The transport properties of LiFePO4, with partial removal of Li from LiFePO4 by annealing polycrystalline samples at various temperatures, were measured. The LiFePO4 was synthesized at low temperatures, and the purity was investigated by X- ray diffraction, scanning electron microscope and Raman spectroscopy. Conductivity measurements were carried out in a symmetrical cell, Ti/LiFePO4/Ti. The conductivity increased gradually by more than 3 orders of magnitude during long-term annealing at 300 to 500C. Upon annealing, the activation energy initially remained constant at 0.65eV but eventually dropped to almost half of this value (0.30eV). This could be consistently explained by a transition from a defect regime characterized by predominant ionic disorder to the regime of pure Li-deficiency (ionic disorder compensated by electronic defects). Further support was offered by magnetization measurements which furnished information on the effective hole concentration, i.e., the Fe3+/Fe2+ ratio. A tentative defect model was proposed which was based upon Li vacancies and holes as native carriers.

Effect of Annealing on Transport Properties of LiFePO4 - Towards a Defect Chemical Model. R.Amin, J.Maier: Solid State Ionics, 2008, 178[35-36], 1831-6