JP7508414B2 - Method for manufacturing positive electrode for secondary battery - Google Patents

Description

The present invention relates to a positive electrode for a secondary battery and a method for manufacturing a positive electrode for a secondary battery.

Secondary batteries are widely used as so-called portable power sources for personal computers and mobile terminals, and as power sources for driving vehicles. Among secondary batteries, lithium-ion secondary batteries, which are lightweight and have high energy density, are particularly suitable for use as high-output power sources for driving vehicles such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles. Lithium-ion secondary batteries are secondary batteries that can be charged and discharged by the movement of lithium ions (charge carriers) in the electrolyte between the positive and negative electrodes.

In order to achieve high input and output, such secondary batteries require a positive electrode for the secondary battery (hereinafter sometimes simply referred to as the "positive electrode") that can smoothly insert and remove charge carriers.

Patent Document 1 discloses a precursor of a positive electrode active material for a secondary battery, the precursor including secondary particles having a single-layer structure in which columnar primary particles oriented radially from the center of the particle toward the surface are aggregated, the secondary particles having a shell shape, and the primary particles including a Ni-Co-Mn composite metal hydroxide represented by the following Chemical Formula 1, and a positive electrode active material produced using the precursor.
[Chemical Formula 1]
_{Ni1-(x+y+z) CoxMyMnz (} OH) ₂

JP 2018-536972 A

However, the technology described in Patent Document 1 has a problem in that if the reaction area contributing to the electrochemical reaction is reduced due to overlapping of particles of the positive electrode active material, the insertion and removal of charge carriers at the interface between the positive electrode active material and the electrolyte is hindered, resulting in poor input/output characteristics of the secondary battery.

The present invention has been made to solve these problems, and aims to provide a positive electrode for a secondary battery that improves the input/output characteristics of the secondary battery, and a method for manufacturing the positive electrode for the secondary battery.

The positive electrode for a secondary battery according to one embodiment has a current collector and a positive electrode composite layer formed on the current collector and containing a positive electrode active material and a conductive material. The positive electrode active material is formed in a substantially spherical crown shape in which a part of a hollow active material particle in a substantially spherical shell shape or a substantially elliptical shell shape made of lithium transition metal oxide is cut off by a cut surface, and has a substantially spherical crown-shaped convex curved surface that protrudes in one direction, and a substantially spherical crown-shaped concave curved surface that exists on the opposite side of the one direction and is recessed in one direction.

The method for manufacturing a positive electrode for a secondary battery according to one embodiment includes the steps of forming hollow active material particles in the shape of a roughly spherical shell or roughly elliptical shell made of lithium transition metal oxide, crushing the hollow active material particles to form a roughly spherical crown-shaped positive electrode active material in which a part of the hollow active material particle is cut off by a cut surface, and forming a positive electrode composite layer containing a positive electrode active material and a conductive material on a current collector, and the positive electrode active material has a roughly spherical crown-shaped convex surface that protrudes in one direction and a roughly spherical crown-shaped concave surface that exists on the opposite side of the one direction and is recessed in one direction.

The present invention provides a positive electrode for a secondary battery that improves the input/output characteristics of the secondary battery, and a method for manufacturing the positive electrode for the secondary battery.

FIG. 2 is a diagram showing a positive electrode for a secondary battery according to the first embodiment. 2 is a flowchart showing a method for manufacturing a positive electrode for a secondary battery according to the first embodiment. 2 is a diagram showing an example of a spherical cap-shaped active material particle contained in the positive electrode shown in FIG. 1 . 2 is a diagram showing another example of a spherical active material particle contained in the positive electrode shown in FIG. 1 . FIG. 2 is a diagram showing another example of a spherical active material particle contained in the positive electrode shown in FIG. 1 . FIG. FIG. 1 shows various positive electrodes each including active material particles with different angles θ. 7 is a graph showing the liquid conductivities of the various positive electrodes shown in FIG. 6. 7 is a graph showing the solid conductivity of the various positive electrodes shown in FIG. 6. 7 is a graph showing the effective reaction areas of the various positive electrodes shown in FIG. 6. 1 is a table illustrating examples and comparative examples. FIG. 1 is a diagram illustrating various positive electrodes used in the evaluation of the reaction area. 12 is a graph showing the results of evaluating the reaction area of active material particles for the various positive electrodes shown in FIG. 11 . FIG. 4 is a diagram illustrating the reaction area of hollow active material particles. FIG. 4 is a diagram illustrating the reaction area of a spherical active material particle. FIG. 1 is a diagram illustrating various positive electrodes used in the evaluation of porosity. 16 is a graph showing the results of evaluating the porosity of the various positive electrodes shown in FIG. 15 . FIG. 2 is a diagram showing a positive electrode for a secondary battery of a comparative example.

First embodiment
Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In order to clarify the description, the following description and drawings are appropriately simplified. In the following description, the same or equivalent elements are given the same reference numerals, and duplicate descriptions are omitted.

Here, unless otherwise specified, the "short axis length" and "long axis length" in this embodiment are measured using the surfaces arranged on the outside or inside of a plurality of arbitrarily selected active material particles from image analysis of a three-dimensional model obtained by FIB-SEM measurement. The average value of the shortest diameter passing through the center of curvature of the active material particles on the outside or inside surface of the active material particles can be obtained as the "short axis length". In addition, the average value of the longest diameter passing through the center of curvature of the active material particles on the outside or inside surface of the active material particles can be obtained as the "long axis length". Furthermore, the "long axis length" on the outside surface of the active material particles corresponds to the "diameter (particle size)" of the active material particles.

In addition, unless otherwise specified, the "thickness" in this embodiment refers to the shortest distance measured from multiple positions on the inner surface of the active material particle to the outer surface in a cross section of an arbitrarily selected active material particle through image analysis of a three-dimensional model obtained by FIB-SEM measurement. The average value of the shortest distances measured at multiple positions on the inner surface of the active material particle can be obtained as the "thickness".

For example, when the active material particles are spherical active material particles 20, the surface disposed on the outside is a convex curved surface 21, and the surface disposed on the inside is a concave curved surface 22. When the active material particles are hollow active material particles 120, the surface disposed on the outside is an outer surface 121, and the surface disposed on the inside is an inner surface 122.

Note that FIB-SEM refers to processing a sample with a focused ion beam (FIB) and observing the exposed cross section of the sample with a scanning electron microscope (SEM). One method of processing a sample is, for example, to cut a sample solidified with an appropriate resin at the desired cross section and perform SEM observation while gradually scraping off the cross section.

In the FIB-SEM technique, a sample is subjected to repeated intermittent processing with an FIB and observation with an SEM, and the resulting SEM images are then used to create a three-dimensional model, which makes it possible to visualize the structure of the material, including its internal structure.

The following describes a preferred embodiment of the positive electrode for a secondary battery according to this embodiment, specifically as a positive electrode for a lithium-ion secondary battery. A lithium-ion secondary battery is a secondary battery that uses lithium ions as a charge carrier and achieves charging and discharging by the movement of charge between a positive electrode (positive electrode plate) and a negative electrode (negative electrode plate). Lithium-ion secondary batteries are used as power sources for driving vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHEVs).

