High-performance lithium iron phosphate with phosphorus-doped carbon layers for lithium ion batteries

A novel composite of LiFePO4 with phosphorus-doped carbon layers has been prepared via a simple hydrothermal method using glucose as the carbon source to generate a carbon coating and triphenylphosphine as the phosphorus source. The effects of phosphorus doping on the phase purity, morphology and electrochemical performance of the materials are studied by the characterizations using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy and electrochemical techniques. It is indicated that phosphorus doping into the carbon layers is beneficial for the graphitization of the carbon. Phosphorus in the carbon layers exists in the form of P-C bonds and its concentration depends on the second calcination temperature. Moreover, the phosphorus-doped carbon layers on the particle surface make the charge transfer resistance decrease remarkably from 156.5 U to 49.1 Omega, which can be ascribed to the free carriers donated by phosphorus. The as-prepared LiFePO4 with phosphorus-doped carbon layers calcined at 600 degrees C shows the best electrochemical performance with a discharge capacity of 124.0 mA h g(-1) at a high rate of 20 C and an excellent retention rate of 91.4% after 50 cycles. The LiFePO4 with phosphorus-doped carbon layers exhibits excellent electrochemical performances, especially at high current rates; thus, it is a promising cathode material for high-performance lithium ion batteries.