Nonaqueous electrolyte for lithium ion secondary battery and lithium ion secondary battery

Abstract

To provide a nonaqueous electrolyte solution for a lithium ion secondary battery, which can protect the surface of a negative electrode active material to increase the battery life.SOLUTION: A nonaqueous electrolyte solution is arranged for use in a lithium ion secondary battery. The nonaqueous electrolyte solution comprises: a nonaqueous solvent; an electrolyte dissolved in the nonaqueous solvent; and a high-molecular weight organic compound of 1,000 or larger in weight average molecular weight. It is preferred that the high-molecular weight organic compound is included at 0.01-10 mass% to 100 mass% of the nonaqueous electrolyte solution.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a nonaqueous electrolyte for a lithium ion secondary battery and a lithium ion secondary battery comprising the nonaqueous electrolyte. [Background technology]

[0002] Lithium-ion secondary batteries are lightweight and have high energy density, so they are widely used as portable power sources for personal computers and mobile devices, as well as power sources for vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHVs). In recent years, in order to further increase the capacity of lithium-ion secondary batteries, the use of silicon-based negative electrode materials as the negative electrode active material has been considered. It is known that silicon-based materials have a theoretical capacity density five times higher than that of graphite, which is widely used as a negative electrode active material, and studies are underway to apply them 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 a property of changing its volume significantly during charging and discharging. This property may cause cracks or fissures in the Si-based negative electrode active material, which may cause it to be isolated from the current collecting network and reduce the battery life. In addition, this property may cause cracks or peeling in the solid electrolyte interphase (SEI) formed on the surface of the negative electrode active material, which may cause lithium ions in the electrolyte to be absorbed in order to re-form the SEI, thereby causing deterioration of the non-aqueous electrolyte. This is a disadvantage. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2014-002972 A [Patent Document 2] JP 2008-071559 A [Patent Document 3] JP 2007-027110 A [Patent Document 4] Special Publication No. 2004-525495 Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, Patent Documents 1 to 4 disclose a technique of adding an additive to a non-aqueous electrolyte in order to improve the battery life or to enhance the safety of the battery. In other words, the battery performance can be improved by adding an additive to the conventional components constituting the non-aqueous electrolyte. Therefore, the present inventors have come up with the idea of ​​protecting the Si-based negative electrode active material, which has a large volume change due to charging and discharging, by adding an additive to the non-aqueous electrolyte. If the surface of the Si-based material can be protected by an additive, it is possible to prevent cracks and fissures caused by the volume change of the Si-based material. As a result of intensive research by the present inventors, it has been found that by adding a predetermined high molecular weight organic compound having a weight average molecular weight of 1,000 or more to the non-aqueous electrolyte, the high molecular weight organic compound is adsorbed on the surface of the negative electrode active material, and the capacity deterioration due to the battery durability test (charge and discharge cycle test) can be suppressed.

[0006] Therefore, an object of the present invention is to provide a non-aqueous electrolyte solution for a lithium ion secondary battery that can protect the surface of a negative electrode active material and improve the battery life. Another object of the present invention is to provide a lithium ion secondary battery including the non-aqueous electrolyte solution for a lithium ion secondary battery. [Means for solving the problem]

[0007] The nonaqueous electrolyte solution for lithium ion secondary batteries disclosed herein is a nonaqueous electrolyte solution for use in lithium ion secondary batteries, and is characterized in that it contains a nonaqueous solvent and an electrolyte dissolved in the nonaqueous solvent, and contains a high molecular weight organic compound having a weight average molecular weight of 1,000 or more. According to this configuration, the high molecular weight organic compound contained in the nonaqueous electrolyte solution is adsorbed to and protects the surface of the negative electrode active material in the negative electrode, thereby making it possible to suppress a decrease in the capacity retention rate during charge-discharge cycles.

[0008] In a preferred embodiment of the nonaqueous electrolyte solution for lithium ion secondary batteries disclosed herein, the nonaqueous electrolyte solution for lithium ion secondary batteries contains 0.01% by mass to 10% by mass of the high molecular weight organic compound when the nonaqueous electrolyte solution is taken as 100% by mass. When the non-aqueous electrolyte contains the high molecular weight organic compound in such a ratio, the capacity retention rate during charge / discharge cycles of the lithium ion secondary battery is effectively improved.

