DE102021125560A1 - Electrode, battery cell and method for producing an electrode of a battery cell - Google Patents

Abstract

Electrode (1) for a battery cell (2), at least comprising a carrier material (4) coated with a coating material (3), which comprises at least one active material (5) for the incorporation of lithium ions and a polyelectrolyte (6), the coating material (3 ) has a framework structure (7) formed by the polyelectrolyte (6), through which a multiplicity of conduction paths (8) for the lithium ions are formed. A corresponding battery cell (2) and a method for producing an electrode (1) of a battery cell (2) are also specified.

Description

The invention relates to an electrode of a battery cell, a battery cell and a method for producing an electrode of a battery cell. The electrode comprises at least one carrier material which is coated with a coating material and has at least one active material for the incorporation of lithium ions. The carrier material comprises in particular a carrier material in the form of a strip.

Batteries, in particular lithium-ion batteries, are increasingly being used to drive motor vehicles. Batteries are typically assembled from battery cells, each battery cell having a stack of anode, cathode and separator sheets. At least some of the anode and cathode sheets are designed as current collectors to divert the current provided by the battery cell to a consumer arranged outside the battery cell.

In the production of a lithium-ion battery cell, what is known as a carrier material, in particular a strip-shaped carrier material, e.g. B. a carrier film, coated on both sides with a slurry (a slip or a suspension) via an application tool. The slurry consists of several components, including an active material, conductive carbon black, binder, solvents and other additives, if necessary. After the coating has been carried out on one side in each case, the coated carrier material is fed into a drying process in order to evaporate the solvent it contains and to bond the remaining components firmly to the carrier film. The carrier film forms a current collector for the battery cell. In particular, copper or copper alloys are used as carrier material for anodes. In particular, aluminum or aluminum alloys are used as the carrier material for cathodes.

During the drying process, the final electrode structure is formed with a specific porosity and preferred orientation of the resulting pores. The pores later serve as a reservoir for the battery electrolyte. In particular, an attempt is made to set a density of the coating material in such a way that the electrode has a high volumetric energy density. The porosity of the coating allows lithium ions to pass through the coating, so that lithium ions can be distributed as evenly as possible and thus be embedded in large numbers throughout the coating material.

The porosity of the coating affects the availability of electrolyte/active material particle interfaces available for lithium ion transfer. The reduction in the electrolyte/active material particle interfaces poses a problem for the electrolyte wetting of the coating material, which would result in a deterioration in the electrochemical performance of the electrode.

Especially for the use of lithium-ion batteries in fast-charging applications, the lithium-ions have to quickly penetrate the entire existing coating material, e.g. B. an anode and can be stored there. In order to make this process possible, layer thicknesses of the coating material, comprising an active material of an anode, ie in particular graphite or graphite-silicon mixtures, can be made as thin as possible on the carrier material. However, so that a high gravimetric and volumetric energy density can be achieved in the battery cell, greater layer thicknesses of the coating material are necessary, since this improves the ratio of active material to non-active material in a battery cell. However, if the porosity of the coating material is low, the transport of lithium ions to the entire depth of the coating material, i. H. from the surface of the coating material to the carrier material, cannot be fully realized. In order to enable this transport of lithium ions, channels or oriented structures must be provided in the coating material.

It is Z. B. known to produce such channels in the coating material in the context of calendering. During calendering, the coating material is compacted to a predetermined density. To produce the channels, the calender rolls can be fitted with needle-like elements which penetrate into the coating material.

From the U.S. 2019/190010 A1 and U.S. 2021/002496 A1 It is known to produce or align channels by subjecting the coating material to a magnetic field. For this purpose, however, appropriate devices must be integrated into the process for producing an electrode.

From the WO 2018/108455 A1 electrodes are known which have an electrolyte. The electrolyte comprises a metal-organic framework and a single-ion conducting polyelectrolyte. The Framework connection serves to provide porosity in the coating of the electrode. The polyelectrolyte acts as an ion conductor for the lithium ions.

The addition of a pore-forming agent to a coating material affects at least the chemistry and viscosity of the active material.

From the DE 10 2019 210 865 B3 a method for producing a polymer layer on a composite electrode layer is known.

The object of the present invention is to at least partially solve the problems cited with reference to the prior art. In particular, a method is to be proposed by which a porosity of an active material can be adjusted in an advantageous manner.

