Novel aramid nanofiber-coated polypropylene separators for lithium ion batteries

Aramid nanofiber (ANF)-coated separators were successfully prepared by the dip-coating of a cationized polypropylene (PP) porous separator in an ANF dispersion in DMSO. The ANFs were successfully coated onto the surface of the cationized PP separator as demonstrated by FT-IR and XPS measurements and the ANFs could be directly observed on the surface of the composite separator via SEM and AFM. The ANF-based coating layers became more uniform and denser as more dip-coating cycles were employed. The gas permeabilities of the separators were strongly influenced by the concentrations of the ANF dispersion and the number of dip-coating cycles. The porosity decreased and a narrower pore size distribution was obtained after the ANFs were coated onto the cationized PP separator. The ANF-coated separators were found to exhibit higher dimensional stabilities than the pristine PP separator. The separators exhibited almost identical endothermic peaks in the DSC experiment and a similar shrink temperature in the DMA experiment but the ANF-coated separator exhibited a higher rupture temperature. The ANF-coated separator retained a comparable mechanical strength with that of the pristine PP separator. The ANF coating layer was mechanically stable and durable in the electrolyte. The ANF-coated separator exhibited comparable C-rate performance and cycling performance in LMO/Li cell systems to that of the PDA-PP separator, and showed significantly better C-rate performance and cycling performance than that of the pristine PP separator. The ANF-coated PP separators exhibited improved safety in a hot oven test in comparison with the pristine PP separator. Thus the ANF-coated separators have great potential for use in lithium ion batteries.