Positive electrode active material particles, method for manufacturing positive electrode active material particles, positive electrode, and all-solid-state battery

Abstract

Provided are positive electrode active material particles with which it is possible to achieve both long life and low resistance. Each positive electrode active material particle comprises a core particle and a coating layer that coats the surface of the core particle. The core particle is formed of a first composite oxide. The first composite oxide contains lithium and at least one element selected from the group consisting of manganese, cobalt, nickel, aluminum, iron, and phosphorus. The coating layer is formed of a second composite oxide. The second composite oxide contains lithium and at least one element selected from the group consisting of niobium, boron, phosphorus, zirconium, titanium, aluminum, lanthanum, and tungsten. The coverage of the coating layer with respect to the core particles as calculated by observing the positive electrode active material particles by using a scanning electron microscope is 90.0 area% or more. The average thickness of the coating layer is 15.0 nm or less, and the maximum thickness of the coating layer is 15.0 nm or less.

Description

Positive electrode active material particles, method for manufacturing positive electrode active material particles, positive electrode, and all-solid-state battery

[0001] This disclosure relates to positive electrode active material particles that can be used in secondary batteries, a method for producing positive electrode active material particles, a positive electrode, and an all-solid-state battery.

[0002] Lithium-ion batteries are secondary batteries with excellent energy density and cycle characteristics. In recent years, all-solid-state batteries, which use a solid instead of a conventional organic solvent as the electrolyte, have been developed and are expected to achieve higher safety than conventional batteries. In particular, sulfide-based solid electrolytes containing sulfur have achieved high conductivity, but a challenge remains in that a high-resistance layer is formed at the interface due to a side reaction between the positive electrode active material and the sulfide-based solid electrolyte, which reduces the lifespan. To address these challenges, Patent Document 1 discloses a measure to suppress the side reaction with the sulfide solid electrolyte by providing a coating layer on the positive electrode active material.

[0003] Japanese Patent Publication No. 2010-170715

[0004] However, the inventors have recognized that the coating state of the coating layer significantly affects the performance of the battery. Specifically, if the coating layer has a low coverage rate, the effect of suppressing side reactions between the positive electrode active material and the solid electrolyte is reduced, which may decrease the battery life. On the other hand, if the coating layer is thick, the resistance increases, which may decrease the conductivity of lithium ions.

[0005] This disclosure provides positive electrode active material particles that can achieve both long lifespan and low resistance by having a coating layer. This disclosure also provides a method for manufacturing the above positive electrode active material particles. Furthermore, this disclosure provides a positive electrode and an all-solid-state battery using the above positive electrode active material particles.

[0006] This disclosure relates to positive electrode active material particles having core particles and a coating layer covering the surface of the core particles, wherein the core particles are formed of a first composite oxide, the first composite oxide comprises lithium and at least one selected from the group consisting of manganese, cobalt, nickel, aluminum, iron, and phosphorus, the coating layer is formed of a second composite oxide, the second composite oxide comprises lithium and at least one selected from the group consisting of niobium, boron, phosphorus, zirconium, titanium, aluminum, lanthanum, and tungsten, and when the backscattered electron image obtained by observing the positive electrode active material particles with a scanning electron microscope at 50,000x magnification is binarized into the portion corresponding to the coating layer and the portion corresponding to the core particles, the ratio of the area of ​​the portion corresponding to the coating layer to the sum of the areas of the portion corresponding to the coating layer and the portion corresponding to the core particles (area %) is defined as the coverage rate of the coating layer over the core particles, and the coverage rate is 90.0 area % or more. This invention relates to positive electrode active material particles in which the average thickness of the coating layer is 15.0 nm or less, and the maximum thickness of the coating layer is 15.0 nm or less.

[0007] This disclosure relates to a method for producing positive electrode active material particles, the method comprising: (i) mixing an alkoxide monomer and / or its oligomer with an organic solvent to obtain a mixture 1; (ii) mixing the mixture 1 with the core particles to obtain a mixture 2; and (iii) mixing the mixture 2 with a hydrolyzing agent to hydrolyze the alkoxide monomer and / or its oligomer contained in the mixture 2, dehydrate and condense it, precipitate it on the surface of the core particles to form the coating layer, and obtain the positive electrode active material particles, wherein the alkoxide monomer and / or its oligomer comprises lithium and at least one selected from the group consisting of niobium, boron, phosphorus, zirconium, titanium, aluminum, lanthanum, and tungsten.

[0008] This disclosure relates to a positive electrode for an all-solid-state battery, wherein the positive electrode is a molded body of a positive electrode composite containing positive electrode active material particles, and the positive electrode active material particles are the positive electrode active material particles described above.

[0009] This disclosure relates to an all-solid-state battery, the all-solid-state battery having at least a positive electrode layer, a solid electrolyte layer and a negative electrode layer in that order, wherein the positive electrode layer contains the positive electrode active material particles.

[0010] This disclosure provides positive electrode active material particles that can achieve both long lifespan and low resistance. Furthermore, this disclosure provides a method for manufacturing the above positive electrode active material particles. Finally, this disclosure provides a positive electrode and an all-solid-state battery using the above positive electrode active material particles.

[0011] Figure 1 shows an example of a cross-sectional image and line analysis of positive electrode active material particles. Figure 2 shows an example of a profile obtained by line analysis of elements contained in the coating layer.

[0012] In this disclosure, descriptions of numerical ranges such as "XX or greater and YY or less" or "XX to YY" mean a numerical range that includes the lower and upper limits, unless otherwise specified. When numerical ranges are described in steps, the upper and lower limits of each numerical range can be any combination. In addition, in this disclosure, a description such as "at least one selected from the group consisting of XX, YY, and ZZ" means any of the following: XX, YY, ZZ, a combination of XX and YY, a combination of XX and ZZ, a combination of YY and ZZ, or a combination of XX, YY, and ZZ. Note that if XX is a group, multiple values ​​may be selected from XX, and the same applies to YY and ZZ.

