JP7717490B2 - Method for producing transition metal-containing lithium phosphate and method for producing transition metal-containing lithium phosphate carbon composite - Google Patents

Description

The present invention relates to a method for producing transition metal-containing lithium phosphate, which is useful as a positive electrode material for lithium secondary batteries, all-solid-state batteries, etc.

Lithium-ion batteries are used in portable devices and laptop computers. They are generally considered to have excellent capacity and energy density. They are also expected to be used in hybrid and electric vehicles.

Olivine-type phosphates such as lithium cobalt phosphate (LiCoPO ₄ ) are attracting attention as positive electrode active materials for lithium secondary batteries for automobiles, all-solid-state batteries, and other batteries because their robust structure prevents oxygen release even at high temperatures, making them highly safe.

As a method for producing lithium cobalt phosphate, for example, Patent Document 1 proposes a method in which lithium carbonate, cobalt trioxide, and phosphorus pentoxide are dry-mixed and the mixture is calcined in air at 780°C. Patent Document 2 proposes a method in which lithium acetate, cobalt acetate, and ammonium hydrogen phosphate are added to water and the pH is adjusted to 1.5 or less with concentrated nitric acid to obtain a solution, glycolic acid is then added as a chelating agent to suppress particle growth, the solvent is removed, and the resulting precursor is dried and calcined at 600°C in an argon atmosphere. Patent Document 3 proposes a method in which lithium hydroxide and ammonium hydrogen phosphate are dissolved in water bubbled with _N2 , an aqueous solution containing cobalt sulfate is added to obtain a mixed solution, the solvent is removed, and the resulting precursor is calcined at 600°C in a nitrogen atmosphere.

The present inventors have also previously proposed an industrially advantageous method for producing lithium cobalt phosphate (Patent Document 4).

Furthermore, Patent Document 5 proposes an example method for producing lithium iron phosphate with an olivine structure by mixing lithium metaphosphate, an Fe compound, and a reducing agent and firing the mixture in a hydrogen-argon atmosphere.

JP-A-9-134724, paragraph 0008 JP 2015-88266 A, paragraph 0044 JP 2015-170464 A, paragraph 0066 International Publication No. WO2020/012970 Pamphlet International Publication WO2011/043255 Pamphlet, Examples

However, in the method of dry mixing of raw materials as in Patent Document 1, it is difficult to obtain single-phase lithium cobalt phosphate in X-ray diffraction analysis. Moreover, the example in Patent Document ₅ obtains LiFePO4, and simply using the method of Patent Document 5 makes it difficult to obtain single-phase lithium phosphate containing Co, Mn, or Ni in X-ray diffraction analysis.

On the other hand, the method of Patent Document 4 is an industrially advantageous method for obtaining lithium cobalt phosphate that is single-phase as measured by X-ray diffraction. However, the method of Patent Document 4 has the problem that when a slurry containing the pulverized raw material is spray-dried using a spray dryer, the slurry containing the pulverized raw material adheres to the spray dryer, resulting in a low yield of lithium cobalt phosphate.

In addition, lithium phosphates containing transition metals, such as lithium cobalt phosphate, have attracted attention as safe positive electrode active materials, and there is a need for their development using more industrially advantageous methods.

Lithium metaphosphate is a compound represented by _LiPO3 and contains Li and P, and further contains Li and P in equimolar amounts. Therefore, compared to other compounds containing Li and P, when used as a raw material for production of a compound containing at least Li and P in an atomic molar ratio of 1:1, it has advantages such as ease of composition adjustment. However, industrially available lithium metaphosphate is a coarse particle with problems in reactivity, and, except for Patent Document 5, has rarely been used as a raw material for production of compounds containing Li and P in fields where the presence of heterogeneous phases in X-ray diffraction is undesirable.

Therefore, an object of the present invention is to provide an industrially advantageous method for obtaining X-ray diffraction single-phase transition metal-containing lithium phosphate in high yield.

In view of the above circumstances, the present inventors have conducted extensive research and have found that a compound represented by the following general formula (1):
Li _x M _1-y A _y PO ₄ (1)
(In the formula, 0.8≦x≦1.2, 0.0≦y≦0.5. M represents one or more transition metal elements selected from Co, Mn, and Ni. A represents one or more metal elements selected from Mg, Zn, Cu, Fe, Cr, Al, B, Na, K, F, Cl, Br, I, Ca, Sr, Ba, Ti, Zr, Hf, Nb, Ta, Y, Yb, Si, S, Mo, W, V, Bi, Te, Pb, Ag, Cd, In, Sn, Sb, Ga, Ge, La, Ce, Nd, Sm, Eu, Tb, Dy, and Ho.)
The inventors have discovered that in a method for producing a transition metal-containing lithium phosphate represented by the formula (I), lithium metaphosphate is used as the phosphorus source and the lithium source, and the lithium metaphosphate and the M source are mixed in an aqueous solvent and wet-pulverized to produce a slurry that exhibits little adhesion to a spray-drying apparatus, and that therefore, by spray-drying a slurry containing a pulverized raw material obtained by wet-pulverization by a spray-drying method and then calcining the resulting spray-dried powder, a transition metal-containing lithium phosphate that is single-phase as measured by X-ray diffraction can be obtained in high yield, leading to the completion of the present invention. Furthermore, the inventors have discovered that by pulverizing the transition metal-containing lithium phosphate thus obtained, a product with a uniform particle size can be obtained.

