JP7717491B2 - Method for producing transition metal-containing lithium pyrophosphate and method for producing transition metal-containing lithium pyrophosphate carbon composite - Google Patents

Description

The present invention relates to a method for producing transition metal-containing lithium pyrophosphate, 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.

Pyrophosphates such as lithium cobalt pyrophosphate (Li ₂ CoP ₂ O ₇ ) have attracted attention as positive electrode active materials for high-capacity and high-energy density lithium secondary batteries (e.g., Patent Documents 1 and 2) and as positive electrode active materials for all-solid-state batteries with high battery and capacity (e.g., Patent Documents 3 and 4).

As methods for producing lithium cobalt pyrophosphate, for example, Patent Document 1 proposes a method in which a lithium source, a cobalt source, and a phosphorus source are mixed in a ball mill, the resulting particle mixture is further pelletized, and the pelletized mixture is fired. Patent Document 3 also proposes a method in which a lithium source, a cobalt source, and a phosphorus source are mixed, the resulting mixture is pre-fired in an air atmosphere, and the pre-fired product is then fired in an air atmosphere at a temperature higher than that used for pre-fired firing, at 650 to 680°C for 20 to 30 hours.

The present inventors have also previously proposed an industrially advantageous method for producing lithium cobalt pyrophosphate (Patent Document 5).

Special Publication No. 2006-523930 JP 2016-38996 A JP 2017-192949 A Japanese Patent Application Laid-Open No. 2017-228453 International Publication No. WO2020/031690 Pamphlet

However, with methods such as those described in Patent Documents 1 and 2, in which the raw materials are mixed dry, it is difficult to obtain single-phase lithium cobalt pyrophosphate in X-ray diffraction analysis.

The method of Patent Document 5 is an industrially advantageous method for obtaining lithium cobalt pyrophosphate that is single-phase as measured by X-ray diffraction. However, the method of Patent Document 5 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 pyrophosphate.

In addition, lithium cobalt pyrophosphate has attracted attention as a safe positive electrode active material, and there is a need for its development in a more industrially advantageous manner.

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 in the form of coarse particles and has reactivity problems, and 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 pyrophosphate 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 P ₂ O ₇ (1)
(In the formula, 1.7≦x≦2.2, 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 present inventors have discovered that in a method for producing a transition metal-containing lithium pyrophosphate 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 using a spray-drying method and then calcining the resulting spray-dried powder, a transition metal-containing lithium pyrophosphate 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 present inventors have discovered that by pulverizing the transition metal-containing lithium pyrophosphate 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 P ₂ O ₇ (1)
(In the formula, 1.7≦x≦2.2, 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 pyrophosphate 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 transition metal-containing lithium pyrophosphate, which comprises the steps of:

The present invention (2) also provides the method for producing transition metal-containing lithium pyrophosphate 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) provides the method for producing transition metal-containing lithium pyrophosphate 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 transition metal-containing lithium pyrophosphate 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 transition metal-containing lithium pyrophosphate according to any one of (1) to (4), wherein M in the general formula (1) is Co.

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

The present invention (8) also provides a method for producing transition metal-containing lithium pyrophosphate according to any one of (1) to (6), characterized in that in the fourth step, the spray-dried powder is calcined at 600 to 700°C in an air atmosphere.

The present invention (9) also provides a method for producing any one of the transition metal-containing lithium pyrophosphates (1) to (8), further comprising a fifth step of pulverizing the transition metal-containing lithium pyrophosphate obtained by carrying out the fourth step.

The present invention (10) also provides a method for producing a transition metal-containing lithium pyrophosphate-carbon composite, comprising step A of mixing a transition metal-containing lithium pyrophosphate obtained by carrying out any one of the methods (1) to (9) for producing a transition metal-containing lithium pyrophosphate with a conductive carbon material source that precipitates carbon upon thermal decomposition to obtain a mixture of the transition metal-containing lithium pyrophosphate 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 pyrophosphate-carbon composite.

The method for producing transition metal-containing lithium pyrophosphate of the present invention provides an industrially advantageous method for obtaining X-ray diffraction single-phase transition metal-containing lithium pyrophosphate in high yield.

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

The method for producing transition metal-containing lithium pyrophosphate of the present invention is a method for producing a transition metal-containing lithium pyrophosphate by the reaction of a compound represented by the following general formula (1):
Li _x M _1-y A _y P ₂ O ₇ (1)
(In the formula, 1.7≦x≦2.2, 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 pyrophosphate 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 transition metal-containing lithium pyrophosphate, the method comprising the steps of:

The transition metal-containing lithium pyrophosphate obtained by the method for producing transition metal-containing lithium pyrophosphate of the present invention is represented by the following general formula (1):
Li _x M _1-y A _y P ₂ O ₇ (1)
(In the formula, 1.7≦x≦2.2, 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 transition metal-containing lithium pyrophosphate is represented by the formula:

In general formula (1), x is 1.7 or more and 2.2 or less, preferably 1.8 or more and 2.1 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 transition metal-containing lithium pyrophosphate of the present invention is to mix 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 15.0 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 0.30 to 0.70, and particularly preferably 0.40 to 0.60. 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 transition metal-containing lithium pyrophosphate 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 pyrophosphate 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. In other words, in the method for producing transition metal-containing lithium pyrophosphate 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 A source mixed is preferably an amount such that 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 (A/(A+M)) is 0.10 to 0.20.

