Pore-filling solvent-free polymer electrolytes based on porous P(VdF-HFP)/P(EO-EC) membranes for rechargeable lithium batteries

Highly porous membranes consisting of poly(vinylidene fluoride-co-hexafluoropropylene) [P(VdF-HFP)] and poly(ethylene oxide-co-ethylene carbonate) [P(EO-EC)] were prepared by a phase inversion method using acetone as a solvent and ethylene glycol as a nonsolvent. The presence of viscous P(EO-EC) in the membranes not only contributes to their flexibility but also results in increases in their pore size and porosity. Pore-filling solvent-free polymer electrolytes (SPEs) were then prepared by filling the pores of the porous membranes with viscous P(EO-EC) complexed with LiCF3SO3. The ionic conductivities of the SPEs were found to exhibit Arrhenius temperature dependences, with the highest value of 3.7 × 10−5 S cm−1 at 298 K for E-V6E4; ‘E-VxEy’ denotes an electrolyte with a P(VdF-HFP)/P(EO-EC) matrix (x/y by wt%) filled with P(EO-EC)/LiCF3SO3 (in the case of E-V6E4, ca. 61 wt%). The temperature dependences of ln τc (τc is the correlation time) obtained from 7Li NMR linewidth measurements were found for all the SPEs to consist of two distinct regions, above and below the temperature of slope change, Tsc, with linear Arrhenius behavior in each region. Above Tsc, the temperature region in which the conductivity measurements were carried out, the correlation times, τc, and the corresponding activation energies, Ea, were found to decrease with increases in the amount of P(EO-EC)/LiCF3SO3 electrolyte.