Positive electrode active material and non-aqueous electrolyte secondary battery

Abstract

To provide a positive electrode active material and a non-aqueous electrolyte secondary battery in which an increase in output resistance and an increase in the amount of storage gas generation are suppressed.SOLUTION: The positive electrode active material includes a small particle group and a large particle group. The small particle group includes multiple small particles. The large particle group includes multiple large particles. When particle sizes at which the cumulative particle volume from the small particle side in the volume-based particle size distribution of the small particle group is 90%, 50%, and 10% of the total particle volume are set to D1(90), D1(50), and D1(10), respectively, a formula: 1.0≤[D1(90)-D1(10)] / D1(50) is satisfied. The small particles include single particles. The BET specific surface area of the small particles is 0.6 or more and 0.85 or less. The large particles include agglomerated particles. The agglomerated particles are formed by agglomeration of a plurality of primary particles. The ratio of the c-axis length to the a-axis length of the crystal lattice of the primary particles is 4.9620 or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a positive electrode active material and a non-aqueous electrolyte secondary battery including the same.

Background Art

[0002] Patent Document 1 (Japanese Patent Application Laid-Open No. 2020-35625) proposes a positive electrode active material for a secondary battery that forms a particle size distribution width having a predetermined spread for the purpose of suppressing fluctuations in volume resistivity due to the size of the particle diameter of the positive electrode active material particles for a secondary battery.

[0003] Patent Document 2 (Japanese Patent Application Laid-Open No. 2018-125305) proposes a production method for obtaining a positive electrode active material including lithium transition metal oxide particles that are composed of single particles or have a reduced number of primary particles constituting one secondary particle.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] When the positive electrode active materials described in Patent Documents 1 and 2 are used for the positive electrode of a non-aqueous electrolyte secondary battery, the output resistance of the non-aqueous electrolyte secondary battery may increase, and the amount of gas generated during storage may increase.

[0006] An object of the present disclosure is to provide a positive electrode active material and a non-aqueous electrolyte secondary battery in which an increase in output resistance and an increase in the amount of gas generated during storage are suppressed.

Means for Solving the Problems

[0007] The present disclosure provides the following positive electrode active material and non-aqueous electrolyte secondary battery. [1] A positive electrode active material including a small particle group and a large particle group, wherein the small particle group is composed of a plurality of small particles, the large particle group is composed of a plurality of large particles, when the particle diameters at which the cumulative particle volume from the small particle diameter side in the volume-based particle size distribution of the small particle group becomes 90%, 50%, and 10% of the total particle volume are D1(90), D1(50), and D1(10), respectively, the following formula (1): (1) 1.0 ≦ [D1(90) - D1(10)] / D1(50) is satisfied, the small particles include single particles, the BET specific surface area of the small particles is 0.6 or more and 0.85 or less, the large particles include aggregated particles, the aggregated particles are formed by aggregation of a plurality of primary particles, the ratio of the c-axis length to the a-axis length of the crystal lattice of the primary particles is 4.9620 or more, the positive electrode active material. [2] The following formula (2): (2) [D1(90) - D1(10)] / D1(50) ≦ 1.4 is satisfied, the positive electrode active material according to [1]. [3] The positive electrode active material according to [1] or [2], wherein D1(50) is 3 μm or more and 6 μm or less. [4] When the particle diameter at which the cumulative particle volume from the small particle diameter side in the volume-based particle size distribution of the large particle group becomes 90% of the total particle volume is D2(50), the positive electrode active material according to any one of [1] to [3], wherein D2(50) is 14 μm or more. [5] A non-aqueous electrolyte secondary battery including the positive electrode active material according to any one of [1] to [4].

Advantages of the Invention

[0008] According to the present disclosure, it is possible to provide a positive electrode active material and a non-aqueous electrolyte secondary battery in which an increase in output resistance and an increase in the amount of gas generated during storage are suppressed.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

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

[0011] <Positive electrode active material> The positive electrode active material of the present embodiment is for a non-aqueous electrolyte secondary battery (hereinafter also referred to as a battery). Details of the battery will be described later. The positive electrode active material includes a small particle group and a large particle group. The small particle group has a smaller D(50) than the large particle group. D(50) refers to the particle diameter at which the cumulative particle volume from the small particle size side becomes 50% of the total particle volume in the volume-based particle size distribution. The D(50) of the small particle group and the D(50) of the large particle group are also referred to as D1(50) and D2(50), respectively. D1(50) and D2(50) will be described later. In the volume-based particle size distribution of the positive electrode active material, the ranges of the particle size distributions of the small particle group and the large particle group do not overlap with each other. The volume-based particle size distribution of the positive electrode active material may be bimodal. The volume-based particle size distribution is measured according to the measurement method described in the Examples section below.