[0009] In a preferred embodiment of the non-aqueous electrolyte for a lithium ion secondary battery disclosed herein, the high molecular weight organic compound has a polar functional group, the polar functional group being at least one type of 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 has a concentration of 0.1 mmol / g or more. According to this configuration, the high molecular weight organic compound has increased stability in the non-aqueous electrolyte solution and is easily adsorbed to the negative electrode active material, thereby further improving the battery capacity retention rate.

[0010] In a preferred embodiment of the nonaqueous electrolyte for a lithium ion secondary battery disclosed herein, the high molecular weight organic compound contains a copolymer compound obtained by copolymerizing a polymerizable unsaturated monomer. According to this configuration, the high molecular weight organic compound is more stable in the non-aqueous electrolyte solution and is more easily adsorbed onto the negative electrode active material, thereby further improving the battery capacity retention rate.

[0011] The lithium ion secondary battery disclosed herein comprises an electrode assembly having a negative electrode including a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, a positive electrode including a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and a separator, and the nonaqueous electrolyte solution for lithium ion secondary batteries. According to this configuration, the high molecular weight organic compound is adsorbed onto the surface of the negative electrode active material, making it difficult for the negative electrode active material to crack or break, and thus a lithium ion secondary battery with improved battery life can be provided.

[0012] In a preferred embodiment of the lithium ion secondary battery disclosed herein, the lithium ion secondary battery is characterized in that the negative electrode active material layer contains, as a negative electrode active material, a Si-based negative electrode active material that has Si as a constituent element and is capable of reversibly absorbing and releasing lithium ions. According to this configuration, the high molecular weight organic compound protects the surface of the Si-based negative electrode active material, making it less likely for the Si negative electrode active material to crack or break due to volumetric changes caused by charging and discharging, thereby improving the durability of the lithium ion secondary battery.

[0013] In a preferred embodiment of the lithium ion secondary battery disclosed herein, the lithium ion secondary battery is characterized in that, when the negative electrode active material layer is taken as 100 mass %, the lithium ion secondary battery contains 0.01 mass % to 20 mass % of the Si-based negative electrode active material and 50 mass % or more of a carbon-based negative electrode active material. According to this configuration, it is possible to provide a lithium-ion secondary battery with improved battery capacity while suppressing the risk of reduced battery life caused by cracks or fissures due to volumetric changes in the Si negative electrode active material caused by charging and discharging. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a cross-sectional view that illustrates a schematic internal structure of a lithium-ion secondary battery using a nonaqueous electrolyte according to one embodiment. [Diagram 2] FIG. 1 is a schematic diagram showing the configuration of a wound electrode body of a lithium ion secondary battery using a nonaqueous electrolyte according to one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, the embodiments of the present invention will be described. Note that matters other than those specifically mentioned in this specification and necessary for implementation can be understood as design matters for a person skilled in the art based on the prior art in the field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the field. In this specification, when a numerical range is described as A to B (where A and B are arbitrary numerical values), this is generally interpreted as meaning A or more and B or less.

[0016] In this specification, the term "secondary battery" refers to a general storage device that can be repeatedly charged and discharged, and includes storage batteries and storage elements such as electric double-layer capacitors. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as a charge carrier and realizes charging and discharging by the transfer of charge associated with lithium ions between positive and negative electrodes.

[0017] In this specification, when a high molecular weight organic compound (resin) contains a raw material monomer X, unless otherwise specified to the contrary, it means that the high molecular weight organic compound (resin) is a (co)polymer of raw material monomers including the monomer X. In addition, in this specification, a (co)polymer means a polymer or a copolymer. In addition, in this specification, "(meth)acrylate" means acrylate and / or methacrylate, "(meth)acrylic acid" means acrylic acid and / or methacrylic acid, "(meth)acryloyl" means acryloyl and / or methacryloyl, and "(meth)acrylamide" means acrylamide and / or methacrylamide.

[0018] The nonaqueous electrolyte for lithium ion secondary batteries according to this embodiment includes a nonaqueous solvent and an electrolyte dissolved in the nonaqueous solvent, and also contains the following high molecular weight organic compound having a weight average molecular weight of at least 1,000. In this specification, durability refers to a long life performance that can withstand a decrease in battery capacity due to charging and discharging of the lithium ion secondary battery.

