JP2023137478A - Positive electrode for secondary battery and manufacturing method of positive electrode for secondary battery - Google Patents

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

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

Secondary batteries are widely used as so-called portable power sources for personal computers, mobile terminals, etc., and as power sources for driving vehicles. Among secondary batteries, lithium ion secondary batteries, which are lightweight and provide high energy density, are particularly suitable for use as high-output power sources for driving vehicles such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles. A lithium ion secondary battery is a secondary battery that can be charged and discharged by moving lithium ions (charge carriers) in an electrolyte between a positive electrode and a negative electrode.

An electrode used in a secondary battery such as a lithium ion secondary battery includes a conductive current collector and a composite material layer containing an electrode material such as an active material held by the current collector. In order to achieve high input/output in such secondary batteries, a positive electrode (hereinafter sometimes simply referred to as "positive electrode") for secondary batteries that can smoothly insert and remove charge carriers is required. Desired.

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

Special table 2018-536972 publication

By the way, for the purpose of improving battery characteristics, there is a technology in which a large number of through holes are provided in a current collector, a composite material layer is also formed in the through holes, and the electrolyte and charge carriers can flow through the through holes. It has been known. According to such a technique, not only the peel strength of the composite material layer is improved, but also the volume fraction of the electrolyte in the electrode is increased, and the permeation of the electrolyte from the bulk can be promoted.

On the other hand, if the active material filled in the through-hole blocks the through-hole, the flow of the electrolyte through the through-hole may deteriorate. For example, the technique described in Patent Document 1 does not assume that the electrolyte flows through the through-holes, so if the positive electrode active material described in Patent Document 1 is also filled into the through-holes, the through-holes become blocked. There is a problem in that the battery may come off and sufficient battery characteristics may not be obtained.

The present disclosure has been made to solve such problems, and provides a positive electrode for a secondary battery that reduces internal resistance and provides a secondary battery with good cycle characteristics, and a positive electrode for a secondary battery. The purpose is to provide a manufacturing method.

A positive electrode for a secondary battery according to an embodiment includes a porous current collector provided with a plurality of first through holes penetrating in the thickness direction, and a porous current collector provided with a plurality of first through holes penetrating the porous current collector on the surface of the porous current collector and in the first through holes. a positive electrode composite material layer formed and containing a positive electrode active material, the positive electrode active material being a part of hollow active material particles having a substantially spherical shell shape or an approximately elliptical spherical shell shape composed of a lithium transition metal oxide. A spherical cap shell part formed in a roughly spherical crown shape cut out by a cutting surface, a roughly spherical crown-shaped convex curved surface that protrudes in one direction, and a roughly spherical shell that exists on the opposite side of the one direction side and is concave in one direction. The first through-hole is filled with at least the spherical crown-shaped active material particles.

A method for manufacturing a positive electrode for a secondary battery according to an embodiment includes the steps of forming hollow active material particles in the shape of a substantially spherical shell or substantially elliptic spherical shell made of a lithium transition metal oxide; a step of pulverizing the hollow active material particles to form a positive electrode active material including spherical active material particles having a spherical cap shell portion formed in a substantially spherical cap shape with a part of the hollow active material particles cut away by a cut surface; forming a positive electrode composite layer containing a positive electrode active material on the surface of a porous current collector provided with a plurality of first through holes penetrating in the direction and in the first through holes; The pores are filled with at least spherical active material particles.

According to the present disclosure, it is possible to provide a positive electrode for a secondary battery and a method for manufacturing a positive electrode for a secondary battery, which can provide a secondary battery with reduced internal resistance and good cycle characteristics.

1 is a diagram showing a positive electrode according to Embodiment 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 shown in FIG. 1. FIG. FIG. 2 is a cross-sectional view showing spherical active material particles included in the positive electrode shown in FIG. 1. FIG. 3 is a flowchart showing a method for manufacturing a positive electrode according to Embodiment 1. FIG. 4 is a cross-sectional view showing hollow active material particles that are the material of the spherical active material particles shown in FIG. 3. FIG. FIG. 2 is a diagram showing an example of spherical active material particles. FIG. 7 is a diagram showing another example of spherical active material particles. FIG. 7 is a diagram showing another example of spherical active material particles. It is a table explaining examples, comparative examples, and reference examples. It is a graph which shows the resistance reduction rate (%) of the half cell for evaluation. FIG. 2 is a diagram showing a positive electrode using a positive electrode active material containing approximately spherical active material particles. 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. 11. FIG.

Embodiment 1
Embodiments of the present disclosure will be described below with reference to the drawings. Further, in order to clarify the explanation, the following description and drawings are simplified as appropriate. In the following description, the same or equivalent elements are given the same reference numerals, and overlapping description will be omitted.

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

In addition, unless otherwise specified, the "thickness" of the active material particles in this embodiment is determined based on image analysis of a three-dimensional model obtained by FIB-SEM measurement. The shortest distance from a plurality of positions on a surface placed inside a material particle to a surface placed on the outside is measured. The average value of the shortest distances measured at a plurality of positions on the inner surface of the active material particle can be determined as the "thickness".

For example, when the active material particles are spherical 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 the outer surface 133, and the surface disposed on the inside is the inner surface 134.

In addition, the "pore diameter" of the active material particles in this embodiment is the diameter of the narrowest part of a plurality of arbitrarily selected second through holes 36 from image analysis of a three-dimensional model obtained by FIB-SEM measurement. It can be determined as an average value.

Note that FIB-SEM means processing a sample using a focused ion beam (FIB) and observing the exposed cross section of the sample using a scanning electron microscope (SEM). do. As a method for processing a sample, for example, a sample hardened with a suitable resin may be cut at a desired cross section, and the cross section may be ground little by little while SEM observation is performed.

In the method using FIB-SEM, a sample is repeatedly processed intermittently by FIB and observed by SEM, and a three-dimensional model is constructed from the obtained SEM images to obtain a three-dimensional model. It is possible to visualize the structure of substances, including

Hereinafter, as one of the preferred embodiments of the positive electrode for a secondary battery according to the present 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 is charged and discharged by the movement of charge between a positive electrode 1 (positive electrode plate) and a negative electrode (negative electrode plate). Lithium ion secondary batteries are used, for example, as a power source for driving vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHEVs).

First, the configuration of the 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. Note that in FIGS. 1 and 2, illustrations of components other than the porous current collector 10 and the positive electrode active material 40 (spherical active material particles 30) are omitted.

As shown in FIGS. 1 and 2, the positive electrode 1 is a porous current collector provided with a first through hole 11 that penetrates in the thickness direction from one surface (one surface 12) to the other surface (other surface 13). 10, and a positive electrode composite material layer 20 that is formed on the surface of the porous current collector 10 and in the first through hole 11 and includes a positive electrode active material 40. This positive electrode active material 40 includes spherical active material particles 30 . The positive electrode 1 has a configuration in which the first through hole 11 is filled with at least spherical active material particles 30 .

