coating liquid

Abstract

To provide a coating liquid capable of having a high coating capacity.SOLUTION: A coating liquid is used for forming a coating film onto a front surface of a positive electrode active material, and the coating liquid contains a solvent and a solute. The solvent contains water, and the solute contains an M element. The M element contains at least one kind of phosphorus and boron selected from a group of them. A percentage content of the M element in the coating liquid is 0.05 mass% or more and 3.0 mass%. The percentage content of lithium in the coating liquid is less than 0.1 mass%, and an absorbance in a wavelength of 660nm of the coating liquid is 0.1 or less. A front surface energy of the coating liquid is 50 mN / m or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to coating fluids. [Background technology]

[0002] Patent Document 1 (JP 2022-47501 A) discloses that in order to form a uniform coating layer on the surface of a positive electrode active material, a slurry containing a positive electrode active material and a coating liquid is formed into droplets, and the slurry droplets are dried by airflow. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-47501 Summary of the Invention [Problem to be solved by the invention]

[0004] The method disclosed in Patent Document 1 has advantages over conventional rolling flow methods, such as improved processing speed. However, the method disclosed in Patent Document 1 has poor wettability between the positive electrode active material and the coating liquid, which makes it difficult to increase the coverage of the coating film.

[0005] An object of the present disclosure is to provide a coating liquid that can have high coating ability. [Means for solving the problem]

[0006] [1] A coating liquid used to form a coating film on the surface of a positive electrode active material, the coating liquid includes a solvent and a solute; The solvent contains water, the solute contains an M element, the M element includes at least one selected from the group consisting of phosphorus and boron, the content of the M element in the coating liquid is 0.05% by mass or more and 3.0% by mass or less, the lithium content in the coating liquid is less than 0.1% by mass, The coating liquid has an absorbance of 0.1 or less at a wavelength of 660 nm, The coating liquid has a surface energy of 50 mN / m or less.

[0007] The surface energy can be used to evaluate the coverage of the coating liquid. High coverage indicates that the coverage rate is likely to increase. Low surface energy of the coating liquid means that the wettability between the coating liquid and the positive electrode active material is good. If the wettability between the coating liquid and the positive electrode active material is good, when the coating liquid is applied to the surface of the positive electrode active material, the coating liquid will wet and spread evenly, resulting in a uniform coating film and improved coverage.

[0008] The coverage of the coating liquid can also be evaluated by its absorbance. A low absorbance of the coating liquid means that the concentration of fine particles contained in the coating liquid is low, i.e., there is little residual solute or insoluble components. If the concentration of fine particles contained in the coating liquid is low, when the coating liquid is applied to the surface of the positive electrode active material, the unevenness of the formed coating film is reduced, and the unevenness of the thickness is also suppressed, which is thought to improve the coverage rate.

[0009] Furthermore, the lithium content in the coating solution of the present disclosure is less than 0.1% by mass, which is thought to suppress an increase in the pH of the solution, promote dissolution of the M element in the coating solution, and reduce the surface energy.

[0010] The coating liquid of the present disclosure has a surface energy of 50 mN / m or less, an absorbance at a wavelength of 660 nm of 0.1 or less, and a lithium content of less than 0.1 mass %, and in this case, the coating liquid can have excellent coating ability.

[0011] [2] The coating liquid further contains a surfactant, The coating liquid according to [1], wherein the content of the surfactant in the coating liquid is 0.01% by mass or more and 1.5% by mass or less.

[0012] [3] The coating liquid according to [1] or [2], wherein the surface energy of the coating liquid is 15 mN / m or more and 30 mN / m or less.

[0013] [4] A coating liquid used to form a coating film on the surface of a positive electrode active material, the coating liquid comprises a solvent, a solute, and a surfactant; The solvent contains water, the solute contains an M element, the M element includes at least one selected from the group consisting of phosphorus and boron, the content of the M element in the coating liquid is 1.5% by mass or more and 2.6% by mass or less, the lithium content in the coating liquid is less than 0.1% by mass, The coating liquid has an absorbance of 0.1 or less at a wavelength of 660 nm, the surface energy of the coating liquid is 15 mN / m or more and 30 mN / m or less; The coating liquid has a surfactant content of 0.1% by mass or more and 1.0% by mass or less. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a graph showing the relationship between the surface energy value and the coverage rate for Nos. 1 to 3. [Figure 2] FIG. 2 is a graph showing the relationship between the surface energy value and the coverage rate for Nos. 4 to 10. [Figure 3] FIG. 3 is a graph showing the relationship between the surface energy value and the coverage rate for Nos. 11 and 12. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the present disclosure (hereinafter may be abbreviated as "the present embodiment") and an example of the present disclosure (hereinafter may be abbreviated as "the present example") will be described. However, the present embodiment and the example do not limit the technical scope of the present disclosure.

