Positive electrode for secondary battery, and secondary battery

Abstract

This positive electrode for a secondary battery comprises a positive electrode current collector, and a positive electrode mixture layer provided on the surface thereof. The positive electrode mixture layer contains a positive electrode active material, a conductive auxiliary agent, and a binder. The conductive auxiliary agent contains carbon black. The positive electrode active material contains first and second metal composite oxides respectively having first and second particle size distributions. The volume-based median diameters D1, D2 of the first and second metal composite oxides satisfy D1 > D2, the first particle size distribution satisfies 6.5 µm ≤ D1 ≤ 16 µm, the second particle size distribution satisfies 1.5 µm ≤ D2 ≤ 6 µm, and the D1 / D2 ratio is 2.5 to 4.5. The ratio of the mass of the first metal composite oxide and the mass of the second metal composite oxide contained in the positive electrode mixture layer is 9:1 to 2:8, and the content of the conductive auxiliary agent in the positive electrode mixture layer is 0.1 to 1.1 mass%.

Description

Positive electrode for secondary batteries and secondary batteries Cross-reference of related applications

[0001] This disclosure claims priority rights to Japanese Patent Application No. 2024-229194, filed with the Japan Patent Office on 25 December 2024, and the entirety of the said patent application is incorporated herein by reference.

[0002] This disclosure relates to a positive electrode for a secondary battery and a secondary battery.

[0003] Patent Document 1 proposes a mixed positive electrode active material comprising a large-particle positive electrode active material having an average diameter of 10 μm or more, and a small-particle positive electrode active material having an average diameter of 5 μm or less, wherein the large-particle positive electrode active material and the small-particle positive electrode active material are each coated with different substances from lithium triborate and metal oxides.

[0004] Special table 2018-505508 publication

[0005] To improve the capacity of secondary batteries, it is effective to increase the mass of active material per unit area of ​​the current collector. However, various difficulties can arise when attempting to manufacture such electrodes using conventional processes. Electrodes for non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, are formed by applying a slurry containing an electrode mixture to the surface of a current collector, drying the slurry coating, and then rolling the dried coating. The larger the mass of active material per unit area of ​​the current collector, the greater the pressure required to roll the electrode mixture layer. Rolling the coating at high pressure can cause the current collector to stretch or the active material particles to crack, making it difficult to obtain electrodes that meet the desired conditions.

[0006] One aspect of the present disclosure comprises a long positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material, a conductive additive, and a binder, wherein the conductive additive comprises carbon black, and the positive electrode active material comprises a first metal composite oxide having a first particle size distribution and a second metal composite oxide having a second particle size distribution, wherein the volume-based median diameter D1 of the first metal composite oxide and the volume-based median diameter D2 of the second metal composite oxide satisfy D1 > D2, and the first The present invention relates to a positive electrode for a secondary battery, wherein the particle size distribution satisfies 6.5 μm ≤ D1 ≤ 16 μm, the second particle size distribution satisfies 1.5 μm ≤ D2 ≤ 6 μm, the ratio of the median diameter D1 to the median diameter D2 (D1 / D2) is 2.5 to 4.5, the ratio of the mass of the first metal composite oxide contained in the positive electrode mixture layer to the mass of the second metal composite oxide contained in the positive electrode mixture layer is 9:1 to 2:8, and the content of the conductive additive in the positive electrode mixture layer is 0.1% by mass or more and 1.1% by mass or less.

[0007] Another aspect of this disclosure relates to a secondary battery comprising the above-described positive electrode for a secondary battery, a separator, a negative electrode facing the positive electrode via the separator, and a non-aqueous electrolyte.

[0008] The positive electrode for secondary batteries according to this disclosure is easy to manufacture because the positive electrode mixture has a structure that is easily compressible. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings.

[0009] This is a schematic longitudinal section of a secondary battery according to one embodiment of the present disclosure.

[0010] The embodiments relating to this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following descriptions, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "greater than or equal to numerical value A and less than or equal to numerical value B". In the following descriptions, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either the given lower limit and either the given upper limit may be arbitrarily combined, as long as the lower limit does not exceed the upper limit. In the following descriptions, when examples of components or methods are listed, unless otherwise specified, only one of the listed examples may be used, or multiple of the listed examples may be used in combination.

[0011] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0012] The secondary battery according to this disclosure is typically a non-aqueous electrolyte secondary battery comprising a wound electrode body. The wound electrode body is constructed by winding a positive electrode and a negative electrode with a separator in between. The electrode body, together with the non-aqueous electrolyte, is housed in, for example, a bottomed cylindrical case. The opening of the bottomed cylindrical case is sealed with a sealing body.

[0013] Non-aqueous electrolyte secondary batteries include lithium-ion secondary batteries, lithium metal secondary batteries, and solid-state batteries containing gel electrolytes or solid electrolytes. In other words, non-aqueous electrolyte secondary batteries may be liquid-type secondary batteries containing an electrolyte, or all-solid-state secondary batteries containing a solid electrolyte.

[0014] (Positive electrode for secondary battery) The positive electrode for a secondary battery according to this embodiment (hereinafter also referred to as "positive electrode (P)") comprises a positive electrode current collector and a positive electrode mixture layer. The positive electrode mixture layer is provided on the surface of the positive electrode current collector. The positive electrode mixture layer is composed of a positive electrode mixture, which includes a positive electrode active material, a conductive additive, and a binder.

