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Novel method to incorporate Si into monodispersed mesoporous carbon spheres
Accepted Manuscript Novel Method to Incorporate Si into Monodispersed Mesoporous Carbon Spheres Kazuhisa Yano, Narihito Tatsuda, Takashi Masuda, Tatsuya Shimoda PII: DOI: Reference:
S0021-9797(16)30387-3 http://dx.doi.org/10.1016/j.jcis.2016.06.028 YJCIS 21338
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
22 April 2016 9 June 2016
Please cite this article as: K. Yano, N. Tatsuda, T. Masuda, T. Shimoda, Novel Method to Incorporate Si into Monodispersed Mesoporous Carbon Spheres, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/ 10.1016/j.jcis.2016.06.028
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 proof before it is published in its final 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.
Novel Method to Incorporate Si into Monodispersed Mesoporous Carbon Spheres
Kazuhisa Yano a*, Narihito Tatsudaa, Takashi Masudab, and Tatsuya Shimodab
a
Toyota Central R&D Labs., Inc., Nagakute, Aichi, 480-1192, Japan
b
Green Devices Research Center, Japan Advanced Institute of Science and Technology,
Nomi, Ishikawa, 923-1211, Japan
* Corresponding author. TEL: +81 561 71 7570/ Fax: +81 561 63 6156 E-mail address: [email protected] (K. Yano)
Abbreviations: CPS, cyclopentasilane; MMCS, monodispersed mesoporous carbon spheres; NP, nanoparticles
Abstract Liquid silicon precursor is used as a silicon source and very simple and easy method for the incorporation of Si into mesoporous carbon spheres is presented. By using capillary condensation, the liquid precursor, Cyclopentasilane, penetrates into mesopores of carbon spheres homogeneously and subsequent heating brings the decomposition of the precursor and the formation of silicon inside meso-channels of carbon even though the decomposition is done much higher than the boiling point of the precursor. The homogeneous distribution of silicon is verified by EDX mapping of the composite as well as SEM observation of the calcined one. More than 45 wt% of Si can be incorporated into mesopores by just one operation. The Si@mesoporous carbon composite works as an anode for a Lithium ion battery.
Keywords: Silicon, Mesoporous carbon, Cyclopentasilane, Incipient wetness, Li ion battery
1. Introduction Mesoporous materials have uniform-sized pores and are suited to incorporate nanomaterials. We have been incorporating various types of nanomaterials such as, magnetic particles [1-3], luminescence materials [4-6], and precious metals [7-9] to extend their properties. Tin oxide incorporated mesoporous carbon works as an anode for lithium ion battery, exhibiting high capacity and durability [10]. Recently, Si, that has much higher capacity than tin oxide, has been paid much attention. Si is capable of storing three to ten folds of Lithium ions if compared to other conventional carbonaceous materials. This feature is advantageous to increase power density of Li ion batteries. However, Si shows large volume expansion of 300-400% while doping [11], leading to degradation in cycle performance caused by the deformation. Both to prevent the deformation and to increase conductivity, carbon coating of Si nanoparticles (Si NPs) has been conducted. Spray pyrolysis technique [12,13], hydrothermal carbonization of glucose [14] or other methods were employed for the carbon coating [15-17]. Hybridization of Si nanoparticles with graphene is also useful to achieve these objectives. Homogeneous dispersion was made from graphene oxide and Si NPs by sonicating them in water [18]. Discharge plasma assisted milling was applied to the mixture of graphene and Si NPs directly [19]. Incorporation of Si NPs into pores of mesoporous carbon is considered to be effective to prevent the degradation of the cycle performance because Si NP can be confined in a narrow space even after it is expanded. Some methods have been conducted to fabricate the hybrid material in which Si NPs are incorporated into mesoporous carbon. Hierarchical structure was achieved by a bottom-up approach using chemical vapor deposition (CVD) of Si and carbon [20].
