Thermally and Oxidatively Stable Polymer Electrolyte for Lithium Batteries Enabled by Phthalate Plasticization

Hydrogenated nitrile butadiene rubber (HNBR) blended with lithium bis(trifluorosulfonyl)imide (LiTFSI) has been shown to be stable up to 5.3 V vs Li/Li+, making it a promising solid polymer electrolyte material for state-of-the-art high voltage cathodes. However, its relatively low room temperature ionic conductivity (3.6 × 10–8 S cm–1) limits its application in practical lithium battery cells. To address this issue, phthalate esters were selected as plasticizers to improve the conductivity. The low volatility, low flammability, outstanding thermal stability, and expected high oxidative stability of phthalates make them a compelling class of plasticizers for HNBR-based polymer electrolytes. A homologous series of dialkyl phthalate esters blended with HNBR/LiTFSI was studied. The oxidation potential of the plasticized HNBR/LiTFSI exceeded 5.2 V vs Li/Li+, demonstrating that phthalates do not substantially compromise the oxidative stability. The conductivity of the electrolyte (N/Li = 5) was 4.4 × 10–5 S cm–1 at 70 °C. FTIR revealed that phthalate esters not only increase segmental mobility of the polymer chains but also participate in the solvation of lithium salt by coordinating Li+. Electrochemical impedance analysis showed that a resistive interface developed between HNBR-based electrolytes and lithium metal, indicating a chemical incompatibility between HNBR and lithium metal. However, this problem was readily addressed through a laminated electrolyte structure where a layer of PEO/LiTFSI physically separated plasticized HNBR/LiTFSI from direct contact with lithium metal. The effectiveness of this structure was confirmed by over 2000 h of reversible galvanostatic cycling of symmetric lithium metal electrodes at 70 °C.