Electrode active material, electrode, and lithium ion secondary battery

Abstract

The purpose of the present invention is to improve the charge / discharge capacity and the capacity retention rate of an electrode active material constituting an electrode of a lithium ion secondary battery. Provided is an electrode active material comprising particles containing an organic sulfur compound, wherein when the oxygen content (mass %) in the electrode active material is AO and the sulfur content (mass %) is AS, AO and AS satisfy the following formulas. (1) AO > 9.0, (2) AS > 45.0, (3) AO × AS > 550

Description

Electrode active material, electrode, and lithium ion secondary battery

[0001] The present invention relates to a novel electrode active material, an electrode comprising the electrode active material, and a lithium ion secondary battery comprising the electrode.

[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 obtained by heat treating high-cis-butadiene rubber together with sulfur and a vulcanization accelerator, and Patent Document 2 describes an electrode active material obtained by baking polymethyl methacrylate together with sulfur.

[0004] International Publication No. 2015 / 050086 Japanese Patent Application Laid-Open No. 2021-172814

[0005] However, the active materials of Patent Documents 1 and 2 only have a low oxygen content, and there is room for improvement in the charge / discharge capacity and capacity retention rate.

[0006] The present invention provides a novel electrode active material that can improve charge / discharge capacity and capacity retention rate, an electrode comprising the electrode active material, i.e., a positive electrode or a negative electrode, and a lithium ion secondary battery comprising the electrode.

[0007] The present invention relates to the following electrode active material: An electrode active material comprising particles containing an organic sulfur compound, wherein the oxygen content (mass %) in the electrode active material is A O and the sulfur content (mass%) is A S In the case where A O and A S and an electrode active material that satisfies the following formula: (1) A O >9.0 (2) A S >45.0 (3) A O ×A S >550

[0008] According to the present invention, it is possible to provide a novel electrode active material that can improve charge / discharge capacity and capacity retention rate, an electrode comprising the electrode active material, i.e., a positive electrode or a negative electrode, and a lithium ion secondary battery comprising the electrode.

[0009] While not intending to be bound by theory, the following is believed to be the reason why the present invention can improve the charge / discharge capacity and capacity retention rate. That is, sulfur functions as an active material capable of repeated charge / discharge by undergoing a reversible electrochemical reaction with lithium. In the present invention, as in conventional technology, sulfur is mixed with a polymer and fixed to the polymer through heat treatment, which is thought to suppress elution and contribute to the reversible reaction. Therefore, it is believed that the more sulfur atoms there are, the greater the charge / discharge capacity. On the other hand, in the present invention, oxygen atoms bond with carbon or sulfur to form C—O bonds, etc. Therefore, (1) the active material contains these polar bonds, which improves the dispersibility of the active material in an electrode slurry using water as a solvent, resulting in the production of a uniform electrode, which is thought to make it easier to maintain the cycle capacity even after expansion and contraction associated with charge / discharge cycles. Furthermore, (2) the high oxygen content and its presence in the active material framework can more stably maintain S—S bonds within the active material structure, which is thought to enhance the ability to maintain sulfur within the active material structure during the reaction between sulfur and lithium associated with the insertion and desorption of lithium ions during charge / discharge. Therefore, it is believed that the cycle capacity is likely to be maintained even after undergoing electrochemical reactions accompanying charge-discharge cycles.

[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.

[0011] FIG. 1 is a cross-sectional view schematically showing a reaction apparatus used in the production of an electrode active material in an example of the present invention.

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

[0013] One embodiment of the present invention is an electrode active material comprising particles containing an organic sulfur compound, wherein the oxygen content (mass%) in the electrode active material is A O and the sulfur content (mass%) is A S In the case where A O and A S and are electrode active materials that satisfy the following formula: (1) A O >9.0 (2) A S >45.0 (3) A O ×A S >550

[0014] The right side of formula (1) is preferably 11.0. The right side of formula (2) is preferably 50.0. The right side of formula (3) is preferably 570. It is believed that the effects of the present invention can be more effectively achieved by satisfying stricter conditions for at least one of the formulas.

[0015] The active material preferably further contains a metal compound containing at least one metal selected from the group consisting of iron, molybdenum, vanadium, and titanium.

[0016] The metal compound is preferably an iron compound.

[0017] Another embodiment of the present invention is an electrode comprising the electrode active material.

[0018] The electrode includes a current collector, the current collector includes a metal foil, and the coating density (mg / cm) of the electrode active material on the current collector is 2) is D, then D and A O and A S It is preferable that D×(A O ×A S ) > 1000

[0019] It is believed that by making the product of the coating density, the oxygen content, and the sulfur content exceed a predetermined value, the performance of the electrode and / or the battery can be improved.

[0020] The electrode includes a current collector, the current collector includes a metal foil, and the coating density (mg / cm) of the electrode active material on the current collector is 2 ) is D and the thickness of the metal foil is T (μm), D, T and A O and A S It is preferable that D×(A O ×A S ) / T>60

[0021] It is believed that by ensuring that the coating density, oxygen content, sulfur content, and metal foil thickness satisfy the above formula, the performance of the electrode and / or battery can be improved.

[0022] The electrode includes a current collector, the current collector includes a metal foil, and the coating density (mg / cm) of the electrode active material on the current collector is 2 ) is D, D is 2.50 mg / cm 2 Preferably, it is greater than 1000 .mu.m.

[0023] The electrode is preferably a positive electrode.

[0024] It is believed that the use of the positive electrode can improve the performance of the electrode and / or the battery.

[0025] Another embodiment of the present invention is a lithium ion secondary battery comprising the electrode.

[0026] The lithium ion secondary battery further comprises an electrolyte, the volume of the electrolyte being V (mL), and the coating density of the electrode active material on the current collector (mg / cm 2 ) is D, then D, V and A O and A SIt is preferable that D×(A O ×A S ) / V>4000

[0027] It is believed that by ensuring that the coating density, oxygen content, sulfur content, and volume of the electrolyte satisfy the above formula, the performance of the electrode and / or battery can be improved.

[0028] Another embodiment of the present invention is a method for producing an electrode active material, comprising: (1) a mixing step of mixing raw materials containing cellulose and sulfur in an amount equal to or greater than the mass of the cellulose to obtain a calcined raw material; (2) a calcination step of calcining the calcined raw material to obtain a calcined product; and (3) a granulation step of pulverizing the calcined product to obtain particles of the calcined product.

[0029] The cellulose is preferably chemically unmodified cellulose.

[0030] The absence of chemical modification is believed to prevent the reaction between sulfur and cellulose from becoming insufficient due to chemical modification, and as a result, prevent a decrease in the sulfur content.

[0031] <Definition> "Particles" refers to an electrode active material that has been sufficiently finely divided to be suitable for mixing with other materials for the purposes of the present invention. The size of the particles that make up the electrode active material is not particularly limited as long as the mixing can be carried out suitably. For example, when the median diameter of "particles" is expressed as a particle size, it can be in the range of 1 nm to 1000 μm.

[0032] "Particle size" is expressed as the median size (d50) unless otherwise specified.

[0033] "Electrode active material" refers to one of the electrode materials in a battery, and refers to a substance that is involved in the reaction that generates electricity. Electrode active materials are divided into positive electrode active materials and negative electrode active materials.

[0034] The "oxygen content in the electrode active material" is the amount (mass %) of oxygen element contained in the electrode active material.

[0035] The "sulfur content in the electrode active material" is the amount (mass %) of elemental sulfur contained in the electrode active material.

[0036] The term "active material" refers to a substance that is responsible for the oxidation-reduction reaction that takes place for energy conversion in a lithium ion secondary battery.

[0037] "Coating density" refers to the density per unit area (cm ) of the active material coated on the current collector. 2 ) is the mass (mg) per

[0038] "Electrolyte volume" refers to the total volume of the electrolyte solution containing the solute or the solid electrolyte, expressed in mL.

[0039] Unless otherwise specified, the "initial discharge capacity" refers to the second discharge capacity.

