JP5765798B2 - Cathode active material for Li-ion battery and method for producing the same - Google Patents

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, along with the development of portable electronic devices such as mobile phones and notebook computers, and the practical application of electric cars and hybrid cars, secondary batteries with small size and light weight and high capacity are required. In addition, secondary batteries with high capacity and good cycle characteristics are demanded by the battery industry and heavy industry for use as backup power sources for natural energy such as solar power generation and wind power generation, and power supply sources in earthquake disasters. 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 order to obtain fine lithium iron phosphate particles. Requires a pulverization step after firing. 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

As a result of intensive research, the inventors have calcinated the raw material mixture, and then mixed the polymer material with the calcined product, followed by calcining, and carbon fine particles are present on the surface of LiFePO _4. It has been found that a fine cathode active material for a Li-ion battery has a very high discharge capacity and good cycle characteristics.

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) (A) A step of preparing a raw material mixture by mixing a lithium compound, an iron compound excluding iron oxide , and a phosphoric acid compound, and (B) a raw material mixture in an inert atmosphere, a reducing atmosphere or a vacuum atmosphere. (C) without pulverizing the calcined product, mixing the polymer material dissolved in a polar solvent or a non-polar solvent not containing water , A particle size range of 15 includes: a step of producing an object to be fired; and (D) a step of firing the object to be fired in an inert atmosphere, a reducing atmosphere or a vacuum atmosphere in this order. is to 400 nm, method for producing a cathode active material for Li-ion batteries exist fine carbon particles to the surface of LiFePO _4.
(2) The manufacturing method of the positive electrode active material for Li ion batteries of said (1) description whose polymer material is polyethyleneglycol, polystyrene, or polyvinyl alcohol.
(3) Temporary baking temperature in (B) process is 250-400 degreeC, The temperature of baking in (D) process is 600-800 degreeC, The said (1) or (2) description A method for producing a positive electrode active material for a Li-ion battery.
(4) Any of the above (1) to (3), 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.
(5) an iron compound, iron citrate, iron oxalate, iron phosphate is at least one selected from iron sulfate, and carbonated iron or Ranaru group, any of the above (1) to (4) The manufacturing method of the positive electrode active material for Li ion batteries of these.
(6) The Li according to any one of (1) to (5), 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 a positive electrode active material for an ion battery.
(7) Lithium ion battery positive electrode active material produced by the method for producing a lithium ion battery positive electrode active material according to any one of (1) to (6) above , at 34 mA with respect to 1 g, based on metallic Li A positive electrode active material for a Li ion battery, characterized in that carbon fine particles are present on the surface of LiFePO ₄ having a voltage range of 4.0 to 2.5 V and a discharge capacity of 130 mAh / g or more.

According to the present invention (1), a positive electrode active material for a Li-ion battery in which carbon fine particles are present on the surface of LiFePO ₄ having a very high discharge capacity and good cycle characteristics can be easily obtained without using expensive equipment and raw materials. Can be manufactured. Here, it is considered that the carbon fine particles not only contribute to the conductivity of the positive electrode active material for the Li ion battery but also bring about a remarkable effect of improving the discharge capacity.

  According to the present invention (7), a Li-ion battery having a very high discharge capacity and good cycle characteristics can be easily produced.

2 is a scanning electron micrograph of a positive electrode active material for a Li ion battery produced in Example 1. FIG. 2 is a particle size distribution measurement result of a positive electrode active material for a Li-ion battery produced in Example 1. FIG. It is a photograph of an agate container and an agate ball used in the examples. 2 is an X-ray diffraction pattern of a positive electrode active material for a Li-ion battery produced in Example 1. FIG. 6 is an X-ray diffraction diagram of a positive electrode active material for a Li-ion battery produced in Example 4. FIG. 6 is an X-ray diffraction diagram of a positive electrode active material for a Li-ion battery produced in Example 5. FIG. 2 is an X-ray diffraction pattern of a positive electrode active material for a Li ion battery produced in Comparative Example 1. FIG. 6 is an X-ray diffraction pattern of a positive electrode active material for a Li-ion battery produced in Comparative Example 4. 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 1C 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 in 0.2C of the positive electrode active material for Li ion batteries obtained in Example 3. FIG. It is a figure which shows the charging / discharging result in 1C of the positive electrode active material for Li ion batteries obtained in Example 3. 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 0.2C of the positive electrode active material for Li ion batteries obtained in Example 4. 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 5. 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 2. It is a figure which shows the charging / discharging result in 0.2C of the positive electrode active material for Li ion batteries obtained by the comparative example 3. 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 by the comparative example 4.

