Battery system with improved lifetime property and method for operating battery system

1. A battery system comprising one or two or more secondary battery cells

wherein the battery system includes

a first state where an electric power is generated by a secondary battery cell having a structure where a first electrolyte solution and an electrode assembly are contained in a battery case and the battery case is sealed;

a second state where an electric power is generated by a secondary battery cell having a 20% to 60% reduced maximum capacity compared with the first state; and

a third state where an electric power is generated by a secondary battery cell wherein a second electrolyte solution having a different composition from a first electrolyte solution is additionally added into the battery case in the second state where the maximum capacity is reduced,

wherein the secondary battery cell in the third state to which the second electrolyte solution is added has a 10% to 60% lower internal resistance than the secondary battery cell in the second state,

wherein the first electrolyte solution includes a lithium salt, non-aqueous based solvent and electrode passivation-film forming agent;

wherein the second electrolyte solution includes a lithium salt and a non-aqueous based solvent; and

wherein the second electrolyte solution does not include an electrode passivation-film forming agent,

wherein the battery case includes a perforated opening at a portion near to heat-fused external circumference sides, and the opening is sealed and an insulation tape is added in a state of injecting the second electrolyte solution into the battery case through the opening, and

wherein the opening is perforated in the second state and has a diameter of 0.001 mm to 0.01 mm.

2. The battery system of claim 1 , wherein the battery case is a laminate sheet including a metal layer and resin layer; and

the secondary battery cells in the first state and the second state have a structure where external circumference sides of the laminate sheet are heat-fused in a state where the electrode assembly and the first electrolyte solution are contained in the laminate sheet.

3. The battery system of claim 1 , wherein the battery case is a laminate sheet including a metal layer and resin layer; and

the secondary battery cell in the third state has a structure where external circumference sides of the laminate sheet are heat-fused in a state where the electrode assembly, the first electrolyte solution, and the second electrolyte solution are contained in the laminate sheet.

4. The battery system of claim 3 , wherein the battery case includes a perforated opening at a portion near to heat-fused external circumference sides and circumference sides of the opening is sealed in a form of heat-fusion in a state of injecting the second electrolyte solution into the battery case through the opening.

5. The battery system of claim 1 , wherein the second electrolyte solution in the third state is 3% to 50% of the total weight of the first electrolyte solution in the first state.

6. The battery system of claim 2 , wherein the secondary battery cell in the second state shows 25% to 45% reduced maximum capacity compared with that of the secondary battery cell in the first state.

7. The battery system of claim 1 , wherein at least two secondary battery cells are electrically coupled in series and/or in parallel and supply an external device with electric power.

8. The battery system of claim 1 , wherein the first electrolyte solution comprises vinylene carbonate, propane sultone, and LiPF 6 .

9. The battery system of claim 1 , wherein the first electrolyte solution further comprises a solvent system of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate.

10. The battery system of claim 1 , wherein the second electrolyte solution comprises LiPF 6 and a solvent system of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate.