Solid-State Preparation and Electrochemical Properties of Mg2+- doped LiFe0.7Mn0.3PO4/C as Cathode Material for Lithium-Ion Batteries

A series of LiFe0.7Mn0.3-xMgxPO4/C (x = 0, 0.02, 0.05, and 0.08) cathode materials were synthesized a high-temperature solid-phase method and characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical tests to determine the structural, morphological, and electrochemical properties of the synthesized materials. The experimental results showed that the small amount of Mg2+ doping not only had no effect on the crystalline structure of material but also significantly improved the electrochemical properties of the material. LiFe0.7Mn0.25Mg0.05PO4/C material achieved specific discharge capacities of 163.2, 155.2, 149.1, 142.0 mAh/g at 0.1C, 0.2C, 0.5C, and 1C, respectively, and a capacity retention rate of 98.6% after cycles at 0.1C. It exhibited excellent rate performances and cycling stability. Electrochemical alternating-current impedance showed that the lithium-ion diffusion coefficient of LiFe0.7Mn0.25Mg0.05PO4/C sample reached 1.19 x 10-13 cm2/s, which was 4.1 times higher than that the pre-modified sample. Combined with the cyclic voltammetry test results, it was found that appropriate amount of Mg2+ doping could reduce the impedance and polarization of the material as as increase the conductivity and the lithium-ion diffusion rate, ultimately improving the electrochemical properties.