[0015] The positive electrode active material layer is formed by applying a positive electrode slurry in which a positive electrode active material is dispersed in a liquid component (dispersion medium) onto the surface of a positive electrode current collector, drying the coating film of the positive electrode slurry, and then rolling the coating film.

[0016] The liquid component (dispersion medium) is not particularly limited, and water, an organic solvent, or a mixed solvent thereof may be used. Examples of the organic solvent include protic solvents such as alcohol (e.g., ethanol), ethers (e.g., tetrahydrofuran), amides (e.g., dimethylformamide), and aprotic solvents such as N-methyl-2-pyrrolidone (NMP).

[0017] The positive electrode current collector is, for example, in the form of a long sheet, and a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punching sheet) is used. The thickness of the positive electrode current collector may be, for example, 3 μm or more and 20 μm or less. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, titanium, etc. The positive electrode active material layer may be formed on only one side of the positive electrode current collector, but it is preferable to form it on both sides of the positive electrode current collector from the viewpoint of increasing the capacity.

[0018] Carbon black is used as the conductive assistant. Although carbon nanotubes (hereinafter also referred to as "CNT") are known as another conductive assistant, when the positive electrode active material contains CNT, the viscosity of the positive electrode slurry tends to increase significantly, and the load during kneading the positive electrode slurry also tends to increase. In addition, CNT is more expensive than carbon black. Furthermore, CNT is not easily dispersed in the positive electrode slurry, and a process of pre-dispersing it in a predetermined resin component as a dispersant is required. Therefore, it is desired to construct an inexpensive and highly productive positive electrode manufacturing process by making good use of carbon black without using CNT.

[0019] However, carbon black has a structure in which carbon nanoparticles are linked together, resulting in a very large apparent volume relative to its mass, making it a bulky material. Compared to carbon nanotubes (CNTs), a larger amount of carbon black is required to effectively act as a conductive additive. When such bulky carbon black becomes heavily entangled on the surface of the positive electrode active material particles, the coating of the positive electrode mixture becomes hard and difficult to compress. If such a coating is compressed under high pressure, the positive electrode current collector may stretch or the particles of the positive electrode active material may crack, making it difficult to obtain a positive electrode that meets the desired conditions. Furthermore, in order to obtain a high-capacity positive electrode, the larger the mass of positive electrode mixture supported per unit area of ​​the positive electrode current collector, the greater the pressure required to roll the coating.

[0020] As described above, when using carbon black as a conductive additive, it is crucial to reduce the pressure applied when rolling the coating of the cathode mixture during the cathode manufacturing process.

[0021] The coating of the positive electrode mixture is typically rolled by passing it between a pair of nip rollers. Hereafter, the "pressure applied when rolling the coating of the positive electrode mixture" may be referred to as the "linear pressure" applied by the nip rollers.

[0022] To reduce linear pressure in the rolling process, it is effective to satisfy the following conditions:

[0023] (A) The positive electrode active material comprises a first metal composite oxide having a first particle size distribution and a second metal composite oxide having a second particle size distribution, wherein the volume-based median diameter D1 of the first metal composite oxide and the volume-based median diameter D2 of the second metal composite oxide satisfy D1 > D2.

[0024] (B) The first particle size distribution satisfies 6.5 μm ≤ D1 ≤ 16 μm, and the second particle size distribution satisfies 1.5 μm ≤ D2 ≤ 6 μm.

[0025] (C) The ratio of the median diameter D1 to the median diameter D2 (D1 / D2) satisfies 2.5 ≤ D1 / D2 ≤ 4.5.

[0026] (D) The ratio of the mass of the first metal composite oxide (W1) contained in the positive electrode mixture layer to the mass of the second metal composite oxide (W2) contained in the positive electrode mixture layer satisfies W1:W2 = 9:1 to 2:8 (i.e., 2 / 8 ≤ W1 / W2 ≤ 9 / 1).

[0027] (E) The content of the conductive additive in the positive electrode mixture layer is 0.1% by mass or more and 1.1% by mass or less.

[0028] Condition (A): The first metal composite oxide constitutes large active material particles, and the second metal composite oxide constitutes small active material particles. When large and small active material particles are mixed, provided that the other conditions (B) to (E) are also met, the packing efficiency of the positive electrode active material is expected to improve significantly. This is because, during rolling, the small active material particles fill the gaps between the large active material particles. As a result, the gaps in the positive electrode mixture layer are reduced, and the content of positive electrode active material in the positive electrode mixture layer is increased. This not only reduces the linear pressure in the rolling process but also increases the capacity of the positive electrode.

[0029] Condition (B) D1 may be 6.5 μm or larger, but may also be 7 μm or larger, 9 μm or larger, 11 μm or larger, or 13 μm or larger. D1 may be 16 μm or smaller, but preferably 15 μm or smaller, and may also be 14 μm or smaller. The first particle size distribution may satisfy 6.5 μm ≤ D1 ≤ 16 μm, but preferably 7 μm ≤ D1 ≤ 16 μm, may also satisfy 9 μm ≤ D1 ≤ 15.8 μm, or may satisfy 13 μm ≤ D1 ≤ 15 μm.

