Positive electrode and secondary battery

Abstract

To provide a positive electrode having an excellent balance between a diffusibility and an ion resistance in a surface direction of a metal ion, and provide a secondary battery containing the positive electrode.SOLUTION: A secondary battery includes: a collector; and a positive electrode layer that is arranged onto the collector, and contains an active material particle. An uneven level of the active material particle contained in a region that is farthest from the collector when the positive electrode layer is divided into two or more regions in a thickness direction is larger than the uneven level of the active material particle contained in a region that is closest to the collector, and the uneven level is expressed by the following formula (1). In formula (1), a BET expresses a value (m2 / g) of a specific surface area by a BET method of the active material particle, ρ expresses a density (g / m3) of the active material particle, r1 is an outer diameter*0.5(m) of the active material particle, and r2 expresses an inner diameter*0.5(m) of the active material particle.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] 2. Description of the Related Art Secondary batteries such as lithium ion secondary batteries are used in a wide range of fields, including electronic devices, electric vehicles, and power storage facilities. A typical secondary battery has an electrode assembly including a positive electrode and a negative electrode disposed with a separator therebetween.

[0003] If a metallic foreign object is contained in the positive electrode of a secondary battery, metal ions originating from the metallic foreign object may pass through the separator and migrate to the negative electrode side, forming dendrites and causing chemical short circuits (micro-short circuits). Therefore, from the viewpoint of improving the safety of secondary batteries, it is desirable to suppress the migration of metallic ions contained in the positive electrode to the negative electrode side. For example, Patent Document 1 describes an electrode having a multi-layered active material-containing layer, the active material-containing layer being composed of a surface layer disposed farthest from a current collector and a lower layer consisting of one or more layers disposed between the surface layer and the current collector, in which the curvature of the surface layer is greater than that of the lower layer. The curvature here is a value calculated by L' / L, where L is a vertical line A in the cross section of each layer, and L' is the sum of the maximum diameters of the active material particles that overlap with the line A in the horizontal direction (the direction perpendicular to the line A). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2008-251250 A Summary of the Invention [Problem to be solved by the invention]

[0005] In order to suppress the migration of metal ions originating from metallic foreign matter in the positive electrode to the negative electrode, it is believed that it is effective to increase the diffusivity of metal ions in the plane direction in the positive electrode. On the other hand, increasing the diffusivity of metal ions in the plane direction increases ion resistance, so it is necessary to consider the balance between the two. The invention described in Patent Document 1 is directed to suppressing the sudden migration of lithium ions to the negative electrode, and does not consider the relationship between the diffusibility of metal ions in the positive electrode and ionic resistance. An object of the present disclosure is to provide a positive electrode that has an excellent balance between the diffusivity of metal ions in the planar direction and ion resistance, and a secondary battery including this positive electrode. [Means for solving the problem]

[0006] Means for solving the above problems include the following means. <1> a current collector; and a positive electrode layer disposed on the current collector and including active material particles; A positive electrode, wherein the degree of unevenness of active material particles included in a region farthest from a current collector when the positive electrode layer is divided into two or more regions in a thickness direction is greater than the degree of unevenness of active material particles included in a region closest to the current collector, and the degree of unevenness is expressed by the following formula (1):

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[0007] According to the present disclosure, there are provided a positive electrode having an excellent balance between the diffusivity of metal ions in the planar direction and the ionic resistance, and a secondary battery including this positive electrode. [Brief description of the drawings]

[0008] [Figure 1] FIG. 2 is a cross-sectional view showing the structure of an active material particle without irregularities used in calculating the degree of irregularity. [Diagram 2] FIG. 2 is a cross-sectional view showing the configuration of a simulation model of a positive electrode produced in an example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] 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 do not 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.

[0010] Each component may contain multiple types of the corresponding substance. When referring to the amount of each component in a composition, if there are multiple substances corresponding to each component in the composition, unless otherwise specified, it means the total amount of those multiple substances present in the composition. In the present disclosure, the cross-sectional image of the positive electrode layer may be obtained by a known measurement method such as a scanning electron microscope (SEM).

