Precise in situ atomic force microscopy (AFM) is used to monitor the formation of the solid electrolyte interphase (SEI) on Si electrodes. The stability of these passivation films on negative electrodes is critically important in rechargeable Li-ion batteries, and high capacity materials such as Si present substantial challenges because of the large volume changes that occur with Li insertion and removal. The results reported here show that the initial rapid SEI formation can be stabilized before significant Li insertion into the Si begins and that the rate at which this occurs varies significantly with the nature of the surface. The initial cycling conditions also have a substantial impact on the SEI that forms, with faster rates leading to a smoother, thinner SEI film. To quantitatively interpret the SEI measurements, irreversible expansion of the Si during the first cycle was also monitored in situ with specifically designed specimen configurations. On the basis of the experimental results, relatively simple models were also used to describe the initial formation and stabilization of the SEI and to describe the relationship between the SEI thickness and expected SEI degradation mechanisms.

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).

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