Polar Organic Cations at Electrified Metal with Superconcentrated Ionic Liquid Electrolyte and Implications for Sodium Metal Batteries

Understanding the potential-induced changes across the electrode/electrolyte interface, the so-called electric double layer (EDL), is essential to adjust the working properties of energy-storage devices. Electrolytes with a high molar ratio of metal salt to solvent (1:1 salt:solvent), e.g., superconcentrated ionic liquids (ILs), enable uniform metal deposition, formation of stable solid-electrolyte interphase (SEI), and higher redox stability, which make them attractive for battery applications. However, the presence of an organic IL cation and its interactions with metal salt complexes can significantly impact the mechanism of charge transfer at an electrode compared with conventional ether/ester-based electrolytes. The competition between IL and metal cations to enter the electrified interface affects interfacial chemistry, a key determinant of metal deposition potential and the nature of the SEI. This, in turn, is also affected by IL cation and anion chemistries, which are not yet fully understood. This letter demonstrates that the polarity of an organic IL cation, which is expressed through its dipole moment (μ), and its redox stability can serve as a predictive descriptor for EDL structure in superconcentrated IL electrolytes and the implications for charge transfer. We showed that, in the family of pyrrolidinium cations, a less polar organic cation with a small μ, e.g. N-methyl-N-ethylpyrrolidinium [C2mpyr]+, packs tighter and in a greater number at a negatively charged electrode/electrolyte interface in comparison to more polar IL cations with greater μ, e.g. N-methyl-N-propylpyrrolidinium [C3mpyr]+ and N-methyl-N-methoxymethylpyrrolidinium [C2O1mpyr]+. This IL cation-rich interface results in a greater overpotential for Na deposition, whereas the nature of the SEI and sodium anode cycling behavior correlate with both the dipole moment and the reductive stability of the IL cation.