WO2024095670A1

WO2024095670A1 - Positive electrode for secondary batteries, and secondary battery

Info

Publication number: WO2024095670A1
Application number: PCT/JP2023/036210
Authority: WO
Inventors: 林太郎名取; 昂輝守田; 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: 2023-10-04
Publication date: 2024-05-10

Abstract

A positive electrode (11) as an embodiment comprises a positive electrode core (30) and positive electrode mixture layers (31) disposed on the positive electrode core (30). The positive electrode mixture layers (31) each comprise first regions (35) and second regions (36) which are alternately arranged in the length direction and/or the width direction of the positive electrode core (30). The first regions (35) and the second regions (36) include a positive electrode active material, a binder and a conductive agent. The content ratio of the conductive agent in the second regions (36) is higher than the content ratio of the conductive agent in the first regions (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 the amount of conductive agent contained in the mixture layer is greater on the core side than on the surface side.

JP 2022-37549 A

Patent Document 1 describes the effect that a sufficient conductive path can be formed in the positive electrode mixture layer, and the porosity of the mixture layer can be increased. However, if the amount of conductive agent in the mixture layer is increased, the capacity decreases and the permeability of the electrolyte deteriorates, which may also decrease the rapid charging performance. In particular, if the mixture layer is formed to be high density in order to improve the 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 arranged alternately in at least one of the length and width directions of the positive electrode core, the first and second regions including a positive electrode active material, a binder, and a conductive agent, and the content of the conductive agent in the second region is greater than the content of the conductive agent in 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 conductive agent contents 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. The function of the second region, which has a high conductive agent content, is to form a good conductive path in the mixture layer, improving rapid charging performance.

On the other hand, the first region of the mixture layer, which has a low content of conductive agent, contributes to high capacity and improves the permeability of the electrolyte. Since the electrolyte is also supplied to the second region from the adjacent first region, excellent rapid charging performance is obtained due to the synergistic effect of the first and second regions. 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, since 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 focus on 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.

The lithium metal composite oxide may be used alone or in combination of two or more types. The lithium metal composite oxide may be, for example, a secondary particle formed by agglomeration of primary particles having an average particle size of 50 nm to 5 μm, or may be a non-agglomerated single particle. In this specification, a non-agglomerated single particle means a single primary particle (a single crystal particle having no internal grain boundaries) or a secondary particle formed by agglomeration of five or fewer primary particles. Compared to secondary particles formed by agglomeration of many primary particles, a single particle is characterized by being harder and less likely to be crushed by compression during the manufacture of the positive electrode.

The volume-based median diameter (hereinafter sometimes referred to as "D50") of the lithium metal composite oxide is, for example, 2 μm or more and 30 μm or less. In this specification, D50 means the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size. The particle size distribution can be measured using a laser diffraction particle size distribution measuring device (for example, 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 circumscribing circle of the particles in a cross-sectional image of the positive electrode mixture layer 31, and this average particle size can be applied instead of D50. The cross-section of the positive electrode mixture layer 31 can be prepared by a cross polisher (CP) method, and the cross-sectional image is taken with a scanning electron microscope (SEM). The average particle size can be calculated by averaging the particle sizes of 100 particles selected from the SEM image. The average particle size of primary particles is calculated by measuring the diameters of the circumscribing circles of 100 primary particles extracted by analysis of the SEM image of the cross-section of the secondary particles, and averaging the measured values.

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.

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 the positive electrode mixture layer 31 (positive electrode 11) is viewed from the front.

The first region 35 and the second region 36 are regions with different conductive agent contents, and the conductive agent content in the second region 36 is greater than the conductive agent content in the first region 35. As a result of the inventors' studies, it was found that by providing the positive electrode mixture layer 31 with the first region 35 and the second region 36 with different conductive agent contents and arranging the regions alternately in the longitudinal direction of the positive electrode core 30, a cylindrical battery 10 with high capacity and excellent rapid charging performance can be realized. Note that, within the scope of the purpose of the present disclosure, the first region 35 and the second region 36 may differ from each other in physical properties other than the conductive agent content. Furthermore, the positive electrode mixture layer may have three or more regions with different conductive agent contents.

In the positive electrode 11, the second region 36, which has a high content of conductive agent, functions to form a good conductive path in the positive electrode mixture layer 31, improving rapid charging performance. On the other hand, the first region 35, which has a low content of conductive agent, contributes to high capacity and improves the permeability of the electrolyte. Since the electrolyte is also supplied to the second region 36 from the adjacent first region 35, the synergistic effect of the first region 35 and the second region 36 provides excellent rapid charging performance. In addition, since the electrolyte is smoothly supplied to the entire positive electrode mixture layer 31, a uniform battery reaction occurs over a wide area of the positive electrode mixture layer 31, improving cycle characteristics.

