Electrode composition for electrodes of electrochemical energy storage devices

Abstract

The invention relates to an electrode composition for an electrode of a lithium-sulfur based electrochemical energy storage device, wherein the electrode composition comprises a solvent, wherein a binder and an electrically conductive carbon material are provided, wherein the electrically conductive carbon material comprises carbon nanotubes, and wherein the electrode composition further comprises a dispersing aid for dispersing the carbon nanotubes in the composition, wherein the dispersing aid comprises a hydrogenated nitrile-butadiene rubber, and wherein the hydrogenated nitrile-butadiene rubber has a molecular weight Mn of > 10 kg / mol to < 83 kg / mol, a molecular weight Mw of > 10 kg / mol to < 245 kg / mol, and an acrylonitrile content in an amount of < 39 wt.-%.

Description

Technical Field

[0001] This invention relates to electrode compositions for electrochemical energy storage devices, particularly for lithium-sulfur (LiS) batteries. The invention further relates to electrodes (such as cathodes) formed from such compositions and to electrochemical energy storage devices comprising corresponding electrodes, particularly electrodes serving as cathodes. Background Technology

[0002] Since their introduction around 1991 as small, lightweight, and high-capacity batteries, lithium-ion rechargeable batteries have been widely used as power sources for portable devices. Other applications include energy storage devices for electrically powered vehicles.

[0003] Lithium-sulfur (LiS) batteries, in particular, have attracted considerable attention due to their high theoretical energy density. However, the practical commercial application of LiS batteries is limited by the low conductivity of sulfur and discharge products, severe polysulfide shuttle effects, and large volume changes during cell operation, due to technical challenges different from those of standard Li-ion batteries. Various nanostructured carbon materials have been used as conductive additives to overcome conductivity problems, among which carbon nanotubes (CNTs) are superior to other nanostructured carbon materials. Carbon nanotubes tend to aggregate into bundles and more geometrically complex aggregates. The aggregation of carbon nanotubes into long and thick bundles leads to an increased sedimentation rate of dispersed carbon nanotube aggregates, resulting in reduced stability of the dispersion or cathode paste. Reduced dispersion stability shortens the storage time of the dispersion, leading to limitations in possible logistics and technical solutions, increasing the risk of inhomogeneity in subsequent mixing, and thus contributing to defects in lithium-ion or lithium-sulfur batteries.

[0004] In the prior art, solutions for improving the stability of CNT dispersions for lithium-ion batteries are known by using dispersants to prevent the aggregation of dispersed particles (such as carbon nanotubes). However, the preparation of dispersed CNTs for LiS batteries can be carried out in various ways, typically through CNT modification, filtration, or coating. Modification of the CNT surface with a small amount of polymer (less than 10%) can achieve good chemisorption, which effectively enhances the cycle performance of lithium-sulfur batteries, but requires additional preparation steps to attach specific PAA or PEG-based polymers to the CNT surface. Therefore, there is a need for improved polymer dispersants for CNTs that provide a simple preparation method for CNT dispersions used in LiS batteries.

[0005] EP 3 333 946 A1 provides a conductive material dispersion and a secondary battery made using the same, the conductive material dispersion comprising: a conductive material comprising bundled carbon nanotubes; a dispersant comprising hydrogenated nitrile rubber; and a dispersion medium wherein, when measured by a rheometer at a frequency of 1 Hz, the complex modulus (|G| at 1 Hz) is... The Pa value ranges from 20 to 500. This conductive material dispersion exhibits a controlled complex modulus to demonstrate excellent dispersibility and powder resistivity characteristics, and thus can significantly improve the output characteristics of the battery. No examples or information regarding the application of LiS batteries are provided.

[0006] WO 2022 / 136314 A relates to carbon / sulfur composite materials comprising carbon nanotubes or carbon nanofibers and elemental sulfur that can be identified by their X-ray spectra, their preparation, and their use as positive electrode active materials, and to electrochemical elements comprising said materials.

[0007] WO 2014 / 082296 A1 discloses a cathode material for Li-S batteries. This cathode material comprises a hydrogenated acrylonitrile-based polymer, sulfur, and graphene nanolayers (GNS), wherein the cathode material particles are spherical, the content of the hydrogenated acrylonitrile-based polymer is 20-79 wt%, the content of sulfur is 20-79 wt%, and the content of GNS is 1-30 wt%. Methods for preparing the cathode material, a cathode made from the cathode material, and a Li-S battery comprising the cathode are also provided.

[0008] WO 2012 / 040934 A1 discloses a cathode material for Li-S batteries, comprising a hydrogenated acrylonitrile copolymer, sulfur, and carbon nanotubes, wherein the weight percentages of these components are as follows: 20% ≤ hydrogenated acrylonitrile copolymer ≤ 80%, 20% ≤ sulfur ≤ 80%, and 1% ≤ carbon nanotubes ≤ 30%. A cathode, a Li-S battery using said cathode material, and a method for preparing said cathode material are also provided.

[0009] WO 2016 / 053063 A1 relates to a positive electrode active material slurry, a positive electrode comprising a positive electrode active material layer formed therefrom, and a lithium secondary battery comprising the positive electrode. The amorphousness of the positive electrode active material slurry is adjusted by including a predetermined proportion of a rubber-based binder. The positive electrode active material layer correspondingly formed from the positive electrode active material slurry exhibits enhanced flexibility and hot-rolling properties, and the lithium secondary battery using the positive electrode comprising the positive electrode active material layer can suppress internal short circuits, high-voltage defects, and reduced capacity.

[0010] JP 2020187866 A describes a conductive material dispersion for non-aqueous electrolyte secondary batteries, comprising a conductive material (A), a dispersion medium (B), and a dispersant (C). The dispersant (C) is a copolymer containing (meth)acrylonitrile source units and conjugated diene monomer source units. The copolymer contains 15-50% by mass of (meth)acrylonitrile source units. The conductive material dispersion has a weight-average molecular weight of 5000-400000 g / mol.

