JP2023000753A - Positive electrode for secondary battery and method for manufacturing positive electrode for secondary battery - Google Patents

Abstract

To provide a positive electrode for a secondary battery and a method for manufacturing the positive electrode for the secondary battery which achieve improved input/output characteristics of the secondary battery.SOLUTION: A positive electrode for a secondary batteries includes a current collector, and a positive electrode mixture layer 10 formed on the current collector and containing a positive electrode active material and a conductive material 30. The positive electrode active material has a convex curved surface 21 that is formed into a substantially spherical cap shape by cutting off a part of substantially spherical shell-shaped or substantially elliptical spherical shell-shaped hollow active material particles composed of lithium transition metal oxide along a cutting surface 23, the convex curved surface protruding in one direction; and a substantially spherical cap-shaped concave curved surface 22 that exists on the opposite side of the one direction side and is depressed in the one direction.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a positive electrode for secondary batteries and a method for manufacturing a positive electrode for secondary batteries.

Secondary batteries are widely used as so-called portable power sources for personal computers, mobile terminals, and the like, and as power sources for driving vehicles. Among secondary batteries, lithium-ion secondary batteries, which are lightweight and provide 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. A lithium ion secondary battery is a secondary battery that can be charged and discharged by moving lithium ions (charge carriers) in an electrolyte between a positive electrode and a negative electrode.

In such a secondary battery, in order to achieve high input/output, a positive electrode for a secondary battery (hereinafter sometimes simply referred to as a "positive electrode") capable of smoothly inserting and removing charge carriers is provided. Desired.

Patent Document 1 discloses a secondary particle having a single layer structure in which columnar primary particles radially oriented from the center of the particle toward the surface are agglomerated, and the secondary particle has a shell shape. , the primary particles include a Ni-Co-Mn composite metal hydroxide represented by Chemical Formula 1 below, a precursor of a cathode active material for a secondary battery, and a cathode active material prepared using the same.
[Chemical Formula 1]
Ni _1-(x+y+z) Co _x MyMn _z (OH) ₂

Japanese translation of PCT publication No. 2018-536972

However, in the technique described in Patent Document 1, when the reaction area that contributes to the electrochemical reaction is reduced due to, for example, overlapping of particles of the positive electrode active material, charge carriers are intercalated and desorbed at the interface between the positive electrode active material and the electrolyte. is hindered, and the input/output characteristics of the secondary battery do not become good.

The present invention was made to solve such problems, and an object of the present invention is 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. and

A positive electrode for a secondary battery according to one embodiment includes a current collector, and a positive electrode mixture layer formed on the current collector and containing a positive electrode active material and a conductive material, wherein the positive electrode active material is lithium. A substantially spherical-shell-shaped or substantially elliptical-spherical-shell-shaped hollow active material particle composed of a transition metal oxide is formed into a substantially spherical crown shape by cutting a part of the hollow active material particle by a cut surface, and a substantially spherical crown-shaped convex protrudes in one direction. It has a curved surface and a substantially spherical crown-shaped concave curved surface that exists on the opposite side of the one-way side and is depressed in one direction.

A method for manufacturing a positive electrode for a secondary battery according to one embodiment comprises the steps of: forming substantially spherical shell-shaped or substantially elliptical shell-shaped hollow active material particles made of a lithium transition metal oxide; pulverizing to form a positive electrode active material in which a part of the hollow active material particles are cut off by the cut surface to form a substantially spherical crown shape; and forming a material layer, wherein the positive electrode active material has a substantially spherical crown-shaped convex curved surface that protrudes in one direction and a substantially spherical crown-shaped recess that exists on the opposite side of the one direction side and is depressed in one direction. a curved surface;

ADVANTAGE OF THE INVENTION By this invention, the manufacturing method of the positive electrode for secondary batteries which improves the input-output characteristic of a secondary battery, and the positive electrode for secondary batteries can be provided.

1 is a diagram showing a positive electrode for a secondary battery according to Embodiment 1; FIG. 4 is a flow chart showing a method for manufacturing a positive electrode for a secondary battery according to Embodiment 1. FIG. 2 is a diagram showing an example of spherical crown-shaped active material particles contained in the positive electrode shown in FIG. 1. FIG. 2 is a diagram showing another example of spherical crown-shaped active material particles contained in the positive electrode shown in FIG. 1. FIG. 2 is a diagram showing another example of spherical crown-shaped active material particles contained in the positive electrode shown in FIG. 1. FIG. FIG. 4 shows various positive electrodes each containing active material particles with different angles θ; FIG. 7 is a graph showing the liquid conductivities of various positive electrodes shown in FIG. 6. FIG. 7 is a graph showing the solid conductivity of various positive electrodes shown in FIG. 6; 7 is a graph showing the effective reaction area of various positive electrodes shown in FIG. 6; It is a table|surface explaining an Example and a comparative example. It is a figure explaining various positive electrodes used for evaluation of a reaction area. FIG. 12 is a graph showing the results of evaluating the reaction area of active material particles for various positive electrodes shown in FIG. 11. FIG. It is a figure explaining the reaction area of a hollow active material particle. FIG. 4 is a diagram for explaining the reaction area of spherical crown-shaped active material particles; It is a figure explaining various positive electrodes used for porosity evaluation. 16 is a graph showing the results of evaluating the porosity of various positive electrodes shown in FIG. 15. FIG. It is a figure which shows the positive electrode for secondary batteries of a comparative example.

Embodiment 1
BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings. Also, for clarity of explanation, the following description and drawings are simplified as appropriate. In the following description, the same or equivalent elements are denoted by the same reference numerals, and overlapping descriptions are omitted.

