Electrode and lithium-ion secondary battery

Abstract

The present application provides an electrode for a lithium ion secondary battery, the electrode having an active material layer containing a particulate active material, the active material comprising an organic sulfur compound and an iron compound, A S , A F and M satisfy the following inequality (1) A S × A F × M > 1600, where A S represents the sulfur element content (mass %) in the active material, A F represents the iron element content (mass %) in the active material, and M represents the median particle diameter (μm) of the active material. The present application aims to improve the comprehensive performance of the charge-discharge capacity and capacity retention rate of a lithium ion secondary battery.

Description

TECHNICAL FIELD

[0001] The present application relates to a novel electrode and a lithium ion secondary battery including the same. BACKGROUND

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

[0003] Patent Document 1 describes an active material for an electrode, which is obtained by firing a raw material containing a polymer and sulfur, the polymer containing methacrylonitrile as a monomer component.

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

[0005] PRIOR ART DOCUMENTS

[0006] PATENT DOCUMENT

[0007] Patent Document 1: JP 2020-167144 A SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] However, there is still room for further improvement in electrodes using active materials such as that of Patent Document 1.

[0010] An object of the present application is to provide an electrode (i.e., a positive electrode or a negative electrode) for a lithium ion battery that can achieve a high comprehensive performance in terms of charge-discharge capacity and capacity retention rate, and a lithium ion secondary battery including the same.

[0011] MEANS FOR SOLVING THE PROBLEM

[0012] That is, the present application relates to an electrode.

[0013] An electrode for a lithium ion secondary battery,

[0014] The electrode has an active material layer containing a particulate active material,

[0015] The active material contains an organic sulfur compound and an iron compound,

[0016] wherein A S , A F and M satisfy the following inequality (1),

[0017] (1) A S x A F x M > 1600,

[0018] wherein A S represents the content of sulfur element in the active material, unit: mass%, A F represents the content of iron element in the active material, unit: mass%, M represents the median particle diameter of the active material, unit: μm.

[0019] Effects of the Invention

[0020] According to the present application, it is possible to provide an electrode (i.e., a positive electrode or a negative electrode) of a lithium ion battery, which can realize a high comprehensive performance of charge-discharge capacity and capacity retention rate, and a lithium ion secondary battery including the electrode. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 A cross-sectional view schematically showing a reaction apparatus for preparing an active material in an embodiment of the present application. DETAILED DESCRIPTION

[0022] The present application will be described in detail below. In addition, the numerical values of the upper limit and the lower limit related to "above" and "below" for explaining the numerical range can be arbitrarily combined values, and the numerical values in the embodiments can also be these upper limit and lower limit. In addition, unless it is contrary to the purpose of the present application, the numerical range shown to include both end values should be interpreted as also disclosing a numerical range not including one of the both end values and a numerical range not including both end values.

[0023] One embodiment of the present application is the following electrode.

[0024] An electrode for a lithium ion secondary battery,

[0025] The electrode has an active material layer containing a particulate active material,

[0026] The active material contains an organic sulfur compound and an iron compound,

[0027] wherein A S , A F and M satisfy the following inequality (1),

[0028] (1) A S × A F × M > 1600,

[0029] wherein A S represents the content of sulfur element in the active material (mass%), A F represents the content of iron element in the active material (mass%), M represents the median particle diameter of the active material (μm).

[0030] While not wishing to be bound by theory, the reason why the electrode of the present application can improve the comprehensive performance of charge-discharge capacity and capacity retention rate is as follows.

[0031] Sulfur easily generates a soluble compound together with lithium during charge-discharge. Therefore, there is a disadvantage that repeated charge-discharge causes gradual decrease in charge-discharge capacity. In addition, as the active material swells or shrinks during charge-discharge, the volume of the electrode mixture layer changes. Therefore, cracks appear in the mixture material, the mixture material peels off from the current collector, and the like, thereby decreasing the capacity. In contrast, the present application is characterized in that the active material contains iron, and the product of the sulfur content ratio in the active material, the iron content ratio in the active material, and the median particle diameter (μm) of the active material is kept at a certain value or more. Under such conditions, the sulfur and iron remaining on the electrode interact with each other under given conditions, and it is considered that the comprehensive performance of charge-discharge capacity and capacity retention rate can be improved.

[0032] The value on the right side of inequality (1) is preferably 2400.

[0033] Sulfur element content A in active material S It is preferably more than 65.0 mass %.

[0034] Iron element content A in active material F It is preferably more than 15.0 mass %.

[0035] Median particle diameter M is preferably 1.0 μm or more.

[0036] When the electrode as described above is used as a positive electrode, the initial discharge capacity is expressed as DC3 (mAh / g), and DC3 is preferably more than 400 mAh / g, and more preferably more than 600 mAh / g.

[0037] The electrode as described above has a current collector, and the current collector has a metal foil, and when the thickness of the metal foil is expressed as T (μm), A S , M, and T preferably satisfy the following inequality (2),

[0038] (2) A S x M / T > 6.0.

[0039] The quotient obtained by dividing the product value of the amount of sulfur and the median particle diameter by the thickness value of the current collector is a certain value or more, and each value is mutually restricted, and it is considered that the performance of the electrode and / or the battery is improved.

[0040] When the coating density of the active material is expressed as D (mg / cm 2 ), A S , A F , and D preferably satisfy the following inequality (3),

[0041] (3) AS × A F > 7000.

[0042] When the product of the sulfur content ratio, the iron content ratio, and the coating density (mg / cm 2 ) of the active material remains a certain value or more, the sulfur and iron remaining on the electrode interact with each other under given conditions, and it is considered that the performance of the electrode and / or the battery is improved.

[0043] When the electrode as described above is used as a positive electrode, the 10th discharge capacity is expressed as DC 10 (mAh / g), and DC 10 is preferably greater than 350 mAh / g.

[0044] Another embodiment of the present application is a lithium ion secondary battery including the above-described electrode.

[0045] The lithium ion secondary battery preferably further has an electrolyte, and when the volume of the electrolyte is expressed as V (mL), A S , M, and V satisfy the following inequality (4),

[0046] (4) A S × M / V > 60.

[0047] The quotient obtained by dividing the product value of the amount of sulfur and the median particle diameter by the volume value of the electrolyte is a certain value or more, and each value is mutually restricted, and it is considered that the performance of the electrode and / or the battery is improved.

[0048] <Definitions>

[0049] The term "granular" refers to a state in which the active material is made fine enough to be suitable for mixing with another material for the purpose of the present application. As for the size of the particles constituting the active material, the size is not particularly limited as long as the size makes the mixing properly proceed.

[0050] The "active material" refers to a material that plays a role of an oxidation-reduction reaction for energy conversion in a lithium ion secondary battery.

[0051] The "active material layer" refers to a layer formed on a current collector constituting an electrode, which is made of an electrode material (including an active material).

[0052] The "coating density" refers to the mass (mg) per unit area (cm 2 ) of the active material coated on the current collector.

[0053] In the present specification, unless otherwise specified, the "initial discharge capacity" refers to the 3rd discharge capacity.

[0054] <Measurement Methods>

[0055] The "sulfur element content" is the mass ratio (%) of sulfur element in the active material, which is measured by the elemental analysis method described in the present specification below.

[0056] The "iron element content" is the mass ratio (%) of iron element in the active material, which is measured by the thermogravimetric analysis method described in the present specification below.

[0057] The "median particle diameter" is the size at which 50% by volume (median particle diameter d 50 ) is accumulated on the volume basis, which is measured with a laser diffraction / scattering type particle size distribution analyzer (particle size distribution analyzer PSA1090L manufactured by Anton Paar GmbH) using water as the dispersion medium.

[0058] The electrode for a lithium-ion secondary battery of the present application and the lithium-ion secondary battery of the present application will be described below.

[0059] <Electrode for a lithium-ion secondary battery>

[0060] The electrode for a lithium-ion secondary battery of the present application has an active material layer containing particulate active material, which comprises an organic sulfur compound and an iron compound, wherein, S A F and M satisfy the following inequality (1),

[0061] (1) A S x A F x M > 1600,

[0062] In the formula, A S represents the sulfur element content (mass %) in the active material, A F represents the iron element content (mass %) in the active material, and M represents the median particle diameter (μm) of the active material.

[0063] [Active material layer]

[0064] The active material layer according to the present application comprises particulate active material. The active material layer is composed of an electrode material (i.e., a positive electrode material or a negative electrode material) which comprises the particulate active material described below. Herein, the phrase "active material is particulate" refers to a state in which the active material is made fine enough to be suitable for mixing with another material constituting an electrode for the purpose of the present application. Furthermore, the meaning of the phrase is not intended to be particularly limited as long as the active material is in the state. For example, when the term "particulate" is expressed by the median particle diameter, it can range from 1 nm to 1000 μm.

[0065] The median particle diameter (median particle diameter d 50) is preferably about 1.0 pm or more and 40.0 pm or less. The median particle diameter is more preferably 1.5 pm or more, further preferably 2.0 pm or more, further preferably 2.4 pm or more. In addition, the median particle diameter is more preferably 30.0 pm or less, further preferably 25.0 pm or less, further preferably 20.0 pm or less, further preferably 15.0 pm or less. The median particle diameter can be measured by the method described in the following Examples section.

[0066] [Active material]

[0067] The active material related to the present application contains an organic sulfur compound and an iron compound.

[0068] The organic sulfur compound is a compound in which at least a carbon atom, a hydrogen atom, and a sulfur atom are combined with each other. Since the active material is obtained by a step of firing a raw material containing a polymer (for example, rubber, resin, etc.), an iron ion-containing compound, and sulfur, it is difficult to analyze the detailed structure of the product obtained by this step, but it can be confirmed that the carbon atom, the hydrogen atom, and the sulfur atom are combined with each other. In this case, it is presumed that the organic sulfur compound contains a carbon-sulfur structure, such as a long-chain type polythiophene and benzene structure (in which a thiophene ring is condensed to form a chain), a structure similar thereto, or the like. In addition, depending on other raw materials used for firing or a gas atmosphere used, other atoms (for example, a nitrogen atom, etc.) can further be combined with the organic sulfur compound.

