Identifying Electrolyte Reduction Intermediates in Lithium Metal Batteries with Spin Trapping

Despite promising developments in liquid electrolyte engineering for lithium metal batteries, solid electrolyte interphase (SEI) formation mechanisms from electrolyte components remain challenging to study. Here we establish an approach employing spin trapping to identify radical intermediates formed during electrolyte reduction. We use electron paramagnetic resonance spectroscopy to identify three distinct radical intermediates formed during fluoroethylene carbonate reduction that confirm a ring-opening mechanism. Our spin trapping approach further enables confirmation of radical identity with liquid-chromatography mass spectrometry and nuclear magnetic resonance spectroscopy. The resulting SEI formed in the spin trapped system is investigated with titration and spectroscopic techniques to observe how stunting organic SEI formation reduces SEI stability and increases anion decomposition. Overall, this is the first time radical intermediates in battery interphase formation reactions have been directly characterized, affording insight into the complex mechanisms of electrolyte reduction and establishing a new approach for studying SEI formation and function.