First, the configuration of the positive electrode for a secondary battery (positive electrode 1) according to this embodiment will be described with reference to FIG. 1. FIG. 1 is a diagram showing the positive electrode for a secondary battery according to the first embodiment. FIG. 1 shows a three-dimensional model of the positive electrode 1. The positive electrode 1 has a current collector (positive electrode current collector) and a positive electrode composite layer 10 formed on the positive electrode current collector.

The positive electrode current collector is formed in a plate or foil shape and is made of a metal with good electrical conductivity. For example, aluminum, an aluminum alloy, nickel, titanium, stainless steel, etc. can be used as the positive electrode current collector used for the positive electrode 1.

The positive electrode mixture layer 10 has at least a positive electrode active material (spherical active material particles 20) and a conductive material 30. The positive electrode mixture layer 10 may also contain additives such as a dispersant and a binder.

The conductive material 30 is a material for forming a conductive path in the positive electrode composite layer 10. By mixing an appropriate amount of the conductive material 30 into the positive electrode composite layer 10, the electronic conductivity inside the positive electrode 1 can be increased, and the charge/discharge efficiency and input/output characteristics of the battery can be improved. As the conductive material 30, for example, various carbon blacks (e.g., acetylene black, ketjen black) and carbon fibers (e.g., carbon nanotubes, carbon nanofibers) and other carbon materials can be used.

Figure 1 illustrates a positive electrode composite layer 10a using acetylene black (AB) as the conductive material 30, and a positive electrode composite layer 10b using carbon nanotubes (CNT) as the conductive material 30, and shows the distribution of the conductive materials 30 in the positive electrode composite layers 10a and 10b. In this embodiment, when AB is used as the conductive material 30, it is preferable to use AB with an average particle size of 20 to 50 μm. When CNT is used as the conductive material 30, it is preferable to use CNT with an outer diameter of 5 to 30 nm and an aspect ratio of 15 to 300.

Examples of dispersants include polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), polyacrylate, polymethacrylate, polyoxyethylene alkyl ether, polyalkylene polyamine, benzimidazole, etc. Examples of binders that can be used include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyacrylic acid, polyacrylate, etc.

Each positive electrode active material has a particle form and is formed into a roughly spherical crown shape. The positive electrode active material has a roughly spherical crown-shaped convex surface 21 that protrudes in one direction, and a roughly spherical crown-shaped concave surface 22 that is on the opposite side of the one direction and is recessed in one direction. In the following explanation, the positive electrode active material contained in the secondary battery positive electrode (positive electrode 1) according to this embodiment will be referred to as a spherical crown-shaped active material particle 20.

The spherical cap-shaped active material particles 20 contain a lithium transition metal oxide having a layered crystal structure. The lithium transition metal oxide contains one or more specific transition metal elements in addition to Li (lithium). The transition metal element contained in the lithium transition metal oxide is preferably at least one of Ni, Co, and Mn. A suitable example of the lithium transition metal oxide is a lithium transition metal oxide that contains all of Ni, Co, and Mn.

The spherical active material particles 20 may contain one or more additional elements in addition to the transition metal element (i.e., at least one of Ni, Co, and Mn). The additional elements may include any of the elements belonging to Group 1 (alkali metals such as sodium), Group 2 (alkaline earth metals such as magnesium and calcium), Group 4 (transition metals such as titanium and zirconium), Group 6 (transition metals such as chromium and tungsten), Group 8 (transition metals such as iron), Group 13 (metalloid elements such as boron or aluminum), and Group 17 (halogens such as fluorine) of the periodic table.

In a preferred embodiment, the spherical active material particles 20 may have a composition (average composition) represented by the following general formula (1).
Li ₁ + _x Ni _y Co _z Mn _(1-y-z) MA _α MB _β O ₂ ... (1)

In the above formula (1), x may be a real number satisfying 0≦x≦0.2. y may be a real number satisfying 0.1<y<0.6. z may be a real number satisfying 0.1<z<0.6. MA is at least one metal element selected from W, Cr, and Mo, and α is a real number satisfying 0<α≦0.01 (typically 0.0005≦α≦0.01, for example 0.001≦α≦0.01). MB is one or more elements selected from the group consisting of Zr, Mg, Ca, Na, Fe, Zn, Si, Sn, Al, B, and F, and β may be a real number satisfying 0≦β≦0.01. β may be substantially 0 (i.e., an oxide that does not substantially contain MB). In addition, in the chemical formula showing the lithium transition metal oxide with a layered structure, the composition ratio of O (oxygen) is shown as 2 for convenience, but this value should not be interpreted strictly, and some variation in the composition (typically within the range of 1.95 to 2.05) is acceptable.

The spherical crown-shaped active material particle 20 is formed by cutting a part of a hollow active material particle 120 having a roughly spherical shell shape or a roughly elliptical shell shape at a cutting surface 23, into a roughly spherical crown shape. In other words, the hollow active material particle 120, which is the material for the spherical crown-shaped active material particle 20, is composed of a lithium transition metal oxide having the layered crystal structure described above.

Now, an example of the method for manufacturing the above-mentioned positive electrode 1 will be described with reference to FIG. 2. FIG. 2 is a flowchart showing the method for manufacturing a positive electrode for a secondary battery according to the first embodiment. As shown in FIG. 2, the method for manufacturing the positive electrode 1 according to this embodiment includes steps S1 to S6. Note that steps S3 and S5 can be omitted if the quality of the manufactured positive electrode 1 is ensured, and can be replaced by other methods.

Step S1 is a process for forming hollow active material particles 120. Step S2 is a process for crushing the hollow active material particles 120 to form a positive electrode active material (spherical crown-shaped active material particles 20). Step S3 is a process for checking whether the physical properties of the positive electrode active material (spherical crown-shaped active material particles 20) obtained in step S2 are appropriate or not compared with predetermined values. Step S4 is a process for preparing a paste for forming the positive electrode composite layer 10. Step S5 is a process for checking whether the viscosity of the paste prepared in step S4 is appropriate or not compared with a predetermined value. Step S6 is a process for applying the paste onto a positive electrode current collector to form the positive electrode composite layer 10.

Each of the above steps will be described in detail. First, in step S1, hollow active material particles 120 are formed as the material for the spherical active material particles 20. The hollow active material particles 120 have a hollow structure with a shell 123 and a hollow portion 124 formed inside the shell 123, and have a roughly spherical shell shape or a roughly elliptical shell shape. That is, the outer shape of the hollow active material particles 120 is roughly spherical or elliptical (slightly distorted spherical shape, etc.). The hollow active material particles 120 also have an outer surface 121 and an inner surface 122, both of which are formed in a spherical or elliptical spherical shape.

The shell 123 is a secondary particle formed by connecting primary particles of lithium transition metal oxide to form an approximately spherical shell or an approximately elliptical shell. Inside the shell 123, a hollow portion 124 is formed. In the thickness direction of the shell 123, the primary particles may be a single layer or multiple layers. In one preferred embodiment, the hollow active material particle 120 is configured in such a manner that the primary particles are connected in a substantially single layer throughout the entire shell 123.

