The multiphase-field method has great potential to advance and accelerate research on battery materials as it can capture phase transformations driven by chemical, mechanical and electrical forces. The state-of-the-art electrode materials LiFePO₄, LiCoO₂ and graphite, but also many promising candidates for sodium-ion cathodes exhibit phase separation upon charge and discharge. This gives rise to a multitude of energetically favorable states (combinations of inter- and intra-particle phase separation) governed by the rich interplay of anisotropic diffusion, coherency strain and surface reaction. Furthermore, there is a strong dependency on the particle morphology.
In the present work, a multiphase-multicomponent Allen-Cahn framework combined with the grand-chemical potential formalism is employed to study the multi-grain interaction during (de-)intercalation and coupled phase transformations. The main focus is on the sodium-ion cathode material Na_2/3Ni_1/3Mn_2/3O₂ which undergoes a first order phase transition (O2 to P2 phase) and several ordered states over the range of its theoretical capacity. We study the strongly coupled effects of electro-chemical surface reaction and diffusion potentials in the layered oxide to identify possible rate-limiting factors. Furthermore, links to the DFT method and Newman-type models are discussed to pave the way for a fully digital simulation chain.