Comparative study of different membranes as separators for rechargeable lithium-ion batteries

Membranes of polypropylene (PP), PP coated with nano-Al2O3 , PP electrospun with polyvinylidene fluoride- hexafluoropropylene (PVdF-HFP), and trilayer laminates of polypropylene-polyethylene-polypropylene (PP/PE/PP) were comparatively studied. Their physical properties were characterized by means of thermal shrinkage test, liquid electrolyte uptake, and field emission scanning electron microscopy (FESEM). Results show that, for the different membranes as PP, PP coated with nano-Al2O3 , PP electrospun with PVdF-HFP, and PP/PE/PP, the thermal shrinkages are 14%, 6%, 12.6%, and 13.3%, while the liquid electrolyte uptakes are 110%, 150%, 217%, and 129%, respectively. In addition, the effects on the performance of lithium-ion batteries (LiFePO4 and LiNi1/3 Co1/3 Mn1/3O2 as the cathode material) were investigated by AC impedance and galvanostatic charge/discharge test. It is found that PP coated with Al2O3 and PP electrospun with PVdF-HFP can effectively increase the wettability between the cathode material and liquid electrolyte, and therefore reduce the charge transfer resistance, which improves the capacity retention and battery performance.