Intrinsic origin of intra-granular cracking in Ni-rich layered oxide cathode materials

Abstract

Mechanical degradation phenomena in layered oxide cathode materials during electrochemical cycling have limited their long-term usage because they deteriorate the structural stability and result in a poor capacity retention rate. Among them, intra-granular cracking inside primary particles progressively degrades the performance of the cathode but comprehensive understanding of its intrinsic origin is still lacking. In this study, the mechanical properties of the primary particle in a Ni-rich layered oxide cathode material (LiNi_0.8Co_0.1Mn_0.1O₂) are investigated under tensile and compressive deformation towards both in-plane and out-of-plane directions within the density functional theory framework. The Young's modulus and maximum strength values indicate that the pristine structure is more vulnerable to tensile deformation than compression. In addition, delithiation significantly deteriorates the mechanical properties regardless of the direction of deformation. In particular, a substantial degree of anisotropy is observed, indicating that the mechanical properties in the out-of-plane direction are much weaker than those in the in-plane direction. Particular weakness in that direction is further confirmed using heterogeneously delithiated structures as well as by calculating the accumulated mechanical stress values inside during delithiation. A comparison of the mechanical properties of the structure with a lower Ni content (Ni = 33%) demonstrates that the Ni-rich material is slightly weaker and hence its intra-granular cracking could become accelerated during cycling.