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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‐induced Mn dissolution was confirmed, but 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°C, of a protonated λ‐MnO2 phase partially inactive with respect to lithium intercalation. This result sheds light on how 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 be oxidized further, and a protonic ion‐exchange reaction takes place at the expense of the delithiation reaction. The resulting protonated λ‐Mn2−yO4 phase has a reduced capacity with respect to lithium, thereby accounting for some of the irreversible capacity loss experienced at 55°C for such a material. © 1999 The Electrochemical Society. All rights reserved.

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