DE102016216555A1 - Shaped electrochemical cell - Google Patents

Abstract

The present invention relates to a composite electrode structure for a lithium ion cell comprising a solid electrolyte material having a porous foam structure, a conductivity additive incorporated in the pores of the foam structure, and an active material incorporated in the pores of the foam structure.

Description

Technical area

The present invention relates to an electrode for a lithium ion cell comprising a foamed inorganic or polymeric lithium ion conductor and a corresponding lithium ion cell comprising the electrode.

Technical background

At present, liquid electrolyte lithium ion batteries are generally used, which basically comprise a negative electrode (anode), a positive electrode (cathode), and an intermediate separator impregnated with a non-aqueous liquid electrolyte. The anode and cathode each comprise an anode or cathode active material, which is optionally applied to a current collector using a binder and / or an additive for improving the electrical conductivity. The liquid electrolyte used is a polar aprotic solvent, usually a mixture of organic carbonic esters in which a conducting salt such as LiPF _{6 is} dissolved. The electrode structure of such a cell is generally porous, so that the liquid electrolyte comes into contact with the active material particles and an exchange of lithium ions is possible.

However, such liquid electrolyte cells are subject to certain limitations. Thus, due to the limited stability of the electrolyte, the maximum cell voltage is currently limited to about 4.3 to 4.4 volts. Irreversible reactions between electrolyte solvent and electrode can also lead to a loss of capacity with increasing number of cycles. Furthermore, the electrolyte solvents used are highly flammable organic compounds, which in the event of a malfunction, for example in the event of overheating of the cell due to an internal short circuit, represents a fire hazard. In addition, LiPF ₆ , which is commonly used as conductive salt, can be decomposed in the event of fire to form highly toxic, corrosive species such as HF and POF ₃ .

The use of solid electrolytes is one way to avoid these risks on the one hand and to further increase the energy density and durability on the other hand. Solid-state electrolytes on the one hand glassy or ceramic inorganic compounds are understood to have conductivity for lithium ions. On the other hand, polymer electrolytes are also counted among the solid-state electrolytes.

In the prior art, solid-state lithium ion batteries in thin-film construction with inorganic solid electrolyte and capacities in the range of a few μAh to mAh are known, which can be used, for example, for the power supply of very small consumers such as smart cards or the like. Such cells are usually single-layered and can be produced, for example, by vapor deposition techniques. The problems associated with solid electrolyte in terms of limited lithium ion conductivity and interface effects are at least partially compensated by the thin layer thicknesses. For higher capacities, e.g. However, such a construction is impracticable to drive vehicles are necessary. Instead, electrodes with a certain minimum of active material loading are required to provide the necessary capacity. While in the thin-film cells practically all the active material is in direct contact with both the current collector and the electrolyte and the effect of the layer thickness can be largely neglected, with correspondingly larger layer thicknesses, the electrical conductivity and the lithium ion conductivity in the interior ("bulk") of the layer relevant factor. The electrical conductivity within the solid-state electrode can be achieved by adding a conductivity additive, such as in conventional liquid electrolyte cells with porous electrodes. Ensure conductive soot if required. However, unlike the liquid electrolyte cells, lithium ion conductivity must be provided by the electrode structure itself.

This can be achieved by using a composite material of active material, solid electrolyte and optionally an electrically conductive additive and optionally by a binder. The solid electrolyte, which is in the form of particles or can also form a uniform matrix in the case of polymer electrolytes or glassy inorganic solid electrolytes, provides lithium ion conductivity within the composite. It thus assumes the role of the liquid electrolyte which has penetrated into the pore structure in the case of conventional cells. Such a composite material can be produced, for example, by sintering and / or pressing, depending on the materials used, and preferably has the lowest possible porosity, since the presence of voids brings about a deterioration in the contact between active material and solid electrolyte.

Summary of the invention

Problem to be solved by the invention

As described above, for cells having a solid electrolyte, composite electrodes, the active material particles, solid electrolyte particles for imparting lithium ion conductivity are usually used and optionally, a conductivity additive for imparting electrical conductivity used.

A disadvantage of such composite materials is often that a relatively high proportion of solid electrolyte is necessary, resulting in a correspondingly reduced proportion of active material. Thus, the ratio of active material to passive material can become unfavorable and the volume and weight-related energy density decreases.

Another disadvantage may be that due to the volume changes in the active material due to the intercalation and Deinterkalation of lithium ions mechanical stresses can form. This can further limit the loading of active material and also lead to increased wear of the electrode with increasing number of charging cycles. If a conversion material is to be used instead of an intercalation active material, this effect is even more pronounced.

