Binder for metal-ion secondary battery

Abstract

To provide a binder for a metal-ion secondary battery that exhibits excellent binding properties with an active material and excellent adhesion to a current collector and that increases Coulombic efficiency while suppressing decomposition of an electrolytic solution.SOLUTION: The binder includes a fumaric-acid-based polymer having a repeating unit represented by Formula (I) and a nitrogen-cation-containing polymer having a repeating unit represented by Formula (II). (In Formula (I), R1 and R2 independently indicate H or an alkali-metal atom.)SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a binder for metal ion secondary batteries. More specifically, the present invention relates to a binder for metal ion secondary batteries, a metal ion secondary battery having a negative electrode containing the binder for metal ion secondary batteries, and a nitrogen cation-containing polymer that can be suitably used for the binder for metal ion secondary batteries. [Background technology]

[0002] Polyacrylic acid and polyvinylidene fluoride are commonly used as negative electrode binders for metal ion secondary batteries such as lithium ion secondary batteries, but polyacrylic acid is more widely used than polyvinylidene fluoride because it has better performance than polyvinylidene fluoride (see, for example, Patent Documents 1 to 3).

[0003] However, since polyacrylic acid has carbon chains between its functional groups, it is not possible to introduce functional groups into the polyacrylic acid molecule at a high density, which limits its ability to bind to the active material and its adhesion to the current collector. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-152123 [Patent Document 2] Japanese Patent Application Publication No. 2019-175629 [Patent Document 1] International Publication No. 2019 / 212040 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been made in view of the above-mentioned conventional technology, and an object of the present invention is to provide a binder for metal ion secondary batteries which, compared to conventional binders using polyacrylic acid, has excellent binding properties with an active material and adhesion with a current collector, and which can increase Coulomb efficiency while suppressing decomposition of an electrolyte, and a metal ion secondary battery having a negative electrode containing the binder for metal ion secondary batteries. [Means for solving the problem]

[0006] The present invention provides (1) Formula (I):

[0007] [ka] (In the formula, R 1 and R 2 each independently represents a hydrogen atom or an alkali metal atom) and a fumaric acid-based polymer having a repeating unit represented by formula (II):

[0008] [ka]

[0009] (In the formula, X + is N + (This refers to a cation with a ring structure containing a binder for a metal ion secondary battery, comprising at least one binder polymer selected from the group consisting of nitrogen cation-containing polymers having a repeating unit represented by the formula: (2) A metal ion secondary battery having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the negative electrode contains the binder for a metal ion secondary battery according to (1) above; and (3) Formula (II):

[0010] [ka]

[0011] (In the formula, X + is N + (This refers to a cation with a ring structure containing The present invention relates to a nitrogen cation-containing polymer having a repeating unit represented by the formula: [Effects of the Invention]

[0012] According to the present invention, there are provided a binder for metal ion secondary batteries which has excellent binding properties with active materials and adhesion with current collectors, as compared with conventional binders which use polyacrylic acid, and which can increase Coulombic efficiency while suppressing decomposition of the electrolyte, and a metal ion secondary battery having a negative electrode containing the binder for metal ion secondary batteries. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a graph showing the H-NMR spectrum and the C-NMR spectrum of the fumaric acid homopolymer obtained in Example 1. [Figure 2] 1 is a graph showing the FT-IR spectrum of the fumaric acid homopolymer obtained in Example 1. [Figure 3] 1 is a graph showing the results of mass spectrometry of the fumaric acid homopolymer obtained in Example 1. [Figure 4] 1 is a graph showing the adhesive strength of fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride used as a binder. [Figure 5] 1 is a graph showing cyclic voltammograms measured using half cells in which fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride is used as the binder. [Figure 6] 1 is a graph showing Nyquist plots before and after cyclic voltammetry measurements using half cells in which fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride is used as the binder. [Figure 7]1 is a graph showing Arrhenius plots measured using half-cells in which fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride is used as the binder. [Figure 8] 1 is a graph showing the change in specific capacity measured using half cells in which fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride is used as the binder. [Figure 9] 1 is a graph showing the results of measuring the internal resistance of a battery during lithiation and delithiation, measured using half cells in which fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride is used as a binder. [Figure 10] 1 is a graph showing cyclic voltammograms measured using half-cells in which fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride is used as the binder. [Figure 11] 1 is a graph showing Nyquist plots before and after cyclic voltammetry measurements using half cells in which fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride is used as the binder. [Figure 12] 1 is a graph showing Arrhenius plots measured using half-cells in which fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride is used as the binder. [Figure 13] 1 is a graph showing the change in specific capacity measured using half cells in which fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride is used as the binder. [Figure 14] 1 is a graph showing the results of measuring the internal resistance of a battery in a sodiated and desodium state, measured using half cells in which a fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride is used as a binder. [Figure 15] 1 is a graph showing the 13C-NMR spectrum of an oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer. [Figure 16]1 is a graph showing the FT-IR spectrum of an oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer. [Figure 17] 1 is a graph showing cyclic voltammograms measured using a half cell in which oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer is used as a binder. [Figure 18] 1 is a graph showing Nyquist plots before cyclic voltammetry using half cells in which oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer or polyvinylidene fluoride is used as the binder. [Figure 19] 1 is a graph showing an Arrhenius plot using a half cell in which oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer or polyvinylidene fluoride is used as the binder. [Figure 20] This graph shows the results of investigating the specific capacity stepwise from 0.1 C to 2 C using a half-cell that uses oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer as a binder and a half-cell that uses polyvinylidene fluoride. [Figure 21] 1 is a graph showing the results of examining the change in specific capacity using half cells in which oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer or polyvinylidene fluoride is used as a binder. [Figure 22] 1 is a graph showing the results of measuring the internal resistance of a battery during lithiation and delithiation, measured using half cells in which oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer or polyvinylidene fluoride is used as a binder. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will now be described in detail, but the present invention is not limited to the embodiments described below, and various changes and modifications can be made by those skilled in the art within the scope of the technical ideas described in this specification.

[0015] 1. Binders for metal-ion secondary batteries The binder for a metal ion secondary battery of the present invention has the formula (I):

[0016] [ka]

[0017] (In the formula, R 1 and R 2 each independently represents a hydrogen atom or an alkali metal atom) and a fumaric acid-based polymer having a repeating unit represented by formula (II):

[0018] [ka]

[0019] (In the formula, X + is N + (This refers to a cation with a ring structure containing One of the features of the present invention is that it contains at least one binder polymer selected from the group consisting of nitrogen cation-containing polymers having a repeating unit represented by the following formula:

[0020] The binder for metal ion secondary batteries of the present invention has excellent binding properties with the active material and adhesion with the current collector, compared to conventional binders that use polyacrylic acid, and exhibits the excellent effect of being able to increase the Coulomb efficiency while suppressing decomposition of the electrolyte.

[0021] The binder for metal ion secondary batteries of the present invention can be used as either a positive electrode binder or a negative electrode binder, but is preferably used as a negative electrode binder.

[0022] The fumaric acid-based polymer having a repeating unit represented by formula (I) and the nitrogen cation-containing polymer having a repeating unit represented by formula (II) may be used either alone or in combination.

[0023] (1) Fumaric acid polymer As described above, the fumaric acid-based polymer is a polymer represented by formula (I). Research by the present inventors has revealed that the fumaric acid-based polymer can prevent cracks from occurring in electrodes during use of a metal-ion secondary battery, suppresses decomposition of the electrolyte, and improves the electrochemical performance of the metal-ion secondary battery.

[0024] In formula (I), R 1 and R 2 are each independently a hydrogen atom or an alkali metal atom. Examples of alkali metal atoms include lithium, sodium, and potassium atoms. Among these alkali metal atoms, lithium and sodium atoms are preferred. R 1 and R 2 Among these, hydrogen atoms are preferred from the viewpoints of improving the binding strength with the active material and the adhesion strength with the current collector, suppressing decomposition of the electrolyte, and increasing the Coulomb efficiency.

[0025] In formula (I), R 1 and R 2 When both are hydrogen atoms, the fumaric acid-based polymer represented by formula (I) is a fumaric acid homopolymer. The fumaric acid homopolymer can be suitably used in the present invention from the viewpoints of improving the binding strength with the active material and the adhesion with the current collector, suppressing decomposition of the electrolyte, and increasing the Coulomb efficiency.

[0026] Fumaric acid homopolymers can be easily prepared by using fumaric acid as a raw material.