[0030] D2 may be 6 μm or less, but may also be 5.5 μm or less, or 5 μm or less. D2 may be 1.5 μm or more, but preferably 2 μm or more, may also be 2.5 μm or more, or may also be 3 μm or more. The second particle size distribution may satisfy 1.5 μm ≤ D2 ≤ 6 μm, but preferably 1.5 μm ≤ D2 ≤ 5.5 μm, and may also be 2 μm ≤ D2 ≤ 5 μm.

[0031] Condition (C) The D1 / D2 ratio must satisfy 2.5 ≤ D1 / D2 ≤ 4.5, but it may also satisfy 3.0 ≤ D1 / D2 ≤ 4.5, 2.5 ≤ D1 / D2 ≤ 4, or 3.0 ≤ D1 / D2 ≤ 3.5.

[0032] Condition (D) The W1 / W2 ratio should satisfy 2 / 8 ≤ W1 / W2 ≤ 9 / 1, but it is preferable that it satisfies 5 / 5 ≤ W1 / W2 ≤ 7 / 3, and it may also satisfy 5 / 5 ≤ W1 / W2 ≤ 6 / 4. Up to W1 / W2 = 1, the more small the particle size of the second metal composite oxide, the lower the pressure (linear pressure) when rolling the coating film of the positive electrode mixture tends to be.

[0033] Condition (E) The content of the conductive additive in the positive electrode mixture layer may be 0.1% by mass or more and 1.1% by mass or less, but from the viewpoint of improving the conductivity of the positive electrode, 0.2% by mass or more is preferable, 0.3% by mass or more is more preferable, and 0.4% by mass or more is even preferable. On the other hand, from the viewpoint of reducing the pressure (linear pressure) when rolling the coating film of the positive electrode mixture, the content of the conductive additive in the positive electrode mixture layer may be 1.0% by mass or less, 0.9% by mass or less, or 0.8% by mass or less. From the viewpoint of ensuring sufficient conductivity, a small amount of CNT may be used in combination with carbon black. The content of the conductive additive in the positive electrode mixture layer may be 0.1% by mass to 1.0% by mass, 0.3% by mass to 1.0% by mass, 0.5% by mass to 1.0% by mass, or 0.3% by mass to 0.8% by mass.

[0034] The binder content in the positive electrode mixture layer is preferably 0.1% by mass or more and 1.0% by mass or less. From the viewpoint of kneading load and peel strength during positive electrode slurry preparation, a higher binder content in the positive electrode mixture layer is preferable, and may be 0.15% by mass or more, or 0.2% by mass or more. On the other hand, from the viewpoint of ensuring a high positive electrode capacity, a lower binder content in the positive electrode mixture layer is preferable, and may be 0.8% by mass or less, 0.6% by mass or less, or 0.5% by mass or less. The binder content in the positive electrode mixture layer may be 0.1% by mass to 0.6% by mass, or 0.2% by mass to 0.5% by mass.

[0035] (Positive Electrode Active Material) The following will further explain the first metal composite oxide having a first particle size distribution (hereinafter also referred to as the "first particle group") and the second metal composite oxide having a second particle size distribution (hereinafter also referred to as the "second particle group") contained in the positive electrode active material.

[0036] The volume-based median diameter D1 of the first metal composite oxide and the volume-based median diameter D2 of the second metal composite oxide may be measured by separating the positive electrode active material from the positive electrode mixture layer, or by image analysis of a cross-sectional SEM image of the positive electrode mixture layer. Either method yields approximately the same (without significant difference) median diameter (D).

[0037] When separating the positive electrode active material from the positive electrode mixture layer, the positive electrode mixture layer may be peeled off the positive electrode (P), immersed in a suitable solvent to dissolve or swell components other than the active material particles, such as conductive additives and binders, and then separated by centrifugation, one or more times. When the separated sample of positive electrode active material is analyzed with a laser diffraction scattering particle size distribution analyzer, a volume-based particle size distribution can be obtained. The particle diameter of the peak with the largest area in the obtained particle size distribution is the median diameter of either the first particle group or the second particle group, and the particle diameter of the peak with the second largest area is the median diameter of the other. The larger median diameter is D1, and the smaller one is D2. If the peaks overlap, peak separation can be performed by image analysis. Furthermore, the volume V1 of the first particle group and the volume V2 of the second particle group can be calculated from the area of ​​each peak.

[0038] When performing image analysis of a cross-sectional SEM image of a positive electrode mixture layer, first, the positive electrode mixture layer and the positive electrode current collector are simultaneously cut along the width direction of the positive electrode to obtain a cross-sectional sample of the positive electrode in the thickness direction. At this time, the cross section may be processed with a cross-section polisher (CP) to obtain the cross-sectional sample. Next, the cross section of the positive electrode mixture layer in the cross-sectional sample is observed using a scanning electron microscope (SEM).

[0039] From the contour images of the active material particles in the SEM image, the area enclosed by the contour is determined. The diameter of a circle (equivalent circle) having the same area as the area enclosed by the contour of the active material particle is determined and taken as the particle size of each particle i. Then, the volume of a sphere having the same diameter as the equivalent circle is considered as the volume Vi of each particle i. By determining the diameter and volume of the equivalent circles for any 100 or more (preferably 1000 or more) particles, a volume-based particle size distribution can be obtained. From the obtained particle size distribution, the median diameters D1 and D2, and the volumes V1 of the first particle group and V2 of the second particle group can be calculated, similar to the case when separating the positive electrode active material from the positive electrode mixture layer.

