Negative electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

To provide a negative electrode for a nonaqueous electrolyte secondary battery which can improve coulomb efficiency while omitting a negative electrode active substance.SOLUTION: A negative electrode for a nonaqueous electrolyte secondary battery has a current collector, and a layer which is arranged on the current collector and contains a polyurethane resin and a single layer carbon nanotube, and does not contain a negative electrode active substance.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Embodiments of the present invention relate to a negative electrode for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery using the same. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, are used as high-voltage, high-energy-density energy storage devices, for example, as power sources for electronic devices. The negative electrode of a non-aqueous electrolyte secondary battery is usually manufactured by coating and drying a mixture containing a negative electrode active material, a conductive additive, and a binder onto a current collector to form a negative electrode active material layer, with the negative electrode active material being an essential component.

[0003] For example, Patent Document 1 describes that the composite electrode corresponding to the negative electrode active material layer formed on the current collector includes a negative electrode active material, carbon nanotubes as a conductive additive, and a binder such as polyvinylidene fluoride. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2015-153714 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] As described above, the negative electrode active material has conventionally been considered an essential component in the negative electrode of a non-aqueous electrolyte secondary battery. However, if the negative electrode active material could be removed from the negative electrode, the mass of the negative electrode could be reduced, leading to a lighter non-aqueous electrolyte secondary battery.

[0006] The embodiment of the present invention aims to provide a negative electrode for a non-aqueous electrolyte secondary battery that can improve Coulomb efficiency while omitting the negative electrode active material. [Means for solving the problem]

[0007] The present invention includes embodiments shown below. [1] A negative electrode for a non-aqueous electrolyte secondary battery, comprising a current collector and a layer disposed on the current collector containing polyurethane resin and single-walled carbon nanotubes, and not containing a negative electrode active material. [2] The negative electrode for a non-aqueous electrolyte secondary battery according to [1], wherein the polyurethane resin comprises a lithium salt and / or a sodium salt of an anionic polyurethane resin. [3] The negative electrode for a non-aqueous electrolyte secondary battery according to [1] or [2], wherein the polyurethane resin comprises an alicyclic polyisocyanate as a raw material. [4] The negative electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [3], wherein the mass ratio (A) / (B) of the polyurethane resin (A) to the single-walled carbon nanotube (B) is 99.9 / 0.1 to 97.0 / 3.0. [5] The negative electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [4], wherein the thickness of the layer containing the polyurethane resin and the single-walled carbon nanotube is 0.5 to 2 μm. [6] A non-aqueous electrolyte secondary battery comprising a negative electrode for a non-aqueous electrolyte secondary battery as described in any one of items [1] to [5]. [7] A non-aqueous electrolyte secondary battery as described in [6], using an ionic liquid as the electrolyte. [Effects of the Invention]

[0008] According to embodiments of the present invention, it is possible to provide a negative electrode for a non-aqueous electrolyte secondary battery that can improve Coulomb efficiency (charge / discharge efficiency) while omitting the negative electrode active material. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic diagram of a non-aqueous electrolyte secondary battery according to one embodiment. [Figure 2] This is an exploded perspective view of the evaluation cell used in the example. [Modes for carrying out the invention]

[0010] The negative electrode for a non-aqueous electrolyte secondary battery according to this embodiment has a current collector and a layer containing a polyurethane resin and single-walled carbon nanotubes disposed on the current collector (hereinafter sometimes referred to as a "CNT layer").

[0011] [Current collector] The current collector is not particularly limited as long as it has electronic conductivity and can extract current to the outside. The current collector can be formed of, for example, a metal material such as copper, stainless steel, aluminum, nickel, titanium, a conductive polymer, a conductive glass, or the like. In one embodiment, the current collector may be a metal layer formed of a metal material having electronic conductivity. The metal layer may be, for example, a metal layer such as copper whose surface is treated with tin, nickel, titanium, silver, or the like, or a metal layer whose surface is oxidized.

[0012] The shape of the current collector is not particularly limited. For example, the current collector may be in the form of a film such as a foil of a metal material, or may be a molded body such as a porous body or a foam in addition to a sheet shape or a plate shape. The current collector may also be, for example, a metal layer formed on a resin film or sheet.

[0013] The thickness of the current collector is not particularly limited, and may be, for example, 1 to 1000 μm, 5 to 100 μm, or 10 to 50 μm.

[0014] [CNT layer] The CNT layer is a layer containing single-walled carbon nanotubes as a conductive material and a polyurethane resin as a binder, and has conductivity. The CNT layer is a layer formed on the current collector (that is, the surface of the current collector) instead of the conventional negative electrode active material layer and does not contain a negative electrode active material. The CNT layer formed on the current collector may be provided on one side of the current collector or on both sides.

[0015] A single-walled carbon nanotube is a carbon nanotube having a shape formed by cylindrically winding a six-membered ring network (graphene sheet) composed of carbon, and having a single-layer structure, and is also referred to as a single-walled carbon nanotube (hereinafter sometimes abbreviated as SWCNT).

[0016] The diameter (fiber diameter) of the single-walled carbon nanotube is not particularly limited, and the average diameter may be, for example, 0.4 to 100 nm, 0.5 to 10 nm, or 1.0 to 5.0 nm. The length of the single-walled carbon nanotube is not particularly limited, and the average length may be, for example, 50 nm to 1 mm, or 0.5 to 100 μm. The aspect ratio of the single-walled carbon nanotube (that is, the ratio of the average length to the average diameter) is not particularly limited, and may be, for example, 10 or more, or 100 or more.

