Positive electrode and non-aqueous electrolyte secondary battery

Abstract

To provide a technique of realizing a non-aqueous electrolyte secondary battery which attains an excellent output characteristic and an excellent storage characteristic at the same time.SOLUTION: The positive electrode disclosed herein includes: a positive electrode collector 52; and a positive electrode active material layer 54 arranged in the surface of the positive electrode collector 52. The positive electrode active material layer 54 includes: first positive electrode active material particles 100; and second positive electrode active material particles 200. The first positive electrode active material particles 100 are hollow particles with an average particle diameter based on a laser diffraction diffusing method of 1 μm to 7 μm, both inclusive. The second positive electrode active material particles 200 have an outside shell part 210 and a hollow part 220 surrounded by the outside shell part 210, and are hollow particles with an average particle diameter based on a laser diffraction diffusing method of 8 μm to 14 μm, both inclusive. The mass M1 of the first positive electrode active material particles 100 and the mass M2 of the second positive electrode active material particles 200 satisfy the formula of 0<M1 / (M1+M2)≤0.6.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Positive electrode active material particles having a hollow structure are known as positive electrode active materials used in non-aqueous electrolyte secondary batteries. Positive electrode active material particles having a hollow structure have an increased specific surface area, which is expected to improve output characteristics, for example. Patent Documents 1 and 2 disclose positive electrode active materials having a hollow structure. Patent Documents 3 and 4 disclose positive electrodes including positive electrode active material particles having a hollow structure and positive electrode active material particles having a solid structure. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-077577 [Patent Document 2] International Publication No. 2014-181891 [Patent Document 3] Japanese Patent Application Publication No. 2017-123236 [Patent Document 4] Japanese Patent Publication No. 2021-120937 Summary of the Invention [Problem to be solved by the invention]

[0004] However, while hollowing out the positive electrode active material particles improves output characteristics, it also increases the specific surface area of ​​the positive electrode active material particles, which can lead to a trade-off between the positive electrode active material particles and the electrolyte solution, making it easier for the positive electrode active material particles to react with each other and generating gas inside the case. Therefore, a technology that can achieve both improved output characteristics and storage characteristics is desired.

[0005] Therefore, a main object of the present disclosure is to provide a technology that can realize a nonaqueous electrolyte secondary battery that combines excellent output characteristics and storage characteristics.

Means for Solving the Problem

[0006] One aspect of the technology disclosed herein is a positive electrode used in a non-aqueous electrolyte secondary battery. This positive electrode includes a current collector and an active material layer disposed on the surface of the current collector. The active material layer includes first positive electrode active material particles and second positive electrode active material particles. The first positive electrode active material particles have an average particle diameter based on the laser diffraction scattering method of 1 μm or more and 7 μm or less, and a porosity measured based on a cross-sectional SEM image of less than 10%. The second positive electrode active material particles include an outer shell portion and a hollow portion surrounded by the outer shell portion, have an average particle diameter based on the laser diffraction scattering method of 8 μm or more and 14 μm or less, and a ratio of voids measured based on a cross-sectional SEM image of 10% or more, and 80% or more of the voids are present inside the outer shell portion. The mass M1 of the first positive electrode active material particles and the mass M2 of the second positive electrode active material particles satisfy the following formula: 0 < M1 / (M1 + M2) ≤ 0.6.

[0007] Such a positive electrode includes first positive electrode active material particles that are relatively solid particles with few voids and second positive electrode active material particles that are relatively hollow particles with many voids. The first positive electrode active material particles having a solid structure are smaller in size than the second positive electrode active material particles and have a relatively small particle diameter (for example, 7 μm or less), so that the lithium diffusion distance in the first positive electrode active material particles is shortened, and the output characteristics of the secondary battery can be improved. In addition, the second positive electrode active material particles having a hollow structure are larger in size than the first positive electrode active material particles and have a relatively large particle diameter (for example, 8 μm or more), so that the specific surface area of the second positive electrode active material particles is reduced and gas generation can be suppressed, and thus the storage characteristics are improved. By adjusting the mixing ratio of such first positive electrode active material particles and second positive electrode active material particles to an appropriate range, a non-aqueous electrolyte secondary battery with excellent output characteristics and storage characteristics can be realized.

Brief Description of the Drawings

[0008] [Figure 1] FIG. 1 is a cross-sectional view schematically showing a positive electrode sheet according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view schematically showing the structure of a second positive electrode active material particle according to one embodiment. [Figure 3] FIG. 3 is a cross-sectional view schematically showing the configuration of a lithium ion secondary battery according to one embodiment. [Figure 4] FIG. 4 is a schematic exploded view showing the configuration of a wound electrode body. [Figure 5] FIG. 5 is a schematic diagram showing the general configuration of a crystallizer. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the technology disclosed herein will be described with reference to the drawings. Matters other than those specifically mentioned in this specification that are necessary for implementing the technology (e.g., the general configuration and manufacturing process of a non-aqueous electrolyte secondary battery that do not characterize the technology) can be understood as design matters for a person skilled in the art based on conventional technology in the relevant field. The technology can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. The drawings are schematic, and dimensional relationships (length, width, thickness, etc.) do not necessarily reflect actual dimensional relationships. In the drawings described below, components and parts that perform the same function are designated by the same reference numerals, and redundant descriptions may be omitted or simplified.

[0010] In this specification, when a numerical range is described as "A to B (where A and B are any numerical values)," it means "A or more and B or less," and also encompasses the meanings of "greater than A and less than B," "greater than A and B or less," and "greater than A and less than B."