[0027] Fumaric acid is a white crystalline powder that is readily available commercially and is used in food additives, pharmaceutical ingredients, bath additives, and oral treatments for psoriasis. It is also an intermediate in the citric acid cycle, a cellular process that generates energy in mitochondria. It can be produced or isolated from plants (Fumaria genus) and fungi (Rhizopus genus), making it environmentally friendly.

[0028] Fumaric acid homopolymers can be prepared using fumaric acid as a raw material, and therefore can be considered to be environmentally friendly bio-based polymers. For example, fumaric acid homopolymers can be easily prepared by preparing a fumaric acid diester such as diethyl fumarate from fumaric acid, polymerizing the fumaric acid diester, and treating the resulting fumaric acid diester polymer with alkali.

[0029] The fumaric acid diester can be prepared, for example, by reacting fumaric acid with an aliphatic monohydric alcohol. Examples of the aliphatic monohydric alcohol include aliphatic monohydric alcohols having 1 to 4 carbon atoms, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and tert-butyl alcohol, but the present invention is not limited to these examples.

[0030] The fumaric acid diester polymer can be prepared by polymerizing a fumaric acid diester. Examples of polymerization methods for fumaric acid diester polymers include bulk polymerization, solution polymerization, emulsion polymerization, and suspension polymerization, but the present invention is not limited to these examples. Among these polymerization methods, bulk polymerization and solution polymerization are preferred because they allow fumaric acid diester polymerization to be easily carried out, and bulk polymerization is more preferred because it does not require the use of a solvent.

[0031] When the fumaric acid diester polymer is polymerized by solution polymerization, for example, the fumaric acid diester can be polymerized by dissolving the fumaric acid diester in an organic solvent and adding a polymerization initiator to the resulting solution while stirring the solution, or the fumaric acid diester can be polymerized by dissolving a polymerization initiator in an organic solvent and adding the fumaric acid diester to the resulting solution while stirring the solution.

[0032] Examples of the organic solvent include ethyl acetate, butyl acetate, toluene, tetrahydrofuran, cyclopentane, ethyl acetoacetate, and propylene glycol monomethyl ether acetate, but the present invention is not limited to these examples. These organic solvents may be used alone or in combination of two or more. Among the organic solvents, ethyl acetate, butyl acetate, toluene, and tetrahydrofuran are preferred. The amount of the organic solvent is not particularly limited, but is generally preferably 50 to 400 parts by mass, more preferably 100 to 350 parts by mass, per 100 parts by mass of the fumaric acid diester.

[0033] When polymerizing a fumaric acid diester, it is preferable to use a polymerization initiator. Examples of polymerization initiators include 2,2'-azobisisobutyronitrile, azobisdimethylvaleronitrile, methyl azoisobutyrate, benzoyl peroxide, potassium persulfate, ammonium persulfate, benzophenone derivatives, phosphine oxide derivatives, benzoketone derivatives, phenylthioether derivatives, azide derivatives, diazo derivatives, and disulfide derivatives, but the present invention is not limited to these examples. These polymerization initiators may be used alone or in combination of two or more. It is preferable to select and use from these polymerization initiators one that is suitable for the polymerization method of a fumaric acid diester polymer. The amount of polymerization initiator is not particularly limited, but is usually preferably about 0.05 to 20 parts by mass per 100 parts by mass of the fumaric acid diester.

[0034] Furthermore, when polymerizing the fumaric acid diester, a chain transfer agent may be used to control the molecular weight. The chain transfer agent can usually be used by mixing it with the fumaric acid diester. Examples of the chain transfer agent include mercaptan group-containing compounds such as lauryl mercaptan, dodecyl mercaptan, and thioglycerol, and inorganic salts such as sodium hypophosphite and sodium hydrogen sulfite, but the present invention is not limited to these examples. These chain transfer agents may be used alone or in combination of two or more. The amount of the chain transfer agent is not particularly limited, but is usually about 0.01 to 10 parts by mass per 100 parts by mass of the fumaric acid diester.

[0035] There are no particular limitations on the polymerization reaction temperature and atmosphere when polymerizing a fumaric acid diester. Typically, the polymerization reaction temperature is about 60 to 120°C. To avoid the effects of oxygen contained in the air, the polymerization reaction is preferably carried out in an inert gas atmosphere such as nitrogen gas or argon gas. The polymerization reaction time for a fumaric acid diester cannot be determined in general because it varies depending on factors such as the polymerization reaction temperature, but is typically about 3 to 20 hours.

[0036] By polymerizing the fumaric acid diester in the manner described above, a fumaric acid diester polymer is obtained.

[0037] Next, the fumaric acid diester polymer obtained above is dissolved in a monohydric aliphatic alcohol such as ethyl alcohol, and an aqueous solution of an alkali metal hydroxide such as potassium hydroxide is added to the obtained solution to hydrolyze the fumaric acid diester polymer, thereby obtaining a reaction mixture containing a fumaric acid-based polymer.

[0038] The reaction mixture containing the produced fumaric acid-based polymer is acidified with an acid such as hydrochloric acid, and then dialyzed to obtain the fumaric acid-based polymer represented by the formula (I), 1 and R 2In addition, a fumaric acid homopolymer in which R is a hydrogen atom can be obtained. 1 and R 2 A fumaric acid-based polymer in which is an alkali metal atom can be obtained by adding an aqueous solution of a hydroxide of the alkali metal to an aqueous solution of a fumaric acid homopolymer, and neutralizing the aqueous solution of the fumaric acid homopolymer.

[0039] The number-average molecular weight of the fumaric acid-based polymer obtained as described above is not particularly limited, but from the viewpoints of excellent binding ability with the active material and adhesion with the current collector, increasing Coulomb efficiency while suppressing decomposition of the electrolyte, improving durability against charge and discharge when used as a negative electrode binder for a metal ion secondary battery, and increasing discharge capacity, it is preferably 1,000 to 100,000, and more preferably 2,000 to 50,000. The number-average molecular weight of the fumaric acid-based polymer is a value measured according to the method described in the following examples.

[0040] The fumaric acid-based polymer may contain repeating units other than the repeating unit represented by formula (I) within the scope of not impairing the object of the present invention.

[0041] (2) Nitrogen-containing cation-containing polymer The nitrogen cation-containing polymer has a repeating unit represented by formula (II). + is N + represents a cation having a ring structure containing

[0042] N + Examples of cations having a ring structure containing the formula (IIa):

[0043] [ka]

[0044] (In the formula, R 3 and R 4 each independently represents an alkyl group or an allyl group having 1 to 4 carbon atoms. an imidazolium ring-containing cation represented by formula (IIb):

[0045] [ka]

[0046] (In the formula, R 5 and R 6 each independently represents an alkyl group having 1 to 4 carbon atoms. a piperidinium ring-containing cation represented by formula (IIc):

[0047] [ka]

[0048] (In the formula, R 5 and R 6 is the same as above) However, the present invention is not limited to these examples. + The cations having a ring structure containing the following may be used alone or in combination of two or more kinds.

[0049] The nitrogen cation-containing polymer having the repeating unit represented by formula (II) can be prepared by using N + It can be prepared using a cyclic compound containing N + Examples of cyclic compounds containing N include imidazolium compounds, piperidinium compounds, and pyrrolidinium compounds, but the present invention is not limited to these examples. + The cyclic compounds containing the following may be used alone or in combination of two or more kinds.

[0050] Representative examples of imidazolium compounds include 1-allyl-3-methylimidazolium and 1-allyl-3-methylimidazolium hydroxide, but the present invention is not limited to these examples. 1-Allyl-3-methylimidazolium hydroxide can be easily prepared, for example, by ion-exchanging 1-allyl-3-methylimidazolium using a column packed with an ion-exchange resin (manufactured by Organo Corporation, trade name: Amberlite IRN78, etc.).

[0051] Typical examples of pyrrolidinium compounds include dialkylpyrrolidiniums in which the alkyl group has 1 to 4 carbon atoms, but the present invention is not limited to these examples.

[0052] Typical examples of piperidinium compounds include alkylpiperidiniums in which the alkyl group has 1 to 4 carbon atoms, but the present invention is not limited to these examples.

[0053] Next, the fumaric acid homopolymer and N + The fumaric acid homopolymer and the cyclic compound containing N are dissolved in a solvent capable of dissolving the cyclic compound containing N. + The cyclic compound containing the compound (II) is dissolved in the polymer, the resulting solution is thoroughly stirred, the solvent is removed, and the resulting product is washed with an organic solvent such as acetone and dried, thereby obtaining a nitrogen cation-containing polymer.

