JP2007213819A - Secondary battery for large-current discharge - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a secondary battery for large-current discharge capable of preventing leakage of electrolyte liquid and reducing a power loss at the large-current discharge. <P>SOLUTION: The lithium secondary battery has a metallic upper lid 10 caulked and fixed to a metallic battery can.An electrode group is housed in the battery can. The upper lid 10 has a caulking part 5 where a flange part of a dish-shaped lid case 2 made of aluminum is folded toward upside of the flange part of the nickel-plated metallic lid cap 4 and caulked. Aluminum having a melting point lower than that of iron used as a material for the lid cap 4 is used as the material for the lid case 2. Friction stir welding is applied on four points of the caulking part 5 from upside of the flange part of the folded lid case 2. Formation of pinholes is prevented at lower face side of the lid case 2, and corrosion of the lid cap 4 is prevented because non-aqueous electrolyte liquid does not touch the lid cap 4. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a secondary battery for large current discharge, and more particularly, to a secondary battery for large current discharge in which a plate-like lid case and a metal sealing lid with a lid cap caulked and a metal battery can are fixed by caulking. .

  2. Description of the Related Art Conventionally, a secondary battery for discharging a large current is used as a power source for an electric vehicle, an electrically assisted bicycle or the like to assist the start of the electric vehicle. As such a secondary battery for large current discharge, an assembled battery (battery module) in which a large number (for example, 40 to 100 in the case of an electric vehicle) secondary batteries (unit cells) are connected in series is used. A large current (for example, 60 to 200 amperes) is supplied from each unit cell.

  Generally, in such a unit cell, a metal sealing lid that also serves as a positive electrode terminal is caulked and fixed to a metal battery can that also serves as a negative electrode terminal via a gasket. The sealing lid is electrically connected by caulking a dish-like lid case and a lid cap, and the potential collected from the positive electrode is guided to the lid cap through the lid case. However, since the electrical resistance of about 2 milliohms is present in the sealed lid, the electrical resistance when it is an assembled battery reaches a total of 80 to 200 milliohms even with the sealed lid alone, so that it is 4.8 to 40 V due to large current discharge. Power loss (voltage drop) will occur. In order to reduce the power loss, for example, when the conduction cross-sectional area of the external terminal is increased, there is a problem that the weight of the unit cell and the assembled battery is increased. In order to reduce the electrical resistance of the hermetic lid, the present inventors have provided a secondary battery using a hermetic lid in which a flange portion of a dish-shaped aluminum lid case and a flange portion of an iron lid cap are joined by spot welding. (Refer to Patent Document 1). Further, a secondary battery using a sealed lid in which the flange portion of the lid case and the flange portion of the lid cap are joined by spot welding and then the flange portions are caulked is disclosed (for example, see Patent Document 2). ).

JP 2000-90892 A JP 2002-170531 A

However, in spot welding, an aluminum (alloy) lid case having a melting point lower than that of iron is used because it is joined by energization between electrodes in contact with the lower surface of the flange portion of the lid case and the upper surface of the flange portion of the lid cap In other words, defects such as pinholes may be generated on the lower surface of the battery, that is, inside the battery, and welding marks may remain. Even if a pinhole or the like is formed in the aluminum lid case, it is difficult to form a pinhole or the like in the iron lid cap. Therefore, even if an airtight inspection of the battery lid is performed, the pinhole in the lid case cannot be detected. For this reason, when using the battery, the electrolyte in the battery may enter the pinhole of the lid case and come into contact with the iron lid cap. As a result, there is a problem that the lid cap is corroded and the electrolyte solution leaks.

  In view of the above-described case, an object of the present invention is to provide a secondary battery for large current discharge that can prevent leakage of an electrolyte and reduce power loss due to large current discharge.

