Lithium secondary battery

Abstract

PROBLEM TO BE SOLVED: To provide a technique for suppressing deterioration of output characteristics and productivity of a lithium secondary battery. This lithium secondary battery comprises: (a) a first laminate that is provided with a first end exposed from a first electrode; (b) an intermediate laminate; (c) a second laminate that is provided with a second end exposed from a second electrode; (d) a metal sheet that is disposed between the first end and the second end; and (e) an electrode tab that constitutes a bonding region with the first end, the metal sheet, and the second end, and is electrically connected to the first laminate and the second laminate. The bonding region includes a first region configured by integrally laminating a pair of first conductive layers, a metal sheet, and a pair of second conductive layers in a cross section in the lamination direction.

Description

[Technical Field]

[0001] Exemplary embodiments of the present disclosure relate to lithium secondary batteries. [Background technology]

[0002] Patent Document 1 discloses that the safety of a battery cell is improved by using a current collector in which metal layers are formed on both sides of a resin film. The front and back of the resin film are separated by an insulating resin layer, preventing electrical continuity. Therefore, when connecting an electrode film to an electrode tab for drawing out wiring, electrical continuity cannot be achieved between the front and back of the electrode, or between multiple electrodes and the electrode tab. In this regard, Patent Document 2 discloses that multiple current collectors are folded and stacked on each metal layer in order to connect each metal layer separated by a resin layer to the electrode tab for drawing out wiring. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-102711 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-016321 Summary of the Invention [Problem to be solved by the invention]

[0004] The present disclosure provides a technique for suppressing a decrease in output characteristics and productivity of lithium secondary batteries. [Means for solving the problem]

[0005] In one exemplary embodiment of the present disclosure, there is provided a lithium secondary battery comprising: (a) a first laminate including a first current collector having a first insulating layer sandwiched between a pair of first conductive layers; and a first electrode disposed on the first current collector, the first current collector having a first end exposed from the first electrode; (b) an intermediate laminate including an electrode having a polarity different from that of the first electrode and a separator; and (c) a second laminate disposed apart from the first laminate in the stacking direction via the intermediate laminate, the second current collector having a second insulating layer sandwiched between a pair of second conductive layers; and a second electrode disposed on the second current collector and having the same polarity as the first electrode; The current collector comprises: (d) a second laminate having a second end exposed from the second electrode; (d) a metal sheet disposed between the first end and the second end; and (e) an electrode tab that forms a bonding area with the first end, the metal sheet, and the second end and is electrically connected to the first laminate and the second laminate, wherein the bonding area comprises a first area and a second area in a cross section in the stacking direction, the first area being formed by integrally stacking a pair of first conductive layers, a metal sheet, and a pair of second conductive layers, and the second area being formed by including a pair of first conductive layers sandwiching a first insulating layer, a metal sheet, and a pair of second conductive layers sandwiching the second insulating layer. [Effects of the Invention]

[0006] According to one exemplary embodiment of the present disclosure, it is possible to provide a technique for suppressing a decrease in the output characteristics and productivity of a lithium secondary battery. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is an exploded perspective view illustrating an example of the configuration of a lithium secondary battery according to one embodiment. [Figure 2A] FIG. 2 is a diagram showing an example of the cross-sectional structure of a first positive electrode stacking body. [Figure 2B] FIG. 4 is a diagram showing an example of the cross-sectional structure of a second positive electrode stacking body. [Figure 3] 10 is a plan view illustrating the positional relationship between the end of the current collector and the metal sheet. FIG. [Figure 4] FIG. 2 is a diagram schematically illustrating a cross section of a bonding region. [Figure 5] 1 is a flowchart showing an example of a method for manufacturing a lithium secondary battery. [Figure 6A] 10A and 10B are diagrams illustrating an example of a cross-sectional structure of a fiber-shaped buffer function layer. [Figure 6B] FIG. 10 is a diagram showing an example of a state of the buffer function layer during charging. [Figure 6C] FIG. 2 is a diagram showing an example of a cross-sectional structure of a fiber. [Figure 7] FIG. 10 is a cross-sectional view of a main part for explaining another example of the configuration of a lithium secondary battery. [Figure 8] FIG. 10 is a cross-sectional view of a main part for explaining another example of the configuration of a lithium secondary battery. [Figure 9] FIG. 10 is a perspective view illustrating another example of the configuration of the positive electrode laminate. [Figure 10] FIG. 10 is a perspective view illustrating another example of the configuration of the positive electrode laminate. [Figure 11] 1 is a diagram showing the configurations and results of an example and a comparative example. [Figure 12] 1 is a diagram showing stacking patterns of metal sheets in Examples and Comparative Examples. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, each embodiment of the present disclosure will be described.

[0009] In one exemplary embodiment, a lithium secondary battery includes: (a) a first laminate including a first current collector having a first insulating layer sandwiched between a pair of first conductive layers; and a first electrode disposed on the first current collector, the first current collector having a first end exposed from the first electrode; (b) an intermediate laminate including an electrode having a polarity different from that of the first electrode and a separator; and (c) a second laminate disposed apart from the first laminate in the stacking direction via the intermediate laminate, the second current collector having a second insulating layer sandwiched between a pair of second conductive layers; and a second electrode disposed on the second current collector and having the same polarity as the first electrode; a second laminate having a first end exposed from the second electrode; (d) a metal sheet disposed between the first end and the second end; and (e) an electrode tab forming a bonding area with the first end, the metal sheet, and the second end and electrically connected to the first laminate and the second laminate, wherein the bonding area comprises a first area and a second area in a cross section in the stacking direction, the first area being formed by integrally stacking a pair of first conductive layers, a metal sheet, and a pair of second conductive layers, and the second area being formed by including a pair of first conductive layers sandwiching a first insulating layer, a pair of second conductive layers sandwiching a metal sheet, and a second insulating layer.

[0010] In one exemplary embodiment, a plurality of first stacks and second stacks are arranged alternately in the stacking direction with an intermediate stack sandwiched between them.

[0011] In one exemplary embodiment, the first laminate is formed from a flat sheet, and the second laminate is formed from a flat sheet separate from the first laminate.

[0012] In one exemplary embodiment, the first and second laminates are constructed by folding or rolling a single sheet.

[0013] In one exemplary embodiment, a total of 10 or more layers of the first laminate and the second laminate are arranged.

[0014] In one exemplary embodiment, a plurality of first ends and second ends are alternately arranged in the stacking direction, and a metal sheet is disposed at least at one of the plurality of first ends and second ends.

[0015] In one exemplary embodiment, the number of metal sheets is not more than three times the total number of the first ends and the second ends.

[0016] In one exemplary embodiment, based on the positions of the first ends and the second ends in the stacking direction, the number or thickness of the metal sheets disposed between the first ends and the second ends is set.

[0017] In one exemplary embodiment, a metal sheet is further disposed between the electrode tab and the first end or the second end facing the electrode tab.

[0018] In one exemplary embodiment, when the total thickness of the pair of first conductive layers, the pair of second conductive layers, and the metal sheet is X, and the total thickness of the first insulating layer and the second insulating layer is Y, the relationship 0.85 < X / Y < 2.3 holds.

[0019] In one exemplary embodiment, the thickness of the metal sheet is 3 μm or more and 15 μm or less.

[0020] In one exemplary embodiment, the metal sheet is made of the same material as the first conductive layer and the second conductive layer.

[0021] In one exemplary embodiment, the maximum thickness of the first region is not more than half of the maximum thickness of the second region.

[0022] In one exemplary embodiment, in a cross-section in the stacking direction, the first region is disposed between two second regions.

[0023] In one exemplary embodiment, the first electrode and the second electrode are lithium as the positive electrode.

[0024] In one exemplary embodiment, the metal sheet is soft aluminum foil.

[0025] In one exemplary embodiment, the electrode tabs are hard aluminum.

[0026] In one exemplary embodiment, the resistance of the bonded region is 5.0 mΩ or less.

[0027] In one exemplary embodiment, the first electrode and the second electrode are negative electrodes.

[0028] In one exemplary embodiment, the metal sheet is formed from at least one selected from the group consisting of metals that do not react with Cu, Ni, Ti, Fe, and Li, and alloys thereof, and stainless steel.

