JP7144785B2 - Method for producing lithium vanadium phosphate - Google Patents

Description

The present invention relates to a method for producing lithium vanadium phosphate, which is particularly useful as a positive electrode active material for lithium secondary batteries.

Lithium-ion batteries are used as batteries for mobile devices, laptop computers, electric vehicles, and hybrid vehicles. Lithium ion batteries are generally considered to be excellent in capacity and energy density, and currently LiCoO ₂ is mainly used for the positive electrode, but LiMnO ₂ , LiNiO ₂ and the like are also being actively developed due to Co resource problems. ing.
At present, _LiFePO4 is drawing attention as a further alternative material, and research and development are proceeding in various institutions. Fe is an excellent resource, and although LiFePO ₄ using Fe has a slightly low energy density, it is expected to be used as a positive electrode material for lithium-ion batteries for electric vehicles because of its excellent high-temperature characteristics.

However, _LiFePO4 has a rather low operating voltage, and lithium vanadium phosphate ( _{Li3V2 (} PO4) ₃ ) having a NASICON ₍ Na Super Ionic Conductor) structure using V instead of Fe has attracted attention. there is

As a method for producing lithium vanadium phosphate, for example, a method has been proposed in which a lithium source, a vanadium compound and a phosphorus source are pulverized and mixed, the resulting uniform mixture is formed into pellets, and then the formed product is fired ( For example, see Patent Documents 1 and 2). Further, in Patent Document 3 below, vanadium (V) oxide is dissolved in an aqueous solution containing lithium hydroxide, a phosphorus source and carbon and/or a nonvolatile organic compound are added, and the resulting raw material mixed solution is dried. A method of obtaining a precursor and heat-treating this precursor in an inert atmosphere to obtain a composite of Li ₃ V ₂ (PO ₄ ) ₃ and a conductive carbon material has been proposed.

The present applicant also previously described in Patent Document 4 below that a lithium source, a pentavalent or tetravalent vanadium compound, a phosphorus source, and a conductive carbon material source from which carbon is generated by thermal decomposition are mixed in an aqueous solvent. A first step of preparing a raw material mixed solution, a second step of heating the raw material mixed solution to perform a precipitation reaction to obtain a reaction solution containing a precipitated product, and a media mill for preparing a reaction solution containing the precipitated product. A third step of obtaining a slurry containing the pulverized product by wet pulverizing, a fourth step of spray-drying the slurry containing the pulverized product to obtain a reaction precursor, and A method for producing a lithium vanadium phosphate-carbon composite was proposed by firing at 600 to 1300° C. in an active gas atmosphere or in a reducing atmosphere.

Further, in Patent Document 5 below, a vanadium compound, a phosphorus source, and a conductive carbon material source in which carbon is generated by thermal decomposition are subjected to a heat treatment in an aqueous solvent, preferably at 60 to 100 ° C., and then reacted at room temperature. After cooling to , a lithium source is further added to the liquid after heat treatment to react, the resulting reaction liquid is spray-dried to obtain a reaction precursor, and the reaction precursor is placed in an inert gas atmosphere or a reducing atmosphere. It is disclosed to produce lithium vanadium phosphate by calcining in.

Japanese Patent Publication No. 2001-500665 Japanese Patent Publication No. 2002-530835 Japanese Patent Application Laid-Open No. 2008-052970 International Publication No. 2012/043367 Pamphlet International Publication No. 2014/006948 pamphlet

Li ₃ V ₂ (PO ₄ ) ₃ is known to have a theoretical capacity as high as 197 mAhg ⁻¹ .
However, the conventional lithium secondary battery using Li ₃ V ₂ (PO ₄ ) ₃ as a positive electrode active material has a low discharge capacity. However, in order to obtain a highly reactive reaction precursor, a precipitation reaction and a pulverization treatment with a media mill are required, which complicates the production process and is not industrially advantageous.
In addition, Patent Document 5 does not indicate any specific reaction conditions, and after cooling to room temperature, the reaction was obtained by simply adding a lithium source to the liquid after heat treatment. Even if the spray-dried liquid is used as a reaction precursor, it is difficult to obtain a high discharge capacity.

Accordingly, an object of the present invention is to provide, by an industrially advantageous method, a single-phase lithium vanadium phosphate in terms of X-ray diffraction, which is particularly useful as a positive electrode active material for lithium secondary batteries.

The present inventors have made intensive studies in view of the above problems, and found that by heat-treating an acidic mixed slurry containing vanadium pentoxide, phosphoric acid and reducing sugar, a reduction reaction of vanadium pentoxide occurs and the solution to be able to In addition, when a solution containing lithium hydroxide is added while maintaining a reduction reaction solution obtained by heat-treating an acidic mixed slurry containing vanadium pentoxide, phosphoric acid and reducing sugar at a certain temperature or less, The solution state can be maintained for a while after the addition, while the reaction between the component and lithium hydroxide is suppressed. In addition, the reaction precursor obtained by spray-drying the solution after addition of the solution containing lithium hydroxide has excellent reactivity, and a single-phase highly crystalline lithium vanadium phosphate can be obtained even at a low temperature of about 600 ° C. thing. In addition, the inventors have found that a lithium secondary battery using lithium vanadium phosphate obtained by using the reaction precursor as a positive electrode active material has excellent battery performance, and have completed the present invention. rice field.

