Double-edge X-ray absorption study of LiFeNiPO cathode materials

X-ray absorption spectroscopy was used to study the evolution of the local structure induced by nickel doping in LiFeNiPO solid solutions, functional materials for Li-ion cells. Samples with increasing Ni content (x = 0, 0.03, 0.05, 0.10, 0.15) were measured and analyzed by extended X-ray absorption fine structure (EXAFS) at both Ni and Fe K-edges. Experiments were carried out at room temperature and at various temperatures in the 23-300 K range. Fe-O and Ni-O nearest neighbor distances were accurately measured by EXAFS double-edge data-analysis. The Ni and Fe octahedral sites have been found to be largely distorted in the solid solutions, with two atoms at longer distances. Mean square relative displacements of neighboring atoms (O and P) are found to be larger indicating the presence of an increased level of configurational disorder. EXAFS shows that the basic connectivity structure between Fe/Ni- and P-based polyhedra is preserved, but differences in local geometry and distance distributions are found. Possible connections among the presence of a distribution of distorted octahedra of different size in the structure and different electrochemical performances of the material as a function of doping are briefly discussed.