NCM-based active material for positive electrode, positive electrode, and battery

Abstract

To provide an active material based on NCM for a positive electrode which can exhibit a high heat stability and an improved cycle characteristic.SOLUTION: In the active material based on NCM for a positive electrode, the size of crystallites is at least 460Å.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to an NCM-based active material for a positive electrode (hereinafter also referred to as an active material), a positive electrode, and a battery.

Background Art

[0002] Japanese Patent Application Laid-Open No. 2017-162790 (Patent Document 1) discloses a positive electrode active material for a non-aqueous electrolyte secondary battery in which nickel atoms at 3b sites of a lithium metal composite oxide are substituted with additive elements to shorten the bond distance between oxygen atoms and transition metal atoms.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] A positive electrode containing a lithium nickel composite oxide tends to generate heat and release oxygen during charge and discharge, and has a low thermal stability. When nickel elements are substituted with additive elements for the purpose of improving thermal stability, the active material containing the additive elements tends to have lower cycle characteristics than the active material not containing the additive elements.

[0005] An object of the present invention is to provide an NCM (nickel·cobalt·manganese)-based active material for a positive electrode that can exhibit high thermal stability and improved cycle characteristics.

Means for Solving the Problems

[0006] The present invention provides the following NCM-based active material for a positive electrode, a positive electrode, and a battery. [1] An NCM-based active material for a positive electrode having a crystallite size of 460 Å or more. [2] The NCM-based active material for a positive electrode according to [1], wherein the crystallite size is 570 Å or more. [3] The NCM-based active material for a positive electrode according to [1] or [2], wherein the crystallite size is 710 Å or less. [4] The NCM-based active material for a positive electrode according to any one of [1] to [3], comprising agglomerated particles having a D50 diameter of the secondary particles of 14 μm or more and 18 μm or less. [5] The NCM-based active material for a positive electrode according to any one of [1] to [4], wherein the weight loss amount in thermogravimetric analysis at 120 to 600 °C and a heating rate of 5 °C / min is 12% by mass or less. [6] The NCM-based active material for a positive electrode according to any one of [1] to [5], wherein the weight loss rate per minute in thermogravimetric analysis at 120 to 600 °C and a heating rate of 5 °C / min is 0.46% by mass / min or less. [7] A positive electrode comprising the NCM-based active material for a positive electrode according to any one of [1] to [6]. [8] A battery comprising the positive electrode according to [7].

Advantages of the Invention

[0007] According to the present invention, it is possible to provide an NCM-based active material for a positive electrode that can exhibit high thermal stability and improved cycle characteristics.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0009] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiments. In all the following drawings, the scale is appropriately adjusted for easy understanding of each component, and the scale of each component shown in the drawings does not necessarily match the scale of the actual component.

[0010] <Active material> The active material contains one or more nickel, cobalt, and manganese-containing compounds. The nickel content ratio with respect to the metal elements excluding lithium in the active material may be, for example, 50 mol% or more and 60 mol% or more, preferably 60 mol% or more and 70 mol% or more, respectively. The nickel, cobalt, and manganese-containing compounds preferably include nickel cobalt manganese composite hydroxide, more preferably lithium nickel cobalt manganese composite oxide. The nickel cobalt manganese composite hydroxide may be obtained, for example, by a coprecipitation method or the like. The nickel cobalt manganese composite hydroxide is, for example, represented by the general formula: Ni x Co y Mn z (OH)2 (where x + y + z = 1). The molar ratio of lithium to nickel, cobalt, and manganese in the lithium nickel cobalt manganese composite oxide, Li:(Ni + Co + Mn), can be, for example, 1.0 to 1.2:1.0. The active material can be used for a lithium ion battery. Details of the lithium ion battery will be described later.

[0011] The active material may contain, for example, a first layered metal oxide. The first layered metal oxide is represented by the formula (1): Li 1-a1 Ni x1 Me 1 1-x1 O2(1) In the formula (1), "a1" satisfies the relationship of "-0.3 ≤ a1 ≤ 0.3", "x1" satisfies the relationship of 0.5 ≤ x1 < 1.0, and "Me 1 " represents at least one selected from the group consisting of Co, Mn, Al, Zr, Ti, V, Cr, Fe, Cu, Zn, B, Mo, Sn, Ge, Nb, and W.

