Separatormaterial

Abstract

Polymer-based separator material for a galvanic element, characterized in that the separator material contains at least one polymer compound in which the polymer base is formed by one or more polyolefin compounds or by one or more derivatives of polyolefin compounds, wherein the polymer compound has polar groups and the polar groups are acid groups and / or acid anhydride groups, wherein 0.25 to 3% of possible attachment points for acid groups are occupied by a polar acid group and / or acid anhydride group.

Description

[0001] The invention relates to a polymer-based separator material for a galvanic element.

[0002] In galvanic elements, for example lithium-ion cells - particularly for use in the automotive industry - porous separator materials based on polymers or preferably on polyolefins are used due to high safety requirements, see for example Z. Zhang and P. Ramadass in M. Yoshio, RJ Brodd, A. Kozawa Lithium-Ion Batteries: Science and Technologies, Springer, 2009, Chapter 20 or in T. Wöhrle in R.

[0003] Korthauer, Handbook of Lithium-Ion Batteries, Springer Verlag Berlin Heidelberg 2013, Chapter 9. These include film separators made of polypropylene (PP), polyethylene (PE) or with mixed layer sequences of these materials.

[0004] WO 2015 / 027919 A1 describes a separator for a lithium-ion battery, comprising a substrate, a coating, and a middle layer formed between the substrate and the coating. The middle layer consists of a portion of the substrate and a portion of the coating. The substrate contains a base polymer, a first polymer, and a first inorganic material. The coating contains a second polymer and a second inorganic material. The first polymer and the second polymer independently contain an acid residue in a side chain. The first inorganic material reacts with the first polymer via a first neutralization reaction, and the second inorganic material reacts with the second polymer via a second neutralization reaction. Furthermore, a method for producing a separator for a lithium-ion battery and a lithium-ion battery are described.

[0005] DE 102012000910 A1 relates to a separator for an electrochemical cell comprising a porous layer comprising at least one block copolymer with three or more polymer blocks and at least one aluminum oxide or hydroxide, a lithium-ion battery comprising such a separator and a method for producing such a separator.

[0006] US 4434215 A discloses a composition suitable for use as a battery separator comprising a substantially homogeneous mixture of a copolymer of ethylene and acrylic acid having 10 to 23 mole percent acrylic acid and a melt index of about 0.1 to 5 in combination with a copolymer of ethylene and acrylic acid having at least 25 mole percent acrylic acid and a melt index of at least about 10. Further disclosed is a process for producing a sheet product from the present composition and the use of the produced product as a battery separator.

[0007] A disadvantage of these separator materials is their relatively poor wettability with the commonly used liquid, non-aqueous electrolyte solvents such as ethylene carbonate or diethyl carbonate. This is due to the relatively low surface tension of polyolefin separators.

[0008] It is an object of the invention to provide an improved polymer-based separator material for a galvanic element.

[0009] This object is achieved by a polymer-based separator material for a galvanic element according to claim 1. Advantageous embodiments and further developments of the invention emerge from the dependent claims.

[0010] According to the invention, the separator material comprises at least one polymer compound having polar groups.

[0011] Since known polymer-based separator materials are predominantly nonpolar, the separator material according to the invention with polar groups is also referred to as a polar-modified separator material. Compared to an unmodified, nonpolar polymer separator, e.g., compared to a prior-art polyolefin separator, the surface tension of the modified separator material is significantly increased by the polar groups. This can increase the wetting of the separator with non-aqueous solvents. Consequently, the electrolyte, in which a conductive salt is usually dissociated, can penetrate the pores of the separator more evenly and effectively. The wettability and surface tension are proportional.

[0012] The polymer base itself comprises one or more polyolefin compounds or polar derivatives of polyolefin compounds. Particularly advantageous are polar derivatives of polyolefin compounds formed by grafting the polyolefin with acids, i.e., by graft copolymerization. In this case, the graft copolymer is understood to be the polymer base of the separator material within the scope of this document. Polypropylene is particularly suitable for this purpose.

[0013] According to the invention, the polar groups are acid groups and / or acid anhydride groups. The polar polymer thus contains free acid groups, some of which can also develop into acid anhydride groups, or alternatively, exclusively acid anhydride groups.

[0014] The acid groups and / or acid anhydride groups can be based on acrylic acid and / or methacrylic acid and / or derivatives of acrylic acid and / or methacrylic acid. The polar polymer characterized by these groups is preferably formed by graft copolymerization of polyolefins. These resulting compounds thus form a polar polymer base for the separator material.

