Electrode active material, electrode, and lithium ion secondary battery

Abstract

The purpose of the present invention is to improve the charge / discharge capacity and capacity retention rate of an electrode active material constituting a lithium ion secondary battery electrode. The present invention provides an electrode active material comprising particles containing an organic sulfur compound, in which AO and AS satisfy the following inequality: (1) AOgt; 9.0 (2) ASgt; 45.0 (3): AO * ASgt; in the formula 550, AO represents the oxygen content in the electrode active material in percentage by mass, and AS represents the sulfur content in the electrode active material in percentage by mass.

Description

Technical Field

[0001] This invention relates to a novel electrode active material, an electrode containing the electrode active material, and a lithium-ion secondary battery containing the electrode. 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, with further performance improvements expected.

[0003] Patent document 1 describes a positive electrode active material obtained by heat-treating high cis-butadiene rubber with sulfur and a vulcanization accelerator, and patent document 2 describes an electrode active material obtained by calcining polymethyl methacrylate with sulfur.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: WO2015 / 050086

[0007] Patent Document 2: JP2021-172814A Summary of the Invention

[0008] The problem the invention aims to solve

[0009] However, the active materials in patent documents 1 and 2 have only low oxygen content, and there is still room for improvement in their charge / discharge capacity and capacity retention.

[0010] The purpose of this invention is to provide a new electrode active material that can improve charge and discharge capacity and capacity retention, an electrode (i.e., a positive or negative electrode) containing the electrode active material, and a lithium-ion secondary battery containing the electrode.

[0011] Problem Solving Methods

[0012] This invention relates to the following electrode active materials:

[0013] An electrode active material comprising particles containing organic sulfur compounds.

[0014] Among them, A O and A S The following inequalities must be satisfied:

[0015] (1) A O >9.0

[0016] (2) A S >45.0

[0017] (3) A O ×AS >550

[0018] In the formula, A O Indicates the oxygen content in the electrode active material, expressed as a percentage by mass, A S This indicates the sulfur content in the electrode active material, expressed as a percentage by mass.

[0019] Invention Effects

[0020] According to the present invention, a novel electrode active material capable of improving charge-discharge capacity and capacity retention rate, an electrode (i.e., a positive or negative electrode) containing the electrode active material, and a lithium-ion secondary battery containing the electrode can be provided.

[0021] Although not intended to be limited to theory, the following can be considered as the reasons for the improved charge / discharge capacity and capacity retention in this invention. That is, sulfur, as an active material, can be repeatedly charged and discharged through a reversible electrochemical reaction with lithium. Furthermore, in this invention, similar to the prior art, sulfur is mixed with a polymer and fixed onto the polymer by heat treatment to suppress dissolution, which is believed to help achieve a reversible reaction. Therefore, it can be considered that the more sulfur atoms there are, the greater the charge / discharge capacity. On the other hand, in this invention, oxygen atoms are bonded to carbon or sulfur to form CO bonds, etc. Therefore, (1) the active material containing these polar bonds improves the dispersibility of the active material in an electrode slurry with water as the solvent, thereby preparing a uniform electrode; accordingly, it is believed that the cycle capacity is easily maintained even after experiencing expansion and contraction associated with charge / discharge cycles. Furthermore, (2) the active material framework contains a high oxygen content, which allows the SS bonds to be more stably maintained inside the active material structure; this is further believed to make the active material structure more capable of retaining sulfur when sulfur reacts with lithium during lithium-ion insertion and extraction processes caused by charge and discharge. Therefore, it can be assumed that the cycle capacity is easily maintained even after undergoing electrochemical reactions associated with charge-discharge cycles.

[0022] In this article, the term "cycle characteristics" refers to the ability of a secondary battery to retain its charge and discharge capacity after repeated charging and discharging. 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

[0023] Figure 1 This is a cross-sectional schematic diagram of the reaction apparatus used to prepare electrode active materials in an embodiment of the present invention. Detailed Implementation

[0024] The following describes embodiments relating to the present invention. Furthermore, the upper and lower limits of numerical ranges, related to terms such as "above," "below," "higher than," and "lower than," can be arbitrarily combined, and the values ​​in the embodiments can also serve as these upper and / or lower limits. Moreover, unless contrary to the purpose of the present invention, numerical ranges including lower or upper limits are understood to simultaneously disclose numerical ranges excluding upper or lower limits; conversely, unless contrary to the purpose of the present invention, numerical ranges excluding lower or upper limits are understood to simultaneously disclose numerical ranges including lower or upper limits.

[0025] One embodiment of the present invention is an electrode active material comprising particles containing an organic sulfur compound, wherein A O and A S The following inequalities must be satisfied:

[0026] (1) A O >9.0

[0027] (2) A S >45.0

[0028] (3) A O ×A S >550

[0029] In the formula, A O Indicates the oxygen content in the electrode active material, expressed as a percentage by mass, A S This indicates the sulfur content in the electrode active material, expressed as a percentage by mass.

[0030] The right side of inequality (1) is preferably 11.0. The right side of inequality (2) is preferably 50.0. The right side of inequality (3) is preferably 570. It can be considered that satisfying at least any one of the inequalities under more stringent conditions can further achieve the effects of the present invention.

[0031] Preferably, the active material further comprises a metal compound, said metal compound comprising at least one metal selected from iron, molybdenum, vanadium and titanium.

[0032] The preferred metal compound is an iron compound.

[0033] Another embodiment of the present invention is an electrode comprising an electrode active material.

[0034] Preferably, the electrode has a current collector, the current collector having a metal foil, and D, A O and A S The following inequalities must be satisfied:

[0035] (4) D×(A) O ×A S >1000,

[0036] In the formula, D represents the coating density of the electrode active material on the current collector, with units of mg / cm³. 2 .

[0037] It is believed that the performance of the electrode and / or battery can be improved by setting the product of coating density, oxygen content, and sulfur content to a value exceeding a predetermined value.

[0038] Preferably, the electrode has a current collector, the current collector having a metal foil, and D, T, A O and A S The following inequalities must be satisfied:

[0039] (5) D×(A) O ×A S ) / T>60

[0040] In the formula, D represents the coating density of the electrode active material on the current collector, with units of mg / cm³. 2 T represents the thickness of the metal foil, in μm.

[0041] It is believed that the performance of electrodes and / or batteries can be improved by satisfying the above inequalities in coating density, oxygen content, sulfur content, and metal foil thickness.

[0042] Preferably, the electrode has a current collector, which has a metal foil and a density (D) greater than 2.50 mg / cm². 2 Where D represents the coating density of the electrode active material on the current collector, in mg / cm³. 2 .

[0043] The electrode is preferably a positive electrode.

[0044] It is believed that using the electrode as the positive electrode can improve the performance of the electrode and / or the battery.

[0045] Another embodiment of the present invention is a lithium-ion secondary battery including electrodes.

[0046] Preferably, the lithium-ion secondary battery further includes an electrolyte, and D, V, A O and A S The following inequalities must be satisfied:

[0047] (6) D×(A) O ×A S ) / V>4000

[0048] In the formula, V represents the volume of the electrolyte in mL, and D represents the coating density of the electrode active material on the current collector in mg / cm³. 2 .

[0049] It is believed that the performance of electrodes and / or batteries can be improved by satisfying the above inequalities in coating density, oxygen content, sulfur content, and metal foil thickness.

[0050] Another embodiment of the present invention is a method for preparing electrode active materials, the method comprising:

[0051] (1) Mixing step: The raw materials containing cellulose and sulfur are mixed to obtain calcined raw materials, wherein the mass of sulfur is equal to or greater than the mass of cellulose;

[0052] (2) Calcination step: Calcining the raw material to obtain the calcined material; and

[0053] (3) Crushing step: The calcined material is crushed to obtain calcined material particles.

[0054] Cellulose is preferably unmodified cellulose.

[0055] This is because it is believed that not chemically modified can avoid insufficient reaction between sulfur and cellulose due to chemical modification, thereby preventing a decrease in sulfur content.

[0056] <Definition>

[0057] "Particles" refers to the state in which the electrode active material is refined to a point where it can be mixed with other materials to achieve the objectives of this invention. There are no particular limitations on the size of the particles constituting the electrode active material, as long as the aforementioned mixing can be carried out appropriately. For example, if "particles" are expressed in terms of median particle size, the range can be from 1 nm to 1000 μm.

