Lithium ion secondary battery negative electrode slurry composition, lithium ion secondary battery negative electrode, and lithium ion secondary battery

Abstract

To provide a negative electrode slurry composition for a lithium ion secondary battery, which can provide a negative electrode and a secondary battery having excellent binding properties between a current collector and a negative electrode layer.SOLUTION: A negative electrode slurry composition for a lithium ion secondary battery includes a negative electrode active material, a binder, and an additive, the additive is a heterocyclic compound containing a nitrogen atom, and the heterocyclic compound has a substituent containing a thiol group or a -SM group (M represents a metal element).SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a negative electrode slurry composition for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

Background Art

[0002] Lithium ion secondary batteries have a high energy density and a high capacity, and thus are widely used as a driving power source for mobile information terminals. In recent years, their use in industrial applications such as being mounted in electric and hybrid vehicles that require a large capacity has also been spreading, and studies are being conducted to further increase the capacity and performance.

[0003] As one of such attempts, for example, silicon, tin, or an alloy containing these, which has a large lithium storage amount per unit volume, is used to increase the charge and discharge capacity. However, when using an active material with a large charge and discharge capacity such as silicon, tin, or an alloy containing these, the volume change of the negative electrode active material due to the lithium ion storage and release reactions during charge and discharge is extremely large. Therefore, it is known that the negative electrode layer containing the negative electrode active material peels off from the negative electrode current collector, and the negative electrode active material is likely to desorb.

[0004] Conventionally, polyvinylidene fluoride (PVDF), which has been used as a binder, needs to be used in a large amount because of its low binding force and flexibility. Furthermore, since PVDF dissolves only in organic solvents, a binder with a reduced environmental load has been demanded.

[0005] As an aqueous binder that can reduce the environmental load without reducing the binding force, the use of styrene-butadiene rubber (SBR), which is a rubbery polymer, has been studied. However, there are problems with the binding force when using an active material with large expansion and contraction, such as a negative electrode using silicon-containing particles. In order to solve this problem, studies have been conducted on improving the binding force of the negative electrode layer to the negative electrode current collector in order to develop a battery having a high energy density and excellent cycle characteristics.

[0006] For example, in Patent Document 1, a method of suppressing the destruction of the electrode structure has been proposed by using a partially crosslinked polyacrylic acid as a binder. Further, in Patent Document 2, a method of exhibiting excellent binding properties has been proposed by using a copolymer of acrylic acid and polyvinyl alcohol as a binder. However, with the binders described in Patent Documents 1 and 2 above, the cycle characteristics at high and low temperatures when used as a secondary battery were not always sufficient.

[0007] In addition, studies have also been conducted to improve the peel strength between the current collector and the negative electrode layer by adding an additive to the negative electrode slurry. In Patent Document 3, lignin substances and humic acid substances are added, and in Patent Document 4, a water-soluble additive is added, and in both cases, an improvement in peel strength has been confirmed. However, with the techniques described in Patent Documents 3 and 4, the peel strength and charge-discharge cycle characteristics are not always sufficient, and there is still room for improvement.

[0008] Furthermore, in Patent Documents 5 and 6, studies have also been conducted to improve the dispersion stability and coating film adhesion by adding a triazine derivative. However, in Patent Documents 5 and 6, measurements regarding the peel strength have not been performed, and it is not certain whether a sufficient effect has been obtained regarding the adhesive strength. Also, in Patent Document 6, since it is essential to use in combination with a resin having a basic functional group, there are restrictions on the raw materials that can be used.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0010] An object of the present invention is to provide a negative electrode slurry composition for a lithium ion secondary battery, from which a negative electrode and a secondary battery having excellent adhesion between a negative electrode layer containing a negative electrode active material and a current collector can be obtained. Further, an object of the present invention is to provide a negative electrode and a secondary battery formed using the negative electrode slurry composition.

Means for Solving the Problems

[0011] As a result of intensive studies by the present inventors, it has been found that the above problems can be solved by including a specific additive in the negative electrode slurry composition, and the present invention has been achieved.

[0012] That is, the present invention relates to the following. [1] A negative electrode slurry composition for a lithium ion secondary battery, comprising a negative electrode active material, a binder, and an additive, wherein the additive is a heterocyclic compound containing a nitrogen atom, and the heterocyclic compound has a substituent containing a thiol group or -SM group (M represents a metal element). [2] The negative electrode slurry composition for a lithium ion secondary battery according to [1], wherein at least one of the substituents represents a thiol group or -SM group and is directly bonded to the heterocyclic compound. [3] The negative electrode slurry composition for a lithium ion secondary battery according to [1] or [2], wherein the additive is a heterocyclic compound represented by the following general formula (1).

[0013]

