HK1192193B - Method and device for friction stir welding an electric energy storage assembly - Google Patents

Description

TECHNICAL FIELD

The present invention relates to the technical field of electrical energy storage systems.

In the context of the present invention, the term "electrical energy storage unit" refers either to a capacitor (i.e., a passive system comprising electrodes and an insulator), or to a supercapacitor (i.e., a passive system comprising at least two electrodes, an electrolyte, and at least one separator), or to a battery (i.e., a system comprising an anode, a cathode, and an electrolyte solution between the anode and the cathode), for example, a lithium-based battery.

Such energy storage systems are most often combined into energy storage modules, enabling them to meet the specific requirements of certain applications.

STATE OF THE ART

In the state of the art, a power storage module is known, comprising a plurality of energy storage units, each energy storage unit being contained in a cylindrical housing. The cylindrical housings are arranged side by side within the module and are electrically connected to each other in series by connection bars.

Each connection strip is placed on one of the essentially flat end walls of the cylinder and on the corresponding end wall of the adjacent cylinder.

In the state of the art, the connection bar is welded onto each housing by a laser welding process, as for example described in document FR 2 915 626.

Laser welding is a transparent welding process that creates a very localized melting point in both materials being welded. The melted materials then mix together and the entire structure solidifies again almost instantaneously, due to the highly localized nature of the fusion. However, the achievement of the weld depends on the precise adjustment of a number of parameters, including the laser's focal length, its power, etc.

Alternatively, the connection of the energy storage unit's housing to the connection strip is performed after the electrolyte has been introduced into the housing and the impregnation hole has been sealed. Therefore, it is very important to keep the housing hermetically sealed, in order to avoid introducing impurities into the electrolyte of the energy storage unit, which could substantially reduce its lifespan or even lead to its disposal.

The U.S. Patent 2011/086255 describes an assembly process for a storage unit comprising a step of securing an electrically conductive member between two battery cells. The step of securing the member to the battery cells can be carried out by arc welding, gas welding, energy beam welding, friction welding, induction welding, or ultrasonic welding.

The U.S. document 2011/081568 describes a method for manufacturing a battery module (see abstract). In US 2011/081568, a connection bar is welded to the batteries using conventional welding techniques. Connection bars are welded together by friction stir welding.

It was observed that the laser welding process was difficult to repeat due to the various parameters of the process influencing the weld zone fusion, and could potentially cause perforation of the cover during the fusion process if the beam is too powerful or too focused.

In order to obtain a reliable welding process, it is therefore necessary to provide a storage unit housing with a relatively thick wall and/or a special design, thus preventing even a slight misalignment of the laser parameters from degrading the housing's hermeticity. Alternatively, it is necessary to monitor the various laser parameters using highly precise means, allowing the barrette to be welded onto the storage unit housing. This leads to a significant increase in manufacturing costs.

A goal of the present invention is to provide a simple and low-cost manufacturing process for an energy storage module, while still maintaining the hermetic sealing of each energy storage unit.

SUMMARY OF THE INVENTION

To this end, a method for connecting two energy storage units is proposed according to claim 1.

In the context of the present invention, the term "extreme face" refers to the upper or lower face of a housing intended to be adjacent to the module. In the case of a housing having a longitudinal axis (such as a cylindrical housing), the extreme faces are, for example, the faces perpendicular to the longitudinal axis of the housing.

Each storage unit preferably comprises at least two electrodes and an electrolyte. The units can be identical, either with the same shape and same type (supercapacitors, batteries, etc.), or else with different shapes and/or types.

Such a process is very advantageous. Indeed, friction stir welding is a welding technique in which a tool is rotated very rapidly against the parts to be welded. The tool penetrates into the material, heating it until it reaches a pasty state. The combination of heating and stirring allows the material forming the parts to be welded to be joined. The welding is performed in the pasty state, and the tool only heats the material that is in contact with it.