The positive electrode 1 can constitute a secondary battery by combining with, for example, a negative electrode, an electrolyte containing lithium ions such as a non-aqueous electrolyte, and, if necessary, an insulating separator through which lithium ions can pass.

The porous current collector 10 is formed into a plate shape or a foil shape, and is made of a metal with good conductivity. For the porous current collector 10 used in the positive electrode 1, aluminum, aluminum alloy, nickel, titanium, stainless steel, etc. can be used, for example. Among these, aluminum is preferably used because it has 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, problems such as when applying a composite layer forming paste to the porous current collector 10 or when winding an electrode in the manufacturing process of a wound battery, etc. There is a high possibility that the porous current collector 10 will be torn due to stress that may be applied during manufacturing or use. When 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. , the energy density of secondary batteries shows a decreasing trend.

The porous current collector 10 is a porous current collector in which a plurality of first through holes 11 are provided. The first through hole 11 provided in the porous current collector 10 is formed, for example, in the shape of a slit. 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 an etched foil in which the first through holes 11 are formed by mechanical punching or other processing. It may also be made of expanded metal, punched metal, or the like.

The open area ratio (%) of the first through holes 11 in the porous current collector 10 is preferably 10% or less in terms of area ratio, and more preferably 1% or more and 5% or less. If the aperture ratio is too high than necessary, the abundance ratio of the electrical conductor in the positive electrode 1 decreases, so that the resistance increases and the cycle characteristics associated with charging and discharging tend to decrease. If the aperture ratio is too low than necessary, there is a possibility that the flow of the electrolytic solution and the lithium ions in the electrolytic solution through the first through hole 11 will be impaired.

The first through hole 11 is formed in a size that allows the spherical active material particles 30 to enter therein. The shape, arrangement, number, etc. of the first through-holes 11 are such that the electrolytic solution and lithium ions in the electrolytic solution can flow through the first through-holes 11 well while suppressing a decrease in the strength of the porous current collector 10. It is set appropriately so that

It is preferable that the lower limit of the length of the short side of the first through hole 11 is at least twice the average diameter R of the convex curved surface 33 of the spherical active material particle 30 and at most 1 mm. Details of the spherical active material particles 30 will be described later. Further, it is preferable that the lower limit of the length of the long side of the first through hole 11 is at least twice the length of the short side and 10 mm or less. Here, the long side of the first through hole 11 is arranged along the longitudinal direction of the porous current collector 10. The longitudinal direction of the porous current collector 10 is the direction in which the composite layer forming paste is applied, and in the case of a wound type battery, the direction in which the electrode is wound. The short side of the first through hole 11 is arranged along the width direction of the porous current collector 10, which is perpendicular to the long side.

By arranging the long side of the first through-hole 11 in the coating direction (or winding direction), it is possible to prevent damage during manufacturing or use, such as when coating a paste for forming a composite material layer or when winding an electrode. It is possible to suppress breakage of the porous current collector 10 that may occur due to stress and the like. On the other hand, if the long side of the first through hole 11 is larger than necessary, the strength of the porous current collector 10 will be insufficient, and the porous current collector 10 will be easily torn.

A positive electrode mixture layer 20 including at least spherical active material particles 30 is formed in the first through hole 11 . By forming the positive electrode composite material layer 20 not only on the surface of the porous current collector 10 but also in the first through hole 11, the peel strength of the positive electrode composite material layer 20 is improved. As a result, the storage stability and cycle characteristics of the secondary battery are improved.

The first through-hole 11 serves as a permeation path for an electrolytic solution included in the secondary battery constituted by the positive electrode 1. In particular, in the case of a secondary battery configured such that the positive electrode 1 in which the positive electrode composite layer 20 is formed on one side 12 of the porous current collector 10 faces the negative electrode with a separator in between, the surface of the negative electrode current collector The flow of the electrolytic solution is improved on the porous current collector 10 side of the positive electrode 1 that does not face the negative electrode composite material layer formed on the positive electrode 1 . Thereby, during discharge of the secondary battery, unreacted electrolyte can be supplied into the positive electrode composite layer 20 through the first through hole 11. Therefore, it is possible to obtain a secondary battery having a reduced internal resistance and good cycle characteristics in which a decrease in battery capacity is suppressed during charge/discharge cycles. As described above, the positive electrode 1 according to the present embodiment is suitable for a positive electrode 1 in which the positive electrode composite material layer 20 is formed on one side 12 of the porous current collector 10.

Note that the shape of the porous current collector 10 described above can be determined, for example, by SEM observation of the 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 from the positive electrode 1. I can do it.

The positive electrode composite material layer 20 includes at least a positive electrode active material 40 . In addition, the positive electrode composite material layer 20 may also contain a conductive material and, if necessary, additives such as a dispersant and a binder. The positive electrode composite material layer 20 is formed on at least one side 12 of the porous current collector 10 . In this embodiment, the positive electrode 1 will be described by taking as an example a case where the positive electrode composite material layer 20 is formed on one side 12 of the porous current collector 10, but depending on the purpose, one side of the porous current collector 10 may be A configuration may be adopted in which the positive electrode composite material layer 20 is formed on both surfaces 12 and 13.

The conductive material is a material for forming a conductive path within the positive electrode composite material layer 20. By mixing an appropriate amount of a conductive material into the positive electrode composite layer 20, it is possible to increase the electronic conductivity inside the positive electrode 1 and improve the charging/discharging efficiency and input/output characteristics of the battery. Examples of the conductive material include carbon materials such as various carbon blacks (e.g., acetylene black (AB), Ketjen black), carbon fibers (e.g., carbon nanotubes (CNT), carbon nanofibers (CNF)), etc. 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 using CNTs as the conductive material, it is preferable to use CNTs with an outer diameter of 5 to 30 nm and an aspect ratio of 15 to 300.

Examples of the dispersant include polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), polyacrylate, polymethacrylate, polyoxyethylene alkyl ether, polyalkylene polyamine, benzimidazole, and the like. As the binder, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyacrylic acid, polyacrylate, etc. can be used.

The positive electrode active material 40 will be explained with reference to FIG. 3. FIG. 3 is a cross-sectional view showing spherical active material particles included in the positive electrode shown in FIG. The positive electrode active material 40 is capable of inserting and releasing lithium ions, which are charge carriers. The spherical crown-shaped active material particles 30 included in the positive electrode active material 40 are spheres formed in a substantially spherical crown shape, with a portion of the hollow active material particles 130 having a substantially spherical shell shape or an approximately elliptical spherical shell shape cut off by the cut surface 31. It has a crown shell part 32. The spherical crown-shaped active material particles 30 have a substantially spherical crown-shaped convex curved surface 33 that protrudes in one direction, and a substantially spherical crown-shaped concave curved surface 34 that is present on the opposite side of the one direction and is depressed in one direction. Further, in the spherical crown-shaped active material particles 30, a concave portion 35 is formed in which the vicinity of the center of the end face is depressed in a substantially spherical crown shape toward the convex curved surface 33.

