Positive electrode for secondary battery, method of preparing the same, and lithium secondary battery including the positive electrode

1. A positive electrode for a secondary battery, the positive electrode comprising:

a positive electrode collector; and

a positive electrode material mixture layer coated on at least one side of the positive electrode collector,

wherein the positive electrode material mixture layer comprises a positive electrode active material, a conductive agent, a binder, and bimodal pores composed of first pores and second pores which have different average diameters, and

the conductive agent and the positive electrode active material are included at a volume ratio (K 1 ) of 0.08:1 to 0.32:1,

wherein a volume ratio of the conductive agent to the total pores is in a range of 0.1:1 to 0.33:1,

a porosity is in a range of 30 vol % to 45 vol %,

maximum diameters of the first pores and the second pores are less than 1 μm, and

an average diameter ratio (k) of the first pores to the second pores is in a range of 0.13:1 to 0.27:1.

2. The positive electrode for a secondary battery of claim 1 , wherein the positive electrode active material has an average particle diameter (D 50 ) of 3 μm to 20 μm.

3. The positive electrode for a secondary battery of claim 1 , wherein the positive electrode active material comprises a single material selected from the group consisting of lithium-manganese-based oxide, lithium-cobalt-based oxide, lithium-nickel-based oxide, lithium-nickel-manganese-based oxide, lithium-nickel-cobalt-based oxide, lithium-manganese-cobalt-based oxide, lithium-nickel-manganese-cobalt-based oxide, and lithium-nickel-cobalt-transition metal oxide, or a mixture of two or more thereof.

4. The positive electrode for a secondary battery of claim 1 , wherein the conductive agent has an average particle diameter (D 50 ) of 5 nm to 30,000 nm.

5. The positive electrode for a secondary battery of claim 1 , wherein the conductive agent comprises a single material selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, conductive fibers, fluorocarbon, aluminum, nickel powder, conductive whiskers, conductive metal oxide, and polyphenylene derivatives, or a mixture of two or more thereof.

6. The positive electrode for a secondary battery of claim 1 , wherein the first pores are pores between the conductive agent-the conductive agent which are formed by being surrounded by adjacent conductive agent particles, and

the second pores are pores between the conductive agent-the positive electrode active material which are formed by being surrounded by the adjacent conductive agent and positive electrode active material,

wherein outer surfaces of the first and second pores are respectively nonlinear along surfaces of pluralities of adjacent conductive agent and positive electrode active material particles.

7. The positive electrode for a secondary battery of claim 1 , wherein the average diameter of the first pores is in a range of 1 nm to 100 nm, and

the average diameter of the second pores is in a range of 100 nm to 500 nm.

8. The positive electrode for a secondary battery of claim 7 , wherein the average diameter of the second pores is in a range of 200 nm to 400 nm.

9. The positive electrode for a secondary battery of claim 1 , wherein the positive electrode has an energy density per unit area of 0.8 mAh/cm 2 to 1.8 mAh/cm 2 .

10. A method of preparing a positive electrode for a secondary battery, the method comprising:

preparing a positive electrode active material slurry having a solid content of 60 wt % to 90 wt % by mixing a conductive agent with a positive electrode active material at a volume ratio of 0.08:1 to 0.32:1;

preparing a positive electrode including a positive electrode material mixture layer having a porosity of 55 vol % to 65 vol % by coating a positive electrode collector with the positive electrode active material slurry and drying the coated positive electrode collector;

performing primary rolling of the positive electrode to prepare a positive electrode including a positive electrode material mixture layer having a porosity of 53 vol % to 57 vol %;

performing secondary rolling of the positive electrode prepared after the primary rolling to prepare a positive electrode including a positive electrode material mixture layer having a porosity of 30 vol % to 45 vol %; and

performing tertiary rolling of the positive electrode prepared after the secondary rolling to prepare a positive electrode including a positive electrode material mixture layer having a porosity of 30 vol % to 45 vol %.

11. The method of claim 10 , wherein the positive electrode active material slurry is coated with a loading amount of 2 mg/cm 2 to 15 mg/cm 2 .

12. The method of claim 10 , wherein the primary rolling is performed under conditions in which a gap between two top rolls and a bottom roll at room temperature is (total thickness of the positive electrode before the primary rolling+total thickness of the positive electrode targeted after the tertiary rolling)/2.

13. The method of claim 10 , wherein the secondary rolling and the tertiary rolling are performed under conditions in which a gap between two top rolls and a bottom roll at room temperature is the same as a total thickness of the positive electrode targeted after the tertiary rolling.

14. The method of claim 10 , further comprising allowing the positive electrode prepared after the primary rolling to be left standing for 30 minutes to 2 hours before the secondary rolling.

15. The method of claim 10 , further comprising allowing the positive electrode prepared after the secondary rolling to be left standing for 30 minutes to 2 hours before the tertiary rolling.

16. A lithium secondary battery comprising:

a positive electrode;

a negative electrode;

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

a non-aqueous electrolyte solution,

wherein the positive electrode comprises the positive electrode of claim 1 .