Disproportionation of stoichiometric LiMn 2O 4 on annealing in oxygen

Abstract

The evolution of stoichiometric LiMn 2O 4 upon annealing under oxygen pressures in the range 0.2–5 atm at moderate temperature (450°C) was studied with a combination of thermogravimetry, X-ray and neutron diffraction. It is shown that such treatments result in a slight, but significant mass increase. Structural analyses show that the resulting spinel is a manganese-deficient spinel phase with lower cell parameter and higher manganese valence, and that the expelled manganese forms Mn 2O 3. The presence of this second phase, which was not identified in a recent study of oxygen annealing on this compound (Nakamura and Kajiyama, Solid State Ionics 133 (2000) 195), is compatible with the initial stoichiometry and does not require any oxygen vacancies in the initial LiMn 2O 4, as supposed earlier. The most likely formula of the resulting lithium-rich spinel with increased manganese valence is Li(Mn 2− ε □ ε )O 4 with ε in the range 0.02–0.03 at 5 atm O 2.