Effect of annealing on transport properties of LiFePO4: Towards a defect chemical model

As a part of the program to construct a defect model for LiFePO4 we measured the transport properties of LiFePO4 with partial removal of lithium from LiFePO4 by annealing the polycrystalline sample at different temperatures. LiFePO4 was synthesized at low temperature and purity of the prepared LiFePO4 investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy. The conductivity measurements have been carried out in the symmetrical cell Ti/LiFePO4/Ti. The conductivity increases gradually by more than three order of magnitude on long-time annealing in the range of 300 to 500 °C. Upon annealing the activation energy initially stays constant at 0.65 eV, but eventually drops to almost half of its value (0.30 eV). This can 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 for the discussion, is achieved by magnetization measurements which gives information on the effective hole concentration, i.e., the Fe3+/Fe2+ ratio. A tentative defect model is proposed based on lithium vacancies and holes as native carriers which needs further confirmation by doping experiments.