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

The cycle‐life behavior of a Li/1 + EC/DMC(1:2 by volume)/ 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 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 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., ) 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.