WO2024095686A1

WO2024095686A1 - Positive electrode for secondary battery, and secondary battery

Info

Publication number: WO2024095686A1
Application number: PCT/JP2023/036330
Authority: WO
Inventors: 昂輝守田; 林太郎名取; 大河深堀
Original assignee: パナソニックエナジー株式会社
Priority date: 2022-10-31
Filing date: 2023-10-05
Publication date: 2024-05-10

Abstract

A positive electrode (11), which is an example of an embodiment, comprises a positive electrode core (30) and a positive electrode mixture layer (31) that is disposed on the positive electrode core (30). The positive electrode mixture layer (31) includes a first region (35) and a second region (36) that are alternately disposed in at least one direction from among the length direction and the width direction of the positive electrode core (30). The porosity in the second region (36) is greater than the porosity in the first region (35).

Description

Positive electrode for secondary battery and secondary battery

This disclosure relates to a positive electrode for a secondary battery and a secondary battery using the positive electrode.

In recent years, secondary batteries such as lithium-ion batteries have been widely used in applications requiring high capacity, high durability, and rapid charging performance, such as in-vehicle applications and power storage applications. The positive electrode, which is a major component of secondary batteries, has a significant effect on these performances, and so much research has been done on the positive electrode. For example, Patent Document 1 discloses a positive electrode in which the physical properties of the positive electrode mixture layer are different in the thickness direction, and there is more active material with a large specific surface area on the core side than on the surface side of the mixture layer.

Patent No. 5929183

Patent Document 1 describes the effect of improving discharge capacity and cycle characteristics. However, it is believed that if the specific surface area on the surface side of the mixture layer is reduced, the electrolyte will not penetrate sufficiently into the core side, resulting in a decrease in rapid charging performance. In particular, if the mixture layer is formed to be dense in order to improve energy density, the decrease in rapid charging performance becomes more noticeable. The purpose of this disclosure is to provide a positive electrode that can realize a secondary battery with high capacity and excellent rapid charging performance.

The positive electrode for a secondary battery according to the present disclosure comprises a positive electrode core and a positive electrode mixture layer disposed on the positive electrode core, the positive electrode mixture layer including first and second regions alternately disposed in at least one of the length and width directions of the positive electrode core, and the porosity of the second region is greater than the porosity of the first region.

The secondary battery disclosed herein comprises the above-mentioned positive electrode, a negative electrode, and an electrolyte.

The positive electrode disclosed herein can provide a secondary battery with high capacity and excellent rapid charging performance.

1 is a cross-sectional view of a secondary battery according to an embodiment; FIG. 2 is a front view of a positive electrode according to an embodiment of the present invention. 3 is a cross-sectional view taken along line AA in FIG. 2. FIG. 13 is a diagram showing a modified example of a positive electrode. FIG. 13 is a diagram showing a modified example of a positive electrode.

As a result of extensive research into the above-mentioned problems, the inventors have succeeded in realizing a secondary battery that has high capacity and excellent rapid charging performance by providing first and second regions with different porosities in the positive electrode mixture layer and arranging the first and second regions alternately in at least one of the length and width directions of the positive electrode core. Due to the function of the second region with a large porosity, the permeability of the electrolyte throughout the entire mixture layer is greatly improved, and the electrolyte also quickly permeates in the thickness direction of the mixture layer. As a result, excellent rapid charging performance is obtained.

On the other hand, the first region of the mixture layer, which has a small porosity, contributes to high capacity. The first region is also supplied with electrolyte from the adjacent second region. A secondary battery using the positive electrode according to the present disclosure can achieve both high capacity and excellent rapid charging performance to a high degree. In addition, because the electrolyte is smoothly supplied to the entire mixture layer, a uniform battery reaction occurs over a wide area of the mixture layer, and cycle characteristics are also improved.

Below, with reference to the drawings, a detailed description will be given of an example of an embodiment of a positive electrode for a secondary battery according to the present disclosure, and a secondary battery using the positive electrode. Note that configurations that selectively combine the components of the multiple embodiments and modified examples described below are included within the scope of the present disclosure.

In the embodiment described below, a cylindrical battery 10 in which a wound electrode body 14 is housed in a cylindrical exterior can 16 with a bottom is exemplified as a secondary battery, but the exterior body of the battery is not limited to a cylindrical exterior can. Other embodiments of the secondary battery according to the present disclosure include a prismatic battery with a prismatic exterior can, a coin battery with a coin-shaped exterior can, and a pouch-type battery with an exterior body composed of a laminate sheet including a metal layer and a resin layer. In addition, the electrode body is not limited to a wound type, and may be a laminated type electrode body in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators between them. In addition, the electrolyte may be an aqueous electrolyte, but a nonaqueous electrolyte is used in this embodiment.

1 is a schematic diagram showing an axial cross section of a cylindrical battery 10 according to an embodiment of the present invention. As shown in FIG. 1, the cylindrical battery 10 includes a wound electrode body 14, a non-aqueous electrolyte, and an outer can 16 that contains the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container with a bottom that is open at one axial end, and the opening of the outer can 16 is closed by a sealing body 17. In the following description, for convenience of explanation, the sealing body 17 side of the battery is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. For example, esters, ethers, nitriles, amides, and mixed solvents of two or more of these are used as the non-aqueous solvent. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted product (e.g., fluoroethylene carbonate, etc.) in which at least a part of the hydrogen of these solvents is replaced with a halogen atom such as fluorine. For example, a lithium salt such as _LiPF6 is used as the electrolyte salt.

