Coated positive electrode active substance for lithium secondary battery, and lithium secondary battery

Abstract

Provided is a coated positive electrode active substance for a lithium secondary battery, which is used in a positive electrode of an all-solid state battery in which an electrolyte is a solid electrolyte. The coated positive electrode active substance comprises a positive electrode active substance and a coating layer disposed on the surface of the positive electrode active substance. The positive electrode active substance contains nickel and cobalt and has a layered crystal structure. The coating layer has a layered structure that includes a first coating layer and a second coating layer. The first coating layer contains niobium, lithium and oxygen. The second coating layer contains a lithium-containing carboxylic acid salt.

Description

Coated cathode active material for lithium secondary batteries, lithium secondary batteries

[0001] The present invention relates to a coated positive electrode active material for a lithium secondary battery and a lithium secondary battery.

[0002] In recent years, with the widespread use of portable electronic devices such as mobile phones and laptop computers, there has been a strong demand for the development of small, lightweight lithium secondary batteries with high energy density, as well as for use in electric vehicles.

[0003] In recent years, all-solid-state batteries have attracted attention as a lithium secondary battery that can meet these requirements. All-solid-state batteries are composed of a positive electrode layer, a solid electrolyte layer, a negative electrode layer, etc., and are highly anticipated for practical use due to their high energy density, high output, high voltage, and high stability compared to conventional batteries that use electrolytes (electrolytic solutions) such as organic solvents.

[0004] However, current all-solid-state batteries are insufficient in both output characteristics and high voltage resistance, and one of the reasons for this is the formation of a high-resistance layer at the contact interface between the solid electrolyte and the positive electrode active material.

[0005] For example, Patent Document 1 discloses an invention aimed at providing an electrode body capable of reducing interface resistance. Patent Document 1 discloses an electrode body characterized by containing a positive electrode active material having an active material and a first solid electrolyte covering 70% or more of the surface of the active material, and a second solid electrolyte. Patent Document 1 also discloses that the first solid electrolyte is lithium niobate and the second solid electrolyte is a sulfide.

[0006] However, when a lithium secondary battery using the electrode assembly disclosed in Patent Document 1 is charged at a high potential, the battery capacity may decrease.

[0007] Japanese Patent Application Publication No. 2009-193940

[0008] In view of the problems associated with the above-described conventional techniques, an object of the present invention is to provide a positive electrode active material for a coated lithium secondary battery that can be applied to an all-solid-state battery and can suppress a decrease in battery capacity even when the battery is charged at a high potential.

[0009] In order to solve the above-mentioned problems, one aspect of the present invention provides a positive electrode active material for a coated lithium secondary battery used in a positive electrode of an all-solid-state battery in which the electrolyte is a solid electrolyte, the positive electrode active material having a positive electrode active material and a coating layer disposed on a surface of the positive electrode active material, wherein the positive electrode active material contains nickel and cobalt and has a layered crystal structure, the coating layer has a laminated structure including a first coating layer and a second coating layer, the first coating layer contains niobium, lithium, and oxygen, and the second coating layer contains a carboxylate containing lithium.

[0010] According to one aspect of the present invention, it is possible to provide a positive electrode active material for a coated lithium secondary battery that can be applied to an all-solid-state battery and can suppress a decrease in battery capacity even when charged at a high potential.

[0011] FIG. 1A is a cross-sectional schematic diagram of a coated positive electrode active material for a lithium secondary battery according to one embodiment of the present disclosure. FIG. 1B is a cross-sectional schematic diagram of a coated positive electrode active material for a lithium secondary battery according to one embodiment of the present disclosure. FIG. 1C is a cross-sectional schematic diagram of a lithium secondary battery according to one embodiment of the present disclosure. FIG. 2 is a cross-sectional schematic diagram of a lithium secondary battery. FIG. 3 is a cross-sectional TEM image of the coated positive electrode active material for a lithium secondary battery obtained in Example 1. FIG. 4 is a cross-sectional TEM image of the coated positive electrode active material for a lithium secondary battery obtained in Example 2.

