Polyimide compound, and lithium ion secondary battery negative electrode material, lithium ion secondary battery negative electrode, and lithium ion secondary battery using the same

Abstract

Provided is a polyimide that is used as an electrode material and makes it possible to markedly improve the cycle characteristics of a lithium ion secondary battery. Specifically provided is a polyimide used as an electrode material for a lithium ion secondary battery, wherein the diamine component includes an aromatic diamine compound represented by formula (1).

Description

[Technical Field]

[0001] The present invention relates to polyimides used as electrode materials for lithium ion secondary batteries. [Background technology]

[0002] In recent years, electronic devices such as smartphones have become increasingly smaller, and this has led to active development of lithium-ion secondary batteries that are small, lightweight, and capable of achieving high energy density. Silicon (Si)-based materials are used as the negative electrode active material in lithium-ion secondary batteries, as they have high discharge capacity, excellent initial charge / discharge efficiency, and excellent cycle characteristics. However, silicon expands and contracts significantly during charge and discharge, and repeated use can lead to problems such as disconnection of the conductive paths between the negative electrode active material and peeling between the current collector and the negative electrode active material layer.

[0003] In view of the above problems with silicon-based materials, the use of polyimide compounds as binder resins, instead of carboxymethyl cellulose and the like that have been used conventionally, as materials constituting negative electrodes, has been investigated (see Patent Document 1, etc.).

[0004] However, there is room for improvement in the initial charge-discharge efficiency and cycle characteristics, and there has been a demand for polyimide compounds that can further improve the initial charge-discharge efficiency and cycle characteristics. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. WO2017 / 138604 Brochure Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above problems, and an object of the present invention is to provide a polyimide to be used as an electrode material that can significantly improve the cycle characteristics of lithium ion secondary batteries. Another object of the present invention is to provide a negative electrode material for a lithium ion secondary battery containing the polyimide, a negative electrode made of the negative electrode material, and a lithium ion secondary battery including the negative electrode. [Means for solving the problem]

[0007] The present inventors have discovered that by using a specific aromatic diamine as the diamine component, it is possible to suppress electrode expansion and significantly improve cycle characteristics, even when a silicon-based negative electrode active material is used. The present invention is based on this discovery. Specifically, the gist of the present invention is as follows.

[0008] [1] A polyimide used as an electrode material for a lithium-ion secondary battery, The diamine component is represented by the following formula (1): [ka] (In the formula, R1 to R4 are hydrogen atoms, any one of R5 to R8 is an aromatic group having 6 to 10 carbon atoms, a phenoxy group, a benzyl group, or a benzyloxy group, and the remaining R5 to R8 are hydrogen atoms. [2] The polyimide according to [1], wherein the acid anhydride component contains at least one of an alicyclic acid anhydride and an aromatic acid anhydride. [3] The polyimide according to [1] or [2], wherein the compound of formula (1) is contained in the diamine component in a proportion of 20 mol % or more. [4] A negative electrode material for a lithium ion secondary battery, comprising the polyimide according to any one of [1] to [3] and a silicon-based negative electrode active material as a negative electrode active material. [5] The negative electrode material according to [4], wherein the silicon-based negative electrode active material is contained in an amount of 30 parts by mass or more per 100 parts by mass of the negative electrode active material. [6] A negative electrode for a lithium ion secondary battery comprising a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, The negative electrode for a lithium ion secondary battery, wherein the negative electrode active material layer is made of the negative electrode material according to [4] or [5]. [7] A lithium ion secondary battery comprising the negative electrode according to [6], a positive electrode, and a non-aqueous electrolyte. [Effects of the Invention]

[0009] By using the polyimide of the present invention as an electrode material for a lithium ion secondary battery, even when a silicon-based negative electrode active material is used, it is possible to suppress electrode expansion and significantly improve cycle characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Definition] In the present invention, "solvent-soluble" means that 5 g or more of the compound dissolves in 100 g of an organic solvent. The term "aromatic group" also includes substituents that are bonded to the main skeleton via an oxygen atom, a nitrogen atom, or a carbon atom, and also includes heteroaromatic groups such as a pyrrole group.

[0011] [Polyimide] The polyimide of the present invention is a reaction product of a diamine component containing an aromatic diamine compound represented by the following formula (1) and an acid anhydride component. [ka] (In the formula, R1 to R4 are hydrogen atoms, Any one of R5 to R8 is an aromatic group having 6 to 10 carbon atoms, a phenoxy group, a benzyl group, or a benzyloxy group, and the remaining R5 to R8 are hydrogen atoms.

[0012] From the viewpoints of ease of synthesis and application in the field of electronic component materials, the aromatic group having 6 to 10 carbon atoms is preferably unsubstituted, but may have a substituent, for example, an alkyl group, a halogen group such as a fluoro group or a chloro group, an amino group, a nitro group, a hydroxyl group, a cyano group, a carboxyl group, a sulfonic acid group, etc. The alkyl group and aromatic group may have one or more of these substituents, or two or more of these substituents.

