Prismatic sealed battery and method of manufacturing the same

A metallic outer case has an opening portion. A first corner portion is provided at an outer periphery of the opening portion. A power generating element is stored within the outer case. The power generating element has a positive electrode and a negative electrode opposed to each other, with a separator interposed. An end face of the opening portion is sealed by a metallic cover member. The cover member has a second corner portion corresponding to the first corner portion of the outer case, and welded to the outer case, with a coupled portion between the cover member and the outer case irradiated with a laser beam. Those portions of the first corner portion of the outer case and the second corner portion of the cover member, which are irradiated with the laser beam, are shaped as angular portions of substantially the same shape.

BACKGROUND OF THE INVENTION
 The present invention relates to a prismatic sealed battery obtained by
 coupling a cover member to an opening portion of a prismatic outer case
 and welding the cover member and the opening portion, and to a method of
 manufacturing the prismatic sealed battery.
 Recently, because of an increasing demand of portable OA devices and
 communication devices, there is a great demand for prismatic sealed
 batteries as power supplies. In particular, nickel metal hydride
 rechargeable batteries and lithium ion rechargeable batteries, the
 prismatic sealed batteries are small in size and can be mounted in the
 above devices with high volume efficiency.
 In general, the prismatic sealed battery has an outer case 1A provided with
 an opening portion 3A, as shown in FIG. 1. A cover member 2A is coupled to
 the opening portion 3A of outer case 1A, and a coupling portion between
 these members is hermetically welded by a laser beam L.
 When the laser beam L is radiated on the lateral side of the outer case 1A
 and scanned along the coupling portion between the outer case 1A and cover
 member 2A, the scan direction of laser beam L is controlled in the X- and
 Y-directions along the outer peripheral surface of outer case 1A in FIG.
 1.
 In a case where the laser beam L is incident on the lateral side of outer
 case 1A and scanned in the X- and Y-directions for welding, however, it is
 difficult to treat a corner portion R of outer case 1A and a corner
 portion r of cover member 2A. Specifically, if the laser beam L is scanned
 linearly in the X- and Y-directions, the laser beam L is not incident
 perpendicular to the outer periphery of the outer case 1A and cover member
 2A at the corner portions R and r of outer case 1A and cover member 2A.
 For example, when the radius of curvature of corner portion R, r is set at
 about 2.0 mm, an angle .theta. between the direction of radiation of laser
 beam L and a plane normal to a tangential line (including a surface to be
 welded) of the corner portion, R, r is 45.degree. at maximum. At this
 time, the area of radiation of laser beam L increases 1. 41 times (=1/cos
 45.degree.).
 Consequently, the radiation energy density (fluence: J/cm.sup.2) of laser
 beam L decreases by about 40%, compared to the case where beam L is
 incident perpendicular to the surface to be welded. In general, the
 allowance in a laser welding process with respect to a variation in the
 fluence is .+-.10%. Under the above condition, the depth of weld
 penetration also decreases and the strength of bonding deteriorates. As a
 result, cracks may occur at the welded portion.
 In particular, in a case where the outer case 1A and cover member 2A is
 formed of aluminum or aluminum alloy in order to reduce weight, cracks
 will easily occur and the yield of manufactured products will considerably
 deteriorate. In order to achieve good welding, the condition, cos
 .theta.&gt;0.9, needs to be satisfied. Therefore, it is desirable to meet the
 condition, .theta.&lt;25.degree..
 In a case where the outer case 1A and cover member 2A are formed of steel
 or stainless steel, the radius of curvature of the entire outer case
 including corner portions R, r is set to a small value, e.g. about 1.0 to
 1.3 mm. Thereby, a deviation in angle of radiation of the laser beam L on
 the outer periphery of the corner portion R, r is decreased, and the
 strength of bonding is prevented from deteriorating.
 In the case of using aluminum or aluminum alloy, however, if the radius of
 curvature of the corner portion R, r is decreased, the stress will
 concentrate at the corner portion R, r when the pressure in side the outer
 case 1A increases, and the rigidity thereof decreases. Since the strength
 of the aluminum or aluminum alloy itself is low, the radius of curvature
 of the corner portion R, r needs to be increased. Because of this, a
 deviation in angle of radiation of the laser beam L increases and the
 depth of welding penetration at the welded portion decreases. As a result,
 the welding strength decreases.
 If the scan direction of the laser beam L is turned in accordance with the
 curvature of the corner portion R, r, such problem will not arise.
 However, if the scan direction of the laser beam L is turned along the
 corner portion R, r, the control for scanning the laser beam L becomes
 intricate and the cost of the scan device increases. Furthermore, a
 process time increases. Therefore, this measure is not practical.
 On the other hand, the cross section of the outer case 1A of the prismatic
 sealed battery, which is parallel to the opening portion 3A, is formed
 prismatic (including a curve of the corner portion R), and the wall
 thickness (i.e. thickness of material forming the outer case) of a
 long-side portion 4A is equal to that of a short-side portion 5A.
 If the wall thickness is not sufficient, the rigidity of the outer case 1A
 decreases. Thus, if a gas is produced by a chemical reaction of an
 electrolyte while a power generating element in the secondary battery is
 being charged, the internal pressure of the outer case 1 increases and, as
 indicated by solid-line arrows in FIG. 3, the inner surface of the
 long-side portion 4A of outer case 1A is pressurized in the direction of
 expansion and deformed.
 Consequently, a bending moment is applied at the corner portions R, as
 indicated by broken-like arrows, and the short-side portions 5A are
 deformed inward by the bending moment and the entire outer case 1A is
 deformed.
