Ionic mobilities of PVDF-based polymer gel electrolytes as studied by direct current NMR

In order to estimate the ionic transference number precisely the equipment of the direct current (DC) NMR measurement (the pulsed-gradient spin-echo (PGSE) NMR by applying a direct electric field) was improved with an electric field control technique using a four-terminal electrophoretic sample cell. It was found that this technique was appropriate to achieve the constant electric field condition at the NMR detected section. The ionic mobilities of the cation and anion of the lithium gel electrolyte, 1 M LiN(CF3SO2)2–EC/DEC–PVdF-HFP, were measured individually using the modified apparatus. The transport number of lithium ion obtained from the ionic mobilities of lithium ion and counter anion group was different from the apparent value estimated from the diffusion coefficients of the lithium and anion species. This disagreement was due to the ion-pair component in diffusion coefficients. It indicated that the DC-NMR was more precise technique for estimating the transference number of ions than the simple PGSE-NMR.