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 oxycarbide (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. 7Li 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 7Li NMR resonance was observed when the polysilanes alone were pyrolyzed. Although there was no decrease in the capacities, the voltage plateau disappeared with electrochemical cycling. Furthermore, different 7Li 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.