Free-standing electrode membranes for electrochemical cells and methods of making same

Abstract

A free-standing electrode membrane (16) for an electrochemical cell (12) includes a binder system having a first binder (20) and a second binder (22), where the first binder (20) is a fluorine-free binder and the second binder (22) is a fluorine-free binder, and an active material (18) housed in the binder system. And wherein the second binder (22) is a fibril-free binder having an average particle size d50 in the range of 40 to 200 nm. The binder system is free of a binder based on polytetrafluoroethylene (PTFE) and / or cellulose. The invention further relates to an electrode (10), to an electrochemical cell (12), and to a method for producing a free-standing electrode membrane (16).

Description

Technical Field

[0001] The present invention relates to a freestanding electrode membrane for an electrochemical cell, an electrode for an electrochemical cell having such an electrode membrane, an electrochemical cell having such an electrode, and a method for preparing the freestanding electrode membrane. Background Technology

[0002] To prepare electrodes for electrochemical cells, methods are known in the prior art that involve applying a solvent-based coating material (a so-called "slurry") onto a current collector (e.g., a metal current collector foil), wherein the coating material comprises an electrochemically active material, an electrode binder, and additives (such as conductive additives and a carrier solvent). Such methods require a drying step to remove the solvent used. This results in additional costs and significant technical and energy consumption.

[0003] Freestanding electrode films are known from US 10 741 843 B2, prepared by a method that forgoes the use of a carrier solvent. This electrode film comprises a composite binder containing polytetrafluoroethylene (PTFE) and poly(ethylene oxide) (PEO), which are fibrillable or form fibrils. However, the use of PTFE is associated with high cost due to its high fluorine content. Furthermore, PTFE-based binders are not chemically, and especially electrochemically, stable enough in all applications. For example, using a PTFE-based binder in the anode of an electrochemical cell based on lithium-ion graphite or silicon or metallic lithium may cause PTFE to decompose into lithium fluoride (LiF) and amorphous carbon. For example, as described in Guobao Li et al., “The influence of polytetrafluorethylene reduction on the capacity loss of the carbon anode for lithium ion batteries” (Solid State Ionics, Vol. 90 (1-4), pp. 221-225, 2019, doi:10.1016 / S0167-2738(96)00367-0), PTFE, used as a binder in the electrodes of lithium-ion batteries, makes an irreversible contribution to the formation loss of lithium-ion batteries because lithium present in lithium-ion batteries is partially converted into lithium fluoride (LiF) by reacting with PTFE and is thus consumed. This is associated with the irreversible capacity loss of electrochemical batteries, which reduces the energy density or specific energy of the electrochemical batteries. In addition, from an ecological perspective, PTFE is increasingly undesirable because the preparation process of PTFE relies on toxic chemicals and PTFE can only degrade extremely slowly in the environment.

[0004] US 11,545,666 B2 also discloses dried-processed, freestanding composite electrode membranes having both profibrillable and particulate nonfibrillable binders. The profibrillable binder is traced back to PTFE, while the nonfibrillable binder is cellulose-based. However, the use of cellulose-based binders results in unsatisfactory adhesion between particles (especially active materials and conductive additives) within the electrode membrane and between the composite electrode membrane and conventional current collectors. Summary of the Invention

[0005] Therefore, the object of the present invention is to provide freestanding electrode films and electrodes and electrochemical cells comprising said freestanding electrode films, which have mechanically and chemically, especially electrochemically, stable electrode films and good adhesion properties, and can be prepared, in particular, by a dry processing method. In particular, the use of PTFE should be eliminated.

[0006] The object of this invention is achieved by a freestanding electrode membrane for an electrochemical cell, the freestanding electrode membrane comprising a binder system having a first binder and a second binder, and an active material contained within the binder system, wherein the first binder is a fluorine-free binder, and wherein the second binder has an average particle size d in the range of 40 to 200 nm. 50 A non-fibrillated binder. The binder system does not contain binders based on polytetrafluoroethylene (PTFE) and / or cellulose.

