Non-aqueous electrolyte secondary battery

Abstract

To provide a non-aqueous electrolyte secondary battery which is improved in high temperature cycle characteristics.SOLUTION: In a non-aqueous electrolyte secondary battery, an anode contains as anode active materials: a lithium transition metal composite oxide (A), of which D50 of a volume reference is 0.6 μm to 3 μm, consisting of secondary particle aggregating primary articles, of which the average particle diameter is 0.5 μm or more, or a practically single particle; and a lithium transition metal composite oxide (B), of which D50 of a volume reference is 6 μm to 25 μm, of a secondary particle aggregating primary particles of which the average particle diameter is 0.3 μm or less. In the lithium transition metal composite oxide (B), a molar fraction (B1) of boron with respect to a total mol number of metal elements excluding Li in the first particle of which the particle diameter is larger than D70 of a volume reference is larger than a molar fraction (B2) of boron with respect to the total mol number of metal elements excluding Li in a second particle of which the particle diameter is smaller than D30 of a volume reference.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, lithium transition metal composite oxides with a high Ni content have been attracting attention as a positive electrode active material with a high energy density. For example, Patent Document 1 describes a positive electrode active material containing Ni and Li as main components, and having the general formula Li x Ni 1-p-q-r Co p Al q A r O 2-y (A is at least one element selected from the group consisting of Ti, V, In, Cr, Fe, Sn, Cu, Zn, Mn, Mg, Ga, Ni, Co, Zr, Bi, Ge, Nb, Ta, Be, Ca, Sr, Ba, and Sc) and a nonaqueous electrolyte secondary battery containing a composite oxide composed of single-crystal primary particles having an average particle size of 2 μm to 8 μm. Patent Document 1 describes that the positive electrode active material has properties such as low reactivity with the electrolyte, low internal resistance when used in a battery, and high resistance to pressure during positive electrode molding.

[0003] Furthermore, Patent Document 2 discloses a positive electrode active material whose particle surfaces are coated with boron oxide. Patent Document 2 states that by using this positive electrode active material, side reactions between the active material and the electrolyte are suppressed, and gas generation inside the battery is suppressed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-54159 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-40382 Summary of the Invention [Problem to be solved by the invention]

[0005] In non-aqueous electrolyte secondary batteries such as lithium-ion batteries, particle cracking of the positive electrode active material can occur during charge and discharge, resulting in increased resistance and a decrease in battery capacity. In particular, when large particles composed of many primary particles crack, many primary particles are isolated from the conductive path, resulting in a large rate of increase in resistance. This cycle degradation becomes more pronounced as the battery temperature during charge and discharge increases.

[0006] Furthermore, by having a boron compound on the particle surface of the positive electrode active material, side reactions between the active material and the electrolyte are suppressed, which is expected to improve the resistance retention rate during high-temperature cycling. However, it is anticipated that the boron compound will form a resistive layer, which will deteriorate the rate characteristics.

[0007] An object of the present disclosure is to improve the high-temperature cycle characteristics of a high-capacity nonaqueous electrolyte secondary battery containing a lithium transition metal composite oxide with a high Ni content as a positive electrode active material, without impairing other battery characteristics. [Means for solving the problem]

[0008] A non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a positive electrode having a positive electrode active material containing Li and Ni, a negative electrode, and a non-aqueous electrolyte. The positive electrode contains, as the positive electrode active material, a particle group (A) of lithium transition metal composite oxide having a volume-based median diameter (D50) of 0.6 μm to 3 μm, which is composed of secondary particles formed by agglomeration of primary particles having an average particle diameter of 0.5 μm or more, or single particles, and a particle group (B) of lithium transition metal composite oxide having a volume-based median diameter (D50) of 6 μm to 25 μm, which is composed of secondary particles formed by agglomeration of primary particles having an average particle diameter of 0.3 μm or less. The particle surface of the particle group (A) is smoother than the particle surface of the particle group (B), the sphericity of the particle group (A) is 0.7 or more and 0.9 or less, the particle group (B) includes first particles which are particles having a particle size larger than a 70% particle size on a volume basis (D70) and second particles which are particles having a particle size smaller than a 30% particle size on a volume basis (D30), and the molar fraction (B1) of boron relative to the total number of moles of metal elements excluding Li in the first particles is larger than the molar fraction (B2) of boron relative to the total number of moles of metal elements excluding Li in the second particles. [Effects of the Invention]

