Battery pack

Abstract

[Problem] To provide a battery pack suitably connected by laser welding regardless of the presence or absence of a gap between metal members being overlap-welded, or even when the metal members are of different types. [Solution] The battery pack has: a battery module having a plurality of unit cells each having a positive electrode tab and a negative electrode tab; and a bus bar welded and connected to at least one of the positive electrode tab or the negative electrode tab. The material of either the positive electrode tab or the negative electrode tab is different from the material of the bus bar. A welded portion 73 between at least one of the positive electrode tab or the negative electrode tab and the bus bar is connected by laser welding in which a laser beam 70 is scanned. The trajectories of the laser beam form a shape in which a first portion 71 having overlapping trajectories and a second portion 72 having non-overlapping trajectories are repeated. In addition, the connection strength at the welded portion is obtained at intervals in either the first portion or the second portion.

Description

battery pack

[0001] The present invention relates to a battery pack.

[0002] The battery pack includes a battery module having a plurality of cells and a plurality of bus bars. Each cell has a positive electrode tab and a negative electrode tab. The bus bar is welded to at least one of the positive electrode tab and the negative electrode tab. The positive electrode tab is made of, for example, aluminum, and the negative electrode tab is made of, for example, copper.

[0003] When connecting unit cells in series, for example, if an aluminum bus bar is used, the positive electrode tab and the bus bar are made of the same material. Therefore, if the positive electrode tab and the bus bar are overlapped and laser welded, sufficient welding quality, such as connection strength, can be ensured. However, the negative electrode tab and the bus bar are made of different materials. Therefore, even if the negative electrode tab and the bus bar are overlapped and laser welded, it is difficult to ensure welding quality, such as connection strength.

[0004] Therefore, when connecting unit cells in series, a busbar made of a clad material is used. This busbar has a first busbar portion made of aluminum and a second busbar portion made of copper. The first busbar portion and the second busbar portion are joined by pressure welding. The positive electrode tab is welded to the first busbar portion by laser welding, and the negative electrode tab is welded to the second busbar portion by laser welding. Although the positive electrode tab and the negative electrode tab are made of dissimilar metals, the welds to the busbar are made of the same metals. This ensures welding quality, such as connection strength.

[0005] However, bus bars made of clad material are expensive, which is one of the factors that hinders cost reduction of battery packs.

[0006] As a technique for joining different materials, the techniques disclosed in Patent Documents 1 and 2 listed below are known.

[0007] Patent Document 1 discloses a technique for welding and joining dissimilar metals, which involves repeated laser spot welding at intervals.

[0008] Patent Document 2 discloses a technique for improving adhesion between a metal part and a resin part. This technique involves adjusting the spacing between adjacent roughened surfaces and the ratio of the depth to the width of the irregularities that form the roughened surface when roughened surfaces are formed in rows on the surface of the metal part using a laser.

[0009] JP 2018-505058 A JP 2014-117724 A

[0010] With the technology disclosed in Patent Document 1, if there is a gap between the metal members to be lap-welded, it is difficult to obtain sufficient connection strength.

[0011] The technology disclosed in Patent Document 2 relates to roughening the surface of metal parts, and does not take into consideration application to laser welding of dissimilar metals.

[0012] An object of the present invention is to provide a battery assembly that is suitably connected by laser welding, regardless of whether or not there is a gap between the metal members to be lap-welded, and even if the metal members are dissimilar.

[0013] To achieve the above object, one aspect of the present invention provides a battery module including a plurality of cells, each having a positive electrode tab and a negative electrode tab, and a bus bar welded to at least one of the positive electrode tab and the negative electrode tab. The material of either the positive electrode tab or the negative electrode tab is different from the material of the bus bar. The welded portion between the bus bar and at least one of the positive electrode tab and the negative electrode tab is connected by laser welding, in which a laser beam is scanned, and the trajectory of the laser beam has a shape in which a first portion where the trajectory overlaps and a second portion where the trajectory does not overlap are repeatedly generated. The connection strength at the welded portion is obtained at intervals in either the first portion or the second portion.

[0014] Another aspect of the present invention, which aims to achieve the above object, provides a battery module including a plurality of cells, each having a positive electrode tab and a negative electrode tab. The positive electrode tab of one cell is connected to the negative electrode tab of another cell by welding. The material of the positive electrode tab is different from the material of the negative electrode tab. The welded portion between the positive electrode tab and the negative electrode tab is connected by laser welding, which scans a laser beam, and the trajectory of the laser beam has a shape in which a first portion where the trajectory overlaps and a second portion where the trajectory does not overlap are repeatedly generated. The connection strength at the welded portion is obtained at intervals in either the first portion or the second portion.

[0015] According to the present invention, the welding depth can be appropriately changed by wobbling the laser beam. The connection strength at the weld can be obtained at intervals in either the first portion or the second portion. Regardless of whether there is a gap between the metal members to be lap-welded, or even if the metal members are dissimilar, a battery pack can be provided that is appropriately connected by laser welding.

[0016] 1 is a perspective view showing a main part of a battery pack; FIG. 2 is a perspective view showing a cell; FIG. 3 is a side view showing a main part of an embodiment, in which a positive electrode tab of one cell and a negative electrode tab of another cell are welded and connected to a bus bar; FIG. 4 is a perspective view showing a state in which metal members are connected to each other by lap welding using laser welding; FIG. 5 is a view showing the trajectory of a laser beam, in which overlapping and non-overlapping portions of the trajectory are repeatedly generated; FIG. 6 is a cross-sectional view schematically showing a state in which connection strength at a welded portion is obtained at intervals depending on the welding depth in portions where the trajectories of the laser beam do not overlap; FIG. 7 is a cross-sectional view schematically showing a state in which connection strength at a welded portion is obtained at intervals depending on the welding depth in portions where the trajectories of the laser beam overlap; FIG. 8 is a side view showing a main part of a modified example, in which a positive electrode tab of one cell and a negative electrode tab of another cell are welded and connected;

[0017] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The embodiments shown here are merely examples for embodying the technical concept of the present invention and are not intended to limit the present invention. Therefore, all other embodiments, examples, and operational techniques that can be conceived by those skilled in the art without departing from the spirit of the present invention are included within the scope and spirit of the present invention, as well as within the scope of the inventions set forth in the claims and their equivalents.

[0018] In addition, for the convenience of illustration and ease of understanding, the drawings attached to this specification may be represented schematically with the scale, aspect ratio, shape, etc. appropriately changed from the actual product, but these are merely examples and do not limit the interpretation of the present invention.

[0019] In this specification, ordinal numbers such as "first" and "second" may be used. However, unless otherwise specified, these ordinal numbers are used to identify components for the sake of convenience, and do not specify the number or order.

[0020] 1 and 2 , a battery pack 10 includes a battery module 30 having a plurality of cells 20, and a plurality of bus bars 40. The cells 20 have positive electrode tabs 21 and negative electrode tabs 22. The bus bar 40 is connected to at least one of the positive electrode tabs 21 and the negative electrode tabs 22 by welding.

[0021] The battery module 30 has a stacked structure in which a plurality of unit cells 20 are stacked. Each unit cell 20 has a flat shape and is held in place by a spacer 31. The unit cell 20 has a flat battery body 23 including a power generating element, and a positive electrode tab 21 and a negative electrode tab 22 extending from the battery body 23. The leading ends 21 a, 22 a of the positive electrode tab 21 and the negative electrode tab 22 are bent along the direction in which the unit cells 20 are stacked. The bus bar 40 has a flat plate shape. The bus bar 40 is welded to the leading ends 21 a, 22 a of the positive electrode tab 21 and the negative electrode tab 22 while facing the leading ends 21 a, 22 a of the positive electrode tab 21 and the negative electrode tab 22. FIG. 1 shows a state in which the positive electrode tabs 21 of three unit cells 20 and the negative electrode tabs 22 of three unit cells 20 are connected to one bus bar 40. That is, two sets of cell groups each including three cells 20 are connected in series via one bus bar 40. Note that, in order to form a desired electrical path, the bus bar 40 can be connected to only the positive electrode tab 21 or only the negative electrode tab 22.

[0022] The material of the positive electrode tab 21 is different from the material of the negative electrode tab 22. Furthermore, the material of either the positive electrode tab 21 or the negative electrode tab 22 is different from the material of the bus bar 40. Specifically, the positive electrode tab 21 is made of, for example, aluminum, the negative electrode tab 22 is made of, for example, copper, and the bus bar 40 is made of, for example, aluminum.

