DE102021211598A1 - Current collector for a battery cell - Google Patents

Abstract

The invention relates to a current collector (16) for a battery cell (2), having a polymer film (22) made of a polymer material (26) and at least one metal layer (20) applied to the polymer film (22), the polymer material (26) having a for the battery cell (2) safety-related function implemented.

Description

The invention relates to a current collector for a battery cell, having a polymer film made from an electrically non-conductive polymer material and at least one metal layer made from a metal material applied to the polymer film. The invention also relates to a battery cell with such a current collector.

Motor vehicles that are driven or can be driven electrically or by an electric motor, such as electric or hybrid vehicles, generally include an electric motor with which one or both vehicle axles can be driven. To supply electrical energy, the electric motor is usually connected to a vehicle-internal (high-voltage) battery system as an electrical energy store.

A vehicle battery is to be understood in particular as an electrochemical battery or what is known as a secondary battery (secondary battery) of the motor vehicle. In such a (secondary) vehicle battery, chemical energy that has been consumed can be restored by means of an electrical (charging) charging process. Such vehicle batteries are designed, for example, as electrochemical accumulators, in particular as lithium-ion accumulators.

In order to generate or provide a sufficiently high operating voltage, such vehicle batteries typically have at least one battery module (battery cell module) in which several individual battery cells are connected in a modular manner. Alternatively, a so-called Cell2Pack design is possible, in which the battery cells are connected directly to the vehicle battery, in particular connected in parallel, and are not combined into modules in advance.

The battery cells are designed, for example, as electrochemical (thin) layer cells. The thin-film cells have a layered structure with a cathode layer (cathode) and with an anode layer (anode) and with a separator layer (separator) introduced in between. These components are permeated, for example, by a liquid electrolyte (liquid electrolyte), which creates an ionic connection between the components or a charge balance.

To produce battery cells, for example, an active material or electrode material layer is applied to a film-like current collector (current conductor, current collector). The current collectors are often designed as metal foils and are used for making electrical contact with the electrode material, with copper foils typically being used for the anode layers and aluminum foils for the cathode layers.

Here and in the following, a “gravimetric energy density” of the battery cell means in particular how much energy can be stored in the battery cell per weight (mass). The gravimetric energy density is given in kilowatt hours per kilogram (kWh/kg). The higher the gravimetric energy density, the lighter a battery cell is with the same amount of stored energy. Battery cells with a high gravimetric energy density are particularly desirable for electromobility.

To improve the gravimetric energy density, it is possible, for example, to make the current collectors thinner and more structured. However, the processing of the increasingly thin metal foils is more difficult, so that waste in battery cell production is increasing because the thin current collectors tear more easily during processing. The structuring of the current collector usually requires an additional process step.

In order to achieve an improved gravimetric energy density of the battery cell, the purely metallic current collector of the anode and/or the cathode can also be replaced by a current collector made of a polymer with a (thin) metal layer, with the safety and service life of the battery cell being optimized by a suitable choice of polymers can be. The materials of polymer or plastic foils have a lower density than a corresponding metal foil, so that a corresponding reduction in structural weight is achieved. In this case, the metal layer or metal coating realizes the electrical contacting. Such current collectors are, for example, in U.S. 2020/0052339 A1 , U.S. 2021/0159507 A1 , and in the US 2021/0159490 A1 disclosed.

To reduce current heat losses, the interconnected battery modules or battery cells have the lowest possible electrical internal resistance. Due to the low internal resistance, however, comparatively high short-circuit currents occur in the event of an electrical short-circuit. Due to the high packing or energy density in vehicle batteries, there is a risk that a self-reinforcing chain reaction will occur, for example due to an internal short circuit or overcharging of the battery cell, which is referred to as thermal runaway becomes.

