JP5673953B2 - Cathode active material - Google Patents

Description

  The present invention relates to a method for producing a lithium iron phosphate-based positive electrode active material useful as a positive electrode material for a lithium ion secondary battery.

Secondary batteries used for portable electronic devices, hybrid cars, electric cars, and the like have been developed, and lithium ion secondary batteries are particularly widely known. The lithium ion battery basically includes a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator. LiCoO ₂ is widely used as a positive electrode material, and LiNiO ₂ , LiMn ₂ O _{4 and the} like have been developed. However, these lithium-based metal oxides have a problem that the capacity is low although the voltage is high.

  On the other hand, recently, it has been proposed to use a phosphate compound such as lithium iron phosphate having an olivine structure for the positive electrode (Patent Document 1). However, this method of synthesizing lithium iron phosphate is a solid phase method, which requires firing and pulverization in an inert gas atmosphere, and the operation is complicated.

  Therefore, attempts have been made to produce lithium iron phosphate by a hydrothermal reaction (Patent Documents 2 and 3, Non-Patent Document 1). In these methods, a lithium compound, an iron compound, and a phosphoric acid compound are hydrothermally reacted in a pressure resistant vessel.

Japanese Patent Laid-Open No. 9-171827 JP 2002-151082 A JP 2004-95385 A

Electrochemistry Communications 3 (2001) 505-508

According to these conventional methods for producing lithium iron phosphate by hydrothermal reaction, particles having a uniform particle size can be obtained as compared with the solid phase method, but side reactions occur and impurities are mixed into the product. It has been found that there is a problem that large ones may be mixed in and the discharge characteristics of the obtained battery are not sufficient.
Accordingly, there has been a demand for a method for producing lithium iron phosphate having a finer and more uniform particle size and excellent charge / discharge characteristics of the obtained secondary battery with high purity and high yield.

Therefore, as a result of various studies on the hydrothermal reaction conditions of the iron compound, phosphate compound and lithium compound, the present inventor mixed lithium compound, phosphate compound and water first, and adjusted the mixture to a constant reducing atmosphere. Then, if a divalent iron compound is added and subjected to a hydrothermal reaction, a high-purity lithium iron phosphate having a fine and uniform particle diameter is easy to operate without side reactions or condensation during the reaction. The inventors have found that a lithium ion secondary battery having a high capacity and excellent charge / discharge characteristics can be obtained if the obtained lithium iron phosphate is used as a positive electrode material, and the present invention has been completed.
That is, in the present invention, after mixing a lithium compound, a phosphoric acid compound and water, a reducing compound is added to make the oxidation-reduction potential of the mixture less than -1 mV, and then a divalent iron compound is added to add water. The present invention provides a method for producing lithium iron phosphate characterized by thermal reaction.
In the present invention, after mixing a lithium compound, a phosphoric acid compound and water, a reducing compound is added to make the oxidation-reduction potential of the mixture less than -1 mV, a carbon source is added, and then divalent iron is added. The present invention provides a method for producing a lithium iron phosphate-based positive electrode active material characterized in that a compound is added, subjected to a hydrothermal reaction, and then fired in an inert gas or a reducing atmosphere.
Moreover, this invention provides the lithium ion secondary battery which contains the lithium iron phosphate obtained by said manufacturing method as a positive electrode material.

  According to the method of the present invention, the operation is easy and no side reaction occurs. Further, according to the method of the present invention, high purity lithium iron phosphate having a fine and uniform particle size can be obtained in high yield. When lithium iron phosphate obtained by the method of the present invention is used as a positive electrode material, a lithium ion secondary battery having a high capacity and excellent charge / discharge characteristics can be obtained.

It shows an SEM image of LiFePO ₄ obtained in Example 1. It shows the XRD chart of LiFePO ₄ obtained in Example 1. It shows an SEM image of LiFePO ₄ obtained in Example 2. It shows the XRD chart of LiFePO ₄ obtained in Example 2. It shows the SRM image of LiFePO ₄ produced in Comparative Example 1. It shows the XRD chart of LiFePO ₄ obtained in Comparative Example 1.

