Failure progression in the solid electrolyte interphase (SEI) on silicon electrodes

Maintaining an electrochemically and mechanically stable solid electrolyte interphase (SEI) is of fundamental importance to the performance of high capacity anode materials such as silicon. In this study, a novel approach is utilized to apply controlled strains to SEI films on patterned Si electrodes. Mechanical failure mechanisms of SEI are investigated with integrated in situ AFM, ex situ FIB measurements and finite element modeling. Cross-sectional images reveal that the SEI has a bilayer structure, and through-thickness cracks appear inside of the outer SEI and arrest at the outer and inner SEI interface. The absence of cracking of the inner SEI layer implies that it has a high fracture toughness, and that it is possible to create an inner SEI which exhibits excellent strain tolerance compared to the outer layer. Interfacial delamination occurs between the outer and inner SEI layers while the inner layer is still well adhered to the underlying Si. The experimental and modeling results indicate that the inner SEI layer is sufficient for passivation of the Si surface. More broadly, the present work provides important guidelines for producing inner SEI layers that can simultaneously satisfy both electrochemical and mechanical criteria for long term passivation of silicon electrode surfaces.