Lithium-ion batteries (LIBs) are widely chosen for energy storage owing to their high coulombic efficiency and energy density. Within the positive electrode materials of LIBs, the structural integrity of secondary particles, composed of randomly oriented single-crystal primary particles, is crucial for sustained performance. These particles can fracture as a result of both mechanical stress and chemical interactions within the solid. Modelling LIBs is a complex task involving electro-chemo-mechanical phenomena and their interactions on different length scales. This study proposes a numerical modeling framework to investigate the active particle degradation and its impact on electrochemical performance. The model integrates mechanical and electrochemical processes, tracking crack evolution and mechanical failure through phase field damage. The coupled time-dependent non-linear partial differential equations are solved in a finite element framework using COMSOL Multiphysics. The model offers numerical insights into intergranular and transgranular fracture within secondary particles. The electrolyte infiltration into cracks reduces the electrochemical overpotential due to the increase in electrochemically active surface area, positively affecting performance. However, prolonged cycling with particle cracking poses severe threat to the battery performance and capacity. This comprehensive numerical modeling approach provides valuable insights into the intricate interplay of mechanical and electrochemical factors governing LIB performance and degradation.

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.