JP2020004643A - Power storage device - Google Patents

Abstract

To provide a power storage device capable of improving bonding strength.SOLUTION: The secondary battery includes: an electrode assembly having a tab group 15 on which tabs 26 are stacked; a terminal 16 for connecting the electrode assembly to an external device; and a joint 32 formed by integrating the tab group 15 and the terminal 16. When a portion where the tab group 15 and the terminal 16 are separated from each other outside the joint 32 is defined as a non-joint 31, the secondary battery has a heat-affected zone 33, having a different crystal structure from both the joint 32 and the non-joint 31, located between the joint 32 and the non-joint 31. Crystal grain sizes forming the heat-affected zone 33 are smaller than crystal grain sizes forming the non-joint 31.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a power storage device including a joint where an uncoated part group and a terminal are integrated.

  2. Description of the Related Art Conventionally, vehicles such as an EV (Electric Vehicle) and a PHV (Plug in Hybrid Vehicle) are equipped with a lithium ion secondary battery, a nickel hydrogen secondary battery, or the like as a power storage device for storing power supplied to an electric motor or the like. . The secondary battery disclosed in Patent Literature 1 includes an electrode assembly in which a plurality of electrodes are stacked. The electrode has a metal foil, an active material layer present on at least one surface of the metal foil, and an uncoated portion (core exposed portion) where the active material layer is not present and the metal foil is exposed. The electrode assembly includes an uncoated portion group in which uncoated portions are stacked. The secondary battery includes a terminal (a current collecting member) for connecting the electrode assembly and an external device, and a joint in which the uncoated portion group and the terminal are integrated by laser welding.

JP 2011-76776 A

  In such a secondary battery, when the bonding strength is low, for example, when the vehicle vibrates after the secondary battery is mounted on the vehicle, the bonding between the uncoated part group and the terminal and the uncoated part group that constitutes the uncoated part group are not performed. The bonding between the coated parts may be released. When the connection between the uncoated part group and the terminal is released, it becomes impossible to take out electricity from the electrode assembly. Further, when the bonding between the uncoated portions constituting the uncoated portion group is released, the electric resistance in the uncoated portion group increases. Therefore, improvement in bonding strength is desired.

  SUMMARY An advantage of some aspects of the invention is to provide a power storage device that can improve bonding strength.

  The power storage device for solving the above problems is a metal foil, an active material layer present on at least one surface of the metal foil, the active material layer does not exist, the uncoated portion where the metal foil is exposed. An electrode assembly comprising a group of uncoated portions in which a plurality of electrodes having the following are stacked and the uncoated portion is stacked; a terminal for connecting the electrode assembly to an external device; and the uncoated portion. A power storage device comprising: a joint in which a group and the terminal are integrated with each other, wherein a part where the uncoated part group and the terminal are separated outside a non-joined part outside the joint. When, between the bonded portion and the non-bonded portion, both the bonded portion and the non-bonded portion has a heat-affected portion having a different crystal structure from the non-bonded portion, the crystal grain size constituting the heat-affected portion The gist of the invention is that it is smaller than the crystal grain size constituting the non-bonded portion.

  According to this, compared with the case where the crystal grain size of the heat-affected zone is larger than the crystal grain size of the non-joined zone, the strength of the region from the junction to the heat-affected zone is improved. As a result, after the power storage device is mounted on the vehicle, even if the vehicle vibrates, the joining between the uncoated part group and the terminal, and the joining between the uncoated parts constituting the uncoated part group are not easily released. Become.

  Further, in the power storage device, the L / S ratio of the crystal grain size in a portion of the heat-affected zone adjacent to the joint is 2 to 20% of the L / S ratio of the crystal grain size in the non-joined portion. Is preferred.

  Laser welding when welding is performed under welding conditions such that the L / S ratio of the crystal grain size of the portion adjacent to the joint in the heat-affected zone is larger than 20% of the L / S ratio of the crystal grain size of the non-joined portion In this case, the heat input to the uncoated portion increases, so that the metal foil is melted. On the other hand, when welding is performed under welding conditions such that the L / S ratio of the crystal grain size of the portion of the heat-affected zone close to the joint is less than 2% of the L / S ratio of the crystal grain size of the non-joined portion. In some cases, the uncoated portion group and the terminal may not be properly joined due to insufficient heat. From the above, by setting the L / S ratio of the crystal grain size of the portion of the heat-affected zone close to the joint to 2 to 20% of the L / S ratio of the crystal grain size of the non-joined portion, The uncoated part group and the terminal can be satisfactorily joined while suppressing fusing.

  Further, regarding the power storage device, when the joining portion and the heat-affected zone are combined to form a welded portion, the welded portion is formed by a laser welding of a keyhole method, and the welded portion in a surface direction of the uncoated portion is formed. Is preferably gradually increased as the distance from the terminal increases in a direction in which the uncoated portion group and the terminal overlap.

  According to this, since the welded portion is formed by the laser welding of the keyhole method, the penetration depth of the welded portion is easily increased as compared with the case of being formed by the laser welding of the heat conduction method, for example. Therefore, it is easy to secure the penetration depth necessary for joining the uncoated part group and the terminal. Further, when joining the uncoated part group and the terminal, among the plurality of uncoated parts constituting the uncoated part group, the uncoated part located on the opposite side to the terminal is thermally shrunk. It is easy to lift up from other uncoated portions, but by joining so that the area of the welded portion on the opposite side to the terminal increases as the terminal gets away from the terminal, the lifting of the uncoated portion can be suppressed.

  According to the present invention, the joining strength can be improved.

FIG. 2 is an exploded perspective view of the secondary battery of the embodiment. FIG. 2 is a cross-sectional view of the secondary battery of the embodiment. The perspective view which shows a welding part. (A) is a schematic view of a microstructure of a cross-sectional metal structure of a weld in a rolling direction, (b) is an enlarged view of a non-joined part in (a), (c) is an enlarged view of a heat-affected zone in (a), (D) is an enlarged view of the joining part in (a). FIG. 6 is a cross-sectional view illustrating another example of a secondary battery. FIG. 10 is an exploded perspective view of another example of an electrode assembly. The schematic diagram which observed the cross-section metallographic structure of the welding part of another example under the microscope. The schematic diagram which observed the cross-section metallographic structure of the welding part of another example under the microscope.

Hereinafter, an embodiment in which the power storage device is embodied as a secondary battery will be described with reference to FIGS. 1 to 4.
As shown in FIG. 1, a secondary battery 10 as a power storage device includes a case 11 and an electrode assembly 12 housed in the case 11. The case 11 has a rectangular parallelepiped case body 13 and a rectangular flat lid 14 for closing the opening 13 a of the case body 13. The case main body 13 and the lid 14 constituting the case 11 are both made of metal (for example, stainless steel or aluminum). Further, the secondary battery 10 of the present embodiment is a prismatic battery having a rectangular appearance. Further, the secondary battery 10 of the present embodiment is a lithium ion battery.

  As shown in FIG. 2, the electrode assembly 12 includes a plurality of positive electrodes 21, negative electrodes 22, and a separator 23. The electrode assembly 12 has a layered structure in which a separator 23 is interposed between the positive electrode 21 and the negative electrode 22 and is insulated from each other. The direction in which the positive electrode 21 and the negative electrode 22 are stacked is referred to as a stacking direction.

  The positive electrode 21 has a positive metal foil 24 as a rectangular metal foil, and a positive electrode active material layer 25 as an active material layer present on both surfaces of the positive metal foil 24. The material of the positive electrode metal foil 24 of the present embodiment is aluminum (thermal conductivity: 236 [W / m · K]). The positive electrode metal foil 24 is formed by rolling an aluminum base material. The positive electrode 21 includes a tab-side edge 21a at one of the edges along the longitudinal direction. The positive electrode 21 has a rectangular positive electrode tab 26 protruding from a part of the tab side edge 21a. The longitudinal direction of the positive electrode tab 26 coincides with the lateral direction of the positive electrode 21, and the lateral direction of the positive electrode tab 26 coincides with the longitudinal direction of the positive electrode 21. The tab 26 of the positive electrode is an uncoated portion where the positive electrode active material layer 25 does not exist and the positive electrode metal foil 24 is exposed. As shown in FIG. 4B, in the positive electrode metal foil 24, the aluminum crystal has an elongated shape and extends in the longitudinal direction of the tab 26.