An electrode having the features according to patent claim 1, a battery cell having the features according to patent claim 7 and a method according to patent claim 8 contribute to the solution of these tasks. Advantageous developments are the subject matter of the dependent patent claims. The features listed individually in the patent claims can be combined with one another in a technologically meaningful manner and can be supplemented by explanatory facts from the description and/or details from the figures, with further embodiment variants of the invention being shown.

An electrode for a battery cell is proposed, at least comprising a carrier material coated with a coating material, which comprises at least one active material for the incorporation of lithium ions and a polyelectrolyte, the coating material having a framework structure formed by the polyelectrolyte, through which a large number of conduction paths for the Lithium ions are formed.

The electrode to be produced is intended in particular for use in a lithium-ion battery cell. In particular, the electrode comprises a carrier material, e.g. B. a copper or aluminum foil. The carrier material is coated with a coating material at least on one of the largest side surfaces, possibly also on the largest side surfaces lying opposite one another.

In particular, the active material or the active material layer comprises at least one of carbon black, NMC (lithium nickel cobalt manganese as a lithium-storing active material, e.g. the anode), graphite (as a lithium-storing active material, e.g. the cathode), CNT (carbon nano tubes), SBR (styrene butadiene rubber as binder), CMC (carboxymethyl cellulose polymer), PVDF (polyvinylidene fluoride) and porous graphite. The components of the material that are not binding material are in particular assigned to the active material.

The binder material content (solids content) of the coating material, ie in particular the proportion of the polyelectrolyte and optionally also the binder material, is in particular between 0.5 and 5% by weight.

A polyelectrolyte can be understood in particular as meaning a polymer which consists of a macroion which carries firmly bound charges on the chain or side and its low-molecular counterions which bring about the electroneutrality of the entire molecule. The respective charge can therefore be bound in particular to a main chain (on the chain) and/or a side chain (on the side).

In contrast, a polymer electrolyte comprises an uncharged polymer, or a polymer that has no ions, and a salt, e.g. B. a lithium-based salt.

Polyelectrolytes are known in principle. The use of polyelectrolytes to adjust porosity in a coating material is proposed here for the first time.

In particular, the coating material does not include any metal-organic framework compound, but exclusively the polyelectrolyte to form the framework structure.

The creation of pore volumes or pore channels within a coating material of an electrode, which can lead to an improvement in the conduction of lithium ions when filled with a liquid electrolyte, is decisive for improving the rapid charging capability while maintaining the cell capacity. The ability to influence the polymer structure is particularly characteristic of polyelectrolytes ture (near order) by the choice of the solvent used for the coating. In particular, the polarity of the solvent influences close structuring depending on the functionality of the respective polymer. This allows z. B. build up targeted local agglomerates in the polymer structure. The porosity of an electrode or the coating material can be influenced by this agglomerate formation. The preferred orientation in the solution is retained when the coating dries and forms the corresponding directional porosity.

The utilization of these effects is z. B. known from the production of fuel cell electrodes.

For the formation of the framework structure in the coating material, the pH of the coating material in particular can be set in a specific way (depending on the selected polyelectrolyte). In particular, however, a pH value adjustment is not necessary within the coating material, since the framework structure is also formed by van der Waals forces.

In particular, the swelling behavior of silicon-containing anodes during charging/discharging processes can be specifically adapted by selecting the polyelectrolyte and by adjusting the spatial arrangement of the polyelectrolyte by selecting a suitable solvent.

The polyelectrolyte is used here in particular as an electrode binder, ie as a binding material. A secondary function of the polyelectrolyte used is the provision of additional lithium ions (in the case of polyanion) and e.g. B. FSI anions (in the case of polycation), each of which is present as a counterion. In particular, the polyelectrolyte is not included in the coating material in order to act as an electrolyte. In any case, a liquid electrolyte is used for the ionic contact in the battery cell in which the electrode is arranged. As an alternative, intrinsic lithium ion conduction via the polyelectrolyte without an additional liquid electrolyte is conceivable as an expansion stage.

The skeletal structures in the coating material obtained by admixing the polyelectrolyte are in particular channel-like and therefore particularly suitable for rapid transport of the lithium ions into the depth of the coating material due to the possibility of targeted arrangement of the liquid electrolyte.

