Lithium polymer batteries (LPBs) are considered excellent candidates for the next generation power sources since their high energy density and flexible characteristics. Nevertheless, the performance of LPBs is limited by the low ionic conductivity of the solvent-free polymer electrolytes at room temperature. A very promising approach for overcoming this drawback is represented by the incorporation of ionic liquids (ILs) into the polymer electrolytes. In the last years, it was successfully demonstrated [1] that incorporation of N-alkyl-N-methylpyrrolidinium perfluorosulfonylimide ILs enhances the room temperature ionic conductivity of solid polymer electrolytes (SPEs) above 10-4 Scm-1 with a good compatibility with respect to the lithium anode even after prolonged storage time.
However, faster transport properties are required, especially in practical devices operating at high current rates. In the frame of the MARS-EV project we have developed SPEs based on the polyethyleneoxide (PEO) or the polymethylmetaacrylate (PMMA) and incorporating the ionic liquid N-methyl-N-propylpyrrolidinium bis (trifluoromethanesulfonylimide, PYR13FSI. This latter material is well known for its high ion conduction, low melting point and good-film forming ability on lithium and graphite anodes [2]. Also, the addition of proper additive (EC) in small amount, aiming to improve the interface with graphite anodes, was considered.
Results and Discussions
The PEO-LiTFSI-PYR13FSI and PMMA-LiTFSI-PYR13FSI electrolytes were prepared accordingly to a solvent-free procedure route reported elsewhere [1]. PYR13:Li mole ratio was ranged from 0.5 to 7.0.
Appreciable mechanical properties were exhibited even at high ionic liquid contents (Figure 1) in conjunction good chemical stability as indicated by the absence of feature change of the AC plots upon prolonged storage periods (Figure 2). The impedance measurements evidenced a progressive reduction of the electrolyte resistance during the initial storage days, likely due to internal structural reorganization of the SPE samples, followed by stable ion conduction upon long period times.
The incorporation of PYR13FSI results in large conductivity increase, especially at room temperature or below. For instance, conduction values approaching 10-3 S cm-1 and overcoming 10-4 S cm-1 at 20 and -20 °C, respectively, were detected. In addition, the IL addition was seen to prevent the polymer host crystallization, in good agreement with the DSC results.
Acknowledgements
The authors wish to thank the financial support of the European Commission for the MARS-EV project within the 7th Framework Program (Grant agreement n°: 609201).
References
[1] S. Passerini, M. Montanino, G.B. Appetecchi, Lithium Polymer Batteries based on Ionic Liquids, in
Polymers for Energy Storage and Conversion, Vikas Mittal editor, John Wiley and Scriverner Publishing, USA, 2013.
[2] G.B. Appetecchi, M. Montanino, S. Passerini, Ionic Liquid-based Electrolytes for High-Energy Lithium Batteries, in Ionic Liquids Science and Applications, ACS Symposium Series1117, A.E. Visser, N.J. Bridges and R.D. Rogers editors, Oxford University Press, Inc., American Chemical Society, Washington, DC, USA, 2013.