Anode foil for an electrode foil arrangement and method for producing an anode foil

Abstract

The invention relates to an anode foil (1) for an electrode foil arrangement of an electrochemical single cell, comprising a copper foil (1.1) coated with a silicon-carbon-graphite layer (1.2). According to the invention, the copper foil (1.1) additionally comprises a graphite layer (1.3). Furthermore, the invention relates to a method for producing an anode foil (1).

Description

[0001] The invention relates to an anode foil for an electrode foil arrangement of an electrochemical single cell, comprising a copper foil coated with a silicon-carbon-graphite layer. The invention further relates to a method for producing an anode foil.

[0002] In general, an electrochemical single cell is known, comprising a cell housing and an electrode array arranged within it, with anode and cathode foils, each electrically insulated from the other by a separator. Such an anode foil is formed from a coated copper foil, which acts as the current collector. To achieve a high volumetric energy density, the anode foil has a predetermined silicon content. Silicon-carbon composites typically have a lower density than graphite. Therefore, a high calendering pressure is required during the production of an anode foil to achieve a specific volumetric electrode density. Such a high calendering pressure can lead to the embedding of silicon particles in the current collector of the copper foil.Expansion, or swelling, of the silicon particles can also generate stresses in the anode foil, leading to deformation and cracking of the copper foil. Furthermore, relatively strong adhesion of a binder to the silicon-carbon-graphite layer can cause the copper foil to wrinkle and / or crack when the silicon-carbon particles change volume during operation of the individual cell. Such cracking can result in a short circuit and accelerate the aging of the individual cell.

[0003] The invention is based on the objective of providing an anode foil for an electrode foil arrangement of an electrochemical single cell and a method for producing such an anode foil.

[0004] The problem is solved according to the invention by an anode foil which has the features specified in claim 1, and by a method which has the features specified in claim 6.

[0005] Advantageous embodiments of the invention are the subject of the dependent claims.

[0006] An anode foil for an electrode foil arrangement of an electrochemical single cell comprises a copper foil coated with a silicon-carbon-graphite layer as a substrate, which according to the invention additionally has a graphite layer.

[0007] The additional graphite layer significantly reduces the risk of silicon particles from the silicon-carbon-graphite layer penetrating the copper foil. Since this prevents silicon particles, which are comparatively hard, from penetrating the copper foil, especially during calendering, it largely prevents the silicon particles, which expand during operation of the individual cell (i.e., during charging), from causing deformation, particularly wrinkling, and / or cracking of the anode foil.

[0008] The volumetric energy density of the individual cell is not affected by the additional graphite layer.

[0009] In one embodiment, the graphite layer is arranged between a surface side of the copper foil and the silicon-carbon graphite layer, so that penetration of silicon particles from the silicon-carbon graphite layer into the copper foil, especially during calendering, can be avoided.

[0010] Another design of the anode foil provides that the area of ​​the graphite layer corresponds to the area of ​​the silicon-carbon-graphite layer, with the silicon-carbon-graphite layer being applied congruently to the graphite layer. This largely ensures that, even under a comparatively high calendering force acting on the silicon-carbon-graphite layer, no silicon particles penetrate the copper foil, thus largely preventing wrinkling and / or cracking of the copper foil and consequently of the anode foil as an electrode, particularly during operation of the individual cell.

[0011] In one embodiment, the copper foil has a thickness between 4.5 µm and 6 µm, for example, 5 µm or 5.5 µm. The thickness serves as an indicator for setting the calendering force applied to the coating during calendering to achieve a predetermined density of the coating on the anode foil, which consists of the graphite layer and the silicon-carbon-graphite layer. The density of the coating on the copper foil after calendering is defined as its density.

[0012] The invention further relates to a method for producing an anode foil for an electrode foil arrangement of an electrochemical single cell, wherein a graphite layer is additionally applied to the copper foil. The graphite layer serves in particular to prevent comparatively hard silicon particles from the silicon-carbon-graphite layer, especially through calendering, from penetrating the copper foil and thus potentially causing wrinkling and / or cracking during subsequent operation of the single cell.

[0013] In one embodiment, the graphite layer is applied to one surface of the copper foil, so that the graphite layer is then positioned between the copper foil and the silicon-carbon-graphite layer. This effectively prevents silicon particles from the silicon-carbon-graphite layer from penetrating the copper foil and thus avoiding the risk of wrinkling and / or cracking of the copper foil during operation of the individual cell.

[0014] In another embodiment of the process, the silicon-carbon-graphite layer is applied in a wet state to the graphite layer, which is also wet. This ensures that the coating surface of the copper foil is smooth and uniform, and intermediate drying is not required.

[0015] Another embodiment provides that a calendering force is set depending on the thickness of the copper foil and / or depending on a predetermined pressure density of a coating of the copper foil, in particular to prevent silicon particles from the silicon-carbon-graphite layer from penetrating the copper foil.

[0016] Exemplary embodiments of the invention are explained in more detail below with reference to drawings.

[0017] This shows: Fig. 1. Schematic cross-sectional view of an enlarged section of an anode foil with silicon particles partially embedded in a copper foil. Fig. 2 schematically a cross-sectional view of an enlarged section of an anode foil with particles of a silicon-carbon-graphite layer strongly fixed by means of a binder, Fig. 3 schematically a cross-sectional view of an enlarged section of a double-coated copper foil for the formation of an anode foil and Fig. 4 schematically a side view of an anode foil for an electrode foil arrangement of an electrochemical single cell.

[0018] Corresponding parts are marked with the same reference symbols in all figures.

[0019] Fig. Figure 1 shows a highly simplified cross-sectional view of an enlarged section of an anode foil 1, which has a copper foil 1.1 provided with a silicon-carbon-graphite layer 1.2. Such an anode foil 1 is intended for arrangement in an electrode foil assembly (not shown) of an electrochemical single cell.

[0020] A relatively high silicon content in the silicon-carbon-graphite layer 1.2 is required to achieve the desired volumetric energy density of the individual cell. Volumetric energy density refers to how much electrical energy per unit volume can be extracted from the individual cell and is expressed in watt-hours per liter. The higher the volumetric energy density, the smaller the dimensions of the individual cell for the same amount of stored electrical energy.

[0021] The silicon-carbon-graphite layer 1.2 is applied to the copper foil 1.1 using a wet coating process and calendered after drying. During calendering, a comparatively high calendering force acts on a mixture of a binder (not shown) with silicon-carbon particles S and graphite particles G to achieve a predetermined density of the subsequent silicon-carbon-graphite layer 1.2. This calendering force presses the relatively hard silicon-carbon particles S into the copper foil 1.1. During operation of the individual cell, i.e., during charging and discharging, the silicon-carbon particles S, in particular, change their volume. When the individual cell is charged, the silicon-carbon particles S expand, and when the individual cell is discharged, their volume decreases.Since silicon-carbon particles S were embedded in the copper foil 1.1 during calendering, there is a risk of wrinkling and cracking of the copper foil 1.1 due to a change in the volume of the silicon-carbon particles S. Such cracking poses a risk of short circuits within the individual cell and accelerates the aging of the individual cell.

[0022] Fig. Figure 2 shows a cross-sectional view of an enlarged section of an anode foil 1 with silicon-carbon particles S of the silicon-carbon graphite layer 1.2 of the anode foil 1, which are at least strongly fixed by means of a binder. Due to the volume changes occurring during operation of the individual cell, in particular of the silicon-carbon particles S fixed by means of the binder, there is also a risk of wrinkling and cracking.

[0023] To reduce the risk of wrinkling and cracking of the coated copper foil 1.1, the anode foil 1, as shown by the Fig. 3 and Fig. 4 is described in more detail below.

[0024] The embedding of silicon-carbon particles S in the copper foil 1.1 is to be largely avoided by means of an additional graphite layer 1.3. The graphite layer 1.3 is arranged between a surface of the copper foil 1.1 and the silicon-carbon-graphite layer 1.2.

[0025] An electrochemically active material of the anode foil 1 is applied to the copper foil 1.1 in two layers. A first layer, in the form of the graphite layer 1.3, is arranged on the surface side of the copper foil 1.1 and contains only graphite particles G as active materials. A second layer, the silicon-carbon-graphite layer 1.2, is arranged on top of the first layer, the graphite layer 1.3, and contains silicon-carbon particles S as active materials. The graphite particles G of the graphite layer 1.3 can become embedded in the copper foil 1.1 during calendering, but only to a relatively shallow degree. Furthermore, the volume increase of graphite during charging of the individual cell is much smaller. Thus, the mechanical stress generated on the copper foil 1.1 is lower and has less of an effect on the copper foil 1.1 and therefore on the anode foil 1. In other words, by means of a material embedded in the Fig. 3 and Fig.In the structure of the anode foil 1 shown in Figure 4, the embedding of silicon-carbon particles S in the copper foil 1.1 can be largely avoided, thereby essentially preventing deformation and cracking of the copper foil 1.1.

[0026] Additionally, to reduce cracking of the anode foil 1 caused by silicon expansion (i.e., an increase in volume) occurring during charging of the individual cell, a copper foil 1.1 of a predetermined thickness can be used. If a copper foil 1.1 of a predetermined thickness is used, where the predetermined thickness exceeds a conventional thickness of copper foil 1.1, calendering can be carried out with a conventionally set calendering force to coat the copper foil 1.1. A comparatively thicker copper foil 1.1 can withstand the mechanical stress generated by silicon expansion and thus largely prevent cracking. In particular, the predetermined thickness can be at least 8 µm to prevent cracking. To still achieve a comparatively high energy density, the predetermined thickness can be at least 4.5 µm and at most 6 µm.

[0027] If a copper foil 1.1 of conventional thickness is used, it may be possible to reduce the calendering force during calendering, thereby reducing the compaction density of the coating of the copper foil 1.1 formed by the graphite layer 1.3 and the silicon-carbon-graphite layer 1.2. In the production of the anode foil 1, a lower calendering force, i.e., a lower calendering pressure, is thus required to achieve a reduced compaction density of the coating, thereby reducing the penetration depth of the silicon-carbon particles S into the copper foil 1.1.

[0028] A drastic reduction in the coating density of the copper foil 1.1 allows for a reduction in the volume increase of the anode foil 1 during a single cell charging process. In particular, the thickness increase of the single cell can be reduced by 42 percent along a vertical axis, with this reduction depending not only on the coating density but also significantly on the silicon content of the coating.

[0029] The pressing density can possibly be reduced to decrease an initial increase in volume of the anode foil 1 and mechanical stress on the copper foil 1.1.

[0030] Furthermore, it may be possible to increase the true density, also called skeletal density, of the silicon-carbon particles S in the silicon-carbon graphite layer 1.2.

[0031] The additional graphite layer 1.3 leaves the volumetric energy density of the individual cell essentially unaffected. Further modifications, such as reducing the electrode density and / or using a thicker copper foil 1.1, can reduce wrinkling and cracking at the expense of the individual cell's volumetric energy density. However, this diminishes the advantages of using active silicon material for the anode foil 1.

[0032] During the charging process of the individual cell, electrons are transferred from the current collector towards the second layer, i.e., the silicon-carbon-graphite layer 1.2, which is the active anode material. The closer to the current collector, the higher the electric current. The graphite layer 1.3 has a comparatively low electrical resistance, which is advantageous in this process. Furthermore, during the charging process of the individual cell, lithium ions are transferred from the electrolyte to the anode foils 1. Therefore, a higher lithium-ion flux is required on each surface side of the anode foil 1. An anode foil 1 with a higher capacitance on the surface side reduces the ion diffusion resistance. Thus, the two-layer coating of the copper foil 1.1 optimizes the charging and discharging rate of the individual cell. Reference symbol list 1 anode foil 1.1 Copper foil 1.2 Silicon-carbon-graphite layer 1.3 Graphite layer S silicon-carbon particles G Graphite particles

Claims

Anode foil (1) for an electrode foil arrangement of an electrochemical single cell, comprising a copper foil (1.1) coated with a silicon-carbon graphite layer (1.2), characterized in that the copper foil (1.1) additionally has a graphite layer (1.3). Anode foil (1) according to claim 1, characterized in that the graphite layer (1.3) is arranged between a surface side of the copper foil (1.1) and the silicon-carbon graphite layer (1.2). Anode foil (1) according to claim 1 or 2, characterized in that a planar dimension of the graphite layer (1.3) corresponds to a planar dimension of the silicon-carbon graphite layer (1.2), wherein the silicon-carbon graphite layer (1.2) is applied congruently to the graphite layer (1.3). Anode foil (1) according to one of the preceding claims, characterized in that the copper foil (1.1) has a thickness between 4.5 µm and 6 µm. Method for producing an anode foil (1) according to one of the preceding claims, wherein a graphite layer (1.3) is additionally applied to the copper foil (1.1). Method according to claim 5, characterized in that the graphite layer (1.3) is applied to a surface side of the copper foil (1.1). Method according to claim 5 or 6, characterized in that the silicon-carbon graphite layer (1.2) is applied in a wet state to the graphite layer (1.3) which is in a wet state. Method according to one of claims 5 to 7, characterized in that a calendering force is set as a function of the thickness of the copper foil (1.1) and / or as a function of a predetermined pressing density of a coating of the copper foil (1.1) formed by means of the silicon-carbon graphite layer (1.2) and the graphite layer (1.3).

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