TW201446706A - Fabricating method of lithium iron phosphate - Google Patents

Abstract

The fabricating method of this invention can be obtained by following steps: an aquaous solution preparing step for preparing an aquaous solution including phosphoric acid and hydroxy carboxylic acid, a first fabricating step for adding iron particles containing 0.1 to 2 mass% of oxygen into the aquaous solution and reacting the phosphoric acid, the hydroxy carboxylic acid and the iron particles of the aquaous solution in an oxidation ambience to fabricate a first reacting solution; a second fabricating step for adding a lithium source into the first reacting solution to fabricate a second reacting solution; a third fabricating step for adding a carbon source into the second reacting solution to fabricate a third reacting solution; a precursor forming step for drying the third reacting solution to form a lithium iron phosphate precursor; and a calcining step for calcining the lithium iron phosphate precursor in a non-oxidizable ambience to obtain a lithium iron phosphate.

Description

Method for producing lithium iron phosphate

The present invention relates to a method for producing lithium iron phosphate.

A small-sized lithium ion battery, which is a secondary battery widely used in mobile devices, can be improved in performance by improvement of a negative active material or an electrolytic solution. On the other hand, regarding the cathode active material, lithium cobaltate (LiCoO ₂ ) or ternary oxide (LiNi _x Co _y Mn _z O ₂ ) mainly containing cobalt of a rare rare metal is used. ;x+y+z=1). These active materials can be said to be expensive, have insufficient thermal stability or chemical stability, and have a risk of releasing oxygen at a high temperature exceeding about 180 ° C to ignite the organic electrolyte, leaving a problem in terms of safety.

Therefore, in order to develop an expanded lithium ion battery that focuses on the use of large-scale equipment, it is necessary to develop a positive electrode active material which is not only cheaper than existing active materials containing rare metals, but also thermally stable, chemically stable, and safe. high.

At present, a new positive electrode active material which is considered to be a powerful substitute for an active material containing a rare metal is an olivine type lithium iron phosphate (LiFePO ₄ , hereinafter referred to as "lithium iron phosphate") which is an iron-based active material. It has low resource constraints, low toxicity and high safety.

Since the lithium iron phosphate does not release oxygen until it reaches about 400 ° C by the strong PO bond in the crystal structure, it is an active material having high safety and further excellent in long-term stability or high-speed charging characteristics. .

However, when lithium iron phosphate is used as the positive electrode active material, in order to ensure positive The high-speed charge/discharge characteristic required for the polar active material is required to improve the electron conductivity of lithium iron phosphate and to shorten the diffusion distance of lithium ions.

As a solution to the above-described requirements, it is effective to coat the surface of the lithium iron phosphate particles with a conductive material and to refine the lithium iron phosphate particles to a particle diameter of about 100 nm to increase the reaction surface area. Further, there is also a report that doping other elements in the lithium iron phosphate particles is effective for improving the electron conductivity or stabilizing the crystal structure.

Therefore, as described above, development of low-cost and stable production of surface coatings has been developed. The method of the fine lithium iron phosphate particles of the conductive material is important for realizing the practical use of lithium iron phosphate as a positive electrode active material.

As a method for producing lithium iron phosphate for the purpose of cost reduction, a method of using inexpensive iron particles (metal iron, iron powder, or the like) as an iron source has been known.

For example, Patent Document 1 describes a method of first preparing a compound in which a metal iron and a phosphate ion are in a free state in an aqueous solution, and then adding lithium carbonate or lithium hydroxide to prepare a precursor of lithium iron phosphate. (precursor) Drying is carried out, and the dried product is once calcined in a temperature range of 300 ° C to 450 ° C. Further, after primary calcination, a substance which generates conductive carbon by thermal decomposition is added, and the second calcination is carried out at 500 ° C to 800 ° C.

Patent Document 2 describes a method in which iron powder, lithium salt, and phosphate group are used. The compound was dissolved in an aqueous solution of citric acid to prepare a precursor, and then the precursor was spray-dried and then calcined at a temperature of 500 ° C or higher.

Patent Document 3 describes the following method: First, it contains phosphoric acid and lemon The iron powder is reacted in an aqueous solution of citric acid, and then lithium hydroxide is added, and then a metal oxide or a metal salt which becomes a conductive oxide by calcination is added to prepare a precursor and dried, and finally The dried product of the precursor is calcined.

Patent Document 4 describes a method in which an iron powder is reacted in an oxidizing atmosphere in an aqueous solution containing phosphoric acid and citric acid, and then lithium carbonate is added, and then a carbon source is added to prepare a precursor and dried. Finally, the dried product of the precursor is calcined.