[0069] The iron compound is a compound in which an iron atom is combined with an atom other than the iron atom. Examples of the atom other than the iron atom include, for example, a sulfur atom. Examples of the iron compound include iron (II) sulfide (FeS), iron (III) sulfide (Fe2S3), iron disulfide (FeS2), etc. Among them, iron disulfide (FeS2) is typical or preferred.

[0070] Therefore, in the present application, the expression "the particulate active material contains an organic sulfur compound" can be expressed as "the particulate active material contains at least a carbon atom and a sulfur atom", or the expression "the particulate active material contains an organic sulfur compound and an iron compound" can be expressed as "the particulate active material contains at least a carbon atom, a sulfur atom, and an iron atom". In addition, in the case of containing a carbon atom, a sulfur atom, and an iron atom, the carbon element content, the sulfur element content, and the iron element content are measured by the method described in the following Examples section.

[0071] (Sulfur element content (A S (in mass %)

[0072] The sulfur content (mass%) in the active material is preferably greater than 50.0% by mass, more preferably greater than 55.0% by mass, even more preferably greater than 60.0% by mass, even more preferably greater than 61.0% by mass, even more preferably 61.2% by mass or more, even more preferably greater than 62.0% by mass, even more preferably greater than 63.0% by mass, even more preferably 63.6% by mass or more, even more preferably greater than 64.0% by mass, even more preferably 65.0% by mass or more, even more preferably greater than 65.0% by mass, even more preferably greater than 66.0% by mass, even more preferably greater than 70.0% by mass. The upper limit of the sulfur content is usually about 80% by mass, but there is no particular limitation.

[0073] Iron content (A) F (in terms of mass%)

[0074] The iron content (mass%) in the active material is preferably greater than 10.0% by mass, more preferably greater than 11.0% by mass, even more preferably greater than 12.0% by mass, even more preferably greater than 13.0% by mass, even more preferably greater than 13.5% by mass, even more preferably greater than 14.0% by mass, even more preferably greater than 14.5% by mass, even more preferably 14.9% by mass or more, even more preferably greater than 15.0% by mass, even more preferably 15.4% by mass or more, even more preferably 15.5% by mass or more, even more preferably greater than 16.0% by mass, even more preferably greater than 16.1% by mass, even more preferably greater than 17.0% by mass, even more preferably greater than 17.3% by mass. The upper limit of the iron content is usually about 25.0% by mass, but there is no particular limitation.

[0075] (Coating density (D, in mg / cm³) 2 )

[0076] Coating density of active material on electrode (mg / cm²) 2 Preferred concentration is greater than 2.5 mg / cm³. 2 More preferably greater than 6.2 mg / cm 2 Further preferred concentration is greater than 6.5 mg / cm³. 2 Further preferred concentration is greater than 6.6 mg / cm³. 2 Further preferred concentrations are greater than 7.0 mg / cm³. 2 Further preferred concentrations are greater than 8.0 mg / cm³. 2 Further preferred concentrations are greater than 8.5 mg / cm³. 2 Further preferred is 8.6 mg / cm³ 2 The above is further preferred to be 9.0 mg / cm³. 2 The above is further preferred to be 9.2 mg / cm³. 2 The above is further preferred, with a concentration greater than 9.5 mg / cm³.2 The upper limit of the coating density is not particularly limited, and the higher the upper limit, the more preferable. Thus, although the upper limit of the coating density is not particularly meaningful, the upper limit can be assumed to be about 15.0 mg / cm 2 , only as a reference value.

[0077] (carbon element content)

[0078] The carbon element content (mass %) in the active material is preferably greater than 15.0 mass %, more preferably greater than 17.0 mass %, and further preferably greater than 18.0 mass %. On the other hand, the carbon element content (mass %) is preferably less than 27.0 mass %, more preferably less than 25.0 mass %, and further preferably less than 24.0 mass %.

[0079] (hydrogen element content)

[0080] Since hydrogen atoms in the polymer react with sulfur to produce hydrogen sulfide by firing, the hydrogen sulfide is released to the outside of the system, thereby reducing the hydrogen element content, and thus the hydrogen element content (mass %) in the active material is relatively small. The hydrogen element content in the active material is preferably 1.6 mass % or less. The value is less than 1.5 mass %, more preferably 1.2 mass % or less, further preferably less than 1.1 mass %, further preferably less than 1.0 mass %, further preferably less than 0.5 mass %, further preferably 0.4 mass % or less, further preferably 0.3 mass % or less, and further preferably 0.2 mass % or less. On the other hand, the lower limit of the hydrogen element content (mass %) can be 0.1 mass %, can be less than 0.1 mass %, or can be below the detection limit.

[0081] (median particle diameter)

[0082] The active material is pulverized to have a prescribed particle diameter, and is preferably a particle having a size suitable for the production of an electrode. From the viewpoint of improving the performance of the electrode and / or the battery, the preferred particle size distribution of the active material particles is preferably a median particle diameter d 50 The median particle diameter is more preferably 1.5 μm or more, further preferably 2.0 μm or more, and further preferably 2.4 μm or more. In addition, the median particle diameter is more preferably 30.0 μm or less, further preferably 25.0 μm or less, further preferably 20.0 μm or less, and further preferably 15.0 μm or less. The median particle diameter can be measured by the method described in the following Examples section.

[0083] [Inequality (1)]

[0084] For the electrode of the present application, as described in Inequality (1) above, the sulfur element content A S(mass%), Iron content A in active materials F The product of (mass%) and the median particle size M (μm) of the active material is greater than 1600. The right side of inequality (1) is preferably 2000, more preferably 2400, even more preferably 2800, even more preferably 2900, even more preferably 3600, even more preferably 3700, even more preferably 3800, even more preferably 10000, even more preferably 10800. The upper limit of the value on the left side of inequality (1) is not particularly limited, but is typically about 20000, about 15000, or about 12000.

[0085] [Inequality (2)]

[0086] For the electrode of the present invention, as described in the above inequality (2), the sulfur content A in the active material is... S The quotient obtained by dividing the product of the mass percentage and the median particle size M (μm) of the active material by the thickness T (μm) of the metal foil constituting the current collector is preferably greater than 6.0. The right side of inequality (2) is more preferably 7.0, further preferably 7.9, further preferably 8.1, further preferably 10.0, further preferably 11.0, further preferably 12.0, further preferably 12.4, further preferably 15.0, further preferably 20.0, and further preferably 30.0. There is no particular upper limit to the value on the left side of inequality (2), but a higher upper limit is preferred. Therefore, although mentioning the upper limit of the value on the left side of inequality (2) is not particularly meaningful, the upper limit is usually about 90.0, about 60.0, or about 50.0, and is only used as a reference value.

[0087] The thickness T (μm) of the metal foil is preferably in the range of 5 μm or more, and more preferably in the range of 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.

[0088] [Inequality (3)]

[0089] For the electrode of the present invention, as described in the above inequality (3), the sulfur content A in the active material is... s (mass%), Iron content A in active materials F (mass%) and coating density D of active material (mg / cm³) 2The product of the left side of inequality (3) is preferably greater than 7000. The right side of inequality (3) is more preferably 7500, further preferably 8000, further preferably 8500, further preferably 9000, further preferably 9100, further preferably 9200, further preferably 9400, further preferably 9500, further preferably 10000. The upper limit of the left side of inequality (3) is not particularly limited, and the higher the upper limit is, the more preferable it is. Thus, although the upper limit of the left side of inequality (3) is not particularly meaningful, it can also be assumed to be about 20000, about 15000, or about 12000, as a reference value.

[0090] [charge / discharge capacity]

[0091] The electrode of the present application exhibits an excellent charge / discharge capacity. Further, in the following description, unless otherwise specified, the initial discharge capacity refers to the 3rd discharge capacity (DC3). Herein, the 3rd discharge capacity is the discharge capacity at the time when three times of charge and discharge are performed after the electrode and the battery are prepared (the definition of the 3rd discharge is the following cycle: 1st discharge, 1st charge, 2nd discharge, 2nd charge, 3rd discharge, 3rd charge), in which the discharge termination voltage is 1.0 V and the charge termination voltage is 3.0 V. In the case of discharge, when the discharge is performed at a constant current (the current value corresponding to 50 mA per 1 g of the positive electrode active material), the voltage is finally reduced from 3.0 V to 1.0 V. The total time (hour (h)) taken for the measured voltage to be reduced from 3.0 V to 1.0 V is measured, the capacity (mAh) is obtained by multiplying the measured total time by the applied current (mA), and the specific capacity (mAh / g) is obtained by dividing the capacity by the weight of the active material. On the other hand, in the case of charge, the charge is performed at a constant current, whereby the voltage is rather increased, and when the voltage finally reaches 3.0 V, the charge is terminated. The same applies to the 10th discharge capacity as described below.

[0092] (initial discharge capacity (DC3))

[0093] In the case where the electrode of the present application is used as a positive electrode, the initial discharge capacity (DC3) (mAh / g) is preferably greater than 400 mAh / g. The initial discharge capacity is preferably greater than 500 mAh / g, more preferably greater than 600 mAh / g, further preferably 634 mAh / g or more, further preferably 681 mAh / g or more, further preferably 690 mAh / g or more, further preferably greater than 700 mAh / g, further preferably greater than 730 mAh / g. The upper limit of the initial discharge capacity is not particularly limited, and the higher the upper limit is, the more preferable it is. Thus, although the upper limit of the initial discharge capacity is not particularly meaningful, it can also be assumed to be, for example, about 1000 mAh / g, as a reference value.

[0094] (10th discharge capacity (DC 10 ))

[0095] In the case where the electrode of the present application is used as a positive electrode, the discharge capacity at the time of repeating charging and discharging 10 times (i.e., the 10th discharge capacity (DC 10 )) (mAh / g) is preferably greater than 350 mAh / g. The discharge capacity is preferably greater than 400 mAh / g, more preferably greater than 500 mAh / g, further preferably greater than 600 mAh / g, further preferably greater than 650 mAh / g, further preferably 681 mAh / g or greater, further preferably greater than 690 mAh / g, further preferably 723 mAh / g or greater, further preferably 743 mAh / g or greater. The upper limit of the discharge capacity is not particularly limited, and the higher the upper limit is, the more preferable it is. Thus, although it is not meaningful to mention the upper limit of the discharge capacity, it can be generally assumed that the upper limit is, for example, about equal to the value of the initial discharge capacity or about 900 mAh / g, as a reference value.