 BRIEF SUMMARY OF THE INVENTION
 An object of the present invention is to provide a prismatic sealed battery
 and a method of manufacturing the same, wherein when an outer case and a
 cover member are welded, a decrease in depth of weld penetration at corner
 portions due to a large variation in radiation angle of a laser beam is
 prevented.
 Another object of the invention is to provide a prismatic sealed battery
 and a method of manufacturing the same, wherein an outer case is not
 easily deformed due to a rise in internal pressure.
 According to the present invention, there is prismatic sealed battery
 comprising:
 a metallic outer case having an opening portion and a first corner portion
 at an outer periphery of the opening portion;
 a power generating element stored within the outer case and having a
 positive electrode and a negative electrode opposed to each other, with a
 separator interposed;
 a metallic cover member coupled to an end face of the opening portion of
 the outer case, provided with a second corner portion corresponding to the
 first corner portion of the outer case, and welded to the outer case, with
 a coupled portion between the cover member and the outer case irradiated
 with a laser beam; and
 an electrode element electrically connected to the power generating
 element.
 Additional objects and advantages of the invention will be set forth in the
 description which follows, and in part will be obvious from the
 description, or may be learned by practice of the invention. The objects
 and advantages of the invention may be realized and obtained by means of
 the instrumentalities and combinations particularly pointed out
 hereinafter.

DETAILED DESCRIPTION OF THE INVENTION
 Embodiments of the present invention will now be described with reference
 to the accompanying drawings.
 FIGS. 4 to 11 show a first embodiment of the invention. A prismatic sealed
 battery according to the first embodiment has a structure as shown in FIG.
 1. Specifically, the prismatic sealed battery has a metallic outer case 1
 having a bottomed prismatic shape with an upper end face opened. The outer
 case 1 serves also as a positive electrode terminal, and an insulating
 film 6 is disposed on an inner face of a bottom portion of the case 1.
 An electrode member 7 serving as a power generating element is housed
 within the outer case 1. In the case of a lithium ion secondary battery,
 the electrode member 7 is formed in the following manner. A negative
 electrode 8 of aluminum thin film, on both sides of which an active
 substance containing a carbonaceous material is coated, a separator of
 porous polypropylene sheet, and a positive electrode 10 of copper thin
 film, on both sides of which an active substance containing lithium nickel
 oxide or lithium cobalt oxide is coated, are spirally wound with an oval
 cross section such that the positive electrode 10 is situated on the
 outermost side.
 The outer case 1 and electrode member 7 are electrically connected via a
 cover member (described below). Specifically, a positive electrode lead 11
 is extended from the positive electrode 10, and the positive electrode
 lead 11 is connected to that face of the cover member 2, which is opposed
 to the electrode member 7. A through-hole (not shown) for passage of a
 negative electrode terminal 12 is formed in a central portion of the cover
 member 2. An injection port 14 of electrolyte is provided at a
 predetermined distance from the through-hole. A spacer 13 of synthetic
 resin, which serves as an electric insulator is provided between the cover
 member 2 and an upper end of the electrode member 7 within the outer case
 1.
 The cover member 2 is coupled to an opening portion 3 at the upper end of
 the outer case 1 by means of welding or the like. The negative electrode
 terminal 12 projecting from the central area of the cover member 2 is
 hermetically sealed in the through-hole by means of a glass or resin
 insulator 15. A negative electrode lead 16 is connected at one end to a
 lower end face of the negative electrode terminal 12, and the other end of
 the negative electrode lead 16 is connected to the negative electrode 8.
 After the electrolyte is filled in the outer case 1, the injection port 14
 is sealed by a seal cover 14A of a metallic plate which is hermetically
 welded on the cover member 2. Alternatively, after the electrolyte is
 filled in the outer case 1, the upper opening of the outer case 1 may be
 sealed by the cover member 2, without forming the injection port 14 in the
 cover member 2.
 In the case of the lithium ion secondary battery, the electrolyte is an
 organic solvent consisting of ethylene carbonate or propylene carbonate,
 which contains an electrolyte substance such as lithium perchlorate,
 lithium boro-fluoride, lithium hexafluoride or phosphor lithium
 hexafluoride.
 The entire outer surface of the cover member including the seal cover 14A
 is covered with an upper insulation paper sheet 17. A lower insulation
 paper sheet 19 having a slit 18 is provided on an inner bottom face of the
 outer case 1. One side portion of a two-fold PTC (Positive Thermal
 Coefficient) element 20 is interposed between the bottom face of the outer
 case 1 and the lower insulation paper sheet 19, and the other side portion
 of the PTC element 20 is extended to the outside through the slit 18.
 An outer tube 21 is provided to extend from the side faces of outer case 1
 to peripheral portions of the upper and lower insulation paper sheets 17
 and 19. The upper and lower insulation paper sheets 17 and 19 are fixed to
 the outer case 1. With this structure of the outer tube 21, the
 above-mentioned outward-extending portion of the PTC element 20 is bent
 toward the bottom surface of lower insulation paper 19.
 The cover member 2 is hermetically welded to the opening portion 3 of
 prismatic outer case 1 by means of laser welding. The method of fixing the
 cover member 2 to the prismatic outer case 1 by laser welding has been put
 into practice since the opening portion 3 of prismatic outer case 1 can be
 closed, minimizing the loss of the volume efficiency.
 A cross section of the outer case 1, which is parallel to the opening
 portion 3 that is to be coupled to the cover member 2 after the power
 generating element has been inserted, is prismatic. The wall thickness of
 a long-side portion 4 of the case 1 is set at 0.5 mm, and that of a
 short-side portion 5 is set at 0.7 mm. The outer case 1 is formed of an
 aluminum-based metal containing 0.05 wt % or less of Mg and 0.2 wt % or
 less of Cu.
