Sodium-ion electrolytes based on poly-ethylene oxide oligomers in dual-carbon cells: Anion size drives the charging behavior

Sodium-ion dual-carbon electrochemical cells are an environmentally friendly technology for energy storage purposes. In such systems, the migration of mutual ions in both electrodes is crucial for performance and determines the cell’s behavior. To investigate this effect, we have performed force-field molecular dynamics simulations of dual-carbon sodium-ion systems containing poly-ethylene oxide (PEO6) as the solvent and two different anions, bis(trifluoromethanesulfonyl)imide (TFSI−) and bis(fluorosulfonyl)imide (FSI−). We used a constant potential model that allows simulating the charging behavior of these cells at the potential differences, ΔΨ=1, 2, 3, and 5 volt (V). Our results showed that considering the neat solution, NaTFSIPEO6 exhibited the best transport properties. However, NaFSIPEO6 is a better choice of electrolyte to be employed in dual-carbon cells since it provides higher and faster charge accumulation. Structural analysis also confirmed that the system containing NaFSIPEO6 stored more ions inside their respective electrodes with Na+ cations having fewer molecules composing their first solvation shell. Overall, our findings suggest that the smaller molecular volume of the FSI− anion is the key factor to properly fill the positive electrode, and the NaFSIPEO6 combination is the best choice for electrolytes of dual-carbon cells.