[0058] Unless otherwise stated, “particle size” is expressed as median particle size (d50).

[0059] "Electrode active materials" are one type of battery electrode material, referring to materials that participate in the power generation reaction. Electrode active materials include positive electrode active materials and negative electrode active materials.

[0060] "Oxygen content in electrode active materials" refers to the content of oxygen element in electrode active materials, expressed as a percentage by mass.

[0061] "Sulfur content in electrode active materials" refers to the content of sulfur element in electrode active materials, expressed as a percentage by mass.

[0062] "Active materials" refer to the materials responsible for the redox reactions that occur during energy conversion in lithium-ion secondary batteries.

[0063] "Coating density" refers to the density of a single unit area (in cm²) on the current collector. 2 The mass of the active material coated (in mg).

[0064] "Electrolyte volume" refers to the total volume of the electrolyte or solid electrolyte containing the solute. The unit is mL.

[0065] Unless otherwise stated, “initial discharge capacity” refers to the second discharge capacity.

[0066] <Measurement Method>

[0067] The content of oxygen, sulfur, etc., in the electrode active material was determined using the methods described in the examples. Specifically, the content of carbon, hydrogen, nitrogen, and sulfur was determined using an Elementar vario MICROcube fully automated elemental analyzer via oxygen flow combustion-infrared absorption. The content of oxygen was determined using an EMGA-930 oxygen / nitrogen / hydrogen analyzer manufactured by Horiba Ltd. via inert gas melting-infrared absorption.

[0068] "Particle size distribution" was measured using a laser diffraction / scattering particle size analyzer (PSA1090L particle size analyzer manufactured by Anton Paar GmbH) with water as the dispersion medium.

[0069] Unless otherwise stated, “median particle size” refers to the particle size that accounts for 50% of the total particle size distribution on a volume basis, in μm.

[0070] The electrode active material, electrode, and lithium-ion secondary battery in this embodiment will be described below.

[0071] <Electrode Active Materials>

[0072] The electrode active material is an electrode active material composed of particles containing organic sulfur compounds, as shown in the above inequalities (1) to (3). The oxygen content A of the electrode active material is... O (Unit: mass percentage) and sulfur content A S (Unit: percentage by mass) all exceeded the predetermined value, and A O and A S The product also exceeded the predetermined value.

[0073] Organic sulfur compounds refer to compounds formed by combining sulfur with organic compounds such as cellulose under a non-oxidizing atmosphere. In this embodiment, there are no particular restrictions on organic sulfur compounds as long as the organic sulfur compounds form particles and the electrode active material composed of such particles satisfies the above inequalities (1) to (3).

[0074] (Inequalities (1) to (3))

[0075] A O and A S The following inequalities must be satisfied:

[0076] (1) A O >9.0

[0077] (2) A S >45.0

[0078] (3) A O ×A S >550

[0079] In the formula, A O Indicates the oxygen content in the electrode active material, expressed as a percentage by mass, A S This indicates the sulfur content in the electrode active material, expressed as a percentage by mass.

[0080] The right side of inequality (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, even more preferably 18.0. A O There is no specific upper limit to the value, but it can be assumed that its reference value is approximately 30.00.

[0081] By using organic compounds containing more oxygen atoms as calcination raw materials, the efficiency of A can be improved. O Conversely, by using organic compounds containing fewer oxygen atoms as calcination raw materials, the amount of A can be reduced. O Examples of organic compounds containing a large number of oxygen atoms include cellulose.

[0082] (Inequality (2))

[0083] The right side of inequality (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 There is no specific upper limit to the value, but it can be assumed that its reference value is approximately 70.0.

[0084] By using more sulfur as a calcination feedstock, A can be improved. S Conversely, by using less sulfur as a calcination feedstock, A can be reduced. S .

[0085] (Inequality (3))

[0086] The right side of inequality (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, even more preferably 840. A O ×A S There is no specific upper limit, but it can be assumed that its reference value is about 1500.

[0087] A O ×A S You can adjust A separately O and A S Adjust the value accordingly.

[0088] (Elements other than oxygen and sulfur)

[0089] Electrode active materials can contain elements other than oxygen and sulfur. Examples of such elements include carbon, hydrogen, and nitrogen.

[0090] [Carbon content]

[0091] From the perspective of improving electrode and / or battery performance, the carbon content is preferably greater than 5.0% by mass, more preferably greater than 10.0% by mass, and even more preferably greater than 15.0% by mass. On the other hand, the content of this element is preferably less than 50.0% by mass, more preferably less than 45.0% by mass, and even more preferably less than 40.0% by mass.

[0092] [Hydrogen content]

[0093] Hydrogen (H) in organic compounds reacts with sulfur during calcination to form hydrogen sulfide, which is then released to the outside of the system. Therefore, the hydrogen content in the electrode active material is preferably less than 1.0% by mass, more preferably less than 0.7% by mass, and even more preferably less than 0.5% by mass. When it is less than 1.0% by mass, the calcination (sulfidation reaction) is often sufficient. Therefore, under these conditions, the charge-discharge capacity tends to improve.

[0094] [Nitrogen content]

[0095] If a nitrogen source is not used as a raw material, the nitrogen content (in mass percentage) in the electrode active material can be 0% by mass. For example, if a compound containing nitrogen atoms is used as an organic compound, nitrogen can be detected.

[0096] (Metal element content)

[0097] Preferably, the electrode active material further comprises a metal compound, said metal compound comprising at least one metal selected from iron, molybdenum, vanadium, and titanium. Furthermore, the metal compound is preferably an iron compound. More than one metal compound may be used.

[0098] When the electrode active material contains a metal compound, from the perspective of improving electrode and / or battery performance, the content of the metal element (in mass percentage) is preferably greater than 10.0% by mass, more preferably greater than 15.0% by mass, and even more preferably greater than 20.0% by mass, while the content 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. In this document, when the metal compound contains multiple metal elements, the content of the metal element refers to the total amount of the multiple metal elements.

[0099] (Median particle size)

[0100] The electrode active material is composed of particles with a particle size suitable for electrode fabrication. From the perspective of improving electrode and / or battery performance, the preferred range for the particle size of the electrode active material, expressed as median particle size (median particle size d50), is greater than 1.0 μm and less than 40.0 μm. More preferably, the median particle size is greater than 1.5 μm, further preferably greater than 2.0 μm, and even more preferably greater than 3.0 μm. Furthermore, more preferably, the median particle size is less than 30.0 μm, further 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 particle size can be determined by the method described in the Examples section below.

[0101] (Other components)

[0102] The electrode active material of this embodiment may contain the same material as the material described in the preparation method section below, and its usage method is also the same as that described in that section.

[0103] <Electrode>

[0104] One embodiment of the present invention relates to an electrode comprising the aforementioned electrode active material. The electrode is preferably prepared by mixing the aforementioned electrode active material with other electrode materials such as conductive additives and binders as needed, and then coating the mixture onto a current collector.

[0105] The electrode described in this embodiment can be used in lithium-ion secondary batteries. It can be manufactured using the materials described in the preparation method section below and configured as described therein. Specifically, when the electrode is used as a positive electrode, the conductive additives, binders, current collectors, etc., described in the preparation method section below can be used to construct the positive electrode of the lithium-ion secondary battery as described therein. Similarly, when the electrode is used as a negative electrode, the conductive additives, binders, current collectors, etc., described in the preparation method section below can be used to construct the negative electrode of the lithium-ion secondary battery as described therein. Therefore, the description in the preparation method section below can be considered a description of this electrode.

[0106] Preferably, the electrode in this embodiment has a current collector, the current collector having a metal foil, and D, A O and A S The following inequalities must be satisfied:

[0107] (4) D×(A) O ×A S >1000

[0108] In the formula, D represents the coating density of the electrode active material on the current collector, with units of mg / cm³. 2 .

[0109] (Inequality (4))

[0110] The right side of inequality (4) is more preferably 1500, further preferably 1700, further preferably 1900, further preferably 2100, further preferably 2200, and further preferably 2300. In addition, the higher the value on the left side of inequality (4), the better, and setting an upper limit is not practically meaningful, but it can be assumed that its reference value is about 5000.

