Positive electrode active material for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery

Abstract

The positive electrode active material contains a lithium transition metal composite oxide containing Ni at 85 mol% or more relative to the total molar amount of metal elements other than Li. The lithium transition metal composite oxide contains at least one metal element M1 selected from the group consisting of Mg, Ti, Nb, Zr, and V at 3 mol% or less relative to the total molar amount of metal elements other than Li. In addition, the ratio of the concentration of the metal element M1 contained in the shell of the primary particles of the composite oxide to the concentration of the metal element M1 contained in the core is 1.01 or more and 20 or less, and at least one metal element M2 selected from the group consisting of Ca and Sr is present on the surface of the primary particles at 1 mol% or less relative to the total molar amount of metal elements other than Li.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a positive electrode active material for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery using the same. BACKGROUND

[0002] In a nonaqueous electrolyte secondary battery such as a lithium ion battery, a positive electrode active material has a large influence on battery performance such as power characteristics, capacity, cycle characteristics, and storage characteristics. Generally, a lithium transition metal composite oxide composed of secondary particles in which primary particles containing metal elements such as Ni, Co, Mn, and Al are aggregated is used as the positive electrode active material. The positive electrode active material greatly differs in properties depending on its composition, particle shape, and the like, and thus, a lot of research has been conducted on various positive electrode active materials.

[0003] For example, Patent Literature 1 discloses a positive electrode active material containing Li, Ni, Co, Mn, and W, the ratio of Ni being 30 mol% or more and 60 mol% or less, the ratio of Co being 15 mol% or more and 35 mol% or less, the ratio of Mn being 15 mol% or more and 35 mol% or less, the ratio of W being more than 0 mol% and 5 mol% or less, and W being segregated to the surface layer of the positive electrode active material. In addition, Patent Literature 1 describes that, by using this positive electrode active material, the power characteristics and cycle characteristics of the battery are improved.

[0004] In addition, Patent Literature 2 discloses a positive electrode active material having a layered crystal structure Li 1+a M 1-a O 2± b M’ k S m (-0.03 < a < 0.06, b ≈ 0), M is a transition metal compound formed of at least 95% of any one or more elements in a group of Ni, Mn, Co, and Ti, and M’ is a specific element present on the surface of the oxide. Patent Literature 2 describes that, by using this positive electrode active material, the performance as a cathode of a lithium battery can be improved.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent Application Publication No. 2016-91626

[0008] Patent Literature 2: Japanese Patent Application Publication No. 2010-535699 SUMMARY

[0009] The positive electrode active material having a high Ni content is advantageous for high capacity of the battery, but there is a problem that the capacity retention rate after storage of the battery is greatly reduced. Note that the battery using the positive electrode active material of Patent Literatures 1 and 2 has room for improvement in terms of storage characteristics.

[0010] The positive electrode active material for a nonaqueous electrolyte secondary battery of the present disclosure contains a lithium transition metal complex oxide containing Ni in an amount of 85 mol% or more relative to the total molar amount of metal elements other than Li, the lithium transition metal complex oxide is secondary particles in which primary particles are aggregated, contains at least one metal element M1 selected from Mg, Ti, Nb, Zr, and V in an amount of 3 mol% or less relative to the total molar amount of metal elements other than Li, and in a case where a range of 10 nm in thickness from the surface of the primary particle is defined as a shell and a range closer to the inside than the shell is defined as a core, the ratio of the concentration of the metal element M1 contained in the shell relative to the concentration of the metal element M1 contained in the core is 1.01 or more and 20 or less, and at least one metal element M2 selected from Ca and Sr is present on the surface of the primary particle in an amount of 1 mol% or less relative to the total molar amount of metal elements other than Li.

[0011] The nonaqueous electrolyte secondary battery of the present disclosure includes a positive electrode containing the above-described positive electrode active material, a negative electrode, and a nonaqueous electrolyte.

[0012] The nonaqueous electrolyte secondary battery using the positive electrode active material of the present disclosure has excellent storage characteristics. The positive electrode active material of the present disclosure has a high Ni content, is advantageous for high capacity of the battery, and can sufficiently suppress reduction in the capacity retention rate after storage of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 Cross-sectional view of the nonaqueous electrolyte secondary battery as an example of the embodiment. DETAILED DESCRIPTION

[0014] The lithium transition metal complex oxide having a high Ni content, as described above, is a useful positive electrode active material that is advantageous for high capacity / high energy density of the battery, but on the contrary, there is a problem that the capacity retention rate in a storage test is greatly reduced.

