JP2011204638A - POSITIVE ELECTRODE ACTIVE MATERIAL FOR Li-ION CELL AND METHOD OF MANUFACTURING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material for a Li-ion cell with a high discharged capacity as well as superior cycle characteristics, and to provide a method of manufacturing the same.SOLUTION: The positive electrode active material for the Li-ion cell includes rod-shaped LiFePOpowder and carbon nanofiber or nanocarbon, wherein the carbon nanofiber exists in the interior and/or on the surface of the rod-shaped LiFePOpowder or nanocarbon exists on the surface of the rod-shaped LiFePOpowder. The LiFePOcontained in the positive electrode active material can be synthesized in an aqueous solution containing a lithium compound, a metal compound, a phosphate compound, and carbon nanofiber in an inert atmosphere, a reducing atmosphere or a vacuum atmosphere by a microwave hydrothermal method.

Description

The present invention relates to LiFePO ₄ used as a positive electrode active material for a Li ion battery, and a method for producing the same.

In recent years, with the development of portable electronic devices such as mobile phones and laptop computers, and the practical application of electric vehicles, secondary batteries with small size and light weight and high capacity are required. Currently, lithium ion secondary batteries using a lithium-containing transition metal oxide such as LiCoO ₂ as a positive electrode material of a high capacity secondary battery that meets this requirement and using a carbon-based material as a negative electrode active material have been commercialized.

However, although the lithium ion battery using LiCoO ₂ has excellent performance as a small battery, there are few resource reserves due to the small amount of cobalt reserves in the raw material, and in addition to severe price fluctuations, When there is an internal short circuit for some reason or overcharge, there is a problem that severe heat generation occurs due to release of oxygen from LiCoO ₂ , and there is a risk of burning the electrolyte and exploding the battery. Yes.

Considering the situation where the development of environmentally friendly vehicles such as electric vehicles and hybrid vehicles will become important in the future, safe and inexpensive cathode materials for lithium ion batteries will be required. Under such circumstances, an iron-based material rich in raw materials, particularly LiFePO ₄ , is expected as a positive electrode material for a Li-ion battery.

However, since LiFePO ₄ has very low electronic conductivity, it is not sufficient to simply form a positive electrode by coexisting a conductive additive, and it is difficult to ensure excellent battery characteristics. Therefore, a technique for increasing the electron conductivity in a positive electrode material for a lithium ion battery using LiFePO ₄ has been studied.

  For example, it is manufactured by a method including a raw material mixing step of mixing a lithium compound, an iron compound, a phosphorus-containing ammonium salt, and carbon material fine particles to obtain a mixture, and a baking step of baking the mixture at a temperature of 600 ° C. or higher and 750 ° C. or lower. In addition, a carbon-containing lithium iron composite oxide is shown (Patent Document 1). Moreover, it has the process of manufacturing a carbon-iron phosphate complex by precipitation, the process of manufacturing the coprecipitate containing the said carbon-iron phosphate complex and lithium phosphate, and the process of baking the said coprecipitate. A method for producing carbon-olivine type lithium iron phosphate composite particles is shown (Patent Document 2).

  In addition, carbon as an electron conductive substance is added to the raw material of lithium iron phosphate, the precursor mixture or the precursor suspension is dispersed or pulverized, and then reacted under hydrothermal conditions. A manufacturing method is shown (Patent Document 3).

  However, the carbon materials used in the above production method are all powders, and it is difficult to produce each raw material and the carbon material in a uniform state. In addition, in the production method using the coprecipitate containing the carbon-iron phosphate complex and lithium phosphate, carbon nanofibers are exemplified as the carbon powder (paragraph 0022 of Patent Document 2). ) Since the dispersibility of the carbon nanofibers is poor, the carbon nanofiber aggregates are formed in the positive electrode material simply by mixing and firing the carbon nanofibers with each raw material, and sufficient effects cannot be exhibited. In addition, in the method (Patent Document 3) in which the precursor mixture or the precursor suspension is dispersed or pulverized and then reacted under hot water conditions (Patent Document 3), the dispersibility of the carbon material is increased by the dispersion or pulverization. Although there is a possibility, there is a problem that it is difficult to take advantage of the merit of the iron-based material that the process is complicated, complicated, costly, and inexpensive.