Other methods, such as nanoparticle-seeded dispersion polymerization of acrylonitrile on methacryloxy-modified Si NPs [21], polymerization of phenol resin with Si NPs [22,23], magnesiothermic reduction of silicas were employed [24-28]. We will report on the simple and facile synthesis of Si NPs/mesoporous carbon composite. As an Si precursor, Cyclopentasilane (CPS), is used [29,30]. Since CPS is a liquid-state material and not viscous, it could be homogeneously incorporated into mesopores through capillary condensation when CPS is dropped on mesoporous carbon powder. CPS can be thermally converted to Si NPs. The conversion temperature of CPS is over 350 °C which is much higher than the boiling point of CPS: 194.3°C. Whether mesopore-filled CPS could be converted to Si NPs without significant loss by the evaporation of CPS was a big question. However, thermal conversion in a covered vessel is eventually found to be effective to incorporate Si NPs into mesoporous carbons. As for a mesoporous carbon, monodispersed mesoporous carbon spheres (MMCS) that we have been investigating is used [31,32]. MMCS has uniform particle size and mesopores are connected continuously through the particle. These features are advantageous for higher packing ratio and easy entry/discharge of lithium ions [32]. In addition, since MMCS has uniform particle size, it is extremely easy to distinguish Si NPs if they form outside MMCS. This feature is highly useful to judge whether Si NPs deposition occurs on the surface or inside MMCS.
2. Experimental 2.1 Synthesis of Si@MMCS composites Monodispersed mesoporous carbon spheres (MMCS) were synthesized according to the literature [8]. The introduction of Si NPs into mesopores of MMCS was done by
incipient wetness technique which utilizes Capillary condensation. MMCS was added to a glass tube. CPS with the same volume of mesopores was dropped onto MMCS powder. The bottle was sealed with a plastic cap and tapped several times. Then the plastic cap was replaced with a glass plate and, the glass bottle with the plate was heated at 400 ºC for 30 min to transform CPS to Si NPs. Since CPS reacts with oxygen immediately, these operations were done in a glove box under nitrogen atmosphere. MMCSs with different mesopore size were prepared, and Si NPs was incorporated. Samples are denoted as 0701-Si and 0702-Si.
2.2 Characterization Scanning electron micrographs (SEMs) were obtained with a SU3500 (Hitachi). Nitrogen adsorption/desorption isotherms were measured using a BELSORP-mini II (Bell-Japan) at -196 ºC. The sample was evacuated at 150 ºC under 0.13 Pa before the measurement. Pore volume was estimated from the amount of adsorbed nitrogen at the relative pressure of 0.98. Pore diameter was calculated by the BJH method for the desorption branch. Thermo-gravimetric Analysis was conducted with a Rigaku THERMO PLUS in air. Transmission electron micrograph was obtained with a Jeol 2100F TEM using an acceleration voltage of 200 kV.
2.3 Battery Testing Electrochemical properties of Si@MMCS anode were investigated in non-aqueous lithium half cells. A film consisting of 70 wt% Si@MMCS, 25 wt% carbon black, and 5 wt% PTFE powder were pressed on a stainless steel mesh to obtain Si@MMCS electrode. Porous polyethylene membrane (Tonen General Sekiyu K. K.) was used as a
separator. 1.0 M LiPF6 in EC/DEC (1:1 by vol.)), and Li metal were employed as an electrolyte, and a counter electrode, respectively. The discharge and charge tests were performed at 25 ºC at a current density of 200 mA/g, in a range between 0.02 and 2.0 V.
3. Results and Discussion Two types of MMCS with different mesopore size (0701, 0702) were synthesized according to the literature [8]. Physical properties of MMCS are listed in Table 1, and SEM images are shown in Figure S1. CPS equal to the pore volume of MMCS was dropped onto MMCS, and introduced into mesopores by Capillary condensation. After heated at 400 ºC, amorphous Si NPs were formed inside mesopores. Figure 1 illustrates a scheme for the Si NPs incorporation process. The samples are denoted as 0701-Si and 0702-Si originated from the name of MMCS used. From Figure 2 showing SEM images of Si@MMCS composites, it is obvious that spherical shape and monodispersity of MMCS are retained after Si NPs are incorporated. Since no other small particles are observed, it is inferred that Si NPs preferentially form inside mesopores. The distribution of Si NPs and carbon elements is visualized with SEM-EDX mapping. Images are shown in Figure 3. Aggregated Si@MMCS particles were observed for easy viewing. Locations where Si and carbon are detected coincide with the MMCS particles in the SEM images. To visually investigate Si NPs incorporation, TEM-EDX was used for the mapping. 0702-Si was embedded into epoxy resin, and the resin was sliced to a thin film for the observation. Figure 4 shows a TEM image and corresponding carbon and Si mapping images. In case Si NPs are introduced only near the surface of MMCS, core/shell structure should be observed. Since such a structure is not seen in Figure 4 (c), Si NPs are thought to be uniformly distributed through MMCS. To directly confirm
the incorporation of Si NPs into mesoporous carbon, Si NPs incorporation was done to CMK-3 [34]. Since mesopores of CMK-3 are highly ordered, it is quite easy to see how Si NPs are located.
