Lithium secondary battery and method for manufacturing lithium secondary battery

Abstract

The present invention provides: a lithium secondary battery that achieves weight reduction, cost reduction, and an excellent capacity retention rate; and a method for manufacturing a lithium secondary battery. The present invention pertains to a lithium secondary battery including: a positive electrode; a separator; and a negative electrode including a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector. The negative electrode current collector includes a resin layer and a metal layer provided on both surfaces of the resin layer. The negative electrode active material layer faces the positive electrode via the separator. The separator includes an adhesive layer on the negative electrode-side surface. A peel strength x at 25°C between the resin layer and the metal layer is 0.05 N / cm or greater. A peel strength y at 25°C between the negative electrode active material layer and the separator satisfies x < y.

Description

Lithium secondary battery and method of manufacturing the same

[0001] The present invention relates to a lithium secondary battery and a method for manufacturing the lithium secondary battery.

[0002] In recent years, technologies for converting natural energy such as solar or wind power into electrical energy have been attracting attention. In addition, with the widespread use of devices that utilize electrical energy, such as electric vehicles, various secondary batteries have been developed as lightweight power storage devices that can store large amounts of electrical energy.

[0003] Among these, lithium secondary batteries, which are charged and discharged by the movement of lithium ions between a positive electrode and a negative electrode, are known to exhibit high voltage and high energy density.

[0004] For example, Patent Document 1 discloses a current collector in which metal layers are formed on both sides of a resin layer.

[0005] Japanese Patent Application Publication No. 11-102711

[0006] It has been found that when a current collector having a metal layer formed on both sides of a resin layer is used in a lithium secondary battery, problems such as a decrease in the capacity retention rate of the battery occur.

[0007] The present invention has been made in consideration of the above problems, and aims to provide a lithium secondary battery that is prevented from increasing in mass and has an improved capacity retention rate, and a method for manufacturing such a lithium secondary battery.

[0008] A lithium secondary battery according to one embodiment of the present invention comprises a positive electrode, a separator, and a negative electrode including a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector, wherein the negative electrode current collector comprises a resin layer and metal layers provided on both sides of the resin layer, the negative electrode active material layer faces the positive electrode via the separator, the separator comprises an adhesive layer on the surface facing the negative electrode, a peel strength x between the resin layer and the metal layer at 25°C is 0.05 N / cm or more, and a peel strength y between the negative electrode active material layer and the separator at 25°C satisfies x<y.

[0009] A method for manufacturing a lithium secondary battery according to one embodiment of the present invention includes: a negative electrode current collector preparation step of preparing a negative electrode current collector including a resin layer and metal layers provided on both sides of the resin layer; a negative electrode fabrication step of forming a negative electrode active material layer on the negative electrode current collector to fabricate a negative electrode; a stack fabrication step of arranging the negative electrode, the positive electrode, and the separator to fabricate a stack so that the negative electrode active material layer faces a positive electrode via a separator including an adhesive layer and the adhesive layer is adjacent to the negative electrode active material layer; and a heat press step of applying pressure to the stack while heating, wherein in the lithium secondary battery, a peel strength x between the resin layer and the metal layer at 25°C is 0.05 N / cm or more, and a peel strength y between the negative electrode active material layer and the separator at 25°C satisfies x<y.

[0010] In this lithium secondary battery, the negative electrode current collector includes a resin layer and metal layers provided on both sides of the resin layer. Because this negative electrode current collector has a lower density than a current collector including only a metal layer, using this negative electrode current collector allows for a reduction in the weight of the lithium secondary battery. Furthermore, the inventors have found that in a lithium secondary battery, the negative electrode active material layer faces the positive electrode via a separator, the separator includes an adhesive layer on the negative electrode side, and the peel strength x between the resin layer and the metal layer at 25°C is 0.05 N / cm or greater, and the peel strength y between the negative electrode active material layer and the separator at 25°C satisfies the relationship x<y, thereby enabling the lithium secondary battery to have a good capacity retention rate. Therefore, it is presumed that the above-described configurations achieve both a reduction in weight and a good capacity retention rate for the lithium secondary battery.

[0011] According to the present invention, it is possible to provide a lithium secondary battery that achieves both light weight and a good capacity retention rate, and a method for manufacturing the lithium secondary battery.

[0012] 1 is a schematic cross-sectional view of a lithium secondary battery according to an embodiment of the present invention.

[0013] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail with reference to the drawings as necessary. However, the present invention is not limited to this embodiment, and various modifications are possible without departing from the spirit of the present invention. In the drawings, the same elements are given the same reference numerals, and redundant explanations will be omitted. Furthermore, positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings unless otherwise specified. Furthermore, the dimensional ratios of the drawings are not limited to those shown in the drawings.

[0014] 1. Lithium Secondary Battery The type of the lithium secondary battery 100 of the present embodiment is not particularly limited as long as it is charged and discharged by an oxidation-reduction reaction of lithium, and examples thereof include a lithium ion battery, a lithium metal battery, a lithium sulfur battery, a lithium oxygen battery, and a lithium air battery.

[0015] The shape of the battery of the lithium secondary battery 100 of this embodiment is not particularly limited, and may be, for example, a sheet type, a laminated sheet type, a thin shape, a cylindrical shape with a bottom, a prismatic shape with a bottom, etc. From the viewpoint of more effectively and reliably achieving the effects of this embodiment, the sheet type, laminated sheet type, or thin shape is preferred.

[0016] Hereinafter, the lithium secondary battery of this embodiment will be described in detail as needed, taking as an example a lithium ion secondary battery, which is an example of the lithium secondary battery of this embodiment.

[0017] <Lithium-ion secondary battery> FIG. 1 is a schematic cross-sectional view of a lithium secondary battery 100 according to this embodiment. The lithium secondary battery 100 of this embodiment includes a positive electrode 110, a separator 130, and a negative electrode 120 including a negative electrode current collector 121 and a negative electrode active material layer 122 on the negative electrode current collector 121, the negative electrode current collector 121 including a resin layer 121b and metal layers 121a provided on both sides of the resin layer 121b, the negative electrode active material layer 122 facing the positive electrode 110 via the separator 130, the separator 130 including an adhesive layer 131 on the surface facing the negative electrode 120, a peel strength x at 25°C between the resin layer 121b and the metal layer 121a being 0.05 N / cm or more, and a peel strength y at 25°C between the negative electrode active material layer 122 and the separator 130 satisfying x<y.

[0018] The adhesive layer 131 may be provided only on the negative electrode side surface of the separator body 132 as shown in FIG. 1, or may be provided on both surfaces of the separator body 132.

[0019] 1 , the positive electrode active material layer 112 may be provided on one surface of the positive electrode current collector 111, or on both surfaces of the positive electrode current collector 111. In addition, the negative electrode active material layer 122 may be provided on one surface of the negative electrode current collector 121, or on both surfaces of the negative electrode current collector 121, as shown in FIG.

[0020] The positive electrode current collector 111 may have a single layer structure as shown in FIG. 1, or may have a multi-layer structure including a resin layer and metal layers provided on both sides of the resin layer.

[0021] Furthermore, the negative electrode current collector 121 contains a resin that has a lower density than metal, and therefore has a smaller weight per volume than a current collector that has only a metal layer. Therefore, a lithium secondary battery 100 that uses a negative electrode current collector 121 that has a resin layer 121b and metal layers 121a provided on both sides of the resin layer 121b tends to have an excellent energy density per weight.

[0022] As described above, the use of a current collector comprising a resin layer and metal layers provided on both sides of the resin layer as a current collector tends to improve the energy density per unit weight of a lithium secondary battery. However, the use of such a current collector can cause problems such as a decrease in the capacity retention rate of the battery. The cause of this is not particularly limited, but is presumed to be as follows: In a current collector comprising a resin layer and metal layers provided on both sides of the resin layer, the adhesive strength between the metal layer and the resin layer may be weak. In particular, the adhesive strength between the metal and resin used in the negative electrode current collector of a lithium secondary battery tends to be weak. Therefore, when the negative electrode current collector includes a resin layer and metal layers provided on both sides of the resin layer, the expansion and contraction of the lithium secondary battery that occurs when charge and discharge cycles are repeated causes peeling between the resin layer and the metal layer of the negative electrode current collector, which in turn causes gaps to form in various locations in the lithium secondary battery, such as between the positive electrode and the separator and between the negative electrode and the separator, causing the shape of the lithium secondary battery to collapse, and as a result, problems such as a decrease in the capacity retention rate of the battery may occur.

