Patent Application
20230187636
This application claims the benefit under 35 USC § 119(a)-(d) of German Application No. 10 2021 133 008.4 filed Dec. 14, 2021, the entirety of which is incorporated herein by reference.
The present invention relates to a method for producing an electrode, more particularly, a negative electrode, for lithium-ion batteries, and also to such an electrode and to a battery cell.
WO 2019/166 899 A1 from the prior art discloses a method enabling production of negative electrodes for lithium-ion batteries, by coating a copper carrier foil with a paste containing graphite particles. These particles are oriented in a temporally and/or locally mutative magnetic field to shorten the distances the ions have to travel during charging/discharging, with possible resultant advantages such as reduced cell resistance and higher discharge rates, a shorter charge time or reduction in heat given off by the cells. With conventional electrodes, however, it is possible in certain cases for the local contact resistance between coating and copper foil to be surprising high.
It is an object of the present invention conversely to provide a method which allows the production of electrodes with improved quality but without performance loss.
In the production of such electrodes, for example, the carrier foils are coated generally with a layer of a suspension which as well as graphite particles includes carboxymethylcellulose (CMC) and a styrene-butadiene rubber (SBR binder). CMC is a surface modifier which allows effective dispersing of the particles in water but which also modifies the rheology and so enables a paste of stable viscosity to form, that can be applied optimally to the foil without bubbles, but does not allow sedimentation. The SBR binder ensures adhesion and elasticity of the coating. In certain cases, especially when silicon/silicon oxide-containing negative electrodes are used, a polyacrylic acid-based binder system may also be employed. The particles are subsequently aligned, generally in a temporally and/or locally mutative magnetic field. After the alignment, or often even during the alignment, a drying process begins, for drying and fixing the layers on the carrier foil. The coated carriers, finally, are rolled. The present patent application is concerned here essentially with a coating operation of this kind for the coating of the carrier foil.
For the specific application of a lithium-ion battery (or lithium-ion storage battery), for example, the anode comprises a graphite layer into which lithium ions undergo intercalation. During discharging, this assembly gives up electrons, which flow to the cathode via the external current circuit to be fed through the cell. At the same time, lithium cations from the intercalation layer move through the electrolyte of the cell to the cathode. In order to be able to recharge the storage battery later, the operation is reversed, and so the lithium cations have to move from the cathode back in the direction of the anode. The layered structure of the graphite used is composed of graphite particles, often present in platelet form. Following the application of the graphite layers, there is usually a parallel alignment of the graphite particles to the surface on which they have been applied. As the lithium cations move through this layer, the lithium ions have to travel around these platelets, so leading to tortuous pore channels and to comparatively long pathways on diffusion of the lithium ions.
With the method of the present invention, accordingly, the initial starting point is a carrier in the form of a carrier foil. This may, for example, be a copper foil. According to the present invention, this foil is provided with a multiple coating comprising at least two layers. The flat, foil-like carrier is coated at least on one side of the two sides. More particularly, the layers are applied separately—that is, there is no application of a layer which splits into, for example, two or more phases only as a result of subsequent treatment after the coating process.
The first layer, applied directly on the carrier surface, consists of a first coating material and has functions including that of an adhesion layer. It is, therefore, possible to prevent the coating inadvertently detaching. In addition, a consistently low contact resistance between carrier foil and coating is enabled. The second layer, which is applied in turn on the first layer, consists of a second coating material. It may, for example, critically determine the ionic resistance.
The quality of the electrodes to be produced can be improved, consequently, because of the marked reduction in the likelihood of detachment of the coating.
Depending on specific application, the multiple coating may also comprise more than two layers, allowing the ionic resistance of the electrode to be optimized, in particular. With preference three layers may be provided, and may be configured in such a way that the adhesion properties with respect to the carrier foil increase gradually, while the ionic flow to the side remote from the carrier becomes higher and is improved.
In one exemplary embodiment, for instance, it is conceivable for the first layer applied to be an adhesion layer, which has no paste comprising graphite particles, instead comprising initially a dispersion, more particularly, a polymer dispersion, which can be used as a bonding layer, for advantageously providing the coating with particular stability.
A layer, more particularly, the layer envisaged as an adhesion layer and applied in the form of a (polymer) dispersion, may preferably further comprise conductive carbon black or other additives for increasing the conductivity as well.
At least one of the coating materials may take the form of a paste comprising particles, more particularly graphite particles, and the particles may be aligned for improved conduction of the ions. Two or more coating materials composed of a paste with particles may already differ in that the particles in the respective layers have a different shape or size and/or a different volume on average by comparison with one another.
Hence in one variant embodiment (even without a polymer-based adhesive), the adhesion in the first layer, applied directly on the carrier, may already be improved in that this layer contains spherical particles or optionally these particles at least on average are more spherical than the particles of the mixed layer located above it. To be expected in particular is that a layer composed of a paste comprising flakelike particles oriented perpendicularly to the carrier surface is not so resistant and stable in its adhesion as a layer comprising spherical particles, because spherical particles offer a larger contact area. Flakelike particles have a more strongly anionotropic shape than spherical particles. The particles in the layer over the adhesion layer composed of spherical particles may, for example, be flakelike in configuration and may be oriented later in a locally or temporally mutative magnetic field in order to shorten the distances traveled by the flowing ions.
