A method for producing a chalcogenide active material

Abstract

The object of the present invention is to improve the volumetric energy density while maintaining the capacity retention of active materials constituting lithium-ion secondary batteries. The present invention provides a method for producing a sulfur-based active material, the method comprising the steps of: (1) mixing an acrylic resin, sulfur, and an iron compound containing divalent or trivalent iron ions to obtain a calcination raw material; and (2) calcining the calcination raw material, wherein the heating history of the calcination raw material is one time.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for producing a chalcogen active material. BACKGROUND

[0002] Lithium ion secondary batteries are mainly used as batteries for portable electronic devices because of their large charge and discharge capacity. In addition, lithium ion secondary batteries are increasingly used as electric vehicle batteries, and their performance is expected to improve.

[0003] Patent Document 1 discloses a positive electrode active material for a lithium ion secondary battery, which is obtained by heating a raw material powder containing sulfur powder and polyacrylonitrile powder under a non-oxidizing atmosphere. In addition, Patent Document 2 aims to provide a low-cost positive electrode active material by using industrial rubber.

[0004] On the other hand, as a negative electrode active material, in order to improve the battery capacity of a lithium ion secondary battery, it has been proposed to use a material capable of absorbing and releasing more lithium ions, such as silicon (Si), tin (Sn), etc.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT DOCUMENTS

[0007] Patent Document 1: WO 2010 / 044437

[0008] Patent Document 2: JP 2015-92449 A SUMMARY

[0009] PROBLEMS TO BE SOLVED BY THE INVENTION

[0010] However, polyacrylonitrile is expensive, and its quality (especially particle size) affects the performance of the battery, such as charge and discharge capacity, cycle characteristics, etc. Therefore, there is a problem that in order to obtain polyacrylonitrile with stable quality, the cost will be further increased. In addition, although industrial rubber is inexpensive, there are problems in improving its cycle characteristics. The above-mentioned negative electrode active material has a large volume change during absorption and release of lithium ions, and therefore has a problem of poor cycle characteristics when repeating charge and discharge. Although carbon materials such as graphite, hard carbon, etc. are also used, their capacity has almost reached the theoretical limit, and a significant improvement in capacity is basically not possible.

[0011] Further, the general sulfur-based active material tends to add excess sulfur to the raw material, which causes a problem in that unreacted sulfur or low-molecular sulfur compound remains in the material after heat treatment. The unreacted sulfur is an insulating material, is an impurity in the battery, and the low-molecular sulfur compound is greatly deteriorated during the cycle. Therefore, in order to prepare a sulfur-based active material having good cycle characteristics, a step of removing the unreacted sulfur and the low-molecular sulfur compound, i.e., a so-called desulfurization step, is required. Since the desulfurization step requires high temperature, there are disadvantages in terms of energy and cost.

[0012] Method for solving the problem

[0013] The present application relates to a method for preparing a sulfur-based active material, the method comprising the steps of:

[0014] (1) mixing an acrylic resin, sulfur, and an iron compound containing divalent or trivalent iron ions to obtain a calcination raw material; and

[0015] (2) calcining the calcination raw material,

[0016] wherein the calcination raw material undergoes a heating history of one time.

[0017] Effects of the present application

[0018] The present application relates to a method for preparing a sulfur-based active material, which can obtain a sulfur-based active material having improved cycle characteristics despite the heating history of one time. That is, in the present application, a sulfur-based active material prepared by adding an iron complex to an acrylic polymer and sulfur and heat-treating the same can obtain a material having good cycle characteristics even without performing a step of removing unreacted sulfur or low-molecular sulfur compound. The method requires only one heating treatment, and thus has advantages in terms of cost and energy.

[0019] In the present specification, the "cycle characteristics" refer to a characteristic of maintaining the charge and discharge capacity of a secondary battery even in the case of repeating charge and discharge. Therefore, in the process of repeating charge and discharge, a secondary battery having a large degree of decrease in charge and discharge capacity and a low capacity retention rate has poor cycle characteristics, and vice versa, a secondary battery having a small degree of decrease in charge and discharge capacity and a high capacity retention rate has excellent cycle characteristics. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a cross-sectional view schematically showing a reaction apparatus for preparing a sulfur-based active material in an embodiment of the present application. DETAILED DESCRIPTION

[0021] The present application will be described in detail below. Furthermore, the upper and lower limit values involved in the "above", "below", "greater than", "less than", and the like in relation to the numerical range recited are values that can be arbitrarily combined, and the numerical values in the examples can also serve as the upper and / or lower limits. In addition, the numerical range expressed to include the lower limit or the upper limit can be interpreted as disclosing the numerical range not including the lower limit or the upper limit, and vice versa, without departing from the spirit of the present application.

[0022] One embodiment of the present application is a method for producing a chalcogen active material, the method comprising the steps of: (1) mixing an acrylic resin, sulfur, and an iron compound containing a divalent or trivalent iron ion to obtain a calcination raw material; and (2) calcining the calcination raw material, the calcination raw material having experienced a heating history of one time.

[0023] In the calcination raw material, the content of the iron compound of the divalent or trivalent iron ion is preferably 10 parts by mass or more and 300 parts by mass or less, based on 100 parts by mass of the acrylic resin.

[0024] Preferably, the step (1) includes a sub-step of mixing the acrylic resin, the sulfur, and the iron compound containing the divalent or trivalent iron ion in a powder state.

[0025] Preferably, the calcination temperature in the step (2) is higher than 250°C and lower than 500°C.

[0026] Preferably, the calcination temperature in the step (2) is higher than the thermal decomposition temperature of the iron compound containing the divalent or trivalent iron ion.

[0027] Preferably, the ratio of the amount of sulfur to the total amount of the acrylic resin and the iron compound in the calcination raw material is 1.5 or more and 5.0 or less, in terms of mass ratio.

[0028] Preferably, the acrylic resin is a polymer obtained by polymerizing at least one monomer selected from the group consisting of acrylic compounds represented by the following formula (1), or a polymer obtained by polymerizing at least one monomer selected from the group consisting of acrylic compounds represented by the following formula (1) and at least one monomer selected from the group consisting of diacrylate compounds represented by the following formula (2).

[0029] CH2=C(R 11 ) COOR 12 (1)

[0030] (Here, R 11 is a hydrogen atom or a methyl group, and R 12 is an alkyl group.)

[0031] CH2=C(R21 )COO-Y-OCO(R 22 )C=CH2 (2)

[0032] (in the formula, R 21 and R 22 are the same or different, each being a hydrogen atom or a methyl group, and Y is a straight-chain hydrocarbylene group which can have at least one substituent selected from a hydroxyl group and an alkyl group, and the carbon skeleton constituting the hydrocarbylene group can have an ether bond of an oxygen atom, when there are two or more ether bonds, there are always two or more carbon atoms interposed between adjacent oxygen atoms.)

[0033] Preferably, R 12 is an alkyl group having 1 or more and 6 or less carbon atoms, Y is a straight-chain hydrocarbylene group having 2 or more and 6 or less carbon atoms, the number of substituents in the hydrocarbylene group is 1 or more and 4 or less, the number of carbon atoms of the alkyl group as the substituent is 1 or more and 4 or less, and the number of ether bonds of the carbon skeleton constituting the hydrocarbylene group is 1 or more and 2 or less.

[0034] Preferably, the iron compound is one or more compounds selected from ferrates and iron complexes.

[0035] Preferably, the iron compound is one or more compounds selected from organic acid salts of iron, inorganic acid salts of iron, neutral iron complexes, and iron complex salts.

[0036] Preferably, the iron compound is one or more compounds selected from ferrous oxalate (II), ferrous acetate (II), ferric nitrate (III), and ferrocene.

[0037] Preferably, the heating history temperature is higher than 100°C.

[0038] Another embodiment of the present application is a method of producing an electrode, which further includes a step (3) of producing an electrode by a conventional method using the chalcogen-based active material after the chalcogen-based active material is produced by the method of producing a chalcogen-based active material.

[0039] Still another embodiment of the present application is a method of producing a lithium-ion secondary battery, which further includes a step (4) of producing a lithium-ion secondary battery by a conventional method using the chalcogen-based active material after the electrode is produced by the method of producing an electrode.

[0040] <Raw material for calcination>

[0041] (acrylic resin)

[0042] In the present application, the acrylic resin is a polymer obtained by polymerizing at least one monomer selected from the group consisting of acrylic ester compounds represented by the following formula (1), or a polymer obtained by polymerizing at least one monomer selected from the group consisting of acrylic ester compounds represented by the following formula (1) and at least one monomer selected from the group consisting of diacrylic ester compounds represented by the following formula (2).

[0043] CH2=C(R 11 ) COOR 12 (1)

[0044] (Wherein, R 11 is a hydrogen atom or a methyl group, and R 12 is an alkyl group.)

[0045] CH2=C(R 21 ) COO-Y-OCO (R 22 ) C=CH2 (2)

[0046] (Wherein, R 21 and R 22 are the same or different, each being a hydrogen atom or a methyl group, and Y is a straight-chain alkylene group which can have at least one substituent selected from the group consisting of a hydroxyl group and an alkyl group, and the carbon skeleton constituting the alkylene group can have an ether bond with an oxygen atom, and when there are two or more ether bonds, there are always two or more carbon atoms interposed between adjacent oxygen atoms.)

[0047] In formula (1), R 11 is preferably a methyl group, and R 12 is preferably an alkyl group having 1 or more and 6 or less carbon atoms, more preferably an alkyl group having 1 or more and 4 or less carbon atoms, and among them, a methyl group, a n-butyl group, an iso-butyl group or a t-butyl group is more preferable. Examples of the compound represented by formula (1) include methyl (meth)acrylate, butyl (meth)acrylate and the like, and more preferably, methyl methacrylate and butyl methacrylate. Here, "(meth)acrylate" in methyl (meth)acrylate and butyl (meth)acrylate means "acrylate" or "methacrylate" (also applicable hereinafter). Further preferable examples of the compound represented by formula (1) are butyl methacrylate.

[0048] In formula (2), R 21 and R 22The preferred examples of the acrylic resin include a homopolymer of (meth)acrylic acid methyl ester, a homopolymer of (meth)acrylic acid butyl ester, a copolymer of (meth)acrylic acid methyl ester and ethylene glycol di(meth)acrylate, a copolymer of (meth)acrylic acid butyl ester and ethylene glycol di(meth)acrylate, and the like. Among them, as the acrylic resin, a methacrylate type acrylic resin is preferred. A more preferred example of the acrylic resin is a copolymer of methacrylic acid butyl ester and ethylene glycol dimethacrylate.

[0049] -(CH2) l -(CH2CH2O) m -(CH2CH2CH2O) n -(3)

[0050] (In the formula, 1 is 0 or more and 6 or less, m is 0 or more and 3 or less, and n is 0 or more and 2 or less. However, 1, m, and n cannot be 0 at the same time.)

[0051] Preferably, in the formula (3), 1 is 1, 2, 3, 4, 5, or 6, and m and n are 0; m is 1, 2, or 3, and 1 and n are 0; or n is 1 or 2, and 1 and m are 0.

[0052] Examples of the compound represented by the formula (2) include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerol di(meth)acrylate, and the like. Among them, ethylene glycol dimethacrylate is preferred.

[0053] The preferred examples of the acrylic resin include a homopolymer of (meth)acrylic acid methyl ester, a homopolymer of (meth)acrylic acid butyl ester, a copolymer of (meth)acrylic acid methyl ester and ethylene glycol di(meth)acrylate, a copolymer of (meth)acrylic acid butyl ester and ethylene glycol di(meth)acrylate, and the like. Among them, as the acrylic resin, a methacrylate type acrylic resin is preferred. A more preferred example of the acrylic resin is a copolymer of methacrylic acid butyl ester and ethylene glycol dimethacrylate.

[0054] One or more kinds of acrylic resins can be used.

[0055] [Form of Acrylic Resin]

[0056] In the present application, the acrylic resin is preferably in a fine particle form. Fine particles refer to particles having a particle diameter of less than 300.0 μm. The particle diameter is preferably less than 270.0 μm, more preferably less than 200.0 μm, further preferably less than 100.0 μm, further preferably less than 50.0 μm, further preferably less than 20.0 μm, further preferably less than 15.0 μm, further preferably less than 13.0 μm, further preferably less than 10.0 μm, further preferably less than 6.0 μm. On the other hand, the lower limit of the particle diameter is not particularly limited, but is generally greater than 0.1 μm, preferably greater than 1.0 μm. The particle diameter is a value measured by a precision particle size distribution measuring device Multisizer 3 manufactured by Beckman Coulter, Inc.

[0057] The acrylic resin can be a spherical fine particle or a porous fine particle. When the acrylic resin has porosity, the oil absorption amount thereof is preferably greater than 100 mL / 100 g, more preferably greater than 110 mL / 100 g, further preferably greater than 120 mL / 100 g, further preferably greater than 130 mL / 100 g, further preferably greater than 140 mL / 100 g. The oil absorption amount is a value measured according to JIS K 5101-13-2:2004. More specifically, the oil absorption amount can be measured according to the method disclosed in paragraph 0069 of JP 2017-88501 A.

[0058] [Weight average molecular weight (Mw) of the acrylic resin]

[0059] The Mw of the acrylic resin is not particularly limited as long as having the above structure. However, the Mw of the acrylic resin is generally in the range of greater than 2000 and less than 1500000. The Mw is a value measured by gel permeation chromatography (GPC) using polystyrene calibration.

[0060] [Acquisition or production of the acrylic resin]

[0061] The acrylic resin is commercially available or produced by a conventional method within the knowledge of those skilled in the art. Commercially available acrylic resins include, for example, an acrylic resin manufactured by Sekisui Kasei Co., Ltd.

[0062] (Sulfur)

[0063] As the sulfur, various forms of sulfur can be used, such as, for example, powdered sulfur, insoluble sulfur, precipitated sulfur, colloidal sulfur, and the like. Among them, precipitated sulfur and colloidal sulfur are preferred.

[0064] As for sulfur, from the viewpoint of the effect of the present application, the ratio of the amount of sulfur in the calcined raw material to the total amount of the acrylic resin and the iron compound (sulfur ratio) is preferably 1.5 or greater and 5.0 or less in terms of mass ratio. The ratio is more preferably 2.0 or greater, further preferably 2.5 or greater, further preferably greater than 2.5, further preferably greater than 2.6, further preferably greater than 2.7, further preferably greater than 2.8, further preferably greater than 2.9. On the other hand, the ratio is more preferably less than 4.0, further preferably 3.5 or less, further preferably less than 3.4, further preferably less than 3.3, further preferably less than 3.2, further preferably less than 3.1.

