Li2Mn(VO3)4·2H2O: synthesis, crystal structure, thermal behavior and lithium insertion/deinsertion properties

Lithium manganese (II) vanadate dihydrate, Li2Mn(VO3)4·2H2O, has been prepared hydrothermally and characterized structurally from single-crystal diffraction data. It crystallizes in space group C2/m of the monoclinic system with Z=2 in a cell of dimensions a=19.286(5) Å, b=3.523(1) Å, c=8.164(2) Å and β=105.38(3)°. The structure consists of [Mn(VO3)4]2− layers, built up from MnO6 octahedra and VO5 square pyramids, separated by Li+ ions and water molecules. If the discharge rate is slow enough, Li2Mn(VO3)4·2H2O can insert reversibly up to 2 Li per formula unit between OCVs of 2.6 V and 2.15 V, while a new, unidentified, phase is obtained after insertion of 4 Li. Upon oxidation, lithium deinsertion occurs at high voltage (above 4.4 V), leading to LixMn(VO3)4·2H2O materials with x≤0.7. It is hidden by an important electrolyte oxidation. The Mn3+/Mn2+ redox couple, associated to the lithium deinsertion/insertion process, was estimated at a high voltage value (between 3.5 and 4.5 V), due to the inductive effect of VV on the Mn–O bonds.