Lithium secondary battery and method for manufacturing the same

Abstract

Provided is a technology that improves the energy density and safety of lithium secondary batteries. This lithium secondary battery comprises: an electrode tab; and a plurality of positive electrode stacks disposed separated from each other in the stacking direction via an interposed negative electrode and separator. Each positive electrode stack has: a current collector that is constituted of an insulating layer sandwiched by conductive layers and that is electrically connected to the electrode tab; and a positive electrode disposed on each side of the current collector. At least one of the current collectors of the plurality of positive electrode stacks is joined to the electrode tab at a position that is different, when viewed from the stacking direction, from at least one other current collector.

Description

[Technical Field]

[0001] Exemplary embodiments of this disclosure relate to lithium secondary batteries and methods for manufacturing the same. [Background technology]

[0002] In recent years, technologies that convert natural energy sources such as solar or wind power into electrical energy have attracted attention. Accordingly, various secondary batteries have been developed as energy storage devices that are highly safe and capable of storing large amounts of electrical energy.

[0003] Among these, lithium-ion secondary batteries, which perform charging and discharging by the movement of lithium ions between the positive and negative electrodes, are known to exhibit high voltage and high energy density. A typical lithium-ion secondary battery is the lithium-ion battery (LIB), which has active materials for holding lithium elements in both the positive and negative electrodes. In lithium-ion secondary batteries, charging and discharging occur through the exchange of lithium ions between the positive electrode active material and the negative electrode active material.

[0004] Furthermore, in order to achieve higher energy density, lithium secondary batteries (lithium metal batteries; LMBs) have been developed that use lithium metal as the negative electrode active material instead of materials that can insert lithium ions, such as carbon materials. For example, Patent Document 1 discloses a rechargeable battery that uses a lithium metal-based electrode as the negative electrode.

[0005] Furthermore, lithium secondary batteries using negative electrodes without negative electrode active materials such as carbon materials or lithium metal have been developed with the aim of further increasing energy density and improving productivity. For example, Patent Document 2 discloses a lithium secondary battery comprising a positive electrode, a negative electrode, a separation membrane interposed between them, and an electrolyte, wherein the negative electrode has metal particles formed on a negative electrode current collector, which are moved from the positive electrode by charging, forming lithium metal on the negative electrode current collector within the negative electrode. Patent Document 2 discloses that such a lithium secondary battery can solve the problems caused by the reactivity of lithium metal and the problems that occur during the assembly process, and can provide a lithium secondary battery with improved performance and lifespan. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Special Publication No. 2006-500755 [Patent Document 2] Special Publication No. 2019-505971 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] This disclosure provides a technology to improve the energy density and safety of lithium secondary batteries. [Means for solving the problem]

[0008] In one exemplary embodiment of the present disclosure, a lithium secondary battery is provided, comprising an electrode tab and a plurality of positive electrode stacks spaced apart from each other in the stacking direction via a negative electrode and a separator, each of the positive electrode stacks having a current collector comprising an insulating layer sandwiched between conductive layers and electrically connected to the electrode tab, and positive electrodes provided on both sides of the current collector, wherein at least one of the current collectors of the plurality of positive electrode stacks is joined to the electrode tab at a position different from that of at least one other current collector with respect to the stacking direction. [Effect of the Invention]

[0009] According to one exemplary embodiment of the present disclosure, a technique for improving the energy density and safety of a lithium secondary battery can be provided. [Brief Description of the Drawings]

[0010] [Figure 1] It is an exploded perspective view for explaining a configuration example of a lithium secondary battery according to an embodiment. [Figure 2] It is a diagram showing an example of a cross-sectional structure of a positive electrode laminate. [Figure 3A] It is a front view showing an example of a current collector. [Figure 3B] It is a front view showing an example of a current collector. [Figure 4] It is a diagram schematically showing a joined state of a notch and an electrode tab. [Figure 5] It is a flowchart showing an example of a method for manufacturing a lithium secondary battery. [Figure 6A] It is a front view showing another example of a current collector. [Figure 6B] It is a front view showing another example of a current collector. [Figure 6C] It is a front view showing another example of a current collector. [Figure 7A] It is a front view showing another example of a current collector. [Figure 7B] It is a front view showing another example of a current collector. [Figure 8A] It is a diagram showing an example of a cross-sectional structure of a fibrous buffer functional layer. [Figure 8B] It is a diagram showing an example of a state of the buffer functional layer during charging. [Figure 8C] It is a diagram showing an example of a cross-sectional structure of a fiber. [Figure 9] It is a cross-sectional view of a main part for explaining another configuration example of a lithium secondary battery. [Figure 10] It is a cross-sectional view of a main part for explaining another configuration example of a lithium secondary battery. [Figure 11]This figure shows the configuration of the current collector and various evaluation results for the examples and comparative examples. [Modes for carrying out the invention]

[0011] The embodiments of this disclosure are described below.

[0012] In one exemplary embodiment, a lithium secondary battery is provided, comprising an electrode tab and a plurality of positive electrode stacks arranged spaced apart from each other in the stacking direction via a negative electrode and a separator, each positive electrode stack having a current collector configured by sandwiching an insulating layer between conductive layers and electrically connected to the electrode tab, and positive electrodes provided on both sides of the current collector, wherein at least one of the current collectors of the plurality of positive electrode stacks is joined to the electrode tab at a different position from that of at least one other current collector in the stacking direction.

[0013] In one exemplary embodiment, in each of the plurality of positive electrode stacks, the current collector has a notch that extends in a direction parallel to the main surface of the current collector and is joined to an electrode tab.

[0014] In one exemplary embodiment, the notch is located above or below the electrode tab in the stacking direction.

[0015] In one exemplary embodiment, the area of ​​the notch is smaller than the area of ​​the electrode tab.

[0016] In one exemplary embodiment, the notch of at least one current collector is located at a different position from the notch of another at least one current collector when viewed in the stacking direction.

[0017] In one exemplary embodiment, in each of the multiple positive electrode stacks, the current collector notches have openings, and the openings of at least one current collector are located at different positions from the openings of at least one other current collector when viewed from the stacking direction.

[0018] In one exemplary embodiment, the shape of at least one current collector differs from the shape of at least one other current collector.

[0019] In one exemplary embodiment, 20 to 30 positive electrode stacks are provided.

[0020] In one exemplary embodiment, 4 to 15 current collectors of a positive electrode stack are bonded to electrode tabs at the same position when viewed from the stacking direction.

[0021] In one exemplary embodiment, 4 to 15 current collectors of a positive electrode stack are bonded to an electrode tab at a different position from at least one other current collector in the stacking direction.

[0022] In one exemplary embodiment, the energy density is 300 Wh / kg or more.

[0023] In one exemplary embodiment, the rated capacity is 1.5A or more.

[0024] In one exemplary embodiment, the rated capacity is 5A or more.

[0025] In one exemplary embodiment, a negative electrode laminate is formed by alternately bending a sheet having a negative electrode and separators arranged on both sides of the negative electrode multiple times at acute angles, and each positive electrode laminate is positioned between the opposing separators of the negative electrode laminate.

[0026] In one exemplary embodiment, a negative electrode laminate is constructed by winding a sheet having a negative electrode and separators arranged on both sides of the negative electrode in a manner that involves multiple folds, and each positive electrode laminate is positioned between the opposing separators of the negative electrode laminate.