[0040] <Measurement method>

[0041] The "contents of oxygen, sulfur, and the like in the electrode active material" are measured by the methods described in the Examples. That is, the elemental amounts of carbon, hydrogen, nitrogen, and sulfur are measured by oxygen flow combustion-infrared absorption method using a fully automatic elemental analyzer, Vario MICRO Cube, manufactured by Elementar. The elemental amount of oxygen is measured by inert gas fusion-infrared absorption method using an oxygen / nitrogen / hydrogen analyzer, EMGA-930, manufactured by Horiba, Ltd.

[0042] The "particle size distribution" is measured using a laser diffraction / scattering particle size distribution analyzer (Anton Paar PSA1090L particle size distribution analyzer) with water as the dispersion medium.

[0043] Unless otherwise specified, the "median diameter" is measured as the volume-based cumulative 50% diameter (μm) in the particle size distribution.

[0044] The electrode active material, electrode, and lithium ion secondary battery of this embodiment will be described below.

[0045] <Electrode active material> The electrode active material is an electrode active material made of particles containing an organic sulfur compound, and as shown in the above formulas (1) to (3), the oxygen content (mass %) in the electrode active material A O and sulfur content (mass%) AS and exceed a predetermined value, and A O and A S The product of these electrode active materials exceeds a predetermined value.

[0046] The organic sulfur compound is a compound formed by calcining an organic compound, such as cellulose, with sulfur in a non-oxidizing atmosphere, thereby incorporating sulfur. In the present embodiment, the organic sulfur compound is not particularly limited as long as it constitutes particles and the electrode active material made of the particles satisfies the above formulas (1) to (3).

[0047] (Equation (1) to Equation (3)) The oxygen content (mass%) in the electrode active material is A O and the sulfur content (mass%) is A S In the case where A O and A S A satisfies the following formula: (1) A O >9.0 (2) A S >45.0 (3) A O ×A S >550

[0048] The right side of formula (1) is preferably 10.0, more preferably 11.0, even more preferably 12.0, even more preferably 13.0, even more preferably 14.0, even more preferably 15.0, even more preferably 16.0, even more preferably 17.0, and even more preferably 18.0. O Although there is no particular limit to the upper limit of the value, it can be assumed to be about 30.00 as a reference value.

[0049] A O can be increased by using an organic compound containing many oxygen atoms as a raw material for calcination, and conversely, can be decreased by using an organic compound containing few oxygen atoms as a raw material for calcination. An example of an organic compound containing a relatively large number of oxygen atoms is cellulose.

[0050] (Formula (2)) The right side of formula (2) is preferably 46.0, more preferably 47.0, even more preferably 48.0, even more preferably 49.0, even more preferably 50.0, even more preferably 51.0, even more preferably 52.0, even more preferably 53.0, even more preferably 54.0, even more preferably 55.0, even more preferably 56.0, even more preferably 57.0, even more preferably 58.0, even more preferably 59.0. A S Although the upper limit of the value is not particularly limited, it can be assumed to be about 70.0 as a reference value.

[0051] A S can be increased by using more sulfur as a calcination raw material, and conversely, can be decreased by using less sulfur as a calcination raw material.

[0052] (Formula (3)) The right side of formula (3) is preferably 560, more preferably 570, even more preferably 600, even more preferably 650, even more preferably 700, even more preferably 750, even more preferably 800, even more preferably 830, and even more preferably 840. A O ×A S Although there is no particular upper limit to the value, it can be assumed to be about 1500 as a reference value.

[0053] A O ×A S A O and the value of A S can be adjusted by adjusting the values ​​of

[0054] (Elements Other Than Oxygen and Sulfur) The electrode active material may contain elements other than oxygen and sulfur. Such elements include carbon, hydrogen, nitrogen, and the like.

[0055] [Carbon Content] From the viewpoint of improving the performance of the electrode and / or battery, the carbon element content is preferably more than 5.0 mass%, more preferably more than 10.0 mass%, and even more preferably more than 15.0 mass%, while the carbon element content is preferably less than 50.0 mass%, more preferably less than 45.0 mass%, and even more preferably less than 40.0 mass%.

[0056] [Hydrogen Content] By calcination, hydrogen (H) in the organic compound reacts with sulfur to become hydrogen sulfide and is released outside the system. Therefore, the hydrogen content of the electrode active material is preferably less than 1.0 mass%, more preferably less than 0.7 mass%, and even more preferably less than 0.5 mass%. When the hydrogen content is less than 1.0 mass%, the calcination (sulfurization reaction) tends to be sufficient. Therefore, in this case, the charge / discharge capacity tends to be improved.

[0057] [Nitrogen Content] The amount of nitrogen element (% by mass) in the electrode active material may be 0% by mass if no nitrogen source is used as a raw material. For example, if a compound containing a nitrogen atom is used as the organic compound, nitrogen element can be detected.

[0058] (Amount of Metal Element) The electrode active material preferably further contains a metal compound containing at least one metal selected from the group consisting of iron, molybdenum, vanadium, and titanium. The metal compound is preferably an iron compound. One or more metal compounds may be used.

[0059] When the electrode active material contains a metal compound, the amount of the metal element (% by mass) is preferably more than 10.0% by mass, more preferably more than 15.0% by mass, and even more preferably more than 20.0% by mass, from the viewpoint of improving the performance of the electrode and / or battery, while the amount of the metal element is preferably less than 30% by mass, more preferably less than 25.0% by mass, and even more preferably less than 24.0% by mass. Here, when the metal compound contains multiple metal elements, the amount of the metal element means the total amount of the multiple metal elements.

[0060] (Median diameter) The electrode active material is composed of particles, and the size thereof is suitable for the production of an electrode. From the viewpoint of improving the performance of the electrode and / or battery, the preferred range of particle size of the electrode active material is a median diameter (median diameter d50) of preferably more than 1.0 μm and less than 40.0 μm. The median diameter is more preferably more than 1.5 μm, even more preferably more than 2.0 μm, and even more preferably more than 3.0 μm. The median diameter is more preferably less than 30.0 μm, even more preferably less than 25.0 μm, even more preferably less than 20.0 μm, even more preferably less than 15.0 μm, even more preferably less than 10.0 μm, and even more preferably less than 8.0 μm. The median diameter can be measured by the method described in the Examples section below.

[0061] (Other Components) The electrode active material of this embodiment can contain the materials described in the production method section below, in the same manner as described in the same section.

[0062] <Electrode> An electrode according to one embodiment of the present invention is an electrode comprising the electrode active material described above. The electrode is preferably an electrode obtained by mixing the electrode active material with other electrode materials such as a conductive additive and a binder as necessary, and applying the mixture to a current collector.

[0063] The electrode according to this embodiment can be used in a lithium-ion secondary battery, and the electrode can be constructed using the materials described in the manufacturing method section below in the same manner as described in the same section. That is, when the electrode is used as a positive electrode, the conductive additive, binder, current collector, etc. described in the manufacturing method section below can be used in the same manner as described in the same section to form a positive electrode for a lithium-ion secondary battery. When the electrode is used as a negative electrode, the conductive additive, binder, current collector, etc. described in the manufacturing method section below can be used in the same manner as described in the same section to form a negative electrode for a lithium-ion secondary battery. In this way, the explanations in the manufacturing method section below can be taken into consideration as explanations of this electrode.

[0064] The electrode of this embodiment includes a current collector, and the current collector includes a metal foil. The electrode active material is applied to the current collector at a coating density (mg / cm 2 ) is D, then D and AO and A S It is preferable that D×(A O ×A S ) > 1000

[0065] (Equation (4)) The right side of equation (4) is more preferably 1500, even more preferably 1700, even more preferably 1900, even more preferably 2100, even more preferably 2200, and even more preferably 2300. Note that the higher the value of the left side of equation (4), the more preferable it is, and there is no significance in setting an upper limit, but as a reference value, a value of about 5000 can be assumed.

[0066] (Coating density) The coating density D (mg / cm) of the electrode active material on the electrode 2 ) is 2.50 mg / cm 2 More preferably, it is greater than 3.00 mg / cm 2 More preferably, greater than 3.50 mg / cm 2 More preferably, greater than 3.80 mg / cm 2 More preferably, greater than 3.90 mg / cm 2 The higher the coating density value, the more preferable it is, and there is no point in setting an upper limit. However, as a reference value, 15.0 mg / cm 2 It can also be assumed to be the extent.