That is, the present invention (1) relates to a compound represented by the following general formula (1):
Li _x M _1-y A _y PO ₄ (1)
(In the formula, 0.8≦x≦1.2, 0.0≦y≦0.5. M represents one or more transition metal elements selected from Co, Mn, and Ni. A represents one or more metal elements selected from Mg, Zn, Cu, Fe, Cr, Al, B, Na, K, F, Cl, Br, I, Ca, Sr, Ba, Ti, Zr, Hf, Nb, Ta, Y, Yb, Si, S, Mo, W, V, Bi, Te, Pb, Ag, Cd, In, Sn, Sb, Ga, Ge, La, Ce, Nd, Sm, Eu, Tb, Dy, and Ho.)
A method for producing a transition metal-containing lithium phosphate represented by the formula:
A first step of mixing at least lithium metaphosphate and an M source with an aqueous solvent to obtain a raw material mixture;
a second step of wet-pulverizing the raw material mixture to obtain a slurry containing the pulverized raw material;
a third step of spray-drying the slurry containing the pulverized raw material by a spray-drying method to obtain a spray-dried powder;
a fourth step of calcining the spray-dried powder;
The present invention provides a method for producing a transition metal-containing lithium phosphate, which comprises the steps of:

The present invention (2) also provides the method for producing a transition metal-containing lithium phosphate according to (1), characterized in that in the first step, an A source (A represents one or more metal elements selected from Mg, Zn, Cu, Fe, Cr, Al, B, Na, K, F, Cl, Br, I, Ca, Sr, Ba, Ti, Zr, Hf, Nb, Ta, Y, Yb, Si, S, Mo, W, V, Bi, Te, Pb, Ag, Cd, In, Sn, Sb, Ga, Ge, La, Ce, Nd, Sm, Eu, Tb, Dy, and Ho) is further added to the aqueous solvent.

The present invention (3) also provides the method for producing a transition metal- containing lithium phosphate according to (1) or (2), characterized in that in the second step, the wet grinding treatment is carried out using a media mill.

The present invention (4) also provides a method for producing a transition metal-containing lithium phosphate according to any one of (1) to (3), characterized in that in the second step, the average particle size of the solid content in the slurry containing the ground raw material after wet grinding is 10.0 μm or less.

The present invention (5) also provides a method for producing a transition metal-containing lithium phosphate according to any one of (1) to (4), characterized in that M in the general formula (1) is Co.

The present invention (6) also provides a method for producing a transition metal-containing lithium phosphate according to any one of (1) to (5), wherein the M source is cobalt hydroxide.

The present invention (7) also provides a method for producing a transition metal-containing lithium phosphate according to any one of (1) to (6), characterized in that the firing temperature in the fourth step is 600 to 900°C.

The present invention (8) also provides a method for producing a transition metal-containing lithium phosphate according to any one of (1) to (7), further comprising a fifth step of pulverizing the transition metal-containing lithium phosphate obtained by carrying out the fourth step.

The present invention (9) also provides a method for producing a transition metal-containing lithium phosphate according to any one of (1) to (7), further comprising a fifth step of pulverizing the transition metal-containing lithium phosphate obtained by carrying out the fourth step, and a sixth step of re-firing the pulverized material obtained by carrying out the fifth step.

The present invention (10) also provides a method for producing a transition metal-containing lithium phosphate carbon composite, comprising step A of mixing a transition metal-containing lithium phosphate obtained by any one of the methods (1) to (9) for producing a transition metal-containing lithium phosphate with a conductive carbon material source that precipitates carbon upon thermal decomposition to obtain a mixture of the transition metal-containing lithium phosphate and the conductive carbon material source, and then heat-treating the mixture to thermally decompose the conductive carbon material source, thereby obtaining a transition metal-containing lithium phosphate carbon composite.

The method for producing a transition metal-containing lithium phosphate of the present invention can provide an industrially advantageous method by which a transition metal-containing lithium phosphate that is single-phase as measured by X-ray diffraction can be obtained in high yield.

1 is an X-ray diffraction diagram of a transition metal-containing lithium phosphate sample obtained in Example 1. 1 is an X-ray diffraction diagram of a transition metal-containing lithium phosphate sample obtained in Example 2. 1 is an X-ray diffraction diagram of a transition metal-containing lithium phosphate sample obtained in Example 3. 1 is an X-ray diffraction diagram of a transition metal-containing lithium phosphate sample obtained in Comparative Example 1.

The method for producing a transition metal-containing lithium phosphate of the present invention is a method for producing a transition metal-containing lithium phosphate by reacting a compound represented by the following general formula (1):
Li _x M _1-y A _y PO ₄ (1)
(In the formula, 0.8≦x≦1.2, 0.0≦y≦0.5. M represents one or more transition metal elements selected from Co, Mn, and Ni. A represents one or more metal elements selected from Mg, Zn, Cu, Fe, Cr, Al, B, Na, K, F, Cl, Br, I, Ca, Sr, Ba, Ti, Zr, Hf, Nb, Ta, Y, Yb, Si, S, Mo, W, V, Bi, Te, Pb, Ag, Cd, In, Sn, Sb, Ga, Ge, La, Ce, Nd, Sm, Eu, Tb, Dy, and Ho.)
A method for producing a transition metal-containing lithium phosphate represented by the formula:
A first step of mixing at least lithium metaphosphate and an M source with an aqueous solvent to obtain a raw material mixture;
a second step of wet-pulverizing the raw material mixture to obtain a slurry containing the pulverized raw material;
a third step of spray-drying the slurry containing the pulverized raw material by a spray-drying method to obtain a spray-dried powder;
a fourth step of calcining the spray-dried powder;
The present invention relates to a method for producing a lithium phosphate containing a transition metal, the method comprising the steps of:

The transition metal-containing lithium phosphate obtained by the method for producing a transition metal-containing lithium phosphate of the present invention is a transition metal-containing lithium phosphate having an olivine structure, and is represented by the following general formula (1):
Li _x M _1-y A _y PO ₄ (1)
(In the formula, 0.8≦x≦1.2, 0.0≦y≦0.5. M represents one or more transition metal elements selected from Co, Mn, and Ni. A represents one or more metal elements selected from Mg, Zn, Cu, Fe, Cr, Al, B, Na, K, F, Cl, Br, I, Ca, Sr, Ba, Ti, Zr, Hf, Nb, Ta, Y, Yb, Si, S, Mo, W, V, Bi, Te, Pb, Ag, Cd, In, Sn, Sb, Ga, Ge, La, Ce, Nd, Sm, Eu, Tb, Dy, and Ho.)
It is a transition metal-containing lithium phosphate represented by the formula:

In general formula (1), x is 0.8 or more and 1.2 or less, preferably 0.9 or more and 1.1 or less. y is 0.0 or more and 0.5 or less, preferably 0.0 or more and 0.4 or less. In the formula, M represents one or more transition metal elements selected from Co, Mn, and Ni, and Co is preferred in that it results in a positive electrode active material that can be used to obtain high-capacity, high-energy-density lithium secondary batteries and all-solid-state batteries. In the formula, A is a metal element that is optionally included to improve battery characteristics. A represents one or more metal elements selected from Mg, Zn, Cu, Fe, Cr, Al, B, Na, K, F, Cl, Br, I, Ca, Sr, Ba, Ti, Zr, Hf, Nb, Ta, Y, Yb, Si, S, Mo, W, V, Bi, Te, Pb, Ag, Cd, In, Sn, Sb, Ga, Ge, La, Ce, Nd, Sm, Eu, Tb, Dy, and Ho.

