Negative electrode for lithium ion secondary battery and lithium ion secondary battery

Abstract

To provide a negative electrode for a lithium ion secondary battery that protects the negative electrode from breakage and cracks due to expansion and contraction of a Si-based negative electrode active material, and can withstand repeated charging and discharging.SOLUTION: A negative electrode used in a lithium-ion secondary battery includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, and the negative electrode active material layer contains a Si-based negative electrode active material that contains Si as a negative electrode active material as an element and is capable of reversibly intercalating and deintercalating lithium ions. A high-molecular-weight organic compound having a weight-average molecular weight of 1,000 or more is added to the negative electrode, which can improve the cycle characteristics of the lithium-ion secondary battery.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery. Specifically, it relates to a negative electrode for a lithium ion secondary battery containing a high molecular weight organic compound.

Background Art

[0002] Lithium ion secondary batteries are widely used as portable power sources for personal computers, mobile terminals, etc., and power sources for vehicle drives such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHVs) because they are lightweight and can obtain a high energy density. In recent years, for further increasing the capacity of lithium ion secondary batteries, it has been expected to use Si-based materials as negative electrode active materials. Conventionally, graphite has been used as the negative electrode active material, but it is known that the theoretical capacity density of Si-based materials is more than five times larger than that of graphite, and application studies are being advanced as a new negative electrode active material to replace graphite.

[0003] However, a negative electrode active material containing a Si-based material (hereinafter referred to as a Si-based negative electrode active material) has a high theoretical capacity density, but has the property of greatly changing in volume during charge and discharge. Due to such a property, cracks and fissures may occur in the negative electrode active material layer formed on the negative electrode current collector. If such cracks and fissures occur and the portions become isolated from the current collection network, the battery life may be reduced. At the same time, cracks may occur in the SEI (Solid Electrolyte Interphase) film formed on the negative electrode surface, and lithium ions in the electrolytic solution may be taken in to reform the SEI film, which may cause deterioration of the electrolytic solution.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0005] To address such problems, various methods have been proposed to suppress the expansion and contraction of Si-based negative electrode active materials. For example, Patent Document 1 discloses a method of forming voids in the negative electrode to follow the expansion and contraction of Si fine powder, Patent Document 2 discloses a method of uniformly dispersing an Si compound and conductive carbon, Patent Document 3 discloses a method of doping Si particles into graphite, and Patent Document 4 discloses a method of forming a carbonized film on an Si core, etc.

[0006] However, although the above methods can suppress the occurrence of cracks and fissures in the negative electrode active material layer due to the expansion and contraction of the Si-based negative electrode active material, there is still room for further improvement in terms of maintaining the durability of the battery.

[0007] Therefore, the present invention has been made in view of such points, and an object thereof is to provide a negative electrode for a lithium-ion secondary battery that protects the negative electrode from cracks and fissures due to the expansion and contraction of an Si-based negative electrode active material and can withstand repeated charge and discharge. Another object is to provide a lithium-ion secondary battery using the negative electrode disclosed herein.

Means for Solving the Problems

[0008] The negative electrode used in the lithium-ion secondary battery disclosed herein includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode active material layer contains an Si-based negative electrode active material that has Si as a constituent element and can reversibly occlude and release lithium ions, and a high molecular weight organic compound having a weight average molecular weight of 1,000 or more that can improve the durability of the lithium-ion secondary battery (hereinafter also referred to as a durability improver) is added.

[0009] The negative electrode contains the high molecular weight organic compound in the negative electrode active material layer. With such a configuration, the high molecular weight organic compound is adsorbed on the surface of the negative electrode active material layer, and the surface of the negative electrode active material layer can be preferably protected from the expansion and contraction of the Si-based negative electrode active material.

[0010] In a preferred embodiment, the high molecular weight organic compound added to the negative electrode has at least one polar functional group selected from the group consisting of an amino group, a sulfonic acid group, a carboxyl group, a phosphoric acid group, a polyalkylene ether group, an amide group, a hydroxyl group, an epoxy group, and an alkoxysilyl group, and the polar functional group concentration is 0.1 mmol / g or more; has at least one ionic functional group selected from the group consisting of an amino group, a sulfonic acid group, a carboxyl group, a phosphoric acid group, and an amide group, and the ionic functional group concentration is 0.1 mmol / g or more; and includes a copolymer compound obtained by copolymerizing a polymerizable unsaturated monomer. With such a configuration, the surface of the negative electrode active material layer can be more preferably protected from the expansion and contraction of the Si-based negative electrode active material.