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

[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 (A) a step of mixing a lithium compound, an iron compound, and a phosphoric acid compound to produce a raw material mixture, and (B) a raw material mixture in an inert atmosphere. A step of pre-baking in a reducing atmosphere or a vacuum atmosphere to produce a pre-fired product, (C) a step of mixing a polymer material with the pre-fired product to produce a fired product, and (D) firing And firing the product in an inert atmosphere, a reducing atmosphere or a vacuum atmosphere in this order.

<< (A) Process >>
The lithium compound serves as a lithium source for LiFePO ₄ , and examples of the lithium compound include lithium hydroxide, lithium citrate, lithium oxalate, lithium phosphate, and lithium carbonate. These lithium compounds may be used alone or in combination of two types. You may mix and use the above. Lithium hydroxide, lithium phosphate, and lithium carbonate are preferable, and lithium carbonate is more preferable from the viewpoint that generation of a gas decomposition component promotes micronization of decomposition products. The purity is preferably that which is commercially available as a special grade from a reagent manufacturer.

Iron compounds become a source of iron LiFePO _4, as the iron compound, iron citrate, iron oxalate, iron phosphate, include iron sulfate, and carbonated iron mixed singly or two or more It may be used. Preferred are iron citrate, iron oxalate, and iron phosphate, and more preferred is iron oxalate from the viewpoint that generation of a gas decomposition component promotes micronization of decomposition products. This is because iron oxalate is a material that easily releases anions.

The phosphoric acid compound serves as a phosphoric acid source for LiFePO ₄ , and examples of the phosphoric acid compound include ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid, which are used alone or in combination of two or more. May be. Preferably, it is ammonium dihydrogen phosphate. This is because the cation is easily desorbed.

  As for content of a lithium compound, an iron compound, and a phosphoric acid compound, 7-9 mass parts and 6-5 mass parts of iron ions are preferable with respect to lithium ion: 1 mass part.

  Examples of the mixing method include a ball mill, a mixer, and a mortar, and a ball mill is preferable from the viewpoint that it can be mixed uniformly and can also be pulverized. Here, the mixing is preferably performed by a wet method from the viewpoint of easy uniform mixing. Moreover, when mixing, it is preferable to use containers, such as agate, an alumina, a zirconia, a ball | bowl, etc. from a viewpoint of contamination prevention.

<< (B) Process >>
The atmosphere at the time of pre-baking is in an inert atmosphere, a reducing atmosphere or a vacuum atmosphere from the viewpoint of preventing the oxidation of iron ions, and weak reducing properties such as argon gas containing 3 to 5% hydrogen gas. The atmosphere is preferable. The temperature is preferably 250 to 400 ° C, more preferably 300 to 350 ° C. When the temperature is lower than 250 ° C., a preferable pre-fired product (precursor) is not completely generated. When the temperature is higher than 400 ° C., the growth of LiFePO 4 particles becomes remarkable. The time is preferably 180 to 600 minutes, more preferably 240 to 360 minutes. If it is shorter than this, the production of a preferable precursor is incomplete, and if it is longer than this, the productivity is poor, and the particle growth of LiFePO 4 to be produced becomes remarkable. When pre-baking, it is preferable to carry the raw material mixture on an alumina boat from the viewpoint of preventing contamination.

