# Patent

## Title
Sealing plate for prismatic secondary battery and prismatic secondary battery using the sealing plate

## Abstract

A prismatic secondary battery includes a prismatic hollow outer body having a mouth and a bottom; a flat electrode assembly, a positive electrode collector, a negative electrode collector, and an electrolyte, all of which are stored in the prismatic outer body; a sealing plate sealing up the mouth of the prismatic outer body; and a positive electrode terminal attached to the sealing plate in an electrically insulated manner. The sealing plate includes a gas release valve and an electrolyte pour hole and further includes, on the front face, a concaved flat face having an identification code. With the prismatic secondary battery of the invention, a jig for assembly or the like is unlikely to come into contact with the identification code during an assembly process of the prismatic secondary battery, hence the identification code is unlikely to be abraded, and the traceability is unlikely to be lost.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a Continuation of copending U.S. patent application Ser. No. 13/550,882, filed on Jul. 17, 2012.


TECHNICAL FIELD
The present invention relates to a sealing plate for a prismatic secondary battery and a prismatic secondary battery using the sealing plate.
BACKGROUND ART
Alkaline secondary batteries typified by a nickel-hydrogen battery and nonaqueous electrolyte secondary batteries typified by a lithium ion battery are widely used as power supplies for driving portable electronic equipment such as cell phones including smartphones, portable computers, PDAs, and portable music players. In addition, alkaline secondary batteries and the nonaqueous electrolyte secondary batteries are also widely used for power supplies for driving electric vehicles (EVs) and hybrid electric vehicles (HEVs, PHEVs) and in stationary storage battery systems for suppressing the variation in output power of photovoltaic generation, wind power generation, and the like, and for peak shifts in system power in order to store electric power during the night time and to use the electric power during daytime.
In particular, the batteries for EVs, HEVs, and PHEVs and for the stationary storage battery system are required to have high capacity and high output characteristics, and hence each battery is upsized and a number of batteries are connected in series or parallel when used. To address this, in these applications, prismatic secondary batteries are generally used from the viewpoint of space efficiency. A prismatic secondary battery that further needs physical strength commonly employs, as an outer body of the battery, a metal prismatic outer body having a mouth and a metal sealing plate for sealing up the mouth.
Such a prismatic secondary battery, for example, a prismatic nonaqueous electrolyte secondary battery, is produced as follows. For example, both faces of a positive electrode substrate made from, for example, a long sheet of aluminum foil, are coated with a positive electrode active material mixture containing a positive electrode active material to prepare a positive electrode sheet. Separately, both faces of a negative electrode substrate made from, for example, a long sheet of copper foil, are coated with a negative electrode active material mixture containing a negative electrode active material to prepare a negative electrode sheet.
Next, the positive electrode sheet and the negative electrode sheet are stacked interposing a separator made from, for example, a microporous polyethylene film therebetween, and the positive electrode sheet and the negative electrode sheet are spirally wound on a cylindrical winding core while insulating the positive electrode sheet and the negative electrode sheet from each other through the separator to prepare a cylindrical wound electrode assembly. Then, the cylindrical wound electrode assembly is pressed with a pressing machine to form a flat wound electrode assembly. Next, a positive electrode collector electrically connected to the positive electrode sheet is electrically connected to a positive electrode terminal that is insulated from a sealing plate, while a negative electrode collector electrically connected to the negative electrode sheet is electrically connected to a negative electrode terminal that is insulated from a sealing plate. Then, the flat wound electrode assembly is wrapped with a member having insulating characteristics and stored in a metal prismatic outer body; a mouth portion of the prismatic outer body is sealed with a sealing plate; an electrolyte is poured from a electrolyte pour hole provided on the sealing plate; and finally the electrolyte pour hole is sealed to produce the prismatic nonaqueous electrolyte secondary battery.
Such a prismatic secondary battery required to have high capacity and high output characteristics is required to have much higher safety than that of secondary batteries for portable small equipment. Especially, in the case of a nonaqueous electrolyte secondary battery that uses a material having very high reactivity, for example, as shown in US Patent Publication No. 2010/0233529 (US2010/0233529 (A1)) and U.S. Pat. No. 7,781,088 specification (U.S. Pat. No. 7,781,088 (B2)), this nonaqueous electrolyte secondary battery is equipped with a gas release valve for releasing internal pressure when the pressure in a battery outer body is increased and a current interruption mechanism for interrupting electrical connection between an external terminal and an electrode assembly in the outer body.
The metal sealing plate used for the prismatic secondary battery includes at least a mouth for attaching a positive electrode terminal, a mouth for attaching a negative electrode terminal, a gas release valve, and an electrolyte pour hole. The metal sealing plate commonly has a rectangular shape, a chamfered rectangular shape, a rounded rectangular shape, or an oval shape. The mouth for attaching a positive electrode terminal and the mouth for attaching a negative electrode terminal are arranged on both end sides in a longitudinal direction of the sealing plate, and each of the gas release valve and the electrolyte pour hole is provided between the negative electrode terminal and the positive electrode terminal on the sealing plate.
