Method for manufacturing positive electrode active material

Abstract

To provide a method for producing a positive electrode active material in which carbon is uniformly permeated even within the material. [Solution] The method for producing a positive electrode active material includes (a) preparing secondary particles which are aggregates of primary particles, and (b) permeating molten carbon, obtained by melting a carbon material, between the primary particles, wherein the primary particles contain an olivine-type phosphate compound and carbon is attached to at least a portion of the surface of the primary particles.

Description

[Technical Field]

[0001] This disclosure relates to a method for producing a positive electrode active material. [Background technology]

[0002] Japanese Patent Publication No. 2003-203628 (Patent Document 1) discloses that conductive paths made of carbon are incorporated inside the particles of a positive electrode active material containing an olivine-type phosphate compound, and that conductive paths made of carbon are incorporated by calcining a mixture of the olivine-type phosphate compound material and an organic substance. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2003-203628 [Overview of the project] [Problems that the invention aims to solve]

[0004] Olivine-type phosphate compounds are being investigated as cathode active materials. Conventionally, it has been proposed to enhance the electronic conductivity of olivine-type phosphate compounds by coating the primary particles with carbon.

[0005] However, the method described in Patent Document 1 makes it difficult to penetrate carbon into the interior when producing high-density olivine-type phosphate compounds. As a result, poor conductivity may lead to a deterioration of battery characteristics.

[0006] The purpose of this disclosure is to provide a method for producing a positive electrode active material in which carbon is uniformly permeated even within the material. [Means for solving the problem]

[0007] The technical configuration and effects of this disclosure are described below. However, the mechanism of action of this disclosure includes assumptions. The mechanism of action does not limit the technical scope of this disclosure.

[0008] [1] (a) Prepare secondary particles which are aggregates of primary particles, (b) Permeating the molten carbon obtained by melting the carbon material between the primary particles, Includes, The primary particles contain an olivine-type phosphate compound. A method for producing a positive electrode active material, wherein carbon is attached to at least a portion of the surface of the primary particles.

[0009] By using molten carbon, it is expected that a positive electrode active material with uniform carbon permeation throughout can be obtained. Furthermore, it is expected that the capacity degradation associated with battery use will be suppressed in batteries using such a positive electrode active material.

[0010] [2] The carbon material is pitch, the method for producing a positive electrode active material according to [1].

[0011] Any carbon material is acceptable as long as it contains carbon. Examples of carbon materials include pitch.

[0012] [3] The method for producing a positive electrode active material according to [1] or [2], wherein (b) comprises heating the mixture of the carbon material and the secondary particles under a reduced pressure environment.

[0013] By heating a mixture of carbon material and secondary particles under reduced pressure, it is expected that carbon will permeate more uniformly.

[0014] [4] The method for producing a positive electrode active material according to any one of [1] to [3], wherein the maximum Ferret diameter of the primary particles is 100 nm or less.

[0015] [5] The method for producing a positive electrode active material according to any one of [1] to [4], wherein the amount of the carbon material is 0.5% or more and 5% or more by mass fraction with respect to the secondary particles.

[0016] By using a specific amount of carbon material for the secondary particles, it is expected that carbon will penetrate uniformly.

[0017] Hereinafter, embodiments of the present disclosure (hereinafter may be abbreviated as "the present embodiment") will be described. However, the present embodiment does not limit the technical scope of the present disclosure. The present embodiment is illustrative in all respects. The present embodiment is non-limiting. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the description of the claims. For example, any configurations are extracted from the present embodiment and the present examples, and their arbitrary combinations are also initially planned.

Brief Description of the Drawings

[0018] [Figure 1] It is a schematic flowchart of the method for manufacturing a positive electrode active material in the present embodiment. [Figure 2] It is a table showing the manufacturing conditions and experimental results of the positive electrode active material in the examples.