[0011] WO 2019 / 0246068 A1 provides a rechargeable alkali metal-sulfur battery cell, comprising an anode layer, an electrolyte and a porous membrane, a cathode layer, and a discrete anode protection layer disposed between the anode layer and the membrane and / or a discrete cathode protection layer disposed between the membrane and a cathode active material layer; wherein the anode protection layer or cathode protection layer comprises a conductive sulfonated elastomer composite material having 0.01% to 50% by weight of conductive reinforcing material dispersed in a sulfonated elastomer matrix material, and the protection layer having a thickness of 1 nm to 50 μm, 2% to 500% fully recoverable tensile strain, and 10 -7 S / cm up to 5×10 -2 Lithium-ion conductivity S / cm and 10 -7 Conductivity from S / cm to 100 S / cm.

[0012] Q. Zu’s scientific publication “Carbon nanotube-based materials for lithium-sulfur batteries” in J. Mater. Chem. A, 2019, 7, 17204-17241 provides a review of the different applications of CNTs in LiS batteries.

[0013] However, the existing solutions still leave room for improvement and do not provide a solution for stable, easily prepared CNT dispersions and their use in LiS cells. Summary of the Invention

[0014] Purpose of the invention

[0015] The object of this invention is to provide an electrode composition for use in an electrode that at least partially overcomes at least one problem of the prior art. In particular, the object of this invention is to provide a measure for improving the dispersion of carbon nanotubes in a LiS electrode composition, which allows for easy CNT processing and improved LiS cell characteristics.

[0016] means to achieve an objective

[0017] These objectives are achieved, at least in part, by an electrode composition having the features of claim 1. This objective is further achieved by an electrode having the features of claim 12 and an electrochemical energy storage device having the features of claim 13. Preferred embodiments of the invention are described in the dependent claims, specification, or examples, wherein additional features described or shown in the dependent claims, specification, or examples may constitute the object of the invention individually or in any combination, unless the contrary is clearly concluded from the context.

[0018] An electrode composition for an electrode used in a lithium-sulfur-based electrochemical energy storage device is described, wherein the electrode composition comprises a solvent, a binder and a conductive carbon material comprising carbon nanotubes, and wherein the electrode composition further comprises a dispersing agent for dispersing the carbon nanotubes. in The dispersing agent comprises hydrogenated acrylonitrile-butadiene rubber (HNBR), and wherein the hydrogenated acrylonitrile-butadiene rubber has Molecular weights Mn > 10 kg / mol to < 83 kg / mol; Molecular weights Mw > 10 kg / mol to < 245 kg / mol; and Acrylonitrile content < 39 wt.%

[0019] Surprisingly, the electrode composition as defined above significantly improves the dispersion of carbon nanotubes in the composition, thereby allowing the electrode and energy storage device to have very good electrical properties when equipped with or formed from such electrode composition.

[0020] Therefore, the present invention relates to an electrode composition for an electrode used in a lithium-sulfur-based electrochemical energy storage device. This electrode composition can particularly be a dispersion that is applied to a current collector and can then form an electrode. Further steps may be performed on the composition before it is applied to the current collector. Thus, the composition can be a starting point for the production of an electrode layer. In other words, the composition can be, or can be used to, form a slurry that can be applied to a current collector. According to embodiments, the electrode composition may be free of active materials. However, as described below, active materials may also be added to the composition.

[0021] This electrode composition is particularly suitable for forming a cathode as an electrode. This electrode can be used specifically in lithium-sulfur batteries (LiS), also known as lithium-sulfur-based electrochemical energy storage devices.

[0022] The electrode composition contains a solvent, in which a binder and a conductive carbon material are provided.

[0023] Generally, these compounds are not restricted, as long as they can be used in LiS batteries.

[0024] Regarding the solvent, it can include a single material or a combination of different materials. N-methyl-2-pyrrolidone (NMP) may be preferred as the solvent.

[0025] Regarding the adhesive, it can comprise a single material or a combination of different materials. It is likely preferred to use polyvinylpyrrolidone (PVP), polyacrylic acid (PAA), or polyvinylidene fluoride (PVDF) as the adhesive.

[0026] Conductive carbon materials can comprise a single material or a combination of different materials. However, according to the invention, it is important that carbon nanotubes form at least a portion of the conductive carbon material. For example, the conductive carbon material can be composed of carbon nanotubes. This is particularly advantageous when the composition is used to form a LiS battery. In this regard, it should be noted that the practical commercial application of LiS batteries is limited by the low conductivity of sulfur and discharge products, severe polysulfide shuttle effects, and large volume changes during cell operation, due to different technical challenges compared to standard Li-ion batteries. Various carbon materials have been used as conductive additives to overcome these problems, among which carbon nanotubes (CNTs) are superior to other carbon materials. Therefore, the use of carbon nanotubes, especially in LiS batteries, can provide advantageous electrical properties for electrodes and batteries thus formed.

[0027] However, potential problems when using carbon nanotubes as conductive materials can be seen in their tendency to bundle and aggregate into more complex geometries. The aggregation of carbon nanotubes into long, thick bundles leads to an increased settling rate of the dispersed carbon nanotube aggregates, resulting in reduced stability of the dispersion or cathode paste.

[0028] To overcome this effect, the composition according to the invention further comprises a dispersing agent for dispersing carbon nanotubes in a solvent. Regarding the dispersing agent, the invention provides that the dispersing agent comprises HNBR, wherein the HNBR has…

[0029] Molecular weights Mn > 10 kg / mol to < 83 kg / mol; Molecular weights Mw > 10 kg / mol to < 245 kg / mol; and Acrylonitrile content < 39 wt.%

[0030] Surprisingly, this dispersant significantly improved the dispersion of carbon nanotubes, thereby allowing for highly favorable electrical properties, especially for LiS batteries.