[0023] Therefore, in the lithium secondary battery 100 of this embodiment, a peel strength x at 25°C is specified between the metal layer 121a and the resin layer 121b that constitute the negative electrode current collector 121. This makes it difficult for peeling to occur between the metal layer 121a and the resin layer 121b in the negative electrode current collector 121 even if the lithium secondary battery 100 repeatedly expands and contracts.

[0024] Additionally, in the lithium secondary battery 100 of this embodiment, the peel strength y between the negative electrode active material layer 122 and the separator 130 at 25°C is specified to be greater than the peel strength x at 25°C. Even if slight peeling occurs between the metal layer 121a and the resin layer 121b in the negative electrode current collector 121, this rarely immediately causes a significant deterioration in battery performance. However, peeling between the negative electrode active material layer 122 and the separator 130 tends to hinder the movement of lithium ions within the lithium secondary battery 100, leading to deterioration in battery performance such as a decrease in capacity retention. In this regard, by making the peel strength y at 25°C greater than the peel strength x at 25°C, peeling between the negative electrode active material layer 122 and the separator 130 is less likely to occur. Furthermore, even if peeling occurs between the metal layer 121a and the resin layer 121b, this can be prevented from causing gaps to form in various locations in the lithium secondary battery 100, which could cause the shape of the lithium secondary battery 100 to collapse. Therefore, it is believed that the lithium secondary battery 100 of this embodiment has a good capacity retention rate. However, the factors that cause the lithium secondary battery 100 of this embodiment to have a good capacity retention rate are not limited to those described above.

[0025] Each component of the lithium secondary battery 100 of this embodiment will be described in detail below.

[0026] 1.1 Negative Electrode The negative electrode 120 of this embodiment includes a negative electrode current collector 121 and a negative electrode active material layer 122 on the negative electrode current collector 121 .

[0027] The average thickness of the negative electrode 120 is not particularly limited, but is, for example, 5.0 μm to 120.0 μm. From the viewpoint of improving the capacity and / or energy density of the battery, it is preferably 20.0 μm to 100.0 μm, or 50.0 μm to 80.0 μm.

[0028] 1.1.1. Negative Electrode Current Collector The negative electrode current collector 121 of this embodiment includes a resin layer 121b and metal layers 121a provided on both sides of the resin layer. This allows the lithium secondary battery 100 of this embodiment to be lightweight, and tends to have excellent energy density per weight. Furthermore, because resin tends to be cheaper than metal, the lithium secondary battery 100 using the negative electrode current collector 121 of this embodiment tends to be less expensive than a current collector including only a metal layer.

[0029] 1.1.1.1. Metal Layer The metal constituting the metal layer 121a of this embodiment is not particularly limited as long as it can be used as a current collector, and examples thereof include copper, nickel, titanium, iron, other metals that do not react with lithium, alloys thereof, and stainless steel (SUS), with copper, nickel, alloys thereof, and SUS being preferred. Among these metals, copper and SUS are preferred, with copper being more preferred, from the viewpoint of oxidation-reduction potential. The metals may be used alone or in combination of two or more. In this specification, the term "metal that does not react with lithium" refers to a metal that does not react with lithium ions or lithium metal to form an alloy under the operating conditions of a lithium secondary battery.

[0030] The metal content is preferably 80% by mass or more and 100% by mass or less, 90% by mass or more and 100% by mass or less, 95% by mass or more and 100% by mass or less, or 100% by mass, based on the total amount of the metal layer 121a. It is also preferable that the metal layer 121a is substantially made of metal.

[0031] The thickness of the metal layer 121a is preferably 0.1 μm to 4.0 μm, 0.2 μm to 3.5 μm, 0.3 μm to 3.3 μm, 0.4 μm to 3.0 μm, or 1.0 μm to 3.0 μm.

[0032] The metal layer 121a is formed on both sides of the resin layer 121b by vapor deposition, sputtering, plating such as electrolytic plating, or by bonding with an adhesive. Among these, vapor deposition and plating are preferred, and plating after vapor deposition is more preferred. In this case, the metal layer 121a comprises a vapor deposition layer provided on the resin layer 121b and a plating layer provided on the vapor deposition layer.

[0033] When the metal layer 121a is formed by plating after vapor deposition, the thickness of the vapor-deposited layer formed by vapor deposition is preferably 0.07 μm to 0.30 μm, or 0.10 μm to 0.25 μm, and the thickness of the plated layer formed by plating is preferably 0.50 μm to 2.00 μm, or 0.75 μm to 1.50 μm.

[0034] 1.1.1.2. Resin Layer The resin layer 121b of this embodiment is an insulator and prevents electrical conduction between the metal layers 121a provided on both sides of the resin layer 121b. The resin constituting the resin layer 121b is not particularly limited, but may be, for example, a sheet-like (film-like) or fibrous resin. Examples of resins include, but are not limited to, polyolefin resins such as polypropylene (PP) and polyethylene (PE); thermoplastic resins such as polystyrene, polyvinyl chloride, and polyamide; and polyethylene terephthalate (PET). Among these, PP and PE are preferred, with PP being more preferred. PP tends to have better stability against the electrolyte of lithium secondary batteries. When PP is used as the resin constituting the resin layer 121b, copper is preferably used as the metal constituting the metal layer 121a. Resins may be used alone or in combination.

[0035] In addition to the resins described above, the resin layer 121b may contain other additives as appropriate depending on the desired physical properties. Examples of the other additives include, but are not limited to, colorants, flame retardants, surfactants, etc.

[0036] The resin content is not particularly limited, but may be, for example, 60% by mass to 100% by mass, 80% by mass to 100% by mass, 90% by mass to 100% by mass, 95% by mass to 100% by mass, or 100% by mass, relative to the total amount of the resin layer 121b. Alternatively, the resin layer 121b may be substantially composed of resin.

[0037] The thickness of the resin layer 121b is preferably 1 μm or more and 10 μm or less, 2 μm or more and 9 μm or less, or 3 μm or more and 8 μm or less.

[0038] 1.1.2. Negative Electrode Active Material Layer The negative electrode active material layer 122 of this embodiment includes a negative electrode active material. 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. Specifically, the negative electrode active material of this embodiment includes lithium metal and a host material for lithium element (lithium ion or lithium metal). The host material for lithium element refers to a material provided to retain lithium ion or lithium metal in the negative electrode. Examples of such retention mechanisms include intercalation, alloying, and metal cluster absorption, and intercalation is typically used.

[0039] The negative electrode active material is not particularly limited, but examples thereof include lithium metal and alloys containing lithium metal, carbon-based materials, metal oxides, metals that alloy with lithium, and alloys containing such metals. The carbon-based materials are not particularly limited, but examples thereof include artificial graphite, graphene, graphite, hard carbon, and carbon nanotubes. The metal oxides are not particularly limited, but examples thereof include titanium oxide-based compounds and cobalt oxide-based compounds. The metals that alloy with lithium are not particularly limited, but examples thereof include silicon, germanium, tin, lead, aluminum, and gallium. The negative electrode active materials may be used alone or in combination.

[0040] The content of the negative electrode active material is preferably 60.0 mass % or more and 100 mass % or less, 70.0 mass % or more and 99.5 mass % or less, or 80.0 mass % or more and 99.0 mass % or less, relative to the total amount of the negative electrode active material layer 122.

[0041] The negative electrode active material layer 122 may contain a binder, a conductive aid, and other additives in addition to the negative electrode active material.

