Determining the Phase Diagram of Li_xFePO_4

The phase diagram for Li_xFePO_4 has been determined for different lithium concentrations and temperatures. X-ray diffraction analysis was used to determine phase fractions at temperatures between room temperature and 400 °C for Li_xFePO_4 at different states of lithiation. The two low- temperature phases, heterosite and triphylite, have previously been shown to transform to a disordered solid solution at elevated temperatures. This disordered phase allows for a continuous transition between the heterosite and triphylite phases and is stable at relatively low temperatures. At intermediate temperatures the proposed phase diagram resembles a eutectoid system, with eutectoid point at around x=0.6 and 200 °C. Kinetics of mixing and unmixing transformations are reported, including the hysteresis between heating and cooling. The enthalpy of this transition is at least 700 J/mol.