USE OF PEDOT/PSS IN A CATHODE OF A LITHIUM-SULFUR ELECTROCHEMICAL CELL

Abstract

The present invention relates to a liquid composition comprising a) at least one cationic polythiophene; b) at least one polymeric counterion; c) sulfur; d) at least one solvent having a boiling point of 80 C. or more; e) at least one conductivity improving agent; wherein the liquid composition comprises less than 10 wt.-% of components having a boiling point of less than 80 C., based on the total weight of the liquid composition, and wherein the boiling point in each case is determined at a pressure of 1013 mbar. The present invention also relates to a powdered composition comprising components a), b) and c), wherein the cationic polythiophene and the at least one polymeric counterion are present in the form of a cationic polythiophene:polymeric counterion-complex, to a process for preparing a liquid or powdered composition, a liquid or powdered composition obtainable by this process, to a lithium sulfur electrochemical cell and to the use of the liquid or the powdered composition.

Claims

1. A liquid composition comprising a) at least one cationic polythiophene; b) at least one polymeric counterion; c) sulfur; d) at least one solvent having a boiling point of 80 or more; e) at least one conductivity improving agent; wherein the liquid composition comprises less than 10 wt.-% of components having a boiling point of less than 80 C., based on the total weight of the liquid composition, and wherein the boiling point in each case is determined at a pressure of 1013 mbar.

2. The liquid composition according to claim 1, wherein the composition comprises the at least one conductivity improving agent e) in an amount of at least 0.1 wt.-%, based on the total amount of the liquid composition.

3. The liquid composition according to claim 1, wherein solvent d) is water.

4. The liquid composition according to claim 1, wherein the conductivity improving agent is liquid at 20 C. and 1013 mbar and has a boiling point of more than 100 C.

5. The liquid composition according to claim 1, wherein the conductivity improving agent is solid at 20 C. and 1013 mbar.

6. The liquid composition according to claim 1, wherein the conductivity improving agent e) is selected from the group consisting of ethylene glycol, propylene glycol, butylene glycol, hexylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, tetraethylene glycol, tetrapropylene glycol, propylene glycol methyl ether, dipropylene glycol methyl ether, tripropylene glycol methyl ether, propylene glycol propyl ether, dipropylene glycol propyl ether, tripropylene glycol propyl ether, propylene glycol butyl ether, dipropylene glycol butyl ether, tripropylene glycol butyl ether, propylene glycol phenyl ether, dipropylene glycol dimethyl ether, diethylene glycol ethyl ether, diethylene glycol methyl ether, diethylene glycol butyl ether, diethylene glycol hexyl ether, ethylene glycol propyl ether, ethylene glycol butyl ether, ethylene glycol hexyl ether, triethylene glycol methyl ether, triethylene glycol ethyl ether, triethylene glycol butyl ether, ethylene glycol phenyl ether, ethylene glycol ethyl ether, tetrahydrofuran, butyrolactone, valerolactone, N-methyl caprolactam, N,N-dimethyl acetamide, N-methyl acetamide, N,N-dimethyl formamide (DMF), N-methyl formamide, N-methyl formanilide, N-methyl pyrrolidone (NMP), N-octyl pyrrolidone, pyrrolidone, sulpholane (tetramethylene sulphone), dimethyl sulphoxide (DMSO), glycerol, diglycerol, triglycerol, tetraglycerol, sucrose, glucose, fructose, lactose, sorbitol, mannitol, 2-furancarboxylic acid, 3-furancarboxylic acid or a mixture of at least two thereof.

7. The liquid composition according to claim 1, wherein the liquid composition comprises less than 5 wt.-%, based on the total weight of the liquid composition, of a particulate material based on elemental carbon.

8. The liquid composition according to claim 1, wherein sulfur c) is elemental sulfur.

9. The liquid composition according to claim 1, wherein the cationic polythiophene a) and the polymeric counterion b) are present in the form of a cationic polythiophene:polymeric counterion-complex.

10. The liquid composition according to claim 1, wherein the liquid composition is free of components having a flash point of less than 25 C.

11. A powdered composition comprising a) at least one cationic polythiophene; b) at least one polymeric counterion; c) sulfur; wherein the cationic polythiophene and the at least one polymeric counterion are present in the form of a cationic polythiophene:polymeric counterion-complex.

12. A process of preparing a composition comprising the steps: i) providing a liquid pre-composition comprising at least one cationic polythiophene a), at least one polymeric counterion b) and at least one solvent d) having a boiling point of 80 C. or more, wherein the at least one cationic polythiophene a) and the at least one polymeric counterion b) are present in the form of a cationic polythiophene:polymeric counterion-complex; ii) mixing the liquid pre-composition obtained in process step i) with sulfur c); iii) mixing the liquid pre-composition obtained in process step i) or the composition obtained in process step ii) with at least one conductivity improving agent e); wherein the boiling point in each case is determined at a pressure of 1013 mbar and wherein the composition obtained by the process comprises less than 10 wt.-% of components having a boiling point of less than 80 C., based on the total weight of the composition.

13. The process according to claim 12, wherein during or after process step ii) the mixture is subjected to shearing.

14. The process according to claim 12, wherein the composition obtained by the process comprises less than 10 wt.-% of components with a flash point of less than 25 C., based on the total weight of the composition.

15. The process according to claim 12, wherein the process further comprises the step: iv) removing at least part of the solvent d) from the composition that is obtained by the process to obtain a powdered composition.