That is, the present invention provides a method for producing lithium vanadium phosphate having a NASICON structure,
A first step of mixing vanadium pentoxide, phosphoric acid and a reducing sugar in an aqueous solvent to prepare an acidic mixed slurry, and then a second step of obtaining a reduction reaction solution by heating the mixed slurry to make it into a solution. Then, while maintaining the reduction reaction solution at 10 to 25 ° C. , a third step of adding a solution containing lithium hydroxide to the reduction reaction solution to prepare a dark blue raw material mixed solution, then the dark blue A fourth step of obtaining a reaction precursor by spray drying the raw material mixed solution of No. 4, and then firing the reaction precursor at 500 to 1300 ° C. in an inert gas atmosphere or a reducing atmosphere to obtain lithium vanadium phosphate. A method for producing lithium vanadium phosphate characterized by having five steps is provided.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide, by an industrially advantageous method, a single-phase lithium vanadium phosphate in terms of X-ray diffraction, which is particularly useful as a positive electrode active material for lithium secondary batteries. A lithium secondary battery using lithium vanadium oxide as a positive electrode active material has excellent battery performance.

1 is an X-ray diffraction pattern of reaction precursors obtained in Example 1, Comparative Example 1 and Comparative Example 2. FIG. SEM photograph of the reaction precursor obtained in Example 1. FIG. 1 is an X-ray diffraction pattern of lithium vanadium phosphate samples obtained in Example 1 and Comparative Example 1. FIG. 1 is a SEM photograph of lithium vanadium phosphate obtained in Example 1. FIG. SEM photograph of the reaction precursor obtained in Example 5. FIG. 3 is an X-ray diffraction diagram of a lithium vanadium phosphate sample obtained in Example 5. FIG. SEM photograph of lithium vanadium phosphate sample obtained in Example 5. FIG. 3 is an X-ray diffraction diagram of the reaction precursor obtained in Comparative Example 3. FIG. 3 is an X-ray diffraction diagram of a lithium vanadium phosphate sample obtained in Comparative Example 3. FIG.

The present invention will be described below based on its preferred embodiments.
The method for producing lithium vanadium phosphate of the present invention is a method for producing lithium vanadium phosphate having a NASICON structure (hereinafter simply referred to as "lithium vanadium phosphate").

Lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention has the following general formula (1):
_{LixVy (} PO4) ₃ ( ₁ )
(In the formula, x is 2.5 or more and 3.5 or less, and y is 1.8 or more and 2.2 or less.)
Lithium vanadium phosphate represented by or lithium vanadium phosphate represented by general formula (1), if necessary, Me element (Me represents a metal element having an atomic number of 11 or more other than V or a transition metal element. ) is doped lithium vanadium phosphate.

x in the general formula (1) is preferably 2.5 or more and 3.5 or less, particularly preferably 2.8 or more and 3.2 or less. y is preferably 1.8 or more and 2.2 or less, particularly preferably 1.9 or more and 2.1 or less.

When lithium vanadium phosphate contains Me element, Me element to be doped is Sr, Ba, Sc, Y, Hf, Ta, W, Ru, Os, Ag, Zn, Si, Ga, Ge, Sn, Bi , S, Se, Te, Cl, Br, I, Na, K, Mg, Ca, Al, Mn, Co, Ni, Fe, Ti, Zr, Bi, Cr, Nb, Mo and Cu, or Two or more types are mentioned. Among these, the Me element is preferably one or more selected from Mg, Ca, Al, Mn, Co, Ni, Fe, Ti, Zr, Bi, Cr, Nb, Mo and Cu. .

The method for producing lithium vanadium phosphate of the present invention includes the first step of mixing vanadium pentoxide, phosphoric acid and reducing sugar in an aqueous solvent to prepare an acidic mixed slurry, and then heating the mixed slurry. A second step of obtaining a reduction reaction solution by adding a solution containing lithium hydroxide to the reduction reaction solution while maintaining the reduction reaction solution at 35 ° C. or less to prepare a raw material mixture solution. 3 steps, then a fourth step of spray-drying the raw material mixed solution to obtain a reaction precursor, then firing the reaction precursor at 500 to 1300 ° C. in an inert gas atmosphere or a reducing atmosphere to obtain phosphoric acid It has a fifth step of obtaining lithium vanadium.

The first step in the method for producing lithium vanadium phosphate of the present invention is a step of mixing vanadium pentoxide, phosphoric acid and reducing sugar in an aqueous solvent to obtain an acidic mixed slurry in which the raw materials are mixed.

In the first step, the amount of vanadium pentoxide and phosphoric acid mixed is 0.50 to 0.80, preferably in terms of the molar ratio (V/P) of V atoms in vanadium pentoxide to P atoms in phosphoric acid. A range of 0.60 to 0.73 is preferable from the viewpoint of facilitating the production of single-phase lithium vanadium phosphate as the final product.

