The lifetime and performance of lithium-ion batteries depend significantly on the formation and evolution of the solid electrolyte interphase (SEI) layer. Si has almost 10 times theoretical specific capacity compared to current state-of-the-art graphite anode materials in Li-ion batteries, which is highly attractive for electric vehicle applications. However, this energy advantage comes with a ~300% volume change during voltage cycling, which causes substantial deformations to SEI. The mechanical integrity of SEI on Si electrodes is highly important in developing commercial Si-based batteries. For R&D, directly probing the SEI evolution is challenging due to 1) SEI thickness is from 10 to 100s of nm; 2) SEI is fragile, which requires a gentle probing force; 3) in situ and operando measurements in electrolyte and under ambient control are needed; 4) multimodal information to clarify the convoluted topographical, mechanical, electrical and electrochemical properties is desired; and 5) a well-defined specimen configuration is needed to interpret the deformation. We have addressed these issues by applying controlled strains to the SEI layer using patterned Si island electrodes and by monitoring these electrodes during voltage cycling on a PeakForce electrochemical AFM system housed inside an Ar-filled glove box (< 1ppm O₂ and H₂O contents). From this study, we verified that 1) SEI crack formation occured during lithiation, which has been predicted but not directly observed previously; 2) additional SEI formation at low potentials did not fill these cracks, which directly contradicts some prior speculation; 3) SEI had a layered structure, the individual layer of which could be differentiated by the quantitative nanomechanical probing. 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.

Figure 1.(A)-(D) AFM images of surface topographic evolution of the SEI layer during lithiation-dethiation cycles. (E) SEM image of a PeakForce SECM nanoelectrode probe. The probe is fully insulated with only the Pt-coated tip apex exposed.

Reference:

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

[2] Tokranov et al., Adv. Energy Mater., 2016, 6, 1502302

[3] Kumar et al., ACS Energy Lett., 2016, 1, 689