[0009] The nonaqueous electrolyte secondary battery according to the present disclosure has excellent high-temperature cycle characteristics, a high energy density, and good other battery characteristics such as rate characteristics. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; [Figure 2] FIG. 2 is a cross-sectional view of a positive electrode according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] The inventors conducted extensive research to solve the above-mentioned problems and succeeded in achieving both high energy density and excellent high-temperature cycle characteristics by using lithium transition metal composite oxides (A) and (B) in combination and by increasing the molar fraction of boron contained in the first particles with larger particle size in composite oxide (B) compared to the molar fraction of boron contained in the second particles with smaller particle size. Because the particle surface of composite oxide (A) is smooth, it is believed that this allows for efficient packing of each composite oxide in the composite layer while mitigating stresses caused by pressure during rolling of the positive electrode composite layer and volumetric changes in the composite layer during charge and discharge. This suppresses cracking of the composite oxide particles and increases the density of the composite layer.

[0012] Furthermore, by incorporating a large amount of boron, which suppresses side reactions with the electrolyte, into the first particles, which are large particles that are significantly affected by particle cracking, the cracking of the first particles can be further suppressed. On the other hand, by incorporating less boron into the second particles, which are small particles, than into the first particles, good rate characteristics can be ensured. It is believed that the nonaqueous electrolyte secondary battery according to the present disclosure achieves high energy density and excellent high-temperature cycle characteristics through the synergistic effect of the above-mentioned mitigation effect of the composite oxide (A) and the adjustment of the amount of boron added depending on the particle size of the composite oxide (B).

[0013] An example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail below. Hereinafter, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom will be exemplified. However, the outer can is not limited to a cylindrical outer can and may be, for example, a rectangular outer can or an outer can made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly may be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0014] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As illustrated in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the electrolyte. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container that is open on one axial side and has a bottom, and the opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience of explanation, the sealing member 17 side of the battery is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0015] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. The electrolyte salt may be, for example, a lithium salt such as LiPF6. The electrolyte is not limited to a liquid electrolyte, and may be a solid electrolyte using a gel polymer or the like.

[0016] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all strip-shaped, long bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal direction and width direction (short direction). The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0017] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0018] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure airtightness inside the battery. The outer can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0019] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0020] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14, and in particular the positive electrode active material that constitutes the positive electrode 11, will be described in detail below.

[0021] [Positive electrode] FIG. 2 is a diagram illustrating a portion of a cross section of the positive electrode 11. As illustrated in FIG. 2, the positive electrode 11 includes a positive electrode core 30 and a positive electrode composite layer 31 provided on the surface of the positive electrode core 30. The positive electrode core 30 can be made of a foil of a metal, such as aluminum, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on its surface. The positive electrode composite layer 31 contains a positive electrode active material, a binder, and a conductive material, and is preferably provided on both sides of the positive electrode core 30 except for the portion to which the positive electrode lead 20 is connected. The positive electrode 11 can be produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a binder, a conductive material, etc. to the surface of the positive electrode core 30, drying the coating, and then compressing it to form the positive electrode composite layer 31 on both sides of the positive electrode core 30.

[0022] Examples of the conductive material contained in the positive electrode mixture layer 31 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode mixture layer 31 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), and the like.

[0023] Positive electrode 11 contains, as positive electrode active materials, a lithium transition metal composite oxide (A) having a volume-based median diameter (D50) of 0.6 μm to 3 μm and a lithium transition metal composite oxide (B) having a D50 of 6 μm to 25 μm (hereinafter simply referred to as "composite oxides (A) and (B)"). In this embodiment, only composite oxides (A) and (B) are contained as the positive electrode active material. However, positive electrode mixture layer 31 may contain composite oxides other than composite oxides (A) and (B) or other compounds, as long as the object of the present disclosure is not impaired.

[0024] The volume-based median diameter (D50) is the particle size at which the cumulative frequency of the smallest particle size in the volume-based particle size distribution is 50%, and is also called the median diameter. The particle size and particle size distribution of complex oxides can be measured using a laser diffraction particle size distribution analyzer (e.g., Microtrack Bell's MT3000II) with water as the dispersion medium.

