WO2013079570A1 - Electrodes for lithium ion batteries and the production thereof - Google Patents

Abstract

A process for producing electrodes for lithium ion batteries, which comprises a coating step in which a tape-like current collector runs through a coating apparatus in which one or more strips of an electrode material are applied to at least one side of the collector tape (in the direction of travel), where the electrode material is applied to the collector tape in such a way that at least one region of the collector tape remains uncoated beside and/or between the strips, a calendering step in which the electrode material applied to the current collector is compacted in a calendering apparatus and a treatment step in which at least one of the uncoated regions is converted either directly into a longitudinally extended state or into a state in which the peripheral region is more pliable in respect of extension in the longitudinal direction, is described. Furthermore, tape-like current collectors occurring in this process and also an electrode which can be produced by the process are described.

Description

 description

Electrodes for lithium-ion batteries and their manufacture

The present invention relates to a method for the production of electrodes for lithium-ion batteries and to current collectors for such electrodes and to electrodes which can be produced according to the method. The term "battery" originally meant several galvanic cells connected in series, but today also individual galvanic cells are often referred to as "battery." During the discharge of a galvanic cell, an energy-supplying chemical reaction takes place, which consists of two electrically coupled but spatially separated At the negative electrode, electrons are released in an oxidation process, resulting in an electron current via an external load to the positive electrode, from which a corresponding amount of electrons is taken in. At the positive electrode, a reduction process takes place. At the same time, an ion current corresponding to the electrode reaction occurs inside the cell, this ion current being ensured by an ion-conducting electrolyte, whereas in secondary cells and batteries this discharge reaction is reversible, so there is the possibility to reverse the conversion of chemical energy into electrical energy during discharge.

Among the known secondary cells and batteries comparatively high energy densities are achieved in particular of lithium-ion batteries. Lithium-ion batteries often contain a stack of cells consisting of several single cells. However, especially batteries with very high capacities usually have winding cells (coils). These can be produced at a very high speed and, associated with this, at comparatively low costs. Winding cell technology is suitable for the construction of prismatic cells as well as round cells. The cells of a lithium-ion battery are usually a composite of electrode and separator foils with the sequence positive electrode / separator / negative electrode. Frequently, such individual cells are produced as so-called bicellas with the possible sequences negative electrode / separator / positive electrode / separator / negative electrode or positive electrode / separator / negative electrode / separator / positive electrode. The electrodes usually comprise metallic current collectors, which are usually in the form of flat structures. In the case of the positive electrode, these are usually nets or films of aluminum, for example of expanded aluminum metal or of a perforated aluminum foil. On the side of the negative electrode, networks or films of copper are usually used as collectors.

In general, the cells described for lithium-ion batteries are produced in a multi-stage process. Such methods are known, for example, from DE 10 2010 032 770 A1 or from US 2010/0081052 A1.

In most common processes, the mentioned electrode films are produced in a first step and then subsequently combined with one or more separator films to form the mentioned electrode-separator composites. In most cases, electrodes and separators are connected to one another in a lamination step.

To produce the electrodes, a flat strip of a paste-shaped electrode material (in short: electrode paste) is usually applied to a suitable collector. In terms of production technology, this is usually achieved by providing the collectors as quasi-endless bands (in short collector plates) which pass through a coating device in which they are coated with the electrode paste by means of a doctor blade or by means of a nozzle. However, usually only one center strip of the collector belt is coated, in contrast to the strip areas of the collector belts remain free of Electrode material. These uncoated areas can be used to connect current conductors to the collectors.

It is also possible to apply two or more strips of electrode material parallel to each other on a collector belt. In this case, areas of the collector belt generally remain uncoated between the applied strips. In a subsequent singulation step, the strips of the electrode material can be separated from one another by a cut through these uncoated regions, for example by means of a laser.

For improved mechanical and electrical contacting of the electrode components, the coated collector strips are then usually subjected to calendering. Calendering refers to the pressurization of the collector tape coated with the electrode material, whereby two rolls are used which form a gap which is smaller than the starting thickness of the coated collector tape. Through this gap, the coated collector belt is guided and compacted. It is also possible to set specifically defined porosities of the coating.

However, the calender rollers often exert such a strong pressure on the coated collector belt during calendering that the collector belt is deformed, in particular lengthened in the longitudinal direction. The problem with this is that this deformation occurs only in the areas coated with electrode material areas of the collector belt, since only on these areas, the pressure of the calender rolls is transmitted. The areas of the collector belt that are not covered by electrode material can pass undeformed through the calender rollers, since their thickness is less than the width of the gap between the rollers. This is problematic insofar as the ribbon-shaped electrode issuing from the calender can buckle and distort to reduce stresses that occur due to the partial deformation. This can be Working the electrode to electrode winding cause great problems.