Thus, this process is very easy to control because there is no risk of material polluting the electrolyte due to the welding, provided the tool itself does not penetrate the enclosure. Therefore, process control essentially amounts to a simple check of the tool's movement. It is also not necessary to increase the thickness of the cover as much as in the case of laser welding.

Since welding is performed in the pasty state, it should also be noted that there is no risk of hot cracking, which could potentially compromise the integrity of the enclosure.

A method according to the invention thus allows connecting the energy storage units together in a simple and inexpensive way while maintaining the sealing of each storage unit.

The method according to the invention also presents many other advantages.

In particular, it allows for increased electrical conductivity between different energy storage units, without significantly increasing the cycle time.

Indeed, it is observed that with the method according to the invention, the weld bead corresponds to the size of the friction stir tool, which is not limited by power constraints of the tool, unlike in the prior art for laser welding (where power is greatly related to the beam size). Therefore, it is possible to increase the width of the weld bead or spot welds by increasing the tool size, thereby improving the electrical conductivity between the busbar and each housing, without needing to perform multiple passes of the welding tool over the area to be welded, and thus maintaining a short manufacturing time.

For the same reasons (width of the ribbon), thermal conduction from the top part of the storage unit to the memory stick is improved. Therefore, heat dissipation is optimized by the method according to the invention.

Moreover, the bar can be more economical because of its simpler shape. Indeed, there is no need to hollow out the bar to locally reduce the thickness, which is done in the current state of the art to facilitate laser welding. Using a non-hollowed bar also allows maximizing the surface area of the bar in contact with the conductor pad of a module, which also ensures better heat dissipation from inside the module to the outside.

Finally, the method according to the invention is carried out by heating the envelope material much less than in the prior art. Therefore, the risks of damaging the electrolyte inside the envelope or the energy storage element itself are reduced compared to laser welding.

The method according to the invention may also include one or more of the following characteristics: the strip and the housing can be welded over a distance of at least 1 cm, which allows increasing the electrical conductivity of the entire assembly. The housing and the strip can be overlapped according to a contact face normal to the predetermined axis, notably the end face, so that during the welding step, the strip is pierced to reach the housing. The strip and the housing can be in contact according to a contact face including the predetermined axis, so that the strip and the housing are welded edge to edge. Therefore, the tool can penetrate simultaneously into the material of both the housing and the strip.This method of implementation is generally applied when the housing includes a protrusion forming positioning means for the bar, the bar can be made of a conductive material, notably copper, the housing can be made at least partially of a metallic material, notably aluminum, the outer face of the housing can be made of a first material, while the bar is made of a second material different from the first material. The method according to the invention indeed allows having a bar and a housing made of different materials. Indeed, since the welding is performed in the pasty state of the material, the method enables to weld two different materials easily.Having different properties. In the current state of the art, it is difficult to weld two different materials (using the laser welding technique), especially if their respective melting temperatures are quite far from each other. This is particularly problematic when one wishes to weld aluminum, which is favored for the manufacture of the housing due to its rigidity and lightness, with copper, which has excellent electrical conductivity and can be advantageously used to manufacture the busbar. Indeed, significantly different melting temperatures can lead to numerous technical difficulties, including a difference in contraction that prevents obtaining a reliable and durable weld between the two materials.Thanks to the method according to the invention, it is possible to optimize the selection of materials used for the different components to be assembled (for example, copper for the busbar, aluminum for the housing), as the method does not impose any limiting constraints on this choice. Similarly, the method according to the invention can be used to weld alloyed aluminum (such as type 6000 series, for example), which has advantageous mechanical properties but is difficult to weld using a laser welding process.

The invention also relates to a module comprising at least two energy storage units, each energy storage unit including a sealed housing, the units being connected pairwise using a connecting bar, in accordance with the method described above.

The weld seam of the bar on the housing is larger than 3 mm in size, specifically 5 mm in all directions within the plane of the outer face.