[0096] Furthermore, when the 3rd discharge capacity and the 10th discharge capacity are measured using the electrode of the present application as a positive electrode, in order to enable the electrode to sufficiently exhibit the performance related to the discharge capacity of the positive electrode, the 3rd discharge capacity and the 10th discharge capacity are determined by the structure of the positive electrode using a negative electrode and an electrolyte which can be used as a lithium ion secondary battery for a long time (so that lithium is not immediately consumed) within the range of technical common sense. For example, as to the negative electrode, it is sufficient to use a negative electrode in which the amount (molar amount) of lithium is greater than one percent (preferably one-tenth, more preferably one-half) of the amount (molar amount) of sulfur in the positive electrode. In the example described below, the weight of the mixture layer containing the positive electrode active material is 103.4 mg to 144 mg (3 cm x 4 cm x coating density), and the mixture layer further contains other components other than sulfur, and thus the amount (molar amount) of sulfur is necessarily lower than 0.004491 (the value is 71.5% or less). On the other hand, the weight of the negative electrode is 534 mg (4 cm x 5 cm x 0.5 mm x specific gravity of lithium), and the amount (molar amount) of lithium is 0.076934 (≈0.534 / 6.941), and thus the amount (molar amount) of lithium in the negative electrode is sufficiently large. Furthermore, for example, as to the electrolyte, it is sufficient that the amount (molar amount) of lithium is greater than one percent (preferably one-tenth, more preferably one-half) of the amount (molar amount) of sulfur in the positive electrode. In the example described below, the amount (molar amount) of lithium in the electrolyte is 0.002 = the volume V (=2.0 mL) of the electrolyte x the concentration (=1.0 mol / L), and thus the amount (molar amount) of lithium in the electrolyte is sufficiently large. As described above, when a negative electrode and an electrolyte containing lithium (molar amount) in a sufficiently large amount are used, the discharge capacity of the positive electrode can be sufficiently exhibited. Herein, the volume V (mL) of the electrolyte means the total volume of the electrolyte containing solutes or a solid electrolyte.

[0097] [Application]

[0098] The electrode for a lithium-ion secondary battery of the present application can be used as a positive electrode or a negative electrode of a lithium-ion secondary battery. Furthermore, the electrode for a lithium-ion secondary battery of the present application is preferably used as a positive electrode of a lithium-ion secondary battery.

[0099] The electrode for a lithium-ion secondary battery can be produced in the same manner as described in the production method section below by using the materials described in the production method section below. That is, in the case where the electrode for a lithium-ion secondary battery as described above is used as a positive electrode, the conductive aid, the binder, the current collector, and the like described in the production method section below are used in the same manner as described in the production method section below, so that the electrode for a lithium-ion secondary battery as described above can be produced as a positive electrode; in the case where the electrode for a lithium-ion secondary battery as described above is used as a negative electrode, the negative electrode, the electrolyte, the separator, and the like described in the production method section below are used in the same manner as described in the production method section below, so that the electrode for a lithium-ion secondary battery as described above can be produced as a negative electrode. As described above, the following description of the production method section below can be regarded as a description of the electrode for a lithium-ion secondary battery.

[0100] < Lithium-ion secondary battery >

[0101] The lithium-ion secondary battery of the present application is a lithium-ion secondary battery including the electrode for a lithium-ion secondary battery as described above.

[0102] The lithium-ion secondary battery can be produced in the same manner as described in the production method section below by using the materials described in the production method section below. That is, in the case where the electrode for a lithium-ion secondary battery as described above is used as a positive electrode, the negative electrode, the electrolyte, the separator, and the like described in the production method section below are used in the same manner as described in the production method section below, so that the lithium-ion secondary battery can be produced. On the other hand, in the case where the electrode for a lithium-ion secondary battery as described above is used as a negative electrode, the positive electrode, the electrolyte, the separator, and the like described in the production method section below are used in the same manner as described in the production method section below, so that the lithium-ion secondary battery can be produced. As described above, the following description of the production method section below is regarded as a description of the lithium-ion secondary battery.

[0103] [Inequality (4)]

[0104] For the lithium-ion secondary battery of the present application, the sulfur element content A in the active material is controlled to be in the range of 0.0001 to 0.1% as described in the above inequality (4). SThe quotient obtained by multiplying the product of the mass % of the active material and the median particle diameter M (μm) of the active material by the volume V (mL) of the electrolyte is preferably greater than 60. The right side of Inequality (4) is more preferably 70, further preferably 79, further preferably 81, further preferably 100, further preferably 110, further preferably 120, further preferably 150, further preferably 200, further preferably 250, further preferably 300. The upper limit of the left side of Inequality (4) is not particularly limited, and the higher the upper limit, the more preferable. Thus, although the upper limit of the left side of Inequality (4) is not particularly meaningful, the upper limit is usually about 900, about 600, or about 500, as a reference value.

[0105] Since the volume V (mL) of the electrolyte varies depending on the size of the battery, the range is not particularly limited, and the minimum amount that exhibits the performance of the active material and allows the battery to operate sufficiently can be used. As a reference value, for example, in the case of the laminated battery described in the Examples, the volume V is preferably 1.0 mL or more, more preferably 1.2 mL or more, and further preferably 1.5 mL or more. On the other hand, the volume V is preferably 4.0 mL or less, more preferably 3.0 mL or less, and further preferably 2.5 mL or less.

[0106] [Application]

[0107] The lithium ion secondary battery of the present application can be used as a lithium ion secondary battery having improved comprehensive performance in terms of charge-discharge capacity and capacity retention rate.

[0108] [Production method]

[0109] Hereinafter, the production method of the electrode and the lithium ion secondary battery of the present application will be described in the order of the production method of the active material constituting the electrode.

[0110] [Production of active material]

[0111] The active material to which the present application relates can be produced by the following production method, which comprises: (1) mixing raw materials containing a polymer, an iron ion-containing compound, and sulfur, thereby obtaining a sintering raw material; and (2) sintering the sintering raw material in a non-oxidizing atmosphere.

[0112] (Polymer)

[0113] The polymer is not particularly limited, provided that it is a compound containing at least carbon and hydrogen atoms, and is a compound that absorbs sulfur to form an organic sulfur compound when fired together with an iron ion-containing compound and sulfur under a non-oxidizing atmosphere. In addition, the polymer can be a polymer containing a hetero atom (e.g., a nitrogen atom, a sulfur atom, etc.). Specific examples of the polymer include, for example, a polymer of an unsaturated chain hydrocarbon-based monomer, a condensate of a substituted aromatic hydrocarbon and chlorinated sulfur, etc. The polymer can be used alone, or two or more kinds thereof can be used in combination.

[0114] <<unsaturated chain hydrocarbon-based monomer polymer>>

[0115] Examples of the polymer of an unsaturated chain hydrocarbon-based monomer include, for example, a resin (e.g., an acrylic resin, etc.). In addition, examples of the polymer of an unsaturated chain hydrocarbon-based monomer include a diene-based rubber, etc. One or more kinds of the polymer of an unsaturated chain hydrocarbon-based monomer can be used.

[0116] Examples of the acrylic resin include, for example, at least one polymer selected from the group consisting of a polymer obtained by polymerizing at least one monomer selected from the group consisting of acrylic ester compounds represented by the following chemical formula (1); or a polymer obtained by polymerizing at least one monomer selected from the group consisting of acrylic ester compounds represented by the following chemical formula (1) and at least one monomer selected from the group consisting of diacrylic ester compounds represented by the following chemical formula (2). One or more kinds of the acrylic resin can be used, and one or more kinds of the acrylic ester compound can be used.

[0117] CH2=C(R 11 )COOR 12 (1)

[0118] (incidentally, R 11 is a hydrogen atom or a methyl group, and R 12 is an alkyl group.)

[0119] CH2=C(R 21 )COO-Y-OCO(R 22 )C=CH2(2)

[0120] (incidentally, R 21 and R 22 are the same or different, R 21 and R 22 are each a hydrogen atom or a methyl group, Y is a linear hydrocarbylene group, Y can have at least one substituent selected from the group consisting of a hydroxyl group and an alkyl group, and a carbon skeleton constituting the hydrocarbylene group can form an ether bond with an oxygen atom, wherein when the ether bond is two or more, there must be two or more intervening carbon atoms between any adjacent oxygen atoms.)

[0121] In the chemical formula (1), R 11 is preferably a methyl group, and R 12alkyl group having 1 to 4 carbon atoms, of which, methyl, n-butyl, isobutyl, t-butyl are more preferable. Examples of the compound represented by Chemical Formula (1) include, for example, methyl (meth)acrylate, butyl (meth)acrylate, and the like, of which, methyl methacrylate and butyl methacrylate are more preferable. Herein, "(meth)acrylate" in methyl (meth)acrylate and butyl (meth)acrylate means "acrylate" or "methacrylate" (the same applies hereinafter). Further preferable examples of the compound represented by Chemical Formula (1) are butyl methacrylate.

[0122] In Chemical Formula (2), R 21 and R 22 are each preferably methyl. The number of carbon atoms of the alkylene group (straight chain) of Y is preferably 2 to 6, more preferably 2 or 3. The number of substituents in Y is preferably 1 to 4, more preferably 1 or 2. As the substituents in Y, one or more substituents selected from the group consisting of a hydroxyl group and an alkyl group having 1 to 4 carbon atoms are preferable; as the alkyl group having 1 to 4 carbon atoms, methyl is preferable. For example, when the carbon skeleton of Y forms an ether bond with an oxygen atom, the moiety corresponding to -Y-O- is preferably represented by the following Chemical Formula (3) (wherein Chemical Formula (3) does not take into account the substituents in Y).

[0123] -(CH2) I -(CH2CH2O) m -(CH2CH2CH2O) n -(3)

[0124] (In the formula, I is 0 to 6, m is 0 to 3, and n is 0 to 2. However, I, m, and n cannot be 0 at the same time.)

