JP2016149297A - Manufacturing method of positive electrode active material of lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a positive electrode active material of a lithium secondary battery capable of more easily heighten tap density than before.SOLUTION: The manufacturing method of a positive electrode active material of a lithium secondary battery includes: a primary sintering step (Step S102) for preparing primary particles of lithium iron phosphate by sintering a mixture of a lithium raw material, an iron raw material, and a phosphorus raw material; and a secondary sintering step (Step S106) for preparing secondary particles of carbon-coated lithium iron phosphate by sintering a mixture containing the primary particles of the lithium iron phosphate obtained in the primary sintering step and citric acid.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a positive electrode active material for a lithium secondary battery.

  When producing a positive electrode by applying and drying a slurry containing a positive electrode active material on a positive electrode plate, attempts have been made to increase the tap density of the positive electrode material from the viewpoint of increasing the drying time (Patent Document 1).

JP2013-191516A (particularly paragraph 0041)

  In order to increase the tap density of the positive electrode material, for example, a so-called spray drying method in which a slurry is sprayed and dried in an atmosphere of 70 ° C. to 250 ° C. may be used (paragraph 0050 of Patent Document 1). However, in the spray drying method, since the slurry particles are dropped and dried like a shower, the apparatus is easily increased in size and the operation is complicated. Furthermore, in the spray drying method, it is difficult to control the voids formed between the primary particles of the positive electrode active material contained in the positive electrode material to be dried. Therefore, there is room for improvement in improving the tap density of the positive electrode active material.

  The present invention has been made in view of such problems, and the problem to be solved by the present invention is a method for producing a positive electrode active material for a lithium secondary battery that can increase the tap density more easily than in the past. Is to provide.

  As a result of intensive studies to solve the above problems, the present inventors have found the following findings. That is, the gist of the present invention is a primary firing step of preparing a primary particle of lithium iron phosphate by firing a mixture of a lithium raw material, an iron raw material, and a phosphorus raw material, and phosphoric acid obtained in the primary firing step. A secondary firing step of firing a primary particle mixture containing primary particles of lithium iron and citric acid to prepare secondary particles of carbon-coated lithium iron phosphate; The present invention relates to a method for producing a positive electrode active material for a secondary battery.

  At this time, it is preferable that polyvinyl alcohol is contained in the primary particle mixture.

  Moreover, it is preferable that content of the said citric acid in the said primary particle mixture is 5 mass% or more and 15 mass% or less with respect to content of the primary particle of lithium iron phosphate contained in the said primary particle mixture.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the positive electrode active material of a lithium secondary battery which can raise a tap density more easily than before can be provided.

It is a flowchart which shows the manufacturing method of the positive electrode active material of the lithium secondary battery of this embodiment. It is the graph which plotted the relationship between the citric acid content in the baked mixture, and the tap density of the obtained lithium iron phosphate. It is the graph which plotted the relationship between the citric acid content in the baked mixture, and the bulk density of the obtained lithium iron phosphate.

  Hereinafter, a form for carrying out the present invention (this embodiment) will be described with reference to the drawings as appropriate. In the following description, lithium iron phosphate is cited as an example of the positive electrode active material. However, the positive electrode active material is not limited to lithium iron phosphate, and any positive electrode active material such as lithium manganate or lithium cobaltate can be used. In addition, it can also be used for negative electrode active materials such as lithium titanate.

  FIG. 1 is a flowchart showing a method for producing a positive electrode active material of a lithium secondary battery according to this embodiment. In the present embodiment, secondary particles of carbon-coated lithium iron phosphate are obtained by the flowchart shown in FIG. And the obtained secondary particle of lithium iron phosphate can be used as a positive electrode active material of a lithium secondary battery. Hereinafter, a method for producing carbon-coated lithium iron phosphate will be described while being divided into steps.

