The structure of tavorite LiFePO4(OH) from diffraction and GGA + U studies and its preliminary electrochemical characterization

Abstract

Pure tavorite LiFePO₄(OH) was synthesized through a hydrothermal route. A fine structural analysis was done by X-ray and neutron diffraction techniques. The structure consists of a three-dimensional network with iron(III) octahedra (FeO₆) sharing corners, forming chains that run along the b direction. These chains are interconnected by PO₄ tetrahedra, such as the resulting framework encloses tunnels of two different sizes running along the a and c axis. The lithium and hydrogen atoms were precisely localized in these tunnels. Theoretical (GGA + U) calculations performed for LiFePO₄X materials (X = OH, F) confirmed our results and revealed that a unique lithium position is expected in LiFePO₄(OH), as experimentally observed. For the first time, lithium intercalation was shown to occur in LiFePO₄(OH) through the reduction of Fe³⁺ to Fe²⁺ at an average voltage of ∼2.3 V (vs. Li⁺/Li) with a good cyclability.