Method for producing positive electrode active material

Abstract

The present disclosure is a method for producing a positive electrode active material. The method for producing a positive electrode active material includes the following steps: (a) preparing secondary particles, the secondary particles being an aggregate of primary particles; and (b) permeating a molten carbon, which is formed by melting a carbon material, between the primary particles, the primary particles including an olivine-type phosphate compound, and carbon being attached to at least a part of a surface of the primary particles.

Description

Technical Field

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

[0002] Japanese Patent Application Publication No. 2003-203628 discloses the following: a conductive pathway made of carbon is embedded inside the particles of a positive electrode active material containing an olivine-type phosphate compound; the conductive pathway made of carbon is embedded by sintering a mixture of an olivine-type phosphate compound material and an organic material. Summary of the Invention

[0003] Olivine-type phosphate compounds were investigated as positive electrode active materials. Previously, a scheme was proposed to improve the electronic conductivity by coating the primary particles of olivine-type phosphate compounds with carbon.

[0004] However, in the method described in Japanese Patent Application Publication No. 2003-203628, it is difficult to allow carbon to penetrate into the interior when manufacturing high-density olivine-type phosphate compounds. Therefore, poor conductivity may lead to deterioration of battery characteristics.

[0005] The purpose of this disclosure is to provide a method for manufacturing a positive electrode active material in which carbon is uniformly permeated even inside.

[0006] The technical structure and effects of this disclosure are described below. However, the mechanism of action of this disclosure includes conjecture. The mechanism of action does not limit the technical scope of this disclosure.

[0007] [1] A method for manufacturing a positive electrode active material, comprising the following steps:

[0008] (a) Prepare secondary particles, said secondary particles being an aggregate of primary particles; and

[0009] (b) Allowing molten carbon, formed by melting carbon material, to permeate between the primary particles.

[0010] The primary particles contain olivine-type phosphate compounds.

[0011] At least a portion of the surface of the primary particle is covered with carbon.

[0012] By using molten carbon, it is expected that a positive electrode active material with carbon uniformly permeated even internally can be obtained. Furthermore, for batteries using such a positive electrode active material, it is expected that capacity reduction associated with battery use can be suppressed.

[0013] [2] According to the method for manufacturing the positive electrode active material described in [1],

[0014] The carbon material is asphalt.

[0015] Any carbon material can be considered as long as it contains carbon. Examples of carbon materials include asphalt.

[0016] [3] The method for manufacturing the positive electrode active material according to [1] or [2],

[0017] Step (b) includes heating the mixture of the carbon material and the secondary particles under reduced pressure.

[0018] By heating a mixture of carbon materials and secondary particles under reduced pressure, more uniform carbon penetration can be expected.

[0019] [4] The method for manufacturing the positive electrode active material according to any one of [1] to [3],

[0020] The maximum Ferete diameter of the primary particle is less than 100 nm.

[0021] [5] The method for manufacturing the positive electrode active material according to any one of [1] to [4],

[0022] The amount of carbon material relative to the secondary particles is more than 0.5% and less than 5% by mass fraction.

[0023] By using a specific amount of carbon material relative to secondary particles, uniform carbon penetration can be expected.

[0024] The following describes embodiments of this disclosure (hereinafter referred to as "this embodiment"). However, this embodiment does not limit the technical scope of this disclosure. This embodiment is illustrative in all respects. This embodiment is non-limiting. The technical scope of this disclosure includes all modifications within the meaning and scope equivalent to the claims. For example, it is pre-established from the outset that any configuration can be extracted from this embodiment and these embodiments and combined arbitrarily. Attached Figure Description

[0025] The features, advantages, and technical and industrial significance of typical embodiments of the present invention will now be described with reference to the accompanying drawings, in which the same reference numerals denote the same elements, wherein:

[0026] Figure 1 This is a simplified flowchart of the method for manufacturing the positive electrode active material in this embodiment;

[0027] Figure 2 This is a table showing the manufacturing conditions and experimental results of the positive electrode active material in the examples. Detailed Implementation

[0028] Terms and phrases

[0029] "Possess," "include," "have," and their variations are open-ended expressions. An open-ended expression can include additional elements beyond the necessary ones, or it can omit additional elements.

