Electrodes and secondary batteries

Abstract

Provided is an electrode comprising a current collector, a conductive coating layer that is formed on the current collector, and a mixture layer that is formed on the conductive coating layer, wherein the average thickness of the conductive coating layer is not less than 0.5 μm, the tortuosity τ at the interface of the conductive coating layer and the mixture layer is not less than 1.05, the mixture layer includes an active material, and the average particle diameter d50 of the active material is not less than 2 μm. The purpose of the present invention is to increase charge / discharge capacity while having a high basis weight.

Description

[Technical Field]

[0001] The present invention relates to a novel electrode and a secondary battery, particularly a lithium-ion secondary battery, comprising said electrode. [Background technology]

[0002] Secondary batteries, especially lithium-ion secondary batteries, have a large charge / discharge capacity and are therefore primarily used as batteries for portable electronic devices. Lithium-ion secondary batteries are also increasingly being used in electric vehicles, and their performance is expected to improve.

[0003] Patent Document 1 describes the use of sulfur-modified polyacrylonitrile as a positive electrode active material in the electrodes of lithium ion secondary batteries.

[0004] It has also been proposed to increase battery capacity by using, as the negative electrode active material, materials capable of absorbing and releasing more lithium ions, such as silicon (Si) and tin (Sn). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2010 / 044437 Summary of the Invention [Problem to be solved by the invention]

[0006] However, while organic active materials containing sulfur-based active materials, such as those described in Patent Document 1, have attracted attention as active materials for next-generation batteries, they are softer than conventional transition metal oxides and therefore cannot penetrate into the current collector when pressed, resulting in only a point-like contact between the composite layer containing the active material and the current collector. This results in problems such as high electrode resistance, preventing the active material from achieving its full charge / discharge capacity, and poor adhesion of the composite layer, which easily peels off from the current collector during electrode fabrication and charge / discharge. These problems are particularly pronounced when thickening electrodes to increase battery capacity, or when fabricating so-called high-area-area electrodes.

[0007] The present invention provides an electrode, i.e., a positive electrode or a negative electrode, which has a high charge / discharge capacity despite its high basis weight, and a secondary battery, particularly a lithium ion secondary battery, comprising the electrode. [Means for solving the problem]

[0008] That is, the present invention relates to the following electrode. An electrode comprising a current collector, a conductive coating layer formed on the current collector, and a composite layer formed on the conductive coating layer, The conductive coating layer has an average thickness of 0.5 μm or more, The interface tortuosity τ between the conductive coating layer and the composite layer is 1.05 or more, The mixture layer contains an active material, and the active material has an average particle diameter d 50 The electrode, wherein the thickness is 2 μm or more. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide an electrode, i.e., a positive electrode or a negative electrode, which has a high charge / discharge capacity despite having a high basis weight, and a secondary battery, particularly a lithium ion secondary battery, including the electrode. [Brief explanation of the drawings]

[0010] [Figure 1]FIG. 1 is a cross-sectional view schematically showing a reaction apparatus used in the production of an active material in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] The configuration of the present invention will be described in detail below. The upper and lower limit values ​​of "greater than or equal to" and "less than or equal to" in the description of a numerical range can be arbitrarily combined, and the numerical values ​​in the examples can also be used as the upper and lower limits. Furthermore, a numerical range indicated as including both end values ​​is understood to also disclose a numerical range that does not include either of the end values, and a numerical range that does not include either of the end values, as long as it does not contradict the spirit of the present invention.

[0012] One embodiment of the present invention is the following electrode. An electrode comprising a current collector, a conductive coating layer formed on the current collector, and a composite layer formed on the conductive coating layer, The conductive coating layer has an average thickness of 0.5 μm or more, The interface tortuosity τ between the conductive coating layer and the composite layer is 1.05 or more, The mixture layer contains an active material, and the active material has an average particle diameter d 50 The electrode, wherein the thickness is 2 μm or more.

[0013] Although not intending to be bound by theory, the reason why the electrode of the present invention can improve charge / discharge capacity despite having a high basis weight is thought to be as follows.

[0014] That is, in the electrode of the present invention, a highly flexible conductive coating layer is first provided on the current collector, and then a composite layer is applied thereon and pressed. As a result, the composite layer penetrates into the conductive coating layer, and the contact between the conductive coating layer and the composite layer becomes a surface rather than a point. This is thought to reduce the resistance of the electrode, improve the adhesion of the composite layer, and improve the charge / discharge capacity even with a high basis weight.

[0015] For each of any five particles of the active material, when the particle diameter is d (μm) and the test force at the moment when the particle breaks in a microcompression test is P (mN), it is preferable that the average value of the breaking strength Cs (MPa) calculated by the following formula is less than 70.0. Cs=2.48×P / πd 2 (where d is the average diameter of the particle measured in two perpendicular directions, and π is the constant of the circumference of the particle (3.14).)

[0016] This is because it has a typical feature of the present invention, namely low breaking strength.

[0017] The average value of the breaking strength Cs (MPa) is preferably less than 10.0.

[0018] This is because it has a typical feature of the present invention, namely low breaking strength.

[0019] It is preferable that the active material contains at least two elements selected from the group consisting of carbon, sulfur, nitrogen, and oxygen, and that the content of the elements in the active material is more than 50.0 mass %.

[0020] This is because it is a typical example of an active material having the characteristics of the present invention.

[0021] It is preferable that the active material contains sulfur, and that the content of sulfur in the active material is 45.0 mass % or more.

[0022] This is because it is a typical example of an active material having the characteristics of the present invention.

[0023] The carbon content of the mixture layer is preferably 95 mass % or less relative to the total mass of the mixture layer.

[0024] The sulfur content relative to the total mass of the mixture layer is preferably 5 mass % or more.

[0025] The volume resistivity of the conductive coating layer when coated on the surface of the current collector is 1.0×10 6It is preferably Ωcm or less.

[0026] When the electrode is used as a positive electrode, the initial discharge capacity is preferably more than 650 mAh / g.

[0027] Another aspect of the present invention is a secondary battery comprising the electrode.

[0028] The secondary battery is preferably a lithium ion secondary battery.

[0029] <Definition> The "conductive coating layer" is a layer that contributes to reducing the resistance of the electrode by allowing the composite layer formed on the conductive coating layer to penetrate into the conductive coating layer when pressed during electrode fabrication. The volume resistivity of the conductive coating layer is 1.0 x 10 6 It is preferably Ωcm or less. The "volume resistivity of the conductive coating layer" is a value measured in accordance with JIS K 7194-1994.

[0030] The "composite layer" refers to a layer made of an electrode material (composite material) containing an active material, and is a member that constitutes an electrode together with a current collector.

[0031] Unless otherwise specified, the term "initial discharge capacity" in this specification refers to the third discharge capacity.

[0032] <Measurement method> The "average thickness of the conductive coating layer" is determined by measuring the total thickness, including the current collector, after the conductive coating layer has been formed at any five points using a Mitutoyo Digimatic Indicator, and then calculating the arithmetic mean of the values ​​obtained by subtracting the thickness of the current collector from the measured values.

[0033] In the present invention, the "interface curvature τ" means the interface curvature between the conductive coating layer and the composite material layer constituting the electrode. The interface curvature is represented as b / a (where a < b), where a is the distance (a μm) between two predetermined points and b is the length (b μm) of the interface on the composite material layer side in the cross-section of the electrode by a plane perpendicular to the plane in which the conductive coating layer and the composite material layer form a layer. In the present invention, the interface curvature τ is the average value of the results measured five times, once for each of any five cross-sections of the electrode. Also, when the electrode has a front surface and a back surface, the average value is calculated five times for each surface, and then the average value of both is taken. Also, the values of a and b can be obtained from a microscopic image. When obtaining the values of a and b from a microscopic image, image analysis software can be used. As the image analysis software, for example, Image-Pro (image analysis software manufactured by Media cybernetics) or ImageJ (https: / / imagej.nih.gov / ij / index.html) can be used.

[0034] The "amount of carbon element in the active material" is measured by the elemental analysis method described below.

[0035] The "amount of sulfur element in the active material" is measured by the elemental analysis method described below.

[0036] The "amount of nitrogen element in the active material" is measured by the elemental analysis method described below.

[0037] The "amount of oxygen element in the active material" is measured by the elemental analysis method described below.

[0038] "Average particle size d 50 " is the volume-based cumulative 50% diameter (median diameter) measured using a laser diffraction / scattering particle size distribution measuring device (PSA1090L particle size distribution measuring device manufactured by Anton Paar) with water as the dispersion medium.

[0039] (Fracture strength Cs) The breaking strength Cs (MPa) is a value that can be determined by a microcompression test described in the Examples section below. However, the breaking strength Cs (MPa) can be measured from five active material particles before they are applied to an electrode, or it can be measured by extracting five active material particles that have been applied to an electrode and still retain their particle shape, i.e., are not broken, from the electrode.

