DE102016215047A1 - Reference electrode for Li-ion cells - Google Patents

Abstract

The present invention relates to a flexible conductor reference electrode for a lithium-ion cell and a lithium-ion cell comprising the same. The reference electrode according to the invention comprises a foil conductor which comprises at least one electrically conductive foil embedded between an insulating substrate foil and an insulating cover foil, the foil conductor having at least one connection point at which the conductive foil is exposed in order to provide an electrical connection possibility, and also has at least one active site at which the conductive film is exposed, and the exposed location is coated with an active material.

Description

Technical area

The present invention relates to a reference electrode for use in lithium-ion cells and a lithium-ion cell comprising the same.

Technical background

Galvanic cells, such as lithium-ion cells, comprise two working electrodes of different potential, the one with the more negative potential on discharge being the negative electrode (anode) and the other the positive electrode (cathode). The cell voltage corresponds to the potential difference between the electrodes.

The voltage of the battery may decrease with increasing current, depending on the kinetics of the reactions involved, the limited lithium ion conductivity, and other factors. The extent of the voltage drop as a function of the current flow can thereby draw conclusions about the state of the battery (SOH), since, for example, damage or degradation in the electrodes, reduced conductivity of the electrolyte or damage to the separator and the like, in general change the kinetics of cell reactions.

However, if only the voltage between the positive and negative electrodes can be measured, it is not possible to determine whether a voltage drop is due to the positive electrode, negative electrode or other component of the cell.

In order to be able to examine the condition of the two electrodes individually, it is possible to use a third electrode which functions as a reference electrode. Thus, the voltage can then be measured not only between the first and the second electrode, but in addition also between the first electrode and the reference electrode and / or between the second electrode and the reference electrode. The voltage measurement between the reference electrode and the working electrode is ideally currentless. This makes it possible to independently diagnose the state of the first and second electrodes.

Describes this WO 2009/036444 a lithium-ion cell having two working electrodes and one or more reference electrodes, and an associated battery management system. The reference electrode comprises as active material a redox system with a potential of 1 to 4 V with respect to Li / Li ⁺ , as well as a coating of a porous, non-conductive ceramic material such as SiO ₂ , Al ₂ O ₃ , MgO or TiO ₂ and a binder.

Description of the invention

task

One difficulty with the use of reference electrodes is the integrability in the cell. When using a reference electrode based on a wire coated with active material, there is a risk of short circuits with the working electrodes, so that additional separators may have to be provided. The thickness of the reference electrode should be as thin as possible in order to avoid mechanical pressure on the working electrodes and inhomogeneities. In addition, reference electrodes can interfere with the ion flux between the working electrodes. Another difficulty, especially when using thin wires, concerns the handling in the production. The invention has for its object to solve the above problems.

Disclosure of the invention

• – einen Folienleiter, der mindestens eine elektrisch leitfähige Folie umfasst, die zwischen eine isolierende Substratfolie und eine isolierende Deckfolie eingebettet ist, so dass die leitfähige Folie von der Umgebung isoliert ist;
• – wobei der Folienleiter mindestens eine Anschlussstelle aufweist, an der die elektrisch leitfähige Folie freigelegt ist, um eine elektrische Anschlussmöglichkeit bereitzustellen;
• – und der Folienleiter zudem mindestens eine Aktivstelle aufweist, an der die elektrisch leitfähige Folie freigelegt ist, und die freigelegte Stelle mit einem Aktivmaterial beschichtet ist.

According to the task is solved by the use of a flex foil. Flex conductors are foil conductors which, on the one hand, have a very small layer thickness in comparison with a wire of the same cross section, and on the other hand are easy to handle owing to their strip or strip shape. Thus, the invention provides a flex conductor reference electrode comprising:

• A film conductor comprising at least one electrically conductive film embedded between an insulating substrate film and an insulating cover film so that the conductive film is isolated from the environment;
• - wherein the foil conductor has at least one connection point at which the electrically conductive foil is exposed in order to provide an electrical connection possibility;
• - And the film conductor also has at least one active point at which the electrically conductive film is exposed, and the exposed position is coated with an active material.

Description of the drawings

1 1 schematically shows an embodiment of the foil conductor used for the production of the reference electrode (FIG. 1 ) before the formation of the active site in supervision and side view. The foil conductor contains a strip of the electrically conductive metal foil ( 2 ), the connection point ( 3 ) is designed as a solder pad. The active site ( 4 ) is uncovered by cutting off the drawn line. In the side view, the embedding of the conductive foil strip ( 2 ) between substrate film ( 1a ) and cover sheet ( 1b ) of the film conductor.

