Method for manufacturing a homogenized mixture composed of carbon, sulfur, and PTFE

Abstract

The present invention relates to a method for manufacturing a homogenized mixture composed of carbon, sulfur, and PTFE, characterized in that sulfur is liquefied, and then the liquid sulfur is milled together with carbon for the first time, such that the liquid sulfur is absorbed by the pores of the carbon particles and forms a preferably powdery composite with the carbon particles, then PTFE is added, and subsequently the mixture of the composite and PTFE is milled for the second time and thereby homogenized.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a homogenized mixture and an apparatus for performing the method. Background Art

[0002] Lithium-sulfur batteries (Li-Schwefel-Akkus) are currently considered a promising option for meeting the ever-increasing demand for more efficient batteries. See, for example, the DLR publication "Lithium-Sulfur Batteries: New Developments and Discoveries (Lithium-Schwefel Batterien: Neue Entwicklungen und Erkenntnisse)", N. Wagner, M. Schwan, B. Sievert, B. Milow, F. Warth, December 04, 2018, 2018 Materials Colloquium, retrievable on August 18, 2020 from the server of the German Aerospace Center known as DLR, the web address of which is www.dlr.de 1 . (Full web address: https: / / www.dlr.de / wf / Portaldata / 23 / Resources / dokumente / werkstoff-kolloquium / wsk-2018 / Wagner_-_Lithium-Schwefel_Batterien-Neue_Entwicklungen_und_Erkenntnisse.pdf)

[0003] For such Li-S batteries, a Li-S cathode is required.

[0004] Traditionally, such cathodes are manufactured by mixing an active material, a conductive material, and a binder with a solvent to produce a slurry. The slurry is to be applied to a carrier film. And after coating, the carrier film must be dried. This is an energy-consuming step and is considered a "bottleneck" in production. In addition, the solvents used for manufacturing the cathode are harmful to the environment.

[0005] Therefore, there is a need to improve the manufacturing according to the so-called "powder to roll" process. This process is characterized in that, first, a powder mixture of sulfur (S) and carbon is uniformly mixed with a suitable binder, and then calendered. Thereby, the powder is roll-pressed into a fabric-like film. Then, this film can be laminated onto a metal film (usually aluminum foil) by rolling. Thereby, a Li-S cathode in the desired form can be manufactured by curling or folding. For the existing method, the raw materials - sulfur as the active material and carbon as the conductive additive - are mixed with each other in a dry form and milled in a mill. Then, the mixture is delivered to a furnace so that the sulfur melts and is absorbed into the pores of the carbon. Here, a carbon-sulfur agglomerate that must be milled again is produced. In the next step, the powder is mixed with PTFE (polytetrafluoroethylene) as the binder in a dry form and milled again.

[0006] In order to be able to manufacture a particularly high-quality cathode by means of this method, it is important that the raw materials are distributed as uniformly as possible in the particle collective.

[0007] So far, the dry mixing cannot be carried out continuously, but only batch by batch. Summary of the Invention

[0008] Therefore, the object of the present invention is to improve a method by means of which a more homogeneous mixture can be manufactured continuously and with fewer process steps compared to the techniques used so far.

[0009] Solution According to the Invention

[0010] This object is achieved by the method according to the invention.

[0011] The method relates to a method for manufacturing a homogenized mixture composed of carbon, sulfur, and PTFE.

[0012] The method is characterized in that sulfur is liquefied and then the liquid sulfur is milled together with carbon or carbon particles for the first time. This enables the sulfur and carbon to be very uniformly distributed. Here, the liquid sulfur is absorbed by the pores of the carbon particles that can be contacted from the surface. This absorption is enhanced by the milling process because new surfaces are always exposed during the milling process, and the carbon particles can absorb the liquid sulfur on this surface. The sulfur and carbon form a composite material or composite particles, that is, a composite composed of carbon particles (usually in powder form) and sulfur that has entered the pores of the corresponding carbon particles. Since the milling process occurs simultaneously with the entry into the pores, the individual composite particles remain separate and do not agglomerate at this stage.

