₁₃C Solid State NMR Suggests Unusual Breakdown Products in SEI Formation on Lithium Ion Electrodes

This study investigates the organic components of the solid electrolyte interphase (SEI) on the carbonaceous anodic electrode in LiCoO₂batteries using solid-state nuclear magnetic resonance (NMR) techniques. The electrolyte solvents, ethylene carbonate and diethyl carbonate, were enriched on the carbonyl carbon prior to cell assembly and conditioning in an attempt to trace the breakdown pathways of these components. A strong signal was seen, demonstrating that the carbonyl carbons of both electrolytes survive in some reasonable quantity as insoluble breakdown products on the rinsed electrode surface. This confirms that the fate of these carbons as consumed in the formation of CO₂is either not unique, or not as final product. Furthermore the central carbonyl carbon survives, not as an intact carbonyl species, but instead in the form of compounds in which the carbon-oxygen double bond has been disrupted. Formation of this class of species is likely initiated by a nucleophilic attack on the carbonyl carbon by one or more radical, alkoxy, carbanion or fluorine-containing ionic species formed from solvent and/or salt breakdown. These results suggest a new family of electrolyte breakdown products, predominantly consisting of binary, tertiary and/or quaternary ether-type compounds (i.e., orthocarbonates and orthoesters), as well as fluorine-containing alkoxy compounds.