WO2024231267A1 - Process for producing sulfur-carbon composites - Google Patents

Abstract

The invention relates to a process for producing sulfur-carbon composites, in particular sulfur-carbon aerogel composites, which are produced by way of extrusion at a temperature of at least 300°C.

Description

Process for the preparation of sulfur-carbon composites

The invention relates to a process for producing sulfur-carbon composites, in particular sulfur-carbon aerogel composites, which are produced by extrusion.

In a metal-sulfur battery, sulfur is an electrochemically active material. At room temperature, sulfur is known to exist as crown-shaped cyclooctasulphur (a - S8). At 95.5 °C, the orthorhombic sulfur (a - S8) is converted into monoclinic ß-sulfur; this melts at 119.6 °C (Meyer B (1976) Elemental sulfur. Chem. Rev. 76: 367. D01 : 10.1021/cr60301a003). Above 120 °C, sulfur rings of various sizes (Sn, n=5-33) are present in the sulfur melt. At temperatures above 159 °C, the melt viscosity increases abruptly due to radical ring-opening polymerization. On further heating, the viscosity decreases again from 243 °C and short (Sn, n<6) sulfur chains are formed. The boiling point is 444 °C. In the gaseous state, sulfur molecules are present in the form of S2-8, with the equilibrium shifting towards S2 species as the temperature increases. In the gaseous state at 600 °C, sulfur is present in the form of S6 (about 58.8%) and about 16.4% in the form of S2 molecules. These short-chain sulfur radicals can deposit in micropores of a carbon material (e.g. activated carbon) and also form covalent bonds with carbon (Korpiel JA, Vidic RD (1997) Effect of Sulfur Impregnation Method on Activated Carbon Uptake of Gas-Phase Mercury; Environmental Science & Technology 31 : 2319. DOI: 10.1021/es9609260). There are several methods for producing such composites, often called infiltration or impregnation. One method, gas infiltration, usually takes place at 400-600 °C, so that the sulfur is in the gas phase, and takes about two to six hours (Liu W, Vidic RD, Brown TD (1998) Optimization of Sulfur Impregnation Protocol for Fixed-Bed Application of Activated Carbon-Based Sorbents for Gas-Phase Mercury Removal. Environmental Science & Technology 32: 531. D01 : 10.1021/es970630 ; Nojabaee M, Sievert B, Schwan M, et al. (2021) Ultramicroporous carbon aerogels encapsulating sulfur as the cathode for lithium-sulfur batteries. J. Mater. Chem. A 9: 6508. DOI : 10.1039/D0TA11332H). With this process, a large part of the sulfur is also introduced into the micropores. However, the entire process with heating and cooling phases can take over 10 hours and is therefore very time and energy intensive. In addition, only very small quantities of sample can be processed in the processes established so far. In addition, the processes are carried out as batch processes. For processing large quantities, the effort increases so much that the processes are not economically feasible.

Sulfur-carbon composites can also be produced using melt infiltration. For this, a sulfur-carbon mixture is brought to the melting temperature or higher and held at this temperature for several hours. The following process parameters are known from the literature: T=280 °C, holding time 12 hours (Balakumar K, Kalaiselvi N (2015) High sulfur loaded carbon aerogel cathode for lithium-sulfur batteries. RSC Advances 5: 34008. DOI: 10.1039/C5RA01436K], T=150-300°C); (Wei S, Xu S, Agrawral A, et al. (2016) A stable room-temperature sodium-sulfur battery. Nature Communications 7: 11722. DOI : 10.1038/ncommsll722]) or T=155 °C with holding time 5 hours (Choudhury S, Krüner B, Massuti-Ballester P, et al. (2017) Microporous novolac-derived carbon beads/sulfur hybrid cathode for lithium-sulfur batteries. J. Power Sources 357: 198. DOI: https://doi.Org/10.1016/j.jpowsour.2017.05.005). Another method for The production of sulfur-carbon composites is by milling infiltration, in which a sulfur-carbon mixture is milled in a mill (e.g. ball mill) for 15 min at 550 rpm (Häcker J, Danner C, Sievert B, et al. (2020) Investigation of Magnesium-Sulfur Batteries using Electrochemical Impedance Spectroscopy. Electrochim. Acta 338: 135787.

