Multivalent battery cell

Abstract

A multivalent battery cell (1) is described. This battery cell (1) has paired electrodes (2, 4) configured for the exchange of multivalent ions. At least one of the electrodes (2, 4) has an active material that exhibits one-dimensional diffusion pathways for the multivalent ions and is electrically conductive. The active material is present in particle form at least in one intermediate manufacturing step, the particles (10) of which are at least partially connected to a matrix material (12) that is conductive for the multivalent ions and also electrically conductive.

Description

[0001] The invention relates to a multivalent battery cell, in particular such a battery cell for a vehicle traction battery.

[0002] In the field of secondary batteries, i.e., rechargeable batteries, lithium-ion technology is currently widespread. This typically involves graphite electrodes (usually as anodes) paired with counter electrodes (usually as cathodes) made of lithium-containing materials, such as lithium cobaltate, lithium manganate, or lithium nickel manganese cobaltate (also known as "NMC"). These electrodes are separated by a separator for electrical insulation and are saturated with a typically liquid electrolyte that allows the migration of lithium ions between the electrodes. Lithium-ion batteries currently offer the advantage of achieving a comparatively high energy density.

[0003] A disadvantage of lithium-ion technology, however, is the availability and / or extraction of lithium. For this reason, research is being conducted on alternative materials to address potential future lithium shortages. Multivalent batteries, which utilize magnesium, aluminum, calcium, zinc, or similar ions, have proven particularly promising. These are at least bivalent. For these materials, active materials with one-dimensional (or tunnel-like) diffusion pathways for the ions are of particular interest, as they allow for a higher diffusion rate compared to branched, multidimensional pathways.

[0004] However, such materials have the disadvantage that material defects can lead to a "blockage" of the diffusion path, thus reducing potential battery performance. While solid active material has the advantage of providing approximately direct diffusion paths, statistically speaking, such "long" diffusion paths (i.e., extending across the thickness of the electrode or active material) are more prone to defects than shorter diffusion paths. Therefore, the active material is often milled into "fine" particles to keep diffusion paths as short as possible. However, a disadvantage here is that such small particles are comparatively prone to agglomeration. Such agglomerates, in turn, have the disadvantage that interfaces between individual particles can again block diffusion paths.

[0005] US 2013 / 0302697 A1 describes a magnesium ion cell comprising (a) a cathode comprising a carbon or graphite material as the active cathode material with a surface for capturing and storing magnesium, wherein the cathode has a mesoporous structure with a pore size of 2 nm to 50 nm and a specific surface area of ​​more than 50 m² 2 (g) forms; (b) an anode comprising an anode current collector alone or a combination of an anode current collector and an active anode material; (c) a porous separator arranged between the anode and the cathode; (d) an electrolyte in ionic contact with the anode and the cathode; and (e) a magnesium ion source arranged in the anode to obtain an open-circuit voltage (OCV) of 0.5 volts to 3.5 volts when the cell is manufactured.

[0006] The invention is based on the objective of providing an improved, multivalent battery cell.

[0007] This problem is solved according to the invention by a battery cell having the features of claim 1. Further advantageous and partly inventive embodiments and developments of the invention are set out in the dependent claims and the following description.

[0008] The multivalent battery cell according to the invention has pairs of electrodes (in particular, an anode and a cathode) configured for the exchange of multivalent ions. At least one of the electrodes (in particular, the cathode) comprises an active material that has one-dimensional diffusion pathways for the multivalent ions and is electrically conductive. This active material is present in particle form (i.e., in the form of a plurality of individual particles) at least in one intermediate manufacturing step. The particles of the active material are at least partially bonded to a matrix material that is conductive for the multivalent ions and also electrically conductive. In the intended final manufacturing state of the at least one electrode, the particles of the active material are therefore preferably present in a bound form.

[0009] The matrix material advantageously serves to provide ion-conducting interfaces between the individual particles, so that particle boundaries do not represent diffusion barriers for the ions. At least in part, the matrix material also advantageously prevents the particles from agglomerating, or at least minimizes agglomeration. Agglomerates in which matrix material is located between the individual particles are not disadvantageous, since in this case the matrix material enables or promotes diffusion between the individual particles.

[0010] Preferably, the active material is also redox-active, meaning it can participate in redox reactions. Furthermore, the active material has the highest possible (ion) storage capacity. This latter feature enables high electrical performance of the battery cell. Preferably, the storage capacity of the matrix material is close to that of the active material, thus expediently having the same value. Since the amount, i.e., in particular the volume, of the matrix material between the individual particles is smaller in relation to that of the active material, the matrix material can also have a lower or even no storage capacity, as this is compensated for by the active material.

[0011] According to a preferred embodiment, the active material comprises a transition metal silicate.

