Positive electrode for secondary battery and method for manufacturing positive electrode for secondary battery

Abstract

To provide a positive electrode for a secondary battery capable of obtaining a secondary battery reduced in internal resistance and good in cycle characteristics, and a manufacturing method of the positive electrode for the secondary battery.SOLUTION: A positive electrode for a secondary battery includes: a porous current collector 10 which is provided with a plurality of first through holes 11 penetrating in a thickness direction; and a positive electrode mixture layer 20 which is formed on a surface of the porous current collector 10 and within the first through holes 11, and contains a positive electrode active material 40. The positive electrode active material 40 includes spherical cap-shaped active material particles 30 each of which has: a spherical shell part which is formed into a substantially spherical cap shape by cutting off a part of a substantially spherical shell-shaped or substantially elliptical spherical shell-shaped hollow active material particle composed of lithium transition metal oxide, along a cutting surface; a substantially spherical cap-shaped convex surface protruding in one direction; and a substantially spherical cap-shaped concave surface which exists on the opposite side on one direction side and is depressed in the one direction. At least the spherical cap-shaped active material particles 30 are filled in the first through holes 11.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode for a secondary battery and a method for manufacturing a positive electrode for a secondary battery. [Background technology]

[0002] Secondary batteries are widely used as so-called portable power sources for personal computers, mobile terminals, etc., 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 an electrolyte between a positive electrode and a negative electrode.

[0003] Electrodes used in secondary batteries such as lithium-ion secondary batteries include a conductive current collector and a composite layer containing an electrode material such as an active material held on the current collector. In order to achieve high input / output, such secondary batteries require a positive electrode for secondary batteries (hereinafter sometimes simply referred to as a "positive electrode") that can smoothly insert and remove charge carriers.

[0004] Patent Document 1 discloses a precursor for a positive electrode active material for a secondary battery, which includes secondary particles with a single layer structure formed by aggregation of columnar primary particles oriented radially from the center of the particle toward the surface, 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 same. [Chemical formula 1] Ni 1-(x+y+z) Co x MyMn z (OH)2 [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2018-536972 Summary of the Invention [Problem to be solved by the invention]

[0006] For the purpose of improving battery performance, a technique is known in which a current collector is provided with a large number of through-holes, a composite layer is formed in the through-holes, and the electrolyte and charge carriers are allowed to flow through the through-holes. This technique not only improves the peel strength of the composite layer but also increases the volume fraction of the electrolyte in the electrode, thereby facilitating the penetration of the electrolyte from the bulk.

[0007] On the other hand, if the active material filled in the through-holes blocks the through-holes, the flow of electrolyte through the through-holes may be impaired. For example, the technology described in Patent Document 1 does not anticipate the flow of electrolyte through the through-holes, so if the positive electrode active material described in Patent Document 1 is also filled into the through-holes, there is a problem in that the through-holes may be blocked, preventing sufficient battery characteristics from being obtained.

[0008] The present disclosure has been made to solve such problems, and aims to provide a positive electrode for a secondary battery that reduces internal resistance and provides a secondary battery with good cycle characteristics, and a method for manufacturing a positive electrode for a secondary battery. [Means for solving the problem]

[0009] A positive electrode for a secondary battery according to one embodiment includes a porous current collector having a plurality of first through holes penetrating through the current collector in a thickness direction, and a positive electrode composite layer formed on the surface of the porous current collector and within the first through holes and containing a positive electrode active material. The positive electrode active material includes spherical-crown-shaped active material particles each having a spherical-crown shell portion formed by cutting off a portion of a hollow active material particle having a substantially spherical shell or a substantially ellipsoidal shell shape made of a lithium transition metal oxide, the spherical-crown-shaped active material particle having a substantially spherical-crown-shaped convex surface protruding in one direction and a substantially spherical-crown-shaped concave surface present on the opposite side of the one direction and recessed in one direction, and at least the first through holes are filled with the spherical-crown-shaped active material particles.

[0010] A method for manufacturing a positive electrode for a secondary battery according to one embodiment includes the steps of: forming hollow active material particles having a substantially spherical shell shape or a substantially oval-spherical shell shape made of a lithium transition metal oxide; pulverizing the hollow active material particles to form a positive electrode active material containing spherical-crown-shaped active material particles having a spherical-crown shell portion formed by cutting off a portion of the hollow active material particle along a cut surface; and forming a positive electrode composite layer containing the positive electrode active material on the surface of a porous current collector having a plurality of first through holes penetrating in the thickness direction and within the first through holes, with at least the spherical-crown-shaped active material particles filled within the first through holes. [Effects of the Invention]

[0011] The present disclosure can provide a positive electrode for a secondary battery that reduces internal resistance and provides a secondary battery with good cycle characteristics, and a method for producing a positive electrode for a secondary battery. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram showing a positive electrode according to the first embodiment. [Figure 2] 2 is a cross-sectional view schematically showing the vicinity of a first through-hole provided in a porous current collector included in the positive electrode shown in FIG. 1. FIG. [Figure 3] 2 is a cross-sectional view showing a spherical cap-shaped active material particle contained in the positive electrode shown in FIG. 1. FIG. [Figure 4] 1 is a flowchart showing a method for manufacturing a positive electrode according to the first embodiment. [Figure 5] FIG. 4 is a cross-sectional view showing a hollow active material particle that is a raw material for the spherical crown-shaped active material particle shown in FIG. [Figure 6] FIG. 2 is a diagram showing an example of a spherical cap-shaped active material particle. [Figure 7] FIG. 10 is a diagram showing another example of spherical crown-shaped active material particles. [Figure 8] FIG. 10 is a diagram showing another example of spherical crown-shaped active material particles. [Figure 9] 1 is a table illustrating examples, comparative examples, and reference examples. [Figure 10] 1 is a graph showing the resistance reduction rate (%) of an evaluation half cell. [Figure 11] FIG. 1 is a diagram showing a positive electrode using a positive electrode active material containing substantially spherical active material particles. [Figure 12] 12 is a cross-sectional view schematically showing the vicinity of a first through-hole provided in a porous current collector included in the positive electrode shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0013] Embodiment 1 Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. For clarity, the following description and drawings have been simplified as appropriate. In the following description, identical or equivalent elements are designated by the same reference numerals, and redundant description will be omitted.

[0014] Here, unless otherwise specified, the "minor axis length" and "major axis length" of the active material particles in this embodiment are measured using the outer or inner surfaces of a plurality of arbitrarily selected active material particles from image analysis of a three-dimensional model obtained by FIB-SEM measurement. The "minor axis length" can be determined as the average value of the shortest diameters passing through the center of curvature of the active material particles on the outer or inner surface of the active material particles. Furthermore, the "major axis length" can be determined as the average value of the longest diameters passing through the center of curvature of the active material particles on the outer or inner surface of the active material particles. Furthermore, the "average diameter (average particle size)" of the active material particles is the average value of the "major axis length" and "minor axis length" on the outer surface of the active material particles.

[0015] Furthermore, unless otherwise specified, the "thickness" of the active material particle in this embodiment is measured by measuring the shortest distances from multiple positions on the inner surface of the active material particle to the outer surface in an arbitrarily selected cross section of the active material particle from image analysis of a three-dimensional model obtained by FIB-SEM measurement. The "thickness" can be determined by averaging the shortest distances measured at multiple positions on the inner surface of the active material particle.

[0016] For example, when the active material particles are spherical crown-shaped active material particles 30, the surface disposed on the outside is a convex curved surface 33, and the surface disposed on the inside is a concave curved surface 34. When the active material particles are hollow active material particles 130, the surface disposed on the outside is an outer surface 133, and the surface disposed on the inside is an inner surface 134.

[0017] Furthermore, the "pore diameter" of the active material particles in this embodiment can be determined as the average value of the diameters of the narrowest parts of a plurality of arbitrarily selected second through holes 36 from image analysis of a three-dimensional model obtained by FIB-SEM measurement.