Thermal runaway is an exothermic chemical reaction in which, on the one hand, a high level of heat energy is released and, on the other hand, a (cell) gas is also formed in the affected battery cells, in particular due to decomposition of the electrolyte, which causes high internal pressure in the battery cell becomes. As a result, the battery cell can deform, burn or even explode in gas pressure. Such a thermal runaway or such a fire due to self-ignition of the vehicle battery can also occur if the battery system is damaged, for example in the course of a vehicle accident or vehicle crash.

In the case of a vehicle battery, therefore, fire prevention or fire protection with regard to self-ignition or thermal runaway is of particular importance with regard to operational safety and general personal protection. The gas flow (gas volume flow) emerging from the battery cells has a high volume flow and a high (gas) temperature. The gas flow contains combustible, conductive and glowing substances/particles. There is a risk that the glowing and conductive particles contained in the gas flow will cause further electrical short circuits if the gas flow hits a high-voltage component, such as a module connector, so that the fire will spread further.

Battery cells therefore often have additional components or parts that implement safety-related functions, particularly in the event of a thermal runaway. However, the additional components or parts disadvantageously reduce the gravimetric energy density of the battery cell.

The invention is based on the object of specifying a particularly suitable current collector for a battery cell. In particular, the gravimetric energy density of the battery cell should be improved. The invention is also based on the object of specifying a particularly suitable battery cell.

With regard to the current collector, the object is achieved with the features of claim 1 and with regard to the battery cell with the features of claim 10. Advantageous refinements and developments are the subject of the dependent claims. The advantages and configurations listed with regard to the current collector can also be transferred to the battery cell and vice versa.

The current collector according to the invention is intended for a battery cell, in particular for a lithium-ion cell, and is suitable and set up for it. Here and below, a current collector is to be understood in particular as a current collector or current conductor, i.e. a cell component of the battery cell to which an active material or electrode material or lithium is applied and which is used for electrical contacting of the active or electrode material or the lithium. Applying lithium is understood here to mean both placing a lithium foil on the current collector and depositing lithium on the current collector.

According to the invention, the current collector is designed as a layered system. In this case, the current collector has a polymer film made of an electrically non-conductive polymer material, and at least one metal layer made of a metal material applied to the polymer film. A polymer material is also to be understood here as meaning a polymer composite material. The metal layer is applied to the polymer film, for example, by electroless galvanic deposition or by means of powder coating. However, the metal layer can also be applied by physical vapor deposition (PVD) or chemical vapor deposition (CVD). The metal material is, for example, copper or nickel when the current collector is for an anode, or aluminum or nickel when the current collector is for a cathode.

In this case, the polymer material has a lower density than the metal material of the metal layer, the film thickness of the polymer film preferably being dimensioned greater than the layer thickness of the metal layer.

For example, the polymer film is coated on both sides with the metal layer. This means that the metal layer is arranged around the polymer film in a closed manner at the edge.

According to the invention, a safety-relevant function for the battery cell is implemented by the polymer material of the polymer film. A "safety-relevant function" is understood to mean a functionality or property of the polymer material or the polymer film that improves the operational safety of the battery cell during operation, especially in the event of a fault, such as thermal runaway.

The structural weight of the current collector is reduced by the polymer foil, with less metal being used than for a metal foil due to the metal layer. As a result, the cell reduces costs in relation to the use of materials, while also increasing the gravimetric energy density of the battery cell. By integrating a safety-relevant function in the current collector, additional components or components for this function can be omitted from the battery cell, so that the gravimetric energy density is further improved. A suitable choice of the processing and properties of the plastic or polymer film can thus be used to implement the desired function in a simple and cost-reduced manner. A particularly suitable current collector is realized as a result.

In a preferred embodiment, the safety-relevant function is a temperature control function, a flame retardant or fire protection function, or a gas absorption function. In other words, depending on the requirement, the polymer material is designed for a temperature control function, a flame retardant or fire protection function, or a gas absorption function.

A “temperature control function” is to be understood here in particular as a heating or cooling function for temperature control of the battery cell, by means of which the battery cell is to be kept at a temperature that is as constant as possible. For example, the polymer material integrated in the current collector dissipates heat when the battery cell heats up during operation, so that the temperature of the battery cell is controlled or heated or cooled.

