JP5776573B2 - Positive electrode active material for lithium secondary battery and method for producing the same, precursor of the positive electrode active material and method for producing the same, and lithium secondary battery using the positive electrode active material - Google Patents

Description

  The present invention relates to a positive electrode active material for a lithium secondary battery, a production method thereof, a precursor of the positive electrode active material, a production method thereof, and a lithium secondary battery using the positive electrode active material.

Lithium-ion secondary batteries are lightweight and have high energy density, so they are widely used in small batteries such as mobile phones, laptop computers, and other IT devices. For these applications, LiCoO ₂ , LiCoNiMnO ₂ , A layered rock salt positive electrode material such as LiNiAlO ₂ is used. The demand is growing on a global scale with the development and popularization of IT equipment.
In addition to these small batteries, industrial large batteries can also be used for hybrid vehicles (HEV), plug-in hybrid vehicles (PHEV), electric vehicles (EV), power leveling, power storage, etc. In addition, the demand is expected to expand in various fields, and research and development are actively conducted.

Under such circumstances, high safety, high life, high output, and low price are required for the positive electrode material as a problem for full-scale commercialization of large industrial batteries. Among them, lithium iron phosphate (LiFePO ₄ ), which exhibits high safety and excellent cycle performance and can be manufactured at a low price, has attracted attention as an alternative positive electrode material such as LiCoO ₂ and LiMn ₂ O ₄ .
Since LiFePO ₄ has a strong skeleton of phosphoric acid, it is a safe material with a high discharge capacity and a good cycle life. In addition, Fe is cheaper than Co and has much fewer resource constraints. However, the current situation is not necessarily superior to the conventional positive electrode material in terms of performance and cost, and is not sufficiently accepted by the market. In such a situation, providing low-cost and high-performance LiFePO ₄ has a great social significance for the practical use of large batteries.

Since the synthesis of LiFePO ₄ is unstable because divalent iron is easily oxidized, various methods have been studied. Moreover, since LiFePO ₄ has low electron conductivity, good electrode characteristics cannot be obtained with particles of several microns or more. For this reason, it is known that fine particles can be refined to several tens to several hundreds of nanometers, and further conductivity can be imparted by coating / compositing with a conductive material such as graphite to obtain good characteristics. Yes.
However, the additional processing increases the manufacturing cost of LiFePO ₄ and loses the material superiority. Therefore, it is important to obtain fine LiFePO ₄ particles by a method as simple as possible and to be able to combine conductive materials.
As a general method for producing LiFePO ₄ , iron oxalate, which is a divalent iron raw material, is mixed with lithium carbonate and ammonium dihydrogen phosphate and baked, so that high purity lithium iron phosphate is obtained by solid-phase reaction. It is known to be obtained (for example, refer to Patent Document 1).
However, since the price of iron oxalate is very high and the yield upon firing is low, the resulting LiFePO ₄ is very expensive and is not suitable for large batteries in terms of price. Moreover, since iron oxalate is toxic, it is not preferable in terms of human body and environment, and a method for synthesizing a large amount has not been established.
Furthermore, in general, in the solid phase method, since the temperature required for the reaction is high, the primary particles are easily coarsened and agglomerated, and the particle diameter becomes large. Further, as described above, LiFePO ₄ has low electrical conductivity, and thus LiFePO ₄ obtained in this way requires a powerful refinement process.
A manufacturing method using lithium phosphate and iron phosphate as raw materials has also been proposed (see, for example, Patent Document 2). Since harmless phosphate is used and only H ₂ O is produced as a by-product during firing, it is industrially advantageous. However, this proposal is also a solid-phase reaction, and Fe ₃ (PO ₄ ) ₂ · nH ₂ O, which is a raw material for the synthesis of LiFePO ₄ , has low reactivity with a lithium raw material, and further requires high-temperature firing. The coarsening is remarkable. A powerful micronization process is required. In the fine particle treatment, it is common to use alumina or zirconia spheres as a medium in a wet mill, but impurities are likely to be mixed during pulverization, and a drying treatment after treatment is also required. There is a risk.
As described above, the solid-phase reaction method has a very good efficiency in the firing step, but there is a problem that the coarsening of the particles due to the high-temperature treatment cannot be avoided, and it takes much time and cost.

Moreover, the method of using an iron phosphate ammonium salt as a reaction precursor is disclosed (for example, refer patent document 3). Since ammonium salt is used, there is no residue of sodium (Na), and high-quality LiFePO ₄ is obtained. However, since it is mixed and baked with lithium salts such as lithium hydroxide and lithium carbonate, Grain growth is inevitable.
In addition, LiFePO ₄ having low conductivity needs to be provided with conductivity by a conductive material such as graphite, and it is desirable that the conductive material uniformly coats the surface of the LiFePO ₄ particles. For this purpose, it is necessary to refine the LiFePO ₄ , add a carbon source, and perform heat treatment again, which increases the process and costs. In the solid-phase reaction method, if a carbon source is added at the precursor stage, the synthesis reaction of LiFePO ₄ is inhibited. Therefore, it is extremely difficult to perform conductive carbon complexation by one-step heat treatment.