[0013] The embodiments of this disclosure will be described in more detail below, but this disclosure is not limited to these embodiments.

[0014] This disclosure relates to positive electrode active material particles having core particles and a coating layer covering the surface of the core particles, wherein the core particles are formed of a first composite oxide, the first composite oxide comprises lithium and at least one selected from the group consisting of manganese, cobalt, nickel, aluminum, iron, and phosphorus, the coating layer is formed of a second composite oxide, the second composite oxide comprises lithium and at least one selected from the group consisting of niobium, boron, phosphorus, zirconium, titanium, aluminum, lanthanum, and tungsten, and when the backscattered electron image obtained by observing the positive electrode active material particles with a scanning electron microscope at 50,000x magnification is binarized into the portion corresponding to the coating layer and the portion corresponding to the core particles, the ratio of the area of ​​the portion corresponding to the coating layer to the sum of the areas of the portion corresponding to the coating layer and the portion corresponding to the core particles (area %) is defined as the coverage rate of the coating layer over the core particles, and the coverage rate is 90.0 area % or more. This invention relates to positive electrode active material particles in which the average thickness of the coating layer is 15.0 nm or less, and the maximum thickness of the coating layer is 15.0 nm or less.

[0015] The inventors believe that the effects of this disclosure can be obtained by satisfying the above conditions as follows: As mentioned above, the coating state of the coating layer can have a significant impact on the performance of the battery. If the coating layer has a low coverage rate, the effect of suppressing side reactions between the positive electrode active material and the solid electrolyte is low, which may reduce the battery life. On the other hand, if the thickness of the coating layer is high, the resistance increases, which may reduce the conductivity of lithium ions.

[0016] Patent Document 1 discloses a positive electrode active material having a reaction suppression portion with a coverage rate of 90% and an average thickness of 9.5 nm in an example. However, when the present inventors examined the positive electrode active material described in Patent Document 1, they found that the resistance value remained high even when a coating layer with a high coverage rate was provided. It is thought that even if the average thickness is controlled, areas with a thick coating layer remain, resulting in high resistance. The present inventors conducted further investigations and found that in order to achieve both a long lifespan and low resistance of positive electrode active material particles, it is not sufficient to control only the coverage rate and average thickness of the coating layer, but it is also important to control the maximum thickness.

[0017] This disclosure relates to positive electrode active material particles having core particles and a coating layer covering the surface of the core particles. The positive electrode active material particles are observed using a scanning electron microscope (SEM), and in a 50,000x backscattered electron image obtained, the portion corresponding to the coating layer and the portion corresponding to the core particles are binarized. The ratio of the area of ​​the portion corresponding to the coating layer to the sum of the areas of the portions corresponding to the coating layer and the core particles (area %) is defined as the coverage rate of the coating layer over the core particles. In this case, it is necessary that the coverage rate of the coating layer over the core particles is 90.0 area % or more, the average thickness of the coating layer is 15.0 nm or less, and the maximum thickness of the coating layer is 15.0 nm or less.

[0018] By providing a coating layer with a coverage rate of 90.0 area or more over the core particles of the coating layer, side reactions at the contact interface between the positive electrode active material and the solid electrolyte can be suppressed. As a result, the capacity retention rate after repeated charge-discharge tests is high, and a longer lifespan can be achieved. Furthermore, by controlling the average thickness of the coating layer to 15.0 nm or less, and the maximum thickness to 15.0 nm or less, low resistance can be maintained, thereby suppressing a decrease in lithium ion conductivity.

[0019] If the coverage rate of the coating layer over the core particles is less than 90.0 area%, the area of ​​direct contact between the core particles and the solid electrolyte increases, leading to the formation of a resistive layer due to side reactions at the interface, and shortening the lifespan. From the viewpoint of increasing the capacity retention rate after repeated charge-discharge tests and extending the lifespan, the coverage rate of the coating layer over the core particles is preferably 92.0 area% or more, more preferably 93.0 area% or more, and even more preferably 94.0 area% or more.

[0020] A higher coverage rate is preferable, and there is no particular upper limit, but preferred coverage rates include 100.0 area% or less, 99.0 area% or less, 98.0 area% or less, and 97.0 area% or less. Preferred coverage rates are, for example, 90.0 to 100.0 area%, 92.0 to 100.0 area%, 93.0 to 99.0 area%, 94.0 to 98.0 area%, and 94.0 to 97.0 area%.

[0021] The coverage rate of the coating layer can be determined by the manufacturing method used to coat the core particles, the method of preparing the coating material, specifically the amount of coating material added, the type of solvent used, etc. The coverage rate can be easily increased by employing the manufacturing methods described in steps (i) to (iii) below, or by increasing the amount of coating material added.

[0022] Furthermore, if the average thickness of the coating layer is greater than 15.0 nm, the coating layer will increase the resistance. Also, even if the average thickness of the coating layer can be controlled to be 15.0 nm or less, if the maximum thickness of the coating layer is greater than 15.0 nm, the resistance value will increase, making it difficult to achieve both long lifespan and low resistance. This is thought to be because even if the average thickness is controlled to be low, the resistance will increase in areas where the coating layer is thicker.

[0023] Therefore, the average thickness of the coating layer is 15.0 nm or less, and the maximum thickness of the coating layer is 15.0 nm or less. From the viewpoint of making it easier to achieve low resistance, the average thickness is preferably 10.0 nm or less, and more preferably 9.0 nm or less. The average thickness of the coating layer is, for example, 5.0 to 15.0 nm, preferably 6.0 to 10.0 nm, and more preferably 7.5 to 9.0 nm.

[0024] Furthermore, from the viewpoint of making it easier to achieve low resistance, the maximum thickness of the coating layer is preferably 12.0 nm or less, more preferably 11.0 nm or less, and even more preferably 9.8 nm or less. The maximum thickness of the coating layer is, for example, 6.0 to 15.0 nm, preferably 7.0 to 12.0 nm, more preferably 7.5 to 11.0 nm, and even more preferably 7.5 to 9.8 nm.