[0013] Then, granular, preferably powdered, PTFE is added as a solid.

[0014] At this time, the mixture composed of composite particles and PTFE particles is subjected to a second milling.

[0015] The second milling is used to fully mix or homogenize all the particles of the mixture, while crushing all the agglomerates that may be generated during the melting process. And at the same time, it can cause a very strong swirl of the particles of the mixture, so that the particles are rapidly cooled, and thus the tendency of these particles to form new agglomerates is significantly reduced or completely stopped.

[0016] Alternative method of the present invention

[0017] Ideally, carbon particles in the form of carbon black particles composed of solids are used. The carbon black particles are composed of carbon particles with a very large free surface, and are therefore particularly suitable for forming a composite material with sulfur.

[0018] Particularly preferably, liquid sulfur is introduced into the first mill via an injector.

[0019] Ideally, the liquid sulfur (until it is first milled together with the carbon particles) is in the temperature range between 110 °C and 220 °C. Since the viscosity of liquid sulfur shows a significant correlation with temperature, temperature control of sulfur is required so that it basically always reaches the mill or the first mill with the same viscosity, so that reproducible results can be produced here.

[0020] Optionally, the method can also be carried out with only one mill. In this case, carbon and PTFE are provided metrologically, and sulfur is provided via an injection system (which is marked only for completeness).

[0021] Particularly advantageously, the injector operates with nitrogen under overpressure. The overpressure generates a directional and fast enough flow, which enables the liquid sulfur to reliably enter the mill, and the sulfur will not deposit too much on the inner wall of the pipe feeding the first mill and then no longer provide a fine and homogeneous mixture with carbon. Here, nitrogen is the preferred injection gas because nitrogen is inert with respect to carbon and sulfur and is basically non-toxic.

[0022] To support the temperature control of sulfur, ideally the injection gas is temperature-controlled. The injection gas is heated so that sulfur exists in a liquid state in nitrogen (other inert gases are also conceivable) during transportation, without crystallization and ideally reaches the milling area where it is milled together with the carbon particles at the temperature in the above temperature range.

[0023] It has proven particularly advantageous for the injection gas to be separated from the carbon-sulfur composite and discharged immediately after the milling of the carbon particles with sulfur. Thereby, an appreciably colder atmosphere is provided immediately after the first milling, in which the goal of cooling the current particles significantly faster without further caking can be achieved significantly faster. Generally, it is advantageous for the temperature of the carbon-sulfur composite to decrease after the common first milling.

[0024] Particularly advantageously, the first milling is carried out in an impact mill and in particular in a rod mill. In an impact mill and in particular in a rod mill, the milling material is not squeezed for a long time or "over-crushed" and compacted by the grinding bodies. Alternatively, the milling effect is based on the particles to be milled - when they collide with each other or with the pins of the milling tool with a corresponding force - being thrown around the milling chamber with high kinetic energy and being broken up. This prevents new caking caused by milling.

[0025] Particularly advantageously, the second milling is carried out in an impact mill and in particular in a rod mill. Even when thorough mixing is preferred over further comminution during the second milling, the use of an impact mill and in particular a rod mill also works particularly advantageously. Because the strong vortex in the milling chamber also has a positive effect, mixing very thoroughly while cooling rapidly.

[0026] Here, it is very advantageous for mixing to be accompanied by milling, because the tendency of the initially still hot composite material particles to cake together is counteracted. Even if such larger particles are produced by caking, they will be broken up again by the milling process after a very short time.

[0027] Particularly advantageously, three separate metering operations are carried out for sulfur, carbon particles and PTFE. These metering operations are preferably set such that the homogenized mixture contains 50 m% to 70 m% sulfur, 25 m% to 40 m% carbon particles (in particular in the form of carbon black) and 1 m% to 10 m% PTFE.

