Chalcogenide active materials, electrodes and lithium-ion secondary batteries

Abstract

The purpose of this invention is to provide a novel sulfide-based active material that improves cycle characteristics, an electrode (i.e., a positive or negative electrode) having said sulfide-based active material, and a lithium-ion secondary battery having said electrode. This invention provides a sulfide-based active material obtained by calcining a mixture containing an acrylic monomer and sulfur, wherein the acrylic monomer has a boiling point above 230°C.

Description

Technical Field

[0001] This invention relates to sulfide-based active materials, electrodes, and lithium-ion secondary batteries. Background Technology

[0002] Lithium-ion rechargeable batteries are primarily used in portable electronic devices due to their large charge / discharge capacity. Furthermore, they are increasingly being used in electric vehicles, where their performance is expected to improve.

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

[0004] On the other hand, as negative electrode active materials, it has been proposed to use materials that can absorb and release more lithium ions, such as silicon (Si) and tin (Sn), in order to improve the battery capacity of lithium-ion secondary batteries.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: WO 2010 / 044437

[0008] Patent Document 2: JP 2015-92449A Summary of the Invention

[0009] The technical problem solved by the invention

[0010] However, polyacrylonitrile (PAC) is inherently expensive, and its quality (especially particle size) affects battery performance, such as charge / discharge capacity and cycle characteristics. Therefore, attempts to obtain consistently high-quality PAC face the problem of further cost increases. Furthermore, while industrial rubber is inexpensive, it also presents challenges in improving cycle characteristics. The materials proposed above as negative electrode active materials undergo significant volume changes during lithium ion absorption and release, resulting in poor cycle characteristics during repeated charging and discharging. Although carbon materials such as graphite and hard carbon have been used, their capacity has almost reached its theoretical limit, making significant capacity improvements unrealistic.

[0011] From a materials synthesis perspective, reactions using only solid raw materials such as polyacrylonitrile, rubber, and sulfur exhibit slower diffusion rates compared to liquid and gaseous systems, resulting in slower reaction progress. To efficiently advance the reaction, methods such as liquefaction or vaporization of solid raw materials, or the use of liquid or gaseous raw materials, are preferred. However, liquefaction or vaporization of solid raw materials requires considerably high temperatures, which is disadvantageous from a production cost and process perspective. On the other hand, using rapidly vaporizing raw materials presents the problem of them being released outside the system before participating in the reaction.

[0012] Problem Solving Methods

[0013] This invention relates to:

[0014] A sulfur-based active material is obtained by calcining a mixture containing an acrylic-based monomer and sulfur, wherein the acrylic-based monomer has a boiling point above 230°C.

[0015] Invention Effects

[0016] This invention provides a novel sulfide-based active material with excellent cycle characteristics, an electrode (i.e., a positive or negative electrode) having the sulfide-based active material, and a lithium-ion secondary battery having the electrode.

[0017] In this article, "cycle characteristics" refers to the ability of a secondary battery to maintain its charge / discharge capacity under repeated charging / discharging conditions. Therefore, secondary batteries with a high degree of capacity degradation and low capacity retention during repeated charging and discharging have poor cycle characteristics, while secondary batteries with a low degree of capacity degradation and high capacity retention have excellent cycle characteristics. Attached Figure Description

[0018] Figure 1 This is a cross-sectional schematic diagram of the reaction apparatus used to prepare sulfide-based active materials in an embodiment of the present invention. Detailed Implementation

[0019] The present invention will be described in detail below. Furthermore, the upper and lower limits of numerical ranges related to "above," "below," "higher than," and "lower than" can be arbitrarily combined, and the values ​​in the embodiments can also be these upper and / or lower limits. Moreover, unless contrary to the purpose of the present invention, a numerical range including a lower or upper limit is understood to simultaneously disclose a numerical range excluding a lower or upper limit; conversely, unless contrary to the purpose of the present invention, a numerical range excluding a lower or upper limit is understood to simultaneously disclose a numerical range including a lower or upper limit.

[0020] One embodiment of the present invention is a sulfur-based active material obtained by calcining a mixture containing acrylic monomers and sulfur, wherein the acrylic monomers have a boiling point of 230°C or higher.

[0021] Another embodiment of the present invention is an electrode having the sulfide-based active material.

[0022] Another embodiment of the present invention is a lithium-ion secondary battery having the aforementioned electrodes.

[0023] Another embodiment of the present invention is a method for preparing sulfide-based active materials, the method comprising the following steps:

[0024] (1) Mix acrylic monomers and sulfur to prepare raw materials.

[0025] (2) Roasting the raw materials;

[0026] The acrylic monomer has a boiling point of 230°C or higher.

[0027] Another embodiment of the present invention is a method for preparing an electrode, wherein after preparing a sulfide-based active material by a method for preparing sulfide-based active materials, the method further includes the following steps:

[0028] (3) The electrode is prepared using the sulfide active material by conventional methods.

[0029] Another embodiment of the present invention is a method for preparing a lithium-ion secondary battery, the method comprising the following steps after preparing an electrode by an electrode preparation method:

[0030] (4) Using the electrode, prepare a lithium-ion secondary battery by conventional methods.

[0031] The preferred roasting temperature is above 250℃ and below 500℃.

[0032] The roasting is preferably carried out under a non-oxidizing atmosphere.

[0033] The acrylic monomer is preferably selected from at least one of the acrylate compounds represented by formula (1) and the diacrylate compounds represented by formula (2):

[0034] CH2=C (R 11 COOR 12 (1)

[0035] In the formula, R 11 It is a hydrogen atom or a methyl group, R 12 It is an alkyl group.

