Binder composition for an electrochemical energy storage device and method of preparing the same

Abstract

The present invention relates to a binder composition for forming a binder of an electrode of an electrochemical energy storage device, the binder composition comprising a) hydrogenated carboxylated nitrile rubber, wherein the hydrogenated carboxylated nitrile rubber has a content of carboxylic acid groups in a range of > 0,4 mol% to < 6 mol%; and b1) at least one amino-functional silane, wherein the amino-functional silane comprises a silane group with at least two alkoxy substituents; and b2) optionally at least one tetraalkoxy silane.

Description

[0001] Binder composition for an electrochemical energy storage device and method of preparing the same

[0002] The present invention relates to a binder composition for forming a binder of an electrode of an electrochemical energy storage device. The present invention further relates to a method of forming a binder based on such a binder composition and to an electrode comprising such a binder. The present invention particularly relates to a binder composition which comprises a hydrogenated carboxylated nitrile rubber and a specific amino-functional silane.

[0003] Lithium secondary batteries (LiB) have been widely used as power sources for portable and other devices since their inception around 1991 as small, lightweight, and large- capacity batteries. In recent years the demand has increased strongly due to the usage of batteries in electric vehicles.

[0004] The cathode of Lithium secondary batteries mainly comprises a cathode active material making up more than 95wt.-% of all solid materials used.

[0005] The active material of the cathode of a lithium-ion battery allows reversible insertion and removal of lithium ions into and from this cathode. The higher the mass fraction of this active material in the cathode, the higher is its capacity for charging and discharging.

[0006] Further, generally conductive materials are used which are basically various allotropes of high purity carbon. As these materials are solid powders, flexible polymer binders are thus required to allow coating on the current collector as a layer that is stable enough and that can be further handled by cutting, slicing and rolling together with the anode film to form the desired battery cell geometry. As the uniformity of the coating and the even distribution of all materials is of significant importance for the battery performance, good dispersion of all particles must be ensured. Nitrile rubber, especially the more stable hydrogenated version of nitrile rubber, HNBR, has been shown to be a very suitable dispersion aid for all kind of carbon materials, e.g. carbon coated active materials, such a lithium-iron-phosphate, LPF-C. Insufficient dispersion will result in gelling and agglomeration of conductive materials. The conductive materials are further used as an important component in the electrode to enable the conduction of electricity through the electrode. Hydrogenated nitrile rubber are very suitable polymeric dispersion aids to disperse nano-sized carbon materials in organic solvents to create cathode slurries.

[0007] For binding the particulate components of the cathode of Lithium secondary batteries, fluorinated polymers have been used for a long time. The most common types of binder are based on polyvinylidene fluoride (PVDF) which sometimes may contain further monomer units. Useful PVDF grades have a very high molecular weight (Mw) to improve bonding efficiency. PVDF has a low swell in the battery electrolyte fluid which is believed to be important for the integrity of the binder film in battery service.

[0008] However, according to the prior art, there are problems with the usage of PVDF as binder when active materials are chosen that possess highly active alkaline components on the surface. This can be case for example for nickel-rich active materials such as for nickel-cobalt-manganese oxides in their lithiated form and nickel-cobalt-aluminium oxides. Strongly basic surfaces can cause the dehydro-fluorination of PVDF which leads to the gelation of this polymer. Per consequence, the cathode slurries tend to have less stable viscosities upon storage. The problem occurs especially with very small-scale active materials.

[0009] In W. Bauer et.al (Ceramics International 40 (2014) 4591-4598) it is supposed that the very high molecular weight of PVDF binder may cause flocculation of the small active material particle.

[0010] Fluorinated polymers as binders may cause difficulties in recycling of batteries due to the organic fluorine content. In case of technologies based on pyrolytic processes, the binder polymer will generate hydrogen fluoride (HF) which is extremely toxic and needs to be removed from the waste gas. At the same time, hydrogen fluoride is very corrosive to all metallic materials in a recycling plant. In case of hydrothermal recovery of battery materials, still polymer deposits will be created which still needs treatment which can lead to the formation of hydrogen fluoride.

[0011] More recently, the usage of perfluorinated alkyl substances (PFAS) as emulsifiers in the manufacturing process of fluorinated polymers has been found to be very problematic due to the persistence of these emulsifiers when entering the environment for example via waste-water stream. Thus, it has become desirable to find binder polymers which do not contain organic fluorine and thus to avoid the further accumulation of PFAS in the environment. KR 102329520 B1 describes the addition of small amounts of organic diacids like oxalic acid to lessen the gelation effect while a certain portion of the PVDF is replaced by HNBR. In case of LPF-C, PVDF can be combined with HNBR to allow dispersion of the carbon coated small sized LFP-C particles. Still gelation of PVDF can occur.

[0012] JP 2013073779 discloses the introduction of carbon dioxide into slurry storage containers in order to prevent formation of LiOH and subsequent gelation. However, there are still concerns about the reproducibility.

[0013] Another aspect of the binder is the secure bonding of the cathode materials to the current collector, which typically is a thin aluminum foil. Insufficient bonding can cause the cathode to peel off when the cathode is bent in the core of a cylindrical cell. This can cause the loss of electrical contact and thus can lead to battery failure. Another issue can arise from uneven electrode edges when material is lost during slitting or cutting into size. This can even lead to a short cut between the cathode current collector and the anode current collector.

[0014] It may further be important to allow very thin coatings of carbon materials to equip current collector foils with an electrically conductive layer before applying the electrode slurry. These coatings are also known as conductive primer or undercoats. Such thin coatings can be based on carbon black, carbon nanotubes or graphene wherein still the binder should allow proper bonding. The polymer binder can be advantageously HNBR provided the adhesion performance is good enough while a certain degree of crosslinking is desirable to make the binder more swell resistant in the electrolyte fluid.

[0015] Polymer binders with carboxylic groups which can be obtained by copolymerizing suitable monomer compositions with unsaturated acids like acrylic acid and methacrylic acid for example, and which can be hydrogenated to enhance the electrochemical stability tend to improve the adhesion of the cathode material on the current collector. However, the presence of free carboxylic acid can cause reactions with the highly alkaline active material surfaces so that gelation may occur after short storage of the cathode slurry.

