Effect of battery fast cyclic charging on the mechanical and fracture behavior of the lithium-ion battery separator

The effect of cyclic fast charging on the mechanical and physicochemical properties of the lithium-ion battery separator is investigated. For that purpose, six battery pouch cells were cycled at 4C charge and 0.5C discharge rates for up to 400, 800, and 1600 cycles. Once the fast charging cycles were completed, the battery pouch cells were fully discharged and carefully disassembled. The physical and chemical microstructural properties of pristine and cycled separators were investigated using scanning electron micrographs (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). Additionally, the mechanical behavior of the pristine and cycled separators was also investigated through uniaxial tensile and biaxial punch tests. While the XPS and FTIR analyses of the cycled separator surfaces did not reveal the presence of chemical degradation, the mechanical properties presented a decrease in toughness and ductility with an increased number of charging cycles. A thorough discussion of the possible degradation mechanisms of the cycled separator is provided by combining the results of our analysis and of those reported in the literature.