We are investigating a new class of materials termed “molecular ionic composites” (MICs), that show promise as next-generation solid electrolytes for batteries and other applications. MICs form when mixing a rigid polyanion with purely ionic fluids, and they behave mechanically as a solid but contain ions that move nearly as in a neat liquid. Specifically, prototypical MICs are based on solutions of the rigid-rod polyelectrolyte poly(2,2’-disulfonyl-4,4’-benzideneterephthalamide) (PBDT), which forms a double helix in water, combined with imidazolium-based ionic liquids (ILs). The IL comprises 75-97 wt% of the final solid, even though the Young’s modulus can reach ~ 2 GPa at 80 wt% IL. In order to understand the MIC self-assembly process, we have tracked the rate of composite formation in bulk MIC “ingots” through simple time-lapse photography. We hypothesize that the formation mechanism involves self-assembly of IL ions among PBDT rods to form a partially ordered (liquid crystalline) gel. We observe dramatic changes in self-assembly rate when changing the initial concentration and type of IL and, the amount and molecular weight of PBDT, and we measure the transport of all mobile ions within the MIC and the internal composition using NMR. Finally, we observe a time-independent self-assembly rate, strongly indicating that the MIC formation mechanism is akin to crystal formation and not driven by self-diffusion or a concentration gradient.

Molecular ionic composites: Toward understanding internal structure and self-assembly mechanism

ACS Fall 2020 Virtual Meeting

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