Comparison of structural characteristics and electrochemical properties of LiMPO4 (M=Fe, Mn, and Co) olivine compounds

Abstract

Olivine-type compounds of structural formula LiMPO4 (M=Fe, Mn, and Co) were prepared by sol-gel method. In the entire composite, no secondary phase was detected, and the sponge-like particle morphology was found to be suitable for use as a cathode material. Rietveld refinement shows that all materials have the same orthorhombic olivine structure in the Pnma space group, with lithium occupying octahedra into 4c sites and Mn2+ and Co2+ replacing Fe2+ at the octahedral sites. The a, b, and c cell parameters and cell volume increase with Mn substitution. Local geometric changes in MO6 octahedra occur with substitution of different transition metals. Distortion of the PO4 tetrahedra occurs in LiMnPO4 composite. LiFePO4 shows good electrochemical properties, whereas LiCoPO4 displays poor reversible capacity and unstable cycle performance.