In state-of-the-art lithium ion batteries, energy dense anodes and cathodes are separated by tens of micrometers and are infiltrated with highly flammable, aprotic liquid electrolytes. The development of solid state batteries, where the flammable liquid electrolyte is replaced with a nonflammable (or significantly less flammable) solid electrolyte, is one approach to improving battery safety, but many material challenges remain. Polymer electrolytes are an important class of solid electrolytes due to their processability and low cost, which allows them to be blended readily with electrode powders and conductive additives and then cast into solid composite electrodes. However, archetypical polymer electrolyte compositions based on the ethylene oxide-bearing structural units are limited by relatively low oxidative stability of approximately 4V vs. Li/Li⁺. For high voltage cathodes, oxidatively stable yet conductive materials are required. Polyacrylonitrile (PAN) has high oxidative stability, but Li⁺ transport is limited in PAN due to its semicrystalline nature and high glass transition temperature (T_g > 95°C). In this study, hydrogenated poly(acrylonitrile_x-co-butadiene_y) (HNBR, T_g­ = -19°C) was investigated as a novel polymer electrolyte composition. The copolymer contains acrylonitrile units to solvate lithium cations, but the chain mobility is enhanced (T_g is reduced) and crystallinity is inhibited through the incorporation of butane residues in the polymer backbone. The objective of this study was to characterize the essential physical, chemical, and electrochemical properties of HNBR mixed with lithium salt in order to assess the practical utility of this material in solid state lithium ion batteries.

Hydrogenated nitrile butadiene rubber (HNBR) was donated by Zeon Chemicals, Inc. (Louisville, KY). HNBR was combined with bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and dissolved in acetone. Polymer electrolyte films were prepared by solution casting; the solution was evaporated at 50°C for 12h under a blanket N₂ flow for protection from moisture absorption. The resulting films was then vacuum-dried at 80°C for another 6h to completely remove residual acetone. The polymer chemistry (HNBR + LiTFSI) was characterized using Fourier transform infrared spectroscopy (FTIR). Ionic conductivities were measured using electrochemical impedance spectroscopy and electrochemical stabilities were assessed using linear sweep voltammetry. Differential scanning calorimetry was utilized to measure the glass transition temperature of the polymer-salt mixtures and reveal any crystallinity in the polymer electrolytes. Mechanical properties were measured using dynamic mechanical analysis.

FTIR analysis of the nitrile group’s vibrational stretch revealed that the HNBR interacts strongly with the Li⁺ cations of the salt, effectively solvating the salt. Consequently, large concentrations of salt were soluble in HNBR, down to a N/Li ratio of 5. It is likely that even higher salt concentrations were soluble, but they were not prepared. Because the Li⁺ is solvated by multiple nitrile groups, this complex acts as a physical crosslink, resulting in glass transition temperatures that are 2 – 6°C higher than in pure HNBR. Significant crystallinity was not observed, and the resulting films were highly flexible – capable of 200% tensile strain without fracturing. Lithium ion conductivities of HNBR reached 1x10^-7 S/cm at 20°C, which is comparable to conventional PEO-LiTFSI compositions and three to four orders of magnitude more conductive than dry polymer electrolytes based on polyacrylonitrile. The electrochemical stability of HNBR based electrolyte was measured with LSV and compared with corresponding PAN based electrolyte. The results suggest that HNBR electrolytes can withstand oxidation potential >5V. These results suggest that HNBR is a potential matrix for oxidatively stable, fully solid electrolyte (liquid/plasticizer-free) for lithium batteries.

This work was supported by the OPEN 2015 program of ARPA-E, U.S. Department of Energy, award DE-AR0000653.