[0030] "D50" indicates the particle size at which the cumulative value in the volumetric particle size distribution (cumulative distribution) reaches 50%. D50 can be measured, for example, using a laser diffraction particle size distribution measuring device.

[0031] The "maximum Feret diameter" refers to the length of the longer side of the circumscribed rectangle (rectangle or square) of a particle. When the circumscribed rectangle is a square, the length of the longer side represents the side length.

[0032] Method for manufacturing positive electrode active material

[0033] Figure 1 This is a simplified flowchart of the method for manufacturing the positive electrode active material in this embodiment. Hereinafter, "the method for manufacturing the positive electrode active material in this embodiment" may be abbreviated as "this manufacturing method". This manufacturing method includes "(a) preparation" and "(b) permeation". "(a) preparation" in this manufacturing method may further include, for example, "(a1) slurry formation", "(a2) granulation", and "(a3) first firing", etc. This manufacturing method may further include, for example, "(c) second firing".

[0034] (a) Preparation

[0035] This manufacturing method includes a step of preparing secondary particles, which are aggregates of primary particles. The primary particles comprise olivine-type phosphate compounds. At least a portion of the surface of the primary particles is coated with carbon.

[0036] The positive electrode active material contains secondary particles. The positive electrode active material can be an aggregate of multiple secondary particles. That is, the positive electrode active material can be a powder. The D50 of the powder can be, for example, 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more. Alternatively, the D50 can be, for example, less than 30 μm, less than 25 μm, less than 20 μm, or less than 15 μm.

[0037] Secondary particles can have any shape. For example, they can be spherical, rod-shaped, angular, etc. By making secondary particles spherical, improvements in filling properties can be expected, for example. The sphericity of secondary particles can be, for example, 0.85 or higher, 0.90 or higher, or 0.95 or higher. Alternatively, the sphericity of secondary particles can be, for example, less than 1, less than 0.95, or less than 0.90. "Sphericity" refers to the roundness in an SEM (Scanning Electron Microscope) image (two-dimensional image). Sphericity (roundness) is calculated using the following formula.

[0038] ψ=4πS / L 2

[0039] ψ: Sphericity (Circularity)

[0040] π: Pi

[0041] S: Cross-sectional area of ​​the secondary particle (the area of ​​the region enclosed by the outline of the secondary particle).

[0042] L: The perimeter of the secondary particle (the length of the outline of the secondary particle).

[0043] Sphericity is represented by the arithmetic mean of 30 secondary particles.

[0044] Secondary particles are aggregates of primary particles. Primary particles can have any shape. For example, they can be spherical, rod-shaped, angular, etc. Primary particles can be nanoparticles. The maximum Feret diameter of a primary particle can be, for example, less than 100 nm. Alternatively, the maximum Feret diameter can be less than 80 nm or less than 60 nm. Or, the maximum Feret diameter can be greater than 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, or 80 nm. The maximum Feret diameter of a primary particle represents the arithmetic mean of 30 primary particles.

[0045] At least a portion of the surface of the primary particles is covered with carbon. The carbon can form a carbon layer. The amount of carbon attached, for example, relative to the secondary particles, in mass fraction, can be more than 0.1%, more than 0.5%, more than 1%, more than 2%, more than 3%, or more than 4%. The amount of carbon attached, for example, relative to the secondary particles, in mass fraction, can be less than 5%, less than 4%, or less than 3%.

[0046] Primary particles contain olivine-type phosphate compounds. "Olivine-type" indicates a crystal structure belonging to space group Pnma. Space groups can be identified by powder X-ray diffraction (XRD). Primary particles can, for example, be single-phase compounds. Once a primary particle contains an olivine-type crystalline phase, it can further contain phases belonging to other space groups. Primary particles can, for example, further contain amorphous phases, etc.

