Nickel-rich layered Li transition metal oxides are the most promising cathode materials for high-energy-density Li-ion batteries. However, they exhibit rapid capacity degradation induced by transition metal dissolution and structural reconstruction, which are associated with hydrofluoric acid (HF) generation from lithium hexafluorophosphate decomposition. The potential for thermal runaway during the working process poses another challenge. Separators are promising components to alleviate the aforementioned obstacles. Herein, an ultrathin double-layered separator with a 10 \mu \mathrm{m} polyimide (PI) basement and a 2 \mu \mathrm{m} polyvinylidene difluoride (PVDF) coating layer is designed and fabricated by combining a nonsolvent induced phase inversion process and coating method. The PI skeleton provides good stability against potential thermal shrinkage, and the strong PI-PVDF bonding endows the composite separator with robust structural integrity; these characteristics jointly contribute to the extraordinary mechanical tolerance of the separator at elevated temperatures. Additionally, unique HF-scavenging effects are achieved with the formation of - \mathrm{C} \mathrm{O} \cdots \mathrm{H} - \mathrm{F} hydrogen bonds for the abundant \mathrm{H} \mathrm{F} coordination sites provided by the imide ring; hence, the layered Ni-rich cathodes are protected from HF attack, which ultimately reduces transition metal dissolution and facilitates long-term cyclability of the Ni-rich cathodes. Li||NCM811 batteries (where "NCM" indicates {\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{i}}_x {\mathrm{C}\mathrm{o}}_y {\mathrm{M}\mathrm{n}}_{1-x-y} {\mathrm{O}}_2) with the proposed composite separator exhibit a 90.6 \mathrm{\%} capacity retention after 400 cycles at room temperature and remain sustainable at {60}^{\circ } \mathrm{C} with a 91.4 \mathrm{\%} capacity retention after 200 cycles. By adopting a new perspective on separators, this study presents a feasible and promising strategy for suppressing capacity degradation and enabling the safe operation of Ni-rich cathode materials.