[0018] 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). For example, a sample can be processed by cutting a sample solidified with an appropriate resin at the desired cross section and then gradually scraping the cross section while performing SEM observation.

[0019] In the FIB-SEM technique, a sample is repeatedly processed with an FIB and observed with an SEM. The resulting SEM images are then reconstructed into a three-dimensional model, which allows the structure of the material, including its internal structure, to be visualized.

[0020] Hereinafter, as one preferred embodiment of the positive electrode for a secondary battery according to this embodiment, a positive electrode 1 for a lithium-ion secondary battery will be specifically described. A lithium-ion secondary battery is a secondary battery that uses lithium ions as a charge carrier and achieves charging and discharging by the transfer of charge between a positive electrode 1 (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).

[0021] First, the configuration of a positive electrode 1 according to this embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 is a diagram showing a positive electrode according to the first embodiment. The perspective view of FIG. 1 shows a three-dimensional model of the positive electrode 1. The cross-sectional view of FIG. 1 shows a cross-sectional SEM image of the positive electrode 1. FIG. 2 is a cross-sectional view schematically showing the vicinity of a first through-hole provided in a porous current collector included in the positive electrode 1 shown in FIG. 1. Note that in FIGS. 1 and 2, components other than the porous current collector 10 and the positive electrode active material 40 (spherical crown-shaped active material particles 30) are not shown.

[0022] 1 and 2, the positive electrode 1 includes a porous current collector 10 having first through holes 11 that penetrate the thickness direction from one surface (one side 12) to the other surface (other side 13), and a positive electrode composite layer 20 that is formed on the surface of the porous current collector 10 and in the first through holes 11 and that contains a positive electrode active material 40. The positive electrode active material 40 includes spherical-crown-shaped active material particles 30. The positive electrode 1 has a configuration in which at least the first through holes 11 are filled with the spherical-crown-shaped active material particles 30.

[0023] The positive electrode 1 can be combined with, for example, a negative electrode, an electrolyte containing lithium ions such as a non-aqueous electrolyte solution, and, if necessary, an insulating separator through which lithium ions can pass, to form a secondary battery.

[0024] The porous current collector 10 is formed in a plate or foil shape and is made of a metal with good conductivity. The porous current collector 10 used in the positive electrode 1 can be made of, for example, aluminum, an aluminum alloy, nickel, titanium, or stainless steel. Among these, aluminum is preferred because of its excellent durability, light weight, and low cost. The thickness T1 of the porous current collector 10 is, for example, 5 μm or more and 15 μm or less. If the thickness T1 of the porous current collector 10 is less than 5 μm, the porous current collector 10 is more likely to break due to stress that may be applied during manufacturing or use, such as when applying a composite layer-forming paste to the porous current collector 10 or when winding electrodes in the manufacturing process of a wound-type battery. If the thickness T1 of the porous current collector 10 exceeds 15 μm, the influence of metal resistance increases when a large current is applied to the secondary battery, and the volume occupied by the porous current collector 10 increases, resulting in a tendency for the energy density of the secondary battery to decrease.

[0025] The porous current collector 10 is a porous current collector provided with a plurality of first through holes 11. The first through holes 11 provided in the porous current collector 10 are formed, for example, in the shape of slits. Such a porous current collector 10 may be, for example, an etched foil in which the first through holes 11 are formed by etching or the like, or may be an expanded metal or a punched metal in which the first through holes 11 are formed by mechanical punching or other processing.

[0026] The aperture ratio (%) of the first through holes 11 in the porous current collector 10 is preferably 10% or less, and more preferably 1% or more and 5% or less, in terms of area ratio. If the aperture ratio is higher than necessary, the abundance ratio of the electrical conductor in the positive electrode 1 decreases, which increases the resistance and tends to reduce the cycle characteristics during charge and discharge. If the aperture ratio is lower than necessary, there is a risk that the circulation of the electrolyte and lithium ions in the electrolyte through the first through holes 11 may be poor.

[0027] The first through holes 11 are formed to a size that allows the spherical-crown-shaped active material particles 30 to enter therein. The shape, arrangement, number, etc. of the first through holes 11 are appropriately set so as to suppress a decrease in the strength of the porous current collector 10 while allowing good circulation of the electrolyte solution and lithium ions in the electrolyte solution through the first through holes 11.

[0028] The first through holes 11 preferably have a shorter side whose lower limit is at least twice the average diameter R of the convex curved surfaces 33 of the spherical-crown-shaped active material particles 30 and no greater than 1 mm. Details of the spherical-crown-shaped active material particles 30 will be described later. The first through holes 11 preferably have a longer side whose lower limit is at least twice the shorter side and no greater than 10 mm. Here, the longer sides of the first through holes 11 are arranged along the longitudinal direction of the porous current collector 10. The longitudinal direction of the porous current collector 10 is the coating direction of the composite layer-forming paste, which, in the case of a wound-type battery, is the winding direction of the electrodes. The shorter sides of the first through holes 11 are arranged along the width direction of the porous current collector 10, which is perpendicular to the longer sides.

[0029] By arranging the long sides of the first through holes 11 in the coating direction (or the winding direction), it is possible to prevent the porous current collector 10 from breaking due to stress that may be applied during manufacturing or use, such as when applying the composite layer forming paste or when winding the electrode. On the other hand, if the long sides of the first through holes 11 are larger than necessary, the strength of the porous current collector 10 will be insufficient, making the porous current collector 10 more likely to break.

[0030] A positive electrode mixture layer 20 containing at least spherical-crown-shaped active material particles 30 is formed in the first through holes 11. The positive electrode mixture layer 20 is formed not only on the surface of the porous current collector 10 but also in the first through holes 11, thereby improving the peel strength of the positive electrode mixture layer 20. As a result, the storage stability and cycle characteristics of the secondary battery are improved.

[0031] The first through-holes 11 serve as permeation paths for the electrolyte in a secondary battery formed with the positive electrode 1. In particular, in a secondary battery in which the positive electrode 1, in which the positive electrode composite layer 20 is formed on one surface 12 of the porous current collector 10, faces the negative electrode via a separator, the electrolyte flows smoothly on the porous current collector 10 side of the positive electrode 1 that does not face the negative electrode composite layer formed on the surface of the negative electrode current collector. This allows unreacted electrolyte to be supplied into the positive electrode composite layer 20 through the first through-holes 11 during discharge of the secondary battery. This reduces internal resistance and provides a secondary battery with favorable cycle characteristics in which the decrease in battery capacity over charge / discharge cycles is suppressed. Thus, the positive electrode 1 according to this embodiment is suitable for a positive electrode 1 in which the positive electrode composite layer 20 is formed on one surface 12 of the porous current collector 10.

[0032] The shape of the porous current collector 10 can be determined, for example, by SEM observation of a cross section of the positive electrode 1 or the surface of the porous current collector 10 from which the positive electrode composite layer 20 has been removed.

[0033] The positive electrode mixture layer 20 contains at least a positive electrode active material 40. The positive electrode mixture layer 20 may further contain a conductive material and, as necessary, additives such as a dispersant and a binder. The positive electrode mixture layer 20 is formed on at least one surface 12 of the porous current collector 10. In this embodiment, the positive electrode 1 will be described using an example in which the positive electrode mixture layer 20 is formed on one surface 12 of the porous current collector 10, but the positive electrode mixture layer 20 may be formed on both the one surface 12 and the other surface 13 of the porous current collector 10 depending on the purpose.