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

In addition, unless otherwise specified, the “thickness” in the present embodiment is arbitrarily selected from image analysis of a three-dimensional model obtained by FIB-SEM measurement, in the cross section of the active material particle. The shortest distances from a plurality of positions on the surface arranged on the outside to the surface arranged on the outside are measured. An average value of the shortest distances measured at a plurality of 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 crown-shaped active material particles 20 , the outer surface is the convex curved surface 21 and the inner surface is the concave curved surface 22 . When the active material particles are hollow active material particles 120 , the outer surface is the outer surface 121 and the inner surface is the inner surface 122 .

Note that FIB-SEM means processing a sample with a focused ion beam (FIB) and observing an exposed cross section of the sample with a scanning electron microscope (SEM). do. As a method for processing the sample, for example, a sample hardened with an appropriate resin is cut at a desired cross section, and SEM observation is performed while gradually scraping the cross section.

In the method using FIB-SEM, intermittent processing by FIB and observation by SEM are repeated for the sample, and the internal structure is obtained by obtaining a three-dimensional model constructed three-dimensionally from the obtained SEM image. It is possible to visualize the structure of substances including

Hereinafter, as one preferred embodiment of the positive electrode for a secondary battery according to the present embodiment, a positive electrode for a lithium ion secondary battery will be specifically described. A lithium-ion secondary battery is a secondary battery that utilizes lithium ions as charge carriers and is charged and discharged by charge transfer between a positive electrode (positive electrode plate) and a negative electrode (negative electrode plate). Lithium-ion secondary batteries are used, for example, as power sources for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHEV).

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

The positive electrode current collector is formed in a plate-like or foil-like shape, and is made of a highly conductive metal. For the positive electrode current collector used for the positive electrode 1, for example, aluminum, an aluminum alloy, nickel, titanium, stainless steel, or the like can be used.

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

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

FIG. 1 illustrates a positive electrode mixture layer 10a using acetylene black (AB) as the conductive material 30 and a positive electrode mixture layer 10b using carbon nanotubes (CNT) as the conductive material 30, respectively. The distribution of each conductive material 30 in the material layers 10a, 10b is illustrated. 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. Also, when CNT is used as the conductive material 30 . It is preferable to use CNTs having an outer diameter of 5 to 30 nm and an aspect ratio of 15 to 300.

Dispersants include, for example, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), polyacrylates, polymethacrylates, polyoxyethylene alkyl ethers, polyalkylenepolyamines, and benzimidazole. Examples of binders that can be used include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyacrylic acid, and polyacrylate.

Each positive electrode active material has a particle form and is formed in a substantially spherical crown shape. Moreover, the positive electrode active material 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 side and is recessed in one direction. In the following description, the positive electrode active material contained in the secondary battery positive electrode (positive electrode 1) according to the present embodiment is referred to as spherical crown-shaped active material particles 20 for explanation.

Spherical crown-shaped active material particles 20 contain a lithium transition metal oxide having a layered crystal structure. Lithium transition metal oxides contain one or more predetermined 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 containing all of Ni, Co and Mn.

The spherical crown-shaped active material particles 20 may additionally contain one or more elements in addition to the transition metal element (that is, at least one of Ni, Co and Mn). Additional elements include group 1 of the periodic table (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 (chromium , transition metals such as tungsten), Group 8 (transition metals such as iron), Group 13 (metals such as boron or aluminum which are metalloid elements) and Group 17 (halogens such as fluorine) It can contain elements.

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

In the above formula (1), x may be a real number that satisfies 0≦x≦0.2. y can be a real number that satisfies 0.1<y<0.6. z can 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 0<α≤0.01 (typically 0.0005≤α≤0.01, for example 0.001≤ is a real number that satisfies 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 is 0≦β≦0. It can be a real number that satisfies 01. β may be substantially 0 (ie, an oxide substantially free of MB). In the chemical formula showing the layered structure lithium transition metal oxide, the composition ratio of O (oxygen) is indicated as 2 for convenience, but this numerical value should not be interpreted strictly, and some variation in the composition ( typically included in the range of 1.95 or more and 2.05 or less).

The spherical crown-shaped active material particles 20 are formed in a substantially spherical crown shape by cutting a part of the substantially spherical shell-shaped or substantially elliptical spherical shell-shaped hollow active material particles 120 along the cutting surface 23 . That is, the hollow active material particles 120, which are the material of the spherical crown-shaped active material particles 20, are composed of the lithium transition metal oxide having the layered crystal structure described above.

Therefore, an example of the method for manufacturing the positive electrode 1 will be described with reference to FIG. FIG. 2 is a flow chart showing a method for manufacturing a positive electrode for a secondary battery according to Embodiment 1. FIG. As shown in FIG. 2, the method for manufacturing the positive electrode 1 according to this embodiment includes steps S1 to S6. The steps S3 and S5 can be omitted if the quality of the positive electrode 1 to be manufactured is ensured, and can be replaced by other methods.

Step S<b>1 is a step of forming hollow active material particles 120 . Step S2 is a step of pulverizing the hollow active material particles 120 to form the positive electrode active material (spherical crown-shaped active material particles 20). Step S3 is a step of comparing physical properties of the positive electrode active material (spherical crown-shaped active material particles 20) obtained in step S2 with predetermined values to confirm whether or not they are appropriate values. Step S<b>4 is a step of preparing a paste for forming the positive electrode mixture layer 10 . Step S5 is a step of confirming whether or not the viscosity of the paste prepared in step S4 is an appropriate value compared with a predetermined value. Step S6 is a step of forming the positive electrode mixture layer 10 by applying the paste onto the positive electrode current collector.

Each of the above steps will be described in detail. First, in step S<b>1 , hollow active material particles 120 as materials for spherical crown-shaped active material particles 20 are formed. The hollow active material particles 120 have a particle form with a hollow structure having a shell portion 123 and a hollow portion 124 formed inside the shell portion 123, and have a substantially spherical shell shape or a substantially elliptical shell shape. That is, the outer shape of the hollow active material particles 120 is generally spherical or oval (slightly distorted spherical, etc.). In addition, the hollow active material particles 120 have an outer surface 121 and an inner surface 122 which are both spherical or ellipsoidal.

