Electrolyte for lithium secondary battery and lithium secondary battery including the same

1. An electrolyte for a secondary battery, the electrolyte comprising:

a lithium salt;

a nonaqueous organic solvent; and

a difluorophosphite olefin compound represented by the following Chemical Formula 1:

wherein

R 1 to R 3 are independently of one another hydrogen, *—(CH 2 ) n —OPF 2 , C1-C10 alkyl, or C2-C10 alkenyl;

L is C1-C10 alkylene, and the alkylene of L may be further substituted by one or more selected from the group consisting of C1-C10 alkyl, *—(CH 2 ) a —OPF 2 , and

R′ is hydrogen or —OPF 2 ;

a and b are independently of each other an integer of 0 to 5;

n is an integer of 0 to 5; and

m is an integer of 0 to 5.

2. The electrolyte for a secondary battery of claim 1 , wherein in Chemical Formula 1, R 1 and R 2 are independently of each other hydrogen, *—(CH 2 ) n —OPF 2 , C1-C7 alkyl, or C2-C7 alkenyl, n is an integer of 1 to 3, and R 3 is hydrogen or —OPF 2 .

3. The electrolyte for a secondary battery of claim 1 , wherein the difluorophosphite olefin compound is a difluorophosphite olefin compound represented by the following Chemical Formula 3 or Chemical Formula 4:

wherein

R 1 to R 3 are as defined in Chemical Formula 1 of claim 1 ;

L 1 is C 1 -C 3 alkylene;

L 2 is a single bond or C1-C3 alkylene;

R 11 and R 12 are independently of each other hydrogen, C1-C7 alkyl, *—(CH 2 ) c —OPF 2 or

R″ is hydrogen or —OPF 2 ; and

c and d are independently of each other an integer of 1 to 3,

wherein

R 1 to R 3 are as defined in Chemical Formula 1 of claim 1 ;

L 3 and L 4 are independently of each other a single bond or C1-C3 alkylene;

R 13 and R 14 are independently of each other hydrogen, C1-C7 alkyl, *—(CH 2 ) e —OPF 2 or

R′″ is hydrogen or —OPF 2 ; and

e and f are independently of each other an integer of 0 to 3.

4. The electrolyte for a secondary battery of claim 3 , wherein in Chemical formula 3, R 1 and R 2 are independently of each other hydrogen or C1-C4 alkyl, and R 3 is hydrogen.

5. The electrolyte for a secondary battery of claim 3 , wherein in Chemical Formula 4, R 1 and R 2 are independently of each other hydrogen, —(CH 2 ) n —OPF 2 , or C2-C4 alkenyl, R 3 is hydrogen or —OPF 2 , and n is an integer of 1 to 3.

6. The electrolyte for a secondary battery of claim 1 , wherein the difluorophosphite olefin compound is one or more selected from the following structures:

7. The electrolyte for a secondary battery of claim 1 , wherein the difluorophosphite olefin compound is comprised at 0.1 to 5.0 wt %, based on a total weight of the electrolyte.

8. The electrolyte for a secondary battery of claim 1 , further comprising: one or two or more additives selected from the group consisting of oxalatoborate-based compounds, oxalatophosphate-based compounds, fluorine-substituted carbonate-based compounds, vinylidene carbonate-based compounds, and sulfinyl group-containing compounds.

9. The electrolyte for a secondary battery of claim 8 , wherein the sulfinyl group-containing compound is one or two or more selected from the group consisting of sulfone-based compounds, sulfite-based compounds, sulfonate-based compounds, sultone-based compounds, and sulfate-based compounds.

10. The electrolyte for a secondary battery of claim 1 , further comprising: two or more additives selected from the group consisting of fluorine-substituted carbonate-based compounds, vinylidene carbonate-based compounds, sultone-based compounds, and sulfate-based compounds.

11. The electrolyte for a secondary battery of claim 1 , wherein the nonaqueous organic solvent is selected from the group consisting of cyclic carbonate-based solvents, linear carbonate-based solvents, and mixed solvents thereof.

12. The electrolyte for a secondary battery of claim 11 , wherein the nonaqueous organic solvent has a mixed volume ratio of a linear carbonate solvent to a cyclic carbonate solvent of 1:1 to 9:1.

13. A lithium secondary battery comprising: a cathode, an anode, and the electrolyte for a secondary battery of claim 1 .

14. The lithium secondary battery of claim 13 , wherein the cathode includes a composite metal oxide of at least one transition metal selected from the group consisting of cobalt, manganese, and nickel with lithium as a cathode active material.

15. The lithium secondary battery of claim 14 , wherein the cathode active material is a lithium-nickel-cobalt-manganese-based composite oxide.