Secondary battery

A negative-electrode core laminate of a portion of a negative-electrode core on which no negative-electrode active material layer is formed is bonded to a negative-electrode current collector by ultrasonic bonding. A core recess is formed in a bonding region of the negative-electrode core laminate bonded to the negative-electrode current collector by ultrasonic bonding, a region of the negative-electrode core laminate in which the core recess is formed includes a solid-state bonding layer and a central layer, the solid-state bonding layer being formed by solid-state bonding between layers of the negative-electrode core, the central layer being disposed between the solid-state bonding layers formed on both faces of the negative-electrode core. The average grain size of metal crystal grains constituting the solid-state bonding layer is smaller than the average grain size of metal crystal grains constituting the central layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention application claims priority to Japanese Patent Application No. 2018-245220 filed in the Japan Patent Office on Dec. 27, 2018, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a secondary battery.

Description of Related Art

Non-aqueous electrolyte secondary batteries, such as alkaline secondary batteries and lithium-ion batteries, are used as power supplies mounted on electric vehicles and hybrid electric vehicles. A known secondary battery includes a flat wound electrode assembly and an electrolyte in an exterior, wherein a long positive-electrode sheet and a long negative-electrode sheet with a separator interposed therebetween are wound to form the wound electrode assembly. Such a battery including a wound electrode assembly includes a positive-electrode core laminate at one end of the wound electrode assembly and a negative-electrode core laminate at the other end of the wound electrode assembly to couple the wound electrode assembly to a current collector. The positive-electrode core laminate is composed of layers of a positive-electrode core on which no positive-electrode active material layer is formed, and the negative-electrode core laminate is composed of layers of a negative-electrode core on which no negative-electrode active material layer is formed.

The positive-electrode core laminate and the negative-electrode core laminate are bonded to a positive-electrode current collector and a negative-electrode current collector, respectively, by a welding technique.

Japanese Published Unexamined Patent Application No. 2010-282846 (Patent Document 1) discloses that when a negative-electrode core laminate is bonded to a negative-electrode current collector by ultrasonic bonding, to improve the bonding strength between a wound electrode assembly and the current collector (current collector terminal), the number of welding recesses on the front side of the negative-electrode core laminate is decreased compared with that on the negative-electrode current collector side, and the welding recess is deepened compared with that on the negative-electrode current collector side.

When a copper or copper alloy core laminate is bonded to a copper or copper alloy current collector by ultrasonic bonding according to a method described in Patent Document 1, however, a crack is often formed between a bonding region of the core laminate in which welding recesses are formed and a non-bonding region surrounding the bonding region.

BRIEF SUMMARY OF THE INVENTION

Accordingly, one aspect of the present disclosure aims to decrease cracks in a core laminate in a secondary battery including a copper or copper alloy core laminate bonded to a copper or copper alloy current collector by ultrasonic bonding.

A secondary battery according to one aspect of the present disclosure includes an electrode assembly including a first electrode sheet and a second electrode sheet, the first electrode sheet and the second electrode sheet having different polarities, and

a first electrode current collector electrically connected to the first electrode sheet,

wherein the first electrode sheet includes a first electrode core and a first electrode active material layer on the first electrode core,

the first electrode core is made of copper or a copper alloy,

the first electrode current collector is made of copper or a copper alloy,

the electrode assembly includes a first electrode core laminate of the first electrode core,

the first electrode core laminate is bonded to the first electrode current collector by ultrasonic bonding,

a core recess is formed in a bonding region of the first electrode core laminate bonded to the first electrode current collector by ultrasonic bonding,

a region of the first electrode core laminate in which the core recess is formed includes a solid-state bonding layer and a central layer, the solid-state bonding layer being formed by solid-state bonding of an interface between layers of the first electrode core, the central layer being disposed between the solid-state bonding layers formed on both faces of the first electrode core, and

metal crystal grains constituting the solid-state bonding layer have a first average grain size smaller than a second average grain size of metal crystal grains constituting the central layer.

One aspect of the present disclosure can decrease cracks in a core laminate in a secondary battery including a copper or copper alloy core laminate bonded to a copper or a copper alloy current collector by ultrasonic bonding.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have found the following when a copper or copper alloy negative-electrode core laminate is bonded to a copper or copper alloy negative-electrode current collector by ultrasonic bonding described in Patent Document 1.

Ultrasonic bonding between the copper or copper alloy negative-electrode core laminate and the copper or copper alloy negative-electrode current collector by a method described in Patent Document 1 transformed copper crystal grains into fine crystal grains in the entire bonding region of the negative-electrode core laminate in which a welding recess is formed. However, large copper crystal grain sizes before the ultrasonic bonding were maintained in a non-bonding region outside the bonding region of the negative-electrode core laminate. Thus, it was found that in the negative-electrode core laminate bonded to the negative-electrode current collector by ultrasonic bonding, the state of crystal grains (hereinafter referred to as the crystal grain state), such as grain sizes, in the bonding region was significantly different from the crystal grain state of the non-bonding region, and consequently this increased the risk of cracking due to a lattice defect between the bonding region and the non-bonding region.

