A new type of nanostructure in Si/C composite electrodes for lithium-ion batteries

The composition and structure of thin-film Si/C composite anodes produced by alternately depositing controlled amounts of silicon and carbon using magnetron plasma sputtering have been determined by atomic force microscopy, x-ray diffraction, and optical spectroscopy (Raman and UV through IR specular reflectance spectra). The silicon-to-carbon volume ratio in the films was varied from 39.5: 60.5 to 87: 13, and their thickness ranged from 100 to 480 nm. The surface of the films was found to have a nanogranular structure, which had not been reported previously for Si/C composites. This morphology is atypical of structureless silicon layers deposited under the same conditions but is similar to the nanostructure of a thin carbon film consisting of grains uniform in shape and size (Dav = 20–25 nm). Reducing the carbon content of the composites from 60 to 36% increases the grain size from 25 to 45–50 nm. At high silicon contents (near 80%), the nanostructure of the composites is less homogeneous: in addition to nanograins, there are structureless silicon zones. The homogeneity of the nanostructure depends on the Si: C ratio and the sequence and thicknesses of the deposited Si and C layers. Thin (104–173 nm) films containing more than 30% carbon (they have isolated silicon clusters) reveal the highest activity for the lithium intercalation-deintercalation process. Their Raman spectra show strong luminescence characteristic of silicon nanoparticles less than 5–6 nm in size. This effect is missing in the thicker films, in which the silicon forms an infinite cluster and which have a stronger tendency to degrade.