DOI: https://doi.Org/10.1016/j.electacta.2020.135787) or 30 min at Fe- dorkova AS, Kazda T, Gavalierova K, Gomez-Romero P, Shembel E (2018) Synthesis and Characterization of Porous Sulfur/MWCNTs Composites with Improved Performance and Safety as Cathodes for Li-S Batteries. Int. J. Electro- chem. Sci. 13: 551. DOI: 10.20964/2018.01.67. In both melt infiltration and mill infiltration, sulfur is still present in longer chains due to the lower temperatures, so that the micropores can only be infiltrated to a small extent.

The main disadvantages of producing sulfur-carbon composites by gas infiltration are the small amount of material, the long duration and the high energy consumption. Melt infiltration and milling infiltration can be carried out with larger amounts and in a shorter time, but in contrast to gas infiltration, they do not achieve sufficient infiltration of the sulfur into the micropores of the carbon.

DE 10 2014 210 249 Al discloses the extrusion of sulfur and carbon, in particular conductive carbon black, to produce a cathode for a battery cell. This is not a microporous carbon. The aim of the process is to mix sulfur and carbon material and not to infiltrate the sulfur into the pores of the porous carbon material.

In the publication US 2017 / 0 313 844 Al, a corresponding mixture is also introduced into an extruder to produce a masterbatch made of carbon nanofibers and sulfur. Here, too, it is not a microporous carbon. The aim of the process is also a Mixing of sulfur and carbon material and not infiltration of sulfur into the pores of the porous carbon material.

EP 2 966 708 Bl also describes a combination of carbon material with sulfur in an extruder. This is also not a microporous carbon. The aim of the process is to mix sulfur and carbon material with an ion-conductive polymer binder and not to infiltrate the sulfur into the pores of the porous carbon material.

The above-mentioned processes only describe the mixing and combination of sulfur with carbon.

The object of the present invention in view of the aforementioned prior art relates to the optimization of the production of sulfur-carbon composites, in particular sulfur-carbon aerogel composites, in particular to significantly increase the material throughput and at the same time to improve the duration and energy consumption, in particular the infiltration of the sulfur into the micropores of the carbon material. Temperature also plays an important role here. To melt the sulfur, as described in the previous applications, a temperature of 115 °C is sufficient; therefore, these processes are carried out at well below 200 °C.

However, the invention relates to a batch or continuous process for producing a sulfur-carbon composite in which the sulfur is incorporated into micropores of the carbon, in particular a carbon aerogel. This is not possible with the previously known processes.

A first embodiment of the present invention therefore relates to a process for producing sulphur-carbon composites by introducing and reacting sulfur and a carbon material at a temperature of 300 °C or more in an extruder.

