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 ratio c / a of the length of an a-axis to the length of a c-axis in a crystal structure obtained from an X-ray diffraction pattern analysis result is 4.9625 at largest.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 replaced with additive elements to shorten the bond distance between oxygen atoms and transition metal atoms.

Prior Art Document

Patent Document

[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 is likely to generate heat and release oxygen during charge and discharge, and tends to have low thermal stability. When nickel elements are replaced 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, wherein the ratio c / a of the length of the a-axis to the length of the c-axis in the crystal structure obtained from the X-ray diffraction pattern analysis result is 4.9625 or less. [2] The NCM-based active material for a positive electrode according to [1], wherein the ratio c / a is 4.9619 or more. [3] The NCM-based active material for a positive electrode according to [1] or [2], comprising agglomerated particles having a diameter (D50) of the secondary particles 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], wherein the weight loss amount in thermogravimetric analysis at a temperature of 120 to 600 °C and a heating rate of 5 °C / min is 12% by mass or less. [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 a temperature of 120 to 600 °C and a heating rate of 5 °C / min is 0.40% by mass / min or less. [6] A positive electrode comprising the NCM-based active material for a positive electrode 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 cycle characteristics. [Brief Description of the Drawings]

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

[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 other than 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 by, for example, a coprecipitation method. 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 Li:(Ni + Co + Mn) of lithium to nickel, cobalt, and manganese in the lithium nickel cobalt manganese composite oxide 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 include, for example, a first layered metal oxide. The first layered metal oxide is represented by formula (1): Li 1-a1 Ni x1 Me 1 1-x1 O2(1) In 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 "-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, 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 ratio c / a (hereinafter also referred to as ratio c / a) of the length of the a-axis to the length of the c-axis in the crystal structure obtained from the X-ray diffraction pattern analysis result of 4.9625 or less. By the ratio c / a being within the above range, excellent thermal stability can be exhibited. The X-ray diffraction pattern analysis can be carried out according to the method described in the column of Examples below. The ratio c / a is preferably 4.9619 or more 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 including 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 ratio c / a 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 respectively. 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 visually confirmed 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 a single 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, for example, an average primary particle diameter of 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 can have a weight loss rate per minute of 0.40 mass% / min or less 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 can 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> Figure 1 is a schematic diagram showing an example of a lithium-ion battery according to the present embodiment. The battery 100 shown in Figure 1 may be a lithium-ion battery, such as the 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 directly or indirectly formed 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-described 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 the present embodiment includes preparation (A), coating (B), and rolling (C) of the positive electrode slurry as shown in FIG. 4. In the preparation (A) of the positive electrode slurry, the 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 active material layer 12 is formed by applying the positive electrode slurry onto the surface of the positive electrode substrate 11. 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 mass % and parts by mass, respectively, unless otherwise specified.

[0028] [Fabrication of Positive Electrode for Cycle Characteristics Evaluation] 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 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 were mixed, 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.

[0029] [Synthesis of 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 subjected to a first firing step and a second firing step to obtain a second positive electrode active material (lithium nickel composite oxide having a particle size of 3 to 6 μm). The average particle size D50 of the second positive electrode active material was 4.0 μm.

[0030] [Fabrication of 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 mixture slurry. The slurry was applied to a current collector made of aluminum foil to form a positive electrode mixture layer. 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.

[0031] [Fabrication of Negative Electrode] The negative electrode active material, carboxymethyl cellulose (CMC) as a thickener, and styrene-butadiene rubber (SBR) as a binder were weighed so as to have a mass ratio of 98:1:1, and these were dispersed in water to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to a current collector made of copper foil to form a negative electrode mixture layer. 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.

[0032] [Preparation of 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 at 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.

[0033] [Evaluation of c / a Ratio] The powder sample of the first positive electrode active material was filled into the 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. Cu was used as the tube target (target element), the tube voltage was 45 kV, and the tube current was 200 mA. The crystal peak data obtained from the X-ray diffraction pattern was subjected to Rietveld analysis to calculate the a-axis length and c-axis length in the crystal (crystal system: trigonal system, space group: R-3m), and the c / a ratio was determined.

[0034] [Evaluation of cycle characteristics] The positive electrode and negative electrode obtained by the method for producing a positive electrode plate for evaluating cycle characteristics were laminated with the positive electrode and negative electrode via 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. For 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 , and the charging capacity was determined. After a pause of 10 minutes, constant current discharging was performed at a current density of 0.2 mA / cm 2 until a voltage of 3.0 V was reached, and the discharging capacity was determined.

[0035] 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 discharging 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

[0036] [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 the positive electrode were wound with a polyolefin separator interposed therebetween 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.

[0037] (Method for creating a sampling for TG measurement) The above cell was subjected to constant current charging at a current density of 0.2 mA / cm 2 until it reached 4.35 V, and then further subjected to constant voltage charging at 4.35 V until the current density reached 0.04 mA / cm 2 . After that, constant current discharging was performed until the current density reached 0.04 mA / cm 2 and the voltage reached 2.5 V. After repeating the above charge-discharge conditions for one cycle, constant current charging was performed at a current density of 0.2 mA / cm 2 until it reached 4.40 V, and then further subjected to constant voltage charging at 4.40 V until the current density reached 0.04 mA / cm 2 . 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 of the corresponding positive electrode plate to obtain a sample for TG measurement.

[0038] (TG measurement method) 20 mg of the above sample was weighed into a TG pan of the measuring device (measured with a DTG-60AH manufactured by SHIMADZU) and set in the device. Measurement was performed under an Ar atmosphere at a heating rate of 5 °C / min up to a temperature limit of 600 °C. The weight loss amount and the weight loss rate per minute were calculated in the range of 120 °C to 600 °C.

[0039] <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 size D50 of the first positive electrode active material was 16.0 μm, and the ratio c / a of the crystal lattice c-axis to the a-axis was 4.9619. <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 c / a was controlled by controlling the parameters of the first and second firing steps in Example 1. The results are shown in Table 1.

[0040]

Table 1

[0041] In Table 1, cases where the cycle characteristics were 93.3% or more, the weight reduction amount was 12% or less, and the weight reduction rate per minute was 0.40% / min or less were judged as ○, and others were judged as ×. In Examples 1 to 4 according to the present invention, it was possible to achieve both high thermal stability and improved cycle characteristics.

Explanation of Signs

[0042] 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 ratio c / a of the length of the a-axis to the length of the c-axis in the crystal structure obtained from the X-ray diffraction pattern analysis result is 4.9610 or more and 4.9625 or less, represented by the following formula (1): Li 1-a1 Ni x1 Me 1 1-x1 O 2 (1) wherein in formula (1), "a1" satisfies the relationship of "-0.3 ≤ a1 ≤ 0.3", "x1" satisfies the relationship of 0.55 ≤ 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]. The NCM-based active material for a positive electrode contains a first layered composite oxide represented by the formula.

2. The NCM-based active material for a positive electrode according to claim 1, wherein the ratio c / a is 4.9619 or more.

3. The NCM-based active material for a positive electrode according to claim 1, which contains agglomerated particles having an average particle diameter (D50) of secondary particles of 14 μm or more and 18 μm or less.

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

5. The NCM-based active material for a positive electrode according to claim 1, wherein the weight loss rate per minute in thermogravimetric analysis at a temperature of 120 to 600 °C and a heating rate of 5 °C / min is 0.40% by mass / min or less.

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

7. A battery containing the positive electrode according to claim 6.

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