Here, the spherical crown is a side portion of a sphere or an elliptical sphere cut out at an arbitrary plane (cut plane 31 in this embodiment). The shape of the spherical active material particles 30 can also be said to be approximately bowl-shaped. In this way, when the convex curved surface 33 and the concave curved surface 34 have a curved shape, bulky spherical crown-shaped active material particles 30 are formed. Moreover, overlapping of the spherical active material particles 30 and another active material particle can be suppressed.

The spherical 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 a lithium transition metal oxide is a lithium transition metal oxide containing all of Ni, Co, and Mn.

The spherical active material particles 30 may additionally contain one or more elements in addition to the transition metal element (ie, at least one of Ni, Co, and Mn). Additional elements include 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), and Group 6 (chromium) of the periodic table. , transition metals such as tungsten), Group 8 (transition metals such as iron), Group 13 (metalloids such as boron or aluminum), and Group 17 (halogens such as fluorine). Can contain elements.

In one preferred embodiment, the spherical active material particles 30 may have a composition (average composition) represented by the following general formula (1).
Li ₁ + _x Ni _y Co _z Mn _(1-y-z) MA _α MB _β O ₂ … (1)

In the above formula (1), x may be a real number satisfying 0≦x≦0.2. y may be a real number satisfying 0.1<y<0.6. z may be a real number satisfying 0.1<z<0.6. MA is at least one metal element selected from W, Cr and Mo, and α is 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 is 0≦β≦0. It can be a real number satisfying 01. β may be substantially 0 (ie, an oxide that does not substantially contain MB). In addition, in the chemical formula showing the layered structure lithium transition metal oxide, the composition ratio of O (oxygen) is shown as 2 for convenience, but this value should not be interpreted strictly, and some compositional fluctuations ( (typically within the range of 1.95 or more and 2.05 or less) is acceptable.

Each spherical active material particle 30 has a particle form. The spherical crown shell portion 32 constituting the spherical crown-shaped active material particle 30 is a secondary particle formed in a substantially spherical crown shape by connecting primary particles of lithium transition metal oxide. Here, the primary particle refers to a particle that is considered to be a unit particle (ultimate particle) judging from its apparent geometric form. In the spherical active material particles 30, the primary particles are typically an aggregate of crystallites of lithium transition metal oxide. The shape of the spherical active material particles 30 can be observed using an image obtained by SEM observation.

The average diameter R of the convex curved surface 33 of the spherical active material particle 30 is preferably about 2 μm or more, and more preferably about 3 μm or more, for example. Further, the average diameter R of the spherical active material particles 30 is preferably 25 μm or less, more preferably 15 μm or less. From the viewpoint of productivity, in one preferred embodiment, the average diameter R of the convex curved surface 33 is 4 μm or more and 6 μm or less.

The ratio (T1/R) of the average diameter R of the convex curved surface 33 to the thickness T1 of the porous current collector 10 is preferably 0.5 or more and 1.5 or less. If the ratio of the thickness T1 to the average diameter R (T1/R) is outside the above range and is smaller than necessary, the voids formed between the spherical active material particles 30 in the first through hole 11 may be insufficient. This may make it difficult for the voids to function as permeation passages for the electrolyte. Furthermore, as the average diameter R becomes smaller with respect to the thickness T1, the tortuosity (degree of curvature) of the movement path of lithium ions in the electrolytic solution increases in the first through hole 11. There is a risk that the diffusion resistance of lithium ions moving through the lithium ions may increase.

The thickness T2 of the spherical crown shell 32 is preferably 3.0 μm or less, more preferably 2.5 μm or less, even more preferably 2.0 μm or less. The smaller the thickness T2 of the spherical crown shell 32, the lower the diffusion resistance of lithium ions, and the easier it is for lithium ions to be emitted from the inside of the spherical crown shell 32 (the central part of the thickness T2) during charging. When the battery is discharged, lithium ions are easily absorbed into the spherical crown shell 32. 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 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.

In the thickness direction of the spherical crown shell portion 32, the primary particles may have a single layer or multiple layers. The spherical crown-shaped active material particles 30 according to a preferred embodiment are configured such that primary particles are substantially continuous in a single layer over the entire spherical crown shell portion 32 .

Furthermore, it is preferable that the spherical crown-shaped active material particles 30 are provided with a plurality of second through holes 36 that penetrate the spherical crown shell 32 from the outside to the recess 35 . The second through hole 36 is formed as a gap between the plurality of primary particles forming the spherical crown shell 32 . It is preferable that the hole diameter D of the second through hole 36 is 0.3 μm or more. In one preferred embodiment, the pore diameter D of at least some of the second through holes 36 provided in the spherical active material particles 30 is 0.5 μm or more. By providing the second through-holes 36 with such a hole diameter D in the spherical crown-shaped active material particles 30, the durability of the spherical crown-shaped active material particles 30 is ensured, and the connection between the outside and the recess 35 is made through the second through-holes 36. The electrolyte and lithium ions in the electrolyte can smoothly flow between the two.

In addition, the spherical crown-shaped active material particles 30 provided with the second through holes 36 have a volume ratio of 0, which is the ratio of the volume of the spherical crown shell 32 to the total volume of the spherical crown shell 32 and the second through hole 36. It is preferably .3 or more and 0.7 or less, and more preferably 0.4 or more and 0.5 or less. Here, the total volume of the spherical crown shell 32 and the second through holes 36 is the sum of the volumes of all the primary particles constituting one spherical crown-shaped active material particle 30 and the volume of all the second through holes 36. This is the volume. On the other hand, the volume of the spherical crown shell portion 32 is the volume of all the primary particles constituting one spherical crown-shaped active material particle 30. By configuring the volume ratio in such a range, the first through hole 11 becomes porous, spherical active material particles 30 while ensuring durability against stress, etc. that may be applied during manufacturing or use. The flow of the electrolyte and the lithium ions in the electrolyte within the electrolyte is promoted.

If the volume ratio is outside the above range and is smaller than necessary, the durability of the spherical active material particles 30 may decrease. Furthermore, if the volume ratio is out of the above range and is larger than necessary, it becomes difficult to form the first through holes 11 necessary for smooth flow of the electrolyte and the lithium ions in the electrolyte. There is a possibility that the permeation of the electrolytic solution through the cathode material layer 20 will be delayed, making it difficult for lithium ions to diffuse into the positive electrode composite material layer 20.

