Positive electrode active material for lithium ion secondary battery

Abstract

Provided is a positive electrode active material for a lithium ion secondary battery, which maintains good battery characteristics and has excellent particle strength. A positive electrode active material for a lithium ion secondary battery, the positive electrode active material comprising a lithium metal composite oxide, the lithium metal composite oxide comprising secondary particles obtained by aggregating primary particles, or comprising both the primary particles and the secondary particles, the positive electrode active material being characterized in that: the positive electrode active material is a positive electrode active material for a lithium ion secondary battery; the lithium metal composite oxide contains lithium, nickel, manganese and cobalt, the particle strength of the lithium metal composite oxide is 10-50 MPa, and the secondary particles comprise a core part occupying the inside of the particles and a shell part surrounding the core part and covering the outside. The secondary particle has the following forms (a) to (c) observed by imaging the cross section of the secondary particle. (a) the core part has a porous structure, and the porosity of the core part is 20-60%; (b) The shell part has a solid structure, and the porosity of the shell part is 5% or less. And (c) the overall porosity of the secondary particles is 10%-50%.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery. Background Art

[0002] Recently, from the viewpoint of global environmental protection, clean energy vehicles friendly to the global environment such as hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and electric vehicles (EV) with low carbon dioxide emissions are becoming popular. Among the batteries used for these clean energy vehicles, the development of lithium ion secondary batteries has been ongoing due to their excellent output characteristics and charge-discharge cycle characteristics.

[0003] In addition, in the case where it is difficult to newly build a thermal power station that burns fossil fuels, in order to cope with the increase in power demand, as one of the effective methods of power utilization, surplus night power is stored in a lithium ion secondary battery installed in ordinary households and used during the day when power consumption is high, and load balancing for load equalization is performed.

[0004] Furthermore, measures to use a household storage battery composed of a lithium ion secondary battery together with a solar power generation system are gradually expanding. The stored clean power can be used not only during the day but also at night. In addition, it is highly expected as a backup power source in the event of a power outage caused by disasters such as earthquakes and typhoons.

[0005] As the positive electrode active material of such a lithium ion secondary battery, a lithium ion secondary battery using a lithium metal composite oxide having a layered or spinel-type crystal structure can obtain a voltage of up to 4V level. Therefore, as a secondary battery showing a high energy density, it is being put into practical use.

[0006] As such a lithium metal composite oxide, lithium cobalt composite oxide (LiCoO2) which is relatively easy to manufacture is the first, and lithium nickel composite oxide (LiNiO2) using nickel which is cheaper than cobalt, lithium manganese composite oxide (LiMn2O4), lithium nickel manganese composite oxide (LiNi 0.5 Mn 0.5 O2), lithium nickel manganese cobalt composite oxide (for example, LiNi 1 / 3Mn 1 / 3 Co 1 / 3 O2), lithium nickel cobalt aluminum composite oxide (for example, LiNi 0.75 Co 0.15 Al 0.10 O2), etc. have been proposed.

[0007] Among these lithium metal composite oxides, a positive electrode active material composed of a lithium nickel cobalt aluminum composite oxide (NCA) containing nickel, cobalt, and aluminum at a specific ratio and a positive electrode active material composed of a lithium nickel manganese cobalt composite oxide (NMC) containing nickel, manganese, and cobalt at a specific ratio are particularly attracting attention as materials with excellent thermal stability, high capacity, good cycle characteristics, low resistance, and high output.

[0008] In addition, in the case of being used for a lithium ion secondary battery, for the purpose of obtaining a positive electrode active material with high performance, several techniques focusing on the particle structure of the above-mentioned lithium metal composite oxides have been disclosed.

[0009] Patent Document 1 discloses a positive electrode active material for a non-aqueous electrolyte secondary battery capable of reducing the positive electrode resistance, which is characterized in that it is a positive electrode active material composed of a lithium nickel manganese composite oxide. The lithium nickel manganese composite oxide is represented by the general formula Li 1+u Ni x Mn y Co z M t O2 (-0.05 ≤ u ≤ 0.50, x + y + z + t = 1, 0.3 ≤ x ≤ 0.7, 0.1 ≤ y ≤ 0.55, 0 ≤ z ≤ 0.4, 0 ≤ t ≤ 0.1, M is an additive element and is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, W), and is composed of a hexagonal system lithium-containing composite oxide having a layered structure. The average particle diameter of the positive electrode active material is 2 to 8 μm, [(d90 - d10) / average particle diameter], which is an index representing the width of the particle size distribution, is 0.60 or less, and the specific surface area is 1 to 2 m 2 / g, and has a hollow structure composed of a hollow part inside the particle and a shell part outside it. The thickness of the shell part is 0.5 to 2.5 μm.

[0010] Patent Document 2 discloses a positive electrode active material for a non-aqueous electrolyte secondary battery with excellent Coulomb efficiency and reaction resistance, which is characterized in that it is composed of a lithium nickel manganese composite oxide. The lithium nickel manganese composite oxide is represented by the general formula Li a Ni x Mn y Co z M tO2 (where 0.95 ≤ a ≤ 1.20, 0.2 ≤ x ≤ 0.8, 0 ≤ y < 0.3, 0.07 < z ≤ 0.8, 0 ≤ t ≤ 0.1, x + y + z + t = 1, and M is at least one element selected from Mg, Ca, Ba, Sr, Al, Ti, V, Cr, Zr, Mo, Hf, Ta, and W) is represented and has a hollow or porous structure. The sulfate content of the positive electrode active material is 0.4% by mass or less, and the sodium content is 0.035% by mass or less.

[0011] In Patent Document 3, a positive electrode active material for a lithium battery is disclosed. It is a particle having a porous structure in which pores are distributed in the entire internal region of secondary particles formed by agglomerates of primary particles. The pores are radially distributed from the center of the secondary particles, thereby having a large specific surface area. As a result, the electrolyte easily flows into the pores, the migration resistance of lithium ions decreases, and a secondary battery with high output characteristics can be manufactured.

