Exploring the Interaction Between Particle Cracking and Expansion in NMC811 Using an Electro-Chemo-Mechanical Model

Ni-rich cathode materials, such as NMC811, enable high-performance lithium-ion batteries but their polycrystalline structure predisposes them to particle cracking. Previous experimental research identified particle cracking as a significant degradation mechanism in Ni-rich cathodes, yet the counterintuitive volume expansion of secondary particles upon delithiation remained unexplained. This study employs a 3D electro-chemo-mechanical model with phase field fracture to investigate particle cracking's impact on electrochemical performance and volumetric expansion. Our analysis reveals that intergranular fracture, initiated by anisotropic deformation at grain interfaces, significantly impacts battery performance. Comparison between 2D and 3D modelling approaches establishes that three-dimensional models are essential for accurately capturing crack propagation patterns and their electrochemical consequences. Particle cracking temporarily reduces overpotential by increasing the electrochemically active surface area but may eventually promote side reactions, increasing resistance and reducing capacity. Furthermore, cracking leads to unexpected volumetric expansion of secondary particles despite unit-cell contraction during delithiation. Finally, the study examines mechanisms potentially driving this expansion: dynamic fracture events, electrolyte capillary forces, CEI formation, and residual stresses, concluding that residual stress release, particularly from electrode manufacturing processes, is the primary contributor. These findings show the critical need for controlling anisotropic strains and residual stresses to mitigate particle cracking and improve electrode stability.