JP5531247B2 - Method for producing lithium iron phosphate or lithium iron silicate - Google Patents

Description

  The present invention relates to a method for producing lithium iron phosphate or lithium iron silicate 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 a certain capacity is low at a high voltage.

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

  Therefore, attempts have been made to produce lithium iron phosphate and lithium iron silicate by a hydrothermal reaction (Patent Documents 1 and 2, 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.

JP 2002-151082 A JP 2004-95385 A

Electrochemistry Communications 3 (2001) 505-508

According to these conventional methods for producing lithium iron phosphate or lithium iron silicate by hydrothermal reaction, a uniform particle size can be obtained compared to the solid phase method, but iron is oxidized during the hydrothermal reaction. Therefore, there is a problem that the battery performance of the obtained positive electrode material is lowered. In order to solve such a problem, in Patent Document 1, an inert gas such as nitrogen or argon is sealed in a pressure vessel to carry out the reaction. However, it takes time to enclose the inert gas, and a large amount of inert gas is required, so that there is a problem that the price of the obtained positive electrode material increases.
Therefore, there has been a demand for a method for producing lithium iron phosphate or lithium iron silicate having a finer and more uniform particle size with high purity and high yield by an inexpensive and simple means.

  Therefore, as a result of various studies on the hydrothermal reaction conditions of an iron compound, a phosphoric acid compound or a silicic acid compound, and a lithium compound, the present inventor was manufactured by heating water having a dissolved oxygen concentration of 1.0 mg / L or less. If the hydrothermal reaction is carried out in a steam autoclave, which is a saturated steam, there is no need to fill the autoclave with an inert gas, iron oxidation can be prevented, and it is cheaper and simpler, with a small particle size and uniform phosphorus content. It is found that lithium iron oxide or lithium iron silicate can be obtained, and further, if the obtained product is used as a positive electrode material, a lithium ion secondary battery having high capacity and excellent charge / discharge characteristics can be obtained Was completed.

That is, the present invention provides a mixture slurry containing (A) an iron compound, (B) a phosphoric acid compound or silicic acid compound, (C) a lithium compound, and (D) water, and a heat source having a dissolved oxygen concentration of 1.0 mg / The present invention provides a method for producing lithium iron phosphate or lithium iron silicate, wherein a hydrothermal reaction is carried out in an autoclave, which is a saturated steam produced by heating water of L or less.
The present invention also provides a lithium ion secondary battery containing lithium iron phosphate or lithium iron silicate obtained by the above production method as a positive electrode material.

  According to the method of the present invention, uniform lithium iron phosphate or lithium iron silicate having a small particle size can be obtained by a simple hydrothermal synthesis reaction without using a large amount of inert gas. Moreover, the lithium ion secondary battery containing the obtained lithium iron phosphate or lithium iron silicate as a positive electrode material has high capacity and excellent charge / discharge characteristics.

A schematic view of a steam autoclave used in the method of the present invention is shown. The SEM image of the lithium iron phosphate powder obtained in Example 1 is shown. The XRD chart of the lithium iron phosphate powder obtained in Example 1 is shown. The charging / discharging capacity | capacitance curve of the battery obtained in the Example is shown.

  The method for producing lithium iron phosphate or lithium iron silicate of the present invention is a mixture slurry containing (A) an iron compound, (B) a phosphoric acid compound or a silicate compound, (C) a lithium compound, and (D) water. Is a hydrothermal reaction in an autoclave which is a saturated steam produced by heating water having a dissolved oxygen concentration of 1.0 mg / L or less. (A) As the iron compound, a divalent iron compound is used, and examples thereof include iron halides such as iron fluoride, iron chloride and iron iodide, iron sulfate, and organic acid iron. Among these, as organic acid iron, C1-C20 organic acid iron and C2-C12 organic acid iron are preferable. More preferably, iron dicarboxylic acid such as iron oxalate and iron fumarate, iron hydroxycarboxylate such as iron lactate, and fatty acid iron such as iron acetate.

(B) 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 (B) silicic acid compound is not particularly limited as long as it is a reactive silica compound, and amorphous silica, Na ₄ SiO ₄ (for example, Na ₄ SiO ₄ .H ₂ O) or the like is used.

  (C) Examples of the lithium compound 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. This is preferable.