The reducing sugar used in the first step is thermally decomposed in an inert gas atmosphere or a reducing atmosphere at least during the baking in the fifth step, and carbon is isolated, that is, converted to carbon. The reducing sugar promotes the reduction reaction of vanadium pentoxide in the second step, and makes the reduction reaction solution obtained by performing the second step a reaction solution having a good viscosity that can be stirred. It becomes a necessary component to prevent oxidation of vanadium in the process. Since the excess reducing sugar is converted to conductive carbon by the firing in the fifth step, in the method for producing lithium vanadium phosphate of the present invention, an excessive amount of reducing sugar is added to impart electrical conductivity to lithium vanadium phosphate. It can also function as a component of the conductive carbon source.

Examples of the reducing sugar used in the first step include glucose, fructose, lactose, maltose, and sucrose. Among these, lactose and sucrose are preferable from the viewpoint of obtaining a reaction precursor having excellent reactivity.

In the first step, the amount of the reducing sugar to be mixed is preferably 0.3 to 40 parts by mass in terms of C atoms with respect to 100 parts by mass of lithium vanadium phosphate to be produced. In the calcination in the fifth step, the amount of C atoms contained in the reducing sugar tends to decrease after calcination compared to before calcination. Therefore, in the first step, if the mixed amount of the reducing sugar is 0.3 to 40 parts by mass in terms of C atoms with respect to 100 parts by mass of lithium vanadium phosphate to be produced, the reducing sugar is converted The content of conductive carbon tends to be 0 to 20 parts by mass in terms of C atoms with respect to 100 parts by mass of lithium vanadium phosphate to be produced. Since the amount of reducing sugar mixed with respect to 100 parts by mass of lithium vanadium phosphate to be produced is within the above range, when lithium vanadium phosphate is used as a positive electrode active material for a lithium secondary battery, it is excellent for lithium secondary batteries. performance can be given. In particular, in the first step, the mixed amount of reducing sugar is 0.5 to 40 parts by mass, preferably 5 to 30 parts by mass in terms of C atoms, with respect to 100 parts by mass of lithium vanadium phosphate to be produced. By doing so, the content of the conductive carbon converted from the reducing sugar is 0.1 to 20 parts by weight, preferably 0.5 to 15 parts by weight, with respect to 100 parts by weight of lithium vanadium phosphate to be produced. Since the conductive carbon can impart sufficient conductivity, the internal resistance of the lithium secondary battery can be lowered and the discharge capacity per mass or volume can be increased. On the other hand, if the amount of the reducing sugar mixed with respect to 100 parts by mass of lithium vanadium phosphate to be produced is less than the above range, it tends to be difficult to obtain single-phase lithium vanadium phosphate. The discharge capacity per mass or volume tends to be low. In the first step, the amount of lithium vanadium phosphate to be produced is calculated from the amounts of vanadium pentoxide and phosphoric acid added as raw materials.

Although the production history of vanadium pentoxide, phosphoric acid and reducing sugar in the first step does not matter, it is preferable that the content of impurities is as low as possible in order to produce lithium vanadium phosphate of high purity. .

Examples of the aqueous solvent used in the first step include water and a mixed solvent of water and a hydrophilic organic solvent.

In the first step, the order of adding vanadium oxide, phosphoric acid and reducing sugar to the water solvent and the mixing means are not particularly limited, and the above raw materials are uniformly dispersed to form an acidic mixed slurry. will be

In the second step of the method for producing lithium vanadium phosphate of the present invention, the acidic mixed slurry obtained in the first step is heat-treated to perform a reduction reaction of at least vanadium pentoxide, so that each component of the mixed slurry is It is a step of converting into a dissolved solution to obtain a reduction reaction solution. The temperature of the heat treatment in the second step is 60-100°C, preferably 80-100°C. The reason for this is that if the heat treatment temperature is lower than 60°C, the reaction time becomes longer, which is industrially disadvantageous. and is not industrially advantageous. Therefore, the second step can be carried out under atmospheric pressure.

The heat treatment time in the second step is not particularly limited, and generally 0.2 hours or more, preferably 0.5 to 4 hours, and particularly preferably 0.5 to 2 hours, to obtain a reduction reaction solution. be able to.

If the reduction reaction solution is kept at a temperature exceeding 35° C. for 4 hours or longer after the second step is completed, the reaction proceeds, and precipitates such as (VO)(HPO ₄ )(H ₂ O) ₄ tend to form. Therefore, it is preferable to quickly cool the reduction reaction solution to a temperature of 35° C. or less after the second step from the viewpoint of preparing a uniform raw material mixture solution. After completion of the second step, the reduction reaction solution is usually cooled to 35° C. or less, preferably 5 to 35° C., particularly preferably 10 to 25° C. within 4 hours. More preferably, after completion of the second step, the reduction reaction solution is cooled to 35° C. or lower, preferably 5 to 35° C., particularly preferably 10 to 25° C. within 3 hours.

