Functionalized separator and compound separator

Abstract

A separator for a battery cell comprising a mixture of a ceramic material, a metal oxide different from the ceramic material, and a binder; a composite separator for a battery cell comprising an inner layer between two outer layers, each of the outer layers being a separator and the inner layer being different from each of the outer layers; a battery cell comprising the separator or composite separator.

Description

Field of invention

[0001] The present invention relates to a separator for a battery cell, a multi-layer composite separator for a battery cell and a battery cell containing the separator or composite separator. State of the art

[0002] A battery cell contains an anode, a cathode, an electrolyte, and a separator. The separator separates the anode from the cathode and, due to its high electrical resistance, prevents an internal short circuit. At the same time, the separator's porosity allows the liquid electrolyte to pass through, thus ensuring the ionic charge transport between the anode and cathode.

[0003] In sulfur-containing battery cells, elemental sulfur is used as the cathode material, which in a conversion reaction converts the ionic charge carrier (e.g. Li) +) reversibly binds. In this conversion reaction, the sulfur is gradually reduced via partially soluble polysulfides. The solubility of the polysulfides in the electrolyte leads to a depletion of sulfur at the cathode and to a parasitic precipitation of insoluble sulfides in the anode region, resulting in a rapid loss of cell capacity. This phenomenon is known as the polysulfide shuttle and is considered the crucial aging mechanism in sulfur-containing cells. It occurs in conventional composite cathodes where the sulfur is bound in the pores of a carbon matrix. One mechanism to reduce this phenomenon is to covalently bind the sulfur in a polymer, thus effectively preventing the migration of polysulfides. However, such cathode materials are complex and expensive to produce. Problems to be solved by the invention

[0004] One object of the present invention is to provide an improved separator with slower capacity loss.

[0005] Furthermore, it is a task to provide a separator that is inexpensive, safe, and enables rapid production with a wide range of variations. Summary of the invention

[0006] The problems are solved by providing a functionalized separator in a first aspect of the invention and a composite separator with a sandwich structure in a second aspect of the invention.

[0007] The subject matter of the present invention is as follows: [1] Separator for a battery cell, containing a mixture of a ceramic material, a metal oxide different from the ceramic material and a binder. [1-1] Separator according to point [1], wherein 100 parts by mass of the mixture consist of 50 to 90 parts by mass of ceramic material, 0.1 to 20 parts by mass of metal oxide and 5 to 40 parts by mass of binder. [1-2] Separator according to any of the preceding points, wherein the thickness of the separator is 10 µm to 500 µm, preferably 10 µm to 200 µm. [1-3] Separator according to any of the preceding points, wherein the ceramic material is not a metal oxide. [1-4] Separator according to any of the preceding points, wherein the ceramic material and the metal oxide are embedded in the binder. [2] Separator according to point [1], wherein the mass ratio of the ceramic material to the metal oxide is at least 5:1. [2-1] Separator after point [2], wherein the mass ratio is at least 10:1. [3] Separator according to any of the preceding points, wherein the metal oxide is at least one selected from TiO2, SiO2, MnO2, Al2O3 and SnO2. [3-1] Separator according to point [3], wherein the metal oxide is TiO2 or SiO2. [4] Separator according to any of the above points, wherein the ceramic material is lithium aluminum titanium phosphate. [5] Separator according to any of the above points, wherein the binder is a polymer. [5-1] Separator according to any of the preceding points, wherein the polymer is a polymer electrolyte. [6] Separator according to one of the above points in the form of a membrane. [7] Method for producing a separator according to any of the preceding points, comprising mixing particles of a ceramic material, a metal oxide and a binder. [8] Composite separator for a battery cell, which has an inner layer between two outer layers, each of the outer layers being a separator and the inner layer being different from each of the outer layers. [8-1] Composite separator according to point [8], wherein the inner layer differs in its composition from each of the outer layers. [8-2] Composite separator according to point [8] or [8-1], wherein the thickness of the three layers is 10 µm to 500 µm. [8-3] Compound separator after one of points [8] to [8-2], wherein each of the three layers is a separator. [8-4] Compound separator after one of the points [8] to [8-3], wherein the compound separator consists of the three layers. [8-5] Composite separator according to one of points [8] to [8-4] wherein the outer layers are independently based on a material selected from ceramic materials, paper, polymers or nonwovens. [8-6] Composite separator according to one of points [8] to [8-5], wherein at least one of the outer layers is based on a ceramic material. [8-7] Composite separator according to any of points [8] to [8-6], wherein at least one of the outer layers contains a mixture of a ceramic material, a binder and optionally a metal oxide different from the ceramic material, wherein 100 parts by mass of the mixture consist of 50 to 90 parts by mass of ceramic material, 0 to 10 parts by mass of metal oxide and 5 to 40 parts by mass of binder. [8-8] Composite separator according to one of points [8] to [8-7], wherein the thickness of the outer layers is 10 µm to 200 µm and the thickness of the inner layer is 10 µm to 500 µm. [8-9] Compound separator after one of points [8] to [8-8], wherein the outer layers are the same. [8-10] Compound separator according to one of points [8] to [8-9], wherein the inner layer is thicker than any of the outer layers. [8-11] Compound separator according to a combination of points [8-1] and [8-9], preferably [8-1] and [8-10]. [8-12] Compound separator according to one of points [8] to [8-11], wherein the inner layer can absorb more electrolyte than any of the outer layers. [8-13] Composite separator according to one of points [8] to [8-12], wherein the inner layer has a higher void content than any of the outer layers. [8-14] Composite separator according to one of the points [8] to [8-13], wherein the density of the inner layer is less than the density of each of the outer layers. [8-15] Composite separator according to one of points [8] to [8-14], wherein the density of the inner layer is less than 1000 kg / m³ 3 , preferably less than 800 kg / m³ 3 , preferably less than 500 kg / m³ 3 is. [8-16] Compound separator after a combination of points [8-10] and [8-13] or [8-10] and [8-14]. [9] Compound separator according to one of points [8] to [8-16], wherein at least one of the outer layers is a separator according to one of points [1] to [6]. [9-1] Compound separator according to point [9], wherein each of the outer layers is a separator according to one of points [1] to [6].

[10] Composite separator according to one of points [8] to [9-1], wherein the inner layer is not based on a ceramic material. [10-1] Compound separator according to a combination of points [9-1] and [8-10].

[11] Composite separator according to one of points [8] to [10-1], wherein the inner layer is a woven fabric or a nonwoven fabric. [11-1] Composite separator according to point

[11] wherein the nonwoven fabric is based on a polymer, on paper or on glass fiber. [11-2] Compound separator according to a combination of points

[11] and [8-11].

[12] Composite separator according to one of points [8] to [11-2], wherein the two outer layers and the inner layer are arranged directly on top of each other. [12-1] Compound separator according to a combination of points

[12] , [8-3] and [8-4].

[13] Battery cell containing a separator after one of points [1] to [6] or a compound separator after one of points [8] to [12-1].

[14] Battery cell according to point

[13] , wherein in the composite separator the concentration of the electrolyte in the inner layer is higher than in any of the outer layers. [14-1] Battery cell according to point

[13] or

[14] , wherein the amount of electrolyte in the inner layer is at least twice as high as in each of the outer layers.

[15] Battery cell according to point

[13] or

[14] based on sulfur technology. Advantages of the invention

[0008] The functionalization of the separators according to the invention enables the adaptation of the mechanical properties of the separators. Furthermore, the chemical stability of the interface between the electrode and the separator can be modified.

[0009] The composite separators according to the invention can be integrated into the cell during the manufacturing process, thus enabling cost-effective, safe, and rapid production. The properties of the composite separator can be easily varied by changing individual layers.

[0010] The sandwich concept of the composite separator according to the invention is transferable to all types of separator materials. A coating of the separator with functional layers is not necessary. The sandwich concept is also transferable to other cell applications. For example, thermally stable separators can be produced that prevent thermal runaway of the cell.

[0011] The compound separator can reduce the polysulfide shuttle, thereby reducing capacity loss in metal-sulfur cells and increasing their lifespan. Embodiments of the invention

[0012] In the present invention, the separator or composite separator according to the invention is sometimes simply referred to as "the separator" or "the composite separator". The two outer layers and the inner layer of the composite separator can together be referred to as "the three layers".