Meanwhile, the prismatic secondary battery is mass-produced and thus is preferred to have a sealing plate with any identification code for providing traceability during an assembly process and after the assembly. Such an identification code can be easily formed by printing, laser marking, or seal-affixing on the sealing plate. However, the sealing plate is required to be subjected to an attachment process of a positive electrode terminal and a negative electrode terminal, an attachment process of a negative electrode collector and a positive electrode to the positive electrode terminal and the negative electrode terminal, respectively, an attachment process of the collectors to a prismatic outer body, a pouring process of an electrolyte, and the like until the completion of a prismatic secondary battery. Due to these processes, even when an identification code is simply applied by printing, laser marking, seal-affixing, or the like onto a surface of the sealing plate, such an identification code is abraded by the contact of a jig for assembly or the like during the assembly process of the prismatic secondary battery, and this increases the possibility that the identification code cannot be read thereby causing traceability to be lost.
SUMMARY
An advantage of some aspects of the present invention is to provide, by achieving a structure in which a jig for assembly or the like is unlikely to come into contact with an identification code during an assembly process of a prismatic secondary battery, a sealing plate for a prismatic secondary battery having the structure by which the identification code is unlikely to be abraded and the traceability is unlikely to be lost and a prismatic secondary battery using the sealing plate.
According to an aspect of the invention, a sealing plate for a prismatic secondary battery includes a pair of mouths for attaching a positive electrode terminal and a negative electrode terminal, one mouth being formed on one end side in a longitudinal direction of the sealing plate, and the other mouth being formed on the other end side, and a gas release valve and an electrolyte pour hole provided between the pair of mouths. In the sealing plate for a prismatic secondary battery, the sealing plate includes a front face having a concaved flat face, and an identification code formed on the concaved flat face.
The sealing plate for a prismatic secondary battery of the invention includes the concaved flat face that is formed on the front face of the sealing plate and that has the flat face having a height less than that of the peripheral portions of the sealing plate and includes the identification code formed on the concaved flat face. Hence, even when a jig for assembling the prismatic secondary battery or the like comes into contact with the sealing plate, such a jig is unlikely to come into contact with the identification code part. On this account, with the sealing plate for a prismatic secondary battery of the invention, the identification code is unlikely to be abraded, and therefore the traceability is unlikely to be lost during an assembly process and after the assembly of the prismatic secondary battery. The sealing plate usable in the invention may have, for example, a rectangular shape, a chamfered rectangular shape, a rounded rectangular shape, or an oval shape in a planar view.
The “front face” of the sealing plate for a prismatic secondary battery of the invention represents a portion positioned on the outer side of the prismatic secondary battery, while a portion of the sealing plate positioned on the inner side of the prismatic secondary battery is expressed as a “back face”. The “concaved flat face” in the sealing plate for a prismatic secondary battery of the invention may include a so-called “groove” including a flat face that has a height less than that of the other portions and that is formed across the whole width of the sealing plate, however, preferably includes a flat face having a height less than that of the peripheral portions of the sealing plate. On this account, with the sealing plate for a prismatic secondary battery of the invention, the reduction in flexural strength with respect to the longitudinal direction of the sealing plate is small in the concaved portion. This can suppress the deformation of the sealing plate when the internal pressure of the prismatic secondary battery is increased and can stabilize working pressure of the gas release valve. Moreover, the sealing plate for a prismatic secondary battery of the invention does not generate a level difference in the fitting portion between the sealing plate and the battery outer body, and this enables uniform laser-welding of the fitting portion between the sealing plate and the battery outer body.
In the sealing plate for a prismatic secondary battery of the invention, an applying mode of the identification code may be any known mode, for example, printing, laser marking, and seal-affixing. Among them, laser marking is preferably adopted because the identification code applied is unlikely to be abraded or peeled, and is unlikely to be misread. Examples of the usable types of identification codes include a well-known code including a simple code such as numbers, characters, and symbols, a one-dimensional code such as a bar-code, and a two-dimensional code such as a stacked bar-code. Among them, the two-dimensional code is desirable because it can contain a large amount of information and is misread less frequently even when the code has been abraded or peeled away.
In the sealing plate for a prismatic secondary battery, it is preferable that the sealing plate include a back face having a convex portion at a position corresponding to the concaved flat face.
When the concaved flat face is formed by forging, the convex portion can be easily formed on the back face of the sealing plate at a position corresponding to the concaved flat face. In addition, the presence of the concaved portion on the front face and the convex portion on the back face increases the flexural strength of the sealing plate in both the longitudinal direction and the width direction. Hence, such a structure can further suppress the deformation of the sealing plate when the internal pressure of the prismatic secondary battery is increased and can further stabilize working pressure of the gas exhaust valve gas release valve.
In the sealing plate for a prismatic secondary battery according to the aspect, an additional identification code may be applied on at least one side adjacent to the convex portion in the longitudinal direction of the sealing plate.