[0036] CH2=C (R 21 COO-Y-OCO (R) 22C=CH2 (2)

[0037] In the formula, R 21 and R 22 They may be the same or different, and each is either a hydrogen atom or a methyl group; Y is a straight-chain hydrocarbylene group, which may have at least one substituent selected from hydroxyl and alkyl groups. The carbon skeleton constituting the straight-chain hydrocarbylene group may have an ether bond containing an oxygen atom, provided that when there are more than two ether bonds, there are always more than two carbon atoms sandwiched between adjacent oxygen atoms.

[0038] Preferred R 12 Y is an alkyl group having 12 to 30 carbon atoms, and Y is a straight-chain alkylene group having 2 to 6 carbon atoms. In the straight-chain alkylene group, the number of substituents is 1 to 4, the number of carbon atoms of the alkyl group as substituents is 1 to 4, and the number of ether bonds in the carbon skeleton constituting the straight-chain alkylene group is 1 to 2.

[0039] Acrylic monomers are preferably in liquid form.

[0040] The sulfur content in the sulfur-based active material is preferably 55.0% by mass or more.

[0041] The amount of sulfur is preferably 50 parts by mass and 1000 parts by mass or less relative to 100 parts by mass of acrylic monomer.

[0042] <Acrylic monomers>

[0043] In this invention, there are no particular limitations on the acrylic monomers, as long as they are monomers having an acrylic group and a boiling point of 230°C or higher. Examples of such acrylic monomers include, for example, acrylate compounds represented by formula (1) and diacrylate compounds represented by formula (2).

[0044] The acrylic monomer is preferably selected from at least one of the acrylate compounds represented by formula (1) and the diacrylate compounds represented by formula (2):

[0045] CH2=C (R 11 COOR 12 (1)

[0046] In the formula, R 11 It is a hydrogen atom or a methyl group, R 12 It is an alkyl group.

[0047] CH2=C (R 21 COO-Y-OCO (R) 22 C=CH2 (2)

[0048] In the formula, R 21 and R22 They may be the same or different, and each is either a hydrogen atom or a methyl group; Y is a straight-chain alkylene group, which may have at least one substituent selected from hydroxyl and alkyl groups. The carbon skeleton constituting the straight-chain alkylene group may have an ether bond containing an oxygen atom, provided that when there are more than two ether bonds, there are always more than two carbon atoms sandwiched between adjacent oxygen atoms.

[0049] In equation (1), R 11 The preferred form is methyl, R 12 Preferably, it is an alkyl group having 12 or more carbon atoms, wherein the alkyl group includes straight-chain alkyl groups and branched-chain alkyl groups. 12 Examples of alkyl groups include dodecyl (C12), stearyl (C18), triacontyl (C30), etc. Examples of compounds represented by formula (1) include, for example, dodecyl (meth)acrylate, stearyl (meth)acrylate, triacontyl (meth)acrylate, etc., more preferably dodecyl methacrylate and stearyl methacrylate. Here, "(meth)acrylate" in methyl (meth)acrylate and butyl (meth)acrylate means "acrylate" or "methacrylate" (as applicable below).

[0050] In equation (2), R 21 and R 22 Methyl is preferred. The number of carbon atoms in the straight-chain alkylene group of Y is preferably 2 to 6, more preferably 2 or 3. The number of substituents in Y is preferably 1 to 4, more preferably 1 or 2. As a substituent in Y, it is preferred to have one or more substituents selected from hydroxyl and alkyl groups having 1 to 4 carbon atoms; as an alkyl group having 1 to 4 carbon atoms, it is preferably methyl. When the carbon skeleton of Y has an ether bond formed by an oxygen atom, for example, the part corresponding to -YO- is preferably represented by the following formula (3) (provided that the substituents in Y are not considered in formula (3)):

[0051] -(CH2) l -(CH2CH2O) m -(CH2CH2CH2O) n -(3)

[0052] In the formula, l ranges from 0 to 6, m ranges from 0 to 3, and n ranges from 0 to 2. However, l, m, and n cannot all be 0 at the same time.

[0053] In equation (3), it is preferred that l is 1, 2, 3, 4, 5 or 6, and m and n are 0; m is 1, 2 or 3, and l and n are 0; or n is 1 or 2, and l and m are 0.

[0054] Examples of compounds 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, neopentyl glycol di(meth)acrylate, glycerol di(meth)acrylate, etc. Ethylene glycol dimethacrylate is preferred.

[0055] Acrylic monomers can be used alone or in combination of two or more.

[0056] (Boiling points of acrylic monomers)

[0057] The boiling point of acrylic monomers is 230°C or higher. When the boiling point of acrylic monomers is lower than 230°C, during the roasting process, the acrylic monomers evaporate rapidly as the temperature rises, and are thus discharged outside the system, resulting in insufficient reaction with sulfur. Preferably, the boiling point of acrylic monomers is 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 above 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 acrylic monomers. Furthermore, as will be mentioned below, from the perspective of diffusion to and mixing with sulfur, acrylic monomers preferably have liquid properties at room temperature; therefore, from this perspective, the boiling point is preferably below a predetermined value. For example, the boiling point of acrylic monomers is preferably below 400°C, more preferably below 395°C, even more preferably below 390°C, even more preferably below 385°C, and even more preferably below 380°C.

[0058] (Characteristics of acrylic monomers)

[0059] Acrylic monomers are preferably in liquid form at room temperature (e.g., 20°C). This is because they readily diffuse into and mix with sulfur.

[0060] (Obtaining or preparing acrylic monomers)

[0061] Acrylic monomers are commercially available and can also be prepared by conventional methods known to those skilled in the art. Examples of commercially available acrylic monomers include those produced by Tokyo Chemical Industry Co., Ltd. and Sekisui Kasei Co., Ltd.

