Structural study of LiFePO4-LiNiPO4 solid solutions

Modifications of the local structure and lattice parameters in LiFe1x NixPO4 (0 < x < 0.15,0.9,1.0) olivine-type solid solutions have been studied by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Samples have been synthesized and characterized in our laboratory, and a preliminary electrochemical characterization shows that Ni doping increases slightly the performances in Li-ion cells at low Ni concentrations x. Results of both XRD and XAS techniques are consistent and indicate ordering of Li in the M1 site, and of Fe and Ni in the M2 site of the olivine structure. Ni doping is found to induce an anisotropic shrinking of the unit cell with both Fe and Ni six-coordinated with oxygens, occupying distorted octahedral sites. The local structure measured by XAS shows that average Fe-O and Ni-O distances do not change appreciably with Ni doping indicating that the reduction of cell size is mainly associated with the presence of shorter NieO distances at M2 sites. 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.