Electrode and secondary battery

Abstract

The invention provides an electrode. The electrode comprises a current collector, a conductive coating formed on the current collector and a composite material layer formed on the conductive coating. Wherein the average thickness of the conductive coating is more than 0.5 mu m, and the interface tortuosity tau between the conductive coating and the composite material layer is more than 1.05. Wherein the composite material layer contains an active material, and the average particle size d50 of the active material is 2 [mu] m or more. The purpose of the present invention is to improve charge / discharge capacity even at a high basis weight.

Description

Technical Field

[0001] The present invention relates to a novel electrode and a secondary battery comprising the electrode, in particular to a lithium ion secondary battery. Background Art

[0002] Secondary batteries, particularly lithium-ion secondary batteries, are primarily used as batteries for portable electronic devices due to their high charge and discharge capacity. Furthermore, lithium-ion secondary batteries are increasingly being used as batteries for electric vehicles, and further improvements in their performance are desired.

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

[0004] In addition, in order to increase the capacity of the battery, it has been proposed to use a material capable of absorbing and releasing more lithium ions as a negative electrode active material, such as silicon (Si), tin (Sn), etc.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: WO2010 / 044437 Summary of the Invention

[0008] Technical problem to be solved by the invention

[0009] However, as described in Patent Document 1, organic active materials containing sulfur-based active substances have attracted attention as active materials for next-generation batteries. However, because they are softer than traditional transition metal oxides, they cannot interlock when pressed onto the current collector. This results in the composite material layer containing the active material and the current collector being in point contact only. As a result, the resistance of the electrode increases, making it impossible to achieve the inherent charge and discharge capacity of the active material. In addition, the adhesion of the composite material layer is low, resulting in the composite material layer easily peeling off from the current collector during electrode production or charge and discharge processes. These problems are particularly evident when thickening the electrode to increase battery capacity, that is, producing so-called high-basis-weight electrodes.

[0010] The object of the present invention is to provide an electrode, ie a positive electrode or a negative electrode, which can maintain a high charge and discharge capacity even at a high basis weight, and a secondary battery comprising the electrode, in particular a lithium ion secondary battery.

[0011] Means of solving the problem

[0012] The present invention relates to the following electrode:

[0013] An electrode comprises a current collector, a conductive coating formed on the current collector, and a composite material layer formed on the conductive coating, wherein:

[0014] The average thickness of the conductive coating is more than 0.5 μm;

[0015] The interface tortuosity τ between the conductive coating and the composite material layer is greater than 1.05; and

[0016] The composite material layer contains active materials, and the average particle size d of the active materials is 50 2μm or more.

[0017] Effects of the Invention

[0018] According to the present invention, an electrode (ie, a positive electrode or a negative electrode) having a high charge and discharge capacity even at a high basis weight, and a secondary battery, particularly a lithium ion secondary battery, comprising the electrode can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic cross-sectional view of a reaction device for preparing active materials in an embodiment of the present invention. DETAILED DESCRIPTION

[0020] The specific embodiments of the present invention will be described in detail below. In addition, the upper and lower limits of "above" and "below" involved in the numerical range description are numerical values that can be combined arbitrarily, and the numerical values in the embodiments can also be used as upper and lower limits. In addition, unless otherwise inconsistent with the purpose of the present invention, the numerical range shown to include both end values should be interpreted as also disclosing a numerical range that does not include one of the two end values, and a numerical range that does not include any one of the two end values.

[0021] One embodiment of the present invention is the following electrode:

[0022] An electrode comprises a current collector, a conductive coating formed on the current collector, and a composite material layer formed on the conductive coating, wherein:

[0023] The average thickness of the conductive coating is more than 0.5 μm;

[0024] The interface tortuosity τ between the conductive coating and the composite material layer is greater than 1.05; and

[0025] The composite material layer contains active materials, and the average particle size d of the active materials is 50 2μm or more.

[0026] While not wishing to be bound by theory, the following factors are believed to be the reason why the electrode of the present invention can improve the charge and discharge capacity even at a higher basis weight.

[0027] Specifically, the electrode of the present invention has a highly flexible conductive coating pre-applied to the current collector, and a composite material layer applied to the conductive coating and then pressed. This allows the composite material layer to interlock with the conductive coating, resulting in surface contact between the conductive coating and the composite material layer, rather than point contact. This reduces the resistance of the electrode and enhances the adhesion of the composite material layer, thereby increasing the charge and discharge capacity even at high basis weights.

[0028] For each of any five particles of the active material, when the particle size is d (in μm) and the experimental force at the moment of particle fracture in the microcompression test is P (in mN), the average value of the fracture strength Cs in MPa calculated by the following formula is preferably less than 70.0,

[0029] Cs=2.48×P / πd 2

[0030] (d is the average of the particle diameters measured in two perpendicular directions, and π is the circumference of a circle (3.14).)

[0031] This is because such a structure comprises a typical feature of the present invention, namely, low breaking strength.

[0032] The average value of the breaking strength Cs in MPa is preferably less than 10.0.

[0033] This is because such a structure comprises a typical feature of the present invention, namely, low breaking strength.

[0034] Preferably, the active material contains at least two or more elements selected from carbon, sulfur, nitrogen, and oxygen, and the content of the elements in the active material is greater than 50.0% by mass.

[0035] This is because this structure is a typical example of the active material comprising the features of the present invention.

[0036] Preferably, the active material contains sulfur, and the content of sulfur in the active material is 45.0% by mass or more.

[0037] This is because this structure is a typical example of the active material comprising the features of the present invention.

[0038] The carbon content is 95% by mass or less based on the total mass of the composite material layer.

[0039] The sulfur content is preferably 5% by mass or more based on the total mass of the composite material layer.

[0040] When the conductive coating is coated on the surface of the current collector, the volume resistivity of the conductive coating is preferably 1.0×10 6 Ω·cm or less.

[0041] When the electrode is used as a positive electrode, the initial discharge capacity of the electrode is preferably 650 mAh / g or more.

[0042] Another aspect of the present invention is a secondary battery including the electrode.

[0043] The secondary battery is preferably a lithium ion secondary battery.

[0044] <Definition>

[0045] The term "conductive coating" refers to a layer that helps reduce the resistance of the electrode by allowing the composite material layer formed on the conductive coating to be well interlocked with the conductive coating during pressing during the production process of the electrode. The volume resistivity of the conductive coating is preferably 1.0×10 6 The volume resistivity of the conductive coating layer is measured in accordance with JIS K 7194-1994.

[0046] The term "composite material layer" refers to a layer that is a component constituting an electrode together with a current collector and is composed of an electrode material (composite material) containing an active material.

[0047] In this specification, the term "initial discharge capacity" refers to the third discharge capacity unless otherwise specified.

[0048] <Measurement method>

[0049] The average thickness of the conductive coating is measured using a digital micrometer produced by Mitutoyo Corporation. After the conductive coating is formed, the total thickness including the current collector is measured at any five points. The thickness of the current collector is then subtracted from the measured value to calculate the arithmetic mean of the obtained values.

[0050] In the present invention, the "interface tortuosity τ" refers to the interface tortuosity between the conductive coating constituting the electrode and the composite material layer. The interface tortuosity is represented by b / a, where a is the distance (in μm) between two given points on the electrode cross-section taken on a plane perpendicular to the flat surface on which the conductive coating and the composite material layer are laminated; b is the interface length on the side of the composite material layer (here a < b). In the present invention, the interface tortuosity τ is defined as follows: it is measured once on each of any five cross-sections of the electrode, for a total of 5 measurements, and the average value of the measurement results is taken. In addition, if the electrode has a front side and a back side, the five average values are calculated separately for each side, and then the average value of the two is further calculated. In addition, the values of a and b can also be obtained from a microscope image. Image analysis software can be used to obtain the values of a and b from the microscope image. As the image analysis software, for example, Image-Pro (image analysis software manufactured by Media Cybernetics), ImageJ (https: / / imagej.nih.gov / ij / index.html), etc. can be used.

[0051] The "content of carbon element in the active material" is determined by the following elemental analysis method.

[0052] The "content of sulfur element in the active material" is determined by the following elemental analysis method.

[0053] The "content of nitrogen element in the active material" is determined by the following elemental analysis method.

[0054] The "content of oxygen element in the active material" is determined by the following elemental analysis method.

[0055] "Average particle size d 50 " is the size (median particle size) at which the cumulative volume is 50% measured using a laser diffraction / scattering particle size distribution analyzer (particle size distribution analyzer PSA1090L manufactured by Anton Paar GmbH) with water as the dispersion medium.

[0056] (Fracture strength Cs)

[0057] The fracture strength Cs, with the unit of MPa, is a value that can be determined by the micro-compression test described in the Examples section below. The fracture strength Cs (unit: MPa) can be measured from 5 active material particles before the active material particles are coated on the electrode; it can also be measured by extracting 5 active material particles from the electrode that still maintain their particle shape (i.e., not broken) even after being coated on the electrode.

[0058] The electrode and secondary battery of the present invention will be described below.

[0059] <Electrode>

[0060] The electrode of the present invention comprises a current collector, a conductive coating formed on the current collector, and a composite material layer formed on the conductive coating. The conductive coating has an average thickness of 0.5 μm or more, an interface tortuosity τ between the conductive coating and the composite material layer is 1.05 or more, the composite material layer contains an active material, and the average particle size d of the active material is 0.5 μm or more. 50 2μm or more.

[0061] [current collector]

[0062] The current collector is a chemically inert electronic conductor that continuously applies current to the electrodes during battery discharge or charge. Examples of current collector materials include at least one selected from silver, copper, gold, aluminum, magnesium, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, and molybdenum, as well as metals such as stainless steel. Aluminum is preferred.

[0063] The surface of the current collector may have a known protective layer or may be treated by a known method.

[0064] The current collector can be in the form of a foil, sheet, film, wire, rod, mesh, or the like. For example, metal foils such as copper foil, nickel foil, aluminum foil, and stainless steel foil can be used as appropriate. The thickness of the foil-shaped current collector is preferably 1 μm to 100 μm.

[0065] [Conductive coating]

[0066] When the conductive coating is coated on the surface of the current collector, its volume resistivity is preferably 1.0×10 6 Ω·cm or less. During the electrode production process, the composite material layer formed on the conductive coating can be well bitten into the conductive coating during pressing, thereby helping to reduce the resistance of the electrode. Its volume resistivity is preferably 1.0×10 5 Ω·cm or less, more preferably 1.0×10 4 Ω·cm or less, more preferably 1.0×10 3 Ω·cm or less.

[0067] The material constituting the conductive coating is not particularly limited as long as it can make the interface tortuosity τ with the composite material layer reach 1.05 or more. Therefore, among the materials commonly used to coat the surface of the current collector, any material that can make the interface tortuosity τ reach 1.05 or more can be used. Examples of materials that can be used for the coating include: carbon materials such as graphite, acetylene black, Ketjen black, carbon nanotubes, graphene, etc.; metal materials such as silver, copper, gold, aluminum, magnesium, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, stainless steel, etc.; and conductive polymer materials such as polyvinylidene fluoride, poly (3,4-ethylenedioxythiophene) (PEDOT), polythiophene, polyaniline, polypyrrole, polyacetylene, etc. These materials can be used alone or in combination of two or more. These materials can be fibrous or beaded. For example, fibrous metal materials or beaded metal materials can be used.

[0068] The average thickness of the conductive coating is 0.5 μm or greater. When the average thickness of the conductive coating is less than 0.5 μm, it is believed that achieving the aforementioned interface tortuosity τ may be difficult, which is disadvantageous. The average thickness of the conductive coating is preferably 1 μm or greater, more preferably 2 μm or greater, even more preferably 4 μm or greater, and even more preferably greater than 5 μm. The upper limit of the average thickness of the conductive coating is not particularly limited and may be, for example, approximately 30 μm. The average thickness of the conductive coating can be measured using the above-described method.

[0069] [Composite material layer]

[0070] The composite material layer according to the present invention contains the following active material: As described below, the composite material layer is formed of an electrode material (ie, a positive electrode material or a negative electrode material) containing the active material.

[0071] (Carbon content)

[0072] The carbon content is preferably 95% or less based on the total mass of the composite material layer.

[0073] The mass ratio of carbon is preferably less than 90 mass%, more preferably less than 80 mass%, and further preferably less than 70 mass%. On the other hand, the mass ratio of carbon is preferably greater than 5 mass%, more preferably greater than 10 mass%, and further preferably greater than 15 mass%. The carbon content can be measured, for example, by energy dispersive X-ray spectroscopy using a scanning electron microscope or a transmission electron microscope (SEM-EDX or TEM-EDX).

[0074] (Sulfur content)

[0075] The sulfur content is preferably 5% by mass or more based on the total mass of the composite material layer.

[0076] The mass ratio of sulfur is preferably greater than 10% by mass, more preferably greater than 20% by mass, and further preferably greater than 30% by mass. The sulfur content can be measured, for example, by energy dispersive X-ray spectroscopy using a scanning electron microscope or a transmission electron microscope (SEM-EDX or TEM-EDX).

[0077] [Active Materials]

[0078] The active material of the present invention is not particularly limited as long as its average particle size d 50 The active material can be 2 μm or more and can achieve an interface tortuosity τ of 1.05 or more between the conductive coating layer and the composite material layer. Among them, relatively soft active materials can be typically used.

[0079] The active material preferably contains at least two elements selected from carbon, sulfur, nitrogen, and oxygen, with the content of these elements in the active material exceeding 50.0% by mass. This is because such compounds are relatively soft, allowing for the use of a conductive coating, a feature of the present invention. The active material preferably contains carbon and sulfur, and more preferably contains carbon, sulfur, and oxygen.

[0080] The content of these elements in the active material is preferably greater than 60.0 mass%, more preferably greater than 70.0 mass%, further preferably greater than 75.0 mass%, and further preferably greater 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 further preferably less than 85.0 mass%.

[0081] (Sulfur content)

[0082] The sulfur content in the active material is preferably 45.0% by mass or more. This is because the compound is relatively soft and can take advantage of the characteristics of the present invention, namely, the use of a conductive coating.

[0083] The sulfur content in the active material is preferably greater than 50% by mass, more preferably greater than 55.0% by mass, and even more preferably greater than 60.0% by mass. On the other hand, the sulfur content is preferably less than 75.0% by mass, more preferably less than 70.0% by mass, and even more preferably less than 65.0% by mass.

[0084] (Carbon content)

[0085] The carbon content in the active material is preferably greater than 10.0% by mass, more preferably greater than 13.0% by mass, and further preferably greater than 15.0% by mass. On the other hand, the carbon content is preferably less than 23.0% by mass, more preferably less than 21.0% by mass, and further preferably less than 19.0% by mass.

[0086] (Hydrogen content)

[0087] The relatively low hydrogen content in the active material is due to the fact that hydrogen atoms in the raw materials combine with other elements during the sintering process to form gases, which are then released outside the system and reduced. The hydrogen content in the active material is preferably less than 1.0% by mass. The hydrogen content is more preferably less than 0.5% by mass, further preferably less than 0.3% by mass, and even more preferably less than 0.2% by mass. Alternatively, the lower limit of the hydrogen content may be 0.1% by mass, or less than 0.1% by mass, or may be the detection limit or lower.

[0088] (Nitrogen content)

[0089] The nitrogen content in the active material may be below the detection limit, but may also be complexed. When complexed with nitrogen, the nitrogen content is preferably greater than 5.0% by mass, more preferably greater than 8.0% by mass, and even more preferably greater than 10.0% by mass. On the other hand, the nitrogen content is preferably less than 30.0% by mass, more preferably less than 25.0% by mass, and even more preferably less than 20.0% by mass.

[0090] (Oxygen content)

[0091] The oxygen content in the active material is preferably greater than 0.5% by mass, more preferably greater than 1.0% by mass, even more preferably greater than 1.5% by mass, even more preferably greater than 2.0% by mass, and even more preferably greater than 2.5% by mass. On the other hand, the oxygen content 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.

[0092] (Average particle size d 50 )

[0093] Average particle size d in active material 50 When the average particle size d 50 When the particle size is less than 2 μm, the specific surface area of the active material increases, and the amount of the binder and conductive additive required to constitute the electrode also increases, resulting in a relative decrease in the content of the active material in the electrode, which is disadvantageous.

[0094] Average particle size d 50 It is preferably larger than 3 μm, more preferably larger than 4 μm, further preferably larger than 5 μm, further preferably larger than 6 μm, further preferably larger than 7 μm, further preferably larger than 8 μm. 50 It is preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 30 μm or less.

[0095] (Breaking strength Cs (MPa))

[0096] The average value of the fracture strength Cs in MPa measured for each of five randomly selected particles of the active material is preferably less than 70.0. The fracture strength Cs (in MPa) is a value that can be determined by the microcompression test described in the Examples section below. Furthermore, the fracture strength Cs of LiCoO2 measured by this method was 72.25 MPa.

[0097] From the effects of the present invention, the average value of the breaking strength Cs (in MPa) is preferably less than 60.0 MPa, more preferably less than 30.0 MPa, further preferably less than 15.0 MPa, further preferably less than 10.0 MPa, further preferably less than 10.0 MPa, further preferably less than 5.0 MPa. In addition, "less than 5.0 MPa" also includes the case where Cs cannot be measured. Cs is an indicator of hardness, and the larger its value, the higher the hardness. However, if the hardness is too soft, Cs cannot be measured. The present invention preferably has soft active material particles, so as described above, the preferred range is described. However, even if the active material particles are very soft and Cs cannot be measured, it is preferred. Therefore, "less than 10.0 MPa" also includes the case where Cs cannot be measured.