<< (C) Process >>
Examples of the polymer material include polyethylene glycol, polystyrene, polyvinyl alcohol and the like. From the viewpoint that carbon fine particles are appropriately deposited on the surface of LiFePO ₄ and that the atomic ratio of H to C is relatively small, polyethylene glycol is advantageous. Polystyrene and polyvinyl alcohol are preferred. Here, in the case of polyethylene glycol, the weight average molecular weight is preferably 6,000 to 20,000, and in the case of polystyrene, the weight average molecular weight is preferably 1,000 to 3,000. If the weight average molecular weight is too low, it becomes liquid, and if the weight average molecular weight is too high, it takes time to dissolve in the solvent. The polymer material is preferably used after being dissolved in a solvent from the viewpoint of uniformly dispersing the polymer material on the surface of the precursor, and drying a slurry obtained by mixing the solution in which the polymer material is dissolved in the solvent and the precursor. Thus, it is desirable to uniformly disperse the polymer material on the precursor surface. For this reason, a polymer material having the above-mentioned characteristics is preferable. Here, the polymer material may be any material that can form carbon fine particles on the surface of LiFePO ₄ at the time of firing in the step (D). As the polymer material, a chemical formula such as polyethylene glycol, polystyrene, polyvinyl alcohol, etc., which is composed of only C, H, or only C, H, O, and is preferably solid at room temperature, is moreover a polymer material having polarity. In the case of a nonpolar polymer material such as methanol or ethanol, those that are soluble in a nonpolar solvent such as toluene or xylene are more preferable.

  The polymer material is preferably 5 to 40 parts by mass, and more preferably 15 to 25 parts by mass, with respect to 100 parts by mass of the calcined product. If the amount is less than 5 parts by mass, the amount of precipitated carbon is small and not uniformly dispersed. If the amount is more than 40 parts by mass, the reducing atmosphere is strengthened and the product is reduced.

  The method of mixing the polymer material with the temporarily fired product is the same as in step (A).

<< (D) Process >>
As in the step (B), firing is performed in an inert atmosphere, a reducing atmosphere, or a vacuum atmosphere. The temperature is preferably 600 to 800 ° C, more preferably 700 to 750 ° C. This is because when the temperature is lower than 600 ° C., the crystallinity of LiFePO ₄ is significantly lowered, and when the temperature is higher than 800 ° C., the particle growth of LiFePO ₄ becomes remarkable. Further, the time is preferably 180 to 1200 minutes, and more preferably 480 to 720 minutes. If the length is shorter than this, the crystallinity of LiFePO ₄ is significantly reduced. If the length is longer than this, the carbon fine particles are reduced, the productivity is poor, and the growth of LiFePO ₄ particles becomes remarkable. When firing, it is preferable to place the material to be fired on an alumina boat from the viewpoint of preventing contamination.

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.
Li ₂ CO ₃ +2 (FeC ₂ O ₄ .2H ₂ O) + 2NH ₄ H ₂ PO ₄
→ 2LiFePO ₄ + 5CO ₂ ↑ + 2NH ₃ ↑ + 5H ₂ O ↑ + 2H ₂ ↑

  In the above reaction, since the desired effect can be obtained by preventing the oxidation of Fe, in an inert atmosphere such as argon gas or nitrogen gas; in a reducing atmosphere such as hydrogen or carbon monoxide; or in a vacuum atmosphere Do in the middle.

[Positive electrode active material for Li-ion battery]
The positive electrode active material for Li ion batteries of the present invention is characterized in that carbon fine particles are present on the surface of LiFePO ₄ having a discharge capacity of 130 mAh / g or more produced by the method for producing a positive electrode active material for Li ion batteries. And

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, a part of the Fe site in the crystal structure may be substituted with another element such as Co, Ni, or Mn.

For example, Mn, Ni, and Co have an ionic radius that is approximately the same as that of Fe, and are oxidized and reduced at a potential different from that of Fe. Therefore, the energy density of the lithium iron composite oxide can be improved by replacing 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 at least selected from Mn, Ni, and Co), in which part of the Fe site is substituted with another element M. It is desirable that it be one type and y = 0 to 1.0). In particular, the substitution element M is preferably Mn because it is abundant in terms of resources and is inexpensive.

FIG. 1 shows a scanning electron micrograph of the Li-ion battery positive electrode active material produced in Example 1, and FIG. 2 shows the results of particle size distribution measurement of the Li-ion battery positive electrode active material produced in Example 1. . As can be seen from FIG. 1 and FIG. 2, the positive electrode active material for Li-ion batteries is in the submicron order and has a particle size range of 15 to 400 nm, despite being manufactured by a so-called solid phase method. When the particle diameter at the cumulative 50% of the particle size distribution is considered as the average particle diameter, it is 60 nm. In addition, peaks are observed at a particle size distribution frequency of 36% and a particle size distribution frequency of 64%, and the values are 35 nm and 80 nm, respectively, which indicates that there are many particles having the diameters of these values. Moreover, the specific surface area of the positive electrode active material for Li ion batteries measured by BET method is about 10-30 m < ² > / g, and it is thought that carbon exists with fine particle.

