Nonaqueous electrolyte secondary battery positive electrode and nonaqueous electrolyte secondary battery

Abstract

A positive electrode for a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode core and a positive electrode mixture layer arranged on at least a surface of the positive electrode core, the positive electrode mixture layer including at least a positive electrode active material, a conductive additive, and a nitrile group-containing rubber. The positive electrode active material includes a first positive electrode active material and a second positive electrode active material, the first positive electrode active material being a lithium-containing transition metal phosphate having an olivine structure and containing a specific element, and the second positive electrode active material being a lithium transition metal composite oxide having a layered rock salt structure and containing a specific element. The conductive additive includes at least one selected from the group consisting of carbon black, carbon nanotubes, and porous carbon, and the carbon nanotubes include at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes.

Description

Positive electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

[0001] This invention relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

[0002] A non-aqueous electrolyte secondary battery comprises a positive electrode, a negative electrode, and a separator interposed between the positive and negative electrodes. In such a non-aqueous electrolyte secondary battery, the positive electrode contains a positive electrode active material.

[0003] Patent Document 1 discloses a positive electrode comprising a first active material and a second active material as the positive electrode active material. Furthermore, Patent Document 1 discloses a first active material having the formula LiFe １－ｘ Mn ｘ PO ４ The invention discloses a lithium iron phosphate-based material represented by the formula (wherein x is 0 to 0.8), wherein the second active material is one or more of lithium nickelate, lithium manganeseate, lithium cobaltate, lithium nickel cobalt manganeseate, lithium nickel cobalt aluminate, lithium-rich manganese system, and lithium vanadium phosphate, and the amount of the second active material used is 10 to 70% of the total mass of the first and second active materials.

[0004] Special Publication No. 2024-510692

[0005] When the positive electrode contains a lithium iron phosphate-based material represented by the above formula as the first active material, and at least one of lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminate as the second active material, the cycle characteristics of a secondary battery having such a positive electrode may deteriorate. In other words, when the first active material is a lithium transition metal phosphate having an olivine structure and containing at least one of Fe and Mn, and the second active material is a lithium transition metal composite oxide having a layered rock salt-type structure and containing at least one element selected from the group consisting of Co, Mn, and Al, and Ni, the cycle characteristics of a secondary battery may deteriorate.

[0006] Therefore, an object of the present disclosure is to provide a positive electrode for a non-aqueous electrolyte secondary battery that can suppress a decrease in cycle characteristics of the non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery including such a positive electrode for a non-aqueous electrolyte secondary battery.

[0007] One aspect of the present invention has a positive electrode core and a positive electrode mixture layer disposed on at least the surface of the positive electrode core. The positive electrode mixture layer contains at least a positive electrode active material, a conductive assistant, and a nitrile group-containing rubber. The positive electrode active material includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material is a lithium-containing transition metal phosphate having an olivine structure and containing at least one of Fe and Mn. The second positive electrode active material is a lithium transition metal composite oxide having a layered rock salt structure and containing at least one element selected from the group consisting of Co, Mn, and Al and Ni. The conductive assistant includes at least one selected from the group consisting of carbon black, carbon nanotubes, and porous carbon. The carbon nanotubes include at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes. It relates to a positive electrode for a non-aqueous electrolyte secondary battery.

[0008] Another aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode for a non-aqueous electrolyte secondary battery described above.

[0009] According to the present disclosure, it is possible to provide a positive electrode for a non-aqueous electrolyte secondary battery that can suppress a decrease in cycle characteristics of the non-aqueous electrolyte secondary battery. It is also possible to provide a non-aqueous electrolyte secondary battery including such a positive electrode for a non-aqueous electrolyte secondary battery.

[0010] It is a schematic perspective view of a part of a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, with a part cut away.

[0011] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values, materials, etc. may be applied as long as the effects of this disclosure are obtained. Notwithstanding, known components may be applied to components of parts that are characteristic of this disclosure. In this specification, when "the range of numerical values ​​A to numerical values ​​B" is used, that range includes numerical values ​​A and B.

[0012] In the following explanation, when examples are given of lower and upper limits for specific physical properties or conditions, any combination of either of the given lower limits and any of the given upper limits is permitted, as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, unless otherwise specified, one type may be selected and used alone, or two or more types may be used in combination.

[0013] This disclosure includes any combination of two or more claims that can be arbitrarily selected from the claims set forth in the attached claims. In other words, any combination of two or more claims that can be arbitrarily selected from the claims set forth in the attached claims is possible, as long as it does not result in a technical inconsistency.

[0014] [Positive electrode for non-aqueous electrolyte secondary battery] A positive electrode for a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure comprises a positive electrode core and a positive electrode mixture layer disposed on at least the surface of the positive electrode core, wherein the positive electrode mixture layer comprises at least a positive electrode active material, a conductive additive, and a nitrile group-containing rubber.

[0015] In the positive electrode for a non-aqueous electrolyte secondary battery according to the embodiments of this disclosure, the positive electrode active material comprises a first positive electrode active material and a second positive electrode active material, wherein the first positive electrode active material is a lithium-containing transition metal phosphate having an olivine structure and containing at least one of Fe and Mn, and the second positive electrode active material is a lithium transition metal composite oxide having a layered rock salt type structure and containing at least one element selected from the group consisting of Co, Mn, and Al, and Ni.

[0016] In the positive electrode for a non-aqueous electrolyte secondary battery according to the embodiments of this disclosure, the conductive additive comprises at least one selected from the group consisting of carbon black, carbon nanotubes, and porous carbon, and the carbon nanotubes comprises at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes.

[0017] In the positive electrode for a non-aqueous electrolyte secondary battery according to the embodiments of this disclosure, it is important that (i) the positive electrode mixture layer comprises at least a positive electrode active material, a conductive additive, and a nitrile group-containing rubber; (ii) the first positive electrode active material is a lithium-containing transition metal phosphate having an olivine structure; and (iii) the conductive additive comprises at least one selected from the group consisting of carbon black, carbon nanotubes, and porous carbon. The reasons for these are explained below.

[0018] Lithium-containing transition metal phosphates having an olivine structure, for example, general formula: LiFe １－ｘ Mn ｘ PO ４ In lithium-containing transition metal phosphates represented by the formula (where 0 ≤ x ≤ 1), the bond between phosphorus atoms (P) and oxygen atoms (O) is strong, so even when the inside of a non-aqueous electrolyte secondary battery becomes hot during charging and discharging, the bond between phosphorus atoms (P) and oxygen atoms (O) is less likely to break, and oxygen (O) ２ This makes it less likely for ) to occur. Therefore, by using a lithium-containing transition metal phosphate with an olivine structure as the positive electrode active material, the safety of non-aqueous electrolyte secondary batteries can be improved.

