Effect of SnCl2 addition on the structure and lithium storage performance of SiOC anodes

Sn/SiOC composites with varying Sn content were prepared by the sol–gel method with phenyltriethoxysilane (PhTES) and Tin(II) chloride dihydrate (SnCl2·2H2O) as precursors. The effect of SnCl2 addition on the microstructure, morphology and lithium storage performance of SiOC anodes was investigated. Incorporation of SnCl2 not only reduces carbon-contained units of SiOC ceramics, but also increases the oxygen-containing functional groups on the surface of carbon during the pyrolysis conversion. The formation of Sn involves carbothermal reduction reaction of SnO2 with the free carbon, and the degree of carbothermal reduction increases with the increase of Sn/Si ratio, which results in the reduction of carbon content and destruction of SiOC matrix. The size of Sn nanograins increases from 20 nm to 450 nm with the increasing Sn/Si ratio. The rate capability of Sn/SiOC composites increases first and then decreases with the increase of Sn content. The SiOC and Sn-PhTES-0.15 derived Sn/SiOC composite exhibit better electrochemical performance, which delivers a stable reversible capacity of 485 mAh g−1 and 411 mAh g−1, respectively at 186 mA g−1 after 250 cycles. The uniform dispersed Sn nanograins (below 20 nm), SiOC structure units and the free carbon phase both contribute to better electrochemical properties. However, the free carbon provides a higher reversible capacity than that of Sn and SiOC units, which results in the inferior lithium storage capacity of Sn/SiOC composites.