[0015] To solve this problem, the inventors conducted in-depth research and found that in lithium transition metal composite oxides with high Ni content, a specified amount of at least one metal element M1 selected from Mg, Ti, Nb, Zr and V is dissolved in solid solution, the ratio of the concentration of metal element M1 in the shell of the primary particle to the concentration of metal element M1 in the core is 1.01 or more and 20 or less, and a specified amount of at least one metal element M2 selected from Ca and Sr is fixed on the surface of the primary particle, thereby significantly improving the battery's storage characteristics and specifically suppressing the decrease in capacity retention rate during storage tests.

[0016] A coating containing metal element M2 is uniformly formed on the surface of the primary particles constituting the positive electrode active material of this disclosure. It is believed that metal element M2 reacts / bonds with metal element M1, which is present at a high concentration on the particle surface (shell), thereby uniformly forming a firmly bonded coating on the surface of the primary particles. Therefore, it is envisioned that oxygen desorption caused by side reactions between the active material and the electrolyte during storage tests is suppressed, maintaining a good crystal structure of the active material, thereby improving storage properties.

[0017] Hereinafter, with reference to the accompanying drawings, an example of an embodiment of the positive electrode active material for a non-aqueous electrolyte secondary battery of the present disclosure and a non-aqueous electrolyte secondary battery using the positive electrode active material will be described in detail. It should be noted that it was initially conceived to selectively combine the various embodiments and modifications described below.

[0018] The following example illustrates a cylindrical battery formed by housing a wound electrode body 14 within a bottomed cylindrical outer casing 16. However, the outer casing of the battery is not limited to a cylindrical outer casing; for example, it can be a square outer casing (square battery), a coin-shaped outer casing (coin-shaped battery), or an outer casing made of a laminate containing a metal layer and a resin layer (laminated battery). Furthermore, the electrode body can be a laminated electrode body formed by alternately stacking multiple positive electrodes and multiple negative electrodes separated by separators.

[0019] Figure 1 This is a cross-sectional view of a non-aqueous electrolyte secondary battery 10, as an example of an embodiment. Figure 1 As shown, the non-aqueous electrolyte secondary battery 10 includes: a wound electrode body 14, a non-aqueous electrolyte, and an outer casing 16 for housing the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 has 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 wound in a spiral shape with the separator 13 in between. The outer casing 16 is a bottomed cylindrical metal container with an opening on one axial side, and the opening of the outer casing 16 is blocked by a sealing body 17. Hereinafter, for ease of explanation, the side of the battery with the sealing body 17 is referred to as the top, and the bottom side of the outer casing 16 is referred to as the bottom.

[0020] The nonaqueous electrolyte contains a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. The nonaqueous solvent includes, for example, esters, ethers, nitriles, amides, and mixed solvents of two or more of these. The nonaqueous solvent can contain a halogen-substituted product in which at least a part of the hydrogen of the solvent is replaced with a halogen atom such as fluorine. The electrolyte salt includes, for example, lithium salts such as LiPF6. Note that the nonaqueous electrolyte is not limited to a liquid electrolyte, but can be a solid electrolyte.

[0021] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the electrode body 14 are each a long-size body in a strip shape, and are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed to have a size larger than the positive electrode 11 by one turn to prevent precipitation of lithium. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the length direction and the width direction (width direction). Two separators 13 are formed to have a size larger than the positive electrode 11 by at least one turn, and are arranged, for example, so as to sandwich the positive electrode 11. The electrode body 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.

[0022] The insulating plates 18 and 19 are arranged above and below the electrode body 14, respectively. Figure 1 In the illustrated example, the positive electrode lead 20 extends to the side of the sealing body 17 through a through-hole of the insulating plate 18, and the negative electrode lead 21 extends to the bottom side of the outer can 16 outside the insulating plate 19. The positive electrode lead 20 is connected to the lower surface of an internal terminal plate 23 of the sealing body 17 by welding or the like, and the top plate of the sealing body 17, that is, the lid 27, becomes a positive electrode terminal. The negative electrode lead 21 is connected to the inner surface of the bottom of the outer can 16 by welding or the like, and the outer can 16 becomes a negative electrode terminal.

