FR3146029A1 - MECHANICAL PRESSURE MAINTENANCE SYSTEM FOR HELICAL ELECTRODE - Google Patents

Abstract

The invention relates to an electrochemical battery (1) enclosing a housing (2) comprising at least one electrochemical cell (3), which electrochemical cell (3) comprises electrodes (4), wherein said housing (2) comprises, at the end of said electrochemical cell (3) and/or around said electrochemical cell (3), a prestressing means (5) at least axial relative to a cell axis (D), and said prestressing means (5) forms a pressure element between said electrodes (4) and the inner wall of said housing (2) in order to maintain a constant pressure between said electrodes (4). The invention also relates to a motor vehicle (100) comprising such a battery (1). Figure 1

Description

MECHANICAL PRESSURE MAINTENANCE SYSTEM FOR HELICAL ELECTRODE

The invention relates to a battery containing at least one housing itself comprising at least one electrochemical cell, which electrochemical cell comprises at least two electrodes, one positive and the other negative.

The invention also relates to an electric or hybrid motor vehicle, comprising at least one such battery.

The invention relates to the field of batteries, in particular but not limited to batteries for electric or hybrid motor vehicles. For example for any other use in a vehicle, truck, motorcycle or other, or for a portable power tool, or other.

The invention relates more particularly to the field of the internal structure of electrochemical battery cells.

Four main topologies for designing electrodes for batteries are known from the prior art, each type being associated with a particular casing shape: simple stacking (known as "stacked" in English), Z stacking, cylindrical spiral winding, prismatic winding.

These four types of windings and their variants always present an inhomogeneity from a mechanical point of view. Indeed, their stacked or wound character constitutes a hierarchy of successive layers each having its own mechanical conditions. The windings or layers close to the middle do not experience uniform pressure.

It is accepted by the scientific literature that electrochemical cells (batteries) undergo a volume expansion during their cycling, due to their lithium impregnation. This expansion can go as far as twisting the casing, thus removing the pressure that this casing must exert on the electrodes in order to keep them in contact during a subsequent contraction phase.

The invention seeks to avoid the detachment (delamination) of active materials on the current collectors, or with the separator, in an electrochemical cell, via the application of a homogeneous pressure internal to the housing.

Indeed, if the electrode becomes detached even locally from its current collector or the separator, this can cause points of inhomogeneity which can cause significant concentration gradients involving the deposition of metallic lithium on the negative electrode, or even dendritic formations endangering the safety of the cells.

This risk of detachment is even more problematic for the new technology of all-solid-state batteries, the electrochemical cell must absolutely be put under stress, which causes many problems when it comes to designing a battery pack without significant loss of volume density.

Sometimes this detachment is induced by the formation of gas in the casing, which involves the same type of problem.

The aim of the present invention is to overcome these drawbacks by proposing the insertion of a pressure element such as a spring between the spiral electrode and the wall of the housing in order to maintain a constant and effective pressure between the electrodes.

To achieve this objective, the invention proposes a battery containing at least one housing itself comprising at least one electrochemical cell, which electrochemical cell comprises at least two electrodes, one positive and the other negative.

According to the invention, said housing comprises, at the end of said electrochemical cell and/or around said electrochemical cell, a means of prestressing at least axially relative to a cell axis, and said prestressing means forms a pressure element between said electrodes and the internal wall of said housing in order to maintain a constant pressure between said electrodes.

Thanks to the invention, the electrodes experience uniform pressure, and the flows passing through them are more homogeneous.

In a preferred embodiment, said prestressing means is an axial prestressing means along the direction of said cell axis.

This ensures both good pressure distribution in the electrodes and good maintenance of the geometry of the electrochemical cell.

Advantageously, said prestressing means forms a pressure element between said electrodes and a separator interposed between said electrodes.

This prevents any separation between an electrode and its separator, and avoids mechanical damage linked to the expansion of the electrodes.

Advantageously, a dielectric compression element is interposed between said prestressing means and said electrodes.

This improves insulation and pressure distribution.

Advantageously, said prestressing means comprises at least one spring.

This arrangement is a particularly economical and reliable solution. A suitable spring can also allow for a correction of geometry or dimensions between a housing and a cell of a shape that is not directly complementary.

In another embodiment, said prestressing means comprises at least one foam capable of being subjected to active or passive mechanical stress.

A foam offering physical properties similar to those of a spring, or any other object subjected to active or passive mechanical stress, can fulfill the mechanical function of prestressing, offering similar mechanical work.

More particularly, in a first variant, said spring is a toric cylindrical compression spring.