[0012] According to a preferred further development, the active material comprises MgMSiO4, where M is selected from iron, manganese, or cobalt. In other words, the active material includes MgFeSiO4, MgMnSiO4, and / or MgCoSiO4. These materials have proven to be particularly suitable (one-dimensional) conductors for multivalent ions.

[0013] According to a particularly suitable embodiment, the matrix material includes a component selected from a group comprising a Chevrel phase (in particular an Mg xMo6S8), lambda-MnO2, and defect-containing layered metal oxides. Optionally, the matrix material can also consist of a mixture of these materials. These materials are ion conductors, the Chevrel phase being particularly effective for magnesium ions. In particular, these materials exhibit three-dimensional diffusion pathways for the multivalent ions. This makes them especially suitable as components of the matrix material, as it facilitates ionic conduction between the individual particles of the active material.

[0014] According to a preferred embodiment, the active material particles have dimensions between 2 nm (nanometers) and 3 µm (micrometers), preferably between 30 nm and 300 nm. In particular, the particles are single-crystal, preferably produced by breaking, grinding, or similar processes of comparatively large single crystals. The aforementioned dimensions advantageously allow for comparatively short diffusion paths, which are statistically subject to a lower risk of defects preventing the diffusion of multivalent ions.

[0015] Preferably, the particles – in particular a powder formed by the particles – are subject to a uniform, i.e., especially a narrowband, particle size distribution. In other words, as many particles as possible of the same size are present, as opposed to a wide dispersion of particle sizes. For example, this can be achieved by sieving the powder, whereby powder fractions below and above a predetermined lower or upper limit are discarded – or otherwise reused.

[0016] According to one embodiment of the invention, the particles are shaped like discs with a diffusion direction running transversely to the plane of the disc. Here and in the following, "disc shape" is understood to mean, in particular, that the particles have a planar extent that is many times, at least twice, but preferably at least five times, larger than their thickness. The diffusion direction runs transversely to the plane of the disc, i.e., exactly or at least roughly (e.g., with an angle of attack of less than 45 degrees) in the direction of thickness. This results in particularly short diffusion paths relative to the overall dimensions of the particles, which reduces the statistical risk of diffusion path blockage.

[0017] The surface area of ​​the disc-shaped particles is preferably between 150 nm and 3 µm and the thickness between 30 nm and 300 nm.

[0018] Alternatively, the particles can also have other shapes, e.g. cuboid, spherical or polyhedral.

[0019] In the case of particle aspect ratios that indicate a longitudinal or planar extent (in particular a rod-like or the aforementioned disc-like shape), the diffusion direction is preferably chosen primarily in the direction of the shorter extent. In the case of rods, a diffusion direction transverse to the rod axis is therefore advantageous (and in particular chosen as such) to enable the shortest possible diffusion paths.

[0020] According to an optional embodiment, the particles are (at least partially) agglomerated into secondary particles with dimensions of up to 3 µm, preferably up to 1 µm. Thus, the secondary particles are preferably only present up to the aforementioned maximum size of the (primary or individual) particles (e.g., due to the sieving process mentioned).

[0021] According to a further, optionally additional or alternative, embodiment of the invention, the particles are coated with the matrix material. At least the coating is applied to a side surface of the particles that intersects the direction of diffusion; in the case of disc-shaped particles, to one, preferably both, flat sides. Optionally, such coating with the matrix material is carried out by deposition processes or using solvents. Preferably, the electrode is pressed from the powdered active material whose particles are coated with the matrix material. Adhesion of the particles to one another is preferably mediated by the matrix material.

[0022] Alternatively, or optionally additionally, the active material particles are embedded in the matrix material. In this case, the powdered active material can be used as a filler during extrusion of the matrix material.

[0023] According to another practical embodiment, the particles are coated with a contact layer (also: adhesive layer) for improved bonding of the matrix material.

[0024] Preferably, the matrix material contains, in addition to the aforementioned materials (components), a preferably polymeric binder as well as conductive carbon black (often also referred to as "carbon black"). The latter serves to increase the electrical conductivity in the matrix material or to make it possible in the first place.

[0025] In particular, in the case of the coating of the particles with the matrix material described above, the material thickness on a particle is less than 10 nm.

[0026] According to a suitable design, the anode is designed as a metal anode, in the case of the magnesium-containing cathode described above, made of metallic magnesium (i.e., in particular a magnesium foil) or a magnesium alloy.

[0027] Exemplary embodiments of the invention are described in more detail below with reference to a drawing. The drawing shows: Fig. 1 in a schematic representation a battery cell, Fig. 2 in a schematic representation a cross-section through an active material of an electrode of the battery cell, Fig. 3 in a schematic side view a particle of the active material, Fig. 4 in view according to Fig. 2 an alternative embodiment of the active material, Fig. 5 in a schematic side view a single particle, and Fig. 6 in view according to Fig. 4 another alternative embodiment.

