Secondary battery and negative electrode current collector

Abstract

The present invention provides: a secondary battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity, wherein: the negative electrode is provided with a negative electrode current collector; the negative electrode current collector is provided with a resin film, and a transition metal layer laminated with the resin film; the resin film includes a base material resin layer and a surface resin layer; and the surface resin layer includes an oxygen-containing resin.

Description

Secondary battery and negative electrode current collector CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2024-054195, filed on March 28, 2024, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a secondary battery and a negative electrode current collector.

[0003] The anode electrode structure proposed in Patent Document 1 includes a current collector containing copper. The anode electrode structure further includes a lithium metal film formed on the current collector. The anode electrode structure further includes a solid electrolyte interface (SEI) film stack formed on the lithium metal film. In one embodiment, at least one of the current collectors 110, 160 includes a polyethylene terephthalate film coated with a metallic material.

[0004] Special Publication No. 2021-502671

[0005] The polyethylene terephthalate (PET) film coated with a metal material proposed in Patent Document 1 is prone to deterioration. It is known that when a transition metal comes into contact with a polymer, metal ions migrate to the polymer and cleave the polymer bonds through a radical reaction, resulting in embrittlement of the polymer. For example, when copper comes into contact with a PET film, copper ions migrate to the PET film and embrittle the PET (copper damage). Therefore, laminating a transition metal layer on the surface of a resin film gradually embrittles the resin film. While embrittlement of resin films often progresses slowly, it is desirable to suppress such embrittlement as much as possible.

[0006] On the other hand, when a negative electrode current collector having a transition metal layer laminated on the surface of a resin film is used in a lithium metal secondary battery (hereinafter referred to as a "lithium secondary battery"), embrittlement of the resin film is significantly accelerated. In the negative electrode of a lithium secondary battery, lithium metal precipitates during charging and dissolves in the non-aqueous electrolyte during discharge. This means that when a transition metal in contact with the resin film comes into contact with lithium metal, embrittlement of the resin film is significantly accelerated. No reported examples of this phenomenon have been found in previous literature.

[0007] One aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity, wherein the negative electrode includes a negative electrode current collector, and the negative electrode current collector includes a resin film and a transition metal layer laminated with the resin film, the resin film includes a base resin layer and the surface resin layer, and at least the surface resin layer includes an oxygen-containing resin.

[0008] Another aspect of the present disclosure relates to a negative electrode current collector including a resin film and a transition metal layer laminated with the resin film, the resin film including a base resin layer and the surface resin layer, and at least the surface resin layer including an oxygen-containing resin.

[0009] Yet another aspect of the present disclosure relates to a negative electrode for a secondary battery, including the above-mentioned negative electrode current collector and a metal layer containing lithium metal laminated on a surface of the transition metal layer.

[0010] According to the present disclosure, embrittlement of a negative electrode current collector containing a resin film in a secondary battery is significantly suppressed. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0011] 1 is a longitudinal sectional view schematically illustrating a lithium secondary battery according to an embodiment of the present disclosure, and FIG. 2 is an enlarged sectional view of a region II in FIG. 1. FIG. 3 is an enlarged sectional view of a region III in FIG.

[0012] Hereinafter, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Note that components other than those characteristic of the present disclosure may be components of known secondary batteries. In this specification, when a "range from numerical value A to numerical value B" is mentioned, the range includes numerical value A and numerical value B.

[0013] The present disclosure relates to a secondary battery. The secondary battery includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity. The secondary battery may be, for example, a lithium ion secondary battery that uses a material that reversibly absorbs and releases lithium ions as a negative electrode active material, a lithium secondary battery in which lithium metal precipitates at the negative electrode during charging and dissolves during discharge, or a non-aqueous electrolyte secondary battery such as an all-solid-state battery.

[0014] In a lithium secondary battery, for example, 70% or more of the rated capacity is achieved by the deposition and dissolution of lithium metal. The movement of electrons at the negative electrode during charging and discharging is mainly due to the deposition and dissolution of lithium metal at the negative electrode. Specifically, 70 to 100% (e.g., 80 to 100% or 90 to 100%) of the movement of electrons (or current from another perspective) at the negative electrode during charging and discharging is due to the deposition and dissolution of lithium metal. In other words, the negative electrode of a lithium secondary battery differs from a negative electrode in which the movement of electrons at the negative electrode during charging and discharging is mainly due to the absorption and release of lithium ions by the negative electrode active material (e.g., graphite).

[0015] In a battery in which lithium metal is deposited on the negative electrode during charging, the open circuit potential (OCV) of the negative electrode at full charge is, for example, 70 mV or less relative to lithium metal (lithium dissolution and deposition potential). A fully charged state refers to a state in which the battery is charged to a state of charge (SOC) of, for example, 0.98×C or more, where C is the rated capacity of the battery. The open circuit potential (OCV) of the negative electrode at full charge can be measured by disassembling a fully charged battery under an argon atmosphere, removing the negative electrode, and assembling a cell with lithium metal as the counter electrode. The nonaqueous electrolyte of the cell may have the same composition as the nonaqueous electrolyte in the disassembled battery. Alternatively, the nonaqueous electrolyte of the cell may be a mixture of 1,2-dimethoxyethane and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether in a volume ratio of 1:2, and dissolving LiPF ６ at a concentration of 1 mol / L, LiBF ２ (C ２ O ４ ) at a concentration of 0.1 mol / L may be dissolved in the non-aqueous electrolyte.

[0016] Hereinafter, each component of the secondary battery will be described in order.

[0017] [Negative Electrode] The negative electrode includes a negative electrode current collector. The negative electrode current collector includes a resin film and a transition metal layer laminated on the resin film. The resin film includes a base resin layer and a surface resin layer. The transition metal layer is also a layer in contact with the resin film or the surface resin layer.

[0018] Resin films are lightweight and can easily increase the energy density of secondary batteries. Resin films are resistant to tearing when transported in a roll and are easy to handle. Resin films do not become brittle, even at low negative electrode potentials, unlike common negative electrode current collectors such as copper foil. Resin films are highly resistant to stresses caused by electrode expansion and contraction, making them excellent current collector materials because they are less likely to break. In particular, lithium secondary batteries tend to experience significant expansion of the negative electrode during charging due to the deposition of lithium metal on the negative electrode. "Negative electrode expansion" refers to an increase in the combined volume of the negative electrode and the volume of the deposited lithium metal. In particular, when lithium metal is deposited in a dendritic form, the amount of expansion is even greater. This can easily cause stress in the negative electrode.

[0019] The main surface of the resin film may be smooth, may be roughened, or may be subjected to plasma treatment, corona treatment, or the like. When the main surface of the resin film is smooth, the maximum height roughness Rz of the main surface is less than 2.0 μm. When the main surface of the resin film is roughened, the maximum height roughness Rz of the main surface is, for example, 2.0 μm or more, 2.5 μm or more, or 8 μm or more. The maximum height roughness Rz is measured in accordance with JIS B 0601:2013. The main surface of the resin film refers to the surface other than the end surfaces of the resin film, and refers to the two surfaces having the largest areas. In this specification, "surface" usually means "main surface."

[0020] From the viewpoint of increasing the energy density of the secondary battery, the thickness of the resin film is preferably as thin as possible while ensuring mechanical strength. An example of a preferred range of the thickness of the resin film is 1.5 μm to 30 μm. The thickness of the resin film may be determined by measuring the thickness at any 10 points on the cross section of the resin film using a scanning electron microscope (SEM) and calculating the average value.

[0021] In the resin film, at least the surface resin layer contains an oxygen-containing resin. The oxygen-containing resin contains oxygen atoms (O). 90% by mass or more of the surface resin layer may be composed of the oxygen-containing resin. The oxygen-containing resin may be a polymer having oxygen atoms in the main chain or side chain. The mass content of oxygen atoms (O) contained in the oxygen-containing resin (hereinafter also referred to as "O content") may be 3% by mass or more.

[0022] The oxygen-containing resin may further contain nitrogen atoms (N). The mass content of nitrogen atoms (N) contained in the oxygen-containing resin (hereinafter also referred to as "N content") may be 3 mass% or more. The ratio of the O content to the N content (O content / N content) is preferably greater than 1.

[0023] Based on the surface resin layer, the mass content of oxygen atoms contained in the surface resin layer may be 2.5 mass% or more, and the mass content of nitrogen atoms contained in the surface resin layer may be 2.5 mass% or more.

