The potential distribution through plastic Li-ion cells during electrochemical testing was monitored by means of three-or four-electrode measurements in order to determine the origin of the poor electrochemical performance (namely, premature cell failure, poor storage performance in the discharged state) of LiMn,O,/C Li-ion cells encountered at 55°C. Several approaches to insert reliably one or two reference electrodes that can be either metallic lithium or an insertion compound such as Li,Ti,O,, into plastic Li-ion batteries are reported. Using a reference electrode, information regarding the evolution of (i) the state of charge of each electrode within a Li-ion cell, (ii) their polarization, and (iii) their rate capability can be obtained. From these three-electrode electrochemical measurements, coupled with chemical analyses, X-ray diffraction, and microscopy studies, one unambiguously concludes that the poor 55°C performance is mainly due to the instability of the LiMn,O, phase toward Mn dissolution in LiPF,-type electrolytes. A mechanism, based on Mn dissolution, is proposed to account for the poor storage performance of LiMn,O,/C Li-ion cells.

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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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