Si has been widely recognized as one of the most promising anode materials for its high energy density; however, critical drawbacks such as large irreversible capacity decay and poor reliability/durability over extended cycles have significantly constrained its commercialization in many long duration, heavy duty applications. Aforementioned drawbacks are partially due to loss of electrolyte by reduction at electrode surface (formation of Solid Electrolyte Interphase (SEI)) and the chemical/mechanical instability of SEI through cycling. For a better understanding on the mechanism of Si electrode failure during battery cycling, we design and develop a novel in-situ ATR-FTIR spectra-electrochemical cell, and study the SEI chemistry using this in-situ ATR-FTIR technique.

In this work, we investigate the chemical composition of SEI on single crystal silicon electrode (100), which was selected as an ideal model surface. By tuning the probing depth in vibrational spectra of molecules near the electrode surface, we specifically address the dependence of SEI chemical composition on applied potential and penetration depth from interface to bulk. The experimental observation indicates that soluble species like DEDOHC is formed at silicon surface covered by native oxide, which is related to the chemical instability of SEI and contributes to the early failures of Si anode. In contrast, the main component of SEI on Li₁₅Si₄ is LiEDC, which is responsible for the passivation of freshly cracked surface during the long-term cycling. Si-F species are visible at inner interphase, while the organic product mainly locates at the outer portion of SEI. These clues pave the SEI formation paths on silicon anode, showing how the chemical side reactions couples with mechanical degradation. These failure modes understanding would also benefit other alloy host electrodes, which suffer from multiple fold degradation.

Acknowledgement:

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Freedom CAR and Vehicle Technologies of the U.S. Department of Energy under contract No. DE-AC02-OSCH11231.