Sulfur-based active material, electrode and lithium-ion secondary battery

Abstract

To provide a new sulfur-based active material which can improve cycle characteristics, an electrode which includes the sulfur-based active material, specifically, a positive electrode or a negative electrode, and a lithium ion secondary battery which includes the electrode.SOLUTION: A sulfur-based active material is obtained by firing a mixture containing an acrylic monomer and a sulfur, wherein the boiling point of the acrylic monomer is 230°C or higher.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a sulfur-based active material, an electrode, and a lithium-ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries have a large charge / discharge capacity and are primarily used as batteries for portable electronic devices. Their use in electric vehicles is also increasing, and improvements in their performance are expected.

[0003] Patent Document 1 describes a positive electrode active material for lithium ion secondary batteries obtained by heating raw material powders containing sulfur powder and polyacrylonitrile powder in a non-oxidizing atmosphere. Patent Document 2 aims to provide a low-cost positive electrode active material by using industrial rubber.

[0004] On the other hand, it has been proposed to increase the battery capacity of lithium-ion secondary batteries by using materials that can absorb and release more lithium ions, such as silicon (Si) and tin (Sn), as the negative electrode active material. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2010 / 044437 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-92449 Summary of the Invention [Problem to be solved by the invention]

[0006] However, polyacrylonitrile is inherently expensive, and its quality (particularly particle size) affects battery performance, such as charge / discharge capacity and cycle characteristics. Therefore, obtaining polyacrylonitrile of consistent quality increases costs. While industrial rubber is inexpensive, improving cycle characteristics presents challenges. The materials proposed as negative electrode active materials suffer from poor cycle characteristics after repeated charge / discharge due to large volume changes associated with the absorption and release of lithium ions. Carbon materials such as graphite and hard carbon are also used, but these materials have already reached their theoretical capacity, making significant capacity improvements unlikely.

[0007] From the viewpoint of material synthesis, reactions using only solid raw materials such as polyacrylonitrile, rubber, and sulfur tend to proceed more slowly than in liquid or gaseous systems because the diffusion of materials is slower than in liquid or gaseous systems. To efficiently promote reactions, it is preferable to employ methods such as liquefying or vaporizing the solid raw materials or to use liquid or gaseous raw materials. However, liquefying or vaporizing solid raw materials requires extremely high temperatures, which is disadvantageous from the viewpoint of manufacturing costs and processes. On the other hand, there is also the problem that raw materials that vaporize quickly are discharged from the system before participating in the reaction. [Means for solving the problem]

[0008] That is, the present invention provides: A sulfur-based active material obtained by baking a mixture containing an acrylic monomer and sulfur, wherein the boiling point of the acrylic monomer is 230°C or higher. Regarding. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a novel sulfur-based active material having excellent cycle characteristics, an electrode comprising the sulfur-based active material, i.e., a positive electrode or a negative electrode, and a lithium-ion secondary battery comprising the electrode.

[0010] In this specification, the term "cycle characteristics" refers to the ability of a secondary battery to maintain its charge / discharge capacity despite repeated charge / discharge. Therefore, a secondary battery that experiences a large decrease in charge / discharge capacity and a low capacity retention rate with repeated charge / discharge has poor cycle characteristics, whereas a secondary battery that experiences a small decrease in charge / discharge capacity and a high capacity retention rate has excellent cycle characteristics. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view schematically showing a reaction apparatus used in the production of a sulfur-based active material in an example of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] The configuration of the present disclosure will be described in detail below. The upper and lower limit values ​​associated with terms such as "greater than or equal to," "less than or equal to," "greater than," and "less than" used to describe a numerical range can be arbitrarily combined, and the numerical values ​​in the examples can also be used as the upper and / or lower limits. Furthermore, a numerical range that includes a lower or upper limit is understood to also disclose a numerical range that does not include that lower or upper limit, unless it is contrary to the spirit of the present disclosure. Conversely, a numerical range that does not include a lower or upper limit is understood to also disclose a numerical range that includes that lower or upper limit, unless it is contrary to the spirit of the present disclosure.

[0013] One embodiment of the present disclosure is a sulfur-based active material obtained by baking a mixture containing an acrylic monomer and sulfur, wherein the acrylic monomer has a boiling point of 230° C. or higher.

[0014] Another embodiment of the present disclosure is an electrode comprising the sulfur-based active material.

[0015] Another embodiment of the present disclosure is a lithium ion secondary battery comprising the electrode.

[0016] Another embodiment of the present disclosure is a method for producing a sulfur-based active material, (1) preparing a raw material by mixing an acrylic monomer and sulfur; (2) A step of firing the raw materials comprising In the method for producing a sulfur-based active material, the boiling point of the acrylic monomer is 230°C or higher.

[0017] Another embodiment of the present disclosure is a method for manufacturing an electrode, comprising: After producing a sulfur-based active material by the method for producing a sulfur-based active material, (3) A step of preparing an electrode using the sulfur-based active material by a conventional method. The method for producing an electrode comprises:

[0018] Another embodiment of the present disclosure is a method for manufacturing a lithium ion secondary battery, comprising: After the electrode is manufactured by the electrode manufacturing method, (4) A step of producing a lithium ion secondary battery using the electrode by a conventional method. The method for producing a lithium ion secondary battery includes the steps of:

[0019] The firing temperature is preferably higher than 250°C and lower than 500°C.

[0020] The firing is preferably carried out in a non-oxidizing atmosphere.

[0021] The acrylic monomer is preferably at least one selected from the group consisting of acrylate compounds represented by the following formula (1) and diacrylate compounds represented by the following formula (2). CH2=C(R 11 )COOR 12 (1) (where R 11 is a hydrogen atom or a methyl group, and R 12 is an alkyl group. CH2=C(R 21 )COO-Y-OCO(R22 )C=CH2(2) (where R 21 and R 22 are the same or different and are a hydrogen atom or a methyl group, and Y is a straight-chain hydrocarbylene group, which may have at least one substituent selected from the group consisting of a hydroxyl group and an alkyl group, and the carbon skeleton constituting the straight-chain hydrocarbylene group may have an ether bond via an oxygen atom. However, when there are two or more ether bonds, there are always two or more carbon atoms between adjacent oxygen atoms.