[0028] In most cases, the homogenized mixture is produced continuously in a mill and not batchwise via a plurality of successive mill chargings. In this way, it is possible to produce a consistent quality more easily - especially in the case of monitoring and tracking one or more main process parameters - that is independent of the batch.

[0029] Particularly advantageously, the first milling is hot gas milling. The formation of the composite material, i.e. the formation of the carbon-sulfur complex, is promoted during the first milling.

[0030] Ideally, the second milling is substantially or mainly used to cool the final product provided after the method according to the present invention is completed. Since the process is a milling process, it prevents the product from undergoing unwanted coalescence when cooled to a temperature range where it will no longer significantly agglomerate. Description of the Drawings

[0031] Figure 1 Shows a device-related design solution required for a preferred embodiment of a method according to the present invention.

[0032] Figure 2 Shows the rod disc of a rod mill, which is preferably used for milling here, for example. Detailed Description of the Invention

[0033] Figure 1 Shows a device for performing a method according to the present invention. According to the present invention, the method employs a two-stage milling process. The significant advantage of this two-stage milling process is to prevent the formation of small lumps (Klümpchenbildung) in the sulfur / carbon black composite during cooling. At the same time, this two-stage milling process cools the final product in the second mill or (in the case of using only a single mill) during the second round of milling. In this way, fine homogenization of the PTFE powder can be ensured by using a two-stage milling process. The characteristic of the device is that the device performs the process according to the present invention as a continuous process. Different from the past, it is not set to work batch by batch, for example, working in batches preset by the filling amount of the mill. This improves the quality. It eliminates the typical quality fluctuations that used to occur between batches.

[0034] The homogenized mixture produced by the method according to the present invention is a mixture composed of carbon, sulfur, and PTFE, where PTFE is also known as polytetrafluoroethylene. Carbon is mostly used in the form of carbon black.

[0035] In Figure 1 the carbon storage or carbon black storage 1 can be clearly seen. The carbon black therein is metered out in a weight metering manner via a metering gauge or a metering screw 2. Preferably, the metering is set such that in the completed homogenized mixture, the carbon black has a content of about 25 m% to 40 m% (i.e., mass percentage). The carbon black preferably reaches the mill inlet line 4 described in detail below under the influence of gravity via a flap gate 3.

[0036] In Figure 1The sulfur storage section 5 can also be clearly seen therein. The powdered sulfur preferably reaches the metering device or the metering screw 6 from this sulfur storage section under the influence of gravity. Preferably, the metering is set such that in the homogenized mixture, sulfur has a content of approximately 50 m% to 70 m%. The metering device or the metering screw 6 is heated. For this purpose, usually, the metering screw is equipped with one or more heating elements on the metering pipe, preferably temperature-controlled or temperature-regulated. These heating elements are set to a specific temperature such that during the metering process, the state of sulfur is changed from powdered to liquid by the action of temperature. Thereafter, sulfur leaves the metering device in a completely liquid state.

[0037] Precise metering is crucial for the stable product quality of the later-stage battery. Therefore, the temperature control or temperature regulation of the metering pipe must be very precise. This is because sulfur exhibits special melt viscosity characteristics (Schmelz- ). Sulfur has a melting temperature of approximately 119 °C. When sulfur melts, a low-viscosity liquid is first formed. When heated further, the viscosity increases until it reaches a maximum at 187 °C. When heated further, the viscosity decreases again. Therefore, in order to achieve precise metering, the temperature of sulfur must be carefully controlled.

[0038] Preferably, sulfur reaches the ejector 7 under the action of gravity.

[0039] Heated injection gas under pressure is supplied from the gas reservoir 8 to the ejector 7. In most cases, the injection gas is input in a preheated form, and this injection gas has a temperature of 100 °C to 200 °C.

