Investigation of olivine LiFePO4 cathode substitutted by magnesium at Li-site and Fe-site for secondary lithium ion batteries

Mg-substituted olivine LiFePO4 at Li-site and Fe-site, namely, Li0.98FeMg0.02PO4/C, LiFe0.98Mg0.02PO4/C and Li0.99Fe0.99Mg0.02PO4/C, were synthesize via a two-step method with pre-heating. X-ray diffraction (XRD) patterns indicate all of the samples have pure olivine structures. Scanning electron microscopy (SEM) imply that the Li-site doping will lead to an anomalistic apparent morphology with a particle size in the range of 1–2µm; while the Fe-site doping will promote a spherelike morphology with a smaller particle size. At the current density of 0.2mA/cm², the initial specific discharge capacity of Li0.98FeMg0.02PO4/C, LiFe0.98Mg0.02PO4/C and Li0.99Fe0.99Mg0.02PO4/C are 106mAh/g, 123mAh/g and 112mAh/g, respectively. The cyclic voltammetry (CV) curves and the electrochemical impedance spectroscopy (EIS) profiles indicate that LiFe0.98Mg0.02PO4/C has the best reversibility and the smallest charge transfer resistance, which are in favor of the good electrochemical performances.