The third step in the method for producing lithium vanadium phosphate of the present invention is a step of adding a solution containing lithium hydroxide to the reduction reaction solution obtained in the second step to obtain a raw material mixed solution.

A solution containing lithium hydroxide is a solution of lithium hydroxide dissolved in water.

The concentration of lithium hydroxide in the solution containing lithium hydroxide is 5 to 20 mass %, preferably 10 to 15 mass %. When the concentration of lithium hydroxide in the solution containing lithium hydroxide is within the above range, it is preferable from the viewpoint of improving the production efficiency by controlling the heat generation associated with the preparation of a uniform raw material mixed solution and the addition of the solution containing lithium hydroxide.

The amount of the solution containing lithium hydroxide added is such that the molar ratio (Li/P) of Li atoms in the lithium source to P atoms in the phosphorus source is 0.70-1.30, preferably 0.83-1. 17 is the amount added. When the amount of the solution containing lithium hydroxide to be added is within the above range, it is preferable from the viewpoint that single-phase lithium vanadium phosphate is easily obtained as the final product. Further, in the third step, a solution containing lithium hydroxide is added so that the raw material mixed solution has a pH of 3 to 7, preferably 4 to 6, and particularly preferably 4 to 5. It is particularly preferable from the viewpoint of preparation of the raw material mixed solution.

In the third step of the method for producing lithium vanadium phosphate of the present invention, the production history of lithium hydroxide does not matter, but in order to produce high-purity lithium vanadium phosphate, the impurity content is as low as possible. It is preferable to be

When a solution containing lithium hydroxide is added to the reduction reaction solution obtained by performing the second step, heat is generated. Therefore, in the third step, a solution containing lithium hydroxide is added to the reduction reaction solution while maintaining the reduction reaction solution obtained by performing the second step at 35° C. or lower. In the third step, by adding a solution containing lithium hydroxide to the reduction reaction solution while maintaining the reduction reaction solution at 35° C. or less, a reaction precursor having a uniform composition distribution and excellent reactivity is obtained. easier to obtain. In addition, the addition temperature of the solution containing lithium hydroxide in the third step is preferably 5 to 35° C., particularly 10 to 25° C., from the viewpoint of obtaining a more uniform reaction precursor with excellent reactivity. is particularly preferred. On the other hand, in the third step, when the addition temperature of the solution containing lithium hydroxide exceeds 35° C., the components in the reduction reaction solution react with lithium hydroxide to form a precipitate, which is spray-dried as it is. This is not preferable from the viewpoint of forming a reaction precursor in which each component is non-uniformly distributed.

Moreover, in the third step, it is preferable to add the solution containing lithium hydroxide at a constant rate from the viewpoint of obtaining stable quality.

Further, in the third step, after the solution containing lithium hydroxide is added, the reaction between the components in the reduction reaction solution and lithium hydroxide proceeds very slowly at 35° C. or lower. At a temperature of 35°C, the dark blue solution state is maintained for about 200 minutes without deposits, but after 200 minutes, deposits are gradually observed, and the color of the solution changes from dark blue to light blue and then to green. Change. The raw material mixed solution obtained by performing the third step is a dark blue solution, and in the fourth step described later, the dark blue raw material mixed solution is subjected to a spray drying treatment as it is to have a uniform composition distribution. , is preferable from the viewpoint of obtaining a reaction precursor having excellent reactivity.

The fourth step in the method for producing lithium vanadium phosphate of the present invention is a step of spray-drying the raw material mixed solution obtained by performing the third step to obtain a reaction precursor.

Methods other than the spray drying method are known for drying the liquid, but in the present invention, this drying method is adopted based on the knowledge that it is advantageous to select the spray drying method. Specifically, since the spray-drying method yields granules in which each component is uniformly dispersed and densely packed, the granules are used as the reaction precursor in the method for producing lithium vanadium phosphate of the present invention. By firing the reaction precursor in the fifth step described later, it is possible to obtain a single-phase lithium vanadium phosphate in terms of X-ray diffraction.

In the spray-drying method, a reaction precursor is obtained by atomizing a liquid by a predetermined means and drying the resulting fine droplets. Liquid atomization includes, for example, a method using a rotating disc and a method using a pressure nozzle. Either method can be used in the fifth step.

In the spray drying method, the size of droplets of the atomized liquid affects stable drying and the properties of the resulting dry powder. From this point of view, the size of the atomized droplets is preferably 5-100 μm, particularly preferably 10-50 μm. In spray drying, it is desirable to determine the amount of liquid to be supplied to the spray drying apparatus in consideration of this point of view.

The reaction precursor obtained by the spray drying method is subjected to calcination in the fifth step. It will take over most of the body. For this reason, the spray-drying is preferably carried out so that the secondary particles of the reaction precursor have a particle size determined by scanning electron microscope (SEM) observation of 5 to 100 μm, particularly 10 to 50 μm. It is preferable from the viewpoint of controlling the particle size of the lithium vanadium oxide. Regarding the drying temperature in the spray dryer, the hot air inlet temperature is adjusted to 180 to 250°C, preferably 200 to 240°C, and the powder temperature is 90 to 150°C, preferably 100 to 130°C. Adjustment is preferable because it prevents the powder from absorbing moisture and facilitates the recovery of the powder.

