Formation mechanism of LiFePO<sub>4</sub> in crystallization of lithium iron phosphate glass particles

The formation mechanism of LiFePO4 crystals in lithium iron phosphate glass (33.3Li2O–33.3Fe2O3–33.3P2O5) particles with a diameter of <63 µm was investigated by using X-ray diffraction (XRD) and high resolution transmission electron microscope (HRTEM) techniques. LiFePO4 was mainly crystallized by heat treatments at 350–800°C for 30 min under 7%H2/Ar atmosphere. At low temperature heat treatments, nano-scaled (∼40 nm) LixFePO4 crystallites are formed through a homogeneous nucleation process. With increasing temperature, a contribution of the reduction of Fe3+ to Fe2+ becomes larger, and LiFePO4 grows from the surface toward the inside of particles. Li3Fe2(PO4)3 and Fe2O3 crystals with Fe3+ are formed in the inside. In the heat treatment at 800°C, the formation of LiFePO4 crystals is largely promoted through the reduction of Fe3+ and the phase transition of Li3Fe2(PO4)3 and Fe2O3 crystals. An existence of amorphous layer with a thickness of several nm between the crystallites is confirmed even in the well crystallized sample from HRTEM observations.