[0098] For example, when a polymer is used as the raw material for the active material, the average value of the breaking strength Cs (unit: MPa) can be adjusted. By using a polymer with a lower degree of crosslinking or a polymer with a higher porosity, the average value of the breaking strength Cs can be reduced. Conversely, by using a polymer with a higher degree of crosslinking or a polymer with a lower porosity, the average value of the breaking strength Cs can be increased. In this way, the breaking strength Cs (MPa) can be adjusted.

[0099] [Interface tortuosity τ]

[0100] The interface tortuosity τ between the conductive coating and the composite material layer is greater than 1.05. If the interface tortuosity is less than 1.05, the composite material layer cannot be fully engaged with the conductive coating, and the conductive coating and the composite material layer cannot be fully contacted, which is disadvantageous.

[0101] The interface tortuosity τ can be adjusted by adjusting the softness (or hardness) of the conductive coating and the composite material layer. Specifically, the interface tortuosity τ can be increased or decreased by adjusting the softness (or hardness) relationship between the composite material layer and the conductive coating so that the composite material layer bites into the conductive coating.

[0102] The interface tortuosity τ is preferably 1.10 or greater, more preferably 1.15 or greater, even more preferably 1.20 or greater, even more preferably 1.25 or greater, even more preferably 1.30 or greater, even more preferably 1.35 or greater, even more preferably 1.40 or greater, even more preferably 1.45 or greater, even more preferably 1.50 or greater. The upper limit of the interface tortuosity τ is not particularly limited, but as a reference value only, it is, for example, approximately 1.70.

[0103] [Electrode coating amount]

[0104] Electrode coating amount, in mg / cm 2 , is the value defined in the following Examples section. The electrode coating amount is preferably greater than 1 mg / cm 2 , more preferably 3 mg / cm 2 More than 5 mg / cm 2 On the other hand, the electrode coating amount is preferably less than 20 mg / cm 2 , more preferably 15 mg / cm 2 Below, more preferably less than 13 mg / cm 2 .

[0105] [Electrode porosity]

[0106] The electrode porosity, expressed in %, is defined in the Examples section below. The electrode porosity is preferably greater than 15%, more preferably greater than 22%, and even more preferably greater than 25%. On the other hand, the electrode porosity is preferably less than 60%, more preferably less than 52%, and even more preferably less than 50%.

[0107] [Electrode adhesion]

[0108] Electrode adhesion, expressed in mN / mm, is 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 greater than 31 mN / mm. 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 less than 35 mN / mm.

[0109] [Charge and discharge capacity]

[0110] The electrode of the present invention exhibits excellent charge and discharge capacity. In addition, in the following description, unless otherwise stated, the initial discharge capacity refers to the third discharge capacity (DC3). Here, the third discharge capacity refers to the discharge capacity when the electrode and the battery are produced and three times charged and discharged with a discharge end voltage of 1.0V and a charge end voltage of 3.0V (referring to the third discharge when the charge and discharge are repeated according to the first discharge, the first charge, the second discharge, the second charge, the third discharge, and the third charge). During discharge, when discharged with a constant current (equivalent to a current value of 50mA per gram of positive electrode active material), the voltage eventually drops from 3.0V to 1.0V. The total time (unit: hour) required for the voltage to drop from 3.0V to 1.0V is measured, multiplied by the applied current (unit: mA) to obtain the capacity (unit: mAh), and then divided by the weight of the active material to obtain the specific capacity (unit: mAh / g). On the other hand, in the case of charging, due to the use of constant current charging, the voltage rises instead, and when the voltage finally reaches 3.0V, charging is terminated.

[0111] (First discharge capacity (DC1))

[0112] The first discharge capacity (DC1, unit: mAh / g) of the electrode of the present invention is preferably 650mAh / g or more. The first discharge capacity is preferably greater than 650mAh / g, more preferably greater than 700mAh / g, further preferably greater than 750mAh / g, further preferably greater than 800mAh / g, further preferably greater than 850mAh / g, further preferably greater than 875mAh / g, further preferably greater than 880mAh / g, further preferably greater than 890mAh / g, further preferably greater than 900mAh / g. There is no particular limit to the upper limit of the first discharge capacity, and the higher the capacity, the less preferred. Therefore, it is not meaningful to mention the upper limit of the first discharge capacity, but when it is usually only used as a reference value, it can also be set to, for example, about 950mAh / g.

[0113] (Initial discharge capacity (DC3))

[0114] The initial discharge capacity (DC3, unit: mAh / g) of the electrode of the present invention is preferably 650mAh / g or more. The initial discharge capacity is preferably greater than 650mAh / g, more preferably greater than 700mAh / g, further preferably greater than 710mAh / g, further preferably greater than 720mAh / g, further preferably greater than 730mAh / g, further preferably greater than 740mAh / g, further preferably greater than 750mAh / g, further preferably greater than 750mAh / g. There is no particular limit to the upper limit of the initial discharge capacity, and the higher the capacity, the more preferred. Therefore, it is not meaningful to mention the upper limit of the initial discharge capacity, but when generally used only as a reference value, it can also be set to, for example, about 800mAh / g.

[0115] In addition, when the electrode of the present invention is used as a positive electrode, its discharge capacity depends on the structure of the positive electrode, provided that a negative electrode and an electrolyte (i.e., when Li is not exhausted) in a known technology that can be used as a lithium ion secondary battery for a long time are used, so as to give full play to the performance related to the discharge capacity of the positive electrode. For example, for the negative electrode, the amount of lithium used is preferably more than 2 times the content (molar amount) of sulfur in the positive electrode, more preferably more than 5 times, further preferably more than 10 times, and further preferably more than 50 times (molar amount). In addition, for example, for the electrolyte, when the amount (microliter) of the electrolyte is preferably more than 10 times the content (unit: mg) of sulfur in the positive electrode, more preferably more than 20 times, and further preferably more than 50 times, the discharge capacity of the positive electrode can be fully utilized, thereby extending the battery life. On the other hand, considering the energy density of the battery, the amount of the electrolyte is preferably less. For example, the amount (microliter) of the electrolyte is preferably less than 5 times the content (unit: mg) of sulfur in the positive electrode, more preferably less than 3 times, and further preferably less than 1 times. In the present invention, the volume V (unit: mL) of the electrolyte refers to the total volume of the electrolyte solution including the solute. In addition, the electrolyte can be in the form of an electrolyte solution, a solid (solid electrolyte) form, or a combination of the two.

[0116] [application]

[0117] The electrode of the present invention can be used as a positive electrode or a negative electrode of a secondary battery, in particular a positive electrode or a negative electrode of a lithium ion secondary battery. In addition, the electrode of the present invention is preferably used as a positive electrode.

[0118] The electrodes can be constructed using the materials described in the "Manufacturing Method" section below and in the same manner as described therein. Specifically, when the electrodes are used as positive electrodes, they can be formed using the conductive additives, binders, current collectors, and other materials described therein in the same manner. When the electrodes are used as negative electrodes, they can be formed using the conductive additives, binders, current collectors, and other materials described therein in the same manner. Therefore, the description in the "Manufacturing Method" section below serves as a reference for the relevant explanation of this electrode.

[0119] <Secondary Battery>

[0120] The secondary battery of the present invention is a secondary battery including the above-mentioned electrode, and particularly a lithium ion secondary battery including the above-mentioned electrode.

[0121] The secondary battery can be manufactured by conventional methods. That is, when the above-mentioned electrode is used as the positive electrode, a secondary battery can be constructed by additionally using a negative electrode, an electrolyte, a separator, etc. On the other hand, when the above-mentioned electrode is used as the negative electrode, a secondary battery can be constructed by additionally using a positive electrode, an electrolyte, a separator, etc. Specifically, when the above-mentioned electrode is used as the positive electrode of a lithium-ion secondary battery, the lithium-ion secondary battery can be constructed in the manner described in the manufacturing method section below using the negative electrode, electrolyte, separator, etc. described in this section. On the other hand, when the above-mentioned electrode is used as the negative electrode of a lithium-ion secondary battery, the lithium-ion secondary battery can be constructed in the same manner using the positive electrode, electrolyte, separator, etc. described in this section.

[0122] [application]

[0123] The secondary battery of the present invention is useful as a secondary battery having improved charge and discharge capacity even at a higher basis weight.

[0124] <Manufacturing method>

[0125] The following describes methods for manufacturing the electrode and secondary battery of the present invention, starting with a method for manufacturing an active material constituting the electrode.

[0126] [Preparation of active materials]

[0127] The active material of the present invention can be obtained by the following preparation method, which comprises the following steps:

[0128] (1) mixing an acrylic resin, sulfur, and an iron compound containing divalent or trivalent iron ions to obtain a roasting raw material; and

[0129] (2) calcining the calcined raw material.