[0042] The binder is not particularly limited, but examples thereof include polyvinylidene fluoride; modified polyvinylidene fluoride obtained by introducing functional groups such as hydroxyl groups, amino groups, carbonyl groups, carboxyl groups, phenyl groups, and methyl groups into polyvinylidene fluoride; polytetrafluoroethylene; modified polytetrafluoroethylene obtained by introducing functional groups such as hydroxyl groups, amino groups, carbonyl groups, carboxyl groups, phenyl groups, and methyl groups into polytetrafluoroethylene; block copolymers, random copolymers, and graft copolymers having tetrafluoroethylene as a constituent unit; styrene butadiene rubber; carboxymethyl cellulose; acrylic resins; polyimide resins, etc. The binders may be used alone or in combination of two or more.

[0043] The content of the binder relative to the total amount of the negative electrode active material layer 122 is preferably 0.5% by mass or more and 10.0% by mass or less, 0.7% by mass or more and 8.0% by mass or less, 1.0% by mass or more and 6.0% by mass or less, or 1.0% by mass or more and 4.0% by mass or less.

[0044] The conductive additive is not particularly limited, but examples thereof include carbon black, single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), carbon nanofibers (CF), and acetylene black. The conductive additives may be used alone or in combination of two or more.

[0045] The content of the conductive additive is preferably 0.5% by mass or more and 30.0% by mass or less with respect to the total amount of the negative electrode active material layer 122 .

[0046] The thickness of the negative electrode active material layer 122 is preferably 3.0 μm or more and 100.0 μm or less, 10.0 μm or more and 70.0 μm or less, or 20.0 μm or more and 60.0 μm or less.

[0047] 1.2. Separator The separator 130 of this embodiment includes an adhesive layer 131 on the surface of the separator body 132 facing the negative electrode 120. The separator 130 is not particularly limited as long as it has the function of physically and / or electrically isolating the positive electrode 110 and the negative electrode 120 and the function of ensuring ionic conductivity of lithium ions. To achieve these functions, the separator body 132 may be made of, for example, an insulating porous material, a polymer electrolyte, a gel electrolyte, or an inorganic solid electrolyte. Typically, the separator body 132 is made of at least one material selected from the group consisting of an insulating porous material, a polymer electrolyte, and a gel electrolyte. Furthermore, the separator body 132 may be made of one material alone, or two or more materials may be used in combination.

[0048] Preferably, an insulating porous material, a polymer electrolyte, or a gel electrolyte is used singly or in combination as the separator body 132. When an insulating porous material is used alone as the separator body 132, the lithium secondary battery 100 must further include an electrolytic solution.

[0049] The polymer electrolyte is not particularly limited, but examples thereof include solid polymer electrolytes mainly containing a polymer and an electrolyte, and semi-solid polymer electrolytes mainly containing a polymer, an electrolyte, and a plasticizer. The gel electrolyte is not particularly limited, but examples thereof include those mainly containing a polymer and a liquid electrolyte (i.e., a solvent and an electrolyte).

[0050] Polymers that may be contained in polymer electrolytes and gel electrolytes include, but are not limited to, polymers containing functional groups containing oxygen atoms such as ethers and esters, halogen groups, and polar groups such as cyano groups. Specific examples include resins having ethylene oxide units in the main chain and / or side chains such as polyethylene oxide (PEO), resins having propylene oxide units in the main chain and / or side chains such as polypropylene oxide (PPO), acrylic resins, vinyl resins, ester resins, nylon resins, polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polysiloxane, polyphosphazene, polymethyl methacrylate, polyamide, polyimide, aramid, polylactic acid, polyurethane, polyacetal, polysulfone, polyethylene carbonate, polypropylene carbonate, and polytetrafluoroethylene. The above-mentioned resins may be used alone or in combination of two or more.

[0051] Examples of electrolytes contained in the polymer electrolyte and gel electrolyte include salts of Li, Na, K, Ca, and Mg. Typically, in this embodiment, the polymer electrolyte and gel electrolyte contain a lithium salt. The lithium salt is not particularly limited, but examples thereof include LiI, LiCl, LiBr, LiF, and LiBF. ４ , LiPF ６ , LiAsF ６ , LiSO ３ CF ３ , LiN(SO ２ F) ２ , LiN(SO ２ CF ３ ) ２ , LiN(SO ２ CF ３ CF ３ ) ２ , LiB(O ２ C ２ H ４ ) ２ , LiB(C ２ O ４ ) ２ , LiB(O ２ C ２ H ４ ) F ２ , LiB(OCOCF３ ) ４ , LiNO ３ , and Li ２ SO ４ and preferably LiPF ６ , LiN(SO ２ F) ２ , LiN(SO ２ CF ３ ) ２ , and LiN(SO ２ CF ３ CF ３ ) ２ The salt or lithium salt may be used singly or in combination of two or more thereof.

[0052] The compounding ratio of the polymer to the lithium salt in the polymer electrolyte and the gel electrolyte may be determined by the ratio of the polar group of the polymer to the lithium atom of the lithium salt. For example, when the polymer contains oxygen atoms, the compounding ratio may be determined by the ratio ([Li] / [O]) of the number of oxygen atoms of the polymer to the number of lithium atoms of the lithium salt. In the polymer electrolyte and the gel electrolyte, the compounding ratio of the polymer to the lithium salt can be adjusted so that the ratio ([Li] / [O]) is, for example, 0.02 to 0.20, 0.03 to 0.15, or 0.04 to 0.12.

[0053] The solvent contained in the gel electrolyte is not particularly limited, and for example, the solvents that can be contained in the electrolyte solution described below can be used alone or in combination of two or more. Examples of preferred solvents are the same as those in the electrolyte solution described below. The plasticizer contained in the semi-solid polymer electrolyte is not particularly limited, and examples include the same components as the solvents that can be contained in the gel electrolyte and various oligomers.

[0054] When the separator body 132 includes an insulating porous member, the pores of the member are filled with an ion-conductive substance, which allows the member to exhibit ion conductivity. Therefore, in this embodiment, the pores are filled with, for example, the electrolyte solution of this embodiment or a gel electrolyte containing the electrolyte solution of this embodiment.

[0055] 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 body 132 may be a porous polyethylene (PE) film, a porous polypropylene (PP) film, or a laminated structure thereof.

[0056] 1.2.1 Adhesive Layer The separator body 132 has an adhesive layer 131 on the surface of the separator body 132 facing the negative electrode 120. It is also preferable that the separator body 132 has adhesive layers 131 on both surfaces thereof.

[0057] The adhesive layer 131 is porous and is not particularly limited, but is preferably made of a material containing a binder such as polyvinylidene fluoride (PVdF), polyamide, a mixture of styrene butadiene rubber and carboxymethyl cellulose (SBR-CMC), or polyacrylic acid (PAA), and among these, PVdF is more preferred. The adhesive layer 131 may contain inorganic particles such as silica, alumina, titania, zirconia, or magnesium hydroxide added to the binder.

[0058] The content of the binder relative to the total amount of the adhesive layer 131 is not particularly limited, and is, for example, 80% by mass to 100% by mass, 90% by mass to 100% by mass, or 100% by mass. The adhesive layer 131 may consist essentially of a binder. The adhesive layer 131 may also contain inorganic particles such as alumina, titania, zirconia, or magnesium hydroxide. When inorganic particles are contained, the content of the binder relative to the total amount of the adhesive layer 131 is preferably 10.0% by mass to 90.0% by mass, 20.0% by mass to 80.0% by mass, or 30.0% by mass to 70.0% by mass.

[0059] The thickness of the adhesive layer 131 is preferably 0.1 μm or more and 5.0 μm or less, 0.3 μm or more and 4.0 μm or less, or 0.5 μm or more and 3.0 μm or less.

