JP6388340B2 - Method for producing lithium iron phosphate positive electrode active material - Google Patents

Description

  The present invention relates to a method for producing a lithium iron phosphate positive electrode active material.

As an electrode material of a lithium ion secondary battery, a general formula LiMPO such as lithium iron phosphate (LiFePO ₄ ), lithium manganese phosphate (LiMnPO ₄ ), lithium cobalt phosphate (LiCoPO ₄ ), nickel nickel phosphate (LiNiPO ₄ ), etc. ₄ (M = Fe, Mn, Co, Ni) The transition metal lithium compound represented by this is mentioned.

Among them, lithium iron phosphate having an olivine type crystal structure has a large theoretical capacity (170 mAh / g) and has a relatively high electromotive force (vs. Li / Li ⁺ about 3.4 to 3.5 V at the negative electrode). Furthermore, since it is thermodynamically stable and hardly releases oxygen or generates heat up to about 400 ° C., it can be said to be a preferable positive electrode material from the viewpoint of safety.
Further, since it can be produced at low cost from resource-rich iron, phosphorus, etc., it is expected as a promising positive electrode material.

Lithium iron phosphate (LiFePO ₄ ) is generally synthesized by mixing three raw materials, a raw material that becomes a Li source, a raw material that becomes an Fe source, and a raw material that becomes a PO ₄ source. Is a dry method (also called a solid phase method: for example, Patent Document 1).
In the dry method, as shown in Patent Document 1, a solid precursor (powder) raw material is mixed and pulverized, and a calcined precursor is calcined to produce a lithium iron phosphate positive electrode active material.
In the dry method, only three raw materials are mixed in the state of the firing precursor, and the reaction hardly proceeds. When heat is applied in the step of firing the firing precursor (hereinafter referred to as the firing step), the three raw materials react to produce a lithium iron phosphate positive electrode active material.

  On the other hand, as shown in Patent Document 2 and Patent Document 3, a raw material dispersed in a liquid (for example, an organic solvent such as acetone) is mixed and pulverized by a bead mill or the like, and the solvent is dried and removed. There is a method for producing a lithium iron phosphate positive electrode active material by firing as a precursor. This method is called a wet method.

JP 2003-292308 A JP 2008-4317 A Japanese Patent No. 558065

  Here, in the dry method, since most of the reaction proceeds in the firing step, the firing time is long.

  In addition, in the wet method using a bead mill or the like, a large amount of solvent is required to uniformly disperse the raw material and beads. Therefore, it is necessary to remove the solvent by drying or the like after pulverizing the raw material and before firing. Therefore, energy for removing the solvent is required, and there are problems of complicated manufacturing processes, large equipment, and high costs.

  In addition, when the step of removing (drying) the solvent is performed, depending on the drying conditions and the like, the calcined precursor may be in a state where the bulk density is as low as, for example, powder. If the bulk density of the calcined precursor is low, the amount charged into a calcining furnace or the like for performing calcining is reduced, so that productivity is lowered.

  Therefore, an object of the present invention is to provide a method for producing a lithium iron phosphate positive electrode active material that is simple, energy-saving, and low in cost and has high productivity.

In view of the above problems, a method for producing a lithium iron phosphate positive electrode active material according to the first aspect of the present invention is a method for producing a lithium iron phosphate positive electrode active material represented by the general formula LiFePO ₄ , comprising lithium carbonate (Li ₂ CO ₃ ), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) and iron (II) oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O) are mixed and pulverized. A granulation step of adding water (H ₂ O) to the raw material mixture after the pulverization step to obtain a calcined precursor, and calcining the granulated calcined precursor to obtain lithium iron phosphate. And a firing step to be generated.

The amount of water (H ₂ O) added to the pulverized raw material mixture is an amount that enables the pulverized powder raw material mixture to be granulated (granulated).
According to this aspect, since the firing precursor is granulated by adding water (H ₂ O) to the raw material mixture after the pulverization step, a firing precursor having a high bulk density can be stably obtained. Accordingly, it is possible to produce a stable lithium iron phosphate positive electrode active material with higher productivity.

In addition, since the granulated firing precursor contains water, the synthesis reaction starts in the state of the firing precursor.
This shortens the firing time when completing the reaction in the subsequent firing step. In addition, when the specific reaction formula of the said reaction is shown, it will become like the following (1) Formula and (2) Formula

As described above, the amount of water added in the granulation step is an amount that allows the raw material mixture of the pulverized powder to be granulated, and thus the water is removed from the granulated firing precursor. Even if it is used for the baking process, it evaporates immediately, and there is little risk that much energy is used for evaporation of the water during the baking process.
Therefore, it is not necessary to remove the water added in the granulation step by drying or the like, the manufacturing process can be reduced to shorten the manufacturing time, and the cost and energy required for the equipment required for drying or the like can be suppressed.