This solution is well suited to most cells whose projection along a plane perpendicular to the cell axis is substantially circular, as well as to axial support applications.

More particularly, in a second variant, said spring is a compression spring, the projection of which according to a direction of development of the turns which it comprises, is in the shape of a polygon.

This configuration makes it possible to handle cases of special cell shapes, for example with an external shape of substantially rectangular or square section, or other.

The prestressing means, in particular a spring, takes the shape suggested by the casing (circular, polygon, in particular rectangle, square, triangular, hexagonal, octagonal, or other) both according to its external section and its internal section, given that the electrochemical cell is toric (hollowed out along the main axis).

Advantageously, said electrodes are wound in a double helix in a flat spiral.

This configuration allows for great compactness of the electrochemical cell, and above all great homogeneity of the flows within the electrodes, at which the pressure is thus maintained in a compact and homogeneous manner.

The invention also relates to an electric or hybrid motor vehicle, comprising at least one such battery.

• illustre schématiquement un véhicule selon l’invention, comportant une batterie selon l’invention, laquelle contient plusieurs boîtiers renfermant des cellules électrochimiques réalisées selon une forme bi-hélicoïdale, et toutes munies d’un système de maintien de pression mécanique passif des électrodes, avec des électrodes spiralées à plat maintenues axialement, au travers d’un élément diélectrique de compression, par un moyen de précontrainte formé par un ressort en métal déployé ;
• illustre schématiquement, en perspective éclatée selon un axe de cellule, un boîtier (dit « casing ») cylindrique, dont le couvercle n’est pas représenté, qui renferme une cellule électrochimique avec des électrodes spiralées ensemble à plat maintenues axialement, au travers d’un élément diélectrique de compression en forme de rondelle, par un moyen de précontrainte axial formé par un ressort en métal déployé ;
• illustre schématiquement, en perspective, différentes variantes de ressorts convenant pour former des moyens de précontrainte correspondant à différentes géométries de cellules électrochimiques : ressort de compression cylindrique hélicoïdal à boudin, rondelle-ressort festonnée, ressort de compression hélicoïdal à boudin de forme rectangulaire, anneau cylindrique en métal déployé.

The invention will be further detailed by the description of non-limiting embodiments, and on the basis of the appended figures illustrating variants of the invention, in which:

• schematically illustrates a vehicle according to the invention, comprising a battery according to the invention, which contains several housings enclosing electrochemical cells produced in a bi-helical shape, and all provided with a passive mechanical pressure maintenance system for the electrodes, with flat spiral electrodes held axially, through a dielectric compression element, by a prestressing means formed by an expanded metal spring;
• schematically illustrates, in exploded perspective along a cell axis, a cylindrical casing, the cover of which is not shown, which contains an electrochemical cell with electrodes spiraled together flat and held axially, through a dielectric compression element in the form of a washer, by an axial prestressing means formed by an expanded metal spring;
• schematically illustrates, in perspective, different variants of springs suitable for forming prestressing means corresponding to different geometries of electrochemical cells: helical cylindrical compression spring with coil, scalloped spring washer, helical compression spring with rectangular coil, cylindrical ring in expanded metal.

It is illustrated schematically in a 100 electric or hybrid motor vehicle according to the invention, which comprises an electrochemical battery 1 according to the invention, which contains several housings 2 (also called "casings"). Each housing 2 comprises at least one electrochemical cell 3, which electrochemical cell 3 comprises at least two electrodes 4, one positive, the other negative. More particularly and non-limitingly, each electrode 4 comprises a current collector on which active material is deposited, and is connected to a connection tab, these electrodes are preferably separated by a separator film 6, and are bathed in particular in an electrolyte, and may comprise different additives.

According to the invention, this housing 2 comprises, at the end of the electrochemical cell 3 and/or around the electrochemical cell 3, a prestressing means 5 at least axially relative to a cell axis D. This cell axis D is formed by a winding axis when the electrodes 4 are wound or formed into a cylindrical or flat spiral.

And this prestressing means 5 forms a pressure element between the electrodes 4 and the internal wall of the housing 2 in order to maintain a constant pressure between the electrodes 4. In particular between the positive electrode 41 and the negative electrode 42, and in particular through separator films 6.

In a first embodiment, this prestressing means 5 is an axial prestressing means in the direction of the cell axis D, as visible in And .

More particularly, this prestressing means 5 forms a pressure element between the electrodes 4 and a separator 6 interposed between the electrodes 4 as visible in And .

More particularly, a dielectric compression element 7 is interposed between the prestressing means 5 and the electrodes 4 as visible in And .