[0067] (Formula (5)) The electrode of this embodiment includes a current collector, and the current collector includes a metal foil. The coating density (mg / cm) of the electrode active material on the current collector is 2 ) is D and the thickness of the metal foil is T (μm), D, T and A O and A S It is preferable that D×(A O ×A S ) / T>60

[0068] The right side of formula (5) is more preferably 80, even more preferably 90, even more preferably 110, even more preferably 120, and even more preferably 130. The higher the value of the left side of formula (5), the more preferable it is, and there is no point in setting an upper limit, but as a reference value, a value of about 300 can be assumed.

[0069] (Thickness of Metal Foil) The thickness T (μm) of the metal foil is preferably 5 μm or more, more preferably 10 μm or more, while T is preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 25 μm or less.

[0070] (Charge / Discharge Capacity) The electrode of this embodiment exhibits excellent charge / discharge capacity. The second discharge capacity (DC2), which is the initial discharge capacity, is the discharge capacity when the electrode and battery are charged and discharged twice after fabrication, with the discharge end voltage being 1.0 V and the charge end voltage being 3.0 V (this is the second discharge when repeating the cycle of first discharge, first charge, second discharge, and second charge). In the case of discharge, when discharged at a constant current (a current value equivalent to 50 mA per 1 g of positive electrode active material), the voltage of 3.0 V eventually drops to 1.0 V. The total time (h) from 3.0 V to 1.0 V is measured and multiplied by the applied current (mA) to obtain the capacity (mAh), which is then divided by the weight of the active material to obtain the specific capacity (mAh / g). On the other hand, in the case of charging, the voltage rises conversely when charging at a constant current, and when it finally reaches 3.0 V, the charging is completed. The same applies to the 10th and 20th discharge capacities, which will be described later.

[0071] [Initial Discharge Capacity (DC2)] When the electrode of this embodiment is used as a positive electrode, the initial discharge capacity DC2 (mAh / g) is preferably greater than 455 mAh / g. DC2 is more preferably greater than 460 mAh / g, even more preferably greater than 470 mAh / g, even more preferably greater than 480 mAh / g, even more preferably greater than 490 mAh / g, even more preferably greater than 500 mAh / g, and even more preferably greater than 550 mAh / g. There is no particular limit to the upper limit of the initial discharge capacity, and the higher the better. Therefore, although there is little point in mentioning the upper limit of the initial discharge capacity, it can be assumed to be, for example, about 1000 mAh / g as a reference value.

[0072] [20th discharge capacity (DC 20 ) )] When the electrode of this embodiment is used as a positive electrode, the discharge capacity after 20 cycles of charge and discharge, that is, the 20th discharge capacity DC 20(mAh / g) is preferably greater than 375 mAh / g. 20 is more preferably greater than 400 mAh / g, even more preferably greater than 430 mAh / g, even more preferably greater than 450 mAh / g, even more preferably greater than 480 mAh / g, even more preferably greater than 490 mAh / g, even more preferably greater than 500 mAh / g, and even more preferably greater than 550 mAh / g. There is no particular limit to the upper limit of the discharge capacity, and the higher the better. Therefore, although there is little meaning in mentioning the upper limit of the discharge capacity, it can be assumed, for example, as a reference value, to be about the value of the initial discharge capacity or about 900 mAh / g.

[0073] The second and twentieth discharge capacities when the electrode of this embodiment is used as a positive electrode are determined by the configuration of the positive electrode, provided that the negative electrode and electrolyte are within the technical knowledge required for a lithium-ion secondary battery (i.e., that Li is not depleted) and are measured so that the performance related to the discharge capacity of the positive electrode can be fully demonstrated. For example, for the negative electrode, the amount of lithium used is preferably at least 2 times, more preferably at least 5 times, even more preferably at least 10 times, and even more preferably at least 50 times the amount (molar amount) of sulfur in the positive electrode. Furthermore, for example, for the electrolyte, the amount of electrolyte (microliters) is preferably at least 10 times, more preferably at least 20 times, and even more preferably at least 50 times the amount (mg) of sulfur in the positive electrode, so that the discharge capacity of the positive electrode can be fully demonstrated and the battery life can be extended. On the other hand, considering the energy density of the battery, a small amount of electrolyte is preferable. For example, the amount of electrolyte (microliters) is preferably 5 times or less, more preferably 3 times or less, and even more preferably 1 time or less, relative to the amount of sulfur (mg) in the positive electrode. Here, the volume V (mL) of the electrolyte refers to the total volume of the electrolyte including the solute. The electrolyte may be in the form of an electrolyte solution or a solid (solid electrolyte), or a combination of both.

[0074] (Use) The electrode of this embodiment can be used as a positive electrode or a negative electrode of a lithium ion secondary battery, and is preferably used as a positive electrode of a lithium ion secondary battery.

[0075] <Lithium-ion secondary battery> A lithium-ion secondary battery according to one embodiment of the present invention is a lithium-ion secondary battery including the above-described electrode.

[0076] The lithium ion secondary battery of this embodiment can be constructed using the materials described in the manufacturing method section below in the same manner as described in the same section. That is, when the above electrode is used as a positive electrode, a lithium ion secondary battery can be constructed using the negative electrode, electrolyte, separator, etc. described in the manufacturing method section below in the same manner as described in the same section. On the other hand, when the above electrode is used as a negative electrode, a lithium ion secondary battery can be constructed using the positive electrode, electrolyte, separator, etc. described in the manufacturing method section below in the same manner as described in the same section. In this way, these explanations in the manufacturing method section below can be taken into consideration as explanations of this lithium ion secondary battery.

[0077] The lithium ion secondary battery of this embodiment further contains an electrolyte, the volume of which is V (mL), and the coating density of the electrode active material on the current collector (mg / cm 2 ) is D, then D, V and A O and A S It is preferable that D×(A O ×A S ) / V>4000

[0078] The right side of formula (6) is more preferably 4500, even more preferably 5000, even more preferably 5500, even more preferably 6000, even more preferably 7000, even more preferably 7500, and even more preferably 8000. The higher the value of the left side of formula (6), the more preferable it is, and there is no significance in setting an upper limit, but as a reference value, a value of about 20000 can be assumed.

[0079] (Electrolyte Volume V) The range of the electrolyte volume V (mL) can vary depending on the size of the battery, so it is not generally specified. It is sufficient to use the minimum amount that brings out the performance of the electrode active material and allows the battery to function satisfactorily. For example, as a reference value, in the case of a coin-type battery presented in the examples, the volume is preferably 0.1 mL or more, more preferably 0.12 mL or more, and even more preferably 0.15 mL or more. Meanwhile, the volume V is preferably 0.40 mL or less, more preferably 0.30 mL or less, even more preferably 0.28 mL or less, and even more preferably 0.25 mL or less.

[0080] (Applications) The lithium ion secondary battery of the present embodiment is useful as a lithium ion secondary battery with improved overall performance in terms of charge / discharge capacity and capacity retention rate, and can be used in mobile information terminals such as smartphones and notebook personal computers, mobile electronic devices such as music players and digital cameras, medical devices, as well as batteries for next-generation clean energy automobiles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs).

[0081] <Production Method> The methods for producing the electrode active material, electrode, and lithium ion secondary battery of this embodiment will be described below in order.

[0082] (Production of Electrode Active Material) The electrode active material of this embodiment can be produced by various methods. For example, the following method can be mentioned as a method using cellulose as an organic compound serving as a raw material.

[0083] That is, the electrode active material of this embodiment can be produced by a production method including: (1) a mixing step of mixing raw materials containing cellulose and sulfur in an amount equal to or greater than the mass of the cellulose to obtain a calcined raw material; (2) a calcination step of calcining the calcined raw material to obtain a calcined product; and (3) a granulation step of pulverizing the calcined product to obtain particles of the calcined product.

[0084] [Raw Materials] Raw materials used as firing raw materials will be described below.

[0085] Cellulose (Cellulose, Cell-OH, Ce) is a natural polymer that is the main component of the cell walls and fibers of plant cells. 12 H 20 O 10 ) n Cellulose has the following chemical structural formula: In this chemical structural formula, n, which indicates the average number of repeating units, is a number of 1 or more, preferably 10 to 10,000, and more preferably 50 to 2,000.