[0027] In one exemplary embodiment, a buffering layer is provided between the negative electrode and separators positioned on both sides of the negative electrode.

[0028] In one exemplary embodiment, the negative electrode substantially lacks negative electrode active material.

[0029] In one exemplary embodiment, the negative electrode 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.

[0030] In one exemplary embodiment, a method for manufacturing a lithium secondary battery is provided, comprising the steps of (a) arranging a plurality of positive electrode stacks spaced apart from each other in the stacking direction via a negative electrode and a separator, wherein each positive electrode stack has a current collector composed of an insulating layer sandwiched between conductive layers and positive electrodes provided on both sides of the current collector, and (b) joining each current collector of the plurality of positive electrode stacks to an electrode tab, wherein in step (b), at least one of the current collectors of the plurality of positive electrode stacks is joined to the electrode tab at a different position from that of at least one other current collector when viewed from the stacking direction.

[0031] In one exemplary embodiment, in step (b), the current collector is joined to the electrode tab using ultrasonic welding, laser welding, or resistance welding.

[0032] Hereinafter, each embodiment of this disclosure will be described in detail with reference to the drawings. In each drawing, the same or similar elements are denoted by the same reference numeral, and redundant explanations are omitted. Unless otherwise specified, positional relationships such as top, bottom, 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 actual ratios are not limited to those shown.

[0033] <Example configuration of a secondary battery> Figure 1 is an exploded perspective view illustrating an example of the configuration of a lithium secondary battery 1 (hereinafter also referred to as "secondary battery 1") according to one embodiment. As shown in Figure 1, the secondary battery 1 is composed of a plurality of positive electrode stacks 30 (30A, 30B) and positive electrode tabs 40 electrically joined to the positive electrode stacks 30. Each positive electrode stack 30 is spaced apart from each other in the stacking direction (z direction in Figure 1) via a negative electrode 10 and separators 20 placed on both sides of the negative electrode 10. The configurations will be described in detail below.

[0034] (Negative electrode) In one embodiment, the negative electrode 10 is composed of at least one material selected from the group consisting of Cu, Ni, Ti, Fe, and other metals that do not react with Li, alloys thereof, and stainless steel (SUS). The "metal that does not react with Li" may be any metal that does not react with lithium ions or lithium metal to form an alloy when the secondary battery 1 is in operation.

[0035] The negative electrode 10 may function as a current collector. As shown in Figure 1, the negative electrode 10 may have a negative electrode terminal 12. The negative electrode terminal 12 may be a notch extending from the surface and / or side of the negative electrode 10 in a direction parallel to the main surface of the negative electrode 10, as part of the negative electrode 10. The negative electrode terminal 12 may be a separate component from the negative electrode 10 and may be a conductive member electrically connected to the negative electrode 10. The negative electrode terminal 12 of each negative electrode 10 is electrically connected to an electrode tab (not shown) for the negative electrode.

[0036] In one embodiment, the negative electrode 10 substantially lacks negative electrode active material. In one embodiment, the thickness of the negative electrode active material layer deposited on the negative electrode 10 at the end of discharge (for example, when the open-circuit voltage of the battery is 2.5V or more and 3.6V or less) is 25 μm or less. In one embodiment, the thickness of the negative electrode active material layer 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 it may be 0 μm. By substantially lacking negative electrode active material in the negative electrode 10, the energy density per unit volume can be improved in addition to the energy density per unit 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."

[0037] The negative electrode active material is a substance that undergoes an electrode reaction, i.e., an oxidation reaction and a reduction reaction, at the negative electrode. The negative electrode active material may be, for example, lithium metal and alloys containing lithium metal, carbon-based materials, metal oxides, and metals that alloy with lithium and alloys containing said metal. The carbon-based materials may be, for example, graphene, graphite, hard carbon, carbon nanotubes, etc. The metal oxides may be, for example, titanium oxide compounds, cobalt oxide compounds, etc. The metals that alloy with lithium may be, for example, silicon, germanium, tin, lead, aluminum, and gallium.

[0038] In one embodiment, the negative electrode 10 does not have negative electrode active material before the initial charge of the battery (the state from when the battery is assembled until the first charge). That is, the secondary battery 1 may be charged and discharged after the initial charge by the deposition of lithium metal on the negative electrode and the electrolytic dissolution of the deposited lithium metal. In this case, the volume and mass occupied by the negative electrode active material are suppressed, the overall volume and mass of the battery become smaller, and the energy density becomes higher in principle. Note that "deposition of lithium metal on the negative electrode" includes not only the deposition of lithium metal on the surface of the negative electrode, but also the deposition of lithium metal on the surface of the solid electrolyte interface (SEI) layer or on the surface or inside the buffer function layer, which will be described later.

[0039] In one embodiment, the mass of lithium metal deposited on the negative electrode at a voltage of 4.2V is M. 4.2 Let M be the same mass with a voltage of 3.0V. 3.0 In that case, M 3.0 / M 4.2 This may be 40% or less or 35% or less. In one embodiment, ratio M 3.0 / M 4.2 This may be 1.0% or more, 2.0% or more, 3.0% or more, or 4.0% or more.

[0040] In one embodiment, the average thickness of the negative electrode 10 may be 1.0 μm or more and 30 μm or less. This can reduce the volume occupied by the negative electrode 10 and improve the energy density. The average thickness of the negative electrode 10 may also 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.

[0041] In one embodiment, the negative electrode 10 may be coated with a compound (hereinafter also referred to as "negative electrode coating agent") containing an aromatic ring in which two or more elements selected from the group consisting of N, S, and O are independently bonded to at least a portion of the surface facing the positive electrode laminate 30. The negative electrode coating agent can be retained on the negative electrode 10 by the above elements coordinating with the metal atoms constituting the negative electrode 10. According to this embodiment, the non-uniform deposition reaction of lithium metal on the surface of the negative electrode 10 can be suppressed, and the growth of lithium metal deposited on the negative electrode 10 in a dendritic form can be suppressed.

[0042] In one embodiment, the negative electrode coating agent is coated on at least a portion of the surface of the negative electrode 10. In one embodiment, 10% or more of the surface area may be covered with the negative electrode coating agent, and 20% or more, 40% or more, 60% or more, or 80% or more of the surface area may be covered with the negative electrode coating agent.

[0043] In one embodiment, the aromatic ring contained in the negative electrode coating agent may be aromatic hydrocarbons such as benzene, naphthalene, azulene, anthracene, and pyrene, as well as heteroaromatic compounds 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.

[0044] In one embodiment, the negative electrode coating agent may be composed of an aromatic ring to which one or more nitrogen atoms are bonded. 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 independently bonded. Using a compound in which a nitrogen atom is bonded to an aromatic ring as a negative electrode coating agent can improve the battery's cycle characteristics.

[0045] The negative electrode coating agent may be at least one selected from the group consisting of, for example, benzotriazole, benzimidazole, benzimidazole thiol, benzoxazole, benzoxazole thiol, benzothiazole, and mercaptobenzothiazole, as well as their derivatives. In one example, the negative electrode coating agent is at least one selected from the group consisting of benzotriazole, benzimidazole, benzoxazole, and mercaptobenzothiazole, as well as their derivatives.