[0013] The separator, or composite separator, is suitable for use in a battery cell. It must therefore possess certain functional properties: firstly, high electronic resistance to prevent internal short circuits, and secondly, good wettability by the electrolyte and porosity to allow the passage of the liquid electrolyte and thus the ionic charge transport between the anode and cathode. The separator, or composite separator, is not limited to use in battery cells but can be used in any electrochemical cell where these functional properties are beneficial. separator

[0014] According to the first aspect of the invention, a functionalized separator is provided. The separator contains a mixture of a ceramic material, a metal oxide different from the ceramic material, and a binder. The functionalization of the ceramic material as the base material of the separator is achieved by the addition of the metal oxide.

[0015] Examples of ceramic materials include ionically conductive powders such as lithium aluminum germanium phosphate (LAGP), lithium aluminum titanium phosphate (LATP), or lithium lanthanum titanium oxide (LLTO). If the ceramic material is a metal oxide, a different metal oxide is used for functionalization, resulting in the separator containing two different metal oxides.

[0016] The metal oxide may be at least one selected from TiO2, SiO2, MnO2, Al2O3 and SnO2, whereby TiO2 or SiO2 may be preferred.

[0017] The separator contains a binder in which the other materials can be embedded. The binder can be a polymer electrolyte, such as polyethylene (PEO), polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), or polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP). Due to the high binder content, the separator can form a flexible membrane.

[0018] The separator can be produced by mixing particles of a ceramic material, a metal oxide and a binder, wherein the binder is preferably in polymer into which the other components are embedded.

[0019] The functionalization of the separator with the metal oxide can be carried out as follows: Metal oxides are added to a slurry of the separator material. This substitutes a portion of the ceramic base material (e.g., LATP) with corresponding amounts of metal oxide. The mixture is dissolved in acetone along with a binder, e.g., PVDF, and processed into a lacquer, which is then applied to a substrate in a defined layer thickness using a doctor blade. After drying, the functionalized separator can be peeled off the substrate as a film and used further. For example, a functionalized LATP separator with a thickness of 50 µm can be obtained in this way. The separator can have, for example, the following composition: material Mass % base ceramics LATP 70 - 80 metal oxide 0 - 10 Binder PVDF15 - 20 Compound separator

[0020] According to the second aspect of the invention, a composite separator is provided. The composite separator comprises at least three layers and preferably consists of three layers. The layers are stacked on top of each other in a planar, sandwich-like structure, with an inner layer embedded between two outer layers. The layers are preferably stacked directly on top of each other, i.e., without intermediate layers.

[0021] Each of the outer layers of the composite separator can be any type of separator. Examples of a base material for a separator include a ceramic material, a polymer such as polyethylene or polypropylene, paper, a woven fabric, or a nonwoven fabric, for example, a glass fiber fleece. In a particular embodiment, at least one of the outer layers is a separator according to the first aspect.

[0022] The inner layer differs from each of the outer layers. This difference can be due to variations in thickness, density, structure, composition, or other characteristics. For example, each of the outer layers might have a compact structure, while the inner layer could be a woven fabric with a high void content. By appropriately combining the outer and inner layers, the desired physical and electrochemical properties of the composite separator can be easily adjusted.

[0023] In one embodiment, the composite separator can be suitable for increased electrolyte uptake. Sulfur-based cells require a high electrolyte concentration to achieve sufficient performance, with an electrolyte / sulfur ratio of 3:1 being characteristic. A large portion of the electrolyte is absorbed by the separator, which necessitates high electrolyte uptake capacity and good wettability of the separator. Both a pure LATP separator and a functionalized LATP separator exhibit excellent wettability.To further increase the electrolyte uptake of the separator and thus obtain a separator that has both properties and at the same time exhibits a significantly reduced polysulfide shuttle, the composite structure is provided in which a separator is combined with an intermediate layer, preferably a glass fiber fleece, to form a composite in which the intermediate layer is sandwiched between two outer layers of the separator.

