What Makes Fluoroethylene Carbonate Different?

Rechargeable lithium-ion batteries containing silicon-based negative electrodes have the potential to revolutionize electrical energy storage, but the cyclic and acyclic organic carbonate solvents (such as ethylene and propylene carbonates) that are commonly used in graphite Li-ion batteries yield unsatisfactory performance when used with such Li alloying electrodes. It has been found by trial and error that additions of the closely related carbonate additive, fluoroethylene carbonate (FEC), to conventional electrolytes yields a robust solid electrolyte interphase (SEI) on the LixSiy alloy surface. Several mechanisms for this protective action have been considered in the literature and modeled theoretically; however, at present these mechanisms remain hypothetical. In this study, we use radiolysis, laser photoionization, electron paramagnetic resonance, and transient absorption spectroscopy to establish the redox chemistry of FEC. While the oxidation chemistry is similar to that of other organic carbonates, the reduction chemistry of the fluorinated molecule is strikingly different. Specifically, one-electron reduction of bulk FEC causes the fission of two (instead of one) C-O bonds, resulting in concerted defluorination and decarboxylation; in contrast, the reduction of the ethylene and propylene carbonate results in ring-opening and the formation of a radical anion. For FEC, the reduction yields the vinoxyl radical that can abstract an H atom from another FEC molecule, initiating both the chain reaction causing FEC decomposition and radical polymerization involving the reaction products. The resulting polymer can further defluorinate yielding the interior radicals that migrate and recombine to produce a highly cross-linked network This feature implies that the outer SEI resulting from FEC reduction may exhibit elastomeric properties, which would account for its cohesion during expansion and contraction of silicon particles in the course of Li alloying/dealloying cycling.