[0011] In order to achieve the above object, a negative electrode for a lithium ion secondary battery to which the high molecular weight organic compound disclosed herein is added is provided. With such a configuration, the negative electrode is preferably protected from cracks and fissures caused by the expansion and contraction of the Si-based negative electrode active material, and the durability of the lithium ion secondary battery can be improved.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0013] Hereinafter, embodiments according to the present invention will be described. Note that matters other than those specifically mentioned in this specification and necessary for the implementation of the present invention can be understood as design matters of those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the content disclosed in this specification and common general technical knowledge in the relevant field. In this specification, when a numerical range is described as A to B (where A and B are arbitrary numerical values), it has the same general interpretation and means A or more and B or less.

[0014] In this specification, the term "secondary battery" generally refers to a rechargeable power storage device, and is a term that includes so-called storage batteries and power storage elements such as electric double layer capacitors. Further, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and realizes charge and discharge by the movement of charges associated with lithium ions between the positive and negative electrodes.

[0015] The negative electrode for a lithium ion battery according to this embodiment includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode active material layer contains a Si-based negative electrode active material that has Si as a constituent element as the negative electrode active material and can reversibly occlude and release lithium ions. Further, a high molecular weight organic compound that can improve the durability of the lithium ion secondary battery, which will be described in detail later, is added to the negative electrode active material layer. In this specification, durability means long-life performance that can withstand a decrease in battery capacity due to charge and discharge of the lithium ion secondary battery.

[0016] As the negative electrode current collector, a foil-like body made of a metal with good conductivity (for example, copper, nickel, titanium, stainless steel, etc.) can be used, and preferably, a copper foil is used. Examples of the Si-based negative electrode active material contained in the negative electrode active material layer include SiO, Si, etc., which have Si as a constituent element and can occlude and release lithium ions. Note that the negative electrode active material contained in the negative electrode active material layer is not limited to the Si-based negative electrode active material, and may include carbon-based negative electrode active materials such as graphite (natural graphite, artificial graphite), low-crystalline carbon (hard carbon, soft carbon), etc. In addition, as long as the effects of the present invention are not significantly impaired, the negative electrode active material layer may contain components other than the active material, such as a binder and a thickener, in addition to the above-described negative electrode active material and durability improver. As the binder, for example, styrene butadiene rubber (SBR) etc. can be used. As the thickener, for example, carboxymethyl cellulose (CMC) etc. can be used. In the negative electrode active material layer (negative electrode layer), when the solid content of the negative electrode layer is 100% by mass, the blending amount of the above negative electrode active material is usually 50 to 99.8% by mass, preferably 80 to 99% by mass in terms of solid content, and the blending amount of the above binder is usually 0.05 to 10% by mass, preferably 0.1 to 5.0% by mass in terms of solid content, and the blending amount of the above thickener is usually 0.05 to 10% by mass, preferably 0.1 to 5.0% by mass in terms of solid content, which is preferable.

[0017] The negative electrode for a lithium ion battery according to the present embodiment can be produced by a known method. For example, a negative electrode composite paste is prepared by mixing a negative electrode active material, a binder, a thickener, and a solvent, and then mixing a durability improver to be described in detail later. Further, the negative electrode can be produced by applying the negative electrode composite paste to a negative electrode current collector and drying it. Mixing, coating, and drying can be performed by known methods. As the solvent, an aqueous solvent is preferably used. The aqueous solvent only needs to show water-based properties as a whole, and water or a mixed solvent mainly composed of water can be preferably used. In this specification, "paste" is used as a term that also includes forms called "slurry" and "ink".

[0018] The addition amount of the high molecular weight organic compound in the negative electrode according to the present embodiment is not particularly limited as long as the effects of the present invention are exhibited. If the addition amount is too low, it becomes difficult to obtain the effects of the present invention. Therefore, when the solid content of the negative electrode composite paste before mixing the durability improver is 100% by mass, the addition amount is preferably 0.01% to 10% by mass, for example, 0.1% to 5% by mass, and for example, 0.6% to 1.5% by mass.

[0019] In this specification, when the high molecular weight organic compound (resin) contains the monomer X as its raw material, unless otherwise specified, it means that the high molecular weight organic compound (resin) is a (co)polymer of the raw material monomers containing the monomer X. Further, in this specification, the (co)polymer means a polymer or a copolymer. Further, in this specification, “(meth)acrylate” means acrylate and / or methacrylate, “(meth)acrylic acid” means acrylic acid and / or methacrylic acid. Also, “(meth)acryloyl” means acryloyl and / or methacryloyl. Also, “(meth)acrylamide” means acrylamide and / or methacrylamide.

