Filled carbon nanotubes (CNTs), methods of synthesizing the same, and lithium-ion batteries comprising the same are provided. In situ methods (e.g., chemical vapor deposition techniques) can be used to synthesize CNTs (e.g., multi-walled CNTs) filled with metal sulfide nanowires. The CNTs can be completely (or nearly completely) and continuously (or nearly continuously) filled with the metal sulfide fillers up to several micrometers in length. The filled CNTs can be synthesized on a carbon substrate. A lithium-ion battery can comprise a cathode, an anode comprising filled CNTs as described herein, and an electrolyte in contact with the cathode and/or the anode.