Non-aqueous 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 suppress gas generation owing to the decomposition of a nonaqueous electrolyte solution.SOLUTION: A nonaqueous electrolyte solution is arranged for use in a lithium ion secondary battery in which a negative electrode active material in a negative electrode includes at least a Si-based negative electrode active material including Si as a constituent element or a graphite-based carbon negative electrode active material, which is capable of reversibly occluding and releasing lithium ions. The nonaqueous electrolyte solution comprises: a cyclic carbonate solvent; and high-molecular weight organic compound of 1,000 or larger in weight average molecular weight.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a non-aqueous electrolyte for a lithium-ion secondary battery and a lithium-ion secondary battery provided with the non-aqueous electrolyte.

Background Art

[0002] Lithium-ion secondary batteries are widely used as portable power sources for personal computers, mobile terminals, etc., and as power sources for vehicle drive 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, the use of Si-based anode materials as anode active materials has been studied. It is known that Si-based materials have a theoretical capacity density more than five times greater than that of graphite, which is widely used as an anode active material, and application studies as an anode active material to replace graphite are underway.

[0003] However, an anode active material containing an Si-based material (hereinafter referred to as an Si-based anode 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 Si-based anode active material, isolating it from the current collector network and causing a reduction in battery life. Further, due to such a property, cracks and peeling of the SEI (Solid Electrolyte Interphase) formed on the surface of the anode active material occur, and lithium ions in the electrolyte are taken in for the reformation of the SEI, which is cited as a demerit for causing deterioration of the non-aqueous electrolyte.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

[0005] By the way, Patent Documents 1 to 4 disclose techniques of adding an additive to a non-aqueous electrolyte in order to improve battery life or enhance battery safety. Patent Document 5 discloses that adding fluoroethylene carbonate to the electrolyte of a lithium-ion secondary battery having a carbon-based negative electrode active material and an Si-based negative electrode active material improves the battery life. Since fluoroethylene carbonate has a high redox potential and is easily reductively decomposed, it can preferably form an SEI and prevent the electrolyte and the active material from directly contacting and reacting with each other.

[0006] However, fluoroethylene carbonate has a problem of generating gas during SEI formation. Cyclic carbonates such as fluoroethylene carbonate and ethylene carbonate (EC) are likely to cause gas generation during high-temperature storage. Such gas generation causes an increase in the internal pressure of the lithium-ion secondary battery. Therefore, when gas generation increases due to long-term use or leaving at a high temperature, the internal pressure greatly increases, and the battery life may be shortened due to deformation of the battery case, early operation of a pressure-sensitive safety mechanism such as a current cutoff mechanism or a safety valve. In addition, gas generation may inhibit sufficient penetration of the electrolyte and cause a decrease in battery performance. Therefore, a technique for suppressing gas generation due to decomposition of a non-aqueous electrolyte including fluoroethylene carbonate is desired in order to improve battery life.

[0007] Therefore, an object of the present invention is to provide a non-aqueous electrolyte for a lithium-ion secondary battery that can suppress gas generation due to decomposition of the non-aqueous electrolyte. Another object of the present invention is to provide a lithium-ion secondary battery using the non-aqueous electrolyte for a lithium-ion secondary battery.

Means for Solving the Problems

[0008] The non-aqueous electrolyte for a lithium-ion secondary battery disclosed herein is a non-aqueous electrolyte used in a lithium-ion secondary battery containing at least one of a Si-based negative electrode active material or a graphite-based carbon negative electrode active material in which the negative electrode active material in the negative electrode can reversibly occlude and release lithium ions with Si as a constituent element, and includes a non-aqueous solvent and an electrolyte dissolved in the non-aqueous solvent, and contains a cyclic carbonate and a high molecular weight organic compound having a weight average molecular weight of 1,000 or more.

[0009] According to such a configuration, gas generation due to decomposition of the non-aqueous electrolyte can be suppressed, and the battery life (capacity retention rate) of the lithium secondary battery can be improved.

[0010] In a preferred embodiment of the non-aqueous electrolyte for a lithium-ion secondary battery disclosed herein, the cyclic carbonate is at least one of ethylene carbonate (EC) or monofluoroethylene carbonate (FEC). According to such a configuration, SEI formation on the surface of the negative electrode active material is performed, and the capacity retention rate can be further improved.

