LiCoO2 the active cathode material in commercial rechargeable lithium batteries, is shown to be a p-type semiconductor, associated with the presence of a small concentration of Co4+ ions. Its conductivity at room temperature can be increased by over two orders of magnitude, to similar to 0.5 S cm(-1), by partial substitution of Co3+ by Mg2+ and compensating hole creation. The electrochemical performance of LiMg0.05Co0.95O2 is comparable to that of LiCoO2; a small reduction in capacity, associated with a reduction in Co3+ content, occurs but good reversibility is retained and, in contrast to LiCoO2, the Mg-doped material is single phase throughout the charge/discharge cycle.