The main electrochemical properties of 3 V manganese oxides Li0.45MnO2.1 and Li0.45Mn0.85CO0.15O2.3 synthesized via a solution technique are reported. These materials are characterized by an attractive cycle life with a stable specific capacity in the voltage range 4.2-2 V between 165 and 195 mAh g(-1) depending the C rate and the compound. Impedance spectroscopy is used to evaluate the chemical diffusion coefficient of lithium in both cathodic materials. D-Li is found to be twofold higher in the Co-doped compound and little affected by Li concentration in the composition range 0.45 < x < 0.85. Analysis of impedance diagrams supports the existence of a passivating layer (Li+-ion conducting layer) onto the electrode when propylene carbonate is used as solvent or cosolvent. The presence of cobalt promotes the formation of this surface layer.