Charge-storage performance of Li/LiFePO4 cells with additive-incorporated ionic liquid electrolytes at various temperatures

Butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) ionic liquid (IL) with LiTFSI solute is used as a base electrolyte for Li/LiFePO4 cells. Three kinds of electrolyte additive, namely vinylene carbonate (VC), gamma-butyrolactone (gamma-BL), and propylene carbonate (PC), with various concentrations are introduced. The thermal stability, flammability, and electrochemical properties of the electrolytes are investigated. At 25 degrees C, the additives (gamma-BL is found to be the most effective) can significantly improve the capacity, high-rate performance, and cyclability of the cells. With an increase in temperature to 50 degrees C, the benefits of the additives gradually become insignificant. At 75 degrees C, the additives even have adverse effects. At such an elevated temperature, in the plain IL electrolyte (without additives), a LiFePO4 capacity of 152 mAh g(-1) is found at 0.1 C. 77% of this capacity can be retained when the rate is increased to 3 C. These values are superior to those found for the additive-incorporated IL and conventional organic electrolytes. Moreover, negligible capacity loss is measured after 100 charge discharge cycles at 75 degrees C in the plain, IL electrolyte. (C) 2014 Elsevier B.V. All rights reserved.