Li₃V(MoO₄)₃: A New Material for Both Li Extraction and Insertion

Abstract

Besides topotactical Li insertion, Li deinsertion also is possible for the new potential electrode material Li3V(MoO4)3, in contrast to isostructural Li3Fe(MoO4)3. This new phase crystallizes in an orthorhombic NASICON-type structure with large channels along the crystallographic a-axis, which are half-filled by Li atoms. A reversible reduction of Li3V(MoO4)3 occurs at 2.1 V, with the formation of isostructural Li4V(MoO4)3, whereas an oxidation to Li2V(MoO4)3 is found at 3.7 V, preserving the space-group symmetry. The first charge/discharge cycle differs significantly from the following ones. According to X-ray photoelectron spectroscopy (XPS) measurements, Li extraction is associated with vanadium oxidation (V3+ ⇒ V4+) and Li intercalation is accompanied mainly by molybdenum reduction (Mo6+ ⇒ Mo5+), but also is partially associated with vanadium reduction (V3+ ⇒ V2+). In situ synchrotron diffraction on a LixV(MoO4)3 cathode with x ≈ 3 during the first discharge/charge cycle revealed a monotonous increase of the lattice volume and of the lattice parameter a during Li insertion from x = 3 to x = 3.5, but a multiphase mechanism for 3.5 ≤ x ≤ 4. At x ≤ 2.95, a monotonous decrease of the lattice volume was observed. The paramagnetic moments of LixV(MoO4)3 (x = 2, 3, 4) are smaller than the theoretical values, calculated for spin-only contributions from V4+, V3+, and V2+, respectively, which might indicate the partially delocalized character of the V valence electrons.