This problem is compounded in composite electrodes by the fact that the electrode generally has to be very compact in order to achieve a high lithium ion conductivity in order to provide maximum contact between active material and solid electrolyte. Therefore, unlike electrodes for liquid electrolyte cells, the electrode hardly contains any void volume that could trap the volume change of the active material.

The present invention is therefore based on the object to provide a composite electrode material which allows an increased loading of active material and also has an improved resistance to mechanical stresses.

Summary of the invention

• – Ein Festkörperelektrolytmaterial mit poröser Schaumstruktur und Lithiumionenleitfähigkeit;
• – Einen Leitfähigkeitszusatz mit elektrischer Leifähigkeit, der in die Poren der Schaumstruktur eingebracht ist; und
• – Ein Aktivmaterial, das in die Poren der Schaumstruktur eingebracht ist.

To achieve the above objects, the present invention provides a composite electrode structure for a lithium ion cell, comprising:

• A solid electrolyte material having a porous foam structure and lithium ion conductivity;
• - A conductivity additive with electrical conductivity, which is introduced into the pores of the foam structure; and
• An active material incorporated in the pores of the foam structure.

Optionally, the composite electrode structure may further comprise a binder also incorporated in the pores of the foam structure.

The solid electrolyte material may be, in particular, an inorganic lithium ion conductor or a polymer electrolyte.

Another aspect of the present invention relates to a lithium-ion cell employing this composite electrode structure.

According to the invention, the porous foam structure of the solid electrolyte functions here as a kind of framework which provides stability against mechanical stresses due to volume changes of the active material. Furthermore, the foam structure makes it possible to simultaneously provide a high lithium ion conductivity and to provide a void volume which acts as a buffer volume and thus reduces the mechanical stresses.

Description of the drawings

1 Fig. 12 schematically shows the principle of manufacturing a composite electrode structure according to a first embodiment in which the foamed solid electrolyte material is mixed with the conductivity additive and the active material.

2 schematically shows the first step of producing a composite electrode structure according to a second embodiment, in which the foamed solid electrolyte material is first coated with the conductivity additive.

3 is a schematic view of the thus obtained, coated with conductivity additive foamed solid electrolyte.

4 shows the second step of manufacturing the composite electrode structure according to the second embodiment, in which the coated foamed solid electrolyte material is mixed with the active material.

5 Fig. 12 schematically shows the production of electrodes by connecting a current collector foil to the composite electrode structure according to the first or second embodiment.

6 shows the configuration of solid-state lithium ion cells according to the invention, which are constructed of two electrodes according to the first embodiment or two electrodes according to the second embodiment. The two half-cells are each separated by a solid electrolyte layer, which acts as a separator.

7 shows the configuration of another solid-state lithium-ion cell according to the invention, which includes an electrode according to the first and one Using electrode according to the second embodiment.

8th shows the configuration of hybrid cells according to the invention, in which an electrode according to the invention according to the first embodiment or second embodiment is combined with a conventional electrode with liquid electrolyte.

Detailed description of the invention

In the following, the individual features and components of the present invention will be described in more detail.

Solid electrolyte

By using a solid electrolyte having a porous foam structure, it becomes possible to maximize the contact area between active material and solid electrolyte. Thus, with comparatively low weight fraction of solid electrolyte, a very good lithium ion transfer to the active material can be achieved. The foam structure also forms a far-reaching network of ion-conducting paths, which further improves ionic conductivity within the electrode structure, as effects such as interfacial resistances in the transition between individual solid electrolyte particles are minimized. It is possible to use both inorganic solid-state electrolytes, for example ceramic solid-state electrolytes or glass-like solid-state electrolytes, and also polymer electrolytes as the solid-state electrolyte.

For the provision of inventively employable inorganic solid electrolytes with foam structure, basically known methods can be used from the state of the art, as are used, for example, in the production of foamed ceramics or glasses for filter purposes. In particular, template methods can be used, such as in DE 30 20 630 . DE 35 40 451 or EP 0 260 826 described.

In this case, a polymer foam such as polyurethane foam, which acts as a template, impregnated with precursor compounds (precursor compounds) of the inorganic solid electrolyte. The template and the precursor compounds are decomposed by subsequent pyrolysis, and from the precursors, a ceramic material is formed, which holds the foam structure of the template. In this production method, the pore size can be controlled by the porosity of the template.

