Elastically deformable carbon aerogels as matrix material in sulfur electrodes

Description

[0001] The present invention relates to an electrode for a battery, comprising an active mass (AM) consisting of sulfur and a carbon matrix material in the form of an elastically deformable microporous carbon aerogel, in particular for use in a metal (e.g. lithium, magnesium, aluminum)-sulfur battery, corresponding improved batteries, a method for producing the electrode according to the invention, and the use of elastically deformable microporous carbon aerogels as a matrix material for an electrode, in particular a sulfur electrode.

[0002] To meet the increasing demand for energy storage technologies, there is growing interest in novel batteries, particularly those using sulfur electrodes. A lithium-sulfur (Li-S) battery was first patented by Herbert and Ulan in 1962 (US 3,043,896). The near-unlimited availability of sulfur and the high specific capacity of the Li-S cell (1672 mAh / g(sulfur)) make lithium-sulfur batteries especially attractive. Nevertheless, this capacity is often underutilized in real-world applications, such as... electric vehicleThis target capacity is not achieved because, depending on the design of the sulfur electrode, additional additives (conductive additives, binders, and the like) must be used. The cell's capacity must then be related to the specific capacity of the entire cell, resulting in significantly lower capacities. To improve the specific capacity, the proportion of passive materials in the sulfur electrode must therefore be reduced, and the proportion of the active material, sulfur, maximized. A particular challenge here is that, despite its insulating nature, sulfur must be embedded in an electrically conductive matrix.

[0003] The proportion of passive materials, such as conductive additives, can be reduced by infiltrating sulfur into conductive matrices. For conductive matrices, the prior art specifically describes the use of porous carbon materials.

[0004] DE 199 38 822 A1 describes a process for producing a lithium ion intercalation electrode from open-pore monolithic carbons, preferably carbon aerogels, wherein a carbon layer is deposited on the surface of a skeletal material at temperatures between 500 and 2000 °C.

[0005] EP 1 610 404 B1 discloses an electrochemical element made of liquid material, comprising a metal anode and a carbon-based cathode, wherein the cathode comprises a carbon aerogel.

[0006] From EP 2 250 692 B1, a material comprising carbon and sulfur, as well as an electrode formed from this material, is known. The carbon is present in the form of a porous matrix with nanoporosity in the form of nanopores and nanochannels with an average diameter between 1 and 50 nm, with the sulfur positioned in the nanoporosity in the form of nanoparticles.

[0007] FR 2 872 347 A describes a galvanic cell comprising a metallic anode and a carbon-based cathode, wherein the carbon cathode comprises a carbon aerogel.

[0008] A manufacturing process for carbon aerogels was first described by Richard Pekala in 1990 (Pekala RW, Alviso CT, LeMay JD. Organic aerogels: microstructural dependence of mechanical properties in compression. J Non-Cryst Solids. 1990;125:67-75. Doi:http: / / dx.doi.org / 10.1016 / 0022-3093(90)90324-F). According to this method, carbon aerogels can be obtained from organic aerogels, such as resorcinol-formaldehyde, melamine-formaldehyde, phenol-formaldehyde aerogels, and the like, by carbonization.

[0009] US Patent 2018 / 0183067A1 provides a method for manufacturing a rope-shaped alkali metal battery, comprising: (a) providing a first electrode comprising a conductive porous rod and a mixture of a first electrode active material and a first electrolyte located in the pores of the first porous rod; (b) providing a porous separator encasing the first electrode to form a separator-protected first electrode; (c) providing a second electrode comprising a conductive porous rod with a mixture of a second electrode active material and a second electrolyte located in the pores of the second porous rod; (d) combining the separator-protected first electrode and the second electrode to form a braid or thread with a twisted or spiral electrode;and (e) wrapping or covering the braid or yarn with a protective sheath or casing to form the rope battery.;

[0010] DE 102012 218 548 A1 describes a resorcinol-formaldehyde aerogel. The aerogel is at least partially elastically deformable. Independent claims are also included for: (1) a carbon aerogel obtained by pyrolysis of the aerogel; and (2) preparation of the aerogel, comprising (i) preparing a solution containing distilled water, resorcinol, formaldehyde, and sodium carbonate, (ii) adjusting the pH of the solution to 5.3–5.6, (iii) gelling at a temperature of 70–90 °C, (iv) cooling the gel to room temperature and washing with an aprotic organic solvent, (v) drying the gel at elevated temperature, wherein the formaldehyde is present in a stoichiometric excess with respect to resorcinol, and the molar ratio of resorcinol to water is 0.006–0.01.

