Lithium secondary battery

1. A lithium secondary battery comprising:

a negative electrode;

a positive electrode; and

an electrolyte disposed between the negative electrode and the positive electrode,

wherein the negative electrode comprises

i) a silicon composite comprising

a core of silicon (Si),

a layer of silicon oxide of formula SiO x wherein 0<x<2 disposed directly on the silicon (Si) core, and

graphene grown directly on the silicon oxide layer; or

ii) a carbonaceous composite comprising

the silicon composite, and

a carbonaceous material, which is different from the graphene, and

wherein at least one of the negative electrode and the electrolyte comprises an ionic liquid containing a fluorosulfonyl imide anion and a lithium salt.

2. The lithium secondary battery of claim 1 , wherein the fluorosulfonyl imide anion comprises (FSO 2 ) 2 N − , (CF 3 SO 2 ) 2 N − , (C 2 F 5 SO 2 ) 2 N − , (C 2 F 5 SO 2 )(CF 3 SO 2 )N − , or (FSO 2 )(CF 3 SO 2 )N − .

3. The lithium secondary battery of claim 1 , wherein the ionic liquid containing a fluorosulfonyl imide anion comprises

at least one cation selected from an ammonium cation, a pyrrolidinium cation, a pyridinium cation, a pyrimidinium cation, an imidazolium cation, a piperidinium cation, a pyrazolium cation, an oxazolium cation, a pyridazinium cation, a phosphonium cation, a sulfonium cation, and a triazolium cation; and

at least one anion selected from (FSO 2 ) 2 N − , (CF 3 SO 2 ) 2 N − , (C 2 F 5 SO 2 ) 2 N − , (C 2 F 5 SO 2 )(CF 3 SO 2 )N − , and (FSO 2 )(CF 3 SO 2 )N.

4. The lithium secondary battery of claim 1 , wherein the ionic liquid containing a fluorosulfonyl imide anion is at least one selected from N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide, N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide, N-butyl-N-methylpyrrolidinium bis(3-trifluoromethylsulfonyl)imide, N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium bis(fluorosulfonyl)imide, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide.

5. The lithium secondary battery of claim 1 , wherein an amount of the ionic liquid containing a fluorosulfonyl imide anion is in a range of about 1 part by weight to about 95 parts by weight, based on 100 parts by weight of the electrolyte.

6. The lithium secondary battery of claim 1 , wherein an amount of the lithium salt is in a range of about 0.2 molar to about 2 molar.

7. The lithium secondary battery of claim 1 , wherein the lithium salt is at least one selected from LiSCN, LiN(CN) 2 , LiClO 4 , LiBF 4 , LiAsF 6 , LiPF 6 , LiCF 3 SO 3 , Li(CF 3 SO 2 ) 2 N, LiSbF 6 , Li(CF 3 SO 2 ) 3 C, LiPF 3 (CF 2 CF 3 ) 3 , LiPF 3 (CF 3 ) 3 , LiB(C 2 O 4 ) 2 , Li(FSO 2 ) 2 N, LiN(SO 2 C 2 F 5 )(CF 3 SO 2 ), LiN(SO 2 C 2 F 5 ) 2 , LiPF 3 (CF 2 CF 3 ) 3 , LiCl, and LiI.

8. The lithium secondary battery of claim 1 , wherein the graphene is in a form of a nanosheet or a film.

9. The lithium secondary battery of claim 1 , wherein an amount of the graphene is in a range of about 0.001 part by weight to about 90 parts by weight, based on 100 parts by weight of the silicon composite.

10. The lithium secondary battery of claim 1 , wherein a distance between the graphene and a silicon of the silicon oxide layer is about 10 nanometers or less,

the graphene comprises at least one to 20 graphene layers,

a total thickness of the graphene is in a range of about 0.6 nanometer to about 12 nanometers, and

the graphene is oriented at an angle ranging from about 0° to about 90° with respect to a primary axis of the silicon.

11. The lithium secondary battery of claim 1 , wherein the graphene extends from a silicon of the silicon oxide layer by a distance of about 1 nanometer or less, and the graphene is oriented at an angle from about 0° to about 90° with respect to a primary axis of the silicon.

12. The lithium secondary battery of claim 1 , wherein a thickness of the silicon oxide layer is about 300 micrometers or less.

13. The lithium secondary battery of claim 1 , wherein a thickness of the silicon oxide layer is in a range of about 0.1 nanometer to about 10 nanometers.

14. The lithium secondary battery of claim 1 , wherein a 2D peak of the graphene, obtained by Raman analysis, is present at about 2,600 inverse centimeters to about 2,800 inverse centimeters.

15. The lithium secondary battery of claim 1 , wherein a D/G intensity ratio of the graphene, obtained by Raman analysis, is in a range of about 1 to about 4.

16. The lithium secondary battery of claim 1 , wherein a G/2D intensity ratio of the graphene, obtained from Raman analysis, is in a range of about 1 to about 6.

17. The lithium secondary battery of claim 1 , wherein a viscosity of the electrolyte is in a range of about 50 centipoise to about 300 centipoise.

18. The lithium secondary battery of claim 1 , wherein the lithium secondary battery is a flexible battery.

19. The lithium secondary battery of claim 1 , further comprising at least one metal oxide selected from a magnesium oxide, a manganese oxide, an aluminum oxide, a titanium oxide, a zirconium oxide, a tantalum oxide, a tin oxide, and a hafnium oxide.

20. The lithium secondary battery of claim 1 , wherein the silicon has an average particle diameter of from about 40 nanometers to about 40 micrometers.

21. The lithium secondary battery of claim 1 , wherein the carbonaceous material in the carbonaceous composite is at least one selected from graphene, graphite, and carbon nanotubes.

22. The lithium secondary battery of claim 1 , wherein the silicon composite further comprises silicon carbide on the silicon oxide.