NCM-based active materials for positive electrodes, positive electrodes and batteries

Abstract

To provide an active material based on NCM for a positive electrode which can exhibit a high heat stability and an improved low-temperature low-SOC output 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 technology]

[0002] Japanese Patent Laid-Open Publication No. 2017-162790 (Patent Document 1) discloses a positive electrode active material for a non-aqueous electrolyte secondary battery in which the nickel atom at the 3b site of a lithium metal composite oxide is substituted with an additive element, thereby shortening the bond distance between the oxygen atom and the transition metal atom. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-162790 Summary of the Invention [Problem to be solved by the invention]

[0004] Positive electrodes containing lithium-nickel composite oxides tend to generate heat and release oxygen during charge and discharge, resulting in poor thermal stability. When nickel is replaced with an additive element to improve thermal stability, the active material containing the additive element tends to have poor low-temperature, low-SOC output characteristics compared to active materials that do not contain the additive element.

[0005] The 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 low-temperature and low-SOC output characteristics. [Means for solving the problem]

[0006] The present invention provides the following NCM-based active material for a positive electrode, a positive electrode, and a battery. [1] NCM-based active material for positive electrodes with a crystallite size of 460 Å or more. [2] The crystallite size is 580 Å belowThe NCM-based active material for a positive electrode according to [1], [3] The NCM-based active material for a positive electrode according to [1] or [2], which contains agglomerated particles having a secondary particle diameter D50 of 14 μm or more and 18 μm or less. [4] The NCM-based active material for a positive electrode according to any one of [1] to [3], which has a weight loss of 12% by mass or less in thermogravimetric analysis at 120 to 600°C at a heating rate of 5°C / min. [5] The NCM-based active material for a positive electrode according to any one of [1] to [4], wherein the weight loss rate per minute in thermogravimetric analysis at 120 to 600°C at a heating rate of 5°C / min is 0.40 mass% / min or less. [6] A positive electrode comprising the NCM-based positive electrode active material according to any one of [1] to [5]. [7] A battery comprising the positive electrode according to [6]. [Effects 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 low-temperature, low-SOC output characteristics. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram showing an example of a lithium ion battery according to this embodiment. [Figure 2] FIG. 2 is a schematic diagram showing an example of an electrode body in this embodiment. [Figure 3] FIG. 3 is a conceptual diagram showing the positive electrode in this embodiment. [Figure 4] FIG. 4 is a schematic flowchart of the method for producing a positive electrode in this embodiment. DETAILED DESCRIPTION OF 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 of the drawings, the scales of the components are appropriately adjusted to make them easier to understand, and the scales of the components shown in the drawings do not necessarily match the scales of the actual components.

[0010] <Active material> The active material contains one or more nickel-, cobalt-, and manganese-containing compounds. The nickel content ratios relative to the metal elements excluding lithium in the active material may be, for example, 50 mol% or more and 60 mol% or more, respectively, and are preferably 60 mol% or more and 70 mol% or more, respectively. The nickel-, cobalt-, and manganese-containing compound preferably contains 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 may be, for example, a compound represented by the general formula: Ni x Co y Mn z The active material may be a compound represented by (OH)2 (wherein x+y+z=1). The lithium nickel cobalt manganese composite oxide may have a molar ratio of lithium to nickel, cobalt, and manganese, Li:(Ni+Co+Mn), of, for example, 1.0 to 1.2:1.0. The active material may be for use in a lithium ion battery. Details of the lithium ion battery will be described later.

[0011] The active material may include, for example, a first layered metal oxide represented by formula (1): Li 1-a1 Ni x1 Me 1 1-x1 O2(1) In formula (1), "a1" satisfies the relationship "-0.3≦a1≦0.3", "x1" satisfies the relationship 0.5≦x1<1.0, and "Me 1 " indicates 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 "-0.3 ≤ a2 ≤ 0.3", "x2" satisfies the relationship 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 "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 O2.