[0040] The electrode and secondary battery of the present invention will be described below.

[0041] <Electrode> The electrode of the present invention is an electrode comprising a current collector, a conductive coating layer formed on the current collector, and a composite layer formed on the conductive coating layer, wherein the conductive coating layer has an average thickness of 0.5 μm or more, the interface tortuosity τ between the conductive coating layer and the composite layer is 1.05 or more, the composite layer contains an active material, and the active material has an average particle diameter d 50 is 2 μm or more.

[0042] [Current collector] The current collector is a chemically inert electronic conductor that maintains the flow of current to the electrode during charging or discharging of the battery. Examples of materials for the current collector include at least one metal selected from silver, copper, gold, aluminum, magnesium, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, and molybdenum, as well as stainless steel. Among these, aluminum is preferred.

[0043] The surface of the current collector may have a known protective layer, or may be treated by a known method.

[0044] The current collector can be in the form of a foil, sheet, film, wire, rod, mesh, etc. Suitable current collectors include metal foils such as copper foil, nickel foil, aluminum foil, and stainless steel foil. The foil current collector preferably has a thickness in the range of 1 μm to 100 μm.

[0045] [Conductive coating layer] The conductive coating layer has a volume resistivity of 1.0 x 10 when coated on the collector surface. 6 The conductive coating layer preferably has a volume resistivity of 1.0×10 Ωcm or less. The conductive coating layer contributes to a reduction in the resistance of the electrode by allowing the composite layer formed on the conductive coating layer to penetrate well into the conductive coating layer when pressed during electrode fabrication. The volume resistivity is preferably 1.0×10 5 Ωcm or less, more preferably 1.0×10 4 Ωcm or less, more preferably 1.0×10 3 It is less than Ωcm.

[0046] The material constituting the conductive coating layer is not particularly limited as long as it can achieve an interface tortuosity τ of 1.05 or greater with the composite layer. Therefore, any material commonly used to coat the current collector surface can be used as long as it can achieve an interface tortuosity τ of 1.05 or greater. Materials that can be used for such coating layers include, for example, carbon materials such as graphite, acetylene black, ketjen black, carbon nanotubes, and graphene; metal materials such as silver, copper, gold, aluminum, magnesium, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, and stainless steel; and conductive polymer materials such as polyvinylidene fluoride, poly(3,4-ethylenedioxythiophene) (PEDOT), polythiophene, polyaniline, polypyrrole, and polyacetylene. These materials can be used alone or in combination. These materials may be in the form of fibers or beads. For example, a fibrous metal material or a bead-shaped metal material can be used.

[0047] The average thickness of the conductive coating layer is 0.5 μm or more. If the average thickness of the conductive coating layer is less than 0.5 μm, it may be difficult to achieve the interface tortuosity τ, which is undesirable. The average thickness of the conductive coating layer is preferably 1 μm or more, more preferably 2 μm or more, even more preferably 4 μm or more, and even more preferably more than 5 μm. There is no particular upper limit to the average thickness of the conductive coating layer, but it may be, for example, about 30 μm. The average thickness of the conductive coating layer can be measured by the above-mentioned method.

[0048] [Mixture layer] The composite layer according to the present invention contains an active material described below. As described below, the composite layer is formed using an electrode material (i.e., a positive electrode material or a negative electrode material) containing the active material.

[0049] (carbon content) The carbon content of the mixture layer is preferably 95 mass % or less relative to the total mass of the mixture layer.

[0050] The carbon content is preferably less than 90% by mass, more preferably less than 80% by mass, and even more preferably less than 70% by mass. On the other hand, the carbon content is preferably greater than 5% by mass, more preferably greater than 10% by mass, and even more preferably greater than 15% by mass. The carbon content is measured, for example, by scanning or transmission electron microscope energy dispersive X-ray spectroscopy (SEM-EDX or TEM-EDX).

[0051] (Sulfur content) The sulfur content relative to the total mass of the mixture layer is preferably 5 mass % or more.

[0052] The mass proportion of sulfur is preferably more than 10 mass%, more preferably more than 20 mass%, and even more preferably more than 30 mass%. ,example For example, it can be measured by scanning or transmission electron microscope energy dispersive X-ray spectroscopy (SEM-EDX or TEM-EDX).

[0053] [Active material] The active material according to the present invention has an average particle size d 50 There are no particular limitations on the active material, and various materials can be used as long as the active material has a thickness of 2 μm or more and can achieve an interface tortuosity τ of 1.05 or more between the conductive coating layer and the composite layer. In particular, a relatively soft active material can be mentioned as a typical example.

[0054] The active material preferably contains at least two elements selected from the group consisting of carbon, sulfur, nitrogen, and oxygen, and the content of the elements in the active material is preferably greater than 50.0 mass%. This is because such compounds are relatively soft, allowing for the use of a conductive coating layer, a feature of the present invention. The active material preferably contains carbon and sulfur, and more preferably contains carbon, sulfur, and oxygen.

[0055] The content of the element in the active material is preferably more than 60.0 mass%, more preferably more than 70.0 mass%, even more preferably more than 75.0 mass%, and even more preferably more than 80.0 mass%. On the other hand, the content is preferably less than 95.0 mass%, more preferably less than 90.0 mass%, and even more preferably less than 85.0 mass%. .0 quality The amount is less than %.

[0056] (Sulfur element content) The amount of elemental sulfur in the active material is preferably 45.0% by mass or more, because such compounds are relatively soft and can take advantage of the advantage of the present invention, that is, the use of a conductive coating layer.

[0057] The amount of elemental sulfur in the active material is preferably more than 50.0 mass%, more preferably more than 55.0 mass%, and even more preferably more than 60.0 mass%, while the amount of elemental sulfur is preferably less than 75.0 mass%, more preferably less than 70.0 mass%, and even more preferably less than 65.0 mass%.

[0058] (carbon element amount) The carbon element content in the active material is preferably more than 10.0 mass%, more preferably more than 13.0 mass%, and even more preferably more than 15.0 mass%, while the carbon content is preferably less than 23.0 mass%, more preferably less than 21.0 mass%, and even more preferably less than 19.0 mass%.

[0059] (amount of hydrogen element) The amount of hydrogen element in the active material is relatively small because the hydrogen atoms in the raw materials are released outside the system by combining with other elements and becoming gas during firing. The amount of hydrogen element in the active material is preferably less than 1.0% by mass. The amount of hydrogen element is more preferably less than 0.5% by mass, even more preferably less than 0.3% by mass, and even more preferably less than 0.2% by mass. Meanwhile, the lower limit of the amount of hydrogen element may be 0.1% by mass, or may be less than 0.1% by mass, or may be below the detection limit.

[0060] (nitrogen element content) The amount of nitrogen element in the active material may be below the detection limit, but may be contained, and when the active material contains nitrogen element, the amount of nitrogen element is preferably more than 5.0 mass%, more preferably more than 8.0 mass%, and even more preferably more than 10.0 mass%, while the amount of nitrogen element is preferably less than 30.0 mass%, more preferably less than 25.0 mass%, and even more preferably less than 20.0 mass%.

[0061] (oxygen element content) The amount of oxygen element in the active material is preferably more than 0.5% by mass, more preferably more than 1.0% by mass, even more preferably more than 1.5% by mass, even more preferably more than 2.0% by mass, and even more preferably more than 2.5% by mass, while the amount of oxygen element is preferably less than 5.0% by mass, more preferably less than 4.5% by mass, even more preferably less than 4.0% by mass, and even more preferably less than 3.5% by mass.

[0062] (Average particle size d 50 ) Average particle size of active material d 50 is 2 μm or more. Average particle size d 50If the specific surface area of ​​the active material is less than 2 μm, the amount of binder and conductive additive required to constitute the electrode increases, which undesirably leads to a relative decrease in the amount of active material in the electrode.

[0063] Average particle size d 50 is preferably more than 3 μm, more preferably more than 4 μm, even more preferably 5 μm or more, even more preferably more than 6 μm, even more preferably more than 7 μm, and even more preferably more than 8 μm. 50 is preferably 50 μm or less, more preferably 40 μm or less, and further preferably 30 μm or less.

[0064] (Breaking strength Cs (MPa)) The average value of the breaking strength Cs (MPa) measured for any five particles of the active material is preferably less than 70.0. The breaking strength Cs (MPa) can be determined by the microcompression test described in the Examples section below. The breaking strength Cs of LiCoO2 measured by this method is 72.25 MPa.