[0024] For example, the composite contains two functionalized LATP layers as outer layers and a glass fiber fleece (Whatman GF / D) as an inner layer. These layers can be spot-bonded with a compatible PVDF adhesive. One example of a composite is a sandwich consisting of two layers of the separator, each 40 µm thick, and a glass fiber fleece 150 µm thick. Battery cell

[0025] The battery cell according to the invention comprises an anode, a cathode, an electrolyte and a separator, which is a separator according to the first aspect or a composite separator according to the second aspect.

[0026] The battery cell can be based on any of the technologies known in the prior art. Systems and materials known in the prior art can be used as the anode and cathode. Likewise, known electrolytes, e.g., ionic liquids, can be used. Examples of electrolytes are anhydrous, organic electrolyte solutions.

[0027] The separator or composite separator is configured according to the requirements of the battery cell technology. For example, a composite separator as described above with increased electrolyte uptake is provided to enable its use in a sulfur-based battery cell. Examples Example 1

[0028] In an experimental setup, two liquid containers were connected via a connector in a measuring cell. The separator under test was inserted into the connector, with a concentrated polysulfide solution placed in the first container. Migration through the separator into the second container was indicated by color change. The intensity of the color change is proportional to the polysulfide ion concentration, and the time until a certain degree of color change is reached is called the retention time. The longer the retention time, the greater the separator's ability to reduce polysulfide migration. It was observed that when using the pure LATP separator, a significant increase in the polysulfide concentration in the second container was noticeable after only 6 hours. When using a functionalized LATP separator, comparable concentrations were only observed after approximately 36 hours.The test results demonstrate the effectiveness of the functionalized separators in suppressing polysulfide migration. Example 2

[0029] Sulfur-based cells (1) to (3) were fabricated using sulfur-carbon cathodes against metal anodes. The cells contained different separators and exhibited the following cycle stabilities: (1) The cell, which contained a separator made of pure glass fiber fleece (Whatman GF / D), showed a continuous decrease in discharge capacity of about 50% after 25 cycles, indicating a significant sulfur loss from the cell. (2) The cell, which contained a composite separator with unmodified LATP separators as outer layers and an embedded glass fiber fleece, also showed a decrease in discharge capacity of about 50% after 25 cycles. (3) The cells containing a composite separator with LATP separators as outer layers and an embedded glass fiber fleece, the separators being modified with TiO2, SiO2, MnO2, Al2O3 or SnO2, exhibited a significantly more stable discharge capacity compared to cell (2). In particular, the LATP separators modified with TiO2 or SiO2 showed very high cycle stability, with a decrease in discharge capacity of approximately 10% after 25 cycles.

Claims

[1] Separator for a battery cell containing a mixture of a ceramic material, a metal oxide different from the ceramic material and a binder. [2] The separator according to claim 1, wherein the mass ratio of the ceramic material to the metal oxide is at least 5:

1. [3] A separator according to any one of the preceding claims, wherein the metal oxide is at least one selected from TiO2, SiO2, MnO2, Al2O3 and SnO2. [4] Separator according to one of the preceding claims, wherein the ceramic material is lithium aluminum titanium phosphate. [5] Separator according to one of the preceding claims, wherein the binder is a polymer. [6] Separator according to one of the preceding claims in the form of a membrane. [7] A method of manufacturing a separator according to any preceding claim, which comprises mixing particles of a ceramic material, a metal oxide and a binder. [8] A composite separator for a battery cell having an inner layer between two outer layers, each of the outer layers being a separator and the inner layer being different from each of the outer layers. [9] A composite separator according to claim 8, wherein at least one of the outer layers is a separator according to any one of claims 1 to 6. [10] A composite separator according to claim 8 or 9, wherein the inner layer is not based on a ceramic material. [11] A composite separator according to any one of claims 8 to 10, wherein the inner layer is a woven fabric or a nonwoven fabric. [12] Composite separator according to one of claims 8 to 11, wherein the two outer layers and the inner layer are arranged directly on top of one another. [13] A battery cell comprising a separator according to any one of claims 1 to 6 or a composite separator according to any one of claims 8 to 12. [14] A battery cell according to claim 13, wherein in the composite separator the concentration of the electrolyte in the inner layer is higher than in each of the outer layers. [15] Battery cell according to claim 13 or 14, which is based on sulphur technology.