(Invited) Polymer Electrolytes Based on Ionic Liquids for PEMFC and Lithium Batteries
Authors: Iojoiu Cristina , Judeinstein Patrick , Lyonnard Sandrine 
Published: February 2020

Ionic liquids (IL) are considered as hi-tech new media with emerging applications as solvents for organic reactions or as electrolytes. The organization degree in these complex fluids is actually very challenging since it dictates microscopic as well as macroscopic properties. However in both electrochemical devises, i.e. the Proton Exchange Membrane Fuel Cells (PEMFC) and Lithium Polymer Batteries (LBP), polymer electrolytes with high conductivities and good mechanical properties are obtained by blending the IL with an appropriate polymer. We have shown recently that the membrane properties are guided by the IL structure, i.e. by its interactions with the host polymer1-5. We propose in this work a deep evaluation of the impact of IL structure on the membranes morphology and functional properties. The work is divided in two parts, discussing membranes for PEMFC and LPB.

In the PEMFC, one class of ionic liquids with interesting properties, i.e. Protic Ionic Liquids (PIL), is extensively used. Our study is focused more specifically on a series of PIL obtained by reacting alkylamine with various acids6. In this work we show that well-chosen PIL incorporated in the membrane can act as smart nanoscopic probes able to reveal interesting information on the polymer organization. As example, the morphology of Nafion® membranes doped with PIL based on triethylammonium (TEA) and perfluorosulfonates with varying perfluoroalkyl chain length (C1, C4, C8) was investigated by small-angle neutron scattering (SANS). The membranes were carefully designed by solution casting method. The evolution of the SANS spectra of C1-TEA containing membranes is characteristic to the membranes swollen with polar species e.g. water as revealed by the shift of the ionomer peak toward small angles. In contrast, the introduction of C4-TEA and C8-TEA ionic liquids into Nafion® matrix, even in pretty large quantities, does not result in structural modifications at the nanoscale. Depending on the PIL structure and concentration, the resulting doped membranes exhibit very different functional properties (conductivity, water uptake, gas permeability coefficients, etc.).

With respect with the safety, LPB solvent free polymer electrolyte films are very attractive in particular those made from poly(oxyethylene) (POE) host polymer. POE has outstanding ability to solvate lithium cation. It suffers however of various handicaps, namely (i) a high crystallinity that penalizes the electrolyte conductivity below the melting temperature (ii) a mechanical strength that vanishes following the polymer electrolyte melting and (iii) a stability in oxidation limited to roughly 3.9 V vs Li/Li+. In addition to the high performances such as the thermo mechanical, chemical, electrochemical properties, a special attention has to be paid to ion transport. The use of a battery based on solvent-free polymer electrolyte is partly conditioned by its melting temperature as the conductivity occurs mainly at the amorphous state. The architecture modification of POE is one of the routes to decrease the crystallinity but does not allow overcoming the bottleneck of a low cationic transference number, T+, in POE electrolytes. The use of IL such as tetraalkyl ammoniums is one of the routes to get sufficient conductivities at low temperatures but, once again, T+ remains low. Furthermore these ionic liquids, which benefit to the conductivity, do not participate in the electrode reactions. We have therefore directed our researches towards lithium-conducting ionic liquids. Hence, this work proposes to address these issues by designing new ionic liquid lithium salts containing the oxyethylene moiety (mPEG) and the perfluorosulfonate/ sulfonimide anion. Consequently, a large number of new ionic liquids were synthetized through original methods. The impact of (i) the number of oxyethylene units (ii) the different chemical structures and (iii) the anion on their intrinsic performances and those of the related polymer electrolytes was investigated.

High cation transference numbers were obtained in host polymers based on linear POE or cross-linked POE, and its values increase with the lengthening of the mPEG. The adding of mPEG chain on the anion leads generally to greater conductivity of polymer electrolytes that increases with increasing mPEG lengths.

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