Submicron-sized silicon oxycarbide spheres as anodes for alkali ion batteries

Abstract

Submicron-sized silicon oxycarbide (SiCO) spheres were prepared by using a 2-step acid/base catalyzed sol–gel process of triethoxyphenylsilane (PhTES) with subsequent carbonization at 1000 °C under an argon atmosphere. To prevent the organosilica spheres from sintering during heating, small amounts of tetraethoxysilane (TEOS) were cocondensed with carbosilane. The resulting SiCO material retains the spherical morphology (average particle diameter of around 200–300 nm) of the organosilica material upon heating in contrast to the SiCO obtained from pure PhTES or from the cocondensation of PhTES with methyltriethoxysilane (MTES). X-ray photoelectron spectroscopy (XPS) measurements of the SiCO spheres revealed an absolute carbon content of 41 wt%, which is only slightly lower than the carbon content of the SiCO obtained from pure PhTES with 46 wt%. Together with the O/Si ratio, we determined the following composition for the SiCO spheres: SiC_0.3O_1.4 + 2.89C_free. In order to elucidate the potential of the material as an anode material for sodium and lithium ion batteries, galvanostatic charge/discharge measurements were conducted and compared to the other SiCO materials. For the lithium system, capacities as high as 858 mA h g⁻¹ at a lower current of 50 mA g⁻¹ have been achieved. The spheres show significantly improved rate capability compared to the other SiCO samples. For instance, the material delivers reversible capacities of around 500 mA h g⁻¹ at a specific current of 500 mA g⁻¹. It is noteworthy that the spheres show the highest first cycle coulombic efficiency (73%) compared to the other SiCO materials (down to 51%) prepared throughout this work, which might be attributed to the higher material utilization due to the nanoscopic morphology. The SiCO nanostructure also significantly improved the sodium insertion properties compared to bulk SiCO. For the SiCO spheres we obtained a promising high reversible capacity of 200 mA h g⁻¹ at a lower current of 25 mA g⁻¹ (1st cycle efficiency of 47%). When increasing the current to 200 mA g⁻¹, the material still delivered 111 mA h g⁻¹.