DE102023102564A1 - Electrode for a lithium ion battery comprising a hybrid electrolyte and lithium ion battery - Google Patents

Abstract

An electrode (10) for a lithium ion battery (12) comprises a composite material (16) with a hybrid electrolyte (18) and an active material (20) accommodated in the hybrid electrolyte (18). The hybrid electrolyte (18) 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 an electrode for a lithium-ion battery, with a composite material comprising a hybrid electrolyte, and to a lithium-ion battery comprising such an electrode.

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 isolate them.

It is known to use lithium-ion batteries with a liquid electrolyte that wets the electrodes and the separator and enables 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 include 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 (Ionics, 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 an electrode and a lithium-ion battery with improved performance. In particular, the electrode or the lithium-ion battery should be robust against mechanical, thermal and/or electrical stress.

The object is achieved according to the invention by an electrode for a lithium ion battery, with a composite material comprising a hybrid electrolyte and an active material accommodated in the 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 electrode - and thus in a lithium-ion battery with such an electrode - 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 electrodes 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 electrodes 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 applied to a current collector in particular. The current collector serves as an electrically conductive carrier and for electrical contact with the electrode. Depending on whether the electrode is a cathode or an anode, the current collector is made of aluminum or copper, for example.

Preferably, the electrode consists of the composite material and the current collector. In other words, the electrode is preferably free of unbound liquid electrolyte.

The electrode has in particular a porosity of 8% or less, preferably 4% or less, particularly preferably 2% or less. In this way, a high volumetric energy density of the electrode is achieved. In addition, it is not necessary to additionally impregnate the electrode with a liquid electrolyte in order to provide sufficient lithium ion conductivity within the electrolyte.

The porosity can be determined using mercury porosimetry.

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

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 assumes the function of the electrode binder for holding the particles in the electrode together 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 the 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 electrode can be minimized or the use of a liquid electrolyte can even be completely dispensed with. In this way, the electrode or a lithium-ion battery with a sol chen electrode is particularly robust against mechanical, thermal and/or electrical stress. In particular, in the event of mechanical damage, no solvent can escape from the electrode 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 escape 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 (e.g. polymethyl methacrylate), polyvinylpyrrolidone (PVP), styrene-butadiene rubber (SBR) 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) and mixtures thereof.

The composite material can contain between 4 and 25% by weight of gel electrolyte, based on the total weight of the composite material. A proportion of less than 4% by weight can lead to too low a lithium ion conductivity within the composite material, while a proportion of more than 25% by weight would mean that the proportion of the inorganic lithium ion conductor or the proportion of the active material would have to be reduced too much, which could have a negative effect on the performance of the electrode or a lithium ion battery with such an electrode.

The composite material preferably comprises from 5 to 20 wt.% of gel electrolyte, particularly preferably from 6 to 18 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 from 0 to 5 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 between 4 and 50% by weight of inorganic lithium ion conductor, based on the total weight of the composite material. A proportion of less than 4% by weight can lead to too low a lithium ion conductivity within the composite material, while a proportion of more than 50% by weight would mean that the proportion of the gel electrolyte or the active material would have to be reduced too much, which could have a negative effect on the performance of the electrode or a lithium ion battery with such an electrode.

The composite material preferably comprises from 5 to 25 wt.% of inorganic lithium ion conductor, particularly preferably from 6 to 15 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.

Active material

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 desired.

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

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

The composite material can comprise from 46 to 92% by weight of active material, based on the total weight of the composite material. At a content of less than 46% by weight, the achievable capacity of a lithium-ion battery with such an electrode is increasingly too low, while at a content of more than 92% by weight, the proportion of hybrid electrolyte is reduced to such an extent that the active material is increasingly no longer in sufficient contact with the hybrid electrolyte, which in turn has a negative effect on the performance of the electrode.

The composite material preferably comprises from 60 to 90 wt.% of active material, particularly preferably from 65 to 86 wt.%, in each case based on the total weight of the composite material.

Electrical conductive additive

The composite material may further contain an electrically conductive additive to further increase the electrical conductivity of the composite material.

Especially in the case that the electrode is a cathode, the electrically conductive additive is used.

The conductive additive can be selected from the group consisting of conductive carbon black, carbon nanotubes, graphene, graphite, expanded graphite and carbon nanofibers (CNT), porous carbons, vapor grown carbon fibers (VGCF) and combinations thereof.

The composite material may comprise from 0 to 5 weight percent of the electrically conductive additive based on the total weight of the composite material.

The composite material preferably comprises from 0 to 4 wt.% of conductive additive, particularly preferably from 0 to 3 wt.%, in each case based on the total weight of the composite material.

Manufacturing the electrode

Methods for producing an electrode according to the invention are known in principle in the prior art, whereby only coating compositions adapted to the desired composite material are used.

The electrode can be prepared using a carrier solvent or without the use of a carrier solvent.

For example, the electrode 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 Ionics, 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 onto the current collector and the acetonitrile is evaporated. The result is a homogeneous electrode film made of the composite material.

Alternatively, the electrode can be produced without the use of a carrier solvent using a continuous kneader, as in the EP 1 277 250 B1 The coating mass produced in the kneader is applied directly to the current collector.

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

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

Preferably, more than one electrode of the lithium-ion battery is an electrode as described above, particularly preferably all electrodes of the lithium-ion battery.