The content of the carbon fine particles is preferably 1 to 5 parts by mass and more preferably 1 to 3 parts by mass with respect to 100 parts by mass of the positive electrode active material for Li ion batteries. If it is less than 1 part by mass, carbon cannot be distributed over the entire particle surface, and if it is more than 5 parts by mass, it is useless. Here, the carbon fine particle content is determined by immersing the produced positive electrode active material for Li ion battery in 4 mol / dm ³ hydrochloric acid to dissolve LiFePO ₄ , suction filtration, and drying. The residual mass at this time is determined as the amount of carbon fine particles and is compared with the mass of the positive electrode active material for the Li ion battery before treatment. The content of the carbon fine particles can be controlled mainly by the content of the polymer material to be added, and can also be controlled by the firing temperature and firing time in the step (D).

  The discharge capacity of the positive electrode active material for a Li-ion battery of the present invention is a value measured at a discharge rate of 0.2C. Here, 0.2 C refers to a rate that takes 5 hours to charge and discharge the theoretical capacity. As specific measurement conditions, the current value is 1 g of the positive electrode active material for Li-ion batteries. On the other hand, it is 34 mA, metal lithium is used for the negative electrode, and the voltage range is 4.0 to 2.0 V with respect to metal lithium. The discharge capacity of the positive electrode active material for a Li-ion battery of the present invention is 130 mAh / g or more, preferably 140 mAh or more, more preferably 150 mAh / g or more.

  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. Further, the manufacturing method of the present invention does not require a large-scale apparatus and can be easily synthesized, so that the manufacturing cost can be suppressed.

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

[Example 1]
<< (A) Process >>
FIG. 3 shows an agate container (internal volume: about 80 cm ³ ) and an agate ball used. To an agate vessel containing ethanol, lithium carbonate: 0.2342 g, iron oxalate dihydrate: 1.1404 g, and ammonium dihydrogen phosphate: 0.7292 g were added as raw materials for Li, Fe, and P, respectively. A raw material mixture was prepared by mixing for 2 hours with a ball mill (model number: P-6).
<< (B) Process >>
The raw material mixture was put into an electric furnace and pre-baked at about 350 ° C. for 10 hours while flowing an argon gas containing 5% hydrogen gas to prepare about 1.3 g of a pre-fired product.
<< (C) Process >>
To the calcined product, 0.2 g of polyethylene glycol (PEG) dissolved in ethanol was added as a polymer material and mixed for 2 hours with the above ball mill to prepare a product to be fired.
<< (D) Process >>
Firing was performed at 700 ° C. for 10 hours while flowing an argon gas not containing hydrogen gas, to produce a positive electrode active material for a Li-ion battery of Example 1.

[Example 2]
In the step (C), the positive electrode active for Li-ion battery of Example 2 was performed in the same manner as in Example 1 except that 0.2 g of polystyrene dissolved in ethanol was added as a polymer material to the temporarily fired product. The material was manufactured.

Example 3
In the step (B), a positive electrode active material for a Li-ion battery of Example 3 was produced in the same manner as in Example 2 except that calcination was performed at 320 ° C. for 5 hours.

[Comparative Example 1]
In the step (C), a positive electrode active material for a Li-ion battery of Comparative Example 1 was produced in the same manner as in Example 1 except that polyethylene glycol was not added.

Example 4
In the step (D), a positive electrode active material for a Li-ion battery of Example 4 was produced in the same manner as in Example 1 except that calcination was performed at 650 ° C. for 10 hours.

Example 5
In the step (D), a positive electrode active material for a Li-ion battery of Example 4 was produced in the same manner as in Example 1 except that calcination was performed at 750 ° C. for 10 hours.

[Comparative Example 2]
After adding 0.2 g of polyethylene glycol dissolved in ethanol to the positive electrode active material for Li ion battery produced in Comparative Example 1, the mixture was mixed for 2 hours by a ball mill to obtain the positive electrode active material for Li ion battery of Comparative Example 2. Manufactured.

[Comparative Example 3]
Acetylene black (AB): 0.2 g was added to the Li-ion battery positive electrode active material produced in Comparative Example 1, and then mixed for 2 hours in a ball mill to produce the Li-ion battery positive electrode active material of Comparative Example 3. did.