[0015] The active material has a crystallite size of 460 Å or more. A crystallite size within the above range can exhibit excellent thermal stability. The crystallite size can be determined according to the method described in the Examples section below. From the perspective of low-temperature, low-SOC output characteristics, the active material preferably has a crystallite size of 580 Å or less.

[0016] When the active material contains a lithium nickel cobalt manganese composite oxide, the active material can be synthesized through two firing steps: a first firing step in which a first mixture containing a lithium compound and a transition metal compound is fired, and a second firing step in which a second mixture containing the fired product obtained in the first firing step and the transition metal compound is fired. The crystallite size can be controlled as desired by controlling the firing parameters (e.g., firing temperature and firing time) in the first and second firing steps.

[0017] The active material may contain agglomerated particles with an average secondary particle diameter D50 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 is 50% of the total particle volume. The average particle diameter can be measured by laser diffraction / scattering.

[0018] The primary particles constituting the secondary particles are particles whose grain boundaries cannot be visually identified in SEM images of the particles. The average primary particle diameter is the distance between the two most distant points on the outline of the primary particle. The average primary particle diameter of the primary particles may be, for example, 0.05 μm to 0.7 μm or 0.1 μm to 0.6 μm. When 10 or more primary particles randomly sampled from an SEM image of a single agglomerate particle have an average primary particle diameter of 0.05 μm to 0.2 μm, all of the primary particles contained in the agglomerate are considered to have 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 may further include 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 may have a weight loss of 12% by mass or less in thermogravimetric analysis at a temperature of 120 to 600°C and a heating rate of 5°C / min. The active material may also have a weight loss rate per minute of 0.40% by mass / min or less in thermogravimetric analysis at a temperature of 120 to 600°C and a heating rate of 5°C / min. 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 low-temperature, low-SOC output resistance of, for example, 4.664 Ω or less, and preferably 4.586 Ω or less. The low-temperature, low-SOC output resistance is measured 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 or a power source for power assist of an electric vehicle.

[0023] The battery 100 includes an exterior body 90. The exterior body 90 houses an electrode assembly 50 and an electrolyte (not shown). The electrode assembly 50 is connected to a positive electrode terminal 91 by a positive electrode current collecting member 81. The electrode assembly 50 is connected to a negative electrode terminal 92 by a negative electrode current collecting member 82. FIG. 2 is a schematic diagram showing an example of an electrode assembly in this embodiment. The electrode assembly 50 is a wound type. The electrode assembly 50 includes a positive electrode 20, a separator 40, and a negative electrode 30. That is, the battery 100 includes a 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, a positive electrode 20 may have a positive electrode active material layer 22 formed directly or indirectly on one or both sides of a 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 contains the above-mentioned NCM-based active material for a positive electrode.

[0025] The positive electrode active material layer 22 may have a thickness of, for example, 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 is, for example, 3.5 g / cm. 3 may be 3.6 g / cm or more, 3 or more than 3.7g / cm 3 The positive electrode active material layer 22 may have a density of, for example, 4.0 g / cm 3 It may have the following densities:

[0026] <Positive electrode manufacturing method> The manufacturing method of the positive electrode 20 in this embodiment includes (A) preparation of a positive electrode slurry, (B) application, and (C) rolling, as shown in FIG. 4 . In (A) preparation 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 (B) application, the positive electrode slurry is applied to the surface of the positive electrode substrate 11 to form a positive electrode active material layer 12. In (C) rolling, the positive electrode active material layer 12 and the positive electrode substrate 11 are rolled together to manufacture the positive electrode 10. A raw sheet of the positive electrode 10 is manufactured by rolling. The raw sheet can be cut to a predetermined planar size according to the specifications of the battery 100. [Example]

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

[0028] [Preparation of positive electrodes for evaluating low-temperature, low-SOC output characteristics] The positive electrode active material 1 prepared in the examples and comparative examples was mixed with the positive electrode active material 2 prepared as described below in a 1:1 ratio, and the mixture was mixed to a concentration of 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. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was then 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. The slurry was then dried and rolled using a rolling roller until the positive electrode composite density reached 3.55 g / cm. 3 The resultant was compressed to a predetermined size, cut into pieces, and an aluminum tab was attached to form a positive electrode.

