The Impact of Initial SEI Formation Conditions on Strain-Induced Capacity Losses in Silicon Electrodes

The solid electrolyte interphase (SEI) that passivates silicon surfaces in Li ion batteries is subjected to extremely large mechanical strains during electrochemical cycling. The resulting degradation of these SEI films is a critical problem that limits the cycle life of silicon-based electrodes. With the complex multiphase microstructure in conventional porous electrodes, it is not possible to directly measure the impact of these strains on SEI formation and capacity loss. To overcome this limitation a new in situ method is presented for applying controlled mechanical strains to SEI during electrochemical cycling. This approach uses patterned silicon films with different sized islands that act as model electrode particles. During lithiation/delithiation, the lateral expansion/contraction of the island edges applies in plane strains to the SEI. Detailed analysis of the island size effect then provides quantitative measurements of the impact of strain on the excess capacity losses that occur in different potential ranges. One key finding is that the applied strains lead to large capacity losses during lithiation only (during all cycles). Also, employing fast and slow SEI formation (first cycle) leads to large differences in the strain-induced losses that occur during subsequent cycling.