Figure S2 shows TEM images of Si@CMK-3. White portion in
the image represents Si NPs incorporated, and it is understood that Si NPs are homogeneously incorporated into mesopores of CMK-3. It is expected that the incorporation of Si NPs into mesopores change adsorption properties significantly. Nitrogen adsorption/desorption measurements were conducted, and isotherms and corresponding pore size distribution curves are shown in Figure 5. Large decrease in pore volume due to Si NPs incorporation is observed for both large pore MMCS (0701-Si) and small one’s (0702-Si). The peak top in each pore size distribution curve shifts to lower number, indicating decrease in pore size. These adsorption property changes reveal that Si NPs are incorporated into mesopores of MMCS. The amount of Si NPs incorporated in MMCS was determined with thermo-gravimetric analysis in air. TGA curves are shown in Figure S3. The increase in weight observed between R.T. and 500 ºC can be attributed to the oxidation of Si to SiO2. After this, the weight decreases because of the oxidation and elimination of carbon. The weight becomes constant at the temperature higher than 600 ºC. The amount of Si NPs incorporated is determined by assuming that the residue is SiO2. The calculated values are 47.3 wt% for 0701-Si and 45.7 wt% for 0702-Si. If all of the CPS incorporated into mesopores is assumed to be transformed to Si NPs, the values should be 61.5 wt% or 55.4 wt% in case 0701 or 0702 is used. The difference between the experimental and the estimated value is bigger for 0701-Si, mentioning easy CPS evaporation in larger pore-sized mesoporous carbon. CPS could be kept more in the mesopores with smaller pore size. From the above results, it turns out that a large
amount of Si NPs can be incorporated into MMCS by just one simple and easy operation. As mentioned in Introduction, the decomposition of CPS was done at 400 ºC that is much higher than the boiling point of CPS: 194.3°C. During the conversion reaciton, dimer or trimer CPS are generated by multimerization of gaseous CPS. Those multimers could be preferentially adsorbed onto mesopores of carbon, making it possible to accumulate Si NPs mainly inside carbon spheres. It can be expected that the amount of Si NPs incorporated be increased if the incorporation process is repeated. The process was repeated for 0702-Si and the sample is denoted as 0702-2-Si. Nitrogen adsorption isotherm is shown in Figure 5(d). The decrease in the pore volume is significant and it looks like the most of mesopores are filled with Si NPs. TGA analysis was conducted for 0702-2-Si (Figure S3(c)) and the weight ratio of Si NPs is calculated to be 53.6 wt% indicating the increase in the amount of Si NPs incorporated. Because the value is closed to the theoretical value of 55.4 wt% for a fully CPS filled composite, the result of the nitrogen adsorption measurement can be well understood. If the composite is oxidized in air, only SiO2 NPs remains. It is quite interesting to know if the residual SiO2 particles after the oxidation retain monodispersed spherical shape or not. SEM observation was conducted with Si@MMCS composites calcined in air at 550 ºC for 6h to remove carbon and oxidize Si NPs, and SEM images are shown in Figure 6. All of the calcined particles keep spherical shape and monodispersity is retained. The calcined 0702-Si was embedded in an epoxy resin, and the thin-sliced surface was observed by SEM (Figure S4). It is obvious from the image that SiO2 is distributed throughout particles, which indicates that Si NPs are uniformly incorporated into mesopores in the composite. Si@MMCS composites were heated at 400 ºC to convert CPS to Si NPs. To investigate
whether Si NPs are in amorphous or crystalline phase, XRD measurements were conducted. XRD patterns are shown in Figure S5. No sharp peaks are observed in the patterns. It is concluded that Si NPs exist in amorphous phase in mesopores. A small broad peak appeared around 28º when the sample was heated at 800 ºC (data not shown). Given the fact that polycrystalline structure was obtained when a film made from CPS was heated at 540 ºC [30], the crystallization of Si might be suppressed inside mesopores. One of the most expected applications of Si@MMCS composites is an anode for a Lithium ion battery. A half-cell was fabricated with Si@MMCS and charge-discharge cycle property was evaluated (Figure 7). The initial capacity is 3240 mAh/g, but some degradation is observed. However, it can be expected that capacity and degradation issue could be drastically improved by optimizing the amount of incorporated Si, pore diameter, and particle diameter of MMCS. Because of the large volume change of Si, it should be important to precisely control the ratio of Si solid volume to mesopore space. In addition, it can be suggested that PTFE, which is used as a binder, produces HF that reacts with Si, leading to big capacity fading. We are now trying to tune the amount of Si NPs precisely and to establish a new electrode composition without a PTFE binder.