[0019] On the other hand, lithium-containing transition metal phosphates with an olivine structure contribute less to increasing the capacity of non-aqueous electrolyte secondary batteries and have a lower energy density compared to lithium transition metal composite oxides with a layered rock salt structure. Therefore, in non-aqueous electrolyte secondary batteries, lithium-containing transition metal phosphates with an olivine structure and lithium transition metal composite oxides with a layered rock salt structure are often used in combination to increase capacity and energy density while improving safety. In other words, the positive electrode for non-aqueous electrolyte secondary batteries often contains a lithium-containing transition metal phosphate with an olivine structure as the first positive electrode active material and a lithium transition metal composite oxide with a layered rock salt structure as the second positive electrode active material.

[0020] Lithium-containing transition metal phosphates with an olivine structure tend to have higher internal resistance because their lithium diffusion coefficient within the active material is smaller compared to lithium transition metal composite oxides with a layered rock salt structure. Therefore, in lithium-containing transition metal phosphates with an olivine structure, such as the first positive electrode active material, it is conceivable to prevent an increase in internal resistance by reducing the distance over which lithium diffuses within the active material. Accordingly, lithium-containing transition metal phosphates with an olivine structure, such as the first positive electrode active material, are often used in a finely milled form to prevent such an increase in internal resistance. The average particle size of the first positive electrode active material is, for example, 0.1 μm to 1 μm. The second positive electrode active material may be used with an average particle size of 3 μm to 30 μm.

[0021] A positive electrode for a non-aqueous electrolyte secondary battery typically comprises a positive electrode core and a positive electrode mixture layer disposed on at least the surface of the positive electrode core. The positive electrode mixture layer is formed by dispersing a positive electrode mixture containing a positive electrode active material and conductive additives such as carbon materials in a solvent to obtain a positive electrode mixture slurry, applying this slurry to the surface of the positive electrode core to form a coating, and then drying and rolling the coating. In this case, if the positive electrode active material includes a lithium-containing transition metal goldate having an olivine structure as the first positive electrode active material and a lithium transition metal composite oxide having a layered rock salt-type structure as the second positive electrode active material, the conductive additive may not be sufficiently dispersed and may become unevenly distributed in the positive electrode mixture slurry due to the fineness of the first positive electrode active material. In such cases, there may be areas with high internal resistance in the positive electrode mixture layer, which degrades the cycle characteristics of the non-aqueous electrolyte secondary battery. This phenomenon is more pronounced when the conductive additive is a fibrous carbon material with a small diameter, such as a single-walled carbon nanotube.

[0022] However, in the positive electrode for a non-aqueous electrolyte secondary battery according to the embodiment of this disclosure, the positive electrode mixture layer contains nitrile group-containing rubber. For reasons that are not entirely clear, nitrile group-containing rubber exhibits extremely high affinity with carbon materials. Therefore, when the positive electrode mixture slurry contains a carbon material as a conductive additive and also contains nitrile group-containing rubber, the conductive additive can be dispersed in the positive electrode mixture slurry while enhancing its affinity with the nitrile group-containing rubber. In other words, the dispersibility of the conductive additive, which is a carbon material, can be increased in the positive electrode mixture slurry. As a result, even when the positive electrode active material contains the finely milled first positive electrode active material as described above, the existence of high-resistance regions in the positive electrode mixture layer can be suppressed. Therefore, a decrease in the cycle characteristics of the non-aqueous electrolyte secondary battery can be suppressed.

[0023] The configuration of the positive electrode for a non-aqueous electrolyte secondary battery according to the embodiments of this disclosure will be described in more detail below.

[0024] The positive electrode for a non-aqueous electrolyte secondary battery includes a positive electrode core and a positive electrode mixture layer disposed at least on the surface of the positive electrode core. In an example of the method for forming the positive electrode mixture layer, first, a positive electrode mixture slurry is prepared by dispersing the constituent components of the positive electrode mixture in a dispersion medium. As the dispersion medium, for example, an organic solvent such as N-methyl-2-pyrrolidone can be used. Next, the prepared positive electrode mixture slurry is applied to the surface of the positive electrode core to form a coating film, and then the positive electrode mixture layer can be formed by drying this coating film. The dried coating film may be rolled as necessary. In the positive electrode for a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, the positive electrode mixture layer contains at least a positive electrode active material, a conductive auxiliary agent, and a nitrile group-containing rubber.

[0025] The positive electrode core preferably has a strip shape (long shape) in plan view. As the positive electrode core, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, punching sheet) is used. Examples of the material of the positive electrode core include metal materials such as Al, Al alloy, Ti, Ti alloy, and Fe alloy. The Fe alloy may be stainless steel. The thickness of the positive electrode core is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 20 μm, and still more preferably 10 to 20 μm.

[0026] (Positive electrode active material) In the positive electrode for a non-aqueous electrolyte secondary battery of the present disclosure, the positive electrode active material includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material is a lithium-containing transition metal phosphate having an olivine structure, and this lithium-containing transition metal phosphate contains at least one of Fe and Mn. The second positive electrode active material is a lithium transition metal composite oxide having a layered rock salt structure and contains at least one element selected from the group consisting of Co, Mn, and Al and Ni.

[0027] The first positive electrode active material has the general formula: LiFe １－ｘ Mn ｘ PO ４It is preferable that the lithium-containing transition metal phosphate is represented by the formula above. That is, it is preferable that it be lithium iron manganese phosphate (LMFP). In the above formula, 0.2 ≤ x ≤ 1. In the lithium-containing transition metal phosphate represented by the above general formula, the bond between the phosphorus atom (P) and the oxygen atom (O) is strong, so even if the inside of the non-aqueous electrolyte secondary battery becomes hot during charging and discharging, the bond between the phosphorus atom (P) and the oxygen atom (O) is not easily broken and oxygen (O ２ This makes it less likely for ) to occur. Therefore, by using a lithium-containing transition metal phosphate represented by the above general formula as the first positive electrode active material, the safety of non-aqueous electrolyte secondary batteries can be improved.

[0028] As mentioned above, the first positive electrode active material has lower conductivity than lithium transition metal composite oxides having a layered rock salt-type structure like the second positive electrode active material. Therefore, from the viewpoint of suppressing an increase in the internal resistance of the positive electrode for non-aqueous electrolyte secondary batteries, it may be used in a finely milled form. Accordingly, the average particle diameter of the first positive electrode active material is, for example, 1 μm or less. The lower limit of the average particle diameter of the first positive electrode active material may be, for example, 0.1 μm. The first positive electrode active material may also be secondary particles formed by the aggregation of multiple primary particles. The average particle diameter of the first positive electrode active material is usually measured as the average particle diameter of the secondary particles.

[0029] The average particle size of the first positive electrode active material is the cumulative 50% particle size (median) in the volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. For example, the Microtrac series MT3300 manufactured by Nikkiso Co., Ltd. is used as the laser diffraction / scattering particle size distribution analyzer.