The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode body 14 are all long, strip-like bodies that are wound in a spiral shape and stacked alternately in the radial direction of the electrode body 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium precipitation. That is, the negative electrode 12 is formed to be longer in the length direction and width direction than the positive electrode 11. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

Insulating plates

18, 19 are arranged above and below the electrode body 14. In the example shown in FIG. 1, the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom inner surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior can 16 has a grooved portion 22 formed with a portion of the side surface that protrudes inward to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior can 16, and supports the sealing body 17 on its upper surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 that is crimped against the sealing body 17.

The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks so as to push the upper valve body 26 toward the cap 27, thereby cutting off the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure increases further, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode body 14 will be described in detail below, with particular reference to the positive electrode 11.

[Positive electrode]
The positive electrode 11 has a positive electrode core 30 and a positive electrode mixture layer 31 arranged on the positive electrode core 30. For the positive electrode core 30, a foil of a metal stable in the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film having the metal arranged on the surface, can be used. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core 30 except for the portion to which the positive electrode lead 20 is connected. A protective layer containing inorganic particles and a binder may be arranged between the positive electrode core 30 and the positive electrode mixture layer 31, or on the positive electrode mixture layer 31.

The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core 30, drying the coating, and then compressing it to form a positive electrode mixture layer 31 on both sides of the positive electrode core 30. For example, N-methyl-2-pyrrolidone (NMP) is used as the dispersion medium for the positive electrode mixture slurry. The positive electrode mixture slurry may be prepared by adding a positive electrode active material to a conductive agent paste containing a conductive agent, a binder, and a dispersion medium. The positive electrode mixture slurry and the conductive agent paste may contain a dispersant.

Although details will be described later, the positive electrode mixture layer 31 includes first and second regions, which have different physical properties and are arranged alternately in at least one of the length direction and width direction of the positive electrode core 30. The positive electrode mixture layer 31 is formed using at least two types of positive electrode mixture slurries.

A lithium metal composite oxide is used as the positive electrode active material. Metal elements contained in the lithium metal composite oxide include Li, Ni, Co, Mn, Al, Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, Bi, etc. A suitable lithium metal composite oxide contains Li, Ni, and Co, and also contains at least one of Mn and Al.

The lithium metal composite oxide has, for example, a layered rock salt structure. Examples of layered rock salt structures include layered rock salt structures belonging to space group R-3m and layered rock salt structures belonging to space group C2/m. Among these, from the viewpoints of high capacity and stability of the crystal structure, layered rock salt structures belonging to space group R-3m are preferred. The content of elements in the composite oxide can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), or an energy dispersive X-ray analyzer (EDX).

The lithium metal composite oxide preferably has a Ni ratio of 50 mol% or more, more preferably 80 mol% or more, relative to the total number of moles of metal elements excluding Li. By making the Ni content 50 mol% or more, a high-capacity battery can be obtained, and the effect of adopting the configuration of the present disclosure becomes more pronounced. The Ni content may be 85 mol% or more, or may be 90 mol% or more, relative to the total number of moles of metal elements excluding Li. The upper limit of the Ni content is, for example, 95 mol%.

When the lithium metal composite oxide contains Co, the Co content is preferably 1 mol% to 25 mol% relative to the total number of moles of metal elements excluding Li, and more preferably 2 mol% to 7 mol%. In this case, high capacity and high durability can be achieved while keeping material costs down. When the lithium metal composite oxide contains Mn, the Mn content is, for example, mol% to 20 mol% relative to the total number of moles of metal elements excluding Li. In this case, it becomes easier to achieve both high capacity and high durability. When the lithium metal composite oxide contains Al, the Al content is, for example, 0.1 mol% to 7 mol% relative to the total number of moles of metal elements excluding Li.

FIG. 2 is a front view of the positive electrode 11, and shows a schematic diagram of the positive electrode 11 in an expanded state. FIG. 3 is a cross-sectional view taken along line AA in FIG. 2. As shown in FIGS. 2 and 3, the positive electrode mixture layer 31 includes first regions 35 and second regions 36 arranged alternately in the longitudinal direction of the positive electrode core 30 (positive electrode 11). The first regions 35 and second regions 36 have different physical properties, and in this embodiment, are arranged alternately along the longitudinal direction of the positive electrode core 30, and are formed in a striped pattern when viewed from the front of the positive electrode mixture layer 31 (positive electrode 11).

In the positive electrode 11, the porosity of the second region 36 is greater than the porosity of the first region 35. As a result of the inventors' investigations, it was found that by providing the positive electrode mixture layer 31 with the first region 35 and the second region 36, which have different porosities, and arranging these regions alternately in the longitudinal direction of the positive electrode core 30, a cylindrical battery 10 can be realized that has high capacity and excellent rapid charging performance. Note that the first region 35 and the second region 36 may differ from each other in physical properties other than porosity, as long as this does not impair the objective of the present disclosure. Furthermore, the positive electrode mixture layer may have three or more regions with different porosities.

The positive electrode 11 achieves excellent rapid charging performance because the second region 36, which has a large porosity, greatly improves the permeability of the electrolyte throughout the entire mixture layer. Furthermore, the first region 35, which has a small porosity, contributes to high capacity. The first region 35 is also supplied with electrolyte from the adjacent second region 36, so the positive electrode 11 can achieve both high capacity and excellent rapid charging performance to a high degree. The second region 36 functions as a supply path for the electrolyte, and the electrolyte is smoothly supplied to the entire positive electrode mixture layer 31, so that a uniform battery reaction occurs over a wide area of the positive electrode mixture layer 31, and cycle characteristics are also improved.

The BET specific surface area of the first region 35 and the second region 36 may be substantially the same, for example, but preferably the BET specific surface area of the second region 36 is larger than the BET specific surface area of the first region 35. The BET specific surface area of each region is measured by the BET method using nitrogen gas and a mixture layer (each region) peeled off from the positive electrode core 30 as a sample, and can be measured by a commercially available measuring device such as Macsorb's HM model-1201.