Prior art literature

Patent literature

Patent Document 1: Japanese Patent No. 4448976

Patent Document 2: Japanese Patent No. 4234463

Patent Document 3: Japanese Patent Laid-Open Publication No. 2007-305585

Patent Document 4: Japanese Patent Laid-Open Publication No. 2013-001605

However, in the method described in Patent Document 1, when a precursor is prepared, 99.9% or more of high-purity metal iron is reacted with phosphoric acid, and thus iron phosphate (Fe ₃ (PO ₄ ) ₂ ) which is a poorly soluble divalent iron compound is produced. The aggregated particles of .8H ₂ O) are formed and grown, and the solution becomes a creamy, high-viscosity substance which is white to light blue. As a result, there is a problem that the stirring of the solution is insufficient and the unreacted metallic iron is likely to remain, and the raw materials are not uniformly mixed or the like.

Further, Patent Document 1 discloses that an acid such as hydrochloric acid or oxalic acid is added to promote the reaction of unreacted metallic iron. However, in the case of adding hydrochloric acid, the product is easily oxidized, and in the case where oxalic acid is added, stable iron oxalate is formed by using a monomer as a precipitate, and it is difficult to prepare a uniform precursor.

Further, in the method described in Patent Document 1, since it is necessary to perform calcination twice, the number of steps is increased.

Further, the method described in Patent Document 2 is a lithium salt and a phosphoric acid compound. A method of dissolving iron powder in a coexisting aqueous citric acid solution has a problem that it is uneconomical to dissolve for a long time, and it is necessary to oxidize iron by using an organic acid or a mixed organic acid to form an effective divalent iron. On the one hand, it is difficult to make the divalent iron stably exist only by mixing the raw materials.

Further, in the method described in Patent Document 2, when the lithium salt is lithium nitrate, there is a problem that the nitrate ions act as an oxidizing agent during firing, and when the lithium salt is lithium acetate, since the raw material price is high, the raw material price is high. There are problems in achieving cost reduction.

Further, in the method described in Patent Document 3, iron powder and phosphoric acid are generated. At the time of the reaction, citric acid does not function effectively as a chelating agent, and thus there is a problem in that iron in the precursor is oxidized to trivalent to form iron phosphate as a trivalent iron compound.

Further, in the method described in Patent Document 4, the iron in the precursor is 2 The valence compound is present in a state of a valence compound, but the granulated powder formed by spray drying has deformed particles having depressions or voids, and thus has a problem of poor filling properties and electrode coating properties in the electrode.

As described above, as a method of producing lithium iron phosphate, iron is used in an iron source. In the case of particles, in the prior art, the reaction of the iron particles cannot be sufficiently controlled, and even if it is controllable, the granulated powder is deformed in the spray drying step, so that the filling property and the electrode coating property in the electrode are problematic.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an iron phosphate A method for producing lithium, which is prepared by controlling the reaction of iron particles and uniformly mixing at an atom level, and preparing a granulated powder of a solid spherical lithium iron phosphate precursor, which is inexpensive and excellent in discharge capacity. .

The inventors conducted active research in order to achieve the above object. As a result, it has been found that in order to produce a lithium iron phosphate having high performance and a high discharge capacity as a positive electrode active material from an inexpensive iron particle raw material, it is effective to make not only a hydroxycarboxylic acid when reacting the iron particles with phosphoric acid. Coexisting and reacting in an oxidizing atmosphere, thereby continuously replenishing oxygen chemically bonded to the surface of the iron particles.

Moreover, it was found that uniform dispersion in an aqueous solution was obtained by the above reaction. In the case of a chelate of iron phosphate, when a lithium source is continuously added, a precursor of lithium iron phosphate which is obtained by uniformly mixing raw materials (such as iron particles, a phosphorus source, and a lithium source) at an atomic level is obtained.

Further, it has been found that by adding a dispersant to the solution of the lithium iron phosphate precursor, the spray-dried granulated powder is solidly spheroidized, and finally calcined in a non-oxidizing atmosphere, whereby a high-capacitance phosphoric acid can be produced. Iron lithium. Further, in order to make up the conductive path between the particles of lithium iron phosphate, a conductive material may be added to the solution of the lithium iron phosphate precursor.

That is, the present invention provides the following 1 to 5.

A method for producing lithium iron phosphate, comprising: an aqueous solution preparation step of preparing an aqueous solution containing phosphoric acid and a hydroxycarboxylic acid; and a first production step of adding 0.1% by mass to 2% by mass to the aqueous solution; The oxygen-containing iron particles react the phosphoric acid and the hydroxycarboxylic acid in the aqueous solution with the iron particles in an oxidizing atmosphere to prepare a first reaction liquid, and in the second production step, a lithium source is added to the first reaction liquid. a second reaction liquid is prepared; in the third production step, an ammonium salt of polyacrylic acid and/or a naphthalenesulfonic acid formalin condensate as a dispersing agent is added to the second reaction liquid to prepare a third reaction liquid; and a precursor production step a granulated powder obtained by spray-drying the third reaction liquid to form a lithium iron phosphate precursor; and a calcination step of calcining the lithium iron phosphate precursor in a non-oxidizing atmosphere to obtain lithium iron phosphate.