[0016] With this regard, JP 7031655 describes a polymer which contains copolymerized carboxylic acid groups, hydrogenated conjugated diene groups, up to 20% acrylonitrile groups and in most examples various amounts of acrylate units. This polymer is said to be able to bond to a cationic group. In the experiments, various diamines and polyamines are used together with the polymer in cathode slurry formulations with the purpose of suppressing a too high swell in the electrolyte fluid. However, to achieve the desired effect, the cationic group containing compound needs to have more than 2 cationic groups which means more than 2 amino groups. As amino groups are relatively unstable against electrochemical oxidation, such a high amino-group content is not desirable. Too many amino groups can also lead to gelling if these groups interact with the carboxylic groups of different polymer chains the polymer.

[0017] According to JP 5678419 B2 one can learn that active materials for the LiB anode such as silicone particles can be reacted with various amino-functional silanes in water suspension, to get a functional anode material after filtering off the water and drying. This material is further used in anode coating formulations with polymer binders including those based on polyamic acids with carboxylic groups. This document mentions the hydrolysis of the alkoxy-silane groups, likely under the influence of water. Therefore, the mechanism is still the conventional means of coupling inorganic materials with silanes to improve the interaction with organic binders.

[0018] In CN 114551804, functional silanes are suggested as additives together with PVDF as binder in cathode slurries. However, an organo-fluorine binder is still used, and the silane are used in their form as hydroxy-silanes which tend to be very unstable.

[0019] In US 20020122950, amino silanes are mixed with alcohols and used in a binder solution comprising a fluorinated polymer, dissolved in NMP, mixed with graphite and applied on a copper foil for an adherent coating. However, the pre-reaction of the mentioned silanes with alcohols is not compatible with the intended application in cathode slurries and the usage of fluorinated polymers is not desired in the present invention.

[0020] In JP 2011049046, amino-silanes are used to treat silicone particles for the anode of a battery. Interactions with binders is not reported.

[0021] Therefore, there is still the need to find a binder for a cathode which can suppress the swelling in electrolyte, and which can improve the bonding to the current collectors. Object of the invention

[0022] It is thus an object of the present invention to overcome at least one disadvantage of the prior art at least in part. It is particularly an object of the present invention to find a binder for a cathode of an electrochemical energy storage device which can suppress the swelling in electrolyte, which is advantageous with regard to its bonding properties to the current collector and / or which is free of halogenated compounds.

[0023] Means to solve the objective

[0024] The above-described objects are solved at least in part by a binder composition having the features of independent claim 1 . This object is further at least in part solved by a method having the features of independent claim 10, by an electrode material composition having the features of independent claim 12, by an electrode having the features of independent claim 13 and by an electrochemical energy storage device having the features of independent claim 14, as well as by an electrochemical energy storage device according to claim 15. Advantageous embodiments are given in the dependent claims, in the further description as well as in the figures, wherein the described embodiments can, alone or in any combination of the respective embodiments, provide a feature of the present invention unless not clearly excluded. Further, features and advantages as described in respective embodiments can be transferred to further embodiments.

[0025] The present invention provides a binder composition for forming a binder of an electrode of an electrochemical energy storage device, the binder composition comprising a) hydrogenated carboxylated nitrile rubber, wherein the hydrogenated carboxylated nitrile rubber has a content of carboxylic acid units in a range of > 0,4 mol% to < 6 mol%; and b1) at least one amino-functional silane, wherein the amino-functional silane comprises a silane group with at least two alkoxy substituents; and b2) optionally at least one tetraalkoxy silane. Such a binder composition has significant advantages over solutions of the prior art especially with regard to a binder for a cathode of an electrochemical energy storage device being formed therefrom.

[0026] The binder composition as described is thus useful for forming a binder of an electrode of an electrochemical energy storage device. This can essentially be realized by a respective heat treatment as outlined in more details below.

[0027] The binder composition comprises two main components, namely a hydrogenated carboxylated nitrile rubber (HXNBR) and further a specific amino-functional silane.

[0028] The hydrogenated carboxylated nitrile rubber is defined as component a). With regard to this hydrogenated carboxylated nitrile rubber, it should be noted that this compound is found to be superior with regard to the non-carboxylated HNBR. With this respect, as HNBR is a flexible rubber-like material, it allows to create softer cathode films and will in general give higher peel forces compared to PVDF. However, the swell of HNBR in the LiB electrolyte is higher than those of PVDF. This can give electrolyte swollen polymer layers in the cathode where the ion conductivity is slowed down.

[0029] The introduction of carboxylic groups into the polymer chain of HNBR to obtain hydrogenated carboxylated nitrile rubber can increase the adhesion of this polymer to active materials and to current collectors and other substrates. Therefore, the present invention is clearly superior over solutions of the prior art using non-modified HNBR as main binder component with this regard.

[0030] With regard to the used HXNBR, it may be preferred that it has a nitrile content e.g. in terms of acrylonitrile in the range of > 30 to < 40 wt.-%, relating to the HXNBR. Preferably, it has a nitrile content of > 32 to < 37 wt.-%. > 32,6 bis < 36,7 wt.-%

[0031] With regard to the residual double-bond content (measured as RDB) of the hydrogenated carboxylated nitrile rubber, it may be preferred that the RDB value is in the range of < 10 %, such as > 1 % to < 5 %, such as in the range of > 1.4 % to < 4.6 %, for example > 1 ,5 to < 4,5.

[0032] It may further be preferred that the hydrogenated carboxylated nitrile rubber has a Mooney viscosity of < 200 MU, such as < 170 MU. This embodiment improves processing of the respective composition and further improves the adhesive strength of the binder formed from said composition. The amount of carboxylic acid units is in a range of > 0,4 mol% to < 6 mol%, such as > 1 mol% to < 5 mol%, for example of > 1 ,9 mol% to < 4,8 mol%.

[0033] In a specific preferred embodiment, it may be provided the hydrogenated carboxylated nitrile rubber contains, relating to the hydrogenated carboxylated nitrile rubber, copolymerized unsaturated carboxylic acid units in an amount of > 1 mol% to < 5 mol%, and unsaturated nitrile monomers in an amount of > 30 wt.-% to < 40 wt.-%, wherein the sum of hydrocarbon-based monomer units based on butadiene or isoprene is less than 70 wt.-%, and wherein the hydrogenated carboxylated nitrile rubber has a residual double-bond content between 0 and 10%.

