Structure rearrangements induced by lithium insertion in metal alloying oxide mixed spinel structure studied by x-ray absorption near-edge spectroscopy

Carbon-coated ZnFe2O4 (ZFO-C) spinel ferrite nanoparticles can be used in electrodes for Li-ion batteries and are known to show capacities larger than those calculated for an ideal spinel structure. In this work, the local structure evolution and reordering of this material upon lithium insertion are studied using K-edge and L-edge x-ray absorption near edge spectroscopy (XANES). XANES simulations corresponding to different lithiation stages are performed using full multiple scattering (Fe, Zn K-edge, Zn L-edge) and ligand field multiplet (LFM) calculations (Fe L-edge). XANES simulations are compared with experimental spectra obtained on ZFO-C nanoparticles previously characterized by electrochemical measurements. It is shown that a satisfactory agreement for the XANES Fe and Zn K-edges of pristine ZFO-C bulk nanoparticles can be obtained introducing a mixed spinel structure with Fe and Zn partially occupying tetrahedral and octahedral sites. Upon lithiation, changes in the XANES spectra are interpreted introducing displacements of the cations as an effect of occupation of Li into empty lattice sites. In particular, comparison of the simulations with the XANES data indicates that reversible Li insertion is accompanied by a migration of the Zn and Fe atoms from tetrahedral to octahedral sites. Furthermore, by studying L-edge XANES spectra, we show that the relocation and valence change of metal ions occur at earlier lithiation stages at the surface of the active material, gradually extending to the bulk for larger Li uptakes.