JP2018037379A - Lithium ion secondary battery electrode assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode assembly which is increased in the connection strength between an electrode current collector and an electrode lead, and which enables the production of a reliable battery with a long life.SOLUTION: An electrode assembly of the present invention has an electrode active material layer provided on a metal electrode current collector, and is joined to an electrode lead. In the electrode assembly, the electrode current collector has at least a rust inhibitor coating, and a metal oxide coating. The ratio (mg/dm/nm) of a basis weight of the rust inhibitor coating to a thickness of the metal oxide coating is equal to or higher than 0.007 and lower than 0.025.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electrode assembly used for a nonaqueous electrolyte battery, particularly a lithium ion secondary battery.

  Nonaqueous electrolyte batteries have been put to practical use as automobile batteries including hybrid cars and electric cars. Lithium ion secondary batteries are used as such on-vehicle power supply batteries. Lithium ion secondary batteries are required to have various characteristics such as output characteristics, energy density, capacity, lifetime, and high temperature stability. In particular, various improvements have been made to the electrode in order to improve the stability and life of the battery.

  For example, a negative electrode assembly in which a negative electrode active material is applied to the surface of a copper foil that is an electrode current collector and a negative electrode lead is bonded is used as the negative electrode. The negative electrode lead is usually ultrasonically welded to the surface of a copper foil which is a negative electrode current collector. However, if an oxide film is formed on the copper foil surface, sufficient bonding strength may not be obtained between the negative electrode lead and the copper foil surface. When such a negative electrode assembly is used as a battery for an automobile, the negative electrode lead may be detached due to vibration of the vehicle body, which is not preferable.

  Patent Document 1 discloses a copper foil having a specific dynamic friction coefficient and composed of an oxide film and / or a rust preventive film. In Patent Document 1, when ultrasonic vibration is applied to the laminated copper foil, the surface oxide film or rust preventive film is physically removed between the overlapping copper foils, and a new surface appears. It is described that the smaller the dynamic friction coefficient, the easier the movement between the copper foils and the generation of a new surface over a wide range, so that the ultrasonic weldability is improved.

JP2012-99351

  In order to improve the bonding strength between the copper foil as the negative electrode current collector and the negative electrode lead, the formation of the oxide film on the surface of the copper foil may be reduced, but it is difficult to completely eliminate the oxide film. In order to suppress the formation of oxide film, it is possible to apply a rust preventive to prevent oxidation, but by applying a large amount of the rust preventive to prevent oxidation, the bonding with the negative electrode lead is hindered. Sometimes.

  In view of the above, an object of the present invention is to provide an electrode assembly in which a bonding strength between an electrode current collector and an electrode lead is improved, and a battery having high reliability and a long life can be manufactured.

In the electrode assembly in the embodiment of the present invention, an electrode active material layer is provided on an electrode current collector made of metal, and the electrode current collector and the electrode lead are joined. Here, the electrode current collector has at least a rust preventive coating and a metal oxide coating, and the ratio (mg / dm ² / nm) between the basis weight of the rust preventive coating and the thickness of the metal oxide coating is 0.00. 007 or more and less than 0.025.

  The electrode assembly of the present invention has high bonding strength between the electrode current collector and the electrode lead. For this reason, batteries manufactured using the electrode assembly of the present invention, in particular lithium ion secondary batteries, have high reliability and safety because the electrodes are not damaged by vibration during battery use. It becomes a long battery.

FIG. 1 is a schematic cross-sectional view illustrating a configuration of an electrode assembly according to an embodiment of the present invention.

  Embodiments of the present invention will be described below. The electrode assembly of the embodiment includes an electrode current collector, an electrode active material layer, and an electrode lead. The electrode current collector is preferably made of a metal having electrical conductivity and has a foil shape. As the electrode current collector, an aluminum alloy foil such as a copper foil, an aluminum foil, and a nickel aluminum alloy can be suitably used from the viewpoints of electrical conductivity, workability, lightness, and corrosion resistance.

  The electrode current collector has at least a rust preventive film and a metal oxide film. The rust preventive coating is provided to prevent the metal used as the electrode current collector from being oxidized by oxygen and moisture (water) in the air to form an oxide coating. As the rust preventive agent, a substance capable of suppressing metal corrosion can be used. As such substances, inorganic compounds such as nitrites, chromates, silicates, polyphosphates, organic compounds such as carboxylic acids, metal carboxylate soaps, sulfonates, amine salts, esters, phosphate esters, etc. Can be used. It is preferable to use a compound containing chromium as a rust inhibitor. In particular, chromate treatment using a chromate containing hexavalent chromium or trivalent chromium is a typical method for forming a rust preventive film.

