Stability of LiMn2O4 and new high temperature phases in air, O2 and N2

We investigate the reversible and irreversible transformations, the Li, Mn spine] undergoes under different atmospheres (air, O-2, and N-2) when heated up to 1050 T. In air and 02, a substantial reversible cation exchange occurs. For T greater than or equal to 800 T, Mn2+ ions substitute Li+ at the tetrahedral (8a) position, and Li+ shifts to interstitial octahedral (16c) site. Charge balance is achieved by a decrease in the Mn3+ fraction, which is partially reduced to Mn2+ in the regular octahedral (16d) site, according to the charge distribution (Li1-2y+Mn2y2+)(8a)[Li-3y(+)](16c)[Mn5y2+Mn1-5y3+Mn14+](16d)O-4 (y increases with T). Under N2 flow, a first decomposition occurs between 600 and 800 T and yields Mn3O4, orthorhombic LiMnO2 and O-2. A minor O-2 release occurs when heating above 900 T, which is accompanied by the transformation of o-LiMnO2 into a cubic LixMn1-xO solid solution (x less than or equal to 0.5), consistent with a decrease in the average oxidation state of Mn ions. This cubic phase is stable at high temperature and decomposes upon cooling (T less than or equal to 800 degreesC) leaving just the Mn3O4 and o-LiMnO2 phases. (C) 2002 Published by Elsevier Science Ltd.