Hence, the present inventors arrived at the present invention in which the ultrasonic bonding conditions are controlled to form a solid-state bonding layer containing finer crystal grains near a bonding surface formed by solid-state bonding between surfaces of a negative-electrode core in a bonding region of a negative-electrode core laminate in which a welding recess is formed and to form a central layer within each negative-electrode core (a central portion between solid-state bonding layers formed on both surfaces of each negative-electrode core), transformation of crystal grains in the central layer being suppressed. Thus, in the bonding region of the negative-electrode core laminate bonded to a negative-electrode current collector by ultrasonic bonding, solid-state bonding between layers of a negative-electrode core ensures bonding strength and decreases bonding resistance, and the central layer within each negative-electrode core in which transformation of crystal grains is suppressed ensures the continuity of the crystal grain state between the bonding region and the non-bonding region and can reduce the occurrence of cracking due to a lattice defect between the regions.

A secondary battery according to an embodiment of the present invention is described below with reference to the accompanying drawings. The scope of the present invention is not limited to the following embodiments and can be changed within the scope of the technical idea of the present invention.

First, a rectangular secondary battery according to an embodiment is described below.

FIG.1is a front view of the interior of a rectangular secondary battery100according to the present embodiment except the front of a battery case and the front of an insulating sheet.FIG.2is a top view of the rectangular secondary battery100.

As illustrated inFIGS.1and2, the rectangular secondary battery100includes a rectangular exterior1with an upward opening and a sealing plate2for sealing the opening. The rectangular exterior1and the sealing plate2constitute a battery case200. The rectangular exterior1and the sealing plate2are made of a metal, for example, aluminum or an aluminum alloy. The rectangular exterior1contains a flat wound electrode assembly3and a non-aqueous electrolyte (not shown). The flat wound electrode assembly3is formed by winding a long positive-electrode sheet (not shown) and a long negative-electrode sheet (not shown) with a long separator (not shown) interposed therebetween. The positive-electrode sheet includes a positive-electrode active material layer containing a positive-electrode active material formed on a metallic positive-electrode core and includes a positive-electrode core exposing portion through which the positive-electrode core is exposed in the longitudinal direction. The negative-electrode sheet includes a negative-electrode active material layer containing a negative-electrode active material formed on a metallic negative-electrode core and includes a negative-electrode core exposing portion through which the negative-electrode core is exposed in the longitudinal direction. The positive-electrode core is made of aluminum or an aluminum alloy, for example. The negative-electrode core is made of copper or a copper alloy, for example.

A portion of the positive-electrode core4aon which no positive-electrode active material layer is formed (a positive-electrode core exposing portion) is layered at one end of the wound electrode assembly3in the winding axis direction. The positive-electrode core4ais wound without a separator or a negative-electrode sheet and is layered. The layered positive-electrode core4a(hereinafter also referred to as a positive-electrode core laminate) is coupled to a positive-electrode current collector6. The positive-electrode current collector6is made of aluminum or an aluminum alloy, for example.

A portion of the negative-electrode core5aon which no negative-electrode active material layer is formed (a negative-electrode core exposing portion) is layered at the other end of the wound electrode assembly3in the winding axis direction. The negative-electrode core5ais wound without a separator or a positive-electrode sheet and is layered. The layered negative-electrode core5a(hereinafter also referred to as a negative-electrode core laminate) is coupled to a negative-electrode current collector8. The negative-electrode current collector8is made of copper or a copper alloy, for example.

A positive-electrode terminal7has a rim7aon the outer surface of the sealing plate2and an insert for a through-hole in the sealing plate2. The positive-electrode terminal7is made of a metal, for example, aluminum or an aluminum alloy. A negative-electrode terminal9has a rim9aon the outer surface of the sealing plate2and an insert for a through-hole in the sealing plate2. The negative-electrode terminal9is made of a metal, for example, copper or a copper alloy. The negative-electrode terminal9may have an aluminum or aluminum alloy portion and a copper or copper alloy portion. In this case, the aluminum or aluminum alloy portion may protrude from the sealing plate2, and the copper or copper alloy portion may be coupled to the negative-electrode current collector8.

The positive-electrode current collector6is fixed to the sealing plate2with an inner insulating member10made of a resin interposed therebetween, and the positive-electrode terminal7is fixed to the sealing plate2with an outer insulating member11made of a resin interposed therebetween. Thus, the inner insulating member10is disposed between the sealing plate2and the positive-electrode current collector6, and the outer insulating member11is disposed between the sealing plate2and the positive-electrode terminal7. The negative-electrode current collector8is fixed to the sealing plate2with an inner insulating member12made of a resin interposed therebetween, and the negative-electrode terminal9is fixed to the sealing plate2with an outer insulating member13made of a resin interposed therebetween. Thus, the inner insulating member12is disposed between the sealing plate2and the negative-electrode current collector8, and the outer insulating member13is disposed between the sealing plate2and the negative-electrode terminal9.