Japanese Laid-Open Patent Publication No. 2003-34534 Japanese Laid-Open Patent Publication No. 2007-35295 Special table 2007-511458 public information

Thus, as a result of intensive studies, the inventors have added a carbon nanofiber dispersion to a raw material solution, and then manufactured LiFePO ₄ using a microwave hydrothermal synthesis to produce a fine LiFePO ₄ having a specific shape. When a positive electrode active material for a Li ion battery, in which carbon nanofibers exist inside and / or on the surface of LiFePO ₄ powder or nanocarbon exists on the surface of the rod-like LiFePO ₄ powder, LiFePO ₄ is synthesized. It has been found that the capacity and cycle characteristics of can be significantly improved.

The present invention relates to a positive electrode active material for a Li-ion battery, which has solved the above problems with the following configuration, and a method for producing the same.
(1) The rod-shaped LiFePO ₄ powder and carbon nanofiber or nanocarbon, and the carbon nanofiber is present inside and / or on the surface of the rod-shaped LiFePO ₄ powder, or the rod-shaped LiFePO ₄ A positive electrode active material for a Li-ion battery, characterized in that nanocarbon is present on the powder surface.
(2) The average minor axis diameter of the rod-shaped LiFePO ₄ powder is 20 to 300 nm, the average major axis diameter is 100 to 2000 nm, the average minor axis diameter of the carbon nanofiber is 1 to 100 nm, and the aspect ratio is 5 or more. The positive electrode active material for a Li ion battery according to (1), wherein the nanocarbon has a thickness of 1 to 25 nm.
(3) LiFePO _{4 is} added to an aqueous solution containing a lithium compound, an iron compound, a phosphate compound, and carbon nanofibers in an air atmosphere, an inert atmosphere, a reducing atmosphere, or a vacuum atmosphere by a microwave hydrothermal method. A method for producing a positive electrode active material for a Li-ion battery, characterized by comprising:
(4) The method for producing a positive electrode active material for a Li ion battery according to (3), wherein the carbon nanofibers have an average minor axis diameter of 1 to 100 nm and an aspect ratio of 5 or more.
(5) The carbon nanofibers may be Fe ions in an aqueous solution: 2.24 × 10 ⁻⁵ to 2.24 parts by mass with respect to 1 part by mass, and / or 3.05 × 10 6 with respect to Li ions in the aqueous solution. ^-5 ～3.05 containing parts by mass, (3) or (4) the method of producing the Li ion positive electrode active material for battery according.
(6) Microwave hydrothermal synthesis is performed at a temperature of 100 to 250 ° C. and a pressure of 0.2 to 4.0 MPa, and the positive electrode active for Li ion battery according to any one of (3) to (5) above A method for producing a substance.
(7) Any of (3) to (6) above, wherein the lithium compound is at least one selected from the group consisting of lithium hydroxide, lithium citrate, lithium oxalate, lithium phosphate, and lithium carbonate The manufacturing method of the positive electrode active material for Li ion batteries of description.
(8) The above (3) to (7), wherein the iron compound is at least one selected from the group consisting of iron citrate, iron oxalate, iron phosphate, iron sulfate, iron oxide, and metal iron The manufacturing method of the positive electrode active material for any one of Li ion batteries.
(9) The Li according to any one of (3) to (8), wherein the phosphate compound is at least one selected from the group consisting of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid. A method for producing a positive electrode active material for an ion battery.

  According to the present invention (1), a positive electrode active material for a Li-ion battery having a high material discharge capacity and significantly improved conductivity with carbon nanofibers can be obtained, whereby the discharge capacity is high, A Li ion battery having good cycle characteristics can be easily manufactured.

According to the present invention (3), LiFePO ₄ having a high capacity and good cycle characteristics can be easily produced in an aqueous solution with energy saving. Here, the carbon nanofiber serves as a starting point for synthesis of LiFePO ₄ during microwave hydrothermal synthesis and contributes as a conductivity imparting agent to the produced positive electrode active material. It is believed that it absorbs heat and overheats to promote the synthesis reaction, promotes crystallization of LiFePO ₄ , and provides a remarkable effect that enables the synthesis of a stable material that can withstand charge and discharge in a short time. It is done. Here, the microwave is considered to contribute to the crystallization of LiFePO ₄ .

