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 deg. 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 deg. 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 deg. C was 8.24 A and expanded to 8.31 A at 700 deg. 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 1.2x10-5 deg. 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 deg. 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 deg. C). - Graphical abstract: Non-linear variation of lattice parameters of the Li excess LiMn2O4 spinels was observed during cooling process from 700 to 25 deg. C.