Chaoping Liang1 2 Fantai Kong2 Roberto C. Longo2 Chenxi Zhang2 Yifan Nie2 Yongping Zheng2 Kyeongjae Cho2

1, State Key Laboratory of Powder Metallurgy, Central South University, Changsha, , China
2, The University of Texas at Dallas, Richardson, Texas, United States

Achieving the energy limit of LiNi1-2xCoxMnxO2 (NCM) (x=0~1/3) cathodes has raised great research interests in recent years. In order to obtain longer cycle and calendar life, current NCM cathodes deliver far less than their theoretical energy density, even after intense modifications such as cation/anion doping, coating, core-shell structure, and concentration-gradient design. By a throughout and careful literature survey, we summarized five individual phenomena observed at the end of charge: O3-O1 phase reaction, crack propagation, Li-Ni exchange, layered-spinel phase transition, and Oxygen evolution. These five phenomena have been reported independently by different researchers using novel ex situ and in situ characteristic techniques, and which one leads to the degradation of NCM cathode is not clear.
In this study, we perform a comprehensive study of LiNi1-2xCoxMnxO2 (NCM) (x=0~1/3) cathodes, using first-principle calculations within the DFT+U framework and a bond model based on the effective interaction of transition metal (TM) ions. 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 (DOC). Based on our findings, the optimal composition for a good electrochemical performance of NCM cathode materials is found within the region of 1/10<x<1/4 (50-80% of Ni). We also proposed separate solutions for each Ni concentration to prevent degradation at high voltage/capacity. For 1/4≤x≤1/3, a feasible way to reduce the Oxygen evolution during charge would be through doping with high valence TM ions, in order to reduce strongly covalent Co-O bonds. On the other hand, for 0≤x≤1/4, the use of coating materials or novel materials design like core-shell and concentration-gradient structures could restrict the lattice distortion along the charge process. The key factors found in present work will help researchers, especially the newcomers to understand the obstacles toward unity efficiency, and also help them to rationally design NCM cathode materials with high-energy density through possible solution mechanisms, such as doping, coating or novel nanostructures, like core-shell or concentration gradient cathodes.

Related publications:
C. P. Liang, Roberto C. Longo, et al. Obstacles toward unity efficiency of LiNi1-2xCoxMnxO2 (x=0~1/3) (NCM) cathode materials: Insights from ab initio calculations. Journal of Power Sources, 2017, 340: 217-228.
C. P. Liang, F.T. Kong, et al. Unraveling the Origin of Instability in Ni-Rich LiNi1−2xCoxMnxO2 (NCM) Cathode Materials. The Journal of Physical Chemistry C, 2016, 120 (12): 6383–6393.
C. P. Liang, F.T. Kong, et al. Site Dependent Multicomponent Doping Strategy for Ni–rich LiNi1–2yCoyMnyO2 (y = 1/12) Cathode Materials for Li–Ion Batteries. In revision.
C. P. Liang, Roberto C. Longo, et al. Ab initio study on Surface Segregation and Anisotropy of Ni-rich LiNi1-2xCoxMnxO2 (NCM) (x ≤ 0.1) Cathodes. Submitted.