[0065] The content of sulfur is preferably 50 parts by mass or greater, more preferably 100 parts by mass or greater, further preferably 300 parts by mass or greater, further preferably 400 parts by mass or greater, further preferably 500 parts by mass or greater, further preferably greater than 500 parts by mass, based on 100 parts by mass of the acrylic resin. When the content is 50 parts by mass or greater, there is a tendency for the charge-discharge capacity and the cycle characteristics to improve. On the other hand, the upper limit of the content of sulfur is not particularly limited, but is generally less than 1000 parts by mass, preferably less than 900 parts by mass, further preferably less than 800 parts by mass, further preferably less than 700 parts by mass. When the content is less than 1000 parts by mass, there is a tendency for there to be an advantage in terms of cost.

[0066] As the sulfur, any one of various allotropes can be used, but a sulfur containing S8 sulfur that is a solid at normal temperature and pressure is preferably used, and S8 sulfur alone is more preferably used.

[0067] (Iron compound containing divalent or trivalent iron ions)

[0068] The iron compound containing divalent or trivalent iron ions is not particularly limited as long as it is decomposed during calcination and reacts with sulfur to produce iron disulfide, and various such iron compounds can be used. Examples of such iron compounds include ferrites, iron complexes, and the like. Examples of the ferrite include organic acid salts of iron and inorganic acid salts of iron. On the other hand, examples of the iron complex include neutral iron complexes and iron complex ion salts (iron complex salts).

[0069] Examples of the organic acid salt of iron include, for example, salts of divalent iron (Fe 2+ ) and organic acids, and salts of trivalent iron (Fe 3+salts with organic acids, and the like. Among them, salts with divalent iron and organic acids are preferred. The organic acid is, but not particularly limited to, an acid having a carboxyl group (-COOH), an acid having a sulfonic acid group (-SO3H), and the like. Among them, an acid having a carboxyl group is preferred. Specific examples of the organic acid include fatty acids, oxalic acid, tartaric acid, citric acid, malic acid, succinic acid, and the like. Specific examples of the fatty acid include, for example, a fatty acid having 1 or more and 6 or less carbon atoms, such as acetic acid, propionic acid, butyric acid, and the like. Among them, acetic acid, oxalic acid, and the like are preferred. Preferred examples of the organic acid salt include ferrous (II) acetate, ferrous (II) oxalate, and the like. They can be hydrates. One or more kinds of the organic acid salt of iron can be used.

[0070] Examples of the inorganic acid salt of iron include, for example, divalent iron (Fe 2+ ) and inorganic acid salts, trivalent iron (Fe 3+ ) and inorganic acid salts, and the like. Among them, salts with trivalent iron and inorganic acids are preferred. Specific examples of the inorganic acid include hydrochloric acid, sulfuric acid, nitric acid, and the like. Among them, nitric acid and the like are preferred. Examples of the inorganic acid salt of iron preferably include ferrous (II) chloride, ferric (III) chloride, ferrous (II) sulfate, ferric (III) sulfate, ferrous (II) nitrate, ferric (III) nitrate, and the like. They can be hydrates. One or more kinds of the inorganic acid salt of iron can be used.

[0071] Examples of the iron complex include, for example, divalent iron (Fe 2+ ) complexes, trivalent iron (Fe 3+ ) complexes, and the like. The iron complex can be in the form of a neutral complex or in the form of a complex salt. The ligand coordinated to the iron ion is not particularly limited, and examples thereof include, for example, halogen atoms (such as a chlorine atom, a bromine atom, and the like), a cyano group, dicyclopentadienyl, N,N'-bis(salicylidene)ethylenediamine, and the like. Examples of the iron complex include, for example, potassium hexacyanoferrate (II) ([Fe(CN)6]K4), potassium hexacyanoferrate (III) ([Fe(CN)6]K3), sodium tetrachloroferrate (III) ([FeCl4]Na), dicyclopentadienyl iron (II) (ferrocene), N,N'-bis(salicylidene)ethylenediamine chloroferrate (III), and the like. One or more kinds of the iron complex can be used.

[0072] As the iron compound containing a divalent or trivalent iron ion, at least one selected from the group consisting of the above-described organic acid salt of iron, the inorganic acid salt of iron, the neutral iron complex, and the iron complex salt can be used. Among them, the organic acid salt of iron, the inorganic acid salt of iron, or the neutral iron complex is preferred.

[0073] From the effects of the present application, the mass of the iron compound containing divalent or trivalent iron ions is preferably 10 parts by mass or more, 300 parts by mass or less, more preferably 20 parts by mass or more, further preferably more than 20 parts by mass, further preferably more than 30 parts by mass, further preferably more than 40 parts by mass, further preferably 50 parts by mass or more, further preferably more than 60 parts by mass, further preferably more than 70 parts by mass, further preferably more than 75 parts by mass, based on 100 parts by mass of the acrylic resin. On the other hand, the above content is preferably less than 300 parts by mass, more preferably less than 250 parts by mass, further preferably less than 200 parts by mass, further preferably less than 150 parts by mass, further preferably 100 parts by mass or less.

[0074] (electrically conductive carbon material)

[0075] The calcination raw material can contain an electrically conductive carbon material. This can improve the electrical conductivity of the sulfur-based active material. As such an electrically conductive carbon material, a carbon material having a graphite structure is preferred. As the carbon material, a carbon material having a condensed aromatic ring structure, such as carbon black, acetylene black, ketjen black, graphite, carbon nanotube (CNT), carbon fiber (CF), graphene, fullerene, or the like, can be used. One or more electrically conductive carbon materials can be used.

[0076] Among them, acetylene black, carbon black, and ketjen black, which are inexpensive and have good dispersibility, are preferred. In addition, a small amount of CNT and graphene, etc. can be used together with acetylene black, carbon black, or ketjen black. By such a combination system, the cycle characteristics of the lithium ion secondary battery can be further improved without significantly increasing the cost. Furthermore, the combined amount of CNT or graphene is preferably 8% or more of the total amount of the electrically conductive carbon material, more preferably more than 8%, and at the same time, preferably 12% or less, more preferably less than 12%.

[0077] The content of the electrically conductive carbon material is preferably 5 parts by mass or more, more preferably more than 5 parts by mass, further preferably more than 10 parts by mass, based on 100 parts by mass of the acrylic resin. When the content is 5 parts by mass or more, it is more likely to achieve the tendency to further improve the charge and discharge capacity and the cycle characteristics. On the other hand, the content of the electrically conductive carbon material is preferably 50 parts by mass or less, more preferably less than 50 parts by mass, further preferably less than 40 parts by mass. When the content is 50 parts by mass or less, the proportion of the sulfur-containing structure in the sulfur-based active material does not decrease relatively, and it is more likely to achieve the tendency to further improve the charge and discharge capacity and the cycle characteristics.

[0078] (other materials)

[0079] The calcination raw material can appropriately contain other materials commonly used in the art as needed.

[0080] <Preparation of sulfur-based active material>

[0081] In the present application, the sulfur-based active material can be prepared by calcining a calcination raw material obtained by mixing an acrylic resin, sulfur, and an iron compound containing a divalent or trivalent iron ion.

[0082] (Step (1))

[0083] Step (1) is a step of mixing an acrylic resin, sulfur, and an iron compound containing a divalent or trivalent iron ion to obtain a calcination raw material for calcination. There is no particular restriction on the mixing, as long as the components are mixed sufficiently, but in the present application, examples of the preferred mixing method can include at least the following mixing by a wet method with a solvent or mixing by a dry method without a solvent.

