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

Abstract

To provide a cathode for secondary batteries with which it is possible to obtain a secondary battery that achieves both high energy density and high input / output characteristics, and a method for manufacturing the cathode for secondary batteries.SOLUTION: The cathode for secondary batteries comprises a cathode current collector 10, and a cathode mixture layer 20 which is formed on the cathode current collector 10 and includes a conductive material and a cathode active material 30. The cathode active material 30 contains large-diameter active material particles 40, each composed of lithium transition metal oxide, and small-diameter active material particles 50 whose diameters are smaller than those of the large-diameter active material particles 40. The large-diameter active material particles 40 are formed in a substantially globe shape or a substantially elliptic spherical shape, and the small-diameter active material particles 50 include a spherical crown shell part which is formed in a shape of a substantially spherical crown, with some of substantially spherical shell or substantially elliptic spherical shell hollow active material particles being cut off by a cross section, a substantially spherical crown convex surface that projects in one direction, and a substantially spherical crown concave surface that exists on the opposite side in the one direction and is recessed in the one direction.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] A positive electrode active material capable of absorbing and releasing charge carriers is contained in a positive electrode mixture layer of a positive electrode constituting a secondary battery. In order to improve the performance of such secondary batteries, various improvements have been made to the positive electrode active material.

[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] In order to increase the energy density of secondary batteries, a technique is known in which a positive electrode active material with adjusted particle size is used to increase the packing rate of the positive electrode active material and improve the electrode density of the positive electrode. Therefore, it is conceivable to manufacture a positive electrode using a positive electrode active material that is a mixture of large and small active material particles with different particle sizes.

[0007] However, for example, when the positive electrode active material disclosed in Patent Document 1 is used as the active material particles with smaller particle sizes, while the reduction in particle size is expected to improve the filling rate of the positive electrode active material contained in the positive electrode, simply using active material particles with smaller particle sizes may result in the active material particles with smaller particle sizes occupying voids in the positive electrode. In secondary batteries constructed with such positive electrodes, the reduced particle size of the active material particles increases the tortuosity (bending degree) of the lithium ion migration path, and the increased number of contact points between the active material particles reduces the reaction fields that can contribute to the electrochemical reaction and the conductive paths in the positive electrode, resulting in insufficient input / output characteristics.

[0008] The present disclosure has been made to solve such problems, and aims to provide a positive electrode for a secondary battery that can provide a secondary battery that has both high energy density and high input / output 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 positive electrode current collector and a positive electrode composite layer formed on the positive electrode current collector and containing a conductive material and a positive electrode active material. The positive electrode active material includes large-sized active material particles and small-sized active material particles having a smaller diameter than the large-sized active material particles, each of which is composed of a lithium transition metal oxide. The large-sized active material particles are formed in a substantially spherical or ellipsoidal shape. The small-sized active material particles have a spherical crown shell portion formed in a substantially spherical crown shape by cutting off a portion of a hollow active material particle having a substantially spherical shell shape or a substantially ellipsoidal shell shape along a cut surface, a substantially spherical crown-shaped convex curved surface protruding in one direction, and a substantially spherical crown-shaped concave curved surface present on the opposite side of the one direction and recessed in one direction.

[0010] A method for manufacturing a positive electrode for a secondary battery according to one embodiment includes the steps of: forming a positive electrode active material containing large-sized active material particles and small-sized active material particles having a smaller diameter than the large-sized active material particles, each composed of a lithium transition metal oxide; and forming a positive electrode composite layer containing the large-sized active material particles, small-sized active material particles, and a conductive material on a positive electrode current collector. The step of forming the positive electrode active material includes the steps of forming large-sized active material particles having a substantially spherical or substantially ellipsoidal shape; forming hollow active material particles having a substantially spherical shell shape or a substantially ellipsoidal shell shape; and pulverizing the hollow active material particles to form small-sized active material particles having a substantially spherical crown shape in which a portion of the hollow active material particle is cut off along a cut surface. [Effects of the Invention]

[0011] The present disclosure can provide a positive electrode for a secondary battery that can provide a secondary battery that achieves both high energy density and high input / output characteristics, and a method for manufacturing 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 showing large particle size active material particles contained in the positive electrode shown in FIG. [Figure 3] 2 is a cross-sectional view showing small particle diameter active material particles contained in the positive electrode shown in 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 material for the small particle size active material particle shown in FIG. [Figure 6] FIG. 2 is a diagram showing an example of small particle diameter active material particles. [Figure 7] 10A and 10B are diagrams showing other examples of small particle diameter active material particles. [Figure 8] 10A and 10B are diagrams showing other examples of small particle diameter active material particles. [Figure 9] 1 is a table illustrating examples, reference examples, and comparative examples. [Figure 10] 10 is a graph showing the amount of voltage drop in various test battery cells. [Figure 11] FIG. 10 is a diagram illustrating a configuration of a reference example. 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" 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 the "minor axis length" on the outer surface of the active material particles.

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

[0016] For example, when the active material particles are small-diameter active material particles 50, the surface disposed on the outside is a convex curved surface 53, and the surface disposed on the inside is a concave curved surface 54. When the active material particles are hollow active material particles 150, the surface disposed on the outside is an outer surface 153, and the surface disposed on the inside is an inner surface 154.

[0017] Furthermore, the "hole diameter" in this embodiment can be determined as the average value of the diameters of the narrowest parts of a plurality of arbitrarily selected through-holes 56 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 visualization of the material's structure, including its internal structure.

[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 FIG. 1. FIG. 1 is a diagram showing a positive electrode according to the first embodiment. FIG. 1 shows a three-dimensional model of the positive electrode 1. As shown in FIG. 1, the positive electrode 1 has a positive electrode current collector 10 and a positive electrode composite layer 20 formed on the positive electrode current collector 10. Note that the three-dimensional model shown in FIG. 1 omits illustration of components other than the positive electrode current collector 10, large-particle-size active material particles 40, and small-particle-size active material particles 50.

[0022] The positive electrode current collector 10 is formed in a plate or foil shape and is made of a metal with good conductivity. For the positive electrode current collector 10 used in the positive electrode 1, for example, aluminum, an aluminum alloy, nickel, titanium, stainless steel, etc. can be used.

[0023] Positive electrode mixture layer 20 contains at least positive electrode active material 30 and a conductive material. Positive electrode mixture layer 20 may also contain additives such as a dispersant and a binder.