If it is important, in one embodiment of the present invention, to minimize the cell resistance by means of a first adhesion layer as well, then this layer may also contain flakelike particles on the carrier surface. The layers differ, for example, in the size and/or volume of the flakelike particles. Because of the larger particles contained therein, therefore, the top layer or top layers also have a greater pore volume. Moreover, polarization effects can be prevented (less lithium plating). More rapid charging of the cell is enabled as well. The particles in the individual layers may additionally have different alignments. Depending on their size, the effectiveness in the context of the alignment of the particles in the magnetic field may change, thus producing, in terms of the particle alignment, an anisotropy of the layers comprising particles of different sizes.
Depending on variant embodiment, accordingly, distinguishing features between the layers may be as follows:
shape of the particles, in particular spherical through to flakelike, and/or
the size and/or volume of the particles, and/or
the alignment of the particles.
The adhesion may be influenced by the size of the particles, since smaller particles are generally associated with an increase in the contact area. Instead, an increase in the adhesion may in turn be achieved by using a polymer dispersion as adhesive.
There may be a tradeoff between the use of spherical particles with greater adhesion effect and particles having good conduction properties for conducting the ions in that the layer in contact with the carrier may be provided with flakelike particles which, however, are smaller or have a smaller volume than the flakelike particles in the layer located above it:
on the one hand, in the case of aligned flakes, especially in the case of an alignment perpendicular to the carrier surface, the distance traveled by the ions through the layer is reduced and hence the cell resistance is reduced, the charge time is shortened, the discharge power is increased, and the heat given off is reduced.
On the other hand, smaller particles can, in principle, be packed more closely and enable a larger contact area with greater adhesion.
In one particularly preferred development of the present invention, the carrier is coated with at least three layers, and more preferably at least one of the layers takes the form of an adhesion layer. For this purpose, the adhesion layer may be configured either as a polymer dispersion having bonding properties, or a paste is used comprising particles, thus having an active material, where the particles are smaller than in another layer composed of a paste comprising particles and/or are spherical in form, so that they are arranged more closely packed and/or form a larger contact area. In order to be able to prevent detachment directly from the carrier, the adhesion layer may be located directly on the carrier. Alternatively, it is conceivable for an adhesion layer, with or without particles, to be arranged between two layers with active material, in order to produce a layered construction with a greater level of stability.
For the purpose of increasing the conductivity, advantageously, a layer, more particularly of a particle-comprising paste, may be admixed with an admixture composed of conductive material, such as conductive granules, more preferably of conductive fibers or carbon nanotubes. Immediately above the carrier foil, such an admixture for increasing the conductivity may be omitted. This first layer may have a thickness in the region of a few micrometers, more particularly about 1 μm.
In the coating process, the layers in the case of one embodiment of the present invention may in principle be applied sequentially. Since the paste is applied per layer but is only dried later during and/or after the alignment of the particles, the layer already applied may be distorted at the upper interface in the case of sequential application. This distortion effect may at least be reduced if the layers are applied simultaneously. This can be done using a nozzle which has a plurality of channels in succession in the direction of application, so that the channel which is first in the direction of application applies the bottommost layer, the subsequent channel the layer lying above it, and so on. As a result of the temporally parallel application it is also possible to shorter the production time.
The advantages of the present invention can be utilized in a corresponding electrode or cell.
Exemplary embodiments of the present invention are represented in the drawings and are elucidated in more detail below with indication of further details and advantages.
The embodiment of the electrode E2 according to
The layers 1 and 2 or 1a and 2 may be applied by means of a specific nozzle 10, as also represented in
The first layer I on the carrier T has the function of an adhesion layer. The two layers II and III above it possess graphite particles, the sizes of which increase in average from layer I to layer III. The pore structure is improved, with increasingly larger pores as the distance from the carrier goes up. The ion flow as well can be channeled more effectively in this way. Layer II comprises spherical particles, which by virtue of the large contact area are also able to make a large contribution to the adhesion. Arranged in the third layer III above, in turn, are flakelike particles, which have been oriented substantially perpendicularly to the surface of the carrier T. An embodiment of this kind is able to offer a particularly stable construction.
In a further variant embodiment of the present invention, the first layer I may instead be configured as a layer comprising active material, hence possessing graphite particles. In order to obtain good adhesion properties, the graphite particles of the first layer I may, for example, be small and spherical, thus affording a large contact area and hence an effective hold. Conductive carbon nanotubes may be integrated in this layer, for example, according to the adaptation of the conductivity properties.
Furthermore, a further layer II may have been applied, comprising flakelike particles, or an adhesion layer composed of a polymer dispersion, for example, while large, flakelike particles are arranged in turn as a third layer III. The alignment of the flakelike particles shortens the distance the ions must travel during charging or discharging, with possible advantageous consequences for the performance of a battery, for the charge time, and for a reduction in the heat produced.
Frequently, however, pastes with particles are used that also comprise nanotubes or other conductive material. In this case, in one embodiment, at least two layers possess the same concentration of conductive material.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2021 133 008.4 | Dec 2021 | DE | national |