  Even if a rust preventive film is provided on the electrode current collector, it is difficult to completely suppress the formation of an oxide film on the surface of the metal electrode current collector. Therefore, an oxide film corresponding to the metal foil used as the electrode current collector is formed on the surface of the electrode current collector. That is, a copper oxide film is formed on the copper foil, and an aluminum oxide film is formed on the aluminum foil.

  The electrode active material layer includes an electrode active material that is a substance that performs a redox reaction and plays a central role in the battery reaction. There are electrode active materials for positive and negative electrodes. For example, a carbon material can be used as a negative electrode active material for a lithium ion secondary battery. As a positive electrode active material for a lithium ion secondary battery, a lithium / nickel composite oxide, a lithium / manganese composite oxide, or the like can be used.

  In addition to the electrode active material, a mixture of a conductive aid for improving conductivity, a binder for binding the electrode active material, etc. is usually used to form a slurry or paste as an electrode active material mixture The electrode active material layer can be formed by coating this on the surface of the electrode current collector. Examples of the conductive assistant include carbon fibers such as carbon nanofibers, carbon blacks such as acetylene black and ketjen black, carbon materials such as activated carbon, graphite, mesoporous carbon, fullerenes, and carbon nanotubes. In addition, as binders, fluoropolymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF), conductive polymers such as polyanilines, polythiophenes, polyacetylenes, and polypyrroles, styrene butadiene rubber Synthetic rubbers such as (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), acrylonitrile butadiene rubber (NBR), or polysaccharides such as carboxymethyl cellulose (CMC), xanthan gum, guar gum and pectin. Can be used. In addition, electrode additives generally used for electrode formation, such as thickeners, dispersants, and stabilizers, can be appropriately used for the electrode active material layer.

  In the embodiment, the electrode lead is a terminal for taking in and out electricity between the positive electrode or the negative electrode in the battery and the outside. A nickel conductor or a copper conductor plated with nickel can be used as the negative electrode lead of the lithium ion secondary battery, and an aluminum conductor can be used as the positive electrode lead. The electrode current collector and the electrode lead described above are usually joined by welding including ultrasonic welding. A portion where the electrode current collector and the electrode lead are bonded forms a bonded portion.

Here, in the electrode current collector, the ratio (mg / dm ² / nm) between the basis weight of the rust preventive agent film and the thickness of the metal oxide film is preferably 0.007 or more and less than 0.025. Here, the weight per unit area of the rust preventive coating is the weight of the rust preventive existing per unit area of the electrode current collector. In this embodiment, the basis weight of the rust preventive film is estimated in units of mg / dm ² . That is, the weight (mg) of the rust inhibitor applied per 1 dm ^{2 of the} electrode current collector is the basis weight of the rust inhibitor film. The thickness of the metal oxide film is the thickness of the metal oxide film used as the electrode current collector described above, and is estimated in units of nm (nanometers). The basis weight of the rust inhibitor can be estimated by an existing method such as atomic absorption spectrometry. The thickness of the metal oxide film can be measured by elemental analysis using an XPS apparatus. When the ratio between the basis weight of the rust preventive coating and the thickness of the metal oxide coating is in the range of 0.007 or more and less than 0.025, the bonding strength of the bonding portion between the electrode current collector and the electrode lead is increased. Even when the value of the ratio of the basis weight of the rust preventive coating and the thickness of the metal oxide coating is less than 0.007 (that is, the basis weight of the rust preventive coating is small or the thickness of the metal oxide coating is large), It was also found that the bonding strength is low when the ratio value is 0.025 or more (that is, the basis weight of the rust preventive film is large or the thickness of the metal oxide film is small). There is an appropriate balance between the amount of the rust preventive coating applied and the amount of the metal oxide coating formed to maintain a high bonding strength between the electrode current collector and the electrode lead. It is considered that the bonding strength of the part cannot be maintained if the amount is too much or too little. The ratio between the basis weight of the rust preventive coating and the thickness of the metal oxide coating is more preferably 0.01 or more and less than 0.02.

The basis weight (mg / dm ² ) of the rust preventive film is preferably 0.029 or more and 0.04 or less. That is, it can be said that the thickness of the metal oxide film is preferably in the range of 1.6 to 4.14 nm.

