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C–graphite composite anode materials with improved cyclic performance
Author’s Accepted Manuscript Ultrafine Si/C-graphite composite anode materials with improved cyclic performance Mengjie Pan, Xuelian Liu, Hongbo Liu, Yuxi Chen
www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)30743-1 http://dx.doi.org/10.1016/j.matlet.2016.05.018 MLBLUE20829
To appear in: Materials Letters Received date: 24 January 2016 Revised date: 18 April 2016 Accepted date: 4 May 2016 Cite this article as: Mengjie Pan, Xuelian Liu, Hongbo Liu and Yuxi Chen, Ultrafine Si/C-graphite composite anode materials with improved cyclic performance, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.05.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultrafine Si/C‒graphite composite anode materials with improved cyclic performance Mengjie Pan, Xuelian Liu, Hongbo Liu, Yuxi Chen* College of Materials and Engineering/Hunan Province Key Laboratory for Spray Deposition Technology and Application, Hunan University, Changsha 410082, P.R. China *
Corresponding
author.
Yuxi
Chen,
Professor,
Tel:
+86-731-88664018,
Fax:+86-731-88823554, [email protected] Abstract Si is of great interest as anode candidate for high energy-density lithium ion batteries. However, the rapid capacity fading and low first-cycle coulombic efficiency strongly hinder its applications. Ultrafine Si/C nanoparticles mixed graphite powders as composite anode materials have been synthesized through magnesiothermic reduction of ultrafine SiO2/C, and then mixed with graphite. Electron microscopy investigation indicates that the ultrafine Si/C particles with size ca. 10 nm are attached on the graphite surface. Electrochemical evaluation demonstrates that the cyclic performance of the ultrafine Si/C‒graphite composite materials has been improved compared with Si and Si/C, in which the reversible capacity can reach 662.4 mAh g‒1 with the first-cycle coulombic efficiency as high as 90.9%. Meanwhile, the capacity retention can reach 90.8% over 100 cycles. These ultrafine Si/C‒graphite composite materials are promising anode candidate with combined electrochemical properties to meet multiple requirements of high energy-density lithium ion batteries. Keywords: energy storage and conversion; lithium ion batteries; silicon; composites; electron microscopy
1
1. Introduction Si is of great interest as anode materials for high energy-density lithium ion batteries (LIBs) [1-3], since it can present a specific capacity of ca. 4200 mAh g−1, which is more than 10 times higher than that of the commercial graphite anode materials (372 mAh g−1). However, the large volume variation of Si (> 300%) during lithiation/delithiation strongly hinders its applications. To minimize this volume variation, many novel Si-based materials for improving cyclic stability have been synthesized, such as Si nanotubes [4,5], Si@SiOx/C nanocomposites [6], Si@C hollow core-shell heterostructures [7-10], SiCN/BN [11], Si-Al-C-N/carbon nanotubes [12], as well as porous Si [13,14]. These methods have been demonstrated to be effective in improving cyclic performance of Si. However, up to date, a big challenge of Si attracts much less attention and still remains, i.e., the low first-cycle coulombic efficiency (CE). In general, the first-cycle CE of the nanosized Si is ca. 80% or less [15], which is far below the requirements of LIBs. Combination of the nannosized Si with the commercial graphite is expected to be an effective way [16]. However, previous investigation demonstrates that extensive milling of Si and graphite may induce high first-cycle irreversible capacity [17,18]. To improve the first-cycle CE and the cyclic stability of Si-based anode simultaneously, we report systematic investigation of ultrafine Si/C‒ graphite composite anode materials with combined electrochemical performance, which are believed to meet multiple requirements of high energy-density LIBs. 2. Experimental 0.06 g lysine was dissolved into 70 mL deionized water, and then 3.2 mL