The BET specific surface area of the second region 36 is preferably 1.1 to 3.0 times, and more preferably 1.2 to 2.0 times, the BET specific surface area of the first region 35. For example, the BET specific surface area of the first region ³⁵ is 1.5 to 2.5 ^m2 /g, and the BET specific surface area of the second region 36 is ^2.0 to ^3.0 m2/g.

As described above, the first regions 35 and the second regions 36 are alternately arranged in the longitudinal direction of the positive electrode core 30 and are formed in a striped pattern. There are no particular limitations on the shape of the first regions 35 and the second regions 36 when viewed from the front, but in the example shown in FIG. 2, they are formed in a rectangular shape when viewed from the front. The shape and size of each first region 35 do not have to be the same, but it is preferable that they are substantially the same from the standpoint of uniformity of the electrode reaction (the same applies to the second regions 36). Note that if only a small portion of the regions have different shapes and sizes, the same effect is achieved as when the regions have uniform shapes and sizes.

The first region 35 and the second region 36 can be formed, for example, by using different positive electrode mixture slurries. After the first positive electrode mixture slurry that forms the first region 35 is intermittently applied to the surface of the positive electrode core 30 along the length direction of the positive electrode core 30, the second positive electrode mixture slurry that forms the second region 36 is intermittently applied to the portion where the first positive electrode mixture slurry is not applied, thereby forming the stripes illustrated in FIG. 2. It is also possible to simultaneously apply the first and second positive electrode mixture slurries to different locations on the surface of the positive electrode core 30 to form stripes. For example, a positive electrode active material that tends to have a small porosity is added to the first positive electrode mixture slurry, and a positive electrode active material that tends to have a large porosity is added to the second positive electrode mixture slurry.

The physical properties such as porosity are preferably substantially the same in the multiple first regions 35. By forming each first region 35 using the same first positive electrode mixture slurry, the porosity of each first region 35 is substantially the same. Similarly, the physical properties such as porosity are preferably substantially the same in the multiple second regions 36. Furthermore, the porosity is substantially the same in the thickness direction of the first region 35, and for example, the porosity is substantially the same in the vicinity of the positive electrode core 30 of the first region 35 and in the vicinity of the surface of the first region 35 away from the positive electrode core 30 (the same applies to the second region 36). The thicknesses of the first region 35 and the second region 36 are preferably approximately the same.

The length of the first region 35 along the length direction of the positive electrode core 30 may be equal to or less than the length of the second region 36 along the length direction of the positive electrode core 30, but is preferably longer than the length of the second region 36. In this case, high capacity and excellent rapid charging performance can be more highly compatible. In this embodiment, each of the first region 35 and the second region 36 extends longer in the width direction than in the length direction of the positive electrode core 30. Hereinafter, the length of the first region 35 along the length direction of the positive electrode core 30 is referred to as "width W1", and the length of the second region 36 along the length direction of the positive electrode core 30 is referred to as "width W2".

The first region 35 and the second region 36 are preferably formed over the entire width of the positive electrode core 30. In this case, high capacity and excellent rapid charging performance can be more effectively combined. The widths W1 and W2 of the regions may vary in the width direction of the positive electrode core 30, but in this embodiment, each region is formed with substantially the same width along the width direction of the positive electrode core 30. Furthermore, the width W1 of each first region 35 is substantially the same, and the width W2 of each second region 36 is also substantially the same, so that stripes of the first regions 35 and the second regions 36 are regularly repeated in the length direction of the positive electrode core 30.

The width W1 of the first region 35 is preferably 1.1 times or more, more preferably 1.5 times or more, and particularly preferably 2.0 times or more or 2.5 times or more, of the width W2 of the second region 36. The upper limit of the ratio (W1/W2) of the width W1 to the width W2 is not particularly limited, but examples are 10.0 times, 9.0 times, or 8.0 times. Examples of suitable ranges of the ratio (W1/W2) are 1.1 times or more and 10.0 times or less, 1.5 times or more and 9.0 times or less, 1.5 times or more and 8.0 times or less, or 2.0 times or more and 8.0 times or less. The suitable ratio (W1/W2) varies depending on the porosity of each region, but as long as the ratio (W1/W2) is generally within this range, high capacity and excellent rapid charging performance can be achieved at the same time to a higher degree.

The width W1 of the first region 35 is, for example, 1 mm or more and 30 mm or less, more preferably 2 mm or more and 20 mm or less. The width W2 of the second region 36 is, for example, 0.5 mm or more and 20 mm or less, more preferably 1 mm or more and 15 mm or less. The length of the positive electrode 11 varies depending on the size of the cylindrical battery 10, and is, for example, 40 mm or more and 4000 mm or less. In this case, the first region 35 and the second region 36 are arranged on one side of the positive electrode core 30 in the longitudinal direction of the positive electrode 11, for example, in the number of 1 to 2000. The number of each of the first regions 35 and second regions 36 arranged in the longitudinal direction of the positive electrode 11 is determined from the above widths W1 and W2 and the length of the positive electrode 11.

If the porosity of the positive electrode mixture layer 31 satisfies the condition that the porosity of the second region 36 is greater than the porosity of the first region 35, it contributes to achieving both high capacity and excellent rapid charging performance, but the lower limit of the porosity of each region is preferably greater than 11%, and more preferably greater than 12%. That is, the porosity of the first region 35 is preferably greater than 11%. Furthermore, the upper limit of the porosity of each region is preferably 35%, and more preferably 30%. That is, it is preferable to satisfy the condition that the porosity of the second region 36 is greater than the porosity of the first region 35 within the range of 10% to 35%.

The porosity of the first region 35 is preferably 10% to 30%, more preferably 12% to 25%, and particularly preferably 15% to 20%. The porosity of the second region 36 is preferably 15% to 35%, more preferably 17% to 30%, and particularly preferably 20% to 25%. If the porosity of each region is within this range, and the ratio (W1/W2) is within the above range, it is possible to achieve a high degree of compatibility between high capacity and excellent rapid charging performance.

