In-situ XAS study on Li2MnSiO4 and Li2FeSiO4 cathode materials

Li2FeSiO4/C and Li2MnSiO4/C cathode materials were a subject of in-situ characterisations during the first cycle of oxidation/reduction by complementary techniques of X-ray absorption spectroscopy and Mössbauer spectroscopy, whereby the changes in the local environment of Fe (and Mn) are monitored. Both techniques demonstrate the reversibility of monitored parameters of Fe environment during lithium exchange, and hence confirm the cycling stability of the Li2FeSiO4/C. The position of Fe in slightly distorted oxygen tetrahedra shifts with the change of valence accompanying Li removal. Outside of the immediate oxygen envelope, in more distant coordination shells, only minor variation in the neighbour distances is detected, apparently due to a slight relaxation of the structure. The oxidation state of Mn in Li2MnSiO4/C cathode material changes from the initial Mn(II) to Mn(III) in the potential range from 4.0 to 4.5 V vs. lithium reference but not beyond. During the oxidation, the short MnO bond decreases from 2.04 to 1.87 Å in the correlation with the increase of Mn valence state, while the larger MnO distance in the MnO4 tetrahedron remains unchanged. The reduction is not completely reversible as evident from the distribution of O atoms in the MnO4 tetrahedron after one oxidation/reduction cycle.