[0011] The proportion of sulfur in the sulfur electrode can be increased by increasing the pore volume of the matrix material used. A large pore volume allows for the infiltration of a significant amount of active material (sulfur). Not only pore volume, but also pore size and pore size distributions play a crucial role. Carbon aerogels are particularly suitable materials not only because of their high electrical conductivity, but primarily due to their high porosity (up to 99 vol.%), large specific surface area (500 to 3500 m² / g), and large pore volume (2 to 3 cm³ / g). The pore sizes can be precisely controlled both during synthesis and carbonization. Carbon aerogels can be produced with mono-, bi-, and / or multimodal pore size distributions.

[0012] A particular challenge for the use of sulfur electrodes with porous carbon materials in batteries is the so-called polysulfide shuttle. This process occurs during cell discharge, when polysulfides are formed as intermediate products. These intermediate products exhibit very high solubility in the electrolyte. The solubility of long-chain polysulfides, in particular, is very high and leads to the polysulfide shuttle. The polysulfide shuttle describes the diffusion of polysulfides between the cathode and anode. This mechanism causes the active material (sulfur) required for energy storage to leave the cathode compartment. The sulfur then reacts at the anode, for example, at the metallic lithium anode, to form Li₂S. The anode is consumed by the formation of a passivating layer.These processes reduce the capacity of the Li-S battery, as the charging reaction is inhibited by the passivating layer and the sulfur irreversibly leaves the cathode.

[0013] Another challenge in lithium-sulfur cells is the change in sulfur density within the cathode. The sulfur initially present in the form of α-S₈ has a density of 2.06 g / cm³. However, the final discharge product, Li₂S, has a density of 1.66 g / cm³. This density difference causes an 80% volume expansion during cell discharge and leads to additional mechanical stress within the cell. This stress results in degradation of the lithium-sulfur cell due to microcracking within the cathode.

[0014] The present invention is therefore based on the objective of providing a sulfur electrode for accumulators, in particular for lithium-sulfur accumulators, which avoids the disadvantages of the prior art described above. In particular, the specific capacity is to be improved by increasing the sulfur content of the electrode as permanently as possible. In addition, the cycle stability is to be improved, in particular, by avoiding or reducing the polysulfide shuttle and by avoiding / reducing microcracking due to volume expansion in the electrode.

[0015] In a first embodiment, this problem is solved by an electrode for a lithium-sulfur battery, comprising sulfur and a porous carbon material, wherein the pores of the porous carbon material are infiltrated with the sulfur, characterized in that the porous carbon material comprises an elastically deformable microporous carbon aerogel, according to independent claim 1.

[0016] According to the invention, the porous carbon material consists of the elastically deformable microporous carbon aerogel.

[0017] The electrode according to the invention differs from the prior art in particular in that a (flexible) carbon material is used as the matrix material, which predominantly has micropores (smaller than 2 nm) and is also elastically deformable.

[0018] A carbon aerogel that can be used according to the invention is described in particular in DE 10 2012 218 548 A1, without the present invention being limited to the material disclosed therein.

[0019] Surprisingly, it has been shown that the use of an elastically deformable microporous carbon aerogel can increase both the proportion of the active material sulfur and thus the specific capacity of the battery, as well as suppress the so-called polysulfide shuttle. Surprisingly, it has also been shown that microcracking due to volume expansion during cell discharge can be reduced by using an elastically deformable microporous carbon aerogel as a matrix.

[0020] Fig. 2Figure 1 shows, by way of example, the pore size distribution for a carbon aerogel used according to the invention and an activated carbon material used in the prior art (Ketjenblack®), determined using the Barrett-Joyner-Halenda (BJH) method and density functional theory (DFT). The pore size distribution for mesopores (2 to 50 nm) and macropores in the range of 50 to 110 nm was calculated based on the BJH method using the Kelvin equation. The DFT model was used to determine the pore size distribution in the range of 0.4 to 2 nm (Groen JC, Peffer LAA, Pérez-Ram). rez J. Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis. Microporous Mesoporous Mater. 2003; 60:1-17. Doi:https: / / doi.org / 10.1016 / S1387-1811(03)00339-1; IUPAC Technical Report Matthias Thommes*, Katsumi Kaneko, Alexander V. Neimark, James P. Olivier, Francisco Rodriguez-Reinoso, Jean Rouquerol and Kenneth S.W. Sing Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report) Pure Appl. Chem. 2015).