[0034] The conductive material is a material for forming a conductive path within the positive electrode composite layer 20. By mixing an appropriate amount of conductive material into the positive electrode composite layer 20, the electronic conductivity within the positive electrode 1 can be increased, thereby improving the charge / discharge efficiency and input / output characteristics of the battery. As the conductive material, for example, carbon materials such as various carbon blacks (e.g., acetylene black (AB) and ketjen black) and carbon fibers (e.g., carbon nanotubes (CNT) and carbon nanofibers (CNF)) can be used. For example, when AB is used as the conductive material, it is preferable to use AB with an average particle size of 20 to 50 μm. When CNT is used as the conductive material, it is preferable to use CNT with an outer diameter of 5 to 30 nm and an aspect ratio of 15 to 300.

[0035] 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.

[0036] The positive electrode active material 40 will be described with reference to FIG. 3. FIG. 3 is a cross-sectional view of a spherical-crown-shaped active material particle contained in the positive electrode shown in FIG. 1. The positive electrode active material 40 is capable of absorbing and releasing lithium ions, which serve as charge carriers. The spherical-crown-shaped active material particle 30 contained in the positive electrode active material 40 has a spherical-crown shell portion 32 formed in a substantially spherical crown shape by cutting a portion of a hollow active material particle 130 having a substantially spherical shell or a substantially ellipsoidal shell shape along a cut surface 31. The spherical-crown-shaped active material particle 30 has a substantially spherical-crown-shaped convex surface 33 that protrudes in one direction and a substantially spherical-crown-shaped concave surface 34 on the opposite side that is concave in one direction. Furthermore, the spherical-crown-shaped active material particle 30 has a recess 35 that is recessed in a substantially spherical crown shape near the center of the end face toward the convex surface 33.

[0037] Here, the term "spherical crown" refers to the side surface of a spherical notch formed by cutting a portion of a sphere or oval sphere along any plane (cut plane 31 in this embodiment). The shape of the spherical crown-shaped active material particle 30 can also be described as being roughly bowl-shaped. When the convex curved surface 33 and the concave curved surface 34 are curved in this way, a bulky spherical crown-shaped active material particle 30 is formed, which makes it possible to suppress overlapping between the spherical crown-shaped active material particles 30 and between the spherical crown-shaped active material particles 30 and other active material particles within the positive electrode composite layer 20.

[0038] The spherical-crown-shaped active material particles 30 contain a lithium transition metal oxide having a layered crystal structure. The lithium transition metal oxide contains 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.

[0039] The spherical-crown-shaped active material particles 30 may contain one or more additional elements in addition to a 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.

[0040] In a preferred embodiment, the spherical cap-shaped active material particles 30 can have a composition (average composition) represented by the following general formula (1). Li1+ x Ni y Co z Mn (1-y-z) MA α MB β O2…(1)

[0041] In the above formula (1), x can be a real number satisfying 0 ≦ x ≦ 0.2. y can be a real number satisfying 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 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 can be a real number satisfying 0 ≦ β ≦ 0.01. β may be substantially 0 (that is, an oxide substantially free of MB). In the chemical formula indicating the layered lithium transition metal oxide, for the sake of convenience, the composition ratio of O (oxygen) is shown as 2, but this numerical value should not be strictly interpreted and some compositional variations (typically included in the range of 1.95 or more and 2.05 or less) can be tolerated.

[0042] Each spherical coronal active material particle 30 has a particle form. The spherical coronal shell portion 32 constituting the spherical coronal active material particle 30 is a secondary particle formed by connecting primary particles of a lithium transition metal oxide in a substantially spherical crown shape. Here, the primary particle refers to a particle that can be considered an ultimate particle judged from the geometric form in appearance. In the spherical coronal active material particle 30, the primary particle is typically an aggregate of crystallites of a lithium transition metal oxide. The shape observation of the spherical coronal active material particle 30 can be performed by an image obtained by SEM observation.

[0043] The average diameter R of the convex surface 33 of the spherical coronal active material particle 30 is preferably, for example, approximately 2 μm or more, and more preferably 3 μm or more. Also, the average diameter R of the spherical coronal active material particle 30 is preferably 25 μm or less, and more preferably 15 μm or less. From the viewpoint of productivity, in a preferred embodiment, the average diameter R of the convex surface 33 is 4 μm or more and 6 μm or less.

[0044] And the ratio of the average diameter R of the convex surface 33 to the thickness T1 of the porous current collector 10 ( R / T1) is preferably 0.5 or more and 1.5 or less. The ratio ( R / T1 ) is outside the above range and is smaller than necessary, the voids formed between the spherical-crown-shaped active material particles 30 in the first through holes 11 cannot be sufficiently secured, and these voids may not function well as permeation paths for the electrolyte solution. Furthermore, as the average diameter R relative to the thickness T1 becomes smaller, the tortuosity (degree of bending) of the migration path of the lithium ions in the electrolyte solution in the first through holes 11 increases, which may increase the diffusion resistance of the lithium ions migrating in the positive electrode mixture layer 20.

[0045] The thickness T2 of the spherical crown shell portion 32 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 T2 of the spherical crown shell portion 32, the lower the diffusion resistance of lithium ions, making it easier for lithium ions to be released from the interior of the spherical crown shell portion 32 (the central portion of thickness T2) during charging and for lithium ions to be absorbed into the interior of the spherical crown shell portion 32 during discharging of the secondary battery. The lower limit of the thickness T2 of the spherical crown shell portion 32 is preferably 0.1 μm or more. From the viewpoint of achieving both an internal resistance reduction effect and durability, in one preferred embodiment, the thickness T2 of the spherical crown shell portion 32 is 0.4 μm or more and 1.5 μm or less.

[0046] The primary particles may be in a single layer or in multiple layers in the thickness direction of the spherical crown shell portion 32. In a preferred embodiment, the spherical crown-shaped active material particle 30 is configured such that the primary particles are connected in a substantially single layer throughout the entire spherical crown shell portion 32.

[0047] Furthermore, the spherical crown-shaped active material particle 30 preferably has a plurality of second through holes 36 penetrating the spherical crown shell portion 32 from the outside to the recesses 35. The second through holes 36 are formed as gaps between the plurality of primary particles that make up the spherical crown shell portion 32. The diameter D of the second through holes 36 is preferably 0.3 μm or more. In a preferred embodiment, the diameter D of at least some of the second through holes 36 provided in the spherical crown-shaped active material particle 30 is 0.5 μm or more. By providing the spherical crown-shaped active material particle 30 with second through holes 36 having such a diameter D, the durability of the spherical crown-shaped active material particle 30 can be ensured while the electrolyte and lithium ions in the electrolyte can smoothly circulate between the outside and the recesses 35 through the second through holes 36.

[0048] Furthermore, in the spherical crown-shaped active material particle 30 provided with the second through holes 36, the volume ratio, which is the ratio of the volume of the spherical crown shell portion 32 to the total volume of the spherical crown shell portion 32 and the second through holes 36, is preferably 0.3 to 0.7, more preferably 0.4 to 0.5. Here, the total volume of the spherical crown shell portion 32 and the second through holes 36 is the sum of the volume of all primary particles constituting one spherical crown-shaped active material particle 30 and the volume of all second through holes 36. Meanwhile, the volume of the spherical crown shell portion 32 is the volume of all primary particles constituting one spherical crown-shaped active material particle 30. By configuring this volume ratio within this range, the spherical crown-shaped active material particle 30 is porous while ensuring durability against stresses that may be applied during production or use, and therefore the flow of the electrolyte and lithium ions in the electrolyte within the first through holes 11 is promoted.

[0049] If the volume ratio is outside the above range and is smaller than necessary, this may reduce the durability of the spherical-crown-shaped active material particles 30. If the volume ratio is outside the above range and is larger than necessary, it may be difficult to form the first through holes 11 necessary for the smooth flow of the electrolyte solution and lithium ions in the electrolyte solution, which may hinder the permeation of the electrolyte solution through the first through holes 11 and make it difficult for the lithium ions to diffuse into the positive electrode mixture layer 20.