[0031] LiS batteries are so-called "post-lithium-ion batteries" with the potential to supplement or replace lithium-ion batteries in the medium to long term. Conductive carbon, especially CNTs, should be used in these battery types, and therefore specific dispersants or methods for these types have not yet been developed. Regarding the general use of CNTs in batteries, no specific investigation or analysis has been conducted to select suitable dispersants for LiS batteries and their potential impact.

[0032] According to the present invention, low molecular weight hydrogenated nitrile butadiene rubber with a moderate ACN content has been found to be advantageously used as a CNT dispersant, also known as a dispersant, for use in LiS batteries. However, the application of carbon nanotube dispersions (CNT dispersions) to LiS batteries requires a tailored dispersant based on a specific grade of low molecular weight HNBR with a particular ACN content, which is more specific than standard CNT dispersants to date. The direct impact of the dispersant used on battery capacity and stability can be demonstrated, and the low molecular weight HNBR grade with a moderate ACN content exhibits optimal performance. When using low molecular weight hydrogenated nitrile butadiene rubber with a moderate ACN content, a simple dispersion method can therefore be established to obtain LiS batteries with high capacity and high capacity retention.

[0033] Generally, as is known in the art, hydrogenated nitrile butadiene rubber is a copolymer formed from acrylonitrile and 1,3-butadiene, wherein the double bonds of the NBR units are hydrogenated to at least a high degree.

[0034] Furthermore, regarding the degree of hydrogenation of HNBR, it is preferable that the degree of hydrogenation, as measured by RDB, is less than 1%, preferably less than 0.9%.

[0035] To achieve favorable results, HNBR used as a dispersing agent for the provided carbon nanotubes has different properties.

[0036] In detail, the HNBR used according to the present invention should have the following characteristics.

[0037] HNBR should have a molecular weight Mn of > 10 kg / mol to < 83 kg / mol.

[0038] HNBR should further have a molecular weight Mw of > 10 kg / mol to < 245 kg / mol.

[0039] HNBR should have an acrylonitrile content of < 39 wt.%

[0040] According to a preferred embodiment, the HNBR should have a molecular weight Mn of >12 to <75 kg / mol.

[0041] According to a preferred embodiment, the HNBR should have a molecular weight Mw of ≥15 kg / mol to <240 kg / mol.

[0042] According to a preferred embodiment, the HNBR should have an acrylonitrile content of 25 to 38 wt.%

[0043] Therefore, it may be particularly preferred if the HNBR used as a dispersant for carbon nanotubes in the electrode composition has any, two, or all of the following properties: Molecular weight Mn ranging from >15 kg / mol to <75 kg / mol; and / or Molecular weight Mw ranging from ≥ 20 kg / mol to < 240 kg / mol; and / or Acrylonitrile content of 30 to 38 wt.%

[0044] It may be further preferred that the HNBR used as a dispersant for carbon nanotubes in the electrode composition has any, two, or all of the following properties: Molecular weight Mn ranging from >15 kg / mol to <65 kg / mol; and / or Molecular weight Mw ranging from ≥ 20 kg / mol to < 210 kg / mol; and / or Acrylonitrile content of 32 to 36 wt.%

[0045] As previously mentioned, it is also possible to provide that the composition further comprises an active material. Since the compositions of the present invention are used in LiS batteries, it is preferred that the active material comprises a lithium-sulfur-based active material. For example, Li₂S can be used as the active material. However, generally, any active material that can be used in LiS batteries can be used, as this will not degrade the dispersion of carbon nanotubes.

[0046] According to another preferred embodiment, the dispersing agent is present in the electrode composition in an amount of ≥2 wt.-% to ≤150 wt.-% relative to carbon nanotubes, such as ≥3 wt.-% to ≤100 wt.-% or, for example, ≥5 wt.-% to ≤75 wt.-% relative to carbon nanotubes. Results show that this amount of HNBR is sufficient to provide good dispersion of carbon nanotubes in the solvent.

[0047] According to another preferred embodiment, the dispersing agent is present in the electrode composition in an amount of ≥ 10 wt.-% to ≤ 50 wt.-% relative to carbon nanotubes, preferably 15 to 25 wt.-%.

[0048] According to another preferred embodiment, the dispersing agent is present in the electrode composition in an amount of ≥ 0.05 wt.-% to ≤ 10 wt.-% relative to the electrode composition, preferably in an amount of ≥ 0.05 wt.-% to ≤ 5.0 wt.-% relative to the electrode composition, such as in an amount of ≥ 0.1 wt.-% to ≤ 2.5 wt.-% relative to the electrode composition.

[0049] Perhaps even more preferred are multi-walled carbon nanotubes (MWCNTs). Results show that such nanotubes possess particularly excellent mechanical properties and, especially, electrical properties. Therefore, when using such nanotubes, a combination of good dispersion and excellent electrical properties can be achieved particularly effectively.

[0050] According to another preferred embodiment, in which the electrode composition forming the carbon nanotube dispersion comprises

[0051] - Solvent amount ≥ 85 to ≤ 97 wt.%; - Dispersing agent in amounts ≥ 0.05 to ≤ 10 wt.%; - ≥ 0.5 to ≤ 10 wt.% of carbon nanotubes; and - ≥ 0 to ≤ 10 wt.% of adhesive, The dispersing agent comprises HNBR, wherein the HNBR has -> Molecular weight Mn from 10 kg / mol to < 83 kg / mol; -> Molecular weight Mw from 10 kg / mol to < 245 kg / mol; and - < 39 wt.% acrylonitrile content; The total amount of the solvent, the binder, the carbon nanotubes, and the dispersing agent is ≥ 98 wt.-%, preferably 100 wt.-%.