[0084] [Wet method]

[0085] In the present application, the wet method includes sub-steps for preparing the calcination raw material:

[0086] (1-a-1) adding an acrylic resin and an iron compound containing a divalent or trivalent iron ion to an organic solvent and mixing them to obtain a liquid mixture;

[0087] (1-a-2) removing the organic solvent from the liquid mixture to obtain a dry mixture; and

[0088] (1-a-3) mixing the above dry mixture with sulfur.

[0089] In sub-step (1-a-1), there is no particular restriction on the method of adding the acrylic resin and the iron compound containing a divalent or trivalent iron ion to the organic solvent, as long as they can be mixed to obtain a liquid mixture. For example, (1) the acrylic resin and the iron compound containing a divalent or trivalent iron ion can be added to the organic solvent at the same time and mixed, (2) the acrylic resin can be added to the organic solvent and mixed first, and then the iron compound containing a divalent or trivalent iron ion can be further added and mixed, or (3) the iron compound containing a divalent or trivalent iron ion can be added to the organic solvent and mixed first, and then the acrylic resin can be further added and mixed.

[0090] In sub-step (1-a-1), as the organic solvent, an organic solvent commonly used in the art can be used, and examples of the solvent include, for example, N-methyl-2-pyrrolidone, N,N-dimethylformamide, an alcohol, hexane, water, acetone, an ether such as tetrahydrofuran, and the like. In addition, as the organic solvent, a solvent that can dissolve the acrylic resin is preferred. This is because it contributes to good mixing. One or more solvents can be used.

[0091] The mixing of the acrylic resin and / or the iron compound containing divalent or trivalent iron ions with the organic solvent can be achieved by stirring them in a container such as a beaker.

[0092] In the substep (1-a-2), the removal of the organic solvent can be achieved by a conventional method, a method involving heating, or a drying method not involving heating. For example, the removal can be performed by subjecting the liquid mixture to a drying method such as drying under reduced pressure.

[0093] Preferably, the obtained dried mixture is pulverized before the next step is performed. This is because, by this means, it is expected that the mixing in the substep (1-a-3) can be performed better. The pulverization can be performed by a conventional method, for example, using a grinder, a freeze pulverizer, or the like. In particular, after rough pulverization with a grinder, fine pulverization is preferably performed using a freeze pulverizer.

[0094] For the pulverized product, the particle size is preferably a median diameter of 1 μm or more, more preferably greater than 1 μm, further preferably greater than 2 μm, further preferably greater than 3 μm, further preferably greater than 4 μm. In addition, the median diameter is preferably 40 μm or less, more preferably less than 40 μm, further preferably less than 30 μm, further preferably less than 20 μm, further preferably less than 15 μm, further preferably less than 10 μm. From the effect of the present application, the median diameter is preferably within the above range. The median diameter can be measured by the method described below.

[0095] In the substep (1-a-3), the mixing of the dried mixture and sulfur can be performed by a conventional method, examples of which can include, for example, a method of mixing them using a stirrer, or the like.

[0096] [Dry process]

[0097] In the present application, the dry process includes the substeps for preparing a calcined raw material:

[0098] (1-b) mixing the acrylic resin, sulfur, and the iron compound containing divalent or trivalent iron ions in a powder state.

[0099] Here, the powder refers to a state in which each solid material has been sufficiently refined to be suitable for the mixing purpose of the present application. As for the size of the particles constituting the powder, the size of the particles is not particularly limited as long as the mixing can be properly performed, but the median diameter is usually 1 μm or more and 40 μm or less. From the effect of the present application, the size of the particles is preferably more than 1 μm, more preferably more than 2 μm, further preferably more than 3 μm, further preferably more than 4 μm, while it is preferably 40 μm or less, more preferably less than 40 μm, more preferably less than 30 μm, further preferably less than 20 μm, further preferably less than 15 μm, further preferably less than 10 μm. The median diameter can be measured by the following method.

[0100] The above mixing can be achieved by a conventional method, for example, in the same mixing manner as in the above sub-step (1-a-3).

[0101] Whether the wet method or the dry method is employed, it is desirable to sufficiently mix the calcination raw material in advance. In addition, at the time of adding the electrically conductive carbon material or the like to the calcination raw material, these additives can also be mixed in advance before the calcination so as to be previously contained in the calcination raw material.

[0102] The calcination raw material thus obtained can be used as it is in the next step (2), or it can be molded into a pellet according to the need for use in step (2).

[0103] (Step (2))

[0104] Step (2) is a step of calcining the calcination raw material obtained above. The calcination of the calcination raw material can be performed by a conventional method, for example, by first heating the calcination raw material at a predetermined temperature raising rate to a predetermined temperature, maintaining at the predetermined temperature for a predetermined time, and then naturally cooling.

[0105] [Non-oxidizing atmosphere]

[0106] The calcination is preferably performed in a non-oxidizing atmosphere. The non-oxidizing atmosphere refers to an atmosphere substantially free of oxygen, which is used to suppress the oxidative deterioration and excessive pyrolysis of the constituent components. Specifically, it refers to an inert gas atmosphere such as nitrogen gas, argon gas, a sulfur gas atmosphere, and the like. Thereby, the modification is performed in a quartz tube under an inert gas atmosphere, for example.

[0107] [Temperature raising rate]

[0108] Preferably, the temperature raising rate is in the range of 50°C / h or more and 500°C / h or less. The temperature raising rate is preferably higher than 100°C / h, more preferably higher than 150°C / h. On the other hand, the temperature raising rate is more preferably lower than 400°C / h, further preferably lower than 350°C / h. When the temperature raising rate is in this range, the tendency to achieve the purpose of improving the charge and discharge capacity and the cycle characteristics is easy.

[0109] [Firing temperature / time]

[0110] The calcination temperature refers to the temperature after the calcination raw material is heated, and the calcination raw material is kept at this temperature for a certain period of time. The above temperature is preferably in the range of higher than 250°C and lower than 500°C. When the temperature is higher than 250°C, there is a tendency to avoid insufficient sulfurization reaction and prevent the charge and discharge capacity of the material from decreasing. On the other hand, when the temperature is lower than 500°C, there is a tendency to avoid decomposition of the calcination raw material, prevent a decrease in yield and a decrease in charge and discharge capacity. The above temperature is more preferably higher than 300°C, further preferably higher than 350°C, further preferably higher than 380°C. On the other hand, the above temperature is more preferably lower than 480°C, further preferably lower than 450°C.

[0111] From the perspective of the effects of the present invention, the calcination temperature in step (2) is preferably higher than the thermal decomposition temperature of the iron compound containing divalent or trivalent iron ions.

[0112] The hold time of above-mentioned roasting temperature can be suitably set according to the kind of roasting raw material, roasting temperature etc. For example, under the situation represented by this treatment mode of giving an example in the embodiment part, roasting time is preferably more than 1 hour, below 6 hours.When roasting time is more than 1 hour, there is the tendency that roasting can fully advance, and when roasting time is below 6 hours, there is the tendency that prevents the excessive thermal decomposition of constituent.Above-mentioned time is preferably greater than 1 hour, more preferably greater than 1.5 hours.On the other hand, above-mentioned time is preferably less than 6 hours, more preferably less than 4 hours.

[0113] [Installation]

[0114] Calcination can be carried out by, for example, a muffle furnace ( Figure 1 ) can also be carried out using a continuous device such as a twin-screw extruder. The advantage of using a continuous device is that the raw materials in the device can be kneaded, crushed, mixed, etc. through a series of operations such as roasting and simultaneously preparing the sulfur-based active material.