[0021] The aerogel described here has a predominantly microporous structure. This structure enables the suppression of the polysulfide shuttle, as the contact between sulfur and electrolyte is significantly reduced due to the microporosity. In contrast, the structure of the activated carbon material used in the prior art (Ketjenblack®, EC 600-JD from AkzoNobel) consists predominantly of mesopores (pores in the range of 2 to 100 nm). With such structures, contact between sulfur and electrolytes cannot be avoided. Properties (analysis method) Ketjenblack ®< Carbon aerogel Mesopore volume (BJH), cm 3< / g 2,34 0,04 Micropore volume (t-plot), cm 3< / g 0,02 0,18 Micropore area (t-plot), m² / g 18 464 External surface area (t-plot), m² < g 1317 97 Specific surface area (BET), m² / g 1335 561 Skeletal density (He pycnometry), g / cm³ < 2,5 2,4 Enveloping density (pycnometry), g / cm³ < - 0,04 Porosity, % - 98

[0022] Furthermore, the carbon aerogel used according to the invention has a large micropore volume of, for example, 0.18 cm³ / g, while a comparable prior art material (carbon black) (Ketjenblack®) has a micropore volume of only 0.02 cm³ / g. The micropore volume of the carbon aerogel allows for an increase in the sulfur content at the cathode, which leads to an increase in capacity.

[0023] Fig. 3 Figure 1 shows an example of a compression-release curve of a carbon aerogel used according to the invention. Due to its elasticity, the carbon aerogel can compensate for the volume expansion of the cathode. This elasticity reduces or even prevents the formation of microcracks in the matrix, thus preventing cell degradation and the associated loss of capacity.

[0024] The carbon aerogel used according to the invention is elastically deformable. Flexibility or elastic deformability is the property of a body to return to its original state after deformation as soon as the deforming force is removed. If the elastic deformability is, for example, 25%, this means that the material, compressed uniaxially by 25%, returns completely to its original state after the force is removed. For this invention, the elastic deformability should preferably have a value of at least 10%. An elastic deformability of at least 15% is particularly preferred. Most preferably, the carbon aerogel has an elastic deformability of at least 25%. The elastic deformability is preferably not more than 30%.

[0025] The carbon aerogel used according to the invention preferably has a specific surface area in the range of 400 to 2000 m² / g. A specific surface area in the range of 400 to 1500 m² / g, and particularly 500 to 700 m² / g, is especially preferred.

[0026] According to the invention, at least 50% of the total surface area of ​​the carbon aerogel according to the invention is formed by pores with a diameter of 2 nm or less. Particularly preferably, the proportion is at least 75%, and most preferably at least 85%. The proportion can be up to 98%, preferably 99%, and most preferably up to 100%.

[0027] According to the invention, the carbon aeorgan has a microporous structure, with predominantly pores having a diameter of less than 2 nm. The volume of the pores with a diameter of less than 2 nm is preferably 0.1 to 3 cm³ / g, more preferably 0.2 to 3 cm³ / g, and most preferably 0.3 to 2 cm³ / g. If the pore volume is too low, there is insufficient space available for active material, which reduces the capacity of the accumulator. A high pore volume can positively influence the performance of the accumulator.

[0028] The pore space of the carbon aerogel is largely infiltrated with sulfur. For use in a battery, the sulfur is preferably present in the form of elemental sulfur and / or in the form of a sulfur compound. Known sulfur compounds include, besides Li₂S, for example Li₂S₆. If the electrode is used in a lithium-sulfur battery, the sulfur is preferably present in the form of elemental sulfur in a fully charged battery and in the form of lithium sulfide (Li₂S) in a fully discharged battery. In between, sulfur is present in the form of polysulfides Li₂Sₗ (2 ≤ x ≤ 8), depending on the state of charge of the battery. The simplified reaction equations during the charging / discharging process are given in the following equations: Anode: 2 Li ⇌ 2 Li + + 2 e − Cathode: S 8 + 2 e − ⇌ S 8 2 − 3 / 2 S 8 2 - + e - ⇌ 2 S 6 2 - S62−+e−⇌3 / 2S42− 1 / 2S42−+e−⇌S22− 1 / 2S22−+e−⇌S2− 2Li++S2−⇌Li2S (End product during discharge)

