Investigation of olivine LiFePO₄ cathode substitutted by magnesium at Li-site and Fe-site for secondary lithium ion batteries effects of the doping sites on the structures and the electrochemical performances

Mg-substituted olivine LiFePO₄ at Li-site and Fe-site, namely, Li_0.98FeMg_0.02PO₄/C, LiFe_0.98Mg _0.02PO₄/C and Li_0.99Fe_0.99Mg _0.02PO₄/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 Li _0.98FeMg_0.02PO₄/C, LiFe_0.98Mg _0.02PO₄/C and Li_0.99Fe_0.99Mg _0.02PO₄/C are 106mAh/g, 123mAh/g and 112mAh/g, respectively. The cyclic voltammetry (CV) curves and the electrochemical impedance spectroscopy (EIS) profiles indicate that LiFe_0.98Mg _0.02PO₄/C has the best reversibility and the smallest charge transfer resistance, which are in favor of the good electrochemical performances.