[0024] The conductive material is a material for forming a conductive path in the positive electrode composite layer 20. By mixing an appropriate amount of conductive material into the positive electrode composite layer 20, it is possible to increase the electronic conductivity in the positive electrode 1 and improve 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), ketjen black) and carbon fibers (e.g., carbon nanotubes, carbon nanofibers) can be used.

[0025] 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.

[0026] The positive electrode active material 30 is capable of absorbing and releasing lithium ions, which are charge carriers. The positive electrode active material 30 includes large-sized active material particles 40 and small-sized active material particles 50, which are smaller in size than the large-sized active material particles 40. The large-sized active material particles 40 and the small-sized active material particles 50 are each composed of a lithium transition metal oxide. 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.

[0027] Each of the large-diameter active material particles 40 and the small-diameter active material particles 50 may additionally contain one or more elements in addition to a transition metal element (i.e., at least one of Ni, Co, and Mn). The additional element may include any element 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 element boron or metals such as aluminum), and Group 17 (halogens such as fluorine) of the periodic table.

[0028] In a preferred embodiment, each of the large-diameter active material particles 40 and the small-diameter active material particles 50 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)

[0029] 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.

[0030] The filling rate of the positive electrode active material 30 contained in the positive electrode 1 is preferably 50% or more, and more preferably 60% or more. Thereby, a secondary battery with a high discharge capacity can be obtained. In FIG. 1, a positive electrode 1 with a filling rate of the positive electrode active material 30 of 75% is shown.

[0031] The positive electrode active material 30 has at least two peaks in its particle size distribution. The positive electrode active material 30 of the present embodiment contains two types of active material particles, namely large particle size active material particles 40 and small particle size active material particles 50, such that its particle size distribution has two peaks. The large particle size active material particles 40 and the small particle size active material particles 50 may be compounds having the same composition or compounds having different compositions from each other.

[0032] Here, referring to FIGS. 2 and 3, the details of the large particle size active material particles 40 and the small particle size active material particles 50 will be described. FIG. 2 is a cross-sectional view showing an example of the large particle size active material particles contained in the positive electrode shown in FIG. 1. FIG. 3 is a cross-sectional view showing an example of the small particle size active material particles contained in the positive electrode shown in FIG. 1.

[0033] First, as shown in FIG. 2, large-sized active material particles 40, which correspond to the large-sized peak in the particle size distribution of the positive electrode active material 30, are formed in a substantially spherical or substantially oval spherical shape. Each large-sized active material particle 40 has a particle form. The structure of the large-sized active material particles 40 is not particularly limited, but a solid structure is preferable from the viewpoint of achieving a high energy density. The large-sized active material particles 40 may be primary particles or secondary particles formed by aggregation of primary particles. In a preferred embodiment, the large-sized active material particles 40 are primary particles.

[0034] Here, the term "primary particle" refers to a particle that can be considered as a unit particle (ultimate particle) based on its apparent geometric shape. In the large-sized active material particles 40, the primary particles are typically aggregates of lithium transition metal oxide crystallites. The shape of the large-sized active material particles 40 can be observed using images obtained by SEM observation.

[0035] The average diameter R1 of the large-sized active material particles 40 is preferably 5 μm or more, more preferably 7 μm or more. If the average diameter R1 of the large-sized active material particles 40 is too small, the gaps between the large-sized active material particles 40 become small, making it difficult for the small-sized active material particles 50 to enter the gaps between the large-sized active material particles 40. As a result, the filling rate of the positive electrode active material 30 decreases, which may result in a decrease in the electrode density of the positive electrode 1. Furthermore, the average diameter R1 of the large-sized active material particles 40 is preferably 20 μm or less, more preferably 15 μm or less. If the average diameter R1 of the large-sized active material particles 40 is too large, the diffusion resistance of lithium ions within the large-sized active material particles 40 increases, which may result in a decrease in the input / output characteristics of a secondary battery including the positive electrode 1.

[0036] 3, small-diameter active material particles 50 corresponding to the small-diameter peak in the particle size distribution of positive electrode active material 30 have a spherical crown shell portion 52 formed in a substantially spherical crown shape by cutting a portion of a hollow active material particle 150 having a substantially spherical shell or a substantially ellipsoidal shell shape along a cut surface 51. These small-diameter active material particles 50 have a substantially spherical crown-shaped convex surface 53 that protrudes in one direction, and a substantially spherical crown-shaped concave surface 54 on the opposite side that is concave in one direction. Furthermore, small-diameter active material particles 50 have a recess 55 that is concave in a substantially spherical crown shape near the center of the end face toward convex surface 53.

[0037] Here, the spherical cap refers to the side surface of a spherical notch formed by cutting a portion of a sphere or an oval sphere along any plane (cut plane 51 in this embodiment). The shape of the small-particle-size active material particles 50 can also be said to be roughly bowl-shaped. When the convex curved surface 53 and the concave curved surface 54 are curved in this way, bulky small-particle-size active material particles 50 are formed, and therefore, overlapping between the small-particle-size active material particles 50 and between the large-particle-size active material particles 40 and the small-particle-size active material particles 50 in the positive electrode mixture layer 20 can be suppressed.

[0038] As a result, in a positive electrode 1 using small-diameter active material particles 50 as the small-diameter active material particles, the reduction in the reaction field and the conductive path due to the increase in contact points between the active material particles (between the large-diameter active material particles 40 and the small-diameter active material particles 50, and between the small-diameter active material particles 50) is suppressed. By ensuring the reaction field of the active material particles, the resistance to lithium ion migration at the solid-liquid interface can be reduced. Furthermore, the increase in conductive paths can improve the electronic conductivity within the positive electrode 1. Furthermore, even when the positive electrode 1 is densified, sufficient voids can be ensured between the active material particles to serve as permeation paths for the electrolyte. As a result, the input / output characteristics of a secondary battery including the positive electrode 1 are improved.

[0039] Each small-diameter active material particle 50 has a particle shape. The spherical crown shell portion 52 constituting the small-diameter active material particle 50 is a secondary particle formed by a series of primary particles of a lithium transition metal oxide, forming a roughly spherical crown shape. In the small-diameter active material particle 50, the primary particles are typically aggregates of lithium transition metal oxide crystallites. The shape of the small-diameter active material particle 50 can be observed using images obtained by SEM observation.