[0030] The average particle diameter of the first positive electrode active material may be measured from a cross-section obtained by cutting the laminate of the positive electrode mixture layer and the positive electrode core in the thickness direction. The cross-section may be formed using a cross-section polisher (CP). In this case, the positive electrode mixture layer may be embedded with a thermosetting resin (such as epoxy resin). The average particle diameter from the cross-section can be measured using a scanning electron microscope (SEM) image of the cross-section. As the SEM image, an image taken so that 10 or more first positive electrode active materials are observed can be used. Then, the equivalent circle diameter of the cross-sections of 10 or more first positive electrode active materials is determined by image processing, and the average value of these is determined as the average particle diameter. Here, the equivalent circle diameter means the diameter of a circle having the same area as the area of ​​the cross-section of the first positive electrode active material (the area of ​​the positive electrode active material observed in the cross-section of the positive electrode mixture layer). Note that the average particle diameter of the first positive electrode active material obtained using a particle size distribution analyzer and the average particle diameter of the first positive electrode active material obtained from the cross-section are equivalent values.

[0031] The second positive electrode active material has the general formula: Li α Ni(1-x1-x2-x3-y)Co ｘ１ Mn ｘ２ Al ｘ３ M ｙ O （２＋β） It is preferable that the material is a lithium transition metal composite oxide represented by the formula above. In the above formula, 0.95 ≤ α ≤ 1.05, 0 < 1 - x1 - x2 - x3 - y < 1, 0 ≤ x1 ≤ 0.1, 0 ≤ x2 < 1, 0 ≤ x3 ≤ 0.1, 0 ≤ y ≤ 0.1, -0.05 ≤ β ≤ 0.05, and M is at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, and Y. Furthermore, α, which represents the atomic ratio of lithium, increases or decreases with charging and discharging.

[0032] The second positive electrode active material is a lithium transition metal composite oxide having a layered rock salt-type structure, and reversible insertion and removal of Li ions is possible between the layers of the layered rock salt-type structure. Furthermore, because the second positive electrode active material contains Ni as described above, Li ions are more easily extracted during charging, thereby increasing the capacity of the non-aqueous electrolyte secondary battery. Moreover, the higher the proportion of Ni in the second positive electrode active material, the easier it is to extract more Li ions during charging, thus further increasing the capacity of the non-aqueous electrolyte secondary battery. In addition, Co, Mn, and Al contribute to stabilizing the layered rock salt-type crystal structure in the second positive electrode active material. The contribution of Co, Mn, and Al to stabilizing the crystal structure becomes more pronounced as the proportion of Ni in the second positive electrode active material increases. However, from the viewpoint of reducing manufacturing costs, a low proportion of Co is preferable.

[0033] The second positive electrode active material may be secondary particles formed by the aggregation of multiple primary particles. The average particle diameter of the second positive electrode active material may be 3 μm or more, or 5 μm or more. The average particle diameter of the second positive electrode active material may be 30 μm or less, or 25 μm or less. The average particle diameter of the second positive electrode active material is usually measured as the average particle diameter of the secondary particles. The average particle diameter of the second positive electrode active material can be measured in the same manner as the first positive electrode active material.

[0034] The positive electrode active material may include other positive electrode active materials besides the first and second positive electrode active materials. Examples of other positive electrode active materials include lithium iron phosphate (LiFePO4). ４ (LFP), Lithium Cobalt Oxide (LiCoO) ２ ), and lithium manganese (LiMn ２ O ４ Examples include the following. Note that lithium manganate is a lithium transition metal composite oxide having a spinel-type structure. The proportion of the first and second positive electrode active materials in the positive electrode active material is, for example, 90% by mass or more, may be 95% by mass or 100% by mass.

[0035] In the positive electrode active material, when the proportion of the first positive electrode active material is R1 and the proportion of the second positive electrode active material is R2, R1 and R2 may satisfy the relationship R1:R2 = 1:9 to 9:1.

[0036] The proportions of each element in the first and second cathode active materials can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), or an energy dispersive X-ray spectrometer (EDX).

[0037] (Conductive additive) The conductive additive includes at least one selected from the group consisting of carbon black, carbon nanotubes, and porous carbon (PC). The carbon nanotubes include at least one of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). Examples of carbon black include acetylene black (AB) and Ketjenblack. It is preferable to use acetylene black as the carbon black. Examples of porous carbon include Knobel® manufactured by Toyo Tanso Co., Ltd., Triporous® manufactured by Sony Corporation, Maxsorb® manufactured by MC Evatec Co., Ltd., NORIT® or DARCO® manufactured by Cabot Corporation, and activated carbon. The conductive additive may be used alone or in combination of two or more. It is preferable that the conductive additive is contained in the positive electrode mixture layer in an amount of 0.01 parts by mass or more and 5 parts by mass or less per 100 parts by mass of positive electrode active material. By including a conductive additive in the positive electrode mixture layer within this range, the conductivity of the positive electrode mixture layer can be improved.

[0038] Porous carbon has pores, and when a non-aqueous electrolyte permeates these pores, ion conduction occurs efficiently in non-aqueous electrolyte secondary batteries. Furthermore, the greater the number of pores in the porous carbon, in other words, the larger the specific surface area of ​​the porous carbon, the more efficiently ion conduction occurs in non-aqueous electrolyte secondary batteries. On the other hand, as the specific surface area of ​​porous carbon increases, its conductivity decreases. Therefore, from the viewpoint of efficiently generating electron transfer and ion conduction while suppressing an excessive decrease in conductivity, porous carbon is 100 m ２ / g or more 4000m ２ It is preferable that the specific surface area is less than or equal to 1 / g. The specific surface area of ​​porous carbon can be determined by the BET method. The BET specific surface area is measured by the gas adsorption method. Nitrogen gas is used as the gas. Details of the BET method should be in accordance with JIS Z8830:2013. Specifically, the amount of nitrogen adsorbed onto porous carbon at liquid nitrogen temperature is measured. The porous carbon sample is filled into a sample tube of a measuring device (for example, the Shimadzu Corporation's automatic specific surface area / pore distribution analyzer "Tristar II 3020"), the sample tube is cooled to -196°C, the pressure is reduced, and then nitrogen (99.999% purity) is adsorbed onto the sample at the desired relative pressure, and the adsorption isotherm is measured. From the obtained adsorption isotherm, the BET specific surface area is determined by a multi-point method (for example, 3 points) in the relative pressure range of 0.05 to 0.1.

[0039] A single-walled carbon nanotube is a cylindrical carbon nanostructure composed of a single layer of graphene sheet, while a multi-walled carbon nanotube is a cylindrical carbon nanostructure composed of two or more layers of graphene sheet stacked concentrically. A graphene sheet refers to a layer in which the carbon atoms of sp2 hybrid orbitals constituting the graphite crystal are located at the vertices of a regular hexagon. Carbon nanotubes typically have a cylindrical shape. However, carbon nanotubes may also have a coil shape in which the cylindrical tube is wound in a spiral.