The first step in the method for producing a transition metal-containing lithium phosphate of the present invention is a step of mixing at least lithium metaphosphate and an M source with an aqueous solvent to obtain a raw material mixture.

The lithium metaphosphate (LiPO ₃ ) used in the first step is not particularly limited as long as it is industrially available. Usually, industrially available lithium metaphosphate is hard and coarse particles having an average particle diameter of 200 μm or more as determined by a laser scattering/diffraction method, but in the present invention, even such hard and coarse lithium metaphosphate particles can be used without any problems.

Examples of the M source used in the first step include oxides, hydroxides, carbonates, organic acid salts, nitrates, and phosphates containing the M element. From the standpoints of industrial availability and ease of handling, oxides, hydroxides, and carbonates containing the M element are preferred as the M source.

As the M source for the first step, Co-containing compounds, such as Co-containing oxides, hydroxides, carbonates, organic acid salts, nitrates, and phosphates, are preferred because they can produce positive electrode active materials that can be used to produce high-capacity and high-energy density lithium secondary batteries and all-solid-state batteries. Cobalt hydroxide is particularly preferred because of its excellent reactivity.

The aqueous solvent used in the first step may be water alone or a mixed solvent of water and a hydrophilic solvent.

In the first step, at least lithium metaphosphate and an M source are added to an aqueous solvent, and the raw material mixture is obtained by mixing at least lithium metaphosphate and an M source with the aqueous solvent.

In the first step, the amount of lithium metaphosphate mixed into the aqueous solvent is preferably 5.0 to 40.0 parts by mass, and particularly preferably 8.0 to 30.0 parts by mass, per 100.0 parts by mass of the aqueous solvent. By mixing the amount of lithium metaphosphate in the first step within this range, it is possible to increase the milling efficiency or suppress an increase in the viscosity of the slurry after milling.

In the first step, the amount of M source mixed into the aqueous solvent is such that the molar ratio of M atoms in the M source to phosphorus atoms in the lithium metaphosphate (M/P) is preferably 0.80 to 1.2, and particularly preferably 0.90 to 1.1. When the molar ratio of M atoms in the M source to phosphorus atoms in the lithium metaphosphate (M/P) is within the above range, it becomes easier to obtain a transition metal-containing lithium phosphate that is single-phase in terms of X-ray diffraction.

In this way, a raw material mixture is obtained in the first step. In the method for producing transition metal-containing lithium phosphate of the present invention, if necessary, source A (A represents one or more metal elements selected from Mg, Zn, Cu, Fe, Cr, Al, B, Na, K, F, Cl, Br, I, Ca, Sr, Ba, Ti, Zr, Hf, Nb, Ta, Y, Yb, Si, S, Mo, W, V, Bi, Te, Pb, Ag, Cd, In, Sn, Sb, Ga, Ge, La, Ce, Nd, Sm, Eu, Tb, Dy, and Ho) can be further mixed into the aqueous solvent in the first step. That is, in the method for producing a transition metal-containing lithium phosphate of the present invention, in addition to lithium metaphosphate and an M source, an A source is added, if necessary, to an aqueous solvent, and the lithium metaphosphate, M source, and A source are mixed with the aqueous solvent, so that the raw material mixture in the first step can contain the A source in addition to lithium metaphosphate and an M source.

Examples of A sources include oxides, hydroxides, carbonates, organic acid salts, nitrates, and phosphates containing A elements.

The amount of the A source mixed is the molar ratio of A atoms in the A source to the total molar ratio of M atoms in the M source and A atoms in the A source.
The amount is preferably such that the atomic molar ratio (A/(A+M)) is 0.10 to 0.20.

It is preferable that the lithium metaphosphate, M source, and A source mixed as needed in the first step are high-purity products, as this allows for the production of high-purity transition metal-containing lithium phosphate.

The second step in the method for producing transition metal-containing lithium phosphate of the present invention is to wet-grind the raw material mixture obtained by carrying out the first step to obtain a slurry containing the pulverized raw material.

In the second step, the solids concentration in the raw material mixture when wet-pulverizing is preferably 5 to 40% by mass, and particularly preferably 10 to 30% by mass. Having a solids concentration in the raw material mixture when wet-pulverizing within this range ensures good operability and efficient pulverization. Therefore, after performing the first step, if necessary, the solids concentration of the raw material mixture may be adjusted to the above solids concentration before performing the second step.

Then, in the second step, the raw material mixture is wet-pulverized. The wet-pulverization in the second step can be carried out using, for example, a media mill, a grinding-type pulverizer that utilizes the mill principle, a wet jet mill, an ultrasonic pulverizer that pulverizes using ultrasound, or a pulverizer that uses the supersonic droplet collision dispersion method, but it is preferable to carry out the wet-pulverization using a media mill. Wet-pulverizing the raw material mixture using a media mill allows the solids contained in the raw material mixture to be pulverized even finer, which is industrially advantageous. Therefore, spray-drying the mixture using a spray-drying method in the third step allows for the production of a spray-dried powder with even greater reactivity.

Examples of media mills include bead mills, ball mills, paint shakers, attritors, and sand mills, with bead mills being preferred. When using a bead mill, the operating conditions and the type and size of the beads should be appropriately selected depending on the size of the equipment and the processing volume.