[0040] The average diameter R2 of the convex curved surfaces 53 of the small-sized active material particles 50 is preferably 2.5 μm or more, more preferably 3 μm or more. As the average diameter R2 of the convex curved surfaces 53 decreases, the specific surface area of ​​the small-sized active material particles 50 increases. However, in a positive electrode 1 having a high filling rate of the positive electrode active material 30, the small-sized active material particles 50 occupy the voids between the large-sized active material particles 40. In this state, the number of contact points between the active material particles increases more than necessary, making it difficult to form conductive paths. Therefore, when the average diameter R2 of the convex curved surfaces 53 is relatively small, a larger amount of conductive material is required to ensure sufficient electronic conductivity in the positive electrode mixture layer 20 than when the average diameter R2 of the convex curved surfaces 53 is relatively large. Furthermore, when the number of contact points between the active material particles increases more than necessary, there is a risk that the reaction fields that can contribute to the electrochemical reaction of the positive electrode active material 30 may decrease.

[0041] The average diameter R2 of the convex curved surface 53 is less than 10 μm, preferably 8 μm or less, and more preferably 5 μm or less. If the average diameter R2 of the convex curved surface 53 is too large, the small-sized active material particles 50 will have difficulty entering the gaps between the large-sized active material particles 40. As a result, the filling rate of the positive electrode active material 30 will decrease, and the electrode density of the positive electrode 1 may decrease. From the viewpoint of productivity, in a preferred embodiment, the average diameter R2 of the convex curved surface 53 is 4 μm or more and 6 μm or less.

[0042] When the average diameter R2 of the convex curved surface 53 is 2.5 μm or more, the diameter ratio, which is the ratio of the average diameter R2 of the convex curved surface 53 to the average diameter R1 of the large-particle-size active material particles 40, is preferably 0.8 or less. With such a diameter ratio, the small-particle-size active material particles 50 can easily enter the gaps between the large-particle-size active material particles 40. As a result, the filling rate of the positive electrode active material 30 is increased, and the electrode density of the positive electrode 1 is improved.

[0043] Furthermore, the volume ratio, which is the ratio of the volume of the small-sized active material particles 50 to the volume of the large-sized active material particles 40 in the positive electrode mixture layer 20, is preferably 0.2 or more and 0.9 or less. By configuring this volume ratio within this range, the small-sized active material particles 50 can efficiently fill the gaps between the large-sized active material particles 40. As a result, the filling rate of the positive electrode active material 30 is increased, and the electrode density of the positive electrode 1 is improved. If the proportion of either the large-sized active material particles 40 or the small-sized active material particles 50 is higher than necessary, the filling rate of the positive electrode active material 30 decreases, which is not preferable.

[0044] The thickness T of the spherical crown shell portion 52 is less than 2.0 μm, preferably 1.5 μm or less, and more preferably 1.0 μm or less. The thinner the thickness T of the spherical crown shell portion 52, 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 52 (the central portion of the thickness T) during charging of the secondary battery and easier for lithium ions to be absorbed into the interior of the spherical crown shell portion 52 during discharging of the secondary battery. From the perspective of durability, the lower limit of the thickness T of the spherical crown shell portion 52 is preferably 0.5 μm or more. If the thickness T of the spherical crown shell portion 52 is less than 0.5 μm, durability against stresses that may be applied during manufacturing or use decreases. From the perspective of achieving both an internal resistance reduction effect and durability, in one preferred embodiment, the thickness T of the spherical crown shell portion 52 is 0.5 μm or more but less than 1.0 μm.

[0045] The primary particles may be in a single layer or in multiple layers in the thickness direction of the spherical cap shell portion 52. In a preferred embodiment, the small-diameter active material particles 50 are configured such that the primary particles are connected in a substantially single layer throughout the entire spherical cap shell portion 52.

[0046] Furthermore, the small-particle-size active material particles 50 preferably have a plurality of through-holes 56 that penetrate the spherical crown shell portion 52 from the outside to the recessed portion 55. The through-holes 56 are formed as gaps between the plurality of primary particles that make up the spherical crown shell portion 52. The diameter D of the through-holes 56 is preferably 0.1 μm or greater. By providing the small-particle-size active material particles 50 with through-holes 56 having such a diameter D, the electrolyte can smoothly flow between the outside and the recessed portion 55 through the through-holes 56.

[0047] Furthermore, the volume ratio of the small-diameter active material particle 50 having the through holes 56, which is the ratio of the volume of the spherical cap shell portion 52 to the total volume of the spherical cap shell portion 52 and the through holes 56, is preferably 0.5 to 0.8, more preferably 0.7 to 0.8. Here, the total volume of the spherical cap shell portion 52 and the through holes 56 is the sum of the volume of all the primary particles constituting one small-diameter active material particle 50 and the volume of all the through holes 56. Meanwhile, the volume of the spherical cap shell portion 52 is the volume of all the primary particles constituting one small-diameter active material particle 50. By configuring the volume ratio within this range, the small-diameter active material particle 50 becomes porous with an appropriate porosity, thereby facilitating electrolyte penetration even in a highly densified positive electrode 1. Furthermore, sufficient conductive paths are formed between the active material particles.

[0048] If the volume ratio is outside the above range and is smaller than necessary, it may be difficult to form conductive paths between the active material particles. On the other hand, if the volume ratio is outside the above range and is larger than necessary, it may be difficult to form the through holes 56 necessary for the smooth flow of the electrolyte, which may reduce the permeability of the electrolyte in the positive electrode 1.

[0049] Characteristic values ​​such as volume ratios can be calculated based on measurements of a plurality of arbitrarily selected active material particles using image analysis of a 3D model obtained by FIB-SEM measurement. For example, 3D model data of active material particles 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. Furthermore, in the case of small-sized active material particles 50, the total volume of the spherical crown shell portion 52 and the through-holes 56 can be calculated for each particle based on the 3D model data. Furthermore, the connected voids within the small-sized active material particles 50 can be extracted from the 3D model data of the small-sized active material particles 50, and the volume of the through-holes 56 can be calculated based on the extracted data. The volume of the spherical crown shell portion 52 can then be calculated based on the 3D model data of the small-sized active material particles 50 from which the extracted connected voids have been removed.

[0050] 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 S7. Note that steps S4 and S6 can be omitted if the quality of the manufactured positive electrode 1 can be ensured, and can also be replaced by other methods.

[0051] Step S1 is a step of forming large-particle-size active material particles 40. Step S2 is a step of forming hollow active material particles 150. Step S3 is a step of pulverizing the hollow active material particles 150 to form small-particle-size active material particles 50. Step S4 is a step of confirming whether the physical properties of the small-particle-size active material particles 50 obtained in step S3 are appropriate compared to predetermined values. Step S5 is a step of preparing a composite layer-forming paste for forming the positive electrode composite layer 20. Step S6 is a step of confirming whether the viscosity of the composite layer-forming paste prepared in step S5 is appropriate compared to a predetermined value. Step S7 is a step of applying the composite layer-forming paste onto the positive electrode current collector 10 to form the positive electrode composite layer 20.

