Enhancement of Lithium-Ion Conductivity in Liquid Crystalline Block Copolymer Electrolyte by Electric Field Alignment

Abstract

A block copolymer electrolyte (BCPE) with a liquid crystal and a lithium-ion conductive phase is investigated to assess the influence of an external applied electric field on the bulk morphology and the resulting electrochemical performance. For this purpose, the controlled synthesis of poly(10-[(4-cyano-4′-biphenyl)oxy] decatyl methacrylate)-block-(methoxy-poly(ethylene glycol) methacrylate-co-glycidyl methacrylate) [P(MALC)-b-P(PEGMA-co-GM)] block copolymer is performed by reversible addition–fragmentation transfer polymerization. The BCPE containing lithium bis(trifluoromethanesulfonyl)imide as the salt is drop-cast and crosslinked inside an alternating or direct current electric field. Transmission electron microscopy and small-angle X-ray scattering are utilized to study the phase behavior of BCPE and assess the influence of the electric field on the spatial orientation of the microdomains. A hierarchical lam-in-CYL nanostructure with the perpendicular orientation of the mesogenic smectic layers (lam) with respect to the BCPE cylinders (CYL) long axis is identified. Interestingly, the BCPE cast with electric field treatment gives rise to highly ordered cylindrical structures in comparison to the same BCPE without electric field treatment, which in turn exhibits a poorly ordered worm-like morphology. Consequently, a consistent improvement of the ionic conductivity is observed for the electric field-treated polymer, reaching ionic conductivities up to 4.7·10^–5 S·cm^–1 at 60 °C, compared to 6.1·10^–6 S·cm^–1 at the same temperature for the polymer electrolyte cast without an electric field. Surprisingly, for most of the investigated systems, the BCPE microstructure aligns perpendicular to the applied stimuli, which is explained by the movement of the whole liquid crystalline layer rather than by individual mesogen reorientation.