[0060] The thickness of the separator 130 including the adhesive layer 131 is not particularly limited, but is, for example, 1.0 μm or more and 40.0 μm or less. In the lithium secondary battery 100, from the viewpoint of reducing the volume occupied by the separator 130 in the battery while reliably isolating the positive electrode 110 and the negative electrode 120, the thickness of the separator 130 is preferably 3.0 μm or more and 30.0 μm or less, 4.0 μm or more and 25.0 μm or less, or 5.0 μm or more and 20.0 μm or less.

[0061] In the lithium secondary battery 100 of this embodiment, the peel strength x between the resin layer 121b and the metal layer 121a at 25°C is 0.05 N / cm or more, preferably 0.05 N / cm to 2.00 N / cm, 0.06 N / cm to 1.50 N / cm, 0.07 N / cm to 1.45 N / cm, 0.08 N / cm to 1.40 N / cm, or 0.09 N / cm to 1.35 N / cm. When the peel strength x at 25°C is within the above range, the capacity retention rate of the lithium secondary battery 100 tends to be improved.

[0062] The peel strength x at 25°C can be adjusted by the method for providing the metal layer 121a on the resin layer 121b, the type and composition of the resin of the resin layer 121b, etc. The adjustment of the method is not particularly limited, but examples thereof include providing the metal layer 121a by multiple methods such as vapor deposition and plating, and adjusting the time and order for performing the multiple methods.

[0063] In the lithium secondary battery 100 of this embodiment, the peel strength y at 25°C between the negative electrode active material layer 122 and the separator 130 is preferably 0.06 N / cm or more, 0.07 N / cm or more, 0.06 N / cm to 2.00 N / cm, 0.07 N / cm to 1.75 N / cm, 0.08 N / cm to 1.50 N / cm, 0.09 N / cm to 1.40 N / cm, 0.10 N / cm to 1.35 N / cm, 0.11 N / cm to 1.30 N / cm, 0.12 N / cm to 1.25 N / cm, 0.13 N / cm to 1.20 N / cm, or 0.14 N / cm to 1.10 N / cm. When the peel strength y at 25° C. is within the above range, the capacity retention rate of the lithium secondary battery 100 tends to be further improved.

[0064] The peel strength y at 25° C. can be adjusted by the type and composition of the material contained in the negative electrode active material layer 122 and the type and composition of the material contained in the adhesive layer 131, etc.

[0065] In the lithium secondary battery 100 of this embodiment, the peel strength z between the metal layer 121a and the negative electrode active material layer 122 at 25°C is preferably 0.30 N / cm or more, 0.30 N / cm to 3.00 N / cm, or 0.35 N / cm to 2.50 N / cm. When the peel strength z at 25°C is within the above range, the capacity retention rate of the lithium secondary battery 100 tends to be further improved.

[0066] The peel strength z at 25° C. can be adjusted by the type and composition of the material contained in the negative electrode active material layer 122 and the type and composition of the metal contained in the metal layer 121a.

[0067] In the lithium secondary battery 100 of this embodiment, the peel strength y at 25° C. between the negative electrode active material layer 122 and the separator 130 satisfies the relationship x<y with respect to the peel strength x at 25° C. When the peel strength x at 25° C. and the peel strength y at 25° C. satisfy the above relationship, the capacity retention rate of the lithium secondary battery 100 tends to be improved.

[0068] In addition, in the lithium secondary battery 100 of this embodiment, the peel strength y at 25° C. preferably satisfies the relationship y>0.1+0.5x with the peel strength x at 25° C. When the peel strength x at 25° C. and the peel strength y at 25° C. satisfy the above relationship, the capacity retention rate of the lithium secondary battery 100 tends to be further improved.

[0069] Furthermore, in the lithium secondary battery 100 of this embodiment, the peel strengths x at 25°C, y at 25°C, and z at 25°C preferably satisfy the relationship x<y<(z / 2). In the negative electrode 120 of the lithium secondary battery 100, electrons are exchanged between the metal layer 121a and the negative electrode active material layer 122. If peeling occurs between these layers, properties such as the capacity retention rate of the battery tend to deteriorate significantly. Therefore, if the peel strength z at 25°C is sufficiently greater than the peel strength x at 25°C and the peel strength y at 25°C, peeling between the metal layer 121a and the negative electrode active material layer 122 can be prevented, and the capacity retention rate of the lithium secondary battery 100 tends to be further improved.

[0070] When the peel strength x at 25°C, the peel strength y at 25°C, and the peel strength z at 25°C satisfy x≧0.05 N / cm and x<y<(z / 2), x is preferably 0.09 N / cm or more, 0.05 N / cm or more to 2.00 N / cm or less, 0.06 N / cm or more to 1.50 N / cm or less, 0.07 N / cm or more to 1.45 N / cm or less, 0.08 N / cm or more to 1.40 N / cm or less, or 0.09 N / cm or more to 1.35 N / cm or less.

[0071] Furthermore, when the peel strength x at 25°C, the peel strength y at 25°C, and the peel strength z at 25°C satisfy x≧0.05 N / cm and x<y<(z / 2), z is preferably 0.30 N / cm or more, 0.30 N / cm or more and 3.00 N / cm or less, or 0.35 N / cm or more and 2.50 N / cm or less.

[0072] The peel strength x at 25°C, the peel strength y at 25°C, and the peel strength z at 25°C can be measured at a temperature of 25°C in accordance with the 180-degree peel adhesive strength test method of JIS K 6854-2:1999.

[0073] In this embodiment, the temperature at which the peel strength is measured is 25°C. In this regard, depending on the specifications of the lithium secondary battery, it is expected that the temperature during charging or discharging of the lithium secondary battery may be higher or lower than 25°C. However, even if the actual temperature during charging or discharging of the lithium secondary battery is not 25°C but is higher or lower than this, if the peel strength measured with 25°C as the reference falls within a predetermined range, the adhesion between the resin layer 121b and the metal layer 121a, the adhesion between the negative electrode active material layer 122 and the separator 130, and the adhesion between the metal layer 121a and the negative electrode active material layer 122 are improved, and as a result, it can be said that the capacity retention rate of the lithium secondary battery 100 is improved.

[0074] The positive electrode 110 of this embodiment is not particularly limited as long as it is a material generally used in lithium secondary batteries, and known materials can be appropriately selected depending on the application of the lithium secondary battery. From the viewpoint of improving the stability and output voltage of the battery, the positive electrode 110 preferably includes a positive electrode current collector 111 and a positive electrode active material layer 112.

[0075] The average thickness of the positive electrode 110 is not particularly limited, but is, for example, 10 μm to 100 μm, 20 μm to 90 μm, 30 μm to 80 μm, or 40 μm to 70 μm, although the average thickness of the positive electrode 30 can be adjusted appropriately depending on the desired battery capacity.

[0076] 1.3.1. Positive Electrode Current Collector The positive electrode current collector 111 of this embodiment is not particularly limited as long as it contains a metal that does not react with lithium in a lithium secondary battery. Examples of such metals include aluminum. The positive electrode current collector 111 of this embodiment may have a single-layer structure including only a metal layer containing the above-described metal, or a multi-layer structure including a resin layer and metal layers containing the above-described metal provided on both sides of the resin layer.

[0077] When the positive electrode current collector 111 has a multi-layer structure, the metal constituting the metal layer is not particularly limited as long as it is a metal that does not react with lithium in a lithium secondary battery, and an example of such a metal is aluminum.

[0078] The metal content is preferably 80% by mass or more and 100% by mass or less, 90% by mass or more and 100% by mass or less, 95% by mass or more and 100% by mass or less, or 100% by mass, based on the total amount of the metal layer. It is also preferable that the metal layer consists essentially of metal.

[0079] When the positive electrode current collector 111 has a multi-layer structure, the thickness of the metal layer is preferably 0.1 μm or more and 4.0 μm or less, 0.2 μm or more and 3.0 μm or less, 0.3 μm or more and 2.5 μm or less, or 0.4 μm or more and 2.0 μm or less.

