Li⁺ and Oxidant Addition To Control Ionic and Electronic Conduction in Ionic Liquid-Functionalized Conjugated Polymers

Abstract

Conduction of ions and charge (electrons) often follow distinct materials design rules, presenting a significant challenge for the development of homogeneous materials that are good at both. The fundamental interactions that dictate ionic and electronic conduction in mixed conductors are still unclear. Here, we characterize the ionic and electronic conduction of a class of mixed polymeric conductors in which ionic liquid groups are tethered to an electron-conducting conjugated polymer backbone. A model conjugated polymeric ionic liquid, poly{3-[6′-(N-methylimidazolium)hexyl]thiophene}BF₄⁻ (P3HT-IM), is synthesized and shown to have significant long-range ordering. Chemical oxidation of the polymer results in a room-temperature electronic conductivity of 10⁻² S cm⁻¹. The polymer is also capable of dissolving Li⁺ salt up to a concentration of r_(salt) = 1 [moles of salt]/[moles of monomer]. The polymer displays a monotonic increase in ionic conductivity with salt concentration, reaching a maximum room-temperature ionic conductivity of 10⁻⁵ S cm⁻¹ at the highest concentration of r_(salt) = 1. Notably, this is among the first studies to characterize both ionic conductivity and electronic conductivity of an ionic liquid-functionalized conjugated polymer upon the addition of an oxidant and salt. All-atom molecular dynamics (MD) simulations indicate that the imidazolium side chains promote the formation of a percolated network of solvation sites at high salt concentrations, which facilitates ion transport. Pulsed-field gradient nuclear magnetic resonance diffusivity measurements and MD indicate a lithium transference number around 0.5, suggesting that the percolated solvation network promotes lithium transport in a way that is unique from many ion-conducting systems. These results suggest that the addition of diffuse, ionic liquid-like groups to a conjugated polymer backbone serves as an effective design approach to facilitate simultaneous lithium-ion conduction and electronic conduction in the absence of a solvent.

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