  In order to solve the above-mentioned problems, the present invention provides a secondary battery for large current discharge in which a metal-sealed lid and a metal battery can with a dish-shaped lid case and a lid cap are caulked and fixed. The case is made of a metal having a melting point lower than that of the lid cap, and the flange portion of the lid cap and the flange portion of the lid case folded back above the flange portion of the lid cap are joined at a plurality of locations. It is joined by friction stir welding accompanied by plastic flow of the part, and the friction stir welding is performed from the flange portion side of the folded lid case.

  In the present invention, the flange portion of the lid cap and the flange portion of the lid case folded back above the flange portion of the lid cap are joined by friction stir welding with plastic flow of the joint portion, and the friction Since the stir welding is applied from the flange side of the folded lid case, no pinhole is formed on the lower surface side of the lid case, which has a lower melting point than the lid cap. Since corrosion of the battery is prevented, leakage of the electrolyte can be prevented, and since there are multiple joints by friction stir welding, the electrical resistance between the lid case and the lid cap is reduced. The power loss in the part can be reduced.

  In this case, the lid case may be made of aluminum or aluminum alloy. Further, the lid cap may be made of carbon steel, stainless steel, or nickel. The cover cap may be made of nickel-plated carbon steel.

  According to the present invention, the flange portion of the lid cap and the flange portion of the lid case folded back upward from the flange portion of the lid cap are joined by friction stir welding with plastic flow of the joining portion, and Since the friction stir welding is performed from the flange part side of the folded lid case, no pinhole is formed on the lower surface side of the lid case, so that the electrolyte solution does not contact the lid cap and prevents corrosion of the lid cap Therefore, leakage of the electrolyte can be prevented and the electrical resistance between the lid cap and lid case is reduced because there are multiple joints by friction stir welding, reducing power loss during large current discharge. It is possible to obtain the effect of being able to.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of a lithium secondary battery for large current discharge to which the present invention is applied will be described with reference to the drawings.

(Constitution)
As shown in FIG. 1, a lithium secondary battery 20 for discharging a large current according to this embodiment includes a metal battery can 1 having a bottomed cylindrical shape and an upper side sealed with a metal upper lid (sealing lid) 10 and a belt-like positive electrode. The negative electrode has an electrode group 11 wound around an axial core in a spiral shape in cross section via a separator.

  On the upper side of the electrode group 11, a positive electrode current collection ring for collecting a potential from the positive electrode is disposed on a substantially extension line of the shaft core. The positive electrode current collecting ring is fixed to the upper end portion of the shaft core. The edge part of the positive electrode lead piece led out from the positive electrode is joined by ultrasonic welding to the periphery of the flange that integrally projects from the periphery of the positive electrode current collecting ring. A disc-shaped upper lid 10 is disposed above the positive electrode current collecting ring. One end of a positive electrode lead plate 12 formed by stacking a plurality of aluminum ribbons is fixed to the upper portion of the positive electrode current collecting ring, and the other end of the positive electrode lead plate 12 is joined to the lower surface of the upper lid 10 by welding. Yes.

  On the other hand, a negative electrode current collection ring for collecting a potential from the negative electrode is disposed below the electrode group 11. The outer peripheral surface of the lower end portion of the shaft core is fixed to the inner peripheral surface of the negative electrode current collecting ring. The end of the negative electrode lead piece led out from the negative electrode is joined to the outer peripheral edge of the negative electrode current collecting ring by welding. A copper negative electrode lead plate for electrical conduction is welded to the lower part of the negative electrode current collecting ring, and the negative electrode lead plate is joined to the inner bottom surface of the battery can 1 by welding.