[0012] In Patent Document 4, a positive electrode active material for a lithium secondary battery is disclosed. As a positive electrode active material for a lithium secondary battery having excellent rate characteristics, it includes secondary particles formed by agglomerating a plurality of primary particles of a lithium composite metal oxide. The secondary particles have pores formed inside and through holes connecting the pores to the surface of the secondary particles, and all of the following (i) to (iii) are satisfied.

[0013] (i) In the cross-section of the secondary particle, the ratio (B / A) of the short axis length B of the figure surrounded by the outer edge of the cross-section to the long axis length A of the figure is 0.75 or more and 1.0 or less.

[0014] (ii) The proportion of the total area of the pores exposed in the cross-section to the area of the figure is 2.0% or more and 40% or less.

[0015] (iii) The proportion of the area of the pores present in the central portion of the secondary particle among the pores exposed in the cross-section to the total area of the pores exposed in the cross-section is 60% or more and 99% or less. (Here, the long axis length is the longest diameter among the diameters of the figure passing through the centroid position of the figure in the figure.) The central portion is the portion surrounded by a circle with the centroid position of the figure as the center and a radius r calculated by the formula "r = (S / π) 0.5 / 2".

[0016] Prior Art Documents

[0017] Patent Documents

[0018] Patent Document 1: Japanese Patent Application Laid-Open No. 2012-254889.

[0019] Patent Document 2: International Publication No. WO2015 / 146598

[0020] Patent Document 3: Japanese Patent Application Laid-Open No. 2017-533571

[0021] Patent Document 4: Japanese Patent Application Laid-Open No. 2019-096406 Summary of the Invention

[0022] Problems to be Solved by the Invention

[0023] However, in the technologies of the above Patent Documents 1 to 4, in order to improve battery characteristics, a particle structure such as high porosity, hollow, porous, or multilayer is provided in the lithium metal composite oxide. As a result, there is a problem that the strength of the particles themselves decreases and sufficient durability cannot be obtained. Nevertheless, a solution to this problem has not been clearly known so far.

[0024] Therefore, the present inventors conducted intensive studies and found that the above problems can be solved by forming a shell portion having excellent particle strength with respect to a core portion having a particle structure such as high porosity, hollow, porous, or multilayer.

[0025] The present invention has been completed in view of the above problems, and an object thereof is to provide a positive electrode active material for a lithium ion secondary battery that maintains good battery characteristics and has excellent particle strength.

[0026] Means for Solving the Problems

[0027] That is, according to one aspect of the present invention for solving the above problems, a first aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, which is a positive electrode active material for a lithium ion secondary battery composed of a lithium metal composite oxide. The lithium metal composite oxide is composed of secondary particles aggregated from primary particles or composed of both the primary particles and the secondary particles. The lithium metal composite oxide contains lithium, nickel, manganese, and cobalt. The particle strength of the lithium metal composite oxide is 10 to 50 MPa. The secondary particles are composed of a core portion occupying the inside of the particles and a shell portion surrounding the core portion and covering the outside. The secondary particles have the following morphologies (a) to (c) observed by photographing a cross section of the secondary particles.

[0028] (a) The core portion has a porous structure, and the core porosity is 20% to 60%.

[0029] (b) The shell portion has a solid structure, and the shell porosity is 5% or less.

[0030] (c) The overall porosity of the secondary particles is 10% to 50%.

[0031] The second aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, characterized in that, in the invention of the first aspect, the porous structure in the particle structure observed by photographing the cross section of the secondary particles of the lithium metal composite oxide is a cross-sectional state showing two or more pores in the particle cross section, and the solid structure is a cross-sectional state showing a filled state of the particle cross section.

[0032] The third aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, characterized in that, in the invention of the first or second aspect, the lithium metal composite oxide has an average particle diameter of 3 to 8 μm, and the thickness of the shell portion is 0.1 to 2 μm.

[0033] The fourth aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, characterized in that, in the invention of the first or second aspect, the lithium metal composite oxide has a specific surface area of 1.2 to 4 m 2 / g and a tapped density of 0.8 to 2.2 g / cm 3 , and the oil absorption amount of the lithium metal composite oxide is 25 to 55 mL / 100 g.

[0034] The fifth aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, characterized in that, in the invention of the first or second aspect, the lithium metal composite oxide is represented by the general formula Li a Ni 1-x-y-z Mn x Co y M z O 2+α (satisfying 0.95 ≤ a ≤ 1.3, 0.01 ≤ x ≤ 0.5, 0.01 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.1, and -0.1 ≤ α ≤ 0.2, and M contains one or more elements selected from W, Mo, V, Ca, Mg, Sr, Ba, Ti, Cr, Zr, Al, Nb, Ta, Si, P, B, S).

[0035] The sixth aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, characterized in that, in the invention of the first or second aspect, the pressing force for vertically pressing the surface of the powder composed of the lithium metal composite oxide is 20 to 200 N.

[0036] The seventh aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, characterized in that, in the invention of the first or second aspect, the viscosity of the positive electrode composite paste at a paste temperature of 20 to 25 °C is 20000 mPa ·Below s, the positive electrode composite paste is composed of a solid component containing a lithium metal composite oxide and a solvent, and the mass ratio of the solid component to the solvent (solid component / solvent) is 1 to 2.5.

[0037] Effects of the Invention

[0038] It is possible to provide a positive electrode active material for a lithium ion secondary battery that maintains good battery characteristics and has excellent particle strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic cross-sectional view of the metal composite hydroxide (precursor) of the present invention.

[0040] Figure 2 It is a schematic cross-sectional view of the lithium metal composite oxide of the present invention.

