Composition for gel polymer electrolyte including siloxane oligomer and styrene-based oligomer, gel polymer electrolyte prepared therefrom, and lithium secondary battery including the same

Abstract

The present invention provides a composition for a gel polymer electrolyte, the composition including a first oligomer represented by Formula 1, a second oligomer including a first repeating unit represented by Formula 2a derived from a styrene monomer, a polymerization initiator, a lithium salt, and a non-aqueous solvent. The present invention also provides 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: a first oligomer represented by Formula 1; a second oligomer comprises a unit represented by Formula 2-1; a polymerization initiator; a lithium salt; and a non-aqueous solvent, ##STR00023## wherein, in Formula 1, A and A′ are each independently a unit including a (meth)acrylate group, B and B′ are each independently a unit including an amide group, C and C′ are each independently a unit including an oxyalkylene group, D is a unit including a siloxane group, and k is an integer of 1 to 100, ##STR00024## wherein, in Formula 2-1, d and e are each independently an integer of 1 to 50, and * is a connected part between the same or different atoms or between terminal parts of Formula 2-1.

2. The composition for a gel polymer electrolyte according to claim 1, wherein the first oligomer and the second oligomer are included at a mass ratio of 97.5:2.5 to 2.5:97.5.

3. The composition for a gel polymer electrolyte according to claim 1, wherein the weight-averaged molecular weight (Mw) of the first oligomer is 1,000 to 10,000.

4. The composition for a gel polymer electrolyte according to claim 1, wherein the weight-averaged molecular weight (Mw) of the second oligomer is 300 to 10,800.

5. The composition for a gel polymer electrolyte according to claim 1, wherein A and A′ each independently comprises at least one unit represented by Formulae A-1 to A-5, ##STR00025## wherein, in Formulae A-1 to A-5, R.sub.1 is each independently one selected from the group consisting of hydrogen and an alkyl group having 1 to 6 carbon atoms, and * is a connected part between the same or different atoms or between terminal parts of Formulae A-1 to A-5.

6. The composition for a gel polymer electrolyte according to claim 1, wherein the first oligomer comprises at least one compound selected from the group consisting of compound represented by Formulae 1-1 to 1-5, ##STR00026## wherein, in Formulae 1-1 to 1-5, n, o, and p are each independently an integer of 1 to 30, and q is an integer of 1 to 100.

7. A composition for a gel polymer electrolyte, the composition comprising: a first oligomer represented by Formula 1; a second oligomer comprises a repeating unit represented by Formula 2c, a polymerization initiator; a lithium salt; and a non-aqueous solvent, ##STR00027## wherein, in Formula 1, A and A′ are each independently a unit including a (meth)acrylate group, B and B′ are each independently a unit including an amide group, C and C′ are each independently a unit including an oxyalkylene group, D is a unit including a siloxane group, and k is an integer of 1 to 100, ##STR00028## wherein, in Formula 2c, R.sub.12, R.sub.13, and R.sub.14 are each independently one selected from the group consisting of hydrogen and an alkyl group having 1 to 5 carbon atoms, and c is an integer of 1 to 50, and * is a connected part between the same or different atoms or between terminal parts of Formula 2c.

8. The composition for a gel polymer electrolyte according to claim 7, wherein the first oligomer and the second oligomer are included at a mass ratio of 97.5:2.5 to 2.5:97.5.

9. The composition for a gel polymer electrolyte according to claim 7, wherein the weight-averaged molecular weight (Mw) of the first oligomer is 1,000 to 10,000.

10. The composition for a gel polymer electrolyte according to claim 7, wherein the weight-averaged molecular weight (Mw) of the second oligomer is 300 to 10,800.

11. The composition for a gel polymer electrolyte according to claim 7, wherein the second oligomer comprises a unit represented by Formula 2-2, ##STR00029## wherein, in Formula 2-2, f, g, and h are each independently an integer of 1 to 50, and * is a connected part between the same or different atoms or between terminal parts of Formula 2-2.

12. The composition for a gel polymer electrolyte according to claim 7, wherein A and A′ each independently comprises at least one unit represented by Formulae A-1 to A-5, ##STR00030## wherein, in Formulae A-1 to A-5, R.sub.1 is each independently one selected from the group consisting of hydrogen and an alkyl group having 1 to 6 carbon atoms, and * is a connected part between the same or different atoms or between terminal parts of Formula A-1 to A-5.

13. The composition for a gel polymer electrolyte according to claim 7, wherein the first oligomer comprises at least one compound selected from the group consisting of compound represented by Formulae 1-1 to 1-5, ##STR00031## wherein, in Formulae 1-1 to 1-5, n, o, and p are each independently an integer of 1 to 30, and q is an integer of 1 to 100.

14. A gel polymer electrolyte prepared using the composition for a gel polymer electrolyte according to claim 1.

15. 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 9.

16. A gel polymer electrolyte prepared using the composition for a gel polymer electrolyte according to claim 7.

17. 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 16.

Description

EXAMPLES

1. Example 1

(1) (1) Preparation of Composition for Gel Polymer Electrolyte

(2) 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, adding 0.7 M of LiPF.sub.6 and 0.5 M of LiFSI to prepare a mixed solvent, and then adding, into 100 g of the prepared mixed solvent, 2.5 g of the oligomer represented by Formula 1-5 (weight-averaged molecular weight of 5,000), 2.5 g of the oligomer represented by Formula 2-1 (weight-averaged molecular weight of 3,000), 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.