  First, a raw material mixture is obtained by mixing a phosphorus raw material, an iron raw material, and a lithium raw material (step S101). Among these raw materials, the phosphorus raw material becomes phosphorus constituting lithium iron phosphate manufactured below. Similarly, the iron raw material becomes iron and the lithium raw material becomes lithium. The phosphorus raw material is, for example, ammonium dihydrogen phosphate. The iron raw material is, for example, iron oxalate dihydrate. Further, the lithium raw material is lithium hydroxide or lithium carbonate. One compound may also serve as two or more raw materials.

The mixing ratio of the raw materials is not particularly limited, but it is preferable to mix the raw materials so that each of phosphorus, iron, and lithium is included in an equal amount by molar ratio. Lithium iron phosphate is represented by the composition formula LiFePO ₄ and contains equimolar amounts of phosphorus, iron, and lithium, so by using such a mixing ratio, raw materials can be used without excess or deficiency. .

  Next, the raw material mixture is subjected to primary firing, thereby producing crystals (primary particles) of lithium iron phosphate (step S102, primary firing step). The firing conditions at the time of primary firing are not particularly limited, but from the viewpoint of avoiding excessive increase of crystals and excessive reduction in the BET specific surface area of lithium iron phosphate (conditions that can relatively suppress crystal growth) It is preferable that Specifically, for example, it is preferable to set the temperature at 300 to 500 ° C. for about 4 to 5 hours.

Then, by cooling the crystals obtained by the primary firing (step S103), primary particles formed by aggregation of a plurality of lithium iron phosphate crystals are obtained (step S104). Since these primary particles are subjected to secondary firing (main firing) described later, they are also referred to as “firing precursors”. In addition, along with the generation of primary particles of lithium iron phosphate, lithium phosphate (Li ₃ PO ₄ ) is usually generated as a by-product. This by-product is preferably removed, but in this embodiment, it is removed in step S105 described later.

  Next, the obtained primary particles of lithium iron phosphate are mixed with citric acid (step S105). The mixing at this time is preferably performed in a state containing water. By doing so, the pH of the mixture tends to be weakly acidic. As a result, the impurity lithium phosphate generated in step S104 is dissolved, and the lithium phosphate is easily removed. However, since citric acid absorbs some moisture, citric acid usually contains water. Therefore, citric acid is preferably added to the primary particles of lithium iron phosphate in the form of an aqueous solution, but may be added to the primary particles of lithium iron phosphate in the form of citric acid (solid). The mixing is also preferably performed while pulverizing the primary particles. Thereby, it is suppressed that the particles of lithium iron phosphate are excessively increased during secondary firing (step S106) described later, and secondary particles of lithium iron phosphate having a larger BET specific surface area are obtained.

  The content of citric acid (that is, the content of citric acid at the time of secondary firing) is not particularly limited, but the content of citric acid relative to the content of lithium iron phosphate is 5% by mass to 15% by mass. It is preferable that By making the content of citric acid within this range, lithium iron phosphate sufficiently excellent in both tap density and bulk density can be obtained.

  Moreover, it is preferable that polyvinyl alcohol (PVA) is contained in this mixture (primary particle mixture). By including polyvinyl alcohol, it is possible to form a “crosslinking portion” that crosslinks between primary particles of lithium iron phosphate during firing. And by forming this bridge | crosslinking part, the electrical conductivity of the primary particle | grains of lithium iron phosphate improves, and the electroconductivity of lithium iron phosphate which tends to be insufficient normally can be supplemented. Moreover, lithium iron phosphates approach by forming a bridge | crosslinking part, and, thereby, the density of the secondary particle which is an aggregate | assembly of primary particles, such as lithium iron phosphate, can be improved.

  The reason why such a crosslinked part is formed is considered that polyvinyl alcohol is a straight-chain polymer having a long molecular chain while polyvinyl alcohol contains many functional groups according to the study of the present inventors.

  Here, in order to facilitate the explanation, the “crosslink” formed is considered as a “bridge”, and the formed crosslink will be described. The “bridge” is composed of a “ridge part” (a bridge crossing start part and a crossing end part) and a “length part” connecting them. The functional groups contained in the polyvinyl alcohol are likely to interact with each other, and it is considered that the above-mentioned “saddle portion” is formed when firing (carbonization) proceeds in a state in which this function is performed. Furthermore, since polyvinyl alcohol is a linear polymer compound, it is considered that a “length portion” is formed starting from the “ridge portion”.