As shown in FIGS. 2 and 3, the upper lid 10 has a dish-shaped aluminum lid case 2 and a nickel-plated iron lid cap 4 superimposed thereon, and the lid portion 2 has a flange portion and a lid cap. The four flange portions are caulked to form a caulking portion 5. The lid case 2 is made of aluminum having a melting point lower than that of the iron used for the lid cap 4. In the caulking portion 5, the flange portion of the lid case 2 is folded back to the upper side of the lid cap 4. The lid case 2 is formed with a V-shaped groove cleaving valve 3 that cleaves when the battery internal pressure increases. The lid cap 4 has a convex portion protruding in the center on the side opposite to the electrode group 11. Vents 8 for exhausting the gas inside the battery when the cleavage valve 3 is operated are formed on the upper surface side and the side surface side of the convex portion. The caulking portion 5 is subjected to friction stir welding, for example, at four locations. In friction stir welding, a rotating tool (joint rotating tool) whose tip is rotated is used, and the rotating tool is pressed against the caulking portion 5 from above the flange portion of the folded lid case 2 (in the direction of arrow A). Applied.

As shown in FIG. 1, the upper lid 10 and the battery can 1 are caulked and fixed via an insulating and heat resistant resin gasket 13. At this time, the battery can 1 is subjected to a stepping process for forming a stepped portion for placing the upper lid 10 on the inner surface side of the battery can 1 containing the electrode group 11 and above the electrode group 11. The upper lid 10 is caulked and fixed on the upper side. For this reason, the inside of the lithium secondary battery 20 is sealed, the battery can 1 also serves as the negative electrode external terminal, and the upper lid 10 also serves as the positive electrode external terminal. In addition, a non-aqueous electrolyte is injected into the battery can 1. The non-aqueous electrolyte includes 1 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). Is used.

  In the electrode group 11, a positive electrode and a negative electrode are wound around (outside) the shaft core via a separator so that the positive and negative electrodes do not directly contact each other. In this example, a polyethylene porous membrane having a thickness of 25 μm and a width of 100 mm is used as the separator. The positive electrode lead piece and the negative electrode lead piece are respectively disposed on both end surfaces of the electrode group 11 on the opposite sides. Insulation coating is applied to the entire circumference of the collar surface of the electrode group 11 and the positive electrode current collector ring. For the insulation coating, an adhesive tape in which a hexamethacrylate adhesive is applied to one side of a polyimide base material is used. The adhesive tape is wound one or more times from the peripheral surface of the collar portion to the outer peripheral surface of the electrode group 11.

  The negative electrode constituting the electrode group 11 has a copper foil having a thickness of 10 μm as a negative electrode current collector. A negative electrode mixture containing carbon particles having an average particle diameter of 20 μm as a negative electrode active material is coated on both surfaces of the copper foil. In the negative electrode mixture, for example, relative to 90 parts by weight of carbon particles, a binder (binder) polyvinylidene fluoride (manufactured by Kureha Chemical Co., Ltd., trade name: KF # 120) (hereinafter abbreviated as PVDF). .) 10 parts by weight is blended. When the negative electrode mixture is applied to the copper foil, a dispersion solvent N-methyl-2-pyrrolidone (hereinafter abbreviated as NMP) is used. An uncoated portion of the negative electrode mixture is formed on the side edge on one side in the longitudinal direction of the copper foil. The uncoated part is notched in a comb shape, and a negative electrode lead piece is formed in the notch remaining part. The negative electrode is pressed after drying and cut to a width of 94.5 mm.

  On the other hand, the positive electrode has an aluminum foil having a thickness of 20 μm as a positive electrode current collector. On both surfaces of the aluminum foil, a positive electrode mixture containing lithium manganate having an average particle diameter of 10 μm is applied as a positive electrode active material. In the positive electrode mixture, for example, 10 parts by weight of carbon powder having an average particle size of 3 μm and 5 parts by weight of PVDF as a binder are blended with respect to 85 parts by weight of lithium manganate. When applying the positive electrode mixture to the aluminum foil, a dispersion solvent NMP is used. An uncoated portion of the positive electrode mixture is formed on the side edge on one side in the longitudinal direction of the aluminum foil, as in the negative electrode, and a positive electrode lead piece is formed. After drying, the positive electrode is pressed in the same manner as the negative electrode, and is cut to a width of 94 mm.

