Solid State Synthesis of lithiated manganese oxides from mechanically activated Li2CO3-Mn3O4 mixtures

The solid state formation of lithium manganese oxides has been studied from the thermal decomposition of mixtures Li2CO3/Mn3O4 with XLi (lithium cationic fraction) = 0.33 (LiMn2O4), 0.50 (LiMnO2) and 0.66 (Li2MnO3). The analysis of the reactivity has been performed mainly by thermoanalytical (TG/DSC) and diffractometric (XRPD) techniques either on physical mixtures and on mixtures subjected to mechanical activation by high energy milling. At XLi/0.33, the cubic lithium manganese spinel oxide (LiMn2O4) forms in air. TG measurements showed that the reaction starts at a considerably lower temperature in the activated mixture. By variable temperature X-ray diffraction it has been assessed that, upon mechanical activation, LiMn2O4 forms directly and its formation is completed within 700 °C whereas, starting from a physical mixture, the formation goes through Mn2O3 and is complete only at 800 8C. At T>820 °C LiMn2O4 reversibly decomposes to LiMnO2 and Mn3O4 with an enthalpy of 30.05 kJ mol-1 of LiMn2O4. At XLi=0.50, by annealing under nitrogen flow for 6 h at 650 °C the activated mixture, the orthorhombic LiMnO2 is formed. Such a formation goes through a mixture of LiMnO2 and LiMn2O4. The enthalpy of LiMnO2 solid state formation from the activated mixture has been determined to be 57.4 kJ mol-1 of LiMnO2. At XLi=0.66 in air the mechanical activation considerably lowers the temperature within the monoclinic phase Li2MnO3 forms. Besides the reaction enthalpy could be determined as 40.13 kJ mol-1 of Li2MnO3. The reaction, when performed under nitrogen flow, goes through the formation of LiMnO2. Such a first stage of the reaction is affected by the temperature of reaction rather than by mechanical activation. The activation greatly enhances the second stage of the reaction leading from LiMnO2 to Li2MnO3.