JP7824092B2 - Lithium-inorganic acid stabilized niobate sol - Google Patents

Description

The present invention relates to a lithium-inorganic acid-stabilized niobic acid sol.

In recent years, niobium-based sols have been attracting attention in the field of battery materials as a surface coating material for positive and negative electrodes.

The applicant first invented technology related to ammonium niobate sol, a niobium-based sol, as described in Patent Document 1, and then invented the amine compound-stabilized niobic acid sol described in Patent Document 2. Furthermore, they invented alkali metal-stabilized niobic acid sols, as described in Patent Document 3, one of which is a lithium-stabilized niobic acid sol.

Patent No. 5441264 Patent No. 6156876 Patent No. 6774158

The lithium-stable niobic acid sol obtained by the manufacturing method disclosed in Patent Document 3 is a sol in which relatively small particles having an average particle size of about 10 to 30 nm are dispersed.
Controlling particle size has been required from the viewpoint of expanding application areas.

The objective of this invention is to develop a niobic acid sol in which fine particles with larger particle sizes are stably dispersed. In particular, the objective is to develop a niobic acid sol with an average particle size of 50 nm or more.

After extensive research into the above-mentioned problems, the inventors surprisingly discovered that the above-mentioned problems could be solved by using lithium and an inorganic acid as a dispersion stabilizer, and based on this finding, they have completed the present invention.

That is, the present invention is as follows.
[1] A lithium-inorganic acid stabilized niobic acid sol containing lithium and an inorganic acid as a dispersion stabilizer.
[2] The lithium-inorganic acid-stable niobic acid sol according to the above [1], wherein the inorganic acid is at least one selected from the group consisting of nitric acid, hydrochloric acid, sulfuric acid, and perchloric acid.
[3] (1) First step of mixing ammonium niobate sol with inorganic acid
(2) A second step of mixing the mixture obtained in the first step with lithium hydroxide.
(3) A method for producing a lithium-inorganic acid stabilized niobic acid sol containing lithium and an inorganic acid as a dispersion stabilizer, comprising a third step of heating and/or washing the liquid obtained in the second step.

The present invention will be described in detail below based on preferred embodiments, but the present invention is not limited to the following embodiments and various modifications are possible within the scope of the claims.
In the present invention, the expression "numeric value 1 to numerical value 2" in relation to a numerical range means a numerical range including both the numerical values 1 and 2, with numerical value 1 being the lower limit and numerical value 2 being the upper limit, and is synonymous with "numerical value 1 or more and numerical value 2 or less."

The present invention relates to a lithium-inorganic acid-stabilized niobic acid sol (hereinafter referred to as "the sol of the present invention") that contains lithium and an inorganic acid as dispersion stabilizers. The sol of the present invention can also be said to be a niobic acid sol stabilized with lithium and an inorganic acid.

In the sol of the present invention, at least a portion of the lithium is thought to directly bond to or adsorb onto dispersed particles of niobic acid, thereby contributing to the dispersion stabilization of the dispersed particles, similar to the lithium-stable niobic acid sol described in Patent Document 3. On the other hand, the remaining lithium, together with the coexisting inorganic acid, is thought to indirectly contribute to the dispersion stabilization of the dispersed particles.

The Li/Nb (molar ratio) in the sol of the present invention is preferably in the range of 0.25 to 1.5 (Li/Nb ₂ O ₅ (molar ratio) is in the range of 0.5 to 3.0). In this range, it is believed that dispersion stabilization is achieved by the majority of lithium being bound to or adsorbed onto the dispersed particles of niobic acid. The lower limit of the Li/Nb (molar ratio) is more preferably 0.3 or more. The upper limit of the Li/Nb (molar ratio) is more preferably 1.25 or less, and even more preferably 1.0.

The presence of an inorganic acid allows for an increase in particle size. It also stabilizes the sol, which is thought to be because the inorganic acid inhibits the precipitation of lithium niobate. The type of inorganic acid is preferably at least one selected from the group consisting of nitric acid, hydrochloric acid, sulfuric acid, and perchloric acid.