[0125] In Chemical Formula (3), preferably, I is 1, 2, 3, 4, 5, or 6, and m and n are 0; m is 1, 2, or 3, and I and n are 0; or n is 1 or 2, and I and m are 0.

[0126] Examples of the compound represented by Chemical Formula (2) include, for example, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerol di(meth)acrylate, and the like. Of these, ethylene glycol dimethacrylate is preferable.

[0127] Preferred examples of the acrylic resin include a homopolymer of methyl (meth)acrylate, a homopolymer of butyl (meth)acrylate, a copolymer of methyl (meth)acrylate and ethylene glycol di(meth)acrylate, a copolymer of butyl (meth)acrylate and ethylene glycol di(meth)acrylate, and the like. Among them, as the acrylic resin, an acrylic resin of the methacrylate type is preferred. More preferred examples of the acrylic resin include a copolymer of methyl methacrylate and ethylene glycol dimethacrylate.

[0128] In the present application, the acrylic resin is preferably in the form of fine particles. In the present context, fine particles refer to particles having a particle diameter of 300.0 μm or less. The particle diameter is preferably 270.0 μm or less, more preferably 200.0 μm or less, further preferably 100.0 μm or less, further preferably 50.0 μm or less, further preferably 20.0 μm or less, further preferably 15.0 μm or less, further preferably 13.0 μm or less, further preferably 10.0 μm or less, further preferably 6.0 μm or less. On the other hand, the lower limit of the particle diameter is not particularly limited, but is generally, for example, 0.1 μm or more, preferably 1.0 μm or more. The particle diameter is a value (median particle diameter) measured by a particle diameter distribution measuring device (PSA1090L manufactured by Anton Paar GmbH).

[0129] The acrylic resin can be a spherical fine particle or a porous fine particle. When the acrylic resin is porous, the oil absorption amount thereof is preferably 100 mL / 100 g or more, more preferably 110 mL / 100 g or more, further preferably 120 mL / 100 g or more, further preferably 130 mL / 100 g or more, further preferably 140 mL / 100 g or more. The oil absorption amount is a value measured according to JIS K 5101-13-2:2004. More specifically, the measurement can be performed by the method described in paragraph 0069 of JP 2017-88501 A.

[0130] The acrylic resin is not particularly limited as long as it has the structure as described above, and the Mw thereof is not particularly limited. However, the Mw of the acrylic resin is generally in the range of 2000 to 1500000. The Mw is a value measured by gel permeation chromatography (GPC) (calibrated with polystyrene).

[0131] The acrylic resin is commercially available or can be produced by a conventional method within the knowledge of those skilled in the art. Examples of commercially available acrylic resins include, for example, acrylic resins manufactured by Sekisui Kasei Co., Ltd.

[0132] Examples of the diene-based rubber include, for example, natural rubber, isoprene rubber, butadiene rubber (e.g., high-cis polybutadiene rubber, etc.), and the like. The diene-based rubber is commercially available, or can be produced by a conventional method within the knowledge of one skilled in the art.

[0133] < Condensate of substituted aromatic hydrocarbon and chlorinated sulfur >

[0134] Examples of the condensate of substituted aromatic hydrocarbon and chlorinated sulfur include, for example, a condensate of alkylphenol and chlorinated sulfur, and the like. Specific examples of the condensate of alkylphenol and chlorinated sulfur include, for example, TACKIROL V200, TS3108, and TS3109 manufactured by Taoka Chemical Co., Ltd., Vultac 3 manufactured by Arkema, and the like. One or more condensates of substituted aromatic hydrocarbon and chlorinated sulfur can be used.

[0135] (Sulfur)

[0136] As the sulfur, various forms of sulfur such as powdered sulfur, insoluble sulfur, precipitated sulfur, colloidal sulfur, and the like can be used. Among them, precipitated sulfur and colloidal sulfur are preferred. One or more sulfurs can be used.

[0137] The content of the sulfur is preferably greater than 50 parts by mass, more preferably greater than 100 parts by mass, further preferably greater than 300 parts by mass, further preferably greater than 400 parts by mass, further preferably 500 parts by mass or more, based on 100 parts by mass of the polymer. When the content is greater than 50 parts by mass, there is a tendency that the charge / discharge capacity and the cycle characteristics can be improved. On the other hand, the content of the sulfur has no upper limit, but it is generally preferably less than 1000 parts by mass, more preferably less than 900 parts by mass, further preferably less than 800 parts by mass, further preferably less than 700 parts by mass. When the content is less than 1000 parts by mass, there is a tendency that there is an advantage in terms of cost. Furthermore, in the present specification, the term "cycle characteristics" refers to a characteristic in which the charge / discharge capacity of the secondary battery is maintained despite the repetition of charging / discharging. Thus, a secondary battery in which the degree of decrease in the charge / discharge capacity is high and the capacity retention rate is low as the repetition of charging and discharging progresses has poor cycle characteristics, in contrast, a secondary battery in which the degree of decrease in the charge / discharge capacity is low and the capacity retention rate is high has excellent cycle characteristics.

[0138] As the sulfur, any one of various allotropes can be used, but it is preferred that the sulfur include S8 sulfur which is a solid at normal temperature and pressure, and more preferably a monomer of S8 sulfur.

[0139] (Iron ion-containing compound)

[0140] The iron ion-containing compound is not particularly limited as long as it is a compound that forms an iron compound when fired with the polymer and sulfur under a non-oxidizing atmosphere. Examples of such an iron ion-containing compound can include ferrite salts, iron complexes, and the like that contain divalent or trivalent iron ions. Examples of the ferrite salt can include organic acid salts of iron and inorganic acid salts of iron. On the other hand, examples of the iron complex can include neutral iron complexes and iron complex salt (iron complex salt).

[0141] Examples of the organic acid salt of iron include, for example, a salt of divalent iron (Fe 2+ ) with an organic acid, a salt of trivalent iron (Fe 3+ ) with an organic acid, and the like. Among them, a salt of divalent iron with an organic acid is preferred. The organic acid is not particularly limited and is, for example, an organic acid having a carboxyl group (-COOH), an organic acid having a sulfonic group (-SO3H), or the like. Among them, an organic acid having a carboxyl group is preferred. Specific examples of the organic acid include aliphatic acids, oxalic acid, tartaric acid, citric acid, malic acid, succinic acid, and the like. Specific examples of the aliphatic acid include, for example, aliphatic acids having 1 to 6 carbon atoms, such as acetic acid, propionic acid, butyric acid, and the like. Among them, acetic acid, oxalic acid, and the like are preferred. Preferred examples of the organic acid salt of iron include iron (II) acetate, iron (II) oxalate, and the like. They can be hydrates. One or more kinds of the organic acid salt of iron can be used.

[0142] Examples of the inorganic acid salt of iron include, for example, a salt of divalent iron (Fe 2+ ) with an inorganic acid, a salt of trivalent iron (Fe 3+ ) with an inorganic acid, and the like. Among them, a salt of divalent iron with an inorganic acid is preferred. Specific examples of the inorganic acid include hydrochloric acid, sulfuric acid, nitric acid, and the like. Among them, nitric acid and the like are preferred. Preferred examples of the inorganic acid salt of iron include iron (II) chloride, iron (III) chloride, iron (II) sulfate, iron (III) sulfate, iron (II) nitrate, iron (III) nitrate, and the like. They can be hydrates. One or more kinds of the inorganic acid salt of iron can be used.

[0143] Examples of the iron complex include, for example, a divalent iron (Fe 2+ ) complex, a trivalent iron (Fe 3+ ) complex, and the like. The iron complex can be in the form of a neutral complex, and can also be in the form of a complex salt. The ligand coordinated to the iron ion is not particularly limited and examples include, for example, a halogen atom (e.g., a chlorine atom, a bromine atom, and the like), a cyano group, a dicyclopentadienyl group, N,N’-bis(salicylidene)ethylenediamine, and the like. Examples of the iron complex include potassium hexacyanoferrate (II) ([Fe(CN)6]K4), potassium hexacyanoferrate (III) ([Fe(CN)6]K3), sodium iron (III) chloride ([FeCl4]Na), dicyclopentadienyl iron (II) (ferrocene), N,N’-bis(salicylidene)ethylenediamine iron (III) chloride, and the like. One or more kinds of the iron complex can be used.

[0144] As the iron ion-containing compound, at least one selected from the group consisting of the organic acid salt of iron, the inorganic acid salt of iron, the neutral iron complex, and the iron complex salt as described above can be used. Among them, the organic acid salt of iron, the inorganic acid salt of iron, or the neutral iron complex is preferred.

[0145] The particle diameter of the iron ion-containing compound is not particularly limited, but the particle diameter (e.g., the median particle diameter) is preferably 1 μm or more, more preferably 2 μm or more. In addition, the particle diameter is preferably 40 μm or less, further preferably 30 μm or less, further preferably 20 μm or less, further preferably 15 μm or less. The median particle diameter can be measured by the method described in the Examples section described below.

[0146] From the viewpoint of improving the performance of the electrode and / or the battery, the content of the iron ion-containing compound is preferably 50 parts by mass or more and 300 parts by mass or less, based on 100 parts by mass of the polymer. The content is more preferably greater than 50 parts by mass, further preferably greater than 60 parts by mass, further preferably greater than 70 parts by mass, further preferably greater than 75 parts by mass. On the other hand, the content is more preferably less than 250 parts by mass, further preferably less than 200 parts by mass, further preferably less than 150 parts by mass, further preferably less than 125 parts by mass.

[0147] (Iron ion-containing compound dispersed polymer)

[0148] In the present application, the polymer and the iron ion-containing compound can be used as an iron ion-containing compound dispersed polymer obtained by previously dispersing the iron ion-containing compound in the polymer. Such an iron ion-containing compound dispersed polymer can be produced by causing the polymer to undergo a polymerization reaction in a state in which the iron ion-containing compound is previously dispersed in a monomer constituting the polymer. The polymerization reaction can be performed by a conventional method.