[0034] It was, however, found that the swell in the electrolyte may still be present by a so far unknown mechanism when using hydrogenated carboxylated nitrile rubber.

[0035] In order to overcome this problem, it was surprisingly found that the carboxyl groups can be neutralized in an advantageous manner by the features of the present invention.

[0036] Generally, however, by adding a suitable base, the deprotonation of carboxy groups may lead to a loss of adhesion to solid substrates such as the active material or the surfaces of the cathode current collector, typically an Al-foil which may contain bonding friendly AI-OH groups on the surface. Therefore, to add common bases was not found to be successful.

[0037] It was found, however, that specific amino compounds are active to improve adhesion properties. Such amino compounds may form a binder composition with the polymer while still in solution. The combination of these functions leads to the concept of using amino-functional silanes as part of the binder composition.

[0038] Functional silanes according to the prior art are generally known as coupling agents where in most cases, a hydrolysis must take place to activate the bonding mechanism. Company Gelest (in: Silane Coupling Agent: Connecting Across Boundaries, 2006 Gelest Inc.) explains a 4-step process involving the formation of silanol groups in the presence of water moisture. Under anhydrous conditions extended reaction times of 4- 12 hours and elevated temperatures (50-120 C) are useful for the alkoxysilanes to function; only methoxy-silanes are effective without catalysis according to Gelest (same source).

[0039] However, according to the present invention, moisture may be excluded by using carefully dried solvents and dried polymer binders. This might be important in order to avoid drying processes as moisture may be detrimental with regard to the cell chemistry.

[0040] The amino compounds according to the present invention will be referred to as component b1) and comprise at least one amino-functional silane, wherein the aminofunctional silane comprises a silane group with at least two alkoxy substituents. It was surprisingly found that these components overcome the problem of reduces adhesive strength. At the same time using component b1) neutralizes the carboxylic groups of the polymer binder and lessens its reaction with the highly alkaline surface of the active material while it still gives a high adhesion.

[0041] With this regard, while tris-alkoxysilanes with one primary amino-function are the most preferred structures for component b 1 ), many other types of amino-silanes can be used. The following silane may be used in an exemplary manner: mono-aminosilanes such as 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 4- aminobutyltriethoxysilane, 4-aminobutyltrimethoxysilane, 3-aminopropyltris- (methoxyethoxy-ethoxysilane, 3-aminopropylmethyldimethoxysilane; diamino-silanes such as N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3- aminopropyltriethoxysilane, N-(6-aminhexyl)-3-aminopropyltriethoxysilane, N-(6- aminhexyl)-3-aminopropyltrimethoxysilane, N-(2-aminethyl)-3- aminopropylmethyldimethoxysilane, N-(2-aminethyl)-3- aminobutylmethyldimethoxysilane.

[0042] It is understood that the number of alkoxy-groups attached to the Si-atom should preferably at least 2 so that a crosslinking can take place, while mono-alkoxy-silanes still can contribute to adhesion through the action of new bonds.

[0043] Additionally, to the compounds as described above and especially additionally to compounds a) and b1), the binder composition may comprise further components which are known per se for the person skilled in the art. However, it may also be provided that the binder composition according to the present invention consists of components a), b1) and optionally b2). Further to the above defined components a) and b1), component b2) may optionally further be added. Component b2) comprises at least one tetraalkoxy silane. The tetraalkoxy-silanes as component b2) can preferably be selected from tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS).

[0044] An advantage by using component b1) and b2) can come from the tendency of the trisalkoxy groups to condense to form crosslinked particles which may resemble so called POSS, polyhedral oligomerric silseqsquioxanes. This effect may lessen the content of free crosslinker and minimize any diffusion of free crosslinker molecules so that the electrochemical stability is maintained. It may thus be preferred that the hydrogenated carboxylated nitrile rubber is crosslinked. This might be the case in the composition but of course also at the binder being produced from this composition.

[0045] The crosslinking should not happen to any significant extent when the crosslinking agent is added to the organic solution containing the polymer as this will cause gelation or strong increase of viscosity. The crosslinking should set in during the drying process when the organic solvent is removed at elevated temperature and optionally when a vacuum is applied.

[0046] The optional component b2) further allows scavenging traces of moisture which can be present in the cathode slurry if dry working conditions cannot be fully implemented, from traces of moisture in the materials used and from possible reactions of amino groups with protonated carboxylic groups when amide groups are formed. It is assumed that TEOS and TMOS for example can form 3-dimensional networks alone and together with the amino silane. The amino silane component b1) provides the link of these networks to the polymer which is then included into the silanol condensation network.

[0047] It may generally be preferred that the binder composition has a water content of less than 0.3 wt.-%, relating to the binder composition. Advantageously, the binder composition may be free of water. The absence of water or at least the significantly reduced amount of water allows avoiding later drying processes as water, or moisture, respectively, should be avoided in a formed battery cell.

[0048] Trialkoxy-alkylsilanes can also be used as crosslinker or component b2), respectively, but they are less preferred as crosslinkers as they tend to render coating layers more hydrophobic. At the same time the amino functionality can bond itself to the carboxylated polymer binder or to the hydroxyl groups of the current collector for example. The alkoxy group in TEOS may favor bonding and promote crosslinking.

[0049] It is understood that the binder composition as described can also be used to bond ceramic particles on separator webs, to bond conductive carbon materials on active material particles and to bond conductive carbon materials on current collectors. All of these applications, however, are found to be binder applications.

[0050] It was further found that it may be preferred that the amino-functional silane is present in the composition in an amount of > 0.5 molar equivalent, preferably > 2 molar equivalent, relating to the carboxylic acid units of the hydrogenated carboxylated nitrile rubber.

[0051] It is further preferred that the hydrogenated carboxylated nitrile rubber is crosslinked. Like stated above, crosslinking allows in a very efficient manner to increase the swell resistance of the binder formed from the binder composition according to the invention. With this regard, crosslinking may be realized in a desired manner by choosing respective compounds b1). With this regard, the number of alkoxy-groups attached to the Si-atom of compound b1) should preferably at least 2 so that a crosslinking can take place. It is possible to introduce a high level of crosslinking to create an elastic polymer binder film that ensures the mechanical stability of the cathode during the swelling and shrinking from the charging-discharging cycles. Such a crosslinking should happen during the normal drying procedure of the coated cathode layer and should not need undue long times or too high temperatures. Most desirably the conventional drying operations should be carried out as usual while the binder develops into crosslinked form deposited on the active material particles in association with the conductive carbon materials used.