The general formula LiFePO ₄ produced in the present specification includes lithium (Li), iron (Fe), and phosphoric acid (PO ₄ ) in a stoichiometric ratio of approximately 1: 1: 1. It is not limited to the case where each component is strictly contained at 1: 1: 1. Moreover, it does not require that no other components or impurities are contained.

  In the first aspect of the method for producing a lithium iron phosphate positive electrode active material according to the second aspect of the present invention, the amount of water added in the granulation step is 25 wt% or less with respect to the raw material mixture. It is characterized by this.

  According to this aspect, by setting the amount of water added in the granulation step to 25 wt% or less with respect to the raw material mixture, the granular firing precursor can be easily formed and the bulk density can be increased. . Moreover, it is possible to form a firing precursor having a uniform particle size, and uniform firing can be performed during the subsequent firing step. Therefore, the reaction can be progressed uniformly to produce a more stable quality lithium iron phosphate positive electrode active material.

  The method for producing a lithium iron phosphate positive electrode active material according to the third aspect of the present invention is characterized in that, in the first aspect or the second aspect, the particle size distribution of the raw material mixture is D50 ≦ 15 μm. It is.

  According to this aspect, since the raw material mixture is pulverized into fine particles having a particle size distribution D50 ≦ 15 μm, the reaction proceeds more efficiently and a calcined precursor having a stable composition can be obtained. Further, even if the raw material mixture is finely pulverized to this extent, it is granulated by adding water, so that the bulk density as the firing precursor can be easily increased. Therefore, when a lithium ion secondary battery is created using the lithium iron phosphate positive electrode active material produced by this method, better battery characteristics can be obtained.

  The method for producing a lithium iron phosphate positive electrode active material according to the fourth aspect of the present invention is the granulated particle of the calcined precursor granulated in the granulation step according to any one of the first to third aspects. The diameter is 10 mm or less.

In the present specification, the “granulated particle diameter” is the diameter of the calcined precursor granulated in the granulation step. In addition, when the surface of the granulated firing precursor has irregularities, the spherical shape including the convex portion is defined as a virtual spherical shape, and the diameter of the virtual spherical shape is defined as the diameter of the firing precursor. Further, when the granulated particle diameter has a major axis and a minor axis as in the case where the cross section of the calcining precursor is, for example, an ellipse, the average of the major axis and the minor axis is used as the diameter of the calcining precursor.
According to this aspect, when the granulated particle diameter of the calcined precursor is 10 mm or less, heat is reliably transmitted to the central portion of the calcined precursor in the calcining step, and the reaction can proceed more effectively. .

  In the method for producing a lithium iron phosphate positive electrode active material according to the fifth aspect of the present invention, in any one of the first aspect to the fourth aspect, the firing step is performed at room temperature to 360 ° C. or more and 450 ° C. or less. It includes firing at a temperature.

  According to this aspect, since the reaction proceeds in the state of the granulated firing precursor, and the firing process is performed on the firing precursor in the course of the reaction, the product (iron phosphate) is more efficiently produced. Lithium positive electrode active material) can be obtained.

  Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples, and various modifications are possible within the scope of the invention described in the claims, They are also included within the scope of the present invention.

[Outline of production method of lithium iron phosphate positive electrode active material]
Below, the outline | summary of the manufacturing method of the lithium iron phosphate positive electrode active material which concerns on this invention is demonstrated. The lithium iron phosphate positive electrode active material according to the present invention is a material represented by the general formula LiFePO ₄ .

Lithium carbonate (Li ₂ CO ₃ ) is used as a raw material for introducing component Li. As a raw material for introducing the component PO _4, ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) is used. As the raw material for introducing the component Fe, iron (II) oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O) is used.

(1) Grinding step Mixing raw material lithium carbonate, ammonium dihydrogen phosphate and iron (II) oxalate dihydrate (hereinafter sometimes simply referred to as iron oxalate) obtain. Lithium carbonate, ammonium dihydrogen phosphate, and iron oxalate are mixed so that the stoichiometric ratio of lithium, iron, and phosphorus is 1: 1: 1. The mixing of the raw materials can be performed using a known mixing apparatus such as a planetary mixer. The pulverization can be performed using a known pulverizer such as a ball mill.

  Further, the pulverization in the pulverization step is preferably performed so that the particle size distribution of the mixed raw material mixture after pulverization satisfies D50 ≦ 15 μm. By pulverizing the raw material into fine particles having a particle size distribution of D50 ≦ 15 μm, the synthesis reaction of the lithium iron phosphate positive electrode active material can be performed with higher efficiency. In addition, a calcined precursor having a stable composition can be obtained.