[0062] Sulfur

[0063] Various forms of sulfur can be used, such as powdered sulfur, insoluble sulfur, precipitated sulfur, and colloidal sulfur. Precipitated sulfur and colloidal sulfur are preferred. The mixing amount of sulfur relative to 100 parts by mass of acrylic monomer 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. When the mixing amount is 50 parts by mass or more, there is a tendency for improved charge / discharge capacity and cycle characteristics. On the other hand, there is no specific upper limit to the mixing amount of sulfur, but it is generally 1000 parts by mass or less, preferably less than 750 parts by mass, and more preferably less than 500 parts by mass. When the mixing amount is 1000 parts by mass or less, there is a cost advantage.

[0064] <Conductive Carbon Materials>

[0065] When modifying acrylic monomers with sulfur, a conductive carbon material can be added to the acrylic monomers beforehand. This is because it can improve the conductivity of the sulfur-based active material. Preferably, the conductive carbon material is a carbon material with a graphite structure. As the carbon material, carbon materials with condensed aromatic ring structures can be used, such as carbon black, acetylene black, Ketjen black, graphite, carbon nanotubes (CNTs), carbon fibers (CF), graphene, fullerenes, etc. One or more conductive carbon materials can be used.

[0066] Acetylene black, carbon black, and Ketjen black are preferred because they are inexpensive and have good dispersibility. Furthermore, small amounts of CNTs, graphene, or the like can be used in combination with acetylene black, carbon black, or Ketjen black. This combined system can further improve the cycle characteristics of lithium-ion secondary batteries without significantly increasing costs. Moreover, the total amount of CNTs or graphene is preferably 8% by mass or more and 12% by mass or less of the total conductive carbon material.

[0067] The amount of conductive carbon material mixed with 100 parts by mass of acrylic monomer is preferably greater than 5 parts by mass, more preferably greater than 7 parts by mass, and even more preferably greater than 10 parts by mass. When the mixing amount is greater than 5 parts by mass, it is generally easier to further improve the charge-discharge capacity and cycle characteristics. On the other hand, the mixing 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 mixing 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 it is also easier to further improve the charge-discharge capacity and cycle characteristics.

[0068] <Other Materials>

[0069] When modifying acrylic monomers with sulfur, other materials commonly used in the art can be added to the acrylic monomers beforehand if necessary.

[0070] <Preparation of Chalcogenide Active Materials>

[0071] In this invention, sulfur-based active materials can be prepared by calcining a mixture containing a predetermined acrylic monomer and sulfur.

[0072] (Preparation of raw materials)

[0073] The raw materials used for calcination include acrylic monomers and sulfur. It is preferable to thoroughly mix the acrylic monomers and sulfur beforehand. If additives such as conductive carbon materials are added to the acrylic monomers beforehand, these additives are also mixed together. Mixing can be carried out using conventional methods, such as using a high-speed mixer. Alternatively, the raw materials used for calcination can be made into granules.

[0074] (Non-oxidizing atmosphere)

[0075] Calcination is preferably carried out in a non-oxidizing atmosphere. A non-oxidizing atmosphere is an atmosphere that is essentially free of oxygen, used to suppress the oxidative degradation and excessive pyrolysis of the components. Specifically, a non-oxidizing atmosphere refers to an inert gas environment (such as nitrogen, argon, etc.) or a sulfur gas environment. Therefore, the denaturation can be carried out, for example, in a quartz tube under an inert gas environment.

[0076] (Roasting method)

[0077] Calcination can be carried out using conventional methods. For example, it can be performed by heating the raw materials (including acrylic monomers and sulfur, and additives if necessary) at a predetermined heating rate until a predetermined temperature is reached, holding them at the predetermined temperature for a predetermined time, and then allowing them to cool naturally.

[0078] [Heating rate]

[0079] The heating rate is preferably in the range of, for example, 50°C / h or higher and 500°C / h or lower. The heating rate is preferably greater than 100°C / h, more preferably greater than 150°C / h. On the other hand, the heating rate is preferably less than 400°C / h, and even more preferably less than 350°C / h. When the heating rate is within this range, it is often easy to achieve the goal of improving charge / discharge capacity and cycle characteristics.

[0080] [Roasting temperature / time]

[0081] The roasting temperature refers to the temperature at which the raw materials are heated to a certain point, and this temperature is maintained for a period of time to roast the raw materials. This temperature is preferably in the range of above 250°C and below 500°C. When the temperature is above 250°C, incomplete sulfurization reaction can be avoided, and a decrease in the charge / discharge capacity of the target material can be prevented. On the other hand, when the temperature is below 500°C, decomposition of the raw materials can be avoided, preventing a decrease in yield and a reduction in charge / discharge capacity. This temperature is preferably above 270°C, more preferably above 300°C. On the other hand, this temperature is preferably below 450°C, more preferably below 400°C, and even more preferably below 380°C. The holding time can be appropriately set according to the type of raw materials, roasting temperature, etc., but is preferably more than 1 hour and less than 6 hours. When the time is more than 1 hour, there is a tendency for sufficient roasting, while when the time is less than 6 hours, there is a tendency to prevent excessive pyrolysis of the components. This time is preferably greater than 1.3 hours, more preferably greater than 1.5 hours. On the other hand, this time is preferably less than 4 hours, more preferably less than 3 hours.

[0082] [equipment]

[0083] Firing can be achieved through, for example, a muffle furnace ( Figure 1 It can be carried out either by means of a single process or by using continuous equipment such as a twin-screw extruder. Using continuous equipment has the following advantages: sulfur-based active materials can be produced continuously through a series of operations, such as kneading, crushing, mixing, and calcining the raw materials simultaneously within the equipment.