[0052] A binder composition as described above achieves significant advantages over solutions of the prior art. In detail, a binder which is formed from the binder composition as described allows suppressing the swelling in an electrolyte, very good bonding properties to the current collector and has further no need for the addition of fluorinated compounds.

[0053] Apart from that, the present invention is advantageous in that the respective binder composition is free of halogenated compounds. Like described above, this allows that the binder formed from the respective binder composition is superior to respective binders according to the prior art with respect to recycling properties. Especially recycling of the binder or the electrodes formed therefrom can be done much more effective.

[0054] With respect to further advantages and technical features of the binder composition it is referred to the description of the method, the binder, the electrode material composition, the electrode, the electrochemical energy storage device, the figures, the examples and vice versa.

[0055] Further described is a method of preparing a binder for an electrode of an electrochemical energy storage device, wherein the method comprises the steps of: i) Providing a binder composition as described above; and ii) Subjecting the binder composition to a heat treatment.

[0056] Forming a binder from the binder composition is thus very straight forward and possible without problems for the person skilled in the art. Simply, a heat treatment of the binder composition as described in detail above has to be performed. This can be realized by conventional heating means.

[0057] It may be advantageous that step ii) is performed for a time period of > 30 minutes and at a temperature of > 100 °C, particularly at a temperature of > 120 °C. It was found that under these conditions, a binder can be produced which is clearly superior over solutions of the prior art. Especially, the advantages as described above may be achieved.

[0058] However, in some cases higher temperatures than 100 °C might also be used. For example, in case crosslinking should be performed, it was found that curing takes place especially effectively when using a temperature for the heat treatment of 120 °C for at least 30 minutes.

[0059] Thus, the method as described provides a binder which has good adhesion properties and suppresses swelling effectively, thereby having no need of fluorinated products.

[0060] Consequently, also provided is a binder for an electrode of an electrochemical energy storage device, wherein the binder is formed by a method as described above, the binder having the advantages as described. With respect to further advantages and technical features of the method and the binder, it is referred to the description of the binder composition, the electrode material composition, the electrode, the electrochemical energy storage device, the figures, the examples and vice versa.

[0061] Further described is an electrode material composition for an electrode of an electrochemical energy storage device, wherein the composition comprises a solvent, and the binder composition as described above or a binder formed from said binder composition, and wherein the composition further comprises an active material, conductive carbon material and optionally one or more further binders.

[0062] Such an electrode material composition is thus formed based on the binder composition as described above or the binder as described above. It thus comprises the binder composition, or the binder, respectively, as described. Additionally, it comprises respective components which may generally be used according to the prior art. With this regard, it should be noted that the binder composition as described does not show requirements being strict with regard to the further components. This allows that the binder composition, or the binder, respectively, may be used in known processes and in known systems without problems.

[0063] Additionally to the binder composition, or the binder, respectively, the electrode material composition comprises a solvent, an active material, conductive carbon material and optionally one or more further binders. These materials may generally be chosen according to the prior art.

[0064] With regard to the solvent, for example, N-Methyl-2-pyrrolidone (NMP) may be used.

[0065] As an active material, especially as a cathode active material, for example graphite, carbon nanotubes or further carbon materials, such as carbon black may be used.

[0066] Suitable active materials comprise inter alia Lithium Nickel Cobalt Manganese Oxide (LiNiCoMnO2, NCM), Lithium Iron Phosphate (LiFePO C, LFP), Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAIO2, NCA), Lithium Manganese Oxide (LiMn2O4, LMO) or Lithium Cobalt Oxide (LiCoO2, LOO).

[0067] Additionally, a further binder may be included, which may be chosen according to the desired need, if required.

[0068] The electrode material composition as described provides a solution which has good adhesion properties and suppresses swelling effectively, thereby having no need of fluorinated products.

[0069] With respect to further advantages and technical features of the electrode material composition, it is referred to the description of the binder composition, the method, the binder, the electrode, the electrochemical energy storage device, the figures, the examples and vice versa.

[0070] Further described is an electrode for an electrochemical energy storage device, wherein the electrode comprises a current collector and an active material composition coated on the current collector, wherein the active material composition comprises a binder as described above.

[0071] In particular the electrode may comprise an active material composition as described above wherein, however, the solvent may be removed.

[0072] With regard to the binder, the latter may be cross-linked. Such a cross-linking reaction may generally be realized by condensation reaction of alkoxysilyl groups.

[0073] Correspondingly, further described is an electrochemical energy storage device, wherein the electrochemical energy storage device comprises an anode and a cathode, wherein at least one of the anode and the cathode is arranged like described above.

[0074] To summarize, the electrochemical energy storage device and its electrode particularly has the advantages that the binder provides advantageous adhesive properties to the current collector, which might be an aluminium foil, especially in case the electrode is arranged as a cathode. Further, the electrode has advantages with regard to recycling as no fluorinated compounds are present and swelling is effectives suppressed.

[0075] With respect to further advantages and technical features of the electrode and the electrochemical energy storage device, it is referred to the description of the electrode material composition, the binder composition, the method, the binder, the figures, the examples and vice versa.

[0076] Examples and figures

[0077] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0078] In the figures:

[0079] Figure 1 shows anhydrous bonding mechanism of an alkoxy-silane on hydroxylcontaining surfaces;

[0080] Figure 2 shows stress-strain curves of H-XNBR films from examples 1), example 2 and comp, example 3;

[0081] Figure 3 shows the weight swell of samples from examples 1 to 3;

[0082] Figure 4 shows flow curves of solutions of (comparative) examples 4 to 9;

[0083] Figure 5 shows peel test data, dosages of APTES in parts per 8 parts of H-XNBR (comparative) examples 4 to 9, with increasing silane dosage from 0 to 1 .65 parts;

[0084] Figure 6 shows adhesion tests with APTES and TEOS according to examples 10, 11 and 12;

[0085] Figure 7 shows Cure curves with APTES according to examples 13 and 14 with 9.5 and 19 parts APTES, respectively; and

[0086] Figure 8 shows flow curves according to comparative example 15 (340x), 16 (340x+APTES) and 17 (XT+APTES).