  The negative electrode 22 has a negative electrode metal foil 27 as a rectangular metal foil and a negative electrode active material layer 28 as an active material layer present on both surfaces of the negative electrode metal foil 27. The material of the negative electrode metal foil 27 of the present embodiment is copper (thermal conductivity: 398 [W / m · K]). The negative electrode metal foil 27 is formed by rolling a copper base material. The negative electrode 22 includes a tab-side edge 22a at one of the edges along the longitudinal direction. The negative electrode 22 has a rectangular negative electrode tab 26 protruding from a part of the tab side edge 22a. The longitudinal direction of the negative electrode tab 26 matches the short direction of the negative electrode 22, and the short direction of the negative electrode tab 26 matches the long direction of the negative electrode 22. The negative electrode tab 26 is an uncoated portion where the negative electrode active material layer 28 does not exist and the negative electrode metal foil 27 is exposed. As shown in FIG. 4B, in the negative electrode metal foil 27, the copper crystal has an elongated shape and extends in the longitudinal direction of the tab 26.

The separator 23 is made of a rectangular sheet-shaped insulating material. The separator 23 insulates the positive electrode 21 and the negative electrode 22.
The electrode assembly 12 includes a tab group 15 as a group of uncoated portions of the positive electrode in which the tabs 26 of the positive electrodes of the respective positive electrodes 21 are gathered to one end side in the laminating direction, and a tab of the negative electrode of each of the negative electrodes 22. 26 is provided with a tab group 15 as an uncoated portion group of the negative electrode which is gathered and stacked on one end side in the laminating direction. The tab group 15 of the positive electrode and the tab group 15 of the negative electrode are arranged side by side at intervals in the direction along the tab side edges 21a, 22a. The electrode assembly 12 has a tab-side end surface 12a on the end surface where the tab group 15 exists. The electrode assembly 12 is housed in the case 11 with the tab group 15 bent in the longitudinal direction of the tab 26.

  As shown in FIG. 1, the secondary battery 10 includes terminals 16 of each polarity for extracting electricity from the electrode assembly 12. The material of the positive electrode terminal 16 is aluminum, and the material of the negative electrode terminal 16 is copper. Each terminal 16 has a rectangular plate portion 16a and a shaft portion 16b protruding from the plate portion 16a. The plate portion 16a of the positive terminal 16 is joined to the tab group 15 of the positive electrode, and the plate portion 16a of the negative terminal 16 is joined to the tab group 15 of the negative electrode. Each shaft portion 16 b penetrates through the through hole 14 a of the lid 14 and projects outside the case 11. A bus bar as an external device (not shown) for electrically connecting the secondary batteries 10 to each other can be fixed to the tip of each shaft portion 16b. Each terminal 16 electrically connects the electrode assembly 12 and the bus bar. The secondary battery 10 includes an insulating ring 17 for insulating the lid 14 from the shaft 16b of each terminal 16.

  The secondary battery 10 includes an insulating sheet 18 that covers the electrode assembly 12. The insulating sheet 18 covers five surfaces of the electrode assembly 12 excluding the tab-side end surface 12a. The insulating sheet 18 insulates the five surfaces excluding the tab-side end surface 12a of the electrode assembly 12 and the inner surface of the case body 13.

  As shown in FIG. 4A, the secondary battery 10 has a positive electrode tab group 15 and a positive electrode terminal 16 having a plate portion 16 a joined to form an integrated positive electrode joint portion 32 and a negative electrode tab group. 15 and the plate portion 16a of the terminal 16 are joined to each other, and an integrated portion 32 of the negative electrode is provided. The electrode assembly 12 is integrated with the terminal 16 by the joint 32.

  Each joint 32 is formed by a keyhole type laser welding. The joining portion 32 changes the metal structure by solidification after the tab group 15 and the plate portion 16a of the terminal 16 are melted by the heat of the laser welding, and the tab group 15 and the plate portion 16a of the terminal 16 are integrally formed. It is the part that became. In the direction in which the tab group 15 and the terminal 16 overlap, the deepest portion 32a of the joining portion 32 reaches about half of the thickness direction of the plate portion 16a of the terminal 16. In the tab portion 16 a of each tab group 15 and each terminal 16, a portion outside the joint portion 32 and separated as before the welding is a non-joint portion 31.