[0011] In this specification, the term "nonaqueous electrolyte secondary battery" refers to an electricity storage device that uses a nonaqueous electrolyte as a charge carrier and can be repeatedly charged and discharged by the movement of charge carriers between positive and negative electrodes. Also, in this specification, the term "lithium ion secondary battery" refers to an electricity storage device that uses lithium ions as a charge carrier and achieves charging and discharging by the movement of charge associated with lithium ions between positive and negative electrodes.

[0012] The positive electrode disclosed herein is used in a non-aqueous electrolyte secondary battery, and is preferably used in a lithium-ion secondary battery. One embodiment of the positive electrode disclosed herein will be specifically described below with reference to FIG. 1. FIG. 1 is a cross-sectional view schematically illustrating a positive electrode sheet 50 according to this embodiment, taken along the thickness and width directions. The positive electrode sheet 50 can be used as a positive electrode for a lithium-ion secondary battery.

[0013] 1, the positive electrode sheet 50 includes a positive electrode current collector 52 and a positive electrode active material layer 54 disposed on the surface of the positive electrode current collector 52. Here, the positive electrode active material layer 54 is disposed directly on the surface of the positive electrode current collector 52, but it may also be disposed indirectly. The positive electrode active material layer 54 may be disposed on only one surface of the positive electrode current collector 52, or may be disposed on both surfaces of the positive electrode current collector 52 as shown in FIG.

[0014] 1, the positive electrode current collector 52 may have a positive electrode current collector exposed portion 52a at the widthwise end of the positive electrode sheet 50, where the positive electrode active material layer 54 is not provided. The positive electrode current collector exposed portion 52a can function as a current collector. In some embodiments, the positive electrode current collector exposed portion 52a may be provided at the end of the positive electrode current collector 52 that has been processed into a tab shape.

[0015] In this embodiment, the positive electrode current collector 52 has a foil (or sheet) shape, but is not limited thereto. The positive electrode current collector 52 may have various shapes, such as a rod, a plate, or a mesh. As with conventional lithium-ion secondary batteries, the positive electrode current collector 52 can be made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.), and aluminum is particularly preferred. The positive electrode current collector 52 is preferably made of aluminum foil.

[0016] The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on the battery design. When an aluminum foil is used as the positive electrode current collector 52, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 6 μm to 20 μm.

[0017] The positive electrode active material layer 54 includes first positive electrode active material particles 100 and second positive electrode active material particles 200.

[0018] The first positive electrode active material particle 100 is a solid particle having a solid structure. In this specification, the term "solid particle" refers to a particle in which, in a cross-sectional SEM image of the particle, the proportion of voids (porosity) is less than 10% of the entire cross section of the particle.

[0019] The first positive electrode active material particles 100 have a porosity of less than 10%, preferably 7% or less, and more preferably 5% or less, as measured by cross-sectional SEM images. This can improve capacity characteristics. In this specification, "porosity" refers to the arithmetic average of the porosities of at least 100 randomly selected particles.

[0020] The shape of the first positive electrode active material particles 100 is not particularly limited, but may be, for example, spherical or approximately spherical. In some embodiments, the first positive electrode active material particles 100 may have an irregular shape. As used herein, "substantially spherical" refers to a shape that can be considered roughly spherical overall, with an average aspect ratio based on cross-sectional electron microscope images of approximately 1 to 2, preferably 1 to 1.5, and more preferably 1 to 1.2. Furthermore, as used herein, "aspect ratio" refers to the value obtained by dividing the long side of the rectangle with the smallest area among rectangles circumscribing the particle outline by the short side. Furthermore, "average aspect ratio" refers to the arithmetic mean of the aspect ratios of at least 100 randomly selected particles.

[0021] The average particle diameter of the first positive electrode active material particles 100 is, for example, 1 μm or more, preferably 2 μm or more, and more preferably 3 μm or more. The average particle diameter of the first positive electrode active material particles 100 is, for example, 7 μm or less, preferably 6 μm or less, and more preferably 5 μm or less. The smaller the average particle diameter of the first positive electrode active material particles 100, which are solid particles, the shorter the lithium diffusion distance, and therefore the more improved the output characteristics of the secondary battery. In this specification, the term "average particle size" refers to the particle size (D50) corresponding to 50% cumulative particle size in the volume-based particle size distribution measured by a laser diffraction / scattering particle size analyzer.

[0022] The BET specific surface area of ​​the first positive electrode active material particles 100 is, for example, 0.5 m 2 / g or more and 0.8m 2 / g or more, 1.5m 2 / g or more, or 1.8m 2 The BET specific surface area of ​​the first positive electrode active material particles 100 is, for example, 3 m 2 / g or less and 2.5m 2 / g or less, 2.4m 2 / g or less, 2.3m 2 / g or less, or 2.2m 2 / g or less. The higher the BET specific surface area, the more improved the output characteristics tend to be, and the lower the BET specific surface area, the more improved the storage characteristics tend to be. When the first positive electrode active material particles 100 have a BET specific surface area within the range defined by the above-mentioned upper and lower limits, the balance between the output characteristics and the storage characteristics can be suitably adjusted, and a higher level of compatibility between the output characteristics and the storage characteristics can be achieved.

[0023] The first positive electrode active material particles 100 may be, for example, a lithium transition metal composite oxide, a lithium transition metal phosphate compound, etc. The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, etc.

[0024] The lithium transition metal composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element, and specific examples thereof include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0025] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0026] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0027] In this embodiment, the first positive electrode active material particles 100 are represented by the following general formula (i): Li d Ni (1-x-y-z) Co x Mn y A z O2(i) It has a composition represented by the following. However, in the general formula (i), it is assumed that 0 ≦ x ≦ 0.5, 0 ≦ y ≦ 0.5, 0 ≦ z ≦ 0.1, and 0.95 ≦ d ≦ 1.2. In the general formula (i), A is one or more elements selected from the group consisting of Al, Ti, Zr, Nb, Mo, and W.