The porosity of the first region 35 and the second region 36 can be measured by the following method.
(1) Using an ion milling device, a cross section of the positive electrode mixture layer 31 is exposed.
(2) Using a scanning electron microscope (SEM), a reflected electron image of the cross section of the exposed positive electrode mixture layer 31 is taken. The magnification when taking the reflected electron image is, for example, 1000 to 5000 times.
(3) The SEM image of the cross section of the positive electrode mixture layer 31 is input into a computer, and is color-coded into three colors based on contrast using image analysis software (for example, ImageJ manufactured by the National Institutes of Health), with intermediate colors representing voids.
(4) A measurement area is selected from the processed image, the total area of voids within that area is found, and the proportion of voids in the measurement area (porosity) is calculated.

The porosity of the first region 35 and the second region 36 can be adjusted to the desired range, for example, by using the same material and changing the compression force of each region, or by forming each region using different materials. Specifically, at least one selected from the positive electrode active material, the conductive agent, and the binder is changed in the first region 35 and the second region 36. In particular, it is preferable from the standpoint of battery performance, productivity, etc. to use different types of positive electrode active material in the first region 35 and the second region 36. Different positive electrode active materials may be used in the first region 35 and the second region 36, and the compression force of the coating may be different.

The positive electrode mixture layer 31 contains, as the positive electrode active material, a first lithium metal composite oxide, which is a secondary particle formed by agglomeration of primary particles having an average particle size of 50 nm to 5 μm, and a second lithium metal composite oxide, which is a non-aggregated particle. The first lithium metal composite oxide contains a large number of primary particles. The second lithium metal composite oxide may be a single primary particle (a single crystal particle having no grain boundaries inside), or a secondary particle formed by agglomeration of five or less primary particles. The second lithium metal composite oxide is characterized by having a smaller number of primary particles and being harder than the first lithium metal composite oxide. The first lithium metal composite oxide is easily crushed during the compression process of the positive electrode mixture layer 31, for example, and is packed at a high density.

The first region 35 contains, for example, more of the first lithium metal composite oxide than the second lithium metal composite oxide. That is, in the first region 35, the content (mass ratio) of the first lithium metal composite oxide relative to the total mass of the positive electrode active material is greater than the content of the second lithium metal composite oxide. On the other hand, the second region 36 contains, for example, more of the second lithium metal composite oxide than the first lithium metal composite oxide. As the positive electrode active material, the first region 35 may contain only the first lithium metal composite oxide, and the second region 36 may contain only the second lithium metal composite oxide.

The volumetric median diameter (hereinafter sometimes referred to as "D50") of the lithium metal composite oxide contained in the first region 35 is, for example, 7 μm or more and 30 μm or less. In this specification, D50 means the particle size at which the cumulative frequency in the volumetric particle size distribution is 50% from the smallest particle size. The D50 of the lithium metal composite oxide contained in the second region 36 is, for example, 1 μm or more and 10 μm or less. Note that when the first region 35 contains only the first lithium metal composite oxide, the D50 of the first lithium metal composite oxide is equal to the D50 of the lithium metal composite oxide contained in the first region 35.

The particle size distribution of the positive electrode active material can be measured using a laser diffraction particle size distribution measuring device (e.g., MT3000II, manufactured by Microtrack-Bell Co., Ltd.) with water as the dispersion medium. The average particle size of the positive electrode active material can also be determined by measuring the diameter of the circumscribed circle of the particles in a cross-sectional image of the positive electrode mixture layer 31. If it is difficult to measure the particle size distribution, this average particle size can be applied instead of D50. The cross-section of the positive electrode mixture layer 31 can be prepared by the cross polisher (CP) method, and the cross-sectional image is captured by SEM. The average particle size can be calculated by averaging the particle sizes of any 100 particles from the SEM image.

The average particle size of the primary particles constituting the positive electrode active material (lithium metal composite oxide) is, for example, 50 nm or more and 5.0 μm or less, and preferably 50 nm or more and 1.0 μm or less. The average particle size of the primary particles is calculated by measuring the diameters of the circumscribed circles of 100 primary particles extracted by analyzing SEM images of the cross sections of the secondary particles, and averaging the measured values.

The first region 35 may contain, for example, two types of lithium metal composite oxides with different D50 as the positive electrode active material. By using a mixture of large and small particles, the packing density of the positive electrode active material is increased and the porosity of the first region 35 is reduced. In this case, the lithium metal composite oxide contained in the first region 35 has a particle size distribution with two peaks. On the other hand, the second region 36 contains, for example, one type of lithium metal composite oxide. In this case, the lithium metal composite oxide contained in the second region 36 has a particle size distribution with one peak. Note that, as long as the porosity of the first region 35 and the second region 36 can be adjusted to the desired range, the combination of positive electrode active materials used in each region is not particularly limited.

Lithium metal composite oxide can be synthesized, for example, by mixing and firing a composite oxide raw material containing Ni and Co and at least one of Mn and Al, and a Li raw material such as lithium hydroxide (LiOH). The fired product may be crushed, classified, etc., and may be washed with water. A composite oxide raw material containing Ni, Co, etc. can be obtained, for example, by precipitating (co-precipitating) a composite hydroxide containing Ni, Co, etc., and then heat-treating the composite hydroxide.

The above composite hydroxides can be synthesized, for example, by dropping an alkaline solution such as sodium hydroxide into a stirred solution of metal salts containing Ni, Co, etc., and adjusting the pH to the alkaline side (for example, 8.5 to 12.5). The particle size of the composite hydroxide tends to be smaller as the pH during synthesis is higher, and can also be controlled by adjusting the amount of metal salt solution added. Lithium metal composite oxides with different particle sizes can be produced by controlling the particle size of the composite hydroxide.