[0025] The average and maximum thickness of the coating layer can be controlled by the manufacturing method used to coat the core particles, the method of preparing the coating material, specifically the amount of coating material added, the type of solvent, etc. As a means of controlling the average and maximum thickness of the coating layer to 15.0 nm or less, the manufacturing methods described in steps (i) to (iii) below are recommended. In coating methods using a rolling fluidized bed, as in Patent Document 1, even if the average thickness can be controlled to 15.0 nm or less, the maximum thickness of the coating layer may exceed 15.0 nm.

[0026] As described above, by setting the coating layer on the positive electrode active material particles to 90.0 area or more, and controlling both the average and maximum thickness of the coating layer to 15.0 nm or less, it is possible to achieve both a longer lifespan and lower resistance for the positive electrode active material particles.

[0027] Furthermore, the difference between the maximum thickness and the minimum thickness of the coating layer (maximum thickness - minimum thickness) is, for example, 6.5 nm or less, preferably 5.0 nm or less, more preferably 4.2 nm or less, even more preferably 2.5 nm or less, and even more preferably 1.5 nm or less. The lower limit is not particularly limited as a smaller value is preferable, and examples include 0 nm or more and 0.1 nm or more.

[0028] By keeping the difference between the maximum and minimum thickness of the coating layer to 5.0 nm or less, it is possible to suppress non-uniformity in the lithium ion conductivity path between relatively thick and thin areas of the coating layer, thereby making it easier to achieve lower resistance. The difference between the maximum and minimum thickness of the coating layer can be controlled by the manufacturing method when coating the core particles, the method of adjusting the coating material, specifically the amount of coating material added, the type of solvent, etc.

[0029] The content of the coating layer with respect to 100 parts by mass of the core particles is, for example, 1.20 parts by mass or less, preferably 1.00 parts by mass or less, more preferably 0.90 parts by mass or less, still more preferably 0.70 parts by mass or less, and even more preferably 0.50 parts by mass or less. When the ratio of the coating layer to the core particles is 1.00 parts by mass or less, the ratio of the coating layer to the core particles can be reduced, making it easier to lower the resistance, and thus it becomes easier to maintain a high lithium ion conductivity.

[0030] The content of the coating layer with respect to 100 parts by mass of the core particles is, for example, 0.10 to 1.20 parts by mass, preferably 0.20 to 1.00 parts by mass, more preferably 0.20 to 0.90 parts by mass, still more preferably 0.20 to 0.70 parts by mass, and even more preferably 0.25 to 0.50 parts by mass. The ratio of the coating layer to the core particles can be adjusted by the addition amount of the coating material when coating the core particles.

[0031] Next, the core particles formed of the first composite oxide in the positive electrode active material particles will be described. The core particles formed of the first composite oxide contain lithium and at least one selected from the group consisting of manganese, cobalt, nickel, aluminum, iron, and phosphorus. Preferably, the core particles contain at least one selected from the group consisting of manganese, cobalt, and nickel and lithium.

[0032] The core particles formed of the first composite oxide are, for example, LiCoO ２ , ２ , ４ , ４ , ２ ,

[0032] , ２ , ２ , ２ , LiNiO ２ , LiMnO ２ , LiMn ２ O ４ , Li(NiCoMn)O ２ , Li(NiCoAl)O ２ and LiFePO ４ and the like, and at least one selected from the group consisting thereof can be mentioned. Note that "Li(NiCoMn)O ２ " and "Li(NiCoAl)O ２In the above, "(NiCoMn)" and "(NiCoAl)" indicate that the sum of the composition ratios in parentheses is 1. As long as the sum is 1, the amounts of individual components are arbitrary. Li(NiCoMn)O ２ For example, Li(Ni １／３ Co １／３ Mn １／３ ) O ２ , Li(Ni ０．５ Co ０．２ Mn ０．３ ) O ２ , Li(Ni ０．８ Co ０．１ Mn ０．１ ) O ２ It may include the following:

[0033] The core particle is Li(NiCoMn)O ２ Li(NiCoMn)O is preferred. ２ Commercially available products can be used, for example, LiNi ０．８ Co ０．１ Mn ０．１ O ２ (Example: PO5038 (product name) manufactured by MSE Supplies)

[0034] The coating layer is formed of a second composite oxide. The second composite oxide contains lithium and also contains at least one selected from the group consisting of niobium, boron, phosphorus, zirconium, titanium, aluminum, lanthanum, and tungsten. For example, from the viewpoint of reducing resistance, the second composite oxide is preferably a composite oxide containing lithium and niobium. The second composite oxide may contain, for example, lithium niobate.

[0035] A preferred method for producing positive electrode active material particles is described below. The method for producing positive electrode active material particles includes, for example, the steps of: (i) mixing an alkoxide monomer and / or its oligomer with an organic solvent to obtain a mixture 1; (ii) mixing the mixture 1 with core particles formed from the first composite oxide to obtain a mixture 2; and (iii) mixing the mixture 2 with a hydrolyzing agent to hydrolyze the alkoxide monomer and / or its oligomer contained in the mixture 2, dehydrate and condense it, and precipitate it on the surface of the core particles to form the coating layer and obtain the positive electrode active material particles. The alkoxide monomer and / or its oligomer comprises lithium and at least one selected from the group consisting of niobium, boron, phosphorus, zirconium, titanium, aluminum, lanthanum, and tungsten.

[0036] In the above manufacturing method, after obtaining mixture 2 in step (ii), a hydrolyzing agent is added in step (iii) to initiate hydrolysis. This manufacturing method allows the coating reaction to begin from a state where the core particles and the alkoxide monomers and / or their oligomers, which are the materials for the coating layer, are uniformly dispersed. Therefore, it becomes easier to obtain positive electrode active material particles with high coating density, minimal thickness variations in the coating layer, and small differences between the maximum thickness and the minimum thickness of the coating layer.