Chemical Formula

[0014] (In general formula (1), Q 1 represents a -SH group or -SM group (M represents a metal element). Q2 represents a -SH group, -SM group (where M represents a metal element), -H group, -OH group, -NH 2 group, halogen group, halogenated alkyl group, alkyl group, nitro group, cyano group, acyl group, -O-R 1 , -S-R 1 , or -NH-R 1 . However, R 1 represents an alkyl group which may have a substituent, an alkenyl group which may have a substituent, or an aromatic substituent which may have a substituent. X 1 , X 2 is each independently a carbon atom or a nitrogen atom. Y 1 represents a -SH group, -SM group (where M represents a metal element), -H group, -OH group, -NH 2 group, halogen group, halogenated alkyl group, alkyl group, nitro group, cyano group, acyl group, -O-R 2 group, -S-R 2 group, or -NH-R 2 group. However, R 2 represents an alkyl group which may have a substituent, an alkenyl group which may have a substituent, or an aromatic substituent which may have a substituent.) [4] The negative electrode slurry composition for a lithium ion secondary battery according to [2], wherein the additive is at least one selected from 1,3,5-triazine-2,4,6-trithiol, trisodium salt hydrate of 1,3,5-triazine-2,4,6-trithiol, disodium salt hydrate of 1,3,5-triazine-2,4,6-trithiol, monosodium salt hydrate of 1,3,5-triazine-2,4,6-trithiol, 2-(dibutylamino)-1,3,5-triazine-4,6-dithiol, disodium salt of 2-(dibutylamino)-1,3,5-triazine-4,6-dithiol, monosodium salt of 2-(dibutylamino)-1,3,5-triazine-4,6-dithiol, 2-amino-1,3,5-triazine-4,6-dithiol, disodium salt of 2-amino-1,3,5-triazine-4,6-dithiol, monosodium salt of 2-amino-1,3,5-triazine-4,6-dithiol, 2-(N-carboxymethyl-N-phenyl)amino-4,6-dimercapto-1,3,5-triazine, disodium salt of 2-(N-carboxymethyl-N-phenyl)amino-4,6-dimercapto-1,3,5-triazine, sodium N-(4,6-dimercapto-1,3,5-triazine)-N-phenylglycinate, 2-mercaptopyrimidine, sodium salt of pyrimidine-2-thiol, 4-amino-2-mercaptopyrimidine, and sodium salt of 4-amino-2-mercaptopyrimidine. [5] The negative electrode slurry composition for a lithium ion secondary battery according to any one of [1] to [4], wherein when a copper foil for a negative electrode current collector is immersed in an aqueous solution or an ethanol solution containing the additive, the additive adsorbs on the surface of the copper foil. [6] When a copper foil for a negative electrode current collector is immersed in an aqueous solution or an ethanol solution containing the additive at a concentration of 1.0 to 10.0 mmol / L at room temperature for 6 to 24 hours, the adsorption amount of the additive to the copper foil is 1.0 μmol / dm 2 or more. The negative electrode slurry composition for a lithium ion secondary battery according to any one of [1] to [5]. [7] The negative electrode slurry composition for a lithium ion secondary battery according to any one of [1] to [6], wherein the additive is contained in an amount of 0.5 to 5.0% by mass based on the total mass of the solid content of the negative electrode slurry composition. [8] A negative electrode for a lithium ion secondary battery, comprising a negative electrode layer formed using the negative electrode slurry composition for a lithium ion secondary battery according to any one of [1] to [7]. [9] A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to [8].

Effect of the Invention

[0015] By using the negative electrode slurry composition for a lithium ion secondary battery of the present invention, a negative electrode and a secondary battery with good adhesion between the current collector and the negative electrode layer can be obtained. Further, the cycle characteristics of the secondary battery of the present invention can be improved thereby.

Mode for Carrying Out the Invention

[0016] Hereinafter, embodiments of the present invention will be described in detail. In this specification, "mass" is synonymous with "weight". Further, in this specification, "~" indicating a numerical range is used to mean including the numerical values described before and after it as the lower limit value and the upper limit value unless otherwise specified.

[0017] <Negative Electrode Slurry Composition for Lithium Ion Secondary Battery> The negative electrode slurry composition for a lithium ion secondary battery of the present embodiment (hereinafter, simply referred to as "negative electrode slurry composition") contains a negative electrode active material, a binder, and an additive, and the additive is a heterocyclic compound containing a nitrogen atom, and the heterocyclic compound is characterized by having a substituent containing a thiol group or a -SM group (M represents a metal element).

[0018] [Negative Electrode Active Material] The type of the negative electrode active material constituting the negative electrode slurry composition of the present embodiment is not particularly limited. For example, silicon, SiO negative electrode material, SiOC negative electrode material, carbon-based negative electrode material, oxide-based negative electrode material, metal-based negative electrode material, etc. may be mentioned, and one kind or two or more kinds may be used in combination. Among these, from the viewpoints of electrode expansion and energy density, it is preferable to contain at least one of a carbon-based negative electrode material, a SiOC negative electrode material, and a SiO negative electrode material, and a combination of a carbon-based negative electrode material and a SiOC negative electrode material, or a combination of a carbon-based negative electrode material and a SiO negative electrode material is more preferable.

[0019] The above-mentioned SiO negative electrode material is a material containing SiO (silicon monoxide) as a main component that exhibits charge-discharge characteristics. In this specification, the "main component" means that it is contained in an amount of 51% or more by mass ratio. The SiO negative electrode material may contain silicon particles, carbon, etc. that exhibit charge-discharge characteristics in addition to SiO (silicon monoxide), and may contain silicon oxycarbide (SiOC). Note that these components may be present not alone but in plural.

[0020] The above-mentioned SiOC negative electrode material is a material containing SiOC (silicon oxycarbide) as a main component that exhibits charge-discharge characteristics. The SiOC negative electrode material may contain silicon particles, carbon, etc. that exhibit charge-discharge characteristics in addition to SiOC (silicon oxycarbide), and may contain SiO (silicon monoxide). Note that these components may be present not alone but in plural.

[0021] Examples of the above-mentioned carbon-based negative electrode material include natural graphite, artificial graphite, expanded graphite, activated carbon, carbon fiber, coke, soft carbon, hard carbon, etc. Among these, artificial graphite is preferable from the viewpoint of improving cycle characteristics.

[0022] The content of the negative electrode active material is preferably 80 to 97 parts by mass, more preferably 90 to 96 parts by mass, based on 100 parts by mass of the negative electrode slurry composition of the present embodiment. When the content of the negative electrode active material is within the above range, an optimal mixing ratio with the binder and the conductive assistant can be achieved.