[0040] The compositions of the first metal composite oxide and the second metal composite oxide are independent of each other and may be the same or different. Even with the same composition, the volume-based median diameter (D) can be arbitrarily controlled by the synthesis conditions of the positive electrode active material.

[0041] The particles of the positive electrode active material are usually aggregates of primary particles. The particle size of the primary particles is, for example, 0.2 μm to 1.2 μm. The second group of particles may also be primary particles, or for example, aggregates of 10 or fewer primary particles.

[0042] The compositions of the first metal composite oxide and the second metal composite oxide are, independently, given by the general formula: Li ｙ Ni ｘ M （１－ｘ） O ２ It may also be expressed as follows: However, the above general formula satisfies 0.8 ≤ x and 0 < y ≤ 1.2, and element M is at least one selected from the group consisting of Co, Mn, Al, Fe, Zr, Ti, Sr, Ca, and B.

[0043] The composite oxide having the composition represented by the above general formula has a high Ni content and is promising as a high-capacity positive electrode active material. It is more preferable that the above general formula satisfies 0.85 ≤ x.

[0044] Such a composite oxide containing Ni and Li (hereinafter, also referred to as "composite oxide (N)") is advantageous for increasing capacity and reducing cost. From the viewpoint of obtaining a high capacity, the proportion of Ni in the metal elements other than Li contained in the composite oxide (N) is preferably 80 atomic% or more, may be 85 atomic% or more, and may be 90 atomic% or more, as described above. The proportion of Ni in the metal elements other than Li is, for example, preferably 95 atomic% or less.

[0045] The composite oxide (N) can be a lithium transition metal composite oxide having a layered rock salt-type crystal structure. The composite oxide (N) may contain Co as the element M, or may contain at least one of Mn and Al. Co, Mn, and Al contribute to the stabilization of the crystal structure of the composite oxide (N) having a high Ni content.

[0046] Among them, the composite oxide (N) containing Co and Mn as the element M has a high hardness and is likely to have a high linear pressure in the rolling process. In other words, the composite oxide (N) containing Co and Mn as the element M is likely to exhibit the effect of reducing the linear pressure in the rolling process due to the above configuration.

[0047] However, from the viewpoint of reducing manufacturing costs, the element M preferably contains at least Mn, and the lower the Co content, the more desirable, and it may not contain Co. From the viewpoint of reducing manufacturing costs, in the composite oxide (N), it is desirable to keep the proportion of Co in the metal elements other than Li at less than 5 atomic%.

[0048] The composite oxide (N) is, for example, represented by the general formula: Li ｙ Ni1−x1−x2−x3−zCo ｘ１ Mn ｘ２ Al ｘ３ Me ｚ O ２＋βIt may also be expressed as follows: However, the general formula satisfies 0.95 ≤ y ≤ 1.05, 0 ≤ x1 ≤ 0.2, 0 ≤ x2 ≤ 0.5, 0 ≤ x3 ≤ 0.1, 0 ≤ z ≤ 0.1, 0.8 ≤ 1-x1-x2-x3-z and -0.05 ≤ β ≤ 0.05, where Me is an element other than Li, Ni, Mn, Al, Co and oxygen. x1, which represents the Co content, may satisfy 0.03 ≤ x1 ≤ 0.2 or 0.04 ≤ x1 ≤ 0.15. x2, which represents the Mn content, may satisfy 0.01 ≤ x2 ≤ 0.10 or 0.02 ≤ x2 ≤ 0.08.

[0049] As for Me, at least one selected from the group consisting of Nb, Zr, B, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Si, Ti, Fe, and Cr may be used, from the viewpoint of stabilizing the crystal structure of the composite oxide (N).

[0050] The elemental content of the active material particles 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).

[0051] The greater the mass of the positive electrode active material per unit area of ​​the positive electrode current collector, the greater the pressure required to roll the positive electrode mixture layer. In other words, the greater the mass of the positive electrode mixture layer per unit area of ​​the positive electrode current collector, the more pronounced the effect of reducing the linear pressure in the rolling process becomes with the above configuration. For example, the mass of the positive electrode mixture layer per unit area of ​​the positive electrode current collector is 240 g / m². ２ The above is also acceptable, 260 g / m ２ The above is also acceptable, 280 g / m ２ Anything above that is also acceptable; for example, 240 g / m² ２ 350g / m or more ２ It may fall within the following range.

[0052] The greater the thickness of the positive electrode mixture layer, the greater the pressure required to roll the positive electrode mixture layer. In other words, the greater the thickness of the positive electrode mixture layer, the more apparent the effect of the above configuration in reducing the linear pressure in the rolling process becomes. The thickness of the positive electrode mixture layer may be, for example, in the range of 50 μm to 250 μm, or it may be 100 μm or more and 200 μm or less.

[0053] The longer the positive electrode current collector is in the longitudinal direction, the greater the elongation of the positive electrode current collector during rolling of the positive electrode mixture layer. In other words, the longer the positive electrode current collector is in the longitudinal direction, the more pronounced the effect of the above configuration in suppressing the elongation of the positive electrode current collector during the rolling process becomes. Therefore, the length of the positive electrode current collector in the longitudinal direction may be, for example, 3000 mm or more. Furthermore, the longer the positive electrode current collector, the greater the linear pressure required for rolling of the positive electrode mixture layer tends to be. This is thought to be because the elongation of the positive electrode current collector makes rolling of the positive electrode mixture layer more difficult.