[0012] The positive electrode active material may substantially consist of a small particle group and a large particle group. The positive electrode active material may consist of a small particle group and a large particle group. The positive electrode active material may be a powder. In this specification, the description of "substantially consisting of..." indicates that additional components may be included in addition to the essential components, as long as the object of the present disclosure is not impaired. For example, components that are normally assumed in the art (such as inevitable impurities, etc.) may be included as additional components.

[0013] By configuring the positive electrode active material from small particle groups and large particle groups, the generation of storage gas tends to be easily suppressed. The positive electrode active material can contain, for example, the small particle groups and the large particle groups in a mass ratio of 4:6 to 6:4 (small particle groups:large particle groups). From the viewpoint of output resistance and the amount of storage gas generation, the positive electrode active material preferably contains the small particle groups and the large particle groups in a mass ratio of 1:1 (small particle groups:large particle groups).

[0014] (Small particle group) A small particle group is made up of multiple small particles. A small particle group is an aggregate of small particles. Small particles can have any shape. Small particles may be, for example, spherical, cylindrical, or lumpy. In this specification, elements expressed in the singular can also include the plural unless otherwise specified. For example, a "particle" can mean not only "one particle" but also "an aggregate of particles (powder, powder, particle group)."

[0015] In the volume-based particle size distribution of the small particle group, when the particle diameters at which the cumulative particle volume from the small particle size side accounts for 90%, 50%, and 10% of the total particle volume are defined as D1(90), D1(50), and D1(10), respectively, the following formula (1): (1) 1.0≦[D1(90)-D1(10)] / D1(50) The volumetric particle size distribution is measured according to the method described in the Examples section below. The [D1(90)-D1(10)] / D1(50) on the right side of formula (1) is sometimes referred to as the particle size distribution width. When the positive electrode active material satisfies formula (1), the proportion of small particles, which are considered to be advantageous for the diffusion of lithium (Li) within the solid, increases, and as a result, the increase in output resistance tends to be more easily suppressed. The particle size [D1(50)] and particle size distribution width of the small particles can be controlled, for example, by selecting the particle size (D50) and particle size distribution width of the transition metal compound that is the raw material for the small particles.

[0016] From the viewpoint of output resistance, the left side of the formula (1) is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.3 or more.

[0017] The small particle group can, for example, satisfy the following formula (2): (2) [D1(90) - D1(10)] / D1(50) ≤ 1.4 and can further satisfy this condition.

[0018] D1(50) can be, for example, 5 μm or less, and can be, for example, 3 μm or more. D1(50), for example, can be controlled by selecting the particle diameter (D50) and the particle size distribution width of the transition metal compound serving as the raw material for the small particles.

[0019] The small particles include single particles. The single particles are relatively large primary particles. By using single particles as the small particles, the generation of the storage gas tends to be easily suppressed. The single particles can have any shape. The single particles can be, for example, spherical, columnar, massive, etc. The small particles can be formed from one single particle. The small particles can also be formed by aggregation of 2 to 10 single particles.

[0020] The number of single particles contained in the small particles is measured in the SEM (scanning electron microscope) image of the small particles. The magnification of the SEM image is appropriately adjusted according to the size of the particles. The magnification of the SEM image can be, for example, 10,000 times to 30,000 times.

[0021] In the SEM image of the particles, for example, when two single particles overlap, the particle on the back side may not be confirmed. However, in this embodiment, the number of single particles that can be confirmed in the SEM image is regarded as the number of single particles contained in the small particles. The same applies to the aggregated particles described later. The small particles can, for example, consist essentially of 1 to 10 single particles. The small particles can, for example, consist of 1 to 10 single particles. The small particles can, for example, consist of 1 to 5 single particles. The small particles can, for example, consist of 1 to 3 single particles. The small particles can, for example, consist of one single particle.

[0022] The "single particle" in this embodiment refers to a particle in the SEM image of the particles, which, in appearance, has no discernible grain boundaries and has a first maximum diameter of 0.5 μm or more. The first maximum diameter indicates the distance between the two farthest points on the contour line of the single particle. In this embodiment, the "contour line of the particle" may be confirmed in the two-dimensional projection image of the particle or in the cross-sectional image of the particle. The contour line of the particle may be confirmed, for example, in the SEM image of the powder or in the cross-sectional SEM image of the particle.

