Radical Species Emerging Prior to Electrolyte Degradation
Hristo Rasheev, Georgi Vassilev, Radostina Stoyanova and Alia Tadjer
Abstract: Electrolytes are key components of rechargeable metal-ion batteries as they allow the transfer of charge-carriers from one electrode to the other. Suitable electrolytes have to satisfy a few essential requirements, the most important being: a high ion conductivity and a wide electrochemical stability window. Preliminary experimental results on standard electrolytes, indicate that prior to degradation, carbonate-based solvents form stable radicals under both oxidative and reductive conditions. Additives such as vinylene carbonate (VC), ceria (CeO2), or both, influence the concentration and lifetime of the generated radicals: VC plays a significant role upon discharge and ceria mainly enhances the radical concentration during the charge stage. Interestingly, the EPR spectra upon oxidation and reduction are almost identical, although different radical species are expected to be formed. In order to uggest an interpretation of the experimental findings, we used the Density Functional Theory (DFT) to calculate the isotropic g-factors of various radicals originating from the solvent molecules (ethylene carbonate (EC) and dimethyl carbonate (DMC)), and from VC. The effect of CeO2 and the electrolyte ions (Li+ and PF6¯) was also estimated. The most likely candidates behind the experimental EPR signal are identified and a two-step mechanism of radical
formation upon oxidation is proposed.
Acknowledgements: The authors appreciate the funding provided by the Bulgarian National Science Fund, project CARiM-VIHREN, grant number КП06-ДВ-6/16.12.2019.
The authors from CARiM’s Research Team are bolded.
VIHREN-CARiM 2019 