Nanoconfinement effects on the dynamics of an ionic liquid-based electrolyte probed by multinuclear NMR

Abstract

The measurement of ion diffusivity inside nanoporous materials by Pulsed-Field Gradient (PFG) NMR is not an easy task due to enhanced NMR relaxation. Here, we employed multinuclear (¹H, ³¹P, and ⁷Li) NMR spectrometry and diffusometry to probe ion dynamics of a fluorine-free battery electrolyte comprising the [P_4,4,4,4][MEEA] ionic liquid (IL) and LiMEEA salt in a 7 : 3 molar ratio, confined in three different nanoporous SiO₂ glasses with pore diameters of 3.7, 7 and 98 nm. Confinement of the electrolyte leads to NMR resonance line broadening and variation in the ³¹P and ⁷Li NMR chemical shifts. The complicated diffusion decays are explained taking into consideration the complex porous structure of the porous glasses, the presence of pore “necks” and the “partially isolated volumes” containing the liquid, which is in a “slow exchange” regime with the rest of the liquid. The mean apparent diffusivity is controlled by the exchange of ions between the “narrow” and the “large” pores and the boundary separating these pores to measure diffusion coefficients by PFG NMR is in the range of pore sizes of Vycor and Varapor. The temperature-dependent ion diffusivities in the “large” pores deviate from the Arrhenius law and the exchange of diffusing units between the “narrow” and the “large” pores leads to abnormal temperature-dependent diffusion coefficients. Like the bulk, diffusivity of the small Li⁺ is slower than that of the larger organic ions in the confinement, demonstrating the solvation of Li⁺ inside the pores.