Low-Cost and Large-Scale Fabricating Technology for High-Performance Lithium-Ion Battery Composite Separators with Connected Nano-Al₂O₃ Coating

The ever-increasing demand for high power density improves lithium-ion batteries. However, the poor microporous structure and inferior compatibility of separators heighten the lithium-ion migration barrier. Based on the cavitation of β-crystal polypropylene (β-iPP), separators with connected nano-Al2O3 coating are prepared by facile extrusion and biaxial stretching processes of β-iPP and nano-Al2O3. The crystal structure and microporous morphology diagnoses indicate that the moderate nano-Al2O3 content refines coarse fibrils and homogenizes pore size dispersion of the separator after biaxial stretching. Also, the connected nano-Al2O3 coating remains on the surface and inner pore wall because pores arise directly at nano-Al2O3/β-iPP interfaces, which endows the separator with superior electrolyte affinity. Assembled batteries further exhibit better C-rate capacity and cycle stability owing to the stable solid electrolyte interface formation and a higher lithium-ion transference number. Especially, the compound separator possesses the highest Li+ transference number of 0.53 and discharge capacity retention of 88.7% after 100 cycles as the nano-Al2O3 content is increased to 6%. This facile fabrication technique ameliorates the porous structure and electrochemical properties of the separator simultaneously, which provides practical guidance for the high-performance separator manufacturing technology in a large scale with low cost.