Elucidating the Morphology of Partially Lithiated Microscale Silicon Particles using Transmission Electron Microscopy

Partial lithiation of silicon active materials for lithium-ion batteries recently gained attention as a promising mitigation strategy for the degradation phenomena associated with the severe volume expansion upon the lithiation, particularly in the case of large, microscale silicon particles. It was suggested that this is caused by the formation of a stabilizing core–shell-like particle structure in the first cycles, consisting of a crystalline core and amorphous silicon shell. In this study, we investigated the microstructure of partially lithiated microscale silicon particles using transmission electron microscopy (TEM). When observed via TEM, the contrast difference in amorphous and crystalline silicon is utilized to reveal previously lithiated areas inside the silicon microparticle. We investigated the influence of lower cutoff potentials and amorphization progress in half-cells. We also examined the changes over prolonged cycling in full-cells with an NCA cathode after 12 and 243 cycles. Silicon particle pulverization was not observed for any sample, even though we found that substantial parts of the particles’ insides had been lithiated. We suggest that the diffusion of Li along grain boundaries and stacking faults plays an essential part in the amorphization and cycling of microscale Si particles but does not lead to their cracking or pulverization.