[0022] R 12 is an alkyl group having 12 to 30 carbon atoms, and Y is a straight-chain hydrocarbylene group having 2 to 6 carbon atoms, and it is preferred that the straight-chain hydrocarbylene group has 1 to 4 substituents, the alkyl group as the substituent has 1 to 4 carbon atoms, and the carbon skeleton constituting the straight-chain hydrocarbylene group has 1 or 2 ether bonds.

[0023] The acrylic monomer is preferably a liquid.

[0024] The sulfur content in the sulfur-based active material is preferably 55.0% by mass or more.

[0025] The amount of sulfur relative to the acrylic monomer is preferably 50 parts by mass or more and 1000 parts by mass or less of sulfur relative to 100 parts by mass of the acrylic monomer.

[0026] <Acrylic monomer> In the present disclosure, the acrylic monomer is not particularly limited as long as it is a monomer having an acrylic group and has a boiling point of 230° C. or higher. Examples of such acrylic monomers include an acrylate compound represented by the following formula (1) and a diacrylate compound represented by the following formula (2).

[0027] The acrylic monomer is preferably at least one selected from the group consisting of acrylate compounds represented by the following formula (1) and diacrylate compounds represented by the following formula (2). CH2=C(R 11 )COOR 12 (1) (where R 11 is a hydrogen atom or a methyl group, and R 12 is an alkyl group. CH2=C(R 21 )COO-Y-OCO(R 22 )C=CH2(2) (where R 21 and R 22 are the same or different and are a hydrogen atom or a methyl group, and Y is a straight-chain hydrocarbylene group, which may have at least one substituent selected from the group consisting of a hydroxyl group and an alkyl group, and the carbon skeleton constituting the straight-chain hydrocarbylene group may have an ether bond via an oxygen atom. However, when there are two or more ether bonds, there are always two or more carbon atoms between adjacent oxygen atoms.

[0028] In formula (1), R 11 is preferably a methyl group, and R 12 R is preferably an alkyl group having 12 or more carbon atoms, and the alkyl group includes both a straight-chain alkyl group and a branched-chain alkyl group. 12Examples of the alkyl group include dodecyl (C12), stearyl (C18), and triacontyl (C30). Examples of the compound represented by formula (1) include dodecyl (meth)acrylate, stearyl (meth)acrylate, and triacontyl (meth)acrylate, and more preferably, dodecyl methacrylate and stearyl methacrylate. Here, the "(meth)acrylate" of methyl (meth)acrylate and butyl (meth)acrylate refers to either "acrylate" or "methacrylate" (the same applies hereinafter).

[0029] In equation (2), R 21 and R 22 are preferably methyl groups. The linear hydrocarbylene group of Y preferably has 2 to 6 carbon atoms, more preferably 2 or 3. The number of substituents on Y is preferably 1 to 4, more preferably 1 or 2. The substituents on Y are preferably one or more substituents selected from the group consisting of hydroxyl groups and alkyl groups having 1 to 4 carbon atoms, and the alkyl groups having 1 to 4 carbon atoms are preferably methyl groups. When the carbon skeleton of Y has an ether bond via an oxygen atom, for example, the portion corresponding to -YO- is preferably one represented by the following formula (3) (however, in formula (3), the substituents on Y are not taken into consideration). -(CH2) l -(CH2CH2O) m -(CH2CH2CH2O) n - (3) (Here, l is a number between 0 and 6, m is a number between 0 and 3, and n is a number between 0 and 2. However, l, m, and n cannot all be 0 at the same time.)

[0030] In formula (3), it is preferred that l is 1, 2, 3, 4, 5 or 6, and m and n are 0; alternatively, m is 1, 2 or 3, and l and n are 0; or alternatively, n is 1 or 2, and l and m are 0.

[0031] Examples of the compound represented by formula (2) include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyne glycol di(meth)acrylate, glycerin di(meth)acrylate, etc. Among these, ethylene glycol dimethacrylate is preferred.

[0032] One or more acrylic monomers can be used.

[0033] (Boiling point of acrylic monomer) The boiling point of the acrylic monomer is 230°C or higher. If the boiling point of the acrylic monomer is lower than 230°C, it will vaporize quickly as the temperature rises during baking, resulting in it being discharged outside the system and not reacting sufficiently with sulfur. The boiling point of the acrylic monomer is preferably higher than 230°C, more preferably higher than 235°C, even more preferably higher than 250°C, even more preferably higher than 270°C, even more preferably higher than 300°C, even more preferably higher than 320°C, and even more preferably higher than 350°C. On the other hand, there is no particular upper limit to the boiling point of the acrylic monomer. As will be described later, it is preferable for the acrylic monomer to be liquid at room temperature from the viewpoint of diffusing and mixing with sulfur, and from this viewpoint, it is preferable that the boiling point be below a predetermined value. The boiling point of the acrylic monomer is, for example, preferably lower than 400°C, more preferably lower than 395°C, even more preferably lower than 390°C, even more preferably lower than 385°C, and even more preferably lower than 380°C.

[0034] (Properties of acrylic monomers) The acrylic monomer is preferably a liquid at room temperature (for example, 20° C.), since it tends to be easily diffused and mixed into the sulfur.

[0035] (Acquisition or production of acrylic monomers) The acrylic monomers are commercially available or can be prepared by conventional methods within the knowledge of those skilled in the art, such as those manufactured by Tokyo Chemical Industry Co., Ltd. and Sekisui Plastics Co., Ltd.

[0036] <Sulfur> Sulfur can be used in various forms such as powdered sulfur, insoluble sulfur, precipitated sulfur, and colloidal sulfur, with precipitated sulfur and colloidal sulfur being preferred. The amount of sulfur added is preferably 50 parts by mass or more, more preferably more than 100 parts by mass, even more preferably more than 150 parts by mass, even more preferably more than 200 parts by mass, and even more preferably more than 250 parts by mass, per 100 parts by mass of the acrylic monomer. An amount of sulfur of 50 parts by mass or more tends to improve charge / discharge capacity and cycle characteristics. On the other hand, there is no particular upper limit to the amount of sulfur added, but it is usually 1000 parts by mass or less, preferably less than 750 parts by mass, and more preferably less than 500 parts by mass. An amount of sulfur of 1000 parts by mass or less tends to be advantageous in terms of cost.

[0037] <Conductive carbon materials> When modifying an acrylic monomer with sulfur, a conductive carbon material may be added to the acrylic monomer in advance. This is because the conductivity of the sulfur-based active material can be improved. As such a conductive carbon material, a carbon material having a graphite structure is preferred. Examples of carbon materials that can be used include those having a condensed aromatic ring structure, such as carbon black, acetylene black, ketjen black, graphite, carbon nanotubes (CNT), carbon fiber (CF), graphene, and fullerene. One or more conductive carbon materials can be used.

[0038] Among these, acetylene black, carbon black, and ketjen black are preferred because they are inexpensive and have excellent dispersibility. Furthermore, small amounts of CNTs or graphene may be used in combination with acetylene black, carbon black, or ketjen black. Such a combination system can further improve the cycle characteristics of lithium-ion secondary batteries without significantly increasing costs. The amount of CNTs or graphene used in combination is preferably 8% by mass or more and 12% by mass or less of the total amount of conductive carbon material.

[0039] The amount of the conductive carbon material is preferably more than 5 parts by mass, more preferably more than 7 parts by mass, and even more preferably more than 10 parts by mass, relative to 100 parts by mass of the acrylic monomer. When the amount is more than 5 parts by mass, the objective of further improving the charge / discharge capacity and cycle characteristics tends to be easily achieved. On the other hand, the amount is preferably less than 50 parts by mass, more preferably less than 45 parts by mass, and even more preferably less than 40 parts by mass. When the amount is less than 50 parts by mass, the proportion of sulfur-containing structures in the sulfur-based active material does not decrease relatively, and the objective of further improving the charge / discharge capacity and cycle characteristics tends to be easily achieved.

[0040] <Other ingredients> When the acrylic monomer is modified with sulfur, other materials commonly used in this field may be added to the acrylic monomer in advance, if desired.

[0041] <Production of sulfur-based active materials> In the present disclosure, the sulfur-based active material can be produced by firing a mixture containing a predetermined acrylic monomer and sulfur.

[0042] (Preparation of raw materials) The raw materials for firing include an acrylic monomer and sulfur. The acrylic monomer and sulfur are preferably mixed thoroughly in advance. If additives such as a conductive carbon material are added to the acrylic monomer in advance, these additives are also mixed together. The mixing can be carried out by a conventional method, for example, using a high-speed blender. The raw materials for firing can also be formed into pellets.

[0043] (Non-oxidizing atmosphere) The firing is preferably carried out in a non-oxidizing atmosphere. A non-oxidizing atmosphere is an atmosphere substantially free of oxygen, and is employed to suppress oxidative degradation and excessive thermal decomposition of the constituent components. Specifically, this refers to an inert gas atmosphere such as nitrogen or argon, or a sulfur gas atmosphere. Therefore, the modification is carried out, for example, in a quartz tube under an inert gas atmosphere.

[0044] (Baking method) The calcination can be carried out by a conventional method, for example, by heating the raw materials (including the acrylic monomer, sulfur, and, if desired, an additive) at a predetermined temperature increase rate until the temperature reaches a predetermined temperature, maintaining the temperature for a predetermined time, and then allowing the mixture to cool naturally.

[0045] [Temperature increase rate] The temperature rise rate is preferably, for example, in the range of 50°C / h to 500°C / h. The temperature rise rate is preferably greater than 100°C / h, more preferably greater than 150°C / h. On the other hand, the temperature rise rate is more preferably less than 400°C / h, and even more preferably less than 350°C / h. When the temperature rise rate is within such a range, it tends to be easier to achieve the objective of improving charge / discharge capacity and cycle characteristics.

[0046] [Baking temperature and time] The calcination temperature refers to the temperature after the raw materials have been heated and maintained for a certain period of time to calcinate the raw materials. The temperature is preferably in the range of greater than 250°C and less than 500°C. A temperature greater than 250°C tends to prevent insufficient sulfurization reaction and a decrease in the charge / discharge capacity of the target product. On the other hand, a temperature less than 500°C tends to prevent decomposition of the raw materials, a decrease in yield, and a decrease in charge / discharge capacity. The temperature is more preferably greater than 270°C, and even more preferably greater than 300°C. On the other hand, the temperature is more preferably less than 450°C, even more preferably less than 400°C, and even more preferably 380°C or less. The time for maintaining the temperature can be appropriately set depending on the type of raw materials, the calcination temperature, etc., but is preferably, for example, greater than 1 hour and less than 6 hours. A temperature of 1 hour or more tends to allow sufficient calcination, while a temperature of 6 hours or less tends to prevent excessive thermal decomposition of the constituent components. The time is more preferably greater than 1.3 hours, and even more preferably greater than 1.5 hours. On the other hand, the time is more preferably less than 4 hours, and even more preferably less than 3 hours.

[0047] [Device] The calcination can be carried out, for example, in a muffle furnace (FIG. 1), or can be carried out using a continuous apparatus such as a twin-screw extruder. The use of a continuous apparatus has the advantage that the sulfur-based active material can be produced continuously through a series of operations, such as kneading, pulverizing, and mixing the raw materials while calcining them in the apparatus.

