Battery cell having double sealing structure

1. A battery cell in which an electrode assembly with a first electrolyte therein is accommodated in a state of being connected to electrode terminals protruding from a battery case, the battery cell comprising a first sealing portion formed at an outer circumferential surface of the battery case by thermal bonding and a second sealing portion further formed between the electrode assembly and the first sealing portion at at least one side surface of the first sealing portion,

wherein a space is formed between the first sealing portion and the second sealing portion and opposite to the electrode assembly, wherein the electrode assembly is not accommodated in said space,

wherein the space has a volume of 5% to 50% based on a total volume of an inside of the battery cell,

wherein second electrolyte is included in the space for being introduced into the battery cell via the second sealing portion when the second sealing portion is damaged by gas generated, and

wherein the second sealing portion has a lower sealing strength than the first sealing portion.

2. The battery cell according to claim 1 , wherein the second sealing portion is formed at a surface at which the electrode terminals are not positioned.

3. The battery cell according to claim 1 , wherein the second sealing portion is formed at a surface to which greatest internal pressure is applied.

4. The battery cell according to claim 1 , wherein the second electrolyte is included in an amount of 5% to 50% based on a total volume of the space.

5. The battery cell according to claim 1 , wherein the second sealing portion has a sealing strength of 1.0 kg/cm 2 to 1.9 kg/cm 2 .

6. The battery cell according to claim 1 , wherein the first sealing portion has a sealing strength of 2.0 kg/cm 2 to 4.5 kg/cm 2 .

7. The battery cell according to claim 1 , wherein the second sealing portion has a lower area than the first sealing portion.

8. The battery cell according to claim 1 , wherein the electrode assembly comprises a cathode, an anode, and a separator disposed between the cathode and the anode.

9. The battery cell according to claim 8 , wherein the cathode comprises a lithium transition metal oxide represented by Formula 1 or 2 below as a cathode active material:

Li x M y Mn 2-y O 4-z A z   (1)

wherein M is at least one element selected from the group consisting of Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, Ti, and Bi;

A is at least one monovalent or divalent anion; and

0.9≦ x≦ 1.2, 0< y< 2, and 0≦ z< 0.2,

(1 −x )LiM′O 2-y A y - x Li 2 MnO 3-y′ A y′   (2)

wherein M′ is Mn a M b ;

M is at least one selected from the group consisting of Ni, Ti, Co, Al, Cu, Fe, Mg, B, Cr, Zr, Zn, and Period 2 transition metals;

A is at least one selected from the group consisting of PO 4 , BO 3 , CO 3 , F and NO 3 anions; and

0< x< 1, 0< y≦ 0.02, 0< y′≦ 0.02, 0.5≦ a≦ 1.0, 0≦ b≦ 0.5, and a+b= 1.

10. The battery cell according to claim 8 , wherein the anode comprises a carbon-based material and/or Si as an anode active material.

11. The battery cell according to claim 1 , wherein the battery cell is a lithium ion battery, a lithium ion polymer battery, or a lithium polymer battery.

12. A battery module comprising the battery cell according to claim 1 as a unit cell.

13. A battery pack comprising the battery module according to claim 12 .

14. A device comprising the battery pack according to claim 13 as a power source.

15. The device according to claim 14 , wherein the device is an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a system for storing power.