[0052] Each of the above steps will be described in detail. First, in step S1, the method for forming the large-sized active material particles 40 may be any method capable of forming large-sized active material particles 40 having a desired average diameter R1 and a desired composition and a substantially spherical or elliptical shape. For example, the large-sized active material particles 40 can be obtained by pulverizing raw materials having a desired composition using a pulverizer such as a jet mill. Furthermore, by controlling the pulverization conditions during pulverization, large-sized active material particles 40 having a desired average diameter R1 can be formed.

[0053] In step S2, hollow active material particles 150 are formed as materials for the small-diameter active material particles 50. Fig. 5 is a cross-sectional view showing an example of a hollow active material particle as a material for the small-diameter active material particles shown in Fig. 3.

[0054] 5, the hollow active material particle 150 has a particle form with a hollow structure having a spherical shell portion 152 and a hollow portion 155 formed inside the spherical shell portion 152, and is formed in the shape of a substantially spherical shell or a substantially ellipsoidal shell. That is, the outer shape of the hollow active material particle 150 is generally spherical or ellipsoidal (e.g., a slightly distorted spherical shape). The hollow active material particle 150 also has an outer surface 153 and an inner surface 154, both of which are formed in the shape of a sphere or an ellipsoidal sphere.

[0055] The spherical shell 152 is a secondary particle formed by connecting primary particles of a lithium transition metal oxide to form an approximately spherical or elliptical shell shape. A hollow 155 is formed inside the spherical shell 152. In the thickness direction of the spherical shell 152, the primary particles may be a single layer or multiple layers. In a preferred embodiment, the hollow active material particle 150 is configured such that the primary particles are connected in a substantially single layer throughout the entire spherical shell 152. In the hollow active material particle 150, the primary particles are typically an aggregate of crystallites of the lithium transition metal oxide. The shape of the hollow active material particle 150 can be observed using images obtained by SEM observation.

[0056] The average diameter R2 of the outer surface 153 of the hollow active material particle 150 is approximately equal to the average diameter R2 of the convex curved surface 53. The outer surface 153 is the surface that corresponds to the outline of the hollow active material particle 150. The thickness T of the spherical shell portion 152 is approximately equal to the thickness T of the spherical crown shell portion 52.

[0057] The diameter of the inner surface 154 of the hollow active material particle 150 can be obtained by subtracting the thickness T of the spherical shell portion 152 from the average diameter R2 of the outer surface 153. The diameter of the inner surface 154 is approximately the same as the diameter of the concave curved surface 54. The inner surface 154 is the surface that corresponds to the outline of the hollow portion 155.

[0058] Furthermore, the hollow active material particle 150 preferably has a through-hole 56 that penetrates the spherical shell portion 152 from the outside to the hollow portion 155. As described above, this through-hole 56 serves as a permeation path for the electrolyte in the small particle size active material particle 50.

[0059] The method for forming the hollow active material particles 150 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 150 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 reduced compared to 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 150 (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 150, which are the target product.

[0063] The firing step is a step in which the mixture is fired to obtain hollow active material particles 150. 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, whereby the primary particles are sintered together to form hollow active material particles 150 in the shape of a roughly spherical shell or roughly oval spherical shell, and hollow portions 155 are formed inside the hollow active material particles 150. The hollow active material particles 150 thus formed are composed of a lithium transition metal oxide having a layered crystal structure.

[0065] Returning to Fig. 4, in step S3, the hollow active material particles 150 obtained in step S2 are fed into a pulverizer, and a shearing force is applied to the hollow active material particles 150, thereby forming small-diameter active material particles 50 in which a portion of the hollow active material particles 150 is cut off along a cutting surface 51. A jet mill is preferably used as the pulverizer used to pulverize the hollow active material particles 150. The pulverization method may be either a dry method or a wet method.

[0066] Here, the structure of the small particle size active material particles 50 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 small particle size active material particle. Figure 7 is a diagram showing another example of a small particle size active material particle. Figure 8 is a diagram showing another example of a small particle size active material particle.

[0067] 6 to 8 are intended to schematically represent the convex curved surfaces 53 of the small-diameter active material particles 50. Furthermore, the dashed lines in FIGS. 6 to 8 are intended to schematically represent the outer surfaces 153 of the hollow active material particles 150 that are the raw material for the small-diameter active material particles 50. FIGS. 6 to 8 show a case in which the small-diameter active material particles 50 and the hollow active material particles 150 are arranged overlapping each other so that the entire small-diameter active material particle 50 coincides with a part of the hollow active material particle 150. The convex curved surfaces 53 and the outer surfaces 153 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 150 coincides with the center of curvature O of the small particle size active material particle 50. The center of curvature O of the hollow active material particle 150 is the same as the center of curvature O of the outer surface 153 and the center of curvature O of the inner surface 154, and the center of curvature O of the small particle size active material particle 50 is the same as the center of curvature O of the convex curved surface 53 and the center of curvature O of the concave curved surface 54.

[0069] As shown in FIGS. 6 to 8 , the cut surface 51 dividing the hollow active material particle 150 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 51 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 53 on the cut surface 51 side and the major axis of the convex curved surface 53 is θ. In this embodiment, the angle θ is preferably within ±15°. That is, the small-diameter active material particle 50 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 cut surface 51 is a plane passing through the center of curvature O, and the small-diameter active material particles 50 are formed in a hemispherical shape. When θ=15°, the cut surface 51 is a plane located above the center of curvature O and spaced apart, and the small-diameter active material particles 50 are a solid with a relatively large volume that includes the center of curvature O. When θ=−15°, the cut surface 51 is a plane located below the center of curvature O and spaced apart, and the small-diameter active material particles 50 are 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 51 may be flat or may be non-flat, such as uneven. In this way, the shape of the small-diameter active material particles 50 is determined depending on the position and shape of the cut surface 51.

[0073] Furthermore, in this embodiment, the proportion of active material particles formed with an angle θ of ±15° among the small-diameter active material particles 50 contained in the positive electrode composite layer 20 is preferably 30% by mass or more and 100% by mass or less, and more preferably 50% by mass or more and 100% by mass or less. This proportion further enhances the effect of improving the input / output characteristics of a secondary battery including the positive electrode 1. The positive electrode composite layer 20 may contain other active material particles in addition to the small-diameter active material particles 50 formed with an angle θ of ±15° or less. 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.