[0023] [Binder] As the binder (binding agent) constituting the negative electrode slurry composition of the present embodiment, a known binder can be used. Specifically, for example, styrene-butadiene rubber copolymer (SBR); ethylenically unsaturated carboxylic acid esters (e.g., methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, (meth)acrylonitrile, and hydroxyethyl (meth)acrylate, etc.), and ethylenically unsaturated carboxylic acids (e.g., acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, etc.) consisting of (meth)acrylic copolymers; polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, polyamideimide, carboxymethyl cellulose (CMC) and other polymer compounds can be mentioned. Among these, from the viewpoint of versatility (good battery characteristics can be exhibited regardless of the type of active material), it is preferable to use carboxymethyl cellulose (CMC) and styrene-butadiene copolymer (SBR) in combination.

[0024] In the present embodiment, the binder may contain at least one of water and organic solvents such as N-methyl-2-pyrrolidone (NMP) in order to dissolve the above polymer compound.

[0025] The content of the binder is preferably 2.5 to 20% by mass with respect to the negative electrode slurry composition of the present embodiment. When the content of the binder is within the above range, sufficient binding force can be exhibited. The content of the binder is more preferably 3 to 12% by mass, still more preferably 3 to 8% by mass, and particularly preferably 3 to 5% by mass.

[0026] [Additive] The additive that constitutes the negative electrode slurry composition of the present embodiment is a heterocyclic compound containing a nitrogen atom, and the heterocyclic compound is characterized by having a substituent containing a thiol group or -SM group (M represents a metal element). The inventors of the present invention have found that by mixing and using the above specific additive with a binder, the thiol group or -SM group in the additive dissociates, and the sulfur atoms contained therein chemisorb to the metal constituting the current collector, thereby increasing the binding force of the binder to the current collector. As a result, when the negative electrode slurry composition of the present embodiment is applied to a secondary battery, the binding force between the current collector and the negative electrode layer can be increased, and excellent cycle characteristics can be obtained.

[0027] The additive in the present embodiment may have at least one substituent containing a thiol group or -SM group, but it is preferable to have a plurality of such substituents. From the viewpoints of water solubility and improvement of the binding force, the number of substituents containing a thiol group or -SM group for the heterocyclic compound is preferably 1 to 3, and more preferably 2 to 3. When having a plurality of such substituents, each substituent may be the same or different, but it is preferable to have both a substituent containing a thiol group and a substituent containing -SM group, and it is more preferable that all substituents contain -SM group.

[0028] Specific examples of the substituent containing a thiol group or -SM group include an alkylthiol group, an arylthiol group, and metal salts thereof.

[0029] In the additive of the present embodiment, it is preferable that at least one of the above substituents represents a thiol group or -SM group, and further, the substituent is directly bonded to the heterocyclic compound. That is, it is preferable that a thiol group or -SM group is bonded to the heterocyclic compound. By using a heterocyclic compound in which a thiol group or -SM group is directly bonded as an additive, the binding force between the current collector and the negative electrode layer can be further improved.

[0030] Specifically, the additive preferably has a structure represented by the following general formula (1).

[0031]

Chem.

[0032] In the above general formula (1), Q 1 represents a -SH group or a -SM group (M represents a metal element). Q 2 represents a -SH group, a -SM group (M represents a metal element), a -H group, a -OH group, -NH 2 group, a halogen group, a halogenated alkyl group, an alkyl group, a nitro group, a cyano group, an acyl group, -O-R 1 , -S-R 1 , or -NH-R 1 represents. However, R 1 represents an alkyl group which may have a substituent, an alkenyl group which may have a substituent, or an aromatic substituent which may have a substituent. X 1 , X 2 are each independently a carbon atom or a nitrogen atom. Y 1 represents a -SH group, a -SM group (M represents a metal element), a -H group, a -OH group, -NH 2 group, a halogen group, a halogenated alkyl group, an alkyl group, a nitro group, a cyano group, an acyl group, -O-R 2 group, -S-R 2 group, or -NH-R 2 group. However, R 2 represents an alkyl group which may have a substituent, an alkenyl group which may have a substituent, or an aromatic substituent which may have a substituent.

[0033] Regarding the M (metal element) of the -SM group in the above Q 1 , Q 2 and Y 1 , examples of the metal element M of the -SM group include lithium, sodium, potassium, calcium, barium, magnesium, aluminum, nickel, or cobalt, etc. Among these, from the viewpoint of solubility in the binder, sodium, potassium, or lithium is preferred.

[0034] Regarding the above Q 2 and Y 1Examples of the halogen group in [the compound] include a fluoro group, a chloro group, a bromo group, and an iodo group.

[0035] The above Q 2 and Y 1 Examples of the halogenated alkyl group in [the compound] include a group in which some or all of the hydrogen atoms of the alkyl group are substituted with halogen atoms. The alkyl group is preferably an alkyl group having 1 to 3 carbon atoms, and examples of the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. Specific examples of the halogenated alkyl group include, for example, a trifluoromethyl group, a trifluoroethyl group, and a perfluoroethyl group.

[0036] The above Q 2 and Y 1 Examples of the alkyl group in [the compound] include an alkyl group having 1 to 3 carbon atoms. The alkyl group may be linear or branched. Specific examples of the alkyl group include, for example, a methyl group, an ethyl group, and an isopropyl group.

[0037] The above Q 2 and Y 1 In [the compound], the alkyl group which may have a substituent represented by R 1 and R 2 and the alkenyl group which may have a substituent are preferably those having 1 to 3 carbon atoms, more preferably those having 1 to 2 carbon atoms. The alkyl group or alkenyl group having a substituent means a group in which some or all of the hydrogen atoms of the alkyl group or alkenyl group are substituted with a halogen atom such as a fluoro group, a chloro group, or a bromo group, an amino group, a hydroxyl group, a mercapto group, a carboxy group, a nitrile group, or the like.