[0013] Examples of the aromatic group having 6 to 10 carbon atoms include a phenyl group, a tolyl group, a methylphenyl group, a dimethylphenyl group, an ethylphenyl group, a diethylphenyl group, a propylphenyl group, a butylphenyl group, a fluorophenyl group, a pentafluorophenyl group, a chlorophenyl group, a bromophenyl group, a methoxyphenyl group, a dimethoxyphenyl group, an ethoxyphenyl group, a diethoxyphenyl group, an aminophenyl group, a nitrophenyl group, a nitrobenzyl group, a cyanophenyl group, a cyanobenzyl group, a phenethyl group, a phenylpropyl group, a phenylamino group, a diphenylamino group, a biphenyl group, a naphthyl group, a phenylnaphthyl group, a diphenylnaphthyl group, an anthryl group, and an anthrylphenyl group. Examples of heteroaromatic groups include a phenyl group, a phenylanthryl group, a naphthacenyl group, a phenanthryl group, a phenanthrylphenyl group, a phenylphenanthryl group, a pyrenyl group, a phenylpyrenyl group, a fluorenyl group, a phenylfluorenyl group, a naphthylethyl group, a naphthylpropyl group, an anthracenylethyl group, a phenanthrylethyl group, a pyrrole group, an imidazole group, a thiazole group, an oxazole group, a furan group, a thiophene group, a triazole group, a pyrazole group, an isoxazole group, an isothiazole group, a pyridine group, a pyrimidine group, a benzofuran group, a benzothiophene group, a quinoline group, an isoquinoline group, an indolyl group, a benzothiazolyl group, and a carbazolyl group.

[0014] Among the above aromatic groups, phenyl and methylphenyl groups are preferred in terms of availability of starting materials and synthesis costs.

[0015] Two or more aromatic diamine compounds represented by the above formula (1) may be used in combination.

[0016] Specifically, a compound represented by the following formula (2) can be preferably used as a diamine compound satisfying the above formula (1). [ka]

[0017] The aromatic diamine compound represented by the above formula (1) can be obtained by reacting a compound represented by the following formula (3) with a compound represented by the following formula (4), and then reducing the nitro group. [ka] [ka]

[0018] In the above formula, R1' to R4' are hydrogen atoms, any of R5' to R8' is an aromatic group having 6 to 10 carbon atoms, a phenoxy group, a benzyl group or a benzyloxy group, and the remaining R5' to R8' are hydrogen atoms.

[0019] Furthermore, X represents a hydroxyl group or a halogen group selected from a fluoro group, a chloro group, a bromo group, and an iodo group. From the viewpoint of reactivity with the compound represented by general formula (4), X is preferably a halogen group, and particularly preferably a chloro group or a bromo group.

[0020] When X in formula (3) is a hydroxyl group, the reaction of the compounds represented by formulas (3) and (4) is preferably carried out in the presence of a dehydration condensation agent such as N,N'-dicyclohexylcarbodiimide (DCC) or an organic acid catalyst such as p-toluenesulfonic acid. When X in formula (3) is a halogen group, the reaction of the compounds represented by formulas (3) and (4) is preferably carried out in the presence of an acid acceptor such as triethylamine.

[0021] Specifically, the diamine compound represented by the above formula (2) can be obtained by reacting compounds represented by the following formulas (5) and (6).

[0022] [ka] [ka]

[0023] The compound represented by the formula (4) can be obtained by nitrating a commercially available or synthesized compound represented by the formula (7) below. The nitration of the compound represented by the formula (7) below can be carried out by a conventionally known nitration method using a mixed acid of concentrated sulfuric acid and concentrated nitric acid, nitric acid, fuming nitric acid, an alkali metal salt of an acid in concentrated sulfuric acid, acetyl nitrate, a nitronium salt, a nitrogen oxide, or the like. [ka]

[0024] Any one of R5'' to R8'' is an aromatic group having 6 to 10 carbon atoms, a phenoxy group, a benzyl group, or a benzyloxy group, and the remaining R5'' to R8'' are hydrogen atoms.

[0025] In the present invention, the diamine component may contain a diamine compound other than the above-mentioned aromatic diamine compounds. Examples thereof include m-phenylenediamine, p-phenylenediamine, 2,4-diaminotoluene, 2,4(6)-diamino-3,5-diethyltoluene, 5(6)-amino-1,3,3-trimethyl-1-(4-aminophenyl)-indan, 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl, 4,4'-diamino-2,2'-ditrifluoromethyl-1,1'-biphenyl, 4,4'-diamino-3,3'-dimethyl-1,1'-biphenyl, 3,4'-diaminodiphenyl ether, 4,4'- Diaminodiphenyl ether, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfide, 4-aminophenyl-4-aminobenzoate, 4,4'-(9-fluorenylidene)dianiline, 9,9'-bis(3-methyl-4-aminophenyl)fluorene, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 2,2-bis(4-aminophenyl) )propane, 2,2-bis(3-methyl-4-aminophenyl)propane, 4,4'-(hexafluoroisopropylidene)dianiline, 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane, 2,2-bis(3-methyl-4-aminophenyl)benzene, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, α,α-bis[4-(4-aminophenoxy)phenyl]-1,3-diisopropyl Benzene, α,α-bis[4-(4-aminophenoxy)phenyl]-1,4-diisopropylbenzene, 3,7-diamino-dimethyldibenzothiophene, bis[4-(4-aminophenoxy)phenyl]sulfone, 5,5-dioxide, bis(3-carboxy-4-aminophenyl)methylene, 3,3'-diamino-4,4'-dihydroxy-1,1'-biphenyl, 4,4'-diamino-3,3'-dihydroxy-1,1'-biphenyl, 2,2-bis(3-amino-4-hydroxyphenyl)propane, 2,Examples include 2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 1,3-bis(3-hydroxy-4-aminophenoxy)benzene, 2,2-bis(3-hydroxy-4-aminophenyl)benzene, and 3,3'-diamino-4,4'-dihydroxydiphenylsulfone.