[0130] (Acrylic resin)

[0131] In the present invention, the acrylic resin is at least one polymer selected from the following: a polymer obtained by polymerizing at least one monomer selected from the group consisting of the acrylate compounds represented by the following formula (1); or a polymer obtained by polymerizing at least one monomer selected from the group consisting of the acrylate compounds represented by the following formula (1) and at least one monomer selected from the group consisting of the diacrylate compounds represented by the following formula (2).

[0132] CH2=C(R 11 )COOR 12 (1)

[0133] (Among them, R 11 is a hydrogen atom or a methyl group, R 12 is an alkyl group.)

[0134] CH2=C(R 21 )COO-Y-OCO(R22 )C=CH2(2)

[0135] (Among them, R 21 and R 22 (a) is the same or different, each being a hydrogen atom or a methyl group; Y is a straight-chain alkylene group, which may have at least one substituent selected from a hydroxyl group and an alkyl group. The carbon skeleton constituting the alkylene group may have an ether bond with an oxygen atom; however, when there are two or more ether bonds, any adjacent oxygen atoms are always separated by two or more carbon atoms.

[0136] In formula (1), R 11 Preferably, methyl, R 12 It is preferably an alkyl group having 1 or more and 6 or less carbon atoms, more preferably an alkyl group having 1 or more and 4 or less carbon atoms, and more preferably a methyl group, an n-butyl group, an isobutyl group, or a tert-butyl group. The compound represented by formula (1) includes, for example, 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 means "acrylate" or "methacrylate" (the same below). The compound represented by formula (1) is further preferably butyl methacrylate.

[0137] In formula (2), R 21 and R 22 are preferably methyl. The number of carbon atoms of the alkylene group (straight chain) of Y is preferably 2 or more and 6 or less, more preferably 2 or 3. The number of substituents in Y is preferably 1 or more and 4 or less, more preferably 1 or 2. The substituents in Y are preferably one or more substituents selected from hydroxyl groups and alkyl groups having 1 or more and 4 or less carbon atoms, wherein the alkyl group having 1 or more and 4 or less carbon atoms is preferably methyl. For example, when the carbon skeleton of Y has an ether bond formed by an oxygen atom, the portion corresponding to -YO- is preferably represented by the following formula (3) (the substituents in Y are not considered in formula (3)):

[0138] -(CH2) l -(CH2CH2O) m -(CH2CH2CH2O) n -(3)

[0139] (Herein, l is 0 or more and 6 or less, m is 0 or more and 3 or less, and n is 0 or more and 2 or less. However, l, m, and n cannot all be 0 at the same time.)

[0140] In formula (3), preferably, l is 1, 2, 3, 4, 5 or 6, and m and n are 0; or preferably, m is 1, 2 or 3, and l and m are 0; or n is 1 or 2, and l and m are 0.

[0141] 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, neopentyl glycol di(meth)acrylate, and glycerol di(meth)acrylate. Among them, ethylene glycol dimethacrylate is preferred.

[0142] 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. Among these, methacrylate-based acrylic resins are preferred. More preferred examples of acrylic resins include copolymers of butyl methacrylate and ethylene glycol di(meth)acrylate.

[0143] One or more acrylic resins may be used.

[0144] <Availability or Preparation Method of Acrylic Resin>

[0145] Acrylic resins are commercially available or can be prepared by conventional methods within the knowledge of those skilled in the art. Examples of commercially available acrylic resins include, for example, acrylic resins produced by Sekisui Kasei Co., Ltd.

[0146] (sulfur)

[0147] As sulfur, various forms of sulfur can be used, such as powdered sulfur, insoluble sulfur, precipitated sulfur, colloidal sulfur, etc. Among them, precipitated sulfur and colloidal sulfur are preferred.

[0148] The sulfur content, based on 100 parts by mass of acrylic resin, is preferably greater than 50 parts by mass, more preferably greater than 70 parts by mass, and even more preferably greater than 90 parts by mass. When the content exceeds 50 parts by mass, the charge-discharge capacity and cycle characteristics tend to be improved. On the other hand, there is no upper limit to the sulfur content, but it is generally 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. A content of 1000 parts by mass or less tends to be cost-effective.

[0149] As sulfur, various allotropes can be used, but it is preferable to include an allotrope of S8 sulfur that is solid at normal temperature and normal pressure, and it is more preferable to use S8 sulfur alone.

[0150] (iron compounds containing divalent or trivalent iron ions)

[0151] The iron compound containing divalent or trivalent iron ions is not particularly limited, as long as it decomposes during the roasting process and reacts with sulfur to form iron disulfide, and various such iron compounds can be used. Examples of such iron compounds include ferrites, iron complexes, etc. Examples of ferrites include organic acid salts of iron and inorganic acid salts of iron. On the other hand, examples of iron complexes include neutral iron complexes and iron complex ion salts (iron complex salts).

[0152] Examples of organic acid salts of iron include, for example, ferrous iron (Fe 2+ ) and organic acid salts, trivalent iron (Fe 3+ ) and salts of organic acids, etc. Among them, salts of divalent iron and organic acids are preferred. There are no particular restrictions on the organic acid, such as organic acids with a carboxyl group (-COOH), organic acids with a sulfonic group (-SO3H), etc. Among them, organic acids with a carboxyl group are preferred. Specific examples of organic acids include fatty acids, oxalic acid, tartaric acid, citric acid, malic acid, succinic acid, etc. Specific examples of fatty acids include, for example, fatty acids with more than 1 and less than 6 carbon atoms, such as acetic acid, propionic acid, butyric acid, etc. Among them, acetic acid, oxalic acid, etc. are preferred. Preferred examples of organic acid salts of iron include iron (II) acetate, iron (II) oxalate, etc. The organic acid salt of iron may be in the form of a hydrate. One or more organic acid salts of iron may be used.

[0153] Examples of inorganic acid salts of iron include, for example, ferrous iron (Fe 2+ ) and inorganic acid salts, trivalent iron (Fe 3+ ) with an inorganic acid. Specific examples of inorganic acids include hydrochloric acid, sulfuric acid, nitric acid, etc. Among them, nitric acid, etc. are 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, iron (III) nitrate, etc. The inorganic acid salt of iron may be a hydrate. One or more inorganic acid salts of iron may be used.

[0154] Examples of iron complexes include ferrous iron (Fe 2+ ) complex, ferric iron (Fe 3+ ) complexes, etc. The iron complex may be in the form of a neutral complex or in the form of a complex salt. The ligand coordinated to the iron ion is not particularly limited, and may be, for example, a halogen atom such as a chlorine atom, a bromine atom, a cyano group, a dicyclopentadienyl group, N,N'-bis(salicylicaldehyde)ethylenediamine, etc. Examples of iron complexes include, for example, potassium hexacyanoferrate (II) ([Fe(CN)6]K4), potassium hexacyanoferrate (III) ([Fe(CN)6]K3), sodium tetrachloroferrate (III) ([FeCl4]Na), dicyclopentadienyl iron (II) (ferrocene), N,N'-bis(salicylicaldehyde)ethylenediamine iron (III) chloride, etc. One or more iron complexes may be used.

[0155] As the iron compound containing divalent or trivalent iron ions, 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 can be used. Among them, organic acid salts of iron, inorganic acid salts of iron, and neutral iron complexes are preferred.

[0156] From the perspective of the effects of the present invention, the content of the iron compound containing divalent or trivalent iron ions is preferably 50 parts by weight or more and 300 parts by weight or less based on 100 parts by weight of the acrylic resin. This content is more preferably greater than 50 parts by weight, further preferably greater than 60 parts by weight, further preferably greater than 70 parts by weight, and further preferably greater than 75 parts by weight. On the other hand, this content is more preferably less than 250 parts by weight, further preferably less than 200 parts by weight, further preferably less than 150 parts by weight, and further preferably less than 100 parts by weight.

[0157] (Other materials)

[0158] The raw materials may contain other materials commonly used in the art as needed. Examples of such materials include, for example, conductive carbon materials. Conductive carbon materials can improve the conductivity of the active material.

[0159] (Conductive carbon material)

[0160] As the conductive carbon material, a carbon material having a graphite structure is preferred. As the carbon material, a carbon material having a fused aromatic ring structure, such as carbon black, acetylene black, Ketjen black, graphite, carbon nanotubes (CNTs), carbon fibers (CFs), graphene, fullerene, etc. can be used. One or more conductive carbon materials can be used.

[0161] Among them, acetylene black, carbon black, and Ketjen black are preferred due to their low price and excellent dispersibility. In addition, small amounts of CNTs, graphene, etc. can also be used in combination with acetylene black, carbon black, or Ketjen black. This combination system can further improve the cycle characteristics of lithium-ion secondary batteries without significantly increasing costs. In addition, the combined amount of CNTs or graphene is preferably 8% by mass or more and 12% by mass or less of the total amount of the conductive carbon material.