2
tetraethylorthosilicate were added under continuous stirring at 60oC for 21 h. Then, white SiO2 nanoparticles were obtained. SiO2 was mixed with deionized water, alcohol and sucrose with appropriate weight ratio, and stirred at 60oC for 3 h. The products was filtered and dried overnight, and then calcined at 800oC for 2 h. The carbon-coated SiO2/C was obtained. The carbon content was measured by oxidizing the SiO2/C at 850oC for 3 h in a muffle furnace. Stoichiometric magnesium powders were mixed with the SiO2/C, and then heated at 650oC for 6 h under Ar gas flow. After that, the products were mixed with 1 M HCl, and stirred for 2 h. The products were filtered and then dried in vacuum over overnight. The carbon-coated Si/C was obtained. Finally, the Si/C was mixed with graphite and ground for 2 h with weight ratio of 1:3, 1:2, 1:1 and 2:1, respectively, which are donated as SCG-1, SCG-2, SCG-3 and SCG-4, respectively. The microstructure of the products were characterized by X-ray diffraction (XRD) (Bruker D8), scanning electron microscope (SEM) (JSM 6700F) and transmission electron microscope (TEM) (JEM 2100). Electrodes were prepared by drying a slurry (a mixture of 78 wt.% active materials, 12 wt.% acetylene black and 10 wt.% polyvinylidene fluoride) at 120 oC for 12 h under vacuum. Then, CR2032 coin-type cells were assembled with lithium metal as counter electrodes in an argon-filled glove box. The weight of the active materials in the electrode is ca. 5‒6 mg. The electrolyte was composed of 1M LiPF6 dissolved in a mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (MEC) with volume ratio of 1:1:1.The discharge−charge measurements were carried out on a battery cycler in a voltage range between 0.05 V and 3.00 V (CT 2001A). Electrochemical impedance spectroscopy (EIS)
3
measurements were performed on an electrochemical workstation (CHI660D) in a frequency range from 100 kHz to 0.01 Hz. 3. Results and discussion XRD pattern shown in Fig. 1 indicates that the product obtained by magnesiothemic
Fig. 1. XRD patterns of Si/C, and SCG-1 to SCG-4. reduction of SiO2/C is Si/C (JCPDS No. 27-1402). The absence of reflection peaks of carbon implies that carbon is in amorphous state. It can be seen that the reflection intensity ratio of Si to graphite increases with increase of Si/C to graphite ratio. The SiO2 content in SiO2/C was measured to be 71 wt.%. Therefore, the Si contents in SCG-1 to 4 are calculated to be ca. 13 wt.%, ca. 18 wt.%, ca. 27 wt.% and ca. 35 wt.%, respectively. Fig. 2(a) is a SEM image exhibiting monodispersed ultrafine Si/C nanoparticles. The corresponding TEM image (Fig. 2(b)) demonstrates carbon coating feature of the ultrafine Si/C nanoparticles with Si particle size ca. 10 nm. The electron diffraction rings in bottom left corner confirm presence of Si. The graphite powders display ellipsoid-like morphology with size ca. 10 μm (Fig. 2(c)). Fig. 2(d) is a representative SEM image of SCG-2, which indicates that the ultrafine Si/C nanoparticles are uniformly attached on the graphite surface. Only very 4
few isolated Si/C nanoparticles are observed. Voltage profiles of SCG-1 to SCG-4 at current density of 0.1 mA cm‒2 are exhibited in
Fig. 2. (a) SEM and (b) TEM images of the ultrafine Si/C. Inset in bottom left corner is electron diffraction rings of the ultrafine Si particles. Black arrows denote carbon layers. SEM images of (c) graphite and (d) SCG-2.