[0023] The single particle may have a first maximum diameter of, for example, 1 μm to 7 μm. The single particle may have a first maximum diameter of, for example, 2 μm to 5 μm. The single particle may have a first maximum diameter of, for example, 2.5 μm to 3.8 μm. The average value of the first maximum diameter may be, for example, 1 μm to 5 μm. The average value of the first maximum diameter is calculated from the first maximum diameters of 100 single particles. The 100 single particles are randomly extracted.

[0024] The BET specific surface area of the small particles is 0.6 or more and 0.85 or less. When the BET specific surface area of the small particles including the single particle is within the above range, it becomes easier to suppress the increase in the amount of stored gas accompanying the increase in the proportion of small particles in the positive electrode active material. As a result, the increase in the output resistance of the battery is suppressed and the decrease in the amount of stored gas is also likely to be suppressed. The BET specific surface area of the small particles is preferably 0.6 or more and 0.8 or less, more preferably 0.6 or more and 0.7 or less, from the viewpoint of the amount of stored gas generated. The BET specific surface area in this embodiment is measured by the BET multipoint method. The BET specific surface area is measured according to the method described in the column of the examples below.

[0025] (Large particle group) The large particle group is composed of multiple large particles. The large particle group is an aggregate of large particles. When the particle diameter at which the cumulative particle volume from the small particle side in the volume-based particle size distribution of the large particle group is 90% of the total particle volume is defined as D2(50), D2(50) may be, for example, 12 μm or more, or, for example, 20 μm or less. D2(50) can be controlled, for example, by selecting the particle diameter (D50) and particle size distribution width of the transition metal compound that is the raw material for the large particles.

[0026] The large particles may have any shape. The large particles may be, for example, spherical, columnar, or lumpy. The large particles include aggregated particles. The large particles may, for example, consist essentially of aggregated particles. The large particles may, for example, consist of aggregated particles. The aggregated particles may be formed by aggregating 50 or more primary particles. The number of primary particles contained in the aggregated particles is measured in an SEM image of the aggregated particles. The magnification of the SEM image may, for example, be 10,000 to 30,000 times. The aggregated particles may, for example, be formed by aggregating 100 or more primary particles. There is no upper limit to the number of primary particles in the aggregated particles. The aggregated particles may, for example, be formed by aggregating 10,000 or fewer primary particles. The aggregated particles may, for example, be formed by aggregating 1,000 or fewer primary particles. The primary particles may have any shape. The primary particles may, for example, be spherical, columnar, or lumpy.

[0027] The agglomerated particles may have a particle diameter of, for example, 10 μm or more and 25 μm or less. The particle diameter of the agglomerated particles is the distance between the two most distant points on the contour line of the agglomerated particles. The particle diameter of the agglomerated particles is measured on an SEM image of the agglomerated particles. The agglomerated particles may have a particle diameter of, for example, 12 μm or more and 20 μm or less. The average particle diameter of the agglomerated particles may be, for example, 14 μm or more and 20 μm or less. The average particle diameter of the agglomerated particles is calculated from the particle diameters of 100 agglomerated particles. The 100 agglomerated particles are randomly selected.

[0028] In this embodiment, the term "primary particles" refers to particles in which grain boundaries cannot be visually identified in an SEM image of the particles and which have a second maximum diameter of less than 0.5 μm. Primary particles contained in large particles have a smaller particle size than the individual particles contained in small particles. The second maximum diameter refers to the distance between the two most distant points on the outline of a primary particle. Primary particles may have a second maximum diameter of, for example, 0.05 μm to 0.2 μm. When 10 or more primary particles randomly sampled from an SEM image of a single agglomerate particle have a second maximum diameter of 0.05 μm to 0.2 μm, all of the primary particles contained in the agglomerate are considered to have a second maximum diameter of 0.05 μm to 0.2 μm. Primary particles may have a second maximum diameter of, for example, 0.1 μm to 0.2 μm. The average value of the second maximum diameters may be, for example, 0.1 μm to 0.3 μm. The average value is calculated from the second maximum diameters of 100 primary particles. 100 primary particles are randomly selected.

[0029] The aggregated particles may further contain primary particles having a maximum diameter of 0.5 μm or more, so long as the aggregated particles contain 50 or more primary particles having a maximum diameter of less than 0.5 μm.

[0030] The ratio of the c-axis length to the a-axis length of the crystal lattice of the primary particles (hereinafter also referred to as the c / a ratio) is 4.9620 or more. When the c / a ratio is within the above range, the amount of storage gas generation tends to be easily suppressed. This is presumably because, when the c / a ratio is large, the amount of Li ions at the Li site is relatively small, and the Li that does not enter the crystal forms a larger coating layer containing Li compounds on the positive electrode surface, which easily suppresses side reactions with the electrolyte that cause storage gas generation. The upper limit of the c / a ratio may be, for example, 4.9660 or less, 4.9650 or less, or 4.9640 or less. The c / a ratio is measured according to the method described in the Examples section below. The c / a ratio can be controlled, for example, by adjusting the firing parameters (e.g., firing time and firing temperature) of the firing step described below.

