Self-Assembled Nanostructures in Aprotic Ionic Liquids Facilitate Charge Transport at Elevated Pressure

BB Yao and M Paluch and J Paturej and S McLaughlin and A McGrogan and M Swadzba-Kwasny and J Shen and B Ruta and M Rosenthal and JL Liu and D Kruk and Z Wojnarowska, ACS APPLIED MATERIALS & INTERFACES (2023).

DOI: 10.1021/acsami.3c08606

Ionic liquids (ILs),revealing a tendency to form self- assemblednanostructures, have emerged as promising materials in various applications,especially in energy storage and conversion. Despite multiple reportsdiscussing the effect of structural factors and external thermodynamicvariables on ion organization in a liquid state, little is known aboutthe charge-transport mechanism through the self-assembled nanostructuresand how it changes at elevated pressure. To address these issues,we chose three amphiphilic ionic liquids containing the same tetra(alkyl)-phosphoniumcation and anions differing in size and shape, i.e., thiocyanate SCN(-), dicyanamide DCA(-), and tricyanomethanideTCM(-). From ambient pressure dielectric and mechanicalexperiments, we found that charge transport of all three examinedILs is viscosity-controlled at high temperatures. On the other hand,ion diffusion is much faster than structural dynamics in a nanostructuredsupercooled liquid (at T < 210 & PLUSMN; 3 K), whichconstitutes the first example of conductivity independent from viscosityin neat aprotic ILs. High-pressure measurements and MD simulationsreveal that the created nanostructures depend on the anion size andcan be modified by compression. For small anions, increasing pressureshapes immobile alkyl chains into lamellar-type phases, leading toincreased anisotropic diffusivity of anions through channels. Bulkyanions drive the formation of interconnected phases with continuous3D curvature, which render ion transport independent of pressure.This work offers insight into the design of high-density electrolyteswith percolating conductive phases providing efficient ion flow.

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