Patent Description:
There has been known a secondary battery in which a negative electrode constituting an electrode assembly is electrically connected through a negative electrode current collector to a negative electrode terminal on a sealing plate or the like. Typically, a copper foil is used as the core of the negative electrode, and the negative electrode current collector is welded to the copper foil. For example, Patent Literature <NUM> discloses a negative electrode core made of a copper foil having a dynamic friction coefficient of <NUM> or less between one surface and the other surface of the copper foil, with an oxide film and/or a rust-proof coating having a thickness of from <NUM> to <NUM> formed on a surface of the copper foil. By using such a negative electrode core, Patent Literature <NUM> has achieved the improved weldability with the negative electrode current collector. Improving the weldability with the negative electrode current collector has also been proposed by controlling the surface roughness, glossiness, and so on (see, for example, Patent Literature <NUM> and <NUM>).

Another method for improving the weldability between the negative electrode core and the negative electrode current collector has been known, in which a protruding portion called a projection is formed on the surface of the negative electrode current collector contacting the negative electrode core (see, for example, Patent Literature <NUM>). The projection enables concentration of the current at the tip of the projection during resistance-welding, reducing the reactive current and achieving efficient and excellent resistance-welding.

There is a case where voids are generated at the welded portion between the negative electrode core and the negative electrode current collector. In particular, the possibility of generating the voids increases according to the increase of the number of layers of the negative electrode core to be welded to the negative electrode current collector. When the voids are generated, drawbacks such as unwelded portions or high resistance may occur at the welded portion. It is therefore desired to minimize the generation of the voids. With respect to the existing techniques disclosed in Patent Literature <NUM> to <NUM>, there is still room for improvement in terms of suppressing the voids.

Accordingly, it is an object of the present disclosure to provide a method for securely welding a negative electrode core and a negative electrode current collector while minimizing the generation of voids at a welded portion of the core and the current collector.

A method for manufacturing a secondary battery according to an aspect of the present disclosure is a method for manufacturing a secondary battery including an electrode assembly and a negative electrode current collector, the electrode assembly including a positive electrode, a negative electrode, and a separator, and formed by stacking the positive electrode and the negative electrode with the separator interposed therebetween, the negative electrode including a negative electrode core made of a copper foil having a surface roughness of <NUM>µητ or less and a glossiness of from <NUM> to <NUM>, and a negative electrode mixture layer formed on a surface of the negative electrode core except for an exposure region where a surface of the negative electrode core is exposed, the electrode assembly including a core stacked portion formed by stacking a plurality of the exposure regions of the negative electrode, the negative electrode current collector including a projection having a height of from <NUM> to <NUM> on at least one of a first member and a second member constituting the negative electrode current collector, the method for manufacturing the secondary battery includes resistance-welding the negative electrode current collector and the core stacked portion in a state where the core stacked portion is sandwiched between the first member and the second member from both sides, and the projection is in contact with the core stacked portion.

A secondary battery according to an aspect of the present disclosure is a secondary battery including an electrode assembly and a negative electrode current collector, the electrode assembly including a positive electrode, a negative electrode, and a separator, and formed by stacking the positive electrode and the negative electrode with the separator interposed therebetween, in which the negative electrode includes a negative electrode core made of a copper foil having a surface roughness of <NUM> or less and a glossiness of from <NUM> to <NUM>, and a negative electrode mixture layer formed on a surface of the negative electrode core except for an exposure region where a surface of the negative electrode core is exposed, the electrode assembly includes a core stacked portion formed by stacking a plurality of the exposure regions of the negative electrode, the core stacked portion is sandwiched between a first member and a second member, which constitute the negative electrode current collector, from both sides, and welded with the first member and the second member to obtain a nugget formed by the welding, and no void having a length of at least <NUM> is present at an interface between the core stacked portion and the negative electrode current collector, while a maximum diameter of the nugget is at least <NUM>.

In the method for manufacturing the secondary battery according to the present disclosure, it is possible to minimize the generation of the voids at the welded portion of the negative electrode core and the negative electrode current collector, enabling secure welding of the core and the current collector. The secondary battery according to the present disclosure achieves a high strength and low resistance welded portion between the negative electrode core and the negative electrode current collector.

An example of an embodiment of the present disclosure will be described in detail below. The drawings referred to in the description of the embodiment are schematically illustrated, and the dimensional proportions and the like of the components drawn in the drawings may differ from the actual components. Specific dimensional ratios and the like should be determined by referring to the following description. In the specification, the phrase "from numerical value A to numerical value B" means "numerical value A or higher and numerical value B or lower," unless otherwise specified.

