COMPOSITION FOR GEL POLYMER ELECTROLYTES, GEL POLYMER ELECTROLYTE PREPARED THEREFROM, AND LITHIUM SECONDARY BATTERY COMPRISING SAME

Abstract

The present invention provides a composition for a gel polymer electrolyte, the composition including: an oligomer represented by Formula 1; an additive; a polymerization initiator; a lithium salt; and a non-aqueous solvent, the additive including at least one compound selected from the group consisting of a substituted or unsubstituted phosphate-based compound and a substituted or unsubstituted benzene-based compound, a gel polymer electrolyte prepared using the same, and a lithium secondary battery.

Claims

1. A composition for a gel polymer electrolyte, the composition comprising: an oligomer represented by Formula 1 below; an additive; a polymerization initiator; a lithium salt; and a non-aqueous solvent, wherein the additive comprises at least one compound selected from the group consisting of a substituted or unsubstituted phosphate-based compound and a substituted or unsubstituted benzene-based compound, ##STR00018## wherein, in Formula 1, A is a unit including an alkylene group having 1 to 5 carbon atoms which is substituted with at least one fluorine or unsubstituted, B and B are each independently a unit including an amide group, C and C are each independently a unit including a (meth)acrylate group, and m is an integer of 1 to 100.

2. The composition according to claim 1, wherein the substituted or unsubstituted phosphate-based compound comprises a compound represented by Formula 2 below, ##STR00019## wherein, in Formula 2, R.sub.3, R.sub.4, and R.sub.5 are each independently at least one selected from the group consisting of an alkyl group having 1 to 5 carbon atoms, which is substituted with at least one halogen group, and a silyl group.

3. The composition according to claim 2, wherein R.sub.3, R.sub.4, and R.sub.5 are each independently one selected from the group consisting of substituents represented by Formula 2a and Formula 2b below, ##STR00020## wherein, in Formula 2a, x.sub.1 is an integer selected from among integers of 0 to 4, y.sub.1 and z.sub.1 are each an integer selected from among integers of 0 to 3, and y.sub.1+z.sub.1=3, ##STR00021## wherein, in Formula 2b, R.sub.6, R.sub.7, and R.sub.8 are each independently one selected from the group consisting of hydrogen, or an alkyl group having 1 to 5 carbon atoms.

4. The composition according to claim 1, wherein the substituted or unsubstituted phosphate-based compound comprises at least one compound selected from the group consisting of compounds represented by Formula 2-1 and Formula 2-2 below. ##STR00022##

5. The composition according to claim 1, wherein the substituted or unsubstituted benzene-based compound comprises a compound represented by Formula 3 below, ##STR00023## wherein, in Formula 3, R.sub.9 is at least one selected from the group consisting of halogen group and a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms, and X.sub.2 is an integer selected from among 1 to 6.

6. The composition according to claim 1, wherein the substituted or unsubstituted benzene-based compound comprises at least one selected from the group consisting of compounds represented by Formulae 3-1 and 3-2 below. ##STR00024##

7. The composition according to claim 1, wherein the additive is included in an amount of 1 to 30 parts by weight with respect to 100 parts by weight of the composition.

8. The composition according to claim 1, wherein the unit A comprises at least one of units represented by Formulae A-1 to A-6 below, ##STR00025## wherein, in Formulae A-1 to A-6, n1 to n6 are each independently an integer of 1 to 30.

9. The composition according to claim 1, wherein the oligomer comprises at least one selected from the group consisting of compounds represented by Formulae 1-1 and 1-6 below, ##STR00026## wherein n1 to n6 are each independently an integer of 1 to 30, and m is an integer of 1 to

100.

10. A gel polymer electrolyte prepared using the composition according to claim 1.

11. A lithium secondary battery comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and the gel polymer electrolyte according to claim 10.

Description

EXAMPLES

1. Example 1

[0145] (1) Preparation of Composition for Gel Polymer Electrolyte

[0146] A composition for a gel polymer electrolyte was prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 3:7, and adding 0.7 M of LiPF.sub.6 and 0.5 M of LiFSI to prepare a mixed solvent, and then adding, into 76.98 g of the prepared mixed solvent, 5 g of the oligomer represented by Formula 1-5 (weight-average molecular weight of 4,000), 10 g of the compound represented by Formula 2-1 as an additive, 0.02 g of a polymerization initiator (AIBN), and, as other additives, 1.5 g of VC, 0.5 g of PS, and 1 g of ESa.

[0147] (2) Manufacture of Lithium Secondary Battery

[0148] A positive electrode mixture slurry was prepared by adding, into N-methyl-2-pyrrolidone (NMP) which was a solvent, 97.5 wt % of LiNi.sub.0.8Co.sub.0.1Mn.sub.0.1O.sub.2 (NCM) as a positive electrode active material, 1.5 wt % of carbon black as a conductive agent, and 1 wt % of PVDF as a binder. An aluminum (Al) thin film having a thickness of about 20 as a positive electrode current collector, was coated with the positive electrode mixture slurry and dried, and then, roll-pressed to prepare a positive electrode.

[0149] Next, an artificial graphite electrode was used as a negative electrode.

[0150] An electrode assembly was prepared by using the positive electrode, the negative electrode, and a separator formed of three layers of polypropylene/polyethylene/polypropylene (PP/PE/PP), the prepared composition for a gel polymer electrolyte was injected into the electrode assembly, the resultant mixture was left standing for 2 days, and the battery was heated at a temperature of 60 C. for 24 hours to manufacture a lithium secondary battery including the gel polymer electrolyte.