[0024] In the resin film, the surface resin layer may contain, for example, an oxygen-hydrogen bond (O—H bond), an oxygen-carbon bond (C—O bond, C═O bond, etc.), an ester bond (O—C═O bond), an ether bond (C—O—C bond), an amide bond (NH—C═O bond), an N═C═O bond, etc. The presence or absence of these bonds can be determined by infrared absorption spectrometry. For example, the infrared absorption peaks specific to an O—H bond, a C—O bond, a C═O bond, and an N═C═O bond are 3396, 1278, 1726, and 2260 cm －１ It can be observed nearby.

[0025] A sample of the surface resin layer or resin film may be prepared by scraping or wet-removing the active material from the negative electrode current collector, and then dissolving the transition metal layer in a diluted aqueous solution of distilled water and 65% nitric acid at a volume ratio of 1:1. In this case, the outermost surface resin layer may be analyzed by infrared absorption spectroscopy. Example analysis conditions are shown below. FT-IR (ATR method) Measurement device: Varian 670 FTIR (manufactured by Varian) Measurement mode: Attenuated total reflection Light source: Special ceramics Detector: DLaTGS (deuterated L-alanine-doped triglycine sulfate) Resolution: 4 cm－１ Number of integrations: 256 IRE: Ge Incident angle: 60 degrees Attachment: Single reflection ATR attachment (Seagull)

[0026] Furthermore, each bond can also be analyzed by chemical shifts in X-ray photoelectron spectroscopy (XPS).

[0027] The oxygen-containing resin may be a polymer of a compound containing at least one functional group selected from the group consisting of an epoxy group, a (meth)acryloyl group, a peroxy group, a hydroxy group, an ester bond, an ether bond, and a (meth)acrylate group. Such a polymer may be a polymer modified with a urethane group, a urea group, a melamine group, an amide group, an aramid group, an imide group, or the like.

[0028] The polymer of the above compound may contain, for example, an oxygen-hydrogen bond (O—H bond), an oxygen-carbon bond (C—O bond, C═O bond, etc.), an ester bond (O—C═O bond), an ether bond (C—O—C bond), an amide bond (NH—C═O bond), etc. Among these, a β-hydroxyether structure (C—O—C—C—OH) derived from an epoxy group is excellent as a current collector material in that it has a high oxygen content, is highly flexible, has high resistance to stress caused by the expansion and contraction of the electrode, and is resistant to fracture.

[0029] The polymer of the above compound may be an acrylic resin. The acrylic resin is a polymer of an acrylic monomer. The acrylic monomer may be (meth)acrylates such as acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, isobutyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, methyl methacrylate, and n-butyl methacrylate. Copolymers of acrylic acid with acrylonitrile, butadiene, vinyl acetate, or the like may also be used.

[0030] The polymer of the above compound may be a polyester acrylate resin, a polyether acrylate resin, a polyurethane acrylate resin, a polyester polyol resin, a polyether polyol resin, a polyurethane polyol resin, a polyester polyurethane resin, a polyether polyurethane resin, or the like.

[0031] The oxygen-containing resin may be a polymer of a compound having an unsaturated hydrocarbon structure. The compound having an unsaturated hydrocarbon structure may be a polyester having an unsaturated bond. That is, the oxygen-containing resin may be an unsaturated polyester resin. The unsaturated polyester resin may be a polymer of a mixture of a polyester obtained by the condensation reaction of an unsaturated organic acid (e.g., maleic anhydride, phthalic anhydride, etc.) with a polyol (e.g., glycols such as ethylene glycol and propylene glycol, polyether polyols, polyester polyols, polycarbonate polyols, polyurethane polyols, acrylic polyols, etc.) and a vinyl monomer (e.g., styrene, acrylic monomer). The vinyl monomer functions as a diluent and crosslinker. The unsaturated polyester resin may also contain a polymerization inhibitor (e.g., hydroquinone), a catalyst (e.g., benzoyl peroxide, ethyl methyl ketone peroxide, cyclohexanone peroxide, lauryl peroxide, azobisisobutyronitrile), or an accelerator (e.g., dimethylaniline, cobalt naphthenate, etc.).

[0032] The oxygen-containing resin may be a cured product of a curable resin composition. The curable resin composition has a three-dimensional network structure or a cross-linked structure. Therefore, when a transition metal contacts a polymer, metal ions are less likely to migrate to the surface resin layer, effectively suppressing embrittlement of the resin film due to radical reactions. The curable resin composition may be thermosetting or photocurable.

[0033] The curable resin composition may contain a base resin and a curing agent. As the base resin, for example, an epoxy resin is preferred because it generates a β-hydroxy ether structure (C—O—C—C—OH) with a high oxygen content and also provides a high crosslink density in the cured product.

[0034] The type of epoxy resin is not particularly limited, but examples include bisphenol A epoxy resins, bisphenol F epoxy resins, bisphenol AD ​​epoxy resins, bisphenol S epoxy resins, bisphenol fluorene epoxy resins, biscresol fluorene epoxy resins, naphthalene epoxy resins, biphenyl epoxy resins, tetramethylbiphenyl epoxy resins, p-aminophenol epoxy resins, aniline epoxy resins, toluidine epoxy resins, glycidylamine epoxy resins, alicyclic epoxy resins, dicyclopentadiene epoxy resins, trimethylolalkane epoxy resins, polyether epoxy resins, silicone-modified epoxy resins, and novolac epoxy resins. These may be used alone or in combination of two or more. Among these, bisphenol A epoxy resins, bisphenol F epoxy resins, bisphenol AD ​​epoxy resins, naphthalene epoxy resins, and novolac epoxy resins are preferred. These may be used alone or in combination of two or more.

[0035] As the curing agent, for example, at least one selected from the group consisting of phenolic resins, amine compounds, acid anhydrides, peroxides, styrene, isocyanates, formaldehyde, and the like can be used.

[0036] The phenol resin is not particularly limited, but phenol novolac resin is preferred. Phenol novolac resin is obtained by condensation polymerization of a phenol compound or a naphthol compound with formaldehyde. Specifically, phenol novolac resin, cresol novolac resin, aralkylphenol novolac resin, biphenylphenol novolac resin, terpene phenol novolac resin, α-naphthol novolac resin, β-naphthol novolac resin, etc. may be used. These may be used alone or in combination of two or more.

[0037] The amine compound is not particularly limited, but examples thereof include primary, secondary, and tertiary aliphatic polyamines (e.g., polyethylene polyamine, diethylene triamine, metaxyliresin amine, polyoxypropylene diamine), alicyclic polyamines (e.g., isophorone diamine, 1,3-bisaminomethyl cyclohexane, etc.), aromatic amines (e.g., diaminodiphenylmethane, diaminodiphenyl sulfone, 2,4-diethyltoluene diamine, 2,6-diethyltoluene diamine), N-aminoethylpiperazine, triethyltetramine, diethylaminopropylamine, metaphenylenediamine, dicyandiamide, and BF ３ Examples include monoethylamine, polyamidoamine, imidazole compounds, etc. These may be used alone or in combination of two or more.

[0038] Examples of acid anhydrides include phthalic anhydride, hexahydrophthalic anhydride, methylnadic anhydride, dodecenylphthalic anhydride, pyromellitic anhydride, maleic anhydride mixture, chlorendic anhydride (head acid anhydride), etc. These may be used alone or in combination of two or more.

[0039] Examples of the isocyanate include hexamethylene diisocyanate (HMDI), m-xylylene diisocyanate (XDI), methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), etc. These may be used alone or in combination of two or more.

[0040] When the main component is polymerized by cationic polymerization, anionic polymerization, radical polymerization, or the like, a photopolymerization initiator, a photocationic polymerization initiator, or the like may be used as the curing agent.

[0041] Examples of photopolymerization initiators include 2,2-dimethoxy-1,2-diphenylethan-1-one, 1-hydroxy-cyclohexyl-phenyl-ketone, 2-hydroxy-2-methyl-1-phenyl-propan-1-one, 1-(4-(2-hydroethoxy)-phenyl)-2-hydroxy-2-methyl-1-propan-1-one, oligo(2-hydroxy-2-methylvinyl)phenyl)propane, 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, 2,4,6-trimethylbenzoyl-phenylethoxy Photoradical polymerization initiators such as chi-phosphine oxide and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide; and photocationic polymerization initiators such as triarylsulfonium hexafluorophosphate, triarylsulfonium hexafluoroantimonate, triarylsulfonium iodonium salt, (4-methylphenyl)(4-(2-methylpropyl)phenyl)-hexafluorophosphate(1-)tolylcumyliodonium tetrakispentafluorophenylborate can be used.

