DE102019118207A1 - Electrode with spacers - Google Patents

Abstract

The present invention relates to an electrode for a lithium ion cell, which comprises a current collector made of metal foil and an active layer applied thereon, the active layer comprising active material particles and compressible spacers introduced between the active material particles, wherein the active material particles have a maximum volume expansion ΔV of 33% or 33% when lithium is absorbed have more.

Description

Technical area

The present invention relates to an electrode for a lithium ion cell which has reversible, deformable spacers.

Technical background

For the negative electrode (anode) of a lithium ion cell, carbon-based intercalation materials such as graphite are typically used at present, which, together with at least one electrode binder, are applied to a copper foil as a current conductor.

To increase the energy density, attempts are being made to replace the carbon-based active material with an active material with a higher specific capacity, for example alloying agents such as silicon or conversion active materials with a low potential for Li / Li ⁺ such as metal oxides, phosphides, nitrides and sulfides, e.g. Fe ₂ 0 ₃ . The use of a purely metallic lithium-based anode is also possible. Optionally, combinations of these materials and / or composite materials in conjunction with carbon are also contemplated.

For the positive electrode (cathode), too, attempts are being made to replace the currently common intercalation materials such as transition metal oxides with a layered structure, spinnelle or phosphates with an olivine structure by materials with a higher specific capacity. Conversion materials such as E.g. FeF ₃ into consideration.

A disadvantage of these alloy formers or conversion materials is the large volume difference ΔV between the lithiated and delithiated state. With silicon, for example, ΔV is around 300%, with FeF ₃ around 100%. In contrast, the difference for layer oxides is a few percent and for graphite approx. 10%.

Due to this change in volume, the active material layer or the entire assembled electrode is subjected to considerable mechanical stresses during cycling. These can lead to a degradation of the active material due to "internal grinding", to damage to the SEI layer as well as to breaks and detachment of the active material coating from the current collector. Effects such as microscopic decoupling of the conductive soot from the active material as a failure mechanism can also occur, which leads to a reduction in service life.

In addition, alloy formers and conversion materials are typically used in very small layer thicknesses to improve the kinetics, so that the current collector must also be selected to be correspondingly thin in order to save weight and not destroy the advantage of the increased specific capacity. In the case of very thin current collector foils in particular, however, there is a risk that the current collector itself will be deformed due to the change in volume of the active material layer and that it may also tear or completely tear.

Summary of the invention

In view of the above problem, the present invention provides an electrode for a lithium ion cell, which comprises a current collector made of metal foil and an active layer applied thereon, the active layer comprising active material particles as well as compressible spacers introduced between the active material particles, wherein the active material particles when lithium is absorbed a maximum Have volume expansion ΔV of 33% or more.

• - Bereitstellung einer Metallfolie als Stromkollektor;
• - Aufbringung einer Struktur aus komprimierbaren Abstandshaltern mit Zwischenräumen, die für die Aufnahme von Aktivmaterialpartikeln vorgesehen sind; und

Einbringung von Aktivmaterialpartikeln mit mindestens einem Elektroden-Binder zwischen die Abstandshalter, worin die Aktivmaterialpartikel bei Lithiumaufnahme eine maximale Volumenexpansion ΔV von 33% oder mehr aufweisen.The invention further relates to a method for producing the above electrode, comprising:

• - Provision of a metal foil as a current collector;
• - Application of a structure made of compressible spacers with interspaces, which are provided for receiving active material particles; and

Introduction of active material particles with at least one electrode binder between the spacers, wherein the active material particles have a maximum volume expansion ΔV of 33% or more when lithium is absorbed.

wobei bei V_max und Vmin das Volumen des Materials der aktiven Schicht bei maximaler bzw. minimaler Lithiumbeladung darstellen.Here, ΔV is defined as: $$\mathrm{\Delta }\text{V}=\text{100\% *}\left({\text{V}}_{\text{Max}}-{\text{V}}_{\text{min}}\right)\mathrm{/}{\text{V}}_{\text{min}}$$

where V _max and Vmin represent the volume of the material of the active layer with maximum and minimum lithium loading, respectively.

