Capacity Fading on Cycling of 4 V Li / LiMn2 O 4 Cells

The cycle-life behavior of a Li/1 M-LiPF 6 + EC/DMC(1:2 by volume)/LiMn 2 O 4 cell was investigated at various temperatures (0, 25, and 50°C). The capacity fades faster on cycling at high rather than low temperatures. The mechanisms responsible for the capacity fading of the spinel LiMn 2 O 4 during cycling were extensively investigated by chemical analysis of the dissolved Mn in combination with in situ x-ray diffraction, Rietveld analysis, and ac impedance techniques. Chemical analytical results indicated that the capacity loss caused by the simple dissolution of Mn 3+ accounted for only 23 and 34% of the overall capacity losses cycling at room temperature and 50°C, respectively. In situ x-ray diffraction results showed that the two-phase structure coexisting in the high-voltage region persists during lithium-ion insertion/extraction at low temperatures during cycling. By contrast, this two-phase structure was effectively transformed to a more stable, one-phase structure, accompanied by the dissolution of Mn and the loss of oxygen (e.g., Mn 2 O 3 .MnO) at the high temperature; this dominated the overall capacity-loss process. AC impedance spectra revealed that the capacity loss at the high temperature was also due in part to the decomposition of electrolyte solution at the electrode.