[0023] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure the airtightness of the inside of the battery. A groove portion 22 for supporting the sealing body 17 is formed in the outer can 16 so that a part of the side surface portion is bulged inward. The groove portion 22 is preferably formed in a ring shape along the circumferential direction of the outer can 16, and supports the sealing body 17 from the upper surface thereof. The sealing body 17 is fixed to the upper portion of the outer can 16 by the groove portion 22 and the open end portion of the outer can 16 that is tightly connected to the sealing body 17.

[0024] The seal 17 has a structure in which an inner terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a lid 27 are stacked in this order from the electrode body 14 side. Each member constituting the seal 17 has, for example, a circular plate shape or a ring shape, and each member except the insulating member 25 is electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at central portions thereof, and the insulating member 25 is interposed between the peripheral edge portions thereof. When the internal pressure of the battery increases due to abnormal heat release, the lower valve body 24 is deformed to break in a manner that the upper valve body 26 is pushed toward the lid 27 side, and thus the current passage between the lower valve body 24 and the upper valve body 26 is blocked. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening portion of the lid 27.

[0025] Hereinafter, the positive electrode 11, the negative electrode 12, the separator 13, and particularly the positive electrode active material constituting the positive electrode 11 will be described in detail.

[0026] [Positive Electrode]

[0027] The positive electrode 11 has a positive electrode core and a positive electrode composite material layer provided on the surface of the positive electrode core. As the positive electrode core, a foil of a metal such as aluminum or an aluminum alloy that is stable in the potential range of the positive electrode 11, a thin film in which the metal is provided on the surface layer, or the like can be used. The positive electrode composite material layer contains a positive electrode active material, a conductive material, and a binding material, and is preferably provided on both surfaces of the positive electrode core. The positive electrode 11 can be produced, for example, by applying a positive electrode composite material slurry containing a positive electrode active material, a conductive material, a binding material, and the like on the positive electrode core, drying the coating film, and compressing the positive electrode core, and thus the positive electrode composite material layer is formed on both surfaces of the positive electrode core.

[0028] As the conductive material contained in the positive electrode composite material layer, a carbon material such as carbon black, acetylene black, ketjen black, or graphite can be exemplified. As the binding material contained in the positive electrode composite material layer, a fluorine resin such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVdF), a polyacrylonitrile (PAN), a polyimide resin, an acrylic resin, a polyolefin resin, or the like can be exemplified. These resins can be used in combination with a cellulose derivative such as carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like.

[0029] The positive electrode active material contains a lithium transition metal composite oxide containing 85 mol% or more of Ni relative to the total molar amount of metal elements other than Li. The lithium transition metal composite oxide having a high content of Ni, as described above, can be used as a useful positive electrode active material that is advantageous for high capacity and high energy density of the battery, but there is a problem of greatly reducing the capacity retention rate of the battery after storage. In the lithium transition metal composite oxide that constitutes the positive electrode active material of the present embodiment, the metal element M1 such as Nb is biased toward the surface of the particles and its vicinity, and the metal element M2 such as Ca is dispersed on the surface of the particles, thereby greatly improving the storage characteristics of the battery.

[0030] The positive electrode active material contains the above-described lithium transition metal composite oxide as a main component. Here, the main component refers to a component having the largest mass ratio among the materials that constitute the positive electrode active material. In the positive electrode composite material layer, as the positive electrode active material, a composite oxide other than the above-described lithium transition metal composite oxide can be contained within a range that does not impair the object of the present disclosure, but the ratio of the above-described lithium transition metal composite oxide is preferably 50% by mass or more, more preferably 80% by mass or more, and can be substantially 100% by mass. Note that the positive electrode active material can be composed of two or more kinds of composite oxides.

[0031] The lithium transition metal composite oxide preferably contains other metal elements in addition to Li, Ni, and the metal elements M1 and M2. As the other metal elements, Co, Al, Mn, B, Cr, Fe, Cu, Zn, Ga, In, Sn, Ta, W, and the like can be given. The composite oxide preferably contains at least one kind selected from Co, Al, and Mn among these. The content of the metal elements other than Li, Ni, and the metal elements M1 and M2 contained in the lithium transition metal composite oxide is preferably 15 mol% or less, more preferably 10 mol% or less, relative to the total molar amount of metal elements other than Li.

[0032] The lithium transition metal composite oxide preferably contains at least one kind selected from Al and Mn. In this case, the content of, for example, Ni is 90 mol% or more, the content of Al is 7 mol% or less, and the content of Mn is 5 mol% or less, relative to the total molar amount of metal elements other than Li. In addition, an example of a suitable lithium transition metal composite oxide is a composite oxide that does not substantially contain Co. Co is scarce and expensive, and therefore, by not using Co, it is possible to reduce the manufacturing cost of the battery. Co is assumed to be mixed as an impurity, but in this case, the content of Co is less than 0.5 mol%.

[0033] The lithium transition metal composite oxide is a secondary particle in which primary particles are aggregated. The average particle diameter of the primary particle is, for example, 200 nm or more and 500 nm or less. The average particle diameter of the primary particle is obtained by analyzing an SEM image of a cross section of a particle observed by a scanning electron microscope (SEM). For example, the positive electrode 11 is embedded in a resin, and a cross section is processed using a cross section polisher (CP), and the cross section is photographed using an SEM. From the SEM image, 30 primary particles are randomly selected, the grain boundaries are observed, the average of the average diameters (the longest diameters) of the 30 primary particles is obtained, and the average is taken as the average particle diameter.

[0034] The median particle diameter (hereinafter, referred to as "D50") of the secondary particle (lithium transition metal composite oxide) on a volume basis is, for example, 1 μm or more and 30 μm or less, and preferably 5 μm or more and 20 μm or less. D50 means the particle diameter at which the cumulative frequency becomes 50% from the smaller particle diameter in the particle size distribution on a volume basis, and is also referred to as the median diameter. The particle size distribution of the secondary particle can be measured using a particle size distribution measuring device of a laser diffraction type (for example, MT3000II manufactured by Microtrac BEL Corp.) using water as a dispersion medium.

[0035] The lithium transition metal composite oxide contains at least one metal element M1 selected from Mg, Ti, Nb, Zr, and V in an amount of 3 mol% or less with respect to the total mole amount of the metal elements other than Li. If the content of the metal element M1 exceeds 3 mol%, there is a tendency that the energy density of the battery decreases. Even if the metal element M1 is in a very small amount, it is advantageous to improve the storage characteristics compared to the case where the metal element M1 is not contained, but the content of the metal element M1 is, for example, 0.01 mol% or more.

[0036] The content of the metal element M1 is preferably 0.05 mol% or more, and more preferably 0.1 mol% or more, with respect to the total mole amount of the metal elements other than Li. The upper limit of the content of the metal element M1 is preferably 2.5 mol%, and more preferably 2 mol%. An example of a suitable content of the metal element M1 is 0.05 mol% or more and 2 mol% or less, 0.1 mol% or more and 2 mol% or less, 0.1 mol% or more and 1.5 mol% or less, or 0.1 mol% or more and 1 mol% or less.

[0037] In the lithium transition metal composite oxide, one element can be contained as the metal element M1, or two or more elements can be contained as the metal element M1. In addition, the metal element M1 is solid-solved with other metal elements such as Li, Ni, and Al. For example, at least one selected from Ti, Nb, and Zr is solid-solved in the lithium transition metal composite oxide. Among them, Nb, Zr, and more preferably Nb are preferable.

[0038] For the lithium transition metal complex oxide, in a case where a thickness range of 10 nm from the surface of the primary particles is defined as a shell, and a region closer to the inside thereof is defined as a core, the ratio (R-M1) of the concentration of the metal element M1 contained in the shell to the concentration of the metal element M1 contained in the core is 1.01 or more and 10 or less. That is, the metal element M1 is solid-solved at a high concentration closer to the surface or in the vicinity thereof from the inside of the primary particles of the complex oxide. Note that the concentration ratio (R-M1) of the metal element M1 can be obtained by measuring the element distribution of the cross section of the primary particles using a transmission electron microscope-energy dispersive X-ray spectroscopy (TEM-EDX).

[0039] The concentration ratio (R-M1) of the metal element M1 is preferably 1.5 or more, more preferably 2 or more, if it is a value greater than 1, high energy density can be ensured and the storage characteristics are improved compared to a case where it is 1 or less. From the viewpoint of charge transfer resistance, the upper limit value of the concentration ratio (R-M1) is preferably 18 or less, more preferably 15 or less. An example of a suitable concentration ratio (R-M1) is 1.5 or more and 18 or less, 1.5 or more and 15 or less, or 2 or more and 15 or less.

