Cathode, method of preparing the same, and lithium battery including the cathode

1. A cathode comprising:

a current collector;

a first cathode active material layer disposed on the current collector; and

a second cathode active material layer disposed on a surface of the first cathode active material layer opposite the current collector,

wherein the first cathode active material layer comprises a lithium transition metal oxide having a layered structure, and the second cathode active material layer comprises a lithium transition metal oxide having a spinel structure and an average working potential of 4.5 V or more,

wherein the lithium transition metal oxide having the spinel structure is represented by Formula 2 below:

Li 1+a Ni 0.5+b Mn 1.5+c M d O 4−e M′ e   <Formula 2>

in which −0.2≦a≦0.2, −0.2≦b≦0.2, −0.2≦c≦0.2, 0≦d≦0.5, and 0≦e≦0.1; M includes at least one metal selected from the group consisting of aluminum (Al), magnesium (Mg), titanium (Ti), zinc (Zr), vanadium (V), niobium (Nb), iron (Fe), zinc (Zn), silicon (Si), and tin (Sn); and M′ includes at least one element selected from the group consisting of nitrogen (N), sulfur (S), and fluorine (F),

wherein the cathode has a coulombic efficiency of greater than 94% after 100 cycles when cycled at a temperature higher than 25° C., and

wherein the lithium transition metal oxide of the second cathode active material layer is not charged or discharged during charging and discharging of the cathode.

2. The cathode of claim 1 , wherein the lithium transition metal oxide having the layered structure is represented by Formula 1 below:

Li 1+x Me 1−x−y A y O 2   <Formula 1>

in which −0.1≦x≦0.3 and 0≦y≦0.1; Me includes at least one metal selected from the group consisting of nickel (Ni), cobalt (Co), and manganese (Mn); and A includes at least one selected from the group consisting of aluminum (Al), chromium (Cr), magnesium (Mg), titanium (Ti), zirconium (Zr), and molybdenum (Mo).

3. The cathode of claim 1 , wherein the lithium transition metal oxide having the layered structure comprises at least one metal oxide selected from the group consisting of LiCoO 2 , LiNiO 2 , and LiNi v Co w Al y O 2 (0≦v≦0.95, 0≦w≦0.5, and 0≦y≦0.1).

4. The cathode of claim 1 , wherein the lithium transition metal oxide having the layered structure comprises LiNi 1−x−y Co x Mn y O 2 (0≦x≦0.5 and 0≦y≦0.5).

5. The cathode of claim 1 , wherein the lithium transition metal oxide having the layered structure comprises xLi 2 MO 3 .(1−x)LiMeO 2 in which 0≦x≦1, M includes at least one metal selected from the group consisting of manganese (Mn), titanium (Ti), zinc (Zr), tin (Sn), and molybdenum (Mo), and Me includes at least one metal selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), aluminum (Al), magnesium (Mg), zinc (Zn), copper (Cu), chromium (Cr), vanadium (V), and niobium (Nb).

6. The cathode of claim 1 , wherein the lithium transition metal oxide having the spinel structure comprises LiNi 0.5 Mn 1.5 O 4 .

7. The cathode of claim 1 , wherein the thickness of the second cathode active material layer is 1/10 or less of that of the first cathode active material layer.

8. The cathode of claim 1 , wherein the second cathode active material layer completely covers the first cathode active material layer.

9. A method of forming a cathode, the method comprising:

forming a first cathode active material layer on a current collector; and

forming a second cathode active material layer on a surface of the first cathode active material layer opposite the current collector,

wherein the first cathode active material layer comprises a lithium transition metal oxide having a layered structure, and the second cathode active material layer comprises a lithium transition metal oxide having a spinel structure and an average working potential of 4.5 V or more,

wherein the lithium transition metal oxide having the spinel structure is represented by Formula 2 below:

Li 1+a Ni 0.5+b Mn 1.5+c M d O 4−e M′ e   <Formula 2>

in which −0.2≦a≦0.2, −0.2≦b≦0.2, −0.2≦c≦0.2, 0≦d≦0.5, and 0≦e≦0.1; M includes at least one metal selected from the group consisting of aluminum (Al), magnesium (Mg), titanium (Ti), zinc (Zr), vanadium (V), niobium (Nb), iron (Fe), zinc (Zn), silicon (Si), and tin (Sn); and M′ includes at least one element selected from the group consisting of nitrogen (N), sulfur (S), and fluorine (F), and

wherein the cathode has a coulombic efficiency of greater than 94% after 100 cycles when cycled at a temperature higher than 25° C., and

wherein the lithium transition metal oxide of the second cathode active material layer is not charged or discharged during charging and discharging of the cathode.

10. The method of claim 9 , wherein the lithium transition metal oxide having the layered structure is represented by Formula 1 below:

Li 1+x Me 1−x−y A y O 2   <Formula 1>

in which −0.1≦x≦0.3 and 0≦y≦0.1; Me includes at least one metal selected from the group consisting of nickel (Ni), cobalt (Co), and manganese (Mn); and A includes at least one metal selected from the group consisting of aluminum (Al), chromium (Cr), magnesium (Mg), titanium (Ti), zirconium (Zr), and molybdenum (Mo).

11. The method of claim 9 , wherein the lithium transition metal oxide having the spinel structure comprises LiNi 0.5 Mn 1.5 O 4 .

12. A lithium battery comprising the cathode of claim 1 .

13. A lithium battery comprising the cathode of claim 2 .

14. A lithium battery comprising the cathode of claim 3 .

15. A lithium battery comprising the cathode of claim 4 .

16. A lithium battery comprising the cathode of claim 5 .

17. A lithium battery comprising the cathode of claim 6 .

18. A lithium battery comprising the cathode of claim 7 .

19. A lithium battery comprising the cathode of claim 8 .