Here, the primary particle refers to a particle that is considered to be an ultimate particle based on its apparent geometric shape. In the hollow active material particles 120 disclosed herein, the primary particles are typically an aggregate of crystallites of lithium transition metal oxide. The shape of the hollow active material particles 120 can be observed from images obtained by SEM observation.

The diameter (particle size) of the outer surface 121 of the hollow active material particle 120 is, for example, preferably approximately 2 μm or more, and more preferably 3 μm or more. The diameter of the hollow active material particle 120 is preferably 25 μm or less, and more preferably 15 μm or less. From the viewpoint of productivity, in a preferred embodiment, the diameter of the hollow active material particle 120 is 4 μm or more and 6 μm or less. The outer surface 121 is the surface that corresponds to the outline of the hollow active material particle 120.

The thickness of the shell portion 123 is preferably 3.0 μm or less, more preferably 2.5 μm or less, and even more preferably 2.0 μm or less. The smaller the thickness of the shell portion 123, the easier it is for lithium ions to be released from the inside of the shell portion 123 (the center of the thickness) during charging, and the easier it is for lithium ions to be absorbed into the inside of the shell portion 123 during discharging of the lithium ion secondary battery. The lower limit of the thickness of the shell portion 123 is preferably 0.1 μm or more. From the viewpoint of achieving both the effect of reducing internal resistance and durability, in one preferred embodiment, the thickness of the shell portion 123 is 0.8 μm or more and 1.5 μm or less.

The diameter of the inner surface 122 of the hollow active material particle 120 can be calculated by subtracting the thickness of the shell portion 123 from the diameter of the outer surface 121. The inner surface 122 is the surface that corresponds to the outline of the hollow portion 124.

For the outer surface 121 of the hollow active material particle 120, the ratio of the long axis length L1 to the short axis length L2 (long axis length/short axis length, L1/L2) is preferably 1.0 or more and less than 1.8. For the inner surface 122 of the hollow active material particle 120, the ratio of the long axis length to the short axis length (long axis length/short axis length) is preferably 1.0 or more and less than 1.5. When the spherical crown-shaped active material particle 20 is formed using the hollow active material particle 120 configured in such a ratio, the front and back surfaces (convex curved surface 21 and concave curved surface 22) of the spherical crown-shaped active material particle 20 become curved, and the particle becomes bulky.

The method for forming the hollow active material particles 120 includes, for example, a raw hydroxide production process, a mixing process, and a firing process. Each of these processes will be described in detail. However, the method for forming the hollow active material particles 120 is not limited to this.

The raw hydroxide production process is a process in which ammonium ions ( _NH4 ⁺ ) are supplied to an aqueous solution of a transition metal compound to precipitate transition metal hydroxide particles from the aqueous solution. Here, the aqueous solution contains at least one transition metal element that constitutes the lithium transition metal oxide.

The raw hydroxide production process preferably includes a nucleation stage in which transition metal hydroxide is precipitated from an aqueous solution, and a particle growth stage in which transition metal hydroxide particles are grown in a state in which the pH of the aqueous solution is reduced from that in the nucleation stage. In the particle growth stage, the structure of the hollow active material particles 120 (the arrangement distance between primary particles, the particle porosity, etc.) can be changed by adjusting the precipitation rate of the transition metal hydroxide by changing the pH and ammonium ion concentration.

In the mixing step, the transition metal hydroxide particles produced in the raw hydroxide production step are separated from the reaction liquid, washed, filtered, and dried. The transition metal hydroxide thus obtained is mixed with a lithium compound to prepare a mixture. The transition metal hydroxide and lithium compound are preferably mixed as uniformly as possible in a predetermined ratio. In the mixing step, the lithium compound and transition metal hydroxide particles are typically mixed in a quantitative ratio that corresponds to the composition of the hollow active material particles 120, which are the target product.

The firing process is a process in which the mixture is fired to obtain hollow active material particles 120. The firing process is carried out, for example, in an oxidizing atmosphere (for example, in an air atmosphere). The firing temperature is, for example, 700°C or higher and 1100°C or lower. The firing process may also include multiple steps in which firing is carried out at different temperature ranges. After firing, it is preferable to crush the fired product and classify it to adjust the particle size, if necessary.

In this process, the sintering reaction of the primary particles of the lithium transition metal oxide is allowed to proceed. As a result, the primary particles are sintered together to form hollow active material particles 120 in the shape of a roughly spherical shell or roughly elliptical shell, and a hollow portion 124 is formed inside the hollow active material particles 120.

Next, in step S2, the hollow active material particles 120 obtained in step S1 are fed into a grinder, and a shearing force is applied to the hollow active material particles 120 to form spherical crown-shaped active material particles 20 in which a portion of the hollow active material particles 120 is cut off by a cutting surface 23. It is preferable to use a jet mill as the grinder used to grind the hollow active material particles 120. The grinding method may be either a dry method or a wet method.

In the case of grinding using a jet mill, the degree of grinding is adjusted by appropriately setting grinding conditions such as the grinding gas pressure, the supply speed of the hollow active material particles 120, and the number of grinding times. It is preferable that the grinding conditions are set so that the hollow active material particles 120 can be cut at a cut surface 23 along the major axis, which is the longest diameter passing through the center of curvature O of the hollow active material particles 120. In other words, by controlling the grinding conditions, the desired spherical crown-shaped active material particles 20 can be formed.

Here, the spherical crown refers to the side portion of a notch formed by cutting a portion of a sphere or an oval sphere at any plane (cut plane 23 in this embodiment). The shape of the spherical crown-shaped active material particle 20 can be said to be roughly bowl-shaped. The center of curvature O of the hollow active material particle 120 coincides with the center of curvature O of the spherical crown-shaped active material particle 20. In addition, the center of curvature O of the hollow active material particle 120 is the same as the center of curvature O of the outer surface 121 and the center of curvature O of the inner surface 122, and the center of curvature O of the spherical crown-shaped active material particle 20 is the same as the center of curvature O of the convex curved surface 21 and the center of curvature O of the concave curved surface 22.

The grinding conditions are not particularly limited and vary depending on the grinding machine and hollow active material particles 120 used. After grinding, the ground material is classified to remove fine powder, and spherical crown-shaped active material particles 20 having the desired particle size can be obtained. Classification of the ground material may be performed using a jet mill equipped with a classification function, or may be performed using a classifier other than the jet mill.

The spherical crown-shaped active material particles 20 obtained by crushing the hollow active material particles 120 are formed into a substantially spherical crown shape having a predetermined thickness, and have a convex curved surface 21, a concave curved surface 22, and a cut surface 23. The convex curved surface 21 corresponds to the outer surface 121 of the hollow active material particles 120, and a part of the outer surface 121 is cut off by the cut surface 23. The convex curved surface 21 is formed into a substantially spherical crown shape that protrudes in one direction. The concave curved surface 22 corresponds to the inner surface 122 of the hollow active material particles 120, and a part of the inner surface 122 is cut off by the cut surface 23. The concave curved surface 22 is formed into a substantially spherical crown shape that is recessed in the same direction as the protruding direction of the convex curved surface 21. The concave curved surface 22 is present on the opposite side of the protruding direction side of the convex curved surface 21, and the concave curved surface 22 and the convex curved surface 21 face each other.