  Outside the joint 32, a heat affected zone 33 is further present between the joint 32 and the non-joint 31. The heat-affected zone 33 is a portion where the metallographic structure of the tab group 15 and the plate portion 16a of the terminal 16 has changed, though the metal structure has not been melted by the heat of the laser welding. That is, the above-described joint portion 32 and heat-affected zone 33 are portions where the metal structures of the tab group 15 and the plate portion 16a of the terminal 16 have changed by laser welding. The joint 32 and the heat-affected zone 33 are combined to form a weld 30.

  As shown in FIG. 3, each welded portion 30 of the present embodiment has a line shape, and the length of the welded portion 30 extends in the longitudinal direction of the plate portion 16 a of the terminal 16. The welded portion 30 is not visible from the surface of the plate portion 16a of the terminal 16 on the side of the shaft portion 16b. In the heat-affected zone 33, although the tab group 15 and the plate portion 16 a of the terminal 16 are not integrated by melting, the tab 26 located on the terminal 16 side of the tabs 26 constituting the tab group 15, There is a case where the end face of the plate portion 16a on the tab group 15 side is solid-phase bonded.

  The width W of the welded portion 30 in the short direction of the plate portion 16a of the terminal 16 is substantially constant in the longitudinal direction of the plate portion 16a, and gradually increases as the distance from the terminal 16 increases in the direction in which the tab group 15 and the terminal 16 overlap. Become. That is, the area of the welded portion 30 along the surface of the tab 26 increases in the direction in which the tab group 15 and the terminal 16 overlap, as the distance from the terminal 16 increases.

  FIGS. 4B to 4D show cross-sectional metal structures (collections of metal crystals) in the non-joined portion 31, the joined portion 32, and the heat-affected zone 33. As shown in FIGS. 4B to 4C, the crystal structure of the non-bonded portion 31, the crystal structure of the bonded portion 32, and the crystal structure of the heat-affected portion 33 are different from each other. As shown in FIG. 4B, the crystal of the non-bonded portion 31 has an elongated shape. As shown in FIG. 4C, the crystals of the heat-affected zone 33 are granular. As shown in FIG. 4D, the crystal of the bonding portion 32 has a smaller grain size than the crystal of the heat-affected zone 33. The crystal grain diameters of the non-joined portion 31, the joined portion 32, and the heat-affected portion 33 increase in the order of the joined portion 32, the heat-affected portion 33, and the non-joined portion 31 (the crystal grain size of the joined portion 32 <heat The crystal grain size of the affected portion 33 <the crystal grain size of the non-bonded portion 33). Note that the crystal grain size refers to the major axis of the crystal.

  The L / S ratio of the crystal grain size of the heat-affected zone 33 and the bonded portion 32 is smaller than the L / S ratio of the crystal grain size of the non-bonded portion 31. That is, the L / S ratio of the crystal grain size of the heat-affected zone 33 and the bonded portion 32 is closer to 1 than the L / S ratio of the crystal grain size of the non-bonded portion 31. In the heat-affected zone 33, the L / S ratio of the crystal grain size of the portion close to the joint 32 is larger than the L / S ratio of the crystal grain size of the portion close to the non-joint portion 31. The L / S ratio of the crystal grain size of the portion of the heat-affected zone 33 close to the joint 32 is 2 to 20% of the L / S ratio of the crystal grain size of the non-joined portion 31.

  The L / S ratio of the crystal grain size is measured by the following method. After the formation of the welding portion 30, the tab 26 (metal foil) constituting the tab group 15 is sliced by a focused ion beam device (FIB: Focused Ion Beam), and the rolling direction (the longitudinal direction of the tab 26 in the present embodiment) is determined. A cross section at 45 ° and parallel to the plate thickness direction was obtained, and a scanning ion (SIM) image of this cross section was obtained at a magnification of 10,000 times. The crystal grain size in the thickness direction of this image is measured by a cutting method defined in JIS H0501 at n = 5, and the maximum length of the crystal grain size at the center position in the thickness direction is L, and the crystal grain perpendicular to it is L. Assuming that the minimum length of the diameter is S, the L / S ratio was determined, and the arithmetic average L / S ratio of each n = 5 was defined as the average L / S ratio.

Next, a part of a method for manufacturing the secondary battery 10 will be described.
The method of manufacturing the secondary battery 10 includes a foil collecting step of collecting a plurality of tabs 26 included in the electrode assembly 12 to form the tab group 15, and joining the tab group 15 and the plate portion 16 a of the terminal 16, The method includes a joining step of forming the welded portion 30 and a housing step of housing the electrode assembly 12 in the case 11.