  Like these, it is thought that the bridge | crosslinking part between lithium iron phosphates is mainly formed through two steps of reaction. Among them, such a reaction is unlikely to occur in a compound having a cyclic structure, a compound having a short straight chain portion, or the like, specifically, for example, saccharides, and is considered to be a reaction peculiar to polyvinyl alcohol.

  Furthermore, this mixture may contain sucrose. This is because sucrose has a non-aromatic cyclic structure in addition to being easily dispersed because it is water-soluble carbon. That is, when sucrose is carbonized, its own cyclic structure remains to some extent, so that a carbon coating layer in which lithium ions can easily enter and exit is obtained. And by obtaining such a carbon coating layer, a lithium secondary battery having a large discharge capacity can be obtained.

  In step S105, primary particles of lithium iron phosphate, citric acid, and if necessary, polyvinyl alcohol are mixed, and then secondary firing is performed (step S106, secondary firing step). The secondary firing is preferably performed in an inert atmosphere (for example, in nitrogen). Here, the citric acid is contained in the mixture in which the secondary firing is performed as described above. Therefore, by firing in an inert atmosphere, primary particles of lithium iron phosphate are covered with carbon constituting citric acid. Moreover, secondary particles of lithium iron phosphate formed by aggregation of primary particles of carbon-coated lithium iron phosphate are obtained by secondary firing.

  The conditions for the secondary firing are not particularly limited, but can be, for example, about 600 ° C. to 800 ° C. for about 3 hours to 6 hours.

  After the secondary firing, the fired lithium iron phosphate is cooled (step S107), and carbon-coated lithium iron phosphate secondary particles are obtained (step S108). The secondary particles of lithium iron phosphate can be applied to a metal plate after being mixed with an arbitrary solvent to form a slurry, for example. Thereby, the positive electrode of a lithium secondary battery is obtained.

  Further, as in this embodiment, by subjecting the mixture containing citric acid to secondary firing, lithium iron phosphate having a large tap density can be obtained without depending on the conventionally performed spray drying method. The reason is considered to be a decrease in porosity generated in the secondary particles.

  That is, in the conventional spray drying method, after the particles are aggregated as water droplets, the aggregate of the particles is dried. On the other hand, in this embodiment, lithium iron phosphate is tumbled and granulated, for example, while lithium phosphate cross-linking lithium iron phosphate is dissolved by weakly acidic citric acid. For this reason, the lithium iron phosphate that is broken evenly after the cross-linking is agglomerated, and when the primary particles are formed, the aggregability of the lithium iron phosphate is enhanced. Then, secondary particles are formed from the primary particles of lithium iron phosphate with improved cohesiveness. In the secondary particles formed in this way, the void density is reduced, so that the tap density can be improved. Furthermore, the electrode density can be improved by increasing the tap density of lithium iron phosphate. The tap density can be measured using a measuring apparatus based on JIS 2512: 2012 (metal powder tap density measuring method).

  Furthermore, according to the manufacturing method of the present embodiment, the bulk density of lithium iron phosphate can be increased in addition to the tap density. This is presumably because the same effect as the above tap density occurs with respect to the bulk density. And the handling property at the time of transporting lithium iron phosphate etc. can be improved by enlarging the bulk density of lithium iron phosphate, and the reduction of the manufacturing cost of a lithium secondary battery can be aimed at. The bulk density can be measured using a measuring device based on JIS 2504: 2012 (metal powder apparent density measuring method).

  Hereinafter, the present embodiment will be described in more detail with reference to specific examples. Lithium iron phosphate was produced according to the manufacturing method of this embodiment, and the tap density and bulk density of the obtained lithium iron phosphate were measured.

<Example 1>
Lithium carbonate (Li ₂ CO ₃ ), iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), and ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) were mixed. Next, the mixture was subjected to primary firing at 405 ° C. to obtain lithium iron phosphate not coated with carbon. The firing time was 4 hours.

  200 g of the obtained lithium iron phosphate (primary particles of lithium iron phosphate) and 60 g of citric acid (manufactured by Kishida Chemical Co.) 50% by mass solution (15% by mass as the content of citric acid with respect to 200 g of lithium iron phosphate) And the mixture was heated at 720 ° C. for 6 hours to perform secondary firing. After secondary firing, cooling, crushing and classification (excluding those having a diameter of 45 μm or more by sieving) yielded carbon-coated lithium iron phosphate.

When the tap density of the obtained carbon-coated lithium iron phosphate was measured using MULTI TESTER (manufactured by Seishin Enterprise Co., Ltd., MT-1001), the tap density was 1.59 g / cm ³ .

Furthermore, when the bulk density of the obtained carbon-coated lithium iron phosphate was measured using MULTI TESTER (manufactured by Seishin Enterprise Co., Ltd., MT-1001), the bulk density was 1.14 g / cm ³ .

<Example 2>
Instead of “60 g of 50% by mass of citric acid (made by Kishida Chemical Co., Ltd.)”, “46 g of polyvinyl alcohol 10% by mass solution (PVA-205 made by Kuraray Co., Ltd.) and 50% by mass of citric acid (made by Kishida Chemical Co., Ltd.) A carbon-coated lithium iron phosphate was obtained in the same manner as in Example 1 except that 13.2 g of the solution (3.3% by mass as the citric acid content relative to 200 g of lithium iron phosphate) was used. It was.

When the tap density of the obtained carbon-coated lithium iron phosphate was measured in the same manner as in Example 1, the tap density was 1.13 g / cm ³ . Furthermore, when the bulk density of the obtained carbon-coated lithium iron phosphate was measured in the same manner as in Example 1, the bulk density was 0.82 g / cm ³ .

<Example 3>
Instead of “60 g of 50% by mass of citric acid (manufactured by Kishida Chemical Co., Ltd.)”, “34 g of polyvinyl alcohol 10 mass% solution (manufactured by Kuraray Co., Ltd., PVA-205) and 50% by mass of citric acid (manufactured by Kishida Chemical Co., Ltd.) The carbon-coated lithium iron phosphate was obtained in the same manner as in Example 1 except that 26.8 g of the solution (9.7% by mass as the citric acid content relative to 200 g of lithium iron phosphate) was used. It was.

When the tap density of the obtained carbon-coated lithium iron phosphate was measured in the same manner as in Example 1, the tap density was 1.53 g / cm ³ . Furthermore, when the bulk density of the obtained carbon-coated lithium iron phosphate was measured in the same manner as in Example 1, the bulk density was 1.28 g / cm ³ .

<Example 4>
Instead of “60 g of 50% by weight citric acid (made by Kishida Chemical Co., Ltd.)”, “20 g of polyvinyl alcohol 10% by weight solution (PVA-205 made by Kuraray Co., Ltd.) and 50% by mass of citric acid (made by Kishida Chemical Co., Ltd.) Carbon-coated lithium iron phosphate was obtained in the same manner as in Example 1 except that 40 g of the solution (10 mass% as the citric acid content relative to 200 g of lithium iron phosphate) was used.

When the tap density of the obtained carbon-coated lithium iron phosphate was measured in the same manner as in Example 1, the tap density was 1.61 g / cm ³ . Furthermore, when the bulk density of the obtained carbon-coated lithium iron phosphate was measured in the same manner as in Example 1, the bulk density was 1.25 g / cm ³ .

<Comparative Example 1>
Instead of “60 g of a 50% by mass solution of citric acid (manufactured by Kishida Chemical Co., Ltd.)”, 7 g of “coal pitch (manufactured by JFE Chemical Co., Ltd., MCP110C)” (0% by mass as the content of citric acid with respect to 200 g of lithium iron phosphate; A carbon-coated lithium iron phosphate was obtained in the same manner as in Example 1 except that no acid was used.

When the tap density of the obtained carbon-coated lithium iron phosphate was measured in the same manner as in Example 1, the tap density was 0.77 g / cm ³ . Furthermore, when the bulk density of the obtained carbon-coated lithium iron phosphate was measured in the same manner as in Example 1, the bulk density was 0.38 g / cm ³ .

