Enhancement of Li Ion Conductivity by Electrospun Polymer Fibers and Direct Fabrication of Solvent-Free Separator Membranes for Li Ion Batteries

Poly(ethylene oxide) (PEO)-based polymer fibers, containing different amounts of the conductive salt LiBF4 and the plasticizer succinonitrile, were prepared by an electrospinning process. This process resulted in fiber membranes of several square centimeters area and an overall thickness of similar to 100 mu m. All membranes are characterized by scanning electron microscopy, differential scanning calorimetry, X-ray diffraction, impedance spectroscopy, cyclic voltammetry (CV), and solid-state NMR spectroscopy, to evaluate the influence of the preparation process and the composition on the conductivity of the materials. Impedance spectroscopy was used to measure the conductivities and activation barriers for the different membranes. The highest conductivity of 2 x 10(-4) S/cm at room temperature and 9 X 10(-4) S/cm at 328 K is reached for a PEO/SN/LiBF4 (36:8:1) membrane, featuring an activation energy of 31 kJ/mol. Li mobilities, as deduced from the evaluation of the temperature dependence of the Li-7 NMR line width and the overall electrochemical performance, are found to be distinctively superior to nonspun samples, synthesized via conventional solution casting. The same trend was found for the conductivities. NMR spectroscopy clearly substantiated that the mobility of the PEO segments drastically increases with the addition of succinonitrile pushing the conductivity to reasonable high values. In CV experiments the reversible Li transport through the dry membrane was evaluated and proved. This study shows that electrospinning provides a direct synthesis of solvent-free solid-state electrolyte membranes, ready to use in electrochemical applications.