[0084] muffle furnace ( Figure 1 A furnace is a type of furnace separated by a hot plate or similar structure, so that the heat source (heater) is not exposed inside the furnace, thus preventing sample contamination. Figure 1 In the muffle furnace 1, a heater 2 is located at the bottom of the furnace, which is separated by a hot plate. A furnace lid 3 is mounted on the front surface of the furnace (left side in the figure), giving the furnace a structure that allows the interior to maintain an inert gas atmosphere 4. A thermocouple (not shown) is mounted on the furnace lid, allowing for measurement of the furnace's internal temperature during roasting. Two layers of stainless steel (SUS) parallelepiped trays 5 and 6 are mounted on the upper and lower layers of the furnace, respectively, for roasting the raw materials.

[0085] The internal structure of the furnace allows gases (such as inert gases like argon (Ar)) to be continuously introduced and discharged to the outside through the gas inlet pipe 7 and the gas outlet pipe 8. The gas outlet pipe 8 is connected to a collection tank 10 containing an aqueous sodium hydroxide solution 9. Waste gas discharged from the muffle furnace 1 through the gas outlet pipe 8 is discharged to the outside after passing through the aqueous sodium hydroxide solution 9 in the collection tank 10. Therefore, even if the waste gas contains hydrogen sulfide gas generated in the reaction, the hydrogen sulfide gas will be neutralized by the aqueous sodium hydroxide solution and removed from the waste gas.

[0086] (Steps to remove residue)

[0087] The processed product obtained after roasting may contain residual unreacted sulfur and other substances that precipitate after cooling from the sublimated sulfur produced during roasting. Since these residues may deteriorate cycle characteristics, it is best to remove them as much as possible. Residue removal can be carried out using conventional methods, such as vacuum drying, hot air drying, and solvent washing.

[0088] (Grinding / Grading)

[0089] The obtained sulfide-based active material can be pulverized into particles with a predetermined particle size and classified to obtain particles with a particle size suitable for electrode production. From the perspective of the effects of this invention, the preferred particle size distribution is a median particle size of about 1 μm or more and 40 μm or less. The median particle size is preferably greater than 1 μm, more preferably greater than 2 μm, further preferably greater than 3 μm, further preferably greater than 4 μm, and further preferably greater than 5 μm. Furthermore, the median particle size is preferably less than 40 μm, more preferably less than 30 μm, further preferably less than 20 μm, further preferably less than 15 μm, and further preferably less than 10 μm. Using a laser diffraction / scattering type particle size distribution analyzer (HORIBA Ltd. LA-960) with water as the dispersion medium, the median particle size can be calculated by measuring the cumulative 50% particle size (median particle size D50) on a volume basis.

[0090] Furthermore, in the calcination method using a twin-screw extruder as described above, the sulfur-based active material can be pulverized by shearing during the kneading process while it is being prepared.

[0091] <Sulfur-based active materials>

[0092] The resulting sulfur-based active materials are mainly composed of carbon and sulfur. A higher sulfur content often improves charge / discharge capacity and cycle characteristics. Therefore, a higher sulfur content is better. Generally, the preferred range for sulfur content in sulfur-based active materials 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 mixed with conductive carbon materials, due to the influence of carbon constituting the conductive carbon materials, even a slightly lower sulfur content can be expected to improve charge / discharge capacity and cycle characteristics. In this case, the sulfur content can be about 5.0% by mass less than the aforementioned sulfur content.

[0093] Lithium-ion secondary batteries

[0094] The sulfide-based active material of this invention can be used as an electrode active material in lithium-ion secondary batteries, i.e., as a positive electrode active material or a negative electrode active material. In other words, except for the use of the sulfide-based active material, the preparation method of the lithium-ion secondary battery electrode is the same as that of conventional lithium-ion secondary battery electrodes; furthermore, except for the use of the lithium-ion secondary battery electrode, the preparation method of the lithium-ion secondary battery is the same as that of conventional lithium-ion secondary batteries. The lithium-ion secondary battery manufactured in this way exhibits a large charge-discharge capacity and excellent cycle characteristics.

[0095] 1. Using sulfide-based active materials as positive electrode active materials

[0096] In addition to the positive electrode containing the aforementioned sulfide-based active material (positive electrode active material), the lithium-ion secondary battery of the present invention can also be manufactured using a negative electrode, an electrolyte, and components such as a separator (if necessary) according to conventional methods.

[0097] (positive electrode)

[0098] Aside from using the aforementioned sulfide-based active materials as positive electrode active materials, the preparation method for lithium-ion secondary battery positive electrodes is the same as that for conventional lithium-ion secondary battery positive electrodes. For example, a paste-like positive electrode material is prepared by mixing particulate sulfide-based active materials with conductive additives, binders, and solvents; this positive electrode material is then coated onto a current collector and dried to produce the positive electrode. Alternatively, the positive electrode can be produced by, for example, kneading sulfide-based active materials with conductive additives, binders, and a small amount of solvent together in a mortar or mortar to form a film, and then pressing it onto a current collector using a press or similar device.

[0099] [Conductive additives]

[0100] Examples of conductive additives include, for instance, vapor-grown carbon fiber (VGCF), carbon powder, carbon black (CB), acetylene black (AB), Ketjen black (KB), graphite, or fine metal powders stable at positive electrode potentials, such as aluminum and titanium. One or more of these conductive additives may be used.

[0101] [Adhesive]

[0102] Examples of adhesives include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamide-imide (PAI), carboxymethyl cellulose (CMC), polyvinyl chloride (PVC), acrylic monomers, methacrylic acid resin (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), and polypropylene (PP). One or more of these adhesives may be used.

