Secondary battery

1. A secondary battery comprising:

an electrode assembly in which an electrode and a separator are alternately stacked; and

a battery case accommodating the electrode assembly therein,

wherein the battery case comprises a stepped part that is disposed to be stepped,

an outer surface of the battery case is sealed to allow a folded part to be seated on the outer surface of the stepped part, and

the electrode assembly comprises a stepped protrusion that is stepped in a shape corresponding to an inner surface of the stepped part disposed on the battery case

wherein the stepped part of the battery case is disposed on an upper lateral portion of the battery case.

2. The secondary battery of claim 1 , wherein the stepped part has an outer surface that is stepped in a shape corresponding to the folded part.

3. The secondary battery of claim 1 ,

wherein the stepped protrusion of the electrode assembly is disposed on an upper lateral portion of the electrode assembly, and

the electrode assembly has a narrow width so that a width of the electrode and the separator, which are disposed on an upper end of the electrode assembly, corresponds to the stepped part of the battery case.

4. The secondary battery of claim 3 , wherein an outer surface of the stepped protrusion disposed along a circumference of the electrode assembly has a shape corresponding to the inner surface of the stepped part of the battery case, and

the outer surface of the stepped protrusion of the electrode assembly and the inner surface of the stepped part of the battery case are closely attached to each other.

5. The secondary battery of claim 1 , wherein the folded part is attached to the outer surface of the stepped part through a tape or an adhesive.

6. The secondary battery of claim 1 , wherein folded part comprises a bent part, and

the outer surface of the battery case is manufactured by sealing and folding a portion of the folded part except for the bent part.

7. The secondary battery of claim 6 , wherein the portion of the folded part except for the bent part is thermally fused and fixed to the outer surface of the stepped part.

8. The secondary battery of claim 7 , wherein the battery case comprises a lower case comprising an accommodation part communicating the electrode assembly and an upper case covering the accommodation part, and

outer surfaces of the upper and lower cases are folded together to provide the folded part and are seated on the stepped part.

9. The secondary battery of claim 1 , wherein the stepped part has a stepped height corresponding to a height of the folded part.

10. The secondary battery of claim 1 , wherein the folded part is folded in two stages to sequentially provide a first bent part and a second bent part.

11. The secondary battery of claim 10 , wherein a hook protrusion that protrudes in a direction of the folded part to catch an end of the folded part and prevent an end of the folded part from being separated from the stepped part is disposed on an upper end of the stepped part.

12. The secondary battery of claim 11 , wherein the hook protrusion is rounded, and the second bent part disposed on the end of the folded part is coupled to the hook protrusion to prevent the second bent part from being separated from the stepped part.

13. The secondary battery of claim 1 , wherein the stepped part of the battery case comprises a first stepped part disposed on an upper lateral portion of the battery case and a second stepped part disposed on a lower rear end of the battery case, and

the stepped protrusion of the electrode assembly comprises a first stepped protrusion disposed on an upper lateral portion of the electrode assembly and a second stepped protrusion disposed on a lower rear end of the electrode assembly.

14. The secondary battery of claim 13 , wherein the upper lateral portion of the electrode assembly has a shape corresponding to the first stepped part, and

the lower rear end of the electrode assembly has a shape corresponding to the second stepped part.

15. The secondary battery of claim 13 , wherein the folded part comprises:

a first folded part that is primarily folded to be seated on the first stepped part of the battery case;

a second folded part that is secondarily folded to face a rear end surface of the battery case; and

a third folded part that is tertiarily folded to be seated on the second stepped part of the battery case.

16. The secondary battery of claim 1 , wherein the stepped part of the battery case is disposed on a lower rear end of the battery case, and

the stepped protrusion of the electrode assembly is disposed on a lower rear end of the electrode assembly.

17. The secondary battery of claim 16 , wherein the folded part comprises:

a first folded part that is primarily folded to face a rear end surface of the battery case; and

a second folded part that is secondarily folded to be seated on the stepped part of the battery case.