[0111] (Coating density)

[0112] The coating density D of the electrode active material on the electrode (unit: mg / cm³) 2 Preferred concentration is greater than 2.50 mg / cm³. 2 More preferably greater than 3.00 mg / cm³ 2 Further preferred concentrations are greater than 3.50 mg / cm³. 2 Further preferred concentrations are greater than 3.80 mg / cm³. 2 Further preferred concentrations are greater than 3.90 mg / cm³. 2 A higher coating density value is preferred, and setting an upper limit is not practically meaningful, but a reference value of approximately 15.0 mg / cm³ can be assumed. 2 .

[0113] (Inequality (5))

[0114] Preferably, the electrode in this embodiment has a current collector, the current collector having a metal foil, and D, T, A O and A S The following inequalities must be satisfied:

[0115] (5) D×(A) O ×A S ) / T>60

[0116] In the formula, D represents the coating density of the electrode active material on the current collector, with units of mg / cm³. 2 T represents the thickness of the metal foil, in μm.

[0117] The right side of inequality (5) is preferably 80, more preferably 90, even more preferably 110, even more preferably 120, and even more preferably 130. The higher the value on the left side of inequality (5), the better, and setting an upper limit is not practically meaningful, but it can be assumed that its reference value is about 300.

[0118] (Thickness of the metal foil)

[0119] The thickness T (in μm) of the metal foil is preferably 5 μm or more, more preferably 10 μm or more. On the other hand, T is preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 25 μm or less.

[0120] (charge and discharge capacity)

[0121] The electrode of this embodiment exhibits excellent charge-discharge capacity. The second discharge capacity (DC2), i.e., the initial discharge capacity, refers to the discharge capacity after two charge-discharge cycles (1.0V discharge termination voltage and 3.0V charge termination voltage) following electrode and battery fabrication (i.e., the second discharge measured when the charge-discharge sequence is performed: first discharge, first charge, second discharge, second charge). During discharge, a constant current (50mA per gram of positive electrode active material) is applied, and the voltage eventually drops from 3.0V to 1.0V. The total time (in hours) required for the voltage to drop from 3.0V to 1.0V is measured, multiplied by the applied current (in mA) to obtain the capacity (in mAh), and then divided by the weight of the active material to obtain the specific capacity (in mAh / g). Conversely, during charging, the voltage increases with constant current charging, and charging terminates when it finally reaches 3.0V. This also applies to the tenth discharge capacity, twentieth discharge capacity, etc., mentioned below.

[0122] [Initial discharge capacity (DC2)]

[0123] When the electrode of this embodiment is used as the positive electrode, its initial discharge capacity (DC2) (in 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 upper limit to the initial discharge capacity; the higher the better. Therefore, mentioning an upper limit to the initial discharge capacity is not particularly meaningful, but it can generally be assumed to be, for example, about 1000 mAh / g, only as a reference value.

[0124] [20th discharge capacity (DC)] 20 )]

[0125] When the electrode of this embodiment is used as the positive electrode, the discharge capacity after 20 repeated charge-discharge cycles, i.e., the discharge capacity (DC) after the 20th cycle. 20 (Unit: mAh / g) Preferably greater than 375 mAh / g. DC 20 More preferably, it is 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 upper limit to the discharge capacity; the higher the better. Therefore, mentioning the upper limit of the discharge capacity is not very meaningful, but it can usually be used only as a reference value, assumed to be approximately the initial discharge capacity value, or approximately 900 mAh / g.

[0126] Furthermore, when the electrode of this embodiment is used as the positive electrode, its second or twentieth discharge capacity is determined by the composition of the positive electrode, when a negative electrode and electrolyte that are durable enough to function as a lithium-ion secondary battery (i.e., ensuring that lithium is not depleted) and are within the scope of common knowledge in the art are used, and the performance related to the discharge capacity of the positive electrode is fully utilized. For example, for the negative electrode, the lithium content (molar amount) used is preferably 2 times or more than the sulfur content (molar amount) in the positive electrode, more preferably 5 times or more, further preferably 10 times or more, and even more preferably 50 times or more. In addition, for example, for the electrolyte, when the electrolyte solution content (μL) is preferably 10 times or more than the sulfur content (mg) in the positive electrode, more preferably 20 times or more, and even more preferably 50 times or more, the discharge capacity of the positive electrode can be fully demonstrated, thereby extending the battery life. On the other hand, considering the energy density of the battery, the electrolyte solution content is preferably low. For example, the electrolyte solution content (μL) is preferably 5 times or less than the sulfur content (mg) in the positive electrode, more preferably 3 times or less, and even more preferably 1 time or less. In this article, the volume V (mL) of an electrolyte refers to the total volume of the electrolyte solution containing the solute. Furthermore, the electrolyte can be in the form of an electrolyte solution or a solid (solid electrolyte), or a combination of both.

[0127] (application)

[0128] The electrode of this embodiment can be used as the positive or negative electrode of a lithium-ion secondary battery. Furthermore, the electrode of this embodiment is preferably used as the positive electrode of a lithium-ion secondary battery.

[0129] Lithium-ion secondary batteries

[0130] One embodiment of the present invention provides a lithium-ion secondary battery comprising the electrodes described above.

[0131] The lithium-ion secondary battery of this embodiment can be manufactured using the materials described in the preparation method section below, and configured in the manner described therein. That is, when the electrode described above is used as the positive electrode, the negative electrode, electrolyte, separator, etc., described in the preparation method section below can be used, and the lithium-ion secondary battery can be configured in the manner described therein. Conversely, when the electrode described above is used as the negative electrode, the positive electrode, electrolyte, separator, etc., described in the preparation method section below can be used, and the lithium-ion secondary battery can be configured in the manner described therein. Therefore, the description in the preparation method section below can be considered a description of this lithium-ion secondary battery.

[0132] (Inequality (6))

[0133] Preferably, the lithium-ion secondary battery of this embodiment further includes an electrolyte, and D, V, A O and A S The following inequality (6) must be satisfied:

[0134] (6) D×(A) O ×A S ) / V>4000

[0135] In the formula, V represents the volume of the electrolyte in mL, and D represents the coating density of the electrode active material on the current collector in mg / cm³. 2 .

[0136] The right side of inequality (6) is more preferably 4500, further preferably 5000, further preferably 5500, further preferably 6000, further preferably 7000, further preferably 7500, and further preferably 8000. The higher the value on the left side of inequality (6), the better, and setting an upper limit is not practically meaningful, but it can be assumed that its reference value is about 20000.

[0137] (Electrolyte volume V)

[0138] Since the volume V (in mL) of the electrolyte varies depending on the battery size, it cannot be uniformly defined. It is sufficient to use the minimum amount that allows the electrode active materials to perform optimally and enables the battery to function fully. For example, for the button cell shown in the embodiment, as a reference only, 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. On the other hand, 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.

[0139] (application)

[0140] The lithium-ion secondary battery of this embodiment is practical as a lithium-ion secondary battery with improved overall performance in terms of charge / discharge capacity and capacity retention. It can be used in portable information terminals such as smartphones and laptops, portable electronic devices such as music players and digital cameras, as well as medical devices, and batteries for next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs).

[0141] <Preparation Method>

[0142] The preparation methods of the electrode active material, electrode, and lithium-ion secondary battery of this embodiment will be described in turn below.

[0143] (Preparation of electrode active materials)

[0144] The electrode active material of this embodiment can be prepared by various methods, such as the following methods using cellulose as a raw material organic compound.

[0145] That is, the electrode active material of this embodiment can be prepared by the following preparation method:

[0146] (1) Mixing step: The raw materials containing cellulose and sulfur are mixed to obtain calcined raw materials, wherein the mass of sulfur is equal to or greater than the mass of cellulose;

[0147] (2) Calcination step: Calcining the raw material to obtain the calcined material; and

[0148] (3) Crushing step: The calcined material is crushed to obtain calcined material particles.

[0149] [raw material]

[0150] The raw materials used as calcination feedstock will be described below.