In particular, the active material can be an alloy former such as silicon or a conversion active material. Purely metallic lithium can also be used.

The compressible spacers yield when the active material particles expand in volume, so that they can expand without interfering with one another.

This prevents mechanical stresses from occurring between the active material particles and / or between the active layer and the current collector. This can prevent degradation of the active layer and deformation of the current collector, which increases the service life and reliability of the cell.

Figure list

• 1 illustrates the principle of operation of the invention. Without spacers, the volume expansion of the active material in the lithiated state leads to mechanical stresses between the active material particles and between them and with respect to the current collector, which can ultimately deform the current collector and lead to cracks (top). With spacers, the mechanical tension is reduced and deformation of the current collector is prevented (below).

Detailed description

Active material particles

The active material to be used according to the invention has a volume expansion ΔV of 33% or more. If an active material is used that has a defined density and composition in both the lithiated and the delithiated state, ΔV can be calculated directly from the known material parameters (i.e. density and molecular weight of the delithiated or lithiated form) of the respective pure substances. Alternatively, ΔV can also be measured directly, for example dilatometrically or by comparing light microscopic or electron microscopic images of the charged and uncharged electrodes.

Examples of anode active materials with great volume or length expansion during the cycling include alloying agents such as silicon (ΔV = 300%), aluminum, gallium, germanium, tin, silver, alloys of these, or phosphorus, or even metallic lithium itself. Optionally, these materials can also be used as a composite material with carbon. Silicon and silicon / carbon composite materials are preferred. As further examples, conversion active materials with a low potential against Li / Li + can be cited, such as metal oxides, phosphides, nitrides and sulfides, eg Fe ₂ O ₃ .

Examples of cathode active materials with a large ΔV or ΔL are, in particular, fluoride-based conversion materials, such as FeF ₃ (ΔV = 100%), or also sulfides with a high potential for Li / Li +, such as CuS.

All of these anode and cathode active materials are able to stoichiometrically absorb a high ratio of lithium per formula unit and release it again, which explains the large change in volume.

The volume-averaged particle diameter d of the active material particles for the delithiated state is typically 0.005 to 40 μm, preferably 0.05 to 20 μm, particularly preferably 0.1 to 10 μm. If the active material is already in particle form during production, the particle size can by laser scattering ISO 13320 be determined. Otherwise, for example in the case of in-situ generation of the active material particles from precursor compounds, the volume-averaged particle size can also be determined using an electron microscope, for example by measuring 50 particles and calculating the mean value, where each contribution is related to the particle volume (or to the third power of the Diameter) is weighted.

Spacers

The spacers according to the invention are introduced between the active material particles and are made of a compressible material. When the active material particles expand as a result of the uptake of lithium, the spacers give way and thus absorb the volume increase. This can prevent the particles from touching each other and thus creating mechanical stresses that lead to degradation of the electrode. 1 (below) illustrates this concept.

Any polymer materials that are inert to the electrolyte under the operating conditions of the cell and do not enter into any undesired chemical or electrochemical reactions can be used as the material for the spacers. For example, materials can be used that are also used as binders for electrode layers, such as polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexa-fluoro-propylene (PVdF-HFP), SBR or binders based on cellulose ethers such as CMC and acrylate-based. Electrically conductive additives such as carbon black, graphite or carbon nanotubes can optionally be added to these binders.