[0041] Figure 3 It is a schematic cross-sectional view of a coin-type battery used in the evaluation of the battery characteristics of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0042] Hereinafter, the positive electrode active material for a lithium ion secondary battery of the present embodiment will be described in detail.

[0043] It should be noted that the present invention is not limited to the content described below, and various modification examples and substitution examples can be included without departing from the gist of the present invention.

[0044] <Positive Electrode Active Material for Lithium Ion Secondary Battery>

[0045] In the present invention, there is provided a positive electrode active material for a lithium ion secondary battery, which is a positive electrode active material for a lithium ion secondary battery composed of a lithium metal composite oxide. The lithium metal composite oxide is composed of secondary particles aggregated from primary particles or composed of both the primary particles and the secondary particles. The lithium metal composite oxide contains lithium, nickel, manganese, and cobalt. The particle strength of the lithium metal composite oxide is 10 to 50 MPa. The secondary particles are composed of a core part occupying the inside of the particles and a shell part surrounding the core part and covering the outside. The secondary particles have the following morphologies (a) to (c) observed by photographing the cross-section of the secondary particles.

[0046] (a) The core part has a porous structure, and the core part porosity is 20% to 60%.

[0047] (b) The shell part has a solid structure, and the shell part porosity is 5% or less.

[0048] (c) The overall porosity of the secondary particles is 10% to 50%.

[0049] (1) Composition

[0050] The lithium metal composite oxide as a positive electrode active material according to an embodiment of the present invention contains lithium, nickel, manganese, and cobalt. Specifically, it is represented by the general formula Li a Ni 1-x-y-z Mn x Co y M z O 2+α (wherein they are numbers satisfying 0.95 ≤ a ≤ 1.3, 0.01 ≤ x ≤ 0.5, 0.01 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.1, and -0.1 ≤ α ≤ 0.2, and M contains one or more elements selected from W, Mo, V, Ca, Mg, Sr, Ba, Ti, Cr, Zr, Al, Nb, Ta, Si, P, B, and S). In addition, its critical significance is as follows.

[0051] "a" representing the molar ratio of lithium (Li) is preferably in the range of 0.95 to 1.3, more preferably in the range of 0.96 to 1.25, and particularly preferably in the range of 0.97 to 1.2. If within the above range, a positive electrode active material for a lithium ion secondary battery with excellent output characteristics and battery capacity, and a manufacturing method thereof can be provided. In contrast, when "a" is less than 0.95, the positive electrode resistance increases and the output characteristics cannot be improved. If it is greater than 1.3, in addition to a decrease in the initial discharge capacity, the positive electrode resistance also increases.

[0052] "1 - x - y - z" representing the molar ratio of nickel (Ni) is preferably in the range of 0.2 to 0.98, more preferably in the range of 0.25 to 0.95, and particularly preferably in the range of 0.3 to 0.93. If within the above range, when used as a positive electrode active material, high potential and high capacity of the lithium ion secondary battery can be achieved. In contrast, when "1 - x - y - z" is less than 0.2, high potential and high capacity cannot be fully achieved. When it is greater than 0.98, the molar ratio of other elements decreases and their effects cannot be fully obtained.

[0053] "x" representing the molar ratio of manganese (Mn) is preferably in the range of 0.01 to 0.5, more preferably in the range of 0.05 to 0.45, and particularly preferably in the range of 0.1 to 0.4. If within the above range, when used as a positive electrode active material, the thermal stability of the lithium ion secondary battery can be improved. In contrast, when "x" is less than 0.01, the thermal stability cannot be fully improved. When it is greater than 0.5, manganese dissolves out from the positive electrode active material during high - temperature operation, and the cycle characteristics may deteriorate.

[0054] "y", which represents the molar ratio of cobalt (Co), is preferably in the range of 0.01 to 0.5, more preferably in the range of 0.05 to 0.45, and particularly preferably in the range of 0.1 to 0.4. If it is within the above range, when used as a positive electrode active material, the cycle characteristics of the lithium ion secondary battery can be improved. In contrast, when "y" is less than 0.01, the effect of reducing the expansion and contraction behavior of the crystal lattice is insufficient, and the cycle characteristics cannot be sufficiently improved. If it is greater than 0.5, the addition amount of cobalt is excessive, and the initial discharge capacity significantly decreases.

[0055] M is an element that contributes to the output characteristics, durability, stability, etc. of the lithium ion secondary battery and can be added arbitrarily. The value of "z", which represents the molar ratio of M, can be 0, but in order to fully obtain the above effects, it is preferably set such that "z" is greater than 0 and 0.1 or less, more preferably greater than 0 and 0.05 or less, and particularly preferably greater than 0 and 0.01 or less. When the value of "z" is 0, the output characteristics, durability, stability, etc. of the lithium ion secondary battery cannot be improved. On the other hand, when the value of "z" is greater than 0.1, the molar ratio of other elements decreases, and the effects cannot be fully obtained.

[0056] As such an M, one or more selected from tungsten (W), molybdenum (Mo), vanadium (V), calcium (Ca), magnesium (Mg), strontium (Sr), barium (Ba), titanium (Ti), chromium (Cr), zirconium (Zr), aluminum (Al), niobium (Nb), tantalum (Ta), silicon (Si), phosphorus (P), boron (B), and sulfur (S) can be used.

[0057] (2) Particle strength

[0058] The particle strength of the lithium metal composite oxide as the positive electrode active material according to the embodiment of the present invention is preferably in the range of 10 to 50 MPa, more preferably in the range of 11 to 45 MPa, and particularly preferably in the range of 12 to 40 MPa. If it is within the above range, a positive electrode active material for a lithium ion secondary battery and a method for manufacturing the same that can provide good battery characteristics and excellent particle strength can be provided.

[0059] In contrast, when the particle strength is less than 10 MPa, in the subsequent crushing process of the lithium metal composite oxide, secondary particles are pulverized to generate fine powder. If it is greater than 50 MPa, the filling property during the production of the positive electrode film deteriorates.