[0017] Here, the average diameter and average length of the single-walled carbon nanotube can be obtained by measuring the dimensions of 50 randomly selected single-walled carbon nanotubes in an atomic force microscope image and taking the arithmetic mean. For lengths on the order of mm that cannot be measured by an atomic force microscope, measurement may be performed using an image obtained by a microscope.

[0018] [[ID=eleven]] As the polyurethane resin, various aqueous polyurethane resins such as an anionic polyurethane resin, a cationic polyurethane resin, or a nonionic polyurethane resin can be used.

[0019] An anionic polyurethane resin is an aqueous polyurethane resin that can be dispersed in water and has an anionic group. Examples of the anionic group include a carboxy group, a sulfonic acid group, a phosphoric acid group, etc., and salts thereof. These anionic groups may be used alone or in combination of two or more. Examples of the salt include alkali metal salts such as lithium salt, sodium salt, potassium salt, ammonium salts, and amine salts such as primary amines, secondary amines, and tertiary amines.

[0020] Examples of anionic polyurethane resins include those obtained by reacting an active hydrogen-containing compound having two or more active hydrogen groups with an organic polyisocyanate and a compound having both active hydrogen groups and anionic groups. For example, an isocyanate-containing urethane prepolymer can be synthesized from an active hydrogen-containing compound having two or more active hydrogen groups, an organic polyisocyanate, and a compound having both active hydrogen groups and anionic properties. The urethane prepolymer can then be neutralized with a neutralizing agent, emulsified in water, and chain-extended with a polyvalent amine compound to obtain an aqueous dispersion of anionic polyurethane resin.

[0021] Here, an active hydrogen group is a group containing active hydrogen that reacts with an isocyanate group, such as a hydroxyl group, a primary amino group (-NH2), or a secondary amino group (-NHR).

[0022] The above-mentioned active hydrogen-containing compounds with two or more active hydrogen groups are compounds having two or more active hydrogen groups in their molecule, and those known in the field of the urethane industry can be used. For example, those having two or more hydroxyl groups and / or amino groups at the molecular terminals or within the molecule. Preferably, they are polyol compounds having two or more hydroxyl groups at the molecular terminals.

[0023] Specific examples of active hydrogen-containing compounds with two or more active hydrogen groups include ethylene glycol, propylene glycol, propanediol, butanediol, pentanediol, 3-methyl-1,5-pentanediol, hexanediol, neopetyl glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, dipropylene glycol, tripropylene glycol, 1,4-cyclohexanedimethanol, bisphenol A, bisphenol F, bisphenol S, hydrogenated bisphenol A, dibromobisphenol A, 1,4-cyclohexanedimethanol, dihydroxyethyl terephthalate, and hydroxy Examples include polyhydric alcohols such as non-dihydroxyethyl ether, trimethylolpropane, glycerin, and pentaerythritol, their alkylene oxide adducts, or esterified products of the polyhydric alcohol or its alkylene oxide adduct with polyhydric carboxylic acids, polyhydric carboxylic acid anhydrides, or polyhydric carboxylic acid esters, polycarbonate polyols, polycaprolactone polyols, polyester polyols, polyether polyols, polyacetal polyols, polytetramethylene glycol, polybutadiene polyols, castor oil polyols, soybean oil polyols, fluorine polyols, and silicone polyols, as well as modified versions thereof. Examples of alkylene oxides include ethylene oxide, propylene oxide, and butylene oxide.

[0024] Specific examples of active hydrogen-containing compounds with two or more active hydrogen groups include aliphatic polyamine compounds such as ethylenediamine, trimethylenediamine, propylenediamine, diethylenetriamine, and triethylenetetramine; aromatic polyamine compounds such as metaxylenediamine, tolylenediamine, and diaminodiphenylmethane; alicyclic polyamine compounds such as piperazine and isophoronediamine; and polyhydrazide compounds such as hydrazine and adipic acid dihydrazide.

[0025] Any one of the active hydrogen-containing compounds listed above may be used, or two or more may be used in combination.

[0026] As the organic polyisocyanate, those known in the field of the urethane industry can be used, such as aliphatic polyisocyanates, alicyclic polyisocyanates, and aromatic polyisocyanates, and either one of these or two or more may be used in combination.

[0027] Examples of aliphatic polyisocyanates include tetramethylene diisocyanate, dodecamethylene diisocyanate, hexamethylene diisocyanate (HDI), 2,2,4-trimethylhexamethylene diisocyanate, and lysine diisocyanate.

[0028] Examples of alicyclic polyisocyanates include isophorone diisocyanate (IPDI), dicyclohexylmethane diisocyanate (hydrogenated MDI), hydrogenated xylylene diisocyanate, and 1,4-cyclohexane diisocyanate.

[0029] Examples of aromatic polyisocyanates include tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), polymeric MDI, xylylene diisocyanate (XDI), and naphthalene diisocyanate.

[0030] Furthermore, isocyanurate, adduct, biuret, allophenate, and carbodiimide forms of these polyisocyanates may also be used. Additionally, one or more of these organic polyisocyanates may be used, or two or more may be used in combination.

[0031] Examples of compounds having active hydrogen groups and anionic groups include hydroxy acids such as glycolic acid, malic acid, glycine, aminobenzoic acid, alanine, dimethylolpropionic acid, and dimethylolbutanoic acid, compounds having carboxyl groups such as aminocarboxylic acids and polyhydric hydroxy acids, aminosulfonic acids such as aminoethylsulfonic acid and 2-hydroxyethanesulfonic acid, and compounds having sulfonic acid groups such as hydroxysulfonic acids. These may be used individually or in combination of two or more. Examples of neutralizing agents for neutralizing these anionic groups include alkali metal hydroxides such as lithium hydroxide, potassium hydroxide, and sodium hydroxide, and tertiary amine compounds such as ammonia, trimethylamine, and triethylamine. These may be used individually or in combination of two or more.

[0032] Examples of polyhydric amine compounds used as chain extenders include the aliphatic polyamine compounds, aromatic polyamine compounds, alicyclic polyamine compounds, and polyhydrazide compounds mentioned above.

[0033] Nonionic polyurethane resins are water-dispersible, uncharged, aqueous polyurethane resins that do not have anionic or cationic groups. Examples of nonionic polyurethane resins include those obtained by synthesizing an isocyanate group-containing urethane prepolymer from active hydrogen-containing compounds with two or more active hydrogen groups, organic polyisocyanates, and monoalcohols or polyhydric alcohols, either from ethylene oxide alone or from ethylene oxide and propylene oxide adducts, emulsifying the urethane prepolymer in water, and then extending the chain with a polyhydric amine compound.

[0034] Details and specific examples of active hydrogen-containing compounds with two or more active hydrogen groups, organic polyisocyanates, and polyhydric amine compounds used in the synthesis of nonionic polyurethane resins are as described above for anionic polyurethane resins.

[0035] Cationic polyurethane resins are aqueous polyurethane resins that are dispersible in water and contain cationic groups. Examples of cationic groups include quaternary ammonium groups.

[0036] Cationic polyurethane resins include those obtained by reacting an active hydrogen-containing compound having two or more active hydrogen groups with an organic polyisocyanate and a compound having both active hydrogen groups and cationic groups. For example, an isocyanate group-containing urethane prepolymer can be synthesized from an active hydrogen-containing compound having two or more active hydrogen groups, an organic polyisocyanate, and a compound having both active hydrogen groups and cationic salt-forming groups. A salt-forming agent can then be added to the prepolymer, emulsified in water, and chain elongation with a polyvalent amine compound to obtain an aqueous dispersion of cationic aqueous urethane resin.

[0037] Details and specific examples of active hydrogen-containing compounds, organic polyisocyanates, and polyvalent amine compounds with two or more active hydrogen groups used in the synthesis of cationic polyurethane resins are as described above for anionic polyurethane resins.

[0038] Compounds having active hydrogen groups and cationic groups include, for example, N-alkyldialkanolamines such as N,N-dimethylethanolamine, N-methyldiethanolamine, and N-ethyldiethanolamine, which have active hydrogen groups and salt-forming groups. These may be used individually or in combination of two or more. Corresponding salt-forming agents include, for example, organic acids such as formic acid, acetic acid, propionic acid, butyric acid, lactic acid, malic acid, malonic acid, adipic acid, dimethyl sulfate, diethyl sulfate, methyl chloride, and benzyl chloride, inorganic acids such as formic acid, hydrochloric acid, phosphoric acid, and nitric acid, and compounds having reactive halogen atoms.

[0039] Among the various water-based polyurethane resins described above, it is preferable to use an anionic polyurethane resin from the viewpoint of dispersion stability of the negative electrode coating liquid composition obtained by dispersing single-walled carbon nanotubes and polyurethane resin in water. Furthermore, from the viewpoint of reducing nucleation overpotential, it is preferable to use an alkali metal salt of an anionic polyurethane resin as the polyurethane resin, and more preferably to use a lithium salt and / or sodium salt of an anionic polyurethane resin. Moreover, it is even more preferable to use a lithium salt of anionic polyurethane resin because it can enhance the effect of improving Coulomb efficiency while reducing nucleation overpotential.

[0040] In one embodiment, the polyurethane resin preferably contains a lithium salt and / or sodium salt of an anionic polyurethane resin (i.e., at least one of a lithium salt and a sodium salt). More preferably, the polyurethane resin contains a lithium salt of an anionic polyurethane resin. In these cases, the amount of lithium salt and / or sodium salt of the anionic polyurethane resin (preferably the amount of lithium salt of the anionic polyurethane resin) per 100% by mass of the polyurethane resin is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 100% by mass.

[0041] In one embodiment, the polyurethane resin preferably contains an alicyclic polyisocyanate as a raw material. That is, it is preferable to use an alicyclic polyisocyanate as the organic polyisocyanate, thereby improving the dispersion stability of the negative electrode coating liquid composition. In this case, the amount of alicyclic polyisocyanate in 100% by mass of organic polyisocyanate is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 100% by mass.

[0042] In one embodiment, it is preferable to use a lithium salt and / or sodium salt of an anionic polyurethane resin containing an alicyclic polyisocyanate as a raw material as the polyurethane resin. More preferably, a lithium salt and / or sodium salt of an anionic polyurethane resin obtained by reacting a polyol containing a polycarbonate polyol and / or polybutadiene polyol with an organic polyisocyanate containing an alicyclic polyisocyanate and a carboxyl group-containing polyol is used. Even more preferably, a lithium salt and / or sodium salt of an anionic polyurethane resin obtained by chain elongation with a polyhydric amine compound of an isocyanate group-containing urethane prepolymer obtained by reacting a polyol containing a polycarbonate polyol and / or polybutadiene polyol with an organic polyisocyanate containing an alicyclic polyisocyanate and a carboxyl group-containing polyol is used.

[0043] In the CNT layer, the mass ratio (A) / (B) of polyurethane resin (A) to single-walled carbon nanotubes (B) is preferably 99.9 / 0.1 to 97.0 / 3.0. More preferably, the mass ratio (A) / (B) is 99.7 / 0.3 to 97.5 / 2.5, and even more preferably 99.5 / 0.5 to 98.0 / 2.0.

[0044] The polyurethane resin content in the CNT layer is not particularly limited, but is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 97% by mass or more.

[0045] The CNT layer may consist only of polyurethane resin and single-walled carbon nanotubes, but it may also contain other components as long as the effects of this embodiment are not impaired. Other components include, for example, conductive materials other than single-walled carbon nanotubes, such as multi-walled carbon nanotubes (MWCNTs), and carbon blacks such as acetylene black and Ketjen black.

[0046] In addition, other components may include binders other than polyurethane resin, dispersants for dispersing single-walled carbon nanotubes in water, and thickeners for adjusting the viscosity of the anode coating liquid composition. Specifically, examples include polyvinylidene fluoride, styrene-butadiene rubber, hydroxymethylcellulose, carboxymethylcellulose, and their alkali metal salts, methylcellulose, ethylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, polyacrylic acid, sodium polyacrylate, polyvinylpyrrolidone, polyvinyl alcohol, sodium alginate, xanthan gum, carrageenan, guar gum, agar, and starch.

[0047] In one embodiment, the content of a dispersant such as carboxymethylcellulose and / or its salt may be 10 to 300 parts by mass, 50 to 200 parts by mass, or 70 to 150 parts by mass per 100 parts by mass of single-walled carbon nanotubes.

[0048] The thickness of the CNT layer is preferably 0.5 to 2 μm. That is, the thickness of the CNT layer may be less than 0.5 μm or greater than 2 μm, but from the viewpoint of improving the processability of the CNT layer and the effects of this embodiment, it is preferably 0.5 to 2 μm, and more preferably 0.7 to 1.5 μm.

[0049] [Negative electrode] The negative electrode of the non-aqueous electrolyte secondary battery according to this embodiment can be manufactured, for example, by applying a negative electrode coating liquid composition, which consists of single-walled carbon nanotubes and polyurethane resin dispersed in water, onto a current collector and evaporating the water as a dispersion medium.

[0050] The negative electrode coating liquid composition comprises single-walled carbon nanotubes, polyurethane resin, and water as a dispersion medium, and may optionally contain other components. The dispersion medium may be water alone, or it may contain water along with polar organic solvents such as alcohol or ketone solvents.

[0051] In this embodiment, the negative electrode of the non-aqueous electrolyte secondary battery does not contain negative electrode active material. By not including negative electrode active material, the mass of the negative electrode can be reduced. Furthermore, by providing the above-mentioned CNT layer that does not contain negative electrode active material, the Coulomb efficiency can be improved while reducing the mass of the negative electrode.

[0052] A negative electrode active material is a substance that allows for the insertion / deinsertion of metallic lithium or lithium ions, and is commonly used as a negative electrode active material in non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries. Specific examples of negative electrode active materials include natural graphite, artificial graphite, non-graphitizable carbon, easily graphitizable carbon, silicon oxide (SiO), silicon, silicon alloys, and lithium titanate.

[0053] [Nonaqueous electrolyte secondary battery] The non-aqueous electrolyte secondary battery according to this embodiment is equipped with the above-mentioned negative electrode. Here, the non-aqueous electrolyte secondary battery is a secondary battery that uses a non-aqueous electrolyte such as an organic solvent or an ionic liquid as the electrolyte, and examples include lithium-ion secondary batteries, sodium-ion secondary batteries, potassium-ion secondary batteries, calcium-ion secondary batteries, magnesium-ion secondary batteries, etc. Among these, lithium-ion secondary batteries are preferred.

[0054] A non-aqueous electrolyte secondary battery comprises a negative electrode, a positive electrode, and an electrolyte, and may also include a separator disposed between the positive electrode and the negative electrode. In this embodiment, the non-aqueous electrolyte secondary battery uses an electrode as the negative electrode, in which a CNT layer containing polyurethane resin and single-walled carbon nanotubes is formed on the above-mentioned current collector and the electrode does not contain a negative electrode active material. For components other than the negative electrode, known non-aqueous electrolyte secondary battery configurations can be adopted.

[0055] Figure 1 is a schematic diagram showing the configuration of a non-aqueous electrolyte secondary battery 10 according to one embodiment. The non-aqueous electrolyte secondary battery 10 comprises a negative electrode 12, a positive electrode 14, a separator 16 disposed between the negative electrode 12 and the positive electrode 14, and an electrolyte 18. The negative electrode 12 comprises a current collector 20 for the negative electrode and a CNT layer 22 provided on the current collector 20, and the negative electrode of the above embodiment can be used. The positive electrode 14 comprises a current collector 24 for the positive electrode and a positive electrode active material layer 26 containing positive electrode active material provided on the current collector 24.

[0056] As the current collector used in the positive electrode, those commonly used in lithium-ion secondary batteries can be used, such as foils made of metal materials like aluminum, titanium, stainless steel, and nickel. Alternatively, metals whose surfaces have been treated with carbon, nickel, titanium, silver, etc., may also be used.

