DE102016216549A1 - Solid-state cell with adhesion-promoting layer - Google Patents

Abstract

The present invention relates to a solid state (all-solid-state) lithium battery comprising a positive electrode, a negative electrode an intermediate solid electrolyte layer as a separator, wherein between the positive electrode and the separator and / or between the negative electrode and the Separator an adhesion promoting layer of a polymeric lithium ion conductor is introduced.

Description

Technical area

The present invention relates to a solid state (all solid state) lithium ion battery having at least one adhesion promoting layer.

In the following description, the terms "lithium ion battery", "lithium ion rechargeable battery" and "lithium ion secondary battery" are used interchangeably. The terms also include the terms "lithium battery", "lithium-ion battery" and "lithium-ion cell" as well as all lithium or alloy batteries. Thus, the term "lithium ion battery" is used as a generic term for the conventional terms used in the prior art. In particular, a "battery" in the context of the present invention also includes a single or single "electrochemical cell".

Technical background

At present, lithium or lithium ion batteries are mostly used with liquid electrolyte, which essentially comprise a negative electrode (anode), a positive electrode (cathode) and an intermediate separator impregnated with a nonaqueous liquid electrolyte. The anode or cathode each comprise at least one 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 acid esters in which a lithium conducting salt such as lithium hexafluorophosphate (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 electrochemical 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 hydrogen fluoride (HF) and POF ₃ .

The use of solid electrolytes is a way to avoid these risks on the one hand and to further increase the energy density, reliability and durability on the other hand. Solid-state electrolytes on the one hand glassy or ceramic inorganic compounds are understood to have sufficient conductivity for lithium ions. On the other hand, the solid electrolytes also classical polymer electrolytes (for example based on polyethylene oxide (PEO) with Li-Leitsalz without solvent) are expected.

In the prior art solid state lithium-ion batteries are known in thin film construction with inorganic solid electrolyte and capacities in the range of some μAh to mAh, which can be used, for example, to power micro 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 or composite electrode loading are required to provide the necessary capacity. While in the thin-film cells virtually all of 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 greater layer thicknesses, the electrical conductivity and the lithium ion conductivity in the interior ("bulk") of the layer relevant and limiting 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 or composition itself.

This can be achieved by using a composite material of active material, solid electrolyte and optionally an electrically conductive additive. The solid electrolyte, which is in the form of particles or in the case of polymer electrolytes or glassy inorganic Also, solid state electrolytes can form a uniform matrix, providing lithium ion conductivity within the composite material. 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, which approaches zero, since the presence of voids brings about a deterioration of the contact between active material and solid electrolyte.

Summary of the invention

Problem to be solved by the invention

In the production of lithium or lithium-ion cells with liquid electrolyte, the electrode-separator ensembles are usually wound. On the other hand, the electrodes and separators (solid electrolyte layers) of solid-state cells are generally too rigid to be wound and therefore need to be stacked. For this purpose, electrodes or electrolyte layers are first placed loosely on top of each other. This can result in a shift of the layers, resulting in a reduction in productivity and increased rejects (e.g., internal fine defects due to faulty electrode positioning).

Furthermore, because of increased contact resistances between the interfaces, the performance and lifetime of the cell may be reduced.

According to the invention, these problems are to be solved by the use of a special binder or adhesion promoter, which has lithium-ion conductivity and increases the adhesion between electrode and separator and lowers the contact resistance.

Disclosure of the invention

• – eine positive Elektrode;
• – eine negative Elektrode; und
• – eine dazwischenliegende Festkörperelektrolytschicht aus einem anorganischen Festkörperelektrolyten als Separator;

wobei zwischen die positive Elektrode und den Separator und/oder zwischen die negative Elektrode und den Separator eine Haftungsvermittlungsschicht aus einem polymeren Lithiumionenleiter eingebracht ist. The present invention thus relates to a solid-state lithium-ion cell (all-solid-state cell), comprising:

• A positive electrode;
• A negative electrode; and
• An intermediate solid electrolyte layer made of an inorganic solid electrolyte as a separator;

wherein between the positive electrode and the separator and / or between the negative electrode and the separator, a bonding layer made of a polymer lithium ion conductor is introduced.

Another aspect of the present invention relates to the use of a polymeric lithium ion conductor as an adhesion promoting layer between the positive electrode and the separator and / or between the negative electrode and the separator in a solid state lithium ion cell.

A special case is a solid state cell with metallic lithium (e.g., lithium foil): Here, the coupling agent may be applied between lithium anode and inorganic lithium ion conductor. Cells with a metallic anode can be called a lithium cell.

Description of the drawing

1 outlines the structure of a solid state lithium ion cell according to the invention. The compound cathode schematically shown on the left side is composed of a current collector (Al foil) and a composite material composed of solid electrolyte particles ( 1 ) and cathode active material particles ( 2 ) built up. Similarly, the composite anode made of solid electrolyte particles shown on the right ( 1 ) and anode active material particles ( 3 ), which are applied to a copper foil as a current collector. In between there is a separator made of solid electrolyte particles ( 1 ). Between the composite cathode and the separator and between the separator and the composite anode, an adhesion-promoting layer of a polymer electrolyte according to the invention is introduced in each case.

Detailed description of the invention

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

electrodes

As a positive and negative electrode electrodes of basically known type can be used in the solid-state cell according to the invention.

Unlike a conventional liquid electrolyte cell, in a composite electrode for a solid state cell, the electrode structure must provide not only electrical conductivity but also lithium ion conductivity. For this purpose, composites of active material, solid electrolyte for lithium ion conduction and conductivity additive such as, for example, are generally used. Leitruß or Leitgraphit or carbons such as Carbo-Nano-Tubes (CNT) or graphene used for electrical conductivity.

As a solid electrolyte in such a composite electrode are inorganic lithium ion conductors into consideration, in which the ion conduction through Movement of lithium ions between vacancies or vacancies of the solid state structure takes place. These inorganic solid state electrolytes for lithium ion batteries are generally classified into amorphous (glassy), semi-crystalline and crystalline solid state electrolytes. A preferred example is garnet-type oxide lithium-ion conductors such as Li ₅ La ₃ M ₂ O ₁₂ (where M = Nb, Ta), Li ₇ La ₃ Zr ₂ O _12, or doped derivatives thereof such as Li ₆ La ₂ BaTa ₂ O ₁₂ . Further oxide lithium ion conductors are compounds of the type Li _{2 + 2x} Zn _1-x GeO ₄ , which are known by the name "LISICON" (Li super ionic conductor). Examples of sulfide-based solid-state electrolytes include Li ₂ SP ₂ S ₅ , which may be amorphous (as glass) or partially crystalline. Crystalline sulfide lithium ion conductors are also known, for example, Li _3.25 Ge _0.25 P _0.75 S ₄ and related compounds known as "thio-LISICON".

A composite material can be produced, for example, by sintering (in particular in the case of oxides) and / or pressing (in the case of sulfides). Preferably, the composite electrode structure is as compact and non-porous as the presence of voids brings about a deterioration of the contact between active material and solid electrolyte and increases the interfacial resistances.

As the active material for the positive electrode, the same materials are used, which are also used for cells with liquid electrolyte. These are usually compounds which can intercalate and deintercalate lithium ions under reduction or oxidation. Typical representatives are transition metal oxides with layer structure of the type LiMO ₂ (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 (NMC, with x + y + z = 1, eg x = y = z = 0.33) or LiCo _0.85 Al _0.15 O ₂ (NCA), including lithium-rich layer oxides (OLO), 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). According to the invention, LCO or an LCO-containing mixture of active materials is preferred. Alternatively, the use of conversion materials such as FeF ₃ comes into consideration.

As the negative electrode active material, the materials already known for liquid electrolyte cells may also be used, for example, carbon-based materials such as graphite, hard carbon, soft carbon, silicon, silicon alloy, carbon / silicon or lithium titanate. Since the dentrite deposition is inhibited in solid-state cells, metallic lithium, which is used for example in the form of a film, is also suitable for the negative electrode instead of the composite material.

The electrodes typically also include a current collector to which the composite material or metallic lithium is applied. Preferably, for electrodes having a potential to Li / Li ⁺ of more than 1 V, aluminum foil is used for anodes having a potential of 1 V or less to Li / Li ⁺ copper foil. The layer thickness of the composite material layer is generally 30 to 300 μm, preferably 50 to 200 μm, more preferably 80 to 150 μm. In the case of metallic lithium, the layer thickness may be in the range of 5 to 100 microns, preferably 15 to 50 microns.

