JP2016081866A - Lithium ferromanganese phosphate positive electrode active material and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ferromanganese phosphate positive electrode active material the surface of which is uniformly and efficiently covered with carbon, and which is capable of further improving the battery characteristics.SOLUTION: Provided is a lithium ferromanganese phosphate positive electrode active material, in which carbon is carried on a lithium ferromanganese phosphate compound represented by the following formula (A): LiFeMnMPO(in the formula, M represents Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd, with a, b and c satisfying 0<a<0.5, 0.5<b<1, and 0≤c≤0.2, and satisfying 2a+2b+(valence of M)×c=2), and which is obtained by subjecting to a hydrothermal reaction a carbon source mixture including a lithium compound, a phosphate compound, an iron compound, a manganese compound, and polyvinylpyrrolidone or a carbon source mixture including polyvinylpyrrolidone and glucose.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a lithium iron manganese phosphate positive electrode active material obtained using a carbon source containing polyvinylpyrrolidone and a method for producing the same.

Secondary batteries used for portable electronic devices, hybrid cars, electric cars, and the like have been developed. In particular, lithium ion secondary batteries are widely known. Under these circumstances, lithium-containing olivine-type metal phosphates such as Li (Fe, Mn) PO ₄ are not greatly affected by resource constraints, and can exhibit high safety, resulting in high output. Thus, it is an optimum positive electrode material for obtaining a large capacity lithium secondary battery. However, these compounds have the property that it is difficult to sufficiently increase the conductivity due to the crystal structure, and there is room for improvement in the diffusibility of lithium ions. Has been made.

  For example, in Patent Document 1, an attempt is made to improve the performance of the obtained battery by making primary crystal particles ultrafine particles and shortening the lithium ion diffusion distance in the olivine-type positive electrode active material. In Patent Document 2, a conductive carbonaceous material is uniformly deposited on the particle surface of the positive electrode active material, and a regular electric field distribution is obtained on the particle surface to increase the output of the battery.

JP 2010-251302 A JP 2001-15111 A

  These positive electrode active materials can further improve battery characteristics if carbon can be efficiently coated while suppressing the exposure of the compound constituting the positive electrode active material on the surface. It is difficult to accurately determine the covering state, and there is still room for improvement.

  Accordingly, an object of the present invention is to provide a lithium manganese iron phosphate positive electrode active material in which carbon is uniformly and efficiently coated on the surface and the battery characteristics can be further improved.

  Therefore, the present inventors have found that if a Karl Fischer moisture meter is used under specific measurement conditions, it is possible to accurately estimate the degree of coating of the active material surface with carbon. In order to complete the present invention, it is found that a positive electrode active material whose surface is uniformly and efficiently coated with carbon can be obtained by subjecting it to a hydrothermal reaction using a carbon source containing polyvinylpyrrolidone and the like together with the compound. It came.

That is, the present invention provides the following formula (A):
LiFe _a Mn _b M _c PO ₄ (A)
(In the formula, M represents Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd. A, b and c are 0 <a <0.5, 0 .5 <b <1, and 0 ≦ c ≦ 0.2, and 2a + 2b + (valence of M) × c = 2.
Carbon is supported on a lithium manganese iron phosphate compound represented by the formula: a lithium compound, a phosphate compound, an iron compound, a manganese compound, a carbon source containing polyvinylpyrrolidone, or a carbon source containing polyvinylpyrrolidone and glucose. The present invention provides a lithium manganese iron phosphate positive electrode active material obtained by a thermal reaction.
The present invention also includes a step (I) of obtaining a trilithium phosphate aqueous solution by dropping a phosphate compound into slurry water obtained by mixing a lithium compound and water.
A step of adding an iron compound, a manganese compound and a carbon source containing polyvinylpyrrolidone or a carbon source containing polyvinylpyrrolidone and glucose to the aqueous trilithium phosphate solution obtained in step (I) to obtain an aqueous dispersion (II ), And subjecting the aqueous dispersion obtained in step (II) to a hydrothermal reaction (III)
The manufacturing method of the said lithium iron manganese phosphate positive electrode active material provided with this is provided.

  According to the lithium iron manganese phosphate positive electrode active material of the present invention, the carbon derived from the carbon source containing polyvinylpyrrolidone on the surface is uniform and efficient, as can be confirmed by using a Karl Fischer moisture meter under specific measurement conditions. Therefore, it is extremely useful as a positive electrode material capable of dramatically improving battery characteristics.

It is a schematic diagram which shows the profile for estimating the extent of the coating | cover of the active material surface by carbon obtained by the measuring method using a Karl Fischer moisture meter.

Hereinafter, the present invention will be described in detail.
The lithium manganese iron phosphate positive electrode active material of the present invention has the following formula (A):
LiFe _a Mn _b M _c PO ₄ (A)
(In the formula, M represents Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd. A, b and c are 0 <a <0.5, 0 .5 <b <1, and 0 ≦ c ≦ 0.2, and 2a + 2b + (valence of M) × c = 2.
Carbon is supported on a lithium iron manganese phosphate compound represented by the formula:

The lithium manganese iron phosphate compound represented by the above formula (A) contains at least manganese (Mn) and iron (Fe) which are transition metals. In the formula (A), M represents Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd, and is preferably Mg or Zr. a is 0 <a <0.5, and preferably 0.1 ≦ a ≦ 0.3. b is 0.5 <b <1, and preferably 0.7 ≦ b ≦ 0.9. c satisfies 0 ≦ c ≦ 0.2, and preferably 0 ≦ c ≦ 0.1. These a, b and c are numbers satisfying 2a + 2b + (M valence) × c = 2. Specific examples of lithium iron manganese phosphate compounds represented by the above formula (A) include LiMn _0.8 Fe _0.2 PO ₄ , LiMn _0.75 Fe _0.15 Mg _0.1 PO ₄ , LiMn _0.75 Fe _0.19 Zr _0.03 PO _{4 and the} like. Among them, LiMn _0.8 Fe _0.2 PO ₄ is preferable.

  The amount of carbon supported on the lithium manganese iron phosphate compound is preferably 1 to 15% by mass, more preferably 1.5 to 10% by mass in the positive electrode active material for a lithium secondary battery, More preferably, it is 2-7 mass%. The amount of carbon supported on the lithium manganese iron phosphate compound can be determined by measurement using a carbon / sulfur analyzer.

  The lithium manganese iron phosphate positive electrode active material of the present invention includes a lithium compound, a phosphate compound, an iron compound, a manganese compound, a carbon source containing polyvinylpyrrolidone, or a carbon source containing polyvinylpyrrolidone and glucose (hereinafter referred to as “polyvinylpyrrolidone etc. It is also obtained by subjecting to a hydrothermal reaction. Thus, since the lithium iron manganese phosphate positive electrode active material of the present invention is obtained by subjecting a carbon source containing polyvinyl pyrrolidone or the like to a hydrothermal reaction, a carbon source containing polyvinyl pyrrolidone or the like. Carbon obtained by carbonizing is firmly supported on a lithium iron manganese phosphate compound. Such polyvinyl pyrrolidone has high solubility in solvents such as water and exhibits an appropriate viscosity, and therefore, it is considered that the polyvinyl pyrrolidone can exhibit good dispersibility together with the raw material compound of lithium iron manganese phosphate compound. Therefore, by subjecting them both to a hydrothermal reaction, the surface of the lithium manganese iron phosphate compound can be uniformly coated while the polyvinylpyrrolidone contained in the carbon source is carbonized, and the resulting battery performance is effective. It is speculated that a useful positive electrode active material that can be increased to a high level can be obtained.

That is, in the method for producing a lithium iron manganese phosphate positive electrode active material of the present invention, the lithium compound and water are added to the slurry water obtained by mixing the lithium compound and water to obtain a trilithium phosphate aqueous solution (I) ,
A step of adding an iron compound, a manganese compound and a carbon source containing polyvinylpyrrolidone or a carbon source containing polyvinylpyrrolidone and glucose to the aqueous trilithium phosphate solution obtained in step (I) to obtain an aqueous dispersion (II ), And subjecting the aqueous dispersion obtained in step (II) to a hydrothermal reaction (III)
Is provided.

Step (I) is a step of obtaining a trilithium phosphate aqueous solution by dropping a phosphate compound into slurry water obtained by mixing a lithium compound and water.
Examples of the lithium compound that can be used include lithium oxide and lithium hydroxide. Specific examples include lithium hydroxide, lithium carbonate, lithium sulfate, lithium nitrate, lithium oxide, lithium oxalate, and lithium acetate. These may be used alone or in combination of two or more. Of these, lithium hydroxide is preferable from the viewpoint of improving battery characteristics. Specifically, for example, a hydrate such as LiOH.H ₂ O can be used.

  Examples of phosphoric acid compounds that can be used include phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. These may be used alone or in combination of two or more. Among these, phosphoric acid is preferable from the viewpoint of improving battery characteristics, and it is preferably used as an aqueous solution having a concentration of 70 to 90% by mass, for example.

  The slurry water obtained by mixing the lithium compound and water preferably contains 20 to 50 parts by mass of lithium compound with respect to 100 parts by mass of water, and preferably contains 25 to 45 parts by mass of lithium compound. More preferred. Next, the stirring speed when the phosphoric acid compound is dropped into the obtained slurry water is preferably 200 to 700 rpm, more preferably 250 to 600 rpm. The stirring time of the slurry water is preferably 1 to 24 hours, more preferably 5 to 15 hours, and the temperature of the slurry water is preferably 20 to 90 ° C, more preferably 20 to 70 ° C. It is. Moreover, it is preferable to purge nitrogen with respect to the trilithium phosphate aqueous solution after mixing a phosphoric acid compound, and it is preferable to make dissolved oxygen concentration 0.5 mg / L or less.

  In step (II), the aqueous solution of trilithium phosphate obtained in step (I) is added with a carbon source containing an iron compound, a manganese compound, and polyvinyl pyrrolidone, or a carbon source containing polyvinyl pyrrolidone and glucose, and dispersed in water. This is a step of obtaining a liquid.

  Examples of iron compounds that can be used include iron acetate, iron nitrate, iron sulfate, and hydrates thereof. These may be used alone or in combination of two or more. Among these, iron sulfate and hydrates thereof are preferable from the viewpoint of improving battery characteristics.

  Examples of manganese compounds that can be used include manganese acetate, manganese nitrate, manganese sulfate, and hydrates thereof. These may be used alone or in combination of two or more. Of these, manganese sulfate and hydrates thereof are preferable from the viewpoint of improving battery characteristics.

  The use molar ratio of these manganese compound and iron compound (manganese compound: iron compound) is preferably 99: 1 to 51:49, more preferably 95: 5 to 70:30, and still more preferably 90:10. ~ 75: 25.

Furthermore, you may use metal (M) compounds other than a manganese compound and an iron compound as needed. M in the metal (M) compound is Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd, and is synonymous with M in the above formula (A). As such metal (M) compounds, halides, sulfates, organic acid salts, hydrates thereof, and the like can be used. These may be used alone or in combination of two or more. Especially, it is preferable to use the metal (M) compound whose M is Mg or Zr from a viewpoint of improving a battery characteristic.
When these metal (M) compounds are used, the total amount of iron compound, manganese compound, and metal (M) compound is preferably 0.99 to 1.01 mol, more preferably 1 mol of lithium compound. Is 0.995 to 1.005 mol.