[0023] As shown in Fig. 3 , the weld 73 between the positive electrode tab 21 and the bus bar 40 of one cell 20 is connected by lap welding using laser welding, which scans a laser beam 70. The weld 73 between the negative electrode tab 22 and the bus bar 40 of another cell 20 is also connected by lap welding using laser welding. In the embodiment, the welding between the positive electrode tab 21 and the bus bar 40 is welding between metals of the same kind, and the welding between the negative electrode tab 22 and the bus bar 40 is welding between metals of different kinds. In Fig. 1 , the weld between metals of the same kind is indicated by a line with reference numeral 51, and the weld between metals of different kinds is indicated by a line with reference numeral 52.

[0024] FIG. 4 shows how metal members 61 and 62 are connected to each other by lap welding using laser welding, and FIG. 5 shows a trajectory 70 a of laser light 70 .

[0025] As shown in Figures 4 and 5, in laser welding, the trajectory 70a of the laser beam 70 has a shape that includes a first portion 71 where the trajectory 70a overlaps, a second portion 72 where the trajectory 70a does not overlap, and a portion 74 where no trajectory 70a exists. As shown in Figure 5, by wobbling the laser beam 70 to create a portion 71 where the trajectory 70a overlaps multiple times, a non-overlapping portion 72, and a portion 74 where no trajectory 70a exists, the welding depth can be appropriately changed. The connection strength of the welded portion 73 is obtained at intervals in either the first portion 71 or the second portion 72. Here, "the connection strength is obtained at intervals" does not mean that the connection strength is obtained at all positions in the welded portion 73, but rather that the connection strength is obtained partially at intervals in either the first portion 71 or the second portion 72.

[0026] This will be explained in more detail with reference to Figures 6 and 7. Figure 6 shows a state in which the connection strength at the welded portion 73 is obtained by spacing the second portion 72. Figure 7 shows a state in which the connection strength at the welded portion 73 is obtained by spacing the first portion 71. Of the overlapping metal members 61, 62 in Figures 4, 6, and 7, the upper metal member 61 is referred to as the first metal member 61, and the lower metal member 62 is referred to as the second metal member 62. Figure 6 shows a state in which no gap S exists between the first metal member 61 and the second metal member 62, while Figures 4 and 7 show a state in which a gap S exists between the first metal member 61 and the second metal member 62. In this specification, "a state in which no gap S exists" means a state in which the spacing between the first metal member 61 and the second metal member 62, which are overlapped for welding, is maintained at or below the maximum design spacing. The phrase "equal to or less than the maximum distance" also includes a state in which the first metal member 61 and the second metal member 62 are in contact with each other or in close contact due to external pressure or the like. In this specification, "a state in which the gap S exists" refers to a state in which the distance between the first metal member 61 and the second metal member 62 is maintained at a distance greater than the maximum distance in design (a distance greater than "a state in which the gap S does not exist"). The maximum distance in design is determined taking into consideration the manufacturing tolerances of the positive electrode tab 21 and the negative electrode tab 22, misalignment that occurs when the cell 20 is set for laser welding, the surface roughness and amount of waviness of the positive electrode tab 21 and the negative electrode tab 22, and the like.

[0027] A first portion 71 of the trajectory 70a of the laser beam 70 can be welded to a relatively deep depth, and a second portion 72 can be welded to a relatively shallow depth (see FIG. 4 ). The difference in weld depth (symbol d in FIG. 4 ) is set to be equal to or greater than the maximum design distance between the first metal member 61 and the second metal member 62. In this embodiment, the welding conditions (laser output, wobbling magnitude, scanning speed, etc.) are kept constant even when the objects to be lap welded (same metals or dissimilar metals) change. In cases where the required difference in weld depth cannot be set due to a change in the objects to be lap welded, the welding conditions can be changed each time the objects change.

[0028] The operation will be described when the first metal member 61 is the bus bar 40 and the second metal member 62 is the positive electrode tab 21. The welding between the positive electrode tab 21 and the bus bar 40 is welding between metals of the same kind.