The polyelectrolyte serves in particular to produce pores in the coating material of an electrode that is used in lithium-ion battery cells. The pores are produced in particular by creating a local order through partial or complete alignment of the functional groups (ionomer side chains) of the film-forming polyelectrolyte depending on the degree of solvation by the solvent used (electrostatic interaction).

The polyelectrolyte is used in particular for the targeted formation of more hydrophilic or more hydrophobic polymer channels inside the coating material by varying the degree of solvation through the solvent or solvent mixture used.

The polyelectrolyte enables in particular the targeted creation of channels in the coating material of the electrode by using the different attachment tendencies of the polyelectrolyte to the active material depending on the solvent and the resulting agglomeration of the active materials and control of their spatial extent and density.

The formation of the porosity can be controlled by means of the electrostatic interaction according to the polarity/dielectric constant of the solvent used and the respective functional group.

The polyelectrolyte toughness (extensibility) can be controlled in a targeted manner by influencing the crystallinity of the polyelectrolyte through the choice of solvent and type of processing conditions (drying temperature/drying rate). This can also be used, among other things, to compensate for the expansion of the electrodes during charging cycles (such a teaching can be found, for example, in DE 10 2015 118 206 A1 disclosed).

In particular, the polyelectrolyte comprises a polymer anion with a lithium ion as a counter ion.

In particular, an (electrical) charge of a polymer strand of the polyelectrolyte is arranged in a side chain or a main chain of the polymer strand.

In particular, there are positive and negative charges on a polymer strand of the polyelectrolyte, with positive and negative counterions being mobile along the polymer strand. The mobility of the counterions runs essentially via the electrolyte.

In particular, the coating material is binder-free, with the polyelectrolyte having binding properties.

In particular, the polyelectrolyte has at least one of the following groups: a sulfonic acid (organic sulfur compound: R—SO ₂ —OH with R as a radical), an amino group (R—NH ₂ , with R as a radical), a phosphonic acid group (R—PO— uh) ₂ . The examples shown below show e.g. B. polyelectrolytes PAA, PSS and others. Instead of the Cl shown ^, the FSI mentioned as an example is provided. Other anions such. B. TFSI ^- or BF4- are also conceivable.

A battery cell is also proposed, at least comprising an anode and a cathode as electrodes, a separator material arranged between the anode and the cathode, and a liquid electrolyte. At least one of the electrodes is designed as the electrode described above or has a carrier material coated with a coating material, which comprises at least one active material for the incorporation of lithium ions and a polyelectrolyte. The coating material has a framework structure formed by the polyelectrolyte, through which a multiplicity of conduction paths for the lithium ions are formed. The liquid electrolyte is arranged in the conduction paths and the lithium ions are conducted via the liquid electrolyte.

In particular, the battery cell additionally includes a housing in which the aforementioned components are arranged.

The battery cell is in particular a pouch cell (with a deformable housing consisting of a pouch film) or a prismatic cell (with a dimensionally stable housing).

A pouch film is a well-known deformable housing part that is used as a housing for so-called pouch cells. It is a composite material, e.g. B. comprising a plastic and aluminum.

The battery cell is in particular a lithium ion battery cell with liquid electrolyte. The liquid electrolyte is used here to conduct the lithium ions. The polyelectrolyte of the at least one electrode is used to form porosity or channels within the coating material, so that the liquid electrolyte can penetrate into the pores or channels.

In contrast to the liquid electrolyte, the polyelectrolyte of the battery cell is in a solid state.

A battery cell is an electricity storage device that B. is used in a motor vehicle for storing electrical energy. In particular, z. B. a motor vehicle has an electric machine for driving the motor vehicle (a traction drive), wherein the electric machine can be driven by the electrical energy stored in the battery cell.

1. a) Bereitstellen des Trägermaterials;
2. b) Bereitstellen des Beschichtungsmaterials, dass zumindest ein Aktivmaterial zur Einlagerung von Lithiumionen sowie ein Polyelektrolyt umfasst;
3. c) Auftragen des Beschichtungsmaterials auf das Trägermaterial;
4. d) Trocknen des beschichteten Trägermaterials zur Bildung der Elektrode.

