Electrolyte for lithium secondary batteries with improved low temperature performance and lithium secondary battery including the same

1. An electrolyte for lithium secondary batteries, consisting of:

a non-aqueous solvent and a lithium salt,

wherein the non-aqueous solvent consists of:

25 to 45% by weight of a cyclic carbonate compound; and

55 to 75% by weight of a linear ester compound,

the linear ester compound is a mixture of ethyl propionate and propyl propionate, and

the content of ethyl propionate is 25 to 35% by weight, and the content of propyl propionate is 30 to 40% by weight, based on the total weight of the non-aqueous solvent.

2. The electrolyte according to claim 1 , wherein the cyclic carbonate compound comprises at least one selected from the group consisting of ethylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, fluorethylene carbonate and vinylene carbonate.

3. The electrolyte according to claim 2 , wherein the cyclic carbonate compound comprises ethylene carbonate.

4. The electrolyte according to claim 1 , wherein the electrolyte has an ionic conductivity at −10° C. equal to or higher than 5.5 mS/cm and equal to or lower than 6.0 mS/cm and a viscosity at −10° C. equal to or higher than 4.5 cP and equal to or lower than 5.9 cP.

5. The electrolyte according to claim 1 , wherein the electrolyte has an ionic conductivity at −10° C. equal to or higher than 5.5 mS/cm and equal to or lower than 6.0 mS/cm and a viscosity at −10° C. equal to or higher than 5.0 cP and equal to or lower than 5.9 cP.

6. A lithium secondary battery comprising:

the electrolyte according to claim 1 ;

a positive electrode;

a negative electrode; and

a polymer membrane,

wherein an electrode assembly having a structure in which the polymer membrane is interposed between the positive electrode and the negative electrode is impregnated with the electrolyte and accommodated in a battery case.

7. The lithium secondary battery according to claim 6 , wherein the positive electrode comprises, as a positive electrode active material, at least one selected from the group consisting of a compound represented by the following Formul 1 and a compound represented by the following Formula 2:

Li x MO 2-z A z   (1)

Li a M′(PO 4-c )X c   (2)

wherein 0.9≤x≤1.2, 0≤z≤0.2, 0.5≤a≤1.5, 0≤c≤0.1;

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

M′ comprises at least one element selected from the group consisting of Fe, Mn, Co, Ni, Cu, An, Mg, Cr, V, Mo, Ti, Al, Nb, B, and Ga;

A comprises at least one anion with a charge of 1 or −2; and

X comprises at least one element selected from F, S and N.

8. The lithium secondary battery according to claim 7 , wherein the compound represented by Formula 1 is represented by the following Formula 3 and the compound represented by Formula 2 is represented by the following Formula 4:

Li x Co y M 1-y O 2-z A z   (3)

Li a Fe 1-b M′ b (PO 4-c )X c   (4)

wherein 0.9≤x≤1.2, 0≤y≤1, 0≤z≤0.2, 0.5≤a≤1.5, 0≤b≤0.5, 0≤c≤0.1;

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

M′ comprises at least one element selected from the group consisting of Mn, Co, Ni, Cu, Zn, Mg, Cr, V, Mo, Ti, Al, Nb, B, and Ga;

A comprises at least one anion with a charge of −1 or −2; and

X comprises at least one element selected from F, S and N.

9. The lithium secondary battery according to claim 8 , wherein the compound represented by Formula 3 is LiCoO 2 and the compound represented by Formula 4 is LiFePO 4 .

10. The lithium secondary according to claim 7 , wherein the positive electrode active material comprises the compound of Formula 1 and the compound of Formula 2, and a weight mix ratio of the compound of Formula 1 and the compound of Formula 2 is 60:40 to 99:1.

11. The lithium secondary battery according to claim 10 , wherein the weight mix ratio of the compound of Formula 1 and the compound of Formula 2 is 70:30 to 99:1.

12. The lithium secondary battery according to claim 6 , wherein the negative electrode comprises, as a negative electrode active material, a carbon-based material.

13. A battery pack comprising the lithium secondary battery according to claim 6 as a unit battery.

14. A device comprising the battery pack according to claim 13 .

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.