 Thereby, in case solidification by cooling occurs after laser welding,
 formation of cracks at the coagulation point is suppressed. For example,
 materials of AA standards 3003, 3004, 1050, 1100 and 1200 are preferable.
 These materials are pressed by deep drawing or cold shock process in the
 shape of outer case 1.
 As is shown in FIG. 6, the cover member 2 has a prismatic thin-plate shape
 and includes a stepped portion 2a of 0.3 mm at its periphery. The stepped
 portion 2a is coupled to the upper end face of the opening portion 3 of
 outer case 1.
 The outer peripheral face of the stepped portion 2a is continuous with the
 outer peripheral face of outer case 1 with no stepped portion. It is
 desirable that the cover member 2 have a thickness of 0.8 mm or more, and
 preferably 0.9 to 1.5 mm. If the thickness is less than 0.8 mm, the
 strength decreases.
 If the cover member 2 is coupled to the opening portion 3 of outer case 1,
 a laser beam L.sub.1 from a YAG laser is incident onto the coupled portion
 (i.e. faces to be welded) between the outer case 1 and cover member 2, and
 the laser beam L.sub.1 is scanned in the direction of an arrow in FIG. 5.
 A convergence diameter (laser spot diameter) of the laser beam L.sub.1 is
 0.4 to 0.5 mm.
 In this manner, the coupled portion is seam-welded so that a welded portion
 with a diameter of about 0.8 mm is continuously formed. The outer case 1
 is thus sealed by the cover member 2. The conditions for the YAG laser
 (wavelength: 1.06 .mu.m) are as follows: repetition rate=20 to 30 Hz;
 pulse width=3 to 5 ms; movement (scan) speed=5 to 10 mm/s; and overlap
 ratio=70 to 80%. The coupled portion is welded while nitrogen gas is being
 applied thereto, thereby preventing formation of porosities due to
 oxidation of structural members of outer case 1 during welding.
 The YAG laser is used to emit the laser beam L.sub.1 because the YAG laser
 has a shorter wavelength than a carbon dioxide laser and has a lower
 reflectance on aluminum or an alloy thereof than the carbon dioxide laser.
 Accordingly, the YAG laser can achieve more efficient welding. The laser
 beam L.sub.1 is emitted from the laser oscillator (not shown), passing
 through an optical fiber and a lens (both not shown) and converged onto
 the coupled portion. The laser beam L.sub.1 is used for welding, with the
 optical axis of the laser beam L.sub.1 being set in a range of 20.degree.
 with respect to a line normal to the coupled portion. The laser beam is
 scanned while the lens is situated to keep a predetermined angle and
 distance with respect to the coupled portions of the long-side portion 4
 and short-side portion 5 of outer case 1.
 The inventor has conducted experiments in order to set the shape of the
 outer case 1 which is not easily deformed due to internal pressure and
 does not incur an increase in weight. The inventor has obtained results as
 illustrated in FIGS. 7 and 8. Table 1 (below) shows dimensions of the
 outer case 1 which were used as reference dimensions in the experiments.
 In Table 1, "width" refers to a length of the long-side portion 4,
 "thickness" to a length of the short-side portion, and "height" to a
 length between the bottom of outer case 1 and the coupled portion of the
 cover member 2.
 FIG. 7 shows a relationship between the corner portion R shown in FIG. 5
 and the expansion of long-side portion 4 of outer case 1, which was
 obtained by using the outer case 1 with the above dimensions. The internal
 pressure in the outer case 1 was set at 0.1913 MPa (atmospheric pressure
 +0.09 MPa). The degree of expansion is defined as a maximum outward
 displacement of the long-side portion 4, as shown in FIG. 7, with the
 shape of outer case 1 at the time of atmospheric pressure of 0.1013 MPa
 being adopted as a reference shape. It is understood, from FIG. 7, that as
 the radius of curvature of corner portion R increases, the degree of
 expansion decreases and deformation of outer case 1 can be effectively
 suppressed. At the same time, the surface area of outer case 1 decreases
 and accordingly the weight decreases.
 FIG. 8 shows a relationship between the wall thickness of the short-side
 portion 5 and the expansion of long-side portion 4 of outer case 1, which
 was obtained by using the outer case 1 with the above dimensions. In this
 case, too, the internal pressure in the outer case 1 was set at 0.1913 MPa
 (atmospheric pressure +0.09 MPa). The degree of expansion is defined as a
 maximum outward displacement of the long-side portion 4, as shown in FIG.
 8, with the shape of outer case 1 at the time of atmospheric pressure of
 TABLE 1
 Standard Container Dimensions (unit: mm)
 Thickness of Thickness of Radius of
 Thick- short-side long-side curvature of
 Height Width ness portion portion corner portion
 45 29.6 6.0 0.5 0.5 0.5
 TABLE 2
 Examples of battery outer case dimensions (unit: mm)
 Thickness of Thickness of Radius of
 short-side long-side curvature of
 portion portion corner portion
 0.7 0.5 2.0
 0.6 0.5 2.0
 0.6 0.4 2.0
 0.6 0.4 2.0
 In the present embodiment, the radius of curvature of the corner portion R
 of opening portion 3 is increased for the purpose of suppressing the
 expansion of outer case 1. Then, as described in the "BACKGROUND OF THE
 INVENTION", the angle of incidence of the laser beam L.sub.1 on the welded
 portion may increase. As a result, the welded portion between the cover
 member 2 and opening portion 3 may become defective.