[0029] The total pore volume of the carbon aerogel is preferably infiltrated with sulfur to a volume of at least 65 vol%. Particularly preferably, the pore volume is infiltrated with sulfur to a volume of at least 85 vol%, and most preferably, to a volume of at least 88 vol%. Preferably, the total pore volume is infiltrated with sulfur to a volume of no more than 90 vol%.

[0030] The sulfur content of the active material in the electrode is preferably at least 30 wt.%, particularly preferably at least 70 wt.%, and most preferably at least 80 wt.%. Preferably, the sulfur content of the active material is not more than 90 wt.%. If the sulfur content is too low, the energy density decreases; if the sulfur content is too high, the electrical conductivity is too low.

[0031] In an alternative embodiment, the problem underlying the invention is solved by an accumulator comprising a sulfur electrode according to the invention, a separator, an electrolyte, and a further electrode. The two electrodes are surrounded by the electrolyte. The separator divides the electrolyte space into two separate compartments and prevents a direct current flow between the electrodes through the electrolyte.

[0032] The further electrode can in particular be a lithium electrode known per se, so that the accumulator according to the invention is a lithium-sulfur accumulator. Fig. 1 The structure and function of a lithium-sulfur accumulator according to the invention are illustrated by way of example.

[0033] The other electrode (anode) can alternatively also include an electrode made of Mg, Al, Si, Sn and their alloys.

[0034] The electrolyte can, for example, comprise a solvent and dissolved electrolyte compounds. The electrolyte compounds can include, for example, lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate (LiPF6), or mixtures thereof. Solvents can include, for example, ethylene glycol dimethyl ether, 1,3-dioxolane, ethylene carbonate (EC), dimethyl carbonate (DEC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and mixtures thereof.

[0035] Suitable separator materials include polypropylene, polyethylene, or fiberglass fleece. These separators can be used in single or multiple layers, or with a coating.

[0036] In a further alternative embodiment, the problem according to the invention is solved by a method for producing a sulfur electrode according to the invention, comprising the following steps i) Providing an elastically deformable microporous carbon aerogel and elemental sulfur, ii) Mixing and grinding the carbon aerogel with the sulfur, iii) Infiltrating the micropores of the carbon aerogel with the sulfur, in particular by means of gas-phase infiltration, wherein the sulfur-aerogel mixture is heated under vacuum, iv) Mixing the sulfur-infiltrated aerogel with a binder suspension and v) Applying the resulting suspension to a support material (current collector), in particular to a foil, for example an aluminum foil.

[0037] In a first step, an elastically deformable microporous carbon aerogel is provided, which exhibits the properties defined above and can be used in the sulfur electrode according to the invention. Any suitable aerogel synthesis method can be used for this purpose. A carbon aerogel according to the invention is preferably produced by pyrolysis of an organic aerogel with the corresponding porosity and elastic properties. In particular, the organic aerogel is a resorcinol-formaldehyde-based aerogel.

[0038] Carbonization for the production of the aerogel can be carried out, for example, under an argon atmosphere at a temperature of at least 900 °C, preferably 950 °C, and up to 1000 °C. Preferably, the temperature is 1000 °C. Temperatures above 1000 °C lead to significant shrinkage and a reduction in pore volume. Temperatures below 900 °C result in incomplete carbonization. Carbonation preferably takes place within one hour. If necessary, the carbon aerogel obtained by carbonization can be etched with CO₂ to increase the pore volume.

[0039] A suitable organic aerogel as a starting material for the carbon aerogel obtained by pyrolysis is described, for example, in DE 10 2012 218 548 A1, to which reference is made in full. In a preferred embodiment, an elastically deformable microporous organic aerogel based on resorcinol-formaldehyde is used for pyrolysis, which is obtainable by a process comprising the steps: i) Preparing a solution containing distilled water, resorcinol, formaldehyde and a base, in particular sodium carbonate, wherein formaldehyde is present in stoichiometric excess with respect to resorcinol and the molar ratio of resorcinol to water is in the range of 0.006 to 0.01, ii) Adjusting the pH of the solution to a range of 5.3 to 5.6, iii) Gelation at a temperature of 70 to 90 °C, iv) Cooling the gel to room temperature and washing with an aprotic organic solvent, and v) Drying the gel at a temperature elevated above room temperature.