[0115] Muffle furnace Figure 1 ) is a furnace separated by a hot plate, etc., so that the heat source (heater) is not exposed inside the furnace to prevent the sample from being contaminated. Figure 1 In the figure, a heater 2 is located at the bottom of a muffle furnace 1, separated by a hot plate. A lid 3 is installed on the front surface of the furnace (the left end in the figure), which provides a structure that can maintain an atmosphere of inert gas 4 inside the furnace. A thermocouple (not shown) is connected to the lid to measure the temperature inside the furnace during the roasting process. Two layers of stainless steel (SUS) rectangular trays 5 and 6 are installed on the upper and lower layers of the furnace, respectively, for roasting the raw materials.

[0116] The furnace is configured so that a gas (for example, an inert gas such as argon (Ar) gas or the like) can be continuously supplied and discharged from the outside through the gas introduction pipe 7 and the gas discharge pipe 8. The gas discharge pipe 8 is connected to a trap tank 10 that contains a sodium hydroxide aqueous solution 9, and the exhaust gas is discharged from the muffle furnace 1 through the gas discharge pipe 8, released to the outside after passing through the sodium hydroxide aqueous solution 9 in the trap tank 10. Thus, even if the exhaust gas contains hydrogen sulfide gas generated by the reaction, it is neutralized by the sodium hydroxide aqueous solution and removed from the exhaust gas.

[0117] (Step of removing unreacted sulfur)

[0118] In the processed product obtained after the calcination, unreacted sulfur and the like are precipitated from the sublimed sulfur during the calcination process and then remain as a residue after cooling, but the method of the present application does not require removal of these residues. However, removal of the residues can be performed by a method that does not involve heating, such as reduced pressure drying, solvent washing, and the like.

[0119] (Pulverization / classification)

[0120] The obtained sulfur-based active material can be pulverized to a predetermined particle size and classified to obtain particles of a size suitable for manufacturing an electrode. From the effects of the present application, the preferred particle size distribution is a median diameter of about 1 μm or more and 40 μm or less. The median diameter is preferably greater than 1 μm, more preferably greater than 2 μm, further preferably greater than 3 μm, further preferably greater than 4 μm, further preferably greater than 5 μm. Also, the median diameter is preferably less than 40 μm, more preferably less than 30 μm, further preferably less than 20 μm, further preferably less than 15 μm, further preferably less than 10 μm. Furthermore, the median diameter is calculated by measuring the volume-based cumulative 50% size (median diameter D50) using a laser diffraction / scattering type particle size distribution analyzer (LA-960 manufactured by HORIBA, Ltd.).

[0121] Furthermore, in the above-described calcination method using a twin-screw extruder, the prepared sulfur-based active material can be pulverized by shearing during the kneading process while the sulfur-based active material is being prepared.

[0122] (Heating history)

[0123] In the present application, the heating history refers to a heat treatment history to which the calcined raw material is subjected. The end point temperature of the heating history is usually higher than 50°C, more usually higher than 60°C, further usually higher than 70°C, further usually higher than 80°C, further usually higher than 90°C, further usually higher than 100°C, further usually higher than 110°C, further usually higher than 120°C, further usually higher than 130°C, further usually higher than 140°C, further usually higher than 150°C. In the production method of the present application, the calcined raw material is not subjected to any heating history other than the calcination in the above-mentioned step (2).

[0124] <Chalcogen-based active material>

[0125] The above-mentioned produced chalcogen-based active material will be described below. The chalcogen-based active material obtained as above contains a sulfur-modified acrylic resin (sulfurized acrylic resin) and iron disulfide, wherein the active material in which they are compounded contains unreacted sulfur, and the chalcogen-based active material contains at least carbon, sulfur, and iron as constituent elements. The sulfur content as shown below is the amount contained in the chalcogen-based active material. Further, the fact that the obtained chalcogen-based active material contains iron disulfide is based on a comparison with the distribution of diffraction intensity peaks of iron disulfide (pyrite) determined by X-ray diffraction.

[0126] (Sulfur content)

[0127] The range of the amount of sulfur in the thus obtained chalcogen-based active material will be described below. In the case where the unreacted sulfur is not removed after calcination, the amount of sulfur in the chalcogen-based active material is usually greater than 55.0 mass%, more usually greater than 60.0 mass%, further usually greater than 61.0 mass%, further preferably greater than 62.0 mass%, further usually greater than 63.0 mass%. On the other hand, the sulfur content is usually less than 70.0 mass% or less than 65.0 mass%. However, when compounded with a conductive carbon material, the sulfur content can decrease due to the influence of carbon in the conductive carbon material, but in this case, the sulfur content can be considered to be about 5.0 mass% lower than the above-mentioned amount of sulfur. Further, in the case where the unreacted sulfur is removed without a heating history after calcination, the sulfur content decreases depending on the amount of removal of the unreacted sulfur.

[0128] <Lithium ion secondary battery>

[0129] The sulfur-based active material of the present application can be used as an electrode active material, i.e., a positive electrode active material or a negative electrode active material, of a lithium ion secondary battery. That is, a method of producing the electrode of the lithium ion secondary battery is the same as a method of producing a general electrode of a lithium ion secondary battery except that the sulfur-based active material is used, and further, a method of producing the lithium ion secondary battery is the same as a method of producing a general lithium ion secondary battery except that the electrode of the lithium ion secondary battery is used. The lithium ion secondary battery thus produced has a large charge and discharge capacity and excellent cycle characteristics.

[0130] 1. Use of a sulfur-based active material as a positive electrode active material

[0131] The lithium ion secondary battery of the present application can be produced according to a conventional method using a negative electrode and an electrolyte and a positive electrode containing the above-mentioned sulfur-based active material (positive electrode active material), and, as needed, a member such as a separator can also be used.

[0132] (Positive electrode)

[0133] The positive electrode for a lithium ion secondary battery can be produced in the same manner as a general positive electrode of a lithium ion secondary battery except that the above-mentioned sulfur-based active material is used as a positive electrode active material. For example, a particulate sulfur-based active material is mixed with a conductive aid, a binder and a solvent to produce a paste-like positive electrode material, and then the positive electrode material is coated onto a current collector, which is then dried to produce a positive electrode. As another method, a positive electrode can also be produced by, for example, kneading a sulfur-based active material with a conductive aid, a binder and a small amount of a solvent together and forming a thin film using a mortar or the like, and then pressing it onto a current collector using a press or the like.

[0134] [Conductive aid]

[0135] Examples of the conductive aid include, for example, vapor grown carbon fiber (VGCF), carbon powder, carbon black (CB), acetylene black (AB), ketjen black (KB), graphite or a fine powder of a metal (such as aluminum, titanium or the like) stable at a positive electrode potential. One or more kinds of conductive aids can be used.

[0136] [Binder]

[0137] Examples of the binder include, for example, polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamide-imide (PAI), carboxymethyl cellulose (CMC), polyvinyl chloride (PVC), acrylic resin, methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene ether (PPO), polyethylene oxide (PEO), polyethylene (PE), polypropylene (PP) or the like. One or more kinds of binders can be used.

[0138] [Solvent]

[0139] Examples of the solvent include, for example, N-methyl-2-pyrrolidone, N,N-dimethylformamide, an alcohol, hexane, water, and the like. One or more solvents can be used.

