In this paper, the electrochemical performance of two nitrogen-based ionic liquids (ILs), 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPyr-TFSI) and 2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide (BMMI-TFSI), with different concentrations of NaTFSI, as electrolytes for the Na0.67Ni0.33Mn0.67O2 (NNM) positive electrode for sodium-ion batteries (SIBs) were compared with the conventional 1.0 mol L-1 NaClO4 in carbonate electrolyte. Moreover, the influence of salt concentration on the physicochemical properties of both ILs was evaluated. Amidst the neat ILs, BMPyr-TFSI showed better transport properties than BMMI-TFSI, whereas, for NaTFSI-mixtures, adding salt was detrimental to the ILs' properties. The poorer transport properties of the ILs compared to those of the carbonate electrolyte negatively impact the NNM electrode performance. At C/10, the highest discharge capacity obtained in IL mixtures was 40 mA h g-1 for BMPyr-TFSI with 0.5 mol L-1 of NaTFSI, compared to 59 mA h g-1 for NNM in NaClO4 electrolyte. Lowering the current density improved the performance of NNM in both BMPyr and BMMI-based mixtures, achieving specific capacities and Coulombic efficiencies above 53 mA h g-1 and 96%, respectively, at C/50. This approach has proven effective in overcoming the kinetic limitations due to the poorer transport properties displayed by ILs, encouraging the implementation of these electrolytes in SIBs.
Domingues et al. (Wed,) studied this question.