The spherical crown-shaped active material particle 20 also has a recess 24 that is recessed in a roughly spherical crown shape near the center of the end face toward the convex curved surface 21. The recess 24 corresponds to the hollow portion 124 of the hollow active material particle 120, and is defined by the concave curved surface 22. The cut surface 23 is the portion that connects the leading edge of the convex curved surface 21 and the leading edge of the concave curved surface 22. The cut surface 23 has a roughly circular ring shape that surrounds the opening of the recess 24.

In this way, when the convex curved surface 21 and the concave curved surface 22 are curved, bulky spherical crown-shaped active material particles 20 are formed, and overlapping of the spherical crown-shaped active material particles 20 in the positive electrode mixture layer 10 can be suppressed. As a result, in the positive electrode 1 using the spherical crown-shaped active material particles 20, the reduction in the reaction area caused by the overlapping of the active material particles is suppressed, and gaps that serve as permeation paths for the electrolyte are secured between the spherical crown-shaped active material particles 20. As a result, the input/output characteristics of the secondary battery are improved.

Next, in step S3, the physical properties of the spherical-crown-shaped active material particles 20 obtained in step S2 are measured, and it is confirmed whether the measured values of the physical properties of the spherical-crown-shaped active material particles 20 are greater than a predetermined value. For example, the oil absorption and BET specific surface area can be used as the physical properties of the spherical-crown-shaped active material particles 20. In addition, as the predetermined values in step S3 , the oil absorption and BET specific surface area of the hollow active material particles 120 that are the material for the spherical-crown-shaped active material particles 20 can be used.

In this step, at least one of the oil absorption and the BET specific surface area is measured for the spherical crown-shaped active material particles 20. If the result of the measurement (measured value) of the physical property of the spherical crown-shaped active material particles 20 is greater than the result of the measurement (predetermined value) of the same physical property of the hollow active material particles 120 (step S3; YES), proceed to the next step S4. On the other hand, if the result of the measurement (measured value) of the physical property of the spherical crown-shaped active material particles 20 is equal to or less than the predetermined value (step S3; NO), it is considered that the desired spherical crown-shaped active material particles 20 have not been obtained due to insufficient pulverization or other reasons, and so return to step S2.

Now, with reference to Figures 3 to 5, the structure of the spherical crown-shaped active material particle 20 that is desirable in this embodiment will be described in detail. Figure 3 is a diagram showing an example of a spherical crown-shaped active material particle contained in the positive electrode shown in Figure 1. Figure 4 is a diagram showing another example of a spherical crown-shaped active material particle contained in the positive electrode shown in Figure 1. Figure 5 is a diagram showing another example of a spherical crown-shaped active material particle contained in the positive electrode shown in Figure 1.

The solid lines shown in Figures 3 to 5 are intended to represent the convex curved surface 21 of the spherical crown-shaped active material particle 20. The dashed lines shown in Figures 3 to 5 are intended to represent the outer surface 121 of the hollow active material particle 120 that is the material for the spherical crown-shaped active material particle 20. Figures 3 to 5 show the case where the spherical crown-shaped active material particle 20 and the hollow active material particle 120 are arranged in an overlapping manner so that the entire spherical crown-shaped active material particle 20 coincides with a part of the hollow active material particle 120. The convex curved surface 21 and the outer surface 121 are arranged so that their major axes extend horizontally, and their minor axes extend vertically.

As shown in Figures 3 to 5, the cut surface 23 that divides the hollow active material particle 120 may be a surface that passes through the center of curvature O, or may be a surface located at a position away from the center of curvature O. The cut surface 23 is formed at a position where the angle between the direction from the center of curvature O toward the tip of the cut surface 23 side of the convex curved surface 21 and the major axis of the convex curved surface 21 is angle θ. In this embodiment, it is preferable that the angle θ is within ±15°. In other words, it is preferable that the spherical crown-shaped active material particle 20 has a central angle of 150° or more and 210° or less.

Specifically, when θ = 0°, the cut surface 23 is a surface that passes through the center of curvature O, and the spherical crown-shaped active material particle 20 is formed in a hemispherical shape. When θ = 15°, the cut surface 23 is a surface that is spaced above the center of curvature O, and the spherical crown-shaped active material particle 20 is a solid with a relatively large volume that includes the center of curvature O. When θ = -15°, the cut surface 23 is a surface that is spaced below the center of curvature O, and the spherical crown-shaped active material particle 20 is a solid with a relatively small volume that does not include the center of curvature O.

Furthermore, when the angle θ is within ±15°, the shape of the cut surface 23 may be flat, as shown in Figures 3 and 4, or may be non-flat, such as uneven, as shown in Figure 5. In this way, the shape of the spherical crown-shaped active material particle 20 is determined according to the position and shape of the cut surface 23.

In this embodiment, the proportion of the spherical crown-shaped active material particles 20 formed with an angle θ of ±15° among all the active material particles contained in the positive electrode composite layer 10 is preferably 30% by mass or more and 100% by mass or less, more preferably 50% by mass or more and 100% by mass or less. By being configured in such a proportion, the effect of improving the input/output characteristics of the secondary battery is further enhanced. In addition to the spherical crown-shaped active material particles 20 formed with the angle θ of ±15° or less described above, the positive electrode composite layer 10 may contain other active material particles. The other active material particles are, for example, active material particles with a solid structure, active material particles with a hollow structure, spherical crown-shaped active material particles 20 formed with an angle θ other than ±15°, etc.

Next, in step S4, a paste for forming the positive electrode composite layer 10 is prepared using the spherical-crown-shaped active material particles 20 whose physical properties have been confirmed in step S3. The paste containing the spherical-crown-shaped active material particles 20 is obtained by kneading the spherical-crown-shaped active material particles 20, the conductive material 30, other additives, and a solvent (aqueous solvent, non-aqueous solvent, or a mixture thereof) in a predetermined ratio.

In step S5, the viscosity of the paste obtained in step S4 is measured to check whether the measured viscosity of the paste containing the spherical crown-shaped active material particles 20 is smaller than a predetermined value. The predetermined value in step S5 can be the viscosity of the paste containing the hollow active material particles 120 that are the material for the spherical crown-shaped active material particles 20. The paste containing the hollow active material particles 120 is obtained by kneading the hollow active material particles 120, the conductive material 30, other additives, and the solvent in the same materials and proportions as those for the spherical crown-shaped active material particles 20, except for the type of active material particles.

In this step, the viscosity of the paste containing the spherical-crown-shaped active material particles 20 is measured. If the result of measuring the viscosity of the paste containing the spherical-crown-shaped active material particles 20 (measured value) is smaller than the result of measuring the viscosity of the paste containing the hollow active material particles 120 (predetermined value) (step S5; YES), proceed to the next step S6. On the other hand, if the result of measuring the viscosity of the paste containing the spherical-crown-shaped active material particles 20 (measured value) is equal to or greater than the predetermined value (step S5; NO), it is considered that the desired spherical-crown-shaped active material particles 20 have not been obtained due to insufficient grinding or other reasons, so return to step S2.

In step S6, a paste containing spherical active material particles 20 is applied to the surface of the positive electrode current collector, and the paste is dried to volatilize the solvent, and the dried product is pressed as necessary. This allows the positive electrode composite layer 10 to be formed on the positive electrode current collector.