2. The method for producing lithium iron phosphate according to the above 1, wherein a conductive material and/or a conductive material precursor are further added to the third reaction liquid.

3. The method for producing lithium iron phosphate according to the above 1 or 2, wherein the amount of the dispersant added is 0.1% by mass to 2% by mass in the third reaction liquid.

4. The method for producing lithium iron phosphate according to any one of the above 1 to 3, wherein the amount of the conductive material and/or the conductive material precursor added is 0.1% by mass to 2% by mass in the third reaction liquid.

5. The method for producing lithium iron phosphate according to any one of the above 1 to 4, wherein the lithium iron phosphate is a positive electrode active material for a secondary battery.

According to the present invention, it is possible to provide a method for producing a spherical lithium iron phosphate powder which is inexpensive, has excellent discharge capacitance, and is excellent in filling property in an electrode.

Fig. 1 is an electron micrograph of lithium iron phosphate powder (granulated powder) in Inventive Example 1 and Comparative Example 1.

The method for producing lithium iron phosphate according to the present invention (hereinafter also referred to as "the production method of the present invention") is a method for producing lithium iron phosphate comprising the following steps: an aqueous solution preparation step, preparing an aqueous solution containing phosphoric acid and a hydroxycarboxylic acid; In the production step, iron particles containing 0.1% by mass to 2% by mass of oxygen are added to the aqueous solution, and the phosphoric acid and the hydroxycarboxylic acid in the aqueous solution and the iron particles are generated in an oxidizing atmosphere. The first reaction liquid is prepared by the reaction; in the second production step, a lithium source is added to the first reaction liquid to prepare a second reaction liquid; and in the third production step, a dispersant is added to the second reaction liquid to prepare a third reaction. a precursor; a precursor forming step of spray-drying the third reaction liquid to form a granulated powder of a lithium iron phosphate precursor; and a calcining step of calcining the granulated powder of the lithium iron phosphate precursor in a non-oxidizing atmosphere And lithium iron phosphate is obtained.

Hereinafter, the production method of the present invention will be described in detail.

[Aqueous solution preparation step]

The aqueous solution preparation step is a step of preparing an aqueous solution containing phosphoric acid and a hydroxycarboxylic acid. Details of phosphoric acid and hydroxycarboxylic acid contained in the aqueous solution prepared in the aqueous solution preparation step will be described later. Further, the water contained in the aqueous solution is not particularly limited, and for example, ion-exchanged water or distilled water is preferably used.

[1st production step]

The first production step is a step of adding iron particles containing 0.1% by mass to 2% by mass of oxygen to the aqueous solution prepared in the aqueous solution preparation step, and making the phosphoric acid and the hydroxycarboxylic acid in the aqueous solution under an oxidizing atmosphere. The first reaction liquid was prepared by reacting with iron particles.

In the first production step, iron particles containing 0.1% by mass to 2% by mass of oxygen are added to an aqueous solution containing phosphoric acid and a hydroxycarboxylic acid, and reacted in an oxidizing atmosphere to form a chelate of iron phosphate. Things.

As the iron particles, for example, iron powder can be used. Specific examples of the iron powder include reduced iron powder obtained by reducing rust skin (iron oxide) by coke, and high utilization. The atomized iron powder obtained by pulverizing and cooling the molten steel, and the electrolytic iron powder obtained by electrolyzing and depositing an aqueous solution of the iron salt to the cathode.

In terms of the average particle size of the iron powder, due to the promotion and subsequent phosphoric acid and hydroxyl groups The reaction of the carboxylic acid to form a chelate compound of iron phosphate is advantageous, and therefore it is preferably 100 μm or less, more preferably 30 μm to 80 μm. Further, the average particle diameter was determined in accordance with Japanese Industrial Standards (JIS) M 8706.

The usual general industrial iron powder has an average particle diameter of 70 μm to 80 μm, but since it also contains particles having a maximum particle diameter of 150 μm to 180 μm, it is preferred to remove coarse particles by a screen or mechanically pulverize as needed. The coarse particle is made fine, and the reaction area is increased to be used.

In addition, in the present specification, the "iron particles" used in the present invention may be appropriately referred to as "iron", "metal iron" or "iron powder".

In the present invention, the oxygen contained in the iron particles means oxygen chemically bonded to iron. By setting the oxygen content of the iron particles to 0.1% by mass to 2% by mass, preferably 0.6% by mass to 2% by mass, the formation of the iron phosphate chelate compound at the initial stage of the reaction is promoted.

On the other hand, when the oxygen content of the iron particles is less than 0.1% by mass, direct reaction of metallic iron with phosphoric acid is preferred, and iron phosphate (Fe ₃ (PO ₄ ) ₂ .8H ₂ O) which is a poorly soluble divalent iron compound is preferable. Condensed particles are easy to generate. growing up. Therefore, the aqueous solution may be a creamy high-viscosity substance which is white to light blue. As a result, there is an obstacle that the stirring of the aqueous solution is insufficient and the unreacted metallic iron is likely to remain, and the raw materials are not uniformly mixed or the like.