Modes for Carrying Out the Invention

[0019] <Terms and Phrases> "Comprising", "including", "having", and their modifications are open-ended expressions. A configuration expressed in an open-ended manner may further include additional elements in addition to the essential elements, or may not include them.

[0020] "D50" indicates the particle diameter at which the integrated value becomes 50% in the volume-based particle size distribution (integrated distribution). D50 is measured, for example, by a laser diffraction particle size distribution measuring device.

[0021] "Maximum Feret diameter" indicates the length of the long side of the circumscribed rectangle (rectangle or square) of the particle. When the circumscribed rectangle is a square, the length of the long side indicates the length of one side.

[0022] <Method for Manufacturing Positive Electrode Active Material> Figure 1 is a schematic flowchart of the method for producing the positive electrode active material in this embodiment. Hereinafter, "the method for producing the positive electrode active material in this embodiment" may be abbreviated as "this manufacturing method". This manufacturing method includes "(a) preparation" and "(b) infiltration". "(a) preparation" in this manufacturing method may further include, for example, "(a1) slurry formation", "(a2) granulation", and "(a3) first calcination". This manufacturing method may further include, for example, "(c) second calcination".

[0023] (a) Preparation This manufacturing method involves preparing secondary particles, which are aggregates of primary particles. The primary particles contain an olivine-type phosphate compound. At least a portion of the surface of the primary particles has carbon attached to it.

[0024] The positive electrode active material includes secondary particles. The positive electrode active material may be an aggregate of multiple secondary particles. That is, the positive electrode active material may be a powder. The D50 of the powder may be, for example, 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more. The D50 may be, for example, 30 μm or less, 25 μm or less, 20 μm or less, or 15 μm or less.

[0025] Secondary particles can have any shape. For example, secondary particles may be spherical, rod-shaped, angular, etc. If the secondary particles are spherical, for example, improved packing performance can be expected. The sphericity of the secondary particles may be, for example, 0.85 or higher, 0.90 or higher, or 0.95 or higher. The sphericity of the secondary particles may be, for example, 1 or less, 0.95 or less, or 0.90 or less. "Sphericity" refers to the circularity in an SEM (Scanning Electron Microscope) image (two-dimensional image). Sphericity (circularity) is calculated by the following formula. ψ = 4πS / L 2 ψ: Sphericity (Circularity) π: Pi S: Cross-sectional area of ​​secondary particle (area of ​​the region enclosed by the contour line of secondary particle 2) L: Circumference of secondary particle (length of the outline of secondary particle 2) Sphericity represents the arithmetic mean of 30 secondary particles.

[0026] Secondary particles are aggregates of primary particles. Primary particles can have any shape. Primary particles may be spherical, rod-shaped, angular, etc. Primary particles may be nanoparticles. The maximum Ferret diameter of primary particles may be, for example, 100 nm or less. The maximum Ferret diameter of primary particles may be, for example, 80 nm or less, or 60 nm or less. The maximum Ferret diameter of primary particles may be, for example, 10 nm or more, 20 nm or more, 30 nm or more, 40 nm or more, 50 nm or more, 60 nm or more, 70 nm or more, or 80 nm or more. The maximum Ferret diameter of primary particles represents the arithmetic mean of 30 primary particles.

[0027] Carbon is attached to at least a portion of the surface of the primary particles. The carbon may form a carbon layer. The amount of carbon attached may be, for example, 0.1% or more, 0.5% or more, 1% or more, 2% or more, 3% or more, or 4% or more by mass fraction relative to the secondary particles. The amount of carbon attached may also be, for example, 5% or less, 4% or less, or 3% or less by mass fraction relative to the secondary particles.

[0028] The primary particles contain an olivine-type phosphate compound. "Olivine-type" refers to a crystalline structure belonging to the space group Pnma. The space group is identified by powder X-ray diffraction (XRD) measurement. The primary particles may be, for example, a single-phase compound. The primary particles may further contain phases belonging to other space groups, as long as they contain an olivine-type crystalline phase. The primary particles may further contain, for example, an amorphous phase.