The amount of the positive electrode mixture layer 10 applied per unit area on the positive electrode current collector is not particularly limited, but from the viewpoint of ensuring a sufficient conductive path, it is preferably 3 mg/ ^cm2 or more, more preferably 5 mg/ ^cm2 or more, and particularly preferably 6 mg/ ^cm2 or more per one side of the positive electrode current collector. The application amount is the application amount converted into the solid content of the paste.

The coating amount per side of the positive electrode current collector is preferably 45 mg/ ^cm2 or less, more preferably 28 mg/ ^cm2 or less, and particularly preferably 15 mg/ ^cm2 or less. The density of the positive electrode mixture layer 10 is not particularly limited, but is preferably 1.0 g/ ^cm3 to 3.8 g/ ^cm3 , more preferably 1.5 g/ ^cm3 to 3.0 g/ ^cm3 , and particularly preferably 1.8 g/ ^cm3 to 2.4 g/ ^cm3 .
By the above-described manufacturing method, the positive electrode for a secondary battery according to this embodiment can be manufactured.

Next, with reference to Figures 6 to 9, the optimal range of angle θ for the spherical active material particles 20 will be explained based on the results of simulating the liquid conductivity, solid conductivity, and effective reaction area using a three-dimensional model of the positive electrode.

Figure 6 shows various positive electrodes each containing active material particles with different angles θ. Figure 7 is a graph showing the liquid conductivity of the various positive electrodes shown in Figure 6. Figure 8 is a graph showing the solid conductivity of the various positive electrodes shown in Figure 6. Figure 9 is a graph showing the effective reaction area of the various positive electrodes shown in Figure 6.

As shown in FIG. 6, when measuring the liquid conductivity, solid conductivity, and effective reaction area by simulation, various positive electrodes were manufactured using various active material particles designed with angles θ of 90°, 15°, 0°, and -15°. Specifically, a paste containing each of the active material particles, a conductive material 30, other additives, and a solvent was applied to a positive electrode current collector, which was then dried and pressed, and the electrolyte was allowed to penetrate into the positive electrode mixture layer to manufacture various positive electrodes. The active material particles with angles θ = -15°, 0°, and 15° are spherical crown-shaped active material particles 20, respectively. The active material particles with angle θ = 90° are hollow active material particles 120.

Furthermore, three-dimensional models of various positive electrodes were obtained by FIB-SEM measurement. Then, image analysis was performed using the three-dimensional models of various positive electrodes, and the liquid conductivity, solid conductivity, and effective reaction area were calculated for each, and the graphs shown in Figures 7 to 9 were created.

The horizontal axis of the graph shown in Figure 7 indicates the angle θ [°], and the vertical axis indicates the liquid conductivity [S/m]. The liquid conductivity is the electrical conductivity of the electrolyte. As shown in Figure 7, it was found that when the angle θ is in the range of -90° to 45°, the smaller the angle θ, the higher the liquid conductivity. On the other hand, when the angle θ is in the range of 45° to 90°, the liquid conductivity tends to decrease as the angle θ increases from 45°, but when the angle θ becomes 90°, the liquid conductivity increases. In other words, it was shown that when the angle θ is 45° or less, the smaller the angle θ, the lower the electrical resistance.

The horizontal axis of the graph shown in FIG. 8 indicates the angle θ [°], and the vertical axis indicates the solid conductivity [S/m]. The solid conductivity is the electrical conductivity of the active material particles. As shown in FIG. 8, it was found that when the angle θ is in the range of -90° to 90°, the larger the angle θ, the higher the solid conductivity. On the other hand, it was found that if the angle θ is too small, the solid conductivity decreases. Since active material particles with a small angle θ are relatively small and divided particles, it was shown that a larger amount of conductive material 30 is required in the positive electrode mixture layer to ensure sufficient electrical conductivity compared to active material particles with a large angle θ.

Next, the horizontal axis of the graph shown in Fig. 9 indicates the angle θ [°], and the vertical axis indicates the effective reaction area [m ² ]. The effective reaction area is the reaction area of the active material particles that actually contributes to the electrochemical reaction at the interface between the active material particles and the electrolyte. As shown in Fig. 9, it was found that when the angle θ is in the range of -15° to 90°, the effective reaction area increases as the angle θ becomes smaller. On the other hand, it was found that when the angle θ is too large, the effective reaction area decreases. In other words, it was shown that the surface area increases in the particles that are divided into relatively small pieces, and therefore the effective reaction area increases.

From the results shown in Figures 7 to 9, it was determined that if the angle θ is within ±15°, a positive electrode 1 with good performance and a good balance between liquid conductivity, solid conductivity, and effective reaction area can be obtained, and the optimal range for the angle θ was determined.

Next, Examples 1 to 3 and Comparative Examples 1 to 7 will be described with reference to Fig. 10. Note that the present invention is not limited to the Examples. Fig. 10 is a table for explaining the Examples and Comparative Examples.
Ten types of positive electrodes having the configuration shown in FIG. 10 were manufactured, and the manufactured positive electrodes, negative electrodes, and two separators were stacked to manufacture an electrode body. This was housed in a battery case having a liquid inlet. Then, an electrolyte was injected from the liquid inlet of the battery case, and the liquid inlet was sealed airtight. In this manner, ten types of evaluation batteries were manufactured. A polyolefin porous film was used as the separator. A nonaqueous electrolyte solution in which lithium hexafluorophosphate (LiPF ₆ ) was dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the electrolyte solution.

In Examples 1 to 3, the positive electrodes 1 were manufactured according to the flow shown in FIG. 2. In the positive electrodes 1 of Examples 1 to 3, spherical crown-shaped active material particles 20 with a particle size of 5 μm and a thickness of 0.8 to 1.5 μm were used as the active material particles of the positive electrode 1. As the material for the spherical crown-shaped active material particles 20, hollow active material particles 120 with a particle size of 5 μm, a thickness of 0.8 to 1.5 μm, a major axis length/minor axis length (L1/L2) of 1.0 to 1.5 on the outer surface 121, and a major axis length/minor axis length of 1.0 to 1.8 on the inner surface 122 were used. The hollow active material particles 120 were pulverized using a jet mill equipped with a classification function, and the pulverized product was classified to obtain spherical crown-shaped active material particles 20 with an angle θ = 0°.

The oil absorption and BET specific surface area of the obtained spherical crown-shaped active material particles 20 were measured. It was confirmed that the measured oil absorption of the spherical crown-shaped active material particles 20 was greater than the predetermined value obtained by measuring the oil absorption of the hollow active material particles 120. It was also confirmed that the measured BET specific surface area of the spherical crown-shaped active material particles 20 was greater than the predetermined value obtained by measuring the BET specific surface area of the hollow active material particles 120.

In the positive electrodes of Comparative Examples 1 to 4, hollow active material particles 120, which are the material of the spherical active material particles 20 used in Examples 1 to 3, were used as the active material particles of the positive electrodes. In the positive electrodes of Comparative Examples 5 to 7, small-diameter active material particles formed to a particle size of 3 μm were used as the active material particles of the positive electrodes. The small-diameter active material particles used here have a solid structure and are secondary particles formed by the aggregation of primary particles.