[0080] When the positive electrode current collector 111 has a multi-layer structure, the resin constituting the resin layer may be the same as that of the resin layer 121b of the negative electrode current collector 121. The resin layer may contain other additives as appropriate depending on the desired physical properties, and examples of the other additives may be the same as those of the resin layer 121b of the negative electrode current collector 121. The resin content in the resin layer may be the same as that of the resin layer 121b of the negative electrode current collector 121. The thickness of the resin layer may be the same as that of the resin layer 121b of the negative electrode current collector 121.

[0081] 1.3.2. Positive Electrode Active Material Layer The positive electrode active material layer 112 of this embodiment contains a positive electrode active material. The positive electrode active material is a material that causes an electrode reaction, i.e., an oxidation reaction and a reduction reaction, at the positive electrode. Specifically, the positive electrode active material of this embodiment is a host material for lithium element (typically, lithium ions).

[0082] Such positive electrode active materials are not particularly limited, but include, for example, metal oxides and metal phosphates. Metal oxides are not particularly limited, but include, for example, cobalt oxide-based compounds, manganese oxide-based compounds, and nickel oxide-based compounds. Metal phosphates are not particularly limited, but include, for example, iron phosphate-based compounds and cobalt phosphate-based compounds. Typical positive electrode active materials include LiCoO ２ , LiNi ｘ Co ｙ Mn ｚ O ２ (x+y+z=1), LiNi ｘ Co ｙ Al ｚ O ２ (x+y+z=1), LiNi ｘ Mn ｙ O ２ (x+y=1), LiNiO ２ , LiMn ２ O ４ , LiFePO ４ , LiCoPO ４ , LiFeOF, LiNiOF, and LiTiS ２ The positive electrode active materials may be used singly or in combination of two or more kinds.

[0083] The content of the positive electrode active material relative to the total amount of the positive electrode active material composition is not particularly limited, but is, for example, 60% by mass or more and 100% by mass or less, 70% by mass or more and 99% by mass or less, 80% by mass or more and 99% by mass or less, 85% by mass or more and 99% by mass or less, or 90% by mass or more and 99% by mass or less.

[0084] The positive electrode active material layer 112 of this embodiment may contain a binder. By containing a binder, the positive electrode active material layer 112 tends to be more easily bound to the positive electrode current collector 111.

[0085] The binder of this embodiment is not particularly limited, but examples thereof include polyvinylidene fluoride; modified polyvinylidene fluoride obtained by introducing functional groups such as hydroxyl groups, amino groups, carbonyl groups, carboxyl groups, phenyl groups, and methyl groups into polyvinylidene fluoride; polytetrafluoroethylene; modified polytetrafluoroethylene obtained by introducing functional groups such as hydroxyl groups, amino groups, carbonyl groups, carboxyl groups, phenyl groups, and methyl groups into polytetrafluoroethylene; block copolymers, random copolymers, and graft copolymers having tetrafluoroethylene as a constituent unit; styrene butadiene rubber; carboxymethyl cellulose; acrylic resins; polyimide resins, etc. The binders may be used alone or in combination of two or more.

[0086] The content of the binder relative to the total amount of the positive electrode active material layer 112 is not particularly limited, but is, for example, 0.5 mass% or more and 10.0 mass% or less, 0.5 mass% or more and 8.0 mass% or less, or 0.5 mass% or more and 6.0 mass% or less.

[0087] The positive electrode active material layer 112 of this embodiment may contain a conductive additive. Examples of the conductive additive include, but are not limited to, carbon black, carbon nanotubes, carbon nanofibers (CF), and acetylene black. The conductive additives may be used alone or in combination.

[0088] The content of the conductive additive relative to the total amount of the positive electrode active material layer 112 is not particularly limited, but is, for example, 0.5 mass % or more and less than 30.0 mass %.

[0089] The lithium secondary battery 100 of this embodiment may include a laminate in which a plurality of sets of the positive electrode 110, the separator 130, and the negative electrode 120 are stacked. By including a laminate in which a plurality of such sets are stacked, the performance of the battery tends to be improved.

[0090] 1.4. Electrolyte The lithium secondary battery 100 of this embodiment preferably contains an electrolyte. The electrolyte is a liquid containing a solvent and an electrolyte, and is not particularly limited as long as it has ion conductivity. The electrolyte may be impregnated into the separator 130, or the lithium secondary battery 100 may be completed by sealing the electrolyte together with a laminate of the negative electrode 120, the separator 130, and the positive electrode 110.

[0091] The electrolyte contained in the electrolytic solution may be any of the electrolytes that can be contained in polymer electrolytes and gel electrolytes, particularly the lithium salts described above, which may be used alone or in combination of two or more. Preferred lithium salts are the same as those used in the polymer electrolyte and gel electrolyte.

[0092] The solvent contained in the electrolytic solution is not particularly limited, but examples thereof include chain carbonates, cyclic carbonates, chain ethers, and other solvents other than these solvents. The solvents may be used alone or in combination of two or more.

[0093] The chain carbonate is a carbonate that does not have a cyclic structure such as an aromatic ring, an alicyclic ring, a homocyclic ring, a heterocyclic ring, etc. The chain carbonate is not particularly limited, but examples thereof include dimethyl carbonate (DMC), diethyl carbonate, ethyl methyl carbonate (EMC), and compounds in which some or all of the hydrogen atoms in these carbonates have been substituted with fluorine atoms.

[0094] The cyclic carbonate is a carbonate having a cyclic structure such as an aromatic ring, an alicyclic ring, a homocyclic ring, a heterocyclic ring, etc. The cyclic carbonate is not particularly limited, but examples thereof include ethylene carbonate (EC), propylene carbonate, chloroethylene carbonate, and compounds in which some or all of the hydrogen atoms in these carbonates have been substituted with fluorine atoms.

[0095] The chain ether is an ether that does not have a cyclic structure such as an aromatic ring, an alicyclic ring, a homocyclic ring, a heterocyclic ring, etc. The chain ether is not particularly limited, but examples thereof include 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, dimethoxyethane, diethoxyethane, dimethoxypropane, dimethoxybutane, and diethylene glycol dimethyl ether.

[0096] The other solvents are not particularly limited, but examples thereof include acetonitrile, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, 12-crown-4, trimethyl phosphate, triethyl phosphate, derivatives of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, derivatives of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, and derivatives of 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether.

[0097] The total amount of the chain carbonate and the cyclic carbonate relative to the total amount of the solvent is not particularly limited, but is, for example, 40% by volume or more and 100% by volume or less, 60% by volume or more and 100% by volume or less, or 80% by volume or more and 100% by volume or less.

[0098] The content of the chain ether relative to the total amount of the solvent is not particularly limited, but is, for example, 5% by volume to 40% by volume, or 10% by volume to 30% by volume. In addition, the solvent may not contain a chain ether.

[0099] The content of the other solvents relative to the total amount of the solvents is not particularly limited, but is, for example, 1% by volume or more and 10% by volume or less. In addition, the solvent does not necessarily contain other solvents.

[0100] The electrolyte solution of the present embodiment preferably contains an additive. The additive is not particularly limited, but examples thereof include vinylene carbonate (VC), lithium difluorophosphate (LiPO ２ F ２ ), lithium bis(fluorosulfonyl)imide (LiFSI).

[0101] The content of the additives relative to the total amount of the solvent is not particularly limited, but is, for example, 0.1% by mass or more and 5.0% by mass or less.

[0102] 2. Use of Lithium Secondary Battery An exemplary use of the lithium secondary battery 100 will be described. In an exemplary use of the lithium secondary battery 100, a positive electrode terminal and a negative electrode terminal for connecting the battery to an external circuit are joined to the positive electrode current collector 111 and the negative electrode current collector 121, respectively. The lithium secondary battery 100 is charged and discharged by connecting the negative electrode terminal to one end of the external circuit and the positive electrode terminal to the other end of the external circuit. The lithium secondary battery 100 is charged by applying a voltage to the positive electrode terminal and the negative electrode terminal such that a current flows from the negative electrode terminal (negative electrode) through the external circuit to the positive electrode terminal (positive electrode). After charging, the lithium secondary battery is discharged by connecting the positive electrode terminal and the negative electrode terminal via a desired external circuit.

[0103] 3. Manufacturing Method of Lithium Secondary Battery The manufacturing method of the lithium secondary battery 100 of this embodiment includes the following steps: a negative electrode current collector preparation step of preparing a negative electrode current collector 121 including a resin layer 121b and metal layers 121a provided on both sides of the resin layer 121b; a negative electrode production step of forming a negative electrode active material layer 122 on the negative electrode current collector 121 to manufacture a negative electrode 120; and a negative electrode production step of forming a separator 130 having an adhesive layer 131 between the negative electrode active material layer 122 and the positive electrode 110, the adhesive layer 131 being disposed between the negative electrode active material layer 122 and the positive electrode 110. 2, and a heat pressing step of applying pressure to the laminate while heating the laminate; and in the lithium secondary battery 100, a peel strength x between the resin layer 121b and the metal layer 121a at 25°C is 0.05 N / cm or more, and a peel strength y between the negative electrode active material layer 122 and the separator 130 at 25°C satisfies x<y.

[0104] A method for manufacturing the lithium secondary battery 100 of this embodiment will be described in detail below.

[0105] 3.1. Negative Electrode Current Collector Preparation Step In the negative electrode current collector preparation step of this embodiment, a negative electrode current collector 121 including a resin layer 121b and metal layers 121a provided on both sides of the resin layer 121b is prepared. The negative electrode current collector 121 may be a commercially available product or may be prepared from various materials. Methods for preparing the negative electrode current collector 121 include forming the metal layers 121a on both sides of the resin layer 121b by vapor deposition, sputtering, plating such as electroplating, or using an adhesive. Among these methods, vapor deposition and plating are preferred, and plating after vapor deposition is more preferred. It is preferable to prepare the negative electrode current collector 121 by vapor-depositing metal on both sides of the resin layer 121b and then plating to provide the metal layers 121a on both sides of the resin layer 121b.

[0106] The method for depositing the metal layer 121a on the resin layer 121b is not particularly limited, but examples thereof include vacuum deposition, sputtering, and ion plating.

[0107] 3.2. Negative Electrode Fabrication Process In the negative electrode fabrication process of this embodiment, the negative electrode active material layer 122 is formed on the negative electrode current collector 121. The method for forming the negative electrode active material layer 122 on the negative electrode current collector 121 is not particularly limited, and examples thereof include the following method. First, a negative electrode active material and, if necessary, a binder, a conductive additive, and other additives other than the negative electrode active material are mixed to obtain a negative electrode active material composition. The negative electrode active material or the obtained negative electrode active material composition is applied to both sides or one side of the negative electrode current collector 121 and press-molded to form the negative electrode active material layer 122 on both sides or one side of the negative electrode current collector 121, thereby obtaining a molded body. The obtained molded body is punched into a predetermined shape by a punching process to produce the negative electrode 120 of this embodiment.

[0108] Other methods for forming the anode active material layer 122 on the anode current collector 121 include, for example, a method in which a thermosetting compound is added to the anode active material composition and the composition is heated to harden it, a method in which a photocurable compound is added to the anode active material composition and the composition is hardened by irradiating it with light, and a method in which the anode active material composition is a two-component hardening composition and the two components are mixed together to harden it.

[0109] 3.3. Stack Fabrication Process In the stack fabrication process of this embodiment, a stack is fabricated by arranging the negative electrode 120, the positive electrode 110, and the separator 130 so that the negative electrode active material layer 122 faces the positive electrode 110 via the separator 130 having the adhesive layer 131, and so that the adhesive layer 131 is adjacent to the negative electrode active material layer 122.

[0110] The separator 130 preferably has adhesive layers 131 on both sides, in which case one adhesive layer 131 is adjacent to the negative electrode active material layer 122 and the other adhesive layer 131 is adjacent to the positive electrode active material layer 112 .

[0111] 3.4. Heat Pressing Process In the heat pressing process of this embodiment, the laminate is heated and pressurized. Examples of heat pressing processes include a pre-liquid injection heat pressing process, which is performed before the laminate is enclosed in a sealed container together with the electrolyte; a post-liquid injection heat pressing process, which is performed after the laminate is enclosed in a sealed container together with the electrolyte; and a charging heat pressing process, which is performed after the laminate is enclosed in a sealed container together with the electrolyte, while charging at a temperature of 60°C or less. Performing the heat pressing process tends to further improve the adhesive strength between the layers of the lithium secondary battery 100.

[0112] In the heat pressing step, the battery is preferably heated to a surface temperature of 50°C or higher and 150°C or lower, more preferably 55°C or higher and 120°C or lower.

[0113] In the heat pressing step, the pressure is preferably 250 kPa or more and 2000 kPa or less, and more preferably 300 kPa or more and 1750 kPa or less.

[0114] 3.5. Sealing Step The method for manufacturing the lithium secondary battery 100 of this embodiment may include a sealing step. In the sealing step, the laminate is sealed in a sealed container to obtain the lithium secondary battery 100. The sealed container is not particularly limited, but examples thereof include a laminate film. When sealing the laminate in the sealed container, an electrolyte may be sealed in together with the laminate.

[0115] 3.6. Positive Electrode Fabrication Step The manufacturing method for the lithium secondary battery 100 of this embodiment may include a positive electrode fabrication step. In the positive electrode fabrication step, a positive electrode active material layer 112 is formed on a positive electrode current collector 111. The method for forming the positive electrode active material layer 112 on the positive electrode current collector 111 is not particularly limited, and examples thereof include the following method. First, a positive electrode active material composition is obtained by mixing a binder, a conductive additive, and other additives, as needed, in addition to the positive electrode active material and negative electrode active material. The positive electrode active material or the obtained positive electrode active material composition is applied to one or both sides of the positive electrode current collector 111 and press-molded to form the positive electrode active material layer 112 on one or both sides of the positive electrode current collector 111, thereby obtaining a molded body. The obtained molded body is punched into a predetermined shape by a punching process to produce the positive electrode 110 of this embodiment.

[0116] Other methods for forming the positive electrode active material layer 112 on the positive electrode current collector 111 include, for example, a method in which a thermosetting compound is added to a positive electrode active material composition and the composition is heated to harden it, a method in which a photocurable compound is added to a positive electrode active material composition and the composition is hardened by irradiating it with light, and a method in which the positive electrode active material composition is a two-component hardening composition and the two components are mixed to harden it.

[0117] When the positive electrode current collector 111 includes a resin layer and metal layers provided on both sides of the resin layer, a commercially available product may be used for the positive electrode current collector 111, or the positive electrode current collector 111 may be made from various materials. The positive electrode current collector 111 may be made by a method similar to the method for making the negative electrode current collector 121.

[0118] 3.7 Other Steps The method for manufacturing the lithium secondary battery 100 of this embodiment may include other steps. The other steps are not particularly limited, but may include, for example, a step of forming a functional layer to impart a desired function to the lithium secondary battery 100.

[0119] The present invention will be described in more detail below using examples and comparative examples. The present invention is not limited to the following examples and comparative examples. Unless otherwise specified, the experiments were carried out at room temperature (25°C) and 1 atmosphere.

[0120] 1. Fabrication of Lithium Secondary Battery A lithium secondary battery of Example 1 was fabricated as follows.

[0121] 1.1. Preparation of Negative Electrode A negative electrode current collector CC1 was prepared by forming a 1.0 μm-thick metal layer, Cu, on both sides of a 4.5 μm-thick polypropylene (PP) resin layer. A 0.10 μm-thick Cu layer was formed by vacuum deposition, followed by a 0.90 μm-thick Cu layer by electroplating, resulting in a 1.0 μm-thick Cu layer.