[0012] The active material may contain, for example, a second-layered metal oxide. The second-layered metal oxide has the formula (2): Li 1-a2 Ni x2 Me 2 1-x2 O2(2) which is represented by. In formula (2), "a2" satisfies the relationship of "-0.3 ≤ a2 ≤ 0.3", "x2" satisfies the relationship of 0.6 ≤ x2 ≤ 1.0, and "Me 2 " represents at least one selected from the group consisting of Co, Mn, Al, Zr, Ti, V, Cr, Fe, Cu, Zn, B, Mo, Sn, Ge, Nb, and W.

[0013] In formulas (1) and (2), for example, the relationship of "x1 < x2" may be satisfied.

[0014] The active material may contain, for example, at least one selected from the group consisting of LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.7 Co 0.2 Mn 0.1 O2, LiNi 0.7 Co 0.1 Mn 0.2 O2, LiNi 0.6 Co 0.3 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.6 Co 0.1 Mn 0.3 O2 and LiNi 0.55 Co 0.20 Mn 0.25 O2. For example, the primary particles and single particles described later are LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.7 Co 0.2 Mn 0.1 O2, LiNi 0.7 Co 0.1 Mn 0.2 O2 and LiNi0.55 Co 0.20 Mn 0.25 It may contain at least one selected from the group consisting of O2.

[0015] The active material has a crystallite size of 460 Å or more. By having the crystallite size within the above range, excellent thermal stability can be exhibited. The crystallite size can be determined according to the method described in the Examples section below. The active material is preferably 570 Å or more from the viewpoint of the weight loss rate per minute in thermogravimetric analysis at 120 to 600 °C and a heating rate of 5 °C / min. The active material preferably has a crystallite size of 710 Å or less from the viewpoint of cycle characteristics.

[0016] When the active material contains a lithium nickel cobalt manganese composite oxide, the active material can be synthesized through a two-step firing process, for example, a first firing step of firing a first mixture containing a lithium compound and a transition metal compound, and a second firing step of firing a second mixture containing the fired product obtained in the first firing step and a transition metal compound. The crystallite size can be arbitrarily controlled by controlling the firing parameters (such as firing temperature and firing time, etc.) in the first firing step and the second firing step.

[0017] The active material can contain agglomerated particles with an average particle diameter D50 of the secondary particles of 14 μm or more and 18 μm or less. The average particle diameter D50 represents the particle diameter at which the cumulative particle volume from the small particle size side in the volume-based particle size distribution becomes 50% of the total particle volume. The average particle diameter can be measured by the laser diffraction / scattering method.

[0018] The primary particles that make up the secondary particles are particles in which grain boundaries cannot be confirmed visually in the SEM image of the particles. The average primary particle diameter indicates the distance between the two farthest points on the contour line of the primary particle. The average primary particle diameter of the primary particles may be, for example, 0.05 μm or more and 0.7 μm or less, or 0.1 μm or more and 0.6 μm or less. When 10 or more primary particles randomly extracted from the SEM image of one agglomerated particle have an average primary particle diameter of 0.05 μm to 0.2 μm, all of the primary particles contained in the agglomerated particle are regarded as having an average primary particle diameter of 0.05 μm to 0.2 μm. The primary particles may have an average primary particle diameter of, for example, 0.2 μm to 0.5 μm.

[0019] The active material can further contain single particles. The average particle diameter D50 of the single particles may be, for example, 1 μm or more and 20 μm or less, preferably 1 μm or less and 10 μm or less, and more preferably 1 μm or more and 5 μm or less. The single particles may be surface-coated.

[0020] The active material can have a weight loss amount of 12 mass% or less in thermogravimetric analysis at a temperature of 120 to 600 °C and a heating rate of 5 °C / min. Further, the active material may have a weight loss rate per minute of 0.46 mass% / min or less, preferably 0.40 mass% / min, in thermogravimetric analysis at a temperature of 120 to 600 °C and a heating rate of 5 °C / min. The thermogravimetric analysis is performed according to the method described in the Examples section below.