16. A liquid composition obtainable by the process according to claim 12.

17. A powdered composition, obtainable by the process according to claim 15.

18. A lithium-sulfur electrochemical cell comprising a first electrode that has been produced by superimposing a substrate with the liquid composition according to claim 1 and by subsequently removing at least a part of solvent d), thereby obtaining a substrate that is superimposed with an electrically conductive layer.

19. The use of the liquid composition according to claim 1 for the production of an electrode in a lithium-sulfur electrochemical cell.

20. The use of the powdered composition according to claim 11 for the production of an electrode in a lithium-sulfur electrochemical cell.

Description

[0178] The invention is now explained in more detail with the aid of non-limiting figures and examples.

[0179] FIG. 1 is a plot showing cathode voltage versus time that is obtained when subjecting the lithium-sulfur electrochemical cell that has been prepared in Example 4 to multiple discharge/charge cycles in the voltage range of 1.5 to 3.0 V and a constant current of 10 mA/g.

EXAMPLES

Example 1: Preparation of a Liquid Composition According to the Present Invention

[0180] A dissolver equipped with bead milling adaption and a 500 mL water-cooled stainless steel container (Dispermat CV/S with APS bead mill from VMA-Getzmann GmbH) was charged with 380 g cerium-stabilized zirconium dioxide beads (d=0.7-1.2 mm), 200 g of an aqueous PEDOT:PSS dispersion (PEDOT:PSS=1:2.5, solid content=4.45%) and 10 g of ethylene glycol (>98%, Applichem). While milling the mixture at 1000 rpm 80 g of sulfur (99.5%, Aldrich) were added in 10 g portions over one minute. The resulting slurry was further milled at 3500 rpm for 20 min. By using a grindometer (BYK-Gardner, No. 1512) the particle size was determined to be smaller than 10 m. After sieving off the zirconium dioxide beads the viscosity of the slurry was 1800 mPa.Math.s (Haake Viscotester VT-02, 62.5 rpm, rotor No. 1). The pH value of a diluted sample (1.1 g slurry+4.0 g deionized water) was determined to be 1.9 (pH meter, Schott CG817T).

[0181] The undiluted slurry was coated onto a glass plate using a 500 m doctor blade. The layer was dried for 5 h at room temperature and then for 16 h at 110 C. in a drying oven. The smooth layer had a sheet resistance of 20 Ohm/sq (four point probe, Mitsubishi Chemical Analytech, Loresta-AX MCP-T370)

Example 2: Preparation of a Powdered Composition According to the Present Invention

[0182] 50 g of the dispersion of example 1 was transferred into a glass dish (r=4 cm) and dried in a drying oven at 110 C. for 5 h. The remaining solid was milled in a mortar grinder (Retsch RM 100) for 10 min at low fineness and 2 min at high fineness for 15 min. The ground powder was dried further at 110 C. for 16 h.

[0183] The resulting powder was pressed into a pellet (r=7 mm, h=1.3 mm, density 1.84 g/cm.sup.3). Metal electrodes of known dimensions were pressed against the bottom and top of the pellet and the resistivity was determined to be 8.9 Ohm.Math.cm.

Example 3: Preparation of a Liquid Composition According to the Present Invention

[0184] A dissolver equipped with bead milling adaption and a 500 mL water-cooled stainless steel container (Dispermat CV/S with APS bead mill from VMA-Getzmann GmbH) was charged with 380 g cerium-stabilized zirconium dioxide beads (d=0.7-1.2 mm), 200 g of an aqueous PEDOT:PSS dispersion (PEDOT:PSS=1:2.5, solid content=4.45%) and 10 g of ethylene glycol (>98%, Applichem). While milling the mixture at 1000 rpm 75.7 g of sulfur (99.5%, Aldrich) were added in 10 g portions over one minute. Then 4.6 g of carbon black (Super C65 from Timcal) were added. The resulting slurry was further milled at 3500 rpm for 20 min. By using a grindometer (BYK-Gardner, No. 1512) the particle size was determined to be smaller than 10 m. After sieving off the zirconium dioxide beads the viscosity of the slurry was 2300 mPa.Math.s (Haake Viscotester VT-02, 62.5 rpm, rotor No. 1). The pH value of a diluted sample (1.0 g slurry+4.5 g deionized water) was determined to be 2.1 (pH meter, Schott CG817T).

[0185] The undiluted slurry was coated onto a glass plate using a 500 m doctor blade. The layer was dried for 5 h at room temperature and then 16 h at 110 C. in a drying oven. The smooth layer had a sheet resistance of 7 Ohm/sq (four point probe, Mitsubishi Chemical Analytech, Loresta-AX MCP-T370).

Example 4: Preparation of a Lithium-Sulfur Cell According to the Present Invention

[0186] The slurry of example 1 was coated onto an argon-plasma treated aluminium foil via doctor blading (100 m slid width, 1 mm/s). The layer was dried by applying infrared light for 3 h and further drying in vacuum for 15 h at 60 C. The layer thickness was 25 m. After drying, coin cells were assembled in inert atmosphere comprising a lithium anode, lithium reference electrode, polyolefin separator (Celgard), and a liquid electrolyte consisting of 1 mol/L lithiumbis(trifluoromethylsulphonyl)imid in a mixture (v/v=4/1) of 1,2-dimethoxyethane and 1,3-dioxolane.

[0187] The coin cells were subjected to multiple discharge/charge cycles in the voltage range of 1.5 to 3.0 V and a constant current of 10 mA/g (see FIG. 1). The capacity of first discharge was determined to be 45 mAh/g.