[0054] (Conductive additive) The conductive additive contains at least carbon black. Examples of carbon black include acetylene black, Ketjen black, and furnace black, and the type of carbon black is not particularly limited. However, among carbon blacks, acetylene black has a large specific surface area, excellent dispersibility, and is suitable for forming a conductive network with high mechanical strength. Therefore, acetylene black can impart particularly high conductivity to the positive electrode mixture layer.

[0055] The conductive additive may contain materials other than carbon black. For example, carbon black and CNTs may be used in combination as the conductive additive. However, in order to construct a cheaper and more productive cathode manufacturing process, it is preferable that 90% or more by mass of the conductive additive be carbon black, and more preferably that 99% or more by mass (more preferably 99.9% or more by mass) of the conductive additive be carbon black.

[0056] (Binder) It is preferable to use a vinylidene fluoride resin as the binder (binding agent). The vinylidene fluoride resin has a strong effect in bonding active material particles to each other or to the positive electrode current collector. The vinylidene fluoride resin contains vinylidene fluoride units as monomer units. The vinylidene fluoride resin may also be a copolymer of vinylidene fluoride and other monomers. Examples of vinylidene fluoride resins include polyvinylidene fluoride (PVDF) and polymers of vinylidene fluoride units and other monomer units (tetrafluoroethylene, hexafluoropropylene, etc.). The content of vinylidene fluoride units in the polymer is, for example, 30 mol% or more.

[0057] The weight-average molecular weight of the vinylidene fluoride resin may be 1 million or more, 1.1 million or more, 1.2 million or more, or 1.3 million or more. The weight-average molecular weight of the vinylidene fluoride resin may be 2 million or less, or 1.8 million or less. By setting the weight-average molecular weight to 1 million or more, a high binder effect can be obtained with a small amount.

[0058] The content, composition, and type of binders and conductive additives other than the positive electrode active material contained in the positive electrode mixture layer can be determined using a sample of the positive electrode mixture. A sample of the positive electrode mixture is obtained by the following procedure: First, a discharged secondary battery is disassembled and the positive electrode is removed. Next, the positive electrode is washed with an organic solvent, then vacuum dried, and then only the positive electrode mixture is removed. The sample of the positive electrode mixture can be analyzed using TG-DTA, NMR, pyrolysis GC-MS, micro-Raman spectroscopy, etc.

[0059] (Secondary Battery) A secondary battery comprises a positive electrode for secondary batteries as described above, a separator, a negative electrode facing the positive electrode via the separator, and an electrolyte. A secondary battery may also include an outer casing. Examples of secondary batteries include lithium-ion secondary batteries and lithium metal secondary batteries. The components other than the positive electrode mixture layer are not particularly limited, and known components may be used. Examples of components of a secondary battery other than the positive electrode are described below.

[0060] (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 current collector and a negative electrode mixture layer disposed on the negative electrode current collector. However, in the case of lithium metal secondary batteries, a negative electrode current collector capable of depositing lithium metal or lithium alloy is used for the negative electrode. The negative electrode of a lithium metal secondary battery does not necessarily have a negative electrode mixture layer.

[0061] The negative electrode mixture layer is composed of a negative electrode mixture, which contains a negative electrode active material as an essential component. The negative electrode mixture may also contain optional components such as binders, thickeners, and conductive additives. These optional components may be those exemplified as components of the positive electrode.

[0062] The negative electrode mixture layer may be formed by dispersing the components of the negative electrode mixture in a dispersion medium, applying the resulting negative electrode slurry to the surface of the negative electrode current collector, and drying it. The dried coating may be rolled if necessary. The dispersion medium may be one of the dispersion media exemplified for the positive electrode slurry. The negative electrode mixture layer may be formed on one side of the negative electrode current collector or on both sides of the negative electrode current collector.

[0063] (Negative Electrode Active Material) The negative electrode active material is selected according to the type of secondary battery. An example of a negative electrode active material is a material capable of intercalating and releasing lithium ions. Examples of such materials include carbonaceous materials and Si-containing materials. The negative electrode active material may contain Si-containing materials or may be a Si-containing material. Metallic lithium, lithium alloys, etc., may be used as the negative electrode active material. The negative electrode may contain one type of negative electrode active material or a combination of two or more types.

[0064] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). Carbonaceous materials may be used individually or in combination of two or more. Graphite is preferred because of its excellent charge-discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0065] Examples of silicon-containing materials include elemental silicon, 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). An example of silicon oxide is SiO ｘ Particles are included. x may be, for example, 0.5 ≤ x < 2, and 0.8 ≤ x ≤ 1.6. The lithium ion conducting phase is SiO ２ At least one selected from the group consisting of phase, silicate phase, and carbon phase may be used.

[0066] Metal foil may be used for the negative electrode current collector. The negative electrode current collector may also be porous. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0067] (Non-aqueous electrolytes) Non-aqueous electrolytes may be liquid electrolytes (electrolyte solutions), gel electrolytes, or solid electrolytes.

[0068] The gel-like electrolyte comprises a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, and polyethylene oxide.

[0069] The solid electrolyte may be an inorganic solid electrolyte. As an inorganic solid electrolyte, for example, materials known for use in all-solid-state lithium-ion secondary batteries, etc. (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.) can be used.