  In the method for producing lithium iron phosphate of the present invention, first, a lithium compound, a phosphate compound and water are mixed.

  The synthetic raw material of lithium iron phosphate is basically a divalent iron compound, a lithium compound and a phosphate compound. In the present invention, a mixture of a lithium compound, a phosphate compound and water is prepared first. Finally, the addition of a divalent iron compound is important for preventing side reactions and allowing the reaction to proceed easily. When a divalent iron compound, a phosphoric acid compound and water are mixed first, a carbon source is added to this, and nitrogen gas is added after introducing nitrogen gas, condensation occurs during the reaction, which makes stirring impossible and makes it special. A simple stirring device is required. On the other hand, when a divalent iron compound, lithium carbonate, and water are first mixed, a carbon source is added, a nitrogen gas is introduced, and then a phosphoric acid compound is added, stirring becomes difficult due to excessive foaming and condensation occurs. In contrast, when the raw materials are added in the order as in the present invention, no agglomeration occurs, stirring is easy, and the reaction proceeds smoothly.

  Examples of the lithium compound used as a raw material include lithium metal salts such as lithium fluoride, lithium chloride, lithium bromide, and lithium iodide, lithium hydroxide, lithium carbonate, etc., but it is inexpensive to use lithium carbonate. It is preferable in a certain point.

  As the phosphoric acid compound, orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, ammonium hydrogen phosphate and the like are used.

  The amount of the lithium compound and phosphate compound used is preferably 2.5: 1 to 3.7: 1 in terms of molar ratio of lithium ion and phosphate ion, and is approximately 2.8: 1 to 3.3-1. Is more preferable.

  The amount of water used is preferably 10 to 50 mol, more preferably 13 to 30 mol with respect to 1 mol of phosphorus ion of the phosphoric acid compound, from the viewpoints of solubility of the raw material compound, easiness of stirring, synthesis efficiency, and the like. 15-20 mol is particularly preferable.

  The order of addition of the lithium compound, phosphate compound and water is not particularly limited, and the mixing time of these raw materials is not limited. These raw materials may be mixed at room temperature, for example, 10 to 35 ° C.

  In the method of the present invention, a reducing substance is added to a mixture of a lithium compound, a phosphoric acid compound and water so that the oxidation-reduction potential of the mixture is reduced to less than -1 mV. Thus, by making a mixture into a reducing atmosphere, oxidation to the trivalence of the bivalent iron compound added later is suppressed, and highly purified lithium iron phosphate is obtained efficiently.

  Examples of the reducing substance used include ammonia and an ammonia generating compound. More specifically, ammonia, urea, methylamine, and the like can be mentioned. Ammonia is more preferable from the viewpoint of efficiency for reducing the oxidation-reduction potential to less than -1 mV.

  The amount of the reducing substance added is more preferably an amount that makes the oxidation-reduction potential of the mixture less than -1 mV, an amount that is less than -5 mV, and an amount that is -10 to -500 mV is more preferred. When using ammonia, 0.01 mol or more is preferable as an ammonia amount with respect to 1 mol of a divalent iron compound, and 0.05 mol or more is more preferable.

  In the method of the present invention, it is preferable to introduce nitrogen gas into the mixture before adding the divalent iron compound. The introduction of nitrogen gas is preferable from the viewpoint of reducing the amount of dissolved oxygen in the reaction solution and preventing oxidation of a divalent iron compound added later. The amount of nitrogen gas introduced is preferably carried out until the dissolved oxygen concentration in the solution is 1.0 mg / L or less, more preferably 0.5 mg / L or less. Nitrogen gas is preferably introduced by bubbling nitrogen gas into the solution.

  In the method of the present invention, an antioxidant may be added at this point in order to prevent oxidation of a divalent iron compound added later. The antioxidant may be added at the time of introduction of nitrogen gas and at that time, or before, during or after these operations. Examples of the antioxidant include ascorbic acid, ascorbic acid ester, ascorbate, isoascorbic acid, aldehydes, hydrogen gas, sulfite and the like. The amount of these antioxidants to be used is preferably 0.001 mol to 0.1 mol, and more preferably 0.005 mol to 0.05 mol, per 1 mol of iron ions of the divalent iron compound.