[0029] Olivine-type phosphate compounds may include, for example, lithium iron phosphate (LFP), lithium manganese phosphate (LMP), etc. In LMP, some of the manganese (Mn) may be substituted with iron (Fe). The Fe-substituted form of LMP is also written as lithium iron manganese phosphate (LMFP). LMP may have a composition represented by, for example, the following general formula. Li 1-a Mn1-x Fe x PO4 For example, the relationships -0.5 ≤ a ≤ 0.5 and 0 ≤ x ≤ 1 may also be satisfied.

[0030] (a1) Formation of slurry This method may include forming a slurry by mixing, for example, a lithium compound, a manganese compound, an iron compound, a phosphate compound, a carbon source, and a solvent. For example, a compound with the composition formula "Li 1-a Mn 1-x Fe x Lithium compounds, manganese compounds, phosphate compounds, and iron compounds may be weighed out to achieve the composition ratio (mole ratio) shown in PO4 (-0.5 ≤ a ≤ 0.5, 0 ≤ x ≤ 1). The lithium compound may include, for example, lithium carbonate, lithium hydroxide, etc. The manganese compound may include, for example, manganese carbonate, etc. The phosphate compound may include, for example, phosphoric acid, lithium dihydrogen phosphate, etc. The iron compound may include, for example, iron oxalate, ferric phosphate, etc.

[0031] The carbon source is a raw material for carbon that adheres to the surface of the primary particles. The carbon source may include, for example, sugars, organic acids, etc. The carbon source may include, for example, glucose, sucrose, fructose, citric acid, lactic acid, etc. The carbon source may include, for example, sugars. The amount of carbon source added may be, for example, 1 to 20% by mass fraction relative to the raw material mixture. The carbon source may also be added after forming a granule using a raw material mixture of other raw materials. In this case, the amount of carbon source added may be, for example, 1 to 20% by mass fraction relative to the granule.

[0032] In addition to the carbon material described later, it is preferable to include a carbon source in this process. By doing so, carbon can be attached to the surface of the primary particles in advance, and then carbon can be attached to the surface of the primary particles in step (b), which is expected to result in more uniform carbon penetration.

[0033] The solvent may include, for example, water. The solid content concentration of the slurry may be, for example, 20-40% by mass fraction.

[0034] The particle size in the slurry may be adjusted by wet grinding. For example, wet grinding may be performed so that D50 is between 0.10 and 1 μm.

[0035] (a2) Granulation This method may include, for example, granulating secondary particles by drying the slurry. For example, secondary particles may be granulated by spray drying. The secondary particles formed by the granulation operation are also called "granulated bodies." In other words, secondary particles may be referred to as granulated bodies.

[0036] (a3) First firing This method may include forming precursor particles by subjecting secondary particles to heat treatment (first calcination). Any heat treatment furnace (e.g., electric furnace, muffle furnace, etc.) can be used. The heat treatment atmosphere may be, for example, an inert atmosphere. The inert atmosphere may be, for example, a nitrogen atmosphere. The heat treatment temperature may be, for example, 400 to 700°C. The heat treatment time may be, for example, 1 to 6 hours.

[0037] (b) Penetration This manufacturing method involves impregnating the primary particles with molten carbon obtained by melting a carbon material.

[0038] Any carbon material is acceptable as long as it contains carbon. Examples of carbon materials include pitch. The carbon material may have a softening point of, for example, 50 to 150°C, or 100 to 120°C.

[0039] The melting temperature can be adjusted as appropriate depending on the carbon material. For example, the melting temperature may be higher than the softening point of the carbon material. For instance, if the softening point of the carbon material is 110°C, melting may be carried out at around 150°C.

[0040] The melting of the carbon material may be carried out, for example, under reduced pressure. The melting of the carbon material may also be carried out, for example, under vacuum. By carrying out this process under such conditions, it is expected that the carbon will penetrate more uniformly.