[0040] Nitrogen is preferably used as the injection gas. The advantage of nitrogen is that it is not prone to significant reaction with sulfur even under heating conditions. However, other inert gases can also be used as an alternative. However, for reasons of process economy, nitrogen is clearly preferred.

[0041] In addition, as Figure 1 shown, a heating element 9 is provided upstream of the ejector 7, and this heating element 9 adjusts the temperature of the injection gas accordingly. The injection gas leaves the ejector 7 via the mill input line 4 and flows in the direction of the first mill. Here, the temperature of the injection gas is adjusted such that sulfur remains liquid in the injection gas during transportation and does not crystallize. The pressure of the injection gas is selected such that sulfur is transported from the ejector to the first mill 10. Before the sulfur-carrying injection gas reaches the first mill, carbon black is fed into the mill input line 4 at the pipe intersection 4a. At this time, the liquid sulfur and carbon black reach the first mill 10 or its grinding chamber by the carrying of the injection gas. Usually, the first mill 10 is an impact mill. Ideally, a so-called rod mill is used.

[0042] For a rod mill, a grinding disc is used as the grinding tool, and the grinding disc is equipped with pins positioned at intervals from each other, as Figure 2 shown in a preferred embodiment. Usually, at least one fixed pin disc 23 and at least one rotating pin disc 24 are used, and counter-rotating pin discs are also conceivable. The material to be ground is mostly fed in at the pinless center Z of the pin discs 23, 24 or preferably the stationary pin disc. From here, the material is thrown outwards by centrifugal force and is then usually also conveyed outwards. Here, the grinding process is achieved by the collision with the pins 25 and the relative particle impact.

[0043] Usually, the rotational speed of the rotating pin disc is controlled or regulated. Usually, the faster the pin disc rotates, the finer the grinding result.

[0044] Through grinding, sulfur and carbon black are finely distributed and well mixed. By grinding in the first mill 10 under hot gas working conditions, the liquid sulfur is absorbed by the pores of the carbon black. It becomes a composite in the above sense.

[0045] After the first grinding, the composite formed by carbon black and sulfur falls into the storage part 11 under the mill.

[0046] As can be clearly seen according to Figure 1 the injection gas (usually nitrogen) runs cyclically. After the first grinding, the excess injection gas is purified by the filter 12 and led out of the grinding equipment through the ejector 13. Preferably, the injection gas is re-supplied to the injection gas storage 8 or the nitrogen storage (not shown here). It should also be noted that the discharge of the injection gas can also be achieved by a blower not shown here.

[0047] The sulfur / carbon black composite falls from the storage part 11 into the impeller gate 14 whose rotational speed is usually regulated or controlled. The composite is discharged from here into the storage container 15. In this storage container 15, free-flowing PTFE is metered out from the PTFE storage part 17 via the metering gauge or metering screw 16. The PTFE powder falls into the metering gauge or metering screw 16 under the influence of gravity. From here, the PTFE powder is conveyed into the storage container 15.

[0048] The carbon black / sulfur / PTFE mixture is continuously fed into the second impact mill 19 from the storage container 15 via another impeller gate 18 whose rotational speed is regulated or controlled. Here, the impact mill 19 is preferably also implemented as a rod mill. The above description applies accordingly.

[0049] However, in this second impact mill, hot gas milling is not carried out. Instead, the carbon black / sulfur mixture is not only homogenized with PTFE, but is also cooled during the milling process for further processing. By cooling during this second milling step, it causes the sulfur lumps formed during the cooling process to break immediately, so that no coarse and hard sulfur lumps are produced during the cooling process. Additionally, the PTFE powder is evenly and finely distributed.

[0050] The final product in the form of a homogenized mixture composed of carbon, sulfur, and PTFE is discharged via another impeller gate 20. The excess process gas is filtered through a filter 21 and discharged from the milling equipment through another ejector 22. Alternatively, a vacuum blower can also be used for discharging.

