Gel Polymer Electrolyte Made of an Amorphous Polyacrylonitrile-Based Elastomer

Abstract

Lithium-ion batteries (LiBs) are a mature technology which has attracted much attention over the last three decades with the development of portable devices. With the increasing number of LiBs and their use in electric vehicles, there are persistent efforts to enhance their safety by identifying suitable substitutes for liquid non-aqueous electrolytes, which are highly flammable, volatile, and can cause leakage. To this end, gel polymer electrolytes (GPEs) represent a promising alternative. To be suitable in applications and maximize ionic conductivity, polymers must meet specific requirements, the most important ones being a low glass transition temperature (Tg) and a low crystallinity, as ions move faster in amorphous phase. Moreover, these polymers need to be electrochemically stable and provide mechanical resistance as well as flexibility to the GPE. To increase the conductivity, polymers with polar functional groups that can dissolve salts are required. Several polymers, like PEO or PAN have already been studied but one limitation is their high Tg (above ambient temperature) as well as a high crystallinity which make them brittle and limit the ionic conductivity.To circumvent this limitation, we studied an amorphous polymer based on acrylonitrile monomers. This elastomer, HNBR (Hydrogenated Nitrile Butadiene Rubber), was previously studied by our group and was shown to be thermally cross-linkable, leading to a polymer chemically resistant to electrolytes and electrochemically stable over a wide potential range. Hence, we propose to investigate thermally cross-linked HNBRs to gain a better understanding on the effect of interactions between polar nitrile functions and lithium ions and finally use this system as a GPE for Li-ion battery. To do so, a three-component system comprising HNBR:solvent:LiTFSI was studied to pinpoint the correct ratio to provide the GPE with competitive conductivity. Infrared spectroscopy was used to shed light on the interactions between nitriles and lithium ions while PFG-NMR was utilized to obtain spin-lattice relaxation times (T1) and diffusion coefficients of 7Li and 19F for various HNBR-based GPEs. These results were correlated to EIS measurements and among the GPEs tested, those composed of 2M LiTFSI in propylene carbonate and HNBR with an acrylonitrile content of 50% are the most promising. This study highlights the benefits of high acrylonitrile content in the polymer and the use of a solvent with moderate donor number to promote interactions between nitriles and Li+.