Method for manufacturing a battery cell, as well as a battery cell

Abstract

A method for producing a battery cell (2) comprising an anode (4) and a cathode (6) is described, wherein the anode (4) is produced from an anode slurry, wherein the cathode (6) is produced from a cathode slurry, wherein SBR particles (8) of predominantly styrene-butadiene rubber (SBR) are provided, wherein the SBR particles (8) are coated with a coating (20) of a carbon-containing material, and wherein the SBR particles (8) thus coated are added to the anode paste and / or the cathode paste.

Description

The invention relates to a method for manufacturing a battery cell, as well as a battery cell. A battery cell serves to provide electrical energy to a consumer. In a traction battery, also called a drive battery, several battery cells are appropriately connected and controlled to supply consumers, usually electric motors, with electrical energy. Such traction batteries are often designed as lithium-ion batteries. A lithium-ion battery cell typically has two electrodes, an anode and a cathode, as well as an electron-impermeable separator for galvanic isolation of the electrodes. An electrolyte is also usually present to improve lithium ion transport between the electrodes. In the production of such a battery cell, an electrode is often made from an electrode paste (slurry). This electrode paste consists of at least a particulate electrode active material, a solvent, and usually also a particulate binder. The electrode paste is applied to a conductive substrate (receptor), dried, and compacted. Two electrodes produced in this way, with different electrical potentials (cathode and anode), are then placed on top of each other, separated by a separator, and formed into a shape appropriate for the battery cell. A subsequent manufacturing step often involves inserting the substrates into a housing, filling it with the electrolyte, and then sealing it. While the solvent evaporates during drying, the binder remains in the electrode, ensuring spatial cohesion of the active material particles and, in particular, adhesion to the support film. Styrene-butadiene rubber (SBR) is often added as a binder to an anode paste (anode slurry) for the production of an anode. A coating of active material is described in DE 10 2012 109 641 A1 , WO 2023 / 079315 A1 , and in US 2023 0 006 204 A1. WO 2021 / 259084 A1 describes an improvement to styrene-butadiene rubber. It proposes adding a polar end group to the styrene-butadiene rubber via a sulfur bridge to improve its ionic conductivity. SBR as a component of electrode paste is also known from CN 119 419 213 A and CN 118 983 394 A. The invention is based on the objective of providing a particularly suitable method for manufacturing a battery cell, and a particularly suitable battery cell. The problem is solved according to the invention with regard to the method by the features of claim 1, and with regard to the battery cell by the features of claim 10. Advantageous embodiments and further developments are the subject of the dependent claims and the following description. The advantages and embodiments listed below with regard to the method are also applicable to the battery cell and vice versa. In the inventive process for manufacturing a battery cell comprising an anode and a cathode, the anode is produced from an anode slurry and the cathode is produced from a cathode slurry. SBR particles, predominantly made of styrene-butadiene rubber (SBR), are provided and subsequently coated with a carbon-containing material. In particular, the carbon in the coating is present in its native elemental form. The SBR particles thus coated are added to the anode slurry and / or the cathode slurry, in particular as a binder or as a component of a binder. In the context of the SBR particles, "predominantly" here and in the following refers in particular to the fact that the SBR particles are made up of more than half styrene-butadiene rubber, and preferably the SBR particles are made entirely of this material. The conjunction “and / or” is to be understood here and in the following as meaning that the features linked by means of this conjunction can be both common and alternative to each other. The use of styrene-butadiene rubber in the anode and / or cathode can improve the flexibility of the respective electrode. This flexibility is advantageous so that the electrode can tolerate any deformations, particularly during the battery cell manufacturing process. Furthermore, styrene-butadiene rubber can help improve the bonding between the individual particles in the electrode as well as its adhesion to a substrate. Since styrene-butadiene rubber is an electrical insulator and would therefore generally increase the internal electrical resistance of the respective electrode, the invention—namely, coating the SBR binder particles with the carbon-containing material—improves the electrical conductivity of the binder and the electrode formed from the anode paste and cathode paste, respectively, due to the electrical conductivity of carbon. The coating, which is electrically conductive due to the carbon, thus advantageously contributes to electron transport within the respective electrode. Electrons can therefore be transported more effectively within the electrode and between a consumer and an active material of the electrode. In particular, this allows for a significant reduction in the internal electrical resistance of the battery cell. The cathode is electrically connected, in particular, to a positive terminal of the battery cell, while the anode is electrically connected, in particular, to a negative terminal of the battery cell. The coating is produced, in particular, by mixing the SBR particles with sucrose, glucose, or carbon black at a preferred weight fraction of 5% (+ / - 1%) in an aqueous solution. Distilled water is particularly suitable for this purpose, and its pH is adjusted to a preferably 10 using ammonium hydroxide, also known as ammonia solution. Lithium hydroxide or sodium hydroxide can be used as alternatives to ammonium hydroxide. The solid that settles after stirring is primarily composed of the coated SBR particles. The SBR particles have a maximum diameter in the range of 0.5 µm to 2 µm, while the coating has a layer thickness in the range of 1 nm to 10 nm. The SBR particles are thus provided with a carbon nanocoating.Subsequent heating evaporates the water and ammonium hydroxide. In particular, the coated SBR particles remain as an agglomerated solid. Preferably, sucrose or glucose is used to produce the coating. In particular, after the water and ammonium hydroxide have evaporated, the coated particles are reduced by heat treatment in a temperature range of 130 °C to 150 °C and under exclusion of oxygen. During this process, water is evaporated, and elemental carbon is formed. According to an alternative process variant to the aqueous coating described above, the coating is produced by mixing the SBR particles with carbon dioxide gas and hydrogen gas. Specifically, in a reaction tank, carbon dioxide is reduced by hydrogen to elemental carbon for the coating, with water acting as a solvent. This preferably takes place in a temperature range of 10 °C to 150 °C, particularly between 100 °C and 150 °C. The water is then evaporated, leaving behind the coated SBR particles, preferably agglomerated, as a solid. Preferably, the water is removed from the solution by spray drying with heated air. This is particularly advantageous for controlling the size and shape of the resulting coated SBR particles. In a preferred further development of the process, lithium is applied to the coated SBR particles. This is done, in particular, with a mass fraction of 0.5 to 2%, and especially of about 1% (i.e., ± 0.25%) relative to the SBR particles, and particularly before they are added to the anode and / or cathode paste. Preferably, elemental lithium is applied to the coated SBR particles in their dry, optionally agglomerated, solid form. Preferably, powdered lithium, especially in the form of nanoparticles, is used. The application process oxidizes the elemental lithium and reduces the carbon in the coating, resulting in the formation of intercalated LiC6 compounds in the coating. Alternatively or additionally, covalent C-Li bonds can also form as functional groups by applying elemental lithium. In this case, one lithium atom is covalently bonded to one carbon atom of the coating. The intercalated LiC6 compounds are advantageous because, in addition to the active material, they provide a lithium ion source for the anode and / or cathode. Lithium ions, which can (and should) migrate between the anode and cathode during subsequent operation of the battery cell, are lost, particularly during the battery cell's formation process. Alternatively or additionally, the styrene-butadiene rubber can also be functionalized with a covalent C-Li bond as a functional group by applying elemental lithium. The formation of the intercalated LiC6 compounds and / or the covalent C-Li bonds is actively promoted in particular by baking in a rotary kiln at preferably 150 °C. The application of lithium, particularly in the small amounts described above, can