[0047] Olivine-type phosphate compounds may include, for example, lithium iron phosphate (LFP) and lithium manganese phosphate (LMP). In LMP, a portion of the manganese (Mn) can be replaced by iron (Fe). The Fe-substituted form of LMP is also denoted as lithium manganese iron phosphate (LMFP). LMP may, for example, have a composition represented by the following general formula.

[0048] Li 1-a Mn 1-x Fe x PO4

[0049] For example, it can also satisfy the relationship -0.5≤a≤0.5 and 0≤x≤1.

[0050] (a1) Formation of slurry

[0051] This method may include, for example, a step of forming a slurry by mixing lithium compounds, manganese compounds, iron compounds, phosphate compounds, a carbon source, and a solvent. For example, it may be formulated with the formula "Li". 1-a Mn 1-x Fe x The lithium compound, manganese compound, phosphoric acid compound, and iron compound are weighed according to the composition ratio (molar ratio) shown in "PO4 (-0.5≤a≤0.5, 0≤x≤1)". Lithium compounds may include, for example, lithium carbonate and lithium hydroxide. Manganese compounds may include, for example, manganese carbonate. Phosphoric acid compounds may include, for example, phosphoric acid and lithium dihydrogen phosphate. Iron compounds may include, for example, ferric oxalate and ferric phosphate.

[0052] A carbon source is a raw material containing carbon that adheres to the surface of primary particles. Carbon sources can include, for example, sugars and organic acids. Examples of carbon sources include glucose, sucrose, fructose, citric acid, and lactic acid. The amount of carbon source added relative to the raw material mixture, by mass fraction, can be, for example, 1 to 20%. The carbon source can also be added after the raw material mixture using other raw materials has been formed into granules. In this case, the amount of carbon source added relative to the granules, by mass fraction, can be, for example, 1 to 20%.

[0053] 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 pre-attached to the surface of the primary particles, and further attached to the surface of the primary particles in process (b), resulting in more uniform carbon penetration.

[0054] Solvents may include, for example, water. The concentration of solids in the slurry, by mass fraction, may be, for example, 20–40%.

[0055] The particle size in the slurry can be adjusted by performing wet milling. For example, wet milling can be performed to make the D50 0.10 to 1 μm.

[0056] (a2) Granulation

[0057] This method may include, for example, a step of granulating a slurry to form secondary particles. For instance, spray drying can be used to granulate the secondary particles. The secondary particles formed through the granulation operation are also referred to as "granules." That is, secondary particles can also be referred to as granules in other words.

[0058] (a3) First firing

[0059] This method may include a step of forming precursor particles by heat-treating the secondary particles (first sintering). Any heat treatment furnace (e.g., electric furnace, muffle furnace, etc.) can be used. The heat treatment atmosphere can be, for example, an inactive atmosphere. An inactive atmosphere can be, for example, a nitrogen atmosphere. The heat treatment temperature can be, for example, 400–700°C. The heat treatment time can be, for example, 1–6 hours.

[0060] (b) Infiltration

[0061] This manufacturing method includes a process of permeating molten carbon, formed by melting carbon materials, into the spaces between primary particles.

[0062] As long as it contains carbon, the carbon material can be any type. Examples of carbon materials include asphalt. Carbon materials can have softening points of 50–150°C or 100–120°C.

[0063] The melting temperature can be adjusted appropriately based on the carbon material. For example, the melting temperature can be higher than the softening point of the carbon material. For instance, if the softening point of the carbon material is 110°C, melting can be carried out at a temperature of around 150°C.

[0064] The melting of carbon materials can be carried out, for example, under reduced pressure. Alternatively, it can be carried out under vacuum. By performing this process under such conditions, more uniform carbon penetration is expected.