More particularly, the prestressing means 5 comprises at least one spring 50, as visible in , And .

More particularly, this spring 50 is a toric cylindrical compression spring, as seen in , And .

More particularly, the spring 50 is a compression spring, the projection of which according to a direction of development of the coils which it comprises, is in the shape of a polygon, in particular a rectangle, or other, as visible in the lower part of the .

More particularly, the electrodes 4 are wound in a double helix in a flat spiral, as visible in And .

An advantageous embodiment of the invention uses the concept of a double spiral helix electrode, in order to insert inside the housing a pressure element such as a spring in order to maintain a constant pressure on the electrodes in a compact and homogeneous manner.

This electrode topology reduces mechanical inhomogeneities by applying a uniform pressure in a geometry oriented on the compressed axis. Due to this new geometry, the layers maintain optimal contact with each other, regardless of their level of lithiation and therefore volume expansion. This technology dampens and channels the pressure normally exerted by the electrodes on the walls of the housing towards an element capable of absorbing this expansion while maintaining their contact.

The invention consists of inserting a pressure element such as a spring over a dielectric compression ring between the spiral electrode and the end of the casing, before filling with electrolyte and sealing the cell.

A first variant concerns toric-cylindrical shaped cells, for which a toric cylindrical spring is suitable.

A second variant concerns cells of toric-cubic or flat-faced shape, for which a spring fitting into a prism of rectangular, in particular square, section is appropriate, or even according to any other internal and external shape independently (triangle, hexagonal, octagonal, or other).

The invention provides numerous advantages: the homogeneity of the pressure applied to the electrodes of the electrochemical cell, the capacity to absorb cyclic extensions and aging of the cells without external constraints to the housing, and therefore ease of assembly, and effective maintenance of contact between electrodes and separator.

This topology makes it possible to channel the mechanical pressures linked to the expansion of cells, both on the scale of a cycle and that of aging, in a single direction in which this expansion can be absorbed by a spring.

Consequently this induces a reduction in inhomogeneous constraints leading to safety problems (dendritic formation, among others) or to an accelerated loss of capacity (aging).

Furthermore, battery chemistries are moving towards high expansion technologies (silicon), the spiral shape (spring) offers new flexibility to the electrodes, which until now have tended to degrade with their over-stressed expansion.

In short, the issue of battery optimization is at the heart of the concerns of automobile manufacturers and equipment manufacturers. One of the difficulties specific to electrochemical cells is their geometric inhomogeneity causing gradients in thermal, electrochemical, and mechanical behavior, which can lead to the destruction of the electrochemical cell by thermal runaway or dendritic short circuit.

This new electrode shape offers the advantage of a homogeneous distribution of the electrodes according to all these physical dynamics. Thus making it possible to go beyond current performances, particularly in fast charging power, while maintaining or even slightly improving the mass and volume energy density of the battery pack.

Claims (9)

Electrochemical battery (1) containing at least one housing (2) itself comprising at least one electrochemical cell (3), which electrochemical cell (3) comprises at least two electrodes (4), one positive, the other negative, characterized in that said housing (2) comprises, at the end of said electrochemical cell (3) and/or around said electrochemical cell (3), a prestressing means (5) at least axial relative to a cell axis (D), and in that said prestressing means (5) forms a pressure element between said electrodes (4) and the internal wall of said housing (2) in order to maintain a constant pressure between said electrodes (4). Electrochemical battery (1) according to claim 1 characterized in that said prestressing means (5) is an axial prestressing means in the direction of said cell axis (D). Electrochemical battery (1) according to any one of claims 1 to 2, characterized in that said prestressing means (5) forms a pressure element between said electrodes (4) and a separator (6) interposed between said electrodes (4). Electrochemical battery (1) according to any one of claims 1 to 3, characterized in that a dielectric compression element (7) is interposed between said prestressing means (5) and said electrodes (4). Electrochemical battery (1) according to any one of claims 1 to 4, characterized in that said prestressing means (5) comprises at least one spring. Electrochemical battery (1) according to any one of claims 1 to 4, characterized in that said prestressing means (5) comprises at least one foam capable of being placed under active or passive mechanical stress. Electrochemical battery (1) according to claim 5 characterized in that said spring (50) is a compression spring which is toroidal cylindrical, or a compression spring whose projection according to a direction of development of the turns which it comprises, is in the shape of a polygon. Electrochemical battery (1) according to any one of claims 1 to 7, characterized in that said electrodes (4) are wound in a double helix in a flat spiral. Electric or hybrid motor vehicle (100), comprising at least one electrochemical battery (1) according to any one of claims 1 to 8.