To make the wet grinding process more efficient, a dispersant may be added to the raw material mixture. The dispersant is selected appropriately depending on the type and characteristics of the slurry. Examples of dispersants include various surfactants and ammonium polycarboxylate salts. The concentration of the dispersant in the slurry is preferably 0.01 to 10% by mass, and particularly preferably 0.1 to 5% by mass, in order to obtain a sufficient dispersion effect.

In the second step, wet milling using a media mill is carried out until the average particle size of the solids in the slurry containing the ground raw material, as determined by laser scattering and diffraction, is preferably 10.0 μm or less, and particularly preferably 1.0 to 8.0 μm, as measured by _D50 . Having the average particle size of the solids in the slurry containing the ground raw material within this range facilitates the production of a spray-dried powder with excellent reactivity. Note that _D50 measured by laser scattering and diffraction refers to the particle size at 50% of the volumetrically integrated particle size on a particle size distribution curve measured by laser scattering and diffraction using, for example, a Microtrackbell MT3300.

In this way, by performing the second step, a slurry containing the ground raw material can be obtained.

In the method for producing transition metal-containing lithium phosphate of the present invention, the first step of mixing lithium metaphosphate, the M source, and optionally the A source with an aqueous solvent may be carried out in an apparatus or container separate from the wet grinding treatment device in which the second step is carried out.

Furthermore, the method for producing a transition metal-containing lithium phosphate of the present invention may include a first step of mixing lithium metaphosphate, the M source, and optionally the A source with an aqueous solvent in a wet-milling apparatus that performs the second step, and a second step of wet-milling using the wet-milling apparatus. For example, when a media mill is used as the wet-milling apparatus, media, lithium metaphosphate, the M source, and optionally the A source, and an aqueous solvent may be added to the media mill that performs the second step, and then the lithium metaphosphate, the M source, the aqueous solvent, and optionally the A source may be mixed in the media mill to obtain a raw material mixture, and then the resulting raw material mixture may be wet-milled. Alternatively, media, lithium metaphosphate, the M source, and optionally the A source, and an aqueous solvent may be added to the media mill that performs the second step, and then operation of the media mill may be started to mix lithium metaphosphate, the M source, and optionally the A source, and an aqueous solvent, and then the resulting raw material mixture may be wet-milled.

The third step in the method for producing transition metal-containing lithium phosphate of the present invention is to spray-dry the slurry containing the ground raw material obtained in the second step to obtain a spray-dried powder.

While methods other than spray drying are known for drying the slurry, the spray drying method is found to be advantageous for the production method of the transition metal-containing lithium phosphate of the present invention, and this drying method is therefore adopted. Specifically, the spray drying method makes it possible to obtain a dry powder in which the particles are packed tightly together. By calcining the resulting dry powder in the fourth step, it is possible to obtain a transition metal-containing lithium phosphate that is single-phase in terms of X-ray diffraction.

In the third step, spray drying, a slurry containing the pulverized raw material is atomized by a specified means, and the resulting fine droplets are dried to obtain a spray-dried powder. Slurry atomization can be achieved, for example, by using a rotating disk or a pressure nozzle. Either method can be used in the third step.

In the spray drying method used in the third step, the relationship between the size of the atomized slurry droplets and the particle size of the ground raw material contained therein affects stable drying and the properties of the resulting dried powder. Specifically, if the particle size of the ground raw material is too small compared to the droplet size, the droplets become unstable, making successful drying difficult. From this perspective, the size of the atomized droplets is preferably 5 to 50 μm, with 10 to 40 μm being particularly preferred. It is preferable to determine the amount of slurry supplied to the spray drying apparatus taking this into consideration.

The spray-dried powder obtained by spray drying in the third step is calcined in the fourth step, but the powder properties of the resulting transition metal-containing lithium phosphate, such as the average particle size, largely inherit those of the spray-dried powder. Therefore, in order to control the particle size of the desired transition metal-containing lithium phosphate, spray drying in the third step is preferably performed so that the secondary particle size of the spray-dried powder is 5 to 50 μm, and particularly preferably 10 to 40 μm, as determined by observation with a scanning electron microscope (SEM).

In the spray drying method of the third step, the drying temperature is preferably adjusted so that the hot air inlet temperature of the spray dryer is 170 to 320°C, preferably 190 to 300°C, and the hot air outlet temperature is 80 to 140°C, as this prevents the powder from absorbing moisture and makes it easier to recover the powder.

In this way, by carrying out the third step, spray-dried powder is obtained, which is then subjected to calcination in the fourth step.

The fourth step in the method for producing a transition metal-containing lithium phosphate of the present invention is a step of calcining the spray-dried powder obtained by carrying out the third step to obtain an X-ray single-phase transition metal-containing lithium phosphate.

The firing temperature in the fourth step is 600 to 900°C, preferably 650 to 850°C. If the firing temperature is below this range, the firing time required to reach a single phase as determined by X-ray diffraction will be long, which is industrially disadvantageous. On the other hand, if the firing temperature exceeds this range, the transition metal-containing lithium phosphate will become a hard sintered body, which is undesirable.

The firing atmosphere in the fourth step is air, an oxidizing atmosphere such as an oxygen gas atmosphere, or an inert gas atmosphere. In the fourth step, firing in an oxidizing atmosphere is preferred from the viewpoint of promoting crystal growth and achieving the desired particle size at a low temperature in a short time. Furthermore, if the firing in the fourth step contains an M source or A source that must be prevented from oxidation, it is preferable to use an inert gas atmosphere for the firing atmosphere.

The firing time in the fourth step is not particularly limited, but is at least 2 hours, preferably 4 to 10 hours. In the fourth step, firing for at least 2 hours, preferably 4 to 10 hours, will result in the production of a transition metal-containing lithium phosphate that is single-phase in terms of X-ray diffraction.

In the fourth step, the transition metal-containing lithium phosphate obtained after the first calcination may be calcined multiple times, if necessary.

After performing the fourth step, the transition metal-containing lithium phosphate obtained from the fourth step may be further classified, if necessary.