[0048] A muffle furnace (Figure 1) is a furnace partitioned by a hot plate or the like to prevent the heat source (heater) from being exposed inside the furnace in order to prevent sample contamination. In Figure 1, muffle furnace 1 has heater 2 at the bottom of the furnace, which is partitioned by a hot plate. A lid 3 is installed on the front of the furnace (on the left end in the figure), and the furnace is designed to maintain an atmosphere of inert gas 4 inside. A thermocouple (not shown) is attached to the lid, allowing the temperature inside the furnace to be measured during firing. Inside the furnace, two tiers of stainless steel rectangular trays 5 and 6 are installed, one on the upper tier and one on the lower tier, for firing the raw materials.

[0049] Gas (e.g., an inert gas such as argon (Ar) gas) can be continuously supplied to and discharged from the inside of the furnace through a gas inlet pipe 7 and a gas outlet pipe 8. The gas outlet pipe 8 is connected to a trap tank 10 containing an aqueous sodium hydroxide solution 9, and exhaust gas from the muffle furnace 1 attempting to exit through the gas outlet pipe 8 to the outside first passes through the aqueous sodium hydroxide solution 9 in the trap tank 10 before being released to the outside. Therefore, even if the exhaust gas contains hydrogen sulfide gas generated by the reaction, it is neutralized by the aqueous sodium hydroxide solution and removed from the exhaust gas.

[0050] (Residue removal process) The treated product obtained after calcination contains unreacted sulfur, which is the sulfur that sublimed during calcination and then cooled and precipitated. These residues can cause deterioration in cycle characteristics, so it is desirable to remove them as much as possible. The removal of residues can be carried out by conventional methods, such as reduced-pressure heating drying, hot air drying, and solvent washing.

[0051] (Crushing, classification) The obtained sulfur-based active material can be pulverized to a predetermined particle size and classified to obtain particles of a size suitable for electrode production. From the viewpoint of the effects of the present disclosure, the particle size distribution of the particles is preferably a median diameter of approximately 1 μm or more and 40 μm or less. The median diameter is preferably greater than 1 μm, more preferably greater than 2 μm, even more preferably greater than 3 μm, even more preferably greater than 4 μm, and even more preferably greater than 5 μm. The median diameter is preferably less than 40 μm, more preferably less than 30 μm, even more preferably less than 20 μm, even more preferably less than 15 μm, and even more preferably less than 10 μm. The median diameter can be determined by measuring the volume-based cumulative 50% diameter (median diameter D50) using a laser diffraction / scattering particle size distribution analyzer (LA-960 manufactured by Horiba, Ltd.) with water as the dispersion medium.

[0052] In the firing method using the twin-screw extruder described above, the shearing during kneading can produce the sulfur-based active material and simultaneously pulverize the produced sulfur-based active material.

[0053] <Sulfur-based active material> The sulfur-based active material thus obtained is primarily composed of carbon and sulfur, and a higher sulfur content tends to improve charge / discharge capacity and cycle characteristics. Therefore, a higher sulfur content is preferable. Generally, the preferred range of sulfur content in the sulfur-based active material is 55.0% by mass or more, more preferably greater than 58.0% by mass, even more preferably greater than 60.0% by mass, even more preferably greater than 61.0% by mass, and even more preferably greater than 62.0% by mass. However, when a conductive carbon material is added, even if the sulfur content is slightly lower, due to the influence of the carbon constituting the conductive carbon material, improvements in charge / discharge capacity and cycle characteristics may be expected. In such cases, the sulfur content may be approximately 5.0% by mass lower than the above-mentioned sulfur content.

[0054] <Lithium-ion secondary battery> The sulfur-based active material of the present disclosure can be used as an electrode active material for a lithium-ion secondary battery, i.e., as a positive electrode active material or a negative electrode active material. That is, a lithium-ion secondary battery electrode can be fabricated in the same manner as a general lithium-ion secondary battery electrode except for using the sulfur-based active material. Furthermore, a lithium-ion secondary battery can be fabricated in the same manner as a general lithium-ion secondary battery except for using the lithium-ion secondary battery electrode. The lithium-ion secondary battery fabricated in this manner has a large charge / discharge capacity and excellent cycle characteristics.

[0055] 1. When sulfur-based active materials are used as positive electrode active materials The lithium ion secondary battery of the present disclosure can be produced in accordance with a conventional method by using a positive electrode containing the above-described sulfur-based active material (positive electrode active material), a negative electrode, an electrolyte, and, if desired, components such as a separator.

[0056] (positive electrode) The positive electrode for a lithium-ion secondary battery can be fabricated in the same manner as a general positive electrode for a lithium-ion secondary battery, except that the sulfur-based active material is used as the positive electrode active material. For example, the positive electrode can be fabricated by mixing particulate sulfur-based active material with a conductive additive, a binder, and a solvent to prepare a paste-like positive electrode material, applying the positive electrode material to a current collector, and then drying the paste. Alternatively, the positive electrode can be fabricated by kneading the sulfur-based active material with the conductive additive, the binder, and a small amount of solvent in a mortar or the like, forming a film, and then pressing the film onto a current collector using a press or the like.

[0057] [Conductive additive] Examples of the conductive additive include vapor grown carbon fiber (VGCF), carbon powder, carbon black (CB), acetylene black (AB), ketjen black (KB), graphite, or fine powder of a metal that is stable at a positive electrode potential, such as aluminum or titanium. These conductive additives can be used alone or in combination.

[0058] [Binder] Examples of binders include polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamide-imide (PAI), carboxymethyl cellulose (CMC), polyvinyl chloride (PVC), acrylic monomers, methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), polypropylene (PP), etc. These binders can be used alone or in combination of two or more.

[0059] [solvent] Examples of the solvent include N-methyl-2-pyrrolidone, N,N-dimethylformaldehyde, alcohol, hexane, water, etc. These solvents can be used alone or in combination of two or more.

[0060] [Amount] The amounts of the materials constituting the positive electrode are not particularly limited, but for example, it is preferable to mix 2 parts by mass or more and 100 parts by mass or less of a conductive additive, 2 parts by mass or more and 50 parts by mass or less of a binder, and an appropriate amount of a solvent with respect to 100 parts by mass of a sulfur-based active material.