 Accordingly, it is considered that if the radius of curvature of the corner
 portion R of outer case 1 is increased to enhance the strength of the case
 1 and the outer periphery of the opening portion 3 in the height direction
 of outer case 1 is shaped as an angular portion Q, defective welding
 between the cover member 2 and case 1 may be prevented. Needless to say,
 the corner portion r of cover member 2 is also shaped as an angular
 portion q, similarly with the corner portion R of outer case 1.
 In this case, the angular portion Q, q must 0.1013 MPa being adopted as a
 reference shape.
 It is understood, from FIG. 8, that if the wall thickness of the short-side
 portion 5 is increased, for example, from 0.5 mm to 0.7 mm, the degree of
 expansion of long-side portion 4 decreases by about 20%. In this case, as
 regards the dimensions shown in Table 1, the weight of entire outer case 1
 was increased only by 0.1 g, i.e. from 4.5 g to 4.6 g. This contributes to
 reducing the weight of the battery including a power generating element.
 In actual products, about 8% of the length of the short-side portion 5
 (thickness of outer case 1) is allowed as expansion of the long-side
 portion 4 (width of outer case 1). Thus, as the thickness of outer case 1
 increases, the wall thickness of short-side portion 5 may be smaller. The
 dimensions in Table 2 (below), in which the wall thickness of short-side
 portion 5 is greater than that of the long-side portion 4, may be regarded
 as an example of proper dimensions.
 TABLE 2
 Examples of battery outer case dimensions (unit: mm)
 Height Width Thickness
 44.5 29.6 6.0
 44.5 28.6 8.1
 46.5 21.6 5.3
 46.5 21.6 4.8
 include a portion with a smaller radius of curvature than the corner
 portion R, r. It contributes to decrease the incident angle of the laser
 beam on the welded portion.
 As is shown in FIG. 11, the angular portion Q, q is formed such that two
 flat portions 17a and 17b are continuously formed on the outer periphery
 of the corner portion R, r at an angle .theta. with respect to the
 long-side portion and short-side portion.
 FIG. 9 is a cross-sectional view of the opening portion 3 of outer case 1,
 taken along line IX--IX in FIG. 5, and the outer periphery of the opening
 portion 3 is provided with angular portions Q. When the wall thickness of
 long-side portion 4 is a and the wall thickness of short-side portion 5 is
 b, the condition, a&lt;b, is satisfied in order to reduce the expansion due
 to internal pressure of outer case 1, as described above. A hatched area
 indicates a cross section of the opening portion 3 with angular portions
 Q. An area defined by broken lines indicates a cross section of the
 portion of outer case 1, other than the opening portion 3, which has the
 corner portions R with a radius of curvature. FIG. 10 shows the cross
 sectional view of the portion of outer case 1, other than the opening
 portion 3.
 The angular portion Q was formed such that the angle .theta. between the
 long-side portion 4 or short-side portion 5 and the angular portion Q
 shown in FIG. 9 is 15.degree.. From geometrical calculations, the angle
 between the long-side portion 4 or short-side portion 5 and the flat
 portion 17a, 17b of angular portion Q is defined as .theta., as shown in
 FIG. 11. The angle .theta. is equal to an angle between a plane 18
 (indicated by a broken line) normal to the flat portion 17a, 17b and the
 laser beam L incident on the flat portion 17a, 17b.
 Accordingly, the welding can be performed in the state in which the angle
 .theta. shown in the enlarged view of FIG. 11 meets the condition
 described in the "BACKGROUND OF THE INVENTION", .theta.&lt;25.degree., i.e.
 cos .theta.&gt;0.9.
 When the angular portion Q, q was processed under the above-mentioned
 condition of the wall thickness and with the angle .theta. of 15.degree.
 and 150 pairs of outer cases 1 and cover members 2 were welded, no cracks
 were produced in the welded members. By contrast, when the outer cases 1
 and cover members 2 were welded without angular portions Q, q, cracks were
 produced in all welded members.
 The angular portion Q, q is formed by a pressing process such as deep
 drawing. Specifically, after the outer case 1 and cover member 2 are
 formed, the corner portion R, r is pressed by a mold, and then the angular
 portion Q, q is formed. The method of forming the angular portion Q, q is
 not limited to the pressing process, and the angular portion Q, q may be
 formed by chamfering.
 In this case, to keep constant the condition for welding the cover member 2
 to opening portion 3 by means of the laser beam L, the relationship in
 thickness between the long-side portion 4 and short-side portion 5 of the
 opening portion 3 may be set to a=b, while it should be a&lt;b in the portion
 of the outer case 1.
 The ratio of the opening portion 3 to the outer case 1 in the height
 direction is small. Thus, with respect to the opening portion 3, even if
 the long-side portion 4 and short-side portion 5 are pressed to have the
 same wall thickness, the pressure-resistance properties of the outer case
 1 are not substantially influenced.
 When the outer case 1 and cover member 2 are formed of aluminum alloys, the
 materials are chosen such that the average Mg (magnesium) content of the
 aluminum alloys for the outer case 1 and than for the cover member is 1.0
 wt % or less on average and the Mg content in the outer case 1 is greater
 than that in the cover member 2.
 If the Mg content in the aluminum alloy is increased, the tensile strength
 increases. For example, when the Mg content is 2.8 wt %, the tensile
 strength is 285 to 290 MPa.
 On the other hand, the boiling point of Mg is about 1100.degree. C., which
 is for lower than that of aluminum, 2500.degree. C. Consequently, during
 welding, Mg is selectively vaporized, resulting in a splash and porosities
 in the welded portion. Thus, the quality of welding is degraded.
 By specifying the quantity of Mg in the outer case 1 and cover member 2, as
 described above, the quality of welding can be enhanced. In addition,
 since the Mg content in the outer case 1 is made greater than that in the
 cover member 2, the outer case 1 can have a great strength.
