Interfacial properties of the a-Si/Cu:active-inactive thin-film anode system for lithium-ion batteries

Amorphous silicon thin films deposited on copper foil have been observed to exhibit near theoretical capacity for a limited number of cycles. The films, however, eventually delaminate, leading to failure of the anode. In order to better understand the mechanism of capacity retention and the ultimate failure mode of a model brittle active: elastic/plastic inactive anode system, the films were subjected to in situ adhesion tests while observing the film surface using scanning electron microscopy. Atomic force and transmission electron microscopy, and electrochemical cycling were conducted to analyze the emerging morphology of the films during cycling. The adhesion of the as-deposited Si film to the Cu substrate was measured to similar to 7.7 J/m(2), reflecting a weak interface adhesion strength. Plastic deformation of the underlying Cu substrate combined with a ratcheting mechanism is proposed to occur in the Si: Cu system, with delamination failure mode occurring only after the formation of an interface imperfection. From the analysis of slow rate cycling experiments, nucleation of a lithium compound based on the interdiffusion of Si and Cu is identified as the most probable cause of the ultimate delamination failure of the deposited film. (C) 2006 The Electrochemical Society.