The demands for novel approaches that ensure stability in lithium-ion batteries are increasing and have led to the development of new materials and fabrication strategies. In this study, sandwich structure-like polysulfonamide (PSA)/polyacrylonitrile (PAN)/polysulfonamide (PSA) composite nanofibrous membranes were prepared via an electrospinning method and used as a separator in lithium-ion batteries. The spinning time of each polymer nanofiber layer of the composite membranes was respectively and precisely controlled to maximize the merits of each component. It was found that the PSA/PAN/PSA composite nanofibrous membranes exhibited superior thermal stability and excellent porosity, liquid electrolyte uptake and ionic conductivity, showing obvious enhancement as compared to those of the commercial microporous polyolefin separator (Celgard 2400), pure PSA and pure PAN membranes. In addition, they were evaluated in the assembled Li/LiFePO₄ cells with an electrolyte solution, and good cycling performance and C-rate capacity were obtained; especially for the case of the PP6P membrane, the first discharge capacity of the battery reached 152 mA h g⁻¹, and the discharge capacity retention ratio was 85.94% from 0.2C to 2C; moreover, the battery displayed highest capacity retention ratio after 70 cycles, which was found to be 96.2% of its initial discharge capacity. Therefore, the PSA/PAN/PSA composite nanofibrous membranes can be regarded as a promising candidate for application in lithium-ion batteries.