Method for manufacturing external cladding for laminate battery

In method for manufacturing an external cladding for a laminate battery according to the present invention, austenitic stainless steel foil having a thermoplastic resin layer on one of a front surface and a rear surface and a lubricating film on the other surface is used as a material, the stainless steel foil is disposed such that the surface provided with the thermoplastic resin layer opposes a punch, and drawing is implemented on the stainless steel foil without using lubricating oil in a condition where an annular region of the stainless steel foil, which is contacted by a shoulder portion of the punch, is set at a temperature of 20° C. or lower, and an exterior region on an exterior of the annular region is set at a temperature between 40° C. and 100° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national phase application filed under 35 U.S.C. §371 of International Application PCT/JP2012/056839, filed on Mar. 16, 2012, designating the United States, which claims priority from Japanese Patent Application No. 2011-071667, filed Mar. 29, 2011, which are hereby incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a method for manufacturing an external cladding for a laminate battery, and more particularly to a novel improvement for enabling the accommodation of a battery capacity increase and the reduction of the possibility of infiltration of contamination by impurities in the interior of the laminate battery by using as a material austenitic stainless steel foil having a thermoplastic resin layer on one of a front surface and a rear surface and a lubricating film on the other of the front surface and the rear surface, and performing drawing under temperature conditions appropriate for the material without using lubricating oil.

BACKGROUND ART

Recently, laminate batteries in which a battery element is sealed by external cladding (laminate sheets) have drawn attention as a form of a secondary battery such as a lithium battery. Japanese Patent Application Publication No. 2004-052100 A discloses a method for manufacturing the external cladding on which a projecting portion for housing the battery element is formed by using austenitic stainless steel foil as a material, and by implementing drawing on the stainless steel foil at room temperature. By employing the external cladding made of the stainless steel foil in this manner, a strong and lightweight laminate battery can be constructed.

This type of laminate battery is applied to electric automobiles or the like, and therefore requires a large capacity in order to extend the range of electric automobiles for instance. To increase the capacity of the laminate battery, a space that can accommodate a larger battery element must be secured. With the configuration described above, however, drawing is implemented on the stainless steel foil at room temperature, and therefore molding defects such as cracks occur when an attempt is made to form a deep projecting portion.

Japanese Patent Application Publication No. 2009-113058 A discloses a configuration for realizing deep drawing, when drawing is implemented on austenitic stainless steel sheet, by performing warm working in which a region of the stainless steel foil that contacts a punch is cooled, a region on an exterior thereof is heated, and the punch is pressed into the stainless steel foil while supplying lubricating oil.

The present applicant considered the application of drawing such as that described in Japanese Patent Application Publication No. 2009-113058 A to manufacture external claddings for laminate batteries such as that described in Japanese Patent Application Publication No. 2004-052100 A, but found that the following problems arise. The method described in Japanese Patent Application Publication No. 2009-113058 A requires the performance of washing for degreasing after the drawing because the drawing is performed while supplying lubricating oil. When a laminate battery is constructed using external claddings manufactured by this drawing process, however, the lubricating oil as well as dust and the like adhered to the lubricating oil may infiltrate interior of the battery, causing the battery to malfunction. If the lubricating oil is simply not used, on the other hand, the material can no longer be moved smoothly, and therefore deep drawing cannot be realized.

SUMMARY OF THE INVENTION

The present invention has been designed to solve the problems described above, and an object thereof is to provide a method for manufacturing an external cladding for a laminate battery for enabling the accommodation of a battery capacity increase and the reduction a possibility of infiltration of contamination by impurities in the interior of the laminate battery.

A method for manufacturing external cladding for a laminate battery according to the present invention is a method wherein;

an external cladding for a laminate battery, in which a projecting portion for housing a battery element is formed is manufactured by

using as a material austenitic stainless steel foil having a thermoplastic resin layer on one of a front surface and a rear surface and a lubricating film on the other of the front surface and the rear surface;

disposing the stainless steel foil such that the surface provided with the thermoplastic resin layer is opposite a punch; and

implementing drawing on the stainless steel foil without using lubricating oil in a condition where an annular region of the stainless steel foil, which is contacted by a shoulder portion of the punch, is set at a temperature of 20° C. or lower, and an exterior region on an exterior of the annular region is set at a temperature between 40° C. and 100° C.