  In the foil collecting step, all the tabs 26 of the electrode assembly 12 are arranged on the plate portion 16a of the terminal 16 placed on a metal work table (not shown). Next, all the tabs 26 are pressed from the opposite side of the plate portion 16a across the tabs 26 by a foil collecting device (not shown) to collect the foils, thereby forming the tab group 15. The surface of the plate portion 16a opposite to the surface on which the shaft portion 16b protrudes faces the tab 26 located at one end in the stacking direction among the plurality of tabs 26 constituting the tab group 15.

  In the joining step, first, the tab group 15 is pressed toward the plate portion 16a of the terminal 16 by using a metal jig (not shown) arranged above the tab group 15. The portion pressed in the tab group 15 is a portion surrounding the portion to be the welded portion 30, and the portion to be the welded portion 30 is not pressed. Thereby, the tabs 26 that are adjacent to each other in the stacking direction and constitute the tab group 15 and the tab group 15 and the plate portion 16a are in close contact with each other. Next, in a state where the tabs 26 and the tab group 15 and the plate portion 16a are brought into close contact with each other with a jig, a laser is irradiated from the tab group 15 side to the tab group 15 and the plate portion 16a by a laser irradiation device (not shown). I do. The laser irradiation device of the present embodiment moves from one end in the longitudinal direction of the plate portion 16a to the other end while irradiating the laser.

  In the portion where the laser is irradiated in the tab group 15 and in the vicinity thereof, the melting of the tab group 15 progresses rapidly, and the molten metal is expanded by the recoil force of the metal vapor to form a keyhole. The laser penetrates into the formed keyhole and undergoes multiple reflections within the keyhole. Thereby, the depth of the keyhole is further increased. Then, when the deepest portion of the keyhole reaches the plate portion 16a of the terminal 16, a fused portion in which the tab group 15 and the plate portion 16a of the terminal 16 are melted is formed. After the laser beam passes, the melted portion solidifies to form a joint portion 32 in which the tab group and the plate portion 16a of the terminal 16 are integrated. When the fusion zone is formed, a heat-affected zone 33 is formed between the non-bonded zone 31 and the fusion zone. That is, the welded portion 30 is formed. The keyhole formed by the laser irradiation is filled by the surface tension of the molten metal before the molten portion is solidified to become the bonding portion 32. The laser irradiation conditions are set so that the laser does not penetrate the plate portion 16a of the terminal 16. The laser irradiation conditions refer to the laser output, the spot diameter, the position of the focal point in the thickness direction of the plate 16a of the terminal 16, the moving speed of the laser irradiation device, and the like. The welding part 30 of the tab group 15 and the terminal 16 is naturally cooled after the laser irradiation. The growth of the crystal in the heat-affected zone 33, that is, the coarsening of the crystal grain size becomes slower by cooling.

  Here, as described above, the material of the tab 26 and the terminal 16 is aluminum or copper, and the thermal conductivity of aluminum or copper is, for example, the thermal conductivity of steel (thermal conductivity: 67 [W / m · K]). High) compared to For this reason, the cooling rate of the tab 26 and the terminal 16 of this embodiment is higher than the cooling rate when the material of the tab 26 and the terminal 16 is steel. It is generally known that the crystal grain size increases as the cooling time increases. Therefore, the crystal grain size of the heat-affected zone 33 of the present embodiment is smaller than the crystal grain size of the heat-affected zone 33 when the material of the tab 26 and the terminal 16 is steel. As a result, when the material of the tab 26 and the terminal 16 is steel, the crystal grain size of the heat-affected zone 33 is larger than the crystal grain size of the non-joined zone 31, whereas the heat-affected zone 33 of the present embodiment is larger. Is smaller than the crystal grain size of the non-bonded portion 31.

  As described above, the non-joined portion 31, the joined portion 32, and the heat-affected zone 33 can be distinguished mainly by the difference in the crystal structure constituting the metal structure. Further, from the viewpoint of the manufacturing process, the non-joined portion 31 is a portion that does not change before and after laser welding, the joined portion 32 is a portion that is melted once at the time of laser welding and then solidified, and the heat-affected zone 33 is Although it is not melted during laser welding, it is a portion where the metal structure has changed due to the heat of laser welding.

