Acrylic binder for lithium-sulfur secondary battery cathode

Abstract

The present invention relates to a binder for a lithium-sulfur secondary battery cathode, and a composition containing the same. The binder of the present application enables a cathode active material electrolyte to have excellent resistance.

Claims

1. An acrylic binder for a lithium-sulfur secondary battery cathode, comprising a polymerized units of a polymerizable monomer having a polar functional group interacting with a cathode active material, wherein the polar functional group comprises an alkylene oxide group, and optionally further comprises at least one selected from the group consisting of a nitrogen-containing functional group, a hydroxy group and an alkoxysilyl group.

2. The acrylic binder for a lithium-sulfur secondary battery cathode according to claim 1, wherein the polymerizable monomer having a polar functional group is contained in the polymerized unit in an amount of 30 to 100 parts by weight.

3. The acrylic binder for a lithium-sulfur secondary battery cathode according to claim 1, further comprising a polymerized unit of alkyl (meth)acrylate.

4. The acrylic binder for a lithium-sulfur secondary battery cathode according to claim 3, wherein the alkyl (meth)acrylate is contained in the polymerized unit in an amount of 5 to 30 parts by weight.

5. The acrylic binder for a lithium-sulfur secondary battery cathode according to claim 1, having a particle diameter of 10 nm or less.

6. The acrylic binder for a lithium-sulfur secondary battery cathode according to claim 1, having a glass transition temperature in a range of 80 C. to 50 C.

7. The acrylic binder for a lithium-sulfur secondary battery cathode according to claim 1, having a weight average molecular weight in a range of 5,000 to 3,000,000.

8. A composition for forming a cathode active layer of a lithium-sulfur secondary battery comprising the acrylic binder according to claim 1, a cathode active material and a conductive material.

9. The composition for forming a cathode active layer of a lithium-sulfur secondary battery according to claim 8, wherein the acrylic binder is contained in an amount of 0.01 to 10 parts by weight, relative to 100 parts by weight of a solid content of the composition.

10. The composition for forming a cathode active layer of a lithium-sulfur secondary battery according to claim 8, wherein the cathode active material is a sulfur-carbon composite.

11. The composition for forming a cathode active layer of a lithium-sulfur secondary battery according to claim 8, wherein the cathode active material is contained in an amount of 30 to 95 parts by weight, relative to 100 parts by weight of a solid content of the composition.

12. The composition for forming a cathode active layer of a lithium-sulfur secondary battery according to claim 8, further comprising a non-acrylic binder.

13. The composition for forming a cathode active layer of a lithium-sulfur secondary battery according to claim 8, wherein the conductive material is contained in an amount of 2 to 70 parts by weight, relative to 100 parts by weight of a solid content of the composition.

14. A cathode for a lithium-sulfur secondary battery having: a current collector; and an active layer formed on the current collector and comprising the acrylic binder according to claim 1.

15. The cathode for a lithium-sulfur secondary battery according to claim 14, wherein the active layer has a thickness in a range of 1 to 200 m.

16. A lithium-sulfur secondary battery comprising the cathode of claim 14.

17. The cathode for a lithium-sulfur secondary battery according to claim 14, wherein a cathode active material has a compound comprising a sulfur element.

18. The cathode for a lithium-sulfur secondary battery according to claim 17, wherein the cathode active material is a sulfur-carbon composite.

19. The cathode for a lithium-sulfur secondary battery according to claim 17, wherein the polar functional group of the acrylic binder interacts with the sulfur element.

Description

BEST MODE

(1) Hereinafter, the present invention will be described with reference to examples thereof, but the following examples are intended to illustrate the present application, and it is apparent to those having ordinary skill in the art that the scope of the present application is not limited by the following examples.

(2) The physical properties shown in these examples and comparative examples were evaluated in the following manner.

(3) [1. Method of Measuring Conversion Rate of Binder]

(4) Analysis Equipment Gas chromatography (PerkinElmer)

(5) Analysis Conditions Solvent: Tetrahydrofuran Initial temperature: 3 minutes at 50, Ramp: 30/min at 200 Injection volume: 0.5 l

(6) Analysis Procedure

(7) The reaction product is diluted in a solvent at a concentration of 20 mg/mL and 5 mg/mL of toluene is added as a standard substance, and then the gas chromatography is measured. The conversion rate is calculated by the ratio change of the monomer peak size, relative to the toluene peak.
Conversion rate (%)=(A.sub.iniA.sub.fin)/A.sub.ini100

(8) A.sub.ini: an area relative ratio of the monomer peak at the start of the reaction, relative to the toluene peak

(9) A.sub.fin: an area relative ratio of the monomer peak at the end of the reaction, relative to the toluene peak

(10) [2. Evaluation of Molecular Weight of Binder]

(11) A weight average molecular weight (Mw) and molecular weight distribution (PDI) were measured using GPC under the following conditions, and the measurement results were converted by using standard polystyrene of the Agilent system in production of calibration curves.

(12) <Measurement Conditions>

(13) Measuring instrument: Agilent GPC (Agilent 1200 series, U.S.)

(14) Column: Two PL Mixed B connected

(15) Column temperature: 40 C.

