An investigation of capacity fading of manganese spinels stored at elevated temperature

The storage characteristics of a manganese spinel at various discharge depths and 80 degrees C were examined in 1 M LiPF6 ethylene carbonate/dimethyl carbonate (1:2 by volume) electrolyte. The quantities of Mn dissolution and discharge-capacity loss were measured after the cathode at each discharge depth had been exposed to the electrolyte at 80 degrees C. The quantities of dissolved manganese in the solution were less than 1.2% of the total manganese in all cathodes examined. Little capacity fading (3%) was found in the fully charged cathode, but a 59% capacity loss was observed in the fully discharged cathode. Correlations of the capacity loss with the X-ray diffraction peak widths were found, and the amount of capacity loss increased with broadening peak width. On the other hand, no correlation between the amount of Mn dissolution and the capacity loss was found. From these results, we propose a mechanism of the capacity fading of the spinel LiMn2O4 stored at elevated temperatures as follows: lattice defects in the spinel due to Mn dissolution cause disordered crystal structures and as a result, the Li insertion-extraction paths are blocked, leading to capacity fading.