On the other hand, if the oxygen content of the iron particles exceeds 2% by mass, the surface of the iron powder is biased. The scale of the iron oxide is precipitated, thereby preventing the reaction of the phosphoric acid with an aqueous solution of the hydroxycarboxylic acid.

The iron content of the iron particles is, for example, a reduced iron powder obtained by reducing only rust skin (iron oxide) by coke, or an atomized iron powder obtained by drying only the molten steel with high pressure water and cooling. In the case of the iron particles, the oxygen content is usually 0.5% by mass to 1.5% by mass. Further, the oxygen content at the time of hydrogen reduction of the iron particles is often 0.1% by mass to 0.4% by mass.

The iron particles used in the present invention do not particularly need to be a high-cost hydrogen reduction site. The resulting iron particles.

Further, the oxygen content of the iron particles is determined by a vacuum melting infrared absorption method according to JIS Z 2613 (1992) and using TC436 manufactured by LECO.

Occurring by adding iron particles to an aqueous solution containing phosphoric acid and a hydroxycarboxylic acid The atmosphere at the time of the reaction needs to be an oxidizing atmosphere.

When a chelate-forming reaction (hereinafter also referred to as "chelation reaction") is carried out and oxygen on the surface of the iron particles is consumed, the chelation reaction cannot be continued, and the direct reaction between the metal iron and the phosphate ion is preferred, so that it is poorly soluble. Aggregate particle formation of iron phosphate. growing up.

Therefore, in the present invention, the chelating reaction is continued by supplementing the atmosphere in which the surface of the iron particles is appropriately oxidized and chemically bonded to the iron particles by setting the atmosphere during the reaction to an oxidizing atmosphere.

The oxidizing atmosphere in the present invention is a table capable of bringing iron particles in an aqueous solution a state in which the surface is moderately oxidized, and the state is achieved, for example, by a method of bringing an interface of an aqueous solution into contact with an oxygen-containing gas, a method of blowing an oxygen-containing gas into an aqueous solution, or the like. Specific examples of the methods include, for example, stirring in an air atmosphere or bubbling using air to supply oxygen.

The phosphoric acid is preferably an aqueous solution of orthophosphoric acid (H ₃ PO ₄ ), and a high-order aqueous solution of condensed phosphoric acid (H _n+2 P _n O3 _n+1 ) can also be used. The orthophosphoric acid is usually obtained as an industrial product, that is, an aqueous solution of 75% by mass to 85% by mass.

In terms of the amount of phosphoric acid added, the stoichiometric equivalent is 1 mol with respect to 1 mol of iron, but an additional 0.1 mol may be added.

A hydroxycarboxylic acid is a carboxylic acid having a hydroxyl group and a carboxyl group in one molecule, and is formed as a carboxylic acid. A chelating agent in the case of a chelate of iron phosphate functions.

The hydroxycarboxylic acid to be used in the present invention is an aliphatic hydroxycarboxylic acid, and examples thereof include glycolic acid, gluconic acid, lactic acid, tartaric acid, malic acid, and citric acid having strong chelating power to iron. Among them, preferred are chelating. Citric acid which is strong in combination and forms a chelate which is not easily oxidized. Since the aromatic hydroxycarboxylic acid such as salicylic acid lowers the hydrophilicity of the formed chelate compound, the sustainability of the formation reaction of the chelate compound is lowered, which is not preferable.

Hydroxycarboxylic acid produces carbon residue when calcined, thus It also functions as a reducing agent. In order to exhibit this function, in the present invention, the residual carbon ratio of the hydroxycarboxylic acid is preferably 3% by mass or more. This is because if the residual carbon ratio is less than 3% by mass, the obtained lithium iron phosphate precursor may be oxidized by a trace amount of oxygen in the atmosphere. In addition, when the residual carbon ratio exceeds 20% by mass, the amount of residual carbon after calcination becomes excessive, and thus the residual carbon ratio is preferably 20% by mass or less.

Among them, a preferred residual carbon ratio of the hydroxycarboxylic acid is glycolic acid: 6% by mass, gluconic acid: 20% by mass, lactic acid: 3% by mass, tartaric acid: 4% by mass, and apple. Acid: 3% by mass, and citric acid: 5% by mass.

In addition, the "residual carbon ratio" in the present invention is a value obtained by quantifying carbon remaining after calcination according to JIS G 1211 (1995) high-frequency induction heating furnace combustion-infrared absorption method. The value obtained from the amount of the original hydroxycarboxylic acid.

The amount of the hydroxycarboxylic acid to be added is preferably from 0.1 mol% to 0.5 mol%, more preferably from 0.1 mol% to 0.5 mol, based on 1 mol% to 5 mol% of the iron in the iron particles. Preferably, it is 0.15 mol~0.3 mol.