 The results of experiments will now be described.
 In Experiment 1, the outer case 1 was formed of an aluminum alloy
 comprising 1.0 to 1.5 wt % of Mn, 0.6 wt % or less of Si, 0.7 wt % or less
 of Fe, 0.25% or less of Cu, and 0.8 to 1.3 wt % of Mg. The cover member 2
 was formed of an aluminum alloy comprising 1.0 to 1.5 wt % of Mn, 0.6 wt %
 or less of Si, 0.7 wt % or less of Fe, 0.25% or less of Cu, and 0.05 or
 less of wt % of Mg.
 The coupled portion between the outer case 1 and cover member 2 was welded
 by the laser beam L. The Mg concentration in the welded portion was about
 0.4 to 0.65 wt %, and welding was carried out without cracks.
 In Experiment 2, the outer case 1 was formed of an aluminum alloy
 comprising 1.0 to 1.5 wt % of Mn, 0.3 wt % or less of Si, 0.7 wt % or less
 of Fe, 0.25% or less of Cu, and 0.8 to 1.3 wt % of Mg. The cover member 2
 was formed of an aluminum alloy comprising 0.05 wt % or less of Mn, 0.25
 wt % or less of Si, 0.4 wt % or less of Fe, 0.05% or less of Cu, and 0.05
 or less of wt % of Mg.
 The coupled portion between the outer case 1 and cover member 2 was welded
 by the laser beam L. The Mg concentration in the welded portion was about
 0.4 to 0.65 wt %, and in this case, too, welding was carried out without
 cracks.
 In Experiment 3, the outer case 1 was formed of an aluminum alloy
 comprising 0.8 to 1.4 wt % of Mn, 0.6 wt % or less of Si, 0.8 wt % or less
 of Fe, 0.25% or less of Cu, and 0.8 to 1.3 wt % of Mg. The cover member 2
 was formed of an aluminum alloy comprising 1.0 to 1.5 wt % of Mn, 0.6 wt %
 or less of Si, 0.7 wt % or less of Fe, 0.25% or less of Cu, and 0.05 or
 less of wt % of Mg.
 The coupled portion between the outer case 1 and cover member 2 was welded
 by laser beam L. The Mg concentration in the welded portion was about 0.4
 to 0.65 wt %, and welding was carried out without cracks.
 In order to increase the strength of the outer case 1, the Mg content in
 the aluminum alloy of the outer case 1 may be made greater than that in
 the above-described experiments. It was confirmed that when the Mg content
 was 1.9 wt % or less, the strength of the outer case 1 can be increased
 with no welding defects.
 FIGS. 12 and 13 show a second embodiment of the invention. In the second
 embodiment, the radius of curvature of the inner periphery of the corner
 portion R of the opening portion 3 of the outer case 1 is made less than
 the radius of curvature of the other part of the outer case 1.
 Specifically, as shown in FIG. 13, a shape indicated by broken lines is
 the shape of the inner periphery of that portion of outer case 1, which is
 other than the opening portion 3. A shape indicated by solid lines is the
 shape of the inner periphery of the opening portion 3.
 The outer periphery of the corner portion R and the corner portion r of the
 cover member 2 corresponding to the corner portion R are provided with
 angular portions Q and q, like the first embodiment.
 FIG. 13 shows a cross section of the opening portion 3 of outer case 1. The
 angle .theta. between the long-side portion 4 and short-side portion 5 of
 the angular portion of the opening portion 3 meets the above-described
 condition, .theta.&lt;25.degree.. Accordingly, if the condition, cos
 .theta.&gt;0.9, is satisfied, good welding can be performed.
 The angular portion Q is formed by a pressing process such as deep drawing.
 Specifically, after the outer case 1 is formed, the angular portion Q is
 formed by an extrusion pressing process. In this embodiment, too, the
 relationship between the thickness a of long-side portion 4 and the
 thickness b of short-side portion 5 is set at a&lt;b. This relationship may
 be set at a=b.
 In this embodiment, a pulsed laser beam is used for welding. However, even
 if a laser beam of continuous wave (CW) is used, the same effect can be
 obtained if the radiation energy is set at a value suitable for welding.
 In each of the above embodiments, the lithium ion rechargeable battery is
 applied to the outer case 1 for the prismatic sealed battery, by way of
 example. However, other types of rechargeable batteries, such as nickel
 metal hydride rechargeable batteries, may be used. Furthermore, in view of
 the advantage of the present invention, i.e. the increased strength, the
 prismatic sealed battery of this invention can be applied to the primary
 battery.
 A third embodiment of the present invention will now be described with
 reference to FIGS. 14 to 18. The prismatic sealed battery, as shown in
 FIG. 4, has a sealed container structure of an outer case 41 and a cover
 member 42. The cover member 42 is provided with a negative electrode 50
 and a seal cover 51 for sealing an injection port for electrolyte.
 FIG. 14 is a cross-sectional view illustrating the main feature of this
 embodiment, and shows a welded portion of the prismatic sealed battery.
 The wall thickness of the outer case 1 is 0.6 mm and the cover member 42
 with a wall thickness of 1 mm is engaged with the opening portion of the
 outer case 41. The cover member 42 has a prismatic thin-plate shape and
 its peripheral portion is provided with a stepped portion of 0.3 mm. At
 the engagement portion between the cover member 42 and outer case 41, the
 end face in the thickness direction of the outer stepped portion is formed
 flush with the outer face of the outer case 41.
