Secondary battery

1. A secondary battery, comprising:

an electrode assembly, the electrode assembly including a first electrode, a second electrode, and a separator;

a case accommodating the electrode assembly therein;

a cell cover sealing the case; and

a first terminal unit having one end that is electrically connected to the first electrode of the electrode assembly and having another end extracted to an outside of the case,

wherein the first terminal unit includes:

a first collector in the case and electrically connected to the first electrode of the electrode assembly, the first collector having a meltable fuse area; and

an arc cutting block adjacent to the meltable fuse area of the first collector, the arc cutting block:

being movable to a position previously occupied by the meltable fuse area in an event that the meltable fuse area is melted and cut off, and

blocking an arc generation path,

wherein the arc cutting block includes an insulating block on a top of the meltable fuse area; and a plate on a bottom of the meltable fuse area and coupled with the insulating block such that the meltable fuse area is between the insulating block and the electrode assembly and the position previously occupied by the meltable fuse area in an event that the meltable fuse area is melted and cut off is closer to the electrode assembly than a position of the insulating block when the meltable fuse area is in an unmelted state, and

wherein the plate defines a gas exhaust hole space surrounding the meltable fuse area that is open or closed according to a location of the plate along a vertical direction, a width of the insulation block being wider than that of the gas exhaust hole.

2. The secondary battery as claimed in claim 1 , wherein the meltable fuse area has a sectional area that is smaller than a sectional area of another region of the first collector.

3. The secondary battery as claimed in claim 1 , wherein the first terminal unit includes a first electrode terminal having one side exposed at a top surface of the cell cover, and having another side that is electrically connected to the first collector.

4. The secondary battery as claimed in claim 3 , wherein the first collector has a structure in which the first electrode terminal is coupled with a plate-shaped side thereof, and another side is bent, extends downwardly, and is electrically connected to the first electrode of the electrode assembly.

5. The secondary battery as claimed in claim 3 , wherein the first electrode terminal includes:

an external electrode on the cell cover; and

a rivet terminal having:

a lower end that contacts the first collector, and

an upper end that is coupled with the external electrode by passing through the cell cover.

6. The secondary battery as claimed in claim 1 , wherein the first terminal unit includes:

an upper insulating member on a top of the cell cover; and

a lower insulating member on a bottom of the cell cover, the lower insulating member having one opened side through which a portion of the first collector is mounted.

7. The secondary battery as claimed in claim 6 , wherein the first terminal unit includes an insert molding portion being coupled with the lower insulating member, covering the meltable fuse area of the first collector, and having the gas exhaust hole aligned with the meltable fuse area.

8. The secondary battery as claimed in claim 7 , wherein the plate seals the gas exhaust hole of the insert molding portion.

9. The secondary battery as claimed in claim 8 , wherein the insulating block is movable into the position that was occupied by the meltable fuse area in response to pressure applied to the plate by arc gas generated when the meltable fuse area is damaged.

10. The secondary battery as claimed in claim 8 , wherein the insulating block includes one or more coupling projections protruding therefrom, the one or more coupling projections being coupled and fixed to one or more coupling holes in the plate.