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

Abstract

Disclosed are an electrolyte for lithium secondary batteries including 10 wt % to 90 wt % of an ester based solvent and 10 wt % to 90 wt % of a carbonate based solvent with respect to the total weight of a non-aqueous solvent, and a lithium secondary battery including the same.

Claims

1. An electrolyte for lithium secondary batteries comprising a lithium salt and a non-aqueous solvent, wherein the non-aqueous solvent comprises an ester based solvent, a carbonate based solvent, and an ether based solvent wherein the ester based solvent is present in an amount ranging from 20 wt % to 30 wt %-, the carbonate based solvent is present in an amount ranging from 20 wt % to 30 wt %-, and the ether based solvent is present in an amount ranging from 40 wt % to 60 wt % with respect to a total weight of the non-aqueous solvent, and wherein the ester based solvent is at least one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate (EP), γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, and ε-caprolactone, and wherein the lithium salt consists of at least one selected from the group consisting of LiCl, LiBr, LiI, LiClO.sub.4, LiBF.sub.4, LiB.sub.10Cl.sub.10, LiPF.sub.6, LiCF.sub.3SO.sub.3, LiCF.sub.3CO.sub.2, LiAsF.sub.6, LiSbF.sub.6, LiPF.sub.6, LiAlCl.sub.4, CH.sub.3SO.sub.3Li, CF.sub.3SO.sub.3Li, (CF.sub.3SO.sub.2).sub.2NLi, chloroborane lithium, lithium tetraphenyl borate, and imides.

2. The electrolyte according to claim 1, wherein the ester based solvent comprises at least one of methyl propionate (MP) or ethyl propionate (EP).

3. The electrolyte according to claim 2, wherein a mixing ratio of the methyl propionate (MP) to ethyl propionate (EP) is 10:90 to 90:10 based on the total weight of the ester based solvent.

4. The electrolyte for lithium secondary batteries according to claim 1, wherein the carbonate based solvent comprises a cyclic carbonate, and the cyclic carbonate is at least one of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, or 2,3-pentylene carbonate.

5. The electrolyte for lithium secondary batteries according to claim 4, wherein the carbonate based solvent comprises the cyclic carbonate and a linear carbonate, and the linear carbonate is at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), or ethyl propyl carbonate (EPC), and the cyclic carbonate and the linear carbonate are mixed in a weight ratio of 1:4 to 4:1.

6. The electrolyte for lithium secondary batteries according to claim 1, wherein a concentration of the lithium salt in the electrolyte is 0.5 to 3 M.

7. A lithium secondary battery comprising the electrolyte for lithium secondary batteries according to claim 1.

8. The lithium secondary battery according to claim 7, wherein the lithium secondary battery comprises: a cathode comprising a lithium metal phosphate according to Formula 1 below, as a cathode active material; and an anode comprising amorphous carbon, as an anode active material,
Li.sub.1+aM(PO.sub.4−b)X.sub.b  (1) wherein M is at least one selected from metals of Groups II to XII, X is at least one selected from F, S and N, −0.5≦a≦+0.5, and 0≦b≦0.1.

9. The lithium secondary battery according to claim 8, wherein the lithium metal phosphate is a lithium iron phosphate having an olivine crystal structure according to Formula 2 below:
Li.sub.1+aFe.sub.1−xM′.sub.x(PO.sub.4−b)X.sub.b  (2) wherein M′ is at least one selected from Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, X is at least one selected from F, S, and N, and −0.5≦a≦+0.5, 0≦x≦0.5, and 0≦b≦0.1.

10. The lithium secondary battery according to claim 9, wherein the lithium metal phosphate having the olivine crystal structure is LiFePO.sub.4.

11. The lithium secondary battery according to claim 10, wherein the lithium iron phosphate having the olivine crystal structure is coated with conductive carbon.

12. The lithium secondary battery according to claim 8, wherein the amorphous carbon is hard carbon and/or soft carbon.

13. A battery module comprising the lithium secondary battery according to claim 7 as a unit cell.

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

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

16. The device according to claim 15, wherein the device is a hybrid electric vehicle, a plug-in hybrid electric vehicle, or an energy storage system.

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 drawing, in which:

(2) FIG. 1 is a graph illustrating relative resistances of secondary batteries according to Experimental Example 1 of the present invention;

(3) FIG. 2 is a graph illustrating room-temperature output characteristics of secondary batteries according to Experimental Example 2 of the present invention; and

(4) FIG. 3 is a graph illustrating relative resistances and relative output characteristics of secondary batteries according to Experimental Example 3 of the present invention.

MODE FOR INVENTION

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

Example 1

(6) 88 wt % of Li(Ni0.4Mn0.3Co0.3)O.sub.2 as a cathode active material, 8.5 wt % of Super-P as a conductive material, and 3.5 wt % of PVdF as a binder were added to NMP to prepare a cathode mixture slurry. The resulting cathode mixture slurry was coated, dried, and pressed over one side of aluminum foil to prepare a cathode.