 In this state, a coupled portion 45 between the outer case 41 and cover
 member 42 is seam-welded so that a welded portion 44 with a weld diameter
 (d) of about 0.8 mm is continuously formed by radiation of a YAG laser
 beam L.sub.2 having a convergence diameter (D) of 0.45 mm. The outer case
 41 is thus sealed by the cover member 42. In this case, the conditions for
 the YAG laser L.sub.2 are as follows: repetition rate=20 to 30 Hz; pulse
 width=3 to 5 ms; and movement speed=5 to 10 mm/s. Nitrogen gas is applied
 to the welded portion, thereby preventing formation of porosities due to
 oxidation during welding.
 Specifically, in order to prevent a crack from being produced during
 welding, the thickness (t) as measured from the outside face of the cover
 member 42 is set at 0.3 mm, and the condition, t&lt;d/2, is met. Thus, a
 welded portion 44 reaching the edge portion is formed even with low
 welding energy, and a prismatic sealed battery having the structure
 wherein cracks are not easily produced in the welded portion can be
 obtained.
 As is shown in FIG. 17, cracks are produced in the welded portion due to
 concentration of stress at a welded boundary portion, which results from a
 volumetric shrinkage occurring when the welded portion 44 coagulates. In
 the normal weld state, as shown in FIG. 17, the welded portion 44 does not
 reach outer edges on both sides. At the time of coagulation, since right
 and left sides (in the Figure) of the welded portion 44 are restricted, a
 stress is caused in directions of arrows A.sub.1 and A.sub.2, resulting in
 severe cracks.
 On the other hand, if one side of the welded portion 44 reaches the outer
 edge of the plate, as shown in FIG. 16, a stress acts in the direction of
 arrow A.sub.1 at the weld boundary portion. At the time of coagulation,
 one side of the welded portion 44 is not restricted and made free.
 Compared to the case where both sides are restricted, the stress caused at
 the weld boundary portion is low, suppressing the cracks produced.
 In the above embodiment, as shown in FIGS. 14 and 15, the radius of welded
 portion (d/2) is made greater than the thickness t of welded portion as
 measured from the outside edge of cover member 42 or the wall thickness t
 of outer case 41. Thereby, the welded portion is made to reach the outer
 edge of the plate, and one side of the welded portion 44 is made free.
 Thus, the stress caused at the weld boundary portion during coagulation is
 reduced.
 When the scan center of the laser beam L.sub.2 is made to agree with the
 coupled portion 45 between the cover member 42 and outer case 41, whole of
 the laser beam L.sub.2 is incident on the cover member 42 and outer case
 41. For this purpose, it is desirable in this embodiment that the spot
 diameter (D) of YAG laser beam L.sub.2 is set at D/2&lt;t.
 Accordingly, a proper range of weld energy is increased and a stable
 process can be performed even if the laser radiation energy varies. The
 YAG laser is used as an oscillation source of the laser beam because the
 YAG laser has a shorter wavelength than a carbon dioxide laser and has a
 lower reflectance on aluminum or an alloy thereof than the carbon dioxide
 gas laser. Accordingly, the YAG laser can achieve the welding with higher
 efficiency.
 Table 3 (below) shows measures results on the relationship among the
 thickness of the weld portion, as measured from the outer edge of cover
 member 42, the corresponding proper range of weld energy, and the crack
 defect ratio at the laser weld portion.
 TABLE 3
 Crack defect ratio
 Thickness t of weld portion
 measured from outer
 Energy (J) edge of cover member 2
 (present invention) 0.3 mm 0.4 mm 0.5 mm
 4.4 18% 62% 100%
 4.8 0% 2% 18%
 5.3 0% 0% 0%
 5.8 0% 0% 0%
 When the thickness (t) of the weld portion, as measured from the outer edge
 of cover member 42, is 0.5 mm, the range of weld energy, which produces no
 crack, is 5.3 to 5.8J (.+-.15%). The pulse energy exceeding 5.8J is
 improper since it causes thermal damage to the electrode element. On the
 other hand, when the thickness (t) of the weld portion, as measured from
 the outer edge of cover member 42, is 0.3 mm, the range of weld energy,
 which produces no crack, is 4.8 to 5.8J (.+-.10%). Even if the radiation
 energy slightly varied, highly stable weld was made. Since the weld
 diameter of the weld portion is 0.8 mm and the convergence diameter of
 laser beam L.sub.2 is 0.45 mm, when the thickness (t) of the weld portion,
 as measured from the outer edge of cover member 42, is 0.3 mm, the
 relationship
EQU D/2&lt;t&lt;d/2
 is satisfied and the weld portion 44 reaches the outer edge of the cover
 member 42.
 Since the relationship, d/2&lt;t, is present when the thickness of the weld
 portion, as measured from the outer edge of cover member, is 0.5 mm, the
 weld portion 44 does not reach the outer edge of cover member 42,
 resulting in severe cracks.
 The direction of the coupled face is changed in various directions in
 accordance with the relationship of coupling between the outer case 1 and
 cover member 42. FIG. 15 shows a modification of the third embodiment. In
 the embodiment shown in FIG. 14, the coupled face between the outer case
 41 and cover member 42 is perpendicular to the direction of the opening of
 the outer case 41. In the modification shown in FIG. 15, the coupled face
 is parallel to the direction of the opening. The structures in FIGS. 14
 and 15 are different only with respect to the direction of the coupled
 face, and the elements denoted by reference numerals are the same or have
 the same functions. Therefore, the same advantage can be obtained with the
 structures shown in FIGS. 14 and 15.
 In this embodiment, a pulsed laser beam is used as laser beam L.sub.2.
 However, even if a laser beam of continuous wave (CW) is used, the same
 effect can be obtained if the radiation energy is set at a value suitable
 for welding.
