Apparent microstructurally induced phase separation in porous LiNi1/3Mn1/3Co1/3O2 cathodes

A thermodynamically consistent phase field framework is presented to analyze the combined effects of internal grain microstructure and the particle size polydispersity on the microstructural mechanisms that control lithium transport and intercalation kinetics in porous LiNi1/3Mn1/3Co1/3O2 (NMC111) cathodes. Physically, Fickian, chemomechanical, and interfacial reaction driving forces are identified and summarized into simple dimensionless numbers, amenable to define different regimes of behavior. Microstructurally NMC111 cathodes comprised of a monodispersed distribution of secondary particle sizes display a unimodal lithium population in the absence of transport limitations in the electrolyte. In contrast, cathodes with a bimodal distribution of secondary particle sizes display two clearly distinguishable populations of lithiated particles due local to differences in the chemomechanical and Fickian transport mechanisms in large and small particles. For larger particles, lithium transport is driven by stress-induced chemomechanical driving forces that preferably lithiate the grain boundaries leading to the formation of lithium-rich networks. For smaller particles, lithiation is driven by Fickian diffusion from the surface inwards due to a lack of chemomechanical driving forces. The compound microstructural and particle size contributions to lithium intercalation result in the development of macroscopic populations of lithium distributions that may appear at first glance as two-phase in character, and highlight the possibility of simultaneously engineering the interior of the particle and the particle size distribution.