A survey of the chemical stability of high-surface area LiMn2O4 in various Li-based electrolytes was performed as a function of temperature. The evidence for an acidic-indued Mn dissolution was confirmed, hut more importantly we identified, by means of combined infrared spectroscopy, thermogravimetric analysis, and X-ray diffraction measurements, the growth, upon storage of LiMn2O4 in the electrolyte at 100 degrees C, of a protonated lambda-MnO2 phase partially inactive with respect to Lithium intercalation. This result sheds light on bow the mechanism of high temperature irreversible capacity loss proceeds. Mn dissolution first occurs, leading to a deficient spinel having all the Mn in the +4 oxidation state. Once this composition is reached, Mn cannot he oxidized further, and a protonic ion-exchange reaction takes place at the expense of the delithiation reaction. The resulting protonated lambda-Mn2-yO4 phase has a reduced capacity with respect to lithium, thereby accounting for some of the irreversible capacity loss experienced at 55 degrees C for such a material