[0029] As shown in FIG. 6 , when laser welding is performed in a state where there is no gap S between the positive electrode tab 21 and the bus bar 40, the positive electrode tab 21 and the bus bar 40 abut at the second portion 72. In this specification, "abutting" includes not only a state where the positive electrode tab 21 and the bus bar 40 are in contact with each other, but also a state where the distance between the positive electrode tab 21 and the bus bar 40 is equal to or less than the maximum distance designed. While the welding depth in the first portion 71 is too deep and may result in a hole defect, the welding depth in the second portion 72 is a relatively shallow and appropriate depth. Therefore, the connection strength at the weld 73 between the positive electrode tab 21 and the bus bar 40 is obtained in terms of spacing depending on the welding depth in the second portion 72. As a result, when there is no gap S between the positive electrode tab 21 and the bus bar 40, the positive electrode tab 21 and the bus bar 40 can be suitably connected by laser welding.

[0030] As shown in Figure 7, when laser welding is performed in a state in which a gap S exists between the positive electrode tab 21 and the bus bar 40, a gap S remains between the positive electrode tab 21 and the bus bar 40 in the second portion 72. In this state, the welding depth in the first portion 71 is a relatively shallow, appropriate depth. Therefore, the connection strength at the weld 73 between the positive electrode tab 21 and the bus bar 40 is obtained in a spaced manner depending on the welding depth in the first portion 71. As a result, even when a gap S exists between the positive electrode tab 21 and the bus bar 40, the positive electrode tab 21 and the bus bar 40 can be suitably connected by laser welding.

[0031] In this way, whether or not a gap S exists between the positive electrode tab 21 and the bus bar 40 to be lap-welded, the positive electrode tab 21 and the bus bar 40 can be welded together without changing the welding conditions, and the required welding quality can be ensured.

[0032] The operation will be described below when the first metal member 61 is the bus bar 40 and the second metal member 62 is the negative electrode tab 22. The welding between the negative electrode tab 22 and the bus bar 40 is welding between dissimilar metals.

[0033] As shown in Fig. 6 , when laser welding is performed in a state in which there is no gap S between the negative electrode tab 22 and the bus bar 40, the negative electrode tab 22 and the bus bar 40 abut against each other at the second portion 72. Although the welding depth in the first portion 71 is too deep and there is a risk of a hole defect occurring, the welding depth in the second portion 72 is a relatively shallow, appropriate depth. Therefore, the connection strength at the welded portion 73 between the negative electrode tab 22 and the bus bar 40 is obtained in a spaced manner depending on the welding depth in the second portion 72. As a result, when there is no gap S between the negative electrode tab 22 and the bus bar 40, the negative electrode tab 22 and the bus bar 40 can be suitably connected by laser welding.

[0034] A welded connection between the same metals (the positive electrode tab 21 and the bus bar 40) can ensure sufficient weld quality, such as connection strength. On the other hand, a welded connection between dissimilar metals (the negative electrode tab 22 and the bus bar 40) inevitably generates an intermetallic compound at the connection interface. The generation of the intermetallic compound leads to a decrease in connection strength. Although a large amount of intermetallic compound is generated in the first portion 71, the intermetallic compound is not generated in the entire welded portion 73. Therefore, the negative electrode tab 22 and the bus bar 40 are connected at intervals in the second portion 72 with sufficient strength.

[0035] As shown in Fig. 7 , when laser welding is performed in a state in which a gap S exists between the negative electrode tab 22 and the bus bar 40, a gap S remains between the negative electrode tab 22 and the bus bar 40 in the second portion 72. In this state, the welding depth in the first portion 71 is a relatively shallow, appropriate depth. Therefore, an intermetallic compound is not generated in the entire welded portion 73, and the connection strength of the welded portion 73 between the negative electrode tab 22 and the bus bar 40 is obtained in intervals depending on the welding depth in the first portion 71. As a result, even when a gap S exists between the negative electrode tab 22 and the bus bar 40, the negative electrode tab 22 and the bus bar 40 can be suitably connected by laser welding.

[0036] In this way, regardless of whether a gap S exists or does not exist between the negative electrode tab 22 and the bus bar 40 to be lap-welded, the negative electrode tab 22 and the bus bar 40 can be welded together without changing the welding conditions, and the required welding quality can be ensured.