The assembly of the lithium secondary battery 20 can be performed as follows. First, the upper lid 10 is produced. That is, the flange portion of the lid case 2 and the flange portion of the lid cap 4 are overlapped, and the caulking portion 5 is formed by folding back the peripheral edge of the flange portion of the lid case 2 to the upper side of the lid cap 4. Next, a rotary tool having a flat tip surface slightly swelled in a dome shape or a spherical shape at the center and a backup member (anvil) that supports the caulking portion 5 of the joining portion from the lower side are used. A rotary tool is pressed from the direction of the arrow A and friction stir welding is performed on the caulking portion 5, thereby joining the two together to produce the upper lid 10. On the other hand, after the electrode group 11 produced by winding the positive electrode and the negative electrode through a separator is inserted into the battery can 1, the negative electrode side is connected, and the upper lid 10 and the electrode group 11 are connected via the positive electrode lead plate 12. Connect electrically. After pouring the non-aqueous electrolyte, the battery can 1 and the upper lid 10 are sealed by caulking and fixing via a gasket 13. In this example, the lithium secondary battery 20 is set to have a diameter of 40 mm, a height of 114 mm, and a capacity of 6 Ah.

(Operation, etc.) Next, the operation and the like of the lithium secondary battery 20 of the present embodiment will be described.

In the lithium secondary battery 20 of the present embodiment, friction stir welding is applied to the caulking portion 5 where the lid case 2 and the lid cap 4 are caulked. In the friction stir welding, the lid case 2 constituting the caulking part 5 and a part on the upper surface side of the lid cap 4 are plastically flowed by frictional heat by pressing the caulking part 5 while rotating the tip of the rotary tool. Are integrated by mixing (the flange portion of the folded lid case 2 and the flange portion of the lid cap 4 are joined). For this reason, since the dissimilar metal contact surface between the lid case 2 and the lid cap 4 is eliminated at the integrated joint portion, the electrical resistance between the lid case 2 and the lid cap 4 during energization can be reduced. Further, in the lithium secondary battery 20 of the present embodiment, joining by friction stir welding is performed at four places of the caulking portion 5. For this reason, since an increase in electrical resistance is suppressed even during large current discharge, power loss during large current discharge can be reduced.

Further, in the lithium secondary battery 20 of the present embodiment, the friction stir welding of the caulking portion 5 is performed by pressing a rotary tool against the flange portion of the lid case 2 that is folded back to the upper side of the lid cap 4. For this reason, formation of a pinhole etc. accompanying welding is prevented on the lower surface side of the lid case 2 located inside the battery after the battery is assembled. Thereby, in the lithium secondary battery 20, since the non-aqueous electrolyte does not contact the lid cap 4 and corrosion of the lid cap 4 is prevented, leakage of the non-aqueous electrolyte can be prevented, and peripheral devices and the like can be prevented. Can reduce damage to the Therefore, the lithium secondary battery 20 of the present embodiment, which reduces power loss during large current discharge and prevents leakage of the nonaqueous electrolyte, has the reliability of joining the lid case 2 and the lid cap 4 of the upper lid 10. It is an improved battery.

Furthermore, in the lithium secondary battery 20 of this embodiment, since the material of the lid case 2 has a lower melting point than the material of the lid cap 4, the lid case 2 and the positive electrode lead plate 12 can be easily joined by welding. it can. In addition, since the lid cap 4 has a higher melting point than the lid case 2, it is possible to suppress temperature rise during battery use, deformation during use in a high temperature environment, and the like.