[0042] As a result of producing an oxygen-containing resin from the above materials, the oxygen-containing resin can contain at least one selected from the group consisting of a structure derived from an epoxy resin, a structure derived from a phenol resin, a structure derived from an amine compound, a structure derived from an acid anhydride, a structure derived from a peroxide, a structure derived from a polyol, and a structure derived from styrene. The oxygen-containing resin preferably has, for example, a three-dimensional network structure derived from an epoxy resin.

[0043] The resin film may contain a filler. At least one of the substrate resin layer and the substrate resin layer may contain a filler. It is desirable for the resin film to have improved adhesion with a transition metal layer or a metal layer containing lithium metal (hereinafter also referred to as the "lithium metal layer"). By incorporating a filler into the resin film, when the lithium metal layer is attached to a negative electrode current collector, the adhesion between the negative electrode current collector and the lithium metal layer is significantly improved. Furthermore, it has been shown that a negative electrode current collector with the lithium metal layer retained becomes a negative electrode with uniform resistance overall, improving the cycle characteristics of secondary batteries. For example, adding a filler such as silica or alumina to the surface resin layer creates irregularities in the surface resin layer, increasing the surface area and improving adhesion through an anchoring effect. The filler is not particularly limited as long as it can roughen the surface of the surface resin layer or the substrate resin layer and does not degrade battery performance by side reactions with the nonaqueous electrolyte in the battery. Particles of resin, metal oxide, ceramic, metal, etc. may be used as the filler.

[0044] The main surface of the negative electrode current collector may be smooth, but is preferably roughened, as in the case of incorporating a filler into a resin film. The maximum height roughness Rz of the main surface of the negative electrode current collector may be 2.0 μm or more, 2.5 μm or more, or even more than 2.5 μm, or even 8 μm or more. The maximum height roughness Rz of the main surface of the negative electrode current collector is measured in accordance with JIS B 0601:2013, as in the case of the main surface of the resin film. The main surface of the negative electrode current collector refers to the surface other than the end surface of the negative electrode current collector, and refers to the two surfaces with the largest areas. In addition, the arithmetic mean roughness Ra is preferably 0.15 μm or more.

[0045] The average particle diameter of the filler is, for example, 0.5 μm or more and 20 μm or less, and may be 1 μm or more and 10 μm or less. The average particle diameter of the filler is determined by forming a cut surface of the surface resin layer, observing the cross section with a scanning electron microscope (SEM), and averaging the diameter of an equivalent circle having the same area as the area surrounded by the outlines of any 30 particles observed. However, since excessively large irregularities can affect the charge / discharge reaction, the maximum diameter of the filler is preferably 30 μm or less. The particle size distribution of the filler may be broad.

[0046] The surface resin layer may be formed as a coating layer by applying an oxygen-containing resin to the surface of the base resin layer. In this case, the oxygen-containing resin may be a thermoplastic resin dissolved in a solvent, a thermosetting resin (resin composition), or a photocurable or UV-curable resin (resin composition). The oxygen-containing resin may be diluted with a solvent and applied to the surface of the base resin layer.

[0047] The thickness of the surface resin layer is, for example, 5 μm or less, and a preferred range of the thickness is 0.05 μm to 1.5 μm. The thickness of the surface resin layer may be determined by measuring the thickness of any 10 points of the surface resin layer on a cross section of the negative electrode, negative electrode current collector, or resin film using a scanning electron microscope (SEM) and calculating the average value of the measured thicknesses.

[0048] When a filler is added to the surface resin layer, the particle size of the filler often exceeds the thickness of the surface resin layer. In such cases, the thickness of the surface resin layer is measured in a portion where there are no protruding large filler particles.

[0049] Most resin films are not electrically conductive. The transition metal layer serves to impart good electrical conductivity to the negative electrode current collector. The transition metal layer may be any layer that contains a transition metal and has electronic conductivity. The transition metal layer desirably contains a transition metal in a metallic state that has electronic conductivity due to free electrons.

[0050] When the negative electrode current collector is used in a lithium-ion secondary battery or a lithium secondary battery, the transition metal layer preferably contains copper, nickel, chromium, titanium, iron, silver, gold, tin, or the like, as this facilitates ensuring corrosion resistance and electrical conductivity. The transition metal layer preferably contains at least one of copper, a copper alloy, stainless steel, nickel, a nickel alloy, or the like, and is particularly preferably copper or a copper alloy, which has excellent electrical conductivity. Since the transition metal layer may discolor due to corrosion, a chromium oxide coating (chromate coating) may be formed or an anti-rust treatment using benzotriazole or the like may be performed as needed.

[0051] Transition metals have the effect of embrittling resin films. Resin film embrittlement is particularly likely when the transition metal layer contains copper. A new, yet unreported finding has been that lithium metal significantly accelerates transition metal-induced resin film embrittlement (e.g., copper damage). For example, when a lithium metal layer is attached to a negative electrode current collector in which copper is vapor-deposited on a polyethylene terephthalate (PET) film, the appearance of the negative electrode current collector deteriorates within one month, becoming uneven and clearly visible to the naked eye. That is, in lithium secondary batteries in which lithium metal is deposited on the negative electrode, significant deterioration of the resin film due to the transition metal layer can occur. In contrast, by incorporating an oxygen-containing resin into at least the surface resin layer of the resin film, embrittlement of the resin film is significantly suppressed.

[0052] A nickel-chromium alloy layer is a metal layer that blocks the migration of copper ions into the resin film. However, in lithium secondary batteries, the nickel-chromium alloy layer does not provide any shielding effect. This is presumably because when easily ionized metallic lithium comes into contact with a transition metal such as copper, the ionization of the transition metal is accelerated, accelerating the cleavage of polymer bonds by the transition metal ions. On the other hand, a surface resin layer containing an oxygen-containing resin has a significant effect of suppressing polymer cleavage.

[0053] The most significant factor in embrittlement of a resin film is the rupture of carbon-carbon bonds. When a resin film contains an oxygen-containing resin, the oxygen-containing resin contains at least a carbon-oxygen bond. It is believed that the carbon-oxygen bond is less susceptible to the influence of transition metals or their ions. It is possible that the transition metal is stabilized by oxygen atoms, thereby suppressing the rupture of carbon-carbon bonds.

[0054] The transition metal layer may be formed by depositing it on the surface of the surface resin layer using a liquid phase method or a gas phase method. Examples of liquid phase methods include electrodeposition methods such as electrolytic plating and electroless plating. Examples of gas phase methods include vapor deposition, sputtering, and atomic layer deposition (ALD). It is also possible to form a base layer using a sputtering method, and then form a thick transition metal layer thereon using a wet electrolytic plating method. That is, it is also possible to apply a combination of multiple methods. The transition metal layer may also be formed using a lamination method. However, the method for forming the transition metal layer is not particularly limited.

[0055] The thickness of the transition metal layer is, for example, 5 μm or less, and may be 3 μm or less. An example of a preferred range for the thickness of the transition metal layer is 0.05 μm to 1.5 μm, and may be 0.1 μm to 1.5 μm. The thickness of the transition metal layer may be determined by measuring the thickness of any 10 points of the transition metal layer on a cross section of the negative electrode or negative electrode current collector using a scanning electron microscope (SEM) and calculating the average value. The transition metal layer may have multiple layers made of different metals.

[0056] The substrate resin layer is the main part of the negative electrode current collector and is usually thicker than the surface resin layer and the transition metal layer. At least 51% by mass of the substrate resin layer is composed of a resin or an organic material. To improve adhesion with the transition metal layer or the metal layer containing lithium metal (lithium metal layer), the substrate resin layer may contain an inorganic material such as inorganic particles. The substrate resin layer may be a stretched film, a nonporous film (a film without holes), or a film having a plurality of regularly arranged holes. The substrate resin layer may be insulating, conductive, or non-conductive. The shape and physical properties of the substrate resin layer are not particularly limited.

[0057] The substrate resin layer is formed, for example, by molding raw materials such as general-purpose plastics and general-purpose engineering plastics into a sheet. Examples of raw materials include polyester resins, polyolefin resins, polyphenylene sulfide resins, acrylic resins, polycarbonate resins, polyether ether ketone resins, polysulfone resins, polyphenylsulfone resins, polyethersulfone resins, polyamide resins, polyimide resins, polyetherimide resins, polybenzimidazole resins, liquid crystal polymer resins, polyacetal resins, polyvinyl chloride resins, polyarylate resins, silicone resins, nylon resins, polyvinylidene chloride resins, ethylene-vinyl alcohol copolymers, polyvinyl alcohol resins, polystyrene resins, epoxy resins, urethane resins, phenolic resins, melamine resins, urea resins, and unsaturated polyester resins. The resins contained in the substrate resin layer may be used alone or in combination of two or more.