  In the accommodation step, the tab group 15 is bent so that the welded portion 30 faces the tab-side end surface 12a of the electrode assembly 12, and the electrode assembly 12 is inserted into the case body 13. Next, the shaft portion 16b of each terminal 16 is inserted into the through hole 14a of the lid 14, and the opening 13a of the case body 13 is closed by the lid 14. Then, the case body 13 and the lid 14 are joined by welding. Thereby, the secondary battery 10 is completed.

The operation and effect of the present embodiment will be described.
(1) In the tab group 15 and the plate portion 16 a of the terminal 16, the non-bonded portion 31, the bonded portion 32, and the heat-affected portion 33 have different crystal grain sizes. In the present embodiment, the crystal grain size of the heat-affected zone 33 is smaller than the crystal grain size of the non-joined zone 31. Therefore, the strength of the region from the joint 32 to the heat-affected zone 33 is improved as compared with the case where the crystal grain size of the heat-affected zone 33 is larger than the crystal grain size of the non-joined zone 31. As a result, after the secondary battery 10 is mounted on the vehicle, even if the vehicle vibrates, the joining between the tab group 15 and the terminal 16 and the joining between the tabs 26 constituting the tab group 15 are not easily released.

  (2) In the non-joined portion 31 of the tab 26, the aluminum or copper crystal has an elongated shape extending in the longitudinal direction of the tab 26. Therefore, when the tab group 15 is bent, stress is applied in the longitudinal direction of the tab 26, and the tab 26 may be broken. Therefore, the crystal grain size of the heat-affected zone 33 is smaller than the crystal grain size of the non-joined zone 31, and the strength of the region from the joined zone 32 to the heat-affected zone 33 is improved, so that breakage of the tab 26 can be suppressed. .

  (3) The L / S ratio of the crystal grain size of the portion of the heat-affected zone 33 close to the joint 32 is 2 to 20% of the L / S ratio of the crystal grain size of the non-joined portion 31. When welding is performed under welding conditions such that the L / S ratio of the crystal grain size of the portion of the heat-affected zone 33 close to the joint portion 32 is larger than 20% of the L / S ratio of the crystal grain size of the non-joined portion 31. In the welding process, the amount of heat input to the tab 26 increases, so that the tab 26 is melted. On the other hand, welding is performed under welding conditions such that the L / S ratio of the crystal grain size of the portion of the heat-affected zone 33 close to the joining portion 32 is less than 2% of the L / S ratio of the crystal grain size of the non-joining portion 31. In this case, the tab group 15 and the plate portion 16a of the terminal 16 may not be satisfactorily joined due to insufficient heat input. From the above, by setting the L / S ratio of the crystal grain size of the portion of the heat-affected zone 33 close to the bonding portion 32 to 2 to 20% of the L / S ratio of the crystal grain size of the non-bonding portion 31, The tab group 15 and the terminal 16 can be satisfactorily joined while suppressing the fusing of the tab 26.

  (4) As one type of laser welding, a heat conduction method is known. When joining the tab group 15 and the terminal 16 by laser welding of a heat conduction method, the first side spreading in the direction orthogonal to the laser irradiation direction is located on the near side of the laser irradiation direction (the side opposite to the terminal 16). A welded portion is formed, and a second welded portion extending in the laser irradiation direction is formed on the back side (terminal 16 side) in the laser irradiation direction. For this reason, the welded portion has a so-called wine cup shape. In such a wine cup-shaped welded portion, the area of the welded portion in the surface direction of the tab 26 increases as the distance from the terminal 16 increases in the direction in which the tab group 15 and the terminal 16 overlap, as in the above embodiment. However, it differs from the above-described embodiment in that the width of the weld changes abruptly at the boundary between the first weld and the second weld.

  Of the plurality of tabs 26 constituting the tab group 15, the tab 26 located on the side opposite to the terminal 16 is thermally shrunk at the time of melting and easily rises from other tabs 26. For this reason, by increasing the area of the welded portion on the near side in the laser irradiation direction by the first welded portion, it is possible to suppress the lifting of the tab 26 located on the side opposite to the terminal 16. However, in the heat conduction method, it is difficult to increase the penetration depth of the welded portion. For this reason, when trying to join the tab group 15 and the terminal 16 by the heat conduction method, the terminal 16 of the plurality of tabs 26 constituting the tab group 15 must be The tab 26 located on the opposite side is melted.

