LiFe0.9Mg0.1PO4 material was prepared by mechanical milling method, followed by heat treatment. The equilibrium potential-composition isotherm of LiFe0.9Mg0.1PO4 and charge–discharge kinetics of LiFe0.9Mg0.1PO4 were measured using galvanostatic intermittent titration technique (GITT), potential-step chronoamperometry (PSCA), and electrochemical impedance spectroscopy (EIS). The rate performance of the cathode is controlled by the charge-transfer kinetics, electronic conductivity, Li-ion diffusion capability, and phase transformation rate. Since LiFe0.9Mg0.1PO4 has a fast charge-transfer reaction and high electronic and ionic diffusivity, the phase transformation between LiFe0.9Mg0.1PO4 and Li0.1Fe0.9Mg0.1PO4 begins to play a more important role in the charge–discharge process, as is evident by an inductive loop induced by the phase transformation in the low frequency region of EIS. The phase purity and morphology of LiFe0.9Mg0.1PO4 were also observed using X-ray diffraction (XRD) and scanning electron microscopy (SEM).