The firing process for the mixture of the composite oxide raw material and the Li raw material may be a multi-stage firing process including a first firing process and a second firing process at a higher temperature than the first firing process. The mixture is fired in an oxygen atmosphere, and the oxygen concentration is set to 85% or more, for example. A suitable first firing temperature varies somewhat depending on the composition of the mixture, but an example is 500°C or higher and 750°C or lower. A suitable second firing temperature is, for example, 800°C or higher and 1150°C or lower. It is preferable that there is a temperature difference of 50°C or more between the temperatures of each firing process.

Non-aggregated single particles can be synthesized, for example, by increasing the pH of the alkaline aqueous solution used to synthesize the composite hydroxide compared to the case of synthesizing secondary particles formed by agglomeration of many primary particles. Alternatively, instead of or in addition to increasing the pH of the alkaline aqueous solution, they can also be synthesized by increasing the firing temperature. An example of a suitable pH for the alkaline aqueous solution when synthesizing single particles is 10 to 11, and an example of a suitable firing temperature is 950 to 1100°C. When synthesizing secondary particles, for example, an alkaline aqueous solution with a pH of 9 to 10 is used, and the firing temperature is set to 950°C or lower.

Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, conductive whiskers, etc. One type of conductive agent may be used alone, or multiple types may be used in combination. The first region 35 and the second region 36 may contain, for example, the same type of conductive agent, but may also contain different conductive agents.

Examples of the binder contained in the positive electrode mixture layer 31 include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer, and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. These resins may also be used in combination with carboxymethylcellulose (CMC) or a salt thereof, polyethylene oxide (PEO), and the like. One type of binder may be used alone, or multiple types may be used in combination. The first region 35 and the second region 36 may contain, for example, the same type of binder, but may also contain different binders.

The conductive agent and binder content is, for example, 0.1 mass % or more and 5 mass % or less, respectively, relative to the mass of the positive electrode mixture layer 31. The conductive agent amount in the first region 35 and the second region 36 may, for example, be substantially the same, but the conductive agent content in each region may be different. One example is that the conductive agent content in the second region 36 is greater than the conductive agent content in the first region 35. Similarly, the binder amount in each region may, for example, be substantially the same, but the binder content in each region may be different.

FIGS. 4 and 5 are front views showing modified examples of the positive electrode 11. In the example shown in FIG. 2, the first regions 35 and the second regions 36 are alternately arranged only along the length direction of the positive electrode core 30, but as shown in FIGS. 4 and 5, the first regions 35 and the second regions 36 may be alternately arranged in both the length direction and the width direction of the positive electrode core 30. The first regions 35 and the second regions 36 may be arranged in a random, irregular pattern, but from the viewpoints of stabilizing the battery performance and uniforming the battery reaction, it is preferable that they are arranged in a regular pattern.

The first regions 35 and second regions 36 may be alternately arranged only along the width direction of the positive electrode 11, but in the case of a striped shape, the shape shown in FIG. 2 is preferable. In a cylindrical battery 10, when the volume change of the electrode body 14 due to charging and discharging becomes large, the electrolyte may be pushed out in the axial direction of the electrode body 14. For this reason, a striped shape in which the second regions 36, which have a high porosity and good electrolyte permeability, are formed along the axial direction allows for smoother supply of electrolyte to the electrode body 14.

As shown in FIG. 4, the second region 36 may be arranged in a lattice pattern when viewed from the front of the positive electrode 11. In the example shown in FIG. 4, the lattice of the second region 36 is aligned along the width and length directions of the positive electrode 11, but the lattice may be formed along a direction that is inclined relative to the width and length directions. The first region 35 surrounded by the lattice of the second region 36 has a square shape when viewed from the front, but may also have a rectangular shape. In this case, the lattice-shaped second region 36 also functions as a supply path for the electrolyte, and a portion of the electrolyte is supplied to the first region 35 via the second region 36.

As shown in FIG. 5, the first regions 35 may be arranged in a dot pattern when viewed from the front of the positive electrode 11. The first regions 35 have, for example, a perfect circle shape when viewed from the front. In the example shown in FIG. 5, the first regions 35 are the same size and are arranged at equal intervals in the length direction of the positive electrode 11. In the width direction of the positive electrode 11, the first regions 35 are densely arranged so that the dots fill the recesses of the dots of the two first regions 35. Then, the second regions 36 are formed so as to fill the spaces between the dots of the first regions 35. In this case as well, the second regions 36 function as a supply path for the electrolyte.

The second region 36 may be formed in a honeycomb shape (hexagonal shape) when viewed from the front of the positive electrode 11, or may have a shape other than a circle, a square, or a hexagon. The first region 35 may also be formed in a lattice or honeycomb shape, or the second region 36 may be formed in a dot shape.

[Negative electrode]
As shown in FIG. 1, the negative electrode 12 has a negative electrode core 40 and a negative electrode mixture layer 41 arranged on the negative electrode core 40. For the negative electrode core 40, a foil of a metal stable in the potential range of the negative electrode 12, such as copper, a copper alloy, stainless steel, nickel, or a nickel alloy, or a film having the metal arranged on the surface can be used. The negative electrode mixture layer 41 contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core 40 except for the portion to which the negative electrode lead 21 is connected. A protective layer containing inorganic particles and a binder may be arranged between the negative electrode core 40 and the negative electrode core 40 or on the negative electrode mixture layer 41.

The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the surface of the negative electrode core 40, drying the coating, and then compressing it to form a negative electrode mixture layer 41 on both sides of the negative electrode core 40. Water, for example, is used as a dispersion medium for the negative electrode mixture slurry. The negative electrode mixture layer 41 may contain a conductive agent such as CNT, and the same conductive agent as in the case of the positive electrode 11 can be used as the conductive agent. The negative electrode mixture slurry may contain a dispersant.