<FIG> is a perspective view illustrating an appearance of a secondary battery <NUM> of an example of an embodiment. <FIG> is a perspective view of an electrode assembly <NUM> and a sealing plate <NUM> constituting the secondary battery <NUM> of the example of the embodiment. The secondary battery <NUM> illustrated in <FIG> is a rectangular battery with a rectangular outer can <NUM>. The outer body of the battery may not be the outer can <NUM> and may be made of, for example, a laminate sheet including a metal layer and a resin layer, or may be a cylindrical outer can.

As illustrated in <FIG>, the secondary battery <NUM> includes the electrode assembly <NUM>, an electrolyte, and the rectangular outer can <NUM> housing the electrode assembly <NUM> and the electrolyte. The outer can <NUM> is a flat rectangular metal container with an opening. The electrode assembly <NUM> is a wound electrode assembly in which a positive electrode <NUM> and a negative electrode <NUM> are wound spirally through a separator <NUM> and formed into a flat shape. The positive electrode <NUM>, the negative electrode <NUM>, and the separator <NUM> are all long and belt-shaped components. The secondary battery <NUM> also includes a positive electrode current collector <NUM> connected to the positive electrode <NUM> and a negative electrode current collector <NUM> connected to the negative electrode <NUM>. The electrode assembly may be a stacked electrode assembly in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked on top of the other through the separator <NUM>.

The electrolyte may be an aqueous electrolyte or a nonaqueous electrolyte. In the present embodiment, a nonaqueous electrolyte is used. The nonaqueous electrolyte includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. As the nonaqueous solvent, there may be used, for example, esters, ethers, nitriles, amides, and a mixed solvent or the like of two or more thereof. The nonaqueous solvent may include a halogen substitute obtained by substituting at least a part of hydrogen of the above solvent with halogen atoms such as fluorine. As the electrolyte salt, a lithium salt such as LiPF<NUM> or the like is used. The electrolyte solution may not be a liquid electrolyte, and may be a solid electrolyte for which a gel-like polymer or the like is used.

The secondary battery <NUM> includes a positive electrode terminal <NUM> which is electrically connected to the positive electrode <NUM> through the positive electrode current collector <NUM>, and a negative electrode terminal <NUM> which is electrically connected to the negative electrode <NUM> through the negative electrode current collector <NUM>. The secondary battery <NUM> also includes a sealing plate <NUM> that closes the opening of the outer can <NUM>. The outer can <NUM> and the sealing plate <NUM> are made of a metal material that mainly contains, for example, aluminum.

In the present embodiment, the sealing plate <NUM> has an elongated rectangular shape, with the positive electrode terminal <NUM> located on one end side of the sealing plate <NUM> in the longitudinal direction and the negative electrode terminal <NUM> located on the other end side. The positive electrode terminal <NUM> and the negative electrode terminal <NUM> are external connection terminals connected to other secondary batteries <NUM> or loads, and are fixed to the sealing plate <NUM> through an insulating member. The sealing plate <NUM> usually includes a gas discharge valve <NUM> and an electrolyte injection portion <NUM>.

The electrode assembly <NUM> includes a flat portion and a pair of curved portions. The electrode assembly <NUM> is housed in the outer can <NUM> in such a manner that the winding axis of the electrode assembly <NUM> is in the same direction as the lateral direction of the outer can <NUM> (direction in which the positive electrode terminal <NUM> and the negative electrode terminal <NUM> are disposed), and a width of the electrode assembly <NUM>, in which the pair of curved portions is located, is in the same direction as the height direction of the secondary battery <NUM> (direction orthogonal to the lateral direction and the thickness direction of the outer can <NUM>). As will be described in detail later, a core stacked portion <NUM> of the positive electrode <NUM> is formed at one end of the electrode assembly <NUM> in the axial direction, and a core stacked portion <NUM> of the negative electrode <NUM> is formed at the other end of the electrode assembly in the axial direction. The core stacked portions are each electrically connected to the corresponding external connection terminal through the current collector. An insulating electrode assembly holder (insulating sheet) may be placed between the electrode assembly <NUM> and an inner surface of the outer can <NUM>.

