Pyrrolidinium and Imidazolium-Based Ionic Liquids as Electrolytes for Na _0.67 Ni _0.33 Mn _0.67 O ₂ Positive Electrode for Sodium–Ion Batteries

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 Na_0.67Ni_0.33Mn_0.67O₂ (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.