A "flame protection or fire protection function" is to be understood here in particular as a fire or flame prevention with regard to self-ignition of the battery cell. Here and in the following, a fire is to be understood in particular as a process in which the battery modules and/or the battery cells decompose in an undesired chemical reaction with the generation of heat. Fires in this sense are in particular exothermic chemical reactions of the battery cells, such as a self-reinforcing chain reaction or a thermal runaway. In this case, the polymer material is therefore designed to suppress or prevent fires within the battery cell. For example, the polymer material has fire-retardant or fire-extinguishing properties. For example, a flame retardant is added to the polymer material.

A “gas absorption function” is to be understood here in particular as meaning that the polymer material binds cell gases that occur during operation. The polymer material is therefore a gas scavenger or a gas scavenger material. A “gas scavenger” or a “gas scavenger material” is to be understood here and in the following in particular as a material which absorbs and/or adsorbs cell gases (in particular carbon dioxide and oxygen) that occur, i.e. physically and/or chemically binds them to itself. In other words, a gas scavenger can absorb or adsorb the cell gases that occur or also chemically convert them in a reaction and thus bind them.

During the operation of lithium-ion cells, decomposition reactions occur within the battery cell, which outgas, i.e. produce a gaseous reaction or formation product (degradation product). This gas formation or gas development occurs increasingly during a charging process, in particular during a first charging step. These (cell) gases depend on the electrolyte material, with oxygen (O ₂ ) and carbon dioxide (CO ₂ ) generally being formed, with flammable or harmful gases such as ethene (C ₂ H ₄ ), hydrogen (H ₂ ), or hydrogen fluoride (HF) can be formed.

The resulting gases accumulate inside the battery cell and cause an increase in volume or volume expansion of the battery cell and/or other undesirable side reactions. Battery cells are usually arranged inside a cell housing to protect them from external influences. Multiple battery cells are often stacked to form a cell stack and held in place by a bracing system and accommodated in a common cell housing. The gas-related increase in volume of the or each battery cell can lead to damage (cracks) or complete destruction of the battery or the cell housing, or an intended bursting membrane of the cell housing can open.

The formation of cell gases typically cannot be prevented. The gas absorption function can thus largely avoid or at least reduce the harmful effects of the cell gases, such as a pressure build-up inside the battery cell.

In an expedient embodiment, the metal layer has a certain porosity. This means that the metal layer has a number of pores or recesses (holes, through-openings), so that the metal layer is designed to be permeable or permeable at least partially or in certain areas. As a result, the polymer film is not completely sealed or enclosed by the metal layer, but is at least partially in contact with the environment. As a result, external influences can react with the polymer material, so that the security relevant function is reliably implemented. This is particularly advantageous with regard to the gas absorption function, in which the cell gases can penetrate into the polymer film via the porosity of the metal layer and be bound there.

The diameter of the pores in the metal layer is, for example, a few micrometers (μm) or millimeters (mm). In principle, however, pore diameters in the nanometer range (nm) to the centimeter range (cm), in particular the nanometer range to the micrometer range, are possible. The pore size is, for example, 1 nm to 1 mm, preferably 100 nm to 0.1 mm, particularly preferably 1 μm to 10 μm. The pores in a copper layer have a diameter of about 1 mm (millimeters), for example, with the pore diameter of an aluminum layer being dimensioned to be 0.8 mm to 1.5 mm, for example.

In a conceivable embodiment, the porosity is introduced into the metal layer by structuring. This means that the porosity is subsequently introduced into the metal layer by machining, so the porosity does not already result from the coating of the polymer film with the metal layer. For example, a number of pores, holes or openings are introduced into the metal layer, for example by means of a laser beam or an electron beam. This makes it possible to adapt the porosity in a targeted manner to a respective application or with regard to a safety-relevant property/function of the plastic film that is to be implemented in each case.