[0065] The penetration of molten carbon into the primary particles can be achieved, for example, by mixing secondary particles (precursor particles) with molten carbon.

[0066] The infiltration of molten carbon into the primary particles and the melting of the carbon material can be carried out simultaneously. For example, carbon material can be added to precursor particles to form a mixture, which is then heated (melted). The heating of the mixture can be carried out under reduced pressure or under vacuum. By simultaneously carrying out the infiltration of molten carbon into the primary particles and the melting of the carbon material under such conditions, this process can be expected to achieve more uniform carbon infiltration.

[0067] The amount of carbon material relative to secondary particles (precursor particles), expressed as a mass fraction, can be, for example, 0.5% or more and 5% or less. Alternatively, the amount of carbon material relative to secondary particles (precursor particles), expressed as a mass fraction, can be 1% or more and 4% or less, or 1% or more and 3% or less.

[0068] (c) Second firing

[0069] This manufacturing method may include a further heat treatment (second firing) process.

[0070] The heat treatment atmosphere can be, for example, an inactive atmosphere. An inactive atmosphere can be, for example, a nitrogen atmosphere. The heat treatment temperature can be, for example, 400–700°C. The heat treatment time can be, for example, 1–6 hours.

[0071] other

[0072] In this embodiment, for example, the relaxation rate and Li dissolution amount measured using the methods described in the embodiments described later can be used to evaluate the penetration of carbon into the interior of the positive electrode active material. For example, if the relaxation rate of the positive electrode active material manufactured using conventional technology is set to 1, the relaxation rate of the positive electrode active material obtained by this manufacturing method can be 2 or higher. For example, if the Li dissolution amount of the positive electrode active material manufactured using conventional technology is set to 1, the Li dissolution amount of the positive electrode active material obtained by this manufacturing method can be 0.7 or lower.

[0073] Manufacturing of positive electrode active materials

[0074] No.1

[0075] To become the compositional formula "Li1Mn 0.8 Fe 0.2Lithium carbonate, manganese carbonate, iron oxalate, and phosphoric acid were weighed according to the composition ratio shown in "PO4". These raw materials were dispersed in water and slurried. The slurry was then milled for 30 minutes using a bead mill with 0.3 mm diameter beads at a circumferential speed of 10 m / s, followed by calcination to obtain granules. 10% fructose was added relative to the granules by mass fraction for blending. After drying, the blended solution was heated to a calcination temperature of 650°C at a heating rate of 5°C / min under an inactive atmosphere and maintained for 1 hour. It was then cooled to room temperature to obtain precursor particles.

[0076] The obtained precursor particles were heated again in an inactive atmosphere to a firing temperature of 600°C at a heating rate of 5°C / min, maintained for 1 hour, and then cooled to room temperature, thereby producing the positive electrode active material (LMFP).

[0077] No.2

[0078] The precursor particles were formed using the same process as No.1. The precursor particles were measured in mass fraction... Figure 2 The recorded mass fraction of asphalt (softening point: 110℃) was added to form a mixture. The mixture was then stirred in a mixer (product name "Awatori Rentaro", manufactured by Thinky) at a stirring speed of 2000 rpm for 1 minute.

[0079] The resulting mixture was heated to a firing temperature of 600°C at a heating rate of 5°C / min under an inactive atmosphere, maintained for 1 hour, and then cooled to room temperature, thereby producing the positive electrode active material.

[0080] No. 3

[0081] The same process as No. 2 was used to form and mix the mixture. While the mixture was evacuated in a vacuum drying container, it was heated (melted) at 150°C. Then, the mixture was calcined under the same conditions as No. 2, thereby producing the positive electrode active material.

[0082] No. 4

[0083] Relative to precursor particles, in mass fraction Figure 2 The recorded mass fraction of asphalt was added, and otherwise, the positive electrode active material was manufactured in the same manner as No. 3.