In the longitudinal direction of the sealing plate, both sides adjacent to the convex portion are positions at which a jig for assembling the prismatic secondary battery or the like is unlikely to physically come into contact. The back face of the sealing plate is positioned inside of the prismatic secondary battery and is not exposed to the outside after the completion of the prismatic secondary battery. Thus, with the sealing plate for a prismatic secondary battery of the invention, the additional identification code is unlikely to be abraded and hence the traceability is unlikely to be lost, at least during the assembly process of the prismatic secondary battery.
The additional identification code may be the same as or different from the identification code applied onto the concaved flat face. However, the additional identification code is not exposed to the outside after the completion of the prismatic secondary battery and is entirely used to ensure the traceability during an assembly process. Thus, the additional identification code is preferably different from the identification code applied onto the concaved flat face.
In the sealing plate for a prismatic secondary battery according to the aspect, it is desirable that the convex portion be provided at a position adjacent to the gas release valve and that the additional identification code be formed at a position opposite to the gas release valve with respect to the convex portion.
The presence of the convex portion at a position adjacent to the gas release valve further increases the flexural strength of the sealing plate in both the longitudinal direction and the width direction. Hence, such a structure can further suppress the deformation of the sealing plate when the internal pressure of the prismatic secondary battery is increased and can further stabilize working pressure of the gas release valve.
According to another aspect of the invention, a prismatic secondary battery includes: a prismatic hollow outer body having a mouth and a bottom; a flat electrode assembly including a positive electrode sheet and a negative electrode sheet, a positive electrode collector electrically connected to the positive electrode sheet, a negative electrode collector electrically connected to the negative electrode sheet, and an electrolyte, all being stored in the prismatic outer body; a sealing plate sealing up the mouth of the prismatic outer body; and a positive electrode terminal and a negative electrode terminal attached to the sealing plate in a manner electrically insulated from the sealing plate, while electrically connected to the positive electrode collector and the negative electrode collector, respectively. In the prismatic secondary battery, on the sealing plate, the positive electrode terminal is fixed on one end side in a longitudinal direction of the sealing plate and the negative electrode terminal is fixed on the other end side, a gas release valve and an electrolyte pour hole are formed between the positive electrode terminal and the negative electrode terminal, the sealing plate includes a front face having a concaved flat face, and an identification code formed on the concaved flat face.
In the prismatic secondary battery of the invention, the concaved flat face is formed on the front face of the sealing plate and has a flat face having a height less than that of the peripheral portions of the sealing plate, and the identification code is formed on the concaved flat face. Hence, even when a jig for assembling the prismatic secondary battery or the like comes into contact with the sealing plate, such a jig is unlikely to come into contact with the identification code part. On this account, with the prismatic secondary battery of the aspect, the identification code is unlikely to be abraded, and therefore the traceability is unlikely to be lost after the assembly of the prismatic secondary battery. The sealing plate usable in the prismatic secondary battery may be a metal plate having, for example, a rectangular shape, a chamfered rectangular shape, a rounded rectangular shape, or an oval shape. The sealing plate may also include a first region having a flat surface, disposed between the positive electrode terminal and the gas release valve or between the negative electrode terminal and the gas release valve in the direction parallel to the long side of the sealing assembly. The sealing plate may also include a second region and a third region, the first region being provided between the second region and the third region in a direction parallel to the short side of the sealing assembly, and the second region and the third region being farther from the bottom of the prismatic outer hollow body than the first region.
Moreover, with the prismatic secondary battery of the aspect, the reduction in flexural strength with respect to the longitudinal direction of the sealing plate is small in the concaved portion, and this can suppress the deformation of the sealing plate when the internal pressure of the prismatic secondary battery is increased and can stabilize working pressure of the gas release valve. Furthermore, the prismatic secondary battery of the aspect does not generate a level difference in the fitting portion between the sealing plate and the battery outer body, and this enables uniform laser-welding of the fitting portion between the sealing plate and the battery outer body. In addition, when the concaved flat face is formed by forging, a convex portion is formed on the back face. The concaved portion increases the flexural strength of the sealing plate in the longitudinal direction, and this can further stabilize working pressure of the gas release valve.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements.
FIG. 1A is a front view of a prismatic nonaqueous electrolyte secondary battery of an embodiment and FIG. 1B is a plan view of the prismatic nonaqueous electrolyte secondary battery.
FIG. 2A is a partial cross-sectional view taken along the line IIA-IIA in FIG. 1B and FIG. 2B is a partial cross-sectional view taken along the line IIB-IIB in FIG. 2A.
FIG. 3A is a plan view of a sealing plate of the embodiment, FIG. 3B is a cross-sectional view taken along the line IIIB-IIIB in FIG. 3A, and FIG. 3C is a bottom view of the sealing plate of the embodiment.



