Liquid crystalline lithium-ion electrolytes derived from biodegradable cyclodextrin

Abstract

The first family of lithium-ion electrolytes based on thermotropic liquid crystalline (LC) cyclodextrin (CD) is reported. The new electrolytes consist of composites of self-assembling and environment-friendly amphiphilic β-CDs with lithium bistrifluoromethanesulfonimidate (LiTFSI). The unique geometry of the CD scaffold allows for efficient control of the amphiphilicity of the molecule through chemical derivatizations. The introduction of non-polar alkyl chains at the primary face of CD and multiple short and polar nitrile-terminated tetraethylene glycol chains at the other face provides a new class of LC materials capable of self-organization into homeotropically-aligned smectic mesophases, which retain long-range order over a wide temperature window. The assembly of numerous ethoxy groups at the secondary face of CD serves as an elegant pre-organization of the ion-chelating groups to form two-dimensional ion-conducting pathways, facilitating the directional motion of metal ions. Studies of several LC composites with LiTFSI have revealed promising thermal stability and ionic conductivity. Solid-state NMR studies have uncovered an activation energy of 0.14 eV for lithium diffusion, while cyclic voltammetry confirmed the composites remain electrochemically stable over a potential range of up to ∼3 V versus Li/Li⁺. These results demonstrate the great potential of this new class of organic electrolytes for different metal ions.