From the viewpoint of achieving a higher degree of compatibility between high capacity and rapid charging performance, the difference in the conductive agent content between the first region 35 and the second region 36 is preferably 10 times or less, more preferably 8 times or less, and particularly preferably 5 times or less. The ratio (B/A) of the conductive agent content (B) in the second region 36 to the conductive agent content (A) in the first region 35 may be 3 times or less. The lower limit of the ratio (B/A) should be greater than 1, but if the difference is too small, the effect will be reduced, so it is preferably 1.2 or more, and more preferably 1.5 or more.

An example of a suitable ratio (B/A) is 1<(B/A)≦5 (greater than 1 and equal to or less than 5), 1<(B/A)≦3, 1.2<(B/A)≦3, or 1.5<(B/A)≦3. If the ratio (B/A) is within this range, it is possible to achieve a high degree of compatibility between high capacity and excellent rapid charging performance.

The first region 35 and the second region 36 may differ from each other in terms of porosity, BET specific surface area, particle size distribution of the positive electrode active material contained therein, and the like. For example, in the first region 35, the content (mass ratio) of the secondary particle type lithium metal composite oxide relative to the total mass of the positive electrode active material may be greater than the content of the monoparticle type lithium metal composite oxide. On the other hand, the second region 36 may contain more monoparticles than secondary particles. As the positive electrode active material, the first region 35 may contain only secondary particles, and the second region 36 may contain only monoparticles. In this case, for example, the porosity of the second region 36 is greater than the porosity of the first region 35.

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 positive electrode mixture slurries with different amounts of conductive agent added. 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 of the positive electrode core 30, and then the second positive electrode mixture slurry that forms the second region 36 is intermittently applied to the parts where the first positive electrode mixture slurry is not applied, thereby forming the stripes shown in FIG. 2. It is also possible to form stripes by simultaneously applying the first and second positive electrode mixture slurries to different places on the surface of the positive electrode core 30.

The physical properties, such as the conductive agent content, of the multiple first regions 35 are preferably substantially the same. By forming each first region 35 using the same first positive electrode mixture slurry, the conductive agent content of each first region 35 is substantially the same. Similarly, the physical properties, such as the conductive agent content, of the multiple second regions 36 are preferably substantially the same. Furthermore, the conductive agent content in the thickness direction of the first region 35 is substantially the same, and for example, the conductive agent content 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 state 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 or more and 2000 or less. 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.

Examples of binders 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 contain different binders. The binder content is, for example, 0.1 mass% or more and 5.0 mass% or less with respect to the mass of the positive electrode mixture layer 31. The amount of binder in the first region 35 and the second region 36 may be, for example, substantially the same, but the binder content in each region may be different. As an example, the binder content in the second region 36 may be greater than the binder content in the first region 35.

Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon black such as acetylene black, ketjen black, and furnace black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, and conductive whiskers. The conductive agent may be used alone or 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. The conductive agent preferably contains at least one type selected from carbon black and CNT, and is particularly preferably at least CNT.

As described above, the conductive agent content in the positive electrode mixture layer 31 may be any content ratio (B/A) in each region that is greater than 1, but the upper limit of the conductive agent content in each region is preferably 5.0 mass%, more preferably 3.0 mass%, relative to the mass of the positive electrode mixture layer 31. The lower limit of the conductive agent content in each region is preferably 0.05 mass%, more preferably 0.1 mass%, relative to the mass of the positive electrode mixture layer 31.

The conductive agent content (A) in the first region 35 is, for example, 0.05 mass% or more and 1.0 mass% or less, preferably 0.1 mass% or more and 0.8 mass% or less, and more preferably 0.2 mass% or more and 0.5 mass% or less. The conductive agent content (B) in the second region 36 is, for example, 0.3 mass% or more and 3.0 mass% or less, preferably 0.4 mass% or more and 2.0 mass% or less, and more preferably 0.5 mass% or more and 1.0 mass% or less. The suitable range of the conductive agent content varies slightly depending on the type of conductive agent, but it is preferable that the content be within this range, especially when CNTs are used alone.

CNTs are conductive carbon fibers with an outer tube diameter of several tens of nanometers or less, and have an extremely large aspect ratio (ratio of fiber length to fiber diameter). The average aspect ratio of CNTs is, for example, 20 times or more, and preferably 50 times or more. CNTs with a high aspect ratio form linear contact with the active material and core material rather than point contact. For this reason, a good conductive path can be formed with the addition of a small amount.

The average fiber diameter of CNT is, for example, 50 nm or less, preferably 20 nm or less, and more preferably 15 nm or less. The fiber diameter means the length in the direction perpendicular to the fiber length direction. The lower limit of the average fiber diameter of CNT is not particularly limited, but an example is 1 nm. The average fiber diameter of CNT is determined by image analysis using a transmission electron microscope (TEM). The average fiber diameter of CNT is determined by measuring the fiber diameter of 100 randomly selected CNTs and taking the arithmetic average of the measured values.

