Reaction Mechanisms of Fluoroethylene Carbonate Degradation, an Additive of Lithium-Ion Batteries, Unraveled by Radiation Chemistry

Numerous additives are used in electrolytes of lithium-ion batteries, especially for the formation of efficient solid electrolyte interphase at the surface of the electrodes. It is, therefore, necessary to elucidate the degradation processes of these compounds since it directly affects the lifetime of the battery. These mechanisms can be obtained through radiolysis. In this work, we investigated the degradation mechanisms induced by irradiation in fluoroethylene carbonate (FEC), a cyclic carbonate, which is an additive commonly used in lithium-ion batteries. The first reaction steps were studied by pulse radiolysis. At long timescales, the radiolytic yields of produced gases (H$_2$, CO, and CO$_2$) were quantified. Pulse radiolysis experiments evidenced the formation of the FEC$^{●-}$ radical anion, characterized by an absorption band centered ca. 430 nm. The radical anion is not detected when FEC is solubilized in other solvents: ethanol, diethylcarbonate, etc. This radical is indeed stabilized in neat FEC, whereas the ring opens to form more stable radical anions when FEC is a solute in other solvents, as confirmed by calculations. A multi-species deconvolution of the spectrum measured in pure FEC revealed a small absorption band centered around 560 nm, attributed to the solvated electron, decaying in ca. 100 ps. In neat FEC, excess electrons primarily undergo attachment compared to solvation. Together with gas chromatography coupled to mass spectrometry measurements, all these observations have allowed us to propose a reaction scheme for both the oxidizing and reducing pathways at stake in irradiated FEC. This work gives clues for the reaction mechanisms undergone by FEC present in electrolytes of lithium-ion batteries and evidences that the nature of the primary species formed in FEC depends on the amount of FEC in the solution