The wound electrode assembly3is covered with an insulating sheet14and is placed in the rectangular exterior1. The sealing plate2is welded to an opening edge of the rectangular exterior1by laser welding. The sealing plate2has an electrolyte solution inlet16, which is sealed with a sealing plug17after an electrolyte solution is poured into the rectangular exterior1. The sealing plate2has a gas release valve15for releasing gas if the internal pressure of the battery exceeds a predetermined value.

A method for producing the wound electrode assembly3is described below.

FIG.3Ais a plan view of a positive-electrode sheet4according to the present embodiment. As illustrated inFIG.3A, the positive-electrode sheet4includes a positive-electrode active material layer4bcontaining a positive-electrode active material on a positive-electrode core4a, for example, made of aluminum and has a positive-electrode core exposing portion with a predetermined width at an end thereof in the short side direction. The positive-electrode active material layer4bis not formed on the positive-electrode core exposing portion.

The positive-electrode sheet4illustrated inFIG.3Ais produced by the following method. First, a positive-electrode mixture slurry containing a positive-electrode active material, for example, lithium nickel cobalt manganese composite oxide, an electrically conductive agent, a binder, and a dispersion medium is prepared. The positive-electrode mixture slurry is then applied to both faces of the positive-electrode core4a, for example, made of belt-like aluminum foil 15 μm in thickness. The positive-electrode mixture slurry is then dried to remove the dispersion medium. Thus, the positive-electrode active material layer4bis formed on both faces of the positive-electrode core4a. The positive-electrode active material layer4bis then pressed to a predetermined packing density to complete the positive-electrode sheet4.

FIG.3Bis a plan view of a negative-electrode sheet5according to the present embodiment. As illustrated inFIG.3B, the negative-electrode sheet5includes a negative-electrode active material layer5bcontaining a negative-electrode active material on a negative-electrode core5a, for example, made of copper and has a negative-electrode core exposing portion with a predetermined width at an end thereof in the short side direction. The negative-electrode active material layer5bis not formed on the negative-electrode core exposing portion.

The negative-electrode sheet5illustrated inFIG.3Bis produced by the following method. First, a negative-electrode mixture slurry containing a negative-electrode active material, for example, a graphite powder, a binder, and a dispersion medium is prepared. The negative-electrode mixture slurry is then applied to both faces of the negative-electrode core5a, for example, made of belt-like copper foil 8 μm in thickness. The negative-electrode mixture slurry is then dried to remove the dispersion medium. Thus, the negative-electrode active material layer5bis formed on both faces of the negative-electrode core5a. The negative-electrode active material layer5bis then pressed to a predetermined packing density to complete the negative-electrode sheet5.

The positive-electrode sheet4and the negative-electrode sheet5produced by these methods are placed such that the positive-electrode core exposing portion and the negative-electrode core exposing portion do not overlap the active material layer of the facing electrode, are wound with a porous separator, for example, made of polyethylene interposed therebetween, and are flattened. Thus, the wound electrode assembly3is produced that includes a positive-electrode core laminate of the positive-electrode core4a(positive-electrode core exposing portion) at one end and a negative-electrode core laminate of the negative-electrode core5a(negative-electrode core exposing portion) at the other end.

<Attachment of Component to Sealing Plate>

The positive-electrode current collector6, the positive-electrode terminal7, the negative-electrode current collector8, and the negative-electrode terminal9are attached to the sealing plate2by the following method.

First, on the positive electrode side, the outer insulating member11is placed on the outer surface of the sealing plate2, and the inner insulating member10and the positive-electrode current collector6are placed on the inner surface of the sealing plate2. An insert of the positive-electrode terminal7is put into a through-hole in the outer insulating member11, the sealing plate2, the inner insulating member10, and the positive-electrode current collector6from the outside, and the tip of the insert of the positive-electrode terminal7is bent on the positive-electrode current collector6. Thus, the positive-electrode terminal7, the outer insulating member11, the sealing plate2, the inner insulating member10, and the positive-electrode current collector6are integrally fixed. The tip of the insert of the positive-electrode terminal7thus bent may be welded to the positive-electrode current collector6.

Likewise, on the negative electrode side, the outer insulating member13is placed on the outer surface of the sealing plate2, and the inner insulating member12and the negative-electrode current collector8are placed on the inner surface of the sealing plate2. An insert of the negative-electrode terminal9is put into a through-hole in the outer insulating member13, the sealing plate2, the inner insulating member12, and the negative-electrode current collector8from the outside, and the tip of the insert of the negative-electrode terminal9is bent on the negative-electrode current collector8. Thus, the negative-electrode terminal9, the outer insulating member13, the sealing plate2, the inner insulating member12, and the negative-electrode current collector8are integrally fixed. The tip of the insert of the negative-electrode terminal9thus bent may be welded to the negative-electrode current collector8.

