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.