The present invention has for its object to find a solution to this problem.

This object is achieved by the method having the features of claim 1 and by the band-shaped current collector having the features of claim 8. Preferred embodiments of the method according to the invention are specified in the dependent claims 2 to 7. Preferred embodiments of the current collector according to the invention can be found in the dependent claims 9 or 10. Also, the electrode with the features of claim 1 1 is encompassed by the present invention. The wording of all claims is hereby incorporated by reference into the content of this specification.

The inventive method is used in particular for the production of electrodes for lithium-ion batteries, ie of batteries, in which during charging and discharging processes lithium ions migrate from one electrode to another.

It always comprises a coating step in which a strip-shaped current collector (in short: collector strip) passes through a coating device in which a strip of an electrode material is applied to at least one side of the collector strip. Preferably strips of electrode material are applied to both sides of the collector belt. Of course, the inventive method also includes the initially mentioned variant, according to which two or more strips of electrode material are applied in parallel side by side on the collector belt. The application of the strip or strips is usually carried out by means of a doctor device or a nozzle. The electrode material must be be provided speaking form, for example as Rakel or printable paste. The corresponding procedural details that must be observed when coating collector tapes with electrode material, are known in the art and need not be explained in detail in the context of the present application.

The current collector used is usually a band of an electrically conductive material, in particular of a metal. Pastes comprising electrochemically active particles, an electrode binder and a solvent or suspending agent are generally used as the electrode material.

With the method according to the invention, both positive and negative electrodes can be produced. In a preferred embodiment of the method according to the invention, the current collector is a current collector made of aluminum and the electrode material is a material for the positive electrode of lithium-ion batteries. In a further preferred embodiment of the method according to the invention, the current collector is a current collector made of copper and the electrode material is material for the negative electrode of a lithium-ion battery.

An example of a material for the negative electrode of a lithium-ion battery would be a paste comprising graphite particles as electrochemical active material, sodium carboxymethylcellulose as electrode binder and possibly carbon black as a conductivity additive. A corresponding paste for the positive electrode could comprise, for example, lithium cobalt oxide as electrochemical active material and also polyvinylidene fluoride as electrode binder and carbon black as conductivity additive.

As in the case of the methods mentioned at the beginning, which are known from the prior art, the electrode material is preferably also applied to the collector band in such a way that at least one region of the collector band is arranged next to and / or between the strip or strips. the uncoated remains. For example, a central region of the electrode strip is coated with an electrode strip, while uncoated edge regions remain next to the electrode strip, which can subsequently serve for electrically contacting the electrode. In the event that two or more strips of electrode material are applied parallel to one another on the collector belt, areas of the collector belt (intermediate areas) usually remain uncoated between the strips, as a rule in addition to the uncoated edge areas.

As a further step, the method according to the invention comprises a laminating step, in which the electrode material applied to the current collector is compressed in a calendering device (calender). Calendering devices and their use for compaction of electrodes are familiar to the person skilled in the art and likewise need not be described in detail in the context of the present application. It is only important that a calendering device is used, in which the coated collector belt is guided through a gap between two calender rolls, which has a gap width which is smaller than the thickness of the electrode material coated collector belt, but larger than the thickness of the (uncoated) collector belt.

In particular, the method according to the invention is characterized by a processing step in which at least one of the uncoated regions of the collector belt, ie

- e.g. one of the uncoated edge areas,

 - e.g. both uncoated edge areas,

- e.g. one or more uncoated intermediate regions between two electrode strips arranged parallel to one another on the collector band or

for example, at least one of the edge regions and one or more uncoated intermediate regions between two electrode strips arranged parallel to one another on the collector strip, is placed either directly in a longitudinally extended state or in a state in which the at least one uncoated area is more resilient to elongation in the longitudinal direction.

More yielding to elongation in the longitudinal direction is intended to mean that the at least one uncoated region can be more easily deformed after forming by a tensile force in the longitudinal direction of the collector belt than before. A longitudinally extended state is to be understood as meaning that the at least one uncoated region is stretched in the longitudinal direction.

The processing step is preferably a forming step, that is to say that preferably the forming of the uncoated edge region takes place. Alternatively or additionally, the processing step may also include a material-removing step and / or a separation process, or the processing step is such a step. A method for producing electrodes with such a processing step is not known from the prior art. The processing step is intended to prevent or at least reduce the stresses occurring during the calendering as a result of the deformation of the region of the collector belt covered with electrode material, so that the aforementioned bulges of the electrode emerging from the calender do not occur. For this purpose, the uncoated regions are to be deformed such that either the deformation of the collector region coated with electrode material leads to a stress compensation or build up in the uncoated regions no tensions due to the longitudinal expansion of the coated collector regions during calendering, as the uncoated regions go along the longitudinal strain can.