<Attachment of Current Collector to Electrode Assembly>

A method for attaching the negative-electrode current collector8to the negative-electrode core laminate of the wound electrode assembly3is described below.

The negative-electrode current collector8made of copper 0.8 mm in thickness is placed on the outer surface of the negative-electrode core laminate, for example, composed of 62 layers of the negative-electrode core5a, for example, made of copper 8 μm in thickness. The negative-electrode current collector8and the negative-electrode core laminate are placed between a horn90and an anvil91of an ultrasonic bonding apparatus illustrated inFIG.4, for example. The horn90is in contact with the outer surface of the layers of the negative-electrode core5a, and the anvil91is in contact with a surface of the negative-electrode current collector8opposite the surface of the negative-electrode current collector8in contact with the negative-electrode core5a.

The horn90is then vibrated for bonding between the layers of the negative-electrode core5aand for bonding between the negative-electrode core5aand the negative-electrode current collector8. The ultrasonic bonding conditions may include, but are not limited to, a horn load in the range of 1000 to 2500 N (100 to 250 kgf), a frequency in the range of 19 to 30 kHz, and a bonding time in the range of 300 to 800 ms. At a frequency of 20 kHz, the horn amplitude may range from 60% to 95% of the maximum amplitude (for example, 50 μm).

Ultrasonic vibration applied to the layered negative-electrode core5aand the negative-electrode current collector8removes an oxide film from the surfaces of the negative-electrode core5aand the negative-electrode current collector8by friction and enables solid-state bonding between the layers of the negative-electrode core5aand between the negative-electrode core5aand the negative-electrode current collector8, thus resulting in strong bonding between the layered negative-electrode core5a, that is, the negative-electrode core laminate and the negative-electrode current collector8.

As illustrated inFIG.4, a surface of the horn90to come into contact with the negative-electrode core5ahas a plurality of horn protrusions90a, and ultrasonic bonding is performed while the horn protrusions90aare engaged in the layered negative-electrode core5a.

As illustrated inFIG.4, a surface of the anvil91to come into contact with the negative-electrode current collector8has a plurality of anvil protrusions91a, and ultrasonic bonding is performed while the anvil protrusions91aare engaged in the negative-electrode current collector8.

FIGS.5A and5Bshow the layered negative-electrode core5a(negative-electrode core laminate) on the negative-electrode current collector8.FIG.5Ashows the front side of the negative-electrode core laminate, andFIG.5Bshows the front side of the negative-electrode current collector8.

As illustrated inFIG.5A, the layered negative-electrode core5ais bonded to the negative-electrode current collector8by ultrasonic bonding, and the layered negative-electrode core5ahas a bonding region81bonded to the negative-electrode current collector8. The bonding region81has recessed and raised portions. More specifically, the bonding region81has core recesses81xcorresponding to the horn protrusions90a. Each of the core recesses81xmay have a flat portion81x1at its bottom. The boundary between adjacent core recesses81xmay be raised.

The flat portion81x1at the bottom of each core recess81xpromotes friction behavior in the bonding region81during ultrasonic bonding and forms a strong bond between the layers of the negative-electrode core5aand between the negative-electrode core5aand the negative-electrode current collector8. The flat portion81x1may have an area in the range of 0.01 to 0.16 mm2.

As illustrated inFIG.5B, a surface of the negative-electrode current collector8in the region bonded to the layered negative-electrode core5aopposite the layered negative-electrode core5ahas current collector recesses8xcorresponding to the anvil protrusions91a. No flat portion may be formed at the bottom of each current collector recess8x, or a flat portion smaller than the flat portion81x1may be formed at the bottom of each current collector recess8x.

Any number of core recesses81xmay be formed in the bonding region81, and any number of current collector recesses8xmay be formed in the negative-electrode current collector8. For example, the number of current collector recesses8xmay be larger than the number of core recesses81x.

FIG.6is a cross-sectional view taken along the line VI-VI ofFIG.5A.

As illustrated inFIG.6, the bonding region81of the layered negative-electrode core5a(negative-electrode core laminate) includes a first region81awith a thickness smaller than Tn1and a second region81bwith a thickness larger than Tn1, wherein Tn1denotes the product of the thickness of a layer of the negative-electrode core5ain a region not bonded to the negative-electrode current collector8(a non-bonding region86inFIG.5A) and the number of layers of the negative-electrode core5ain the bonding region81. The bonding region81with such a structure in the negative-electrode core laminate can prevent the negative-electrode core5afrom being damaged or broken and form a strong bond between the negative-electrode core5aand the negative-electrode current collector8, as described later. In particular, the bonding strength (peel strength) between the layers of the negative-electrode core5ain the first region81ahigher than the bonding strength (peel strength) between the layers of the negative-electrode core5ain the second region81bcan result in a strong bond between the layered negative-electrode core5aand the negative-electrode current collector8and more effectively prevent the negative-electrode core5afrom being damaged or broken.