2 is a transmission electron micrograph of a positive electrode active material for a Li-ion battery produced in Example 1. FIG. 2 is a scanning electron micrograph of a positive electrode active material for a Li ion battery produced in Example 1. FIG. 3 is a transmission electron micrograph of a positive electrode active material for a Li-ion battery produced in Example 2. FIG. 3 is a transmission electron micrograph of a positive electrode active material for a Li-ion battery produced in Example 2. FIG. It is a transmission electron micrograph of carbon nanofiber. 2 is a scanning electron micrograph of a positive electrode active material for a Li-ion battery produced in Comparative Example 1. It is an example of the schematic of the container used by the microwave hydrothermal method. 2 is an X-ray diffraction pattern of a positive electrode active material for a Li-ion battery produced in Example 1. FIG. It is a block diagram of the electrochemical cell used in order to measure the active material synthesize | combined in the Example. It is a figure which shows the charging / discharging result in 0.2C of the positive electrode active material for Li ion batteries obtained in Example 1. FIG. It is a figure which shows the charging / discharging result in 0.2C of the positive electrode active material for Li ion batteries obtained in Example 2. FIG. It is a figure which shows the charging / discharging result at 0.2C of the positive electrode active material for Li ion batteries obtained by the comparative example 1. FIG. It is a figure which shows the charging / discharging result in 0.5C of the positive electrode active material for Li ion batteries obtained in Example 1. FIG. It is a figure which shows the charging / discharging result in 0.5C of the positive electrode active material for Li ion batteries obtained by the comparative example 1. FIG.

  Hereinafter, the present invention will be specifically described based on embodiments. Unless otherwise indicated,% is% based on mass unless otherwise specified.

[Positive electrode active material for Li-ion battery]
The positive electrode active material for a Li ion battery of the present invention includes rod-shaped LiFePO ₄ powder and carbon nanofibers or nanocarbon, and the carbon nanofibers are present inside and / or on the surface of the rod-shaped LiFePO ₄ powder. Or the nanocarbon is present on the surface of the rod-shaped LiFePO ₄ powder.

LiFePO ₄ is an olivine type, and a preferred composition is Li _x FePO ₄ (wherein x = 0 to 1). Here, quantitative analysis of Li, Fe, P, and O is performed by ICP mass spectrometry. For example, part of the Fe site in the crystal structure may be replaced with other elements such as Co, Ni, Al, Mg, Cu, Zn, and Ge.

For example, Mn, Mg, Ni, Co, Cu, Zn, and Ge have an ionic radius substantially the same as that of Fe, and are oxidized and reduced at a potential different from that of Fe. Therefore, the crystal structure of the lithium iron composite oxide can be stabilized by substituting a part of the Fe site with one or more of these elements. Therefore, the lithium iron composite oxide has a composition formula LiFe _1-y M _y PO ₄ (where M is Mn, Mg, Ni, Co, Cu), in which a part of the Fe site is substituted with another element M. , Zn, and Ge, and y = 0 to 0.2 is desirable. In particular, the substitution element M is preferably Mn because it is abundant in terms of resources and is inexpensive.

FIG. 1 shows a transmission electron micrograph of the positive electrode active material for Li ion battery produced in Example 1, and FIG. 2 shows a scanning electron micrograph of the positive electrode active material for Li ion battery produced in Example 1. . As can be seen from FIG. 1, carbon nanofibers are present inside and / or on the surface of the rod-shaped LiFePO ₄ powder. Specifically, the carbon nanofibers are present in the surface of the rod-shaped LiFePO ₄ powder, it can be seen that the electron beam overlap rod LiFePO ₄ powder and the carbon nanofibers is part that transmitted. Here, the rod shape means a rod shape as shown in a dark color in FIG.

3 and 4 show transmission electron micrographs of the Li-ion battery positive electrode active material produced in Example 2. FIG. As can be seen from FIG. 4, nanocarbon is present in layers on the surface of the rod-shaped LiFePO ₄ powder. This nanocarbon is formed by heating the rod-shaped LiFePO ₄ powder in which carbon nanofibers exist inside and / or on the surface under specific conditions. Here, nanocarbon means carbon nanofibers whose shape has been changed by heating.

For reference, FIG. 5 shows a transmission electron micrograph of carbon nanofibers, and FIG. 6 shows a scanning electron micrograph of the positive electrode active material for Li-ion battery of Comparative Example 1 synthesized without adding carbon nanofibers. . From comparison of FIG. 1, FIG. 5, and FIG. 6, it can be confirmed that what is shown in dark color in FIG. 1 is rod-shaped LiFePO ₄ powder.