  In the method of the present invention, the carbon source may be added after the hydrothermal reaction, but it is preferable to add it before the hydrothermal reaction from the viewpoint of obtaining a uniform positive electrode active material. Examples of the carbon source include glucose, fructose, polyethylene glycol, polyvinyl alcohol, carboxymethylcellulose, saccharose, starch, dextrin, and citric acid, and starch and glucose are particularly preferable from the viewpoint of low cost. The amount of the carbon source used is 0. 0 with respect to the mixture weight of the lithium compound, phosphate compound, divalent iron compound and water from the viewpoint of charge / discharge characteristics when the obtained lithium iron phosphate is used as the positive electrode material. It is preferably 1 to 15% by weight, more preferably 0.5 to 10% by weight, and particularly preferably 1.5 to 5% by weight.

  The order of addition of the reducing substance, introduction of nitrogen gas, and addition of the carbon source is not limited and may be simultaneous.

  Next, in the method of the present invention, a divalent iron compound is added. Examples of the divalent iron compound used include iron halides such as iron fluoride, iron chloride, iron bromide, and iron iodide, iron sulfate, iron oxalate, and iron acetate. The amount of the divalent iron compound used is preferably about equimolar to the phosphate ion as the iron ion from the viewpoint of obtaining high purity lithium iron phosphate.

  In the method of the present invention, the mixture is preferably stirred from the viewpoint of obtaining lithium iron phosphate having a fine and uniform particle size, and more preferably mixed for 30 minutes or more. In mixing, it is preferable to stir. The stirring time is 30 minutes or more, preferably 30 to 120 minutes, and more preferably 60 to 120 minutes. The stirring reaction may be performed at room temperature, and is preferably performed at 10 to 35 ° C. Stirring can be performed by ordinary stirring means such as propeller stirring and pump circulation stirring.

  The reaction mixture is then subjected to a hydrothermal reaction. In the hydrothermal reaction, since water is present in the reaction mixture, the reaction mixture may be sealed in a pressure vessel and heated to 150 ° C or higher. A more preferable reaction temperature is 150 to 200 ° C, and further preferably 180 to 200 ° C. The pressure only needs to be sealed and heated in a pressure-resistant container, and is theoretically about 1.0 to 1.5 MPa. The heating time is preferably 1 to 10 hours, more preferably 2 to 5 hours. In addition, it is preferable to stir the reaction liquid during the hydrothermal reaction.

  After completion of the hydrothermal reaction, the produced lithium iron phosphate is preferably collected by filtration and washed. Washing is preferably performed with water using a filtration device having a cake washing function. The obtained crystals are dried if necessary. Examples of the drying means include spray drying, vacuum drying, freeze drying and the like.

The obtained lithium iron phosphate becomes a lithium iron phosphate useful as a positive electrode material by firing in an inert gas or a reducing atmosphere. If the carbon source is not added before the hydrothermal reaction, the carbon source is added here. Examples of the inert gas include Ar and N ₂ . In addition, hydrogen gas may be introduced to obtain a reducing atmosphere. Firing conditions are preferably 600 ° C. or higher, more preferably 600 to 900 ° C., and particularly preferably 600 to 800 ° C. The firing time is preferably 0.5 hours to 5 hours, more preferably 1 hour to 3 hours.

The lithium iron phosphate obtained by the method of the present invention has a chemical composition represented by LiFePO ₄ and is useful as a positive electrode active material because it is coated with carbon. The obtained lithium iron phosphate has a feature that the average particle size is as fine as 1 μm or less and the particle size distribution is narrow. The average particle size calculated from the SEM image is 1000 nm or less, and the particle size distribution is preferably 100 to 800 nm, more preferably 200 to 600 nm, and particularly preferably 300 to 500 nm. The average particle size is preferably 600 nm or less, and particularly preferably 500 nm or less.

  The lithium iron phosphate-based positive electrode active material obtained by the method of the present invention is useful as a positive electrode material for lithium ion secondary batteries because the particle size is fine and uniform. Next, a lithium ion secondary battery containing the lithium iron phosphate positive electrode active material obtained by the method of the present invention as a positive electrode material will be described.