A carbon material that reversibly absorbs and releases lithium ions is generally used as the negative electrode active material. Elements that alloy with Li, such as Si and Sn, and materials containing these elements may also be used as the negative electrode active material. Of these, materials containing Si are preferred. Lithium titanate, which has a higher charge/discharge potential relative to metallic lithium than carbon materials, may also be used as the negative electrode active material. One type of negative electrode active material may be used alone, or multiple types may be used in combination.

The carbon material that functions as the negative electrode active material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. Among them, it is preferable to use, as the carbon material, at least artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, and earthy graphite, or a mixture of these. The volume-based D50 of the carbon material is, for example, 1 μm or more and 30 μm or less, and preferably 5 μm or more and 25 μm or less.

Examples of silicon-containing materials that function as negative electrode active materials include silicon alloys, silicon compounds, and composite materials containing Si. Among these, composite materials containing Si are preferred. A suitable composite material is a composite particle containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase. The ion-conducting phase is, for example, at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The Si phase is formed by dispersing Si in the form of fine particles. The ion-conducting phase is a continuous phase composed of a collection of particles finer than the Si phase. The volume-based D50 of the silicon-containing material is, for example, 1 μm to 20 μm, or 1 μm to 15 μm.

As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer 41 may be fluororesin, olefin resin, PAN, polyimide, polyamide, acrylic resin, etc., but generally, polyvinyl acetate, styrene-butadiene rubber (SBR), etc. are used. Of these, it is preferable to use SBR. One type of binder may be used alone, or multiple types may be used in combination. In addition, it is preferable that the negative electrode mixture layer 41 contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. These also function as thickeners in the negative electrode mixture slurry.

[Separator]
A porous sheet having ion permeability and insulation is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. The material of the separator 13 is preferably a polyolefin such as polyethylene or polypropylene, or cellulose. The separator 13 may have a single layer structure or a multilayer structure. The separator 13 may have, for example, a multilayer structure including a thermoplastic resin layer such as a polyolefin and a cellulose fiber layer, a two-layer structure of polyethylene (PE)/polypropylene (PP), or a three-layer structure of PE/PP/PE.

A filler layer containing an inorganic filler may be disposed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphate compounds. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13. In addition, a resin layer (heat-resistant layer) having high heat resistance such as aramid resin may be disposed on the surface of the separator 13. The separator 13 may have, for example, a substrate made of a porous sheet and a filler layer or a heat-resistant layer disposed on the substrate.

The present disclosure will be further explained below with reference to examples, but the present disclosure is not limited to these examples.

Example 1
[Synthesis of first lithium metal composite oxide]
A composite hydroxide containing Ni, Co, and Al in a molar ratio of 85:10:5 was synthesized by coprecipitation, and heat-treated at 600°C to obtain a composite oxide. In the synthesis of the composite hydroxide, the pH and the amount of the metal salt solution were adjusted so that the D50 of the finally obtained lithium metal composite oxide was about 15 to 20 μm. The obtained composite oxide and lithium hydroxide were mixed so that the molar ratio (Li/Me ratio) of the metal element (Me) in the composite oxide to Li in the lithium hydroxide was 1:1.020. This mixture was placed in a calcination furnace and calcined in two stages.

In the calcination step, the mixture was heated from room temperature to 650 ^° C (first calcination temperature) at a temperature increase rate of 3°C/min (first temperature increase rate) under an oxygen flow with an oxygen concentration of 95% (flow rate of 2mL/min per 10 cm3 and 5L/min per 1 kg of the mixture). The mixture was then heated from 650°C to 750°C (second calcination temperature) at a temperature increase rate of 1°C/min (second temperature increase rate), and held at 750°C for 3 hours. The calcined product was pulverized and washed with water to obtain a first lithium metal composite oxide.

The volume-based D50 of the first lithium metal composite oxide measured using an MT3000II manufactured by Microtrac-Bell, Inc. and water as a dispersion medium was 17 μm. From the SEM image, it was confirmed that the composite oxide was a secondary particle formed by the aggregation of primary particles having an average particle size of 500 nm.

[Preparation of first positive electrode mixture slurry]
The first lithium metal composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed in a solid content mass ratio of 98:1:1, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a first positive electrode mixture slurry.

[Synthesis of second lithium metal composite oxide]
A second lithium metal composite oxide was obtained in the same manner as the first lithium metal composite oxide, except that the pH and the amount of the metal salt solution during the synthesis of the composite hydroxide were adjusted so that the D50 of the finally obtained lithium metal composite oxide would be about 1 to 5 μm, and 10 mass % of KOH was added to the composite oxide that had been subjected to the two-stage calcination, and the composite oxide was calcined at 750° C. for 40 hours in an oxygen stream.

The volume-based D50 of the second lithium metal composite oxide measured using an MT3000II manufactured by Microtrac Bell Inc. and water as a dispersion medium was 3 μm. From the SEM image, the composite oxide was a single primary particle, or a non-aggregated single particle composed of 5 or less primary particles.

[Preparation of second positive electrode mixture slurry]
A second positive electrode mixture slurry was prepared in the same manner as the first positive electrode mixture slurry, except that the second lithium metal composite oxide was used as the positive electrode active material.

[Preparation of Positive Electrode]
The first positive electrode mixture slurry was intermittently applied to both sides of a positive electrode core made of aluminum foil to form a first coating film, and the coating film was dried. Then, the second positive electrode mixture slurry was applied to the portion where the first coating film was not present to form a second coating film, and the coating film was dried. At this time, the first and second positive electrode mixture slurries were applied so that the first and second coating films were alternately formed in the length direction of the positive electrode core, that is, in a striped shape as shown in FIG. 2. The first coating film became the first region of the positive electrode mixture layer, and the second coating film became the second region of the positive electrode mixture layer. In this example, the ratio of the width of the first region to the width of the second region was adjusted to 75:25. The average value of the width of the first region was 7.5 mm, and the average value of the width of the second region was 2.5 mm.

