Simulation of NMC cathode particle fracture based on the multiphase-field method

Intergranular cracking is considered a major cause of capacity fading in layered battery cathodes [2]. During intercalation and deintercalation of Li-ions, the highly anisotropic chemo-mechanical expansion of LiNi$_{0.1}$Mn$_{0.1}$Co$_{0.8}$O$_{2}$ (NMC811) primary grains within the agglomerate causes complex mechanical stress fields, triggering crack formation particularly during the initial cell charge cycle (cathode discharge) [2] [3, p.14] [4] [5, p.9]. A promising approach in grain engineering is correlated primary grain structures called rod-shaped morphologies, to which researchers [5, p.17] [6] allocate reduced anisotropic stress and cracking compared to common gravel-shaped morphologies. Building on previous work by Daubner et al. [1], this study examines spherical agglomerates in a 2D framework with a diameter of 10 μm. Using the multiphase-field method, it simulates chemo-mechanical lattice expansion and resulting cracking, beginning with an idealized, spatially homogeneous lithiation. This enables to analyze the effects of primary grain orientation and grain boundaries. Subsequently, Li-ion diffusion is modeled through a Potentiostatic Intermittent Titration Technique (PITT), with mechanical simulations conducted at times of interest. ... mehrIt is shown that strong grain misorientation causes tensile stress peaks in grain boundaries, while approximately parallel grain orientations instead lead to a local stress-reduction. This confirms the hypothesis of the above mentioned research and is reflected as well in the crack results, wherein cracking primarily occurs between unaligned grains. As a result, these regions have high intergranular crack densities, causing shattering into many fragments of the gravel-shaped agglomerate, while radially-aligned grains mostly stay coherent with lower crack densities.


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