A polymer blend of a partially-branched phenyl-substituted polysilane, (Ph2Si)0.85(PhSi)0.15, and polystyrene (1:1 by weight) has been prepared to produce silicon oxycarbide (Si–O–C) composite materials through pyrolysis in the temperature range 700–1200 °C under an argon atmosphere. According to elemental analysis results, carbon is a major constituent in a series of Si–O–C composite materials obtained in this study. Completely amorphous features were observed for the composite materials obtained between 700 and 1100 °C, while a clearly discernible crystalline evolution of silicon carbide (SiC) phases was found in a glass network of the composite material obtained at 1200 °C. This paper also deals with electrochemical lithiation and delithiation for the series of Si–O–C composite materials. The first delithiation capacities of these composite materials were highly dependent on pyrolysis temperatures. The composite material obtained at 700 °C had the maximum delithiation capacity of ca. 800 mA h g−1, while the composite material obtained at 1200 °C showed the minimum delithiation capacity of ca. 330 mA h g−1. The crystalline evolution of SiC phases is thought to cause such a drastic decrease in delithiation capacity at 1200 °C.