[0026] When the diamine component of the polyimide contains the aromatic diamine represented by the above formula (1) and other diamines, in consideration of suppressing electrode expansion and improving recycling characteristics, the aromatic diamine compound represented by the above formula (1) is preferably contained in a proportion of 20 mol % or more, more preferably 50 mol % or more, of the total diamine compounds.

[0027] The acid anhydride constituting the polyimide of the present invention is not particularly limited, and various acid anhydrides used in polyimides can be used, such as oxydiphthalic acid, pyromellitic dianhydride, 3-fluoropyromellitic dianhydride, 3,6-difluoropyromellitic dianhydride, 3,6-bis(trifluoromethyl)pyromellitic dianhydride, 1,2,3,4-benzenetetracarboxylic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and 3,3',4,4'-biphenylsulfonetetracarboxylic acid. Acid dianhydride, 4,4'-(4,4'-isopropylidenediphenoxy)bisphthalic dianhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, 3,3'',4,4''-terphenyltetracarboxylic dianhydride, 3,3''',4,4'''-quaterphenyltetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, methylene-4,4'-diphthalic dianhydride, 1,1-ethynylidene-4,4'-diphthalic dianhydride, 2,2-propylidene-4,4'-diphthalic acid Dianhydride, 1,2-ethylene-4,4'-diphthalic dianhydride, 1,3-trimethylene-4,4'-diphthalic dianhydride, 1,4-tetramethylene-4,4'-diphthalic dianhydride, 1,5-pentamethylene-4,4'-diphthalic dianhydride, 1,3-bis[2-(3,4-dicarboxyphenyl)-2-propyl]benzene dianhydride, 1,4-bis[2-(3,4-dicarboxyphenyl)-2-propyl]benzene dianhydride, bis[3-(3,4-dicarboxyphenoxy)phenyl]methane dianhydride, bis[4 -(3,4-dicarboxyphenoxy)phenyl]methane dianhydride, 2,2-bis[3-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride, difluoromethylene-4,4'-diphthalic dianhydride, 1,1,2,2-tetrafluoro-1,2-ethylene-4,4'-diphthalic dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, oxy-4,4'-diphthalic dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, thio-4,4'-diphthalic dianhydride, sulfonyl-4,4'-diphthalic dianhydride, 1,3-bis(3,4-dicarboxyphenyl)benzene dianhydride, 1,4-bis(3,4-dicarboxyphenyl)benzene dianhydride, 1,3-bis(3,4-dicarboxyphenoxy)benzene dianhydride, 1,4-bis(3,4-dicarboxyphenoxy)benzene dianhydride, bis(3,4-dicarboxyphenoxy)dimethylsilane dianhydride, 1,3-bis(3,4-dicarboxyphenoxy)-1,1,3 ,3-Tetramethyldisiloxane dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, 2,3,6,7-anthracenetetracarboxylic dianhydride, 1,2,7,8-phenanthrenetetracarboxylic dianhydride, 1,2,3,4-butanetetracarboxylic dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, cyclopentanetetracarboxylic dianhydride, cyclohexane-1,2,3,4-tetracarboxylic acid Acid dianhydrides, cyclohexane-1,2,4,5-tetracarboxylic dianhydride, 3,3',4,4'-bicyclohexyltetracarboxylic dianhydride, carbonyl-4,4'-bis(cyclohexane-1,2-dicarboxylic) dianhydride, methylene-4,4'-bis(cyclohexane-1,2-dicarboxylic) dianhydride, 1,2-ethylene-4,4'-bis(cyclohexane-1,2-dicarboxylic) dianhydride, oxy-4,4'-bis(cyclohexane-1,2-dicarboxylic) dianhydride, thio-4,4'-bis(cyclohexane-1,2-dicarboxylic) ) dianhydride, sulfonyl-4,4'-bis(cyclohexane-1,2-dicarboxylic acid) dianhydride, 3,3',5,5'-tetrakis(trifluoromethyl)oxy-4,4'-diphthalic dianhydride, 3,3',6,6'-tetrakis(trifluoromethyl)oxy-4,4'-diphthalic dianhydride, 5,5',6,6'-tetrakis(trifluoromethyl)oxy-4,4'-diphthalic dianhydride, 3,3',5,5',6,6'-hexakis(trifluoromethyl)oxy-4,4'-diphthalic dianhydride, 3,3'-difluorosulfonyl-4,4'-Diphthalic dianhydride, 5,5'-difluorosulfonyl-4,4'-diphthalic dianhydride, 6,6'-difluorosulfonyl-4,4'-diphthalic dianhydride, 3,3',5,5',6,6'-hexafluorosulfonyl-4,4'-diphthalic dianhydride, 3,3'-bis(trifluoromethyl)sulfonyl-4,4'-diphthalic dianhydride, 5,5'-bis(trifluoromethyl)sulfonyl-4,4'-diphthalic dianhydride, 6,6'-bis(trifluoromethyl)sulfonyl-4,4'-diphthalic dianhydride, 3,3',5,5'-tetrakis (Trifluoromethyl)sulfonyl-4,4'-diphthalic dianhydride, 3,3',6,6'-tetrakis(trifluoromethyl)sulfonyl-4,4'-diphthalic dianhydride, 5,5',6,6'-tetrakis(trifluoromethyl)sulfonyl-4,4'-diphthalic dianhydride, 3,3',5,5',6,6'-hexakis(trifluoromethyl)sulfonyl-4,4'-diphthalic dianhydride, 3,3'-difluoro-2,2-perfluoropropylidene-4,4'-diphthalic dianhydride, 5,5'-difluoro-2,2-perfluoropropylidene-4 ,4'-Diphthalic dianhydride, 6,6'-difluoro-2,2-perfluoropropylidene-4,4'-diphthalic dianhydride, 3,3',5,5',6,6'-hexafluoro-2,2-perfluoropropylidene-4,4'-diphthalic dianhydride, 3,3'-bis(trifluoromethyl)-2,2-perfluoropropylidene-4,4'-diphthalic dianhydride, ethylene glycol bistrimellitate dianhydride, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, methylhimic anhydride, hexahydrophthalic anhydride, tetrahydrophthalic anhydride Water phthalic acid, trialkyltetrahydrophthalic anhydride, methylcyclohexene dicarboxylic anhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, bicyclo[2.2.2]octane-2,3,5,6-tetracarboxylic dianhydride, 3-(carboxymethyl)1,2,4-cyclopentanecarboxylic acid 1,4:2,3-dianhydride, 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, 4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,Examples include 4-tetrahydronaphthalene-1,2-dicarboxylic anhydride, 5,5'-(1,4-phenylene)bis(hexahydro-4,7-methanoisobenzofuran-1,3-dione, octahydro-3H,3''H-dispiro[4,7-methanoisobenzofuran-5,1'-cyclopentane-3',5''-[4,7]methanoisobenzofuran]-1,1'',2',3,3''(4H,4''H)-pentaone. These may be used alone or in combination of two or more.