[0037] As described above, in this embodiment, the trajectory 70a of the laser beam 70 has a shape in which the first portion 71 where the trajectory 70a overlaps and the second portion 72 where the trajectory 70a does not overlap are repeatedly formed. The connection strength at the welded portion 73 is achieved at intervals in either the first portion 71 or the second portion 72. This configuration allows for a battery assembly 10 that is suitably welded and connected by laser welding, regardless of whether a gap S exists between the metal members 61, 62 to be lap-welded, and regardless of whether the metal members are the same type (e.g., the positive electrode tab 21 and the bus bar 40) or different types (e.g., the negative electrode tab 22 and the bus bar 40). Furthermore, even if the objects to be lap-welded (the same type or different types of metals) change, the required welding quality can be ensured without changing the welding conditions (e.g., laser output, wobbling magnitude, scanning speed). This allows for efficient laser welding, resulting in a battery assembly 10 that is more cost-effective than a battery assembly 10 that uses a clad bus bar 40.

[0038] In this embodiment, in order to prepare for a situation in which a gap S occurs between the metal members 61, 62 to be lap-welded, the laser beam 70 is wobbled to change the melt depth even when there is no gap S between the metal members 61, 62. Even when there is no gap S between the metal members 61, 62 as a whole (see FIG. 6 ), the welding strength is not obtained at all positions in the melted portion, but is obtained at intervals (partially at intervals). In this respect, this method differs from a technique in which laser spot welding is performed repeatedly at intervals.

[0039] 8 is a side view showing a main portion of a modified example, in which the positive electrode tab 21 of one cell 20 and the negative electrode tab 22 of another cell 20 are welded together. The battery pack 10 of the modified example has a portion where the positive electrode tab 21 and the negative electrode tab 22, which are made of different metals, are welded together without using a bus bar 40. The remaining configuration is the same as that of the battery pack 10 of the embodiment.

[0040] 4 and 5 , in the same manner as in the embodiment, in laser welding, a trajectory 70a of a laser beam 70 has a shape in which a first portion 71 where the trajectory 70a overlaps and a second portion 72 where the trajectory 70a does not overlap are repeatedly generated. As shown in the trajectory 70a of the laser beam 70 shown in FIG. 5 , by wobbling the laser beam 70, the welding depth can be appropriately changed. The connection strength of the welded portion 73 is obtained at intervals in either the first portion 71 or the second portion 72.

[0041] The modified example will be described in more detail with reference to Figures 6 and 7. In the modified example, of the overlapping metal members 61 and 62 shown in Figures 6 and 7, the first metal member 61 is the positive electrode tab 21 and the second metal member 62 is the negative electrode tab 22. The welding between the positive electrode tab 21 and the negative electrode tab 22 is welding between dissimilar metals.

[0042] As shown in Fig. 6 , when laser welding is performed in a state in which there is no gap S between the positive electrode tab 21 and the negative electrode tab 22, the positive electrode tab 21 and the negative electrode tab 22 abut against each other at the second portion 72. While the welding depth in the first portion 71 is too deep and may result in a hole defect, the welding depth in the second portion 72 is a relatively shallow and appropriate depth. Therefore, the connection strength at the welded portion 73 between the positive electrode tab 21 and the negative electrode tab 22 is obtained in a spaced manner depending on the welding depth in the second portion 72. As a result, when there is no gap S between the positive electrode tab 21 and the negative electrode tab 22, the positive electrode tab 21 and the negative electrode tab 22 can be suitably connected by laser welding.

[0043] When dissimilar metals (positive electrode tab 21 and negative electrode tab 22) are welded together, the formation of intermetallic compounds at the connection interface is inevitable. The formation of intermetallic compounds leads to a decrease in connection strength. Although a large amount of intermetallic compounds is formed in the first portion 71, intermetallic compounds are not formed in the entire welded portion 73. Therefore, the positive electrode tab 21 and the negative electrode tab 22 are connected with strength at intervals in the second portion 72.

[0044] As shown in Figure 7, when laser welding is performed in a state in which a gap S exists between the positive electrode tab 21 and the negative electrode tab 22, a gap S remains between the positive electrode tab 21 and the negative electrode tab 22 in the second portion 72. In this state, the welding depth in the first portion 71 is a relatively shallow, appropriate depth. Therefore, an intermetallic compound is not generated in the entire welded portion 73, and the connection strength of the welded portion 73 between the positive electrode tab 21 and the negative electrode tab 22 is obtained in intervals depending on the welding depth in the first portion 71. As a result, even when a gap S exists between the positive electrode tab 21 and the negative electrode tab 22, the positive electrode tab 21 and the negative electrode tab 22 can be suitably connected by laser welding.