  On the other hand, the welded portion 30 of the present embodiment is formed by keyhole laser welding. In the laser welding of the keyhole method, the penetration depth of the welded portion 30 is easily increased. Therefore, it is easy to secure the penetration depth necessary for joining the tab group 15 and the terminal 16. Further, the area where the welded portion 30 is formed in the plane direction of the tab 26 increases as the distance from the terminal 16 increases in the direction in which the tab group 15 and the terminal 16 overlap. Therefore, it is possible to prevent the tab 26 located on the side opposite to the terminal 16 from being lifted during welding.

  (5) The work table on which the terminals 16 are mounted and the jig for pressing the tab group 15 are each made of metal. For this reason, the heat applied to the tab group 15 and the terminal 16 in the welding process is easily released via the work table and the jig. Therefore, the cooling rate of the tab group 15 and the terminals 16 is increased, and the crystal grain size of the heat-affected zone 33 can be made smaller. As a result, the strength of the region from the joint 32 to the heat-affected zone 33 is further improved.

This embodiment can be implemented with the following modifications. The present embodiment and the following modifications can be implemented in combination with each other within a technically consistent range.
As shown in FIGS. 5 to 7, the electrode assembly 12 has a winding type in which a strip-shaped positive electrode 21, a strip-shaped separator 23, and a strip-shaped negative electrode 22 are stacked in this order and wound. An electrode assembly may be used. The electrode assembly 12 has a layered structure in which a positive electrode 21, a separator 23, and a negative electrode 22 are stacked.

  As shown in FIG. 6, the positive electrode 21 does not have the positive electrode active material layer 25 and has an uncoated portion 29 where the positive metal foil 24 is exposed at one end in the short direction. The negative electrode 22 has an uncoated portion 29 in which the negative electrode active material layer 28 does not exist and the negative electrode metal foil 27 is exposed, at one end in the lateral direction. The uncoated portion 29 exists over the entire length of the positive electrode 21 and the negative electrode 22 in the longitudinal direction. The electrode assembly 12 includes a positive electrode uncoated portion group 29a in which a positive electrode uncoated portion 29 is laminated on one end of the winding axis, and a negative electrode uncoated portion on the other end of the winding axis. 29 is provided with a group of uncoated portions 29 a of the negative electrode on which the negative electrode 29 is laminated.

  As shown in FIG. 5, the secondary battery 10 includes terminals 16 of each polarity for extracting electricity from the electrode assembly 12. Each terminal 16 is located at a plate portion 16a, a shaft portion 16b protruding from the plate portion 16a, an extension portion 16c extending from the plate portion 16a toward the electrode assembly 12, and a distal end portion of the extension portion 16c. It has a rectangular plate-like connecting portion 16d. As shown in FIG. 7, the secondary battery 10 includes a joining portion 32 in which an uncoated portion group 29 a having the same polarity and a connecting portion 16 d of the terminal 16 are integrated, and a joining portion 32 and a non-joining portion 31. And a heat affected zone 33 formed therebetween. Further, the secondary battery 10 has a non-joined portion 31 in which the uncoated portion group 29a and the connecting portion 16d of the terminal 16 are separated outside the joined portion 32. The joining portion 32 and the heat-affected portion 33 are formed by irradiating a laser from the uncoated portion group 29a side in a state where the uncoated portion group 29a and the connection portion 16d of the terminal 16 are overlapped. In addition, the crystal grain diameters of the non-bonded portion 31, the bonded portion 32, and the heat-affected portion 33 increase in the order of the bonded portion 32, the heat-affected portion 33, and the non-bonded portion 31.

  Although not shown, the uncoated portion group 29a and the connection portion 16d of the terminal 16 are connected to the connection portion 16d of the terminal 16 in a state where the connection portion 16d of the terminal 16 is inserted into the electrode assembly 12 so as to be a winding axis of the electrode assembly 12. May be joined.

  In the positive electrode 21, the positive electrode active material layer 25 may be present on one side of the positive electrode metal foil 24. Similarly, in the negative electrode 22, the negative electrode active material layer 28 may be present on one side of the negative electrode metal foil 27.