4. Conclusion In conclusion, simple and easy method to incorporate Si NPs into mesoporous carbon is achieved by using a liquid Si monomer, CPS, as a precursor. By utilizing Capillary condensation, as much as 47 wt% of Si NPs can be incorporated into mesopores by just one operation. Si NPs are homogeneously incorporated into mesopores, which is verified by TEM observation and EDX mapping of the composite as well as SEM
observation of the calcined one. The Si@mesoporous carbon shows high initial capacity as an anode for Lithium ion battery although fading of capacity is observed. Development of high performance Lithium ion battery anode with Si@MMCS is underway by tuning the amount of Si nanoparticles and optimizing battery system.
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[9] K. Yano, S. Zhang, X. Pan, N. Tatsuda, J. Colloid Interface Sci., 421 (2014) 22-26. [10] J. Chen, K. Yano, ACS Appl. Mater. Interfaces, 5 (2013) 7682-7687. [11] B. A. Boukamp, G. C. Lesh, R. A. Huggins, J. Electrochem. Soc., 128 (1981) 725-729. [12] S-H. Ng, J. Wang, D. Wexler, K. Konstaninov, Z-P. Guo, H-K Liu, Angew. Chem. Int Ed., 45 (2006) 6896-6899. [13] C. Paireau, S. Jouanneau, M. R. Ammar, P. Simon, F. Beguin, E. Raymundo-Pinero, Electrochim. Acta, 174 (2015) 361-368. [14] Y. S. Hu, R. Demir-Cakan, M. M. Titirici, J. O. Muller, R. Schlogl, M. Antonietti, J. Majer, Angew. Chem. Int. Ed., 47 (2008) 1645-1649. [15] Y. Chen, Y. Hu, J. Shao, Z. Shen, R. Chen, X. Zhang, X. He, Y. Song, X, Xing, J. Power Sources, 298 (2015) 130-137. [16] S. A. Klankowskia, G. P. Pandeya, G. A Malekb,. J. Wub, R. A. Rojeskic, J. A. Lia, Electrochim. Acta, 178 (2015) 797-805. [17] L. Y. Yang, H. Z. Li, J. Liu, Z. Q. Sun, S. S. Tang, M. Lei, Sci. Rep. 5 (2015) 10908. [18] J. K. Lee, K. B. Smith, C. M. Hayner, H. H. Kung, Chem. Commun., 46 (2010) 2025-2027. [19] W. Sun, R. Hu, H. Liu, M. Zeng, L. Yang, H. Wang, M. Zhu, J. Power. Sources, 268, (2014) 610-618. [20] A. Magasinski, P. Dixon, B. Hertzberg, A. Kvit, J. Ayala, G. Yushin, Nat. Mater., 9 (2010) 353-358.
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Table 1. Properties of MMCS Sample Name
Particle diameter [nm]
Surface area [m2/g]
Mesopore size [nm]
Pore volume [mL/g]
0701
553
1220
6.2
1.9
0702
464
1320
3.3
1.48
Figure 1. A scheme for the incorporation of Si with incipient wetness technique. CPS with the same volume of mesopores of MSCS is dropped on MMCS. Mesopores of MMCS is filled with CPS when incipient wetness is achieved. The mixture is heated at 400 ºC for 30 min to convert CPS to Si.
Figure 2. SEM images of Si@MSCSs: 0701-Si (a) and 0702-Si (b).
Figure 3. SEM images of Si@MSCSs: 0701-Si (a) and 0702-Si (b), and corresponding mappings for Si: (c), (d) and carbon: (e), (f) elements.
Figure 4. TEM image of 0701-Si (a), and corresponding carbon (b), Si (c) mappings.
Figure 5. Nitrogen adsorption-desorption isotherms for 0701-Si (a) , 0702-Si (c) , 0702-2-Si (d) and corresponding original mesoporous carbons (b), (e). Pore size distribution curves for 0701-Si (e) , 0702-Si (g), 0702-2-Si (h) and corresponding mesoporous carbons (f), (i).