[0054] The number-average molecular weight of the nitrogen-cation-containing polymer having a repeating unit represented by formula (II) obtained as described above is not particularly limited, but from the viewpoints of excellent binding ability with the active material and adhesion with the current collector, increasing Coulomb efficiency while suppressing decomposition of the electrolyte, improving durability against charge and discharge when used as a negative electrode binder for a metal ion secondary battery, and increasing discharge capacity, it is preferably 1,000 to 100,000, and more preferably 2,000 to 50,000. The number-average molecular weight of the nitrogen-cation-containing polymer is a value measured according to the method described in the following examples.

[0055] The nitrogen cation-containing polymer may contain repeating units other than the repeating unit represented by formula (II) within the scope of not impairing the object of the present invention.

[0056] [Binder for metal ion secondary batteries] As described above, the binder for metal ion secondary batteries of the present invention contains at least one binder polymer selected from the group consisting of fumaric acid-based polymers having a repeating unit represented by formula (I) and nitrogen cation-containing polymers having a repeating unit represented by formula (II).

[0057] The binder for metal ion secondary batteries of the present invention contains the binder polymer, and therefore has excellent binding properties with the active material and adhesion with the current collector, enhances Coulomb efficiency while suppressing decomposition of the electrolyte, and when used as a negative electrode binder for metal ion secondary batteries, improves durability against charge and discharge, and can increase discharge capacity.

[0058] The binder for metal ion secondary batteries of the present invention can be used by dissolving it in a nonpolar solvent. Examples of nonpolar solvents include saturated hydrocarbon compounds such as hexane, cyclohexane, heptane, octane, nonane, decane, and 2-ethylhexane, aromatic compounds such as toluene and xylene, and halogenated hydrocarbon compounds such as dichloromethane, chloroform, and dichloroethane, but the present invention is not limited to these examples. It is preferable to select and use a nonpolar solvent suitable for the binder for metal ion secondary batteries from among these nonpolar solvents.

[0059] The binder for metal ion secondary batteries of the present invention may be composed only of the binder polymer, or may contain other negative electrode binders such as polyacrylic acid, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, and ethylene propylene diene rubber, as long as the object of the present invention is not impaired.

[0060] 2. Metal-ion secondary batteries As described above, the metal ion secondary battery of the present invention is a metal ion secondary battery having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, and one of the features of the present invention is that the negative electrode is a negative electrode containing the binder for metal ion secondary batteries.

[0061] The metal ion secondary battery of the present invention has a negative electrode containing the binder for metal ion secondary batteries, and therefore has excellent binding properties with the active material and adhesion to the current collector, and can increase coulombic efficiency while suppressing decomposition of the electrolyte.

[0062] The metal ion secondary battery of the present invention has a structure similar to that of commonly used metal ion secondary batteries. Representative examples of metal ion secondary batteries include lithium ion secondary batteries and sodium ion secondary batteries, and both lithium ion secondary batteries and sodium ion secondary batteries can be suitably used in the present invention.

[0063] The metal ion secondary battery of the present invention typically comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. Examples of the shape of the metal ion secondary battery include a cylindrical type and a laminated type, but the present invention is not limited to these examples.

[0064] When the metal ion secondary battery of the present invention is, for example, a CR2025 coin battery, the negative electrode, separator, and non-aqueous electrolyte are housed in a case. In the following, an embodiment in which the metal ion secondary battery is a CR2025 coin battery will be described, but the present invention is not limited to this embodiment.

[0065] The case used in CR2025 coin batteries is hollow, has an opening, and doubles as a positive electrode container. A lid is provided at the opening of the case, and the lid also doubles as a negative electrode lid. A gasket is provided between the case and the lid to maintain an insulating and sealed state between the case and the lid. The space between the case and the lid contains an electrode and a nonaqueous electrolyte.

[0066] (1) Electrode The electrodes include a positive electrode, a separator, and a negative electrode, arranged in this order. The positive electrode is in contact with the inner surface of the case, and the negative electrode is in contact with the inner surface of the lid.

[0067] [Positive electrode] The positive electrode may be the same as a positive electrode used in a commonly used metal ion secondary battery, and the present invention is not limited by the composition and structure of the positive electrode.

[0068] The positive electrode can be produced, for example, by mixing a positive electrode active material, a conductive material, and a binder in a predetermined ratio, adding to the resulting mixture an appropriate amount of a solvent, and if necessary, activated carbon, a viscosity-adjusting additive, and the like, kneading the resulting mixture to prepare a positive electrode composite paste, which is then applied to the surface of a current collector, press-molded if necessary, and dried.

[0069] Representative examples of positive electrode active materials include lithium and lithium-manganese composite oxides, but the present invention is not limited to these examples. Examples of conductive materials include carbon black such as acetylene black, natural graphite, artificial graphite, and expanded graphite, but the present invention is not limited to these examples. Examples of binders include the binder for metal ion secondary batteries of the present invention, as well as polyacrylic acid, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, and ethylene propylene diene rubber, but the present invention is not limited to these examples. Among these binders, the binder for metal ion secondary batteries of the present invention is preferred. Examples of solvents include N-methyl-2-pyrrolidone, but the present invention is not limited to these examples. Examples of current collectors include copper, aluminum, nickel, silver, tin, indium, magnesium, iron, chromium, molybdenum, and alloys thereof, but the present invention is not limited to these examples.

[0070] [Separator] The separator is used between the positive electrode and the negative electrode. The separator separates the positive electrode and the negative electrode, retains the electrolyte, and forms a path for lithium ions to move between the positive electrode and the negative electrode. The separator can be a foil membrane made of a polyolefin resin such as polyethylene or polypropylene, cellulose, glass, or the like. It is preferable that the thin film has a large number of micropores formed therein.

[0071] [Negative electrode] The negative electrode can be produced by, for example, mixing a negative electrode active material, the metal ion secondary battery binder of the present invention, and optionally a conductive additive, adding the resulting mixture to a solvent to prepare a paste-like negative electrode mixture, applying the paste to the surface of a metal foil current collector such as copper foil, drying it, and optionally forming it using a roll press, followed by drying.

[0072] The negative electrode active material is not particularly limited as long as it is an active material capable of inserting and extracting lithium ions. Examples of the negative electrode active material include carbon-based materials such as graphite, silicon, metallic lithium, lithium alloys, metallic sodium, sodium alloys, silicon-containing compounds, silicon-containing alloys, tin, tin-containing alloys, metal oxides, metal sulfides, and metal nitrides, but the present invention is not limited to these examples.

[0073] In order to increase the capacity of a metal-ion secondary battery, it is preferable to use silicon as the negative electrode active material, which expands and contracts significantly during charging and discharging of the metal-ion secondary battery. The silicon may be amorphous silicon or silicon crystallites. Silicon nanoparticles can also be used as the silicon. Silicon nanoparticles are readily available commercially, but can be prepared as follows.

[0074] Alkoxysilane compounds can be used as raw materials for silicon nanoparticles. Examples of alkoxysilane compounds include 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-aminopropylmethyldiethoxysilane, 3-aminopropyldimethylethoxysilane, and N-(2-aminoethyl)aminomethyltrimethoxysilane, but the present invention is not limited to these examples. These alkoxysilane compounds may be used alone or in combination of two or more. The alkoxysilane compounds may be used as a mixture with water.

[0075] It is preferable to reduce the alkoxysilane compound in advance. The reduction of the alkoxysilane compound can be carried out, for example, by adding a reducing agent to a mixture of the alkoxysilane compound and water. The temperature when reducing the alkoxysilane compound is not particularly limited, but is usually about 5 to 80°C. The atmosphere when reducing the alkoxysilane compound is not particularly limited, and may be air or an inert gas such as nitrogen gas or argon gas.

[0076] Examples of reducing agents include ascorbic acid and salts thereof, such as ascorbic acid, sodium ascorbate, and potassium ascorbate; sulfites, such as sodium sulfite, potassium sulfite, sodium bisulfite, aldehyde sodium bisulfite, and potassium bisulfite; and pyrosulfites, such as sodium pyrosulfite, potassium pyrosulfite, sodium bisulfite, and potassium pyrosulfite, but the present invention is not limited to these examples. These reducing agents may be used alone or in combination of two or more.

[0077] The amount of the reducing agent cannot be determined in general because it varies depending on the type of the reducing agent. It is usually preferable to use the reducing agent in an amount necessary to neutralize the alkoxysilane compound.

[0078] The temperature at which the reducing agent is added to the mixture of the alkoxysilane compound and water is not particularly limited, but is usually about 5 to 80° C. After adding the reducing agent to the mixture of the alkoxysilane compound and water, it is preferable to stir the mixture until it has a uniform composition, from the viewpoint of obtaining silicon nanoparticles having a uniform particle size.

