Negative electrode active material, method of preparing the same, and lithium secondary battery including the same

1. A composite negative electrode active material comprising:

a core capable of intercalating and deintercalating lithium ions; and

a plurality of coating layers comprising two or more silicon (Si) layers having different densities formed on a surface of the core,

wherein each of the two or more Si layers include at least about 90 wt % of Si,

wherein the core comprises a non-carbon-based core.

2. The composite negative electrode active material of claim 1 , wherein the non-carbon-based core comprises Si or a non-crystalline or crystalline Si oxide represented by SiO x where 0<x≤2.

3. The composite negative electrode active material of claim 1 , wherein a ratio of an average particle diameter of the core to a total thickness of the plurality of coating layers is 1:0.1 to 0.7.

4. The composite negative electrode active material of claim 1 , wherein the plurality of coating layers comprises a high-density Si layer and a low-density Si layer.

5. The composite negative electrode active material of claim 4 , wherein each of the high-density Si layer and the low-density Si layer comprises one selected from the group consisting of a Si single phase, SiO x where 0<x≤2, and an Si—Z alloy wherein Z is an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, or a combination thereof, except for Si, or a mixture of two or more of these materials.

6. The composite negative electrode active material of claim 4 , wherein the plurality of coating layers has a structure in which two or more of the high-density Si layer and the low-density Si layer are sequentially and repeatedly formed.

7. The composite negative electrode active material of claim 6 , wherein a thickness ratio of the high-density Si layer to the low-density Si layer is 1:1.

8. The composite negative electrode active material of claim 4 , wherein the high-density Si layer has a bulk density of greater than 2.0 g/cm 3 to 2.33 g/cm 3 or less.

9. The composite negative electrode active material of claim 4 , wherein the low-density Si layer has a bulk density of 2.0 g/cm 3 or less.

10. The composite negative electrode active material of claim 1 , wherein the negative electrode active material has an average particle diameter (D50) of 3 μm to 55 μm.

11. The composite negative electrode active material of claim 1 , wherein the negative electrode active material has a specific surface area (BET) of 0.5 m 2 /g to 20 m 2 /g.

12. The composite negative electrode active material of claim 1 , wherein the composite negative electrode active material further comprises a carbon coating layer.

13. A method of preparing the composite negative electrode active material of claim 1 , the method comprising:

forming a high-density Si layer on a surface of a core capable of intercalating and deintercalating lithium ions;

forming a low-density Si layer on the surface of the core with the high-density Si layer formed thereon; and

repeating the forming of the high-density Si layer and the forming of the low-density Si layer a plurality of times.

14. The method of claim 13 , wherein the forming of the high-density Si layer is performed by radio-frequency (RF) sputtering under conditions where an argon (Ar) working gas pressure has been adjusted to 1.0×10 −3 mbar to 2.0×10 −3 mbar.

15. The method of claim 13 , wherein the forming of the low-density Si layer is performed by RF sputtering under conditions where an Ar working gas pressure has been adjusted to 1.0×10 −2 mbar to 2.0×10 −2 mbar.

16. The method of claim 13 , wherein the method further comprises forming a carbon coating layer.

17. A negative electrode comprising:

a current collector; and

an electrode mixture layer comprising the composite negative electrode active material of claim 1 formed on at least one surface of the current collector.

18. A lithium secondary battery comprising a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and a non-aqueous electrolyte,

wherein the negative electrode comprises the negative electrode of claim 17 .