[0028] In some embodiments, the first positive electrode active material particles 100 contain at least Ni as a transition metal. That is, in such embodiments, the general formula (i) further satisfies x + y + z < 1.

[0029] In some embodiments, the proportion of Ni is higher than the total proportion of Co and Mn. That is, in the general formula (i), x + y + z < 0.5 may be satisfied.

[0030] In some embodiments, the first positive electrode active material particles 100 contain Ni, Co, and Mn. That is, in the general formula (i), 0 < x < 0.5 and 0 < y < 0.5 may be satisfied. In some embodiments, 0 < x < 0.3 and 0 < y < 0.3 may be satisfied.

[0031] The second positive electrode active material particles 200 are hollow particles having a hollow structure. FIG. 2 is a cross-sectional view schematically showing the structure of the second positive electrode active material particles 200 according to the present embodiment. The second positive electrode active material particles 200 include an outer shell portion 210 and a hollow portion 220. As used herein, the "hollow particle" refers to a particle in which, in the cross-sectional SEM image of the particle, the proportion of the void portion is 10% or more with respect to the entire cross-section of the particle, and 70% or more of the voids are inside the particle rather than the outer shell portion.

[0032] The porosity measured by the cross-sectional SEM image of the second positive electrode active material particles 200 is, for example, 10% or more, preferably 20% or more, and more preferably 25% or more. As a result, the specific surface area increases and Li ions are more likely to react, so the output characteristics can be improved.

[0033] The shape of the second positive electrode active material particles 200 is not particularly limited, but may be, for example, spherical or approximately spherical. In some embodiments, the second positive electrode active material particles 200 may have an irregular shape.

[0034] The average particle diameter of the second positive electrode active material particles 200 is, for example, 8 μm or more, 9 μm or more, 10 μm or more, or 11 μm or more. The average particle diameter of the second positive electrode active material particles 200 is, for example, 14 μm or less, preferably 13 μm or less, and more preferably 12 μm or less. The larger the average particle diameter of the hollow second positive electrode active material particles 200, the more likely it is that the reactivity between the second positive electrode active material particles 200 and the electrolyte can be suppressed, thereby suppressing gas generation and improving the storage characteristics of the secondary battery. The smaller the average particle diameter of the second positive electrode active material particles 200, the larger the specific surface area, which tends to improve the output characteristics. Having the second positive electrode active material particles 200 have an average particle diameter within the range defined by the above-mentioned upper and lower limits allows for a favorable adjustment of the balance between the output characteristics and the storage characteristics.

[0035] The outer shell 210 includes a plurality of primary particles 212. In this embodiment, the outer shell 210 is formed by agglomerating the plurality of primary particles 212 through physical or chemical bonding forces. In this specification, the term "primary particle" refers to the smallest unit constituting a positive electrode active material particle, and more specifically, refers to the smallest unit determined from the geometric shape of its appearance.

[0036] The outer shell 210 may have a single layer structure, or may have a multi-layer structure in which multiple layers are stacked.

[0037] The average thickness of outer shell 210 may be, for example, 1 / 5 or less, and preferably 1 / 6 or less, of the average particle diameter of second positive electrode active material particles 200. Furthermore, the average thickness of outer shell 210 may be, for example, 1 / 10 or more, or 1 / 8 or more, of the average particle diameter of second positive electrode active material particles 200. By making the average thickness of outer shell 210 thinner than the average particle diameter of second positive electrode active material particles 200, the diffusion resistance of Li ions is reduced and the output characteristics are improved. In this specification, the thickness of the outer shell 210 refers to the thickness measured in a cross-sectional observation image of the second positive electrode active material particle 200 taken with an electron microscope, and the average thickness of the outer shell 210 refers to the arithmetic mean of the thicknesses of the outer shell 210 measured for at least 100 second positive electrode active material particles 200.

[0038] The shape of the primary particles 212 may be, for example, needle-like, plate-like, elliptical, or the like. The average aspect ratio (length of major axis / length of minor axis) of the second positive electrode active material particles 200 based on a cross-sectional observation image taken with an electron microscope is, for example, 2 or more, preferably 3 or more, and more preferably 5 or more. The longer the average aspect ratio of the primary particles 212, the more likely the diffusion resistance of Li ions will be reduced. The average aspect ratio of the primary particles 212 may be, for example, 50 or less, 20 or less, or 10 or less.

[0039] The average length of the major axes of the primary particles 212 is, for example, 0.05 μm or more, 0.1 μm or more, 0.2 μm or more, or 0.3 μm or more, and the average length of the major axes of the primary particles 212 is, for example, 1.2 μm or less, 1 μm or less, or 0.8 μm or less.

[0040] The average length of the minor axes of the primary particles 212 is, for example, 0.01 μm or more, 0.02 μm or more, 0.05 μm or more, or 0.1 μm or more, and the average length of the minor axes of the primary particles 212 is, for example, 0.6 μm or less, 0.4 μm or less, or 0.2 μm or less.

[0041] The average lengths of the major and minor axes of the primary particles 212 are measured based on an electron microscope cross-sectional observation image of the second positive electrode active material particles 200. In this specification, the average lengths of the major and minor axes of the primary particles 212 refer to the arithmetic mean of the major and minor axes of at least 100 primary particles 212.