(3) (2) Manufacture of Lithium Secondary Battery

(4) A positive electrode mixture slurry was prepared by adding, into N-methyl-2-pyrrolidone (NMP) as 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 μm, 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.

(5) In addition, an artificial graphite electrode was used as a negative electrode.

(6) An electrode assembly was prepared 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 60° C. for 24 hours to prepare a lithium secondary battery including the gel polymer electrolyte.

2. Example 2

(7) A lithium secondary battery including a gel polymer electrolyte was manufactured in the same manner as in Example 1 except that 2 g of the compound represented by Formula 2-1 was added unlike Example 1.

3. Example 3

(8) A lithium secondary battery including a gel polymer electrolyte was manufactured in the same manner as in Example 1 except that, as a second oligomer, 3 g of the oligomer represented by Formula 2-1 and 3 g of the oligomer represented by Formula 2-2 were added unlike Example 1.

4. Example 4

(9) A lithium secondary battery including a gel polymer electrolyte was manufactured in the same manner as in Example 1 except that 2 g of the oligomer represented by Formula 2-2 was added unlike Example 3.

5. Example 5

(10) A lithium secondary battery including a gel polymer electrolyte was manufactured in the same manner as in Example 1 except that, as a second oligomer, 1.5 g of the oligomer represented by Formula 2-1 and 1.5 g of the oligomer represented by Formula 2-2 were added unlike Example 1.

COMPARATIVE EXAMPLES

1. Comparative Example 1

(11) (1) Preparation of Electrolyte

(12) An electrolyte was prepared using a solvent mixture 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.

(13) (2) Manufacture of Lithium Secondary Battery

(14) 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 μm, 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.

(15) In addition, an artificial graphite electrode was used as a negative electrode.

(16) A lithium secondary battery was manufactured 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.

2. Comparative Example 2

(17) A lithium secondary battery including a gel polymer electrolyte was manufactured in the same manner as in Example 1 except that 5 g of the oligomer(weight-averaged molecular weight of 5,000) represented by Formula 1-5 was used, but the oligomer represented by Formula 2-1 was not used unlike Example 1.

3. Comparative Example 3

(18) A lithium secondary battery including a gel polymer electrolyte was manufactured in the same manner as in Example 1 except that 5 g of the oligomer(weight-averaged molecular weight of 3000) represented by Formula 2-1 was used, but the oligomer represented by Formula 1-5 was not used unlike Example 1.

4. Comparative Example 4

(19) 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 as an oligomer, instead of the oligomer of Formula 1-5 and the oligomer of Formula 2-1, unlike Example 1.

EXPERIMENTAL EXAMPLES

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

(20) Lithium secondary batteries manufactured according to Examples 1-5 and Comparative Examples 1-4 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 about 120° C. (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 about 150° C. (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 about 200° C. (third temperature maintaining section), the lithium secondary battery was subsequently 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 presented in Table 1 below.

(21) 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 50 100 Example 2 60 105 Example 3 45 95 Example 4 60 107 Example 5 70 120 Comparative Example 1 580 1020 Comparative Example 2 150 210 Comparative Example 3 140 205 Comparative Example 4 505 950

(22) An amount of heat generated was not observed in the first temperature maintaining section in the Examples and the Comparative Examples. It can be ascertained that the batteries prepared according to the Examples exhibit small amounts of heat generated in both the second and third temperature maintaining sections, whereas the batteries prepared according to the Comparative Examples exhibit remarkably large amounts of heat generated in both the second and third temperature maintaining sections.

2. Experimental Example 2: Evaluation of High-Temperature Safety (Measurement of Amount of Gas in Battery)

(23) Lithium secondary batteries prepared according to Examples 1-5 and Comparative Examples 1-4 were charged to SOC of 100% under the condition of a voltage of 4.25 V. The batteries were exposed at 60° C. for 10 weeks, and then the amount of gas generated in the lithium secondary batteries was measured and presented in Table 2 below.

(24) TABLE-US-00002 TABLE 2 Gas amount (ml) after storage at 60° C. for 10 weeks Example 1 150 Example 2 155 Example 3 125 Example 4 150 Example 5 170 Comparative Example 1 1050 Comparative Example 2 290 Comparative Example 3 250 Comparative Example 4 305

(25) If a lithium secondary battery is charged and exposed at high temperature for a long time, carbon monoxide (CO), and carbon dioxide (CO.sub.2) gases, which are products generated by decomposition of the gel polymer electrolyte, are produced. It can be ascertained that less gas is generated in the batteries prepared according to the Examples using the gel polymer electrolyte than in the batteries using a liquid electrolyte (Comparative Example 1).

(26) In addition, it can be ascertained that less gas is generated in the lithium secondary batteries prepared according to the Examples using two types of oligomers in combination than in the batteries of using only one type of oligomer. It seems that this is because when the oligomers having different molecular weights are mixed, the gel polymer is formed more stably, and the characteristics of the polymer (such as suppression of volatility) are expressed.