A vacuum deposited Si film having a Li extraction capacity over 2000 mAh/g with a long cycle life

We have found that a Si film vacuum deposited on a Ni foil has a Li insertion capacity over 2000 mAh/g with cycleability over 1000 cycles, but a great issue was its difficulty to obtain a sufficiently thicker film capable of high current charge/discharge. In the present paper the examination of the high current charge/discharge performance of thicker Si film in relation to the film formation condition. The electrochemical evaluation was performed with cyclic voltammetry (CV) and constant current charge/discharge test with various loading currents in PC containing 1 M LiClO4. A Si film prepared with a rapid deposition rate gave a discharge capacity over 2000 mAh/g even with a very high charge/discharge rate over 10 C. In addition, the surface roughening of the substrate foil was found to play an important role to provide a thick film capable of high current performance. The constant discharge curve gave a wide plateau in the potential range between 200 and 500 mV versus Li/Li+. The XRD pattern of the deposited film gave no peaks due to Si, indicating the film to be amorphous. The SEM image of the deposited film was rather homogeneous, and after 500 cycles it still covered the entire surface of the Ni substrate though the surface became inhomogeneous.