The power-delivery capability of lithium-ion cells based on LiNi0.8Co0.15Al0.05O2-based positive electrodes shows a significant dependence on the cell's state-of-charge. One reason for this behavior is the variation of the positive electrode's impedance with the oxide's lithium content. In this article, an electrochemical model based on concentrated solution porous electrode theory is used to model impedance data obtained on LiNi0.8Co0.15Al0.05O2-based positive electrodes charged to potentials ranging from 3.55 to 4.55V versus Li. The parameters obtained from model fits include the exchange-current density and Li-ion diffusion coefficients in the oxide. The variations in these parameters with oxide potential are correlated with structural changes in the material observed during Li-ion intercalation–deintercalation reactions.