On the other hand, LiFePO ₄ can be obtained by synthesizing raw materials by a hydrothermal method and firing at a low temperature, and production of LiFePO 4 by a method based on hydrothermal synthesis has been proposed (for example, Patent Document 4). reference.). According to this hydrothermal method, LiFePO ₄ can be synthesized by a low-temperature reaction, so that it is clear that fine particles having a particle size of 100 nm or less can be obtained. However, the hydrothermal method has low concentrations of Li, Fe, and P raw materials with respect to the solvent, and uses a high-pressure vessel for the reaction, so that it is not suitable for efficient mass synthesis, and is inexpensive in price. It is not suitable for large batteries that require this.
As described above, the conventional production methods have the above-mentioned problems, and a method for producing fine and high-performance LiFePO ₄ at an industrial cost has not been developed.

JP 2000-294238 A JP 2003-292309 A JP 2006-056754 A JP 2005-276474 A

  An object of the present invention is to provide a uniform fine precursor suitable as a raw material for producing a positive electrode active material for a lithium secondary battery (hereinafter sometimes simply referred to as a positive electrode active material) in view of the above-mentioned problems of the prior art. In addition to providing a positive electrode material for a uniform and fine lithium ion secondary battery, a method for manufacturing the same, and a lithium ion secondary battery having high capacity and high output using the positive electrode material. There is to do.

  In order to solve the above problems, the present inventors have conducted extensive research on a precursor of lithium iron phosphate that can be synthesized by low-temperature heat treatment. As a result, a coprecipitate of iron and phosphoric acid, It has been found that the above precursor of lithium iron phosphate can be obtained by reacting a compound with an alcohol, and the present invention has been completed based on this finding.

According to a first aspect of the present invention, there is provided a method for producing a positive electrode active material for a lithium ion secondary battery, comprising lithium, iron and phosphoric acid, wherein the molar ratio of lithium: iron: phosphoric acid is 1: 0. .8: 0.8 to 1: 1.2: 1.2 A composition comprising lithium iron phosphate having a carbon and nitrogen content of 0.4 or less in terms of molar ratio to iron Including a step of heat-treating a precursor of a positive electrode active material for an ion secondary battery at 200 to 500 ° C. in an inert or reducing atmosphere , the precursor comprising a coexisting raw material solution of divalent iron ions and phosphate ions The first step of obtaining a coprecipitate containing iron and phosphoric acid by adding and coprecipitating in a reaction aqueous solution adjusted to a pH of 7 to 10 and iron in a molar ratio to lithium of 0.8 A with lithium content of ~ 1.2 A second step of reacting the lithium and the co-precipitate with the call solvent slurry, obtained by the production method comprising, method for producing a cathode active material for a lithium ion secondary battery is provided, characterized in that.
According to a second invention of the present invention, the lithium ion secondary battery according to the first invention includes a step of adding a carbon source to the precursor in advance before the heat treatment. A method for producing a positive electrode active material is provided.
Furthermore, according to the third invention of the present invention, the positive electrode active material for a lithium ion secondary battery according to the first invention, further comprising a step of pulverizing the precursor in advance before the heat treatment. A manufacturing method is provided.

Further, according to the fourth aspect of the present invention, obtained by the method for producing a cathode active material for a lithium ion secondary battery according to any one of aspects 1-3, wherein the lithium iron phosphate, the phosphate The average particle diameter of primary particles measured by scanning electron microscope observation of iron lithium is 50 to 200 nm, and is calculated using the Scherrer equation from the peak of lithium iron phosphate in X-ray diffraction with respect to the average particle diameter of primary particles. A positive electrode active material for a lithium ion secondary battery is provided, wherein a ratio of crystallite diameter (crystallite diameter / average particle diameter) is 0.4 or more. Furthermore, according to the fifth aspect of the present invention, there is provided the positive electrode active material for a lithium ion secondary battery according to the fourth aspect , wherein the BET method has a specific surface area of 10 to 50 m ² / g. The

According to a sixth aspect of the present invention, there is provided a lithium ion secondary battery comprising a positive electrode comprising the positive electrode active material for a lithium ion secondary battery according to the fourth or fifth aspect of the invention. .

As described above, the present invention relates to a method for producing a precursor of a positive electrode active material for a lithium ion secondary battery, and the preferred embodiments include the following.
(1) In the first invention, a method for producing a precursor of a positive electrode active material for a lithium ion secondary battery, wherein water-soluble normal phosphoric acid is used as a phosphate ion supply source in the first step. .
(2) The lithium ion according to the first invention, wherein the lithium ion source in the second step uses fine particles having a particle size of a general lithium compound such as lithium hydroxide and lithium carbonate of 200 μm or less. The manufacturing method of the precursor of the positive electrode active material for secondary batteries.
(3) In the first invention, the alcohol in the second step is ethanol or isopropanol having a water content of 10% by mass or less, preferably 5% by mass or less, and a positive electrode for a lithium ion secondary battery A method for producing a precursor of an active material.

According to the method for producing a precursor of a positive electrode active material for a lithium ion secondary battery of the present invention, a suitable uniform fine precursor can be obtained as a raw material for producing a positive electrode active material for a lithium secondary battery. The positive electrode active material can be easily obtained by heat-treating the precursor of the active material.
In addition, according to the method for producing a positive electrode active material for a lithium ion secondary battery of the present invention, since lithium, iron, and phosphoric acid are wet-reacted at the precursor stage, a high synthesis temperature is obtained in the synthesis of the positive electrode active material. This is not necessary, and the lithium iron phosphate particles can be easily produced industrially.
Furthermore, the obtained positive electrode active material is composed of uniform and fine lithium iron phosphate particles. Using the positive electrode active material, a lithium ion secondary battery in which a positive electrode is formed has excellent physical properties such as high capacity and high output. Industrial value is extremely high.