also be described as lithium doping. This has the advantage of preventing excessive, and especially unacceptably strong, reactions of elemental lithium, or lithium present in covalent bonds, with air (oxygen) or water. In this and the following, a formation process is understood to mean, in particular, a number (i.e., one or more) initial charging cycles with a charging voltage. Specifically, during a charging cycle, a charging voltage is applied to the battery cell for a specific charging period, followed by a discharge period. Multiple charging cycles can differ from one another in their respective charging and / or discharging times and / or charging and discharging voltage profiles over the respective charging and discharging periods. The battery cell is then sealed, particularly after the formation process. Furthermore, the lithium-functionalized coating, as described above, advantageously possesses at least one lone pair of electrons, particularly when the coated SBR particles are added to an aqueous electrode paste, allowing a previously intercalated lithium ion to dissolve. The electrode paste contains distilled water as a solvent, especially for the anode paste. Preferably, the cathode paste contains a polar and preferably anhydrous solvent, such as N-methyl-2-pyrrolidone (NMP). Any residual water present can then react advantageously with the lithium and be removed from the NMP. The free electron pair(s) contribute to increasing the negative zeta potential of a coated SBR particle, which in turn advantageously helps to prevent mutual adhesion of individual coated SBR particles in the electrode paste. This results in a particularly homogeneous dispersion of the coated SBR particles in the electrode paste, which does not self-agglomerate and / or settle (sediment). In particular, this eliminates the need for an additional dispersing agent, such as ethyl alcohol. Furthermore, the free electron pair(s) are advantageous for promoting the transport of lithium ions through the electrode during operation (discharging or charging) of the battery cell. This can be described as a kind of "hip-hop" transfer of lithium ions between the accessible free electron pairs. The covalent C-Li bonds, acting as functional groups, are particularly advantageous for reacting with the solvent in the electrode paste as a strong base. For example, lithium hydroxide can be formed by adding the SBR particles to an aqueous electrode paste. The lithium hydroxide formed contributes significantly to the formation of a beneficial solid electrolyte interphase (SEI) layer, especially at the cathode. As mentioned above, this reaction can also occur at the cathode with residual water, such as that contained in an NMP solvent. Optionally, an acid, such as malic or formic acid, is added to the electrode paste to prevent the pH from becoming too high (basic). In addition to or as an alternative to lithium, sodium is applied to the coated SBR particles according to a convenient process variant before they are added to the anode paste and / or cathode paste. The application of sodium is preferably carried out in the same manner as the previously described application of lithium. In particular, this results in the formation of intercalated NaCl compounds and / or covalent C-Na bonds as functional groups on the coating and / or the styrene-butadiene rubber. The corresponding advantages are analogous to those described above. Preferably, if the coated SBR particles are added to the cathode paste, they are added at a concentration of up to 1% by weight. Preferably, the coated SBR particles have been previously functionalized with lithium and / or sodium (i.e., mixed according to the process variants described above), as described above. However, an addition of more than 1% by weight may lead to adverse effects, such as reduced electrical conductivity and ion transport capacity of the cathode. Preferably, if the coated SBR particles are added to the anode paste, they are added at a concentration of up to 2% by weight. Preferably, the coated SBR particles have been previously functionalized with lithium and / or sodium (i.e., mixed in the process variants described above), as described above. An addition of more than 2% by weight may, in some cases, lead to adverse effects, such as reduced electrical conductivity and ion transport capacity of the anode. In a preferred embodiment of the process, a formation process is carried out at least partially at more than 3.8 V, in particular at up to 4.2 or even up to 4.4 V. "At least partially" here refers in particular to at least a temporary (section-by-section) exceeding or reaching of a charging voltage of the aforementioned values. This can occur, in particular, within a single charging cycle and / or within several charging cycles. This oxidizes any styrene-butadiene rubber and / or elemental carbon in the coating at the cathode. The oxidation of styrene-butadiene rubber at the cathode can be further promoted, in particular, by carrying out the formation process at least partially at 50 °C according to an advantageous process variant. Here, "at least partially" is understood to mean that the battery cell reaches a temperature of 50 °C at least temporarily (in sections). This can occur, in particular, within a single charging cycle and / or within several charging cycles. In particular, temperatures of up to a maximum of 60 °C can also be reached, at least partially. During this oxidation process, the styrene-butadiene rubber and / or the carbon in the coating react with oxygen to form, in particular, hydrocarbons, carbon monoxide, and carbon dioxide. Specifically, a polymer chain of the styrene-butadiene rubber is broken down. The oxygen can originate, for example, from a lithium nickel manganese cobalt oxide (NMC), which is used as the active material for the cathode. Particularly at a charging voltage of 4.4 V, nickel ions migrate from the cathode and release oxygen. Alternatively or additionally, the oxygen can originate from the electrolyte. Any C-Li bonds present in the cathode act as functional groups on the coating, reacting with the released oxygen and carbon dioxide to form lithium oxide and / or lithium carbonate (Li₂CO₃) as lithium salts. These lithium salts can then form a beneficial SEI layer on the cathode. Lithium carbonate, in particular, contributes to the formation of this SEI layer. The following reaction equation can be formulated: 2CO₂ + 2Li⁺ + 2e⁻ → Li₂CO₃ + CO (1) The SEI layer at the cathode effectively prevents unwanted reactions between the electrolyte and the cathode throughout the battery cell's lifetime. Specifically, the SEI layer acts as a protective layer around cathode particles (particularly particles of the active material). Any additional titanium oxide and / or aluminum oxide salts that may be present can be incorporated into this SEI layer, thus enhancing its performance. The presence of a C-Li and / or C-Na bond as a functional group is particularly advantageous for capturing released oxygen within the battery cell, especially within the cathode during the formation process, and binding it in a lithium and / or sodium compound. This, in particular, prevents thermal runaway of the battery cell. In particular, carbon dioxide produced during the formation process helps to reduce an electrolyte, for example ethylene carbonate and / or vinylene carbonate, in order to further form a beneficial SEI layer at the cathode. The reduction of ethylene carbonate, using carbon dioxide, primarily yields carboxylates (LiO₂COCH₂CH₂COOLI) and / or alcoholate carboxylates (LiOCH₂CH₂COOLI). Carbon dioxide acts as a catalyst in this process. The following reaction equations can be formulated: C₃H₄O₃ + CO₂ + 2e⁻ + 2Li⁺ → LiO₂COCH₂CH₂COOLI (2) C₃H₄O₃ + CO₂ + 2e⁻ + 2Li⁺ → LiOCH₂CH₂COOLI + CO₂ (3) Other, especially linear, carbonic acid esters are also conceivable as additional or alternative electrolytes. For example, ethyl methyl carbonate and / or diethyl carbonate can be reduced to the corresponding alkoxide and carboxylate. In particular, the alkoxides and / or carboxylates also function as an advantageous organic SEI layer at the cathode. This organic SEI layer consists primarily of polyolefins. The following reaction equation can be formulated, especially with R1 and R2 as linear alkyl groups: R1OCO2R2 + 2Li+ + 2e- → R1COOLi + R2OLi (4) The battery cell according to the invention comprises an anode and a cathode, wherein the anode and / or the cathode comprises SBR particles, which are formed at least predominantly from styrene-butadiene rubber and which preferably serve as a binder for particles of a corresponding (electrode) active material. The SBR particles are coated with a carbon-containing material. The battery cell is thus manufactured by a process according to one of the preceding embodiments. The procedure described above leads in particular to the lithium supplied to the anode or cathode by means of the SBR particles, and in particular contained in the anode paste or cathode paste, being present at less than 1 wt.%. The battery cell preferably exhibits the physical characteristics and advantages described in the preceding method and its advantageous, optional method variants – particularly in the context of optional embodiments corresponding to the method variants – equally. The same applies conversely. In particular, the battery cell comprises an electrolyte consisting of at least ethylene carbonate and / or vinylene carbonate. Preferably, the battery cell has an NMC cathode. The battery cell is especially preferably designed and intended for use in a lithium-ion battery. The invention is explained in more detail below with reference to exemplary embodiments shown in a drawing. These drawings show, in simplified and schematic representations: Fig. 1 a battery cell for a lithium-ion battery, Fig. 2 a flowchart of a process for manufacturing a battery cell, Fig. 3 a process step from Fig. 2 for producing a coating, and Fig. 4 a process step from Fig. 2 for applying lithium. Corresponding parts and sizes are always marked with the same reference symbols in all figures. Figure 1 shows a battery cell 2 comprising two electrodes, namely an anode 4 and a cathode 6, wherein the anode 4 and the cathode 6 contain SBR particles 8 made of styrene-butadiene rubber, which serve as a binder for particles of a corresponding (electrode) active material. It is equally possible that only the anode 4 or the cathode 6 contains the SBR particles 8. For the sake of clarity, the SBR particles 8 are each labeled with a reference numeral only once. The SBR particles 8 are coated with a carbon-containing material, as described in more detail below. The anode 4 and the cathode 6 are galvanically separated from each other by an electron-impermeable separator 10 and are connected to their respective electrically conductive contacts 12. A positive terminal 16 and a negative terminal 18, specifically in the form of contact tabs, protrude from a housing 14 for electrical contact. An electrolyte (not shown in detail here), containing, for example, ethylene carbonate, enables improved lithium ion transport between the electrodes and, due to the porosity caused by the particles, also within the electrodes. The cathode 6 is designed as an NMC cathode. The battery cell 2 shown is designed and intended for use in a lithium-ion battery. Figure 2 shows a flowchart of a process for manufacturing a battery cell 2 comprising an anode 4 and a cathode 6, wherein the anode 4 is produced from an anode slurry and the cathode 6 is produced from a cathode slurry. In a first process step S1, SBR particles 8 made of styrene-butadiene rubber are provided, which in a subsequent process step S2 are coated with a coating 20 made of a carbon-containing material. Fig. 3 shows an embodiment of how the coating 20 is applied to the SBR particles 8 in process step S2. The SBR particles 8 are first placed in a dispersion of carbon black 22 in water, here exemplarily in a reaction vessel 24, and mixed with the carbon black 22. Distilled water is used as a solvent 26, the pH of which is adjusted using ammonium hydroxide, also known as ammonia water. This provides the SBR particles 8 with a nanoscale carbon coating. A solid 28 that settles after stirring is primarily formed by the coated SBR particles 8. Subsequent heating evaporates the solvent 26. The coated SBR particles 8 remain agglomerated as a solid 30 in the reaction vessel 24.For clarity, the SBR particles 8, the carbon black 22, and the coating 20 are each only identified once with a reference symbol. Sucrose or glucose can be used in addition to or as an alternative to carbon black 22. In the case of sucrose or glucose, the coated SBR particles 8 are subjected to an elevated temperature in the absence of oxygen in order to reduce the sucrose or glucose content. As an alternative to the above description for producing the coating 20, the coating 20 can be produced by mixing the SBR particles 8 with carbon dioxide gas and hydrogen gas. In this process, carbon dioxide is reduced by hydrogen to elemental carbon for the coating 20 and water, particularly in a reaction tank. The water is then evaporated as in the embodiment described above, leaving behind the coated SBR particles 8 (optionally agglomerated). In process step S3 (see Fig. 4), lithium is subsequently applied to the coated SBR particles 8. Elemental (also: metallic) lithium powder 32 is applied to the dry, optionally agglomerated, coated SBR particles 8. This is done in a (optionally additional) reaction vessel. This allows covalent C-Li bonds 34 to form as functional groups. In each case, a lithium atom 36 is covalently bonded to a carbon atom of the coating 20. The coating 20 exhibits several covalent C-Li bonds 34. Alternatively or additionally, the elemental lithium can be oxidized