Electrochemical Performance and Thermal Stability of LiMn_1−xFe_xPO₄/C (x= 0.2, 0.3, 0.4) Secondary Particles

Abstract

 LiMn_1−xFe_xPO₄/C secondary particles (x = 0.2, 0.3, 0.4) were synthesized by hydrothermal method followed by carbon coating and evaluated for electrochemical performance and thermal stability. The prepared LiMn_1−xFe_xPO₄/C secondary particles were attributed to the Pnma space group symmetry of an orthorhombic unit cell. They were spherical particles 10μm in diameter comprised with nanoparticles 100 nm in diameter. The differences of the particle size distributions and the specific surface areas of the synthesized LiMn_1−xFe_xPO₄/C were negligible for evaluation of electrochemical performance and thermal stability. The first discharge capacities of the LiMn_1−xFe_xPO₄/C were 155.3–155.7mAh g⁻¹ at a current rate of 34mA g⁻¹, and they did not depend on the Mn content. The LiMn_0.8Fe_0.2PO₄/C exhibited the best energy density of 596Wh kg⁻¹ at a current rate of 34mA g⁻¹. On the other hand, the rate capability was improved with the decrease in the Mn content of the LiMn_1−xFe_xPO₄/C. The LiMn_0.6Fe_0.4PO₄/C (x = 0.4) exhibited the best rate capability, where the discharge capacity and the energy density were 143.5mAh g⁻¹ and 489Wh kg⁻¹, respectively, at a current rate of 850mA g⁻¹. The cycling performances of the LiMn_1−xFe_xPO₄/C were almost equal. The capacity retentions during the 20 cycles of the LiMn_1−xFe_xPO₄/C were 97.5–98.8%. No change was found during the 20 cycles in the crystal structure of the LiMn_1−xFe_xPO₄/C that could cause capacity fading. The thermal stabilities of the LiMn_1−xFe_xPO₄/C were similar to each other regardless of the Mn content, which was much superior to that of commercially available LiNi_0.6Mn_0.2Co_0.2O₂.