DESCRIPTION OF EXEMPLARY EMBODIMENT
Hereinafter, embodiment of the invention will be described in detail with reference to the accompanying drawings. However, the embodiment described below is intended to exemplify the technical spirit of the invention, the invention is not intended to be limited to the embodiment, and the invention may equally be applied to various modified cases without departing from the technical spirit described in the claims. In each drawing used for explanation in the specification, each member is appropriately shown on a different scale so that the member has a recognizable size in each drawing and the members are not necessarily shown in proportion to the actual sizes.
EMBODIMENT
Firstly, as an example of a prismatic secondary battery of the embodiment, a prismatic nonaqueous electrolyte secondary battery will be described with reference to FIG. 1 and FIG. 2. FIG. 1A is a front view of the prismatic nonaqueous electrolyte secondary battery, and FIG. 1B is a plan view of the prismatic nonaqueous electrolyte secondary battery. FIG. 2A is a cross-sectional view taken along the line IIA-IIA in FIG. 1B, and FIG. 2B is a cross-sectional view taken along the line IIB-IIB in FIG. 2A.
A prismatic nonaqueous electrolyte secondary battery 10 includes a flat wound electrode assembly 11 in which a positive electrode sheet and a negative electrode sheet are wound while being insulated from each other through a separator, which are not shown in the drawings. The positive electrode sheet is prepared by coating both faces of a positive electrode substrate made from aluminum foil with a positive electrode active material mixture, then drying and rolling the coated substrate, and slitting the substrate so as to expose the aluminum foil in a strip shape. Separately, the negative electrode sheet is prepared by coating both faces of a negative electrode substrate made from copper foil with a negative electrode active material mixture, then drying and rolling the coated substrate, and slitting the substrate so as to expose the copper foil in a strip shape.
Then, the flat wound electrode assembly 11 is prepared as follows. The positive electrode sheet and the negative electrode sheet obtained as above are stacked interposing a polyolefin microporous separator therebetween so as to displace the aluminum foil exposed portion of the positive electrode sheet and the copper foil exposed portion of the negative electrode sheet from the corresponding counter electrode active material mixtures, and the whole is wound while insulating the positive electrode sheet and the negative electrode sheet from each other through the separator to afford the flat wound electrode assembly 11 including one end with a plurality of stacked positive electrode substrate exposed portions 12 and the other end with a plurality of stacked negative electrode substrate exposed portions 13.
The plurality of stacked positive electrode substrate exposed portions 12 are electrically connected through a positive electrode collector 14 made of aluminum to a positive electrode terminal 15 also made of aluminum, and similarly, the plurality of stacked negative electrode substrate exposed portions 13 are electrically connected through a negative electrode collector 16 made of copper to a negative electrode terminal 17 also made of copper. The positive electrode terminal 15 and the negative electrode terminal 17 are, as shown in FIG. 2A and FIG. 2B, fixed to a sealing plate 20 made of, for example, aluminum through an insulating members 18 and 19, respectively. The positive electrode terminal 15 and the negative electrode terminal 17 are, as necessary, connected to an external positive electrode terminal and an external negative electrode terminal (not shown in the drawings) arranged on the insulating members 18 and 19, respectively.
The flat wound electrode assembly 11 prepared as above is wrapped with a resin sheet (not shown in the drawings) having insulating characteristics except the sealing plate 20 side and is inserted into a prismatic outer body 21 made of, for example, aluminum having an open face. Then, the sealing plate 20 is fitted to the mouth portion of the prismatic outer body 21; a fitting portion 22 between the sealing plate 20 and the outer body 21 is laser-welded; then, a nonaqueous electrolyte is poured from an electrolyte pour hole 23; and the electrolyte pour hole 23 is sealed up to produce the prismatic nonaqueous electrolyte secondary battery 10 of the embodiment.
A current interruption mechanism 24 that works correspondingly to the pressure of gas generated in the battery is provided between the positive electrode collector 14 and the positive electrode terminal 15. The sealing plate 20 also equips a gas release valve 25 that opens when the gas pressure becomes higher than the working pressure of the current interruption mechanism 24. Thus, the inside of the prismatic nonaqueous electrolyte secondary battery 10 is hermetically sealed. The prismatic nonaqueous electrolyte secondary battery 10 is used alone or used with a plurality of the batteries connected in series or parallel, for various applications. When a plurality of the prismatic nonaqueous electrolyte secondary batteries 10 are connected in series or parallel to be used, an external positive electrode terminal and an external negative electrode terminal are preferably provided separately for connecting each battery through bus bars so as to send a large current.
