The electrochemical properties of LiFe5O8-Li4Ti5O12 solid solutions, Li4/3-5y/3(3-y)Fe5y/(3-y)Ti5(1-y)/(3-y)O4, 0 less than or equal to y less than or equal to 1, are investigated. In a low level of Fe doping, y < 0.5, the size of the unit cell is only slightly influenced and gives good discharge/charge properties with a specific capacity nearly to the theoretical values for Fe2+/Fe3+ redox couple. However, the solid solution with y 0.5 mainly features an inverse spinel structure with the extensive occupation of Fe in 8a sites and the decrease of O-O distance, which impede the diffusion of Li+ ions through 8a-16c-8a channels. The crystallographic characteristics would result in a poor Li+ insertion/extraction electrochemistry. From ac impedance analysis, the D-Li(+) decreased with the increase in Fe doping level. It is considered that the impeded Li diffusion caused the electrode polarization that is observed in the highly Fe-doped samples.