[0046] (Separator) As shown in Figure 1, the separators 20 are arranged on both sides of the negative electrode 10. The separators 20 physically and / or electrically isolate the negative electrode 10 from the positive electrode laminate 30 and ensure the 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 a single material or a combination of two or more materials.

[0047] When the separator 20 includes an insulating porous member, the pores of the porous member are filled with an ion-conductive substance (electrolyte, polymer electrolyte, and / or gel electrolyte, etc.). This allows the separator 20 to exhibit ion conductivity. The material constituting the insulating porous member is not particularly limited, and examples 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 laminated structure thereof.

[0048] In one embodiment, the separator 20 may be coated on one or both sides with a separator coating layer. This can improve the cycle characteristics of the secondary battery 1. In one embodiment, the separator coating layer may be a continuous film of uniform thickness over an area of ​​50% or more of the surface of the separator 20. In one embodiment, the separator coating layer may contain a binder such as polyvinylidene fluoride (PVDF), a composite material of styrene-butadiene rubber and carboxymethylcellulose (SBR-CMC), and polyacrylic acid (PAA). In one embodiment, the separator coating layer may be composed of the above binder with inorganic particles such as silica, alumina, titania, zirconia, or magnesium hydroxide added.

[0049] In one embodiment, the average thickness of the separator 20 (including the coating layer if the separator 20 includes a coating layer) may be 3.0 μm or more and 40 μm or less. This allows for the separation of the negative electrode 10 and the positive electrode laminate 30 while reducing the volume occupied by the separator 20. In one embodiment, the average thickness of the separator 20 may be 5.0 μm or more and 30 μm or less, 7.0 μm or more and 10 μm or less, or 10 μm or more and 20 μm or less.

[0050] (Positive electrode stack) Figure 2 shows an example of the cross-sectional structure of the positive electrode laminate 30. As shown in Figure 2, the positive electrode laminate 30 has a current collector 32 and positive electrodes 34 arranged on both sides of the current collector 32.

[0051] The current collector 32 has an insulating layer 320 and a conductive layer 322 formed so as to sandwich the insulating layer 320.

[0052] The insulating layer 320 may be composed of, for example, a sheet-like (film-like) or fibrous resin. The resin may be at least one of the following: polyethylene terephthalate (PET), polyethylene, polypropylene or other polyolefin resins, polystyrene, polyvinyl chloride or other thermoplastic resins. The insulating layer 320 may be composed of multiple layers of at least one of these resins. In one embodiment, the insulating layer 320 is formed from a material having a melting point of 150°C or more and 300°C or less. In one embodiment, the average thickness of the insulating layer 320 may be 3 μm or more and 10 μm or less, preferably 4 μm or more and 8 μm or less.

[0053] The insulating layer 320 may melt in the event of abnormal heat generation due to overcharging or high temperature conditions, damaging the positive electrode laminate 30 and interrupting the short-circuit current inside the battery. This can suppress a rapid rise in temperature inside the secondary battery 1 and prevent the battery from igniting. In other words, the insulating layer 320 can contribute to improving the safety of the secondary battery 1.

[0054] The conductive layer 322 is formed on both sides of the insulating layer 320, sandwiching it between them. The conductive layer 322 is in physical and / or electrical contact with the positive electrode 34 and functions to transfer electrons to and from the positive electrode 34. The conductive layer 322 is composed of a conductor that does not react with lithium ions in the battery. In one embodiment, the conductive layer 322 is composed of at least one material selected from the group consisting of aluminum, titanium, stainless steel, nickel, and alloys thereof. In one example, the conductive layer 322 is aluminum or an aluminum alloy. In one embodiment, the conductive layer 322 is formed by vapor deposition, sputtering, electroplating, or bonding the above material to the surfaces on both sides of the insulating layer 320. In one embodiment, the average thickness of one side of each conductive layer 322 may be 0.5 μm or more and 5 μm or less, preferably 0.7 μm or more and 3 μm or less, and more preferably 0.8 μm or more and 2.0 μm or less.

[0055] As shown in Figure 2, the positive electrode 34 is formed on both sides of the current collector 32. The material of the positive electrode 34 may be appropriately selected from known materials depending on the application. The average thickness of the positive electrode 34 may be appropriately adjusted according to the desired battery capacity and rate characteristics. In one embodiment, the average thickness of each positive electrode 34 is, for example, 20 μm to 150 μm.

[0056] In one embodiment, the positive electrode 34 has a positive electrode active material. The positive electrode active material is a substance for holding a carrier metal in the positive electrode 34, and can also be called a host material for the carrier metal. The positive electrode active material may be a substance for holding lithium ions in the positive electrode 34, in which case lithium ions are filled into and released from the positive electrode active material by charging and discharging the battery. This can improve the battery's stability and output voltage.

[0057] 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 compound, a manganese oxide compound, or a nickel oxide compound. The metal phosphate may be, for example, an iron phosphate compound or a cobalt phosphate compound. In one embodiment, the positive electrode active material is LiCoO2, LiNi x Coy Mn z O(x + y + z = 1), LiNi x Co y Al z O(x + y + z = 1), LiNi x Mn y O(x + y = 1), LiNiO2, LiMn2O4, LiFePO, LiCoPO, LiFeOF, LiNiOF, and LiTiS2, and may be at least one selected from the group consisting of. 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.

[0058] In one embodiment, the positive electrode 34 may contain one or more components other than the positive electrode active material.

[0059] In one embodiment, the positive electrode 34 may contain a sacrificial positive electrode material. It is a lithium-containing compound that causes an oxidation reaction in the charge and discharge potential range of the positive electrode active material and does not substantially cause a reduction reaction.

[0060] In one embodiment, the positive electrode 34 may contain a gel electrolyte. The gel electrolyte can improve the adhesion force between the positive electrode 34 and the current collector 32. In one example, the gel electrolyte contains 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, and a copolymer of polyvinylidene fluoride and hexafluoropropylene, etc.

[0061] In one embodiment, the positive electrode 34 may contain a conductive additive and / or a binder. For example, the conductive additive may be carbon black, single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), carbon nanofibers (CF), etc. For example, the binder may be polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylic resin, polyimide resin, etc. In one embodiment, the content of the conductive additive may be 0.5% by mass or less of 30% by mass of the entire positive electrode 34. In one embodiment, the content of the binder may be 0.5% by mass or less of 30% by mass of the entire positive electrode 34.

[0062] In one embodiment, the positive electrode 34 may contain a polymer electrolyte. In one example, the polymer electrolyte is a solid polymer electrolyte mainly comprising a polymer and an electrolyte, and a semi-solid polymer electrolyte mainly comprising a polymer, an electrolyte, and a plasticizer. In one embodiment, the total content of the polymer electrolyte may be 0.5% by mass or less of the total positive electrode 34.

[0063] Of the multiple positive electrode stacks 30, the current collector 32 of at least one positive electrode stack 30 has a different shape from the current collector 32 of at least one other positive electrode stack 30. For example, as shown in Figure 1, the multiple positive electrode stacks 30 may consist of a first positive electrode stack 30A and a second positive electrode stack 30B. The current collectors 32 (32A, 32B) of the first positive electrode stack 30A and the second positive electrode stack 30B have different shapes.