and the carbon in the coating 20 reduced by the application, resulting in the formation of intercalated LiC6 compounds in the coating 20. In a subsequent process step S4, the coated particles 4 are added to the anode paste and / or cathode paste. Here, the coated SBR particles 8 are added to the cathode paste at a rate of up to 1% by weight. Furthermore, the coated SBR particles 8 are added to the anode paste at a rate of up to 2% by weight. Upon contact with a suitable solvent, particularly water, used to form the anode or cathode paste, an intercalated lithium ion can dissolve, leaving a lone pair of electrons 38 on the coating 20. This lone pair 38 is located in one or more π-orbitals of carbon atoms in the coating 20. The coating 20, in particular, exhibits several lone pairs of electrons 38. After the anode 4 and the cathode 6 have been manufactured from their respective electrode pastes, the battery cell 2 is assembled and filled with electrolyte in process step S5. The battery cell 2 then undergoes a formation process. During this process, a charging voltage of more than 3.8 V, particularly 4.4 V, is applied to the electrodes. The battery cell 2 is also heated to 50 °C. This causes any styrene-butadiene rubber and / or elemental carbon in the coating 20 to be oxidized to carbon dioxide, acting as sacrificial material at the cathode 6. The carbon dioxide acts as a catalyst in a reaction to form the SEI layer. In addition to or as an alternative to lithium, sodium can also be applied to the coated SBR particles 8 before they are added to the anode paste and / or cathode paste. The application of sodium is preferably carried out in the same manner as the previously described application of lithium. In particular, this results in the formation of intercalated NaC6 compounds and / or covalent C-Na bonds as functional groups on the coating 20 and / or the styrene-butadiene rubber. The invention is not limited to the embodiments described above. Rather, other variants of the invention can also be derived by a person skilled in the art within the scope of the disclosed claims without departing from the subject matter of the claimed invention. In particular, all individual features described in connection with the various embodiments can also be combined in other ways within the scope of the disclosed claims without departing from the subject matter of the invention. Reference symbol list 2 Battery cell 4 Anode 6 Cathode 8 SBR particles 10 Separator 12 Collector 14 Housing 16 Positive terminal 18 Negative terminal 20 Coating 22 Carbon black 24 Reaction vessel 26 Solvent 28 Solid 30 Solid 32 Lithium powder 34 Bond 36 Lithium atom 38 Electron pair S1 Process step S2 Process step S3 Process step S4 Process step S5 Process step

Claims

Method for manufacturing a battery cell (2) comprising an anode (4) and a cathode (6), wherein the anode (4) is produced from an anode paste, wherein the cathode (6) is produced from a cathode paste, wherein SBR particles (8) of predominantly styrene-butadiene rubber are provided, wherein the SBR particles (8) are coated with a coating (20) of a carbon-containing material, wherein the SBR particles (8) so coated are added to the anode paste and / or the cathode paste. Method according to claim 1, wherein to produce the coating (20) the SBR particles (8) are mixed with sucrose, glucose or soot (22) in an aqueous solution. Method according to claim 1, wherein the SBR particles (8) are mixed with carbon dioxide gas and hydrogen gas to produce the coating (20). Method according to any one of claims 1 to 3, wherein lithium is applied to the coated SBR particles (8), in particular before they are added to the anode paste and / or cathode paste. Method according to any one of claims 1 to 4, wherein sodium is applied to the coated SBR particles (8), in particular before they are added to the anode paste and / or cathode paste. Method according to any one of claims 1 to 5, wherein the coated SBR particles (8) are added to the cathode paste in a quantity of up to 1 percent by weight. Method according to any one of claims 1 to 6, wherein the coated SBR particles (8) are added to the anode paste in a quantity of up to 2 percent by weight. Method according to any one of claims 1 to 7, wherein a forming process is carried out at least partially with more than 3.8 V, in particular with up to 4.4 V. Method according to any one of claims 1 to 8, wherein the forming process is carried out at least partially at 50 °C. Battery cell (2) manufactured by a method according to any one of claims 1 to 9.

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