[0037] The alkoxide monomer and / or its oligomer is preferably an alkoxide monomer. Step (i) is preferably a mixture of lithium alkoxide, at least one alkoxide selected from the group consisting of niobium, boron, phosphorus, zirconium, titanium, aluminum, lanthanum, and tungsten, and an organic solvent. The number of carbon atoms in the alkoxy group of the alkoxide is, for example, 1 to 6, preferably 1 to 3, and more preferably 1 or 2.

[0038] At least one element selected from the group consisting of niobium, boron, phosphorus, zirconium, titanium, aluminum, lanthanum, and tungsten is preferably niobium, and more preferably niobium. The organic solvent is not particularly limited, and known solvents such as 2-propanol, toluene, THF, methanol, ethanol, and 1-butanol can be used. The purity of the organic solvent is preferably 99.9% or higher, and more preferably 99.99% or higher.

[0039] In step (i), the amount of lithium alkoxide and / or its oligomer is, for example, 50.0 to 500.0 mg or 100.0 to 350.0 mg per 100 ml of organic solvent. In addition, in step (i), the amount of at least one alkoxide and / or its oligomer selected from the group consisting of niobium, boron, phosphorus, zirconium, titanium, aluminum, lanthanum, and tungsten is, for example, 0.10 to 2.00 ml or 0.50 to 1.80 ml per 100 ml of organic solvent, and in terms of mass, for example, 0.13 to 2.60 g or 0.65 to 2.30 g.

[0040] Furthermore, when using multiple alkoxide monomers with different reaction rates, reflux may be added in the process of obtaining the mixture 1 in order to improve the uniformity of the composition of the coating layer. Additionally, a chelating material may be added in order to control the reaction rate of the alkoxide and obtain a uniform coating layer. There are no particular restrictions on the chelating material used, but examples include acetylacetone.

[0041] In step (ii), mixture 1 and core particles formed from the first composite oxide are mixed to obtain mixture 2. The core particles can be those described above. From the viewpoint of material dispersion, it is preferable to mix mixture 1 and core particles while continuing to stir. The amounts of mixture 1 and core particles should be controlled so as to yield the desired amount of coating layer.

[0042] In step (iii), the mixture 2 and the hydrolyzing agent are mixed. For example, the hydrolyzing agent is added to the mixture 2. Then, the alkoxide monomer and / or its oligomer contained in the mixture 2 are hydrolyzed, dehydrated and condensed, and precipitated on the surface of the core particles to form a coating layer. The hydrolyzing agent only needs to be able to hydrolyze the alkoxide monomer and / or its oligomer. The hydrolyzing agent is, for example, water H ２ Includes O.

[0043] The amount of hydrolyzing agent is not particularly limited, as long as it is sufficient to hydrolyze the alkoxy monomer and / or its oligomer. For example, the amount of hydrolyzing agent is 1.00 to 15.00 ml, preferably 1.00 to 10.00 ml, and more preferably 2.00 to 8.00 ml, per 100 ml of organic solvent in step (i).

[0044] From the viewpoint of material dispersion, step (iii) should be carried out while continuing stirring. Also, from the viewpoint of minimizing thickness variations in the coating layer, it is preferable not to hydrolyze the alkoxy monomer and / or its oligomer up to step (iii). For example, it is preferable not to substantially add a hydrolyzing agent up to step (iii).

[0045] "Substantially omitting hydrolysants" means intentionally omitting hydrolysants, and trace amounts of hydrolysants that inevitably become mixed in small amounts during the production of positive electrode active material particles are acceptable. For example, trace amounts of water that are inevitably contained in organic solvents, or amounts of water that are too small to exert a hydrolytic effect, are acceptable. For example, the content ratio of hydrolysants in mixture 1 and mixture 2 is 0.1% by mass or less, preferably 0.05% by mass or less, more preferably 0.01% by mass or less, and even more preferably 0.001% by mass or less. For example, when 2-propanol and ethanol with a purity of 99.9% are used, the frequency of contact between water and alkoxide can be extremely reduced, so it is considered that no hydrolytic effect will occur.

[0046] From the viewpoint of reducing thickness variations in the coating layer, after adding the hydrolyzing agent, the reaction is preferably carried out for 1 to 48 hours or 2 to 24 hours while continuing to stir. Through this reaction, the alkoxide monomer and / or its oligomer are hydrolyzed and precipitated on the core particle surface through dehydration condensation, forming a coating layer and obtaining positive electrode active material particles.

[0047] Furthermore, the obtained positive electrode active material particles may be dried and sintered at approximately 200°C to 800°C for the purpose of removing residual carbon and controlling the crystal structure of the coating layer. The particle size of the positive electrode active material particles is not particularly limited and can be set according to the application. Examples of volume-based median diameters (D50) of the positive electrode active material particles include 1 to 100 μm, 2 to 50 μm, 3 to 20 μm, and 5 to 15 μm.

[0048] (All-Solid-State Battery) This disclosure can provide an all-solid-state battery. The all-solid-state battery has at least a positive electrode layer, a solid electrolyte layer, and a negative electrode layer in this order, wherein the positive electrode layer contains the positive electrode active material particles. Current collectors may be provided in the positive electrode layer and the negative electrode layer.

[0049] The positive electrode layer contains positive electrode active material particles. If necessary, the positive electrode composite material used in the positive electrode layer may contain conductive additives such as carbon black and a solid electrolyte. The same solid electrolyte as that used in the solid electrolyte layer may be used. The amounts of conductive additives and solid electrolytes are not particularly limited and can be set appropriately according to the desired positive electrode.

[0050] This disclosure can provide a positive electrode for an all-solid-state battery. The positive electrode is a molded body of a positive electrode composite containing positive electrode active material particles, wherein the positive electrode active material particles are the positive electrode active material particles described above. The means for obtaining the positive electrode or positive electrode layer are not particularly limited. For example, a positive electrode composite containing positive electrode active material particles, a conductive additive if necessary, and a solid electrolyte may be molded by pressurizing it, for example, with a press machine.

