Design Parameters for Enhanced Performance of Li1+xNi0.6Co0.2Mn0.2O2 at High Voltage: A Phase Transformation Study by In Situ XRD

The electrochemical performance of Al-doped and un-doped Li1+xNi0.6Co0.2Mn0.2O2 (NCM) cathodes was evaluated at a high cut-off voltage up to 4.6 V (vs Li/Li+) using 1 Ah pouch-type full cells and coin-type half cells. The batteries employing Al-doped NCM exhibited lower internal resistance, higher C-rate capability, and better low-temperature performance. In situ X-ray diffraction revealed that the Al-doped cathode followed a one-phase reaction route, which is attributed to enhanced Li+ diffusion due to Al doping and the relatively small and porous secondary particles. The existence of the pristine phase in undoped NCM throughout the cycling is explained by the slow diffusion of lithium in and out of the large and dense particles, leading to phase separation caused by regions with different lithium concentrations. Analysis using time-of-flight secondary ion mass spectrometry combined with a focused ion-beam scanning electron microscopy confirmed that the Li-ion distribution of undoped NCM was less homogeneous than that of the doped NCM.