Conventionally, in a lithium secondary battery, spot welding is used for joining a lid case and a lid cap constituting a hermetic lid. In spot welding, the welding electrode is brought into contact with the lower surface of the flange portion of the lid case and the upper surface of the flange portion of the lid cap, and is joined by energizing between the electrodes. This may cause defects such as pinholes on the lower surface of the lid case, leaving weld marks. Even if a pinhole or the like is formed in the aluminum lid case, it is difficult to form a pinhole or the like in the iron lid cap. Therefore, even if an airtight inspection of the battery lid is performed, the pinhole in the lid case cannot be detected. After the battery is assembled, the lower surface side of the lid case becomes the inside of the battery, and therefore the lower surface of the lid case is exposed to the non-aqueous electrolyte, so that the non-aqueous electrolyte enters the pinhole. For this reason, since the nonaqueous electrolyte solution that has entered the pinhole comes into contact with the iron lid cap, the lid cap is corroded and the nonaqueous electrolyte solution leaks. The non-aqueous electrolyte leaked out of the battery may cause damage such as corrosion not only to the battery but also to adjacent batteries and surrounding devices when assembled batteries are used. On the other hand, since the metal of the lid case and the lid cap are different from each other, the sealing lid has a contact resistance of different metals. For this reason, power loss occurs due to large current discharge. In an assembled battery in which a large number of batteries are connected, the overall resistance increases and the power loss also increases. The present embodiment is a lithium secondary battery for large current discharge that can solve these problems.

In addition, in the lithium secondary battery 20 of this embodiment, although the example which sets the joining location by the friction stir welding of the caulking part 5 as 4 places was shown, this invention is not limited to this, For example, 6 places , 8 locations, etc. may be used. In consideration of a decrease in electrical resistance during large current discharge, it is preferable to use a plurality of joints. Moreover, although the shape of the front-end | tip part of the rotary tool used for friction stir welding was illustrated in this embodiment, this invention is not limited to this, You may change according to the magnitude | size etc. of a joining part.

In the lithium secondary battery 20 of the present embodiment, the lid case 2 is made of aluminum and the lid cap 4 is made of iron with nickel plating, but the present invention is not limited to these. For example, the lid case 2 may be made of an aluminum alloy, and the lid cap 4 may be made of carbon steel, stainless steel, or nickel. In this way, the lid case 2 can have a lower melting point than the lid cap 4. By plating the lid cap 4 with a soft metal such as nickel or copper, the surface oxide film of the lid cap 4 is easily peeled off and the bonding interface is activated to facilitate good bonding. It is preferable to use nickel-plated carbon steel.

  Furthermore, in the lithium secondary battery 20 of the present embodiment, lithium manganate is exemplified as the positive electrode active material, but the present invention is not limited to this. As the positive electrode active material that can be used other than the present embodiment, for example, a lithium transition metal composite oxide such as a lithium nickel composite oxide or a lithium cobalt composite oxide may be used. In this embodiment, carbon particles are exemplified as the negative electrode active material. However, the present invention is not limited to this, and carbon materials such as amorphous carbon and graphite that are usually used in lithium secondary batteries are used. I just need it.

  Furthermore, in the lithium secondary battery 20 of the present embodiment, an example in which about 6 mol / liter of lithium hexafluorophosphate is dissolved in a nonaqueous electrolyte solution in a mixed solvent such as ethylene carbonate is illustrated. There is no particular limitation on the non-aqueous electrolyte that can be used in the above. Any organic solvent or lithium salt may be used as long as it is usually used for lithium ion secondary batteries. For example, a solvent using a single organic solvent or a mixture of organic solvents such as carbonates, sulfolanes, ethers, and lactones. What dissolved lithium salt in it can be used. Further, the mixing ratio of the organic solvent and the content of the lithium salt are not particularly limited.

  Furthermore, in the present embodiment, the lithium secondary battery 20 for discharging a large current with a battery capacity of 6 Ah is exemplified, but the present invention is not limited to this, and is suitably applied to a battery with a battery capacity of 3.5 Ah or more. can do. In addition, for large current discharge, for example, it can be suitably applied to a battery for an electric vehicle that requires 500 amperes or more when the engine is started, and is also suitable for a power source such as a bicycle that assists with power when climbing up. is there. Further, the battery shape, size, etc. are not particularly limited.

Next, examples of the lithium secondary battery 20 manufactured according to the present embodiment will be described. For comparison, the fabricated lithium secondary battery of the comparative example is also shown.

