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

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.