 In the above embodiment, the invention is applied to the outer case 41 for
 the prismatic sealed battery such as the lithium ion batteries. However,
 the invention is also applicable to laser welding of outer cases of
 cylindrical batteries or components of aluminum alloy material of general
 household electric devices or vehicles. Needless to say, the invention can
 be modified variously without departing from the spirit of the invention.
 FIGS. 19 to 25 show a fourth embodiment of the present invention.
 FIG. 19 shows a laser weld apparatus 61 for working the present invention.
 The laser weld apparatus 61 comprises a laser oscillator 62 such as a YAG
 laser for oscillation-outputting a pulsed laser beam L.sub.3. A controller
 63 is connected to the laser oscillator 62. The controller 63 controls a
 pulse waveform and an output peak value P of the pulse laser beam L.sub.3
 oscillation-outputted from the controller 63.
 The pulsed laser beam L.sub.3 oscillation-outputted from the laser
 oscillator 62 is input to an optical fiber 64. The pulsed laser beam
 L.sub.3 emitted from the optical fiber 64 is made incident on a
 convergence lens 65 and is converged through the lens 65. The converged
 beam L.sub.3 is radiated on, and welds, a prismatic sealed battery 71
 shown in FIG. 22 or a member to be welded. In this case, the laser beam
 L.sub.3 is moved at a feed speed of 10 mm/s by a scan mechanism (not
 shown).
 A shield gas is supplied from a nozzle (not shown) to the welded portion of
 the to-be-welded member on which is irradiated with the pulsed laser beam
 L.sub.3. The shield gas is an inert gas such as nitrogen, argon or helium.
 Thereby, formation of porosities due to oxygen is prevented at the welded
 portion of the to-be-welded member.
 The prismatic sealed battery 71 comprises an outer case 72 and a cover
 member 73, as shown in FIG. 22. An injection port 74 formed in the cover
 member 73 is sealed by a seal cover 75. These members 72, 73 and 75 are
 formed of aluminum or aluminum alloys. The aluminum alloy is one
 containing manganese or magnesium, as specified AA standards 3003 and
 5052.
 The outer case 72 and cover member 73 are abutted on each other and welded
 by the pulsed laser beam L, as shown in FIG. 23. The cover member 73 and
 seal member 75 are placed on each other and welded, as shown in FIGS. 24A
 and 24B. In this example, the wall thickness of the outer case 72 is set
 at 0.3 to 0.5 mm, and the wall thickness at the abutted portion of the
 cover member 73 is also set at 0.3 to 0.5 mm. The wall thickness of the
 cover member 73 itself is set at 1.0 mm, and that of the seal cover 75 is
 set at 0.2 mm.
 When the components (outer case 72, cover member 73, and seal cover 75) of
 the prismatic sealed battery 71 are welded, the output peak value P and
 pulse waveform of the pulsed laser beam L.sub.3, which is
 oscillation-outputted at 20 Hz from the laser oscillator 62, are set at
 predetermined states.
 Specifically, it is desirable that the output peak value P of the pulsed
 laser beam L.sub.3 be set at 1.times.10.sup.10 W/m.sup.2 or more, e.g.
 2.times.10.sup.10 W/m.sup.2 in this embodiment. In addition, the pulse
 waveform is set to one of those shown in FIGS. 21A to 21E. The input
 energy of the pulsed laser beam L.sub.3 is 3 to 4J per pulse, and the
 convergence diameter of the beam is 0.45 mm. The pulse interval is not
 less than the pulse width.
 In order to weld the components of the prismatic sealed container 71 with a
 sufficient weld depth, as described above, it is desirable that the output
 peak value P of the pulsed laser beam L.sub.3 be set at 2.times.10.sup.10
 W/m.sup.2 or more. In this embodiment, the weld diameter is set at 0.8 mm.
 FIG. 20 shows experimental results on the relationship between the output
 peak value of the pulsed laser beam L.sub.3 and the weld depth.
 As is understood from FIG. 20, when the peak value P of the pulsed laser
 beam L.sub.3 has reached 1.times.10.sup.10 W/m.sup.2 or more, the energy
 absorption ratio increases and the weld depth sharply increases.
 Accordingly, in this embodiment, the output peak value P of the pulsed
 laser beam L.sub.3 was set at 2.times.10.sup.10 W/m.sup.2, as mentioned
 above, and thus the weld portion could be welded with a weld depth enough
 to obtain a predetermined weld strength.
 The waveform of the pulsed laser beam L is set to, for example, first
 waveform W.sub.1 shown in FIG. 21A among those shown in FIGS. 21A to 21E.
 As regards the first waveform W.sub.1, a time t.sub.1 between a time point
 at which the oscillation begins and a time point at which the output
 reaches peak value P is set at t.sub.1.ltoreq.0.8 ms, and the first
 waveform W.sub.1 is set at 0.7 ms. A time t.sub.2 between a time point at
 which the pulsed laser beam L is output and a time point at which the
 output decreases to 1/2 of the peak value after having passed the peak
 value P is set at t.sub.2.ltoreq.1.0 ms.
 The output of the pulsed laser beam L.sub.3 decreases gradually in a time
 period between a time point at which the output has decreased to 1/2 of
 the peak value P and a time point at which the output has decreased to
 zero. The pulse width T is set at T.gtoreq.2.0 ms or more, and
 specifically at 2.6 ms in the first waveform W.sub.1 of this embodiment.