In principle, the processing step can take place both before the loading step and parallel to it. It would be conceivable even to follow the processing step to the calendering step. However, the collector belt is preferably subjected to the processing step before it enters the calendering device. In particular, it may also be preferable to subject the collector belt to the processing step before the coating step, so that the strip or strips of the electrode material is or will already be applied to a collector belt that already has at least one longitudinally stretched or longitudinally resilient region having.

Regardless of whether a strip of the electrode material is applied to the collector belt in the middle or whether a plurality of parallel strips are applied, it is preferred that both edge regions of the collector belt are stretched longitudinally in the processing step or transferred to the state more yielding in the longitudinal direction than an elongation the stresses caused by the extension of the central area of the collector belt also have an effect on both sides. In the case of a plurality of strips, the uncoated regions lying between the strips are preferably also lengthwise stretched or transferred into the state that is more flexible in the longitudinal direction than an elongation.

The at least one uncoated region is particularly preferably stretched out of the group comprising rolls and embossing in the longitudinal direction or offset in the longitudinally stretched state. This can be done, for example, by a rolling process (rolling out of the at least one uncoated area) in which the uncoated areas or areas are subjected to pressure, such as the areas covered with the electrode strips during calendering. However, it is not easy to match the print parameters during the calendering and during the forming in such a way that the result is a stress-free electrode. Preferably, therefore, the edge region is processed by an embossing tool such as a gear to a deformation perpendicular to the direction of the Bandes bring about. Edge regions resulting from such a process have in preferred embodiments a wave, triangular, rectangular or sawtooth-like profile (in longitudinal section). Such reshaped areas can easily relax in the longitudinal direction when the electrode material coated central area is stretched or stretched in that direction.

If the uncoated areas are stretched in the longitudinal direction prior to calendering or even before the coating step, either by a rolling process or by deformation perpendicular to the running direction of the belt, this can result in stresses within the collector belt, since the area of the collector belt to be coated or coated itself still undeformed. It is therefore preferable to carry out the forming parallel to the calendering step.

Alternatively or additionally, it may also be preferred to structurally weaken the at least one uncoated region by perforation or by introducing weakening or weakening lines, so that it becomes more yielding in the longitudinal direction compared to elongation. In practice, such a structural weakening, for example by means of punching or drilling tools (introducing a perforation) or embossing tools (introduction of folds or creases) are brought about. Of course, perforations or points of weakness or lines of weakness can also be introduced "non-mechanically" into the collector belt, namely by means of a material-removing process, for example by means of a laser C02 laser can be cut easily.

Collector belts which have at least one first and, in parallel thereto, at least one second strip-shaped region can be obtained by the described processing variants, wherein the belts have a larger one in the at least one first region Have stretchability in the longitudinal direction than in the at least one second region or the at least one first region opposite the at least one second region is stretched in the longitudinal direction. In accordance with the above statements, the at least one first region is a region processed in the processing step in the context of a method according to the invention and the at least one second region is an area which is provided for coating with one or more electrode strips.

Such collector strips preferably have as the first region one or more strip-shaped regions which have weakening points or lines of weakness or are perforated. In a further preferred embodiment, such collector tapes have, as the first region, one or more strip-shaped regions which have a wave-shaped, triangular, rectangular or sawtooth-like longitudinal profile. Such collector strips are also the subject of the present invention.

Electrodes made in accordance with the method of the present invention typically stand out clearly from conventional electrodes over the described characteristically processed portions of their collector tapes. Correspondingly, electrodes which comprise one of the band-shaped current collectors described are also the subject of the present invention. Preferably, they comprise one of the band-shaped current collectors described and at least one strip of an electrode material which is applied to the current collector and at least partially covers the at least one second region. In preferred embodiments, the electrodes according to the invention are in a wound form, in particular spirally wound, So as electrode winding. They are particularly preferably part of a wound electrode-separator composite of areal electrodes and separators with the sequence positive electrode / separator / negative electrode. To produce such a composite, the electrodes and separators can be connected to one another in a lamination step and then wound up, but in modern winding machines these steps can also be carried out simultaneously. Further features of the invention will become apparent from the following description of the drawings, in which the invention is illustrated with reference to schematic sketches. It should be emphasized at this point that all facultative aspects of the method according to the invention described in the present application can be implemented individually or in combination with one or more further features in one embodiment of the invention. The preferred embodiment described below is merely illustrative and for a better understanding of the invention and is in no way limiting.

drawings

1 illustrates the problem underlying the present invention based on a schematic illustration of two calender rolls and a collector strip coated with electrode material passing through the gap between the calender rolls.