When the amount of carbon is less than 1% by mass, the conductivity of lithium iron phosphate may be insufficient, and the performance of the lithium iron phosphate particles as the positive electrode active material may not be sufficiently improved. On the other hand, when the amount of carbon exceeds 5% by mass, the apparent discharge capacity may decrease.

In other words, when the amount of the carbon source added is within the above range, the conductivity of lithium iron phosphate is sufficient, and the apparent discharge capacity is also good.

When the amount of the above hydroxycarboxylic acid added is less than 0.1 mol, the effect of chelation of the hydroxycarboxylic acid is reduced, so that the metallic iron reacts directly with the phosphate ion to form agglomerated particles of the poorly soluble iron phosphate. In the case of growing, the aqueous solution is a high-viscosity substance in a creamy state of white to light blue. As a result, there is a problem that the stirring of the aqueous solution is insufficient, and unreacted metallic iron is likely to remain, and the raw materials are not uniformly mixed. .

When the amount added is more than 0.5 mol, the formed chelate of iron phosphate is uniformly dispersed in the aqueous solution (the raw materials are uniformly mixed), but the amount of residual carbon after calcination is excessive, and as a result, the finally obtained phosphoric acid may be obtained. The apparent discharge capacitance of iron lithium decreases. On the other hand, if the above-mentioned addition amount is within the above range, unreacted gold The iron is not easily left, the raw materials are uniformly mixed, and the apparent discharge capacity of the obtained lithium iron phosphate becomes good.

In the chelation reaction in the first production step, the temperature of the aqueous solution is preferably from 10 ° C to 40 ° C, more preferably from 20 ° C to 30 ° C.

When the temperature of the aqueous solution is controlled to be in the range of 10 ° C to 40 ° C, oxygen on the surface of the iron particles is consumed by the above chelation reaction, and then the surface of the newly formed iron particles is brought into contact with dissolved oxygen or air bubbles in the aqueous solution. It is moderately oxidized, whereby a chelate of iron phosphate can be continuously formed.

On the other hand, when the temperature of the aqueous solution is less than 10 ° C, the chelation reaction of the iron particles becomes slow, and it takes a long time until the reaction is completely completed.

On the other hand, when the temperature of the aqueous solution exceeds 40 ° C, the surface of the iron particles which have been consumed by oxygen is too late to undergo oxidation for supplemental oxygen, and therefore, there is a case where the direct reaction of metallic iron with phosphoric acid is preferred and the phosphoric acid which is poorly soluble Iron condensed particles are generated. When grown, the aqueous solution becomes a creamy, high-viscosity substance that is white to light blue.

In the present invention, by adding iron particles to an aqueous solution containing phosphoric acid and a hydroxycarboxylic acid and exposing them to an oxidizing atmosphere, the hydroxycarboxylic acid is chelated with iron via oxygen or a hydroxyl group present on the surface of the iron particles. Further, in the case where the hydroxycarboxylic acid is citric acid, the phosphoric acid forms a chelate of iron phosphate represented by the following chemical formula (1), and it is presumed that the chelate is uniformly obtained. The first reaction liquid dispersed.

[Chemical 1]

[Second production step]

The second production step is a step of preparing a second reaction liquid by adding a lithium source to the first reaction liquid prepared in the first production step.

In the second production step, a lithium source is added to the first reaction liquid in which the chelate compound represented by the above chemical formula (1) is uniformly dispersed, thereby obtaining lithium iron phosphate in which the raw materials are uniformly mixed at an atomic level. Precursor.

The lithium source to be added to the first reaction liquid is not particularly limited as long as it is a water-soluble lithium salt. However, lithium hydroxide or lithium carbonate is preferred because no harmful gas is generated during firing.

When a lithium source is added to the first reaction liquid, the reaction liquid becomes dark green, and a second reaction liquid having a pH of 6 to 7 is obtained. Further, when X-ray diffraction analysis is performed on the dried product obtained by drying the second reaction liquid, it is confirmed that the crystalline compound is not detected, and the chelate compound uniformly mixed at the atomic level causes amorphous. Amorphous phase.

When a lithium source is added to the first reaction liquid to prepare a second reaction liquid, a part of hydrogen in the carboxyl group of the chelate compound represented by the above chemical formula (1) is substituted with lithium. When the hydroxycarboxylic acid is citric acid, it is presumed that a chelate compound of lithium iron phosphate represented by the following chemical formula (2) is produced.

The chelate compound of lithium iron phosphate is dispersed in the second reaction liquid, but a part of the chelate compound may exist as aggregated particles to form a precipitate.

In the above case, in order to achieve homogenization of the precursor solution before drying in the next step, it is preferred to mechanically pulverize and refine the agglomerated particles by a wet method. Further, examples of the wet pulverization method include methods using ultrasonic irradiation, a jet mill, and a beads mill.