The reaction precursor used in Patent Document 4 contains a crystalline substance, whereas the reaction precursor obtained by performing the fourth step according to the method for producing lithium vanadium phosphate of the present invention is amorphous. Quality. The present inventors speculate that since the reaction precursor is amorphous, it is more reactive, sintered at a lower temperature, and has higher crystallinity than the reaction precursor of Patent Document 4. there is It is confirmed by X-ray diffraction analysis that the reaction precursor is amorphous.

The fifth step according to the method for producing lithium vanadium phosphate of the present invention is to bake the reaction precursor obtained by performing the fourth step at 500 to 1300 ° C. to obtain a single-phase lithium vanadium phosphate by X-ray diffraction. It is a process.

The firing temperature in the fifth step is 500-1300°C, preferably 600-1000°C. The reason for this is that when the firing temperature is lower than 500°C, the firing time required to form a single phase becomes longer, while when the firing temperature is higher than 1300°C, the lithium vanadium phosphate melts.

The sintering atmosphere in the fifth step is an inert gas atmosphere or a reducing atmosphere for the reason of preventing vanadium from being oxidized and melting. The inert gas that can be used in the fifth step is not particularly limited, and examples thereof include nitrogen gas, helium gas, argon gas and the like.

In the fifth step, the sintering time is not particularly limited, and sintering for generally 2 hours or more, particularly 3 to 24 hours, can yield single-phase lithium vanadium phosphate in terms of X-ray diffraction.

In the fifth step, lithium vanadium phosphate obtained by firing may be subjected to multiple firings, if necessary.

In the method for producing lithium vanadium phosphate of the present invention, for the purpose of stabilizing the crystal structure of lithium vanadium phosphate and further improving battery performance such as cycle characteristics, if necessary, a Me source (Me is other than V A metal element or transition metal element having an atomic number of 11 or higher) is mixed with an acidic mixed slurry in the first step according to the method for producing lithium vanadium phosphate of the present invention, and subsequently the vanadium phosphate of the present invention By performing the second to fifth steps of the method for producing lithium, lithium vanadium phosphate represented by the general formula (1) doped with Me element can be obtained. In addition, in the present invention, the Me element is substituted at the Li site and/or the V site of the lithium vanadium phosphate represented by the general formula (1).

Me in the Me source is a metal element having an atomic number of 11 or higher other than V or a transition metal element. Preferred Me elements include Sr, Ba, Sc, Y, Hf, Ta, W, Ru, Os, Ag, Zn, Si, Ga, Ge, Sn, Bi, S, Se, Te, Cl, Br, I, Na, K, Mg, Ca, Al, Mn, Co, Ni, Fe, Ti, Zr, Bi, Cr, Nb, Mo, Cu, etc. may be mentioned, and these may be used singly or in combination of two or more.

Me sources include oxides, hydroxides, halides, carbonates, nitrates, carbonates, organic acid salts, etc. containing Me element. When the Me source is mixed in the first step, the Me source may be dissolved in the mixed slurry, or may be present as a solid matter. When the Me source is present as a solid matter in the mixed slurry, it is preferable to use one having an average particle size of 100 μm or less, preferably 0.1 to 50 μm, from the viewpoint of obtaining a reaction precursor having excellent reactivity. preferable. In addition, when the Me source is mixed in the first step, the amount of the Me source mixed depends on the type of Me element to be doped, but in many cases, V atoms of the vanadium compound with respect to P atoms in the phosphorus source and the total of Me atoms in the Me source (V + Me = M), the molar ratio (M / P) is 0.50 to 0.80, preferably 0.60 to 0.73, and the molar ratio of Me / V is The mixing amount is preferably greater than 0 and 0.45 or less, preferably greater than 0 and 0.1 or less.

Further, in the method for producing lithium vanadium phosphate of the present invention, the lithium vanadium phosphate obtained by performing the fifth step is further subjected to heat treatment to adjust the amount of conductive carbon contained in lithium vanadium phosphate. A sixth step can be applied. Specifically, the operation of the sixth step is carried out by subjecting the conductive carbon-containing lithium vanadium phosphate obtained by performing the fifth step to heat treatment to oxidize the conductive carbon.

The heat treatment in the sixth step is preferably performed in an oxygen-containing atmosphere. The oxygen concentration in the atmosphere of the heat treatment in the sixth step is preferably 5 vol % or more, particularly preferably 10 to 30 vol %, from the viewpoint of highly efficient oxidation treatment of the conductive carbon. The temperature of the heat treatment in the sixth step is preferably 250 to 450° C., preferably 300 to 400° C., from the viewpoint of highly efficient oxidation treatment of the conductive carbon. The duration of the heat treatment in the sixth step is not particularly limited, and the longer the duration of the heat treatment, the lower the content of conductive carbon contained in the lithium vanadium phosphate. In the heat treatment in the sixth step, it is preferable to set suitable conditions in advance so as to obtain a desired conductive carbon content.