• - 1 schematisch eine erfindungsgemäße Elektrode, die als Anode ausgebildet ist, und
• - 2 schematisch eine erfindungsgemäße Elektrode, die als Kathode ausgebildet ist.

Further features and characteristics of the invention emerge from the following description of exemplary embodiments, which are not to be understood in a limiting sense, as well as the examples and drawings. In these:

• - 1 schematically an electrode according to the invention, which is designed as an anode, and
• - 2 schematically an electrode according to the invention, which is designed as a cathode.

1 shows schematically an electrode 10 according to the invention, which is used in a lithium-ion battery 12.

The electrode 10 consists of a current collector 14 and a composite material 16 applied to the current collector 14.

The composite material in turn comprises a hybrid electrolyte 18 in which an active material 20 is accommodated.

The hybrid electrolyte 18 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 example composition of the composite material 16 of the electrode 10. Table 1: Composition of the composite material 16 of an electrode according to the invention, weight proportions in each case based on the total weight of the composite material 16. Percentage in wt.% function Synthetic Graphite (SMG-A5) 66 Negative active material PVdF-HFP (Kynar 2801) 2 Gel electrolyte binder PEO 2 Gel electrolyte binder _LiBF4 2 Lithium conductive salt of the gel electrolyte EC 6 Solvent of the gel electrolyte DMC 6 Solvent of the gel electrolyte Li ₇ La ₃ Zr ₂ O ₁₂ (LLZ garnet) 16 inorganic lithium ion conductor

As can be seen from Table 1, the 1 The electrode 10 shown is an anode. The associated current collector 14 is accordingly made in particular of copper.

As in 1 As indicated, the electrode 10 has no free porosity, i.e. it is volumetrically completely compact. This means that all cavities or pores that are present between the particles of the active material 20 are filled by the hybrid electrolyte 18 and all cavities and pores that are present between the particles of the inorganic lithium ion conductor 22 within the hybrid electrolyte 18 are in turn filled by the gel electrolyte 24.

The electrode 10 therefore has no porosity in which unbound solvent could collect. In this way, even if the electrode 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 16 is absorbed and gelled in the gel electrolyte 24.

Fundamentally, however, it is also conceivable that a certain degree of porosity remains in the composite material 16, wherein the electrode 10 in particular has a porosity of 8% or less.

2 shows a further embodiment of the electrode 10 according to the invention, in which the electrode 10 is designed as a cathode.

In the further embodiment, the composite material 16 comprises, in addition to the active material 20 and the hybrid electrolyte 18, a conductive additive 28.

Furthermore, in the further embodiment, the current collector 14 is made of copper.

Table 2 shows an exemplary composition of the composite material 16 of the electrode 10 according to the further embodiment. Table 2: Composition of the composite material 16 of an electrode according to the invention according to the further embodiment, weight proportions in each case based on the total weight of the composite material 16. Percentage in wt.% function NMC 622 66 Positive active material PVdF-HFP (Kynar 2801) 2 Gel electrolyte binder PEO 2 Gel electrolyte binder _LiBF4 2 Lithium conductive salt of the gel electrolyte EC 5 Solvent of the gel electrolyte DMC 6 Solvent of the gel electrolyte Li ₇ La ₃ Zr ₂ O ₁₂ (LLZ garnet) 14 inorganic lithium ion conductor Carbon Black C65 3 Electrical conductive additive

The electrode 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 [0006]
• EP 1277250 B1 [0011, 0057]
• US 8092941 B2 [0037]

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 [0005]
• 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 (Ionics, 2 (1996), pp. 442-445, doi : 10.1007/BF02375824 [0005]
• 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 [0033 ]
• MME Jacob et al: “Effect of PEO addition on the electrolytic and thermal properties of PVDF-LiClO ₄ polymer electrolytes” (Solid State Ionics, 103 (3-4), 1997, pp. 267-276, doi: 10.1016/S0167- 2738(97)00422-0 [0055]

Claims (10)

Electrode (10) for a lithium ion battery (12), with a composite material (16) comprising a hybrid electrolyte (18) and an active material (20) accommodated in the hybrid electrolyte (18), wherein the hybrid electrolyte (18) contains an inorganic lithium ion conductor (22) and a gel electrolyte (24). Electrode (10) after Claim 1 wherein the gel electrolyte (24) comprises a binder (26), a solvent and a lithium conductive salt which are gelled together. Electrode (10) after Claim 2 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 and mixtures thereof. Electrode (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. Electrode (10) according to one of the Claims 2 until 4 , wherein the lithium conducting salt is selected from the group consisting of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium triflate, lithium bis(trifluoromethylsulfonyl)amide and mixtures thereof. Electrode (10) according to one of the preceding claims, wherein the composite material comprises 4 to 25 wt.% of gel electrolyte, based on the total weight of the composite material (16). Electrode (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. Electrode (10) according to one of the preceding claims, wherein the composite material (16) comprises 4 to 50 wt.% of inorganic lithium ion conductor (22), based on the total weight of the composite material (16). Electrode (10) according to one of the preceding claims, wherein the composite material (16) comprises 46 to 92 wt.% of active material, based on the total weight of the composite material (16) and/or 0 to 5 wt.% of an electrically conductive additive (28), based on the total weight of the composite material (16). Lithium ion battery (12) comprising at least one electrode (10) according to one of the preceding claims.

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