| Three different series of silicon-containing carbonaceous materials were synthesized for use as anodes in lithium ion cells. Disordered (or pregraphitic) carbons containing nanodispersed silicon were prepared by the chemical vapour deposition (CVD) of various chlorosilanes (SiCl{dollar}sb4,{dollar} {dollar}rm(CHsb3)sb2Clsb2Si,{dollar} and {dollar}rm(CHsb3)sb3ClSi){dollar} with benzene in two different apparatuses. Silicon oxycarbide glasses were synthesized by the pyrolysis of over 50 silicon-containing polymers at various temperatures, although the principal materials in the study were prepared at 1000{dollar}spcirc{dollar}C. Finally, materials which we believe to be similar to disordered carbons containing nanodispersed silicon were prepared by the pyrolysis of various blends of pitches with polysilanes.; Powder X-ray diffraction was used to learn about the structure of all the materials made. Thermal gravimetric analysis was used to determine the silicon content in the CVD materials and, when coupled to a residual gas analyzer, to study the decomposition process of the polymers. Near edge X-ray absorption spectroscopy measurements of the silicon L- and K-edges of CVD materials and the silicon K-edges of silicon oxycarbides were used to learn about local chemical environments of the silicon atoms.; Lithium metal electrochemical test cells of the silicon-containing CVD materials showed larger capacities (up to 500 mAh/g) than pure carbons prepared in the same way ({dollar}sim{dollar}300 mAh/g). The additional capacity was observed to be centered near 0.4 V on charge, the average voltage observed for the removal of lithium from a silicon-lithium alloy.; Chemical analysis showed that the stoichiometries of materials made by polymer pyrolysis were distributed over a well-defined region in the Si-O-C Gibbs phase diagram. An interesting series of materials is found near the line in the Si-O-C Gibbs triangle connecting carbon to SiO{dollar}sb{lcub}1.3{rcub}.{dollar}; Lithium metal electrochemical test cells made using all the silicon oxycarbides synthesized showed that a stoichiometry of about {dollar}rm Sisb{lcub}.25{rcub}Csb{lcub}.45{rcub}Osb{lcub}.30{rcub}{dollar} gave the maximum reversible capacity (about 900 mAh/g). However, materials near this stoichiometry exhibit large irreversible capacities ({dollar}>{dollar}350 mAh/g) and significant hysteresis (the voltage difference between charge and discharge) in the voltage profile ({dollar}sim{dollar}0.8 V).; In an attempt to reduce the oxygen content in one of the silicon oxycarbide glasses, a sample was washed in a dilute solution of hydrofluoric acid (HF) for times ranging from 2 minutes to 24 hours. The material lost, at most, 40 percent of its initial mass, although there was only a small change in its stoichiometry. In addition to the techniques mentioned above, small angle X-ray scattering and BET surface area measurements were used to study the microscopic pore network that was created by the HF washing.; Lithium metal electrochemical test cells made using the product of pyrolysing pitch-polysilane blends showed that the capacity increased with silicon content from 340 mAh/g for pure carbon to a maximum of 600 mAh/g for samples with about 15 atomic % silicon {dollar}rm(Sisb{lcub}.14{rcub}Osb{lcub}.09{rcub}Csb{lcub}.77{rcub}).{dollar} The capacity then decreased to near zero as the composition approached SiC. These materials contain oxygen which is correlated to irreversible capacity loss. (Abstract shortened by UMI.)... |