In the flat wound electrode assembly 11, as shown in FIG. 2A and FIG. 2B, on the positive electrode sheet side, the plurality of stacked positive electrode substrate exposed portions 12 are divided into two portions, and between the portions, a positive electrode intermediate member 27 made of resin and having a plurality of positive electrode connection conductive members 26, two positive electrode connection conductive members 26 in the embodiment, is interposed. In a similar manner, on the negative electrode sheet side, the plurality of stacked negative electrode substrate exposed portions 13 are divided into two portions, and between the portions, a negative electrode intermediate member 29 made of resin and having two negative electrode connection conductive members 28 is interposed. On the outermost surface of the positive electrode substrate exposed portions 12 positioned on each side of the positive electrode connection conductive member 26, a positive electrode collector 14 is disposed, and on the outermost surface of the negative electrode substrate exposed portions 13 positioned on each side of the negative electrode connection conductive member 28, a negative electrode collector 16 is disposed. The positive electrode connection conductive member 26 is made of aluminum that is the same material as the positive electrode substrate, the negative electrode connection conductive member 28 is made of copper that is the same material as the negative electrode substrate, and the shape of the positive electrode connection conductive member 26 may be the same as or different from that of the negative electrode connection conductive member 28.
The positive electrode collector 14 is resistance-welded to the positive electrode substrate exposed portion 12 (at four points), and the positive electrode substrate exposed portion 12 is resistance-welded to the positive electrode connection conductive member 26 (at four points), for connection. Similarly, the negative electrode collector 16 is resistance-welded to the negative electrode substrate exposed portion 13 (at four points), and the negative electrode substrate exposed portion 13 is resistance-welded to the negative electrode connection conductive member 28 (at four points), for connection. FIG. 2A shows, on the positive electrode side, two weld marks 30 formed by the resistance welding and, on the negative electrode side, two weld marks 31.
Hereinafter, in the flat wound electrode assembly 11 of the Embodiment, the resistance welding method using the positive electrode substrate exposed portions 12, the positive electrode collector 14, and the positive electrode intermediate member 27 having the positive electrode connection conductive members 26 and the resistance welding method using the negative electrode substrate exposed portions 13, the negative electrode collector 16, and the negative electrode intermediate member 29 having the negative electrode connection conductive members 28 will be described in detail. However, in the Embodiment, the shapes of the positive electrode connection conductive member 26 and the positive electrode intermediate member 27 may be substantially the same as the shapes of the negative electrode connection conductive member 28 and the negative electrode intermediate member 29, and each resistance welding method may be substantially the same. Therefore, the method for the positive electrode sheet side will be described below as a typical example.
First, the positive electrode substrate exposed portions 12 of the flat wound electrode assembly 11 prepared as above were divided from the wound center to both side into two portions, and the divided positive electrode substrate exposed portions 12 were gathered to a center as a quarter of the thickness of the electrode assembly. Then, the positive electrode collectors 14 were disposed on both sides of the outermost periphery of the positive electrode substrate exposed portions 12, the positive electrode intermediate member 27 having the positive electrode connection conductive members 26 was inserted between both sides of the bisectional positive electrode substrate exposed portions 12 so that protrusions on both sides of the positive electrode connection conductive member 26 were in contact with the positive electrode substrate exposed portions 12 on the inner periphery thereof. The positive electrode collector 14 is made from, for example, an aluminum plate having a thickness of 0.8 mm.
Here, the positive electrode connection conductive member 26 held with the positive electrode intermediate member 27 in the Embodiment has a column-shaped body with two opposed faces, and a protrusion (projection) having, for example, a truncated cone shape is formed on each face. The positive electrode connection conductive member 26 may have any shape, for example, a prismatic shape and an elliptical column shape, in addition to the cylindrical shape, as long as it is a metal block. Examples of the material usable for forming the positive electrode connection conductive member 26 include copper, a copper alloy, aluminum, an aluminum alloy, tungsten, and molybdenum. In addition to the members made of these metals, for example, a member having a protrusion coated with nickel or a member in which the material of a protrusion and the vicinity of a bottom of the protrusion are changed into a metal capable of accelerating heat generation, such as tungsten and molybdenum and such a protrusion is bonded to the cylindrical-shaped body of the positive electrode connection conductive member 26 made of copper, a copper alloy, aluminum, or an aluminum alloy by brazing or the like may be used.
