Ionic liquid electrolytes with various sodium solutes for rechargeable Na/NaFePO4 batteries operated at elevated temperatures

Nithinai Wongittharom, Chueh Han Wang, Yi Chen Wang, Cheng Hsien Yang, Jeng-Kuei Chang*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

65 Scopus citations


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.

Original languageEnglish
Pages (from-to)17564-17570
Number of pages7
JournalACS Applied Materials and Interfaces
Issue number20
StatePublished - 22 Oct 2014


  • ionic liquid electrolyte
  • Na solute
  • NaFePO4
  • sodium-ion battery
  • temperature

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