[0052] According to this embodiment, if the total defined components are less than 100 wt.%, the remainder to bring the total to 100 wt.% may be additional additives. However, it is preferred that the previously defined total components are > 98 wt.%, and preferably 100 wt.%.

[0053] According to this embodiment, it may be provided that the solvent contains N 2-Methyl-2-pyrrolidone (NMP). However, the solvent should not be limited to this example.

[0054] According to this embodiment, the adhesive may comprise polyvinylpyrrolidone (PVP). However, the adhesive should not be limited to this embodiment.

[0055] According to another preferred embodiment, the dispersing agent has a Mooney viscosity ML(1+4) 100°C, (MU) of ≥ 1 to ≤ 70. Particularly preferred is that the dispersing agent has a Mooney viscosity ML(1+4) 100°C, (MU) of 5-60, such as 8 to 50, for example 10 to 45.

[0056] The components of the composition are described in more detail below.

[0057] carbon nanotubes

[0058] According to the present invention, carbon nanotubes are tubular carbon structures with a diameter of 1 μm or smaller.

[0059] Depending on the number of bonds forming the walls, carbon nanotubes can be classified as single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs).

[0060] Preferably, in the sense of the present invention, carbon nanotubes may include one or more of single-walled, double-walled, and multi-walled carbon nanotubes.

[0061] However, it may be particularly preferred that the carbon nanotubes include multi-walled carbon nanotubes. Exemplarily, the carbon nanotubes used in this invention are composed of multi-walled carbon nanotubes.

[0062] The average strand diameter of the carbon nanotubes used in this invention can be 30 nm or less.

[0063] The length of the carbon nanotubes used according to the present invention can be in the range of 1 to 200 μm.

[0064] More preferably, the carbon nanotubes may have an aspect ratio of 5 to 50,000, more specifically 10 to 20,000, which is defined as the ratio of the length of the carbon nanotube (the length of the long axis passing through the center of the carbon nanotube) to the diameter (the length of the short axis perpendicular to the long axis and passing through the center of the carbon nanotube).

[0065] HNBR

[0066] In the context of this invention, "nitrile-diene copolymer" (nitrile-butadiene copolymer, nitrile rubber, also abbreviated as "NBR") is understood to mean a rubber of a copolymer, trimer, or tetramer of at least one α,β-encapsulated unsaturated nitrile, at least one conjugated diene, and optionally one or more additional copolymerizable monomers. Therefore, the term also covers copolymers having two or more α,β-encapsulated unsaturated nitrile monomer units and two or more conjugated diene monomer units.

[0067] "Hydrogenated nitrile-diene copolymer" ("HNBR") should be understood to mean a copolymer, trimer, or tetramer in which at least some, preferably at least 50%, of the C=C double bonds in the copolymerized diene units have been hydrogenated. For sufficient electrochemical stability, it is preferred that the hydrogenated HNBR rubber is fully hydrogenated.

[0068] The term "fully hydrogenated" means that the degree of hydrogenation of the butadiene unit in the hydrogenated nitrile-diene copolymer is 99.1% to 100%.

[0069] At least, preferably, the content of residual double bonds is < 1%.

[0070] The term "copolymer" covers polymers having more than one monomer unit.

[0071] α,β-olefinic unsaturated nitrile: The α,β-olefinic unsaturated nitrile used (which forms α,β-olefinic unsaturated nitrile units) is acrylonitrile.

[0072] Based on the total amount of all monomer units in the HNBR rubber by weight of 100%, the amount of α,β-olefinic unsaturated nitrile units is in the range of < 39 wt.-%, preferably 20 to 38 wt.-%.

[0073] Conjugated dienes: The conjugated diene forming the conjugated diene unit can be any conjugated diene, especially conjugated C4-C12 dienes. Preferred are 1,3-butadiene, isoprene, 2,3-dimethylbutadiene, 1,3-pentadiene (pentadiene), 2-chloro-1,3-butadiene, or mixtures thereof. Particularly preferred are 1,3-butadiene and isoprene, or mixtures thereof. Very particularly preferred is 1,3-butadiene.

[0074] Based on the total amount of all monomer units in the HNBR rubber by weight, the amount of conjugated diene is typically in the range of 40% to 90% by weight, preferably 50% to 85% by weight, and more preferably 56% to 83% by weight.

[0075] Other comonomers: Hydrogenated nitrile-diene copolymers may contain at least one additional monomer unit in addition to the monomer units described above. Exemplary additional monomer units are defined below.

[0076] Preferred examples of additional epoxy-containing monomers are selected from the group consisting of: 2-ethyl glycidyl acrylate, 2-ethyl glycidyl methacrylate, 2-(n-propyl) glycidyl acrylate, 2-(n-propyl) glycidyl methacrylate, 2-(n-butyl) glycidyl acrylate, 2-(n-butyl) glycidyl methacrylate, glycidyl methyl acrylate, glycidyl methyl methacrylate, glycidyl acrylate and glycidyl methacrylate.

[0077] Most preferably, the epoxy-containing monomer is (alkyl)glycidyl acrylate, preferably glycidyl acrylate and / or glycidyl methacrylate.

[0078] Furthermore, the copolymers used according to the invention may additionally contain repeating units of one or more other copolymerizable monomers known in the art, such as α,β-unsaturated (preferably monounsaturated) monocarboxylic acids, their esters and amides, α,β-unsaturated (preferably monounsaturated) dicarboxylic acids, their monoesters or diesters, and the corresponding anhydrides or amides of said α,β-unsaturated dicarboxylic acids, vinyl esters, vinyl ketones, α-monoolefins, vinyl monomers having hydroxyl groups, and carbon monoxide.

