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

Abstract

Disclosed are an electrolyte for lithium secondary batteries including 10 to 50% by weight of a cyclic carbonate compound, and 50 to 90% by weight of a linear ester compound, based on the total weight of a non-aqueous solvent, wherein a content of ethyl propionate of the linear ester compound is 20 to 60% by weight, based on the total weight of the non-aqueous solvent, and a lithium secondary battery including the electrolyte and exhibiting superior low-temperature characteristics.

Claims

1. An electrolyte for lithium secondary batteries, consisting of: a non-aqueous solvent, a lithium salt, and an additive that is selected from flame retardants, halogen-containing solvents, carbon dioxide gas, fluoro-ethylene carbonate (FEC), or propene sultone (PRS), wherein the non-aqueous solvent consists of 10 to 50% by weight of a cyclic carbonate compound; and 50 to 90% 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 20 to 40% by weight, and the content of propyl propionate is 30 to 50% 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 non-aqueous solvent consists of 20 to 50% by weight of a cyclic carbonate compound; and 50 to 80% 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 20 to 40% by weight, and the content of propyl propionate is 30 to 40% by weight, based on the total weight of the non-aqueous solvent.

5. The electrolyte according to claim 1, wherein, the electrolyte comprises 20 to 40% by weight of the cyclic carbonate compound and 60 to 80% by weight of the linear ester compound, based on the total weight of the non-aqueous solvent.

6. 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 6.5 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.

7. 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.

8. The electrolyte according to claim 1, wherein the additive is at least one selected from the group consisting of pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol, aluminum trichloride, carbon tetrachloride and ethylene trifluoride, carbon dioxide gas, fluoro-ethylene carbonate (FEC), and propene sultone (PRS).

9. 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.

10. The lithium secondary battery according to claim 9, 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 Formula 1 and a compound represented by the following Formula 2:
Li.sub.xMO.sub.2-zA.sub.z  (1)
Li.sub.aM′(PO.sub.4-c)X.sub.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.

11. The lithium secondary battery according to claim 10, 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.sub.xCo.sub.yM.sub.1-yO.sub.2-zA.sub.z  (3)
Li.sub.aFe.sub.1-bM′.sub.b(PO.sub.4-c)X.sub.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.

12. The lithium secondary battery according to claim 11, wherein the compound represented by Formula 3 is LCoO.sub.2 and the compound represented by Formula 4 is LiFePO.sub.4.

13. The lithium secondary battery according to claim 10, 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.

14. The lithium secondary battery according to claim 13, wherein the weight mix ratio of the compound of Formula 1 and the compound of Formula 2 is 70:30 to 99:1.

15. The lithium secondary battery according to claim 9, wherein the negative electrode comprises, as a negative electrode active material, a carbon-based material.

16. A battery pack comprising the lithium secondary battery according to claim 9 as a unit battery.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a graph showing low-temperature intermittent discharge testing at 10° C. for Example 2 and Comparative Example 3 in Test Example 2.

BEST MODE

(3) Now, the present invention will be described in more detail with reference to the following examples. These examples are provided only to illustrate the present invention and should not be construed as limiting the scope and spirit of the present invention.

Example 1

(4) An electrolyte for lithium secondary batteries containing 1M LiPF.sub.6 dissolved in a mixed solvent including ethylene carbonate (EC), ethyl propionate (EP) and propyl propionate (PP) in a weight mix ratio of 3:3:4 was prepared.

Example 2

(5) An electrolyte for lithium secondary batteries was prepared in the same manner as in Example 1, except that ethylene carbonate (EC), propylene carbonate (PC), ethyl propionate (EP) and propyl propionate (PP) were used in a weight mix ratio of 2.5:1:2.5:4.0 for the electrolyte solvent.

Example 3

(6) An electrolyte for lithium secondary batteries was prepared in the same manner as in Example 1, except that ethylene carbonate (EC), propylene carbonate (PC), ethyl propionate (EP) and propyl propionate (PP) were used in a weight mix ratio of 2.5:0.5:2.5:4.5 for the electrolyte solvent.

Comparative Example 1

(7) An electrolyte for lithium secondary batteries was prepared in the same manner as in Example 1, except that ethylene carbonate (EC), propylene carbonate (PC) and propyl propionate (PP) were used in a weight mix ratio of 3:1:6 for the electrolyte solvent.

(8) The compositions of the electrolytes for lithium secondary batteries according to Examples 1 to 3 and Comparative Example 1 are summarized and shown in the following Table 1.

(9) TABLE-US-00001 TABLE 1 EC PC EP PP Example 1 30 0 30 40 Example 2 25 10 25 40 Example 3 25 5 25 45 Comparative 30 10 0 60 Example 1

Test Example 1

(10) The ionic conductivity and viscosity of the electrolytes for lithium secondary batteries according to Examples 1 to 3 and Comparative Example 1 are measured and shown in the following Table 1.

