Phase stability of LiMn2O4 and LiMn1.8Al0.2O4 was investigated to clarify the effects of the lithium content and substitution of aluminum on phase stability. Thermal stability and crystal structures were measured by differential thermal analysis (DTA) and high-temperature X-ray diffraction (XRD). DTA spectra indicated that an exothermic reaction was observed for Li0.79Mn2O4 at around 125-200 degrees C. High-temperature XRD spectra indicated that both the lattice expansion caused by thermal expansion and the lattice shrinkage caused by formation of a manganese-deficient spinel structure occurred in this temperature range. These phenomena were not found for Li0.82Mn1.8Al0.2O4, which might be due to the effect of aluminum substitution. The low durability of Li0.79Mn2O4 was probably related to low phase stability at the specific lithium contents. In addition, when the temperature was raised to 400 degrees C, Li0.79Mn2O4 remained in the spinel structure, but tetragonal Mn3O4 formed and the spinel structure almost disappeared at other lithium contents. This indicates the high phase stability of the manganese-deficient spinel structure formed at 125-200 degrees C. (C) 2018 The Electrochemical Society.