[0057] The positive electrode active material layer is a layer containing the positive electrode active material, and may also contain a conductive additive and a binder together with the positive electrode active material. The positive electrode active material, conductive additive, and binder can be those commonly used in lithium-ion secondary batteries. For example, the positive electrode active material may be a composite oxide of lithium and a transition metal, such as lithium cobaltate, lithium manganeseate, lithium iron phosphate, or lithium nickelate.

[0058] As separators, those commonly used in lithium-ion secondary batteries can be used, such as porous resins made of polyethylene, polypropylene, polyolefin, polytetrafluoroethylene, ceramics, and nonwoven fabrics.

[0059] As the electrolyte, one that is normally used in lithium-ion secondary batteries can be used. Here, the electrolyte is a substance containing ions that act as charge carriers, such as lithium ions, and also includes the medium (solvent, etc.) that dissolves or disperses the ionic crystal. Examples of electrolytes include non-aqueous electrolytes in which ionic crystals such as lithium salts are dissolved in an organic solvent, non-aqueous electrolytes in which ionic crystals such as lithium salts are dissolved in an ionic liquid, solid electrolytes, and gel electrolytes. Among these, it is preferable to use an ionic liquid as the electrolyte because a higher Coulomb efficiency can be obtained, and for example, it is preferable to use an electrolyte in which lithium salts are dissolved in an ionic liquid as the electrolyte.

[0060] Examples of lithium salts include LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC(CF3SO2)3, LiN(CF3SO2)2 (i.e., LiTFSI), LiN(FSO2)2 (i.e., LiFSI), and LiBC4O8. These lithium salts may be used individually or in combination of two or more. LiFSI and LiTFSI are preferred lithium salts.

[0061] Examples of organic solvents for dissolving lithium salts include carbonates such as dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, ethylene glycol dimethyl carbonate, propylene glycol dimethyl carbonate, ethylene glycol diethyl carbonate, and vinylene carbonate; lactones such as γ-butyl lactone; ethers such as dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, and 1,4-dioxane; sulforanes such as sulfolane and 3-methylsulfolane; dioxolanes such as 1,3-dioxolane; ketones such as 4-methyl-2-pentanone; nitriles such as acetonitrile, pyropionitrile, valeronitrile, and bensonitrile; and halogenated hydrocarbons such as 1,2-dichloroethane. Any one of these may be used, or two or more may be used in combination.

[0062] The ionic liquid for dissolving the lithium salt is composed of a cation component and an anion component, is liquid at room temperature (25 °C), has no volatility, and has a relatively high decomposition temperature. Examples of the anion component contained in the ionic liquid include BF4 - , PF6 - , SbF6 - , NO3 - , CF3SO3 - , (FSO2)2N - (that is, FSI anion), (CF3SO2)2N - (that is, TFSI anion), (C2F5SO2)2N - , (CF3SO2)3C - , CF3CO2 - , C3F7CO2 - , CH3CO2 - , (CN)2N - and the like. Any one of these may be used alone or two or more of them may be used in combination.

[0063] Examples of the cation component constituting the ionic liquid include compounds containing elements such as N, P, S, O, C, and Si, and cations having a chain-like or cyclic structure such as a 5-membered ring or a 6-membered ring as a skeleton. Examples of the cyclic structure such as a 5-membered ring or a 6-membered ring include heterocyclic structures such as a furan ring, a thiophene ring, a pyrrole ring, a pyridine ring, an oxazole ring, an isoxazole ring, a thiazole ring, an isothiazole ring, a furazan ring, an imidazole ring, a pyrazole ring, a pyrazine ring, a pyrimidine ring, a pyridazine ring, a pyrrolidine ring, a piperidine ring, a benzofuran ring, an isobenzofuran ring, an indole ring, an isoindole ring, an indolizine ring, and a carbazole ring. Among these cation components, chain-like or cyclic compounds containing a nitrogen element are particularly preferable in terms of being chemically and electrochemically stable. Examples of the cation containing a nitrogen element include alkylammonium such as triethylammonium, 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-methyl-1-propylpyrrolidinium, and methylpropylpiperidinium. Any one of these cations may be used alone or two or more of them may be used in combination.

[0064] The concentration of salts such as lithium salts in the electrolyte is not particularly limited, but is preferably 0.1 to 3.0 mol / L, and more preferably 0.5 to 2.0 mol / L.

[0065] The non-aqueous electrolyte secondary battery according to this embodiment can be formed into cylindrical, coin-type, prismatic, or any other arbitrary shape. The basic structure of the battery is the same regardless of the shape, and it can be designed and implemented according to the purpose. For example, in the case of a cylindrical battery, a wound body in which a negative electrode and a positive electrode are wound with a separator in between is housed in a battery case, a non-aqueous electrolyte is injected, and insulating plates are placed on the top and bottom and sealed. In the case of a coin-type battery, a disc-shaped negative electrode, a separator, a disc-shaped positive electrode, and a stainless steel plate are stacked and housed in a coin-type battery case, a non-aqueous electrolyte is injected, and it is sealed. [Examples]

[0066] The present invention will be described in more detail below based on examples and comparative examples, but it is not limited thereto.

[0067] The details of the current collector, conductive material, binder, and electrolyte used in the example are as follows.

[0068] [Current collector] • Cu foil: 15μm thick copper foil (manufactured by Furukawa Electric Co., Ltd., "NC-WS") • Cu-Sn foil: A 0.5 μm thick tin film is deposited on the surface of a 15 μm thick copper foil using the following film deposition method.