[0074] For the outer surface 153 of the hollow active material particle 150, 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 154 of the hollow active material particle 150, 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 small-diameter active material particles 50 are formed using hollow active material particles 150 having such a ratio, the front and back surfaces (convex curved surface 53 and concave curved surface 54) of the small-diameter active material particle 50 have curved shapes, making the particles bulky.

[0075] In the pulverization using the jet mill in step S3, 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 150, and the number of pulverizations. The pulverization conditions are preferably set so that the hollow active material particles 150 can be cut along a cut surface 51 along the major axis, which is the longest diameter passing through the center of curvature O of the hollow active material particles 150. In other words, by controlling the pulverization conditions, it is possible to form active material particles 50 with the desired small particle size.

[0076] The pulverization conditions are not particularly limited and vary depending on the pulverizer and hollow active material particles 150 used. After pulverization, the pulverized product is classified to remove fine powder, thereby obtaining small-diameter active material particles 50 having the desired average diameter R2. 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.

[0077] In this way, small-diameter active material particles 50 can be obtained by pulverizing hollow active material particles 150. The spherical cap shell portion 52 of the small-diameter active material particle 50 corresponds to the spherical shell portion 152 of the hollow active material particle 150, and is obtained by cutting away a portion of the spherical shell portion 152 by the cut surface 51. The convex curved surface 53 of the small-diameter active material particle 50 corresponds to the outer surface 153 of the hollow active material particle 150, and is obtained by cutting away a portion of the outer surface 153 by the cut surface 51. The concave curved surface 54 of the small-diameter active material particle 50 corresponds to the inner surface 154 of the hollow active material particle 150, and is obtained by cutting away a portion of the inner surface 154 by the cut surface 51.

[0078] The concave curved surface 54 is located on the opposite side of the protruding direction of the convex curved surface 53, and the concave curved surface 54 and the convex curved surface 53 face each other. The recess 55 corresponds to the hollow portion 155 of the hollow active material particle 150, and is defined by the concave curved surface 54. The cut surface 51 is a portion connecting the tip edge of the convex curved surface 53 and the tip edge of the concave curved surface 54. The cut surface 51 has a substantially annular shape surrounding the opening of the recess 55.

[0079] 4, in step S4, the physical properties of the small-diameter active material particles 50 obtained in step S3 are measured to confirm whether the measured values ​​of the physical properties of the small-diameter active material particles 50 are greater than predetermined values. For example, the oil absorption and BET specific surface area can be used as the physical properties of the small-diameter active material particles 50. Furthermore, the predetermined values ​​in step S4 can be the oil absorption and BET specific surface area of ​​the hollow active material particles 150 that are the raw material for the small-diameter active material particles 50.

[0080] In this step, at least one of the oil absorption and the BET specific surface area is measured for the small-diameter active material particles 50. If the measurement result (measured value) for the physical property of the small-diameter active material particles 50 is greater than the measurement result (predetermined value) for the same physical property of the hollow active material particles 150 (step S4; YES), the process proceeds to the next step S5. On the other hand, if the measurement result (measured value) for the physical property of the small-diameter active material particles 50 is equal to or less than the predetermined value (step S4; NO), it is considered that the desired small-diameter active material particles 50 have not been obtained due to insufficient pulverization or other reasons, and the process returns to step S3.

[0081] Next, in step S5, a composite layer-forming paste for forming the positive electrode composite layer 20 is prepared using the large-particle-size active material particles 40 and the small-particle-size active material particles 50 whose physical properties have been confirmed. The composite layer-forming paste prepared here is obtained by kneading at least the positive electrode active material 30 (large-particle-size active material particles 40, small-particle-size active material particles 50), a conductive material, 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 the positive electrode active material 30 (large-particle-size active material particles 40, small-particle-size active material particles 50), a conductive material, a binder, and a solvent in a predetermined ratio.

[0082] In step S6, the viscosity of the composite layer forming paste obtained in step S5 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 S6 can be the viscosity of a reference paste prepared by incorporating hollow active material particles 150 into the composite layer forming paste instead of small active material particles 50. The reference paste is obtained by kneading at least large active material particles 40, hollow active material particles 150, a conductive material, and a solvent in a predetermined ratio that is recognized as being the same 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 within the manufacturing variation range. In this embodiment, the reference paste is obtained by kneading large active material particles 40, hollow active material particles 150, a conductive material, a binder, and a solvent in a predetermined ratio.

[0083] 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 S6; YES), the process proceeds to the next step S7. 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 S6; NO), it is considered that the desired small-diameter active material particles 50 have not been obtained due to insufficient pulverization or other reasons, and the process returns to step S3.

[0084] In step S7, the composite layer forming paste is applied to the surface of positive electrode current collector 10, dried to volatilize the solvent, and the dried product is pressed as necessary. In this way, positive electrode composite layer 20 can be formed on positive electrode current collector 10.

[0085] The amount of positive electrode mixture layer 20 applied per unit area on positive electrode current collector 10 is not particularly limited, but from the viewpoint of ensuring a sufficient conductive path, it is set to 3 mg / cm 2 per one side of positive electrode 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.

[0086] The coating amount per one surface of the positive electrode current collector 10 was 45 mg / cm 2 Less than 28 mg / cm is preferred 2 Less than 15 mg / cm is more preferred, especially 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 3 Preferably, it 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, and particularly 1.8 g / cm 3 More than 2.4g / cm 3 It is preferable to do the following:

[0087] The positive electrode according to this embodiment can be manufactured by the above manufacturing method. Note that steps S1 to S4 are an example of a process for forming the positive electrode active material 30.

[0088] Next, Examples 1 to 9, Reference Examples 1 and 2, and Comparative Examples 1 to 11 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, Reference Examples, and Comparative Examples.

[0089] First, 22 types of positive electrodes were manufactured containing active material particles having the structure shown in Fig. 9. In Examples 1 to 9, Reference Examples 1 and 2, and Comparative Examples 1 to 11, the active material particles contained in each positive electrode were all LiNi (1-X-Y) Co X Mn Y A lithium nickel cobalt manganese composite oxide (NCM) having an average composition expressed as O2, 0≦X≦0.35, and 0≦Y≦0.35 was used. Of the active material particles contained in each positive electrode, the larger active material particles (hereinafter sometimes referred to as large particles) were all large active material particles 40 with an average diameter R1 of 10 μm composed of primary particles with a solid structure, and the smaller active material particles (hereinafter sometimes referred to as small particles) were all different.