[0103] [solvent]

[0104] Examples of solvents include N-methyl-2-pyrrolidone, N,N-dimethylformaldehyde, alcohols, hexane, water, etc. One or more of these solvents may be used.

[0105] [Mix Amount]

[0106] There is no particular limitation on the mixing amount of these materials constituting the positive electrode, but for example, relative to 100 parts by mass of sulfur-based active material, it is preferable to mix 2 or more but less than 100 parts by mass of conductive additive, 2 or more but less than 50 parts by mass of binder, and an appropriate amount of solvent.

[0107] [Collector]

[0108] As a current collector, conventional current collectors used for the positive electrode of lithium-ion secondary batteries can be used. Examples of current collectors include those composed of aluminum foil, aluminum mesh, perforated aluminum sheets, expanded aluminum sheets, stainless steel foil, stainless steel mesh, perforated stainless steel sheets, expanded stainless steel sheets, nickel foam, nickel nonwoven fabric, copper foil, copper mesh, perforated copper sheets, expanded copper sheets, titanium foil, titanium mesh, carbon nonwoven fabric, carbon woven fabric, etc. When using the sulfur-based active material of this invention as the positive electrode active material, a current collector made of carbon nonwoven fabric or carbon woven fabric made of highly graphitized carbon is suitable because it does not contain hydrogen and has low reactivity with sulfur. As raw materials for highly graphitized carbon fibers, various pitches (i.e., byproducts of petroleum, coal, coal tar, etc.), polyacrylonitrile fiber (PAN), etc., used as carbon fiber materials can be used. The current collector can be used alone or in combination of two or more types.

[0109] (negative electrode)

[0110] As a negative electrode material, known materials such as lithium metal, carbon-based materials like graphite, silicon-based materials like silicon thin films, and alloy-based materials like copper-tin and cobalt-tin can be used. When lithium-free materials (e.g., carbon-based materials, silicon-based materials, alloy-based materials, etc.) are used as negative electrode materials, they have the advantage of being less prone to short circuits between the positive and negative electrodes due to dendrite formation. However, when these lithium-free negative electrode materials are used in combination with the positive electrode of this invention, neither the positive nor the negative electrode contains lithium. Therefore, it is necessary to pre-dope lithium into either or both of the negative and positive electrodes. Known methods can be used as methods for pre-doping lithium. For example, when doping lithium in the negative electrode, there is a method of doping lithium by electrolytic doping, which uses lithium metal as the counter electrode to assemble a half-cell and dops lithium by electrochemical methods; there is also a method of doping lithium by adhesive pre-doping, which adhesives lithium metal foil onto the electrode and then leaves it in the electrolyte, utilizing the diffusion of lithium to the electrode for doping. In addition, the above-mentioned electrolytic doping method can also be used when pre-doping lithium in the positive electrode. As a lithium-free anode material, silicon-based anode materials with high capacity are particularly preferred, with thin-film silicon being more preferred as it has a thin electrode thickness and therefore an advantage in capacity per unit volume.

[0111] (electrolytes)

[0112] As the electrolyte used in lithium-ion secondary batteries, an electrolyte obtained by dissolving an alkali metal salt as the electrolyte in an organic solvent can be used. Preferably, at least one of the following non-aqueous solvents is used: ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dimethyl ether, γ-butyrolactone, acetonitrile, etc. The electrolyte can be LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiI, LiClO4, etc. The concentration of the electrolyte can be about 0.5 mol / L or more and about 1.7 mol / L or less. Furthermore, the electrolyte is not limited to a liquid state. For example, when the lithium-ion secondary battery is a lithium polymer secondary battery, the electrolyte is solid (e.g., polymer gel state).

[0113] (Diaphragm)

[0114] In addition to the negative electrode, positive electrode, and electrolyte mentioned above, lithium-ion secondary batteries may also include components such as a separator. The separator is located between the positive and negative electrodes, allowing ions to migrate between them and preventing internal short circuits. If the lithium-ion secondary battery is a sealed type, the separator is required to retain the electrolyte. As the separator, a thin, microporous, or non-woven membrane made of polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide, cellulose, glass, etc., is preferred.

[0115] (shape)

[0116] There are no particular restrictions on the shape of lithium-ion secondary batteries; they can be cylindrical, stacked, coin-shaped, button-shaped, or any other shape.

[0117] 2. Using sulfide-based active materials as negative electrode active materials

[0118] In addition to the negative electrode containing the aforementioned sulfide-based active material (negative electrode active material), the lithium-ion secondary battery of the present invention can also be manufactured using a positive electrode, an electrolyte, and components such as a separator (if necessary) according to conventional methods.

[0119] (negative electrode)

[0120] Aside from using the aforementioned sulfide-based active materials as negative electrode active materials, the preparation method for lithium-ion secondary battery negative electrodes is the same as that for conventional lithium-ion secondary battery negative electrodes. For example, a paste-like negative electrode material is prepared by mixing particulate sulfide-based active materials with conductive additives, binders, and solvents; this negative electrode material is then coated onto a current collector and dried to produce the negative electrode. Alternatively, a positive electrode can be produced by, for example, kneading sulfide-based active materials with conductive additives, binders, and a small amount of solvent in a mortar or mortar to form a film, and then pressing it onto a current collector using a press or similar device.

[0121] As conductive additives, binders, and solvents, the same materials as those used when using sulfide active materials as positive electrode active materials can be used, and this also applies to current collectors.