[0051] The solid electrolyte layer is, for example, made by solidifying solid electrolyte particles. The solid electrolyte particles are not particularly limited, and ion-conducting solids commonly used in all-solid-state batteries can be used. For example, sulfide-based solid electrolytes (Li ６ PS ５Cl, Li ７ P ３ S １１ Li ３ PS ４ Li ８ P ２ S ９ Li １３ GeP ３ S １６ Li １０ GeP ２ S １２ (etc.), Li-B oxide-based solid electrolyte particles, Li-Yb oxide-based solid electrolyte particles, NASCICON-type solid electrolyte particles (LiAlTi(PO ４ ) ３ , LiAlGe (PO ４ ) ３ (e.g.), perovskite-type oxide solid electrolyte particles (Li ｘ La （１－ｘ）／３ TiO ３ LixLa （１－ｘ）／３ NboO ３ (e.g.), garnet-type oxide solid electrolyte particles (Li ７ La ３ Zr ２ O １２ , etc.), Li-P-O-based solid electrolyte particles (Li ３ PO ４ LiPON (Li ３ PO ４ Examples include particles in which some of the oxygen atoms are replaced with nitrogen atoms.

[0052] The negative electrode layer contains a negative electrode active material. The negative electrode active material is not particularly limited, and known materials that can be used in all-solid-state batteries may be used. Examples of negative electrode active materials include carbon materials, Li materials, and Si materials. For example, metals such as metallic lithium, metallic indium, metallic silicon, or metallic aluminum, or alloys thereof, can be used. All-solid-state batteries can be manufactured by molding the materials of the positive electrode layer, solid electrolyte layer, and negative electrode layer using a known press or the like.

[0053] The measurement methods relating to this disclosure are described below. <Method for calculating the coverage of the coating layer on core particles> (Method for obtaining backscattered electron images of the surface of positive electrode active material particles) The coverage of the coating layer on core particles is calculated using backscattered electron images of the surface of positive electrode active material particles. Backscattered electron images of positive electrode active material particles are obtained using a scanning electron microscope (SEM). Backscattered electron images obtained from an SEM are also called "compositional images," and elements with smaller atomic numbers are detected as darker, while elements with larger atomic numbers are detected as brighter.

[0054] The difference in atomic numbers between the elements in the positive electrode active material particles and the elements in the coating layer creates a contrast between the core particle area and the coating layer area. For example, if the core particles are an oxide containing cobalt, nickel, and manganese, and the coating layer is an oxide containing niobium, the backscattered electron image obtained from the SEM will show the core particles as dark areas and the coating layer as bright areas.

[0055] The SEM equipment and observation conditions are as follows: Equipment used: Carl Zeiss Microscopy Co., Ltd. Gemini SEM 560 Acceleration voltage: 1.0 kV WD: 4.0 mm Aperture Size: 20.0 μm Detection signal: EsB (energy-selective backscattered electrons) EsB Grid: 600 V Magnification: 5,000x (adjusted appropriately to a magnification that allows one particle to be observed within the observation field according to the particle size) Contrast: 63.0 ± 5.0% (reference value) Brightness: 38.0 ± 5.0% (reference value) Image size: 1024 pixels × 768 pixels Preprocessing: Cathode active material particles are scattered on carbon tape (no vapor deposition is performed) Contrast and brightness are set appropriately according to the condition of the equipment used. Furthermore, the acceleration voltage and EsB Grid are set to achieve objectives such as acquiring structural information of the outermost surface of the positive electrode active material particles, preventing charge-up in undeposited samples, and selectively detecting high-energy backscattered electrons. The observation field is selected to be near where the curvature of the positive electrode active material particles is small.

[0056] (Method for confirming elements in the parts observed in backscattered electron images) The elements in the parts observed in backscattered electron images are confirmed by superimposing the above backscattered electron image with an elemental mapping image obtained by energy-dispersive X-ray analysis (EDS) acquired with a scanning electron microscope (SEM).

[0057] The SEM / EDS equipment and observation conditions are as follows: Equipment used (SEM): Carl Zeiss Microscopy Co., Ltd. Gemini SEM 560 Equipment used (EDS): Thermo Fisher Scientific Co., Ltd. NORAN System 7 Detector (EDS): Ultra Dry (Windowless type) Acceleration voltage: 1.0 kV WD: 5.0 mm Aperture Size: 30.0 μm Detection signal: SE2 (secondary electrons) Magnification: 5,000x (adjusted appropriately to a magnification that allows one particle to be observed within the field of view according to the particle size) Mode: Spectral Imaging Pretreatment: Cathode active material particles are scattered on carbon tape (no deposition is performed) The mapping image obtained by this method and the backscattered electron image are superimposed, and atoms contained in the bright and dark areas of the backscattered electron image are identified to determine the core particles and coating layer. For example, if the niobium atom in the mapping image coincides with the bright areas in the backscattered electron image, it can be identified that the bright areas in the backscattered electron image are coated with an oxide containing niobium.

[0058] (Coverage of the coating layer over the core particles) To calculate the coverage, first, in the same manner as the method for acquiring backscattered electron images of the surface of the positive electrode active material particles described above, the observation magnification is adjusted appropriately so that there are about 10 particles in the observation field, and backscattered electron images are acquired. Next, for each particle in the observation field, an image is acquired at an observation magnification of 50,000x so that the center of the particle is at the center of the observation field. Using the image processing software ImageJ (developed by Wayne Rashand), the ratio of the area of ​​the part corresponding to the coating layer to the sum of the area of ​​the part corresponding to the coating layer and the area of ​​the part corresponding to the core particles (area %) is calculated as the coverage S. The procedure is shown below.

[0059] First, convert the backscattered electron image to 8-bit using the Type option in the Image menu. Next, reduce image noise by setting the median diameter to 2.0 pixels using the Filters option in the Process menu. Then, use the Straight Line tool on the toolbar to select the scale bar in the observation conditions display area shown at the bottom of the backscattered electron image. With the scale bar selected, select Set Scale from the Analyze menu. A new window will open, and the pixel distance of the selected line will be entered in the Distance in Pixels field.