The amount of (A) an iron compound, (B) a phosphoric acid compound or silicic acid compound, and (C) a lithium compound varies depending on the target product. When the target is lithium iron phosphate (LiFePO ₄ ), the molar ratio of (A) iron compound and (C) lithium compound used is 1: 2.5 to 1: 1 in terms of iron ion and lithium ion. 3.5 is preferable, and approximately 1: 2.8 to 1: 3.3 is more preferable. The molar ratio of (C) lithium compound and (B) phosphate compound used is preferably 2.5: 1 to 3.5: 1 in terms of lithium ion and phosphate ion ratio, and is approximately 2.8: 1 to 3: 1. .3: 1 is more preferable.
On the other hand, when the target product is lithium iron silicate (Li ₂ FeSiO ₄ ), the molar ratio of (A) iron compound and (C) lithium compound used is 1: 2-1 in terms of iron ion and lithium ion. : 3 is preferable, and 1: 2 to 1: 2.5 is more preferable. The molar ratio of (C) lithium compound and (B) silicate compound used is preferably 2: 1 to 3: 1 in terms of lithium ion and silicate ion, and is preferably 2: 1 to 2.5: 1. Is more preferable.

  The amount of water used is preferably 10 to 50 mol with respect to 1 mol of phosphorus ion of the phosphoric acid compound or silicate ion of the silicic acid compound from the viewpoint of solubility of the raw material compound, easiness of stirring, efficiency of synthesis, etc. Furthermore, 13-30 mol is preferable, and 15-20 mol is especially preferable.

  The order of addition of these raw materials is not particularly limited, but when lithium iron phosphate is the target product, a mixture of lithium compound, phosphate compound and water is prepared first, and finally a divalent iron compound is prepared. Is preferably added to prevent side reactions and facilitate the reaction. 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. On the other hand, when the raw materials are added in such an order, no agglomeration occurs, stirring is easy, and the reaction proceeds smoothly.

  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 mixtures may be performed at room temperature, for example, 10 to 35 ° C.

  In this method, it is preferable to add a carbon source and introduce nitrogen gas to a mixture of a lithium compound, a phosphoric acid compound and water. Here, examples of the carbon source include glucose, fructose, polyethylene glycol, polyvinyl alcohol, carboxymethyl cellulose, saccharose, starch, dextrin, and citric acid. Glucose is particularly preferable from the viewpoint of being inexpensive and the viscosity of the mixed solution. 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 introduction of nitrogen gas is important from the viewpoint of reducing the amount of dissolved oxygen in the reaction solution, preventing oxidation of a divalent iron compound to be added later, and reducing the amount of antioxidant added. 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.

  Either the addition of the carbon source or the introduction of nitrogen gas may be performed first or simultaneously.

  In this method, an antioxidant may be added at this point in order to prevent oxidation of the divalent iron compound added later. The antioxidant may be added at the time of addition of the carbon source and the introduction of nitrogen gas, 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 this method, a divalent iron compound is then added.

  In this method, the mixture is then 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.

On the other hand, in the case of using lithium iron silicate as a target product, the order of addition of raw materials is not particularly limited. In addition, an antioxidant may be added to the mixture slurry as necessary, and as the antioxidant, hydrosulfite sodium (Na ₂ S ₂ O ₄ ), aqueous ammonia, sodium sulfite and the like can be used. The content of the antioxidant in the aqueous dispersion is preferably equal to or less than the molar amount relative to the transition metal because the formation of lithium iron silicate is suppressed when added in a large amount. 0.5 or less is more preferable.

It is preferable that the mixture slurry of these components is basic in order to prevent side reactions and dissolve the silicate compound. The pH of the mixture slurry may be basic, but it is 12.0 to 13.5 to prevent side reactions (formation of Fe ₃ O ₄ ), solubility of silicate compounds, and reaction progress. Is particularly preferable. The pH of the mixture slurry may be adjusted by adding a base such as sodium hydroxide, but it is particularly preferable to use Na ₄ SiO ₄ as the silicate compound.

  In the method of the present invention, the mixture slurry is hydrothermally reacted in an autoclave, which is a saturated steam produced by heating water having a dissolved oxygen concentration of 1.0 mg / L or less. That is, for example, as shown in FIG. 1, a hydrothermal reaction is performed using a steam autoclave. Since the steam autoclave receives saturated steam and pressurizes and heats, the steam autoclave is filled with saturated steam. Saturated steam is produced by heating water, but it is preferable to deoxidize the water used in the boiler for heating the water. The water for producing the steam preferably has a dissolved oxygen concentration of 1.0 mg / L or less, and particularly preferably has a dissolved oxygen concentration of 0.5 mg / L or less. Such deoxidized water can be easily produced by, for example, bubbling nitrogen gas in water or using a membrane separation device.

  Since water used for the production of saturated steam does not contain oxygen, iron oxidation can be prevented in the autoclave during hydrothermal synthesis. Further, even if the saturated steam is cooled, no water (drain water) is generated, so that iron is not oxidized in the process. When the saturated steam is introduced into the autoclave and heating is started, it is preferable to push out (replace) the air in the autoclave with saturated steam to further reduce oxygen remaining in the autoclave.