[0038] The above Q 2 and Y 1 In [the compound], the alkyl group which may have a substituent represented by R 1 and R 2Examples of the aromatic substituent which may have a substituent represented by include, specifically, for example, a phenyl group, a benzyl group, a tolyl group, a xylyl group, a styryl group, a cumyl group and the like. Examples of the substituent include an alkyl group having 1 to 3 carbon atoms such as a methyl group, an ethyl group, a propyl group, a phenyl group, and a styryl group. Further, a plurality of these substituents may be present.

[0039] Since the additive is a heterocyclic compound represented by the above general formula (1), the adhesiveness between the current collector and the negative electrode layer when the negative electrode slurry composition of the present embodiment is applied to a secondary battery can be further improved.

[0040] The above additive is a heterocyclic compound containing a nitrogen atom and having a substituent containing a thiol group or -SM group (M represents a metal element), and is not particularly limited as long as it has such a structure. From the perspective of improving the adhesion between the current collector and the negative electrode layer when applied to a secondary battery, 1,3,5-triazine-2,4,6-trithiol, 1,3,5-triazine-2,4,6-trithiol trisodium salt hydrate, 1,3,5-triazine-2,4,6-trithiol disodium salt hydrate, 1,3,5-triazine-2,4,6-trithiol monosodium salt hydrate, 2-(dibutylamino)-1,3,5-triazine-4,6-dithiol, 2-(dibutylamino)-1,3,5-triazine-4,6-dithiol disodium salt, 2-(dibutylamino)-1,3,5-triazine-4,6-dithiol monosodium salt, 2-amino-1,3,5-triazine-4,6-dithiol, 2-amino-1,3,5-triazine-4,6-dithiol disodium salt, 2-amino-1,3,5-triazine-4,6-dithiol monosodium salt, 2-(N-carboxymethyl-N-phenyl)amino-4,6-dimercapto-1,3,5-triazine, 2-(N-carboxymethyl-N-phenyl)amino-4,6-dimercapto-1,3,5-triazine disodium salt, N-(4,6-dimercapto-1,3,5-triazine)-N-phenylglycine sodium, 2-mercaptopyrimidine, pyrimidine-2-thiol sodium salt, 4-amino-2-mercaptopyrimidine, 4-amino-2-mercaptopyrimidine sodium salt, and it is more preferably at least one selected from these.

[0041] In this embodiment, when the copper foil for the negative electrode current collector is immersed in the aqueous solution or ethanol solution containing the above additive, it is preferable that the additive adsorbs on the surface of the copper foil. By using the additive having the above characteristics, the adhesion between the current collector and the negative electrode layer when the negative electrode slurry composition of this embodiment is applied to a secondary battery can be further improved. Note that the adsorption of the additive on the copper foil surface can be confirmed by the method described in the examples below.

[0042] Furthermore, in the present embodiment, when a copper foil for a negative electrode current collector is immersed in an aqueous solution or an ethanol solution containing an additive at a concentration of 1.0 to 10.0 mmol / L at room temperature for 6 to 24 hours, the adsorption amount of the additive to the copper foil is 1.0 μmol / dm 2 or more is preferable. By using the additive having the above characteristics, the adhesion between the current collector and the negative electrode layer when the negative electrode slurry composition of the embodiment is applied to a secondary battery can be further improved. The adsorption amount of the additive under the above conditions is 3.0 μmol / dm 2 or more is more preferable, 5.0 μmol / dm 2 or more is even more preferable, and 8.0 μmol / dm 2 or more is particularly preferable. The upper limit of the adsorption amount of the additive under the above conditions is not particularly limited, but is, for example, 20.0 μmol / dm 2 or less. Note that the adsorption amount of the additive on the current collector copper foil can be calculated by the method described in the examples below.

[0043] The content of the additive is preferably 0.5 to 5.0% by mass based on the total mass of the solid content of the negative electrode slurry composition. When the content of the additive is 0.5% by mass or more, the adhesive force can be sufficiently improved. The content of the additive is more preferably 0.7% by mass or more, even more preferably 1.2% by mass or more, and particularly preferably 1.7% by mass or more. Also, when the content of the additive is 5.0% by mass or less, an increase in resistance can be reduced and the capacity retention rate can be increased. The content of the additive is more preferably 4.0% by mass or less. In the negative electrode slurry composition, the content of the additive can be measured by spectroscopic methods such as IR and NMR.

[0044] <Manufacturing method of additive> The manufacturing method of the additive of the present embodiment can be manufactured by a known method. For example, it can be manufactured with reference to the methods described in JP-A-60-88185, JP-A-63-305173, JP-A-11-199796, etc.

[0045] [Other components] In addition to the above components, the negative electrode slurry composition of this embodiment may contain a conductive auxiliary agent, a solvent, a dispersant, and the like.

[0046] Examples of the conductive auxiliary agent include carbon black, graphite, acetylene black, or oxides and nitrides exhibiting conductivity. These may be used alone or in combination of two or more. The usage amount of the conductive auxiliary agent is, for example, 0.1 to 5% by mass based on the negative electrode slurry composition.

[0047] The solvent is not particularly limited as long as it can disperse the components necessary for constituting the negative electrode slurry composition of this embodiment. For example, an aqueous solvent can be used, and ion-exchanged water is preferable. The proportion of the solvent in the negative electrode slurry composition is preferably 30 to 70% by mass, and more preferably 35 to 55% by mass.

[0048] The solid content concentration of the negative electrode slurry composition of this embodiment is preferably 35 to 70% by mass. If the solid content concentration is 35% by mass or more, it can be uniformly coated on the current collector. Also, if the solid content concentration is 70% by mass or less, it can be uniformly kneaded. The solid content concentration of the negative electrode slurry composition of this embodiment is more preferably 45 to 65% by mass, and particularly preferably 50 to 60% by mass. The solid content concentration of the negative electrode slurry composition can be measured by subtracting the mass after drying the negative electrode slurry composition at 108°C for 2 hours from the mass before drying.

