Positive electrode for secondary battery and secondary battery

Abstract

To provide a positive electrode that suppresses an increase in resistance (i.e., diffusion resistance) associated with the diffusion of an active material, and a secondary battery including the positive electrode.SOLUTION: A positive electrode for a secondary battery includes a positive electrode active material in which the aspect ratio of the primary particles is 1.00 or more and less than 1.30, and which forms secondary particles with a hollow structure, where the porosity is 50% or more, and the following formula (1) is satisfied; and there is provided a secondary battery including the same.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] Secondary batteries are widely used in vehicles, information and communication technology (e.g., personal computers, smartphones, etc.), power storage, etc. In recent years, secondary batteries have been required to be rapidly charged and discharged with a large current (hereinafter referred to as high-rate charge and discharge). In order to achieve high-rate charge and discharge, the use of a positive electrode active material that forms secondary particles with a hollow structure as a positive electrode active material has been considered. For example, Patent Document 1 describes a battery that includes a positive electrode having a positive electrode current collector and a positive electrode mixture layer provided on the positive electrode current collector and containing a positive electrode active material, and a negative electrode having a negative electrode current collector and a negative electrode mixture layer provided on the negative electrode current collector and containing a negative electrode active material, the positive electrode mixture layer having a density of 1.5 g / cm 3 More than 2.8g / cm 3 The positive electrode active material has a hollow structure and a specific surface area of ​​0.8 m or less. 2 / g or more 1.4m 2 / g or less, the average particle size is 3 μm or more, the sphericity is 0.8 or more and less than 1, and the negative electrode mixture layer has a density of 0.8 g / cm 3 More than 1.5g / cm 3 and the negative electrode active material has a sphericity of 0.8 or more and less than 1.

[0003] For example, Patent Document 2 discloses "a positive electrode active material for a non-aqueous electrolyte secondary battery, which comprises a lithium transition metal-containing composite oxide represented by a specific formula and which is constituted by secondary particles formed by agglomeration of a plurality of primary particles, wherein the secondary particles have an outer shell portion formed by agglomeration of primary particles, a central portion constituted by an internal space present inside the outer shell portion, and at least one through hole formed in the outer shell portion and connecting the central portion to the outside, wherein the ratio of the inner diameter of the through hole to the thickness of the outer shell portion is 0.3 or more."

[0004] For example, Patent Document 3 describes an active material particle for a lithium secondary battery, which has a hollow structure including a secondary particle formed of a plurality of aggregates of primary particles of a lithium transition metal oxide and a hollow portion formed inside the secondary particle, and the secondary particle has a through hole penetrating from the outside to the hollow portion, and has a BET specific surface area of ​​0.5 to 1.9 m. 2 / g" is disclosed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2016-095934 A [Patent Document 2] International Publication No. 2018 / 097191 [Patent Document 3] JP 2011-119092 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, in conventional secondary batteries, the resistance associated with the diffusion of the active material (i.e., diffusion resistance) tends to increase during high-rate charging and discharging. In response to this, Patent Documents 1 to 3 propose methods for suppressing the increase in resistance that occurs during high-rate charging and discharging by controlling the density of the positive electrode active material, controlling the size of secondary particles in the positive electrode active material that forms secondary particles with a hollow structure, and the like. As a result of intensive research by the inventors, it was found that in these conventional techniques (e.g., Patent Documents 1 to 3), even if the density of the positive electrode active material or the size of the secondary particles in the positive electrode active material are within a predetermined range, if the shape of the primary particles of the positive electrode active material or the surface unevenness of the secondary particles changes, an increase in resistance occurs during high-rate charging and discharging. In view of the above circumstances, an object of one embodiment of the present disclosure is to provide a positive electrode for secondary batteries that suppresses an increase in resistance (i.e., diffusion resistance) associated with the diffusion of the active material, and a secondary battery including the same. [Means for solving the problem]

[0007] Means for solving the above problems include the following means. <1> The aspect ratio of the primary particles is 1.00 or more and less than 1.30, and the primary particles include a positive electrode active material that forms secondary particles having a hollow structure, A positive electrode for a secondary battery, having a porosity of 50% or more and satisfying the following formula (1):

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[0008] According to one embodiment of the present disclosure, there is provided a positive electrode for a secondary battery that suppresses an increase in resistance (that is, diffusion resistance) associated with diffusion of an active material, and a secondary battery including the same. [Brief description of the drawings]