Thus, lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention is a single-phase lithium vanadium phosphate in terms of X-ray diffraction, and has an average primary particle size of 10 μm or less, preferably Aggregated lithium vanadium phosphate in which a large number of primary particles of 0.01 to 5 μm aggregate to form secondary particles having an average particle size of 5 to 100 μm, preferably 10 to 50 μm, is preferred.

In the method for producing lithium vanadium phosphate of the present invention, the lithium vanadium phosphate to be obtained may be further subjected to crushing treatment or pulverization treatment as necessary, and then to classification.

In addition, the lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention may be a lithium vanadium phosphate carbon composite in which particles are coated with conductive carbon derived from reducing sugar. By using the lithium vanadium phosphate-carbon composite obtained by the method for producing lithium vanadium phosphate of the present invention as a positive electrode active material, a lithium secondary battery with a higher discharge capacity can be obtained.
In addition, the lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention is also used for applications in solid electrolytes.

EXAMPLES The present invention will be described in detail below with reference to Examples, but the present invention is not limited to these Examples.
{Example 1}
<First step>
2 L of ion-exchanged water was placed in a 5 L beaker, 605 g of 85% phosphoric acid, 320 g of vanadium pentoxide and 170 g of sucrose (sucrose) were added thereto and stirred at room temperature (25° C.) to obtain an ocher-colored acidic mixed slurry. (pH 0.9).
<Second step>
The resulting mixed slurry was heated at 95° C. for 1 hour with stirring to carry out a reduction reaction to obtain a dark blue reduction reaction solution (pH 1.7).
<Third step>
The reduction reaction solution was cooled to room temperature (25° C.) in 2 hours. Next, a lithium hydroxide solution was prepared by dissolving 220 g of lithium hydroxide monohydrate in 1.5 L of deionized water. The jacketed reaction vessel was cooled with a cooler, and the lithium hydroxide solution was added to the reaction solution at a constant rate over 60 minutes while maintaining the temperature of the reaction solution at 22°C to obtain a dark blue raw material mixed solution. (pH 4.6).
<Fourth step>
Next, the dark blue raw material mixed solution was supplied to a spray drying apparatus whose outlet temperature was set to 120° C. to obtain a reaction precursor. The average secondary particle size of the reaction precursor obtained by SEM observation was 12 μm.
The obtained reaction precursor was subjected to X-ray diffraction measurement using CuKα rays as a radiation source, and it was confirmed that the reaction precursor was amorphous. Also, FIG. 1 shows an X-ray diffraction pattern of the reaction precursor. An electron micrograph (SEM image) of the reaction precursor is shown in FIG.
<Fifth step>
The obtained reaction precursor was placed in a mullite sagger and fired at 600° C. for 10 hours in a nitrogen atmosphere. As a result of X-ray diffraction analysis of the obtained lithium vanadium phosphate sample, it was confirmed to be single-phase lithium vanadium phosphate. FIG. 3 shows an X-ray diffraction pattern of the obtained lithium vanadium phosphate sample. An electron micrograph (SEM image) of the obtained lithium vanadium phosphate sample is shown in FIG.
The amount of residual carbon in the obtained lithium vanadium phosphate sample was measured with a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation) to determine the content of C atoms.

{Examples 2-4}
In the fifth step, the reaction was carried out in the same manner as in Example 1, except that the sintering temperature was changed to 700 to 900° C. to obtain a lithium vanadium phosphate sample.
Further, as a result of X-ray diffraction analysis of the obtained lithium vanadium phosphate samples, it was confirmed that they were all single-phase lithium vanadium phosphate. Also, the amount of residual carbon was obtained in the same manner as in Example 1.

注）１）第１工程の「Ｖ／Ｐのモル比」は添加したリン酸中のＰ原子に対する五酸化バナジウム中のＶ原子のモル比を示す。
２）第３工程の「Ｌｉ／Ｐのモル比」は、第１工程で添加したリン酸中のＰ原子に対する第３工程で添加した水酸化リチウム中のＬｉ原子のモル比を示す。
３）第３工程の「添加後のｐＨ」は、水酸化リチウムを含む溶液を添加した後の反応液のｐＨを示す。

Note) 1) "V/P molar ratio" in the first step indicates the molar ratio of V atoms in vanadium pentoxide to P atoms in the added phosphoric acid.
2) "Li/P molar ratio" in the third step indicates the molar ratio of Li atoms in the lithium hydroxide added in the third step to P atoms in the phosphoric acid added in the first step.
3) "pH after addition" in the third step indicates the pH of the reaction solution after adding the solution containing lithium hydroxide.