[0060] Enter the value of the scale bar (e.g., 100) in the Known Distance field of the aforementioned window, enter the unit of the scale bar (e.g., nm) in the Unit of Measurement field, and click OK to complete the scale setting. Next, estimate the image center while excluding the observation condition display area shown at the bottom of the backscattered electron image, and use the Rectangle Tool on the toolbar to select a 1.5 μm square area from the image center of the backscattered electron image.

[0061] Next, select Set Measurements from the Analyze menu and check Area. Select Measure from the Analyze menu, check Display Result, and click OK to perform domain analysis. The area of ​​the analyzed portion (Area) obtained from the newly opened Results window is defined as S1, which is the sum of the area of ​​the portion corresponding to the coating layer and the area of ​​the portion corresponding to the core particles. Next, select Threhold from Adjust. In manual operation, select 128, which is the middle grayscale between black and white in the 8-bit image, as the threshold, and click Apply to obtain a binarized image.

[0062] If the coating layer is a niobium-containing oxide, the coating layer is observed as a bright area, and this operation will display the pixels corresponding to the coating domain D in black. Again, after excluding the observation conditions display area shown at the bottom of the backscattered electron image, estimate the image center and use the rectangle tool on the toolbar to select a 1.5 μm square area from the center of the backscattered electron image.

[0063] Next, select Set Measurements from the Analyze menu and check Area. Select Measure from the Analyze menu, check Display Result, and click OK to perform domain analysis. The area (Area) obtained from the newly opened Results window is defined as the area S2 corresponding to the coating layer.

[0064] The coverage rate S is calculated from the sum of the areas S1 corresponding to the coating layer and the core particles, and the area S2 corresponding to the coating layer, using the following formula: S (area %) = S2 / S1 × 100. The above procedure is performed for each particle that can be observed when approximately 10 particles are initially placed in the observation field, and the coverage rate S for each is calculated. This operation is repeated until there are more than 50 positive electrode active material particles, and the arithmetic mean of the coverage rates S of all observed particles is used as the coverage rate.

[0065] <Method for Calculating the Average Thickness, Maximum Thickness, and Difference Between Maximum and Minimum Thickness of the Coating Layer> The average thickness, maximum thickness, and difference between maximum and minimum thickness of the coating layer are calculated using cross-sectional observation images obtained by a transmission electron microscope (TEM). Cross-sectional observation of the positive electrode active material particles is performed by the following method. The specific method for observing the cross-section of the positive electrode active material particles is as follows. First, the positive electrode active material powder is processed using a focused ion / electron beam processing system "Helios G4UC" (manufactured by FE-I Japan Co., Ltd.) under acceleration voltage conditions of 30kV (sampling) and 8kV (finishing) to cut out a thin section sample with a thickness of approximately 100nm. Cross-sectional observation and elemental mapping by EDS are performed on the cut-out sample using a transmission electron microscope. The atoms contained in the core particles and coating layer can be identified from the mapping image.

[0066] The observation conditions are as follows: TEM (TEM): JEOL JEM-2800; EDS detector (EDS detector): JEOL JED-2300T; Dry SD100GV detector (detection element area: 100 mm²) ２ Equipment used (EDS analyzer): Thermo Fisher Scientific NORAN System 7 Acceleration voltage: 200kV Magnification: 1,000,000x Probe size: 1.0mm STEM image size: 1024 pixels x 1024 pixels EDS mapping size: 256 pixels x 256 pixels Number of frames: 1000 Aperture size: 40.0μm Detection signal: STEM-DF

[0067] From the obtained cross-sectional mapping image, line analysis is performed in the direction normal to the particles for the elements contained in the coating layer. The conditions for line analysis are as follows: STEM magnification: 1,000,000x Line length: 100 nm Number of points: 100 An example of a cross-sectional image of the highly active material particles and line analysis is shown in Figure 1. Line analysis is performed on the coating layer in the direction normal to the particles, i.e., in the direction of the arrow in Figure 1. An example of a profile obtained by line analysis is shown in Figure 2. The full width at half maximum of the obtained X-ray intensity peak is taken as the thickness of the coating layer at the corresponding location. The same line analysis is performed at 20 locations for one cross-sectional image. In this case, the locations to be analyzed should be spaced equally apart. The same analysis is performed on cross-sectional images of 50 fields of view, and the arithmetic mean of the locations where a thickness of 0.5 nm or more is observed is taken as the average thickness of the coating layer. Furthermore, the maximum value among the 1000 obtained thicknesses of the coating layer is taken as the maximum thickness, and the minimum value among the locations where a thickness of 0.5 nm or more was observed is taken as the minimum thickness, and the difference is calculated.

[0068] <Method for calculating the ratio of the coating layer to the core particles> The content of the coating layer relative to the core particles is determined by the fundamental parameter (FP) method using fluorescent X-rays. For example, if the core particles are LiNi ０．８ Co ０．１ Mn ０．１ O ２ In the case where the coating layer is lithium niobate, the Ni in the core particles is quantified as nickel oxide, and the Nb in the coating layer is quantified as niobium pentoxide. This is then converted to parts by mass of the coating layer per 100 parts by mass of core particles. If the powder sample can be formed into pellets, the measurement is performed under a vacuum atmosphere. If pellet formation is not possible, the measurement is performed in a helium atmosphere using a special cup and film while the sample remains in powder form. Furthermore, if there are elements that are not detected by fluorescent X-rays (elements lighter than oxygen), the calculation is performed after setting the balance components.