The shell portion 123 is a secondary particle formed by connecting primary particles of a lithium transition metal oxide to form a substantially spherical shell or a substantially elliptical spherical shell. A hollow portion 124 is formed inside the shell portion 123 . In the thickness direction of the shell portion 123, the primary particles may have a single layer or multiple layers. Hollow active material particles 120 according to a preferred embodiment are configured in a form in which primary particles are substantially continuous in a single layer over the entire shell portion 123 .

Here, primary particles refer to particles that are considered ultimate particles judging from their apparent geometrical form. In the hollow active material particles 120 disclosed herein, the primary particles are typically aggregates of lithium transition metal oxide crystallites. Observation of the shape of the hollow active material particles 120 can be performed using an image obtained by SEM observation.

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

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

The diameter of inner surface 122 of hollow active material particle 120 can be obtained by subtracting the thickness of shell portion 123 from the diameter of outer surface 121 . In addition, the inner surface 122 is a surface corresponding to the outline of the hollow portion 124 .

Further, regarding the outer surface 121 of the hollow active material particle 120, the ratio of the length L1 of the major axis to the length L2 of the minor axis (length of the major axis/length of the minor axis, L1/L2) is 1.0 or more. It is preferably less than 1.8. In addition, regarding the inner surface 122 of the hollow active material particle 120, the ratio of the length of the long axis to the length of the short axis (length of the long axis/length of the short axis) is 1.0 or more and less than 1.5. is preferred. When the spherical crown-shaped active material particles 20 are formed using the hollow active material particles 120 having such a ratio, the front and back surfaces (the convex curved surface 21 and the concave curved surface 22) of the spherical crown-shaped active material particles 20 are curved. the particles become bulky.

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

The raw material hydroxide producing step is a step of supplying an ammonium ion (NH ₄ ⁺ ) to an aqueous solution of a transition metal compound to deposit 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 material hydroxide generation step includes a nucleation step of precipitating a transition metal hydroxide from an aqueous solution, and a particle growth step of growing transition metal hydroxide particles in a state where the pH of the aqueous solution is lower than that of the nucleation step. and preferably include In the particle growth stage, the precipitation rate of the transition metal hydroxide is adjusted by changing the pH and the ammonium ion concentration. ) can be changed.

In the mixing step, the transition metal hydroxide particles produced in the raw material hydroxide producing step are separated from the reaction solution, washed, filtered and dried. A mixture is prepared by mixing the transition metal hydroxide and the lithium compound thus obtained. The transition metal hydroxide and the lithium compound are preferably mixed at a predetermined ratio as uniformly as possible. In the mixing step, typically, the lithium compound and the transition metal hydroxide particles are mixed in an amount ratio corresponding to the composition of the target hollow active material particles 120 .

The firing step is a step of firing the mixture to obtain hollow active material particles 120 . The firing process is performed, 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. Also, the firing step may include multiple steps of firing at different temperature ranges. After the firing, it is preferable to adjust the particle size by classifying the fired material pulverized, if necessary.

In this step, the sintering reaction of the primary particles of the lithium transition metal oxide proceeds. As a result, hollow active material particles 120 having a substantially spherical shell shape or a substantially ellipsoidal shell shape, in which the primary particles are sintered and connected to each other, are formed, and a hollow portion 124 is formed inside the hollow active material particles 120 . be.

Subsequently, in step S2, the hollow active material particles 120 obtained in step S1 are put into a pulverizer, and a shearing force is applied to the hollow active material particles 120, thereby partially cutting the hollow active material particles 120. Spherical crown-shaped active material particles 20 cut by faces 23 are formed. A jet mill is preferably used as a pulverizer used for pulverizing the hollow active material particles 120 . The pulverization method may be a dry method or a wet method.

In pulverization by a jet mill, the degree of pulverization is adjusted by appropriately setting pulverization conditions such as the pulverization gas pressure, the supply speed of the hollow active material particles 120, and the number of pulverizations. Pulverization conditions are preferably set so that hollow active material particles 120 can be cut at cutting plane 23 along the long axis, which is the longest diameter passing through center of curvature O of hollow active material particles 120 . That is, the desired spherical crown-shaped active material particles 20 can be formed by controlling the pulverization conditions.

Here, a spherical crown is a side portion of a sphere cut off from a portion of a sphere or an elliptical sphere along an arbitrary plane (the cutting plane 23 in this embodiment). The shape of the spherical crown-shaped active material particles 20 can also be said to be substantially bowl-shaped. The center of curvature O of the hollow active material particles 120 coincides with the center of curvature O of the spherical crown-shaped active material particles 20 . Further, the center of curvature O of the hollow active material particles 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 particles 20 is the same as the curvature center O of the convex surface It is assumed that the center O is the same as the center of curvature O of the concave curved surface 22 .

Pulverization conditions are not particularly limited, and vary depending on the pulverizer and hollow active material particles 120 used. After pulverization, the pulverized material is classified to remove fine powder, thereby obtaining spherical crown-shaped active material particles 20 having a desired particle size. Classification of the pulverized material may be performed using a jet mill having a classifying function, or may be performed using a classifier other than the jet mill.

