Lithium secondary battery

1. A lithium secondary battery comprising:

a positive electrode;

a negative electrode comprising a negative electrode current collector;

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

a lithium metal layer on the negative electrode current collector in the negative electrode,

wherein the electrolyte comprises a first electrolyte layer and a second electrolyte layer disposed on the first electrolyte layer,

wherein the first electrolyte layer faces the negative electrode, and the second electrolyte layer faces the positive electrode;

wherein the first electrolyte layer has higher ion conductivity than the second electrolyte layer, and a difference in the ion conductivity between the first electrolyte layer and the second electrolyte layer is from 2 times to 10 4 times;

wherein the first electrolyte layer comprises LiFSI, and the second electrolyte layer comprises poly(ethylene glycol) methyl ether acrylate (PEGMEA), poly(ethylene glycol) diacrylate (PEGDA), succinonitrile (SN), and LiTFSI;

wherein the lithium metal layer is formed by migration of lithium ions from the positive electrode after charging, and

wherein the positive electrode comprises a lithium metal compound represented by any one of the following Chemical Formula 3 to 8, or mixtures thereof:

LiM 3 e Mn 1-e O 2   [Chemical Formula 3]

in Chemical Formula 3, 0≤e<0.5, and M 3 is an element of one or more types selected from the group consisting of Cr, Al, Ni, Mn and Co,

Li 2 M 4 O 2   [Chemical Formula 4]

in Chemical Formula 4, M 4 is Cu,

Li 3+f Nb 1-g M 5 g S 4-h   [Chemical Formula 5]

in Chemical Formula 5, −0.1≤f≤1, 0≤g≤0.5, −0.1≤h≤0.5, and M 5 is an element of one or more types selected from the group consisting of Mn, Fe, Co, Cu, Zn, Mg and Cd,

LiM 6 i Mn 1-i O 2   [Chemical Formula 6]

in Chemical Formula 6, i is 0.05≤i<0.5, and M 6 is an element of one or more types selected from the group consisting of Cr, Al, Ni, Mn and Co,

LiM 7 2j Mn 2-2j O 4   [Chemical Formula 7]

in Chemical Formula 7, j is 0.05≤j<0.5, and M 7 is an element of one or more types selected from the group consisting of Cr, Al, Ni, Mn and Co, and

Li k -M 8 m -N n   [Chemical Formula 8]

in Chemical Formula 8, M 8 represents an alkaline-earth metal, k/(k+m+n) is from 0.10 to 0.40, m/(k+m+n) is from 0.20 to 0.50, and n/(k+m+n) is from 0.20 to 0.50).

2. The lithium secondary battery of claim 1 , wherein the first electrolyte layer has ion conductivity of 10 −5 S/cm to 10 −2 S/cm, and

the second electrolyte layer has ion conductivity of 10 −6 S/cm to 10 −3 S/cm.

3. The lithium secondary battery of claim 1 , wherein one or more of the first electrolyte layer and the second electrolyte layer are a semisolid electrolyte or a solid electrolyte.

4. The lithium secondary battery of claim 1 , wherein the first electrolyte layer has a thickness of 0.1 μm to 20 μm, and the second electrolyte layer has a thickness of 0.1 μm to 50 μm.

5. The lithium secondary battery of claim 1 , wherein the electrolyte further comprises at least one additional electrolyte layer formed on the second electrolyte layer facing the positive electrode.

6. The lithium secondary battery of claim 5 , wherein the electrolyte layer facing the positive electrode of the at least one additional electrolyte layer formed on the second electrolyte layer has higher ion conductivity than the second electrolyte layer.

7. The lithium secondary battery of claim 6 , wherein the at least one additional electrolyte layer formed on the second electrolyte layer is a third electrolyte layer.

8. The lithium secondary battery of claim 6 , wherein the electrolyte layer facing the positive electrode of the at least one additional electrolyte layer formed on the second electrolyte layer has ion conductivity of 10 −5 S/cm to 10 −2 S/cm.

9. The lithium secondary battery of claim 1 , wherein the electrolyte has a separator provided between the first electrolyte layer and the second electrolyte layer.

10. The lithium secondary battery of claim 9 , wherein the separator is provided in a form of being impregnated into the electrolyte.

11. The lithium secondary battery of claim 1 , wherein the lithium metal layer is formed through a one-time charge in a voltage range of 4.5 V to 2.5 V.

12. The lithium secondary battery of claim 1 , wherein the positive electrode further comprises one or more of positive electrode active material selected from the group consisting of LiCoO 2 , LiNiO 2 , LiMnO 2 , LiMn 2 O 4 , Li(Ni a Co b Mn c )O 2 wherein 0<a<1, 0<b<1, 0<c<1, and a+b+c=1, LiN 1-Y Co Y O 2 , LiCo 1-Y Mn Y O 2 , LiNi 1-Y Mn Y O 2 wherein 0≤Y<1, Li(Ni a Co b Mn c )O 4 wherein 0<a<2, 0<b<2, 0<c<2, and a+b+c=2, LiMn 2-z Ni z O 4 , LiMn 2-z Co z O 4 wherein 0<Z<2, Li x M y Mn 2-y O 4-z A z wherein 0.9≤x≤1.2, 0<y<2, 0≤z<0.2, M is one or more of Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, Ti and Bi, and A is one or more anions of −1 or −2 valency, Li 1+a Ni b M′ 1-b O 2-c A′c wherein 0≤a≤0.1, 0≤b≤0.8, 0≤c<0.2, M′ is one or more types selected from the group consisting of octahedral stable elements such as Mn, Co, Mg or Al, and A′ is one or more anions of −1 or −2 valency, LiCoPO 4 and LiFePO 4 .

13. The lithium secondary battery of claim 1 , wherein the positive electrode comprises a lithium metal compound represented by Chemical Formula 3.

14. The lithium secondary battery of claim 1 , wherein the positive electrode comprises a lithium metal compound represented by Chemical Formula 4.

15. The lithium secondary battery of claim 1 , wherein the positive electrode comprises a lithium metal compound represented by Chemical Formula 5.

16. The lithium secondary battery of claim 1 , wherein the positive electrode comprises a lithium metal compound represented by Chemical Formula 6.

17. The lithium secondary battery of claim 1 , wherein the positive electrode comprises a lithium metal compound represented by Chemical Formula 7.

18. The lithium secondary battery of claim 1 , wherein the positive electrode comprises a lithium metal compound represented by Chemical Formula 8.