The average minor axis diameter of the rod-shaped LiFePO ₄ powder is preferably 5 to 50 nm, and more preferably 10 to 40 nm. The average major axis diameter is preferably 30 to 2000 nm, and more preferably 30 to 500 nm. Here, the average particle diameter is calculated by observing an SEM photograph with a scanning electron microscope (model number: JSM-5900) manufactured by JEOL or using a particle size distribution measuring apparatus (model number: UPA-EX) manufactured by Microtrack. Moreover, 3-70 m < ² > / g is preferable and, as for the specific surface area of the positive electrode active material for Li ion batteries, 6-40 m < ² > / g is more preferable. Here, the specific surface area is measured by the BET method.

  Carbon nanofibers preferably have an average minor axis diameter of 1 to 100 nm and an aspect ratio of 5 or more from the viewpoint of forming a conductive path and imparting good conductivity. The aspect ratio is preferably 10,000 or less from the viewpoint of dispersibility. In addition, from the viewpoint of dispersibility in an aqueous solution, the surface of the carbon nanofiber is preferably subjected to a hydrophilic treatment by acid treatment or the like. The thickness of the nanocarbon is preferably 1 to 25 nm from the viewpoint of imparting good conductivity and securing the Li ion path during charge and discharge. The thickness of the nanocarbon is more preferably 1 to 5 nm, and further preferably 1 to 3 nm.

From the viewpoint of nucleation and reaction promotion of LiFePO ₄ , imparting good conductivity, and good cycle characteristics, carbon nanofibers and nanocarbons are 0.01 to 0.1 parts by mass with respect to 100 parts by mass of a positive electrode active material for Li ion batteries. The amount is preferably 30 parts by mass, and more preferably 0.03 to 10 parts by mass.

[Method for producing positive electrode active material for Li-ion battery]
The method for producing a positive electrode active material for a Li-ion battery according to the present invention includes an aqueous solution containing a lithium compound, an iron compound, a phosphate compound, and carbon nanofibers in an air atmosphere, an inert atmosphere, a reducing atmosphere, or a vacuum. In the atmosphere, LiFePO ₄ is synthesized by a microwave hydrothermal method.

The lithium compound only needs to be a lithium source of LiFePO ₄ and soluble in water. Examples of such lithium compounds include lithium hydroxide, lithium citrate, lithium oxalate, lithium phosphate, and lithium carbonate. These lithium compounds may be used alone or in admixture of two or more. Lithium hydroxide, lithium phosphate, and lithium carbonate are preferable, and lithium hydroxide is more preferable from the viewpoint of water solubility. Although it is not known in detail, it seems preferable that the aqueous solution containing lithium hydroxide becomes alkaline. The purity is preferably that which is commercially available as a special grade from a reagent manufacturer.

The iron compound only needs to be an iron source of LiFePO ₄ and soluble in water. Examples of such iron compounds include iron citrate, iron oxalate, iron phosphate, iron sulfate, iron oxide, and metals. Iron etc. are mentioned, You may use these individually or in mixture of 2 or more types. Of these, iron sulfate and iron phosphate are preferable. Iron sulfate is more preferable because it is easily dissolved in water and iron ions are easily eluted, and the separated sulfate ion does not inhibit the synthesis of the battery material or is not mixed with the battery material. Further, iron sulfate is more preferable because it is inexpensive.

The phosphoric acid compound only needs to be a phosphoric acid source for LiFePO ₄ and soluble in water. Examples of such phosphoric acid compounds include ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid. These may be used alone or in admixture of two or more. Preferably, it is phosphoric acid. This is because it is liquid at normal temperature and pressure and is easy to mix, and also from the viewpoint of reacting with lithium.

The carbon nanofibers are as described above, but a dispersion of carbon nanofibers in advance is preferable because a dispersion or pulverization treatment is unnecessary. After this carbon nanofiber is added, ultrasonic irradiation is performed, whereby LiFePO ₄ particles can be made fine and further crystallized uniformly. Although the details are not clear, it is considered that the carbon nanofibers not only act as a conduction aid but also act as a nucleation agent and particle growth inhibitor for LiFePO ₄ . Carbon nanofibers are also expected to absorb microwaves and promote hydrothermal synthesis.

  Examples of water include tap water, distilled water, ion exchange water, pure water, and the like, and ion exchange water, pure water, and the like are preferable from the viewpoint of avoiding mixing of inorganic ion impurities.

The concentration of lithium ions in the aqueous solution is preferably 0.08 to 10 g / dm ³ . If it is lower than 0.08 g / dm ^3, the progress of the reaction is slow, and the synthesis takes time. If it is higher than 10 g / dm ^3, there is a high possibility of causing iron oxidation.

