Encapsulated sulfur cathode for lithium ion battery

1. A method of making a cathode element for an electrochemical cell, the method comprising:

providing hollow carbon nanotubes and a sulfur source in a closed environment;

depositing the sulfur within an interior of the hollow carbon nanotubes;

cleaning an exterior surface of the carbon nanotubes; and

incorporating the carbon nanotubes into a cathode element.

2. The method of claim 1 , wherein the carbon nanotubes comprise a stacked-cone structure.

3. The method of claim 1 , wherein the carbon nanotubes comprise an average inner core diameter of from about 50 nm to about 150 nm.

4. The method of claim 1 , wherein the carbon nanotubes comprise an average aspect ratio of from about 500 to about 5,000.

5. The method of claim 1 , comprising providing the sulfur source in an amount that includes from about 10 wt % to about 15 wt % in excess of the theoretical amount of elemental sulfur required to fill the interior of the hollow carbon nanotubes.

6. The method of claim 1 , wherein the sulfur source is selected from the group consisting of elemental sulfur, ammonium sulfate, ammonium sulfide, and combinations thereof.

7. The method of claim 1 , wherein depositing the sulfur comprises filling at least about ⅔ of each hollow carbon nanotube with sulfur.

8. The method of claim 1 , wherein depositing the sulfur comprises a technique selected from the group consisting of sublimation, chemical vapor deposition, physical vapor deposition, and combinations thereof.

9. The method of claim 8 , wherein depositing the sulfur comprises heating the sulfur to a temperature greater than its sublimation point.

10. The method of claim 1 , wherein depositing the sulfur comprises a technique selected from the group consisting of liquid deposition, melt impregnation, and combinations thereof.

11. The method of claim 1 , wherein the cleaning comprises heat treating the carbon nanotubes to a temperature sufficient to remove substantially all deposited sulfur from the exterior surface of the carbon nanotubes.

12. The method of claim 1 , wherein the cleaning comprises washing the carbon nanotubes in a solvent bath and removing substantially all deposited sulfur from the exterior surface of the carbon nanotubes.

13. A lithium ion cell comprising the cathode prepared according to claim 1 .

14. The lithium ion cell according to claim 13 , comprising a specific discharge capacity of from about 500 to about 1,200 mAh/g.

15. A method of making a cathode for a lithium-sulfur electrochemical cell, the method comprising:

providing hollow, stacked-cone structure carbon nanotubes and a sulfur source in a closed, inert environment at ambient pressure;

heating the sulfur source to a temperature greater than its sublimation point and depositing the sulfur within an interior of the hollow carbon nanotubes;

cleaning an exterior surface of the carbon nanotubes; and

incorporating the sulfur filled carbon nanotubes into a cathode element for a lithium-sulfur electrochemical cell.

16. The method of claim 15 , wherein depositing the sulfur comprises filling at least about ⅔ of each hollow carbon nanotube with sulfur using a technique selected from the group consisting of sublimation, chemical vapor deposition, physical vapor deposition, and combinations thereof.

17. A cathode material for an electrochemical cell, comprising:

a plurality of stacked-cone carbon nanotubes, each nanotube defining a hollow interior and having a substantially continuous exterior surface area; and

elemental sulfur disposed within the hollow interior of each nanotube.

18. The cathode material of claim 17 , wherein the hollow interior of each nanotube has an average inner core diameter of from about 50 nm to about 150 nm.

19. The cathode material of claim 17 , wherein the hollow interior of each nanotube is at least about ⅔ filled with elemental sulfur.

20. The cathode material of claim 17 , wherein the exterior surface area of each nanotube is substantially free of elemental sulfur.