[0040] Among the various conductive additives mentioned above, it is preferable to use single-walled carbon nanotubes. In other words, it is preferable that the positive electrode mixture layer contains single-walled carbon nanotubes as a conductive additive. Since single-walled carbon nanotubes have high conductivity, the inclusion of single-walled carbon nanotubes as a conductive additive in the positive electrode mixture layer can significantly reduce the DC resistance (DCR) of a non-aqueous electrolyte secondary battery.

[0041] The positive electrode mixture layer may contain single-walled carbon nanotubes and carbon black as conductive additives. This effectively reduces electronic resistance by compounding the conductive pathways. The positive electrode mixture layer may also contain single-walled carbon nanotubes and multi-walled carbon nanotubes as conductive additives. Even in this case, electronic resistance can be effectively reduced by compounding the conductive pathways. The positive electrode mixture layer may also contain single-walled carbon nanotubes and porous carbon as conductive additives. Even in this case, electronic resistance can be effectively reduced by compounding the conductive pathways.

[0042] When the positive electrode mixture layer contains single-walled carbon nanotubes and carbon black as conductive additives, it is preferable that the single-walled carbon nanotubes and carbon black are included in a ratio of single-walled carbon nanotubes:carbon black = 3:7 to 7:3. Similarly, when the positive electrode mixture layer contains single-walled carbon nanotubes and multi-walled carbon nanotubes as conductive additives, it is preferable that the single-walled carbon nanotubes and multi-walled carbon nanotubes are included in the same ratio as described above. Similarly, when the positive electrode mixture layer contains single-walled carbon nanotubes and porous carbon as conductive additives, it is preferable that the single-walled carbon nanotubes and porous carbon are included in the same ratio as described above.

[0043] It is preferable that the single-walled carbon nanotubes have a length (average length) of 1 μm or more. The single-walled carbon nanotubes may also have a length of 2 μm or more. They may also have a length of less than 10 μm. By having the length of the single-walled carbon nanotubes within the above numerical range, sufficient contact with the positive electrode active material and the positive electrode core can be ensured. This ensures a sufficient conductive network in the positive electrode. Multiple single-walled carbon nanotubes may exist in a bundled state within the positive electrode mixture layer. In such cases, the length and diameter of the single-walled carbon nanotubes refer to the length and diameter of a single single-walled carbon nanotube (a single single-walled carbon nanotube) present within the bundled single-walled carbon nanotubes.

[0044] The average length of single-walled carbon nanotubes (WWs) is determined by image analysis using a scanning electron microscope (SEM). This is achieved by randomly selecting 100 WWs, measuring their lengths, and then taking the arithmetic mean. The length refers to the length of the WW when the nanotube is stretched in a straight line.

[0045] It is preferable that the single-walled carbon nanotubes have a diameter (average diameter) of 3 nm or less. The lower limit of the diameter of the single-walled carbon nanotubes may be 1 nm. In other words, it is preferable that the single-walled carbon nanotubes have a relatively small diameter. This allows the aspect ratio (length / diameter) of the single-walled carbon nanotubes to be made extremely large when the single-walled carbon nanotubes have a length of 1 μm or more as described above. Furthermore, since single-walled carbon nanotubes with a large aspect ratio are more likely to come into linear contact with the positive electrode active material and the positive electrode core, an even more sufficient conductive network can be secured at the positive electrode.

[0046] The average diameter of single-walled carbon nanotubes can be determined by image analysis using a transmission electron microscope (TEM). The average diameter of single-walled carbon nanotubes can be measured by the following method: First, 100 single-walled carbon nanotubes are arbitrarily selected, and the diameter (outer diameter) at any one point on each is measured. Then, the average diameter is obtained by taking the arithmetic mean of the measured diameters.

[0047] (Nitrile Group-Containing Rubber) Nitrile group-containing rubber contains nitrile groups. Nitrile group-containing rubber functions as a dispersant. In addition, nitrile group-containing rubber functions as a binder in the positive electrode mixture layer. Examples of nitrile group-containing rubber include copolymers of monomers containing acrylonitrile and diene (e.g., butadiene). Specifically, examples of nitrile group-containing rubber include nitrile rubber (NBR), hydrogenated nitrile rubber (HNBR), and modified versions thereof. The positive electrode mixture layer preferably contains hydrogenated nitrile rubber (HNBR) among the nitrile group-containing rubbers. The reason is not clear, but hydrogenated nitrile rubber (HNBR) shows particularly high affinity for conductive additives, which are carbon materials, among the nitrile group-containing rubbers. Therefore, by including hydrogenated nitrile rubber (HNBR) as the nitrile group-containing rubber in the positive electrode mixture layer, the dispersibility of the conductive additives, which are carbon materials, can be further enhanced.

[0048] The nitrile group-containing rubber may have a weight-average molecular weight in the range of 5,000 to 500,000. The weight-average molecular weight of the nitrile group-containing rubber may be 40,000 or more. A weight-average molecular weight of 40,000 or more makes it easier to achieve both improved dispersibility of various carbon materials used as conductive additives and a reduction in the viscosity of the positive electrode mixture slurry. The weight-average molecular weight of the nitrile group-containing rubber can be measured, for example, by ultra-high temperature gel permeation chromatography (GPC). Note that the weight-average molecular weight of the nitrile group-containing rubber is measured as a polystyrene equivalent.

[0049] The measurement conditions for ultra-high temperature GPC are shown below. An ultra-high temperature GPC SSC-7110 is used as the measuring apparatus. One TSKguardcolumnHHR(S)HT column and two TSKgelGMHHR-H(S)HT (7.8 mm I.D. × 30 cm) columns are used. A differential refractometer (RI detector) is used as the detector. As sample pretreatment, a nitrile group-containing rubber sample is weighed, a predetermined amount of 1-chloronaphthalene (1-CN) is added as the solvent, and it is heated and dissolved at 250°C for 1 hour. After that, heating filtration is performed using a PTFE filter with a pore size of 0.5 μm. 500 μL of a solution with a sample concentration of 2 mg / mL is injected as the sample injection volume. The flow rate is 1.0 mL / min, the measurement temperature is set to the column temperature of 210°C, and the pre-oven temperature is set to 250°C. Polystyrene is used as the molecular weight standard.