[0050] Characteristic values ​​such as volume ratio can be calculated based on measurements of a plurality of arbitrarily selected active material particles using image analysis of the 3D model obtained by FIB-SEM measurement. For example, 3D model data of spherical-crown-shaped active material particles 30, which are divided into individual particles using an algorithm such as Watershed, can be obtained from the 3D model data of the positive electrode 1, and the volume of each particle can be calculated. The total volume of the spherical-crown-shaped shell portion 32 and the second through holes 36 for each particle can be calculated based on the 3D model data of the spherical-crown-shaped active material particles 30. Furthermore, the connected voids within the spherical-crown-shaped active material particles 30 can be extracted from the 3D model data of the spherical-crown-shaped active material particles 30, and the volume of the second through holes 36 can be calculated based on the extracted data. The volume of the spherical-crown-shell portion 32 can then be calculated based on the 3D model data of the spherical-crown-shaped active material particles 30 from which the extracted connected voids have been removed.

[0051] Next, an example of a method for manufacturing the above-described positive electrode 1 will be described with reference to Fig. 4. Fig. 4 is a flowchart showing a method for manufacturing a positive electrode according to the first embodiment. As shown in Fig. 4, the method for manufacturing a 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 can be ensured, and can also be replaced by other methods.

[0052] Step S1 is a step of forming hollow active material particles 130. Step S2 is a step of pulverizing the hollow active material particles 130 to form a positive electrode active material 40 including spherical crown-shaped active material particles 30. Step S3 is a step of confirming whether the physical properties of the positive electrode active material 40 obtained in step S2 are appropriate compared to predetermined values. Step S4 is a step of preparing a composite layer-forming paste for forming the positive electrode composite layer 20. Step S5 is a step of confirming whether the viscosity of the composite layer-forming paste prepared in step S4 is appropriate compared to a predetermined value. Step S6 is a step of applying the composite layer-forming paste onto the surface of a porous current collector 10 having first through holes 11 penetrating through the thickness direction and into the first through holes 11 to form the positive electrode composite layer 20.

[0053] Each of the above steps will be described in detail. First, in step S1, hollow active material particles 130 are formed as raw materials for the spherical-crown-shaped active material particles 30. Fig. 5 is a cross-sectional view showing an example of a hollow active material particle as a raw material for the spherical-crown-shaped active material particles shown in Fig. 3.

[0054] 5, the hollow active material particle 130 has a particle form with a hollow structure having a spherical shell portion 132 and a hollow portion 135 formed inside the spherical shell portion 132, and is formed in a substantially spherical or ellipsoidal shell shape. That is, the outer shape of the hollow active material particle 130 is substantially spherical or ellipsoidal (e.g., a slightly distorted spherical shape). The hollow active material particle 130 also has an outer surface 133 and an inner surface 134, both of which are formed in a spherical or ellipsoidal shape.

[0055] The spherical shell 132 is a secondary particle formed by connecting primary particles of a lithium transition metal oxide to form an approximately spherical or ellipsoidal shell shape. A hollow 135 is formed inside the spherical shell 132. In the thickness direction of the spherical shell 132, the primary particles may be a single layer or multiple layers. In a preferred embodiment, the hollow active material particle 130 is configured such that the primary particles are connected in a substantially single layer throughout the entire spherical shell 132. In the hollow active material particle 130, the primary particles are typically an aggregate of crystallites of the lithium transition metal oxide. The shape of the hollow active material particle 130 can be observed using images obtained by SEM observation.

[0056] The average diameter R of the outer surface 133 of the hollow active material particle 130 is approximately equal to the average diameter R of the convex curved surface 33. The outer surface 133 is the surface that corresponds to the outline of the hollow active material particle 130. The thickness T2 of the spherical shell portion 132 is approximately equal to the thickness T2 of the spherical crown shell portion 32.

[0057] The diameter of the inner surface 134 of the hollow active material particle 130 can be obtained by subtracting the thickness T2 of the spherical shell portion 132 from the average diameter R of the outer surface 133. The diameter of the inner surface 134 is approximately the same as the diameter of the concave curved surface 34. The inner surface 134 is the surface that corresponds to the outline of the hollow portion 135.

[0058] Furthermore, the hollow active material particle 130 preferably has a second through hole 36 penetrating the spherical shell portion 132 from the outside to the hollow portion 135. As described above, the second through hole 36 serves as a permeation path for the electrolyte in the spherical crown-shaped active material particle 30.

[0059] The method for forming the hollow active material particles 130 includes, for example, a raw material hydroxide production step, a mixing step, and a firing step. Each of these steps will be described in detail. However, the method for forming the hollow active material particles 130 is not limited to this.

[0060] The raw hydroxide production process involves adding ammonium ions (NH4 +) to precipitate particles of a transition metal hydroxide from the aqueous solution, where the aqueous solution contains at least one transition metal element that constitutes the lithium transition metal oxide.

[0061] The raw hydroxide production process preferably includes a nucleation stage in which a transition metal hydroxide is precipitated from an aqueous solution, and a particle growth stage in which transition metal hydroxide particles are grown under conditions in which the pH of the aqueous solution is lower than that in the nucleation stage. In the particle growth stage, the pH and ammonium ion concentration are changed to adjust the precipitation rate of the transition metal hydroxide, thereby changing the structure of the hollow active material particles 130 (such as the spacing between primary particles and particle porosity).

[0062] In the mixing step, the transition metal hydroxide particles produced in the raw hydroxide production step are separated from the reaction solution, washed, filtered, and dried. The resulting transition metal hydroxide is mixed with a lithium compound to prepare a mixture. The transition metal hydroxide and lithium compound should be 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 ratio corresponding to the composition of the hollow active material particles 130, which are the target product.

[0063] The firing step is a step in which the mixture is fired to obtain hollow active material particles 130. The firing step is carried out, for example, in an oxidizing atmosphere (for example, in the air atmosphere). The firing temperature is, for example, 700°C or higher and 1100°C or lower. The firing step 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.

[0064] In this step, a 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 130 in the shape of a roughly spherical shell or roughly oval spherical shell, and hollow portions 135 are formed inside the hollow active material particles 130. The hollow active material particles 130 thus formed are composed of a lithium transition metal oxide having a layered crystal structure.

[0065] Returning to Fig. 4, in step S2, the hollow active material particles 130 obtained in step S1 are fed into a pulverizer, and a shearing force is applied to the hollow active material particles 130, thereby forming spherical crown-shaped active material particles 30 in which a portion of the hollow active material particles 130 is cut off along a cut surface 31. A jet mill is preferably used as the pulverizer used to pulverize the hollow active material particles 130. The pulverization method may be either a dry method or a wet method.

[0066] Here, the structure of the spherical-crown-shaped active material particle 30 desirable in this embodiment will be described in detail with reference to Figures 6 to 8. Figure 6 is a diagram showing an example of a spherical-crown-shaped active material particle. Figure 7 is a diagram showing another example of a spherical-crown-shaped active material particle. Figure 8 is a diagram showing another example of a spherical-crown-shaped active material particle.

[0067] 6 to 8 are intended to schematically represent the convex curved surface 33 of the spherical crown-shaped active material particle 30. The dashed lines in FIGS. 6 to 8 are intended to schematically represent the outer surface 133 of the hollow active material particle 130, which is the raw material for the spherical crown-shaped active material particle 30. FIGS. 6 to 8 show a case in which the spherical crown-shaped active material particle 30 and the hollow active material particle 130 are arranged in an overlapping manner so that the entire spherical crown-shaped active material particle 30 coincides with a portion of the hollow active material particle 130. The convex curved surface 33 and the outer surface 133 are arranged so that their major axes extend horizontally, and their minor axes extend vertically.

[0068] The center of curvature O of the hollow active material particle 130 coincides with the center of curvature O of the spherical-crown-shaped active material particle 30. The center of curvature O of the hollow active material particle 130 coincides with the center of curvature O of the outer surface 133 and the center of curvature O of the inner surface 134, and the center of curvature O of the spherical-crown-shaped active material particle 30 coincides with the center of curvature O of the convex curved surface 33 and the center of curvature O of the concave curved surface 34.