[0040] Preferred embodiments of such an organic elastically deformable organic aerogel based on resorcinol-formaldehyde as the starting point for pyrolysis to a carbon aerogel used according to the invention are known in the prior art and are described in particular in DE 10 2012 218 548 A1.

[0041] The carbon aerogel thus prepared is ground with elemental sulfur, for example, in a ball mill during mixing. The mass ratio of carbon aerogel to sulfur is in the range of 10:90 to 70:30, preferably in the range of 10:90 to 30:70, and particularly preferably in the range of 10:90 to 20:80. In particular, the mass ratio can be 44:56. A low sulfur content reduces the storage capacity of the battery, while an excessively high sulfur content increases the electrical resistance and thus the voltage drop during operation.

[0042] The mixture of carbon aerogel and sulfur is milled after mixing. This milling can be carried out, for example, in a ball mill using zirconium dioxide balls for 3 to 7 minutes at 500 to 800 rpm, with 2 to 4 repetitions. In particular, milling can be performed in a ball mill for 5 minutes at 700 rpm with 3 repetitions.

[0043] In a further step, the micropores of the carbon aerogel are infiltrated with sulfur. For this purpose, the ground mixture of sulfur and carbon aerogel can be heated, resulting in gas-phase infiltration of the carbon aerogel. Heating can take place, for example, under reduced pressure, particularly under vacuum. The temperature must reach at least the boiling point of sulfur at the selected pressure. In particular, the gas-phase infiltration is carried out under vacuum at a temperature of at least 400 °C and up to 800 °C, preferably at least 500 °C, and most preferably at a temperature of 600 °C. The gas-phase infiltration takes place over a period of at least 4 hours, particularly 5 hours. Specifically, the ground mixture of carbon aerogel and sulfur is heated under vacuum to 600 °C for 6 hours.To carry out gas phase infiltration, the sulfur-aerogel mixture can, for example, be melted in a glass ampoule and / or heated in a (closed) oven.

[0044] After gas-phase infiltration, the excess sulfur on the surface can be evaporated. This is achieved primarily by increasing the pressure, particularly by adding an inert gas such as argon. Heating or reducing the pressure of the mixture is also possible for evaporating the sulfur. Specifically, the mixture is heated to 330 °C for 1.5 hours under an inert gas atmosphere, preferably an argon atmosphere. Alternative procedures are possible, but it must be ensured that excess sulfur is removed. For example, excess sulfur can also be removed using a solvent. Suitable solvents include known solvents for sulfur, especially toluene.

[0045] The resulting elastically deformable, sulfur-infiltrated, microporous carbon aerogel is coated with a binder suspension and applied to a support material (current collector) to create a manageable electrode. The support material (current collector) must have sufficient electrical conductivity.

[0046] A suitable binder suspension can be an aqueous dispersion of carboxymethylcellulose (CMC), polyethylene glycol, polyethylene oxide (PEO), styrene-butadiene rubber (SBR), chitosan, guar gum, or mixtures thereof. A dispersion of carboxymethylcellulose and polyethylene glycol in water is preferred, with the mixing ratio of the two components ranging from 2:8 to 4:6, particularly 3:7. For example, polyvinylidene fluoride (PVdF) or polyacrylic acid dissolved in an organic solvent, such as dimethyl sulfoxide (DMSO), can also be used.

[0047] The sulfur-infiltrated carbon aerogel is mechanically mixed with the binder suspension in a homogenizer. The mass ratio between the sulfur-infiltrated carbon aerogel and the solid components of the binder suspension is preferably in the range of 95:5 to 85:15. A mass ratio of 90:10 is particularly preferred. Mixing can be carried out, in particular, in a tumbling mixer for a period of 4 to 6 hours, preferably 5 hours. If the binder content is too low, a homogeneous, mechanically stable layer is not produced; if the binder content is too high, the electrical resistance increases.

[0048] In a final step, the resulting suspension is applied to a substrate (current collector). Application can be carried out, for example, using a doctor blade, suspension spraying, spin coating, ultrasonic spraying, dip coating, or similar methods. Preferably, an aluminum foil, carbon-coated aluminum foil, or other corrosion-resistant materials such as titanium or nickel can be used as the substrate (current collector).