The electrode can be produced by the raw material mixture, for example by so-called slurry coating applied directly to the current collector, dried and pressed or sintered. It is also possible to make a composite electrode as a freestanding film or on a plastic film and then laminate the film onto a metallic current collector foil (e.g., by lamination at elevated temperature and or pressure).

separator

Between the positive and the negative electrode, a solid electrolyte layer is introduced, which acts as a separator. Preferably, the same solid electrolyte material is used, which is also used in the composite material electrodes. The layer thickness of the separator layer is preferably thin with respect to the energy density and the internal resistance, usually 1 to 100 μm, preferably 2 to 50 μm, particularly preferably 5 to 25 μm.

The separator may, for example, likewise be produced as a film by sintering or pressing and then laminated onto one of the two electrodes, if appropriate using the adhesion-promoting layer described below. Alternatively, the separator may be applied as a further layer directly during the manufacture of the electrode, for example under pressing and sintering, or it may be deposited on the electrode surface by vapor deposition techniques such as sputtering.

Liability network layer

In the solid-state cell according to the invention, at least one adhesion-promoting layer is used, which is introduced between the positive electrode and the separator and / or between the negative electrode and the separator. In a preferred embodiment, an adhesion promoting layer is incorporated between both the positive electrode and the separator and between the negative electrode and the separator.

The adhesion promoting layer comprises a polymeric lithium ion conductor. In principle, known polymer electrolytes can be used for this purpose. Such a 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). Fluorine-containing polymers such as PVdF, PVdF-HFP and PTFE may also provide lithium ion conduction in combination with a conducting salt. Therefore, as a polymer electrolyte according to the invention, it is also possible to use fluorine-containing polymers in combination with a conducting salt. A preferred polymer electrolyte is PEO in conjunction with LiFSI or LiTFSI.

The adhesion promoting layer is usually used in a layer thickness of 0.001 to 10 μm, preferably 0.1 to 8, more preferably 0.2 to 5 μm. By using the adhesion promoting layer, the stacking of cell components (positive electrode, separator, negative electrode) in cell production can be facilitated and offset of the layers can be prevented, thereby reducing the generation of scrap and the risk of product defects. In addition, the adhesion promoting layer can improve the contact between the electrode surface and the separator surface and minimize interface resistances. This results in a cell with low internal resistance and a long service life.

Lithium-ion cells

The lithium ion cells of the present invention comprise stacked positive electrode separator, separator and negative electrode assemblies, wherein the adhesion promoting layer is interposed between the positive electrode and the separator and / or the negative electrode and the separator. 1 schematically illustrates such a cell.

• a. Bereitstellung einer Verbundkathode, in der ein Verbund-Kathodenaktivmaterial auf einer Stromkollektorfolie bereitgestellt ist;
• b. Aufbringung einer ersten Haftungsvermittlungsschicht durch Darüberlegen einer freistehenden Folie, alternativ durch Aufrakeln, Tränkung mit Gravurwalze oder Aufsprühen des Polymerelektrolyt-Materials und anschließendes Trocknen bzw. Entfernung des Trägerlösemittels;
• c. Aufbringung einer Separator-Schicht;
• d. Aufbringung einer zweiten Haftungsvermittlungsschicht auf die gleiche Weise wie die erste Haftungsvermittlungsschicht, d.h., durch Darüberlegen einer freistehenden Folie, alternativ durch Aufrakeln oder Aufsprühen des Polymerelektrolyt-Materials und anschließendes Trocknen;
• e. Aufbringen einer Anode durch Darüberlegen einer Anodenfolie, die ein auf eine Stromkollektorfolie aufgebrachtes Anoden-Aktivmaterial umfasst.

The preparation can be done by the following steps:

• a. Providing a composite cathode in which a composite cathode active material is provided on a current collector foil;
• b. Applying a first adhesion promoting layer by overlying a free-standing film, alternatively by knife coating, impregnation with gravure roll or spraying of the polymer electrolyte material and subsequent drying or removal of the carrier solvent;
• c. Application of a separator layer;
• d. Applying a second adhesion promoting layer in the same manner as the first adhesion promoting layer, that is, by overlaying a free-standing film, alternatively by doctoring or spraying the polymer electrolyte material and then drying;
• e. Depositing an anode by overlying an anode foil comprising an anode active material applied to a current collector foil.