[0065] From the viewpoint of the effects of the present invention, the average value of the breaking strength Cs (MPa) is preferably less than 60.0 MPa, more preferably less than 30.0 MPa, even more preferably less than 15.0 MPa, even more preferably 10.0 MPa or less, even more preferably less than 10.0 MPa, and even more preferably less than 5.0 MPa. Note that a value of less than 5.0 MPa also includes cases where Cs cannot be measured. Cs is an index of hardness, with a larger value indicating a harder particle, but if the particle is very soft, it becomes impossible to measure. In the present invention, since the active material particles are preferably soft, the preferred range has been described above, but active material particles that are so soft that they cannot be measured are also preferred. Therefore, a value of less than 10.0 MPa also includes cases where measurement is impossible.

[0066] For example, when a polymer is used as the raw material for the active material, the average breaking strength Cs (MPa) can be adjusted by using a polymer with a lower degree of cross-linking or a more porous polymer, or conversely, by using a polymer with a higher degree of cross-linking or a less porous polymer. In this way, the breaking strength Cs (MPa) can be adjusted.

[0067] [Interface curvature ratio τ] The interface tortuosity τ between the conductive coating layer and the composite layer is 1.05 or more. If the interface tortuosity τ is less than 1.05, the composite layer does not sufficiently penetrate into the conductive coating layer, and the conductive coating layer and the composite layer do not come into sufficient contact with each other, which is undesirable.

[0068] The interface tortuosity τ can be adjusted by adjusting the softness (or hardness) of the conductive coating layer and the composite layer. In other words, it can be increased or decreased by adjusting the relationship between the softness (or hardness) of the composite layer so that the composite layer penetrates into the conductive coating layer.

[0069] The interface tortuosity τ is preferably 1.10 or more, more preferably 1.15 or more, even more preferably 1.20 or more, even more preferably 1.25 or more, even more preferably 1.30 or more, even more preferably 1.35 or more, even more preferably 1.40 or more, even more preferably 1.45 or more, and even more preferably 1.50 or more. There is no particular limitation on the upper limit of the interface tortuosity τ, but if shown merely as a reference value, it is, for example, about 1.70.

[0070] [Electrode coating weight] Electrode coating weight (mg / cm 2 ) is a value defined in the Examples section below. The electrode coating weight is 1 mg / cm 2 More preferably, more than 3 mg / cm 2 More preferably, 5 mg / cm 2 On the other hand, the electrode coating weight is 20 mg / cm 2 Preferably less than 15 mg / cm2 or less, more preferably 13 mg / cm 2 is less than.

[0071] [Electrode porosity] The electrode porosity (%) is a value defined in the Examples section below. The electrode porosity is preferably greater than 15%, more preferably 22% or greater, and even more preferably greater than 25%. On the other hand, the electrode porosity is preferably less than 60%, more preferably 52% or less, and even more preferably less than 50%.

[0072] [Electrode adhesion] The electrode adhesion (mN / mm) is a value defined in the Examples section below. The electrode adhesion is preferably greater than 10 mN / mm, more preferably greater than 25 mN / mm, and even more preferably 31 mN / mm or greater. On the other hand, the electrode porosity is preferably less than 50 mN / mm, more preferably less than 40 mN / mm, and even more preferably 35 mN / mm or less.

[0073] [Charge / discharge capacity] The electrode of the present invention exhibits excellent charge / discharge capacity. In the following, unless otherwise specified, the term "initial discharge capacity" refers to the third discharge capacity (DC3). Here, the third discharge capacity refers to the discharge capacity when the electrode and battery are fabricated and charged / discharged three times with a discharge cutoff voltage of 1.0 V and a charge cutoff voltage of 3.0 V (the third discharge, which is the third discharge, when the cycle is repeated: first discharge, first charge, second discharge, second charge, third discharge, and third charge). When discharging at a constant current (equivalent to 50 mA per gram of positive electrode active material), the voltage of 3.0 V eventually drops to 1.0 V. The total time (h) required for the voltage to drop from 3.0 V to 1.0 V is measured and multiplied by the applied current (mA) to obtain the capacity (mAh). Dividing this value by the weight of the active material gives the specific capacity (mAh / g). On the other hand, when charging, the voltage rises due to charging with a constant current, and when it finally reaches 3.0V, charging is complete.

[0074] (1st discharge capacity (DC1)) The first discharge capacity (DC1) (mAh / g) of the electrode of the present invention is preferably 650 mAh / g or more. The first discharge capacity is preferably greater than 650 mAh / g, more preferably greater than 700 mAh / g, even more preferably greater than 750 mAh / g, even more preferably greater than 800 mAh / g, even more preferably greater than 850 mAh / g, even more preferably greater than 875 mAh / g, even more preferably greater than 880 mAh / g, even more preferably greater than 890 mAh / g, and even more preferably greater than 900 mAh / g. There is no particular upper limit to the first discharge capacity; the higher the capacity, the better. Therefore, although there is little point in mentioning the upper limit of the first discharge capacity, it can usually be assumed to be around 950 mAh / g, for example, as a reference value.

[0075] (Initial discharge capacity (DC3)) The initial discharge capacity (DC3) (mAh / g) of the electrode of the present invention is preferably 650 mAh / g or more. The initial discharge capacity is preferably greater than 650 mAh / g, more preferably greater than 700 mAh / g, even more preferably greater than 710 mAh / g, even more preferably greater than 720 mAh / g, even more preferably greater than 730 mAh / g, even more preferably greater than 740 mAh / g, even more preferably greater than 750 mAh / g, and even more preferably greater than 750 mAh / g. There is no particular upper limit to the initial discharge capacity; the higher the better. Therefore, although there is little point in mentioning the upper limit of the initial discharge capacity, it can usually be assumed to be, for example, about 800 mAh / g, as a reference value.

[0076] The discharge capacity when the electrode of the present invention is used as a positive electrode is determined by the configuration of the positive electrode, provided that the negative electrode and electrolyte are within the technical standards for durability (i.e., that Li is not depleted) and are within the range of acceptable standards for lithium-ion secondary batteries, and the discharge capacity of the positive electrode is fully demonstrated. For example, the amount of lithium used in the negative electrode is preferably at least 2 times, more preferably at least 5 times, even more preferably at least 10 times, and even more preferably at least 50 times the amount (molar amount) of sulfur in the positive electrode. Furthermore, for example, the amount of electrolyte (microliters) is preferably at least 10 times, more preferably at least 20 times, and even more preferably at least 50 times the amount (mg) of sulfur in the positive electrode, thereby fully demonstrating the discharge capacity of the positive electrode and extending the battery life. On the other hand, considering the energy density of the battery, a small amount of electrolyte is preferable. For example, the amount of electrolyte (microliters) is preferably at most 5 times, more preferably at most 3 times, and even more preferably at most 1 time the amount (mg) of sulfur in the positive electrode. Here, the volume V (mL) of the electrolyte means the total volume of the electrolyte solution including the solute. The electrolyte may be in the form of an electrolyte solution or a solid (solid electrolyte), or a combination of both.

[0077] [Application] The electrode of the present invention can be used as a positive electrode or a negative electrode of a secondary battery, particularly a lithium ion secondary battery, and is preferably used as a positive electrode.

[0078] The electrode can be constructed using the materials described in the manufacturing method section below in the same manner as described in the same section. That is, when the electrode is used as a positive electrode, the conductive additive, binder, current collector, etc. described in the manufacturing method section below can be used in the same manner as described in the same section to form a positive electrode, and when the electrode is used as a negative electrode, the conductive additive, binder, current collector, etc. described in the manufacturing method section below can be used in the same manner as described in the same section to form a negative electrode. In this way, the explanations in the manufacturing method section below can be taken into consideration as explanations of this electrode.

[0079] <Secondary battery> The secondary battery of the present invention is a secondary battery comprising the above-described electrode, and in particular a lithium ion secondary battery comprising the above-described electrode.

[0080] The secondary battery can be manufactured by a conventional method. That is, when the electrode is used as a positive electrode, a secondary battery can be constructed using a negative electrode, an electrolyte, a separator, etc. In contrast, when the electrode is used as a negative electrode, a secondary battery can be constructed using a positive electrode, an electrolyte, a separator, etc. In particular, when the electrode is used as a positive electrode of a lithium ion secondary battery, a lithium ion secondary battery can be constructed using the negative electrode, electrolyte, separator, etc. described in the manufacturing method section below in the same manner as described in the same section. In contrast, when the electrode is used as a negative electrode of a lithium ion secondary battery, a lithium ion secondary battery can be constructed using the positive electrode, electrolyte, separator, etc. described in the manufacturing method section below in the same manner as described in the same section.

[0081] [Application] The secondary battery of the present invention is useful as a secondary battery having an improved charge / discharge capacity despite having a high basis weight.

[0082] <Production method> The method for producing the electrode and secondary battery of the present invention will be described below in order, starting with the method for producing the active material that constitutes the electrode.

