What Makes Fluoroethylene Carbonate Different?

Abstract

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 Li x Si y 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.

Consumption of Fluoroethylene Carbonate (FEC) on Si-C Composite Electrodes for Li-Ion Batteries

“…The latter is reported as SEI component in the literature, but different formation mechanisms were proposed. 30,46 Li 2 CO 3 could also be formed by the reaction of Li 2 O with CO 2 , which might also explain why it was not considered an energetically feasible FEC reduction product in the calculations by Leung et al 28 Even though we currently have no experimental evidence for it, we believe that in accordance with the reports by Shkrob et al 49 and Markevich et al 46 a cross-linked 49 and oxygen-poor 46 polymer is formed. Therefore we assume that further direct reduction of the reactive ethenyl-radical (4 th electron transfer) yields lithium-ethenyl.…”

“…22,[26][27][28]30,31,36,[46][47][48][49] A mechanism which would be consistent with this β-value will be presented below. Another important aspect of the FEC consumption analysis in Si-Li vs. Li-Li cells is that the FEC consumption per electrode during charge/discharge cycling only depends on the overall exchanged coulombs.…”

“…30 Even numerous, partially contradicting mechanisms have been proposed, there is a common finding. In particular, LiF was observed or proposed independently by several authors, 22,[26][27][28]30,36,[46][47][48][49] whereby the study by Schroder et al indicates that the LiF content of the SEI is higher in the presence of FEC. 36 Formation of LiF also is consistent with our observations since we did not see any soluble fluorine containing decomposition products in the electrolyte by 19 F-NMR, even though we cannot exclude the formation of other fluoride containing solids.…”

In Situ DRIFTS Analysis of Solid Electrolyte Interphase of Si-Based Anode with and without Fluoroethylene Carbonate Additive

“…27 Schroder et al proposed that the electroreduction of FEC leads to the formation of LiF, Li 2 CO 3 , C 2 H 2 , LiF, CH 3 CH 2 OLi and methylenedioxyl ion. 25 Nie et al, using binder free Si-electrodes, proposed the formation of LiF and unstable organic radical species, which they suggested were transformed to either poly (alkenes) or poly (carbonate), 51 as reported by Nakai et al 58 and Chen et al 54 Shkrob et al argued that the reduction of FEC would produce a highly crosslinked network via the formation of vinoxyl radical, based on EPR experiments, 57 which is consistent with previous theoretical modelling by Leung et al 43 Recently, Jung et al proposed four electron reduction of FEC leading to CO 2 , LiF, Li 2 O, Li 2 CO 3 , H 2 and a partially cross-linked polymer. 28 We observed an onset potential of SEI formation corresponding to the reduction potential of FEC in the 1 st lithiation cycle, and similar SEI species continuously formed without onset potential in subsequent cycles.…”

Performance of Full Cells Containing Carbonate-Based LiFSI Electrolytes and Silicon-Graphite Negative Electrodes

“…18 An electrolyte salt that has generated significant interest in recent years is lithium bis (fluorosulfonyl) …”