[0028] Among the above-mentioned acid anhydrides, alicyclic acid anhydrides and aromatic acid anhydrides are preferably used from the viewpoint of increasing the proportion of the aromatic diamine compound represented by formula (1) in the diamine component of the polyimide.

[0029] The polyimide of the present invention can be obtained by reacting a diamine component containing the aromatic diamine compound represented by the above formula (1) with an acid anhydride component to obtain a polyamic acid, followed by a cyclization dehydration reaction to convert it into a polyimide. The obtained polyimide is solvent-soluble, and can be easily formed into a film by dissolving it in an appropriate organic solvent to form a varnish.

[0030] The mixing ratio of the diamine component and the acid anhydride component is preferably 0.5 to 1.5 mol %, more preferably 0.9 to 1.1 mol %, of the total amount of the diamine component relative to 1 mol % of the total amount of the acid anhydride.

[0031] The reaction between the diamine component and the acid anhydride component is preferably carried out in an organic solvent. The organic solvent is not particularly limited as long as it does not react with the diamine compound and the acid anhydride and can dissolve the reaction product of the diamine compound and the acid anhydride. Examples of the organic solvent include N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, N,N'-dimethylimidazolidinone, γ-butyrolactone, dimethyl sulfoxide, sulfolane, 1,3-dioxolane, tetrahydrofuran, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, dipropylene glycol dimethyl ether, diethylene glycol dibutyl ether, dibenzyl ether, methyl lactate, ethyl lactate, butyl lactate, methyl benzoate, ethyl benzoate, triglyme, tetraglyme, toluene, and xylene. From the viewpoint of the solubility of the diamine compound of the above formula (1), N-methyl-2-pyrrolidone, N,N'-dimethylimidazolidinone, and γ-butyrolactone are preferred.

[0032] The reaction temperature between the diamine compound and the acid anhydride is preferably 40°C or lower in the case of chemical imidization, and is preferably 150 to 220°C, more preferably 170 to 200°C, in the case of thermal imidization.

[0033] An imidization catalyst may be used during the cyclization dehydration reaction, and examples of such catalysts include methylamine, ethylamine, trimethylamine, triethylamine, propylamine, tripropylamine, butylamine, tributylamine, tert-butylamine, hexylamine, triethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, triethylenediamine, N-methylpyrrolidine, N-ethylpyrrolidine, aniline, benzylamine, toluidine, trichloroaniline, pyridine, collidine, lutidine, picoline, quinoline, isoquinoline, and valerolactone. Also, if necessary, an azeotropic dehydrating agent such as toluene, xylene, or ethylcyclohexane, or an acid catalyst such as acetic anhydride, propionic anhydride, butyric anhydride, or benzoic anhydride may be used.

[0034] In the reaction between the diamine compound and the acid anhydride, a sealing agent such as benzoic acid, phthalic anhydride, or hydrogenated phthalic anhydride can be used.

