Charge/discharge behavior of Li[Ni0.20Li0.20Mn0.60]O2 and Li[Co0.20Li0.27Mn0.53]O2 cathode materials in lithium secondary batteries

Layered cathode materials Li[Ni0.20Li0.20Mn0.60]O2 and Li[Co0.20Li0.27Mn0.53]O2 were prepared by a simple combustion method and their structural changes on charge/discharge cycling were investigated by ex situ X-ray diffraction (XRD) method and galvanostatic charge/discharge cycling. Rietveld refinement showed that both compounds adopt the same crystal structure, isostructural with Li2MnO3. A structural difference is that a small amount of Ni2+ ions occupies the lithium layers, resulting in [Li0.98Ni0.02]3a[Ni0.18Li0.22Mn0.60]3b[O2]6c, while all the Co3+ ions the transition metal layers. After the 1st charge, the crystal structure of Li[Co0.20Li0.27Mn0.53]O2 was transformed from layered into spinel phase. Interestingly, the Li[Co0.20Li0.27Mn0.53]O2 exhibited an exceptionally high 1st charge capacity of 360 mA h/g and large irreversible capacity loss of 153 mA h/g. The discharge capacity of Li[Co0.20Li0.27Mn0.53]O2 was also largely decreased from 207 to 110 mA h/g at the 30th cycle while that of Li[Ni0.20Li0.20Mn0.60]O2 was stabilized from 288 to 213 m Ah/g. These results indicate the charge/discharge process is clearly different from each other and related to the structural difference.