[0019] <High molecular weight organic compounds> The weight average molecular weight of the high molecular weight organic compound that can be used in the present invention is usually 1,000 or more, preferably 1,000 to 100,000, more preferably 2,000 to 50,000, and even more preferably within the range of 3,000 to 30,000, from the viewpoint of battery capacity retention rate. In this specification, the number average molecular weight and the weight average molecular weight are values ​​obtained by converting the retention time (retention volume) measured using a gel permeation chromatograph (GPC) into the molecular weight of polystyrene using the retention time (retention volume) of a standard polystyrene with a known molecular weight measured under the same conditions. Specifically, the gel permeation chromatograph is "HLC8120GPC" (trade name, manufactured by Tosoh Corporation), and four columns, "TSKgel G-4000HXL", "TSKgel G-3000HXL", "TSKgel G-2500HXL" and "TSKgel G-2000HXL" (trade names, all manufactured by Tosoh Corporation), are used, and the measurements can be performed under the conditions of a mobile phase of tetrahydrofuran, a measurement temperature of 40°C, a flow rate of 1mL / min, and a detector RI.

[0020] The type of the high molecular weight organic compound is not particularly limited, but specific examples include acrylic resins, polyester resins, epoxy resins, polyether resins, alkyd resins, urethane resins, silicone resins, polycarbonate resins, silicate resins, chlorine-based resins, fluorine-based resins, polyvinyl alcohol, polyvinyl acetal, polyvinylpyrrolidone, and composite resins thereof. One type may be used alone, or two or more types may be used in combination. In particular, from the viewpoint of maintaining battery capacity (including stability in a non-aqueous electrolyte solution and adsorption to the negative electrode active material), 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 type of 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.

[0021] The polar functional group concentration in the 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, from the viewpoint of battery capacity retention rate. In particular, the concentration of the ionic polar functional group is usually 0.1 mmol / g or more, preferably 0.2 to 25 mmol / g, and more preferably 0.3 to 10 mmol / g, from the viewpoint of maintaining the battery capacity.

[0022] In this specification, the polar functional group concentration is calculated by counting one polar functional group as one. For example, when one polymerizable unsaturated monomer has two polar functional groups, the concentration is calculated as two.

[0023] Moreover, the high molecular weight organic compound is preferably a hydrophilic (highly polar) compound due to a polar functional group, and is preferably soluble in water. In the present invention, "soluble in water" means that when mixed with water to form a 5% aqueous solution, it is not in an emulsified state but is in a dissolved or semi-dissolved state. However, such water solubility indicates a preferred property of the high molecular weight organic compound, and is not intended to mean that it is preferable for the electrolyte of the lithium ion secondary battery in this embodiment to contain water.

[0024] From the viewpoint of maintaining battery capacity (including stability in a non-aqueous electrolyte and adsorption to a negative electrode active material), the high molecular weight organic compound is preferably a copolymer compound obtained by copolymerizing a polymerizable unsaturated monomer.

[0025] <Copolymer compound> The polymerizable unsaturated monomer used as a raw material for the copolymer compound can be any monomer having a polymerizable unsaturated group capable of radical polymerization, and examples of the polymerizable unsaturated group include a (meth)acryloyl group, a (meth)acrylamide group, a vinyl group, an allyl group, a (meth)acryloyloxy group, and a vinyl ether group. Among these, it is preferable that the copolymer compound contains a copolymer having, as a constituent component, a polymerizable unsaturated monomer having a polar functional group.

[0026] <Polymerizable unsaturated monomer having a polar functional group> The polymerizable unsaturated monomer having a 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, and examples thereof include monoesters of (meth)acrylic acid and a dihydric alcohol having 2 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; p) ε-caprolactone modified monoester of acrylic acid and dihydric alcohol having 2 to 8 carbon atoms, N-hydroxymethyl(meth)acrylamide, allyl alcohol, hydroxyl group-containing polymerizable unsaturated monomers such as (meth)acrylate 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, β-carboxyethyl acrylate; (meth)acrylamide, N,N-dimethylaminoethyl(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate Polymerizable unsaturated monomers having an amino group and / or an amide group, such as 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; glycidyl (meth)acrylate, β-methylglycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate epoxy group-containing polymerizable unsaturated monomers such as acrylate, 3,4-epoxycyclohexylethyl (meth)acrylate, 3,4-epoxycyclohexylpropyl (meth)acrylate, and allyl glycidyl ether; (meth)acrylates having a polyoxyethylene chain whose molecular terminal is an alkoxy group; polymerizable unsaturated monomers having a sulfonic acid group such as 2-acrylamido-2-methylpropanesulfonic acid, 2-sulfoethyl (meth)acrylate, allylsulfonic acid, 4-styrenesulfonic acid, and the sodium and ammonium salts of these sulfonic acids;Polymerizable unsaturated monomers having a phosphoric acid group, such as 2-acryloyloxyethyl acid phosphate, 2-methacryloyloxyethyl acid phosphate, 2-acryloyloxypropyl acid phosphate, and 2-methacryloyloxypropyl acid phosphate; polymerizable unsaturated monomers having an alkoxysilyl group, such as vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, γ-(meth)acryloyloxypropyltrimethoxysilane, and 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, and ethoxypolyethylene glycol (meth)acrylate, are exemplified. CH2=C(R1)COO(C n H 2n O) m -R2...Equation (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, and n is an integer of 2 to 3, and 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 the battery capacity maintenance 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.