The smallest thickness Tn2in the first region81aand the largest thickness Tn3in the second region81bare controlled via the horn load, frequency, horn amplitude, bonding time, or the like of the ultrasonic bonding apparatus such that the bonding region81of the negative-electrode core laminate can have appropriate bonding strength, conductivity, and appearance. In particular, the thickness Tn2and the thickness Tn3can be controlled via the horn amplitude.

For the negative-electrode core5amade of copper, for example, Tn2/Tn1may range from 0.70 to 0.95, and Tn3/Tn1may range from 1.10 to 1.98, preferably 1.27 to 1.42. This can more reliably prevent the negative-electrode core5afrom being damaged or broken and can form a stronger bond between the negative-electrode core5aand the negative-electrode current collector8. The difference between the thickness Tn3and the thickness Tn2(Tn3−Tn2) may range from 0.2 to 0.4 mm.

Ultrasonic bonding may be performed such that the layer of the negative-electrode core5ain the first region81afarthest from the negative-electrode current collector8has an elongation percentage X of 20% or less by the ultrasonic bonding. This can more reliably prevent the negative-electrode core5afrom being damaged or broken. The “elongation percentage” is calculated by (the length of the negative-electrode core5aafter ultrasonic bonding—the length of the negative-electrode core5abefore ultrasonic bonding)/(the length of the negative-electrode core5abefore ultrasonic bonding)×100.

Ultrasonic bonding may be performed such that the layer of the negative-electrode core5ain the second region81bfarthest from the negative-electrode current collector8has an elongation percentage Y smaller than the elongation percentage X by the ultrasonic bonding. This can more reliably prevent the negative-electrode core5afrom being damaged or broken. For example, ultrasonic bonding may be performed such that the layer of the negative-electrode core5ain the second region81bfarthest from the negative-electrode current collector8has an elongation percentage Y of 5% or less by the ultrasonic bonding.

The thickness Tn2(the smallest thickness in the first region81a) may be larger than the smallest thickness Tn4of the portion of the negative-electrode current collector8bonded to the negative-electrode core5a. The difference between the largest thickness Tn5and the smallest thickness Tn4(Tn5−Tn4) of the portion of the negative-electrode current collector8bonded to the negative-electrode core5amay be larger than the difference between the thickness Tn1(the product of the thickness of a layer of the negative-electrode core5ain the non-bonding region86and the number of layers of the negative-electrode core5ain the bonding region81) and the thickness Tn2(Tn1-Tn2).

In the first region81a, the layers of the negative-electrode core5aare bonded together by solid-state bonding. More specifically, as illustrated inFIG.7, in the first region81a, layers of the negative-electrode core5abonded together by solid-state bonding form a solid-state bonding layer51, and a central layer52is disposed in a central portion of a layer of the negative-electrode core5ain the thickness direction (a portion between the solid-state bonding layers51formed on both faces of the negative-electrode core5a). In the central layer52, transformation of crystal grains is suppressed during ultrasonic bonding. The average grain size of copper crystal grains constituting the solid-state bonding layer51is smaller than the average grain size of copper crystal grains constituting the central layer52. More specifically, copper crystal grains constituting the solid-state bonding layer51have an aspect ratio (short diameter:long diameter) in the range of 1:1 to 1:2 and an average grain size (long diameter) in the range of 0.1 to 1.5 μm, and copper crystal grains constituting the central layer52have an aspect ratio (short diameter:long diameter) in the range of 1:1 to 1:4 and an average grain size (long diameter) in the range of 2 to 5 μm. Thus, the average grain size (long diameter) of copper crystal grains constituting the solid-state bonding layer51may be 75% or less, more specifically in the range of approximately 2% to 75%, of the average grain size (long diameter) of copper crystal grains constituting the central layer52.

The average grain size of copper crystal grains constituting the central layer52is smaller than the average grain size of copper crystal grains constituting the non-bonding region86of the negative-electrode core laminate (seeFIG.5A). More specifically, copper crystal grains constituting the central layer52have an aspect ratio (short diameter:long diameter) in the range of 1:1 to 1:4 and an average grain size (long diameter) in the range of 2 to 5 as described above, and copper crystal grains constituting the non-bonding region86have an aspect ratio (short diameter:long diameter) in the range of 1:1 to 1:4 and an average grain size (long diameter) in the range of 2.5 to 6 The average grain size (long diameter) of copper crystal grains constituting the central layer52is approximately 85% or less of the average grain size (long diameter) of copper crystal grains constituting the non-bonding region86. The average grain size (long diameter) of copper crystal grains constituting the central layer52preferably ranges from 70% to 95%, more preferably 75% to 90%, of the average grain size (long diameter) of copper crystal grains constituting the non-bonding region86. This can ensure the continuity of the crystal grain state between the bonding region81and the non-bonding region86.

