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

We investigate the reversible and irreversible transformations, the Li, Mn spinel undergoes under different atmospheres (air, O2, and N2) when heated up to 1050 °C. In air and O2, a substantial reversible cation exchange occurs. For T≥800 °C, 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[Li3y+]16c[Mn5y2+Mn1−5y3+Mn14+]16dO4 (y increases with T). Under N2 flow, a first decomposition occurs between 600 and 800 °C and yields Mn3O4, orthorhombic LiMnO2 and O2. A minor O2 release occurs when heating above 900 °C, which is accompanied by the transformation of o-LiMnO2 into a cubic LixMn1−xO solid solution (x≤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≤800 °C) leaving just the Mn3O4 and o-LiMnO2 phases.