[0083] [Production of active materials] The active material of the present invention is (1) a step of mixing an acrylic resin, sulfur, and an iron compound containing divalent or trivalent iron ions to obtain a firing raw material; and (2) A step of firing the firing raw material The composition can be produced by a production method including the steps of:

[0084] (acrylic resin) In the present invention, the acrylic resin is a polymer obtained by polymerizing at least one compound selected from the group consisting of acrylate compounds represented by the following formula (1), or at least one polymer selected from the group consisting of polymers obtained by polymerizing at least one compound selected from the group consisting of acrylate compounds represented by the following formula (1) and at least one compound selected from the group consisting of diacrylate compounds represented by the following formula (2). CH2=C(R 11 )COOR 12 (1) (where R 11 is a hydrogen atom or a methyl group, and R 12 is an alkyl group. CH2=C(R 21 )COO-Y-OCO(R 22 )C=CH2(2) (where R 21 and R 22 are the same or different and are a hydrogen atom or a methyl group, and Y is a linear hydrocarbylene group, which may have at least one substituent selected from the group consisting of a hydroxyl group and an alkyl group, and the carbon skeleton constituting the hydrocarbylene group may have an ether bond via an oxygen atom. However, when there are two or more ether bonds, there are always two or more carbon atoms between adjacent oxygen atoms.

[0085] In formula (1), R 11 is preferably a methyl group, and R 12is preferably an alkyl group having 1 to 6 carbon atoms, more preferably an alkyl group having 1 to 4 carbon atoms, and among these, a methyl group, an n-butyl group, an i-butyl group, or a t-butyl group is preferred. Examples of compounds represented by formula (1) include methyl (meth)acrylate, butyl (meth)acrylate, etc., and more preferably methyl methacrylate and butyl methacrylate. Here, the "(meth)acrylate" in methyl (meth)acrylate and butyl (meth)acrylate refers to either "acrylate" or "methacrylate" (the same applies hereinafter). An even more preferred example of a compound represented by formula (1) is butyl methacrylate.

[0086] In equation (2), R 21 and R 22 are preferably a methyl group. The number of carbon atoms in the hydrocarbylene 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. The substituents in Y are preferably one or more substituents selected from the group consisting of hydroxyl groups and alkyl groups having 1 to 4 carbon atoms, and the alkyl groups having 1 to 4 carbon atoms are preferably methyl groups. When the carbon skeleton of Y has an ether bond via an oxygen atom, for example, the portion corresponding to -YO- is preferably represented by the following formula (3) (however, in formula (3), the substituents in Y are not taken into consideration). -(CH2) l -(CH2CH2O) m -(CH2CH2CH2O) n - (3) (Here, l is between 0 and 6, m is between 0 and 3, and n is between 0 and 2. However, l, m, and n cannot all be 0 at the same time.)

[0087] In formula (3), it is preferred that l is 1, 2, 3, 4, 5 or 6, and m and n are 0; alternatively, m is 1, 2 or 3, and l and n are 0; or alternatively, n is 1 or 2, and l and m are 0.

[0088] Examples of the compound represented by formula (2) include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyne glycol di(meth)acrylate, glycerin di(meth)acrylate, etc. Among these, ethylene glycol dimethacrylate is preferred.

[0089] Preferred examples of acrylic resins include homopolymers of methyl (meth)acrylate, homopolymers of butyl (meth)acrylate, copolymers of methyl (meth)acrylate and ethylene glycol di(meth)acrylate, and copolymers of butyl (meth)acrylate and ethylene glycol di(meth)acrylate. Of these, methacrylate-type acrylic resins are preferred. More preferred examples of acrylic resins include copolymers of butyl methacrylate and ethylene glycol dimethacrylate.

[0090] One or more types of acrylic resins can be used.

[0091] <<Acquisition or production of acrylic resin>> The acrylic resins are commercially available or can be prepared by conventional methods within the knowledge of one skilled in the art, such as those manufactured by Sekisui Plastics Co., Ltd.

[0092] (sulfur) Sulfur can be used in various forms such as powdered sulfur, insoluble sulfur, precipitated sulfur, and colloidal sulfur, with precipitated sulfur and colloidal sulfur being preferred.

[0093] The sulfur content is preferably more than 50 parts by mass, more preferably more than 70 parts by mass, and even more preferably more than 90 parts by mass, relative to 100 parts by mass of the acrylic resin. When it is more than 50 parts by mass, charge / discharge capacity and cycle characteristics tend to be improved. On the other hand, there is no particular upper limit for the sulfur content, but it is usually preferably less than 1000 parts by mass, more preferably less than 500 parts by mass, and even more preferably less than 300 parts by mass. 1000 parts by mass less than This tends to be advantageous in terms of cost.

[0094] Although various allotropes of sulfur can be used, sulfur containing S8 sulfur, which is solid at room temperature and pressure, is preferred, and simple S8 sulfur is more preferred.

[0095] (iron compounds containing divalent or trivalent iron ions) The iron compound containing divalent or trivalent iron ions is not particularly limited as long as it decomposes during firing and reacts with sulfur to produce iron disulfide, and various compounds can be used. Examples of such iron compounds include iron acid salts and iron complexes. Examples of iron acid salts include both organic and inorganic iron acid salts. On the other hand, examples of iron complexes include neutral iron complexes and salts of iron complex ions (iron complex salts).

[0096] Examples of organic acid salts of iron include divalent iron (Fe 2+ ) and organic acid salts, and trivalent iron (Fe 3+) and an organic acid salt. Of these, a salt of divalent iron and an organic acid is preferred. The organic acid is not particularly limited, and may be one having a carboxyl group (-COOH) or one having a sulfo group (-SO3H), but one having a carboxyl group is preferred. Specific examples of organic acids include fatty acids, oxalic acid, tartaric acid, citric acid, malic acid, and succinic acid. Specific examples of fatty acids include those having 1 to 6 carbon atoms, such as acetic acid, propionic acid, and butyric acid. Of these, acetic acid and oxalic acid are preferred. Preferred examples of iron organic acid salts include iron(II) acetate and iron(II) oxalate. These may be hydrates. One or more types of iron organic acid salts can be used.

[0097] Examples of inorganic salts of iron include divalent iron (Fe 2+ ) and inorganic acid salts, and trivalent iron (Fe 3+ ) and an inorganic acid. Specific examples of inorganic acids include hydrochloric acid, sulfuric acid, and nitric acid. Of these, nitric acid is preferred. Preferred examples of inorganic acid salts of iron include iron(II) chloride, iron(III) chloride, iron(II) sulfate, iron(III) sulfate, iron(II) nitrate, and iron(III) nitrate. These may also be hydrates. One or more inorganic acid salts of iron can be used.

[0098] As an example of iron complexes, divalent iron (Fe 2+ ) complexes and trivalent iron (Fe 3+) complexes. The iron complex may be in the form of a neutral complex or a complex salt. The ligand coordinated to the iron ion is not particularly limited, and examples thereof include halogen atoms such as chlorine and bromine atoms, cyano groups, dicyclopentadienyl groups, and N,N'-bis(salicylidene)ethylenediamine. Examples of iron complexes include potassium hexacyanidoferrate(II) ([Fe(CN)6]K4), potassium hexacyanidoferrate(III) ([Fe(CN)6]K3), sodium tetrachloroferrate(III) ([FeCl4]Na), dicyclopentadienyl iron(II) (ferrocene), and N,N'-bis(salicylidene)ethylenediaminatoiron(III) chloride. One or more iron complexes can be used.

[0099] The iron compound containing divalent or trivalent iron ions can be at least one selected from the group consisting of the above-mentioned organic acid salts of iron, inorganic acid salts of iron, neutral iron complexes, and iron complex salts. Of these, organic acid salts of iron, inorganic acid salts of iron, and neutral iron complexes are preferred.

[0100] From the viewpoint of the effects of the present invention, the content of the iron compound containing divalent or trivalent iron ions is preferably 50 parts by mass or more and 300 parts by mass or less per 100 parts by mass of the acrylic resin. The content is more preferably more than 50 parts by mass, even more preferably more than 60 parts by mass, even more preferably more than 70 parts by mass, and even more preferably more than 75 parts by mass. On the other hand, the content is more preferably less than 250 parts by mass, even more preferably less than 200 parts by mass, even more preferably less than 150 parts by mass, and even more preferably 100 parts by mass or less.

[0101] (Other ingredients) The raw materials may optionally contain other materials commonly used in this field, such as conductive carbon materials, which can improve the conductivity of the active material.

[0102] (Conductive carbon materials) The conductive carbon material is preferably a carbon material having a graphite structure. Examples of the carbon material that can be used include carbon black, acetylene black, ketjen black, graphite, carbon nanotubes (CNT), carbon fibers (CF), graphene, and fullerenes, which have a condensed aromatic ring structure. One or more conductive carbon materials can be used.

[0103] Among these, acetylene black, carbon black, and ketjen black are preferred because they are inexpensive and have excellent dispersibility. Furthermore, small amounts of CNTs or graphene may be used in combination with acetylene black, carbon black, or ketjen black. Such a combination system can further improve the cycle characteristics of lithium-ion secondary batteries without significantly increasing costs. The amount of CNTs or graphene used in combination is preferably 8% by mass or more and 12% by mass or less of the total amount of conductive carbon material.