[0070] The liquid electrolyte (electrolyte solution) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The electrolyte salt contains at least a lithium salt. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0071] Any known material can be used as the non-aqueous solvent. Examples of non-aqueous 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). The non-aqueous solvent may be used alone or in combination of two or more types.

[0072] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO2). ４ LiAlCl ４ LiB １０ Cl １０ (e.g.), lithium salts of fluorine-containing acids (LiPF) ６ LiPF ２ O ２ LiBF ４ LiSbF ６ LiAsF ６ LiCF ３ SO ３ LiCF ３ CO ２ (etc.), lithium salts of fluorine-containing acidimides (LiN(FSO) ２ ) ２ ,LiN(CF ３ SO ２ ) ２ ,LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２These include lithium halides (LiCl, LiBr, LiI, etc.), etc. Lithium salts may be used individually or in combination of two or more types.

[0073] The concentration of lithium salt in the electrolyte may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By setting the lithium salt concentration within the above range, an electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained.

[0074] The electrolyte may contain known additives. Examples of additives include 1,3-propanesaltone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0075] (Separator) The separator is placed between the positive electrode and the negative electrode. The separator preferably has high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc., can be used. Examples of separator materials include polyolefins (polypropylene, polyethylene, etc.) and other resins.

[0076] (Outer casing) The outer casing (battery case) houses the electrode body and the non-aqueous electrolyte. The outer casing is not particularly limited, and known outer casings may be used. The electrode body consists of a positive electrode, a negative electrode, and a separator. The configuration of the electrode body is not particularly limited, and it may be a wound type or a laminated type. A wound type electrode body is formed by winding the positive electrode and negative electrode with a separator in between. A laminated type electrode body is formed by stacking the positive electrode and negative electrode with a separator in between. The form of the non-aqueous electrolyte secondary battery is not particularly limited, and may be cylindrical, prismatic, coin-shaped, button-shaped, laminated, etc.

[0077] Figure 1 is a longitudinal cross-sectional view of a cylindrical secondary battery 10 (hereinafter also simply referred to as "battery 10") according to the present disclosure. However, the present disclosure is not limited to the following configuration.

[0078] In Figure 1, the battery 10 comprises an electrode body 18, a non-aqueous electrolyte (not shown), and a bottomed cylindrical battery case (metal can) 22 that houses these components. A sealing body 11 is crimped and fixed to the opening of the battery case 22 via a gasket 21. This seals the inside of the battery 10. The sealing body 11 is an internal pressure-operated safety valve that cuts off the current when the internal pressure of the battery rises excessively and ruptures if necessary. Specifically, the sealing body comprises a valve body 12 having a thin-walled portion, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are electrically connected to each other at their respective centers. The positive electrode lead 15L, which is led out from the positive electrode 15, is connected to the metal plate 13. Therefore, the valve body 12 functions as both an external terminal for the positive electrode 15 and a safety valve. When the internal pressure of the battery rises, the connection between the valve body 12 and the metal plate 13 is broken, interrupting the current. Furthermore, when the thin-walled portion ruptures, gas is released to the outside, ensuring safety. The negative electrode lead 16L, which is led out from the negative electrode 16, is connected to the inner surface of the bottom of the battery case 22. An annular groove 22a is formed near the open end of the battery case 22. A first insulating plate 23 is placed between one end face of the electrode body 18 and the annular groove 22a. A second insulating plate 24 is placed between the other end face of the electrode body 18 and the bottom of the battery case 22. The electrode body 18 is formed by winding the positive electrode 15 and the negative electrode 16 in a cylindrical shape via a separator 17. The outermost circumference of the electrode body 18 is composed of the exposed portion 16D of the negative electrode current collector on the winding end side of the negative electrode 16.

[0079] (Note) The following technologies are disclosed by the above description. (Technology 1) A long positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material, a conductive additive, and a binder, wherein the conductive additive comprises carbon black, the positive electrode active material comprises a first metal composite oxide having a first particle size distribution and a second composite metal oxide having a second particle size distribution, the volume-based median diameter D1 of the first metal composite oxide and the volume-based median diameter D2 of the second metal composite oxide satisfy D1 > D2, the first particle size distribution satisfies 6.5 μm ≤ D1 ≤ 16 μm, the second particle size distribution satisfies 1.5 μm ≤ D2 ≤ 6 μm, and the ratio of the median diameter D1 to the median diameter D2 (D1 / D2) is 2.5 to 4.5. A positive electrode for a secondary battery, wherein the ratio of the mass of the first metal composite oxide contained in the positive electrode mixture layer to the mass of the second metal composite oxide contained in the positive electrode mixture layer is 9:1 to 2:8, and the content of the conductive additive in the positive electrode mixture layer is 0.1% by mass or more and 1.1% by mass or less. (Technical 2) The positive electrode for a secondary battery according to Technical 1, wherein the content of the binder in the positive electrode mixture layer is 0.1% by mass or more and 1.0% by mass or less. (Technical 3) The first metal composite oxide and the second metal composite oxide are each independent and have the general formula: Li ｙ Ni ｘ M （１－ｘ） O ２(Technology 4) A positive electrode for a secondary battery according to Technology 1 or 2, wherein the general formula satisfies 0.8 ≤ x and 0 < y ≤ 1.2, and M is at least one selected from the group consisting of Co, Mn, Al, Fe, Zr, Ti, Sr, Ca, and B. (Technology 5) A positive electrode for a secondary battery according to any one of Technology 1 to 4, wherein the carbon black contains acetylene black. (Technology 6) A positive electrode for a secondary battery according to any one of Technology 1 to 5, wherein the longitudinal length of the positive electrode current collector is 3000 mm or more. (Technology 7) A positive electrode for a secondary battery according to any one of Technology 1 to 6, wherein the thickness of the positive electrode mixture layer is 100 μm or more and 200 μm or less. (Technology 8) A secondary battery comprising a positive electrode for a secondary battery described in any one of Technologies 1 to 7, a separator, a negative electrode facing the positive electrode via the separator, and a non-aqueous electrolyte.