(11) TABLE-US-00002 TABLE 2 Ionic Conductivity (mS/cm) Viscosity (cP) −10° −10° 25° C. 10° C. 0° C. −3° C. C. 25° C. C. Example 1 9.27 7.72 6.89 6.07 5.66 2.66 5.67 Example 2 9.42 7.74 6.87 6.01 5.57 2.71 5.99 Example 3 8.89 7.45 6.67 5.8 5.44 2.72 5.94 Comparative 8.47 6.72 5.84 5.02 4.58 3.28 8.13 Example 1

(12) As can be seen from Table 2, the electrolytes according to Examples 1 to 3 show significant difference in ionic conductivity and viscosity at −10° C. from the electrolyte according to Comparative Example 1.

(13) Specifically, Examples 1 to 3 exhibit an ionic conductivity of 5.44 mS/cm or more and a viscosity of 5.99 cP or less at −10° C., whereas Comparative Example 1 exhibits ionic conductivity of 4.58 mS/cm and high viscosity of 8.13 cP at −10° C.

(14) In addition, Examples 1 to 3 exhibit a high ionic conductivity and a low viscosity even at 25° C., as compared to Comparative Example 1 and the electrolyte according to the present invention exhibits excellent wetting properties at low temperature as well as at room temperature.

(15) In addition, when comparing linear ester compounds, Examples 1 and 2 include the same amount of PP, Example 3 includes slightly increased amount of PP, and Comparative Example 1 includes only PP, without EP. It can be seen that as PP content increases, ionic conductivity and viscosity are deteriorated.

(16) That is, as to the composition of the electrolyte, as ethyl propionate (EP) proportion increases and propyl propionate (PP) proportion decreases, low temperature performance can be effectively improved. In particular, the electrolyte of Example 1 including 30% by weight of EP and 40% by weight of PP exhibits a 9.5% increase in ionic conductivity at 25° C. and a 18% increase in ionic conductivity at 0° C., and a 2% decrease in viscosity at room temperature and a 40% decrease in viscosity at −10° C., as compared to the electrolyte of Comparative Example 1 including only 70% by weight of PP as a linear ester compound.

Example 4

(17) A negative electrode active material (graphite), a conductive material (Denka black) and a binder (PVdF) were mixed in NMP in a weight ratio of 96.25:1:2.75 to prepare a negative electrode mixture, and a 20 μm thickness copper foil was coated to a thickness of 200 μm with the negative electrode mixture, followed by pressing and drying to fabricate a negative electrode.

(18) In addition, a positive electrode active material composed of LiCoO.sub.2 and LiFePO.sub.4 (in a weight mix ratio of 97.5:2.5), a conductive material (Denka black) and a binder (PVdF) were mixed in NMP in a weight ratio of 96:2:2, and a 20 μm thickness aluminum foil was coated to a thickness of 200 μm with the formed positive electrode mixture, followed by pressing and drying to fabricate a positive electrode.

(19) As a separation membrane, a polyethylene membrane (Celgard, thickness: 20 μm) was interposed between the negative electrode and the positive electrode, and a lithium secondary battery was produced using the electrolyte for lithium secondary batteries prepared in Example 1.

Comparative Example 2

(20) A lithium secondary battery was produced in the same manner as in Example 4, except that the electrolyte for lithium secondary batteries prepared in Comparative Example 1 was used.

Test Example 2

(21) The secondary batteries of Example 4 and Comparative Example 2 were subjected to low-temperature intermittent discharge testing under the following conditions at −10° C. and results are shown in FIG. 1.

(22) 1) Charge: CC (600 mA)-CV (4.35V), 50 mA cut-off

(23) 2) Rest: 10 min

(24) 3) Discharge: CC (600 mA), 5 min

(25) 4) Rest: 20 min

(26) 5) Repeat 3) until 3.4V

(27) As can be seen from FIG. 1, the battery according to Example 4 uses an electrolyte exhibiting a high ionic conductivity and a low viscosity at low temperature and exhibits a 9% increase in intermittent discharge characteristics at −10° C., as compared to the battery of Comparative Example 2.

INDUSTRIAL APPLICABILITY

(28) As apparent from the fore-going, the electrolyte for secondary batteries according to the present invention includes predetermined cyclic carbonate and linear ester compounds in a certain ratio, and thus has an improved ionic conductivity and a low viscosity, more specifically, ionic conductivity at −10° C. equal to or higher than 5.5 mS/cm and equal to or lower than 6.5 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. As a result, wettability of the electrolyte can be improved. The lithium secondary battery including the electrolyte can exhibit excellent electrochemical properties such as rate characteristics at low temperature.