[0069] As shown in FIGS. 6 to 8, the cut surface 31 dividing the hollow active material particle 130 may be a surface passing 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 31 is formed at a position where the angle between the direction from the center of curvature O toward the tip of the convex curved surface 33 on the cut surface 31 side and the major axis of the convex curved surface 33 is θ. In this embodiment, the angle θ is preferably within ±15°. That is, the spherical crown-shaped active material particle 30 preferably has a substantially spherical crown shape with a central angle of 150° or more and 210° or less.

[0070] Specifically, when θ=0°, the cross section 31 is a plane passing through the center of curvature O, and the spherical-crown-shaped active material particle 30 is formed in a hemispherical shape. When θ=15°, the cross section 31 is a plane located above the center of curvature O and spaced apart, and the spherical-crown-shaped active material particle 30 is a solid with a relatively large volume that includes the center of curvature O. When θ=−15°, the cross section 31 is a plane located below the center of curvature O and spaced apart, and the spherical-crown-shaped active material particle 30 is a solid with a relatively small volume that does not include the center of curvature O.

[0071] When the angle θ is within ±15°, an effective reaction field for the active material particles is ensured, while a positive electrode 1 having good performance can be obtained that achieves high levels of both the electrical conductivity of the electrolyte and the electrical conductivity of the active material particles.

[0072] Furthermore, when the angle θ is within ±15°, the shape of the cut surface 31 may be flat or may be non-flat, such as uneven. In this way, the shape of the spherical-crown-shaped active material particle 30 is determined depending on the position and shape of the cut surface 31.

[0073] In this embodiment, the proportion of spherical crown-shaped active material particles 30 formed with an angle θ of ±15° or less in the positive electrode active material 40 contained in the positive electrode composite layer 20 is preferably 80% by mass or more and 100% by mass or less, and more preferably 90% by mass or more and 100% by mass or less. The closer to 100% by mass the proportion of spherical crown-shaped active material particles 30 formed with an angle θ of ±15° or less is, the more preferable it is.

[0074] This ratio allows the spherical-crown-shaped active material particles 30 to efficiently enter the first through-holes 11, facilitating the penetration of the electrolyte through the first through-holes 11. As a result, the effect of improving the input / output characteristics of a secondary battery including the positive electrode 1 is further enhanced. The positive electrode active material 40 may contain other active material particles in addition to the spherical-crown-shaped active material particles 30 formed with the angle θ within ±15°. Examples of other active material particles include active material particles with a solid structure, active material particles with a hollow structure, and active material particles formed with an angle θ outside the ±15° range.

[0075] For the outer surface 133 of the hollow active material particle 130, the ratio of the major axis length L1 to the minor axis length L2 (major axis length / minor axis length, L1 / L2) is preferably 1.0 or more and less than 1.5. For the inner surface 134 of the hollow active material particle 130, the ratio of the major axis length to the minor axis length (major axis length / minor axis length) is preferably 1.0 or more and less than 1.8. When the spherical-crown-shaped active material particle 30 is formed using hollow active material particles 130 having such a ratio, the front and back surfaces (convex curved surface 33 and concave curved surface 34) of the spherical-crown-shaped active material particle 30 have curved shapes, making the particle bulky.

[0076] In the jet mill pulverization in step S2, the degree of pulverization is adjusted by appropriately setting pulverization conditions such as the pulverization gas pressure, the supply rate of the hollow active material particles 130, and the number of pulverizations. The pulverization conditions are preferably set so that the hollow active material particles 130 can be cut along a cross section 31 along the major axis, which is the longest diameter passing through the center of curvature O of the hollow active material particles 130. In other words, by controlling the pulverization conditions, it is possible to form the desired spherical crown-shaped active material particles 30.

[0077] The pulverization conditions are not particularly limited and vary depending on the pulverizer and hollow active material particles 130 used. After pulverization, the pulverized product is classified to remove fine powder, thereby obtaining spherical crown-shaped active material particles 30 having a desired average particle size. The pulverized product may be classified using a jet mill equipped with a classification function, or may be classified using a classifier separate from the jet mill.

[0078] By pulverizing the hollow active material particles 130 in this manner, a positive electrode active material 40 containing at least spherical-crown-shaped active material particles 30 can be obtained. The spherical-crown-shaped shell portion 32 of the spherical-crown-shaped active material particle 30 corresponds to the spherical shell portion 132 of the hollow active material particle 130, and is obtained by cutting away a portion of the spherical shell portion 132 along the cut surface 31. The convex curved surface 33 of the spherical-crown-shaped active material particle 30 corresponds to the outer surface 133 of the hollow active material particle 130, and is obtained by cutting away a portion of the outer surface 133 along the cut surface 31. The concave curved surface 34 of the spherical-crown-shaped active material particle 30 corresponds to the inner surface 134 of the hollow active material particle 130, and is obtained by cutting away a portion of the inner surface 134 along the cut surface 31.

[0079] The concave curved surface 34 is located on the opposite side of the protruding direction of the convex curved surface 33, and the concave curved surface 34 and the convex curved surface 33 face each other. The recess 35 corresponds to the hollow portion 135 of the hollow active material particle 130, and is defined by the concave curved surface 34. The cut surface 31 is a portion connecting the tip edge of the convex curved surface 33 and the tip edge of the concave curved surface 34. The cut surface 31 has a substantially annular shape surrounding the opening of the recess 35.

[0080] 4, in step S3, the physical properties of the positive electrode active material 40 obtained in step S2 are measured to determine whether the measured physical properties of the positive electrode active material 40 are greater than predetermined values. Examples of the physical properties of the positive electrode active material 40 include oil absorption and BET specific surface area. The predetermined values ​​in step S3 may include the oil absorption and BET specific surface area of ​​the hollow active material particles 130 that are used to form the spherical crown-shaped active material particles 30.

[0081] In this step, at least one of the oil absorption and the BET specific surface area of ​​the positive electrode active material 40 is measured. If the measurement result (measured value) of the physical property of the positive electrode active material 40 is greater than the measurement result (predetermined value) of the same physical property of the hollow active material particles 130 (step S3; YES), the process proceeds to the next step S4. On the other hand, if the measurement result (measured value) of the physical property of the positive electrode active material 40 is equal to or less than the predetermined value (step S3; NO), it is considered that the desired spherical crown-shaped active material particles 30 have not been obtained due to insufficient pulverization or other reasons, and the process returns to step S2.

[0082] Subsequently, in step S4, a composite layer-forming paste for forming positive electrode composite layer 20 is prepared using positive electrode active material 40. The composite layer-forming paste prepared here is obtained by kneading at least positive electrode active material 40 and a solvent (aqueous solvent, non-aqueous solvent, or a mixture thereof) in a predetermined ratio. The composite layer-forming paste of this embodiment is obtained by kneading positive electrode active material 40, a conductive material, a binder, and a solvent in a predetermined ratio.

[0083] In step S5, the viscosity of the composite layer forming paste obtained in step S4 is measured to determine whether the measured viscosity of the composite layer forming paste is smaller than a predetermined value. The predetermined value in step S5 can be the viscosity of a reference paste prepared by incorporating hollow active material particles 130 into the composite layer forming paste instead of the positive electrode active material 40. The reference paste is obtained by kneading at least the hollow active material particles 130 and a solvent in a predetermined ratio that is recognized as being the same materials and proportions as the composite layer forming paste, except for the type of active material particles. Note that the "same ratio" allows for a range of deviations equivalent to manufacturing variations. The reference paste of this embodiment is obtained by kneading the hollow active material particles 130, a conductive material, a binder, and a solvent in a predetermined ratio.