(16) Eluent: tetrahydrofuran or N,N-dimethylformaldehyde

(17) Flow rate: 1.0 mL/min

(18) Concentration: 1 mg/mL (100 l injection)

(19) [3. Formation of Cathode Active Layer]

(20) A carbon-sulfur composite was obtained through a wet ball milling process of a mixture in which a weight ratio of carbon powder:sulfur is 10:90. A slurry was prepared by adding a composition of the carbon-sulfur composite 75.0 mass %: Super-P (conductive material) 20.0 mass %: a binder 5.0 mass % to water as a solvent, and then coated on an aluminum current collector having a thickness of about 20 m to prepare a cathode having a loading amount of 2.0 mAh/cm.sup.2.

(21) [4. Manufacture of Lithium-Sulfur Secondary Battery]

(22) The cathode prepared according to the method of the present application was used, a lithium foil having a thickness of about 150 m was used as an anode, and a polyolefin membrane (Celgard 2400) was used as a separation membrane. An electrolyte mixing TEGDME (Tetraethylene glycol dimethyl ether), DOL (1,3-dioxolane) and DME (dimethoxyethane), where 1M LiN(CF.sub.3SO.sub.2).sub.2) and 0.1M LiNO.sub.3 were dissolved, was used as the electrolyte to complete the manufacture of a lithium-sulfur secondary battery.

(23) [5. Evaluation of Cycle Characteristics]

(24) Instrument: Charger-discharger of 100 mA class

(25) Charge: 0.1 C, constant current/constant voltage mode

(26) Discharge: 0.1 C, constant current mode (1.5V)

(27) Cycle temperature: 25 C.

[Resin Production Example 1]Production of Acrylic Binder (A1)

(28) In a 250 mL round bottom flask, 7.5 g of polyethyleneoxide methylether methacrylate, 6.0 g of N-vinyl-2-pyrrolidone, 1.5 g of acrylonitrile and 60 g of water were charged and the inlet was sealed. Oxygen was removed through nitrogen bubbling for 30 minutes, the reaction flask was immersed in an oil bath heated to 60 C., and then 0.03 g of V-50 (Wako Chemical) was added to initiate the reaction. In 24 hours when the monomer conversion was 87%, the reaction was terminated to obtain an acrylic binder having a weight average molecular weight of about 300,000.

[Resin Production Examples 2 to 4]Production of Acrylic Binders (A2, A3, A4)

(29) An acrylic binder was prepared in the same manner as in Production Example 1, except that the kinds and contents of the monomers used upon the polymerization were adjusted as in Table 1 below.

(30) TABLE-US-00001 TABLE 1 A1 A2 A3 A4 PEOMA (part by 50 50 50 50 weight) VP (part by 40 25 40 weight) DMAA (part by 40 25 weight) AN (part by 10 10 weight) MMA (part by 10 weight) Mw 300,000 350,000 500,000 400,000 PEOMA: Poly(ethylene oxide) methyl ether methacrylate VP: N-vinyl-2-pyrrolidone DMAA: N,N-dimethylacrylamide AN: acrylonitrile MMA: Methyl methacrylate

[Example 1]Manufacture of Lithium-Sulfur Secondary Battery

(31) A lithium-sulfur secondary battery was manufactured using a cathode having an active layer comprising an acrylic binder (A1) prepared according to Production Example 1 above. The charge/discharge was evaluated with 0.1 C/0.1 C by 50 cycles between 1.5 V and 2.8 V, and then the remaining capacity in the second cycle and the remaining capacity in the 50th cycle were calculated relative to the initial capacity to measure the capacity retention rate and the result was shown in Table 2.

[Examples 2 to 4]Manufacture of Lithium-Sulfur Secondary Battery

(32) A battery was manufactured in the same manner as in Example 1, except that a lithium-sulfur secondary battery was produced using a cathode having an active layer comprising each acrylic binder (A2, A3, A4) prepared according to Production Examples 2 to 4, and the capacity retention rate was evaluated and shown in Table 2.

[Comparative Examples 1 and 2]Manufacture of Lithium-Sulfur Secondary Battery

(33) A battery was manufactured in the same manner as in Example 1, except that a polyvinylidene fluoride (PVDF) binder or a mixture of styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) in a ratio of 1:1 was used as a cathode binder instead of acrylic binders (A1, A2, A3, A4), and the capacity retention rate was evaluated and shown in Table 2.

(34) TABLE-US-00002 TABLE 2 Example Comparative Example 1 2 3 4 1 2 Binder A1 A2 A3 A3 PVDF SBR + CMC Capacity retention rate (%) 86 85 88 88 75 80

(35) As shown in Table 2, in the case of the lithium-sulfur secondary battery according to Examples, the phenomenon that the sulfur was dissoluted into the electrolyte could be inhibited by comprising the acrylic binder containing the polymerized units of the polymerizable monomer having the polar functional group interacting with sulfur, which is the cathode active material, in the active layer, and thus it was shown that the capacity retention ratio according to the cycle progress was high.

INDUSTRIAL APPLICABILITY

(36) The present application can provide an acrylic binder contained in a cathode active layer of a lithium-sulfur secondary battery, which can effectively prevent dissolution of a cathode active material and ultimately ensure excellent cycle characteristics, and a composition comprising the same.

(37) Also, the present application can provide a composition for forming a cathode active layer of a lithium-sulfur secondary battery, which can comprise an appropriate amount of a conductive material by having excellent dispersion characteristics, and a cathode comprising the active layer formed therefrom.