[0058] The resin contained in the substrate resin layer is preferably a resin having an aromatic ring, a resin not containing a fluorine atom, or an olefin resin. When the resin has an aromatic ring (e.g., a benzene ring) in its molecule, the affinity between the substrate resin layer and the lithium metal layer is increased, and the adhesive strength between them is improved.

[0059] As the polyester resin, aromatic polyester is preferred, and unstretched polyethylene terephthalate, biaxially oriented polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, etc. are preferred. As the acrylic resin, polymethyl methacrylate, etc. can be mentioned. As the polyimide resin, aromatic polyimide is preferred. As the polyamide resin, aromatic polyamide (aramid resin) is preferred. As the polyolefin resin, unstretched polypropylene, biaxially oriented polypropylene, low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, metallocene polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl acrylate copolymer, ethylene-ethyl acrylate copolymer, ethylene-methyl methacrylate copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, ionomer, etc. are preferred.

[0060] The extrusion method for the substrate resin layer may be a T-die method or an inflation method, and may be unstretched, uniaxially stretched, sequentially biaxially stretched, or simultaneously biaxially stretched. The combination of monomers, which is the bonding mode of the resin, may be a homopolymer, a copolymer, or a terpolymer. The arrangement of the monomers is not limited, and may be a random copolymer or a block copolymer. Two or more types of substrate resin layers may be combined. For example, two or more substrate resin layers may be laminated. The crystalline resin may be in a crystalline state, or in an amorphous state produced by, for example, rapid cooling, or a mixture of both. A polymer alloy containing two or more of the above-mentioned resins may also be used.

[0061] The surface of the base resin layer may be subjected to a corona treatment or plasma treatment to ensure adhesion to other layers such as vapor-deposited films. The base resin layer may be provided with irregularities to improve adhesion to the surface resin layer. In this case, a filler such as ceramic, resin, or metal may be added to the base resin layer to form irregularities on the surface of the base resin layer.

[0062] The tensile strength of the base resin layer is, for example, 100 MPa or more, or may be 200 MPa or more, 250 MPa or more, or 300 MPa or more. The upper limit of the tensile strength of the base resin layer is not particularly limited, but may be, for example, 500 MPa or less from the viewpoint of ensuring sufficient flexibility for relaxing stress.

[0063] The tensile strength of the substrate resin layer is calculated from the maximum load when a test piece of the substrate resin layer is pulled at a constant speed in a direction perpendicular to its cross section, using the following formula. That is, the tensile strength is the nominal stress obtained by dividing the maximum load by the initial cross-sectional area. The tensile strength is measured using a method in accordance with JIS C 2151. The test piece is prepared by punching out a predetermined shape from the substrate resin layer. The shape of the test piece is Type 2 test piece of JIS C 2151. The thickness of the test piece is the thickness of the substrate resin layer.

[0064] ρ = P ｍａｘ / A ０ ρ: tensile strength (MPa) P ｍａｘ :Maximum load (N) A ０： Initial cross-sectional area (mm2 )

[0065] The elongation at break of the base resin layer is, for example, 20% or more, or may be 80% or more, 100% or more, or 200% or more. The upper limit of the elongation at break of the base resin layer is not particularly limited, but may be, for example, 400% or less from the viewpoint of ensuring sufficient mechanical strength.

[0066] The breaking elongation of the substrate resin layer is determined by the following formula from the length at break and the initial length when a test piece of the substrate resin layer is pulled at a constant rate in a direction perpendicular to its cross section. The breaking elongation is measured by a method in accordance with JIS K 7127. The shape of the test piece is test piece type 2 of JIS K 7127. The thickness of the test piece is the thickness of the substrate resin layer.

[0067] Breaking elongation δ (%) = 100 × (length at break L - initial length Lo) / initial length Lo δ: Breaking elongation (%) L: length at break Lo: initial length

[0068] The thickness of the substrate resin layer is, for example, 20 μm or less, and may be 4 μm to 12 μm. The thickness of the substrate resin layer may be determined by measuring the thickness of any 10 points of the substrate resin layer on a cross section of the negative electrode, negative electrode current collector, or resin film using a scanning electron microscope (SEM) and calculating the average value of these thicknesses.

[0069] The negative electrode may further include a metal layer (lithium metal layer) containing lithium metal laminated on the surface of the transition metal layer. In this case, the metal layer is in contact with the transition metal layer. The metal layer usually promotes embrittlement of the resin film due to the transition metal. On the other hand, when the resin film includes a surface resin layer containing an oxygen-containing resin, such embrittlement is significantly suppressed. In other words, the negative electrode current collector according to the present disclosure is particularly suitable for use in lithium secondary batteries in which lithium metal precipitates at the negative electrode during charging and dissolves during discharge.

[0070] However, even when the negative electrode is made of a negative electrode active material other than lithium metal, such as a carbon material such as graphite, or a silicon-containing material, it may be desirable to use the negative electrode current collector according to the present disclosure in consideration of long-term reliability.

[0071] The lithium metal layer can have a current collecting function. During charging, lithium ions contained in the non-aqueous electrolyte receive electrons on the lithium metal layer to become lithium metal, which then deposits on the surface of the lithium metal layer. Upon discharging, the lithium metal deposited on the surface of the lithium metal layer dissolves as lithium ions in the non-aqueous electrolyte.

[0072] The lithium metal layer may be a layer formed of at least one of lithium metal and a lithium alloy. The lithium alloy preferably contains magnesium. The magnesium content in the lithium alloy is, for example, 0.1% by mass or more, preferably 0.5% by mass or more or 1% by mass or more, and may be 3% by mass or more or 5% by mass or more. When the magnesium content in the lithium alloy is within this range, the effect of suppressing dendritic precipitation of lithium metal is obtained. In addition, an SEI coating with excellent film properties is easily formed on the surface of the lithium metal layer. The magnesium content in the lithium alloy may be, for example, 30% by mass or less, and may be 15% by mass or less or 10% by mass or less. When the magnesium content in the lithium alloy is within this range, magnesium is easily dissolved in lithium, and a stable lithium alloy is easily formed.

[0073] The lithium alloy may contain a third element other than lithium (first element) and magnesium (second element). Examples of the third element include aluminum, indium, calcium, lead, hydrogen, sodium, bismuth, gold, silver, copper, and zinc. The lithium alloy may contain one type of third element, or two or more types of third elements. The content of the third element in the lithium alloy may be, for example, 10% by mass or less, 1% by mass or less, or less than 0.1% by mass.

[0074] The lithium metal layer may be formed by laminating a lithium metal foil onto the surface of the transition metal layer. The lithium metal layer may be formed by depositing the lithium metal layer on the surface of the transition metal layer using a liquid phase method or a vapor phase method. Examples of the liquid phase method include electrodeposition. Examples of the vapor phase method include vapor deposition and sputtering. However, the method for forming the lithium metal layer is not particularly limited.

[0075] The thickness of the lithium metal layer may be, for example, 1 μm or more, or 5 μm or more, in a discharged state with a depth of discharge (DOD) of 90% or more. On the other hand, from the viewpoint of increasing the energy density of the secondary battery, the thickness of the lithium metal layer may be 30 μm or less, or 25 μm or less, in a discharged state with a depth of discharge of 90% or more. An example of a preferred range of the thickness of the lithium metal layer is 5 μm to 20 μm. Note that a discharged state with a depth of discharge (DOD) of 90% or more is synonymous with a state of charge (SOC) of 0.1 × C or less, where C is the rated capacity of the battery. The thickness of the lithium metal layer may be determined by measuring the thickness of any 10 points of the lithium metal layer on the cross section of the negative electrode using a scanning electron microscope (SEM) and calculating the average value.

[0076] The surface resin layer and the transition metal layer may be formed on only one side or both sides of the base resin layer, and similarly, the lithium metal layer may be formed on only one side or both sides of the base resin layer.

[0077] When a surface resin layer and a transition metal layer are laminated on both sides of the substrate resin layer, respectively, the total thickness of the substrate resin layer, surface resin layer, and transition metal layer may be, for example, 10 μm to 80 μm. Such a negative electrode current collector is suitable for forming a wound electrode assembly formed by winding a positive electrode and a negative electrode with a separator interposed therebetween. Stress is likely to occur in wound electrode assemblies during charge and discharge, and copper foil negative electrode current collectors, which are commonly used, are particularly susceptible to breakage due to embrittlement. Furthermore, large tensile tension may be applied to the negative electrode on the outer periphery of the wound electrode assembly. On the other hand, when the negative electrode current collector according to the present disclosure is used, because the negative electrode current collector includes a resin film, breakage of the negative electrode is significantly suppressed even when large tensile tension is applied to the negative electrode on the outer periphery of the wound electrode assembly.

