Crystalline Fe3PO7 as an electrode material for lithium secondary batteries

A crystalline Fe3PO7 was synthesized by the solid-state reaction method and investigated using the X-ray diffraction (XRD) method, scanning electron microscopy (SEM), cyclic voltammetry (CV) and a galvanostatic discharge–charge cycler for its application as an electrode material in lithium secondary batteries. Rietveld refinement of the powder XRD patterns yielded lattice constants of a=8.006(3) and c=6.863(3) Å with the space group R3m. The lithium insertion into the Fe3PO7 electrode showed a large first-discharge capacity of about 800 mA h g−1, accompanied by a reversible charge capacity as high as 500 mA h g−1 in a voltage window of 3.5–0.5 V. During the first discharge–charge cycle, an ex situ XRD method was applied for the lithium inserted–extracted electrodes, LixFe3PO7, to investigate the structural changes. As the electrode was discharged to 0.5 V, the crystal structure of the Fe3PO7 collapsed into an amorphous phase because of the reduction of the Fe3+ to a metallic Fe. Despite the structural collapse, a crystalline phase was partially recovered in the subsequent charge.