  The lithium ion secondary battery to which the positive electrode material of the present invention can be applied is not particularly limited as long as it has a positive electrode, a negative electrode, an electrolytic solution, and a separator as essential components.

  Here, as long as lithium ions can be occluded at the time of charging and released at the time of discharging, the material structure is not particularly limited, and a known material structure can be used. For example, a carbon material such as lithium metal, graphite, or amorphous carbon. It is preferable to use an electrode formed of an intercalating material capable of electrochemically inserting and extracting lithium, particularly a carbon material.

  The electrolytic solution is obtained by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent that is usually used for an electrolyte solution of a lithium ion secondary battery. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones An oxolane compound or the like can be used.

The type of the supporting salt is not particularly limited, but an inorganic salt selected from LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ , a derivative of the inorganic salt, LiSO ₃ CF ₃ , LiC (SO ₃ CF ₃ ) ₂ and LiN (SO ₃ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), and organic salt derivatives It is preferable that it is at least 1 type of these.

  The separator plays a role of electrically insulating the positive electrode and the negative electrode and holding the electrolytic solution. For example, a porous synthetic resin film, particularly a polyolefin polymer (polyethylene, polypropylene) porous film may be used.

  EXAMPLES Next, although an Example is given and this invention is demonstrated further in detail, this invention is not limited to this.

Example 1
19.9 g of Li ₂ CO ₃ , 17.6 g of H ₃ PO ₄ and 60.0 g of water were mixed. To this, 0.11 g of 28% ammonia water was added, the oxidation-reduction potential was adjusted to -10 mV, 3.0 g of starch was added, then nitrogen gas was bubbled, and the dissolved oxygen concentration became less than 0.1 mg / L. It was confirmed. This was mixed with 50.0 g of FeSO ₄ .7H ₂ O and stirred at 23 ± 2 ° C. with a propeller type stirring device for 60 minutes.
The mixture stirred for 60 minutes was placed in an autoclave and heated at 200 ° C. for 3 hours. Stirring was continued during heating. The internal pressure of the autoclave was 1.5 MPa. The formed crystals were filtered and then washed with water. The crystals were vacuum dried at 60 ° C. and 1 Torr. The obtained crystal was baked at 700 ° C. for 1 hour in a tubular electric furnace in which 3% of hydrogen was introduced into argon gas to obtain a fine powder of lithium iron phosphate. FIG. 1 shows an SEM image of the obtained powder, and FIG. 2 shows an XRD chart. The particle diameter of the obtained lithium iron phosphate was in the range of 200 to 400 nm, and it was confirmed that high purity lithium iron phosphate was obtained.

Example 2
19.9 g of Li ₂ CO ₃ , 17.6 g of H ₃ PO ₄ and 60.0 g of water were mixed. After adding 0.55 g of 28% aqueous ammonia and setting the redox potential to −31 mV, 3.0 g of starch was added, and then nitrogen gas was bubbled, so that the dissolved oxygen concentration was less than 0.1 mg / L. It was confirmed. This was mixed with 50.0 g of FeSO ₄ .7H ₂ O and stirred at 23 ± 2 ° C. with a propeller type stirring device for 60 minutes.
The mixture stirred for 60 minutes was placed in an autoclave and heated at 200 ° C. for 3 hours. Stirring was continued during heating. The internal pressure of the autoclave was 1.5 MPa. The formed crystals were filtered and then washed with water. The crystals were vacuum dried at 60 ° C. and 1 Torr. The obtained crystal was baked at 700 ° C. for 1 hour in a tubular electric furnace in which 3% of hydrogen was introduced into argon gas to obtain a fine powder of lithium iron phosphate. FIG. 3 shows an SEM image of the obtained powder, and FIG. 4 shows an XRD chart. The particle diameter of the obtained lithium iron phosphate was in the range of 100 to 300 nm, and it was confirmed that high purity lithium iron phosphate was obtained.

