DE102023102563A1 - Separator for a lithium-ion battery comprising a hybrid electrolyte and lithium-ion battery - Google Patents

Abstract

A separator (10) for a lithium ion battery (12) comprises a composite material (18) with a hybrid electrolyte (20). The hybrid electrolyte (20) contains an inorganic lithium ion conductor (22) and a gel electrolyte (24). Furthermore, a lithium ion battery (12) is specified.

Description

The invention relates to a separator for a lithium-ion battery, with a composite material comprising a hybrid electrolyte, and to a lithium-ion battery comprising such a separator.

In the following, the term "lithium ion battery" is used synonymously for all terms commonly used in the state of the art for galvanic elements and cells containing lithium, such as lithium battery, lithium cell, lithium ion cell, lithium polymer cell, lithium ion polymer cell and lithium ion accumulator. In particular, rechargeable batteries (secondary batteries) are included. The terms "battery" and "electrochemical cell" are also used synonymously with the term "lithium ion battery".

A lithium-ion battery has at least two different electrodes, a positive (cathode) and a negative electrode (anode). Each of these electrodes has at least one active material, optionally together with additives such as electrode binders and electrical conductivity additives, which is applied to an electrically conductive carrier of the respective electrode, which is also referred to as a current collector. Solid conductor foils made of aluminum (for the positive electrode) or copper (for the negative electrode) are used as electrically conductive carriers, for example in the EP 3 714 078 B1 Porous current collectors are also known, such as perforated foils, expanded metals or fabric foils as in the DE 10 2018 000 272 A1 described.

A separator is arranged between the two electrodes to electrically insulate them. The separator is made of a polyolefin, such as polypropylene (PP) and/or polyethylene (PE), polyester or polyimide, for example, and generally has a porosity in the range of 20 to 70%.

It is known to use lithium-ion batteries with a liquid electrolyte that fills the porosity of the separator, thereby wetting it and enabling sufficient lithium ion conductivity within the lithium-ion battery so that lithium ions can be exchanged between the positive and negative electrodes during charging and discharging processes. Liquid electrolytes usually comprise organic carbonates such as ethylene carbonate (EC) and dimethyl carbonate (DMC) as well as a lithium conducting salt dissolved therein, for example LiPF ₆ or LiBF ₄ . The disadvantage of using such liquid electrolytes is that organic solvents can escape when the lithium-ion battery is subjected to mechanical, thermal and/or electrical stress.

As an alternative to liquid electrolytes, so-called “solid electrolytes” based on ceramic compounds are also known. These are inorganic lithium ion conductors and are described, for example, in the review article by P. Knauth: “Inorganic solid Li ion conductors: An overview” (Solid State lonics, 180 (2009), pp. 911-916, doi: 10.1016/j.ssi.2009.03.022 ). Solid electrolytes have the advantage that no organic solvents escape if the lithium-ion battery leaks. The disadvantage of solid electrolytes, however, is that although they have a high lithium ion conductivity in the expanded solid body (also known as the “bulk”), for example within a grain of the solid electrolyte, high resistances occur at grain boundaries. Therefore, solid electrolytes often only achieve sufficient lithium ion conductivity at comparatively high temperatures, as for example in T. Wöhrle et al.: Sol-Gel Synthesis of the Lithium-Ion Conducting Perovskite La0.57Li0.3TiO3 - Effect of Synthesis and Thermal Treatments on the Structure and Conducting Properties (lonics, 2 (1996), pp. 442-445, doi: 10.1007/BF02375824 ). This leads to a reduced performance of the lithium-ion battery, especially with regard to the high current capability.

In the EN 10 2014 226 390 A1 A hybrid electrolyte is proposed which is a combination of a ceramic and a liquid electrolyte, with the liquid electrolyte being present in a smaller proportion by weight and volume than the ceramic electrolyte. In this way, the proportion of liquid electrolyte is reduced, but in principle it can still be released immediately in the event of mechanical, thermal and/or electrical stress.

The object of the invention is to provide a separator and a lithium-ion battery with improved performance.