[0087] Materials used:

[0088] HNBR-1 : HNBR with a nitrile content of 34%, a residual double-bond content of <0.9% (IR Spectroscopy, RDB 100% means not hydrogenated) and Mooney viscosity (ML1+4, 100°C, ISO 289 / ASTM D 1646) of 90 MU. This polymer does not contain hydrophilic groups.

[0089] HNBR-2: HNBR with a nitrile content of 34%, a residual double-bond content of <0.9% and Mooney viscosity (ML1+4, 100°C) of 39 MU. This polymer does not contain hydrophilic groups. H-XNBR-1 : Carboxylated HNBR, with a nitrile content of 33%, a residual double-bond content of 3.5% and Mooney viscosity (ML1+4, 100°C) of 77 MU. The content of carboxylic groups is 3.2 mol%. The manufacture of the parent XNBR and the subsequent hydrogenation is carried out under the conditions of low pH. Thus, the degree of protonation was determined by titration as 80% of the total carboxylic group content.

[0090] H-XNBR-2 to HXNBR-6: Produced as in H-XNBR-1 but with the variation of the nitrile content, RDB and Mooney values and amounts of carboxylic groups as shown in table 3.

[0091] APTES: amino-propyl-triethoxy silane, CAS 919-30-3, obtained as KH-550 from Shanghai Aladdin Biochemical Technology Co. Ltd, China, mol-weight 221.4 g / mol.

[0092] TEOS: tetraethoxysilane, CAS 78-10-4, mol-weight 208.3 g / mol

[0093] KH-563: triethoxy(3-glycidyl-oxypropyl)silane, CAS 2602-34-8, molweight 278.42 g / mol NCM: active material NCM (Ni-Co-Mn) type 523, from BASF China

[0094] LFP: active material LiFePO4carbon coated, A8-4E (provided by Wanrun New Energy, China)

[0095] Battery black: Super P®, provided by Imerys, BET nitrogen surface area surface 62 m2 / g taken from technical datasheet of supplier

[0096] PVDF: Solef 5130, provided by Solvay, ultrahigh molecular weight PVDF homopolymer

[0097] Aziridine crosslinker: 1 ,1-isophthaloyol-bis(2-methylaziridine, provided by Tianyun, Yinkou, China.

[0098] General testing methods:

[0099] MDR curves: Moving die rheometer, at 1 arc and a temperature as given in the examples, measured according to ISO 6502-3:2028 with MDR 2000 from Alpha technologies using a PA-film.

[0100] Stress-strain curves: Polymer films were used to punched out test specimen according to ASTM D412-2016 Die C and measured with a Zwick 010 Tester.

[0101] BET surface can be determined according to M. Thommes et.al. , IUPAC Technical Report, Pure Appl. Chem. 87, 1051 (2015). It is further noted that all percentages referred to are weight-%, relating to the respective material, until not explicitly described otherwise.

[0102] Specific testing methods will be specified with the trial examples.

[0103] Crosslinking of HNBR binder films (examples 1 to 3)

[0104] Example 1: H-XNBR-1 , 8 parts, was cut into pieces, dried, and dissolved in 92 parts NMP to get an 8% solids solution. Then 0.82 parts APTES was added at room temperature under stirring to get a homogeneous solution, then the mix was stored for 24 h. Films were prepared by casting on PET liner foil, dried at RT and heated to 130°C for 30 minutes. The films were cut into test specimen, taken off the liner and tested to get stress-strain curves. Another set of cast films was cut into small rectangular pieces and place into a vial with bath of 1M LiPFs / ethylene carbonate (EC) I ethyl-methyl-carbonate (EMC) 30 / 70 v / v, 2% vinyl-carbonate (VC), the vials were lightly closed and kept at 60°C to avoid evaporation of the lower boiling linear carbonate. The dosage of APTES was set to allow full neutralization of the protonated carboxylic groups.

[0105] The degree of swell was determined from the initial weight of the test specimen (1x15x15 mm) and the weight after immersion in electrolyte supported on a Ni-wire mesh. The specimen were gently wiped with a tissue to remove from electrolyte. The swell was the weight gain in %.

[0106] Example 2: example 2 was carried out like example 1 but with added 1 .65 parts of APTES to the solution. The dosage of APTES was set to 2 times of the full neutralization of the protonated carboxylic groups.

[0107] Comparative example 3: Comparative example 3 was carried out like example 1 but without using APTES

[0108] Examples 1 and 2 contained approximately enough primary amino-groups from APTES for Vi and full neutralization of the carboxylic groups in the H-XNBR-1 when assuming a protonation of 80%. In figure 2 the stress-strain curves are shown, wherein lines A relate to example 1 , lines B relate to example 2 and lines 3 relate to comparative example 3. Figure 2 clearly indicate that the polymer has been crosslinked under the influence of APTES. The crosslinking increases the Youngs modulus (the initial slope of the stressstrain curves) and renders the binder film in the cathode elastic to exert a retention force during volume increase in battery cycling. The exact mechanism for the observed polymer crosslinking is not known in detail. However, potentially first ammonium salts are formed in solution. The conversion of ammonium salts into amides is conventionally know in that it requires heating to higher temperatures than used in these examples. The formation of amides alone cannot explain the crosslinking as no diamines were used. It might be possible that the trisalkoxy groups activates the amide formation which will release water. This amount of water may then trigger the hydrolysis of an alkoxy-silyl group with subsequent condensation. This step is still surprising as usually tin-organyl catalysts are needed for the hydrolysis of alkoxy-silanes.

[0109] The swell tests on the same polymer films were carried out for up to 7days to monitor change during immersion time, see figure 3, in which the swelling degree in terms of weight change is shown over the time by using an electrolyte, namely 1M / LiPF6in EC / EMC of 30 / 70 (v / v) with 2 wt.% VC. Points A relate to example 1 , points B to example 2 and points C to comparative example 3.

[0110] It becomes obvious that the observed crosslinking process reduced the swell in a typical lithium-ion battery fluid. However, the reaction of the carboxylic groups of the H-XNBR with the APTES will also change these groups into ammonium carboxylate units or to amide bonds. Therefore, the observed decrease of swell may be a combined effect of crosslinking and a chemical transformation.