  The carbon source to be used may be one containing polyvinyl pyrrolidone alone, further using glucose together, and may contain both polyvinyl pyrrolidone and glucose. A carbon source containing polyvinyl pyrrolidone and glucose is used. More preferably it is used. Specifically, the mass ratio of polyvinylpyrrolidone and glucose (polyvinylpyrrolidone: glucose) in such a carbon source is carbon from the viewpoint of efficiently covering the surface of the lithium iron manganese phosphate compound while firmly supporting the carbon. The conversion amount is preferably 100: 0 to 10:90, more preferably 85:15 to 15:85, and still more preferably 75:25 to 25:75.

  Moreover, the addition amount of the carbon source is the total amount of the iron compound and the manganese compound, or other than the manganese compound and the iron compound from the viewpoint of efficiently covering the surface of the lithium iron manganese phosphate compound while firmly supporting the carbon. When using a metal (M) compound, it is preferably 0.5 to 15 parts by mass, more preferably 1 to 15 parts by mass with respect to 100 parts by mass of the total amount of these iron compound, manganese compound, and metal (M) compound. 10 parts by mass, more preferably 2 to 7 parts by mass.

There are no particular restrictions on the order of addition of these iron compounds, manganese compounds, and carbon sources including polyvinylpyrrolidone, or metal (M) compounds other than manganese compounds and iron compounds. Moreover, while adding these compounds, you may add antioxidant as needed. As such an antioxidant, sodium sulfite (Na ₂ SO ₃ ), hydrosulfite sodium (Na ₂ S ₂ O ₄ ), aqueous ammonia and the like can be used. From the viewpoint of preventing the generation of lithium manganese iron phosphate compound from being excessively added, the amount of addition of the antioxidant is 1 mol in total for the manganese compound, the iron compound, and the metal (M) compound. On the other hand, it is preferably 0.01 to 1 mol, more preferably 0.03 to 0.5 mol.

  The amount of water used in the aqueous dispersion is contained in the aqueous dispersion from the viewpoints of the solubility of the iron compound, manganese compound and metal (M) compound, ease of stirring, and efficiency of synthesis. Preferably it is 10-30 mol with respect to 1 mol of phosphate ions, More preferably, it is 12.5-25 mol.

  Step (III) is a step of subjecting the aqueous dispersion obtained in step (II) to a hydrothermal reaction. By passing through this process, carbon obtained by carbonizing a carbon source containing polyvinylpyrrolidone or the like can be effectively carried on the lithium iron manganese phosphate compound. The hydrothermal reaction should just be 100 degreeC or more, 130-250 degreeC is preferable and 140-230 degreeC is more preferable. The hydrothermal reaction is preferably performed in a pressure vessel. When the reaction is performed at 130 to 200 ° C, the pressure at this time is 0.3 to 1.5 MPa, and the pressure when the reaction is performed at 140 to 180 ° C is 0. 4 to 1.0 MPa. The hydrothermal reaction time is preferably 10 minutes to 3 hours, more preferably 10 minutes to 1 hour.

  Thereafter, the lithium iron manganese phosphate compound can be isolated as primary particles by washing, filtering and drying. In addition, the usage-amount of the water at the time of wash | cleaning becomes like this. Preferably it is 5-100 mass parts with respect to 1 mass part of lithium iron manganese phosphate compounds, More preferably, it is 5-50 mass parts. As the drying means, freeze drying or vacuum drying is used, and freeze drying is preferable.

  The method for producing a lithium manganese iron phosphate positive electrode active material of the present invention preferably includes a step (IV) of firing in a reducing atmosphere or an inert atmosphere after passing through the step (III). Thereby, the positive electrode active material for lithium secondary batteries which carbonizes carbon source containing polyvinylpyrrolidone etc. and carries | supports carbon concerning a lithium manganese iron phosphate compound firmly can be obtained. The firing conditions are preferably 400 ° C. or higher, more preferably 400 to 800 ° C., preferably 10 minutes to 3 hours, more preferably 0.5 to 1.5 hours in a reducing atmosphere or an inert atmosphere. Good.

  The positive electrode active material for lithium secondary batteries has been found by the present inventors to be able to accurately estimate the degree of coverage of the active material surface with carbon if a Karl Fischer moisture meter is used under specific measurement conditions. did. By this method, it can be confirmed that the surface of the positive electrode active material for a lithium secondary battery of the present invention is efficiently coated with carbon.