[0079] Acrylic acid and methacrylic acid are preferred as α,β-unsaturated monocarboxylic acids.

[0080] Esters of α,β-unsaturated monocarboxylic acids, preferably alkyl esters and alkoxyalkyl esters, can also be used. Alkyl esters of α,β-unsaturated monocarboxylic acids, particularly C1-C18 alkyl esters, are preferred. Particularly preferred are alkyl esters of acrylic acid or methacrylic acid, particularly C1-C18 alkyl esters, especially methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, n-dodecyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, and 2-ethylhexyl methacrylate. Hydroxyalkyl acrylates and hydroxyalkyl methacrylates can also be used, wherein the number of carbon atoms in the hydroxyalkyl group is 1-12; 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, and 3-hydroxypropyl acrylate are preferred; α,β-unsaturated carboxylic acid esters containing amino groups, such as dimethylaminomethyl acrylate, N-(2-hydroxyethyl)acrylamide, N-(2-hydroxymethyl)acrylamide, (meth)carbamate, and diethylaminoethyl acrylate, can also be used. Acetylacetyloxyethyl methacrylate is another example of a preferred additional monomer.

[0081] α,β-unsaturated dicarboxylic acids (preferably maleic acid, fumaric acid, crotonic acid, itaconic acid, citraconic acid, and mesocarboxylic acid) can also be used as additional copolymerizable monomers.

[0082] Monoesters or diesters of α,β-unsaturated dicarboxylic acids may also be used. These α,β-unsaturated dicarboxylic acid monoesters or diesters may be, for example, alkyl (preferably C1-C10-alkyl, especially ethyl, n-propyl, isopropyl, n-butyl, tert-butyl, n-pentyl, or n-hexyl) monoesters or diesters, alkoxyalkyl (preferably C2-C12-alkoxyalkyl, particularly preferably C3-C8-alkoxyalkyl) monoesters or diesters, hydroxyalkyl (preferably C1-C12-hydroxyalkyl, particularly preferably C3-C8-hydroxyalkyl) monoesters or diesters, cycloalkyl (preferably C5-C12-cycloalkyl, particularly preferably C6-C12-cycloalkyl) monoesters or diesters, alkylcycloalkyl (preferably C6-C12-alkylcycloalkyl, particularly preferably C7-C10-alkylcycloalkyl) monoesters or diesters, aryl (preferably C6-C14-aryl) monoesters or diesters, wherein in each case the diester may also be a mixed ester.

[0083] In a particularly preferred embodiment, the fully or partially hydrogenated HNBR rubber contains (O1-O4)-alkyl methacrylate, most preferably butyl acrylate.

[0084] The manufacture of this type of rubber is well known in the industry and is described, for example, in the Handbook of Synthetic Rubber published by ARLANXEO Deutschland GmbH, Germany in 2020. For rubber, it is generally described in Chapter 5, pages 67-86, and for HNBR, it is specifically described in Chapter 14, pages 295-314.

[0085] Monomers used in polymerization are typically obtained from fossil sources, such as through crude oil cracking, but monomers from sustainable sources can also be used. Using monomers from sustainable sources has the advantage of reducing the carbon dioxide footprint of the polymer and the products made from them. Monomers from sustainable sources include monomers derived from biological sources, including plants, fungi, or bacteria. These monomers are chemically identical to those from fossil sources but have a higher carbon-14 isotope content, which allows them to be identified. Monomers from sustainable sources also include monomers derived from recycled waste, including recycled biological waste (e.g., wood pulp), recycled rubber waste (e.g., from the pyrolysis of tires), and recycled plastic waste. Monomers obtained from recycled materials also include ISCC+ certified monomers, which can be used, for example, in so-called mass balance methods to reduce the total CO2 content of the production chain (see, for example, Pete Spanos et al., “Sustainable Keltan EPDM,” RUBBERWORLD.COM, April 2023, where the method is described for EPDM polymers, but the principles illustrated there can also be applied accordingly to other polymers). To provide monomers of the same purity, monomers from sustainable sources may or may not need to be purified differently from monomers obtained from fossil sources.

[0086] solvent

[0087] Organic solvents allow binders or binder composites to dissolve. This further allows for the dispersion of conductive materials such as carbon nanotubes. Typical solvents include NMP, DMF, valproic acid, butyrolactone, xylene and other aromatic compounds, various esters, ketones, alcohols and ethers, and alkanes. Low-polarity solvents are primarily used in solid-state batteries where the process solvent needs to be compatible with the solid electrolyte.

[0088] According to the present invention, NMP is particularly preferred.

[0089] The process solvent must be selected to allow the binder to dissolve, and the solubility of a particular HNBR grade depends on its monomer composition.

[0090] adhesive

[0091] Polymer binders are a crucial component of electrodes and are used to help process the active and conductive materials in the composition, stabilizing these materials in the slurry during cathode fabrication and giving the smooth electrode, such as the cathode, a well-defined pore structure. During use, the cohesiveness of the cathode and its adhesion to the current collector are critical and strongly influenced by the type of binder used and the corresponding functional parts of its polymer. Adhesion and cohesiveness are key properties of polymer binders, determining the final performance of the lithium-ion battery, especially its long-term performance. A good polymer binder ensures uniform dispersion of the active and conductive materials and stable bonding with the metal current collector.

[0092] Many types of polymer binders can be used. For example, binders based on PVDF (polyvinylidene fluoride), PTFE, high molecular weight HNBR, PIB, CMC, PAA, PEDOT and PSS, PEO, SBR or PVP (which are readily compatible with cathodes operating at high operating voltages) or mixtures thereof can be used.