A plurality of the positive electrode connection conductive members 26 of the Embodiment, for example, two positive electrode connection conductive members are integrally held with the positive electrode intermediate member 27 made of a resin material. In this case, the positive electrode connection conductive members 26 are held so as to be parallel to each other. The positive electrode intermediate member 27 may have any shape, for example, a prismatic shape and a column shape. However, the shape is preferably a prismatic shape having a long lateral length in order to be fixed between the bisectional positive electrode substrate exposed portions 12 while being stably positioned. However, corners of the positive electrode intermediate member 27 are preferably chamfered so as not to scratch or deform the soft positive electrode substrate exposed portion 12 even when the corner comes into contact with the positive electrode substrate exposed portion 12. At least a part to be inserted between the bisectional positive electrode substrate exposed portions 12 may be chamfered.
The length of the prismatic positive electrode intermediate member 27 varies depending on the size of the prismatic nonaqueous electrolyte secondary battery 10, but may be 20 mm to several tens of mm. The width of the prismatic positive electrode intermediate member 27 may be substantially the same as the height of the positive electrode connection conductive member 26, but the prismatic positive electrode intermediate member 27 may have a width of which at least both ends of the positive electrode connection conductive member 26 to be welded are exposed. Both ends of the positive electrode connection conductive member 26 preferably protrude from the surface of the positive electrode intermediate member 27, but may not necessarily protrude. With such a structure, the positive electrode connection conductive member 26 is held with the positive electrode intermediate member 27 and the positive electrode intermediate member 27 is disposed between the bisectional positive electrode substrate exposed portions 12 while being stably positioned.
Next, between a pair of resistance welding electrode rods (not shown in the drawings), the positive electrode collector 14 and the flat wound electrode assembly 11 with the positive electrode intermediate member 27 holding the positive electrode connection conductive members 26 are disposed. The pair of the resistance welding electrode rods are brought into contact with the positive electrode collectors 14 disposed on both outermost periphery sides of the positive electrode substrate exposed portions 12. Then, an appropriate pressure is applied between the pair of the resistance welding electrode rods to perform resistance welding in a previously determined constant condition. During the resistance welding, the positive electrode intermediate member 27 is disposed while being stably positioned between the bisectional positive electrode substrate exposed portions 14. This improves the dimensional precision between the positive electrode connection conductive member 26 and the pair of electrode rods, enables the resistance welding in an exact and stable condition, and can suppress variation in the welding strength.
Next, the specific structure of the sealing plate of the invention will be described with reference to FIG. 3. FIG. 3A is a plan view of a sealing plate of the embodiment, FIG. 3B is a cross-sectional view taken along the line IIIB-IIIB in FIG. 3A, and FIG. 3C is a bottom view of the sealing plate of the embodiment.
The sealing plate 20 has, for example, a rectangular shape so as to be fitted onto the mouth portion of the prismatic outer body 21 to seal electrolyte therein. The sealing plate 20 has a main surface which is exposed to the outward when the sealing plate 20 is fitted onto the prismatic outer body 21. The main surface is provided with a positive electrode terminal 15 and a negative electrode terminal 17. More precisely, The main surface of the sealing plate 20 includes a mouth 34 for attaching the positive electrode terminal 15 and a mouth 35 for attaching the negative electrode terminal 17 on each end side, and includes an electrolyte pour hole 23, a gas release valve 25, and a concaved flat face 32 between the mouths 34 and 35. The concaved flat face 32 has a flat face having a height less than that of the peripheral portions in the sealing plate 20 and being partially formed in a width direction (short side direction) of the sealing plate 20. That is to say, the main surface of the sealing plate 20 is formed with a step so that the concaved flat face is formed at the height less than that of the main surface of the sealing plate 20. When the concave portion 32 is formed across the whole width of the sealing plate 20, it becomes a so-called “groove.” Such a groove largely reduces the flexural strength of the sealing plate 20 in the longitudinal direction as well as generating a level difference between the sealing plate 20 and the end face of the outer body 21 thereby to interfere with uniform laser-welding.