[0086]

[0087] As is clear from the above chemical structural formula, cellulose has multiple hydroxyl groups, and therefore exists as an esterified form in which all or part of the hydroxyl groups are esterified with an esterifying agent. The esterifying agent is not particularly limited as long as it can impart carboxyl groups to the hydrophilic groups of cellulose, and various esterifying agents can be used. For example, carboxylic acid compounds can be used, such as compounds having two or more carboxyl groups or acid anhydrides of compounds having two or more carboxyl groups. In this embodiment, cellulose also includes such esterified forms.

[0088] Cellulose as a plant material is composed of cellulose fibers of 20 to 40 μm in size, and these cellulose fibers are bundles of cellulose microfibrils, which are in turn bundles of cellulose molecular chains. Therefore, it is preferable to defibrate such cellulose as a plant material before use.

[0089] There are two types of defibration treatment: mechanical defibration and chemical defibration, and either type can be used in this embodiment. Here, mechanical defibration treatments include the high-pressure homogenizer method, the microfluidizer method (counter jet collision method), the grinder method, the ball mill crushing method, the bead mill crushing method, and the freeze crushing method. Of these, the freeze crushing method is preferred. Furthermore, chemical defibration treatments include the TEMPO method, the phosphate esterification method, the phosphite esterification method, the carboxymethylation method, the sandating method, the sulfonation method, the enzyme hydrolysis method, the acid hydrolysis method, and the ionic liquid selective dissolution method.

[0090] When cellulose is defibrated by mechanical defibration treatment, it is not chemically modified, but when cellulose is defibrated by chemical defibration treatment, it is chemically modified. In this embodiment, from the viewpoint of improving the performance of the electrode and / or battery, cellulose that has been defibrated by mechanical defibration treatment and has not been chemically modified is preferred.

[0091] Prior to the defibration treatment, the cellulose is preferably subjected to a chemical or enzymatic pretreatment.

[0092] One or more types of cellulose can be used.

[0093] Sulfur can be used in various forms such as powdered sulfur, insoluble sulfur, precipitated sulfur, and colloidal sulfur, among which precipitated sulfur and colloidal sulfur are preferred. One or more types of sulfur can be used.

[0094] From the viewpoint of improving the performance of the electrode and / or battery, the sulfur content in the calcined raw material is preferably 100 parts by mass or more, more preferably more than 100 parts by mass, even more preferably 200 parts by mass or more, even more preferably more than 200 parts by mass, and even more preferably 300 parts by mass or more, relative to 100 parts by mass of cellulose. On the other hand, there is no particular upper limit for the sulfur content, but it is preferably less than 1000 parts by mass, more preferably less than 900 parts by mass, even more preferably less than 800 parts by mass, even more preferably less than 700 parts by mass, even more preferably less than 600 parts by mass, even more preferably less than 500 parts by mass, and even more preferably 400 parts by mass or less. A sulfur content of less than 1000 parts by mass tends to be advantageous in terms of cost.

[0095] Although various allotropes of sulfur can be used, sulfur containing S8 sulfur, which is solid at room temperature and pressure, is preferred, and simple S8 sulfur is more preferred.

[0096] <<Raw Metal Compound>> When the electrode active material further contains a metal compound containing at least one metal selected from the group consisting of iron, molybdenum, vanadium, and titanium, a raw material metal compound can be further used as a raw material for firing. Examples of the raw material metal compound include the following raw material metal compounds. It is preferable to use a raw material iron compound as the raw material metal compound.

[0097] <<Raw Iron Compound>> The raw iron compound may be an iron compound containing divalent or trivalent iron ions, but is not particularly limited as long as it decomposes during firing, reacts with sulfur, and produces iron disulfide, and various compounds can be used. Examples of the raw iron compound include iron acid salts, iron complexes, etc. Examples of iron acid salts include both organic iron acid salts and inorganic iron acid salts. On the other hand, examples of iron complexes include neutral iron complexes and salts of iron complex ions (iron complex salts). Of these, organic iron acid salts, inorganic iron acid salts, and neutral iron complexes are preferred. One or more iron compounds can be used.

[0098] Examples of organic acid salts of iron include divalent iron (Fe 2+) and organic acid salts, and trivalent iron (Fe 3+ ) and an organic acid salt. Of these, a salt of divalent iron and an organic acid is preferred. The organic acid is not particularly limited, and may include those having a carboxyl group (-COOH) or a sulfo group (-SO3H), but those having a carboxyl group are preferred. Specific examples of organic acids include fatty acids, oxalic acid, tartaric acid, citric acid, malic acid, and succinic acid. Specific examples of fatty acids include those having 1 to 6 carbon atoms, such as acetic acid, propionic acid, and butyric acid. Of these, acetic acid and oxalic acid are preferred. Preferred examples of iron organic acid salts include iron(II) acetate and iron(II) oxalate. These may be hydrates. One or more iron organic acid salts can be used.

[0099] Examples of inorganic salts of iron include divalent iron (Fe 2+ ) and inorganic acid salts, and trivalent iron (Fe 3+ ) and an inorganic acid. Specific examples of inorganic acids include hydrochloric acid, sulfuric acid, and nitric acid. Of these, nitric acid is preferred. Preferred examples of inorganic acid salts of iron include iron(II) chloride, iron(III) chloride, iron(II) sulfate, iron(III) sulfate, iron(II) nitrate, and iron(III) nitrate. These may also be hydrates. One or more inorganic acid salts of iron can be used.

[0100] Examples of iron complexes include divalent iron (Fe 2+ ) complexes and trivalent iron (Fe 3+) complexes. The iron complex may be in the form of a neutral complex or a complex salt. The ligand coordinating with the iron ion is not particularly limited and includes, for example, halogen atoms such as chlorine and bromine atoms, cyano groups, dicyclopentadienyl groups, and N,N'-bis(salicylidene)ethylenediamine. Examples of iron complexes include potassium hexacyanidoferrate(II) ([Fe(CN)6]K4), potassium hexacyanidoferrate(III) ([Fe(CN)6]K3), sodium tetrachloroferrate(III) ([FeCl4]Na), dicyclopentadienyl iron(II) (ferrocene), and N,N'-bis(salicylidene)ethylenediaminatoiron(III) chloride. One or more iron complexes can be used.

[0101] <<Raw Material Molybdenum Compound>> Examples of the raw material molybdenum compound include molybdenum trioxide (VI), sodium molybdate (VI), hexaammonium heptamolybdate (VI), diammonium molybdate (VI), calcium molybdate (VI), molybdic acid (VI), phosphomolybdic acid (VI), molybdenum disulfide (VI), etc. One or more types of molybdenum compounds can be used.

[0102] <<Raw Vanadium Compound>> Examples of the raw vanadium compound include vanadium pentoxide (V), ammonium metavanadate (V), vanadium oxytrichloride (V), sodium metavanadate (V), potassium vanadate (V), sodium vanadate (V), vanadium tetrachloride (IV), vanadium oxysulfate (IV), vanadium oxydichloride (IV), vanadium oxide (IV), vanadium trichloride (IV), vanadium oxide (III), and hexavanadium trioxide (IV, V). One or more vanadium compounds can be used.

[0103] <<Raw Titanium Compound>> Examples of the raw titanium compound include titanium oxide, titanium dioxide, titanium trioxide, titanium tetrachloride, etc. One or more types of titanium compounds can be used.

[0104] <<Content of Raw Metal Compound>> From the viewpoint of improving the performance of the electrode and / or battery, the content of the raw metal compound in the calcined raw material is preferably 50 parts by mass or more and 300 parts by mass or less per 100 parts by mass of cellulose. The content is more preferably more than 50 parts by mass, even more preferably more than 60 parts by mass, even more preferably more than 70 parts by mass, and even more preferably more than 75 parts by mass. On the other hand, the content is more preferably less than 250 parts by mass, even more preferably less than 200 parts by mass, even more preferably less than 150 parts by mass, and even more preferably 100 parts by mass or less.

