Anisotropic model to describe chemo-mechanical response of Ni-rich cathode materials

In the context of lithium-ion battery technology, LiNixMnyCozO2 (x ≥ 0.8) (Ni-rich NMC) is considered a promising cathode material for next-generation batteries due to its high energy density. However, rapid capacity fade caused by mechanical failure hinders commercialization in the battery market. Anisotropic deformation in the basal and normal directions is the main source of mechanical damage to Ni-high NMC. In this study, we developed a fully coupled chemo-mechanical model that rigorously accounts for the anisotropic features of anisotropic diffusivity, anisotropic stress-induced diffusion and anisotropic mechanical deformation. Using the model, we investigate the effects of anisotropic and concentration-dependent lattice strains on the chemo-mechanical behavior of a single-crystalline Ni-rich NMC particle through a comparative study with the cases of a fully isotropic model, constant properties and an isotropic diffusion approximation. Numerical simulations reveal that due to the anisotropic features, Ni-high NMC materials generate larger concentration gradients and associated higher stresses than isotropic materials, thereby enhancing the possibility of mechanical fractures. The comparative study reveals that the anisotropy in diffusivity plays an important role in high stress generation of anisotropic electrode materials. On the other hand, the anisotropy in stress-induced diffusion has small effect on the stress level of anisotropic materials; thus, isotropic approximation of the stress-induced diffusion term is reasonably allowed in the anisotropic chemo-mechanical models. The simulation results indicate that although anisotropic electrode materials are used in a single crystal structure, they are not advantageous compared to isotropic materials in terms of mechanical stability. This study provides fundamental insight into the role of the anisotropic chemo-mechanical behavior of Ni-rich cathode materials, which will help in the better design of robust, high-capacity batteries.