[0042] At least 40% of the primary particles 212 contained in the outer shell 210 are radially oriented from the center of the second positive electrode active material particle 200 toward the surface of the second positive electrode active material particle 200. In a preferred embodiment, the oriented primary particles 212 account for at least 45%, at least 55%, at least 65%, or at least 70% of the primary particles 212. As more primary particles 212 are radially oriented, the flexibility of the second positive electrode active material particle 200 improves, making it less susceptible to breakage. The percentage of the primary particles 212 mentioned above indicates a percentage based on the number, and indicates the percentage of the number of radially oriented primary particles 212 out of the total number (at least 100) of measured primary particles 212. In this specification, "primary particles are radially oriented" means that the angle formed by the long axis direction of the primary particle 212 with respect to a reference line connecting the center of the primary particle 212 and the center of the second positive electrode active material particle 200 is 30° or less. Note that this measurement can be performed by processing a cross section of the second positive electrode active material particle 200 with an ion beam and observing the cross section with an electron microscope. Note that as another method, the orientation can also be confirmed by electron backscatter diffraction (EBSD).

[0043] The hollow portion 220 is a space surrounded by the outer shell portion 210. The hollow portion 220 does not have to be completely surrounded by the outer shell portion 210, and includes embodiments in which a portion of the outer shell portion 210 is missing. In a preferred embodiment, the number of primary particles in the hollow portion 220 is fewer than that in the outer shell portion 210, or there are no primary particles in the hollow portion 220. That is, the proportion of primary particles 212 contained in the outer shell portion 210 among the primary particles contained in the entire second positive electrode active material particle 200 can be, for example, 90% or more, 95% or more, 98% or more, or 100%. This makes it possible to obtain an active material that reacts well with Li during firing. In this specification, the proportion of such primary particles is measured by area ratio in a cross-sectional SEM.

[0044] The composition of the second positive electrode active material particles 200 may be the same as that of the first positive electrode active material particles 100 described above. The first positive electrode active material particles 100 and the second positive electrode active material particles 200 may have the same composition or may have different compositions. In a preferred embodiment, the second positive electrode active material particles 200 are a lithium transition metal composite oxide containing at least lithium and nickel.

[0045] In the present embodiment, the second positive electrode active material particles 200 are a lithium transition metal composite oxide containing lithium and a transition metal, and have a composition represented by the above general formula (i).

[0046] In some embodiments, the second positive electrode active material particles 200 contain Ni, Co, and Mn. That is, in the general formula (i), 0 < x < 0.5 and 0 < y < 0.5 may be satisfied. Also, in some embodiments, 0 < x < 0.3 and 0 < y < 0.3 may be satisfied.

[0047] In the embodiment shown in FIG. 2, the second positive electrode active material particles 200 include an outer shell portion 210 and a hollow portion 220, but in other embodiments, they may further include a central portion formed by aggregation of primary particles. The central portion is located inside the outer shell portion. A space where no primary particles are present may be provided between the central portion and the outer shell portion. Also, the central portion may have a solid structure or a hollow structure.

[0048] The ratio of the first positive electrode active material particles 100 and the second positive electrode active material particles 200 contained in the positive electrode active material layer 54 is, for example, when the mass of the first positive electrode active material particles 100 is M1 and the mass of the second positive electrode active material particles 200 is M2, the formula: 0 < M1 / (M1 + M2) ≦ 0.6 is satisfied. Also, M1 / (M1 + M2) is preferably 0.05 or more, more preferably 0.1 or more. Also, M1 / (M1 + M2) is preferably 0.3 or less, more preferably 0.2 or less. Thereby, the balance between the first positive electrode active material particles 100 which are solid particles and the second positive electrode active material particles 200 which are hollow particles becomes suitable, and both the output characteristics and the storage characteristics are exhibited well.

[0049] The total proportion of the first positive electrode active material particles 100 and the second positive electrode active material particles 200 contained in the positive electrode active material layer 54 is preferably 80 wt % or more, more preferably 85 wt % or more, and even more preferably 95 wt % or more of the entire positive electrode active material layer 54. Although not particularly limited, the total proportion of the first positive electrode active material particles 100 and the second positive electrode active material particles 200 contained in the positive electrode active material layer 54 can be 99 wt % or less of the entire positive electrode active material layer 54.

[0050] The positive electrode active material layer 54 may further contain a conductive material, a binder, and the like. Suitable conductive materials include, for example, carbon black such as acetylene black (AB) and other carbon materials (carbon nanotubes, graphite, and the like). Suitable binders include, for example, polyvinylidene fluoride (PVDF). The proportion of the conductive material is, for example, 5 wt % or less, preferably 3 wt % or less, and more preferably 1 wt % or less of the entire positive electrode active material layer 54, although not particularly limited thereto. The proportion of the binder is, for example, 5 wt % or less, preferably 3 wt % or less, and more preferably 1 wt % or less of the entire positive electrode active material layer 54, although not particularly limited thereto.

[0051] The positive electrode sheet 50 can be produced according to a known method. Specifically, for example, a slurry containing the components of the positive electrode active material layer 54 is prepared. This slurry is applied to the positive electrode current collector 52 and dried to form the positive electrode active material layer 54 on the positive electrode current collector 52. Note that the positive electrode active material layer 54 may be subjected to a press treatment, if necessary.

[0052] The first positive electrode active material particles and the second positive electrode active material particles used in the present technology may be purchased commercially, or may be processed or manufactured using a known method. An example of a method for manufacturing the second positive electrode active material particles 200 described above will be described in the test examples below.

[0053] In the positive electrode sheet 50 according to this embodiment, the first positive electrode active material particles 100 having a solid structure are smaller in size than the second positive electrode active material particles 200, and have a relatively small particle size (e.g., 7 μm or less). This shortens the lithium diffusion distance in the first positive electrode active material particles 100, thereby improving the output characteristics of the secondary battery. Furthermore, the second positive electrode active material particles 200 having a hollow structure are larger in size than the first positive electrode active material particles 100, and have a relatively large particle size (e.g., 8 μm or more). This reduces the specific surface area of ​​the second positive electrode active material particles 200, suppressing gas generation and improving storage characteristics. By adjusting the mixing ratio of the first positive electrode active material particles 100 and the second positive electrode active material particles 200 to an appropriate range (e.g., greater than 0 and less than 0.6), a nonaqueous electrolyte secondary battery that combines excellent output characteristics and storage characteristics can be realized.

