Safe sodium‐ion battery using hybrid electrolytes of organic solvent/pyrrolidinium ionic liquid

Ionic liquids (ILs) have been considered as an alternative class of electrolytes compared to conventional carbonate solvents in rechargeable lithium/sodium batteries. However, the drawbacks of ILs are their reducing ionic conductivity and their large viscosity. Therefore, mixtures of alkyl carbonate solvents with an IL and a sodium bis(trifluoromethane sulfonyl)imide (NaTFSI) have been investigated to develop new electrolytes for sodium-ion batteries. In this work, N-Butyl-N-methylpyrrolidinium bis(trifluoro-methanesulfonyl) imide (Py14TFSI) was used as co-solvent mixing with commercial electrolytes based on the carbonate, i.e. EC-PC (1:1), EC-DMC (1:1), and EC-PC-DMC (3:1:1). The addition of ionic liquid in the carbonate-based electrolyte solution results in (i) enhancing ionic conductivity to be comparable with a solvent-free IL-based electrolyte, (ii) maintaining the electrochemical stability window, and (iii) IL acted as a retardant rather than a flame-inhibitor based on the self-extinguish time (SET) of the mixed electrolyte mixture when exposed to a free flame. All mixed electrolyte systems have been tested in sodium-coin cells versus Na0.44MnO2 (NMO) and hard carbon (HC) electrodes. The cells show good performances in charge/discharge cycling with a retention > 96 % after 30 cycles (∼90 mAh.g-1 for NMO and 180 mAh.g-1 for HC, respectively) demonstrating good interfacial stability and highly stable discharge capacities.