Dimensional Reduction: Synthesis and Structure of Layered Li5M(PO4)2F2 (M = V, Cr)

A new lithium vanadium fluorophosphate Li 5 V(PO 4 ) 2 F 2 has been synthesized by incorporation of LiF into α-Li 3 V 2 (PO 4 ) 3, and its structure has been determined by single-crystal X-ray diffraction. The sample crystallizes in monoclinic symmetry, space group P 2 1 / c (No. 14), with lattice parameters a = 6.3589(4), b = 10.7795(6), c = 10.3836(6), β = 90.019(1)°, and Z = 4. The refinement was performed using a total of 3094 unique reflections, resulting in agreement factors of R and wR( F 2 ) of 0.0422 and 0.0741, respectively. The structure of Li 5 V(PO 4 ) 2 F 2 consists of layers of vanadium fluorophosphate interleaved with lithium ions. Lithium ions also reside in the channels along the a axis and in the interstitial space within the framework. The synthesis of the title compound from α-Li 3 V 2 (PO 4 ) 3 represents the first example of framework reduction for a metal phosphate, from a three-dimensional to a two-dimensional structure. Bulk phases of Li 5 V(PO 4 ) 2 F 2 and its chromium analogue, Li 5 Cr(PO 4 ) 2 F 2, were also prepared from the direct reaction of MPO 4 (M = V, Cr) with LiF and Li 3 PO 4 . Reversible electrochemical cycling of one Li from Li 5 V(PO 4 ) 2 F 2 was observed at a potential of 4.15 V.