A method for producing an electrode of a battery cell, in particular the battery cell described, is also proposed, the electrode comprising at least one carrier material coated with a coating material. The procedure comprises at least the following steps:

1. a) providing the carrier material;
2. b) providing the coating material that comprises at least one active material for the storage of lithium ions and a polyelectrolyte;
3. c) applying the coating material to the carrier material;
4. d) drying the coated substrate to form the electrode.

The coating material has a framework structure formed by the polyelectrolyte, through which a multiplicity of conduction paths for the lithium ions are formed.

The above (non-exhaustive) division of the method steps into a) to d) is primarily intended to serve only as a distinction and does not impose any order and/or dependency. The frequency of the process steps can also vary. It is also possible for method steps to at least partially overlap one another in terms of time. Steps a) and b) are preferably carried out before steps c) and d). Step d) is preferably carried out after step c). In particular, steps a) to d) are carried out in the order given.

According to step a), in particular, the carrier material of an electrode is provided. The carrier material is provided in particular as a continuous material in the form of a strip. The endless material in strip form is conveyed in particular along a conveying direction, at least during steps b) and c), possibly additionally during step d).

In particular, the coating material is produced at least from the active material and polyelectrolyte components mentioned.

The application of the coating material takes place in particular in a continuous process in which the carrier material is conveyed through a coating device as a continuous material in the form of a strip.

The drying takes place in particular on the coated endless material or on individually coated carrier materials.

In particular, the coating material in step b) additionally comprises at least one solvent, by means of which agglomerates are produced in a targeted manner in the polymer structure of the polyelectrolyte, which in turn influence a porosity of the coating material present after step d). The solvent is removed from the coating material in particular by step d), preferably completely.

In particular, the solvent can have special properties with regard to viscosity, dielectric constant and surface tension and can be specifically selected as a function thereof. Depending on the solvent selected or used, there is in particular a specific addition of the polyelectrolyte to the active materials and thus a specifically controllable aggregation of the polyelectrolyte. A different morphology of the polyelectrolyte can thus be set. These properties are e.g. B. in M. Dixit, Journal of the Electrochemical Society, 165 (5) F264-F271 (2018).

This interaction between solvent and polyelectrolytes is also explained in P. Müller, Investigation into the interaction of microparticle systems with fiber suspensions, 2001, TU Darmstadt.

From T. Ngo, Journal of Power Sources225 (2013); 293-303 and also J. Huang, The blessing and course of ionomer, Front. Energy 2017, 11(3); 334-364, e.g. It is known, for example, that the functional groups of the binder (of the polyelectrolyte) are oriented towards or away from the active materials, depending on the hydrophilicity of the solvent. This effect can be used for the targeted adjustment of a short-range order of the polymer.

In step b), in particular, a pH value of the coating material is adjusted as a function of the polyelectrolyte used.

In particular, the pH is adjusted by a solvent that is contained in the coating material at least in step b).

In particular, it is proposed to produce an electrode that is particularly suitable for rapid charging and has a high energy density, the coating material in step b) of the method z. B. still referred to as slurry or slip to add a polyelectrolyte. The polyelectrolyte is in particular a polyanion with a lithium ion as counterion.

The charge of the polymer strand can be located in the side chain or in the main chain of the polymer. The polyelectrolyte can in particular be designed as a so-called zwitterion, i. H. Positive and negative charges are located on the polymer chain and positive as well as negative counterions are movably arranged along the polymer chain or migrate across the liquid electrolyte.

The coating material is applied in particular to the carrier material and the coated carrier material formed in this way is dried in step d) in a drying device, e.g. B. dried in a drying oven. In particular, as a result of the polyelectrolytes during drying, a framework structure, a so-called scaffold, is formed, which remains in particular after the solvent has been removed. This framework structure forms the conduction paths (such as flow paths, penetration channels, etc.) for the lithium ions deep into the coating material, i. H. up to the carrier material.

In particular, an additional binder can be dispensed with in step b). The polyelectrolyte preferably has binding properties. Similar binding materials are known from polyelectrolyte (PEM) fuel cell applications. There, a polyelectrolyte is used in the electrodes, usually polymeric perfluorosulfonic acid, which holds both the active particles (carbon-supported platinum) together and takes over the charge transport (protons) in the electrode.