[0122] (positive electrode)

[0123] There are no particular restrictions on the cathode material, as long as it is a lithium-containing transition metal oxide or solid solution oxide, or a substance capable of electrochemically absorbing and releasing lithium ions. Examples of lithium-containing transition metal oxides include, for example, Li-Co based composite oxides, such as LiCoO2; and Li-Ni-Co-Mn based composite oxides, such as LiNi. x Co y Mn z O2, etc.; Li-Ni based composite oxides, such as LiNiO2, etc.; Li-Mn based composite oxides, such as LiMn2O4, etc. Examples of solid solution oxides 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 combination.

[0124] For the electrolyte, separator, and shape of the lithium-ion secondary battery, the same materials can be used as in the case where sulfide active materials are used as positive electrode active materials.

[0125] Example

[0126] Although the present invention will be described with reference to embodiments, it is not limited to the embodiments.

[0127] The various chemicals used in the examples and comparative examples are listed below. Each chemical was purified as needed using conventional methods.

[0128] <Materials used in the experiment>

[0129] Acrylic monomer 1: Dodecyl methacrylate (manufactured by Tokyo Chemical Industry Co., Ltd., boiling point: 320°C)

[0130] Acrylic monomer 2: Stearyl methacrylate (manufactured by Tokyo Chemical Industry Co., Ltd., boiling point: 380℃)

[0131] Acrylic monomer 3: Ethylene glycol dimethacrylate (manufactured by Tokyo Chemical Industry Co., Ltd., boiling point: 235°C)

[0132] Acrylic monomer 4: Methyl methacrylate (manufactured by Tokyo Chemical Industry Co., Ltd., boiling point: 101℃)

[0133] Acrylic monomer 5: Hexyl methacrylate (manufactured by Tokyo Chemical Industry Co., Ltd., boiling point: 210°C)

[0134] Sulfur: Precipitated sulfur, manufactured by Tsurumi Chemical Industry Co., Ltd.

[0135] Example 1

[0136] <Preparation of Chalcogenide Active Materials>

[0137] (Preparation of raw materials)

[0138] According to the proportions in Table 1, various materials are mixed in a mixer to obtain raw materials for roasting.

[0139] (Reaction apparatus)

[0140] muffle furnace ( Figure 1It is used for roasting raw materials. Figure 1 The muffle furnace in the middle is as described above.

[0141] (Roasting step)

[0142] First, the atmosphere in the muffle furnace was purged three times with argon using a vacuum pump, and the raw material was placed in a SUS container. Then, argon was continuously supplied from the gas inlet tube at a flow rate of 100 mL / min, and heating of the muffle furnace began 30 minutes after the supply started. The temperature was increased at a rate of 300 °C / h, and when the raw material temperature reached 400 °C, it was calcined at 400 °C for 2 hours. Next, while adjusting the argon flow rate, the temperature of the calcined material was allowed to cool naturally to 25 °C under an argon atmosphere, and then the calcined material was removed from the muffle furnace.

[0143] (Removal of unreacted sulfur)

[0144] To remove unreacted sulfur (free sulfur) remaining in the product after the calcination step, the following steps were performed: The product was pulverized in a mortar, and then the pulverized product was placed in a glass tube furnace and heated at 250°C for 3 hours under vacuum to obtain a sulfur-based active material in which unreacted sulfur had been removed (or only trace amounts of unreacted sulfur were present). The heating rate was set to 10°C / min.

[0145] (Grinding step)

[0146] The roasted material after removing unreacted sulfur was shredded using a shredder (Labonect's free-speed shredder, FS-20).

[0147] (Grading process)

[0148] To remove coarse particles from the calcined material, a 32μm stainless steel sieve was used to classify the calcined material in order to obtain sulfur-based active materials.

[0149] <Preparation of Lithium-ion Secondary Batteries>

[0150] The preparation process of lithium-ion secondary batteries is as follows.

[0151] (positive electrode)

[0152] The sulfur-based active material obtained above was used as the active material, acetylene black as the conductive additive, and acrylic monomers as the binder. The ratio of active material:conductive additive:binder was weighed to 85:10:5 (mass%), placed in a container, and mixed using a planetary centrifugal mixer (THINKY CORPORATION ARE-250). MilliQ water was used as the dispersant to prepare a homogeneous slurry. The slurry was coated onto a 20μm aluminum foil using a 60μm slit width applicator, then pressed using a roller press to obtain the electrode. The electrode was then heated at 120°C for 3 hours, dried, and then stamped into shape. The electrode (positive electrode) is obtained. Then, the weight of the electrode is measured, and the content of active material in the electrode is calculated according to the above ratio.

[0153] (negative electrode)

[0154] For the negative electrode, lithium metal foil (disc-shaped, 14 mm in diameter and 500 μm thick, manufactured by Honjo Metal Co., Ltd.) was used.

[0155] (electrolytes)

[0156] The electrolyte used is a non-aqueous electrolyte in which LiPF6 is dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate. The ethylene carbonate and diethyl carbonate are mixed in a 1:1 volume ratio. The concentration of LiPF6 in the electrolyte is 1.0 mol / L.

[0157] (Lithium-ion secondary battery)

[0158] A coin cell was prepared using the aforementioned positive electrode, negative electrode, and electrolyte. Specifically, in a drying chamber, a separator (Celgard 2400 polypropylene microporous membrane with a thickness of 25 μm manufactured by Celgard LLC) and a glass nonwoven fabric filter (GA100 manufactured by ADVANTEC with a thickness of 440 μm) were filled between the positive and negative electrodes, and an electrolyte was injected to form an electrode body battery. This electrode body battery was housed in a battery casing made of stainless steel (a component for a CR2032 type coin cell manufactured by Hohsen). The aforementioned electrolyte was injected into the battery casing. The battery casing was sealed using a caulking machine to obtain the lithium-ion secondary battery of Example 1.