  The hydrothermal reaction may be sealed in a steam autoclave and heated to 130 ° C. or higher. A more preferable reaction temperature is 130 to 220 ° C, and further preferably 140 to 200 ° C. The pressure only needs to be sealed in an autoclave and heated with steam, and is theoretically about 1.0 to 1.6 MPa. The heating time is preferably 1 to 24 hours, more preferably 2 to 12 hours.

  After completion of the hydrothermal reaction, the produced lithium iron phosphate or lithium iron silicate 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. 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 already coated with carbon, so that it is useful as a positive electrode material. 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. Moreover, the average particle diameter of lithium iron silicate is as fine as 1 μm or less, and its particle size distribution is also narrow. The average particle size calculated from the SEM image is 50 nm or less, and the particle size distribution is preferably 10 to 100 nm.

The obtained lithium iron silicate (Li ₂ FeSiO ₄ ) can be used as a positive electrode material for a lithium ion battery by carrying carbon and then firing it. Carbon support may be obtained by adding a carbon source such as glucose, fructose, polyethylene glycol, polyvinyl alcohol, carboxymethyl cellulose, saccharose, starch, dextrin, citric acid, and water to Li ₂ FeSiO ₄ and then baking. The firing conditions are 400 ° C. or higher, preferably 400 to 800 ° C. for 10 minutes to 3 hours, preferably 0.5 to 1.5 hours under an inert gas atmosphere or reducing conditions. By this treatment, a positive electrode material in which carbon is supported on the surface of Li ₂ FeSiO ₄ can be obtained. The amount of the carbon source used is preferably 3 to 15 parts by mass as carbon contained in the carbon source and more preferably 5 to 10 parts by mass as carbon contained in the carbon source with respect to 100 parts by mass of Li ₂ FeSiO ₄ .

  The lithium iron phosphate or lithium iron silicate obtained by the method of the present invention is useful as a positive electrode material for a lithium ion secondary battery because the particle size is fine and uniform. Next, a lithium ion secondary battery containing lithium iron phosphate or lithium iron silicate 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
Li ₂ CO ₃ 3.5 kg, H ₃ PO ₄ 4.1 kg and water 9.5 kg were mixed. To this was added 550 g of glucose, and then nitrogen gas was bubbled to confirm that the dissolved oxygen concentration was less than 0.1 mg / L. This was mixed with 9.2 kg of FeSO ₄ .7H ₂ O, and stirred at 23 ± 2 ° C. with a propeller type stirring device for 60 minutes. At this time, the viscosity of the slurry was 45 mPa · s, and the stirring was smooth.
The mixture stirred for 60 minutes was placed in the steam autoclave of FIG. The inside of the autoclave was heated at 180 ° C. for 3 hours using a saturated vapor obtained by heating water with a dissolved oxygen concentration of less than 0.5 mg / L by a membrane separator. Stirring was continued during heating. The internal pressure of the autoclave was 1.1 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 calcined 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. 2 shows an SEM image of the obtained powder, and FIG. 3 shows an XRD chart. The particle diameter of the obtained lithium iron phosphate was in the range of 300 to 500 nm, and it was confirmed that high purity lithium iron phosphate was obtained.

Comparative Example 1
A raw material slurry was prepared in the same procedure as in Example 1, and placed in a steam autoclave. Subsequently, saturated steam generated using water not subjected to a treatment for reducing dissolved oxygen (dissolved oxygen concentration: 7 to 10 mg / L) was introduced into an autoclave and heated at 180 ° C. for 3 hours. The produced crystal was treated in the same procedure as in Example 1 to obtain a fine powder of lithium iron phosphate. An XRD chart of the fine powder obtained is shown in FIG. The obtained lithium iron phosphate was a composite containing an impurity phase.

Example 2
A battery was fabricated using the material obtained in Example 1 as the positive electrode material.
The fired product obtained in Example 1, ketjen black (conductive agent), and polyvinylidene fluoride (binding agent) were mixed at a weight ratio of 75:15:10, and this was mixed with N-methyl-2-pyrrolidone. Was added and kneaded sufficiently 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.
The discharge characteristics are shown in FIG. 4 from the results of the charge / discharge test. As a result, although the battery using the positive electrode material of Example 1 showed an excellent charge / discharge capacity, the charge / discharge capacity of the battery using the material of Comparative Example 1 was not sufficient.

Claims (3)

  (A) An iron compound, (B) a phosphoric acid compound or silicic acid compound, (C) a lithium compound, and (D) water containing a mixture of water and a water source having a dissolved oxygen concentration of 1.0 mg / L or less are heated. A method for producing lithium iron phosphate or lithium iron silicate, wherein a hydrothermal reaction is carried out in a steam autoclave, which is a saturated steam produced as described above.   The manufacturing method according to claim 1 whose dissolved oxygen concentration in said mixed slurry is 1.0 mg / L or less.   The method according to claim 1 or 2, wherein the hydrothermal reaction is performed under conditions of 130 to 220 ° C.