The optimal design of separators is critical for high-performance lithium-ion batteries (LIBs), particularly under fast-charging and high-temperature conditions. Herein, a dual-engineering strategy, integrating porosity control through nonsolvent-induced phase separation (NIPS) and polydopamine (PDA) surface modification, is proposed as an effective approach for separator optimization. The resulting PDA-coated poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), referred to as PHS73@PD, has a uniformly distributed porous structure that provides abundant Li-ion transport pathways and promotes homogeneous ion diffusion. The introduction of PDA enhances the thermal stability (no shrinkage up to 140 °C) and improves the ionic conductivity by 13.5% compared to uncoated PHS73 due to its polar functional groups and strong electrolyte affinity. As a result, in NCM622||graphite full cells, PHS73@PD maintains a stable rate performance even at a high current density of 5 C and fully recovers at 1 C. It also retains 82% of its capacity after 1000 cycles, indicating improved long-term cycling stability. This study demonstrates that the dual-engineering strategy of NIPS structural control and PDA surface modification can be effectively applied for the development of separators for high-performance LIBs.