• a'. Bereitstellung einer Verbundkathode mit aufgebrachter Separatorschicht,
• d. Aufbringung einer Haftungsvermittlungsschicht durch Darüberlegen einer freistehenden Folie, alternativ durch Aufrakeln oder Aufsprühen des Polymerelektrolyt-Materials und anschließendes Trocknen;
• e. Aufbringen einer Anode durch Darüberlegen einer Anodenfolie, die ein auf eine Stromkollektorfolie aufgebrachtes Anoden-Aktivmaterial umfasst. Ein Sonderfall ist eine Lithium-Zelle; hier kann die metallische Lithium-Folie selbst als Stromkollektor fungieren.

Alternatively, it is also possible during the manufacture of the composite cathode to apply the separator layer directly to the active layer in order to provide a composite cathode with integrated separator layer. This simplifies the process and makes it possible to save one of the two adhesion-promoting layers. The sequence is then as follows:

• a '. Providing a composite cathode with a deposited separator layer,
• d. Applying an adhesion promoting layer by overlaying a freestanding film, alternatively by doctoring or spraying the polymer electrolyte material and then drying;
• e. Depositing an anode by overlying an anode foil comprising an anode active material applied to a current collector foil. A special case is a lithium cell; Here, the metallic lithium foil itself can act as a current collector.

Subsequently, an electrical insulator film is placed over the current collector foil of the anode active material and the process is repeated until the desired target capacity or target thickness is reached. Alternatively, the order of steps and thus the resulting stacking may be reversed. In a further alternative, after formation of the first layer by the steps a-e or a ', d and e, the next layer may be formed in the reverse stacking direction to provide duplex cells.

Since the lithium ion conduction in polymer electrolytes is generally associated with an activation energy, the cell is typically designed for operation at elevated temperature, for example 30 to 100 ° C, preferably 50 to 80 ° C.

Claims (9)

A solid-state lithium battery comprising: a positive electrode; A negative electrode; and an intermediate solid electrolyte layer of a solid state inorganic electrolyte as a separator; between the positive electrode and the separator and / or between the negative electrode and the separator is incorporated with an adhesion-imparting layer of a polymeric lithium-ion conductor. The solid state lithium battery according to claim 1, wherein an adhesion promoting layer is incorporated between both the positive electrode and the separator and between the negative electrode and the separator. The solid state lithium battery according to claim 1, wherein the separator is directly applied to the positive electrode, and an adhesion promoting layer is interposed between the negative electrode and the separator. The solid state lithium battery according to any one of claims 1 to 3, wherein the adhesion promoting layer has a thickness of 0.001 to 10 μm. The solid state lithium battery according to any one of claims 1 to 3, wherein the polymeric lithium ion conductor of the adhesion promoting layer comprises polyethylene oxide (PEO) and a lithium-containing conductive salt. A solid-state lithium battery according to claim 5, wherein the principle is selected from LiPF ₆ , LiBF ₄ , LiN (SO ₂ F) ₂ (LiFSI) and LiN (SO ₂ CF ₃ ) ₂ (LiTFSI) or mixtures thereof.  The solid-state lithium battery according to any one of claims 1 to 6, wherein the solid electrolyte of the separator is selected from garnet-type oxide solid electrolytic, ceramic or glass oxide solid oxide electrolytes and sulfided solid electrolytes. The solid state lithium battery according to any one of claims 1 to 7, wherein the positive electrode comprises an active material composed of LiCoO ₂ (LCO), lithium nickel manganese cobalt oxide (NMC), LiCo _0.85 Al _0.15 O ₂ (NCA), spinel or olivine type active materials, and conversion materials such as FeF ₃ . The solid-state lithium battery according to any one of claims 1 to 8, wherein the negative electrode comprises an active material selected from metallic lithium, carbon-based intercalation materials such as graphite, harcarbon or soft carbon, silicon, silicon alloy or silicon / carbon.

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