[0061] [Current collector] The current collector may be one generally used for the positive electrode of a lithium-ion secondary battery. Examples of current collectors include aluminum foil, aluminum mesh, punched aluminum sheet, expanded aluminum sheet, stainless steel foil, stainless steel mesh, punched stainless steel sheet, expanded stainless steel sheet, foamed nickel, nonwoven nickel fabric, copper foil, copper mesh, punched copper sheet, expanded copper sheet, titanium foil, titanium mesh, nonwoven carbon fabric, and woven carbon fabric. Among these, current collectors made of highly graphitized carbon nonwoven fabric and woven carbon fabric are suitable as current collectors when the sulfur-based active material of the present disclosure is used as the positive electrode active material because they do not contain hydrogen and have low reactivity with sulfur. Raw materials for highly graphitized carbon fibers include various pitches (i.e., by-products of petroleum, coal, coal tar, etc.) and polyacrylonitrile fiber (PAN), which are commonly used as carbon fiber materials. One type of current collector may be used, or two or more types may be used in combination.

[0062] (Negative electrode) Examples of anode materials that can be used include known metallic lithium, carbon-based materials such as graphite, silicon-based materials such as silicon thin films, and alloy-based materials such as copper-tin and cobalt-tin. Lithium-free materials, such as carbon-based, silicon-based, and alloy-based materials among the above-mentioned anode materials, are advantageous in that they are less likely to cause short circuits between the positive and negative electrodes due to dendrite formation. However, when these lithium-free anode materials are used in combination with the positive electrode of the present disclosure, neither the positive nor negative electrodes contain lithium. Therefore, a lithium pre-doping process is required to pre-insert lithium into either or both of the negative and positive electrodes. Known methods for pre-doping lithium can be used. For example, when doping lithium into the anode, lithium can be inserted into a half-cell using metallic lithium as the counter electrode by electrochemical doping, or by pre-doping, in which metallic lithium foil is attached to the electrode and then left in an electrolyte solution to allow lithium to diffuse into the electrode. The above-mentioned electrolytic doping method can also be used when pre-doping the positive electrode with lithium. As a lithium-free negative electrode material, a silicon-based material, which is a high-capacity negative electrode material, is particularly preferred, and among them, thin-film silicon, which has a thin electrode thickness and is advantageous in terms of capacity per volume, is more preferred.

[0063] (electrolyte) The electrolyte used in a lithium ion secondary battery can be an organic solvent in which an electrolyte alkali metal salt is dissolved. The organic solvent is preferably 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, and acetonitrile. The electrolyte can be LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiI, or LiClO4. The electrolyte concentration may be approximately 0.5 mol / L or more and 1.7 mol / L or less. The electrolyte is not limited to a liquid. For example, when the lithium ion secondary battery is a lithium polymer secondary battery, the electrolyte is solid (e.g., polymer gel).

[0064] (separator) In addition to the negative electrode, positive electrode, and electrolyte described above, a lithium-ion secondary battery may also include components such as a separator. The separator is interposed between the positive electrode and negative electrode, allowing ions to move between the positive electrode and negative electrode and preventing internal short-circuiting between the positive electrode and negative electrode. If the lithium-ion secondary battery is a sealed type, the separator is also required to have the function of retaining the electrolyte. As the separator, a thin, microporous or nonwoven membrane made of a material such as polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide, cellulose, or glass is preferably used.

[0065] (shape) The shape of the lithium ion secondary battery is not particularly limited, and it can be in various shapes such as a cylindrical type, a laminated type, a coin type, a button type, and the like.

[0066] 2. When sulfur-based active materials are used as negative electrode active materials The lithium ion secondary battery of the present disclosure can be produced in accordance with a conventional method by using a negative electrode containing the above-described sulfur-based active material (negative electrode active material), a positive electrode, an electrolyte, and, if desired, components such as a separator.

[0067] (Negative electrode) The negative electrode for a lithium-ion secondary battery can be fabricated in the same manner as a general negative electrode for a lithium-ion secondary battery, except that the sulfur-based active material is used as the negative electrode active material. For example, the negative electrode can be fabricated by mixing particulate sulfur-based active material with a conductive additive, a binder, and a solvent to prepare a paste-like negative electrode material, applying the paste to a current collector, and then drying the paste. Alternatively, the negative electrode can be fabricated by kneading the sulfur-based active material with the conductive additive, the binder, and a small amount of solvent in a mortar or the like, forming a film, and then pressing the film onto a current collector using a press or the like.

[0068] The conductive additive, binder, and solvent can be the same as those used in the above case where a sulfur-based active material is used as the positive electrode active material, and the same current collector can also be used.

[0069] (positive electrode) The positive electrode material is not particularly limited as long as it is, for example, a lithium-containing transition metal oxide or solid solution oxide, or a substance that can electrochemically absorb and release lithium ions. Examples of lithium-containing transition metal oxides include Li-Co composite oxides such as LiCoO2, LiNi x Co y Mn z Examples of the solid solution oxide include Li·Ni·Co·Mn-based composite oxides such as LiO2, Li·Ni-based composite oxides such as LiNiO2, and Li·Mn-based composite oxides such as LiMn2O4. 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 z O2 (0.300≦x≦0.850, 0.100≦y≦0.300, 0.100≦z≦0.300), LiMn 1.5 Ni 0.5O4, etc. These compounds may be used alone or in combination.

[0070] The electrolyte, separator, and shape of the lithium ion secondary battery can also be the same as those used in the above case where a sulfur-based active material is used as the positive electrode active material. [Example]

[0071] The present disclosure will be described based on examples, but the present disclosure is not limited to only the examples.

[0072] The various chemicals used in the examples and comparative examples are listed below. The various chemicals were purified according to conventional methods as necessary.