[0015] Preferred polymer compounds containing polar groups are in particular ethylene-acrylic acid copolymer compounds or ethylene-methacrylic acid copolymer compounds.

[0016] Such a polar-modified separator material has the advantage of exhibiting advantageous basic properties of simple, non-polar separator materials based exclusively on polyolefins. A small amount of acid groups is required to increase the surface area of the separator material with regard to adhesion and surface tension. At a small amount, 0.25 to 3% of the polyolefin's potential reactive sites for acid groups are occupied by acid groups.

[0017] Increasing this proportion beyond 10% only slightly increases the surface tension, but can adversely affect the other chemical and electrical properties of the material.

[0018] The graft copolymers are also particularly advantageous because their surface tension can be adjusted with the proportion of polar groups by the amount of acid added during the polymer-forming chemical reaction.

[0019] Furthermore, such polar-modified polyolefins are extremely chemically and electrochemically stable in a galvanic cell. Therefore, the separator material according to the invention is particularly suitable for use as a separator in galvanic cells or for producing a separator for galvanic cells. This applies particularly to lithium-ion cells. The advantages of a polar-modified polyolefin separator can be optimally utilized when used in combination with a polar electrolyte. This supports the wetting of the polar separator with the polar electrolyte through the electrostatic Coulomb interaction of the respective polar groups.

[0020] A suitable polar electrolyte is a mixture of ethylene carbonate with a low-boiling or low-viscosity organic carbonate such as diethyl carbonate. A lithium conducting salt such as lithium hexafluorophosphate (LiPF6) is dissolved in this mixture at an approximately 1 molar concentration. Ethylene carbonate dissociates the lithium conducting salt and, in combination with a graphite anode in a lithium-ion cell, is particularly suitable for forming the corrosion-protective coating layer of electrolyte decomposition products (known in the art as "solid electrolyte interphase") required for graphite anodes. Since pure ethylene carbonate has too high a viscosity as an electrolyte, diethyl carbonate is added as a low-boiling or low-viscosity organic carbonate.

[0021] Alternative electrolyte compositions include mixtures of ethylene carbonate, diethyl carbonate, dimethyl carbonate, or ethyl methyl carbonate. Dissociable conductive salts include lithium hexafluorophosphate, lithium bis(trifluoromethane)sulfonimide, and lithium tetrafluoroborate.

[0022] The wetting tendency of a non-aqueous solvent in the form of a mixture of, for example, ethylene carbonate and diethyl carbonate with the lithium conducting salt LiPF6 is particularly pronounced on a polyolefin separator modified with polar acid groups.

[0023] Good wettability reduces the penetration or residence time of the liquid electrolyte during cell production. With current technology, cell production often requires a tempering step (sometimes under additional pressure). Good wettability thus enables increased cycle times and reduces the energy and cost intensity of cell production, for example, for lithium-ion cells. If the electrolyte is evenly and completely distributed in the separator, this also results in high cell reliability and a longer service life for the cell user.

[0024] The invention is based on the following considerations: In lithium (Li)-ion technology, polyolefin-based separators are frequently used. These are porous and typically have a thickness of approximately 20µm. See, for example, T. Wöhrle in Chapter 9, R. Korthauer, Handbook of Lithium-Ion Batteries, Springer Verlag Berlin Heidelberg 2013. The porosity is approximately 50%. The spaces formed by the pores contain a liquid electrolyte consisting of polar solvents that dissociate a lithium conducting salt (e.g., LiPF6). This conducting salt causes the actual active Li-ion migration during charging or discharging of the secondary Li-ion cell. The plastic separator itself is not a Li-ion conductor. The surface tension of the nonpolar polyolefin is approximately 32–34 mN / m. By introducing polar groups, improved surface tensions of up to approximately 50 mN / m can be achieved.

[0025] Plastic separators, especially polyolefin-based ones, have a very low surface tension and are very difficult to wet, especially with polar liquids. A polyolefin-based separator, for example, is a polypropylene (PP) or polyethylene (PE) separator. The separator insulates the electrodes from each other.

[0026] A minimum degree of electrolyte wetting must be ensured through complex process steps such as vacuum impregnation, standstill times, and annealing. These process steps are usually very costly according to the current state of the art. Incomplete electrolyte wetting can result in, for example, production rejects or a reduced service life of the Li-ion cell.

[0027] It is proposed to use a polar-modified polyolefin as the separator material instead of simple polyolefin. This increases the surface tension or can be optimally adjusted to achieve maximum wetting by the liquid electrolyte. This allows the electrolyte to penetrate completely and evenly into the pores of the separator. The polar modification can be achieved by incorporating approximately 1 wt.% acid or anhydride groups, using a polypropylene separator as an example. For polyethylene-based materials, suitable copolymers can be used, such as ethylene-acrylic acid copolymers (EAA), which already contain polar groups in their structure.

