5 V Stable Nitrile-Bearing Polymer Electrolyte with Aliphatic Segment as Internal Plasticizer

The electrochemical properties of polymer electrolytes based on poly(acrylonitrile-co-butadiene) and hydrogenated poly(acrylonitrile-co-butadiene) blended with lithium bis(trifluorosulfonyl)imide (LiTFSI) were characterized, showing that copolymerization of acrylonitrile with aliphatic comonomers is an effective means to develop conductive, yet oxidatively stable, polymer electrolytes. The copolymer compositions were both 40 wt % acrylonitrile, and the copolymer glass transition temperatures of NBR and HNBR were determined to be −17.7 and −24.3 °C, respectively. Differential scanning calorimetry (DSC) showed that the polymer electrolytes are amorphous when mixed with LiTFSI to a concentration of at least five nitrogen atoms for every one Li cation (N/Li = 5), and Fourier-transform infrared spectroscopy analysis of the nitrile stretch mode confirmed that the nitrile functional groups coordinate Li cations. At N/Li = 5, the ionic conductivity of HNBR:LiTFSI was determined to be 3.1 × 10–7 S/cm at 25 °C and 9.0 × 10–5 S/cm at 100 °C. In addition to dramatically enhancing the conductivity relative to polyacrylonitrile electrolytes, good oxidative stability was maintained. Linear scanning voltammetry showed that HNBR:LiTFSI was stable against oxidation to 5.4 V vs Li/Li+. Li–Li cell cycling and impedance evolution of the HNBR:LiTFSI–Li interface revealed that a substantial, yet stable, passivation layer is formed.