[0009] [Figure 1]FIG. 1 is a schematic cross-sectional view of an example of a secondary particle with a hollow structure that can be formed by the positive electrode active material contained in the positive electrode of the present disclosure. [Diagram 2] 2(A) to (D) are examples of schematic cross-sectional views of secondary particles with a hollow structure that can be formed by the positive electrode active material contained in the positive electrode of the present disclosure. [Diagram 3] FIG. 3 is a graph showing the relationship between the first direct current resistance (DCIR) (Ω) and the porosity. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment that is an example of the present disclosure will be described. These descriptions and examples are merely illustrative of the embodiment, and are not intended to limit the scope of the invention. In the present specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range. In addition, in the present specification, the upper or lower limit of the numerical range may be replaced with a value shown in the examples.

[0011] Each component may contain multiple types of the corresponding substance. When referring to the amount of each component in a composition, if the composition contains multiple substances corresponding to each component, the amount refers to the total amount of those multiple substances present in the composition, unless otherwise specified. The term "process" includes not only independent processes, but also processes that cannot be clearly distinguished from other processes as long as the intended effect of the process is achieved.

[0012] <Positive electrode for secondary batteries> The positive electrode for a secondary battery according to the present disclosure is a positive electrode for a secondary battery, which contains a positive electrode active material having a primary particle aspect ratio of 1.00 or more and less than 1.30, which forms secondary particles with a hollow structure, has a porosity of 50% or more, and satisfies the following formula (1):

[0013]

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[0014] According to the present disclosure, an increase in resistance (i.e., diffusion resistance) associated with the diffusion of an active material is suppressed. Although the mechanism of this action is not entirely clear, it is presumed to be as follows. In the positive electrode of the present disclosure, the relationship among the specific surface area of ​​the secondary particles of the positive electrode active material, the outer diameter and inner diameter of the hollow secondary particles, and the density of the positive electrode active material satisfies the above-mentioned formula (1).

[0015] As a result of intensive research by the inventors, it was found that not only the shape of the primary particles of the positive electrode active material but also the surface roughness of the secondary particles formed by the positive electrode active material is a factor for adjusting the diffusion resistance. The surface roughness of the secondary particles formed by the positive electrode active material is expressed by the following formula (P1).

[0016]

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[0017] In formula (P1), the "specific surface area (calculated value) of hollow particles without irregularities" is expressed by the following formula (P2).

[0018]

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[0019] In formula (P2), the "specific surface area of ​​the hollow outer shell and the specific surface area of ​​the hollow inner shell are each taken as theoretical values, and when the density and volume of the positive electrode active material are substituted, the "specific surface area of ​​the hollow particles without irregularities" is expressed by the following formula (P3).

[0020]

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[0021] By cancelling out the numerator and denominator of formula (P3) and substituting this into formula (P1), the degree of unevenness is expressed by the following formula (P4).

[0022] JPEG2024178042000006.jpg1977

[0023] As described above, satisfying formula (1) means that the value of the specific surface area of ​​the secondary particles is appropriately high, and the secondary particles have a hollow structure due to the moderate surface unevenness. Therefore, the increase in pressure loss during flow in the electrode is suppressed, and the decrease in the liquid flow of the electrolyte is also reduced. In addition, if the porosity is 50% or more, the increase in diffusion resistance is suppressed. In other words, the increase in diffusion resistance is suppressed, and the effect of the unevenness of the secondary particles formed by the positive electrode active material on the diffusion resistance is suppressed from becoming small. As a result, it is considered that the increase in resistance (i.e., diffusion resistance) due to the diffusion of the active material is suppressed even during high-rate charge and discharge.

[0024] [Electrode configuration] The positive electrode according to the present disclosure has, for example, a positive electrode current collector and a positive electrode mixture layer provided on the positive electrode current collector. The positive electrode mixture layer contains a positive electrode active material. The positive electrode active material is capable of forming secondary particles with a hollow structure, and the aspect ratio of the primary particles is 1.00 or more and less than 1.30.

[0025] [Positive electrode current collector] The positive electrode current collector is not particularly limited, and may be a current collector used in a known positive electrode for a secondary battery. Examples of the material constituting the positive electrode current collector include aluminum, aluminum alloy, nickel, titanium, stainless steel, etc. The shape of the positive electrode current collector is, for example, a foil shape or a mesh shape.