[0021] The positive electrode containing the active material may have a cycle capacity retention rate of, for example, 93.3% or more, preferably 93.8% or more. The cycle capacity retention rate is performed according to the method described in the Examples section below.

[0022] <Lithium Ion Battery> FIG. 1 is a schematic diagram showing an example of a lithium ion battery according to the present embodiment. The battery 100 shown in FIG. 1 may be a lithium ion battery such as a main power source of an electric vehicle or a power assist power source.

[0023] The battery 100 includes an exterior body 90. The exterior body 90 houses an electrode body 50 and an electrolyte (not shown). The electrode body 50 is connected to the positive electrode terminal 91 by a positive electrode current collector member 81. The electrode body 50 is connected to the negative electrode terminal 92 by a negative electrode current collector member 82. FIG. 2 is a schematic diagram showing an example of the electrode body in the present embodiment. The electrode body 50 is of a wound type. The electrode body 50 includes a positive electrode 20, a separator 40, and a negative electrode 30. That is, the battery 100 includes the positive electrode 20. The positive electrode 20 includes a positive electrode active material layer 22 and a positive electrode substrate 21. The negative electrode 30 includes a negative electrode active material layer 32 and a negative electrode substrate 31.

[0024] <Positive electrode> As shown in FIG. 3, in the positive electrode 20, the positive electrode active material layer 22 may be formed directly or indirectly on one or both sides of the positive electrode substrate 21. The positive electrode substrate 21 may be a conductive sheet made of, for example, an Al alloy foil, a pure Al foil, or the like. The positive electrode active material layer 22 contains an active material 11. The positive electrode active material layer 22 may further contain a conductive material, a binder, and the like. The active material 11 includes the above-mentioned NCM-based active material for the positive electrode.

[0025] The positive electrode active material layer 22 may have a thickness of, for example, from 10 μm to 200 μm. The positive electrode active material layer 22 may have a high density. The density of the positive electrode active material layer 22 may be, for example, 3.5 g / cm 3 or more, 3.6 g / cm 3 or more, or 3.7 g / cm 3 or more. The positive electrode active material layer 22 may have a density of, for example, 4.0 g / cm 3 or less.

[0026] <Method for manufacturing the positive electrode> The manufacturing method of the positive electrode 20 in this embodiment includes the preparation (A), coating (B), and rolling (C) of the positive electrode slurry. In the preparation (A) of the positive electrode slurry, a positive electrode slurry containing the above-described active material is prepared. The positive electrode slurry is prepared by dispersing the active material in a dispersion medium. In the coating (B), the positive electrode slurry is coated on the surface of the positive electrode substrate 11 to form the positive electrode active material layer 12. In the rolling (C), the positive electrode 10 is manufactured by rolling the positive electrode active material layer 12 and the positive electrode substrate 11. The raw sheet of the positive electrode 10 is manufactured by rolling. The raw sheet can be cut into a predetermined planar size according to the specifications of the battery 100.

Example

[0027] Hereinafter, the present invention will be described in more detail with reference to examples. In the examples, “%” and “parts” are by mass% and parts by mass, respectively, unless otherwise specified.

[0028] [Preparation of Positive Electrode for Cycle Characteristic Evaluation] The positive electrode active material 1 prepared in the examples and the comparative examples was mixed with the positive electrode active material 2 prepared as described below at a ratio of 1:1, and the mixture was 97.5 parts by mass, 1.5 parts by mass of carbon black as a conductive material, and 1.0 part by mass of polyvinylidene fluoride (PVdF) as a binder. They were mixed so that, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a positive electrode composite slurry. The slurry was applied to a current collector made of aluminum foil to form a positive electrode composite layer. Then, it was dried and compressed using a rolling roller so that the density of the positive electrode composite material became 3.55 g / cm 3 and cut into a predetermined size and an aluminum tab was attached to obtain a positive electrode. Synthesis of the second positive electrode active material: LiOH and Ni 0.60 Co 0.20 Mn 0.20 The transition metal compound represented by (OH)2 was mixed, and a second positive electrode active material (lithium nickel composite oxide having a particle diameter of 3 to 6 μm) was obtained through a first firing step and a second firing step. The average particle diameter D50 of the second positive electrode active material was 4.0 μm.