1 is an X-ray diffraction chart of a precursor obtained in Example 1 according to the present invention. It is a figure which shows the particle size distribution of the volume integration of the silver powder in Example 1 which concerns on this invention.

  Hereinafter, the present invention will be described in detail for each item.

1. Method for Producing Precursor of Positive Electrode Active Material for Lithium Ion Secondary Battery The method for producing a precursor of a positive electrode active material for lithium ion secondary battery according to the present invention is a first step of obtaining a coprecipitate containing iron and phosphoric acid. And a second step of reacting lithium and the coprecipitate in an alcohol solvent slurry.

In the said 1st process, the coexisting raw material solution of a bivalent iron ion and phosphate ion is added and coprecipitated in the reaction aqueous solution adjusted to the range of pH 7-10. Since the raw material solution is acidic, a coprecipitate can be obtained by dropping an alkali into the reaction aqueous solution and dropping the raw material solution while maintaining the pH in the above range.
In the raw material solution in which the phosphate anion and the iron ion are mixed, the composition becomes completely uniform, and this is dropped into the reaction aqueous solution controlled to pH 7 to 10, preferably 7 to 9, so that the iron and phosphoric acid are mixed. The ratio is strictly controlled, and a coprecipitate having a uniform composition can be obtained. Since the molar ratio of iron and phosphoric acid in the coprecipitate coincides with the molar ratio of the raw material solution, it may be adjusted in the raw material solution. The molar ratio is phosphoric acid with respect to iron with a stoichiometric ratio of 1 as the center. The ratio can be in the range of 0.6 to 1.5, preferably 0.9 to 1.1.

In adjusting the pH of the reaction aqueous solution, one or more selected from sodium hydroxide, lithium hydroxide or ammonia can be used as an alkali source.
Sodium hydroxide is an inexpensive alkali source, and when it is maintained at 9 or less, which is the pH control range, there is little residual sodium (Na) in the coprecipitate. The alkali source is preferably used as an aqueous solution because pH control is easy.
When lithium hydroxide is used, basically, lithium does not precipitate, but it does not matter if a small amount of lithium remains in the coprecipitate.
Furthermore, at a pH of 9 or less, it is possible to adjust the pH with ammonia. However, if the ammonium salt remains, the crystallization in the heat treatment during the production of the positive electrode active material becomes slow, so the nitrogen content is 0 in terms of a molar ratio to iron. .4 or less is preferable.

  After the coprecipitation reaction in the first step, washing can be performed as necessary from the viewpoint of pH adjustment and impurity reduction of the obtained coprecipitate. When sodium hydroxide is used for pH adjustment in the coprecipitation reaction, it is preferable to reduce Na impurities by washing with water. In addition, when ammonia is used for pH adjustment, washing is preferably performed in order to reduce the residual ammonium salt.

  The water used in the first step is preferably water that does not cause impurities to be mixed in either the aqueous reaction solution or the washing, and more preferably pure water, distilled water, or ion-exchanged water is used.

As the divalent iron ion supply source, it is preferable to use ferrous sulfate (FeSO ₄ ). Ferrous sulfate is easy to obtain and contains less impurities in the coprecipitate. The phosphate ion supply source may be any water-soluble source, and normal phosphoric acid can be preferably used, but is not particularly limited.

Next, in the second step, a precursor of lithium iron phosphate (LiFePO ₄ ) is obtained by reacting the coprecipitate obtained in the first step with lithium in an alcohol solvent slurry.
Here, it is important to use alcohol as the solvent of the slurry. As the lithium source, common lithium compounds such as lithium hydroxide and lithium carbonate can be used. However, these lithium compounds are water-soluble, and when water is used as the solvent of the slurry, the slurry is strongly alkaline. Then, iron is oxidized and becomes trivalent. When trivalent iron is generated, it becomes difficult to obtain LiFePO _{4 in} which iron is in a divalent state. On the other hand, since the solubility of a lithium compound is very low in alcohol, iron oxidation does not occur even in the slurry, and divalent iron can be retained.

  As the alcohol used for the solvent, inexpensive ethanol, isopropanol, or the like can be used. These alcohols may increase the solubility of lithium due to the water content, but there is no problem as long as the water content is unavoidable, and the water content is preferably 10% by mass or less, preferably 5% by mass. % Or less is more preferable.

In the slurry, a precursor of lithium iron phosphate (LiFePO ₄ ) is generated by the reaction between the fine lithium compound and the coprecipitate.
The details of the reaction are unknown, but when the fine particles are used, the lithium compound is activated and the reaction between lithium, iron, and phosphoric acid is promoted. Conceivable. In order to further promote the reaction, it is preferable to pulverize in a solvent in advance to form a new surface on the lithium compound fine particles. The lithium compound preferably has a particle size of 200 μm or less, more preferably 100 μm or less. Any pulverization method may be used as long as wet pulverization is possible. In addition to a ball mill, a planetary mill, a bead mill, or the like can be used.

  The slurry may be prepared by adding the coprecipitate and the lithium compound to an alcohol solvent, mixing and stirring the mixture, and individually mixing the coprecipitate and the lithium compound with an alcohol to form a slurry. The slurry may be mixed and stirred. The pulverization of the lithium compound may be performed either after mixing the coprecipitate and the lithium compound or after slurrying the individual lithium compounds.