[0011] In a preferred embodiment of the non-aqueous electrolyte for a lithium-ion secondary battery disclosed herein, when the non-aqueous electrolyte for a lithium-ion secondary battery is 100% by mass, it contains 5% by mass or more of ethylene carbonate (EC) and / or 0.1% by mass or more of monofluoroethylene carbonate (FEC). According to such a configuration, the capacity retention rate of the lithium-ion secondary battery can be further improved.

[0012] In a preferred embodiment of the non-aqueous electrolyte for a lithium-ion secondary battery disclosed herein, the non-aqueous electrolyte for a lithium-ion secondary battery contains 0.01% by mass to 10% by mass of the above high molecular weight organic compound with respect to the non-aqueous electrolyte. By containing the above high molecular weight organic compound in the non-aqueous electrolyte at such a ratio, the capacity retention rate of the lithium-ion secondary battery can be suitably improved.

[0013] In a preferred embodiment of the non-aqueous electrolyte for a lithium-ion secondary battery disclosed herein, the above 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, and the polar functional group concentration in the high molecular weight organic compound is 0.1 mmol / g or more. According to such a configuration, the stability of the above high molecular weight organic compound in the non-aqueous electrolyte is increased, and it becomes easier to adsorb to the negative electrode active material, so that the capacity retention rate can be improved.

[0014] In a preferred embodiment of the non-aqueous electrolyte for a lithium-ion secondary battery disclosed herein, the above high molecular weight organic compound contains a copolymer compound obtained by copolymerizing a polymerizable unsaturated monomer. According to such a configuration, the stability of the above high molecular weight organic compound in the non-aqueous electrolyte is increased, and it becomes even easier to adsorb to the negative electrode active material, so that the capacity retention rate can be further improved.

[0015] The lithium-ion secondary battery disclosed herein includes an electrode body having a negative electrode, a positive electrode, and a separator, and the non-aqueous electrolyte for a lithium-ion secondary battery described above. According to such a configuration, it is possible to provide a lithium-ion secondary battery that suppresses gas generation due to decomposition of the non-aqueous electrolyte and has an improved capacity retention rate.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0017] Hereinafter, embodiments according to the present invention will be described. In addition, matters other than those specifically mentioned in this specification and necessary for implementation can be grasped 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 knowledge in the relevant field. In this specification, when a numerical range is described as A~B (where A and B are arbitrary numerical values), it has the same general interpretation and means A or more and B or less.

[0018] In this specification, the term "secondary battery" generally refers to a rechargeable power storage device, and is a term encompassing 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.

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

[0020] The non-aqueous electrolyte for a lithium-ion secondary battery according to this embodiment contains a non-aqueous solvent containing a cyclic carbonate solvent and an electrolyte dissolved in the non-aqueous solvent, and further contains a high molecular weight organic compound having a weight average molecular weight of 1,000 or more as described below.

[0021] <High molecular weight organic compound> 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 in the range of 3,000 to 30,000, which is preferable from the viewpoint of the 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 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, “HLC8120GPC” (trade name, manufactured by Tosoh Corporation) is used as the gel permeation chromatograph, and four columns “TSKgel G-4000HXL”, “TSKgel G-3000HXL”, “TSKgel G-2500HXL” and “TSKgel G-2000HXL” (trade name, 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.

[0022] The types of the above-mentioned high molecular weight organic compounds are not particularly limited. Specifically, for example, 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, polyvinyl pyrrolidone, and composite resins thereof can be mentioned, and one type can be used alone or two or more types can be used in combination. Among them, from the viewpoint of maintaining battery capacity (including stability in non-aqueous electrolytes and adsorptivity to negative electrode active materials), 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.

[0023] From the viewpoint of the battery capacity retention rate, the concentration of the polar functional group in the above-mentioned 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. Particularly, 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, which is preferable from the viewpoint of maintaining battery capacity.

[0024] In this specification, the polar functional group concentration is calculated with one polar functional group. For example, if two polar functional groups are present in one polymerizable unsaturated monomer, they are calculated as two.