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

The second step in the method for producing transition metal-containing lithium pyrophosphate 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 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 laser scattering and diffraction. 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 the _D50 measured by laser scattering and diffraction refers to the particle size at 50% of the volumetrically integrated particle size in the 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 pyrophosphate 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.

In addition, the method for producing transition metal-containing lithium pyrophosphate of the present invention may also 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 for performing 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 can be added to the media mill for performing the second step, and then the lithium metaphosphate, the M source, the aqueous solvent, and optionally the A source can be mixed in the media mill to obtain a raw material mixture, and then the resulting raw material mixture can be wet-milled. Alternatively, media, lithium metaphosphate, the M source, and optionally the A source, and an aqueous solvent can be added to the media mill for performing the second step, and then the operation of the media mill can 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 can be wet-milled.

The third step in the method for producing transition metal-containing lithium pyrophosphate 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 slurries, the spray drying method is found to be advantageous for the manufacturing method of transition metal-containing lithium pyrophosphate of the present invention, and this drying method is therefore adopted. Specifically, by using the spray drying method, a dry powder in which the particles are packed tightly can be obtained, and by calcining the resulting dry powder in the fourth step, a single-phase transition metal-containing lithium pyrophosphate can be obtained 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 resulting transition metal-containing lithium pyrophosphate largely inherits the powder properties, such as the average particle size, of the spray-dried powder. Therefore, in order to control the particle size of the desired transition metal-containing lithium pyrophosphate, 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 scanning electron microscope (SEM) observation.

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 transition metal-containing lithium pyrophosphate of the present invention is to calcinate the spray-dried powder obtained by carrying out the third step to obtain X-ray single-phase transition metal-containing lithium pyrophosphate.

The firing temperature in the fourth step is 600°C or higher, preferably 600 to 700°C, and particularly preferably 650 to 700°C. By keeping the firing temperature within this range, a transition metal-containing lithium pyrophosphate that is single-phase in terms of X-ray diffraction is obtained. On the other hand, if the firing temperature is below this range, the reaction is unlikely to be completed, and transition metal-containing lithium pyrophosphate tends not to be obtained.

The firing atmosphere in the fourth step is either air or an inert gas atmosphere. When firing in the fourth step is performed at a high temperature exceeding 700°C, firing in an air atmosphere will result in a molten product, not a powdered product. Therefore, when firing at a temperature exceeding 700°C, it is preferable to perform the firing in an inert gas atmosphere. Examples of inert gases include argon gas, helium gas, and nitrogen gas. Of these, nitrogen gas is preferred from the perspective of being inexpensive and industrially advantageous.

In the method for producing transition metal-containing lithium pyrophosphate of the present invention, it is preferable to calcinate the spray-dried powder in the fourth step at 600 to 700°C in an air atmosphere, as this is industrially advantageous.

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

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

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

In the method for producing transition metal-containing lithium pyrophosphate of the present invention, in the first step, lithium metaphosphate is used as the phosphorus source and lithium source, and is mixed with an M source in an aqueous solvent. In the second step, the raw material mixture containing lithium metaphosphate and the M source is wet-pulverized to produce a slurry containing the 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 pyrophosphate that is single-phase in terms of X-ray diffraction after spray drying by the spray drying method in the third step and calcination in the fourth step. As a result, transition metal-containing lithium pyrophosphate 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 pyrophosphate of the present invention, a fifth step can be carried out, if necessary, in which the transition metal-containing lithium pyrophosphate obtained in the fourth step is pulverized. In the method for producing transition metal-containing lithium pyrophosphate of the present invention, pulverization of the transition metal-containing lithium pyrophosphate obtained in the fourth step is preferred, as this allows for the production of fine transition metal-containing lithium pyrophosphate with a narrow 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 pyrophosphate obtained from step 5 may be further classified, if necessary.

The transition metal-containing lithium pyrophosphate obtained by the method for producing the transition metal-containing lithium pyrophosphate of the present invention in this manner is a single-phase transition metal-containing lithium pyrophosphate in terms of X-ray diffraction, and in addition, has an average particle size determined by SEM observation of preferably 10.0 μm or less, particularly preferably 1.0 to 5.0 μm, and a BET specific surface area of preferably 1.0 m ² /g or more, particularly preferably 2.0 to 10.0 ^{m 2} /g.

Furthermore, in the method for producing transition metal-containing lithium pyrophosphate of the present invention, if necessary, the transition metal-containing lithium pyrophosphate obtained by performing either step 4 or step 5 can be subjected to step A, which will be described later.

Step A is a process in which the transition metal-containing lithium pyrophosphate obtained by performing either Step 4 or Step 5 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 pyrophosphate 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 pyrophosphate 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 pyrophosphate. By forming a composite of conductive carbon and transition metal-containing lithium pyrophosphate, lithium secondary batteries that use the transition metal-containing lithium pyrophosphate-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 are 0.1 to 20.0 mass%, and preferably 0.5 to 15.0 mass%, relative to the transition metal-containing lithium pyrophosphate, from the viewpoint of improving the discharge capacity and cycle characteristics of lithium secondary batteries that use the transition metal-containing lithium pyrophosphate carbon composite as the positive electrode active material.

