First principles design of ionomers for facile ion transport

Ab initio calculations have been performed for the association of lithium and sodium cations with various carboxylate, sulfonate, phosphonate and borate anions, with a particular emphasis on the effects of perfluorinating the anions. Fluorinating the benzene ring on benzene carboxylate, benzene sulfonate or benzene phosphonate makes the pair and positive triple ion binding less favorable by 5-10% due to the electron-withdrawing F placing more of the negative charge on the ring, effectively softening these anions. However, fluorinating the four benzene rings of tetraphenyl borate has a significantly stronger electron-withdrawing effect, destabilizing the pair and positive triple ion energies by 20-30%. We also explore two methods to account for the effects of a surrounding polar medium on ion interactions. The polarizable continuum model was studied with six ion pairs to account for the dielectric constant of the surroundings. We also model specific local solvation of poly(ethylene oxide) on Li⁺ and Na⁺ with two anions (benzene sulfonate and triflate) and also their ion pairs, by surrounding these with explicit dimethyl ether (DME) molecules. We find a strong local solvation effect on the cations that is particularly strong for Li+ with four DME in the first solvation shell. There is very little specific solvation of anions by DME and the ion pairs fill their first solvation shell with three DME, with all four ion pairs studied showing very similar solvated pair interactions.