[0029] [Synthesis of second positive electrode active material] LiOH and Ni 0.60 Co 0.20 Mn 0.20 A transition metal compound represented by (OH)2 was mixed and subjected to a first firing step and a second firing step to obtain a second positive electrode active material (lithium nickel composite oxide with a particle diameter of 3 to 6 μm). The average particle diameter D50 of the second positive electrode active material was 4.0 μm.

[0030] [Preparation of positive electrode for TG measurement] A positive electrode composite slurry was prepared by mixing 89.0 parts by weight of the positive electrode active material 1 prepared in the Examples and Comparative Examples, 1.0 part by weight of carbon black as a conductive material, and 10 parts by weight of polyvinylidene fluoride (PVdF) as a binder, and then adding an appropriate amount of N-methyl-2-pyrrolidone (NMP). The slurry was applied to a current collector made of aluminum foil to form a positive electrode composite layer. The mixture was then dried, rolled to a predetermined thickness using a rolling roller, cut to a predetermined size, and attached with an aluminum tab to form a positive electrode.

[0031] [Preparation of negative electrode] The negative electrode active material, carboxymethyl cellulose (CMC) as a thickener, and styrene-butadiene rubber (SBR) as a binder were weighed out in a mass ratio of 98:1:1 and dispersed in water to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was applied to a copper foil current collector to form a negative electrode mixture layer. The negative electrode mixture layer was then dried, rolled to a predetermined thickness using a rolling roller, cut to a predetermined size, and attached with a nickel tab to form a negative electrode.

[0032] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 30:30:40. Lithium hexafluorophosphate (LiPF6) was added to the mixed solvent to a concentration of 1.15 mol / L. Furthermore, vinylene carbonate (VC) was added to the mixed solvent at a concentration of 1.0 mass% relative to the total mass of the mixed solvent to prepare a nonaqueous electrolyte.

[0033] [Evaluation of crystallite size] A powder sample of the first positive electrode active material was placed in a dedicated folder of a fully automated multipurpose X-ray diffractometer (Rigaku SmartLab) and measured using the reflection method to obtain an X-ray diffraction pattern. A Cu tube (target element) was used, with a tube voltage of 45 kV and a tube current of 200 mA. From the measurement results, the crystallite size was calculated from the half-width (d) of the crystal peak (104 plane) appearing at 2θ = 44 to 45°.

[0034] [Evaluation of low temperature and low SOC output characteristics] The low-temperature, low-SOC output resistance was measured by the following procedure. The positive electrode for evaluating low-temperature output characteristics prepared as described above and the negative electrode were laminated with a polyolefin separator between them to prepare a laminated electrode body. This electrode body was housed in an exterior body made of an aluminum laminate sheet, and after the nonaqueous electrolyte was injected, the opening of the exterior body was sealed to obtain a test cell. For the above test cell, a current of 0.2 mA / cm was measured at a temperature of 25°C. 2The battery was charged at a constant current density of 0.04 mA / cm until the voltage reached 4.25 V. 2 After a 10-minute break, the battery was charged at a constant voltage of 0.2 mA / cm 2 The battery was discharged at a constant current until the current density reached 3.0 V, and the discharge capacity was determined.

[0035] The battery measurement environment temperature was lowered to -30°C, and the above cell was subjected to a current of 0.2mA / cm 2 The battery was charged at a constant current density of 0.04 mA / cm until the voltage reached 3.44 V. 2 After charging at a constant voltage until the voltage reached 0.4, 0.6, 0.8, 1.0, and 1.2 mA / cm, the battery was discharged at a constant current of 0.2 mA / cm for 10 seconds, and the voltage was measured after 10 seconds. 2 Measurements were taken at each current density, and the 10-second DC resistance was calculated from the slope of the current and voltage values.