[0084] Measurement

[0085] relaxation rate

[0086] The positive electrode active material of each No. was dispersed in a solvent after the electrolyte was removed, and measurements were performed using time-domain nuclear magnetic resonance (TD-NMR). The normalized relaxation rate was determined by comparing the time it took for the initial magnetization to decrease to 20%. The results are presented below. Figure 2 In the middle. Furthermore, Figure 2 The relaxation rate in the figure is a relative value when the relaxation rate of No.1 is set to 1.

[0087] Li dissolution amount

[0088] 0.1 g of the positive electrode active material of each No. 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 through a 0.2 μm membrane filter, yielding a filtrate. The Li concentration in the filtrate was determined by ICP-OES, and the Li dissolution amount was calculated accordingly. The results are shown below. Figure 2 In the middle. Furthermore, Figure 2 The Li dissolution amount in the figure is a relative value when the Li dissolution amount of No.1 is set to 1.

[0089] Battery characteristics

[0090] The making of a coin cell (coin battery)

[0091] A mixture was formed by combining 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 solids concentration of the paste was 50% by mass fraction. The positive electrode layer was formed by coating the paste onto the surface of aluminum foil and drying it. The density of the positive electrode layer was adjusted to 1.8 g / cm³ by rolling. 3 This process forms the positive electrode coil. The positive electrode coil was then subjected to vacuum drying at 120°C for 12 hours. After drying, a disc sample (diameter: 14 mm) was obtained from the positive electrode coil through punching.

[0092] A coin cell battery was assembled inside the glove box. The battery configuration is shown below.

[0093] Working electrode: Disk sample (positive electrode)

[0094] Counter electrode: Li foil

[0095] Separator: Polymer porous membrane

[0096] Electrolyte: "Ethylene carbonate / dimethyl carbonate = 3 / 7 (volume ratio)", LiPF6 (1 mol / L)

[0097] Capacity maintenance rate

[0098] The obtained coin battery was used to calculate the 1C discharge capacity based on the discharge capacity inferred from the coating weight. "C" is a symbol representing the current rate (time rate). At a 1C rate, the theoretical capacity is discharged in 1 hour. Under 25°C conditions, CCCV charging was performed with a charging rate of 0.1C, an upper limit voltage of 4.3V, and a charging termination condition of 0.01C. The 0.1C discharge capacity (as the "first discharge capacity") was then calculated when the discharge termination potential was set to 3V. The battery was then fully charged again and kept at 60°C for 14 days. The 0.1C discharge capacity was calculated again (as the "second discharge capacity"). The capacity retention rate was calculated as (second discharge capacity / first discharge capacity) × 100%. The results are shown below. Figure 2 The Chinese side believes that a higher capacity retention rate indicates better durability.

[0099] result

[0100] like Figure 2 As shown, under the manufacturing conditions of this disclosure, a tendency was observed that the relaxation time increased and the amount of Li dissolved decreased. That is, it is believed that carbon is uniformly permeated even inside. In addition, under the manufacturing conditions of this disclosure, a tendency was observed that the capacity retention rate increased.

Claims

1. A method for manufacturing a positive electrode active material, comprising the following steps: (a) Prepare secondary particles, said secondary particles being an aggregate of primary particles; and (b) Allowing molten carbon, formed by melting carbon material, to permeate between the primary particles. The primary particles contain olivine-type phosphate compounds. At least a portion of the surface of the primary particle is covered with carbon.

2. The method for manufacturing the positive electrode active material according to claim 1, The carbon material is asphalt.

3. The method for manufacturing the positive electrode active material according to claim 1 or 2, Step (b) includes heating the mixture of the carbon material and the secondary particles under reduced pressure.

4. The method for manufacturing the positive electrode active material according to claim 1 or 2, The maximum Ferete diameter of the primary particle is less than 100 nm.

5. The method for manufacturing the positive electrode active material according to claim 1 or 2, The amount of carbon material relative to the secondary particles is more than 0.5% and less than 5% by mass fraction.

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