Origin of the irreversible capacity of the Fe0.5TiOPO4 anode material

Fe0.5TiOPO4 oxyphosphate was prepared directly by solid state reaction. It crystallizes in the monoclinic system with P21/c space group and the use of Fe0.5TiOPO4 as an active material in lithium-ion batteries shows two potential plateaus and an irreversible discharge capacity of ~ 200 mAh/g. To understand the origin of this irreversibility, a series of LixFe0.5TiOPO4 (x = 0.06, 0.22, 0.76, 1.14) were chemically prepared and analyzed by X-ray diffraction. A structure amorphization of the LixFe0.5TiOPO4 phosphates takes place during the lithiation process. Magnetization and Mِssbauer spectroscopy studies of the LixFe0.5TiOPO4 samples clearly show the formation of iron metal which induces a deterioration of the crystal structure of the studied electrode materials. The lithiation process leads thus to a conversion reaction which explains the irreversibility of the electrochemical process during the first discharge.