[0007] This invention is based on the fundamental concept of providing a stable and particularly dryable electrode membrane by utilizing a combination of fibrillation-forming binders and non-fibrillation-forming binders. To this end, the non-fibrillation-forming binders are present in the electrode membrane with a particle size at which they can function as a dot-like adhesive, additionally fixing other components present in the electrode membrane to each other, in addition to the fibrillation-forming binders. Correspondingly, according to the invention, the non-fibrillation-forming binders are not present in the active material as a coil surrounding the active material and other components (such as conductive additives) in the electrode membrane.

[0008] Furthermore, the present invention eliminates the use of binder systems based on PTFE, cellulose, or a combination of PTFE and cellulose. The electrode membrane according to the present invention correspondingly does not contain PTFE or cellulose. This results in improved adhesion between the components of the electrode membrane compared to cellulose-containing electrode membranes, while simultaneously avoiding the use of ecologically problematic PTFE. Additionally, there is no concern about the decomposition of PTFE within the electrode membrane, which advantageously affects the performance, energy density, and reliability of electrochemical cells using the electrode membrane of the present invention. The improved adhesion of the binder system used according to the present invention also enhances the performance, lifespan, and reliability of the electrochemical cell.

[0009] "Freestanding electrode membrane" should be understood herein and hereinafter as a mechanically shaped, especially self-stable, electrode membrane that can be operated and handled independently, that is, it does not necessarily have to be applied to an additional carrier material (e.g., a polyester film), and especially does not necessarily have to have been applied to a current collector foil.

[0010] The difference between a binder that forms fibrils and one that does not is that a binder that forms fibrils forms fibrils under shear force. In particular, as the shear force increases and the duration of shear force action on the binder increases, a greater number of fibrils and / or fibrils with greater length are produced.

[0011] Average particle size d 50 This represents the number mean of the particle size distribution, where 50% of the particles are larger than this value (d). 50 The value is 50% and 50% of the particles are smaller than this d. 50 The particle size can be determined through image analysis using direct light microscopy and optical microscopy.

[0012] The fluorine-free fibrillating binder is particularly a high molecular weight polyolefin, preferably high molecular weight polyethylene (PE). It has been shown that high molecular weight polyolefins, especially high molecular weight polyethylene, possessing the fibrillability required for preparing electrode films are mechanically stable in electrochemical applications and chemically compatible with materials commonly used in electrochemical cells.

[0013] It is easy to understand that this also includes copolymers of high molecular weight polyolefins used accordingly, such as copolymers of high molecular weight polyethylene.

[0014] High molecular weight polyolefins, in particular, have a crystallinity of 60% or higher. Crystallinity can be determined by dynamic differential calorimetry (DSC) using the enthalpy of melt of the high molecular weight polyolefin.

[0015] The thickness of the high molecular weight polyolefin is particularly between 0.920 and 0.996 g / cm. 3 Within the range.

[0016] The high molecular weight polyolefin may have a molecular weight M in the range of 1,000,000 to 9,000,000 g / mol. n The high molecular weight polyolefin may correspond to ultra-high molecular weight polyolefin, such as PE-UHMW. The molecular weight can be determined by gel permeation chromatography (GPC), especially according to DIN EN ISO 16014-5.

[0017] The non-fiber-forming binder may be selected from the group consisting of polyvinylidene fluoride (PVDF), polar-modified PVDF (e.g., as described in EP 2 147 029 B1), PVDF copolymers, and combinations thereof. PVDF and PVDF copolymers such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVFD-HFP) exhibit the desired adhesion behavior to the non-fiber-forming binder when preparing the electrode membrane, i.e., are particularly suitable as point-to-point adhesives to create “point-to-point” bonded connections between the components of the freestanding electrode membrane.

[0018] Compared to PTFE, PVDF, polar-modified PVDF, and PVDF copolymers have lower fluorine content, enabling more cost-effective preparation methods that require less toxic chemicals compared to PTFE. Furthermore, PVDF, polar-modified PVDF, and PVDF copolymers degrade better than PTFE, do not exhibit swelling behavior in liquid electrolytes, are hydrophobic, and provide high mechanical strength and good adhesion between the electrode and the corresponding electrode film.

[0019] To further improve the adhesion of the components forming the electrode film, the melting point of the non-fiber-forming binder can be in the range of 130 to 180°C.

[0020] In one variant, the non-fibrillating binder has an average particle size d in the range of 40 to 200 nm. 50 In this way, a particularly good and customized dotted bond can be achieved using a second adhesive.