The sealing plate 20 of the embodiment further includes anti-rotation protrusions 38 and 40 and auxiliary protrusions 39 and 41 on both sides of the mouth 34 for the positive electrode and the mouth 35 for the negative electrode, respectively, in order to attach the external positive electrode terminal and the external negative electrode terminal (not shown in the drawings) used when a plurality of the prismatic nonaqueous electrolyte secondary batteries are connected in series or parallel. The anti-rotation protrusions 38 and 40 and the auxiliary protrusions 39 and 41 are fitted to the external positive electrode terminal and the electrode negative external terminal that are provided as necessary, and function so that the external positive electrode terminal and the external negative electrode terminal are unlikely to rotate.
For the sealing plate 20 of the embodiment, a rectangular aluminum plate is punched out to form the mouths 34 and 35 and the electrolyte pour hole 23 by forging, and the gas release valve 25, the concaved flat face 32, the anti-rotation protrusions 38 and 40, and the auxiliary protrusions 39 and 41 are simultaneously formed. In the sealing plate 20, concaves 38a, 40a, 39a, and 41a formed on the front face around the anti-rotation protrusions 38 and 40 and the auxiliary protrusions 39 and 41 as well as concaves 38b, 40b, 39b, and 41b formed in the back face are formed by forging so that each of the anti-rotation protrusions 38 and 40 and the auxiliary protrusions 39 and 41 has a height larger than that of the front face of the sealing plate 20. A convex portion 36 is formed on the back face of the sealing plate 20 corresponding to the concaved flat face 32 formed position.
The formation of the convex portion 36 on the back face of the sealing plate 20 corresponding to the concaved flat face 32 formed position as above increases the flexural strength of the sealing plate 20 in both the longitudinal direction and the width direction, compared with the case without the convex portion 36. Therefore, a prismatic nonaqueous electrolyte secondary battery 10 produced using the sealing plate 20 suppresses the deformation of the sealing plate 20 when the internal pressure is increased, and this can stabilize working pressure of the gas release valve 25.
In the sealing plate 20 of the embodiment, a two-dimensional code as a first identification code 33 is formed on the flat face as the bottom of the concaved flat face 32 on the front face by, for example, laser marking, while, a second identification code 37 is similarly formed by laser marking on a surface of a side adjacent to the convex portion 36 on the back face.
The first identification code 33 is formed on the concaved flat face having a height less than that of the peripheral portions that are formed on the front face of the sealing plate 20. Thus, even when a jig for assembling the prismatic nonaqueous electrolyte secondary battery 10 or the like comes into contact with the sealing plate 20, such a jig is unlikely to physically come into contact with the first identification code 33. Therefore, the first identification code 33 is unlikely to be abraded, and the traceability is unlikely to be lost during the assembly process and after the assembly of the prismatic nonaqueous electrolyte secondary battery 10. The second identification code 36 is not exposed to the outside after the assembly of the prismatic nonaqueous electrolyte secondary battery 10, but is used for tracing during an assembly process of the prismatic nonaqueous electrolyte secondary battery 10. The first identification code 33 may be the same as or different from the second identification code 36.
Here, the laser marking is exemplified for explaining the application of the first identification code 33 and the second identification code 36, but well-known printing or seal-affixing may be adopted. However, the laser marking is preferably adopted because an applied code is unlikely to be abraded or peeled and is unlikely to be misread. Examples of the usable type of the identification code includes, in addition to the two-dimensional code, a simple code such as numbers, characters, and symbols, a one-dimensional code such as a bar-code. Among them, the two-dimensional code is desirable because it can contain a large amount of information and is misread less frequently even when the code has been abraded or peeled away.
As the sealing plate 20 of the embodiment, the exemplified sealing plate has the convex portion 36 on the back face corresponding to the concaved flat face 32 that is formed on the front face, but the convex portion 36 is not a necessary component. For example, when the concaved flat face 32 is formed on the front face by cutting work, the convex portion is not formed. However, a sealing plate without such a convex portion does not have the advantage of abrasion resistance or peel resistance with respect to the identification code formed on the back face as well as having a smaller flexural strength in the longitudinal direction of the sealing plate 20. To address this, the convex portion 36 is preferably formed on the back face corresponding to the concaved flat face 32 that is formed on the front face as in the case of the sealing plate of the embodiment.
The embodiment describes the prismatic nonaqueous electrolyte secondary battery as an example of the prismatic secondary battery. However, the invention is also applicable to a prismatic secondary battery using an aqueous electrolyte, such as a nickel-hydrogen secondary battery.