[0073] <Materials used in the test> Acrylic monomer 1: dodecyl methacrylate (Tokyo Chemical Industry Co., Ltd., boiling point: 320°C) Acrylic monomer 2: stearyl methacrylate (Tokyo Chemical Industry Co., Ltd., boiling point: 380°C) Acrylic monomer 3: ethylene glycol dimethacrylate (Tokyo Chemical Industry Co., Ltd., boiling point: 235°C) Acrylic monomer 4: methyl methacrylate (Tokyo Chemical Industry Co., Ltd., boiling point: 101°C) Acrylic monomer 5: hexyl methacrylate (Tokyo Chemical Industry Co., Ltd., boiling point: 210°C) Sulfur: Precipitated sulfur manufactured by Tsurumi Chemical Industry Co., Ltd.

[0074] Example 1 <Preparation of sulfur-based active material> (Preparation of raw materials) According to the formulation in Table 1, the materials were mixed in a blender to obtain a raw material for firing.

[0075] (Reaction Apparatus) The raw materials were fired in a muffle furnace (Fig. 1). The muffle furnace in Fig. 1 was as explained above.

[0076] (Firing process) First, the raw materials were placed in a SUS container, and the atmosphere in the muffle furnace was replaced with Ar gas three times using a vacuum pump. Then, Ar gas was continuously supplied from the gas inlet tube at a flow rate of 100 mL / min, and 30 minutes after the start of supply, heating of the muffle furnace was started. The temperature was increased at a rate of 300 ° C. / h, and when the temperature of the raw materials reached 400 ° C., the material was fired for 2 hours while maintaining the temperature at 400 ° C. Next, the temperature of the fired material was naturally cooled to 25 ° C. under an Ar gas atmosphere while adjusting the flow rate of Ar gas, and then the fired material was removed from the muffle furnace.

[0077] (Removal of unreacted sulfur) To remove unreacted sulfur (free elemental sulfur) remaining in the product after the firing process, the following process was performed. The product 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 evacuating to obtain a sulfur-based active material from which unreacted sulfur had been removed (or which contained only a trace amount of unreacted sulfur). The heating rate was 10°C / min.

[0078] (Crushing process) The calcined product from which unreacted sulfur had been removed was pulverized using a cutter mill (free speed mill, FS-20, manufactured by Labnect Co., Ltd.).

[0079] (Classification work) In order to remove coarse particles from the fired product, the fired product was classified using a stainless steel sieve with a mesh of 32 μm to obtain a sulfur-based active material.

[0080] <Fabrication of lithium-ion secondary batteries> A lithium ion secondary battery was fabricated as follows.

[0081] (positive electrode) The active material was the sulfur-based active material obtained above, the conductive additive was acetylene black, and the binder was an acrylic monomer. These were weighed to a ratio of 85:10:5 (mass%) of active material, conductive additive, and binder, and placed in a container. MilliQ water was used as a dispersant and the mixture was stirred and mixed using a centrifugal mixer (ARE-250, manufactured by Thinky Corporation) to produce a uniform slurry. The resulting slurry was applied to a 20 μm aluminum foil using an applicator with a 60 μm slit width. The compressed electrode was heated in a dryer at 120 °C for 3 hours, dried, and punched out to a diameter of 11 mm to obtain an electrode (positive electrode). The electrode was then weighed, and the amount of active material in the electrode was calculated from the above ratio.

[0082] (Negative electrode) The negative electrode used was a metallic lithium foil (disk-shaped, 14 mm in diameter and 500 μm in thickness, manufactured by Honjo Metals Co., Ltd.).

[0083] (electrolyte) The electrolyte used was a non-aqueous electrolyte prepared by dissolving LiPF6 in a mixed solvent of ethylene carbonate and diethyl carbonate. The ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1:1. The concentration of LiPF6 in the electrolyte was 1.0 mol / L.

[0084] (lithium-ion secondary battery) A coin battery was fabricated using the above positive electrode, negative electrode, and electrolyte. Specifically, in a dry room, a separator (Celgard 2400, a 25 μm thick polypropylene microporous membrane manufactured by Celgard Corporation), a glass nonwoven fabric filter (440 μm thick, GA100 manufactured by Advantec Corporation), and an electrolyte were filled between the positive electrode and negative electrode to form an electrode assembly battery. This electrode assembly battery was housed in a battery case (CR2032 type coin battery material, manufactured by Hosen Co., Ltd.) made of a stainless steel container. The above electrolyte was poured into the battery case. The battery case was sealed using a crimping machine to obtain the lithium ion secondary battery of Example 1.

[0085] Examples 2 to 3 and Comparative Examples 1 to 4 Except for making appropriate changes according to the formulations and conditions in Table 1, the same treatment as in Example 1 was carried out to prepare the respective raw materials, sulfur-based active materials, and lithium-ion secondary batteries.

[0086] However, in Comparative Examples 1 and 2, the material obtained by firing was mostly unreacted sulfur, and the material was almost completely lost during the process of removing the unreacted sulfur. As a result, it was impossible to fabricate a battery, and subsequent evaluation was not possible. For the other Examples and Comparative Examples, batteries were fabricated and the following evaluations were performed.

[0087] <Measurement of discharge capacity and capacity retention rate> The coin-shaped lithium-ion secondary batteries prepared in each Example and Comparative Example were charged and discharged at a current value equivalent to 50 mA (0.1 C) per 1 g of positive electrode active material at a test temperature of 30° C. The discharge end voltage was 1.0 V, and the charge end voltage was 3.0 V. Charging and discharging were repeated, and the battery discharge capacity (mAh) was observed after 1, 2, and 10 cycles.

[0088] The second discharge capacity (mAh / g) was taken as the initial capacity. The larger the initial capacity, the greater the charge / discharge capacity of the lithium-ion secondary battery, and the more preferable it can be evaluated. 10 (mAh / g) and the second discharge capacity DC2 (mAh / g), the following formula (a): Capacity retention rate (%)=(DC 10 / DC2)×100 (a) As explained above, the higher the capacity retention rate, the better the cycle characteristics of the lithium ion secondary battery.

[0089] <Sulfur mass ratio (%)> The mass ratio (%) of sulfur in the sulfur-based active material was determined by calculating the proportion of sulfur in the total mass of the sulfur-based active material from the mass of sulfur measured using a Dionex ion chromatograph DX-320 and a Dionex column (IonPac AS12A).