[0069] An example of measurement is shown below. Equipment used: X-ray fluorescence analyzer Zetium (manufactured by Spectris Co., Ltd.) Sample preparation A Mylar film with a thickness of 6.0 μm (manufactured by Chemplex Industries, Inc.) was mounted in a P2 plastic liquid sample cup with an inner diameter of 29 mm (manufactured by Spectris Co., Ltd.), and 2.0 g of the sample was placed in it, leveled to a uniform thickness. Measurement conditions X-ray tube: Rh target tube Output: 2.4 kW Measurement diameter: 27 mm Atmosphere: Helium Analysis conditions FP quantification software: Omnian Element: Oxide balance component: Li

[0070] The present disclosure will be described in detail below with reference to examples and comparative examples, but the present disclosure is not limited to these examples.

[0071] Examples relating to the positive electrode active material particles of this disclosure will be described below. <Example of production of positive electrode active material particles 1> [Preparation of mixed solution 1] 100.0 ml of 2-propanol (super dehydrated) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) is added to a container equipped with a stirring device, and while stirring, LiOC is added as lithium alkoxide. ２ H ５ 260.0 mg of (manufactured by High Purity Chemicals Co., Ltd.), and as niobium alkoxide, Nb(OC ２ H ５ ) ５ 1.25 ml of (manufactured by High Purity Chemicals Co., Ltd.) was added. Then, it was refluxed at 85°C overnight. This was designated as mixture 1.

[0072] [Preparation of Mixture 2] While continuing to stir, LiNi is added to the mixture 1 as core particles containing the first composite oxide. ０．８ Co ０．１ Mn ０．１ O ２ 30g of (PO5038, manufactured by MSE Supplies) was added to obtain mixture 2.

[0073] Next, while continuing to stir, H is added to the mixture 2 as a hydrolyzing agent. ２4.50 ml of O was added and stirring was continued overnight to hydrolyze the alkoxide monomer and deposit it on the surface of the core particles with dehydration condensation to form a coating layer. Then, it was recovered by filtration, vacuum dried at 80 °C for 4 hours, and sintered at 350 °C for 1 hour to obtain the cathode active material particles 1 having a core-shell structure. The physical property values of the cathode active material particles 1 are shown in Table 2.

[0074] <Production Examples of Cathode Active Material Particles 2 to 8> In the production example of the cathode active material particles 1, except that the solvent species, the addition amount of Li alkoxide LiOC ２ H ５ , Nb alkoxide Nb(OC ２ H ５ ) ５ and H ２ O were changed as shown in Table 1, the cathode active material particles 2 to 8 were obtained in the same manner. The physical property values of the cathode active material particles 2 to 8 are shown in Table 2.

[0075] <Cathode Active Material Particles 9> LiNi ３ coated with LiNbO ０．８ Co ０．１ Mn ０．１ O ２ (PO0185 manufactured by MSE Supplies) was used as the cathode active material particles 9.

[0076] <Production Example of Cathode Active Material Particles 10> 100.0 ml of ethanol (ultra-dehydrated) (99.5) (manufactured by Fujifilm Wako Pure Chemical Corporation) was added to a container having a stirring device, and while stirring, as the alkoxide of lithium, LiOC ２ H ５ (manufactured by Kanto Chemical Co., Inc.) 816.4 mg, and as the alkoxide of niobium, Nb(OC ２ H ５ ) ５ (manufactured by Kanto Chemical Co., Inc.) 3.92 ml was added. While continuing stirring, 30 g of PO5038 manufactured by MSE Supplies was added thereto. Further, 1.00 ml of H ２ O was added and stirring was continued overnight. Then, ethanol was evaporated and sintered at 500 °C for 3 hours to obtain the cathode active material particles 10. The physical property values of the cathode active material particles 10 are shown in Table 2.

[0077] 〈Production Example of Positive Electrode Active Material Particles 11〉 For 1.00 kg of ethanol (ultra-dehydrated) (99.5) (manufactured by Fujifilm Wako Pure Chemical Corporation), 8.16 g of LiOC ２ H ５ and 39.2 ml of Nb(OC ２ H ５ ) ５ are added and stirred to obtain a coating raw material composition. The raw material composition is applied onto the surface of PO5038 manufactured by MSE Supplies using a coating apparatus with a rolling fluidized bed. At this time, the ratio of the coating raw material composition to the core particles is adjusted so that the coating raw material composition is 0.0525 mg with respect to 1 g of the core particles. Thereafter, a sintering treatment is performed at 350 °C to obtain positive electrode active material particles 11. The physical property values of the positive electrode active material particles 11 are shown in Table 2.

[0078] 〈Positive Electrode Active Material Particles 12〉 PO5038 manufactured by MSE Supplies is used as the positive electrode active material particles 12.

[0079]

[0080]

[0081] In the table, the amount of the coating layer indicates the content (parts by mass) of the coating layer with respect to 100 parts by mass of the core particles.

[0082] [Measurement of Resistance Value and Measurement of Capacity Retention Rate] For the prepared positive electrode active material particles 1 and 9, all-solid-state batteries were prepared by the following method, and the resistance value and the capacity retention rate were measured. The results of the measured resistance value and capacity retention rate are shown in Table 3. For the positive electrode active material particles 2 to 8 and 10 to 12, all-solid-state batteries were also prepared by the same method, and the resistance value and the capacity retention rate were measured. The resistance value and the capacity retention rate are shown in Table 3.

[0083] 〈Production of All-Solid-State Battery〉 The following operations were performed inside a glove box under an argon atmosphere. (Preparation of Positive Electrode Composite Material) 65.0 mg of the positive electrode active material particles obtained by the above method, 5.0 mg of a conductive assistant (acetylene black), and 30.0 mg of a solid electrolyte (Li ６ PS ５ Cl) were mixed in a mortar for 5 minutes to prepare a positive electrode composite material.

[0084] (Battery cell fabrication) Next, solid electrolyte (Li) is placed inside a solid-state battery cell (Housensha HSSC-05, electrode size Φ10 mm) ６ PS ５ 85.0 mg of Cl was added, and a solid electrolyte layer was formed by pressurizing it at 30 MPa using a press machine.