[0021] The binder system may contain up to 50% by weight, particularly 5 to 30% by weight, and preferably 10 to 25% by weight, of a second binder, in each case relative to the total weight of the binder system. At a second binder content greater than 50% by weight, the mechanical stability of the freestanding electrode membrane may decrease again because there is less and less first binder in the binder system, causing the particles of the active material to be held together by the fibrils provided by the first binder.

[0022] The adhesive system is particularly composed of the first adhesive and the second adhesive.

[0023] The weight ratio of the first adhesive to the second adhesive can be in the range of 50:50 to 95:5, preferably 75:25 to 90:10.

[0024] In order to provide sufficient stability for the freestanding electrode membrane on the one hand, and to avoid excessively negatively impairing the specific energy density of the electrochemical cell having the freestanding electrode membrane on the other hand, the binder system is present in the freestanding electrode membrane, particularly in a weight percentage of 0.25 to 5, preferably in a weight percentage of 0.5 to 3, relative to the total weight of the freestanding electrode membrane.

[0025] According to the present invention, the type of active material is not further limited and all materials known in the prior art can be used.

[0026] It is readily understood that the freestanding electrode membrane may have at least one active material contained in the binder system, that is, it may also have more than one active material. For example, a blend formed of multiple active materials may be used.

[0027] If the electrode film is used in the anode, the active material can be selected from the group consisting of: carbon-containing materials, silicon, nano-silicon, silicon composite materials, low-valence oxides of silicon, silicon alloys, lithium, lithium alloys, aluminum alloys, indium, indium alloys, tin, tin alloys, cobalt alloys, niobium pentoxide, titanium dioxide, titanates such as lithium titanate (Li4Ti5O) 12 Tin dioxide and mixtures thereof. The active material is preferably selected from the group consisting of: synthetic graphite, natural graphite, graphene, mesophase carbon, doped carbon, hard carbon, soft carbon, fullerene, silicon-carbon composites, silicon, surface-coated silicon, low-valence silicon oxides, silicon alloys, lithium, aluminum alloys, indium, tin alloys, cobalt alloys and mixtures thereof.

[0028] If, conversely, the electrode film is used in the cathode, the active material can be selected from the group consisting of: lithium transition metal oxides such as lithium cobalt oxide (LiCoO2), lithium nickel cobalt manganese compounds (known by the abbreviations NCM or NMC), lithium nickel cobalt aluminum oxide (NCA), lithium olivine (such as lithium iron phosphate (LFP)), lithium spinel (such as lithium manganese oxide spinel (LMO)), or combinations thereof. So-called overlithiated layered oxides (OLO) can also be used.

[0029] In addition to the binder system and the active material, the freestanding electrode membrane may also contain additives, such as conductive additives.

[0030] The conductive additive can be selected from the group consisting of: conductive carbon black, carbon nanotubes, graphene, graphite, expanded graphite, carbon nanofibers (CNTs), especially vapor-phase prepared carbon nanofibers (VGCF), porous carbon, and combinations thereof. The conductive additive can improve the conductivity of the freestanding electrode membrane, thereby resulting in improved power performance of the freestanding electrode membrane, as well as electrodes and electrochemical cells having such electrode membranes. Additionally, the additive can improve the processability of the freestanding electrode membrane. Carbon-based conductive additives can also improve mechanical properties, such as tear resistance.

[0031] Preferably, the freestanding electrode membrane is prepared by a dry processing method. This means that the freestanding electrode membrane is preferably obtained using dried particles, thereby eliminating the time-consuming, costly, and energy-intensive drying step. The freestanding electrode membrane is particularly free of carrier solvent. The object of the invention is also achieved by an electrode for an electrochemical cell comprising the freestanding electrode membrane described above.

[0032] The features and properties of the freestanding electrode membrane according to the invention are correspondingly applicable to the electrode according to the invention, and vice versa, with reference to the detailed description above.

[0033] The electrode can be an anode or a cathode.

[0034] In addition to the freestanding electrode membrane, the electrode also has a current collector, particularly a current collector foil, onto which the freestanding electrode membrane is applied.

[0035] The current collector is, for example, a solid foil made of aluminum or copper (referred to as "solid foils" in English). It is also possible to use a porous current collector, such as a metal mesh or a metallized fabric foil.

[0036] Furthermore, the objective of this invention is achieved by an electrochemical cell having the aforementioned electrodes.

[0037] The features and properties of the electrodes according to the invention are correspondingly applicable to the electrochemical cells according to the invention, and vice versa, with reference to the detailed description above.

[0038] The electrochemical cell has a cathode and an anode as electrodes, wherein at least one of these electrodes is an electrode as described above.

[0039] A diaphragm is also arranged between the cathode and the anode. The type of diaphragm is not further limited, so all known diaphragms can be used, provided that they are chemically, and especially electrochemically, compatible with the electrode according to the invention.

[0040] Electrochemical batteries, especially lithium-ion batteries.

[0041] The term "lithium-ion battery" is used as a synonym for all names commonly used in the prior art for lithium-containing galvanic elements and batteries, such as lithium battery, lithium cell, lithium-ion cell, lithium-polymer cell, lithium-ion battery cell, lithium-ion polymer cell, and lithium-ion rechargeable battery. This includes, in particular, rechargeable batteries (secondary batteries). The terms "battery" and "electrochemical cell" are also used as synonyms for the terms "lithium-ion battery" and "lithium-ion cell."

[0042] However, it is also possible that the electrochemical battery is a different type of electrochemical battery from lithium-ion batteries, such as a sodium-ion battery.

[0043] The object of the present invention is also achieved by a method for preparing a freestanding electrode membrane for an electrochemical battery, the method comprising the steps of: providing a first binder, a second binder, and an active material, wherein the first binder is a fluorine-free, fibrillating binder and the second binder is a non-fibrillating binder, and wherein the first binder and the second binder are free of binders based on polytetrafluoroethylene (PTFE) and / or cellulose. The first binder, the second binder, and the active material are mixed to form an intermediate electrode membrane, and the intermediate electrode membrane is pressed into the freestanding electrode membrane.

[0044] The freestanding electrode membrane that can be obtained by the method of the present invention corresponds particularly to the freestanding electrode membrane described above, and with reference to the above-described features and properties, these features and properties are also similarly applicable to the method of the present invention.

[0045] It is readily understood that other components may be added during the mixing of the first binder, the second binder, and the active material. Preferably, additives, such as conductive additives, are provided and are incorporated into the intermediate electrode film in the same mixing step.

[0046] Basically, methods for preparing freestanding electrode films are known in the prior art. For example, the preparation of the freestanding electrode film is similar to the preparation method described in US 11 545 666 B2, however, it utilizes the first and second binders according to the invention.

[0047] High molecular weight polyolefins, preferably high molecular weight polyethylene, are provided as the first binder. The high molecular weight polyolefins can be provided in powder form, which are fibrillated by the shear forces that occur during mixing and / or pressing.

[0048] Powders formed from high molecular weight polyolefins can have an average particle size d in the range of 3 to 500 μm. 50The average particle size is determined, in particular, by means of laser diffraction or sieving analysis, for example, according to ISO 13320 or ISO 2591-1.

[0049] The high molecular weight polyolefin does not exhibit "swelling" behavior with the liquid electrolyte, is hydrophobic, and contributes to the mechanical strength and good binding of the freestanding electrode membrane. The polyolefin is chemically and electrochemically stable under normal operating conditions of the electrochemical cell and is compatible with other commonly used components.

[0050] The mixing of the first binder, the second binder, and the active material can be carried out using a "low-shear" mixing process or a "high-shear" mixing process, i.e., using a mixing process with lower or higher shear forces. The choice of shear force used during mixing allows for targeted influence on fibril formation through the first binder. For example, using higher shear forces and / or longer mixing times can help form fibrils, thereby improving the mechanical stability of the intermediate electrode film and matching the operability required for other processing steps.

[0051] The intermediate electrode film is pressed into the freestanding electrode film, especially by means of calendering, for example by means of heated calendering rolls.

[0052] Preferably, the shear force applied during mixing of the first binder, the second binder, and the active material is matched to the calendering conditions in order to produce the desired fibrillation behavior of the first binder and the desired adhesive behavior of the second binder. For example, the smaller the shear force used during mixing, the higher the rolling pressure and / or rolling temperature can be selected during calendering.