[0031] The small particles (single particles) and large particles (primary particles) of the present embodiment may have any crystal structure. The single particles and primary particles may each independently have, for example, a layered structure, a spinel structure, an olivine structure, or the like.

[0032] The small particles (single particles) and large particles (primary particles) of the present embodiment may each independently have any chemical composition. The small particles (single particles) may have the same chemical composition as the large particles (primary particles). The small particles (single particles) may have a chemical composition different from that of the large particles (primary particles). For example, the single particles and primary particles may each independently contain at least one selected from the group consisting of LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li(NiCoMn)O2, Li(NiCoAl)O2, and LiFePO4. Here, descriptions such as "(NiCoMn)" in a composition formula such as "Li(NiCoMn)O2" indicate that the total of the composition ratios within the parentheses is 1.

[0033] For example, both the small particles (single particles) and the large particles (primary particles) may contain a layered metal oxide. The layered metal oxide has a layered structure. The layered metal oxide may contain, for example, Ni. The layered metal oxide containing Ni may have a large specific capacity.

[0034] The single particles (small particles) may contain, for example, a first layered metal oxide. The first layered metal oxide has the formula (i): Li a1 Ni 1-b1 M 1 b1 O2(i) represented by In formula (i), a1 satisfies the relationship 0.7 ≦ a1 ≦ 1.3. b1 satisfies the relationship 0 < b1 ≦ 0.5. M 1 represents at least one selected from the group consisting of Co, Mn, Al, Zr, Ti, V, Cr, Fe, Cu, Zn, Mo, Sn, Ge, Nb, W, B, Mg, Na, K, Ba, Sr, Ca, and Si.

[0035] The first layered metal oxide is preferably represented by the formula (i'): Li a1’ Ni 1-c1-d1 Co c1 Mn d1 M 1’ e1 O2 (i') wherein. In the formula (i'), a1' satisfies the relationship of 1.00 ≤ a1' ≤ 1.10. c1 satisfies the relationship of 0.18 ≤ c1 ≤ 0.22. d1 satisfies the relationship of 0.18 ≤ c1 ≤ 0.22. e1 satisfies the relationship of 0 ≤ e1 ≤ 0.10. M 1’ represents at least one selected from the group consisting of Al, Zr, Ti, V, Cr, Fe, Cu, Zn, Mo, Sn, Ge, Nb, W, B, Mg, Na, K, Ba, Sr, Ca, and Si.

[0036] The primary particles (large particles) may contain, for example, the second layered metal oxide. The second layered metal oxide is represented by the formula (ii): Li a2 Ni 1-b2 M 2 b2 O2 (ii) wherein. In the formula (ii), a2 satisfies the relationship of 0.7 ≤ a2 ≤ 1.3. b2 satisfies the relationship of 0 < b2 ≤ 0.5. M 2 represents at least one selected from the group consisting of Co, Mn, Al, Zr, Ti, V, Cr, Fe, Cu, Zn, Mo, Sn, Ge, Nb, W, B, Mg, Na, K, Ba, Sr, Ca, and Si.

[0037] The second layered metal oxide is preferably represented by the formula (ii'): Li a2’ Ni 1-c2-d2 Co c2 Mnd2 M 2’ e2 O2(ii’) is represented by In formula (ii’), a2’ satisfies the relationship of 1.00 ≦ a2’ ≦ 1.10. c2 satisfies the relationship of 0.18 ≦ c2 ≦ 0.30. d2 satisfies the relationship of 0.18 ≦ d2 ≦ 0.30. e2 satisfies the relationship of 0 ≦ e2 ≦ 0.10. M 2’ represents at least one selected from the group consisting of Al, Zr, Ti, V, Cr, Fe, Cu, Zn, Mo, Sn, Ge, Nb, W, B, Mg, Na, K, Ba, Sr, Ca, and Si.

[0038] The single particles (small particles) and primary particles (large particles) are independent of each other, for example, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.7 Co 0.2 Mn 0.1 O2, LiNi 0.7 Co 0.1 Mn 0.2 O2, LiNi 0.55 Co 0.2 Mn 0.25 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, and LiNi 0.6 Co 0.1 Mn 0.3 may contain at least one selected from the group consisting of O2.

