PVdF-HFP and Ionic-Liquid-Based, Freestanding Thin Separator for Lithium-Ion Batteries

Abstract

Lithium-ion batteries are the key for modern electricity-based transportation systems and more generally for sustainable large-scale energy applications. However, typical commercial batteries seldom meet safety regulations because of the presence of organic, flammable, and volatile liquid electrolytes, and viable alternatives need to be found. Ionic liquids (IL) are considered to be one of the most promising candidates, when combined with a polymer matrix to form the so-called gel polymer electrolyte, working as separator. Along this line, a new, purely water-based methodology has been developed in this work to produce thin separators. This involves the formation of fractal polymer clusters (PCs) through intense shear-driven gelation of poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP) nanoparticles in water, impregnation of the IL (Pyr13TFSI-LiTFSI) solution into the dried PCs, and hot-pressing to form continuous, porous, and transparent membranes. Because of the large amount of pores generated in the fractal structures with well-defined pore dimensions, the impregnated IL solution forms a continuous phase in the PC-IL matrix without any dead volume, thus forming a bicontinuous structure and presenting good ionic conductivity. The formed membrane has been used as the separator to assemble a half-coin cell having LiFePO4 and Li as the cathode and the counter electrode, respectively, which is tested at 60 °C. The electrochemical performances of the cell are excellent not only at low but also at high current densities. The parameters affecting the performance of the membrane have been investigated, and the proper optimal preparation conditions have been proposed.