[0053] To achieve dotted adhesion using a second adhesive, the intermediate electrode film can be heated to a temperature in the range of 100 to 180°C, preferably 110 to 160°C. Heating can be performed during the pressing process using heated calendering rolls to form a freestanding electrode film.

[0054] Other features and performances will be derived from the following description of exemplary embodiments and the accompanying drawings, and these embodiments should not be construed as limiting. Attached Figure Description

[0055] In the attached diagram:

[0056] - Figure 1 The electrode according to the invention, formed as the anode, is schematically shown, and

[0057] - Figure 2 An electrode according to the invention, formed as a cathode, is schematically shown.

[0058] Figure 1 An electrode 10 according to the invention is schematically shown, which is used in an electrochemical cell 12 according to the invention.

[0059] Electrode 10 has a current collector 14 on which a freestanding electrode film 16 according to the invention is applied.

[0060] The current collector 14 is, for example, a copper foil with a thickness of about 10 μm or an aluminum foil with a thickness of about 15 μm and is used as the current collector of the electrode 10.

[0061] The freestanding electrode membrane 16 contains active material 18, whose particles are connected to each other by means of a binder system.

[0062] The independent electrode film 16 may also include (in) Figure 1 Other components (not shown in the schematic diagram), especially additives, such as conductive additives, for example, carbon nanotubes (CNTs).

[0063] The adhesive system comprises a first adhesive 20 and a second adhesive 22 and does not contain adhesives based on polytetrafluoroethylene (PTFE) and / or cellulose.

[0064] The first binder 20 is a fluorine-free binder that forms fibrils, such as a high molecular weight polyolefin, like high molecular weight polyethylene (PE). Figure 1 The binder forming the fibrils is schematically shown only as fibrils that connect the particles of active material 18 and extend substantially in a straight line. It is easy to understand that the first binder 20 can wrap around the particles of active material 18 in the sense of "wrinkled clumps of thread" to connect them to each other.

[0065] Conversely, the second binder 22 has an average particle size d of 20 nm to 5 μm, preferably 50 to 150 nm. 50 The smaller size of the second adhesive 22 means that it is not aggregated around the particles of the active material 18 in the form of a coil as in the first adhesive 20, but is fixed in a basically dotted area on the surface of the active material 18.

[0066] In this way, the second binder 22 creates "point-to-point" connections between the particles of the active material 18, which helps to mechanically stabilize the freestanding electrode membrane 16. In other words, the second binder 22 acts as a dot-like adhesive. Further "point-to-point" connections can be achieved between other components (not shown) of the freestanding electrode membrane 16, particularly between the particles of the conductive additives, through the second binder 22.

[0067] The second binder 22 is particularly selected from the group consisting of: polyvinylidene fluoride (PVDF), polar modified polyvinylidene fluoride, polyvinylidene fluoride copolymers (such as polyvinylidene fluoride hexafluoropropyl (PVDF-HFP)) and combinations thereof.

[0068] The adhesive system has a weight ratio of 50:50 to 95:5, preferably 75:25 to 90:10, between the first adhesive and the second adhesive relative to the total weight of the adhesive system.

[0069] The second adhesive 22 is correspondingly used to further improve the adhesion of other components of the freestanding electrode film 16 in addition to the effect provided by the first adhesive 20.

[0070] The total content of the binder system in the freestanding electrode membrane 16 is 0.25 to 5% by weight, preferably 0.5 to 3% by weight, relative to the total weight of the freestanding electrode membrane.

[0071] Figure 2 A block diagram illustrating a method for preparing the freestanding electrode film 16 as described above according to the present invention is shown.

[0072] First, a first adhesive 20, a second adhesive 22, and an active material 18 are provided (step S1).

[0073] The first binder 20 is pre-placed in powder form, that is, in its unfiberized form. The powder of the first binder 20 particularly has a particle size d in the range of 3 to 500 μm. 50 .

[0074] At this point in time, the particle size of the second adhesive 22 can be compared with that of the previous adhesive. Figure 1 The particle sizes described are different. At this point in time, the second binder 22 has a smaller average particle size d than that in the finished freestanding electrode film 16. 50 .

[0075] The first binder 20, the second binder 22, and the active material 18 are then mixed together to form an intermediate electrode film (not shown) (step S2). Due to the shear forces that occur during the mixing of the components, the first binder 20 begins to fibrillate and the particles of the active material 18 bind together.

