Obstacles toward unity efficiency of LiNi1-2xCoxMnxO2 (x=0 ∼ 1/3) (NCM) cathode materials: Insights from ab initio calculations

In this work, we perform a comprehensive study of five phenomena of LiNi1-2xCoxMnxO2 (NCM) (x = 0 -1/3) cathodes at the end of charge (phase reaction, crack propagation, Li-Ni exchange, phase transition, and oxygen evolution), using first-principle calculations within the DFT + U framework. Based on our results, we have located the obstacles toward unity efficiency and revealed that the degradation strongly depends on the Ni concentration and the depth of charge. The threshold capacities for degradation of LiNi1-2xCoxMnxO2 are 130-140 mA.hg(-1) (y < 0.5) for 1/4 <= x = 1/3 (33.33-50% of Ni), and 200 -210 mA.hg(-1) (y < 0.25) for 0 <= x = 1/4 (50-100% of Ni), respectively. For 1/4 <= x = 1/3, our results show that the origin of the degradation is the oxidation of O2-, which is the result of the pining of O-p and Ni-d bands at the valence band edge. For 0 <= x = 1/4, lattice distortion and Li-Ni exchange are the mechanisms responsible for the degradation of the cathode material, leading to severe structural instabilities in the Ni-rich region (x = 0.1). Our findings will help to rationally design NCM cathode materials with high-energy density, also providing possible solution mechanisms to the degradation factors, such as doping, coating or novel nanostructures, like core-shell or concentration gradient cathodes. (C) 2016 Elsevier B.V. All rights reserved.