Lithium carbonate is commonly found on the surfaces of lithiated cathode active materials that have been exposed to air. Long-term exposure of LiNi0.8Co0.15Al0.05O2 electrodes produced a dense Li2CO3 coating, approximately 10 nm in thickness that severely reduced both the accessible capacity and the rate at which the electrodes could be cycled. Ex situ X-ray diffraction patterns of cycled electrodes revealed a bimodal distribution of active material in different states of charge. Particle isolation is proposed to contribute to both capacity and power losses, and possible mechanisms related to the formation of Li2CO3 are discussed. (C) 2004 Elsevier B.V. All rights reserved.