High-temperature X-ray diffraction study of crystallization and phase segregation on spinel-type lithium manganese oxides

To study crystallization process of spinel-type Li1+xMn2−xO4, in-situ high-temperature X-ray diffraction technique (HT-XRD) was utilized for the mixture consisting of Li2CO3 and Mn2O3 as starting material in the temperature range of 25–700 °C. In-situ HT-XRD analysis directly revealed that crystallization process of Li1+xMn2−xO4 was significantly affected by the difference in the Li/Mn molar ratio in the precursor. Single phase of stoichiometric LiMn2O4 formed at 700 °C. The formation of single phase of spinel was achieved at the lower temperature than the stoichiometric sample as Li/Mn molar ratio in the precursor increased. Lattice parameter of the stoichiometric LiMn2O4 at 25 °C was 8.24 Å and expanded to 8.31 Å at 700 °C, which corresponds to the approximately 3% expansion in the unit cell volume. From the slope of the lattice parameter change as a function of temperatures, linear thermal expansion coefficient of the stoichiometric LiMn2O4 was calculated to be 1.2×10−5 °C−1 in this temperature range. When the Li/Mn molar ratio in Li1+xMn2−xO4 increased (x > 0.1), the spinel phase segregated into the Li1+yMn2−yO4 (x > y) and Li2MnO3 during heating, which involved the oxygen loss from the materials. During the cooling process from 700 °C, and the segregated phase merged into Li1+xMn2−xO4 with oxygen incorporation. Such trend directly observed by in-situ HT-XRD was supported by thermal gravimetric analysis as reversible weight (oxygen) loss/gain at higher temperature (500–700 °C).