To solve the above problem, the invention provides a method in which an extruder commercially available in the plastics industry is used and the carbon is homogenized and heated with sulfur in a continuous process. This scalable process can be operated batchwise or continuously on systems established in the mass production industry, which makes it possible to process significantly larger quantities with a significantly reduced throughput time and lower energy consumption. A specialist can easily set the appropriate process parameters, such as temperature, residence time, speed, L/D ratios, based on his knowledge. At temperatures below 159 °C, the sulfur is present in S8 rings or long-chain molecules, as described at the beginning. Only at a temperature of at least 300 °C do short-chain sulfur molecules (S6, S4 and S2) also form in sufficient numbers so that they can fully penetrate the small pores of the carbon. This is not possible for the large sulfur molecules at lower temperatures below 300 °C. Therefore, complete infiltration of the pores (especially micropores) of the carbon cannot take place at lower temperatures. This can be seen in the TGA curves of the samples that were processed at different temperatures. The lower the temperature at which sulphur is released, i.e. a mass loss is visible in the TGA curve, the less energy is required to evaporate sulphur from the carbon. More energy is required to free small pores, especially micropores, of sulphur than for surface sulphur. Therefore, the mass loss takes place at significantly higher temperatures than when sulphur is in large pores and on the surface of the carbon particles. Therefore, after processing the composite in the extruder at 200 °C, two stages are shown in the TGA curve of the material obtained (Fig. 1). The stage in the mass loss between 200 °C and 300 °C is caused by the sulfur in large pores or on the surface. The mass loss from around 350 °C is caused by the sulfur in the smaller pores (especially micropores). When processed at 350 °C, only one step is seen in the TGA curve. Only at over 300 °C does this sample lose significant mass. This shows that the sulfur is completely in the pores (especially micropores), as intended by the process according to the invention.

In a preferred embodiment of the present invention, the aforementioned process is characterized in that both sulfur and the optionally porous carbon material are introduced separately into the extruder. The mixture formation and conversion thus takes place in the extruder. Alternatively, it is also possible to first produce a homogeneous mixture of sulfur and the carbon material in a first step and to introduce the aforementioned mixture into the extruder in a second step. Here, too, the conversion takes place in the extruder.

A large number of extruders of all sizes are commercially available. According to the invention, a single-screw or twin-screw extruder is used in particular.

The feedstock materials used are commercially available sulfur in powder form and the carbon material used is, in particular, a conventional carbon aerogel.

The material introduced into the extruder preferably goes through one or more temperature steps. The throughput time in the extruder is preferably set in the range of 0.1 to 10 minutes. If the throughput time is too short, sufficient conversion will not be achieved.

In a further embodiment of the invention, it is possible to further process, but also when applying the base materials, in particular After the first temperature treatment steps, additional materials in solid form, such as thickeners or binders, conductive additives, or liquid form, such as solvents, can be added.

Preferably, in the sense of the invention, different nozzle geometries are used in the discharge zone for further processing in the further course of the process. For extrusion of strands, round strand nozzle heads with screw-in nozzles or pasta nozzle heads with different diameters can be used. For extrusion coatings, wide slot nozzles with adjustable coating thicknesses can be used. For more precise temperature control, modular cooling nozzles with independent temperature control of heating and cooling zones can be installed along the nozzle. Extruders without nozzles in the discharge zone can also be used.

To the best of the inventors' knowledge, many processes for infiltrating carbon with sulfur are known, but no patent or other publication is known in which an extruder is used to infiltrate the sulfur into the micropores of the carbon material.

With the help of extruders, sulfur-carbon composites can be produced batchwise and, in particular, continuously, which was not possible with previously used gas or melt infiltration methods. The carbon and sulfur can either be added individually or a premixed carbon-sulfur mixture is dosed into the extruder. This is moved by the screw(s) in the extruder and passes through an area where the temperature is increased to at least 300 °C so that the sulfur diffuses into the micropores of the porous carbon (Wu HB, Wei S, Zhang L, Xu R, Hng HH, Lou XW (2013) Embedding Sulfur in MOF- Derived Microporous Carbon Polyhedrons for Lithium-Sulfur Batteries. Chemistry - A European Journal 19: 10804.

DOI :https://doi.org/10.1002/chem.201301689). The Sulfur-carbon composite cooled again. The sample throughput time is in the range of a few seconds to minutes and is therefore significantly lower than the times used for previous batch processes of up to more than 10 hours. With this process, throughputs of more than 200 g/h can be achieved; in previous processes, the batch sizes are on the gram scale. Since extruders are already established in other applications in industry, the devices are available in a wide variety of scales, which means that the process can be scaled up as required without great effort.

In order to obtain a powder using the process according to the invention, it is possible to actively cool the last section of the extruder to temperatures of 20°C or less, whereby the material becomes brittle and exits the extruder as a powder. This means that a subsequent grinding step can be omitted.