Figure 6. SEM images of silica particles obtained from 0701-Si (a) and 0702-Si (b) by calcining them at 550 ºC for 6 h in air.
Figure 7. Charge-Discharge cycle property of 0701-Si as Li battery anode.
Supporting material
Novel Method to Incorporate Si into Monodispersed Mesoporous Carbon Spheres
Kazuhisa Yano, Narihito Tatsuda, Takashi Masuda, and Tatsuya Shimoda
Figure S1. SEM images of MSCSs for 0701 (a) and 0702 (b). Average particle size and standard deviation are presented in parentheses.
Figure S2. TEM images of Si@CMK-3.
Figure S3. Thermal gravimetric analysis curves for 0701-Si (a), 0702-Si (b) and 0702-2-Si (c).
Figure S4. SEM image of thin sections of silica particles obtained from 0702-Si embedded in epoxy resin.
Figure S5. XRD patterns of Si@MSCSs: 0701-Si (a) and 0702-Si (b).
*3: Graphical Abstract
Si
Si
Si
Si Si MMCS
Incipient Wetness
△
Si@MMCS MMCS
Si@MMCS
S0021-9797(16)30387-3 http://dx.doi.org/10.1016/j.jcis.2016.06.028 YJCIS 21338
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
22 April 2016 9 June 2016
Please cite this article as: K. Yano, N. Tatsuda, T. Masuda, T. Shimoda, Novel Method to Incorporate Si into Monodispersed Mesoporous Carbon Spheres, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/ 10.1016/j.jcis.2016.06.028
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 proof before it is published in its final 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.
Novel Method to Incorporate Si into Monodispersed Mesoporous Carbon Spheres
Kazuhisa Yano a*, Narihito Tatsudaa, Takashi Masudab, and Tatsuya Shimodab
a
Toyota Central R&D Labs., Inc., Nagakute, Aichi, 480-1192, Japan
b
Green Devices Research Center, Japan Advanced Institute of Science and Technology,
Nomi, Ishikawa, 923-1211, Japan
* Corresponding author. TEL: +81 561 71 7570/ Fax: +81 561 63 6156 E-mail address: [email protected] (K. Yano)
Abbreviations: CPS, cyclopentasilane; MMCS, monodispersed mesoporous carbon spheres; NP, nanoparticles
Abstract Liquid silicon precursor is used as a silicon source and very simple and easy method for the incorporation of Si into mesoporous carbon spheres is presented. By using capillary condensation, the liquid precursor, Cyclopentasilane, penetrates into mesopores of carbon spheres homogeneously and subsequent heating brings the decomposition of the precursor and the formation of silicon inside meso-channels of carbon even though the decomposition is done much higher than the boiling point of the precursor. The homogeneous distribution of silicon is verified by EDX mapping of the composite as well as SEM observation of the calcined one. More than 45 wt% of Si can be incorporated into mesopores by just one operation. The Si@mesoporous carbon composite works as an anode for a Lithium ion battery.
Keywords: Silicon, Mesoporous carbon, Cyclopentasilane, Incipient wetness, Li ion battery
1. Introduction Mesoporous materials have uniform-sized pores and are suited to incorporate nanomaterials. We have been incorporating various types of nanomaterials such as, magnetic particles [1-3], luminescence materials [4-6], and precious metals [7-9] to extend their properties. Tin oxide incorporated mesoporous carbon works as an anode for lithium ion battery, exhibiting high capacity and durability [10]. Recently, Si, that has much higher capacity than tin oxide, has been paid much attention. Si is capable of storing three to ten folds of Lithium ions if compared to other conventional carbonaceous materials. This feature is advantageous to increase power density of Li ion batteries. However, Si shows large volume expansion of 300-400% while doping [11], leading to degradation in cycle performance caused by the deformation. Both to prevent the deformation and to increase conductivity, carbon coating of Si nanoparticles (Si NPs) has been conducted. Spray pyrolysis technique [12,13], hydrothermal carbonization of glucose [14] or other methods were employed for the carbon coating [15-17]. Hybridization of Si nanoparticles with graphene is also useful to achieve these objectives. Homogeneous dispersion was made from graphene oxide and Si NPs by sonicating them in water [18]. Discharge plasma assisted milling was applied to the mixture of graphene and Si NPs directly [19]. Incorporation of Si NPs into pores of mesoporous carbon is considered to be effective to prevent the degradation of the cycle performance because Si NP can be confined in a narrow space even after it is expanded. Some methods have been conducted to fabricate the hybrid material in which Si NPs are incorporated into mesoporous carbon. Hierarchical structure was achieved by a bottom-up approach using chemical vapor deposition (CVD) of Si and carbon [20].