The concentration of iron ions in the aqueous solution is preferably 0.2 to 110 g / dm ³ . If it is lower than 1 g / dm ^3, the progress of the reaction is slow and it takes time to synthesize, and if it is higher than 100 g / dm ³ , the target LiFePO ₄ becomes difficult to synthesize. Moreover, the concentration of iron ions in the aqueous solution is preferably 0.1 to 4 times the concentration of lithium. If it is lower than 0.1 times, the progress of the reaction is slow and it takes time to synthesize. If it is higher than 4 times, the target LiFePO ₄ is hardly synthesized.

The concentration of phosphorus in the aqueous solution is preferably 0.1 to 68 g / dm ³ . The progress of the reaction is lower than 0.1 g / dm ³ it takes time to slow synthesis, LiFePO ₄ which is higher than 68 g / dm ³ interest is not easily synthesized. The concentration of the phosphoric acid compound in the aqueous solution is preferably 0.02 to 7 times the concentration of the lithium compound. If it is lower than 0.02 times, the progress of the reaction is slow and it takes time to synthesize. If it is higher than 7 times, the target LiFePO ₄ is difficult to be synthesized.

The concentration of the carbon nanofibers in the aqueous solution is preferably 2.50 × 10 ^{−3 to} 25 g / dm ³ . If it is lower than 2.50 × 10 ⁻³ g / dm ³ , the progress of the reaction is slow and it takes time to synthesize, and the conductivity imparting to LiFePO ₄ is not sufficient, and if it is higher than 25 g / dm ³ , the intended LiFePO ₄ is difficult to synthesize. Further, the carbon nanofibers may be used in an amount of 2.24 × 10 ⁻⁵ to 2.24 parts by mass with respect to 1 part by mass of Fe ions in the aqueous solution and / or 3 parts by mass of Li ions in the aqueous solution. It is preferable to contain 05 * 10 < ^-5 > -3.05 mass part. It is preferable that the carbon nanofibers are contained in an amount of 7.92 × 10 ^{−5 to} 0.792 parts by mass with respect to 1 part by mass of LiFePO _{4 to be} produced. A coaxial probe was inserted into a 20 ml solution at room temperature, and the dielectric loss (2.45 GHz) was measured with a network analyzer. In the case of water only: 9.82, in the case of water and ascorbic acid: 11.29, In the case of water and carbon nanofibers: 12.79, and it can be seen that the absorption rate of electromagnetic waves is improved by the addition of carbon nanofibers. For this reason, it is considered that the carbon nanofibers become a heat source and contribute to the nucleus of LiFePO ₄ synthesis and the promotion of the reaction.

  The aqueous solution can be prepared by adding the above lithium compound, iron compound and phosphoric acid compound in water and then dissolving with stirring, adding the carbon nanofibers preferably in the state of a dispersion, and stirring. . Stirring may be performed by a conventional method such as propeller stirring. Further, the aqueous solution is preferably degassed or deoxygenated from the viewpoint of preventing oxidation of Fe during synthesis.

  FIG. 7 shows an example of a schematic diagram of a container used in the microwave hydrothermal method. 1 is a pressure vessel, 2 is a lid, 3 is a locking lid, 4 is a rupturable plate, and 5 is an exhaust direction. This container is installed in a microwave generator, and a reaction is performed by a microwave hydrothermal method. Examples of the microwave generator include a microwave hydrothermal apparatus and a microwave oven that is a general household appliance. An apparatus for carrying out the present invention is not particularly limited as long as it can irradiate an aqueous solution with microwaves and perform hydrothermal synthesis.

The manufacturing method of the positive electrode active material for Li ion batteries of this invention is considered to occur by the following reaction formula, for example.
3LiOH · H ₂ O + FeSO ₄ · 7H ₂ O + H ₃ PO ₄
→ LiFePO ₄ + Li ₂ SO ₄ + 13H ₂ O

  The above reaction may be performed in an air atmosphere, but since a desired effect can be obtained by preventing oxidation of Fe, in an inert atmosphere such as argon gas or nitrogen gas; hydrogen, monoxide More preferably, it is carried out in a reducing atmosphere such as carbon; or in a vacuum atmosphere.

  The microwave frequency of the microwave hydrothermal method is preferably 2.45 to 28.00 GHz, and more preferably 2.45 GHz. Exceeding this range is not preferable because the water absorption efficiency decreases and the reaction rate decreases.

