Characterization of electrospun PVdF fiber-based polymer electrolytes

Abstract

Porous PVdF fiber-based membranes with a three-dimensional network structure, high porosity, large electrolyte solution uptake, and adequate mechanical properties were prepared by an electrospinning technique using various mixed-solvent compositions with poly(vinylidene fluoride) (PVdF). Their physical properties, including surface morphology, average fiber diameter, pore size, and electrolyte solution uptake, strongly depended on the composition of the polymer solution used for electrospinning. From X-ray diffraction and FT-Raman data, we found the PVdF membranes to have mixed-crystal structure sof Form II (alpha-type) and Form III (gamma-type). Electrospun PVdF fiber-based polymer electrolytes were prepared by immersing porous PVdF membranes into 1 M LiPF6 electrolyte solutions. On the basis of FT-Raman data of the PVdF fiber-based polymer electrolytes, it was shown that ethylene carbonate molecules mainly participated in the solvation of the lithium salt. Moreover, with the exception of diethyl carbonate, these aliphatic carbonate molecules strongly interacted with the PVdF chain. The polymer electrolytes exhibited high ionic conductivities up to 1.0 x 10(-3) S/cm at room temperature, and wide electrochemical stability windows of 0.0 to 4.5 V vs Li/Li+. The ionic conductivity of the PVdF fiber-based polymer electrolytes depended on the physicochemical properties of the 1 M LiPF6 electrolyte solution inside the pores, whereas their electrochemical properties were enhanced by the interaction between the PVdF chain and the aliphatic carbonate molecules. Thus, prototype cells with PVdF fiber-based polymer electrolytes showed a range of different charge/discharge properties according to the solvent composition of the 1 M LiPF6 electrolyte solutions and the C-rate. In addition, the cycling performances depended on the electrochemical and spectroscopic properties of the electrospun PVdF fiber-based polymer electrolytes.