[0079] By reducing the alkoxysilane compound with a reducing agent in the manner described above, an aqueous dispersion of silicon nanoparticles can be obtained.

[0080] The average particle size of the silicon nanoparticles obtained above is preferably 1 to 300 nm, more preferably 2 to 250 nm, and even more preferably 3 to 200 nm, from the viewpoints of improving dispersion stability and durability against charge and discharge of metal ion secondary batteries and increasing discharge capacity. Note that the average particle size of silicon nanoparticles means a value obtained by randomly selecting 100 silicon nanoparticles from an image taken with a scanning electron microscope, calculating the average values ​​of the vertical and horizontal axes for each silicon nanoparticle, summing the average values ​​for the 100 silicon nanoparticles, and dividing the total by 100.

[0081] Examples of the solvent include water and organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, diethyltriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran; however, the present invention is not limited to these examples.

[0082] The negative electrode composite may contain a conductive additive as needed. Examples of conductive additives include carbon blacks such as ethylene black, ketjen black (conductive carbon black), channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; fluorinated carbon; metal powders such as copper and nickel; and organic conductive materials such as polyphenylene derivatives. However, the present invention is not limited to these examples. The content of the conductive additive in the negative electrode composite is usually preferably 10 mass% or less.

[0083] Furthermore, the negative electrode mixture may contain an appropriate amount of an inclusion compound, if necessary. The inclusion compound has the property of absorbing gas generated within the battery. Examples of the inclusion compound include cyclodextrins such as α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin, and crown ethers such as 12-crown-4, 15-crown-5, and 18-crown-6, but the present invention is not limited to these examples. The content of the inclusion compound in the negative electrode mixture is usually preferably about 3 to 10 mass%.

[0084] Examples of current collectors include copper, aluminum, nickel, silver, tin, indium, magnesium, iron, chromium, molybdenum, and alloys thereof, but the present invention is not limited to these examples.

[0085] (2) Nonaqueous electrolyte Examples of non-aqueous electrolytes include cyclic carbonates such as ethylene carbonate, diethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; and ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane, but the present invention is not limited to these examples. These non-aqueous electrolytes may be used alone or in combination of two or more.

[0086] The metal ion secondary battery of the present invention uses the binder for metal ion secondary batteries of the present invention, and therefore the metal ion secondary battery of the present invention has excellent binding properties with the active material and adhesion to the current collector, compared to batteries using conventional binders that use polyacrylic acid, and can increase coulombic efficiency while suppressing decomposition of the electrolyte. [Example]

[0087] Next, the present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0088] 1. Raw materials Diethyl fumarate (purity: 98%), hydrochloric acid (12N), 2,2'-azobis(isobutyronitrile) (AIBN) as a radical polymerization initiator, poly(vinylidene difluoride) (PVDF) binder (average molecular weight: 540,000), polyacrylic acid, 1.0 M LiPF6 in ethylene carbonate / diethyl carbonate (volume ratio: 50 / 50), electrolyte, and battery-grade graphite were obtained from Sigma-Aldrich.

[0089] Potassium hydroxide, ethanol (purity: 99.5%), and N-methylpyrrolidone (NMP) were obtained from Fujifilm Wako Pure Chemical Industries, Ltd. Conductive carbon black (Super P conductive carbon black) was obtained from Alfa Aesar as a conductive additive. Regenerated cellulose dialysis tubes were used as dialysis tubes.

[0090] 2. Physical Properties (1) Number-average molecular weight of polymer (MALDI-TOF mass spectrometry) The number average molecular weight of the polymer was measured using a mass spectrometer (Bruker Daltonics, trade name: microflex LRF) with 2,5-dihydroxybenzoic acid (DHB) as the matrix and sodium iodide as the cationic reagent.

[0091] (2) Nuclear magnetic resonance spectrum Polymer Nuclear Magnetic Resonance ( 1 H-NMR and 13 C-NMR was measured using a nuclear magnetic resonance spectrometer (manufactured by Bruker, trade name: AVANCE II NMR Spectrometer 400 MHz) using D2O as a solvent.

[0092] (3) Fourier transform infrared absorption spectrum (FT-IR spectrum) The Fourier transform infrared absorption spectrum of the polymer was examined using a Fourier transform infrared spectrometer (PerkinElmer).

[0093] (4) Thermogravimetric analysis Thermogravimetric analysis of the polymer was carried out using a thermal analysis system (manufactured by Hitachi High-Tech Science Corporation, product number: STA7200) at a heating rate of 10°C / min and a nitrogen gas flow rate of 200 mL / min.

[0094] 3.Battery performance (1) Cyclic voltammetry (CV) Cyclic voltammetry was measured at room temperature using a potentiostat (Biological, product number: VSM) with the coin battery. The potential range during cyclic voltammetry measurement was 0.01 to 2.1 V, and the scan rate was adjusted to 0.1 mV / s, 0.2 mV / s, 0.4 mV / s, 0.6 mV / s, or 0.8 mV / s.

[0095] (2) Electrochemical Impedance Spectroscopy (EIS) Electrochemical impedance spectroscopy was measured before and after the cyclic voltammetry measurement at a frequency of 1.0 MHz to 0.1 Hz and an AC voltage of 10 mV.

[0096] (3) Dynamic Electrochemical Impedance Spectroscopy (EIS) Dynamic electrochemical impedance spectroscopy was measured after 700 charge / discharge cycles at a frequency of 1.0 MHz to 0.1 Hz and an AC voltage of 10 mV.

[0097] (4) Measurement of constant current cycle The constant current cycle was examined at a constant current of 0.01 to 2.1 V using an automatic battery evaluation device (manufactured by Electrofield, trade name: ABE 1024).

[0098] (5) Activation energy After the cycle test, the potential of the half-cell was set to 0.2 V, the potential at which lithium intercalates in graphite, and then electrochemical impedance spectroscopy was used to calculate the potential. The potential was maintained at the same value, and electrochemical impedance spectroscopy was performed over a range of temperatures.

[0099] (6) Rheology of Slurry and Electrodes The rheology of the slurry and electrode was evaluated by viscosity and peel tests, respectively. The viscosity was measured at 25°C using a viscometer (Toki Sangyo Co., Ltd., Model No. TV-35). The peel test was performed using a universal testing machine (Instron, Model No. 3342) for a 40 mm long electrode. Adhesive tape was attached to the coated surface of the roll-pressed electrode at 25°C, and the adhesive tape was peeled off at a 90° angle at a constant speed of 1 mm / min to examine the adhesive strength.

[0100] (7) Analysis after long-term cycle testing X-ray photoelectron spectroscopy was performed after the long-term cycle test using an X-ray photoelectron spectrometer (Fisons Instruments, product name: S-PROBE 2803). The morphology of the electrode after the long-term cycle test was also examined using a field emission scanning electron microscope (Hitachi High-Tech, product number: S-4500) at an accelerating voltage of 1.0 kV.

[0101] Example 1 (1) Preparation of fumaric acid homopolymer

[0102] [ka]

[0103] Diethyl fumarate (2 g, 11.6 mmol) was polymerized in the presence of 2,2'-azobisisobutyronitrile (AIBN) (38 mg, 0.23 mmol) at 70°C in a nitrogen gas atmosphere for 24 hours. The resulting polymer was added to an excess of hexane (50 mL) with stirring, and the hexane was then distilled off. The polymer was then dried under reduced pressure for approximately 10 hours.

[0104] Next, the polymer obtained above was hydrolyzed in a mixed solvent of ethanol:water (2:1) using an excess amount of potassium hydroxide under reflux for 24 hours, and the resulting reaction mixture was concentrated and acidified with 1N hydrochloric acid (pH: 4.6). Subsequently, the reaction mixture was dialyzed using a cellulose acetate tube and then dried to obtain the polymer represented by formula (I), 1 and R 2 are hydrogen atoms, 1.1 g of fumaric acid homopolymer was obtained (yield: 82%).

[0105] (2) 1 H-NMR and 13 C-NMR The fumaric acid homopolymer obtained above 1 H-NMR spectrum and 13 The C-NMR spectra are shown in Figure 1(a) and (b), respectively.

[0106] As shown in Figure 1(a) 1 In the H-NMR spectrum, the peak p at 3.25 ppm is due to the CH of the methylene group present in the main chain of the fumaric acid homopolymer, as shown in Figure 1(b). 13 In the C-NMR spectrum, the characteristic peaks q and r at 170.8 ppm and 62.4 ppm were identified as being due to the carbonyl group and methylene carbon of the fumaric acid homopolymer, respectively.