[0090] The diameter ratio A shown in the table of Fig. 9 is the ratio of the average diameter of the outer surface of the small particles to the average diameter of the large particles. The volume ratio B shown in the table of Fig. 9 is the ratio of the volume of the small particles to the volume of the large particles in the positive electrode mixture layer. The volume ratio C shown in the table of Fig. 9 is the ratio of the volume of the shell to the total volume of the spherical crown-shaped shell and through holes 56 that make up the small particles.

[0091] Each positive electrode was manufactured as follows. First, large particles (large-diameter active material particles 40) and small particles (small-diameter active material particles 50, solid active material particles 140, or spherical-crown-shaped active material particles) were mixed in the volume ratio B shown in Figure 9 to obtain a positive electrode active material. The obtained positive electrode active material was mixed with AB as a conductive material and PVdF as a binder in a mass ratio of 90:5:5, and N-methyl-2-pyrrolidone was added as a solvent and kneaded to prepare a paste. The prepared paste was applied to one side of aluminum foil, which served as a positive electrode current collector 10, to a specified thickness and coating weight, and then dried and pressed to produce 22 types of sheet-shaped positive electrodes.

[0092] Then, for each of the produced positive electrodes, the positive electrode composite layer side of the positive electrode was placed opposite a lithium metal foil (lithium metal counter electrode) and a separator was placed between them. The electrode body was then housed inside an aluminum laminate exterior material, and an electrolyte solution was added and sealed to construct a laminated evaluation battery cell (half cell).

[0093] The separator of each test battery cell was a polyolefin porous film, and the electrolyte of each test battery cell 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.

[0094] The active material particles contained in each positive electrode will be described.

[0095] (Reference examples 1~2) With reference to FIG. 11, the positive electrodes constituting the test battery cells of Reference Examples 1 and 2 will be described. FIG. 11 is a diagram illustrating the configuration of the reference example. The three-dimensional model shown in the upper part of FIG. 11 shows the positive electrode constituting the test battery cell of Reference Example 1. Note that the three-dimensional model shown in the upper part of FIG. 11 omits components other than the positive electrode current collector 10, the large-particle-size active material particles 40, and the solid active material particles 140. The three-dimensional model shown in the lower part of FIG. 11 shows the positive electrode constituting the test battery cell of Reference Example 2. Note that the three-dimensional model shown in the lower part of FIG. 11 omits components other than the large-particle-size active material particles 40 and the solid active material particles 140.

[0096] As shown in Fig. 11, in Reference Examples 1 and 2, large-diameter active material particles 40 were used as the large particles, and solid active material particles 140 were used as the small particles. Each of the solid active material particles 140 used in Reference Examples 1 and 2 is a substantially spherical primary particle having a solid structure. Specifically, in Reference Example 1, solid active material particles 140 with an average diameter of 5 µm were used. In Reference Example 2, solid active material particles 140 with an average diameter of 1 µm were used. Reference Examples 1 and 2 have the same configuration except that the average diameters of the solid active material particles 140 are different from each other.

[0097] Examples 1 to 9 Examples 1 to 9 are test battery cells constructed with positive electrodes 1 manufactured according to the flow shown in Fig. 4. In Examples 1 to 9, large-diameter active material particles 40 were used as the large particles, and small-diameter active material particles 50 were used as the small particles. The positive electrodes 1 constituting the test battery cells of Examples 1 to 9 each conform to the following conditions 1 to 4 shown in the optimum conditions column of Fig. 9. (Condition 1) Shell thickness: 0.5 μm or more and 1.5 μm or less, (Condition 2) Average diameter of small particles: 2.5 μm or more (diameter ratio A≦0.8), (Condition 3) Volume ratio B: 0.2 or more and 0.9 or less, (Condition 4) Volume ratio C: 0.5 or more and 0.8 or less

[0098] (Comparative Examples 1 to 11) In Comparative Examples 1 to 11, large-diameter active material particles 40 were used as the large particles, and spherical-crown-shaped active material particles were used as the small particles. The positive electrodes constituting the test battery cells of Comparative Examples 1 to 11 did not meet the above-mentioned conditions 1 to 4. The spherical-crown-shaped active material particles used in Comparative Examples 1 to 11 were secondary particles formed by agglomerating primary particles into a roughly spherical-crown shape. Each spherical-crown-shaped active material particle was obtained by appropriately adjusting the manufacturing conditions for forming the hollow active material particles 150 and the small-diameter active material particles 50, as described in the flow chart of FIG. 4.

[0099] The 22 types of test battery cells were evaluated by measuring the voltage drop during high-current discharge, which is the amount of change in battery voltage during charge and discharge. To measure the voltage drop, the 22 types of test battery cells were placed in a 25°C environment and discharged at a current of 100 C from a state of charge of 60% SOC (State of Charge). The voltage drop for each was measured 1 second after the start of discharge. The results are shown in the results section of Figure 9. Figure 10 is a graph showing the voltage drop for each test battery cell. Figure 10 corresponds to the results section of Figure 9. Here, a difference of 0.01 V or more was considered significant. A lower voltage drop indicates a lower internal resistance, which indicates superior battery characteristics.

[0100] From the results shown in Figures 9 and 10, when comparing Examples 1 to 9 with Reference Examples 1 and 2, it was found that the amount of voltage drop during discharge can be reduced by using small-diameter active material particles 50 having an approximately spherical crown shape rather than simply using solid active material particles 140 having an approximately spherical shape and a small average diameter.

[0101] The reason for these results is believed to be that in Reference Example 1, the average diameter of the solid active material particles 140 was too large, increasing the diffusion resistance of lithium ions within the solid active material particles 140. Furthermore, in Reference Example 2, the average diameter of the solid active material particles 140 was too small, resulting in the voids between the large-diameter active material particles 40 being occupied by the solid active material particles 140. In this state, the number of contact points between the active material particles increases more than necessary, making it difficult to form conductive paths. This is believed to have reduced the number of conductive paths in the positive electrode composite layer and increased DC resistance. Furthermore, the tortuosity (degree of bending) of the migration path along which lithium ions in the electrolyte migrate in the voids formed between the active material particles increases, resulting in increased migration resistance of lithium ions.

[0102] Next, it was found that Examples 1 to 9, which met Conditions 1 to 4, were able to reduce the amount of voltage drop during discharge compared to Comparative Examples 1 to 11, which did not meet Conditions 1 to 4. Considering Comparative Examples 1 to 11, first, in Comparative Example 1, the shell thickness of the spherical-crown-shaped active material particles was too thick, which is thought to have increased the diffusion resistance of lithium ions within the spherical-crown-shaped active material particles.

