Optimum Particle Size in Silicon Electrodes Dictated by Chemomechanical Deformation of the SEI

Abstract Nanostructured alloy‐forming anode materials can resist fracture that is caused by extreme volume changes during cycling. However, the higher surface area per unit mass in nanomaterials increases exposure to the electrolyte reduction reactions that form a solid electrolyte interphase (SEI), which implies that capacity loss will increase as particle size decreases. This hypothesis is investigated with composite electrodes using different silicon nanoparticle sizes, and the expected particle size effect is not observed. Instead, there is an optimum particle size where capacity loss per volume is minimized. Finite element modeling demonstrates that the mechanical deformation of the SEI varies significantly with the silicon particle size. Smaller particles lead to the decrease of the tensile hoop strains in the outer portion of the SEI and simultaneously make the overall elastic strains in the inner portion more compressive. These results suggest that the SEI on smaller particles is more resistant to mechanical degradation, even though the higher specific surface areas increase initial SEI formation. The trade‐off between these effects leads to the observed optimum particle size.