Anisotropic mechanical properties of LiNi_xMn_yCo_{1 -x-y}O₂ cathodes for Li-ion batteries: A first-principles theoretical study

LiNi_xMn_yCo_{1 -x-y}O₂ (NMC) is a prominent cathode material for advanced Li-ion batteries. The mechanical integrity of NMC is crucial, as it significantly influences the electrochemical stability of NMC cathodes during cycling. Despite the importance, a comprehensive understanding of the mechanical properties of NMC remains elusive, primarily due to its pronounced structural anisotropy that poses significant challenges for both experimental quantification and theoretical modeling. Herein, we employ first-principles calculations to investigate the evolution of anisotropic elastic properties of NMC (NMC532, NMC622, and NMC811) throughout the delithiation process. Our findings reveal a complex interplay of mechanical properties and structural features of NMC. As delithiation proceedings, the elastic moduli present a reduction at disparate rates along the c-axis versus those directions paralleling the ab-plane, owing to the differential bonding dynamics occurring within and between the transition metal slabs. Therefore, the anisotropy ratio of NMC exhibits exponential increase during delithiation, where the basal plane shows the near-highest Young's modulus and near-lowest shear modulus. Intriguingly, we observe that a higher Ni content in the NMC composition accelerates elastic modulus reduction, a behavior predominantly influenced by the Jahn−Teller distortion. Collectively, our insights bridge the evolution of lattice structure with the corresponding shifts in the elastic properties of NMC, laying the foundational groundwork for future material optimizations.