[0025] The composite oxide (A) is either a secondary particle formed by agglomeration of large primary particles having an average particle size of 0.5 μm or more, or a composite oxide particle substantially composed of a single particle. The composite oxide (A) substantially composed of a single particle means a particle in which the grain boundary of the primary particles cannot be confirmed when observed at an appropriate magnification using a scanning electron microscope (SEM). When the composite oxide (A) is a secondary particle, the average particle size of the primary particles is 0.5 μm to 3 μm. On the other hand, the composite oxide (B) is, for example, a secondary particle formed by agglomeration of small primary particles having an average particle size of 0.3 μm or less.

[0026] When the composite oxide (A) is a secondary particle, the grain boundaries of the primary particles are observed in the particle cross section observed by SEM. For example, the composite oxide (A) is composed of 2 to 5 primary particles, such as 100 or less, several to several tens of primary particles, and the composite oxide (B) is composed of 10,000 to 5,000,000 primary particles. The particle size of the primary particles is measured as the Feret diameter of the region surrounded by the grain boundaries (primary particles) in the SEM image of the cross section of the composite oxide particle. The average particle size of the primary particles is calculated by averaging the particle sizes of 100 primary particles.

[0027] The composite oxide (A) contains 65 mol % or more of Ni relative to the total number of moles of metal elements excluding Li. The composite oxide (B) contains 70 mol % or more of Ni relative to the total number of moles of metal elements excluding Li. By using a positive electrode active material with a high Ni content, a battery with a high energy density can be obtained. The Ni content of the composite oxide (A) may be lower than the Ni content of the composite oxide (B). Furthermore, boron is present at least on the particle surface of the composite oxide (B).

[0028] The composite oxides (A) and (B) may contain metal elements other than Li, Ni, and B. Examples of such metal elements include Co, Mn, Al, Ti, Zr, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Nb, and Si. The compositions of the composite oxides (A) and (B) may be different from each other. The composite oxide (A) may or may not contain boron. On the other hand, the composite oxide (B) contains boron, and boron is present at least on the particle surface of the composite oxide (B).

[0029] An example of a suitable composite oxide (A) is a compound represented by the general formula Li a Ni b Co c Mn d B e O f (wherein 0.8≦a≦1.2, b≧0.70, c≦0.10, 0.03≦d≦0.12, 0≦e≦0.05, 1≦f≦2, b+c+d+e=1) The molar fraction of metal elements in the entire complex oxide particle is measured by inductively coupled plasma (ICP) atomic emission spectroscopy.

[0030] The particle surface of the composite oxide (A) is smoother than that of the composite oxide (B). The sphericity of the composite oxide (A) is not particularly limited, but is preferably 0.7 or more and 0.9 or less. The sphericity can be calculated by the following formula based on particle images taken by irradiating a sample flow containing composite oxide particles with a strobe light.

[0031] Sphericity = (perimeter of a circle with the same area as the particle image) / (perimeter of the particle image) The compressive strength of the composite oxide (A) may be, for example, 250 MPa or more, or 350 MPa or more. In this case, particle cracking due to charge and discharge is suppressed compared to when the above range is not satisfied, contributing to improvement of high-temperature cycle characteristics. The upper limit of the compressive strength of the composite oxide (A) is not particularly limited, but is, for example, 1500 MPa or less. The compressive strength of the composite oxide (A) is measured by the method specified in JIS-R1639-5.

[0032] The composite oxide (A) can be prepared, for example, by the following procedure.

[0033] First, a lithium compound such as lithium hydroxide and an oxide containing nickel and one of the above-mentioned metal elements are mixed in a mixing ratio based on the desired composite oxide (A). A potassium compound is then added to the mixture. Next, the mixture containing the lithium compound, the oxide containing nickel and a metal element, and the potassium or sodium compound is fired in air or an oxygen stream. The fired product is then washed with water to remove any potassium or sodium compounds adhering to the surface of the fired product.

[0034] The composite oxide (A) is synthesized by the above method. Although the detailed theory behind the increase in the particle size of the primary particles is not clear, it is thought that the addition of a potassium compound to the mixture causes the growth of single crystal particles during firing to proceed uniformly throughout the entire mixture phase.