Other methods, such as nanoparticle-seeded dispersion polymerization of acrylonitrile on methacryloxy-modified Si NPs [21], polymerization of phenol resin with Si NPs [22,23], magnesiothermic reduction of silicas were employed [24-28]. We will report on the simple and facile synthesis of Si NPs/mesoporous carbon composite. As an Si precursor, Cyclopentasilane (CPS), is used [29,30]. Since CPS is a liquid-state material and not viscous, it could be homogeneously incorporated into mesopores through capillary condensation when CPS is dropped on mesoporous carbon powder. CPS can be thermally converted to Si NPs. The conversion temperature of CPS is over 350 °C which is much higher than the boiling point of CPS: 194.3°C. Whether mesopore-filled CPS could be converted to Si NPs without significant loss by the evaporation of CPS was a big question. However, thermal conversion in a covered vessel is eventually found to be effective to incorporate Si NPs into mesoporous carbons. As for a mesoporous carbon, monodispersed mesoporous carbon spheres (MMCS) that we have been investigating is used [31,32]. MMCS has uniform particle size and mesopores are connected continuously through the particle. These features are advantageous for higher packing ratio and easy entry/discharge of lithium ions [32]. In addition, since MMCS has uniform particle size, it is extremely easy to distinguish Si NPs if they form outside MMCS. This feature is highly useful to judge whether Si NPs deposition occurs on the surface or inside MMCS.
2. Experimental 2.1 Synthesis of Si@MMCS composites Monodispersed mesoporous carbon spheres (MMCS) were synthesized according to the literature [8]. The introduction of Si NPs into mesopores of MMCS was done by
incipient wetness technique which utilizes Capillary condensation. MMCS was added to a glass tube. CPS with the same volume of mesopores was dropped onto MMCS powder. The bottle was sealed with a plastic cap and tapped several times. Then the plastic cap was replaced with a glass plate and, the glass bottle with the plate was heated at 400 ºC for 30 min to transform CPS to Si NPs. Since CPS reacts with oxygen immediately, these operations were done in a glove box under nitrogen atmosphere. MMCSs with different mesopore size were prepared, and Si NPs was incorporated. Samples are denoted as 0701-Si and 0702-Si.
2.2 Characterization Scanning electron micrographs (SEMs) were obtained with a SU3500 (Hitachi). Nitrogen adsorption/desorption isotherms were measured using a BELSORP-mini II (Bell-Japan) at -196 ºC. The sample was evacuated at 150 ºC under 0.13 Pa before the measurement. Pore volume was estimated from the amount of adsorbed nitrogen at the relative pressure of 0.98. Pore diameter was calculated by the BJH method for the desorption branch. Thermo-gravimetric Analysis was conducted with a Rigaku THERMO PLUS in air. Transmission electron micrograph was obtained with a Jeol 2100F TEM using an acceleration voltage of 200 kV.
2.3 Battery Testing Electrochemical properties of Si@MMCS anode were investigated in non-aqueous lithium half cells. A film consisting of 70 wt% Si@MMCS, 25 wt% carbon black, and 5 wt% PTFE powder were pressed on a stainless steel mesh to obtain Si@MMCS electrode. Porous polyethylene membrane (Tonen General Sekiyu K. K.) was used as a
separator. 1.0 M LiPF6 in EC/DEC (1:1 by vol.)), and Li metal were employed as an electrolyte, and a counter electrode, respectively. The discharge and charge tests were performed at 25 ºC at a current density of 200 mA/g, in a range between 0.02 and 2.0 V.