Fig. 3. (a) Voltage profiles, (b) cyclic performance and (c) coulombic efficiencies of the
5
products at current density of 0.1 mA cm‒2. (d) Rate capability of the products. Fig. 3(a). The lithiation potentials of Si and graphite are very close, which are ca. 0.4 V and 0.1 V, respectively. The lithiation potentials of SCG-1 to SCG-4 display mixed feature, which increase with increase of Si/C content in the composite. The first-cycle discharge/charge capacities of SCG-1 to SCG-4 are 611.6/554.3 mAh g‒1, 728.6/662.4 mAh g‒1, 763.6/676.3 mAh g‒1 and 802.8/698.0 mAh g‒1, respectively. Correspondingly, their first-cycle coulombic efficiency (CE) is 90.6%, 90.9%, 88.6% and 86.9%, respectively, in which the CE of SCG-2 is the highest among the four products and is higher than the previous report about Si/graphite/carbon composite anode (86%) [19]. The cyclic performance of the four products is displayed in Fig. 3(b). As a comparison, the cyclic performance of Si and Si/C at the same testing condition is also exhibited. The first-cycle discharge/charge capacities of Si can reach 2519.6/1872.9 mAh g‒1 (CE = 74.3%). However, the capacity drops rapidly with cycling and the reversible capacity retention is only 21.2% after 50 cycles. Si/C displays lower first-cycle discharge/charge capacities compared with Si, which are 1418.5/1157.0 mAh g‒1 (CE = 81.6%). However, its reversible capacity retention is only 55.2% after 100 cycles. It can be seen that SCG-1 to SCG-4 display much more stable cyclic performance than Si and Si/C, demonstrating effectiveness of the graphite. The reversible capacity of SCG-2 can reaches 601.3 mAh g‒1 over 100 cycles. Correspondingly, its reversible capacity retention is as high as 90.8%. The CEs of Si, Si/C, SCG-1 to SCG-4 are exhibited in Fig. 3(c). It is obvious that SCG-1 and SCG-2 display the best performance among all the products, especially in the starting 20 cycles. The CEs of SCG-2 are over 98% and 99% after 10 and 20 cycles, respectively,
6
demonstrating its excellent lithiation/delithiation efficiency. The enhancement of the first-cycle CEs of Si/C-graphite composites compared with Si/C is mainly due to high first-cycle CE of the commercial graphite. Rate capability of SCG-1 to SCG-4 at different current density is shown in Fig. 3(d). Among the four products, SCG-2 exhibits the best performance, especially at high current density. Its initial reversible capacity is 633.5 mAh g‒1 at 0.2 mA cm‒2, and can reach 161.2 mAh g‒1 at 2 mA cm‒2. The capacity of SCG-2 reaches 614.9 mAh g‒1 when the current density recovers 0.2 mA cm‒2. Its capacity recovery ability is as high as 97%. Fig. 4 displays Nyquist plots of the four products, each of which consists of a semicircle at
Fig. 4. Nyquist plots of the products. high frequency region followed by a straight line in low frequency region. The radius of the semicircle is an indicative of charge transfer resistance Rct happened in the electrode/electrolyte interface. Apparently, Rct of SCG-2 is the lowest, implying the highest electrical conductivity and consequently higher rate capability among the four products. 4. Conclusions In summary, ultrafine Si/C‒graphite composite anode materials have been synthesized through magnesiothemic reduction of SiO2/C and then mixed with graphite powders. SCG-2 with Si content of 16 wt.% and graphite content of 67 wt.% displays the best electrochemical performance, in which the reversible capacity can reach 662.4 mAh g‒1 with capacity
7
retention as high as 90.8% after 100 cycles at current density of
0.1 mA cm‒2. Meanwhile,