  The temperature of the microwave hydrothermal method is preferably 100 to 250 ° C, and more preferably 120 to 200 ° C. When the temperature is lower than 100 ° C., the crystallinity is lowered, and when the temperature is higher than 250 ° C., the crystal particles become larger, which is not preferable.

  The pressure of the microwave hydrothermal method is preferably 0.1 to 4.0 MPa, and more preferably 0.5 to 1.6 MPa. If it is lower than 0.1 MPa, the reaction does not proceed because the temperature does not rise, and if it is higher than 4.0 MPa, the temperature rises and the crystal grains become larger, which is not preferable.

In addition, when LiFePO ₄ is synthesized by the microwave hydrothermal method and then heated at 300 to 800 ° C. in an inert atmosphere, a reducing atmosphere or a vacuum atmosphere, the viewpoint of increasing the capacity of LiFePO ₄ and improving the cycle characteristics To 300 to 500 ° C. is more preferable. If it is lower than 300 ° C., the effect of heating cannot be obtained, and if it is higher than 800 ° C., sintering proceeds and the particle size becomes large. The heating time is preferably 30 to 300 minutes, and more preferably 60 to 180 minutes.

  In order to construct a positive electrode for a lithium ion battery using the positive electrode active material for a Li ion battery produced by the method of the present invention, for example, the positive electrode active material for a Li ion battery is used as an active material as it is, and the others Like the conventionally known positive electrode, a molded body of a positive electrode slurry containing a binder and, if necessary, a conductive aid such as a carbon material may be used. Moreover, what is necessary is just to form these positive electrode slurries as a positive electrode active material layer in the single side | surface or both surfaces of the electroconductive base | substrate used as a collector as needed.

  When a lithium ion battery is constructed using a positive electrode for a lithium ion battery using the positive electrode active material for a Li ion battery produced by the method of the present invention, the negative electrode, separator, non-aqueous electrolyte, exterior body, etc. There is no restriction | limiting in particular about various structures, The structure similar to a conventionally well-known lithium ion battery is employable.

  The positive electrode active material for Li ion batteries produced by the method of the present invention is effectively used as a positive electrode active material for battery electrodes and secondary battery electrodes. In particular, it is extremely effective as a positive electrode active material for non-aqueous electrolyte secondary batteries such as lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries, and is also effective as a positive electrode active material for lithium primary batteries. The non-aqueous electrolyte secondary battery using the electrode active material of the present invention has a large charge / discharge capacity and high energy density, and exhibits excellent cycle characteristics, safety, etc. It can be effectively applied as a positive electrode active material of a secondary battery. In addition, since the manufacturing method of the present invention does not require a large-scale apparatus and can be easily synthesized in a short time, the time and procedure can be shortened and the manufacturing cost can be reduced.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these.

[Adjustment of carbon nanofiber dispersion]
Carbon nanofibers were mixed with a mixed solution of nitric acid (concentration 60%) and sulfuric acid (concentration 95% or more) at a ratio of carbon nanofiber: nitric acid: sulfuric acid = 1 part by weight: 5 parts by weight: 15 parts by weight and heated. Then, filtration and water washing were performed and the hydrophilic treatment was performed. After drying, 5 g of the obtained carbon nanofiber was dispersed in ion exchange water: 95 cm ³ to prepare a carbon nanofiber dispersion.

[Example 1]
LiFePO ₄ was charged in a flask containing 19 cm ^{3 of} degassed ion-exchanged water, and lithium hydroxide monohydrate: 1.0 g and iron sulfate heptahydrate: 1.12 g as raw materials for Li, Fe, and P, respectively. Phosphoric acid: 0.43 g was added and stirred, then carbon nanofiber dispersion: 1.00 cm ³ was added and stirred to prepare an aqueous solution. The mixture was placed in the container shown in FIG. 7 and reacted at 150 ° C. for 20 minutes in an argon atmosphere, and then vacuum dried at 110 ° C. for 60 minutes to synthesize a positive electrode active material.

[Example 2]
The positive electrode active material produced in Example 1 was heated at 400 ° C. for 60 minutes in an atmosphere (97% by volume of argon + 3% by volume of hydrogen).

[Comparative Example 1]
A positive electrode active material was synthesized in the same manner as in Example 1 except that the carbon nanofiber dispersion was not used.

[Test Example 1]
The positive electrode active material obtained in Example 1 was observed with a scanning electron microscope (model number: JSM-5900) manufactured by JEOL. FIG. 2 shows the result. Moreover, the positive electrode active material obtained in Comparative Example 1 was observed. FIG. 6 shows the result.