The positive electrode <NUM> includes a positive electrode core <NUM> and a positive electrode mixture layer (not illustrated) formed on the surface of the positive electrode core <NUM> except for an exposure region <NUM> where a surface of the positive electrode core <NUM> is exposed. For the positive electrode core <NUM>, there is used a metal foil made of, for example, aluminum which is stable in the potential range of the positive electrode <NUM> within the battery operating voltage range, or a film with such a metal disposed on the surface layer. The positive electrode mixture layer includes a positive electrode active material such as a lithium transition metal compound, a conductive material such as acetylene black, and a binder material such as polyvinylidene fluoride. The positive electrode mixture layer is formed on both sides of the positive electrode core <NUM>.

The positive electrode <NUM> includes the exposure region <NUM> where the positive electrode mixture layer is not formed and the surface of the positive electrode core <NUM> is exposed. The exposure region <NUM> is formed like a belt at one end in the width direction of the positive electrode <NUM> along the length of the positive electrode <NUM>. In addition, the exposure region <NUM> is formed on both sides of the positive electrode <NUM> with a substantially fixed width from one end of the positive electrode <NUM> in the width direction. The positive electrode <NUM> is wound so that the exposure regions <NUM> are placed at one end in the axial direction of the electrode assembly <NUM> and the exposure regions <NUM> overlap each other without the separator <NUM> interposed therebetween.

The negative electrode <NUM> includes a negative electrode core <NUM> and a negative electrode mixture layer (not illustrated) formed on the surface of the negative electrode core <NUM> except for an exposure region <NUM> where a surface of the negative electrode core <NUM> is exposed. The negative electrode mixture layer includes a negative electrode active material such as graphite or an Si-containing compound, and a binder material such as styrene-butadiene rubber (SBR). The negative electrode mixture layer is formed on both sides of the negative electrode core <NUM>. A thickness of the negative electrode core <NUM> is, for example, from <NUM> to <NUM>, and preferably <NUM> or less (at least <NUM>).

The negative electrode core <NUM> is made of a copper foil having a surface roughness of <NUM> or less and a glossiness of from <NUM> to <NUM>. The copper foil is mainly composed of Cu and may contain small amounts of metal elements other than Cu, such as Cr. The negative electrode core <NUM> needs to be composed of a material including copper foil having a surface roughness of <NUM> or less and a glossiness of from <NUM> to <NUM>. By using the copper foil with the surface roughness of <NUM> or less and the glossiness of from <NUM> to <NUM> as the negative electrode core <NUM>, a high-strength and low-resistance welded portion of the negative electrode core <NUM> and the negative electrode current collector <NUM> can be formed with a synergistic action of a projection <NUM> which will be described later.

The surface roughness of the negative electrode core <NUM> (copper foil) is preferably from <NUM> to <NUM> on both sides. A preferred example of the surface roughness of the negative electrode core <NUM> is from <NUM> to <NUM>, representing a surface with less irregularities. The surface roughness is determined by a measurement method specified in JIS B <NUM><NUM>, and measured by a surface roughness measuring instrument (model SE1700 α manufactured by Kosaka Laboratory, Ltd. ) using a noncontact method.

The glossiness of the negative electrode core <NUM> (copper foil) is preferably from <NUM> to <NUM> on both sides. A preferable example of the glossiness of the negative electrode core <NUM> is from <NUM> to <NUM>. The glossiness is measured by a surface glossiness measuring instrument (micro-gloss series manufactured by BYK) at an incident angle of <NUM> degrees in accordance with a measurement method specified in JIS (Z8741).

The negative electrode <NUM> includes the exposure region <NUM> where the negative electrode mixture layer is not formed and the surface of the negative electrode core <NUM> is exposed. The exposure region <NUM> is formed like a belt at one end in the width direction of the negative electrode <NUM> along the length of the negative electrode <NUM>. In addition, the exposure region <NUM> is formed on both sides of the negative electrode <NUM> with a substantially fixed width from one end of the negative electrode <NUM> in the width direction. The width of the exposure region <NUM> is, for example, at least <NUM>. The negative electrode <NUM> is wound so that the exposure regions <NUM> are placed at one end in the axial direction of the electrode assembly <NUM> and the exposure regions <NUM> overlap each other without any separator <NUM> being disposed therebetween.