[0111] The following examples 4 to 9 show bonding tests of the respective binder on an Al-foil

[0112] The respective anhydrous bonding mechanism of an alkoxy-silane on hydroxylcontaining surfaces is shown at figure 1 .

[0113] Comparative example 4: A solution of the H-XNBR-1 , 8% in NMP was prepared as in comparative example 3. The viscosity of the solution was measured with a shear sweep from 10 to 1000 s-1 .

[0114] The shear rheology was taken with a Brookfield rheometer RST-CPS in cone-plate geometry with increasing shear rate.

[0115] The polymer bonding force on Al foil was the determined with the method as follows:

[0116] The polymer solution was coated an Al-foil for cathode current collectors to get a polymer film after drying at 100°C for 45 minutes with a thickness of approximately 10 p. Then this polymer film was laminated with another non-coated Al-foil, pressed together with a 2 kg roller to apply the necessary force. A small area on the edges was kept without polymer to allow for These laminates were stored in an oven at 120°C for 0.5 hours and for 1 hour while a comparison sample was kept without oven treatment (0 hours). This treatment simulated a final drying step in an industrial coating process. Peel tests were carried out on 2.5 cm wide strips by peeling at 180° with a Zwick universal testing machine.

[0117] Example 5: Example 5 was performed like comparative example 4 but with the addition of APTES (as in example 1), 0.25 parts, molar ratio silane / protonated -COOH is 0.3.

[0118] Example 6: Example 6 was performed like comparative example 4 but with the addition of APTES (as in example 1), 0.51 parts, molar ratio silane / protonated -COOH is 0.6.

[0119] Example 7: Example 7 was performed like comparative example 4 but with the addition of APTES (as in example 1), 0.76 parts, molar ratio silane / protonated -COOH is 0.9.

[0120] Example 8: Example 8 was performed like comparative example 4 but with the addition of APTES (as in example 1), 1.02 parts, molar ratio silane / protonated -COOH is 1 ,25.

[0121] Example 9: Example 9 was performed like comparative example 4 but with the addition of APTES (as in example 1), 1.65 parts, molar ratio silane / protonated -COOH is 2.0.

[0122] The rheology of the solutions from comparative example 4 and examples 5 to 9 is shown in figure 4 by showing flow curves in terms of viscosity over shear rate. Until around 1 part added APTES which is slightly above the amount needed to neutralize all protonated carboxyl groups the viscosities remain mostly unchanged and most importantly no gel formations was observed. At even higher dosages of APTES an increase of solution viscosity is observed.

[0123] The results of peeling tests with regard to the peeling strength are shown in figure 5 in which different temperature treatments were examined wherein the different bars show different additions of APTES in terms of wt.-%. No heat treatment is indicated as “120°C 0 hrs”. It becomes obvious that simply drying the polymer film and laminating it on an Al- foil does not give a high peel strength. Under these conditions, the addition of APTES does not improve the adhesion. Without being bound to a theory it is believed that a neutralization of the carboxyl-groups has taken place first to give ammonium carboxylate groups. However, the heat treatment at the moderate temperature of 120°C doubled the adhesion forces. Further, the addition of APTES caused the peel strength to increase compared to the un-modified H-XNBR-1. It appears that just 30 minutes are needed to obtain the observed effect. The adhesion is optimal if the amount of added silane is enough to neutralize the carboxyl groups.

[0124] Examples 10 to 12 show bonding tests of the binder on Al-foil (ex. 10 to 12), using APTES and TEOS

[0125] Example 10: 8 parts of H-XNBR-1 were dissolved in 92 parts of NMP, then 1.02 parts of APTES was added, molar ratio amino-silane / protonated -COOH is 1 ,25.

[0126] Example 11 : 8 parts of H-XNBR-1 were dissolved in 92 parts of NMP, then 0.51 parts of APTES and 0.48 parts of TEOS was added, molar ratio amino-silane / protonated -COOH is 0.58.

[0127] Example 12: 8 parts of H-XNBR-1 were dissolved in 92 parts of NMP, then 1.02 parts of APTES and 0.48 parts of TEOS was added.

[0128] Solutions of examples 10 to12 were used for coating on Al-foil to determine the bonding forces on Al-foil as described above. The data are shown in figure 6 in which each left bar represents example 10, each right bar represents example 12 and each middle bar represents example 11 .

[0129] The experiments confirm that short heating time of 0.5 h at 120°C is sufficient to create a high adhesion of the binder films on Al-foil. Further, at least 1 of the amino silane can be replaced by TEOS. It is assumed that TEOS can form inorganic crosslinks when two ethoxy functions are hydrolyzed and condensed. It is also possible that at least one ethoxy function can create bonds with hydroxyl groups of metal surfaces to give the desired adhesive binding.

[0130] Examples 13 and 14 show cure experiments

[0131] Experiment 13: Dried H-XNBR-1 , 100 parts, were soaked with 9.5 parts of APTES overnight under the conditions of a dry room, then mixed on a two-roll mill at RT, the molar ratio of amino-silane / protonated -COOH was 0.9. Experiment 14: Dried H-XNBR-1 , 100 parts, were soaked with 19 parts of APTES overnight under the conditions of a dry room, then mixed on a two-roll mill at RT, the molar ratio of amino-silane I protonated -COOH was 1 .8.

[0132] The results are shown in figure 7. As one can see in the MDR curves of figure 7, in which line A represents example 13 at 120 °C, line B represents example 13 at 130 °C, line C represents example 14 at 120 °C, and line D represents example 14 at 130 °C, there is a crosslinking of the H-XNBR polymer when amino-silane as was added close to the neutralization of the protonated carboxy groups. One can see that the cure takes place at 120°C and that the temperature of 130°C will not increase the cure anymore. One can also see that the cure does not require added water, further the cure torque is already high at the beginning of the MDR which is an indication for a cure reaction either during mill mixing or during the heating up to the MDR temperature. That means that the desired reaction of the amino-silane with the H-XNBR will take place already during the drying step of the cathode and thus, no separate additional process step is needed.

[0133] Experiments 15 to 17 show battery trials with Lithium iron phosphate (LFP).