[0103] In Comparative Example 2, the shell thickness of the spherical crown-shaped active material particles was too thin, so the fired material disappeared during the firing step during production, and spherical crown-shaped active material particles could not be obtained, making it impossible to produce the desired positive electrode.

[0104] In Comparative Example 3, the average diameter of the spherical-crown-shaped active material particles is too small, so similar to Reference Example 2, the voids between the large-diameter active material particles 40 are occupied by the spherical-crown-shaped active material particles.

[0105] In Comparative Example 4, the spherical crown-shaped active material particles arranged in the gaps between the large particle size active material particles 40 were crushed by pressing during production, and therefore a desired positive electrode could not be produced.

[0106] In Comparative Examples 5 to 8, it is believed that the filling rate of the positive electrode active material was reduced because the proportion of either the large particle size active material particles 40 or the spherical crown-shaped active material particles was too high.

[0107] In Comparative Example 9, the spherical crown-shaped active material particles did not have through-holes, which is thought to have reduced the permeability of the electrolyte solution in the positive electrode.

[0108] In Comparative Example 10, the volume ratio C was too small, which resulted in the porosity of the spherical-crown-shaped active material particles being higher than necessary, and it is thought that the conductive material was unable to form sufficient conductive paths between the active material particles.

[0109] In Comparative Example 11, it is believed that the diameter of the through-holes was too small, which hindered the flow of the electrolyte through the through-holes.

[0110] The following considerations are made regarding Examples 1 to 9. From the results of Examples 1 to 3, it is clear that the diffusion resistance of lithium ions in the small particle size active material particles 50 is reduced, and therefore within the range of Condition 1, the thinner the thickness T of the spherical crown shell portion 52, the better.

[0111] From the results of Examples 1 and 4, it is clear that the small-diameter active material particles 50 can easily enter the gaps between the large-diameter active material particles 40, and therefore within the range of Condition 2, the smaller the diameter ratio A, the better.

[0112] From the results of Examples 5 and 6, it is clear that within the range of Condition 3, the larger the volume ratio B, the better, since the small particle size active material particles 50 efficiently enter the gaps between the large particle size active material particles 40 .

[0113] The results of Examples 7 and 8 show that the through holes 56 promote the penetration of the electrolyte and facilitate the formation of conductive paths between the active material particles, and therefore, when the hole diameter D of the through holes 56 is constant within the range of Condition 4, a larger volume ratio C is preferable.

[0114] From the results of Examples 1 and 9, the penetration of the electrolyte is further promoted by the through holes 56, and therefore, within the range of at least 0.1 μm to 0.3 μm, the larger the hole diameter D, the more preferable.

[0115] As described above, the positive electrode for a secondary battery according to this embodiment includes a positive electrode current collector 10 and a positive electrode composite layer 20 formed on the positive electrode current collector 10 and containing a conductive material and a positive electrode active material 30. The positive electrode active material 30 includes large-sized active material particles 40 and small-sized active material particles 50, each of which is composed of a lithium transition metal oxide. The large-sized active material particles 40 are formed in a substantially spherical or ellipsoidal shape. The small-sized active material particles 50 each have a substantially spherical crown-shaped portion 52 formed by cutting a portion of a hollow active material particle 150 having a substantially spherical or ellipsoidal shell shape along a cut surface 51, a substantially spherical crown-shaped convex surface 53 that protrudes in one direction, and a substantially spherical crown-shaped concave surface 54 that is present on the opposite side of the one direction and is recessed in one direction.

[0116] With this configuration, the particle size distribution of the positive electrode active material 30 has at least two peaks, which increases the packing rate of the positive electrode active material 30 and improves the electrode density of the positive electrode 1. Therefore, a secondary battery including the positive electrode 1 can achieve a high energy density.

[0117] In addition, because the small-sized active material particles 50 are bulky and curved, using the small-sized active material particles 50 as the positive electrode active material 30 can suppress the reduction in reaction fields and conductive paths due to the increase in contact points between the active material particles, compared to when simply using smaller spherical particles as the small-sized active material particles. Furthermore, even in a high-density positive electrode 1, the bulkiness of the small-sized active material ensures sufficient voids between the active material particles to serve as electrolyte penetration paths, and reduces the tortuosity of the lithium ion migration path. For these reasons, this embodiment can improve the input / output characteristics of a secondary battery including the positive electrode 1.

[0118] Furthermore, in the positive electrode for a secondary battery according to this embodiment, the thickness T of the spherical crown shell portion 52 is preferably 0.5 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.

[0119] Furthermore, in the positive electrode for a secondary battery according to this embodiment, it is preferable that the average diameter R2 of the convex curved surfaces 53 is 2.5 μm or more, and the ratio of the average diameter R2 of the convex curved surfaces 53 to the average diameter R1 of the large active material particles 40 is 0.8 or less. With this configuration, the small active material particles 50 can easily enter the gaps between the large active material particles 40, thereby increasing the filling rate of the positive electrode active material 30 and improving the electrode density of the positive electrode 1.

[0120] Furthermore, in the positive electrode for a secondary battery according to this embodiment, the ratio of the volume of the small-sized active material particles 50 to the volume of the large-sized active material particles 40 in the positive electrode composite layer 20 is preferably 0.2 or more and 0.9 or less. With this configuration, the small-sized active material particles 50 efficiently fill the gaps between the large-sized active material particles 40, thereby further increasing the filling rate of the positive electrode active material 30.

[0121] Furthermore, in the positive electrode for a secondary battery according to this embodiment, the small-diameter active material particles 50 preferably have through-holes 56 that penetrate the spherical crown shell portion 52 and have a hole diameter D of 0.1 μm or more, and the ratio of the volume of the spherical crown shell portion 52 to the total volume of the spherical crown shell portion 52 and the through-holes 56 is preferably 0.5 to 0.8. This configuration promotes electrolyte penetration into the positive electrode 1, even in a highly densified positive electrode 1. It also prevents a reduction in the number of conductive paths within the positive electrode 1. As a result, the input / output characteristics of a secondary battery including the positive electrode 1 can be improved.

[0122] Furthermore, in the positive electrode for a secondary battery according to this embodiment, it is preferable that the angle θ between the direction from the center of curvature O of the convex curved surface 53 toward the tip of the convex curved surface 53 and the major axis of the convex curved surface 53 of the small particle size active material particles 50 is within ±15°. With this configuration, it is possible to obtain a positive electrode 1 with excellent balance of performance, in which the bulkiness of the small particle size active material particles 50 results in good electronic conductivity and a reaction field for the positive electrode active material 30.

