Structural Refinement of Ni doped LiFePO4 Materials used in Energy Storage Devices

New materials with advanced functional properties are needed to improve the performances of energy storage devices like rechargeable lithium ion (Li-ion) batteries, currently used in a variety of applications. In this work we have concentrated our efforts in the synthesis and detailed characterization of promising cathode materials such as metal-doped LiFePO4 Olivines. We have succesfully synthesized Ni-doped solid solutions LiFe1−xNixPO4 (0≥x≤0.15,0.9,1.0) and the novel materials were characterized by elemental analysis and scanning electron microscopy. Some materials were electrochemically tested by cyclic voltammetry, showing that Ni doping may play a role in reducing electrode polarization increasing slightly the performances in Liion cells at low Ni concentrations x. The solid solutions were deeply studied by X-ray diffraction (XRD) and X-ray Absorption Spectroscopy (XAS), in order to check modifications of the lattice and of the local structure as well as possible correlations with the functional properties. Rietveld refinement of XRD data showed that Ni doping induces an anisotropic contraction of the unit cell which mainly concerns the volume of the M2 octahedral sites. Results of both XRD and XAS techniques are consistent and indicates the formation of homogeneous impurity-free solid solutions for x ≤ 0.15, ordering of Li in the (octahedral) 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 Ni-O 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. Most results of this work, obtained in the framework of an extended collaboration, were published on an international scientific journal (Journal of Power Source, vol. 213, 287-295 (2012)