[0159] Examples 2 and 3 and Comparative Examples 1 to 4

[0160] Except for appropriate changes to the proportions and conditions in Table 1, each raw material, sulfide active material, and lithium-ion secondary battery was produced in the same manner as in Example 1.

[0161] However, in Comparative Examples 1 and 2, the material obtained by calcination was almost entirely unreacted sulfur, and therefore almost completely disappeared during the unreacted sulfur removal step. This made it impossible to produce a battery and thus impossible to conduct subsequent evaluations. For the other examples and comparative examples, batteries were produced and evaluated as follows.

[0162] <Measuring Discharge Capacity and Capacity Retention>

[0163] Under a test temperature of 30°C, the button lithium-ion secondary batteries produced in the examples and comparative examples were charged and discharged at a current value of 50mA (0.1C) per 1g of positive electrode active material. The discharge termination voltage was set to 1.0V, and the charge termination voltage was set to 3.0V. The first, second, and tenth discharge capacities (mAh) of the batteries were observed during repeated charging and discharging.

[0164] The second discharge capacity (mAh / g) is defined as the initial capacity. A larger initial capacity indicates a larger charge / discharge capacity of the lithium-ion secondary battery, which is considered superior. Furthermore, based on the tenth discharge capacity (DC)... 10 The capacity retention rate (%) can be calculated from the second discharge capacity DC2 (mAh / g) and the second discharge capacity DC2 (mAh / g) using the following formula (a):

[0165] Capacity retention rate (%) = (DC) 10 / DC2)×100(a)

[0166] As mentioned above, it can be said that the higher the capacity retention rate, the better the cycle characteristics of the lithium-ion secondary battery.

[0167] <Sulfur mass ratio (%)>

[0168] The mass percentage (%) 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 by measuring the mass of sulfur using a Dionex IonPac AS12A column manufactured by Dionex for an ion chromatograph DX-320 manufactured by Dionex.

[0169]

[0170]

[0171] <Implementation Method>

[0172] The preferred implementation is shown below.

[0173] [1] A sulfur-based active material obtained by calcining a mixture comprising an acrylic monomer and sulfur, wherein the boiling point of the acrylic monomer is 230°C or higher, 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, even more preferably higher than 350°C, or preferably 230°C or higher and lower than 400°C, more preferably higher than 230°C and lower than 395°C, even more preferably 235°C or higher and lower than 390°C, even more preferably higher than 250°C and lower than 385°C, even more preferably higher than 270°C and lower than 380°C.

[0174] [2] According to the sulfur-based active material described in [1], the calcination 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.

[0175] [3] According to the sulfur-based active material described in [1] or [2] above, the calcination is carried out in a non-oxidizing atmosphere.

[0176] [4] The sulfur-based active material according to any one of [1]-[3] above, wherein the acrylic monomer is at least one selected from the acrylate compound represented by formula (1) and the diacrylate compound represented by formula (2):

[0177] CH2=C (R 11 COOR 12 (1)

[0178] In the formula, R 11 It is a hydrogen atom or a methyl group, R 12 It is an alkyl group.

[0179] CH2=C (R 21 COO-Y-OCO (R) 22 C=CH2 (2)

[0180] In the formula, R 21 and R 22 They may be the same or different, and each is either a hydrogen atom or a methyl group; Y is a straight-chain alkylene group, which may have at least one substituent selected from hydroxyl and alkyl groups. The carbon skeleton constituting the straight-chain alkylene group may have an ether bond containing an oxygen atom, provided that when there are more than two ether bonds, there are always more than two carbon atoms sandwiched between adjacent oxygen atoms.

[0181] [5] The sulfide-based active material according to any one of [1]-[4] above, wherein R 12Y is an alkyl group having 12 to 30 carbon atoms, preferably 12 to 18 carbon atoms, and Y is a straight-chain alkylene group having 2 to 6 carbon atoms, preferably 2 or 3 carbon atoms. In the straight-chain alkylene group, the number of substituents is 1 to 4, preferably 1 or 2, the number of carbon atoms of the alkyl group as a substituent is 1 to 4, preferably 1, and the number of ether bonds in the carbon skeleton constituting the straight-chain alkylene group is 1 to 2.

[0182] [6] The sulfur-based active material according to any one of [1]-[5] above, wherein the acrylic monomer is in liquid state.

[0183] [7] The sulfur-based active material according to any one of [1]-[6] above, wherein the sulfur content in the sulfur-based active material is 55.0% by mass or more, preferably greater than 58.0% by mass, 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.

[0184] [8] An electrode having any one of the sulfide active materials described in any one of [1]-[7] above.

[0185] [9] A lithium-ion secondary battery having the electrodes described above [8].

[0186]

[10] A method for preparing chalcogenide active materials, the method comprising the following steps:

[0187] (1) Mix acrylic monomers and sulfur to prepare raw materials.

[0188] (2) Roasting the raw materials;

[0189] Wherein, the boiling point of the acrylic monomer is above 230°C, preferably above 230°C, more preferably above 235°C, even more preferably above 250°C, even more preferably above 270°C, even more preferably above 300°C, even more preferably above 320°C, even more preferably above 350°C, or preferably above 230°C and below 400°C, more preferably above 230°C and below 395°C, even more preferably above 235°C and below 390°C, even more preferably above 250°C and below 385°C, even more preferably above 270°C and below 380°C.

[0190]

[11] According to the method described in

[10] above, the calcination 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.

[0191]

[12] According to the method described in

[10] or

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

[0192]

[13] The method according to any one of

[10] -

[12] above, wherein the acrylic monomer is at least one selected from the acrylate compound represented by formula (1) and the diacrylate compound represented by formula (2):

[0193] CH2=C (R 11 COOR 12 (1)

[0194] In the formula, R 11 It is a hydrogen atom or a methyl group, R 12 It is an alkyl group.