[0105] <<Median diameter of raw metal compound>> The raw metal compound is preferably pulverized before use as a firing raw material. The median diameter (d50) of the metal compound is preferably 12.00 μm or less, more preferably 10.00 μm or less, more preferably 8.00 μm or less, even more preferably 6.00 μm or less, even more preferably 4.00 μm or less, and even more preferably 3.00 μm or less. On the other hand, there is no particular restriction on the lower limit of the median diameter, but it is usually about 0.10 μm or more, and may be about 1.00 μm or about 2.00 μm. The median diameter can be measured by the method described above.

[0106] <Specific surface area of ​​raw metal compound> The specific surface area of ​​the raw metal compound is 1.0 m 2 / g or more, and more preferably 2.0m 2 / g or more, more preferably 3.0m 2 / g or more, more preferably 4.0m 2 / g or more, more preferably 4.5m 2 On the other hand, there is no particular upper limit to the specific surface area, but it is usually 40.0 m 2 / g or less, and 2 / g or less, and 2 The specific surface area can be measured using a fully automatic specific surface area measuring device Macsorb (HM-model 1201, manufactured by Mountec Co., Ltd.).

[0107] The raw metal compound having the above-mentioned median diameter or specific surface area can be prepared by a conventional method, for example, by pulverizing the raw metal compound using a pulverizer, such as a pulverizer manufactured by Japan Analytical Industry Co., Ltd. (e.g., JFC-2000).

[0108] <<Other Materials>> The raw material may contain other materials commonly used in this field, as desired. Examples of such raw materials include carbon materials.

[0109] <<Carbon Material>> In the electrode active material of this embodiment, the carbon material preferably has a graphite structure. Furthermore, the carbon material is preferably conductive. Examples of carbon materials include porous carbon materials such as activated carbon, graphite, carbon black, acetylene black, and ketjen black, as well as carbon fibers such as carbon fiber, vapor-grown carbon fiber (VGCF), carbon nanotubes (CNT), and carbon nanofiber, and nanocarbon materials in a form other than carbon fiber, such as graphene and fullerene. Among these, carbon fibers such as carbon fiber, vapor-grown carbon fiber (VGCF), CNT, and carbon nanofiber are preferred, with CNT being particularly preferred. One or more types of carbon materials can be used.

[0110] When the carbon material is carbon fiber, the form of the fibers constituting the carbon fiber is preferably such that the average fiber length is a predetermined value or more and the average fiber diameter is a predetermined value or less, from the viewpoint of improving the performance of the electrode and / or battery. This is because it is believed that the conductivity of the electrode active material can be improved. The average fiber length is preferably greater than 1 μm, more preferably greater than 1.5 μm, and even more preferably greater than 2 μm. There is no particular upper limit to the average fiber length, and it may be 100 μm, 50 μm, or 20 μm. In addition, the average fiber diameter is preferably less than 100 nm, more preferably less than 50 nm, and even more preferably less than 10 nm. There is no particular lower limit to the average fiber diameter, but it is usually about 1 nm.

[0111] The aspect ratio of the carbon material is preferably greater than 10, more preferably greater than 100, and even more preferably greater than 1,000, and is preferably less than 100,000, more preferably less than 50,000, and even more preferably less than 10,000.

[0112] From the viewpoint of the effect of the present invention, the carbon material has a specific surface area of ​​400 m 2 / g or more, and 2 The specific surface area is preferably 500 m / g or less. 2 / g or more is more preferable, and 600m 2 On the other hand, the specific surface area is preferably 2000 m / g or more. 2 / g or less is more preferable, and 1800m 2 The specific surface area is measured by the BET multipoint method.

[0113] From the viewpoint of the effects of the present invention, the carbon material preferably has a G / D ratio of 10 or more. The G / D ratio is more preferably 20 or more, even more preferably 30 or more, and even more preferably 40 or more. On the other hand, there is no particular upper limit to the G / D ratio, but a ratio of 50 or more can be said to be a carbon material with extremely few defects. Here, the G / D ratio refers to the ratio of a representative Raman shift peak in the Raman spectrum of the carbon material, and more specifically, the ratio of the G-band peak derived from the graphite structure to the D-band peak derived from defects. The Raman spectrum was measured using a RAMANTouch (excitation wavelength λ=532 nm, grating: 1200 gr / mm, resolution: 1.2 cm-1) manufactured by Nanophoton Co., Ltd.

[0114] From the viewpoint of the effects of the present invention, the carbon material preferably has a metal impurity content of 5% by mass or less. The metal impurity content is preferably 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less. The lower the metal impurity content, the better, and for example, 0.1% by mass is a sufficiently low metal impurity content. Note that the metal impurities referred to here refer to metal elements other than iron, molybdenum, vanadium, and titanium.

[0115] From the viewpoint of improving the performance of the electrode and / or battery, the content of the carbon material in the calcined raw material is preferably less than 5 parts by mass relative to 100 parts by mass of the cellulose. The content is more preferably less than 1 part by mass, and even more preferably less than 0.5 parts by mass. On the other hand, the content is preferably more than 0.05 parts by mass, more preferably more than 0.07 parts by mass, and even more preferably 0.10 parts by mass or more.

[0116] (Production Process) [Mixing Step (1)] The mixing step is a step of mixing raw materials containing cellulose and sulfur in an amount equal to or greater than the mass of the cellulose to obtain a calcined raw material.

[0117] The mixing method is not particularly limited as long as it is a method that sufficiently mixes these components, and can be carried out by a conventional method. In this embodiment, for example, a method can be used in which a raw material containing powdered cellulose and powdered sulfur in a mass equal to or greater than the mass of the cellulose is mixed directly using a blender or the like. Furthermore, when an optional component such as a raw metal compound or a carbon material is added to the raw material, the optional component can also be mixed together using a blender or the like.

[0118] Here, the term "powder" refers to a state in which the solid raw materials are sufficiently finely divided to be suitable for mixing for the purposes of the present invention. The size of each particle constituting the powder is not particularly limited as long as mixing is carried out appropriately, but is usually, for example, in the range of 1 μm to 40 μm. From the viewpoint of improving the performance of the electrode and / or battery, the particle size, in terms of median diameter, is preferably 2 μm or more, more preferably 3 μm or more, even more preferably 4 μm or more, and preferably 30 μm or less, even more preferably 20 μm or less, even more preferably 15 μm or less, and even more preferably 10 μm or less. The median diameter can be measured by the method described above.

[0119] The raw material thus obtained may be used as it is in the next firing step, or may be formed into pellets as desired and then used in the next step.

[0120] [Firing step (2)] The firing step is a step of firing the firing raw material obtained above to obtain a fired product. Firing can be performed by a conventional method, for example, by heating the firing raw material at a predetermined temperature increase rate until it reaches a predetermined temperature, maintaining the predetermined temperature for a predetermined time, and then allowing it to cool naturally.

[0121] <Non-oxidizing atmosphere> The firing is preferably carried out in a non-oxidizing atmosphere. A non-oxidizing atmosphere refers to an atmosphere that is substantially free of oxygen and is adopted to suppress oxidative degradation of the constituent components and excessive thermal decomposition. Specifically, the non-oxidizing atmosphere refers to an inert gas atmosphere such as nitrogen or argon, a sulfur gas atmosphere, an ammonia gas atmosphere, etc. Therefore, the firing can be suitably carried out, for example, in a quartz tube under an inert gas atmosphere.

[0122] <<Heating Rate>> The heating rate is preferably, for example, in the range of 50°C / h or more and 500°C / h or less. The heating rate is preferably 80°C / h or more, more preferably 100°C / h or more, and even more preferably 120°C / h or more. On the other hand, the heating rate is more preferably 400°C / h or less, even more preferably 300°C / h or less, and even more preferably 200°C / h or less. When the heating rate is within such a range, it tends to be easier to achieve the objective of improving the charge / discharge capacity and cycle characteristics.

[0123] <<Firing temperature and time>> The firing temperature refers to the temperature after the temperature rise of the raw materials is completed, and is maintained for a certain period of time to fire the raw materials. The temperature is preferably in the range of more than 250°C and less than 550°C. A temperature above 250°C tends to avoid insufficient sulfurization reaction and prevent a decrease in the charge / discharge capacity of the target product. On the other hand, a temperature below 550°C tends to prevent decomposition of the raw materials and prevent a decrease in yield and a decrease in charge / discharge capacity. The temperature is more preferably more than 270°C, even more preferably more than 290°C, and even more preferably 300°C or higher. On the other hand, the temperature is more preferably less than 500°C, even more preferably less than 470°C, and even more preferably 450°C or lower.