[0016] <Coating liquid> The coating liquid includes a solvent and a solute. The solvent includes water. The solute includes an M element. The M element includes at least one element selected from the group consisting of phosphorus (P) and boron (B). The coating liquid may further include, for example, a suspended solid (insoluble component), a precipitate, etc. Note that the "solute" in this disclosure does not include a surfactant, which will be described later.

[0017] "solvent" The solvent contains water. Preferably, the solvent contains water as a main component. The solvent may consist of water. The solvent may contain, for example, ion-exchanged water. Preferably, the solvent contains, for example, ion-exchanged water as a main component. The solvent may consist of, for example, ion-exchanged water. The solvent may contain any component as long as the solute dissolves in it. The solvent may contain, for example, alcohol, etc. The solvent may contain, for example, ethanol, etc. Note that "containing water as a main component" means that the water content in the solvent exceeds 50 mass%.

[0018] The water content in the coating liquid is preferably 90% by mass or more and 97% by mass or less. If the water content in the coating liquid is less than 90% by mass, it may be difficult to coat the positive electrode active material particles with the coating liquid. If the water content in the coating liquid is more than 97% by mass, the solute concentration in the coating liquid may be low, and the coverage of the coating film may not be improved. The water content in the coating liquid may be, for example, 91% by mass or more, or 93% by mass or more. The solvent content in the coating liquid may be, for example, 96% by mass or less, or 95% by mass or less.

[0019] The water content in the coating liquid is measured by volumetric titration using a Karl Fischer moisture meter. For example, a Karl Fischer moisture meter (product name MKA-610) manufactured by Kyoto Electronics Manufacturing Co., Ltd. is prepared. A titrant and a solvent are prepared to measure the water content. For example, the titrant may be "Composite 5K" (manufactured by Honeywell) and the solvent may be "Medium K" (manufactured by Honeywell). The solvent in the titration flask is anhydrous with the titrant, and the coating liquid is then added to measure the water content.

[0020] 《Solute》 P is contained as a phosphorus compound. Examples of phosphorus compounds include phosphorus compounds. The phosphorus compound may contain at least one selected from the group consisting of phosphoric anhydride (PO), orthophosphoric acid, pyrophosphoric acid, metaphosphoric acid, polyphosphoric acid, and their sodium salts, lithium salts, and potassium salts. Preferred phosphorus compounds are metaphosphoric acid, polyphosphoric acid, and their sodium salts, lithium salts, and potassium salts. Metaphosphoric acid and polyphosphoric acid may have longer molecular chains than other phosphorus compounds. When the solute contains at least one of metaphosphoric acid and polyphosphoric acid, it is expected that a continuous coating film will be easily formed. This is expected to result in, for example, an improved coverage.

[0021] B is included as a boron compound. Examples of the boron compound include boric acid compounds. The boric acid compound may include, for example, at least one selected from the group consisting of orthoboric acid, metaboric acid, and tetraboric acid.

[0022] The solute preferably contains P and B. When the solute contains P and B, a further reduction in surface energy is expected.

[0023] The content of the M element in the coating liquid is 0.05% by mass or more and 3.0% by mass or less. If the content of the M element in the coating liquid is less than 0.05% by mass, the concentration of the M element in the coating liquid is low, and the coverage of the coating film may not be improved. If the content of the M element in the coating liquid is more than 3.0% by mass, the M element may not be completely dissolved in the coating liquid. The content of the M element in the coating liquid may be, for example, 0.2% by mass or more, 0.5% by mass or more, 1.0% by mass or more, or 1.5% by mass or more. The content of the M element in the coating liquid may be 2.9% by mass or less, 2.8% by mass or less, 2.7% by mass or less, or 2.6% by mass or less.

[0024] The lithium (Li) content in the coating solution is less than 0.1% by mass. If the Li content is 0.1% by mass or more, the pH of the solution may increase, which may prevent the M element from being completely dissolved in the coating solution. Note that, as long as the lithium (Li) content in the coating solution can be less than 0.1% by mass, the solute may contain a lithium compound. The lithium compound may include, for example, lithium hydroxide, lithium nitrate, lithium carbonate, etc.