Characteristic values such as volume ratio can be calculated based on measured values for each of a plurality of arbitrarily selected active material particles using image analysis on a three-dimensional model obtained by FIB-SEM measurement. . For example, it is preferable to obtain data on a three-dimensional model of the spherical active material particles 30 divided into single particles using an algorithm such as Watershed from data on a three-dimensional model of the positive electrode 1, and calculate the volume of each particle. Based on the data of the three-dimensional model of the spherical crown-shaped active material particles 30, the total volume of the spherical crown shell 32 and the second through hole 36 for each particle can be calculated. Furthermore, the connected pores in the spherical crown-shaped active material particles 30 are extracted from the data of the three-dimensional model of the spherical crown-shaped active material particles 30, and the volume of the second through hole 36, etc. is calculated based on the extracted data. I can do it. Then, based on the data of the three-dimensional model of the spherical crown-shaped active material particles 30 from which the extracted connecting pores have been removed, the volume of the spherical crown shell portion 32, etc. can be calculated.

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 the positive electrode 1 according to this embodiment includes steps S1 to S6. Note that the steps S3 and S5 can be omitted if the quality of the positive electrode 1 to be manufactured is ensured, and can be replaced by other methods.

Step 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 the positive electrode active material 40 including the spherical active material particles 30. Step S3 is a step of checking whether the physical properties of the positive electrode active material 40 obtained in step S2 are appropriate values compared with predetermined values. Step S4 is a step of preparing a composite material layer forming paste for forming the positive electrode composite material layer 20. Step S5 is a step of checking whether the viscosity of the composite material layer forming paste produced in step S4 is an appropriate value compared with a predetermined value. In step S6, a paste for forming a composite material layer is applied on the surface of the porous current collector 10 provided with the first through holes 11 penetrating in the thickness direction and in the first through holes 11 to form a positive electrode composite material. This is a step of forming layer 20.

Each of the above steps will be explained in detail. First, in step S1, hollow active material particles 130, which are the material of the spherical active material particles 30, are formed. Therefore, FIG. 5 is a cross-sectional view showing an example of hollow active material particles that are the material of the spherical active material particles shown in FIG. 3.

As shown in FIG. 5, the hollow active material particles 130 have a particle shape with a hollow structure including a spherical shell part 132 and a hollow part 135 formed inside the spherical shell part 132, and have a substantially spherical shell shape or It is formed into a roughly elliptical shell shape. That is, the outer shape of the hollow active material particles 130 is approximately spherical or ellipsoidal (a slightly distorted spherical shape, etc.). Further, the hollow active material particles 130 have an outer surface 133 and an inner surface 134, both of which are formed in a spherical or ellipsoidal shape.

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

The average diameter R of the outer surface 133 of the hollow active material particle 130 substantially matches the average diameter R of the convex curved surface 33. Note that the outer surface 133 is a surface that corresponds to the outline of the hollow active material particles 130. Further, the thickness T2 of the spherical shell portion 132 substantially matches the thickness T2 of the spherical crown shell portion 32.

The diameter of the inner surface 134 of the hollow active material particle 130 can be determined 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 substantially matches the diameter of the concave curved surface 34 . Note that the inner surface 134 is a surface that corresponds to the outline of the hollow portion 135.

Furthermore, it is preferable that the hollow active material particle 130 is provided with a second through hole 36 that penetrates the spherical shell part 132 from the outside to the hollow part 135. As described above, this second through hole 36 serves as a permeation path for the electrolyte in the spherical active material particles 30.

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

The raw material hydroxide generation step is a step of supplying ammonium ions (NH ₄ ⁺ ) to an aqueous solution of a transition metal compound to precipitate transition metal hydroxide particles from the aqueous solution. Here, the aqueous solution contains at least one transition metal element constituting the lithium transition metal oxide.

The raw material hydroxide generation process includes a nucleation stage in which transition metal hydroxide is precipitated from an aqueous solution, and a particle growth stage in which transition metal hydroxide particles are grown in a state where the pH of the aqueous solution is lower than that in the nucleation stage. It is preferable to include. In the particle growth stage, the structure of the hollow active material particles 130 (interval between primary particles, particle porosity, etc.) is adjusted by adjusting the precipitation rate of the transition metal hydroxide by changing the pH and ammonium ion concentration. ) can be changed.

In the mixing step, the transition metal hydroxide particles produced in the raw material hydroxide production step are separated from the reaction solution, washed, filtered, and dried. A mixture is prepared by mixing the transition metal hydroxide thus obtained and a lithium compound. The transition metal hydroxide and the lithium compound are preferably mixed as uniformly as possible at a predetermined ratio. In the mixing step, the lithium compound and the transition metal hydroxide particles are typically mixed in a quantitative ratio corresponding to the composition of the hollow active material particles 130 that are the target object.

The firing step is a step of firing the mixture to obtain hollow active material particles 130. The firing step is performed, for example, in an oxidizing atmosphere (for example, in an air atmosphere). The firing temperature is, for example, 700°C or higher and 1100°C or lower. Further, the firing step may include a plurality of steps of firing at different temperature ranges. After firing, it is preferable to crush the fired product and classify it to adjust the particle size, if necessary.

In this step, a sintering reaction of primary particles of lithium transition metal oxide is allowed to proceed. As a result, hollow active material particles 130 having a substantially spherical or elliptical shell shape in which the primary particles are sintered and connected are formed, and a hollow portion 135 is formed inside the hollow active material particles 130. Ru. The hollow active material particles 130 thus formed are composed of a lithium transition metal oxide having a layered crystal structure.

Returning to FIG. 4, in step S2, the hollow active material particles 130 obtained in step S1 are put into a pulverizer, and by applying a shearing force to the hollow active material particles 130, some of the hollow active material particles 130 are forms a spherical active material particle 30 cut by a cutting surface 31. As the pulverizer used to pulverize the hollow active material particles 130, it is preferable to use a jet mill. The pulverization method may be a dry method or a wet method.

Here, the structure of the spherical active material particles 30 desirable in this embodiment will be described in detail with reference to FIGS. 6 to 8. FIG. 6 is a diagram showing an example of spherical active material particles. FIG. 7 is a diagram showing another example of spherical active material particles. FIG. 8 is a diagram showing another example of spherical active material particles.

Note that the solid lines shown in FIGS. 6 to 8 schematically represent the convex curved surface 33 of the spherical active material particle 30. Further, the broken lines shown in FIGS. 6 to 8 schematically represent the outer surface 133 of the hollow active material particle 130 that is the material of the spherical active material particle 30. 6 to 8 show the case where the spherical crown-shaped active material particles 30 and the hollow active material particles 130 are arranged one on top of the other so that all of the spherical crown-shaped active material particles 30 coincide with a part of the hollow active material particles 130. ing. The long axes of the convex curved surface 33 and the outer surface 133 extend in the horizontal direction, and the short axes of the convex curved surface 33 and the outer surface 133 extend in the vertical direction.

Note that the center of curvature O of the hollow active material particles 130 coincides with the center of curvature O of the spherical active material particles 30 . Further, the center of curvature O of the hollow active material particle 130 is the same as 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 active material particle 30 is the same as the center of curvature O of the convex curved surface 33. It is assumed that the center O is the same as the center of curvature O of the concave curved surface 34.