The electrode assembly <NUM> includes the core stacked portion <NUM> formed by stacking a plurality of exposure regions <NUM> of the positive electrode <NUM>, and the core stacked portion <NUM> formed by stacking a plurality of exposure regions <NUM> of the negative electrode <NUM>. As described above, the exposure region <NUM> of the positive electrode <NUM> is formed at one end of the electrode assembly <NUM> in the axial direction, and the exposure region <NUM> of the negative electrode <NUM> is formed at the other end of the electrode assembly <NUM> in the axial direction. The positive electrode <NUM> and the negative electrode <NUM> are arranged so that the positive electrode mixture layer and the negative electrode mixture layer face each other through the separator <NUM>, but the positive and negative electrodes are displaced from each other in the axial direction of the electrode assembly <NUM> so that the exposure region <NUM> of the positive electrode <NUM> does not face the negative electrode <NUM> and the exposure region <NUM> of the negative electrode <NUM> does not face the positive electrode <NUM>.

The core stacked portions <NUM>, <NUM> are each formed by stacking, for example, more than forty layers of the positive electrode core <NUM> and the negative electrode core <NUM>, respectively. The number of the stacked layers of the core stacked portions <NUM>, <NUM> depends on the number of turns of the positive electrode <NUM> and the negative electrode <NUM>. As the number of turns of the positive electrode <NUM> and the negative electrode <NUM> increases, the number of stacked layers increases. Increased number of turns of the positive electrode <NUM> and the negative electrode <NUM> leads to higher capacity and output of the secondary battery <NUM>. On the other hand, the increased number of stacked layers of the core in the core stacked portions <NUM>, <NUM> often causes welding defects, such as generation of more voids at the interface with the current collectors, due to the varied surface condition of the core. In particular, the welding of the core stacked portion <NUM> made of a copper foil and the negative electrode current collector <NUM> becomes a problem.

In the following, the welded portion of the core stacked portion and the current collector will be described using the negative electrode <NUM> as an example. The same configuration can be applied to the welded portions of the core stacked portion <NUM> and the positive electrode current collector <NUM> of the positive electrode <NUM> as in the case of the negative electrode <NUM> described below. Alternatively, a conventionally known configuration may be applied to the welded portion of the core stacked portion <NUM> and the positive electrode current collector <NUM>.

The negative electrode current collector <NUM> is composed of, for example, a metal mainly containing copper. The negative electrode current collector <NUM> preferably includes a first member <NUM> and a second member <NUM>. The core stacked portion <NUM> is sandwiched between the first member <NUM> and the second member <NUM> from both sides in the thickness direction of the electrode assembly <NUM> and welded to the first member <NUM> and the second member <NUM>. The core stacked portion <NUM> is compressed in the thickness direction of the electrode assembly <NUM>, and the overlapping exposure regions <NUM> are brought into contact with each other.

The first member <NUM>, which constitutes the negative electrode current collector <NUM>, is welded to one side of the core stacked portion <NUM>, extending to the sealing plate <NUM> side and is connected to the negative electrode terminal <NUM>. The second member <NUM> is a rectangular-shaped plate member, and its end portion may be bent to the side opposite to the core stacked portion <NUM> from the viewpoint of, for example, preventing generation of spatters during welding. The second member <NUM> is welded to the other side of the core stacked portion <NUM> and not connected to other members. Therefore, the first member <NUM> is the member having the current collector function of electrically connecting the negative electrode terminal <NUM> to the negative electrode <NUM>. The second member <NUM> is regarded as a receiving member to ensure excellent weldability of the core stacked portion <NUM> and the negative electrode current collector <NUM> by sandwiching the core stacked portion <NUM> with the first member <NUM>.

<FIG> is a sectional view of the welded portion and its vicinity between the core stacked portion <NUM> and the negative electrode current collector <NUM>. As illustrated in <FIG>, a nugget <NUM> is formed by welding at the welded portion of the core stacked portion <NUM> and the negative electrode current collector <NUM>. The nugget <NUM> is a lump-like region where the negative electrode core <NUM>, which forms the core stacked portion <NUM>, and the negative electrode current collector <NUM> are melted. A large nugget <NUM> is formed at the welded portion of the core stacked portion <NUM> and the negative electrode current collector <NUM>, with a maximum diameter (X) of preferably at least <NUM>. The nugget <NUM> is formed in a spherical shape, for example, with its center located at the center portion of the core stacked portion <NUM> in the thickness direction, although the diameter usually somewhat varies. The maximum diameter (X) of the nugget <NUM> refers to a maximum span of the diameter of the nugget <NUM>.

