A defect chemistry approach was used to analyze the observed concentration dependence of the Li in-diffusion rate. An ambipolar diffusion model was used in which the Li and Nb diffusion rates were coupled, and therefore caused an internal electric field to develop during the process. Both Li and Nb diffusion were found to proceed via simple vacancy mechanisms. The internal electric field forced the more rapid Li diffusion to be reduced to a level which was consistent with the slower Nb diffusion process. This coupling led to a concentration dependence which matched the linear increase in interdiffusion rate which was observed as the total cation vacancy concentration decreased.

D.P.Birnie, P.F.Bordui: Journal of Applied Physics, 1994, 76[6], 3422-8