Identifying the Structural Basis for the Increased Stability of the Solid Electrolyte Interphase Formed on Silicon with the Additive Fluoroethylene Carbonate

TL;DR

This study uses advanced NMR techniques to reveal how FEC additive enhances the stability of the SEI on silicon anodes by forming branched polymers, unlike the linear oligomers in standard electrolytes.

Contribution

It provides a detailed structural analysis of the SEI formed with FEC, showing the formation of branched polymers that improve stability, a novel insight into electrolyte additive effects.

Findings

FEC suppresses soluble PEO-like oligomers in the SEI.

FEC is defluorinated to form vinylene carbonate and vinoxyl species.

Branched ethylene-oxide polymers are formed with FEC, unlike in standard electrolytes.

Abstract

To elucidate the role of fluoroethylene carbonate (FEC) as an additive in the standard carbonate-based electrolyte for Li-ion batteries, the solid electrolyte interphase (SEI) formed during electrochemical cycling on silicon anodes was analyzed with a combination of solution and solid-state NMR techniques, including dynamic nuclear polarization. To facilitate characterization via 1D and 2D NMR, we synthesized 13C-enriched FEC, ultimately allowing a detailed structural assignment of the organic SEI. We find that the soluble PEO-like line- ar oligomeric electrolyte breakdown products that are observed after cycling in the standard ethylene carbonate (EC)-based electrolyte are suppressed in the presence of 10 vol % FEC additive. FEC is first defluorinated to form soluble vinylene carbonate and vinoxyl species, which react to form both soluble and insoluble branched ethylene-oxide based polymers. No evidence for branched polymers are observed in the absence of FEC.