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− y Mn y PO 4 (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 Nb 5+ -doped olivine-structured LiFe 0.5 Mn 0.5− x Nb x PO 4 (LFMP- x Nb, 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. Graphical Nb 5+ doping shortens the TM  O and P  O bond lengths, suppressing the Jahn–Teller aberration and the solvation of Mn 2+ ions. Consequently, after 1000 charge-discharge cycles at 1 C rate under 25 °C and 55 o C temperatures, the LiFe 0.5 Mn 0.49 PO 4 −1%Nb cathode material exhibits capacity retention rates of 78.71% and 73.14%, respectively.