{Comparative example 1}
Put 3.5 L of ion-exchanged water in a 5 L beaker, add 220 g of lithium hydroxide monohydrate, 320 g of vanadium pentoxide, 605 g of 85% phosphoric acid, and 170 g of sucrose, and stir at 95° C. for 1 hour. Heating gave a green slurry. Then, the slurry was supplied to a spray drying apparatus whose outlet temperature was set at 120° C. to obtain a reaction precursor. When the obtained reaction precursor was subjected to X-ray diffraction analysis, a clear diffraction peak was confirmed for the reaction precursor. An X-ray diffraction pattern of the reaction precursor is also shown in FIG.
The resulting reaction precursor was placed in a mullite sagger and fired at 600° C. for 10 hours in a nitrogen atmosphere to obtain a lithium vanadium phosphate sample.
An X-ray diffraction analysis diagram of the obtained lithium vanadium phosphate sample is also shown in FIG. As a result of X-ray diffraction analysis, the presence of a heterogeneous phase other than lithium vanadium phosphate was confirmed. Further, when the residual carbon content was obtained in the same manner as in Example 1, the residual carbon content was 1.9% by mass.

{Comparative example 2}
2 L of ion-exchanged water was placed in a 5 L beaker, and 252 g of lithium hydroxide monohydrate was added and dissolved. 364 g of vanadium pentoxide was added to this solution and stirred for 1 hour. 72 g of glucose (glucose) and 692 g of 85% phosphoric acid were added to this liquid, and the mixture was stirred for 1 hour to obtain a raw material mixed liquid. Next, the raw material mixed liquid was supplied to a spray drying apparatus whose outlet temperature was set at 120° C. to obtain a reaction precursor. An X-ray diffraction pattern of the reaction precursor is also shown in FIG.
The resulting reaction precursor was placed in a mullite sagger and fired at 900° C. for 12 hours in a nitrogen atmosphere. The fired product was pulverized by a jet mill to obtain a lithium vanadium phosphate sample. As a result of X-ray diffraction analysis of the obtained lithium vanadium phosphate sample, it was confirmed to be a single-phase lithium vanadium phosphate. Further, when the residual carbon content was determined in the same manner as in Example 1, the residual carbon content was 0.1% by mass.

{Example 5}
<First step>
Put 2.5 L of ion-exchanged water in a 5 L beaker, add 864.7 g of 85% phosphoric acid, 457.7 g of vanadium pentoxide, and 156.6 g of lactose monohydrate, and stir at room temperature (25°C). An ocher-colored acidic mixed slurry was obtained (pH 0.9).
<Second step>
The resulting mixed slurry was heated at 95° C. for 1 hour with stirring to carry out a reduction reaction to obtain a dark blue reduction reaction solution (pH 1.7).
<Third step>
The reduction reaction solution was cooled to room temperature (25° C.) in 2 hours. Next, a lithium hydroxide solution was prepared by dissolving 314.7 g of lithium hydroxide monohydrate in 1.5 L of deionized water. The jacketed reaction vessel was cooled with a cooler, and the lithium hydroxide solution was added to the reaction solution at a constant rate for 60 minutes while maintaining the temperature of the reaction solution at 22°C to obtain a dark blue raw material mixed solution. (pH 4.6).
<Fourth step>
Next, the dark blue raw material mixed solution was supplied to a spray drying apparatus whose outlet temperature was set to 120° C. to obtain a reaction precursor. The obtained reaction precursor was subjected to X-ray diffraction measurement using CuKα rays as a radiation source, and it was confirmed that the reaction precursor was amorphous. An electron micrograph (SEM image) of the reaction precursor is shown in FIG.
<Fifth step>
The obtained reaction precursor was placed in a mullite sagger and fired at 800° C. for 10 hours in a nitrogen atmosphere. As a result of X-ray diffraction analysis of the obtained lithium vanadium phosphate sample, it was confirmed to be single-phase lithium vanadium phosphate. Also, FIG. 6 shows an X-ray diffraction diagram of a lithium vanadium phosphate sample. An electron micrograph (SEM image) of the obtained lithium vanadium phosphate sample is shown in FIG.
The amount of residual carbon in the obtained lithium vanadium phosphate sample was measured with a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation) and found to be 1.5% by mass. Also, the BET specific surface area was 9.8 m ² /g.

{Comparative example 3}
After completion of the second step, the reaction solution was cooled to room temperature (25° C.) in 2 hours, and in the third step, the lithium hydroxide solution was added at a constant rate over 15 minutes without cooling the reaction vessel with a cooler. A lithium vanadium phosphate sample was obtained in the same manner as in Example 1 except for the above. The temperature of the reaction solution rose from 25°C to 40°C during the addition of the lithium hydroxide solution. Moreover, the raw material mixed solution after the third step was a blue slurry (pH 4.7).
Next, in the fourth step, the blue slurry was supplied to a spray drying apparatus whose outlet temperature was set at 120° C. to obtain a reaction precursor. When the obtained reaction precursor was subjected to X-ray diffraction measurement, a clear diffraction peak was confirmed for the reaction precursor. Also, FIG. 8 shows the X-ray diffraction pattern of the reaction precursor.
Next, the resulting reaction precursor was placed in a mullite sagger and fired at 600° C. for 10 hours in a nitrogen atmosphere to obtain a lithium vanadium phosphate sample.
FIG. 9 shows an X-ray diffraction analysis diagram of the obtained lithium vanadium phosphate sample. As a result of X-ray diffraction analysis, the presence of a heterogeneous phase other than lithium vanadium phosphate was confirmed. Further, when the residual carbon content was determined in the same manner as in Example 1, the residual carbon content was 1.8% by mass.