The process according to the invention can be carried out on single-screw and twin-screw extruders. The expert can set the appropriate process parameters, such as temperature, residence time, speed, b/D ratios.

The sulfur-carbon, in particular sulfur-carbon aerogel composites obtainable according to the invention can be used, for example, in metal-sulfur batteries or for the adsorption of mercury from aqueous solutions.

Example:

Preparation of sulfur-carbon aerogel composites as an example of a sulfur-carbon composite • 60 g of a carbon aerogel (molar ratios: R/W=0.008, R/C=50 and R/F=0.5, pH=5.65) (Marina Schwan, Lorenz Ratke (DE) (2019); Process for the production of flexible aerogels based on resorcinol-formaldehyde and aerogels obtainable by this process DE 10 2012 218548 Al) was mixed in particular with 40 g of sulfur (20 to 95 wt.%) to form a homogeneous mixture.

extrusion process

• The sulfur-carbon aerogel mixture was added to the extruder using a dosing device at a fixed dosing rate of 200 g/h. The material was fed through various temperature ranges in a twin-screw extruder with temperatures up to 350 °C and then cooled to 10 °C.

• The processing time was in the range of 1 to 10 minutes, during which the material was heated up to 350 °C.

The amount of sulfur in the sulfur-carbon aerogel composite could be investigated by thermogravimetric analysis (Fig. 1). Fig. 1 shows a thermogravimetric analysis of sulfur-carbon aerogel composites using an extruder in comparison with the gas, melt and mill infiltration methods.

The following samples were analyzed:

• CA10+S Vmicro Gas:

Gas-infiltrated sample, sulfur amount adjusted to the volume of the micropores

• CA10+S Vmicro mill:

Milled infiltrated sample, amount of sulfur adjusted to the volume of the micropores io

• CA10+S Vmicro+meso Gas:

Gas-infiltrated sample, amount of sulfur adjusted to the volume of the micro- and mesopores combined

• CA10+S Vmicro+meso mill:

Mill-infiltrated sample, amount of sulfur adjusted to the volume of the micro- and mesopores combined

• CA10+S Vmicro Extruder:

Sample infiltrated with the extruder, amount of sulfur adjusted to the volume of the micropores

• CA10+S Vmicro+meso melt infiltration:

Melt-infiltrated sample, amount of sulfur adjusted to the volume of the micro- and mesopores combined

Fig. 1 shows that the amount of sulfur differed slightly in the various infiltration methods. The crucial sulfur content, which was only released at higher temperatures from 300 °C, was at least as high for the sample processed in the extruder as for the other methods. Fig. 2 shows an extruder in which the sulfur-carbon aerogel composite was processed.

Claims

Patent claims: 1. Process for producing sulfur-carbon composites by introducing and reacting sulfur and a carbon material at a temperature of at least 300 °C in an extruder. 2. Process according to claim 1, characterized in that both sulfur and the carbon material are introduced separately into the extruder. 3. Process according to claim 1, characterized in that in a first step a homogeneous mixture of sulfur and the carbon material is first prepared and in a second step the aforementioned mixture is introduced into the extruder. 4. Process according to one of claims 1 to 3, characterized in that a single-screw or twin-screw extruder is used. 5. Process according to one of claims 1 to 4, characterized in that a porous carbon material, in particular carbon aerogel, is used as the carbon material. 6. Process according to one of claims 1 to 5, characterized in that the introduced material in the extruder passes through one or more temperature steps. 7. Process according to claim 1 or 2, characterized in that the throughput time in the extruder is set in the range of 0.1 to 10 minutes. 8. Process according to one of claims 1 to 7, characterized in that in the further course of the process, after the first temperature treatment steps, additional materials in solid or liquid form are added. 9. Method according to one of claims 1 to 8, characterized in that different nozzle geometries are used for further processing in the further course of the process.