(Example 1) In Example 1, the material of the lid case 2 is 0.4 mm thick aluminum, and the material of the lid cap 4 is approximately 5 μm thick nickel plated 0.6 mm thick iron. It was. The melting point of the lid case 2 is lower than the melting point of the lid cap 4. In the production of the upper lid 10, the diameter D of the tip is 3.2 mm, the diameter d of the bulge at the center of the tip surface of the rotary tool is 1.6 mm which is 1/2 of the diameter D, and the height h of the bulge is 0.1 mm. The rotary tool set to is used. Joining portions by friction stir welding were formed at four places of the caulking portion 5.

(Comparative example 1) In the comparative example 1, the lithium secondary battery was produced using the upper lid produced by giving the same friction stir welding as Example 1 from the lower surface side of the lid case.

(Test and evaluation) The resistance values of the upper lids of the lithium secondary batteries produced in Example 1 and Comparative Example 1 were measured, and pinholes formed on the lower surface (case surface) of the lid case of the caulking portion 5 were counted. Thus, the probability of occurrence of pinholes was obtained. Table 1 shows the measurement results of pinhole occurrence probability and resistance value.

As shown in Table 1, the lithium secondary battery of Comparative Example 1 in which the friction stir welding was performed from the lower surface side of the lid case, and the friction stir welding was performed from the flange portion side of the lid case folded back to the upper side of the lid cap 4. In any of the lithium secondary batteries 20 of Example 1, the resistance value of the upper lid was approximately 0.10 to 0.14 mΩ, and no significant difference was recognized. From this, it can be confirmed that the same continuity can be obtained as compared with the case of joining by resistance (spot) welding, and the battery resistance can be reduced from the upper lid simply by caulking the lid case and the lid cap. It was. Further, in the battery of Comparative Example 1, the probability of occurrence of pinholes on the case surface was 0.012%. On the other hand, in the battery of Example 1, no pinhole was observed. This is considered that when the upper lid used in Example 1 was manufactured, the lower surface side of the lid case was only supported by the backup member, and no pinhole was generated in principle. From this, it was found that the non-aqueous electrolyte solution can be prevented from leaking because the contact of the non-aqueous electrolyte solution with the lid cap 4 is prevented and corrosion of the lid cap is prevented.

Since the present invention provides a secondary battery for large current discharge that prevents leakage of electrolyte and can reduce power loss due to large current discharge, and contributes to the manufacture and sale of secondary batteries for large current discharge. Have industrial applicability.

It is sectional drawing which shows the lithium secondary battery for large current discharge of embodiment to which this invention is applied. It is sectional drawing which shows typically the position of the friction stir welding of the cover case and cover cap of the upper cover used for the lithium secondary battery for large current discharge of embodiment, and the direction which presses a rotary tool. It is a top view which shows the upper cover used for the lithium secondary battery for large current discharge of embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Battery can 2 Lid case 4 Lid cap 5 Caulking part 10 Upper lid (sealing lid) 11 Electrode group 20 Lithium secondary battery for large current discharge (secondary battery for large current discharge)

Claims (4)

  In a secondary battery for high-current discharge in which a metal-sealed lid and a metal battery can with a plate-like lid case and a lid cap are caulked and fixed, a metal having a lower melting point than the lid cap is placed on the lid case. The flange portion of the lid cap and the flange portion of the lid case folded back above the flange portion of the lid cap are joined at a plurality of locations by friction stir welding with plastic flow of the joint portion. And the friction stir welding is performed from the flange portion side of the folded lid case. The secondary battery for high current discharge according to claim 1, wherein the lid case is made of aluminum or an aluminum alloy. The secondary battery for high-current discharge according to claim 1 or 2, wherein the lid cap is made of carbon steel, stainless steel, or nickel. The secondary battery for high current discharge according to claim 1, wherein the lid cap is made of nickel-plated carbon steel.

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