As described above, the core stacked portion <NUM> is sandwiched between the first member <NUM> and the second member <NUM> and welded from both sides, and includes the nugget <NUM> formed by welding. A contact length (Y) between the nugget <NUM> of the core stacked portion <NUM> and the first and second members <NUM> and <NUM>, respectively, is preferably at least <NUM>. There is no void, like the one illustrated in <FIG> described later, having the length (maximum span length) of at least <NUM> at the interface between the core stacked portion <NUM> and the negative electrode current collector <NUM> (the first member <NUM> and the second member <NUM>). This means that the core stacked portion <NUM> and the negative electrode current collector <NUM> are welded together at a high strength and a low resistance.

A ratio of the contact length (Y) between the nugget <NUM> and the negative electrode current collector <NUM> to the maximum diameter (X) of the nugget <NUM> (Y/X) is preferably at least <NUM>%, more preferably at least <NUM>%, and most preferably at least <NUM>%. For example, assuming that maximum diameter (X) of the nugget <NUM> is the same, the contact length (Y) becomes longer and the Y/X becomes higher when less voids are present at the interface between the core stacked portion <NUM> and the negative electrode current collector <NUM>.

Although not illustrated in <FIG> (see <FIG> above <FIG> below), it is preferable to provide an insulating sheet <NUM> with a hole <NUM> having a diameter of from <NUM> to <NUM> between the core stacked portion <NUM> and the negative electrode current collector <NUM>. Such a diameter leads to prevention of melting of the insulating sheet during resistance-welding.

In the following, by referring to <FIG>, an example of a method for manufacturing the secondary battery <NUM> having the above configuration will be described in detail. <FIG> illustrates the welding process of the core stacked portion <NUM> and the negative electrode current collector <NUM>, and <FIG> illustrate the second member <NUM> constituting the negative electrode current collector <NUM> before being welded to the core stacked portion <NUM>.

As illustrated in <FIG>, in the manufacturing process of the secondary battery <NUM>, a pair of electrode rods <NUM> is used to resistance-weld the core stacked portion <NUM> and the negative electrode current collector <NUM>. The manufacturing process of the secondary battery <NUM> includes the following steps:.

The manufacturing process of the secondary battery <NUM> further includes fabricating the positive electrode <NUM>, fabricating the negative electrode <NUM>, fabricating the electrode assembly <NUM>, welding the current collector and the external connection terminal, and assembling the components of the secondary battery <NUM>. The negative electrode <NUM> can be fabricated by coating both sides of the negative electrode core <NUM> made of, for example, a long copper foil with a negative electrode mixture slurry containing a negative electrode active material, a binder material, and the like, except for the belt-like exposure region <NUM> along the longitudinal direction, followed by drying and rolling the coated film to form the negative electrode mixture layer on both sides of the negative electrode core <NUM>. The positive electrode <NUM> can also be fabricated in the same way as the negative electrode <NUM> using the mixture slurry.

The electrode assembly <NUM> is fabricated by spirally winding the positive electrode <NUM> and the negative electrode <NUM> through the separator <NUM> to form the core stacked portions <NUM>, <NUM>, followed by press-forming into a flat shape, thus fabricating the electrode assembly <NUM>. It is also possible to fabricate the electrode assembly <NUM> by winding the positive electrode <NUM> and the negative electrode <NUM> in a flat shape. The positive electrode <NUM> and the negative electrode <NUM> are made to overlap each other through the separator <NUM> so that the exposure regions <NUM> and <NUM> are located on opposite sides, the exposure region <NUM> does not overlap the negative electrode <NUM> and the separator <NUM>, and the exposure region <NUM> does not overlap the positive electrode <NUM> and the separator <NUM>. After that, the positive electrode <NUM> and the negative electrode <NUM> are wound using a predetermined winding core to fabricate the electrode assembly <NUM>.

In the example illustrated in <FIG>, the projection <NUM> is formed on the second member <NUM> that constitutes the negative electrode current collector <NUM>. The projection <NUM> is a protruding portion that contacts the core stacked portion <NUM> and protrudes toward the core stacked portion <NUM> side. The projection <NUM> formed on the negative electrode current collector <NUM> enables concentration of current at the tip of the projection <NUM> during resistance-welding and decreases a reactive current, thus achieving efficient and excellent resistance-welding. The projection <NUM> may be formed only on the first member <NUM>, or on both the first member <NUM> and the second member <NUM>.

