Sulfur based active material for a positive electrode

1. A sulfur based active material, comprising:

a core-shell structure, including:

a hollow core; and

a porous carbon shell surrounding the hollow core;

sulfur present in a portion of the hollow core; and

a polymer shell coating formed on the porous carbon shell, the polymer shell coating including nitrogen atoms that bond to carbon atoms of the porous carbon shell so that the porous carbon shell is a nitrogen-confused porous carbon shell.

2. The sulfur based active material as defined in claim 1 wherein the polymer shell coating is made of polydopamine, polypyrrole, polyaniline, polyamides, polyacrylonitrile, polyurethane, or combinations thereof.

3. The sulfur based active material as defined in claim 1 , wherein the sulfur is in the form of a plurality of nanoparticles, each of which has a size of less than 500 nm.

4. The sulfur based active material as defined in claim 1 , wherein:

a size of the hollow core is less than 1μm;

a maximum amount of the sulfur present in the hollow core occupies less than 90% of the hollow core; and

an other portion of the hollow core is a void.

5. The sulfur based active material as defined in claim 1 , further comprising a polysulfide ion cross-linked to the polymer shell coating.

6. The sulfur based active material as defined in claim 1 , wherein the polymer shell coating includes pores having a size to transport lithium ions and to block transport of polysulfide ions.

7. A positive electrode, comprising:

a sulfur based active material, including:

a core-shell structure with a hollow core and a porous carbon shell surrounding the hollow core;

sulfur present in a portion of the hollow core; and

a polymer shell coating formed on the porous carbon shell, the polymer shell coating including nitrogen atoms that bond to carbon atoms of the porous carbon shell so that the porous carbon shell is a nitrogen-confused porous carbon shell;

a conductive carbon material; and

a binder.

8. The positive electrode as defined in claim 7 , wherein the polymer shell coating is made of polydopamine, polypyrrole, polyaniline, polyamides, polyacrylonitrile, polyurethane, or combinations thereof.

9. The positive electrode as defined in claim 7 , wherein:

the conductive carbon material is acetylene black;

the binder is water soluble poly(vinyl alcohol); and

a total amount of the sulfur based active material in the positive electrode is up to 90% by weight.

10. The positive electrode as defined in claim 7 , wherein the sulfur is in the form of a plurality of nanoparticles, each of which has a size of less than 500 nm.

11. The positive electrode as defined in claim 7 , further comprising a polysulfide ion cross-linked to the polymer shell coating.

12. A method for making a sulfur based active material, the method comprising:

forming a porous carbon shell having a hollow core;

incorporating sulfur into the hollow core, thereby forming a hollow carbon-sulfur composite;

dispersing the hollow carbon-sulfur composite in an aqueous solution including a nitrogen-containing monomer and having an alkaline pH, thereby forming a dispersion; and

stirring the dispersion at a reaction temperature for a time sufficient to polymerize the nitrogen-containing monomer and to form a polymer shell coating on the porous carbon shell of the hollow carbon-sulfur composite, whereby at least some of the nitrogen atoms of the polymer shell coating bond to carbon atoms of the porous carbon shell to form a nitrogen-confused porous carbon shell.

13. The method as defined in claim 12 , wherein forming the porous carbon shell and incorporating sulfur into the hollow core are accomplished by:

depositing a carbon precursor onto and into a sacrificial porous template;

pyrolyzing the deposited carbon precursor, thereby forming the porous carbon shell on the sacrificial porous template;

removing the sacrificial porous template, thereby forming the hollow core of the porous carbon shell;

mixing sulfur with the porous carbon shell having the hollow core to form a mixture; and

heat treating the mixture, thereby impregnating the sulfur into the hollow core.

14. The method as defined in claim 13 , wherein:

the carbon precursor is a polydopamine carbon precursor;

the sacrificial porous template includes a plurality of porous silica nanoparticles; and

the nitrogen-containing monomer is selected from the group consisting of dopamine, acrylamide, pyrrole, acrylonitrile, pyridine, urethane, and aniline.

15. The method as defined in claim 14 , wherein:

pyrolyzing takes place at a temperature ranging from about 600° C. to about 1000° C. for about 4 hours under an inert gas;

removing the sacrificial porous template is accomplished with an acid or a base; and

stirring is accomplished from about 3 hours to about 20 hours.

16. A lithium-sulfur battery, comprising:

a positive electrode, including:

a sulfur based active material, including:

a core-shell structure with a hollow core and a porous carbon shell surrounding the hollow core;

sulfur present in a portion of the hollow core; and

a polymer shell coating formed on the porous carbon shell, the polymer shell coating including nitrogen atoms that bond to carbon atoms of the porous carbon shell so that the porous carbon shell is a nitrogen-confused porous carbon shell;

a conductive carbon material; and

a binder;

a negative electrode;

a porous membrane positioned between the positive electrode and the negative electrode; and

an electrolyte in contact with at least the porous membrane.

17. The lithium-sulfur battery as defined in claim 16 , wherein the sulfur is in the form of a plurality of nanoparticles, each of which has a size of less than 500 nm.

18. The lithium-sulfur battery as defined in claim 16 , wherein an other portion of the hollow core is a void.

19. The lithium-sulfur battery as defined in claim 16 , further comprising a polysulfide ion cross-linked to the polymer shell coating.