[0011] ≪Positive electrode≫ The positive electrode according to the present disclosure has a current collector and a positive electrode layer disposed on the current collector and containing active material particles, wherein when the positive electrode layer is divided into two or more regions in the thickness direction, the degree of unevenness of the active material particles contained in the region farthest from the current collector is greater than the degree of unevenness of the active material particles contained in the region closest to the current collector, and the degree of unevenness is expressed by the following formula (1):

[0012]

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[0013] As shown in the examples described later, a positive electrode that satisfies formula (1) has an excellent balance between the diffusibility of metal ions in the planar direction and the ionic resistance.

[0014] In the present disclosure, the number of regions into which the positive electrode layer is divided in the thickness direction is not particularly limited as long as it is two or more, and may be, for example, two to five. In the present disclosure, when the positive electrode layer is divided in the thickness direction, the thicknesses of the respective regions may be the same or different.

[0015] In the present disclosure, the ratio (X / Y) of the unevenness X of the active material particles contained in the region farthest from the current collector to the unevenness Y of the active material particles contained in the region closest to the current collector is a value greater than 1. From the viewpoint of increasing the diffusibility of metal ions in the plane direction, X / Y is preferably 1.2 or more, more preferably 1.3 or more, and even more preferably 1.4 or more. From the viewpoint of suppressing an increase in ionic resistance, X / Y is preferably 2.0 or less, more preferably 1.9 or less, and even more preferably 1.8 or less.

[0016] From the viewpoint of increasing the diffusibility of metal ions in the planar direction, X is preferably 2.0 or more, more preferably 3.0 or more, and even more preferably 5.0 or more. From the viewpoint of suppressing an increase in ionic resistance, X is preferably 12.0 or less, more preferably 10.0 or less, and even more preferably 9.0 or less.

[0017] From the viewpoint of increasing the diffusibility of metal ions in the planar direction, Y is preferably 1.0 or more, more preferably 2.0 or more, and even more preferably 3.0 or more. From the viewpoint of suppressing an increase in ionic resistance, Y is preferably 10.0 or less, more preferably 8.0 or less, and even more preferably 6.0 or less.

[0018] In formula (1) showing the degree of unevenness of an active material particle, r1 is a value obtained by multiplying the outer diameter of a smooth active material particle by 0.5, and r2 is a value obtained by multiplying the inner diameter of a smooth active material particle by 0.5. The smooth active material particles are spherical and have a cross section as shown in FIG. The "outer diameter of an active material particle without irregularities" corresponding to the active material particle to be measured is the diameter of a circle having the same area as the projected image of the active material particle (equivalent circle diameter). The "inner diameter of an active material particle without irregularities" corresponding to the active material particle to be measured is the diameter of a circle having the same area as the projected image of the hollow part of the active material particle (circle equivalent diameter). When the active material particles have a hollow portion, r2 is a positive number smaller than r1. When the active material particles do not have a hollow portion, r2 is 0.

[0019] The values ​​of r1 and r2 used in calculating the degree of unevenness are determined based on the average values ​​of the outer diameter and inner diameter of the active material particles contained in the target region of the positive electrode layer. Specifically, the "outer diameter of the active material particle without irregularities" and the "inner diameter of the active material particle without irregularities" are measured for 100 active material particles randomly selected from the cross-sectional image of the target region of the positive electrode, and the average values ​​obtained are multiplied by 0.5 to obtain r1 and r2, respectively.

[0020] The degree of unevenness of the active material particles represented by formula (1) is a value obtained by dividing the specific surface area (BET) of the actual active material particles by the specific surface area (calculated value) of active material particles without unevenness, as shown in the following formula. The specific surface area (BET) of the actual active material is determined by a nitrogen gas adsorption method.

[0021]

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[0022] The "specific surface area (calculated value) of the active material particles without irregularities" in the above formula is calculated by the following formula.

[0023]

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[0024] From the viewpoint of effectively increasing the diffusibility of metal ions in the planar direction, the active material particles contained in the positive electrode preferably contain secondary particles. In the present disclosure, a secondary particle refers to a particle consisting of an aggregate of a plurality of primary particles. When the active material particles are secondary particles, the above-mentioned irregularity of the active material particles refers to the irregularity of the secondary particles. The aspect ratio of the primary particles constituting the secondary particles may be, for example, not less than 1.00 and less than 1.30.