[0029] Hereinafter, each embodiment of the present disclosure will be described in detail with reference to the drawings. In each drawing, the same or similar elements are designated by the same reference numerals, and redundant explanations will be omitted. Unless otherwise specified, the positional relationships, such as up, down, left, and right, will be described based on the positional relationships shown in the drawings. The dimensional ratios in the drawings do not represent actual ratios, and the actual ratios are not limited to the ratios shown in the drawings.

[0030] As mentioned above, Patent Document 2 proposes folding the current collector multiple times and stacking it on each metal layer to connect each metal layer separated by a resin layer to an electrode tab for wiring. However, this method requires a new device mechanism for stacking the current collector while folding back the end of the current collector for each metal layer. Furthermore, the end of the current collector must be folded back in conjunction with the stacking, significantly reducing productivity. Furthermore, even if this method can mechanically join the electrode tab, the current collector, and each metal layer, the resistance of the joint increases, resulting in reduced output characteristics. A lithium secondary battery 1 (hereinafter also referred to as "secondary battery 1") according to one embodiment can solve these problems.

[0031] <Example of secondary battery configuration> Fig. 1 is an exploded perspective view illustrating a configuration example of a secondary battery 1. As shown in Fig. 1, the secondary battery 1 includes a negative electrode 10, a separator 20, a first positive electrode laminate 30A, a second positive electrode laminate 30B, a metal sheet MS, a positive electrode electrode tab 40, and a negative electrode electrode tab 42. Each component will be described in detail below.

[0032] (Negative electrode) In one embodiment, the negative electrode 10 includes a negative electrode current collector and a negative electrode active material disposed on the negative electrode current collector. In one embodiment, the negative electrode current collector may include a negative electrode insulating layer and a pair of negative electrode conductive layers disposed so as to sandwich the negative electrode insulating layer. In one embodiment, the negative electrode insulating layer may be formed of a sheet-like (film-like) or fibrous resin. In one embodiment, the negative electrode conductive layer is formed of at least one metal selected from the group consisting of metals that do not react with Cu, Ni, Ti, Fe, and Li, alloys thereof, and stainless steel. In one example, the negative electrode conductive layer is Cu. By including the negative electrode insulating layer, the negative electrode 10 can be made lighter than when the negative electrode 10 is formed only of a conductive layer while maintaining the necessary thickness (rigidity) of the negative electrode 10. Note that in one embodiment, the negative electrode current collector may be formed only of a negative electrode conductive layer without including the negative electrode insulating layer.

[0033] The negative electrode active material is a material that causes an electrode reaction, i.e., an oxidation reaction and a reduction reaction, at the negative electrode. Examples of the negative electrode active material include lithium metal and alloys containing lithium metal, carbon-based materials, metal oxides, and metals that can be alloyed with lithium and alloys containing such metals. Examples of the carbon-based materials include graphene, graphite, hard carbon, and carbon nanotubes. Examples of the metal oxides include titanium oxide-based compounds and cobalt oxide-based compounds. Examples of the metals that can be alloyed with lithium include silicon, silicon oxide, germanium, tin, lead, aluminum, and gallium, as well as those pre-doped with lithium.

[0034] As shown in FIG. 1 , the negative electrode 10 has a negative electrode end portion 12. The negative electrode end portion 12 extends in a direction parallel to the main surface of the negative electrode 10. No negative electrode active material is formed on the negative electrode end portion 12. The negative electrode end portion 12 may be made of the same material as the negative electrode current collector, or may be made of a material separate from the negative electrode current collector. The negative electrode 10 is electrically connected to a negative electrode electrode tab 42 via the negative electrode end portion 12.

[0035] (separator) The separator 20 is disposed on the negative electrode 10. In the example shown in FIG. 1, the separators 20 are disposed on both sides of the negative electrode 10. The separator 20 physically and / or electrically isolates the negative electrode 10 from the positive electrode laminate 30 and ensures ionic conductivity of lithium ions. In one embodiment, the separator 20 may be at least one selected from the group consisting of an insulating porous material, a polymer electrolyte, a gel electrolyte, and an inorganic solid electrolyte. The separator 20 may be formed of one material alone or a combination of two or more materials.

[0036] When the separator 20 includes an insulating porous member, the pores of the porous member are filled with an ion-conductive substance (such as an electrolytic solution, a polymer electrolyte, and / or a gel electrolyte). This allows the separator 20 to exhibit ion conductivity. The material constituting the insulating porous member is not particularly limited, and examples thereof include insulating polymer materials, specifically polyethylene (PE) and polypropylene (PP). That is, the separator 20 may be a porous polyethylene (PE) film, a porous polypropylene (PP) film, or a laminate structure thereof.

[0037] In one embodiment, one or both surfaces of the separator 20 may be coated with a separator coating layer. This may improve the cycle characteristics of the secondary battery 1. In one embodiment, the separator coating layer may be a continuous film with a uniform thickness over 50% or more of the surface area of ​​the separator 20. In one embodiment, the separator coating layer may include a binder such as polyvinylidene fluoride (PVDF), a styrene butadiene rubber-carboxymethyl cellulose mixture (SBR-CMC), and polyacrylic acid (PAA). In one embodiment, the separator coating layer may be formed by adding inorganic particles such as silica, alumina, titania, zirconia, or magnesium hydroxide to the binder.

[0038] In one embodiment, the thickness of the separator 20 (including the coating layer if the separator 20 includes the coating layer) may be 3.0 μm or more and 40 μm or less. This allows the negative electrode 10 and the positive electrode laminate 30 to be separated while reducing the volume occupied by the separator 20. In one embodiment, the thickness of the separator 20 may be 5.0 μm or more, 7.0 μm or more, or 10 μm or more. In one embodiment, the thickness of the separator 20 may be 30 μm or less, 20 μm or less, or 10 μm or less.

[0039] (Intermediate laminate) In one embodiment, the negative electrode 10 and the separator 20 constitute an intermediate laminate LM. The intermediate laminate LM may have a structure in which the separator 20, the negative electrode 10, and the separator 20 are stacked in this order in a stacking direction (the direction indicated by the arrow z in FIG. 1). The secondary battery 1 includes a plurality of intermediate laminates LM. In one embodiment, the plurality of intermediate laminates LM may each be configured as a single flat sheet, as shown in FIG. 1. In one embodiment, the plurality of intermediate laminates LM may be configured as a single sheet (an example of such an embodiment will be described later with reference to FIGS. 7 and 8).

[0040] (Positive electrode laminate) The first positive electrode laminate 30A includes a current collector 32A and a positive electrode 34A. The first positive electrode laminate 30A is an example of a first laminate. In one embodiment, the first positive electrode laminate may have a structure in which a positive electrode 34A, a current collector 32A, and a positive electrode 34A are stacked in this order in the stacking direction. The current collector 32A has a first end P1 exposed from the positive electrode 34A. In other words, the positive electrode 34A is not formed on the first end P1. The first end P1 extends, as part of the current collector 32A, from a side surface of the current collector 32A in a direction parallel to the main surface of the current collector 32A.

[0041] The second positive electrode laminate 30B includes a current collector 32B and a positive electrode 34B. The second positive electrode laminate 30B is an example of a second laminate. In one embodiment, the second positive electrode laminate 30B may have a structure in which a positive electrode 34B, a current collector 32B, and a positive electrode 34B are stacked in this order in the stacking direction. The current collector 32B has a second end P2 exposed from the positive electrode 34B. In other words, the positive electrode 34B is not formed on the second end P2. The second end P2 extends, as part of the current collector 32B, from the side surface of the current collector 32B in a direction parallel to the main surface of the current collector 32B.

[0042] The first positive electrode laminate 30A and the second positive electrode laminate 30B are alternately stacked in the stacking direction with intermediate laminates LM interposed therebetween (hereinafter, when there is no need to distinguish between the first positive electrode laminate 30A and the second positive electrode laminate 30B, they are collectively referred to as the "positive electrode laminate 30"). In one embodiment, the multiple positive electrode laminates 30 may each be configured as a single flat sheet, as shown in FIG. 1. In one embodiment, the multiple positive electrode laminates 30 may be configured as a single sheet (an example of such an embodiment will be described later with reference to FIGS. 9 and 10).

[0043] In one embodiment, the total number of positive electrode laminates 30 (first positive electrode laminates 30A and second positive electrode laminates 30B) included in the secondary battery 1 may be 5 or more, 10 or more, or 20 or more. In one embodiment, the total number of positive electrode laminates 30 included in the secondary battery 1 may be 50 or less, 40 or less, or 30 or less. In one embodiment, the energy density of the secondary battery 1 may be 300 Wh / kg or more. In one embodiment, the rated capacity of the secondary battery 1 may be 1.5 Ah or more, or 5 Ah or more.