Spherical crown-shaped active material particles 20 obtained by pulverizing hollow active material particles 120 are formed in a substantially spherical crown shape with a predetermined thickness, and have convex surfaces 21 , concave surfaces 22 , and cut surfaces 23 . Convex curved surface 21 corresponds to outer surface 121 of hollow active material particle 120 , and a portion of outer surface 121 is cut off by cut surface 23 . The convex curved surface 21 is formed in a substantially spherical crown shape protruding in one direction. Concave curved surface 22 corresponds to inner surface 122 of hollow active material particle 120 , and part of inner surface 122 is cut off by cutting surface 23 . The concave curved surface 22 is formed in a substantially spherical crown shape recessed in the same direction as the projecting direction of the convex curved surface 21 . The concave curved surface 22 exists on the opposite side of the projecting direction side of the convex curved surface 21, and the concave curved surface 22 and the convex curved surface 21 face each other.

Also, in the spherical crown-shaped active material particles 20 , a concave portion 24 is formed in which the vicinity of the center of the end surface of the spherical crown-shaped active material particle 20 is depressed in a substantially spherical crown shape toward the convex curved surface 21 . Concave portion 24 corresponds to hollow portion 124 of hollow active material particle 120 and is defined by concave curved surface 22 . The cut surface 23 is a portion that connects the tip edge of the convex curved surface 21 and the tip edge of the concave curved surface 22 . The cut surface 23 has a substantially annular shape surrounding the opening of the recess 24 .

When the convex curved surface 21 and the concave curved surface 22 are curved in this way, the spherical crown-shaped active material particles 20 are formed with high bulk. can be suppressed. As a result, in the positive electrode 1 using the spherical crown-shaped active material particles 20, the decrease in the reaction area due to the overlapping of the active material particles is suppressed, and the gap between the spherical crown-shaped active material particles 20 becomes a permeation path for the electrolytic solution. A void is ensured. As a result, the input/output characteristics of the secondary battery are improved.

Subsequently, 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 or not the measured value of the physical properties of the spherical crown-shaped active material particles 20 is greater than a predetermined value. . As physical properties of the spherical crown-shaped active material particles 20, for example, oil absorption and BET specific surface area can be used. As the predetermined values in step S4, the oil absorption amount and the BET specific surface area of the hollow active material particles 120, which are the material of 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 of the spherical crown-shaped active material particles 20 is measured. Then, when the result (measured value) of the measurement of the physical properties of the spherical crown-shaped active material particles 20 is larger than the result (predetermined value) of the measurement of the same physical properties of the hollow active material particles 120 (step S3; YES), the following to step S4. On the other hand, if the result (measurement value) of the measurement of the physical properties of the spherical crown-shaped active material particles 20 is equal to or less than the predetermined value (step S3; NO), the desired spherical crown-shaped active material is not produced due to reasons such as insufficient pulverization. Since it is considered that the particles 20 have not been obtained, the process returns to step S2.

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

3 to 5 schematically represent the convex surface 21 of the spherical crown-shaped active material particle 20. As shown in FIG. 3 to 5 schematically represent the outer surface 121 of the hollow active material particle 120 that is the material of the spherical crown-shaped active material particle 20. As shown in FIG. 3 to 5 show cases where spherical crown-shaped active material particles 20 and hollow active material particles 120 are arranged so that all of spherical crown-shaped active material particles 20 coincide with part of hollow active material particles 120. FIG. ing. Long axes of the convex curved surface 21 and the outer surface 121 extend in the horizontal direction, and short axes of the convex curved surface 21 and the outer surface 121 extend in the vertical direction.

As shown in FIGS. 3 to 5, the cutting plane 23 dividing the hollow active material particles 120 may be a plane passing through the center of curvature O, or may be a plane located away from the center of curvature O. . The position where the cut surface 23 is formed is formed at a position where the angle between the direction from the center of curvature O to the tip of the convex curved surface 21 on the side of the cut surface 23 and the major axis of the convex curved surface 21 forms an angle θ. . In this embodiment, the angle θ is preferably within ±15°. That is, the spherical crown-shaped active material particles 20 preferably have a spherical crown shape with a central angle of 150° or more and 210° or less.

Specifically, when θ=0°, the cut plane 23 is a plane passing through the center of curvature O, and the spherical crown-shaped active material particles 20 are formed in a hemispherical shape. When θ=15°, the cut surface 23 is a surface located at a position spaced upward from the center of curvature O, and the spherical crown-shaped active material particles 20 have a relatively large volume including the center of curvature O. It is three-dimensional. When θ=−15°, the cut surface 23 is a surface spaced downward from the center of curvature O, and the spherical crown-shaped active material particle 20 has a relatively small volume that does not include the center of curvature O. It is a solid of the direction of .

Furthermore, within ±15° of the angle θ, the shape of the cut surface 23 may be flat as shown in FIGS. can be Thus, the shape of the spherical crown-shaped active material particles 20 is determined according to the position and shape of the cut surface 23 .

In the present embodiment, the proportion of the spherical crown-shaped active material particles 20 formed at an angle θ of ±15° or less in all the active material particles contained in the positive electrode mixture layer 10 is preferably 30% by mass or more. It is 100% by mass or less, more preferably 50% by mass or more and 100% by mass or less. With such a ratio, the effect of improving the input/output characteristics of the secondary battery is further enhanced. The positive electrode mixture layer 10 may contain other active material particles in addition to the spherical crown-shaped active material particles 20 formed with the angle θ within ±15°. Other active material particles include, 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°, and the like.

Subsequently, in step S4, a paste for forming the positive electrode mixture layer 10 is prepared using the 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 prepared 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 mixed solvent thereof) at a predetermined ratio. obtained by

In step S5, by measuring the viscosity of the paste obtained in step S4, it is confirmed whether or not the measured value of the viscosity of the paste containing spherical crown-shaped active material particles 20 is smaller than a predetermined value. As the predetermined value in step S5, the viscosity of the paste containing the hollow active material particles 120 that are the material of the spherical crown-shaped active material particles 20 can be used. The paste containing the hollow active material particles 120 contains the hollow active material particles 120, the conductive material 30, other additives, and the solvent in the same materials and proportions as in the case of the spherical crown-shaped active material particles 20, except for the type of the active material particles. It is obtained by kneading with

In this step, the viscosity of the paste containing spherical crown-shaped active material particles 20 is measured. When the viscosity measurement result (measured value) of the paste containing the spherical crown-shaped active material particles 20 is smaller than the viscosity measurement result (predetermined value) of the paste containing the hollow active material particles 120 (step S5; YES), the process proceeds 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), the desired spheres may not be obtained due to reasons such as insufficient pulverization. Since it is considered that the coronal active material particles 20 have not been obtained, the process returns to step S2.