In particular, the polyelectrolyte is selected with regard to suitable functional groups. Such suitable groups include e.g. B. a sulfur, nitrogen, or phosphorus-containing acid group, z. B. sulfonic acid, a phosphonic acid group and / or a base group, z. B. an amino group. In particular, these groups enable attachment and transfer of lithium ions.

In the fuel cell application, corresponding polymeric binders, e.g. B. under the trade name Nafion (also referred to as 2-[1-[difluoro[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2-tetrafluoroethanesulfonic acid). Comparable materials are also used industrially for electrolysis applications, e.g. B. used in chlor-alkalium electrolysis. The use of corresponding materials as porosity-forming and thus lithium-ion-conducting components in lithium-ion battery cells has not been known to date. The structural formula of Nafion is as follows, with the proton H ⁺ replaced by a Li ⁺ ion:

In particular, the polyelectrolyte can also be used to conduct the lithium ions. In particular, depending on the strength of the lithium ion conduction of the polyelectrolyte, liquid electrolytes can also be used, which are located in the framework structure or the pores created by the polyelectrolyte and transport lithium ions in addition to the intrinsic conduction of the polyelectrolyte.

In particular, no adaptation of the previous methods for producing electrodes is necessary here, since only the coating material is changed. In particular, only the number of process steps is increased because of the mixing of the coating material with the polyelectrolytes. In particular, the coating and drying can be carried out as they are.

The otherwise well-known introduction of pore-forming agents involves the risk that residues of the materials remain in the electrode, or that there are undesirable side effects with the active materials of a battery cell.

A motor vehicle is also proposed, at least comprising a traction drive and a battery with at least one described battery cell, wherein the traction drive can be supplied with energy by the at least one battery cell.

The use of indefinite articles (“a”, “an”, “an” and “an”), particularly in the claims and the description reflecting them, is to be understood as such and not as a numeral. Correspondingly introduced terms or components are to be understood in such a way that they are present at least once and in particular can also be present several times.

As a precaution, it should be noted that the numerals used here (“first”, “second”, ...) primarily (only) serve to distinguish between several similar objects, sizes or processes, i.e. in particular no dependency and/or sequence of these objects, sizes or make processes mandatory for each other. Should a dependency and/or order be necessary, this is explicitly stated here or it is obvious to the person skilled in the art when studying the specifically described embodiment. If a component can occur several times (“at least one”), the description of one of these components can apply equally to all or part of the majority of these components, but this is not mandatory.

• 1: eine Elektrode in einer Seitenansicht im Schnitt; und
• 2: eine Batteriezelle in einer Seitenansicht im Schnitt.

The invention and the technical environment are explained in more detail below with reference to the accompanying figures. It should be pointed out that the invention should not be limited by the exemplary embodiments given. In particular, unless explicitly stated otherwise, it is also possible to extract partial aspects of the facts explained in the figures and to combine them with other components and findings from the present description. In particular, it should be pointed out that the figures and in particular the proportions shown are only schematic. Show it:

• 1 : an electrode in a sectional side view; and
• 2 : a battery cell in a side view in section.

The 1 shows an electrode 1 in a side view and partially in section. 2 shows a battery cell 2 in a side view in section. The 1 and 2 are described together below.

The battery cell 2 comprises a housing 13 in which at least one anode 9 and one cathode 10 are arranged as electrodes 1, a separator material 11 arranged between the anode 9 and the cathode 10, and a liquid electrolyte 12.

The electrode 1 has a carrier material 4 coated with a coating material 3 , which includes an active material 5 for the incorporation of lithium ions and a polyelectrolyte 6 . The coating material 3 has a framework structure 7 formed by the polyelectrolyte 6, through which a multiplicity of conduction paths 8 for the lithium ions are formed. The liquid electrolyte 12 is arranged in the conduction paths 8 and the lithium ions are conducted via the liquid electrolyte 12.

The polyelectrolyte 6 is not included in the coating material 3 to function as an electrolyte. In any case, for the ionic contacting, a liquid electrolyte 12 is used in the battery cell 2 in which the electrode 1 is arranged.

The framework structures 7 obtained by adding the polyelectrolyte 6 in the coating material 3 are designed like channels and are therefore particularly suitable for rapid transport of the lithium ions deep into the coating material 3 due to the possibility of arranging the liquid electrolyte 12 in the channels.