[0054] Hereinafter, a nonaqueous electrolyte secondary battery including a positive electrode sheet 50 will be described using a lithium ion secondary battery as an example. FIG. 3 is a cross-sectional view schematically showing the configuration of a lithium ion secondary battery 1 according to one embodiment. The lithium ion secondary battery 1 in FIG. 3 is a sealed battery in which a flat wound electrode assembly 20 and a nonaqueous electrolyte (not shown) are housed in a flat, rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 that is designed to release the internal pressure of the battery case 30 when the internal pressure rises above a predetermined level. The positive and negative electrode terminals 42 and 44 are electrically connected to positive and negative electrode current collector plates 42a and 44a, respectively. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

[0055] FIG. 4 is a schematic exploded view showing the configuration of the wound electrode body 20. As shown in FIGS. 3 and 4, the wound electrode body 20 has a configuration in which the above-mentioned positive electrode sheet 50 and negative electrode sheet 60 are overlapped with two long separators 70 interposed therebetween and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long negative electrode current collector 62. The positive electrode current collector exposed portion 52a and the negative electrode current collector exposed portion 62a (i.e., portions where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed) are formed so as to protrude outward from both ends in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the wound electrode body 20. A positive electrode current collector plate 42a and a negative electrode current collector plate 44a are joined to the positive electrode current collector exposed portion 52a and the negative electrode current collector exposed portion 62a, respectively.

[0056] The negative electrode current collector 62 constituting the negative electrode sheet 60 may be a known negative electrode current collector used in lithium ion secondary batteries, such as a sheet or foil made of a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.). Copper foil or the like is preferred as the negative electrode current collector 62. The dimensions of the negative electrode current collector 62 are not particularly limited and may be determined appropriately depending on the battery design. When copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0057] The negative electrode active material layer 64 includes a negative electrode active material. Examples of the negative electrode active material that can be used include carbon materials such as graphite, hard carbon, and soft carbon. The negative electrode active material layer 64 can further include a binder, a thickener, and the like. Examples of the binder that can be used include styrene butadiene rubber (SBR). Examples of the thickener that can be used include carboxymethyl cellulose (CMC).

[0058] The separator 70 can be made of various porous sheets similar to those conventionally used in lithium-ion secondary batteries, including porous resin sheets made of resins such as polyethylene (PE) and polypropylene (PP). Such porous resin sheets may have a single-layer structure or a multi-layer structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). The separator 70 may also have a heat-resistant layer (HRL).

[0059] The nonaqueous electrolyte can be the same as that used in conventional lithium-ion secondary batteries, and typically, a supporting salt can be used in an organic solvent (nonaqueous solvent). The nonaqueous solvent can be an aprotic solvent such as carbonates, esters, or ethers. Among these, carbonates are preferred because they are particularly effective in reducing low-temperature resistance due to the positive electrode material. Examples of carbonates include ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). These nonaqueous solvents can be used alone or in appropriate combinations of two or more. Suitable supporting salts include lithium salts such as LiPF6, LiBF4, lithium bis(fluorosulfonyl)imide (LiFSI), etc. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0060] The nonaqueous electrolyte may contain components other than the nonaqueous solvent and supporting salt described above, such as various additives such as a gas generating agent, a film-forming agent, a dispersant, and a thickener, as long as the effects of the present invention are not significantly impaired.

[0061] The above describes a prismatic lithium ion secondary battery having a flat wound electrode assembly as one embodiment. However, the lithium ion secondary battery can also be configured as a lithium ion secondary battery having a stacked electrode assembly (i.e., an electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked). The lithium ion secondary battery can also be configured as a cylindrical lithium ion secondary battery, a laminated case lithium ion secondary battery, or the like. Furthermore, an all-solid-state secondary battery using a solid electrolyte as the electrolyte can also be constructed.

[0062] The lithium ion secondary battery 1 can be used for a variety of purposes. Suitable applications include a driving power source mounted on vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium ion secondary battery 1 can also be used as a storage battery for small power storage devices and the like. The lithium ion secondary battery 1 can also be used in the form of an assembled battery in which a plurality of batteries are connected in series and / or parallel.

[0063] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: A positive electrode used in a non-aqueous electrolyte secondary battery, A current collector; an active material layer disposed on the surface of the current collector; Equipped with the active material layer includes first positive electrode active material particles and second positive electrode active material particles, The first positive electrode active material particles are The average particle size based on the laser diffraction scattering method is 1 μm or more and 7 μm or less, The porosity measured based on cross-sectional SEM images is less than 10%; The second positive electrode active material particles are The device has an outer shell and a hollow portion surrounded by the outer shell, Based on the laser diffraction scattering method, the average particle size is 8 μm or more and 14 μm or less, the ratio of voids measured based on the cross-sectional SEM image is 10% or more, 80% or more of the voids are present inside the outer shell part, the mass M1 of the first positive electrode active material particles and the mass M2 of the second positive electrode active material particles satisfy the following formula: a positive electrode that satisfies 0 < M1 / (M1 + M2) ≤ 0.6. Item 2: The BET specific surface area of the first positive electrode active material particles is 1.5 m 2 / g or more and 2.3 m 2 / g or less, the positive electrode according to Item 1. Item 3: The mass M1 of the first positive electrode active material particles and the mass M2 of the second positive electrode active material particles satisfy the following formula: 0 < M1 / (M1 + M2) ≤ 0.2, the positive electrode according to Item 1 or 2. Item 4: 90% or more of the primary particles contained in the second positive electrode active material particles as a whole are present in the outer shell part, the positive electrode according to any one of Items 1 to 3. Item 5: 40% or more of the primary particles present in the outer shell part are such that the angle formed by the major axis direction of the primary particles with respect to the reference line connecting the center of the primary particles and the center of the second positive electrode active material particles is 30° or less, the positive electrode according to any one of Items 1 to 4. Item 6: The first positive electrode active material particles and the second positive electrode active material particles each have a general formula: Li d Ni (1-x-y-z) Co x Mn y A z O2 (where 0 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.1, 0.95 ≤ d ≤ 1.2, A is one or more additive elements selected from the group consisting of Al, Ti, Zr, Nb, Mo, W) a compound represented by, the positive electrode according to any one of Items 1 to 5. Item 7: A non-aqueous electrolyte secondary battery comprising the positive electrode according to any one of Items 1 to 6.