[0140] [Blending amount]

[0141] The blending amount of the positive electrode material is not particularly limited, but, for example, it is preferable that the conductive aid is blended in an amount of 2 parts by mass or more and 100 parts by mass or less, the binder is blended in an amount of 2 parts by mass or more and 50 parts by mass or less, and an appropriate amount of the solvent, based on 100 parts by mass of the sulfur-based active material.

[0142] [Current collector]

[0143] As the current collector, a material commonly used for the positive electrode of a lithium ion secondary battery can be used. Examples of the current collector include, for example, an aluminum foil, an aluminum mesh, a punched aluminum sheet, an expanded aluminum sheet, a stainless steel foil, a stainless steel mesh, a punched stainless steel sheet, an expanded stainless steel sheet, a nickel foam, a nickel nonwoven fabric, a copper foil, a copper mesh, a punched copper sheet, an expanded copper sheet, a titanium foil, a titanium mesh, a carbon nonwoven fabric, and a carbon woven fabric, and the like. Among them, when the sulfur-based active material of the present application is used as the positive electrode active material, the current collector composed of the carbon nonwoven fabric and the carbon woven fabric made of carbon having a high degree of graphitization is suitable as the current collector because it does not contain hydrogen and has low reactivity with sulfur. As the raw material of the carbon fiber having a high degree of graphitization, various pitches (i.e., by-products of petroleum, coal, coal tar, and the like) and polyacrylonitrile fibers (PAN) used as carbon fiber materials, and the like can be used. The current collector can be used alone or two or more of them can be used in combination.

[0144] (Negative electrode)

[0145] As the negative electrode material, known metal lithium, carbon-based materials such as graphite, silicon-based materials such as silicon thin film, and alloy-based materials such as copper-tin and cobalt-tin can be used. When a lithium-free material such as a carbon-based material, a silicon-based material, and an alloy-based material is used as the negative electrode material among the above-mentioned negative electrode materials, it is advantageous to avoid short-circuiting between the positive electrode and the negative electrode due to the generation of dendrites. However, when these lithium-free negative electrode materials are used in combination with the positive electrode of the present application, neither the positive electrode nor the negative electrode contains lithium. Therefore, a lithium pre-doping step is required in which lithium is inserted in advance into one or both of the negative electrode and the positive electrode. The method of pre-doping lithium can employ an existing method. For example, when lithium is doped in the negative electrode, one is a method of inserting lithium by electrolytic doping in which metal lithium is assembled as a counter electrode into a half-cell and lithium is doped by an electrochemical method, and the other is a method of inserting lithium by adhesive pre-doping in which a metal lithium foil is attached to an electrode, and then it is placed in an electrolyte to dope it by allowing lithium to diffuse into the electrode. In addition, when lithium is pre-doped in the positive electrode, the above-mentioned electrolytic doping method can also be used. As the lithium-free negative electrode material, a silicon-based material is particularly preferable as a high-capacity negative electrode material, and among them, a thin film silicon which is thin in electrode thickness and has an advantage in terms of capacity per unit volume is more preferable.

[0146] (electrolyte solution)

[0147] As the electrolyte solution for the lithium ion secondary battery, an electrolyte solution obtained by dissolving an alkali metal salt as an electrolyte in an organic solvent can be used. As the organic solvent, at least one selected from non-aqueous solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl ether, γ-butyrolactone, acetonitrile, and the like is preferably used. As the electrolyte, LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiI, LiClO4, and the like can be used. The concentration of the electrolyte solution is about 0.5 mol / L or more and 1.7 mol / L or less. In addition, the electrolyte solution is not limited to a liquid state. For example, when the lithium ion secondary battery is a lithium polymer secondary battery, the electrolyte solution is in a solid state (for example, a polymer gel state).

[0148] (separating membrane)

[0149] The lithium ion secondary battery can include components such as a separating membrane, in addition to the above-mentioned negative electrode, positive electrode, and electrolyte solution. The separating membrane is interposed between the positive electrode and the negative electrode, allows ions to migrate between the positive electrode and the negative electrode, and prevents internal short-circuiting between the positive electrode and the negative electrode. If the lithium ion secondary battery is a sealed type, the separating membrane is required to have a function of retaining the electrolyte solution. As the separating membrane, a thin and microporous or non-woven fabric film made of polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide, cellulose, glass, or the like is preferably used.

[0150] (shape)

[0151] The shape of the lithium ion secondary battery is not particularly limited, and various shapes such as a cylindrical shape, a stacked shape, a coin shape, a button shape, and the like can be adopted.

[0152] 2. Use of a sulfur-based active material as a negative electrode active material

[0153] The lithium ion secondary battery of the present application can be produced in accordance with a conventional method using a positive electrode and an electrolyte solution and a negative electrode containing the above-described sulfur-based active material (negative electrode active material), and, if necessary, members such as a separator and the like can also be used.

[0154] (Negative electrode)

[0155] The negative electrode of the lithium ion secondary battery can be produced in the same manner as a general negative electrode of a lithium ion secondary battery, with the exception that the above-described sulfur-based active material is used as a negative electrode active material. For example, a sulfur-based active material in a particulate form is mixed with a conductive aid, a binder, and a solvent to produce a paste-like negative electrode material, the negative electrode material is coated on a current collector, and then dried to produce a negative electrode. As another method, a negative electrode can also be produced by, for example, kneading a sulfur-based active material with a conductive aid, a binder, and a small amount of a solvent together and forming a thin film using a mortar or the like, and then pressing on a current collector using a press or the like.

[0156] As the conductive aid, the binder, and the solvent, the same binder material as when the above-described sulfur-based active material is used as a positive electrode active material can be used, and can also be applied to the current collector.

[0157] (Positive electrode)

[0158] The positive electrode material is not particularly limited, and is, for example, a transition metal oxide or a solid solution oxide containing lithium, or a substance capable of electrochemically absorbing and releasing lithium. Examples of the transition metal oxide containing lithium include, for example, Li-Co-based composite oxides such as LiCoO2and the like, Li-Ni-Co-Mn-based composite oxides such as LiNi x Co y Mn z O2and the like, Li-Ni-based composite oxides such as LiNiO2and the like, and Li-Mn-based composite oxides such as LiMn2O4and the like. Examples of the solid solution oxide include, for example, Li a Mn x Co y Ni z O2(1.150≤a≤1.430, 0.450≤x≤0.600, 0.100≤y≤0.150, 0.200≤z≤0.280), LiMn x Co y Ni zO2(0.300≤x≤0.850, 0.100≤y≤0.300, 0.100≤z≤0.300), LiMn 1.5 Ni 0.5 O4, etc. These compounds can be used alone or in a mixture of a plurality of compounds.

[0159] As for the electrolyte, the separator, and the shape of the lithium ion secondary battery, those used when the sulfur-based active material is used as the positive electrode active material can be used.

[0160] Example

[0161] Although the present application will be described based on examples, it is not limited to the examples.

[0162] Each chemical used in the examples and comparative examples is shown below. Each chemical was purified according to a conventional method as needed.

[0163] <Materials used for the test>

[0164] Acrylic resin 1: spherical acrylic resin made of a homopolymer of methyl methacrylate (TECHPOLYMER MB-4 manufactured by Sekisui Kasei Co., Ltd., particle diameter: 4 μm)

[0165] Acrylic resin 2: spherical acrylic resin made of a homopolymer of methyl methacrylate (PARAPET GF-P manufactured by Kuraray Co., Ltd., particle diameter: 270 μm)

[0166] Iron compound 1 (organic acid salt): ferrous (II) oxalate dihydrate (ferrous (II) oxalate dihydrate manufactured by KANTO CHEMICAL Co., Ltd., ultra-pure)

[0167] Iron compound 2 (organic acid salt): ferrous (II) acetate (ferrous (II) acetate manufactured by KANTO CHEMICAL Co., Ltd.)