[0124] When a raw material metal compound is used as a raw material, the calcination temperature in the calcination step is preferably higher than the temperature at which the raw material metal compound thermally decomposes, from the viewpoint of improving the performance of the electrode and / or battery.

[0125] The time for maintaining the calcination temperature may be set appropriately depending on the type of raw material, the calcination temperature, etc., but is preferably, for example, 0.5 hours or more and 6 hours or less. A time of 0.5 hours or more tends to allow the calcination to proceed sufficiently, and a time of 6 hours or less tends to prevent excessive thermal decomposition of the constituent components. The time is more preferably 0.6 hours or more, and even more preferably 0.7 hours or more. On the other hand, the time is more preferably 4 hours or less, and even more preferably 2 hours or less.

[0126] <Apparatus> The calcination can be carried out, for example, using a muffle furnace (FIG. 1), or can be carried out using a continuous apparatus such as a twin-screw extruder. When a continuous apparatus is used, there is an advantage that the sulfur-based electrode active material can be continuously produced by a series of operations, such as kneading, pulverizing, and mixing the raw materials in the apparatus while calcining.

[0127] 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 the furnace. A thermocouple (not shown) is attached to the lid, allowing the temperature inside the furnace to be measured during firing. Inside the furnace, two trays 5 and 6, which are rectangular parallelepiped stainless steel reaction vessels for firing the raw materials, are installed on the upper and lower levels.

[0128] 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 attempting to exit the muffle furnace 1 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.

[0129] [Residue Removal Step] The residue removal step is a step of removing residues, such as unreacted sulfur that precipitates when the sulfur sublimated during the calcination cools, from the calcined product. These residues can cause deterioration in cycle characteristics, so it is desirable to remove as much of the residue as possible, if any. The residue removal can be carried out, for example, by subjecting the calcined product to a conventional method such as vacuum heating drying, hot air drying, or solvent washing.

[0130] [Pulverization step (3)] The pulverization step is a step of pulverizing the fired product to obtain particles of the fired product. The fired product is preferably pulverized to obtain particles of a size suitable for producing an electrode. The preferred size range of the particles of the electrode active material is as described above.

[0131] The pulverization can be carried out by a conventional method, for example, by subjecting the mixture to a pulverization treatment under predetermined conditions using a pulverizer such as a cutter mill or a jet mill. The pulverization conditions vary depending on the mill used, but for example, when using a cutter mill (e.g., a free speed mill, FS-20, manufactured by Labnect Co., Ltd.), the treatment can be carried out at a rotation speed of 20,000 rpm to 30,000 rpm and for 1 second to 30 seconds. Furthermore, when using a dry jet mill (e.g., a nanojetmizer, NJ-30, manufactured by Aisin Nano Technologies Co., Ltd.), the treatment can be carried out at a treatment speed of 1 g / min to 3 g / min and a pulverization pressure of 0.5 MPa to 2.0 MPa.

[0132] The particles of the pulverized fired product obtained above may be classified as desired to further make the particle size uniform. Classification can be carried out, for example, using a sieve with a desired mesh size.

[0133] In the firing method using the twin-screw extruder described above, the electrode active material can be produced and simultaneously pulverized into particles by shearing during kneading.

[0134] (Production of Electrode for Lithium-Ion Secondary Battery) Using the electrode active material obtained above, a lithium-ion secondary battery electrode having an electrode active material layer containing the electrode active material can be produced by a conventional method. That is, the electrode can be obtained in the same manner as in the production of a general electrode for a lithium-ion secondary battery, except that the electrode active material described above is used as the active material.

[0135] [When the electrode active material is used as the positive electrode active material] The positive electrode for a lithium ion secondary battery can be produced in the same manner as a general positive electrode for a lithium ion secondary battery, except that the above-mentioned electrode active material is used as the positive electrode active material. For example, the positive electrode can be produced by mixing the electrode 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 it. Alternatively, the positive electrode can be produced by kneading the electrode active material with the conductive additive, the binder, and a small amount of solvent in a mortar or the like, forming it into a film, and then pressing it onto a current collector using a press or the like.

[0136] <Conductive Aid> Examples of conductive aids include vapor-grown carbon fiber (VGCF), carbon powder, carbon black (CB), acetylene black (AB), ketjen black (KB), graphite, and fine powders of metals that are stable at a positive electrode potential, such as aluminum and titanium. Furthermore, conductive carbon materials among the above-mentioned carbon materials can also be used as the conductive aid. One or more of these conductive aids can be used.

[0137] <Binder> Examples of binders include polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamideimide (PAI), carboxymethyl cellulose (CMC), polyvinyl chloride (PVC), acrylic resin, methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), polypropylene (PP), etc. One or more of these binders can be used.

[0138] <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.

[0139] <<Bundling Amount>> The blending amounts of these materials constituting the positive electrode are not particularly limited, but for example, it is preferable to blend 2 to 100 parts by mass of a conductive additive, 2 to 50 parts by mass of a binder, and an appropriate amount of a solvent with respect to 100 parts by mass of an electrode active material.

[0140] <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 metal foils such as aluminum foil, aluminum mesh, punched aluminum sheet, aluminum expanded sheet, stainless steel foil, stainless steel mesh, punched stainless steel sheet, stainless steel expanded sheet, foamed nickel, nickel nonwoven fabric, copper foil, copper mesh, punched copper sheet, copper expanded sheet, titanium foil, and titanium mesh, as well as carbon nonwoven fabric and carbon woven fabric. Among these, current collectors containing metal foil are preferred. One type of current collector may be used, or two or more types may be used in combination. The surface of the current collector may be coated with carbon or the like. A specific example of such a current collector whose surface is coated with carbon or the like is carbon-coated aluminum foil. In this case, the current collector includes a carbon-coated portion.

[0141] [When the electrode active material is used as the negative electrode active material] The negative electrode for a lithium ion secondary battery can be produced in the same manner as a general negative electrode for a lithium ion secondary battery, except that the above-mentioned electrode active material is used as the negative electrode active material. For example, the negative electrode can be produced by mixing the electrode active material with a conductive additive, a binder, and a solvent to prepare a paste-like negative electrode material, applying the negative electrode material to a current collector, and then drying it. Alternatively, the negative electrode can be produced by kneading the electrode active material together with the conductive additive, the binder, and a small amount of solvent in a mortar or the like, forming it into a film, and then pressing it onto a current collector using a press or the like.

[0142] The conductive additive, binder, and solvent can be the same as those used in the above case where the electrode active material is used as the positive electrode active material, and the same current collector can also be used.

[0143] (Manufacturing of Lithium-Ion Secondary Battery) The lithium-ion secondary battery of this embodiment can be manufactured in the same manner as in the case of manufacturing a general lithium-ion secondary battery, except that the lithium-ion secondary battery electrode obtained above is used.

[0144] [When the electrode active material is used as a positive electrode active material] The lithium ion secondary battery of the present embodiment can be produced according to a conventional method by using a positive electrode containing the electrode active material (positive electrode active material), a negative electrode, an electrolyte, and, if desired, components such as a separator.

[0145] <Negative Electrode> As the negative electrode material, 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 can be used. When a lithium-free material is used as the negative electrode material, for example, a carbon-based material, silicon-based material, or alloy-based material among the above-mentioned negative electrode materials, it is advantageous in that it is less likely to cause a short circuit between the positive and negative electrodes due to the generation of dendrites. However, when these lithium-free negative electrode materials are used in combination with the positive electrode of this embodiment, neither the positive electrode nor the negative electrode contains lithium. For this reason, a lithium pre-doping process is required to pre-insert lithium into either the negative electrode or the positive electrode, or both. Known methods can be used for pre-doping lithium. For example, when doping lithium into the negative electrode, a method is used in which a half-cell is assembled using metallic lithium as the counter electrode and lithium is inserted by an electrolytic doping method in which lithium is electrochemically doped, or a method is used in which metallic lithium foil is attached to the electrode and then left in an electrolyte solution to dope lithium into the electrode by utilizing the diffusion of lithium 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.

