Insertion of fluorine into a LiFePO 4 electrode material by gas-solid fluorination.

In this study, the insertion of fluorine into LiFePO 4 was carried out under molecular fluorine F 2 at different temperatures. The reactivity strongly depends on the applied fluorination temperature, leading to very different products: core delithiation of the material is observed at low temperatures with the formation of a LiF shell around particles, while the material decomposes to gradually form a mixture of α-FeF 3 and α-Li 3 FeF 6 iron fluorides at higher temperatures. A second thermal treatment under N 2 leads to the formation of LiFePO 4 F in a new way that has not been reported before. Supported by X-ray diffraction, Raman, infrared, Mössbauer spectroscopies, 7 Li nuclear magnetic resonance and electrochemical characterization of the different materials, this report demonstrates various fluorination mechanisms for LiFePO 4 , from chemical delithiation to the stable pure fluorinated form LiFePO 4 F and illustrates an innovative method that can be extended to obtain the triphylite form of NaFePO 4 .