[0025] The concentration of solutes in a coating solution is measured using the following procedure: 0.1 g of coating solution is mixed with pure water and hydrochloric acid and heated. After cooling, hydrogen peroxide is added and the mixture is heated. After cooling, the volume is adjusted to 100 mL to prepare the sample solution. An inductively coupled plasma atomic emission spectroscopy (ICP-AES) device is prepared. For example, the concentration of solutes in a coating solution is measured using an ICP-AES device (product name CP-720) manufactured by Agilent Technologies.

[0026] <Surfactants> The coating liquid may further contain a surfactant. By including a surfactant in the coating liquid, an improvement in coverage is expected. The surfactant may be an insoluble component. The surfactant may be a soluble component (solute). The surfactant may, for example, be water-soluble. The dissolved surfactant may improve coverage. The surfactant may be cationic, anionic, amphoteric, or nonionic. The nonionic surfactant may improve coverage. The surfactant may contain, for example, at least one selected from the group consisting of polyether-modified silicone and polyethylene glycol alkyl ether. Examples of polyether-modified silicone include "Product Name: Silsurf C208 (manufactured by SILTECH)" and "Product Name: KF-945 (manufactured by Shin-Etsu Chemical Co., Ltd.)." Examples of polyethylene glycol alkyl ether include polyethylene glycol monolauryl ether (manufactured by Tokyo Chemical Industry Co., Ltd.).

[0027] The surfactant content in the coating liquid is preferably 0.01% by mass or more and 1.5% by mass or less. If the surfactant content in the coating liquid is less than 0.01% by mass, the effect of improving the coverage due to the inclusion of the surfactant may not be obtained. If the surfactant content in the coating liquid is more than 1.5% by mass, precipitation may occur or the amount of insoluble components may increase. The surfactant content in the coating liquid may be, for example, 0.1% by mass or more, 0.2% by mass or more, or 0.5% by mass or more. The surfactant content in the coating liquid may be, for example, 1.4% by mass or less, 1.2% by mass or less, or 1.0% by mass or less.

[0028] 《Absorbance》 The absorbance of the coating solution at a wavelength of 660 nm is 0.1 or less. Here, the absorbance at a wavelength of 660 nm indicates the intensity of scattered light caused by the fine particles present in the coating solution. A higher absorbance value indicates a higher concentration of fine particles present in the coating solution, i.e., the presence of residual solute or a large amount of insoluble components (see, for example, JIS-K0101). When a coating solution with a high concentration of fine particles is applied to a positive electrode active material, the fine particles adhere to the surface of the positive electrode active material, causing the coating film to become uneven and the thickness of the coating film to become non-uniform. This non-uniformity in the thickness of the coating film results in areas where the coating film is insufficiently thick or is not coated, leading to a decrease in coverage.

[0029] When the absorbance of the coating liquid at a wavelength of 660 nm is 0.1 or less, the concentration of fine particles in the coating liquid is low, the thickness of the coating film becomes uniform, and an improvement in the coverage rate is expected. The absorbance of the coating liquid at a wavelength of 660 nm may be 0.

[0030] The absorbance value is measured using the following procedure. 3.5 mL of coating liquid is placed in a quartz cell (10 mm x 10 mm x 45 mm). The absorbance of the coating liquid is measured using a UV-Vis spectrophotometer. For example, a Shimadzu UV-Vis spectrophotometer (product name: UV-1280) is prepared. The temperature of the coating liquid during measurement is 25°C. The lower limit of absorbance is 0.000. However, the detection limit of the UV-Vis spectrophotometer used for absorbance measurement is 0.001. The zero point of absorbance can be determined by measuring the absorbance using ultrapure water at 25°C with an electrical conductivity of 17 MΩ·cm or higher in the measurement cell.

[0031] Surface Energy The surface energy of the coating liquid is 50 mN / m or less. Here, surface energy is synonymous with surface tension. By reducing the surface energy of the coating liquid, the wettability with the positive electrode active material improves, and when the coating liquid is applied to the surface of the positive electrode active material, it spreads evenly. As a result, the formed coating film becomes more uniform, and areas with insufficient coating film thickness or uncoated areas are reduced, suppressing a decrease in coverage rate.

[0032] When the surface energy of the coating liquid is 50 mN / m or less, the thickness of the coating film becomes uniform, and it is thought that an improvement in coverage rate can be expected. The surface energy of the coating liquid may be 45 mN / m or less, or may be 41 mN / m or less. The surface energy of the coating liquid is preferably 30 mN / m or less, and more preferably 26 mN / m or less. The surface energy of the coating liquid is more preferably 20 mN / m or less.

[0033] On the other hand, if the surface energy of the coating solution is too small compared to the surface energy of the positive electrode active material, the coating solution tends to peel off easily from the surface of the positive electrode active material. As a result, the formed coating film may become non-uniform and the coverage may decrease. Therefore, the surface energy is preferably 10 mN / m or more.