[0044] 2A is a diagram showing an example of the cross-sectional structure of a first positive electrode laminate. In one embodiment, the first positive electrode laminate 30A may have a current collector 32A and positive electrodes 34A disposed on both sides of the current collector 32A. The current collector 32A has an insulating layer 320A and conductive layers 322A formed to sandwich the insulating layer 320A.

[0045] 2B is a diagram showing an example of the cross-sectional structure of a second positive electrode laminate. In one embodiment, the second positive electrode laminate 30B may have a current collector 32B and positive electrodes 34B disposed on both sides of the current collector 32B. The current collector 32B has an insulating layer 320B and conductive layers 322B formed to sandwich the insulating layer 320B.

[0046] In one embodiment, the first positive electrode laminate 30A and the second positive electrode laminate 30B may have the same configuration. Hereinafter, when there is no need to distinguish between the configurations of the first positive electrode laminate 30A and the second positive electrode laminate 30B, they will be described together. In this regard, the "current collector 32A" and the "current collector 32B" will be collectively referred to as the "current collector 32," the "insulating layer 320A" and the "insulating layer 320B" will be collectively referred to as the "insulating layer 320," the "conductive layer 322A" and the "conductive layer 322B" will be collectively referred to as the "conductive layer 322," and the "positive electrode 34A" and the "positive electrode 34B" will be collectively referred to as the "positive electrode 34."

[0047] The insulating layer 320 of the current collector 32 may be made of, for example, a sheet-like (film-like) or fibrous resin. The resin may be, for example, at least one of polyolefin resins such as polyethylene terephthalate (PET), polyethylene, and polypropylene, and thermoplastic resins such as polystyrene, polyvinyl chloride, and polyamide. The insulating layer 320 may be made by laminating at least one of the resins. In one embodiment, the insulating layer 320 is made of a material having a melting point of 150°C or higher and 300°C or lower. In one embodiment, the thickness of the insulating layer 320 may be 3 μm or higher and 10 μm or lower, or 4 μm or higher and 8 μm or lower.

[0048] The insulating layer 320 can melt in the event of abnormal heat generation due to overcharge or high temperature conditions, damaging the positive electrode laminate 30 and interrupting short-circuit current inside the battery. This can suppress a sudden temperature rise inside the secondary battery 1 and prevent the battery from catching fire. In other words, the insulating layer 320 can contribute to improving the safety of the secondary battery 1.

[0049] The conductive layers 322 of the current collector 32 are formed on both sides of the insulating layer 320 so as to sandwich the insulating layer 320. The conductive layers 322 are in physical and / or electrical contact with the positive electrode 34 and function to donate and receive electrons to and from the positive electrode 34. The conductive layers 322 are made of a conductor that does not react with lithium ions in a battery. In one embodiment, the conductive layers 322 are made of at least one material selected from the group consisting of aluminum, titanium, stainless steel, nickel, and alloys thereof. In one example, the conductive layers 322 are aluminum or aluminum alloys. In one embodiment, the conductive layers 322 are formed by vapor deposition, sputtering, electroplating, or laminating the above materials on both surfaces of the insulating layer 320. In one embodiment, the thickness of each conductive layer 322 may be 0.5 μm to 5 μm, 0.7 μm to 3 μm, or 0.8 μm to 2.0 μm.

[0050] The positive electrodes 34 are formed on both sides of the current collector 32. The material of the positive electrodes 34 may be appropriately selected from known materials depending on the application. The thickness of the positive electrodes 34 may be appropriately adjusted depending on the desired capacity and rate characteristics of the battery. In one embodiment, the thickness of each positive electrode 34 is, for example, 20 μm or more and 150 μm or less.

[0051] In one embodiment, the cathode 34 includes a cathode active material. The cathode active material is a material for holding a carrier metal in the cathode 34 and can also be referred to as a host material for the carrier metal. The cathode active material may be a material for holding lithium ions in the cathode 34, in which case lithium ions are loaded into and deloaded from the cathode active material as the battery is charged and discharged. This can improve the stability and output voltage of the battery.

[0052] In one embodiment, the positive electrode active material is a metal oxide or a metal phosphate. The metal oxide may be, for example, a cobalt oxide-based compound, a manganese oxide-based compound, or a nickel oxide-based compound. The metal phosphate may be, for example, an iron phosphate-based compound or a cobalt phosphate-based compound. In one embodiment, the positive electrode active material is LiCoO, LiNi x Co y Mn z O(x+y+z=1), LiNi x Co y Al z O(x+y+z=1), LiNi x Mn y The positive electrode active material may be at least one selected from the group consisting of LiFePO (x+y=1), LiNiO2, LiMn2O4, LiFePO4, LiCoPO4, LiFeOF, LiNiOF, and LiTiS2. The positive electrode active material may be used alone or in combination of two or more. In one embodiment, the content of the positive electrode active material in the positive electrode 34 may be 50% by mass or more and 100% by mass or less with respect to the entire positive electrode 34.

[0053] In one embodiment, positive electrode 34 may include one or more components other than the positive electrode active material.

[0054] In one embodiment, the positive electrode 34 may include a sacrificial positive electrode material, which is a lithium-containing compound that undergoes an oxidation reaction and does not substantially undergo a reduction reaction in the charge / discharge potential range of the positive electrode active material.

[0055] In one embodiment, the positive electrode 34 may include a gel electrolyte. The gel electrolyte may improve adhesion between the positive electrode 34 and the current collector 32. In one example, the gel electrolyte includes a polymer, an organic solvent, and a lithium salt. The polymer in the gel electrolyte may be, for example, a copolymer of polyethylene and / or polyethylene oxide, polyvinylidene fluoride, or a copolymer of polyvinylidene fluoride and hexafluoropropylene.

[0056] In one embodiment, the positive electrode 34 may include a conductive additive and / or a binder. In one example, the conductive additive is carbon black, single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), carbon nanofibers (CF), or the like. In one example, the binder is polyvinylidene fluoride, polytetrafluoroethylene, styrene butadiene rubber, acrylic resin, polyimide resin, or the like. In one embodiment, the content of the conductive additive is 0.5% by mass to 30% by mass or less with respect to the entire positive electrode 34. In one embodiment, the content of the binder may be 0.5% by mass to 30% by mass or less with respect to the entire positive electrode 34.

[0057] In one embodiment, the positive electrode 34 may include a polymer electrolyte. For example, the polymer electrolyte may be a solid polymer electrolyte primarily containing a polymer and an electrolyte, or a semi-solid polymer electrolyte primarily containing a polymer, an electrolyte, and a plasticizer. In one embodiment, the total content of the polymer electrolyte may be 0.5% by mass to 30% by mass or less of the total positive electrode 34.

[0058] (metal sheet) As shown in FIG. 1, the metal sheet MS is disposed between at least one first end P1 of the current collector 32A and a second end P2 of the current collector 32B (hereinafter, when there is no need to distinguish between the two, they will be collectively referred to as "end P").

[0059] In one embodiment, metal sheets MS may be placed between all of the end portions P. In another embodiment, metal sheets MS may be placed between some of the end portions P, and no metal sheets MS may be placed between the remaining end portions P. For example, metal sheets MS may be placed every other plurality of end portions P.

[0060] In one embodiment, the number and thickness of metal sheets arranged between the end portions P may be set based on the position of the end portions P in the stacking direction. For example, two (or more) metal sheets MS may be arranged between the end portions P at the center in the stacking direction of the secondary batteries 1, and one metal sheet may be arranged between the end portions P at the top and bottom in the stacking direction. Also, for example, the thickness of the metal sheet MS arranged between the end portions P at the center in the stacking direction of the secondary batteries 1 may be made larger than the thickness of the metal sheet arranged between the end portions P at the top and bottom in the stacking direction. This can suppress variations in resistance between the end portions P at the center.

[0061] In one embodiment, the metal sheet MS may also be disposed between the electrode tab 40 and the end P for the positive electrode.

[0062] In one embodiment, the number of metal sheets MS may be three times or less than the total number of end parts P. In one embodiment, the number of metal sheets MS may be the same as the total number of end parts P, or may be less than the total number of end parts P, for example, half or less of the total number of end parts P.

