Ionic Mobility Measurements Applying a Controlled Direct Electric Field on Pulsed Gradient Spin Echo Nuclear Magnetic Resonance

The ionic mobilities of the cation and anion in lithium electrolytes composed of LiN(CF₃SO₂)₂ in ethylene carbonate/diethyl carbonate have been measured using the pulsed gradient spin echo nuclear magnetic resonance (NMR) technique with application of a direct electric field. The NMR cells were composed of four-terminal electrodes. The direct potential was applied by controlling the detected potential at the center of the sample as a constant during the spin echo measurement. The estimated mobilities of the cation and the anion were 7.2 × 10^-5 and 1.4 × 10^-4 cm² s^-1 V^-1, respectively. These values imply mobility multiplied by the probability of existence of the species. Compared with the diffusion values obtained from the measurements of the self-diffusion coefficient without an electric field, the diffusion value under the electric field according to the Einstein relation was estimated to be larger, although the self-diffusion values additionally include the diffusion component of the undissociated ion pairs. This is due to the coulombic interactions operating among the charged species that restrict the free diffusion following the random walk of the isolated species. From this result, we can conclude that the mobility measurement by applying the electric field reflects the real migration properties of each ion.