Nb Doping Enhancing Structure Stability and High-Temperature Electrochemical Performances of LiFe0.5Mn0.5PO4 Cathode Material in Lithium-Ion Batteries.

The application of LiFe_1-yMn_yPO₄ (0 < y < 1) cathode materials with high voltage and high specific energy for lithium-ion batteries is restrained by capacity decay, structure degradation, and low Li⁺/electronic conductivity, especially at high temperature. In this study, the Nb⁵⁺-doped olivine-structured LiFe_0.5Mn_0.5-xNb_xPO₄ (LFMP-xNb, x = 0%, 0.5%, 1%, 2%, and 3%) are synthesized and characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, and electrochemical techniques. The lattice parameters (a, b, c, and V), PO and TMO bond lengths, and particle size are reduced with Nb doping, and the structure stability, electronic/Li⁺ transport rate, and electrochemical performances are improved. The preferred LFMP-1%Nb sample delivers an initial discharge specific capacity of 160.57 mAh g^-1 at 0.1 C with a coulombic efficiency of 91.38%, and a specific capacity of 91.23 mAh g^-1 at 10 C, and a capacity retention rate of 78.71% with a residual capacity of 106.10 mAh g^-1 after 1000 cycles at 1 C. Especially at 55 °C temperature, it can give a discharge specific capacity of 142.9 mAh g^-1 and a capacity retention rate of 73.14% after 1000 cycles at 1 C.