[0034] The surface energy value is measured using a contact angle meter by the pendant drop method. For example, a contact angle meter "DMo-502" manufactured by Kyowa Interface Science Co., Ltd. is prepared. Diiodomethane and n-hexadecane are prepared as probe liquids. The coating liquid is extruded from the capillary tube into the probe liquid, and image capture begins 1000 ms later. Images are then captured 300 times at 1000 ms intervals. The captured images are read into the included multi-functional integrated analysis software, FAMAS, and analyzed based on the Young-Laplace method to calculate the surface energy. The temperature of the coating liquid during measurement is 25°C.

[0035] 《Application》 The coating liquid of the present disclosure is used to form a coating film on the surface of a positive electrode active material. The positive electrode active material may consist of a single particle or may contain two or more particles. The positive electrode active material may have an average particle size (D50) of, for example, 1 μm or more and 30 μm or less. "D50" refers to the particle size at which the cumulative total is 50% in a volume-based particle size distribution (cumulative distribution). The particle size distribution can be measured by a laser diffraction method.

[0036] The positive electrode active material may contain any component. The positive electrode active material may contain, for example, at least one selected from the group consisting of Li(NiCoMn)O2 and Li(NiCoAl)O2. For example, "(NiCoMn)" in "Li(NiCoMn)O2" indicates that the sum of the composition ratios in the parentheses is 1. Li(NiCoMn)O2 is, for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 Li(NiCoAl)O2 may contain, for example, LiNi 0.8 Co 0.15 Al 0.05 It may contain O2 etc.

[0037] <Method of manufacturing the coating liquid> The method for producing the coating liquid of the present disclosure is not particularly limited. For example, the coating liquid is produced by dissolving a solute in a solvent. When the solute contains both P and B as the M element, they may be mixed into the solvent simultaneously or separately. Furthermore, when a surfactant is added, they may be mixed into the solvent simultaneously or separately. [Example]

[0038] <Sample preparation> Coating solutions and coated positive electrode active materials according to Nos. 1 to 12 were produced as follows. Hereinafter, for example, "coating solution according to No. 1" may be abbreviated as "No. 1."

[0039] No. 1 A coating liquid was prepared by dissolving 6.0 g of metaphosphoric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) in 92.14 g of ion-exchanged water.

[0040] As the positive electrode active material, NCM (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2) was prepared. A slurry (solid content: 69 mass%) was prepared by dispersing the powder of the positive electrode active material in the coating liquid. A spray dryer manufactured by BUCHI, product name Mini Spray Dryer B-290, was prepared. The slurry was supplied to the spray dryer to produce a powder of the coated positive electrode active material. The supply air temperature of the spray dryer was 200°C, and the supply air volume was 0.45 m 3 The coated positive electrode active material was heat-treated in air at a temperature of 200° C. for 5 hours.

[0041] No.2 A coating solution was prepared by dissolving polyethylene glycol monolauryl ether (manufactured by Tokyo Chemical Industry Co., Ltd.) as a polyethylene glycol alkyl ether in the coating solution No. 1 to a concentration of 1.0 mass %. Thereafter, a coated positive electrode active material was prepared in the same manner as in No. 1.

[0042] No.3 A coating solution was prepared by dissolving polyether-modified silicone (KF-945, manufactured by Shin-Etsu Chemical Co., Ltd.) in the coating solution No. 1 to a concentration of 1.0 mass %. Then, a coated positive electrode active material was prepared in the same manner as in No. 1.

[0043] No.4 A phosphoric acid solution was prepared by dissolving 2.15 g of metaphosphoric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) in 66.05 g of ion-exchanged water. Boric acid (manufactured by Nacalai Tesque, Inc.) was then dissolved in the phosphoric acid solution so that the molar ratio of P to B was 1:1, thereby preparing a coating solution. A coated positive electrode active material was then prepared in the same manner as in No. 1.

[0044] No.5-6 Coating solution No. 5 was prepared by dissolving polyethylene glycol alkyl ether in coating solution No. 4 to a concentration of 0.5 mass %, and coating solution No. 6 was prepared by dissolving polyethylene glycol alkyl ether in coating solution No. 4 to a concentration of 1.0 mass %. Then, each coated positive electrode active material was prepared in the same manner as No. 1.