[0149] As the iron ion-containing compound dispersed polymer, for example, an iron ion-containing compound dispersed acrylic resin obtained by dispersing an iron ion-containing compound in an acrylic resin can be appropriately used. Preferred examples of the acrylic resin used for this purpose include a homopolymer of methyl (meth)acrylate, a copolymer of methyl methacrylate and ethylene glycol dimethacrylate, and the like. In addition, preferred examples of the iron ion-containing compound used for this purpose include ferrous oxalate (II), and the like.

[0150] (Other materials)

[0151] The raw material can appropriately contain other materials commonly used in the art as needed. Examples of such materials include, for example, an electrically conductive carbon material, and the like.

[0152] <<Electrically conductive carbon material>>

[0153] The raw material can contain an electrically conductive carbon material. This is because it can improve the electrical conductivity of the active material. A carbon material having a graphite structure is preferred as the electrically conductive carbon material. As the carbon material, for example, porous carbon materials (e.g., activated carbon, etc.), graphite, carbon black, acetylene black, ketjen black, carbon fiber (CF), and nano carbon materials (e.g., carbon nanotube (CNT), carbon nanofiber, graphene, fullerene, etc.) can be used. One or more electrically conductive carbon materials can be used.

[0154] Among them, acetylene black, carbon black, and ketjen black are preferred because they are inexpensive and have excellent dispersibility. Furthermore, a small amount of CNT, graphene, etc. can be used in combination with acetylene black, carbon black, or ketjen black. Such a combination system makes it possible to further improve the cycle characteristics of the lithium ion secondary battery without significantly increasing the cost. Furthermore, the total amount of CNT or graphene is preferably 8 mass% or more and 12 mass% or less of the total amount of the electrically conductive carbon material.

[0155] The content of the electrically conductive carbon material is preferably more than 5 parts by mass and more preferably more than 10 parts by mass, based on 100 parts by mass of the polymer. When the content is more than 5 parts by mass, there is a tendency to easily achieve the purpose of further improving the charge and discharge capacity and the cycle characteristics. On the other hand, the content is preferably less than 50 parts by mass and more preferably less than 40 parts by mass. When the content is less than 50 parts by mass, the proportion of the sulfur-containing structure in the active material does not decrease relatively, and there is a tendency to easily achieve the purpose of further improving the charge and discharge capacity and the cycle characteristics.

[0156] (Mixing Step (1))

[0157] The mixing step is a step of preparing a sintering raw material.

[0158] The mixing step can be performed by mixing the polymer, the iron ion-containing compound, sulfur, and any other components. In this case, the iron ion-containing compound-dispersed polymer described above can also be used instead of the polymer and the iron ion-containing compound.

[0159] The mixing described above can be performed by a conventional method without particular limitation as long as the mixing is a method in which the components are sufficiently mixed. In the present application, examples of the preferred mixing method can include at least mixing by a wet (WET) method or mixing by a dry (DRY) method, which will be mentioned below.

[0160] <<WET method>>

[0161] In the present application, the WET method includes the following steps for preparing a raw material,

[0162] (a-1) adding a polymer and an iron ion-containing compound to a solvent (e.g., an organic solvent, etc.) to obtain a mixture, or adding a monomer capable of forming a polymer and an iron ion-containing compound and performing a polymerization reaction to obtain a mixture containing a polymer, wherein the iron ion-containing compound is dispersed inside and on the surface of the polymer;

[0163] (a-2) removing the solvent from the mixture to obtain a dry mixture; and

[0164] (a-3) mixing the dry mixture with sulfur.

[0165] In sub-step (a-1), the method of adding a polymer or a monomer, an iron ion-containing compound, etc. to an organic solvent is not particularly limited as long as a mixture can be obtained by mixing them. For example, (1) the polymer (or monomer) and the iron ion-containing compound can be simultaneously added to the organic solvent and mixed, (2) the polymer (or monomer) can be added to the organic solvent and mixed, and then the iron ion-containing compound can be added and mixed, or (3) the iron ion-containing compound can be added to the organic solvent and mixed, and then the polymer (or monomer) can be added and mixed.

[0166] In sub-step (a-1), as the organic solvent, an organic solvent commonly used in the art can be used, examples of which include, for example, N-methyl-2-pyrrolidone, N,N-dimethylformamide, alcohols, hexane, water, acetone, ethers (e.g., tetrahydrofuran, etc.), etc. Furthermore, as the organic solvent, an organic solvent capable of dissolving a polymer is preferred. This is because it contributes to good mixing. One or more of these solvents can be used. Furthermore, in the case of using a liquid monomer in sub-step (a-1), a solvent is not essential, and when a solvent is not used, sub-step (a-2) can be omitted.

[0167] Sub-step (a-1) can be performed by stirring in a container (e.g., a beaker, etc.).

[0168] In sub-step (a-2), the removal of the organic solvent can be performed by a conventional method. For example, the removal can be performed by a drying method (e.g., heat drying, reduced pressure drying, reduced pressure and heat drying, etc.) on the mixture in sub-step (a-1).

[0169] Preferably, the dry mixture thus obtained is pulverized before the next step is performed. This is because, in this way, it can be expected that the mixing in sub-step (a-3) will be more appropriately performed.

[0170] In sub-step (a-3), the mixing of the dry mixture and sulfur can be performed by a conventional method, examples of which can include, for example, a method of mixing them using a blender, etc.

[0171] < Dry method >

[0172] In the present application, the dry method includes the following steps for preparing the raw material,

[0173] (b-1) mixing the polymer, the iron ion-containing compound, and the sulfur in a powder state, or dispersing the iron ion-containing compound in the polymer and the sulfur in a powder state.

[0174] In this context, the powder refers to a state in which each of the raw materials of the solid has been made sufficiently fine for use in mixing for the purpose of the present application. The size of each of the particles constituting the powder is not particularly limited as long as mixing can be appropriately performed, but is generally in the range of, for example, 1 μm or more to 40 μm or less. From the viewpoint of improving the performance of the electrode and / or the battery, 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, further preferably 10 μm or less, in terms of the median particle diameter. The median particle diameter can be measured by the method described in the Examples section below.

[0175] The mixing can be performed by a conventional method, for example, in the same manner as the mixing in sub-step (a-3) as described above.

[0176] In the wet method and the dry method, it is preferable to mix the raw materials sufficiently in advance for the firing. Furthermore, when an electrically conductive carbon material or the like is added to the raw materials, these additives can also be mixed in advance for the firing, so that they are included in the raw materials in advance.

[0177] The raw material thus obtained can be used directly for the next firing step, or it can be formed into a pellet as needed and used for the next step.

[0178] (Firing step (2))

[0179] The firing step is a step of firing the raw material obtained as described above. The firing of the raw material can be performed by a conventional method, for example, by heating the raw material at a predetermined temperature- increasing rate until a predetermined temperature is reached, holding it at the predetermined temperature for a predetermined time, and then allowing it to cool naturally.

[0180] < Non-oxidizing atmosphere >

[0181] The firing is preferably performed under a non-oxidizing atmosphere. The non-oxidizing atmosphere refers to an atmosphere substantially free of oxygen, which is used for suppressing the oxidative deterioration and excessive pyrolysis of the constituent components. Specifically, the non-oxidizing atmosphere refers to an inert gas atmosphere (e.g., nitrogen, argon, etc.), a sulfur gas atmosphere, an ammonia gas atmosphere, etc. Thus, the firing can be appropriately performed under an inert gas atmosphere, for example, in a quartz tube.

[0182] <<Heating Rate>>

[0183] The heating rate is preferably in the range of, for example, 50°C / h or higher and 500°C / h or lower. The heating rate is preferably 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 tends to be easier to achieve the goal of improving charge / discharge capacity and cycle characteristics.

[0184] <<Firing Temperature / Time>>

[0185] The firing temperature refers to the temperature at which the raw material is heated to its final temperature, and this temperature is maintained for a certain period of time for firing the raw material. This temperature is preferably in the range of 250°C or higher and 550°C or lower. When the temperature is higher than 250°C, it tends to avoid incomplete sulfidation reaction and prevent a decrease in the charge / discharge capacity of the material. On the other hand, when the temperature is lower than 550°C, it tends to prevent raw material decomposition, prevent a decrease in yield, and prevent a decrease in charge / discharge capacity. This temperature is more preferably higher than 300°C, even more preferably higher than 350°C, and even more preferably higher than 370°C. On the other hand, this temperature is more preferably lower than 500°C, and even more preferably lower than 450°C.

[0186] From the perspective of improving the performance of electrodes and / or batteries, the firing temperature in the firing step is preferably higher than the temperature at which iron-containing compounds undergo thermal decomposition.

[0187] The holding time at the firing temperature can be set appropriately according to the type of raw materials and the firing temperature, for example, preferably more than 1 hour and less than 6 hours. When it is more than 1 hour, it tends to promote firing sufficiently, and when it is less than 6 hours, it tends to prevent excessive pyrolysis of the constituent components.

[0188] <<Device>>

[0189] Firing can be performed using, for example, a muffle furnace. Figure 1 The process can be carried out either directly or using a continuous equipment (such as a twin-screw extruder). When using a continuous equipment, there are advantages such as the ability to continuously prepare sulfur-based active materials through a series of operations (such as kneading, crushing, mixing raw materials, and firing simultaneously within the equipment).

[0190] muffle furnace ( Figure 1 The furnace is separated by hot plates, etc., so that the heat source (heater) is not exposed inside the furnace, thus preventing sample contamination. Figure 1In this embodiment, the muffle furnace 1 is provided with a heater 2 in the lower portion of the furnace, which is separated by a heating plate. The front side (left side in the drawing) of the furnace is provided with a cover 3, which maintains the atmosphere of the interior of the furnace in an inert gas 4. A thermocouple is connected to the cover (not shown), so that the temperature of the interior of the furnace during the firing process can be measured. The upper and lower portions of the interior of the furnace are provided with two layers of stainless steel (SUS) rectangular cuboid trays 5, 6 for firing the raw material.