(2) Granulation step Water (H ₂ O) is added to the raw material mixture of the powder pulverized in the pulverization step, and granulated in a granular form to form a calcined precursor.

The amount of water added is preferably 25 wt% or less with respect to the raw material mixture. This makes it possible to easily form a firing precursor having a high bulk density.
In addition, since a firing precursor having a uniform particle size can be formed, uniform firing can be performed during the subsequent firing step. Since the uniform firing is possible, the reaction proceeds uniformly, so that the quality of the product lithium iron phosphate positive electrode active material can be stabilized.

  In addition, the granulated particle diameter of the calcined precursor to be granulated is desirably 10 mm or less. More preferably, it is 1.0 mm to 10 mm. As a result, heat is reliably transmitted to the center of the firing precursor during firing, and the reaction can proceed efficiently.

(3) Firing step The firing step is a step in which the firing precursor obtained in the granulation step is fired to produce LiFePO ₄ . In this description, the firing process performed by the following processes (3-1) to (3-4) will be described.

(3-1) First-stage firing step The firing precursor obtained by the granulation step is fired in a firing apparatus such as a firing furnace.
The firing temperature is preferably raised from room temperature to a temperature of from 300 ° C. to 450 ° C., more preferably from 360 ° C. to 450 ° C. The firing time is several hours (for example, about 1 to 8 hours).

(3-2) Carbon precursor addition step In the carbon precursor addition step, a carbon precursor is added to the fired product in the fired product crushing step (hereinafter referred to as a primary fired product). Conductive carbon can be deposited on the surface of lithium iron phosphate by adding a carbon precursor and performing the second-stage firing step described later. An electrode material having conductive carbon deposited on the surface can exhibit even higher charge / discharge characteristics than when no carbon is deposited.
As the carbon precursor, in addition to organic acids such as citric acid, bitumens such as coal pitch and saccharides can be added.

(3-3) Volume reduction process of calcined product In the volume reduction process of the calcined product, the primary calcined product after the carbon precursor addition step is pulverized to reduce the volume.
The pulverization can be performed using equipment such as a turbo mill or a jet mill. The median diameter after pulverization is preferably 100 μm or less, and more preferably 50 μm or less.

(3-4) Second-stage firing step After the volume reduction step of the fired product, the temperature is raised from room temperature to 600 ° C to 800 ° C (more preferably from 650 ° C to 780 ° C) by a firing furnace or the like. And firing. The “room temperature” at this time is the room temperature after the carbon precursor addition step. After raising the temperature in the furnace, firing is performed for several hours (for example, about 1 to 8 hours).
By performing the second stage baking step, a product as a lithium iron phosphate positive electrode active material having conductive carbon on the surface is obtained.

By producing the lithium iron phosphate positive electrode active material by the method described above, the following effects can be obtained.
Since water (H ₂ O) is added to the raw material mixture after the pulverization step and granulated to obtain a calcined precursor, a calcined precursor having a high bulk density can be stably obtained. When the bulk density is high, it becomes possible to charge a large number of firing precursors into the firing furnace at a time, thereby improving productivity.

Furthermore, when the granulated firing precursor contains water, the synthesis reaction starts in the state of the firing precursor. That is, the reaction can be advanced before the firing step.
This shortens the firing time when the reaction is completed in the firing step.

Further, the water added in the granulation step is water for granulating the raw material mixture of the powder, and since it is a small amount, it can be used for the firing step without removing the water from the granulated firing precursor. Evaporates immediately. That is, much energy is not required for the evaporation of the water.
Therefore, a baking process can be performed without removing the water added in the granulation process, and manufacturing time can be shortened. In addition, the cost and energy required for equipment necessary for drying and the like can be reduced.

  As described above, the production of a lithium iron phosphate positive electrode active material with high productivity can be performed simply, energy saving and at low cost.

  In the above description, the case of performing the two-stage firing of the first-stage firing process and the second-stage firing process has been described. However, the firing process may be performed by one-stage firing without dividing the firing process into two stages. Moreover, when performing a two-step baking, the case where at least one of a carbon precursor addition process and a baked material volume reduction process is not performed is also included in the scope of the present invention.

[Concrete example]
A lithium iron phosphate positive electrode active material was produced by the following procedure, a lithium ion secondary battery was prepared using the produced active material, and the battery characteristics were examined (Example 1). As a comparative example, a lithium iron phosphate positive electrode active material was produced by a conventional dry method (Comparative Example 1) and a wet method (Comparative Example 2).

Example 1
As raw materials, lithium carbonate, 73.8909 g, ammonium dihydrogen phosphate, 115.0257 g, iron (II) oxalate dihydrate, 179.8946 g are weighed, mixed and pulverized using a pulverizer, A raw material mixture having a particle size distribution of D50 ≦ 15 μm was obtained (grinding step). The pulverized raw material mixture was put in a granulator and granulated while adding 20 wt% of water to obtain a calcined precursor (granulation step). The granulated particle diameter of the firing precursor was 10 mm.
After performing the first stage firing step (calcination temperature: 400 ° C., holding time: 4 hours) on the granulated firing precursor, 3.5 wt% of coal pitch as a carbon source is added to the first stage fired product, The mixture was pulverized with a pulverizer and subjected to a second stage baking process (calcination temperature: 760 ° C., holding time: 6 hours) to obtain a product as a lithium iron phosphate positive electrode active material (calcination process).

(Comparative Example 1: Dry method)
As raw materials, lithium carbonate, 73.8909 g, ammonium dihydrogen phosphate, 115.0257 g, iron (II) oxalate dihydrate, 179.8946 g, weighed, mixed and pulverized using a pulverizer Was used as a firing precursor. The firing precursor was subjected to the same firing step as in Example 1.

(Comparative Example 2: Wet method)
As raw materials, lithium carbonate, 73.8909 g, ammonium dihydrogen phosphate, 115.0257 g, iron (II) oxalate dihydrate, 179.8946 g are weighed, and the raw materials are dispersed in an organic solvent to bead mill. The mixture was pulverized and slurried. A solvent evaporation process was performed on the obtained slurry, and the organic solvent was removed to obtain a calcined precursor.
The firing precursor was subjected to the same firing step as in Example 1.

  Table 1 shows the manufacturing time and the bulk density of each baking precursor when manufacturing the baking precursor in Example 1, Comparative Example 1, and Comparative Example 2.

Next, using the lithium iron phosphate positive electrode active material of Example 1, Comparative Example 1, and Comparative Example 2, a lithium ion secondary battery was created under the following conditions, and a charge / discharge test was performed.
Cell type: CR2032
Positive electrode composition: LFP: AB: PVdF = 91: 4: 5
Cathode loading: 16 mg / cm ²
Negative electrode: Li-Metal
Electrolyte: 1M LiPF ₆ in EC / EMC
Temperature: 25 ° C
Charging conditions: 0.1C (CC-CV): 4.3V
1C (CC-CV): 4.0V
Discharge conditions: 0.1, 1C (CC), 2.0V
LFP = lithium iron phosphate, AB = acetylene black, PVdF = polyvinylidene fluoride, LiPF ₆ = lithium hexafluorophosphate, EC = ethylene carbonate, and EMC = ethyl methyl carbonate.

  Table 2 shows the test results of the battery capacity of the lithium ion secondary batteries prepared using the lithium iron phosphate positive electrode active material of Example 1, Comparative Example 1, and Comparative Example 2.

As shown in Table 2, the lithium iron phosphate positive electrode active material of Example 1 was able to create a secondary battery having a larger battery capacity than Comparative Example 1 and Comparative Example 2.
In addition, as shown in Table 1, Comparative Example 1 (dry method) takes a shorter time to obtain a calcined precursor than Example 1, but under the same calcining conditions as in Example 1, the battery capacity is sufficient. A secondary battery could not be produced.

Claims (4)

A method for producing a lithium iron phosphate positive electrode active material represented by the general formula LiFePO4,
Mixed ammonium dihydrogen phosphate and lithium carbonate (Li2CO3) (NH4H2PO4) and iron oxalate (II) · dihydrate (FeC2O4 · 2H2O), a milling step of the powder was ground without the addition of water,
A granulation step of adding water (H2O) to the raw material mixture after the pulverization step and granulating to obtain a calcined precursor;
A firing step of generating a lithium iron phosphate by burning granulated the calcination precursor, was closed,
The method for producing a lithium iron phosphate positive electrode active material , wherein the amount of water added in the granulation step is 25 wt% or less with respect to the raw material mixture . In the manufacturing method of the lithium iron phosphate positive electrode active material of Claim 1 ,
The method for producing a lithium iron phosphate positive electrode active material, wherein a particle size distribution of the raw material mixture is D50 ≦ 15 μm. In the manufacturing method of the lithium iron phosphate positive electrode active material of Claim 1 or Claim 2 ,
The method for producing a lithium iron phosphate positive electrode active material, wherein a granulated particle diameter of a calcined precursor granulated in the granulation step is 10 mm or less. In the manufacturing method of the lithium iron phosphate positive electrode active material as described in any one of Claims 1-3 ,
The method for producing a lithium iron phosphate positive electrode active material, wherein the firing step includes firing at a temperature from room temperature to 360 ° C. to 450 ° C.