[0063] In one embodiment, the metal sheet MS is made of at least one material selected from the group consisting of aluminum, titanium, stainless steel, nickel, and alloys thereof. For example, the metal sheet MS is a hard aluminum foil. For example, the metal sheet MS is a soft aluminum foil. The soft aluminum foil may be formed by subjecting the hard aluminum foil to a high-temperature heat treatment (around 400°C). In one embodiment, the metal sheet MS may be made of the same material as the conductive layer 322.

[0064] In one embodiment, the thickness of the metal sheet MS may be set based on the thickness of the insulating layer 320 and the thickness of the conductive layer 322. For example, when the sum of the total thickness (A) of each metal sheet MS and the total thickness (B) of each conductive layer 322 is set as X (= A + B), and the total thickness of each insulating layer 322 is set as Y, the thickness of the metal sheet MS may be set so that the relationship 0.85 < X / Y < 2.3 holds. In one embodiment, 1.0 < X / Y < 2.0 may be satisfied. In one embodiment, the thickness of the metal sheet MS may be greater than the thickness of the insulating layer 320. In one embodiment, all of the metal sheets MS may have the same thickness, or some of them may have different thicknesses. In one embodiment, the thickness of the metal sheet MS may be 3 μm or more, 5 μm or more, or 7 μm or more. In one embodiment, the thickness of the metal sheet MS may be 15 μm or less, 12 μm or less, or 10 μm or less.

[0065] FIG. 3 is a plan view for explaining the positional relationship between the end portion of the current collector and the metal sheet. In one embodiment, the metal sheet MS may be configured to cover only a part of the end portion P of the current collector 32, rather than the entire end portion. For example, as shown in FIG. 3, the metal sheet MS may be disposed at a position separated from the positive electrode 34 formed on the current collector 32 by a distance D. At this time, an insulating layer may be provided in a region RS on the conductive layer 322 of the end portion P where the metal sheet MS is not disposed. Thereby, when the separator 20 is damaged or the like, the short circuit between the negative electrode 10 and the positive electrode 34 through the conductive layer 322 and / or the metal sheet MS of the end portion P can be suppressed. Thereby, the safety of the secondary battery 1 can be improved. The insulating layer provided in the region RS may be formed of, for example, a sheet (film) or fibrous resin. The resin may be at least one of, for example, polyolefin resins such as polyethylene terephthalate (PET), polyethylene, and polypropylene, thermoplastic resins such as polystyrene, polyvinyl chloride, or polyamide. In one embodiment, the metal sheet MS may be configured to cover the entire surface of the end portion P of the current collector.

[0066] (Electrode tab for positive electrode) As shown in FIG. 1 , the electrode tab 40 for the positive electrode is arranged to be aligned with each end P (P1, P2) of each current collector 32 (32A, 32B) and each metal sheet MS in the stacking direction (the direction indicated by z in FIG. 1 ). In one embodiment, the electrode tab 40 for the positive electrode may be arranged above or below the end P of each current collector 32 and each metal sheet MS. In one embodiment, the electrode tab 40 for the positive electrode may be arranged between an end P and an adjacent end P.

[0067] The electrode tab 40 for the positive electrode has a surface 40A that overlaps with each end P of each current collector 32 and each metal sheet MS when viewed from the stacking direction, and is joined to each end P and each metal sheet MS at this surface 40A. This electrically connects the electrode tab 40 for the positive electrode to each positive electrode 34 of each positive electrode laminate 30. The electrode tab for the positive electrode is made of a conductive material, and may be made of aluminum or an aluminum alloy, for example. In one example, the electrode tab 40 may be made of hard aluminum. In one embodiment, the thickness of the electrode tab 40 may be 0.05 mm or more and 1 mm or less, or 0.1 mm or more and 0.5 mm or less.

[0068] In one embodiment, the electrode tab 40 for the positive electrode may be joined to each end P and each metal sheet MS by welding. The welding may be, for example, ultrasonic welding, laser welding, resistance welding, or spot welding. In one example, the welding is ultrasonic welding. The joint between the electrode tab 40 for the positive electrode and the metal sheet MS or the end P, the joint between the end Ps, and the joint between the end P and the metal sheet MS may exist as one or more points (spots), or may exist as a continuous surface, as long as they are electrically connected.

[0069] 4 is a diagram schematically illustrating cross sections of the bonding regions between the positive electrode tab 40 and each end P and each metal sheet MS. In one embodiment, at least one cross section of the bonding region includes a first region R1 and a second region R2.

[0070] As shown in FIG. 4 , in the first region R1, the conductive layers 322 (322A, 322B) and the metal sheet MS are integrally stacked and bonded to the positive electrode tab 40. In one embodiment, being integrally stacked includes a state in which the conductive layers 322 and the metal sheet MS are partially or completely fused together by heat or the like. The first region R1 provides a physical path for electrical connection between the electrode tab 40 and the conductive layers 322 and the metal sheet MS. In one embodiment, the first region R1 may be substantially free of the insulating layer 320 along the stacking direction (z direction in FIG. 4 ). In the second region R2, a pair of conductive layers 322A sandwiching the insulating layer 320A, the metal sheet MS, and a pair of conductive layers 322B sandwiching the insulating layer 320B are stacked. That is, the second region R2 is a region that includes the insulating layer 320 along the stacking direction (z direction in FIG. 4 ).

[0071] In one embodiment, the first region R1 may be configured between two second regions R2, as shown in Figure 4. In one embodiment, the maximum thickness of the first region R1 may be less than or equal to half the maximum thickness of the second region R2.

[0072] In one embodiment, the first region R1 and the second region R2 may be formed by welding. When the positive electrode tab 40, the end P, and the metal sheet MS are pressed and welded in the stacking direction, the insulating layer 320 softens at the welded portion and is pushed outward in the width direction (left and right direction in FIG. 4 ). At the welded portion, each conductive layer 322 and the metal sheet MS are thermally fused and integrated. This may form the first region R1 and the second region R2.

[0073] As described above, the insulating layer 320 can prevent a sudden temperature rise inside the secondary battery 1 and prevent the battery from catching fire in the event of abnormal heat generation due to overcharge or high temperature conditions. With a current collector sandwiching an insulating layer between conductive layers, it becomes more difficult to bond the current collector end to the electrode tab and ensure stable bonding quality (controlling variations) at each layer as the number of current collectors increases or the insulating layer thickness increases. For example, if a strong pressure is applied to weld all of the current collector end portions to the electrode tabs, the conductive layer at the end portion may be damaged or broken, especially if the conductive layer is thin. Furthermore, welding the electrode tabs with a force that does not damage the conductive layer may result in an insufficient bond, increasing the resistance between the current collector end portion and the electrode tab.

[0074] In this regard, in the secondary battery 1 according to one embodiment, a metal sheet MS is disposed between at least one end P. The metal sheet MS functions as an additional conductive layer in the bonding region, increasing the ratio of conductive layers to insulating layers in the bonding region. This reduces the resistance of the bonding region, thereby improving the output characteristics of the secondary battery 1. The metal sheet MS can also function as a protective layer for the conductive layer 322 of the end P when bonding the positive electrode tab 40 to the end P. This allows for strong pressure bonding between the positive electrode tab 40 and each end P while suppressing damage or breakage of the conductive layer 322, even when the total number (number of stacked positive electrode bodies 30) of the secondary battery 1 is large. This improves the productivity of the secondary battery 1. Furthermore, an increase in resistance in the bonding region is suppressed, thereby improving the output characteristics of the secondary battery 1. In one embodiment, the resistance of the bonding region may be 5.0 mΩ or less, 3.0 mΩ or less, 1.0 mΩ or less, or 0.5 mΩ or less.

[0075] (negative electrode tab) As shown in FIG. 1 , the electrode tab 42 for the negative electrode is arranged to be aligned with each negative electrode end portion 12 in the stacking direction (the direction indicated by z in FIG. 1 ). In one embodiment, the electrode tab 42 for the negative electrode may be arranged above or below each negative electrode end portion 12. In one embodiment, the electrode tab 40 for the negative electrode may be arranged between a certain negative electrode end portion 12 and an adjacent negative electrode end portion 12. The electrode tab 42 for the negative electrode has a surface 42A that overlaps with the negative electrode end portion 12 when viewed from the stacking direction, and is joined to the negative electrode end portion 12 at this surface 42A. This electrically connects the electrode tab 42 for the negative electrode and each negative electrode 10.

