Method to predict phase formation and specific capacity for lithium in codeposited silicon-transition metal thin films

A modified version of the effective heat of formation (EHF) model of Pretorius et al. is presented to predict phase formation in codeposited silicon-transition metal films. The EHF model predicts that the first Si - M (transition metal) phase to form in thin-film diffusion couples is the phase with the most negative EHF at the composition of the lowest temperature eutectic (i.e., at the growth interface). Combinatorial thin-film libraries of codeposited Si - M produced using the modified composition spread method of Dahn et al. consist of intimately mixed Si and M atoms. We propose the entire film can be considered as the growth interface and that the Si - M phase with the most negative EHF at a given composition is present. Film nanostructure as a function of composition can then be directly determined from EHF diagrams (assuming regions of Si are amorphous). This information, when combined with the assumption that all Si - M phases are inactive, can be used to predict the specific capacity of all Si-(transition or rare-earth metal) systems in Li/ Si1- xMx cells as a function of composition. (c) 2007 The Electrochemical Society.