A blend of phenyl-substituted, branched polysilane, (Ph(2)Si)(0.85)(PhSi)(0.15), and polystyrene (1:1 in weight) has been transformed into a composite material consisting of graphene layers, Si-O-C glasses, and micropores through a pyrolytic polymer-to-ceramic conversion. Several analytical techniques have been employed to characterize the Si-O-C composite material, demonstrating the presence of the three components in its host framework. The Si-O-C composite material performs well in electrochemical operations with a characteristic voltage plateau, offering a capacity of more than 600 mA h g(-1). When polystyrene is not blended, the resulting comparative material is even less porous and shows a shorter voltage plateau in electrochemical operations. A broad resonance in the (7)Li NMR spectrum recorded at low temperature can be deconvoluted into three components in the fully lithiated state of the Si-O-C composite material derived from the polymer blend. This result indicates that the Si-O-C composite material electrochemically stores lithium species in interstitial spaces or edges of the graphene layers, directly or indirectly the Si-O-C glass phase, and the micropores. However, both the Si-O-C glass phase and micropores are minor as electrochemically active sites for lithium storage, and interstitial spaces or edges of the graphene layers act as major electrochemically active sites in this composite material. Despite the excellent cyclability of the Si-O-C composite material, the voltage plateau disappeared over cycling. This phenomenon suggests that the microstructure is delicate for repetitive lithium insertion and extraction and that newly formed sites must generate the nearly equal capacity.

The work described herein deals with efforts to make a persuasive correlation between structural characteristics and electrochemical lithium storage for a silicon oxycarbide prepared from poly(methylhydrogensiloxane) and divinylbenzene. Structural characterization reveals that the silicon oxycarbide includes excess free carbon in an amorphous network. The reversibility of lithiation and delithiation in the silicon oxycarbide reaches 74% between 0.005 and 3 V relative to lithium at the first cycle but falls to only ca. 30% between 0.4 and 3 V. We found two resonances at 0 and 2.4 ppm in the (7)Li magic angle spinning nuclear magnetic resonance spectrum of the silicon oxycarbide lithiated to 0.4 V, whose contributions are 67 and 33%, respectively, and thus are in good agreement with the reversibility observed between 0.4 and 3 V. The fully lithiated silicon oxycarbide shows a single resonance at ca. 3-9 ppm, which tends to broaden at lower temperatures to -120 °C, whereas the fully delithiated silicon oxycarbide has a single resonance at 0 ppm. These results indicate that both reversible and irreversible lithium species have ionic natures. The Li K edge in electron energy loss spectroscopy does not show clearly any identified near-edge fine structures in the inner part of the silicon oxycarbide after delithiation. Near the surface, on the other hand, LiF and oxygen- and phosphorus-containing compounds were found to be the major constituents of a solid electrolyte interface (SEI) layer. Over repeated lithiation and delithiation, the SEI layer appears to become thick, which should in part trigger capacity fading.