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

Abstract

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.

Phosphate Framework Electrode Materials for Sodium Ion Batteries

“…Ionic liquid electrolytes with various sodium solutes were tested for olivine NaFePO 4 by Chang's group. At elevated temperature (50 and 75 °C), the ionic liquid electrolytes assist NaFePO 4 in delivering higher capacity with stable cycle life 47, 48…”

A Safer Sodium‐Ion Battery Based on Nonflammable Organic Phosphate Electrolyte

“…NaFSI‐C 1 C 3 pyrFSI electrolyte is report to have good compatibility with a practical anode material such as hard carbon at 90 °C 47. And the cathode material (NaFePO 4 ) is also discussed in butylmethylpyrrolidinium−bis(trifluorometh‐anesulfonyl)imide (BMP−TFSI) ionic liquid, which performed a capacity of 152 mAh g −1 at 75 °C 48. A promising Na + ionic conductor based on crystalline NASICON compounds is reported to exhibit an initial discharge capacity of 330 and 131 mA h g −​1 for a hard carbon anode and a NaFePO 4 cathode at a 0.2C‐rate of room temperature 49.…”

Thermally-responsive, nonflammable phosphonium ionic liquid electrolytes for lithium metal batteries: operating at 100 degrees celsius

“…Although SPEs offer a number of important advantages, we are focused on ionic liquids as these materials usually possess higher ionic conductivity and better interfacial contact with the electrodes 17,19,30,31. Within this family of materials, imidazolium, piperidinium, and ammonium cations have been extensively studied for room temperature applications,24,30,32–34 however the investigation of their performance at elevated temperature is rare and satisfactory battery cycling above 60 °C has not been demonstrated 29,35,36. Phosphonium ionic liquids are less studied and exhibit both better chemical and thermal stability than imidazolium, pyrrolidinium or other nitrogen based ionic liquid cations, but with low conductivity at room temperature 33,37–41.…”