[0064] Figure 3A is a front view showing an example of a current collector 32A of the first positive electrode stack 30A. Figure 3B is a front view showing an example of a current collector 32B of the second positive electrode stack 30B. As shown in Figure 3A, the current collector 32A has a notch NA extending in a direction parallel to the main surface of the current collector 32A. The notch NA is located on one side in the width direction of the current collector 32A (the right side when facing the plane of the paper in Figure 3A). As shown in Figure 3B, the current collector 32B has a notch NB extending in a direction parallel to the main surface. The notch NB is located on the other side in the width direction of the current collector 32B (the left side when facing the plane of the paper in Figure 3B). In other words, the front view shape (shape viewed from the stacking direction) of the current collector 32A and the current collector 32B is different due to the different extension positions of the notches (NA, NB). Note that the shapes of the notches NA and NB themselves may be the same or different.

[0065] The arrangement of the first positive electrode stack 30A and the second positive electrode stack 30B (the order in which they are arranged via the negative electrode 10 and separator 20 in the stacking direction) can be selected as appropriate. For example, as shown in Figure 1, the first positive electrode stack 30A and the second positive electrode stack 30B may be arranged alternately, one at a time. Alternatively, for example, sets of one or more first positive electrode stacks 30A and sets of one or more second positive electrode stacks 30B may be arranged alternately, one or more times.

[0066] In one embodiment, the number of the first positive electrode stacks 30A and the second positive electrode stacks 30B may be approximately the same (about half the total number of positive electrode stacks 30). In one embodiment, the number of the first positive electrode stacks 30A and the second positive electrode stacks 30B may be different. In one embodiment, the difference between the number of the first positive electrode stacks 30A and the number of the second positive electrode stacks 30B may be 10 or less, 5 or less, or 2 or less.

[0067] As shown in Figure 1, the electrode tab 40 for the positive electrode is positioned on one side (the lower side in Figure 1) of the stacking direction (z-direction in Figure 1) of the secondary battery 1. The electrode tab 40 may be made of the same material as the conductive layer 322 of the current collector 32, and in one example, it may be aluminum or an aluminum alloy. The electrode tab 40 may have a first junction region 40A and a second junction region 40B on its surface. As shown in Figure 1, the first junction region 40A and the second junction region 40B may be completely separate regions with no overlapping parts. Alternatively, the first junction region 40A and the second junction region 40B may have overlapping parts. The surface area of ​​the electrode tab 40 may be larger than the area of ​​the first notch NA and the second notch NB, respectively.

[0068] The first current collector 32A of the first positive electrode stack 30A and the current collector 32B of the second positive electrode stack 30B are joined to the electrode tab 40 at different positions when viewed from the stacking direction (z direction in Figure 1). More specifically, the first notch NA of the first current collector 32A is joined to the electrode tab 40 on the first joining region 40A on the surface of the electrode tab 40. The second notch NB of the second current collector 32B is joined to the electrode tab 40 on the second joining region 40B on the surface of the electrode tab 40. In one embodiment, the first notch NA and the second notch NB may be joined to the electrode tab 40 without overlapping when viewed from the stacking direction. In one embodiment, the first notch NA and the second notch NB may be joined to the electrode tab 40 such that a portion of them overlap when viewed from the stacking direction.

[0069] Figure 4 is a schematic diagram showing an example of the bonding state between the notches and the electrode tabs. As shown in Figure 4, the first notches NA (NA1 to NAn) are each positioned opposite to each other on the first bonding region 40A of the electrode tab 40 and bonded to the electrode tab 40. The second notches NB (NB1 to NBm) are each positioned opposite to each other on the second bonding region 40B of the electrode tab 40 and bonded to the electrode tab 40. The first notches NA and the second notches NB are bonded to the electrode tab 40 at different positions when viewed from the stacking direction (z direction in Figure 4).

[0070] As shown in Figure 4, each notch (NA, NB) is pressed in the stacking direction (z direction in Figure 4) when joined to the electrode tab 40, causing the insulating layer 320 to be pushed outward in the width direction, and the conductive layers 322, 322 sandwiching the insulating layer 320 to be electrically connected to each other. As a result, all positive electrodes 34 included in the multiple positive electrode stacks 30 (30A, 30B) are electrically connected to the electrode tab 40 (via the conductive layer 322 of the current collector 32 to which each positive electrode 34 is connected). In one embodiment, the joining of the notches to the electrode tab 40 may be done by ultrasonic welding, laser welding, resistance welding, or spot welding. The joining points between the notches and the joining points between the notches and the electrode tab 40 may exist as one or more points (spots) or as a continuous surface, as long as they are electrically connected.

[0071] As described above, the insulating layer 320 can suppress a rapid rise in temperature inside the secondary battery 1 in the event of abnormal heat generation during overcharging or high-temperature conditions, thereby suppressing battery ignition. In current collectors with such an insulating layer sandwiched between conductive layers, as the number of current collectors increases, it becomes more difficult to connect with the electrode tabs, and the resistance when connections are made tends to increase. In this regard, in the secondary battery 1 according to one embodiment, as described above, the first notch NA and the second notch NB are connected to the electrode tab 40 at positions that are different from each other when viewed from the stacking direction (z direction in Figure 4). That is, the connection points between the notches (NA, NB) and the electrode tab 40 are distributed to two locations when viewed from the stacking direction. Therefore, even if the total number of positive electrode stacks 30 in the secondary battery 1 (number of stacks) increases, the number of notches joined to the electrode tabs 40 at the same location when viewed from the stacking direction is suppressed (for example, if the number of first notches NA and second notches NB is the same, the number of notches joined at the same location when viewed from the stacking direction is half the total number of positive electrode stacks 30).

[0072] By reducing the number of notches joined at the same location when viewed from the stacking direction, the insulating layer 320 of each notch is pushed outward in the width direction when joining the notches (NA, NB) to the electrode tab 40, making it easier to ensure conductivity between the conductive layers 322, 322. As a result, an increase in the resistance of the electrode tab 40 and excessive heat generation can be suppressed. Furthermore, by reducing the number of notches joined at the same location when viewed from the stacking direction, the pressing force required when joining the notches (NA, NB) to the electrode tab 40 can be suppressed. This can prevent the notches (NA, NB) from being excessively pressed and damaged or broken during joining.

[0073] As described above, it is possible to increase the total number of positive electrode stacks 30 (number of stacks) included in the secondary battery 1 while suppressing the increase in electrical resistance of the current collector 32 and electrode tabs 40. This can improve the energy density, capacity, and cycle characteristics of the secondary battery 1.

[0074] In one embodiment, the total number of positive electrode stacks 30 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 stacks 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. The rated capacity of the lithium secondary battery may be 1.5 Ah or more, or 5 Ah or more.

[0075] (electrolyte) In one embodiment, the secondary battery 1 may contain an electrolyte. The electrolyte is a liquid containing a solvent and an electrolyte, and is ionic conductive. The electrolyte may also be called a liquid electrolyte, and acts as a conductive pathway for lithium ions. Therefore, when the secondary battery 1 contains an electrolyte, the internal resistance decreases, and the energy density, capacity, and cycle characteristics can be improved.