Examples of usable solid state inorganic electrolytes include ceramic or glassy oxide, sulfide or phosphate based solid state electrolytes. Preferred examples of the oxide solid electrolytes include garnet type solid electrolytes such as Li _x La ₃ M ₂ O ₁₂ (where M = Nb, Ta, Zr and x = 15-2 * [valence of M], usually x 5 to 7) or doped or substituted derivatives thereof, such as Li ₆ La ₂ BaTa ₂ O ₁₂ , Li ₇ La ₃ Zr ₂ O ₁₂ or Al-stabilized variants, such as Li _6.24 La ₃ Zr ₂ Al _0.24 O _11.98 , LISICON type lithium ion conductors , or perovskites like LLTO. As precursors, for example, oxides, hydroxides, carbonates or nitrates can be used. For example, the synthesis can be carried out by impregnating the template with a precursor solution, precipitating the corresponding hydroxides, and then drying and sintering to obtain the solid oxide electrolyte.

Examples of solid state sulfidic electrolytes include Li ₂ S - P ₂ S ₅ or Li ₂ S - SiS ₂ - Li ₃ N in varying mixing ratios. As precursors, for example, Li ₂ S and P ₂ S ₅ can be used dissolved in suitable solvents such as NMF. As a phosphate-based solid electrolyte, for example, lithium-aluminum-titanium phosphates (LATP) into consideration, for example, with the stoichiometry Li _1.5 Al _0.5 Ti _1.5 (PO ₄ ) ₃ or Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ . Such solid-state electrolytes can be obtained by sol-gel synthesis, as precursors CH ₃ COOLi, Al (C ₃ H ₇ O) ₃ , Ti (C ₃ H ₇ O) ₄ and (NH ₄ ) ₂ HPO ₄ , optionally with C ₃ H ₇ OH and H ₂ O are used.

As an alternative to the inorganic solid electrolyte, it is also possible to use a polymer electrolyte. The polymer electrolyte comprises a polymer material having lithium ion conductivity, such as polyethylene oxide (PEO), polymethyl methacrylate (PMMA), polyphenylene ether (PPO), phosphazene polymers such as MEEP or polyacrylonitrile (PAN). To adjust the lithium ion conductivity, the polymer electrolytes usually contain a conductive salt such as lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, LiN (SO ₂ F) ₂ (LiFSI) or LiN (SO ₂ CF ₃ ) ₂ (LiTFSI). A preferred polymer electrolyte is PEO in conjunction with LiN (SO ₂ F) ₂ (LiFSI) or LiN (SO ₂ CF ₃ ) ₂ (LiTFSI). For further adjustments of the properties of the polymer, it is also possible to use copolymers or to make changes to the basic structure of the polymer.

The foaming of the polymer electrolyte may be effected by known extrusion or spraying methods. For this purpose, a mixture of polymer, conductive salt and foaming agent is melt extruded. The foaming agent is not specifically limited. For example, volatile hydrocarbons or halogenated hydrocarbons can be used, which evaporate during extrusion or spraying and thus cause foaming. Compounds which decompose upon heating to form gas also come into consideration. Alternatively, foaming may be effected by introducing gases during the polymerization.

The solid electrolyte foams thus obtained have, for example, a porosity of 50 to 99%, preferably 80 to 97%. The porosity can be determined, for example, by means of mercury porosimetry or pycnometry. If the foam is in a form whose external dimensions can be readily determined (eg as a block, sheet or foil), then the porosity can also be simply determined by the apparent density, which is determined from the dimensions and weight of the foam, and Density of the solid electrolyte material that composes the foam can be calculated.

The mean pore size is for example in the range of 0.01 to 500 .mu.m, preferably 0.1 to 50 microns. The mean pore size here refers to the pore volume median pore diameter, which can be determined for example by mercury porosimetry.

The foam may be provided, for example, in the form of particles, which may then be loaded with active material and conductivity additive by mixing or using precursor techniques, as described below. In this case, the volume-average particle diameter is, for example, 0.05 to 50 μm, preferably 0.1 to 30 μm, more preferably 0.5 to 5 μm, and the average pore size is correspondingly smaller than the particle size.

Alternatively, the foam may also be provided in the form of a layer or film and loaded, for example, by impregnation or precursor techniques.

active material

The active material is not particularly limited, and usual cathode and anode active materials may be used for lithium-ion batteries.

Examples of usable cathode active materials are LiMO ₂ type layered metal oxides (M = Co, Ni, Mn) such as LiCoO ₂ (LCO), LiNiO ₂ , LiMnO ₂ or mixed oxides such as LiNi _x Mn _y Co _z O ₂ (with x + y + z = 1, NMC) or LiCo _0.85 Al _0.15 O ₂ (NCA), spinels such as LiMn ₂ O ₄ (LMO) or olivine type crystallizing phosphates such as LiFePO ₄ (LFP) or LiFe _{0, 15} Mn _0.85 PO ₄ (LFMP). Another class of cathode materials are conversion materials such as transition metal fluorides such as FeF ₃ , NiF ₂ , CoFeF _3, etc. The use of a mixture of two or more of these materials is also contemplated.

