# Pyrrolidinium and Imidazolium-Based Ionic Liquids as Electrolytes for Na0.67Ni0.33Mn0.67O2 Positive Electrode for Sodium–Ion Batteries

## TL;DR

This study compares pyrrolidinium and imidazolium-based ionic liquids as electrolytes for sodium-ion batteries, finding that they perform worse than traditional electrolytes but can still be improved at lower current densities.

## Contribution

The novel contribution is the evaluation of two specific ionic liquids with varying NaTFSI concentrations for their suitability in sodium-ion battery systems.

## Key findings

- BMPyr-TFSI showed better transport properties than BMMI-TFSI in neat ionic liquid form.
- Adding NaTFSI salt reduced the performance of both ionic liquids compared to carbonate electrolytes.
- Lower current densities improved performance, achieving over 53 mA h g–1 and 96% Coulombic efficiency at C/50.

## Abstract

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.