Interplay between local structure and transport properties in iron-doped LiCoPO4 olivines

LiCoPO4 (LCP) is a challenging high voltage positive electrode material for next-generation secondary Li-ion cells. Doping the LCP olivine lattice with iron and annealing the material at high temperature result in improved and stable performances in lithium cells. Here we investigate the structural effects of iron doping and annealing at high temperature by advanced synchrotron X-ray techniques (X-ray diffraction and absorption) in close comparison with the corresponding performances in lithium cells (lithium de-insertion/insertion) and the ionic diffusion coefficients evaluated by galvanostatic intermittent titration tests. The partial substitution of cobalt ions in the olivine lattice with iron ions, 2+ or 3+, strongly affects the long range crystal structure as well as the short range atomic coordination. These structural changes alter the concentration of anti-site defects, the natural concentration of lithium vacancies, and the size of the lithium diffusion channels along the [010] direction as well as their local distortion. The balancing between these competitive effects modulate the lithium transport properties in the lattice.