Redox processes in electrolytes for rechargeable batteries
G. Vassilev, H. Rasheev, A. Tadjer, R. Stoyanova
Abstract: The electrolyte LP30 is composed of 1M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with molar ratio 1:1 and is widely used in lithium-ion batteries (LIBs). On the other hand, propylene carbonate (PC) is the main solvent choice for sodium-ion batteries (SIBs) combined with NaPF6 as a salt. Preliminary experimental evidence suggests that during both the oxidative and reductive phases, within the electrochemical stability window (ESW) of the LP30 solvents, stable radicals are formed. Surprisingly, they are characterized with different concentration but identical electron paramagnetic resonance (EPR) signal in both stages, which implies that the same radical is formed. The results obtained for the electrolyte suitable for SIBs are different. In this case the EPR signals during the oxidative and reductive scans do not match, suggesting the presence of different radical species. In this research we used Density Functional Theory to provide interpretation of the observed EPR spectra and outline the main differences between the behavior of EC and PC. The gfactors of various radicals were calculated and the effects of traditional additives were also examined. All cation-radicals are unstable and they would deprotonate to a neutral radical whose g-value is similar to the experimentally found one. However, the lack of hyperfine structure in the sample containing LP30 refutes this possibility which led us to the assumption that an anion-radical should be formed even in the oxidative stage, due to electron leakage at the electrode surface. Homo- and hetero-dimerization of the reduced cyclic carbonates and the complex formation, as a result of Li+ and Na+ solvation, were also investigated.
Acknowledgement: The study is supported by Project CARiM/Vihren, grant KP-06-DV-6/2019
The authors from CARiM’s Research Team are bolded.
VIHREN-CARiM 2019 