Mössbauer study of LiFeVPO_x as a new cathode material for lithium-ion battery

The Mössbauer spectrum of LiFeVPO_x , LiFeV _0.5PO_x and LiFePO_x glasses prepared by conventional melt-quenching method for cathode active material is composed of a doublet due to distorted Fe^IIIO₄ tetrahedra. The Mössbauer spectrum of LiFePO_x glass has an additional doublet due to distorted Fe^IIO₆ octahedra. Heat treatment of LiFeVPO_x and LiFeV_0.5PO_x glasses at a given temperature close to each crystallization temperature causes a marked decrease in the value of Δ, reflecting a decrease in the distortion or an increase in the local symmetry of distorted Fe^IIIO₄ tetrahedra. Heat treatment of LiFeVPO_x glass causes an increase in the electric conductivity from the order of 10^-7 to 10^-3 S·cm^-1, together with an increase in the specific discharge-and charge-capacity of a coin-type Li-ion cell from 50 to 150 mAh·g^-1. These results prove that structural relaxation of the glass network causes an increase in the electric conductivity and an increase in the energy density of the Li-ion cell.