In the method for producing transition metal-containing lithium phosphate of the present invention, in the first step, lithium metaphosphate is used as the phosphorus source and lithium source and mixed with an M source in an aqueous solvent. In the second step, the raw material mixture slurry containing lithium metaphosphate and the M source is wet-pulverized to obtain a slurry containing a pulverized raw material. This slurry is less likely to adhere to the spray drying apparatus during the spray drying step in the third step, and is converted into transition metal-containing lithium phosphate that is single-phase in terms of X-ray diffraction after spray drying by the spray drying method in the third step and firing in the fourth step. As a result, transition metal-containing lithium phosphate that is single-phase in terms of X-ray diffraction can be obtained in high yield.

In addition, in the method for producing transition metal-containing lithium phosphate of the present invention, a fifth step can be carried out, if necessary, in which the transition metal-containing lithium phosphate obtained in step four is pulverized. In the method for producing transition metal-containing lithium phosphate of the present invention, pulverization of the transition metal-containing lithium phosphate obtained in step four is preferred, as this allows for the production of transition metal-containing lithium phosphate that is fine and has a sharp particle size distribution.

The grinding process in the fifth step may be a dry grinding process or a wet grinding process. Examples of wet grinding devices include a ball mill and a bead mill. Examples of dry grinding devices include known grinding devices such as a jet mill, a pin mill, a roll mill, a ball mill, and a bead mill.

After performing step 5, the transition metal-containing lithium phosphate obtained from step 5 may be further classified, if necessary.

Furthermore, in the method for producing a transition metal-containing lithium phosphate of the present invention, if the fourth step involves calcining at a temperature exceeding 700°C, preferably 750 to 850°C, and particularly preferably 800 to 850°C, and grain growth is performed so that the average particle size of the resulting transition metal-containing lithium phosphate is 2 μm or greater, sintering of the transition metal-containing lithium phosphate proceeds. Then, if the fifth step involves pulverization, chipping may result in the inclusion of a large amount of fine particles. Therefore, after the fifth step, it is preferable to perform a sixth step in which the pulverized product obtained by the fifth step is re-fired to reduce the fine particle content. For example, in the method for producing a transition metal-containing lithium phosphate of the present invention, the spray-dried powder obtained by the third step is calcined at a temperature exceeding 700°C, preferably 750 to 850°C, and particularly preferably 800 to 850°C, in the fourth step. The fifth step involves pulverizing the transition metal-containing lithium phosphate obtained by the fourth step, and then the sixth step involves re-firing the pulverized product obtained by the fifth step.

By re-firing in this sixth step, the fine particles of the transition metal-containing lithium phosphate are incorporated into larger particles of the transition metal-containing lithium phosphate, thereby reducing the fine particle content.

The re-firing temperature in the sixth step is 400 to 800°C, preferably 500 to 700°C. If the firing temperature is below this range, the effect of reducing fine particles will be reduced. On the other hand, if the re-firing temperature exceeds this range, excessive sintering will tend to occur, increasing the number of coarse particles.

The re-firing atmosphere in the sixth step is an air atmosphere, an oxidizing atmosphere such as an oxygen gas atmosphere, or an inert gas atmosphere. In the sixth step, firing in an oxidizing atmosphere is preferred because it makes it easier for the fine particles of the transition metal-containing lithium phosphate to be incorporated into larger particles of the transition metal-containing lithium phosphate. Furthermore, if the firing in the sixth step contains an M source or an A source that must be prevented from oxidation, it is preferable to use an inert gas atmosphere for the firing atmosphere.

The re-calcination time in Step 6 is not particularly limited, but is at least 2 hours, preferably 4 to 10 hours. In Step 6, lithium cobalt phosphate with satisfactory physical properties can be obtained by performing the calcination for at least 2 hours, preferably 4 to 10 hours.

After Step 6 is completed, the transition metal-containing lithium phosphate obtained from Step 6 can be crushed, classified, etc., if necessary.

The transition metal-containing lithium phosphate obtained by the method for producing a transition metal-containing lithium phosphate of the present invention in this manner is a single-phase transition metal-containing lithium phosphate in terms of X-ray diffraction, and in addition, has an average particle size determined by SEM observation of preferably 0.1 to 10 μm, particularly preferably 0.5 to 5.0 μm, and a BET specific surface area of preferably 0.5 to 10.0 ^{m 2} /g, particularly preferably 1.0 to 8.0 m ² /g.

Furthermore, in the method for producing transition metal-containing lithium phosphate of the present invention, if necessary, the transition metal-containing lithium phosphate obtained by carrying out any of steps 4, 5, and 6 can be subjected to step A, which will be described later.

Step A is a process in which the transition metal-containing lithium phosphate obtained by performing any of Steps 4, 5, and 6 is mixed with a conductive carbon material source that precipitates carbon upon thermal decomposition (hereinafter also referred to simply as the "conductive carbon material source") to obtain a mixture of the transition metal-containing lithium phosphate and the conductive carbon material source, and then the mixture is heat-treated to thermally decompose the conductive carbon material source and obtain a transition metal-containing lithium phosphate carbon composite.

The conductive carbon material source used is one that undergoes heat treatment in at least step A, whereby it is thermally decomposed to precipitate carbon. The conductive carbon material source is a component that imparts conductivity to the transition metal-containing lithium phosphate. By forming a composite of conductive carbon and transition metal-containing lithium phosphate, lithium secondary batteries that use the transition metal-containing lithium phosphate carbon composite as the positive electrode active material are expected to have improved discharge capacity and cycle characteristics.

Sources of conductive carbon materials include, for example, coal tar pitch ranging from soft pitch to hard pitch; petroleum heavy oils such as coal-based heavy oils such as carbonized liquefied oil, atmospheric residual oil, DC heavy oils such as vacuum residual oil, and cracked heavy oils such as ethylene tar produced as a by-product during the thermal cracking of crude oil and naphtha; aromatic hydrocarbons such as acenaphthylene, decacyclene, anthracene, and phenanthrene; polyphenylenes such as phenazine, biphenyl, and terphenyl; polyvinyl chloride; water-soluble polymers such as polyvinyl alcohol, polyvinyl butyral, and polyethylene glycol, and insolubilized products thereof; and nitrogen-containing polyacrylonitrile Examples of suitable materials include organic polymers such as polypyrrole; sulfur-containing organic polymers such as polythiophene and polystyrene; natural polymers such as sugars such as glucose, fructose, lactose, maltose, and sucrose; thermoplastic resins such as polyphenylene sulfide and polyphenylene oxide, and thermosetting resins such as phenol-formaldehyde resins and imide resins. Of these, sugars are preferred because they are commercially available at low cost and improve the discharge capacity and cycle characteristics of lithium secondary batteries that use the final transition metal-containing lithium phosphate carbon composite as a positive electrode active material.

The conductive carbon material source is preferably added in such a proportion that the carbon atoms in the conductive carbon material source account for 0.1 to 20.0 mass%, and preferably 0.5 to 15.0 mass%, of the transition metal-containing lithium phosphate, from the viewpoint of improving the discharge capacity and cycle characteristics of a lithium secondary battery using the transition metal-containing lithium phosphate carbon composite as the positive electrode active material.

In step A, the transition metal-containing lithium phosphate and the conductive carbon material source can be mixed using a dry or wet method.

In Step A, the dry mixing method is preferably carried out by mechanical means, as this results in a uniform mixture. There are no particular restrictions on the equipment used for dry mixing, as long as it can produce a uniform mixture. Examples include high-speed mixers, super mixers, turbosphere mixers, Eirich mixers, Henschel mixers, Nauta mixers, ribbon blenders, V-type mixers, conical blenders, jet mills, cosmomizers, paint shakers, bead mills, and ball mills. At the laboratory level, a household mixer is sufficient.

In addition, as a method for performing a wet mixing process in Step A, a mixture of the transition metal-containing lithium phosphate and the conductive carbon material source can be obtained by adding the transition metal-containing lithium phosphate and the conductive carbon material source to an aqueous solvent so that the solids content is 10 to 80% by mass, preferably 20 to 70% by mass, and mixing this by mechanical means to prepare a slurry. The slurry is then either left to dry or spray-dried to dry, thereby obtaining a mixture of the transition metal-containing lithium phosphate and the conductive carbon material source.

There are no particular limitations on the equipment used for wet mixing, as long as it can produce a uniform slurry. Examples include stirrers, blade mixers, three-roll mills, ball mills, disper mills, homogenizers, vibration mills, sand grind mills, attritors, and powerful mixers. Wet mixing is not limited to the mechanical mixing methods exemplified above. During wet mixing, a surfactant may be added to the slurry before mixing.

Next, the mixture of the transition metal-containing lithium phosphate and the conductive carbon material source prepared as described above is heat-treated. The heat treatment must be carried out at a temperature at which the conductive carbon material source is thermally decomposed to precipitate carbon, and the heating temperature is 180 to 900°C, preferably 210 to 800°C. By using a heating temperature within the above range, it is possible to uniformly coat the particle surfaces with carbon while suppressing aggregation. The heating time for the heat treatment is 0.2 hours or more, preferably 0.5 to 5 hours. An inert gas atmosphere is preferred for the heat treatment, as it can suppress carbon oxidation. Furthermore, in the heat treatment in the production method of the transition metal-containing lithium phosphate of the present invention, the mixture is first heated to a temperature above the melting point of the conductive carbon material source used to melt the conductive carbon material source, and then heat-treated within the above range to precipitate carbon from the conductive carbon material source, which is preferred for the purpose of uniformly coating the particle surfaces with carbon.

The transition metal-containing lithium phosphate and transition metal-containing lithium phosphate carbon composite obtained by the manufacturing method of the present invention are suitable for use as positive electrode materials in lithium secondary batteries, all-solid-state batteries, and the like.

The present invention will be described in more detail below using examples, but the present invention is not limited to these examples.

Example 1
<1st process/2nd process>
2.3 kg of φ5 mm zirconia balls, 81.1 g of high-purity lithium metaphosphate (average particle size 300 μm, manufactured by Nippon Chemical Industry Co., Ltd.), 90 g of cobalt hydroxide (average particle size 0.24 μm), 855 g of pure water, and 11 g of dispersant (ammonium polycarboxylate) were placed in a 2.4 L polypropylene pot, and the mill was started at a rotation speed of 200 rpm, and pulverization was carried out for 20 hours. After pulverization, the zirconia balls were separated using a 1 mm sieve to obtain a slurry containing the pulverized raw material.
The average particle size (D ₅₀ ) of the solid content in the slurry containing the pulverized raw material, determined by laser scattering and diffraction, was 5.5 μm.
<3rd process>
The slurry containing the pulverized raw material was supplied to a spray dryer with a hot air inlet temperature set to 220° C. at a supply rate of 2.4 L/h to obtain 170 g of spray-dried powder. Visual inspection of the inside of the spray dryer after spray drying revealed that there was little internal deposit, and the recovery rate was 95.5% based on the solid content.
<4th process/5th process>
The resulting spray-dried powder was then calcined in an air atmosphere at 650° C. for 4 hours to obtain a calcined product.
Next, the fired product was pulverized in a laboratory jet mill to obtain a pulverized product.
X-ray diffraction analysis of the obtained pulverized material revealed that the pulverized material was single-phase _LiCoPO4 . This was used as a transition metal-containing lithium phosphate sample. The X-ray diffraction pattern of the obtained transition metal-containing lithium phosphate sample is shown in Figure 1.