[0151] <<Cellulose>>

[0152] Cellulose (Cell-OH, Ce) is a natural polymer, a major component of plant cell walls or fibers, and a compound composed of (C... 12 H 20 O 10 ) n The carbohydrate represented is cellulose, which has the following chemical structural formula. In this chemical structural formula, n represents the average number of repeats, and n is a value of 1 or more, preferably 10 to 10000, more preferably 50 to 2000.

[0153]

[0154] As can be seen from the above chemical structural formula, cellulose has multiple hydroxyl groups, therefore, it contains an ester, wherein all or some of the hydroxyl groups have undergone esterification with an esterifying agent. There are no particular limitations on the type of esterifying agent, as long as it can introduce carboxyl groups into the hydrophilic groups of cellulose; various types can be used, such as carboxylic acid compounds, compounds having two or more carboxyl groups, acid anhydrides of compounds having two or more carboxyl groups, etc. In this embodiment, cellulose also includes such an ester.

[0155] Cellulose, as a plant material, is composed of cellulose fibers with a size of 20 to 40 μm. Each cellulose fiber is a bundle-like aggregate of cellulose microfibers, and each cellulose microfiber is itself a bundle-like aggregate of cellulose molecular chains. Therefore, it is preferable to defibrinate this type of cellulose used as a plant material before use.

[0156] There are two types of defiberization: mechanical defiberization and chemical defiberization, and either type can be used in this embodiment. Examples of mechanical defiberization include high-pressure homogenization, microjet method (opposed jet collision method), grinding mill method, ball milling method, bead milling method, and cryogenic grinding method. Cryogenic grinding is preferred. Examples of chemical defiberization include TEMPO method, phosphorylation method, phosphite esterification method, carboxymethylation method, xanthate esterification method, sulfonation method, enzymatic hydrolysis method, acid hydrolysis method, and selective dissolution method using ionic liquids.

[0157] If cellulose is defibriled by mechanical defibrilation, no chemical modification occurs; however, if cellulose is defibriled by chemical defibrilation, chemical modification occurs. In this embodiment, from the perspective of improving electrode and / or battery performance, cellulose that has been defibriled by mechanical defibrilation and has not undergone chemical modification is preferred.

[0158] In addition, cellulose is preferably pretreated chemically or enzymatically before defibrillation.

[0159] More than one type of cellulose can be used.

[0160] <<Sulfur>>

[0161] 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. More than one type of sulfur can be used.

[0162] From the perspective of improving electrode and / or battery performance, the sulfur content in the calcined raw material is preferably 100 parts by mass or more, more preferably more than 100 parts by mass, further 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 to 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 less than 400 parts by mass. When the content is less than 1000 parts by mass, it is often more advantageous in terms of cost.

[0163] Various allotropes can be used for sulfur, but allotropes containing S8 sulfur that is solid at room temperature and pressure are preferred, and S8 sulfur alone is even more preferred.

[0164] <<Raw Metal Compounds>>

[0165] When the electrode active material also contains a metal compound (the metal compound comprising at least one metal selected from iron, molybdenum, vanadium, and titanium), the raw material metal compound can be further used as a calcination raw material. Examples of raw material metal compounds include the following. Iron compounds are preferred as raw material metal compounds.

[0166] <<Raw Iron Compounds>>

[0167] Examples of raw iron compounds include iron compounds containing divalent or trivalent ferric ions; however, there are no particular limitations as long as they decompose during calcination and react with sulfur to form iron disulfide, and various compounds can be used. Examples of raw iron compounds include ferrates, iron complexes, etc. Examples of ferrates include organic acid salts of iron and inorganic acid salts of iron. On the other hand, examples of iron complexes include neutral iron complexes and iron complex ion salts (iron complex salts). Among these, organic acid salts of iron, inorganic acid salts of iron, or neutral iron complexes are preferred. More than one type of iron compound can be used.

[0168] Examples of organic acid salts of iron include ferrous iron (Fe2+). 2+ ) and salts of organic acids, ferric iron (Fe) 3+Salts of ferrous iron and organic acids, etc. Salts of ferrous iron and organic acids are preferred. The organic acids can be, but are not particularly limited to, organic acids having a carboxyl group (-COOH), organic acids having a sulfonic acid group (-SO3H), etc. Organic acids having a carboxyl group are preferred. Specific examples of organic acids include fatty acids, oxalic acid, tartaric acid, citric acid, malic acid, succinic acid, etc. Specific examples of fatty acids include, for example, fatty acids having one or more but less than six carbon atoms, such as acetic acid, propionic acid, butyric acid, etc. Acetic acid, oxalic acid, etc., are preferred. Preferred examples of organic acid salts of iron include ferric acetate (II), ferric oxalate (II), etc. They can be hydrates. More than one type of organic acid salt of iron can be used.

[0169] Examples of inorganic salts of iron include ferrous iron (Fe2+). 2+ ) and salts of inorganic acids, ferric iron (Fe3+) 3+ Salts of inorganic acids, etc. Specific examples of inorganic acids include hydrochloric acid, sulfuric acid, nitric acid, etc. Among them, nitric acid is preferred. Preferred examples of inorganic acid salts of iron include ferric chloride (II), ferric chloride (III), ferric sulfate (II), ferric sulfate (III), ferric nitrate (II), ferric nitrate (III), etc. They can be hydrates. More than one type of inorganic acid salt of iron can be used.

[0170] Examples of iron complexes include divalent iron (Fe2+). 2+ Complexes, trivalent iron (Fe) 3+ Iron complexes can be in the form of neutral complexes or complex salts. There are no particular restrictions on the ligands coordinating with the iron ion; examples include halogen atoms such as chlorine and bromine, cyano groups, dicyclopentadienyl groups, and N,N'-bis(salicylate)ethylenediamine. Examples of iron complexes include potassium hexacyanoferrate(II) ([Fe(CN)6]K4), potassium hexacyanoferrate(III) ([Fe(CN)6]K3), sodium ferric chloride(III) ([FeCl4]Na), dicyclopentadienylferric(II) (ferrocene), and N,N'-bis(salicylate)ethylenediamine ferric chloride(III). More than one type of iron complex may be used.

[0171] <<Molybdenum Compounds as Raw Materials>>

[0172] Examples of raw material molybdenum compounds 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. More than one type of molybdenum compound may be used.

[0173] <<Raw Vanadium Compounds>>

[0174] Examples of vanadium compounds used as raw materials include vanadium pentoxide (V), ammonium metavanadate (V), vanadium oxychloride (V), sodium metavanadate (V), potassium vanadate (V), sodium vanadate (V), vanadium tetrachloride (IV), vanadium oxysulfate (IV), vanadium dichloride (IV), vanadium oxide (IV), vanadium trichloride (IV), vanadium oxide (III), and vanadium tridecyloxide (IV, V), etc. More than one type of vanadium compound may be used.

[0175] <<Raw Titanium Compounds>>

[0176] Examples of raw material titanium compounds include titanium oxide, titanium dioxide, titanium trioxide, and titanium tetrachloride. More than one type of titanium compound may be used.

[0177] <<Content of raw material metal compounds>>

[0178] From the perspective of improving electrode and / or battery performance, the content of raw material metal compounds in the calcined raw material is preferably 50 parts by mass and 300 parts by mass or less relative to 100 parts by mass of cellulose. This content is more preferably greater than 50 parts by mass, even more preferably greater than 60 parts by mass, even more preferably greater than 70 parts by mass, and even more preferably greater than 75 parts by mass. On the other hand, this 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 less than 100 parts by mass.

[0179] <<Median Particle Size of Raw Material Metal Compounds>>

[0180] When using raw metal compounds as calcination feedstocks, it is preferable to pre-crush them. The median particle size (d) of the metal compound... 50 The median particle size is preferably 12.00 μm or less, more preferably 10.00 μm or less, even 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, the lower limit of the median particle size is not particularly limited, but it is generally about 0.10 μm or more, and may also be about 1.00 μm or about 2.00 μm. The median particle size can be determined by the method described above.

