Lithium insertion chemistry of phosphate phases with the lipscombite structure

The lithium insertion chemistry of an iron phosphate with the lipscombite structure, Fe1.19PO4F0.11(OH)0.46(H2O)0.43, was investigated by X-ray diffraction (XRD), galvanostatic cycling, and potentiostatic intermittent titration. The compound, prepared by a simple hydrothermal method, contains interconnecting chains of face-sharing FeO6 octahedra with about 60% Fe occupancy. Assuming that all the iron may be reduced, the theoretical capacity is about 180 mAh g−1, similar to that of olivine-type LiFePO4. Reversible intercalation was found to proceed via a single-phase reaction at an average potential of 2.8 V versus Li+/Li. Good structural stability upon intercalation/deintercalation was observed. The unit cell volume increased linearly and isotropically with increasing lithium content, reaching 10% for a Li:Fe ratio of 0.96. XRD peak widths increased on lithiation, presumably due to disorder created by conversion of Fe3+ to the larger Fe2+, but decreased on subsequent delithiation. The rate capability of this material appears to be diffusion-limited, and may benefit from a decrease in particle size. The lithium insertion behavior of a related compound, Ti5O4(PO4)4, was also investigated.