Combined XRD, EXAFS, and Mössbauer Studies of the Reduction by Lithium of α-Fe[sub 2]O[sub 3] with Various Particle Sizes

Abstract

The electrochemical reduction of hematite with various particle sizes by metallic lithium has been studied by means of X-ray diffraction (XRD) Mössbauer and extended X-ray absorption fine structure (EXAFS) spectroscopy. Previous in situ XRD analysis coupled with electrochemical data showed that lithium can be inserted in the nanosized sample up to 1 Li per Fe2O3 whereas bulk material undergoes an irreversible Li-driven transformation from an hexagonal anionic packing to a close cubic packed framework as soon as 0.03 Li is inserted in the corundum structure. The present data show that only 0.6 Li per formula unit are actually inserted in the structure of small particles. The remaining lithium (0.4) is engaged in irreversible reduction of surface groups, or capacitive behavior. Beyond the solid solution domains, both samples are multiphase, and consist of Li2Fe2O3, Fe0 clusters (10-15 Å) and inserted α-Fe2O3, which proportions are used to calculate the mean iron oxidation state in the electrode as the reaction proceeds. From these data, we found that electrolyte decomposition can occur at very different steps of the reduction depending on the texture of the active materials. In addition, during the reduction process, we evidenced a reaction of disproportionation (3Fe2+ → 2Fe3 + Fe0), an intense electrochemical grinding of the hematite particles and the formation of extremely fine metallic surface clusters. For the first time, the EXAFS/X-ray absorption near-edge structure signature of the divalent intermediate Li2Fe2O3 phase is obtained