The electrolytes for lithium batteries, are an important area of research as it is believed that small changes in either the composition or the concentration  is expected impact performance. Current electrolytes are limited in terms of their thermal stability, operation temperature window, metal dissolution and high potential operation. Room temperature Ionic liquids (RTILs) are potential electrolyte ingredients as they can exhibit better electrochemistry in a wider potential window and high thermal stability¹. However their use is hampered by the interfacial issues with graphite electrodes and also their viscosity². The current study is focused towards electrolytes being a mixture of ionic liquids in conventional electrolyte components³. The studies were carried out with an optimized amount of ~40%   N-Methyl-N-Propyl Pyrrolidinium bis (trifluoromethanesulfonyl) imide[C₃mpyr][TFSI] in a mixture of LiPF₆and the alkyl carbonates, also known as Hybrid Electrolytes(HE), and was focussed towards understanding the synergistic benefits evolved at the interface upon ionic liquid addition. The viscous effects of ionic liquids were alleviated by the alkyl carbonates in the solution.

As the ionic liquids, are known for their thermal stability, their addition produces a positive impact on the thermal stability of the hybrid electrolytes. Lithium symmetric cell studies were carried out in the conventional electrolyte (LP30), pure ionic liquid and the hybrid combination to look into the synergy brought by this combination. The combination lowers the overpotential in the symmetric cells as shown in Figure 1a. The combination also widens the temperature of operation of these batteries both on the low and high temperature scales. Further studies were carried out of the electrochemical performance of hybrid electrolytes with commercial battery cathodes where the performance of these combination gives better performance than the conventional system. The effect of metal dissolution on LiFePO₄cathodes on these electrolytes were compared upon storage, and a clear distinction was observed on the separators from the aged cells dismantled after 30 days as shown in the Figure 1b & 1c inset. The EDAX analysis of such separators also confirms the traces of iron as shown in Figure 1b&1c.

Further morphological studies were also carried out on delithiated cathodes and it appears that the ionic liquid forms a protective layer on the cathode which helps in achieving a better cyclic stability of the electrodes. On the whole, addition of ionic liquid in the alkyl carbonate system helps in the thermal stability, interfacial kinetics and better cyclic stability.

References

1.    MacFarlane, D. R. et al,Acc. Chem. Res. 40, 1165–1173 (2007).

2.    Navarra, M. A., MRS Bull. 38, 548–553 (2013).

3.    Theivaprakasam, S., MacFarlane, D. R. & Mitra, S.,Electrochim. Acta 180, 737–745 (2015).