  Specifically, the measurement method using a Karl Fischer moisture meter uses 0.5 g of the positive electrode active material as a measurement sample, is exposed to an atmosphere of 20 ° C. and 50% relative humidity for 2 hours to absorb moisture, and the nitrogen flow rate. Under the conditions of 250 mL / min and a temperature of 250 ° C., the moisture content from the start of measurement to the lapse of 20 minutes is measured. Next, based on these measurement results, for example, as shown in FIG. 1, a profile is created with the vertical axis representing the moisture content and the horizontal axis representing the measurement time. The time T from the start of measurement to the appearance of the maximum water content peak α, which is read based on such a profile, is an index for estimating the degree of coverage of the active material surface with carbon. The smaller the time T is, the more effectively the exposure of the lithium iron manganese phosphate compound on the surface is reduced, and the carbon obtained by carbonizing a carbon source containing polyvinylpyrrolidone or the like is efficiently coated while the manganese phosphate is coated. It means that it is firmly supported on an iron lithium compound.

  The lithium secondary battery to which the positive electrode for a lithium secondary battery including the positive electrode active material for a lithium secondary battery of the present invention can be applied is not particularly limited as long as the positive electrode, the negative electrode, the electrolytic solution, and the separator are 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 Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples.

[Example 1]
12.72 g of LiOH.H ₂ O and 90 mL of water were mixed to obtain slurry water. Next, 11.53 g of 85% phosphoric acid aqueous solution was dropped at 35 mL / min while stirring the resulting slurry water at a temperature of 25 ° C. for 2 to 3 minutes, followed by stirring at a speed of 400 rpm for 12 hours. As a result, an aqueous trilithium phosphate solution was obtained. Such an aqueous solution contained 2.97 moles of lithium per mole of phosphorus. The obtained aqueous solution was purged with nitrogen to have a dissolved oxygen concentration of 0.5 mg / L.

Next, 19.29 g of MnSO ₄ .5H ₂ O and 4.17 g of FeSO ₄ .7H ₂ O (manganese compound: iron compound (molar ratio)) = 80: with respect to the total amount of the aqueous solution containing the obtained trilithium phosphate. 20), 0.32 g of polyvinylpyrrolidone and 0.56 g of glucose (about 3.8 parts by mass, polyvinylpyrrolidone: glucose = 50: 50 (carbon conversion amount) with respect to 100 parts by mass of the total amount of manganese compound and iron compound)) The carbon source containing was added and the liquid mixture was obtained.
Subsequently, the obtained mixed liquid was put into an autoclave, and a hydrothermal reaction was performed at 170 ° C. for 1 hour. The pressure in the autoclave was 0.8 MPa. The produced crystal was filtered, and then washed with 12 parts by mass of water with respect to 1 part by mass of the crystal. The washed crystal was lyophilized at −50 ° C. for 12 hours to obtain lithium manganese iron phosphate (LiFe _0.2 Mn _0.8 PO ₄ ).
The obtained lithium iron manganese phosphate was confirmed to be a single phase by powder X-ray diffraction measurement.

  Then, the obtained lithium iron manganese phosphate is fired at 700 ° C. for 1 hour in an argon hydrogen atmosphere (hydrogen concentration: 3%) for a lithium secondary battery in which carbon is supported on the surface and uniformly coated. A positive electrode active material (carbon content = 2.0 mass%) was obtained.

[Example 2]
As a carbon source, 0.16 g of polyvinylpyrrolidone and 0.84 g of glucose (about 4.3 parts by mass with respect to 100 parts by mass of the total amount of manganese compound and iron compound, polyvinylpyrrolidone: glucose = 25: 75 (carbon equivalent) ) Was used in the same manner as in Example 1 to obtain a positive electrode active material (carbon content = 2.0% by mass) for a lithium secondary battery in which carbon was uniformly coated on the surface.