[3rd production step]

The third production step is a step of preparing a third reaction liquid by adding a dispersant to the second reaction liquid prepared in the second production step.

The dispersing agent to be added to the second reaction liquid is an ammonium salt of a polyacrylic acid and/or a naphthalenesulfonic acid formalin condensate.

The dispersing agent is adsorbed on the surface of the precursor particles, and acts to generate a charge or a three-dimensional repulsive force between the particles so that the distance between the particles is kept constant, so that the particles in the droplets are deviated during spray drying described later. It is inhibited and granulated into a spherical shape. Therefore, the molecular weight of the dispersant is preferably in the range of 1,000 to 10,000, more preferably in the range of 3,000 to 6,000, because the particle size of the dispersed particles is submicron. Further, the dispersant is preferably added to the third reaction liquid in an amount of 0.1% by mass to 2% by mass. This is because the granulated powder is spheroidized by satisfying the range. That is, when it is less than 0.1% by mass, the dispersing agent adsorbed on the surface of the dispersed particles is insufficient, so that the dry granulated powder cannot be spheroidized, and when it exceeds 2% by mass, the dispersing agent becomes excessive, and thus the remaining amount may be on the battery. Characteristics cause adverse effects.

In the present invention, a conductive material and/or a conductive material may be added to the third reaction liquid. Material precursor. In this case, the conductive material and/or the conductive material precursor may be added in a total amount of 0.1% by mass to 2% by mass in the third reaction liquid. This is because the conductivity between the particles is increased by satisfying the range. That is, when it is less than 0.1% by mass, there is a problem that the conductive path is insufficient, and in particular, the high-rate charge and discharge characteristics are deteriorated. When the amount exceeds 2% by mass, the battery characteristics are saturated, and on the other hand, the conductive material is excessive, and thus the apparent discharge capacity is lowered.

The conductive material added to the third reaction liquid may be at least one selected from the group consisting of carbon black, carbon nanotubes, and graphite in order to complement the conductive path between the particles of lithium iron phosphate. The form is preferably an aqueous dispersion. As the carbon black, acetylene black, ketjen black, or the like can be used. As the carbon nanotube, a single-wall type, a multi-wall type, or the like can be used as the carbon black tube. For graphite, artificial graphite, natural graphite, or the like can be used.

Further, at least one of the organic material nanofibers selected from the group consisting of cellulose nanofibers and aromatic polyamide fibers can be used as the conductive material precursor to be added to the third reaction liquid. Jia is an aqueous dispersion.

[Precursor generation step]

The precursor formation step is a step of spray-drying the third reaction liquid prepared in the third production step to form a granulated powder of a lithium iron phosphate precursor.

The lithium iron phosphate precursor which is a dry granulated powder of the third reaction liquid has a spherical shape, and the particle diameter thereof is not particularly limited, but is preferably 5 μm to 30 μm from the viewpoint of easy handling.

As a method of drying the third reaction liquid, it is based on the principle of good drying efficiency. A spray drying method is employed. This is because the spray drying method is a method of spraying a sample solution onto a high-temperature heated air and drying it, thereby obtaining a granulated powder having a uniform shape.

Here, the particle size of the dry granulated powder can be determined by the solid concentration of the solution, It is adjusted as a rotary disc or nozzle type of the spray method. The inlet temperature (heated air temperature) of the spray drying apparatus used in the spray drying method is preferably set to 100 ° C to 250 ° C. This is because if the inlet temperature is set to 100 ° C to 250 ° C, the temperature at which the dried product is reached depends on the equilibrium relationship with the amount of liquid supplied, and if it is 80 ° C to 150 ° C, the above-mentioned required content can be obtained. shape.

[calcination step]

The calcination step is a step of calcining a lithium iron phosphate precursor (granulated powder) produced in the precursor formation step in a non-oxidizing atmosphere to obtain lithium iron phosphate.

The lithium iron phosphate precursor formed in the precursor formation step is calcined in a non-oxidizing atmosphere, and the hydroxyl group and the organic substance contained in the lithium iron phosphate precursor are thermally decomposed to be H ₂ O, CO ₂ , H ₂ and a hydrocarbon are removed, and a dried product having an amorphous phase is crystallized to become a crystal of lithium iron phosphate having an olivine structure, and the thermally decomposed carbon is precipitated on the surface of the particles of lithium iron phosphate.

In addition, in the non-oxidizing atmosphere, for example, the oxygen concentration is 1000 ppm. In the case of an inert gas atmosphere such as nitrogen or argon, a reducing gas atmosphere containing a reducing gas such as hydrogen or carbon monoxide is used. Calcination is carried out in a non-oxidizing atmosphere to prevent oxidation.

The calcination temperature in the calcination step is preferably 300 ° C or higher, more preferably 500 ° C The above is further preferably 600 ° C to 800 ° C.

