Ionic and electronic transport in single crystalline LiFePO4 grown by optical floating zone technique
The present work is devoted to a systematic investigation of ionic and electronic conductivity as well as chemical Li-diffusivity in single crystalline LiFePO4 as a function of crystallographic orientation over an extended temperature range. Besides chemical analysis of the elemental ratio of the crystal, we also determined the Fe occupancy on the lithium lattice positions by single crystal X-ray diffraction and synchrotron X-ray radiation followed by Rietveld refinement. AC impedance as well as improved DC polarization/depolarization measurements have been carried out using electronically as well as ionically blocking cells. The activation energies obtained for electronic and ionic conductivities are in the range of 0.55-0.59 eV and of 0.62-0.74 eV, respectively, depending on the orientations. The ionic conductivity is much smaller than the electronic conductivity along all three axes and the electronic conductivity, ionic conductivity and chemical diffusivity of Li ion are found to be effectively two-dimensional (i.e., isotropic in the b-c plane).