Alternatively, the porosity is realized in that no metal layer is applied to the polymer film in certain or predetermined areas. This eliminates subsequent treatment or structuring.

In a suitable embodiment, the metal layer is made as thin as possible, so that the greatest possible weight saving is achieved. In this case, however, the metal layer is sufficiently thick for reliable and operationally safe electrical conduction and contacting. In this case, the metal layer preferably has a layer thickness of between a few nanometers and a few micrometers. For example, the metal layer has a layer thickness of 10 nm to 10 μm, preferably 0.1 μm to 5 μm, particularly preferably 0.5 μm to 3 μm.

In an expedient embodiment, the polymer film is designed to be as thin as possible to reduce weight, but also sufficiently thick to ensure sufficient mechanical stability of the current collector, so that rejects in battery cell production are reduced. In this case, the polymer film also has a sufficient film thickness to reliably and safely implement the desired functionality. Depending on the functionality and polymer material, the polymer film has a film thickness in the nanometer range to the micrometer range. For example, the polymer film has a film thickness of 100 nm to 20 μm, preferably 0.5 μm to 10 μm, particularly preferably 1 μm to 5 μm.

In one configuration, in which the polymer film or the polymer material implements a temperature control function, the polymer material is designed as a phase change material, in particular as a solid-solid phase change material (solidsolid PCM). A solid-solid phase change material is to be understood here as a phase change material which, at a specific temperature, completes a phase change from a solid phase to a (different) solid phase. The phase change material is intended and suitable for absorbing or giving off heat depending on the phase change. Depending on the cell chemistry, the phase transition temperature of the phase change material is set just above the ideal battery cell temperature, or the polymer or phase change material is selected according to its phase transition temperature. For example, the phase change material has a phase transition temperature between 20°C and 80°C, in particular between 30°C and 60°C.

In a conceivable embodiment, the polymer film is made of polyurethane with a film thickness of 5 μm, with an unstructured/non-porous nickel-copper alloy (Ni-Cu) being applied to the polymer film as a metal layer with a layer thickness of 2 μm. The temperature control functionality is designed for a temperature range between 55 °C and 60 °C, for example.

In a further conceivable embodiment, the polymer film is, for example, completely enveloped or surrounded by the metal layer (metal envelope). In this case, a phase change material with a solid-liquid phase change can also be used since the polymeric material cannot leak or leach out. For example, a 10 μm thick polymer film made of paraffin can be used here, since the liquid phase cannot flow away due to the pressure on the layers. The metal layer or metal shell here has a thickness of about 5 μm, with no porosity being provided. This embodiment has a temperature control function in the range between 45°C and 55°C.

In one configuration, in which the polymer film or the polymer material implements a flame retardant or fire protection function, a flame retardant is embedded in the polymer material.

In one possible embodiment, the polymer film is designed as a polyolefin, in particular as a polyethylene, to which a fire retardant or flame retardant, for example aluminum hydroxide, has been added. In this case, the polymer film has, for example, a film thickness of approximately 8 μm, with an approximately 2 μm thick metal layer made of copper being applied. In this case, the metal layer is not structured, for example. The flame protection or fire protection function is realized here in particular by a cooling release of water from the aluminum hydroxide.

In an alternative embodiment that is also possible, the polymer film is made of a polyetherimide. The polymer film preferably has a film thickness of about 5 μm, with an approximately 3 μm thick aluminum metal layer being applied to the polymer film. In this case, the aluminum layer is preferably not structured. The flame retardant or fire protection function is realized here in particular by the fact that the polymer is flame retardant and thus has a fire retardant effect.

In one configuration, in which the polymer film or the polymer material implements a gas absorption function, graphene is embedded in the polymer material.

