Impedance and lithium-7 NMR studies of polymer electrolytes based on poly(vinylidene fluoride)

Abstract

Composite polymer electrolytes consisting of at least 35 mol% poly(vinylidene fluoride) (PVdF) and mixtures of propylene carbonate (PC) and dimethyl formamide (DMF), lithium salts (LiAsF 6, LiN(CF 3SO 2) 2, or LiC(CF 3SO 2) 3 and small amounts (~ 5 mol%) of plasticizer, either diethylphthalate (DEP) or dibutylphthalate (DBP), were investigated by differential scanning calorimetry, complex impedance analysis and 7Li NMR spectroscopy. Despite the composite nature of these materials, they all exhibit a single glass transition temperature T g . Significant variations of T g with the two different plasticizers were noted. The sample containing the LiC(CF 3SO 2) 3 salt has the highest electrical conductivity of the series, ~ 3× 10 −4 S/cm at 25 °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 T g 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 T g . Lithium-7 spin-lattice relaxation measurements exhibit non-exponential recovery profiles below T g , with about ~ 15% of the Li species relax differently than the majority. Activation energies extracted from 7Li T 1, data show that localized Li motion is significantly impeded in the DEP-containing samples, relative to those prepared with DBP.