[0045] No.7-10 Coating liquid No. 4 was prepared by dissolving polyether-modified silicone in a concentration of 0.01% by mass, coating liquid No. 7 was prepared by dissolving polyether-modified silicone in a concentration of 0.1% by mass, coating liquid No. 8 was prepared by dissolving polyether-modified silicone in a concentration of 0.5% by mass, and coating liquid No. 9 was prepared by dissolving polyether-modified silicone in a concentration of 1.0% by mass to produce coating liquid No. 10. Each coated positive electrode active material was then produced in the same manner as No. 1.

[0046] No. 11 A phosphoric acid solution was prepared by dissolving 6.0 g of metaphosphoric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) in 109.0 g of ion-exchanged water. Boric acid (manufactured by Nacalai Tesque, Inc.) was then dissolved in the phosphoric acid solution so that the molar ratio of P to B was 1:1, thereby preparing a coating solution. A coated positive electrode active material was then prepared in the same manner as in No. 1.

[0047] No.12 A coating solution was prepared by dissolving polyethylene glycol alkyl ether in the coating solution No. 11 so that the concentration was 0.5 mass %. Thereafter, a coated positive electrode active material was prepared in the same manner as in No. 1.

[0048] <Evaluation> <Coating liquid> (moisture content) The amount of water contained in the coating solution was measured by volumetric titration using a Karl Fischer moisture meter, and the results are shown in Table 1.

[0049] (M element) The concentration (mass%) of the M element contained in the coating solution was measured by ICP-AES. The results are shown in Table 1. The concentration of Li contained in each coating solution was also measured, but it could not be measured because it was not present or was below the detection limit.

[0050] (absorbance) The absorbance of the coating solution was measured using a UV-visible spectrophotometer, and the results are shown in Table 1.

[0051] (surface energy) The surface energy of the coating solution was measured using a contact angle meter by the pendant drop method, and the results are shown in Table 1.

[0052] <Coated Positive Electrode Active Material> (coverage rate) The coverage of the coated positive electrode active material was measured by X-ray photoelectron spectroscopy (XPS). An XPS instrument manufactured by ULVAC-PHI, Inc., called "PHI X-tool," was prepared. A sample powder of the coated positive electrode active material was placed in the XPS instrument. Narrow scan analysis was performed with a pass energy of 224 eV. The measurement data was processed using analysis software manufactured by ULVAC-PHI, Inc., called "MulTiPak."

[0053] By analyzing the measurement data, the ratio of each element (element concentration) was calculated from the peak areas of Li1s, B1s, P2p, Ni2p3, Co2p3, and Mn2p3. The coverage was calculated using the following formula (1).

[0054] θ=M / (M+Ni+Co+Mn)×100...(1) In the above formula (1), θ represents the coverage (%). M, Ni, Co, and Mn represent the ratio of each element. M represents P in the coated positive electrode active materials Nos. 1 to 3, and P and B in the coated positive electrode active materials Nos. 4 to 12.

[0055] The coverage rates are shown in Table 1 and Figures 1 to 3 as relative values ​​with Nos. 1, 4, and 11, which do not contain surfactant, as the reference values. That is, the values ​​of Nos. 2 and 3 are relative values ​​with No. 1 as the reference value, the values ​​of Nos. 5 to 10 are relative values ​​with No. 4 as the reference value, and the value of No. 12 is relative values ​​with No. 11 as the reference value.

[0056] [Table 1]

[0057] <Result> 1 to 3, it is believed that a sufficient coverage rate for the positive electrode active material can be achieved by using the coating liquids according to Nos. 2, 3, 5, 7 to 10, and 12. Furthermore, it is believed that the inclusion of a surfactant reduces the surface energy, resulting in a higher coverage rate for the positive electrode active material.

[0058] The present embodiment and examples are illustrative in all respects. The present embodiment and examples are not limiting. The technical scope of the present disclosure encompasses all modifications within the meaning and scope equivalent to the claims. For example, it is also intended from the beginning that any configuration may be extracted from the present embodiment and examples and that they may be combined in any desired manner.

Claims

[Claim 1] A coating liquid used to form a coating film on the surface of a positive electrode active material, the coating liquid comprises a solvent, a solute, and a surfactant; The solvent contains water, the solute contains an M element, the M element includes phosphorus and boron; The surfactant is nonionic, the water content in the coating liquid is 90% by mass or more and 97% by mass or less, the content of the M element in the coating liquid is 1.5 mass % or more and 2.6 mass % or less, the lithium content in the coating liquid is less than 0.1% by mass, the coating liquid has an absorbance of 0.1 or less at a wavelength of 660 nm; the surface energy of the coating liquid is 10 mN / m or more and 30 mN / m or less; The coating liquid has a surfactant content of 0.1% by mass or more and 1.0% by mass or less.

Images