[0040] At least one metal element M2 selected from Ca and Sr is present on the surface of the primary particles of the lithium transition metal complex oxide in an amount of 1 mol% or less with respect to the total molar amount of the metal elements other than Li. If the content of the metal element M2 exceeds 1 mol%, there is a tendency for the energy density of the battery to decrease. Even if the metal element M2 is in an extremely small amount, it is advantageous for improving the storage characteristics compared to a case where the metal element M2 is not contained, but the content of the metal element M2 is, for example, 0.01 mol% or more.

[0041] The metal element M2 is present, for example, uniformly dispersed on the surface of each of the primary particles including the surface and the inside of the secondary particles (interfaces of the primary particles with each other). That is, it can be said that the surface of the primary particles is covered with a coating layer of the metal element M2. Note that the coating layer can contain elements other than the metal element M2 without impairing the purpose of the present disclosure. In a case where the concentration ratio (R-M1) of the metal element M1 is 1.01 or more and 20 or less, the metal element M1 and M2 easily react / bond, and the metal element M2 (coating layer) is firmly bonded to the surface of the primary particles. It is considered thereby that the oxygen desorption due to the side reaction is suppressed, and the storage characteristics of the battery are improved.

[0042] The metal element M2 is not solid-solved like the metal element Ml, but is fixed to the surface of the primary particles. In other words, the metal element M2 is substantially not present in the inside of the primary particles. The content of the metal element M2 is preferably 0.03 mol% or more, more preferably 0.05 mol% or more, with respect to the total molar amount of the metal elements other than Li. The upper limit of the content of the metal element M2 is preferably 0.8 mol%. An example of the content of the metal element M2 is 0.03 mol% or more and 0.8 mol% or less, 0.05 mol% or more and 1 mol or less, or 0.05 mol% or more and 0.8 mol% or less.

[0043] An example of the suitable lithium transition metal composite oxide is a composite oxide represented by the composition formula Li α Ni β Co x Al y Mn z Ml a M2 b O2 (in the formula, 0.9 < a < 1.2, 0.85 < β < 0.95, 0 < x < 0.05, 0 < y < 0.07, 0 < z < 0.05, 0.0005 < a < 0.01, 0.0005 < b < 0.01). As indicated by the composition formula, it is preferable to set the upper limit of the content of Ni to 0.95 mol%, and to add at least a prescribed amount of Al. In this case, the crystal structure is stabilized, and the preservation characteristics are improved. The contents of the metal elements Ml and M2 can be substantially the same, or Ml > M2, or Ml < M2, but it is preferable that the contents be less than those of Ni, Co, Al, and Mn.

[0044] The lithium transition metal composite oxide of the present embodiment can be synthesized, for example, by mixing a composite oxide containing Ni, Co, Al, and Mn, a compound containing the metal element Ml such as niobium hydroxide, a compound containing the metal element M2 such as calcium hydroxide, and a lithium compound, and performing firing at a high temperature at which the maximum temperature is 700°C to 850°C, whereby it can be synthesized. In addition, the holding time at the maximum temperature is 1 hour or more and 10 hours or less. The metal element Ml such as Nb is an element that is easily solid-solved with Ni and the like, and thus easily enters the inside of the primary particles, whereas the metal element M2 such as Ca is an element that is not easily solid-solved with Ni and the like, and thus is fixed to the surface of the primary particles. The concentration ratio (R-Ml) of the metal element Ml has a tendency to decrease if the firing temperature is increased, and to increase if the firing temperature is decreased, and thus the concentration ratio (R-Ml) can be adjusted to the target range by controlling the firing temperature.

[0045] [Negative electrode]

[0046] The negative electrode 12 has a negative electrode core and a negative electrode composite material layer provided on the surface of the negative electrode core. A foil of a metal stable in the potential range of the negative electrode 12, such as copper, or a thin film provided with the metal on the surface layer, can be used in the negative electrode core. The negative electrode composite material layer preferably contains a negative electrode active material and a binder and is provided on both surfaces of the negative electrode core. The negative electrode 12 can be produced, for example, by coating the surface of the negative electrode core with a negative electrode composite material slurry containing a negative electrode active material, a conductive material, a binder, and the like, drying the coating film, and compressing the negative electrode core to form the negative electrode composite material layer on both surfaces of the negative electrode core.

