Ambient and high-pressure structures of LiMnVO4 and its Mn3+/Mn2+ redox energy

LiMnVO4 has been prepared and shown, by Rietveld analysis, to be orthorhombic isostructural with Li1.2In0.6VO4; it has a melting point of 784 degrees C, rand it transforms under a pressure of 55 kbar at 850 degrees C to a 7.7% more dense cubic spinel phase V[LiMn]O-4. The orthorhombic phase contains linear, ferromagnetic chains of Mn2+ ions in edge-shared oxygen octahedra, The spinel has mostly octahedral Mn2+ ions with evidence, below 50 K, of ferrimagnetic clusters associated with some terahedral-site Mn2+. Electrochemical delithiation of the spinel reveals a Mn3+/Mn2+ couple located at similar to 3.8 eV below the Fermi energy of elemental lithium, which indicates that the (VO4)(3-) tetrahedron behaves as a polyanion, This energy compares with 4.2 and 48 eV for the Co3+/Co2+ and Ni3+/Ni2+ couples in V[LiCo]O-4 and V[LiNi]O-4, respectively. (C) 1997 Academic Press.