As the anode active material, for example, carbon-based intercalating materials, lithium titanate materials, lithium, silicon or carbon / silicon may be used.

Loaded with a mixture of active material and conductivity additive

In the pore structure of the foamed solid electrolyte active material and conductivity additive can be introduced as a mixture.

In a first embodiment, particles of solid electrolyte foam, active material particles and conductivity additive are mechanically mixed, as in 1 shown schematically. The mixing can be done in the dry state, or as a dispersion in a solvent.

The mean particle sizes of the active material particles and of the conductivity additive are smaller than the average pore size of the solid electrolyte foam. Typical volume average particle sizes of the active material particles are in the range of 10 nm to 20 μm, preferably 50 nm to 5 μm, more preferably 0.1 to 1 μm.

As the conductive additive, a carbon material having a particle size of preferably 1 nm to 10 μm, more preferably 10 nm to 5 μm, especially 50 nm to 1 μm, such as carbon black or graphite particles, may be used. Also contemplated is the use of carbon nanotubes (CNTs) or graphene.

The volume ratio of active material and conductivity additive is usually in the range of 70:30 to 99: 1, preferably 80:20 to 95: 5. The ratio of active material and conductivity additive to the solid electrolyte foam can be selected in consideration of the porosity of the solid electrolyte foam so that the pore volume of the solid electrolyte is filled.

The coating of the pores can be done by dry mixing, or the active material and the conductive additive can be dispersed in a suitable solvent and mixed with the particles of the solid electrolyte foam or the solid state foam layer. Optionally, a binder such as PVdF may be added. The resulting mixture may optionally be screened to isolate the loaded particles of the solid electrolyte foam.

Subsequently, the mixture or the isolated, loaded particles can be pressed and / or sintered to produce the composite electrode.

If the foamed solid electrolyte is in the form of a film or a film, the coating of the pores can be carried out by impregnation with a dispersion of the active material and the conductivity additive in a suitable solvent, optionally with the addition of a binder. For uniform impregnation, the ratio of the median pore diameter of the solid electrolyte foam to the volume median diameter of the dispersed active material and the conductivity additive is preferably 2: 1 or greater, more preferably 5: 1 or greater, even more preferably 10: 1 or greater.

As an alternative to mechanical mixing or impregnation with a dispersion, it is also possible to introduce the active material and / or the conductivity additive into the pore structure using precursor compounds. For this purpose, the solid electrolyte foam is impregnated with the precursors, which are then subsequently converted into the active material or the conductivity additive, which are deposited in the interior of the pore structure.

For active materials based on oxide or phosphate, for example, soluble salts or alcoholates are suitable as precursors, which are precipitated in the pores after impregnation, for example by controlled hydrolysis or pH change. In addition, thermally decomposable precursors such as carbonates or nitrates can be used, in particular in conjunction with temperature-resistant solid electrolyte.

For carbon-based anode active materials, pyrolyzable organic compounds can be used, for example, sugar materials such as sucrose or polysaccharides, hydrocarbons or polymer solutions which are converted to carbon by thermal decomposition. For anode-active materials based on silicon come as precursors silicon compounds such as silanes, eg SiH ₄ into consideration. The pyrolysis of these carbon or silicon compounds is preferably carried out under inert gas or under reducing atmosphere.

In the case of temperature-stable solid-state electrolytes, a precursor process is also suitable for introducing the conductivity additive. For this purpose, the solid electrolyte is impregnated with a carbon-containing compound, which is then pyrolyzed, preferably under an inert gas atmosphere, to deposit carbon. The procedure is essentially the same as that described above for carbon-based anode-active materials. Suitable precursors are, for example, sugars such as sucrose, which are dissolved in a suitable solvent.

The use of precursors allows a particularly intense adhesion between the active material particles and the solid electrolyte, so that optionally the binder can be saved. For the loading of solid electrolyte in film or film form, the use of precursor methods is particularly preferred.

Coating with conductivity additive, then loading with active material

In a second embodiment, the solid electrolyte foam is first coated with a conductivity additive, and then the active material and optionally further conductivity additive is introduced into the pore structure. Thus, a continuous electrically conductive layer can be applied to the extended network structures of the solid electrolyte, which brings with it the advantage of a very high electrical conductivity even with a comparatively small mass fraction of the conductivity additive.