The calender rolls 101 and 102 shown in longitudinal section form the gap 103, which is traversed by the collector strip 105 coated on both sides with electrode material 104a and 104b. The terminal edge portions 106a and 106b of the collector belt are uncoated and do not contact the calender rollers 101 and 102 because of their small thickness. Accordingly, the calender rolls 101 and 102 exert pressure only on the region of the collector belt 105 coated with electrode material and can cause them to be deformed. men. Since the collector belt 105 is drawn through the gap 103 between the rollers 101 and 102 and thus a tensile force acts on the collector belt 105, a deformation results, in particular in the longitudinal direction and leads to a corresponding longitudinal expansion of the coated region of the collector belt 105.

Fig. 2 is a plan view of a collector belt processed according to the present invention (schematic representation). This has a second strip-shaped region 201, which is coated with electrode material, and the uncoated first strip-shaped regions (edge regions) 202 and 203. The uncoated edge region 202 has a plurality of perforation lines 204, which are either perpendicular or at an angle between 30 and 60 ° are aligned with the longitudinal direction of the collector belt. If a collector belt treated in this way passes through a calendering device, in which longitudinal stretching of the coated region 201 occurs, the stresses occurring between the coated region and the uncoated region 202 are counteracted by the fact that the latter is caused by the perforation lines 204 in the longitudinal direction (arrow direction). can relax.

In Fig. 3 is a plan view of a processed according to the present invention collector band (schematic representation), are applied to the three parallel strips 301 a, 301 b and 301 c made of electrode material. The collector band has the uncoated edge regions 302a and 302b and, between the strips 301a, 301b and 301c, the uncoated intermediate regions 303a and 303b. The uncoated regions 302a and 302b as well as 303a and 303b have a plurality of perforation lines 304a-d, which are aligned perpendicular to the longitudinal direction of the collector belt. With a tensile load in the longitudinal direction, the uncoated regions generally expand more easily than the regions of the collector band which are covered with the electrode strips 301 a, 301 b and 301 c. A separation of the three electrode strips may be made by cuts along lines 305 and 306.

Claims

claims A method of manufacturing electrodes for lithium-ion batteries, comprising a coating step in which a band-shaped current collector passes through a coating device in which one or more strips of an electrode material are applied on at least one side of the collector band (in the running direction), the electrode material being applied to the collector band in such a way that adjacent and / or at least one area of the collector belt remains uncoated between the strip or strips,  a calendering step in which the electrode material applied to the current collector is compressed in a calendering device, and  - A processing step in which at least one of the uncoated areas is either placed directly in a longitudinally extended state or in a state in which it is more resilient to elongation in the longitudinal direction. 2. The method according to claim 1, characterized in that the processing step upstream of the calendering step, in particular also the coating step, or is performed in parallel to at least one of these steps. 3. The method according to claim 1 or claim 2, characterized in that the processing step comprises a deformation, a material-removing step and / or a separation process. 4. The method according to any one of the preceding claims, characterized in that the one or more uncoated areas be offset by a rolling or an embossing process in the longitudinally extended state. 5. The method according to claim 4, characterized in that the at least one uncoated area after the transfer in the longitudinally extended state has a wave-like longitudinal section. 6. The method according to any one of the preceding claims, characterized in that the at least one uncoated area is structurally weakened by perforation or by introducing weakening points or lines of weakness (folds, fold lines). 7. The method according to claim 6, characterized in that the perforation or the introduction of weakening points or lines of weakness by means of a laser or a punching device. 8. A strip-shaped current collector for electrodes of lithium-ion batteries, which has at least one first and, in parallel, at least one second strip-shaped region, wherein the current collector in the at least one first region has a greater extensibility in the longitudinal direction than in the at least a second region or the at least one first region is stretched in the longitudinal direction relative to the at least one second region. 9. belt-shaped current collector according to claim 8, characterized in that it comprises as a first region one or more strip-shaped areas having weakening points or lines of weakness and / or perforated. 10. Band-shaped current collector according to one of claims 8 or 9, characterized in that it has as a first region one or more strip-shaped regions which have a wave, triangular, rectangular or sawtooth-like longitudinal profile. 1 1. An electrode for a lithium-ion battery, comprising a band-shaped current collector according to any one of claims 8 to 10, and at least one strip of an electrode material which is applied to the current collector and at least partially covers the at least one second region.