Preparation and characterization of electrospun nanofiber-coated membrane separators for lithium-ion batteries

Nanofiber-coated membrane separators were prepared by electrospinning polyvinylidene fluoride-co-chlorotrifluoroethylene (PVDF-co-CTFE) nanofibers onto three different microporous membrane substrates. The nanofibers on the membrane substrates showed uniform morphology with average fiber diameters ranging from 129 to 134 nm. Electrolyte uptakes, ionic conductivities, and interfacial resistances were studied by soaking the nanofiber-coated membrane separators with a liquid electrolyte solution of 1 M lithium hexafluorophosphate in ethylene carbonate/dimethylcarbonate/ethylmethyl carbonate (1:1:1 by volume). Compared with uncoated membranes, nanofiber-coated membranes had greater electrolyte uptakes and lower interfacial resistances to the lithium electrode. It was also found that after soaking in the liquid electrolyte solution, nanofiber-coated membranes exhibited higher ionic conductivities than uncoated membranes. In addition, lithium-ion half cells containing nanofiber-coated membranes were evaluated with a LiFePO₄ cathode for charge–discharge capacities and cycle performance. The cells containing a nanofiber-coated separator membrane showed high discharge specific capacities and good cycling stability at room temperature. Results demonstrated that coating PVDF-co-CTFE nanofibers onto microporous membrane substrates is a promising approach to obtain new and high-performance separators for rechargeable lithium-ion batteries.