[0049] An electrode according to the invention, produced according to the inventive method, can be used as an electrode in a battery according to the invention. In particular, the electrode can be used in a lithium-sulfur battery.

[0050] In another alternative embodiment, the problem according to the invention is solved by using the elastically deformable microporous carbon aerogel as a matrix material for an electrode.

[0051] The carbon aerogel used according to the invention corresponds to the carbon aerogel described, which is part of the sulfur electrode according to the invention.

[0052] In a preferred embodiment, the elastically deformable microporous carbon aerogel is used as a matrix material for a sulfur electrode. Examples of implementation Production of a carbon aerogel Setting up the solution:

[0053] First, 300 ml of distilled water (W) were weighed into a beaker. Then, 15 g of resorcinol (R) were added, and the solution was stirred until the resorcinol was completely dissolved (5 to 7 minutes). Next, 22.1 g of formaldehyde (F) (37 wt% solution stabilized with 10 wt% methanol) were added, and the solution was stirred for another 5 minutes. Then, 0.29 g of Na₂CO₃ (C) (solid) was added to the solution. After stirring for another 5 minutes, the pH was adjusted to between 5.4 and 5.6 using dilute 2N nitric acid. The solution was stirred for a further 60 minutes at room temperature. Composition: Molar ratios R / C 50; R / F 0.5; R / W 0.008 Gelation and aging

[0054] The solution was placed in a tightly sealable container and allowed to gel in an oven at 80°C for one week. Solvent exchange / washing

[0055] After one week, the resulting gel was removed from the oven and cooled to room temperature. The gel was carefully removed from the container and placed in a container filled with acetone. It was washed for three days, with the acetone being changed twice a day. After three days of washing, the washed gel was dried. Dry

[0056] Drying took place in an autoclave using supercritical CO2. The process parameters were: 83 bar, 50 °C, mass flow rate in the range of 10 to 14 kg / h.

[0057] After drying, an orange-brown, flexible resorcinol-formaldehyde aerogel was obtained. Carbonization

[0058] The flexible resorcinol-formaldehyde aerogel was carbonized in an argon atmosphere at 1000 °C for 1 hour and then etched with CO2. Infiltration of sulfur into a carbon aerogel shredding

[0059] 440 mg of the flexible carbon aerogel obtained above and 560 mg of elemental sulfur were mixed and ground in a ball mill for 5 minutes at 700 rpm with 3 repetitions. Gas-phase infiltration of sulfur into the micropores

[0060] The ground sulfur aerogel mixture was melted under vacuum in a glass ampoule and heated in an oven at 600 °C for 6 h. Evaporation of excess sulfur

[0061] The sulfur-aerogel mixture was heated to 330 °C for 1.5 h in an argon atmosphere in a reactor with a cold trap. Electrode manufacturing Binder suspension

[0062] 26.7 mg of carboxymethylcellulose and 62.2 mg of polyethylene glycol were mixed and dispersed in 4938 mg of water. suspension

[0063] 800 mg of the sulfur-infiltrated aerogel were added to the binder suspension and mixed in a tumbling mixer for 5 h. coating

[0064] The suspension was applied to an aluminum foil using a squeegee with a gap of 200 µm and dried for 6 h at 60 °C. Battery cell structure Electrodes and separator

[0065] The aerogel-sulfur electrode was tested against pure lithium in a half-cell setup. A commercially available Celgard®< 2500 polypropylene separator from Celgard was used. electrolyte

[0066] Commercially available 1M lithium bis(trifluoromethanesulfonyl)imide dissolved in ethylene glycol dimethyl ether and 1,3-dioxolane in a volume ratio of 1:1. characterization

[0067] The sample was heated to 1200 °C under an Ar / O 2 atmosphere in an Al 2 O 3 measuring crucible at 5 K / min.

[0068] In Fig. 4The results of the thermogravimetric analysis of the sulfur aerogel after gas-phase filtration are shown. The thermogravimetric measurement demonstrates that the carbon aerogel has the ability to bind the sulfur within its micropores. This results in this portion of the sulfur evaporating at a higher temperature than the sulfur located at the surface. The electrochemical characterization of the battery cell

[0069] The battery cell was discharged and charged 100 times at an ambient temperature of 25 °C and a C-rate of 0.3 C. The battery cell was charged to a final charging voltage of 3.3 V and discharged to a final discharge voltage of 1.0 V.

