Diffusion of Li in Li (x) Sn alloys was investigated using a density functional theory in order to fully understand the lithiation process in these types of Li ion batteries. Variation of the calculated open-circuit voltages of the Li (x) Sn alloys was found to agree well with experimental results. Diffusion coefficients of the Li in the Li (x) Sn alloys were calculated to be in the range between 6.6 x 10(-8) and 5.6 x 10(-7) cm(2) s(-1) at room temperature, which is within the range between 8.0 x 10(-8) and 5.9 x 10(-7) cm(2) s(-1) obtained from the experimental measurement.