Si-O-C composite materials with various compositions are prepared from a phenyl-substituted polysiloxane. The pyrolyzing temperature and atmosphere is varied to determine the effect that these parameters have on the final composition. The compositional effect of using a divinylbenzene (DVB) as an alternative carbon source is also investigated. Materials prepared at either 800 or 1000 degrees C under a hydrogen atmosphere have a significantly larger reversible capacity than those prepared at the same temperature under an argon atmosphere. Utilizing DVB as the carbon source further increases the reversible capacity of the Si-O-C material. The specific capacity increases with an increase of the C/Si ratio and with a decrease of O/Si ratio of the source materials when the O/Si ratio is in the range of 1.0-2.0. A model based on the nanostructure of the Si-O-C material is employed to express the relationship between the specific reversible capacity and the structure of the Si-O-C phase. According to the utilized model, the reversible capacity of lithium in the Si-O-C phase is much greater than that in carbon, and the capacity increases with the increase of the p value in SiO2(1-p)Cp. The "free carbon" in the synthesized materials was found to store twice as many lithium ions as the same amount of graphite. (C) 2011 Elsevier Ltd. All rights reserved.