  The composition ratio of the obtained lithium iron phosphate precursor can be controlled by the composition of the coprecipitate and the amount of lithium added to the slurry. The amount of lithium added to the slurry is preferably adjusted so that iron is 0.8 to 1.2 in a molar ratio to lithium. The molar ratio of lithium: iron: phosphoric acid in the slurry is preferably adjusted to be in the composition range of 1: 0.8: 0.8 to 1: 1.2: 1.2. When the composition range is exceeded, phases other than the ordered olivine phase increase, and battery characteristics may be deteriorated.

  The slurry temperature during the reaction is preferably 30 ° C. or higher. By setting the temperature to 30 ° C. or higher, the reaction is promoted, and the precursor can be obtained efficiently. If it is less than 30 degree | times, in order to fully advance reaction, a long time is required and productivity may fall. Although the slurry temperature should just be below the boiling point of an alcohol solvent, in order to suppress volatilization of alcohol, it is preferable to set it as 80 degrees C or less, and it is more preferable to set it as 60 degrees C or less.

  The product of the above reaction is washed with alcohol and separated into solid and liquid, if necessary, and then dried in a non-oxidizing atmosphere, specifically in an inert atmosphere, a reducing atmosphere or a vacuum atmosphere. It is preferred that The non-oxidizing atmosphere means an atmosphere in which iron in the reactive organism is not oxidized from divalent to trivalent, and may be a reduced-pressure atmosphere that can prevent oxidation of iron. As conditions necessary for volatilization of alcohol, it is preferable to dry at 80-200 degreeC for 3 to 10 hours. Volatile alcohol can be recovered by a condenser or the like, and is preferably reused for cost reduction.

During or after the reaction in the second step, organic substances such as sucrose may be mixed and contained, and used as a carbon source for imparting conductivity to lithium iron phosphate.
In any of the coprecipitation reaction and the reaction in the slurry, it is preferable to carry out in an inert gas atmosphere such as nitrogen or argon gas in order to suppress oxidation of iron. Can be sufficiently suppressed.
The reaction vessel to be used is preferably provided with a stirring device such as a stirring blade in order to make the reaction uniform, but preferably has a structure in which an upper lid is attached to the vessel and the above atmosphere control is possible.

2. Precursor of positive electrode active material for lithium ion secondary battery The precursor of the positive electrode active material for lithium ion secondary battery of the present invention is a precursor of a positive electrode active material for lithium ion secondary battery comprising lithium iron phosphate. A composition obtained by the above production method, composed of lithium, iron and phosphoric acid, wherein the molar ratio of lithium: iron: phosphoric acid is 1: 0.8: 0.8 to 1: 1.2: 1.2 The content of carbon and nitrogen is 0.4 or less in terms of molar ratio to iron.

The above precursor has the above composition of lithium iron phosphate composed of substantially uniform lithium, iron, and phosphoric acid, and there is a possibility that residual carbon resulting from alcohol or the like may inhibit the formation of the LiFePO ₄ crystal phase. small.
However, when the carbon content exceeds 0.4 in terms of a molar ratio with respect to iron, the amount of the active material in the unit volume decreases, and the volume energy density decreases. Further, if the nitrogen content exceeds 0.4 in terms of a molar ratio with respect to iron and remains in the form of an ammonium salt or the like, crystallization in the heat treatment during the production of the positive electrode active material becomes slow.

Further, the precursor is a lithium-iron-phosphate compound close to the final active material composition, but although it may contain some unreacted substances, it has a clear diffraction peak in X-ray diffraction. The status is not detected.
That is, the precursor is presumed to be in an amorphous state, has excellent reactivity, and is easily crystallized by firing at a low temperature to produce fine lithium iron phosphate particles.
Here, the state where the diffraction peak is not detected means that the peak is in a broad state and the diffraction angle is not specified, that is, a state where it is not mechanically detected by the X-ray diffraction apparatus.

By heat-treating such a precursor at 200 to 500 ° C. in an inert or reducing atmosphere, lithium iron phosphate (LiFePO ₄ ) having good crystallinity can be obtained. This is because, in the solid phase reaction method, a temperature of about 700 ° C. or higher is required for the synthesis of LiFePO ₄ , but it can be synthesized at a significantly low temperature, the particle growth is suppressed, and it is fine and has excellent battery characteristics. A substance can be obtained.
On the other hand, a precursor with high crystallinity that can detect the diffraction peak is low in reactivity to produce lithium iron phosphate particles, and is fired at a high temperature as in the case of the solid-phase reaction method. Is required and the particles are also coarsened.
The precursor does not have an olivine structure and has an amorphous structure, but the crystalline peak of Fe ₃ (PO ₄ ) ₂ or Li ₃ PO ₄ is partially weak There is. However, even in this case, each phase is very finely dispersed, so that the reactivity is high.

3. The manufacturing method of the positive electrode active material for lithium ion secondary batteries The manufacturing method of the positive electrode active material for lithium ion secondary batteries of this invention heat-processes the said precursor at 200-500 degreeC in inert or reducing atmosphere. is there. As described above, since the precursor has extremely high reactivity, it can be crystallized at 200 to 500 ° C., which is significantly lower than the heat treatment temperature in the solid-phase reaction method, to synthesize LiFePO ₄ .
You may grind | pulverize as needed before the heat processing of the said crystallization. Thereby, finer LiFePO ₄ particles can be obtained. When strong pulverization is performed after the crystallization heat treatment, distortion may be introduced into the LiFePO ₄ particles, which may affect the battery characteristics.