[0195] CH2=C (R 21 COO-Y-OCO (R) 22 C=CH2 (2)

[0196] In the formula, R 21 and R 22 They may be the same or different, and each is either a hydrogen atom or a methyl group; Y is a straight-chain alkylene group, which may have at least one substituent selected from hydroxyl and alkyl groups. The carbon skeleton constituting the straight-chain alkylene group may have an ether bond containing an oxygen atom, provided that when there are more than two ether bonds, there are always more than two carbon atoms sandwiched between adjacent oxygen atoms.

[0197]

[14] The method according to any one of

[10] -

[13] above, wherein R 12 Y is an alkyl group having 12 to 30 carbon atoms, preferably 12 to 18 carbon atoms, and Y is a straight-chain alkylene group having 2 to 6 carbon atoms, preferably 2 or 3 carbon atoms. In the straight-chain alkylene group, the number of substituents is 1 to 4, preferably 1 or 2, the number of carbon atoms of the alkyl group as a substituent is 1 to 4, preferably 1, and the number of ether bonds in the carbon skeleton constituting the straight-chain alkylene group is 1 to 2.

[0198]

[15] The method according to any one of

[10] -

[14] above, wherein the acrylic monomer is in liquid state.

[0199]

[16] In the method according to any one of

[10] -

[15] above, the amount of sulfur relative to 100 parts by mass of acrylic monomer is 50 parts by mass or more and 1000 parts by mass or less, preferably more than 100 parts by mass and less than 750 parts by mass, more preferably more than 150 parts by mass and less than 500 parts by mass, even more preferably more than 200 parts by mass and less than 500 parts by mass, and even more preferably more than 250 parts by mass and less than 500 parts by mass.

[0200]

[17] A method for preparing an electrode, the method comprising, after preparing a chalcogenide active material by any one of the methods described in

[10] -

[16] above, the following steps:

[0201] (3) The electrode is prepared using the sulfide active material by conventional methods.

[0202]

[18] A method for preparing a lithium-ion secondary battery, the method comprising, after preparing the electrode by the method described in

[17] above, the following steps:

[0203] (4) Using the electrode, prepare a lithium-ion secondary battery by conventional methods.

[0204] List of reference numerals

[0205] 1. Muffle furnace

[0206] 2. Heater

[0207] 3. Furnace lid

[0208] 4. Inert gases

[0209] 5. Pallet (top layer)

[0210] 6. Tray (lower layer)

[0211] 7. Gas inlet tube

[0212] 8. Gas exhaust pipe

[0213] 9. Sodium hydroxide aqueous solution

[0214] 10. Collection tank

Claims

1. A chalcogenide active material obtained by calcining a mixture containing an acrylic monomer and sulfur, wherein, The boiling point of the acrylic monomer is 230°C or higher and lower than 400°C. The baking temperature is higher than 250°C and lower than 500°C 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) wherein R 11 is a hydrogen atom or a methyl group, R 12 is an alkyl group having 12 to 30 carbon atoms, CH2=C (R 21 ) COO-Y-OCO (R 22 ) C=CH2 (2) wherein R 21 and R 22 are the same or different, each being a hydrogen atom or a methyl group; Y is a linear hydrocarbylene group having 2 to 6 carbon atoms, which can have at least one substituent selected from a hydroxyl group and an alkyl group, the number of the substituent being 1 to 4, the number of carbon atoms of the alkyl group as the substituent being 1 to 4, the number of ether bonds possessed by the carbon skeleton constituting the linear hydrocarbylene group being 1 to 2, The sulfur content in the sulfur-based active material is 55.0% by mass or more.

2. The chalcogenide active material of claim 1, wherein, The baking is performed in a non-oxidizing atmosphere.

3. The chalcogenide active material of claim 1 or 2, wherein, The acrylic monomer is in a liquid state.

4. An electrode having the sulfur-based active material according to any one of claims 1 to 3.

5. A lithium-ion secondary battery having the electrode according to claim 4.

6. A method for producing a sulfur-based active material, the method comprising the steps of: (1) mixing an acrylic monomer and sulfur to produce a raw material, (2) baking the raw material; The baking temperature is higher than 250°C and lower than 500°C 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) wherein R 11 is a hydrogen atom or a methyl group, R 12 is an alkyl group having 12 to 30 carbon atoms, CH2=C (R 21 ) COO-Y-OCO (R 22 ) C=CH2 (2) wherein R 21 and R 22 are the same or different, each being a hydrogen atom or a methyl group; Y is a linear hydrocarbylene group having 2 to 6 carbon atoms, which can have at least one substituent selected from a hydroxyl group and an alkyl group, the number of said substituent being 1 to 4, the number of carbon atoms of the alkyl group as the substituent being 1 to 4, the number of ether bonds possessed by the carbon skeleton constituting the linear hydrocarbylene group being 1 to 2, The boiling point of the acrylic monomer is 230°C or higher and lower than 400°C. The amount of sulfur is 50 parts by mass or more and 1000 parts by mass or less with respect to 100 parts by mass of the acrylic monomer.

7. The method of claim 6, wherein, The baking is performed in a non-oxidizing atmosphere.

8. The method of claim 6 or 7, wherein, The acrylic monomer is in a liquid state.

9. A method for producing an electrode, the method comprising the step of: (3) producing an electrode using the sulfur-based active material by a conventional method after the sulfur-based active material is produced by the method according to any one of claims 6 to 8.

10. A method for producing a lithium-ion secondary battery, the method comprising the step of: (4) producing a lithium-ion secondary battery using the electrode by a conventional method after the electrode is produced by the method according to claim 9.

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