[Test Example 2]
The positive electrode active materials obtained in Examples 1 and 2 were observed with a transmission electron microscope (model number: JEM-1200EX) manufactured by JEOL. FIG. 1 shows the results of Example 1, and FIGS. 3 and 4 show the results of Example 2. FIG.

[Test Example 3]
The positive electrode active material obtained in Example 1 was subjected to X-ray diffraction measurement in the range of 2θ: 10 to 60 ° using an X-ray diffractometer manufactured by Max Science Co., Ltd. The result is shown in FIG.

[Test Example 5]
In FIG. 9, the block diagram of the electrochemical cell used for battery characteristic evaluation is shown. In FIG. 9, 10 is a working electrode, 11 is a positive electrode and a current collector, 12 is a nonwoven fabric, 13 is a separator, 14 is a negative electrode, 15 is a counter electrode, and 16 is an electrolytic solution. The electrode area was 1 cm ² . The positive electrode 11 was prepared by mixing the synthesized LiFePO ₄ powder, acetylene black (conducting aid) and polytetrafluoroethylene (binder) in a mass ratio of 70: 25: 5 (total amount: 0.1 g). For the negative electrode 14, metallic lithium was used, and for the electrolytic solution 16, an organic solvent in which 1 mol / dm ³ of LiClO ₄ was dissolved as an electrolyte in a mixed solution of polycarbonate and dimethoxyethane at a volume ratio of 1: 1 was used. The current collector 11 was a nickel mesh, the separator 13 was a separator manufactured by JGC Chemicals, and the nonwoven fabric 12 was a polypropylene nonwoven fabric manufactured by Mitsui Petrochemical Industries.

  The charge / discharge measurement was performed using a charge / discharge measuring device (HJ-101 SM6 manufactured by Hokuto Denko Co., Ltd.). The measurement conditions are a temperature condition of 20 ° C., a two-terminal method, a charge / discharge rate of 0.2 C (a rate that takes 5 hours to charge / discharge the theoretical capacity), and a voltage range of 2.5-4. Charging / discharging was repeated 10 times at 0V. 10 shows the result of the positive electrode active material obtained in Example 1, FIG. 11 shows the result of the positive electrode active material obtained in Example 2, and FIG. 12 shows the result of the positive electrode active material obtained in Comparative Example 1. The results are shown. The measurement was also performed at a charge / discharge rate of 0.5 C (a rate at which the time taken to charge / discharge the theoretical capacity was 2 hours). FIG. 13 shows the results in Example 1, and FIG. 14 shows the results in Comparative Example 1.

  Table 1 shows the initial discharge capacity, the discharge capacity after 10 cycles, and the capacity retention rate. Here, the capacity retention rate is [(discharge capacity after 10 cycles) / (initial discharge capacity)] (unit is “%”).

From FIG. 1, the average minor axis diameter of the rod-shaped LiFePO ₄ powder is 10 to 40 nm, the average major axis diameter is 30 to 500 nm, the average minor axis diameter of the carbon nanofiber is 5 to 20 nm, and the aspect ratio is 5 to 60. I found out that From FIG. 6, in Comparative Example 1, only LiPO ₄ particles (confirmed by X-ray diffraction) were observed. From FIG. 8, it was confirmed that the positive electrode active material of Example 1 was a LiPO ₄ single phase.

As can be seen from Table 1, improvement of the carbon nanofibers added synthesized initial capacity of Example 1 was the 138 mAh · g ^-1, as compared with Comparative Example 1 without addition of the carbon nanofibers 7 mAh · g ^-1 Was confirmed. Furthermore, the cycle characteristics were greatly improved, and 128 mAh · g ⁻¹ was exhibited even after 10 cycles. The discharge voltage was 3.35 V. As can be seen from FIGS. 10 and 12, the plateau region was wider than that of Comparative Example 1 with no additive. This is considered that the electron conductivity was improved by the addition of carbon nanofibers. Although not described in Table 1, as a result of measuring the specific surface area by the BET method, Example 1 to which carbon nanofibers were added was about 29 m ² / g, and Comparative Example 1 to which no carbon nanofibers were added was 15 m ² / g. The addition of carbon nanofibers is considered to contribute greatly to the fact that the surface area is remarkably increased by the addition. The positive electrode active material of Example 2 in which LiFePO ₄ synthesized by adding carbon nanofibers was heated at 400 ° C. for 60 minutes in an atmosphere (97% by volume of argon + 3% by volume of hydrogen) has little deterioration due to repeated cycles, Even after 10 cycles, 97% of the initial capacity was maintained. This is considered to be due to the fact that the crystallinity improved by heating and the appearance of nanocarbon on the particle surface. 13 and 14, when charging / discharging is performed at a high charge rate of 0.5 C (each charging / discharging time is 2 hours each), charge obtained at a rate of 0.2 C in both Example 1 and Comparative Example 1 is obtained. Compared with the discharge characteristics, the capacity and cycle characteristics decreased. However, since Example 1 synthesized by adding carbon nanofibers had a smaller polarization than Comparative Example 1 in which carbon nanofibers were not added, improvement in electronic conductivity by carbon nanofibers was observed.