[0012] Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiments, and various modifications and substitutions can be made to the following embodiments without departing from the scope of the present invention. [Positive Electrode Active Material for Coated Lithium Secondary Battery] The positive electrode active material for a coated lithium secondary battery of this embodiment (hereinafter also referred to as "coated positive electrode active material") can be used in the positive electrode (positive electrode layer) of an all-solid-state battery in which the electrolyte is a solid electrolyte.

[0013] 1A to 1C are cross-sectional schematic diagrams of the coated cathode active material of this embodiment. Note that FIGS. 1A to 1C are merely schematic views. Therefore, the cross-sectional shapes of the particles of coated cathode active material 10, coated cathode active material 100, coated cathode active material 110, and cathode active material 11 are not limited to circular shapes, and can have any shape. Furthermore, coating layer 12 does not need to have a constant thickness. Because coated cathode active material 100 and coated cathode active material 110 shown in FIGS. 1B and 1C are modified examples of the configuration of coating layer 12, they will be described primarily using FIG. 1A , with FIGS. 1B and 1C being used as needed.

[0014] As shown in FIG. 1A , a coated positive electrode active material 10 of this embodiment can have a positive electrode active material 11 and a coating layer 12 disposed on the surface of the positive electrode active material 11 .

[0015] Hereinafter, each component contained in the coated positive electrode active material 10 of this embodiment will be described. (1) Positive Electrode Active Material The positive electrode active material 11 contained in the coated positive electrode active material of this embodiment may be any positive electrode active material that can insert and extract lithium (Li) by an electrochemical reaction.

[0016] The positive electrode active material 11 can contain, for example, nickel and cobalt.

[0017] Examples of the positive electrode active material 11 include composite oxides containing lithium, nickel, and cobalt. Examples of the positive electrode active material 11 include composite oxides containing lithium (Li), nickel (Ni), cobalt (Co), and element M (M) in a ratio of Li:Ni:Co:M = a:x:y:z in terms of the amount of substances. Note that a, x, y, and z satisfy x + y + z = 1, 0.8≦a≦1.2. x and y can be selected so that their sum with z is 1, and may be 0<x<1, 0<y<1. The element M can be at least one selected from the group consisting of manganese (Mn), magnesium (Mg), aluminum (Al), titanium (Ti), iron (Fe), copper (Cu), zinc (Zn), zirconium (Zr), niobium (Nb), molybdenum (Mo), tantalum (Ta), and tungsten (W). Since the element M is an optional additive element, 0≦z<1 can be satisfied.

[0018] The composite oxide has the general formula: Li ａ Ni ｘ Co ｙ M ｚ O ２＋α It is preferable that α in the general formula satisfies −0.2≦α≦0.2. As x, y, z, a, and element M in the general formula have been explained, their explanation will be omitted.

[0019] The positive electrode active material 11 contained in the coated positive electrode active material 10 of this embodiment may be a mixture of multiple types of positive electrode active materials with different compositions.

[0020] The positive electrode active material preferably has a layered structure, i.e., a layered crystalline structure. This is because a positive electrode active material having a layered crystalline structure can particularly improve the output characteristics when applied to a lithium secondary battery. When the positive electrode active material has a layered crystalline structure, the positive electrode active material has a layered rock salt structure (α-NaFeO ２ The mold structure may be formed.

[0021] As described above, it is preferable that the positive electrode active material 11 contains at least cobalt (Co). When the positive electrode active material 11 contains cobalt, the structure of the positive electrode active material 11 is more likely to be stabilized during charging and discharging, and therefore deterioration of the positive electrode active material 11 during high-voltage use of the battery can be suppressed.

[0022] The structure of the positive electrode active material can be identified by analytical techniques such as X-ray diffraction, electron beam diffraction, etc. The ratio of the amounts of elements contained in the positive electrode active material can be determined by analytical techniques such as X-ray fluorescence analysis, ICP (Inductively Coupled Plasma) emission spectroscopy, etc.