[0039] (Method for manufacturing a positive electrode active material) The positive electrode active material can be manufactured, for example, by first synthesizing small particles and large particles and then mixing the synthesized small particles and large particles. The small particles and large particles can be synthesized, for example, through a two-step firing process. The method for manufacturing the positive electrode active material can include, for example, a first firing step of firing a first mixture containing a lithium compound and a transition metal compound, and a second firing step of firing a second mixture containing the fired product obtained in the first firing step and a transition metal compound. The crystal structures of the small particles and large particles can be arbitrarily controlled by adjusting the firing parameters (such as firing temperature and firing time, etc.) in the first firing step and the second firing step.

[0040] The transition metal compound used for synthesizing the small particles and large particles can include, for example, one or more compounds containing nickel (Ni), cobalt (Co), and manganese (Mn) (hereinafter also referred to as NCM compounds). The NCM compound preferably contains a nickel cobalt manganese composite hydroxide. The nickel cobalt manganese composite hydroxide can be obtained, for example, by a coprecipitation method or the like. The nickel cobalt manganese composite hydroxide can be, for example, a compound represented by the general formula: NixCoyMnz(OH)2 (where x + y + z = 1).

[0041] The positive electrode active material is obtained by mixing the synthesized small particles and large particles at a mass ratio of 4:6 to 6:4 (small particle group: large particle group), preferably at a mass ratio of 1:1.

[0042] <Non-aqueous electrolyte secondary battery> FIG. 1 is a schematic diagram showing an example of the battery in this embodiment. The battery 100 shown in FIG. 1 can be used for any application. The battery 100 can be used, for example, as a main power source or a power assist power source in an electric vehicle. A battery module or a battery pack may be formed by connecting a plurality of batteries 100. The battery 100 may be a lithium-ion battery.

[0043] The battery 100 includes an exterior body 90. The exterior body 90 is rectangular (flat cuboid shape). However, the rectangular shape is just an example. The exterior body 90 may be, for example, cylindrical or pouch-shaped. The exterior body 90 may be made of, for example, an aluminum (Al) alloy. The exterior body 90 houses the electrode body 50 and an electrolyte (not shown). The electrode body 50 is connected to the positive electrode terminal 91 by the positive electrode current collector member 81. The electrode body 50 is connected to the negative electrode terminal 92 by the negative electrode current collector member 82.

[0044] FIG. 2 is a schematic diagram showing an example of the electrode body in the present embodiment. The electrode body 50 is a wound type. The electrode body 50 includes a positive electrode 10, a separator 30, and a negative electrode 20. That is, the battery 100 includes the positive electrode 10. The positive electrode 10, the separator 30, and the negative electrode 20 are all strip-shaped sheets. The electrode body 50 may include two separators 30. The electrode body 50 is formed by laminating the positive electrode 10, the separator 30, and the negative electrode 20 in this order and winding them in a spiral shape. The electrode body 50 is formed in a flat shape after winding. Note that the wound type is just an example. The electrode body 50 may be, for example, a stacked type.

[0045] (Positive electrode) The positive electrode 10 includes a positive electrode active material layer 12 and a positive electrode substrate 11. The positive electrode active material layer 12 is formed on the surface of the positive electrode substrate 11. The positive electrode active material layer 12 may be directly formed on the surface of the positive electrode substrate 11. For example, an intervening layer (not shown) may be formed between the positive electrode active material layer 12 and the positive electrode substrate 11. In the present embodiment, even when the intervening layer is formed, the positive electrode active material layer 12 is regarded as being formed on the surface of the positive electrode substrate 11. The intervening layer may have a thickness smaller than that of the positive electrode active material layer 12. The intervening layer may include, for example, a conductive material, an insulating material, etc. The positive electrode active material layer 12 may be formed on only one side of the positive electrode substrate 11. The positive electrode active material layer 12 may be formed on both the front and back surfaces of the positive electrode substrate 11.

[0046] The positive electrode active material layer 12 may have a thickness of, for example, from 10 μm to 200 μm. The positive electrode active material layer 12 may have a high density. The positive electrode active material layer 12 may have a density of, for example, 3.5 g / cm 3 or more, or may have a density of 3.6 g / cm 3 or more, or may have a density of 3.7 g / cm 3 or more. The upper limit of the density is arbitrary. The positive electrode active material layer 12 may have a density of, for example, 3.8 g / cm 3 or less.