[0122] A negative electrode active material composition was prepared by mixing 99.0 parts by mass of artificial graphite as a negative electrode active material, 0.5 parts by mass of styrene butadiene rubber (SBR) and 0.5 parts by mass of carboxymethyl cellulose (CMC) as binders with water as a solvent. This negative electrode active material composition was applied to one side of a negative electrode current collector in a manner that the basis weight was 15 mg / cm. ２ The negative electrode active material layer was formed on one side of the negative electrode current collector by applying the coating and pressing the coating so that the coating was as follows: a negative electrode active material layer was formed on one side of the negative electrode current collector, and a molded body was obtained. The molded body was then cut into a predetermined size (4.5 cm x 4.5 cm). This gave a negative electrode.

[0123] 1.2. Preparation of Positive Electrode A 12 μm thick aluminum foil was used as a positive electrode current collector. LiNi was dissolved in N-methyl-pyrrolidone (NMP) as a solvent. ０．８ Co ０．１ Mn ０．１ O ２A positive electrode active material composition was prepared by mixing 96 parts by mass of the above-mentioned cellulose acetate copolymer, 1 part by mass each of acetylene black and carbon nanotubes as a conductive additive, and 2 parts by mass of polyvinylidene fluoride (PVdF) as a binder. This positive electrode active material composition was applied to one side of a positive electrode current collector in a manner that the basis weight was 23 mg / cm. ２ The positive electrode active material layer was formed on one side of the positive electrode current collector by coating with the paste and pressing it, and a molded body was obtained. This molded body was cut out to a predetermined size (4 cm x 4 cm).

[0124] 1.3. Separator Preparation A polyethylene microporous membrane sheet (thickness: 8 μm, length and width: 5 cm × 5 cm) was prepared as the separator body. Then, a 2 μm thick adhesive layer containing polyvinylidene fluoride (PVdF) was formed on both sides of the separator body. The separator thickness was 12 μm.

[0125] 1.4. Preparation of Electrolyte Solution 30% by volume of ethylene carbonate (EC), 35% by volume of dimethyl carbonate (DMC), and 35% by volume of ethyl methyl carbonate (EMC) were mixed to prepare a 100% by volume mixed solvent. LiPF was added to this mixed solvent to a concentration of 1M. ６ and then dissolving vinylene carbonate (VC) and LiPO ２ F ２ The electrolyte solution was obtained by dissolving VC in the total mass of the mixed solvent (i.e., VC, LiPO ２ F ２ and LiPF ６ The total mass of EC, DMC, and EMC not including LiPO was 2% by mass. ２ F ２ was 1% by mass relative to the total mass of the mixed solvent.

[0126] 1.5. Battery Assembly Next, the positive electrode, separator, and negative electrode were arranged so that the negative electrode active material layer faced the positive electrode active material layer via the separator to obtain a laminate. Furthermore, a 100 μm Al terminal and a 100 μm Ni terminal were ultrasonically welded to the positive electrode current collector and the negative electrode current collector, respectively, and then inserted into a laminate outer casing. The above-mentioned electrolyte solution was then injected into the outer casing. The outer casing was sealed to obtain a lithium secondary battery.

[0127] 1.6 Heat Pressing The lithium secondary battery obtained as described above into which the electrolyte had been injected was subjected to heat pressing using a high-temperature heat press machine after the electrolyte was injected. The heat pressing after the electrolyte was injected was performed by applying a pressure of 350 kPa and heating for 3 minutes after the temperature of the battery surface reached 60°C.

[0128] The lithium secondary batteries of Examples 2 to 8 and Comparative Examples 1 to 7 were obtained in the same manner as in Example 1, except that the negative electrode current collectors and negative electrode active material compositions shown in Table 1 were used, that the adhesive layers shown in Table 1 were used when adhesive layers were used, and that the heat pressing was performed under the heat pressing conditions shown in Table 1 when heat pressing was performed after injection.

[0129] The negative electrode current collectors CC2 to CC5 were prepared as follows.

[0130] CC2 was prepared by forming a 1.0 μm thick metal layer of Cu on both sides of a 4.5 μm thick polypropylene (PP) resin layer. A 0.20 μm thick Cu layer was formed by vacuum deposition, and then a 0.80 μm thick Cu layer was formed by electroplating, resulting in a 1.0 μm thick Cu layer.

[0131] CC3 was prepared by forming a 1.0 μm thick metal layer of Cu on both sides of a 4.5 μm thick polyethylene (PE) resin layer. A 0.20 μm thick Cu layer was formed by vacuum deposition, and then a 0.80 μm thick Cu layer was formed by electroplating, resulting in a 1.0 μm thick Cu layer.

[0132] CC4 was prepared by forming a 1.0 μm-thick Cu layer on both sides of a 4.5 μm-thick polyethylene terephthalate (PET) resin layer. A 0.20 μm-thick Cu layer was formed by vacuum deposition, and then a 0.80 μm-thick Cu layer was formed by electroplating, resulting in a 1.0 μm-thick Cu layer.

[0133] CC5 was prepared by forming a 1.0 μm thick metal layer of Cu on both sides of a 4.5 μm thick polypropylene (PP) resin layer. A 0.05 μm thick Cu layer was formed by vacuum deposition, and then a 0.95 μm thick Cu layer was formed by electroplating, resulting in a 1.0 μm thick Cu layer.

[0134] The peel strengths x, y, and z at 25°C were measured in an environment at 25°C in accordance with the peel adhesion strength test method using 180-degree peeling in JIS K 6854-2:1999. Specifically, the measurements were performed as follows. When heat pressing was performed, the measurement was performed using the lithium secondary battery after heat pressing. When heat pressing was not performed, the measurement was performed using the lithium secondary battery after assembly. If necessary, the lithium secondary battery was disassembled to measure the peel strengths x, y, and z at 25°C. In measuring the peel strength x at 25°C, a 15 mm wide tape was attached to the metal layer, and the tape was bent 180° and pulled to peel the metal layer from the resin layer. The peel strength x (N / cm) was calculated from the force applied when peeling the metal layer from the resin layer. In measuring the peel strength y at 25 ° C, a 15 mm wide tape was attached to the negative electrode active material layer, and the tape was bent 180 ° and pulled to peel the negative electrode active material layer from the separator. The peel strength y (N / cm) was calculated from the force applied when peeling the negative electrode active material layer from the separator. In measuring the peel strength z at 25 ° C, a 15 mm wide tape was attached to the negative electrode active material layer, and the tape was bent 180 ° and pulled to peel the negative electrode active material layer from the metal layer. The peel strength z (N / cm) was calculated from the force applied when peeling the negative electrode active material layer from the metal layer. Note that the tape used in the peel strength test had sufficiently strong adhesive strength, and when peeling one layer from the other, the tape was completely adhered to one layer. For example, when peeling the metal layer from the resin layer to measure the peel strength x at 25 ° C, the tape was completely adhered to the metal layer.

[0135] In the table, peel strength relational expression 1 is x<y, and peel strength relational expression 2 is x<y<(z / 2). In addition, in the table, the column for vapor deposition layer / plated layer thickness indicates the thickness of the vapor deposition layer and the plated layer in the metal layer of the negative electrode current collector.

[0136] 2. Evaluation Hereinafter, the initial CC charging of the fabricated lithium secondary battery will be referred to as "initial charging," and the initial CC discharging will be referred to as "initial discharging." CC charging refers to charging at a constant current value, and CC discharging refers to discharging at a constant current value. A 1C current is a current calculated based on the theoretical capacity of the battery, which can fully charge the battery in one hour. A 2C current is twice as large as a 1C current.

[0137] 2.1. Capacity Retention Rate The lithium secondary batteries of the Examples and Comparative Examples were initially charged at a current of 0.1 C under pressure at 350 kPa in an environment at 25°C until the voltage reached 4.2 V, and then initially discharged at a current of 0.1 C until the voltage reached 3.0 V. Next, the lithium secondary batteries of the Examples and Comparative Examples were subjected to CC charging at a current of 1.0 C without pressure in an environment at 25°C until the voltage reached 4.2 V, and then CC discharging at a current of 1.0 C until the voltage reached 3.0 V. The discharge capacity (1st cycle) was determined from this CC discharge. This cycle was repeated 100 times, and the discharge capacity (100th cycle) was determined from the CC discharge at the 100th cycle. The discharge capacity (100th cycle) was then divided by the discharge capacity (1st cycle) and multiplied by 100 to determine the capacity retention rate at the 100th cycle. This value is shown in the capacity retention rate column of Table 1.

