A method for making a lithium-sulfur battery separator includes providing a separator substrate comprising a first surface and a second surface opposite to the first surface; and forming a functional layer on at least one of the first surface and the second surface. A method of forming the functional layer includes providing a carbon nanotube layer comprising a plurality of carbon nanotubes; etching the carbon nanotube layer to form defects on surfaces of the plurality of carbon nanotubes; and forming a hafnium oxide layer on the defects to form a carbon nanotube/hafnium oxide composite layer.