Characterization of poly(vinylidene fluoride) flat sheet membranes prepared in various ratios of water/ethanol for separator of li-ion batteries: Morphology and other properties

PVDF, poly(vinylidene fluoride), membranes were prepared and investigated by a scanning electron microscope, a universal testing machine, and capillary porometer for its potential use as a separator in lithium ion batteries. The membranes were prepared by phase inversion with different polymer types, concentrations of solution, amounts of additive, and nonsolvent ratios of water/ethanol. The morphology of membranes is affected by the ratio of both the coagulation bath (water/ethanol) and a low molecular weight additive (polymer/solvent/additive). The results showed that significant variations in the membrane were detected when adding an additive to the casting solution or ethanol to the coagulation bath. With an increased concentration of ethanol, the upper structure was found to be transformed into a sponge-like arrangement. In the case of Solef®1015 of the same polymer concentration, despite the higher molecular weight of 1015, a relatively small sized nucleus is formed, resulting in a denser network and relatively uniform membrane structure being formed. Mechanical testing showed that the tensile strength of the PVDF membranes increased when added to a 25 wt % ethanol coagulation bath, whereas it is decreased when added to higher concentrations of ethanol in the bath or additives in the casting solution. In a bath condition of water/ethanol = 75/25 wt % (Bath no. 2), the value of tensile strength is 7.11 and 7.52 MPa, for Solef®6010 20 wt % and Solef®1015 17 wt %, respectively. The thickness of the prepared membrane is 21-34 μm and the porosity is up to 50%. The electrolyte absorption changes of the fabricated membranes at different conditions are measured from 151 to 223 ± 15%.