Origin of self-discharge mechanism in LiMn2O4-based Li-ion cells: A chemical and electrochemical approach

LiMn₂O₄-based Li-ion cells suffer from a limited cycle-life and a poor storage performance at 55 °C, both in their charged and discharged states. To get some insight on the origin of the poor 55 °C storage performance, the voltage distribution through plastic Li-ion cells during electrochemical testing was monitored by means of 3-electrode type measurements. From these measurements, coupled with chemical analysis, X-ray diffraction and microscopy studies, one unambiguously concludes that the poor performance of LiMn₂O₄/C-cells at 55 °C in their discharged state is due to enhanced Mn dissolution that increases with increasing both the temperature and the electrolyte HF content. These results were confirmed by a chemical approach which consists in placing a fresh LiMn₂O₄ electrode into a 55 °C electrolyte solution. A mechanism, based on an ion-exchange reaction leading to the Mn dissolution is proposed to account for the poor storage performance of LiMn₂O₄/C Li-ion cells in their discharged state. In order to minimize the Mn dissolution, two surface treatments were performed. The first one consists in applying an inorganic borate glass composition to the LiMn₂O₄ surface, the second one in using an acetylacetone complexing agent. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997