The surface of the negative electrode current collector <NUM> (first member <NUM> and second member <NUM>) that contacts the core stacked portion <NUM> (hereinafter may be referred to as the "contact surface") is substantially flat except for the portion where the projection <NUM> is formed. For example, the thickness of the first member <NUM> is from <NUM> to <NUM>, and the thickness of the second member <NUM> is from <NUM> to <NUM>. As in the present embodiment, when the projection <NUM> is formed only on one of the two members sandwiching the core stacked portion <NUM>, it is preferable to decrease the thickness of the second member <NUM> where the projection <NUM> is formed compared to the thickness of the first member <NUM> where the projection <NUM> is not formed. The difference in thickness stabilizes the overall thermal balance and in turn leads to stabilization of the welding.

In the present embodiment, the projection <NUM> having a height (h) of from <NUM> to <NUM> is formed on the contact surface of the second member <NUM>. By controlling the height (h) of the projection <NUM> within the above range, the generation of the voids between the core stacked portion <NUM> and the negative electrode current collector <NUM> is largely suppressed compared to the case where the height (h) is outside the above range, so that a well-formed nugget <NUM> can be obtained.

The height (h) of the projection <NUM> is preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>, and most preferably from <NUM> to <NUM>. The height (h) of the projection <NUM> refers to the length along the thickness direction of the negative electrode current collector <NUM> from the flat region of the contact surface of the negative electrode current collector <NUM> to the tip of the projection <NUM>. The contact surface of the second member <NUM> is flat except for the region where the projection <NUM> is formed. The above range is set because, if the height of the projection <NUM> exceeds <NUM>, the electrode assembly <NUM> may tilt by the pressure applied prior to the resistance-welding, causing a change in contact resistance between the projection <NUM> and the core stacked portion <NUM>.

The projection <NUM> may be formed in, for example, a substantially trapezoidal shape with a flat tip in a cross-sectional view, but is preferably formed in a rounded-hill shape. By forming the projection <NUM> in a rounded-hill shape, it is possible to increase the concentration of the current at the tip of the projection <NUM>, enabling more efficient and better resistance-welding. A diameter (d) of the rounded-hill shaped projection <NUM> is preferably controlled in a range from <NUM> to <NUM>. By controlling the diameter (d) within this range, the generation of the voids is minimized compared to the case where the diameter (d) is outside the above range, so that the well-formed nugget <NUM> can be obtained.

A plurality of projections <NUM> may be formed on the contact surface of the second member <NUM>, but it is preferable to form one projection on the second member <NUM> from the viewpoint of current concentration during resistance-welding. The projection <NUM> may be formed on each contact surface of the first member <NUM> and second member <NUM>. The projection <NUM> may be formed on any part of the contact surface of the second member <NUM> on the condition that the above dimensions are satisfied and the welding operation is not interfered. When the projection <NUM> is formed on each of the first member <NUM> and the second member <NUM>, the projections <NUM> are formed to face each other across the core stacked portion <NUM>.

The projection <NUM> is formed, for example, by pressing the second member <NUM> from the surface opposite to the contact surface. A recess portion <NUM> is formed, therefore, in the second member <NUM> on the surface opposite to the projection <NUM> (contact surface) at a position where the projection <NUM> and the second member <NUM> overlap in the thickness direction. Although the projection <NUM> melts and collapses during the resistance-welding of the core stacked portion <NUM> and the negative electrode current collector <NUM>, the shape of the recess portion <NUM> remains, so that the shape and dimensions of the projection <NUM> can be estimated from the shape and dimensions of the recess portion <NUM>, the thickness of the second member <NUM>, and the like.

A diameter (D) of the recess portion <NUM> is, for example, from <NUM> to <NUM>, and preferably from <NUM> to <NUM>. As illustrated in <FIG>, the recess portion <NUM> is formed in a substantially trapezoidal shape in cross-sectional view, with the diameter decreasing toward the projection <NUM> side. In this case, the diameter (D) means the maximum diameter at the entrance of the recess portion <NUM>. A depth (H) of the recess portion <NUM> is, for example, from <NUM> to <NUM>, and preferably from <NUM> to <NUM>. When the projection <NUM> is formed on the first member <NUM>, the recess portion <NUM> is formed in the first member <NUM>.

