Electrochemical Properties of LiFe[sub 5]O[sub 8]-Li[sub 4]Ti[sub 5]O[sub 12] Solid Solution

The electrochemical properties of LiFe 5 O 8 ­ Li 4 Ti 5 O 12 solid solutions, Li 4 / 3 − 5 y / 3 ( 3 − y ) Fe 5 y / ( 3 − y ) Ti 5 ( 1 − y ) / ( 3 − y ) O 4 , 0 ≦ y ≦ 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 Fe 2 + / Fe 3 + 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.