[0050] In the positive electrode for a non-aqueous electrolyte secondary battery of this disclosure, the positive electrode mixture layer preferably contains 5 to 500 parts by mass of nitrile group-containing rubber per 100 parts by mass of the conductive additive. When the conductive additive is a single-walled carbon nanotube or a multi-walled carbon nanotube, the positive electrode mixture layer preferably contains 25 to 200 parts by mass of nitrile group-containing rubber per 100 parts by mass of the single-walled carbon nanotube or multi-walled carbon nanotube. When the conductive additive is carbon black or a combination of single-walled carbon nanotube and carbon black, the positive electrode mixture layer preferably contains 5 to 200 parts by mass of nitrile group-containing rubber per 100 parts by mass of the carbon black or the combination of single-walled carbon nanotube and carbon black. When the conductive additive is a combination of single-walled carbon nanotube and multi-walled carbon nanotube, the positive electrode mixture layer preferably contains 10 to 200 parts by mass of nitrile group-containing rubber per 100 parts by mass of the total of the single-walled carbon nanotube and multi-walled carbon nanotube. When the conductive additive is a combination of single-walled carbon nanotubes and porous carbon, the positive electrode mixture layer preferably contains nitrile group-containing rubber in an amount of 50 parts by mass or more and 200 parts by mass or less per 100 parts by mass of the total of single-walled carbon nanotubes and porous carbon.

[0051] The positive electrode mixture layer may further contain a cellulose derivative such as cellulose as a binder other than nitrile group-containing rubber. The inclusion of a cellulose derivative in the positive electrode mixture slurry further improves the dispersibility of the conductive additive within the slurry. This further suppresses the existence of high-resistance regions in the positive electrode mixture layer, even when the positive electrode active material contains the finely milled first positive electrode active material as described above. Therefore, the deterioration of the cycle characteristics of the non-aqueous electrolyte secondary battery can be further suppressed. The positive electrode mixture layer may contain 1 to 100 parts by mass of the cellulose derivative per 100 parts by mass of the conductive additive.

[0052] (Binding Agent) In the positive electrode for a non-aqueous electrolyte secondary battery of this disclosure, the positive electrode mixture layer may contain a binding agent other than nitrile group-containing rubber. Examples of binding agents other than nitrile group-containing rubber include fluororesins (e.g., PVDF), polyolefin resins, polyamide resins, polyimide resins, and acrylic resins. In the binding agent, the mass ratio of nitrile group-containing rubber is preferably 70% by mass or more, may be 80% by mass or more, may be 90% by mass or more, or may be 100% by mass.

[0053] Examples of cellulose derivatives include alkylcellulose, hydroxyalkylcellulose, carboxyalkylcellulose, and salts thereof (such as alkali metal salts and ammonium salts). Examples of alkylcellulose include methylcellulose, ethylcellulose, and ethylmethylcellulose. Examples of hydroxyalkylcellulose include hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, and hydroxypropylmethylcellulose. Examples of carboxyalkylcellulose include carboxymethylcellulose and carboxyethylcellulose. Examples of alkali metals that form alkali metal salts include sodium and potassium. Among these, methylcellulose, ethylcellulose, or hydroxypropylmethylcellulose are preferred. The cellulose derivative may have a weight-average molecular weight in the range of 1,000 to 1,000,000. The weight-average molecular weight of the cellulose derivative may be in the range of 10,000 to 1,000,000, or in the range of 10,000 to 500,000. The weight-average molecular weight of cellulose derivatives can be measured, for example, by ultra-high temperature gel permeation chromatography (GPC), similar to the weight-average molecular weight of nitrile group-containing rubber.

[0054] [Non-aqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to an embodiment of this disclosure comprises a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. In the non-aqueous electrolyte secondary battery according to an embodiment of this disclosure, the positive electrode is a positive electrode for a non-aqueous electrolyte secondary battery according to an embodiment of this disclosure. In addition to the positive electrode, negative electrode, and separator, the non-aqueous electrolyte secondary battery according to an embodiment of this disclosure may also include a non-aqueous electrolyte and an outer casing. Examples of non-aqueous electrolyte secondary batteries include lithium-ion secondary batteries and lithium metal secondary batteries. The components other than the positive electrode will be described below.

[0055] (Negative electrode) The negative electrode typically includes a negative electrode mixture layer containing a negative electrode active material. The negative electrode may also include a negative electrode core and a negative electrode mixture layer disposed on at least the surface of the negative electrode core. When the non-aqueous electrolyte secondary battery is a lithium metal secondary battery, the negative electrode uses a negative electrode core on which lithium metal or a lithium alloy can be deposited.

[0056] The negative electrode mixture layer contains a negative electrode active material as an essential component. The negative electrode mixture layer may also contain optional components such as binders, thickeners, and conductive additives. Optional components may include those exemplified as components of the positive electrode.

[0057] The negative electrode mixture layer may be formed by dispersing the components of the negative electrode mixture layer in a liquid medium (dispersion medium), applying the resulting negative electrode mixture slurry to the surface of the negative electrode core to form a coating, and then drying this coating. The dried coating may be rolled as needed. As the dispersion medium, organic solvents such as N-methyl-2-pyrrolidone, as well as water, can be used.

[0058] (Negative electrode active material) The negative electrode active material is selected according to the type of non-aqueous electrolyte secondary battery. An example of a negative electrode active material is a material capable of intercalating and releasing lithium ions. Such materials include carbonaceous materials and Si-containing materials. Metallic lithium and lithium alloys may also be used as negative electrode active materials. The negative electrode may contain one type of negative electrode active material or two or more types of negative electrode active materials.

[0059] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). One type of carbonaceous material may be used alone, or two or more types may be used in combination. Graphite is preferred as the carbonaceous material due to its excellent charge-discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0060] Examples of Si-containing materials include elemental Si, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which a silicon phase is dispersed within a lithium-ion conductive phase (matrix phase). Examples of silicon oxides include SiO ｘ Particles are an example. x may satisfy the relationship 0.5 ≤ x < 2 or 0.8 ≤ x ≤ 1.6. As a lithium ion conducting phase, for example, SiO ２ At least one selected from the group consisting of phase, silicate phase, and carbon phase can be used.

[0061] The negative electrode core can be made of a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as mesh, net, or perforated sheet). Examples of materials for the negative electrode core include metallic materials such as Ni, Ni alloys, Cu, Cu alloys, and Fe alloys. Fe alloys may be stainless steel. The thickness of the negative electrode core is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0062] The negative electrode mixture layer may be formed on both main surfaces of the negative electrode core, or on only one main surface. If the negative electrode core is a porous conductive substrate as described above, the negative electrode mixture layer may be formed such that at least a portion of it is embedded in the pores of the porous conductive substrate.

[0063] (Non-aqueous electrolytes) Non-aqueous electrolytes (non-aqueous electrolyte solutions) contain a solvent (non-aqueous solvent) and a solute dissolved in the solvent. Examples of solutes include lithium salts. Non-aqueous electrolytes may also contain various additives.

[0064] Various known materials can be used as solvents. Examples of solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One solvent may be used alone, or two or more solvents may be used in combination.

