In Situ and Operando Investigations of Failure Mechanisms of the Solid Electrolyte Interphase on Silicon Electrodes

The lifetime of rechargeable lithium ion batteries is closely related to the formation and evolution of the solid electrolyte interphase (SEI). These passivation films undergo substantial deformations when the underlying electrode particles expand and contract during cycling. Directly probing these changes is extremely challenging. In this study, we demonstrate a new approach for applying controlled strains to SEI films with patterned Si electrodes, in conjunction with direct observations of mechanical degradation using in operando atomic force microscopy. Monitoring both strain and crack formation in SEI provides new in-depth understanding of SEI fracture. These results verify that crack formation occurs during lithiation (this has been predicted previously but not directly observed). Additional SEI formation at low potentials did not fill these cracks, which directly contradicts prior speculation. These experiments also made it possible to estimate the fracture toughness of the SEI (a key value that has not been previously measured).