2 shows the formation of the active site ( 4 by (A) cutting the distal end to expose the cross-sectional area of the conductive foil; (B) etching to ablate the cross-sectional area to rearward the active site relative to the cut edge, and (C) coating with lithium as the active material ( 5 ), to obtain a rearwardly offset active site with protruding substrate and cover sheet. Subsequently, the reference electrode can be integrated into the cell. Alternatively, the lithiation can also take place after integration into the cell by electrochemical deposition.

3 shows the principle of making the in 1 shown foil conductor.

4 schematically shows the integration of the reference electrode in a stacked-type lithium ion cell in pouch construction.

LIST OF REFERENCE NUMBERS

(1)

 foil conductor

(1a)

 substrate film

(1b)

 cover sheet

(2)

 Conductive foil

(3)

 junction

(4)

 active site

(5)

 active material

foil conductor

Film conductors which can be used according to the invention are laminated multilayer films made of an electrically conductive film, in particular of metal, which is embedded between insulating, flexible plastic films. Such film conductors, also commonly referred to as flex conductors, are generally well known in the art and are widely used for electrical or electronic equipment in which the thinnest possible, space-saving, yet easy to handle and mechanically flexible wiring is needed, for example for the Connecting printed circuit boards or connecting displays of laptops and mobile phones. The present invention is based on the idea of providing a reference electrode for a lithium-ion cell on the basis of such a foil conductor.

Usually, the film conductor which can be used according to the invention is in the form of a strip or a strip, wherein one or more strips of electrically conductive film are embedded between substrate and cover film, so that they are electrically isolated from the environment (apart from active and connection point). For the provision of the reference electrode is basically sufficient a single embedded strip of conductive films, which is exposed at the junction or active point. For example, the junction may be at one end of the foil conductor (proximal end) and the active site at the other end (distal end).

In certain cases, for example for the provision of a plurality of measuring points, it is also possible for there to be a plurality of conductive foil strips which are jointly embedded between the substrate foil and cover foil and in turn exposed at the connection points and active sites.

Not necessarily each of the embedded conductive foil strips must be provided with an active site. Alternatively, it is also possible for the film conductor to provide, in addition to one or more conductive film strips provided with active location, further conductive film strips for the connection of other sensors, such as temperature sensors or pressure sensors, in order to be able to supply additional measurement data on the state of the cell in addition to the function as a reference electrode ,

The conductive foil is preferably a metal foil, preferably a metal with high ductility and good electrical conductivity. Examples of suitable metals are copper, nickel, aluminum, or alloys of these. Particularly preferred is copper or a copper alloy. When selecting the material, the potential of the active material to be used must also be considered. For example, aluminum is particularly suitable for active materials having a potential to Li / Li ⁺ of 1 V or more, since alloying with metallic lithium can take place at lower potentials. For active material with potentials to Li / Li ⁺ less than 1 V, copper is preferred. The thickness of the conductive film is generally 100 μm or less, preferably 50 μm or less, still more preferably 30 μm or less. For example, copper foils that can be used are commercially available in thicknesses of about 70 and 30 microns. Usually, the conductive film is used as a strip having a width of 10 mm or less, preferably 5 mm or less, more preferably 2 mm or less, for example, about 1 mm.

The substrate film and the cover sheet may be made of any plastic material that is stable to the electrolyte under the conditions of use. Examples are polyimide, polyethylene naphthalate or polyolefins such as polyethylene or polypropylene. The thickness of the substrate and cover sheets is generally 100 μm or less, preferably 50 μm or less. For example, films with thicknesses in the range of 30 to 50 microns can be used.

To facilitate a reliable and trouble-free integration into the lithium-ion cell, the film conductor is preferably as thin as possible. Considering the above-mentioned thicknesses for the substrate and cover sheets as well as the conductive sheet, the total thickness is typically 300 μm or less, preferably 200 μm or less, more preferably 150 μm or less, and even more preferably 100 μm or less.

As far as the geometric shape is concerned, the foil conductor may, for example, have a substantially rectangular shape. Generally, however, the shape is arbitrary and may, for example, also have bends or angles. If multiple strips of the conductive foil are embedded, they may also be branched or fanned at the distal end to provide multiple measurement sites at different locations. Thus, the shape of the foil conductor is not particularly limited, and may in practice depend on the environment of the insert, the layout of the battery module, and the arrangement of the cells and controllers.