As described above, in the microwave hydrothermal synthesis, by adding carbon nanofibers, a positive electrode active material having a large specific surface area in which carbon nanofibers are present in and / or on the surface of LiFePO ₄ powder can be synthesized, and charge / discharge characteristics can be improved. It can be greatly improved. In addition, by using a dispersion-type carbon nanofiber solution, the uniformly dispersed carbon nanofiber absorbs microwaves, and it is considered that the reaction proceeds efficiently using it as a heat source. Further, when the positive electrode active material in which the carbon naon fiber is present in and / or on the surface of the LiFePO ₄ powder is heated at about 400 ° C. in a nitrogen atmosphere, nanocarbon can be seen, which causes the powder surface to be layered. It was confirmed that the cycle characteristics were improved by covering.

DESCRIPTION OF SYMBOLS 1 Pressure vessel 2 Lid 3 Locking lid 4 Rupture plate 5 Exhaust direction 10 Working electrode 11 Positive electrode and current collector 12 Nonwoven fabric 13 Separator 14 Negative electrode 15 Counter electrode 16 Electrolyte

Claims (9)

Rod-shaped LiFePO ₄ powder and carbon nanofiber or nanocarbon, and the carbon nanofiber is present inside and / or on the surface of the rod-shaped LiFePO ₄ powder, or on the surface of the rod-shaped LiFePO ₄ powder. A positive electrode active material for a Li-ion battery, characterized in that nanocarbon is present. The average minor axis diameter of the rod-shaped LiFePO ₄ powder is 5 to 50 nm, the average major axis diameter is 30 to 2000 nm, the average minor axis diameter of the carbon nanofibers is 1 to 100 nm, and the aspect ratio is 5 or more, Or the thickness of nanocarbon is 1-25 nm, The positive electrode active material for Li ion batteries of Claim 1. LiFePO ₄ is synthesized by an microwave hydrothermal method in an air atmosphere, an inert atmosphere, a reducing atmosphere or a vacuum atmosphere in an aqueous solution containing a lithium compound, an iron compound, a phosphoric acid compound, and carbon nanofibers. The manufacturing method of the positive electrode active material for Li ion batteries characterized by the above-mentioned.   The manufacturing method of the positive electrode active material for Li ion batteries of Claim 3 whose average minor axis diameter of carbon nanofiber is 1-100 nm, and whose aspect-ratio is 5 or more. The carbon nanofibers are Fe ions in an aqueous solution: 2.24 × 10 ⁻⁵ to 2.24 parts by mass with respect to 1 part by mass, and / or 3.05 × 10 ⁻⁵ to Li ions in an aqueous solution. The manufacturing method of the positive electrode active material for Li ion batteries of Claim 3 or 4 which contains 3.05 mass parts.   The method for producing a positive electrode active material for a Li-ion battery according to any one of claims 3 to 5, wherein the microwave hydrothermal synthesis is performed at a temperature of 100 to 250 ° C and a pressure of 0.2 to 4 MPa.   The Li ion according to any one of claims 3 to 6, wherein the lithium compound is at least one selected from the group consisting of lithium hydroxide, lithium citrate, lithium oxalate, lithium phosphate, and lithium carbonate. A method for producing a positive electrode active material for a battery.   The iron compound is at least one selected from the group consisting of iron citrate, iron oxalate, iron phosphate, iron sulfate, iron oxide, and metal iron, according to any one of claims 3 to 7. A method for producing a positive electrode active material for a Li-ion battery.   The positive electrode for a Li ion battery according to any one of claims 3 to 8, wherein the phosphoric acid compound is at least one selected from the group consisting of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid. A method for producing an active material.