Ultraviolet-Cured Al₂O₃-Polyethylene Terephthalate/Polyvinylidene Fluoride Composite Separator with Asymmetric Design and Its Performance in Lithium Batteries

An Al2O3/polyethylene terephthalate (Al2O3/PET) film can be used as a practical battery separator with superior thermostability, but particle shedding inhibits wide application of this material. We developed a high-security Al2O3/PET film by incorporating a photoactive oligomer, poly(ethylene glycol)diacrylate (PEGDA), into the Al2O3 coating, followed by ultraviolet (UV) irradiation. One side of the as-prepared film was coated with aqueous polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) to obtain a UV-Al2O3-PET/PVDF film. Optimizing the added PEGDA content is key to achieving an ideal balance between the strength and ionic conductivity of the UV-cured composite film. A PEGDA content of 18 wt % significantly enhances the adhesion of the inorganic coating after UV irradiation, and the corresponding film remains intact even after 1000 bends. The optimized UV-Al2O3-PET/PVDF separator has an average pore size of 0.6 μm and a breakdown voltage above 250 V. The separator exhibits good electrochemical performance (δ = 1.03 mS cm–1, tLi+ = 0.71, and electrochemical window = 5.5 V). Using the UV-Al2O3-PET/PVDF separator in batteries results in good charge retention at room temperature and 55 °C. The discharge capacity of a lithium iron phosphate (LFP)||Li battery is 137.3 mA h g–1 (84.1%) after 300 cycles at 0.2 C. The PVDF-HFP coating enhances the stability of the interface between the separator and the lithium metal anode.