Probing Reduction Intermediates and Products of Localized High Concentration Electrolytes

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. 1 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. Recently, solvation structure engineering via high concentration (HCE) and localized high concentration electrolytes (LHCEs) has proven to be effective in promoting an inorganic-rich SEI and enabling >99% Coulombic efficiencies. 2–4 Yet, the mechanisms by which electrolyte components are consumed in SEI formation reactions and the nature of these reactions remains largely unknown, especially given recent literature suggesting that diluents in LHCEs may play a role in SEI formation 5,6 , which is in conflict with the previously accepted idea that diluents are inert. In this research, we employ electron paramagnetic resonance (EPR) spectroscopy with spin traps to study the radical intermediates that are generated during electrolyte reduction in LHCEs and HCEs. We reduce electrolyte within an electrochemical cell which also contains a spin trap. The EPR spectra of trapped radicals enables identification of key radical intermediates generated during SEI formation. We apply this approach to study high performance fluorinated electrolyte solvents, including 1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (TTE) and bis(2,2,2-trifluoroethyl) ether (BTFE), which have been employed as diluents in localized high concentration electrolytes (LHCEs). Further, we use 19 F and 7 Li solution NMR to probe changes in solvation structure and consumption of electrolyte components during cycling. Taken together, this work provides insight to the mechanisms that underpin SEI formation in high Coulombic efficiency systems, enabling next generation electrolyte design. References K. G. Gallagher et al., Energy Environ. Sci. , 7 , 1555–1563 (2014). X. Cao et al., Proceedings of the National Academy of Sciences , 118 , e2020357118 (2021). X. Cao et al., Nat Energy , 4 , 796–805 (2019). X. Fan et al., Chem , 4 , 174–185 (2018). R. May, J. C. Hestenes, N. A. Munich, and L. E. Marbella, Journal of Power Sources , 553 , 232299 (2023). Y. Zheng et al., J. Mater. Chem. A , 7 , 25047–25055 (2019).