Development of coarse-grained model for anisotropic lithium transport in polycrystalline active material in lithium-ion batteries

The macroscopic lithium transport behavior in lithium-ion battery (LIB) electrodes is significantly influenced by the anisotropy of lithium diffusion and the polycrystalline structure of active materials. However, electrode-scale numerical simulations that simultaneously account for both effects remain scarce due to the large disparity in length scales between primary particles and the electrode thickness. In this study, a coarse-grained model is developed to estimate the apparent lithium transport properties in polycrystalline active materials with various anisotropy strengths and primary particle sizes. Two-dimensional steady-state lithium transport simulations are performed on a wide range of virtual polycrystalline structures, and the correlation between microscopic flux distribution and primary particle configuration is discussed. The results show that stronger anisotropy suppresses lithium transport in the stacking direction of primary particles, leading to a reduction in macroscopic flux. Larger particle sizes and higher anisotropy strengths increase the complexity of lithium transport pathways and cause greater variation in transport characteristics across structures. A strong correlation is found between lithium flux and the stacking direction, underscoring the importance of microstructural orientation. Based on the simulation results, a dimensionless anisotropy factor is introduced to quantify the apparent transport properties. This factor reflects both the intrinsic anisotropy of lithium diffusion and the randomness in primary particle configuration. Its statistical distribution is modeled as a function of anisotropy strength and primary particle size in a probabilistic manner instead of a deterministic manner. The resulting coarse-grained model provides a computationally efficient and physically grounded framework for representing anisotropic lithium transport in polycrystalline structures. This makes the model suitable as a component model in large-scale LIB simulations to analyze the behavior of LIBs across multiple length scales.