[0162] The conductive carbon material content is preferably 5 parts by mass or greater, more preferably 10 parts by mass or greater, based on 100 parts by mass of acrylic resin. When the content is 5 parts by mass or greater, further improvements in charge-discharge capacity and cycle characteristics are more easily achieved. On the other hand, the content is preferably 50 parts by mass or less, more preferably 40 parts by mass or less. When the content is 50 parts by mass or less, the proportion of sulfur-containing structures in the sulfur-based active material does not decrease, and further improvements in charge-discharge capacity and cycle characteristics are more easily achieved.

[0163] (Step (1))

[0164] Step (1) is a step of mixing an acrylic resin, sulfur, and an iron compound containing divalent or trivalent iron ions to obtain a calcined raw material. The mixing method is not particularly limited, as long as the components are thoroughly mixed. In the present invention, preferred mixing methods include at least wet mixing using a solvent or dry mixing without a solvent, which will be mentioned below.

[0165] <Wet method>

[0166] In the present invention, the wet method includes the following steps for preparing the raw materials:

[0167] (1-a-1) adding an acrylic resin and an iron compound containing divalent or trivalent iron ions to an organic solvent and mixing them to obtain a liquid mixture;

[0168] (1-a-2) removing the organic solvent from the liquid mixture to obtain a dry mixture; and

[0169] (1-a-3) The dry mixture is mixed with sulfur.

[0170] In sub-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 a liquid mixture can be obtained by mixing. For example, (1) the acrylic resin and the iron compound containing divalent or trivalent iron ions can be added to the organic solvent simultaneously and mixed; (2) the acrylic resin can be added to the organic solvent and mixed, and then the iron compound containing divalent or trivalent iron ions is added and mixed; or (3) the iron compound containing divalent or trivalent iron ions can be added to the organic solvent and mixed, and then the acrylic resin is added and mixed.

[0171] In sub-step (1-a-1), as the organic solvent, any organic solvent commonly used in the art can be used. Examples include N-methyl-2-pyrrolidone, N,N-dimethylformamide, alcohols, hexane, water, acetone, ethers such as tetrahydrofuran, and the like. Furthermore, an organic solvent that can dissolve the acrylic resin is preferred because this facilitates good mixing. These solvents can be used alone or in combination.

[0172] The acrylic resin and / or the iron compound containing divalent or trivalent iron ions and the organic solvent can be mixed by stirring in a container such as a beaker.

[0173] In sub-step (1-a-2), the organic solvent can be removed by conventional methods, for example, by drying the mixed solution by heating, drying under reduced pressure, or drying under reduced pressure and heating.

[0174] The dry mixture thus obtained is preferably pulverized before proceeding to the next step. This is because, in this way, it can be expected that the mixing in sub-step (1-a-3) will be more sufficient. The pulverization can be carried out using conventional methods, for example, using a pulverizer. In particular, it is preferred that after coarse pulverization with a pulverizer, fine pulverization is carried out using a frozen pulverizer.

[0175] In the sub-step (1-a-3), the mixing of the dry mixture and sulfur may be performed by a conventional method, examples of which may include, for example, mixing using a stirrer, and the like.

[0176] <Dry method>

[0177] In the present invention, the dry method includes the step (1-b) of mixing a powdery acrylic resin, sulfur, and an iron compound containing divalent or trivalent iron ions for preparing a raw material.

[0178] Here, "powder" refers to a state in which various solid materials are refined to a state suitable for the mixing purpose of the present invention. The particle size constituting the powder is not particularly limited as long as it can be properly mixed, but for example, the median particle size is generally in the range of, for example, 1 μm or more and 40 μm or less. From the perspective of the effect of the present invention, from the perspective of the median particle size, the particle size is preferably greater than 1 μm, more preferably greater than 2 μm, further preferably greater than 3 μm, further preferably greater than 4 μm, and preferably less than 40 μm, more preferably less than 40 μm, more preferably less than 30 μm, further preferably less than 20 μm, further preferably less than 15 μm, further preferably less than 10 μm. The median particle size can be measured by the method described in the Examples section below.

[0179] The mixing can be performed by conventional methods, for example, in the same manner as in the above-mentioned sub-step (1-a-3).

[0180] Whether it is a wet method or a dry method, it is desirable to fully mix the raw materials in advance. In addition, if a conductive carbon material or the like is added to the raw materials, these additives can also be mixed before calcination so that they are contained in the raw materials in advance.

[0181] The raw material thus obtained can be used directly in the next step (2), or, if necessary, can be granulated and then used in step (2).

[0182] (Step (2))

[0183] Step (2) is the step of calcining the raw material obtained above. The calcination of the raw material can be carried out by conventional methods, for example, heating the raw material to a predetermined temperature at a predetermined heating rate, maintaining the temperature at the predetermined temperature for a predetermined time, and then cooling naturally.

[0184] <Non-oxidizing atmosphere>

[0185] Calcination is preferably performed in a non-oxidizing atmosphere. A non-oxidizing atmosphere is an atmosphere that is substantially free of oxygen, and is intended to suppress oxidative degradation and excessive thermal decomposition of the constituent components. Specifically, a non-oxidizing atmosphere includes an inert gas atmosphere such as nitrogen or argon, or a sulfur-containing gas atmosphere. Therefore, for example, modification can be performed in a quartz tube under an inert gas atmosphere.

[0186] <Heating rate>

[0187] The heating rate is preferably within a range of, for example, 50°C / h to 500°C / h. The heating rate is preferably higher than 50°C / h, and more preferably higher than 100°C / h. On the other hand, the heating rate is preferably lower than 500°C / h, more preferably lower than 400°C / h, further preferably lower than 300°C / h, and further preferably lower than 200°C / h. When the heating rate is within this range, it is easier to achieve improvements in charge-discharge capacity and cycle characteristics.

[0188] <Firing temperature / time>

[0189] The calcination temperature refers to the temperature after the raw material is heated, and the temperature is maintained for a certain period of time to calcine the raw material. The temperature is preferably in the range of higher than 250°C and lower than 550°C. When the temperature is higher than 250°C, it tends to avoid insufficient sulfurization reaction and prevent the charge and discharge capacity of the target object from decreasing. On the other hand, when the temperature is lower than 550°C, it tends to prevent the decomposition of the raw material and prevent the decrease in yield and charge and discharge capacity. The temperature is more preferably higher than 300°C, further preferably higher than 350°C, and further preferably higher than 380°C. On the other hand, the temperature is preferably lower than 500°C, further preferably lower than 480°C, and further preferably lower than 450°C.

[0190] From the perspective of the effects of the present invention, the calcination temperature in step (2) is preferably higher than the thermal decomposition temperature of the iron compound containing divalent or trivalent iron ions.

[0191] The holding time at the calcination temperature can be appropriately set depending on the type of raw materials, calcination temperature, and other factors, and is preferably, for example, 1 hour or longer and 6 hours or shorter. A holding time of 1 hour or longer tends to allow for sufficient calcination, while a holding time of 6 hours or shorter tends to prevent excessive thermal decomposition of the constituent components. This holding time is preferably greater than 1 hour, more preferably greater than 1.5 hours. On the other hand, this holding time is preferably less than 6 hours, more preferably less than 4 hours.

[0192] <Device>

[0193] Calcination can be carried out by, for example, a muffle furnace ( Figure 1) can also be carried out using continuous equipment such as a twin-screw extruder. The use of continuous equipment has the following advantages: by kneading, crushing and mixing the raw materials in the equipment while performing a series of operations such as roasting, the sulfur-based active material can be produced continuously.

[0194] Muffle furnace Figure 1 ) is a furnace separated by a heating plate, etc., so that the heat source (heater) is not exposed inside the furnace to prevent sample contamination. Figure 1 In the figure, a heater 2 is installed at the bottom of the muffle furnace 1, separated by a heating plate. A cover 3 is installed on the front surface of the furnace (the left end in the figure), which enables the furnace to maintain an inert gas 4 atmosphere. A thermocouple (not shown) is installed on the cover to measure the temperature inside the furnace during the roasting process. Two stainless steel (SUS) rectangular trays 5 and 6 are installed in the upper and lower layers of the furnace, respectively, for roasting the raw materials.

[0195] The interior of the furnace is constructed to allow for the continuous supply of gas (e.g., an inert gas such as argon) from the outside through a gas inlet pipe 7 and a gas outlet pipe 8, which is then discharged to the outside. The gas outlet pipe 8 is connected to a collection tank 10 filled with an aqueous sodium hydroxide solution 9. Exhaust gas from the muffle furnace 1 passes through the gas outlet pipe 8 and first flows through the aqueous sodium hydroxide solution 9 in the collection tank 10 before being discharged to the outside. Therefore, even if the exhaust gas contains hydrogen sulfide gas generated by the reaction, it is neutralized by the aqueous sodium hydroxide solution and removed from the exhaust gas.