 In a case where the pulsed laser beam L.sub.3 of the first waveform W.sub.1
 is radiated to the welded portion of the outer case 1, the time needed for
 the output to reach the peak value P is as short as 0.8 ms or less. Thus,
 even if the material of prismatic sealed battery 71 is aluminum or alloys
 thereof and the thermal diffusion rate is high, the welding can be carried
 out with relatively high efficiency. Specifically, if the time t.sub.1 is
 set at t.sub.1.ltoreq.0.8 ms, in addition to the setting of the output
 peak value P of the pulsed laser beam L at 2.times.10.sup.10 W/m.sup.2,
 the to-be-welded portion can be locally welded with a high efficiency and
 sufficient penetration depth.
 Since the time t.sub.2 between a time point at which the oscillation begins
 and a time point at which the output peak value decreases to 1/2 of the
 peak value after having passed the peak value P is set at 1.0 ms or less,
 the value (t.sub.2 -t.sub.1) can be set to be relatively short and the
 quantity of heat into the prismatic sealed battery 71 can be decreased.
 Thereby, the rise in temperature of the prismatic sealed battery 71 can be
 suppressed, and the separator between the positive and negative
 electrodes, which is formed by winding a low-melting-point resin material
 such as fluororesin, polypropylene or polyethylene, can be put in the
 prismatic sealed battery 71. Accordingly, even if the present invention is
 applied to the welding for the components of the prismatic sealed battery
 such as a lithium ion battery, the resin material is not thermally
 damaged.
 The output is gradually decreased in the time period between the time point
 at which the output has decreased to 1/2 of the peak value P and the time
 point at which the output has decreased to zero. In addition, the pulse
 width T is set at 2.6 ms. Thus, the cooling time period (T-t.sub.1) in
 which the output of pulse laser beam L has decreased from peak value P to
 zero can be made longer than the heating time period t.sub.1 in which the
 output of pulse laser beam L rises from zero to peak value P. Therefore,
 the cooling rate of the welded portion can be made lower than the heating
 rate.
 Accordingly, the cooling rate of the welded portion after welding, is
 decreased, resulting in a suppressing cracks formed in the welded portion.
 Second to fifth pulse waveforms W.sub.2 to W.sub.5 shown in FIGS. 21B to
 21E will now be described. In the second waveform W.sub.2, the time
 t.sub.1 needed for the output to reach peak value P is set at 0.5 ms and
 the peak value P is continued until 0.8 ms. The time t.sub.2 needed for
 the output to fall to 1/2 of peak value P is 1.0 ms and the pulse width T
 is 2.0 ms.
 In the third waveform W.sub.3, the time t.sub.1 needed for the output to
 reach peak value P is set at 0.8 ms and the time t.sub.2 needed for the
 output to fall to 1/2 of peak value P is 1.0 ms.
 Although the pulse width T is set at 3.0 ms, the time period in which the
 output decreases from 1/2 of the peak value P to zero is divided into a
 first decreasing portion D.sub.1 spanning between 1.0 ms and 2.0 ms and a
 second decreasing portion D.sub.2 spanning between 2.0 ms and 3.0 ms. The
 rate of decrease in the first decreasing portion D.sub.1 is lower than
 that in the second decreasing portion D.sub.2. Thus, the cooling rate of
 the welded portion of the to-be-welded component can be made lower.
 In the fourth waveform W.sub.4 the time t.sub.1 needed for the output to
 reach the peak value P is set at 0.8 ms. Before the output reaches peak
 value P, the output is once raised sharply up to about 3/4 of the peak
 value P, and then the output is decreased slightly lower than 1/2 of peak
 value P. Subsequently, the output is raised to peak value P.
 The time t.sub.2 needed for the output to fall from peak value P to 1/2 of
 peak value P is 1.0 ms. The following cooling period is, like the third
 waveform W.sub.3, divided into a first decreasing portion D.sub.1 and a
 second decreasing portion D.sub.2. The first decreasing portion D.sub.1 is
 a time period of 1.0 ms ranging between 1.0 ms and 2.0 ms, and the second
 decreasing portion D.sub.2 is a time period of 2.0 ms ranging between 2.0
 ms and 4.0 ms. Besides, the pulse width T is set at 4.0 ms.
 In the fifth waveform W.sub.5, the time t.sub.1 needed for the output to
 reach peak value P is set at 0.8 ms and the time t.sub.2 needed for the
 output to fall to 1/2 of peak value P is 1.0 ms. The time period in which
 the output decreases from 1/2 of the peak value P to zero is divided into
 a first decreasing portion D.sub.1 spanning between 1.0 ms and 2.0 ms and
 a second decreasing portion D.sub.2 spanning between 2.0 ms and 3.0 ms.
 Accordingly, the pulse width T is set at 3.0 ms. In the fifth waveform
 W.sub.5, the upward line of the output reaching the peak value P is not
 straight but is bent midway.
 The present invention is not limited to the above embodiments, and various
 modifications may be made. For example, the waveform which can be used in
 the present invention is not limited to the above-described first to fifth
 waveforms. Various waveforms can be used if the time t.sub.1 needed for
 the output to reach peak value P, the time t.sub.2 needed for the output
 to fall to 1/2 of peak value P and the pulse width T meet predetermined
 conditions, and the output gradually decreases after it falls to 1/2 of
 peak value P.
 As is shown in FIG. 25, the cover member 73 may be fitted in the opening
 portion of outer case 72 and the coupled portion therebetween may be
 welded by radiation of pulse laser beam L.sub.3.
 The member to be welded is not limited to the prismatic sealed battery, and
 this invention is applicable to welding which requires efficient melting
 and prevention of a crack of the welded portion due to quick cooling.
 Additional advantages and modifications will readily occur to those skilled
 in the art. Therefore, the invention in its broader aspects is not limited
 to the specific details and representative embodiments shown and described
 herein. Accordingly, various modifications may be made without departing
 from the spirit or scope of the general inventive concept as defined by
 the appended claims and their equivalents.