The lifetime and performance of lithium-ion batteries depend significantly on the formation and evolution of the solid electrolyte interphase (SEI) layer. A stable SEI is necessary for utilization of high specific capacity anode materials such as Si which goes through huge volume changes (upto ~400%) during cycling. In this study we report in situ atomic force microscopy (AFM) investigation on the formation and failure mechanisms of SEI layer using patterned Si island structures. Due to the shear lag effect [1], these patterned Si islands go through lateral expansion and contraction. This puts the SEI layer on the top of the island in tension and compression during lithiation and delithiation, respectively. We performed the studies in a glovebox at the environment with < 1 ppm O₂ and H₂O. In addition, the unique PeakForce tapping mode was used to image the extremely fragile SEI layer, which is very challenging for conventional AFM modes in organic solvent. Our in situ studies show for the first time the in operando cracking of SEI layer and its evolution during cycling. To understand the mechanics of the SEI layer, the critical strain at which this cracking starts to evolve was derived from a progression of the AFM images. This observation will be complemented with coin cell data on comparison of irreversible capacities of continuous film and patterned island samples which differ in their mode of deformation. We are also using Scanning electrochemical microscopy (SECM) to detect local electrochemical differences at stable and unstable SEI sites. Our comprehensive studies not only provide a new in-depth understanding of the in operando SEI formation, evolution and its mechanical response, but also offer guidance to tailor passivation layers for optimal battery performance.

Reference:

[1] Tokranov et. al. ACS Appl. Mater. Interfaces, 2014, 6, 6672