[0023] The shape of the positive electrode active material is not particularly limited. For example, the positive electrode active material may be positive electrode active material particles having an average particle diameter of several nanometers to several tens of micrometers and formed in the form of primary particles or secondary particles formed by aggregation of primary particles, or may be a thin-film positive electrode film. Examples of thin-film positive electrode films include positive electrode films formed by PLD (pulsed laser deposition). (2) Coating Layer The coating layer 12 may have a layered structure including a first coating layer 121 and a second coating layer 122. The order in which the first coating layer 121 and the second coating layer 122 are stacked is not particularly limited. The first coating layer 121 and the second coating layer 122 may be stacked in this order from the position closest to the positive electrode active material 11, or the second coating layer 122 and the first coating layer 121 may be stacked in this order from the position closest to the positive electrode active material 11. The coating layer 12 may include layers other than the first coating layer 121 and the second coating layer 122 , or may be composed of only the first coating layer 121 and the second coating layer 122 .

[0024] The coating layer 12 may cover at least a part of the surface of the positive electrode active material 11. However, the higher the coverage rate, which is the area ratio of the region covered with the coating layer 12 to the surface of the positive electrode active material 11, the better.

[0025] The first coating layer 121 and the second coating layer 122 may have a laminated structure in which they are laminated over the entire outer periphery of the positive electrode active material 11, but it is sufficient that the first coating layer 121 and the second coating layer 122 are laminated over at least a portion of the outer periphery of the positive electrode active material 11 to form a laminated structure. (2-1) Composition of the First Coating Layer and the Second Coating Layer (First Coating Layer) The first coating layer 121 may contain niobium, lithium, and oxygen. The elements contained in the first coating layer 121 can be analyzed and identified by EELS (Electron Energy Loss Spectroscopy), EDX (Energy Dispersive X-ray Spectroscopy), or the like.

[0026] The elements contained in the first coating layer 121 may form a compound, or may exist as a simple substance without forming a compound.

[0027] When the niobium, lithium, and oxygen contained in the first coating layer 121 form a compound, the first coating layer 121 may be, for example, lithium niobate (lithium niobium oxide) or lithium oxide (Li ２ The lithium niobate may contain one or more compounds selected from the group consisting of lithium niobate (LiO), niobium oxide (NbO), and the like. ３ NbO ４ , LiNbO ３ , LiNb ３ O ８ , Li ８ Nb ２ O ９ However, the content ratio of lithium, niobium, and oxygen contained in lithium niobate is not limited to the composition ratio of the above chemical formula.

[0028] By including the first coating layer 121 in the coating layer 12, when the coating layer 12 is applied to a lithium secondary battery, the formation of a high-resistance layer between the positive electrode active material 11 and the solid electrolyte can be prevented, and the interfacial resistance can be suppressed.

[0029] The material contained in the first coating layer 121 may be amorphous. By making the material contained in the first coating layer 121 amorphous, lithium ion conductivity can be increased, and the positive electrode resistance in the lithium secondary battery can be particularly reduced.

[0030] The fact that the substance contained in the first coating layer 121 is amorphous can be confirmed, for example, by performing electron diffraction on the first coating layer 121 and finding that no crystalline spots are observed.

[0031] Specifically, for example, the following procedure can be used to evaluate whether the material contained in the first coating layer 121 is crystalline or amorphous. First, the coated positive electrode active material 10 to be evaluated is embedded in a resin or the like, and then a cross-sectional observation sample is prepared by performing focused ion beam processing (FIB (Focused Ion Beam) processing). Then, the cross-sectional observation sample is observed using a transmission electron microscope (TEM), and, if necessary, EELS measurement or EDX measurement is performed to identify the location of the first coating layer 121. Next, electron beam diffraction measurement is performed on the identified first coating layer 121. If no crystalline spots are observed in the obtained diffraction pattern, the material contained in the first coating layer 121 can be evaluated to be amorphous. At this time, a halo pattern may be observed in the diffraction pattern. Furthermore, if crystalline spots are observed in the obtained diffraction pattern, it can be confirmed that the substance contained in the first coating layer 121 is crystalline, i.e., has crystallinity. (Second Coating Layer) The second coating layer 122 can contain a carboxylate containing lithium. The elements contained in the second coating layer 122 can be analyzed and identified by EELS, EDX, or the like.

