Electrolyte solvent for improving safety of battery and lithium secondary battery comprising the same

1. A solvent system for use in an electrolyte for a lithium secondary battery, comprising

a fluoroethylene carbonate solvent and

a linear ester solvent,

wherein the fluoroethylene carbonate solvent is contained at an amount of 20-60% by volume, based on the total volume of the electrolyte solvent, and the linear ester solvent is contained at an amount of 40-80% by volume, based on the total volume of the electrolyte solvent, and

wherein a lithium secondary battery containing an electrolyte comprising the solvent system is able to withstand a temperature of 150° C. for up to 120 minutes without a thermal explosion at a charged state of 4.35 V.

2. The solvent system of claim 1 , wherein the fluoroethylene carbonate solvent is selected from the group consisting of monofluoroethylene carbonate, difluoroethylene carbonate and a mixture thereof.

3. The solvent system of claim 1 , wherein the linear ester solvent is selected from the group consisting of ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, ethyl acetate, methyl propionate, and a mixture of two or more thereof.

4. The solvent system of claim 1 , wherein a lithium secondary battery containing the electrolyte comprising the solvent system is able to withstand a temperature of 150° C. for over 90 minutes without a thermal explosion at a charged state of 4.2 V.

5. An electrolyte for batteries comprising the solvent system of claim 1 and a lithium salt.

6. The electrolyte of claim 5 , wherein the wherein the electrolyte has an ion conductivity of 7.56 mS/cm or more.

7. A lithium secondary battery comprising a positive electrode, a negative electrode and the electrolyte according to claim 5 , wherein the electrolyte further comprises lithium salts.

8. The lithium secondary battery of claim 7 , wherein the fluoroethylene carbonate solvent is selected from the group consisting of monofluoroethylene carbonate, difluoroethylene carbonate and a mixture thereof.

9. The lithium secondary battery of claim 7 , wherein the linear ester solvent is selected from the group consisting of ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, ethyl acetate, methyl propionate, and a mixture of two or more thereof.

10. A solvent system for use in an electrolyte for a lithium secondary battery, comprising:

20-60% by volume of a fluoroethylene carbonate solvent selected from the group consisting of monofluoroethylene carbonate, difluoroethylene carbonate and a mixture thereof, and

40-80% by volume of a linear ester solvent selected from the group consisting of ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, ethyl acetate, methyl propionate, and a mixture of two or more thereof, wherein the volume percentages fluoroethylene carbonate solvent and linear ester solvent are each based on the total volume of the electrolyte solvent,

wherein a lithium secondary battery containing an electrolyte comprising the solvent system is able to withstand a temperature of 150° C. for up to 120 minutes without a thermal explosion at a charged state of 4.35V.

11. An electrolyte for batteries, which comprises:

a solvent system comprising:

20-60% by volume of a fluoroethylene carbonate solvent selected from the group consisting of mono fluoroethylene carbonate, difluoroethylene carbonate and a mixture thereof, and

40-80% by volume of a linear ester solvent selected from the group consisting of ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, ethyl acetate, methyl propionate, and a mixture of two or more thereof, wherein the volume percentages fluoroethylene carbonate solvent and linear ester solvent are each based on the total volume of the electrolyte solvent; and

a lithium salt,

wherein a lithium secondary battery containing the electrolyte is thermally stable to 150° C. for up to 120 minutes at a charged state of 4.35 V.

12. The electrolyte for batteries of claim 11 , wherein the lithium salt comprises lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF 3 SO 2 ) 2 ), lithium bis(perfluoroethylsulfonyl)imide (LiN(C 2 F 5 SO 2 ) 2 ), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF 3 SO 2 ) 3 ), lithium triflate (LiCF 3 SO 3 ), lithium bisoxalatoborate (LiC 4 BO 8 ), or a mixture of two or more thereof.

13. A lithium secondary battery comprising a positive electrode, a negative electrode, and the electrolyte of claim 11 .

14. The lithium secondary battery of claim 13 , wherein the positive electrode comprises active materials including LiCoO 2 ; LiNiO 2 ; LiMn 2 O 4 ; LiMnO 2 ; LiNi 1-X Co X M Y O 2 where M=Al, Ti, Mg or Zr, 0<X≦1, and 0≦Y≦0.2; LiNi X Co Y Mn 1-X-Y O 2 where 0<X≦0.5, and 0<Y≦0.5; LiM x M′ y Mn (2-x-y) O 4 where M, M′=V, Cr, Fe, Co, Ni or Cu, and 0<X≦1, 0<Y≦1); or LiCoPO 4 .

15. The lithium secondary battery of claim 13 , wherein the negative electrode includes materials which absorb, store, and release lithium, and where the negative electrode materials include carbon, silicon, and tin compounds.