the first-cycle CE reaches 90.9%. These ultrafine Si/C‒graphite composite materials with combined electrochemical properties are believed to meet multiple requirements of high energy-density lithium ion batteries. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant no. 51472083). References [1] Y.-S. Hu, R. Demir-Cakan, M.-M. Titirici, J.-O. Müller, R. Schlögl, M. Antonietti, and J. Maier, Angewandte Chemie International Edition 47 (2008) 1645-1649. [2] N. Ding, J. Xu, Y. Yao, G. Wegner, I. Lieberwirth, C. Chen, Journal of Power Sources 192 (2009) 644-651. [3] K.F. Chiu, K.M. Lin, H.C. Lin, C.H. Hsu, C.C. Chen, D.T. Shieh, Journal of the Electrochemical Society 155 (2008) A623-A627. [4] J.K. Yoo, J. Kim, Y.S. Jung and K. Kang, Advanced Materials 24 (2012) 5452-5456. [5] Z. Wen, G. Lu, S. Mao, H. Kim, S. Cui, K. Yu, X. Huang, P.T. Hurley, O. Mao and J. Chen, Electrochemistry Communications 29 (2013) 67-70. [6] Y. S. Hu, R. Demir-Cakan, M.M. Titirici, J.O. Müller, R. Schlögl, M. Antonietti and J. Maier, Angewandte Chemie International Edition 47(2008) 1645-1649. [7] X.Y. Zhou, J.J. Tang, J. Yang, J. Xie and L.L. Ma, Electrochimica Acta 87 (2013) 663-668. [8] Q. Si, K. Hanai, N. Imanishi, M. Kubo, A. Hirano, Y. Takeda, O. Yamamoto, Journal of Power Sources 189 (2009) 761-765. [9] J. Saint, M. Morcrette, D. Larcher, L. Laffont, S. Beattie, J.P. Peres, D. Talaga, M. Couzi, J.M. Tarascon, Advanced Functional Materials 17 (2007) 1765-1774. [10] X. Yang, Z. Wen, X. Zhu, S. Huang, Electrochemical and Solid-State Letters 8 (2005) A481-A483. 8
[11] L. David, S. Bernard, C. Gervais, P. Miele, G. Singh, The Journal of Physical Chemistry C 119 (2015) 2783-2791. [12] L. David, D. Asok, G. Singh, ACS Applied Materials & Interfaces 6 (2014) 16056-16064. [13] H. Kim, B. Han, J. Choo, J. Cho, Angewandte Chemie International Edition 47 (2008) 10151-10154. [14] H. Wu, G. Chan, J.W. Choi, I. Ryu, Y. Yao, M.T. McDowell, S.W. Lee, A. Jackson, Y. Yang, L. Hu, Y. Cui, Nature Nanotechnology 7 (2012) 310-315. [15] W.-J. Zhang, Journal of Power Sources 196 (2011) 13-24. [16] X. Yang, Z. Wen, X. Xu, B. Lin, Z. Lin, Journal of the Electrochemical Society 153 (2006) A1341-A1344. [17] H. Kim, D. Im, S.G. Doo, Journal of Power Sources 174 (2007) 588-591. [18] Y. Liu, K. Hanai, K. Horikawa, N. Imanishi, A. Hirano, Y. Takeda, Materials Chemistry and Physics 89 (2005) 80-84. [19] J.H. Lee, W.J. Kim, J.Y. Kim, S.H. Lim, S.M. Lee, Journal of Power Sources 176 (2008) 353-358.
Highlights
Ultrafine Si/C has been obtained through magnesiothermic reduction of ultrafine SiO2/C.
Cyclic performance of the ultrafine Si/C‒graphite anode has been improved.
The first-cycle coulombic efficiency can reach 90.9%.
The capacity retention can reach 90.8% over 100 discharge‒charge cycles.
9
www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)30743-1 http://dx.doi.org/10.1016/j.matlet.2016.05.018 MLBLUE20829
To appear in: Materials Letters Received date: 24 January 2016 Revised date: 18 April 2016 Accepted date: 4 May 2016 Cite this article as: Mengjie Pan, Xuelian Liu, Hongbo Liu and Yuxi Chen, Ultrafine Si/C-graphite composite anode materials with improved cyclic performance, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.05.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultrafine Si/C‒graphite composite anode materials with improved cyclic performance Mengjie Pan, Xuelian Liu, Hongbo Liu, Yuxi Chen* College of Materials and Engineering/Hunan Province Key Laboratory for Spray Deposition Technology and Application, Hunan University, Changsha 410082, P.R. China *
Corresponding
author.
Yuxi
Chen,
Professor,
Tel:
+86-731-88664018,
Fax:+86-731-88823554, [email protected] Abstract Si is of great interest as anode candidate for high energy-density lithium ion batteries. However, the rapid capacity fading and low first-cycle coulombic efficiency strongly hinder its applications. Ultrafine Si/C nanoparticles mixed graphite powders as composite anode materials have been synthesized through magnesiothermic reduction of ultrafine SiO2/C, and then mixed with graphite. Electron microscopy investigation indicates that the ultrafine Si/C particles with size ca. 10 nm are attached on the graphite surface. Electrochemical evaluation demonstrates that the cyclic performance of the ultrafine Si/C‒graphite composite materials has been improved compared with Si and Si/C, in which the reversible capacity can reach 662.4 mAh g‒1 with the first-cycle coulombic efficiency as high as 90.9%. Meanwhile, the capacity retention can reach 90.8% over 100 cycles. These ultrafine Si/C‒graphite composite materials are promising anode candidate with combined electrochemical properties to meet multiple requirements of high energy-density lithium ion batteries. Keywords: energy storage and conversion; lithium ion batteries; silicon; composites; electron microscopy
1
1. Introduction Si is of great interest as anode materials for high energy-density lithium ion batteries (LIBs) [1-3], since it can present a specific capacity of ca. 4200 mAh g−1, which is more than 10 times higher than that of the commercial graphite anode materials (372 mAh g−1). However, the large volume variation of Si (> 300%) during lithiation/delithiation strongly hinders its applications. To minimize this volume variation, many novel Si-based materials for improving cyclic stability have been synthesized, such as Si nanotubes [4,5], Si@SiOx/C nanocomposites [6], Si@C hollow core-shell heterostructures [7-10], SiCN/BN [11], Si-Al-C-N/carbon nanotubes [12], as well as porous Si [13,14]. These methods have been demonstrated to be effective in improving cyclic performance of Si. However, up to date, a big challenge of Si attracts much less attention and still remains, i.e., the low first-cycle coulombic efficiency (CE). In general, the first-cycle CE of the nanosized Si is ca. 80% or less [15], which is far below the requirements of LIBs. Combination of the nannosized Si with the commercial graphite is expected to be an effective way [16]. However, previous investigation demonstrates that extensive milling of Si and graphite may induce high first-cycle irreversible capacity [17,18]. To improve the first-cycle CE and the cyclic stability of Si-based anode simultaneously, we report systematic investigation of ultrafine Si/C‒ graphite composite anode materials with combined electrochemical performance, which are believed to meet multiple requirements of high energy-density LIBs. 2. Experimental 0.06 g lysine was dissolved into 70 mL deionized water, and then 3.2 mL
2
tetraethylorthosilicate were added under continuous stirring at 60oC for 21 h. Then, white SiO2 nanoparticles were obtained. SiO2 was mixed with deionized water, alcohol and sucrose with appropriate weight ratio, and stirred at 60oC for 3 h. The products was filtered and dried overnight, and then calcined at 800oC for 2 h. The carbon-coated SiO2/C was obtained. The carbon content was measured by oxidizing the SiO2/C at 850oC for 3 h in a muffle furnace. Stoichiometric magnesium powders were mixed with the SiO2/C, and then heated at 650oC for 6 h under Ar gas flow. After that, the products were mixed with 1 M HCl, and stirred for 2 h. The products were filtered and then dried in vacuum over overnight. The carbon-coated Si/C was obtained. Finally, the Si/C was mixed with graphite and ground for 2 h with weight ratio of 1:3, 1:2, 1:1 and 2:1, respectively, which are donated as SCG-1, SCG-2, SCG-3 and SCG-4, respectively. The microstructure of the products were characterized by X-ray diffraction (XRD) (Bruker D8), scanning electron microscope (SEM) (JSM 6700F) and transmission electron microscope (TEM) (JEM 2100). Electrodes were prepared by drying a slurry (a mixture of 78 wt.% active materials, 12 wt.% acetylene black and 10 wt.% polyvinylidene fluoride) at 120 oC for 12 h under vacuum. Then, CR2032 coin-type cells were assembled with lithium metal as counter electrodes in an argon-filled glove box. The weight of the active materials in the electrode is ca. 5‒6 mg. The electrolyte was composed of 1M LiPF6 dissolved in a mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (MEC) with volume ratio of 1:1:1.The discharge−charge measurements were carried out on a battery cycler in a voltage range between 0.05 V and 3.00 V (CT 2001A). Electrochemical impedance spectroscopy (EIS)
3
measurements were performed on an electrochemical workstation (CHI660D) in a frequency range from 100 kHz to 0.01 Hz. 3. Results and discussion XRD pattern shown in Fig. 1 indicates that the product obtained by magnesiothemic
Fig. 1. XRD patterns of Si/C, and SCG-1 to SCG-4. reduction of SiO2/C is Si/C (JCPDS No. 27-1402). The absence of reflection peaks of carbon implies that carbon is in amorphous state. It can be seen that the reflection intensity ratio of Si to graphite increases with increase of Si/C to graphite ratio. The SiO2 content in SiO2/C was measured to be 71 wt.%. Therefore, the Si contents in SCG-1 to 4 are calculated to be ca. 13 wt.%, ca. 18 wt.%, ca. 27 wt.% and ca. 35 wt.%, respectively. Fig. 2(a) is a SEM image exhibiting monodispersed ultrafine Si/C nanoparticles. The corresponding TEM image (Fig. 2(b)) demonstrates carbon coating feature of the ultrafine Si/C nanoparticles with Si particle size ca. 10 nm. The electron diffraction rings in bottom left corner confirm presence of Si. The graphite powders display ellipsoid-like morphology with size ca. 10 μm (Fig. 2(c)). Fig. 2(d) is a representative SEM image of SCG-2, which indicates that the ultrafine Si/C nanoparticles are uniformly attached on the graphite surface. Only very 4
few isolated Si/C nanoparticles are observed. Voltage profiles of SCG-1 to SCG-4 at current density of 0.1 mA cm‒2 are exhibited in
Fig. 2. (a) SEM and (b) TEM images of the ultrafine Si/C. Inset in bottom left corner is electron diffraction rings of the ultrafine Si particles. Black arrows denote carbon layers. SEM images of (c) graphite and (d) SCG-2.
