Nano-fibers and concentration modulation in films of Si and Si-based nano-composite alloys as negative electrode materials for Li secondary battery

As negative electrode materials with high capacity and long cycle lives for advanced Li-ion batteries, Si-based alloy films have been prepared on Cu-foil substrate by use of electron beam deposition method or magnetron-sputtering method at room temperature. In this study, nano-structures of the Si-based alloy films have been investigated by conventional and analytical transmission electron microscopy. Experimental results are summarized as follows: (1) Si-based alloy films consist of a number of nano-fibers of 5 nm in diameter. Electron diffraction patterns from the films show halo. Crystallites are seldom found in high resolution images. (2) One-dimensional modulations of the constituents are formed in the range of 3.6 to 11 nm in the films. These nano-structures of the Si-based alloy films could relax the volume expansion of the Si during Li-insertion, causing the longer cycle lives than that of the Si film as the negative electrodes.