Method for manufacturing an electrode for a lithium-sulfur battery having a large active surface area

Abstract

The invention relates to a method for preparing a positive electrode for a lithium-sulfur battery, comprising the following steps: a) a step of preparing a first mixture by placing a carbon additive such as carbon black and/or activated carbon, a carbon additive chosen from carbon nanotubes, carbon fibres and the mixtures of the two, a carbon organic binder, and a solvent in contact; b) a step of carbonising said mixture, by means of which the result is a powder comprising agglomerates of carbon black and/or activated carbon and of carbon nanotubes and/or carbon fibres; c) a step of placing the powder obtained in b) in contact with sulfur thus forming a second mixture; d) a step of dispersing said second mixture in an organic binder; e) a step of depositing the dispersion thus obtained on a substrate; and f) a step of drying said dispersion thus deposited.

Claims

1. A method for preparing a positive electrode for a lithium-sulfur battery, comprising: a. preparing a first mixture by placing a carbon additive which is carbon black and/or activated carbon; a carbon additive selected from the group consisting of carbon nanotubes, carbon fibres and mixtures thereof; a carbon organic binder, and a solvent in contact; b. carbonising said mixture, to prepare a powder comprising agglomerates of carbon black and/or activated carbon and of carbon nanotubes and/or carbon fibres; c. placing the powder obtained in b) in contact with sulfur thus forming a second mixture; d. dispersing said second mixture in an organic binder; e. depositing the dispersion thus obtained on a substrate; and f. drying said dispersion thus deposited.

2. The method of claim 1, wherein the carbon fibres are selected from the group consisting of ground carbon fibres, carbon fibres obtained in vapour phase and mixtures thereof.

3. The method of claim 1, wherein the carbon organic binder of a) is a phenolic resin.

4. The method of claim 1, wherein the carbonisation is carried out in an inert atmosphere.

5. The method of claim 1, further comprising, between c) and d), allowing the second mixture to warm to a temperature greater than the melting temperature of the sulfur and allowing the second mixture to cool to ambient temperature.

6. The method of claim 1, wherein the organic binder of d) is a polymer binder.

7. The method of claim 6, wherein said polymer is selected from the group consisting of cellulosic polymers.

8. The method of claim 7, wherein said cellulosic polymer is selected from the group consisting of carboxymethylcellulose and methylcellulose.

9. The method of claim 6, wherein said polymer is selected from the group consisting of fluorinated ethylene polymers.

10. The method of claim 9, wherein said fluorinated ethylene polymer is selected from the group consisting of polytetrafluoroethylene.

11. The method of claim 6, wherein said polymer is selected from the group consisting of poly(ethylene oxides).

12. The method of claim 6, wherein said polymer is selected from the group consisting of vinyl polymers.

13. The method of claim 12, wherein said vinyl polymer is selected from the group consisting of polyvinyl alcohol.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph illustrating the change in the potential E (in V) as a function of the capacity C (in a.u).

(2) FIG. 2 is a graph illustrating the change in the discharge capacity C (in mAh/g) as a function of the number of cycles for two batteries implemented according to the example disclosed below.

DETAILED DISCLOSURE OF SPECIFIC EMBODIMENTS

Example

(3) The present example illustrates the preparation of a positive electrode according to the method of the invention.

(4) To do this, a composition is prepared by mixture, in 400 g of ethanol, of Ketjenblack EC-600 JD carbon black (4 g), of phenolic resin (1 g). After stirring of the mixture at 4000 revolutions/min for 15 minutes, 1 g of carbon nanotubes is added. The resulting mixture is stirred for 15 minutes at 4000 revolutions/minute. Then, the ethanol is evaporated in order to obtain a dry material.

(5) The material thus obtained is reduced into grains in order to be carbonised in a tube furnace at 950 C. under argon for 1 hour. This step allows the phenolic resin to be transformed into carbon and the agglomerates comprising the carbon black and the carbon nanotubes to be bound together, these agglomerates entirely consisting of carbon having strong mechanical cohesion and a large active surface area (greater than 1000 m.sup.2/g according to the BET method).

(6) The carbon material thus structured and in the form of a powder is then mixed with elemental sulfur for 1 hour in a jar mill, by means of which a homogenous mixture is obtained. The mixture weight ratio between the sulfur and the carbon is 2/1.

(7) The mixture is then subjected to a heating of 150 C. for 1 hour, which melts the sulfur on the surface of the particles of carbon.

(8) The mixture thus obtained after this heat treatment is used for the manufacturing of a liquid composition (or ink) comprising 95% by weight of said mixture and 5% by weight of carboxymethylcellulose at 2% in water.

(9) The composition thus formulated is coated onto a sheet of aluminium and dried at 80 C. under air, by means of which the result is a positive electrode deposited on a current collector consisting of the sheet of aluminium. The grammage of the electrode obtained is approximately 4.5 mg of sulfur/cm.sup.2.

(10) For comparison, a reference positive electrode is manufactured by blade-coating a sheet of aluminium with a composition comprising: 80% elemental sulfur by weight; 10% Super P carbon black; 10% carboxymethylcellulose.

(11) followed by drying at 80 C. in air.

(12) The grammage of the electrode in terms of sulfur is approximately 4.5 mg of sulfur/cm.sup.2.

(13) Two batteries are assembled with, respectively, the positive electrode corresponding to the method of the invention and the reference electrode.

(14) Each of these batteries comprises: as the negative electrode, an electrode made of metallic lithium; as the electrolyte, a tetraethylene glycol dimethyl ether/dioxolane (TEGDME/DIOX) mixture (1:1 by volume) comprising LiTFSI (1M) and LiNO.sub.3 (0.1M).

(15) The change in the discharge capacity C (in mAh/g) as a function of the number of cycles N was determined for these two batteries, the results being reported in the appended FIG. 2 (curve a) for the battery comprising the positive electrode obtained according to the method of the invention and curve b) for the battery comprising the reference positive electrode).

(16) It follows that the battery comprising the electrode obtained according to the method of the invention has better performance in terms of discharge capacity than the battery comprising the reference electrode.