[0060] It should be noted that the "particle strength" refers to applying a load to a single sample particle and defining the breaking strength when the particle is broken as the "particle strength". When evaluating it, the breaking strengths of a plurality of (for example, 10) particles are obtained, and their average value is calculated as the "particle strength". The measurement method is not particularly limited and is performed by a compression test.

[0061] (3) Particle Morphology / Internal Particle Structure

[0062] In the embodiments of the present invention, the lithium metal composite oxide as the positive electrode active material almost entirely has the form of secondary particles formed by aggregation of plural primary particles, but may also partially contain primary particles that are not aggregated into secondary particles. There is no particular limitation on the shape of the primary particles constituting the above secondary particles and the primary particles existing alone, and various shapes such as spherical, plate-like, needle-like, cuboid, elliptical, rhombohedral, etc. can be adopted. In addition, there is no particular limitation on the aggregation form of plural primary particles. In addition to the form of aggregation in random directions, various forms such as the form of secondary particles that are aggregated substantially evenly and radially from the central part and form a substantially spherical shape or an ellipsoidal shape can also be adopted.

[0063] For the particles of the lithium metal composite oxide having the above form of secondary particles, the core part occupying the interior of the particles has a porous structure, and the shell part surrounding the core part and covering the outside has a solid structure.

[0064] Here, the so-called porous structure refers to a cross-sectional state in which, when observing the cross-sectional particle structure of the lithium metal composite oxide by photographing the cross-section of the secondary particles, the porous structure shows a cross-sectional state having two or more pores in the particle cross-section, that is, a structure in which a large number of pores are dispersed throughout the interior of the secondary particles. On the other hand, the solid structure refers to a cross-sectional state in which the particle cross-section shows a filled state. That is, the secondary particles have the following forms (a) to (c) observed by photographing their cross-sections.

[0065] (a) The core part has a porous structure, and the porosity of the core part is preferably in the range of 20% to 60%, more preferably 25 to 55%, and particularly preferably 30 to 50%.

[0066] (b) The shell part has a solid structure, and the porosity of the shell part is preferably 5% or less, more preferably 4.9% or less, and particularly preferably 4.8% or less.

[0067] (c) The overall porosity of the secondary particles is preferably in the range of 10% to 50%, more preferably 12 to 45%, and particularly preferably 15 to 40%.

[0068] If each porosity is within the above range, a positive electrode active material for a lithium ion secondary battery and a manufacturing method thereof that can provide good battery characteristics and excellent particle strength can be provided.

[0069] (4) Average Particle Diameter (MV)

[0070] For the lithium metal composite oxide as the positive electrode active material in the embodiments of the present invention, the average particle size (MV) of the secondary particles is preferably in the range of 3 to 8 μm, more preferably in the range of 3.5 to 7.5 μm, and particularly preferably in the range of 4 to 7 μm. If within the above range, in a secondary battery in which the positive electrode active material is assembled into the positive electrode, the battery capacity per unit volume can be increased, and the safety is improved and the cycle characteristics are good. In contrast, when the average particle size is less than 3 μm, the packing density of the particles becomes low when manufacturing the positive electrode, and the battery capacity per unit volume of the positive electrode deteriorates. If it is greater than 8 μm, the specific surface area of the positive electrode active material becomes low, and the interface with the electrolyte of the secondary battery decreases. As a result, the resistance of the positive electrode increases and the output characteristics of the battery deteriorate.

[0071] (5) Thickness of the shell

[0072] For the lithium metal composite oxide as the positive electrode active material in the embodiments of the present invention, the thickness of the shell is preferably in the range of 0.1 to 2 μm, more preferably in the range of 0.3 to 1.5 μm, and particularly preferably in the range of 0.5 to 1 μm. If within the above range, it is possible to provide a positive electrode active material for a lithium ion secondary battery and a manufacturing method thereof that maintain good battery characteristics and have excellent particle strength.

[0073] (6) Specific surface area

[0074] The specific surface area of the lithium metal composite oxide as the positive electrode active material in the embodiments of the present invention is preferably 1.2 to 4 m 2 / g, more preferably 1.4 to 3.8 m 2 / g, and particularly preferably 1.6 to 3.6 m 2 / g. If within the above range, when used as the positive electrode active material, the particle contact surface that can contact the electrolyte can be sufficiently ensured. When the specific surface area is less than 1.2 m 2 / g, the particle contact surface becomes too small, and the charge-discharge capacity of the particles with a porous structure cannot be obtained. If it is greater than 4 m 2 / g, the particle contact surface becomes too large, and the surface activity may become too high.

[0075] (7) Tap density

[0076] The tap density of the lithium metal composite oxide as the positive electrode active material in the embodiments of the present invention is preferably 0.8 to 2.2 g / cm 3 , more preferably 0.85 to 2.15 g / cm 3 , and particularly preferably 0.8 to 2.1 g / cm 3 in the range. If within the above range, the battery capacity per unit volume and the cycle characteristics of the secondary battery can be improved simultaneously. When the tap density is less than 0.8 g / cm3 When the value is low, the filling property of the positive electrode active material is low. Therefore, the charge-discharge capacity cannot be sufficiently improved. If it is greater than 2.2 g / cm 3 , the specific surface area of the positive electrode active material decreases. As a result, the reaction area with the electrolyte decreases, and the output characteristics cannot be sufficiently improved.

[0077] (8) Oil absorption amount

[0078] The oil absorption amount of the lithium metal composite oxide as the positive electrode active material in the embodiment of the present invention is preferably in the range of 25 to 55 mL / 100 g, more preferably in the range of 27 to 53 mL / 100 g, and particularly preferably in the range of 30 to 50 mL / 100 g. If it is within the above range, when used as the positive electrode active material, the particle contact surface that can contact the electrolyte can be sufficiently ensured. When the oil absorption amount is less than 25 mL / 100 g, the particle contact surface becomes too small, and the charge-discharge capacity of the particles having a porous structure cannot be obtained. If it is greater than 55 mL / 100 g, the particle contact surface becomes too large, and the surface activity may become too high.

