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

A series of LiFe 0.7 Mn 0.3−x Mg x PO 4 /C (x = 0, 0.02, 0.05, and 0.08) cathode materials were synthesized by 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 Mg 2+ doping not only had no effect on the crystalline structure of the material but also significantly improved the electrochemical properties of the material. The LiFe 0.7 Mn 0.25 Mg 0.05 PO 4 /C material achieved specific discharge capacities of 163.2, 155.2, 149.1, and 142.0 mAh/g at 0.1C, 0.2C, 0.5C, and 1C, respectively, and a capacity retention rate of 98.6% after 50 cycles at 0.1C. It exhibited excellent rate performances and cycling stability. Electrochemical alternating-current impedance showed that the lithium-ion diffusion coefficient of the LiFe 0.7 Mn 0.25 Mg 0.05 PO 4 /C sample reached 1.19 × 10 −13 cm 2 /s, which was 4.1 times higher than that of the pre-modified sample. Combined with the cyclic voltammetry test results, it was found that the appropriate amount of Mg 2+ doping could reduce the impedance and polarization of the material as well as increase the conductivity and the lithium-ion diffusion rate, ultimately improving the electrochemical properties.