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.

The nature of the phases obtained by acid digestion of LiMn2O4 phases prepared at 800°C from a mixture of Mn02 (EMD) and Li2CO3 was investigated. We found that the complete transformation toward a-Mn02 and then -y-Mn02 observed for LiMn2O, treated in 2.5 M H2S04 for 24 h at 05°C is highly dependent on the amount of water in the reaction medium. The ) -. u/-y transformation was found to be the result of a dissolution.-crystallization mechanism that can be completely avoided by adding a soluble Ri, Pb, or TI salt to the reaction medium. By coupling energy dispersive spectroscopy analysis, infrared spectroscopy, and potentiometric titration, we demonstrated the presence of Bi species adsorbed at the surface of the X-MnO, oxide thus modifying its reactivity. In addition, the kinetics of the X -°u/-y phase transformation was found to depend on the amount of added Bi salt, suggesting the complexing role of Bi toward Mn (Ri-Mn complexes), thereby affecting the crystallization step of the reaction. The same treatment was applied to LiMn,04 in the presence of a Bi salt in anhydrous electrolyte (LiPF6/ethylene carbonate/dimethyl carbonate). In this case, the spinel oxide dissolution slows down and Ru1 precipitates. With respect to recent findings about the mechanisms involved in the electrochemical capacity failure at elevated temperature in Li-ion LiMn2O4 cells, these results open new alternatives to solve this recurrent problem.