[0035] Furthermore, a double bond or a triple bond can be introduced into the terminal of the polyimide compound by using maleic anhydride, ethynylphthalic anhydride, methylethynylphthalic anhydride, phenylethynylphthalic anhydride, phenylethynyltrimellitic anhydride, 3- or 4-ethynylaniline, or the like.

[0036] [Anode materials for lithium-ion secondary batteries] The polyimide of the present invention can be suitably used as a negative electrode material for lithium ion secondary batteries (hereinafter sometimes simply referred to as "negative electrode material"), particularly as a binder component for the negative electrode. That is, it can be used as a negative electrode material containing the polyimide as a binder and a negative electrode active material.

[0037] The above-mentioned polyimides may be used alone as the binder, or may contain other resins, such as polyvinylidene fluoride (PVDF). Furthermore, instead of PVDF, other binders may be used, such as PVDF copolymer resins, fluororesins, styrene-butadiene rubber (SBR), ethylene-propylene rubber (EPDM), and styrene-acrylonitrile copolymers. Examples of PVDF copolymer resins include copolymers of PVDF with hexafluoropropylene (HFP), perfluoromethyl vinyl ether (PFMV), or tetrafluoroethylene (TFE). Examples of fluororesins include polytetrafluoroethylene (PTFE), fluororubbers, and the like. Other binders may include polysaccharides such as carboxymethyl cellulose (CMC), and thermoplastic resins such as polyimide resins. Furthermore, two or more of the above binders may be used in combination.

[0038] The polyimide content in the negative electrode material is preferably 1% by mass or more and 20% by mass or less, and more preferably 3% by mass or more and 18% by mass or less. By setting the polyimide content in the negative electrode material within the above range, the cycle characteristics of the lithium ion secondary battery can be further improved.

[0039] From the viewpoint of battery capacity, it is preferable to use a silicon-based material as the negative electrode active material. Examples of the silicon-based material include silicon particles, alloys of silicon with metals such as tin, nickel, iron, copper, silver, cobalt, manganese, and zinc, and compounds of silicon with boron, nitrogen, oxygen, and carbon.

[0040] Examples of silicon-based materials include Si, SiO, SiO2, SiB4, Mg2Si, Ni2Si, CoSi2, NiSi2, Cu5Si, FeSi2, MnSi2, ZnSi2, SiC, Si3N4, and Si2N2O, and among these, Si and SiO are preferred.

[0041] The negative electrode active material may be a material other than a silicon-based material, such as metallic lithium, a metal oxide, or graphite. The negative electrode material may contain two or more negative electrode active materials.

[0042] From the viewpoint of battery capacity, the silicon-based negative electrode active material is preferably contained in an amount of 30 parts by mass or more, more preferably 50 parts by mass or more, and particularly preferably 95 parts by mass or more, per 100 parts by mass of the negative electrode active material.

[0043] In one embodiment, the negative electrode material may contain a conductive agent in addition to the polyimide and negative electrode active material described above. Examples of conductive agents include carbon black (acetylene black, ketjen black, furnace black, etc.), graphite, carbon fiber, carbon flakes, metal fiber and foil, etc. Among these, carbon black is preferred, and acetylene black is more preferred. These conductive agents may be used alone or in combination of two or more.

[0044] The content of the conductive agent in the negative electrode material is preferably 0.1% by mass or more and 25% by mass or less, and more preferably 1% by mass or more and 20% by mass or less, based on the total amount of the negative electrode material. By setting the content of the conductive agent within this range, a good conductive path can be formed in the negative electrode active material layer.

[0045] The negative electrode material may contain additives, such as thickeners and fillers, within the range that does not impair the characteristics of the present invention.

[0046] [Anode for lithium-ion secondary batteries] The negative electrode for a lithium ion secondary battery of the present invention comprises a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, and the negative electrode active material layer is made of the above-mentioned negative electrode material.

[0047] The current collector is not particularly limited, and examples thereof include copper, nickel, stainless steel, gold, iron, aluminum and alloys thereof, nickel-plated steel, and chrome-plated steel.

[0048] The thickness of the negative electrode active material layer is preferably 15 μm or more and 150 μm or less, and more preferably 20 μm or more and 120 μm or less. By setting the thickness of the negative electrode active material layer within the above numerical range, the initial charge / discharge efficiency and cycle characteristics of the lithium ion secondary battery can be further improved.

[0049] The negative electrode for a lithium ion secondary battery of the present invention can be produced by applying a solution in which the above-mentioned negative electrode material is dissolved or dispersed in an organic solvent onto a current collector and drying it.

[0050] As the organic solvent, those mentioned above can be used, and from the viewpoint of the solubility or dispersibility of the resin composition, N-methyl-2-pyrrolidone, N,N'-dimethylimidazolidinone and γ-butyrolactone are preferred.

[0051] The application method is not particularly limited, and examples thereof include a die coater method, a three-roll transfer coater method, a doctor blade method, a dip method, a direct roll method, and a gravure method.

[0052] [Lithium-ion secondary battery] The lithium ion secondary battery of the present invention includes the above-described negative electrode, a positive electrode, and a non-aqueous electrolyte. In one embodiment, the lithium ion secondary battery of the present invention also includes a separator disposed between the negative electrode and the positive electrode.