  The material of the tab 26 and the terminal 16 is not limited to aluminum or copper, and may be a conductive material having a thermal conductivity of 200 [W / m · K] or more. Conductivity: 295 [W / m · K]) or silver plating of aluminum or copper (thermal conductivity: 418 [W / m · K]). If the thermal conductivity of the material of the tab 26 and the terminal 16 is 200 [W / m · K] or more, the crystal grain size of the heat-affected zone 33 is smaller than the crystal grain size of the non-bonded portion 31.

The material of the tab 26 and the material of the terminal 16 may be different.
As shown in FIG. 8, when the material of the tab 26 and the material of the terminal 16 are the same metal, the joining portion 32 reaches the plate portion 16a of the terminal 16 in the direction in which the tab group 15 and the terminal 16 overlap. It does not have to be. In this case, of the plurality of tabs 26 constituting the tab group 15, the tab 26 located on the terminal 16 side and the plate portion 16a of the terminal 16 are solid-phase bonded (diffusion bonded). When welding is performed such that the depth H33 of the heat-affected zone 33 is 5 to 20% of the thickness H16 of the plate portion 16a of the terminal 16 in the direction in which the tab group 15 and the terminal 16 overlap, the tab 26 and the terminal The 16 plate portions 16a are solid-phase bonded. When the depth H33 of the heat-affected zone 33 is less than 5% of the thickness H16 of the plate portion 16a of the terminal 16, there is a possibility that the tab group 15 and the terminal 16 cannot be satisfactorily joined. On the other hand, when the depth H33 of the heat-affected zone 33 is larger than 20% of the thickness H16 of the plate portion 16a of the terminal 16, the joint 32 is also formed on the terminal 16.

  ○ The width W of the welded portion 30 in the short direction of the plate portion 16a of the terminal 16 may be constant in the direction in which the tab group 15 and the terminal 16 overlap, or may gradually decrease as the distance from the terminal 16 increases. .

  The welded portion 30 is not limited to a line shape, and may be, for example, a band shape. In this case, the laser irradiation device that moves from one end to the other end in the longitudinal direction of the plate portion 16a while irradiating the laser is further reciprocated in the short direction of the plate portion 16a.

  The secondary battery 10 may be a lithium ion secondary battery or another secondary battery. The point is that any material can be used as long as ions move between the positive electrode active material and the negative electrode active material and transfer charges.

  The power storage device can be applied to a power storage device other than a secondary battery, such as a capacitor.

DESCRIPTION OF SYMBOLS 10 ... Secondary battery as a power storage device, 12 ... Electrode assembly, 15 ... Tab group as an uncoated part group, 16 ... Terminal, 21 ... Positive electrode, 22 ... Negative electrode, 24 ... Positive metal foil as metal foil, 25 ... Positive electrode active material layer as active material layer, 26 ... Tab as uncoated portion, 27 ... Negative electrode metal foil as metal foil, 28 ... Negative electrode active material as active material layer Layer, 29: uncoated portion, 29a: uncoated portion group, 30: welded portion, 31: non-joined portion, 32: joined portion, 33: heat-affected portion.

Claims (3)

A plurality of electrodes each having a metal foil, an active material layer present on at least one surface of the metal foil, and an uncoated portion where the active material layer does not exist and the metal foil is exposed are stacked, and An electrode assembly including an uncoated part group in which a coating part is laminated,
A terminal for connecting the electrode assembly and an external device;
A joint where the uncoated part group and the terminal are integrated,
A power storage device comprising:
Outside the joining portion, when the uncoated portion group and the terminal are separated from each other as a non-joining portion,
Between the bonded portion and the non-bonded portion, having a heat affected zone having a different crystal structure from both the bonded portion and the non-bonded portion,
A power storage device, wherein a crystal grain size forming the heat-affected zone is smaller than a crystal grain size forming the non-bonded portion.   2. The power storage device according to claim 1, wherein an L / S ratio of a crystal grain size of a portion of the heat-affected zone close to the joint is 2 to 20% of an L / S ratio of a crystal grain size of the non-joined portion. apparatus. When the joint and the heat-affected zone are combined to form a weld,
The welded portion is formed by keyhole laser welding,
The power storage device according to claim 1, wherein an area of the welded portion in a surface direction of the uncoated portion gradually increases as the distance from the terminal increases in a direction in which the uncoated portion group and the terminal overlap. 4. apparatus.