This is because, when the calcination temperature is less than 300 ° C, not only the thermal decomposition of H ₂ O, CO ₂ , H _{2 ,} and hydrocarbons, which are volatile components, may be insufficiently removed, and crystallization may not occur. On the other hand, when the calcination temperature exceeds 800 ° C, the obtained crystal particles may be coarsened to form a by-product such as Fe ₂ P.

However, when the calcination temperature is in the above range, the thermal decomposition removal and crystallization of the volatile component are sufficiently performed, and the coarsening of the crystal particles or the formation of by-products such as Fe ₂ P are also suppressed.

Obtaining a lithium iron phosphate precursor by calcination in a non-oxidizing atmosphere The primary particle diameter of lithium iron phosphate is preferably 200 nm or less, and more preferably 50 nm to 150 nm, for the reason of shortening the diffusion distance of lithium ions. The primary particle diameter is the smallest unit of crystallization from the crystal grain size.

As described above, according to the manufacturing method of the present invention, the raw material is obtained as the original A solution (second reaction liquid) of a lithium iron phosphate precursor which is uniformly mixed in a sub-level, a dispersant is added to the solution, spray-dried, and then calcined in a non-oxidizing atmosphere, whereby a discharge capacitor can be produced. High solid spherical shape of lithium iron phosphate.

In addition, the lithium iron phosphate obtained by the production method of the present invention has a high discharge capacity, and is suitable for use as a positive electrode active material for a secondary battery such as a lithium ion battery. Further, in the present invention, since inexpensive iron particles are used, cost reduction is also achieved.

Further, in the present invention, a conductive material or a conductive material precursor is added, and After spray drying, calcination is carried out in a non-oxidizing atmosphere to complement the conductive path, whereby a solid spherical lithium iron phosphate having a higher discharge capacity can be produced.

Example

Hereinafter, the present invention will be specifically described by way of examples. However, the invention is not limited to these.

[Inventive Example 1]

10 g of 85% by mass of phosphoric acid and 2 mol of citric acid monohydrate were dissolved in 2000 g of distilled water, and 10 mol of iron powder was added to the mixed solution (manufactured by JFE Steel Co., Ltd., oxygen content: 0.68 mass%, average particle diameter: 80 μm), and reacted for one day while stirring at a liquid temperature of 20 ° C and air bubbling. Then, 5 mol of lithium carbonate was added to form lithium iron phosphate, and finally, 8 g of polyacrylic acid (manufactured by Wako Pure Chemical Industries, Ltd., molecular weight: 5000) as a dispersing agent was added to prepare a precursor solution.

Use a spray dryer (manufactured by Okawara Chemical Machinery Co., Ltd. NL5) to The precursor solution was dried at a mouth temperature of 200 ° C to obtain a dried granulated powder having an average particle diameter of about 12 μm as observed by a scanning electron microscope (SEM). The dried granulated powder was calcined at 750 ° C for 5 hours in a nitrogen stream, and finally sieved at 45 μm with a mesh to prepare lithium iron phosphate.

In addition, TC436 manufactured by LECO Corporation was used to quantify the oxygen content of the iron powder.

[Inventive Example 2]

Lithium iron phosphate was prepared in the same manner as in Inventive Example 1, except that 2 mol of lactic acid was used instead of the above citric acid monohydrate.

[Inventive Example 3]

Lithium iron phosphate was prepared in the same manner as in Inventive Example 1, except that 2 mol of malic acid was used instead of the above citric acid monohydrate.

[Inventive Example 4]

Lithium iron phosphate was prepared in the same manner as in Inventive Example 1, except that 2 mol of tartaric acid was used instead of the above citric acid monohydrate.

[Inventive Example 5]

Lithium iron phosphate was prepared in the same manner as in Inventive Example 1, except that 1.8 mol of citric acid monohydrate and 0.02 mol of gluconic acid were used instead of the above citric acid monohydrate.

[Inventive Example 6]

Lithium iron phosphate was prepared in the same manner as in Inventive Example 1, except that iron powder (manufactured by JFE Steel Co., Ltd., oxygen content: 0.41% by mass, average particle diameter: 80 μm) was used instead of the above iron powder.

[Inventive Example 7]

Lithium iron phosphate was prepared in the same manner as in Inventive Example 1, except that 20 g of a sodium salt of a naphthalenesulfonic acid condensate ammonium salt (manufactured by Sannopco, 40% by mass) was used instead of the polyacrylic acid of the above dispersing agent.

[Inventive Example 8]

A sample was prepared in the same manner as in Inventive Example except that a graphite dispersion (solvent: water, dispersant: polyacrylic acid, average particle diameter: 0.8 μm, graphite concentration: 10% by mass) of 80 g of a conductive material was added to the precursor solution. Lithium iron phosphate.

[Comparative Example 1]

Lithium iron phosphate was prepared in the same manner as in Inventive Example 1 except that the dispersant was not added.