[0025] 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 invention, "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. However, such water solubility shows a preferable property of the high molecular weight organic compound and is not intended to imply that the electrolyte of the lithium ion secondary battery in the present embodiment preferably contains water.

[0026] Among them, from the viewpoint of maintaining battery capacity (including stability in a non-aqueous electrolyte and adsorptivity to the negative electrode active material), as the high molecular weight organic compound, a copolymer compound obtained by copolymerizing a polymerizable unsaturated monomer is preferable.

[0027] <Copolymer compound> As the polymerizable unsaturated monomer used as a raw material of 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, it is preferable that the copolymer compound contains a copolymer containing a polymerizable unsaturated monomer having a polar functional group as a constituent component.

[0028] <Polymerizable unsaturated monomer having a polar functional group> Examples of the polymerizable unsaturated monomer having the polar functional group include 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. For example, 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, 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; 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, an adduct of glycidyl (meth)acrylate and amines; polymerizable unsaturated monomers having a urethane bond such as a reaction product of an isocyanate group-containing polymerizable unsaturated monomer and a hydroxyl group-containing compound or a reaction product of a hydroxyl group-containing polymerizable unsaturated monomer and an isocyanate group-containing compound; 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, allyl glycidyl ether; (meth)acrylate 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, sodium salts and ammonium salts of these sulfonic acids, etc.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.

[0029] <Other polymerizable unsaturated monomers> Examples of the polymerizable unsaturated monomer other than the polymerizable unsaturated monomer having the 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, tricyclodecanyl (meth) acrylate, etc.; 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.;

[0030] <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 a polymerizable unsaturated monomer in an organic solvent, but it is not limited thereto. For example, bulk polymerization, emulsion polymerization, suspension polymerization, etc. may also be used. When performing solution polymerization, it may be continuous polymerization or batch polymerization, and the polymerizable unsaturated monomer may be charged all at once, dividedly charged, or added continuously or intermittently.

[0031] 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.

[0032] 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, since it is used in the electrolytic solution, it is preferably water-free 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.

[0033] 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 tank while dropping a polymerization initiator as needed after the polymerization at each stage may be provided.

[0034] As the copolymer compound, particularly from the viewpoints of adsorptivity and stability to the Si-based negative electrode active material, it is preferably a copolymer compound having a graft structure or a block structure divided into two segments, an adsorption part and a steric repulsion part, and particularly preferably a graft structure (comb structure). 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 as the main chain and a hydrophilic functional group in the steric repulsion part as the side chain.

[0035] 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.

[0036] As a method for introducing the side chain of the 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 as the 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.

[0037] Regarding the above-mentioned macromonomer containing a polymerizable unsaturated group, it can be produced by a method known per se. For example, Japanese Patent Publication No. 43-11224 discloses a method of introducing a carboxylic acid group to the end of a polymer chain using a chain transfer agent such as mercaptopropionic acid in the process of producing a macromonomer, and then introducing an ethylenically unsaturated group by adding glycidyl methacrylate 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. Furthermore, Japanese Patent Application Laid-Open No. 7-002954 describes a method of obtaining a macromonomer by radical polymerization of methacrylic acid using 2,4-diphenyl-4-methyl-1-pentene as an addition-cleavage type chain transfer agent.

[0038] The addition amount of the above-mentioned high molecular weight organic compound in the non-aqueous electrolyte for a lithium ion secondary battery according to this embodiment is not particularly limited as long as the effects of the present invention are exhibited. However, if the addition amount is too small, it becomes difficult to obtain the effects of the present invention. Therefore, when the mass of the electrolyte is 100% by mass, the addition amount is typically 0.01% to 10% by mass, preferably 0.1% to 5% by mass, and more preferably 0.6% to 1.5% by mass. By adding the above-mentioned high molecular weight organic compound within such a range, the capacity retention rate in the charge and discharge cycles of the lithium ion secondary battery can be improved more effectively.