[Example 3]
As a carbon source, 0.48 g of polyvinylpyrrolidone and 0.28 g of glucose (about 3.2 parts by mass with respect to 100 parts by mass of the total amount of manganese compound and iron compound, polyvinylpyrrolidone: glucose = 75: 25 (carbon equivalent) ) Was used in the same manner as in Example 1 to obtain a positive electrode active material (carbon content = 2.0% by mass) for a lithium secondary battery in which carbon was uniformly coated on the surface.

[Example 4]
Carbon was uniformly formed on the surface in the same manner as in Example 1 except that 0.64 g of polyvinylpyrrolidone (about 2.7 parts by mass with respect to 100 parts by mass of the total amount of manganese compound and iron compound) was used as the carbon source. A coated positive electrode active material for lithium secondary battery (carbon content = 2.0 mass%) was obtained.

[Example 5]
As a carbon source, 0.06 g of polyvinyl pyrrolidone and 1.01 g of glucose (about 4.6 parts by mass with respect to 100 parts by mass of the total amount of manganese compound and iron compound, polyvinyl pyrrolidone: glucose = 10: 90 (carbon equivalent) ) Was used in the same manner as in Example 1 to obtain a positive electrode active material (carbon content = 2.0% by mass) for a lithium secondary battery in which carbon was uniformly coated on the surface.

[Comparative Example 1]
As the carbon source, carbon was uniformly formed on the surface in the same manner as in Example 1 except that 0.56 g of glucose alone (about 2.4 parts by mass with respect to 100 parts by mass of the total amount of manganese compound and iron compound) was used. A positive electrode active material for lithium secondary batteries (carbon content = 2.0 mass%) was obtained.

[Comparative Example 2]
12.72 g of LiOH.H ₂ O and 90 mL of water were mixed to obtain slurry water. Next, 11.53 g of 85% phosphoric acid aqueous solution was dropped at 35 mL / min while stirring the resulting slurry water at a temperature of 25 ° C. for 2 to 3 minutes, followed by stirring at a speed of 400 rpm for 12 hours. As a result, an aqueous trilithium phosphate solution was obtained. Such an aqueous solution contained 2.97 moles of lithium per mole of phosphorus.

Then, to the aqueous solution the total amount containing phosphate tribasic lithium _{obtained, MnSO 4 · 5H 2 O 19.29g} , was added to FeSO ₄ · 7H ₂ O 4.17g, to obtain a mixed solution.
Subsequently, the obtained mixed liquid was put into an autoclave, and a hydrothermal reaction was performed at 170 ° C. for 1 hour. The pressure in the autoclave was 0.8 MPa. The produced crystal was filtered, and then washed with 12 parts by mass of water with respect to 1 part by mass of the crystal. The washed crystal was lyophilized at −50 ° C. for 12 hours to obtain lithium manganese iron phosphate (LiFe _0.2 Mn _0.8 PO ₄ ).
The obtained lithium iron manganese phosphate was confirmed to be a single phase by powder X-ray diffraction measurement.

  Then, using 0.56 g of glucose alone as a carbon source, adding this to 3 g of lithium iron manganese phosphate and subjecting it to hot air drying at 80 ° C. for 3 hours to obtain a composite mixture, Baking at 700 ° C. for 1 hour under an atmosphere (hydrogen concentration: 3%), a positive electrode active material for lithium secondary battery (carbon content = 7.5% by mass) obtained by uniformly coating the surface with carbon was obtained.

[Test Example 1: Evaluation of coating state of carbon on the surface of the positive electrode active material]
Measurement and evaluation were performed using a Karl Fischer moisture meter (MKC-610, manufactured by Kyoto Electronics Industry Co., Ltd.). Specifically, 0.5 g of the positive electrode active material obtained in Examples 1 to 5 and Comparative Examples 1 and 2 was used as a measurement sample, and the sample was exposed to air at an air temperature of 20 ° C. and a relative humidity of 50% for 2 hours to absorb moisture. Under the conditions of a nitrogen flow rate of 250 mL / min and a temperature of 250 ° C., the moisture content was measured from the start of measurement until 20 minutes had passed. Next, based on each of these measurement results, a profile as shown in FIG. 1 is prepared, the time T from the start of measurement until the maximum peak α of moisture content appears, and relative example 2 is set to 100. The value was determined.
The results are shown in Table 1.

[Test Example 2: Evaluation of charge / discharge characteristics]
Using the positive electrode active materials obtained in Examples 1 to 5 and Comparative Examples 1 and 2, positive electrodes for lithium ion secondary batteries were produced. Specifically, the obtained positive electrode active material, ketjen black, and polyvinylidene fluoride were mixed at a mixing ratio of 75:15:10 by weight, and N-methyl-2-pyrrolidone was added thereto and kneaded sufficiently. A positive electrode slurry was prepared. 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).

Using the manufactured lithium secondary battery, 0.1C discharge capacity (17 mAh / g) and 3C discharge capacity (510 mAh / g) were measured with a discharge capacity measuring device (HJ-1001SD8, manufactured by Hokuto Denko), and 0.1C The ratio of the 3C discharge capacity to the discharge capacity ({3C discharge capacity / 0.1C discharge capacity} × 100) (%) was determined.
The results are shown in Table 2.

  According to the above results, the positive electrode active material for lithium secondary battery of the example has a smaller value of time T than the positive electrode active material for lithium secondary battery of the comparative example. It can be seen that the surface is efficiently coated with carbon obtained by carbonizing a carbon source containing polyvinylpyrrolidone, and the rate characteristics of the obtained lithium secondary battery also have a high ratio of 3C discharge capacity to 0.1C discharge capacity, It turns out to be very good.

Claims (7)

The following formula (A):
LiFe _a Mn _b M _c PO ₄ (A)
(In the formula, M represents Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd. A, b and c are 0 <a <0.5, 0 .5 <b <1, and 0 ≦ c ≦ 0.2, and 2a + 2b + (valence of M) × c = 2.
Carbon is supported on a lithium manganese iron phosphate compound represented by the formula: a lithium compound, a phosphate compound, an iron compound, a manganese compound, a carbon source containing polyvinylpyrrolidone, or a carbon source containing polyvinylpyrrolidone and glucose. A lithium iron manganese phosphate positive electrode active material obtained by a thermal reaction.   2. The lithium manganese iron phosphate positive electrode active material according to claim 1, wherein a mass ratio of polyvinyl pyrrolidone to glucose (polyvinyl pyrrolidone: glucose) in the carbon source is 100: 0 to 10:90 in terms of carbon.   3. The manganese iron phosphate lithium positive electrode active according to claim 1, wherein the amount of carbon supported on the lithium manganese iron phosphate compound is 0.5 to 5 mass% in the lithium iron manganese phosphate positive electrode active material. material.   The lithium manganese iron phosphate positive electrode active material according to any one of claims 1 to 3, obtained by subjecting to a hydrothermal reaction and then firing in a reducing atmosphere or an inert atmosphere.   The positive electrode for lithium secondary batteries containing the manganese iron phosphate lithium positive electrode active material of any one of Claims 1-4. Step (I) of obtaining a trilithium phosphate aqueous solution by dropping a phosphate compound into slurry water obtained by mixing a lithium compound and water,
A step of adding an iron compound, a manganese compound and a carbon source containing polyvinylpyrrolidone or a carbon source containing polyvinylpyrrolidone and glucose to the aqueous trilithium phosphate solution obtained in step (I) to obtain an aqueous dispersion (II ), And subjecting the aqueous dispersion obtained in step (II) to a hydrothermal reaction (III)
The manufacturing method of lithium manganese iron phosphate positive electrode active material of Claims 1-4 provided with these.   The method for producing a lithium manganese iron phosphate positive electrode active material according to claim 5, further comprising a step (IV) of firing in a reducing atmosphere or an inert atmosphere after the step (III).

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