[Comparative Example 2]

Lithium iron phosphate was prepared in the same manner as in Inventive Example 1, except that ammonium polyacrylate having a molecular weight of 13,000 was added as a dispersing agent.

Prepared in Inventive Example 1 to Inventive Example 8 and Comparative Example 1 to Comparative Example 2 Each lithium iron phosphate was subjected to identification analysis (identification of a generated phase) by X-ray diffraction analysis and quantification of carbon. Further, the shape and particle diameter of the granulated powder of each lithium iron phosphate were measured by SEM. In addition, in the test, the measurement was performed by arbitrarily selecting 10 pieces from the SEM screen (a screen observed by 1000 times to 0.04 mm × 0.12 mm) in each test.

The X-ray diffraction analysis was carried out using Ultimate V (X-Ray: Cu-Kα1) manufactured by Rigaku Corporation. The carbon is quantified using EMIA620 manufactured by HORIBA to add the carbon content of lithium iron phosphate. Quantitative. The measurement results of the above-described identification analysis, carbon amount, particle shape, and particle diameter are shown in Table 1.

Further, each of lithium iron phosphate prepared in Inventive Example 1 to Inventive Example 8 and Comparative Example 1 to Comparative Example 2 was measured for discharge capacity by the following method.

For the current collector of the aluminum foil, lithium iron phosphate was coated at 10 mg/cm ² : Vapor-grown carbon fiber (VGCF): polyvinylidene fluoride (KFL #1320 manufactured by Kureha Co., Ltd.) =86: 4:10 (mass ratio) paste, and pressed at 80 kN to make a positive electrode. The negative electrode is a half cell (manufactured by Baoquan) using metal lithium. The electrolyte used was 1M-LiPF ₆ /EC (ethylene carbonate): EMC (ethyl methyl carbonate) = 3:7 (mass ratio). The measurement conditions were as follows: after constant current charging was performed at 0.2 mA/cm ² until 4.0 V, the constant current discharge was performed at 0.2 mA/cm ² until 2.5 V, and the discharge capacity was determined.

The above measurement results are shown together in Table 1.

According to Table 1, it is understood that in the invention examples 1 to 8, all of the olivine-type lithium iron phosphate is obtained, and the amount of carbon of the lithium iron phosphate is 1.7% by mass to 2.3% by mass, and the solid spherical shape is 12 μm to 15 μm. The electrode density was 2.1 g/cm ³ and the discharge capacity was high.

Further, Fig. 1 shows SEM (electron micrograph) photographs of the appearance and cross section of Inventive Example 1 and Comparative Example 1.

As is apparent from Fig. 1, the lithium iron phosphate powder of the present invention has a spherical shape and is solid.

On the other hand, in Comparative Example 1 to Comparative Example 2, the electrode density was low and lithium iron phosphate having a sufficient discharge capacity could not be obtained. This is presumably because in Comparative Examples 1 to 2, the granulated powder was deformed as shown in Fig. 1, and thus the electrode film thus formed became uneven.

Claims (7)

A method for producing lithium iron phosphate, comprising: preparing an aqueous solution, preparing an aqueous solution containing phosphoric acid and a hydroxycarboxylic acid; and in the first production step, adding iron particles containing 0.1% by mass to 2% by mass of oxygen to the aqueous solution, The phosphoric acid and the hydroxycarboxylic acid in the aqueous solution are reacted with the iron particles in an oxidizing atmosphere to prepare a first reaction liquid; and in the second production step, a lithium source is added to the first reaction solution to prepare a second reaction liquid; a third production step, wherein an ammonium salt of polyacrylic acid and/or a naphthalenesulfonic acid formalin condensate as a dispersing agent is added to the second reaction liquid to prepare a third reaction liquid; and a precursor production step a granulated powder obtained by spray-drying the third reaction liquid to form a lithium iron phosphate precursor; and a calcination step of calcining the lithium iron phosphate precursor in a non-oxidizing atmosphere to obtain lithium iron phosphate. The method for producing lithium iron phosphate according to claim 1, wherein a conductive material and/or a conductive material precursor are further added to the third reaction liquid. The method for producing lithium iron phosphate according to the first aspect of the invention, wherein the amount of the dispersant added is 0.1% by mass to 2% by mass in the third reaction liquid. The method for producing lithium iron phosphate according to the second aspect of the invention, wherein the amount of the dispersant added is 0.1% by mass to 2% by mass in the third reaction liquid. The method for producing lithium iron phosphate according to any one of claims 1 to 4, wherein the amount of the conductive material and/or the conductive material precursor added is 0.1 in the third reaction liquid. Mass%~2% by mass. The method for producing lithium iron phosphate according to any one of the preceding claims, wherein the lithium iron phosphate is a positive electrode active material for a secondary battery. The method for producing lithium iron phosphate according to the fifth aspect of the invention, wherein the lithium iron phosphate is a positive electrode active material for a secondary battery.