Example 2
<1st process/2nd process>
In a 2.4 L polypropylene pot, 2.3 kg of φ5 mm zirconia balls, 110.8 g of high-purity lithium metaphosphate (average particle size 300 μm, manufactured by Nippon Chemical Industry Co., Ltd.), 155 g of manganese carbonate (average particle size 17.8 μm), 949 g of pure water, and 15 g of dispersant (ammonium polycarboxylate) were placed, and the mill was started at a rotation speed of 200 rpm, and the mill was pulverized for 20 hours. After pulverization, the zirconia balls were separated using a 1 mm sieve to obtain a slurry containing the pulverized raw material.
The average particle size (D ₅₀ ) of the solid content in the slurry containing the pulverized raw material, determined by laser scattering and diffraction, was 3.3 μm.
<3rd process>
The slurry containing the pulverized raw material was supplied to a spray dryer with a hot air inlet temperature set to 220° C. at a supply rate of 2.4 L/h to obtain 254 g of spray-dried powder. Visual inspection of the inside of the spray dryer after spray drying revealed that there was little internal deposit, and the recovery rate was 95.6% based on the solid content.
<4th process/5th process>
The spray-dried powder was then calcined in an air atmosphere at 650° C. for 4 hours to obtain a calcined product, which was then pulverized in a lab jet mill to obtain a pulverized product.
X-ray diffraction analysis of the obtained pulverized material revealed that the pulverized material was single-phase _LiMnPO4 . This was used as a transition metal-containing lithium phosphate sample. The X-ray diffraction pattern of the obtained transition metal-containing lithium phosphate sample is shown in Figure 2.

Example 3
<1st process/2nd process>
In a 2.4 L polypropylene pot, 2.3 kg of φ5 mm zirconia balls, 131.7 g of high-purity lithium metaphosphate (average particle size 300 μm, manufactured by Nippon Chemical Industry Co., Ltd.), 145 g of nickel hydroxide (average particle size 22.7 μm), 922 g of pure water, and 18 g of dispersant (ammonium polycarboxylate) were placed, and the mill was started at a rotation speed of 200 rpm, and the mill was pulverized for 20 hours. After pulverization, the zirconia balls were separated using a 1 mm sieve to obtain a slurry containing the pulverized raw material.
The average particle size (D ₅₀ ) of the solid content in the slurry containing the pulverized raw material, determined by laser scattering and diffraction, was 5.1 μm.
<3rd process>
The slurry containing the pulverized raw material was supplied to a spray dryer with a hot air inlet temperature set to 220°C at a supply rate of 2.4 L/h to obtain 279 g of spray-dried powder. Visual inspection of the inside of the spray dryer after spray drying revealed that there was little internal deposit, and the recovery rate was 96.1% based on the solid content.
<4th process/5th process>
The spray-dried powder was then calcined at 700°C for 4 hours in an air atmosphere to obtain a calcined product, which was then pulverized in a lab jet mill to obtain a pulverized product.
X-ray diffraction analysis of the obtained pulverized material revealed that the pulverized material was single-phase _LiNiPO4 . This was used as a transition metal-containing lithium phosphate sample. The X-ray diffraction pattern of the obtained transition metal-containing lithium phosphate sample is shown in Figure 3.

(Reference example 1)
1597.7 g of oxalic acid dihydrate was added to 11 L of pure water at room temperature (25°C) and stirred for 30 minutes using a three-one motor stirrer. 227 g of dispersant (ammonium polycarboxylate) was then added. 1200 g of cobalt hydroxide was then added and stirred for 30 minutes. 1455.6 g of 85% by weight phosphoric acid was then added and stirred for 30 minutes. 532.5 g of lithium hydroxide monohydrate was then added and stirred for 1 hour to obtain an aqueous raw material slurry.
Next, this aqueous raw material slurry was fed, while being stirred, into a media stirring type bead mill charged with zirconia beads having a diameter of 0.5 mm, and mixed for 3 hours to carry out wet pulverization.
The slurry was then supplied to a spray dryer with a hot air inlet temperature set to 220° C. at a supply rate of 2.4 L/h to obtain 2,960 g of a reaction precursor. Visual inspection of the inside of the spray dryer after spray drying revealed a large amount of internal deposits, and the recovery rate was 79.7% based on the solid content.
The resulting reaction precursor was then fired at 550° C. for 4 hours in an air atmosphere to obtain a fired product.
Next, the fired product was pulverized in an airflow pulverizer to obtain a pulverized product.
The obtained pulverized product was subjected to X-ray diffraction analysis, and was found to be single-phase LiCoPO _4. This was used as a transition metal-containing lithium phosphate sample.

(Comparative Example 1)
A fired product was prepared in the same manner as in Example 1, except that the slurry containing the pulverized product obtained after the second step was not dried by a spray drying method but was instead dried at 120° C. for 5 hours. Next, the fired product was pulverized using a lab jet mill to obtain a pulverized product.
X-ray diffraction analysis of the obtained pulverized material revealed that the pulverized material contained a different phase in addition to _LiCoPO4 . This was designated as a transition metal-containing lithium phosphate sample. The X-ray diffraction pattern of the obtained transition metal-containing lithium phosphate sample is shown in Figure 4.

<Evaluation of various physical properties>
The average particle size, BET specific surface area, and particle size distribution of the transition metal-containing lithium phosphate samples obtained in the Examples, Reference Examples, and Comparative Examples were measured. The average particle size was measured by observing the samples under a scanning electron microscope at a magnification of 10,000 times, and the average value of the diameters of 50 or more randomly selected particles was calculated as the average particle size. The particle size distribution was evaluated as follows.
(particle size distribution)
The D _{50 and D 90} of the transition metal-containing lithium phosphate samples obtained in the Examples, Reference Examples, and Comparative Examples were measured by a laser diffraction/ _scattering method (manufactured by Nikkiso, product name: Microtrac MT3300EXII particle size analyzer, model: MTEX-SDU).
The smaller the difference between _D50 and _D90 , the more uniform the particle size.

From the results in Table 2, when Example 1 and Comparative Example 1 are compared, it can be seen that in Example 1, the difference between _D50 and _D90 is smaller than that in Comparative Example 1, and particles with a uniform particle size are obtained.

