Influence of polystyrene/phenyl substituents in precursors on microstructures of Si-O-C composite anodes for lithium-ion batteries

Pyrolysis of methylphenyl-substituted branched polysilane-polystyrene polymer blends led to the formation of silicon oxycarbicle (Si-O-C) glasses with microstructures similar to those found in hard carbon. Electrochemical measurements showed that the Si-O-C composite materials had lithium storage capacities of more than 500 mA h g(-1), with a short but characteristic pseudo-voltage plateau at ca. 0.1 V upon delithiation. This voltage plateau indicates the existence of micropores where less-ionic lithium species can be formed, as seen in the case of hard carbon. The micropores were minor electrochemically active sites but led to an increase in the capacities. Li-7 NMR spectra of the Si-O-C composite materials had two resonances in the fully lithiated state, and one resonance could be attributed to less-ionic lithium species in the micropores. No voltage plateau was observed upon delithiation, and only a singular Li-7 NMR resonance was observed when the polysilanes alone were pyrolyzecl. Although there was no decrease in the capacities, the voltage plateau disappeared with electrochemical cycling. Furthermore, different Li-7 NMR resonances from those observed in the first lithiation were clearly seen. These results indicate changes in the lithium environments in the Si-O-C composite materials. (C) 2010 Elsevier B.V. All rights reserved.