Composite negative active material, negative electrode and lithium secondary battery including the composite negative active material, and method of preparing the composite negative active material

1. A composite negative active material comprising:

a core comprising silicon; and

a coating layer disposed on the core,

wherein the coating layer comprises a water-insoluble polymer composite comprising at least one anionic component selected from F − , PO 4 3− , PF 6 − , BO 3 3− , and BF 4 − , wherein the at least one anionic component is chemically bonded to a water-soluble polymer.

2. The composite negative active material of claim 1 , wherein the water-soluble polymer comprises at least one selected from a polyvinyl alcohol homopolymer and a polyvinyl alcohol copolymer.

3. The composite negative active material of claim 2 , wherein the polyvinyl alcohol copolymer comprises at least one selected from poly(vinyl alcohol-co-ethylene glycol), poly(vinyl alcohol-co-ethylene), poly(vinyl alcohol-co-nitrile), polyacrylonitrile-grafted polyvinyl alcohol, poly(vinyl alcohol-co-methyl methacrylate), and poly(vinyl alcohol-co-acrylic acid).

4. The composite negative active material of claim 1 , wherein the coating layer comprises a water-insoluble polymer composite in which the anionic component as a part of a salt is chemically bonded to the water-soluble polymer.

5. The composite negative active material of claim 4 , wherein the salt comprises at least one selected from NH 4 F, NH 2 H 2 PO 4 , NH 4 PF 6 , (NH 4 ) 3 BO 3 , NH 4 BF 4 , and H 3 BO 3 .

6. The composite negative active material of claim 1 , wherein the coating layer is uncarbonized.

7. The composite negative active material of claim 1 , wherein the core comprising the silicon comprises at least one selected from elemental silicon, a blend of silicon and carbon, a composite of silicon and carbon, and a silicon alloy.

8. The composite negative active material of claim 7 , wherein the composite of silicon and carbon comprises a composite of silicon and a carbon nanotube or a composite of silicon and a carbon nanofiber.

9. The composite negative active material of claim 1 , wherein an amount of the coating layer is about 5 parts by weight or less, based on 100 parts by weight of a total weight of the composite negative active material.

10. The composite negative active material of claim 1 , wherein an amount of the water-soluble polymer is about 5 parts by weight or less, based on 100 parts by weight of a total weight of the composite negative active material.

11. The composite negative active material of claim 1 , wherein an amount of the anionic component is about 5 parts by weight or less, based on 100 parts by weight of a total weight of the composite negative active material.

12. The composite negative active material of claim 1 , wherein a molar ratio of the water-soluble polymer to the anionic component is in a range of about 1:0.2 to about 1:3.

13. The composite negative active material of claim 1 , further comprising an inorganic material which is non-reactive with the core.

14. The composite negative active material of claim 13 , wherein the inorganic material comprises at least one selected from MgF 2 , Mg 3 (PO 4 ) 2 , AlF 3 , Al(PO 4 ) 3 , Al 2 O 3 , and MgO.

15. The composite negative active material of claim 13 , wherein an amount of the inorganic material is about 5 parts by weight or less, based on 100 parts by weight of a total weight of the composite negative active material.

16. A negative electrode comprising the composite negative active material of claim 1 .

17. The negative electrode of claim 16 , further comprising a binder.

18. A lithium secondary battery comprising the negative electrode of claim 16 .