[0049] <Method for manufacturing a negative electrode slurry composition for a lithium ion secondary battery> The method for manufacturing the negative electrode slurry composition for a lithium ion secondary battery of this embodiment is not particularly limited, and examples include a method of kneading each component such as a negative electrode active material, a binder, and an additive.

[0050] The kneading method of each component is not particularly limited, and for example, general devices such as a mixer, a kneader, a disperser, a mill, and a rotating device with rotation and revolution can be used.

[0051] <Negative electrode for a lithium ion secondary battery> The negative electrode for a lithium-ion secondary battery of the present embodiment (hereinafter simply referred to as "negative electrode") is characterized by including a negative electrode layer formed using the negative electrode slurry composition of the present embodiment. The negative electrode of the present embodiment may include a current collector in addition to the negative electrode layer.

[0052] The negative electrode of the present embodiment can be obtained, for example, by applying the negative electrode slurry composition of the present embodiment onto a current collector and then drying it to form a thin film as the negative electrode layer. Alternatively, the negative electrode slurry composition may be formed into a sheet shape, pellet shape, etc., and integrated with the current collector to obtain the negative electrode.

[0053] The material and shape of the above current collector are not particularly limited. For example, a strip-shaped material made of copper, nickel, titanium, stainless steel, etc., in the form of foil, perforated foil, mesh, etc. may be used. Also, porous materials such as porous metal (foamed metal) and carbon paper can be used.

[0054] The method of applying the negative electrode slurry composition onto the above current collector is not particularly limited. Examples include known methods such as metal mask printing method, electrostatic coating method, dip coating method, spray coating method, roll coating method, doctor blade method, gravure coating method, and screen printing method. After applying the negative electrode slurry composition, it is preferably subjected to rolling treatment using a flat press, calendar roll, etc. as necessary.

[0055] Also, the integration of the negative electrode slurry composition formed into a sheet shape, pellet shape, etc. and the current collector can be performed by known methods such as rolling, pressing, or a combination thereof. The density of the negative electrode layer after integration is, for example, 1.0 to 1.8 g / cm 3 and preferably 1.1 to 1.7 g / cm 3 is.

[0056] It is preferable to apply and integrate the negative electrode layer on the current collector and then perform heat treatment. The heat treatment conditions are, for example, 80 to 150 °C for 5 to 20 hours. By this heat treatment, removal of the solvent and strengthening due to curing of the binder proceed, and the binding property between the negative electrode active materials contained in the negative electrode layer and between the negative electrode layer and the current collector can be improved. In addition, in order to prevent oxidation of the current collector during the treatment, these heat treatments are preferably performed in an inert atmosphere such as helium, argon, or nitrogen, or in a vacuum atmosphere.

[0057] Also, after heat treatment, it is preferable to press (apply pressure to) the negative electrode. In the negative electrode formed using the negative electrode slurry composition of the present embodiment, the electrode density is preferably 1.0 to 1.8 g / cm 3 and more preferably 1.1 to 1.7 g / cm 3 and even more preferably 1.2 to 1.6 g / cm 3 Regarding the electrode density, the higher it is, the more the adhesion and the volumetric capacity density of the electrode tend to improve. However, if the density is too high, the voids in the electrode decrease, weakening the volume expansion suppression effect of silicon or the like, resulting in a decrease in cycle characteristics.

[0058] <Lithium-ion secondary battery> The lithium-ion secondary battery of the present embodiment includes the negative electrode of the present embodiment. The lithium-ion secondary battery of the present embodiment can be configured, for example, by arranging a positive electrode and the negative electrode of the present embodiment to face each other with a separator interposed therebetween and injecting an electrolytic solution.

[0059] The positive electrode can be obtained by forming a positive electrode layer on the surface of a current collector in the same manner as the negative electrode. In this case, as the current collector, a strip-shaped material made of a metal or alloy such as aluminum, titanium, or stainless steel in the form of a foil, a perforated foil, or a mesh can be used.

[0060] The positive electrode active material used for the positive electrode layer is not particularly limited. For example, a metal compound, a metal oxide, a metal sulfide, or a conductive polymer material capable of doping or intercalating lithium ions can be used. More specifically, lithium cobaltate (LiCoO2 ) Lithium nickelate (LiNiO 2 ), lithium manganate (LiMnO 2 ), and their composite oxides (LiCoxNiyMnzO 2 , x + y + z = 1), lithium manganese spinel (LiMn 2 O 4 ), lithium vanadium compounds, V 2 O 5 ), V 6 O 13 ), VO 2 ), MnO 2 ), TiO 2 ), MoV 2 O 8 ), TiS 2 ), V 2 S 5 ), VS 2 ), MoS 2 ), MoS 3 ), Cr 3 O 8 ), Cr 2 O 5 ), olivine-type LiMPO 4 (M: Co, Ni, Mn, Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, porous carbon, etc. can be used alone or in combination.

[0061] As the separator, for example, non-woven fabrics, cloths, microporous films mainly composed of polyolefins such as polyethylene and polypropylene, or combinations thereof can be used. In addition, when the structure is such that the positive electrode and the negative electrode of the lithium ion secondary battery to be manufactured do not come into direct contact, it is not necessary to use a separator.

[0062] As the electrolyte, for example, LiClO 4 , LiPF 6 , LiAsF 6 , LiBF 4 , LiSO 3 CF 3A so-called organic electrolyte solution in which a lithium salt such as the above is dissolved in a non-aqueous solvent that is a single substance or a mixture of two or more components, such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, etc., can be used.

[0063] The structure of the lithium-ion secondary battery of this embodiment is not particularly limited, but usually, a positive electrode and a negative electrode, and a separator provided as necessary are wound in a flat spiral shape to form a wound electrode plate group, or these are formed into a flat plate shape and laminated to form a laminated electrode plate group, and these electrode plate groups are generally enclosed in an exterior body.