In addition, since LiFePO ₄ particles have poor conductivity, it is preferable to impart conductivity. However, in the solid-phase reaction method, if a carbon source is added before firing for imparting conductivity, the synthesis reaction may be inhibited. There is sex. Therefore, it is necessary to add a carbon source and heat-treat again after the heat treatment of LiFePO ₄ synthesis.
On the other hand, according to the production method of the present invention, since the precursor is a uniform composition close to the final composition and has a very high reactivity, the step of adding the carbon source is performed before the heat treatment for crystallization. Can be incorporated. Thereby, crystallization and conductivity imparting heat treatment can be performed continuously, which is industrially advantageous.
In that case, although depending on the carbon source to be used, graphitization proceeds at 600 ° C. or higher. For example, as a heat treatment for the synthesis, after crystallization by holding at 400 to 500 ° C. for 1 to 10 hours, By carrying out the heat treatment at 600 to 700 ° C. for 1 to 10 hours, LiFePO ₄ fine particles combined with suitable conductive graphite can be obtained as a positive electrode active material for a lithium ion secondary battery.

The heat treatment for imparting conductivity is 600 to 700 ° C., which is higher than the crystallization heat treatment, but since the crystallization of LiFePO ₄ is completed, the coarsening of the particles is suppressed. Therefore, in the present invention, LiFePO ₄ fine particles having good characteristics can be obtained even when the carbon source is added after the crystallization heat treatment and further the heat treatment for imparting conductivity is performed.
Examples of the carbon source include graphite such as natural graphite and artificial graphite, carbon blacks such as acetylene black and ketjen black, carbon fiber, sucrose, ascorbic acid, and other organic compounds that generate carbonaceous matter by decomposition. These can be used alone or in combination of two or more. Among these, acetylene black and sucrose are particularly preferable because they can be easily formed with a conductive carbon material and are easily available industrially.
Further, the amount of carbon atoms contained in the conductive carbon material tends to be smaller than that of the conductive carbon material product by heat treatment. For this reason, the compounding amount of the conductive carbon material product may be increased by 40 to 100% in terms of the mass ratio of carbon contained with respect to the amount of carbon contained after heat treatment in consideration of the amount of carbon contained in the precursor. Preferably, it is more preferably increased by 50 to 100%.
The mixing is preferably sufficiently performed dry using a blender or the like so that the precursor and the conductive carbonaceous material product are uniformly mixed.

The pulverization of the precursor is more preferably performed dry or wet during the mixing or after the mixing. By grinding in the coexistence state with the carbon source, the positive electrode active material obtained by the subsequent heat treatment becomes finer, and the fine LiFePO ₄ particle surface can be uniformly coated with the conductive carbon material. When used as a positive electrode material of a battery, the discharge capacity can be improved.
As in the pulverization of the coprecipitate, the pulverization is carried out by placing the precursor or mixture and a pulverization medium such as balls and beads in a container and colliding the medium with each other. It is preferable to carry out by. By mixing using a pulverizing medium, mixing and pulverization can be performed simultaneously after the addition of the carbon source.
As the pulverizer that can be used, a pulverizer having a shearing force such as a rolling ball mill, a vibration mill, a planetary mill, or a medium stirring mill is preferable. As such an apparatus, a commercially available apparatus can be used.
It is preferable that the particle size of the pulverization medium is 1 to 15 mm because pulverization can be sufficiently performed. Moreover, it is preferable that the material of the medium is zirconia or alumina ceramic because it has high hardness, high wear resistance, and metal contamination of the material to be crushed can be prevented.
Further, the pulverization medium contains the pulverization medium in a container with a space volume of 50 to 90%, and appropriately manages the shearing force and frictional force due to the fluid medium. It is preferable to process.
The precursor is already a stable phosphate, but in the case of a dry mill, it is preferable to introduce an inert gas in order to sufficiently suppress oxidation.

According to the method of the present invention, it is not necessary to use a high-pressure vessel such as a hydrothermal synthesizer, and a positive electrode active material for lithium ion secondary batteries made of lithium iron phosphate can be produced by an industrially easy method. .
As the furnace used for the heat treatment, for example, a general heat treatment furnace such as a batch furnace, a roller hearth kiln, a pusher furnace, a rotary kiln, or a fluidized bed furnace can be used. The atmosphere during the heat treatment is an inert gas atmosphere such as argon or a reducing atmosphere such as a mixed gas of argon and hydrogen in order to suppress oxidation of iron during the heat treatment.

4). Positive electrode active material for lithium ion secondary battery The positive electrode active material for a lithium ion secondary battery of the present invention contains lithium iron phosphate in which secondary particles are composed of fine primary particles, and the average particle size of the primary particles is 50. The average particle size of secondary particles is 1 to 20 μm, and the specific surface area according to the BET method is 10 to 50 m ² / g. The average particle size of the primary particles can be determined by averaging the particle sizes of 100 or more particles measured by scanning electron microscope (SEM) observation.
In the positive electrode active material, since the primary particles are fine and excellent in crystallinity, the crystallite diameter calculated from the peak of lithium iron phosphate in X-ray diffraction using the Scherrer equation is 50 to 200 nm. More preferably, it is 50-150 nm. The larger the crystallite size, the higher the crystallinity and the better battery characteristics. The larger the ratio of the primary particles to the average particle size (crystallite size / average particle size), the better the crystallinity. Therefore, when the ratio of the crystallite diameter to the primary particle diameter is 0.4 or more, the used battery can have a high capacity. Since the crystallite diameter is a scale indicating the size of crystals in the primary particles, it usually does not exceed the size of the primary particles. Therefore, the upper limit of the crystallite diameter ratio is about 1.1 even when measurement errors are taken into account.
When the BET specific surface area is less than 10 m ² / g, when used in a battery, a sufficient reaction area with the electrolytic solution cannot be obtained and the capacity is not increased. Moreover, when the BET specific surface area exceeds 50 m ² / g, the bulk density becomes too low, which is disadvantageous as a battery material whose volume is limited.