Alternatively, the following further polymeric binders can also be used, such as polyethylene oxide (PEO), polymethyl methacrylate (PMMA), polyphenylene ether (PPO), phosphazene polymers such as MEEP or also polyacrylonitrile (PAN) and polyolefin-based plastics.
To improve the lithium ion conductivity in the cell, a lithium salt such as. E.g. lithium hexa-fluoro-phosphate (LiPF ₆ ), lithium triflate, lithium perchlorate, lithium tetrafluoroborate, Lithium hexafluorophosphate, LiN (SO ₂ F) ₂ (LiFSI) or LiN (SO ₂ CF ₃ ) ₂ (LiTFSI) can be added. This embodiment is especially preferred in combination with active materials with slow lithium exchange kinetics such as conversion materials.

As a further alternative, polymers with intrinsic electrical conductivity can be mentioned, such as polyfluorene, polypyrrole, polythiophene, polyaniline or poly (p-phenylenevinylene) (PPV), which may optionally be p- or n-doped. The use of carbon nanofibers (CNF) and conductive black can also be considered.

The mean width b of the spacers is typically at most 150% of the mean diameter d of the active material, preferably 2-100%, in particular 10-90%. If the spacers have a rectangular profile, the mean width is determined as the numerical mean value of the rectangular widths. In the case of different profiles, for example with sloping or rounded flanks, the width corresponds to the numerical mean value of the profile widths at half the height.

The choice of the mean width also depends on the volume expansion ΔV of the active material particles as well as on the type of spacer material, i.e. larger widths of the spacer are preferred for active materials with large volume expansion.

The arrangement of the spacers, also referred to below as the spacer structure, can be regular or irregular. In the case of a regular arrangement, the spacers can be arranged, for example, in the form of a rectangular or hexagonal pattern, the spacer forming the “edges” of the rectangles or hexagons and the empty interior being provided for receiving the active material particles. The “edges” can be in the form of homogeneous wall-like structures, or they can be a chain or an agglomerate of particles of the spacer material.

In contrast to regular arrangements, irregular arrangements have no long-range order, but typically a short-range order consists of, for example, polygonal patterns with the spacers as "edges", with the interior spaces in turn being intended to accommodate the active material particles. Another example of irregular arrangements are fiber structures, for example made of carbon nanofibers, which have pores into which the active material particles can be introduced.

The space between the spacers is dimensioned such that it is suitable for receiving active material particles. The minimum distance between two spacers is therefore preferably approximately 50-500%, more preferably 100-250% of the volume-average particle diameter of the active material particles.

Manufacture of the electrode

The electrode according to the invention can be produced, for example, by first applying the spacer structure to the current collector and then introducing the active material particles into the gaps in the spacer structure, for example by coating the spacer structure with a dispersion of the active material particles, optionally in combination with an electrically conductive additive and / or a binder.

The current collector is typically a metal foil, usually copper for the negative electrode and aluminum for the positive electrode.

The spacer structure and active material can be applied in one or more passes. In one pass, a substantially two-dimensional pattern of spacer and active material is obtained; in the case of several passes, a multilayered three-dimensional pattern is accordingly obtained.

With multiple passes, the compositions and patterns of the individual layers can optionally be varied. For example, a larger spacer width b can be selected for the first layer than for the subsequent layers in order to minimize the load on the current collector, and / or a larger amount of electrically conductive material such as conductive carbon black can be added.

Regular arrangements of the spacer structure can be produced, for example, by printing processes such as inkjet printing, screen printing, spray (e.g. airbrush, squeegee) or pattern printing of a dispersion of the spacer material in a suitable medium on the current collector. 3D printing is also possible.

A dispersion of the active material particles, optionally in combination with a binding agent and / or an electrically conductive additive, is then applied so that the particles are deposited in the voids of the pattern between the spacers.

Irregular spacer structures can, for example, by coating the current collector with a dispersion of the Spacer material can be obtained in the presence of template particles. Extraction of the template particles with a suitable solvent, in which the spacer material is insoluble, then again provides a pattern with voids into which the active material can then be introduced. Alternatively, the arrangement can also be obtained by spray drying a dispersion or solution of the spacer material. In this case, the evaporating solvent takes over the function of the template. The spacer material preferably precipitates on the surface of the evaporating solvent droplets, so that a structure with edges of fine, agglomerated particles of the spacer material and empty spaces for receiving the active material particles is created. The size of the voids and the width of the spacers can be adjusted by a suitable choice of concentration, solvent and spray conditions.