[0104] The content of the conductive carbon material is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, relative to 100 parts by mass of the acrylic resin. A content of 5 parts by mass or more tends to facilitate the achievement of the objective of further improving the charge / discharge capacity and cycle characteristics. On the other hand, the content is preferably 50 parts by mass or less, more preferably 40 parts by mass or less. A content of 50 parts by mass or less keeps the proportion of sulfur-containing structures in the sulfur-based active material from decreasing relatively, and tends to facilitate the achievement of the objective of further improving the charge / discharge capacity and cycle characteristics.

[0105] (Process (1)) Step (1) is a step of mixing an acrylic resin, sulfur, and an iron compound containing divalent or trivalent iron ions to obtain a raw material for firing. The mixing method is not particularly limited as long as it sufficiently mixes these components, but in the present invention, at least the following wet method, in which mixing is performed using a solvent, or the dry method, in which mixing is performed without using a solvent, are preferred mixing methods.

[0106] ≪WET method≫ In the present invention, the WET method is a method for preparing a raw material by: (1-a-1) a step of adding the acrylic resin and the iron compound containing divalent or trivalent iron ions to an organic solvent and mixing them to obtain a liquid mixture; (1-a-2) removing the organic solvent from the liquid mixture to obtain a dry mixture; and (1-a-3) Mixing the dry mixture with sulfur It includes:

[0107] In step (1-a-1), the method for adding the acrylic resin and the iron compound containing divalent or trivalent iron ions to the organic solvent is not particularly limited, as long as they can be mixed to obtain a liquid mixture. For example, (1) the acrylic resin and the iron compound containing divalent or trivalent iron ions may be simultaneously added to the organic solvent and mixed, (2) the acrylic resin may be added to the organic solvent and mixed, and then the iron compound containing divalent or trivalent iron ions may be added and mixed, or (3) the iron compound containing divalent or trivalent iron ions may be added to the organic solvent and mixed, and then the acrylic resin may be added and mixed.

[0108] In step (1-a-1), the organic solvent may be any organic solvent commonly used in this field, and examples of such solvents include N-methyl-2-pyrrolidone, N,N-dimethylformaldehyde, alcohol, hexane, water, acetone, ethers such as tetrahydrofuran, etc. Furthermore, the organic solvent is preferably one that dissolves the acrylic resin, as this contributes to good mixing. One or more of these solvents may be used.

[0109] The organic solvent and the acrylic resin and / or the iron compound containing divalent or trivalent iron ions can be mixed, for example, by stirring in a container such as a beaker.

[0110] In step (1-a-2), the organic solvent can be removed by a conventional method, for example, by subjecting the liquid mixture to a drying method such as heat drying, drying under reduced pressure, or drying under reduced pressure with heat.

[0111] The dry mixture thus obtained is preferably pulverized before being subjected to the next step. This is because it is expected that the mixing in step (1-a-3) can be carried out more efficiently by doing so. The pulverization can be carried out by a conventional method, for example, using a cutter mill. In particular, it is preferable to perform coarse pulverization using a cutter mill and then fine pulverization using a freezing pulverizer.

[0112] In the step (1-a-3), the dry mixture and sulfur can be mixed by a conventional method, for example, by using a blender.

[0113] ≪DRY method≫ In the present invention, the DRY method includes, in preparing a raw material, (1-b) a step of mixing the acrylic resin, the sulfur, and the iron compound containing divalent or trivalent iron ions, all in a powder state.

[0114] Here, powder refers to a state in which the solid raw materials are sufficiently finely divided to be suitable for mixing for the purposes of the present invention. The particle size of the particles constituting the powder is not particularly limited as long as mixing is carried out satisfactorily, but the median diameter is typically, for example, in the range of 1 μm to 40 μm. From the perspective of the effects of the present invention, the particle size is preferably greater than 1 μm, more preferably greater than 2 μm, even more preferably greater than 3 μm, and even more preferably greater than 4 μm, while being preferably 40 μm or less, more preferably less than 40 μm, more preferably less than 30 μm, even more preferably less than 20 μm, even more preferably less than 15 μm, and even more preferably less than 10 μm. The median diameter can be measured by the method described in the Examples section below.

[0115] The mixing can be carried out by a conventional method, for example, in the same manner as in the mixing in the above step (1-a-3).

[0116] In both the wet and dry methods, it is desirable to thoroughly mix the raw materials in advance. Furthermore, if a conductive carbon material or the like is added to the raw materials, these additives may be added to the raw materials before firing.

[0117] The raw material thus obtained may be used in the next step (2) as it is, or may be formed into pellets, if desired, before being used in step (2).

[0118] (Process (2)) Step (2) is a step of calcining the raw materials obtained above. Calcination of the raw materials can be carried out by a conventional method, for example, by heating the raw materials at a predetermined temperature increase rate until they reach a predetermined temperature, maintaining the temperature at that temperature for a predetermined time, and then allowing them to cool naturally.

[0119] <Non-oxidizing atmosphere> The firing is preferably carried out in a non-oxidizing atmosphere. A non-oxidizing atmosphere is an atmosphere substantially free of oxygen, and is employed to suppress oxidative degradation and excessive thermal decomposition of the constituent components. Specifically, this refers to an inert gas atmosphere such as nitrogen or argon, or a sulfur gas atmosphere. Therefore, the modification is carried out, for example, in a quartz tube under an inert gas atmosphere.

[0120] <Heating rate> The temperature rise rate is preferably, for example, in the range of 50°C / h or more and 500°C / h or less. The temperature rise rate is preferably greater than 50°C / h, more preferably greater than 100°C / h. On the other hand, the temperature rise rate is preferably less than 500°C / h, more preferably less than 400°C / h, even more preferably less than 300°C / h, and even more preferably less than 200°C / h. When the temperature rise rate is within such a range, it tends to be easier to achieve the objective of improving the charge / discharge capacity and cycle characteristics.

[0121] <Baking temperature and time> The calcination temperature refers to the temperature after the temperature rise of the raw materials is completed and is maintained for a certain period of time to calcinate the raw materials. The temperature is preferably in the range of more than 250°C and less than 550°C. A temperature above 250°C tends to avoid insufficient sulfurization reaction and prevent a decrease in the charge / discharge capacity of the target product. On the other hand, a temperature below 550°C tends to prevent decomposition of the raw materials and prevent a decrease in yield and a decrease in charge / discharge capacity. The temperature is more preferably above 300°C, even more preferably above 350°C, and even more preferably above 380°C. On the other hand, less than 500°C is more preferably, less than 480°C is even more preferably, and even more preferably less than 450°C.

[0122] From the viewpoint of the effects of the present invention, the firing temperature in the step (2) is preferably higher than the temperature at which the iron compound containing divalent or trivalent iron ions thermally decomposes.

[0123] The time for maintaining the calcination temperature may be set appropriately depending on the type of raw material, the calcination temperature, etc., but is preferably, for example, 1 hour or more and 6 hours or less. A time of 1 hour or more tends to allow the calcination to proceed sufficiently, while a time of 6 hours or less tends to prevent excessive thermal decomposition of the constituent components. The time is preferably more than 1 hour, more preferably more than 1.5 hours. On the other hand, the time is preferably less than 6 hours, more preferably less than 4 hours.

[0124] ≪Device≫ The calcination can be carried out, for example, in a muffle furnace (FIG. 1), or can be carried out using a continuous apparatus such as a twin-screw extruder. The use of a continuous apparatus has the advantage that the sulfur-based active material can be produced continuously through a series of operations, such as kneading, pulverizing, and mixing the raw materials while calcining them in the apparatus.

[0125] A muffle furnace (Figure 1) is a furnace partitioned by a hot plate or the like to prevent the heat source (heater) from being exposed inside the furnace in order to prevent sample contamination. In Figure 1, muffle furnace 1 has heater 2 at the bottom of the furnace, which is partitioned by a hot plate. A lid 3 is installed on the front of the furnace (on the left end in the figure), and the furnace is designed to maintain an atmosphere of inert gas 4 inside. A thermocouple (not shown) is attached to the lid, allowing the temperature inside the furnace to be measured during firing. Inside the furnace, two tiers of stainless steel rectangular trays 5 and 6 are installed, one on the upper tier and one on the lower tier, for firing the raw materials.

[0126] Gas (e.g., an inert gas such as argon (Ar) gas) can be continuously supplied to and discharged from the inside of the furnace through a gas inlet pipe 7 and a gas outlet pipe 8. The gas outlet pipe 8 is connected to a trap tank 10 containing an aqueous sodium hydroxide solution 9, and exhaust gas from the muffle furnace 1 attempting to exit through the gas outlet pipe 8 to the outside first passes through the aqueous sodium hydroxide solution 9 in the trap tank 10 before being released to the outside. Therefore, even if the exhaust gas contains hydrogen sulfide gas generated by the reaction, it is neutralized by the aqueous sodium hydroxide solution and removed from the exhaust gas.