[0047] In the negative electrode composite material layer, for example, a carbon-based active material that reversibly occludes and releases lithium ions is contained as the negative electrode active material. Suitable carbon-based active materials are graphite such as flaky graphite, massive graphite, earthy graphite, and artificial graphite such as massive artificial graphite (MAG) and graphitized mesocarbon microbeads (MCMB). In addition, a Si-based active material composed of at least one of Si and a Si-containing compound can be used as the negative electrode active material, and the carbon-based active material can be used in combination with the Si-based active material.

[0048] As the conductive material contained in the negative electrode composite material layer, a carbon material such as carbon black, acetylene black, ketjen black, and graphite can be used as in the case of the positive electrode 11. As the binder contained in the negative electrode composite material layer, a fluorine resin, PAN, polyimide, an acrylic resin, a polyolefin, or the like can be used as in the case of the positive electrode 11, but a styrene-butadiene rubber (SBR) is preferably used. In addition, the negative electrode composite material layer preferably further contains a CMC or a salt thereof, a polyacrylic acid (PAA) or a salt thereof, a polyvinyl alcohol (PVA), or the like. Among them, the SBR is suitably used in combination with the CMC or a salt thereof and the PAA or a salt thereof.

[0049] [Separator]

[0050] A porous sheet having ion permeability and insulation is used in the separator 13. As specific examples of the porous sheet, a microporous film, a woven fabric, a nonwoven fabric, and the like can be given. As the material of the separator 13, a polyolefin such as polyethylene, polypropylene, and a copolymer of ethylene and an alpha olefin, and cellulose are suitable. The separator 13 can be a single-layer structure or a laminated structure, and both are acceptable. A heat-resistant layer containing inorganic particles, a heat-resistant layer composed of a resin having high heat resistance such as an aromatic polyamide resin, a polyimide, and a polyamide-imide, or the like can be formed on the surface of the separator 13.

[0051] [Embodiments]

[0052] Hereinafter, the present disclosure will be further described based on embodiments, but the present disclosure is not limited to these embodiments.

[0053] [Embodiment 1]

[0054] [Synthesis of positive electrode active material]

[0055] A composite oxide containing Ni, Al, and Mn, niobium hydroxide, calcium hydroxide, and lithium hydroxide were mixed at a prescribed mass ratio, and the mixture was fired at a temperature increasing rate of 0.5°C / min from 650°C to 730°C after being heated at a temperature increasing rate of 2.0°C / min from room temperature to 650°C in a stream of oxygen to obtain a fired product. The fired product was washed with water and dried to obtain a lithium transition metal composite oxide (positive electrode active material) containing the metal elements shown in Table 1. Note that the contents of the metal elements Ml and M2 shown in Table 1 are amounts added externally with respect to the total content of Ni, Co, Al, and Mn.

[0056] The lithium transition metal composite oxide obtained was secondary particles of D50 of 12 μm in which primary particles of average particle diameter of 350 nm were aggregated. The element distribution in the cross section of the primary particles was measured by TEM-EDX, and as a result, it was confirmed that Nb was solid-solved in the composite oxide, and the ratio of the concentration of Nb contained in the shell (10 nm in thickness from the surface of the particle) to the concentration of Nb contained in the core (shell / core concentration ratio) was 9, and that Nb was segregated to the surface of the primary particles. In addition, it was confirmed from the results of the measurement by TEM-EDX that Ca was roughly uniformly fixed to the surface of each of the primary particles contained in the surface and the inside of the secondary particles (a coating layer containing Ca was formed).

[0057] [Production of positive electrode]

[0058] As the positive electrode active material, the above-described lithium transition metal composite oxide was used. The positive electrode active material was mixed with acetylene black and polyvinylidene fluoride at a prescribed solid content mass ratio, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode composite material slurry. Next, the positive electrode composite material slurry was applied to a positive electrode core formed of an aluminum foil, and the coated film was dried and compressed, and then cut to a prescribed electrode size to obtain a positive electrode.

[0059] [Production of negative electrode]

[0060] Graphite was mixed with a dispersant of styrene butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC-Na) at a prescribed solid content mass ratio, and water was used as a dispersion medium to prepare a negative electrode composite material slurry. Next, the negative electrode composite material slurry was applied to both surfaces of a negative electrode core formed of a copper foil, and the coated film was dried and compressed, and then cut to a prescribed electrode size to produce a negative electrode in which a negative electrode composite material layer was formed on both surfaces of the negative electrode core.