[0064] The lithium-ion secondary battery of this embodiment is not particularly limited, but is used as a paper-type battery, button-type battery, coin-type battery, laminated battery, cylindrical battery, square battery, etc. The negative electrode active material of the above-described embodiment of this invention can also be applied to all electrochemical devices having a charge-discharge mechanism of inserting and desorbing lithium ions, such as hybrid capacitors and solid lithium secondary batteries.

[0065] [Characteristics] Since the lithium-ion secondary battery of this embodiment is formed using the negative electrode slurry composition of this embodiment, the binding property between the negative electrode layer and the current collector is good, and the cycle characteristics are excellent. Specifically, the peel strength of the lithium secondary battery of this embodiment is preferably 15 N / m or more, more preferably 19 N / m or more, and still more preferably 25 N / m or more. The upper limit of the peel strength is not particularly limited, but is, for example, 100 N / m. Note that the peel strength can be measured by the method described in the examples below.

[0066] Also, although it depends on the active material used, the capacity retention rate (cycle characteristics) measured under the following conditions for the lithium-ion secondary battery of this embodiment is preferably 78.0% or more. The upper limit of the capacity retention rate is not particularly limited, but for example, it is 99.5%. (Conditions) Attach the secondary battery to a charge-discharge device, leave it for 3 hours at 25°C, and then perform one charge-discharge cycle at 0.1C. Next, repeat 100 charge-discharge cycles at 45°C and 0.5C, and calculate the discharge capacity retention rate at the 100th cycle.

Examples

[0067] Hereinafter, the present invention will be described in detail by way of examples. However, the present invention is not limited to the following examples, and the present invention can be arbitrarily modified and implemented without departing from the gist thereof.

[0068] <Preparation of Additive> (Additive A) Dissolve 2 g of 2-(carboxymethyl-N-phenyl)amino-4,6-dimercapto-1,3,5-triazine (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product code: 328-25692) in 27.2 mL of 0.5 mol / L sodium hydroxide solution (manufactured by Kanto Chemical, product number: 37848-08), and stir with a magnetic stirrer for 10 minutes. Then, freeze-dry for 16 hours using a freeze dryer (FD-81 manufactured by EYELA) to obtain Additive A represented by the following structural formula.

[0069]

Chemical formula

[0070] (Additive B) To 23.8 mL of water, 3.4 mL of 4 mol / L lithium hydroxide solution (manufactured by Fujifilm Wako Chemicals, product code: 1125-02325) was added to prepare a 0.5 mol / L lithium hydroxide solution. 2 g of 2-(carboxymethyl-N-phenyl)amino-4,6-dimercapto-1,3,5-triazine (manufactured by Fujifilm Wako Chemicals, product code: 328-25692) was dissolved and stirred with a magnetic stirrer for 10 minutes. Then, it was freeze-dried with a freeze dryer (FD-81 manufactured by EYELA) for 16 hours to obtain additive B represented by the following structural formula.

[0071] [Chemical formula]

[0072] (Additive C) As additive C, Trithiocyanuric acid trisodium salt hydrate (QH-7587) manufactured by Combi-blocks and represented by the following structural formula was prepared.

[0073] [Chemical formula]

[0074] (Additive D) As additive D, 2-amino-1,3,5-triazine-4,6-dithiol (product number QF-1638 manufactured by COMBI-BLOCKS) represented by the following structural formula was prepared.

[0075] [Chemical formula]

[0076] (Additive E) As additive E, melamine (25093-02) manufactured by Kanto Chemical Co., Inc. and represented by the following structural formula was prepared.

[0077] [Chemical formula]

[0078] <Preparation of Negative Electrode Slurry Composition> (Example 1) Weighed 10.3 parts by mass of SiO negative electrode material (SiO-P-105 manufactured by Nippon NER Co., Ltd.), 82.7 parts by mass of artificial graphite, 1.0 part by mass of acetylene black, and 1.5 parts by mass of sodium carboxymethyl cellulose (CMC, Sunrose MAC350HC manufactured by Nippon Paper Industries Co., Ltd.). Using a planetary mixer (ARE-310 manufactured by Thinky, hereinafter referred to as mixer), stirred at 1000 rpm for rotation and 2000 rpm for revolution for 30 seconds. Next, 3.0 parts by mass of additive A obtained above and 40.0 parts by mass of distilled water were added and mixed until the whole became a paste state. Then, using the above mixer, stirred at 1000 rpm for rotation and 2000 rpm for revolution for 2 minutes. Since heat was generated by stirring, it was cooled to room temperature with ice water. Again, stirred at 1000 rpm for rotation and 2000 rpm for revolution for 2 minutes, and then cooled to room temperature with ice water. 7.5 parts by mass of distilled water was added and mixed until the whole became uniform. Then, using the above mixer, stirred at 1000 rpm for rotation and 2000 rpm for revolution for 2 minutes and cooled to room temperature with ice water. 10 parts by mass of distilled water and 2.97 parts by mass (1.5 parts by mass in terms of non-volatile content) of styrene-butadiene copolymer (SBR) (DS407H manufactured by DIC Co., Ltd., non-volatile content concentration 50.8%) were added. Again, using the above mixer, stirred at 1000 rpm for rotation and 2000 rpm for revolution for 30 seconds to prepare a negative electrode slurry composition.

[0079] (Example 2) The negative electrode slurry composition was adjusted in the same procedure as in Example 1 except that additive A in Example 1 was changed to additive B.

[0080] (Example 3) The negative electrode slurry composition was adjusted in the same procedure as in Example 1 except that additive A in Example 1 was changed to additive C.