Further, the surface of the primary particles is coated with a conductive carbonaceous material of 3 to 10% by mass, preferably 3 to 8% by mass with respect to the total amount of the positive electrode active material for a lithium secondary battery. Preferably it is.
Since the lithium iron phosphate particles constituting the positive electrode active material of the present invention have low conductivity, coating with a conductive carbonaceous material in the above range ensures sufficient conductivity between the particles, Excellent properties can be obtained as an active material.
When the coating amount of the conductive carbonaceous material is less than 3% by mass, sufficient conductivity is not ensured. On the other hand, when the coating amount of the conductive carbonaceous material exceeds 10% by mass, preferably over 8% by mass, the positive electrode active Since the amount of lithium iron phosphate particles per volume of material decreases, the battery capacity per volume of battery decreases, which is disadvantageous.

The positive electrode active material of the present invention can be suitably used as the positive electrode active material of a lithium secondary battery comprising a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte containing a lithium salt, and the resulting lithium ion secondary battery is High capacity, excellent rate characteristics, and high safety.
The positive electrode active material is used in combination with other known lithium cobalt composite oxide, lithium nickel composite oxide, lithium metal composite oxide such as lithium manganese composite oxide, and so on. The safety of the secondary battery can be further improved. In this case, the other lithium metal composite oxide used in combination is not particularly limited, and a general one can be used.

  EXAMPLES The present invention will be described in more detail below with reference to examples of the present invention, but the present invention is not limited to these examples. The analysis and evaluation of the samples used in the examples were performed by the following methods.

(1) Analysis of metal elements:
Analysis was performed by ICP emission analysis using an ICP emission analyzer (Varian, 725ES).
(2) X-ray diffraction:
Using a powder X-ray diffractometer (manufactured by PANALYTICAL, X'Pert PRO MRD), measurement was performed by powder X-ray diffraction using Cu-Kα rays.
(3) Average particle size:
It was determined by measuring the particle size of 50 primary particles by scanning electron microscope observation and averaging the number.
(4) Specific surface area:
It was measured by the BET method by nitrogen gas adsorption.
(5) Carbon (C) content:
It measured using the carbon analyzer (made by LECO).

[Example 1]
1. Step 1 Ferrous sulfate heptahydrate (special grade manufactured by Wako Co., Ltd .: purity 99.5% by mass) 1 mol (278 g) and phosphoric acid (manufactured by Wako Co., Ltd .: purity 85.0% by mass or more) 1 mol (115 g) ) Was added to 1 L of distilled water and stirred well for 1 hour with a stirrer to obtain a raw material solution. Moreover, 25 mass% sodium hydroxide aqueous solution was used as pH adjustment solution.
Next, 1 L of pure water was put into a 5 L separable flask equipped with a stirrer, and the mixture was stirred for 30 minutes while the inside was replaced with nitrogen. The raw material solution was added at a rate of 10 ml per minute while controlling the supply amount of the pH adjusting solution with a pH controller so that the pH of the reaction aqueous solution was 8.0 to 8.2. After completion of the dropping, stirring was continued for 30 minutes while replacing the separable flask with nitrogen to allow the reaction to proceed completely.
The reaction solution was filtered by suction filtration, and washed with distilled water to recover the generated coprecipitate. The recovered coprecipitate was dried at 50 ° C. in a vacuum dryer to obtain a coprecipitate powder.

2. Second Step 250 ml of ethanol was placed in a 2 L separable flask equipped with a stirrer and placed in a warm water bath at a temperature of 40 ° C. and purged with nitrogen. Then, 100 g of the obtained coprecipitate powder was dispersed in ethanol and coprecipitated. A slurry was obtained.
To this coprecipitate slurry, stirring was continued while adding a slurry in which about 20 g of lithium carbonate (deer grade: 99.0%, manufactured by Kanto Chemical Co., Ltd.) was dispersed in 50 ml of ethanol at a rate of 5 ml per minute. After completion of the addition, stirring was continued for 3 hours while replacing the separable flask with nitrogen to allow the reaction to proceed. The slurry after the reaction was filtered by suction filtration, and the reaction product was recovered. This was put into a vacuum dryer and dried at 100 ° C. for 4 hours to obtain a precursor.
When chemical analysis of the obtained precursor was performed, lithium, iron, phosphorus, carbon, and nitrogen were 3.8, 30.2, 16.8, 1.6, and 0% by mass, respectively, lithium: iron : Phosphorus molar ratio was 1.0: 0.99: 0.99. When this precursor was subjected to X-ray diffraction, only a broad amorphous peak was observed, and a clear peak indicating a crystal structure was not confirmed. FIG. 1 shows an X-ray diffraction chart of this precursor.

3. Active material production process This precursor was heated at a heating rate of 10 ° C./min while purging the inside of the furnace with an argon-2 volume% hydrogen mixed gas at a flow rate of 1 L / min using an electric furnace, A heat treatment was performed at 500 ° C. for 5 hours to obtain a positive electrode active material composed of lithium iron phosphate particles.
The obtained positive electrode active material was identified as lithium iron phosphate (LiFePO ₄ ) having an olivine structure by X-ray diffraction. FIG. 2 shows an X-ray analysis chart of the positive electrode active material. The crystallite diameter was calculated from the peak of the (131) plane of LiFePO ₄ in X-ray diffraction using the Scherrer equation, and found to be 82 nm. Moreover, the primary particle diameter of this positive electrode active material was 130 nm, and the specific surface area was 17.3 m ² / g.

[Example 2]
1. First and Second Steps A precursor was obtained in the same manner as in Example 1 except that 25% by mass ammonia water was used for the pH adjustment solution.
The obtained precursor is 3.7, 30.2, 16.6, 1.2, 2.4 mass% of lithium, iron, phosphorus, carbon, and nitrogen, respectively: lithium: iron: phosphorus: carbon: The molar ratio of nitrogen was 1.0: 1.01: 1.01: 0.19: 0.32. When this precursor was subjected to X-ray diffraction, only a broad amorphous peak was observed, and a clear peak indicating a crystal structure was not confirmed.

2. Next, the precursor is heated at a heating rate of 10 ° C./min using an electric furnace while purging the inside of the furnace with an argon-2 volume% hydrogen mixed gas at a flow rate of 1 L / min. Then, heat treatment was performed at 500 ° C. for 5 hours to obtain a positive electrode active material.
The obtained positive electrode active material was identified as lithium iron phosphate (LiFePO ₄ ) having an olivine structure by X-ray diffraction. When the positive electrode active material was evaluated in the same manner as in Example 1, the crystallite diameter was 71 nm, the primary particle diameter was 100 nm, and the specific surface area was 17.9 m ² / g.
To this positive electrode active material, sucrose as a carbon source was added in an equimolar amount (5% by mass with respect to the positive electrode active material) with respect to lithium iron phosphate, and mixed with a mixer for 10 minutes. The obtained mixture was heated at 700 ° C. for 5 hours by heating at 700 ° C. while purging the inside of the furnace with an argon-4 vol% hydrogen mixed gas at a flow rate of 2 L / field in an electric furnace. Conductivity imparting treatment was performed. The amount of carbon (C) in the positive electrode active material after imparting conductivity was 5.1% by mass with respect to the entire positive electrode active material.

4). Evaluation of battery characteristics The battery characteristics of the positive electrode active material imparted with conductivity were evaluated using a C2032 coin-type battery. The coin-type battery was produced by the following procedure.
A positive electrode active material is mixed with 15% by mass of acetylene black as a conductive material, 10% by mass of polyvinylidene fluoride (PVDF) as a binder, and an N-methylpyrrolidone (NMP) solution. A mixture of 15% by mass of the material-15% by mass of PVDF was obtained. This mixture was applied onto an aluminum foil, dried at 80 ° C., punched out to an electrode size of 11 mmφ, and pressed at a press pressure of 98 MPa (1.0 tonf / cm ² ) to produce a positive electrode plate.
Using this positive electrode plate, a coin-type cell 2032 was produced in a glove box.
At this time, a metal lithium foil was used for the negative electrode, and an electrolytic solution in which 1 mol of LiClO ₄ was dissolved in 1 liter of an equal volume mixture of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1 was used.
The produced lithium secondary battery was operated at room temperature at a charge of 0.2 mA / cm ² , a rest of 60 minutes, 4.1 V, a discharge of 0.2 mA / cm ² and 2.0 V, and the initial discharge capacity was measured. The initial charge capacity was 166 mAh / g, the initial discharge capacity was 156 mAh / g, and the initial efficiency was 96%.

[Example 3]
1. Step 1 Ferrous sulfate heptahydrate (special grade manufactured by Wako Co., Ltd .: purity 99.5% by mass) 1 mol (278 g) and phosphoric acid (manufactured by Wako Co., Ltd .: purity 85.0% by mass or more) 1 mol (115 g) ) Was added to 1 L of distilled water and stirred well for 1 hour with a stirrer to obtain a raw material solution. Moreover, 25 mass% ammonia aqueous solution was used as pH adjustment solution.
Next, 1 L of pure water was put into a 5 L separable flask equipped with a stirrer, and the mixture was stirred for 30 minutes while the inside was replaced with nitrogen. The raw material solution was added at a rate of 10 ml per minute while controlling the supply amount of the pH adjusting solution with a pH controller so that the pH of the aqueous reaction solution was 8.0 to 8.2. After completion of the dropping, stirring was continued for 30 minutes while replacing the separable flask with nitrogen to allow the reaction to proceed completely.
The reaction solution was filtered by suction filtration, and washed with distilled water to recover the generated coprecipitate. The recovered coprecipitate was dried at 50 ° C. in a vacuum dryer to obtain a coprecipitate powder.

2. Second Step 30 g of this coprecipitate powder is put into a zirconia pot for a planetary ball mill, and further, 6.0 g of lithium carbonate (Kanto Chemical Co., Ltd. deer special grade: 99.0%), 70 ml of ethanol and zirconia balls (1 mmφ) ) And pulverized at 350 rpm for 30 minutes using a planetary ball mill (manufactured by Fritsch Japan) and reacted. After pulverization, the zirconia balls were removed using a sieve, and the resulting slurry was placed in a vacuum dryer and dried at 100 ° C. for 4 hours to collect the reaction product, which was used as a precursor.
When chemical analysis of the obtained precursor was performed, lithium, iron, phosphorus, carbon, and nitrogen were 3.7, 30.2, 16.4, 1.3, 2.6 mass%, respectively, and lithium: The molar ratio of iron: phosphorus was 1.0: 1.01: 1.02. X-ray diffraction of the precursor was performed, but only a broad amorphous peak was observed, and a clear crystal structure was not observed.

