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

Abstract

Novel polymer electrolytes (LiBF 4/plasticizer/PE) based on the interpenetrating network approach, were obtained starting from functionalized polyether network precursors, in the presence of LiBF 4 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 1H, 19F, 7Li 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 ( Δ NE ) and information on ion associations. Ionic conductivities (σ) were measured for PC and TGME-based membranes at various plasticizer and salt contents as a function of T (60 to – 20 °C). LiBF 4/PC/PE with 9% (w / w) salt and 59% PC has the highest σ (2.4 × 10 −3 and 1.0 × 10 −3 S/cm at 20 °C and −3 °C, respectively). Membranes with PC have higher σ than those of TGME due mainly to the greater amount of ion associations (correlated motions of ions) in TGME systems as evidenced by Δ NE · Li + in PC seems to interact preferentially with the polymer. NMR shows that LiBF 4/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. Δ NE decreases with increasing T indicating that ion associations are favoured at high T. Simple binary electrolytes (LiBF 4/plasticizer) were also studied by conductimetry and PMFG-NMR. The differences between PC and TGME electrolytes regarding σ, Δ NE and t + were the same as found for ternary systems. D data show a larger Li + solvation shell in LiBF 4/PC and on increasing the salt content above 10% (w / w) σ drops rapidly (attributed to increased viscosity and ion associations).