[0028] Corresponding features in all figures are always marked with the same reference symbols.

[0029] Fig. Figure 1 schematically shows the structure of a battery cell 1. This cell has two electrodes, namely an anode 2 and a cathode 4, as well as a separator 6 between them. A housing and an electrolyte that wets the electrodes 2, 4 and the separator 6 are not shown in detail, nor are contact tabs for electrically connecting the electrodes 2, 4. The battery cell 1 is designed as a multivalent battery cell 1, so that at least divalent ions are used as charge carriers. In the present embodiment, magnesium ions (Mg⁻) are used. 2+ ) for use.

[0030] These magnesium ions must diffuse through an active material of the cathode 4 during the operation of battery cell 1 in order to enable the necessary charge transport between electrodes 2 and 4 (indicated in Fig. 1 by a diffusion direction D). For this purpose, one-dimensional diffusion pathways, i.e., straight, elongated, unbranched (i.e., tunnel-like) structures in the active material, have proven to be advantageous, as they enable a comparatively fast charge transport (especially compared to multidimensional diffusion pathways, i.e., with branches).

[0031] In the present embodiment, MgFeSiO4 is selected as the active material for the cathode 4. Alternatively, MgMnSiO4 or MgCoSiO4 can also be chosen as suitable active materials for one-dimensional diffusion of magnesium ions. These materials are also electrically conductive. In this case, the anode 2 is formed as a metal anode made of magnesium or, alternatively, a magnesium alloy.

[0032] However, the active material is not used as a "solid" layer in cathode 4, but rather as a composite material. For this purpose, a semi-finished product – preferably a single crystal – made of the active material is first comminuted until it is in particle form, particularly as a powder. Particle sizes between 30 nm and 300 nm are selected by sieving and used for further processing. As a result, the powder exhibits a comparatively narrow particle size distribution, meaning that many particles of the same (or similar) size are present; significantly larger or smaller particles are either absent or barely present.

[0033] The advantage of these small particles, hereinafter also referred to as primary particles 10 (see below) Fig. 2 or Fig. 3) The advantage lies in the fact that the length of the diffusion path through the primary particles 10 is comparatively short. With a solid electrode (e.g., designed as a single-crystal wafer), the diffusion paths would ideally have a length at least equal to the thickness of the electrode 2 or 4. However, such a length carries a statistically higher risk of the diffusion paths being blocked for the magnesium ions. A blockage would, in turn, reduce the efficiency of the battery cell 1. Therefore, the goal is to provide diffusion paths that are as continuous as possible. From a statistical perspective, this is made possible by the primary particles 10 with short diffusion paths described above. Their production from a single crystal also avoids the possibility that the primary particles 10 might contain grain boundaries, which would again represent a blockage. As a result, each primary particle 10 has an assigned (local) diffusion direction D. PThis means that, within the system, the diffusion paths are oriented differently due to manufacturing processes – in Fig. 2 indicated by differently oriented hatching of the primary particles 10.

[0034] In order to enable diffusion between the individual primary particles 10 as well as towards the environment, i.e. towards the separator 6 and / or into the electrolyte, the primary particles 10 are embedded in a matrix material 12 (see Fig. 2) The matrix material 12 exhibits multidimensional diffusion pathways. The matrix material 12 is formed by a plastic that is electrically conductive and additionally contains a material that enables the multidimensional diffusion of magnesium ions. In the present embodiment, the plastic is filled with conductive carbon black and a powdered Chevrel phase. Alternatively or additionally to the Chevrel phase, lambda-MnO2 or defect-containing layered metal oxides can also be used.

[0035] In principle, the primary particles can have any spatial shape. In the exemplary embodiment according to Fig. However, in 3 a plate- or disk-like shape is chosen. This is characterized by a planar extent that is many times, at least twice, but preferably at least five or ten times larger than its thickness. The dimensions in the planar direction (in the case of a round disk, its diameter) are between 150 nm and 3 µm. The thickness is between 30 nm and 300 nm. Furthermore, the diffusion direction D PThe orientation of such a disc-shaped primary particle 10 is chosen such that it runs exactly or at least approximately (i.e., with a deviation of up to + / - 20 degrees) in the thickness direction. One advantage of the disc-shaped primary particles 10 is that, during the manufacturing of the cathode 4 using a plastic processing method (e.g., by extrusion), they usually align themselves parallel to the longitudinal extent of the cathode 4. Furthermore, if secondary particles form from the primary particles 10 through agglomeration, the disc-shaped primary particles 10 are also highly likely to stack on top of each other with their flat sides facing each other. This allows the diffusion direction D to be uniformly oriented across the entire secondary particle.