[0027] <Other polymerizable unsaturated monomers> Examples of the polymerizable unsaturated monomer other than the above-mentioned polymerizable unsaturated monomer having a 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, and stearyl (meth)acrylate. alkyl or cycloalkyl (meth)acrylates such as 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;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 Examples of the polymerizable unsaturated monomers include those having two or more polymerizable unsaturated groups in one molecule, such as 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, and divinylbenzene. These may be used alone or in combination of two or more.

[0028] <Polymerization method> The copolymer compound can be produced by a conventionally known method. For example, the copolymer compound can be produced by solution polymerization of a polymerizable unsaturated monomer in an organic solvent, but the method is not limited thereto, and may be, for example, bulk polymerization, emulsion polymerization, suspension polymerization, etc. When carrying out solution polymerization, the polymerizable unsaturated monomer may be continuously or batchwise charged, and the polymerizable unsaturated monomer may be charged all at once or in portions, or may be added continuously or intermittently.

[0029] 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 azo-based polymerization initiators 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.

[0030] The solvent used in the polymerization or dilution is not particularly limited, and examples thereof include water, organic solvents, and 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; ethyl acetate, n-butyl acetate, isobutyl acetate, and ethylene glycol monomethyl ether. Examples of such solvents include ester-based solvents such as butyl ether acetate and butyl carbitol acetate; ketone-based solvents such as methyl ethyl ketone, methyl isobutyl ketone and diisobutyl ketone; alcohol-based solvents such as ethanol, isopropanol, n-butanol, sec-butanol and isobutanol; and amide-based solvents such as Equamide (trade name, manufactured by Idemitsu Kosan Co., Ltd.), N,N-dimethylformamide, N,N-dimethylacetamide, N-methylformamide, N-methylacetamide, N-methylpropioamide and N-methyl-2-pyrrolidone. Among them, since it is used in an electrolytic solution, it is preferable that it does not contain water and that it 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.

[0031] In solution polymerization in an organic solvent, a method is used in which a polymerization initiator, a polymerizable unsaturated monomer component, and an organic solvent are mixed and heated while stirring, or a method is used in which an organic solvent is charged into a reaction vessel to suppress a rise in temperature of the system due to the heat of reaction, and the polymerizable unsaturated monomer component and the polymerization initiator are mixed or separately dripped over a specified period of time while stirring at a temperature of 60°C to 200°C and blowing in an inert gas such as nitrogen or argon as necessary. The polymerization can generally be carried out for about 1 to 10 hours. After each polymerization stage, an additional catalyst step may be performed in which the reaction vessel is heated while a polymerization initiator is dropped, as necessary.

[0032] As the above-mentioned copolymer compound, in particular from the viewpoint of the adsorption property and stability to the Si-based negative electrode active material, it is preferable that the copolymer compound has a graft structure divided into two segments, an adsorption portion and a steric repulsion portion, or a block structure, and a graft structure (comb structure) is particularly preferable. The above graft structure (comb structure) preferably has an ionic functional group in the adsorption portion, which is the main chain, and a hydrophilic functional group in the steric repulsion portion, which is the side chain, from the viewpoint of compatibility with the electrolyte.

[0033] As the hydrophilic functional group of the side chain, an ionic functional group or a nonionic functional group can be suitably used, and among these, it is preferable that the hydrophilic functional group contains at least one type of nonionic functional group. The weight average molecular weight of the steric repulsive portion of the side chain is preferably 200 to 30,000, more preferably 300 to 10,000, and further preferably 400 to 10,000. The mass ratio of the main chain to the side chain is preferably from 1 / 99 to 99 / 1, more preferably from 5 / 95 to 95 / 5, and further preferably from 5 / 95 to 50 / 50.

[0034] As a method for introducing a side chain of a steric repulsive portion into a copolymer compound, a method known per se can be suitably used. Specific examples include a method of copolymerizing a polymerizable unsaturated group-containing macromonomer, which is a side chain, with another polymerizable unsaturated group-containing monomer by the polymerization method described above, and a method of copolymerizing a polymerizable unsaturated group-containing monomer and then adding a side chain compound, and any of these can be suitably used.

[0035] The polymerizable unsaturated group-containing macromonomer can be produced by a method known per se. For example, JP-B-43-11224 describes a method of obtaining a macromonomer by introducing a carboxylic acid group to a polymer chain end using a chain transfer agent such as mercaptopropionic acid in a step of producing the macromonomer, and then adding glycidyl methacrylate to introduce an ethylenically unsaturated group. In addition, JP-B-6-23209 and JP-B-7-35411 disclose a method of catalytic chain transfer polymerization (CCTP) using a cobalt complex. Furthermore, JP-A-7-002954 describes a method of obtaining a macromonomer by radically polymerizing methacrylic acid using 2,4-diphenyl-4-methyl-1-pentene as an addition-fragmentation type chain transfer agent.

[0036] The amount of the high molecular weight organic compound added in the non-aqueous electrolyte for lithium ion secondary batteries according to this embodiment is not particularly limited as long as the effects of the present invention are exhibited. However, if the amount added is too small, the effects of the present invention are difficult to obtain, so the amount added is typically 0.01% by mass to 10% by mass, preferably 0.1% by mass to 5% by mass, and more preferably 0.6% by mass to 1.5% by mass, when the mass of the entire electrolyte is taken as 100% by mass. By adding the high molecular weight organic compound in such a range, the capacity retention rate of the lithium ion secondary battery can be improved more effectively.

[0037] The nonaqueous electrolyte solution for a lithium ion secondary battery according to this embodiment has a supporting salt (lithium salt) dissolved or dispersed in a nonaqueous solvent. The type of non-aqueous solvent is not particularly limited as long as it can dissolve the high molecular weight organic compound, and carbonates, ethers, esters, nitriles, sulfones, lactones, etc., which are used in the electrolyte of lithium ion secondary batteries, can be used. Among them, carbonates are preferable. Examples of carbonates include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. These can be used alone or in combination of two or more. In the present invention, the term "nonaqueous electrolyte solution" refers to an electrolyte solution that does not contain water. It is preferable that the electrolyte solution contains as little water as possible. However, a very small amount of moisture may be mixed in from the raw materials or the air (during the manufacturing process). In such a case, the amount of moisture contained may be typically 1% by mass or less, preferably 0.5% by mass or less, and more preferably 0.1% by mass or less.

[0038] The type of lithium salt can be appropriately selected from various types used in general lithium ion secondary batteries. For example, LiPF6, LiBF4, LiClO4, LiAsF6, Li(CF3SO2)2N, LiCF3SO3, etc. can be used, and these can be used alone or in combination of two or more. The concentration of such lithium salt is preferably in the range of 0.7 mol / L to 1.3 mol / L.

[0039] The non-aqueous electrolyte for lithium ion secondary batteries according to this embodiment may contain various additives, etc., as long as they do not impair the characteristics of the lithium ion secondary battery. Such additives may be used as film-forming agents, overcharge additives, etc., for one or more purposes of improving the input / output characteristics of the battery, improving cycle characteristics, improving initial charge / discharge efficiency, improving safety, etc. Specific examples of such additives include film-forming agents such as lithium bis(oxalato)borate (LiBOB), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and fluoroethylene carbonate (FEC); overcharge additives made of compounds that can generate gas during overcharge, such as aromatic compounds such as biphenyl (BP) and cyclohexylbenzene (CHB); surfactants; dispersants; thickeners; and antifreeze agents. The concentration of these additives in the entire nonaqueous electrolyte solution varies depending on the type of additive, but examples of such an additive include a film-forming agent that is usually about 0.1 mol / L or less (typically 0.005 mol / L to 0.05 mol / L) and an overcharge additive that is usually about 6 mass% or less (typically 0.5 mass% to 4 mass%).

[0040] The non-aqueous electrolyte for lithium ion secondary batteries according to the present embodiment can be used in lithium ion secondary batteries according to a known method. In the lithium ion secondary battery, the high molecular weight organic compound contained in the non-aqueous electrolyte for lithium ion secondary batteries is adsorbed onto the surface of the negative electrode active material to protect the negative electrode active material, thereby suppressing a decrease in the capacity retention rate.

[0041] An outline of a configuration example of a lithium ion secondary battery including a nonaqueous electrolyte for lithium ion secondary batteries according to this embodiment will be described below with reference to the drawings. In the following drawings, members and parts having the same functions are denoted by the same reference numerals. Moreover, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships.

[0042] The lithium ion secondary battery 100 shown in FIG. 1 is a sealed battery constructed by housing a flat wound electrode body 20 and an electrolyte 80 in a flat rectangular battery case (i.e., an outer container) 30. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin-walled safety valve 36 that is set to release the internal pressure when the internal pressure of the battery case 30 rises to a predetermined level or higher. The battery case 30 is also provided with an injection port (not shown) for injecting the electrolyte 80. The positive terminal 42 is electrically connected to the positive current collector plate 42a. The negative terminal 44 is electrically connected to the negative current collector plate 44a. The material of the battery case 30 is, for example, a lightweight metal material with good thermal conductivity, such as aluminum.

[0043] As shown in FIGS. 1 and 2, the wound electrode body 20 has a form in which a sheet-like positive electrode 50 in which a positive electrode active material layer 54 is 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 in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides of a long negative electrode current collector 62, are stacked together with two long, sheet-like separators 70 interposed therebetween and wound in the longitudinal direction. In addition, a positive electrode active material layer-free portion 52a (i.e., a portion where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed) and a negative electrode active material layer-free portion 62a (i.e., a portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed) are formed so as to protrude outward from both ends in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the wound electrode body 20, and are joined to a positive electrode current collector 42a and a negative electrode current collector 44a, respectively.

[0044] The positive electrode current collector 52 constituting the positive electrode 50 may be, for example, an aluminum foil. The positive electrode active material contained in the positive electrode active material layer 54 may be, for example, a lithium transition metal oxide (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi 0.5 Mn 1.5O4, etc.), lithium transition metal phosphate compounds (e.g., LiFePO4, etc.), etc.

[0045] 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, for example, carbon black such as acetylene black (AB) or other carbon materials (e.g., graphite, etc.) may be suitably used. As the binder, for example, polyvinylidene fluoride (PVdF) may be used.

[0046] The negative electrode current collector 62 constituting the negative electrode 60 may be, for example, a copper foil. The negative electrode active material contained in the negative electrode active material layer 64 may be, for example, a carbon-based material such as graphite, hard carbon, or soft carbon; lithium titanate (Li4Ti5O 12 : LTO); Sn; Si-based materials, etc. can be used. From the viewpoint of increasing the capacity of the lithium ion secondary battery 100, it is preferable to contain a Si-based negative electrode active material, which has Si as a constituent element and can reversibly absorb and release lithium ions, in the negative electrode. As the Si-based negative electrode active material, for example, SiO, Si, etc. can be used.

[0047] In addition, the components of the negative electrode active material can be used alone or in combination of two or more. From the viewpoint of increasing the capacity of the lithium ion secondary battery 100 and suppressing the decrease in the capacity retention rate, for example, a negative electrode active material containing a Si-based material and a carbon-based material can be used. As the proportion constituting the negative electrode active material, for example, when the negative electrode active material layer is 100 mass%, the Si-based material can be contained at a ratio of 0.01 mass% to 20 mass% and the carbon-based material can be contained at a ratio of 50 mass% or more, more preferably, the Si-based material can be contained at a ratio of 0.1 mass% to 10 mass% and the carbon-based material can be contained at a ratio of 60 mass% or more, and even more preferably, the Si-based material can be contained at a ratio of 1 mass% to 8 mass% and the carbon-based material can be contained at a ratio of 90 mass% or more. By containing a negative electrode active material composed of such a ratio, it is possible to suitably achieve both high capacity and suppression of the decrease in the capacity retention rate.

[0048] The negative electrode active material layer 64 may contain components other than the active material, such as a binder, a thickener, etc. As the binder, for example, styrene butadiene rubber (SBR) or the like may be used. As the thickener, for example, carboxymethyl cellulose (CMC) or the like may be used.

[0049] The separator 70 may be a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which a PP layer is laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0050] The nonaqueous electrolyte for a lithium ion secondary battery according to the present embodiment described above is used for the electrolyte 80. Note that FIG. 1 does not strictly show the amount of electrolyte 80 injected into the battery case 30.

[0051] The lithium ion secondary battery 100 configured as described above can be used for various purposes. Suitable applications include a driving power source mounted on vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHVs). The lithium ion secondary battery 100 can also be used in the form of a battery pack in which a plurality of batteries are typically connected in series and / or parallel.

[0052] As an example, a rectangular lithium ion secondary battery 100 including a flat wound electrode assembly 20 has been described. However, the lithium ion secondary battery can also be configured as a lithium ion secondary battery including a laminated electrode assembly. 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.

[0053] The present invention will now be further described with reference to the following examples. The synthesis method of various compounds, the manufacturing method of secondary batteries, and the evaluation and testing methods are conventionally known in the art, but the present invention is not limited thereto, and various modifications and variations are possible within the scope of the technical idea of ​​the present invention and the scope of the claims. In each example, "parts" indicates parts by mass and "%" indicates % by mass.

[0054] <Production of macromonomers> (Macromonomer 1) In a reaction vessel equipped with a thermometer, a cooling tube, a nitrogen gas inlet 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 stirred while blowing nitrogen at 160°C. Then, a mixed liquid consisting of 100 parts of methacrylamide and 7 parts of ditertiary amyl peroxide was dropped into the reaction vessel over 3 hours, and the mixture was stirred for 2 hours. Then, the mixture 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.

[0055] <Production of high molecular weight organic compounds> (High molecular weight organic compound No.4) A reaction vessel equipped with a thermometer, a cooling tube, a nitrogen gas inlet tube, a stirrer and a dropping device was charged with 40 parts of diethyl carbonate, and after replacing with nitrogen, the temperature was maintained at 120° C. The monomer mixture shown below was added dropwise to the reaction vessel over a period of 4 hours. (Monomer Mixture) Methyl methacrylate 25 parts n-Butyl acrylate 25 parts 2-Hydroxyethyl acrylate 50 parts t-Butyl peroxy-2-ethylhexanoate (polymerization initiator) 9 parts One hour after the end of the dropwise addition, a solution of 0.5 parts of t-butylperoxy-2-ethylhexanoate dissolved in 10 parts of diethyl carbonate was added dropwise over one hour. After the dropwise addition, the mixture was kept at 120°C for another hour. Diethyl carbonate was then added to the mixture so that the solid content was 50%, to obtain a solution of high molecular weight organic compound No. 4 with a solid content of 50%. 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.

[0056] (High molecular weight organic compounds No.5~15) Solutions of high molecular weight organic compounds Nos. 5 to 15 were produced in the same manner as for the high molecular weight organic compound No. 4, except that the monomer compositions and polymerization initiators 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 shown in Table 1 below.

[0057] [Table 1]

[0058] <Electrolyte production> (Example 1) The electrolyte LiPF6 was dissolved at a ratio of 1.0 mol / L in a non-aqueous solvent made by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC:EMC = 30:70. Furthermore, high molecular weight organic compound No. 1 "polyethylene glycol (molecular weight 2,000, functional group concentration 22.7 mmol / g, solid content 100%)" was dissolved at a solid content of 1 mass % to produce an electrolyte solution (Example 1).

[0059] (Examples 2 to 19) Electrolyte solutions (Examples 2 to 19) were produced in the same manner as in Example 1, except that high molecular weight organic compounds No. 2 to No. 16 were dissolved in a non-aqueous solvent in the proportions shown in Table 2 below, instead of the high molecular weight organic compound No. 1 in Example 1. The results of the capacity retention evaluation test described below are also shown. In this specification, if there is even one evaluation result of "x (fail)", the electrolyte is deemed to have failed.

[0060] <Preparation of lithium-ion secondary battery for evaluation> <Preparation of positive electrode> Cathode active material (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2):conductive additive (acetylene black):binder (PVdF) = 87:10:3 (mass %) was mixed with N-methyl-2-pyrrolidone as a dispersion solvent to prepare a paste, which was then applied to aluminum foil and dried to prepare a positive electrode plate.

[0061] <Production of negative electrodes> A paste was prepared by mixing a powder mixture of graphite (average particle size 20μm) and SiO (average particle size 15μm) in a ratio (mass ratio) of graphite:SiO = 95:5 as the negative electrode active material, styrene-butadiene copolymer (SBR) as the binder, carboxymethyl cellulose (CMC) as the thickener, and water as the dispersion solvent in a ratio (mass ratio) of the powder mixture:SBR:CMC = 98:1:1. The paste was then applied onto copper foil and dried to form a negative electrode.

