NMR measurements of self diffusion in polymer gel electrolytes

The pulsed field gradient NMR spin-echo technique has been used to measure the self diffusion coefficients of solvent molecules, Li+ cations and CF3SO3-1 anions in polyvinylidene fluoride (PVDF) gel electrolytes incorporating liquid electrolytes based on either N,N-dimethylformamide (DMF) or tetraglyme (TG) and lithium triflate (LIT). Using the Nernst-Einstein equation with the NMR diffusivities, calculated values for the ionic conductivity were obtained and compared with those determined by ac conductivity measurements to provide estimates of ionic association. These results for the gel electrolytes are compared with those for the corresponding liquid electrolytes, over a range of salt concentrations and temperatures. in general terms, the comparisons support the simple conclusion that regions of liquid electrolyte exist within the gel electrolytes, consistent with limited measurements of the transverse relaxation time T-2 on these systems, which are also reported. There are, however, some small but significant differences between the behaviours of the gel electrolytes and the liquid electrolytes and these are discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.