Process for producing a slurry for a cathode and a battery cell

Abstract

Method for producing a slurry (1) for a cathode (2), which comprises a carrier material (3) and a coating (4) formed by the slurry (1); at least comprising the following steps: a) providing a starting material (5) for the slurry (1), comprising at least an active material, a binder and a conductive material, wherein the starting material (5) has a high first pH value; b) providing an acid compound (6) having a low second pH value; c) mixing the starting material (5) with the acid compound (6) to lower the first pH value to a lower third pH value and form the slurry (1); wherein the acid compound (6) is formed exclusively by acids and comprises at least a first acid which is one of tetrafluoroboric acid HBF4, trifluoromethanesulfonic acid CF3SO3H and bistriflimidic acid C2HF6NO4S2.

Description

[0001] The invention relates to a method for forming a coating on a cathode. The coating is applied, in particular, to a carrier material for producing an electrode foil of a battery cell. The carrier material comprises, in particular, a strip-shaped carrier material. The coating comprises, in particular, an active material. The carrier material consists, in particular, of aluminum or an aluminum alloy (with more than 90 wt.% aluminum).

[0002] Batteries, especially lithium-ion batteries, are increasingly being used to power motor vehicles. Batteries are typically composed of cells, each of which contains a stack of anode, cathode, and separator plates. At least some of the anode and cathode plates are designed as current collectors, diverting the current generated by the cell to a load located outside the cell.

[0003] During the production of a lithium-ion battery cell, a so-called carrier material, in particular a strip-shaped carrier material, e.g., a carrier foil, is coated on both sides with a slurry using an application tool. The slurry consists of several components, including an active material, conductive carbon black, a binder, solvents, and possibly other additives. After each coating on one side, the coated carrier material is subjected to a drying process to evaporate the solvent and firmly bond the remaining components to the carrier foil. The carrier foil forms a current collector for the battery cell.

[0004] The resulting coating is porous. This porosity is reduced by calendering, which densifies the coating. Densification is necessary to increase specific capacitance (relative to volume) and electrical conductivity.

[0005] If the pH of the slurry (the coating material containing the active material) is high (i.e., higher than 8.5), the slurry can react significantly with the aluminum in the support material and damage the microstructures of the support material and the corresponding performance of the battery cell. Therefore, the pH of a prepared slurry, e.g., NMC 811 (LiNi 0.8 Mn 0.1 Co 0.1 O2) should be set lower.

[0006] Therefore, a high pH value is problematic for the coating process of the substrate material as soon as the slurry comes into contact with the substrate. The aluminum surface is protected by an oxide layer that is only stable within the pH range of 4.5 to 8.5. At higher pH values, the oxide layer dissolves, resulting in corrosive attack on the substrate material. This can lead to the formation of water-soluble aluminum compounds, corrosion and pitting of the substrate material, and the formation of gas bubbles trapped within the coating of the substrate material. These undesirable phenomena can be particularly exacerbated during high-temperature processes such as coating and drying steps.

[0007] A high pH of the slurry affects the dispersion homogeneity of NMC811-coated substrates in that the microstructure on top of the coating is homogeneous but exhibits holes. The microstructure of the coating in cross-section also shows numerous holes in the interior of the coating. The presence of these holes indicates that a large amount of gas was generated and trapped during the coating and drying steps, indicating a chemical reaction between the basic slurry and the substrate. If the pH is lowered below 9, the microstructure remains homogeneous, but there are no holes in the interior of the coating.

[0008] If the pH of the slurry drops below 6, dilatant flow behavior of the slurry can be observed, i.e., the viscosity increases with increasing shear rate. This dilatant behavior indicates the presence of highly coagulated powders, which can cause a three-dimensional gel-like structure in the slurry. At a pH of 5.7, the coating of a carrier material exhibits poor homogeneity with severe fractures, and its microstructure shows significant binder aggregates. The poor homogeneity of the coating is caused by the agglomeration of the active material at a pH below 6 (i.e., in the acidic range). The aggregation of the binder is due not only to the agglomeration of its adsorbed powder, but also to the unstable SBR (styrene-butadiene rubber as a binder component of the slurry) when it comes into contact with acid during the pH adjustment process.

[0009] At a pH value below 6, the decreasing binding effect of the aggregated binders, along with poor dispersion homogeneity, can be considered the main reason for the weak adhesion of the coating material to the substrate. Too low pH values lead to the binders becoming unstable and forming aggregates that damage the microstructures, e.g., of the LiCoO2 layers. Measurements of the physical and electrochemical properties showed that the coated substrates exhibit good properties in terms of adhesion, electronic conduction, and discharge rate capability when they are homogeneously dispersed and free of fractures. This is the case at a pH value around 9.In particular, it is known that a coated substrate with stronger adhesion has better electronic conductivity, and this relationship can be explained by the fact that the better adhesion increases the electron transfer between the substrate and the coating.

