Ultrasmall Sn nanodots (1-2 nm) are homogeneously encapsulated in porous N-doped carbon nanofibers using a simple and scalable electrospinning method. The composite nanofibers weave into flexible free-standing membrane and can be directly used as binder-and current collector-free anode for Na-ion batteries, exhibiting excellent electrochemical performance with high reversible capacity, exceptional rate capability, and ultralong cycle life. [GRAPHICS] .