In step S6, a paste containing the spherical crown-shaped active material particles 20 is applied to the surface of the positive electrode current collector, dried to evaporate the solvent, and the dried product is pressed if necessary. Thereby, the positive electrode mixture layer 10 can be formed on the positive electrode current collector.

The coating amount per unit area of the positive electrode mixture layer 10 on the positive electrode current collector is not particularly limited, but from the viewpoint of ensuring a sufficient conductive path, it is 3 mg/cm per one side of the positive electrode current collector. ² or more is preferable, 5 mg/cm ² or more is more preferable, and 6 mg/cm ² or more is particularly preferable. The amount of application is the amount of application in terms of the solid content of the paste.

The coating amount per one side of the positive electrode current collector is preferably 45 mg/cm ² or less, more preferably 28 mg/cm ² or less, and particularly preferably 15 mg/cm ² or less. The density of the positive electrode mixture layer 10 is also not particularly limited, but it is preferably 1.0 g/cm ³ or more and 3.8 g/cm ³ or less, and more preferably 1.5 g/cm ³ or more and 3.0 g/cm ³ or less. Preferably, it is particularly preferably 1.8 g/cm ³ or more and 2.4 g/cm ³ or less.
By the manufacturing method described above, the positive electrode for a secondary battery according to this embodiment can be manufactured.

Next, with reference to FIGS. 6 to 9, liquid conductivity, solid conductivity, and effective reaction area are simulated using a three-dimensional model of the positive electrode for the optimum range of the angle θ in the spherical crown-shaped active material particles 20. We will explain based on the results obtained.

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

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

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

The horizontal axis of the graph shown in FIG. 7 indicates the angle θ [°], and the vertical axis indicates the liquid conductivity [S/m]. In addition, the liquid conductivity is the electrical conductivity of the electrolytic solution. As shown in FIG. 7, it was found that the smaller the angle θ, the higher the liquid conductivity in the range of −90° to 45°. 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 the angle θ = 45°, but the liquid conductivity increases when the angle θ = 90°. found out. That is, when the angle θ is 45° or less, it was shown that the electrical resistance decreased as the angle θ decreased.

Subsequently, 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 the larger the angle θ, the higher the solid conductivity in the range of −90° to 90°. On the other hand, it has been found that if the angle θ is too small, the solid conductivity decreases. Since the active material particles with a small angle θ are relatively small and divided particles, in order to ensure sufficient electrical conductivity, the amount of the conductive material in the positive electrode mixture layer is smaller than that of the active material particles with a large angle θ. The requirement for 30 was shown to be high.

Subsequently, 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 electrolytic solution. As shown in FIG. 9, it was found that the effective reaction area increased as the angle θ decreased in the range of −15° to 90°. On the other hand, it has been found that if the angle θ is too large, the effective reaction area decreases. That is, it was shown that relatively small and fragmented particles have a larger effective reaction area due to an increase in surface area.

From the results shown in FIGS. 7 to 9, when the angle θ is within ±15°, the liquid conductivity, solid conductivity, and effective reaction area are well balanced, and the positive electrode 1 having good performance can be obtained. and determined the optimum range of the angle θ.

Next, Examples 1 to 3 and Comparative Examples 1 to 7 will be described with reference to FIG. In addition, an Example does not limit this invention. FIG. 10 is a table for explaining examples and comparative examples.
Ten types of positive electrodes having the configuration shown in FIG. 10 were manufactured, and the manufactured positive electrode, the negative electrode, and two separators were superimposed to prepare an electrode body. This was housed in a battery case having an injection port. Subsequently, the electrolytic solution was injected through the injection port of the battery case, and the injection port was airtightly sealed. As described above, 10 types of evaluation batteries were produced. A polyolefin porous film was used as the separator. As the electrolytic solution, a non-aqueous electrolytic solution was used in which lithium hexafluorophosphate (LiPF ₆ ) was dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC).

Examples 1 to 3 are positive electrodes 1 manufactured according to the flow shown in FIG. In the positive electrodes 1 of Examples 1 to 3, spherical crown-shaped active material particles 20 having 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 . The material of the spherical crown-shaped active material particles 20 has a particle diameter of 5 μm, a thickness of 0.8 to 1.5 μm, and a major axis length/minor axis length (L1/L2) of the outer surface 121 of 1.0. 1.5, and the length of the major axis/length of the minor axis on the inner surface 122: 1.0 to 1.8. After the hollow active material particles 120 were pulverized by a jet mill equipped with a classifying function, the pulverized material was classified to obtain spherical crown-shaped active material particles 20 with an angle θ of 0°.

The obtained spherical crown-shaped active material particles 20 were measured for oil absorption and BET specific surface area. Then, it was confirmed that the measured value obtained by measuring the oil absorption of the spherical crown-shaped active material particles 20 was larger than the predetermined value obtained by measuring the oil absorption of the hollow active material particles 120 . It was also confirmed that the measured value obtained by measuring the BET specific surface area of the spherical crown-shaped active material particles 20 was larger 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, the hollow active material particles 120, which are the material of the spherical crown-shaped active material particles 20 used in Examples 1 to 3, were used as the active material particles of the positive electrode. In the positive electrodes of Comparative Examples 5 to 7, small-diameter active material particles having a particle size of 3 μm were used as the active material particles of the positive electrode. The small-diameter active material particles used here are secondary particles that have a solid structure and are formed by gathering together primary particles.

