Exploring the use of EMImFSI ionic liquid as additive or co-solvent for room temperature sodium ion battery electrolytes

Sodium ion batteries (SIB) market is limited by their gravimetric and volumetric energies and therefore, search of high voltage redox couples combined with suitable electrolytes is nowadays mandatory. Ionic liquids can bring advantageous properties to SIB, particularly from a safe point of view, but are still quite expensive. In this pioneering work, we explore the use of small amounts of 1-ethyl-3-methylimidazolium bis(fluoromethanesulfonyl)imide (EMImFSI) ionic liquid in the electrolyte formulation, either as an additive (2% in weight ratio to a binary mixture of ethylene carbonate (EC)/propylene carbonate (PC)) or as co-solvent (10% in weight ratio). Results are compared to electrolytes containing classical SIB additive fluoro-ethylenecarbonate (FEC). Physicochemical properties of the electrolytes prepared with NaClO4, NaPF6 and NaFSI are differently influenced by the EMImFSI additive, which contributes with ionic species to the electrolyte ionic conductivity but increases its viscosity. Extension of the sodium plating/stripping is higher for more dissociated salt in un-doped electrolytes. Introduction of EMImFSI results in the occurrence of a SEI that hinders the plating except for the 10% EMImFSI-NaFSI based electrolyte. Electrochemical cycling of a hard carbon electrode reveals that FSI− anion, mainly coming from the added ionic liquid, can introduce new peaks in the cyclic voltammetry or new steps in the galvanostatic discharge. The products of such degradation create an electrode/electrolyte interface which determines the cycling properties. The complex picture of the interplay between sodium salt anion and EMImFSI presence and amount is not completely described in this work. However, it is clear that EMImFSI introduces initially a larger polarization in the discharge/charge curves and that only added to the NaPF6 system helps to stabilize the coulombic efficiency and gain in capacity (+50%). EMImFSI has not beneficial effect on the NaClO4-based formulations and the initial positive effect on the sodium half cells cycled in NaFSI-based electrolytes is cancelled after 25 cycles. Microscopy examinations on cycled electrodes reveal that coulombic efficiency and thus capacity retention are associated to a protective film completely recovering the hard carbon particles without modifying their pristine shape. This is the case for electrodes tested with un-doped NaClO4 and EMImFSI-NaPF6 electrolytes. Our results also show that compared with EMImFSI, FEC added in 2% weight ratio content provides much better or similar electrochemical performances at a lower price.