Battery separators are supposed to be electrical insulators to prevent internal short-circuit failure between electrodes as well as having porous channels to allow ion transport. Here, as a multifunctional membrane strategy to dispel this stereotypical belief about battery separators, a new class of Janus-faced, dual (ion/electron)-conductive/chemically active battery separators (denoted as "Janus separators") based on a heterolayered nanofiber mat architecture is demonstrated. The Janus separator, which is fabricated through in-series, concurrent electrospraying/electrospinning processes, consists of an ion-conductive/metal ion-chelating support layer (a mat of densely packed, thiol-functionalized silica particles spatially besieged by polyvinylpyrrolidone/polyacrylonitrile nanofibers) and a dual-conductive top layer (a thin mat of polyetherimide nanofibers wrapped with multi-walled carbon nanotubes). The support layer acts as a chemical trap that can capture heavy metal ions dissolved in liquid electrolytes and the top layer serves as an upper current collector for cathodes to boost the redox reaction kinetics. Notably, the unusual porous microstructure of the top layer is theoretically elucidated using molecular dynamics simulation. Benefiting from such material/structural uniqueness, the Janus separator enables significant improvements in fast-rate charge/discharge reactions (even for high-mass loading cathodes) and in the high-temperature cycling performance, which lie far beyond those achievable with conventional polyethylene separators.