The average fiber length of the CNTs is, for example, 0.5 μm or more, and may be 1 μm or more. The fiber length means the length of the CNTs when stretched in a straight line. If the average fiber length is 0.5 μm or more, the DC resistance is reduced more effectively. There is no particular upper limit to the average fiber length of the CNTs, but an example is 100 μm. The average fiber length of the CNTs is determined by image analysis using a SEM. The average fiber length of the CNTs is determined by measuring the lengths of 100 randomly selected CNTs and arithmetically averaging the measured values.

CNTs may be either single-walled CNTs (SWCNTs) or multi-walled CNTs (MWCNTs), or SWCNTs and MWCNTs may be used in combination. SWCNTs have a structure in which one graphite sheet is formed into a tube shape, while multi-walled CNTs have a structure in which multiple graphite sheets are formed into a tube shape. One example of a multi-walled CNT is a double-walled CNT, which has a two-layer structure.

The preferred BET specific surface area of CNTs varies slightly depending on the type of CNT, but is, for example, ²⁰⁰ m2/g or more, and more preferably 250 ^m2 /g or more. The upper limit of the BET specific surface area is not particularly limited, but is, for example, 2000 ^m2 /g. The BET specific surface area is measured according to the BET method (nitrogen adsorption method) described in JIS R1626.

The CNTs are supplied to the manufacturing process of the positive electrode 11 in the form of a conductive agent dispersion liquid in which they are dispersed in a liquid containing a dispersant and an aprotic polar solvent (dispersion medium), and are added to the positive electrode mixture slurry. In the dispersion liquid, the dispersant is dissolved in the polar solvent, and the CNTs are dispersed in the polar solvent by the action of the dispersant. The solid content (CNTs and dispersant) concentration of the dispersion liquid is, for example, 0.1 mass% to 20 mass%, and from the viewpoint of balancing the dispersibility of the CNTs and productivity, is preferably 0.2 mass% to 15 mass%, and more preferably 1 mass% to 10 mass%. An aprotic polar solvent such as NMP is used as the dispersion medium.

The D50 of the CNT dispersed in the polar solvent is preferably less than 8 μm, more preferably less than 3 μm. The particle size distribution of the CNT can be measured using a laser diffraction particle size distribution measuring device, similar to the particle size distribution of the positive electrode active material. The dispersion liquid is prepared by mixing a conductive agent, a dispersant, and an aprotic polar solvent. For mixing (kneading) these raw materials, a conventionally known dispersing machine or mixer, such as a planetary mixer, homomixer, pin mixer, high-speed mixer, disperser, roll mill, ball mill, jet mill, kneader, etc., can be used.

The dispersant includes, for example, a nitrile group-containing rubber. The nitrile group-containing rubber is a copolymer of a monomer containing unsaturated nitrile and a conjugated diene as raw materials, and may be a copolymer of substantially only unsaturated nitrile and conjugated diene. The molar ratio of unsaturated nitrile to conjugated diene is, for example, 10:90 to 70:30. The weight average molecular weight of the nitrile rubber is not particularly limited, but an example is 5,000 to 5,000,000. At least a portion of the dispersant may function as a binder for the positive electrode mixture layer 31.

The nitrile group-containing rubber may be a hydrogenated nitrile rubber. The hydrogenated nitrile rubber contains, for example, structural units derived from an unsaturated nitrile, structural units derived from a conjugated diene, and structural units derived from a hydrogenated conjugated diene. An example of a suitable hydrogenated nitrile rubber is a partially hydrogenated nitrile rubber in which 80 mol% or more of the structural units derived from a conjugated diene are hydrogenated. An example of an unsaturated nitrile is acrylonitrile or methacrylonitrile, preferably acrylonitrile. An example of a conjugated diene is a conjugated diene having 3 to 6 carbon atoms, preferably butadiene.

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 the dispersion medium for the negative electrode mixture slurry. The negative electrode mixture layer 41 may contain a conductive agent such as CNT, and the conductive agent may be the same as that of the positive electrode 11. 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 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 lithium metal composite oxide.

The volumetric D50 of the lithium metal composite oxide was measured using an MT3000II manufactured by Microtrac Bell Co., Ltd., with water as the dispersion medium, and was found to be 17 μm. SEM images confirmed that the composite oxide was a secondary particle formed by the aggregation of primary particles with an average particle size of 500 nm.

[Preparation of first positive electrode mixture slurry]
The lithium metal composite oxide was used as the positive electrode active material. The positive electrode active material, carbon nanotubes (CNT), 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 having a CNT solid content concentration of 0.25 mass%. Multi-walled carbon nanotubes were used as the CNT.