In particular, the separator or lithium-ion battery should be robust against mechanical, thermal and/or electrical stress.

The object is achieved according to the invention by a separator for a lithium ion battery, with a composite material comprising a hybrid electrolyte. The hybrid electrolyte contains an inorganic lithium ion conductor and a gel electrolyte.

The invention is based on the basic idea of minimizing the proportion of free liquid electrolyte in the separator - and thus in a lithium-ion battery with such a separator - by using the inorganic lithium ion conductor and the gel electrolyte, while at the same time the lithium ion conductivity is increased by using this combination of electrolyte materials, compared to separators that only use inorganic lithium ion conductors.

Accordingly, the composite material is preferably free of unbound liquid electrolyte.

Lithium-ion batteries that use gel electrolytes (also called “polymer gel electrolytes”) as the sole electrolyte are fundamentally known and are used, for example, in EP 1 277 250 B1 described.

It has now been recognized that gel electrolytes can be combined with inorganic lithium ion conductors to produce high-performance separators or lithium ion batteries. This is attributed to the fact that the contact resistance between the inorganic lithium ion conductor and the gel electrolyte is sufficiently low to enable excellent lithium ion conductivity between the inorganic lithium ion conductor and the gel electrolyte, so that a liquid electrolyte can be dispensed with or at least its amount can be minimized.

The composite material is in particular inherently stable (also referred to as a “self-standing film”). In this way, the use of additional carrier materials, such as a polyester film, for handling and/or process control of the separator according to the invention can be dispensed with.

The separator preferably consists of the composite material. In other words, the separator is preferably free of unbound liquid electrolyte.

Particularly preferably, the separator is completely volumetrically compact. This means that the separator has no pores, apart from gaps that are unavoidable due to manufacturing.

In this way, it is also not necessary to additionally soak the separator with a liquid electrolyte to provide sufficient lithium ion conductivity within the separator.

The separator has in particular an electrical conductivity of 10 ^-10 S/cm. In this way, particularly good electrical insulation of electrodes in a lithium-ion battery can be achieved with a separator according to the invention, while at the same time a high lithium-ion conductivity is provided.

Gel electrolyte

The gel electrolyte comprises in particular a polymeric binder, a solvent and a lithium conductive salt, which are gelled together.

In this context, the polymeric binder simultaneously takes on the function of the electrode binder for holding the particles together in the separator and for gelling the liquid electrolyte components, i.e. the solvent and the lithium conductive salt dissolved in the solvent.

The term "gelled" in this context means that all components of the gel electrolyte that would be liquid in their unbound form at room temperature are bound in the gel electrolyte. In other words, there is no free solvent in the gel electrolyte.

Basically, the gel electrolyte can be understood as a gel with a non-liquid colloidal or polymeric network that is expanded over its entire volume by means of a liquid absorbed in the network, as defined in IUPAC Gold Book (doi: 10.1351/goldbook.G02600) .

In the gel electrolyte, the lithium ion conductivity continues to be essentially determined by the solvent used and the lithium conducting salt, i.e. the “liquid” electrolyte components.

By using the gel electrolyte in the hybrid electrolyte, the amount of unbound liquid electrolyte in the separator can be minimized or the use of a liquid electrolyte can even be completely avoided. In this way, the separator or a lithium-ion battery with such a separator is particularly robust against mechanical, thermal and/or electrical stress. In particular, in the event of mechanical damage, no solvent can escape from the separator or the lithium-ion battery due to the low flow tendency of the gel electrolyte, or at least only a reduced amount of solvent. In addition, the leakage of solvent can at least be delayed.

The binder is in particular selected from the group consisting of polyethylene oxide (PEO), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), cellulose, acrylate (for example polymethyl methacrylate), polyvinylpyrrolidone (PVP), styrene-butadiene rubber (SBR), polyisobutene (PIB) and mixtures thereof.

Any solvent suitable for use in lithium-ion batteries can be used as solvent.