[0047] FIG. 3 is a conceptual diagram showing the positive electrode in the present embodiment. The positive electrode active material layer 12 contains the positive electrode active material described above. The positive electrode active material contains small particles 1 and large particles 2. The small particles 1 include single particles. The large particles 2 include aggregated particles. The small particles 1 and the large particles 2 are densely packed. For example, the large particles 2 may be distributed in an island shape in an aggregate of the small particles 1 spreading in a sponge-like manner. In addition to the positive electrode active material, the positive electrode active material layer 12 may further contain a conductive material (not shown), a binder (not shown), etc. The conductive material may contain any component. The conductive material may contain, for example, acetylene black or the like. The blending amount of the conductive material may be, for example, from 0.1 part by mass to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material. The binder may contain any component. The binder may contain, for example, polyvinylidene fluoride (PVdF) or the like. The blending amount of the binder may be, for example, from 0.1 part by mass to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material.

[0048] The positive electrode substrate 11 is a conductive sheet. The positive electrode substrate 11 may have a thickness of, for example, from 10 μm to 30 μm. The positive electrode substrate 11 may have a thickness of, for example, from 10 μm to 20 μm. The positive electrode substrate 11 may have a thickness of, for example, from 10 μm to 15 μm. The positive electrode substrate 11 may contain, for example, an Al alloy foil, a pure Al foil, etc. The positive electrode substrate 11 may be substantially made of an Al alloy foil, for example. The positive electrode substrate 11 may be made of an Al alloy foil, for example.

[0049] The positive electrode 10 is manufactured by forming a positive electrode active material layer 12 by applying a positive electrode slurry onto the surface of a positive electrode substrate 11, then rolling the positive electrode active material layer 12 and the positive electrode substrate 11 to produce a raw sheet, and subsequently cutting it to a predetermined planar size according to the specifications of the battery 100. The positive electrode slurry is prepared by mixing a positive electrode active material and additional components.

[0050] (Negative electrode) The negative electrode 20 includes a negative electrode active material layer 22 and a negative electrode substrate 21. The negative electrode substrate 21 may contain, for example, a copper foil or the like. The negative electrode active material layer 22 is formed on the surface of the negative electrode substrate 21. The negative electrode active material layer 22 contains a negative electrode active material. The negative electrode active material may contain any components. The negative electrode active material powder may contain, for example, at least one selected from the group consisting of graphite, soft carbon, hard carbon, Si, SiO, Si-based alloys, Sn, SnO, Sn-based alloys, and Li4Ti5O 12 and may contain at least one selected from the group consisting of. The negative electrode active material layer 22 may further contain a binder or the like in addition to the negative electrode active material powder. The binder may contain, for example, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), or the like.

[0051] The negative electrode 20 is manufactured by forming a negative electrode active material layer 22 by applying a negative electrode slurry onto the surface of a negative electrode substrate 21, then rolling the negative electrode active material layer 22 and the negative electrode substrate 21 to produce a raw sheet, and subsequently cutting it to a predetermined planar size according to the specifications of the battery 100. The negative electrode slurry is prepared by mixing a negative electrode active material and other components.

[0052] (Separator) At least a part of the separator 30 is disposed between the positive electrode 10 and the negative electrode 20. The separator 30 separates the positive electrode 10 and the negative electrode 20. The separator 30 is porous. The separator 30 allows the electrolyte to permeate. The separator 30 is electrically insulating. The separator 30 may be made of, for example, polyolefin. In addition, when the electrolyte is solid, the electrolyte may function as a separator in some cases.

[0053] (Electrolyte) The electrolyte includes at least one selected from the group consisting of a liquid electrolyte (electrolyte solution, ionic liquid), a gel electrolyte, and a solid electrolyte. In this embodiment, as an example, an electrolyte solution will be described. The electrolyte solution includes a solvent and a supporting electrolyte. The electrolyte solution may further contain any additive. The supporting electrolyte is dissolved in the solvent. The solvent is aprotic. The solvent may include, for example, at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). The supporting electrolyte is dissolved in the solvent. The supporting electrolyte may contain any component. The supporting electrolyte may include, for example, at least one selected from the group consisting of LiPF6, LiBF4, and LiN(FSO2)2.

[0054] Hereinafter, the present invention will be described in more detail with reference to examples.

Examples

[0055] [Evaluation of output resistance] The test cells fabricated in the examples and comparative examples were cooled to a battery measurement environmental temperature of 0°C, and constant current charging was performed at a current density of 0.2 mA / cm 2 until a voltage of 3.44 V was reached, and then constant voltage charging was performed at 3.49 V until the current density reached 0.04 mA / cm 2 . After that, constant current discharge was performed at a current value of 0.2 mA / cm 2 for 10 seconds, and the voltage at the 10-second mark was measured. Measurements were performed at current densities of 0.4, 0.6, 0.8, 1.0, and 1.2 mA / cm 2 , and the 10-second DC resistance was calculated from the slope of the current value and voltage value.