[0168] Sulfur: precipitated sulfur manufactured by Tsurumi Chemical Industry Co., Ltd.

[0169] Preparation Example 1 (preparation of calcination raw material by dry method)

[0170] According to the compound composition of Table 1, the powdered acrylic resin, sulfur, and iron compound were mixed by a stirrer to obtain a calcination raw material.

[0171] Example 1

[0172] <Preparation of sulfur-based active material>

[0173] (Reaction apparatus)

[0174] The calcined raw material was calcined using a muffle furnace. Figure 1 The muffle furnace in the above-described manner. Figure 1 The muffle furnace in the above-described manner.

[0175] (Calcination step)

[0176] First, the atmosphere in the muffle furnace was replaced with argon three times using a vacuum pump, and the calcined raw material was placed in a SUS container. Then, argon was continuously introduced from a gas introduction tube at a flow rate of 100 mL / min, and the muffle furnace was started to be heated 30 minutes after the start of the argon introduction. The muffle furnace was raised in temperature at a heating rate of 5°C / min, and after the temperature of the calcined raw material reached 400°C, the temperature was maintained at 400°C for two hours to perform heat treatment. Next, while adjusting the argon flow rate, the calcined raw material was naturally cooled to 25°C under an argon atmosphere, and then the calcined raw material was taken out of the muffle furnace.

[0177] (Pulverization step)

[0178] The calcined material was pulverized with a grinder (free speed grinder, FS-20, manufactured by Labonect).

[0179] (Classification step)

[0180] In order to remove coarse particles from the pulverized calcined material, the calcined material was classified using a 32-μm mesh screen to obtain a sulfur-based active material.

[0181] <Manufacture of lithium ion secondary battery>

[0182] The method of manufacturing a lithium ion secondary battery was as follows:

[0183] (Positive electrode)

[0184] Using the sulfur-based active material obtained above as an active material, acetylene black as a conductive aid, and an acrylic resin as a binder, a uniform slurry was prepared in the following manner. The mass ratio of the active material : conductive aid : binder was 85 : 10 : 5 (mass %), and they were placed in a container, stirred and mixed with a planetary centrifugal mixer (ARE-250 manufactured by THINKY), and ultra-pure water (milliQ water) was used as a dispersant. The prepared slurry was coated on a 20-μm-thick aluminum foil using a coater with a slit width of 60 μm, and an electrode was obtained by pressing with a roll press machine, and then dried by heating at 120°C for 3 hours, and punched to φ11 to obtain an electrode (positive electrode). Then, the mass of the electrode was measured, and the amount of the active material in the electrode was calculated according to the above-described ratio.

[0185] (Negative electrode)

[0186] As the negative electrode, a metal lithium foil (disc shape, diameter 14 mm, thickness 500 μm, manufactured by Honjo Metal Co., Ltd.)

[0187] (Electrolyte)

[0188] As the electrolyte, a non-aqueous electrolyte was used, and LiPF6was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate to prepare it. Ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1 : 1. The concentration of LiPF6in the electrolyte was 1.0 mol / L.

[0189] (Lithium-ion secondary battery)

[0190] A coin battery was prepared using the positive electrode, the negative electrode, and the electrolyte. Specifically, in a dry room, a separator (Celgard 2400 manufactured by Celgard LLC, polypropylene microporous film having a thickness of 25 μm) and a glass non-woven filter paper (GA100 manufactured by ADVANTEC, having a thickness of 440 μm) were placed between the positive electrode and the negative electrode, and the electrolyte was filled to obtain an electrode body battery. The electrode body battery was placed in a battery case made of stainless steel (CR2032 type coin battery assembly manufactured by Hohsen Corp.). The above-mentioned electrolyte was injected into the battery case. The battery case was sealed with a caulking machine to obtain the lithium-ion secondary battery of Example 1.

[0191] Examples 2 to 6 and Comparative Examples 1 to 3

[0192] The sulfur-based active material and the method of preparing the lithium-ion secondary battery were the same as in Example 1, except that appropriate adjustments were made in accordance with Table 1. However, in the case where a desulfurization step was present (for example, Comparative Examples 1 and 3), the desulfurization step was performed after the pulverization step described above.

[0193] (Desulfurization step)

[0194] In order to remove residual unreacted sulfur (sulfur alone in a free state) and low-molecular sulfur compounds from the calcined material, the following step was performed. That is, the calcined material was pulverized in a mortar, and the pulverized material was placed in a glass tube oven and heated at 250°C for 3 hours while performing vacuum suction to obtain a calcined material from which unreacted sulfur (or containing only a trace amount of unreacted sulfur) was removed. The temperature increase rate was set to 10°C / min.

[0195] <Measurement of discharge capacity and capacity retention rate>

[0196] Each of the button-type lithium ion secondary battery examples and comparative examples was subjected to charge and discharge at a current value of 50 mA (0.1 C) per 1 g of positive electrode active material at a test temperature of 30°C. The discharge termination voltage was set to 1.0 V, and the charge termination voltage was set to 3.0 V. During repeated charge and discharge, the first, second, third, and tenth battery discharge capacities (mAh) were observed.

[0197] The second discharge capacity (mAh / g) was defined as the initial capacity. The greater the initial capacity, the greater the charge and discharge capacity of the lithium ion secondary battery, and thus it was evaluated as being preferable. The capacity retention rate (%) was calculated from the third discharge capacity DC3 (mAh / g) and the tenth discharge capacity DC10 (mAh / g) according to the following formula (a): 10

[0198] Capacity retention rate (%) = (DC 10 / DC3) x 100 (a)

[0199] As described above, it can be said that the higher the capacity retention rate, the more excellent the cycle characteristics of the lithium ion secondary battery.

[0200] <Mass ratio of sulfur (%)

[0201] The mass ratio of sulfur (%) in the sulfur-based active material was determined by measuring the mass of sulfur using a chromatographic column (IonPac AS12A) manufactured by Dionex for an ion chromatograph DX-320 manufactured by Dionex, and calculating the proportion of sulfur in the total mass of the sulfur-based active material.

[0202]

[0203]

[0204] <Embodiment>

[0205] The preferred embodiment is shown below.

[0206] [1] A method for producing a sulfur-based active material, the method comprising the steps of:

[0207] (1) mixing an acrylic resin, sulfur, and an iron compound containing divalent or trivalent iron ions to obtain a calcination raw material; and

[0208] (2) calcining the calcination raw material,

[0209] wherein the calcination raw material has experienced one heating history.

[0210] ​[2] The method according to [1] above, wherein the mass of the iron compound containing divalent or trivalent iron ions is 10 parts by mass or more and 300 parts by mass or less, preferably 20 parts by mass or more and 250 parts by mass or less, more preferably more than 20 parts by mass and less than 250 parts by mass, further preferably more than 30 parts by mass and less than 200 parts by mass, further preferably more than 40 parts by mass and less than 150 parts by mass, further preferably 50 parts by mass or more and 100 parts by mass or less, based on 100 parts by mass of the acrylic resin.

[0211] [3] The method according to [1] or [2] above, wherein the step (1) includes a sub-step of mixing the acrylic resin, sulfur, and the iron compound containing divalent or trivalent iron ions in a powder state.