[0084] In this step, the viscosity of the composite layer forming paste is measured. If the result of measuring the viscosity of the composite layer forming paste (measured value) is smaller than the result of measuring the viscosity of the reference paste (predetermined value) (step S5; YES), the process proceeds to the next step S6. On the other hand, if the result of measuring the viscosity of the composite layer forming paste (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 30 have not been obtained due to insufficient pulverization or other reasons, and the process returns to step S2.

[0085] In step S6, the composite layer forming paste is applied to one surface 12 of porous current collector 10, dried to volatilize the solvent, and the dried product is pressed as necessary. This allows positive electrode composite layer 20 to be formed on the surface of porous current collector 10 and in first through holes 11.

[0086] Examples of the coating method include a die coater, a slit coater, a comma coater, a gravure coater, and a blade coater. As a drying method, for example, reduced pressure drying is employed, which improves the filling of positive electrode composite layer 20 into first through holes 11. Examples of the pressing method include a flat plate press and a roll press.

[0087] The amount of the positive electrode mixture layer 20 applied to the porous current collector 10 per unit area is not particularly limited, but is preferably 3 mg / cm 2 per one side of the porous current collector 10. 2 More than 5 mg / cm is preferable. 2 More preferably, 6 mg / cm or more is more preferable, and particularly 6 mg / cm 2 The amount of application is the amount of application converted into the solid content of the paste.

[0088] The amount of coating on one side of the porous current collector 10 was 45 mg / cm 2 Less than 28 mg / cm is preferred 2 More preferably, 15 mg / cm 2 The density of positive electrode mixture layer 20 is also not particularly limited, but is preferably 1.0 g / cm 3 More than 3.8g / cm 3It is preferable that the concentration is 1.5 g / cm or less. 3 More than 3.0g / cm 3 More preferably, 1.8 g / cm or less is more preferable. 3 More than 2.4g / cm 3 It is preferable to do the following: The positive electrode 1 according to this embodiment can be manufactured by the above manufacturing method.

[0089] Next, Examples 1 to 5, Comparative Examples 1 and 2, and Reference Examples 1 and 2 will be described with reference to Fig. 9. Note that the present disclosure is not limited to the Examples. Fig. 9 is a table for explaining the Examples, Comparative Examples, and Reference Examples.

[0090] Here, two types of positive electrodes were manufactured for each example using a porous current collector 10 having slit-shaped first through-holes 11 as the positive electrode current collector, and a non-porous current collector not having the first through-holes 11. Furthermore, a laminated half-cell was manufactured using each of the two types of positive electrodes. Hereinafter, the half-cell manufactured using the porous current collector 10 will be referred to as the evaluation half-cell, and the half-cell manufactured using the non-porous current collector will be referred to as the reference half-cell.

[0091] The evaluation half-cells used in Examples 1 to 5, Comparative Examples 1 and 2, and Reference Examples 1 and 2 have the same configuration except for the different types of positive electrode active materials. The reference half-cells used in Examples 1 to 5, Comparative Examples 1 and 2, and Reference Examples 1 and 2 have the same configuration except for the different types of positive electrode active materials.

[0092] [Half-cell fabrication] The half-cell was fabricated as follows. First, the positive electrode active material, CNT as a conductive material, and PVdF as a binder were mixed in a mass ratio of 96:3:1, and N-methyl-2-pyrrolidone was added as a solvent and kneaded to prepare a paste. The paste was applied to one side of a porous current collector 10 to a certain thickness and coating amount, dried, and pressed to produce various sheet-shaped positive electrodes (porosity 50%).

[0093] The electrode assembly, with the positive electrode composite layer side of the positive electrode facing the lithium metal foil (lithium metal counter electrode) and a separator interposed therebetween, was housed inside an aluminum laminate exterior material, and an electrolyte solution was added and sealed to prepare an evaluation half-cell. A reference half-cell was also prepared in the same manner as the evaluation half-cell, except that a non-porous current collector was used instead of the porous current collector 10.

[0094] Here, an aluminum foil having a thickness T1 of 8 μm and having first through holes 11 with a short side of 0.5 mm, a long side of 1 mm, and an aperture ratio of 10% was used as the porous current collector 10. An aluminum foil having a thickness of 8 μm and no first through holes 11 was used as the non-porous current collector.

[0095] The separator was a porous polyolefin film, and the electrolyte was a non-aqueous electrolyte prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:1.

[0096] In Examples 1 to 5, Comparative Examples 1 and 2, and Reference Examples 1 and 2, the positive electrode active materials were all LiNi (1-X-Y) Co X Mn Y A lithium nickel cobalt manganese composite oxide (NCM) with an average composition expressed as O2, 0≦X≦0.35, and 0≦Y≦0.35 was used.

[0097] Next, the positive electrode active materials used in Examples 1 to 5, Comparative Examples 1 and 2, and Reference Examples 1 and 2 will be described.

[0098] Example 1 In Example 1, the average diameter R was 5 μm ( R / T1 = 0.6), thickness T2: 1 μm, pore diameter D: 0.5 μm, and volume ratio: 0.4.

[0099] Example 2 In Example 2, the average diameter R was 5 μm ( R / T1 = 0.6), thickness T2: 1 μm, pore diameter D: 0.3 μm, and volume ratio: 0.5.

[0100] Example 3 In Example 3, the average diameter R was 6 μm ( R / T1 = 0.8), thickness T2: 1 μm, pore diameter D: 0.3 μm, and volume ratio: 0.5.

[0101] Example 4 In Example 4, the average diameter R was 10 μm ( R / T1 = 1.3), thickness T2: 1 μm, pore diameter D: 0.3 μm, and volume ratio: 0.5.

[0102] Example 5 In Example 5, the average diameter R was 12 μm ( R / T1 = 1.5), thickness T2: 1 μm, pore diameter D: 0.3 μm, and volume ratio: 0.5.

[0103] (Comparative Example 1) In Comparative Example 1, the average diameter R was 3 μm ( R / T1 A positive electrode active material was used that contained active material particles (secondary particles) that were approximately spherical crown-shaped, with a pore size of 0.4 (=0.4). The approximately spherical crown-shaped secondary particles used here were obtained by appropriately adjusting the manufacturing conditions for forming the hollow active material particles 130 and the spherical crown-shaped active material particles 30, as described in the flow chart of FIG.

[0104] (Comparative Example 2) In Comparative Example 2, the average diameter R was 14 μm ( R / T1 A positive electrode active material was used that contained active material particles (secondary particles) that were approximately spherical crown-shaped, with a pore size of 1.75 (=1.75). The approximately spherical crown-shaped secondary particles used here were obtained by appropriately adjusting the production conditions for forming the hollow active material particles 130 and the production conditions for forming the spherical crown-shaped active material particles 30, as described in the flow shown in FIG.

[0105] (Reference example 1) In Reference Example 1, a positive electrode active material containing approximately spherical active material particles (primary particles) having a solid structure and an average diameter of 5 μm was used.

[0106] (Reference example 2) In Reference Example 2, a positive electrode active material containing approximately spherical active material particles (primary particles) having a solid structure and an average diameter of 1 μm was used.

[0107] [evaluation] The battery characteristics were evaluated by measuring and comparing the voltage drop during large current discharge, which is the amount of change in battery voltage during charge and discharge, for each of the nine types of test half-cells and reference half-cells. To measure the voltage drop, each half-cell was placed in a 25°C environment and discharged at a current of 100C from a charged state of SOC (State of Charge) of 60%, and the voltage drop was measured 0.1 minutes after the start of discharge.

