The structure and charge-storage capacitance of carbonized films based on silicon-polymer nanocomposites

New film materials for electrodes of lithium batteries were synthesized and studied. Thin-film silicon-polymer composites were prepared by vacuum cocondensation of silicon and the monomer onto a substrate cooled with liquid nitrogen; the polymerization and formation of the nanostructured composite were performed at room temperature. The films were carbonized by vacuum annealing. The film composition and microstructure were studied by AFM, SEM, Raman spectroscopy, and X-ray spectral microanalysis. It was shown that the polymer matrix became almost fully carbonized because of pyrolysis. The silicon concentration in the films varied from 2 to 5 at %. The concentration of silicon nanoparticles on carbonized film surfaces was ∼10⁶ cm⁻². Electrochemical experiments with lithium insertion into the composite films were performed in standard three-electrode cells under galvanostatic conditions. The specific capacitance of the films was measured. It was shown that the samples were capable of long-term cycling; the capacitance decreased by only 6% during the first 200 cycles; after 250 cycles, the capacitance still exceeded 80% of its initial value. The mechanism of lithium insertion into the films was discussed. It was concluded that long-term stability during cycling was caused by the presence of silicon both as nanoparticles and in the atomically dispersed form.