Si-containing disordered carbons prepared by pyrolysis of pitch polysilane blends: effect of oxygen and sulfur

Blends of pitch and polysilane were investigated for the effects of their initial oxygen and sulfur contents on the composition and electrochemical performances of the resulting pyrolyzed Si-containing disordered carbons. We found that the irreversible capacity of these materials (around 170 mAh/g) is caused by the presence of oxygen and sulfur which act as trapping agents for Li during the first discharge. The irreversible capacity is not strongly correlated to the Si content although the reversible capacity is (max. 500 mAh/g). When there are significant amounts of S and O in the blends, appreciable amounts of Si can be incorporated in the char as Si-O-S-C glass. These Si atoms are electrochemically active, and thus result in high reversible capacity. On the other hand, as the O and S contents are reduced, the chars are found to contain less Si and unfortunately most of the Si is in electrochemically inactive Si-C instead of Si-O-S-C active glass. Thus, this results in a dramatic reduction of the irreversible capacity but also of the reversible capacity. Although disordered carbons containing pure silicon clusters were previously prepared by Chemical Vapor Deposition (CVD), this work shows that the pyrolysis of pitch/polysilane blends is less suited for the preparation of similar materials. (C) 1999 Elsevier Science B.V. All rights reserved.