[0026] [Characteristics of the positive electrode mixture layer] ·Porosity In the positive electrode according to the present disclosure, the positive electrode mixture layer has a porosity of 50% or more. The porosity of the positive electrode mixture layer may be 50% or more, 55% or more, or 60% or more. The porosity is calculated by observing the cross section of the positive electrode mixture layer with a scanning electron microscope (SEM). The porosity is calculated by calculating the ratio (percentage) of the area of ​​the void region in the cross section to the entire area of ​​the positive electrode mixture layer confirmed from the cross section (i.e., including the void region).

[0027] ·Equations (1)~(3) The positive electrode active material satisfies the following formula (1), and from the viewpoint of further suppressing an increase in the diffusion resistance of the active material even during high-rate charge and discharge, it more preferably satisfies the following formula (2), and further more preferably satisfies the following formula (3).

[0028]

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[0029]

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[0030]

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[0031] The method for satisfying the formulas (1) to (3) is not particularly limited, and examples thereof include a method of adjusting the type of the positive electrode active material; and a method of adjusting the heating temperature and heating time for firing the positive electrode active material during production of the positive electrode active material (for example, increasing the firing temperature and / or lengthening the firing time of the positive electrode active material).

[0032] In the formulas (1) to (3), BET is the specific surface area (m 2 / g). The value of the specific surface area of ​​the secondary particles of the positive electrode active material is not particularly limited as long as it satisfies the above formula (1); however, for example, from the viewpoint of further suppressing an increase in the diffusion resistance of the active material even during high-rate charging and discharging, the specific surface area is preferably 1.65 or more and 2.10 or less, more preferably 1.70 or more and 2.00 or less, and even more preferably 1.71 or more and 1.95 or less.

[0033] The method for adjusting the specific surface area of ​​the secondary particles of the positive electrode active material is not particularly limited, and examples thereof include a method of adjusting the type of positive electrode active material; and a method of adjusting the heating temperature and heating time for firing the positive electrode active material during production of the positive electrode active material (for example, increasing the firing temperature and / or lengthening the firing time of the positive electrode active material).

[0034] In the formulas (1) to (3), ρ is the density of the positive electrode active material (g / m 3 The density of the positive electrode active material is not particularly limited as long as it satisfies the above formula (1). For example, from the viewpoint of further suppressing an increase in the diffusion resistance of the active material even during high-rate charging and discharging, it is preferably 3.00 (g / m 3 ) or more 5.50 (g / m 3 ) or less, more preferably from 3.50 to 5.00, and even more preferably from 4.00 to 4.80.

[0035] In formulas (1) to (3), r 1 represents the outer diameter (μm) of the hollow-structured secondary particles formed by the positive electrode active material. The outer diameter (μm) of the hollow-structured secondary particles formed by the positive electrode active material is not particularly limited as long as it satisfies the above formula (1), but from the viewpoint of further suppressing an increase in the diffusion resistance of the active material even during high-rate charging and discharging, for example, it is preferably 1.40 μm to 3.00 μm, more preferably 1.50 μm to 2.80 μm, and even more preferably 1.60 μm to 2.30 μm.

[0036] In formulas (1) to (3), r 2 represents the inner diameter (μm) of the hollow secondary particle formed by the positive electrode active material. The inner diameter (μm) of the hollow secondary particle formed by the positive electrode active material is not particularly limited as long as it satisfies the above formula (1), but from the viewpoint of further suppressing an increase in the diffusion resistance of the active material even during high-rate charging and discharging, for example, it is preferably 0.50 μm or more and 2.0 μm or less, more preferably 0.60 μm or more and 1.90 μm or less, and even more preferably 0.65 μm or more and 1.65 μm or less.

[0037] The outer diameter and inner diameter of the hollow secondary particles formed by the positive electrode active material are determined as follows. A cross section cut along the thickness direction of the positive electrode mixture layer is observed by SEM, and image analysis is performed on 10 random points of the secondary particles of the positive electrode active material. As shown in FIG. 1, the longest linear distance r1 when connecting one end to the other end of the outer peripheral surface of the hollow shell of the secondary particle 20 in the longitudinal direction is defined as the outer diameter of the hollow secondary particle 20 formed by the positive electrode active material. As shown in FIG. 1, the longest linear distance r2 when connecting one end to the other end of the inner peripheral surface of the hollow shell of the secondary particle 20 in the lateral direction is defined as the inner diameter of the hollow secondary particle 20 formed by the positive electrode active material.

[0038] The method for adjusting the outer diameter and inner diameter of the hollow-structured secondary particles formed by the positive electrode active material is not particularly limited, and examples thereof include a method for adjusting the type of positive electrode active material; and a method for adjusting the heating temperature and heating time for firing the positive electrode active material during production of the positive electrode active material.

[0039] ·Metsuke The basis weight of the positive electrode mixture layer is not particularly limited. For example, from the viewpoint of further suppressing an increase in the diffusion resistance of the active material even during high-rate charge and discharge, it is preferably 5.0 (mg / cm 2 ) or more 20.0 (mg / cm 2 ) or less, and 8.0 (mg / cm 2 ) or more 15.0 (mg / cm 2 ) or less, and more preferably 10.0 (mg / cm 2) or more 13.0 (mg / cm 2 ) or less is even more preferable.

[0040] [Characteristics of positive electrode active material] Secondary particle morphology The positive electrode active material of the present disclosure forms secondary particles with a hollow structure in the positive electrode mixture layer. The morphology of the secondary particles having a hollow structure will be described in detail with reference to the drawings. 2(A) to (D) are schematic cross-sectional views of examples of hollow-structured secondary particles that can be formed by the positive electrode active material contained in the positive electrode of the present disclosure. The positive electrode active material 10 shown in FIG. 1(A) is formed in the positive electrode mixture layer by the positive electrode active material 10 gathering together to form a secondary particle 20 having a hollow structure with a shell portion and a hollow portion formed inside the shell portion. As shown in FIG. 2(A) to (D), the positive electrode active material 10 can form four main types of secondary particle 20 depending on the direction in which the positive electrode active material 10 gathers together and the number of gatherings. The positive electrode active material 10 shown in FIG. 2(A) forms secondary particles 20 that are oriented so that the area of ​​mutual contact is minimized, that is, so that the specific surface area is increased and so that the unevenness of the surface of the shell is minimized. The positive electrode active material 10 shown in FIG. 2(B) forms secondary particles 20 that are oriented so that the area of ​​mutual contact is minimized, that is, so that the specific surface area is large and so that the surface irregularities of the shell portion are large. The positive electrode active material 10 shown in FIG. 2(C) is folded and in contact with each other, that is, has a small specific surface area and is oriented so as to minimize unevenness on the shell surface, forming secondary particles 20. The positive electrode active material 10 shown in FIG. 2(D) is folded and in contact with each other, that is, oriented so as to have a small specific surface area and large unevenness on the surface of the shell, forming secondary particles 20.

[0041] The morphology of the hollow secondary particles is not particularly limited as long as it satisfies formula (1). From the viewpoint of more suitably satisfying formula (1), for example, it is preferable for the particles to have the morphology shown in FIG. 2(A) and FIG. 2(B), and it is more preferable for the particles to have the morphology shown in FIG. 2(A).

[0042] ·Material The positive electrode active material is not particularly limited as long as it satisfies formula (1) and can form secondary particles with a hollow structure, and known positive electrode active materials can be used. Examples of the positive electrode active material include lithium transition metal oxides with layered structures such as nickel cobalt aluminum lithium oxide (NCA) and nickel cobalt manganese lithium oxide (NCM); sodium transition metal oxides with layered structures; and lithium nickel cobalt manganese oxides (NCM). Among the above, the positive electrode active material preferably contains a lithium transition metal oxide having a layered structure, and more preferably contains a lithium nickel cobalt manganese oxide (NCM).

[0043] Aspect ratio of primary particles The positive electrode active material has a primary particle aspect ratio of 1.00 or more and less than 1.30, and from the viewpoint of further suppressing an increase in the diffusion resistance of the active material even during high-rate charging and discharging, the aspect ratio is preferably 1.05 or more and 1.15 or less, and more preferably 1.06 or more and 1.14 or less.

[0044] The aspect ratio of the primary particles of the positive electrode active material refers to the ratio (long side / short side) of the length of the positive electrode active material in the longitudinal direction (hereinafter also referred to as the long side) to the length of the positive electrode active material in the lateral direction perpendicular to the long side (hereinafter also referred to as the short side). The length of the positive electrode active material in the longitudinal direction refers to the longest linear distance when connecting one end of the positive electrode active material to the other end of the positive electrode active material in the longitudinal direction. The length of the positive electrode active material in the longitudinal direction is not particularly limited as long as it is longer than the length of the positive electrode active material in the lateral direction, but is preferably 0.20 μm or more and 0.50 μm or less from the viewpoint of further suppressing the increase in the diffusion resistance of the active material even during high-rate charging and discharging. The length of the positive electrode active material in the short side direction refers to the longest linear distance when connecting one end of the positive electrode active material to the other end of the short side direction. The length of the positive electrode active material in the short side direction is not particularly limited as long as it is shorter than the length of the positive electrode active material in the longitudinal direction, but is preferably 0.18 μm or more and 0.44 μm or less from the viewpoint of further suppressing the increase in the diffusion resistance of the active material even during high-rate charging and discharging.