As shown in FIGS. 6 to 8, the cutting plane 31 that divides the hollow active material particles 130 may be a plane passing through the center of curvature O, or may be a plane located at a position apart 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 long axis of the convex curved surface 33 is an angle θ. . In this embodiment, the angle θ is preferably within ±15°. That is, it is preferable that the spherical crown-shaped active material particles 30 have a substantially spherical crown shape with a central angle of 150° or more and 210° or less.

Specifically, when θ=0°, the cut plane 31 is a plane passing through the center of curvature O, and the spherical active material particles 30 are formed in a hemispherical shape. When θ=15°, the cut plane 31 is a plane located at a position spaced upward from the center of curvature O, and the spherical crown-shaped active material particles 30 have a relatively large volume including the center of curvature O. It is three-dimensional. When θ=-15°, the cut plane 31 is a plane spaced downward from the center of curvature O, and the spherical crown-shaped active material particles 30 have a relatively small volume that does not include the center of curvature O. It is a three-dimensional object.

When the angle θ is within ±15°, good performance is achieved by ensuring an effective reaction field in the active material particles and achieving a high level of both the electrical conductivity of the electrolytic solution and the electrical conductivity of the active material particles. A positive electrode 1 having the following properties can be obtained.

Further, when the angle θ is within ±15°, the shape of the cut surface 31 may be flat or non-planar such as an uneven shape. In this way, the shape of the spherical active material particles 30 is determined according to the position and shape of the cut surface 31.

Further, in the present embodiment, the proportion of the spherical active material particles 30 formed with the angle θ within ±15° of the positive electrode active material 40 contained in the positive electrode composite material layer 20 is preferably 80% by mass or more. It is 100% by mass or less, more preferably 90% by mass or more and 100% by mass or less. The ratio of the spherical active material particles 30 formed with the angle θ within ±15° is preferably close to 100% by mass.

By configuring at such a ratio, the spherical active material particles 30 efficiently enter the first through-holes 11, so that permeation of the electrolytic solution through the first through-holes 11 can be promoted. As a result, the effect of improving the input/output characteristics of the secondary battery including the positive electrode 1 is further enhanced. Note that the positive electrode active material 40 may include other active material particles in addition to the spherical active material particles 30 formed with the above-mentioned angle θ within ±15°. Other active material particles include, for example, active material particles with a solid structure, active material particles with a hollow structure, active material particles formed with an angle θ outside the range of ±15°, and the like.

Regarding the outer surface 133 of the hollow active material particles 130, the ratio of the length L1 of the long axis to the length L2 of the short axis (length of the long axis/length of the short axis, L1/L2) is 1.0 or more. Preferably it is less than 1.5. Further, regarding the inner surface 134 of the hollow active material particles 130, the ratio of the length of the long axis to the length of the short axis (length of the long axis/length of the short axis) is 1.0 or more and less than 1.8. It is preferable. When the spherical active material particles 30 are formed using the hollow active material particles 130 having such a ratio, the front and back surfaces (the convex curved surface 33 and the concave curved surface 34) of the spherical crown-shaped active material particles 30 have a curved shape. The particles become bulky.

In the pulverization by the jet mill 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 times of pulverization. The pulverization conditions are preferably set so that the hollow active material particles 130 can be cut at a cutting surface 31 along the long axis, which is the longest diameter passing through the center of curvature O of the hollow active material particles 130. That is, by controlling the pulverization conditions, desired spherical active material particles 30 can be formed.

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

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

The concave curved surface 34 exists 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 are opposed to each other. Further, the recessed portion 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 that connects 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 .

Returning to FIG. 4, in step S3, the physical properties of the positive electrode active material 40 obtained in step S2 are measured, and it is confirmed whether the measured value of the physical properties of the positive electrode active material 40 is a value larger than a predetermined value. As the physical properties of the positive electrode active material 40, for example, oil absorption and BET specific surface area can be used. Further, as the predetermined values in step S3, the oil absorption amount and BET specific surface area of the hollow active material particles 130 that are the material of the spherical active material particles 30 can be used.

In this step, at least one of oil absorption and BET specific surface area of the positive electrode active material 40 is measured. If the result of measuring the physical properties of the positive electrode active material 40 (measured value) is larger than the result of measuring the same physical properties of the hollow active material particles 130 (predetermined value) (step S3; YES), the next step is performed. Proceed to S4. On the other hand, if the result (measured value) of the physical properties of the positive electrode active material 40 is below the predetermined value (step S3; NO), the desired spherical active material particles 30 may be Since it is considered that this has not been obtained, the process returns to step S2.

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

In step S5, by measuring the viscosity of the paste for forming a composite material layer obtained in step S4, it is confirmed whether the measured value of the viscosity of the paste for forming a composite material layer is a value smaller than a predetermined value. As the predetermined value in step S5, the viscosity of a reference paste prepared by including hollow active material particles 130 in place of the positive electrode active material 40 in the composite layer forming paste can be used. The reference paste is obtained by kneading at least the hollow active material particles 130 and a solvent in a predetermined ratio that is certified to be the same material and the same ratio as the paste for forming a composite material layer, except for the type of active material particles. Note that the same ratio allows for a deviation in the range to the extent of manufacturing variation. The reference paste of this embodiment is obtained by kneading hollow active material particles 130, a conductive material, a binder, and a solvent in a predetermined ratio.

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. move on. On the other hand, if the result of measuring the viscosity of the composite layer forming paste (measured value) is equal to or higher than the predetermined value (step S5; NO), the desired globular active material Since it is considered that the particles 30 have not been obtained, the process returns to step S2.

In step S6, a paste for forming a composite material layer is applied from one side 12 of the porous current collector 10, the paste is dried to volatilize the solvent, and the dried material is pressed as necessary. Thereby, the positive electrode composite material layer 20 can be formed on the surface of the porous current collector 10 and in the first through hole 11 .

Examples of the coating method include a die coater, slit coater, comma coater, gravure coater, blade coater, and the like. Further, by employing, for example, reduced pressure drying as the drying method, the filling property of the positive electrode composite material layer 20 into the first through hole 11 is improved. Examples of the pressing method include methods such as flat plate pressing and roll pressing.

The amount of the positive electrode composite material layer 20 applied per unit area to the porous current collector 10 is not particularly limited, but is preferably 3 mg/cm 2 or more per side of the porous current collector 10, and 5 mg/cm ² or more per side of the porous current collector 10. It is more preferably at least 6 mg/cm ² , particularly preferably at least 6 mg/cm ² . Note that the amount of application is the amount of application in terms of solid content of the paste.