[0020] <High molecular weight organic compound> As the durability improver added to the negative electrode active material layer, a high molecular weight organic compound having a weight average molecular weight of 1,000 or more is usually used. From the viewpoint of the battery capacity retention rate, the weight average molecular weight of the high molecular weight organic compound is preferably 1,000 to 100,000, more preferably 2,000 to 50,000, and even more preferably 3,000 to 30,000. The number-average molecular weight and weight-average molecular weight are values obtained by converting the retention time (retention volume) measured using gel permeation chromatography (GPC) into the molecular weight of polystyrene based on the retention time (retention volume) of a standard polystyrene with a known molecular weight measured under the same conditions. Specifically, as the gel permeation chromatograph, "HLC8120GPC" (trade name, manufactured by Tosoh Corporation) is used, and as the columns, four columns of "TSKgel G-4000HXL", "TSKgel G-3000HXL", "TSKgel G-2500HXL", and "TSKgel G-2000HXL" (trade names, all manufactured by Tosoh Corporation) are used, and the measurement can be carried out under the conditions of a mobile phase of tetrahydrofuran, a measurement temperature of 40 °C, a flow rate of 1 mL / min, and a detector RI.

[0021] The type of the above high molecular weight organic compound is not particularly limited. Specifically, for example, acrylic resin, polyester resin, epoxy resin, polyether resin, alkyd resin, urethane resin, silicone resin, polycarbonate resin, silicate resin, chlorine-based resin, fluorine-based resin, polyvinyl alcohol, polyvinyl acetal, polyvinyl pyrrolidone, and composite resins thereof can be mentioned, and one kind can be used alone or two or more kinds can be used in combination. Among them, from the viewpoints of battery capacity retention rate, stability in the negative electrode composite paste, adsorptivity to the negative electrode active material, etc., it is preferable that the high molecular weight organic compound has a polar functional group, and it is more preferable that the polar functional group is at least one polar functional group selected from the group consisting of an amino group, a sulfonic acid group, a carboxyl group, a phosphoric acid group, a polyalkylene ether group, an amide group, a hydroxyl group, an epoxy group, and an alkoxysilyl group.

[0022] From the viewpoint of the battery capacity retention rate, the concentration of the polar functional group in the above high molecular weight organic compound is usually 0.1 mmol / g or more, preferably 1 to 30 mmol / g, more preferably 2 to 25 mmol / g, and even more preferably 5 to 22 mmol / g. In particular, the concentration of ionic polar functional groups is usually 0.1 mmol / g or more, preferably 0.2 to 25 mmol / g, more preferably 0.3 to 10 mmol / g, which is preferable from the viewpoint of the battery capacity retention rate.

[0023] In the present specification, the concentration of polar functional groups is calculated with one polar functional group. For example, when two polar functional groups are present in one polymerizable unsaturated monomer, it is calculated as two.

[0024] Further, the high molecular weight organic compound is preferably a hydrophilic (highly polar) compound having a polar functional group and is preferably soluble in water. In the present specification, "soluble in water" means that when it is mixed with water to form a 5% aqueous solution, it is not in an emulsified state but in a dissolved or semi-dissolved state.

[0025] Among them, from the viewpoints of battery capacity retention rate, stability in the negative electrode composite material paste, adsorptivity to the negative electrode active material, etc., the high molecular weight organic compound preferably contains a copolymer compound obtained by copolymerizing a polymerizable unsaturated monomer.

[0026] <Copolymer compound> As the polymerizable unsaturated monomer used as a raw material for the copolymer compound, any monomer having a polymerizable unsaturated group capable of radical polymerization can be used without particular limitation. Examples of the polymerizable unsaturated group include (meth)acryloyl group, (meth)acrylamide group, vinyl group, allyl group, (meth)acryloyloxy group, vinyl ether group, and the like. Among them, from the viewpoints of battery capacity retention rate, stability in the negative electrode composite material paste, adsorptivity to the negative electrode active material, etc., it is preferable that the copolymer compound contains a copolymer having a polymerizable unsaturated monomer having a polar functional group as a constituent component. Further, it is more preferable that the polar functional group is at least one polar functional group selected from the group consisting of an amino group, a sulfonic acid group, a carboxyl group, a phosphoric acid group, a polyalkylene ether group, an amide group, and a hydroxyl group.

[0027] <Polymerizable unsaturated monomer having a polar functional group> Examples of the polymerizable unsaturated monomer having the polar functional group include monoesterified products of (meth)acrylic acid and a divalent alcohol having 2 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, ε-caprolactone-modified products of the monoesterified products of (meth)acrylic acid and a divalent alcohol having 2 to 8 carbon atoms, N-hydroxymethyl (meth)acrylamide, allyl alcohol, and (meth)acrylates having a polyoxyalkylene chain with a hydroxyl group at the molecular end; carboxyl group-containing polymerizable unsaturated monomers such as (meth)acrylic acid, maleic acid, crotonic acid, and β-carboxyethyl acrylate; polymerizable unsaturated monomers having an amino group and / or an amide group, such as (meth)acrylamide, N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, and adducts of glycidyl (meth)acrylate and amines; polymerizable unsaturated monomers having a urethane bond, such as reaction products of isocyanate group-containing polymerizable unsaturated monomers and hydroxyl group-containing compounds or reaction products of hydroxyl group-containing polymerizable unsaturated monomers and isocyanate group-containing compounds; epoxy group-containing polymerizable unsaturated monomers such as glycidyl (meth)acrylate, β-methylglycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, 3,4-epoxycyclohexylethyl (meth)acrylate, 3,4-epoxycyclohexylpropyl (meth)acrylate, and allyl glycidyl ether; (meth)acrylates having a polyoxyethylene chain with an alkoxy group at the molecular end; polymerizable unsaturated monomers having a sulfonic acid group, such as 2-acrylamido-2-methylpropanesulfonic acid, 2-sulfoethyl (meth)acrylate, allyl sulfonic acid, 4-styrenesulfonic acid, and sodium salts and ammonium salts of these sulfonic acids;Polymerizable unsaturated monomers having a phosphate group such as 2-acryloyloxyethyl acid phosphate, 2-methacryloyloxyethyl acid phosphate, 2-acryloyloxypropyl acid phosphate, 2-methacryloyloxypropyl acid phosphate; Polymerizable unsaturated monomers having an alkoxysilyl group such as vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, γ-(meth)acryloyloxypropyltrimethoxysilane, γ-(meth)acryloyloxypropyltriethoxysilane; Polymerizable unsaturated monomers having a polyalkylene ether group represented by the following formula (1) such as polyethylene glycol (meth)acrylate, polypropylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, ethoxypolyethylene glycol (meth)acrylate, etc. are exemplified.; CH2=C(R1)COO(C n H 2n O) m -R2···Formula (1) [In the formula, R1 represents a hydrogen atom or CH3, R2 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, m is an integer of 4 to 60, particularly 4 to 55, n is an integer of 2 to 3, and here, m oxyalkylene units (C n H 2n O) may be the same or different from each other.] The above polymerizable unsaturated monomers can be used alone or in combination of two or more. From the viewpoint of battery capacity retention rate, polymerizable unsaturated monomers having an ionic functional group and / or a polyalkylene ether group are preferred, and polymerizable unsaturated monomers having an ionic functional group are more preferred.

[0028] <Other polymerizable unsaturated monomers> Examples of the polymerizable unsaturated monomer other than the polymerizable unsaturated monomer having the above polar functional group include alkyl (meth) acrylates having 3 or less carbon atoms such as methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, and isopropyl (meth) acrylate; n-butyl (meth) acrylate, i-butyl (meth) acrylate, t-butyl (meth) acrylate, n-hexyl (meth) acrylate, octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, nonyl (meth) acrylate, tridecyl (meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate, isostearyl (meth) acrylate, cyclohexyl (meth) acrylate, methylcyclohexyl (meth) acrylate, t-butylcyclohexyl (meth) acrylate, cyclododecyl (meth) acrylate, and tricyclodecanyl (meth) acrylate; polymerizable unsaturated compounds having an isobornyl group such as isobornyl (meth) acrylate; polymerizable unsaturated compounds having an adamantyl group such as adamantyl (meth) acrylate; aromatic ring-containing polymerizable unsaturated monomers such as benzyl (meth) acrylate, styrene, α-methylstyrene, and vinyltoluene;Polymerizable unsaturated monomers having two or more polymerizable unsaturated groups in one molecule, such as allyl (meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, 1,4-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tetra(meth)acrylate, glycerol di(meth)acrylate, 1,1,1-trishydroxymethylethane di(meth)acrylate, 1,1,1-trishydroxymethylethane tri(meth)acrylate, 1,1,1-trishydroxymethylpropane tri(meth)acrylate, triallyl isocyanurate, diallyl terephthalate, divinylbenzene, etc. These can be used alone or in combination of two or more.;

[0029] <Polymerization method> As the polymerization method of the copolymer compound, a conventionally known method can be used. For example, it can be produced by solution polymerization of the polymerizable unsaturated monomer in an organic solvent, but it is not limited thereto, and for example, bulk polymerization, emulsion polymerization, suspension polymerization, etc. may also be used. When performing solution polymerization, continuous polymerization or batch polymerization may be used, and the polymerizable unsaturated monomer may be charged all at once, charged in portions, or added continuously or intermittently.;

[0030] The radical polymerization initiator used in the polymerization can be any of the conventionally known methods. For example, cyclohexanone peroxide, 3,3,5-trimethylcyclohexanone peroxide, methylcyclohexanone peroxide, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(tert-butylperoxy)cyclohexane, n-butyl-4,4-bis(tert-butylperoxy)valerate, cumene hydroperoxide, 2,5-dimethylhexane-2,5-dihydroperoxide, 1,3-bis(tert-butylperoxy-m-isopropyl)benzene, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, diisopropylbenzene peroxide, tert-butylcumyl peroxide, decanoyl peroxide, lauroyl peroxide, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, peroxide-based polymerization initiators such as di-tert-amyl peroxide, bis(tert-butylcyclohexyl)peroxydicarbonate, tert-butylperoxybenzoate, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, and tert-butylperoxy-2-ethylhexanoate; 2,2'-azobis(isobutyronitrile), 1,1-azobis(cyclohexane-1-carbonitrile) Examples of the azo-based polymerization initiator include azocumene, 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobisdimethylvaleronitrile, 4,4'-azobis(4-cyanovaleric acid), 2-(t-butylazo)-2-cyanopropane, 2,2'-azobis(2,4,4-trimethylpentane), 2,2'-azobis(2-methylpropane), and dimethyl 2,2'-azobis(2-methylpropionate). These can be used alone or in combination of two or more.

[0031] The solvent used for the above polymerization or dilution is not particularly limited, and examples thereof include water, organic solvents, or mixtures thereof. Examples of organic solvents include hydrocarbon solvents such as n-butane, n-hexane, n-heptane, n-octane, cyclopentane, cyclohexane, and cyclobutane; aromatic solvents such as toluene and xylene; ketone solvents such as methyl isobutyl ketone; ether solvents such as n-butyl ether, dioxane, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, and diethylene glycol; ester solvents such as ethyl acetate, n-butyl acetate, isobutyl acetate, ethylene glycol monomethyl ether acetate, and butyl carbitol acetate; ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone, and diisobutyl ketone; alcohol solvents such as ethanol, isopropanol, n-butanol, sec-butanol, and isobutanol; and amide solvents such as Equamide (trade name, manufactured by Idemitsu Kosan Co., Ltd.), N,N-dimethylformamide, N,N-dimethylacetamide, N-methylformamide, N-methylacetamide, N-methylpropionamide, and N-methyl-2-pyrrolidone, and other conventionally known solvents can be mentioned. Among them, it is preferably free of water and preferably contains at least one carbonate solvent selected from diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, propylene carbonate, and ethylene carbonate. These can be used alone or in combination of two or more.

[0032] In solution polymerization in an organic solvent, a method of mixing a polymerization initiator, a polymerizable unsaturated monomer component, and an organic solvent and heating while stirring, a method of charging the organic solvent into a reaction tank to suppress the temperature rise of the system due to the heat of reaction, and stirring at a temperature of 60°C to 200°C while blowing an inert gas such as nitrogen or argon as necessary, and mixing and dropping or separately dropping the polymerizable unsaturated monomer component and the polymerization initiator over a predetermined time are used. The polymerization can generally be carried out for about 1 to 10 hours. An additional catalyst step of heating the reaction vessel while dropping a polymerization initiator as needed after the polymerization at each stage may be provided.

[0033] As the copolymer compound, particularly from the viewpoints of adsorptivity and stability to the Si-based negative electrode active material, a copolymer compound having a graft structure or a block structure divided into two segments of an adsorption part and a steric repulsion part is preferable, and a graft structure (comb structure) is particularly preferable. From the viewpoint of compatibility with the electrolytic solution, it is preferable that the graft structure (comb structure) has an ionic functional group in the adsorption part which is the main chain and a hydrophilic functional group in the steric repulsion part which is the side chain.

[0034] As the hydrophilic functional group of the side chain, an ionic functional group, a nonionic functional group, etc. can be preferably used, and among them, it is preferable to contain at least one nonionic functional group. The weight average molecular weight of the steric repulsion part of the side chain is preferably 200 to 30,000, more preferably 300 to 10,000, and even more preferably 400 to 10,000. The mass ratio of the main chain to the side chain is preferably 1 / 99 to 99 / 1, more preferably 5 / 95 to 95 / 5, and even more preferably 5 / 95 to 50 / 50.

[0035] As a method for introducing a side chain of a steric repulsion part into the copolymer compound, a method known per se can be preferably used. Specifically, for example, a method of copolymerizing a polymerizable unsaturated group-containing macromonomer which is a side chain and other polymerizable unsaturated group-containing monomers by the polymerization method described above, a method of adding a side chain compound after copolymerizing polymerizable unsaturated group-containing monomers, etc. can be mentioned, and any of them can be preferably used.

[0036] Regarding the above-mentioned macromonomer containing a polymerizable unsaturated group, it can be produced by a method known per se. For example, in Japanese Patent Publication No. 43-11224, in the process of producing a macromonomer, a carboxylic acid group is introduced into the polymer chain end using a chain transfer agent such as mercaptopropionic acid, and then glycidyl methacrylate is added to introduce an ethylenically unsaturated group to obtain a macromonomer. Also, a method by catalytic chain transfer polymerization (CCTP) using a cobalt complex is disclosed in Japanese Patent Publication No. 6-23209 and Japanese Patent Publication No. 7-35411. Further, Japanese Patent Application Laid-Open No. 7-002954 describes a method of radical polymerization of methacrylic acid using 2,4-diphenyl-4-methyl-1-pentene as an addition-cleavage type chain transfer agent to obtain a macromonomer.

[0037] FIG. 1 schematically shows an example of the internal structure of a lithium ion secondary battery 100 using a negative electrode according to an embodiment. The lithium ion battery 100 is a sealed battery constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte 80 in a rectangular battery case 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. Further, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. As the material of the battery case 30, for example, a lightweight and highly thermally conductive metal material such as aluminum is used.

[0038] FIG. 2 schematically shows the configuration of the electrode body 20 of the lithium ion secondary battery 100 using a negative electrode according to an embodiment. The wound electrode body 20 has a form in which a sheet-like positive electrode 50 with a positive electrode active material layer 54 formed along the longitudinal direction on one or both sides of a long positive electrode current collector 52 and a sheet-like negative electrode 60 with a negative electrode active material layer 64 formed along the longitudinal direction on one or both sides of a long negative electrode current collector 62 are overlapped via two long separators 70 and wound in the longitudinal direction. Note that a positive electrode current collector 42a and a negative electrode current collector 44a are joined to a positive electrode active material non-formation part 52a and a negative electrode active material non-formation part 62a, respectively, which are formed so as to protrude outward from both ends in the winding axis direction of the wound electrode body 20. In the implementation of the present invention, it is not necessary to limit the electrode body to a wound type as illustrated. For example, a lithium-ion secondary battery including a laminated type electrode body formed by laminating a plurality of sheet-like positive electrodes and negative electrodes via a separator may be used. Also, as is clear from the technical information disclosed in this specification, the shape of the battery is not limited to a rectangular shape.

[0039] The negative electrode 60 uses the negative electrode for a lithium-ion secondary battery according to the above-described embodiment. As the positive electrode 50, those used in conventional lithium-ion secondary batteries can be used without particular limitation. A typical aspect of the positive electrode 50 is shown below.

[0040] Examples of the positive electrode current collector 52 constituting the positive electrode 50 include aluminum foil. Examples of the positive electrode active material included in the positive electrode active material layer 54 include lithium transition metal oxides (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi 0.5 Mn 1.5 O4, etc.), lithium transition metal phosphate compounds (e.g., LiFePO4, etc.). The positive electrode active material layer 54 may contain components other than the active material, such as a conductive material and a binder. As the conductive material, carbon black such as acetylene black (AB) and other carbon materials (e.g., graphite) can be preferably used. As the binder, polyvinylidene fluoride (PVdF) can be used, for example.

[0041] As the separator 70, various microporous sheets similar to those conventionally used in lithium ion secondary batteries can be used. For example, microporous resin sheets made of resins such as polyethylene (PE) and polypropylene (PP) can be mentioned. Such a microporous resin sheet may have a single-layer structure or a multi-layer structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). The separator 70 may be provided with a heat-resistant layer (HRL).

[0042] The non-aqueous electrolyte 80 may contain a non-aqueous solvent and a supporting salt. As the non-aqueous solvent, various organic solvents such as carbonates, ethers, esters, nitriles, sulfones, lactones, etc. used in the electrolytes of general lithium ion secondary batteries can be used without particular limitation. Specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC), etc. Such non-aqueous solvents can be used alone or in appropriate combinations of two or more. As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, LiClO4 (preferably LiPF6) can be preferably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less. The non-aqueous electrolyte 80 may contain components other than the above-described components, for example, gas generators such as biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; and various other additives, as long as the effects as a non-aqueous electrolyte are not significantly impaired.

[0043] The descriptions of the structure, construction materials, etc. of the lithium-ion secondary battery described above are general and do not particularly characterize the present invention. Therefore, further detailed descriptions and illustrations are omitted. A person skilled in the art can easily construct lithium-ion secondary batteries and other secondary batteries in various forms and sizes by adopting conventional materials and manufacturing processes, except for adding the durability improver disclosed herein to the negative electrode active material.

[0044] The lithium-ion secondary battery 100 configured as described above can be used for various applications. Some suitable applications include power sources for driving mounted on vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHVs). The lithium-ion secondary battery 100 can typically be used in the form of a battery pack formed by connecting a plurality of them in series and / or in parallel.

[0045] As an example, a rectangular lithium-ion secondary battery 100 having a flat wound electrode body 20 has been described. However, the lithium-ion secondary battery can also be configured as a lithium-ion secondary battery having a laminated electrode body. Further, the lithium-ion secondary battery can also be configured as a cylindrical lithium-ion secondary battery, a laminated lithium-ion secondary battery, or the like.

[0046] Hereinafter, the present invention will be further described by way of examples. The synthesis methods of various compounds, the manufacturing methods of secondary batteries, the evaluation test methods, etc. use methods conventionally known in the art. However, the present invention is not limited thereto, and various modifications and variations are possible within the equivalent scope of the technical idea of the present invention and the claims. In addition, "parts" in each example indicates parts by mass, and "%" indicates mass %.

[0047] <Manufacture of Macromonomer> (Macromonomer 1) Into a reaction vessel equipped with a thermometer, a cooling tube, a nitrogen gas introduction tube, a stirrer, and a dropping device, 16 parts of ethylene glycol monobutyl ether and 9.15 parts of 2,4-diphenyl-4-methyl-1-pentene were charged, and the mixture was stirred while blowing nitrogen at 160 °C. Next, a mixed solution consisting of 100 parts of methacrylamide and 7 parts of ditertiary amyl peroxide was dropped into this over 3 hours, and the mixture was stirred for 2 hours as it was. Then, it was cooled to 30 °C and diluted with diethyl carbonate to obtain a hydrophilic polymerizable unsaturated group-containing macromonomer (macromonomer 1) solution with a solid content of 60%. The weight average molecular weight of the obtained macromonomer 1 was 2,000, and the polar functional group concentration was 11.8 mmol / g.

[0048] <Production of High Molecular Weight Organic Compounds> (High Molecular Weight Organic Compound No. 4) 40 parts of diethyl carbonate was charged into a reaction vessel equipped with a thermometer, a cooling tube, a nitrogen gas introduction tube, a stirrer, and a dropping device. After nitrogen substitution, the temperature was maintained at 120 °C. Into this, the following monomer mixture was dropped over 4 hours. (Monomer Mixture) 25 parts of methyl methacrylate 25 parts of n-butyl acrylate 50 parts of 2-hydroxyethyl acrylate 9 parts of t-butyl peroxy-2-ethylhexanoate (polymerization initiator) One hour after the completion of dropping, a solution prepared by dissolving 0.5 part of t-butyl peroxy-2-ethylhexanoate in 10 parts of diethyl carbonate was dropped into this over 1 hour. After the completion of dropping, this was further maintained at 120 °C for 1 hour. Then, diethyl carbonate was added to make the solid content 50% to obtain a solution of high molecular weight organic compound No. 4 with a solid content of 50%. The high molecular weight organic compound No. 4 had a weight average molecular weight of 4,000 and a polar functional group concentration of 4.3 mmol / g.

[0049] (High Molecular Weight Organic Compounds No. 5 - 15) A solution of high molecular weight organic compounds Nos. 5 to 15 was produced in the same manner as high molecular weight organic compound No. 4, except that the monomer composition and polymerization initiator were as shown in Table 1 below. The weight average molecular weight, polar functional group concentration (mmol / g), and ionic polar functional group concentration (mmol / g) of each resin are described in Table 1 below.

[0050]

Table 1

[0051] <Manufacture of Non-aqueous Electrolyte Lithium Ion Secondary Battery> <Manufacture of Negative Electrode> (Example 1) A mixed powder obtained by mixing graphite (average particle diameter: 20 μm) and SiO (average particle diameter: 15 μm) as negative electrode active materials at a ratio of graphite:SiO = 95:5 (mass ratio), styrene-butadiene copolymer (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed at a ratio of mixed powder:SBR:CMC = 98:1:1 (mass ratio) using water as a dispersion solvent to prepare a paste. Next, polyethylene glycol (molecular weight: 2,000, functional group concentration: 22.7 mmol / g, solid content: 100%) as high molecular weight organic compound No. 1 was mixed at 1% by mass with respect to the solid content of the paste. The above slurry was applied onto a copper foil to obtain a negative electrode.

[0052] <Manufacture of Positive Electrode> Positive electrode active material (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2): A paste obtained by mixing conductive assistant (acetylene black): binder (PVdF) at a ratio of 87:10:3 (mass ratio) was applied onto an aluminum foil to manufacture a positive electrode.

[0053] <Manufacture of Electrolyte> LiPF6 as an electrolyte was dissolved at a ratio of 1.0 mol / L in a solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 50:50 to obtain a non-aqueous electrolyte.