In Examples 1 to 3 and Comparative Examples 1 to 7, the positive _{electrode active material particles were all made of lithium nickel cobalt manganese composite oxide (NCM) having an average composition represented by LiNi(1-XY)CoXMnYO2 , 0 ≦} X≦0.35, 0≦Y≦0.35.

Various positive electrodes were manufactured as follows. First, active material particles (spherical cap-shaped active material particles 20, hollow active material particles 120, or small particle size active material particles), conductive material 30 (AB or CNT), and other additives were mixed in the mixing ratio shown in FIG. 10, and N-methyl-2-pyrrolidone was added as a solvent and kneaded to prepare a paste. The prepared paste was applied to both sides of aluminum foil, which serves as a positive electrode current collector, to a certain thickness and coating amount, and then dried and pressed to produce 10 types of sheet-shaped positive electrodes. A certain ratio of PVdF was added as a binder to the additives.

Here, the viscosity of the paste containing the spherical crown-shaped active material particles 20 was measured. It was confirmed that the measured viscosity of the paste containing the spherical crown-shaped active material particles 20 was smaller than the predetermined value obtained by measuring the viscosity of the paste containing the hollow active material particles 120. The paste containing the hollow active material particles 120 was made with the same materials and proportions as the spherical crown-shaped active material particles 20, except for the type of active material particles.

The negative electrode was manufactured as follows. A paste for forming the negative electrode composite layer was prepared by kneading particulate natural graphite coated with amorphous carbon as the active material particles of the negative electrode, styrene butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener in ion-exchanged water at a certain mass ratio. The obtained paste was applied to both sides of copper foil as the negative electrode current collector, dried, and pressed to manufacture the negative electrode.

The voltage drop was measured for 10 types of evaluation batteries containing the positive electrodes of Examples 1 to 3 and Comparative Examples 1 to 7. The voltage drop was measured for each of the 10 types of evaluation batteries 10 seconds after the start of discharge when the batteries were discharged from a charged state of SOC (State of Charge) of 60% at a current value of 10 to 40 C in an environment of 25°C. The results of measuring the voltage drop are shown in the results column in Figure 10.

From the results in Figure 10, when the conductive material 30 is AB, comparing Examples 1 and 2 with Comparative Example 1, it was found that in Examples 1 and 2, the amount of voltage drop can be reduced by increasing the amount of AB used. Also, when the conductive material 30 is CNT, comparing Example 3 with Comparative Examples 2 to 7, it was found that, for example, Example 3 can effectively reduce the amount of voltage drop compared to Comparative Examples 2 to 7 even when the amount of CNT used is the same, so there is no need to increase the amount of CNT more than necessary.

The distribution of the conductive material 30 in the positive electrode composite layer 10 will be described with reference to Figs. 1 and 17. Fig. 17 is a diagram showing a comparative example of a positive electrode for a secondary battery. Here, the upper side of Fig. 1 shows a three-dimensional model of the positive electrode 1 according to Example 1, in which AB is used as the conductive material 30. Fig. 17 also shows a three-dimensional model of the positive electrode according to Comparative Example 1, in which AB is used as the conductive material 30. Example 1 and Comparative Example 1 have a common configuration except for the active material particles.

Comparing the distribution of conductive material 30 between Example 1 and Comparative Example 1, it can be seen that in Comparative Example 1, conductive material 30 is not present in the hollow portion 124 of the hollow active material particle 120, and conductive material 30 is unevenly distributed in the gaps formed between the hollow active material particles 120. In contrast, in Example 1, it can be seen that conductive material 30 is distributed so as to be in contact with the entire outer surface, including the convex curved surface 21 and the concave curved surface 22, of the spherical-crown-shaped active material particle 20. Thus, according to this embodiment, the outer surface of the spherical-crown-shaped active material particle 20 can be utilized to the maximum extent, and therefore the effective reaction area can be efficiently secured.

Next, the reason why the results shown in Figure 10 were obtained will be explained based on the results of simulating the reaction area of the active material particles and the porosity of the positive electrode using a three-dimensional model of the positive electrode. Four types of active material particles were prepared to perform these simulations.

The four types of active material particles are spherical crown-shaped active material particles 20, hollow active material particles 120, solid active material particles 220, and plate-shaped active material particles 320. The spherical crown-shaped active material particles 20 have a radius of 2.5 μm (i.e., particle size: 5 μm) and a thickness of 1 μm. The hollow active material particles 120 have a radius of 2.5 μm (i.e., particle size: 5 μm), a thickness of 1 μm, a major axis length/minor axis length (L1/L2) on the outer surface 121 of 1.05, and a major axis length/minor axis length on the inner surface 122 of 1.05. The solid active material particles 220 are approximately spherical secondary particles formed by the aggregation of primary particles, and have a radius of 2.5 μm (i.e., particle size: 5 μm), a major axis length/minor axis length: 1.05. The plate-shaped active material particles 320 are formed in a roughly rectangular plate shape, with a long axis length of 2.5 μm and a short axis length (thickness) of 1 μm. The spherical cap-shaped active material particles 20 used here are obtained by pulverizing hollow active material particles 120 having a radius of 2.5 μm (i.e., particle size: 5 μm), a thickness of 1 μm, a long axis length/short axis length (L1/L2) on the outer surface 121 of 1.05, and a long axis length/short axis length on the inner surface 122 of 1.05.

First, the results of a simulation of the reaction area of active material particles will be described with reference to Figures 11 and 12. Figure 11 is a diagram explaining the various positive electrodes used in the evaluation of the reaction area. Figure 12 is a graph showing the results of evaluating the reaction area of active material particles for the various positive electrodes shown in Figure 11.

As shown in FIG. 11, when measuring the reaction area of the active material particles by simulation, various active material particles were randomly filled in the area of (20 μm) ³ formed by using a 20 μm square case. Specifically, a paste containing each of the active material particles, a conductive material 30, other additives, and a solvent was applied onto a positive electrode current collector, which was then dried and pressed, and the electrolyte was allowed to penetrate into the positive electrode mixture layer. In this way, various positive electrodes formed within the area of (20 μm) ³ were manufactured. The filling rate of each active material particle in each positive electrode was set to 45%.

Example A is a positive electrode 1 containing spherical active material particles 20. Comparative Example A is a positive electrode containing hollow active material particles 120. Reference Example 1A is a positive electrode containing solid active material particles 220. Reference Example 2A is a positive electrode containing plate-shaped active material particles 320.

Furthermore, three-dimensional models of various positive electrodes were obtained by FIB-SEM measurement. Then, image analysis was performed using the three-dimensional models of various positive electrodes, and the theoretical reaction area and effective reaction area were calculated to create the graph shown in Figure 12. Figure 12 shows the results of comparing the reaction areas (theoretical reaction area and effective reaction area) at the interface between the active material particles and the electrolyte in various positive electrodes. The solid line in Figure 12 is an approximation curve calculated from the plotted values of the theoretical reaction area. The dashed line in Figure 12 is an approximation curve calculated using the plotted values of the effective reaction area for Comparative Example A.