[0069] (Method for forming a tin plating film) The copper foil was immersed in a 0.1M hydrochloric acid solution for 1 second to remove oxides from the surface of the copper foil, then washed with pure water and dried. After that, the copper foil was immersed in a tungsten plating solution (Ishihara Chemical Co., Ltd.'s "580M-Z series") at 60°C for 5 minutes. The tungsten-plated copper foil was then washed with pure water and punched out to produce a copper-tungsten foil.

[0070] [Conductive material] • SWCNT: A single-walled carbon nanotube (TUBALL BATT, manufactured by OCSiAl, carbon purity >99%, average diameter = 1.6 nm, average fiber length = 5 μm) is used as a conductive material in an aqueous dispersion prepared by the following aqueous dispersion preparation method, with a single-walled carbon nanotube concentration of 0.99% by mass. • MWCNT: A water dispersion with a multi-walled carbon nanotube concentration of 0.99% by mass, prepared using the following water dispersion preparation method, with multi-walled carbon nanotubes (CNano's "Flotube9110", carbon purity >99%, average diameter = 12 nm, average fiber length = 10 μm) as a conductive material. AB: An aqueous dispersion with an acetylene black concentration of 0.99% by mass, prepared using acetylene black (Denka Co., Ltd.'s "Li-400") as a conductive material, according to the following aqueous dispersion preparation method.

[0071] (Method for preparing aqueous dispersion) 1.0 g of conductive material was mixed in a beaker with 100 g of a 1% by mass aqueous solution of carboxymethylcellulose salt ("Selogen 7A" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.). After stirring, the slurry was circulated using the beaker, an ultrasonic homogenizer ("US-600T" manufactured by Nippon Seiki Seisakusho Co., Ltd.), a circulation unit, and a tube pump, and dispersed at an output of 100 μA for 90 minutes to obtain an aqueous dispersion of the conductive material.

[0072] [Binder] • WPU1: An aqueous dispersion of lithium salt of anionic polyurethane resin obtained by Synthesis Example 1 below. (Synthesis Example 1) In a four-necked flask equipped with a stirrer, reflux condenser, thermometer, and nitrogen inlet, 51.3 parts by mass of polycarbonate polyol (UBE Corporation's "ETERNACOLL UH-100", average hydroxyl value 110.0 mg KOH / g, active hydrogen number 2.0), 5.1 parts by mass of dimethylolpropionic acid (active hydrogen number 2), 2.6 parts by mass of trimethylolpropane (active hydrogen number 2), 41.0 parts by mass of dicyclohexylmethane 4,4'-diisocyanate, and 150 parts by mass of methyl ethyl ketone were added and reacted at 75°C for 4 hours to obtain a methyl ethyl ketone solution of urethane prepolymer with a free isocyanate group content of 3.0% by mass relative to nonvolatile matter. This solution was cooled to 45°C, and an aqueous lithium hydroxide solution (1.59 parts by mass of lithium hydroxide monohydrate and 300 parts by mass of water) was gradually added and emulsified and dispersed using a homogenizer. Subsequently, an aqueous solution of 2.08 parts by mass of ethylenediamine (with 2 active hydrogen groups) diluted with 100 parts by mass of water was added, and a chain extension reaction was carried out for 1 hour. This was then desolvented under reduced pressure and heating at 50°C to obtain an aqueous polyurethane resin dispersion with a non-volatile content of approximately 32% by mass.

[0073] • WPU2: An aqueous dispersion of the sodium salt of the anionic polyurethane resin obtained by Synthesis Example 2 below. (Synthesis Example 2) A polyurethane resin aqueous dispersion with a non-volatile content of approximately 32% by mass was obtained in the same manner as in Synthesis Example 1, except that an aqueous sodium hydroxide solution (1.51 parts by mass of sodium hydroxide, 300 parts by mass of water) was used instead of an aqueous lithium hydroxide solution.

[0074] • WPU3: An aqueous dispersion of lithium salt of anionic polyurethane resin obtained by Synthesis Example 3 below. (Synthesis Example 3) In a four-necked flask equipped with a stirrer, reflux condenser, thermometer, and nitrogen inlet, 74.1 parts by mass of polybutadiene polyol (Evonik "PolyVestHT", average hydroxyl value 46.5 mg KOH / g, active hydrogen number 2.32), 4.2 parts by mass of dimethylolpropionic acid (active hydrogen number 2), 21.7 parts by mass of dicyclohexylmethane 4,4'-diisocyanate, and 150 parts by mass of methyl ethyl ketone were added and reacted at 75°C for 4 hours to obtain a methyl ethyl ketone solution of urethane prepolymer with a free isocyanate group content of 2.15% by mass relative to nonvolatile matter. This solution was cooled to 45°C, and an aqueous lithium hydroxide solution (1.31 parts by mass of lithium hydroxide monohydrate, 300 parts by mass of water) was gradually added and emulsified and dispersed using a homogenizer. Next, an aqueous solution of 1.05 parts by mass of diethylenetriamine (with 3 active hydrogen groups) diluted with 100 parts by mass of water was added, and the chain extension reaction was carried out for 1 hour. This was then desolvated under reduced pressure and heating at 50°C to obtain a polyurethane aqueous dispersion with a non-volatile content of approximately 32% by mass.