Further, the coating amount per side of the porous current collector 10 is preferably 45 mg/cm ² or less, more preferably 28 mg/cm ² or less, particularly preferably 15 mg/cm ² or less. The density of the positive electrode composite layer 20 is also not particularly limited, but it is preferably 1.0 g/cm ³ or more and 3.8 g/cm 3 or less, and more preferably 1.5 g/cm ^{3 or} more and 3.0 g/cm ³ or less. Preferably, it is particularly preferably 1.8 g/cm ³ or more and 2.4 g/cm ³ or less.
By the above manufacturing method, the positive electrode 1 according to this embodiment can be manufactured.

Next, Examples 1 to 5, Comparative Examples 1 to 2, and Reference Examples 1 to 2 will be described with reference to FIG. Note that the examples do not limit the present disclosure. FIG. 9 is a table explaining examples, comparative examples, and reference examples.

Here, a porous current collector 10 provided with a slit-shaped first through hole 11 and a non-porous current collector not provided with the first through hole 11 are each used as a positive electrode current collector. Two types of positive electrodes were produced for each example. Furthermore, a laminate type half cell was produced using each of the two types of positive electrodes. Hereinafter, a half cell manufactured using the porous current collector 10 will be referred to as an evaluation half cell, and a half cell manufactured using a non-porous current collector will be referred to as a reference half cell.

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

[Fabrication of half cell]
A half cell was produced as follows.
First, a positive electrode active material, CNT as a conductive material, and PVdF as a binder are mixed at a mass ratio of 96:3:1, and N-methyl-2-pyrrolidone is added as a solvent and kneaded to form a paste. Created. The prepared paste is applied to one side of the porous current collector 10 to a certain thickness and coating amount, and after drying, it is pressed to produce various sheet-shaped positive electrodes (50% porosity). did.

Then, the electrode body with the positive electrode composite layer side of the positive electrode and the lithium metal foil (lithium metal counter electrode) facing each other and a separator interposed is housed inside the aluminum laminate exterior material, and an electrolyte is added and sealed. In this way, a half cell for evaluation was prepared. Further, a reference half cell was produced in the same manner as the evaluation half cell except that a non-porous current collector was used in place of the porous current collector 10.

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

A porous polyolefin film was used as the separator. In addition, the electrolytic solution contains lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1 mol/L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) mixed at a volume ratio of 1:1. A non-aqueous electrolyte dissolved in

Further, in Examples 1 to 5, Comparative Examples 1 to 2, and Reference Examples 1 to 2, each positive electrode active material was all LiNi _(1-X-Y) Co _X Mn _Y O ₂ , 0≦X≦0. A lithium nickel cobalt manganese composite oxide (NCM) having an average composition expressed by 35, 0≦Y≦0.35 was used.

Next, each positive electrode active material used in Examples 1 to 5, Comparative Examples 1 to 2, and Reference Examples 1 to 2 will be explained.

(Example 1)
In Example 1, a positive electrode active material containing spherical active material particles 30 having an average diameter R: 5 μm (T1/R=0.6), a thickness T2: 1 μm, a pore diameter D: 0.5 μm, and a volume ratio: 0.4 was used. Substance 40 was used.

(Example 2)
In Example 2, a positive electrode active material containing spherical active material particles 30 having an average diameter R: 5 μm (T1/R=0.6), a thickness T2: 1 μm, a pore diameter D: 0.3 μm, and a volume ratio: 0.5 was used. Substance 40 was used.

(Example 3)
In Example 3, a positive electrode active material containing spherical active material particles 30 having an average diameter R: 6 μm (T1/R=0.8), a thickness T2: 1 μm, a pore diameter D: 0.3 μm, and a volume ratio: 0.5 was used. Substance 40 was used.

(Example 4)
In Example 4, a positive electrode active material containing spherical active material particles 30 having an average diameter R: 10 μm (T1/R=1.3), a thickness T2: 1 μm, a pore diameter D: 0.3 μm, and a volume ratio: 0.5 was used. Substance 40 was used.

(Example 5)
In Example 5, a positive electrode active material containing spherical active material particles 30 having an average diameter R: 12 μm (T1/R=1.5), a thickness T2: 1 μm, a pore diameter D: 0.3 μm, and a volume ratio: 0.5 was used. Substance 40 was used.

(Comparative example 1)
In Comparative Example 1, a positive electrode active material containing substantially spherical active material particles (secondary particles) with an average diameter R of 3 μm (T1/R=0.4) was used. For the approximately spherical secondary particles used here, the manufacturing conditions when forming the hollow active material particles 130 and the manufacturing conditions when forming the spherical active material particles 30 explained in the flow shown in FIG. 4 are adjusted as appropriate. It can be obtained by

(Comparative example 2)
In Comparative Example 2, a positive electrode active material containing substantially spherical active material particles (secondary particles) with an average diameter R of 14 μm (T1/R=1.75) was used. For the approximately spherical secondary particles used here, the manufacturing conditions when forming the hollow active material particles 130 and the manufacturing conditions when forming the spherical active material particles 30 explained in the flow shown in FIG. 4 are adjusted as appropriate. It can be obtained by

(Reference example 1)
In Reference Example 1, a positive electrode active material including substantially spherical active material particles (primary particles) having a solid structure with an average diameter of 5 μm was used.

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

[evaluation]
For nine types of evaluation half cells and reference half cells, the battery characteristics were evaluated by measuring and comparing the amount of voltage drop during large current discharge as the amount of voltage change of the battery when charging and discharging. To measure the voltage drop, each half cell was discharged from a state of charge of 60% SOC (State of Charge) at a current value of 100C in an environment of 25°C, 0.1 minute after the start of discharge. The amount of voltage drop for each was measured.

Then, the resistance reduction rate (%) of the evaluation half cell was calculated based on the voltage drop amount of the reference half cell. The results are shown in the results column of FIG. Moreover, FIG. 10 is a graph showing the resistance reduction rate (%) of the half cell for evaluation. FIG. 10 is a graph corresponding to the results column of FIG. 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 being 100%. The larger the value of the resistance reduction rate (%), the more the internal resistance of the battery is reduced, and it can be said that the first through hole 11 is more effective in reducing the diffusion resistance of lithium ions.

From the results shown in FIGS. 9 and 10, in Examples 1 to 5, the resistance reduction rate was higher than that in Comparative Example 1, and it was confirmed that the first through hole 11 had a high effect of reducing the diffusion resistance of lithium ions. . In Comparative Example 1, active material particles having a smaller diameter than those in Examples 1 to 5 are used, so the reaction resistance of the positive electrode active material is lowered, but the number of points of contact between the active material particles is increased. Therefore, as the number of contact points between the active material particles increases, the tortuosity in the first through hole 11 increases, and the diffusion resistance of lithium ions increases. Therefore, it is considered that the effect of reducing the diffusion resistance of lithium ions by the first through hole 11 was not sufficiently achieved.

Furthermore, the active material particles used in Comparative Example 2 were pulverized by a press during production. Therefore, a desired positive electrode could not be manufactured. Therefore, it is difficult to obtain a high-density positive electrode.