[0181] <<Specific Surface Area of ​​Raw Material Metal Compounds>>

[0182] The specific surface area of ​​the raw material metal compound is preferably 1.0 m². 2 / g or more, preferably 2.0m 2 / g or more, further preferably 3.0m 2 / g or more, further preferably 4.0m 2 / g or more, further preferably 4.5m 2 / g or higher. On the other hand, there is no particular upper limit on the specific surface area, but it is typically around 40.0m². 2 Below / g, it can also be approximately 20.0m. 2 / g or less or about 10.0m 2 / g or less. Specific surface area can be measured using a fully automated specific surface area analyzer (HM-Model 1201, manufactured by MOUNTECH Co., Ltd.).

[0183] Raw material metal compounds having the aforementioned median particle size or specific surface area can be prepared by conventional methods, such as by pulverizing the raw material metal compounds using a pulverizer. Such pulverizers can be those manufactured by Japan Analytical Industry Co., Ltd. (e.g., JFC-2000, etc.).

[0184] <<Other Materials>>

[0185] Depending on the need, the raw materials may appropriately include other materials commonly used in the art. Examples of such materials include carbon materials.

[0186] <<Carbon Materials>>

[0187] In the electrode active material of this embodiment, the carbon material preferably has a graphite structure. Furthermore, the carbon material preferably has electrical conductivity. Examples of carbon materials may include porous carbon materials, such as activated carbon; graphite; carbon black; acetylene black; Ketjen black; and carbon fibers, such as carbon fibers, vapor-grown carbon fibers (VGCF), carbon nanotubes (CNTs), carbon nanofibers, etc.; and other forms of nano-carbon materials besides carbon fibers, such as graphene, fullerenes, etc. Among these, carbon fibers, vapor-grown carbon fibers (VGCF), CNTs, carbon nanofibers, etc., are preferred, and CNTs are particularly preferred. More than one type of carbon material may be used.

[0188] When the carbon material is carbon fiber, from the perspective of improving electrode and / or battery performance, the average fiber length of the fibers constituting the carbon fiber is preferably above a predetermined value, and the average fiber diameter is below a predetermined value. This is considered to help improve the conductivity of the electrode active material. The average fiber length is preferably greater than 1 μm, more preferably greater than 1.5 μm, and even more preferably 2 μm or more. There is no particular upper limit to the average fiber length, and it can be 100 μm, 50 μm, or 20 μm. Furthermore, 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 typically about 1 nm.

[0189] The aspect ratio of the carbon material is preferably greater than 10, more preferably greater than 100, even more preferably greater than 1000, and also preferably less than 100000, more preferably less than 50000, and even more preferably less than 10000.

[0190] From the perspective of the effectiveness of this invention, the specific surface area of ​​the carbon material is preferably 400 m². 2 / g or more, and preferably 2400m 2 / g or less. A more preferred specific surface area is 500m². 2 / g or more, further preferably 600m 2 / g or more. On the other hand, a specific surface area of ​​2000m² is more preferable. 2 / g or less, more preferably 1800m 2 Below / g. Furthermore, the specific surface area was determined by the BET multi-point method.

[0191] From the perspective of the effectiveness of this invention, the G / D ratio of the carbon material is preferably 10 or higher. More preferably, it is 20 or higher, even more preferably 30 or higher, and still more preferably 40 or higher. On the other hand, there is no particular upper limit to the G / D ratio, but if the value is not lower than 50, the carbon material can be considered to have very few defects. In this text, the G / D ratio is the ratio of representative Raman shift peaks in the Raman spectrum of the carbon material; more specifically, it is the ratio of the G-band peak originating from the graphite structure to the D-band peak originating from defects. Furthermore, the Raman spectroscopy was performed using a RAMANtouch spectrometer manufactured by Nanophoton (excitation wavelength λ = 532 nm, grating: 1200 gr / mm, resolution: 1.2 cm⁻¹). -1 ) to be measured.

[0192] From the perspective of the effectiveness of this invention, the metal impurity content in the carbon material is preferably 5% by mass or less. More preferably, it is 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, but for example, 0.1% by mass is already considered a low metal impurity content. Furthermore, the metal impurities referred to herein are metallic elements other than iron, molybdenum, vanadium, and titanium.

[0193] From the perspective of improving electrode and / or battery performance, the carbon content in the calcined raw material is preferably less than 5 parts by mass relative to 100 parts by mass of cellulose. This 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, this content is preferably greater than 0.05 parts by mass, more preferably greater than 0.07 parts by mass, and even more preferably 0.10 parts by mass or more.

[0194] (Preparation steps)

[0195] [Mixing Step (1)]

[0196] The mixing step involves mixing a raw material containing cellulose with sulfur of equal or greater mass than cellulose to obtain a calcination feedstock.

[0197] The above-described mixing method is not particularly limited as long as it can ensure thorough mixing of the components, and conventional methods can be used. In this embodiment, for example, the following method can be employed: directly mixing the raw material containing powdered cellulose and powdered sulfur with a mass equal to or greater than that of cellulose using a mixer or similar device. Furthermore, when optional components such as raw material metal compounds or carbon materials are added to the raw material, these optional components can also be mixed together using a mixer or similar device.

[0198] In this document, "powder" refers to a state in which each solid material is refined to a level suitable for mixing with other materials to achieve the objectives of this invention. The size of the particles constituting the powder is not particularly limited, as long as proper mixing is possible, but is typically, for example, from 1 μm to 40 μm. From the perspective of improving electrode and / or battery performance, the particle size is preferably 2 μm or more, more preferably 3 μm or more, further preferably 4 μm or more, and preferably 30 μm or less, further preferably 20 μm or less, further preferably 15 μm or less, and even more preferably 10 μm or less. The median particle size can be determined by the method described above.

[0199] The raw materials obtained in this way can be used directly in the next calcination step, or, if necessary, they can be granulated and used in the next step.

[0200] [Calcination step (2)]

[0201] The calcination step refers to the step of calcining the raw materials obtained above to obtain calcined materials. Calcination can be carried out using conventional methods, such as heating the raw materials to a predetermined temperature at a predetermined heating rate, maintaining that predetermined temperature for a predetermined time, and then allowing it to cool naturally.

[0202] <<Non-oxidizing atmosphere>>

[0203] 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 thermal decomposition of the components. Specifically, this refers to inert gas atmospheres such as nitrogen, argon, sulfur, and ammonia. Therefore, calcination can be suitably carried out, for example, in a quartz tube under an inert gas atmosphere.

[0204] <<Heating Rate>>

[0205] The heating rate is preferably from, for example, 50°C / h to 500°C / h. The heating rate is preferably 80°C / h or higher, more preferably 100°C / h or higher, and even more preferably 120°C / h or higher. On the other hand, the heating rate is more preferably 400°C / h or lower, even more preferably 300°C / h or lower, and even more preferably 200°C / h or lower. When the heating rate is within this range, it is generally easy to achieve the goal of improving charge / discharge capacity and cycle performance.

[0206] <<Calcination Temperature / Time>>

[0207] The calcination temperature refers to the temperature at which the raw material is heated to a certain point and maintained for a period of time to calcine it. This temperature is preferably above 250°C and below 550°C. When the temperature is above 250°C, incomplete sulfidation 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 550°C, the decomposition of the raw material can often be prevented, and a decrease in yield and charge / discharge capacity can be prevented. This temperature is more preferably above 270°C, even more preferably above 290°C, and even more preferably above 300°C. On the other hand, this temperature is more preferably below 500°C, even more preferably below 470°C, and even more preferably below 450°C.

[0208] When using raw metal compounds as raw materials, from the perspective of improving electrode and / or battery performance, the calcination temperature in the calcination step is preferably higher than the thermal decomposition temperature of the raw metal compounds.

[0209] The time for maintaining the calcination temperature can be appropriately set according to the type of raw material and the calcination temperature, but is preferably 0.5 hours or more and 6 hours or less. When the time is 0.5 hours or more, there is a tendency to ensure sufficient calcination; when the time is 6 hours or less, there is a tendency to prevent excessive pyrolysis of the components. This time is more preferably 0.6 hours or more, and even more preferably 0.7 hours or more. On the other hand, this time is more preferably 4 hours or less, and even more preferably 2 hours or less.

[0210] <<Equipment>>

[0211] Calcination can be performed using, for example, a muffle furnace ( Figure 1 It can be carried out either by means of a single process or by continuous equipment such as a twin-screw extruder. Using continuous equipment has the following advantages: sulfide electrode 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.

