Instability of Li-Mn-O ternary phases having a spinel-framework structure especially at temperatures above 50 degrees C is a problem in designing lithium-ion batteries with these materials. This paper describes the solubility of manganese ions from these materials in 1 M LiPF6EC/DNIC (2/1 by volume), X-ray diffraction (XRD) analysis of these materials after solubility tests, and cycle tests of lithium cells with these materials at room temperature or at temperature higher than 50 degrees C. Severe damage was observed in both solubility and cycle tests when Li-Mn-O ternary phases were operated or stored at temperatures higher than 50 degrees C. The XRD examinations indicated that the spinel-framework structure changed in its line shape and location. In order to cope with this problem, we intensively examined the addition of transition metal elements into Li-Mn-O phases and found possible solution on this problem. One of the possible materials is Li[Cr0.1Mn1.9]O-4 (Fd3m) which shows excellent performance on cycling and storage at 50 degrees C.