Considering Examples 1 to 5, from the viewpoint of improving the flow of the electrolytic solution and lithium ions in the electrolytic solution through the second through hole 36, as long as durability is ensured, the second through hole 36 is The larger the pore diameter D is, the more preferable it is. Furthermore, from the viewpoint of reducing the diffusion resistance of lithium ions within the spherical active material particles 30, the thinner the thickness T2 is, the more preferable it is, as long as durability is ensured.

Next, when 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, so the reaction resistance of the positive electrode active material is higher; The increase in contact points between particles is suppressed. This suppresses an increase in the tortuosity in the first through-hole 11 due to an increase in the number of contacts between the active material particles, thereby lowering the diffusion resistance of lithium ions. Therefore, it is considered that the effect of reducing the diffusion resistance of lithium ions by the first through hole 11 is improved.

Since the active material particles used in Reference Example 2 have a smaller diameter than those in Reference Example 1, the reaction resistance of the positive electrode active material decreases, but the number of contact points between the active material particles increases. As a result, as the number of contact points between the active material particles increases, the tortuosity in the first through hole 11 increases, and the diffusion resistance of lithium ions increases. Therefore, it is considered that the effect of reducing the diffusion resistance of lithium ions by the first through hole 11 was not sufficiently achieved.

However, when the porous current collector 10 was used, it was confirmed that in Reference Examples 1 to 2, the amount of voltage drop due to discharge was higher than in Examples 1 to 5, and the internal resistance increased.

Therefore, with reference to FIGS. 11 and 12, problems with the positive electrode 100 using a positive electrode active material containing approximately spherical active material particles will be described. FIG. 11 is a diagram showing a positive electrode using a positive electrode active material containing approximately 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 the first through hole provided in the porous current collector 10 included in the positive electrode shown in FIG. 11.

The positive electrode 100 shown in FIGS. 11 and 12 includes a porous current collector 10 and a positive electrode formed on the surface (one side 12) of the porous current collector 10 and in the first through hole 11, and containing a positive electrode active material 140. It has a composite material layer 120. This positive electrode active material 140 includes hollow active material particles 130 having a substantially spherical shape. By applying the porous current collector 10 to the positive electrode 100, permeation of the electrolytic solution through the first through-holes 11 is promoted, and the diffusion resistance of lithium ions is reduced.

In such a positive electrode 100, small-diameter solid active material particles 150 are contained in the positive electrode composite layer 120 in order to achieve high input/output and high energy density of the secondary battery. When using the porous current collector 10, the first through holes 11 are filled with solid active material particles 150 having a reduced diameter. 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 the curvature of the lithium ion movement path in the first through hole 11 tends to increase. As a result, there is a problem that the diffusion resistance of lithium ions increases. Furthermore, when the solid active material particles 150 having a reduced diameter are packed at a high density, the number of contact points between the solid active material particles increases more than necessary, creating a reaction field that can contribute to an electrochemical reaction. There may also be a problem that there is a possibility that the amount of energy may decrease.

In contrast, the positive electrode for a secondary battery according to the present embodiment includes a porous current collector 10 provided with a plurality of first through holes 11 penetrating in the thickness direction, and A positive electrode composite layer 20 is formed in the first through hole 11 and includes a positive electrode active material 40. The positive electrode active material 40 is a sphere formed into an approximately spherical crown shape, in which a portion of a hollow active material particle 130 made of a lithium transition metal oxide and having an approximately spherical shell shape or an approximately elliptical spherical shell shape is cut off by a cut surface 31. A spherical crown-shaped active material having a crown shell portion 32, a substantially spherical crown-shaped convex curved surface 33 that protrudes in one direction, and a substantially spherical crown-shaped concave curved surface 34 that is present on the opposite side of the one direction and is depressed in one direction. Contains particles 30. The first through hole 11 is filled with at least spherical active material particles 30 .

According to such a configuration, not only the peel strength of the positive electrode composite material layer 20 is improved, but also the first through hole 11 functions well as a permeation path for the electrolyte of the secondary battery, and the positive electrode composite material layer 20 Diffusion resistance of lithium ions moving inside can be reduced. As a result, the storage stability, cycle characteristics, etc. of the secondary battery including the positive electrode 1 can be improved, and high input/output characteristics can be realized.

Furthermore, in the positive electrode for a secondary battery according to the present embodiment, it is preferable that the thickness T2 of the spherical crown shell portion 32 is 0.4 μm or more and 1.5 μm or less. With such a configuration, it is possible to obtain a secondary battery that has both reduced internal resistance and durability.

Furthermore, in the positive electrode for a secondary battery according to the present embodiment, the ratio of the average diameter R of the convex curved surface 33 to the thickness T1 of the porous current collector 10 is preferably 0.5 or more and 1.5 or less. With such a configuration, the first through hole 11 functions well as a permeation path for the electrolyte of the secondary battery, and the diffusion resistance of lithium ions moving within the positive electrode composite layer 20 can be further reduced. . As a result, high input/output characteristics of the secondary battery including the positive electrode 1 can be achieved.

Furthermore, in the positive electrode for a secondary battery according to the present embodiment, the spherical crown-shaped active material particles 30 are provided with a second through hole that penetrates the spherical crown shell 32 and has a hole diameter D of 0.3 μm or more, It is preferable that the ratio of the volume of the spherical crown shell part 32 to the total volume of the spherical crown shell part 32 and the second through hole is 0.3 or more and 0.7 or less. With such a configuration, while ensuring durability, the electrolytic solution and the lithium ions in the electrolytic solution can flow well through the second through hole 36, and the input/output characteristics of the secondary battery including the positive electrode 1 are improved. be able to.

Furthermore, in the positive electrode for a secondary battery according to the present embodiment, the length of the short side of the first through hole 11 is greater than or equal to twice the average diameter R of the convex curved surface 33 and less than or equal to 1 mm, and the long side of the first through hole 11 is It is preferable that the length of the short side is at least twice the length of the short side and at most 10 mm. With such a configuration, it is possible to suppress breakage of the porous current collector 10 that may occur due to stress that may be applied during manufacturing or use, such as when applying the paste for forming a composite material layer or when winding an electrode.

Furthermore, in the positive electrode for a secondary battery according to the present embodiment, it is preferable that the open area ratio of the first through holes 11 is 10% or less in terms of area ratio. With such a configuration, it is possible to obtain a secondary battery that has both high cycle characteristics and high input/output characteristics.

Furthermore, in the positive electrode for a secondary battery according to the present embodiment, the direction from the center of curvature of the convex curved surface 33 toward the tip of the convex curved surface 33 on the cut surface 31 side, and the long axis of the convex curved surface 33 passing through the center of curvature of the convex curved surface 33 It is preferable that the angle formed by and is within ±15°. With such a configuration, it is possible to obtain a cathode 1 that has good electronic conductivity due to the bulk of the spherical active material particles 30 and has an excellent balance of performance in which a reaction field for the cathode active material 40 is secured.