## Claims

1. A prismatic secondary battery comprising:
a prismatic hollow outer body having a mouth and a bottom;
an electrode assembly including a positive electrode sheet and a negative electrode sheet;
a positive electrode collector electrically connected to the positive electrode sheet;
a negative electrode collector electrically connected to the negative electrode sheet;
an electrolyte;
a sealing assembly sealing up the mouth of the prismatic outer body, the sealing assembly having a long side and a short side, the sealing assembly including
a positive electrode terminal electrically connected to the positive electrode collector,
a negative electrode terminal electrically connected to the negative electrode collector,
a first region having a flat surface, and
a gas release valve provided between the positive electrode terminal and the negative electrode terminal in a direction parallel to the long side of the sealing assembly,
a level difference portion having a surface on a far side from the bottom of the prismatic hollow outer body which is farther from the bottom of the prismatic hollow outer body than the first region; and

an identification code formed on the flat surface of the first region,
wherein the first region is spaced from the gas release valve, in a direction parallel to the long side of the sealing assembly,
wherein the level difference portion is provided between the first region and the gas release valve in the direction parallel to the long side of the sealing assembly,
wherein the first region is disposed between the negative electrode terminal and the gas release valve in the direction parallel to the long side of the sealing assembly,
wherein the sealing assembly includes a sealing plate,
wherein the first region is a part of the sealing plate,
wherein the identification code is formed on the sealing plate, and
wherein the sealing plate is directly connected to the prismatic hollow outer body by welding.

2. The prismatic secondary battery according to claim 1,
wherein the sealing assembly further includes a second region and a third region,
wherein the first region provided between the second region and the third region in a direction parallel to the short side of the sealing assembly,
wherein the second region and the third region are farther from the bottom of the prismatic hollow outer body than the first region.

3. The prismatic secondary battery according to claim 2, wherein the sealing assembly further includes an electrolyte pour hole provided between the positive electrode terminal and the gas release valve in the direction parallel to the long side of the sealing assembly.

4. The prismatic secondary battery according to claim 3, wherein the identification code is provided between the negative electrode terminal and the gas release valve in the direction parallel to the long side of the sealing assembly.

5. The prismatic secondary battery according to claim 1,
wherein the sealing plate is a single plate member, and
wherein the sealing plate has a first through hole through which the negative electrode terminal is inserted, a second through hole through which the positive electrode terminal is inserted, and an electrolyte pour hole.

6. The prismatic secondary battery according to claim 1,
wherein the sealing plate has a long side and a short side,
wherein the long side of the sealing plate is parallel to the long side of the sealing assembly, and
wherein the short side of the sealing plate is parallel to the short side of the sealing assembly.

7. The prismatic secondary battery according to claim 1,
wherein a first insulating member is arranged between the sealing plate and the positive electrode terminal, and
wherein a second insulating member is arranged between the sealing plate and the negative electrode terminal.

8. The prismatic secondary battery according to claim 2,
wherein the sealing plate has a first through hole through which the negative electrode terminal is inserted and a second through hole through which the positive electrode terminal is inserted, and
wherein the first region is disposed between the second region and the third region in a direction perpendicular to a straight line connecting a center of the first through hole of the sealing plate and a center of the second through hole of the sealing plate.

9. The prismatic secondary battery according to claim 2,
wherein, in a direction parallel to the long side of the sealing assembly, the second region and the third region are disposed away from the negative electrode terminal and from the positive electrode terminal.