[0076] The electrolyte may be, for example, the solution used to fill the housing (pouch) of the secondary battery 1. Alternatively, the electrolyte may be impregnated into the separator 20, or it may be held by a polymer to constitute a polymer electrolyte or gel electrolyte.

[0077] The electrolyte contained in the electrolyte solution may be, for example, a lithium salt. The lithium salt may be one or more selected from the group consisting of LiI, LiCl, LiBr, LiF, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2CF3CF3)2, LiB(O2C2H4)2, LiB(C2O4)2, LiB(O2C2H4)F2, LiB(OCOCF3)4, LiNO3, and Li2SO4.

[0078] The solvent contained in the electrolyte may be, for example, a non-aqueous solvent containing fluorine atoms (hereinafter referred to as "fluorinated solvent") and a non-aqueous solvent not containing fluorine atoms (hereinafter referred to as "non-fluorinated solvent").

[0079] The fluorinating 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.

[0080] Non-fluorinated solvents may include, 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, ethylmethyl 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.

[0081] The above-mentioned fluorinated solvent and / or non-fluorinated solvent may be used individually or in any combination of two or more in any proportion. The content of the fluorinated solvent and non-fluorinated solvent is not particularly limited, and the proportion of the fluorinated solvent to the total solvent may be 0 to 100% by volume, and the proportion of the non-fluorinated solvent to the total solvent may be 0 to 100% by volume.

[0082] <Manufacturing method for secondary batteries> Figure 5 is a flowchart showing an example of a method for manufacturing a secondary battery 1. This method includes steps ST1 of arranging multiple positive electrode stacks, ST2 of joining current collectors to electrode tabs, and ST3 of sealing them in a sealed container.

[0083] In step ST1, multiple positive electrode stacks are arranged. First, a predetermined number (for example, 4 to 15) of the first positive electrode stack 30A and the second positive electrode stack 30B are prepared. Next, the first positive electrode stack 30A and the second positive electrode stack 30B are arranged in a predetermined order in the stacking direction via the negative electrode 10 and the separator 20. For example, as shown in Figure 1, the first positive electrode stack 30A and the second positive electrode stack 30B may be arranged alternately one by one via the negative electrode 10 and the separator 20. Furthermore, if the negative electrode 10 and the separator 20 are configured in sheet form as described later, the first positive electrode laminate 30A and the second positive electrode laminate 30B may be arranged in a predetermined order between separators 20 formed by folding the sheet in a zigzag (or woven) manner (see Figure 9), or between separators 20 formed by winding the sheet (see Figure 10).

[0084] Next, in step ST2, the current collector 32 of the positive electrode laminate 30 is joined to the electrode tab 40 for the positive electrode. Specifically, the first notch NA of the first current collector 32A is joined so that it faces each other on the first joining region 40A of the electrode tab 40. Also, the first notch NB of the second current collector 32B is joined so that it faces each other on the second joining region 40B of the electrode tab 40. As a result, the first notch NA and the second notch NB are joined to the electrode tab 40 at different positions when viewed from the lamination direction. The negative electrode terminal 12 of the negative electrode 10 is also joined to the electrode tab for the negative electrode. The joining of the current collector and the electrode tab may be done by ultrasonic welding, laser welding, resistance welding, or spot welding.

[0085] Next, in step ST3, the product is sealed into a sealed container. That is, the molded product obtained in step ST2 is sealed into a sealed container such as a laminate film. At this time, the electrolyte may also be sealed into the sealed container. Through this process, the secondary battery 1 is manufactured.

[0086] <How to use rechargeable batteries> The secondary battery 1 is charged and discharged by connecting the positive electrode tab 40 to one end of an external circuit and the negative electrode tab to the other end of the external circuit. The external circuit may be, for example, a resistor, power supply, device, another battery, or potentiostat. Each notch (NA, NB) of the multiple positive electrode stacks 30 may be connected to the external circuit at the same potential. Similarly, each negative electrode terminal 12 of the multiple negative electrodes 10 may be connected to the external circuit at the same potential.

[0087] When a voltage is applied between the positive electrode tab 40 and the negative electrode tab such that a current flows from the negative electrode tab through an external circuit to the positive electrode tab 40, 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 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.

[0088] In one embodiment, the secondary battery 1 may have a solid electrolyte interface layer (SEI layer) 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) after the first charge (initial charge) following the assembly of the battery. The SEI layer may contain, for example, an inorganic compound containing lithium or an organic compound containing lithium. In one embodiment, the average 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 charging and discharging. <Variation> The secondary battery 1 can be modified in various ways without departing from the scope and spirit of this disclosure.

[0089] (Shape of the current collector) In one embodiment, the shape of the current collector 32 included in the secondary battery 1 may be set to three or more types, so that each current collector 32 is joined to the electrode tab 40 at three or more different positions when viewed from the stacking direction. In one example, the shape of the current collector 32 included in the secondary battery 1 may be three, four, or five types. In one embodiment, the number of types of shapes of the current collector 32 included in the secondary battery 1 may be set so that the number of current collectors 32 joined to the electrode tab 40 at the same position when viewed from the stacking direction is 15 or less, 10 or less, 5 or less, or 3 or less.

[0090] Figures 6A to 6C are front views showing other examples of the current collector 32. In one embodiment, one or more current collectors 32C, 32D, and 32E shown in Figures 6A to 6C may be used as the current collector 32.

[0091] As shown in Figure 6A, the notch NC of the current collector 32C is located on one side of the current collector 32C in the width direction (the right side when facing the plane of the paper in Figure 6A). As shown in Figure 6B, the notch ND of the current collector 32D is located in the center of the current collector 32D in the width direction (the center when facing the plane of the paper in Figure 6B). As shown in Figure 6C, the notch NE of the current collector 32E is located on the other side of the current collector 32E in the width direction (the left side when facing the plane of the paper in Figure 6C). In other words, current collectors 32C to 32E have different shapes when viewed from the front (shape when viewed from the stacking direction) due to the different extension positions of the notches (NC to NE).

[0092] In one embodiment, the current collectors 32 of the multiple positive electrode stacks 30 may be any of the current collectors 32C to 32E (hereinafter, each positive electrode stack 30 will be referred to as "stack 30C", "stack 30D", and "stack 30E"). In this case, each current collector 32D to 32E of stacks 30C to 30E is joined to the electrode tab 40 at three different positions when viewed from the stacking direction. This allows the joining points between the notches (NC to NE) and the electrode tab 40 to be distributed to three different positions when viewed from the stacking direction.

[0093] The arrangement of the laminates 30C to 30E (the order in which they are arranged via the negative electrode 10 and separator 20 in the stacking direction) can be selected as appropriate. For example, laminates 30C, 30D, and 30E may be arranged one by one in this order. Alternatively, for example, a set of one or more laminates C, a set of one or more laminates D, and a set of one or more laminates E may be arranged in this order, repeated once or multiple times.

[0094] In one embodiment, the number of laminates 30C to 30E may be approximately the same, i.e., about one-third of the total number of positive electrode laminates 30. In one embodiment, the number of laminates 30C to 30E may be different. In one embodiment, the difference in the number of laminates 30C to 30E may be 10 or less, 5 or less, or 2 or less.

[0095] (Notch shape) In one embodiment, in addition to or instead of varying the extension position of the notch, the shape of the notch in a front view (shape as seen from the stacking direction) may be varied.

