In Situ X-Ray Diffraction during Lithium Extraction from Rhombohedral and Monoclinic Li[sub 3]V[sub 2](PO[sub 4])[sub 3

Abstract

A comparative in situ X-ray diffraction (XRD) study of phase transitions and volume variations that occur during lithium extraction from the rhombohedral and monoclinic forms of Li 3 vg 2 (PO 4 ) 3 is presented. For this purpose, a new in situ XRD cell configuration was designed that enabled us to extract lithium at high voltages (up to 4.65 V vs. Li) without cell deterioration. A thin film of aluminum was deposited onto the surface of a beryllium window as a protective layer to avoid direct electronic contact with the positive electrode. Lithium extraction from NaSICON Li 3 V 2 (P0 4 ) 3 (R-3, cla = 2.709) led to NaSICON LiV 2 (P0 4 ) 3 (R-3c, cla = 2.427) isotypical with LiTi 2 (P0 4 ) 3 in a single two-phase process. Monoclinic Li 3 V 2 (PO 4 ) 3 showed a much more complicated series of four successive two-phase transitions on Li extraction to Li 0 . 1 V 2 ( P0 4 ) 3 between 3.4 and 4.6 V vs. Li. Each pattern of the intermediate compositions was fully indexed in the monoclinic space group P2 1 /n with a unit-cell contraction of ΔV/V = -6.8% between Li 3 V 2 (P0 4 ) 3 and LiV 2 (PO 4 ) 3 and a slight increase upon further oxidation to Li 0 . 1 V 2 (P0 4 ) 3 . Despite these numerous phase transitions and non-negligible volume changes, the monoclinic Li 3 V 2 (PO 4 ) 3 system is structurally highly reversible.