[0081] (Example 4) Weighed 10.3 parts by mass of SiO negative electrode material, 82.7 parts by mass of artificial graphite, and 1.0 part by mass of acetylene black, and stirred them in the above mixer for 30 seconds under the conditions of 1000 rpm of rotation and 2000 rpm of revolution. Next, diluted polyacrylic acid with distilled water and adjusted the aqueous solution to a non-volatile content concentration of 8.0% (3.0 parts by mass in terms of solid content conversion weight), added 3.0 parts by mass of additive A and 5.5 parts by mass of distilled water, and mixed them until the whole became a paste state. Then, stirred in the above mixer for 2 minutes under the conditions of 1000 rpm of rotation and 2000 rpm of revolution. Since heat was generated during stirring, it was cooled to room temperature with ice water. Again, stirred for 2 minutes under the conditions of 1000 rpm of rotation and 2000 rpm of revolution, and then cooled to room temperature with ice water. Added 7.5 parts by mass of distilled water, mixed until the whole became uniform, then stirred in the above mixer for 2 minutes under the conditions of 1000 rpm of rotation and 2000 rpm of revolution, and cooled to room temperature with ice water. Next, in order to adjust the viscosity of the negative electrode slurry composition, measured the viscosity with a B-type viscometer, and appropriately added distilled water so that it was in the range of 2000 - 4000 Pa·s under the condition of 30 rpm. Finally, stirred in the above mixer for 30 seconds under the conditions of 1000 rpm of rotation and 2000 rpm of revolution to prepare the negative electrode slurry composition.

[0082] (Example 5) The negative electrode slurry composition was adjusted in the same procedure as in Example 4 except that additive A in Example 4 was changed to additive B.

[0083] (Example 6) The negative electrode slurry composition was adjusted in the same procedure as in Example 4 except that additive A in Example 4 was changed to additive C.

[0084] (Examples 7, 8) The negative electrode slurry composition was adjusted in the same procedure as in Example 1 except that the content of additive A in Example 1 was changed to the numerical value shown in Table 1.

[0085] (Example 9) The negative electrode slurry composition was adjusted in the same procedure as in Example 1 except that additive A in Example 1 was changed to additive D.

[0086] (Example 10) A negative electrode slurry composition was adjusted in the same procedure as in Example 1, except that the SiO negative electrode material in Example 1 was changed to a SiOC negative electrode material. The SiOC negative electrode material was synthesized by the method described in Example 1 of JP-A-2017-195083.

[0087] (Example 11) A negative electrode slurry composition was adjusted in the same procedure as in Example 10, except that additive A in Example 10 was changed to additive B.

[0088] (Example 12) A negative electrode slurry composition was adjusted in the same procedure as in Example 10, except that additive A in Example 10 was changed to additive C.

[0089] (Comparative Example 1) A negative electrode slurry composition was adjusted in the same procedure as in Example 1, except that the addition amount of the SiO negative electrode material was 10.7 parts by mass and the addition amount of artificial graphite was 85.3 parts by mass, and no additive was added.

[0090] (Comparative Example 2) A negative electrode slurry composition was adjusted in the same procedure as in Example 1, except that additive A in Example 1 was changed to additive E.

[0091] (Comparative Example 3) A negative electrode slurry composition was adjusted in the same procedure as in Example 4, except that additive C was not added.

[0092] (Comparative Example 4) A negative electrode slurry composition was adjusted in the same procedure as in Comparative Example 1, except that the SiO negative electrode material in Comparative Example 1 was changed to a SiOC negative electrode material. The SiOC negative electrode material was produced by the same method as in Example 10.

[0093] <Fabrication of Negative Electrode> The coating amount (areal density) of the dried negative electrode slurry composition was 8.8 mg / cm 2The gap of the bar coater was adjusted to achieve this. Using this bar coater, the negative electrode slurry composition obtained above was respectively coated on a copper foil serving as a current collector. Thereafter, it was dried for 8 minutes using a blow dryer set at 80°C. The dried electrode was cut into strips with a width of 40 mm, and using a roll press machine (small bench roll press SA - 602 manufactured by Tester Sangyo Co., Ltd.), the density of the negative electrode layer was 1.55 g / cm 3 (with a thickness of the negative electrode layer of 66.7 μm) and pressed. After vacuum drying at 110°C for 10 hours, when the density of the negative electrode layer was measured again, all were 1.50 g / cm 3 (with a thickness of the negative electrode layer of 68.6 μm).

[0094] <Fabrication of secondary battery> The negative electrode fabricated above was cut into a 24 mm × 24 mm square with tabs, and the positive electrode shown in the positive electrode fabrication example was cut into a 22 mm × 22 mm square with tabs using a Thomson blade. For the tab parts of the cut electrodes, nickel tab leads were welded to the negative electrode and aluminum tab leads were welded to the positive electrode. Next, a separator (a microporous polyethylene film with a thickness of 25 μm) was cut into a 28 mm × 3.8 cm rectangle using a Thomson blade. The positive electrode and the negative electrode were opposed to each other through the separator, packaged with a laminate film, and the tab parts were fixed by thermocompression bonding. Then, an electrolytic solution (1.0 M LiPF 6 ethylene carbonate / dimethyl carbonate / methyl ethyl carbonate = 30 / 30 / 40 mixed solution (volume ratio) + 1% vinyl carbonate + 5% fluoroethylene carbonate) was added in an amount of 208 μL, and it was completely sealed by vacuum lamination to fabricate a laminated secondary battery.

[0095] <Evaluation> (Peel strength) The negative electrode prepared above was left in a thermo-hygrostat chamber at a temperature of 25°C and a relative humidity of 50% for 6 hours, and then cut into strips with a width of 25 mm and a length of 100 mm. Subsequently, using a double-sided tape (No. 5015 manufactured by Nitto Denko Corporation), it was bonded to a stainless-steel plate with the negative electrode layer as the adherend surface to obtain a sample for the peel strength test. About 10 mm of the copper foil end was peeled off, and a polyimide tape was attached thereto to serve as an attachment part to the peeling tester. The sample for the peel strength test was mounted on a peeling tester (Autograph AG-X Plus manufactured by Shimadzu Corporation), and a 180-degree peel test was performed.

