Stress analysis of solid-electrolyte interphase (SEI) on silicon thin-film anodes: Role of undulations, thickness, and charging cycle

The mechanical degradation of silicon anode resulting from its significant volumetric fluctuations during the insertion/extraction of lithium is a major roadblock in the success of high-capacity Si anode Li-ion batteries. Owing to the extreme deformation of Si, the chemically passivating solid-electrolyte interphase (SEI) layer formed due to electrolyte reduction on the Si anode also suffers from mechanical deterioration through fracture, followed by delamination and spallation. Naturally, the characteristics of the Si-SEI interface play a significant role in the cracking of the SEI layer. In this study, we investigate the possibility of fracture of the SEI layer formed on an undulated surface of the Si film. In agreement with previous experimental observations, the surface undulations are found to play a decisive role in the location of the most deleterious tensile stress in the SEI. During lithiation, the peaks of the undulations have the highest value of tensile stress. However, during delithiation, the valleys are observed to be the location of maximum tensile stress. Furthermore, during the cyclic lithiation/delithiation of the undulated Si film, the evolution of interface morphology and, in turn, the stress in the SEI is found to have a very rich dependence on the SEI thickness. For a very thin SEI, the amplitude of undulations grows with cycling, leading to an increase in the detrimental tensile stress in the SEI as it is cyclically operated. On the other hand, the undulations tend to flatten out for a moderately thick SEI, as the Si film is cyclically charged/discharged, making the tensile stresses progressively less deleterious to the SEI. And, for very thick SEI, the undulations do not undergo any change in their morphology, so the stress varies periodically in the SEI as the silicon is cyclically charged/discharged. In addition to agreeing with simplified theoretical models, our observations qualitatively match previous experimental studies We expect that these findings on the initial interface morphology of the Si-SEI interface would aid in designing a mechanically robust artificial SEI layer that can help reduce the mechanical degradation of the SEI and the Si film.