[0061] [Preparation of nonaqueous electrolyte]

[0062] Ethylene carbonate (EC) was mixed with methyl ethyl carbonate (EMC) and dimethyl carbonate (DMC) in a prescribed volume ratio. LiPF6 was added to the mixed solvent to obtain a nonaqueous electrolyte solution.

[0063] [Manufacture of Test Cell (Nonaqueous Electrolyte Secondary Cell)]

[0064] The above positive electrode with an aluminum positive electrode lead attached thereto and the above negative electrode with a nickel negative electrode lead attached thereto were spirally wound with a polyethylene separator interposed therebetween, molded into a flat shape, and an electrode body of a wound type was manufactured. The electrode body was housed in an outer case body composed of an aluminum laminate sheet, the nonaqueous electrolyte solution was injected, and the opening portion of the outer case body was sealed, and a test cell for evaluation was manufactured.

[0065] [Storage Test]

[0066] The test cell manufactured was subjected to charge-discharge 1 cycle at a temperature environment of 25°C at a current of 0.5 It with a charge termination voltage of 4.2 V and a discharge termination voltage of 2.5 V, and after the discharge capacity was measured, it was left to stand at a high temperature environment of 50°C for 45 days in a state charged to 4.2 V. The test cell after storage was discharged to 2.5 V at a temperature environment of 25°C at a current of 0.5 It, and the discharge capacity was measured.

[0067] The capacity retention rate after the storage test was calculated according to the following formula, and the results are shown in Table 1.

[0068] Capacity retention rate = (discharge capacity after storage test / discharge capacity before storage test) x 100

[0069] [Example 2]

[0070] In the synthesis of the positive electrode active material, strontium hydroxide was added instead of calcium hydroxide so as to become the content of the metal elements shown in Table 1, and otherwise, a test cell was manufactured in the same manner as in Example 1, and a storage test was performed.

[0071] [Example 3]

[0072] In the synthesis of the positive electrode active material, a composite oxide containing Ni, Co, and Al was added instead of a composite oxide containing Ni, Al, and Mn, and zirconium oxide was added instead of niobium hydroxide so as to become the content of the metal elements shown in Table 1, and otherwise, a test cell was manufactured in the same manner as in Example 1, and a storage test was performed.

[0073] [Example 4]

[0074] In the synthesis of the positive electrode active material, a composite oxide containing Ni and Al was added in an amount such that the content of the metal elements shown in Table 1 was obtained instead of a composite oxide containing Ni, Al, and Mn, and a test battery cell was produced in the same manner as in Example 1, except for this, and a storage test was performed.

[0075] <Example 5>

[0076] In the synthesis of the positive electrode active material, titanium oxide was added instead of niobium hydroxide in an amount such that the content of the metal elements shown in Table 1 was obtained, and a test battery cell was produced in the same manner as in Example 4, except for this, and a storage test was performed. The amounts of Nb and Ti added were set to 1:1 in terms of molar ratio.

[0077] <Comparative Example 1>

[0078] In the synthesis of the positive electrode active material, no niobium hydroxide and no calcium hydroxide were added, and a test battery cell was produced in the same manner as in Example 1, except for this, and a storage test was performed.

[0079] <Comparative Example 2>

[0080] In the synthesis of the positive electrode active material, no calcium hydroxide was added, and a test battery cell was produced in the same manner as in Example 1, except for this, and a storage test was performed.

[0081] <Comparative Example 3>

[0082] In the synthesis of the positive electrode active material, no niobium hydroxide was added, and a test battery cell was produced in the same manner as in Example 1, except for this, and a storage test was performed.

[0083] <Comparative Example 4>

[0084] In the synthesis of the positive electrode active material, the maximum temperature during firing was set to 780°C in such a manner that the shell / core concentration ratio became lower than 1.01, and a test battery cell was produced in the same manner as in Example 1, except for this, and a storage test was performed.

[0085] [Table 1]

[0086]

[0087] As shown in Table 1, the capacity retention after the preservation test of the test battery cells of the examples was higher than that of the test battery cells of the comparative examples, and the preservation characteristics were excellent. From the results shown in Table 1, it was found that the capacity retention after the preservation test was significantly reduced in the case of Ca (metal element M2) not being fixed to the surface of the primary particles of the lithium transition metal composite oxide (Comparative Example 2), the case of Nb (metal element M1) not being solid-solved in the composite oxide (Comparative Example 3), the case of neither of the metal elements M1 and M2 being added (Comparative Example 1), and the case of the shell / core concentration ratio of the metal element M1 being 0.9 (Comparative Example 4) as compared with the test battery cell of Example 1.

[0088] In other words, by adding the metal elements M1 and M2 in a prescribed amount to the composite oxide, the metal element M1 is biased to the surface of the primary particles of the composite oxide, thereby specifically improving the preservation characteristics of the battery. It is considered that the metal elements M1 and M2 react / combine, thereby uniformly forming a coating layer containing the metal element M2 that is firmly bound to the surface of the primary particles. It is thus presumed that the oxygen release caused by the side reaction of the active material and the electrolyte during the preservation is suppressed, the good crystal structure of the active material is maintained, and the preservation characteristics are improved.

[0089] <Comparative Example 5>

[0090] In the synthesis of the positive electrode active material, the maximum temperature during the firing was made to be 700°C in such a manner that the shell / core concentration ratio exceeds 20, and otherwise, the test battery cell was produced in the same manner as in Example 1, and the preservation test was performed. Also, for the same test battery cell, after charging to the charge termination voltage 4.2 V at a current of 0.5 It in a temperature environment of 25°C, the alternating current impedance was measured in the range of 10 mHz to 100 kHz, and the Cole-Cole chart was produced. The charge transfer resistance of the positive electrode composite material layer was calculated from the size of the approximately semicircle appearing in the obtained Cole-Cole chart. Also, for Examples 1 and 2, the charge transfer resistance of the positive electrode composite material layer was calculated in the same manner, and the evaluation results are shown in Table 2.

[0091] [Table 2]

[0092]

[0093] The capacity retention after the preservation test of the test battery cell of Comparative Example 5 was the same as that of the test battery cells of the examples. However, as shown in Table 2, the charge transfer resistance of the positive electrode composite material layer was significantly higher than that of Examples 1 and 2, and it was difficult to achieve practical use for the configuration of Comparative Example 5. That is, only in the case where the shell / core concentration ratio of the metal element M1 in the positive electrode active material satisfies the condition of 1.01 or more and 20 or less, it is possible to improve the preservation characteristics without impairing other battery performances such as the charge transfer resistance.

[0094] In this embodiment, Nb, Zr, and Ti are used as the metal element M1, but the same storage property improvement effect can be obtained even when Mg or V is used.

[0095] Explanation of Reference Numerals

[0096] 10 nonaqueous electrolyte secondary battery

[0097] 11 positive electrode

[0098] 12 negative electrode

[0099] 13 separator

[0100] 14 electrode body

[0101] 16 outer can

[0102] 17 sealing body

[0103] 18, 19 insulating plate

[0104] 20 positive electrode lead

[0105] 21 negative electrode lead

[0106] 22 groove portion

[0107] 23 inner terminal plate

[0108] 24 lower valve body

[0109] 25 insulating member

[0110] 26 upper valve body

[0111] 27 lid

[0112] 28 gasket

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a lithium transition metal composite oxide, wherein the lithium transition metal composite oxide contains Ni in a molar amount of 85 mol% or more relative to the total molar amount of metal elements other than Li. The lithium transition metal composite oxide contains at least one element selected from Al and Mn, wherein the content of Al is less than 7 mol% and the content of Mn is less than 5 mol% relative to the total molar amount of metal elements other than Li. The lithium transition metal composite oxide is a secondary particle formed by the aggregation of primary particles. The metal contains at least one metal element M1 selected from Mg, Ti, Nb, Zr, and V in an amount of 3 mol% or less relative to the total molar amount of metal elements other than Li. When the thickness range within 10 nm from the surface of the primary particle is defined as the shell, and the area closer to the interior is defined as the core, the ratio of the concentration of metal element M1 in the shell to the concentration of metal element M1 in the core is 1.01 or more and 20 or less. At least one metallic element M2 selected from Ca and Sr exists on the surface of the primary particles in an amount of less than 1 mol% relative to the total molar amount of metallic elements other than Li.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein, The content of Ni is over 90 mol% relative to the total molar amount of metallic elements other than Li.

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein, The lithium transition metal composite oxide does not actually contain Co.

4. A non-aqueous electrolyte secondary battery comprising: a positive electrode containing the positive electrode active material according to any one of claims 1 to 3, a negative electrode, and a non-aqueous electrolyte.

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