[0075] • WPU4: An aqueous dispersion of the triethylamine salt of the anionic polyurethane resin obtained by Synthesis Example 4 below. (Synthesis Example 4) In a four-necked flask equipped with a stirrer, reflux condenser, thermometer, and nitrogen inlet, 51.3 parts by mass of polycarbonate polyol (UBE Corporation's "ETERNACOLL UH-100", average hydroxyl value 110.0 mg KOH / g, active hydrogen groups 2.0), 5.1 parts by mass of dimethylolpropionic acid (active hydrogen groups 2), 2.6 parts by mass of trimethylolpropane (active hydrogen groups 2), 41.0 parts by mass of dicyclohexylmethane 4,4'-diisocyanate, and 100 parts by mass of methyl ethyl ketone were added. The mixture was then reacted at 75°C for 4 hours to obtain a methyl ethyl ketone solution of polyurethane prepolymer. The free isocyanate group content relative to the nonvolatile content of this solution was 3.0% by mass. This solution was cooled to 45°C, neutralized with 2.9 parts by mass of triethylamine, and then gradually added 300 parts by mass of water and emulsified and dispersed using a homogenizer. Subsequently, an aqueous solution of 2.08 parts by mass of ethylenediamine (with 2 active hydrogen groups) diluted with 100 parts by mass of water was added, and the chain extension reaction was carried out for 1 hour. This was then desolvented under reduced pressure and heating at 50°C to obtain a polyurethane aqueous dispersion with a non-volatile content of approximately 32% by mass.

[0076] • SBR: Styrene-butadiene rubber (SBR) emulsion obtained by the synthesis example 5 below. (Synthesis Example 5) In a flask equipped with a stirrer, reflux condenser, and thermometer, 51 parts by mass of prepared water and 0.2 parts by mass of dodecylbenzenesulfonate were charged, and the temperature was raised to 40°C. Separately, a preliminary emulsion was prepared by emulsifying and dispersing 5 parts by mass of acrylonitrile, 8 parts by mass of methyl methacrylate, 55 parts by mass of styrene, 32 parts by mass of 1,3-butadiene, and 0.95 parts by mass of alkylbenzenesulfonate in 40 parts by mass of prepared water. This preliminary emulsion was added dropwise to the flask from a dropping funnel over 4 hours, and 0.4 parts by mass of a 10% by mass aqueous solution of sodium persulfate initiator was added as a polymerization initiator to start polymerization. After maintaining the reaction temperature at 65°C for 4 hours, the temperature was raised to 80°C and the reaction was continued for another 2 hours to obtain an SBR emulsion. Dilution water was added to this to obtain an SBR emulsion with a non-volatile content of approximately 40% by mass.

[0077] [Electrolyte] • Electrolyte 1: LiFSI / MPPy-FSI (Lithium salt: Lithium bis(fluorosulfonyl)imide, Ionic liquid: 1-Methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide), Lithium salt concentration 1 mol / kg

[0078] • Electrolyte 2:LiFSI / EC:DMC = 1:1 (Lithium salt: Lithium bis(fluorosulfonyl)imide, Organic solvent: Ethylene carbonate / Dimethyl carbonate = 1 / 1 (volume ratio)), Lithium salt concentration 1 mol / L

[0079] (Examples 1-4, Comparative Examples 1-4) Using the conductive material and binder shown in Table 1, the mixture was prepared according to the proportions shown in Table 1. After adjusting the solid content concentration to 12% by mass with water, the mixture was stirred in a mixer for 10 minutes. The binder / conductive material proportions in the table represent the mass ratio of the resin solid content as the binder to the conductive material (amount of single-walled carbon nanotubes, multi-walled carbon nanotubes, or acetylene black as conductive components). A vortex mixer (Scientific Industries' "VORTEX-GENIE 2Mixer") was used as the mixer, but the stirring device is not fundamentally dependent on it. An appropriate amount of the resulting mixture was dropped onto a current collector attached to a glass substrate and coated using a spin coater (Mikasa Corporation's "MIKASA SPINNER") at a rotation speed of 2000 rpm and a rotation time of 20 seconds. Cu foil was used as the current collector. The coated current collector was placed in a forced-air dryer and dried at 100°C for 30 minutes to produce the negative electrode.

[0080] The thickness of the dried coating (a layer containing a binder and conductive material (a CNT layer in this example)) was measured using a micrometer (Mitutoyo MDE-25PJ).

[0081] The Coulomb efficiency was measured for the negative electrodes of Examples 1-4 and Comparative Examples 1-4. For Comparative Example 1, the current collector was used as the negative electrode without forming a coating, and the Coulomb efficiency (charge / discharge efficiency) was measured. The measurement method is as follows.

[0082] (Creation of evaluation cells) The evaluation cell was constructed in a glove box under an argon atmosphere with a dew point of -70°C or lower. As shown in Figure 2, a 13 mm diameter metallic Li foil was attached to the bottom surface of the first body of the two-electrode cell for electrochemical measurement. A 20 mm diameter polyimide separator, impregnated with electrolyte 1 under reduced pressure for 30 minutes, was placed on top of it. The negative electrode, with a diameter of 14 mm, was then placed on top of that, and the cell was sealed with the second body via an electrode holder and spring to create the evaluation cell.

[0083] (Measurement conditions for Coulomb efficiency) A constant current charge / discharge device (Nagano Corporation's "BTS2004W") was used as the measurement device, and the evaluation cell described above was charged and discharged in a constant temperature room at 25°C. The current density was 1.00 mA / cm². 2 Then, 1 mAh of Li was deposited on the negative electrode current collector with a constant current. Subsequently, the cutoff voltage was 1.0V vs. Li / Li. + Constant current discharge was performed until the specified value was reached. This was repeated 50 times, and the average Coulomb efficiency in the Li elution reaction was calculated. The average Coulomb efficiency was calculated as the average value of 100 × discharge capacity / charge capacity (%) for each cycle. For evaluation cells that could not be charged or discharged and therefore did not function as a battery, the Coulomb efficiency was indicated as "Not functional".