[0039] The non-aqueous electrolyte for a lithium ion secondary battery according to this embodiment has a supporting salt (lithium salt) dissolved or dispersed in a non-aqueous solvent. The type of non-aqueous solvent is not particularly limited as long as it can dissolve the above-mentioned high molecular weight organic compound, and carbonates, ethers, esters, nitriles, sulfones, lactones, etc. that are used in the electrolytic solution of lithium-ion secondary batteries can be used. Among them, carbonates are preferred. Examples of carbonates include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), and chain carbonates such as diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). These can be used alone or in combination of two or more. In the present invention, the "non-aqueous electrolytic solution" means an electrolytic solution that does not contain water, and it is preferably as free of water as possible. However, a very small amount of moisture may be mixed in from raw materials or the air (during the manufacturing process). In that case, usually, it can be contained in the range of 1% by mass or less, preferably 0.5% by mass or less, more preferably 0.1% by mass or less.

[0040] Regarding the non-aqueous electrolytic solution for lithium secondary batteries according to this embodiment, it is preferable to use ethylene carbonate (EC) among cyclic carbonates. Ethylene carbonate not only has a high relative dielectric constant but also participates in SEI formation and can improve the stability and / or durability of the negative electrode. If the content of ethylene carbonate in the non-aqueous electrolytic solution is too low, it is difficult to exert the above effects. Therefore, it is preferably contained in the non-aqueous electrolytic solution at a ratio of 5% by mass or more, more preferably 15% by mass or more, and still more preferably 25% by mass or more.

[0041] The type of lithium salt can be appropriately selected from various ones 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 used within the range of 0.7 mol / L or more and 1.3 mol / L or less.

[0042] The non-aqueous electrolyte for a lithium-ion secondary battery according to this embodiment may contain various additives and the like as long as the characteristics of the lithium-ion secondary battery are not impaired. Such additives can be used for one or more purposes among improving the input / output characteristics of the battery, improving the cycle characteristics, improving the initial charge / discharge efficiency, improving the safety, etc., as film-forming agents, overcharge additives, 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 monofluoroethylene carbonate (FEC); overcharge additives composed of compounds that can generate gas during overcharge, represented by aromatic compounds such as biphenyl (BP) and cyclohexylbenzene (CHB); surfactants; dispersants; thickeners; antifreeze agents, etc. Although the concentration of these additives in the entire non-aqueous electrolyte varies depending on the type of additive, it is typically exemplified that the concentration of the film-forming agent is about 0.1 mol / L or less (typically 0.005 mol / L to 0.05 mol / L), and the concentration of the overcharge additive is about 6% by mass or less (typically 0.5% by mass to 4% by mass).

[0043] Among the film-forming agents, it is preferable to use monofluoroethylene carbonate (FEC) in the non-aqueous electrolyte for a lithium-ion secondary battery according to this embodiment. SEI formation is promoted by monofluoroethylene carbonate, which is a cyclic carbonate, and the negative electrode can be suitably protected. The addition amount of monofluoroethylene carbonate to the non-aqueous electrolyte is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more. Also, the upper limit of the addition amount is preferably 10% by mass or less, more preferably 4% by mass or less, and even more preferably 3% by mass or less.

[0044] The non-aqueous electrolyte for a lithium-ion secondary battery according to this embodiment can be used in a lithium-ion secondary battery according to a known method. Since the lithium-ion secondary battery can suppress gas generation due to the decomposition of the non-aqueous electrolyte by the high molecular weight organic compound contained in the non-aqueous electrolyte for a lithium-ion secondary battery according to this embodiment, it is possible to suppress a decrease in the capacity retention rate in the charge and discharge cycle.

[0045] A schematic configuration example of a lithium-ion secondary battery using the non-aqueous electrolyte for a lithium-ion secondary battery according to this embodiment will be described below with reference to the drawings. In the following drawings, members and parts having the same function are denoted by the same reference numerals and will be described. Also, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships.

[0046] 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 exterior container) 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 electrolyte 80. 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.

[0047] As shown in FIGS. 1 and 2, the wound electrode body 20 has a form in which a sheet-shaped 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-shaped 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 and sheet-shaped separators 70 and wound in the longitudinal direction. Note that a positive electrode active material non-formation portion 52a (i.e., a portion where the positive electrode current collector 52 is exposed without the formation of the positive electrode active material layer 54) and a negative electrode active material non-formation portion 62a (i.e., a portion where the negative electrode current collector 62 is exposed without the formation of the negative electrode active material layer 64) formed so as to protrude outward from both ends in the winding axis direction (i.e., the sheet width direction orthogonal to the longitudinal direction) of the wound electrode body 20 are joined with a positive electrode current collector plate 42a and a negative electrode current collector plate 44a, respectively.

[0048] Examples of the positive electrode current collector 52 constituting the positive electrode 50 include aluminum foil. Examples of the positive electrode active material contained 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.).

[0049] 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) or the like can be used.

[0050] Examples of the negative electrode current collector 62 constituting the negative electrode 60 include copper foil. Examples of the negative electrode active material contained in the negative electrode active material layer 64 include graphite-based carbon materials; lithium titanate (Li4Ti5O 12: LTO); Sn; Si-based materials, etc. can be used. Further, it contains at least one of a Si-based material or a graphite-based carbon material. From the viewpoint of increasing the capacity of the lithium-ion secondary battery 100, a Si-based negative electrode active material that contains Si as a constituent element and can reversibly occlude and release lithium ions can be selected as the negative electrode active material. As the Si-based negative electrode active material, for example, SiO, Si, etc. can be used. In this specification, the "graphite-based carbon material" refers to the general term for a carbon material composed only of graphite and a carbon material in which graphite occupies 50 mass% or more (typically 80 mass% or more, for example 90 mass% or more) of the entire material.

[0051] Further, the constituent components of the negative electrode active material can be used singly or in combination of two or more. From the viewpoints 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 graphite-based carbon material can be used. As the ratio of the constituent of the negative electrode active material, for example, when the negative electrode active material layer is 100 mass%, the Si-based material can be used at a ratio of 0.01 mass% to 20 mass%, and the graphite-based carbon material can be used at a ratio of 50 mass% or more.

[0052] The negative electrode active material layer 64 may contain components other than the active material, such as a binder and a thickener. 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.

[0053] Examples of the separator 70 include a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide, etc. 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 PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0054] The non-aqueous electrolyte for a lithium-ion secondary battery according to the above-described embodiment is used for the electrolyte 80. Note that FIG. 1 does not precisely show the amount of the electrolyte 80 injected into the battery case 30.

[0055] The lithium-ion secondary battery 100 configured as described above can be used for various applications. 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 in which a plurality of batteries are connected in series and / or in parallel.

[0056] Note that, as an example, a rectangular lithium-ion secondary battery 100 including 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 including a laminated electrode body. Further, the lithium-ion secondary battery can be configured as a cylindrical lithium-ion secondary battery, a laminate-type lithium-ion secondary battery, or the like.

[0057] Hereinafter, the present invention will be further described with reference to examples. Methods for synthesizing various compounds, manufacturing methods for secondary batteries, evaluation test methods, etc. use methods conventionally known in the relevant technical field. 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 indicate parts by mass, and "%" indicates mass %.

[0058] <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 at 160 °C while blowing nitrogen gas. Next, a mixed solution consisting of 100 parts of methacrylamide and 7 parts of ditertiary amyl peroxide was added dropwise thereto over 3 hours, and the mixture was stirred for 2 hours as it was. 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 having 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.

[0059] <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 added dropwise 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 the dropwise addition, a solution prepared by dissolving 0.5 part of t-butyl peroxy-2-ethylhexanoate in 10 parts of diethyl carbonate was added dropwise thereto over 1 hour. After the completion of the dropwise addition, the mixture was further maintained at 120 °C for 1 hour. Then, diethyl carbonate was added so that the solid content became 50%, and a solution of high molecular weight organic compound No. 4 having a solid content of 50% was obtained. 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.

[0060] (High molecular weight organic compounds No. 5 to 15) A high-molecular-weight organic compound No.5-15 solution was produced in the same manner as the high-molecular-weight organic compound No.4, except that the monomer composition and the polymerization initiator were as shown in Table 1 below. The weight-average molecular weight, the concentration of polar functional groups in mmol / g, and the concentration of ionic polar functional groups in mmol / g of each resin are described in Table 1 below.