[0065] Examples of lithium salts include lithium salts of chlorine-containing acids (e.g., LiClO ４ LiAlCl ４ , and LiB １０ Cl １０ (e.g., lithium salts of fluorine-containing acids (e.g., LiPF)) ６ LiPF ２ O ２ LiBF ４ LiSbF ６ LiAsF ６ LiCF ３ SO ３ , and LiCF ３ CO ２ (e.g., lithium salts of fluorine-containing acidimides (e.g., LiN(FSO)) ２ ) ２ ,LiN(CF ３ SO ２ ) ２ ,LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), and LiN(C ２ F ５ SO ２) ２ Examples include lithium halides (e.g., LiCl, LiBr, and LiI). Lithium salts may be used individually or in combination of two or more types.

[0066] In a non-aqueous electrolyte, the lithium ion concentration may be between 1 mol / L and 2 mol / L, or between 1 mol / L and 1.5 mol / L. By setting the lithium ion concentration within the above range, a non-aqueous electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained.

[0067] The non-aqueous electrolyte may contain various known additives. Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene. Cyclic carbonate esters such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC), which were exemplified as solvents, may also be used as additives.

[0068] (Separator) The separator is interposed between the positive electrode and the negative electrode. The separator preferably has high ion permeability and appropriate mechanical strength and insulating properties. Examples of separators include microporous thin films, woven fabrics, and nonwoven fabrics. Examples of separator materials include polyolefins (e.g., polypropylene, polyethylene, etc.) and other resins.

[0069] (Outer casing) The outer casing (battery case) houses the electrode group. A non-aqueous electrolyte may be housed in the outer casing. Various known materials can be used as the outer casing. The electrode group consists of a positive electrode, a negative electrode, and a separator interposed between the positive and negative electrodes. The configuration of the electrode group is not particularly limited. The electrode group may be wound or laminated. A wound electrode group is formed by winding a laminate of a positive electrode, a negative electrode, and a separator interposed between the positive and negative electrodes. The form of the non-aqueous electrolyte secondary battery is not particularly limited. The form of the non-aqueous electrolyte secondary battery may be cylindrical, prismatic, coin-shaped, button-shaped, or laminated.

[0070] Figure 1 is a schematic perspective view showing a portion of a non-aqueous electrolyte secondary battery 10 according to one embodiment of the present disclosure. As an example, Figure 1 shows a rectangular non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery 10 shown in Figure 1 includes a bottomed rectangular tubular battery case 4, and an electrode group 1 and a non-aqueous electrolyte (not shown) housed inside the battery case 4.

[0071] The electrode group includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed between them. The core of the negative electrode (negative electrode core) is electrically connected to a negative electrode terminal 6 provided on the sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. That is, the positive electrode is electrically connected to a battery case 4, which also serves as the positive electrode terminal. The peripheral edge of the sealing plate 5 is fitted to the open end of the battery case 4, and the fitting portion is laser-welded. An injection hole for a non-aqueous electrolyte is formed in the sealing plate 5. The injection hole is sealed by a seal 8 after the non-aqueous electrolyte is injected.

[0072] The positive electrode includes a positive electrode core and a positive electrode mixture layer disposed on at least the surface of the positive electrode core. The positive electrode used is a positive electrode for a non-aqueous electrolyte secondary battery according to the embodiment of this disclosure.

[0073] (Note) The following technologies are disclosed in accordance with the above description. (Technology 1) A positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode core body and a positive electrode mixture layer disposed on at least the surface of the positive electrode core body, wherein the positive electrode mixture layer comprises at least a positive electrode active material, a conductive additive, and a nitrile group-containing rubber, wherein the positive electrode active material comprises a first positive electrode active material and a second positive electrode active material, wherein the first positive electrode active material is a lithium-containing transition metal phosphate having an olivine structure and comprising at least one of Fe and Mn, wherein the second positive electrode active material is a lithium transition metal composite oxide having a layered rock salt type structure and comprising at least one element selected from the group consisting of Co, Mn, and Al, and Ni, wherein the conductive additive comprises at least one selected from the group consisting of carbon black, carbon nanotubes, and porous carbon, wherein the carbon nanotubes comprise at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes. (Technical 2) The first positive electrode active material has the general formula: LiFe １－ｘ Mn ｘ PO ４ The second positive electrode active material is a lithium-containing transition metal phosphate represented by the formula (wherein 0.2 ≤ x ≤ 1), and the general formula for the second positive electrode active material is Li α Ni(1-x1-x2-x3-y)Co ｘ１ Mn ｘ２ Al ｘ３ M ｙ O （２＋β）(Technology 3) A lithium transition metal composite oxide represented by the formula (wherein 0.95≦α≦1.05, 0<1-x1-x2-x3-y<1, 0≦x1≦0.1, 0≦x2<1, 0≦x3≦0.1, 0≦y≦0.1, -0.05≦β≦0.05, and M is at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, and Y), a positive electrode for a non-aqueous electrolyte secondary battery according to Technology 1. (Technology 3) The nitrile group-containing rubber is hydrogenated nitrile rubber (HNBR), a positive electrode for a non-aqueous electrolyte secondary battery according to Technology 1 or 2. (Technology 4) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 3, wherein the positive electrode mixture layer contains 5 to 500 parts by mass of the nitrile group-containing rubber per 100 parts by mass of the conductive additive. (Technology 5) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 4, wherein the positive electrode mixture layer contains the single-walled carbon nanotube as the conductive additive. (Technology 6) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 5, wherein the positive electrode mixture layer contains the single-walled carbon nanotube and the carbon black as the conductive additive. (Technology 7) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 5, wherein the positive electrode mixture layer contains the single-walled carbon nanotube and the multi-walled carbon nanotube as the conductive additive. (Technology 8) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 5, wherein the positive electrode mixture layer comprises the single-walled carbon nanotube and the porous carbon as the conductive additive. (Technology 9) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 8, wherein the single-walled carbon nanotube has a length of 1 μm or more and a diameter of 3 nm or less. (Technology 10) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 9, wherein the positive electrode mixture layer further comprises a cellulose derivative. (Technology 11) A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 10.

[0074] The present disclosure will be described below in detail based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0075] [Example 1] (1) Preparation of the positive electrode A positive electrode mixture was obtained by mixing a positive electrode active material, a conductive additive, hydrogenated nitrile rubber (HNBR), and polyvinylidene fluoride (PVDF). A positive electrode mixture slurry was obtained by adding an appropriate amount of N-methyl-2-pyrrolidone (dispersion medium) to this positive electrode mixture and stirring. As the positive electrode active material, lithium-containing transition metal phosphate (LiFe) having an olivine structure was used. ０．５ Mn ０．５ PO ４ . First positive electrode active material), and lithium transition metal composite oxide (LiNi) having a layered rock salt type structure ０．８８ Co ０．０７ Mn ０．０５ O ２ The second positive electrode active material was used. When the proportion of the first positive electrode active material was R1 and the proportion of the second positive electrode active material was R2, the ratio of R1 and R2 was set to R1:R2 = 1:1. Acetylene black (AB) was used as the conductive additive. The amount of acetylene black (AB) was 1 part by mass per 100 parts by mass of the positive electrode active material, the amount of hydrogenated nitrile rubber (HNBR) was 10 parts by mass per 100 parts by mass of acetylene black, and the amount of polyvinylidene fluoride (PVDF) was 1 part by mass per 100 parts by mass of the positive electrode active material.