In Examples 1 to 3 and Comparative Examples 1 to 7, the positive electrode active material particles were all LiNi _{(1-XY) CoXMnYO2 , 0≤X≤0.35} , 0≤Y≤0. A lithium-nickel-cobalt-manganese composite oxide (NCM) having an average composition represented by 35 was used.

Various positive electrodes were manufactured as follows. First, active material particles (spherical crown-shaped active material particles 20, hollow active material particles 120, or small-diameter active material particles), conductive material 30 (AB or CNT), and other additives are mixed as shown in FIG. A paste was prepared by mixing at a ratio, adding N-methyl-2-pyrrolidone as a solvent, and kneading. The prepared paste was applied to both sides of an aluminum foil as a positive electrode current collector so as to have a constant thickness and coating amount, dried, and then pressed to produce 10 types of sheet-like positive electrodes. In addition, PVdF as a binder was added to the additive at a constant rate.

Here, the viscosity of the paste containing the spherical crown-shaped active material particles 20 was measured. Then, it was confirmed that the measured value of the 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 is made of the same materials and proportions as the spherical crown-shaped active material particles 20 except for the type of active material particles.

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

The amount of voltage drop was measured for 10 types of evaluation batteries each containing the positive electrodes of Examples 1 to 3 and Comparative Examples 1 to 7 described above. The 10 types of evaluation batteries were discharged at a current value of 10 to 40 C from a state of charge of 60% SOC (State Of Charge) in an environment of 25 ° C. Each voltage after 10 seconds from the start of discharge The amount of descent was measured. The result of measuring the amount of voltage drop is shown in the result column of FIG.

From the results of FIG. 10, when the conductive material 30 is AB, comparing Examples 1 and 2 with Comparative Example 1, it can be seen that in Examples 1 and 2, the amount of voltage drop can be reduced by increasing the amount of AB used. found out. Further, when the conductive material 30 is CNT, when comparing Example 3 and Comparative Examples 2 to 7, Example 3 is compared to Comparative Examples 2 to 7, even if the amount of CNT used is the same. It was found that the amount of CNTs need not be increased more than necessary because the amount of voltage drop can be effectively reduced.

Furthermore, the distribution of the conductive material 30 in the positive electrode mixture layer 10 will be described with reference to FIGS. 1 and 17. FIG. FIG. 17 is a diagram showing a positive electrode for a secondary battery of a comparative example. Here, the upper side of FIG. 1 shows a three-dimensional model of the positive electrode 1 according to Example 1 using AB as the conductive material 30 . Also, FIG. 17 shows a three-dimensional model of the positive electrode according to Comparative Example 1 using AB 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 the conductive material 30 between Example 1 and Comparative Example 1, in Comparative Example 1, the conductive material 30 does not exist in the hollow portions 124 of the hollow active material particles 120, and the particles of the hollow active material particles 120 It can be confirmed that the conductive material 30 is unevenly distributed in the gap formed between them. On the other hand, in Example 1, it can be confirmed that the conductive material 30 is distributed over 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 so as to be in contact therewith. As described above, according to the present embodiment, the outer surface of the spherical crown-shaped active material particles 20 can be utilized to the maximum, so that the effective reaction area can be secured efficiently.

Next, the reason why the results shown in FIG. 10 were obtained will be described based on the result 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 for 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 (that is, a particle size of 5 μm) and a thickness of 1 μm. The hollow active material particles 120 have a radius of 2.5 μm (that is, a particle size of 5 μm), a thickness of 1 μm, and a length of the major axis/length of the minor axis (L1/L2) on the outer surface 121 of 1.05. The major axis length/minor axis length at the inner surface 122 is 1.05. The solid active material particles 220 are substantially spherical secondary particles formed by gathering together primary particles, and have a radius of 2.5 μm (that is, a particle size of 5 μm) and a length of the major axis/length of the minor axis. thickness: 1.05. The plate-like active material particles 320 are formed in a substantially rectangular plate-like shape and have a major axis length of 2.5 μm and a minor axis length (thickness) of 1 μm. The spherical crown-shaped active material particles 20 used here have a radius of 2.5 μm (that is, a particle size of 5 μm), a thickness of 1 μm, and the length of the major axis/the length of the minor axis (L1/L2) on the outer surface 121. : 1.05, and the length of the major axis/length of the minor axis on the inner surface 122: 1.05.

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

As shown in FIG. 11, in measuring the reaction area of the active material particles by simulation, various active material particles were randomly filled in a region (20 μm) ³ formed using a 20 μm square housing. Specifically, a paste containing each of the active material particles, the conductive material 30, other additives, and a solvent is applied on the positive electrode current collector, dried and pressed, and then in the positive electrode mixture layer. was permeated with electrolyte. As a result, various positive electrodes formed within a region 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 crown-shaped active material particles 20 . Comparative Example A is a positive electrode including hollow active material particles 120 . Reference Example 1A is a positive electrode including solid active material particles 220 . Reference Example 2A is a positive electrode containing plate-like active material particles 320 .

Further, three-dimensional models of various positive electrodes were obtained by FIB-SEM measurement. Then, image analysis was performed using three-dimensional models of various positive electrodes, and the theoretical reaction area and effective reaction area were calculated, and the graph shown in FIG. 12 was created. FIG. 12 shows the result of comparing the reaction area (theoretical reaction area and effective reaction area) at the interface between the active material particles and the electrolytic solution in various positive electrodes. The solid line shown in FIG. 12 is an approximated curve calculated from the plotted values of the theoretical reaction area. The dashed line shown in FIG. 12 is an approximate curve calculated using the plotted values of the effective reaction area for Comparative Example A. As shown in FIG.

