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

Besides topotactical Li insertion, Li deinsertion also is possible for the new potential electrode material Li 3 V(MoO 4 ) 3, in contrast to isostructural Li 3 Fe(MoO 4 ) 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 Li 3 V(MoO 4 ) 3 occurs at 2.1 V, with the formation of isostructural Li 4 V(MoO 4 ) 3, whereas an oxidation to Li 2 V(MoO 4 ) 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 (V 3+ ⇒ V 4+ ) and Li intercalation is accompanied mainly by molybdenum reduction (Mo 6+ ⇒ Mo 5+ ), but also is partially associated with vanadium reduction (V 3+ ⇒ V 2+ ). In situ synchrotron diffraction on a Li x V(MoO 4 ) 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 Li x V(MoO 4 ) 3 ( x = 2, 3, 4) are smaller than the theoretical values, calculated for spin-only contributions from V 4+, V 3+, and V 2+, respectively, which might indicate the partially delocalized character of the V valence electrons.