[0123] Furthermore, in the positive electrode for a secondary battery according to this embodiment, it is preferable that the average diameter R2 of the convex curved surface 53 is 4 μm or more and 6 μm or less. With such a configuration, it is possible to reliably obtain high-quality small-diameter active material particles 50. Furthermore, since the small-diameter active material particles 50 efficiently fill the gaps between the large-diameter active material particles 40, it is possible to further increase the filling rate of the positive electrode active material 30.

[0124] Furthermore, the ratio of the major axis length to the minor axis length (L1 / L2) of the outer surface 153 of the hollow active material particle 150 is preferably 1.0 or more and less than 1.5. Furthermore, the ratio of the major axis length to the minor axis length of the inner surface 154 of the hollow active material particle 150 is preferably 1.0 or more and less than 1.8. When small-diameter active material particles 50 are formed using hollow active material particles 150 configured in this manner, the convex curved surface 53 and the concave curved surface 54 of the small-diameter active material particle 50 each have a moderately curved shape, making the particle bulky. This ensures a permeation path for the electrolyte within the positive electrode 1 and suppresses a decrease in the reaction field and conductive path due to an increase in contact points between the active material particles.

[0125] As described above, according to this embodiment, it is possible to provide a positive electrode for a secondary battery that can provide a secondary battery that has both high energy density and high input / output characteristics.

[0126] 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.

[0127] Furthermore, by appropriately measuring the physical properties of the small-diameter active material particles 50 obtained by pulverizing the hollow active material particles 150, it is possible to adjust the pulverization conditions while confirming whether the hollow active material particles 150 have been pulverized appropriately. Furthermore, by appropriately measuring the viscosity of the composite layer forming paste containing the small-diameter active material particles 50, it is possible to adjust the pulverization conditions while confirming whether the hollow active material particles 150 have been pulverized appropriately. With this configuration, it is possible to reliably obtain the desired small-diameter active material particles 50, and the quality of the positive electrode 1 produced using the small-diameter active material particles 50 is improved.

[0128] The present disclosure is not limited to the above-described embodiment, and can be modified as appropriate without departing from the spirit of the present disclosure. For example, in the above-described embodiment, the hollow active material particles 150 are pulverized using a jet mill, but this is not limiting and other pulverization methods may also be used. Examples of other pulverization methods that can be used include a ball mill, a bead mill, and a rod mill.

[0129] Furthermore, for example, when checking the physical properties of the small-diameter active material particles 50, the predetermined value may be the result of measuring the same physical properties of another substance that has a clear relationship with the physical properties of the small-diameter active material particles 50. Similarly, when checking the viscosity of the composite layer-forming paste, the predetermined value may be the 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]

[0130] 1 positive electrode 10 Positive electrode current collector 20 Positive electrode mixture layer 30 Cathode active material 40 Large particle size active material particles 50 Small size active material particles 51 Cut surface 52 Spherical crown shell 53 Convex curved surface 54 Concave Surface 55 recess 56 Through hole 140 Solid active material particles 150 Hollow active material particles 152 Spherical shell part 153 Exterior 154 Inside 155 Hollow part D Pore diameter L1 Long axis length L2 Minor axis length O center of curvature R1, R2 average diameter T Thickness

Claims

1. a positive electrode current collector; a positive electrode mixture layer formed on the positive electrode current collector and containing a conductive material and a positive electrode active material, The positive electrode active material is Each of the large-sized active material particles is made of a lithium transition metal oxide, and the small-sized active material particles are smaller in size than the large-sized active material particles, The large particle size active material particles are It is formed into a substantially spherical or substantially oval spherical shape, The small particle size active material particles are 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 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 The thickness of the spherical crown shell portion is 0.5 μm or more and 1.5 μm or less, The average diameter of the convex curved surface is 2.5 μm or more, the ratio of the average diameter of the convex curved surface to the average diameter of the large-diameter active material particles is 0.8 or less; a ratio of the volume of the small-particle-size active material particles to the volume of the large-particle-size active material particles in the positive electrode mixture layer is 0.2 or more and 0.9 or less; the small-diameter active material particles have through-holes that penetrate the spherical cap shell portion and have a diameter of 0.1 μm or more; A positive electrode for a secondary battery, wherein a ratio of the volume of the spherical crown shell portion to the total volume of the spherical crown shell portion and the through holes is 0.5 or more and 0.8 or less.

2. 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°.

3. 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.

4. 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.

5. 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.

6. forming a positive electrode active material including large-sized active material particles and small-sized active material particles each composed of a lithium transition metal oxide; forming a positive electrode mixture layer on a positive electrode current collector, the positive electrode mixture layer including the large-particle-size active material particles, the small-particle-size active material particles, and a conductive material; and The step of forming the positive electrode active material includes: forming the large-diameter active material particles into a substantially spherical or substantially oval spherical shape; forming hollow active material particles in the shape of a substantially spherical shell or a substantially elliptical shell; The hollow active material particles are pulverized, a substantially spherical crown-shaped spherical crown shell portion formed by cutting a part of the hollow active material particle 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; forming the small size active material particles having and The thickness of the spherical crown shell portion is 0.5 μm or more and 1.5 μm or less, The average diameter of the convex curved surface is 2.5 μm or more, the ratio of the average diameter of the convex curved surface to the average diameter of the large-diameter active material particles is 0.8 or less; a ratio of the volume of the small-particle-size active material particles to the volume of the large-particle-size active material particles in the positive electrode mixture layer is 0.2 or more and 0.9 or less; the small-diameter active material particles have through-holes that penetrate the spherical cap shell portion and have a diameter of 0.1 μm or more; a ratio of the volume of the spherical crown shell portion to the total volume of the spherical crown shell portion and the through hole being 0.5 or more and 0.8 or less.

7. In the step of forming the positive electrode active material, 7. The method for manufacturing a positive electrode for a secondary battery according to claim 6, wherein when a result of measuring at least one of the oil absorption amount and the BET specific surface area of ​​the small-particle-diameter active material particles is larger than a result of measuring the small-particle-diameter active material particles, the method proceeds to the step of forming the positive electrode composite layer.

8. The step of forming the positive electrode composite layer includes: a step of preparing a paste for forming a composite layer containing the large-particle-size active material particles, the small-particle-size active material particles, the conductive material, and a solvent in a predetermined ratio; 7. The method for manufacturing a positive electrode for a secondary battery according to claim 6, 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 large-particle-size active material particles, the hollow active material particles, the conductive material, and the solvent in the predetermined ratio, the composite layer-forming paste is applied onto the positive electrode current collector to form the positive electrode composite layer.

Images