[0084] [Table 1]

[0085] The results are shown in Table 1. Compared to Comparative Example 1, which was a blank using copper foil as the current collector, Examples 1-4, in which a layer containing polyurethane resin and single-walled carbon nanotubes was formed on the copper foil, showed improved Coulomb efficiency despite not containing a negative electrode active material. In contrast, Comparative Example 2, which used styrene-butadiene rubber instead of polyurethane resin as the binder, did not show any improvement in Coulomb efficiency compared to the blank Comparative Example 1. Furthermore, Comparative Examples 3 and 4, which used acetylene black or multi-walled carbon nanotubes instead of single-walled carbon nanotubes as the conductive material, could not be charged or discharged and did not function as batteries.

[0086] As for the type of polyurethane resin used as a binder, Example 1, which uses a lithium salt, showed superior improvement in Coulomb efficiency compared to Example 2, which uses a sodium salt, and Example 4, which uses a triethylamine salt (NEt3).

[0087] (Measurement of nucleation overpotential) For the negative electrodes of Examples 1, 2, and 4, nucleation overpotential was measured and evaluated as an indicator of resistance. In measuring the Coulomb efficiency using the evaluation cells described above with the negative electrodes of Examples 1, 2, and 4, the difference between the minimum voltage at the start of the first charge and the voltage at the end of the first charge was defined as nucleation overpotential. The results are shown in Table 2 below.

[0088] [Table 2]

[0089] As shown in Table 2, among the polyurethane resin salt types, lithium salts and sodium salts exhibited lower nucleation overpotentials and were superior to amine salts. Furthermore, it was found that lithium salts were more advantageous than sodium salts in improving Coulomb efficiency while reducing nucleation overpotentials.

[0090] (Examples 5-9, Comparative Examples 5-6) Negative electrodes were fabricated using Cu-Sn foil instead of Cu foil as the current collector, and the types and mixing ratios of the binder and conductive material were changed as shown in Table 3 below. Otherwise, the procedure was the same as in Examples 1-4 and Comparative Examples 1-4, and the Coulomb efficiency was measured. For negative electrodes with different coating thicknesses in Examples 6 and 7, the solid content concentration of the mixture was changed during fabrication. For Comparative Example 5, the Cu-Sn foil, which is the current collector, was used directly as the negative electrode without forming a coating, and the Coulomb efficiency was measured.

[0091] [Table 3]

[0092] The results are shown in Table 3. Compared to Comparative Example 5, which was a blank using Cu-Sn foil as the current collector, Examples 5-9, in which a layer containing polyurethane resin and single-walled carbon nanotubes was formed on the Cu-Sn foil, showed improved Coulomb efficiency despite not containing negative electrode active material. The improvement in Coulomb efficiency was also observed when the thickness of the coating film and the binder / conductive material mixing ratio were changed. In contrast, in Comparative Example 6, which used styrene-butadiene rubber instead of polyurethane resin as the binder, the Coulomb efficiency decreased compared to the blank Comparative Example 5.

[0093] (Example 10, Comparative Example 7) In Example 10, instead of electrolyte 1, which uses an ionic liquid, electrolyte 2, which uses an organic solvent, was used as the electrolyte, and the Coulomb efficiency was measured in the same manner as in Example 1. For Comparative Example 7, the Cu foil, which is the current collector, was used as the negative electrode without forming a coating film as a blank, and the Coulomb efficiency was measured using electrolyte 2, as in Example 10.

[0094] [Table 4]

[0095] The results are shown in Table 4. Even when using an organic electrolyte 2, Example 10, in which a layer containing polyurethane resin and single-walled carbon nanotubes was formed on Cu foil, showed improved Coulomb efficiency compared to the blank Comparative Example 7, despite not containing a negative electrode active material.

[0096] Furthermore, the various numerical ranges described in this specification can be any combination of their upper and lower limits, and all such combinations are described herein as preferred numerical ranges. Also, the description of a numerical range as "X~Y" means X or greater and Y or less.

[0097] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their omissions, substitutions, and modifications are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of symbols]

[0098] 10...Non-aqueous electrolyte secondary battery, 12...Negative electrode, 14...Positive electrode, 16...Separator, 18...Electrolyte, 20...Current collector, 22...CNT layer, 24...Current collector, 26...Positive electrode active material layer

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery, comprising a current collector and a layer containing polyurethane resin and single-walled carbon nanotubes disposed on the current collector, and not containing a negative electrode active material.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the polyurethane resin comprises a lithium salt and / or a sodium salt of an anionic polyurethane resin.

3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the polyurethane resin contains an alicyclic polyisocyanate as a raw material.

4. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the mass ratio (A) / (B) of the polyurethane resin (A) to the single-walled carbon nanotube (B) is 99.9 / 0.1 to 97.0 / 3.

0.

5. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the thickness of the layer containing the polyurethane resin and the single-walled carbon nanotube is 0.5 to 2 μm.

6. A non-aqueous electrolyte secondary battery comprising a negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5.

7. A non-aqueous electrolyte secondary battery according to claim 6, using an ionic liquid as the electrolyte.

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