Samples of spinel type oxides Li((4-x)/3)Mn((5-2x)/3)Fe(x)O4 (x = 0.5 and x = 0.7) were prepared by combining sol-gel processes and further thermal treatments. The structure and microstructure were characterized by different techniques (XRD, SEM and HRTEM) showing a cubic spinel-type structure, S.G. Fd3m in all cases. Complementary techniques were used to determine composition (EDS, EELS and TGA). Electrochemical properties were studied from charge/discharge measurements and supported by impedance spectroscopy analysis. Optimal results were obtained for the x = 0.7 sample treated at 500 °C that reached a capacity of 185 mAh g(-1) at a charge rate of C/10; this value implies an enhancement of 60% in capacity with respect to the minimum value registered in a similar composition. The observed response has been related both to microstructural aspects, i.e. morphology and particle size, and to the amount of Mn(3+), suggesting that nano-sizes lead to better electrochemical behavior. Both samples used as cathodes showed a good cycling efficiency, only with a slight loss of capacity and a coulombic efficiency close to 95%.