  The electrode assembly of this embodiment can be used as an electrode for a lithium ion secondary battery. The lithium ion secondary battery is a lithium ion secondary battery in which a power generation element including a positive electrode assembly, a negative electrode assembly, a separator, and an electrolytic solution is sealed with an exterior body. Here, the positive electrode assembly is a positive electrode active material layer formed by applying or rolling and drying a mixture of a positive electrode active material and an electrode additive such as a binder and a conductive additive on a positive electrode current collector such as a metal foil. It is one form of the electrode assembly of this embodiment which joined the positive electrode lead to the positive electrode. The negative electrode assembly refers to the present embodiment in which a negative electrode lead is bonded to a negative electrode formed by applying a mixture of a negative electrode active material and an electrode additive such as a binder and a conductive additive to a negative electrode current collector to form a negative electrode active material layer. It is one form of the electrode assembly of a form. The separator is a film-like battery member for separating the positive electrode and the negative electrode and ensuring the conductivity of lithium ions between the negative electrode and the positive electrode. The electrolytic solution is an electrically conductive solution in which an ionic substance is dissolved in a solvent. In this embodiment, a nonaqueous electrolytic solution can be used in particular. The power generation element is a unit of the main constituent member of the battery. Usually, the positive electrode and the negative electrode are stacked (stacked) via a separator, and the stacked body is immersed in the electrolytic solution.

  The lithium ion secondary battery is formed by sealing the power generation element with an exterior body. The term “sealed” means that at least a part of the power generation element is wrapped with an exterior body material described later so as not to come into contact with the outside air. The exterior body is a housing that can seal the power generation element or a bag-shaped one made of a flexible material. The lithium ion secondary battery may be in various forms such as a coin-type battery, a laminate-type battery, and a wound-type battery.

  The electrode assembly of the present embodiment that can be used as a negative electrode assembly of a lithium ion secondary battery includes a negative electrode current collector such as a copper foil obtained by mixing a mixture of a negative electrode active material and an electrode additive such as a binder and a conductive additive. A battery member in which a negative electrode lead is bonded to a negative electrode on which a negative electrode active material layer is formed by coating, rolling and drying on a body. A carbon material is preferably used as the negative electrode active material. Here, the carbon material includes graphite. In particular, when graphite is contained in the negative electrode active material layer, there is an advantage that the output of the battery can be improved even when the remaining capacity (SOC) of the battery is low. Graphite is a carbon material of hexagonal hexagonal plate crystal, and is sometimes referred to as graphite or graphite. The graphite is preferably in the form of particles.

  Graphite includes natural graphite and artificial graphite. Natural graphite can be obtained in large quantities at low cost, has a stable structure and is excellent in durability. Artificial graphite is artificially produced graphite that has high purity (contains almost no impurities such as allotropes) and has low electrical resistance. As the carbon material in the embodiment, both natural graphite and artificial graphite can be suitably used. Natural graphite having a coating with amorphous carbon or artificial graphite having a coating with amorphous carbon can also be used.

  Amorphous carbon is a carbon material that has a structure in which microcrystals are randomly networked, and may be partially similar to graphite. is there. Examples of the amorphous carbon include carbon black, coke, activated carbon, carbon fiber, hard carbon, soft carbon, and mesoporous carbon. When natural graphite particles having a coating with amorphous carbon or artificial graphite having a coating with amorphous carbon is used as the carbon material of the negative electrode active material, the decomposition of the electrolytic solution is suppressed and the durability of the negative electrode is improved.

When artificial graphite is used, the interlayer distance d value (d ₀₀₂ ) is preferably 0.337 nm or more. The crystal structure of artificial graphite is generally thinner than natural graphite. When artificial graphite is used as a negative electrode active material for a lithium ion secondary battery, it is necessary to have an interlayer distance into which lithium ions can be inserted. The interlayer distance at which lithium ions can be inserted / removed can be estimated by a d value (d ₀₀₂ ). If the d value is 0.337 nm or more, lithium ions can be inserted / removed without any problem.

  The negative electrode active material layer is prepared by mixing a carbon material, which is a negative electrode active material, and an electrode additive such as a binder and a conductive additive in a solvent (N-methylpyrrolidone (NMP), water, etc.) at an appropriate ratio. It can be formed by applying or rolling this onto a negative electrode current collector made of a metal foil (such as a copper foil) and heating to evaporate the solvent.