[0035] The firing temperature in the above step is, for example, 600°C to 1050°C, and the higher the temperature, the larger the primary particles tend to be. When the firing temperature is 600 to 1050°C, the firing time is approximately 1 to 100 hours. The composite oxide (A) can also be obtained by growing crystals together with oxides having low melting points, such as Na and K, on ​​the condition that the flux compound is removed by washing with water or the like. Examples of the potassium compound include potassium hydroxide (KOH) and its salts, and potassium acetate. The potassium compound is added, for example, in an amount of 0.1 to 100 mass% or less based on the composite oxide (A) to be synthesized.

[0036] As described above, the composite oxide (B) contains boron. Boron may be present on the surface of the secondary particles of the composite oxide (B) as well as on the surface of the primary particles. Furthermore, some of the boron may also be present inside the primary particles and form a solid solution with other metal elements contained in the composite oxide (B). However, from the viewpoint of suppressing side reactions between the active material and the electrolyte, it is preferable that boron be present in large amounts on the surface of the secondary particles.

[0037] An example of a suitable composite oxide (B) is a compound represented by the general formula Li a Ni b Co c Mn d B e O f (wherein 0.8≦a≦1.2, b≧0.70, c≦0.10, 0.03≦d≦0.12, 0≦e≦0.05, 1≦f≦2, b+c+d+e=1). In the composite oxide (B), the content (number of moles) of boron relative to the total number of moles of metal elements excluding Li is preferably 0.1 to 5 mol%, more preferably 0.2 to 2 mol%. If the amount of boron is within this range, particle cracking can be efficiently suppressed.

[0038] The composite oxide (B) is characterized in that, when particles having a particle size larger than the 70% particle size (D70) on a volume basis are defined as first particles and particles having a particle size smaller than the 30% particle size (D30) on a volume basis are defined as second particles, the molar fraction (B1) of boron in the first particles relative to the total number of moles of metal elements excluding Li is larger than the molar fraction (B2) of boron in the second particles relative to the total number of moles of metal elements excluding Li. The amount of boron contained in the first and second particles is measured by ICP, as with other metal elements.

[0039] That is, the composite oxide (B) is a particle in which the ratio (B1 / B2) of the molar fraction (B1) of boron contained in the first particle to the molar fraction (B2) of boron contained in the second particle is at least 1. By using such a composite oxide (B), side reactions with the electrolyte can be efficiently suppressed, and a synergistic effect with the composite oxide (A) can provide a battery having high energy density and excellent high-temperature cycle characteristics.

[0040] Here, D70 refers to the particle size at which the cumulative frequency is 70% from the smallest particle size in the volumetric particle size distribution. Similarly, D30 refers to the particle size at which the cumulative frequency is 30% from the smallest particle size in the volumetric particle size distribution. For example, D70 is 9 μm to 19 μm, and D30 is 3 μm to 13 μm.

[0041] The ratio (B1 / B2) of the molar fraction (B1) of boron contained in the first particles to the molar fraction (B2) of boron contained in the second particles is preferably 1.1 or more, more preferably 1.5 or more, and may be 3.0 or more. The upper limit of (B1 / B2) is not particularly limited, but is, for example, 10. An example of a suitable range for (B1 / B2) is 1.5 to 3.5, or 2.5 to 3.5.

[0042] As long as the boron molar fractions (B1) and (B2) measured by ICP satisfy the condition (B1) > (B2), the first particles may contain particles whose boron molar fraction (B1) is the same as or lower than the boron molar fraction (B2) of the second particles. The second particles may contain particles whose boron molar fraction (B2) is higher than the boron molar fraction (B1) on the particle surface of the first particles. Preferably, (B2) > 0, and boron is present on the surfaces of both the first and second particles.

[0043] In the composite oxide (B), boron generally exists in the form of a boron compound. The boron compound may contain Li. As will be described later, boron compounds such as boric acid (H3BO3), boron oxide (BO3), and lithium borate (LiBO2, Li2BO7) are used as boron sources. When boric acid or boron oxide is used as the boron source, it may react with Li present on the particle surface during calcination to produce a boron compound containing Li and B.

[0044] The boron compound may be formed in a layer covering the particle surface (secondary particle surface) of the composite oxide (B), or may be scattered on the particle surface, but it is preferable that it does not completely cover the entire particle surface. That is, there are areas on the particle surface of the composite oxide (B) where the boron compound is not attached. When the boron compound is present in particulate form, the particle size of the boron compound is generally smaller than the particle size of the primary particles constituting the composite oxide (B). Note that the boron compound particles can be confirmed by SEM. It is preferable that the boron compound is attached over a wide area of ​​the particle surface of the composite oxide (B), without being concentrated in a certain area.