[0108] The resistance reduction rate (%) of the evaluation half cell was then calculated based on the voltage drop amount of the reference half cell. The results are shown in the results section of Figure 9. Figure 10 is a graph showing the resistance reduction rate (%) of the evaluation half cell. Figure 10 is a graph corresponding to the results section of Figure 9. Figures 9 and 10 show the resistance reduction rate (%) of the evaluation half cell as a relative value, with the voltage drop amount of the reference half cell set to 100%. A larger resistance reduction rate (%) indicates a greater reduction in the internal resistance of the battery, and it can be said that the first through-holes 11 are more effective in reducing the diffusion resistance of lithium ions.

[0109] 9 and 10, it was confirmed that Examples 1 to 5 had a higher resistance reduction rate than Comparative Example 1, and that the first through holes 11 were highly effective in reducing the diffusion resistance of lithium ions. Comparative Example 1 used active material particles with smaller diameters than Examples 1 to 5, and therefore the reaction resistance of the positive electrode active material was lower, but the number of contact points between the active material particles increased. Therefore, as the number of contact points between the active material particles increased, the tortuosity within the first through holes 11 increased, and the diffusion resistance of lithium ions increased. Therefore, it is believed that the first through holes 11 did not sufficiently reduce the diffusion resistance of lithium ions.

[0110] Furthermore, the active material particles used in Comparative Example 2 were crushed by pressing during production, which prevented the production of a desired positive electrode, making it difficult to obtain a high-density positive electrode.

[0111] Considering Examples 1 to 5, from the viewpoint of improving the circulation of the electrolyte and lithium ions in the electrolyte through the second through holes 36, the larger the hole diameter D of the second through holes 36, as long as durability is ensured. Also, from the viewpoint of reducing the diffusion resistance of lithium ions in the spherical crown-shaped active material particles 30, the thinner the thickness T2, as long as durability is ensured.

[0112] Next, comparing Reference Examples 1 and 2, the active material particles used in Reference Example 1 have a larger diameter than those in Reference Example 2, which increases the reaction resistance of the positive electrode active material, but suppresses an increase in the number of contact points between the active material particles. This suppresses an increase in the tortuosity inside the first through holes 11 that accompanies an increase in the number of contact points between the active material particles, thereby lowering the diffusion resistance of lithium ions. This is thought to improve the effect of first through holes 11 in reducing the diffusion resistance of lithium ions.

[0113] The active material particles used in Reference Example 2 have a smaller diameter than those in Reference Example 1, so the reaction resistance of the positive electrode active material is lower, but the number of contact points between the active material particles increases. As a result, the tortuosity inside the first through holes 11 increases with the increase in the number of contact points between the active material particles, and the diffusion resistance of lithium ions increases. Therefore, it is thought that the first through holes 11 did not sufficiently reduce the diffusion resistance of lithium ions.

[0114] However, when the porous current collector 10 was used, it was confirmed that the voltage drop during discharge was larger in Reference Examples 1 and 2 than in Examples 1 to 5, and the internal resistance increased.

[0115] 11 and 12, problems with a positive electrode 100 using a positive electrode active material containing substantially spherical active material particles will be described. FIG. 11 is a diagram showing a positive electrode using a positive electrode active material containing substantially spherical active material particles. The perspective view of FIG. 11 shows a three-dimensional model of the positive electrode 100. The cross-sectional view of FIG. 11 shows a cross-sectional SEM image of the positive electrode 100. FIG. 12 is a cross-sectional view schematically showing the vicinity of a first through-hole provided in a porous current collector 10 included in the positive electrode shown in FIG. 11.

[0116] 11 and 12 includes a porous current collector 10 and a positive electrode composite layer 120 formed on the surface (one side 12) of the porous current collector 10 and in the first through-holes 11, the positive electrode composite layer 120 including a positive electrode active material 140. The positive electrode active material 140 includes hollow active material particles 130 having a substantially spherical shape. By using the porous current collector 10 in the positive electrode 100, permeation of the electrolyte through the first through-holes 11 is promoted, and the diffusion resistance of lithium ions is reduced.

[0117] In this positive electrode 100, reduced-diameter solid active material particles 150 are contained in the positive electrode composite layer 120 to achieve high input / output and high energy density of the secondary battery. When a porous current collector 10 is used, the first through-holes 11 are filled with reduced-diameter solid active material particles 150. However, when the filling rate of the positive electrode active material 140 is constant, the smaller the particle size of the solid active material particles 150, the more tortuosity of the lithium ion migration path in the first through-holes 11 tends to increase. This results in a problem of increased lithium ion diffusion resistance. Furthermore, when the reduced-diameter solid active material particles 150 are densely packed, the number of contact points between the solid active material particles increases more than necessary, which may result in a problem of reduced reaction fields that can contribute to the electrochemical reaction.

[0118] In contrast, the positive electrode for a secondary battery according to this embodiment includes a porous current collector 10 having a plurality of first through holes 11 penetrating the current collector in the thickness direction, and a positive electrode composite layer 20 formed on the surface of the porous current collector 10 and within the first through holes 11, and containing a positive electrode active material 40. The positive electrode active material 40 includes a spherical crown-shaped active material particle 30 having a spherical crown shell portion 32 formed by cutting a portion of a hollow active material particle 130 having a substantially spherical shell or a substantially ellipsoidal shell shape made of a lithium transition metal oxide along a cut surface 31, a substantially spherical crown-shaped convex surface 33 protruding in one direction, and a substantially spherical crown-shaped concave surface 34 present on the opposite side of the first through hole 11 and recessed in one direction. The spherical crown-shaped active material particle 30 is then filled at least within the first through holes 11.

[0119] This configuration not only improves the peel strength of positive electrode mixture layer 20, but also allows first through holes 11 to function well as permeation paths for the electrolyte of the secondary battery, thereby reducing the diffusion resistance of lithium ions moving within positive electrode mixture layer 20. As a result, the storage stability, cycle characteristics, etc. of a secondary battery including positive electrode 1 can be improved, and high input / output characteristics can be achieved.

[0120] Furthermore, in the positive electrode for a secondary battery according to this embodiment, the thickness T2 of the spherical crown shell portion 32 is preferably 0.4 μm or more and 1.5 μm or less. With this configuration, a secondary battery that achieves both reduced internal resistance and durability can be obtained.

[0121] Furthermore, in the positive electrode for a secondary battery according to this embodiment, it is preferable that the ratio of the average diameter R of the convex curved surface 33 to the thickness T1 of the porous current collector 10 is 0.5 or more and 1.5 or less. With such a configuration, the first through holes 11 function well as permeation paths for the electrolyte contained in the secondary battery, and the diffusion resistance of lithium ions moving within the positive electrode mixture layer 20 can be further reduced. As a result, high input / output characteristics of a secondary battery including the positive electrode 1 can be achieved.

[0122] Furthermore, in the positive electrode for a secondary battery according to this embodiment, the spherical crown-shaped active material particles 30 are provided with second through-holes that penetrate the spherical crown shell portions 32 and have a hole diameter D of 0.3 μm or more, and the ratio of the volume of the spherical crown shell portions 32 to the total volume of the spherical crown shell portions 32 and the second through-holes is preferably 0.3 to 0.7. This configuration ensures durability while improving the circulation of the electrolyte and lithium ions in the electrolyte through the second through-holes 36, thereby improving the input / output characteristics of a secondary battery including the positive electrode 1.

[0123] Furthermore, in the positive electrode for a secondary battery according to this embodiment, it is preferable that the length of the short side of each first through hole 11 is at least twice the average diameter R of the convex curved surface 33 and not more than 1 mm, and that the length of the long side of each first through hole 11 is at least twice the length of the short side and not more than 10 mm. This configuration can prevent breakage of the porous current collector 10 that may occur due to stress or the like that may be applied during application of the composite layer-forming paste, during production such as winding the electrode, or during use.

[0124] Furthermore, in the positive electrode for secondary battery according to this embodiment, the aperture ratio of the first through holes 11 is preferably 10% or less in terms of area ratio. With such a configuration, a secondary battery having both high cycle characteristics and high input / output characteristics can be obtained.

