Preparation of carbons from pitch containing polysilane and their anode properties for lithium-ion batteries

Abstract

Recently, we have reported the preparation of metal or metal oxide fine particles dispersed carbons by heat treatment of pitches containing various organometallics, e.g., (C5H5)3Ln (Ln=Y, La, Sm), Cp2Zr(CH3)2, Cp2MoH2, CpWH2, [RhCl(C8H12)]2, PdCl2(C8H12), and Fe(acac)3 [1]. As a result, we found that the crystallinities of the carbons were generally lowered while the carbonization yields increased by the addition of organometallics into pitch [1]. On the other hand, many researchers have investigated the applications of various kinds of carbon materials as the anodes of lithium ion secondary batteries. As a result, it has been reported that some carbon materials of low crystallinity, produced by pyrolysis of phenolic resin [2], poly-p-phenylene [3–6], mesocarbon microbeads at low temperature [7], present much higher charge and discharge capacities than that expected from graphite intercalation (372 mAh g−1). Those values are two or three times higher than that of graphite. From the view-point that our method can disperse fine metal particles in amorphous carbon, we attempted to prepare carbonaceous materials from pitch containing silane polymer, namely poly(dimethyl)silane, poly (methylphenyl)silane, poly(dimethyl-methylphenyl) silane, and poly(diphenyl-methylphenyl)silane, and we measured the lithium ion charge and discharge capacities of the carbons obtained. The pitch was coal tar pitch from the Mitsubishi Chemical Co. (softening point: 98◦C, toluene insoluble 40%, quinoline soluble). Dichlorodimethylsilane, dichlorodiphenylsilane, and dichloromethylphenylsilane were purchased from Tokyo Chemical Industry and purified by distillation. Tetrahydrofuran (THF) (Kanto Chemical) was dried over a Na/K alloy and distilled before use. Polysilane, poly(dimethyl)silane [(SiMe2)n], poly (methylphenyl)silane [(SiMePh)n], poly(dimethylmethyphenyl)silane [(SiMe2)x (SiMePh)y], and poly (diphenyl-methylphenyl)silane [(SiPh2)x (SiMePh)y] were prepared by dechlorination of the corresponding dichlorosilane with sodium [8, 9]. A typical preparation for polysilane is as follows: Dichloromethylphenyl silane was added dropwise to a toluene dispersion of sodium metal in a round-bottomed flask. After refluxing the mixture for 4 h at 130◦C, it was cooled to 0◦C. The resulting polymer was precipitated by pouring excess methanol onto the mixture, and was then dried under vacuum at room temperature. Various carbons were prepared by pyrolysis of pitch containing polysilane. The pitch containing polysilane was obtained by mixing a THF solution of pitch with a THF solution of polysilane. A typical preparation for pitch containing polysilane is as follows: pitch (1.81 g) was placed in a 300 ml two necked round bottom flask and air and volatile components were removed under reduced pressure at 110◦C. The pitch was dissolved in 30 ml of dry THF. Poly(dimethyl-methylphenyl)silane (1.35 g) dissolved in 30 ml of dry THF was added to the pitch solution. After stirring the mixture for 2 h, the THF was removed by flash distillation. Pyrolysis was performed in argon using a combustion furnace (Motoyama MTKW-11-1040). The pitch containing polysilane was heated up at a rate of 1.67◦C min−1, and kept for 2 h at the pyrolysis temperature under an argon atmosphere, and then cooled to room temperature at a cooling rate of −1.67◦C min−1. XRD analyses were performed using a Rigaku RDA1B system with Cu Kα radiation. TEM observation was made on a Topcon EM-002B electron microscope at 200 kV. X-ray photoelectron spectroscopy(XPS) was performed by PERKIN ELMER ESCA 5400MC. Elemental analysis was performed on PERKIN ELMER 2400 CHN analyser. 1H-NMR spectra were recorded on a JEOL EX-400. Gel permeation chromatography (GPC) was used to measure the molecular weight of the polysilane. Lithium ion charge-discharge measurements were performed by using two-electrode test cells. 1 M LiPF6 propylene carbonate solution was used as an electrolysis solution. All operations were carried out in a glove box filled with argon. The cells were charged and discharged in the range of 0 to 1.5 V vs. Li/Li+ at a constant current density. Table I shows the polymerization recipes, Mn and composition of polysilane, (SiMe2)n , (SiMePh)n , (SiMe2)x (SiMePh)y , and (SiPh2)x (SiMePh)y . In spite of the addition of excess sodium to dichlorosilane, the molecular weights of (SiMePh)n and (SiMe2)x (SiMePh)y increased with increasing sodium metal concentration in addition to dichlorosilane concentration, and thus Mn= 8.8× 103 for (SiMe2)x (SiMePh)y was obtained. Consequently, polysilane of Mn= 103– 104 were added to the pitch. The (SiMe2)n was precipitated during the polymerization because these are insoluble in toluene and THF. Table II shows the carbonization results of pitch containing polysilane. Silane contents in the carbons were calculated from the residual amounts after CHN analyses. Carbonization yields generally decrease with an increasing content of polysilane. The lowering of the carbonization yields is supposed to be due to the