[0212] muffle furnace ( Figure 1 A furnace is a type of furnace that uses heating plates or similar partitions to prevent the heat source (heater) from being exposed inside the furnace, thereby avoiding sample contamination. Figure 1In the furnace, a heater 2 is located at the bottom of the muffle furnace 1, and the heater is separated by a heating plate. A cover 3 is installed on the front surface of the furnace (left side in the figure) to maintain an inert gas atmosphere 4 inside the furnace. A thermocouple (not shown in the figure) is attached to the cover for measuring the temperature inside the furnace during calcination. The furnace has two layers, upper and lower, on which stainless steel cuboid reaction vessel trays 5 and 6 are placed to hold the raw materials for calcination.

[0213] The internal structure of the furnace is designed to allow gases (such as inert gases like argon (Ar)) to be continuously introduced from the outside through the inlet pipe 7 and discharged to the outside through the exhaust pipe 8. The exhaust pipe 8 is connected to a collection tank 10 containing an aqueous sodium hydroxide solution 9. When the exhaust gas from the muffle furnace 1 is discharged to the outside through the exhaust pipe 8, it first passes through the aqueous sodium hydroxide solution 9 in the collection tank 10 before being discharged to the outside. Therefore, even if the exhaust gas contains hydrogen sulfide gas generated in the reaction, the hydrogen sulfide gas will be neutralized by the aqueous sodium hydroxide solution and thus removed from the exhaust gas.

[0214] [Residue Removal Steps]

[0215] The residue removal step refers to the process of removing residues from the calcined material, such as unreacted sulfur precipitated upon cooling after sublimation of sulfur during calcination. Since these residues can degrade cycle performance, they should be removed as much as possible. Residue removal can be achieved through conventional methods, such as vacuum drying, hot air drying, and solvent washing of the calcined material.

[0216] <<Pulverizing Step (3)>>

[0217] The pulverization step refers to the process of pulverizing the calcined material to obtain granules of the calcined material. Preferably, the calcined material is pulverized into particle sizes suitable for manufacturing electrodes. The preferred size range for electrode active material particles is as described above.

[0218] Grinding can be carried out using conventional methods, such as shredders or jet mills, under predetermined conditions. The grinding conditions vary depending on the type of shredder used. For example, when using a shredder (e.g., the Free Speed ​​Mill FS-20 manufactured by Labonect), the grinding can be performed at a speed of 20,000 rpm to 30,000 rpm for 1 to 30 seconds. Furthermore, when using a dry jet mill (e.g., the Nano Jetmizer NJ-30 manufactured by Aisin Nano Technologies Co., LTD), the grinding can be performed at a processing speed of 1 g / min to 3 g / min and a grinding pressure of 0.5 MPa to 2.0 MPa.

[0219] The pulverized and calcined material obtained above can be graded as needed to make the particle size more uniform. Grading can be performed, for example, using a sieve with the desired mesh size.

[0220] Furthermore, in the calcination method using a twin-screw extruder described above, while producing electrode active materials, the resulting electrode active materials can also be pulverized into particles through the shearing action during the kneading process.

[0221] (Preparation of lithium-ion secondary battery electrodes)

[0222] Using the electrode active material obtained above, a lithium-ion secondary battery electrode containing an electrode active material layer of this electrode active material can be prepared by conventional methods. That is, except for using the above-mentioned electrode active material as the active material, the electrode can be obtained by the same method as for preparing a conventional lithium-ion secondary battery electrode.

[0223] [Using electrode active materials as positive electrode active materials]

[0224] Aside from using the aforementioned electrode active materials as the positive electrode active materials, the preparation method for the positive electrode of a lithium-ion secondary battery is the same as that for conventional lithium-ion secondary battery positive electrodes. For example, a paste-like positive electrode material can be prepared by mixing the electrode active material with a conductive additive, a binder, and a solvent, then coating this positive electrode material onto a current collector, and finally drying it to produce the positive electrode. Alternatively, the positive electrode can be produced by, for example, kneading the electrode active material together with a conductive additive, a binder, 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.

[0225] <<Conductive Additives>>

[0226] Examples of conductive additives include 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. Furthermore, conductive carbon materials from the aforementioned carbon materials can also be used as conductive additives. One or more of these conductive additives can be used.

[0227] <<Adhesives>>

[0228] 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 resins, methacrylic acid resins (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), polypropylene (PP), etc. One or more of these adhesives may be used.

[0229] <<Solvent>>

[0230] Examples of solvents include N-methyl-2-pyrrolidone, N,N-dimethylformaldehyde, alcohols, hexane, water, etc. One or more of these solvents may be used.

[0231] <<Mixed Amount>>

[0232] There is no particular limitation on the mixing amount of these materials constituting the positive electrode, but for example, it is preferable to mix 2 to 100 parts by mass of conductive additive, 2 to 50 parts by mass of binder and appropriate amount of solvent relative to 100 parts by mass of electrode active material.

[0233] <<Current collector>>

[0234] As the current collector, current collectors conventionally used for the positive electrode of lithium-ion secondary batteries can be used. Examples of current collectors include those composed of metal foils (e.g., aluminum foil, aluminum mesh, perforated aluminum sheet, expanded aluminum sheet, stainless steel foil, stainless steel mesh, perforated stainless steel sheet, expanded stainless steel sheet, nickel foam, nickel nonwoven fabric, copper foil, copper mesh, perforated copper sheet, expanded copper sheet, titanium foil, titanium mesh, etc.) and those composed of carbon nonwoven fabric, carbon woven fabric, etc. Currently, current collectors containing metal foils are preferred. Current collectors can be used alone or in combination of two or more. Furthermore, the surface of the current collector can be coated with carbon or other materials. Specific examples of current collectors with carbon or other material coatings include carbon-coated aluminum foil. In this case, the current collector includes a carbon-coated portion.

[0235] [Using electrode active materials as negative electrode active materials]

[0236] Aside from using the aforementioned electrode active materials as the 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 can be prepared by mixing the electrode active material with a conductive additive, a binder, and a solvent, then coating this negative electrode material onto a current collector, and finally drying it to produce the negative electrode. Alternatively, the negative electrode can be produced by, for example, kneading the electrode active material together with a conductive additive, a binder, 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.

[0237] The conductive additives, binders, and solvents can be the same materials used when the electrode active material is used as the positive electrode active material, and the same applies to the current collector.

[0238] (Preparation of lithium-ion secondary batteries)

[0239] Except for using the lithium-ion secondary battery electrode obtained above, the preparation method of the lithium-ion secondary battery in this embodiment is the same as that of conventional lithium-ion secondary battery preparation methods.

[0240] [Using electrode active materials as positive electrode active materials]

[0241] The lithium-ion secondary battery of this embodiment can be manufactured using conventional methods. In addition to using a positive electrode containing the above-mentioned electrode active material (positive electrode active material), a negative electrode and an electrolyte can also be used, and components such as a separator can also be used as needed.

[0242] <<Negative Electrode>>

[0243] As the negative electrode material, known materials such as lithium metal, 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 lithium-free materials (e.g., carbon-based materials, silicon-based materials, alloy-based materials, etc.) are used as the negative electrode material, 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 embodiment, 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 into 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 into 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.

[0244] <<Electrolytes>>

[0245] The electrolyte compensates for the charge generated when electrons are released to the external circuit during the oxidation / reduction reactions of the active materials on the positive and negative electrodes through ion flow. The electrolyte for lithium-ion secondary batteries can be an alkali metal salt dissolved in an organic solvent. Preferably, at least one of the following non-aqueous solvents is used: dimethoxyethane, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dimethyl ether, γ-butyrolactone, and acetonitrile. As the electrolyte, Li(FSO2)2N, LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiI, and LiClO4 can be used. The electrolyte concentration only needs to be approximately 0.5 mol / L to 5.0 mol / L. 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 can be solid (e.g., polymer gel state).

[0246] <<Septum>>

[0247] In addition to the aforementioned negative electrode, positive electrode, and electrolyte, 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, thin microporous membranes or non-woven membranes made of polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide, cellulose, glass, etc., are preferred.