[Comparative Example 4]
In (A), polyethylene glycol dissolved in ethanol: 0.2 g was further added, and in the step (C), polyethylene glycol was not added. A positive electrode active material for a Li-ion battery was produced.

  In Table 1, the summary of the manufacturing conditions of Examples 1-5 and Comparative Examples 1-4 is shown.

[Quantification of carbon content]
The produced positive electrode active material for Li ion battery was immersed in 4 mol / dm ³ hydrochloric acid to dissolve LiFePO ₄ , suction filtered, and dried. The carbon content in the positive electrode active material for the Li ion battery was determined from the comparison with the mass of the positive electrode active material for the Li ion battery before the treatment, with the residual mass at this time as the amount of carbon fine particles. Table 1 shows the results.

[Measurement of specific surface area]
The specific surface area of the manufactured positive electrode active material for Li ion batteries was measured using a specific surface area measuring device (model number: Flowsorb II-2300) manufactured by Shimadzu Corporation. Table 1 shows the results.

[Scanning electron microscope observation]
The positive electrode active material for Li ion battery obtained in Example 1 was observed with a scanning electron microscope (model number: JSM-5900) manufactured by JEOL. FIG. 1 shows the result.

(Particle size distribution measurement)
The particle size distribution of the positive electrode active material for a positive Li ion battery obtained in Example 1 was measured with a particle size distribution measuring machine (model number: UPA-EX) manufactured by Microtrack. FIG. 2 shows the result. In FIG. 2, the bar graph indicates frequency, and the line graph indicates accumulation.

[X-ray diffraction measurement]
X-ray diffraction measurement of the positive electrode active materials for Li-ion batteries obtained in Examples 1, 4, 5 and Comparative Examples 1, 4 using a Rigaku X-ray diffractometer in the range of 2θ: 10 to 70 °. Went. FIG. 4 shows the results of Example 1, FIG. 5 shows the results of Example 4, FIG. 6 shows the results of Example 5, FIG. 7 shows the results of Comparative Example 1, and FIG. 4 to 8, the vertical axis represents intensity (Intensity, unit: au (Arbitrary unit)), and the horizontal axis represents 2θ (unit: °). The X-ray peak described based on the LiFePO ₄ inorganic crystal structure database is shown for reference. All measured confirmed a single phase of LiFePO ₄ .

[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 . Synthesized positive electrode active material powder for Li-ion battery, acetylene black (conducting aid), polytetrafluoroethylene (binder) mixed in a mass ratio of 70: 25: 5 (total amount: 0.1 g) It was set to 11. 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.). Measurement conditions are a temperature condition of 20 ° C., a two-terminal method, a charge / discharge rate: 0.2 C (34 mA for a positive electrode active material for Li ion battery: 1 g), a voltage range: 2.5 to 4.0 V. Then, charging and discharging were repeated 10 times. Further, in Examples 1 and 3, the measurement was also performed at a charge / discharge rate of 1C (a rate in which the time taken to charge / discharge the theoretical capacity is 1 hour). FIG. 10 shows the result of Example 1 at 0.2C, FIG. 11 shows the result of Example 1 at 1C, FIG. 12 shows the result of Example 2 at 0.2C, and FIG. FIG. 14 shows the result of Example 3 at 0.2C, FIG. 15 shows the result of 1C of Example 3, FIG. 15 shows the result of 0.2C of Comparative Example 1, FIG. FIG. 17 shows the result at 0.2C, FIG. 18 shows the result at 0.2C in Example 5, FIG. 18 shows the result at 0.2C in Comparative Example 2, and FIG. The result at 2C is shown in FIG. 20, and the result at 0.2C in Comparative Example 4 is shown. 10 to 20, the vertical axis represents the voltage (Voltage, unit: V) with respect to the negative electrode Li metal, and the horizontal axis represents the capacity (capacity, unit: mAh / g).

  Table 2 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 “%”). Here, the discharge capacity after 10 cycles of Comparative Example 3 is the value after 2 cycles, and the discharge capacity after 10 cycles of Comparative Example 4 is the value after 3 cycles.