The central layer52in the first region81acan effectively prevent the smallest thickness Tn2in the first region81afrom becoming excessively small and can effectively prevent the negative-electrode core5afrom being damaged or broken. As illustrated inFIG.7, when the thickness Tny of the central layer52of the negative-electrode core5ain the first region81a(the portion between the solid-state bonding layer51on one face of the negative-electrode core5aand the solid-state bonding layer51on the other face) after ultrasonic bonding is, for example, 80% or more of the thickness Tnx of a layer of the negative-electrode core5ain the non-bonding region86(seeFIG.5A) (substantially the same as the thickness of a layer of the negative-electrode core5abefore ultrasonic bonding), this can further effectively prevent the negative-electrode core5afrom being damaged or broken.

The width of the second region81bof the negative-electrode core laminate may decrease with the distance from the negative-electrode current collector8. This allows the second region81bof the negative-electrode core laminate to compensate for the elongation of a metal constituting the first region81aof the negative-electrode core laminate during ultrasonic bonding. This can further effectively prevent the negative-electrode core5afrom being damaged or broken. For example, the second region81bmay include a protrusion between adjacent core recesses81xin the bonding region81.

In the second region81b, the bonding strength between layers of the negative-electrode core5amay decrease with the distance from the negative-electrode current collector8. This can further effectively prevent the negative-electrode core5afrom being damaged or broken. For example, in the second region81b, a space may be formed between layers of the negative-electrode core5anear the top of the layered negative-electrode core5ain the lamination direction.

As described above, in the present embodiment, solid-state bonding between layers of the negative-electrode core5ain the bonding region81of the negative-electrode core laminate bonded to the negative-electrode current collector8by ultrasonic bonding can ensure bonding strength and decrease bonding resistance, and the central layer52within the negative-electrode core5ain which transformation of crystal grains is suppressed can ensure the continuity of the crystal grain state between the bonding region81and the non-bonding region86and can reduce the occurrence of cracking due to a lattice defect between the regions.

FIG.8Ais a schematic view of ultrasonic bonding between the negative-electrode core laminate and the negative-electrode current collector8according to the present embodiment when the horn90is in contact with the negative-electrode core laminate, andFIG.8Bis a schematic view of ultrasonic bonding between the negative-electrode core laminate and the negative-electrode current collector8when the negative-electrode core laminate is being bonded to the negative-electrode current collector8.

As illustrated inFIG.8A, in the present embodiment, when the horn90is in contact with the negative-electrode core laminate (the layered negative-electrode core5a), the horn90is not vibrated, and the layered negative-electrode core5ais only pressed such that the layered negative-electrode core5acan maintain its interface structure. Subsequently, as illustrated inFIG.8B, the horn90is vibrated in the direction parallel to the surface of the negative-electrode core5a. This removes surface oxides from each surface of the negative-electrode core5aand the negative-electrode current collector8by friction while the crystal grain state of each interior of the negative-electrode core5aand the negative-electrode current collector8is maintained. This exposes the crystal lattice {111} plane on each surface of the negative-electrode core5aand the negative-electrode current collector8, and the {111} planes are bonded together at the contact interface between layers of the negative-electrode core5aand at the contact interface between the negative-electrode core5aand the negative-electrode current collector8, thereby forming fine crystal grains and forming strong and stable bonding surfaces in the solid-state bonding state. Meanwhile, the crystal grain state before ultrasonic bonding is maintained in the negative-electrode core5aand the negative-electrode current collector8, and this can ensure the continuity of the crystal grain state between the bonding region81and the non-bonding region86and reduce the occurrence of cracking due to a lattice defect between the regions.

FIG.9Ais a schematic view of ultrasonic bonding between the negative-electrode core laminate and the negative-electrode current collector8according to the comparative example when the horn90is in contact with the negative-electrode core laminate, andFIG.9Bis a schematic view of ultrasonic bonding between the negative-electrode core laminate and the negative-electrode current collector8when the negative-electrode core laminate is being bonded to the negative-electrode current collector8.

As illustrated inFIG.9A, in the comparative example, pressure and vibration are applied to the negative-electrode core laminate (the layered negative-electrode core5a) when the horn90is in contact with the negative-electrode core laminate. Thus, in the portion of the negative-electrode core laminate in which the horn90is engaged, the negative-electrode core5ais damaged or broken. Subsequently, as illustrated inFIG.9B, while the negative-electrode core laminate is pressed and vibrated, the negative-electrode core laminate is bonded to the negative-electrode current collector8. The damage or breakage of the negative-electrode core5aspreads over the entire bonding region81, and ultrasonic vibration breaks crystal grains. Consequently, after the negative-electrode core laminate is bonded to the negative-electrode current collector8, the crystal grain state before bonding is not observed, or alternate lamination of central layers and solid-state bonding layers of the negative-electrode core5ais not observed, and fine crystal grains are distributed over the entire bonding region81. Thus, the crystal grain state of the bonding region81is significantly different from the crystal grain state of the non-bonding region86, and therefore cracking due to a lattice defect tends to occur between the bonding region81and the non-bonding region86.