[0212] [4] The method according to any one of [1] to [3] above, wherein the calcination temperature in the step (2) is higher than 250°C and lower than 500°C, preferably higher than 300°C and lower than 480°C, more preferably higher than 350°C and lower than 450°C, further preferably higher than 380°C and lower than 450°C.

[0213] [5] The method according to any one of [1] to [4] above, wherein the calcination temperature in the step (2) is higher than the thermal decomposition temperature of the iron compound containing divalent or trivalent iron ions.

[0214] [6] The method according to any one of [1] to [5] above, wherein the ratio of the amount of sulfur to the total amount of the acrylic resin and the iron compound in the calcination raw material is 1.5 or more and 5.0 or less, preferably 2.0 or more and 4.0 or less, more preferably 2.5 or more and 3.5 or less, further preferably more than 2.5 and 3.5 or less, further preferably more than 2.6 and less than 3.4, further preferably more than 2.7 and less than 3.3, further preferably more than 2.8 and less than 3.2, further preferably more than 2.9 and less than 3.1, in terms of mass ratio.

[0215] [7] The method according to any one of [1] to [6] above, wherein the acrylic resin is a polymer obtained by polymerizing at least one acrylic compound selected from the group consisting of compounds represented by the following formula (1); or a polymer obtained by polymerizing at least one acrylic compound selected from the group consisting of compounds represented by the following formula (1) and at least one diacrylate compound selected from the group consisting of compounds represented by the following formula (2).

[0216] CH2=C(R 11 )COOR 12 (1)

[0217] (in the formula, R 11 is a hydrogen atom or a methyl group, and R 12 is an alkyl group)

[0218] CH2=C (R 21 ) COO-Y-OCO (R 22 ) C=CH2 (2)

[0219] (Where R 21 and R 22 are the same or different, each being a hydrogen atom or a methyl group, Y being a straight-chain alkylene group having at least one substituent selected from a hydroxyl group and an alkyl group, and the carbon skeleton constituting the alkylene group may have an ether bond containing an oxygen atom, and when there are two or more ether bonds, there are always two or more carbon atoms sandwiched between adjacent oxygen atoms.

[0220] [8] The method of [7] above, wherein the R 12 is an alkyl group having 1 or more and 6 or less carbon atoms, preferably an alkyl group having 1 or more and 4 or less carbon atoms, Y is a straight-chain alkylene group having 2 or more and 6 or less carbon atoms, preferably 2 or 3 carbon atoms, in the alkylene group, the number of substituents is 1 or more and 4 or less, preferably 1 or 2, the number of carbon atoms of the alkyl group as a substituent is 1 or more and 4 or less carbon atoms, preferably 1, and the number of ether bonds possessed by the carbon skeleton constituting the alkylene group is 1 or more and 2 or less.

[0221] [9] The method according to any one of [1] to [8] above, wherein the iron compound is one or more compounds selected from ferrites and iron complexes.

[0222]

[10] The method according to any one of [1] to [8] above, wherein the iron compound is one or more compounds selected from the group consisting of organic acid salts of iron, inorganic acid salts of iron, neutral iron complexes, and iron complex salts.

[0223]

[11] The method of any one of [1] to [8] above, wherein the iron compound is one or more compounds selected from ferrous oxalate, ferrous acetate, ferric nitrate and ferrocene.

[0224]

[12] The method of any one of [1] to

[11] above, wherein the temperature of the heating history is greater than 50°C, more preferably greater than 60°C, further preferably greater than 70°C, further preferably greater than 80°C, further preferably greater than 90°C, greater than 100°C, preferably greater than 110°C, more preferably greater than 120°C, further preferably greater than 130°C, further preferably greater than 140°C, further preferably greater than 150°C.

[0225]

[13] A method for preparing an electrode, which, after preparing a sulfur-based active material by the method for preparing a sulfur-based active material according to any one of [1] to

[12] above, further comprises the following steps:

[0226] (3) using the chalcogen-based active material, an electrode is prepared by a conventional method.

[0227]

[14] A method for producing a lithium ion secondary battery, which comprises the following step after the electrode is produced by the method for producing an electrode described in

[13] above:

[0228] (4) using the electrode, a lithium ion secondary battery is produced by a conventional method.

[0229] List of Reference Signs

[0230] 1. Muffle furnace

[0231] 2. Heater

[0232] 3. Lid

[0233] 4. Inert gas

[0234] 5. Tray (upper layer)

[0235] 6. Tray (lower layer)

[0236] 7. Gas introduction pipe

[0237] 8. Gas output pipe

[0238] 9. Sodium hydroxide aqueous solution

[0239] 10. Trapping tank

Claims

1. A method for producing a sulfur-based active material, the method comprising the steps of: (1) mixing an acrylic resin, sulfur, and an iron compound containing a divalent iron ion to obtain a calcination raw material; and (2) calcining the calcination raw material; the acrylic resin is a polymer obtained by polymerizing at least one of the acrylate compounds represented by the following formula (1), the iron compound is one or more compounds selected from organic acid salts of iron, CH2=C (R 11 ) COOR 12 (1) wherein R 11 is a hydrogen atom or a methyl group, and R 12 is an alkyl group having 1 or more, 6 or less carbon atoms; the content of the iron compound containing a divalent iron ion in the calcination raw material is 10 parts by mass or more and 300 parts by mass or less, based on 100 parts by mass of the acrylic resin, the ratio of the amount of sulfur to the total amount of the acrylic resin and the iron compound in the calcination raw material is 2.5 or more and less than 5.0, in terms of parts by mass, the temperature increase rate in the step (2) is 50°C / h or more and 500°C / h or less, the calcination temperature in the step (2) is higher than 250°C and lower than 500°C the heating history of the calcination raw material is one, the heating history refers to a heating treatment at a terminal temperature of the calcination raw material higher than 50°C. the content of the iron compound containing a divalent iron ion in the calcination raw material is 20 parts by mass or more and 100 parts by mass or less, based on 100 parts by mass of the acrylic resin.

2. The method of claim 1, wherein, the step (1) includes a sub-step of mixing the acrylic resin, sulfur, and the iron compound containing a divalent iron ion in a powder state.

3. The method of claim 1 or 2, wherein, the calcination temperature in the step (2) is higher than 300°C and lower than 480°C.

4. The method of claim 1 or 2, wherein, the calcination temperature in the step (2) is higher than the thermal decomposition temperature of the iron compound containing a divalent iron ion.

5. The method of claim 1 or 2, wherein, the ratio of the amount of sulfur to the total amount of the acrylic resin and the iron compound in the calcination raw material is 2.5 or more and less than 3.1, in terms of parts by mass.

6. The method of claim 1 or 2, wherein, 8. The method according to claim 1 or 2, the acrylic resin is any one selected from the group consisting of a homopolymer of methyl (meth)acrylate, a homopolymer of butyl (meth)acrylate.

7. The method of claim 1, wherein, R 12 is an alkyl group having 1 or more, 4 or less carbon atoms. the iron compound is one or more compounds selected from ferrous oxalate, ferrous acetate, and hydrates of these.

9. The method of claim 1 or 2, wherein, 10. A method for producing an electrode, the method further comprising the step of: (3) producing an electrode using the sulfur-based active material, after the method for producing a sulfur-based active material according to any one of claims 1 to 9 produces the sulfur-based active material.

11. A method for producing a lithium-ion secondary battery, the method further comprising the step of: (4) producing a lithium-ion secondary battery using the electrode, after the method for producing an electrode according to claim 10 produces the electrode. ​

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