Existence of Superstructures Due to Large Amounts of Fe Vacancies in the LiFePO₄-Type Framework

LiFePO 4 has been under intense scrutiny over the past decade because it stands as an attractive positive electrode material for the next generation of Li-ion batteries to power electric vehicles and hybrid electric vehicles, hence the importance of its thermal behavior. The reactivity of LiFePO 4 with air at moderate temperatures is shown to be dependent on its particle size. For nanosized materials, a progressive displacement of Fe from the core structure leading to a composite made of nanosize Fe 2 O 3 and highly defective, oxidized Li x Fe y PO 4 compositions, among which the “ideal” formula LiFe 2/3 PO 4 . Herein we report, from both temperature-controlled X-ray diffraction and electronic diffraction microscopy, that these off-stoichiometry olivine-type compounds show a defect ordering resulting in the formation of a superstructure. Such a finding shows striking similarities with the temperature-driven oxidation of fayalite Fe 2 SiO 4 (another olivine) to structurally defective laihunite, reported in the literature three decades ago.