As can be seen from Table 1, 1.67 to 3.79% by mass of carbon was present in any of Examples 1 to 5, and the specific surface area was very large at 14.55 m ² / g. On the other hand, Comparative Examples 1 to 3 had a specific surface area of 7.68 m ² / g or less. In Examples 1 to 5, the discharge capacity at 0.2 C was 140 mAh / g or more, and the cycle retention after 10 cycles was 95% or more. In particular, in Examples 1 to 3, in which the firing temperature was 700 ° C. and the carbon amount was 1.98 to 2.92%, it was very high at 150 mAh / g or more, and the cycle maintenance rate after 10 cycles was remarkably good. It was. Further, as can be seen from FIGS. 10, 12, and 13, a flat discharge region in the vicinity of 3.4 V is large, which is favorable in terms of use. Further, in Examples 1 and 3, good results were obtained even in 1C discharge. On the other hand, in Comparative Example 1 in which the polymer material was not added in the step (C), the discharge capacity was as small as 113 mA / g, and the cycle characteristics were also poor. In Comparative Example 2 in which the polymer material was added after the step (D), the discharge capacity was as large as 150 mAh / g, but the cycle characteristics were degraded due to the large particle size. Here, for Comparative Example 2, the voltage difference (polarization) between charging and discharging at 120 mAh / g was calculated from FIG. 18 and found to be 0.254V. On the other hand, the polarities at 120 mAh / g of the positive electrode materials synthesized in Example 1, Example 2, and Example 3 are 0.163 V, 0.166 V, and 0.104 V, respectively. Since the polarization of 2 is large, it can be seen that the battery resistance is large. In Comparative Example 3 in which AB was added after the (D) step, the particle growth progressed, so the discharge capacity was as small as 109 mA / g. In Comparative Example 4 in which the polymer material was added in the (A) step, the discharge capacity was also 97 mA. It was as small as / g. In each comparative example, not only the discharge start voltage is low and the discharge capacity is low, but also the amount of discharge energy is low. From these results, the carbon fine particles present on the surface of the positive electrode active material for the Li ion battery of the present invention can significantly increase the discharge capacity in addition to improving the conductivity of the positive electrode active material for the Li ion battery. all right.

From the above, the positive electrode active material for Li ion battery in which carbon fine particles are present on the surface of LiFePO ₄ by the production method of the present invention is remarkably excellent in both discharge capacity and cycle characteristics, and is suitable for Li ion battery.

DESCRIPTION OF SYMBOLS 10 Working electrode 11 Positive electrode and current collector 12 Nonwoven fabric 13 Separator 14 Negative electrode 15 Counter electrode 16 Electrolyte

Claims (7)

(A) A step of mixing a lithium compound, an iron compound excluding iron oxide , and a phosphoric acid compound to produce a raw material mixture; (B) a raw material mixture temporarily in an inert atmosphere, a reducing atmosphere or a vacuum atmosphere; A step of firing and preparing a calcined product, (C) without crushing the calcined product, mixing the calcined product with a polymer material dissolved in a polar solvent or a nonpolar solvent not containing water, And (D) a step of firing the object to be fired in an inert atmosphere, a reducing atmosphere or a vacuum atmosphere in this order, and a particle size range of 15 to 400 nm. A method for producing a positive electrode active material for a Li ion battery, in which carbon fine particles are present on the surface of LiFePO ₄ .   The manufacturing method of the positive electrode active material for Li ion batteries of Claim 1 whose polymer material is polyethyleneglycol, polystyrene, or polyvinyl alcohol. The positive electrode active for Li ion battery according to claim 1 or 2, wherein the temperature of pre-baking in step ( B ) is 250 to 400 ° C, and the temperature of baking in step (D) is 600 to 800 ° C. A method for producing a substance.   The Li ion according to any one of claims 1 to 3, 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. Iron compounds, iron citrate, iron oxalate, iron phosphate is at least one selected from iron sulfate, and carbonated iron or Ranaru group, Li of any of claims 1-4 A method for producing a positive electrode active material for an ion battery.   The positive electrode for a Li ion battery according to any one of claims 1 to 5, 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. The positive electrode active material for a Li ion battery manufactured by the method for manufacturing a positive electrode active material for a Li ion battery according to any one of claims 1 to 6 , wherein the voltage range based on metal Li is 34 mA with respect to 1 g. A positive electrode active material for a Li-ion battery, wherein carbon fine particles are present on the surface of LiFePO ₄ having a discharge capacity of 4.0 to 2.5 V and a discharge capacity of 130 mAh / g or more.