[0041] The penetration of molten carbon between primary particles may be carried out, for example, by mixing secondary particles (precursor particles) with the molten carbon.

[0042] The penetration of molten carbon between primary particles and the melting of the carbon material may be carried out simultaneously. For example, after a mixture is formed by adding a carbon material to precursor particles, the mixture may be heated (melted). For example, the heating of the mixture may be carried out under reduced pressure or under vacuum. By carrying out this process under such conditions, the penetration of molten carbon between primary particles and the melting of the carbon material simultaneously are expected to result in more uniform carbon penetration.

[0043] The amount of carbon material may be, for example, 0.5% to 5% or more in mass fraction relative to the secondary particles (precursor particles). The amount of carbon material may be 1% to 4% or 1% to 3% in mass fraction relative to the secondary particles (precursor particles).

[0044] (c) Second firing This manufacturing method may further include heat treatment (second firing).

[0045] The heat treatment atmosphere may be, for example, an inert atmosphere. The inert atmosphere may be, for example, a nitrogen atmosphere. The heat treatment temperature may be, for example, 400 to 700°C. The heat treatment time may be, for example, 1 to 6 hours.

[0046] (others) In this embodiment, the penetration of carbon into the positive electrode active material may be evaluated, for example, by the relaxation rate and the Li elution amount measured by the method described in the examples described later. For example, the relaxation rate of the positive electrode active material obtained by this manufacturing method may be 2 or more when the relaxation rate of the positive electrode active material manufactured by the prior art is set to 1. For example, the Li elution amount of the positive electrode active material obtained by this manufacturing method may be 0.7 or less when the Li elution amount of the positive electrode active material manufactured by the prior art is set to 1.

Examples

[0047] <Manufacture of positive electrode active material> (No.1) Composition formula "Li1Mn 0.8 Fe 0.2 PO4", lithium carbonate, manganese carbonate, iron oxalate, and phosphoric acid were weighed so as to have the composition ratio shown. After dispersing each of these raw materials in water to form a slurry, it was pulverized with a bead mill at a peripheral speed of 10 m / sec for 30 minutes using beads with a diameter of 0.3 mm, and then fired to obtain granulated bodies. To the granulated bodies, fructose with a mass fraction of 10% was added and formulated. After drying this formulation liquid, it was heated to a firing temperature of 650°C at a heating rate of 5°C / min in an inert atmosphere, maintained for 1 hour, and then cooled to room temperature to obtain precursor particles.

[0048] The obtained precursor particles were heated to a firing temperature of 600°C at a heating rate of 5°C / min again in an inert atmosphere, maintained for 1 hour, and then cooled to room temperature to manufacture a positive electrode active material (LMFP).

[0049] (No.2) Precursor particles were formed in the same process as No.1. To the precursor particles, a pitch (softening point: 110°C) as described in FIG. 2 was added at a mass fraction to form a mixture. The mixture was stirred and mixed for 1 minute at a stirring speed of 2000 rpm with a mixer (product name "Awatori Ren Taro", manufactured by Shinky Co., Ltd.).

[0050] The resulting mixture was heated in an inert atmosphere at a heating rate of 5°C / min to a calcination temperature of 600°C, maintained for 1 hour, and then cooled to room temperature to produce the positive electrode active material.

[0051] (No.3) The mixture was formed and stirred using the same process as in No. 2. The mixture was heated (melted) at 150°C under vacuum conditions in a vacuum drying container. Subsequently, the positive electrode active material was manufactured by calcining the mixture under the same conditions as in No. 2.

[0052] (No.4) The positive electrode active material was manufactured in the same manner as in No. 3, except that pitch was added to the precursor particles in mass fraction as shown in Figure 2.

[0053] <Measurement> (relaxation rate) Each positive electrode active material (No.) was dispersed in a solvent without the electrolyte, and measurements were performed using time-resolved nuclear magnetic resonance (TD-NMR). The normalized relaxation rate was determined by comparing the time it took for the initial magnetization intensity to decrease to 20%. The results are shown in Figure 2. Note that the relaxation rates in Figure 2 are relative values ​​with the relaxation rate of No. 1 set to 1.