[0045] The aspect ratio of the primary particles of the positive electrode active material is the arithmetic mean value of the aspect ratio (long side / short side) between the long side and the short side of the positive electrode active material, which is obtained by observing a cross section cut along the thickness direction of the positive electrode mixture layer by SEM and performing image analysis on the primary particles of the positive electrode active material at any 10 points.

[0046] The method for adjusting the aspect ratio of the primary particles of the positive electrode active material is not particularly limited, and examples thereof include a method for adjusting the type of positive electrode active material.

[0047] The positive electrode mixture layer may further contain other materials other than the positive electrode active material, such as binder resins, conductive assistants, thickeners, surfactants, dispersants, wetting agents, and defoamers, within the scope of the effects of the present disclosure.

[0048] ≪Secondary battery≫ The secondary battery according to the present disclosure is a secondary battery formed by laminating, in this order, the positive electrode for secondary batteries according to the present disclosure, an electrolyte layer, and a negative electrode.

[0049] The electrolyte layer may be a known electrolyte layer, and may be either a solid electrolyte layer or a liquid electrolyte layer. In the case of a liquid electrolyte layer, the electrolyte is sealed in the separator. According to the secondary battery of the present disclosure, an increase in resistance (i.e., diffusion resistance) caused by the diffusion of the active material is suppressed.

[0050] The negative electrode has, for example, a negative electrode current collector and a negative electrode mixture layer provided on the negative electrode current collector. The negative electrode mixture layer contains a negative electrode active material. The negative electrode mixture layer may further contain other materials other than the negative electrode active material. Examples of the other materials include a binder resin, a conductive assistant, a thickener, a surfactant, a dispersant, a wetting agent, and a defoamer. The negative electrode current collector is not particularly limited, and a current collector used in a known negative electrode for a secondary battery can be used. Examples of materials constituting the negative electrode current collector include copper, copper alloy, nickel, titanium, stainless steel, etc. The shape of the negative electrode current collector is, for example, a foil shape or a mesh shape. The negative electrode active material is not particularly limited, and may be any negative electrode active material used in known negative electrodes for secondary batteries. Examples of the negative electrode active material include graphite such as natural graphite and artificial graphite; carbon materials such as hard carbon, soft carbon, and activated carbon; and silicon.

[0051] The secondary battery according to the present disclosure can be applied to various secondary batteries such as lithium ion secondary batteries, sodium ion secondary batteries, and lithium ion polymer secondary batteries, but is preferably a lithium ion secondary battery. EXAMPLES

[0052] Examples will be described below, but the present invention is not limited to these examples. In the following description, unless otherwise specified, all "parts" and "%" are based on mass.

[0053] <Example 1> Preparation of the positive electrode The positive electrode active material (composition formula LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 ) and NMP as a solvent to prepare a coating solution. The coating solution was applied to one side of a positive electrode current collector (aluminum foil), and the coating was dried by heating at 100 to 150°C for 1 to 10 minutes to obtain a coating film. The coating film had a thickness of 35 μm. In this manner, a positive electrode was prepared.

[0054] Preparation of negative electrode A coating solution was obtained by mixing the negative electrode active material (graphite) and water as a solvent. The coating solution was applied to both sides of a negative electrode current collector (stainless steel foil) and dried to obtain a coating film. The coating film had a thickness of 50 μm. In this way, a negative electrode was produced.

[0055] -Secondary battery production A cylindrical electrode body was produced by winding the positive electrode and the negative electrode with a separator (including a laminate in which polypropylene (PP), polyethylene (PE), and polypropylene (PP) were laminated in this order) between them. A flat electrode body was produced by pressing the cylindrical electrode body in a direction perpendicular to the winding axis. The flat electrode body and the non-aqueous electrolyte were housed in a case to obtain a lithium ion secondary battery in which a positive electrode, a separator serving as an electrolyte layer, and a negative electrode were laminated in this order.

[0056] <Example 2, Comparative Examples 1 to 6> In the preparation of the positive electrode, a lithium ion secondary battery was obtained in the same manner as in Example 1, except that the heat drying conditions were adjusted so that the value of formula (1) and each parameter factor were as shown in Table 1.

[0057] The positive electrodes prepared in each example were measured for the following physical properties by the above-mentioned measuring methods. The results of each test and the values ​​of formula (1) derived therefrom are shown in Table 1. Aspect ratio of primary particles of positive electrode active material - Specific surface area of ​​secondary particles of positive electrode active material measured by BET method ·Density of the positive electrode active material ρ The outer diameter r of the hollow secondary particles formed by the positive electrode active material 1 The inner diameter r of the hollow secondary particles formed by the positive electrode active material 2

[0058] <Evaluation of proliferation resistance index> The diffusion resistance index was calculated by subtracting the DC resistance component and the reaction resistance component obtained by impedance measurement from the resistance value obtained by DC-IR measurement. The results are shown in Table 1.

[0059] [Table 1]

[0060] As shown in Table 1, it was found that the lithium ion secondary battery including the positive electrode of the Example suppressed an increase in the diffusion resistance of the active material, compared to the lithium ion secondary battery including the positive electrode of the Comparative Example.

[0061] <Evaluation of the relationship between porosity and diffusion resistance region of DCIR> The relationship between the porosity of the positive electrode mixture layer and the diffusion resistance region (0.1 to 10s) of DCIR is shown below. Preparation of a lithium-ion secondary battery with a porosity of 50% in the positive electrode composite layer The lithium ion secondary battery of Example 1 was prepared. Preparation of a lithium-ion secondary battery with a porosity of 40% in the positive electrode composite layer In the preparation of the positive electrode, a lithium ion secondary battery having a porosity of 40% in the positive electrode mixture layer was obtained in the same manner as in Example 1, except that the pressing pressure was increased by 10 to 20%. Preparation of a lithium-ion secondary battery with a porosity of 45% in the positive electrode composite layer In the preparation of the positive electrode, a lithium ion secondary battery having a porosity of 45% in the positive electrode mixture layer was obtained in the same manner as in Example 1, except that the pressing pressure was increased by 5 to 10%.

[0062] The secondary battery of each example was charged at a constant current and constant voltage of 1C and SOC 60% in a temperature environment of 25°C, and an initial DCIR measurement was performed. Subsequently, the battery was discharged at a constant current of 1C for 10 seconds, and then charged at a constant current of 1C for 10 seconds. The first DC resistance (DCIR) was calculated based on each voltage drop amount at the discharge rate (=voltage before the start of discharge-voltage 10 seconds after the start of discharge) and each current value (i.e., each current value corresponding to a discharge rate of 1C to 3C). FIG. 3 shows a graph showing the relationship between the first DC resistance (DCIR) (Ω) and the porosity. Each data represents the average value of three evaluations, and the error bar represents the standard deviation. As shown in FIG. 3, it was found that when the porosity of the positive electrode mixture layer is 50% or more (in the direction of the arrow in FIG. 3), an increase in the diffusion resistance is suppressed. [Explanation of symbols]

[0063] 10 Primary particles of positive electrode active material, 20 Secondary particles of positive electrode active material

Claims

1. The aspect ratio of the primary particles is equal to or greater than 1.00 and less than 1.30, and the primary particles form secondary particles having a hollow structure, A positive electrode for a secondary battery has a porosity of 50% or more and satisfies the following formula (1): [0010] In the above formula (1), BET is the specific surface area (m2) of the secondary particles measured by the BET (Bentley-Emmett-Teller) method. ２ / g), ρ is the density of the positive electrode active material (g / m ３ ), r １ is the outer diameter (m) of the hollow secondary particle, r ２ and represent the inner diameter (m) of the secondary particle having the hollow structure.

2. 2. The positive electrode for a secondary battery according to claim 1, wherein the aspect ratio of the primary particles is 1.05 or more and 1.15 or less.

3. 2. The positive electrode for a secondary battery according to claim 1, wherein the specific surface area of ​​the secondary particles is 1.70 or more and 2.00 or less.

4. 2. The positive electrode for a secondary battery according to claim 1, wherein the positive electrode active material comprises lithium nickel cobalt manganese oxide (NCM).

5. A positive electrode for a secondary battery according to any one of claims 1 to 4, An electrolyte layer; A negative electrode; A secondary battery is made by stacking the above in this order.

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