Unraveling the degradation mechanism of LiNi0.8Co0.1Mn0.1O2 at the high cut-off voltage for lithium ion batteries

LiNi0.8Co0.1Mn0.1O2 (NCM811) layered oxides have been regarded as promising alternative cathodes for the next generation of high-energy lithium ion batteries (LIBs) due to high discharge capacities and energy densities at high operation voltage. However, the capacity fading under high operation voltage still restricts the practical application. Herein, the capacity degradation mechanism of NCM811 at atomic-scale is studied in detail under various cut-off voltages using aberration-corrected scanning transmission electron microscopy (STEM). It is observed that the crystal structure of NCM811 evolution from a layered structure to a rock-salt phase is directly accompanied by serious intergranular cracks under 4.9 V, which is distinguished from the generally accepted structure evolution of layered, disordered layered, defect rock salt and rock salt phases, also observed under 4.3 and 4.7 V. The electron energy loss spectroscopy analysis also confirms the reduction of Ni and Co from the surface to the bulk, not the previously reported only Li/Ni interlayer mixing. The degradation mechanism of NCM811 at a high cut-off voltage of 4.9 V is attributed to the formation of intergranular cracks induced by defects, the direct formation of the rock salt phase, and the accompanied reduction of Ni2+ and Co2+ phases from the surface to the bulk.