[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 positive electrode active material, CNT, and PVdF were mixed in a solid content mass ratio of 98:1:1 and the solid content concentration of CNT was changed to 0.75 mass%.

[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 50:50. The average value of the width of the first region was 5.0 mm, and the average value of the width of the second region was 5.0 mm.

Then, the coating film (positive electrode mixture layer) was rolled using a roller, and the positive electrode core was cut to a specified electrode size to obtain a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode core. In addition, an exposed portion was provided on part of the positive electrode, where the surface of the positive electrode core was exposed.

[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 98:1:1, 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 produced in the same manner as in Example 1, except that in the preparation of the second positive electrode mixture slurry, the solid content concentration of CNTs was changed to 1.50 mass %.

Example 3
A test cell was produced in the same manner as in Example 1, except that in the preparation of the second positive electrode mixture slurry, the solid content concentration of CNT was changed to 2.50 mass %.

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 60:40.

Example 5
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 70:30.

Example 6
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 40:60.

<Comparative Example 1>
A test cell was produced in the same manner as in Example 1, except that in the preparation of the first positive electrode mixture slurry, the solid content concentration of CNTs was changed to 0.50 mass%, and the positive electrode mixture layer was formed using only the first positive electrode mixture slurry.

<Comparative Example 2>
A test cell was produced in the same manner as in Example 1, except that the positive electrode mixture layer was formed using only the second positive electrode mixture slurry prepared in Example 2.

<Comparative Example 3>
A test cell was produced in the same manner as in Example 1, except that the positive electrode mixture layer was formed using only the first positive electrode mixture slurry.

For each test cell in the examples and comparative examples, the discharge capacity (initial discharge capacity per unit mass of the positive electrode active material), the liquid permeability of the positive electrode, the volume resistance of the positive electrode, and the rapid charging performance were evaluated using the methods described below, and the evaluation results are shown in Table 1.

[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 Liquid Permeability]
Ethylene carbonate (EC) was dropped to a thickness of 3 μm on the surface of the positive electrode mixture layer, and the time (penetration time) until the EC penetrated from the surface of the mixture layer to the inside and disappeared was measured. The values shown in Table 1 are relative values with the evaluation result of Comparative Example 1 taken as 100, and a smaller value means better liquid permeability.

[Evaluation of Volume Resistance]
The volume resistance of the positive electrode was measured using an electrode resistance measurement system (RM2610) manufactured by Hioki E.E. Corporation. The values shown in Table 1 are relative values with the evaluation result of Comparative Example 1 taken as 100, and the smaller the value, the lower the resistance. The boundary between the first region and the second region can also be identified by measuring the volume resistance along the length of the positive electrode. When the volume resistance is measured along the length of the positive electrode, the part where the resistance value changes significantly is the boundary between the first region and the second region.

[Evaluation of rapid charging performance]
This is the combined value of electrolyte permeability and volume resistance. The electrolyte permeability and volume resistance are closely related to the rapid charging performance of the battery, and the smaller this value is, the better the rapid charging performance is.

As shown in Table 1, all of the test cells of the examples have superior rapid charging performance compared to the test cells of Comparative Examples 1 and 3. In addition, all of the test cells of the examples have higher capacity compared to the test cell of Comparative Example 2. The test cells of Comparative Examples 1 and 3 have high capacity but inferior rapid charging performance, while the test cell of Comparative Example 2 has superior 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 first and second regions in the positive electrode mixture layer that have different contents of conductive agent in the positive electrode active material, 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 arranged on the positive electrode core, the positive electrode mixture layer including first regions and second regions arranged alternately in at least one of a length direction and a width direction of the positive electrode core, the first regions and the second regions including a positive electrode active material, a binder, and a conductive agent, and a content of the conductive agent in the second region is greater than a content of the conductive agent in the first region.
Configuration 2: The positive electrode for a secondary battery according to Configuration 1, wherein a ratio (B/A) of a content rate (B) of the conductive agent in the second region to a content rate (A) of the conductive agent in the first region is 1<(B/A)≦3.
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 conductive agent includes at least one selected from carbon black and carbon nanotubes.
Configuration 8: A secondary battery comprising the positive electrode for secondary batteries according to any one of configurations 1 to 7, 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,
the first region and the second region contain a positive electrode active material, a binder, and a conductive agent;
a content of the conductive agent in the second region is greater than a content of the conductive agent in the first region.
The positive electrode for a secondary battery according to claim 1, wherein the ratio (B/A) of the content (B) of the conductive agent in the second region to the content (A) of the conductive agent in the first region is 1<(B/A)≦3.
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 conductive agent includes at least one selected from carbon black and carbon nanotubes.
The positive electrode for a secondary battery according to any one of claims 1 to 7,
A negative electrode;
An electrolyte;
A secondary battery comprising: