Positive electrode and secondary battery

Abstract

To provide a positive electrode and a secondary battery that can improve battery characteristics.SOLUTION: A positive electrode includes a positive electrode active material including a plurality of first particles and a plurality of second particles having an average particle size smaller than that of the plurality of first particles, and when the resistance of the plurality of first particles measured using a scanning spreading resistance microscope is R1 and the resistance of the plurality of second particles is R2, the ratio of the logarithms of the resistance of the plurality of first particles (R1) and the resistance of the plurality of second particles (R2) (logR2 / logR1) is 1.25 or less.SELECTED DRAWING: Figure 3

Description

[Technical Field]

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

[0002] Patent Documents 1 and 2 describe lithium ion secondary batteries that include a positive electrode, a negative electrode, and a non-aqueous electrolyte. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2017 / 208894 [Patent Document 2] Patent No. 7193671 Summary of the Invention [Problem to be solved by the invention]

[0004] Lithium-ion secondary batteries are required to have improved battery characteristics such as cycle characteristics. Patent Documents 1 and 2 disclose techniques for controlling the spreading resistance within an appropriate range when measuring the cross section or surface of a positive electrode active material using a spreading resistance microscope. However, Patent Documents 1 and 2 do not take into consideration the spreading resistance of each particle contained in the positive electrode active material.

[0005] An object of the present invention is to provide a positive electrode and a secondary battery that can improve battery characteristics. [Means for solving the problem]

[0006] A positive electrode according to one embodiment has a positive electrode active material including a plurality of first particles and a plurality of second particles having an average particle size smaller than that of the plurality of first particles, and when the resistance of the plurality of first particles measured using a scanning spreading resistance microscope is R1 and the resistance of the plurality of second particles is R2, the logarithmic ratio (logR2 / logR1) of the resistance of the plurality of first particles (R1) to the resistance of the plurality of second particles (R2) is 1.25 or less.

[0007] A secondary battery according to one embodiment includes the above positive electrode, a negative electrode, and an electrolyte. [Effects of the Invention]

[0008] The positive electrode and secondary battery of the present invention can improve battery characteristics. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view showing the configuration of a secondary battery according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view showing the configuration of a wound electrode body according to the embodiment. [Figure 3] FIG. 3 is a schematic diagram showing a cross section of a positive electrode active material layer according to an embodiment. [Figure 4] FIG. 4 is a schematic diagram showing a cross section of a positive electrode active material layer according to a comparative example. [Figure 5] FIG. 5 is an explanatory diagram for explaining the particle size distribution of the positive electrode active material according to the embodiment. [Figure 6] FIG. 6 is an explanatory diagram for explaining another example of the particle size distribution of the positive electrode active material according to the embodiment. [Figure 7] FIG. 7 is a graph schematically showing the resistance distribution of the first particles and the second particles according to the embodiment and the comparative example. [Figure 8] FIG. 8 is a flowchart illustrating the method for manufacturing the positive electrode active material layer according to Examples 1 and 2. As shown in FIG. [Figure 9] FIG. 9 is a flowchart illustrating a method for manufacturing a positive electrode active material layer according to Comparative Example 1. [Figure 10] FIG. 10 is a flowchart illustrating a method for manufacturing a positive electrode active material layer according to Comparative Example 2. [Figure 11] FIG. 11 is a flowchart illustrating the method for manufacturing the positive electrode active material layer according to Examples 3, 4, 5, and 6 and Comparative Examples 3 and 4. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described, but the present disclosure is not limited to these embodiments.

[0011] (Embodiment) Fig. 1 is a cross-sectional view showing the configuration of a secondary battery according to an embodiment. Fig. 2 is a cross-sectional view showing the configuration of a wound electrode body according to an embodiment. A secondary battery 10 according to an embodiment is a secondary battery that uses lithium as an electrode reactant, and is a lithium-ion secondary battery that obtains battery capacity by utilizing the absorption and desorption of lithium (Li).

[0012] 1, a secondary battery 10 according to the embodiment includes a battery can 11, a pair of insulating plates 12 and 13, a wound electrode body 20, a positive electrode lead 25, and a negative electrode lead 26. The secondary battery 10 is a cylindrical secondary battery, and the wound electrode body 20, which is a battery element, is housed inside the cylindrical battery can 11.

[0013] The battery can 11 is cylindrical and has a hollow structure with one end closed and the other end open. The battery can 11 is made of, for example, iron (Fe), aluminum (Al), or an alloy thereof. The battery can 11 may also be made of iron (Fe) with nickel (Ni) or the like plated on its surface.

[0014] The wound electrode body 20 is housed inside the battery can 11. The wound electrode body 20 is formed by, for example, stacking a positive electrode 21 and a negative electrode 22 (see FIG. 2) with a separator 23 interposed therebetween and then winding them.

[0015] The pair of insulating plates 12 and 13 extend, for example, in a direction perpendicular to the winding circumferential surface of the wound electrode body 20 and are arranged so as to sandwich the wound electrode body 20 between them.

[0016] A battery lid 14, a safety valve mechanism 15, and a thermosensitive resistor (PTC element) 16 are crimped to the open end of the battery can 11 via a gasket 17. This seals the open end of the battery can 11. The battery lid 14 is formed, for example, from the same material as the battery can 11. The safety valve mechanism 15 and the thermosensitive resistor 16 are provided inside the battery lid 14, and the safety valve mechanism 15 is electrically connected to the battery lid 14 via the thermosensitive resistor 16.

[0017] When the internal pressure exceeds a certain level due to an internal short circuit or external heating, the safety valve mechanism 15 reverses the disk plate to cut off the electrical connection between the battery lid 14 and the wound electrode body 20. The thermosensitive resistor element 16 prevents abnormal heat generation due to a large current, and the resistance of the thermosensitive resistor element 16 increases as the temperature rises.

[0018] The gasket 17 is made of, for example, an insulating material, and the surface thereof may be coated with asphalt.

[0019] A center pin 24 is inserted into the center of the winding of the wound electrode body 20. However, the center pin 24 may be omitted.

[0020] A positive electrode lead 25 made of a conductive material such as aluminum is connected to the positive electrode 21. The positive electrode lead 25 is connected to a safety valve mechanism 15 by welding or the like, and is electrically connected to the battery lid 14 via the safety valve mechanism 15. Furthermore, a negative electrode lead 26 made of a conductive material such as nickel is connected to the negative electrode 22. The negative electrode lead 26 is electrically connected to the battery can 11 by welding or the like.

[0021] 2, the positive electrode 21 includes a positive electrode current collector 21A and two positive electrode active material layers 21B provided on both sides of the positive electrode current collector 21A. However, only one positive electrode active material layer 21B may be provided on one side of the positive electrode current collector 21A.

[0022] The positive electrode current collector 21A includes, for example, one or more of conductive materials such as aluminum, nickel, stainless steel, etc. The positive electrode current collector 21A may be a single layer or a multi-layer.

[0023] The positive electrode active material layer 21B includes a positive electrode active material capable of absorbing and releasing lithium. The positive electrode active material layer 21B includes a positive electrode active material, a positive electrode binder, and a positive electrode conductive additive. The positive electrode active material layer 21B is not limited to the above-mentioned materials and may also include a dispersant or the like.

[0024] The positive electrode active material is preferably a lithium-containing compound such as a lithium-containing composite oxide. The lithium-containing composite oxide is an oxide containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing composite oxide has, for example, a layered rock salt type or spinel type crystal structure.

[0025] The positive electrode active material may be one type or a combination of multiple types. In the case of multiple types, the compound type (element composition, coating element, dopant type, etc.) and form (secondary particles, primary particles, etc.) of the combined positive electrode active material may be any. The positive electrode active material is, for example, a nickel-based positive electrode active material. The nickel-based positive electrode active material is, for example, nickel cobalt manganese oxide (NCM), which is a lithium-containing compound containing the transition metal elements nickel (Ni), cobalt (Co), and manganese (Mn) as constituent elements.

[0026] Specific examples of lithium-containing composite oxides include LiNiO2, LiCoO2, and LiCo 0.98 Al 0.01 Mg 0.01 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.8 Co0.15 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O2, Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 )O2, LiMn2O4, LiFePO4, etc.

[0027] The positive electrode binder contained in the positive electrode active material layer 21B may be any material, and may include, for example, one or more of synthetic rubber and polymer compounds. Examples of synthetic rubber include styrene-butadiene rubber, fluorine-containing rubber, and ethylene-propylene-diene. Examples of polymer compounds include polyvinylidene fluoride and polyimide.

[0028] The positive electrode conductive additive contained in the positive electrode active material layer 21B may be any material, including, for example, carbon. Examples of carbon include graphite, carbon black, acetylene black, ketjen black, carbon nanotubes, and graphene. However, the positive electrode conductive additive contained in the positive electrode active material layer 21B is not limited to these materials as long as it is a conductive material, and may be other carbon materials, metal materials, conductive polymers, or the like.

[0029] The negative electrode 22 includes a negative electrode current collector 22A and two negative electrode active material layers 22B provided on both sides of the negative electrode current collector 22A. However, only one negative electrode active material layer 22B may be provided on one side of the negative electrode current collector 22A.

[0030] The negative electrode current collector 22A includes, for example, one or more of conductive materials such as aluminum, nickel, stainless steel, etc. The positive electrode current collector 21A may be a single layer or a multilayer.

[0031] The negative electrode active material layer 22B includes a negative electrode active material capable of absorbing and releasing lithium. The negative electrode active material layer 22B includes a negative electrode active material, a negative electrode binder, and a negative electrode conductive additive. The materials of the negative electrode active material layer 22B are not limited to those listed above, and may also include a dispersant or the like.

[0032] The negative electrode active material includes a material capable of absorbing and releasing lithium, such as a carbon material, a metal, a metalloid, an alloy or compound of silicon, or an alloy or compound of tin (Sn).

[0033] Examples of carbon materials that can be used as the negative electrode active material include graphite, non-graphitizable carbon, and graphitizable carbon. More specifically, carbon materials include pyrolytic carbons, cokes, glassy carbon fiber, fired organic polymer compounds, activated carbon, and carbon blacks. Examples of cokes include pitch coke, needle coke, and petroleum coke. Here, fired organic polymer compounds are carbonized by firing a polymer compound such as a phenolic resin or a furan resin at an appropriate temperature.

[0034] Examples of metals and metalloids that can be used as negative electrode active materials include tin, lead (Pb), aluminum, indium (In), silicon, zinc (Zn), antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y), and hafnium (Hf). Among these, silicon, germanium, tin, and lead are preferred. Silicon and tin are more preferred because they have a high ability to absorb and release lithium and can achieve a high energy density.

[0035] Examples of silicon alloys that can be used as the negative electrode active material include those containing at least one element selected from the group consisting of tin, nickel, copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc, indium, silver, titanium (Ti), germanium, bismuth, antimony, and chromium (Cr) as a second constituent element other than silicon. Examples of silicon compounds that can be used as the negative electrode active material include those containing oxygen (O) or carbon (C), and may contain the above-mentioned second constituent element in addition to silicon.

[0036] Examples of tin alloys that can be used as the negative electrode active material include those containing at least one element selected from the group consisting of silicon, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium as a second constituent element other than tin. Examples of tin compounds that can be used as the negative electrode active material include those containing oxygen or carbon, and may contain the above-mentioned second constituent element in addition to tin.

[0037] The negative electrode binder contained in the negative electrode active material layer 22B may be any material, and may include, for example, one or more of synthetic rubber and polymer compounds. Examples of synthetic rubber include styrene-butadiene rubber, fluorine-containing rubber, and ethylene-propylene-diene. Examples of polymer compounds include polyvinylidene fluoride and polyimide.

[0038] The anode conductive additive contained in the anode active material layer 22B may be any material, including, for example, carbon. Examples of carbon include graphite, carbon black, acetylene black, and ketjen black. However, the anode conductive additive contained in the anode active material layer 22B is not limited to these materials as long as it is a conductive material, and may be a metal material, a conductive polymer, or the like.

[0039] The separator 23 separates the positive electrode 21 and the negative electrode 22, preventing a short circuit of current due to contact between the two electrodes, while allowing lithium ions to pass through. In the example shown in Fig. 2, the separator 23 is provided between the positive electrode active material layer 21B of the positive electrode 21 and the negative electrode active material layer 22B of the negative electrode 22.

[0040] The separator 23 is preferably made of a material that is electrically stable, chemically stable against the positive electrode active material, the negative electrode active material, and the electrolyte, and is insulating. The separator 23 can be made of, for example, a polymer nonwoven fabric, a porous film, or a layer of glass or ceramic fibers. The separator 23 is more preferably made of a porous polyolefin film. This improves battery safety by preventing short circuits and providing a shutdown effect.

[0041] The electrolyte is impregnated into the separator 23. In the example of Fig. 1, the electrolyte fills the space inside the battery can 11. The electrolyte is a non-aqueous electrolyte solution containing an electrolyte salt and a solvent that dissolves the electrolyte salt.

[0042] Examples of electrolyte salts include lithium salts such as lithium perchlorate (LiClO), lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium bis(trifluoromethanesulfonyl)imide (LiN(SOCF)), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SOCF)), and lithium hexafluoroarsenate (LiAsF).

[0043] Examples of the solvent include lactone-based solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone; carbonate-based solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; ether-based solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; nitrile-based solvents such as acetonitrile; sulfolane-based solvents; phosphoric acids; phosphate ester solvents; and pyrrolidones.

[0044] The electrolytic solution may further contain an additive such as a fluorinated carboxylic acid ester, a sulfonic acid ester, a sulfonic acid anhydride, or a carboxylic acid anhydride.

[0045] Next, the detailed configuration of the positive electrode active material contained in the positive electrode active material layer 21B will be described with reference to FIGS. 3 to 7. FIG. 3 is a schematic diagram showing a cross section of a positive electrode active material layer according to an embodiment. FIG. 4 is a schematic diagram showing a cross section of a positive electrode active material layer according to a comparative example. FIG. 5 is an explanatory diagram for explaining the particle size distribution of the positive electrode active material according to an embodiment. FIG. 6 is an explanatory diagram for explaining another example of the particle size distribution of the positive electrode active material according to an embodiment. FIG. 7 is a graph schematically showing the resistance distribution of the first particles and the second particles according to an embodiment and a comparative example.

[0046] 3 and 4 show schematic diagrams of cross sections measured using a scanning spreading resistance microscope (SSRM). Specifically, measurements were performed using a Bruker Dimension ICON under an Ar atmosphere at a scan rate of 0.2 Hz and a scan speed of 16 μm / s.

[0047] The schematic diagrams shown in Figures 3 and 4 are displayed so that the brightness varies depending on the resistance value. That is, in Figures 3 and 4, higher brightness (closer to white display) indicates higher resistance, and lower brightness (closer to black display) indicates lower resistance.

[0048] As shown in FIG. 3, the positive electrode active material according to the embodiment includes a plurality of first particles 31 having a relatively large average particle diameter and a plurality of second particles 32 having an average particle diameter smaller than that of the plurality of first particles 31. The plurality of first particles 31 and the plurality of second particles 32 each contain a nickel-based positive electrode active material.

[0049] The particle diameters of the plurality of first particles 31 and the plurality of second particles 32 are measured by performing cross-sectional processing by ion milling in the discharged state of the positive electrode 21 and observing the cross section. The cross-sectional observation of the positive electrode 21 was performed by imaging three fields of each sample at an angle of view of 40 μm × 40 μm. Then, the area of each particle is calculated from the image analysis of the cross-sectional observation image. Assuming a circle having the same area as the area of the particle, the diameter of the circle is taken as the particle diameter.

[0050] As shown in FIGS. 5 and 6, the positive electrode active material according to the embodiment has a bimodal particle size distribution having a first mode value M1 and a second mode value M2. The particle diameter D1 of the first mode value M1 is smaller than the particle diameter D2 of the second mode value M2 (D1 < D2). The particle diameter D3 with the lowest frequency between the first mode value M1 and the second mode value M2 is defined as the boundary T. Particles with a particle diameter D3 (boundary T) or more are defined as the first particles 31, and particles with a particle diameter smaller than D3 (boundary T) are defined as the second particles 32.

[0051] In the present embodiment, the particle diameter of the plurality of first particles 31 is 8 μm or more, and the particle diameter of the plurality of second particles 32 is smaller than 8 μm.

[0052] The positive electrode active material of the positive electrode 121 according to the comparative example shown in FIG. 4 includes a plurality of first particles 131 and a plurality of second particles 132 having an average particle diameter smaller than that of the plurality of first particles 131, similar to the embodiment. Also in the comparative example, the particle diameter of the plurality of first particles 131 is 8 μm or more, and the particle diameter of the plurality of second particles 132 is smaller than 8 μm.

[0053] In the positive electrode active material according to the embodiment shown in Fig. 3, the positive electrode conductive additive is evenly attached to the surfaces of the plurality of first particles 31 and the plurality of second particles 32. In contrast, in the positive electrode active material according to the comparative example shown in Fig. 4, the attachment of the positive electrode conductive additive is uneven, and the attachment of the positive electrode conductive additive to the plurality of second particles 132 having small particle diameters is insufficient.

[0054] 4, in the comparative example, the variation between the resistance value of the plurality of first particles 131 and the resistance value of the plurality of second particles 132 increases. More specifically, among the plurality of second particles 132, the second particles 132 to which the positive electrode conductive additive is insufficiently attached have a high resistance, which is higher than that of the plurality of first particles 131.

[0055] In contrast, in the positive electrode active material according to the embodiment shown in Fig. 3, the variation between the resistance value of the plurality of first particles 31 and the resistance value of the plurality of second particles 32 is suppressed. That is, compared to the comparative example, the variation between the luminance of the plurality of first particles 31 and the luminance of the plurality of second particles 32 shown in Fig. 3 is small. Furthermore, localized high resistance among the plurality of second particles 32 is suppressed.

[0056] The horizontal axis of the graph shown in Fig. 7 represents the resistance of the first particles 31, 131 and the second particles 32, 132. The vertical axis of the graph shown in Fig. 7 represents the frequency of the particles.

[0057] The resistance of the first particles 31, 131 and the second particles 32, 132 is measured using image analysis software to extract the particle contours in the SSRM images shown in Figures 3 and 4, and calculate the resistance by quantifying the brightness of each particle.

[0058] Next, according to the definitions (particle sizes) of the first particles 31, 131 and the second particles 32, 132 described above, the logarithm of the resistance (logR) and the average value of the logarithm of the resistance (logR) are calculated for each of the first particles 31, 131 and the second particles 32, 132. The relationship between the brightness of the SSRM image and the resistance (more specifically, the logarithm of the resistance (logR)) is set in advance in the scanning spreading resistance microscope used for the measurement.

[0059] 5, in the comparative example, the logarithm of the resistance (logR2) of the second particles 132 is distributed on the high resistance side compared to the logarithm of the resistance (logR1) of the first particles 131. In contrast, in the embodiment, the logarithm of the resistance (logR2) of the second particles 32 is distributed in a resistance range that almost overlaps with the logarithm of the resistance (logR1) of the first particles 31. Furthermore, the logarithm of the resistance (logR2) of the second particles 32 according to the embodiment is distributed on the low resistance side compared to the logarithm of the resistance (logR2) of the second particles 32 according to the comparative example.

[0060] As described above, it has been shown that the positive electrode active material according to this embodiment suppresses the increase in resistance of the second particles 32 and suppresses the variation in the resistance value between the plurality of first particles 31 and the plurality of second particles 32. As a result, the positive electrode 21 according to this embodiment suppresses the occurrence of areas with locally high potential during charging, etc. As a result, the secondary battery 10 can improve its battery characteristics, such as an improved cycle capacity retention rate and suppressed resistance increase.

[0061] (Example) Examples will be described below, but the present invention is not limited to these examples.

[0062] Fig. 8 is a flowchart illustrating a method for manufacturing a positive electrode active material layer according to Examples 1 and 2. Fig. 9 is a flowchart illustrating a method for manufacturing a positive electrode active material layer according to Comparative Example 1. Fig. 10 is a flowchart illustrating a method for manufacturing a positive electrode active material layer according to Comparative Example 2.

[0063] The positive electrode active materials according to Examples 1 and 2 were prepared according to the mixing flow 1 shown in Fig. 8. In Examples 1 and 2, lithium nickel cobalt manganese oxide (NCM) was used for both the first particles and the second particles. The mass ratio of the first particles to the second particles was set to first particles / second particles = 7 / 3.

[0064] As shown in FIG. 8, in Examples 1 and 2, the first particles, carbon black as a positive electrode conductive assistant, polyvinylidene fluoride (PVDF) as a positive electrode binder, and N-methylpyrrolidone (NMP) as a solvent are stirred using a homogenizer (stirring 1).

[0065] Stirring was carried out for 15 minutes at a rotation speed of 3000 rpm in Stirring 1. Stirring was carried out under the same conditions in Stirring 2 and Stirring 3 described below.

[0066] In a separate step from Stirring 1, the second particles, carbon black as a positive electrode conductive additive, polyvinylidene fluoride (PVDF) as a positive electrode binder, and N-methylpyrrolidone (NMP) as a solvent are stirred using a homogenizer (Stirring 2).

[0067] The slurries mixed in the stirring 1 and stirring 2 are stirred using a homogenizer (stirring 3), thereby forming a slurry containing the first particles and the second particles.

[0068] The slurry-like positive electrode mixture obtained in Stirring 3 was uniformly applied to a strip-shaped aluminum foil (positive electrode current collector) to form a coating film. Next, this coating film was dried with hot air and then compression-molded using a roll press to form a positive electrode sheet having a positive electrode active material layer. The produced positive electrode sheet was punched into a predetermined shape and vacuum-dried using a vacuum dryer to obtain the positive electrodes according to Examples 1 and 2.

[0069] Example 1 differs from Example 2 in that a larger amount of positive electrode conductive additive is added together with the second particles during stirring 2. However, the total amount of positive electrode conductive additive added during stirring 1 and stirring 2 is the same in Example 1 and Example 2. That is, the composition of the slurry obtained during stirring 3 in Example 1 is the same as the composition of the slurry obtained during stirring 3 in Example 2.

[0070] The positive electrode active material according to Comparative Example 1 was produced according to the mixing flow 2 shown in Fig. 9. As shown in Fig. 9, in Comparative Example 1, unlike Examples 1 and 2, the first particles and the second particles are simultaneously stirred in a single step. That is, in Comparative Example 1, carbon black as a positive electrode conductive assistant, polyvinylidene fluoride (PVDF) as a positive electrode binder, and N-methylpyrrolidone (NMP) as a solvent are added to the first particles and the second particles, and the mixture is stirred using a homogenizer (stirring 4).

[0071] The positive electrode active material according to Comparative Example 2 was produced according to the mixing flow 3 shown in FIG. 10. As shown in FIG. 10, Comparative Example 2 differs from Comparative Example 1 in that the number of stirring steps is increased to two. That is, in Comparative Example 2, carbon black as a positive electrode conductive additive, polyvinylidene fluoride (PVDF) as a positive electrode binder, and N-methylpyrrolidone (NMP) as a solvent are added to the first particles and the second particles, and the mixture is stirred using a homogenizer (stirring 5). Thereafter, the slurry obtained in stirring 5 is further stirred using a homogenizer (stirring 6).

[0072] Table 1 shows the results of preparing coin cells for Examples 1 and 2 and Comparative Examples 1 and 2, and evaluating the average logarithm of the resistance (logR1) of the first particles 31 of the positive electrode 21, the average logarithm of the resistance (logR2) of the second particles 32, the ratio of the logarithms of the resistance (logR2 / logR1), and the cycle retention rate of the coin cells.

[0073] The logarithms of the resistance of the first particles and the second particles were calculated based on the SSRM images using the method described with reference to Fig. 3 to Fig. 7. The cycle retention rate of the coin cells was calculated as the capacity retention rate (=discharge capacity at the 100th cycle / discharge capacity at the 1st cycle) for each of Examples 1 and 2 and Comparative Examples 1 and 2. Table 1 shows values ​​normalized with the capacity retention rate of Example 1 set to 100.

[0074] [Table 1]

[0075] As shown in Table 1, in Example 1, Example 2, Comparative Example 1, and Comparative Example 2, the logarithm of the resistance of the second particles (logR2) is greater than the logarithm of the resistance of the first particles (logR1). The logarithm of the resistance of the first particles (logR1) in Example 1 and Example 2 is greater than the logarithm of the resistance of the first particles (logR1) in Comparative Examples 1 and 2. Furthermore, the logarithm of the resistance of the second particles (logR2) in Example 1 and Example 2 is smaller than the logarithm of the resistance of the second particles (logR2) in Comparative Examples 1 and 2. That is, in Example 1 and Example 2, the difference between the logarithm of the resistance of the first particles (logR1) and the logarithm of the resistance of the second particles (logR2) is smaller than in Comparative Examples 1 and 2.

[0076] Furthermore, the logarithmic ratios (logR2 / logR1) of the resistances (R1) of the plurality of first particles and the resistances (R2) of the plurality of second particles in Examples 1 and 2 are smaller than the logarithmic ratios of the resistances in Comparative Examples 1 and 2. Specifically, the logarithmic ratios of the resistances (logR2 / logR1) in Examples 1 and 2 are 1.25 or less. In contrast, the logarithmic ratios of the resistances (logR2 / logR1) in Comparative Examples 1 and 2 are 1.30 or more.

[0077] Comparing Example 1 and Example 2, the logarithm of the resistance (logR2) of the second particles in Example 1 is smaller than that in Example 2. In addition, the ratio of the logarithms of the resistance (logR2 / logR1) in Example 1 is smaller than that in Example 2. This shows that even if the total amount of positive electrode conductive additive is the same, the resistance of the second particles can be effectively suppressed by relatively increasing the amount of positive electrode conductive additive added together with the second particles during stirring 2 (see FIG. 8).

[0078] Comparing Comparative Example 1 and Comparative Example 2, the logarithm of the resistance (logR2) of the second particles in Comparative Example 1 is smaller than that in Comparative Example 2. In addition, the ratio of the logarithms of the resistance (logR2 / logR1) in Comparative Example 1 is smaller than that in Comparative Example 2. This means that simply increasing the number of stirring times and the stirring time results in the positive electrode conductive additive being unevenly attached to the surfaces of the first particles, which have larger particle sizes, and it can be said that the effect of suppressing the resistance of the second particles is small.

[0079] Moreover, the cycle retention rates in Examples 1 and 2 are higher than those in Comparative Examples 1 and 2. Specifically, the cycle retention rate in Example 2 maintains a value of 99 or more when normalized with Example 1 as the reference (100). In contrast, the cycle retention rates in Comparative Examples 1 and 2 are 95 or less.

[0080] As described above, in Examples 1 and 2, the first particles and the second particles were separately stirred by stirring 1 and 2 (see FIG. 8 ), respectively, so that the positive electrode conductive additive was evenly attached to the surfaces of the plurality of first particles and the plurality of second particles. This demonstrated that Examples 1 and 2 were able to suppress the increase in resistance of the second particles compared to Comparative Examples 1 and 2. Furthermore, Examples 1 and 2 were shown to have a higher cycle retention rate compared to Comparative Examples 1 and 2.

[0081] FIG. 11 is a flowchart illustrating the method for manufacturing the positive electrode active material layer according to Examples 3, 4, 5, and 6 and Comparative Examples 3 and 4.

[0082] Table 2 shows the order of adding materials and the stirring conditions for Examples 3, 4, 5, and 6 and Comparative Examples 3 and 4. Table 2 also shows the results of evaluating the average value of the logarithm of the resistance (logR1) of the first particles of the positive electrode, the average value of the logarithm of the resistance (logR2) of the second particles, the ratio of the logarithms of the resistance (logR2 / logR1), and the cycle retention rate of the coin cells produced for Examples 3, 4, 5, and 6 and Comparative Examples 3 and 4.

[0083] As shown in FIG. 11 and Table 2, Examples 3, 4, 5, and 6 and Comparative Examples 3 and 4 differ in the order in which the positive electrode conductive additives, carbon black (CB) and carbon nanotubes (CNT), and the positive electrode binder, polyvinylidene fluoride (PVDF), are added to the first particles and the second particles, respectively.

[0084] 11, a first material selected from CB, CNT, and PVDF (hereinafter referred to as material 1) was added to the first and second particles, and the mixture was stirred using a homogenizer (stirring 7). As in Examples 1 and 2, N-methylpyrrolidone (NMP) was used as the solvent.

[0085] Next, a second material (hereinafter referred to as material 2) selected from CB, CNT, and PVDF excluding material 1 above is added to the slurry obtained in stirring 7, and the mixture is stirred using a homogenizer (stirring 8).

[0086] Next, a third material (hereinafter referred to as material 3) among CB, CNT, and PVDF excluding material 1 and material 2 is added to the slurry obtained in stirring 8, and the mixture is stirred using a homogenizer (stirring 9).

[0087] Using the slurry positive electrode mixture obtained in Stirring 9, positive electrodes according to Examples 3, 4, 5, and 6 and Comparative Examples 3 and 4 were obtained in the same manner as in Example 1. The order in which CB, CNT, and PVDF were added (material 1, material 2, material 3) and the stirring conditions for Stirring 7, Stirring 8, and Stirring 9 in Examples 3, 4, 5, and 6 and Comparative Examples 3 and 4 are shown in Table 2.

[0088] [Table 2]

[0089] As shown in Table 2, the logarithm of the resistance (logR2) of the second particles in Examples 3, 4, 5, and 6 is smaller than the logarithm of the resistance (logR2) of the second particles in Comparative Examples 3 and 4. Furthermore, the logarithm ratios (logR2 / logR1) of the resistances (R1) of the plurality of first particles and the resistances (R2) of the plurality of second particles in Examples 3, 4, 5, and 6 are smaller than the logarithm ratios (logR2 / logR1) of the resistances in Comparative Examples 3 and 4. Specifically, the logarithm ratios (logR2 / logR1) of the resistances in Examples 3, 4, 5, and 6 are 1.25 or less. More specifically, the logarithm ratios (logR2 / logR1) of the resistances in Examples 3, 4, 5, and 6 are 1.09 or less. In contrast, the logarithm ratios (logR2 / logR1) of the resistances in Comparative Examples 3 and 4 are 1.28 or more.

[0090] Furthermore, the cycle retention rates in Examples 3, 4, 5, and 6 are higher than those in Comparative Examples 3 and 4. Specifically, the cycle retention rates in Examples 4, 5, and 6 are maintained at 99 or higher when normalized with Example 3 as the reference (100). In contrast, the cycle retention rates in Comparative Examples 3 and 4 are 95 or lower.

[0091] As described above, in Examples 3, 4, 5, and 6, by appropriately setting the order of adding CB, CNT, and PVDF and the stirring conditions for Stirring 7, Stirring 8, and Stirring 9, the positive electrode conductive additive was able to adhere evenly to the surfaces of the plurality of first particles and the plurality of second particles, and high resistance of the second particles could be suppressed compared to Comparative Examples 3 and 4. More specifically, in Examples 3, 4, 5, and 6, by improving the slurry mixing process, the conductive paths around the small particles were improved and the difference in resistance between large and small particles was reduced compared to Comparative Examples 3 and 4. As a result, in Examples 3, 4, 5, and 6, deterioration due to uneven resistance was suppressed, and the cycle capacity retention rate and resistance increase could be improved.

[0092] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention may be modified or improved without departing from the spirit and scope of the present invention, and equivalents thereof are also included in the present invention.

[0093] The present disclosure may also have the following configurations.

[0094] (1) A cathode active material including a plurality of first particles and a plurality of second particles having an average particle size smaller than that of the plurality of first particles, When the resistance of the plurality of first particles measured using a scanning spreading resistance microscope is R1 and the resistance of the plurality of second particles is R2, The ratio (logR2 / logR1) of the logarithms of the resistance (R1) of the plurality of first particles and the resistance (R2) of the plurality of second particles is 1.25 or less. Positive electrode. (2) the particle diameter of the plurality of first particles is 8 μm or more; The particle size of the plurality of second particles is smaller than 8 μm. The positive electrode according to (1). (3) The positive electrode according to (1) or (2), a negative electrode; an electrolyte; Secondary battery. [Explanation of symbols]

[0095] 10 Secondary battery 20 Wound electrode body 21 Positive electrode 21A positive electrode current collector 21B Positive electrode active material layer 22 Negative electrode 22A negative electrode current collector 22B Negative electrode active material layer 31, 131 first particle 32, 132 second particle

Claims

1. a positive electrode active material including a plurality of first particles and a plurality of second particles having an average particle size smaller than that of the plurality of first particles; When the resistance of the plurality of first particles measured using a scanning spreading resistance microscope is R1 and the resistance of the plurality of second particles is R2, The ratio (logR2 / logR1) of the logarithms of the resistance (R1) of the first particles and the resistance (R2) of the second particles is 1.25 or less. Positive electrode.

2. the particle diameter of the plurality of first particles is 8 μm or more; The particle diameter of the plurality of second particles is smaller than 8 μm. The positive electrode according to claim 1 .

3. The positive electrode according to claim 1 or claim 2; a negative electrode; an electrolyte; Secondary battery.

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