A Novel Electrolyte for Lithium-Ion Batteries: Li-Doped N-Methoxyethyl-N-Methylpyrrolidinium Fluorosulfonyl-(trifluoromethanesulfonyl)Imide (PYR12O1 FTFSI) Ionic Liquid

The lithium ion battery (LIB) technology would greatly benefit from the replacement of the highly flammable, thermally unstable and toxic electrolytes currently used. The employment of ionic liquids (ILs) will lead to a safety enhancement as they are non-flammable and non-volatile and possess excellent thermal and electrochemical stabilities [1]. Moreover the modulation of their characteristics by cation-anions combinations permits to design electrolytes for specific LIBs applications. Electrolytes based on the coupling of pyrrolidinium cations with imide anions, mainly bis(trifluoromethanesulfonyl)imide (TFSI) and bis(fluorosulfonyl)imide (FSI), have gained increased attention because they exhibit, among other ILs systems, very large electrochemical stability windows. However, the utilization of these ILs in electrochemical devices is hindered by their tendency to crystallize that limits the working temperature range [1].

To overcome this problem, our group recently synthesized and characterized a new ionic liquid, N-methoxyethyl-N-methylpyrrolidinium fluorosulfonyl-(trifluoromethanesulfonyl)imide (PYR_12O1FTFSI), based on the introduction of an ether functionality in the ammonium cation side chain and the coupling with the asymmetric imide anion which combines the benefits of FSI and TFSI [2-4]. The enhanced asymmetry prevents any detectable crystallization down to -150°C while maintaining conductivity values suitable for lithium ion- battery applications.

The electrochemical performance of PYR_12O1FTFSI doped with LiFTFSI salt is presented. The electrolyte show no evidence of crystalline phases even at low temperatures, possess high ionic conductivity (3.7 *10^-3 S cm^—1 at 20°C) and wide electrochemical stability window (up to 5V). The new electrolyte has been tested in combination with aqueous processed electrode materials showing good electrochemical performance even at room temperature. The electrochemical performance of lab-scale, lithium-ion cells comprising Li₄Ti₅O₁₂ / PYR_12O1FTFSI-LiFTFSI / LiFePO₄ are also presented. The cells exhibit stable cycling behavior at room temperature (with an energy density of 130Wh kg^-1), and improved cyclability at higher temperature (up to 80°C) with respect to conventional electrolytes, demonstrating the feasibility of PYR_12O1FTFSI-based ionic liquids as electrolyte for safer and greener lithium ion batteries.

 

Acknowledgement

This work was supported by BMBF within the project "MEET Hi-END - Materialien und Komponenten für Batterien mit hoher Energiedichte" (Förderkennzeichen: 03X4634A). 

 

References

[1]        G. B. Appetecchi, M. Montanino, S. Passerini, in Ionic Liquids: Science and Application (Eds.: A.E. Visser, N.J. Bridges, R. D. Rogers), American Chemical Society, 2012, pp. 67–128

[2]        J. Reiter, E. Paillard, L. Grande, M. Winter, S. Passerini, Electrochimica Acta 2013, 91, 101–107.

[3]        J. Reiter, S. Jeremias, E. Paillard, M. Winter, S. Passerini, Phys. Chem. Chem. Phys. 2013, 15, 2565–2571.

[4]        G. A. Giffin, N. Laszczynski, S. Jeong, S. Jeremias, S. Passerini, J. Phys. Chem. C 2013, 117, 24206–24212.