[0053] The positive electrode may be any of the conventional positive electrodes used in lithium ion secondary batteries. The positive electrode may be prepared by applying a positive electrode material onto a current collector and drying the applied material.

[0054] The positive electrode forming material contains a positive electrode active material and a binder, and may further contain the above-mentioned conductive agent and additives.

[0055] Examples of the positive electrode active material include lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, and lithium iron phosphate.

[0056] As the binder, the polyimide or binder described above as the negative electrode material can be used.

[0057] Furthermore, the positive electrode is not limited to the above, and lithium foil or the like can also be used.

[0058] The separator may be a conventionally known one, for example, a paper separator, a resin separator such as polyethylene or polypropylene, or a glass fiber separator.

[0059] The positive electrode and negative electrode are placed in a battery container, which is filled with an organic solvent having an electrolyte dissolved therein. The electrolyte is not particularly limited, and examples thereof include non-aqueous electrolytes such as LiPF6, LiClO4, LiBF4, LiClF4, LiAsF6, LiSbF6, LiAlO4, LiAlCl4, CF3SO3Li, LiN(CF3SO2)3, LiCl, and LiI. Among these, LiPF6, LiClO4, and CF3SO3Li, which have a high degree of dissociation, are preferred.

[0060] The organic solvent is not particularly limited either, and examples thereof include carbonate compounds, lactone compounds, ether compounds, sulfolane compounds, dioxolane compounds, ketone compounds, nitrile compounds, and halogenated hydrocarbon compounds. Specific examples include carbonates such as dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, ethylene glycol dimethyl carbonate, propylene glycol dimethyl carbonate, ethylene glycol diethyl carbonate, and vinylene carbonate, lactones such as γ-butyl lactone, ethers such as dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, and 1,4-dioxane, sulfolanes such as sulfolane and 3-methylsulfolane, dioxolanes such as 1,3-dioxolane, ketones such as 4-methyl-2-pentanone, nitriles such as acetonitrile, propionitrile, valeronitrile, and benzonitrile, halogenated hydrocarbons such as 1,2-dichloroethane, and ionic liquids such as other methyl formates, dimethylformamide, diethylformamide, dimethyl sulfoxide, imidazolium salts, and quaternary ammonium salts. Furthermore, mixtures of these may also be used.

[0061] Among the organic solvents mentioned above, carbonate compounds are preferably used because they have low solubility for the polyimide used in the negative electrode and can suppress swelling of the polyimide.

[0062] Lithium-ion secondary batteries can be formed into any shape, such as a cylindrical type, a coin type, a rectangular type, a laminate type, or any other type. The basic configuration of the battery is the same regardless of the shape, and the design can be modified depending on the purpose. For example, in the case of a cylindrical type, a negative electrode formed by applying a negative electrode active material to a negative electrode current collector and a positive electrode formed by applying a positive electrode active material to a positive electrode current collector are wound together via a separator, and the resulting wound body is housed in a battery can, and a nonaqueous electrolyte is poured into it and sealed with insulating plates placed on the top and bottom. In addition, when applied to a coin-type lithium secondary battery, a disc-shaped negative electrode, a separator, a disc-shaped positive electrode, and a stainless steel plate are stacked and housed in a coin-type battery can, and a nonaqueous electrolyte is poured into it and sealed. [Example]

[0063] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following, "parts" and "%" are all by mass unless otherwise specified.

[0064] <Synthesis of Polyimide> [Example 1] A 500 ml separable flask equipped with a nitrogen inlet tube and a stirrer was charged with 64.88 g (150 mmol) of bis[4-(4-aminophenoxy)phenyl]sulfone (BAPS), an aromatic diamine compound represented by the formula below, 15.22 g (50 mmol) of (2-phenyl-4-aminophenyl)-4-aminobenzoate (PHBAAB), an aromatic diamine compound represented by the formula below, 62.04 g (200 mmol) of diphenyl-3,3',4,4'-tetracarboxylic dianhydride (ODPA), represented by the formula below, 868 g of N-methyl-2-pyrrolidone, 4.0 g (50 mmol) of pyridine, and 87 g of toluene. The mixture was reacted under a nitrogen atmosphere at 180°C for 9 hours while removing the toluene from the system during the reaction, thereby obtaining a 15 wt % polyimide solution.

[0065] [ka]

[0066] [Example 2] Using the same apparatus as in Example 1, 43.25 g (100 mmol) of BAPS, 30.44 g (100 mmol) of PHBAAB, 62.04 g (200 mmol) of ODPA, 868 g of N-methyl-2-pyrrolidone, 4.0 g (50 mmol) of pyridine, and 87 g of toluene were added, and the mixture was reacted under a nitrogen atmosphere at 180°C for 9 hours while removing the toluene from the system during the reaction, thereby obtaining a 15 wt % polyimide solution.

[0067] [Comparative Example 1] Using the same apparatus as in Example 1, 86.5 g (200 mmol) of BAPS, 62.04 g (200 mmol) of ODPA, 868 g of N-methyl-2-pyrrolidone, 4.0 g (50 mmol) of pyridine, and 87 g of toluene were added, and the mixture was reacted under a nitrogen atmosphere at 180°C for 9 hours while removing the toluene from the system during the reaction, thereby obtaining a 15 wt % polyimide solution.