In the method for manufacturing external cladding for a laminate battery according to the present invention, the austenitic stainless steel foil having the thermoplastic resin layer on one of the front surface and the rear surface and the lubricating film on the other surface is used as the material, and drawing is performed on the material under appropriate temperature conditions without the use of lubricating oil. Hence, the thermoplastic resin layer and the lubricating film having been softened by heating exhibit the functions of conventionally used lubricating oil, and therefore deep drawing can be realized without the use of lubricating oil. As a result, an increase in battery capacity can be accommodated, and the possibility of infiltration of contamination by impurities in the interior of the laminate battery can be reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 1is a perspective view showing a laminate battery according to a first embodiment of the present invention. In the drawing, a battery element1is stored in an interior of a battery case2. As is well known, the battery element1is a laminated body including a positive electrode, a negative electrode, and a separator, which is submerged in an electrolyte. A pair of tabs3(projecting terminals of the positive electrode and the negative electrode) are connected to the battery element1. The tabs3are drawn out to an exterior of the battery case2and connected to an external power supply or an external load, not shown in the drawing. A plurality of attachment holes2aare provided in the battery case2. The attachment holes2aare used to attach the laminate battery to an object of attachment such as an electric automobile.

FIG. 2is a sectional view taken along a line II-II inFIG. 1. As shown in the drawing, the battery case2includes a flat plate-shaped first external cladding20, and a second external cladding21provided with a projecting portion21a. The battery element1is housed in a housing space22formed by the projecting portion21aof the second external cladding21and the first external cladding20. In other words, the projecting portion21ais used to house the battery element1. As will be described in more detail below using the drawings, the projecting portion21ais formed by drawing.

FIG. 3is a sectional view showing the first and second external claddings20,21ofFIG. 2. As shown in the drawing, stainless steel foil32having a thermoplastic resin layer30(a laminate layer) on one of a front surface and a rear surface and a lubricating film31on the other surface is used as a material of the first and second external claddings20,21.

The thermoplastic resin layer30is a resin layer of approximately 60 μm, which is formed from a resin that melts when heated to approximately 120 to 200° C. The battery case2shown inFIG. 1is formed by overlapping the respective thermoplastic resin layers30of the first and second external claddings20,21and then applying heat to the first and second external claddings20,21while restraining the first and second external claddings20,21so that the respective thermoplastic resin layers30of the first and second external claddings20,21are thermally bonded (heat-sealed) to each other. As the thermoplastic resin layer30, a heat seal insulation film such as a polyethylene film or a polypropylene film may be used individually. Alternatively, the thermoplastic resin layer30may be formed by joining a polyethylene terephthalate film to a joint part of the stainless steel foil32and then laminating a heat seal insulation film such as a polyethylene film or a polypropylene film onto the polyethylene terephthalate film.

The lubricating film31is a layer of approximately 2 μm, which is provided to impart superior moldability and chemical resistance to the external claddings20,21. The lubricating film31may be a resin film disclosed by the present applicant in Japanese Patent Application Publication No. 2008-307092, for example, or more specifically a resin film formed from one or more types of resin selected from polyvinyl alcohol resin, urethane resin, and acrylic resin, wherein the ratio between the weight of OH groups and a total weight of the resin is no less than 0.2.

Austenitic stainless steel sheet of approximately 10 to 400 μm is used as the stainless steel foil32. When strain is applied thereto at room temperature, the austenite is more likely to undergo martensitic transformation in steel grades with more unstable austenite. As a result, austenitic stainless steel has a property of dramatically hardening from transformation hardening in combination with work hardening. Hence, drawing performance can be improved greatly by cooling an annular region of the stainless steel foil32that is contacted by a shoulder portion42dof a punch42(seeFIG. 4), to be described below, in order to maintain high strength while heating a region on an exterior thereof in order to suppress hardening due to martensitic transformation.

FIG. 4is a configuration diagram showing a mold4used to implement a method for manufacturing external cladding for a laminate battery in order to manufacture the second external cladding21ofFIG. 2. As shown in the drawing, the mold4is provided with a lower mold40and an upper mold45disposed so as to sandwich the stainless steel foil32. The lower mold40is provided with a bed41, the punch42which is fixed to the bed41, and a blank holder44that is disposed at an outer peripheral position of the punch42and coupled to the bed41via a cushion pin43. The upper mold45is provided with a slide46, and a die48that is disposed above the blank holder44and fixed to the slide46via a spacer47.

A servo motor, not shown in the drawing, is connected to the slide46. The slide46, the spacer47, and the die48, or in other words the upper mold45, are(is) driven integrally in a direction approaching the lower mold40and a direction moving away from the lower mold40by the driving force of the servo motor. Drawing is implemented by displacing the upper mold45in the direction approaching the lower mold40such that the punch42is pressed into an inner side of the die48together with the stainless steel foil32.