[0138]

[0139] 3. Evaluation Results Table 1 shows that the lithium secondary battery example includes a positive electrode 110, a separator 130, and a negative electrode 120 including a negative electrode current collector 121 and a negative electrode active material layer 122 on the negative electrode current collector 121, wherein the negative electrode current collector 121 includes a resin layer 121b and a metal layer 121a provided on both sides of the resin layer 121b, the negative electrode active material layer 122 faces the positive electrode 110 via the separator 130, the separator 130 includes an adhesive layer 131 on the surface facing the negative electrode 120, and wherein the peel strength x at 25°C between the resin layer 121b and the metal layer 121a is 0.05 N / cm or more and the peel strength y at 25°C between the negative electrode active material layer 122 and the separator 130 satisfies x<y, and the lithium secondary battery example includes a negative electrode 120 including a resin layer 121b and a metal layer 121a provided on both sides of the resin layer 121b, the negative electrode active material layer 122 faces the positive electrode 110 via the separator 130, and the separator 130 includes an adhesive layer 131 on the surface facing the negative electrode 120, and wherein the peel strength x at 25°C between the resin layer 121b and the metal layer 121a is 0.05 N / cm or more and the peel strength y at 25°C between the negative electrode active material layer 122 and the separator 130 satisfies x<y. This example shows a better capacity retention rate than the lithium secondary battery comparative example.

[0140] <Additional Notes> Embodiments of the present disclosure include the following aspects. [1] A lithium secondary battery comprising: a positive electrode, a separator, and a negative electrode including a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector, wherein the negative electrode current collector comprises a resin layer and metal layers provided on both sides of the resin layer, wherein the negative electrode active material layer faces the positive electrode via the separator, and the separator comprises an adhesive layer on the surface facing the negative electrode, wherein a peel strength x at 25°C between the resin layer and the metal layer is 0.05 N / cm or more, and a peel strength y at 25°C between the negative electrode active material layer and the separator satisfies x<y. [2] The lithium secondary battery according to [1], wherein the peel strength y at 25°C is 0.07 N / cm or more. [3] The lithium secondary battery according to [1] or [2], wherein the peel strength x and the peel strength y satisfy y>0.1+0.5x. [4] The lithium secondary battery according to any one of [1] to [3], wherein the resin layer contains polypropylene, and the metal layer contains copper. [5] The lithium secondary battery according to any one of [1] to [4], wherein the adhesive layer contains polyvinylidene fluoride. [6] The lithium secondary battery according to any one of [1] to [5], wherein the peel strength z at 25°C between the negative electrode active material layer and the metal layer, the peel strength x at 25°C, and the peel strength y at 25°C satisfy x<y<(z / 2). [7] The lithium secondary battery according to any one of [1] to [6], wherein the peel strength x at 25°C is 0.09 N / cm or more. [8] The lithium secondary battery according to [6] or [7], wherein the peel strength z at 25°C is 0.30 N / cm or more. [9] The lithium secondary battery according to any one of [1] to [8], comprising a laminate in which a plurality of sets of the positive electrode, the separator, and the negative electrode are stacked.

[10] The lithium secondary battery according to any one of [1] to [9], wherein the metal layer comprises a vapor deposition layer provided on the resin layer and a plating layer provided on the vapor deposition layer.

[11] A method for manufacturing a lithium secondary battery, comprising: a negative electrode current collector preparation step of preparing a negative electrode current collector including a resin layer and metal layers provided on both sides of the resin layer; a negative electrode fabrication step of forming a negative electrode active material layer on the negative electrode current collector to fabricate a negative electrode; a stack fabrication step of arranging the negative electrode, the positive electrode, and the separator to fabricate a stack so that the negative electrode active material layer faces a positive electrode via a separator including an adhesive layer and the adhesive layer is adjacent to the negative electrode active material layer; and a heat press step of applying pressure to the stack while heating, wherein in the lithium secondary battery, a peel strength x between the resin layer and the metal layer at 25°C is 0.05 N / cm or more, and a peel strength y between the negative electrode active material layer and the separator at 25°C satisfies x<y.

[12] The method for producing a lithium secondary battery according to

[11] , wherein in the negative electrode current collector preparation step, metal is vapor-deposited on both surfaces of the resin layer, and then plating is performed to provide metal layers on both surfaces of the resin layer, thereby producing a negative electrode current collector.

[0141] The lithium secondary battery of the present invention is lightweight, inexpensive, and has a good capacity retention rate, and therefore has industrial applicability as an electricity storage device for a variety of uses.

[0142] 100... Lithium secondary battery, 110... Positive electrode, 111... Positive electrode current collector, 112... Positive electrode active material layer, 120... Negative electrode, 121... Negative electrode current collector, 121a... Metal layer, 121b... Resin layer, 122... Negative electrode active material layer, 130... Separator, 131... Adhesive layer, 132... Separator body

Claims

1. A lithium secondary battery comprising a positive electrode, a separator, a negative electrode current collector, and a negative electrode including a negative electrode active material layer on the negative electrode current collector, wherein the negative electrode current collector includes a resin layer and metal layers provided on both surfaces of the resin layer, the negative electrode active material layer faces the positive electrode through the separator, the separator includes an adhesive layer on the surface on the negative electrode side, the peel strength x at 25°C between the resin layer and the metal layer is 0.05 N / cm or more, and the peel strength y at 25°C between the negative electrode active material layer and the separator satisfies x < y.

2. The lithium secondary battery according to claim 1, wherein the peel strength y at 25°C is 0.07 N / cm or more.

3. The lithium secondary battery according to claim 1, wherein the peel strength x and the peel strength y satisfy y > 0.1 + 0.5x.

4. The lithium secondary battery according to claim 1, wherein the resin layer contains polypropylene and the metal layer contains copper.

5. The lithium secondary battery according to claim 1, wherein the adhesive layer contains polyvinylidene fluoride.

6. The lithium secondary battery according to claim 1, wherein the peel strength z at 25°C between the negative electrode active material layer and the metal layer, the peel strength x at 25°C, and the peel strength y at 25°C satisfy x < y < (z / 2).

7. The lithium secondary battery according to claim 6, wherein the peel strength x at 25°C is 0.09 N / cm or more.

8. The lithium secondary battery according to claim 6, wherein the peel strength z at 25°C is 0.30 N / cm or more.

9. The lithium secondary battery according to claim 1, comprising a laminate in which a plurality of sets of the positive electrode, the separator, and the negative electrode are laminated.

10. The lithium secondary battery according to claim 1, wherein the metal layer includes a vapor deposition layer provided on the resin layer and a plating layer provided on the vapor deposition layer.

11. A method for manufacturing a lithium secondary battery, comprising: A negative electrode current collector preparation step of preparing a negative electrode current collector including a resin layer and metal layers provided on both surfaces of the resin layer; A negative electrode manufacturing step of forming a negative electrode active material layer on the negative electrode current collector to manufacture a negative electrode; A laminate manufacturing step of arranging the negative electrode, the positive electrode, and the separator so that the negative electrode active material layer faces the positive electrode through a separator including an adhesive layer, and the adhesive layer is adjacent to the negative electrode active material layer to manufacture a laminate; A heat press step of pressurizing the laminate while heating it; In the lithium secondary battery, The peel strength x at 25° C. between the resin layer and the metal layer is 0.05 N / cm or more, The peel strength y at 25° C. between the negative electrode active material layer and the separator satisfies x < y. A method for manufacturing a lithium secondary battery.

12. The method for manufacturing a lithium secondary battery according to claim 11, wherein in the negative electrode current collector preparation step, metal is vapor-deposited on both surfaces of the resin layer and then plated to provide metal layers on both surfaces of the resin layer to manufacture a negative electrode current collector.

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