Furthermore, in the positive electrode for a secondary battery according to the present embodiment, it is preferable that the average diameter R of the convex curved surface 33 is 4 μm or more and 6 μm or less. With such a configuration, high quality spherical active material particles 30 can be reliably obtained. Moreover, the spherical active material particles 30 can be filled into the first through-hole 11 more reliably.

Furthermore, it is preferable that the ratio of the length L1 of the long axis to the length L2 of the short axis passing through the center of curvature of the outer surface 133 of the hollow active material particle 130 is 1.0 or more and less than 1.5. Further, it is preferable that the ratio of the length of the long axis to the length of the short axis passing through the center of curvature of the inner surface 134 of the hollow active material particle 130 is 1.0 or more and less than 1.8. When the spherical active material particles 30 are formed using the hollow active material particles 130 configured in this way, the convex curved surface 33 and the concave curved surface 34 of the spherical crown-shaped active material particle 30 each have an appropriately curved shape, and the particles become It becomes bulky. As a result, a permeation path for the electrolytic solution is secured in the positive electrode 1, and a decrease in the reaction field due to an increase in the number of contact points between the active material particles is suppressed.

As described above, according to the present embodiment, it is possible to provide a positive electrode for a secondary battery that reduces internal resistance and provides a secondary battery with good cycle characteristics.

Furthermore, according to the method for manufacturing a positive electrode for a secondary battery according to the present embodiment, it is possible to manufacture the positive electrode 1 that exhibits the above-mentioned effects.

In addition, by appropriately measuring the physical properties of the positive electrode active material 40 including the spherical active material particles 30 obtained by pulverizing the hollow active material particles 130, it is confirmed whether the hollow active material particles 130 are properly pulverized. However, the grinding conditions can be adjusted. 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 are appropriately pulverized. According to such a configuration, the desired positive electrode active material 40 can be reliably obtained, and the quality of the positive electrode 1 manufactured using the positive electrode active material 40 is improved.

Note that the present disclosure is not limited to the above embodiments, and can be modified as appropriate without departing from the spirit. For example, in the above embodiment, the hollow active material particles 130 are pulverized using a jet mill, but the present invention is not limited to this, and other pulverization methods may be used. Other pulverization methods that can be used include a ball mill, bead mill, and rod mill.

Further, for example, when confirming the physical properties of the positive electrode active material 40, the results 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 may be used as the predetermined value. Similarly, when confirming the viscosity of the composite layer forming paste, the results of measuring the viscosity of other substances that have a clear relationship with the viscosity of the composite layer forming paste may be used as the predetermined value.

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 Spherical crown-shaped active material particles 31 Cut surface 32 Spherical crown shell 33 Convex curved surface 34 Concave curved surface 35 Concave portion 36 Second through holes 40, 140 Positive electrode active material 130 Hollow active material particles 132 Spherical shell portion 133 Outer surface 134 Inner surface 135 Hollow portion 150 Solid active material particle D Hole diameter L1 Long axis length L2 Short axis length O Center of curvature R Average diameter T1, T2 Thickness

Claims (13)

a porous current collector provided with a plurality of first through holes penetrating in the thickness direction;
a positive electrode composite layer formed on the surface of the porous current collector and in the first through hole and containing a positive electrode active material,
The positive electrode active material is
a spherical crown shell portion formed in a substantially spherical crown shape in which a part of a hollow active material particle in a substantially spherical shell shape or an approximately elliptical spherical shell shape made of a lithium transition metal oxide is cut off by a cutting surface;
a substantially spherical convex curved surface protruding in one direction;
a substantially spherical concave curved surface that is present on the opposite side to the one direction side and is recessed in the one direction;
comprising spherical active material particles having
A positive electrode for a secondary battery, wherein the first through hole is filled with at least the spherical active material particles. The positive electrode for a secondary battery according to claim 1, wherein the thickness of the spherical crown shell is 0.4 μm or more and 1.5 μm or less. 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. The spherical crown-shaped active material particles are provided with a second through hole that penetrates the spherical crown shell and has a hole diameter of 0.3 μm or more,
The positive electrode for a secondary battery according to claim 1, wherein the ratio of the volume of the spherical crown shell to the total volume of the spherical crown shell and the second through hole is 0.3 or more and 0.7 or less. The length of the short side of the first through hole is not less than twice the average diameter of the convex curved surface and not more than 1 mm,
The positive electrode for a secondary battery according to claim 1, wherein the length of the long side of the first through hole is at least twice the length of the short side and at most 10 mm. The positive electrode for a secondary battery according to claim 1, wherein the first through hole has an aperture ratio of 10% or less in terms of area ratio. Claim: The angle between the direction from the center of curvature of the convex curved surface toward the tip of the convex curved surface on the cutting surface side and the long axis of the convex curved surface passing through the center of curvature of the convex curved surface is within ±15 degrees. 1. The positive electrode for a secondary battery according to 1. 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. The ratio of the length of the long axis to the length of the short axis passing through the center of curvature of the outer surface of the hollow active material particle is 1.0 or more and less than 1.5,
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 long axis of the outer surface. The ratio of the length of the long axis to the length of the short axis passing through the center of curvature of the inner surface of the hollow active material particle is 1.0 or more and less than 1.8,
The positive electrode for a secondary battery according to claim 1, wherein the concave curved surface is formed in a substantially spherical crown shape cut by the cutting surface along the long axis of the inner surface. forming hollow active material particles in the shape of an approximately spherical shell or an approximately elliptic spherical shell composed of a lithium transition metal oxide;
The hollow active material particles are pulverized to form a positive electrode active material including spherical active material particles having a spherical cap shell portion formed in a substantially spherical cap shape in which a portion of the hollow active material particles is cut off by a cutting surface. the step of
forming a positive electrode composite layer containing the positive electrode active material on the surface of a porous current collector provided with a plurality of first through holes penetrating in the thickness direction and in the first through holes;
has
A method for manufacturing a positive electrode for a secondary battery, wherein the first through hole is filled with at least the spherical active material particles. In the step of forming the positive electrode active material,
2. The step of forming the positive electrode composite layer is performed when the result of measuring at least one of the oil absorption amount and the BET specific surface area of the positive electrode active material is larger than the result of measuring the hollow active material particles. 12. The method for manufacturing a positive electrode for a secondary battery according to 11. The step of forming the positive electrode composite material layer includes:
A step of preparing a composite layer forming paste containing the positive electrode active material and the solvent in a predetermined ratio,
If the result of measuring the viscosity of the composite layer forming paste is smaller than the result of measuring the viscosity of the reference paste containing the hollow active material particles and the solvent in the predetermined ratio, the porous current collector 12. The method for manufacturing a positive electrode for a secondary battery according to claim 11, wherein the positive electrode composite layer is formed by applying the paste for forming the composite material layer onto the surface of the battery and into the first through hole.

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