The electrode assembly of the present embodiment that can be used as a positive electrode assembly of a lithium ion secondary battery includes a positive electrode current collector such as an aluminum foil obtained by mixing a mixture of a positive electrode active material and an electrode additive such as a binder and a conductive additive. A battery member in which a positive electrode lead is bonded to a positive electrode on which a positive electrode active material layer is formed by applying or rolling and drying on a body. Preferably, the positive electrode active material layer includes a lithium composite oxide such as a lithium / nickel composite oxide or a lithium / manganese composite oxide. Here, the lithium-nickel based composite oxide is a general formula Li _x Ni _y Me _(1-y) O ₂ (where Me is Al, Mn, Na, Fe, Co, Cr, Cu, Zn, Ca, At least one metal selected from the group consisting of K, Mg, and Pb, 1.0 ≦ x ≦ 1.2, and y is a positive number less than 1.0). It is a transition metal composite oxide containing lithium and nickel. For example, a lithium / nickel composite oxide in which Me is Co (cobalt) and Al (aluminum), or a lithium / nickel composite oxide in which Me is Co (cobalt) and Mn (manganese) is used as a component of the positive electrode active material. Can be used as

Examples of the lithium / manganese composite oxide include a zigzag layered lithium manganate (LiMnO ₂ ) and a spinel type lithium manganate (LiMn ₂ O ₄ ). By using a lithium-manganese composite oxide in combination, the positive electrode can be produced at a lower cost. In particular, it is preferable to use spinel type lithium manganate (LiMn ₂ O ₄ ) which is excellent in terms of stability of the crystal structure in an overcharged state. There are trivalent and tetravalent Mn (manganese) in LiMn ₂ O ₄ . Of these, manganese that contributes to the oxidation-reduction of the battery reaction is trivalent. The crystal structure can be stabilized by substituting a part of this trivalent manganese with another element. Examples of elements that can be substituted for a part of trivalent manganese include Li, Mg, B, Al, V, Cr, Fe, Co, Ni, W, and combinations of two or more thereof.

In all the embodiments, the lithium-nickel composite oxide is a lithium nickel manganese cobalt composite oxide having a layered crystal structure represented by the general formula Li _x Ni _y Co _z Mn _(1.0-yz) O _2. It is preferable to contain a product as a positive electrode active material. Here, x in the general formula is 1.0 ≦ x ≦ 1.2, and y and z are positive numbers that satisfy y + z <1.0. The lithium / nickel composite oxide having this general formula is a lithium / nickel / cobalt / manganese composite oxide (hereinafter sometimes referred to as “NCM”). NCM is a lithium-nickel composite oxide that is suitably used to increase the capacity of a battery. For example, in the general formula Li _x Ni _y Co _z Mn _(1.0-yz) O ₂ , a composite oxide of x = 1, y = 0.4, z = 0.3 is referred to as “NCM433”, A composite oxide having x = 1, y = 0.5, and z = 0.2 is referred to as “NCM523”.

  In the embodiment, the positive electrode active material layer is a slurry obtained by mixing the positive electrode active material and an electrode additive such as a binder and a conductive additive in a solvent (N-methylpyrrolidone (NMP), water, etc.) at an appropriate ratio. It can be formed by applying or rolling this onto a positive electrode current collector made of a metal foil (such as an aluminum foil) and heating to evaporate the solvent.

  The separator constituting the lithium ion secondary battery is a film-like battery member for separating the positive electrode and the negative electrode and ensuring the conductivity of lithium ions between the negative electrode and the positive electrode. The separator is composed of an olefin resin layer. The olefin resin layer is a layer composed of polyolefin obtained by polymerizing or copolymerizing α-olefin such as ethylene, propylene, butene, pentene, hexene and the like. In the embodiment, a structure having pores that are closed when the battery temperature rises, that is, a layer composed of a porous or microporous polyolefin is preferable. Since the olefin resin layer has such a structure, even if the battery temperature rises, the separator is closed (shuts down), and the ion flow can be cut off. In order to exert a shutdown effect, it is very preferable to use a porous polyethylene film. The separator may optionally have a heat-resistant fine particle layer. At this time, the heat-resistant fine particle layer provided for preventing abnormal heat generation of the battery is composed of inorganic fine particles having a heat-resistant temperature of 150 ° C. or more and stable in electrochemical reaction. Examples of such inorganic fine particles include inorganic oxides such as silica, alumina (α-alumina, β-alumina, θ-alumina), iron oxide, titanium oxide, barium titanate, zirconium oxide; boehmite, zeolite, apatite, kaolin, Mention may be made of minerals such as spinel, mica and mullite. Thus, a ceramic separator having a heat resistant resin layer can also be used.

  The electrolytic solution used together with the electrode assembly of the embodiment and constituting the lithium ion secondary battery is an electrically conductive solution in which an ionic substance is dissolved in a solvent. In particular, a non-aqueous electrolyte can be used. The power generation element including the positive electrode, the negative electrode, the separator, and the electrolytic solution is a unit of the main constituent member of the battery. Usually, the positive electrode and the negative electrode are laminated via the separator, and this laminate is immersed in the electrolytic solution. Has been.

The electrolytic solution is a non-aqueous electrolytic solution, which is dimethyl carbonate (hereinafter referred to as “DMC”), diethyl carbonate (hereinafter referred to as “DEC”), ethyl methyl carbonate (hereinafter referred to as “EMC”), di-acid. Chain carbonates such as -n-propyl carbonate, di-t-propyl carbonate, di-n-butyl carbonate, di-isobutyl carbonate, or di-t-butyl carbonate, propylene carbonate (PC), ethylene carbonate (hereinafter " It is preferably a mixture containing a cyclic carbonate such as “EC”. The electrolytic solution is obtained by dissolving a lithium salt such as lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), or lithium perchlorate (LiClO ₄ ) in such a carbonate mixture.

  The electrolytic solution preferably includes a combination of PC and / or EC, which is a cyclic carbonate, and DMC and / or EMC, which is a chain carbonate, as appropriate. PC is a solvent having a low freezing point, and is used for improving the output of the battery at a low temperature. However, it is known that PC has a slightly low compatibility with graphite used as a negative electrode. EC is a solvent having a high polarity and a high dielectric constant, and is used as a constituent component of an electrolytic solution for a lithium ion secondary battery. However, since EC has a high melting point (freezing point) and is a solid at room temperature, it may be solidified and precipitated at a low temperature even if it is used as a mixed solvent. DMC is a solvent having a large diffusion coefficient and low viscosity. However, since DMC has a high melting point (freezing point), the electrolyte may solidify at a low temperature. Similarly to DMC, EMC is a solvent having a large diffusion coefficient and low viscosity. Thus, the constituent components of the electrolytic solution have different characteristics. For example, in order to improve the output of the battery at a low temperature, it is important to consider these balances. By adjusting the content ratio of the cyclic carbonate and the chain carbonate, it is possible to obtain an electrolytic solution that has a low viscosity at normal temperature and does not lose its performance even at low temperatures.

  In addition, the electrolytic solution may contain a cyclic carbonate compound different from the above cyclic carbonate as an additive. Examples of cyclic carbonate compounds used as additives include vinylene carbonate (hereinafter referred to as “VC”). Moreover, you may use the cyclic carbonate compound which has a halogen as an additive. These cyclic carbonates are also compounds that form a protective film for the positive electrode and the negative electrode during the charge / discharge process of the battery. Examples of the cyclic carbonate compound having a halogen include fluoroethylene carbonate (hereinafter referred to as “FEC”), difluoroethylene carbonate, trifluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, trichloroethylene carbonate, and the like. Fluoroethylene carbonate, which is a cyclic carbonate compound having halogen, is particularly preferably used.

  The electrolytic solution may further contain a disulfonic acid compound as an additive. The disulfonic acid compound is a compound having two sulfo groups in one molecule, and includes a disulfonate compound in which the sulfo group forms a salt with a metal ion, or a disulfonate compound in which the sulfo group forms an ester. . One or two of the sulfo groups of the disulfonic acid compound may form a salt with the metal ion or may be in an anionic state. Examples of disulfonic acid compounds include methanedisulfonic acid, 1,2-ethanedisulfonic acid, 1,3-propanedisulfonic acid, 1,4-butanedisulfonic acid, benzenedisulfonic acid, naphthalenedisulfonic acid, biphenyldisulfonic acid, and these And salts (lithium methanedisulfonate, lithium 1,3-ethanedisulfonate, etc.) and anions thereof (methanedisulfonate anion, 1,3-ethanedisulfonate anion, etc.). Examples of the disulfonic acid compound include disulfonic acid ester compounds, such as methanedisulfonic acid, 1,2-ethanedisulfonic acid, 1,3-propanedisulfonic acid, 1,4-butanedisulfonic acid, benzenedisulfonic acid, naphthalenedisulfonic acid, Alternatively, chain disulfonic acid esters such as alkyl diesters or aryl diesters of biphenyl disulfonic acid; and cyclic disulfonic acid esters such as methylenemethane disulfonic acid ester, ethylenemethane disulfonic acid ester, and propylene methane disulfonic acid ester are preferably used. Methylenemethane disulfonate (hereinafter referred to as “MMDS”) is particularly preferably used.

  The exterior body used together with the electrode assembly for a lithium ion secondary battery according to the embodiment and constituting the lithium ion secondary battery can be a casing made of a metal material. Alternatively, the exterior body may be a bag-shaped body composed of a laminate in which a coating layer such as a nylon layer, a polyethylene terephthalate layer, a metal base material, an acid-modified polypropylene layer, and a polypropylene layer are laminated. Here, the metal material used as the material of the exterior body may be aluminum, nickel, iron, copper, stainless steel, tin, or the like. Moreover, the metal base material which comprises a laminated body is a base material used suitably as an exterior film of a battery, Preferably metal foil, for example, is foil of aluminum, nickel, iron, copper, stainless steel, and tin. The exterior body has a function of sealing the nonaqueous electrolytic solution inside the exterior body. At least a part of the power generation element composed of the positive electrode assembly, the negative electrode assembly, the separator, and the electrolytic solution can be sealed inside the exterior body that is a metal casing. Or the laminated body is bent and three sides other than the bent portion are heat-sealed, or two laminated bodies are overlapped and the four sides are heat-sealed to form an exterior body. Inside this, a positive electrode, a negative electrode, The power generation element composed of the separator and the electrolytic solution is sealed. At this time, the exterior body can be sealed such that at least a part of the positive electrode lead and the negative electrode lead out of the positive electrode assembly and the negative electrode assembly extends to the outside of the exterior body.

  The “acid-modified polypropylene” in the acid-modified polypropylene layer constituting the laminate means a polypropylene into which an acid is introduced by a graft reaction. In this specification, a propylene / ethylene copolymer, a propylene / ethylene / butene copolymer is used. A copolymer in which propylene is introduced as a copolymerization component, such as a polypropylene / butene copolymer, is also referred to as “acid-modified polypropylene”. Examples of the acid introduced by the graft reaction include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, itaconic acid, itaconic anhydride and the like. Representative maleic anhydride-modified polypropylene, maleic anhydride-modified propylene / ethylene copolymer, maleic anhydride-modified propylene / ethylene / butene copolymer, and maleic anhydride-modified polypropylene / butene copolymer introduced with maleic anhydride “Acid-modified polypropylene”. The acid-modified polypropylene has a function of bonding a metal substrate and a polypropylene layer described later.

  In this specification, “polypropylene” in the polypropylene layer constituting the laminate refers to a propylene homopolymer, a propylene / ethylene copolymer, a propylene / ethylene / butene copolymer, a polypropylene / butene copolymer, It includes all copolymers of propylene and other olefins such as propylene / 4-methylpentene-1 copolymer and propylene / hexene copolymer, and may be a mixture thereof. A polypropylene layer plays the role which gives a softness | flexibility to a laminated body. The polypropylene layer preferably contains a lubricant. The lubricant provides ease of molding when forming the polypropylene layer. As the lubricant, it is preferable to use a higher fatty acid amide containing 18 or more carbon atoms. Examples of higher fatty acid amides containing 18 or more carbon atoms are oleic acid amide, behenic acid amide, erucic acid amide and the like.

  Here, a configuration example of a lithium ion secondary battery manufactured using the positive electrode active material of the embodiment will be described with reference to the drawings. FIG. 1 is a cross-sectional view of a part of a power generation element using the electrode assembly of the embodiment. The negative electrode assembly 10 includes a negative electrode 11 including a negative electrode current collector 101 and a negative electrode active material layer 102 and a negative electrode lead 12 as main components. In FIG. 1, negative electrode active material layers 102 are provided on both surfaces of a negative electrode current collector 101. Four pieces of the negative electrode current collector 101 provided with the negative electrode active material layer 102 are collectively welded to the negative electrode lead 12 to form the joint portion 103. In FIG. 1, the positive electrode 21 includes a positive electrode current collector 201 and a positive electrode active material layer 202. Although not shown in FIG. 1, a plurality of positive electrodes 21 are collectively welded to the positive electrode lead 22 to form the positive electrode assembly 20. The negative electrode 11, the positive electrode 21, and the separator 31 are overlapped with each other. In FIG. 1, four negative electrodes 11, three positive electrodes 21, and six separators 31 are overlapped.