[0107] (3) FT-IR spectrum The FT-IR spectrum of the fumaric acid homopolymer obtained above is shown in FIG.

[0108] In the FT-IR spectrum shown in Figure 2, the wave number is 3500 cm -1 The peak at 2977 cm is due to the stretching of OH of the carboxyl group of the fumaric acid homopolymer. -1 The characteristic absorption peak at 1715 cm is due to the stretching of -CH- in the main chain of the fumaric acid homopolymer. -1 and 1090cm -1The peak at is due to the carbonyl group and ether bond of the fumaric acid homopolymer.

[0109] (4) Number average molecular weight The results of MALDI-TOF mass spectrometry of the fumaric acid homopolymer obtained above are shown in FIG.

[0110] Mass spectrometry of the fumaric acid homopolymer confirmed that the number average molecular weight of the fumaric acid homopolymer was 3000 (m / z).

[0111] (5) Thermogravimetric analysis Thermogravimetric analysis of fumaric acid homopolymer showed an initial weight loss at approximately 100°C. This weight loss is believed to be due to the evaporation of water adsorbed by the carboxyl groups of the fumaric acid homopolymer. The thermal stability of fumaric acid homopolymer was up to approximately 235°C, and at temperatures above this, a steep decomposition rate was observed, which was believed to be due to dehydration through the formation of intramolecular bonds or anhydride bonds.

[0112] From the above, it can be seen that the fumaric acid homopolymer has excellent thermal stability and is therefore suitable as a binder for metal ion secondary batteries that are used at high temperatures.

[0113] Example 2 Using the fumaric acid homopolymer obtained in Example 1, an electrode and a half cell were prepared as follows.

[0114] Graphite, conductive carbon black, and fumaric acid homopolymer were added to water in a mass ratio of 80:10:10, and the resulting mixture was uniformly dispersed with a homogenizer to prepare a slurry (negative electrode mixture).

[0115] The slurry obtained above was applied to a copper foil (length: 20 mm, width: 100 mm, thickness: 20 μm) using a doctor blade to give a coating thickness of approximately 100 μm, and the coating was dried under reduced pressure at 80°C for 12 hours. After that, the coating was roll-pressed by passing it between rollers with a gap of 70 μm at a temperature of 80°C to produce an electrode with a coating thickness of approximately 50 μm.

[0116] The electrode obtained above was compressed in a hot rolling mill and cut into a disk having a diameter of 13 mm, thereby obtaining a disk-shaped electrode.

[0117] The disk-shaped electrode obtained above was incorporated into a coin battery (CR2025), and the coin battery was placed in a glove box filled with argon gas, with oxygen and water vapor concentrations both less than 0.5 ppm. A coin battery (half cell) was fabricated using a lithium disk as the counter electrode, polypropylene Celgard as the separator, and a 1.0 M solution of LiPF6 in ethylene carbonate / diethyl carbonate (volume ratio: 50 / 50) as the electrolyte.

[0118] To compare the battery performance, coin batteries (half cells) were fabricated in the same manner as above using polyacrylic acid or polyvinylidene fluoride as the binder instead of fumaric acid homopolymer. When polyvinylidene fluoride was used, N-methyl-2-pyrrolidone was used as the solvent instead of water.

[0119] (1) Binder adhesive strength The adhesive strength of the binder was evaluated based on the stress when peeling off the adhesive tape. Figure 4 shows the results of measuring the adhesive strength of the binder when fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride was used as the binder.

[0120] In FIG. 4, symbol A indicates the adhesive strength when fumaric acid homopolymer is used, symbol B indicates the adhesive strength when polyacrylic acid is used, and symbol C indicates the adhesive strength when polyvinylidene fluoride is used.

[0121] As shown in Figure 4, the adhesive strength A when fumaric acid homopolymer is used is significantly higher than the adhesive strength B when polyacrylic acid is used and the adhesive strength C when polyvinylidene fluoride is used. As such, since fumaric acid homopolymer has high adhesive strength to the electrode current collector, it is thought to be useful in accommodating the volume expansion during lithiation in the electrode and preventing the collapse of electrode components during delithiation.

[0122] (2) Viscosity of the binder The viscosity of a slurry containing fumaric acid homopolymer, a slurry containing polyacrylic acid, and a slurry containing polyvinylidene fluoride was measured at 25°C. -1 The viscosity of the slurry at 1000 kJ / min was 4.9 Pa·s for the slurry containing fumaric acid homopolymer, 4.2 Pa·s for the slurry containing polyacrylic acid, and 2.9 Pa·s for the slurry containing polyvinylidene fluoride.

[0123] It is generally believed that the higher the viscosity of the slurry, the more uniform the coating formed on the electrode. Therefore, it can be seen that the slurry containing fumaric acid homopolymer has an appropriate viscosity and is superior in forming a uniform coating compared to slurries containing fumaric acid homopolymer and slurries containing polyacrylic acid.

[0124] (3) Electrochemical properties [Electrochemical Stability] The electrochemical stability of the binder in the electrode was evaluated by cyclic voltammograms.

[0125] Using half-cells containing fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride as the binder, cyclic voltammograms were measured at scan rates of 0.1 mV / sec, 0.2 mV / sec, 0.4 mV / sec, 0.6 mV / sec, and 0.8 mV / sec. The results are shown in Figure 5.

[0126] In Figure 5, (a) is a cyclic voltammogram obtained using a half-cell containing fumaric acid homopolymer, (b) is a cyclic voltammogram obtained using a half-cell containing polyacrylic acid, and (c) is a cyclic voltammogram obtained using a half-cell containing polyvinylidene fluoride.

[0127] As shown in Figure 5, regardless of the binder used, a reduction peak of 0.7 V was observed, indicating that decomposition of the electrolyte was suppressed. Furthermore, when polyacrylic acid was used as the binder, the overpotential difference of the delithiation peak was 0.129 V, and when polyvinylidene fluoride was used as the binder, the overpotential difference of the delithiation peak was 0.132 V. However, when fumaric acid homopolymer was used as the binder, the overpotential difference of the delithiation peak was 0.117 V, indicating that the overpotential difference of the delithiation peak between 0.1 mV / sec and 1.0 mV / sec was small.

[0128] From the above results, it is thought that fumaric acid homopolymer has excellent binding properties with the active material, and lithium ions easily penetrate into the SEI layer even at high scanning rates, inducing the diffusion of lithium ions.

[0129] [Lithium ion diffusion coefficient] Randles-Sevcik formula: Ip=2.69×10 5 n 3 / 2 AD Li C Li V 1 / 2 [where Ip is the peak current (A), n is the number of electrons involved in the reaction, and A is the electrode area (cm 2 ), D is the diffusion coefficient of lithium ions (cm 2 / sec), C is molar concentration (mol / cm 3 ), V indicates the scan speed (V / sec) The lithium ion diffusion coefficient was investigated based on the results shown in Table 1. The lithium ion diffusion coefficient was 3.57 × 10 for the fumaric acid homopolymer. -8 cm 2 / sec, and 2.11 × 10 for polyacrylic acid. -8 cm 2 / sec, and 6.59 × 10 for polyvinylidene fluoride -9 cm 2 / sec.

[0130] These results indicate that the fumaric acid homopolymer exhibits weak interaction with lithium ions and promotes the diffusion of lithium within the electrode.

[0131] [Nyquist plot] Nyquist plots were investigated before and after cyclic voltammetry measurements using half cells containing fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride as the binder, and the results are shown in Figure 6.

[0132] In Figure 6, symbol A indicates the Nyquist plot before cyclic voltammetry using the half-cell containing fumaric acid homopolymer, symbol B indicates the Nyquist plot after cyclic voltammetry using the half-cell containing fumaric acid homopolymer, symbol C indicates the Nyquist plot before cyclic voltammetry using the half-cell containing polyacrylic acid, symbol D indicates the Nyquist plot after cyclic voltammetry using the half-cell containing polyacrylic acid, symbol E indicates the Nyquist plot before cyclic voltammetry using the half-cell containing polyvinylidene fluoride, and symbol F indicates the Nyquist plot after cyclic voltammetry using the half-cell containing polyvinylidene fluoride.

[0133] The charge transfer process within the electrode is related to the concave curve of the Nyquist plot occurring at high frequencies followed by the linear curve of the Nyquist plot at low frequencies, i.e., the diffusion-controlled Warburg impedance. The smaller the value of the concave curve of the Nyquist plot, the smaller the charge transfer resistance (RCT).