[0068] [Example 3] Using the same apparatus as in Example 1, 43.26 g (200 mmol) of 4,4'-diaminodiphenyl sulfide (ASD), an aromatic diamine compound represented by the formula below, 60.87 g (200 mmol) of PHBAAB, 89.67 g (400 mmol) of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA) represented by the formula below, 538 g of N-methyl-2-pyrrolidone, 6.3 g (80 mmol) of pyridine, and 54 g of toluene were added, and the mixture was reacted under a nitrogen atmosphere at 180°C for 14 hours while removing toluene from the system during the reaction, thereby obtaining a 25 wt% polyimide solution.

[0069] [ka]

[0070] [Example 4] Using the same apparatus as in Example 1, 121.74 g (400 mmol) of PHBAAB, 89.67 g (400 mmol) of HPMDA, 591 g of N-methyl-2-pyrrolidone, 6.3 g (80 mmol) of pyridine, and 59 g of toluene were added, and the mixture was reacted under a nitrogen atmosphere at 180°C for 14 hours while removing the toluene from the system during the reaction, thereby obtaining a 25 wt% polyimide solution.

[0071] Comparative Example 2 Using the same apparatus as in Example 1, 86.52 g (400 mmol) of ASD, 89.67 g (400 mmol) of HPMDA, 485 g of N-methyl-2-pyrrolidone, 6.3 g (80 mmol) of pyridine, and 49 g of toluene were added, and the mixture was reacted under a nitrogen atmosphere at 180°C for 14 hours while removing the toluene from the system during the reaction, thereby obtaining a 25 wt % polyimide solution.

[0072] [Example 5] Using the same apparatus as in Example 1, 40.05 g (200 mmol) of 4,4'-diaminodiphenyl ether (ODA), an aromatic diamine compound represented by the following formula, 60.87 g (200 mmol) of PHBAAB, 124.09 g (400 mmol) of ODPA, 959 g of N-methyl-2-pyrrolidone, 6.3 g (80 mmol) of pyridine, and 96 g of toluene were added, and the mixture was reacted under a nitrogen atmosphere at 180°C for 7 hours while removing the toluene from the system during the reaction, thereby obtaining an 18 wt% polyimide solution.

[0073] [ka]

[0074] [Example 6] Using the same apparatus as in Example 1, 61.58 g (150 mmol) of 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), an aromatic diamine compound represented by the following formula, 45.65 g (150 mmol) of PHBAAB, 93.07 g (300 mmol) of ODPA, 863 g of N-methyl-2-pyrrolidone, 4.8 g (60 mmol) of pyridine, and 86 g of toluene were added, and the mixture was reacted under a nitrogen atmosphere at 180°C for 7 hours while removing the toluene from the system during the reaction, thereby obtaining an 18 wt % polyimide solution.

[0075] [ka]

[0076] [Example 7] Using the same apparatus as in Example 1, 20.02 g (100 mmol) of ODA, 41.05 g (100 mmol) of BAPP, 30.44 g (100 mmol) of PHBAAB, 93.07 g (300 mmol) of ODPA, 792 g of N-methyl-2-pyrrolidone, 4.8 g (60 mmol) of pyridine, and 79 g of toluene were added, and the mixture was reacted under a nitrogen atmosphere at 180°C for 7 hours while removing the toluene from the system during the reaction, thereby obtaining an 18 wt % polyimide solution.

[0077] [Example 8] Using the same apparatus as in Example 1, 64.87 g (150 mmol) of BAPS, 45.65 g (150 mmol) of PHBAAB, 46.53 g (150 mmol) of ODPA, 48.33 g (150 mmol) of 3,4,3',4'-benzophenonetetracarboxylic dianhydride (BTDA) represented by the following formula, 886 g of N-methyl-2-pyrrolidone, 4.8 g (60 mmol) of pyridine, and 89 g of toluene were added, and the mixture was reacted under a nitrogen atmosphere at 180°C for 4 hours while removing the toluene from the system during the reaction, thereby obtaining an 18 wt % polyimide solution.

[0078] [ka]

[0079] <Measurement of film properties> Each of the polyimide solutions obtained in Examples 1 to 8 and Comparative Examples 1 and 2 was applied to a 10 cm square glass plate by spin coating, and dried for 0.5 hours at 100° C., 0.5 hours at 200° C., and 1 hour at 250° C. Then, the solution was peeled off from the glass plate and cut to obtain a test piece measuring 80 mm long, 10 mm wide, and 15 μm thick.

[0080] For each test piece obtained in this manner, the elastic modulus, tensile strength, and elongation were measured in both the MD and TD directions using a tensile tester (Shimadzu Corporation, product name: AG-Xplus 50kN) at a tension speed of 10 mm / min. The average values ​​of the measured values ​​in the MD and TD directions were calculated as the elastic modulus, tensile strength, and elongation. The measurement results are shown in Table 1. In Table 1, the "measurable" for Examples 3 and 4 indicates that the test piece was not successfully prepared using the above-mentioned method of applying a polyimide solution to a glass plate to obtain a polyimide monolayer film, and therefore the film properties could not be measured.