2. Example 2

[0151] A lithium secondary battery including a gel polymer electrolyte was manufactured in the same manner as in Example 1 except that, as an additive, 10 g of the compound represented by Formula 3-1 was added instead of 10 g of the compound represented by Formula 2-1 unlike Example 1.

COMPARATIVE EXAMPLES

1. Comparative Example 1

[0152] (1) Preparation of Electrolyte Solution

[0153] An electrolyte solution was prepared by using a mixed solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 3:7, and adding 0.7 M of LiPF.sub.6 and 0.5 M of LiFSI. 1.5 g of VC, 0.5 g of PS, and 1 g of Esa were added as the other additives to prepare an electrolyte solution.

[0154] (2) Manufacture of Lithium Secondary Battery

[0155] A positive electrode mixture slurry was prepared by adding 97.5 wt % of LiNi.sub.0.8Co.sub.0.1Mn.sub.0.1O.sub.2 (NCM) as a positive electrode active material, 1.5 wt % of carbon black as a conductive agent, and 1 wt % of PVDF as a binder to N-methyl-2-pyrrolidone (NMP) as a solvent. An aluminum (Al) thin film having a thickness of about 20 m, as a positive electrode current collector, was coated with the positive electrode mixture slurry and dried, and then, roll-pressed to prepare a positive electrode.

[0156] Next, an artificial graphite electrode was used as a negative electrode.

[0157] An electrode assembly was prepared by using the positive electrode, the negative electrode, and a separator formed of three layers of polypropylene/polyethylene/polypropylene (PP/PE/PP), and the prepared electrolyte was injected into the electrode assembly to manufacture a lithium secondary battery.

2. Comparative Example 2

[0158] A lithium secondary battery including a gel polymer electrolyte was manufactured in the same manner as in Example 1 except that an acrylate-based oligomer composed of dipentaerythritol pentaacrylate was used instead of 5 g of the oligomer of Formula 1-6 unlike Example 1.

3. Comparative Example 3

[0159] A lithium secondary battery including a gel polymer electrolyte was manufactured in the same manner as in Example 1 except that the additive was not used unlike Example 1.

EXPERIMENTAL EXAMPLE

1. Experimental Example 1: Evaluation of High-Temperature Safety (Measurement of Amount of Heat Generated)

[0160] Lithium secondary batteries manufactured according to Examples 1-2 and Comparative Examples 1-3 were charged to SOC of 100% under the condition of a voltage of 4.25 V. Thereafter, the temperature was raised at a heating rate of 0.7 C./min, from 25 C., and the temperature was maintained for about 100 minutes in a temperature range of 120 C. vicinity (first temperature maintaining section). Thereafter, the temperature was raised again at a heating rate of 0.7 C/min and maintained in a temperature range of 150 C. vicinity (second temperature maintaining section). Thereafter, the temperature was raised again at a heating rate of 0.7 C./min and maintained in a temperature range of 200 C. vicinity (third temperature maintaining section), and then, the lithium secondary battery was exposed at high-temperature, and thereafter the amount of heat generated of the inside of the lithium secondary battery was measured (measured by a multi module calorimeter (MMC) 274 of NETZSCH Co., Ltd.), and the results thereof are represented in Table 1 below.

TABLE-US-00001 TABLE 1 Amount of heat Amount of heat generated (J/g) in the generated (J/g) in the second temperature third temperature maintaining section maintaining section Example 1 45 95 Example 2 37 84 Comparative Example 1 580 1020 Comparative Example 2 150 210 Comparative Example 3 110 140

[0161] The amount of heat generated was not observed in the first temperature maintaining section in Examples and the Comparative Examples. It can be ascertained that, the batteries manufactured according to Examples exhibit low amount of heat generated in both the second and third temperature maintaining sections whereas the batteries manufactured according to Comparative Examples exhibit remarkably high amount of heat generated in both the second and third temperature maintaining sections.

2. Experimental Example 2: Evaluation of High-Temperature Storage (Measurement of Amount of Gas Generation)

[0162] Lithium secondary batteries manufactured according to Examples 1-2 and Comparative Examples 1-3 were charged to SOC of 100%, and the batteries were exposed at a temperature of 60 C. for 10 weeks, and then, the amount of gas generation in the batteries (the amount of gas generation by the oxidation reaction of the electrolyte on the positive electrode surface) was measured, and the results are presented in Table 2 below. When a large amount of gas, such as carbon monoxide (CO) or carbon dioxide (CO.sub.2), is generated after high-temperature storage, the temperature in the battery rises, and accordingly, exothermic and ignition reactions may occur.

TABLE-US-00002 TABLE 2 Gas amount (ml) after storage at a temperature of 60 C. for 10 weeks Example 1 101 Example 2 110 Comparative Example 1 1050 Comparative Example 2 310 Comparative Example 3 220

[0163] Referring to Table 2, it can be ascertained that the batteries manufactured according to Examples have the small amount of gas generation whereas the batteries manufactured according to Comparative Examples have the remarkably large amount of gas generation. It seems that, in Examples, the electrolyte oxidation reaction on the positive electrode surface is suppressed, thereby reducing the amount of lattice oxygen (in the form of superoxide and peroxide) generated by the reaction, whereas in Comparative Examples, the oxidation reaction is not suppressed, thereby increasing the lattice oxygen and accordingly increasing the gas amount.