[0134] Nyquist plot B after cyclic voltammetry when fumaric acid homopolymer is used shows a smaller value at the concave part of the Nyquist plot than Nyquist plots D and F when polyacrylic acid and polyvinylidene fluoride are used, indicating a smaller charge transfer resistance (RCT).

[0135] Charge transfer resistance (RCT) is related to the concentration of lithium ions on the electrode surface that forms the SEI layer and has a negative effect on cycle life. However, when fumaric acid homopolymer is used, the charge transfer resistance (RCT) is small, which indicates that cycle life is improved.

[0136] [Activation energy] Arrhenius plots were investigated using half-cells containing each binder, and the results are shown in Figure 7. In Figure 7, symbol A represents the Arrhenius plot measured using the half-cell containing fumaric acid homopolymer, symbol B represents the Arrhenius plot measured using the half-cell containing polyacrylic acid, and symbol C represents the Arrhenius plot measured using the half-cell containing polyvinylidene fluoride.

[0137] The activation energy of each binder was calculated from the slope of the Arrhenius plot, and was found to be 36.4 kJ / mol for fumaric acid homopolymer, 48.9 kJ / mol for polyacrylic acid, and 52.8 kJ / mol for polyvinylidene fluoride. This indicates that fumaric acid homopolymer has a lower activation energy than polyacrylic acid and polyvinylidene fluoride, allowing for smooth transport of lithium ions in the electrode.

[0138] [Initial charge / discharge capacity] The initial charge / discharge capacity was measured using half cells containing each binder, and the results are shown in Table 1.

[0139] [Table 1]

[0140] The results shown in Table 1 show that the initial discharge capacity is significantly higher when fumaric acid homopolymer is used as the binder than when polyacrylic acid or polyvinylidene fluoride is used as the binder, indicating that fumaric acid homopolymer has superior coulombic efficiency.

[0141] [Effect of binder on constant current charge / discharge cycles] To evaluate whether this method can actually be applied to lithium-ion batteries, we investigated the effect of the binder on constant-current charge-discharge cycling at a charge capacity of 5 C using half cells. The results are shown in Figure 8. In Figure 8, symbol A represents the change in specific capacity measured using a half cell containing fumaric acid homopolymer, symbol B represents the change in specific capacity measured using a half cell containing polyacrylic acid, and symbol C represents the change in specific capacity measured using a half cell containing polyvinylidene fluoride.

[0142] The results shown in Figure 8 indicate that the half-cells using fumaric acid homopolymer as the binder maintained a high charge capacity even after 1,000 charge-discharge cycles, compared to the half-cells using polyacrylic acid or polyvinylidene fluoride as the binder. This indicates that fumaric acid homopolymer has excellent coulombic efficiency and long-term cycle durability of the electrode.

[0143] [Battery internal resistance] The internal resistance of the battery during lithiation and delithiation was investigated using half-cells containing each binder. The results are shown in Figure 9. In Figure 9, (a) shows the internal resistance during lithiation, and (b) shows the internal resistance during delithiation. Also in Figure 9, symbol A represents data when fumaric acid homopolymer was used, symbol B represents data when polyacrylic acid was used, and symbol C represents data when polyvinylidene fluoride was used.

[0144] The results shown in Figure 9(a) indicate that the internal resistance during lithiation is small when fumaric acid homopolymer is used, similar to that of polyvinylidene fluoride, and the results shown in Figure 9(b) indicate that the internal resistance during delithiation is smallest when fumaric acid homopolymer is used. This indicates that fumaric acid homopolymer has excellent lithium ion conductivity and low activation energy during lithiation and delithiation.

[0145] [Observation of electrodes after cycle test] After 1,000 5C charge-discharge cycles using half-cells containing each binder, the electrode surfaces were observed with a scanning electron microscope. The results showed that no cracks were observed in the electrode containing fumaric acid homopolymer, whereas cracks were observed on the electrode surface in the electrodes containing polyacrylic acid and polyvinylidene fluoride. This indicates that the electrode containing fumaric acid homopolymer has superior charge-discharge cycle characteristics.

[0146] Example 3 Using the fumaric acid homopolymer obtained in Example 1, an electrode and a half cell were prepared as follows.

[0147] Hard carbon, conductive carbon black, and fumaric acid homopolymer were mixed in a mass ratio of 80:10:10 in water to prepare a slurry (negative electrode mixture).

[0148] The slurry obtained above was applied to a copper foil (length: 20 mm, width: 100 mm, thickness: 20 μm) using a doctor blade to give a coating thickness of approximately 100 μm, and the coating was dried under reduced pressure at 80°C for 12 hours. After that, the coating was roll-pressed by passing it between rollers with a gap of 70 μm at a temperature of 80°C to produce an electrode with a coating thickness of approximately 50 μm.

[0149] The electrode obtained above was compressed in a hot rolling mill and cut into a disk having a diameter of 13 mm, thereby obtaining a disk-shaped electrode.

[0150] The disk-shaped electrode obtained above was incorporated into a coin battery (CR2025), and the coin battery was placed in a glove box filled with argon gas and with oxygen and water vapor concentrations both less than 0.5 ppm. A coin battery (half cell) was fabricated using a sodium disk as the counter electrode, polypropylene Celgard as the separator, and a 1.0 M solution of NaClO4 in ethylene carbonate / propylene carbonate (volume ratio: 50 / 50) as the electrolyte.

[0151] To compare the battery performance, coin batteries (half cells) were fabricated in the same manner as above using polyacrylic acid or polyvinylidene fluoride as the binder instead of fumaric acid homopolymer. When polyvinylidene fluoride was used, N-methyl-2-pyrrolidone was used as the solvent instead of water.

[0152] Using half-cells containing fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride as the binder, cyclic voltammograms were measured at scan rates of 0.1 mV / sec, 0.2 mV / sec, 0.4 mV / sec, 0.6 mV / sec, 0.8 mV / sec, and 1.08 mV / sec. The results are shown in Figure 10.

[0153] In FIG. 10, (a) is a cyclic voltammogram when fumaric acid homopolymer was used, (b) is a cyclic voltammogram when polyacrylic acid was used, and (c) is a cyclic voltammogram when polyvinylidene fluoride was used.

[0154] As shown in Figure 10, regardless of the binder used, a reduction peak of 0.3 V associated with the decomposition of the electrolyte is present, indicating that decomposition of the electrolyte is suppressed. Furthermore, when polyacrylic acid is used as the binder, the overpotential difference of the sodium decomposition peak is 0.200 V, and when polyvinylidene fluoride is used as the binder, the overpotential difference of the sodium decomposition peak is 0.237 V. However, when fumaric acid homopolymer is used as the binder, the overpotential difference of the sodium decomposition peak is 0.200 V. This suggests that when fumaric acid homopolymer is used as the binder, sodium ions easily penetrate the SEI layer, inducing their diffusion.

[0155] [Nyquist plot] Nyquist plots were investigated before and after cyclic voltammetry measurements using half-cells containing fumaric acid homopolymer, polyacrylic acid, or polyvinylidene fluoride as the binder, and the results are shown in Figure 11.

[0156] In FIG. 11, symbol A indicates a Nyquist plot before cyclic voltammetry measurement using a half-cell containing fumaric acid homopolymer, symbol B indicates a Nyquist plot after cyclic voltammetry measurement using a half-cell containing fumaric acid homopolymer, symbol C indicates a Nyquist plot before cyclic voltammetry measurement using a half-cell containing polyacrylic acid, symbol D indicates a Nyquist plot after cyclic voltammetry measurement using a half-cell containing polyacrylic acid, symbol E indicates a Nyquist plot before cyclic voltammetry measurement using a half-cell containing polyvinylidene fluoride, and symbol F indicates a Nyquist plot after cyclic voltammetry measurement using a half-cell containing polyvinylidene fluoride.

[0157] The charge transfer process within the electrode is related to the concave curve of the Nyquist plot occurring at high frequencies followed by the linear curve of the Nyquist plot at low frequencies, i.e., the diffusion-controlled Warburg impedance. The smaller the value of the concave curve of the Nyquist plot, the smaller the charge transfer resistance (RCT).

[0158] The Nyquist plot after cyclic voltammetry for the half-cell containing fumaric acid homopolymer is compared to the Nyquist plots for the half-cells containing polyacrylic acid and polyvinylidene fluoride. The smaller values ​​at the concave portions of the Nyquist plot indicate a smaller charge transfer resistance (RCT). The charge transfer resistance (RCT) is related to the concentration of sodium ions on the electrode surface that forms the SEI layer and has a negative effect on cycle life. However, the smaller charge transfer resistance (RCT) for the fumaric acid homopolymer improves cycle life.