[0125] Furthermore, in the positive electrode for a secondary battery according to this embodiment, it is preferable that the angle formed by the direction from the center of curvature of the convex curved surface 33 toward the tip of the cut surface 31 side of the convex curved surface 33 and the major axis of the convex curved surface 33 passing through the center of curvature of the convex curved surface 33 is within ±15°. With this configuration, the bulkiness of the spherical crown-shaped active material particles 30 makes it possible to obtain a positive electrode 1 with excellent balance of performance, in which good electronic conductivity is achieved and a reaction field for the positive electrode active material 40 is ensured.

[0126] Furthermore, in the positive electrode for a secondary battery according to this embodiment, the average diameter R of the convex curved surface 33 is preferably 4 μm or more and 6 μm or less. With this configuration, it is possible to reliably obtain high-quality spherical crown-shaped active material particles 30. Also, it is possible to more reliably fill the first through holes 11 with the spherical crown-shaped active material particles 30.

[0127] Furthermore, the ratio of the length L1 of the major axis to the length L2 of the minor axis passing through the center of curvature of the outer surface 133 of the hollow active material particle 130 is preferably 1.0 or more and less than 1.5. Furthermore, the ratio of the length L2 of the major axis to the length L2 of the minor axis passing through the center of curvature of the inner surface 134 of the hollow active material particle 130 is preferably 1.0 or more and less than 1.8. When a spherical-crown-shaped active material particle 30 is formed using hollow active material particles 130 configured in this manner, the convex curved surface 33 and the concave curved surface 34 of the spherical-crown-shaped active material particle 30 each have a moderately curved shape, resulting in a bulky particle. This ensures a permeation path for the electrolyte within the positive electrode 1 and prevents a decrease in the reaction field due to an increase in contact points between the active material particles.

[0128] As described above, according to this embodiment, it is possible to provide a positive electrode for a secondary battery, which allows a secondary battery to be obtained that has reduced internal resistance and good cycle characteristics.

[0129] Furthermore, according to the method for producing a positive electrode for a secondary battery according to this embodiment, it is possible to produce a positive electrode 1 that exhibits the above-mentioned effects.

[0130] Furthermore, by appropriately measuring the physical properties of the positive electrode active material 40, which includes the spherical crown-shaped active material particles 30 obtained by pulverizing the hollow active material particles 130, it is possible to adjust the pulverization conditions while confirming whether the hollow active material particles 130 have been pulverized appropriately. Furthermore, by appropriately measuring the viscosity of the composite layer-forming paste containing the positive electrode active material 40, it is possible to adjust the pulverization conditions while confirming whether the hollow active material particles 130 have been pulverized appropriately. With this configuration, it is possible to reliably obtain the desired positive electrode active material 40, and the quality of the positive electrode 1 manufactured using the positive electrode active material 40 is improved.

[0131] The present disclosure is not limited to the above-described embodiment, and modifications can be made as appropriate without departing from the spirit of the present disclosure. For example, in the above-described embodiment, the hollow active material particles 130 are pulverized using a jet mill, but 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.

[0132] Furthermore, for example, when checking the physical properties of the positive electrode active material 40, the predetermined value may be a result of measuring the same physical properties of another substance that has a clear relationship with the physical properties of the positive electrode active material 40. Similarly, when checking the viscosity of the composite layer forming paste, the predetermined value may be a result of measuring the viscosity of another substance that has a clear relationship with the viscosity of the composite layer forming paste. [Explanation of symbols]

[0133] 1, 100 positive electrode 10 Porous current collector 11 First through hole 12 one side 13 Other side 20, 120 Positive electrode composite layer 30 Sphere-shaped active material particles 31 Cut surface 32 Spherical crown shell 33 Convex curved surface 34 Concave Surface 35 recess 36 Second through hole 40, 140 Cathode active material 130 Hollow active material particles 132 Spherical shell part 133 Exterior 134 Inside 135 Hollow part 150 Solid active material particles D Pore diameter L1 Long axis length L2 Minor axis length O center of curvature R average diameter T1, T2 thickness

Claims

1. a porous current collector having a plurality of first through holes formed therethrough in a thickness direction; a positive electrode mixture layer formed on the surface of the porous current collector and in the first through holes, the positive electrode mixture layer including a positive electrode active material; The positive electrode active material is a spherical crown shell portion formed in a substantially spherical crown shape by cutting a part of a hollow active material particle in a substantially spherical shell shape or a substantially elliptical spherical shell shape made of a lithium transition metal oxide along a cutting surface; a convex curved surface having a substantially spherical crown shape that protrudes in one direction; a substantially spherical crown-shaped concave curved surface that is present on the opposite side of the one direction side and is recessed in the one direction; and spherical crown-shaped active material particles having The positive electrode for a secondary battery, wherein at least the spherical crown-shaped active material particles are filled in the first through-holes.

2. 2. The positive electrode for a secondary battery according to claim 1, wherein the thickness of the spherical crown shell portion is 0.4 μm or more and 1.5 μm or less.

3. 2. The positive electrode for a secondary battery according to claim 1, wherein the ratio of the average diameter of the convex curved surface to the thickness of the porous current collector is 0.5 or more and 1.5 or less.

4. the spherical-crown-shaped active material particle is provided with a second through-hole that penetrates the spherical-crown shell portion and has a hole diameter of 0.3 μm or more; 2. The positive electrode for a secondary battery according to claim 1, wherein a ratio of the volume of the spherical crown shell portion to the total volume of the spherical crown shell portion and the second through-hole is 0.3 or more and 0.7 or less.

5. the length of a short side of the first through hole is equal to or greater than two times the average diameter of the convex curved surface and equal to or less than 1 mm, 2 . The positive electrode for a secondary battery according to claim 1 , wherein the length of the long side of the first through hole is from twice to 10 mm the length of the short side.

6. 2. The positive electrode for a secondary battery according to claim 1, wherein the opening ratio of the first through holes is 10% or less in terms of area ratio.

7. 2. The positive electrode for a secondary battery according to claim 1, wherein an angle formed between a direction from the center of curvature of the convex curved surface toward the 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°.

8. 2. The positive electrode for a secondary battery according to claim 1, wherein the average diameter of the convex curved surface is 4 μm or more and 6 μm or less.

9. 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 1.5; 2. The 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 cutting surface along the major axis of the outer surface.

10. 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 1.8; 2. The positive electrode for a secondary battery according to claim 1, wherein the concave curved surface is formed in a substantially spherical crown shape cut by the cutting surface along the major axis of the inner surface.

11. forming hollow active material particles having a substantially spherical shell shape or a substantially ellipsoidal shell shape, which are made of a lithium transition metal oxide; a step of crushing the hollow active material particles to form a positive electrode active material including spherical crown-shaped active material particles having a spherical crown shell portion formed in a substantially spherical crown shape by cutting a portion of the hollow active material particle along a cut surface; forming a positive electrode composite layer containing the positive electrode active material on a surface of a porous current collector having a plurality of first through holes penetrating in a thickness direction thereof and in the first through holes; and The method for manufacturing a positive electrode for a secondary battery, wherein at least the spherical crown-shaped active material particles are filled in the first through-holes.

12. In the step of forming the positive electrode active material, 12. The method for manufacturing a positive electrode for a secondary battery according to claim 11, wherein 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 method proceeds to the step of forming the positive electrode composite layer.

13. The step of forming the positive electrode composite layer includes: a step of preparing a paste for forming a composite layer containing the positive electrode active material and a solvent in a predetermined ratio; 12. The method for manufacturing a positive electrode for a secondary battery according to claim 11, wherein, when a result of measuring the viscosity of the composite layer-forming paste is smaller than a result of measuring the viscosity of a reference paste containing the hollow active material particles and the solvent in the predetermined ratio, the composite layer-forming paste is applied onto the surface of the porous current collector and into the first through holes to form the positive electrode composite layer.

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