[0045] The thickness of the boron compound on the particle surface of the composite oxide (B) is preferably 100 nm or less, more preferably 50 nm or less. Furthermore, the thickness of the boron compound is more preferably 10 nm or more. A suitable example of the thickness of the boron compound is 10 nm to 50 nm. If the thickness of the boron compound is within this range, the high-temperature cycle characteristics can be efficiently improved without impairing the rate characteristics, etc.

[0046] Furthermore, the coverage of boron on the surfaces of the first and second particles (hereinafter sometimes referred to as "surface coverage") is preferably 99% or less, more preferably 90% or less, and particularly preferably 70% or less. In this case, the high-temperature cycle characteristics can be efficiently improved without impairing the rate characteristics, etc., of the battery. A surface coverage of 100% means that the entire particle surface is covered with the boron compound. In other words, if the surface coverage is less than 100%, there will be areas on the particle surface where the boron compound is not attached. The lower limit of the surface coverage of boron is, for example, 50%.

[0047] The boron surface coverage is measured by electron probe microanalyzer (EPMA) or X-ray photoelectron spectroscopy (XPS). With EPMA, the boron surface coverage is determined by the ratio of the boron peak area to the total peak area for metal elements excluding Li. With XPS, it is calculated from the molar fraction of boron relative to the total number of moles of metal elements excluding Li (if the boron molar fraction is 1, the surface coverage is 100%). XPS identifies the elements on the particle surface. XPS measurements are performed with an X-ray irradiation spot diameter of 1 mm or more, with the first and second particles selectively placed within the irradiation spot. In this case, several hundred particles are contained within the irradiation spot, so the boron molar fraction is measured as the average value of multiple particles.

[0048] The composite oxide (B) can be prepared, for example, by the following procedure.

[0049] First, two types of nickel compounds (X1) and (X2) containing at least Ni but not Li or B and having different D50s are added with a lithium source such as lithium hydroxide, respectively, and calcined to synthesize lithium nickel composite oxides (Y1) and (Y2) having different D50s (Step 1). An example of the composite compound is a composite oxide or hydroxide containing Ni, Co, and Mn. At this time, one type of lithium nickel composite oxide may be classified to obtain two types of lithium nickel composite oxides having different D50s. A conventionally known method can be used for classification. The obtained lithium nickel composite oxides (Y1) and (Y2) may also be washed with water. Washing with water reduces not only the amount of Li present on the particle surface of the composite oxide but also the amount of Li present inside the particles, resulting in voids inside the washed composite oxide particles.

[0050] Next, a boron source is added to each of the lithium nickel composite oxides (Y1) and (Y2), and the boron is composited onto the particle surface, followed by calcination to synthesize the lithium transition metal composite oxides (Z1) and (Z2) (step 2). The composite oxides (Z1) and (Z2) are then mixed to obtain the composite oxide (B). An example of a boron source is boric acid (H3BO3). A dry particle composite apparatus (e.g., NOB-130, manufactured by Hosokawa Micron Corporation) is used for the composite. At this time, a lithium source, such as lithium hydroxide, may be added together with the boron source.

[0051] In the step 2, by adding more H3BO3 to the lithium nickel composite oxide (Y1) than to the lithium nickel composite oxide (Y2), a state in which the molar fraction (B1) of boron on the surfaces of the first and second particles of the composite oxide (B) is greater than the molar fraction (B2) of boron is obtained. The firing temperature in the step 2 is, for example, 200°C to 500°C.

[0052] The amount and thickness of the boron compound coating on the surfaces of the first and second particles of the composite oxide (B) can be adjusted by adjusting whether or not the composite oxides (Y1) and (Y2) are washed with water and by adjusting the calcination temperature. By calcining the washed composite oxides (Y1) and (Y2) at high temperature together with a boron source, composite oxides with a low surface coverage by the boron compound can be synthesized. Even if the composite oxides (Y1) and (Y2) are washed with water, if they are calcined at a low temperature, boron does not enter the voids inside the particles, and the amount and thickness of the surface coverage by the boron compound are approximately the same as when they are not washed with water. Here, a high temperature is, for example, 350°C to 500°C, and a low temperature is, for example, 200°C to 325°C.