[0045] In this way, regardless of whether a gap S exists or does not exist between the positive electrode tab 21 and the negative electrode tab 22 to be lap-welded, the positive electrode tab 21 and the negative electrode tab 22 can be welded together without changing the welding conditions, and the required welding quality can be ensured.

[0046] As described above, in this modified example, the trajectory 70a of the laser beam 70 has a shape in which the first portion 71 where the trajectory 70a overlaps and the second portion 72 where the trajectory 70a does not overlap are repeatedly formed. The connection strength at the welded portion 73 is achieved at intervals in either the first portion 71 or the second portion 72. This configuration makes it possible to provide a battery assembly 10 in which the connection is suitably welded by laser welding, regardless of whether or not there is a gap S between the dissimilar metals (the positive electrode tab 21 and the negative electrode tab 22) to be lap-welded. Furthermore, even if the objects to be lap-welded (the same metals or dissimilar metals) change, the required welding quality can be ensured without changing the welding conditions (laser output, wobbling magnitude, scanning speed, etc.). This allows for efficient laser welding, and provides a battery assembly 10 that is more cost-effective than a battery assembly using a clad bus bar 40.

[0047] Although the embodiment and modifications of the battery pack 10 of the present invention have been described above, the present invention is not limited to the above-described configurations and can be modified as appropriate based on the claims.

[0048] This application is based on Japanese Patent Application No. 2024-063927, filed on April 11, 2024, the disclosure of which is incorporated by reference in its entirety.

[0049] REFERENCE SIGNS LIST 10 Battery pack 20 Cell 21 Positive electrode tab 21a Tip portion 22 Negative electrode tab 22a Tip portion 23 Battery body 30 Battery module 31 Spacer 40 Bus bar 61 First metal member 62 Second metal member 70 Laser light 70a Trajectory 71 First portion where the trajectories overlap 72 Second portion where the trajectories do not overlap 73 Welded portion 74 Portion where no trajectory exists S Gap

Claims

1. A battery module comprising: a battery module having a plurality of cells each having a positive electrode tab and a negative electrode tab; and a bus bar welded to at least one of the positive electrode tabs and the negative electrode tabs, wherein the material of either the positive electrode tab or the negative electrode tab is different from the material of the bus bar; the welded portion between at least one of the positive electrode tab or the negative electrode tab and the bus bar is connected by laser welding using a scanning laser beam, and the trajectory of the laser beam has a shape in which a first portion where the trajectory overlaps and a second portion where the trajectory does not overlap are repeatedly generated; and the connection strength at the welded portion is obtained at intervals in either the first portion or the second portion.

2. The battery pack according to claim 1, wherein at least one of the positive electrode tab and the negative electrode tab abuts against the bus bar in the second portion, and the connection strength at the welded portion is obtained at intervals depending on the welding depth in the second portion.

3. The battery pack according to claim 1, wherein a gap exists between the bus bar and at least one of the positive electrode tab and the negative electrode tab in the second portion, and the connection strength at the welded portion is obtained at intervals depending on the welding depth in the first portion.

4. A battery module having a plurality of cells each having a positive electrode tab and a negative electrode tab, wherein the positive electrode tab of one of the cells is welded to the negative electrode tab of another of the cells, the material of the positive electrode tab is different from the material of the negative electrode tab, the welded portion between the positive electrode tab and the negative electrode tab is connected by laser welding using a scanning laser beam, the trajectory of the laser beam has a shape in which a first portion where the trajectory overlaps and a second portion where the trajectory does not overlap are repeatedly generated, and the connection strength at the welded portion is obtained at intervals in either the first portion or the second portion.

5. The battery pack according to claim 4, wherein the positive electrode tab and the negative electrode tab abut in the second portion, and the connection strength at the welded portion is obtained at intervals depending on the welding depth in the second portion.

6. The battery pack according to claim 4, wherein a gap exists between the positive electrode tab and the negative electrode tab in the second portion, and the connection strength at the welded portion is obtained at intervals depending on the welding depth in the first portion.

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