[0248] <<Shape>>

[0249] There are no particular restrictions on the shape of lithium-ion secondary batteries; they can be cylindrical, stacked, button-shaped, laminated, coin-shaped, and many other shapes.

[0250] [Using electrode active materials as negative electrode active materials]

[0251] In addition to using a negative electrode containing the aforementioned electrode active material (negative electrode active material), the lithium-ion secondary battery of this embodiment can also be manufactured using a positive electrode, an electrolyte, and components such as a separator (if necessary) according to conventional methods.

[0252] <<Positive Electrode>>

[0253] 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 composite oxides, such as LiCoO2; and Li-Ni-Co-Mn composite oxides, such as LiNi.x Co y Mn z O2, etc.; Li-Ni composite oxides, such as LiNiO2, etc.; Li-Mn 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 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 can be used alone or in combination.

[0254] <<Other>>

[0255] For the electrolyte, separator, and shape of the lithium-ion secondary battery, the same materials can be used as when the electrode active material is used as the positive electrode active material. Example

[0256] Although the present invention will be described with reference to embodiments, it is not limited to the embodiments.

[0257] The various chemicals used in the examples and comparative examples are listed below. All chemicals were purified as needed using conventional methods.

[0258] <Materials used in the experiment>

[0259] Cellulose: Cellulose (prepared by Fujifilm Wako Pure Chemical Corporation, white powder, can pass through a 38μm (400 mesh) sieve)

[0260] Diene rubber: High cis-butadiene rubber (UBEPOL (registered trademark) BR150L, ​​manufactured by UBE Corporation, with cis content (cis-1,4-bonded butadiene unit content): 98% by mass)

[0261] PP: Polypropylene (purchased from Sigma-Aldrich, item number: 428116, melting point: 157℃)

[0262] Crosslinked PMMA: Crosslinked polymethyl methacrylate (TECHPOLYMER MB30X-8, prepared by Sekisui Kasei Co., Ltd., is a spherical acrylic resin composed of methyl methacrylate and ethylene glycol dimethacrylate copolymer, with a particle size of 8 μm)

[0263] Sulfur: Precipitated sulfur prepared by Tsurumi Chemical Industry Co., Ltd.

[0264] Production Example 1 (Raw Material Crushing)

[0265] Before using cellulose and polypropylene as raw materials, the cellulose and polypropylene were pre-crushed for 10 minutes each using a cryogenic grinder (JFC-2000 manufactured by Japan Analysis Industries Co., Ltd.).

[0266] <Preparation of Electrode Active Materials>

[0267] (Mixed steps)

[0268] According to the proportions shown in the table below, the raw materials are put into a mixer (LAB MILL manufactured by Osaka Organic Chemical Co., Ltd.) for mixing to obtain calcined raw materials.

[0269] (Calcination step)

[0270] Use a muffle furnace ( Figure 1 The raw materials are calcined. Figure 1 The muffle furnace shown is as described above.

[0271] First, the stainless steel reaction vessel tray containing the calcining raw material was placed into a muffle furnace, and the atmosphere inside the muffle furnace was purged three times with argon using a vacuum pump. Then, argon was continuously introduced through the inlet pipe at a flow rate of 100 mL / min, and the muffle furnace was heated after 30 minutes of purging. The temperature was increased at a rate of 5 °C / min, and when the temperature of the calcining raw material reached the calcination temperature shown in Table 1, this temperature was maintained for 45 minutes of heat treatment. Next, while adjusting the argon flow rate, 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.

[0272] (Removal of unreacted sulfur)

[0273] To remove unreacted sulfur (free elemental sulfur) remaining in the product after the calcination step, the following steps were performed: The calcined material was ground in a mortar, and the pulverized product was placed in a glass tube furnace and heated at 290°C for 3 hours under vacuum to obtain an electrode active material with unreacted sulfur removed (or containing only trace amounts of unreacted sulfur). The heating rate was set to 10°C / min.

[0274] (Grinding step)

[0275] The calcined material after removing unreacted sulfur was pulverized using a shredder (LAB MILL manufactured by Osaka Organic Chemical Industry Co., Ltd.).

[0276] (Grading process)

[0277] To remove coarse particles from the pulverized calcined material, a 32μm stainless steel sieve was used to classify the calcined material to obtain electrode active material.

[0278] <Physical Properties of Electrode Active Materials>

[0279] The following performance tests were performed on the electrode active material obtained above.

[0280] (Particle size distribution, median particle size)

[0281] The particle size distribution was determined using a laser diffraction / scattering particle size analyzer (PSA1090L particle size analyzer manufactured by Anton Paar GmbH) with water as the dispersion medium, and a particle size distribution curve was obtained. Based on the particle size distribution curve, the cumulative particle size of the volume 50% (median particle size d50) was determined.

[0282] (Elemental composition)

[0283] The contents of carbon, hydrogen, nitrogen, and sulfur were measured by mass using a fully automated elemental analyzer, the vario MICROcube, manufactured by Elt, and the percentage (%) of each element relative to the total mass of the electrode active material was calculated. The contents of oxygen were measured by mass using an oxygen / nitrogen / hydrogen analyzer, the EMGA-930, manufactured by Horiba Ltd., and the percentage (%) of each element relative to the total mass of the electrode active material was calculated.

[0284] <Preparation of Lithium-ion Secondary Batteries>

[0285] The preparation method of lithium-ion secondary batteries is as follows.

[0286] (positive electrode)

[0287] The electrode active material prepared above was used as the active material, acetylene black (HS-100 manufactured by Denka Co., Ltd.) was used as the conductive additive, and acrylic resin (manufactured by Fujifilm and Kojun Pharmaceutical Co., Ltd., average molecular weight: 2700-7500) was used as the binder. The active material, acetylene black, and binder were weighed in a mass percentage ratio of 85:10:5 and placed in a container. Ultrapure water (Milli-Q wayer) was added as a dispersant, and the mixture was stirred using a planetary centrifugal mixer (ARE-250 manufactured by THINKYCORPORATION) to prepare a uniform slurry. The prepared slurry was coated onto a 17μm aluminum foil using a coater with a 100μm slit width. The foil was then pressed using a roller press to obtain the electrode. The electrode was then heated at 120°C for 3 hours, dried in a desiccator, and then stamped into shape. 11 mm, to obtain the electrode (positive electrode). Measure the mass of the electrode and calculate the content of active material in the electrode according to the above proportions.

[0288] (negative electrode)

[0289] 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. The negative electrode current collector was made of stainless steel sheet.

[0290] (electrolytes)

[0291] 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.

[0292] (Lithium-ion secondary battery)

[0293] Button batteries were prepared using the aforementioned positive and negative electrodes. 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 filter membrane (GA100 manufactured by ADVANTEC with a thickness of 440 μm) were sandwiched between the positive and negative electrodes to form an electrode body battery. This electrode body battery was housed in a battery casing made of stainless steel (a component for CR2032 type button batteries manufactured by Hohsen). The aforementioned electrolyte was injected into the battery casing. The amount of electrolyte was 0.28 mL by volume. The battery casing was sealed using a crimping machine to obtain the lithium-ion secondary batteries used in the various embodiments and comparative examples.

[0294] <Evaluation of Lithium-ion Secondary Batteries>

[0295] (Discharge capacity and capacity retention)

[0296] The button-type lithium-ion secondary batteries prepared in the examples and comparative examples were charged and discharged at a test temperature of 30°C for the first to tenth cycles at a current value of 50 mA per gram of positive electrode active material; and for the eleventh to twentieth cycles at a current value of 100 mA per gram of positive electrode active material. The discharge termination voltage was set to 1.0 V, and the charge termination voltage was set to 3.0 V. The discharge capacity (in mAh) of the batteries was observed during repeated charging and discharging at the first, second, tenth, and twentieth cycles. Measurements were performed using a battery performance evaluation device (BLS system manufactured by KEISOKUKI CENTERCO., LTD.).

[0297] The second discharge capacity DC3 (in 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 second discharge capacity DC2 (in mAh / g) and the twentieth discharge capacity DC... 20 (Unit: mAh / g), calculate the capacity retention rate (unit: %) using the following formula. It can be considered that the higher the capacity retention rate, the better the cycle performance of the lithium-ion secondary battery.