3. Results and Discussion Two types of MMCS with different mesopore size (0701, 0702) were synthesized according to the literature [8]. Physical properties of MMCS are listed in Table 1, and SEM images are shown in Figure S1. CPS equal to the pore volume of MMCS was dropped onto MMCS, and introduced into mesopores by Capillary condensation. After heated at 400 ºC, amorphous Si NPs were formed inside mesopores. Figure 1 illustrates a scheme for the Si NPs incorporation process. The samples are denoted as 0701-Si and 0702-Si originated from the name of MMCS used. From Figure 2 showing SEM images of Si@MMCS composites, it is obvious that spherical shape and monodispersity of MMCS are retained after Si NPs are incorporated. Since no other small particles are observed, it is inferred that Si NPs preferentially form inside mesopores. The distribution of Si NPs and carbon elements is visualized with SEM-EDX mapping. Images are shown in Figure 3. Aggregated Si@MMCS particles were observed for easy viewing. Locations where Si and carbon are detected coincide with the MMCS particles in the SEM images. To visually investigate Si NPs incorporation, TEM-EDX was used for the mapping. 0702-Si was embedded into epoxy resin, and the resin was sliced to a thin film for the observation. Figure 4 shows a TEM image and corresponding carbon and Si mapping images. In case Si NPs are introduced only near the surface of MMCS, core/shell structure should be observed. Since such a structure is not seen in Figure 4 (c), Si NPs are thought to be uniformly distributed through MMCS. To directly confirm
the incorporation of Si NPs into mesoporous carbon, Si NPs incorporation was done to CMK-3 [34]. Since mesopores of CMK-3 are highly ordered, it is quite easy to see how Si NPs are located.
Figure S2 shows TEM images of Si@CMK-3. White portion in
the image represents Si NPs incorporated, and it is understood that Si NPs are homogeneously incorporated into mesopores of CMK-3. It is expected that the incorporation of Si NPs into mesopores change adsorption properties significantly. Nitrogen adsorption/desorption measurements were conducted, and isotherms and corresponding pore size distribution curves are shown in Figure 5. Large decrease in pore volume due to Si NPs incorporation is observed for both large pore MMCS (0701-Si) and small one’s (0702-Si). The peak top in each pore size distribution curve shifts to lower number, indicating decrease in pore size. These adsorption property changes reveal that Si NPs are incorporated into mesopores of MMCS. The amount of Si NPs incorporated in MMCS was determined with thermo-gravimetric analysis in air. TGA curves are shown in Figure S3. The increase in weight observed between R.T. and 500 ºC can be attributed to the oxidation of Si to SiO2. After this, the weight decreases because of the oxidation and elimination of carbon. The weight becomes constant at the temperature higher than 600 ºC. The amount of Si NPs incorporated is determined by assuming that the residue is SiO2. The calculated values are 47.3 wt% for 0701-Si and 45.7 wt% for 0702-Si. If all of the CPS incorporated into mesopores is assumed to be transformed to Si NPs, the values should be 61.5 wt% or 55.4 wt% in case 0701 or 0702 is used. The difference between the experimental and the estimated value is bigger for 0701-Si, mentioning easy CPS evaporation in larger pore-sized mesoporous carbon. CPS could be kept more in the mesopores with smaller pore size. From the above results, it turns out that a large
amount of Si NPs can be incorporated into MMCS by just one simple and easy operation. As mentioned in Introduction, the decomposition of CPS was done at 400 ºC that is much higher than the boiling point of CPS: 194.3°C. During the conversion reaciton, dimer or trimer CPS are generated by multimerization of gaseous CPS. Those multimers could be preferentially adsorbed onto mesopores of carbon, making it possible to accumulate Si NPs mainly inside carbon spheres. It can be expected that the amount of Si NPs incorporated be increased if the incorporation process is repeated. The process was repeated for 0702-Si and the sample is denoted as 0702-2-Si. Nitrogen adsorption isotherm is shown in Figure 5(d). The decrease in the pore volume is significant and it looks like the most of mesopores are filled with Si NPs. TGA analysis was conducted for 0702-2-Si (Figure S3(c)) and the weight ratio of Si NPs is calculated to be 53.6 wt% indicating the increase in the amount of Si NPs incorporated. Because the value is closed to the theoretical value of 55.4 wt% for a fully CPS filled composite, the result of the nitrogen adsorption measurement can be well understood. If the composite is oxidized in air, only SiO2 NPs remains. It