As described above, in the manufacturing process of the secondary battery <NUM>, the resistance-welding of the core stacked portion <NUM> and the negative electrode current collector <NUM> is performed in a state where the core stacked portion <NUM> is sandwiched between the first and second members <NUM> and <NUM> and the projection <NUM> is pressed against the core stacked portion <NUM>. At this time, the insulating sheet <NUM> is placed between the core stacked portion <NUM> and the first member <NUM>, and the insulating sheet <NUM> is also placed between the core stacked portion <NUM> and the second member <NUM>. In the resistance-welding, the pair of electrode rods <NUM> is used to pressurize the core stacked portion <NUM> and the negative electrode current collector <NUM> from both sides in the thickness direction, while applying an electric current to generate Joule heat, to melt the components and form the nugget <NUM>.

The resistance-welding is performed preferably when the insulating sheet <NUM> with a hole <NUM> having a diameter of from <NUM> to <NUM> (see <FIG>) is placed between the core stacked portion <NUM> and the negative electrode current collector <NUM>. That is, the core stacked portion <NUM> and the negative electrode current collector <NUM> are resistance-welded through the hole <NUM> of the insulating sheet <NUM>. By providing the insulating sheet <NUM>, it is possible to suppress the scattering of conductive dust generated from spatters during resistance-welding. It is also possible to prevent the portions of the second member <NUM> other than the projection from contacting the core stacked portion <NUM>. This process is performed when the projection <NUM> is placed in the hole <NUM> of the insulating sheet <NUM>.

The following examples further explains the present disclosure, but the present disclosure is not limited to these examples.

A positive electrode mixture slurry was applied to both sides of a positive electrode core made of an aluminum foil having a width of <NUM> (coating width: <NUM>) to form a coating film, and the coating film was dried and compressed. The obtained positive electrode core with the coating film (positive electrode mixture layer) was cut to a specified electrode size to fabricate the positive electrode. The positive electrode included an exposure region where the positive electrode mixture slurry was not applied and the surface of the core was exposed. The exposure region was formed like a belt having a fixed width along the longitudinal direction of the positive electrode.

A negative electrode mixture slurry was applied to both sides of a negative electrode core having a width of <NUM> and a thickness of <NUM> (coating width: <NUM>) to form a coating film, and the coating film was dried and compressed. The obtained negative electrode core with the coating film (negative electrode mixture layer) was cut to a specified electrode size to fabricate the negative electrode. In Example <NUM>, a copper foil having a surface roughness of <NUM> and a glossiness of <NUM> was used as the negative electrode core. The negative electrode included an exposure region where the negative electrode mixture slurry was not coated and the surface of the core was expose. The exposure region was formed like a belt having a fixed width (<NUM>) along the longitudinal direction of the negative electrode.

The fabricated negative electrode and positive electrode were placed on top of each other with a separator having a width of <NUM> interposed therebetween, and spirally wound to form a stack of separator A/negative electrode/separator B/positive electrode/separator A and so on in the radial direction of the spiral winding. After that, the obtained spiral winding was pressed in the radial direction (at temperature <NUM>, press pressure <NUM> kN, and pressing time <NUM>) to obtain a flat wound electrode assembly having a thickness of <NUM>. <NUM> (average thickness of <NUM> units fabricated). At one end of the electrode assembly in the axial direction, there was formed the positive electrode core stacked portion in which the core exposure regions of the positive electrode were stacked. At the other end of the electrode assembly, there was formed the negative electrode core stacked portion in which the core exposure regions of the negative electrode were stacked. Eighty-four layers of the negative electrode core were stacked in the negative electrode core stacked portion.

Next, the first member constituting the negative electrode current collector was crimped to the sealing body and connected to the negative electrode terminal. The negative electrode core stacked portion was compressed by the first member obtained above and the second member of the negative electrode current collector, and the stacked portion was resistance-welded with the negative electrode current collector. The first and second members were composed of copper, the first member having a thickness of <NUM> and the second member having a thickness of <NUM>. In Example <NUM>, the second member of the negative electrode current collector was pressed to form the projection of a rounded-hill shape having a height (h) of <NUM> and a diameter (d) of <NUM>. On the surface of the second member opposite to the contact surface contacting the core stacked portion, a recess portion having a diameter (D) of <NUM> and a depth (H) of <NUM> was formed.

An insulating sheet having a thickness of <NUM> and including a hole having a diameter of <NUM> was placed between the negative electrode core and the first and second members. At this time, the insulating sheet was arranged so that the holes of the two insulating sheets overlap in the thickness direction of the core stacked portion, and the projection of the second member was located at the center of the holes.

