Deciphering the True FEC Reduction Mechanism and Its Implications to the Understanding of the SEI in Li-Ion Batteries
Authors: Surace Yuri , Leanza Daniela , Mirolo Marta , Kondracki Lukasz , Vaz Carlos A. F. , El Kazzi Mario , Novák Petr , Trabesinger Sigita 
Published: February 2024

The main challenges in rechargeable batteries, especially when aiming for high energy density, arise from the limited electrochemical stability window of current state-of-the-art electrolytes. Negative electrode active materials, such as silicon, tin oxide and others, whose capacity is many times higher than that of graphite, often exhibit large volume expansion upon lithiation and therefore the SEI formed on their surface is unstable. The most popular strategy for improving their cycling stability is the use of electrolyte additives that are reduced at potentials more positive than the electrolyte solvents. Their decomposition products are incorporated in the SEI, leading to a more stable electrode/electrolyte interface.
One of the most-investigated electrolyte additives is fluoroethylene carbonate (FEC), because it has been shown to lead to a significant performance enhancement of Si- and Sn-based anodes. Many reaction pathways have been proposed, however there is currently no agreement on the exact type of chemical compounds constituting the decomposition products, nor on the exact mechanism for FEC decomposition. To address these questions, we conducted a systematic study that tracked the morphological and chemical changes during electrochemical decomposition of FEC. We have found that despite FEC often being referred as a “film-forming” additive, the first stage of its decomposition leads instead to the formation of spherical particles, consisting mainly of lithium fluoride (see figure below);1 only later a carbonate-rich film covers the entire electrode, covering as well the LiF-rich spheres. This phenomenon has been overlooked before likely due to very simple reason, disclosed in our contribution.
A detailed investigation using XPEEM—a surface-sensitive analytical technique with high lateral resolution—as a function of electrolyte composition shows that fluorine in the later-formed carbonate film comes from the electrolyte salt, while all fluorine from FEC decomposition resides in the spherical particles, formed upon FEC reduction. In addition, we have found that the size and amount of particles strongly depend on the cell medium, where electrolytes with higher dielectric constant lead to larger particle size, as does the presence of the high-capacity electroactive materials in the electrode, both contributing to the final properties of FEC-derived SEI. The results of this study provide a deeper understanding of how fluorine-containing additives work and enables tuning of the SEI properties to the desired morphological and chemical outcome by using the laws of simple crystal-growth theory by adjusting the inter-cell environment.
This work demonstrates that, with the right analytical tools, the true nature of the even seemingly very-well studied compounds can be revealed and a full understanding of their decomposition mechanisms can be clarified.

Acknowledgement

This research was supported by InnoSuisse (Project number 18254.2)

References

Y. Surace, D. Leanza, M. Mirolo, Ł. Kondracki, C.A.F. Vaz, M. El Kazzi, P. Novák, S. Trabesinger, Energy Storage Materials 2022, 44, 156-167.

Figure 1