[0134] Experiment 15 (comparison): Active material LFP and conductive carbon black Super P were dried in an oven at 120°C for 2 hours, binder HNBR-1 was dried in a vacuum oven at 50°C overnight. The binder compositions were dissolved in NMP (shaker, RT) Vi of the total amount), Super P was added and then mixed with a Thinky mixer (planetary centrifugal mixer, Thinky corporation) for 18 minutes at 2000 rpm. Then the LFP was added with the other 1 of the NMP and further mixed in a Thinky mixer for 18 minutes at 2000 rpm. All mixing steps were done in 6 minutes intervals, followed by cooling to RT. The cathode slurries were then coated on Al-foil and on PET film with a 150 pm gauge, dried with convection oven 120°C, 2 h) for physical testing, vacuum oven (120°C, 4 h) for impedance testing and calendared. The cathode films were cut into disks for coin cells. The thickness was determined before and after calendaring to get the weight, areal density and porosity of the cathode layer from the densities of the used materials and the Al foil weight.

[0135] Coin cells were assembled in an argon filled glove box with the prepared cathode on Al foil, a separator foil (Celgard 2340, 38 pm diameter) and an Li-disk (16 mm diameter, China Energy Lithium Co. Ltd). The coin cell was filled with an electrolyte solution (LiPF6 1 M, EC / DEC 30 / 70 by volume. Cycling tests were performed with a constant current charge / discharge, (0.1 C for 2 cycles, 0.3 C / 0.3 C runs to 100 cycles in the potential range of 2.8 - 4.0 V vs. Li / Li+) at 23°C. AC impedance was measured at a frequency of 0.01 ~4 105Hz, Amplitude: 0.005V, without prior charging-discharging before impedance measurement, then specify cycling is performed at 80% SoC and impedance values were taken at 2 cycles and 100 cycles.

[0136] Example 16 (comparative): example 16 is performed as in experiment 15 but with added APTES O.16%.

[0137] Example 17: example 17 was performed as in experiment 15 but with H-XNBR and added APTES 0.16%, which is sufficient to neutralize the protonated -COOH groups.

[0138] Table 1: Cathode formulations and testing results

[0139] As can be seen from the above table 1 , excellent discharging capacity is found for examples 15 and 16. At the same time inventive example 17 shows excellent peel strength while ex. 15 and 16 show too low peel strength. All cathode slurries give good flow curves while those in ex. 17 show the best storage stability, see figure 8, in which examples 15, 16 and 17 are shown each at time to and after 3 days.

[0140] Experiments 18 to 20 show NCM cathode trials In comparative example 18, cathode slurries were prepared as shown in table 2 where two different reactive silanes were used. It becomes obvious that HXNBR as binder alone gives slurries with an increase of viscosity upon storage. The peel values for the dried cathode layers on Al-foil are still too low in experiment 18.

[0141] According to example 19, the H-XNBR binder is combined with the amino-silane, APTES, resulting in a more stable slurry viscosity. Further the peel values are significantly improved.

[0142] According to comparative experiment 20, the glycidyl-group silane also gives good peel strength, but the slurry shows a steep increase of viscosity. It is suspected that the glycidyl group of the silane might have reacted in solution.

[0143] Table 2: Cathode formulations with NCM

[0144] Examples 21 to 27: NCM cathode experiments

[0145] As in experiments 18 to 20, cathode slurries were prepared with different H-XNBR binders and PVDF as reference. The composition of the cathode slurries was binder

[0146] 1.0%, HNBR-2 as polymeric dispersion aid 0.4%, Super P 3%, NCM 95.7%, dispersed in NMP to 68% solids content. The experiments were not conducted in a dry room to examine the effect of less perfect manufacturing conditions.

[0147] H-XNBR samples were prepared by selecting a suitable carboxylated nitrile rubber and carrying out the hydrogenation as described for example in US 7265185. The XNBR was polymerized in emulsion and obtained as crumbs by coagulation of the latex with a CaCh coagulation salt while keeping the pH between 3 and 5 so that the carboxyl groups are mainly in their protonated form. In experiment 21 the H-XNBR already described in the “materials section” was used. Examples 21 to 27 are performed as outlined below in table 3, wherein AN refers to acrylonitrile, MAS refers to methacrylic acid, ML is the Mooney value at 100°C, in Mooney units.

[0148] Examples 21 to 26 show that an increasing content of copolymerized unsaturated acid gives increased peel strength values. However, as in experiment 26 when at the same time the Mooney value is too high, the peel values are reduced. This is because the very high Mooney value reduced the flow and wetting properties of the binder. Therefore, the Mooney values should be kept below 200 MU while the content of carboxylic acid groups should be below 6mol%. Further, a too high Mooney value can lead to higher solution viscosities so that the cathode slurries is difficult to process in coating.

[0149] A too low content of carboxylic groups such as in experiment 22 causes too low peel values and bad cycling behavior which may come from the insufficient anchorage of the cathode to current collector so that the electrical contents is progressively lost with service. There seem to be a too high impedance as well. Experiment 27 is based on PVDF as binder and gives a too low peel value.

[0150] In examples 21 to 26, methacrylic acid was used to introduce carboxylic monomer units into the hydrogenated nitrile polymer. Methacrylic acid can be very conveniently used in the emulsion copolymerization of emulsion nitrile rubber. Other unsaturated carboxylic acid monomers can also be used such as acrylic acid, fumaric acid and itaconic acid. Preference is given to methacrylic acid. The content of the carboxylic monomer units in the polymer is expressed in mol% to allow an easy choice of the dosage of amino-silane in relation to the carboxylic groups regardless what type of acid had been used in the polymerization.

[0151] Current collector coatings with CNT, examples 28 to 32

[0152] Dried HNBR polymers were dissolved in dry NMP, silanes were added, then mwCNT type CR3000 was added, and the mix was dispersed with a single-ball ball mil (30 minutes, 6 minutes intervals with cooling, 28 Hz, machine provided by Retsch GmbH, Germany). HNBR was 1 % and CNT was used at a level of 4%. The obtained CNT pastes were tested for their viscosity, Al-foils were coated to different thicknesses, dried, and crosslinked at 120°C for 2 hours, calendared and evaluated for the bonding performance.