[0076] In one embodiment, the electrode tab 42 for the negative electrode and each negative electrode end 12 may be joined by welding. The welding may be, for example, ultrasonic welding, laser welding, resistance welding, or spot welding. The joining point between the electrode tab 42 for the negative electrode and the negative electrode end 12 or the joining point between the negative electrode ends 12 may exist as one or more points (spots), or may exist as a continuous surface, as long as they are electrically connected.

[0077] (electrolyte) In one embodiment, the secondary battery 1 may contain an electrolytic solution. The electrolytic solution is a liquid containing a solvent and an electrolyte and has ion conductivity. The electrolytic solution may also be referred to as a liquid electrolyte, and acts as a conductive path for lithium ions. Therefore, when the secondary battery 1 contains an electrolytic solution, the internal resistance can be reduced, and the energy density, capacity, and cycle characteristics can be improved.

[0078] The electrolytic solution may be, for example, a solution that fills the housing (pouch) of the secondary battery 1. Furthermore, for example, the electrolytic solution may be impregnated into the separator 20, or may be held in a polymer to form a polymer electrolyte or a gel electrolyte.

[0079] The electrolyte contained in the electrolytic solution may be, for example, a lithium salt, which may be, for example, one or a combination of two or more selected from the group consisting of LiI, LiCl, LiBr, LiF, LiBF, LiPF, LiAsF, LiSOCF, LiN(SOF), LiN(SOCF), LiN(SOCFCF), LiB(OCOCF), LiB(OCOH), LiB(OCOCF), LiNO, and LiSO.

[0080] As the solvent contained in the electrolytic solution, for example, a non-aqueous solvent containing fluorine atoms (hereinafter referred to as a "fluorinated solvent") and a non-aqueous solvent containing no fluorine atoms (hereinafter referred to as a "non-fluorinated solvent") may be added.

[0081] The fluorinated solvent may be, for example, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether.

[0082] The fluorine-free solvent may be, for example, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,2-dimethoxyethane, dimethoxyethane, dimethoxypropane, dimethoxybutane, diethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, chloroethylene carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, trimethyl phosphate, triethyl phosphate, and 12-crown-4.

[0083] The above fluorinated solvents and / or non-fluorinated solvents may be used alone or in any combination of two or more in any ratio. The contents of the fluorinated solvent and non-fluorinated solvent are not particularly limited, and the ratio of the fluorinated solvent to the total solvent may be 0 to 100% by volume, or the ratio of the non-fluorinated solvent to the total solvent may be 0 to 100% by volume.

[0084] <Secondary battery manufacturing method> 5 is a flowchart showing an example of a method for manufacturing the secondary battery 1. This method includes step ST1 of arranging a plurality of positive electrode laminates, step ST2 of joining current collectors to electrode tabs, and step ST3 of sealing the laminate in a sealed container.

[0085] In step ST1, multiple positive electrode laminates 30 are arranged. First, a predetermined number (e.g., 20) of multiple positive electrode laminates 30 are prepared. Also, a predetermined number (e.g., 4 to 20) of metal sheets MS are prepared. Each metal sheet MS may be joined in advance to an end P of the current collector 32 of the positive electrode laminate 30 by, for example, welding. Next, as shown in FIG. 1, the positive electrode laminates 30 are arranged in the stacking direction via an intermediate laminate LM. Note that, as will be described later, when the negative electrode 10 and the separator 20 are configured in a sheet shape, each positive electrode laminate 30 may be arranged between separators 20 formed by folding the sheet in a zigzag shape (see FIG. 9) or between separators 20 formed by rolling the sheet (see FIG. 10).

[0086] Next, in step ST2, the current collectors 32 of the positive electrode laminate 30 are joined to the electrode tabs 40 for the positive electrode. Specifically, the end P of each current collector 32 is joined to the metal sheet MS so as to form the above-mentioned joining area on the electrode tab 40. The joining may be performed by ultrasonic welding, laser welding, resistance welding, or spot welding. Also, the negative electrode end 12 of the negative electrode 10 is joined to the electrode tab 42 for the negative electrode.

[0087] Next, in step ST3, the molded body obtained in step ST2 is sealed in a sealed container, for example, a laminate film. At this time, an electrolyte may also be sealed in the sealed container. In this way, the secondary battery 1 is manufactured.

[0088] <How to use secondary batteries> The secondary battery 1 is charged and discharged by connecting the electrode tab 40 for the positive electrode to one end of an external circuit and the electrode tab 42 for the negative electrode to the other end of the external circuit. The external circuit may be, for example, a resistor, a power source, an apparatus, a device, another battery, or a potentiostat. The ends P of the multiple positive electrode laminates 30 may be connected to the external circuit at the same potential. The negative electrode ends 12 of the multiple negative electrodes 10 may be connected to the external circuit at the same potential.

[0089] When a voltage is applied between the positive electrode tab 40 and the negative electrode tab 42 such that a current flows from the negative electrode tab 42 to the positive electrode tab 40 through an external circuit, the secondary battery 1 is charged, and lithium metal is deposited on the surface of the negative electrode 10. When the positive electrode tab 40 and the negative electrode tab 42 are connected to the charged secondary battery 1 via a desired external circuit, the secondary battery 1 is discharged, and the lithium metal deposited on the surface of the negative electrode 10 is electrolytically dissolved.

[0090] In one embodiment, a solid electrolyte interface layer (SEI layer) may be formed on the surface of the negative electrode 10 or the surface of the separator 20 (i.e., the interface between the negative electrode 10 and the separator 20) during the first charge (initial charge) after the secondary battery 1 is assembled. The SEI layer may contain, for example, an inorganic compound containing lithium or an organic compound containing lithium. In one embodiment, the thickness of the SEI layer is 1.0 nm or more and 10 μm or less. When an SEI layer is formed in the secondary battery 1, lithium metal is deposited or dissolved at the interface between the negative electrode 10 and / or the separator 20 and the SEI layer during charge and discharge.

[0091] According to the secondary battery 1 described above, the output characteristics and productivity of the battery can be improved.

[0092] <Modification> The secondary battery 1 can be modified in various ways without departing from the scope and spirit of the present disclosure.

[0093] (Configuration of negative electrode end and joining with negative electrode tab) In one embodiment, the joining of the negative electrode end 12 and the negative electrode tab 42 may be performed in the same manner as the joining of the end P and the positive electrode tab 40. That is, a negative metal sheet may be provided between the negative electrode end 12, and the negative electrode end 12 and the negative electrode tab 42 may be joined via the metal sheet. The metal sheet may be formed of at least one selected from the group consisting of metals that do not react with Cu, Ni, Ti, Fe, and Li, alloys thereof, and stainless steel. The metal sheet may be composed of, for example, the same material as the negative electrode layer of the negative electrode 10. The cross section of the joining region between the negative electrode end 12 and the metal sheet and the negative electrode tab 42 may have a first region where the metal sheet and the conductive layer are integrally laminated, and a second region including an insulating layer along the lamination direction, as shown in FIG. 4 .

[0094] (Negative electrode composition) In one embodiment, the negative electrode 10 may be made of at least one material selected from the group consisting of Cu, Ni, Ti, Fe, other metals that do not react with Li, alloys thereof, and stainless steel (SUS). The "metal that does not react with Li" may be a metal that does not react with lithium ions or lithium metal to form an alloy when the secondary battery 1 is in operation. In this case, the negative electrode 10 also functions as a current collector.

[0095] In one embodiment, the negative electrode 10 is substantially free of a negative electrode active material. For example, the thickness of the layer of the negative electrode active material deposited on the negative electrode 10 at the end of discharge (e.g., when the open circuit voltage of the battery is 2.5 V or more and 3.6 V or less) may be 25 μm or less. In one embodiment, the thickness of the layer of the negative electrode active material at the end of discharge may be 20 μm or less, 15 μm or less, 10 μm or less, or 5 μm or less, or may even be 0 μm. Since the negative electrode 10 is substantially free of a negative electrode active material, the energy density per volume can be improved in addition to the energy density per weight. In this case, the secondary battery 1 can also be called an "anode-free lithium battery," a "zero anode lithium battery," or an "anodeless lithium battery."