[0053] As described above, the positive electrode active material is a mixture of composite oxide (A) and composite oxide (B). The content of composite oxide (A) relative to the mass of the positive electrode active material is, for example, 5 to 65 mass%, preferably 10 to 60 mass%, and particularly preferably 20 to 55 mass%. The content of composite oxide (B) relative to the mass of the positive electrode active material is, for example, 35 to 95 mass%, preferably 40 to 90 mass%, and particularly preferably 45 to 80 mass%.

[0054] The density of positive electrode mixture layer 31 is preferably 3.55 g / cc or more, and more preferably 3.60 g / cc or more. When the mixing ratio of composite oxides (A) and (B) is within the above range, the packing property of the particles is improved, and the packing density of positive electrode mixture layer 31 can be increased, resulting in a battery with high energy density and excellent high-temperature cycle characteristics.

[0055] [Negative electrode] The negative electrode 12 has a negative electrode core and a negative electrode composite layer provided on the surface of the negative electrode core. The negative electrode core can be a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface. The negative electrode composite layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core except for the portion to which the negative electrode lead 21 is connected. The negative electrode 12 can be produced, for example, by applying a negative electrode composite slurry containing a negative electrode active material and a binder to the surface of the negative electrode core, drying the coating, and then compressing it to form a negative electrode composite layer on both sides of the negative electrode core.

[0056] The negative electrode mixture layer contains, as the negative electrode active material, for example, a carbon-based active material that reversibly absorbs and releases lithium ions. Suitable carbon-based active materials include natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). The negative electrode active material may be a Si-based active material composed of at least one of Si and a Si-containing compound, or a combination of a carbon-based active material and a Si-based active material.

[0057] The binder contained in the negative electrode mixture layer may be, as in the case of the positive electrode 11, a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but is preferably styrene-butadiene rubber (SBR). The negative electrode mixture layer preferably further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. Among these, it is preferable to use SBR in combination with CMC or a salt thereof, or PAA or a salt thereof.

[0058] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator. [Example]

[0059] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0060] Example 1 [Preparation of composite oxide A] Obtained by coprecipitation, D50 is 15 μm, composition is Ni 0.80 Co 0.10 Mn 0.10 The nickel-cobalt-manganese composite hydroxide (OH)2 was calcined at 500°C to obtain a nickel-cobalt-manganese composite oxide. Next, lithium hydroxide and the prepared nickel-cobalt-manganese composite oxide were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Mn was 1.05:1, and a potassium compound was added to the mixture in an amount of 20 mass% relative to the composite oxide. This mixture was calcined at 750°C for 72 hours in an oxygen atmosphere, then pulverized and washed with water to remove the potassium compound, yielding composite oxide A.

[0061] The composition of composite oxide A was analyzed by ICP, and Li 1.01 Ni 0.80 Co 0. 10 Mn 0.10 O2. The D50 value of composite oxide A was 2.3 μm. SEM observation of the cross section of composite oxide A after CP processing revealed that the average particle size of the primary particles of composite oxide A was 1.4 μm. Approximately 95% or more of all particles in composite oxide A had a single particle structure, with some particles having a pseudo-aggregate structure in which 3 to 10 or so primary particles were bonded together.

[0062] [Preparation of composite oxide B] Co-precipitation of Ni 0.85 Co 0.08 Mn 0.07The nickel-cobalt-manganese composite hydroxide (OH)2 was calcined at 500°C and then sieved using a vibrating sieve (sieve opening: 300 μm, shaking time: 30 minutes) to obtain a nickel-cobalt-manganese composite oxide (X1) with a large particle size and a D50 of 14 μm that remained on the sieve, and a nickel-cobalt-manganese composite oxide (X2) with a small particle size and a D50 of 7 μm that passed through the sieve.

[0063] Next, lithium hydroxide and the large particle size nickel-cobalt-manganese composite oxide (X1) were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Mn was 1.08: 1. This mixture was fired at 700°C in an oxygen atmosphere for 8 hours and then pulverized to obtain a large particle size lithium composite oxide (Y1).

[0064] Lithium hydroxide and a small particle size nickel-cobalt-manganese composite oxide (X2) were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Mn was 1.08: 1. This mixture was fired at 700°C in an oxygen atmosphere for 8 hours and then pulverized to obtain a small particle size lithium composite oxide (Y2).

[0065] Next, the large particle size lithium composite oxide (Y1) was dry-mixed with boric acid (H3BO3) so that the molar ratio of the total amount of Ni, Co, and Mn to B in H3BO3 was 100:1.5, and boron was composited onto the particle surface. This mixture was calcined at 300°C for 8 hours in an oxygen atmosphere and then pulverized to obtain a large particle size lithium composite oxide (Z1) with boron present at least on the particle surface.

[0066] In addition, small-particle lithium composite oxide (Y2) was dry-mixed with H3BO3 so that the molar ratio of the total amount of Ni, Co, and Mn to B in H3BO3 was 100:0.5, and boron was composited onto the particle surface. This mixture was calcined at 300°C for 8 hours in an oxygen atmosphere and then pulverized to obtain small-particle lithium composite oxide (Z2) with boron present at least on the particle surface.

[0067] The lithium composite oxides (Z1) and (Z2) were mixed in a mass ratio of 1:1 to form composite oxide (B), and composite oxide A was mixed in an amount of 50 mass % relative to the total mass of the positive electrode active material to form a positive electrode active material.

[0068] In the composite oxide (B), the molar fraction (B1) of boron in the first particles having a particle size larger than D70 on a volume basis was 0.015, and the molar fraction (B2) of boron on the surface of the second particles having a particle size smaller than D30 on a volume basis was 0.005, with the ratio (B1 / B2) being 3.0. The coverage of boron on the surfaces of the first and second particles was 96%. As described above, the amount (mole fraction) of boron contained in the first and second particles was measured by ICP. The amount (surface coverage) of boron present on the particle surfaces was measured by EPMA.

[0069] The composition of composite oxide B was analyzed by ICP, and Li 1.01 Ni 0.85 Co 0.08 Mn 0.07 B 0.01 The particle size distribution of composite oxide B was D50 11 μm, D70 14 μm, and D30 7.5 μm. The cross section of composite oxide B after CP processing was observed by SEM, and the average particle size of the primary particles of composite oxide B was found to be 0.15 μm.

[0070] [Preparation of positive electrode] The positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed in a solids mass ratio of 96.3:2.5:1.2, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added. The mixture was then kneaded to prepare a positive electrode composite slurry. The positive electrode composite slurry was applied to both sides of a positive electrode core made of aluminum foil, and the coating was dried. The coating was then rolled using a roller and cut to a predetermined electrode size to obtain a positive electrode with a positive electrode composite layer formed on both sides of the positive electrode core. An exposed portion was provided on a portion of the positive electrode, exposing the surface of the positive electrode core.

[0071] [Preparation of negative electrode] Natural graphite was used as the negative electrode active material. The negative electrode active material, carboxymethyl cellulose sodium (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in an aqueous solution at a solids mass ratio of 100:1:1 to prepare a negative electrode composite slurry. The negative electrode composite slurry was applied to both sides of a negative electrode core made of copper foil, and the coating was dried. The coating was then rolled using a roller and cut to a predetermined electrode size to obtain a negative electrode with a negative electrode composite layer formed on both sides of the negative electrode core. An exposed portion was provided in part of the negative electrode, exposing the surface of the negative electrode core.

[0072] [Preparation of non-aqueous electrolyte] Lithium hexafluorophosphate (LiPF6) was dissolved at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4. Furthermore, vinylene carbonate (VC) was dissolved in the mixed solvent at a concentration of 2.0 mass % to prepare a nonaqueous electrolyte solution.

[0073] [Battery construction] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the exposed portion of the negative electrode, and the positive and negative electrodes were spirally wound with a polyolefin separator interposed therebetween, followed by radial press forming to produce a flat wound electrode assembly. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and the nonaqueous electrolyte solution was injected. The opening of the exterior body was then sealed to obtain a nonaqueous electrolyte secondary battery with a design capacity of 650 mAh.

[0074] <Example 2> A positive electrode active material and a nonaqueous electrolyte secondary battery were produced in the same manner as in Example 1, except that in the preparation of composite oxide B, the firing temperature during the synthesis of lithium composite oxides (Z1) and (Z2) was changed to 400°C.

[0075] <Comparative Example 1> A positive electrode active material and a non-aqueous electrolyte secondary battery were produced in the same manner as in Example 1, except that composite oxide A was not used (composite oxide B was 100%).