2 schematically shows the coating of the solid electrolyte foam with the conductivity additive, and 3 Fig. 12 is a schematic view of the thus obtained expanded structure having high electrical conductivity. The subsequent loading with active material is in 4 shown.

The method for loading the solid electrolyte foam may basically be otherwise the same as described above, except that first the conductivity additive and then the active material is introduced. In the case of temperature-stable solid-state electrolytes, it is preferred to use a carbon-forming precursor material, for example based on sugars. In this case, the solid electrolyte is impregnated with a solution of the precursor and then the precursor is pyrolyzed.

Electrode configuration and cell configuration

Another aspect of the present invention relates to a positive or negative electrode for a lithium ion cell employing the composite electrode structure of the present invention.

For this purpose, the composite electrode structure is applied as a layer on a current conductor, as in 5 shown schematically. The current collector is typically metal foil. In the case of a negative electrode having a strong negative potential, copper foil is preferably used since Aluminum tends to form alloys with metallic lithium which can be formed under highly reducing conditions. For the positive electrode, aluminum foil is preferably used.

The preparation of the electrode can be carried out by a slurry process, wherein a slurry of the loaded solid electrolyte foam, optionally together with further active material, conductivity additive and / or a binder is applied to the current conductor, dried and pressed and / or sintered. Alternatively, a composite electrode structure layer may be first prepared by pressing and / or sintering, which is then optionally laminated to the current collector using a binder.

Another aspect of the present invention relates to lithium-ion cells comprising at least one of the electrodes of the invention.

The lithium-ion cells may be pure solid-state cells which comprise the electrodes according to the invention both as an anode and as a cathode. Alternatively, the solid state cell may comprise an electrode according to the invention and a conventional solid state electrode or a cathode according to the invention in conjunction with an anode of metallic lithium.

Between the anode and the cathode in each case a solid electrolyte layer is introduced, which is electrically insulating, but has conductivity for lithium ions, and acts as a separator. Suitable solid electrolyte are the same types of inorganic or polymeric solid electrolyte described for the foamed solid electrolyte. For example, the same electrolyte can be used in the electrode and in the separator layer. 6 shows the configuration of solid state cells with two electrodes according to the invention.

As an alternative to the pure solid-state cells, the electrode according to the invention can also be used for hybrid cells, in which one half-cell comprises a conventional porous electrode and a liquid electrolyte, and the other half-cell comprises the electrode according to the invention. Between the two half-cells again a solid electrolyte layer is introduced, which acts as a separator, or a conventional polymer separator, which is impregnated with liquid electrolyte. 7 schematically shows the configuration of such hybrid cells.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been 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 assumes no liability for any errors or omissions.

Cited patent literature

• DE 3020630 [0027]
• DE 3540451 [0027]
• EP 0260826 [0027]

Claims (12)

 A composite electrode structure for a lithium-ion cell, comprising: A solid electrolyte material having a porous foam structure and lithium ion conductivity; - A conductivity additive with electrical conductivity, which is introduced into the pores of the foam structure; An active material incorporated in the pores of the foam structure; and - Optional a binder. The composite electrode structure according to claim 1, wherein the solid electrolyte material is a polymer electrolyte comprising a polymer material having lithium ion conductivity and a conductive salt. The composite electrode structure according to claim 1, wherein the solid electrolyte material is a lithium ion conductivity inorganic solid electrolyte. The composite electrode structure according to any one of claims 1 to 3, wherein the conductivity additive is a carbon material. The composite electrode structure according to any one of claims 1 to 4, which is obtainable by simultaneously coating the pores of the foam structure with the conductivity additive and the active material. Composite electrode structure according to one of claims 1 to 4, which is obtainable in that the pores of the foam structure are first coated with conductivity additive, and then the active material and optionally further conductivity additive are introduced into the coated pores of the foam structure. The composite electrode structure according to any one of claims 1 to 6, wherein the active material is a cathode active material. The composite electrode structure according to any one of claims 1 to 6, wherein the active material is an anode active material. A composite electrode structure according to any one of claims 1 to 8, further comprising a binder. The composite electrode structure according to any one of claims 1 to 9, wherein the solid electrolyte material has a porosity of 50 to 99% and a median pore diameter of 0.01 to 500 μm. The composite electrode structure according to any one of claims 1 to 10, wherein the volume ratio of active material to conductivity additive is in the range of 70:30 to 99: 1. A lithium-ion cell comprising at least one electrode structure according to any one of claims 1 to 11.

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