<Comparative example 2>
Instead of “60 g of 50% by weight citric acid (made by Kishida Chemical Co., Ltd.)”, “60 g of polyvinyl alcohol 10% by weight solution (manufactured by Kuraray Co., Ltd., PVA-205)” The carbon-coated lithium iron phosphate was obtained in the same manner as in Example 1 except that “0% by mass; not containing citric acid” was used.

When the tap density of the obtained carbon-coated lithium iron phosphate was measured in the same manner as in Example 1, the tap density was 0.8 g / cm ³ . Furthermore, when the bulk density of the obtained carbon-coated lithium iron phosphate was measured in the same manner as in Example 1, the bulk density was 0.51 g / cm ³ .

<Examination>
The above results are summarized in Table 1 and Table 2 below. In addition, the tap density and the bulk density were plotted on a graph with the content of citric acid as the horizontal axis (FIGS. 2 and 3).

  FIG. 2 is a graph plotting the relationship between the citric acid content in the fired mixture and the tap density of the obtained lithium iron phosphate. As shown in FIG. 2, in Examples 1 to 4 in which the mixture containing citric acid was fired, the tap density was higher than in Comparative Examples 1 and 2 in which the mixture containing no citric acid was fired.

  In particular, it has been found that the tap density is sufficiently increased by adding citric acid so that the citric acid content with respect to lithium iron phosphate is 5 mass% or more and 15 mass or less. Specifically, the tap density when the citric acid content is 6.7% (Example 3), 10% (Example 4) and 15% (Example 1) is Comparative Example 1 containing no citric acid. Compared with the tap density of Comparative Example 2, the density increased by 1.5 to 2 times.

  FIG. 3 is a graph plotting the relationship between the citric acid content in the fired mixture and the bulk density of the obtained lithium iron phosphate. As shown in FIG. 3, in Examples 1 to 4 in which the mixture containing citric acid was fired, the bulk density was higher than in Comparative Examples 1 and 2 in which the mixture containing no citric acid was fired.

  In particular, it was found that by adding citric acid so that the citric acid concentration with respect to lithium iron phosphate is 5% by mass or more and 15% by mass or less, the bulk density is also sufficiently increased. Specifically, when the citric acid content is 6.7% (Example 3), 10% (Example 4), and 15% (Example 1), the bulk density is Comparative Example 1 containing no citric acid. Compared with the bulk density of Comparative Example 2, it increased by about 2 to 3.5 times.

  As shown in FIGS. 2 and 3, according to the manufacturing method of the present embodiment, the tap density of the obtained lithium iron phosphate can be increased without depending on the spray drying method. Therefore, according to the manufacturing method of the present embodiment, the electrode density can be controlled and the electrode density can be improved as compared with the conventional case without depending on the spray drying method. The amount (volume, space, etc.) of the positive electrode material that can be used for the lithium secondary battery is limited, and the electrode density increases, so that the mass of the positive electrode material that can be filled in the volume can be increased. High energy density of the secondary battery can be achieved.

  Moreover, according to the manufacturing method of this embodiment, the bulk density of lithium iron phosphate can also be increased. Thereby, handleability can be improved and the production cost of a lithium secondary battery can be reduced.

Claims (3)

A primary firing step of preparing primary particles of lithium iron phosphate by firing a mixture of a lithium raw material, an iron raw material, and a phosphorus raw material;
A secondary firing step of preparing secondary particles of lithium iron phosphate coated with carbon by firing a primary particle mixture containing primary particles of lithium iron phosphate obtained in the primary firing step and citric acid; A method for producing a positive electrode active material for a lithium secondary battery.   The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein the primary particle mixture contains polyvinyl alcohol.   The citric acid content in the primary particle mixture is 5% by mass or more and 15% by mass or less based on the content of primary particles of lithium iron phosphate contained in the primary particle mixture. The manufacturing method of the positive electrode active material of the lithium secondary battery of Claim 1 or 2.

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