Insertion of fluorine into a LiFePO4 electrode material by gas-solid fluorination.

Abstract

In this study, the insertion of fluorine into LiFePO4 was carried out under molecular fluorine F2 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-FeF3 and α-Li3FeF6 iron fluorides at higher temperatures. A second thermal treatment under N2 leads to the formation of LiFePO4F in a new way that has not been reported before. Supported by X-ray diffraction, Raman, infrared, Mössbauer spectroscopies, 7Li nuclear magnetic resonance and electrochemical characterization of the different materials, this report demonstrates various fluorination mechanisms for LiFePO4, from chemical delithiation to the stable pure fluorinated form LiFePO4F and illustrates an innovative method that can be extended to obtain the triphylite form of NaFePO4.