[0196] (Residue Removal Step)

[0197] The treated product obtained after calcination may contain residual unreacted sulfur and other substances that sublime during the calcination process and precipitate upon cooling. Because these residues can degrade cycle performance, it is desirable to remove them as much as possible. Residue removal can be accomplished using conventional methods, such as reduced-pressure heating and drying, hot air drying, or solvent washing.

[0198] (Crushing / Classification)

[0199] The obtained sulfur-based active material is pulverized to a predetermined particle size and classified to obtain a particle size suitable for electrode production.

[0200] Furthermore, in the sintering method using a twin-screw extruder as described above, the sulfur-based active material can be produced while the produced active material is being pulverized by a shearing step during kneading.

[0201] [Preparation of electrodes for secondary batteries]

[0202] The secondary battery electrode of the present invention can be prepared using the active material obtained above. Specifically, the electrode is prepared using the same method as a conventional secondary battery electrode, except that a conductive coating is formed on the current collector, a composite material layer is formed on the conductive coating, and the interface tortuosity τ between the conductive coating and the composite material layer is set to a predetermined value.

[0203] (Using active material as positive electrode active material)

[0204] A positive electrode for a secondary battery can be prepared, for example, by mixing the aforementioned active material with a conductive additive, a binder, and a solvent to form a paste-like positive electrode material; applying the paste to a current collector pre-coated with a conductive coating (a current collector coated with a conductive coating); and, after drying, applying pressure to the paste using a press or the like to achieve a predetermined interface tortuosity. If necessary, the resulting positive electrode can be vacuum-dried before use to remove solvents, etc.

[0205] <Conductive additive>

[0206] Examples of conductive additives include vapor-grown carbon fiber (VGCF), carbon powder, carbon black (CB), acetylene black (AB), Ketjen black (KB), graphite, or fine metal powders that are stable at positive electrode potential, such as aluminum and titanium. Furthermore, the aforementioned conductive carbon materials can also be used as conductive additives. These conductive additives can be used alone or in combination of two or more.

[0207] <Adhesive>

[0208] Examples of binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamide-imide (PAI), carboxymethyl cellulose (CMC), polyvinyl chloride (PVC), acrylic resins (e.g., polyacrylic acid), methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene ether (PPO), polyethylene oxide (PEO), polyethylene (PE), polypropylene (PP), etc. These binders may be used alone or in combination of one or more.

[0209] <Solvent>

[0210] Examples of the solvent include N-methyl-2-pyrrolidone, N,N-dimethylformamide, alcohols, hexane, water, etc. These solvents may be used alone or in combination of one or more.

[0211] <Amount>

[0212] The amounts of these materials constituting the positive electrode are not particularly limited, but 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 are preferably added based on 100 parts by mass of the active material.

[0213] <Current Collector Coated with Conductive Coating>

[0214] The current collector used to manufacture the current collector coated with the conductive coating can use the above-mentioned current collector. The current collector can be used alone or in combination of two or more. In addition, the surface of the current collector can be coated with carbon or the like. Specific examples of current collectors coated with carbon or the like include, for example, carbon-coated aluminum foil. In this case, the current collector includes a portion coated with carbon.

[0215] The current collector coated with the conductive coating can be prepared as follows. For example, the conductive coating material can be mixed with a solvent to form a paste-like conductive coating material, which can then be applied to the surface of the current collector and dried. Any solvent used in preparing positive electrode materials can be used as the solvent.

[0216] (Using active material as negative electrode active material)

[0217] A negative electrode for a secondary battery can be manufactured, for example, by mixing the aforementioned active material with a conductive additive, a binder, and a solvent to form a paste-like negative electrode material; applying the paste to a current collector pre-coated with a conductive coating (i.e., a current collector coated with a conductive coating); and, after drying, pressing the paste together using a press, etc., to achieve a predetermined interface tortuosity. If necessary, the resulting negative electrode can be vacuum-dried before use to remove solvents, etc.

[0218] As the conductive auxiliary agent, binder, and solvent, the same materials as those used in the case of using the active material as the positive electrode active material can be used, and the same applies to the material constituting the current collector coated with the conductive coating.

[0219] [Manufacturing of Secondary Batteries]

[0220] The secondary battery of the present invention can be manufactured in the same manner as a conventional secondary battery, except that the positive electrode obtained above is used. Furthermore, the lithium-ion secondary battery of the present invention can be manufactured in the same manner as a conventional lithium-ion secondary battery, except that the negative electrode obtained above is used. The manufacturing method for a lithium-ion secondary battery will be described below.

[0221] (Active material as positive electrode active material)

[0222] Lithium ion secondary batteries can be prepared according to conventional methods using a positive electrode (positive electrode active material) containing the above-mentioned active material, a negative electrode, an electrolyte solution, and further adding components such as a separator as needed.

[0223] <Negative electrode>

[0224] The negative electrode comprises the above-mentioned current collector and negative electrode material. As the negative electrode material, known metallic lithium, carbon-based materials (such as graphite, etc.), silicon-based materials (such as silicon thin film, etc.) and alloy-based materials (such as copper-tin, cobalt-tin, etc.) can be used. When a material (such as carbon-based materials, silicon-based materials, alloy-based materials, etc.) that does not contain lithium in the above-mentioned negative electrode materials is used as the negative electrode material, its advantage is 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 negative electrode materials that do not contain lithium are used in combination with the positive electrode of the present invention, both the positive electrode and the negative electrode do not contain lithium. Therefore, a lithium pre-doping process is required, that is, lithium is pre-embedded in one or both of the negative electrode and the positive electrode. As a method for pre-doping lithium, it can be carried out according to methods known in the prior art. For example, when doping lithium in the negative electrode, there are the following methods: an electrolytic doping method in which a half-cell is assembled using metallic lithium as a counter electrode and lithium is doped by an electrochemical method; and a coating pre-doping method in which a metallic lithium foil is attached to the electrode, which is then placed in an electrolyte and doped by diffusion of lithium into the electrode. In addition, the above-mentioned electrolytic doping method can also be used when lithium is pre-doped in the positive electrode. As a lithium-free negative electrode material, high-capacity silicon-based negative electrode materials are particularly preferred. Among them, thin-film silicon is more preferred because of its thin electrode thickness and greater advantages in unit volume capacity.

[0225] <Electrolyte>

[0226] The electrolyte compensates for the charge generated by the electrons released to the external circuit due to the oxidation / reduction of the positive and negative active materials by ion flow. For the electrolyte of lithium ion secondary batteries, an electrolyte obtained by dissolving an alkali metal salt as an electrolyte in an organic solvent can be used. As the organic solvent, it is preferred to use at least one of non-aqueous solvents selected from dimethoxyethane, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl ether, γ-butyrolactone, acetonitrile, etc. As the electrolyte, Li(FSO2)2N, LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiI, LiClO4, etc. can be used. The concentration of the electrolyte only needs to be about 0.5 mol / L to 5.0 mol / L. In addition, the electrolyte is not limited to liquid. For example, when the lithium ion secondary battery is a lithium polymer secondary battery, the electrolyte is solid (for example, polymer gel state).

[0227] <Diaphragm>

[0228] In addition to the above-mentioned negative electrode, positive electrode and electrolyte, lithium-ion secondary batteries may also include components such as a separator. The separator is located between the positive electrode and the negative electrode, allowing ions to migrate between the positive electrode and the negative electrode and preventing internal short circuits between the positive electrode and the negative electrode. If the lithium-ion secondary battery is a sealed type, the separator needs to have the function of retaining the electrolyte. As the separator, it is preferred to use a thin film or non-woven film made of polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide, cellulose, glass, etc. that has micropores. In addition, these materials can be coated with aluminum oxide, etc. Examples of such coated separators include, for example, aluminum oxide-coated polypropylene, etc.

[0229] <Shape>

[0230] The shape of the lithium ion secondary battery is not particularly limited, and various shapes such as a cylindrical shape, a stacked shape, a button shape, a laminated shape, and a button shape may be employed.

[0231] (Using the active material as the negative electrode active material)

[0232] Lithium-ion secondary batteries are manufactured according to conventional methods using, in addition to a negative electrode (negative electrode active material) containing the above-mentioned active material, a positive electrode, an electrolyte, and further, if necessary, components such as a separator.

[0233] <Positive electrode>

[0234] There is no particular limitation on the positive electrode material, and it may be, for example, a lithium-containing transition metal oxide or solid solution oxide, or a substance capable of electrochemically absorbing and releasing lithium ions. Examples of lithium-containing transition metal oxides include lithium-cobalt composite oxides such as LiCoO2, lithium-cobalt composite oxides such as LiNi x Co y Mn z O2, etc., lithium nickel cobalt manganese composite oxides, such as LiNiO2, etc., lithium nickel composite oxides, such as LiMn2O4, etc., etc. Examples of solid solution oxides include Li a Mn x Co y Ni z O2 (1.150≤a≤1.430, 0.450≤x≤0.600, 0.100≤y≤0.150, 0.200≤z≤0.280), LiMn x Co y Ni z O2 (0.300≤x≤0.850, 0.100≤y≤0.300, 0.100≤z≤0.300), LiMn 1.5 Ni 0.5 O4, etc. These compounds can be used alone or in combination.

[0235] Regarding the shapes of the electrolyte, separator, and lithium ion secondary battery, the same shapes as those when the active material is used as a positive electrode active material can be used.

[0236] Example

[0237] Although the present invention will be described based on the embodiments, the present invention is not limited to the embodiments.

[0238] The materials used in the examples and comparative examples are described below.

[0239] <Materials used in the experiment>

[0240] Acrylic resin 1: Spherical acrylic resin made of methacrylate homopolymer (TECHPOLYMER MB-4, manufactured by Sekisui Kasei Co., Ltd., particle size: 4 μm)

[0241] Iron compound 1 (organic acid salt): Iron (II) oxalate dihydrate (Iron (II) oxalate dihydrate, manufactured by KANTOCHEMICAL CO., INC., standard reagent)

[0242] Sulfur: Precipitated sulfur manufactured by Tsurumi Chemical Industry Co., Ltd.

[0243] Organic solvent (wet method): acetone (acetone, manufactured by YONEYAMA YAKUHIN KOGYO CO., LTD., special grade)

[0244] Preparation Example 1 (Fine Pulverization of Iron Compound)

[0245] Before the iron compound was subjected to the kneading step, it was previously pulverized for 5 minutes using a freeze pulverizer (Model JFC-2000 manufactured by Japan Analytical Industry Co., Ltd.).

[0246] Preparation Example 2 (preparation of raw materials by wet method)

[0247] Acrylic resin was added to an organic solvent and mixed thoroughly, and then an iron compound was further added to the mixture to obtain a liquid mixture. Next, the organic solvent in the liquid mixture was removed and the mixture was pulverized using a chopper (LAB MILL manufactured by OSAKA CHEMICAL Co., Ltd.) to obtain a dry mixture.

[0248] The dry mixture thus obtained was mixed with sulfur using a stirrer to obtain a roasting raw material.

[0249] Examples and Comparative Examples

[0250] <Preparation of Active Material>

[0251] (Roasting raw materials)

[0252] According to the recipe shown in Table 1, raw materials prepared by the wet method were used as the calcination raw materials.

[0253] (Calcination reaction device)

[0254] Use a muffle furnace ( Figure 1 ) Roasting the raw materials. Figure 1 The muffle furnace is as described above.

[0255] (Calcination step)

[0256] First, the raw materials were placed in a SUS container tray. The atmosphere in the muffle furnace was replaced with argon three times using a vacuum pump. Argon was then continuously introduced through the gas inlet tube at a rate of 100 mL / min. Heating of the muffle furnace began 30 minutes after the start of the process. The temperature was increased at a rate of 5°C / min. When the raw materials reached the temperatures shown in Table 1, they were held at this temperature for 2 hours. Next, the raw materials were allowed to cool naturally to 25°C under an argon atmosphere while adjusting the argon flow rate. The raw materials were then removed from the muffle furnace.

[0257] (Crushing step)

[0258] The calcined material was pulverized as follows.

[0259] Preparation Example 1: Grind with a chopper at 25,000 rpm for 10 seconds

[0260] Preparation Example 2: Grind with a chopper at 25,000 rpm for 60 seconds

[0261] Preparation Example 3: Grind with a chopper at 25,000 rpm for 30 seconds

[0262] Preparation 4: Grind gently in a mortar for 30 seconds

[0263] Preparation Example 5: Grinding with a ball mill for 1 hour

[0264] Preparation Example 6: The calcined material was frozen and then crushed with steel balls for 10 minutes.

[0265] (Grading steps)

[0266] In order to remove coarse particles in the pulverized calcined material, the calcined material was classified using a stainless steel sieve with a mesh size of 32 μm to obtain an active material.

[0267] <Physical Properties of Active Materials>

[0268] (Elemental Analysis)

[0269] The active materials prepared in Examples and Comparative Examples were subjected to elemental analysis.

[0270] The masses of carbon, hydrogen, nitrogen, and sulfur were measured using a VariO MICROcube fully automatic elemental analyzer manufactured by Elementar, and their mass ratios (%) in the total active material were calculated. Furthermore, the mass of oxygen was measured using an EMGA-920 instrument manufactured by Horiba Ltd. using an inert gas pulse heating / fusion-NDIR method, and its mass ratio (%) in the total active material was calculated. The results are listed in Table 1.

[0271] (Median particle size)

[0272] The median particle size distribution was measured using a laser diffraction / scattering particle size analyzer (PSA1090L particle size distribution analyzer manufactured by Anton Paar GmbH) with water as the dispersion medium. 50 ).

[0273] (Breaking strength Cs (MPa))

[0274] For the active material, the test force P (in mN) at which the active material particles fracture was measured using the indenter of a micro-compression tester (MCT-510, manufactured by Shimadzu Corporation). Specifically, a small amount of active material was spread on a sample table. Five random particles were then compressed individually with the indenter. The test force P (in mN) at which each active material particle fractured was measured. The particle diameter d (in μm) of each particle was calculated as the average of the values measured in two perpendicular directions.

[0275] The breaking strength Cs (in MPa) is calculated from the test force P (in mN) and the particle size d (in μm) using the following formula, where π is the ratio of the circumference of a circle to 3.14:

[0276] Cs=2.48×P / πd 2 .

[0277] The average value of the breaking strength Cs (in MPa) of the five particles was calculated and regarded as the breaking strength Cs (in MPa) of the active material.

[0278] The results are shown in Table 1.

[0279] Table 1

[0280]

[0281] <Production of positive electrode and lithium-ion battery>

[0282] According to Tables 2 to 5, a positive electrode was prepared using the active materials obtained above, and a lithium ion secondary battery was further prepared using the positive electrode, as described below.

[0283] (Current collector coated with conductive coating)

[0284] The conductive coating-coated current collector was prepared by adding 55% by mass of polyvinylidene fluoride (KF-#9700, manufactured by KUREHA CORPORATION) and 45% by mass of acetylene black (HS-100, manufactured by Denka Company Limited) and thoroughly dispersing them with N-methylpyrrolidone (NMP) until the slurry viscosity reached approximately 1000 cP. The slurry was passed through a 400-mesh sieve and then coated onto a 15-μm-thick aluminum foil to a predetermined thickness. The resulting conductive coating-coated current collector was then dried.

[0285] (positive electrode)

[0286] The positive electrode was prepared as follows: 10.0% by mass of polyacrylic acid (manufactured by FUJIFILM Wako Pure Chemical Corporation, average molecular weight: 2700 to 7500) and 5.0% by mass of acetylene black (HS-100, manufactured by DENKA HS-100) were added to 85% by mass of the active material obtained in Table 1, the mixture was fully kneaded with pure water for 1 hour, and then coated on the collector coated with the conductive coating obtained above and dried, followed by pressing with a desktop hot roller press (HSRP-60150H, manufactured by Hohsen Corp.) to achieve a predetermined electrode porosity (in %), and vacuum dried at 120°C for 6 hours. Finally, the resulting material was cut into a predetermined size to obtain a positive electrode.

[0287] (negative electrode)

[0288] A 100 μm thick lithium (Li) layer was laminated on a 6 μm thick copper (Cu) foil, and then punched into a predetermined size in a dry atmosphere with a dew point of -50°C or less to prepare a negative electrode.

[0289] (Electrolyte)

[0290] The electrolyte 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.

[0291] (Lithium-ion secondary battery)

[0292] The preparation method of the lithium-ion battery for evaluation is as follows: using an ultrasonic metal bonding machine (manufactured by Nippon Avionics Co., Ltd.), the tabs with sealant are respectively bonded to the uncoated parts of the positive electrode and the lithium negative electrode cut into predetermined sizes. These tabs are placed relative to each other and encapsulated in an aluminum laminate so that the surface of the composite material is sandwiched between an alumina-coated polypropylene diaphragm cut into predetermined sizes (manufactured by Shanghai Dinghao New Material Technology Co., Ltd. with a total thickness of 20 μm). The aluminum laminate is heat-sealed at both ends, leaving an electrolyte injection port. 2.0 mL of electrolyte is then injected from the injection port and vacuum-sealed using a vacuum packaging machine manufactured by TOSEI CORPORATION to obtain a laminated lithium-ion battery for evaluation.