[0078] The surface resin layer is preferably provided on the entire surface of each main surface of the base resin layer, but there may be portions that are not covered by the surface resin layer as long as they account for 10% or less of the area of ​​the main surface of the base resin layer. The main surfaces of the base resin layer are surfaces other than the end faces of the base resin layer, and refer to the two surfaces having the largest areas.

[0079] The transition metal layer is preferably provided on the entire surface of each main surface of the surface resin layer, but there may be a portion not covered with the transition metal layer as long as it occupies 30% or less of the area of ​​the main surface of the surface resin layer. Note that when a portion of the main surface of the base resin layer is not covered with the surface resin layer, it is desirable that the surface of that portion is not in contact with the transition metal layer.

[0080] When forming a lithium metal layer, it is desirable that the lithium metal layer be provided on the entire surface of each main surface of the transition metal layer, and there may be portions that are not covered by the lithium metal layer as long as they account for 30% or less of the area of ​​the main surface of the transition metal layer.

[0081] A negative electrode current collector according to a preferred embodiment has a base resin layer, surface resin layers formed on both sides of the base resin layer, and transition metal layers formed on the surfaces of both surface resin layers (i.e., on both sides of the resin film).

[0082] A preferred embodiment of the negative electrode comprises a substrate resin layer, surface resin layers formed on both sides of the substrate resin layer, transition metal layers formed on the surfaces of both surface resin layers (i.e., on both sides of the resin film), and lithium metal layers formed on the surfaces of both transition metal layers (i.e., on both sides of the negative electrode current collector). The substrate resin layer, surface resin layer, and transition metal layers constitute the negative electrode current collector. The lithium metal layer can function as both the negative electrode current collector and the negative electrode active material layer.

[0083] A preferred embodiment of the negative electrode is suitable for a lithium secondary battery in which lithium metal precipitates at the negative electrode during charging and dissolves in a non-aqueous electrolyte during discharging. The lithium contained in the lithium metal layer may or may not dissolve in the non-aqueous electrolyte during discharging. In other words, the lithium precipitated at the negative electrode during charging and dissolved from the negative electrode during discharging may originate from the positive electrode or from both the positive electrode and the lithium metal layer.

[0084] [Positive Electrode] The positive electrode includes, for example, a positive electrode current collector and a positive electrode composite layer supported on the positive electrode current collector. The positive electrode composite layer can be formed by applying a positive electrode slurry, in which the positive electrode composite is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The positive electrode composite contains a positive electrode active material as an essential component, and may contain a binder, a conductive agent, etc. as optional components. The positive electrode composite layer may be formed on only one surface of the positive electrode current collector, or on both surfaces thereof.

[0085] The positive electrode active material is a material that exhibits capacity by absorbing and releasing lithium ions. Examples of the positive electrode active material include lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides. Among these, lithium-containing transition metal oxides are preferred because of their low production cost and high average discharge voltage.

[0086] Examples of transition metal elements contained in the lithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. The lithium-containing transition metal oxide may contain one or more transition metal elements. The transition metal element may be Co, Ni, and / or Mn. The lithium-containing transition metal oxide may contain one or more typical elements as needed. Examples of typical elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The typical element may be Al, etc.

[0087] The conductive material is, for example, a carbon material, such as carbon black, acetylene black, ketjen black, carbon nanotubes, and graphite.

[0088] Examples of the binder include fluororesin, polyacrylonitrile, polyimide resin, acrylic resin, polyolefin resin, rubber polymer, etc. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, etc.

[0089] The positive electrode current collector may be a foil, a film, or the like. The surface of the positive electrode current collector may be coated with a carbon material. Examples of the material for the positive electrode current collector include metal materials containing Al, Ti, Fe, and the like. The metal material may be Al, an Al alloy, Ti, a Ti alloy, an Fe alloy, or the like. The Fe alloy may be stainless steel (SUS). A material in which a metal material is applied to the surface of a resin film by physical vapor deposition (PVD) or the like may also be used. The thickness of the positive electrode current collector is not particularly limited and is, for example, 5 μm or more and 30 μm or less.

[0090] [Separator] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. As the separator, a microporous thin film, a woven fabric, a nonwoven fabric, etc. can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene, polymethylpentene, polyethylene terephthalate, polybutylene terephthalate, aramid, cellulose, etc. are preferred.

[0091] In order to improve the heat resistance of the separator, a heat-resistant coating layer containing ceramic particles may be provided on one or both surfaces of the separator.

[0092] A single separator may be interposed between the positive electrode and the negative electrode, or multiple separators may be interposed. When multiple separators are interposed between the positive electrode and the negative electrode, multiple sheets of one of a microporous thin film, a woven fabric, and a nonwoven fabric may be stacked, or multiple sheets of at least two of a microporous thin film, a woven fabric, and a nonwoven fabric may be stacked. When a single separator is interposed, the thickness is not particularly limited, but is preferably 5 to 80 μm. When multiple separators are interposed, the thickness is also not particularly limited, but is preferably 5 to 80 μm.

[0093] [Non-aqueous electrolyte] The non-aqueous electrolyte having lithium ion conductivity may be a solid electrolyte or a liquid electrolyte (electrolytic solution). The non-aqueous electrolyte may be a gel electrolyte (polymer electrolyte) containing a matrix polymer that absorbs a non-aqueous solvent and gels.

[0094] The polymer electrolyte contains, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. Examples of the matrix polymer include fluororesin, acrylic resin, and polyether resin.

[0095] As the solid electrolyte, for example, an inorganic solid electrolyte is used, for example, a material known in all-solid-state lithium ion secondary batteries (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halide-based solid electrolyte, etc.) is used.

[0096] The electrolyte solution contains, for example, a non-aqueous solvent and a lithium salt that dissolves in the non-aqueous solvent. The dissolution of the lithium salt in the non-aqueous solvent generates lithium ions and anions.

[0097] As the anion, a known anion used in a non-aqueous electrolyte can be used. Specifically, BF ４ － , ClO ４ － , P.F. ６ － , C.F. ３ SO ３ － , C.F. ３ CO ２ － , anions of imides, anions of oxalate complexes, etc. Examples of the anions of imides include N(SO ２ F) ２ － , N(SO ２ CF ３ ) ２ － The anion of the oxalate complex may contain boron and / or phosphorus. Examples of the anion of the oxalate complex include bisoxalate borate anion, BF ２ (C ２ O ４ ) － , P.F. ４ (C ２ O ４ ) － , P.F. ２ (C ２ O ４ )２ － The non-aqueous electrolyte may contain one type of lithium salt alone, or may contain two or more types of lithium salts.

[0098] In lithium secondary batteries, the non-aqueous electrolyte preferably contains at least an oxalate complex anion from the viewpoint of suppressing dendritic deposition of lithium metal. The interaction between the oxalate complex anion and lithium facilitates uniform deposition of lithium metal in the form of fine particles. This facilitates suppression of localized deposition of lithium metal. The non-aqueous electrolyte may contain an oxalate complex anion and another anion. The other anion may be PF ６ － , anions of imides, etc.

[0099] Examples of non-aqueous solvents include ester compounds, ether compounds, nitrile compounds, and amide compounds. These compounds include halogen-substituted compounds. Examples of halogen-substituted compounds include fluorides. The non-aqueous electrolyte may contain one or more of these non-aqueous solvents.

[0100] The non-aqueous solvent of the lithium secondary battery may contain an ether compound as a main component. The term "main component" means that the content of the ether compound in the non-aqueous solvent is 50% by mass or more, and may be 80% by mass or more. The content of the ether compound in the non-aqueous solvent may be 95% by mass or less, or may be 100% by mass or less. The range of the content of the ether compound in the non-aqueous solvent may be any combination of the above upper and lower limits.

[0101] It is believed that ether compounds have excellent stability (especially resistance to reduction) and suppress the generation of decomposition products on the surface of the negative electrode, thereby suppressing expansion of the negative electrode.

[0102] Examples of ether compounds include cyclic ethers and chain ethers. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of chain ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, diethylene glycol dimethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. Among these, 1,2-dimethoxyethane and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether are preferred. These compounds may be used alone or in combination of two or more.

[0103] Examples of ester compounds include carbonate esters and carboxylic acid esters. Examples of cyclic carbonate esters include ethylene carbonate and propylene carbonate. Examples of chain carbonate esters include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate. Examples of cyclic carboxylic acid esters include γ-butyrolactone and γ-valerolactone. Examples of chain carboxylic acid esters include ethyl acetate, methyl propionate, and methyl fluoropropionate. These may be used alone or in combination of two or more.

[0104] The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 3.5 mol / L or less. The concentration of the anion in the non-aqueous electrolyte may be 0.5 mol / L or more and 3.5 mol / L or less. Furthermore, the concentration of the anion of the oxalate complex in the non-aqueous electrolyte may be 0.05 mol / L or more and 1 mol / L or less.

[0105] The non-aqueous electrolyte may contain an additive. The additive may form a coating on the negative electrode. The formation of a coating derived from the additive on the negative electrode further facilitates suppression of dendrite formation. Examples of the additive include vinylene carbonate, fluoroethylene carbonate (FEC), and vinyl ethyl carbonate (VEC).

[0106] [Lithium Secondary Battery] Hereinafter, the configuration of a lithium secondary battery according to the present disclosure will be described with reference to the drawings, taking as an example a cylindrical battery including a wound electrode group, although the present disclosure is not limited to the following configuration.

[0107] Fig. 1 is a longitudinal cross-sectional view schematically illustrating an example of a lithium secondary battery according to an embodiment of the present disclosure. Fig. 2 is an enlarged view of a portion surrounded by region II in Fig. 1 (a portion including a positive electrode). Fig. 3 is an enlarged view of a portion surrounded by region III in Fig. 1 (a portion including a negative electrode). Note that each figure is a schematic illustration, and the ratios of dimensions (e.g., thickness) of each component may differ from the actual ratios.

[0108] The lithium secondary battery 10 includes a cylindrical battery case, a wound electrode group 14 housed in the battery case, and a non-aqueous electrolyte (not shown). The electrode group 14 is formed by winding a strip-shaped positive electrode 11 and a strip-shaped negative electrode 12 with a separator 13 interposed between the positive electrode 11 and the negative electrode 12.

[0109] The negative electrode 12 in the illustrated example is in a discharged state with a depth of discharge (DOD) of 100%, and is composed of a negative electrode current collector 40 and a lithium metal layer 41. The negative electrode current collector 40 is composed of a base resin layer 42, surface resin layers 43 formed on both surfaces of the base resin layer 42, and transition metal layers 44 formed on the surfaces of both surface resin layers 43. Not limited to the illustrated example, the negative electrode 12 may be in a discharged state with a depth of discharge (DOD) of 100%, and may be composed of only the negative electrode current collector.

[0110] The negative electrode 12 is electrically connected to the case body 15, which also serves as a negative electrode terminal, via a negative electrode lead 20. One end of the negative electrode lead 20 is connected to, for example, an end of the negative electrode 12 in the longitudinal direction, and the other end is welded to the inner bottom surface of the case body 15.

[0111] The positive electrode 11 includes a positive electrode current collector 30 and a positive electrode composite layer 31, and is electrically connected to a cap 26, which also serves as a positive electrode terminal, via a positive electrode lead 19. One end of the positive electrode lead 19 is connected, for example, near the center of the positive electrode 11 in the longitudinal direction. The positive electrode lead 19 extends from the positive electrode 11 through a through-hole (not shown) formed in the insulating plate 17 and reaches the filter 22. The other end of the positive electrode lead 19 is welded to the surface of the filter 22 on the electrode group 14 side.

[0112] The battery case is composed of a case body 15, which is a cylindrical metal container with a bottom, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is disposed between the case body 15 and the sealing body 16, thereby ensuring the airtightness of the battery case. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the winding axis direction.

[0113] Case body 15 has a step 21 formed, for example, by pressing a portion of the side wall of case body 15 from the outside. Step 21 may be formed in an annular shape along the circumferential direction of case body 15 on the side wall of case body 15. In this case, sealing body 16 is supported by the surface of step 21 on the opening side.

[0114] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. These components are stacked in this order in the sealing body 16. The sealing body 16 is attached to the opening of the case body 15 so that the cap 26 is located outside the case body 15 and the filter 22 is located inside the case body 15. The above-mentioned components constituting the sealing body 16 are, for example, disk-shaped or ring-shaped. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, and an insulating member 24 is interposed between their respective peripheral edges. The filter 22 and the lower valve body 23 are connected to each other at their respective peripheral edges. The upper valve body 25 and the cap 26 are connected to each other at their respective peripheral edges. In other words, all components except the insulating member 24 are electrically connected to each other.

[0115] A vent hole (not shown) is formed in the lower valve body 23. Therefore, if the internal pressure of the battery case increases due to abnormal heat generation or the like, the upper valve body 25 bulges toward the cap 26 and separates from the lower valve body 23. This cuts off the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure increases further, the upper valve body 25 breaks, and gas is discharged from an opening (not shown) formed in the cap 26.

[0116] In the illustrated example, a cylindrical lithium secondary battery has been described, but this embodiment is not limited to this case and can be applied to other types. The shape of the secondary battery can be appropriately selected from various shapes such as a cylindrical shape, a coin shape, a square shape, a sheet shape, and a flat shape depending on the application. In addition, in the illustrated example, a wound electrode group is shown, but the shape of the electrode group is not particularly limited, and it may be a stacked electrode group configured by stacking a positive electrode and a negative electrode with a separator interposed therebetween. In addition, known configurations other than the electrode group and non-aqueous electrolyte of the secondary battery can be used without particular limitation.

[0117] (Additional Notes) The above description discloses the following technologies. (Technology 1) A secondary battery comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte having lithium ion conductivity, wherein the negative electrode comprises a negative electrode current collector, and the negative electrode current collector comprises a resin film and a transition metal layer laminated with the resin film, and the resin film comprises a base resin layer and the surface resin layer, and at least the surface resin layer comprises an oxygen-containing resin. (Technology 2) The secondary battery according to Technology 1, in which lithium metal precipitates in the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharging. (Technology 3) The secondary battery according to Technology 1 or 2, in which the oxygen-containing resin comprises a polymer of a compound containing at least one functional group selected from the group consisting of an epoxy group, a (meth)acryloyl group, a peroxy group, a hydroxy group, an ester bond, an ether bond, and a (meth)acrylate group. (Technology 4) The secondary battery according to any one of Technologies 1 to 3, wherein the oxygen-containing resin includes a polymer of a compound having an unsaturated hydrocarbon structure. (Technology 5) The secondary battery according to any one of Technologies 1 to 4, wherein the oxygen-containing resin includes at least one bond selected from the group consisting of an O-H bond, a C-O bond, a C=O bond, an O-C=O bond, a C-O-C bond, an N=C=O bond, and an NH-C=O bond. (Technology 6) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 5, wherein the oxygen-containing resin is a cured product of a curable resin composition. (Technology 7) The nonaqueous electrolyte secondary battery according to Technology 6, wherein the curable resin composition includes a base agent and a curing agent, and the curing agent includes at least one selected from the group consisting of a phenolic resin, an amine compound, an acid anhydride, a peroxide, and styrene. (Technology 8) The secondary battery according to any one of Technologies 1 to 7, wherein the oxygen-containing resin includes at least one selected from the group consisting of a structure derived from an epoxy resin, a structure derived from a phenol resin, a structure derived from an amine compound, a structure derived from an acid anhydride, a structure derived from a peroxide, a structure derived from a polyol, and a structure derived from styrene. (Technology 9) The secondary battery according to any one of Technologies 1 to 8, wherein the surface resin layer includes a filler.(Technology 10) The secondary battery according to Technology 9, wherein the average particle diameter of the filler is 0.5 μm or more and 20 μm or less. (Technology 11) The secondary battery according to any one of Technology 1 to 10, wherein the thickness of the transition metal layer is 3 μm or less. (Technology 12) The secondary battery according to any one of Technology 1 to 11, wherein a metal layer containing lithium metal is laminated on the surface of the transition metal layer. (Technology 13) The secondary battery according to any one of Technology 1 to 12, wherein the thickness of the resin film is 20 μm or less. (Technology 14) The secondary battery according to any one of Technology 1 to 13, wherein the base resin layer contains at least one selected from the group consisting of polyester and polyolefin. (Technology 15) A negative electrode current collector comprising a resin film and a transition metal layer laminated on the resin film, wherein the resin film includes a base resin layer and the surface resin layer, and at least the surface resin layer contains an oxygen-containing resin. (Technology 16) The anode current collector according to Technology 15, wherein the oxygen-containing resin comprises a polymer of a compound containing at least one functional group selected from the group consisting of an epoxy group, a (meth)acryloyl group, a peroxy group, a hydroxy group, an ester bond, an ether bond, and a (meth)acrylate group. (Technology 17) The anode current collector according to Technology 15 or 16, wherein the oxygen-containing resin is a cured product of a curable resin composition. (Technology 18) The anode current collector according to Technology 17, wherein the curable resin composition contains a base agent and a curing agent, and the curing agent contains at least one selected from the group consisting of a phenolic resin, an amine compound, an acid anhydride, a peroxide, and styrene. (Technology 19) The anode current collector according to any one of Technology 15 to 18, wherein the oxygen-containing resin contains at least one selected from the group consisting of a structure derived from an epoxy resin, a structure derived from a phenolic resin, a structure derived from an amine compound, a structure derived from an acid anhydride, a structure derived from a peroxide, a structure derived from a polyol, and a structure derived from styrene. (Technology 20) The negative electrode current collector according to any one of Techniques 15 to 19, wherein the surface resin layer contains a filler. (Technology 21) The negative electrode current collector according to Technique 20, wherein the filler has an average particle diameter of 0.5 μm or more and 20 μm or less.(Technology 22) The negative electrode current collector according to Technology 21, wherein the main surface of the negative electrode current collector has an arithmetic mean roughness Ra of 0.15 μm or more and a maximum height roughness Rz of 2 μm or more. (Technology 23) A negative electrode for a secondary battery, comprising: the negative electrode current collector according to any one of Technology 15 to Technology 22; and a metal layer containing lithium metal laminated on the surface of the transition metal layer.

