Elastic Mechanics Study of Layered Li(Ni_{x}Mn_{y}Co_{z})O_{2}

Abstract

Understanding the elastic mechanical behavior of layered Li(Ni_{x}Mn_{y}Co_{z})O_{2} (NMC) cathode materials is critical for developing strategies to address their prominent cracking issues and mitigate the mechanical degradation of the cathode of lithium-ion batteries (LIBs). So far, a systematic and in-depth investigation into the elastic mechanical properties of NMC materials is still lacking. Hence, both the isotropic and anisotropic mechanical properties, including elastic constants, Young’s modulus, shear modulus, bulk modulus, compressive modulus, linear compressibility, Poisson’s ratio, Pugh’s ratio, Cauchy pressure, and Kube’s log-Euclidean and universal elastic anisotropy indexes, of seven NMC materials with different compositions are systematically studied here through first-principles calculations. The bulk modulus is proved to be an isotropic mechanical quantity, and its proper relationship with the anisotropic linear compressibility is revealed. Then, we investigate how the isotropic and anisotropic mechanical properties of NMC materials can be tuned by the compositions of Ni, Mn, and Co. It is found that the elastic moduli of NMC materials decrease as the Ni content increases, and Mn plays a positive role in not only enhancing the elastic moduli but also in reducing the mechanical anisotropy of NMC materials. Through a quantitative chemical bond analysis, we find that the reason for such a dependence of the elastic moduli on the transition metal (TM) compositions of NMC materials lies in the difference in the strengths of TM–O bonds, and the positive effect of Mn mainly stems from the contribution of the ionic bonding part of the Mn−O bond due to the high average valence state (+4) of Mn ions; this highlights the benefits of doping with other high-valence-state ions for the promotion of the mechanical properties of NMC materials. Besides, the applicability, physical meaning, and proper use of Pugh’s ratio and the Cauchy pressure in metal-oxides like NMC materials, as well as their relations with Poisson’s ratio, are clarified. The analysis of the relative ductility and brittleness of NMC materials with different compositions via Pugh’s ratio and the isotropic Poisson's ratio reveals a negative correlation between the relative ductility and the magnitude of the modulus. Cauchy pressures of layered NMC materials are found to be directional, with an intralayer negative value but an interlayer positive value, which predicts the significant covalent bonding character of the TM–O bonds, but a nearly ionic dominated bonding character of the Li−O bonds, and this qualitative result has been further proved by a quantitative chemical bond analysis. Furthermore, an in-depth investigation of the three-dimensional spatial distribution characteristics of anisotropic mechanical properties of NMC materials is conducted. It is found that, first, the distributions of directions of the extreme values of various anisotropic mechanical properties all exhibit an approximate trigonal symmetry to some extent; second, the anisotropy is most prominent in the {21[over ¯]0} crystal plane family; third, the (104) plane and [4[over ¯]8[over ¯]1] direction are, respectively, prone to be the cleavage plane and cleavage direction, along which the microcracks and even mechanical failures may be susceptible to occur due to stress concentration. The findings presented in this study will provide guidance for the design and development of robust and reliable LIBs.