title: 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.
authors.affiliation: Department of Chemical and Materials Engineering, National Central University , Jhongli 32001, Taiwan.
authors.forename: Nithinai
authors.initials: N
authors.lastname: Wongittharom
authors.forename: Chueh-Han
authors.initials: CH
authors.lastname: Wang
authors.forename: Yi-Chen
authors.initials: YC
authors.lastname: Wang
authors.forename: Cheng-Hsien
authors.initials: CH
authors.lastname: Yang
authors.forename: Jeng-Kuei
authors.initials: JK
authors.lastname: Chang
keywords: Na solute; NaFePO4; ionic liquid electrolyte; sodium-ion battery; temperature