[0096] Figures 7A and 7B are front views showing other examples of the current collector 32, respectively. In one embodiment, one or more current collectors 32F and 32G shown in Figures 7A and 7B may be used as the current collector 32.

[0097] As shown in Figure 7A, the notch NF of the current collector 32F has an opening OP1 on one side in the width direction (the left side when facing the plane of the paper in Figure 7A). As shown in Figure 7B, the notch NG of the current collector 32G has an opening OP2 on the other side in the width direction (the right side when facing the plane of the paper in Figure 7B). In other words, the notches NF and NG of the current collectors 32F and 32G have different opening positions (OP1 and OP2), which results in different shapes when viewed from the front (shape when viewed from the stacking direction).

[0098] In one embodiment, the current collector 32 of the multiple positive electrode stacks 30 may be either current collector 32F or 32G (hereinafter, each positive electrode stack 30 will be referred to as "stack 30F" or "stack 30G"). Since the opening positions of the notches (NF, NG) of the current collectors 32F and 32G are different, they are joined to the electrode tabs 40 at different positions when viewed from the stacking direction. This allows the joining points between the notches (NF, NG) and the electrode tabs 40 to be distributed to two different positions when viewed from the stacking direction.

[0099] The arrangement of the laminates 30F and 30G (the order in which they are arranged via the negative electrode 10 and separator 20 in the stacking direction) can be selected as appropriate. For example, one laminate 30F and one laminate 30G may be arranged alternately. Alternatively, for example, one or more sets of laminates F and one or more sets of laminates G may be arranged alternately once or multiple times.

[0100] In one embodiment, the number of laminates 30F and 30G may be approximately the same, that is, about half the total number of positive electrode laminates 30. In one embodiment, the number of laminates 30F and 30G may be different. In one embodiment, the difference in the number of laminates 30F and 30G may be 10 or less, 5 or less, and 2 or less.

[0101] (buffer layer) In one embodiment, a porous or fibrous buffering layer 50 may be provided between the negative electrode 10 and the separator 20. The buffering layer has a solid portion (including a gel-like portion) that has ionic conductivity and electrical conductivity, and a void portion formed by the gaps between the solid portion. In this case, lithium metal may be deposited on the surface of the negative electrode 10 (the interface between the negative electrode 10 and the buffering layer) and / or inside the buffering layer (the surface of the solid portion of the buffering layer).

[0102] The buffering layer may be, for example, a nonwoven or woven fabric made of fibers. The materials constituting the buffering layer may be inorganic, organic, metallic, or a combination thereof. A material that has been plated to impart conductivity to a framework that does not have electrical conductivity may also be used.

[0103] When a secondary battery 1 having a buffer layer is charged, electrons from the negative electrode 10 and lithium ions from the separator 20 and / or electrolyte are supplied to the solid portion of the buffer layer. As a result, electrons and lithium ions react on the surface of the solid portion of the buffer layer, and lithium metal is deposited in the vacancies (on the surface of the solid portion). This allows the buffer layer to suppress the volume expansion of the battery caused by lithium metal deposition during charging. The buffer layer also contributes to expanding the surface area of ​​the lithium metal deposition site. This allows the reaction rate of lithium metal deposition to be controlled slowly, suppressing dendrite formation and ultimately improving the cycle characteristics of the secondary battery.

[0104] In one embodiment, the porosity of the buffering layer may be, for example, 50% or more, 60% or more, or 70% or more by volume. The porosity of the buffering layer may be, for example, 97% or less, 95% or less, or 90% or less by volume.

[0105] In one embodiment, the average thickness of the buffering layer may be, for example, 100 μm, 50 μm or less, or 30 μm or less. The average thickness of the buffering layer may be, for example, 1 μm, 4 μm or more, or 7 μm or more.

[0106] In one embodiment, if the buffering layer contains a metal that can react with lithium, the sum of the capacities of the negative electrode 10 and the buffering layer may be sufficiently small compared to the capacity of the positive electrode 34, for example, 20% or less, 15% or less, 10% or less, or 5% or less.

[0107] In one embodiment, the weight per surface area of ​​the buffering layer is 3 g / m². 2 More than 20g / m 2 Below, 4g / m 2 More than 15g / m 2 , or 5g / m 2 More than 10g / m 2 The following is fine.

[0108] Figure 8A shows an example of the cross-sectional structure of the fibrous buffering layer 50. Figure 8B shows an example of the state of the buffering layer 50 when charged. Figure 8C shows an example of the cross-sectional structure of the fiber 52.

[0109] As shown in Figure 8A, the buffering layer 50 has fibers 52 (solid portion) and void portions 54 formed by the gaps between the fibers 52. The fibers 52 have ionic conductivity and electrical conductivity.

[0110] As shown in Figure 8B, when the secondary battery 1 having a buffer layer 50 is charged, lithium metal 56 is deposited on the surface of the fibers 52 (solid portion) of the buffer layer, filling the voids 54.

[0111] As shown in Figure 8C, the fiber 52 may consist of a fibrous ion-conducting layer 520 and an electrical conductive layer 522 that covers the surface of the ion-conducting layer 520.

[0112] The average diameter of the ion-conducting layer 520 may be 30 nm to 5000 nm, 50 nm to 2000 nm, 70 nm to 1000 nm, or 80 nm to 500 nm. The average thickness of the electrical-conducting layer 522 may be 1 nm to 300 nm, 5 nm to 200 nm, or 10 nm to 150 nm.

[0113] (Negative electrode laminate) In one embodiment, the negative electrode 10 and the separator 20 arranged on both sides of the negative electrode 10 may be configured in a sheet shape.

[0114] Figures 9 and 10 are cross-sectional views of key parts illustrating other configuration examples of lithium secondary batteries, respectively. As shown in Figures 9 and 10, the negative electrode 10 and the separator 20 arranged on both sides of the negative electrode 10 may be configured as a single sheet SH.

[0115] In the example shown in Figure 9, sheet SH is folded alternately at multiple acute angles to form a negative electrode laminate, and multiple positive electrode laminates 30 (30A, 30B) are arranged between the opposing separators of the negative electrode laminate.

[0116] In the example shown in Figure 10, the sheet SH is wound in a manner that involves multiple folds to form a negative electrode laminate, and multiple positive electrode laminates 30 (30A, 30B) are respectively placed between the opposing separators of the negative electrode laminate.

[0117] In the examples shown in Figures 9 and 10, even when the negative electrode 10 and separator 20 are very thin, they can be handled as a single sheet SH, which can improve battery productivity. Furthermore, in the sheet SH, physical pressure is applied to the negative electrode 10 sandwiched between the separator 20 from both sides, which makes it less likely for wrinkles to form on the negative electrode 10 when the sheet SH is laminated, thereby potentially improving the battery's cycle characteristics.

[0118] <Examples> The present disclosure will be described below using examples and comparative examples. The present disclosure is not limited in any way by the following examples and comparative examples.

[0119] (Example 1) As Example 1, a lithium secondary battery with the structure shown in Figure 9 was fabricated. A sheet-like Cu substrate with a thickness of 8 μm was prepared as the negative electrode 10. Ni terminals were joined to the negative electrode 10 as negative electrode terminals 12 by ultrasonic welding. A sheet (thickness: 15 μm) with a surface coated with a mixture of polyvinylidene fluoride (PVDF) and Al2O3 was prepared as the separator 20. Sheet SH was obtained by sandwiching both sides of the negative electrode 10 with the separator 20 and pressing it.

