Solid Polymer Electrolytes Based on Poly(vinylidene fluoride-trifluoroethylene) and the [N1 1 1 2(OH)][NTf2] Ionic Liquid for Energy Storage Applications

Rechargeable lithium-ion batteries take advantage of solid-state ionic conductors, such as solid polymer electrolytes (SPEs) [1, 2]. Despite the large technological potential of solvent-free SPEs, they suffer from a serious problem: a relatively low ionic conductivity. The addition of ionic liquids (ILs) to SPEs appears as a possible solution due to the enhancement of electrochemical stability (~4-6 V) and ionic conductivity (~10^-3 – 10^-2 S.cm^-1) [3].

ILs has two important features for their specific use in battery applications: high concentration of ions and high ion mobility at room temperature. The use of PVDF polymer and its copolymers together with ILs as SPEs still has to be thoroughly explored.

This work shows the thermal and electrochemical properties of SPEs based on poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), and specific amounts (4wt% to 32wt%) of the ionic liquid N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, [N_{1 1 1 2(OH)}][NTf₂]. The SPE was prepared by thermaly induced phase separation (TIPS).

The addition of ILs in P(VDF-TrFE) affects the microstructure, thermal stability and ionic conductivity of the polymer membrane.

The ionic conductivity increases as the ionic liquid (IL) content increases with a maximum value at room temperature of 1.7x10^-5 S.cm^-1for an IL composition of 32wt%. The temperature behavior of the ionic conductivity follows the Arrhenius equation and the electrical response of the membrane is well represented by a Randles modified equivalent circuit.  The results prove that the developed SPEs are adequate for energy storage devices.

Acknowledgments

This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2011, PEST-C/QUI/UI0686/2011, PEST-OE/EQB/LA0004/2011, projects PTDC/CTM-NAN/121274/2010 and PTDC/QUI-QUI/117340/2010, grants SFRH/BD/68499/2010 (C.M.C.) and SRFH/BD/90366/2012 (R.L.) and FCT Investigator grant (J.M.S.S.E.). The authors also thank funding from Matepro –Optimizing Materials and Processes”, ref. NORTE-07-0124-FEDER-000037”, co-funded by the “Programa Operacional Regional do Norte” (ON.2 – O Novo Norte), under the “Quadro de Referência Estratégico Nacional” (QREN), through the “Fundo Europeu de Desenvolvimento Regional” (FEDER). The authors thank Solvay for kindly supplying the high quality materials.

 References

[1]       P. Arora, Z. Zhang, Chemical Reviews 104(2004) (10) 4419.

[2]       J.M. Tarascon, M. Armand, Nature 414(2001) (6861) 359.

[3]       Y.-S. Ye, J. Rick, B.-J. Hwang, Journal of Materials Chemistry A 1 (2013) (8) 2719.