[0153] As shown in the table below for examples 28 to 32 the two HNBR polymer can disperse mwCNT. The paste viscosities drop when the APTES silane is used together with the unreactive HNBR-2. With H-XNBR-1 (example 30) first a rather high viscosity paste obtained with unstable viscosity. However, when the APTES is included in example 31 , the viscosity drop to a very good level which only slightly increases with storage. This is interpreted by a moderation of the high reactivity from the carboxyl via interaction with the silane in solution. The coating on All-foil with both larger thickness and thinner layers is very good for the paste from example 31 . The peel tests were determined on the coated Al-foils with an estimated thickness of 1 micron to assess the industrial application for CNT-based current collector coatings. With experiment an excellent peel behavior was found with consistent high forces. In example 30, very high peel values were found but with an erratic peel behavior. In example 29, the peel was smooth but achieved only a low level while in example 28, peel tests could not be carried out due to the insufficient coating. Example 32 represents example 28 but with a ball-milled dilution with NMP. The subsequent low viscosity allowed a coating on the Al-foil. Then the peel adhesion was 220 N / m which showed that the non-reactive HNBR-2 provided the lowest adhesion value. Therefore, the reactive polymer H-XNBR-1 in combination with an amino-silane according to the present invention is very well suited to create current collector coating on metal foils to increase the electrical contact between current collector and electrode material. It is believed that the added amino-silane moderates the reactivity of the H- XNBR-1 and surprisingly promotes the high peel adhesion, likely from a crosslinking effect.

[0154] Table 4 It is understood that the overall level of adhesion can be raised by the usage of the inventive components b1) and optionally b2) based in component a), in particular depending on the requirements of the manufacturing process.

[0155] The inventive binder composition allows to avoid the usage of binders which contain organic fluorine such PVDF which can make the recycling of used batteries easier.

[0156] Swell test with aziridine crosslinking

[0157] Comparative example 33

[0158] H-XNBR- 1 was dissolved in NMP to a solid content of 10%. Then 1 ,1-isophthaloyol-bis(2- methylaziridine was added with a dosage of 0.35 parts for 10 parts of H-XNBR. The solutions were cast of a PTFE dish, dried at 60°C to remove solvent. The heated to 120°C for 4 hours. The aziridine dosages correspond to a molar ratio of 0.25 in relation to the bound methacrylic acid in the H-XNBR.

[0159] Comparative example 34: As in comparative example 33 but the aziridine was used with a dosage of 1.05 parts. The dosage corresponds to a molar ratio of aziridine / bound methacrylic acid of 0.75.

[0160] The films were immersed in electrolyte (1 M / L LiPF6 in EC / EMC=30 / 70 (v / v)) after 1 d, 2 d and week at 60°C. The swell results are shown below in wt%.

[0161] Table 4

[0162] The films were also studied is stress-strain tests and the crosslinked nature confirmed by comparison with a non-crosslinked polymer film.

[0163] Although the aziridine used can indeed crosslink H-XNBR the swell in electrolyte was not less. Thus, the reported improvement of swell resistance must be related to the specific usage of an amino-silane.

Claims

Claims1 . A binder composition for forming a binder of an electrode of an electrochemical energy storage device, the binder composition comprising a) hydrogenated carboxylated nitrile rubber, wherein the hydrogenated carboxylated nitrile rubber has a content of carboxylic acid groups in a range of > 0,4 mol% to < 6 mol%; and b1) at least one amino-functional silane, wherein the amino-functional silane comprises a silane group with at least two alkoxy substituents; and b2) optionally at least one tetraalkoxy silane.

2. The binder composition according to claim 1 , wherein the amino-functional silane is present in the composition in an amount of > 0.5 molar equivalent, preferably > 2 molar equivalent, relating to the carboxylic acid units of the hydrogenated carboxylated nitrile rubber.

3. The binder composition according to claim 1 or 2, wherein the hydrogenated carboxylated nitrile rubber is being crosslinked in a drying step.

4. The binder composition according to any of claims 1 to 3, wherein the hydrogenated carboxylated nitrile rubber has a Mooney viscosity of < 200 MU.

5. The binder composition according to any of claims 1 to 4, wherein the hydrogenated carboxylated nitrile rubber has a degree of hydrogenation measured as RDB of < 10%.

6. The binder composition according to any of claims 1 to 5, wherein the binder composition is free of halogenated compounds.

7. The binder composition according to any of claims 1 to 6, wherein at least one tetraalkoxy silane is present and wherein the at least one tetraalkoxy silane is selected from the group consisting of trialkoxyalkylsilanes, tetraethoxysilane, and tetramethoxysilane.

8. The binder composition according to any of claims 1 to 7, wherein the hydrogenated carboxylated nitrile rubber contains copolymerized unsaturated carboxylic acid units in an amount of > 1 mol% to < 5 mol%, and unsaturated nitrile monomers in an amount of > 30 wt.-% to < 40 wt.-%, wherein the sum of hydrocarbon-based monomer units based on butadiene or isoprene is less than 70 wt.-%, and wherein the hydrogenated carboxylated nitrile rubber has a residual double-bond content between 0 and 10%.

9. The binder composition according to any of claims 1 to 8, characterized in that the binder composition has a water content of less than 0,3 wt.-%.

10. Method of preparing a binder for an electrode of an electrochemical energy storage device, wherein the method comprises the steps of: i) Providing a binder composition according to any of claims 1 to 9; and ii) Subjecting the binder composition to a heat treatment.11 . Method according to claim 10, wherein the heat treatment according to step ii) is performed for a time period of > 30 minutes and at a temperature of > 100 °C.

12. Binder for an electrode of an electrochemical energy storage device, wherein the binder is formed by a method according to claim 11 .

13. An electrode material composition for an electrode of an electrochemical energy storage device, wherein the composition comprises a solvent, and the binder composition according to any of claims 1 to 9 or a binder according to claim 12, and wherein the composition further comprises an active material, conductive carbon material and optionally one or more further binders.

14. An electrode for an electrochemical energy storage device, wherein the electrode comprises a current collector and an active material composition coated on the current collector, wherein the active material composition comprises a binder being formed from a binder composition according to any of claims 1 to 9.

15. An electrochemical energy storage device, wherein the electrochemical energy storage device comprises an anode and a cathode, wherein at least one of the anode and the cathode is arranged according to claims 13.

16. The electrochemical energy storage device according to claim 14, wherein the cathode is formed according to claim 13.

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