[0032] The lithium-containing carboxylate contained in the second coating layer 122 may also be crystalline. Therefore, the lithium-containing carboxylate contained in the second coating layer 122 can also be identified by its electron diffraction pattern. The crystalline nature of the lithium-containing carboxylate contained in the second coating layer 122 can be evaluated using the same procedure as when evaluating whether the substance contained in the first coating layer 121 is amorphous. Specifically, the same procedure can be used to identify the location of the second coating layer 122 instead of the first coating layer 121 using TEM observation, except that electron diffraction measurement is performed on the second coating layer 122. If crystalline spots are observed in the diffraction pattern for the second coating layer 122 and match the pattern for the lithium-containing carboxylate, it can be identified that the second coating layer 122 contains a lithium-containing carboxylate. It can also be identified that the lithium-containing carboxylate contained in the second coating layer 122 is crystalline.

[0033] The lithium-containing carboxylate contained in the second coating layer 122 may be one or more selected from the group consisting of lithium oxalate, lithium malonate, lithium succinate, lithium glutarate, and lithium adipate.

[0034] The valence of the carboxylic acid in the lithium-containing carboxylate is not particularly limited, but may be, for example, divalent.

[0035] The second coating layer 122 has lithium ion conductivity but also functions as an electrically insulating layer. Therefore, even when the coated cathode active material 10 of this embodiment is applied to a lithium secondary battery and charging is performed at a high potential, the voltage applied to the first coating layer 121, the cathode active material 11, and the solid electrolyte is suppressed, preventing the application of a high voltage and suppressing oxidative decomposition of the solid electrolyte. This prevents the first coating layer 121, the cathode active material 11, and the solid electrolyte from being decomposed by the application of a voltage, and therefore prevents a decrease in battery capacity for a lithium secondary battery including the coated cathode active material 10 of this embodiment, even when charging at a high potential.

[0036] In this specification, charging at a high potential means charging by applying a voltage of 4.4 V or more based on the Li—In alloy potential, for example.

[0037] As described above, the lithium-containing carboxylate contained in the second coating layer 122 may have crystallinity. When the lithium-containing carboxylate has crystallinity, the electrical insulating properties of the second coating layer 122 can be particularly improved, and decomposition of the first coating layer 121, the positive electrode active material 11, and the solid electrolyte due to application of a voltage can be particularly prevented.

[0038] Furthermore, the lithium-containing carboxylate contained in the second coating layer 122 can have a layered crystal structure and can absorb and release lithium. Therefore, the second coating layer 122 can also function as a positive electrode active material, and when the coated positive electrode active material 10 of this embodiment is applied to a lithium secondary battery, the battery capacity can be increased.

[0039] The second coating layer 122 may contain a substance other than a lithium-containing carboxylate. For example, the second coating layer 122 may contain lithium carbonate (Li ２ CO ３ ) may also be included. Lithium carbonate may be intentionally added to the second coating layer 122, for example. Lithium carbonate may also be included in the second coating layer 122 due to decomposition of a carboxylate containing lithium contained in the second coating layer 122 or due to impurities contained in the positive electrode active material 11. Note that the coating layer 12 may also have a third coating layer containing lithium carbonate in addition to the first coating layer 121 and the second coating layer 122.

[0040] The inclusion of lithium carbonate in the second coating layer 122 can enhance the electrical insulating properties of the second coating layer 122. This particularly prevents the first coating layer 121, the cathode active material 11, and the solid electrolyte from being decomposed by the application of voltage, and particularly prevents a decrease in battery capacity of a lithium secondary battery including the coated cathode active material 10 of this embodiment, even when charged at a high potential. (2-2) Thickness of the First Coating Layer and the Second Coating Layer (Thickness of the First Coating Layer) The thickness T121 of the first coating layer 121 is not particularly limited, and can be, for example, 0.5 nm to 200 nm, and may be 1 nm to 20 nm.

[0041] By making the thickness T121 of the first coating layer 121 0.5 nm or more, when applied to a lithium secondary battery, it is possible to prevent the formation of a high-resistance layer between the positive electrode active material 11 and the solid electrolyte, and particularly suppress the interfacial resistance.