The content of inorganic acid in the sol of the present invention is not particularly limited, but is preferably an amount that ensures at least stability as a sol. The preferred range of inorganic acid content varies depending on the type of inorganic acid. For example, it is preferred that nitric acid has a nitric acid/Li (molar ratio) in the range of 0.40 to 0.50, hydrochloric acid has a hydrochloric acid/Li (molar ratio) in the range of 0.30 to 0.50, sulfuric acid has a sulfuric acid/Li (molar ratio) in the range of 0.15 to 0.25, and perchloric acid has a perchloric acid/Li (molar ratio) in the range of 0.45 to 0.55.

The sol of the present invention is permissible to contain ammonia. The content of ammonia in the sol of the present invention is not particularly limited, but is preferably in the range of 0 or more and less than 0.25 as NH ₃ /Nb (molar ratio). The upper limit of the above range is more preferably less than 0.2, and even more preferably less than 0.1. When ammonia is contained, the lower limit of the above range is preferably, for example, 0.005 or more, but from the viewpoint of even lower content, it is preferably 0.001 or more. When ammonia is contained, the ammonia in the dispersed particles is thought to be bound to or adsorbed to the dispersed particles at the same location as lithium in the dispersed particles.

A preferred form of the sol of the present invention is one having an average particle size of 50 nm or more. There is no particular upper limit to the average particle size as long as the sol maintains its properties without causing precipitation, but a preferred example is 300 nm. Therefore, a preferred range is 50 to 300 nm. The upper limit is more preferably 250 nm, and even more preferably 200 nm.

(Manufacturing method)
A preferred method for producing the sol of the present invention comprises the following steps 1 to 3.
(1) First step of mixing ammonium niobate sol with inorganic acid
(2) A second step of mixing the mixture obtained in the first step with lithium hydroxide.
(3) A third step in which the liquid obtained in the second step is heated and/or washed.

The ammonium niobate sol used as a raw material will now be described.
Ammonium niobate sol and its manufacturing method are described in detail in Patent Document 1, so an outline will be provided here. Ammonium niobate sol is an aqueous dispersion sol in which amorphous ammonium niobate microparticles are dispersed as colloidal particles. When the sol is dried at 100°C for 10 hours, the ammonia and niobic acid have a NH ₃ /Nb (molar ratio) of 0.25 to 0.75 (NH ₃ /Nb ₂ O ₅ (molar ratio) of 0.5 to 1.5). The manufacturing method of ammonium niobate sol involves mixing and reacting an aqueous solution of a niobium compound dissolved in hydrofluoric acid or a mixed acid of hydrofluoric acid and sulfuric acid with an aqueous ammonia solution while maintaining a pH of 8 or higher to obtain a dispersion containing ammonium niobate microparticles, which is then filtered and washed. Commercially available ammonium niobate sols include, for example, "Bailar Nb-G6000" manufactured by Taki Chemical Industry Co., Ltd.

In the first step, ammonium niobate sol and inorganic acid are mixed. There are no particular restrictions on the method for mixing the two, and they can be mixed using a conventional method. For example, inorganic acid is added to ammonium niobate sol under stirring. The mixture obtained by this mixing is usually a semi-solid with very high viscosity. The type of inorganic acid is preferably at least one selected from the group consisting of nitric acid, hydrochloric acid, sulfuric acid, and perchloric acid.

In the second step, the mixture obtained in the first step is mixed with lithium hydroxide. Usually, the viscosity of the mixture decreases when mixed with lithium hydroxide, and the mixture becomes liquid. There are no particular restrictions on the method for mixing the two, and they can be mixed using conventional methods. For example, lithium hydroxide is added to the mixture obtained in the first step while stirring.

In the third step, the liquid obtained in the second step is heated and/or washed. This treatment is preferably carried out for the purpose of removing ammonia. For example, in the sol of the present invention, this treatment is carried out until the _NH3 /Nb (molar ratio) becomes less than 0.25.