[0090] [Table 1]

[0091] <Embodiment> The following describes a preferred embodiment.

[0092] [1] A sulfur-based active material obtained by baking a mixture containing an acrylic monomer and sulfur, wherein the boiling point of the acrylic monomer is 230°C or higher, preferably greater than 230°C, more preferably greater than 235°C, even more preferably greater than 250°C, even more preferably greater than 270°C, even more preferably greater than 300°C, even more preferably greater than 320°C, even more preferably greater than 350°C, or preferably greater than 230°C and less than 400°C, more preferably greater than 230°C and less than 395°C, even more preferably greater than 235°C and less than 390°C, even more preferably greater than 250°C and less than 385°C, even more preferably greater than 270°C and less than 380°C. [2] The sulfur-based active material according to [1] above, wherein the firing temperature is higher than 250°C and lower than 500°C, preferably higher than 270°C and lower than 450°C, more preferably higher than 300°C and lower than 400°C, and even more preferably higher than 300°C and lower than 380°C. [3] The sulfur-based active material according to [1] or [2] above, wherein the firing is carried out in a non-oxidizing atmosphere. [4] The sulfur-based active material according to any one of [1] to [3] above, wherein the acrylic monomer is at least one selected from the group consisting of an acrylate compound represented by the following formula (1) and a diacrylate compound represented by the following formula (2): CH2=C(R 11 )COOR 12 (1) (where R 11 is a hydrogen atom or a methyl group, and R 12 is an alkyl group. CH2=C(R 21 )COO-Y-OCO(R 22 )C=CH2(2) (where R 21 and R 22 are the same or different and are a hydrogen atom or a methyl group, and Y is a straight-chain hydrocarbylene group, which may have at least one substituent selected from the group consisting of a hydroxyl group and an alkyl group, and the carbon skeleton constituting the straight-chain hydrocarbylene group may have an ether bond via an oxygen atom. However, when there are two or more ether bonds, there are always two or more carbon atoms between adjacent oxygen atoms. [5]R 12 is an alkyl group having 12 to 30 carbon atoms, preferably 12 to 18 carbon atoms, Y is a linear hydrocarbylene group having 2 to 6 carbon atoms, preferably 2 or 3 carbon atoms, in which the linear hydrocarbylene group has 1 to 4, preferably 1 or 2, substituents, the alkyl group as the substituent has 1 to 4, preferably 1, carbon atom, and the carbon skeleton constituting the linear hydrocarbylene group has 1 or 2 ether bonds. [6] The sulfur-based active material according to any one of the above [1] to [5], wherein the acrylic monomer is liquid. [7] The sulfur-based active material according to any one of [1] to [6] above, wherein the sulfur content in the sulfur-based active material is 55.0% by mass or more, preferably more than 58.0% by mass, more preferably more than 60.0% by mass, even more preferably more than 61.0% by mass, and even more preferably more than 62.0% by mass. [8] An electrode comprising the sulfur-based active material according to any one of [1] to [7] above. [9] A lithium ion secondary battery comprising the electrode according to [8] above.

[10] A method for producing a sulfur-based active material, (1) preparing a raw material by mixing an acrylic monomer and sulfur; (2) A step of firing the raw materials comprising A method for producing a sulfur-based active material, wherein the boiling point of the acrylic monomer is 230°C or higher, preferably greater than 230°C, more preferably greater than 235°C, even more preferably greater than 250°C, even more preferably greater than 270°C, even more preferably greater than 300°C, even more preferably greater than 320°C, even more preferably greater than 350°C, or preferably 230°C or higher and lower than 400°C, more preferably greater than 230°C and lower than 395°C, even more preferably 235°C or higher and lower than 390°C, even more preferably greater than 250°C and lower than 385°C, even more preferably greater than 270°C and lower than 380°C.

[11] The method for producing a sulfur-based active material according to

[10] above, wherein the firing temperature is higher than 250°C and lower than 500°C, preferably higher than 270°C and lower than 450°C, more preferably higher than 300°C and lower than 400°C, and even more preferably higher than 300°C and lower than 380°C.

[12] The method for producing a sulfur-based active material according to

[10] or

[11] above, wherein the calcination is carried out in a non-oxidizing atmosphere.

[13] The method for producing a sulfur-based active material according to any one of

[10] to

[12] above, wherein the acrylic monomer is at least one selected from the group consisting of an acrylate compound represented by the following formula (1) and a diacrylate compound represented by the following formula (2): CH2=C(R 11 )COOR 12 (1) (where R 11 is a hydrogen atom or a methyl group, and R 12 is an alkyl group. CH2=C(R 21 )COO-Y-OCO(R 22 )C=CH2(2) (where R 21 and R 22are the same or different and are a hydrogen atom or a methyl group, and Y is a straight-chain hydrocarbylene group, which may have at least one substituent selected from the group consisting of a hydroxyl group and an alkyl group, and the carbon skeleton constituting the straight-chain hydrocarbylene group may have an ether bond via an oxygen atom. However, when there are two or more ether bonds, there are always two or more carbon atoms between adjacent oxygen atoms.

[14] R 12 is an alkyl group having 12 to 30 carbon atoms, preferably 12 to 18 carbon atoms, Y is a linear hydrocarbylene group having 2 to 6 carbon atoms, preferably 2 or 3 carbon atoms, in which the linear hydrocarbylene group has 1 to 4, preferably 1 or 2, substituents in the alkyl group as the substituent have 1 to 4, preferably 1, carbon atom, and the carbon skeleton constituting the linear hydrocarbylene group has 1 or 2 ether bonds.

[15] The method for producing a sulfur-based active material according to any one of the above

[10] to

[14] , wherein the acrylic monomer is liquid.