As shown in FIG. 12, the theoretical reaction area is as follows:
Reference Example 1A: 5.71×10 ⁻⁹ [m ² ]
Reference Example 2A: 8.53×10 ⁻⁹ [m ² ]
Example A: 11.2×10 ⁻⁹ [m ² ]
Comparative Example A (outer surface 121 and inner surface 122): 10.3×10 ⁻⁹ [m ² ]

Considering the reaction area of each, in the solid active material particle 220 included in Reference Example 1A, only the outer surface excluding the overlapping of particles corresponds to the reaction area, so the reaction area was smaller than in the other examples. In addition, in the plate-shaped active material particle 320 included in Reference Example 2A, due to the pressing performed during production, both sides in the thickness direction of the plate-shaped active material particle 320 become oriented in the in-plane direction of the positive electrode current collector. As a result, when the plate-shaped active material particle 320 was used, the particles overlapped more, and the reaction area was reduced.

Next, Comparative Example A will be described with reference to FIG. 13. FIG. 13 is a diagram for explaining the reaction area of hollow active material particles. In the hollow active material particles 120 included in Comparative Example A, the outer surface 121 and the inner surface 122 of the hollow active material particles 120 excluding the overlapping of particles correspond to the theoretical reaction area. However, when considering the effective reaction area for Comparative Example A, since there is no conductive material 30 in the hollow portion 124, it is not possible to ensure sufficient electronic conductivity in the hollow portion 124.

In this way, when hollow active material particles 120 are used, hollow portions 124 cannot be fully utilized, and the reactivity at the interface between inner surface 122 and the electrolyte solution decreases. Therefore, when the effective reaction area of Comparative Example A is calculated assuming that only outer surface 121 excluding hollow portions 124 corresponds to the effective reaction area, the effective reaction area is 7.22 × 10 ^-9 [m ² ].

In contrast, the spherical crown-shaped active material particles 20 included in Example A can suppress the decrease in the reaction area. Figure 14 is a diagram explaining the reaction area of the spherical crown-shaped active material particles. As shown in Figure 14, the convex curved surface 21, the concave curved surface 22, and the cut surface 23 of the spherical crown-shaped active material particles 20, excluding the overlapping of the particles, correspond to the theoretical reaction area. Furthermore, in the spherical crown-shaped active material particles 20, the bulkiness of the particles prevents the particles from overlapping. Therefore, when the spherical crown-shaped active material particles 20 are used, the decrease in the reaction area caused by the overlapping of the particles is suppressed compared to, for example, the plate-shaped active material particles 320. In addition, the concave curved surface 22 corresponding to the hollow portion 124 of the hollow active material particles 120 can be effectively used as a reaction field without being excluded from the reaction area.

Next, the results of a simulation of the porosity of the positive electrode will be described with reference to Figures 15 and 16. Figure 15 is a diagram explaining the various positive electrodes used in the evaluation of the porosity. Figure 16 is a graph showing the results of evaluating the porosity of the various positive electrodes shown in Figure 15.

As shown in FIG. 15, when the porosity of the positive electrode was measured by simulation, various active material particles were randomly filled in the area of (16 μm) ³ formed by using a 16 μm square case. Specifically, a paste containing each of the active material particles, a conductive material 30, other additives, and a solvent was applied to a positive electrode current collector, and after drying, the electrolyte was allowed to penetrate into the positive electrode mixture layer. In this way, various positive electrodes formed in the area of (16 μm) ³ were manufactured. The active material particles in each positive electrode were filled from above in the vertical direction in a random manner.

Example B is a positive electrode 1 containing spherical active material particles 20. Comparative Example B is a positive electrode containing hollow active material particles 120. Reference Example 1B is a positive electrode containing solid active material particles 220. Reference Example 2B is a positive electrode containing plate-shaped active material particles 320.

Furthermore, three-dimensional models of various positive electrodes were obtained by FIB-SEM measurement. Then, image analysis was performed using the three-dimensional models of various positive electrodes, and the theoretical porosity and effective porosity were calculated to create the graph shown in FIG. 16. FIG. 16 shows the results of comparing the porosity (theoretical porosity and effective porosity) formed between active material particles in various positive electrodes. The solid line shown in FIG. 16 is an approximation curve calculated from the plotted values of the theoretical porosity. The dashed line shown in FIG. 16 is an approximation curve calculated using the plotted values of the effective porosity for Comparative Example B. The porosity was calculated by calculating the ratio of the volume occupied by the active material particles other than the active material particles from the ratio of the volume of the active material particles occupied in the region of (16 μm) ³ .

As shown in FIG. 16, the theoretical porosity is as follows:
Reference Example 1B: 42.0 [%]
Reference Example 2B: 51.0 [%]
Example B: 53.8%
Comparative Example B (including hollow portion 124): 54.5%

Considering the porosity of each, in Reference Example 1B, the solid active material particles 220 are roughly spherical, so the filling amount is large and the porosity is lower than in the other examples. Also, in Reference Example 2B, the plate-like active material particles 320 that are filled in by chance are randomly oriented, so gaps are easily formed between the particles, and the porosity is improved compared to Reference Example 1B.

Next, in Comparative Example B, the gaps between the particles and the hollow portions 124 of the hollow active material particles 120 correspond to the theoretical porosity, so the theoretical porosity is larger than in the other examples. However, when considering the effective porosity of Comparative Example B, as in the case of the reaction area, the hollow portions 124 cannot be fully utilized because there is no conductive material 30 in them, and the reactivity at the interface between the inner surface 122 and the electrolyte solution decreases. Therefore, if the effective porosity of Comparative Example B is calculated assuming that only the gaps between the particles, excluding the hollow portions 124, correspond to the effective porosity, the effective porosity is 42.0%.

In contrast, in Example B, the spherical crown-shaped active material particles 20, which are filled by chance, are randomly oriented, so that voids are likely to form between the particles. In addition, because the spherical crown-shaped active material particles 20 have a curved shape, the void ratio is further improved compared to the plate-shaped active material particles 320. Therefore, in Example B, sufficient voids can be secured in the positive electrode mixture layer 10, and the permeability of the electrolyte to the positive electrode mixture layer 10 is improved. Therefore, by using the positive electrode 1 of Example B, lithium ions can be smoothly diffused and moved in the positive electrode mixture layer 10, and it is believed that the input/output characteristics of the secondary battery are improved.

In order to improve the input/output characteristics of a secondary battery, it is possible to consider a method of increasing the reaction area by making the active material particles finer, but if the particle size of the active material particles is too small, problems such as reduced productivity and reduced battery characteristics may occur. On the other hand, the spherical crown-shaped active material particles 20 have a certain particle size, so that the positive electrode 1 can be easily manufactured while suppressing the decrease in productivity caused by the particle size being too small.

As described above, the positive electrode for a secondary battery according to this embodiment includes a spherical crown-shaped active material particle 20 formed in a substantially spherical crown shape by cutting a portion of a hollow active material particle 120 in the shape of a substantially spherical shell or a substantially elliptical shell made of a lithium transition metal oxide at a cutting surface 23. This spherical crown-shaped active material particle 20 has a substantially spherical crown-shaped convex curved surface 21 that protrudes in one direction, and a substantially spherical crown-shaped concave curved surface 22 that exists on the opposite side of the one direction and is recessed in one direction.