BRIEF DESCRIPTION OF THE FIGURES

Other features, purposes, and advantages of the present invention will become further apparent from the following description, which is purely illustrative and not restrictive, and should be read in conjunction with the accompanying drawings, wherein: - Figure 1 is a radial cross-sectional view of an energy storage assembly of a module according to one embodiment of the invention, - Figure 2 is a top view of a module comprising two energy storage assemblies, according to the first embodiment, - Figure 3 is a radial cross-sectional representation of a friction-mixing device for implementing the method according to one embodiment of the invention, - Figures 4A and 4B are detailed cross-sectional views of the interfaces of the energy storage assembly and a connection bar once welded, - Figures 5A to 5C are perspective views of modules that do not fall within the scope of the invention.

DETAILED DESCRIPTION OF THE INVENTION

We will now describe in more detail a method for connecting two units, particularly supercapacitors, according to an embodiment of the invention.

Referring to Figure 1, an example of a supercapacitor implementation is illustrated, in which the method according to the invention is applied.

The supercapacitor 10 includes a winding 12 placed inside a housing 14, which in the described embodiment consists of a casing 16 comprising a cylindrical side wall 16A and a bottom 16B, and a cover 18 covering the casing 16 to close it. The housing also contains a liquid electrolyte, not visible in the figure for clarity reasons.

The housing 16 and the cover 18 are glued along their entire periphery to ensure the hermetic sealing of the supercapacitor. The housing 16 and the cover are generally made of aluminum. They are also electrically insulated from each other by the continuous glue line that allows their connection to prevent a short circuit.

The winding 12 consists of a unit comprising two electrode sheets 20, 21 and an insulating separator 22 stacked together, the separator being positioned between the two electrode sheets 20, 21.

Each electrode sheet includes a current collector and an electrode made of an active material, notably activated carbon, deposited on both opposite sides of the current collector. The current collector of each electrode is respectively connected to the housing 16 and the cover 18, so that the housing and the cover form respectively the positive and negative terminals of the energy storage unit.

The cover 18 includes an upper surface 24 and a cylindrical dropped edge 26 following the contour of the housing 16. The upper surface 24 of the cover and the bottom 16B of the housing form the outermost surfaces of the enclosure. The cover also includes a central protrusion 28, referred to hereafter as the centering pin.

As mentioned above, energy storage units are often connected in series to form a dedicated module for a specific application. To connect the different units in series, it is generally necessary, on one hand, to connect the housing 16 of one unit to at least one adjacent unit, and on the other hand, to connect the cover 18 to another adjacent unit. The units connected to the terminals of the module are connected only to one adjacent unit.

Figure 2 shows a top view of a module with two storage units as shown in Figure 1.

It can be seen that the covers 18 of these two assemblies are connected by a connecting strip 30, located on the upper face 24 of each cover. The connecting strip 30 is elongated, flat, and has at each end, along its longitudinal direction, an opening 32 with dimensions complementary to those of the positioning pin 28 of the cover strip 14. Of course, the connecting strip is longer than the diameter of a storage unit. Its length is particularly between one and three times the diameter of the unit. The spacing between the two openings 32 is also greater than the diameter of the unit.

As shown in the figure, the bar is full but could also include cutouts, and its shape is not fixed.

We will now describe the process of attaching the bar to the covers.

In a first step, the strip 30 is positioned on the covers, particularly by positioning each of the holes 32 on a centering pin 28 of the covers 18 of the energy storage units 10. The centering pins 28 are sized to be essentially the same thickness as the strip.

Once the strip is in place, it is welded onto each cover by friction stir welding (FSW), an acronym for Friction Stir Welding.

For this, a friction-stir welding device as described below is used. This friction-stir welding step will be explained once the description of the device is completed.

The device comprises a cylindrical body 50 extending along an axis of revolution A-A'. The material forming the body 50 is, for example, steel or any type of material harder than the material forming the workpiece to be welded.

The device also includes a head 51 extending at one of the axial ends of the body 50. The head has a conical-shaped tip 52 and a peripheral shoulder 54 extending in a plane substantially perpendicular to the rotational axis A-A' of the body 50.

The body 50 and the head 51 of the device are adapted to be rotated around a rotation axis corresponding to the revolution axis A-A' of the body 50. During the welding operation, the head of the device penetrates into the material, heating it until it becomes pasty. The heating combined with kneading allows the material forming the parts to be welded, here the strip 30 and the cover 18, to be welded. After cooling, the welding is complete.

Thanks to the device according to the invention, the heating of the room is reduced because the welding is done in a pasty state. This reduces the risk of damage to the entire storage unit.

The conical shape of the device's head allows to trap the material and thus limit the formation of chips. This conical shape also directs the heated material against the shoulder to fill the hole created by the device's head as it advances.

The device also includes a motor (not shown) for rotating the body 50 and the head 51 of the device. The motor is, for example, capable of rotating the body and the head of the device at a speed ranging from 500 to 5000 revolutions per minute, preferably equal to 1000 revolutions per minute.

Thus, during this welding step, device 50 is activated and placed on the assembly so that the A-A' axis of rotation of the tool coincides with the symmetry axis of the assembly. The device 50 is first positioned so that its head 51 covers both the centering pin 28 of the cover and the bar 30, near the opening 32.

With the two parts (bar 30 and centering pin 28) in contact as shown in Figure 4A, the device 50 is translated along axis A-A', so that it simultaneously enters both parts. Due to its rotational movement, it locally makes the material pasty and mixes the material of the bar and the centering pin. The centering pin 28 and the bar 30 are then joined edge to edge by a weld bead 60.

Once the device 50 has entered both chambers, it is guided so that it moves along the contour of the centering pin 28, and then the device is removed. Once these steps are completed, the material cools and the welding is performed. The bead 60 has the width of the tool's head, i.e., 3 to 5 mm wide, and its path is shown as dashed lines in Figure 2.

Next, to reinforce the welding of the strip and the cover, device 50 is placed around the perimeter of the strip 30. The head of the device is pressed down so that it passes through the strip once its material is heated, reaching the upper surface 24 of the cover and penetrating into it as well. Due to the movement of device 50, the materials of the cover and the strip also mix, as shown in Figure 4B, and device 50 is moved along the perimeter of the strip in the area overlapping with the cover. The weld bead 62, also shown as dashed lines in Figure 2, is continuous and can be several centimeters long.

Once the welding of the barrette 30 with a storage unit 10 has been performed, the operation is repeated with the other storage unit.

The method according to the invention is very advantageous since it allows welding without heating the material and thus without risking damage to the electrolyte located inside the casing.

Moreover, it allows the development of larger weld seams than in the case of laser welding, which is interesting for reducing the electrical resistance of the module.

It should be noted that the invention is not limited to the described embodiment.

For example, the energy storage unit does not necessarily have the shape described above. It can be prismatic and not cylindrical, and/or may include two covers located at each end of an open tube.

Similarly, the method is not limited to what has been described above. The strip can include only one of the two soldering cords described above.

One could also consider implementing the process according to the invention using a welding device having a different configuration from that described.

Finally, a module comprising more than two energy storage units clearly falls within the definition of the invention. The units can also be batteries and/or capacitors, or can be of different types (for example, one unit being a battery and the other being a supercapacitor).