[0061] [Table 1]

[0062] <Production of Electrolyte Solution> (Example 1) As a non-aqueous solvent, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of EC:EMC = 30:70, and LiPF6, which is an electrolyte, was dissolved at a ratio of 1.0 mol / L. Further, 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 so as to be 1% by mass in terms of solid content, and an electrolyte solution (Example 1) was produced.

[0063] (Examples 2-14, 17-18, 21-22) An electrolyte solution (Examples 2-14, 17-18, 21-22) was produced in the same manner as Example 1, except that high-molecular-weight organic compounds No.2-No.16 were dissolved in a non-aqueous solvent at the ratios shown in Table 2 below instead of high-molecular-weight organic compound No.1 in Example 1.

[0064] (Examples 15-16, 19-20) After performing the same operations as in Example 1 except that high-molecular-weight organic compounds No.2-No.16 were dissolved in a non-aqueous solvent at the ratios shown in Table 2 below instead of high-molecular-weight organic compound No.1 in Example 1, monofluoroethylene carbonate (FEC) was added so as to be 1% by mass, and an electrolyte solution (Examples 15-16, 19-20) was produced.

[0065] (Example 25) An electrolyte solution (Example 25) was produced by the process excluding the step of dissolving high-molecular-weight organic compound No.1 in Example 1.

[0066] (Examples 23 - 24) After manufacturing the electrolytic solution in the same process as in Example 25, monofluoroethylene carbonate (FEC) was added at the ratio shown in Table 2 below to manufacture an electrolytic solution (Examples 23 - 24). Also, the results of the evaluation tests described later are presented. In this application, if there is even one evaluation result of "× (failed)" or "E (failed)" in the evaluation, the electrolytic solution is considered failed.

[0067] <Fabrication of Lithium - Ion Secondary Battery for Evaluation> <Fabrication of Positive Electrode> Positive electrode active material (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2): Conductive assistant (acetylene black): Binder (PVdF) = 87:10:3 (mass ratio) was mixed using N - methyl - 2 - pyrrolidone as a dispersion solvent to prepare a paste, which was then applied to and dried on an aluminum foil to fabricate a positive electrode plate.

[0068] <Fabrication of Negative Electrode> As a negative electrode active material, graphite (average particle diameter 20μm) and SiO (average particle diameter 15μm) were mixed at a ratio of graphite:SiO = 95:5 (mass ratio) to form a mixed powder. Styrene - butadiene copolymer (SBR) as a binder and carboxymethyl cellulose (CMC) as a thickener were mixed with the mixed powder:SBR:CMC = 98:1:1 (mass ratio) using water as a dispersion solvent to prepare a paste. Then, the above - mentioned paste was applied to and dried on a copper foil to obtain a negative electrode.

[0069] <Fabrication of Laminate Battery> Using the above - mentioned positive and negative electrodes, they were opposed to each other through a polypropylene / polyethylene / polypropylene three - layer - structured porous membrane with an air permeability of 300 seconds obtained by the Gurley test method to form an electrode body, and then sealed by lamination together with the above - mentioned electrolytic solution to fabricate an evaluation battery.

[0070] <Evaluation Tests> <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 to 3.00 V at a current value of 0.3C by the constant current mode. This was repeated three times.

[0071] <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 constant voltage charging was carried out until the current value during constant voltage charging reached 1 / 50C to reach a fully charged state. Then, the capacity when discharging to 3.00 V at a current value of 0.2C by the constant current mode was taken as the initial capacity.

[0072] <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 charging set value was 4.10 V and the discharging 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 obtained by the following formula. Capacity retention rate (%) = (Battery capacity after 500 cycles / Initial capacity) × 100 The evaluation is as follows. A: The capacity retention rate is 99% or more and 100% or less. B: The capacity retention rate is 97% or more and less than 99%. C: The capacity retention rate is 94% or more and less than 97%. D: The capacity retention rate is 91% or more and less than 94%. E: The capacity retention rate is less than 91%.

[0073] <Capacity retention rate (60°C)> The capacity retention rate was measured in a 60°C constant temperature bath. Note that it was carried out in the same way except that the temperature of the constant temperature bath was changed from 25°C to 60°C.

[0074] <Gas generation amount> The volume was measured using the Archimedes method. The laminated battery was immersed in water at 25°C, and the volume of the laminated battery was measured from the mass change. The volume measurement was performed before and after the start of the 500-cycle test, and the gas generation amount was calculated by the following formula (2). Gas generation amount (%) = [((Volume after 500 cycles) - (Initial volume)) / (Initial volume)] × 100 ··· Formula (2) The evaluation is as follows. 〇: The gas generation amount is less than 60%. △: The gas generation amount is 60% or more and less than 105%. ×: The gas generation amount is 105% or more.

[0075]

Table 2

[0076] As shown in Table 2, in Examples 1 to 21 to which any one of the high molecular weight organic compounds No. 1 to 15 with a weight average molecular weight of 1,000 or more was added, the capacity retention rate was improved compared to Example 25. However, on the other hand, in Example 22 to which the high molecular weight organic compound No. 16 with a weight average molecular weight of 500 was added, the capacity retention rate at 25°C was improved, but the capacity retention rate at 60°C was not improved. In addition, in Examples 13 to 21 to which any one of the high molecular weight organic compounds No. 13 to 15 was added, the gas generation amount was preferably suppressed compared to Example 25.

[0077] From the comparison of Examples 23 to 25, by adding monofluoroethylene carbonate (FEC), while the capacity retention rate was improved, the gas generation amount increased. However, on the other hand, when comparing Examples 15 to 16, 19 to 20 with Example 23, by adding the high molecular weight organic compound No. 13 or No. 14, the capacity retention rate was improved, and furthermore, the gas generation amount was suppressed. Also, when the addition amount of the high molecular weight organic compound No. 13 or No. 14 was 1 mass% rather than 0.5 mass%, the gas generation amount was more suppressed.

[0078] 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 to the specific examples exemplified above.

Explanation of Reference Numerals

[0079] 20-turn 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 active material layer 70 Separator 80 Electrolyte solution 100 Lithium-ion secondary battery

Claims

1. A non-aqueous electrolyte used in a lithium-ion secondary battery, wherein the negative electrode active material in the negative electrode contains at least one of a Si-based negative electrode active material or a graphite-based carbon negative electrode active material capable of reversibly occluding and releasing lithium ions with Si as a constituent element, comprising a non-aqueous solvent and an electrolyte dissolved in the non-aqueous solvent, and containing a cyclic carbonate and a high molecular weight organic compound having a weight average molecular weight of 1,000 or more, wherein the high molecular weight organic compound has a graft structure comprising a main chain having an ionic functional group and a side chain having a hydrophilic functional group. A non-aqueous electrolyte for a lithium-ion secondary battery.

2. The non-aqueous electrolyte for a lithium-ion secondary battery according to claim 1, wherein the cyclic carbonate is at least one of ethylene carbonate (EC) or monofluoroethylene carbonate (FEC).

3. The non-aqueous electrolyte for a lithium-ion secondary battery according to claim 1 or 2, wherein when the non-aqueous electrolyte is 100% by mass, it contains 5% by mass or more of ethylene carbonate (EC) and / or 0.1% by mass or more of monofluoroethylene carbonate (FEC).

4. The non-aqueous electrolyte for a lithium-ion secondary battery according to any one of claims 1 to 3, wherein when the non-aqueous electrolyte is 100% by mass, it contains 0.01% by mass to 10% by mass of the high molecular weight organic compound.

5. The non-aqueous electrolyte for a lithium-ion secondary battery according to any one of claims 1 to 4, wherein 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 phosphate group, a polyalkylene ether group, an amide group, a hydroxyl group, an epoxy group, and an alkoxysilyl group, and the concentration of the polar functional group is 0.1 mmol / g or more.

6. The non-aqueous electrolyte for a lithium-ion secondary battery according to any one of claims 1 to 5, wherein the high molecular weight organic compound contains a copolymer compound copolymerized with a polymerizable unsaturated monomer.

7. A non-aqueous electrolyte lithium-ion secondary battery comprising an electrode body having a negative electrode, a positive electrode, and a separator, and the non-aqueous electrolyte according to any one of claims 1 to 6.

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