After the first and second members of the negative electrode current collector were disposed on both sides of the negative electrode core stacked portion through the insulating sheet in the thickness direction, a pair of electrode rods was pressed against the first and second members to compress the stacked portion (welding pressure <NUM> N) to perform the resistance-welding by applying an electric current by a two-stage energization method. The energization time was <NUM> for the first energization and <NUM> for the second energization.

The welded portion of the negative electrode core stacked portion and the negative electrode current collector were evaluated according to the following procedures:.

The welded portion was observed by the above method, and the formation of a large nugget with a maximum diameter exceeding <NUM> was confirmed, although small voids were recognized at the interface between the core stacked portion and the current collector. There were no voids exceeding <NUM> in length at the interface between the core stacked portion and the current collector, and the contact length between the nugget and the current collector was at least <NUM>. Also, no melting of the insulating sheet was observed.

The strength (peel strength) of the welded portion was measured using Autograph manufactured by SHIMADZU CORPORATION, and the obtained peel strength was <NUM> N. In addition, the resistance of the negative electrode was measured using a resistance measuring instrument manufactured by HIOKI E. CORPORATION, and the obtained resistance value was <NUM> mΩ. The following examples and comparative examples were evaluated in the same manner as in Example <NUM>.

The electrode assembly was fabricated in the same manner as in Example <NUM>, except that a copper foil having a surface roughness of <NUM> and a glossiness of <NUM> was used as the negative electrode core. The negative body core stacked portion and the negative electrode current collector were resistance-welded, and the welded portion was evaluated.

The electrode assembly was fabricated in the same manner as in Example <NUM>, except that the height (h) and the diameter (d) of the projection formed on the second member of the negative electrode current collector were <NUM> and <NUM>, respectively. The negative electrode core stacked portion and the negative electrode current collector were resistance-welded, and the welded portion was evaluated.

The electrode assembly was fabricated in the same manner as in Example <NUM>, except that no projection was formed on the second member of the negative electrode current collector. The negative electrode core stacked portion and the negative electrode current collector were resistance-welded, and the welded portion was evaluated.

The electrode assembly was fabricated in the same manner as in Example <NUM>, except that a copper foil having a surface roughness of <NUM> and a glossiness of <NUM> was used as the negative electrode core. The negative electrode core stacked portion and the negative electrode current collector were resistance-welded, and the welded portion was evaluated.

As shown in Table <NUM>, the generation of voids in the welded portion of the negative electrode core stacked portion and the negative electrode current collector was substantially suppressed in the examples, and the welded portion with high peel strength and low resistance was obtained. In particular, in Example <NUM>, no void was observed in the welded region, and a well-formed nugget was obtained.

Claim 1:
A method for manufacturing a secondary battery (<NUM>) including an electrode assembly (<NUM>) and a negative electrode current collector (<NUM>), the electrode assembly (<NUM>) including a positive electrode (<NUM>), a negative electrode (<NUM>), and a separator (<NUM>), and formed by stacking the positive electrode (<NUM>) and the negative electrode (<NUM>) with the separator (<NUM>) interposed therebetween,
the negative electrode (<NUM>) including a negative electrode core (<NUM>) made of a copper foil having a surface roughness of <NUM> or less and a glossiness of from <NUM> to <NUM>, and a negative electrode mixture layer formed on a surface of the negative electrode core (<NUM>) except for an exposure region (<NUM>) where a surface of the negative electrode core (<NUM>) is exposed, when the surface roughness and glossiness are measured according to JIS B <NUM><NUM> and JIS (Z8741), respectively, as indicated in the description,
the electrode assembly (<NUM>) including a core stacked portion (<NUM>) formed by stacking a plurality of the exposure regions (<NUM>) of the negative electrode (<NUM>),
the negative electrode current collector (<NUM>) including a projection (<NUM>) having a height of from <NUM> to <NUM> on at least one of a first member (<NUM>) and a second member (<NUM>) constituting the negative electrode current collector (<NUM>),
the method for manufacturing the secondary battery (<NUM>), comprising:
resistance-welding the negative electrode current collector (<NUM>) and the core stacked portion (<NUM>) in a state where the core stacked portion (<NUM>) is sandwiched between the first member (<NUM>) and the second member (<NUM>) from both sides and the projection (<NUM>) is in contact with the core stacked portion (<NUM>).