[0293] <Evaluation of Lithium-ion Secondary Batteries>

[0294] (Discharge capacity and capacity retention rate)

[0295] Each laminated lithium-ion secondary battery produced in the Examples and Comparative Examples was charged and discharged at a test temperature of 30°C with a current value corresponding to 50 mA per 1g of positive electrode active material. The discharge end voltage was set to 1.0V, and the charge end voltage was set to 3.0V. While repeatedly charging and discharging, the first, second, and third battery discharge capacities (mAh) were observed. The tests were performed using a battery performance evaluation device (BLS system, manufactured by KEISOKUKI CENTER CO., LTD.).

[0296] (Average thickness of conductive coating (μm))

[0297] After the conductive coating is produced, the total thickness including the current collector is measured at any five points using a digital micrometer produced by Mitutoyo Corporation, and then the thickness of the current collector is subtracted from the measured value, and the arithmetic mean of the subtracted values is calculated to determine the average thickness of the conductive coating.

[0298] (Interface tortuosity τ)

[0299] The interface tortuosity τ between the conductive coating and the composite material layer that constitutes the electrode is expressed as b / a, where a is a given distance (a μm) on a cross-section of the electrode taken along a plane perpendicular to the stacked conductive coating and composite material layers, and b is the interface length on the composite material layer side (b μm) (where a>b). τ is defined as the average of five measurements taken once for each of five arbitrary cross-sections of the electrode. Furthermore, the values of a and b are defined as those obtained from microscopic images using the image analysis software ImageJ (https: / / imagej.nih.gov / ij / index.html).

[0300] (Electrode coating amount (mg / cm 2 ))

[0301] Use Thomson knife to punch out the predetermined area C (unit: cm 2 ) electrode sheet, measure the weight A (in mg) of the punched electrode, and use the same Thomson knife to punch out the current collector and measure its weight B. Then calculate the coating amount D (mg / cm2) of the electrode composite layer according to the following formula 2 ). The higher the coating amount, the thicker the electrode.

[0302] Electrode coating amount D (mg / cm 2 )=(AB) / C

[0303] (Electrode porosity (%))

[0304] (Electrode porosity (%))

[0305] The electrode coating amount D (unit: mg / cm 2 ), electrode thickness E (in μm), current collector thickness F (in μm), and true specific gravity G of the electrode composite material (in g / cm 3 ), the porosity of the electrode composite material layer is calculated by the following formula.

[0306] Electrode porosity (%) = 100 - [{D / 1000} / {(EF) / 10000}] / G × 100

[0307] (Electrode adhesion (mN / mm))

[0308] Electrode adhesion was measured using a 180° peel method using a dynamometer and test stand (manufactured by IMADA CO., LTD). Acrylic foam double-sided tape (H9004, manufactured by Nitto Denko Corporation) was applied to the surface of a 25 mm wide, 120 mm long stamped electrode. The test sample was compacted with a roller and measured at a speed of 100 mm / min. The maximum adhesion value (in mN) was divided by the 25 mm electrode width to obtain the electrode adhesion.

[0309] Table 2

[0310]

[0311] Table 3

[0312]

[0313] Table 4

[0314]

[0315] Table 5

[0316]

[0317] It can be seen from Tables 2 to 5 that the charge and discharge capacities in the examples are improved.

[0318] <Implementation Method>

[0319] Preferred embodiments are as follows.

[0320] [1] An electrode comprising a current collector, a conductive coating formed on the current collector, and a composite material layer formed on the conductive coating, wherein the average thickness of the conductive coating is greater than 0.5 μm, preferably greater than 1 μm, more preferably greater than 2 μm, further preferably greater than 4 μm, and further preferably greater than 5 μm.

[0321] The interface tortuosity τ between the conductive coating and the composite material layer is 1.05 or more, preferably 1.10 or more, more preferably 1.15 or more, further preferably 1.20 or more, further preferably 1.25 or more, further preferably 1.30 or more, further preferably 1.35 or more, further preferably 1.40 or more, further preferably 1.45 or more, further preferably 1.50 or more;

[0322] The composite material layer contains active materials, and the average particle size d of the active materials is 50 It is 2 μm or more, preferably greater than 3 μm, more preferably greater than 4 μm, further preferably greater than 5 μm, further preferably greater than 6 μm, further preferably greater than 7 μm, further preferably greater than 8 μm.

[0323] [2] The electrode according to the above [1], wherein, for each of any five particles of the active material, when the particle size is d, measured in μm, and the test force at the moment of particle fracture in the microcompression test is P, measured in mN, the average value of the fracture strength Cs in MPa calculated by the following formula is less than 70.0, preferably less than 60.0 MPa, more preferably less than 30.0 MPa, and further preferably less than 15.0 MPa.

[0324] Cs=2.48×P / πd 2

[0325] (Wherein, d is the average of the diameters of the particle measured in two perpendicular directions, and π is the circumference of a circle (3.14)).

[0326] [3] The electrode according to [2] above, wherein the average value of the breaking strength Cs in MPa is less than 10.0 MPa.

[0327] [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 carbon, sulfur, nitrogen, and oxygen, and the content of the elements in the active material is greater than 50.0% by mass, preferably greater than 60.0% by mass, more preferably greater than 70.0% by mass, further preferably greater than 75.0% by mass, and further preferably greater than 80.0% by mass.

[0328] [5] The electrode according to any one of [1] to [3] above, wherein the active material contains sulfur, and the sulfur content in the active material is 45.0 mass% or more, preferably greater than 50.0 mass%, more preferably greater than 55.0 mass%, and further preferably greater than 60.0 mass%.

[0329] [6] An electrode according to any one of [1] to [5] above, wherein the carbon content is 95% by mass or less, preferably less than 90% by mass, more preferably less than 80% by mass, and further preferably less than 70% by mass, based on the total mass of the composite material layer.

[0330] [7] An electrode according to any one of [1] to [5] above, wherein the sulfur content is 5% by mass or more, preferably greater than 10% by mass, more preferably greater than 20% by mass, and further preferably greater than 30% by mass, based on the total mass of the composite material layer.

[0331] [8] The electrode according to any one of [1] to [7] above, wherein when the conductive coating is coated on the surface of the current collector, the volume resistivity of the conductive coating is 1.0×10 6 Ω·cm or less, preferably 1.0×10 5 Ω·cm or less, more preferably 1.0×10 4 Ω·cm or less, more preferably 1.0×10 3 Ω·cm or less.

[0332] [9] The electrode according to any one of [1] to [8] above, wherein when the electrode is used as a positive electrode, the initial discharge capacity of the electrode is 650 mAh / g or more, preferably greater than 650 mAh / g, more preferably greater than 700 mAh / g, further preferably greater than 710 mAh / g, further preferably greater than 720 mAh / g, further preferably greater than 730 mAh / g, further preferably greater than 740 mAh / g, further preferably 750 mAh / g or more, further preferably greater than 750 mAh / g.

[0333]

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

[0334]

[11] The secondary battery according to

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

[0335] Reference Symbol List

[0336] 1. Muffle furnace

[0337] 2. Heater

[0338] 3. Lid

[0339] 4. Inert gas

[0340] 5. Tray (upper layer)

[0341] 6. Tray (lower layer)

[0342] 7. Gas inlet pipe

[0343] 8. Gas exhaust pipe

[0344] 9. Sodium hydroxide aqueous solution

[0345] 10.Catch tank

Claims

1. An electrode comprising a current collector, a conductive coating formed on the current collector, and a composite material layer formed on the conductive coating, wherein: The average thickness of the conductive coating is greater than 0.5 μm; The interface tortuosity τ between the conductive coating and the composite material layer is greater than 1.05; and The composite material layer comprises an active material, the average particle size d of the active material 50 2μm or more.

2. The electrode according to claim 1, wherein For each of any five particles of the active material, when the particle size is d, measured in μm, and the test force at the moment of particle fracture in a microcompression test is P, measured in mN, the average value of the fracture strength Cs in MPa calculated by the following formula is less than 70.0 MPa: Cs=2.48×P / πd 2 d is the average of the diameters of the particle measured in two perpendicular directions, and π is the circumference of a circle, 3.

14.

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

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

5. The electrode according to claim 1, wherein The active material includes sulfur, and a content of sulfur in the active material is 45.0% by mass or more.

6. The electrode according to claim 1, wherein The carbon content is 95% by mass or less based on the total mass of the composite material layer.

7. The electrode according to claim 1, wherein The sulfur content is 5% by mass or more based on the total mass of the composite material layer.

8. The electrode according to claim 1, wherein When the conductive coating is coated on the surface of the current collector, the volume resistivity of the conductive coating is 1.0×10 6 Ω·cm or less.

9. The electrode according to claim 1, wherein When the electrode is used as a positive electrode, the electrode has an initial discharge capacity of 650 mAh / g or more. 10 . A secondary battery comprising the electrode according to claim 1 .

11. The secondary battery according to claim 10, wherein The secondary battery is a lithium-ion secondary battery.

Images