Comparative Example 1
Li ₂ CO ₃ 10.0g, was mixed with H ₃ PO ₄ 17.6 g, and water 60.0 g. At this time, the oxidation-reduction potential of the slurry was +73 mV. To this, 3.0 g of starch was added, and then nitrogen gas was bubbled to confirm that the dissolved oxygen concentration was less than 0.1 mg / L. This was mixed with 50.0 g of FeSO ₄ .7H ₂ O and stirred at 23 ± 2 ° C. with a propeller type stirring device for 60 minutes.
The mixture stirred for 60 minutes was placed in an autoclave and heated at 200 ° C. for 3 hours. Stirring was continued during heating. The internal pressure of the autoclave was 1.5 MPa. The formed crystals were filtered and then washed with water. The crystals were vacuum dried at 60 ° C. and 1 Torr. The obtained crystal was baked at 700 ° C. for 1 hour in a tubular electric furnace in which 3% of hydrogen was introduced into argon gas to obtain a fine powder of lithium iron phosphate. FIG. 5 shows an SEM image of the obtained powder, and FIG. 6 shows an XRD chart. The particle diameter of the obtained lithium iron phosphate was in the range of 500 to 800 nm, and lithium iron phosphate having a large particle diameter was obtained.

Example 3
Batteries were produced using the materials obtained in Examples 1 and 2 and Comparative Example 1 as positive electrode materials.
The fired products obtained in Examples 1 and 2 and Comparative Example 1, Ketjen Black (conductive agent), and polyvinylidene fluoride (binding agent) were mixed at a weight ratio of 75:15:10. N-methyl-2-pyrrolidone was added and sufficiently kneaded to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode.
Next, a coin-type lithium ion secondary battery was constructed using the positive electrode. A lithium foil punched to φ15 mm was used for the negative electrode. As the electrolytic solution, a solution obtained by dissolving LIPF ₆ at a concentration of 1 mol / l in a mixed solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 1 was used. As the separator, a known one such as a polymer porous film such as polypropylene was used. These battery components were assembled and housed in a conventional manner in an atmosphere with a dew point of −50 ° C. or lower to produce a coin-type lithium secondary battery (CR-2032).
A charge / discharge test at a constant current density was performed using the manufactured lithium ion secondary battery. Charging conditions at this time were constant current charging with a current of 0.1 CA (17 mA / g) and a voltage of 4.2 V, and discharging conditions were constant current discharging with a current of 0.1 CA and a final voltage of 2.0 V. All temperatures were 30 ° C.
A charge / discharge test was conducted in the same manner except that the discharge conditions were battery 5CA and constant battery discharge with a final voltage of 2.0V.

  Table 1 shows the discharge capacity ratio (5C / 1C), the Fe (III) content of the positive electrode active material, and the ORP (redox potential) of the slurry.

  From Table 1, the lithium iron phosphate obtained by the method of the present invention is uniform and fine particles and has a low Fe (III) content. As a result, a lithium ion secondary battery having a high discharge capacity at a high rate (5C) and excellent discharge characteristics can be obtained.

Claims (6)

  A lithium compound, a phosphoric acid compound and water are mixed, then a reducing compound is added to make the oxidation-reduction potential of the mixture less than -1 mV, and then a divalent iron compound is added to perform a hydrothermal reaction. And manufacturing method of lithium iron phosphate.   After mixing the lithium compound, the phosphoric acid compound and water, the reducing compound is added to make the oxidation-reduction potential of the mixture less than -1 mV, the carbon source is added, and then the divalent iron compound is added to add water. A method for producing a lithium iron phosphate-based positive electrode active material, wherein a thermal reaction is performed, followed by firing in an inert gas or a reducing atmosphere.   The production method according to claim 1 or 2, wherein the reducing compound is ammonia or an ammonia-generating compound.   The method according to any one of claims 1 to 3, wherein nitrogen gas is introduced into the reaction system before the addition of the divalent iron compound.   The method according to any one of claims 1 to 4, wherein the hydrothermal reaction is carried out in a pressure vessel at 150 to 200 ° C.   Firing conditions are 600-800 degreeC, The manufacturing method of any one of Claims 2-5.