Preparation and electrochemical properties of spherical LiFePO4 and LiFe0.9Mg0.1PO4 cathode materials for lithium rechargeable batteries

The spherical LiFePO₄/C and LiFe_0.9Mg_0.1PO₄/C powders were successfully prepared from spherical FePO₄ via a simple uniform-phase precipitation method at normal pressure, using FeCl₃ and H₃PO₄ as the reactants. The FePO₄, LiFePO₄/C, and LiFe_0.9Mg_0.1PO₄/C powders were characterized by scanning electron microscopies (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectrometer (XPS), and tap-density testing. The uniform spherical particles produced are amorphous, but they were crystallized to FePO₄ after calcining above 400 °C. Due to the homogeneity of the basic FePO₄, the final products, LiFePO₄/C and LiFe_0.9Mg_0.1PO₄/C, are also significantly uniform and the particle size is of about 1 μm in diameter. The tap-density of the spherical LiFePO₄/C and LiFe_0.9Mg_0.1PO₄/C are 1.75 and 1.77 g cm⁻³, respectively, which are remarkably higher than the non-spherical LiFePO₄ powders (the tap-density is 1.0–1.3 g cm⁻³). The excellent specific capacities of 148 and 157 mAh g⁻¹ with a rate of 0.1 C are achieved for the LiFePO₄/C and LiFe_0.9Mg_0.1PO₄/C, respectively. Comparison of the cyclic voltammograms of LiFePO₄/C and LiFe_0.9Mg_0.1PO₄/C shows enhanced redox current and reversibility for the sample substituting Mg on the Fe site. LiFe_0.9Mg_0.1PO₄/C exhibits better high-rate and cycle performances than the un-substituted LiFePO₄/C.