[0062] <Preparation of laminated battery> An electrode body was formed by using the above positive and negative electrodes and opposing them via a polypropylene / polyethylene / polypropylene three-layer porous membrane having an air permeability of 300 seconds measured by the Gurley test method, and the electrode body was sealed together with the above electrolyte by lamination to prepare a battery for evaluation.

[0063] (Capacity retention evaluation test) <Activation> The initial charging was performed in a 25° C. thermostatic chamber at a constant current of 0.3 C up to 4.10 V, and then the battery was discharged at a constant current of 0.3 C down to 3.00 V. This was repeated three times.

[0064] <Initial capacity> Using the constant current-constant voltage method, the battery was charged to 4.10 V at a current value of 0.2 C, and then charged at a constant voltage until the current value during constant voltage charging became 1 / 50 C, at which point the battery was fully charged. After that, the battery was discharged to 3.00 V at a current value of 0.2 C using the constant current method, and the capacity at that point was taken as the initial capacity.

[0065] <Capacity retention rate (25℃)> In a thermostatic chamber at 25°C, 500 cycles of charge and discharge were repeated at a current value of 0.5C. The charging set value was 4.10V, and the discharging set value was 3.00V. A rest period of 10 minutes was provided after each charge and discharge. The capacity after the cycle test was then measured in the same manner, and the capacity retention rate was calculated using the following formula. Capacity retention rate (%) = (battery capacity after 500 cycles / initial capacity) x 100 The evaluation is as follows: ⊚: The capacity retention rate is 97% or more and 100% or less. Good: 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%.

[0066] <Capacity retention rate (60℃)> The capacity retention rate was measured in a thermostatic bath at 60° C. The same procedures were carried out except that the temperature of the thermostatic bath was changed from 25° C. to 60° C. The same evaluation criteria were also used.

[0067] [Table 2]

[0068] As shown in Table 2, the capacity retention rates of Examples 18 and 19, which do not contain the high molecular weight organic compound disclosed herein in the electrolyte solution, were evaluated as × (fail). On the other hand, in Examples 1 to 17, in which the high molecular weight organic compound is contained in the electrolyte solution, it was confirmed that the capacity retention rates were improved, and the evaluation test was passed. In particular, Examples 13 and 15 showed high levels of capacity retention rates at 25°C and 60°C of 97% or more and 100% or less.

[0069] Although the specific examples of the present invention have been described in detail above, 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 symbols]

[0070] 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector 44 Negative terminal 44a Negative current collector plate 50 positive electrode 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 negative electrode 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator 80 Electrolyte 100 Lithium-ion secondary battery

Claims

1. A non-aqueous electrolyte solution for use in a lithium ion secondary battery, The electrochemical device includes a non-aqueous solvent and an electrolyte dissolved in the non-aqueous solvent, and also includes a high molecular weight organic compound having a weight average molecular weight of 1,000 or more; the high molecular weight organic compound has a polar functional group, the polar functional group being at least one type of 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 polar functional group concentration being 0.1 mmol / g or more, The high molecular weight organic compound contains a copolymer compound in which a polymerizable unsaturated monomer is copolymerized. A non-aqueous electrolyte for lithium-ion secondary batteries.

2. 2. The nonaqueous electrolyte solution for lithium ion secondary batteries according to claim 1, wherein the high molecular weight organic compound is contained in an amount of 0.01% by mass to 10% by mass, where the nonaqueous electrolyte solution is taken as 100% by mass.

3. a negative electrode including a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector; a positive electrode including a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector; A lithium ion secondary battery comprising an electrode assembly having a separator and the nonaqueous electrolyte solution according to claim 1 or 2.

4. 4. The lithium ion secondary battery according to claim 3, wherein the negative electrode active material layer contains a Si-based negative electrode active material having Si as a constituent element and capable of reversibly absorbing and releasing lithium ions.

5. The lithium ion secondary battery according to claim 4, characterized in that, when the negative electrode active material layer is taken as 100 mass%, the Si-based negative electrode active material is contained in an amount of 0.01 mass% to 20 mass%, and the carbon-based negative electrode active material is contained in an amount of 50 mass% or more.

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