[16] The method for producing a sulfur-based active material according to any one of

[10] to

[15] above, wherein the amount of sulfur relative to the acrylic monomer is 50 parts by mass or more and 1,000 parts by mass or less, preferably more than 100 parts by mass but less than 750 parts by mass, more preferably more than 150 parts by mass but less than 500 parts by mass, even more preferably more than 200 parts by mass but less than 500 parts by mass, and even more preferably more than 250 parts by mass but less than 500 parts by mass, per 100 parts by mass of the acrylic monomer.

[17] A method for manufacturing an electrode, comprising: After producing a sulfur-based active material by the production method according to any one of

[10] to

[16] above, (3) A step of preparing an electrode using the sulfur-based active material by a conventional method. A method for manufacturing an electrode, comprising:

[18] A method for manufacturing a lithium ion secondary battery, comprising: After producing an electrode by the production method of

[17] above, (4) A step of producing a lithium ion secondary battery using the electrode by a conventional method. A method for producing a lithium ion secondary battery comprising the steps of: [Explanation of symbols]

[0093] 1 muffle furnace 2 heaters 3 Lid 4. Inert gas 5 Tray (upper) 6 Tray (lower) 7 Gas inlet pipe 8 Gas exhaust pipe 9. Sodium hydroxide solution 10 Trap tank

Claims

1. A sulfur-based active material obtained by firing a mixture containing an acrylic monomer and sulfur, The boiling point of the acrylic monomer is 230°C or higher, The sulfur-based active material is fired at a temperature of more than 250°C and less than 500°C.

2. The sulfur-based active material according to claim 1, wherein the firing temperature is higher than 270°C and lower than 450°C.

3. The sulfur-based active material according to claim 1 or 2, wherein the firing is carried out in a non-oxidizing atmosphere.

4. The acrylic monomer is at least one selected from the group consisting of an acrylate compound represented by the following formula (1) and a diacrylate compound represented by the following formula (2): The sulfur-based active material according to any one of claims 1 to 3. CH 2 =C(R 11 )COOR 12 (1) (where R 11 is a hydrogen atom or a methyl group, and R 12 is an alkyl group.) CH 2 =C(R 21 )COO-Y-OCO(R 22 )C=CH 2 (2) (where R 21 and R 22 are the same or different and are a hydrogen atom or a methyl group, Y is a straight-chain hydrocarbylene group, which may have at least one substituent selected from the group consisting of a hydroxyl group and an alkyl group, and the carbon skeleton constituting the straight-chain hydrocarbylene group may have an ether bond via an oxygen atom, provided that when there are two or more ether bonds, there are always two or more carbon atoms between adjacent oxygen atoms.

5. R 12 is an alkyl group having 12 to 30 carbon atoms, Y is a linear hydrocarbylene group having 2 to 6 carbon atoms, the linear hydrocarbylene group has 1 to 4 substituents, the alkyl group that is the substituent has 1 to 4 carbon atoms, and the carbon skeleton constituting the linear hydrocarbylene group has 1 to 2 ether bonds. The sulfur-based active material according to claim 4.

6. The sulfur-based active material according to any one of claims 1 to 5, wherein the acrylic monomer is liquid.

7. The sulfur-based active material according to any one of claims 1 to 6, wherein the sulfur content in the sulfur-based active material is 55.0 mass% or more.

8. An electrode comprising the sulfur-based active material according to any one of claims 1 to 7.

9. A lithium ion secondary battery comprising the electrode of claim 8.

10. A method for producing a sulfur-based active material, (1) preparing a raw material by mixing an acrylic monomer and sulfur; (2) A step of firing the raw material comprising The boiling point of the acrylic monomer is 230°C or higher, The method for producing a sulfur-based active material, wherein the firing temperature is higher than 250°C and lower than 500°C.

11. The method for producing a sulfur-based active material according to claim 10, wherein the firing temperature is higher than 270°C and lower than 450°C.

12. The method for producing a sulfur-based active material according to claim 10 or 11, wherein the firing is carried out in a non-oxidizing atmosphere.

13. The acrylic monomer is at least one selected from the group consisting of an acrylate compound represented by the following formula (1) and a diacrylate compound represented by the following formula (2): The method for producing a sulfur-based active material according to any one of claims 10 to 12. CH 2 =C(R 11 )COOR 12 (1) (where R 11 is a hydrogen atom or a methyl group, and R 12 is an alkyl group.) CH 2 =C(R 21 )COO-Y-OCO(R 22 )C=CH 2 (2) (where R 21 and R 22 are the same or different and are a hydrogen atom or a methyl group, Y is a straight-chain hydrocarbylene group, which may have at least one substituent selected from the group consisting of a hydroxyl group and an alkyl group, and the carbon skeleton constituting the straight-chain hydrocarbylene group may have an ether bond via an oxygen atom, provided that when there are two or more ether bonds, there are always two or more carbon atoms between adjacent oxygen atoms.

14. R 12 is an alkyl group having 12 to 30 carbon atoms, Y is a linear hydrocarbylene group having 2 to 6 carbon atoms, the linear hydrocarbylene group has 1 to 4 substituents, the alkyl group that is the substituent has 1 to 4 carbon atoms, and the carbon skeleton constituting the linear hydrocarbylene group has 1 to 2 ether bonds. The method for producing a sulfur-based active material according to claim 13.

15. The method for producing a sulfur-based active material according to any one of claims 10 to 14, wherein the acrylic monomer is liquid.

16. The method for producing a sulfur-based active material according to any one of claims 10 to 15, wherein the amount of sulfur relative to the acrylic monomer is 50 parts by mass or more and 1000 parts by mass or less per 100 parts by mass of the acrylic monomer.

17. A method for manufacturing an electrode, comprising: After producing a sulfur-based active material by the production method according to any one of claims 10 to 16, (3) A step of producing an electrode using the sulfur-based active material A method for manufacturing an electrode, comprising:

18. A method for manufacturing a lithium ion secondary battery, comprising: After producing an electrode by the production method of claim 17, (4) A step of producing a lithium ion secondary battery using the electrode. A method for producing a lithium ion secondary battery comprising the steps of:

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