[0118] Examples The lithium secondary battery according to the present disclosure will be specifically described below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0119] Example 1 (1) Preparation of Positive Electrode A lithium-containing transition metal oxide (NCA; positive electrode active material) containing Li, Ni, Co, and Al, acetylene black (AB; conductive material), and polyvinylidene fluoride (PVdF; binder) were mixed in a mass ratio of NCA:AB:PVdF = 95:2.5:2.5, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and stirred to prepare a positive electrode mixture slurry. Next, the obtained positive electrode mixture slurry was applied to both sides of an Al foil (thickness 15 μm) that served as a positive electrode current collector, and then dried. The coating film of the positive electrode mixture was rolled using a roller. Finally, the obtained laminate of the positive electrode current collector and the positive electrode mixture layer was cut to a predetermined electrode size, and a positive electrode mixture layer (thickness 65 μm, 245 g / m) was applied to both sides of the positive electrode current collector. 2 ) was fabricated.

[0120] (2) Preparation of Negative Electrode (2-1) Negative Electrode Current Collector A thermosetting acrylic resin (polyester acrylate resin) was coated on both sides of a base resin layer (12 μm thick) made of polypropylene (PP) as an oxygen-containing resin, and the resin was heated and cured at a predetermined temperature to form a surface resin layer (1 μm thick). Next, a copper vapor deposition film was formed on the surface of the surface resin layer as a transition metal layer (1 μm thick), thereby obtaining a negative electrode current collector. The polyester acrylate resin was a polyester-type polyurethane-acrylate-acrylamide copolymer.

[0121] (2-2) Negative Electrode A negative electrode active material containing a silicon-containing material (composite particles in which nanosilicon particles are dispersed in a lithium silicate phase at a content of 55% by mass) and graphite particles in a mass ratio of 6:94, sodium carboxymethyl cellulose (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of negative electrode active material:CMC-Na:SBR=97.5:1:1.5, and an appropriate amount of water was added and stirred to prepare a negative electrode composite slurry. Next, the obtained negative electrode composite slurry was applied to both sides of a negative electrode current collector, dried, and the coating film of the negative electrode composite was rolled using a roller. Finally, the obtained laminate of the negative electrode current collector and negative electrode composite layer was cut to a predetermined electrode size, and a negative electrode composite layer (thickness 75 μm, 226 g / m) was applied to both sides of the negative electrode current collector. 2 ) was prepared.

[0122] (3) Preparation of non-aqueous electrolyte: LiPF ６ to a concentration of 1 mol / L, and LiBF ２ (C ２ O ４ A liquid non-aqueous electrolyte was prepared by dissolving each of the above compounds to a concentration of 0.1 mol / L. A mixed solvent of 1,2-dimethoxyethane and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether in a volume ratio of 1:2 was used as the non-aqueous solvent.

[0123] (4) Battery Fabrication An Al tab was attached to the positive electrode. A Ni tab was attached to the lithium metal layer of the negative electrode. A separator was interposed between the positive electrode and the negative electrode, and the positive electrode and the negative electrode were wound together in an inert gas atmosphere to fabricate a wound electrode assembly. A polyethylene microporous thin film (thickness: 25 μm) was used as the separator. The electrode assembly was housed in a bag-shaped exterior body formed of a laminate sheet with an Al layer, and a nonaqueous electrolyte was injected into the exterior body housing the electrode assembly. The exterior body was then sealed to fabricate a lithium secondary battery. In Table 1, Battery E1 is the battery of Example 1.

[0124] Example 2 In an inert gas atmosphere, a negative electrode having no negative electrode mixture layer was produced by pressure-bonding lithium metal foil (thickness: 25 μm) to both sides of a negative electrode current collector produced in the same manner as in Example 1. Using this negative electrode, a lithium secondary battery E2 was produced in the same manner as in Example 1.

[0125] Example 3 A negative electrode was fabricated in the same manner as in Example 2, except that the polypropylene (PP) base resin layer of the negative electrode current collector was replaced with a polyethylene terephthalate (PET) base resin layer (thickness: 12 μm), and silica having an average particle size of 7 μm was mixed as a filler into the acrylic resin of the surface resin layer. 20 parts by mass of silica was mixed per 100 parts by mass of acrylic resin. Using this negative electrode, a lithium secondary battery E3 was fabricated in the same manner as in Example 1.

[0126] Example 4 A negative electrode was produced in the same manner as in Example 3, except that silica was replaced with alumina having an average particle size of 7 μm. Using this negative electrode, a lithium secondary battery E4 was produced in the same manner as in Example 1.

[0127] Example 5 A negative electrode was produced in the same manner as in Example 3, except that a PET substrate resin layer was used instead of the surface resin layer, and silica was contained at a content of 10 mass %. Using this negative electrode, a lithium secondary battery E5 was produced in the same manner as in Example 1.

[0128] Example 6 A negative electrode was produced in the same manner as in Example 3, except that 40 parts by mass of silica was mixed per 100 parts by mass of the acrylic resin of the surface resin layer. Using this negative electrode, a lithium secondary battery E6 was produced in the same manner as in Example 1.

[0129] Example 7 A negative electrode was produced in the same manner as in Example 6, except that the acrylic resin in the surface resin layer was changed to a thermosetting polyether polyurethane resin and cured by heating at a predetermined temperature. A lithium secondary battery E7 was produced using this negative electrode in the same manner as in Example 6.

[0130] Example 8 A negative electrode was produced in the same manner as in Example 6, except that the acrylic resin of the surface resin layer was changed to a photocurable urethane acrylate resin and cured by UV irradiation. A lithium secondary battery E8 was produced using this negative electrode in the same manner as in Example 6.

[0131] Example 9 A negative electrode was produced in the same manner as in Example 6, except that the acrylic resin in the surface resin layer was changed to a thermosetting urethane-modified bisphenol F epoxy resin composition and cured by heating at a predetermined temperature. Using this negative electrode, a lithium secondary battery E9 was produced in the same manner as in Example 6.

[0132] Example 10 A negative electrode was produced in the same manner as in Example 6, except that the acrylic resin in the surface resin layer was changed to a thermosetting aromatic polyester polyurethane resin and cured by heating at a predetermined temperature. A lithium secondary battery E10 was produced using this negative electrode in the same manner as in Example 6.

[0133] Comparative Example 1 A copper vapor deposition film was directly formed as a transition metal layer (thickness 1 μm) on both sides of a polypropylene (PP) base resin layer (thickness 12 μm) without forming a surface resin layer, to obtain a negative electrode current collector. A negative electrode composite slurry was prepared in the same manner as in Example 1, except that a negative electrode active material consisting only of graphite particles was used, and a negative electrode composite layer (thickness 75 μm, 226 g / m) was formed on both sides of the negative electrode current collector using this. 2 Using this negative electrode, a lithium secondary battery R1 was fabricated in the same manner as in Example 1.