The negative electrode current collector 101 has a rust preventive film and a metal oxide film (both not shown). In the rust preventive film and the metal oxide film, the ratio (mg / dm ² / nm) between the basis weight of the rust preventive film and the thickness of the metal oxide film is 0.007 or more and less than 0.025, more preferably 0.8. It is 01 or more and less than 0.02. Here, a copper foil is preferably used as the negative electrode current collector 101, and a carbon material is preferably included as the negative electrode active material layer 102. Rust preventive coatings present on the negative electrode current collector 101 are inorganic compounds such as nitrites, chromates, silicates, polyphosphates, carboxylic acids, metal carboxylate soaps, sulfonates, amine salts, esters It can be formed by applying an organic compound such as phosphoric acid ester. It is preferable to use a rust inhibitor containing a chromium compound as the rust inhibitor. Similarly, the positive electrode current collector 201 has a rust preventive film and a metal oxide film (both not shown). In the rust preventive film and the metal oxide film, the ratio (mg / dm ² / nm) between the basis weight of the rust preventive film and the thickness of the metal oxide film is 0.007 or more and less than 0.025, more preferably 0.8. It is 01 or more and less than 0.02. Here, an aluminum foil is preferably used as the positive electrode current collector 201, and a lithium-based composite oxide is preferably included as the positive electrode active material layer 202. The anticorrosive agent film present on the positive electrode current collector 201 can be formed by applying an oil such as an antirust oil or a rolling oil.

  A battery formed by laminating a plurality of electrode assemblies and separators in this way is packaged by an exterior body (not shown) in such a manner that the welded negative electrode lead 12 and positive electrode lead 22 (not shown) are pulled out. An electrolyte solution (not shown) is injected into the exterior body. It is preferable that the exterior body has a shape in which two laminated bodies are overlapped and the peripheral portion is heat-sealed. In FIG. 1, the negative electrode lead 12 and the positive electrode lead 22 (not shown) are provided on opposite sides of the exterior body (referred to as “both tabs”). A negative electrode lead 12 and a positive electrode lead 22 (not shown) are provided on one side of the outer package (that is, the negative electrode lead 12 and the positive electrode lead 22 are pulled out from one side of the outer package, referred to as “one tab type”). Is also possible.

  Since the electrode assembly for a lithium ion secondary battery according to the embodiment is excellent in the strength of the joint portion, the lithium ion secondary battery using this has improved strength and has a long life. Such a lithium ion secondary battery has high reliability and is particularly conveniently used as a vehicle-mounted battery or a stationary battery.

[Example 1]
A copper foil (width 200 mm × length 250 mm × thickness 10 μm) was prepared as a negative electrode current collector. A treatment liquid mainly composed of trivalent chromic acid was applied to this copper foil so that the chromium oxide was 0.04 mg / dm ^{2 in} terms of chromium content. The chromate-treated copper foil was left in the atmosphere and oxidized to form a 2 nm thick copper oxide film, whereby a negative electrode current collector 1 was obtained. The chromium content was measured by measuring the welding strength described later, peeling off the welded portion, and measuring by atomic absorption method. The thickness of the copper oxide film was determined by elemental analysis using an XPS apparatus. Specifically, elemental analysis was performed in the depth direction of the copper foil while performing argon sputtering, and the range in which oxygen (O) was detected was defined as the thickness of the copper oxide film.

[Example 2]
The same copper foil as in Example 1 was subjected to chromate treatment so that the chromium oxide was 0.029 mg / dm ² as the chromium content. Next, an oxidation treatment was performed in the same manner as in Example 1 to form a copper oxide film having a thickness of 2.5 nm, whereby a negative electrode current collector 2 was obtained.

[Comparative Example 1]
Chromate treatment was performed on the same copper foil as in Example 1 so that the chromium oxide was 0.02 mg / dm ² as the chromium content. Next, an oxidation treatment was performed in the same manner as in Example 1 to form a copper oxide film having a thickness of 3 nm, whereby a negative electrode current collector 3 was obtained.