The heating conditions of temperature and time during the heat treatment can be set as appropriate, but for example, the heating temperature is preferably in the range of 50 to 150°C. The lower limit of the heating temperature is more preferably 80°C, and even more preferably 90°C. The heating time can be set as appropriate depending on the heating temperature, but is, for example, 0.5 to 8 hours. By optimizing the amount of lithium present and the heating conditions, it is possible to reduce the ammonia content in the sol of the present invention to below the detection limit. In the present invention, a content below the detection limit is considered to be 0. Ammonia is measured using the Kjeldahl method.

There are no particular restrictions on the washing method, but ultrafiltration while adding water is preferred. By optimizing the washing method and conditions, it is possible to reduce the ammonia content in the sol of the present invention to below the detection limit. Heating and washing may be performed alone, or both may be used. When using both, for example, washing may be performed after heating, or heating may be performed after washing.

The mechanism by which ammonia is removed by heating and/or washing is presumed to be as follows: In ammonium niobate sol, there are almost no ammonium ions, and it is presumed that ammonia exists bound to or adsorbed onto niobic acid particles. At least a portion of this ammonia is replaced by lithium, thereby generating free ammonia, which is volatilized during heating and discharged outside the system during washing.

After the third step, a filtration step and a concentration adjustment step may be added as needed.

The Nb concentration of the sol of the present invention is preferably in the range of 0.5 to 30% by mass, for example. From the standpoint of economics in terms of production and transportation, the lower limit is more preferably 2% by mass, even more preferably 3% by mass, and even more preferably 4% by mass. The upper limit of 30% by mass was set from the standpoint of avoiding a decrease in handleability due to high viscosity. In consideration of convenience in production and use, the upper limit is more preferably 20% by mass, even more preferably 15% by mass.

When adjusting the concentration of the sol of the present invention, it may be done by conventional methods within a range in which the sol remains stable, such as by concentrating by heating or vacuum, or by diluting with water.

The sol of the present invention is easy to handle and can be used effectively in a variety of applications. Examples include coating solutions for forming transparent thin films containing the sol of the present invention, and additives for secondary batteries and electronic materials.

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

Examples 1 to 4
As the ammonium niobate sol, "Bailar Nb-G6000" manufactured by Taki Chemical Industry Co., Ltd. (Nb=4.3 mass %, pH 8.8, NH ₃ /Nb (molar ratio)=0.6, average particle size: 15 nm) was used.
To 300 g of stirred ammonium niobate sol, an inorganic acid was added, followed by lithium hydroxide (the type of inorganic acid and the amount of each raw material added were designed to achieve the composition shown in Table 1). The resulting solution was then heated at 90°C for 3 hours. Heating was performed in an open atmosphere, and water was added as needed to adjust the concentration to the desired level. Finally, the solution was filtered to remove impurities, and the concentration was adjusted by adding an appropriate amount of water if necessary, to obtain a niobic acid sol stabilized with lithium and an inorganic acid, having the Nb concentration shown in Table 1.

Table 2 shows the physical properties of the obtained sol.
The sol was subjected to analysis in its original form, including the following analysis.
Average particle size: Measured using a dynamic light scattering particle size distribution analyzer "LB-500" manufactured by Horiba Ltd.
Haze and total light transmittance: Measured using a haze meter "COH7700" manufactured by Nippon Denshoku Industries Co., Ltd., at a wavelength of 400 to 700 nm (10 nm intervals) with an optical path length of 10 mm.

All of the sols obtained in Examples 1 to 4 had an average particle size of 50 nm or more, and after storage at 25°C for one month, no precipitate was observed, confirming their storage stability.

Claims (3)

A lithium-inorganic acid stabilized niobic acid sol containing lithium and an inorganic acid as a dispersion stabilizer and having an average particle size of 50 to 300 nm . The lithium-inorganic acid-stable niobate sol according to claim 1, wherein the inorganic acid is at least one selected from the group consisting of nitric acid, hydrochloric acid, sulfuric acid, and perchloric acid. (1) First step of mixing ammonium niobate sol with inorganic acid
(2) A second step of mixing the mixture obtained in the first step with lithium hydroxide.
(3) A method for producing a lithium-inorganic acid-stabilized niobic acid sol containing lithium and an inorganic acid as dispersion stabilizers and having an average particle size of 50 to 300 nm , comprising a third step of heating and/or washing the liquid obtained in the second step.