[0120] As the current collector 32 of the positive electrode laminate 30, a 6 μm thick film of polyethylene terephthalate (PET, insulating layer 320) was coated with 1.5 μm of Al (conductive layer 322) on both sides, and then punched out into two different shapes. This resulted in 5 cm × 10 cm current collectors 32A and 32B having notches NA and NB as shown in Figures 3A and 3B. As the positive electrode 34, N-methyl-pyrrolidone (NMP) was used as the solvent, and LiNi was used as the 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 the current collector 32A and the current collector 32B, respectively, to obtain the first positive electrode laminate 30A and the second positive electrode laminate 30B. Ten of each of the first positive electrode laminate 30A and the second positive electrode laminate 30B were prepared.

[0121] Next, sheet SH was placed in an automatic lamination device, and a negative electrode laminate was formed by automatically folding sheet SH multiple times at sharp angles alternately. At the same time, one first positive electrode laminate 30A and one second positive electrode laminate 30B were alternately inserted between opposing separators 20. Then, the current collectors 32A (10 pieces) of the first positive electrode laminate 30A and the current collectors 32B (10 pieces) of the second positive electrode laminate 30B were joined to the electrode tabs 40 for the positive electrode by ultrasonic welding. At this time, the first notch NA of the current collector 32A and the second notch NB of the current collector 32B were joined to the electrode tabs 40 at positions that are different from each other when viewed from the lamination direction. The negative electrode terminals 12 were also joined to the electrode tabs for the negative electrode by ultrasonic welding. This structure was inserted into a laminate casing and sealed together with the electrolyte to obtain a lithium secondary battery. The electrolyte used was 4M LiN(SO2F)2 dimethoxyethane.

[0122] (Example 2) In Example 2, a lithium secondary battery was obtained in the same manner as in Example 1, except that seven of each of three types of current collectors 32C to 32E, each having notches NC to NE and NB as shown in Figures 6A to 6, were used as the current collector 32 (the total number of positive electrode stacks 30 was 21).

[0123] (Example 3) In Example 3, a lithium secondary battery was obtained in the same manner as in Example 1, except that current collectors 32A and 32B measuring 10 cm × 20 cm were used.

[0124] (Comparative Example 1) In Comparative Example 1, a lithium secondary battery was obtained in the same manner as in Example 1, except that 20 current collectors 32 of the same shape were used, and the notches of each current collector were joined to the electrode tabs at the same position when viewed from the stacking direction.

[0125] (Comparative Example 2) In Comparative Example 2, a lithium secondary battery was obtained in the same manner as in Comparative Example 1, except that a 12 μm thick current collector consisting only of Al foil and without a resin layer was used as the current collector.

[0126] (Evaluation of battery characteristics) Figure 11 shows the configuration of the current collector and various evaluation results for the examples and comparative examples. In Figure 11, "Capacity" is the initial capacity [mAh] when each battery is charged to 4.3V at 0.2C and then discharged to 3.0V at 0.2C (hereinafter referred to as "initial discharge"). "Cell resistance" is the 1kHz impedance [mΩ] after the initial charge and discharge. For Examples 1, 2, Comparative Example 1, and 2, "Capacity retention rate" is the ratio (Q1 / Q2 × 100) [%] between the discharge capacity Q1 at 0.05C charge-7C discharge and the discharge capacity Q2 at 0.05C charge-0.05C discharge. For Example 3, "Capacity retention rate" is the ratio (Q3 / Q2 × 100) [%] between the discharge capacity Q3 at 0.05C charge-5C discharge and the discharge capacity Q2 at 0.05C charge-0.05C discharge. The electrode tab temperature is the temperature of the electrode tab [°C] measured after the initial discharge. The "piercing test" is a test to check whether the battery ignites or explodes when a nail is pierced through each battery to simulate an internal short circuit. "Pass" means that no ignition or explosion occurred, and "fail" means that ignition or explosion occurred.

[0127] As shown in Figure 11, Examples 1 to 3 yielded good results in all aspects, including cell resistance, capacitance retention rate, electrode tab temperature, and the piercing test. In contrast, Comparative Example 1 showed significantly higher cell resistance compared to Examples 1 to 3. This is presumed to be because, in Comparative Example 1, the notches of each current collector were joined to the electrode tabs at the same position when viewed from the stacking direction, resulting in insufficient bonding between the Al foils of each notch. As a result, Comparative Example 1 also showed a significant decrease in capacitance retention rate and an increase in electrode tab temperature. Comparative Example 2 failed the piercing test and ignition occurred. This is presumed to be because the current collector consisted only of Al foil and lacked an insulating layer.

[0128] Embodiments of this disclosure further include the following embodiments:

[0129] (Note 1) It is a lithium secondary battery, Electrode tabs and A plurality of positive electrode stacks arranged spaced apart from each other in the stacking direction via a negative electrode and a separator, wherein each of the positive electrode stacks comprises a current collector formed by sandwiching an insulating layer between conductive layers and electrically connected to the electrode tab, and positive electrodes provided on both sides of the current collector, Equipped with, A lithium secondary battery wherein at least one of the current collectors of the plurality of positive electrode stacks is bonded to the electrode tab at a different position from the other at least one current collector when viewed with respect to the stacking direction.

[0130] (Note 2) The lithium secondary battery as described in Appendix 1, wherein in each of the plurality of positive electrode stacks, the current collector has a notch that extends in a direction parallel to the main surface of the current collector and is joined to the electrode tab.

[0131] (Note 3) The lithium secondary battery as described in Appendix 2, wherein the notch is located above or below the electrode tab in the stacking direction.

[0132] (Note 4) The lithium secondary battery according to Appendix 2 or Appendix 3, wherein the area of ​​the notch is smaller than the area of ​​the electrode tab.

[0133] (Note 5) A lithium secondary battery according to any one of appendices 2 to 4, wherein the notch of at least one current collector is located at a different position from the notch of the other at least one current collector when viewed from the stacking direction.

[0134] (Note 6) In each of the plurality of positive electrode stacks, the notch of the current collector has an opening. A lithium secondary battery according to any one of appendices 2 to 5, wherein the opening of at least one of the current collectors is located at a different position from the opening of the other at least one current collector when viewed from the stacking direction.

[0135] (Note 7) A lithium secondary battery according to any one of the appendices 1 to 6, wherein the shape of at least one of the current collectors differs from the shape of at least one other current collector.

[0136] (Note 8) A lithium secondary battery according to any one of the appendices 1 to 7, wherein 20 to 30 of the aforementioned positive electrode stacks are provided.

[0137] (Note 9) A lithium secondary battery according to any one of Appendix 1 to Appendix 8, wherein the current collectors of 4 to 15 of the positive electrode stacks are bonded to the electrode tabs at the same position when viewed from the stacking direction.

[0138] (Note 10) A lithium secondary battery according to any one of Appendix 1 to 9, wherein the current collectors of the 4 to 15 positive electrode stacks are bonded to the electrode tabs at a different position from at least one other current collector when viewed from the stacking direction.