[0146] <Electrolyte> The electrolyte compensates for the charge generated by the release of electrons into an external circuit due to the oxidation / reduction of the electrode active material at the positive and negative electrodes with the flow of ions. The electrolyte used in lithium-ion secondary batteries can be an electrolyte obtained by dissolving an alkali metal salt in an organic solvent. The organic solvent is preferably at least one selected from non-aqueous solvents such as dimethoxyethane, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl ether, γ-butyrolactone, and acetonitrile. Examples of electrolytes that can be used include Li(FSO2)2N, LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiI, and LiClO4. The electrolyte concentration may be approximately 0.5 mol / L to 5.0 mol / L. The electrolyte is not limited to a liquid form. For example, when the lithium ion secondary battery is a lithium polymer secondary battery, the electrolyte is in a solid state (for example, a polymer gel state).

[0147] <<Separator>> A lithium ion secondary battery may include components such as a separator in addition to the above-mentioned negative electrode, positive electrode, and electrolyte. The separator is interposed between the positive electrode and negative electrode, allowing ion migration 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, it is preferable to use a thin, microporous or nonwoven membrane made of a material such as polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide, cellulose, or glass.

[0148] <<Shape>> The shape of the lithium ion secondary battery is not particularly limited, and various shapes such as a cylindrical shape, a stacked shape, a coin shape, a laminated shape, and a button shape can be used.

[0149] [When the electrode active material is used as a negative electrode active material] The lithium ion secondary battery of the present embodiment can be produced in accordance with a conventional method by using a negative electrode containing the electrode active material (negative electrode active material), a positive electrode, an electrolyte, and, if desired, components such as a separator.

[0150] <<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 LiNiO, LiNiO, LiMn ... 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 0.5 O4, etc. These compounds may be used alone or in combination.

[0151] Others The electrolyte, separator, and shape of the lithium ion secondary battery can be the same as those in the above case where the electrode active material is used as the positive electrode active material.

[0152] The present invention will be described based on examples, but the present invention is not limited to only the examples.

[0153] The various chemicals used in the examples and comparative examples are listed below. The various chemicals were purified according to conventional methods as necessary.

[0154] <Materials used in the test> Cellulose: cellulose (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., white powder, passing 38 μm (400 mesh)) Diene rubber: high cis butadiene rubber (UBEPOL (registered trademark) BR150L manufactured by UBE Corporation, cis content (amount of cis-1,4-bonded butadiene units): 98% by mass) PP: polypropylene (available from Sigma-Aldrich, catalog number 428116, melting point: 157°C) Crosslinked PMMA: crosslinked polymethyl methacrylate (Techpolymer MB30X-8 manufactured by Sekisui Plastics Co., Ltd., spherical acrylic resin consisting of methyl methacrylate and ethylene glycol dimethacrylate copolymer, particle size: 8 μm) Sulfur: precipitated sulfur manufactured by Tsurumi Chemical Industry Co., Ltd.

[0155] Production Example 1 (Pulverization of Raw Materials) Before being used as raw materials, cellulose and polypropylene were each pulverized for 10 minutes in a freeze pulverizer (JFC-2000 manufactured by Japan Analytical Industry Co., Ltd.).

[0156] <Preparation of Electrode Active Material> (Mixing Step) Raw materials were charged according to the blending ratios in the table below and mixed using a blender (LAB MILL manufactured by Osaka Chemical Co., Ltd.) to obtain a fired raw material.

[0157] (Firing step) The raw materials were fired in a muffle furnace (Fig. 1). The muffle furnace in Fig. 1 was as described above.

[0158] First, the firing raw materials were placed in a tray, which was a stainless steel reaction vessel, 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 began. The temperature was increased at a rate of 5 ° C. / min, and when the temperature of the firing raw materials reached the firing temperature listed in Table 1, heat treatment was performed for 45 minutes while maintaining that temperature. Next, while adjusting the flow rate of Ar gas, the temperature of the fired product was naturally cooled to 25 ° C. under an Ar gas atmosphere, and the fired product was then removed from the muffle furnace.

[0159] (Removal of Unreacted Sulfur) To remove unreacted sulfur (free elemental sulfur) remaining in the product after the calcination step, the following step was carried out. That is, the calcined product was pulverized in a mortar, and the pulverized product was placed in a glass tube oven and heated at 290°C for 3 hours while evacuating to obtain an electrode active material from which unreacted sulfur had been removed (or which contained only a trace amount of unreacted sulfur). The temperature increase rate was 10°C / min.

[0160] (Pulverization Step) The calcined product from which the unreacted sulfur had been removed was pulverized using a cutter mill (LAB MILL manufactured by Osaka Chemical Co., Ltd.).

[0161] (Classification Operation) In order to remove coarse particles from the pulverized fired product, the product was classified using a stainless steel sieve with a mesh of 32 μm to obtain an electrode active material.

[0162] <Physical Properties of Electrode Active Material> The electrode active material obtained above was examined for the following properties.

[0163] (Particle size distribution, median diameter) The particle size distribution was measured using a laser diffraction / scattering particle size distribution analyzer (Anton Paar PSA1090L particle size distribution analyzer) with water as a dispersion medium to obtain a particle size distribution curve. From the particle size distribution curve, the volume-based cumulative 50% diameter (median diameter d50) was measured.

[0164] (Elemental Composition) The elemental amounts of carbon, hydrogen, nitrogen, and sulfur were calculated as a mass ratio (%) of the total amount of the electrode active material from the mass measured using a fully automatic elemental analyzer, Vario MICRO Cube, manufactured by Elementar. The elemental amount of oxygen was calculated as a mass ratio (%) of the total amount of the electrode active material from the mass measured using an oxygen / nitrogen / hydrogen analyzer, EMGA-930, manufactured by Horiba, Ltd.

[0165] <Fabrication of Lithium-Ion Secondary Battery> A lithium-ion secondary battery was fabricated as follows.

[0166] (Positive electrode) The electrode active material obtained above was used as the active material, acetylene black (manufactured by Denka Co., Ltd., HS-100) as a conductive additive, and acrylic resin (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., average molecular weight 2700 to 7500) as a binder. These were weighed so that the ratio was active material: acetylene black: binder = 85:10:5 (mass%), placed in a container, and stirred and mixed using a centrifugal mixer (made by Thinky Corporation, ARE-250) using milliQ water as a dispersant to prepare a uniform slurry. The prepared slurry was applied to 17 μm aluminum foil using an applicator with a slit width of 100 μm, and the compressed electrode was heated in a dryer at 120 ° C. for 3 hours, dried, and punched to φ11 mm to obtain an electrode (positive electrode). Thereafter, the mass of the electrode was measured, and the amount of active material in the electrode was calculated from the above ratio.

[0167] (Negative Electrode) A metallic lithium foil (disk-shaped, 14 mm in diameter and 500 μm in thickness, manufactured by Honjo Metals Co., Ltd.) was used as the negative electrode. A stainless steel sheet was used as the negative electrode current collector.

[0168] (Electrolyte) A non-aqueous electrolyte prepared by dissolving LiPF in a mixed solvent of ethylene carbonate and diethyl carbonate was used as the electrolyte. The ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1:1. The concentration of LiPF in the electrolyte was 1.0 mol / L.

[0169] (Lithium-ion secondary battery) A coin battery was fabricated using the above positive and negative electrodes. Specifically, in a dry room, a separator (Celgard 2400, a 25 μm thick polypropylene microporous membrane manufactured by Celgard) and a glass nonwoven fabric filter (440 μm thick, GA100 manufactured by Advantec) were sandwiched between the positive and negative electrodes to form an electrode battery. This electrode 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 solution was poured into the battery case. The volume of the electrolyte was 0.28 mL. The battery case was sealed using a crimping machine to obtain the lithium-ion secondary batteries of each example and comparative example.

[0170] <Evaluation of Lithium-Ion Secondary Battery> (Discharge Capacity, Capacity Retention Rate) The coin-type lithium-ion secondary batteries produced in each Example and Comparative Example were charged and discharged at a current value corresponding to 50 mA per 1 g of positive electrode active material for the first to tenth cycles at a test temperature of 30°C, and at a current value corresponding to 100 mA for the eleventh to twentieth cycles. The discharge end voltage was 1.0 V, and the charge end voltage was 3.0 V. Charge and discharge were repeated, and the battery discharge capacity (mAh) was observed for the first, second, tenth, and twentieth cycles. The measurements were performed using a battery performance evaluation device (BLS System, manufactured by Measuring Instrument Center Co., Ltd.).