In one possible embodiment, the polymer film is designed as a 4 μm thick graphene-polymer composite, with polymethylmetaacrylate (PMMA), for example, being used as the polymer. A 3 μm thick aluminum layer, for example, is applied as the metal layer. In this case, the metal layer has a porosity, which is realized, for example, by means of structuring or by means of in the course of the deposition process. In this case, the current collector is flushed with a liquid electrolyte, so that cell gases that are produced can be adsorbed or absorbed by the polymer film or the graphene.

However, gas-absorbing designs without graphene are also conceivable. In an embodiment that is also possible, the polymer film is designed, for example, as a polymer core made from polystyrene, in particular from a polystyrene foam, to which a 2 μm thick copper layer is applied as a metal layer. In the area of the anode, polymer films made from fluorinated polymers are particularly conceivable.

The battery cell according to the invention is in particular a lithium-ion cell. The battery cell is designed, for example, as a prismatic cell, a cylindrical cell, or as a pouch cell. In this case, the battery cell has at least one current collector as described above. In particular, the battery cell has an electrode stack with a number of (anode and cathode) current collectors. The gravimetric energy density of the battery cell is advantageously increased by the current collector according to the invention. This results in a particularly suitable battery cell, in particular for applications in electromobility.

• 1 in perspektivischer Ansicht eine prismatische Batteriezelle, und
• 2 in Schnittansicht eine Batterieelektrode der Batteriezelle.

Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. It shows in schematic and simplified representations:

• 1 a perspective view of a prismatic battery cell, and
• 2 a sectional view of a battery electrode of the battery cell.

Corresponding parts and sizes are always provided with the same reference symbols in all figures.

The 1 shows a prismatic battery cell 2. The battery cell 2 is a lithium-ion cell, for example. The battery cell 2 in this case has a prismatic cell housing (Zellcan) 4, which in the 1 is shown partially open, so that an electrode coil 6 of the battery cell 2 is visible. Instead of an electrode coil 6, an electrode stack is also possible, for example if the battery cell 2 is designed as a pouch or soft bag cell.

The electrode coil 6 is designed as a wound or rolled layer system and has an anode layer 8 and a cathode layer 10 as well as two separator layers 12 , one separator layer 12 being arranged between the anode layer 8 and the cathode layer 10 . The anode layer 8 and the cathode layer 10 here form the electrodes of the battery cell 2, ie as battery electrodes which are connected to the battery poles 14.

The 2 1 shows the structure of the electrode layers 8, 10 in a schematic and simplified representation. The electrode layer has a foil-like current collector 16, to which an active material coating 18 is applied on both sides.

The current collector 16 is here also as a layer system with a both sides with a Metal layer 20 provided polymer film 22 running. In this case, the active material coating 18 is applied to the metal layer 20 . The metal layer 20 is applied to the polymer film 22, for example, by electroless galvanic deposition or by means of powder coating.

In the case of a current collector 16 of the anode layer 8, the metal material 24 of the metal layer 20 is in particular copper, with the metal material 24 being embodied as aluminum when the current collector 16 of the cathode layer 10 is concerned. Accordingly, the active material coating 18 is an anode material for the anode layer 8 and a cathode material for the cathode layer 10.

The polymer film 22 is made from an electrically non-conductive polymer material 26 which has a lower density than the metal material 24 of the metal layer 20 . In this case, the polymer film 22 has a film thickness 28 which is dimensioned greater than a layer thickness 30 of the metal layer 20 .

The metal layer 20 preferably has a porosity 32 with a number of pores or holes 34, which are introduced into the metal layer 20, for example, by subsequent processing or structuring. Due to the porosity 32, the polymer film 22 is in contact with the environment at least in sections. In particular, the polymer film 22 is in contact with a (liquid) electrolyte of the battery cell 2.

The polymer material 26 of the polymer film 22 implements a safety-relevant function for the battery cell 2, in particular a temperature control function, a flame retardant or fire protection function, or a gas absorption function.

For example, the polymer film 22 is designed here as a polyolefin, in particular as a polyethylene, to which a fire retardant or flame retardant, for example aluminum hydroxide, has been added. The polymer film 22 has, for example, a film thickness 28 of approximately 8 μm, with an approximately 2 μm thick metal layer 20 made of copper being applied. In this case, the current collector 16 is designed in particular for the anode layer 8 .