[0093] However, according to the present invention, PVP and PVDF are particularly preferred as adhesives, with PVP being especially preferred.

[0094] For further technical features and advantages of the electrode composition, refer to the description of the electrode, electrochemical energy storage device and examples.

[0095] An electrode for an electrochemical energy storage device is further described, wherein the electrode is formed of an electrode composition arranged as previously described.

[0096] For example, the electrode composition can be applied to the current collector of the electrode, such as to aluminum foil, aluminum foam, expanded aluminum, or nonwoven carbon. In particular, the electrode can form a cathode.

[0097] Therefore, the electrode can be formed from the composition as described, or it can contain the composition.

[0098] Providing an electrode composition in an electrode (such as in a cathode) particularly offers the advantages described above with respect to the binder composition.

[0099] In particular, it allows for very good dispersion of carbon nanotubes, thereby enabling even better electronic properties.

[0100] Another advantage is the strong bonding of the electrode material to the current collector, which is typically a thin aluminum foil, aluminum foam, expanded aluminum, or nonwoven carbon. Insufficient bonding can lead to cathode peeling when the cathode is bent within the core of a cylindrical battery cell. This can result in loss of electrical contact, leading to battery failure. Another problem can arise from uneven electrode edges when material is lost during slitting or cutting to size. This can even cause a short circuit.

[0101] For further technical features and advantages of the electrode, refer to the description of the composition, electrochemical energy storage device and examples.

[0102] An electrochemical energy storage device is further described, wherein the electrochemical energy storage device includes an anode and a cathode, the cathode being arranged as described above.

[0103] Specifically, the electrochemical energy storage device is a secondary battery, such as a lithium-sulfur battery. It includes an anode, a cathode, and a separator located between the anode and the cathode in a manner known per se. The anode may be formed of graphite or other materials known in the art, and the separator may include polymer foil or ceramic structures as known to those skilled in the art.

[0104] The cathode is arranged as described above, which allows for the corresponding advantages as stated. In particular, very good dispersion of carbon nanotubes can be achieved, thereby further allowing for very good electronic properties.

[0105] For further technical features and advantages of the electrochemical energy storage device, refer to the description of the composition, electrodes, examples and figures.

[0106] Active materials

[0107] Many types of active materials can be used for the cathode of LiS batteries. For example, sulfur-containing active materials used for the cathode can be based on sulfur, copolymerized sulfur, lithium polysulfides, lithium sulfides, lithium sulfide composites, sulfur copolymers (e.g., poly(S-co-DVB)) or carbon-sulfur polymers, sulfur-transition metal oxide composites, sulfur-transition metal sulfide composites, and sulfur-carbon composites. Detailed Implementation

[0108] Example

[0109] The following examples further illustrate the invention in an exemplary manner, but should not be intended to limit its scope.

[0110] If the following materials are used: The monomer unit acrylonitrile (ACN) is from Sigma-Aldrich, and the 1,3-butadiene is from INEOS.

[0111] Fe(II)SO4 solution: The premixed solution contains 0.986 g of Fe(II)SO4 in 400 g of water. 7 H2O and 6.0g Rongalit® C.

[0112] EDTA: As a complexing agent, it is from Sigma-Aldrich.

[0113] Fatty acid: CAS 67701-08-8, emulsifier used in polymerization.

[0114] t-DDM: Molecular weight regulator.

[0115] Glidox® 500 from Renessenz: 2,6,6-trimethylbicyclo[3.1.1]heptyl hydroperoxide, used as an initiator for emulsion polymerization.

[0116] Diethylhydroxylamine: Polymerization terminator, CAS 3710-84-7.

[0117] Hoveyda-Grubbs second-generation catalyst, metathesis catalyst, (1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinedimethyl)dichloro(o-isopropoxyphenylmethylene)ruthenium

[0118] Hydrogen gas from Nippon Gases.

[0119] Lithium sulfide: Active material, Li2S, 99.9% purity, Alfa Aesar.

[0120] Polyvinylpyrrolidone (PVP): Adhesive, PVP K 90, AppliChem.

[0121] Chromium oxide (III): Additive, Cr2O3, 99+%, 60 nm, IoLiTec-Ionic Liquids Technologies GmbH

[0122] Acid-functionalized multi-walled carbon nanotubes: additives, MWCNTs, 95+%, OD: 20-30 nm, L: 10-30 µm, IoLiTec - Ionic Liquid Technology Co., Ltd.

[0123] N-Methylpyrrolidone: Solvent, NMP, 99.5% purity, Acros Organics.

[0124] Nonwoven carbon: current collector, Sigrette 28AA, SGL Carbon SE.

[0125] Lithium foil: anode, 380 μm thick, stamped into ø 14 mm discs, Alfaisa Corporation

[0126] Porous polyolefin membrane: separator, Celgard 2500, stamped into ø 16 mm discs, Celgard.

[0127] Electrolyte: A solution of 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 0.2 M lithium nitrate (LiNO3) in 1,3-dioxolane (DOL) / 1,2-dimethoxyethane (DME) (by weight 1:1), e-Lyte Innovations GmbH

[0128] Button cell battery: casing, CR2025, Hohsen Corp.

[0129] Spacer disc: 16 mm x 0.5 mm, Hosen Co., Ltd.

[0130] Wave-shaped washer: Spring, 15 mm x 1.4 mm, Hosen Co., Ltd.

[0131] Gasket: Polypropylene, Baoquan Co., Ltd.

[0132] Sample F: Fully hydrogenated butadiene-acrylonitrile copolymer with less than 1% residual double bonds (ACN content 34wt.-%); Mooney viscosity ML(1+4) 100°C = 70 MU.

[0133] Sample G: Fully hydrogenated butadiene-acrylonitrile polymer with less than 1% residual double bonds (ACN content 21 wt.-%); Mooney viscosity ML(1+4) 100°C = 39 MU.

