Detailed In Situ Investigation of the Electrochemical Processes in Li[sub 2]FeTiO[sub 4] Cathodes

Abstract

The possible mechanism of lithium exchange from Li 2 FeTiO 4 is discussed on the basis of three different in situ structural characterization methods. The in situ X-ray diffraction XRD showed that the mechanism of lithium exchange was through a solid solution formation whereby the change in volume during the oxidation was 1.4%. The reversibility in terms of charge recuperation was complete; however, after the first reduction, the change in volume was still 0.4%. In situ Mssbauer spectroscopy experiment showed complete FeII oxidization to FeIII, while the reduction was not reversible. After the complete first cycle, the number of local environments in the Mssbauer spectra needed for a successful fit increased by at least two. Several local environments can be explained on the basis of extended X-ray absorption fine structure results. In the as-prepared sample, Fe cations are coordinated to three oxygen atoms at 2.03 and another three at 2.13 . With the oxidation of Fe, the splitting of two oxygen atoms gradually increases to 0.18 , and the octahedra deformation gradually changes from 3 + 3 to 4 + 2. This deformation is not reversible and it can explain changes in the long-range order as observed from in situ XRD.