[0079] (9) Pressing force for vertically pressing the surface of the powder

[0080] For the lithium metal composite oxide as the positive electrode active material in the embodiment of the present invention, the pressing force for vertically pressing the surface of the powder is preferably in the range of 20 to 200 N, more preferably in the range of 35 to 180 N, and particularly preferably in the range of 50 to 160 N. If it is within the above range, a positive electrode active material for a lithium ion secondary battery and a manufacturing method thereof that can maintain good battery characteristics and have excellent durability can be provided. In contrast, when the pressing force for vertically pressing the surface of the powder is less than 20 N, in the subsequent crushing process of the lithium metal composite oxide, the secondary particles are crushed to generate fine powder. If it is greater than 200 N, it takes too much time for crushing, etc., and it is difficult to handle.

[0081] (10) Viscosity of the positive electrode composite paste

[0082] For the lithium metal composite oxide as the positive electrode active material in the embodiment of the present invention, the viscosity of the positive electrode composite paste (20 to 25 °C) is preferably 20000 mPa · s or less, more preferably 10000 mPa · s or less, and particularly preferably 5000 mPa · s or less. If it is within the above range, the gelation when used as the positive electrode composite paste is suppressed, and more excellent battery characteristics can be achieved.

[0083] Examples

[0084] Hereinafter, the present invention will be specifically described using examples and comparative examples. In addition, in the following examples and comparative examples, unless otherwise specified, reagents manufactured by Fujifilm Wako Pure Chemical Corporation are used. Further, the present invention is not limited by any of the following examples and comparative examples.

[0085] It should be noted that the various evaluation methods and battery evaluation methods used in the examples and comparative examples are as follows.

[0086] (Various evaluation methods)

[0087] (1) Composition

[0088] Regarding the composition, the sample is heated and decomposed with an inorganic acid to become an analytical sample solution, and the analytical sample solution is measured using an ICPE-9000 (manufactured by Shimadzu Corporation), which is a multi-functional inductively coupled plasma (ICP) optical emission spectrometer, to obtain it.

[0089] (2) Particle strength

[0090] Regarding the particle strength, using a micro strength evaluation testing machine MCT-500 (manufactured by Shimadzu Corporation), a load is applied to one sample particle with a indenter, and the particle strength at the time of compression and destruction is calculated. Specifically, the sample particle is made to rest on a silicon plate, the position is finely adjusted to align with the center of the indenter, and the indenter is brought into contact to such an extent that no large load is applied to the sample particle for measurement. The measurement conditions are that the test force is set to 150 mN, the loading speed is set to 2.0 mN / second, and the average value of 10 particles is obtained.

[0091] (3) Porosity

[0092] Regarding the porosity, a sample particle is cut using a cross-section polishing machine IB-19530CP (manufactured by JEOL Ltd.), which is a cross-section preparation device, and in addition, the cross-section is observed using a JSM-7001F (manufactured by JEOL Ltd.), which is a Schottky field emission scanning electron microscope (SEM-EDS). Further, using WinRoof6.1.1 (manufactured by Mitani Corporation), which is an image analysis / measurement software, the pore part of the particle cross-section is measured as black, the dense part of the particle is measured as white, and the area of black part / (black part + white part) is calculated for any 20 or more particles to obtain the porosity.

[0093] In addition, based on the above, the core porosity, shell porosity, and overall porosity of the secondary particles are calculated as follows.

[0094] 1) Core porosity

[0095] Core porosity (%) = Area of the porous part (black part) of the core / Area of the entire core (black part + white part) × 100

[0096] 2) Shell porosity

[0097] Shell porosity (%) = Area of the porous part (black part) of the shell / Area of the entire shell (black part + white part) × 100

[0098] 3) Overall porosity of secondary particles

[0099] Overall porosity of secondary particles (%) = Area of the porous part (black part) of the entire secondary particle / Area of the entire secondary particle (black part + white part) × 100

[0100] (4) Particle structure

[0101] Regarding the particle structure, the sample particle was cut using a cross-section polishing machine IB-19530CP (manufactured by JEOL Ltd.), which is a preparation device for cross-sections, and the cross-section was observed using a JSM-7001F (manufactured by JEOL Ltd.), which is a Schottky field emission scanning electron microscope (SEM-EDS).

[0102] (5) Average particle size

[0103] The sample was measured using the laser diffraction / scattering method, and the average particle size (MV) was obtained based on the volume-based distribution. It should be noted that for the measuring device, a Microtrac MT3300EX-II (manufactured by Microtrac Bellcompany), which uses the laser diffraction / scattering method and is an ultrasonic generator built-in type particle size distribution measuring device, was used.

[0104] (6) Thickness of the shell

[0105] Regarding the thickness of the shell, after preprocessing the sample particle using the focused ion beam (FIB) method, the cross-section of the obtained sample particle was photographed using a transmission electron microscope (TEM) to measure the average thickness of the shell. In addition, the above preprocessing was performed using an FB-2000A (manufactured by Hitachi High-Technologies Corporation) as a focused ion beam processing device, and the cross-section was photographed using a JEM-ARM200F (manufactured by JEOL Ltd.) as a transmission electron microscope.

[0106] For the average thickness of the shell part, using WinRoof 6.1.1 (manufactured by Mitani Corporation), which is image analysis / measurement software, for each of the arbitrarily selected 20 measurement target particles, the thicknesses of the shell part at 4 locations where 2 straight lines orthogonal to each other at the approximate center of the particle cross-section intersect with the shell part are measured for the obtained cross-sectional image. After averaging the obtained 4 shell part thicknesses to obtain the thickness of the shell part of one measurement target particle, the arithmetic average of the shell part thicknesses of the above-mentioned 20 measurement target particles is calculated to obtain it.

