Ion dynamics in an iongel electrolyte based on fluorine-free ionic liquid probed by multinuclear NMR

Multinuclear (¹H, ³¹P, and ⁷Li) NMR was applied to understand the ion dynamics in silica-based iongels with a fluorine-free ionic liquid (IL), tetrabutylphosphonium 2-2-(2-methoxyethoxy)ethoxy acetate, [P_4,4,4,4][MEEA], doped with 10 and 30 mol% of LiMEEA. The results were compared with bulk [P_4,4,4,4][MEEA]/LiMEEA electrolytes and those confined in the "hard" silica matrix of a porous glass. It was found that lithium ion (Li⁺) local dynamics and Li⁺ diffusion coefficients are strongly affected by confinements in an iongel and in porous glass, as revealed from the analysis of NMR parameters, such as diffusion decays (DDs) in ⁷Li PFG NMR spectra, broadening of the ⁷Li NMR resonance lines and variations in the ³¹P and ⁷Li chemical shifts. However, NMR diffusometry data do suggest that the studied electrolytes in the iongel confinement have properties like those of bulk electrolytes: (i) high ion diffusivities, (ii) weak alterations of Vogel-Fulcher-Tammann (VFT) parameters for diffusion; and (iii) high transport numbers of ions. The diffusion coefficients of the [MEEA]^- anion and the [P_4,4,4,4]⁺ cation are comparable in the bulk, while they are significantly different in the iongels: the specific interactions of the [P_4,4,4,4]⁺ cations with the negatively charged silica matrix slowed down diffusivities of the cations, while almost no effect of the matrix on diffusivities of the [MEEA]^- anions was noticed. It was also found that the tortuosity of the iongel channels has a negligible effect on diffusivities of ions. The lithium complexation or/and solvation shells of Li⁺ ions remained unaffected. Thus, the ionic liquid-based iongel electrolyte acquired the advantages of a semi-solid phase and offered transport properties of a liquid electrolyte.