[0171] The second discharge capacity DC3 (mAh / g) was taken as the initial capacity. The larger the initial capacity, the larger the charge / discharge capacity of the lithium ion secondary battery, and the more preferable it can be evaluated. In addition, the second discharge capacity DC2 (mAh / g) and the 20th discharge capacity DC 20 The capacity retention rate (%) was calculated from the capacity (mAh / g) using the following formula. The higher the capacity retention rate, the better the cycle characteristics of the lithium ion secondary battery. Capacity retention rate (%) = (DC 20 / DC2) x 100

[0172] The results are shown in Table 1 below.

[0173]

[0174] Examples 1 to 4 showed higher initial discharge capacities than Comparative Examples 2 and 3. This is thought to be because the high sulfur content allows more binding with lithium ions. Note that, despite the high sulfur content of Comparative Example 1, the initial discharge capacity was only lower than that of Examples 1 to 4. Furthermore, Examples 1 to 4 showed higher capacity retention rates than Comparative Examples 1, 3, and 4. This is thought to be because the sulfur content was sufficiently high and the oxygen content was high, suppressing side reactions in the charge-discharge reaction.

[0175] <Embodiments> Preferred embodiments are described below.

[0176] [1] An electrode active material comprising particles containing an organic sulfur compound, wherein the oxygen content (mass%) in the electrode active material is A Oand the sulfur content (mass%) is A S In the case where A O and A S and satisfy the following formula, or at least one of the right sides of formulas (1) to (3) takes a more preferred value, wherein the preferred value for the right side of formula (1) is 10.0, the preferred value for the right side of formula (2) is 46.0, more preferably 47.0, even more preferably 48.0, and even more preferably 49.0, and the preferred value for the right side of formula (3) is 560. O >9.0 (2) A S >45.0 (3) A O ×A S >550 [2] The electrode active material according to the above [1], wherein the right side of formula (1) is 11.0, preferably 12.0, more preferably 13.0, even more preferably 14.0, even more preferably 15.0, even more preferably 16.0, even more preferably 17.0, and even more preferably 18.0. [3] The electrode active material according to the above [1] or [2], wherein the right side of formula (2) is 50.0, preferably 51.0, more preferably 52.0, even more preferably 53.0, even more preferably 54.0, even more preferably 55.0, even more preferably 56.0, even more preferably 57.0, even more preferably 58.0, and even more preferably 59.0. [4] The electrode active material according to any one of the above [1] to [3], wherein the right side of formula (3) is 570, preferably 600, more preferably 650, even more preferably 700, even more preferably 750, even more preferably 800, even more preferably 830, and even more preferably 840. [5] The electrode active material according to any one of [1] to [4] above, wherein the active material further comprises a metal compound containing at least one metal selected from the group consisting of iron, molybdenum, vanadium, and titanium. [6] The electrode active material according to [5] above, wherein the metal compound is an iron compound. [7] An electrode comprising the electrode active material according to any one of [1] to [6] above. [8] The electrode comprises a current collector, the current collector comprises a metal foil, and the coating density (mg / cm) of the electrode active material on the current collector is 2 ) is D, then D and A O and A Sand satisfy the following formula, or the right side of formula (4) is preferably 1500, more preferably 1700, even more preferably 1900, even more preferably 2100, even more preferably 2200, and even more preferably 2300. (4) The electrode according to the above [7], O ×A S )>1000 [9] The electrode includes a current collector, the current collector includes a metal foil, and the coating density (mg / cm 2 ) is D and the thickness of the metal foil is T (μm), D, T and A O and A S and satisfy the following formula, or the right side of formula (5) is preferably 80, more preferably 90, even more preferably 110, even more preferably 120, and even more preferably 130. (5) The electrode according to [7] or [8] above, O ×A S ) / T>60

[10] The electrode includes a current collector, the current collector includes a metal foil, and the coating density (mg / cm 2 ) is D, D is 2.50 mg / cm 2 More than 3.00 mg / cm 2 More preferably, greater than 3.50 mg / cm 2 More preferably, greater than 3.80 mg / cm 2 More preferably, greater than 3.90 mg / cm 2

[11] The electrode according to any one of [7] to [9] above, wherein the electrode is a positive electrode.

[12] A lithium ion secondary battery comprising the electrode according to any one of [7] to

[11] above.

[13] A lithium ion secondary battery further comprising an electrolyte, wherein the volume of the electrolyte is V (mL), and the coating density of the electrode active material on the current collector (mg / cm 2 ) is D, then V, D and A O and A Sand satisfy the following formula, or the right side of formula (6) is preferably 4500, more preferably 5000, even more preferably 5500, even more preferably 6000, even more preferably 7000, even more preferably 7500, and even more preferably 8000. (6) D×(A O ×A S ) / V>4000

[14] A method for producing an electrode active material, comprising: (1) a mixing step of mixing a raw material containing cellulose and sulfur in a mass equal to or greater than the mass of the cellulose to obtain a raw material for calcination, (2) a calcination step of calcining the raw material for calcination to obtain a calcined product, and (3) a granulation step of pulverizing the calcined product to obtain particles of the calcined product.

[15] The production method according to

[14] above, wherein the cellulose is cellulose that has not been chemically modified.

[0177] REFERENCE SIGNS LIST 1 muffle furnace 2 heater 3 lid 4 inert gas 5 tray (upper stage) 6 tray (lower stage) 7 gas inlet pipe 8 gas outlet pipe 9 sodium hydroxide aqueous solution 10 trap tank

Claims

1. An electrode active material comprising particles containing an organic sulfur compound, wherein the oxygen content (mass%) in the electrode active material is A O and the sulfur content (mass%) is A S In the case where A O and A S and an electrode active material that satisfies the following formula. O >9.0 (2) A S >45.0 (3) A O ×A S >550 2. The electrode active material according to claim 1, wherein the right side of formula (1) is 11.

0.

3. The electrode active material according to claim 1, wherein the right side of formula (2) is 50.

0.

4. The electrode active material according to claim 1, wherein the right side of formula (3) is 570.

5. The electrode active material according to any one of claims 1 to 4, wherein the active material further comprises a metal compound containing at least one metal selected from the group consisting of iron, molybdenum, vanadium and titanium.

6. The electrode active material of claim 5, wherein said metal compound is an iron compound.

7. An electrode comprising the electrode active material according to any one of claims 1 to 4.

8. The electrode includes a current collector, the current collector includes a metal foil, and the coating density (mg / cm) of the electrode active material on the current collector is 2 ) is D, then D and A O and A S The electrode according to claim 7, wherein D x (A O ×A S )>1000 9. The electrode comprises a current collector, the current collector includes a metal foil, and the coating density (mg / cm) of the electrode active material on the current collector 2 ) is D and the thickness of the metal foil is T (μm), D, T and A O and A S The electrode according to claim 7, wherein D x (A O ×A S ) / T>60 10. The electrode comprises a current collector, the current collector includes a metal foil, and the coating density (mg / cm) of the electrode active material on the current collector 2 ) is D, D is 2.50 mg / cm 2 The electrode of claim 7 , 11. The electrode of claim 7, wherein said electrode is a positive electrode.

12. A lithium ion secondary battery comprising the electrode according to claim 7.

13. The electrode further comprises an electrolyte, the volume of the electrolyte being V (mL), and the coating density of the electrode active material on the current collector (mg / cm 2 ) is D, then V, D and A O and A S The lithium ion secondary battery according to claim 12, wherein D x (A O ×A S ) / V>4000 14. A method for producing an electrode active material, comprising: (1) a mixing step of mixing a raw material containing cellulose and sulfur in a mass equal to or greater than the mass of the cellulose to obtain a raw material for sintering; (2) a firing step of sintering the raw material for sintering to obtain a sintered product; and (3) a granulation step of pulverizing the sintered product to obtain particles of the sintered product.

15. The method of claim 14, wherein the cellulose is chemically unmodified cellulose.

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