[0064] Test examples relating to the present technology will be described below, but the present technology is not limited to those shown in the following test examples.

[0065] [Preparation of hollow-structured positive electrode active material particles] A crystallizer capable of generating Taylor vortices in a reaction chamber 330 was used. Figure 5 shows the general configuration of the crystallizer 300 used. As shown in Figure 5, the crystallizer 300 includes a cylindrical outer cylinder 310, a cylindrical inner cylinder 320 arranged coaxially with the outer cylinder 310, and a motor 322 connected to the inner cylinder 320. The inner cylinder 320 has a smaller diameter than the outer cylinder 310. The crystallizer 300 includes a reaction chamber 330 between the outer cylinder 310 and the inner cylinder 320. The reaction chamber 330 is a region where a crystallization reaction takes place and where Taylor vortices 332 and 334 are generated. Note that Taylor vortices refer to the flow of a fluid that generates Taylor vortices. Taylor vortices refer to a circular vortex pattern.

[0066] As an example of producing hollow cathode active material particles, first, an oxygen concentration of 25 vol% was adjusted to 25 vol% in a 0.1 L reaction chamber 330, 1 wt% aqueous ammonia was added, and the inner cylinder 320 was rotated around its axis at 2000 rpm to generate Taylor vortices in the aqueous ammonia in the reaction chamber 330. Although FIG. 5 only partially illustrates the Taylor vortices 332 and 334, Taylor vortices were generated throughout the entire reaction chamber 330. Next, a 30 wt% aqueous sodium hydroxide solution was supplied to the reaction chamber 330, and the mixed solution in the reaction chamber 330 was adjusted to a pH of 12 at a liquid temperature of 25°C. Nickel sulfate, cobalt sulfate, and manganese sulfate were dissolved in water to prepare a 1 mol / L aqueous solution of raw metals, with a molar ratio of nickel sulfate:cobalt sulfate:manganese sulfate of 60:20:20. The raw metal aqueous solution, 1 wt % ammonia aqueous solution, and 30 wt % sodium hydroxide aqueous solution were supplied from first supply port 312, second supply port 314, and third supply port 316, respectively, to the mixed liquid in which Taylor vortices were generated within reaction chamber 330, so that the volume ratio was 1:1:1, and crystallization was carried out for 30 minutes under a condition of pH 12. Thereafter, the crystallized product was taken out from outlet 318, washed with water, filtered, and dried, thereby obtaining a powdery composite hydroxide (precursor). The composite hydroxide obtained above was analyzed by ICP emission spectroscopy. 0.6 Co 0.2 Mn 0.2 (OH)2 was confirmed.

[0067] A lithium compound was mixed with the composite hydroxide so that the molar ratio (Li:M) of lithium (Li) to a metal element other than lithium (M) was 1.1:1. This mixture was calcined in a calcination furnace at 700°C under an oxidizing atmosphere for 5 hours. This resulted in a positive electrode active material. The positive electrode active material thus obtained was used as the second active material in some of the following examples.

[0068] [Measurement of the average particle size of positive electrode active material particles] The average particle size of the positive electrode active material was evaluated as the particle size (D50) corresponding to 50% cumulative particle size in the volume-based particle size distribution measured by a laser diffraction scattering particle size analyzer.

[0069] [Evaluation of the structure of the positive electrode active material] To observe the cross-sectional shape of the positive electrode active material particles, the positive electrode active material particles were processed using an ion beam, and SEM images of the cross-sectional shape of the positive electrode active material particles were obtained. Based on these SEM images, the proportion and distribution of voids (the proportion of voids in the cross-sectional area of ​​the particle) were measured. Those with a void ratio of less than 10% were considered to have a solid structure. Those with a void ratio of 10% or more and with 80% or more of the voids located inside the outer shell were considered to have a hollow structure. It was confirmed that the positive electrode active material particles obtained by the above-mentioned method involving Taylor vortices have an outer shell and a hollow portion surrounded by the outer shell, have a porosity of 25% or more, and that, in the cross section of the particle, 90% or more of the primary particles constituting the particle are present in the outer shell.

[0070] [Evaluation of the orientation rate of hollow-structured positive electrode active material] For hollow-structured positive electrode active material particles, the orientation of the primary particles contained in the outer shell was confirmed by SEM observation of the particle cross section. Specifically, primary particles were evaluated as being radially oriented if the angle between the long axis direction of the primary particle and the reference line connecting the center of the primary particle and the center of the positive electrode active material particle was 30° or less. More than 100 primary particles contained in the outer shell were randomly observed, and the proportion of radially oriented primary particles (radial orientation rate) was calculated. The results are shown in Table 1. Cross-sectional SEM images were also taken of the precursor particles of the positive electrode active material particles, and it was confirmed that there was almost no change in the aspect ratio and orientation rate of the primary particles before and after firing.

[0071] [BET specific surface area measurement of positive electrode active material] The BET specific surface area of ​​the positive electrode active material was measured using a flow type gas adsorption specific surface area measuring device.

