Abstract: Separators with superior electrolyte property and thermal stability are in urgent need for lithium-ion batteries (LIBs). This work designed a space-confined polymerization method to fabricate an advanced composite separator, in which poly(cyclotriphosphazene-co-4, 4-sulfonyldiphenol) microspheres (PZSMS) grow directly in poly(vinylidene fluoride) (PVDF) membrane as the substrate. To regulate the size and distribution of PZSMS, triethylamine vapor as acid-binding agent is introduced into PVDF membrane separately from the polymeric monomers. The physical-chemical properties and battery performances of PPCS were systematically characterized, such as the structure, tensile strength, electrolyte property and thermal resistance as well as the charge-discharge performance. The results show that under the optimized conditions, the liquid absorbency and ionic conductivity of the composite membrane reach 433% and 1.47 mS/cm, respectively, the tensile strength is greater than 25 MPa, and the thermal shrinkage rate is lower than 2% at 150℃ and 0.5 h, which is better than that of PVDF based membrane and commercial polyethylene (PE) membrane. The LiCoO₂/graphite cells with optimized separators exhibit satisfactory discharge capacity retention of 76% at 8.0 C compared with that at 0.5 C and preferable cycling stability with a capacity retention of 97% after 200 cycles. Therefore, PZSMS modified PVDF fibrous membrane prepared by space-confined polymerization method shows a good application prospect in lithium-ion batteries.