[0054] <Manufacture of laminated battery> Using the above negative electrode and positive electrode, an electrode body was formed by opposing them through a polypropylene / polyethylene / polypropylene three-layer porous film having an air permeability of 300 seconds obtained by the Gurley test method, and a secondary battery (Example 1) was manufactured by sealing it with a laminate together with an electrolytic solution.

[0055] (Examples 2 to 19) Secondary batteries (Examples 2 to 19) were manufactured in the same manner as in Example 1, except that the high molecular weight organic compound mixed in the paste was as described in Table 2 below. In addition, the results of the evaluation tests described later are presented. In this application, if there is even one "× (failed)" evaluation result in the evaluation, the secondary battery is considered to have failed.

[0056] <Evaluation test> <Activation> In a 25°C constant temperature bath, the first charge was carried out in a constant current mode, charging up to 4.10 V at a current value of 0.3C, and then discharging up to 3.00 V at a current value of 0.3C by the constant current mode. This was repeated three times.

[0057] <Initial capacity> Charging was carried out up to 4.10 V at a current value of 0.2C in a constant current - constant voltage mode, and low voltage charging was carried out until the current value during constant voltage charging reached 1 / 50C to obtain a fully charged state. Then, the capacity when discharging up to 3.00 V at a current value of 0.2C by the constant current mode was taken as the initial capacity.

[0058] <Capacity retention rate (25°C)> In a 25°C constant temperature bath, 500 - cycle charge - discharge was repeated at a current value of 0.5C. The charge set value was 4.10 V and the discharge set value was 3.00 V. Also, a 10 - minute rest time was provided after each charge and discharge. Then, the capacity after the cycle test was measured in the same way, and the capacity retention rate was determined by the following formula. Capacity retention rate (%) = (Battery capacity after 500 cycles / Initial capacity) × 100 The evaluation is as follows. ◎: The capacity retention rate is 97% or more and 100% or less. 〇: The capacity retention rate is 94% or more and less than 97%. △: The capacity retention rate is 91% or more and less than 94%. ×: The capacity retention rate is less than 91%.

[0059] <Capacity retention rate (60 °C)> The capacity retention rate was measured in a constant temperature bath at 60 °C. Note that it was carried out in the same manner except that the temperature of the constant temperature bath was changed from 25 °C to 60 °C.

[0060]

Table 2

[0061] From the comparison between Examples 1 to 17 and Examples 18 and 19, it can be seen that by including the high molecular weight organic compound disclosed herein in the Si-based negative electrode of the lithium ion secondary battery, a decrease in the capacity retention rate of the lithium ion secondary battery can be suppressed.

[0062] As described above, specific examples of the present invention have been described in detail, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples exemplified above.

Explanation of reference numerals

[0063] 20 Electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive current collector 44 Negative electrode terminal 44a Negative current collector 50 Positive electrode 52 Positive current collector 52a Non-formation part of positive electrode active material layer 54 Positive electrode active material layer 60 Negative electrode 62 Negative current collector 62a Non-formation part of negative electrode active material layer 64 Negative electrode composite layer 70 Separator 80 Non-aqueous electrolyte 100 Lithium-ion secondary battery

Claims

1. A negative electrode used in a lithium-ion secondary battery, comprising a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer comprises a Si-based negative electrode active material capable of reversibly occluding and releasing lithium ions with Si as a constituent element as a negative electrode active material, a binder, and a high molecular weight organic compound having a weight average molecular weight of 1,000 or more, which can improve the durability of the lithium-ion secondary battery. The negative electrode contains these components. The high molecular weight organic compound has a graft structure having a nonionic functional group with a weight average molecular weight of 200 or more and 30,000 or less in a side chain. Negative electrode.

2. The high molecular weight organic compound has a polar functional group, and the polar functional group is at least one polar functional group selected from the group consisting of an amino group, a sulfonic acid group, a carboxyl group, a phosphoric acid group, a polyalkylene ether group, an amide group, a hydroxyl group, an epoxy group, and an alkoxysilyl group. The negative electrode according to claim 1, wherein the concentration of the polar functional group is 0.1 mmol / g or more.

3. The high molecular weight organic compound has an ionic functional group, and the ionic functional group is at least one ionic functional group selected from the group consisting of an amino group, a sulfonic acid group, a carboxyl group, a phosphoric acid group, and an amide group. The negative electrode according to claim 1 or 2, wherein the concentration of the ionic functional group is 0.1 mmol / g or more.

4. The negative electrode according to any one of claims 1 to 3, wherein the high molecular weight organic compound includes a copolymer compound obtained by copolymerizing a polymerizable unsaturated monomer.

5. A lithium-ion secondary battery comprising the negative electrode according to any one of claims 1 to 4. ​ ​ ​

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