[0025] The method for adjusting the unevenness of the active material particles is not particularly limited, and examples thereof include a method for adjusting the type and compounding ratio of the precursor of the active material particles; and a method for adjusting the heating temperature and heating time of the precursor of the active material particles.

[0026] The particle size of the active material particles contained in the positive electrode (if the active material particles are secondary particles, the particle size of the secondary particles) is not particularly limited and can be selected, for example, from the range of 3 μm to 20 μm.

[0027] The ratio (D1 / D2) of the particle diameter D1 of the active material particles contained in the region farthest from the current collector to the particle diameter D2 of the active material particles contained in the region closest to the current collector is preferably 0.7 to 1.3, more preferably 0.8 to 1.2, and even more preferably 0.8 to 1.0. When the value of D1 / D2 is within the above range, the variation in particle size of the active material particles in the positive electrode layer is small, and reaction resistance is effectively suppressed.

[0028] In the present disclosure, the particle diameter D1 of the active material particles contained in the region farthest from the current collector and the particle diameter D2 of the active material particles contained in the region closest to the current collector are the average particle diameters of the active material particles contained in each region. Specifically, the particle diameters of 100 active material particles arbitrarily selected from the cross-sectional image of each region of the positive electrode are measured, and the average of the obtained measurements is taken as the "irregularity of the active material particles."

[0029] The type of active material particles contained in the positive electrode is not particularly limited, and known positive electrode active material particles can be used. Specific examples of the positive electrode active material particles include particles of lithium transition metal composite oxides. Examples of the transition metal include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. Specific examples of lithium transition metal composite oxides include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), and their composite oxides (LiCo x Ni y Mnz O2, x+y+z=1), and a double oxide containing an additive element M' (LiCo a Ni b Mn c M' d O2, a+b+c+d=1, M': Al, Mg, Ti, Zr or Ge), spinel-type lithium manganese oxide (LiMn2O4), olivine-type LiMPO4 (M: Co, Ni, Mn, Fe), etc. Among the above, lithium transition metal composite oxides containing at least one selected from Ni, Co, and Mn are preferred, and lithium transition metal composite oxides containing Ni, Co, and Mn (NCM, nickel cobalt manganese oxide) are more preferred.

[0030] The porosity of the positive electrode layer is preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more. From the viewpoint of ensuring sufficient battery capacity, the porosity of the positive electrode layer is preferably 80% or less, more preferably 70% or less, and even more preferably 60% or less. The porosity of the positive electrode layer is calculated by calculating the ratio (percentage) of the area of ​​the void region in the cross section to the entire area (i.e., including the void region) of the positive electrode composite layer confirmed from the cross-sectional image of the positive electrode layer. When the active material particles have a hollow portion, the proportion of the area of ​​the hollow portion is also included in the porosity of the positive electrode layer.

[0031] The thickness of the positive electrode layer is not particularly limited and can be selected, for example, from the range of 10 μm to 100 μm.

[0032] The positive electrode layer may contain materials other than the active material particles, such as a binder resin, a conductive assistant, a thickener, a surfactant, a dispersant, a wetting agent, and a defoamer.

[0033] The current collector contained in the positive electrode is not particularly limited, and a current collector used in a known positive electrode for a secondary battery can be used. Examples of materials constituting the current collector include aluminum, aluminum alloy, nickel, titanium, stainless steel, etc. The shape of the current collector is, for example, a foil shape or a mesh shape.

[0034] ≪Secondary battery≫ A secondary battery according to the present disclosure includes a positive electrode according to the present disclosure. The secondary battery may include an electrode assembly having a positive electrode, a separator, and a negative electrode in this order. The secondary battery may be an all-solid-state battery including a solid electrolyte layer instead of a separator.

