Introduction of Trifluoromethanesulfonamide Groups in Poly(ethylene oxide): Ionic Conductivity of Single-Ion-Conducting Block Copolymer Electrolytes

Abstract

Single-ion-conducting polymer electrolytes (SICPEs) based on polystyrene and poly(ethylene oxide) block copolymers (PS-b-PEO) are promising materials due to the combination of high stability and exclusive charge transport via lithium cations. However, the incorporation of covalently attached anions into the flexible polyether block to yield SICPEs is a synthetic challenge. For this purpose, a polystyrene-b-multifunctional poly(ethylene oxide) block copolymer precursor (Mn = 15.1 kg·mol–1, Đ = 1.09) with randomly distributed multiple hydroxyl functional groups (6%mol) in the polyether block was synthesized by the combination of living carbanionic and anionic ring-opening polymerization (AROP). First, living polystyryl lithium was end-functionalized with ethylene oxide (EO). Subsequently, AROP copolymerization of EO and ethoxy vinyl glycidyl ether (EVGE) was initiated by the alkoxide-functional polystyrene macroinitiator, followed by deprotection of the vinyl ether groups. A novel postpolymerization Mitsunobu reaction was developed to quantitatively substitute the hydroxyl groups with tert-butyloxycarbonyl (BOC)-protected trifluoromethanesulfonamide (TFSA) functionalities. The SICPE material was obtained in a following deprotection and deprotonation step of the TFSA groups. All reaction steps were monitored via detailed NMR, IR, and size-exclusion chromatography (SEC) characterization. The ionic conductivity of the obtained SICPE was compared and contrasted against the established dual-ion conductors PS-b-PEO doped with LiTf and LiTFSI. It was demonstrated to have superior ionic conductivity to PS-b-PEO/LiTf. In addition, the Li-ion conductivity is comparable to the single-ion block copolymer electrolyte poly(ethylene oxide)-b-poly(styrenesulfonyl(lithium trifluoromethylsulfonyl)imide) (PEO-b-PSLiTFSI) within the ordered state.

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