[0107] (7) Specific surface area

[0108] The specific surface area is obtained by analyzing the sample using the nitrogen adsorption / desorption method based on the BET (Brunauer-Emmett-Teller) single-point method, and the measurement is carried out using the Macsorb 1200 series (manufactured by MOUNTECH Co., Ltd), which is a specific surface area measurement device with a gas flow method.

[0109] (8) Tap density

[0110] For the tap density, 12 g of the sample is collected and placed in a graduated cylinder with a capacity of 20 mL. After installing this graduated cylinder on the KRS-409 (manufactured by Kuramoto Scientific Instruments Co., Ltd), which is a vibration specific gravity meter, the operation of freely dropping it from a position 2 cm high is repeated 500 times to obtain it.

[0111] (9) Oil absorption

[0112] For the oil absorption, DBP (dibutyl phthalate, di-n-butyl phthalate) is used as the reagent liquid, and the measurement is carried out using an absorption amount measurement device based on "JIS_K_6217-4:2008 (Carbon black for rubber - Basic characteristics - Part 4: Determination method of oil absorption amount (including compressed sample))", and it is obtained as the DBP absorption amount. Since this measurement result is calculated as the absorption amount per 100 g of the sample, the unit is expressed as "mL / 100 g". In addition, the measurement was carried out using the S-500 (manufactured by ASAHI SOUKEN CORPORATION).

[0113] (10) Pressing force for vertically pressing the surface of the powder

[0114] For the pressing force for vertically pressing the surface of the powder, after evenly spreading the sample in a container, the detection part of a digital force gauge (manufactured by Nidec-Shimpo Corporation) is vertically inserted into 10 locations on the surface of the powder, the pressing force is measured, and the average value of 8 points after removing the maximum value / minimum value is obtained.

[0115] (11) Viscosity of the positive composite paste

[0116] Regarding the viscosity of the positive composite paste, 6 g of acetylene black as a conductive material, 3 g of polyvinylidene fluoride (manufactured by KUREHA CORPORATION, model KF Polymer #1100), and 16 g of N-methyl-2-pyrrolidone (manufactured by Kanto Chemical Co., Inc.) were kneaded into 21 g of the sample to prepare the positive composite paste (ratio of solid content (g) / organic solvent (g) = 1.875). More than 10 g of the positive composite paste was collected in a specified container, the fluid temperature was controlled at 20 [°C] in a water bath, and measurement was performed using Viscomate VM-100A (manufactured by Sekoniccorporation) as a vibrating viscometer to obtain the result.

[0117] (Battery evaluation method)

[0118] (1) Method for manufacturing an evaluation battery (coin-type battery CBA)

[0119] For Figure 3 Regarding the method for manufacturing the evaluation battery (coin-type battery CBA) shown, first, 52.5 mg of the above positive active material, 15 mg of acetylene black, and 7.5 mg of polytetrafluoroethylene were weighed and mixed respectively, and pressed and formed at a pressure of 100 MPa into a diameter of 11 mm and a thickness of 100 μm to manufacture the positive electrode PE (evaluation electrode). Then, the manufactured positive electrode PE was dried in a vacuum dryer at 120 °C for 12 hours, and using this positive electrode PE, a 2032-type coin-type battery CBA was manufactured in a glove box with an argon gas environment with dew point management at -80 °C. In addition, the negative electrode NE used metallic lithium with a diameter of 17 mm and a thickness of 1 mm, the electrolyte used an equimolar mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (manufactured by Toyama Chemical Co., Ltd.) with 1 mol of LiClO4 as a supporting electrolyte, and the spacer SE used a polyethylene porous membrane with a film thickness of 25 μm. It should be noted that the coin-type battery CBA was assembled into a coin-type battery by arranging a gasket GA and a corrugated washer WW between the positive electrode can PC and the negative electrode can NC.

[0120] (2) Initial discharge capacity

[0121] For the initial discharge capacity, after manufacturing Figure 3 the evaluation battery (coin-type battery CBA) shown and leaving it for about 24 hours, after the open-circuit voltage OCV (open_circuit_voltage) was stabilized, the current density of the positive electrode was set to 0.1 mA / cm 2, it is charged to a cut-off voltage of 4.3V to obtain the initial charge capacity, and after a 1-hour interruption, it is obtained as the capacity when discharged to a cut-off voltage of 3.0V. In addition, the measurement was performed using the R6741A (manufactured by ADVANTEST CORPORATION), a multi-channel voltage / current generator.

[0122] (3) Positive electrode resistance

[0123] Regarding the positive electrode resistance, the evaluation battery (coin-type battery CBA) shown in Figure 3 was fabricated. The evaluation battery was charged at a charging potential of 4.1V, and measured by the AC impedance method using a frequency response analyzer and a constant current potentiostat to obtain a Nyquist plot. And this Nyquist plot is represented as the sum of the characteristic curves representing the solution resistance, the negative electrode resistance and its capacitance, and the positive electrode resistance and its capacitance. Based on the Nyquist plot, fitting calculations were performed using an equivalent circuit to calculate the positive electrode resistance.

[0124] Example 1

[0125] (1) Manufacture of metal composite hydroxide (precursor)

[0126] First, 15L of water was placed in a 60L reaction tank, and while stirring, the temperature inside the tank was set to 40°C. Under stirring, a mixed gas with an air-to-argon ratio of 1:6 was continuously introduced into the tank. In this way, by using a mixed gas of air and argon and / or helium, secondary particles with excellent strength can be obtained.