[0010] An increase in pH is often observed in the coating material. One reason for this is the effect of storage conditions on the stability and performance of, for example, NMC cathodes. For example, structural and performance-related changes are observed when NMC 532 (LiNi 0.3 Mn 0.3 - CO 0.2 O2) cathodes are exposed to moisture. The observation is a delithiated surface layer (where the proportion of lithium in the surface layer is reduced), the formation of certain compounds such as LiOH and Li2CO Sin the surface layer and a concomitant decrease in capacity. These changes are attributed to a proton-lithium exchange mechanism, which also occurs in other oxides. Surface contamination and decomposition layers are also observed in Ni-rich compositions such as NMC 811 (LiNi 0,8 Mn 0,8 Co 0,1O2) in humid environments, and these effects were found to correlate with the nickel content. Both LiOH (lithium hydroxide) and Li2CO3 (lithium carbonate) are alkaline in nature and increase the alkalinity of the slurry. The most likely cause of the observed pH changes is a Li+ / H+ exchange between the NMC and water, which is already known. The water serves as a source of H+ and produces OH-, which increases the pH and combines with the exchanged Li+ to form LiOH (lithium hydroxide). The LiOH can then react with CO2 in air to form LiHCO3, and then with LiOH and CO2 to form Li2CO3 (lithium carbonate) and H2O. Lithium hydroxide and lithium carbonate are both alkaline and increase the pH of the slurry (to a value of approximately 12).

[0011] A higher nickel content in the coating material results in a stronger driving force for the Li+ / H+ exchange reaction, making NMC 811 more reactive than NMC 333 (and thus causing a greater pH change). It is also evident that water plays an important role in the reaction responsible for the Li+ loss (delithiation) and the concomitant pH increase.

[0012] Hydroxide formation is therefore primarily caused by a Li+ / H+ exchange reaction, which occurs through the leaching of lithium ions. Leached Li+ ions react rapidly with oxygen or moisture to form Li2O and LiOH, which are subsequently converted to Li2CO3 by dissolved CO2. During storage in a humid atmosphere, a nanometer-thick carbonate layer forms on the particle surface of the coating material, but when the coating material is immersed in water, the hydroxide dissolves in the water. This causes the pH of the aqueous dispersion to rise significantly into the basic (alkaline) range. The resulting pH values depend on the cathode material used, the additional slurry components, and the exposure time.

[0013] The increase in the pH of the slurry is also caused by impurities in the cathode active material (CAM). The active material typically contains impurities in the form of LiOH and Li2CO3. These impurities are caused by excess lithium added to the NMC precursor during the calcination process. The higher the impurities, the higher the pH of the slurry.

[0014] In high-nickel CAM materials such as NMC811, nickel itself forms nickel hydroxide and nickel carbonate upon contact with moisture. This also increases the alkalinity or pH of the slurry.

[0015] Unfortunately, measuring the pH of the powder and determining the amount of acid to add to achieve the correct pH of the slurry is difficult. Hydroxides convert to carbonates in the presence of CO2, causing the pH of the slurry to constantly change over time. However, viscosity, adhesion, aluminum corrosion, and coating failure depend on a narrow pH range, so it is important to adhere to it. If this narrow range is exceeded, the slurry will be either too alkaline or too basic, and its properties will change.

[0016] The pH value can be lowered by adding an acid, e.g., hydrochloric acid, formic acid, phosphoric acid, or acetic acid. Acetic acid is of interest because it evaporates during the electrode drying step, leaving less or no residue in the finished electrode (i.e., the coated substrate). Acetic acid is also less hazardous than formic acid. The acid is added directly to the NMC solution (i.e., a mixture of NMC, carbon black, NMP, and PVDF (binder)). Mixing takes place at, for example, 2000 revolutions per minute for 90 minutes, followed by a 3-minute vacuum phase at the same speed for degassing. The pH value is measured 10 minutes after the end of the mixing process. After the pH value has been measured, the slurry is immediately transferred to the coating device.

[0017] Below a pH value of 9, the viscosity of the slurry increases again. This increase in viscosity is explained by the agglomeration of the active material and the carbon black. Therefore, the acid content must be controlled very effectively within the desired, narrow pH range, otherwise there is a risk of increased viscosity, increased resistance, and reduced adhesion, especially when the pH value drops below 9.

[0018] The reason for the loss of adhesion at pH values below 9 is more likely a consequence of the small molecular size of typical inorganic acids (used to lower pH). This can be demonstrated by using different PAA grades. While PAA with a molecular weight of 2,000 g / mol (PAA-2K) negatively affects the adhesion strength, adhesion at a high molecular weight of 450,000 g / mol (PAA-450K) or 1,250,000 g / mol (PAA-1.25M) is even slightly higher than that of the untreated sample. Compared to the larger PAA grades, the chain length of PAA-2K is more than 200 and 600 times shorter, respectively. PAA-2K has a molecular size more similar to that of other acids such as hydrochloric acid, formic acid, phosphoric acid, or acetic acid. In addition to lowering the pH, the long PAA types also act as binding molecules and thus as adhesives. Unfortunately, the acids used in current technology reduce adhesion.