[0139] (Note 11) A lithium secondary battery having an energy density of 300 Wh / kg or more, as described in any one of the appendices 1 through 10.

[0140] (Note 12) A lithium secondary battery with a rated capacity of 1.5A or more, as specified in one of the appendices 1 through 11.

[0141] (Note 13) A lithium secondary battery with a rated capacity of 5A or more, as specified in one of the appendices 1 through 12.

[0142] (Note 14) A negative electrode laminate is formed by alternately folding multiple sheets at sharp angles, each sheet having a negative electrode and separators arranged on both sides of the negative electrode. A lithium secondary battery according to any one of Appendix 1 to Appendix 13, wherein each of the positive electrode stacks is disposed between the separators of the negative electrode stacks that face each other.

[0143] (Note 15) The negative electrode laminate is constructed by winding a sheet having a negative electrode and separators arranged on both sides of the negative electrode in a manner that involves multiple folds. A lithium secondary battery according to any one of Appendix 1 to Appendix 14, wherein each of the positive electrode stacks is disposed between the separators of the negative electrode stacks that face each other.

[0144] (Note 16) A lithium secondary battery according to any one of appendices 1 to 15, comprising a buffering layer between the negative electrode and the separators arranged on both sides of the negative electrode.

[0145] (Note 17) The lithium secondary battery described in any one of Appendix 1 to Appendix 16, wherein the negative electrode substantially does not have a negative electrode active material.

[0146] (Note 18) The lithium secondary battery according to any one of the appendices 1 to 17, wherein the negative electrode 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.

[0147] (Note 19) A method for manufacturing a lithium secondary battery, (a) A step of arranging a plurality of positive electrode stacks that are spaced apart from each other in the stacking direction via a negative electrode and a separator, wherein each positive electrode stack has a current collector formed by sandwiching an insulating layer between conductive layers, and positive electrodes provided on both sides of the current collector, (b) A step of joining the current collector of each of the plurality of positive electrode stacks to the electrode tab, Includes, A method for manufacturing a lithium secondary battery, wherein in step (b) above, at least one of the current collectors of the plurality of positive electrode stacks is joined to the electrode tab at a position different from that of at least one other current collector when viewed from the stacking direction.

[0148] (Note 20) The method for manufacturing a lithium secondary battery as described in Appendix 19, wherein in step (b) above, the joining of the current collector to the electrode tab is performed using ultrasonic welding, laser welding, or resistance welding.

[0149] The embodiments described above are for illustrative purposes only and are not intended to limit the scope of this disclosure. The embodiments described above can be modified in various ways without departing from the scope and spirit of this disclosure. For example, some components of one embodiment can be added to other embodiments, or some components of one embodiment can be replaced with corresponding components of other embodiments. [Explanation of Symbols]

[0150] 1...Lithium secondary battery, 10...Negative electrode, 20...Separator, 30...Positive electrode laminate, 32...Current collector, 320...Insulating layer, 322...Conductive layer, 34...Positive electrode, 40...Electrode tab, 50...Buffering layer

Claims

1. It is a lithium secondary battery, Electrode tabs and A plurality of positive electrode stacks arranged spaced apart from each other in the stacking direction via a negative electrode and a separator, wherein each of the positive electrode stacks comprises a current collector formed by sandwiching an insulating layer between conductive layers and electrically connected to the electrode tab, and positive electrodes provided on both sides of the current collector, Equipped with, In each of the plurality of positive electrode stacks, the current collector has a notch that extends in a direction parallel to the main surface of the current collector and is joined to the electrode tab. A lithium secondary battery in which at least one of the current collectors of the plurality of positive electrode stacks is welded to the electrode tab via the notch at a position different from that of the other at least one current collector when viewed from the stacking direction.

2. The lithium secondary battery according to claim 1, wherein the notch is located above or below the electrode tab in the stacking direction.

3. The lithium secondary battery according to claim 2, wherein the area of ​​the notch is smaller than the area of ​​the electrode tab.

4. The lithium secondary battery according to any one of claims 1 to 3, wherein the notch of at least one current collector is located at a different position from the notch of the other at least one current collector when viewed from the stacking direction.

5. In each of the plurality of positive electrode stacks, the notch of the current collector has an opening. The lithium secondary battery according to any one of claims 1 to 3, wherein the opening of at least one of the current collectors is located at a different position from the opening of the other at least one of the current collectors when viewed from the stacking direction.

6. A lithium secondary battery according to any one of claims 1 to 3, wherein the shape of at least one of the current collectors is different from the shape of the other at least one of the current collectors.

7. The lithium secondary battery according to any one of claims 1 to 3, wherein 20 to 30 of the positive electrode stacks are provided.

8. The lithium secondary battery according to claim 7, wherein the current collectors of 4 to 15 of the positive electrode stacks are joined to the electrode tabs at the same position when viewed from the stacking direction.

9. The lithium secondary battery according to claim 8, wherein the current collectors of the 4 to 15 positive electrode stacks are joined to the electrode tabs at a position different from that of at least one other current collector when viewed from the stacking direction.

10. A lithium secondary battery according to any one of claims 1 to 3, wherein the energy density is 300 Wh / kg or more.

11. A lithium secondary battery according to any one of claims 1 to 3, wherein the rated capacity is 1.5A or more.

12. A lithium secondary battery according to any one of claims 1 to 3, wherein the rated capacity is 5A or more.

13. A negative electrode laminate is formed by alternately folding multiple sheets at sharp angles, each sheet having a negative electrode and separators arranged on both sides of the negative electrode. The lithium secondary battery according to claim 1, wherein each of the positive electrode stacks is disposed between the separators of the negative electrode stacks that face each other.

14. The negative electrode laminate is constructed by winding a sheet having a negative electrode and separators arranged on both sides of the negative electrode in a manner that involves multiple folds. The lithium secondary battery according to claim 1, wherein each of the positive electrode stacks is disposed between the separators of the negative electrode stacks that face each other.

15. A lithium secondary battery according to any one of claims 1 to 3, comprising a buffering layer between the negative electrode and the separators arranged on both sides of the negative electrode.

16. The lithium secondary battery according to any one of claims 1 to 3, wherein the negative electrode substantially does not have a negative electrode active material.

17. The lithium secondary battery according to any one of claims 1 to 3, wherein the negative electrode 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.

18. A method for manufacturing a lithium secondary battery, (a) A step of arranging a plurality of positive electrode stacks that are spaced apart from each other in the stacking direction via a negative electrode and a separator, wherein each positive electrode stack has a current collector formed by sandwiching an insulating layer between conductive layers, and positive electrodes provided on both sides of the current collector, (b) A step of joining each of the current collectors of the plurality of positive electrode stacks to an electrode tab, wherein the current collector has a notch that extends in a direction parallel to the main surface of the current collector and is joined to the electrode tab, Includes, A method for manufacturing a lithium secondary battery, wherein in step (b) above, at least one of the current collectors of the plurality of positive electrode stacks is welded to the electrode tab via the notch at a position different from that of the other at least one current collector when viewed from the stacking direction.

19. The method for manufacturing a lithium secondary battery according to claim 18, wherein in step (b), the joining of the current collector to the electrode tab is performed using ultrasonic welding, laser welding, or resistance welding.

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