Formation mechanism of LiFePO₄ in crystallization of lithium iron phosphate glass particles

Abstract

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) Li_xFePO₄ 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.