The punch42is provided with an introduction passage42aconnected to an external coolant system, not shown in the drawing, a cooling chamber42binto which a coolant is introduced through the introduction passage42a, and a discharge passage42cfor discharging the coolant from the cooling chamber42b. In other words, the punch42can be cooled by introducing the coolant into the cooling chamber42b. When the cooled punch42contacts the stainless steel foil32, an annular region32aof the stainless steel foil32, which is contacted by the shoulder portion42dof the punch42, is cooled. Note that the cooling range of the stainless steel foil32is not limited to the annular region32aalone, and as long as at least the annular region32ais cooled, a region on the inner side of the annular region32amay also be cooled. In this embodiment, the interior region of the annular region32ais cooled in addition to the annular region32asince the stainless steel foil32is cooled by the punch42.

Although not shown in the drawing, the cooling effect on the stainless steel plate32can be enhanced by disposing a counterpunch coupled to a slide via a spring or the like in a position opposing the punch and providing a cooling chamber into which the coolant is introduced in the counterpunch.

Heaters44a,48aare built respectively into the blank holder44and the die48in order to heat the blank holder44and the die48. By sandwiching the stainless steel foil32between the heated blank holder44and die48, an exterior region32bat the exterior of the annular region32ais heated.

Next, a method for manufacturing the external cladding for a laminate battery using the mold4shown inFIG. 4will be described. To manufacture the second external cladding21having the projecting portion21ashown inFIG. 2, the stainless steel foil32is placed on the punch42and the blank holder44so that the surface thereof provided with the thermoplastic resin layer30opposes the punch42in a condition where the upper mold45is separated from the lower mold40. Next, the upper mold45is lowered to a position where the stainless steel foil32is sandwiched between the blank holder44and the die48. The reason for setting a placement direction of the stainless steel foil32such that the surface provided with the thermoplastic resin layer30opposes the punch42is to ensure that the first external cladding20and the second external cladding21are heat-sealed by the respective thermoplastic resin layers30thereof as shown inFIG. 2. Note that when the punch42is disposed on the upper side and the die48is disposed on the lower side, the stainless steel foil32is placed on the die48.

At this time, the punch42is cooled and the blank holder44and die48are heated until the annular region32aof the stainless steel foil32is no higher than 20° C. and no lower than 0° C., and the exterior region32bof the stainless steel foil32is no lower than 40° C. and no higher than 100° C., preferably no lower than 60° C. and no higher than 100° C., and more preferably no lower than 60° C. and no higher than 80° C.

The reason for setting the annular region32aat no higher than 20° C. is that when the annular region32aexceeds 20° C., it becomes impossible to obtain a sufficient increase in fracture strength of a punch portion through martensitic transformation. Further, the reason for setting the annular region32aat no lower than 0° C. is that when the annular region falls below 0° C., frost adheres to the punch42and the annular region, and as a result, the shape characteristics of a molded article may be impaired. Moreover, the molded article may collapse due to temperature shrinkage when released from the mold.

The reason for setting the exterior region32bat no lower than 40° C. is that when the temperature of the exterior region32bfalls below 40° C., it becomes impossible to obtain a sufficient effect for suppressing hardening due to martensitic transformation. Further, the reason for setting the exterior region32bat no higher than 100° C. is that when the temperature of the exterior region exceeds 100° C., the thermoplastic resin layer30may melt. By preventing the thermoplastic resin layer30from melting, heat seal performance between the first external cladding20and the second external cladding21can be maintained. Furthermore, by setting the temperature of the exterior region between 40° C. and 100° C., the thermoplastic resin layer30can be softened without melting. By softening the thermoplastic resin layer30in this manner, the thermoplastic resin layer30is caused to exhibit a lubricating property.

After setting the respective temperatures of the annular region32aand the exterior region32bat the temperatures described above, the upper mold45is lowered further. As a result, the punch42is pressed into the inner side of the die48together with the stainless steel foil32such that drawing is implemented, whereby the second external cladding21including the projecting portion21a is manufactured. No lubricating oil is used through the entire drawing process.

FIG. 5is an illustrative view showing drawing performance achieved when the method for manufacturing external cladding for a laminate battery according to this embodiment is applied. The present applicant manufactured the external cladding21including the projecting portion21aset at φ40 using a circular mold4configured as shown inFIG. 4under various drawing ratio (diameter of material/diameter of finished article) conditions. A 60 μm polypropylene film (acid-modified polypropylene thickness 30 μm, melting point 120° C.+polypropylene homopolymer thickness 30 μm, melting point 160° C.) was used as the thermoplastic resin layer30, a 2 μm aqueous urethane resin film with 10% added wax was used as the lubricating film31, and austenitic stainless steel foil (SUS304) having a sheet thickness of 100 μm was used as the stainless steel foil32. Further, the diameter of the punch42was set at 40.0 mm, the punch shoulder portion R was set at 2.5 mm, a hole diameter of the die48was set at 40.4 mm, and the die shoulder R was set at 2.0 mm.