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 respective reaction areas, in the solid active material particles 220 included in Reference Example 1A, only the outer surfaces excluding the overlap between the particles correspond to the reaction areas, so the reaction areas were reduced compared to other examples. . In addition, in the plate-like active material particles 320 included in Reference Example 2A, both sides in the thickness direction of the plate-like active material particles 320 were oriented in the in-plane direction of the positive electrode current collector due to pressing performed during manufacturing. Become. As a result, when the plate-like active material particles 320 were used, the portions where the particles overlapped increased, and the reaction area decreased.

Subsequently, Comparative Example A will be described with reference to FIG. 13 . FIG. 13 is a diagram explaining the reaction area of the 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 overlapping between the particles correspond to the theoretical reaction area. However, considering the effective reaction area in Comparative Example A, since the conductive material 30 does not exist in the hollow portion 124, sufficient electronic conductivity in the hollow portion 124 cannot be ensured.

Thus, when the hollow active material particles 120 are used, the hollow portions 124 cannot be fully utilized, and the reactivity at the interface between the inner surface 122 and the electrolytic solution is lowered. Therefore, when the effective reaction area of Comparative Example A is calculated assuming that only the outer surface 121 excluding the hollow portion 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. FIG. 14 is a diagram explaining the reaction area of the spherical crown-shaped active material particles. As shown in FIG. 14, in the spherical crown-shaped active material particles 20, the theoretical reaction area corresponds to the convex curved surface 21, the concave curved surface 22, and the cut surface 23 excluding the overlapping of the particles. Furthermore, in the spherical crown-shaped active material particles 20, the bulkiness of the particles prevents the particles from overlapping each other. Therefore, when the spherical crown-shaped active material particles 20 are used, for example, compared with the plate-shaped active material particles 320, the decrease in the reaction area due to the overlapping of the particles is suppressed. Further, the concave surfaces 22 corresponding to the hollow portions 124 of the hollow active material particles 120 can also be effectively used as reaction fields without being excluded from the reaction area.

Next, results of simulating the porosity of the positive electrode will be described with reference to FIGS. 15 and 16. FIG. FIG. 15 is a diagram illustrating various positive electrodes used for porosity evaluation. FIG. 16 is a graph showing results of porosity evaluation of various positive electrodes shown in FIG.

As shown in FIG. 15, in measuring the porosity of the positive electrode by simulation, various active material particles were randomly filled in a region (16 μm) ³ formed using a 16 μm square housing. Specifically, a paste containing each of the active material particles, the conductive material 30, other additives, and a solvent is applied on the positive electrode current collector, dried, and then electrolyzed in the positive electrode mixture layer. Allow the liquid to permeate. As a result, various positive electrodes formed within a region of (16 μm) ³ were manufactured. Each active material particle in each positive electrode is filled from above in the vertical direction by chance.

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

Further, three-dimensional models of various positive electrodes were obtained by FIB-SEM measurement. Then, image analysis was performed using three-dimensional models of various positive electrodes, and the theoretical porosity and effective porosity were calculated, and the graph shown in FIG. 16 was created. FIG. 16 shows the result 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 approximate curve calculated from the plotted values of the theoretical porosity. The dashed line shown in FIG. 16 is an approximate curve calculated using the plotted values of the effective porosity for Comparative Example B. As shown in FIG. The porosity was calculated by calculating the ratio of the volume other than the active material particles from the ratio of the volume of the active material particles 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 each porosity, in Reference Example 1B, since the solid active material particles 220 were substantially spherical, the filling amount was increased and the porosity was lower than in the other examples. Further, in Reference Example 2B, since the filled plate-like active material particles 320 were randomly oriented, voids were easily formed between the particles, and the porosity was improved as compared with Reference Example 1B.

Subsequently, in Comparative Example B, the theoretical porosity corresponds to the inter-particle space and the hollow portion 124 of the hollow active material particles 120, so the theoretical porosity is larger than in the other examples. However, when considering the effective porosity for Comparative Example B, since the conductive material 30 does not exist in the hollow portion 124 as in the case of the reaction area, the hollow portion 124 cannot be fully utilized, and the inner surface The reactivity at the interface between 122 and the electrolyte decreases. Therefore, when calculating the effective porosity of Comparative Example B assuming that only the voids between particles excluding the hollow portions 124 correspond to the effective porosity, the effective porosity is 42.0%.

On the other hand, in Example B, since the filled spherical crown-shaped active material particles 20 are randomly oriented, voids are likely to be formed between the particles. Moreover, since the spherical crown-shaped active material particles 20 have a curved shape, the porosity is further improved compared to the plate-shaped active material particles 320 . Therefore, in Example B, sufficient voids in the positive electrode mixture layer 10 can be ensured, and the permeability of the electrolyte to the positive electrode mixture layer 10 is improved. Therefore, it is considered that the use of the positive electrode 1 of Example B enables smooth diffusion and movement of lithium ions in the positive electrode mixture layer 10, thereby improving the input/output characteristics of the secondary battery.

By the way, in order to improve the input/output characteristics of a secondary battery, it is conceivable to make the active material particles finer to increase the reaction area. , a problem such as deterioration of battery characteristics may occur. On the other hand, since the spherical crown-shaped active material has a certain particle size, it is possible to easily manufacture the positive electrode 1 while suppressing a decrease in productivity due to too small a particle size.

As described above, in the positive electrode for a secondary battery according to the present embodiment, part of the hollow active material particles 120 made of a lithium transition metal oxide and having a substantially spherical shell shape or a substantially ellipsoidal shell shape has the cut surface 23 . Spherical crown-shaped active material particles 20 formed in a substantially spherical crown shape cut by . 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 depressed in one direction.