[0159] [Activation energy] Arrhenius plots were investigated using half-cells containing each binder, and the results are shown in Figure 12. In Figure 12, symbol A represents the Arrhenius plot for the half-cell containing fumaric acid homopolymer, symbol B represents the Arrhenius plot for the half-cell containing polyacrylic acid, and symbol C represents the Arrhenius plot for the half-cell containing polyvinylidene fluoride.

[0160] The activation energy of each binder was calculated from the slope of the Arrhenius plot, and was found to be 50.7 kJ / mol for fumaric acid homopolymer, 52.9 kJ / mol for polyacrylic acid, and 58.6 kJ / mol for polyvinylidene fluoride. This indicates that the fumaric acid homopolymer has a low activation energy, allowing for smooth transport of sodium ions at the electrode.

[0161] [Initial discharge capacity] Using half cells containing each binder, the initial discharge capacity was measured at a charge rate of 0.25 C. The results are shown in Table 2.

[0162] [Table 2]

[0163] The results shown in Table 2 show that the initial discharge capacity of the half-cells using fumaric acid homopolymer as the binder is significantly superior to that of the half-cells using polyacrylic acid or polyvinylidene fluoride as the binder.

[0164] [Effect of binder on constant current charge / discharge cycles] To evaluate whether this method can actually be applied to sodium-ion batteries, we investigated the effect of the binder on constant-current charge-discharge cycling at a charge capacity of 30 mA / g using half cells. The results are shown in Figure 13. In Figure 13, symbol A represents the change in specific capacity when fumaric acid homopolymer is used, symbol B represents the change in specific capacity when polyacrylic acid is used, and symbol C represents the change in specific capacity when polyvinylidene fluoride is used.

[0165] The results shown in Figure 13 show that the half-cells using fumaric acid homopolymer as the binder maintained a high charge capacity even after 200 charge-discharge cycles, compared to the half-cells using polyacrylic acid or polyvinylidene fluoride as the binder. This indicates that fumaric acid homopolymer has excellent coulombic efficiency and long-term cycle durability of the electrode.

[0166] [Battery internal resistance] The internal resistance of the battery was investigated using half-cells containing each binder in both sodiated and desodium conditions. The results are shown in Figure 14. In Figure 14, (a) shows the internal resistance in the sodiated state, and (b) shows the internal resistance in the desodium state. Also in Figure 14, symbol A represents data when fumaric acid homopolymer was used, symbol B represents data when polyacrylic acid was used, and symbol C represents data when polyvinylidene fluoride was used.

[0167] The results shown in Figure 14(a) indicate that the internal resistance during sodiation is small when fumaric acid homopolymer is used, similar to that of polyvinylidene fluoride, and the results shown in Figure 14(b) indicate that the internal resistance during desodiumation is smallest when fumaric acid homopolymer is used. This indicates that fumaric acid homopolymer has excellent sodium ion conductivity and low activation energy during sodiation and desodiumation.

[0168] [Observation of electrodes after cycle test] After 1,000 5C charge-discharge cycles using half-cells containing each binder, the electrode surfaces were observed with a scanning electron microscope. The results showed that no cracks were observed in the electrode containing fumaric acid homopolymer, whereas cracks were observed on the electrode surface in the electrodes containing polyacrylic acid and polyvinylidene fluoride. This indicates that the electrode containing fumaric acid homopolymer has superior charge-discharge cycle characteristics.

[0169] Example 4 In a 100 mL round-bottom flask, 2 g of the fumaric acid homopolymer prepared in the same manner as in Example 1 was dissolved in 50 mL of distilled water, and a 0.1 M aqueous solution of lithium hydroxide was added dropwise to the resulting aqueous solution at room temperature to adjust the pH of the aqueous solution to 7.0, thereby obtaining a reaction mixture.

[0170] Water was removed from the reaction mixture to obtain a lithium salt of fumaric acid homopolymer, which had a number average molecular weight of 4000 (m / z).

[0171] Next, a half cell was prepared using the lithium salt of the fumaric acid homopolymer obtained above in the same manner as in Examples 2 and 3, and the performance of the obtained half cell was evaluated in the same manner as in Examples 2 and 3. As a result, it was confirmed that the half cell had performance similar to that of the half cells obtained in Examples 2 and 3.

[0172] Example 5 (1) Preparation of imidazolium polymers (polymers containing nitrogen cations) 2 g of 1-allyl-3-methylimidazolium chloride was dissolved in 100 mL of water, and the 1-allyl-3-methylimidazolium aqueous solution was introduced into a column packed with an ion exchange resin (manufactured by Organo Corporation, trade name: Amberlite IRN78, etc.), and the 1-allyl-3-methylimidazolium was subjected to ion exchange, thereby obtaining an aqueous solution of 1-allyl-3-methylimidazolium hydroxide.

[0173] Next, water was evaporated from the aqueous solution of 1-allyl-3-methylimidazolium hydroxide, and the solution was dried under reduced pressure at 60° C. for about 10 hours to recover 1-allyl-3-methylimidazolium hydroxide.

[0174] 200 mg of fumaric acid homopolymer prepared in the same manner as in Example 1 and 482 mg of 1-allyl-3-methylimidazolium hydroxide were dissolved in 100 mL of water, and the resulting aqueous solution was stirred at room temperature for about 10 hours. After evaporating the water, the resulting product was washed with acetone and then dried under reduced pressure to obtain a compound of formula (III):

[0175] [ka]

[0176] (In the formula, R 7 indicates a methylene group) An imidazolium polymer (1-allyl-3-methylimidazolium polymer) having a repeating unit represented by the formula: was obtained (yield: 78%).

[0177] (2) 13 C-NMR The oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer obtained above 13 The C-NMR spectrum is shown in Figure 15.

[0178] As shown in Figure 15 13In the C-NMR spectrum, the characteristic peaks a and c at 62.4 ppm and 30 ppm were identified as being due to the methine group and methylene carbon of the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer, respectively.

[0179] (3) FT-IR spectrum The FT-IR spectrum of the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer obtained above is shown in FIG.

[0180] In the FT-IR spectrum shown in FIG. 16, the wave number of 3500 cm -1 The peak at is the CO of the carboxyl group of the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer. - It is based on the wave number 1725 cm -1 The characteristic absorption peak at wavenumber 1582 cm is due to the C=O group of the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer. -1 CN stretching vibration at wavenumber 1157 cm -1 CH asymmetric bending vibration at wavenumber 3151 cm -1 The presence of C=CH in the compound confirmed its imidazolium-based structure.

[0181] (4) Number average molecular weight As a result of MALDI-TOF mass spectrometry of the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer obtained above, it was confirmed that the number average molecular weight of the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer was 4000 (m / z).

[0182] (5) Thermogravimetric analysis Thermogravimetric analysis of the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer revealed that the polymer experienced an initial weight loss at approximately 150°C. This initial weight loss was higher than that of the fumaric acid homopolymer, indicating that the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer has superior heat resistance (thermal stability) to the fumaric acid homopolymer. Because of its excellent thermal stability, the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer is suitable as a binder for metal-ion secondary batteries used at high temperatures.

[0183] Example 6 An electrode was prepared using the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer obtained in Example 4 as follows.

[0184] A slurry (negative electrode mixture) was prepared by mixing battery-grade graphite (Sigma-Aldrich), conductive carbon (Alfa Aesar), and oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer as a binder in a mass ratio of 80:10:10 in water.

[0185] The slurry obtained above was applied to a copper foil (length: 20 mm, width: 100 mm, thickness: 20 μm) using a doctor blade to give a coating thickness of approximately 100 μm, and then dried under reduced pressure at 80° C. for 24 hours to prepare an electrode. The electrode obtained above was cut into a disk with a diameter of 13 mm to obtain a disk-shaped electrode.

[0186] In the case of a sodium ion secondary battery, the disk-shaped electrode obtained above was incorporated into a coin battery (CR2025), and the coin battery was placed in a glove box filled with argon gas and with oxygen and water vapor concentrations both less than 0.5 ppm. A coin battery (half cell) was fabricated using a sodium disk as the counter electrode, glass fiber (manufactured by Whatman) as the separator, and a 1.0 M solution of NaClO4 in ethylene carbonate / diethyl carbonate (volume ratio: 50 / 50) as the electrolyte.