Next, the coating film (positive electrode mixture layer) was rolled using a roller so that the mixture density of the first region was 3.4 g/cc and the mixture density of the second region was 3.3 g/cc, and the positive electrode core was cut into a predetermined electrode size to obtain a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode core. An exposed portion in which the surface of the positive electrode core was exposed was provided in a part of the positive electrode. The porosity of the first region of the positive electrode mixture layer measured by the above-mentioned method was 17%, and the porosity of the second region was 21%. The BET specific surface area of the first region was 1.9 m ² /g, and the BET specific surface area of the second region was 2.3 m ² /g.

[Preparation of negative electrode]
As the negative electrode active material, a mixture of natural graphite and a silicon-containing material (a composite material in which fine Si phases are dispersed in a silicon oxide phase) in a mass ratio of 98:2 was used. The negative electrode active material, a dispersion of sodium carboxymethylcellulose (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in a solid content mass ratio of 100:1:1, and a negative electrode mixture slurry was prepared using water as a dispersion medium. The negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil, and the coating film was dried. The coating film was then rolled using a roller and cut to a predetermined electrode size to obtain a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode core. An exposed portion in which the surface of the negative electrode core was exposed was provided in a part of the negative electrode.

[Preparation of non-aqueous electrolyte]
A non-aqueous electrolyte solution was prepared by dissolving _LiPF6 at a concentration of 1.2 mol/L in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4 (25 ° C.).

[Preparation of test cell (non-aqueous electrolyte secondary battery)]
An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the exposed portion of the negative electrode, and the positive and negative electrodes were spirally wound with a polyolefin separator interposed therebetween to prepare a wound electrode assembly. This electrode assembly was placed in a cylindrical outer can with a bottom, and the nonaqueous electrolyte was poured into it, and the opening of the outer can was then sealed with a sealer to obtain a test cell.

Example 2
A test cell was fabricated in the same manner as in Example 1, except that the ratio of the width of the first region to the width of the second region of the positive electrode mixture layer was changed to 25:75.

Example 3
A test cell was fabricated in the same manner as in Example 1, except that the ratio of the width of the first region to the width of the second region of the positive electrode mixture layer was changed to 50:50.

Example 4
A test cell was produced in the same manner as in Example 1, except that the ratio of the width of the first region to the width of the second region of the positive electrode mixture layer was changed to 90:10.

Example 5
A test cell was prepared in the same manner as in Example 1, except that the first coating film and the second coating film were rolled separately, and the second coating film was rolled so that the composite density of the second region was 3.1 g/cc.

Example 6
A test cell was produced in the same manner as in Example 1, except that the coatings in the first region and the second region were formed using only the first positive electrode mixture slurry, and the coatings in each region were rolled separately so that the mixture density in the first region was 3.4 g/cc and the mixture density in the second region was 3.6 g/cc.

<Comparative Example 1>
Except for forming the positive electrode mixture layer using only the first positive electrode mixture slurry, a test cell was produced in the same manner as in Example 1. The porosity of the positive electrode mixture layer was 17%.

<Comparative Example 2>
Except for forming the positive electrode mixture layer using only the second positive electrode mixture slurry, a test cell was produced in the same manner as in Example 1. The porosity of the positive electrode mixture layer was 20%.

Each test cell in the examples and comparative examples was evaluated for discharge capacity (initial discharge capacity per unit mass of positive electrode active material), rapid charge performance (permeability of electrolyte), and cycle characteristics (capacity retention rate after cycle test) using the methods described below, and the evaluation results are shown in Table 1. The rapid charge performance shown in Table 1 is a relative value with the evaluation result of the positive electrode in Comparative Example 1 taken as 100, and a smaller value indicates better rapid charge performance.

[Evaluation of Discharge Capacity]
The test cell was charged at a constant current of 0.3 It in a temperature environment of 25° C. until the battery voltage reached 4.2 V, and then charged at a constant voltage of 0.02 It at 4.2 V. Thereafter, the test cell was discharged at a constant current of 0.5 It until the battery voltage reached 2.5 V, and the discharge capacity was determined.

[Evaluation of rapid charging performance]
Ethylene carbonate (EC) was dropped to a thickness of 3 μm on the surface of the positive electrode mixture layer, and the time (permeation time) until the EC penetrated from the surface of the mixture layer to the inside and disappeared was measured. The shorter this permeation time, the better the permeability of the electrolyte in the positive electrode mixture layer. The permeability of the electrolyte is closely related to the rapid charging performance of the battery, and the better the permeability, the higher the rapid charging performance.

[Evaluation of cycle characteristics]
The test cell was charged at a constant current of 0.3 It in a temperature environment of 25° C. until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 0.02 It. Thereafter, the test cell was discharged at a constant current of 0.5 It until the battery voltage reached 2.5 V. This charge/discharge cycle was counted as one cycle, and 400 cycles were performed. The discharge capacity at the first cycle and the discharge capacity at the 300th cycle were determined, and the capacity retention rate was calculated by the following formula.
Capacity retention rate (%)=(300th cycle discharge capacity/1st cycle discharge capacity)×100

As shown in Table 1, all of the test cells of the examples have better electrolyte permeability than the test cell of Comparative Example 1. Therefore, the test cells of the examples have excellent rapid charging performance. In addition, all of the test cells of the examples have higher capacity than the test cell of Comparative Example 2. The test cell of Comparative Example 1 has high capacity but poor rapid charging performance, and the test cell of Comparative Example 2 has excellent rapid charging performance but low capacity. From these results, it can be seen that a secondary battery with high capacity and excellent rapid charging performance can be realized by providing at least two regions with different porosities in the positive electrode mixture layer, for example by arranging the first and second regions alternately in the longitudinal direction of the positive electrode core.

