Ionic mobility measurements applying a controlled direct electric field onpulsed gradient spin echo nuclear magnetic resonance

The ionic mobilities of the cation and anion in lithium electrolytes compos ed of LiN(CF3SO2)(2) in ethylene carbonate/diethyl carbonate have been meas ured using the pulsed gradient spin echo nuclear magnetic resonance (NMR) t echnique with application of a direct electric field. The NMR cells were co mposed of four-terminal electrodes. The direct potential was applied by con trolling the detected potential at the center of the sample as a constant d uring the spin echo measurement. The estimated mobilities of the cation and the anion were 7.2 x 10(-5) and 1.4 x 10(-4) cm(2) 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 measurem ents of the selfdiffusion coefficient without an electric field, the diffus ion value under the electric field according to the Einstein relation was e stimated to be larger, although the self-diffusion values additionally incl ude the diffusion component of the undissociated ion pairs. This is due to the coulombic interactions operating among the charged species that restric t the free diffusion following the random walk of the isolated species. Fro m this result, we can conclude that the mobility measurement by applying th e electric field reflects the real migration properties of each ion. (C) 20 01 The Electrochemical Society.