For example, the solvent is selected from the group consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC), γ-butyrolactone (GBL), vinylene carbonate (VC), fluoroethylene carbonate (FEC) and mixtures thereof. Such organic solvents are tested, established and available worldwide for use in lithium-ion batteries.

Preferred conducting salts are lithium salts which have inert anions and which are preferably non-toxic. Suitable lithium salts are in particular selected from the group consisting of lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium triflate (LiCF ₃ SO ₃ ), lithium bis(trifluoromethylsulfonyl)amide (LiTFSI), lithium bis(fluorosulfonyl)amide (LiFSi), lithium bis(oxalato)borate (LiBOB) and mixtures thereof.

The composite material can contain between 5 and 95% by weight of gel electrolyte, based on the total weight of the composite material. A proportion of less than 5% by weight can lead to too low a lithium ion conductivity within the composite material, while a proportion of more than 95% by weight would mean that the proportion of the inorganic lithium ion conductor would have to be reduced too much, which could have a negative effect on the performance of the separator or a lithium ion battery with such a separator.

The composite material preferably comprises from 22 to 78 wt.% of gel electrolyte, particularly preferably from 25 to 75 wt.%, in each case based on the total weight of the composite material.

The gel electrolyte may comprise from 15 to 60 wt.% binder, from 40 to 85 wt.% solvent and/or up to 10 wt.% lithium conductive salt, each based on the total weight of the gel electrolyte.

In particular, the gel electrolyte consists of 15 to 60 wt.% binder, 40 to 85 wt.% solvent and 0.1 to 5 wt.% lithium conductive salt, each based on the total weight of the gel electrolyte, whereby the contents of all components add up to 100 wt.%.

In order to generate a particularly high lithium ion conductivity within the hybrid electrolyte, the gel electrolyte can have a lithium ion conductivity of at least 10 ^-6 S/cm.

The lithium ion conductivity can be determined using the “GITT” (galvanostatic intermittent titration technique), as used for example in W. Weppner and RA Huggins: “Determination of the Kinetic Parameters of Mixed-Conducting Electrodes and Application to the System” (J. Electrochem. Soc., 124 (1977), pp. 1569-1578, doi: 10.1149/1.2133112 ) is described.

Inorganic lithium ion conductor

The type of inorganic lithium ion conductor is fundamentally not further restricted as long as it can be processed into a hybrid electrolyte with the gel electrolyte used.

In particular, the inorganic lithium ion conductor can be selected from the group consisting of garnets, perovskites, sulfides, oxides or mixtures thereof.

For example, the inorganic lithium ion conductor is selected from the group consisting of garnets, perovskites, sulfidic lithium ion conductors, NASICON-type lithium ion conductors, LISICON-type lithium ion conductors, LiPON-type lithium ion conductors, LISON-type lithium ion conductors, LIPOS-type lithium ion conductors, LiBSO-type lithium ion conductors, LiSIPON-type lithium ion conductors, composite materials of lithium ion-conducting compounds and a mesoporous compound, and mixtures thereof.

Preferred inorganic lithium ion conductors are garnets, such as Li ₇ La ₃ Zr ₂ O ₁₂ (also known as “LLZ garnet”). These are used, for example, in the US 8 092 941 B2 described.

The composite material can contain from 5 to 95% by weight of inorganic lithium ion conductor, based on the total weight of the composite material. A proportion of less than 5% by weight can lead to too low a lithium ion conductivity within the composite material, while a proportion of more than 95% by weight would mean that the proportion of gel electrolyte would have to be reduced too much, which could have a negative effect on the performance of the separator or a lithium ion battery with such a separator.

The composite material preferably comprises from 22 to 78 wt.% of inorganic lithium ion conductor, particularly preferably from 25 to 75 wt.%, in each case based on the total weight of the composite material.

In order to generate a particularly high lithium ion conductivity within the hybrid electrolyte, the inorganic lithium ion conductor can have a lithium ion conductivity of at least 10 ^-6 S/cm.

