Composite negative active material, negative electrode including composite negative active material, and lithium secondary battery including negative electrode

1. A composite negative active material consisting of:

a silicon-containing negative active material; and

a non-metal doped metal phosphate, wherein the non-metal doped metal phosphate comprises two or more metals,

wherein an amount of the non-metal doped metal phosphate is less than 10 parts by weight, based on 100 parts by weight of the silicon-containing negative active material.

2. The composite negative active material of claim 1 , wherein the silicon-containing negative active material comprises at least one selected from elemental silicon , SiO x wherein 0<x<2, a Si—C composite, and a Si alloy.

3. The composite negative active material of claim 2 , wherein the Si alloy comprises at least one selected from Fe 3 Si, Fe 2 Si, Fe 5 Si 3 , FeSi, FeSi 2 , FeSi 3 , AlSi 2 , Cu 5 Si, Mg 2 Si, NiSi, TiSi 2 , and Si 7 Ti 4 Ni 4 .

4. The composite negative active material of claim 1 , wherein a non-metal dopant in the non-metal doped metal phosphate comprises at least one selected from fluorine, sulfur, nitrogen, and boron.

5. The composite negative active material of claim 4 , wherein the non-metal doped metal phosphate is represented by Formula 1:

M x MePO 4 E y   Formula 1

wherein, in Formula 1,

M is at least one selected from lithium, sodium, rubidium, lanthanum, strontium, and radium,

Me is at least one selected from aluminum, titanium, magnesium, zinc, vanadium, manganese, gallium, chromium, iron, rhodium, nickel, indium, and cobalt,

E is a non-metal dopant and is at least one selected from F, S, N, and B,

M is different from Me, and

0.01≤x<2 and 0.01≤y<2.

6. A negative electrode comprising a composite negative active material according to claim 1 .

7. A lithium secondary battery comprising the negative electrode according to claim 6 .

8. The lithium secondary battery of claim 7 , comprising:

a positive electrode;

the negative electrode;

a separator between the positive electrode and the negative electrode; and

an electrolyte.

9. A composite negative active material consisting of:

a core comprising silicon; and

a coating layer on the core, wherein the coating layer comprises a non-metal doped metal phosphate represented by Formula 1

M x MePO 4 E y   Formula 1

wherein, in Formula 1,

M is at least one selected from lithium, sodium, rubidium, lanthanum, strontium, and radium,

Me is at least one selected from aluminum, titanium, magnesium, zinc, vanadium, manganese, gallium, chromium, iron, rhodium, nickel, indium, and cobalt,

E is a non-metal dopant and is at least one selected from F, S, N, and B,

M is different from Me,

0.01≤x<2, and 0.01≤y<2,

wherein an amount of the coating layer is less than 10 parts by weight, based on 100 parts by weight of the core.

10. The composite negative active material of claim 9 , wherein the composite negative active material comprises at least one selected from elemental silicon, SiO x wherein 0<x<2, a Si—C composite, and a Si alloy.

11. A composite negative active material consisting of:

a silicon-containing negative active material comprising silicon; and

a non-metal doped metal phosphate represented by the formula

Li x AlPO 4 F y

wherein 0.01≤x<2and 0.01≤y<2,

wherein an amount of the non-metal doped metal phosphate is less than 10 parts by weight, based on 100 parts by weight of the silicon-containing negative active material.

12. The composite negative active material of claim 11 , wherein the composite negative active material comprises at least one selected from elemental silicon, SiO x wherein 0<x<2, a Si—C composite, and a Si alloy.

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

providing a silicon-containing negative active material;

contacting the silicon-containing negative active material with a solution of a non-metal doped phosphate to form a coated negative active material;

drying the coated negative active material to form a dried negative active material; and

heat-treating the dried negative active material to prepare the composite negative active material,

wherein the composite negative active material consists of

a silicon-containing negative active material, and

a non-metal doped metal phosphate, wherein the non-metal doped metal phosphate comprises two or more metals,

wherein an amount of the non-metal doped metal phosphate is less than 10 parts by weight, based on 100 parts by weight of the silicon-containing negative active material.

14. The method of claim 13 , wherein the silicon-containing negative active material comprises at least one selected from elemental silicon, SiO x wherein 0<x<2, a Si—C composite, and a Si alloy.

15. The method of claim 13 , wherein the heat-treating comprises heat-treating in a nitrogen atmosphere.