The polyelectrolyte 6 thus serves to generate pores or conduction paths 8 in the coating material 3. The pores are created in particular by creating a local order through partial or complete alignment of the functional groups and arrangement of the polymer groups of the film-forming polyelectrolyte 6 depending on the degree of solvation by the solvent used (electrostatic interaction) generated.

The polyelectrolyte 6 is used in particular for the targeted formation of more hydrophilic or more hydrophobic polymer channels, ie conduction paths 8, inside the coating material 3 by varying the degree of solvation through the solvent or solvent mixture used.

The polyelectrolyte 6 thus enables in particular the targeted creation of channels/conduction paths 8 in the coating material 3 of the electrode 1 by using the different attachment tendencies of the polyelectrolyte 6 to the active material 5 depending on the solvent and the resulting agglomeration of the active materials 5 and control of their spatial expansion and Density. In addition, the polyelectrolyte can assume intrinsic conductivity.

Reference List

1

electrode

2

battery cell

3

coating material

4

carrier material

5

active material

6

polyelectrolyte

7

scaffold structure

8th

conduction path

9

anode

10

cathode

11

separator material

12

liquid electrolyte

13

Housing

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US2019190010A1 [0008]
• US 2021002496 A1 [0008]
• WO 2018108455 A1 [0009]
• DE 102019210865 B3 [0011]
• DE 102015118206 A1 [0032]

Claims (11)

Electrode (1) for a battery cell (2), at least comprising a carrier material (4) coated with a coating material (3), which comprises at least one active material (5) for the incorporation of lithium ions and a polyelectrolyte (6), the coating material (3 ) has a framework structure (7) formed by the polyelectrolyte (6), through which a multiplicity of conduction paths (8) for the lithium ions are formed. electrode (1) after Claim 1 , wherein a charge of a polymer strand of the polyelectrolyte (6) is arranged in a side chain or a main chain of the polymer strand. Electrode (1) according to one of the preceding claims, wherein the polyelectrolyte (6) comprises a polymer anion with a lithium ion as a counter ion. Electrode (1) according to one of the preceding claims, positive and negative charges being located on a polymer strand of the polyelectrolyte (6), positive and negative counterions being mobile along the polymer strand. Electrode (1) according to one of the preceding claims, wherein the coating material (3) is free of binding agent, the polyelectrolyte (6) having binding properties. electrode (1) after Claim 5 , wherein the polyelectrolyte (6) has at least one of the following groups: an acid group containing sulphur, nitrogen or phosphorus and/or a base group. Battery cell (2), at least comprising an anode (9) and a cathode (10) as electrodes (1), a separator material (11) arranged between the anode (9) and the cathode (10) and a liquid electrolyte (12); wherein at least one of the electrodes (1) has a carrier material (4) coated with a coating material (3), which comprises at least one active material (5) for the incorporation of lithium ions and a polyelectrolyte (6), wherein the coating material (3) has a Polyelectrolyte (6) formed framework structure (7), through which a plurality of conduction paths (8) for the lithium ions are formed; wherein the liquid electrolyte (12) is arranged in the conduction paths (8) and the conduction of the lithium ions takes place via the liquid electrolyte (12). Method for producing an electrode (1) of a battery cell (2), the electrode (1) comprising at least one carrier material (4) coated with a coating material (3); the method comprising at least the following steps: a) providing the carrier material (4); b) providing the coating material (3) that comprises at least one active material (5) for storing lithium ions and a polyelectrolyte (6); c) applying the coating material (3) to the carrier material (4); d) drying the coated carrier material (4) to form the electrode (1); wherein the coating material (3) has a framework structure (7) formed by the polyelectrolyte (6), through which a multiplicity of conduction paths (8) for the lithium ions are formed. procedure after patent claim 8 , wherein the coating material (3) in step b) additionally comprises at least one solvent, through which agglomerates are produced in a targeted manner in the polymer structure of the polyelectrolyte (6), which in turn, after the coating material (3) has dried, increases the porosity of the coating material present after step d). (3) influence. Method according to any of the preceding patent claims 8 and 9 , wherein in step b) a pH of the coating material (3) depending on the polyelectrolyte (6) used is adjusted. procedure after Claim 10 , wherein the pH is adjusted by a solvent that is contained in the coating material (3) at least in step b).

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