An apparatus includes a first conductive substrate (e.g., a metal foil) having a first surface; a plurality of conductive stalks (e.g., carbon nano-tubes) extending from the first surface; an electrically insulating coating (e.g., sulfur) about the carbon stalks; a second conductive substrate (e.g., a lithium oxide foil); and an electrolyte (e.g., a polymer electrolyte) disposed between the first surface of the first conductive substrate and the second conductive substrate. In various embodiments: the sulfur is disposed at a thickness of about 3 nanometers+/−1 nanometer; the stalks are at a density such that a gap between them as is between 2 and 200 diameters of an ion transported through the electrolyte; and there is a separator layer within the electrolyte having a porosity amenable to passage by such ions. Also detailed is a method for making the foil with the coated carbon nano-tubes.