[0056] [Evaluation of the amount of gas generated during storage] The gas amount of the cell in the above discharge state was taken as the gas amount before storage. Then, under a temperature condition of 25°C, constant current charging was performed at a current density of 0.2 mA / cm 2 until a voltage of 4.25 V was reached, and then at 4.25 V until the current density reached 0.04 mA / cm2 After constant voltage charging until it reached a certain state, it was stored in a thermostat at 60°C for 30 days. Then, at a current density of 0.2 mA / cm 2 it was discharged at a constant current until it reached 3.0 V, and then the gas volume was measured to obtain the gas generation amount after storage. The amount of gas during storage was measured by subtracting the gas before storage from the gas after storage.

[0057] [Measurement of BET specific surface area] The BET specific surface area of the small particles was measured using a fully automatic specific surface area measuring device (Macsorb HM model - 1208 manufactured by MOUNTECH).

[0058] [Measurement of volume - based particle size distribution] The volume - based particle size distributions D1(90), D1(50), D1(10) of the small particles (small particle group) and the volume - based particle size distribution D2(50) of the large particles (large particle group) were measured using a laser diffraction particle size analyzer (Mastersizer - 3000 manufactured by Malvern Panalytical).

[0059] [Measurement of c / a ratio] For the c / a ratio of the large particles, a measuring device (a fully automatic multi - purpose X - ray diffractometer (SmartLab manufactured by Rigaku)) was used. The powder sample of the large particles was filled in a dedicated folder, and the crystal peak data measured by the reflection method under the following measurement conditions was subjected to Rietveld analysis to calculate the a - axis length and c - axis length in the crystal (crystal system: trigonal system, space group: R - 3m), and the c / a ratio was obtained. (Measurement conditions) Tube ball (target element): Cu Tube voltage: 45 kV Tube current: 200 mA

[0060] <Example 1> (Synthesis of large particles) LiOH, and Ni 0.55 Co 0.20 Mn 0.25A first mixture was obtained by mixing with a transition metal compound represented by (OH)₂. After firing the first mixture to obtain a fired product, large particles were synthesized by firing a second mixture containing the obtained fired product and the transition metal compound. The D2(50) of the large particles (large particle group) was 15.8 μm, and the c / a ratio was 4.9620.

[0061] (Synthesis of small particles) Ni with LiOH 0.60 Co 0.20 Mn 0.20 A third mixture was obtained by mixing with a transition metal compound represented by (OH)₂. After firing the third mixture to obtain a fired product, small particles were synthesized by firing a fourth mixture containing the obtained fired product and the transition metal compound. The D1(50) of the small particles (small particle group) was 4.0 μm, the particle size distribution width [[D1(90) - D1(10)] / D1(50)] was 1.367, and the BET specific surface area was 0.68 m 2 / g.

[0062] (Preparation of positive electrode active material) The obtained large particles and small particles were mixed at a mass ratio of 1:1 to prepare a positive electrode active material.

[0063] (Preparation of positive electrode plate) 97.5 parts by mass of the prepared positive electrode active material, 1.5 parts by mass of carbon black as a conductive material, and 1.0 part by mass of polyvinylidene fluoride (PVdF) as a binder were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a positive electrode slurry. The slurry was applied to a current collector made of aluminum foil, and the coating film was dried to form a positive electrode composite layer. Then, it was compressed using a compression roller so that the positive electrode composite density became 3.55 g / cm 3 and cut to a predetermined size and an aluminum tab was attached to obtain a positive electrode plate.

[0064] (Preparation of negative electrode plate) Graphite negative electrode active material, carboxymethyl cellulose (CMC) as a thickener, and styrene butadiene rubber (SBR) as a binder were weighed so that the mass ratio was 98:1:1, respectively, and these were dispersed in water to prepare a negative electrode slurry. By applying this negative electrode mixture slurry to a current collector made of copper foil, a negative electrode mixture layer was formed. Then, it was dried, rolled to a predetermined thickness using a rolling roller, cut to a predetermined size, and a nickel tab was attached to obtain a negative electrode plate.

[0065] (Preparation of non-aqueous electrolyte) Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:30:40. Lithium hexafluorophosphate (LiPF6) was added to the mixed solvent at a concentration of 1.15 mol / liter. Further, vinylene carbonate (VC) was added so that the addition rate was 1.0 mass% with respect to the total mass of the mixed solvent to prepare a non-aqueous electrolyte.