Preferably, both the gel electrolyte and the inorganic ion conductor have a lithium ion conductivity of the same order of magnitude, for example a lithium ion conductivity of at least 10 ^-6 S/cm.

Manufacturing the separator

Methods for producing a separator according to the invention are known in principle in the prior art, whereby only masses or coating masses adapted to the desired composite material are used.

The separator can be manufactured using a carrier solvent or without the use of a carrier solvent.

For example, the separator is prepared using a carrier solvent according to MME Jacob et al: “Effect of PEO addition on the electrolytic and thermal properties of PVDF-LiClO4 polymer electrolytes” (Solid State lonics, 103 (3-4), 1997, pp. 267-276, doi: 10.1016/S0167-2738(97)00422-0 ) manufactured.

For this purpose, a blend of the binder(s) used is produced at boiling temperature at 100 °C in acetonitrile as a carrier solvent. The other components of the composite material are added to the blend to form a coating mass, whereby the other components are optionally also dissolved or suspended in acetonitrile beforehand. The coating mass is poured directly onto an electrode or onto a carrier film, for example made of polyethylene terephthalate (PETP), and the acetonitrile is evaporated. The result is a homogeneous separator film made of the composite material.

Alternatively, the separator can be produced without the use of a carrier solvent by means of a continuous kneader, as in the EP 1 277 250 B1 The coating mass produced in the kneader is applied directly to an electrode or to a carrier film, for example made of polyethylene terephthalate (PETP).

The invention is further solved by a lithium-ion battery comprising at least one separator as described above.

The features and properties of the separator according to the invention apply accordingly to the lithium-ion battery according to the invention and vice versa.

Preferably, more than one separator of the lithium-ion battery is a separator as described above, particularly preferably all separators of the lithium-ion battery.

In addition to the separator, the lithium-ion battery comprises at least one anode and at least one cathode, each of which comprises an active material.

Basically, an “active material” is a material that is able to reversibly absorb or release lithium or lithium ions.

The type of active material is not further restricted, so that all common active materials for anodes or cathodes can be used, depending on the type of electrode required. The only decisive factor is that the active materials used are chemically compatible with the compounds used in the separator.

In one variant, the active material of the anode is selected from the group consisting of synthetic graphite, natural graphite, hard carbon, soft carbon, silicon, silicon oxide, nano-silicon, silicon alloys, silicon-carbon composites, lithium titanate, metallic lithium, lithium alloys and mixtures thereof.

In one variant, the active material of the cathode is selected from the group consisting of layered oxides, spinels, olivines and mixtures thereof.

Further features and characteristics of the invention will become apparent from the following description of an exemplary embodiment, which is not to be understood in a limiting sense, as well as from the example and the drawing.

The single figure shows schematically a lithium-ion battery according to the invention with a separator according to the invention.

The single figure shows schematically a separator 10 according to the invention, which is used in a lithium-ion battery 12.

The separator 10 is arranged between an anode 14 and a cathode 16 and serves for electrical insulation and for the exchange of lithium ions between the anode 14 and the cathode 16.

The composition of the anode 14 and the cathode 16 is not further restricted as long as they are chemically compatible with the materials used in the separator 10.

The separator 10 consists of a composite material 18, which in turn comprises a hybrid electrolyte 20.

The hybrid electrolyte 20 is composed of an inorganic lithium ion conductor 22 and a gel electrolyte 24, wherein the gel electrolyte 24 consists of a binder 26, a solvent and a lithium conductive salt, which are gelled together.

Table 1 shows an exemplary composition of the composite material 16 of the separator 10. Table 1: Composition of the composite material 18 of a separator according to the invention, weight proportions in each case based on the total weight of the composite material 18. Percentage in wt.% function PVdF-HFP (Kynar 2801) 10 Gel electrolyte binder PEO 10 Gel electrolyte binder _LiBF4 10 Lithium conductive salt of the gel electrolyte EC 15 Solvent of the gel electrolyte DMC 30 Solvent of the gel electrolyte Li ₇ La ₃ Zr ₂ O ₁₂ (LLZ garnet) 25 inorganic lithium ion conductor

As indicated in the figure, the separator 10 has no free porosity, i.e. it is completely volumetrically compact. This means that all cavities or pores that are present between the particles in the composite material 18 are filled by the gel electrolyte 24. In other words, all cavities and pores that are present between the particles of the inorganic lithium ion conductor 22 within the hybrid electrolyte 20 are filled by the gel electrolyte 24.