[0127] (Residue removal process) The treated product obtained after calcination contains unreacted sulfur, which is the sulfur that sublimed during calcination and then cooled and precipitated. These residues can cause deterioration in cycle characteristics, so it is desirable to remove them as much as possible. The removal of residues can be carried out by conventional methods, such as reduced-pressure heating drying, hot air drying, and solvent washing.

[0128] (Crushing, classification) The obtained sulfur-based active material is pulverized to a predetermined particle size and classified to obtain particles of a size suitable for producing an electrode.

[0129] In the firing method using the twin-screw extruder described above, the shearing during kneading allows for the production of the sulfur-based active material and the pulverization of the produced active material at the same time.

[0130] [Manufacturing electrodes for secondary batteries] The electrode for a secondary battery of the present invention can be manufactured using the active material obtained above. That is, the electrode can be obtained in the same manner as in the manufacture of a general electrode for a secondary battery, except that a conductive coating layer is formed on a current collector, a composite layer is formed on the conductive coating layer, and the interfacial tortuosity τ between the conductive coating layer and the composite layer is set to a predetermined value.

[0131] (When the active material is used as a positive electrode active material) The positive electrode for a secondary battery can be produced, for example, by mixing the active material with a conductive additive, a binder, and a solvent to prepare a paste-like positive electrode material, applying the positive electrode material to a current collector previously coated with a conductive coating layer (conductive coating layer-coated current collector), drying, and then pressing the current collector with a press or the like so as to achieve a predetermined interfacial curvature. If necessary, the positive electrode thus obtained may be vacuum-dried to remove the solvent or the like before use.

[0132] <Conductive additive> Examples of conductive additives include vapor grown carbon fiber (VGCF), carbon powder, carbon black (CB), acetylene black (AB), ketjen black (KB), graphite, and fine powders of metals such as aluminum and titanium that are stable at the positive electrode potential. Furthermore, the conductive carbon materials described above can also be used as conductive additives. One or more of these conductive additives can be used.

[0133] <Binder> Examples of binders include polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamide-imide (PAI), carboxymethyl cellulose (CMC), polyvinyl chloride (PVC), acrylic resins such as polyacrylic acid, methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), polypropylene (PP), etc. These binders can be used alone or in combination of two or more.

[0134] <Solvent> Examples of the solvent include N-methyl-2-pyrrolidone, N,N-dimethylformaldehyde, alcohol, hexane, water, etc. These solvents can be used alone or in combination of two or more.

[0135] ≪Blend amount≫ The amounts of these materials constituting the positive electrode are not particularly limited, but it is preferable to mix, for example, 2 to 100 parts by mass of a conductive additive, 2 to 50 parts by mass of a binder, and an appropriate amount of a solvent with respect to 100 parts by mass of an active material.

[0136] <Conductive coating layer coated current collector> The current collector used in producing the conductive coating layer-covered current collector can be any of the above-mentioned current collectors. One type of current collector may be used, or two or more types may be used in combination. The surface of the current collector may be coated with carbon or the like. Specific examples of such current collectors whose surfaces are coated with carbon or the like include carbon-coated aluminum foil. In this case, the current collector includes a carbon-coated portion.

[0137] The conductive coating layer-covered current collector can be produced as follows. For example, the materials constituting the conductive coating layer are thoroughly mixed with a solvent to prepare a paste-like conductive coating layer material, and the conductive coating layer material is applied to the surface of the current collector and then dried. Any of the solvents used in preparing the positive electrode material can be used as the solvent.

[0138] (When the active material is used as a negative electrode active material) The negative electrode for a secondary battery can be produced, for example, by mixing the active material with a conductive additive, a binder, and a solvent to prepare a paste-like negative electrode material, applying the negative electrode material to a current collector previously coated with a conductive coating layer (conductive coating layer-coated current collector), drying, and then pressing the current collector with a press or the like so as to achieve a predetermined interfacial curvature. The negative electrode thus obtained may be vacuum-dried, if necessary, to remove the solvent, etc. before use.

[0139] The conductive additive, binder, and solvent can be the same as those used above when the active material is used as a positive electrode active material, and the materials constituting the conductive coating layer-coated current collector can also be the same as those used above.

[0140] [Secondary battery manufacturing] The secondary battery of the present invention can be manufactured in the same manner as in the manufacture of a general secondary battery, except that the electrode serving as the positive electrode obtained above is used. The lithium-ion secondary battery of the present invention can be manufactured in the same manner as in the manufacture of a general lithium-ion secondary battery, except that the electrode serving as the negative electrode obtained above is used. Hereinafter, a manufacturing method for a lithium-ion secondary battery will be described.

[0141] (When the active material is used as a positive electrode active material) A lithium ion secondary battery can be produced in accordance with a conventional method by using a positive electrode containing the above active material (positive electrode active material), a negative electrode, an electrolyte, and, if desired, components such as a separator.

[0142] ≪Negative electrode≫ The negative electrode includes the current collector and a negative electrode material. Examples of the negative electrode material include known metallic lithium, carbon-based materials such as graphite, silicon-based materials such as silicon thin films, and alloy-based materials such as copper-tin and cobalt-tin. When a lithium-free material, such as a carbon-based material, silicon-based material, or alloy-based material among the above-mentioned negative electrode materials, is used as the negative electrode material, it is advantageous in that it is less likely to cause a short circuit between the positive and negative electrodes due to the generation of dendrites. However, when these lithium-free negative electrode materials are used in combination with the positive electrode of the present invention, neither the positive electrode nor the negative electrode contains lithium. Therefore, a lithium pre-doping process is required to insert lithium into either or both of the negative electrode and the positive electrode. Known methods can be used for lithium pre-doping. For example, when doping lithium into the negative electrode, a method of inserting lithium by electrolytic doping, in which a half-cell is assembled using metallic lithium as the counter electrode and lithium is electrochemically doped, or a method of inserting lithium by pre-doping, in which metallic lithium foil is attached to the electrode and then left in an electrolyte solution to dope by utilizing the diffusion of lithium into the electrode. In addition, the above-mentioned electrolytic doping method can also be used when pre-doping lithium into the positive electrode. As a lithium-free negative electrode material, silicon-based materials, which are high-capacity negative electrode materials, are particularly preferred, and among them, thin-film silicon, which has a thin electrode thickness and is advantageous in terms of capacity per volume, is more preferred.

[0143] ≪Electrolyte≫ The electrolyte compensates for the charge generated by the release of electrons to an external circuit due to the oxidation / reduction of the active materials at the positive and negative electrodes with the flow of ions. The electrolyte used in lithium-ion secondary batteries can be an organic solvent in which an electrolyte alkali metal salt is dissolved. The organic solvent is preferably at least one selected from non-aqueous solvents such as dimethoxyethane, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl ether, γ-butyrolactone, and acetonitrile. Examples of the electrolyte that can be used include Li(FSO2)2N, LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiI, and LiClO4. The electrolyte concentration may be approximately 0.5 mol / L to 5.0 mol / L. The electrolyte is not limited to a liquid. For example, when the lithium-ion secondary battery is a lithium polymer secondary battery, the electrolyte is solid (e.g., polymer gel).

[0144] <Separator> In addition to the negative electrode, positive electrode, and electrolyte described above, a lithium-ion secondary battery may also include components such as a separator. The separator is interposed between the positive electrode and negative electrode, allowing ions to move between the positive electrode and negative electrode while preventing internal short-circuiting between the positive electrode and negative electrode. If the lithium-ion secondary battery is a sealed type, the separator is also required to retain the electrolyte. As the separator, a thin, microporous or nonwoven membrane made of polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide, cellulose, glass, or the like is preferably used. These materials may also be coated with alumina or the like. Examples of such coated separators include alumina-coated polypropylene.

[0145] ≪Shape≫ The shape of the lithium ion secondary battery is not particularly limited, and various shapes such as a cylindrical type, a stacked type, a coin type, a laminated type, and a button type can be used.

[0146] (When the active material is used as a negative electrode active material) A lithium ion secondary battery can be produced in accordance with a conventional method by using a negative electrode containing the above active material (negative electrode active material), a positive electrode, an electrolyte, and, if desired, components such as a separator.