[0187] In the case of a lithium ion secondary battery, the disk-shaped electrode obtained above was incorporated into a coin battery (CR2025), and the coin battery was placed in a glove box filled with argon gas, with oxygen and water vapor concentrations both less than 0.5 ppm. A coin battery (half cell) was fabricated using a lithium disk as the counter electrode, polypropylene Celgard as the separator, and a 1.0 M solution of LiPF6 in ethylene carbonate / diethyl carbonate (volume ratio: 50 / 50) as the electrolyte.

[0188] (1) Binder adhesive strength The adhesive strength of the binder was evaluated based on the stress when peeling off adhesive tape. The adhesive strength of the binder when oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer was used as the binder was approximately 11 N. This high adhesive strength suggests that the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer is useful for accommodating the volume expansion during lithiation in the electrode and preventing the collapse of electrode components during delithiation.

[0189] (2) Electrochemical properties [Electrochemical Stability] The electrochemical stability of the binder in the electrode was evaluated by cyclic voltammograms. Using the half-cell of the lithium-ion secondary battery obtained above, cyclic voltammograms were measured at scan rates of 0.1 mV / sec and 0.2 mV / sec. The results are shown in Figure 17.

[0190] When the half-cell was used, the cyclic voltammogram shown in Figure 17 reveals that a reduction peak appears at approximately 0.6 V due to the decomposition of the electrolyte. Furthermore, in the cyclic voltammogram, the overpotential difference of the delithiation peak between 0.1 mV / sec and 1.0 mV / sec was 0.150 V, which was lower than the overpotential difference of 0.196 V observed when polyvinylidene fluoride was used as the binder. This indicates that the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer promotes lithium ion transport and reduces the internal impedance of the battery.

[0191] [Nyquist plot] Nyquist plots were investigated before cyclic voltammetry measurements using half cells of lithium ion secondary batteries in which oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer or polyvinylidene fluoride was used as the binder in the half cells, and the results are shown in Figure 18.

[0192] In FIG. 18, symbol A indicates the Nyquist plot when a half-cell using oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer is used, and symbol B indicates the Nyquist plot when a half-cell using polyvinylidene fluoride is used.

[0193] The charge transfer process within the electrode is related to the concave curve of the Nyquist plot occurring at high frequencies followed by the linear curve of the Nyquist plot at low frequencies, i.e., the diffusion-controlled Warburg impedance. The smaller the value of the concave curve of the Nyquist plot, the smaller the charge transfer resistance (RCT).

[0194] The Nyquist plot after cyclic voltammetry using the half-cell containing oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer shows a smaller value at the depression of the Nyquist plot compared to the Nyquist plot after cyclic voltammetry using the half-cell containing polyvinylidene fluoride, indicating a smaller charge transfer resistance (RCT).

[0195] Charge transfer resistance (RCT) is related to the concentration of lithium ions on the electrode surface that forms the SEI layer and has a negative effect on cycle life. However, when oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer is used, the charge transfer resistance (RCT) is small, which indicates that cycle life is improved.

[0196] [Activation energy] Arrhenius plots were performed using half-cells of lithium-ion secondary batteries using oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer or polyvinylidene fluoride as the binder, and the results are shown in Figure 19.

[0197] In FIG. 19, symbol A represents the Arrhenius plot when a half-cell containing oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer is used, and symbol B represents the Arrhenius plot when a half-cell containing polyvinylidene fluoride is used.

[0198] The activation energy of each binder was calculated from the slope of the Arrhenius plot, and the activation energy was 36.2 kJ / mol for the half-cell using oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer, while the activation energy was 62.1 kJ / mol for the half-cell using polyvinylidene fluoride. This indicates that the activation energy for desolvation of oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer is lower than that of polyvinylidene fluoride, allowing for smooth ion transport at the electrode.

[0199] [Specific capacity] Using half-cells of lithium-ion secondary batteries in which oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer or polyvinylidene fluoride was used as the binder, the specific capacity was investigated stepwise from 0.1 C to 2 C. The results are shown in Figure 20.

[0200] As shown in Figure 20, the specific capacities of the half-cells using the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer were 355 mAh / g, 348 mAh / g, 308 mAh / g, 260 mAh / g, and 110 mAh / g from 0.1 C to 2 C, respectively, whereas the specific capacities of the half-cells using polyvinylidene fluoride were 326 mAh / g, 226 mAh / g, 184 mAh / g, 150 mAh / g, and 664 mAh / g from 0.1 C to 2 C, respectively. This indicates that the oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer promotes lithium ion transport in the electrode and has a higher specific capacity over a wider range of applied current densities than polyvinylidene fluoride.

[0201] [Effect of binder on constant current charge / discharge cycles] To evaluate whether this binder can be practically applied to lithium-ion batteries, we investigated the effect of the binder on constant-current charge-discharge cycling at a charge capacity of 5 C using a half-cell of a lithium-ion secondary battery. The results are shown in Figure 21.

[0202] In Figure 21, symbol A represents the change in specific capacity when a half-cell containing oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer is used, and symbol B represents the change in specific capacity when a half-cell containing polyvinylidene fluoride is used.

[0203] The results shown in Figure 21 indicate that the half-cell using oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer as the binder maintained a high specific capacity of approximately 85 mAh / g even after 1,000 charge-discharge cycles, demonstrating that oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer has excellent coulombic efficiency and long-term cycle durability of the electrode.

[0204] [Battery internal resistance] The internal resistance of the battery during lithiation and delithiation was investigated using half-cells of lithium-ion secondary batteries that use oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer or polyvinylidene fluoride as the binder. The results are shown in Figure 22.

[0205] In Fig. 22, (a) shows the internal resistance during lithiation, and (b) shows the internal resistance during delithiation. Also, in Fig. 22, symbol A shows data when a half-cell using oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer was used, and symbol B shows data when a half-cell using polyvinylidene fluoride was used.

[0206] The results shown in Figure 22 indicate that the half-cell using oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer has lower internal resistance during lithiation and delithiation than the half-cell using polyvinylidene fluoride. This indicates that oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer has excellent lithium ion conductivity and low activation energy during lithiation and delithiation.

[0207] [Observation of electrodes after cycle test] After 1000 5C charge-discharge cycles using half-cells containing oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer and half-cells containing polyvinylidene fluoride, the electrode surfaces were observed with a scanning electron microscope. The results showed that no cracks were observed on the electrode containing oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer, whereas cracks were observed on the surface of the electrode containing polyvinylidene fluoride. This indicates that the electrode containing oxycarbonylmethylene-1-allyl-3-methylimidazolium polymer has excellent charge-discharge cycle characteristics.

[0208] From the above results, it is found that the binder for metal ion secondary batteries of the present invention has excellent binding properties with the active material and adhesion with the current collector, and can increase Coulomb efficiency while suppressing decomposition of the electrolyte. Furthermore, it is found that the binder for metal ion secondary batteries of the present invention has excellent long-term cycle performance and has the potential to realize high-speed charging and high-load charge / discharge electrochemical performance of metal ion secondary batteries.

[0209] The binder for metal ion secondary batteries of the present invention not only has excellent adhesive strength to metal substrates and excellent charge / discharge cycle characteristics, but also prevents cracks from occurring in the electrodes and maintains the shape of the electrodes even after repeated charge / discharge cycles. Therefore, it can be suitably used as a binder for metal ion secondary batteries, and is expected to open the door to expanded applications for rechargeable high-energy batteries. [Industrial Applicability]

[0210] The binder for metal ion secondary batteries of the present invention is expected to be used as a binder for metal ion secondary batteries, as well as dye-sensitized solar cells, capacitors, and light-emitting devices. Furthermore, since metal ion secondary batteries using the binder for metal ion secondary batteries have a high discharge capacity and excellent durability against charge and discharge, they are expected to be used in a wide range of applications, from small portable devices such as smartphones, tablet personal computers, hybrid vehicles, and electric vehicles to transportation systems.

Claims

1. Formula (I): 【Chemistry 1】 (In the formula, R 1 and R 2 each independently represents a hydrogen atom or an alkali metal atom) and a fumaric acid-based polymer having a repeating unit represented by formula (II): 【Chemistry 2】 (In the formula, X + is N + (represents a cation having a ring structure containing A binder for a metal ion secondary battery comprising at least one binder polymer selected from the group consisting of nitrogen cation-containing polymers having a repeating unit represented by the formula:

2. 10. A metal ion secondary battery having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the negative electrode is a silicon negative electrode containing the binder for metal ion secondary batteries according to claim 1.

3. Formula (II): 【Transformation 3】 (In the formula, X + is N + (represents a cation having a ring structure containing A nitrogen cation-containing polymer having a repeating unit represented by the formula:

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