(680e) Identifying Reaction Intermediates of Fluorinated Electrolytes

The transportation sector accounts for the most emissions of any sector in the United States. While electric vehicle sales have increased exponentially in the United States, and the White House has announced a goal of 50% of all new vehicle sales to be electric by 2030, there is a need for safer, more energy dense, and power dense batteries to facilitate this transition.¹ Particularly, lithium metal batteries (LMBs) are a promising technology due to lithium’s high theoretical specific capacity and low reduction potential, which can result in batteries with energy densities surpassing current state of the art lithium-ion batteries.² However, the highly reductive lithium metal anode reacts with the electrolyte during cycling, resulting in capacity loss. These reactions form a solid electrolyte interphase (SEI), and electrolyte design is a key strategy to improve the cycling stability and Coulombic efficiency of LMBs. Many successful electrolyte engineering strategies have involved tuning the SEI to be mechanically and chemically stable to prevent further reactions.

In this research, we develop a novel approach to studying radical intermediates formed during electrolyte reduction using electron paramagnetic resonance (EPR) spectroscopy. We reduce electrolyte within an electrochemical cell which also contains a spin trap. The use of EPR with spin traps enables identification of radical species formed during electrolyte reduction, which we hypothesize are precursors to cross-linked polymer networks that are key to a high performing SEI. We apply this approach to study high performance fluorinated electrolyte solvents, including tris(2,2,2-trifluoroethyl)orthoformate (TFEO) and bis(2,2,2-trifluoroethyl) ether (BTFE), which have been employed as diluents in localized high concentration electrolytes (LHCEs). Although these diluents are expected to poorly solvate Li⁺, resulting in a largely anion-derived SEI, we demonstrate that the diluents are consumed during cycling using a quantitative ¹⁹F solution NMR, indicating that they play a role in SEI formation, which has also been suggested in recent literature.³ We employ EPR spectroscopy to study the reduction mechanisms of these diluents and provide broader insight into the role of electrolyte fluorination in enabling stable interphases. Ultimately, this approach affords insight into molecular mechanisms to form an effective SEI which can both accommodate volume expansion and inhibit parasitic electrolyte consumption. This will guide electrolyte design for high Coulombic efficiency LMBs in the future.

References

1. C. Li et al., Sci Rep, 5, 9213 (2015).
2. K. G. Gallagher et al., Energy Environ. Sci., 7, 1555–1563 (2014).
3. X. Cao et al., Proceedings of the National Academy of Sciences, 118, e2020357118 (2021).