Ionic liquids (ILs) are novel electrolyte systems which have the potential to be compatible to support the electrochemistry of sodium while replacing commonly used organic solvent electrolytes, thus improving the safety of such devices.
Here the physicochemical properties of three bis(fluorosulfonyl)imide (FSI) based ionic liquid electrolytes with small alkylphosphonium cations or an alkyletherammonium cation (P111i4FSI, P1i4i4i4FSI, N2(2O2O1)3FSI) were investigated including the thermal properties, ionic conductivity and viscosities. The phosphonium ILs showed particularly high conductivities (0.94mScm-1 for P111i4FSI at 50°C) while also having the highest decomposition temperatures (320°C). All IL solutions with NaFSI showed very stable Na symmetric cell cycling with low overpotentials (P111i4FSI:NaFSI 100mV at 1mAcm-2) making them promising electrolytes for sodium batteries.
The electrolytes were cycled against NaPO4 (NFP) cathodes and layered transition metal oxides (P2-Na2/3[Fe2/3Mn1/3]O2, O3-Na2/3[Fe2/3Mn1/3]O2, P2-Na2/3[Mn0.8Fe0.1Ti0.1]O2). The highest stability and capacity was observed for the P111i4FSI electrolyte, particular with the O3-Na2/3[Fe2/3Mn1/3]O2 cathode. Surprisingly the NFP electrodes showed very stable cycling.
On selected cells post-cycling analysis was performed. While the electrolyte is not compromised, there is spectroscopic evidence for the formation of a stabilising solid electrolyte interface on the sodium metal anode as well as on the cathode. In addition the cathode morphology changes.
In summary ionic liquid electrolytes match or out perform conventional organic solvent electrolytes in terms of performance at elevated temperatures, capacity, cycle stability and safety.