Lithium transport in a macrocyclic electrolyte

Abstract

We have carried out nuclear magnetic resonance (NMR) studies on a new class of electrolyte consisting of a lithium salt (LiMPSA) and a macrocycle ([2.2.2] cryptand). NMR diffusion coefficient measurements have determined the motion of lithium cations ${(}^{7}\mathrm{Li}\ensuremath{-}\mathrm{NMR}),$ polyatomic anions (MPSA) ${(}^{19}\mathrm{F}\ensuremath{-}\mathrm{NMR}),$ and encapsulating macrocycles ${(}^{1}\mathrm{H}\ensuremath{-}\mathrm{NMR}),$ with the striking result that they are equal over a large temperature range. Diffusion coefficients as low as $(1.0\ifmmode\pm\else\textpm\fi{}0.1)$ ${10}^{\ensuremath{-}9} {\mathrm{cm}}^{2}/\mathrm{s}$ have been measured through the use of a 42 T/m fringe-field gradient. The magnitudes of the anion and cryptand diffusion were found to be consistent with a free-volume picture for translational diffusion. The transport of the lithium cations was interpreted as taking place mainly through association with large anions (MPSA) and encapsulating macrocycles ([2.2.2] cryptand), with averaged residence time fractions of 77 and 23 %, respectively. NMR spin-lattice relaxation time measurements of ${}^{7}\mathrm{Li}$ and ${}^{19}\mathrm{F}$ spins have also been performed.