[0066] (Fabrication of test cell) The positive electrode and the negative electrode were laminated via a polyolefin separator to fabricate a laminated electrode body. This electrode body was housed in an exterior body composed of an aluminum laminate sheet, the non-aqueous electrolyte was injected, and then the opening of the exterior body was sealed to obtain a test cell. Regarding the above test cell, under a temperature condition of 25°C, constant current charging was performed at a current density of 0.2 mA / cm 2 until a voltage of 4.25 V was reached, and then constant voltage charging was performed at 4.25 V until the current density became 0.04 mA / cm 2 to obtain the charging capacity. After a 10-minute pause, constant current discharging was performed at a current density of 0.2 mA / cm 2 until a voltage of 3.0 V was reached to obtain the discharging capacity. The test cell was evaluated for output resistance and amount of gas generated during storage. The results are shown in Table 1.

[0067] <Examples 2 to 4, Comparative Examples 1 to 3> A test cell was fabricated in the same manner as in Example 1 except that the small particles and large particles shown in Table 1 were used. D1(90), D1(50), D1(10) of the small particle group, D2(50) of the large particle group, and the particle size distribution width [[D1(90)-D1(10)] / D1(50)] were controlled by adjusting the particle diameter (D50) and the particle size distribution width of the transition metal compound. The c / a ratio of the crystal lattice of the large particles was controlled by adjusting the firing parameters in the firing process.

[0068]

Table 1

[0069] In Comparative Example 1, the output resistance was higher than that in Example 1. This is presumably because the BET specific surface area and the particle size distribution width [[D1(90)-D1(10)] / D1(50)] of the small particles in Comparative Example 1 were smaller than those in Example 1. In Comparative Example 2, although both the BET specific surface area and the particle size distribution width of the small particles were larger than those in Comparative Example 1, the output resistance was higher than that in Example 1.

[0070] In Example 1, while the BET specific surface area was made comparable to that in Comparative Example 2, the particle size distribution width was increased and the c / a ratio of the large particles was increased, thereby suppressing the increase in output resistance and the generation of storage gas. In Examples 2 and 3, by adjusting the c / a ratio of the large particles with respect to the BET specific surface area and the particle size distribution width of the small particles, the output resistance and the amount of storage gas generated could be further improved.

[0071] In Example 4, although the particle size distribution width of the small particles was small, by increasing the BET specific surface area and the c / a ratio of the large particles, the increase in output resistance and the generation of storage gas could be suppressed. In Comparative Example 4, although the same small particles as in Example 4 were used, since the c / a ratio was small, a large amount of storage gas was generated. It can be seen that the c / a ratio of the large particles contributes to the amount of storage gas generated.

Explanation of Symbols

[0072] 1 Small particle, 2 Large particle, 10 Positive electrode, 11 Positive electrode substrate, 12 Positive electrode active material layer, 20 Negative electrode, 21 Negative electrode substrate, 22 Negative electrode active material layer, 30 Separator, 50 Electrode body, 81 Positive electrode current collector member, 82 Negative electrode current collector member, 90 Exterior body, 91 Positive electrode terminal, 92 Negative electrode terminal, 100 Battery.

Claims

1. A positive electrode active material including a small particle group and a large particle group, wherein the small particle group is composed of a plurality of small particles, the large particle group is composed of a plurality of large particles, in the volume-based particle size distribution of the small particle group, when the particle diameters at which the cumulative particle volume from the small particle size side becomes 90%, 50%, and 10% of the total particle volume are D1(90), D1(50), and D1(10), respectively, the following formula (1): (1) 1.0 ≤ [D1(90) - D1(10)] / D1(50) is satisfied, the small particles include single particles, the BET specific surface area of the small particles is 0.6 or more and 0.85 or less, the large particles include aggregated particles, the aggregated particles are formed by aggregation of a plurality of primary particles, and the ratio of the c-axis length to the a-axis length of the crystal lattice of the primary particles is 4.9620 or more. A positive electrode active material.

2. The following formula (2): (2) [D1(90) - D1(10)] / D1(50) ≤ 1.4 The positive electrode active material according to Claim 1, which satisfies the above condition.

3. The positive electrode active material according to Claim 1, wherein D1(50) is 3 μm or more and 6 μm or less.

4. In the volume-based particle size distribution of the large particle group, when the particle diameter at which the cumulative particle volume from the small particle size side becomes 90% of the total particle volume is D2(50), the positive electrode active material according to Claim 1, wherein D2(50) is 14 μm or more.

5. A non-aqueous electrolyte secondary battery including the positive electrode active material according to Claim 1.

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