[Comparative Example 2]
The same copper foil as in Example 1 was subjected to a chromate treatment so that the chromium oxide was 0.05 mg / dm ² as the chromium content. Subsequently, an oxidation treatment was performed in the same manner as in Example 1 to form a copper oxide film having a thickness of 2 nm, whereby a negative electrode current collector 4 was obtained.

[Welding of negative electrode current collector and negative electrode lead]
Nickel plated copper was used as the negative electrode lead. The negative electrode lead and each of the electrode current collectors 1 to 4 are energized with an energy of 200 J and an amplitude of 90% using a welding machine (Branson) so that the size of the welded portion is 5 mm wide × 50 mm long. Ultrasonic welding was performed under the conditions.

[Measurement of welding strength]
Measurement of the strength of the welded portions of the negative electrode current collectors 1 to 4 welded with the negative electrode lead was performed using a 180 ° C. peel tester. The 180 ° peel strength was measured by a method approximately in accordance with the adhesive tape and spread sheet test method (JISZ0237). A stainless steel test piece and a copper foil side of each negative electrode current collector having a width of 5 mm and a length of 2 cm were bonded with a double-sided tape. Next, a tape was attached as a backing material to the negative electrode lead side to prepare a peel strength test plate. Each test plate is attached to a 180 ° peel test jig, a load cell (ZTS-2N Digital Force Gauge, Imada Co., Ltd.) is attached to one end of the test plate, and a vertical electric measurement stand (MX2-500N, Imada Co., Ltd.) is used. When the substrate was pulled up at an angle of 180 ° with respect to the test plate at a speed of 50 mm / sec, peeling occurred between the negative electrode lead and the copper foil. After starting the pulling up of the test plate, the peel force of about 1 mm was ignored, and then the peel force in the region where the load cell value was stable was averaged to obtain the peel strength between the negative electrode current collector and the negative electrode lead.

  The weld strengths of Examples 1 and 2 and Comparative Examples 1 and 2 are shown in Table 1:

  It can be seen that a joint having a ratio between the basis weight of the rust preventive coating and the thickness of the metal oxide coating within the scope of the present invention has high welding strength and high reliability. The joint portion (Comparative Example 1) where the thickness of the metal oxide film is large (that is, the ratio between the basis weight of the rust preventive coating and the thickness of the metal oxide coating is small) or the basis weight of the rust preventive coating is large (that is, anti-corrosion) Comparative Example 2 where the ratio of the basis weight of the rust coating and the thickness of the metal oxide coating is large 2) The weld strength of the joint is slightly inferior. To increase the strength of the joint between the electrode current collector and electrode lead, do not increase the amount of rust inhibitor unnecessarily, but balance the basis weight of the rust inhibitor film and the thickness of the metal oxide film. It can be said that it is important to maintain the proper range.

  As mentioned above, although the Example of this invention was described, the said Example was only an example of Embodiment of this invention, and in the meaning which limits the technical scope of this invention to specific embodiment or a specific structure. Absent.

DESCRIPTION OF SYMBOLS 10 Negative electrode assembly 11 Negative electrode 12 Negative electrode lead 101 Negative electrode collector 102 Negative electrode active material layer 103 Bonding part 21 Positive electrode 201 Positive electrode collector 202 Positive electrode active material layer 31 Separator

Claims (7)

An electrode assembly in which an electrode active material layer is provided on an electrode current collector made of metal, and the electrode current collector and an electrode lead are joined, the electrode current collector comprising at least a rust preventive coating And the metal oxide film, and the ratio (mg / dm ² / nm) between the basis weight of the anticorrosive film and the thickness of the metal oxide film is 0.007 or more and less than 0.025, Electrode assembly. ^2. The electrode assembly according to claim 1, wherein a ratio (mg / dm ² / nm) between a basis weight of the rust preventive coating and a thickness of the metal oxide coating is 0.01 or more and less than 0.02. Basis weight of rustproof agent coating is 0.029 mg / dm ² or more 0.04 mg / dm ² or less, the electrode assembly according to claim 1 or 2.   The electrode assembly in any one of Claims 1-3 in which this rust preventive agent film contains a chromium compound.   The electrode assembly according to claim 1, wherein the electrode current collector is copper.   The electrode assembly according to claim 1, wherein the electrode active material contains a carbon material. A lithium ion secondary battery in which a power generation element including a positive electrode assembly, a negative electrode assembly, a separator, and an electrolytic solution is sealed with an exterior body,
A lithium ion secondary battery, wherein the negative electrode assembly is the electrode assembly according to any one of claims 1 to 6.