[0081] <Evaluation of cycle characteristics> The obtained polyimide was used as a binder, and silicon monoxide (SiO) and graphite as negative electrode active materials, and a conductive agent were mixed in the following composition to prepare a negative electrode material 1 for a lithium ion secondary battery. Polyimide 10.0% by mass Silicon monoxide 52.2% by mass ·Graphite 34.8% by mass Conductive agent 3.00% by mass

[0082] A 10 μm thick electrolytic copper foil was prepared as a negative electrode current collector, and the negative electrode material 1 obtained as described above was applied to the surface of the electrolytic copper foil and dried to form a 50 μm thick negative electrode active material layer, thereby producing a negative electrode 1.

[0083] In addition, a negative electrode material 2 for a lithium ion secondary battery was prepared in the same manner as above, except that the composition of the negative electrode material 1 for a lithium ion secondary battery was changed as follows, and a negative electrode 2 was produced using this in the same manner as above. Polyimide 10.0% by mass Silicon monoxide 87.0% by mass Conductive agent 3.00% by mass

[0084] Furthermore, a negative electrode material 3 for a lithium ion secondary battery was prepared in the same manner as above, except that the composition of the negative electrode material 1 for a lithium ion secondary battery was changed as follows. Polyimide 15.0% by mass Silicon 80.0% by mass Conductive agent 5.00% by mass A 10 μm thick electrolytic copper foil was prepared as a negative electrode current collector, and the negative electrode material 3 obtained as described above was applied to the surface of the electrolytic copper foil and dried to form a 25 μm thick negative electrode active material layer, thereby producing a negative electrode 3.

[0085] A lithium foil was prepared as the positive electrode, ethylene carbonate and ethyl methyl carbonate as the electrolyte, and a polyolefin single-layer separator (Celgard (registered trademark) 2500, manufactured by Celgard Co., Ltd.) were prepared, and coin cell-type lithium ion secondary batteries were fabricated using each of the negative electrodes obtained as described above.

[0086] The lithium ion secondary battery fabricated as described above was left to stand for 24 hours in an environment at 25° C. Thereafter, the performance of the lithium ion secondary battery was evaluated in an environment at 25° C. as follows.

[0087] The battery was subjected to CC (constant current) charging at a current density equivalent to 0.1 C up to 5 mV, then switched to CV (constant voltage) charging at 5 mV, and charged to a current density equivalent to 0.01 C, followed by CC discharging at a current density equivalent to 0.1 C down to 1.2 V. This cycle was repeated twice at 25°C. The discharge capacity equivalent to 0.1 C in the first cycle was defined as A. Subsequently, CC (constant current) charging was performed at a current density equivalent to 0.2C to 5mV, followed by switching to CV (constant voltage) charging at 5mV, charging until a current density equivalent to 0.02C was reached, followed by CC discharging at a current density equivalent to 0.2C to 1.2V for three cycles at 25 ° C., followed by CC (constant current) charging at a current density equivalent to 0.5C to 5mV, followed by switching to CV (constant voltage) charging at 5mV, charging until a current density equivalent to 0.05CC was reached, followed by CC discharging at a current density equivalent to 0.5C to 1.2V for 30 cycles at 25 ° C. The discharge capacity equivalent to 0.5C at this 30th cycle was designated B. The cycle characteristics are calculated using the following formula: ΔC(%)=(B / A)×100 The cycle characteristics were evaluated as shown in Table 1 below.

[0088] In Table 1, "SiO60%" indicates negative electrode material 1 for lithium ion secondary batteries (a composition in which the negative electrode active material contains 60% by mass of silicon monoxide), "SiO100%" indicates negative electrode material 2 for lithium ion secondary batteries (a composition in which the negative electrode active material contains 100% by mass of silicon monoxide), and "Si100%" indicates negative electrode material 3 for lithium ion secondary batteries (a composition in which the negative electrode active material contains 100% by mass of silicon).

[0089] [Table 1]

Claims

1. A polyimide used as an electrode material for a lithium ion secondary battery, The diamine component is represented by the following formula (1): 【Chemistry 1】 (In the formula, R １ ~R ４ is a hydrogen atom, R ５ ~R ８ is an aromatic group having 6 to 10 carbon atoms, a phenoxy group, a benzyl group, or a benzyloxy group, and any other R ５ ~R ８ is a hydrogen atom.

2. 2. The polyimide of claim 1, wherein the acid anhydride component comprises at least one of an alicyclic acid anhydride or an aromatic acid anhydride.

3. 3. The polyimide according to claim 1, wherein the compound of formula (1) is contained in the diamine component in a proportion of 20 mol % or more.

4. 4. A negative electrode material for a lithium ion secondary battery, comprising the polyimide according to claim 1 and a silicon-based negative electrode active material as a negative electrode active material.

5. The negative electrode material according to claim 4 , wherein the silicon-based negative electrode active material is contained in an amount of 30 parts by mass or more relative to 100 parts by mass of the negative electrode active material.

6. A negative electrode for a lithium ion secondary battery comprising: a negative electrode current collector; and a negative electrode active material layer formed on the negative electrode current collector, 6. A negative electrode for a lithium ion secondary battery, wherein the negative electrode active material layer comprises the negative electrode material according to claim 4 or 5.

7. A lithium ion secondary battery comprising the negative electrode according to claim 6, a positive electrode, and a non-aqueous electrolyte.

Images