(7) 95.8 wt % of Graphite as an anode active material, 1 wt % of Super-P as a conductive material, and 2.2 wt % of SBR as a binder, and 1 wt % of a thickener were added to H.sub.2O as a solvent to prepare an anode mixture slurry. The resulting anode mixture slurry was coated, dried, and pressed over one side of copper foil to prepare an anode.

(8) The cathode and the anode were laminated using Celgard as a separator to prepare an electrode assembly. Subsequently, a lithium non-aqueous electrolyte including 1M LiPF.sub.6 was added to a solvent including 60 wt % of methyl propionate based solvent and 40 wt % of a carbonate based solvent to manufacture a lithium secondary battery.

Example 2

(9) A lithium secondary battery was manufactured in the same manner as in Example 1, except that a solvent including 60 wt % of an ethyl propionate based solvent and 40 wt % of carbonate based solvent was used.

Example 3

(10) A lithium secondary battery was manufactured in the same manner as in Example 1, except that LiFePO.sub.4 as a cathode active material, and a solvent including 50 wt % of ether based solvent, 30 wt % of ethylpropionate, and 20 wt % of ethylcarbonate were used.

Comparative Example 1

(11) A lithium secondary battery was manufactured in the same manner as in Example 1, except that 100 wt % of a carbonate based solvent was used.

Comparative Example 2

(12) A lithium secondary battery was manufactured in the same manner as in Example 1, except that LiFePO.sub.4 as a cathode active material, and a solvent including 80 wt % of an ether based solvent, and 20 wt % of ethylcarbonate were used.

Experimental Example 1

(13) Relative resistances of the lithium secondary batteries manufactured according to Example 1 and Comparative Examples 1 and 2 were measured. Results are illustrated in FIG. 1 below.

(14) As shown in FIG. 1, it can be confirmed that the batteries including the electrolyte composed of propionate and the carbonate solvent according to Examples 1 and 2 of the present invention exhibit lower resistance, when compared with the batteries according to comparative examples.

Experimental Example 2

(15) Room-temperature output characteristics of the lithium secondary batteries manufactured according to Examples 1 and 2 Comparative Example 1 were measured under a condition of 10 s HPPC at 50% SOC. Results are illustrated in FIG. 2 below.

(16) Relative resistance was measured under a condition of 3 cycles (CC discharge.fwdarw.rest for 20 min.fwdarw.CC/CV charge).fwdarw.rest for 30 min.fwdarw.9 cycles (CC discharge at 10% SOC.fwdarw.rest for 1 hr.fwdarw.10 C discharge for 10 s.fwdarw.rest for 30 min.fwdarw.10 C charge for 10 s.fwdarw.rest for 30 min) Subsequently, relative output was measured at 50% SOC according to an output calculation formula below.
Output calculation formula=OCV.sub.SOC50%X(OCV.sub.SOC50%−Vmin)/R.sub.SOC50%

(17) As shown in FIG. 3, it can be confirmed that the batteries including the electrolyte composed of propionate and the carbonate solvent according to Examples 1 and 2 of the present invention exhibit superior room-temperature output characteristics, when compared with the batteries according to comparative examples.

Experimental Example 3

(18) Resistances and room-temperature output characteristics of the batteries according to Example 3 and Comparative Example 2 were measured, and a relative resistance and room-temperature output characteristics of the battery according to Example 3 based on Comparative Example 2 are illustrated in FIG. 3 below.

(19) Relative resistance was measured under a condition of 3 cycles (CC discharge.fwdarw.rest for 20 min.fwdarw.CC/CV charge).fwdarw.rest for 30 min.fwdarw.9 cycles×(CC discharge at 10% SOC.fwdarw.rest for 1 hr.fwdarw.10 C discharge for 10 s.fwdarw.rest for 30 min.fwdarw.10 C charge for 10 s.fwdarw.rest for 30 min) Subsequently, relative output was measured at 50% SOC according to an output calculation formula below.

(20) As shown in FIG. 3 below, it can be confirmed that the battery according to Example 3 exhibits resistance reduced by 17% and output increased by 20%, when compared with the battery according to Comparative Example 1. In this regard, to overcome a drawback of an ether based electrolyte such as electrolyte decomposition due to crystalline carbon, a propionate based electrolyte was used and, as such, high output was realized.

(21) Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

INDUSTRIAL APPLICABILITY

(22) As described above, since a secondary battery according to the present invention includes an electrolyte including a predetermined amount of ester based solvent and carbonate based solvent, ionic conductivity is improved, thereby exhibiting superior output characteristics. In particular, the secondary battery may have superior output characteristics even at low temperature due to a low melting point of the ester based solvent.

(23) When the electrolyte is used with lithium iron phosphate having an olivine crystal structure and amorphous carbon, internal resistance of a battery is reduced. Accordingly, rate characteristics and output characteristics of the battery are further improved and, thus, the battery may be suitably used in hybrid electric vehicles.