The present disclosure is further illustrated by the following embodiments.
Configuration 1: A positive electrode for a secondary battery, comprising: a positive electrode core; and a positive electrode mixture layer disposed on the positive electrode core, the positive electrode mixture layer including first regions and second regions disposed alternately in at least one of a length direction and a width direction of the positive electrode core, and a porosity of the second region being greater than the porosity of the first region.
Configuration 2: The positive electrode for a secondary battery according to configuration 1, wherein the BET specific surface area of the second region is larger than the BET specific surface area of the first region.
Configuration 3: The positive electrode for a secondary battery according to configuration 1 or 2, wherein the first regions and the second regions are alternately arranged in the longitudinal direction of the positive electrode core.
Configuration 4: The positive electrode for a secondary battery according to Configuration 3, wherein a length of the first region along the longitudinal direction of the positive electrode core body is longer than a length of the second region along the longitudinal direction of the positive electrode core body.
Configuration 5: The positive electrode for a secondary battery according to configuration 3 or 4, wherein the first region and the second region are formed across the entire width of the positive electrode core.
Configuration 6: The positive electrode for a secondary battery according to Configuration 1 or 2, wherein at least one of the first region and the second region is arranged in a stripe pattern, a lattice pattern, a dot pattern, or a honeycomb pattern when viewed from the front of the positive electrode mixture layer.
Configuration 7: The positive electrode for a secondary battery according to any one of configurations 1 to 6, wherein the porosity of the first region is greater than 11%.
Configuration 8: The positive electrode for a secondary battery according to any one of Configurations 1 to 7, wherein the positive electrode mixture layer contains a first lithium metal composite oxide which is a secondary particle formed by agglomeration of primary particles having an average particle size of 50 nm or more and 5 μm or less, and a second lithium metal composite oxide which is a non-aggregated primary particle, the first region contains more of the first lithium metal composite oxide than the second lithium metal composite oxide, and the second region contains more of the second lithium metal composite oxide than the first lithium metal composite oxide.
Configuration 9: The positive electrode for a secondary battery according to any one of configurations 1 to 8, wherein the volumetric median diameter of the lithium metal composite oxide contained in the first region is 10 μm or more and 30 μm or less, and the volumetric median diameter of the lithium metal composite oxide contained in the second region is 2 μm or more and 10 μm or less.
Configuration 10: The positive electrode for a secondary battery according to any one of Configurations 1 to 9, wherein the positive electrode mixture layer contains a lithium metal composite oxide, the lithium metal composite oxide containing Li, Ni, and Co, and containing at least one of Mn and Al, and a ratio of Ni to the total number of moles of metal elements excluding Li is 80 mol % or more.
Aspect 11: A secondary battery comprising the positive electrode for secondary batteries according to any one of aspects 1 to 10, a negative electrode, and an electrolyte.

REFERENCE SIGNS LIST 10 Cylindrical battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode core, 31 Positive electrode mixture layer, 35 First region, 36 Second region, 40 Negative electrode core, 41 Negative electrode mixture layer

Claims

A positive electrode core and a positive electrode mixture layer disposed on the positive electrode core,
the positive electrode mixture layer includes first regions and second regions alternately arranged in at least one of a length direction and a width direction of the positive electrode core,
A positive electrode for a secondary battery, wherein the porosity of the second region is greater than the porosity of the first region.
The positive electrode for a secondary battery according to claim 1, wherein the BET specific surface area of the second region is greater than the BET specific surface area of the first region.
The positive electrode for a secondary battery according to claim 1, wherein the first region and the second region are arranged alternately in the longitudinal direction of the positive electrode core.
The positive electrode for a secondary battery according to claim 3, wherein the length of the first region along the length direction of the positive electrode core is longer than the length of the second region along the length direction of the positive electrode core.
The positive electrode for a secondary battery according to claim 3, wherein the first region and the second region are formed over the entire width of the positive electrode core.
The positive electrode for a secondary battery according to claim 1, wherein at least one of the first region and the second region is arranged in a stripe pattern, a lattice pattern, a dot pattern, or a honeycomb pattern when viewed from the front of the positive electrode mixture layer.
The positive electrode for a secondary battery according to claim 1, wherein the porosity of the first region is greater than 11%.
the positive electrode mixture layer includes a first lithium metal composite oxide which is a secondary particle formed by agglomeration of primary particles having an average particle size of 50 nm or more and 5 μm or less, and a second lithium metal composite oxide which is a non-agglomerated primary particle,
the first region contains more of the first lithium metal composite oxide than the second lithium metal composite oxide;
2. The positive electrode for a secondary battery according to claim 1, wherein the second region contains more of the second lithium metal composite oxide than the first lithium metal composite oxide.
The volume-based median diameter of the lithium metal composite oxide contained in the first region is 10 μm or more and 30 μm or less,
2 . The positive electrode for a secondary battery according to claim 1 , wherein the lithium metal composite oxide contained in the second region has a volume-based median diameter of 2 μm or more and 10 μm or less.
The positive electrode mixture layer contains a lithium metal composite oxide,
2. The positive electrode for a secondary battery according to claim 1, wherein the lithium metal composite oxide contains Li, Ni, and Co, and also contains at least one of Mn and Al, and a ratio of Ni to a total number of moles of metal elements excluding Li is 80 mol % or more.
The positive electrode for a secondary battery according to any one of claims 1 to 10,
A negative electrode;
An electrolyte;
A secondary battery comprising: