Ionic liquid based electrolytes for sodium-ion batteries: Na+ solvation and ionic conductivity

Ionic liquid (IL) based sodium-ion (Na+) battery electrolytes obtained by mixing imidazolium-TFSI ILs (EMIm-TFSI and BMIm-TFSI) with the corresponding sodium salt (NaTFSI) have been investigated using a wide range of characterization techniques: dielectric spectroscopy, differential scanning calorimetry, densitometry, viscometry, and Raman spectroscopy. The sodium ion conducting electrolytes exhibit excellent ionic conductivities, up to 5.5 mS cm−1 at room temperature, and a useful thermal window of −86 °C to 150 °C. In more detail, Raman data analysis supported by DFT calculations on Na+-TFSI complexes, allow us to determine the sodium ion solvation and charge carrier nature as a function of salt concentration. The results are compared to data for the corresponding Li systems and while such electrolytes essentially form [Li(TFSI)2]− as the main Li+ carrier, the sodium systems seem to dominantly form [Na(TFSI)3]2− complexes. The effects on conductivity and viscosity and the consequences for sodium-ion battery implementation are discussed.