With this configuration, the entire outer surface of the spherical crown-shaped active material particles 20, which are the positive electrode active material, can be used as an effective reaction field that can contribute to the electrochemical reaction. Furthermore, because the spherical crown-shaped active material particles 20 have a curved shape, the overlapping portions of the spherical crown-shaped active material particles 20 in the positive electrode mixture layer 10 are reduced. This suppresses the reduction in reaction area caused by the overlapping of particles. Furthermore, because gaps that serve as permeation paths for the electrolyte are secured between the spherical crown-shaped active material particles 20, an increase in internal resistance is suppressed. Therefore, according to this embodiment, a secondary battery with high input/output characteristics can be provided.

In addition, in the positive electrode for a secondary battery according to this embodiment, the combination of the spherical crown-shaped active material particles 20 and CNTs makes it possible to ensure sufficient electronic conductivity in the positive electrode composite layer 10 even if the amount of CNT used is reduced, so that the proportion of the spherical crown-shaped active material particles 20 can be relatively increased. Therefore, according to this embodiment, a secondary battery with a high energy density can be provided.

In addition, it is preferable that the angle θ between the direction from the center of curvature O of the convex curved surface 21 toward the tip of the convex curved surface 21 and the major axis of the convex curved surface 21 of the spherical crown-shaped active material particle 20 is within ±15°. With this configuration, it is possible to obtain a positive electrode 1 with good electronic conductivity and excellent balance of performance with a secured reaction field.

In addition, for the inner surface 122 of the hollow active material particle 120 that is the material for the spherical crown-shaped active material particle 20, the ratio of the major axis length to the minor axis length (L1/L2) is preferably 1.0 or more and less than 1.5. In addition, for the outer surface 121 of the hollow active material particle 120 that is the material for the spherical crown-shaped active material particle 20, the ratio of the major axis length to the minor axis length (L1/L2) is preferably 1.0 or more and less than 1.8. When the spherical crown-shaped active material particle 20 is formed using the hollow active material particle 120 configured in this manner, the convex curved surface 21 and the concave curved surface 22 of the spherical crown-shaped active material particle 20 each have a curved shape, and the particle becomes bulky. This makes it possible to suppress overlapping of the spherical crown-shaped active material particles 20 in the positive electrode mixture layer 10.

Furthermore, according to the method for producing a positive electrode for a secondary battery according to this embodiment, by controlling the grinding conditions when grinding the hollow active material particles 120, a positive electrode for a secondary battery having the above-mentioned effects can be produced.

In addition, by appropriately measuring the physical properties of the spherical crown-shaped active material particles 20 obtained by pulverization, it is possible to adjust the pulverization conditions while checking whether the hollow active material particles 120 are properly pulverized. Furthermore, by appropriately measuring the viscosity of the paste containing the spherical crown-shaped active material particles 20, it is possible to adjust the pulverization conditions while checking whether the hollow active material particles 120 are properly pulverized. With this configuration, it is possible to reliably obtain the desired spherical crown-shaped active material particles 20, and the quality of the positive electrode for secondary batteries manufactured using the spherical crown-shaped active material particles 20 is improved.

The present invention is not limited to the above embodiment, and can be modified as appropriate without departing from the spirit of the invention. For example, in the above embodiment, the hollow active material particles 120 are pulverized using a jet mill, but this is not limiting and other pulverization methods may be used. Examples of other pulverization methods that can be used include a ball mill, a bead mill, and a rod mill.

It is also known that the relationship of the oil absorption is hollow active material particles 120 < plate-shaped active material particles 320 < fine powder active material particles < spherical crown-shaped active material particles 20. Here, the fine powder active material particles are active material particles obtained by, for example, grinding the hollow active material particles 120 with a grinder to a finer size than the spherical crown-shaped active material particles 20. Therefore, the results of measuring the oil absorption of the plate-shaped active material particles 320 or the fine powder active material particles may be used as the predetermined value of the oil absorption. Furthermore, it is known that the relationship of the viscosity of the paste is spherical crown-shaped active material particles 20 < hollow active material particles 120 < plate-shaped active material particles 320 depending on the active material particles contained in the paste. Therefore, the results of measuring the viscosity of the paste containing the plate-shaped active material particles 320 may be used as the predetermined value of the viscosity.

1 Positive electrode 10, 10a, 10b Positive electrode mixture layer 20 Spherical crown-shaped active material particle 21 Convex curved surface 22 Concave curved surface 23 Cut surface 24 Concave portion 30 Conductive material 120 Hollow active material particle 121 Outer surface 122 Inner surface 123 Shell portion 124 Hollow portion 220 Solid active material particle 320 Plate-shaped active material particle O Center of curvature θ Angle

Claims (7)

forming hollow active material particles having a substantially spherical or elliptical shell shape composed of a lithium transition metal oxide;
a step of pulverizing the hollow active material particles to form a positive electrode active material having a substantially crown-shaped hollow active material particle formed by cutting a portion of the hollow active material particle along a cut surface;
forming a positive electrode mixture layer containing the positive electrode active material and a conductive material on a current collector;
having
The positive electrode active material has a generally spherical crown-shaped convex surface protruding in one direction, and a generally spherical crown-shaped concave surface present on the opposite side to the one direction and recessed in the one direction. In the step of forming the positive electrode active material,
2. The method for producing a positive electrode for a secondary battery according to claim 1, further comprising the step of forming the positive electrode composite layer when a result of measuring at least one of an oil absorption amount and a BET specific surface area of the positive electrode active material is larger than a result of measuring the hollow active material particles. The step of forming the positive electrode mixture layer includes:
The method includes preparing a paste containing the positive electrode active material, the conductive material, and a solvent in a predetermined ratio,
3. The method for producing a positive electrode for a secondary battery according to claim 1, wherein when a result of measuring a viscosity of the paste is smaller than a result of measuring a viscosity of a paste containing the hollow active material particles, the conductive material, and the solvent in the predetermined ratio , the paste containing the positive electrode active material, the conductive material , and the solvent is applied onto the current collector to form the positive electrode composite layer. The method for producing a positive electrode for a secondary battery according to claim 1 , wherein the conductive material is a carbon nanotube. 5. The method for producing a positive electrode for a secondary battery according to claim 1, wherein an angle between a direction from a center of curvature of the convex curved surface toward a tip of the convex curved surface on the cut surface side and a major axis of the convex curved surface passing through the center of curvature of the convex curved surface is within ±15°. the ratio of the length of the major axis to the length of the minor axis passing through the center of curvature of the inner surface of the hollow active material particle is 1.0 or more and less than 1.5;
The method for producing a positive electrode for a secondary battery according to claim 1 , wherein the concave curved surface is formed into a substantially spherical crown shape cut by the cut surface along the major axis of the inner surface. the ratio of the length of the major axis to the length of the minor axis passing through the center of curvature of the outer surface of the hollow active material particle is 1.0 or more and less than 1.8;
The method for producing a positive electrode for a secondary battery according to claim 1 , wherein the convex curved surface is formed in a substantially spherical crown shape cut by the cut surface along the major axis of the outer surface.

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