No abstract

More than sixty siloxane polymers containing various organofunctional siloxane units were synthesized. The synthesized siloxane polymers were pyrolyzed in inert gas at 1000°C. Chemical analysis showed that the products of pyrolysis were distributed over a well‐defined region in the Si‐C‐O Gibbs phase diagram. The electrochemical and structural properties of these materials were measured using coin‐type test cells and x‐ray powder diffraction, respectively. The most interesting materials are found near the line in the Si‐C‐O Gibbs triangle connecting carbon to Si0.13 . Materials with the largest reversible specific capacity for lithium (about 900 mAh/g) are on this line and were at about 43% carbon, 32% oxygen, and 25% silicon (atomic percent). Materials which were almost pure carbon showed diffraction patterns characteristic of disordered carbons. Along the line from carbon to SiO1.3 the sample structure can be described as a mixture of single or small groups of graphene sheets mixed with regions of Si‐C‐O amorphous glass. The amount and composition of the glass changed according to the overall sample composition. Moving from carbon to SiO1.3 , the reversible capacity first rises from about 340 mAh/g for pure carbon, to a maximum of 900 mAh/g near 50% carbon, and then falls to near zero mAh/g at 0% carbon. This suggests that the amorphous glass can reversibly react with lithium, provided the carbon is present to provide a path for electrons and Li ions. However, the hysteresis in the voltage profile (difference between charge and discharge voltages) and the irreversible capacity increase almost linearly along this line. There is a clear correlation between both the irreversible capacity and hysteresis in these materials with their oxygen content. Along the line connecting carbon to silicon, the reversible capacity rises from 340 mAh/g for pure carbon to about 600 mAh/g for samples with about 15 atomic percent Si. It then decreases to near zero as the composition nears SiC. Along the C‐SiC line, the irreversible capacities remain below about 200 mAh/g. We are quite convinced that optimized silicon‐containing carbons can be good alternatives to pure carbons as anode materials in lithium‐ion batteries.

Different polysiloxanes were pyrolyzed to make silicon oxycarbide glasses which have been shown to be possible candidates for anode materials of Li-ion batteries. The decomposition process during pyrolysis was studied using a thermal gravimetric analyzer connected to a residual gas analyzer for three representative polymers. No silicon-or oxygen-containing gases were observed during pyrolysis. The stoichiometry of all of the final chars was measured and found to agree well with predictions calculated using the initial polymer stoichiometry, the ceramic yield, and the fact that only carbon and hydrogen atoms were eliminated during pyrolysis. For three chars with identical stoichiometry at 1000 °C, prepared from three different polymers, the local chemical environment of the silicon was studied as a function of pyrolysis temperature by X-ray absorption (XAS) near the silicon K-edge. As the samples are heated, the XAS spectra eventually become identical, once the different labile species have been eliminated, suggesting that the properties of the char are dependent mainly on its stoichiometry and not on the initial composition of the polymer precursor. The bulk properties of these three chars prepared at 1000 °C were also studied by X-ray diffraction and by electrochemical methods which showed them to be identical.