[0076] Next, a cathode mixture slurry was applied to both main surfaces (both sides) of the aluminum foil (positive electrode core) to form a coating. After drying the coating, the laminate of the coating and the aluminum foil was rolled. This produced a positive electrode comprising aluminum foil and a cathode mixture layer arranged on both main surfaces of the aluminum foil.

[0077] (2) Fabrication of the negative electrode A mixture of graphite and silicon composite material was used as the negative electrode active material. In the negative electrode active material, the mass ratio of silicon composite material to graphite was set to silicon composite material:graphite = 5:95. The silicon composite material is a composite material in which a silicon phase is dispersed in the lithium ion conducting phase (matrix phase). The negative electrode active material, carboxymethylcellulose sodium (CMC-Na), styrene-butadiene rubber (SBR), and water were mixed in a predetermined mass ratio to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both main surfaces (both sides) of the copper foil (negative electrode core) to form a coating film. After drying the coating film, the laminate of the coating film and the copper foil was rolled. This produced a negative electrode comprising copper foil and negative electrode mixture layers arranged on both main surfaces of the copper foil.

[0078] (3) Preparation of non-aqueous electrolyte (non-aqueous electrolyte solution) Add LiPF to the non-aqueous solvent ６ A non-aqueous electrolyte (non-aqueous electrolyte solution) was prepared by adding (lithium salt). In the non-aqueous electrolyte, LiPF ６ The concentration was set to 1.0 mol / L. As the non-aqueous solvent, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC:EMC = 3:7 was used.

[0079] (4) Fabrication of a non-aqueous electrolyte secondary battery Leads were attached to the positive electrode and negative electrode fabricated as described above. Next, an electrode stack was obtained by interposing a separator between the positive electrode and the negative electrode, and then this electrode stack was wound in a spiral shape to fabricate an electrode group (wound electrode group). Next, this electrode group was placed inside an outer casing. An outer casing made of laminate film containing aluminum foil (barrier layer) was used. Next, the outer casing in which the electrode group was placed was vacuum dried at 105°C for 2 hours, then a non-aqueous electrolyte (non-aqueous electrolyte solution) was injected into the inside of the outer casing, and the opening of the outer casing was sealed. In this way, a non-aqueous electrolyte secondary battery according to Example 1 was fabricated.

[0080] (Example 2) In preparing the positive electrode, a non-aqueous electrolyte secondary battery according to Example 2 was prepared in the same manner as in Example 1, except that 0.5 parts by mass of multi-walled carbon nanotubes (MWCNTs) were added as a conductive additive to 100 parts by mass of the positive electrode active material, and the amount of hydrogenated nitrile rubber (HNBR) added was 50 parts by mass to 100 parts by mass of multi-walled carbon nanotubes (MWCNTs).

[0081] (Example 3) In preparing the positive electrode, a non-aqueous electrolyte secondary battery according to Example 3 was prepared in the same manner as in Example 1, except that in preparing the positive electrode, 0.1 parts by mass of single-walled carbon nanotubes (SWCNTs) were added as a conductive additive to 100 parts by mass of the positive electrode active material, R1 and R2 were set to R1:R2=1:9, and the amount of hydrogenated nitrile rubber (HNBR) was set to 100 parts by mass per 100 parts by mass of single-walled carbon nanotubes (SWCNTs).

[0082] (Example 4) A non-aqueous electrolyte secondary battery according to Example 4 was manufactured in the same manner as in Example 3, except that R1 and R2 were set to R1:R2=2:8 in the preparation of the positive electrode.

[0083] (Example 5) In preparing the positive electrode, 0.1 parts by mass of single-walled carbon nanotubes (SWCNTs) were added as a conductive additive to 100 parts by mass of the positive electrode active material, and the amount of hydrogenated nitrile rubber (HNBR) added was 50 parts by mass to 100 parts by mass of single-walled carbon nanotubes (SWCNTs). Otherwise, a non-aqueous electrolyte secondary battery according to Example 5 was prepared in the same manner as in Example 2.

[0084] (Example 6) In preparing the positive electrode, the amount of hydrogenated nitrile rubber (HNBR) added was 100 parts by mass per 100 parts by mass of single-walled carbon nanotubes (SWCNTs), otherwise the non-aqueous electrolyte secondary battery according to Example 6 was prepared in the same manner as in Example 5.

[0085] (Example 7) In preparing the positive electrode, the amount of hydrogenated nitrile rubber (HNBR) added was 500 parts by mass per 100 parts by mass of single-walled carbon nanotubes (SWCNTs), otherwise the non-aqueous electrolyte secondary battery according to Example 7 was prepared in the same manner as in Example 5.

[0086] (Example 8) A non-aqueous electrolyte secondary battery according to Example 8 was manufactured in the same manner as in Example 3, except that R1 and R2 were set to R1:R2=4:1 in the preparation of the positive electrode.

[0087] (Example 9) In preparing the positive electrode, a non-aqueous electrolyte secondary battery according to Example 9 was prepared in the same manner as in Example 1, except that in preparing the positive electrode, 1 part by mass of single-walled carbon nanotubes (SWCNTs) and acetylene black (AB) was added per 100 parts by mass of the positive electrode active material, and the amount of hydrogenated nitrile rubber (HNBR) added was 10 parts by mass per 100 parts by mass of the conductive additive (SWCNTs + AB). The SWCNTs and AB were blended in a mass ratio of SWCNTs:AB = 1:10.

[0088] (Example 10) In preparing the positive electrode, a non-aqueous electrolyte secondary battery according to Example 10 was prepared in the same manner as in Example 1, except that in preparing the positive electrode, 0.5 parts by mass of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) were added as conductive additives to 100 parts by mass of the positive electrode active material, and the amount of hydrogenated nitrile rubber (HNBR) added was 30 parts by mass to 100 parts by mass of the conductive additive (SWCNTs + MWCNTs). The SWCNTs and MWCNTs were blended in a mass ratio of SWCNTs:MWCNTs = 1:10.

[0089] (Example 11) In preparing the positive electrode, a non-aqueous electrolyte secondary battery according to Example 11 was prepared in the same manner as in Example 1, except that in preparing the positive electrode, 1 part by mass of single-walled carbon nanotubes (SWCNTs) and porous carbon (PC) was added as conductive additives to 100 parts by mass of the positive electrode active material, and the amount of hydrogenated nitrile rubber (HNBR) added was 100 parts by mass to 100 parts by mass of the conductive additive (SWCNTs + PC). The SWCNTs and PC were blended in a mass ratio of SWCNTs:PC = 1:10.