[0070] In Fig. 5The voltage curve of the cell during discharge and charge is shown against its specific capacity. A voltage plateau at 1.7 V can be observed during discharge. Normally, a lithium-sulfur battery exhibits two voltage plateaus, which is due to the individual reduction stages of the sulfur chains.

[0071] This could indicate that the chain lengths do not exceed S4. The cause could be the pore geometry of the carbon aerogel's micropores, which prevents the formation of longer sulfur chains. Only these longer chains would be soluble in the electrolyte, leading to the polysulfide shuttle reaction.

[0072] Fig. 6 shows the course of the Coulomb efficiency over the cycles. Fig. 7This shows the discharge capacity over the cycles. The absence of the polysulfide shuttle is indicated by a high coulomb efficiency, as the usual charge loss during discharge and charge is significantly reduced. This is evident in the cycle measurement results. The coulomb efficiency remains consistently at nearly 100% throughout the cycle measurement. Furthermore, the battery cell exhibits a constant discharge capacity of over 1000 mAh g-1 sulfur for the first 100 cycles. The capacity degradation between the 5th and 100th cycles is only 4.9%.

Claims

1. An electrode for a lithium-sulfur secondary battery, comprising sulfur and a porous carbon material, wherein the pores of said porous carbon material are infiltrated by the sulfur, characterized in that said porous carbon material comprises an elastic deformable microporous carbon aerogel, wherein said carbon aerogel has an elastic deformability of at least 10%, so that the carbon aerogel fully returns to its original state after removal of the force after uniaxial compression of 10%, and wherein said carbon aerogel has a microporous structure, where at least 50% of the total surface area is formed by pores with a diameter of 2 nm or less, wherein the pore size and the uniaxial compression are determined by methods according to the description.

2. The electrode according to claim 1, characterized in that the sulfur is in the form of elemental sulfur and / or in the form of a sulfur compound, especially in the form of lithium sulfide (Li2S).

3. The electrode according to claim 1 or 2, characterized in that the total pore volume of the carbon aerogel is infiltrated with sulfur to at least 65% by volume.

4. The electrode according to any of claims 1 to 3, characterized in that the sulfur content of said electrode is at least 30% by weight.

5. An accumulator, comprising an electrode according to any of claims 1 to 4, a separator, an electrolyte, and another electrode, especially a lithium electrode.

6. A process for preparing an electrode according to any of claims 1 to 4, comprising the steps: i) providing an elastic deformable microporous carbon aerogel and elemental sulfur, ii) mixing and grinding said carbon aerogel and sulfur, iii) infiltrating the micropores of said carbon aerogel by sulfur, especially via vapor infiltration, wherein said sulfur-aerogel mixture is heated in a vacuum, iv) mixing the sulfur-aerogel with a binder suspension, and v) applying the obtained suspension to a support material (current collector), characterized in that said elastic deformable microporous carbon aerogel is obtainable by carbonizing an elastic deformable microporous organic aerogel by pyrolysis, wherein for said pyrolysis an elastic deformable microporous organic aerogel is deployed based on resorcinol-formaldehyde obtainable by a method comprising the following steps: i) preparing a solution containing distilled water, resorcinol, formaldehyde and a base, especially sodium carbonate, wherein formaldehyde is present in a stoichiometric excess over resorcinol, and the molar ratio of resorcinol to water is within a range of from 0.006 to 0.01, ii) adjusting the pH of the solution in a range of from 5.3 to 5.6, iia) stirring the solution at room temperature for 60 minutes, iii) gelling at a temperature of from 70 to 90°C, iv) cooling the gel to room temperature, and washing it with an aprotic organic solvent, and v) drying the gel at a temperature higher than room temperature.

7. Use of an elastic deformable microporous carbon aerogel as a matrix material for an electrode according to any of claims 1-4, wherein said carbon aerogel has an elastic deformability of at least 10%, so that the carbon aerogel fully returns to its original state after removal of the force after uniaxial compression of 10%, and wherein said carbon aerogel has a microporous structure, where at least 50% of the total surface area is formed by pores with a diameter of 2 nm or less.