[0028] Since the separator according to the invention can be better wetted with polar liquid electrolyte, it results in simpler and faster liquid electrolyte dosing. This significantly reduces process costs and waste material. The more uniform electrolyte wetting results in an increased service life and reliability of the Li-ion cell.

[0029] The porous polyolefin separator according to the invention is characterized in that it consists at least partially of a polymer which has acid and / or acid anhydride groups.

[0030] It can be seen that derivatives of polyolefin-based separators containing acid and / or acid anhydride groups exhibit excellent wetting properties towards polar electrolytes (such as a 1 molar solution of LiPF6 in ethylene carbonate (EC) / diethyl carbonate (DEC) = 3 / 7). At the same time, these modified separators are also stable under the electrochemical conditions encountered in a galvanic cell such as a Li-ion cell. Surprisingly, the reactive acid or acid anhydride groups of the modified separator according to the invention do not have any disruptive effects on the electrochemical processes in the cell.

[0031] The polymer containing acid and / or acid anhydride groups is preferably a modified polymer based on acrylic acid and / or methacrylic acid. Ethylene-methacrylic acid copolymers (EMAA) or ethylene-acrylic acid copolymers (EAA) are considered particularly suitable.

[0032] In the following, a preferred embodiment of the invention is described with reference to the accompanying drawings. Further details, preferred embodiments, and developments of the invention will emerge from these. In detail, schematically Fig. 1a the structural formula of polyethylene, Fig. 1b the structural formula of the ethylene-acrylic acid copolymer (EAA) and Fig. 2 the structural formula of the ethylene-methacrylic acid copolymer (EMAA).

[0033] The Fig. 1 b and Fig. 2 show the structural formula of polar polymer compounds. In Fig. 1 b is the ethylene-acrylic acid copolymer (EAA) and in Fig. 2 shows the ethylene-methacrylic acid copolymer (EMAA). EAA is described in more detail below as a compound in a separator material. When polyethylene, the starting material for a separator, is grafted with acid, in this case acrylic acid, the copolymerization produces EAA, which forms the polar polymer base of the exemplary separator material. EAA has a polar anchor group in the form of the COOH group. Compared to non-polar polyethylene (see Fig. 1a) In EAA, one CH bond is replaced by a C-COOH bond. The separator material exhibits optimal properties when approximately 0.25-3% of the potentially replaceable CH bonds are replaced by C-COOH bonds. This can be precisely adjusted by the amount of acrylic acid added during copolymerization.

[0034] In the Fig. 1a, Fig. 1b and Fig. 2, the square brackets indicate a molecular unit continuing with the parameter n to form a macromolecule. The proportions of polar groups indicated throughout the document refer to at least one macromolecule or several macromolecules, ie, not to a molecular unit. This is indicated by the reference "or" in the Fig. 1b and Fig. 2 illustrates this.

[0035] A plastic separator based on such a polymer, in combination with a polar electrolyte in lithium-ion cells, exhibits very good wetting properties. Fig. Figure 2 shows the ethylene-methacrylic acid copolymer EMAA formed by analogous grafting of polypropylene.

Claims

[1] Polymer-based separator material for a galvanic element, characterized by that the separator material contains at least one polymer compound in which the polymer base is formed by one or more polyolefin compounds or by one or more derivatives of polyolefin compounds, wherein the polymer compound has polar groups and the polar groups are acid groups and / or acid anhydride groups, wherein 0.25 to 3% of possible starting points for acid groups are occupied by a polar acid group and / or acid anhydride group. [2] Separator material according to claim 1, characterized by that the acid groups and / or acid anhydride groups are based on acrylic acid and / or methacrylic acid and / or on derivatives of acrylic acid and / or methacrylic acid. [3] Separator material according to claim 2, characterized by that the polymer compound having polar groups is an ethylene-acrylic acid copolymer compound. [4] Separator material according to claim 2, characterized by that the polymer compound having polar groups is an ethylene-methacrylic acid copolymer compound. [5] Galvanic element with a separator made of a material according to one of the preceding claims. [6] Galvanic element according to claim 5, which is designed as a lithium-ion cell. [7] Galvanic element according to claim 6 comprising a polar electrolyte. [8] Galvanic element according to claim 7, wherein the polar electrolyte contains a mixture of at least two organic carbonates in which a lithium conducting salt is dissolved.

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