[0072] Example 1 [Preparation of positive electrode plate (positive electrode sheet)] Average particle size: 3 μm, BET specific surface area: 2.2 m 2 A first active material with a solid structure of 12 μm / g and a second active material with a hollow structure obtained by the above-mentioned method, with an average particle size of 12 μm and a radial orientation rate of 75%, were prepared. The chemical formulas of both the first and second active materials were Li 1.1 Ni 0.6 Co 0.2 Mn 0.2The positive electrode active material indicated by O2 was used. The first active material and the second active material were mixed so that the mass ratio was 20:80, and this was used as the positive electrode active material of Example 1. The obtained positive electrode active material, conductive material, and binder were mixed with N-methyl-2-pyrrolidone (NMP) so that the ratio was 100:1:1 (by weight), to prepare a positive electrode mixture paste. Acetylene black was used as the conductive material, and PVDF was used as the binder. The prepared positive electrode mixture paste was applied to 13 μm aluminum foil and dried, and then pressed to a predetermined thickness and processed to a predetermined size to produce a positive electrode plate.

[0073] [Preparation of negative electrode plate (negative electrode sheet)] The graphite, SBR, and CMC were mixed with water in a ratio of 100:1:1 (by weight) to prepare a negative electrode mixture paste. The prepared negative electrode mixture paste was applied to copper foil, dried, pressed to a specified thickness, and cut to a specified size to produce a negative electrode plate.

[0074] [Fabrication of non-aqueous electrolyte secondary battery] The prepared positive and negative electrode plates were stacked together with a separator interposed between them to prepare a laminated electrode body. Leads were attached to each of the positive and negative electrode plates. The prepared laminated electrode body was housed in an exterior body made of an aluminum laminate sheet, after which a non-aqueous electrolyte was poured into it, and the opening of the exterior body was sealed to prepare a test cell (laminate cell). The non-aqueous electrolyte used contained a solvent consisting of EC:FEC:EMC:DMC = 15:5:40:40 (by volume) and 1M LiPF6 as the Li salt.

[0075] Example 2 Average particle size: 5 μm, BET specific surface area: 1.8 m 2 A first active material with a solid structure of 10 ... 1.1 Ni 0.6 Co 0.2 Mn 0.2The positive electrode active material of Example 2 was prepared by mixing the first active material and the second active material at a mass ratio of 10:90. The subsequent procedures for preparing the test cell were the same as those in Example 1.

[0076] Example 3 Average particle size: 5 μm, BET specific surface area: 1.8 m 2 A first active material with a solid structure of 1 / g and a second active material with a hollow structure and an average particle diameter of 10 μm were prepared. This second active material consisted of secondary particles formed by agglomeration of a plurality of primary particles. The secondary particles had a hollow center, a space outside the center where no primary particles were present, and an outer shell electrically connected to the center. The secondary particles had an average ratio of the outer diameter of the center to the particle diameter of the secondary particles of 30 to 80%. The primary particles constituting the outer shell of the secondary particles had a radial orientation rate of 70%. The chemical formulas of both the first and second active materials were Li 1.1 Ni 0.6 Co 0.2 Mn 0.2 The positive electrode active material of Example 3 was prepared by mixing the first active material and the second active material at a mass ratio of 10:90. The subsequent procedures for preparing the test cell were the same as those in Example 1.

[0077] Example 4 Average particle size: 4 μm, BET specific surface area: 0.8 m 2 A first active material with a solid structure of 1 / g and a second active material with a hollow structure obtained by the above method, with an average particle diameter of 12 μm and a radial orientation rate of 65%, were prepared. The chemical formulas of both the first and second active materials were Li 1.1 Ni 0.6 Co 0.2 Mn 0.2 The positive electrode active material of Example 4 was prepared by mixing the first active material and the second active material at a mass ratio of 20:80. The subsequent procedures up to the preparation of the test cell were the same as in Example 1.

[0078] Example 5 Average particle size: 4 μm, BET specific surface area: 2.4 m2 A first active material with a solid structure of 1 / g and a second active material with a hollow structure obtained by the above method, with an average particle diameter of 12 μm and a radial orientation rate of 65%, were prepared. The chemical formulas of both the first and second active materials were Li 1.1 Ni 0.6 Co 0.2 Mn 0.2 The positive electrode active material of Example 5 was prepared by mixing the first active material and the second active material at a mass ratio of 60:40. The subsequent procedures up to the preparation of the test cell were the same as those of Example 1.

[0079] (Comparative Example 1) Average particle size: 5 μm, BET specific surface area: 2.4 m 2 A first active material with a solid structure of 0.1g / g and a second active material with a hollow structure and an average particle diameter of 5μm were prepared. This second active material consisted of secondary particles formed by agglomeration of a plurality of primary particles. The secondary particles had a hollow center, a space outside the center where no primary particles were present, and an outer shell electrically connected to the center. The average ratio of the outer diameter of the center to the particle diameter of the secondary particles was 30 to 80%. The primary particles constituting the outer shell of the secondary particles had a radial orientation rate of 65%. The chemical formulas of both the first and second active materials were Li 1.1 Ni 0.6 Co 0.2 Mn 0.2 The positive electrode active material of Comparative Example 1 was prepared by mixing the first active material and the second active material at a mass ratio of 20:80. The subsequent procedures up to the preparation of the test cell were the same as in Example 1.

[0080] (Comparative Example 2) Average particle size is 10 μm, BET specific surface area is 3.4 m 2 A first active material with a solid structure of 1 / g and a second active material with a hollow structure obtained by the above method, with an average particle diameter of 12 μm and a radial orientation rate of 55%, were prepared. The chemical formulas of both the first and second active materials were Li 1.1 Ni 0.6 Co 0.2 Mn 0.2Those indicated by O2 were used. They were mixed so that the mass ratio of the first active material to the second active material was 30:70, and the positive electrode active material of Comparative Example 2 was obtained. The procedure until the production of the test cell thereafter was the same as in Example 1.

