Ionic Liquid Electrolytes with Various Sodium Solutes for Rechargeable Na/NaFePO4 Batteries Operated at Elevated Temperatures

NaFePO4 with an olivine structure is synthesized via chemical delithiation of LiFePO4 followed by electrochemical sodiation of FePO4. Butylmethylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (BMP-TFSI) ionic liquid (IL) with various sodium solutes, namely NaBF4, NaClO4, NaPF6, and NaN(CN)(2), is used as an electrolyte for rechargeable Na/NaFePO4 cells. The IL electrolytes show high thermal stability (>350 degrees C) and nonflammability, and are thus ideal for high-safety applications. The highest conductivity and the lowest viscosity of the electrolyte are obtained with NaBF4. At an elevated temperature (above 50 degrees C), the IL electrolyte is more suitable than a conventional organic electrolyte for the sodium cell. At 75 degrees C, the measured capacity of NaFePO4 in a NaBF4-incorporated IL electrolyte is as high as 152 mAh g(-1) (at 0.05 C), which is near the theoretical value (154 mAh g(-1)). Moreover, 60% of this capacity can be retained when the charge-discharge rate is increased to 1 C.