Tonic diffusivity and conductivity of plasticized polymer electrolytes: PMFG-NMR and complex impedance studies

Novel polymer electrolytes (LiBF4/plasticizer/PE) based on the interpenetrating network approach, were obtained starting from functionalized polyether network precursors, in the presence of LiBF, and PC (propylene carbonate) or TGME (tetraethylene-glycol dimethyl-ether) as plasticizers. Self-consistent and easily handled membranes were obtained as thin films by a dry procedure using UV radiation to polymerize and crosslink the network precursors, directly on suitable substrates. These membranes were studied by complex impedance and H-1, F-19, Li-7 PMFG (Pulsed Magnetic Field Gradient)-NMR was used to determine the self-diffusion coefficients (D) of the cation (D-Li), anion (D-F) and plasticizer (D-pl). Comparison of data from the two techniques allowed calculation of the deviation from the Nernst-Einstein equation (Delta(NE)) and information on ion associations. Ionic conductivities (sigma) were measured for PC and TGME-based membranes at various plasticizer and salt contents as a function of T (60 to - 20 degrees C). LiBF4/PC/PE with 9% (w/w) salt and 59% PC has the highest sigma (2.4 X 10(-3) and 1.0 X 10(-3) S/cm at 20 degrees C and - 3 degrees C, respectively), Membranes with PC have higher a than those of TGME due mainly to the greater amount of ion associations (correlated motions of ions) in TGME systems as evidenced by Delta(NE). Li+ in PC seems to interact preferentially with the polymer. NMR shows that LiBF4/TGME/PE have higher cationic transport number(t(+) = ca. 0.5), little affected by changing salt or TGME contents, and faster polymer chain motions than the corresponding PC system. Delta(NE) decreases with increasing T indicating that ion associations are favoured at high T. Simple binary electrolytes (LiBF4/plasticizer) were also studied by conductimetry and PMFG-NMR. The differences between PC and TGME electrolytes regarding sigma, Delta(NE) and t(+) were the same as found for ternary systems. D data show a larger Li+ solvation shell in LiBF4/PC and on increasing the salt content above 10% (w/w) a drops rapidly (attributed to increased viscosity and ion associations).