[0080] The present disclosure will be described in detail below based on examples, but the present disclosure is not limited to the following examples. In these examples, multiple non-aqueous electrolyte secondary batteries with different positive electrodes were fabricated and evaluated.

[0081] 《Battery A1》 Battery A1 was manufactured by the following method: (1) Preparation of the negative electrode A silicon composite material and graphite were mixed in a mass ratio of silicon composite material:graphite = 5:95 and used as the negative electrode active material. 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 slurry. Next, the negative electrode slurry was applied to the surface of a copper foil (negative electrode current collector) to form a laminate including the copper foil and the coating film formed on the copper foil. Next, after drying the coating film, the laminate was rolled. In this way, a negative electrode including a copper foil and a negative electrode mixture layer formed on both sides of the copper foil was formed.

[0082] (2) Preparation of the positive electrode A positive electrode mixture containing a positive electrode active material, a conductive additive (acetylene black (AB)), and a binder (polyvinylidene fluoride (PVDF)) in a predetermined mass ratio was dispersed in N-methyl-2-pyrrolidone (NMP (dispersion medium)) to prepare a positive electrode slurry.

[0083] The positive electrode active material has the composition formula LiNi ０．９０ Co ０．０５ Mn ０．０５ O ２ A first metal oxide (NCM1) represented by the formula LiNi ０．８５ Co ０．１０ Mn ０．０５ O ２ Particles of a second metal composite oxide (NCM2) represented by [formula] were used.

[0084] The particles of the first metal oxide were a first particle group with a median diameter D1 of 15 μm, and the particles of the second metal oxide were a second particle group with a median diameter D2 of 5 μm (D1 / D2 = 3). The first and second particle groups were mixed in a mass ratio of 9:1 (W1 / W2 = 9 / 1).

[0085] The acetylene black (AB) content in the positive electrode mixture layer was set to 1% by mass.

[0086] The content of polyvinylidene fluoride (PVDF) in the positive electrode mixture layer was set to 0.3% by mass. The weight-average molecular weight of polyvinylidene fluoride was set to 1.4 million.

[0087] A coating film was formed on both sides of a 4000 mm long sheet of aluminum foil (positive electrode current collector) by applying a positive electrode slurry. After the coating film was dried, the laminate of the aluminum foil and the dried coating film was rolled with a rolling roller at a predetermined linear pressure. In this way, a positive electrode was manufactured that included aluminum foil and positive electrode mixture layers with a thickness of 120 μm formed on both sides thereof.

[0088] (3) Preparation of electrolyte: LiPF in a non-aqueous solvent ６ The electrolyte was prepared by adding (lithium salt). LiPF in the electrolyte 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.

[0089] (4) Fabrication of the secondary battery Leads were attached to the positive and negative electrodes. Next, the electrode body was fabricated by spirally winding the positive electrode, negative electrode and separator so that a separator was placed between the positive and negative electrodes. Next, the electrode body was vacuum dried at 105°C for 2 hours, then placed in a bottomed cylindrical battery case which also served as the negative electrode terminal, and electrolyte was injected into the battery case. After that, the opening of the battery case was closed using a metal sealing body which also served as the positive electrode terminal. A gasket was interposed between the sealing body and the opening end of the battery case. The positive electrode lead was connected to the sealing body, and the negative electrode lead was connected to the inner bottom surface of the battery case. In this way, battery A1 of Example 1 was fabricated.

[0090] [Evaluation] <Mixing load of positive electrode slurry> When preparing a positive electrode slurry by dispersing the positive electrode mixture in NMP (dispersion medium), the load power (Wh) applied to the mixing device was measured using the installed power consumption meter.

[0091] <Viscosity Change of Positive Electrode Slurry> For the prepared positive electrode slurry, the viscosity μ0 on the day of preparation and the viscosity μ1 after 7 days were measured. Specifically, 100g of slurry was placed in a measuring cup, and the viscosity was measured by rotating the spindle of a rotational viscometer at 20 rpm. The viscosity increase rate was then calculated using the following formula: A lower viscosity increase rate indicates a better positive electrode slurry. "Viscosity Increase Rate (%) = 100 × μ1 / μ0" <Linear Pressure> The linear pressure when rolling the laminate of aluminum foil and dry coating film with a rolling mill roller was measured using the installed pressure sensor.

[0092] <Battery Capacity> The manufactured batteries were subjected to constant current charging at a constant current of 0.2 It until the voltage reached 4.2V under a temperature environment of 25°C. Then, constant voltage charging was performed until the current value was reduced to 1 / 50 It at 4.2V. After that, constant current discharge was performed at a constant current of 0.2 It until the voltage reached 2.5V. This process was repeated twice to confirm the battery capacity.

[0093] The results are shown in section 1. Note that the following battery evaluation results are relative values, with the result for battery A1 set to 100.