[0042] The thickness T122 of the second coating layer 122 is not particularly limited, but may be, for example, 0.5 nm to 200 nm, or may be, for example, 1 nm to 20 nm.

[0043] By setting the thickness T122 of the second coating layer 122 to 0.5 nm or more, even when the coated cathode active material 10 is applied to a lithium secondary battery and charged / discharged at a high potential, the voltage applied to the first coating layer 121, the cathode active material 11, and the solid electrolyte can be suppressed, and the application of a high voltage can be particularly prevented. This particularly prevents the first coating layer 121, the cathode active material 11, and the solid electrolyte from being decomposed by the application of a voltage, and particularly prevents a decrease in battery capacity in a lithium secondary battery including the coated cathode active material 10 of this embodiment, even when the battery is charged at a high potential.

[0044] Furthermore, by setting the thickness T122 of the second coating layer 122 to 200 nm or less, even when applied to a lithium secondary battery and charging and discharging are performed at a high potential, it is possible to prevent the voltage applied to the first coating layer 121, the positive electrode active material 11, and the solid electrolyte from becoming excessively low.

[0045] The thickness T121 of the first coating layer 121 and the thickness T122 of the second coating layer 122 can be measured at any cross section of the coated positive electrode active material 10, for example, at any location of each layer in a TEM image of the cross section. Whether the layer is the first coating layer 121 or the second coating layer 122 can be identified by analyzing the elemental composition of the layer being evaluated, for example, using EELS or EDX. The thicknesses of the first coating layer 121 and the second coating layer 122 can then be measured by measuring the distance between the boundaries of each layer at any position in the TEM image of the observed cross section. (2-3) Regarding the number of layers of the first coating layer 121 and the second coating layer 122 The number and arrangement of the first coating layer 121 and the second coating layer 122 in the coating layer 12 are not particularly limited.

[0046] 1B and 1C, the coating layer 12 may include a structure in which a first coating layer 121 and a second coating layer 122 are alternately stacked. The coating layer 12 may be configured solely from a structure in which the first coating layer 121 and the second coating layer 122 are alternately stacked, or may further include layers other than the first coating layer 121 and the second coating layer 122, or portions that are not regularly alternately stacked.

[0047] The first coating layer 121 and the second coating layer 122 are not limited to the stacking order shown in Figures 1A, 1B, and 1C, and the second coating layer 122 and the first coating layer 121 can also be arranged in order from the position closest to the positive electrode active material 11.

[0048] Furthermore, like the coated positive electrode active material 100 and the coated positive electrode active material 110 shown in Figures 1B and 1C, the coating layer 12 can also include at least two or more units of a unit structure including a first coating layer 121 and a second coating layer 122.

[0049] 1B and 1C show examples in which the coating layer 12 includes two unit structures, a first unit 12A and a second unit 12B, but the coating layer 12 may include three or more unit structures each including a first coating layer 121 and a second coating layer 122.

[0050] The coating layer 12 preferably includes, for example, one to ten first coating layers 121 and one to ten second coating layers 122. It is more preferable that the coating layer 12 includes one to three first coating layers 121 and one to three second coating layers 122.

[0051] When the coating layer 12 contains the first coating layer 121 and the second coating layer 122 in the above-mentioned unit structure, it preferably contains 1 to 10 unit structures, more preferably 1 to 3 unit structures.

[0052] By including one or more first coating layers 121 and one or more second coating layers 122 in the coating layer 12, it is possible to prevent the first coating layer 121, the cathode active material 11, and the solid electrolyte from being decomposed by application of voltage. Therefore, for a lithium secondary battery including the coated cathode active material 10 of this embodiment, it is possible to prevent a decrease in battery capacity even when the battery is charged at a high potential. Furthermore, when the coated cathode active material of this embodiment is applied to a lithium secondary battery, it is possible to prevent a high-resistance layer from being formed between the cathode active material 11 and the solid electrolyte, thereby suppressing interfacial resistance.

[0053] When the coating layer 12 includes 10 or less first coating layers 121 and 122, respectively, the productivity in producing the coated positive electrode active material of this embodiment can be increased.