[0029] [Fabrication of the positive electrode for TG measurement] The positive electrode active material 1 prepared in the examples and comparative examples was mixed so as to be 89.0 parts by mass, carbon black 1.0 part by mass as a conductive material, and polyvinylidene fluoride (PVdF) 10 parts by mass as a binder. Further, an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode composite slurry. By applying the slurry to a current collector made of aluminum foil, a positive electrode composite layer was formed. Then, it was dried, rolled to a predetermined thickness using a rolling roller, cut to a predetermined size, and an aluminum tab was attached to obtain a positive electrode.

[0030] [Fabrication of the negative electrode] The negative electrode active material, carboxymethyl cellulose (CMC) as a thickener, and styrene-butadiene rubber (SBR) as a binder were weighed so that the mass ratio was 98:1:1, respectively, and these were dispersed in water to prepare a negative electrode mixture slurry. By applying this negative electrode mixture slurry to a current collector made of copper foil, a negative electrode mixture layer was formed. Then, it was dried, rolled to a predetermined thickness using a rolling roller, cut to a predetermined size, and a nickel tab was attached to obtain a negative electrode.

[0031] [Preparation of the non-aqueous electrolyte] Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:30:40. Lithium hexafluorophosphate (LiPF6) was added to the mixed solvent so as to have a concentration of 1.15 mol / liter. Further, vinylene carbonate (VC) was added so that the addition rate was 1.0% by mass based on the total mass of the mixed solvent to prepare a non-aqueous electrolyte.

[0032] [Evaluation of the crystallite size] The powder sample of the first positive electrode active material was filled into a dedicated folder of a fully automatic multi-purpose X-ray diffractometer (SmartLab manufactured by Rigaku), measured by the reflection method, and an X-ray diffraction pattern was obtained. The X-ray tube (target element) used was Cu, the tube voltage was 45 kV, and the tube current was 200 mA. From the measurement results, the crystallite size was calculated from the half-value width (d) of the crystal peak (104 plane) appearing at 2θ = 44 to 45°.

[0033] [Evaluation of cycle characteristics] The positive electrode and the negative electrode obtained by the method for producing a positive electrode plate for evaluating cycle characteristics were laminated with a separator made of polyolefin to produce a laminated electrode body. This electrode body was housed in an exterior body composed of an aluminum laminate sheet, and after injecting the non-aqueous electrolyte, the opening of the exterior body was sealed to obtain a test cell. Regarding the above test cell, under a temperature condition of 25 °C, constant current charging was performed at a current density of 0.2 mA / cm 2 until a voltage of 4.25 V was reached, and then constant voltage charging was performed at 4.25 V until the current density became 0.04 mA / cm 2 to obtain the charging capacity. After a pause of 10 minutes, constant current discharge was performed at a current density of 0.2 mA / cm 2 until a voltage of 3.0 V was reached to obtain the discharge capacity.

[0034] The above cell was subjected to constant current charging at a current density of 0.2 mA / cm 2 under a temperature condition of 25 °C until a voltage of 4.18 V was reached, and then further constant voltage charging was performed at 4.18 V until the current density became 0.04 mA / cm 2 . After charging, constant current discharge was performed at a current density of 0.2 mA / cm 2 until a voltage of 3.48 V was reached. The above charge-discharge conditions were regarded as one cycle, and charge-discharge was performed up to 200 cycles, and the capacity retention rate was calculated by the following formula. Capacity retention rate (%) = Discharge capacity at the 200th cycle ÷ Discharge capacity at the first cycle × 100

