CAPACITY FADING ON CYCLING OF 4 V LI LIMN2O4 CELLS/

The cycle-life behavior of a Li/l M-LiPF6 + EC/DMC(1:2 by volume)/LiMn 2O4 cell was investigated at various temperatures (0, 25, and 50 degre es C). The capacity fades faster on cycling at high rather than low te mperatures. The mechanisms responsible for the capacity fading of the spinel LiMn2O4 during cycling were extensively investigated by chemica l analysis of the dissolved Mn in combination with in situ x-ray diffr action, Rietveld analysis, and ac impedance techniques. Chemical analy tical results indicated that the capacity loss caused by the simple di ssolution of Mn accounted for only 23 and 34% of the overall capacity losses cycling at room temperature and 50 degrees C, respectively. In situ x-ray diffraction results showed that the two-phase structure coe xisting in the high-voltage region persists during lithium-ion inserti on/extraction at low temperatures during cycling. By contrast, this tw o-phase structure was effectively transformed to a more stable, one-ph ase structure, accompanied by the dissolution of Mn and the loss of ox ygen (e.g., Mn2O3 . MnO) at the high temperature; this dominated the o verall capacity-loss process. AC impedance spectra revealed that the c apacity loss at the high temperature was also due in part to the decom position of electrolyte solution at the electrode.