Unraveling the Degradation Mechanism of Lini0.8co0.1mn0.1o2 at High Cut-Off Voltage for Lithium Ion Batteries

Abstract

LiNi0.8Co0.1Mn0.1O2 (NCM811) layered oxides have been regarded as promising alternative cathode materials for the next generation of high-energy lithium ion batteries (LIBs) due to their high discharge capacities and energy densities at high operation voltage. However, the degradation mechanism related to structural changes at high cut-off voltage is not fully understood at the atomic scale, and thus the design of advanced battery materials is hindered by the pronounced structural degradation. Herein, the degradation mechanism of NCM811 at atomic-scale is investigated  under various cut-off voltages using aberration-corrected scanning transmission electron microscopy (STEM). It is observed that the crystal structure of NCM811 at atomic-level evolution from a layered structure to rock salt phase directly accompanyed by serious intergranular cracks under 4.9 V, which is distingushed from the structure evolution of layered structure, disordered layered, defect rock salt and rock salt phases under 4.3 and 4.7 V. The electron energy loss spectroscopy analysis also confirmed the reduction of Ni and Co from surface to the bulk not the previously reported only Li/Ni interlayer mixing. The degradation mechanism of NCM811 at high cut-off voltage of 4.9 V is attributed to the formation of intergranular cracks induced by defects, the direct formation of rock salt phase, and the accompanied reduction of Ni2+ and Co2+ phases from the surface to the bulk.

Keywords: Ni-rich layered cathode, electrochemical performance, degradation mechanism, cracks, atomic scale