[0191] The configuration of the interior of the furnace allows a gas (for example, an inert gas such as argon (Ar) or the like) to be continuously supplied and discharged to the outside through a gas introduction pipe 7 and a gas discharge pipe 8. The gas discharge pipe 8 is connected to a trap tank 10 containing an aqueous sodium hydroxide solution 9, and the exhaust gas that exits the muffle furnace 1 through the gas discharge pipe 8 is discharged to the outside once it passes through the aqueous sodium hydroxide solution 9 in the trap tank 10. Thus, even if the exhaust gas contains hydrogen sulfide gas generated by the reaction, the hydrogen sulfide gas is neutralized by the aqueous sodium hydroxide solution and removed from the exhaust gas.

[0192] (Step of removing residues)

[0193] In the processed product obtained after firing, there are cases in which unreacted sulfur that sublimated during the firing process and then precipitated upon cooling remains. In the presence of these residues, it is necessary to remove them as much as possible, because they can cause deterioration of the cycle characteristics. The removal of residues can be performed according to conventional methods, for example, by drying by reduced pressure heating, drying with hot air, washing with a solvent, or the like.

[0194] (Pulverization / classification)

[0195] In order to obtain particles having a size suitable for the production of electrodes, it is preferable to pulverize the obtained active material to a predetermined particle size and perform classification. The preferred size (particle size distribution) of the particulate active material is as described above.

[0196] The pulverization can be performed by a conventional method, for example, can be performed by subjecting the active material to a pulverization treatment under predetermined conditions using a pulverizer such as a cutting pulverizer, an air flow pulverizer, or the like. The pulverization conditions differ depending on the pulverizer used and the like, for example, in the case of using a cutting pulverizer (for example, Free Speed Mill FS-20 manufactured by Labonect Co., Ltd.), the treatment can be performed under conditions of a rotation speed of 20,000 rpm or more and 30,000 rpm or less for 1 second or more and 30 seconds or less. Further, in the case of using a dry-type air flow pulverizer (for example, Nano Jetmizer NJ-30 manufactured by Aishin Nano Technologies CO., LTD.), the treatment can be performed at a processing speed of 1 g / min or more and 3 g / min or less and a pulverization pressure of 0.5 MPa or more and 2.0 MPa or less.

[0197] Further, in the firing method using the twin-screw extruder as described above, the active material prepared can also be pulverized into particles by being subjected to shearing in the kneading process while the active material is being prepared.

[0198] [Preparation of an electrode for a lithium-ion secondary battery]

[0199] Using the particulate active material obtained as described above, an electrode for a lithium-ion secondary battery having an active material layer containing the particulate active material can be prepared by a conventional method. That is, the electrode can be obtained in the same manner as in the case of preparing a general lithium-ion secondary battery, except that the particulate active material as described above is used as the active material.

[0200] (Case in which the particulate active material is used as a positive electrode active material)

[0201] The positive electrode for a lithium-ion secondary battery can be prepared in the same manner as in the case of a general positive electrode for a lithium-ion secondary battery, except that the particulate active material as described above is used as the positive electrode active material. For example, the positive electrode can be prepared by mixing the particulate active material with a conductive aid, a binder, and a solvent to prepare a paste-like positive electrode material, coating the positive electrode material on a current collector, and then performing drying. Further, as another method, the positive electrode can also be prepared by, for example, kneading the particulate active material together with a conductive aid, a binder, and a small amount of a solvent and causing them to form a film using a mortar or the like, and then crimping it to a current collector using a press or the like. However, in this case, the coating of the active material is preferably performed in such a manner that the coating density D (mg / cm2) of the active material satisfies the above inequality (3). 2 ) of the above inequality (3).

[0202] <Conductive aid>

[0203] Examples of the conductive aid include, for example, vapor grown carbon fiber (VGCF), carbon powder, carbon black (CB), acetylene black (AB), ketjen black (KB), graphite, or a fine powder of a metal stable at a positive electrode potential, such as aluminum, titanium, or the like. In addition, as the conductive aid, the electrically conductive carbon material described above can also be used. One or two or more of these conductive aids can be used.

[0204] <Binder>

[0205] Examples of the binder include polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamide-imide (PAI), carboxymethyl cellulose (CMC), polyvinyl chloride (PVC), acrylic resin, methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene ether (PPO), polyethylene oxide (PEO), polyethylene (PE), polypropylene (PP), or the like. One or more of these binders can be used.

[0206] <Solvent>

[0207] Examples of the solvent include N-methyl-2-pyrrolidone, N,N-dimethylformamide, an alcohol, hexane, water, or the like. One or more of these solvents can be used.

[0208] <Mixing amount>

[0209] The mixing amount of these materials that constitute the positive electrode is not particularly limited, but, for example, 2 to 100 parts by mass of the conductive aid, 2 to 50 parts by mass of the binder, and an appropriate amount of the solvent are preferably mixed based on 100 parts by mass of the active material. In this case, for the sulfur element content A S (mass %) and the iron element content A F (mass %) of the active material and the coating density D (mg / cm 2 ) of the active material, the product of A S , A F and D preferably determines to satisfy the inequality (3) as described above.

[0210] <Current collector>

[0211] As the current collector, a current collector commonly used for the positive electrode of a lithium-ion secondary battery can be used. Examples of the current collector include, for example, a current collector composed of a metal foil (e.g., an aluminum foil, an aluminum mesh, a punched aluminum sheet, an expanded aluminum sheet, a stainless steel foil, a stainless steel mesh, a punched stainless steel sheet, an expanded stainless steel sheet, a nickel foam, a nickel nonwoven fabric, a copper foil, a copper mesh, a punched copper sheet, an expanded copper sheet, a titanium foil, a titanium mesh, or the like) and a carbon nonwoven fabric, a carbon woven fabric, or the like. Among them, a current collector containing a metal foil is preferred. The current collector can be used alone, or two or more kinds thereof can be used in combination. Furthermore, the surface of the current collector can be coated with carbon or the like. Specific examples of the current collector having a surface coated with carbon or the like include, for example, a carbon-coated aluminum foil, or the like. In this case, the current collector contains a carbon-coated portion.

[0212] (Case where particulate active material is used as negative electrode active material)

[0213] The negative electrode for a lithium-ion secondary battery can be prepared in the same manner as in the case of a general negative electrode for a lithium-ion secondary battery, except that the particulate active material as described above is used as the negative electrode active material. For example, the negative electrode can be prepared by mixing the particulate active material with a conductive aid, a binder, and a solvent to prepare a paste-like negative electrode material, coating the negative electrode material on a current collector, and then drying. Furthermore, as another method, the negative electrode can also be prepared by, for example, kneading the particulate active material with a conductive aid, a binder, and a small amount of a solvent using a mortar or the like and forming them into a film, and then crimping it to a current collector using a press or the like. However, in this case, the coating of the active material is preferably performed in such a manner that the coating density D (mg / cm2) of the active material satisfies the inequality (3) as described above. 2

[0214] As the conductive aid, the binder, and the solvent, the same conductive aid, the binder, and the solvent as in the case where the particulate active material is used as the positive electrode active material can be used, and they can also be coated on the current collector.

[0215] [Preparation of lithium-ion secondary battery]

[0216] The lithium-ion secondary battery of the present application can be prepared in the same manner as in the case of a general lithium-ion secondary battery, except that the electrode for a lithium-ion secondary battery as obtained above is used.

[0217] (Case where particulate active material is used as positive electrode active material)

[0218] The lithium-ion secondary battery of the present application can be prepared in accordance with a conventional method, using a negative electrode and an electrolyte, and further using, as necessary, a member such as a separator or the like, except that the positive electrode containing the particulate active material (positive electrode active material) as described above is used. ​

[0219] < negative electrode >

[0220] As the negative electrode material, metal lithium, carbon-based materials (e.g., graphite, etc.), silicon-based materials (e.g., silicon thin film, etc.), and alloy-based materials (e.g., copper-tin, cobalt-tin, etc.) known to those skilled in the art can be used. Among the negative electrode materials as described above, when a lithium-free material (e.g., carbon-based material, silicon-based material, alloy-based material, etc.) is used as the negative electrode material, short-circuiting between the positive electrode and the negative electrode due to the generation of dendrites is less likely to occur, which is advantageous. However, when these lithium-free negative electrode materials are used in combination with the positive electrode of the present application, neither the positive electrode nor the negative electrode contains lithium. Therefore, a lithium pre-doping process in which lithium is doped in advance into either or both of the negative electrode and the positive electrode is required. As the lithium pre-doping method, only a known method needs to be used. For example, lithium pre-doping methods in which lithium is doped in the negative electrode include a method in which lithium is embedded by an electrolytic doping method (a half-cell is assembled using metal lithium as a counter electrode, and lithium is electrochemically doped) and a method in which lithium is embedded by a pasting pre-doping method (a metal lithium foil is pasted to an electrode, and then placed in an electrolyte, and doping is performed by utilizing diffusion of lithium into the electrode). Furthermore, when lithium is pre-doped into the positive electrode, the electrolytic doping method as described above can be used. As the lithium-free negative electrode material, a silicon-based material (a negative electrode material having a high capacity) is particularly preferable, and among them, a thin film silicon in which the electrode thickness is thin and thus has an advantage in terms of capacity per volume is more preferable.

[0221] < electrolyte >

[0222] The electrolyte compensates for the electric charge generated by emission of electrons to the outside circuit accompanying oxidation / reduction of the active material at the positive electrode / negative electrode through ion flow. As the electrolyte for a lithium ion secondary battery, an electrolyte obtained by dissolving an alkali metal salt as an electrolyte in an organic solvent can be used. As the organic solvent, at least one selected from non-aqueous solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dimethyl ether, γ-butyrolactone, acetonitrile, etc. is preferably used. As the electrolyte, LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiI, LiClO4, etc. can be used. The concentration of the electrolyte only needs to be about 0.5 to 1.7 mol / L. Furthermore, the electrolyte is not limited to a liquid electrolyte. For example, when the lithium ion secondary battery is a lithium polymer secondary battery, the electrolyte is in a solid state (e.g., a polymer gel state).