[0127] Then, an appropriate amount of an alkali solution (25 mass% aqueous sodium hydroxide solution) and an ammonium ion donor (25 mass% aqueous ammonia) were added to the water in the tank to prepare a reaction solution such that the pH (based on a liquid temperature of 25°C, the same below) became 12.8 and the ammonium ion concentration became 10 g / L.

[0128] On the other hand, nickel sulfate hexahydrate, manganese sulfate monohydrate, and cobalt sulfate heptahydrate were weighed so that the composition ratio of nickel, manganese, and cobalt was Ni:Mn:Co = 50:30:20, and dissolved in water so that the total concentration of nickel, manganese, and cobalt was 2 mol / L to prepare a raw material solution.

[0129] Then, the raw material solution was added to the reaction solution in the tank at a rate of 100 mL / min. At the same time, the alkali solution and the ammonium ion donor were also added to the reaction solution at a specified rate. Nucleation was carried out by performing crystallization for 1 minute while maintaining the pH of the reaction solution at 12.8 (nucleation process pH) and the ammonium ion concentration at 10 g / L.

[0130] After temporarily stopping the supply of the raw material solution, the alkali solution, and the ammonium ion donor, sulfuric acid is added until the pH of the reaction solution becomes 11.6 (particle growth process pH). After reaching 11.6, the supply of the raw material solution, the alkali solution, and the ammonium ion donor is started again, and crystallization is carried out for 4.5 hours while maintaining the pH at 11.6 and the ammonium ion concentration at 10 g / L, thereby growing the particles.

[0131] Through the above crystallization process, a nickel-manganese-cobalt composite hydroxide (Ni 0.5 Mn 0.3 Co 0.2 (OH)2) slurry containing a metal composite hydroxide with a porous structure in the core part and a solid structure in the shell part (precursor) is obtained.

[0132] The slurry containing the obtained metal composite hydroxide is put into a filter press, and the metal composite hydroxide cake is recovered by pressure filtration. The metal composite hydroxide cake is returned to the reaction tank, the tank is filled with an alkali washing liquid (5 mass% sodium hydroxide aqueous solution), stirred for 30 minutes for alkali washing, and then pressure filtration using the filter press is carried out again to recover the alkali-washed cake. The alkali-washed cake is returned to the reaction tank, the tank is filled with water, stirred for 30 minutes for fine washing, and then pressure filtration using the filter press is carried out again to recover the washed cake.

[0133] The recovered washed cake is dried at 150 °C for 5 hours using an electric heating dryer to obtain a metal composite hydroxide.

[0134] (2) Manufacture of metal composite oxide (intermediate)

[0135] The obtained metal composite hydroxide (precursor) is pre-calcined by heating in an air (oxygen concentration: 21 vol%) stream at 450 °C for 2 hours, and then oxidized and roasted by heating in an air (oxygen concentration: 21 vol%) stream at 600 °C for 5 hours to obtain a nickel-manganese-cobalt composite oxide (Ni 0.5 Mn 0.3 Co 0.2 O) which is a metal composite oxide with a porous structure in the core part and a solid structure in the shell part (intermediate).

[0136] (3) Manufacture of lithium metal composite oxide (positive electrode active material)

[0137] The metal composite oxide (intermediate) obtained by oxidative roasting and lithium hydroxide as a lithium compound are weighed and fully mixed so that the ratio of the number of atoms of lithium (Li) to the number of atoms of metals other than lithium (Me) (i.e., the ratio (Li / Me)) becomes 1.03, thereby obtaining a lithium mixture. The lithium mixture is pre-calcined by heating at 450° C. for 10 hours in an oxygen (oxygen concentration: 100 volume %) gas flow, and then calcined by heating at 860° C. for 5 hours in an oxygen (oxygen concentration: 100 volume %) gas flow to obtain a lithium calcined product.

[0138] The aggregates contained in the lithium calcined product are crushed to obtain a lithium nickel manganese cobalt composite oxide (LiNMC) as a positive electrode active material having a porous core structure and a solid shell structure. 1.03 Ni 0.5 Mn 0.3 Co 0.2 O2).

[0139] Table 1 shows the evaluation results of the obtained lithium metal composite oxide.

[0140] Example 2

[0141] The same procedure as in Example 1 was performed except for the following conditions.

[0142] [Manufacturing of Metal Composite Hydroxide (Precursor)]

[0143] In the crystallization step, a mixed gas of air and argon in a ratio of 1:4 was continuously introduced into the reaction vessel under stirring, and crystallization was performed for 4.5 hours.

[0144] [Manufacturing of Metal Composite Oxide (Intermediate)]

[0145] The metal composite oxide (intermediate) was not produced, but the metal composite hydroxide (precursor) was fired as a raw material.

[0146] Table 1 shows the evaluation results of the obtained lithium metal composite oxide.

[0147] Example 3

[0148] The same procedure as in Example 1 was performed except for the following conditions.

[0149] [Manufacturing of Metal Composite Hydroxide (Precursor)]

[0150] In the crystallization step, a mixed gas of air and argon in a ratio of 1:2 was continuously introduced into the reaction vessel under stirring, and crystallization was performed for 4.5 hours.

[0151] [Manufacturing of Metal Composite Oxide (Intermediate)]

[0152] The production of the metal composite oxide (intermediate) was not carried out, and firing was performed using the metal composite hydroxide (precursor) as the raw material.

[0153] It should be noted that the evaluation results of each of the obtained lithium metal composite oxides are shown in Table 1.

[0154] (Comparative Example 1)

[0155] Except for the following conditions, the same steps as in Example 1 were carried out.

[0156] [Production of metal composite hydroxide (precursor)]

[0157] In the crystallization step, while stirring, a mixed gas obtained by mixing air with nitrogen so that the oxygen concentration becomes 7% by volume was continuously introduced into the reaction tank, and crystallization was carried out for 4.5 hours.

[0158] [Production of metal composite oxide (intermediate)]

[0159] The production of the metal composite oxide (intermediate) was not carried out, and firing was performed using the metal composite hydroxide (precursor) as the raw material.