[0090] (Comparative Example 1) A non-aqueous electrolyte secondary battery according to Comparative Example 1 was prepared in the same manner as in Example 3, except that only the second positive electrode active material was used as the positive electrode active material in the preparation of the positive electrode.

[0091] (Comparative Example 2) A non-aqueous electrolyte secondary battery according to Comparative Example 2 was prepared in the same manner as in Example 3, except that only the first positive electrode active material was used as the positive electrode active material in the preparation of the positive electrode.

[0092] (Comparative Example 3) A non-aqueous electrolyte secondary battery according to Comparative Example 3 was prepared in the same manner as in Example 1, except that hydrogenated nitrile rubber (HNBR) was not used in the preparation of the positive electrode.

[0093] (Comparative Example 4) A non-aqueous electrolyte secondary battery according to Comparative Example 4 was prepared in the same manner as in Example 2, except that hydrogenated nitrile rubber (HNBR) was not used in the preparation of the positive electrode.

[0094] (Comparative Example 5) A non-aqueous electrolyte secondary battery according to Comparative Example 5 was prepared in the same manner as in Example 5, except that hydrogenated nitrile rubber (HNBR) was not used in the preparation of the positive electrode.

[0095] [Evaluation] (Initial Capacity) The non-aqueous electrolyte secondary batteries according to each example (Examples 1 to 11 and Comparative Examples 1 to 5) were left in an environment of 25°C, and constant current charging was performed with a current of 0.5 It until the voltage reached 4.2 V. Then, constant voltage charging was performed with a constant voltage of 4.2 V until the current reached 0.02 It. Next, constant current discharge was performed with a current of 1.0 It until the voltage reached 2.5 V. The discharge capacity during the first discharge performed in this manner was defined as the initial capacity C. ０ This was the request.

[0096] (Capacity Retention Rate) After the initial discharge, the non-aqueous secondary batteries for each example were left for 20 minutes, and then the charge-discharge cycle was repeated 300 times. In the charge-discharge cycle, (1) constant current charging was performed with a current of 1.0 It until the voltage reached 4.2 V, followed by constant voltage charging at a constant voltage of 4.2 V until the current reached 0.02 It, and (2) constant current discharge was performed with a current of 1.0 It until the voltage reached 2.5 V, and this was repeated.

[0097] After repeating the charge-discharge cycle 300 times, the discharge capacity at the 300th discharge is defined as discharge capacity C. ３００ It was measured as follows. The obtained initial capacity C ０ and discharge capacity C ３００Using the following formula (1), the capacity retention rate X (%) was calculated. Then, the capacity retention rate X was evaluated as the cycle retention rate. • Capacity retention rate X (%) = (C ３００ / C ０ ) × 100 ... (1)

[0098] The results of evaluating the cycle retention rate for each example of a non-aqueous electrolyte secondary battery are shown in Table 1 below. Table 1 also shows the mass ratio (R1:R2) of the first and second positive electrode active materials, the type of conductive additive, and the content of hydrogenated nitrile rubber (HNBR).

[0099]

[0100] Table 1 shows that the non-aqueous electrolyte secondary batteries in each example (Examples 1 to 11) have a cycle retention rate of 90% or more, while the non-aqueous electrolyte secondary batteries in each comparative example (Comparative Examples 1 to 5) have a cycle retention rate of less than 90%. This is thought to be because, in each example, the positive electrode slurry contains hydrogenated nitrile rubber (HNBR), which improves the dispersibility of the conductive additive, which is a carbon material, in the positive electrode slurry. More specifically, it is thought that in the positive electrode layer prepared by the positive electrode slurry described above, the conductive additive, which is a carbon material, is sufficiently dispersed, and the existence of high-resistance regions is suppressed.

[0101] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0102] The positive electrode for non-aqueous electrolyte secondary batteries relating to this disclosure can be used in applications where it is required to suppress the deterioration of the cycle characteristics of non-aqueous electrolyte secondary batteries.

[0103] 1: Electrode group, 2: Positive electrode lead, 3: Negative electrode lead, 4: Battery case, 5: Sealing plate, 6: Negative electrode terminal, 7: Gasket, 8: Sealing plug, 10: Non-aqueous electrolyte secondary battery

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode core body and a positive electrode mixture layer disposed on at least the surface of the positive electrode core body, wherein the positive electrode mixture layer comprises at least a positive electrode active material, a conductive additive, and a nitrile group-containing rubber, the positive electrode active material comprises a first positive electrode active material and a second positive electrode active material, the first positive electrode active material is a lithium-containing transition metal phosphate having an olivine structure and comprising at least one of Fe and Mn, the second positive electrode active material is a lithium transition metal composite oxide having a layered rock salt type structure and comprising at least one element selected from the group consisting of Co, Mn, and Al, and Ni, the conductive additive comprises at least one selected from the group consisting of carbon black, carbon nanotubes, and porous carbon, and the carbon nanotubes comprises at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes.

2. The first positive electrode active material is represented by the general formula: LiFe １－ｘ Mn ｘ PO ４ (where 0.2 ≦ x ≦ 1) and is a lithium-containing transition metal phosphate, and the second positive electrode active material is represented by the general formula: Li α Ni(1−x1−x2−x3−y)Co ｘ１ Mn ｘ２ Al ｘ３ M ｙ O （２＋β） (where 0.95 ≦ α ≦ 1.05, 0 < 1−x1−x2−x3−y < 1, 0 ≦ x1 ≦ 0.1, 0 ≦ x2 < 1, 0 ≦ x3 ≦ 0.1, 0 ≦ y ≦ 0.1, −0.05 ≦ β ≦ 0.05, and M is at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, and Y).) and is a lithium transition metal composite oxide. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1.

3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the nitrile group-containing rubber is hydrogenated nitrile rubber (HNBR).

4. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the positive electrode mixture layer contains 5 to 500 parts by mass of the nitrile group-containing rubber per 100 parts by mass of the conductive additive.

5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the positive electrode mixture layer contains the single-walled carbon nanotube as the conductive additive.

6. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the positive electrode mixture layer comprises the single-walled carbon nanotube and the carbon black as the conductive additive.

7. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the positive electrode mixture layer comprises the single-walled carbon nanotube and the multi-walled carbon nanotube as the conductive additive.

8. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the positive electrode mixture layer comprises the single-walled carbon nanotube and the porous carbon as the conductive additive.

9. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the single-walled carbon nanotube has a length of 1 μm or more and a diameter of 3 nm or less.

10. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the positive electrode mixture layer further comprises a cellulose derivative.

11. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2.

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