Effects of surface fluorination on the electrochemical properties and thermal stability of LiFePO4 cathode for lithium-ion batteries

The electrochemical properties and thermal stability of carbon coated LiFePO4 cathode were improved by the surface fluorination using NF3 gas at a pressure less than 6.67 kPa at 25 °C. Especially, in case of F1-LiFePO4, the discharge capacity was about 10% higher than that of untreated LiFePO4. Also it indicated the lowest resistance (Rct = 35.55 Ω) and largest exchange current density (i0 = 0.72 mA) from the EIS results. However, in case of F2-LiFePO4, the electrochemical properties were negatively affected by the formation of some resistive fluoride films. In terms of thermal stability, the decomposition temperature (222.0 °C) of F1-LiFePO4 was 14 °C higher than that of untreated LiFePO4. Therefore, the surface fluorination led to the improvement of electrochemical properties and thermal stability of LiFePO4 as a promising cathode in high power lithium-ion cells for HEVs.