Fig. 3. (a) Voltage profiles, (b) cyclic performance and (c) coulombic efficiencies of the
5
products at current density of 0.1 mA cm‒2. (d) Rate capability of the products. Fig. 3(a). The lithiation potentials of Si and graphite are very close, which are ca. 0.4 V and 0.1 V, respectively. The lithiation potentials of SCG-1 to SCG-4 display mixed feature, which increase with increase of Si/C content in the composite. The first-cycle discharge/charge capacities of SCG-1 to SCG-4 are 611.6/554.3 mAh g‒1, 728.6/662.4 mAh g‒1, 763.6/676.3 mAh g‒1 and 802.8/698.0 mAh g‒1, respectively. Correspondingly, their first-cycle coulombic efficiency (CE) is 90.6%, 90.9%, 88.6% and 86.9%, respectively, in which the CE of SCG-2 is the highest among the four products and is higher than the previous report about Si/graphite/carbon composite anode (86%) [19]. The cyclic performance of the four products is displayed in Fig. 3(b). As a comparison, the cyclic performance of Si and Si/C at the same testing condition is also exhibited. The first-cycle discharge/charge capacities of Si can reach 2519.6/1872.9 mAh g‒1 (CE = 74.3%). However, the capacity drops rapidly with cycling and the reversible capacity retention is only 21.2% after 50 cycles. Si/C displays lower first-cycle discharge/charge capacities compared with Si, which are 1418.5/1157.0 mAh g‒1 (CE = 81.6%). However, its reversible capacity retention is only 55.2% after 100 cycles. It can be seen that SCG-1 to SCG-4 display much more stable cyclic performance than Si and Si/C, demonstrating effectiveness of the graphite. The reversible capacity of SCG-2 can reaches 601.3 mAh g‒1 over 100 cycles. Correspondingly, its reversible capacity retention is as high as 90.8%. The CEs of Si, Si/C, SCG-1 to SCG-4 are exhibited in Fig. 3(c). It is obvious that SCG-1 and SCG-2 display the best performance among all the products, especially in the starting 20 cycles. The CEs of SCG-2 are over 98% and 99% after 10 and 20 cycles, respectively,
6
demonstrating its excellent lithiation/delithiation efficiency. The enhancement of the first-cycle CEs of Si/C-graphite composites compared with Si/C is mainly due to high first-cycle CE of the commercial graphite. Rate capability of SCG-1 to SCG-4 at different current density is shown in Fig. 3(d). Among the four products, SCG-2 exhibits the best performance, especially at high current density. Its initial reversible capacity is 633.5 mAh g‒1 at 0.2 mA cm‒2, and can reach 161.2 mAh g‒1 at 2 mA cm‒2. The capacity of SCG-2 reaches 614.9 mAh g‒1 when the current density recovers 0.2 mA cm‒2. Its capacity recovery ability is as high as 97%. Fig. 4 displays Nyquist plots of the four products, each of which consists of a semicircle at
Fig. 4. Nyquist plots of the products. high frequency region followed by a straight line in low frequency region. The radius of the semicircle is an indicative of charge transfer resistance Rct happened in the electrode/electrolyte interface. Apparently, Rct of SCG-2 is the lowest, implying the highest electrical conductivity and consequently higher rate capability among the four products. 4. Conclusions In summary, ultrafine Si/C‒graphite composite anode materials have been synthesized through magnesiothemic reduction of SiO2/C and then mixed with graphite powders. SCG-2 with Si content of 16 wt.% and graphite content of 67 wt.% displays the best electrochemical performance, in which the reversible capacity can reach 662.4 mAh g‒1 with capacity