A specific shape of the reference electrode according to a preferred embodiment is shown in FIGS 1 to 4 shown. In this embodiment, the reference electrode has a widened proximal region with the junction and a narrower distal region with the active site, with a tapered transition region. On the one hand, this facilitates handling when connecting the reference electrode and, on the other hand, enables interference-free integration into the cell.

The width of the active site region is usually determined by the width of the conductive foil and may be generally 5 mm or less, preferably 2 mm or less, for example about 1 mm, as in FIG 1 shown. The pad area is either as wide as the active site area, or it can be broadened as in 1 shown. In this case, the width is generally 10 mm or less, preferably 5 mm or less, for example about 2 mm.

Connection point and active point

At the connection point, the strip of the conductive film may be exposed by removing part of the substrate film or the cover film, so that electrical contact with the surface of the conductive film is possible.

The contacting of the conductive foil with the measuring device can take place at the connection point, for example, via a basically known clamp fastening. Such terminals are particularly suitable when the film conductor has a plurality of conductive film strips and is used as a multiple reference electrode and / or in combination with other sensors, and is to be connected directly to the board of a control unit.

Alternatively, the connection point can be designed as a solder joint, which is soldered, for example, with a wire. For this purpose, the strip of the conductive film is preferably widened to a pad. The pad may additionally be coated with a solder, for example tin or a tin alloy.

At the active site, a portion of the conductive foil is exposed and coated with active material. Here, the electrochemically relevant reactions take place during the operation of the reference electrode.

The exposure of the conductive foil can take place over the cross-sectional area, for example at the edge of the distal end. In this case, in a preferred embodiment, a supernatant of the substrate film and the cover film may additionally be provided in order to avoid electrical short circuits between the active site and the working electrodes of the cell or other components and to protect the active site from mechanical damage. Such an embodiment is in the 1 and 2 shown. The exposed cross-sectional area of the conductive foil is offset backwards relative to the edge, so that the protruding part of the substrate foil and the cover foil prevents the exposed active site from coming into contact with the working electrodes to be positioned above and below the foil conductor, as in US Pat 4 outlined. The offset of the active site relative to the edge in this embodiment is, for example, 1 mm or less, preferably 0.5 mm or less.

As an alternative to exposing the cross-sectional area, a portion of the surface of the conductive foil may also be uncovered on one or both sides by removing a portion of the substrate and / or cover foil in order to provide the active site. This design has the advantage that a larger area compared to the cross-sectional area as an active site is available. However, when exposing the surface at most the side facing away from the exposed site is electrically isolated, so that this design is particularly suitable for use environments in which the active site does not need to be isolated (for example, because other cell components such as the separator isolation can provide or there are no electrically conductive components in the vicinity). If necessary, an additional separator film may also be provided for isolation, or the active site may be coated with a porous material.

The active material is not particularly limited, and any material that can absorb or release lithium ions under reduction or oxidation can be used. For example, metallic lithium or materials that can intercalate and deintercalate lithium ions, as typically used in the working electrodes, are contemplated.

Although the voltage measurement ideally takes place without current, small leakage currents which change the charge state of the active material can not always be excluded. Therefore, the active material preferably has a voltage plateau, i. the redox potential is approximately independent of the charge state over a wide range. In view of this, metallic lithium, lithium titanate and lithium iron phosphate are particularly preferred.

The advantage of using metallic lithium is that the active material need not be provided in advance on the active site but can be deposited in situ after incorporation into the cell. For this purpose, the reference electrode is introduced into the cell with an uncoated active site. Subsequently, a current flow between the reference electrode and one of the working electrodes (optionally the positive or the negative electrode) is effected, wherein the reference electrode is connected as a negative pole to deposit metallic lithium on the active site.

Integration in battery modules

The reference electrode according to the invention can be used in combination with any type of lithium ion cells, in particular in cells with liquid electrolyte of the winding or stack type. Due to its small thickness, the reference electrode can be integrated into the interior of the coil or stack between the working electrodes. Alternatively, integration into the cell housing outside of the roll or stack is also possible. Integration into the center of the roll along the winding axis is also possible.

The reference electrode may be configured as a multiple reference electrode. For this purpose, the foil conductor of the reference electrode includes a plurality of active film strips provided with active site, which may be fanned out and placed at different locations in the cell interior to provide multiple measurement points.

