Nano-FTIR Spectroscopy of the Solid Electrolyte Interphase Layer on a Thin-Film Silicon Li-Ion Anode

Si anodes for Li-ion batteries are notorious for their large volume expansion during lithiation and the corresponding detrimental effects on cycle life. However, calendar life is the primary roadblock for widespread adoption. During calendar life aging, the main origin of impedance increase and capacity fade is attributed to the instability of the solid electrolyte interphase (SEI). In this work, we use ex situ nano-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy to characterize the structure and composition of the SEI layer on amorphous Si thin films after an accelerated calendar aging protocol. The characterization of the SEI on non-washed and washed electrodes shows that brief washing in dimethyl carbonate results in large changes to the film chemistry and topography. Detailed examination of the non-washed electrodes during the first lithiation and after an accelerated calendar aging protocol reveals that PF6- and its decomposition products tend to accumulate in the SEI due to the preferential transport of PF6- ions through polyethylene oxide-like species in the organic part of the SEI layer. This work demonstrates the importance of evaluating the SEI layer in its intrinsic, undisturbed form and new strategies to improve the passivation of the SEI layer are proposed.

Automated summary

Si anodes for Li-ion batteries suffer from large volume expansion during lithiation, which affects cycle life. However, calendar life is the main obstacle for widespread adoption. This study focuses on the instability of the solid electrolyte interphase (SEI) during calendar life aging. Characterization of the SEI layer on amorphous Si thin films after accelerated calendar aging reveals the accumulation of PF6- and its decomposition products in the SEI, which is attributed to preferential transport of PF6- ions through polyethylene oxide-like species in the SEI. This work emphasizes the importance of evaluating the SEI layer in its intrinsic form and proposes strategies to improve SEI passivation.