7
retention as high as 90.8% after 100 cycles at current density of
0.1 mA cm‒2. Meanwhile,
the first-cycle CE reaches 90.9%. These ultrafine Si/C‒graphite composite materials with combined electrochemical properties are believed to meet multiple requirements of high energy-density lithium ion batteries. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant no. 51472083). References [1] Y.-S. Hu, R. Demir-Cakan, M.-M. Titirici, J.-O. Müller, R. Schlögl, M. Antonietti, and J. Maier, Angewandte Chemie International Edition 47 (2008) 1645-1649. [2] N. Ding, J. Xu, Y. Yao, G. Wegner, I. Lieberwirth, C. Chen, Journal of Power Sources 192 (2009) 644-651. [3] K.F. Chiu, K.M. Lin, H.C. Lin, C.H. Hsu, C.C. Chen, D.T. Shieh, Journal of the Electrochemical Society 155 (2008) A623-A627. [4] J.K. Yoo, J. Kim, Y.S. Jung and K. Kang, Advanced Materials 24 (2012) 5452-5456. [5] Z. Wen, G. Lu, S. Mao, H. Kim, S. Cui, K. Yu, X. Huang, P.T. Hurley, O. Mao and J. Chen, Electrochemistry Communications 29 (2013) 67-70. [6] Y. S. Hu, R. Demir-Cakan, M.M. Titirici, J.O. Müller, R. Schlögl, M. Antonietti and J. Maier, Angewandte Chemie International Edition 47(2008) 1645-1649. [7] X.Y. Zhou, J.J. Tang, J. Yang, J. Xie and L.L. Ma, Electrochimica Acta 87 (2013) 663-668. [8] Q. Si, K. Hanai, N. Imanishi, M. Kubo, A. Hirano, Y. Takeda, O. Yamamoto, Journal of Power Sources 189 (2009) 761-765. [9] J. Saint, M. Morcrette, D. Larcher, L. Laffont, S. Beattie, J.P. Peres, D. Talaga, M. Couzi, J.M. Tarascon, Advanced Functional Materials 17 (2007) 1765-1774. [10] X. Yang, Z. Wen, X. Zhu, S. Huang, Electrochemical and Solid-State Letters 8 (2005) A481-A483. 8
[11] L. David, S. Bernard, C. Gervais, P. Miele, G. Singh, The Journal of Physical Chemistry C 119 (2015) 2783-2791. [12] L. David, D. Asok, G. Singh, ACS Applied Materials & Interfaces 6 (2014) 16056-16064. [13] H. Kim, B. Han, J. Choo, J. Cho, Angewandte Chemie International Edition 47 (2008) 10151-10154. [14] H. Wu, G. Chan, J.W. Choi, I. Ryu, Y. Yao, M.T. McDowell, S.W. Lee, A. Jackson, Y. Yang, L. Hu, Y. Cui, Nature Nanotechnology 7 (2012) 310-315. [15] W.-J. Zhang, Journal of Power Sources 196 (2011) 13-24. [16] X. Yang, Z. Wen, X. Xu, B. Lin, Z. Lin, Journal of the Electrochemical Society 153 (2006) A1341-A1344. [17] H. Kim, D. Im, S.G. Doo, Journal of Power Sources 174 (2007) 588-591. [18] Y. Liu, K. Hanai, K. Horikawa, N. Imanishi, A. Hirano, Y. Takeda, Materials Chemistry and Physics 89 (2005) 80-84. [19] J.H. Lee, W.J. Kim, J.Y. Kim, S.H. Lim, S.M. Lee, Journal of Power Sources 176 (2008) 353-358.
Highlights
Ultrafine Si/C has been obtained through magnesiothermic reduction of ultrafine SiO2/C.
Cyclic performance of the ultrafine Si/C‒graphite anode has been improved.
The first-cycle coulombic efficiency can reach 90.9%.
The capacity retention can reach 90.8% over 100 discharge‒charge cycles.
9