[0223] < separator >

[0224] In addition to the negative electrode, the positive electrode, and the electrolyte as described above, the lithium ion secondary battery can also include components such as a separator and the like. The separator is located between the positive electrode and the negative electrode, allows the migration of ions between the positive electrode and the negative electrode, and prevents internal short-circuiting between the positive electrode and the negative electrode. If the lithium ion secondary battery is of the sealed type, the separator needs to have a function of retaining the electrolyte. As the separator, a thin and microporous film or a non-woven fabric film made of polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide, cellulose, glass, or the like is preferably used.

[0225] <<Shape>>

[0226] 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, a button shape, and the like can be used.

[0227] (Case in which a particulate active material is used as a negative electrode active material)

[0228] In addition to the negative electrode containing the particulate active material (negative electrode active material) as described above, a positive electrode and an electrolyte can also be used, and further components such as a separator and the like can be used as necessary, and the lithium ion secondary battery of the present application is produced according to a conventional method.

[0229] <<Positive Electrode>>

[0230] The positive electrode material is not particularly limited, and only needs to be, for example, a lithium-containing transition metal oxide or a solid solution oxide, or a substance capable of electrochemically absorbing and releasing lithium. Examples of the lithium-containing transition metal oxide include, for example, Li-Co-based composite oxides (e.g., LiCoO2and the like), Li-Ni-Co-Mn-based composite oxides (LiNi x Co y Mn z O2and the like), Li-Ni-based composite oxides (e.g., LiNiO2and the like), Li-Mn-based composite oxides (e.g., LiMn2O4and the like), and the like. Examples of the solid solution oxide include, for example, LiaMn 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 O4and the like. These compounds can be used alone, and a plurality of them can also be used in mixture.

[0231] The same electrolyte, separator, and lithium-ion secondary battery shapes as in the case where the particulate active material is used as the positive electrode active material can be adopted for the electrolyte, separator, and lithium-ion secondary battery shapes.

[0232] Examples

[0233] While the present application will be described based on examples, the present application is not limited to the examples.

[0234] The following summary shows various chemicals used in the examples and comparative examples.

[0235] <Materials used in the test>

[0236] Iron ion-containing compound: ferrous (II) oxalate dihydrate prepared in Synthesis Example 1

[0237] Iron ion-containing compound-dispersed polymer: iron ion-containing compound-dispersed acrylic resin (copolymer of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) (MMA:EGDMA = 80:20 (mass ratio) and in a state in which the iron ion-containing compound is dispersed inside or on the surface of the acrylic resin)) prepared in Synthesis Example 2

[0238] Polymer: acrylic resin (MMA:EGDMA = 80:20 (mass ratio)) obtained by polymerization in the same manner as in Preparation Example 2 except that the iron ion-containing compound was not used

[0239] Sulfur: precipitated sulfur manufactured by Tsurumi Chemical Industry Co., Ltd.

[0240] Synthesis Example 1 (iron ion-containing compound)

[0241] A substance obtained by pulverizing ferrous (II) oxalate dihydrate (ferrous (II) oxalate dihydrate manufactured by KANTO CHEMICAL CO., INC., ultrapure) using a freezer mill (JFC-2000 manufactured by Japan Analytical Industry Co., Ltd.) for 10 minutes was used as the iron ion-containing compound. Particle diameter (median particle diameter d 50 ): 2.10 μm, specific surface area: 6.6 m 2 / g.

[0242] Synthesis Example 2 (iron ion-containing compound-dispersed polymer)

[0243] 80 parts by mass of methyl methacrylate (MMA) and 20 parts by mass of ethylene glycol dimethacrylate (EGDMA) were mixed to prepare a mixture, 100 parts of the iron ion-containing compound was further dispersed therein, and the mixture was subjected to a polymerization reaction, thereby obtaining an acrylic resin in which the iron ion-containing compound was dispersed inside or on the surface of the acrylic resin. The polymerization conversion rate was 100%.

[0244] Examples and Comparative Examples

[0245] Preparation of particulate active material

[0246] (Baking raw material)

[0247] According to Table 2, the respective components were mixed with a blender to obtain a raw material for baking.

[0248] (Reaction apparatus for baking)

[0249] The raw material was baked using a muffle furnace (Model: KDF-12-12, manufactured by Advantec Toyo Kaisha, Ltd.). Figure 1 The muffle furnace in the above-described Figure 1

[0250] (Baking step)

[0251] First, the baking raw material was placed in a tray (SUS container), and the atmosphere inside the muffle furnace was replaced with Ar gas three times using a vacuum pump. Then, Ar gas was continuously supplied from a gas introduction tube at a flow rate of 100 mL / min, and after 30 minutes from the start of the supply, the muffle furnace was heated. The temperature was increased at a temperature increase rate of 5°C / h, and when the temperature of the baking raw material reached the baking temperature described in Table 2, heat treatment was performed for 2 hours while maintaining the baking temperature. Then, while adjusting the flow rate of the Ar gas, the temperature of the baked material was naturally cooled to 25°C under an Ar gas atmosphere, and then the baked substance was taken out of the muffle furnace.

[0252] (Pulverization step)

[0253] The baked substance was pulverized using a cutting pulverizer, a dry jet mill, or the like. The pulverization step of Preparation Examples 1 to 7 will be described below.

[0254] For Preparation Examples 1 to 6, as shown in Table 1, the baked substance was pulverized by processing it using a cutting pulverizer (free speed pulverizer, FS-20, manufactured by Labonect) at a predetermined rotation speed for a predetermined period of time.

[0255] Table 1

[0256]

[0257] ​On the other hand, for Preparation Example 7, the fired product was pulverized using a dry jet mill (Nano Jetmizer NJ-30 manufactured by Aishin Nano Corporation) at a processing speed of 2 g / min and a pulverization pressure of 1.1 MPa.

[0258] (Classification step)

[0259] In order to remove coarse particles from the pulverized fired product, the fired product was classified using a stainless steel sieve with a mesh size of 32 μm to obtain an active material.

[0260] <Physical properties of active material>

[0261] (Elemental analysis)

[0262] The active materials prepared in the examples and comparative examples were subjected to elemental analysis.

[0263] For carbon, hydrogen, nitrogen, and sulfur, using a fully automatic elemental analyzer vario MICROcube manufactured by Elementar Corporation, the mass ratio (%) thereof in the total amount of the active material was calculated based on the measured mass. The results are shown in Table 2.

[0264] (Iron element content)

[0265] The active materials prepared in the examples and comparative examples were subjected to thermogravimetric analysis, and based on the obtained measurement results, the amount of each iron element (mass %) was calculated.

[0266] Thermogravimetric analysis was performed using a TGA Q500 manufactured by TA Instruments Corporation. The measurement conditions were such that, after heating the active material to 750°C under an Ar atmosphere, air was introduced to completely decompose the sample. Then, the ash content ratio (mass %) in each active material was calculated from the measured weight reduction ratio (mass %) by the following calculation method.

[0267] Ash content ratio (mass %) = 100 - weight reduction ratio (mass %)

[0268] Further, since it was confirmed that the iron element in the examples and comparative examples of the present application existed as iron disulfide (FeS2), the proportion (mass %) of iron disulfide was calculated from the ash content ratio (mass %) of each of the examples and comparative examples of the present application by the following calculation method. That is, it was seen from the weight reduction ratio (100 mass %) of Preparation Example 8 in Table 2 that the polymer 1 (acrylic resin) was completely decomposed after conversion into sulfide. On the other hand, it was seen from the weight reduction ratio (36 mass %) of Preparation Example 9 that the weight of iron disulfide (FeS2) in the active material was reduced by 36 mass %. Therefore, the proportion (mass %) of iron disulfide (FeS2) in each active material was calculated by the following calculation method,

[0269] Proportion of FeS2 (mass %) = Ash content ratio x {100 / (100-36)}.

[0270] Further, the iron element content (Fe proportion, mass %) was calculated from the proportion of FeS2 (mass %) by the following calculation method,

[0271] Fe proportion (mass %) = Proportion of FeS2 x Atomic mass of Fe / (Atomic mass of Fe + Atomic mass of S x 2),

[0272] (wherein, it is assumed that 55.845 is used as the atomic mass of Fe and 32.065 is used as the atomic mass of S).

[0273] (Median particle diameter)

[0274] A laser diffraction / scattering type particle size distribution analyzer (Particle Size Distribution Analyzer PSA1090L manufactured by Anton Paar GmbH) was used to measure the volume-based cumulative 50% size (median particle diameter d 50 ) with water as the dispersion medium.

[0275] The results are shown in Table 2 below

[0276] Table 2

[0277]

[0278] Preparation of positive electrode and lithium-ion secondary battery

[0279] According to Table 3, a positive electrode was prepared using the active material obtained as described above, and further using this positive electrode, a lithium-ion secondary battery was prepared as follows.

[0280] (Positive electrode)

[0281] As the active material, the active material obtained as described above was used, as the conductive aid, acetylene black was used, and as the binder, an acrylic resin was used. They were weighed so that the ratio of active material: conductive aid: binder was 90:5:5 (mass %), put into a container, and stirred and mixed using a rotation mixer (ARE-250 manufactured by THINKY Co., Ltd.) to prepare a uniform slurry using milliQ water as a dispersant. The prepared slurry was coated on a 20-μm carbon-coated aluminum foil using a coater with a slit width of 200 to 300 μm, and was pressed using a roll press to obtain an electrode, which was then heated and dried at 120°C for 3 hours using a drying device. After drying, the electrode was cut into a shape in which a portion including the mixture layer of the active material had a width of 4 cm and a length of 3 cm, and the electrode also had an exposed portion of the positive electrode current collector as a positive electrode tab, to thereby obtain an electrode (positive electrode). Then, the mass of the electrode was measured, and the amount of the active material in the electrode was calculated from the ratio as described above. In addition, "coating density (mg / cm 2 " described in Table 3 refers to the coating density of the active material.

[0282] (Negative electrode)

[0283] The negative electrode used a metal lithium foil (manufactured by Honjo Metal Co., Ltd., thickness: 0.5 mm), and the negative electrode current collector used a copper foil. The metal lithium foil was cut into a width of 5 mm and a length of 4 cm, and was connected to the copper foil to thereby obtain an electrode (negative electrode).

