An ac impedance spectroscopic study of LiMg0.05Co.95O2, prepared by sol-gel process, is presented. The results are interpreted on the basis of the equivalent circuit proposed for LiCoO2 oxide that takes into account the ionic and electronic transport processes. The results demonstrate that the intercalation/deintercalation reaction occurs through two-steps processes involving an adatom mechanism that may be described in terms of partially desolvated lithium ions adsorbed/desorbed from the electrode surface and insertion/loss of an electron from the conduction band of the host followed by diffusion of Li+ to/from the lattice intercalation site. Mg doping of LiCoO2 affects the electrochemical and electronic properties of the pristine oxide by facilitating the insulator-to-conductor transitions that are responsible of the phase transition in the undoped material. (c) 2004 Elsevier Ltd. All rights reserved.