[0147] ≪Positive electrode≫ The positive electrode material is not particularly limited as long as it is, for example, a lithium-containing transition metal oxide or solid solution oxide, or a substance that can electrochemically absorb and release lithium ions. Examples of lithium-containing transition metal oxides include Li-Co composite oxides such as LiCoO2, LiNi x Co y Mn z Examples of the solid solution oxide include Li·Ni·Co·Mn-based composite oxides such as LiO2, Li·Ni-based composite oxides such as LiNiO2, and Li·Mn-based composite oxides such as LiMn2O4. a Mn x Co y Ni z O2 (1.150≦a≦1.430, 0.450≦x≦0.600, 0.100≦y≦0.150, 0.200≦z≦0.280), LiMn x Co y Ni z O2 (0.300≦x≦0.850, 0.100≦y≦0.300, 0.100≦z≦0.300), LiMn 1.5 Ni 0.5 O4, etc. These compounds may be used alone or in combination.

[0148] The electrolyte, separator, and shape of the lithium ion secondary battery can also be the same as those in the above case where the active material is used as a positive electrode active material. [Example]

[0149] The present invention will be described based on examples, but the present invention is not limited to only the examples.

[0150] Below are examples , reference exampleThe materials used in the comparative examples are also shown.

[0151] <Materials used in the test> Acrylic resin 1: Spherical acrylic resin consisting of a homopolymer of methyl methacrylate (Techpolymer MB-4 manufactured by Sekisui Plastics Co., Ltd., particle size: 4 μm) Iron compound 1 (organic acid salt): Iron(II) oxalate dihydrate (Iron(II) oxalate dihydrate, special grade, manufactured by Kanto Chemical Co., Ltd.) Sulfur: Precipitated sulfur manufactured by Tsurumi Chemical Industry Co., Ltd. Organic solvent (WET method): Acetone (Yoneyama Pharmaceutical Co., Ltd., acetone, special grade)

[0152] Production Example 1 (Fine Grinding of Iron Compounds) Before being subjected to the kneading step, the iron compound was previously pulverized for 5 minutes using a frozen pulverizer (JFC-2000 manufactured by Japan Analytical Industry Co., Ltd.).

[0153] Production Example 2 (Preparation of raw materials by the WET method) After the acrylic resin was added to the organic solvent and thoroughly mixed, the iron compound was further added and mixed to obtain a liquid mixture, and the organic solvent was then removed from the liquid mixture, which was then pulverized using a cutter mill (LAB MILL manufactured by Osaka Chemical Co., Ltd.) to obtain a dry mixture.

[0154] The dry mixture thus obtained was mixed with sulfur using a blender to obtain a raw material for firing.

[0155] Example ·Reference example Comparison example <Production of active materials> (baking ingredients) The raw materials prepared by the WET method as described above were used as the raw materials for firing, with the composition shown in Table 1.

[0156] (Reaction apparatus for firing) The raw materials were fired in a muffle furnace (Fig. 1). The muffle furnace in Fig. 1 was as described above.

[0157] (Firing process) First, the firing raw materials were placed in a tray, which was a SUS container, and the atmosphere in the muffle furnace was replaced with Ar gas three times using a vacuum pump. Then, Ar gas was continuously supplied from the gas inlet tube at a flow rate of 100 mL / min, and 30 minutes after the start of supply, heating of the muffle furnace began. The temperature was increased at a rate of 5°C / min, and when the temperature of the firing raw materials reached the firing temperature listed in Table 1, the temperature was maintained while heat treatment was carried out for 2 hours. Next, the temperature of the fired product was naturally cooled to 25°C under an Ar gas atmosphere while adjusting the flow rate of Ar gas, and then the fired product was removed from the muffle furnace.

[0158] (Crushing process) The fired product was pulverized as follows. Manufacturing Example 1: Using a cutter mill, grind at 25,000 rpm for 10 seconds Manufacturing Example 2: Using a cutter mill, grind at 25,000 rpm for 60 seconds Manufacturing Example 3: Using a cutter mill, grind at 25,000 rpm for 30 seconds Manufacturing Example 4: Lightly grind in a mortar for 30 seconds Manufacturing Example 5: Grinding in a ball mill for 1 hour Manufacturing Example 6: After freezing the baked product, crush it with a steel ball for 10 minutes.

[0159] (Classification process) In order to remove coarse particles from the pulverized fired product, the product was classified using a stainless steel sieve with a mesh of 32 μm to obtain an active material.

[0160] <Physical properties of active materials> (Elemental analysis) Example , reference example The active material produced in the comparative example was subjected to elemental analysis.

[0161] The mass ratio (%) of carbon, hydrogen, nitrogen, and sulfur to the total amount of active material was calculated from the mass measured using a fully automated elemental analyzer, Elementar's Vario MICRO cube. The mass ratio (%) of oxygen to the total amount of active material was calculated from the mass measured using an EMGA-920 analyzer, Horiba Ltd., by impulse heating / melting in an inert gas and NDIR method. The results are shown in Table 1.

[0162] (median diameter) Using a laser diffraction / scattering particle size distribution analyzer (Anton Paar PSA1090L particle size distribution analyzer), the volume-based cumulative 50% diameter (median diameter d 50 ) was measured.

[0163] (Breaking strength Cs (MPa)) For the active material, a microcompression tester (MCT-510, manufactured by Shimadzu Corporation) was used to measure the test force P (mN) at which the active material particles broke using an indenter. That is, after a very small amount of active material was scattered on a sample stage, five random particles were compressed one by one using an indenter, and the test force P (mN) at which the active material particles broke was determined. In addition, the particle diameter d (μm) of each particle was measured in two perpendicular directions and taken as the average value.

[0164] The breaking strength Cs (MPa) was calculated from the test force P (mN) and particle diameter d (μm) using the following formula (where π is the constant of the circumference of a circle (3.14)). Cs=2.48×P / πd 2

[0165] The average value of the breaking strength Cs (MPa) for the five particles was calculated and used as the breaking strength Cs (MPa) of the active material.

[0166] The results are shown in Table 1.

[0167] [Table 1]

[0168] <Preparation of cathode and lithium-ion secondary battery> According to Tables 2 to 5, positive electrodes were fabricated using the active materials obtained above, as follows, and further, lithium ion secondary batteries were fabricated using the positive electrodes.

[0169] (Conductive coating layer coated current collector foil) A conductive coating layer-coated current collector foil was prepared as follows: 55 wt% polyvinylidene fluoride (KF-#9700 manufactured by Kureha Corporation) and 45 wt% acetylene black (HS-100 manufactured by Denka Co., Ltd.) were added, and N-methylpyrrolidone (NMP) was added until the slurry viscosity reached approximately 1000 cP, followed by thorough dispersion. The resulting slurry was passed through a 400-mesh sieve, applied to a 15-μm-thick aluminum (aluminum) foil to the desired coating thickness, and then dried to obtain a conductive coating layer-coated current collector foil.

[0170] (positive electrode) The positive electrode was fabricated as follows: 10.0 wt% of polyacrylic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., average molecular weight 2700-7500) and 5.0 wt% of acetylene black (manufactured by Denka Corporation HS-100) were added to 85 wt% of the active material obtained in Table 1, and the mixture was thoroughly mixed with pure water for 1 hour. The mixture was then applied to the conductive coating layer-covered current collector foil obtained above, dried, and then pressed using a tabletop hot roll press (manufactured by Hosen Corporation HSRP-60150H) to achieve the specified electrode porosity (%). The mixture was then vacuum dried at 120°C for 6 hours and cut to the specified dimensions to obtain a positive electrode.

[0171] (Negative electrode) A 100 μm thick Li (lithium) foil was laminated onto a 6 μm thick Cu (copper) foil, and this was punched out to a predetermined size in a dry atmosphere with a dew point of −50° C. or less to obtain a negative electrode.

[0172] (electrolyte) The electrolyte used was prepared by dissolving lithium bis(fluorosulfonyl)imide (LiFSI, manufactured by Tokyo Chemical Industry Co., Ltd.) in dimethoxyethane (DME) to a concentration of 4 mol / L.

[0173] (lithium-ion secondary battery) The lithium-ion batteries used for evaluation were fabricated as follows. A sealant-equipped tab was bonded to the uncoated portion of the positive electrode and lithium negative electrode, both cut to the specified size, using an ultrasonic metal bonding machine (manufactured by Nippon Avionics Co., Ltd.). These were then placed facing each other with the composite surface sandwiched between alumina-coated polypropylene separators (manufactured by Shanghai Dinghao New Material Technology Co., Ltd., total thickness 20 μm) cut to the specified size. The resulting assembly was then enclosed in an aluminum laminate, and the edges of the aluminum laminate were heat-sealed, leaving an electrolyte inlet. Electrolyte (2.0 mL) was then poured into the inlet, and the battery was vacuum-sealed using a vacuum packaging machine (manufactured by TOSEI Corporation). The laminated lithium-ion battery for evaluation was obtained.