For the application of the active material layer with the introduction of the active material particles into the spaces between the spacers, such methods can be used as are customary in the prior art for the production of electrodes. Wet chemical processes are preferably used in which a dispersion of the active material particles in a solvent, typically in combination with a binder and an electrically conductive additive such as carbon black, is applied to the electrode surface provided with the spacers by means of a doctor blade, or a slot nozzle, and then applied is dried. Alternatively, an extrusion coating without solvents or a dry coating can be used.

The electrode according to the invention is used as an anode or cathode in a lithium ion cell, depending on the type of active material. It is particularly preferred to use it as an anode, in conjunction with a copper foil as a current collector and silicon or a silicon / carbon composite material as the active material.

The type of cell is not specifically restricted, and all cell types which are suitable for the use of the respective active material are possible. In particular, it can be a cell with liquid electrolyte, a cell with gel or polymer electrolyte or a solid-state cell. All housings or packaging forms for the cell are also possible (cells with a hard housing (hardcase) and cells with soft packaging (softpack)). For the state of the art with regard to housing shapes, see Chapter 9, Springer 2013, Handbook Lithium-Ion Batteries (publisher R. Korthauer).

QUOTES INCLUDED IN THE DESCRIPTION

This list of the 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 assumes no liability for any errors or omissions.

Non-patent literature cited

• ISO 13320 [0018]

Claims (12)

Electrode for a lithium ion cell comprising a current collector made of metal foil and an active layer applied thereon, the active layer comprising active material particles and compressible spacers introduced between the active material particles, wherein the active material particles have a maximum volume expansion ΔV of 33% or more when lithium is absorbed. Electrode according to Claim 1 , wherein the active material particles have a volume-average particle diameter d of 0.005 to 40 µm. Electrode according to Claim 1 or 2 for use as an anode, in which the metal foil is a copper foil and the active layer contains an active material which forms a lithium alloy, preferably silicon or a silicon / carbon composite material. Electrode according to Claim 1 or 2 for use as a cathode, in which the metal foil is an aluminum foil and the active layer contains a conversion active material, preferably FeF ₃ . Electrode according to one of the Claims 1 to 4th wherein the metal foil has a thickness of 20 µm or less. Electrode according to one of the Claims 1 to 5 wherein the active layer has a thickness of 5 to 70 µm. Electrode according to one of the Claims 1 to 6th , wherein the spacers are made of polyvinylidene fluoride (PVdF), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyethylene oxide (PEO), polymethyl methacrylate (PMMA), polyphenylene ether (PPO), MEEP, polyacrylonitrile (PAN), polyfluoren, polypyrrole, Polythiophene, polyaniline or poly (p-phenylenevinylene) (PPV), polyolefins or carbon nanofibers (CNF) are made. Electrode according to one of the Claims 1 to 7th wherein the spacers in the uncompressed state have an average width which is 2-100% of the volume-average particle diameter of the active material particles. Lithium ion cell, the at least one electrode according to one of the Claims 1 to 8th includes. A method of manufacturing an electrode for a lithium ion cell, comprising: - Provision of a metal foil as a current collector; - Application of a structure made of compressible spacers with interspaces, which are provided for receiving active material particles; and Introduction of active material particles between the spacers, in which the active material particles have a maximum volume expansion ΔV of 33% or more when lithium is absorbed. Procedure according to Claim 10 wherein the structure of compressible spacers is formed by applying a solution or dispersion of a polymer material, which may optionally contain an electrically conductive additive, by means of a printing process or by spray drying. Procedure according to Claim 10 or 11 wherein the active material particles are introduced between the spacers by applying a dispersion of the active material particles in a solvent which optionally contains an electrically conductive additive and / or a binder.

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