[0284] (Electrolyte)

[0285] As the electrolyte, a nonaqueous electrolyte in which LiPF6 was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate was used. Ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1:1. The concentration of LiPF6 in the electrolyte was 1.0 mol / L.

[0286] (Lithium-ion secondary battery)

[0287] A laminated battery was prepared using the positive electrode and the negative electrode as described above. Specifically, in a dry room, a separator (Celgard 2400 manufactured by Celgard LLC, polypropylene microporous film with a thickness of 25 μm) and a glass nonwoven fabric filter (GA100 manufactured by ADVANTEC Co., Ltd., with a thickness of 440 μm) were sandwiched between the positive electrode and the negative electrode to form an electrode body battery. This electrode body battery was housed in a laminated pack formed of an aluminum laminated film (manufactured by MTI). The electrolyte (2.0 mL) as described above was infused into the laminated pack. The laminated pack was sealed with an air degassing sealer to obtain the lithium-ion secondary battery of Example 1.

[0288] Evaluation of lithium-ion secondary batteries

[0289] (discharge capacity, capacity retention rate)

[0290] Each of the laminated lithium-ion secondary batteries prepared in the examples and comparative examples was charged and discharged at a current value corresponding to 50 mA per 1 g of positive electrode active material at a test temperature of 30°C. The discharge termination voltage was set to 1.0 V, and the charge termination voltage was set to 3.0 V. In addition, while repeating the charge and discharge, the battery discharge capacity (mAh) at the 1st, 2nd, 3rd, and 10th times was observed.

[0291] The 3rd discharge capacity DC3 (mAh / g) was defined as the initial capacity. The larger the initial capacity, the larger the charge-discharge capacity of the lithium-ion secondary battery, and it can be evaluated as preferable. In addition, the capacity retention rate (%) was calculated from the 10th discharge capacity DC10 (mAh / g) and the 3rd discharge capacity DC3 (mAh / g) by the following formula. 10 (mAh / g) and the 3rd discharge capacity DC3 (mAh / g) by the following formula. It can be said that the higher the capacity retention rate, the more excellent the cycle characteristics of the lithium-ion secondary battery.

[0292] Capacity retention rate (%) = (DC 10 / DC3) x 100.

[0293] (charge-discharge capacity index)

[0294] The 10th discharge capacity (DC 10 ) (mAh / g) in each of the examples and comparative examples was expressed as an index using the following formula. The larger the index, the larger the discharge capacity, and the more preferable it is. The measurement was performed using a battery performance evaluation device (BLS system manufactured by KEISOKU CENTER CO., LTD.),

[0295] Charge-discharge capacity index = 10th discharge capacity in each of the examples and comparative examples / discharge capacity in Comparative Example 1 x 100.

[0296] (capacity retention rate index)

[0297] The capacity retention rate (DC 10 / DC3) (%) in each of the examples and comparative examples was expressed as an index using the following formula. The larger the index, the larger the capacity retention rate, and the more preferable it is,

[0298] Capacity retention rate index = capacity retention rate in each of the examples and comparative examples / capacity retention rate in Comparative Example 1 x 100.

[0299] (comprehensive performance index)

[0300] The average of the total values of the charge-discharge capacity index and the capacity retention index is defined as the comprehensive performance index.

[0301] Table 3

[0302]

[0303] According to Table 3, the improvement in the comprehensive performance of the charge-discharge capacity and the capacity retention rate is exhibited in the examples.

[0304] <Embodiment>

[0305] The following shows a preferred embodiment.

[0306] [1] An electrode for a lithium-ion secondary battery,

[0307] The electrode has an active material layer containing a particulate active material,

[0308] The active material contains an organic sulfur compound and an iron compound,

[0309] wherein A s , A F and M satisfy the following inequality (1), and the right side of the following inequality (1) is preferably 2000,

[0310] (1) A s x A F x M > 1600,

[0311] In the formula, A s represents the sulfur element content in the active material, in mass%, A F represents the iron element content in the active material, in mass%, and M represents the median particle diameter of the active material, in pm.

[0312] [2] The electrode of the above [1], wherein the value of the right side of inequality (1) is 2400, preferably 2800, more preferably 2900, further preferably 3600, further preferably 3700, further preferably 3800, further preferably 10000, further preferably 10800.

[0313] [3] The electrode of the above [1] or [2], wherein the sulfur element content A S in the active material is greater than 65.0 mass%, preferably greater than 66.0 mass%, more preferably greater than 70.0 mass%.

[0314] [4] The electrode of any one of the above [1] to [3], wherein the iron element content A Fmore than 16.1 mass%, further preferably more than 17.0 mass%, further preferably more than 17.3 mass%.

[0315] [5] The electrode according to any one of [1] to [4] above, wherein the median particle diameter M is more than 1.0 pm, preferably more than 1.0 pm and 40.0 pm or less, more preferably more than 1.0 pm and 30.0 pm or less, further preferably more than 1.5 pm and 25.0 pm or less, further preferably more than 2.0 pm and 20.0 pm or less, further preferably more than 2.4 pm and 15.0 pm or less.

[0316] [6] The electrode according to any one of [1] to [5] above, wherein the initial discharge capacity DC3 in the case where the electrode is used as a positive electrode is more than 400 mAh / g.

[0317] [7] The electrode according to [6] above, wherein the initial discharge capacity DC3 is more than 600 mAh / g, preferably more than 634 mAh / g, more preferably more than 681 mAh / g, further preferably more than 690 mAh / g, further preferably more than 700 mAh / g, further preferably more than 730 mAh / g.

[0318] [8] The electrode according to any one of [1] to [7] above,

[0319] wherein the electrode has a current collector,

[0320] the current collector has a metal foil,

[0321] A S , M and T satisfy the following inequality (2), and the right side of inequality (2) is preferably 7.0, more preferably 7.9, further preferably 8.1, further preferably 10.0, further preferably 11.0, further preferably 12.0, further preferably 12.4, further preferably 15.0, further preferably 20.0, further preferably 30.0,

[0322] (2) A S x M / T > 6.0,

[0323] In the formula, T represents the thickness of the metal foil, in pm.

[0324] [9] The electrode according to any one of [1] to [8] above, wherein A S , A Fand D satisfy the following inequality (3), and the right side of inequality (3) is preferably 7500, more preferably 8000, further preferably 8500, further preferably 9000, further preferably 9100, further preferably 9200, further preferably 9400, further preferably 9500, further preferably 10000,

[0325] (3) A S x A F x D > 7000,

[0326] In the formula, D represents the coating density of the active material, and the unit is mg / cm 2 .

[0327]

[10] The electrode according to any one of [1] to [9] above, wherein the 10th discharge capacity DC 10 is greater than 350 mAh / g, preferably greater than 400 mAh / g, more preferably greater than 500 mAh / g, further preferably greater than 600 mAh / g, further preferably greater than 650 mAh / g, further preferably 681 mAh / g or greater, further preferably greater than 690 mAh / g, further preferably 723 mAh / g or greater, further preferably 743 mAh / g or greater, when the electrode is used as a positive electrode.

[0328]

[11] A lithium ion secondary battery having the electrode according to any one of [1] to

[10] above.

[0329]

[12] The lithium ion secondary battery according to

[11] above,

[0330] wherein the lithium ion secondary battery further has an electrolyte,

[0331] A S , M, and V satisfy the following inequality (4), and the right side of inequality (4) is preferably 70, more preferably 79, further preferably 81, further preferably 100, further preferably 110, further preferably 120, further preferably 150, further preferably 200, further preferably 250, further preferably 300,

[0332] (4) A S x M / V > 60,

[0333] In the formula, V represents the volume of the electrolyte, and the unit is mL.

[0334] List of Reference Symbols

[0335] 1. Muffle furnace

[0336] 2. Heater

[0337] 3. Lid

[0338] 4. Inert gas

[0339] 5. Tray (upper layer)

[0340] 6. Tray (lower layer)

[0341] 7. Gas introduction pipe

[0342] 8. Gas discharge pipe

[0343] 9. Sodium hydroxide aqueous solution

[0344] 10. Trapping tank

Claims

1. An electrode for a lithium-ion secondary battery, the electrode has an active material layer containing a particulate active material, the active material contains an organic sulfur compound and an iron compound, The sulfur element content A in the active material S greater than 60.0 mass%, The iron element content A in the active material F greater than 14.0 mass%, wherein A S , A F and M satisfy the following inequality (1), (1) A S x A F x M > 2400, wherein A S represents the content of sulfur element in the active material, in mass%, A F represents the content of iron element in the active material, in mass%, M represents the median particle diameter of the active material, in pm.

2. The electrode of claim 1, wherein, Sulfur element content A in the active material S greater than 65.0 mass %.

3. The electrode according to claim 1 or 2, wherein, Iron element content A in the active material F greater than 15.0 mass %.

4. The electrode according to claim 1 or 2, wherein, the median particle diameter M is 1.0 μm or more.

5. The electrode according to claim 1 or 2, wherein, the initial discharge capacity DC3 is greater than 400 mAh / g in the case where the electrode is used as a positive electrode.

6. The electrode of claim 5, wherein, the initial discharge capacity DC3 is greater than 600 mAh / g.

7. The electrode according to claim 1 or 2, wherein, the electrode has a current collector, the current collector has a metal foil, A S M and T satisfy the following inequality (2), (2) A S x M / T > 6.0, in the formula, T represents the thickness of the metal foil, and the unit is μm.

8. The electrode according to claim 1 or 2, wherein, A S , A F and D satisfy the following inequality (3), (3) A S x A F x D > 7000, In the formula, D represents the coating density of the active material, in mg / cm 2 .

9. The electrode according to claim 1 or 2, wherein, 10th discharge capacity DC in the case where the electrode is used as a positive electrode 10 greater than 350 mAh / g.

10. A lithium-ion secondary battery, wherein, the lithium-ion secondary battery has the electrode according to claim 1 or 2.

11. The lithium-ion secondary battery according to claim 10, wherein, the lithium-ion secondary battery further has an electrolyte, A S M and V satisfy the following inequality (4), (4) A S x M / V > 60, in the formula, V represents the volume of the electrolyte, and the unit is mL.

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