[0076] <Comparative Example 2> A positive electrode active material and a non-aqueous electrolyte secondary battery were produced in the same manner as in Example 2, except that composite oxide A was not used (composite oxide B was 100%).

[0077] <Comparative Example 3> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that composite oxide A was not used (composite oxide B was 100%), no sieving was performed during the synthesis of composite oxide B, and the obtained lithium composite oxide and boric acid (HBO) were dry-mixed so that the molar ratio of the total amount of Ni, Co, and Mn to B in HBO was 100:1.

[0078] <Comparative Example 4> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that composite oxide A was not used (composite oxide B was 100%), no sieving was performed during the synthesis of composite oxide B, and the obtained lithium composite oxide and boric acid (HBO) were dry-mixed so that the molar ratio of the total amount of Ni, Co, and Mn to B in HBO was 100:1.

[0079] [High temperature cycle test] Each battery in the Examples and Comparative Examples was charged at a constant current of 0.5 It in a temperature environment of 60°C until the battery voltage reached 4.2 V, and then charged at a constant voltage until the current value reached 1 / 50 It at 4.2 V. Thereafter, the battery was discharged at a constant current of 0.5 It until the battery voltage reached 2.5 V. This charge / discharge cycle was repeated 150 times.

[0080] [Evaluation of capacity retention rate after cycle test] For each battery of the Examples and Comparative Examples, the discharge capacity at the first cycle and the discharge capacity at the 150th cycle of the cycle test were determined, and the capacity retention rate was calculated by the following formula.

[0081] Capacity retention rate (%) = (150th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100 [Evaluation of filling of positive electrode mixture layer] For each positive electrode prepared in the examples and comparative examples, a rectangular electrode plate (200 μm thick) with a total length of 1 m and a composite layer formed on both sides was prepared for packing evaluation. This electrode plate was compressed using a rolling mill (manufactured by NAKNOR Corporation) with a roll diameter of 750 mm while varying the compression conditions (gap value), and after compression, it was punched out to a diameter of 40 mm. For each electrode plate, the packing density under the compression conditions resulting in an electrode plate elongation of 1% is listed in Table 1. In Table 1, a positive electrode composite layer with a high packing density is indicated as ◯ (particularly high is indicated as ⊚), and a positive electrode composite layer with a low packing density is indicated as × (particularly low is indicated as XX).

[0082] [Table 1]

[0083] As shown in Table 1, all of the batteries of the examples have higher capacity retention rates after cycle testing and are superior in high-temperature cycle characteristics compared to the batteries of the comparative examples. [Explanation of symbols]

[0084] 10 Nonaqueous electrolyte secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 16 outer can 17 Sealing body 18,19 Insulating plate 20 Positive lead 21 Negative lead 22 Grooved part 23 Internal terminal board 24 Lower valve body 25 Insulating material 26 Superior valve 27 Cap 28 Gasket 30 Positive electrode core 31 Positive electrode mixture layer

Claims

1. The battery includes a positive electrode having a positive electrode active material containing Li and Ni, a negative electrode, and a non-aqueous electrolyte; The positive electrode includes, as the positive electrode active material, a particle group (A) of a lithium transition metal composite oxide having a volume-based median diameter (D50) of 0.6 μm to 3 μm, which is composed of secondary particles formed by agglomeration of primary particles having an average particle diameter of 0.5 μm or more, or single particles, and a particle group (B) of a lithium transition metal composite oxide having a volume-based median diameter (D50) of 6 μm to 25 μm, which is composed of secondary particles formed by agglomeration of primary particles having an average particle diameter of 0.3 μm or less, the particle surfaces of the particle group (A) are smoother than the particle surfaces of the particle group (B); The sphericity of the particle group (A) is 0.7 or more and 0.9 or less, the particle group (B) includes first particles having a particle size larger than a 70% particle size (D70) on a volume basis, and second particles having a particle size smaller than a 30% particle size (D30) on a volume basis, a molar fraction (B1) of boron with respect to the total number of moles of metal elements excluding Li in the first particles is greater than a molar fraction (B2) of boron with respect to the total number of moles of metal elements excluding Li in the second particles.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the particle group (A) has a compressive strength of 250 MPa or more and 1500 MPa or less.

3. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the particle group (A) contains 65 mol% or more of Ni relative to the total number of moles of metal elements excluding Li, and the Ni content of the particle group (A) is lower than the Ni content of the particle group (B).

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