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

Schematic layout of the surface fluorination between carbon coated LiFePO4 (C/LiFePO4) cathode and NF3 gas. The Fδ− released from F2Nδ+−Fδ− may prefer to attack the Mδ+(M=Li, Fe) site in LiFePO4 more than the Cδ− with higher electron density. [Display omitted] ► Fluorinated LiFePO4 (F-LiFePO4) cathode active materials were prepared using NF3 gas. ► The discharge capacity of F-LiFePO4 was 10% higher than that of untreated LiFePO4. ► F1-LiFePO4 indicated the lowest resistance and largest exchange current density. ► DSC results proved that the surface fluorination can improve the thermal stability of LiFePO4. 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 6.67kPa 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=35.55Ω) and largest exchange current density (i0=0.72mA) 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.