[0036] In Fig. Figure 4 shows an alternative embodiment. Here, the matrix material 12 does not form a composite material with the particles 10 in the sense described above (i.e., in the sense of plastic manufacturing). Rather, the individual primary particles 10 (or, if present, optionally also the secondary particles) are coated with the matrix material 12, for example, by means of deposition processes such as CVD or similar. The coated particles 10 are then pressed against the cathode 4. This causes the surface layers of matrix material 12 of adjacent primary particles 10 to be pressed into one another and thus fused, thereby enabling ionic conductivity (diffusion).

[0037] The coating of the individual particles (primary particles 10 and / or secondary particles) can be carried out completely or only on partial surfaces. In the latter case, at least the partial surface (or all partial surfaces) that corresponds to the diffusion direction D is coated.P of the respective particle cuts (see Fig. 5) Preferably, especially in the case of the disc-shaped primary particles 10, the flat sides that are transverse to the diffusion direction D are P standing, coated.

[0038] In Fig. Figure 6 shows another embodiment. In this embodiment, the primary particles 10 are coated with an adhesive layer 14 (or contact layer) before being coated with – or embedded in – the matrix material 12. The adhesive layer 14 serves to ensure sufficiently high adhesion of the matrix material 12 to the particles.

[0039] In the case of coating the primary particles 10 with the matrix material 12, a preferred layer thickness (material thickness) is up to 10 nm (+ / -3 nm).

[0040] The invention is not limited to the embodiment described above. Rather, further embodiments can be derived by a person skilled in the art from the description and the figures. Reference symbol list 1 battery cell 2 Anode 4 Cathode 6 Separator 10 primary particles 12 Matrix material 14 adhesive layer D, D P Direction of diffusion

Claims

[1] Multivalent battery cell (1) comprising paired electrodes (2, 4) which are configured for the exchange of multivalent ions among each other, wherein at least one of the electrodes (2, 4) comprises an active material which has one-dimensional diffusion pathways for the multivalent ions and is electrically conductive, wherein the active material is present in particle form at least in one intermediate manufacturing step, the particles (10) of which are at least partially connected with a matrix material (12) which is conductive for the multivalent ions as well as electrically, characterized by , that as a shape for the particles (10) a disc shape with diffusion direction running transversely to the disc plane (D P ) is selected. [2] Multivalent battery cell (1) comprising paired electrodes (2, 4) which are configured for the exchange of multivalent ions among each other, wherein at least one of the electrodes (2, 4) comprises an active material which has one-dimensional diffusion pathways for the multivalent ions and is electrically conductive, wherein the active material is present in particle form at least in one intermediate manufacturing step, the particles (10) of which are at least partially connected with a matrix material (12) which is conductive for the multivalent ions as well as electrically, characterized by , that the particles (10) are coated with the matrix material (12), at least on one side, the diffusion direction (D P ) cutting side surface, or that the particles (10) are embedded in the matrix material (12). [3] Multivalent battery cell (1) according to claim 1 or 2, wherein the active material comprises a transition metal silicate. [4] Multivalent battery cell (1) according to claim 3, wherein the active material comprises MgMSiO4 with M selected from iron, manganese or cobalt. [5] Multivalent battery cell (1) according to any one of claims 1 to 4, wherein the matrix material (12) comprises a component selected from the group consisting of a Chevrel phase, Lambda-MnO2 and defect-containing layered metal oxides. [6] Multivalent battery cell (1) according to one of claims 1 to 5, wherein a uniform particle size distribution is selected. [7] Multivalent battery cell (1) according to claim 2 or one of claims 3 to 6 with reference to claim 2, wherein the shape for the particles (10) is a disk shape with a diffusion direction (D) extending transversely to the disk plane. P ) is selected. [8] Multivalent battery cell (1) according to claim 7, wherein the area of ​​the disc-shaped particles (10) is between 150 nm and 3 µm and the thickness is between 30 nm and 300 nm. [9] Multivalent battery cell (1) according to any one of claims 1 to 8, wherein the particles (10) are agglomerated to form secondary particles whose dimensions are up to 3 µm, preferably up to 1 µm. [10] Multivalent battery cell (1) according to claim 1 or any one of claims 3 to 9 with reference to claim 1, wherein the particles (10) are coated with the matrix material (12), at least on one side in the diffusion direction (D P ) cutting side surface. [11] Multivalent battery cell (1) according to claim 1 or any one of claims 3 to 9 with reference to claim 1, wherein the particles (10) are embedded in the matrix material (12). [12] Multivalent battery cell (1) according to one of claims 1 to 11, wherein the particles (10) are coated with a contact layer (14) for improved bonding of the matrix material (12). [13] Multivalent battery cell (1) according to any one of claims 1 to 12, wherein the material thickness of the matrix material (12) on a particle (10) is less than 10 nm.

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