is quite interesting to know if the residual SiO2 particles after the oxidation retain monodispersed spherical shape or not. SEM observation was conducted with Si@MMCS composites calcined in air at 550 ºC for 6h to remove carbon and oxidize Si NPs, and SEM images are shown in Figure 6. All of the calcined particles keep spherical shape and monodispersity is retained. The calcined 0702-Si was embedded in an epoxy resin, and the thin-sliced surface was observed by SEM (Figure S4). It is obvious from the image that SiO2 is distributed throughout particles, which indicates that Si NPs are uniformly incorporated into mesopores in the composite. Si@MMCS composites were heated at 400 ºC to convert CPS to Si NPs. To investigate
whether Si NPs are in amorphous or crystalline phase, XRD measurements were conducted. XRD patterns are shown in Figure S5. No sharp peaks are observed in the patterns. It is concluded that Si NPs exist in amorphous phase in mesopores. A small broad peak appeared around 28º when the sample was heated at 800 ºC (data not shown). Given the fact that polycrystalline structure was obtained when a film made from CPS was heated at 540 ºC [30], the crystallization of Si might be suppressed inside mesopores. One of the most expected applications of Si@MMCS composites is an anode for a Lithium ion battery. A half-cell was fabricated with Si@MMCS and charge-discharge cycle property was evaluated (Figure 7). The initial capacity is 3240 mAh/g, but some degradation is observed. However, it can be expected that capacity and degradation issue could be drastically improved by optimizing the amount of incorporated Si, pore diameter, and particle diameter of MMCS. Because of the large volume change of Si, it should be important to precisely control the ratio of Si solid volume to mesopore space. In addition, it can be suggested that PTFE, which is used as a binder, produces HF that reacts with Si, leading to big capacity fading. We are now trying to tune the amount of Si NPs precisely and to establish a new electrode composition without a PTFE binder.
4. Conclusion In conclusion, simple and easy method to incorporate Si NPs into mesoporous carbon is achieved by using a liquid Si monomer, CPS, as a precursor. By utilizing Capillary condensation, as much as 47 wt% of Si NPs can be incorporated into mesopores by just one operation. Si NPs are homogeneously incorporated into mesopores, which is verified by TEM observation and EDX mapping of the composite as well as SEM
observation of the calcined one. The Si@mesoporous carbon shows high initial capacity as an anode for Lithium ion battery although fading of capacity is observed. Development of high performance Lithium ion battery anode with Si@MMCS is underway by tuning the amount of Si nanoparticles and optimizing battery system.
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Table 1. Properties of MMCS Sample Name
Particle diameter [nm]
Surface area [m2/g]
Mesopore size [nm]
Pore volume [mL/g]
0701
553
1220
6.2
1.9
0702
464
1320
3.3
1.48
Figure 1. A scheme for the incorporation of Si with incipient wetness technique. CPS with the same volume of mesopores of MSCS is dropped on MMCS. Mesopores of MMCS is filled with CPS when incipient wetness is achieved. The mixture is heated at 400 ºC for 30 min to convert CPS to Si.
Figure 2. SEM images of Si@MSCSs: 0701-Si (a) and 0702-Si (b).
Figure 3. SEM images of Si@MSCSs: 0701-Si (a) and 0702-Si (b), and corresponding mappings for Si: (c), (d) and carbon: (e), (f) elements.
Figure 4. TEM image of 0701-Si (a), and corresponding carbon (b), Si (c) mappings.
Figure 5. Nitrogen adsorption-desorption isotherms for 0701-Si (a) , 0702-Si (c) , 0702-2-Si (d) and corresponding original mesoporous carbons (b), (e). Pore size distribution curves for 0701-Si (e) , 0702-Si (g), 0702-2-Si (h) and corresponding mesoporous carbons (f), (i).
Figure 6. SEM images of silica particles obtained from 0701-Si (a) and 0702-Si (b) by calcining them at 550 ºC for 6 h in air.
Figure 7. Charge-Discharge cycle property of 0701-Si as Li battery anode.
Supporting material
Novel Method to Incorporate Si into Monodispersed Mesoporous Carbon Spheres
Kazuhisa Yano, Narihito Tatsuda, Takashi Masuda, and Tatsuya Shimoda
Figure S1. SEM images of MSCSs for 0701 (a) and 0702 (b). Average particle size and standard deviation are presented in parentheses.
Figure S2. TEM images of Si@CMK-3.
Figure S3. Thermal gravimetric analysis curves for 0701-Si (a), 0702-Si (b) and 0702-2-Si (c).
Figure S4. SEM image of thin sections of silica particles obtained from 0702-Si embedded in epoxy resin.
Figure S5. XRD patterns of Si@MSCSs: 0701-Si (a) and 0702-Si (b).
*3: Graphical Abstract
Si
Si
Si
Si Si MMCS
Incipient Wetness
△
Si@MMCS MMCS
Si@MMCS