[0174] <Evaluation of lithium-ion secondary batteries> (discharge capacity, capacity maintenance rate) Each Example , reference example The laminated lithium-ion secondary battery prepared in the comparative example was charged and discharged at a current value equivalent to 50 mA per 1 g of positive electrode active material at a test temperature of 30°C. The discharge cut-off voltage was 1.0 V, and the charge cut-off voltage was 3.0 V. The battery was repeatedly charged and discharged, and the battery discharge capacity (mAh) was observed after the first, second, and third cycles. The measurements were performed using a battery performance evaluation device (BLS system, manufactured by Measuring Instruments Center Co., Ltd.).

[0175] (Average thickness of conductive coating layer (μm)) The average thickness of the conductive coating layer was determined by measuring the total thickness including the current collector foil after the conductive coating layer was formed at any five points using a Digimatic Indicator manufactured by Mitutoyo Corporation, and then subtracting the thickness of the current collector foil from the measured values ​​to calculate the arithmetic mean.

[0176] (interface curvature ratio τ) The interface tortuosity τ between the conductive coating layer and the composite layer constituting the electrode is expressed as b / a, where b is the length (b μm) of the composite layer side interface at a predetermined distance (a μm) in the cross section of the electrode taken along a plane perpendicular to the plane where the conductive coating layer and the composite layer are layered (where a < b). τ was calculated as the average of five measurements taken at five random cross sections of the positive electrode. The values ​​of a and b were obtained from microscope images using the image analysis software ImageJ (https: / / imagej.nih.gov / ij / index.html).

[0177] (Electrode coating weight (mg / cm 2 )) Coating weight D (mg / cm) of the electrode composite layer 2 ) is the area of ​​a given area C (cm 2 The weight A (mg) of the electrode punched out with a Thomson blade was measured, and the weight B (mg) of the current collector foil punched out with the same Thomson blade was measured, and the weight was calculated using the following formula. The greater the coating weight, the thicker the electrode film. Electrode coating weight D (mg / cm 2 )=(AB) / C

[0178] (Electrode porosity (%) ) electric The porosity of the electrode composite layer is determined by the electrode coating weight D (mg / cm 2 ), electrode thickness E (μm), current collecting foil thickness F (μm), and true specific gravity G (g / cm 3 ) can be calculated using the following formula: Electrode porosity (%)=100-[{D / 1000} / {(EF) / 10000}] / G×100

[0179] (Electrode Adhesion (mN / mm)) Electrode adhesion was measured by the 180° peel method using a device combining a force gauge and a measurement stand manufactured by Imada Corporation. Double-sided acrylic foam tape (H9004 manufactured by Nitto Denko Corporation) was attached to the surface of a punched electrode measuring 25 mm wide x 120 mm long, and the test sample was pressed with a roller. Measurements were made at a speed of 100 mm / min. The maximum adhesion strength (mN) was calculated by dividing the electrode width by 25 mm.

[0180] [Table 2]

[0181] [Table 3]

[0182] [Table 4]

[0183] [Table 5]

[0184] As can be seen from Tables 2 to 5, the charge / discharge capacity is improved in the examples.

[0185] <Embodiment> The following describes a preferred embodiment.

[0186] [1] An electrode comprising a current collector, a conductive coating layer formed on the current collector, and a composite layer formed on the conductive coating layer, the average thickness of the conductive coating layer is 0.5 μm or more, preferably 1 μm or more, more preferably 2 μm or more, even more preferably 4 μm or more, and even more preferably more than 5 μm; the interface tortuosity τ between the conductive coating layer and the composite layer is 1.05 or more, preferably 1.10 or more, more preferably 1.15 or more, even more preferably 1.20 or more, even more preferably 1.25 or more, even more preferably 1.30 or more, even more preferably 1.35 or more, even more preferably 1.40 or more, even more preferably 1.45 or more, even more preferably 1.50 or more, The mixture layer contains an active material, and the active material has an average particle diameter d 50 is 2 μm or more, preferably greater than 3 μm, more preferably greater than 4 μm, even more preferably greater than 5 μm, even more preferably greater than 6 μm, even more preferably greater than 7 μm, even more preferably greater than 8 μm. [2] The electrode according to the above [1], wherein the average value of the breaking strength Cs (MPa) calculated by the following formula for each of any five particles of the active material is less than 70.0 MPa, preferably less than 60.0 MPa, more preferably less than 30.0 MPa, and even more preferably less than 15.0 MPa, where d (μm) is the particle diameter and P (mN) is the test force at the moment when the particles break in a microcompression test: Cs=2.48×P / πd 2 (where d is the average diameter of the particle measured in two perpendicular directions, and π is the constant of the circumference of the particle (3.14).) [3] The electrode according to [2] above, wherein the average value of the breaking strength Cs (MPa) is less than 10.0 MPa. [4] The electrode according to any one of [1] to [3] above, wherein the active material contains at least two or more elements selected from the group consisting of carbon, sulfur, nitrogen, and oxygen, and the content of the elements in the active material is greater than 50.0 mass%, preferably greater than 60.0 mass%, more preferably greater than 70.0 mass%, even more preferably greater than 75.0 mass%, and even more preferably greater than 80.0 mass%. [5] The electrode according to any one of [1] to [3] above, wherein the active material contains sulfur, and the content of the sulfur in the active material is 45.0 mass% or more, preferably more than 50.0 mass%, more preferably more than 55.0 mass%, and even more preferably more than 60.0 mass%. [6] The electrode according to any one of [1] to [5] above, wherein the carbon content of the composite layer is 95 mass% or less, preferably less than 90 mass%, more preferably less than 80 mass%, and even more preferably less than 70 mass%, relative to the total mass of the composite layer. [7] The electrode according to any one of [1] to [5] above, wherein the sulfur content relative to the total mass of the composite layer is 5% by mass or more, preferably more than 10% by mass, more preferably more than 20% by mass, and even more preferably more than 30% by mass. [8] The volume resistivity of the conductive coating layer when coated on the surface of the current collector is 1.0 × 10 6 Ωcm or less, preferably 1.0×10 5 Ωcm or less, more preferably 1.0×10 4 Ωcm or less, more preferably 1.0×10 3 The electrode according to any one of the above [1] to [7], which has a resistance of Ωcm or less. [9] The electrode according to any one of [1] to [8] above, wherein the initial discharge capacity when the electrode is used as a positive electrode is 650 mAh / g or more, preferably more than 650 mAh / g, more preferably more than 700 mAh / g, even more preferably more than 710 mAh / g, even more preferably more than 720 mAh / g, even more preferably more than 730 mAh / g, even more preferably more than 740 mAh / g, even more preferably 750 mAh / g or more, even more preferably more than 750 mAh / g.

[10] A secondary battery comprising the electrode according to any one of [1] to [9] above.

[11] The secondary battery according to

[10] above, wherein the secondary battery is a lithium ion secondary battery. [Explanation of symbols]

[0187] 1 muffle furnace 2 heaters 3 Lid 4. Inert gas 5 Tray (upper) 6 Tray (lower) 7 Gas inlet pipe 8 Gas exhaust pipe 9. Sodium hydroxide solution 10 Trap tank

Claims

1. An electrode comprising a current collector, a conductive coating layer formed on the current collector, and a composite layer formed on the conductive coating layer, The conductive coating layer has an average thickness of 2 μm or more, The interface tortuosity τ between the conductive coating layer and the composite layer is 1.05 or more, The mixture layer contains an active material, and the average particle diameter d of the active material 50 is 2 μm or more, The electrode, wherein the active material contains sulfur, and the content of the sulfur in the active material is 45.0 mass % or more.

2. 2. The electrode according to claim 1, wherein the average value of the breaking strength Cs (MPa) calculated by the following formula for each of any five particles of the active material is less than 70.0 MPa, where d (μm) is the particle diameter and P (mN) is the test force at the moment when the particle breaks in a microcompression test. Cs=2.48×P / πd 2 (where d is the average diameter of the particle measured in two perpendicular directions, and π is the constant of the circumference of the particle (3.14).)

3. 3. The electrode according to claim 2, wherein the average value of the breaking strength Cs (MPa) is less than 10.0 MPa.

4. 3. The electrode according to claim 1, wherein the active material contains at least two elements selected from the group consisting of carbon, sulfur, nitrogen, and oxygen, and the content of the elements in the active material is greater than 50.0% by mass.

5. 3. The electrode according to claim 1, wherein the carbon content of the mixture layer is 95 mass % or less relative to the total mass of the mixture layer.

6. 3. The electrode according to claim 1, wherein the mixture layer has a sulfur content of 5 mass % or more relative to the total mass of the mixture layer.

7. The volume resistivity of the conductive coating layer when coated on the surface of the current collector is 1.0×10 6 3. The electrode according to claim 1, wherein the resistance is Ωcm or less.

8. 3. The electrode according to claim 1, wherein the electrode has an initial discharge capacity of 650 mAh / g or more when used as a positive electrode.

9. A secondary battery comprising the electrode according to claim 1 or 2.

10. 10. The secondary battery according to claim 9, wherein the secondary battery is a lithium ion secondary battery.

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