FIG.10Ais a photograph of a cross section of the negative-electrode core laminate before bonding to the negative-electrode current collector8according to the present embodiment, andFIG.10Bis a SEM photograph of the cross section.FIGS.10A and10Bshow that interfaces between layers of the negative-electrode core5a, that is, copper core can be observed before bonding (arrows inFIG.10B).

FIG.11Ais a photograph of a cross section of the negative-electrode core laminate after bonding to the negative-electrode current collector8according to the present embodiment, andFIG.11Bis a SEM photograph of the cross section.FIG.11Bis an enlarged view of the circular area ofFIG.11A.FIG.11Ashows that in the present embodiment the negative-electrode core5ais layered without damage or breakage after bonding.FIG.11Bshows that, after bonding, the solid-state bonding layers (indicated by the arrows) formed by solid-state bonding between layers of copper core can be observed, the central layers the state of which before ultrasonic bonding is almost unchanged can also be observed, and furthermore alternately laminated solid-state bonding layers and central layers can be observed. As compared with the SEM photograph ofFIG.10B, fine crystal grains are distributed on the copper core surface, that is, in the solid-state bonding layers, and the crystal grain state before bonding is almost maintained within the copper core, that is, in the central layers.

FIG.12Ais a photograph of a cross section of the negative-electrode core laminate after bonding to the negative-electrode current collector8according to the comparative example, andFIG.12Bis a SEM photograph of the cross section.FIG.12Bis an enlarged view of the circular area ofFIG.12A.FIG.12Ashows that in the comparative example finer crystal grains are formed in a recessed portion of the negative-electrode core5aformed by ultrasonic bonding in the negative-electrode core laminate, and a discontinuous crystal grain state responsible for cracking is produced between the recessed portion and its exterior.FIG.12Bshows that, after bonding, alternately laminated solid-state bonding layers of the copper core and central layers are not observed. As compared with the SEM photograph ofFIG.10B, fine crystal grains are widely distributed, and a discontinuous crystal grain state responsible for cracking is produced.

FIG.13Ais an image quality (IQ) map of the crystalline state of the negative-electrode core laminate and the negative-electrode current collector8according to the present embodiment,FIG.13Bis a direction map of a {111} plane, andFIG.13Cis a pole figure of the {111} plane.

The IQ map ofFIG.13Ashows that crystallinity is decreased near the solid-state bonding layer51formed between layers of the negative-electrode core5aand near the solid-state bonding layer51formed between the negative-electrode core5aand the negative-electrode current collector8, indicating the presence of many fine crystal grains. The direction map ofFIG.13Bshows the presence of the solid-state bonding layers formed by solid-state bonding between layers of the negative-electrode core5aand the presence of the central layers the state of which before ultrasonic bonding is almost unchanged, indicating the presence of many {111} crystal faces on both sides of each solid-state bonding layer in the alternately laminated solid-state bonding layers and central layers. The pole figure ofFIG.13Cshows a relatively symmetrical distribution and probably indicates that the {111} crystal face is parallel to the solid-state bonding layer.

FIG.14Ais an image quality (IQ) map of the crystalline state of the negative-electrode core laminate and the negative-electrode current collector8according to the comparative example,FIG.14Bis a direction map of a {111} plane, andFIG.14Cis a pole figure of the {111} plane.

The IQ map ofFIG.14Ashows no solid-state bonding layer of the negative-electrode core5aor no solid-state bonding layer formed between the negative-electrode core5aand the negative-electrode current collector8in the comparative example, indicating the wide presence of many fine crystal grains. The direction map ofFIG.14Bshows the random distribution of the {111} crystal faces. The pole figure ofFIG.14Cshows an asymmetrical distribution and indicates that the {111} crystal faces are randomly aligned.

As described above, the crystal grain state of the bonding layer formed between the negative-electrode core laminate and the negative-electrode current collector is clearly different between the present embodiments and the comparative example. The present embodiments, which ensure the continuity of the crystal grain state between the bonding region and the non-bonding region, can reduce the occurrence of cracking due to a lattice defect between the regions, whereas the comparative example, which has a discontinuous crystal grain state between the bonding region and the non-bonding region, tends to have a crack due to a lattice defect between the regions.

In the present embodiments, the ultrasonic bonding conditions for strong bonding can be reliably determined due to the clear target crystal grain state in bonding. By contrast, in the comparative example, without attention to the continuity of the crystal grain state between the bonding region and the non-bonding region, the ultrasonic bonding conditions for strong bonding are difficult to reliably determine due to various variation factors in bonding.