[0035] The negative electrode includes, for example, a current collector and a negative electrode layer provided on the current collector. The negative electrode layer includes a negative electrode active material. The negative electrode current collector is not particularly limited, and a current collector used in a known negative electrode 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. 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. Specific examples of the separator include nonwoven fabric, cloth, and microporous film that are mainly made of polyolefin such as polyethylene and polypropylene.

[0036] 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

[0037] 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.

[0038] (Creating a simulation model of the positive electrode layer) Using simulation software, a simulation model of the positive electrode having the configuration shown in FIG. 2 was created. The thickness of the first region and the second region of the positive electrode layer was 20 μm, and the porosity of the first region and the second region was 50%. Table 1 shows the roughness and particle size of the active material particles contained in the first and second regions of the positive electrode layer, the surface diffusion distance of metal ions (calculated value), and the ion resistance index. The ionic resistance index in Table 1 means the resistance when ions move inside the porous electrode.

[0039] (Calculation of surface diffusion distance of metal ions) In the second region of the positive electrode layer, the surface diffusion distance of the metal ions originating from the metallic foreign matter was calculated under the assumption that the metallic foreign matter exists at a position 10 μm away from the interface with the current collector. Specifically, the surface diffusion distance of the metal ions at the interface between the first region of the positive electrode layer and the separator was calculated by the following procedure. (1) From the relationship between the unevenness of the active material particles and the ionic resistance (experimental data), the relationship between the unevenness and the tortuosity is calculated using the following formula.

[0040]

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[0041] (2) From the calculated tortuosity, the surface diffusion distance is calculated by the following formula, where L is the thickness of the positive electrode layer and x is the tortuosity.

[0042]

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[0043] [Table 1]

[0044] Comparing Example 1 (X / Y>1), Comparative Example 1 (X / Y=1), and Comparative Example 3 (X / Y<1), which have similar ion resistance indices, Example 1 has the longest surface diffusion distance of metal ions. Comparing Example 2 (X / Y>1) and Comparative Example 2 (X / Y=1), which have the same ion resistance index, Example 2 has a larger surface diffusion distance of metal ions. The above results suggest that by ensuring that the roughness of the active material particles contained in the positive electrode layer satisfies the condition X / Y>1, it is possible to increase the diffusivity of metal ions in the planar direction while suppressing an increase in ionic resistance.

[0045] In Example 1 and Comparative Example 2, the ratio (D1 / D2) of the particle diameter D1 of the active material particles contained in the first region to the particle diameter D2 of the active material particles contained in the second region was all within the range of 0.7 to 1.3. The above results suggest that by making the unevenness of the active material particles contained in the positive electrode layer satisfy the condition X / Y>1, it is possible to increase the diffusivity of metal ions in the planar direction while suppressing the variation in particle size of the active material particles in the positive electrode layer.

Claims

1. a current collector; and a positive electrode layer disposed on the current collector and including active material particles; a positive electrode, wherein when the positive electrode layer is divided into two or more regions in a thickness direction, the degree of unevenness of active material particles included in a region farthest from a current collector is greater than the degree of unevenness of active material particles included in a region closest to the current collector, and the degree of unevenness is represented by the following formula (1): [0010] In formula (1), BET is the specific surface area (m ２ / g), ρ is the density of the active material particles (g / m ３ ), r １ is the outer diameter of the active material particle × 0.5 (m), r ２ represents the inner diameter of the active material particle×0.5 (m).

2. 2. The positive electrode according to claim 1, wherein a ratio (X / Y) of a degree of unevenness X of the active material particles included in the region farthest from the current collector to a degree of unevenness Y of the active material particles included in the region closest to the current collector is 1.2 or more.

3. The positive electrode according to claim 1, wherein a ratio (D1 / D2) of a particle diameter D1 of the active material particles contained in the region farthest from the current collector to a particle diameter D2 of the active material particles contained in the region closest to the current collector is 0.7 to 1.

3.

4. The positive electrode according to claim 1 , wherein the positive electrode layer has a porosity of 30% or more.

5. 10. The positive electrode of claim 1, wherein the active material particles comprise a lithium transition metal oxide.

6. A secondary battery comprising the positive electrode according to any one of claims 1 to 5.

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