In step A, the transition metal-containing lithium pyrophosphate 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 transition metal-containing lithium pyrophosphate and a conductive carbon material source can be obtained by adding the transition metal-containing lithium pyrophosphate 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 pyrophosphate 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 pyrophosphate 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 thermally decomposes and carbon precipitates. The heating temperature is 180 to 900°C, preferably 210 to 800°C. By maintaining the heat treatment temperature within the above range, carbon can be uniformly coated on the particle surfaces 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 this suppresses carbon oxidation. Furthermore, in the heat treatment method for producing transition metal-containing lithium pyrophosphate of the present invention, the conductive carbon material source is first heated to a temperature above the melting point of the conductive carbon material source used to melt it, 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 pyrophosphate and transition metal-containing lithium pyrophosphate 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>
In a 2.4 L polypropylene pot, 2.3 kg of φ5 mm zirconia balls, 181.5 g of high-purity lithium metaphosphate (average particle size 300 μm, manufactured by Nippon Chemical Industry Co., Ltd.), 100.0 g of cobalt hydroxide (average particle size 0.24 μm), 950 g of pure water, and 19 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 6.0 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 280 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 96.7% based on the solid content.
<4th process/5th process>
The resulting spray-dried powder was then calcined at 650° C. for 4 hours in an air atmosphere 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 Li1.86CoP2O7} . This was used as a transition metal-containing lithium _{pyrophosphate} sample. The X-ray diffraction pattern of the obtained transition metal-containing lithium pyrophosphate 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, 154.8 g of high-purity lithium metaphosphate (average particle size 300 μm, manufactured by Nippon Chemical Industry Co., Ltd.), 110.0 g of manganese carbonate (average particle size 17.8 μm), 882 g of pure water, and 16 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 4.1 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 261 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 96.4% 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 Li ₂ MnP ₂ O _7. This was used as a transition metal-containing lithium pyrophosphate sample. The X-ray diffraction pattern of the obtained transition metal-containing lithium pyrophosphate sample is shown in Figure 2.

(Reference example 1)
1604.5 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. 228 g of dispersant (ammonium polycarboxylate) was then added. 1200 g of cobalt hydroxide was then added and stirred for 30 minutes. 2922.4 g of 85% by weight phosphoric acid was then added and stirred for 30 minutes. 1068.8 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 2,590 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 44.8% based on the solid content.
The resulting reaction precursor was then fired at 650° 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 a single-phase Li _1.86 CoP ₂ O _7. This was used as a transition metal-containing lithium pyrophosphate 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 resulting pulverized material _revealed that in addition to _Li1.86CoP2O7 , the pulverized material contained other phases such as _LiCoPO4 . This was designated as a transition metal-containing lithium pyrophosphate sample. The X-ray diffraction pattern of the resulting transition metal-containing lithium pyrophosphate sample is shown in Figure 3.

<Evaluation of various physical properties>
The transition metal-containing lithium pyrophosphate samples obtained in the Examples, Reference Examples, and Comparative Examples were measured for average particle size, BET specific surface area, and particle size distribution. The average particle size was measured by observing the particles under a scanning electron microscope at a magnification of 10,000 times, and the average value 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 pyrophosphate 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 3
<Step A>
10 g of Li ₂ CoP ₂ O ₇ obtained in Example 1 and 2 g of lactose were mixed (using a planetary centrifugal mixer at 100 rpm for 1 minute). The mixture was heat-treated 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 Li ₂ CoP ₂ O ₇ carbon composite. Furthermore, observation of the Li ₂ CoP ₂ O ₇ carbon composite using SEM-EDX confirmed that the surfaces of the Li ₂ CoP ₂ O ₇ particles were 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 P ₂ O ₇ (1)
(In the formula, 1.7≦x≦2.2, 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 pyrophosphate 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 transition metal-containing lithium pyrophosphate, comprising: The method for producing transition metal-containing lithium pyrophosphate 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 transition metal-containing lithium pyrophosphate 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 pyrophosphate 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 pyrophosphate 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 pyrophosphate 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 pyrophosphate according to any one of claims 1 to 6, characterized in that the firing temperature in the fourth step is 600°C or higher. The method for producing transition metal-containing lithium pyrophosphate according to any one of claims 1 to 6, characterized in that in the fourth step, the spray-dried powder is calcined at 600 to 700°C in an air atmosphere. The method for producing transition metal-containing lithium pyrophosphate according to any one of claims 1 to 8, further comprising a fifth step of pulverizing the transition metal-containing lithium pyrophosphate obtained by carrying out the fourth step. A method for producing a transition metal-containing lithium pyrophosphate-carbon composite, comprising: mixing the transition metal-containing lithium pyrophosphate obtained by the method for producing a transition metal-containing lithium pyrophosphate 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 pyrophosphate 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 pyrophosphate-carbon composite.