[0076] The intermediate electrode film is then pressed into a freestanding electrode film (step S3).

[0077] For this purpose, the intermediate electrode film is processed using heated calendering rolls. Several effects occur in parallel during this process. The first binder 20 continues to form fibrils due to the shear forces generated during calendering. Simultaneously, the intermediate electrode film is heated to a temperature that causes at least partial melting of the second binder 22.

[0078] For example, the intermediate electrode film is heated to a temperature in the range of 100 to 180°C, preferably 110 to 160°C. At least partially molten particles of the second binder 22 are deposited onto the particles of the active material 18 and a desired "point-to-point" connection is formed between the particles of the active material 18, which contributes to the mechanical stability of the freestanding electrode film 16.

[0079] By at least partially melting, the particles of the initially pre-placed second binder 22 can also be connected to each other, resulting in an average particle size d in the finished freestanding electrode film 16. 50 It is larger than the average particle size at the beginning when the components are mixed into an intermediate electrode film.

[0080] The performance of the present invention will be further illustrated below with the help of experimental examples. Detailed Implementation

[0081] Example: Preparation of freestanding electrode films

[0082] In a low-shear mixing apparatus, 97% by weight of NMC as the active material, 1% by weight of high molecular weight PE as the first binder, 0.5% by weight of PVDF as the second binder, and 1.5% by weight of conductive carbon black (Carbon Black Super C65) as the conductive additive were mixed for 10 minutes relative to the total weight of all components in each case.

[0083] The obtained intermediate electrode film was then calendered at 160°C to obtain a film with a density of 30 mg / cm³. 2 The unit area weight and 3.6 g / cm³ 3 Freestanding cathode membrane with high electrode density.

Claims

1. A freestanding electrode membrane (16) for an electrochemical cell (12) comprising an adhesive system having a first binder (20) and a second binder (22) and an active material (18) contained in the adhesive system. The first adhesive (20) is a fluorine-free adhesive for forming fibrils. The second binder (22) has an average particle size d in the range of 40 to 200 nm. 50 The binder that does not form fibrils, and The adhesive system therein does not contain adhesives based on polytetrafluoroethylene and / or cellulose.

2. The freestanding electrode membrane (16) according to claim 1, wherein the fluorine-free binder forming the fibrils is a high molecular weight polyolefin, preferably a high molecular weight polyethylene.

3. The freestanding electrode membrane (16) according to claim 1 or 2, wherein the non-fiber-forming binder is selected from the group consisting of polyvinylidene fluoride, polar-modified polyvinylidene fluoride, polyvinylidene fluoride copolymers, and combinations thereof.

4. The freestanding electrode membrane (16) according to any one of the preceding claims, wherein the non-fibrillating binder has an average particle size d in the range of 40 to 200 nm. 50 .

5. The freestanding electrode membrane (16) according to any one of the preceding claims, wherein the binder system has up to 50% by weight, particularly 5 to 30% by weight, preferably 10 to 25% by weight, of a second binder, in each case relative to the total weight of the binder system.

6. The freestanding electrode membrane (16) according to any one of the preceding claims, wherein the binder system is present in the freestanding electrode membrane (16) in a proportion of 0.25 to 5% by weight, preferably 0.5 to 3% by weight, relative to the total weight of the freestanding electrode membrane (16).

7. The freestanding electrode membrane (16) according to any one of the preceding claims, wherein the freestanding electrode membrane (16) is prepared by a drying process.

8. An electrode (10) for an electrochemical cell (12) comprising a freestanding electrode membrane (16) according to any one of the preceding claims.

9. An electrochemical cell (12) having an electrode (10) according to claim 8.

10. A method for preparing a freestanding electrode film (16) for use in an electrochemical cell (12), comprising the following steps: - Provides a first adhesive (20), a second adhesive (22), and an active material (18), wherein the first adhesive (20) is a fluorine-free, fibrillating adhesive and the second adhesive (22) is a non-fibrillating adhesive, and wherein the first adhesive (20) and the second adhesive (22) are free of adhesives based on polytetrafluoroethylene and / or cellulose. - The first binder (20), the second binder (22), and the active material (18) are mixed to form an intermediate electrode film, and - The intermediate electrode film is pressed into the freestanding electrode film (16).

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