[0134] Comparative Example 2 A negative electrode was produced in the same manner as in Comparative Example 1, except that the polypropylene (PP) base resin layer of the negative electrode current collector was changed to a polyethylene terephthalate (PET) base resin layer (thickness: 12 μm). Using this negative electrode, a lithium secondary battery R2 was produced in the same manner as in Example 1.

[0135] Comparative Example 3: A copper vapor deposition film was formed directly as a transition metal layer (1 μm thick) on both sides of a polypropylene (PP) base resin layer (12 μm thick) without forming a surface resin layer, to obtain a negative electrode current collector. Lithium metal foil (25 μm thick) was pressure-bonded to both sides of the negative electrode current collector in an inert gas atmosphere to produce a negative electrode without a negative electrode composite layer. Using this negative electrode, a lithium secondary battery R3 was produced in the same manner as in Example 1.

[0136] Comparative Example 4 A negative electrode was produced in the same manner as in Comparative Example 3, except that the polypropylene (PP) base resin layer of the negative electrode current collector was changed to a polyethylene terephthalate (PET) base resin layer (thickness: 12 μm). Using this negative electrode, a lithium secondary battery R4 was produced in the same manner as in Example 1.

[0137] Comparative Example 5 An anode was fabricated in the same manner as in Comparative Example 4, except that, before forming copper vapor-deposited films on both sides of the substrate resin layer, nickel-chromium alloy (NiCr) layers having a thickness of 0.02 μm were formed on both sides of the substrate resin layer, and then copper vapor-deposited films were formed. Using this anode, a lithium secondary battery R5 was fabricated in the same manner as in Example 1.

[0138] The configuration of each negative electrode is summarized in Table 1. Each battery was designed to have approximately the same initial design capacity.

[0139]

[0140] The non-aqueous electrolyte secondary battery thus obtained was subjected to the following charge-discharge cycle test in an environment of 25°C.

[0141] [Charge-Discharge Cycle Test] (Charging) First, the following first to third steps of constant current charging were performed.

[0142] First step: First charging rate X of 15% １ Up to 0.1C (0.55mA / cm ２ ) Second step: Constant current charging at a second charging rate of 50% ２ Up to 0.4C (2.2mA / cm ２ ) Third step: Third charging rate X ３ Up to 0.6C (3.3mA / cm ２ ) constant current charging

[0143] The termination of the first and second steps was controlled by the charging time. The charging time (hr) was calculated as (1 / I) × (X / 100) when charging an amount of electricity corresponding to a charging rate of X (%) at a current value I (C). The termination of the third step was controlled by the voltage. Specifically, in the third step, constant current charging was performed until the voltage reached 4.1 V, which is estimated to be a charging rate of 100%.

[0144] Furthermore, after the above constant current charging, the current was 0.02 C (0.11 mA / cm ２ The battery was charged at a constant voltage of 4.1 V until the battery reached a constant voltage of 0.1 V.

[0145] (Discharge) After 10 minutes of rest, discharge at 0.6 C (3.3 mA / cm) until the voltage reaches 3 V. ２ ) constant current discharge was performed.

[0146] [Evaluation] The above charge / discharge cycle was counted as one cycle and 100 cycles were performed. The ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle was calculated as the capacity retention rate. The relative values ​​when the capacity retention rate of Example 1 was set to 100 are shown in Table 1.

[0147] As shown in Table 1, the capacity retention rate was significantly longer when the surface resin layer was formed from an oxygen-containing resin. This is thought to be because the oxygen-containing resin suppresses embrittlement of the negative electrode current collector containing the resin film, thereby extending the period until cracks occur in the negative electrode current collector.

[0148] The negative electrode current collector and secondary battery of the present disclosure can be used in electronic devices such as mobile phones, smartphones, and tablet terminals, electric vehicles including hybrids and plug-in hybrids, and home storage batteries combined with solar cells.

[0149] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0150] REFERENCE SIGNS LIST 10 Lithium secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode group 15 Case body 16 Sealing body 17, 18 Insulating plate 19 Positive electrode lead 20 Negative electrode lead 21 Step portion 22 Filter 23 Lower valve body 24 Insulating member 25 Upper valve body 26 Cap 27 Gasket 30 Positive electrode current collector 31 Positive electrode composite layer 40 Resin film 41 Lithium metal layer 42 Base resin layer 43 Surface resin layer 44 Transition metal layer

Claims

1. A secondary battery comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte having lithium ion conductivity; wherein the negative electrode comprises a negative electrode current collector; the negative electrode current collector comprises a resin film and a transition metal layer laminated with the resin film; the resin film includes a base resin layer and the surface resin layer; and at least the surface resin layer comprises an oxygen-containing resin.

2. The secondary battery according to claim 1, wherein lithium metal is precipitated on the negative electrode during charging, and the lithium metal is dissolved in the non-aqueous electrolyte during discharging.

3. The secondary battery according to claim 1, wherein the oxygen-containing resin comprises a polymer of a compound containing at least one functional group selected from the group consisting of an epoxy group, a (meth)acryloyl group, a peroxy group, a hydroxy group, an ester bond, an ether bond, and a (meth)acrylate group.

4. The secondary battery according to claim 1, wherein the oxygen-containing resin includes a polymer of a compound having an unsaturated hydrocarbon structure.

5. The secondary battery according to claim 1, wherein the oxygen-containing resin contains at least one bond selected from the group consisting of an O-H bond, a C-O bond, a C=O bond, an O-C=O bond, a C-O-C bond, an N=C=O bond, and an NH-C=O bond.

6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the oxygen-containing resin is a cured product of a curable resin composition.

7. The nonaqueous electrolyte secondary battery according to claim 6, wherein the curable resin composition comprises a base agent and a curing agent, and the curing agent comprises at least one selected from the group consisting of a phenolic resin, an amine compound, an acid anhydride, a peroxide, an isocyanate, and a styrene.

8. The secondary battery according to claim 1, wherein the oxygen-containing resin contains at least one selected from the group consisting of a structure derived from an epoxy resin, a structure derived from a phenol resin, a structure derived from an amine compound, a structure derived from an acid anhydride, a structure derived from a peroxide, a structure derived from a polyol, and a structure derived from styrene.

9. The secondary battery according to claim 1, wherein the surface resin layer contains a filler.

10. The secondary battery according to claim 9, wherein the average particle size of the filler is 0.5 μm or more and 20 μm or less.

11. The secondary battery according to claim 1, wherein the thickness of the transition metal layer is 3 μm or less.

12. The secondary battery according to claim 1, wherein a metal layer containing lithium metal is laminated on the surface of the transition metal layer.

13. The secondary battery according to claim 1, wherein the thickness of the resin film is 20 μm or less.

14. The secondary battery according to claim 1, wherein the base resin layer contains at least one material selected from the group consisting of polyesters and polyolefins.

15. A negative electrode current collector comprising: a resin film; and a transition metal layer laminated on the resin film, wherein the resin film includes a base resin layer and the surface resin layer, and at least the surface resin layer includes an oxygen-containing resin.

16. The negative electrode current collector according to claim 15, wherein the oxygen-containing resin comprises a polymer of a compound containing at least one functional group selected from the group consisting of an epoxy group, a (meth)acryloyl group, a peroxy group, a hydroxy group, an ester bond, an ether bond, and a (meth)acrylate group.

17. The negative electrode current collector according to claim 15, wherein the oxygen-containing resin is a cured product of a curable resin composition.

18. The negative electrode current collector according to claim 17, wherein the curable resin composition comprises a base agent and a curing agent, and the curing agent comprises at least one selected from the group consisting of a phenolic resin, an amine compound, an acid anhydride, a peroxide, and styrene.

19. The negative electrode current collector according to claim 15, wherein the oxygen-containing resin includes at least one selected from the group consisting of a structure derived from an epoxy resin, a structure derived from a phenol resin, a structure derived from an amine compound, a structure derived from an acid anhydride, a structure derived from a peroxide, a structure derived from a polyol, and a structure derived from styrene.

20. The negative electrode current collector according to claim 15, wherein the surface resin layer contains a filler.

21. The negative electrode current collector according to claim 20, wherein the average particle size of the filler is 0.5 μm or more and 20 μm or less.

22. The negative electrode current collector according to claim 21, wherein the principal surface of the negative electrode current collector has an arithmetic mean roughness Ra of 0.15 μm or more and a maximum height roughness Rz of 2 μm or more.

23. A negative electrode for a secondary battery, comprising: the negative electrode current collector according to claim 15; and a metal layer containing lithium metal laminated on the surface of the transition metal layer.

Images