Examples 1 to 3

A negative-electrode core laminate composed of 62 layers of negative-electrode core5amade of copper 8 μm in thickness was bonded to a negative-electrode current collector8made of copper 0.8 mm in thickness by ultrasonic bonding under different conditions.

In a first region81ain a bonding region81of the layered negative-electrode core5a(negative-electrode core laminate), in which core recesses81xwere formed, the thickness Tnx of a layer of the negative-electrode core5ain a non-bonding region86was adjusted to be 8 μm and the thickness Tny of a central layer52of the negative-electrode core5awas adjusted to be 7.2 μm in Example 1, Tny was adjusted to be 6.8 μm in Example 2, and Tny was adjusted to be 6.4 μm in Example 3. An ultrasonic bonding apparatus with a frequency of 20 kHz was used in these examples.

Comparative Example 1

A negative-electrode core laminate composed of 62 layers of negative-electrode core5amade of copper 8 μm in thickness was bonded to a negative-electrode current collector8made of copper 0.8 mm in thickness by ultrasonic bonding under the conditions described in Patent Document 1.

In a first region81ain a bonding region81of the layered negative-electrode core5a(negative-electrode core laminate), in which core recesses81xwere formed, alternately laminated solid-state bonding layers formed by solid-state bonding between layers of the negative-electrode core5aand central layers the state of which before ultrasonic bonding was almost unchanged were not observed, and fine crystal grains were distributed throughout the negative-electrode core laminate. In Comparative Example 1, there was no central layer52, and Tnx=15 μm and Tny=0. The ultrasonic bonding apparatus with a frequency of 20 kHz was also used in Comparative Example 1.

Table 1 lists the ultrasonic bonding conditions, the cross-sectional state of the negative-electrode core laminate after bonding (the cross-sectional state of the first region81a), the presence or absence of foil breakage at the boundary between the bonding region and the non-bonding region (that is, a crack in the negative-electrode core laminate), the electrical resistance of the bonded portion between the negative-electrode core laminate and the negative-electrode current collector8in Examples 1 to 3 and Comparative Example 1. In Table 1, Tnx=8.

Table 1 shows that ultrasonic bonding under the conditions of Examples 1 to 3 ensured the continuity of the crystal grain state between the bonding region and the non-bonding region, and no foil breakage, that is, no crack in negative-electrode core laminate was observed. The thickness Tny of the central layer52could be decreased to reduce the electrical resistance of the bonded portion. By contrast, in the comparative example, no central layer was formed, and the crystal grain state between the bonding region and the non-bonding region was discontinuous. Thus, foil breakage, that is, a crack in the negative-electrode core laminate was observed.

Although the embodiments (including the examples; the same applies hereinafter) of the present invention have been described, the present invention is not limited to these embodiments, and various modifications are possible within the scope of the present invention. These embodiments are only examples and are not intended to limit the present invention, applications thereof, or uses thereof.

For example, although a rectangular secondary battery including a flat wound electrode assembly was exemplified as a secondary battery in the present embodiments, the present invention may also be applied to another electrode assembly including a negative-electrode core laminate, for example, an electrode assembly including positive electrodes and negative electrodes alternately laminated with a separator interposed therebetween, wherein a negative-electrode core laminate composed of a negative-electrode current collector tab protruding from each negative electrode is bonded to a negative-electrode current collector by ultrasonic bonding. The type of secondary battery is also not particularly limited, and the present invention can be applied to various batteries containing different electrode assembly constituent materials or electrolytes as well as lithium secondary batteries. The present invention can also be applied to secondary batteries of various shapes (cylindrical etc.) as well as rectangular batteries. The shape of the electrode assembly, the electrode active materials of the positive electrode and the negative electrode, and the constituent materials of the electrolyte can also depend on the application.

In the present embodiment, the negative-electrode core is made of copper or a copper alloy, and the negative-electrode current collector is made of copper or a copper alloy. However, the positive-electrode core may be made of copper or a copper alloy, and the positive-electrode current collector may be made of copper or a copper alloy. A core made of copper or a copper alloy preferably has a thickness in the range of 5 to 30 μm, more preferably 6 to 15 μm, for example. The number of layers of copper or copper alloy core preferably ranges from 20 to 100, more preferably 40 to 80, for example.

A current collector made of copper or a copper alloy preferably has a thickness in the range of 0.5 to 2.0 mm, more preferably 0.8 to 1.5 mm, for example.

While detailed embodiments have been used to illustrate the present invention, to those skilled in the art, however, it will be apparent from the foregoing disclosure that various changes and modifications can be made therein without departing from the spirit and scope of the invention. Furthermore, the foregoing description of the embodiments according to the present invention is provided for illustration only, and is not intended to limit the invention.