Highly Homogeneous Sodium Superoxide Growth in Na–O₂ Batteries Enabled by a Hybrid Electrolyte

Abstract

Energy storage is a major challenge for modern society, with batteries being the prevalent technology of choice. Within this area, sodium oxygen (Na–O2) batteries have the capability to make a step change, thanks to their high theoretical energy density. In order to facilitate their use, the development of electrolytes is critical to overcome certain limitations that arise because of the technology’s unique chemistry, particularly relating to the stability of superoxide species. In this study, we have demonstrated the importance of selecting a suitable electrolyte to facilitate both a highly homogeneous distribution of the discharge products and to minimize the formation of undesirable reaction products. The combination of pyrrolidinium-based ionic liquid and diglyme can dramatically change the cell performance. The effect of sodium salt concentration as well as the amount of diglyme and N-butyl-N-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide, [C4mpyr]­[TFSI], in Na–O2 batteries has also been comprehensively studied by combination of experimental and simulation techniques.

Unravelling the Role of Speciation in Glyme:Ionic Liquid Hybrid Electrolytes for Na−O2 Batteries

“…The types of coordination were predicted by calculating the radial distribution function (RDF), which shows the probability of finding an atom/molecule at a certain distance from another atom centre when one of them is at the origin. The Na + coordination numbers (CN) also were calculated as in our previous work [13] . In summary, since the 16.6 mol %/IL:DGME system has excess DGME, most Na + ions are solvated by two DGME molecules – [Na(DGME) 2 ] + –, although there is a small amount of TFSI also participating in the Na + nearest to the coordination shell, as shown in Figure 7a.…”

“…Thus, both experimental (FTIR, NMR) and computational approaches are exploited, also considering the presence of superoxide anion which is normally overlooked in the literature. The research presented here provides further information on the reason behind the growth mechanism of the discharge products, building on the battery performance obtained from our previously reported work, [13] and developing the basis of electrolyte design needed to underpin future research in the field.…”

“…The Na + coordination numbers (CN) also were calculated as in our previous work. [13] In summary, since the 16.6 mol %/IL:DGME system has excess DGME, most Na + ions are solvated by two DGME molecules -[Na(DGME) 2 ] + -, although there is a small amount of TFSI also participating in the Na + nearest to the coordination shell, as shown in Figure 7a. In the 35 mol %/IL:DGME system, since the number of TFSI molecules relative to DGME increases, the Na + coordination environment becomes diversified, including [Na…”

“…Our recent research reported the electrochemical performance of NaÀ O 2 batteries using two different salt concentrations (16.6 (0.5 m) and 35 mol % (2.8 m) NaTFSI) in the ([C 4 mpyr] [TFSI]:DGME) hybrid electrolyte, which in both cases showed longer cyclability despite the lower capacity when compared with the pure glyme system (Figure 1). [13] The less concentrated electrolyte mixture led to larger and more homogenous discharge products. Additionally, fewer side products were observed in the less concentrated electrolyte, which is essential for battery reversibility.…”

“…NaF signal around 0 ppm used as reference and (d) peak width of the different electrolytes studied. Adapted figure from Ref [13]. with permission.…”