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 °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 °C), the IL electrolyte is more suitable than a conventional organic electrolyte for the sodium cell. At 75 °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.