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• A dual-network structured nanofiber separator with fast ionic conductor is prepared. • This separator has well-distributed nanopore, high porosity and good heat-resistance. • LLZTO can well anchor free anions and offer fast Li+ channels to boost Li+ migration. • This separator achieves high t Li +, homogenized Li+ flux and uniform Li deposition. • Batteries with this separator show good cycle stability and high C-rate capability. Separator plays an important role in improving the long-term cycle stability and safety performance of high-energy–density lithium-metal batteries (LMBs). Herein, a robust thermotolerant separator is prepared by one-step electrospinning technique, combing fluorinated poly (m-phenylene isophthalamide) nanofibers with Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO), a fast ionic conductor. The synergistically incorporated poly (vinylidene fluoride-co-hexafluoropropylene) and LLZTO trigger off the formation of a fancy dual-network structure with high porosity and well-distributed nanopores, in which ultrafine secondary nanonets are tightly bound with three-dimensional nanofiber scaffold. LLZTO can not only provide sufficient Li+ migration channels, but also immobilize free anions to strengthen ionic transmission. Moreover, this nanofiber separator possesses high mechanical strength, excellent thermal stability and desirable electrolyte affinity. It can gel liquid electrolyte well, leading to high t Li +, homogenized Li+ flux and uniform Li deposition. Consequently, LMBs with this dual-network structured nanofiber separator deliver satisfactory cycle stability and high C-rate capability. This contribution provides new insights into the design and development of high-quality nanofiber separators for dendrite-free and high-safety LMBs. [ABSTRACT FROM AUTHOR]