Nanoscale Mechanics of the Solid Electrolyte Interphase on Lithiated-Silicon Electrodes

The ∼300% volume changes of lithiated silicon electrodes (Li_xSi) during electrochemical cycling lead to cracking of the solid electrolyte interface (SEI). Here, we report how strain is transferred from Li_xSi to two primary inorganic SEI components: LiF and Li₂O. Our first principle calculations show that LiF, effectively bonded on Li_xSi at x > 1, enables the entire interface structure to deform plastically by forming delocalized stable voids. In contrast, Li₂O tightly bonded to Li_xSi is stiffer, and deforms rigidly across all x. Our results explain the significantly improved ductility of SEI with higher LiF versus Li₂O content observed experimentally.