Various compositions in the spinel Li-Mn-O system were prepared and characterized by thermal analysis, X-ray diffraction, and X-ray absorption spectroscopy, with special attention to the Jahn-Teller (JT) instability of Mn3+. Of particular interest are (1) the discovery of ST structural phase transitions around 280 K, demonstrating that LiMn2O4 involves large JT instability in spite of its cubic symmetry at room temperature, (2) the use of transition temperature, T-t, and latent heat, Delta H, as sensitive parameters of JT instability in the practical composition region of the spinel Li-Mn-O ternary system as a 4 V cathode, and (3) the identification of the large, non-cooperative local JT distortion around Mn3+ in the cubic phase.