[0054] (Li elution amount) 0.1 g of each No. positive electrode active material was dispersed in 50 g of water and allowed to stand for 24 hours. After standing, the positive electrode active material was removed by filtration with a 0.2 μm membrane filter to obtain the filtrate. The amount of Li eluted was determined by measuring the Li concentration in the filtrate using ICP-OES (Intracellular Carbon Emission Spectroscopy). The results are shown in Figure 2. Note that the Li elution amounts in Figure 2 are relative values ​​with the Li elution amount of No. 1 set to 1.

[0055] <Battery Characteristics> (Coin cell production) A mixture was formed by mixing the positive electrode active material, conductive material (acetylene black), and binder (PVdF). The mixing ratio (mass ratio) was "positive electrode active material / conductive material / binder = 92 / 5 / 3". A paste was formed by dispersing the mixture in a solvent (N-methyl-2-pyrrolidone). The solid content concentration of the paste was 50% by mass fraction. The positive electrode layer was formed by applying the paste to the surface of aluminum foil and drying it. The density of the positive electrode layer was 1.8 g / cm³ by roll pressing. 3 By adjusting the material, a cathode base was formed. The cathode base was subjected to vacuum drying at 120°C for 12 hours. After drying, a disc sample (diameter: 14 mm) was removed from the cathode base by punching.

[0056] The coin cell was assembled inside the glove compartment. The cell configuration is as follows: Working electrode: Disc sample (positive electrode) Opposite pole: Li foil Separator: Polymer porous membrane Electrolyte: Ethylene carbonate / dimethyl carbonate = 3 / 7 (volume ratio), LiPF6 (1 ml / L)

[0057] (Capacity maintenance rate) The discharge capacity of the obtained coin cells was determined based on the discharge capacity estimated from the coating weight. "C" is a symbol indicating the current rate (time rate). At a rate of 1C, the theoretical capacity is supplied over one hour. Under conditions of 25°C, the discharge capacity at 0.1C (referred to as the "first discharge capacity") was determined after CCCV charging with a charge rate of 0.1C, an upper voltage limit of 4.3V, and a charge termination condition of 0.01C, followed by a discharge termination potential of 3V. After that, the battery was fully charged again and held at a temperature of 60°C for 14 days, and the discharge capacity at 0.1C (referred to as the "second discharge capacity") was determined again. The capacity retention rate was calculated as (second discharge capacity / first discharge capacity) × 100%. The results are shown in Figure 2. A higher capacity retention rate is considered to indicate better durability.

[0058] <Result> As shown in Figure 2, when the manufacturing conditions of this disclosure are met, the relaxation time tends to be longer and the amount of Li elution tends to be reduced. In other words, it is considered that carbon permeates uniformly even inside. Furthermore, when the manufacturing conditions of this disclosure are met, the volume retention rate tends to be high.

Claims

1. (a) Prepare secondary particles, which are aggregates of primary particles. (b) Permeating the molten carbon obtained by melting the carbon material between the primary particles, Includes, The primary particles contain an olivine-type phosphate compound. A method for producing a positive electrode active material, wherein carbon is attached to at least a portion of the surface of the primary particles.

2. The method for producing a positive electrode active material according to claim 1, wherein the carbon material is pitch.

3. The method for producing a positive electrode active material according to claim 1 or 2, wherein (b) comprises heating the mixture of the carbon material and the secondary particles under a reduced pressure environment.

4. The method for producing a positive electrode active material according to claim 1 or claim 2, wherein the maximum Ferret diameter of the primary particles is 100 nm or less.

5. The method for producing a positive electrode active material according to claim 1 or claim 2, wherein the amount of the carbon material is 0.5% or more and 5% or more by mass fraction with respect to the secondary particles.

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