The separator 10 therefore has no porosity in which unbound solvent could collect. In this way, even if the separator 10 is subjected to mechanical, thermal and/or electrical stress, no liquid electrolyte can escape. Rather, all of the solvent present in the composite material 18 is absorbed and gelled in the gel electrolyte 24.

The separator 10 according to the invention is characterized by high performance and at the same time high resistance to mechanical, thermal and electrical stresses.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed 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 accepts no liability for any errors or omissions.

Cited patent literature

• EP 3714078 B1 [0003]
• DE 102018000272 A1 [0003]
• DE 102014226390 A1 [0007]
• EP 1277250 B1 [0013, 0048]
• US 8092941 B2 [0039]

Cited non-patent literature

• P. Knauth: “Inorganic solid Li ion conductors: An overview” (Solid State lonics, 180 (2009), pp. 911-916, doi: 10.1016/j.ssi.2009.03.022 [0006]
• T. Wöhrle et al.: Sol-Gel Synthesis of the Lithium-Ion Conducting Perovskite La _0.57 Li _0.3 TiO ₃ - Effect of Synthesis and Thermal Treatments on the Structure and Conducting Properties (lonics, 2 (1996), pp. 442-445 , doi: 10.1007/BF02375824 [0006]
• IUPAC Gold Book (doi: 10.1351/goldbook.G02600) [0023]
• W. Weppner and R. A. Huggins: “Determination of the Kinetic Parameters of Mixed-Conducting Electrodes and Application to the System” (J. Electrochem. Soc., 124 (1977), pp. 1569-1578, doi: 10.1149/1.2133112 [0035 ]
• MME Jacob et al: “Effect of PEO addition on the electrolytic and thermal properties of PVDF-LiClO ₄ polymer electrolytes” (Solid State lonics, 103 (3-4), 1997, pp. 267-276, doi: 10.1016/S0167- 2738(97)00422-0 [0046]

Claims (9)

Separator (10) for a lithium ion battery (12), comprising a composite material (18) comprising a hybrid electrolyte (20), wherein the hybrid electrolyte (20) contains an inorganic lithium ion conductor (22) and a gel electrolyte (24). Separator (10) after Claim 1 wherein the gel electrolyte (24) comprises a binder (26), a solvent and a lithium conductive salt which are gelled together. Separator (10) after Claim 1 wherein the binder (26) is selected from the group consisting of polyethylene oxide, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, cellulose, acrylate, polyvinylpyrrolidone, styrene-butadiene rubber, polyisobutene and mixtures thereof. Separator (10) after Claim 2 or 3 , wherein the solvent is selected from the group consisting of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, propylene carbonate, γ-butyrolactone, vinylene carbonate, fluoroethylene carbonate and mixtures thereof. Separator (10) according to one of the Claims 2 until 4 , wherein the lithium conductive salt is selected from the group consisting of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium triflate, lithium bis(trifluoromethylsulfonyl)amide, lithium bis(oxalato)borate and mixtures thereof. Separator (10) according to one of the preceding claims, wherein the composite material (18) comprises 5 to 95 wt.% of gel electrolyte, based on the total weight of the composite material (16). Separator (10) according to one of the preceding claims, wherein the inorganic lithium ion conductor (22) is selected from the group consisting of garnets, perovskites, sulfides, oxides or mixtures thereof. Separator (10) according to one of the preceding claims, wherein the composite material (18) comprises 5 to 95 wt.% of inorganic lithium ion conductor (22), based on the total weight of the composite material (16). Lithium ion battery (12) comprising a separator (10) according to one of the preceding claims.

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