A highly fluorinated lithium iron phosphate with interpenetrating lattices: electrochemistry and ionic conductivity

Li₅Fe₂PO₄F₈, a new member of the family of alkali transition metal fluorophosphates, has been synthesized and characterized using single-crystal X-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy and magnetic susceptibility measurements. The existence of an infinite {-[PO₄(FeF₄)₂]-}_∞ tetrahedral network in an inter-penetrated diamond lattice, along with the presence of seven unique Li sites, presents interesting structural features of this structure-type for energy storage applications. The initial results of (de)lithiation reveal that a relatively low fraction of theoretical capacity may be utilized reversibly (0.2 Li⁺ ion per formula unit), possibly due to the lack of available free volume for Li⁺ insertion. The high Li content and the existence of large channels in all 3-dimensions of space also offer opportunities to study this material as a candidate for solid-state electrolytes. The results from electro-impedance measurements reveal the reasonable activation energy of Li diffusion (0.70 eV), which is also supported by theoretical calculations.