[0051] List of reference numerals

[0052] 1 Carbon black storage

[0053] 2 Metering screw for carbon black

[0054] 3 Impeller gate for carbon black

[0055] 4 Mill input pipeline

[0056] 4a Pipeline intersection for feeding carbon black into the mill input pipeline

[0057] 5 Sulfur storage

[0058] 6 Heated metering screw for sulfur

[0059] 7 Ejector

[0060] 8 Injection gas reservoir

[0061] 9 Heating element for injection gas

[0062] 10 First mill

[0063] 11 Storage for sulfur / carbon black composite under the first mill

[0064] 12 Filter

[0065] 13 Ejector

[0066] 14 Impeller gate for sulfur / carbon black composite

[0067] 15 Storage container

[0068] 16 Metering screw for powdered PTFE

[0069] 17 PTFE storage

[0070] 18 Impeller gate for mixture composed of sulfur / carbon black and PTFE

[0071] 19 Second impact mill

[0072] 20 Another impeller gate

[0073] 21 Another filter

[0074] 22 Another injector

[0075] 23 (Normally stationary) pin disk

[0076] 24 (Rotating) pin disk

[0077] 25 Pins of the pin disk or impact pins

[0078] Center of the Z pin disk

Claims

1. A method for manufacturing a homogenized mixture composed of carbon, sulfur, and PTFE, characterized in that, Liquefy sulfur, then perform a first milling of the liquid sulfur together with carbon such that the liquid sulfur is absorbed into the pores of the carbon particles and forms a composite with the carbon particles, then add PTFE, and subsequently perform a second milling of the mixture of the composite and PTFE and thereby homogenize it. wherein the liquid sulfur is fed to an injector (7) which injects the liquid sulfur into a mill, and wherein the first milling is a hot gas milling and the second milling serves to cool the final product.

2. The method according to claim 1, wherein Use carbon particles in the form of carbon black.

3. The method according to claim 1, wherein The liquid sulfur is in a temperature range between 119 °C and 220 °C until it is milled together with the carbon particles.

4. The method according to claim 1, characterized in that The injector (7) operates with an injection gas under overpressure.

5. The method according to claim 1, characterized in that, Heat the injection gas such that sulfur remains liquid in nitrogen during transport without crystallizing.

6. The method according to claim 4, characterized in that, The injection gas is separated from the carbon-sulfur composite and discharged immediately after the carbon particles are milled together with sulfur.

7. The method according to claim 6, characterized in that, The temperature of the carbon-sulfur composite decreases after co-milling.

8. The method according to claim 1, wherein The first milling is carried out in an impact mill.

9. The method according to claim 1, wherein The second milling is carried out in an impact mill.

10. The method according to claim 1, characterized in that, Perform three separate metering (2, 6, 16) for sulfur, carbon particles and PTFE.

11. The method according to claim 1, wherein The homogenized mixture is continuously produced by a mill (10, 19) and not batchwise through a plurality of successively connected mill chargers.

12. The method according to claim 1, characterized in that, The composite is a powdery composite.

13. The method according to claim 3, characterized in that, The liquid sulfur is only at a temperature of 200 °C until it is milled together with the carbon particles.

14. The method according to claim 3, characterized in that, The liquid sulfur is only at a temperature of 190 °C until it is milled together with the carbon particles.

15. The method according to claim 4, wherein The injection gas is in the form of nitrogen.

16. The method according to claim 5, wherein The temperature of sulfur in the temperature range of 119 °C to 220 °C reaches the milling zone where sulfur is milled together with the carbon particles.

17. The method according to claim 8, characterized in that, The first milling is carried out in a rod mill.

18. The method according to claim 9, wherein The second milling is carried out in a rod mill.

19. The method according to claim 10, characterized in that, The metering is set such that the homogenized mixture contains 50 mass% to 70 mass% sulfur, 25 mass% to 40 mass% carbon particles, and 1 mass% to 10 mass% PTFE.

20. The method according to claim 19, characterized in that, The carbon particles are carbon black.

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