Impedance and Lithium-7 NMR Studies of Polymer Electrolytes Based onPoly(Vinylidene Fluoride)

摘要

Composite polymer electrolytes consisting of at least 35 mol% poly(vinylidenefluoride) (PVdF) and mixtures of propylene carbonate (PC)and dimethyl formate (DMF), lithium salts (LiAsF6, LiN(CF3SO2)2, or LiC(CF3SO2)3 and small amounts (approx. 5 mol%) of plasticizer, either diethylphthalate (DEP) or dibutylphthalate (DBP), were investigated by differential scanning calorimetry, complex impedance analysis and (7)Li NMR spectroscopy. Despite the composite nature of these materials, they all exhibit a single glass transition temperature (Tg). Significant variations of Tg with the two different plasticizers were noted. The sample containing the LiC(CF3SO2)3) salt has the highest electrical conductivity of the series, approx. 3 X 10(exp -1)S/cm at 25 deg C. Time-dependent impedance measurements utilizing cells with lithium electrodes revealed the growth of a passivating interface layer comparable to those observed with other polymer electrolytes. The onset of NMR motional line-narrowing is correlated with Tg for all samples prepared with DBP, as is known to occur in 'single component' polyether-based electrolytes. However the onset of narrowing for the samples containing DEP occurs about 30 K above Tg. Lithium-7 spin-lattice relaxation measurements exhibit non-exponential recovery profiles below Tg, with about approx. 15% of the Li species relax differently than the majority. Activation energies extracted from (7)Li T1 data show that localized Li motion is significantly impeded in the DEP-containing samples relative to those prepared with DBP. jg p3.