[0298] Capacity retention rate (%) = (DC) 20 / DC2)×100

[0299] The results are shown in Table 1 below.

[0300] Table 1

[0301]

[0302] Examples 1 to 4 exhibited higher initial discharge capacities compared to Comparative Examples 2 and 3. This is attributed to the higher sulfur content, which allows more sulfur to bind with lithium ions. Furthermore, despite having a high sulfur content, Comparative Example 1 had a lower initial discharge capacity than Examples 1 to 4. Additionally, Examples 1 to 4 exhibited higher capacity retention compared to Comparative Examples 1, 3, and 4. This is attributed to the sufficiently high sulfur and oxygen content, which suppressed side reactions during the charge and discharge process.

[0303] <Implementation Method>

[0304] The following shows a preferred embodiment.

[0305] [1] An electrode active material comprising particles containing an organic sulfur compound,

[0306] Among them, AO and A S The following inequalities are satisfied, or the right side of at least one of inequalities (1) to (3) takes a more preferred value, wherein the preferred value on the right side of inequality (1) is 10.0, the preferred value on the right side of inequality (2) is 46.0, more preferably 47.0, further preferably 48.0, further preferably 49.0, and the preferred value on the right side of inequality (3) is 560:

[0307] (1) A O >9.0

[0308] (2) A S >45.0

[0309] (3) A O ×A S >550

[0310] In the formula, A O Indicates the oxygen content in the electrode active material, expressed as a percentage by mass, A S This indicates the sulfur content in the electrode active material, expressed as a percentage by mass.

[0311] [2] According to the electrode active material described in [1] above, wherein the right side of the inequality (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, even more preferably 18.0.

[0312] [3] According to the electrode active material described in [1] or [2] above, wherein the right side of the inequality (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, even more preferably 59.0.

[0313] [4] The electrode active material according to any one of [1] to [3] above, wherein the right side of the inequality (3) is 570, preferably 600, more preferably 650, even more preferably 700, even more preferably 750, even more preferably 800, even more preferably 830, even more preferably 840.

[0314] [5] The electrode active material according to any one of [1] to [4] above, wherein the active material further comprises a metal compound comprising at least one metal selected from iron, molybdenum, vanadium and titanium.

[0315] [6] According to the electrode active material described in [5] above, wherein the metal compound is an iron compound.

[0316] [7] An electrode comprising an electrode active material according to any one of [1] to [6] above.

[0317] [8] Based on the electrode described in [7] above,

[0318] The electrode has a current collector.

[0319] The current collector has a metal foil, and

[0320] Among them, D and A O and A S The following inequality is satisfied, or the right side of inequality (4) is preferably 1500, more preferably 1700, even more preferably 1900, even more preferably 2100, even more preferably 2200, even more preferably 2300:

[0321] (4) D×(A) O ×A S >1000

[0322] Where D represents the coating density of the electrode active material on the current collector, in mg / cm³. 2 .

[0323] [9] Based on the electrode described in [7] or [8] above,

[0324] The electrode has a current collector.

[0325] The current collector has a metal foil, and

[0326] Among them, D, T, A O and A S The following inequality is satisfied, or the right side of inequality (5) is preferably 80, more preferably 90, even more preferably 110, even more preferably 120, and even more preferably 130:

[0327] (5) D×(A) O ×A S ) / T>60

[0328] Where D represents the coating density of the electrode active material on the current collector, in mg / cm³. 2 T represents the thickness of the metal foil, in μm.

[0329]

[10] The electrode according to any one of [7] to [9] above,

[0330] The electrode has a current collector.

[0331] The current collector has a metal foil, and

[0332] Among them, D is greater than 2.50 mg / cm³. 2 Preferably greater than 3.00 mg / cm³ 2 More preferably greater than 3.50 mg / cm³ 2 Further preferred concentrations are greater than 3.80 mg / cm³. 2 Further preferred concentrations are greater than 3.90 mg / cm³. 2 ,

[0333] Where D represents the coating density of the electrode active material on the current collector, in mg / cm³. 2 .

[0334]

[11] The electrode according to any one of [7] to

[10] above, wherein the electrode is a positive electrode.

[0335]

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

[11] above.

[0336]

[13] According to the lithium-ion secondary battery described in

[12] above, the lithium-ion secondary battery further includes an electrolyte.

[0337] Among them, V, D, A O and A S The following inequality is satisfied, or the right side of inequality (6) is preferably 4500, more preferably 5000, even more preferably 5500, even more preferably 6000, even more preferably 7000, even more preferably 7500, even more preferably 8000:

[0338] (6) D×(A) O ×A S ) / V>4000

[0339] Where V represents the volume of the electrolyte in mL, and D represents the coating density of the electrode active material on the current collector in mg / cm³. 2 .

[0340]

[14] A method for preparing electrode active materials, the method comprising:

[0341] (1) Mixing step: Mix raw materials containing cellulose and sulfur to obtain calcined raw materials, wherein the mass of sulfur is equal to or greater than the mass of cellulose;

[0342] (2) Calcination step: Calcining the raw material to obtain the calcined material; and

[0343] (3) Crushing step: The calcined material is crushed to obtain calcined material particles.

[0344]

[15] According to the method described in

[14] above, wherein the cellulose is unmodified cellulose.

[0345] Reference Symbol List

[0346] 1. Muffle furnace

[0347] 2. Heater

[0348] 3. Lid

[0349] 4. Inert gases

[0350] 5. Pallet (top layer)

[0351] 6. Tray (lower layer)

[0352] 7. Intake pipe

[0353] 8. Exhaust pipe

[0354] 9. Sodium hydroxide aqueous solution

[0355] 10. Collection tank

Claims

1. An electrode active material comprising particles containing an organic sulfur compound, in, A O and A S The following inequalities must be satisfied: （1）A O >9.0 （2）A S >45.0 (3)A O ×A S >550 In the formula, A O Indicates the oxygen content in the electrode active material, expressed as a percentage by mass, A S This indicates the sulfur content in the electrode active material, expressed as a percentage by mass.

2. The electrode active material according to claim 1, wherein, The right side of the inequality (1) is 11.

0.

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

0.

4. The electrode active material according to claim 1, wherein, The right side of the inequality (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, which includes at least one metal selected from iron, molybdenum, vanadium, and titanium.

6. The electrode active material according to claim 5, wherein, The 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 according to claim 7, in, The electrode has a current collector. The current collector has a metal foil. D、A O and A S The following inequalities must be satisfied: (4)D×(A O ×A S )>1000, In the formula, D represents the coating density of the electrode active material on the current collector, with units of mg / cm³. 2 .

9. The electrode according to claim 7, in, The electrode has a current collector. The current collector has a metal foil. D, T, A O and A S The following inequalities must be satisfied: (5)D×(A O ×A S ) / T>60 In the formula, D represents the coating density of the electrode active material on the current collector, with units of mg / cm³. 2 T represents the thickness of the metal foil, in μm.

10. The electrode according to claim 7, in, The electrode has a current collector. The current collector has a metal foil. D > 2.50 mg / cm 2 , Where D represents the coating density of the electrode active material on the current collector, in mg / cm³. 2 .

11. The electrode according to claim 7, wherein, The electrode is a positive electrode.

12. A lithium-ion secondary battery, wherein, The lithium-ion secondary battery has the electrode as described in claim 7.

13. The lithium-ion secondary battery according to claim 12, wherein, The lithium-ion secondary battery also has an electrolyte. V, D, A O and A S The following inequalities must be satisfied: (6)D×(A O ×A S ) / V>4000, In the formula, V represents the volume of the electrolyte in mL, and D represents the coating density of the electrode active material on the current collector in mg / cm³. 2 .

14. A method for preparing electrode active materials, wherein, The method includes: (1) Mixing step: The raw materials containing cellulose and sulfur are mixed to obtain calcined raw materials, wherein the mass of sulfur is equal to or greater than the mass of cellulose; (2) Calcination step: Calcining the raw material to obtain the calcined material; and (3) Crushing step: The calcined material is crushed to obtain calcined material particles.

15. The method according to claim 14, wherein, The cellulose in question is unmodified cellulose.

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