3. Active material manufacturing process This precursor was heat treated at 500 ° C. at a heating rate of 10 ° C./min while purging the inside of the furnace with an argon-2 vol% hydrogen mixed gas at 1 L / min in an electric furnace, and iron phosphate A positive electrode active material composed of lithium particles was obtained.
The obtained positive electrode active material was identified as lithium iron phosphate having an olivine structure by X-ray diffraction. When the positive electrode active material was evaluated in the same manner as in Example 1, the crystallite diameter was 46 nm. Moreover, the primary particle diameter of this positive electrode active material was 100 nm, and the specific surface area was 36.9 m ² / g.

4). Evaluation of Battery Characteristics The battery characteristics of the positive electrode active material were evaluated in the same manner as in Example 2. As a result, the initial charge capacity was 166 mAh / g, the initial discharge capacity was 159 mAh / g, and the initial efficiency was 96%.

[Comparative Example 1]
In the first step, a coprecipitate was obtained in the same manner as in Example 1 except that the pH of the aqueous reaction solution was controlled to 6.5 to 6.7.
When the coprecipitate was solid-liquid separated, the filtrate was colored yellow, and when analyzed, iron and phosphorus were lost to the filtrate by 50% or more, so the composition of the coprecipitate could not be controlled. Canceled.

[Comparative Example 2]
In the first step, a positive electrode active material was obtained in the same manner as in Example 1 except that the pH of the aqueous reaction solution was controlled to 10.5 to 10.7.
During the solid-liquid separation of the coprecipitate in the first step, it was confirmed that the coprecipitate had turned brown and iron was oxidized to trivalent. Further, when the obtained positive electrode active material was subjected to X-ray diffraction, peaks of lithium iron phosphate and iron phosphate having an olivine structure were detected. It can be considered that the oxidized iron was not sufficiently reduced during firing and a heterogeneous phase was generated.

[Comparative Example 3]
In the second step, a positive electrode active material was obtained in the same manner as in Example 1 except that water was used instead of ethanol.
Since lithium carbonate dissolved in water and the pH rose, it was confirmed that the particles in the slurry turned brown and oxidized iron during the reaction in the second step. Further, when the obtained positive electrode active material was subjected to X-ray diffraction, peaks of lithium iron phosphate and iron phosphate having an olivine structure were detected. It can be considered that the oxidized iron was not sufficiently reduced during firing and a heterogeneous phase was generated.

According to the method for producing a precursor for a positive electrode active material for a lithium ion secondary battery of the present invention, a suitable uniform fine precursor can be obtained as a raw material for producing a positive electrode active material for a lithium secondary battery, Since the positive electrode active material precursor can easily obtain a positive electrode active material for a lithium secondary battery by heat treatment, the industrial applicability is high.
Further, the obtained positive electrode active material for a lithium secondary battery of the present invention is composed of uniform fine lithium iron phosphate particles, and the positive electrode active material is used to form a lithium ion secondary battery with a high Excellent physical properties with high capacity and high output, and extremely high industrial value.

Claims (6)

A method for producing a positive electrode active material for a lithium ion secondary battery, comprising:
A composition composed of lithium, iron and phosphoric acid, wherein the molar ratio of lithium: iron: phosphoric acid is 1: 0.8: 0.8 to 1: 1.2: 1.2, A precursor of a positive electrode active material for a lithium ion secondary battery , which is made of lithium iron phosphate (LiFePO ₄ ) having a molar ratio with respect to iron of 0.4 or less, is 200 to 500 in an inert or reducing atmosphere. Including a step of heat treatment at ℃ ,
The precursor is prepared by adding and coprecipitating a coexisting raw material solution of divalent iron ions and phosphate ions into a reaction aqueous solution adjusted to a pH range of 7 to 10, thereby containing iron and phosphoric acid. A first step of obtaining a precipitate, and a second step of reacting lithium and the coprecipitate in an alcohol solvent slurry having a lithium amount such that iron is 0.8 to 1.2 in a molar ratio to lithium. Obtained by the manufacturing method,
The manufacturing method of the positive electrode active material for lithium ion secondary batteries characterized by the above-mentioned. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 1 , further comprising a step of adding a carbon source to the precursor in advance before the heat treatment. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 1 , further comprising a step of pulverizing the precursor in advance before the heat treatment. Obtained by the method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3 ,
A lithium iron phosphate peak in an X-ray diffraction with respect to the average particle diameter of the primary particles, which contains lithium iron phosphate, the average particle diameter of the primary particles measured by scanning electron microscope observation of the lithium iron phosphate is 50 to 200 nm A positive electrode active material for a lithium ion secondary battery, wherein the ratio of crystallite diameter (crystallite diameter / average particle diameter) calculated by using Scherrer's formula is 0.4 or more. 5. The positive electrode active material for a lithium ion secondary battery according to claim 4 , wherein the specific surface area according to the BET method is 10 to 50 m ² / g. A lithium ion secondary battery comprising a positive electrode comprising the positive electrode active material for a lithium ion secondary battery according to claim 4 or 5 .

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