Figures 5A to 5B show examples not covered by the claims. As shown in Figures 5A to 5C, the cover may also have a completely flat end face and may not include a centering pin. Other centering means can optionally replace the centering pin, such as protrusions following the outer contour of the connector. 5 The connection bar may also differ from what has been described. It may not include an opening, as shown in the variants of Figures 5A to 5C. It may also connect two housings together or a housing to a cover.It can also be welded to the lid at other locations besides those described. For example, it could be welded using weld spots 70 (as shown in Figure 5A), using a continuous circular weld bead 80 forming a circle with a radius approximately equal to half the radius of the lid (as shown in Figure 5B), or using multiple V-shaped beads 80A, 80B, 80C, 80D distributed evenly on the extreme face of a lid assembly (as shown in Figure 5C). It should be noted that, in the variants shown in Figures 5A to 5C,The welding cords pass through the bar to reach the cover of the storage unit, and are therefore similar to that shown in figure 4B on page 20.

Claims (7)

1. A method for connecting two capacitors or supercapacitors (10), each of them comprising a sealed housing (14), each housing (14) comprising a tubular element (16) and at least one lid (18) closing the tubular element at its end, this lid (18) comprising a centring pin (28) forming means for positioning the strip (30) thereupon, this method comprising the steps consisting in:

   - on the two lids (18) of two housings (14) placed side by side, a connector strip (30) is positioned sized to be in contact with the lid (18) of each of the housings, this strip (30) comprising two orifices (32) allowing its positioning on the pins (28),

   this method being characterized in that it comprises the steps consisting in:

   - placing in contact with the lid (18) and the strip (30), a friction stir welding device (50) mobile in rotation, translating this device along a predetermined axis normal to said lid (18), so that it enters into the material of the lid (18) and of the strip (30) and displace it in order to weld the strip (30) on the cover (18) by friction stir welding, thus forming a first weld bead (60) along the contour of one of the two centring pins (28),

   - withdrawing said friction stir welding device (50) from the lid (18) and from the strip (30) after the formation of the first weld bead (60),

   - placing in contact with the lid (18) and the strip (30), a friction stir welding device (50) mobile in rotation, translating this device along a predetermined axis normal to said lid (18), so that it enters into the material of the lid (18) and of the strip (30) and displace it in order to weld the strip (30) on the cover (18) by friction stir welding, thus forming a second weld bead (60) along the contour of the other of the two centring pins (28),

   - withdrawing said friction stir welding device (50) from the lid (18) and from the strip (30) after the formation of the second weld bead (60),

   - and/or the steps consisting in:

   - placing in contact with the strip (30), the friction stir welding device (50) mobile in rotation, translating this device along a predetermined axis normal to said lid (18), so that it enters into the material of the strip (30) and of the lid (18) and displace it in order to weld the strip (30) on the cover (18) by friction stir welding, thus forming a weld bead (62) along the contour of the connector strip (30), and

   - withdrawing said friction stir welding device (50) from the lid (18) and from the strip (30) after the formation of the weld bead (62) along the contour of the connector strip (30).
2. The method according to claim 1, characterized in that the strip (30) and the lid (18) are welded over a distance at least 1 cm in length.
3. The method according to claims 1 or 2, characterized in that the strip (30) and the lid (18) are superimposed on a contact surface normal to the predetermined axis so that welding passes through the strip (30) to reach the lid (18).
4. The method according to claims 1 or 2, characterized in that the strip (30) and the lid (18) are in contact on a contact surface comprising the predetermined axis so that the strip (30) and the lid (18) are welded edge to edge.
5. The method according to any of the preceding claims, characterized in that the lid (18) is made in a first material, the strip (30) being made in a second material different from the first material.
6. A module of at least two capacitors or supercapacitors (10), each of them comprising a sealed housing (14), each housing (14) comprising a tubular element (16) and at least one lid (18) closing the tubular element at its end, this lid (18) comprising a centring pin (28) forming means for positioning the strip (30) thereupon, the capacitors or supercapacitors (10) being connected in pairs by means of a connector strip (30) which comprises two orifices (32) allowing its positioning on the pins (28), this connection being done conforming to the method according to any of claims 1 to 5.
7. The module according to the preceding claim, characterized in that the dimensions of at least one weld bead (60, 62) are larger than 3 mm, in particular larger than 5 mm, in all directions of the plane of the lid (18).