Under these conditions, drawing was performed with the temperature of the annular region32a(the punch42) set at 10° C. while varying the temperature of the exterior region32b(the blank holder44and the die48) between room temperature (25° C.) and 100° C. As shown inFIG. 5, at room temperature, molding defects occurred even when drawing was performed at a drawing ratio of 2.1. By setting the temperature of the exterior region32bwithin a range of 40° C. to 100° C., however, molding could be achieved successfully even when drawing was performed at a larger drawing ratio. It is evident from these results that when the temperature conditions according to this embodiment are applied, deep drawing can be realized without the use of lubricating oil.

FIG. 6is an illustrative view showing the drawing performance when lubricating oil is used. As a comparative example, the present applicant implemented drawing on austenitic stainless steel foil (SUS304) having a sheet thickness of 100 μm but not provided with the thermoplastic resin layer30and the lubricating film31while supplying lubricating oil, as in the prior art. As shown inFIG. 6, when lubricating oil was used, an upper limit drawing ratio at which molding could be achieved successfully was lower than when the method according to the inventive embodiment was applied. The assumed reason for this is that in the temperature range according to the inventive embodiment, the thermoplastic resin layer30and the lubricating film31having been softened by heating exhibits a lubricating property superior to lubricating oil. The superiority of the method of using the austenitic stainless steel foil32provided with the thermoplastic resin layer30and the lubricating film31as a material and implementing warm working thereon is evident from these results.

FIG. 7is an illustrative view showing the drawing performance achieved when the method for manufacturing external cladding for a laminate battery according to the inventive embodiment is applied to Ni-free austenitic stainless steel foil. The present applicant investigated the drawing performance in a case where Ni-free austenitic stainless steel foil (16 Cr−2.5 Ni−3 Mn−3 Cu) having a sheet thickness of 100 μm was used as the stainless steel foil32. As shown inFIG. 7, at room temperature, molding defects occurred even when drawing was performed at a drawing ratio of 2.1, but by setting the temperature of the exterior region32bwithin the range of 40° C. to 100° C., molding could be achieved successfully even when drawing was performed at a larger drawing ratio. It is evident from these results that when the temperature conditions according to the inventive embodiment are applied, deep drawing can be realized without the use of lubricating oil even Ni-free austenitic stainless steel foil. Note that apart from the material of the stainless steel foil32, processing conditions of the example shown inFIG. 7were identical to the processing conditions of the example shown inFIG. 5.

FIG. 8is an illustrative view showing the drawing performance when lubricating oil is used on Ni-free austenitic stainless steel foil. Similarly to the comparative example ofFIG. 6, drawing was implemented on Ni-free austenitic stainless steel foil having a sheet thickness of 100 μm, but not provided with the thermoplastic resin layer30and the lubricating film31while supplying lubricating oil, as in the prior art. As shown inFIG. 8, when lubricating oil was used, the upper limit drawing ratio at which molding could be achieved successfully was lower than when the method according to the inventive embodiment was applied. The assumed reason for this is that in the temperature range according to the inventive embodiment, the thermoplastic resin layer30and the lubricating film31, having been softened by heating, exhibit a superior lubricating property to lubricating oil. It is evident from these results that the method of implementing warm working is also superior when Ni-free austenitic stainless steel foil is used as the material.

According to the inventive method for manufacturing external cladding for laminate battery, by employing the austenitic stainless steel foil32having the thermoplastic resin layer30on one of the front surface and the rear surface and the lubricating film31on the other surface as a material and performing drawing on the material under appropriate temperature conditions without the use of lubricating oil, an increase in battery capacity can be accommodated and the possibility of infiltration of contamination by impurities in the interior of the laminate battery can be reduced. Further, the external cladding21can be manufactured such that the projecting portion21athereof is provided at a sufficient depth, and therefore sufficient space can be secured in the housing space22even when the external cladding21is adhered to the flat plate-shaped first external cladding20. Distortion occurs in the external cladding during drawing, and therefore, when an attempt is made to adhere external claddings respectively having projection portions to each other, the distortion may cause an adhesion defect. However, by providing one external cladding20in a flat plate shape, as in this embodiment, the possibility of adhesion defects can be reduced.

Note that in the inventive embodiment, temperature of the annular region32aof the stainless steel foil32is set at no higher than 20° C. and no lower than 0° C. and the exterior region32bof the stainless steel foil32is set at no lower than 40° C. and no higher than 100° C. by cooling the punch42and heating the blank holder44and die48, but the method of setting the respective temperatures of the annular region and the exterior region at predetermined temperatures is not limited thereto, and a method such as heating the entire stainless steel foil serving as the material and then pressing a separate cooling body to the punch against the annular region, for example, may be employed instead.