[0096] In one embodiment, the negative electrode 10 does not have a negative electrode active material before the initial charge of the battery (the state from the time the battery is assembled until the first charge). That is, after the initial charge, the secondary battery 1 may be charged and discharged by depositing lithium metal on the negative electrode and then electrolytically dissolving the deposited lithium metal. In this case, the volume and mass occupied by the negative electrode active material are reduced, reducing the volume and mass of the entire battery and, in principle, increasing the energy density. Note that "lithium metal deposited on the negative electrode" refers not only to lithium metal being deposited on the surface of the negative electrode, but also to lithium metal being deposited on the surface of a solid electrolyte interface (SEI) layer or on or within a buffer functional layer, which will be described later.

[0097] In one embodiment, the mass of lithium metal deposited on the negative electrode when the voltage is 4.2 V is M 4.2 The same mass at a voltage of 3.0 V is M 3.0 In this case, M 3.0 / M 4.2 may be 40% or less, or 35% or less. In one embodiment, the ratio M 3.0 / M 4.2 may be 1.0% or more, 2.0% or more, 3.0% or more, or 4.0% or more.

[0098] In one embodiment, the thickness of the negative electrode 10 may be 1.0 μm or more and 30 μm or less, which can reduce the volume occupied by the negative electrode 10 in the secondary battery 1 and improve the energy density. The thickness of the negative electrode 10 may be 2.0 μm or more and 20 μm or less, 2.0 μm or more and 18 μm or less, or 3.0 μm or more and 15 μm or less.

[0099] In one embodiment, the negative electrode 10 may be coated on at least a portion of the surface facing the positive electrode laminate 30 with a compound (hereinafter also referred to as "negative electrode coating agent") containing an aromatic ring to which two or more elements selected from the group consisting of N, S, and O are independently bonded. The negative electrode coating agent can be held on the negative electrode 10 by the above elements being coordinately bonded to metal atoms constituting the negative electrode 10. According to this embodiment, a non-uniform deposition reaction of lithium metal on the surface of the negative electrode 10 can be suppressed, and the lithium metal deposited on the negative electrode 10 can be suppressed from growing in a dendritic form.

[0100] In one embodiment, the negative electrode coating agent is applied to at least a portion of the surface of the negative electrode 10. In one embodiment, the negative electrode coating agent may be applied to 10% or more of the surface area, or may be applied to 20% or more, 40% or more, 60% or more, or 80% or more of the surface area.

[0101] In one embodiment, the aromatic ring contained in the negative electrode coating agent may be an aromatic hydrocarbon such as benzene, naphthalene, azulene, anthracene, and pyrene, or a heteroaromatic compound such as furan, thiophene, pyrrole, imidazole, pyrazole, pyridine, pyridazine, pyrimidine, and pyrazine. In one example, the aromatic ring is an aromatic hydrocarbon. In one example, the aromatic ring is benzene or naphthalene. In one example, the aromatic ring is benzene.

[0102] In one embodiment, the negative electrode coating agent may be configured with one or more nitrogen atoms bonded to an aromatic ring. In one embodiment, the negative electrode coating agent may be a compound having a structure in which a nitrogen atom is bonded to an aromatic ring and, in addition to the nitrogen atom, one or more elements selected from the group consisting of N, S, and O are each independently bonded. Using a compound in which a nitrogen atom is bonded to an aromatic ring as the negative electrode coating agent can improve the cycle characteristics of the battery.

[0103] The negative electrode coating agent may be, for example, at least one selected from the group consisting of benzotriazole, benzimidazole, benzimidazole thiol, benzoxazole, benzoxazole thiol, benzothiazole, and mercaptobenzothiazole, and derivatives thereof. In one example, the negative electrode coating agent is at least one selected from the group consisting of benzotriazole, benzimidazole, benzoxazole, and mercaptobenzothiazole, and derivatives thereof.

[0104] (buffer layer) In one embodiment, a porous or fibrous buffer functional layer 50 may be provided between the negative electrode 10 and the separator 20. The buffer functional layer has a solid portion (including a gel portion) that has ionic conductivity and electrical conductivity, and a pore portion formed by gaps in this solid portion. In this case, lithium metal may precipitate on the surface of the negative electrode 10 (the interface between the negative electrode 10 and the buffer functional layer) and / or inside the buffer functional layer (the surface of the solid portion of the buffer functional layer).

[0105] The buffer layer may be, for example, a nonwoven fabric or woven fabric made of fibers. The material constituting the buffer layer may be inorganic, organic, metallic, or a combination thereof. A non-electronically conductive skeleton may be plated to impart electrical conductivity.

[0106] When a secondary battery 1 having a buffer functional layer is charged, electrons from the negative electrode 10 and lithium ions from the separator 20 and / or the electrolyte are supplied to the solid portion of the buffer functional layer. As a result, the electrons and lithium ions react on the surface of the solid portion of the buffer functional layer, and lithium metal precipitates in the pores (surface of the solid portion). This allows the buffer functional layer to suppress volume expansion of the battery due to lithium metal precipitation during charging. The buffer functional layer also contributes to increasing the surface area where lithium metal precipitates. This gently controls the reaction rate of lithium metal precipitation, suppresses dendrite formation, and ultimately improves the cycle characteristics of the secondary battery.

[0107] In one embodiment, the porosity of the buffer layer may be, for example, 50% or more, 60% or more, or 70% or more by volume, and may be, for example, 97% or less, 95% or less, or 90% or less by volume.

[0108] In one embodiment, the thickness of the buffer functional layer may be, for example, 100 μm, 50 μm or less, or 30 μm or less.The thickness of the buffer functional layer may be, for example, 1 μm, 4 μm or more, or 7 μm or more.

[0109] In one embodiment, when the buffer functional layer contains a metal that can react with lithium, the total capacity of the negative electrode 10 and the buffer functional layer may be sufficiently small relative to the capacity of the positive electrode 34, for example, 20% or less, 15% or less, 10% or less, or 5% or less.

[0110] In one embodiment, the weight per surface of the buffer functional layer may be 3 g / m 2 or more and 20 g / m 2 or less, 4 g / m 2 or more and 15 g / m 2 or less, or 5 g / m 2 or more and 10 g / m 2 or less.

[0111] Fig. 6A is a diagram showing an example of the cross-sectional structure of a fiber-shaped buffer function layer 50. Fig. 6B is a diagram showing an example of the state of the buffer function layer 50 during charging. Fig. 6C is a diagram showing an example of the cross-sectional structure of a fiber 52.

[0112] 6A, the buffer functional layer 50 has fibers 52 (solid portions) and pore portions 54 formed by gaps between the fibers 52. The fibers 52 have ionic conductivity and electrical conductivity.

[0113] As shown in FIG. 6B, when the secondary battery 1 having the buffer functional layer 50 is charged, lithium metal 56 is deposited on the surface of the fibers 52 (solid portion) of the buffer functional layer so as to fill the pores 54.

[0114] As shown in FIG. 6C, fiber 52 may be composed of a fibrous ion conducting layer 520 and an electrical conducting layer 522 that covers the surface of ion conducting layer 520.

[0115] The diameter of ion conduction layer 520 may be 30 to 5000 nm, 50 to 2000 nm, 70 to 1000 nm, or 80 to 500 nm. The thickness of electrical conduction layer 522 may be 1 to 300 nm, 5 to 200 nm, or 10 to 150 nm.

[0116] (Intermediate laminate) 7 and 8 are cross-sectional views of a main part illustrating another example of the configuration of a lithium secondary battery. As shown in Fig. 7 and Fig. 8, in one embodiment, the negative electrode 10 and the separators 20 disposed on both sides of the negative electrode 10 may be configured as a single sheet SH.

[0117] In one embodiment, as shown in FIG. 7, a sheet SH may be folded multiple times at acute angles to form an intermediate laminate, and each positive electrode laminate 30 (30A, 30B) may be disposed between opposing separators of the intermediate laminate.

[0118] In one embodiment, as shown in Fig. 8, a sheet SH may be folded back and wound multiple times to form an intermediate laminate, and each positive electrode laminate 30 (30A, 30B) may be disposed between opposing separators of the intermediate laminate. In one embodiment, each positive electrode laminate 30 may be formed by winding a single sheet as described below (see Fig. 9).

[0119] 7 and 8, even if the negative electrode 10 and separator 20 are very thin, they can be handled as a single sheet SH, which can improve the productivity of the battery. Furthermore, in the sheet SH, physical pressure is applied from both sides to the negative electrode 10 sandwiched between the separators 20, so wrinkles are less likely to occur in the negative electrode 10 when the sheets SH are stacked, which can improve the cycle characteristics of the battery.

[0120] (Positive electrode laminate) 9 and 10 are perspective views illustrating other configuration examples of a positive electrode laminate. In one embodiment, each positive electrode laminate 30 may be configured by winding a single sheet SH2 including a current collector 32 and positive electrodes 34 disposed on both sides of the current collector 32 multiple times, as shown in FIG. 9. In one embodiment, each positive electrode laminate 30 may be configured by alternately folding a single sheet SH2 multiple times at acute angles, as shown in FIG. 10. In the examples shown in FIGS. 9 and 10, even if the current collector 32 and positive electrodes 34 are very thin, they can be handled as a single sheet SH2, which can improve battery productivity.

[0121] <Example> Next, examples and comparative examples will be described. The present disclosure is not limited in any way by the following examples and comparative examples.

[0122] Fig. 11 shows the configurations and results of the examples and comparative examples. Fig. 12 shows the lamination patterns of the metal sheets in the examples and comparative examples. "Pattern 1" to "Pattern 4" in Fig. 12 correspond to "Pattern 1" to "Pattern 4" shown in "Lamination Pattern" in Fig. 11.

[0123] Example 1 As Example 1, a lithium secondary battery having the structure shown in FIG. 1 was fabricated. First, a negative electrode 10 was prepared. That is, a 6 μm-thick polyethylene terephthalate (PET) sheet with 1.0 μm of Cu vapor-deposited on both sides was prepared as a negative electrode current collector. Then, a mixed material was prepared by mixing 97 parts by mass of graphite as a negative electrode active material, 0.5 parts by mass of carbon black as a conductive aid, and 1.5 parts by mass of carboxymethyl cellulose (CMC) and 1.0 part by mass of styrene-butadiene rubber (SBR) as binders with water as a solvent. This mixed material was applied to both sides of the negative electrode current collector so that the basis weight was 15 mg / cm. 2 The negative electrode 10 was coated with the paste, pressed, and cut out to a predetermined size. This yielded 21 negative electrodes 10. A metal sheet for the negative electrode (4 μm thick copper foil) was attached to the end 12 of each negative electrode 10 by ultrasonic welding. Next, a sheet (thickness: 15 μm) whose surface was coated with a mixture of polyvinylidene fluoride (PVDF) and Al2O3 was prepared as the separator 20. Both sides of the negative electrode 10 were then sandwiched between the separators 20 and pressed to obtain an intermediate laminate LM.

[0124] The current collector 32 of the positive electrode laminate 30 was a 6 μm-thick polyethylene terephthalate (PET) film with an insulating layer 320 and 1.0 μm of Al (conductive layer 322) deposited on both sides. The positive electrode 34 was made of N-methyl-pyrrolidone (NMP) as a solvent and LiNi as a positive electrode active material. 0.8 Co 0.15 Al 0.05 A mixture of 96 parts by mass of O2, 2 parts by mass of carbon black as a conductive additive, and 2 parts by mass of polyvinylidene fluoride (PVDF) as a binder was used. This positive electrode 34 was applied to both sides of the current collector 32 with a basis weight of 23 mg / cm. 2A positive electrode laminate 30 was obtained by applying the paste to the positive electrode laminate 30. Twenty positive electrode laminates 30 were prepared. A metal sheet MS having the configuration shown in FIG. 11 was attached to the end P of the current collector 32 of each of the ten positive electrode laminates 30 by ultrasonic welding. A positive electrode tab 40 (thickness: 0.2 mm) made of the material shown in FIG. 11 was also prepared. Nickel-plated copper having a thickness of 0.2 mm was used as the negative electrode tab 42.

[0125] Next, intermediate laminates LM and positive electrode laminates 30 were alternately stacked. At this time, positive electrode laminates 30 with metal sheets MS attached to their end portions P and positive electrode laminates 30 without metal sheets MS attached were appropriately selected to form stacking pattern 1 shown in FIG. 12 . Then, each end portion P of the current collector 32 was overlapped with the metal sheets MS and joined to the positive electrode tab 40 by ultrasonic welding. The negative electrode end portion 12 was overlapped with the negative electrode metal sheet and joined to the negative electrode tab 42 by ultrasonic welding. This structure was inserted into a laminate outer casing and sealed together with an electrolyte to obtain a lithium secondary battery. The electrolyte used was a 1M electrolyte solution prepared by dissolving lithium hexafluorophosphate (LiPF₆) in a solvent containing a 30:35:35 mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) at a ratio of 30:35:35 by mass, to which 2 parts by weight of vinylene carbonate (VC) was added.

[0126] Examples 2 to 8 In Examples 2 to 8, as shown in FIGS. 11 and 12, lithium secondary batteries were fabricated in the same manner as in Example 1, except that the lamination patterns of the metal sheet MS, the electrode tab 40 for the positive electrode, and the metal sheet MS were different.

[0127] (Comparative Example 1) In Comparative Example 1, a lithium secondary battery was produced in the same manner as in Example 1, except that the metal sheet MS was not used.

[0128] 11, "X / Y" is the value obtained by dividing X by Y, where X (=A+B) is the sum of the total thickness (A) of each metal sheet MS and the total thickness (B) of each conductive layer 322, and Y is the total thickness of each insulating layer 322, for Examples 1 to 8. In Comparative Example 1, no metal sheet MS was used, so X is the total thickness of each conductive layer.

[0129] In FIG. 11 , “resistance [mΩ]” refers to the resistance at the joint region between the positive electrode tab 40 and the end P, and was measured as follows. Specifically, the lithium secondary batteries according to the example and comparative example were each disassembled, and measurements were performed using a four-terminal method. The positive electrode of a clip-type lead of a HIOKI resistance meter BT3561 was connected to the positive electrode tab 40, and the negative electrode was clipped with a clip at a location of one of the 20 positive electrode laminates 30 where the positive electrode active material was not applied, and the impedance at 1 kHz was measured using the four-terminal lead. Next, the negative electrode was connected to another positive electrode laminate, and measurements were performed, and the average value of the 20 positive electrode laminates was calculated. This average value is the “resistance (mΩ)” shown in FIG. 11 . As shown in FIG. 11 , the resistance of the joint region in Examples 1 to 8 was significantly lower than that in Comparative Example 1.

[0130] Furthermore, when a nail penetration test (a test in which a nail is pierced through each battery to simulate an internal short circuit and check whether the battery ignites or explodes) was conducted on the lithium secondary batteries of Examples 1 to 8 and Comparative Example 1, no fire or explosion occurred in any of the batteries.

[0131] (Comparative Example 2) In Comparative Example 2, a lithium secondary battery was produced in the same manner as in Example 1, except that no metal sheet for the negative electrode was used.

[0132] In Example 1 and Comparative Example 2, the resistance in the joint area between the negative electrode tab 42 and the negative electrode end portion 12 was measured in the same manner as described above. In Comparative Example 2, the resistance was 19.4 mΩ. In contrast, in Example 1, the resistance was 0.88 mΩ, which was significantly lower than that of Comparative Example 2.

[0133] Embodiments of the present disclosure further include the following aspects.

[0134] (Appendix 1) A lithium secondary battery, (a) a first laminate including a first current collector having a first insulating layer sandwiched between a pair of first conductive layers, and a first electrode disposed on the first current collector, the first current collector having a first end exposed from the first electrode; (b) an intermediate laminate including an electrode having a polarity different from that of the first electrode and a separator; (c) a second laminate disposed apart from the first laminate in the stacking direction via the intermediate laminate, the second laminate including a second current collector configured by sandwiching a second insulating layer between a pair of second conductive layers, and a second electrode disposed on the second current collector and having the same polarity as the first electrode, the second current collector having a second end exposed from the second electrode; (d) a metal sheet disposed between the first end and the second end; (e) an electrode tab that forms a joining region with the first end, the metal sheet, and the second end and is electrically connected to the first laminate and the second laminate; the bonding region includes a first region and a second region in a cross section in the stacking direction, the first region is configured by integrally stacking the pair of first conductive layers, the metal sheet, and the pair of second conductive layers, the second region includes the pair of first conductive layers sandwiching the first insulating layer, and the pair of second conductive layers sandwiching the metal sheet and the second insulating layer. Lithium secondary battery.