Example 4
<1st process/2nd process>
2.3 kg of φ5 mm zirconia balls, 81.1 g of high-purity lithium metaphosphate (average particle size 300 μm, manufactured by Nippon Chemical Industry Co., Ltd.), 90 g of cobalt hydroxide (average particle size 0.24 μm), 855 g of pure water, and 11 g of dispersant (ammonium polycarboxylate) were placed in a 2.4 L polypropylene pot, and the mill was started at a rotation speed of 200 rpm, followed by pulverization for 20 hours. After pulverization, the zirconia balls were separated using a 1 mm sieve to obtain a slurry containing the pulverized raw material.
The average particle size (D50) of the solid content in the slurry containing the pulverized raw material, determined by laser scattering and diffraction, was 5.5 μm.
<3rd process>
The slurry containing the pulverized raw material was supplied to a spray dryer with a hot air inlet temperature set to 220°C at a supply rate of 2.4 L/h to obtain 170 g of dry powder. Visual inspection of the inside of the spray dryer after spray drying revealed that there was little internal deposit, and the recovery rate was 95.6% based on the solid content.
<4th process/5th process>
The resulting dried powder was then fired at 820° C. for 4 hours in an air atmosphere to obtain a fired product.
Next, the fired product was pulverized in a laboratory jet mill to obtain a pulverized product.
The obtained pulverized product was subjected to X-ray diffraction analysis, and was found to be single-phase LiCoPO _4. Furthermore, the maximum particle size and cumulative frequency of fine particles of 1.0 μm or less were measured for the obtained transition metal-containing lithium phosphate by a laser scattering/diffraction method.
<6th step>
The pulverized transition metal-containing lithium phosphate obtained in the fifth step was re-fired in an air atmosphere at 600° C. for 4 hours. The maximum particle size and the cumulative frequency of particles of 1.0 μm or less were determined for the re-fired transition metal-containing lithium phosphate by a laser scattering/diffraction method. Furthermore, the average particle size, BET specific surface area, and particle size distribution were measured in the same manner as in Example 1.

Example 6
A transition metal-containing lithium phosphate was obtained in the same manner as in Example 5, except that in the fourth step, the firing temperature was 850° C. for 4 hours.

The results in Table 3 show that re-firing in step 6 reduces the amount of fine particles of 1.0 μm or less.

Example 7
<Step A>
10 _g of the LiCoPO4 obtained in Example 1 and 2 g of lactose were mixed in a planetary centrifugal mixer at 100 rpm for 1 minute. The mixture was heated at 220°C for 2 hours in a nitrogen atmosphere, and then heated to 700°C and maintained at that temperature for 4 hours to obtain a LiCoPO4 _carbon composite.
Observation of the _LiCoPO4 carbon composite with SEM-EDX confirmed that the surface of the _LiCoPO4 particles was uniformly coated with carbon. Furthermore, measurement of the carbon content by TOC revealed that the carbon content was 3% by mass.

Claims (10)

The following general formula (1):
Li _x M _1-y A _y PO ₄ (1)
(In the formula, 0.8≦x≦1.2, 0.0≦y≦0.5. M represents one or more transition metal elements selected from Co, Mn, and Ni. A represents one or more metal elements selected from Mg, Zn, Cu, Fe, Cr, Al, B, Na, K, F, Cl, Br, I, Ca, Sr, Ba, Ti, Zr, Hf, Nb, Ta, Y, Yb, Si, S, Mo, W, V, Bi, Te, Pb, Ag, Cd, In, Sn, Sb, Ga, Ge, La, Ce, Nd, Sm, Eu, Tb, Dy, and Ho.)
A method for producing a transition metal-containing lithium phosphate represented by the formula:
A first step of mixing at least lithium metaphosphate and an M source with an aqueous solvent to obtain a raw material mixture;
a second step of wet-pulverizing the raw material mixture to obtain a slurry containing the pulverized raw material;
a third step of spray-drying the slurry containing the pulverized raw material by a spray-drying method to obtain a spray-dried powder;
a fourth step of calcining the spray-dried powder;
1. A method for producing a transition metal-containing lithium phosphate, comprising: The method for producing transition metal-containing lithium phosphate according to claim 1, characterized in that in the first step, an A source (A represents one or more metal elements selected from Mg, Zn, Cu, Fe, Cr, Al, B, Na, K, F, Cl, Br, I, Ca, Sr, Ba, Ti, Zr, Hf, Nb, Ta, Y, Yb, Si, S, Mo, W, V, Bi, Te, Pb, Ag, Cd, In, Sn, Sb, Ga, Ge, La, Ce, Nd, Sm, Eu, Tb, Dy, and Ho) is further added to the aqueous solvent. 3. The method for producing a transition metal- containing lithium phosphate according to claim 1, wherein the wet grinding treatment in the second step is carried out using a media mill. The method for producing transition metal-containing lithium phosphate according to any one of claims 1 to 3, characterized in that in the second step, the average particle size of the solid content in the slurry containing the ground raw material after wet grinding is 10.0 μm or less. The method for producing transition metal-containing lithium phosphate according to any one of claims 1 to 4, characterized in that M in general formula (1) is Co. The method for producing transition metal-containing lithium phosphate according to any one of claims 1 to 5, characterized in that the M source is cobalt hydroxide. The method for producing transition metal-containing lithium phosphate according to any one of claims 1 to 6, characterized in that the firing temperature in the fourth step is 600 to 900°C. The method for producing transition metal-containing lithium phosphate according to any one of claims 1 to 7, further comprising a fifth step of pulverizing the transition metal-containing lithium phosphate obtained by carrying out the fourth step. The method for producing a transition metal-containing lithium phosphate according to any one of claims 1 to 7, further comprising a fifth step of pulverizing the transition metal-containing lithium phosphate obtained by carrying out the fourth step, and a sixth step of re-firing the pulverized material obtained by carrying out the fifth step. A method for producing a transition metal-containing lithium phosphate-carbon composite, comprising: step A: mixing the transition metal-containing lithium phosphate obtained by carrying out the method for producing a transition metal-containing lithium phosphate according to any one of claims 1 to 9 with a conductive carbon material source that precipitates carbon upon thermal decomposition to obtain a mixture of the transition metal-containing lithium phosphate and the conductive carbon material source; and then heat-treating the mixture to thermally decompose the conductive carbon material source, thereby obtaining a transition metal-containing lithium phosphate-carbon composite.