[0085] Next, after releasing the pressure, the upper punch was withdrawn, and 10.6 mg of the aforementioned cathode composite material was placed on top of the solid electrolyte layer formed in the cell. The upper punch was then reinserted, and pressure was applied at 200 MPa using a press machine to form the cathode layer on top of the solid electrolyte layer. After that, the upper punch was withdrawn, a SUS foil (Φ10 mm × 0.5 mm thick) was inserted on top of the cathode layer, and the upper punch was reinserted and pressed in by hand.

[0086] The all-solid-state battery cell was inverted, and the punch on the side opposite the positive electrode layer was removed. A lithium metal foil (200 μm thick) and an indium foil (100 μm thick), punched to a diameter of Φ8 mm, were then inserted sequentially on top of the solid electrolyte layer as the negative electrode layer. A SUS foil (Φ10 mm x 0.5 mm thick) was then inserted on top of the negative electrode, a punch was inserted, and pressure was applied at 90 MPa using a press machine. After depressurization, the case screws were tightened so that the internal restraining pressure of the cell was 200 MPa.

[0087] A glass desiccator with airtightness, capable of connecting electrical wiring to the inside and outside, was prepared. The aforementioned battery cells were placed in the container, and the wiring of each electrode plate of the cell was connected to the desiccator before sealing, thereby fabricating the battery cells. The completed battery cells were removed from the glove box and evaluated as follows.

[0088] <Resistance Measurement> Two charge-discharge cycles were performed on the prototype battery cell under the following conditions: • Equipment: Bio Logic VMP-3e • Temperature: 25°C • Charging potential: 3.6V • Discharge potential: 2.0V • Current rate: 0.1C • Cutoff current rate: 0.01C (or 60 minutes)

[0089] Next, AC impedance measurements were performed under the following conditions to obtain a Cole-Cole plot: • Equipment: VMP-3e manufactured by BioLogic • Temperature: 25°C • SOC 100% (calculated from the discharge capacity of the second cycle) • Measurement frequency: 1 MHz to 10 m Hz An arc was fitted to the obtained Cole-Cole plot, and the distance between the two intersection points of the fitted arc and the real axis was measured as the resistance value.

[0090] <Measurement of Capacity Retention Rate> After resistance measurement, the battery cells were subjected to further charge-discharge tests under the following conditions. The ratio of the discharge capacity after 25 cycles to the discharge capacity after 1 cycle was measured as the capacity retention rate. • Equipment: Bio Logic VMP-3e • Temperature: 25°C • Charging potential: 3.6V • Discharge potential: 2.0V • Current rate: 0.1C • Cutoff current rate: None

[0091] Table 3 shows the resistance values ​​and capacity retention rates of each battery cell containing the positive electrode active material particles described in Table 3, as Examples 1 to 8 and Comparative Examples 1 to 4.

[0092]

[0093] This disclosure is not limited to the embodiments described above, and various modifications and alterations are possible without departing from the spirit and scope of this disclosure. Accordingly, the following claims are attached to make the scope of this disclosure public. This application claims priority based on Japanese Patent Application No. 2024-228798, filed on 25 December 2024, the entire contents of which are incorporated herein by reference.

Claims

1. A positive electrode active material particle having a core particle and a coating layer covering the surface of the core particle, wherein the core particle is formed of a first composite oxide, the first composite oxide comprises lithium and at least one selected from the group consisting of manganese, cobalt, nickel, aluminum, iron, and phosphorus, the coating layer is formed of a second composite oxide, the second composite oxide comprises lithium and at least one selected from the group consisting of niobium, boron, phosphorus, zirconium, titanium, aluminum, lanthanum, and tungsten, and when the positive electrode active material particle is observed using a scanning electron microscope and a backscattered electron image is obtained at 50,000x magnification, and the portion corresponding to the coating layer and the portion corresponding to the core particle are binarized, the ratio of the area of ​​the portion corresponding to the coating layer to the sum of the area of ​​the portion corresponding to the coating layer and the area of ​​the portion corresponding to the core particle (area %) is defined as the coverage rate of the coating layer over the core particle, and the coverage rate is 90.0 area % or more. Positive electrode active material particles characterized in that the average thickness of the coating layer is 15.0 nm or less, and the maximum thickness of the coating layer is 15.0 nm or less.

2. The positive electrode active material particle according to claim 1, wherein the difference between the maximum thickness and the minimum thickness of the coating layer is 5.0 nm or less.

3. The positive electrode active material particle according to claim 1 or 2, wherein the content of the coating layer relative to 100 parts by mass of the core particles is 1.00 part by mass or less.

4. The positive electrode active material particle according to any one of claims 1 to 3, wherein the content of the coating layer relative to 100 parts by mass of the core particles is 0.20 to 0.90 parts by mass.

5. The positive electrode active material particles according to any one of claims 1 to 4, wherein the second composite oxide comprises lithium and niobium.

6. A method for producing positive electrode active material particles according to any one of claims 1 to 5, the method comprising: (i) mixing an alkoxide monomer and / or its oligomer with an organic solvent to obtain a mixed solution 1; (ii) mixing the mixed solution 1 with the core particles to obtain a mixed solution 2; and (iii) mixing the mixed solution 2 with a hydrolyzing agent to hydrolyze the alkoxide monomer and / or its oligomer contained in the mixed solution 2, dehydrate and condense it, precipitate it on the surface of the core particles to form the coating layer, and obtain the positive electrode active material particles, wherein the alkoxide monomer and / or its oligomer comprises lithium and at least one selected from the group consisting of niobium, boron, phosphorus, zirconium, titanium, aluminum, lanthanum, and tungsten.

7. A positive electrode for an all-solid-state battery, wherein the positive electrode is a molded body of a positive electrode composite containing positive electrode active material particles, and the positive electrode active material particles are positive electrode active material particles according to any one of claims 1 to 5.

8. A solid-state battery, wherein the solid-state battery comprises at least a positive electrode layer, a solid electrolyte layer, and a negative electrode layer in that order, and the positive electrode layer contains positive electrode active material particles according to any one of claims 1 to 5.

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