[0160] It should be noted that the evaluation results of each of the obtained lithium metal composite oxides are shown in Table 1.

[0161] (Comparative Example 2)

[0162] Except for the following conditions, the same steps as in Example 1 were carried out.

[0163] [Production of metal composite hydroxide (precursor)]

[0164] In the crystallization step, first, the inside of the reaction tank was controlled to a non-oxidizing environment with an oxygen concentration of 0.1% by volume while stirring, and after nucleation, crystallization was continued for 0.9 hours in the non-oxidizing environment for particle growth.

[0165] Then, the supply of the raw material solution, the alkali solution, and the ammonium ion donor was stopped, the inside of the reaction tank was switched to an oxidizing environment (air environment) while stirring, the supply of the raw material solution, the alkali solution, and the ammonium ion donor was started again, and crystallization was carried out for 0.9 hours in the oxidizing environment.

[0166] Then, the switching between crystallization in the non-oxidizing environment and crystallization in the oxidizing environment as described above was carried out 3 times (a total of 4 times, i.e., non-oxidation → oxidation → non-oxidation → oxidation → non-oxidation).

[0167] [Production of metal composite oxide (intermediate)]

[0168] Instead of manufacturing a metal composite oxide (intermediate), firing was carried out using a metal composite hydroxide (precursor) as a raw material.

[0169] It should be noted that the evaluation results of each of the obtained lithium metal composite oxides are shown in Table 1.

[0170] Table 1

[0171]

[0172] [Comprehensive evaluation]

[0173] It was confirmed that the lithium metal composite oxides (positive electrode active materials) of Examples 1 to 3 within the scope of the present invention had excellent particle strength in any case compared with the lithium metal composite oxides of Comparative Examples 1 and 2 that deviated from the scope of the present invention. In addition, it was also confirmed that when the lithium metal composite oxides of Examples 1 to 3 were used in a lithium ion secondary battery, compared with the lithium metal composite oxides of Comparative Examples 1 and 2, if the overall porosity was considered, the initial discharge capacity was equal to or higher, and the positive electrode resistance was small.

[0174] It should be noted that in Comparative Example 1, although, like Examples 1 to 3, the core part of the secondary particles of the lithium metal composite oxide had a porous structure and the shell part had a solid structure, the particle strength was not improved. As a reason, it can be considered that there were significant problems in the environmental control (composition of the mixed gas, etc.) in the reaction tank during the crystallization process.

[0175] In addition, the technical scope of the present invention is not limited to the manner described in the above-mentioned one embodiment or the like. One or more of the components described in the above-mentioned one embodiment or the like can be omitted. It should be noted that the components described in the above-mentioned one embodiment or the like can be appropriately combined.

[0176] In addition, within the maximum range permitted by law, the contents of all documents cited in this specification are incorporated herein by reference as part of the present disclosure.

[0177] Explanation of reference numerals

[0178] CBA: Coin-type battery (for evaluation).

[0179] PE: Positive electrode (evaluation electrode).

[0180] NE: Negative electrode.

[0181] SE: Spacer.

[0182] GA: Gasket.

[0183] WW: Wave washer.

[0184] PC: Positive electrode can.

[0185] NC: Negative electrode can.

Claims

1. A positive electrode active material for a lithium ion secondary battery, which is a positive electrode active material for a lithium ion secondary battery composed of a lithium metal composite oxide, and the lithium metal composite oxide is composed of secondary particles aggregated from primary particles or composed of both the primary particles and the secondary particles, and is characterized in that, the lithium metal composite oxide contains lithium, nickel, manganese and cobalt, the particle strength of the lithium metal composite oxide is 10 to 50 MPa, the secondary particles are composed of a core part occupying the inside of the particle and a shell part surrounding the core part and covering the outside, the secondary particles have the following morphologies (a) to (c) observed by photographing the cross section of the secondary particles, (a) The core part has a porous structure, and the core part porosity is 20% to 60%; (b) The shell part has a solid structure, and the shell part porosity is 5% or less; (c) The overall porosity of the secondary particles is 10% to 50%.

2. The positive electrode active material for a lithium ion secondary battery according to claim 1, characterized in that, the porous structure in the particle structure observed by photographing the cross section of the secondary particles of the lithium metal composite oxide is a structure showing a cross-sectional state having two or more pores in the particle cross section, and the solid structure is a cross-sectional state showing a filled state in the particle cross section.

3. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, characterized in that, the lithium metal composite oxide has an average particle diameter of 3 to 8 μm, and the thickness of the shell part is 0.1 to 2 μm.

4. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, characterized in that, The lithium metal composite oxide has a specific surface area of 1.2 to 4 m 2 / g and a tapped density of 0.8 to 2.2 g / cm 3 . The oil absorption amount of the lithium metal composite oxide is 25 to 55 mL / 100 g.

5. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, characterized in that, The lithium metal composite oxide is represented by the general formula Li a Ni 1-x-y-z Mn x Co y M z O 2+α where 0.95 ≤ a ≤ 1.3, 0.01 ≤ x ≤ 0.5, 0.01 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.1, and -0.1 ≤ α ≤ 0.2, and M contains one or more elements selected from W, Mo, V, Ca, Mg, Sr, Ba, Ti, Cr, Zr, Al, Nb, Ta, Si, P, B, and S.

6. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, characterized in that, the pressing force for vertically pressing the surface of the powder composed of the lithium metal composite oxide is 20 to 200 N.

7. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, characterized in that, when the paste temperature of the positive electrode composite paste is 20 to 25 °C, the viscosity is 20000 mPa·s or less, and the positive electrode composite paste is composed of a solid component containing the lithium metal composite oxide and a solvent, and the mass ratio of the solid component to the solvent is 1 to 2.5, that is, solid component / solvent is 1 to 2.5.

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