[0134] Sample H: Fully hydrogenated butadiene-acrylonitrile copolymer with less than 1% residual double bonds (ACN content 39 wt.-%); Mooney viscosity ML(1+4) 100°C = 39 MU.

[0135] Sample I: Partially hydrogenated butadiene-acrylonitrile copolymer with 5.5% residual double bonds (ACN content 43wt.-%); Mooney viscosity ML(1+4) 100°C = 61 MU.

[0136] Test methods

[0137] Acrylonitrile content: The nitrogen content used to determine the acrylonitrile content is determined according to the Kjeldahl method (DIN 53 625).

[0138] Determination of molecular weight: Molecular weight was determined by gel permeation chromatography (GPC) in the form of number average molecular weight (Mn) and weight average molecular weight (Mw) according to DIN 55672-1 (Part 1: Tetrahydrofuran THF as solvent).

[0139] Mooney viscosity: The Mooney viscosity (ML 1+4 at 100°C) is determined at 100°C by a shear disc viscometer according to DIN 53523 / 3 or ASTM D 1646.

[0140] Methods for evaluating discharge specific capacity and capacity retention: After the formation process, the lithium-ion secondary battery was charged at a constant current of 1 C until the battery reached a voltage of 2.8 V. Upon reaching the cutoff voltage, a constant voltage step was then performed until the current dropped below 0.083 C. After a 3-minute rest period, the battery was discharged at a constant current of 1 C until the battery voltage reached 1.85 V. This cycle was repeated 100 times. The average of at least three battery cells was taken as the specific discharge capacity. To evaluate capacity retention, the discharge capacity of the 1st and 100th cycles was compared. The evaluation of capacity retention was categorized as follows: A: Capacity retention rate of 95% or higher B: Capacity retention rate is 90% or higher but less than 95%. C: Capacity retention rate is below 90% D: The battery cell failed before reaching 100 cycles.

[0141] Polymer preparation

[0142] The low molecular weight hydrogenated butadiene-acrylonitrile copolymer is produced based on a base polymer having the formulations specified in Table 1, wherein all raw materials are expressed in parts by weight based on a monomer mixture of 100 parts by weight. Table 1 also specifies the respective polymerization conditions.

[0143] Table 1: Components used in the manufacture of acrylonitrile butadiene-based polymers

[0144] The corresponding polymers were produced in batches in a 5 L autoclave equipped with a stirrer system. In each autoclave batch, 1.25 kg of monomer mixture, 2.1 kg of total water, and an equimolar amount of EDTA based on Fe(II) from Fe(II)SO4 were used. 1.9 kg of this amount of water was initially charged into the autoclave with an emulsifier (fatty acid) and purged with a nitrogen stream. t-DDM was added and the reactor was shut off. Once the contents reached the polymerization temperature, a solution of Fe(II)SO4 premix and Glidox were added. ® 500 is used as an initiator to initiate polymerization. The polymerization reaction runs at a given temperature for Glidox. ® The polymerization was monitored between 8°C and 15°C by gravimetric determination of conversion. Polymerization was stopped by adding 25% aqueous diethylhydroxylamine solution when the conversion shown in Table 1 was reached. Unconverted monomers (i.e., unpolymerized monomers) and residual volatiles were removed by steam distillation.

[0145] The antioxidant is mixed with the polymer dispersion and adjusted to a solids content of 17.5% by weight. Then, following procedures known in the art, the resulting dispersion containing additional anti-aging components is subjected to coagulation by adding acid to lower the pH, followed by washing, dehydration, and drying.

[0146] Prior to the hydrogenation of the polymer, a metathesis reaction is performed to reduce the molecular weight of the nitrile rubber. Metathesis is known, for example, from WO-A-02 / 100941 and WO-A-02 / 100905 and can be used to reduce the molecular weight.

[0147] Acrylonitrile butadiene-based polymers were dissolved in monochlorobenzene at a solid concentration of 12%–13% in a 10 L high-pressure reactor. After the nitrile rubber was completely dissolved, 1-hexene was added to the reactor at 4 phr, and the solution was stirred at 22°C for 2 h. At this point, a solution of (1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinedimethyl)dichloro(o-isopropoxyphenylmethylene)ruthenium in monochlorobenzene was added to the reactor. The solution was then stirred at 22°C until the desired molecular weight was achieved.

[0148] After the metathesis reaction was complete, a monochlorobenzene solution consisting of a Wilkinson catalyst and a co-catalyst was added, and the pressure was increased to 8.4 MPa while the reactor temperature was adjusted to 137°C–140°C. Temperature and pressure were kept constant during the reaction. The reaction process was monitored by measuring the residual double bond content using a Perkin Elmer spectrum 100 FT-IR spectrometer. The polymer solution in monochlorobenzene was poured onto a KBr pan and dried to form a film for testing. The hydrogenation conversion was determined by FT-IR analysis according to ASTM D 5670-95.

[0149] The reaction is terminated when the residual double bond content reaches < 5%, preferably < 1%, by releasing hydrogen gas pressure.

[0150] The resulting hydrogenated polymer was separated from the solution by steam condensation. For this purpose, a monochlorobenzene solution was diluted to a polymer content of 7% by weight and continuously metered into a stirred glass reactor containing water and preheated to 100°C. Simultaneously, 0.5 bar of steam was introduced into the condensate. The polymer precipitated into fragments was dehydrated and then dried under vacuum at 55°C.

[0151] The composition, monomer unit content, residual double bond percentage, and molecular weight of the corresponding hydrogenated polymers are shown in Table 2.

