Extended cycle- and calendar-life testing of lithium-ion cells has shown the effect of aluminum concentration in the cathode on the area-specific impedance versus time behavior of the cell. In cells with a LiNi0.8Co0.15Al0.05O2 (5% Al) cathode, the change in mechanism from t 1/2 to t occurred at about 35 weeks. Increasing the aluminum concentration to 10% (LiNi0.8Co0.1Al0.1O2) delayed the change for, approximately, an additional 30 weeks. The factors affecting the rate of increase during the parabolic and linear portions of the curve depended on cathode composition and particle size. The difference in the time it took for the change in mechanism was related to the cathode composition.