Unraveling stress mechanisms in Ni-rich NMC particles: The dominance of anisotropic deformation over diffusion

Abstract

Ni-rich LiNixMnyCozO2 (NMC) materials, such as NMC811, are gaining attention for lithium-ion batteries (LIBs) due to their high capacity. However, rapid capacity fade caused by mechanical deformation remains problematic. High-nickel NMC typically forms secondary particles through agglomeration of primary particles, and weak interparticle bonds often lead to crack formation akin to grain boundary fractures in polycrystalline materials. Here, we investigate stress development in Ni-rich NMC using a polycrystalline NMC811 particle model coupled with chemo-mechanical simulations, incorporating anisotropic deformation and lithium-ion diffusion. Results highlight that anisotropic deformation significantly outweighs anisotropic diffusion in driving stress within polycrystalline particles, contrasting mechanisms observed in single-crystal particles. Specifically, lattice mismatches at grain boundaries greatly amplify stress, marking these boundaries as critical crack initiation sites. Furthermore, stress increases with the number of grains and with secondary particle size but decreases as grain size grows, supporting experimental strategies toward single-crystalline particle development to reduce mechanical degradation. This study advances the understanding of anisotropic diffusion and deformation effects in Ni-rich polycrystalline materials, guiding the design of durable, high-capacity LIBs. © 2025 Elsevier B.V.