[0135] (Appendix 2) 2. The lithium secondary battery according to claim 1, wherein a plurality of the first stacks and the second stacks are alternately arranged in the stacking direction with the intermediate stack sandwiched between them.

[0136] (Appendix 3) The lithium secondary battery according to claim 1 or 2, wherein the first laminate is made of a flat sheet, and the second laminate is made of a flat sheet separate from the first laminate. (Appendix 4) 3. The lithium secondary battery according to claim 1, wherein the first laminate and the second laminate are formed by folding or rolling a single sheet.

[0137] (Appendix 5) 5. The lithium secondary battery according to claim 1, wherein a total of 10 or more layers of the first laminate and the second laminate are arranged.

[0138] (Appendix 6) 6. The lithium secondary battery according to claim 1, wherein the first ends and the second ends are arranged alternately in the stacking direction, and the metal sheet is arranged between at least one of the first ends and the second ends.

[0139] (Appendix 7) 7. The lithium secondary battery according to claim 1, wherein the number of the metal sheets is three times or less the total number of the first end portions and the second end portions.

[0140] (Appendix 8) 8. The lithium secondary battery according to claim 1, wherein the number or thickness of metal sheets disposed between the first end and the second end is set based on the positions of the first end and the second end in the stacking direction.

[0141] (Appendix 9) 9. The lithium secondary battery according to any one of Supplementary Note 1 to Supplementary Note 8, wherein the metal sheet is further disposed between the electrode tab and the first end or the second end opposite the electrode tab.

[0142] (Appendix 10) When the total thickness of the pair of first conductive layers, the pair of second conductive layers, and the metal sheet is X, and the total thickness of the first insulating layer and the second insulating layer is Y, the lithium secondary battery according to any one of Appendices 1 to 9, in which the relationship 0.85 < X / Y < 2.3 holds.

[0143] (Appendix 11) The lithium secondary battery according to any one of Appendices 1 to 10, in which the thickness of the metal sheet is 3 μm or more and 15 μm or less.

[0144] (Appendix 12) The lithium secondary battery according to any one of Appendices 1 to 11, in which the metal sheet is made of the same material as the first conductive layer and the second conductive layer.

[0145] (Appendix 13) The lithium secondary battery according to any one of Appendices 1 to 12, in which the maximum thickness of the first region is half or less of the maximum thickness of the second region.

[0146] (Appendix 14) The lithium secondary battery according to any one of Appendices 1 to 13, in which the first region is disposed between two of the second regions in a cross section in the stacking direction.

[0147] (Appendix 15) The lithium secondary battery according to any one of Appendices 1 to 14, in which the first electrode and the second electrode are positive electrodes.

[0148] (Appendix 16) The lithium secondary battery according to Appendix 15, in which the metal sheet is a soft aluminum foil.

[0149] (Appendix 17) The lithium secondary battery according to Appendix 15 or 16, in which the electrode tab is made of hard aluminum.

[0150] (Appendix 18) 18. The lithium secondary battery according to any one of claims 15 to 17, wherein the resistance of the junction region is 5.0 mΩ or less.

[0151] (Appendix 19) 15. The lithium secondary battery according to any one of claims 1 to 14, wherein the first electrode and the second electrode are negative electrodes.

[0152] (Appendix 20) 20. The lithium secondary battery according to claim 19, wherein the metal sheet is formed from at least one selected from the group consisting of metals that do not react with Cu, Ni, Ti, Fe, and Li, alloys thereof, and stainless steel.

[0153] The above embodiments are described for the purpose of explanation and are not intended to limit the scope of the present disclosure. Various modifications can be made to the above embodiments without departing from the scope and spirit of the present disclosure. For example, some components in one embodiment can be added to other embodiments. Also, some components in one embodiment can be replaced with corresponding components in other embodiments. [Explanation of symbols]

[0154] 1...Lithium secondary battery, 10...Anode, 20...Separator, 30...Positive electrode laminate, 32...Current collector, 320...Insulating layer, 322...Conductive layer, 34...Positive electrode, 40...Electrode tab for positive electrode, 42...Electrode tab for negative electrode, M...Metal sheet, LM...Intermediate laminate, R1...First region, R2...Second region

Claims

1. A lithium secondary battery, (a) a first laminate including: a first current collector having a first insulating layer sandwiched between a pair of first conductive layers; and a first electrode disposed on the first current collector, the first current collector having a first end exposed from the first electrode; (b) an intermediate laminate including an electrode having a polarity different from that of the first electrode and a separator; (c) a second laminate disposed apart from the first laminate in the stacking direction via the intermediate laminate, the second laminate including a second current collector configured by sandwiching a second insulating layer between a pair of second conductive layers, and a second electrode disposed on the second current collector and having the same polarity as the first electrode, the second current collector having a second end exposed from the second electrode; (d) a metal sheet disposed between the first end and the second end; (e) an electrode tab that forms a joining region with the first end, the metal sheet, and the second end and is electrically connected to the first stack and the second stack; The bonding region includes a first region and a second region in a cross section in the stacking direction, the first region is formed by stacking and integrating the pair of first conductive layers, the metal sheet, the pair of second conductive layers, and the electrode tabs adjacent to each other; the second region includes the pair of first conductive layers sandwiching the first insulating layer, and the pair of second conductive layers sandwiching the metal sheet and the second insulating layer; Lithium secondary battery.

2. 2. The lithium secondary battery according to claim 1, wherein one of the first regions is disposed between two of the second regions, one end of the one first region is disposed contiguous with one of the two second regions, and the other end of the one first region is disposed contiguous with the other of the two second regions.

3. The lithium secondary battery according to claim 1 , wherein a plurality of the first stacked bodies and a plurality of the second stacked bodies are alternately arranged in the stacking direction with the intermediate stacked body sandwiched therebetween.

4. 4. The lithium secondary battery according to claim 3, wherein the first laminate is formed of a flat sheet, and the second laminate is formed of a flat sheet separate from the first laminate.

5. 4. The lithium secondary battery according to claim 3, wherein the first laminate and the second laminate are formed by folding or rolling a single sheet.

6. The lithium secondary battery according to claim 3 , wherein a total of 10 or more layers of the first stack and the second stack are arranged.

7. 4. The lithium secondary battery according to claim 3, wherein the first end portions and the second end portions are arranged alternately in the stacking direction, and the metal sheet is arranged at least one between the plurality of first end portions and the plurality of second end portions.

8. 8. The lithium secondary battery according to claim 7, wherein the number of the metal sheets is three times or less the total number of the first end portions and the second end portions.

9. 8. The lithium secondary battery according to claim 7, wherein the number or thickness of the metal sheets disposed between the first end and the second end is set based on the positions of the first end and the second end in the stacking direction.

10. The lithium secondary battery according to claim 7 , wherein the metal sheet is further disposed between the electrode tab and the first end or the second end opposite the electrode tab.

11. 11. The lithium secondary battery according to claim 1, wherein a relationship of 0.85<X / Y<2.3 is satisfied, where X is a total thickness of the pair of first conductive layers, the pair of second conductive layers, and the metal sheet, and Y is a total thickness of the first insulating layer and the second insulating layer.

12. 11. The lithium secondary battery according to claim 1, wherein the metal sheet has a thickness of 3 μm or more and 15 μm or less.

13. 11. The lithium secondary battery according to claim 1, wherein the metal sheet is made of the same material as the first conductive layer and the second conductive layer.

14. 11. The lithium secondary battery according to claim 1, wherein the maximum thickness of the first region is equal to or less than half the maximum thickness of the second region.

15. The lithium secondary battery according to claim 14 , wherein, in the cross section in the stacking direction, the first region is disposed between two of the second regions.

16. 11. The lithium secondary battery according to claim 1, wherein the first electrode and the second electrode are positive electrodes.

17. 17. The lithium secondary battery according to claim 16, wherein the metal sheet is a soft aluminum foil.

18. 18. The lithium secondary battery according to claim 17, wherein the electrode tab is made of hard aluminum.

19. 17. The lithium secondary battery according to claim 16, wherein the resistance of the junction region is 5.0 mΩ or less.

20. 11. The lithium secondary battery according to claim 1, wherein the first electrode and the second electrode are negative electrodes.

21. 21. The lithium secondary battery according to claim 20, wherein the metal sheet is formed from at least one selected from the group consisting of metals that do not react with Cu, Ni, Ti, Fe, and Li, alloys thereof, and stainless steel.

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