[0152] Table 2: Characteristics of the present invention sample (AE) and the comparative sample (FI)

[0153] nd: Not determined

[0154] General methods for manufacturing button cell batteries

[0155] The entire preparation process was carried out in an argon-filled glove box (Uni-Lab, with a heated front chamber, M. Braun Inertgas-Systeme GmbH).

[0156] Step (1) - Preparation of the MWCNT-containing dispersion: A certain amount of HNBR was dissolved in a solvent (NMP) by mixing the two components with magnetic stirring at room temperature for 24 hours. Then, MWCNT was added to the solution, and the mixture was stirred until a homogeneous dispersion was formed, for two hours. Afterward, PVP was added to the dispersion as a binder, and the mixture was stirred for 30 min. The weights of the components used and the procedure remained unchanged when HNBR was not used as a dispersant.

[0157] Table 3: Components of dispersions containing MWCNT

[0158] Step (2) - Preparation of cathode slurry as electrode composition: The solid components Cr2O3, Li2S and the prepared dispersion (step 1) were homogenized using a planetary ball mill (grinding conditions: speed: 500 rpm, duration: 5 min, interval: 30 min, 5 cycles).

[0159] Table 4: Composition of cathode paste

[0160] Step (3) - Preparation of the disc electrode: The slurry described (Step 2) is coated onto the nonwoven carbon current collector using a doctor blade process through a four-fold coating frame. The slit is 30 µm. The coated current collector is transferred to an oven and dried overnight at 90°C under reduced pressure. The dried electrode sheet is then stamped into a disc electrode.

[0161] Step (4) - Button Cell Assembly: The button cell is manufactured in a CR 2025 housing. The lithium disk as the anode, the porous separator (Celgard 2500), the manufactured cathode, spacers, and springs are placed inside the housing. Each component is placed on top of the previous one. During assembly, 40 μl of electrolyte is added to the separator. The button cell is then press-sealed in a button cell sealing machine (Automatic Button Cell Sealing Machine HSACC-10, Hosen Co., Ltd.).

[0162] The initial discharge capacity and capacity retention of the lithium-sulfur battery (LSB) are shown in Figure 5.

[0163] Table 5: Determination of initial discharge capacity and capacity retention of LSB.

[0164]

[0165] When using low molecular weight HNBR samples with a specific acrylonitrile content, high discharge capacity exceeding 500 mAh / g and excellent capacity retention can be achieved.

[0166] Therefore, the unexpectedly advantageous effects of the compositions according to the present invention are clearly demonstrated.

Claims

1. An electrode composition for an electrode in a lithium-sulfur-based electrochemical energy storage device, wherein the electrode composition comprises (i) a solvent, (ii) a binder, (iii) an active material comprising a sulfur-based active material, and (iv) a conductive carbon material comprising carbon nanotubes, and wherein the electrode composition further comprises (v) a dispersing agent for dispersing the carbon nanotubes. Its features are, The dispersing agent comprises hydrogenated nitrile-butadiene rubber, and the hydrogenated nitrile-butadiene rubber has Molecular weights Mn ranging from >10 kg / mol to <83 kg / mol; Molecular weights Mw ranging from >10 kg / mol to <245 kg / mol; and Acrylonitrile content < 39 wt.% 2. The electrode composition according to claim 1, wherein, The hydrogenated acrylonitrile-butadiene rubber has at least one of the following characteristics. Molecular weight Mn ranging from >15 kg / mol to <75 kg / mol; and / or Molecular weight Mw ranging from ≥ 20 kg / mol to < 240 kg / mol; and / or Acrylonitrile content of 30 to 38 wt.% 3. The electrode composition according to claim 1 or 2, wherein, The dispersing agent is present in the electrode composition in an amount of ≥2 wt.-% to ≤150 wt.-% relative to these carbon nanotubes.

4. The electrode composition according to any one of claims 1 to 3, wherein, The dispersing agent is present in the electrode composition in an amount of ≥ 0.05 wt.-% to ≤ 10 wt.-% relative to the electrode composition.

5. The electrode composition according to any one of claims 1 to 4, wherein, These carbon nanotubes are multi-walled carbon nanotubes.

6. The electrode composition according to any one of claims 1 to 5, wherein, The content of residual double bonds in this dispersant is < 1%.

7. The electrode composition according to any one of claims 1 to 6, wherein, The electrode composition contains N-methyl-2-pyrrolidone as a solvent.

8. The electrode composition according to any one of claims 1 to 7, wherein, The electrode composition contains at least one of polyvinylpyrrolidone, polyacrylic acid, or polyvinylidene fluoride as a binder.

9. The electrode composition according to any one of claims 1 to 8, wherein, The composition contains - Solvent amount ≥ 85 to ≤ 97 wt.%; - Dispersing agent in amounts ≥ 0.05 to ≤ 10 wt.%; - ≥ 0.5 to ≤ 10 wt.% of carbon nanotubes; and - ≥ 0 to ≤ 10 wt.% of adhesive, The dispersing agent comprises hydrogenated nitrile-butadiene rubber, wherein the hydrogenated nitrile-butadiene rubber has... -> Molecular weight Mn from 10 kg / mol to < 83 kg / mol; -> Molecular weight Mw from 10 kg / mol to < 245 kg / mol; and - < 39 wt.% acrylonitrile content; The total amount of the solvent, the binder, the carbon nanotubes, and the dispersing agent is ≥ 98 wt.-%, preferably 100 wt.-%.

10. The electrode composition according to any one of claims 1 to 9, wherein, The dispersant has a Mooney viscosity of ≤ 70 ML(1+4) 100°C (MU).

11. An electrode for an electrochemical energy storage device, wherein the electrode is formed of an electrode composition arranged according to any one of claims 1 to 10.

12. An electrochemical energy storage device, wherein the electrochemical energy storage device comprises an anode and a cathode, the cathode being arranged according to claim 11.

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