For example, a current collector 16 with a polymer film 22 made of a polyetherimide is provided for the cathode layer 10 . In this case, the polymer film 22 preferably has a film thickness 28 of approximately 5 μm, with an approximately 3 μm thick metal layer 20 made of aluminum being applied to the polymer film 22 .

In an embodiment in which the polymer film 22 or the polymer material 26 implements a temperature control function, the polymer material 26 is designed as a phase change material, in particular as a solid-solid phase change material.

For example, the polymer film 22 is made of polyurethane with a film thickness 28 of 5 μm, with an unstructured/non-porous nickel-copper alloy (Ni-Cu) being applied as the metal layer 20 with a layer thickness 30 of 2 μm. Alternatively, the polymer film 22 is completely encased or surrounded by the metal layer 20, for example. In this case, a 10 μm thick polymer film 22 made of paraffin can be used. The metal layer 20 or metal shell here has a layer thickness 30 of approximately 5 μm, with no porosity being provided.

In an embodiment in which the polymer film 22 or the polymer material 26 implements a flame retardant or fire protection function, a flame retardant is embedded in the polymer material 26 .

In one configuration, in which the polymer film or the polymer material implements a gas absorption function, graphene is embedded in the polymer material, for example.

In one possible embodiment, the polymer film 22 is designed as a 4 μm thick graphene-polymer composite, with polymethylmetaacrylate (PMMA), for example, being used as the polymer. For example, a 3 μm thick aluminum layer provided with porosity 32 is applied as the metal layer 20 .

The claimed invention is not limited to the embodiments described above. Rather, other variants of the invention can also be derived from this by the 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 of the individual features described in connection with the various exemplary embodiments can also be combined in other ways within the scope of the disclosed claims, without departing from the subject matter of the claimed invention.

Reference List

2

battery cell

4

cell case

6

electrode coil

8th

anode layer

10

cathode layer

12

separator layer

14

battery pole

16

current collector

18

active material coating

20

metal layer

22

polymer film

24

metal material

26

polymer material

28

film thickness

30

layer thickness

32

porosity

34

pore

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was 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.

Patent Literature Cited

• US 2020/0052339 A1 [0009]
• US 2021/0159507 A1 [0009]
• US 2021/0159490 A1 [0009]

Claims (10)

Current collector (16) for a battery cell (2), having - a polymer film (22) made of a polymer material (26), and - at least one metal layer (20) applied to the polymer film (22), - Wherein the polymer material (26) realizes a safety-relevant function for the battery cell (2). Current collector (16) after claim 1 , characterized in that the polymer material (26) of the polymer film (22) realizes a temperature control function, a flame retardant or fire protection function, or a gas absorption function. Current collector (16) after claim 1 or 2 , characterized in that the metal layer (20) has a porosity (32). Current collector (16) after claim 3 , characterized in that the porosity (32) is introduced by structuring in the metal layer (20). Current collector (16) according to one of Claims 1 until 4 , characterized in that the metal layer (20) has a layer thickness (30) between 10 nm and 10 µm. Current collector (16) according to one of Claims 1 until 5 , characterized in that the polymer film (22) has a film thickness (28) between 100 nm and 20 µm. Current collector (16) according to one of Claims 1 until 6 , The polymer material (26) realizes a temperature control function, characterized in that the polymer material (26) is a phase change material. Current collector (16) according to one of Claims 1 until 6 , wherein the polymer material (26) realizes a flame retardant or fire protection function, characterized in that a flame retardant is embedded in the polymer material (26). Current collector (16) according to one of Claims 1 until 6 , wherein the polymer material (26) realizes a gas absorption function, characterized in that graphene is embedded in the polymer material (26). Battery cell (2), in particular lithium-ion cell, having at least one current collector (16) according to one of Claims 1 until 9 .

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