[0036] [Evaluation of thermal stability] (Preparation of test cell) The positive electrode obtained by the method for preparing a positive electrode plate for TG measurement and metallic lithium were wound with a polyolefin separator between them to prepare a wound electrode body. This electrode body was housed in an exterior body made of an aluminum laminate sheet, and the nonaqueous electrolyte was poured into it. The opening of the exterior body was then sealed to obtain a test cell. (How to prepare a sample for TG measurement) The cell was charged at 0.2 mA / cm under a temperature condition of 25°C. 2 The battery was charged at a constant current density of 0.04 mA / cm until the voltage reached 4.35 V. 2 After constant voltage charging until the current density reached 0.04mA / cm 2 After one cycle of the above charge / discharge conditions, the battery was discharged at a constant current of 0.2 mA / cm 2 The battery was charged at a constant current density of 0.04 mA / cm until the voltage reached 4.40 V. 2The cell was disassembled in a glove box, the positive electrode plate was removed, washed with DMC, and vacuum dried. The positive electrode mixture layer was scraped off from the core of the positive electrode plate to prepare a sample for TG measurement. (TG measurement method) 20 mg of the above sample was weighed out and placed in a TG pan of a measuring device (Shimadzu DTG-60AH) and set in the device. Measurements were performed in an Ar atmosphere at a temperature increase rate of 5°C / min up to a maximum temperature of 600°C. The weight loss from 120°C to 600°C and the weight loss rate per minute were calculated.

[0037] Example 1 LiOH and Ni 0.55 Co 0.20 Mn 0.25 A transition metal compound represented by (OH)2 was mixed and subjected to a first baking step and a second baking step to obtain a first positive electrode active material. The first positive electrode active material had an average particle diameter D50 of 16.0 μm and a crystal size of 485 Å. <Examples 2 to 4 and Comparative Examples 1 to 4> A first positive electrode active material was produced in the same manner as in Example 1, except that the crystal size was controlled by controlling the parameters of the first and second baking steps in Example 1. The results are shown in Table 1.

[0038] [Table 1]

[0039] In Table 1, the cases where the low-temperature, low-SOC output characteristics were 4.664Ω or less, the weight loss was 12% or less, and the weight loss rate per minute was 0.40% / min or less were evaluated as O, and the others were evaluated as X. In Examples 1 to 7 according to the present invention, both high thermal stability and improved low-temperature, low-SOC output characteristics were achieved. [Explanation of symbols]

[0040] 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 collecting member, 82 negative electrode current collecting member, 90 outer casing, 91 positive electrode terminal, 92 negative electrode terminal, 100 battery (lithium ion battery).

Claims

1. An NCM-based active material for a positive electrode having a crystallite size of 460 Å or more and 742 Å or less, The secondary particle diameter D50 contains aggregated particles of 14 μm or more and 18 μm or less, LiNi 0.7 Co 0.2 Mn 0.1 O 2 , LiNi 0.7 Co 0.1 Mn 0.2 O 2 , LiNi 0.6 Co 0.3 Mn 0.1 O 2 , LiNi 0.6 Co 0.2 Mn 0.2 O 2 , LiNi 0.6 Co 0.1 Mn 0.3 O 2 and LiNi 0.55 Co 0.20 Mn 0.25 O ２ At least one selected from the group consisting of The crystallite size is measured by a reflection method using a Cu tube at a tube voltage of 45 kV and a tube current of 200 mA from a powder sample of the NCM-based active material for a positive electrode, and is calculated from the half-width (d) of a crystal peak (104 plane) appearing at 2θ=44 to 45° in the resulting X-ray diffraction pattern. NCM-based active material for a positive electrode.

2. 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% by mass or less.

3. 2. 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 0.40 mass % / min or less.

4. A positive electrode comprising the NCM-based active material for positive electrodes according to claim 1 .

5. A battery comprising the positive electrode of claim 4.

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