[0094] For batteries A2-A7 and B1-10, as shown in Table 1, D1, D2, the D1 / D2 ratio, the W1 / W2 ratio, and the content of acetylene black (AB) and polyvinylidene fluoride (PVDF) in the positive electrode mixture layer were changed.

[0095] Furthermore, as shown in Table 1, the length of the aluminum foil (positive electrode current collector) was changed to 4000 mm and the thickness of the positive electrode mixture layer was also changed.

[0096] Aside from the changes described above, batteries A2 to A7 of the examples and batteries B1 to B10 of the comparative examples were manufactured in the same manner as battery A1 of Example 1, and the same evaluations were performed for each example. The results are shown in Table 1.

[0097] Battery B11 was prepared in the same manner as Battery A1 in Example 1, except that CNTs with an average length of 1 μm and an average diameter of 10 nm were used as a conductive additive instead of acetylene black, and was evaluated in the same manner. The results are shown in Table 1. The CNT content in the positive electrode mixture layer was also set to 1% by mass. Table 1 summarizes the general structure of each positive electrode.

[0098]

[0099] Table 1 shows that when D1, D2, the D1 / D2 ratio, and the W1 / W2 ratio are within the appropriate range, the linear pressure in the rolling process can be kept low. Furthermore, the viscosity change of the positive electrode slurry is small, and the kneading load when preparing the positive electrode slurry is also kept low. It can also be seen that a W1 / W2 ratio within the range of 9 / 1 to 5 / 5 is preferable.

[0100] On the other hand, in comparative examples B1 to B6, which used only the first particle group or only the second particle group, the linear pressure was high, and many cracks occurred in the active material particles during the rolling process. Furthermore, when the length of the positive electrode current collector was 3000 mm or more, significant elongation occurred in the positive electrode current collector, making it difficult to obtain a positive electrode as designed. There was a tendency for the linear pressure required for rolling to increase as the length of the positive electrode current collector increased. This was thought to be because the elongation of the positive electrode current collector made rolling of the positive electrode mixture more difficult. In the example where the length of the positive electrode current collector was 1000 mm, the elongation problem that made it difficult to manufacture a positive electrode as designed did not occur.

[0101] Even when using both the first and second particle groups, if the D1 / D2 ratio was not appropriate, the linear pressure increased, and the mixing load also increased significantly.

[0102] When the acetylene black content in the cathode mixture is very low, it becomes necessary to reduce the amount of binder accordingly, which significantly increases the mixing load during cathode slurry preparation.

[0103] When CNTs were used instead of cetylene black, the linear pressure was appropriate, but the viscosity increase rate of the positive electrode slurry was very large, and the mixing load during the preparation of the positive electrode slurry became very large.

[0104] This disclosure can be applied to high-capacity, high-performance cathodes for secondary batteries. The secondary batteries according to this disclosure are useful as main power sources for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0105] 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.

[0106] 10 Secondary battery 11 Sealing body 12 Valve body 13 Metal plate 14 Insulating material 15 Positive electrode 15L Positive electrode lead 16 Negative electrode 16L Negative electrode lead 17 Separator

Claims

1. The device comprises a long positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material, a conductive additive, and a binder, the conductive additive comprises carbon black, the positive electrode active material comprises a first metal composite oxide having a first particle size distribution and a second metal composite oxide having a second particle size distribution, the volume-based median diameter D1 of the first metal composite oxide and the volume-based median diameter D2 of the second metal composite oxide satisfy D1 > D2, the first particle size distribution satisfies 6.5 μm ≤ D1 ≤ 16 μm, the second particle size distribution satisfies 1.5 μm ≤ D2 ≤ 6 μm, and the ratio of the median diameter D1 to the median diameter D2 (D1 / D2) is 2.5 to 4.

5. A positive electrode for a secondary battery, wherein the ratio of the mass of the first metal composite oxide contained in the positive electrode mixture layer to the mass of the second metal composite oxide contained in the positive electrode mixture layer is 9:1 to 2:8, and the content of the conductive additive in the positive electrode mixture layer is 0.1% by mass or more and 1.1% by mass or less.

2. The positive electrode for a secondary battery according to claim 1, wherein the content of the binder in the positive electrode mixture layer is 0.1% by mass or more and 1.0% by mass or less.

3. The first metal composite oxide and the second metal composite oxide are independent of each other and have the general formula: Li ｙ Ni ｘ M （１－ｘ） O ２ The positive electrode for a secondary battery according to claim 1, wherein the general formula satisfies 0.8 ≤ x and 0 < y ≤ 1.2, and M is at least one selected from the group consisting of Co, Mn, Al, Fe, Zr, Ti, Sr, Ca, and B.

4. The positive electrode for a secondary battery according to claim 3, wherein M comprises Co and Mn.

5. The positive electrode for a secondary battery according to claim 1, wherein the carbon black includes acetylene black.

6. The positive electrode for a secondary battery according to claim 1, wherein the longitudinal length of the positive electrode current collector is 3,000 mm or more.

7. The positive electrode for a secondary battery according to claim 1, wherein the thickness of the positive electrode mixture layer is 100 μm or more and 200 μm or less.

8. A secondary battery comprising a positive electrode for a secondary battery as described in claim 1, a separator, a negative electrode facing the positive electrode via the separator, and a non-aqueous electrolyte.

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