Effect of annealing on transport properties of LiFePO 4: Towards a defect chemical model

Abstract

As a part of the program to construct a defect model for LiFePO 4 we measured the transport properties of LiFePO 4 with partial removal of lithium from LiFePO 4 by annealing the polycrystalline sample at different temperatures. LiFePO 4 was synthesized at low temperature and purity of the prepared LiFePO 4 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/LiFePO 4/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 Fe 3+/Fe 2+ ratio. A tentative defect model is proposed based on lithium vacancies and holes as native carriers which needs further confirmation by doping experiments.