With such a 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. Since 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. As a result, reduction in reaction area due to overlapping of particles is suppressed. Furthermore, since the gaps that serve as permeation paths for the electrolytic solution are ensured between the spherical crown-shaped active material particles 20, an increase in internal resistance is suppressed. Therefore, according to this embodiment, it is possible to provide a secondary battery having high input/output characteristics.

In addition, in the positive electrode for a secondary battery according to the present embodiment, the combination of the spherical crown-shaped active material particles 20 and CNTs ensures that the electron conductivity in the positive electrode mixture layer 10 is sufficiently maintained even when the amount of CNTs used is reduced. Since it is possible to secure it, the ratio of the spherical crown-shaped active material particles 20 can be relatively increased. Therefore, according to this embodiment, a secondary battery with high energy density can be provided.

In addition, in the spherical crown-shaped active material particles 20, 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 is preferably within ±15°. . According to such a configuration, it is possible to obtain the positive electrode 1 having good electronic conductivity and excellent performance balance in which a reaction field is ensured.

In addition, regarding the inner surface 122 of the hollow active material particle 120 which is the material of the spherical crown-shaped active material particle 20, the length of the major axis/the length of the minor axis is preferably 1.0 or more and less than 1.5. In addition, regarding the outer surface 121 of the hollow active material particles 120 that are the material of the spherical crown-shaped active material particles 20, the length of the major axis/the length of the minor axis (L1/L2) is 1.0 or more and less than 1.8. is preferred. When the spherical crown-shaped active material particles 20 are formed using the hollow active material particles 120 configured in this manner, the convex curved surface 21 and the concave curved surface 22 of the spherical crown-shaped active material particles 20 become curved, making the particles bulky. Become. As a result, overlapping of spherical crown-shaped active material particles 20 in positive electrode mixture layer 10 can be suppressed.

Furthermore, according to the method for manufacturing a positive electrode for a secondary battery according to the present embodiment, by controlling the pulverization conditions when pulverizing the hollow active material particles 120, a positive electrode for a secondary battery that exhibits the above effects can be produced. be able to.

Further, 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 appropriately 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 grinding conditions while confirming whether the hollow active material particles 120 are appropriately ground. According to such a configuration, the desired spherical crown-shaped active material particles 20 can be reliably obtained, and the quality of the positive electrode for a secondary battery manufactured using the spherical crown-shaped active material particles 20 is improved.

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

In addition, it is known that the relationship of hollow active material particles 120<plate-like active material particles 320<fine-powder active material particles<spherical crown-shaped active material particles 20 holds true for oil absorption. Here, the fine powder active material particles are, for example, active material particles obtained by pulverizing the hollow active material particles 120 with a pulverizer to a finer size than the spherical crown-shaped active material particles 20 . Therefore, the result of measuring the oil absorption of the plate-like active material particles 320 or fine powder active material particles may be used as the predetermined value of the oil absorption. Furthermore, it is known that the viscosity of the paste satisfies the relationship of 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 result of measuring the viscosity of the paste containing the plate-like active material particles 320 may be used as the predetermined viscosity value.

1 Positive electrode 10, 10a, 10b Positive electrode mixture layer 20 Spherical crown active material particle 21 Convex surface 22 Concave 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-like active material particle O Curvature center θ Angle

Claims (8)

a current collector;
a positive electrode mixture layer formed on the current collector and containing a positive electrode active material and a conductive material;
The positive electrode active material is
A substantially spherical shell-shaped or substantially elliptical spherical shell-shaped hollow active material particle composed of a lithium transition metal oxide is partially formed into a substantially spherical crown shape by cutting a cut surface,
a substantially spherical crown-shaped convex curved surface protruding in one direction;
a substantially spherical crown-shaped concave curved surface existing on the opposite side of the one-way side and recessed in the one-way;
A positive electrode for a secondary battery having The positive electrode for a secondary battery according to claim 1, wherein the conductive material is a carbon nanotube. The angle between a direction from the center of curvature of the convex curved surface toward the tip of the convex curved surface on the cutting surface side and the major axis of the convex curved surface passing through the center of curvature of the convex curved surface is within ±15°. 3. The positive electrode for a secondary battery according to 1 or 2. 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 positive electrode for a secondary battery according to any one of claims 1 to 3, wherein the concave curved surface is formed in 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 long axis to the length of the short 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 positive electrode for a secondary battery according to any one of claims 1 to 4, 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. forming substantially spherical shell-shaped or substantially elliptical spherical shell-shaped hollow active material particles composed of a lithium transition metal oxide;
pulverizing the hollow active material particles to form a positive electrode active material having a substantially spherical crown shape in which a portion of the hollow active material particles is cut by a cut surface;
forming a positive electrode mixture layer containing the positive electrode active material and a conductive material on a current collector;
has
The positive electrode active material has a substantially spherical crown-shaped convex curved surface protruding in one direction and a substantially spherical crown-shaped concave curved surface existing on the opposite side of the one direction and recessed in the one direction. A method for manufacturing a positive electrode. In the step of forming the positive electrode active material,
3. When the result of measuring at least one of the oil absorption amount and the BET specific surface area of the positive electrode active material is larger than the result of measuring the hollow active material particles, proceeding to the step of forming the positive electrode mixture layer. 7. The method for manufacturing the positive electrode for a secondary battery according to 6. The step of forming the positive electrode mixture layer includes:
A step of preparing a paste containing the positive electrode active material, the conductive material, and a solvent in a predetermined ratio,
When the result of measuring the viscosity of the paste is smaller than the result of measuring the viscosity of the paste containing the hollow active material particles, the conductive material, and the solvent in the predetermined ratio, on the current collector 8. The method of manufacturing a positive electrode for a secondary battery according to claim 6, wherein the positive electrode mixture layer is formed by applying a paste containing the positive electrode active material, the conductive material, and the solvent.

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