[0019] The acid used to lower the pH of the slurry reacts with alkaline substances such as LiOH and lithium carbonate. This produces neutral salts and water (e.g., LiOH and HCl produce LiCl and H2O) as a byproduct. During the drying process, the remaining acid and byproducts boil and evaporate. The only problem is that the resulting salts, which remain in the cells, are not good ionic conductors.

[0020] Due to the small ionic radius of the lithium ion, most simple lithium salts do not meet the minimum solubility requirements in low-dielectric media (such as the carbonates used in current electrolytes). Examples include the halides LiX (with X as Cl and / or F) or the oxides Li2O. The requirement for chemical inertness excludes a family of lithium salts widely used in primary lithium batteries: LiAlX4 halides.

[0021] Because the Lewis acids of AlX3 bases such as Cl- do not completely neutralize their activity, they would attack most non-aqueous solvents, especially ethers. The AlX4 anions also cause severe corrosion of other cell components such as the separators, which are typically made of polypropylene, and the insulating sealant and metallic packaging materials of a battery cell. Therefore, neutral salts based on milder Lewis acids are preferred. Unfortunately, the acids used today produce salts with a strong Lewis base, so the resulting salt reduces the ionic conductivity and increases the internal resistance of the coating.

[0022] The problems can be summarized as follows: 1. Acidic slurry can cause adhesion problems and agglomeration of active material, which then increases the viscosity of the slurry. 2. Alkaline slurry causes corrosion of the aluminum substrate and gas evolution. 3. The pH of the slurry changes with time and storage conditions, making it difficult to predict the correct amount of acid to add. 4. The acid used today is a strong Lewis base. The resulting neutralized salt has low solubility in the electrolyte solvent, thus reducing ionic mobility. 5. The acid used has a small molecular size, which reduces the adhesion of the coating to the substrate. 6. Storage conditions and the presence of moisture have a strong impact on the alkalinity of the cathode powder, slurry, and coating. A fixed quantity or quality of acid used in the slurry, which then evaporates during the drying process, does not help counteract the subsequent increase in alkalinity. 7. In most cases, water is produced as a by-product during the reaction of the alkaline slurry with the acid, which in turn leads to lithium leaching (delithiation) and hydroxide formation. 8. Existing lithium salts used in the electrolyte are broken down into Li+ cations and anions. Li+ cations are consumed during SEI formation and come from the cathode material to maintain the balance of anions and cations. If the anions are consumed during gas production, there is no way to add these anions. The acid currently used to make the alkaline slurry basic has a different anion base than the lithium salt used in the electrolyte, so no anions from the electrolyte are added. For example, if lithium hexafluorophosphate is used as the lithium salt, fluoride ions are lost during the formation of HF gas. If HCl is used as the acid in the slurry, a chloride-based salt is formed, which does not contribute to adding lost fluoride ions.

[0023] To solve these problems, maleic acid was previously added to the slurry solution, taking into account lithium hydroxide impurities in the CAM powder. Maleic acid, or cis-butenedioic acid, is an organic compound that is a dicarboxylic acid, a molecule with two carboxyl groups. Its chemical formula is HO2CCH=CHCO2H or C4H4O4. Maleic acid is the cis isomer of butenedioic acid, while fumaric acid is the trans isomer. The melting point of maleic acid (135 degrees Celsius) is also much lower than that of fumaric acid (287 degrees Celsius). The low boiling point contributes to the evaporation of maleic acid during the drying process. Maleic acid neutralizes hydroxide and carbonate, forming a neutral salt with water. Maleic acid is water-soluble, so the concentration of maleic acid decreases with the evolution of water.

[0024] However, the use of maleic acid has the following disadvantages: 1. Low molecular weight of maleic acid, which reduces the adhesion of the coating to the substrate. 2. Neutralized salt is not a suitable lithium salt in terms of ion mobility in the electrolyte. 3. Maleic acid is a good lithium ion leacher and can therefore leach lithium ions from the active material. C4H3O4Li is often produced as a leached product.

[0025] Other approaches include influencing the pH value through CO2 pressurized gas treatment or by adding amphoteric oxide additives. To protect the metal surface of the support material, a carbon coating was applied to the aluminum foil. Direct contact of the active material with water was reduced by a protective layer of VOx, metal phosphates, or a binder shell.

[0026] EP 3 416 217 A1 discloses a method for producing a slurry of a cathode.

[0027] DE 696 35 888 T2 discloses a binder solution and an electrode-forming composition for non-aqueous batteries.

[0028] EP 2 677 573 A1 relates to a slurry for lithium-ion secondary battery electrodes formed using a binder for battery electrodes and an active material.

[0029] The object of the present invention is to at least partially solve the problems cited with reference to the prior art. In particular, a process for producing a slurry as a coating material for an electrode is to be proposed. This process involves the use of special acids that form advantageous salts and improve the properties of the electrode.

[0030] A method having the features of claim 1 and a battery cell having the features of claim 10 contribute to solving these problems. Advantageous developments are the subject of the dependent claims. The features listed individually in the claims can be combined with one another in a technologically expedient manner and can be supplemented by explanatory facts from the description and / or details from the figures, whereby further embodiments of the invention are demonstrated.

[0031] A method for producing a slurry for a cathode is proposed. The cathode comprises a carrier material and a coating formed by the slurry. The method comprises at least the following steps: a) providing a starting material for the slurry, comprising at least an active material, a binder and a conductive material, wherein the starting material has a high first pH value; b) providing an acid compound having a low second pH value; c) Mixing the starting material with the acid compound to lower the first pH value to a lower third pH value and forming the slurry.

[0032] The acid compound is formed exclusively by acids and comprises at least a first acid which is (at least) one (selected) of tetrafluoroboric acid (HBF4), trifluoromethanesulfonic acid (CF3SO3H) and bistriflimidic acid (C2HF6NO4S2).

[0033] The above (non-exhaustive) classification of process steps into a) to c) is primarily intended to serve as a distinction and does not enforce a specific order and / or dependency. The frequency of process steps can also vary. It is also possible for process steps to overlap one another, at least partially. Most preferably, steps a) and b) take place at least temporarily in parallel. Step c) occurs, in particular, after steps a) and b). However, steps a) to c) can also be performed at least partially simultaneously.

[0034] The first pH value is in particular greater than 9, preferably greater than 11. The second pH value is in particular less than 4, preferably less than 2. The third pH value is in particular between 8.5 and 9.5, preferably between 8.8 and 9.2, particularly preferably 9.0.

[0035] In particular, the acid compound comprises at least or exactly two of the first acids or additionally perchloric acid (HClO4) as a further first acid. Preferably, the acid compound comprises exactly three or all four of the first acids mentioned.

[0036] In particular, the first acids form at least 50 wt.%, preferably at least 70 wt.%, particularly preferably at least 85 wt.% of the acid compound.

[0037] In particular, the proportion of bistriflimidic acid in the acid compound is the highest.

[0038] In particular, the proportion of trifluoromethanesulfonic acid in the acid compound is the lowest.

[0039] In particular, the acid compound comprises at least a second acid that has a lower boiling point than the first acid. The second acid is, for example, acetic acid (CH3COOH), maleic acid (C4H4O4), or another acid with a correspondingly lower boiling point than the first acid.

[0040] In particular, at least one solvent or water (or both, if necessary) is added to the acid compound before step c) and then mixed with the starting material. The solvent is, for example, NMP (N-methyl-2-pyrrolidone).

[0041] In particular, the starting material comprises at least magnesium hydroxide (Mg(OH)2) or sodium hydroxide (NaOH). These compounds can react with the first acids to form neutral salts, which are initially present in the slurry or in the coating of the carrier material and then later dissolve in an electrolyte of the battery cell.

[0042] In particular, in a subsequent step d), the slurry is applied to a carrier material and a cathode is formed. In particular, in a subsequent step e) the at least one cathode is stacked with at least one anode and at least one separator (in a known manner), and the stack is arranged in a housing. The housing is filled with an electrolyte, which is supplemented by the acid compound (described above).

[0043] Instead of the acids previously used to reduce the pH of the slurry, it is proposed to use an acid compound comprising first acids, which in particular have a large molecular size and whose anions can form electrically conductive lithium salts. This allows for better pH control, and the neutralized salt contributes to ion mobility.

[0044] Either a first acid is used alone or a mixture of several first acids is used to obtain different anions. The following anions are particularly suitable in terms of ion mobility, SEl evolution, oxidation resistance, and temperature resistance: perchlorate (ClO4-), tetrafluoroborate (BF4-), trifluoromethanesulfonate, and bis(trifluoromethanesulfonyl)imide. Each of these anions exhibits particular advantages with regard to ionic mobility. The salts formed by the respective first acid are particularly soluble in the battery cell's electrolytes and can thus act as ion-carrying salts.

[0045] The amount of acid compound is dosed so that a third pH value of around 9 is established in the slurry.

[0046] The acid compound is added in particular with the solvent NMP (N-methyl-2-pyrrolidone).

[0047] The amount of acid compound or first acid added is determined depending on the amount of impurities, the moisture content, the storage conditions and the nickel content of the slurry or the coating of the carrier material.

[0048] The acid reacts particularly with hydroxide to form neutral salts and water. This water evaporates along with the acid during the drying process of the slurry or coating. To allow the remaining first acid to evaporate, second acids, such as acetic acid, are added to lower the boiling point of the acid compound, e.g., to approximately 130 degrees Celsius.

[0049] To compensate for a subsequent increase in the pH value of the coating, an acid compound (or the acid compound used to manufacture the cathode located in the battery cell) can be added as an additive to the battery cell's electrolyte. This helps reduce the increase in the coating's pH value during battery cell operation (i.e., repeated charging and discharging). This can also reduce corrosion of the aluminum-containing substrate material.

[0050] In particular, sodium hydroxide or magnesium hydroxide can be added to the slurry. These hydroxides react with the first acids to form sodium or magnesium salts, respectively. This means that both lithium and sodium ions, or even magnesium ions, can be present in the electrolyte. This can increase the capacity of the electrolyte. In other words, sodium or magnesium ions are introduced into the battery cell as alkaline "impurities" or additives in the slurry and then converted into beneficial salts by the preferred first acids, as mentioned above.

[0051] Various lithium salts are mentioned here and their specific advantages are explained. It also explains which initial acids should be used to prepare these initial acids in combination with lithium hydroxide. Undoubtedly, carbonates also play a role in the (later) pH increase. All lithium salts have a soft Lewis base, which increases their solubility in a medium with low dielectricity such as ethylene carbonate (the main component of the electrolyte). Furthermore, the initial acids mentioned have a high molecular weight, which increases adhesion at low slurry pH.

[0052] Lithium perchlorate (LiClO4) is due to its good solubility and high conductivity (conductivity of approx. 9.0 mScm -1It is a commonly used electrolyte solvent due to its high anodic stability (up to 5.1 volts on a spinel cathode surface in EC / DMC at 20 degrees Celsius, with EC as ethylene carbonate and DMC as demethyl carbonate) and its high anodic stability (up to 5.1 volts on a spinel cathode surface in EC / DMC). SEI films that form in LiClO4 electrolytes on both lithium and carbonaceous anode surfaces exhibit lower impedance than those that form in lithium hexafluorophosphate (LiPF6) or lithium tetrafluoroborate (LiBF4) electrolytes, since no HF is present in the former. This means that the battery cells have low internal resistance. LiClO4 also has the advantage of being relatively non-hygroscopic and stable to ambient humidity.

[0053] The first acid used in conjunction with lithium perchlorate is perchloric acid. Perchloric acid (HClO4) is a mineral acid. Perchloric acid is suitable for producing perchlorate salt when it comes into contact with lithium hydroxide. This first acid is non-hydrolytic. One disadvantage is that the produced salt reacts explosively. Therefore, this first acid is often used in combination with other first acids to produce only a portion of the lithium perchlorate salt, which facilitates the development of SEI. This first acid is notably not used as a 100% acid. It has a boiling point of approximately 203 degrees Celsius and should therefore be mixed with a lower-boiling first or second acid to lower the boiling point to at least the drying temperature used in the cathode drying process. This first acid is miscible with water.

[0054] To prepare the slurry, either a water-diluted version of this first acid (maximum 70 wt% of the first acid) or a version diluted with acetic acid or NMP can be used. It is preferable to use a weak second acid as the solvent to keep the water content to a minimum. Weak second acids also lower the boiling point of the acid compound.

[0055] Lithium tetrafluoroborate (LiBF4) is a salt based on an inorganic superacid anion and exhibits moderate ionic conductivity in non-aqueous solvents. BF4- has the highest mobility and is also less toxic than perchlorate Cl4-.

[0056] The first acid to be used with lithium tetrafluoroborate is fluoroboric acid or tetrafluoroboric acid (HBF4). This first acid is an inorganic compound with the chemical formula [H+][BF4-]. This first acid reacts with lithium hydroxide to form lithium tetrafluoroborate salt. Because it has a lower molecular mass, it is also used in smaller quantities. It has a boiling point of 130 degrees Celsius, meaning this first acid can evaporate at the drying temperature.

[0057] Lithium trifluoromethanesulfonate offers the least conductive solution. This property is primarily due to the combination of its low dissociation constant in low-dielectric media and its moderate ion mobility compared to other salts. On the other hand, sulfonate (-SO3Li) has become the anion of choice because, compared to LiPF6 or LiBF4, it is highly oxidation-resistant, thermally stable, nontoxic, and insensitive to ambient humidity. Lithium trifluoromethanesulfonate-based electrolytes in various solvent mixtures (EC / DMC, PC / DMC, or EC / DME; with PC as propylene carbonate and DME as dimethoxyethane) exhibit better Coulombic efficiency (~98%) and discharge capacity. However, the problem of aluminum corrosion also exists. Therefore, it is particularly preferable to minimize this salt and only utilize its high oxidation resistance.

[0058] The first acid used with lithium trifluoromethanesulfonate is trifluoroacid (CF3SO3H), short for trifluoromethanesulfonic acid. Trifluoromethanesulfonic acid is a sulfonic acid. When reacted with lithium hydroxide, it forms the LiTF salt. This first acid is typically mixed with polar solvents such as acetic acid or water and added to the slurry feedstock. The boiling point of this first acid is approximately 162°C, so secondary acids should be added to lower its boiling point. It is preferable to use a weak, low-boiling second acid as the solvent to prepare the acid compound.

[0059] Lithium bis(trifluoromethanesulfonyl)imide has been proven to be safe, thermally stable, and highly conductive: It melts at 236 degrees Celsius without decomposition (a rarity among lithium salts) and decomposes only at 360 degrees Celsius. Its ionic conductivity is an order of magnitude higher than that of lithium trifluoromethanesulfonate. Lithium bis(trifluoromethanesulfonyl)imide is preferable for use with solvents with a low dielectric constant, such as ethylene carbonate (used in electrolytes). In particular, the bis(trifluoromethanesulfonyl)imide anion in EC / DMC is oxidation-stable up to approximately 5.0 volts versus lithium, an oxidation limit lower than that for LiBF4, but still high enough to be advantageous. The morphology of cyclic lithium in lithium bis(trifluoromethanesulfonyl)imide-based electrolytes is notably better than that in other salt-based electrolytes. Furthermore, it is not hygroscopic.

[0060] The first acid to be used with lithium bis(trifluoromethanesulfonyl)imide is bistriflimide (C2HF6NO4S2), often referred to by the trivial name bistriflimidic acid. When combined with lithium hydroxide, lithium bis(trifluoromethanesulfonyl)imide (Lilm) salt is formed. This first acid has the highest molar mass compared to other first acids. The resulting neutralized salt also has the highest utility, which is why it should contain the largest proportion of the acid compound compared to the other three first acids mentioned.

[0061] It is therefore proposed to provide one or more, if appropriate all, of the four first acids mentioned, either alone or in combination with a weak second acid as an acid compound. If necessary, the acid compound can be supplemented by further addition of water and / or a solvent, e.g., NMP, to provide an acidic solution with a low boiling point. This acid compound, if appropriate with the addition of water and / or solvent, can then be added to the starting material to form the slurry.

[0062] These first four acids combine with alkaline impurities such as lithium hydroxide to form valuable lithium salts as neutral salts. These salts are soluble in low-dielectric solvents (which are part of the electrolyte) and contribute to increasing ionic conductivity.

[0063] The lithium salt, lithium hexafluorophosphate, is the main component of the electrolyte. This preferably remains unchanged. Therefore, no initial acid is added, which converts lithium hydroxide to lithium hexafluorophosphate. Furthermore, lithium hexafluorophosphate is sensitive to temperature and humidity, so it dissolves and releases fluoride ions.

[0064] By using the first acids mentioned above, combinations of many lithium salts are obtained, each offering specific advantages in addition to the main salt, lithium hexafluorophosphate. Water is undoubtedly formed as a reaction product, but this is removed during the cathode drying process.

[0065] These first four acids are added (individually or in combination) in such a way that they are fully utilized by the corresponding alkaline compounds, such as lithium hydroxide and lithium carbonates. The remaining solvent (second acid), acetic acid or maleic acid, is evaporated during the drying process.

[0066] If the pH value increases after the electrolyte has been placed in the battery cell casing due to the storage conditions of the battery cell, these first acids are released, particularly to a small extent, from the coating into the electrolyte, so that the pH value is lowered or remains at a nearly constant value.

[0067] Instead of adding maleic acid, acetic acid, or HCl alone to lower the pH of the slurry, a primary acid or a mixture of primary acids is added, resulting in a neutralized salt that dissolves in the electrolyte and increases ionic conductivity. The primary acids can be added individually or in combination. They are often mixed with polar solvents such as water or a weak (second) acid to form a miscible solution. The solution evaporates at the drying temperature.

[0068] The salts resulting from the neutralization of the alkaline starting material and the inorganic acids form salts with soft Lewis bases. This increases the solubility of the salt in the electrolyte. The first acid or acid compound is added in such a ratio that the pH of the slurry is lowered to approximately 9, which is most beneficial in terms of slurry consistency, viscosity, and corrosion of the support material.

[0069] In order to compensate for the increase of the alkaline solution in the coated electrode, an acid compound or a first acid is added to the electrolyte to produce a neutralized salt with a weak Lewis base.

[0070] The first acids used have, in particular, a high molecular weight, preferably more than 100 g / mol [grams per mole]. This increases the adhesion of the coating or active material to the carrier material.

[0071] The salts produced or producible by the neutralized reaction are, in particular, lithium perchlorate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, and lithium bis(trifluoromethanesulfonyl)imide (Lilm). They offer advantages over lithium hexaflorophosphate, the most important lithium salt in the electrolyte. The impurities lithium hydroxide and lithium carbonate are thus converted into useful salts, particularly with a new acid combination.

[0072] In particular, it is also possible to add hexafluorophosphoric acid to produce a neutral salt of lithium hexafluorophosphate. Other salts have the advantage of being added to the slurry before drying, as they exhibit greater temperature and moisture resistance.

[0073] The content of lithium hexafluorophosphate in the electrolyte should be reduced, as it is now accompanied by additional salts that contribute to the development of SEI and increase ionic conductivity.

[0074] In particular, it is also possible to add impurities or additives such as sodium hydroxide and magnesium hydroxide to the starting material. The starting material contains not only lithium in the active NMC material, but also magnesium and sodium. This means that three energy-transfer ions are present: lithium, magnesium, and sodium. It is also possible to have only one of these ions present.

[0075] Alkaline impurities such as lithium hydroxide, sodium hydroxide, and magnesium hydroxide are converted into neutral salts by adding the four proposed first acids, either alone or in combination. This makes it possible to produce ion-bearing salts in the slurry, which are ultimately dissolved in the electrolyte.

[0076] This ion-bearing salt (containing lithium ions, magnesium ions, and sodium ions) forms SEI, a combination of inorganic and organic compounds of lithium, sodium, and magnesium. This makes it possible to form a stable SEI, which is the biggest problem for sodium and magnesium batteries.

[0077] In particular, alkaline impurities (mainly hydroxide) can be deliberately added to create valuable salts not only of lithium but also of other metals such as sodium or magnesium. This increases the capacity of the battery cell and reduces the consumption of valuable lithium.

[0078] The first acid should be added in particular in proportion to the added alkali hydroxide so that the pH of the slurry is close to the optimal value of about 9.

[0079] In particular, the following advantages are achieved: • The pH value is maintained at a preferred level, particularly permanently (i.e., also in the cathode or battery cell later in use). This reduces corrosion of the substrate material and pitting. In particular, there is no gas evolution due to aluminum corrosion, since the pH value is not at a higher alkaline level. • Furthermore, improved adhesion of the coating material to the substrate is achieved because the first acids used have a high molecular mass due to the heavy anions. The first acids with high molecular mass act particularly as binders. • In particular, there is no or only a reduced risk of agglomeration of the binder and a consequent increase in the viscosity of the slurry in the low pH range (below a value of 8). • The surface of the coating is free of cracks and holes. • The first acid used has a dual function. It lowers the pH to a favorable value and reacts with hydroxide and carbonate to produce a neutralized soft Lewis base salt. These soft Lewis base salts have higher solubility in the low-dielectric solvent used in the electrolyte. • The initial acids used can be used individually or in combination. Valuable neutral salts such as lithium perchlorate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, and lithium bis(trifluoromethanesulfonyl)imide can be formed. They have advantages over lithium hexaflorophosphate, the main lithium salt in the electrolyte. The impurities lithium hydroxide and lithium carbonate are converted into a useful salt using a new acid compound. • By adding sodium and / or magnesium hydroxide to the slurry, not only lithium salts but also sodium and magnesium salts can be produced. This allows valuable salts of these metals to be extracted from alkaline impurities. • The mixing, coating and drying processes previously used in the production of cathodes remain unchanged and can be used like conventional processes. • The addition of the acid compound in small amounts to the electrolyte can help to control the future build-up of alkalinity in the coating (i.e. the increase in pH during operation of the battery cell). • Initially present lithium hydroxide, which is formed due to storage conditions and humidity, can be converted into ion-bearing salts and not into salts with a Lewis base, as in the well-known case with HCL as the acid. • The new salts give the battery cell produced in this way higher ionic conductivity, oxidation resistance and thermal stability, which can increase the window for safe operation of the battery cell. • The electrolyte is inexpensive and has the most advantageous combination of salts, which are also made from alkaline impurities.

[0080] A battery cell is further proposed, at least comprising a housing and at least one cathode arranged therein, which is produced by using a slurry produced by the described method.

[0081] The battery cell comprises in particular a housing enclosing a volume and arranged in the volume at least one cathode, one anode and a separator arranged therebetween as well as a liquid electrolyte.

[0082] The battery cell is, in particular, a pouch cell (with a deformable housing consisting of a pouch film) or a prismatic cell (with a rigid housing). A pouch film is a well-known deformable housing component used as a housing for so-called pouch cells. It is a composite material, e.g., comprising a plastic and aluminum.

[0083] The battery cell is in particular a lithium-ion battery cell.

[0084] A battery cell is an energy storage device used, for example, in a motor vehicle to store electrical energy. In particular, a motor vehicle, for example, has an electric motor for driving the vehicle (a traction drive), which can be driven by the electrical energy stored in the battery cell.

[0085] Furthermore, a motor vehicle is proposed, at least comprising a traction drive and a battery with at least one of the described battery cells, wherein the traction drive can be supplied with energy by the at least one battery cell.

[0086] The carrier material used for the cathode consists primarily of 5 to 15 µm [micrometer] thick aluminum or an aluminum alloy.

[0087] In particular, the starting material comprises at least one of conductive carbon black, NMC (lithium-nickel-cobalt-manganese as lithium-storing active material), graphite (as lithium-storing active material), CNT (carbon nano tubes), SBR (styrene-butadiene rubber as binder), CMC (carboxymethyl cellulose polymer), PVDF (polyvinylidene fluoride) and porous graphite.

[0088] The starting material for the cathode comprises, for example, approximately 2 wt% conductive carbon black, 0.5 wt% CNT, 2 wt% porous graphite, 3 to 4 wt% PVDF and the remainder NMC.

[0089] The method can be carried out in particular by a control unit that is equipped, configured or programmed to carry out the described method. The control unit can at least • the production and / or provision of the starting material; • the manufacture and / or provision of the acid compound; • mixing the starting material with at least the acid compound;

[0090] Furthermore, the method can also be carried out by a computer or with a processor of a control unit.

[0091] Accordingly, a data processing system is also proposed, which comprises a processor adapted / configured to carry out the method or part of the steps of the proposed method.

[0092] A computer-readable storage medium may be provided which comprises instructions which, when executed by a computer / processor, cause the computer / processor to carry out the method or at least some of the steps of the proposed method.

[0093] The statements regarding the method are particularly applicable to the battery cell, the motor vehicle, the control unit and the computer-implemented method (i.e. the computer or processor, the data processing system, the computer-readable storage medium) and vice versa.

[0094] The use of indefinite articles ("a," "an," "one," and "another"), particularly in the patent claims and the description reproducing them, is to be understood as such and not as a numeral. Terms or components introduced accordingly are therefore to be understood as appearing at least once and, in particular, as being able to appear multiple times.

[0095] As a precaution, it should be noted that the numerals used here ("first", "second",...) primarily serve (only) to distinguish between several similar objects, quantities, or processes, and therefore do not necessarily prescribe any interdependence and / or sequence of these objects, quantities, or processes. Should a dependence and / or sequence be necessary, this is explicitly stated here or will be obvious to the person skilled in the art upon studying the specifically described embodiment. To the extent that a component can occur multiple 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.

[0096] The invention and the technical environment are explained in more detail below with reference to the accompanying figures. It should be noted that the invention is not intended to be limited by the exemplary embodiments cited. In particular, unless explicitly stated otherwise, it is also possible to extract partial aspects of the facts explained in the figures and combine them with other components and findings from the present description. In particular, it should be noted that the figures, and in particular the proportions shown, are only schematic. They show: Fig. 1: a method for producing a cathode and a battery cell; and Fig. 2: a battery cell.

[0097] The Fig. 1 shows a method for producing a cathode 2 and a battery cell 12. In a step a), a starting material 5 for the slurry 1 is provided, comprising at least an active material, a binder, and a conductive material, wherein the starting material 5 has a high first pH. In a step b), an acid compound 6 with a low second pH is provided. In a step c), the starting material 5 is mixed with the acid compound 6 to lower the first pH to a lower third pH, and the slurry 1 is formed.

[0098] In a subsequent step d), the slurry 1 is applied to a provided carrier material 3, and a cathode 2 is formed. In a subsequent step e), the at least one cathode 2 is stacked with at least one anode 7 and at least one separator 8, and the stack 9 is arranged in a provided housing 10. The housing 10 is filled with an electrolyte 11, wherein the electrolyte 11 is supplemented by the acid compound 6.

[0099] Fig. Figure 2 shows a battery cell 12. The battery cell 12 comprises a housing 10 enclosing a volume and, arranged within the volume, a cathode 2, an anode 7, and a separator 8 arranged therebetween, which together form a stack 9, as well as a liquid electrolyte 11. The electrolyte is supplemented by an acid compound 6. The cathode comprises the carrier material 3 and the coating 4 arranged thereon. List of reference symbols 1 slurry 2 Cathode 3 Carrier material 4 Coating 5 Source material 6 acid compound 7 Anode 8 Separator 9 stacks 10 housings 11 Electrolyte 12 battery cells

Claims

[1] Method for producing a slurry (1) for a cathode (2), which comprises a carrier material (3) and a coating (4) formed by the slurry (1); at least comprising the following steps: a) providing a starting material (5) for the slurry (1), comprising at least an active material, a binder and a conductive material, wherein the starting material (5) has a high first pH value; b) providing an acid compound (6) having a low second pH value; c) mixing the starting material (5) with the acid compound (6) to lower the first pH value to a lower third pH value and form the slurry (1); wherein the acid compound (6) is formed exclusively by acids and comprises at least a first acid which is one of tetrafluoroboric acid HBF4, trifluoromethanesulfonic acid CF3SO3H and bistriflimidic acid C2HF6NO4S2. [2] Method according to claim 1, wherein the acid compound (6) comprises at least two of the first acids or additionally perchloric acid HClO4 as a further first acid. [3] Method according to one of the preceding claims, wherein the first acids form at least 50% by weight of the acid compound (6). [4] Method according to one of the preceding claims, wherein a proportion of bistriflimidic acid in the acid compound (6) is the highest. [5] Method according to one of the preceding claims, wherein a proportion of trifluoromethanesulfonic acid in the acid compound (6) is the lowest. [6] Method according to one of the preceding claims, wherein the acid compound (6) comprises at least a second acid having a lower boiling point than the first acid. [7] Method according to one of the preceding claims, wherein at least one solvent or water is added to the acid compound (6) before step c) and then mixed with the starting material (5). [8] Method according to one of the preceding claims, wherein the starting material (5) comprises at least magnesium hydroxide (Mg(OH)2) or sodium hydroxide (NaOH). [9] Method according to one of the preceding claims, wherein in a subsequent step d) the slurry (1) is applied to the carrier material (3) and the cathode (2) is formed; wherein in a subsequent step e) the at least one cathode (2) with at least one anode (7) and at least one separator (8) are stacked on top of one another and the stack (9) thus formed is arranged in a housing (10), wherein the housing (10) is filled with an electrolyte (11); wherein the electrolyte (11) is supplemented by the acid compound (6). [10] Battery cell (12), at least comprising a housing (10) and arranged therein at least one cathode (2), which is produced by using a slurry (1) produced by the method according to one of the preceding claims.

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