Furthermore, as described above, in addition to one or more conductive film strips provided with active location, the film conductor can also provide further conductive film strips for the connection of other sensors, which can then be provided with further sensor elements, such as pressure or temperature sensors, for additional data on the film To deliver the operating state of the cell. In this case, the foil conductor is preferably fanned out or assembled so that the other sensor elements can be mounted at a suitable location inside the cell housing but outside the coil or stack in order to avoid mechanical loading of the working electrodes.

The foil conductor of the reference electrode may be led out of the cell to the outside, for example by the lid or by a sealed seam, and be connected directly to the measuring device, for example via a clamp on the circuit board of a corresponding control device. Such a construction is particularly preferred when the film conductor is provided with a plurality of active sites and sensors.

Alternatively, the film conductor can be completely mounted inside the housing and connected at the connection point with a connection element passing through the cover. Such a construction is particularly suitable for single reference electrodes with only one active point.

example

The present invention will now be described by way of example of a single reference electrode having a solder pad as a terminal and a back-shifted active site attached to the cross-sectional area, as in FIGS 1 to 4 shown.

An exposed cross-sectional area offset rearwardly relative to the edge is obtained by cutting off the foil conductor at the distal end to expose the cross-sectional area of the conductive foil at the level of the edge, and then subsequently eroding a portion of the conductive foil by etching. This procedure is in 2 outlined. Subsequently, at the active site, the exposed cross-sectional area can be coated with metallic lithium as active material, for example by electrochemical deposition. Alternatively, lithium deposition may be dispensed with at this point in time, and instead the lithium is deposited over the reference electrode in the course of start-up of the cell by applying a current.

Due to the offset between the edge and the active site, the active site is protected from short-circuits, but also from mechanical damage, by the protruding substrate and cover foil. On the one hand to minimize the risk of short circuits and on the other hand the best possible Contact the active site with the electrolyte to achieve the offset of the active material loaded active site backwards is usually 0.1 to 1 mm, preferably 0.2 mm to 0.5 mm.

A manufacturing process is in 3 shown. For this purpose, the strips of the conductive film provided with the pad at one end are respectively aligned in an alternating antiparallel manner on the substrate film, so that the other ends project respectively. Subsequently, the cover sheet is laminated and sealed by a method known per se, and the contact pads are exposed. Then, the electrodes are cut out in the manner outlined so that the overhanging ends of the conductive foil are cut off and the cross-sectional area is exposed. The conductor is etched off at the exposed cross-sectional area and coated with lithium as the active material to obtain an active site offset rearward relative to the edge.

The reference electrode can be easily integrated into existing lithium-ion cells, in particular in conventional cells with liquid electrolyte. For this purpose, the reference electrode is introduced between the working electrodes, which can be done relative to the separator on the anode side or the cathode side. The reference electrode according to the invention is suitable both for use in wound-type cells and in stack-type cells. 4 shows as an example schematically the introduction into a stacking cell in bag (pouch) construction.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2009/036444 [0006]

Claims (10)

 Flexible conductor reference electrode for a lithium-ion cell, comprising: A film conductor comprising at least one electrically conductive film embedded between an insulating substrate film and an insulating cover film so that the conductive film is isolated from the environment; - wherein the foil conductor has at least one connection point at which the electrically conductive foil is exposed in order to provide an electrical connection possibility; - And the film conductor also has at least one active point at which the electrically conductive film is exposed, and the exposed position is coated with an active material. The reference electrode according to claim 1, wherein the active site is formed by exposing the cross-sectional area of the electrically conductive film to an edge of the film conductor. Reference electrode according to claim 2, wherein at the active site, the cross-sectional area of the electrically conductive film is offset relative to the edge of the film conductor to the rear. Reference electrode according to claim 1, wherein the active site is formed by exposing the surface of the electrically conductive film by partially removing the substrate or cover sheet.  A reference electrode according to any one of claims 1 to 4, wherein the conductive foil is made of copper or a copper alloy. A reference electrode according to any one of claims 1 to 5, wherein the active material is selected from metallic lithium, lithium titanate and lithium iron phosphate. A reference electrode according to any one of claims 1 to 6, wherein the thickness of the foil conductor is 200 μm or less. A reference electrode according to any one of claims 1 to 7, wherein the foil conductor comprises a plurality of strips of the conductive foil each provided with a junction and an active site. A reference electrode according to any one of claims 1 to 8, wherein the foil conductor additionally comprises one or more strips of the conductive foil which are not provided with an active site and are configured for connection of sensor elements. A lithium-ion cell comprising the reference electrode according to any one of claims 1 to 9.

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