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.

Graphical abstractSchematic 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.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► 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.