[0096] (Capacity retention rate (cycle characteristics)) The secondary battery prepared above was attached to a charge-discharge device, left for 3 hours at 25°C, and then charged and discharged once at 0.1C. Next, charge and discharge were repeated 100 times at 45°C and 0.5C, and the discharge capacity retention rate at the 100th cycle was calculated.

[0097] (Adsorption amount of additive) The additive was dissolved in water or ethanol to form a 1.0 mmol / L solution. A copper foil for the negative electrode current collector (manufactured by Fukuda Metal Foil & Powder Co., Ltd., model number CF-LB9-10) was immersed in the solution for 6 to 24 hours, and then the current collector was washed with the solvent used. The value obtained by subtracting the mass of the current collector before immersion from the mass of the current collector after washing was defined as the adsorption amount of the additive.

[0098]

Table 1

[0099] The secondary batteries prepared using the negative electrode slurry compositions of Examples 1 to 12 had excellent adhesion between the negative electrode layer and the current collector, and had higher cycle characteristics compared with the comparative examples. On the other hand, since the negative electrode slurry compositions of Comparative Examples 1, 3, and 4 did not contain an additive, their peel strength was low and their cycle characteristics were inferior. In addition, in Comparative Example 2, since the additive did not have a thiol group or -SM group, no adsorption onto the current collector copper foil was observed, the peel strength was low, and the cycle characteristics were inferior.

Claims

1. A negative electrode slurry composition for a lithium ion secondary battery, comprising a negative electrode active material, a binder, and an additive, wherein the additive is a heterocyclic compound containing a nitrogen atom, and the heterocyclic compound has a substituent containing a thiol group or a -SM group (M represents a metal element).

2. The negative electrode slurry composition for a lithium ion secondary battery according to Claim 1, wherein at least one of the substituents represents a thiol group or a -SM group and is directly bonded to the heterocyclic compound.

3. The negative electrode slurry composition for a lithium ion secondary battery according to Claim 2, wherein the additive is a heterocyclic compound represented by the following general formula (1). 【Chemical 1】 (In general formula (1), Q １ represents a -SH group or a -SM group (M represents a metal element). Q ２ represents a -SH group, a -SM group (M represents a metal element), a -H group, an -OH group, -NH ２ group, a halogen group, a halogenated alkyl group, an alkyl group, a nitro group, a cyano group, an acyl group, -O-R １ group, -S-R １ group, or -NH-R １ group. However, R １ represents an alkyl group which may have a substituent, an alkenyl group which may have a substituent, or an aromatic substituent which may have a substituent. X １ , X ２ are each independently a carbon atom or a nitrogen atom. Y １ represents a -SH group, a -SM group (M represents a metal element), a -H group, an -OH group, -NH ２ group, a halogen group, a halogenated alkyl group, an alkyl group, a nitro group, a cyano group, an acyl group, -O-R ２ group, -S-R ２ group, or -NH-R ２ group. However, R ２ represents an alkyl group which may have a substituent, an alkenyl group which may have a substituent, or an aromatic substituent which may have a substituent.)

4. The additive is at least one selected from 1,3,5-triazine-2,4,6-trithiol, 1,3,5-triazine-2,4,6-trithiol trisodium salt hydrate, 1,3,5-triazine-2,4,6-trithiol disodium salt hydrate, 1,3,5-triazine-2,4,6-trithiol monosodium salt hydrate, 2-(dibutylamino)-1,3,5-triazine-4,6-dithiol, 2-(dibutylamino)-1,3,5-triazine-4,6-dithiol disodium salt, 2-(dibutylamino)-1,3,5-triazine-4,6-dithiol monosodium salt, 2-amino-1,3,5-triazine-4,6-dithiol, 2-amino-1,3,5-triazine-4,6-dithiol disodium salt, 2-amino-1,3,5-triazine-4,6-dithiol monosodium salt, 2-(N-carboxymethyl-N-phenyl)amino-4,6-dimercapto-1,3,5-triazine, 2-(N-carboxymethyl-N-phenyl)amino-4,6-dimercapto-1,3,5-triazine disodium salt, N-(4,6-dimercapto-1,3,5-triazine)-N-phenylglycine sodium, 2-mercaptopyrimidine, pyrimidine-2-thiol sodium salt, 4-amino-2-mercaptopyrimidine, 4-amino-2-mercaptopyrimidine sodium salt. The negative electrode slurry composition for a lithium ion secondary battery according to Claim 2.

5. The negative electrode slurry composition for a lithium ion secondary battery according to Claim 1, wherein when a copper foil for a negative electrode current collector is immersed in an aqueous solution or an ethanol solution containing the additive, the additive adsorbs on the surface of the copper foil.

6. When a copper foil for a negative electrode current collector is immersed in an aqueous solution or an ethanol solution containing the additive at a concentration of 1.0 to 10.0 mmol / L at room temperature for 6 to 24 hours, the adsorption amount of the additive to the copper foil is 1.0 μmol / dm ２ or more, The negative electrode slurry composition for a lithium ion secondary battery according to claim 1.

7. The negative electrode slurry composition for a lithium-ion secondary battery according to claim 1, wherein the additive is contained in an amount of 0.5 to 5.0% by mass based on the total mass of the solid content of the negative electrode slurry composition.

8. A negative electrode for a lithium-ion secondary battery, comprising a negative electrode layer formed using the negative electrode slurry composition for a lithium-ion secondary battery according to any one of claims 1 to 7.

9. A lithium-ion secondary battery comprising the negative electrode for a lithium-ion secondary battery according to claim 8.

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