[0081] <Measurement of initial capacity> In an environment of 25°C, the test cell was charged by CCCV up to 4.2V. The current value of the CC charge at this time was set to C / 3. Thereafter, it was discharged to 3V at a current value of C / 3. The discharge capacity at this time was taken as the initial capacity. Table 1 shows the results based on Comparative Example 1.

[0082] <IV resistance measurement> In an environment of 25°C, the test cell was charged so that the state of charge (SOC) became 50% and left standing for 1 hour. Thereafter, it was discharged at a current of 5C for 10 seconds. When the OCV voltage immediately before discharge was V0 and the voltage at the 10-second discharge time was V1, Formula: IV resistance = (V0 - V1) / 5C current value The IV resistance value was obtained according to the above formula. Table 1 shows the results at a ratio based on Comparative Example 1 described later.

[0083] )]] <Measurement of gas generation amount> In an environment of 25°C, the test cell was charged by CCCV at a charging potential of 4.4V and a charging current of 0.3C. The charging was terminated when the charging current became 0.01C. After charging, the test cell was placed in a constant temperature bath at 60°C and stored for 24 hours. Thereafter, the test cell was allowed to cool in a 25°C atmosphere. After cooling, the volume change of the test cell before and after storage was measured, and the gas generation amount (mL) during storage was determined. The volume of the test cell before or after storage was measured using Archimedes' principle, and the difference was taken as the gas generation amount.

[0084]

Table 1

[0085] <Evaluation of test examples> Comparative Example 1 is a test example in which a secondary battery with low resistance and excellent output characteristics was realized by using relatively small hollow particles with an average particle diameter of 5 μm, but the amount of gas generated was large, leaving room for improvement in storage characteristics. Here, when Examples 1 to 5 are compared with Comparative Example 1, as shown in Table 1, Examples 1 to 5 were able to reduce the amount of gas generated while maintaining the same resistance or even reducing the resistance compared to Comparative Example 1. In other words, it can be seen that Examples 1 to 5 had improved storage characteristics, and secondary batteries were realized that had both excellent output characteristics and storage characteristics.

[0086] Although specific examples of the present technology have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. [Explanation of symbols]

[0087] 1. Lithium-ion secondary battery 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 positive electrode sheet 52 Positive electrode current collector 52a Exposed part of positive electrode current collector 54 Cathode active material layer 60 negative electrode sheet 62 Negative electrode current collector 62a Exposed part of negative electrode current collector 64 Negative electrode active material layer 70 Separator 100 First positive electrode active material particles 200 Second positive electrode active material particles 210 Outer shell 212 Primary particles 220 Hollow part 300 Crystallizer 310 Outer cylinder 320 Inner cylinder 330 Reaction Chamber

Claims

1. A positive electrode for use in a non-aqueous electrolyte secondary battery, A current collector; an active material layer disposed on the surface of the current collector; Equipped with the active material layer includes first positive electrode active material particles and second positive electrode active material particles, The first positive electrode active material particles include The average particle size based on a laser diffraction scattering method is 1 μm or more and 7 μm or less, a porosity measured based on a cross-sectional SEM image of less than 10%; a BET specific surface area of ​​1.5 m 2 / g or more and 2.3 m 2 / g or less; The second positive electrode active material particles include The device has an outer shell and a hollow portion surrounded by the outer shell, The average particle size based on a laser diffraction scattering method is 8 μm or more and 14 μm or less, The proportion of voids measured based on a cross-sectional SEM image is 10% or more; 80% or more of the voids are present inside the outer shell portion, The mass M1 of the first positive electrode active material particles and the mass M2 of the second positive electrode active material particles are expressed by the following formula: A positive electrode that satisfies 0<M1 / (M1+M2)≦0.

6.

2. A positive electrode for use in a non-aqueous electrolyte secondary battery, A current collector; an active material layer disposed on the surface of the current collector; Equipped with the active material layer includes first positive electrode active material particles and second positive electrode active material particles, The first positive electrode active material particles include The average particle size based on a laser diffraction scattering method is 1 μm or more and 7 μm or less, a porosity measured based on a cross-sectional SEM image of less than 10%; The second positive electrode active material particles include The device has an outer shell and a hollow portion surrounded by the outer shell, The average particle size based on a laser diffraction scattering method is 8 μm or more and 14 μm or less, The proportion of voids measured based on a cross-sectional SEM image is 10% or more; 80% or more of the voids are present inside the outer shell portion, The mass M1 of the first positive electrode active material particles and the mass M2 of the second positive electrode active material particles are expressed by the following formula: A positive electrode that satisfies 0<M1 / (M1+M2)≦0.

2.

3. The positive electrode according to claim 1 , wherein 90% or more of the primary particles contained in the entire second positive electrode active material particles are present in the outer shell portion.

4. 4. The positive electrode according to claim 3, wherein 40% or more of the primary particles present in the outer shell portion have an angle of 30° or less between the major axis direction of the primary particle and a reference line connecting the center of the primary particle and the center of the second positive electrode active material particle.

5. The first positive electrode active material particles and the second positive electrode active material particles each have a general formula: LidNi （１－ｘ－ｙ－ｚ） Co ｘ Mn ｙ A ｚ O ２ (wherein 0≦x≦0.5, 0≦y≦0.5, 0≦z≦0.1, 0.95≦d≦1.2, and A is one or more additional elements selected from the group consisting of Al, Ti, Zr, Nb, Mo, and W). The positive electrode according to claim 1 , wherein the compound is represented by the formula:

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

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