Negative active material, lithium secondary battery including the material, and method of manufacturing the material

1. A negative active material comprising: a porous silicon-carbon secondary particle comprising

a plurality of silicon-carbon primary particles comprising

a silicon material, and

a first carbonaceous material,

wherein an apparent density of a silicon-carbon primary particle of the plurality of silicon-carbon primary particles is about 2 grams per cubic centimeter to about 2.3 grams per cubic centimeter; and

a second carbonaceous material,

wherein the second carbonaceous material is disposed on the plurality of silicon-carbon primary particles, and

wherein the silicon-carbon primary particle has a porosity of 0% to about 10%, based on a total volume of the silicon carbon primary particle, and

wherein an apparent density of the silicon-carbon secondary particle is about 1.6 grams per cubic centimeter to about 2 grams per cubic centimeter.

2. The negative active material of claim 1 , wherein the silicon-carbon primary particle has a porosity of about 1% to about 10%, based on a total volume of the silicon carbon primary particle.

3. The negative active material of claim 1 , wherein the silicon-carbon primary particle has an average pore diameter of about 5 nanometers to about 2 micrometers.

4. The negative active material of claim 1 , wherein the silicon-carbon primary particle has a specific surface area of about 6 square meters per gram to about 70 square meters per gram.

5. The negative active material of claim 1 , wherein the silicon-carbon secondary particle has a porosity of about 20% to about 50%.

6. The negative active material of claim 1 , wherein the silicon-carbon secondary particle has a specific surface area of about 3 square meters per gram to about 20 square meters per gram.

7. The negative active material of claim 1 ,

wherein the silicon-carbon primary particle has an average particle diameter of about 0.1 micrometer to about 15 micrometers, and

wherein the silicon-carbon secondary particle has an average particle diameter of about 5 micrometers to about 25 micrometers.

8. The negative active material of claim 1 , wherein an amount of silicon in the silicon-carbon primary particle is about 50 weight percent to about 99 weight percent, based on a total weight of the silicon-carbon primary particle, and

wherein the amount of silicon in the silicon-carbon secondary particle is about 30 weight percent to about 70 weight percent, based on a total weight of the silicon-carbon secondary particle.

9. The negative active material of claim 1 ,

wherein the silicon material has a nanostructure comprising silicon, and

wherein the nanostructure is in the form of a nanoparticle, a nanowire, a nanorod, a nanofiber, a nanoporous body, a nanotemplate, a nanoneedle, or a combination thereof.

10. The negative active material of claim 1 , wherein the first carbonaceous material and the second carbonaceous material each independently comprise a crystalline carbon, an amorphous carbon, or a combination thereof.

11. The negative active material of claim 10 , wherein the crystalline carbon comprises natural graphite, artificial graphite, expandable graphite, graphene, carbon nanotubes, or a combination thereof.

12. The negative active material of claim 1 , further comprising a coating layer on a surface of the silicon-carbon secondary particle.

13. The negative active material of claim 12 , wherein the coating layer comprises a fluoride compound, an inorganic oxide, a phosphate compound, or a combination thereof.

14. The negative active material of claim 12 , wherein the coating layer comprises LiF, AlF 3 , AlPO 4 , Li 3 AlF 6 , Al 2 O 3 , ZrO 2 , ZnO 2 , TiO 2 , SnO 2 , AlPO 4 , or a combination thereof.

15. The negative active material of claim 12 , wherein an amount of the coating layer is about 0.1 part to about 5 parts by weight, based on 100 parts by weight of the silicon-carbon secondary particle.

16. The negative active material of claim 1 , wherein the silicon-carbon secondary particle has a structure in which the silicon-carbon primary particle is dispersed in the second carbonaceous material.

17. A lithium secondary battery comprising the negative active material of claim 1 .

18. The negative active material of claim 1 , wherein an amount of silicon in the silicon-carbon primary particle is about 55 weight percent to about 99 weight percent, based on a total weight of the silicon-carbon primary particle.

19. A method of preparing a negative active material, the method comprising:

contacting a silicon material and a first carbonaceous material to form a silicon-carbon primary particle having an apparent density of about 2 grams per cubic centimeter to about 2.3 grams per cubic centimeter; and

thermally treating the silicon-carbon primary particle a second carbonaceous material, and a foaming agent to form porous silicon-carbon secondary particle,

wherein the silicon-carbon secondary particle comprises a plurality of silicon-carbon primary particles and the second carbonaceous material, and

wherein the silicon-carbon primary particle has a porosity of 0% to about 10%, based on a total volume of the silicon carbon primary particle, and

wherein an apparent density of the silicon-carbon secondary particle is about 1.6 grams per cubic centimeter to about 2 grams per cubic centimeter.

20. The method of claim 19 , wherein the contacting the silicon material and the first carbonaceous material comprises pressing and grinding.

21. The method of claim 20 , wherein the foaming agent comprises citric acid, stearic acid, oleic acid, oxalic acid, adipic acid, salicylic acid, benzoic acid, monochloroacetic acid, dibutyl phthalate, dioctyl phthalate, ethylene carbonate, propylene carbonate, petroleum pitch, coal-tar pitch, or a combination thereof.

22. The method of claim 19 , wherein the thermal treating is performed at about 500° C. to about 1,000° C.

23. The method of claim 19 , further comprising forming a coating layer on a surface of the silicon-carbon secondary particle.