<Evaluation of battery performance>
<Battery performance test>
(I) Preparation of lithium secondary battery;
91% by mass of each sample of lithium vanadium phosphate of Examples 1, 4, 5, Comparative Examples 2 and 3, 6% by mass of graphite powder, and 3% by mass of polyvinylidene fluoride produced as described above was mixed to prepare a positive electrode material, which was dispersed in N-methyl-2-pyrrolidinone to prepare a kneaded paste. The resulting kneaded paste was applied to an aluminum foil, dried, pressed and punched into a disk having a diameter of 15 mm to obtain a positive electrode plate.
Using this positive electrode plate, a lithium secondary battery was manufactured using members such as a separator, a negative electrode, a positive electrode, a collector plate, a mounting bracket, an external terminal, and an electrolytic solution. Among them, a metal lithium foil was used as the negative electrode, and a solution prepared by dissolving 1 mol of LiPF ₆ in 1 liter of a 1:1 mixture of ethylene carbonate and methyl ethyl carbonate was used as the electrolyte.

(2) Evaluation of Battery Performance The lithium secondary batteries produced were operated under the following conditions to evaluate battery performance.
<Evaluation of cycle characteristics>
Charged by constant current constant voltage (CCCV) charging at 0.5 C to 4.2 V followed by holding at 4.2 V for a total charging time of 5 hours, followed by constant discharging to 2.0 V at 0.1 C. Current (CC) discharge was performed, and these operations were regarded as one cycle, and the discharge capacity was measured for each cycle. This cycle was repeated for 20 cycles, and the capacity retention rate was calculated from the discharge capacities of the 1st cycle and the 20th cycle using the following formula. Note that the discharge capacity at the first cycle was defined as the initial discharge capacity.

Capacity retention rate (%) = ((discharge capacity at 20th cycle)/(discharge capacity at 1st cycle)) x 100

{Example 6}
<Sixth step>
The lithium vanadium phosphate sample (residual carbon content: 1.6% by mass) obtained in Example 3 was heat-treated in an electric furnace in an air atmosphere (oxygen concentration: 20 vol%) at 350°C for 15 hours.
As a result of X-ray diffraction analysis of the obtained lithium vanadium phosphate sample, it was confirmed to be single-phase lithium vanadium phosphate. The amount of residual carbon in the obtained lithium vanadium phosphate sample was measured with a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation) and found to be 0.1% by mass. Also, the BET specific surface area was 7.1 m ² /g.

Claims (9)

A method for producing lithium vanadium phosphate having a NASICON structure,
A first step of mixing vanadium pentoxide, phosphoric acid and a reducing sugar in an aqueous solvent to prepare an acidic mixed slurry, and then a second step of obtaining a reduction reaction solution by heating the mixed slurry to make it into a solution. Then, while maintaining the reduction reaction solution at 10 to 25 ° C. , a third step of adding a solution containing lithium hydroxide to the reduction reaction solution to prepare a dark blue raw material mixed solution, then the dark blue A fourth step of obtaining a reaction precursor by spray drying the raw material mixed solution of No. 4, and then firing the reaction precursor at 500 to 1300 ° C. in an inert gas atmosphere or a reducing atmosphere to obtain lithium vanadium phosphate. A method for producing lithium vanadium phosphate, comprising five steps. The method for producing lithium vanadium phosphate according to claim 1, wherein the temperature of the heating treatment in the second step is 60 to 100°C. 3. The method for producing lithium vanadium phosphate according to claim 1, wherein the reducing sugar is one or two selected from sucrose and lactose. 4. The method for producing lithium vanadium phosphate according to any one of claims 1 to 3, wherein the concentration of lithium hydroxide in the solution containing lithium hydroxide is 5 to 20% by mass. Lithium vanadium phosphate according to any one of claims 1 to 4 , wherein in the third step, the solution containing lithium hydroxide is added so that the pH of the raw material mixed solution is 3 to 7. manufacturing method. Claims 1 to 5 , characterized in that in the first step, a Me source (Me represents a metal element having an atomic number of 11 or higher other than V or a transition metal element) is mixed with the acidic mixed slurry. A method for producing lithium vanadium phosphate according to any one of the above items. 7. The method for producing lithium vanadium phosphate according to any one of claims 1 to 6 , further comprising a sixth step of heat-treating the lithium vanadium phosphate obtained by performing the fifth step. 8. The method for producing lithium vanadium phosphate according to claim 7 , wherein the heat treatment in the sixth step is performed in an oxygen-containing atmosphere. The method for producing lithium vanadium phosphate according to any one of claims 7 and 8 , wherein the heat treatment temperature in the sixth step is 250 to 450°C.

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