size of sample prepared by the LB route is in microsize as clearly shown in Fig. 3(b). Fig. 4 compares charge and discharge curves between NanoNiSb and Micro-NiSb. As shown in the figure, the first charge (Li-removal) and discharge (Li-insertion) capacities of NanoNiSb are 491 and 805 mA h g−1 , respectively. For both samples, its first discharge curve is composed of two quasi-plateaus. The first one at about 0.5 V can be attributed to the reduction decomposition and subsequent formation of solid electrolyte interface (SEI) layer [19], and the second one at about 0.3 V can be attributed to the irreversible decomposition of NiSb structure and the formation of Li3 Sb composition since the discharge plateaus in the subsequent cycles are increased to above 0.5 V. Compared to Micro-NiSb, Nano-NiSb exhibits a larger first charge capacity. In our opinion, it is the larger specific surface area of Nano-NiSb that contributes to its larger specific capacity. In addition, after first discharge process, both the charge and discharge plateaus of Nano-NiSb are almost overlapped indicating highly reversible electrochemical lithiation and delithiation reactions. Fig. 5 compares the cycling stability between Nano-NiSb and Micro-NiSb. Note that Nano-NiSb exhibits better cycling stability than Micro-NiSb in the initial several cycles. As suggested by Besenhard et al. [20], intermetallic anodes generally show rapid capacity fade in the initial cycles followed by relatively slow capacity fade. This is due to the fact that the intermetallic compounds undergo large absolute volume changes upon the alloying and dealloying of Li-ions with active center in the initial cycles especially during the first discharge process,
J. Xie et al. / Journal of Alloys and Compounds 441 (2007) 231–235
233
Fig. 3. SEM images of (a) Nano-NiSb and (b) Micro-NiSb.
Fig. 4. Charge and discharge curves of (a) Nano-NiSb and (b) Micro-NiSb for the first three cycles.
during which, severe pulverization and exfoliation of the active materials occur. However, after initial cycles, the intermetallic anode becomes stable as reflected by the relatively slow capacity fade. The better cycle stability in the initial cycles for NanoNiSb means that Nano-NiSb exhibits smaller absolute volume changes compared to Micro-NiSb. As indicated in Fig. 3, the Nano-NiSb powder is composed of nanosized cotton-like particles, which are loosely stacked with nanosized pores between
Fig. 5. Comparison of cycling stability between (a) Nano-NiSb and (b) MicroNiSb.
them. The nanosized pores can act as buffer to mitigate the volume changes resulting in better cycle stability. In contrast, Micro-NiSb powder consists of microsized particles with irregular shapes and the aggregates of the particles. In addition, the sharp angles and edges produced during ball milling are easy to exfoliate upon severe volume changes, leading to a rapid capacity fade. Although Nano-NiSb exhibits better initial cycling stability, it also suffers from rapid capacity fade upon long-term charge and discharge cycles. As seen in Fig. 5, Nano-NiSb shows a capacity fade with a rate even faster than Micro-NiSb after 10 cycles, which indicates that great microstructure changes indeed occur within the Nano-NiSb electrode. To investigate the exact capacity fade mechanism, EIS technique was used. EIS is a powerful tool to study the electrochemistry-related phenomena especially the electrode resistance variation during the electrode reactions. Fig. 6(a) shows the Nyquist plots of Nano-NiSb electrode after desired cycles. The symbols and solid lines represent the experimental points and fitting results using the equivalent circuit (Fig. 6(b)), respectively. The typical impedance spectra show two partially overlapped semicircles in high and middlefrequency range, and a sloping line in low frequency range. According to the literature [21], the symbol Rs in the equivalent circuit represents the resistance of electrolyte solution and cell component, which corresponds to the intercept of highfrequency semicircle at Z -axis. The R1 and CPE1 elements,
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J. Xie et al. / Journal of Alloys and Compounds 441 (2007) 231–235
Table 1 Fitting results of Nyquist plots using the equivalent circuit Rs ()
R1 ()
CPE1 Y
After 10 cycles After 20 cycles
6.73 9.09
6.35 8.97
1.7 × 10−5 1.0 × 10−6
R2 ()
Rw ()
CPE2
n
Y
n
0.607 0.897
3.8 × 10−4
0.822 0.613
7.01 14.78
3.2 × 10−4
39.41 42.59
corresponding to the high-frequency semicircle, represent the SEI layer resistance and the capacitance due to the dielectric relaxation of the SEI layer, respectively. In the equivalent circuit, the CPE element instead of a pure capacitor (C) is used because of the non-homogeneity of the composite electrode system. CPE can be generally expressed as: nπ nπ YCPE = Yωn cos + jYωn sin (4) 2 2
particles during cycling. The large change of Rw indicates that Li-ion diffusion in bulk active material is also hindered due to the profound microstructure changes within the electrode. As a result, the EIS results are in good consistency with the cycling behavior shown in Fig. 5.
where ω is the angular frequency, j = (−1)1/2 , and n are constants. The value of n denotes the degree of distortion of the capacitance–resistance arc and when n = 1, CPE becomes an ideal capacitor [22]. The R2 and CPE2 elements, corresponding to the middle-frequency semicircle, represent the charger transfer resistance between the electrolyte and the active material and the associated double-layer capacitance, respectively, and Rw is referred to the semi-infinite Warburg impedance, which is related to the solid phase diffusion of Li-ions within the bulk electrode. The fitting results are given in Table 1. Note that Rs , R1 , R2 and Rw are all on the increase during 10 charge and discharge cycles, especially for R2 , whose value is doubled. The significant rise of R2 can be ascribed to the growth of the SEI film on the newly formed interfaces between the active material and the electrolyte resulting from the pulverization of the
The nanosized NiSb was successfully synthesized by the ST route and it exhibits a higher charge capacity compared to the microsized one prepared by LB in the initial cycles due to the large specific surface area of the nanomaterial. The rapid capacity fade of this material during the long-term cycling is caused by pulverization (leading to the growth of SEI film) and exfoliation of the active material caused by the severe electrochemical grinding. Further work should be undertaken to optimize microstructure of Nano-NiSb for the practical use of this material in the secondary Li-ion batteries.
4. Conclusions
Acknowledgements The work is supported by National Natural Science Foundation of China (No. 50201014), PFDP of the Education Ministry of China (No. 20010335045), and China Postdoctoral Science Foundation (No. 2005038278). References
Fig. 6. EIS results of Nano-NiSb electrode: (a) Nyquist plots and the fitting results and (b) the equivalent circuit.
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