[0035] [Evaluation of thermal stability] (Fabrication of test cell) The positive electrode obtained by the method for producing a positive electrode plate for TG measurement, metallic lithium, and a separator made of polyolefin were wound with each other via the separator to produce a wound electrode body. This electrode body was housed in an exterior body composed of an aluminum laminate sheet, and after injecting the non-aqueous electrolyte, the opening of the exterior body was sealed to obtain a test cell. (Method for creating a sampling for TG measurement) Under the temperature condition of 25 °C, the above cell was subjected to constant current charging until the voltage reached 4.35 V at a current density of 0.2 mA / cm 2 , and further subjected to constant voltage charging until the current density reached 0.04 mA / cm 2 at 4.35 V. After that, constant current discharging was performed until the voltage reached 2.5 V at a current density of 0.04 mA / cm 2 . After performing the above charge and discharge conditions for one cycle, constant current charging was performed until the voltage reached 4.40 V at a current density of 0.2 mA / cm 2 , and further constant voltage charging was performed until the current density reached 0.04 mA / cm 2 at 4.40 V. The corresponding cell was disassembled in a glove box, the positive electrode plate was taken out, washed with DMC, and vacuum dried. The positive electrode mixture layer was scraped off from the core body of the corresponding positive electrode plate to obtain a sample for TG measurement. (TG measurement method) 20 mg of the above sample was weighed into a TG pan of a measuring device (measured with a SHIMADZU DTG-60AH) and set in the device. Measurement was performed up to a temperature limit of 600 degrees at a heating rate of 5 °C / min under an Ar atmosphere. The weight loss amount and the weight loss rate per minute in the range of 120 °C to 600 °C were calculated.

[0036] <Example 1> Ni was mixed with LiOH 0.55 Co 0.20 Mn 0.25 The transition metal compound represented by (OH)2 was mixed, and the first positive electrode active material was obtained through a first firing step and a second firing step. The average particle diameter D50 of the first positive electrode active material was 16.0 μm, and the crystal size was 578 Å. <Examples 2 to 4 and Comparative Examples 1 to 4> The first positive electrode active material was produced in the same manner as in Example 1, except that the crystal size was controlled by parameter control in the first and second firing steps in Example 1. The results are shown in Table 1.

[0037] [Table 1]

[0038] In Table 1, when the cycle characteristics are 93.3% or more, the weight reduction amount is 12% or less, and the weight reduction rate per minute is 0.40% / min or less, it is judged as ⊚; when the cycle characteristics are 93.3% or more, the weight reduction amount is 12% or less, and the weight reduction rate per minute is 0.46% / min or less, it is judged as ○; otherwise, it is judged as ×. In Examples 1 to 5 according to the present invention, it was possible to achieve both high thermal stability and improved cycle characteristics. [Explanation of Reference Numerals]

[0039] 11 NCM-based active material for positive electrode, 20 positive electrode, 21 positive electrode substrate, 22 positive electrode active material layer, 30 negative electrode, 31 negative electrode substrate, 32 negative electrode active material layer, 40 separator, 50 electrode body, 81 positive electrode current collector member, 82 negative electrode current collector member, 90 exterior body, 91 positive electrode terminal, 92 negative electrode terminal, 100 battery (lithium ion battery).

Claims

1. The crystallite size is 570 Å or more and 710 Å or less, and contains agglomerated particles having a secondary particle diameter D50 of 14 μm or more and 18 μm or less, an NCM-based active material for a positive electrode containing at least one selected from the group consisting of LiNi0.7Co0.2Mn0.1O2, LiNi0.7Co0.1Mn0.2O2, LiNi0.6Co0.3Mn0.1O2, LiNi0.6Co0.2Mn0.2O2, LiNi0.6Co0.1Mn0.3O2, and LiNi0.55Co0.20Mn0.25O2.

2. The NCM-based active material for a positive electrode according to Claim 1, wherein the weight loss amount in thermogravimetric analysis at 120 to 600 °C and a heating rate of 5 °C / min is 12 mass% or less.

3. The NCM-based active material for a positive electrode according to Claim 1, wherein the weight loss rate per minute in thermogravimetric analysis at 120 to 600 °C and a heating rate of 5 °C / min is 0.46 mass% / min or less.

4. A positive electrode containing the NCM-based active material for a positive electrode according to Claim 1.

5. A battery containing the positive electrode according to Claim 4.

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