[0054] The number of layers of the first coating layer 121 and the second coating layer 122 in the coating layer 12 does not need to be the same. For example, like the coated positive electrode active material 110 shown in FIG. 1C , the coating layer 12 may include three first coating layers 121 and two second coating layers 122, and the number of layers may be different.

[0055] When the coating layer 12 includes a plurality of layers for the first coating layer 121, the thickness of the first coating layer 121 and the composition of the substance contained therein may be the same or different, and the same applies to the second coating layer 122. (2-4) Titration Characteristics The content ratios of the first coating layer 121 and the second coating layer 122 contained in the coated positive electrode active material 10 of this embodiment can also be determined by titration with hydrochloric acid, for example.

[0056] Specifically, for example, first, a filtrate to be used in titration can be prepared by the following mixed solution preparation step, stirring and standing step, and filtration step.

[0057] In the mixed solution preparation step, 100 g of pure water is added to 2 g of the coated positive electrode active material to be evaluated to prepare a mixed solution.

[0058] In the stirring and standing step, the mixture obtained in the mixture preparation step can be stirred for 5 minutes by rotating a stirrer at 400 rpm in the mixture, and then allowed to stand for 5 minutes.

[0059] In the filtration step, the supernatant of the mixture after the stirring and standing step can be filtered to obtain a filtrate. For example, a syringe filter can be used for the filtration.

[0060] Then, 60 g of the filtrate obtained through the above steps can be titrated potentiometrically by adding dropwise a 0.1 mol / L hydrochloric acid standard solution.

[0061] In this case, it is preferable that the amount of the hydrochloric acid standard solution added dropwise until the pH value of the filtrate reaches 8.3 is 1.5 mL or more and 4.5 mL or less.

[0062] In addition, it is preferable that the amount of the hydrochloric acid standard solution dropped when the pH value of the filtrate is in the range of 4.5 to 8.3 is 0.5 mL to 2.5 mL.

[0063] The hydrochloric acid standard solution added until the pH value of the filtrate reaches 8.3 is primarily used to react with the substance contained in the second coating layer 122, such as a carboxylate containing lithium. When the amount of hydrochloric acid standard solution added until the pH value of the filtrate reaches 8.3 is 1.5 mL or more, this means that the coated cathode active material 10 contains a sufficient proportion of the second coating layer 122. Therefore, the first coating layer 121, the cathode active material 11, and the solid electrolyte can be prevented from being decomposed by the application of voltage. Furthermore, a lithium secondary battery including the coated cathode active material 10 of this embodiment can be prevented from experiencing a decrease in battery capacity even when charged at a high potential.

[0064] By setting the amount of hydrochloric acid standard solution dropped until the pH value of the filtrate reaches 8.3 to 4.5 mL or less, the proportion of the second coating layer 122 contained in the coated positive electrode active material 10 falls within a particularly appropriate range, and the productivity of the coated positive electrode active material of this embodiment can be increased.

[0065] The hydrochloric acid standard solution dropped when the pH value of the filtrate is in the range of 4.5 to 8.3 is mainly used to react with the substances contained in the first coating layer 121. Therefore, when the amount of hydrochloric acid standard solution dropped when the pH value of the filtrate is in the range of 4.5 to 8.3 is 0.5 mL or more, when the coated cathode active material 10 is applied to a lithium secondary battery, it is possible to prevent the formation of a high-resistance layer between the cathode active material 11 and the solid electrolyte. Furthermore, it is possible to suppress the interfacial resistance between the cathode active material 11 and the solid electrolyte.

[0066] Furthermore, by setting the amount of hydrochloric acid standard solution dropped to 2.5 mL or less when the pH value of the filtrate is in the range of 4.5 to 8.3, the proportion of the first coating layer 121 contained in the coated cathode active material 10 is within an appropriate range, thereby improving productivity. [Method for Manufacturing a Cathode Active Material for a Coated Lithium Secondary Battery] The method for manufacturing a coated cathode active material for a coated lithium secondary battery of this embodiment is not particularly limited. According to the method for manufacturing a coated cathode active material of this embodiment, the coated cathode active material according to one aspect of the present disclosure can be manufactured, and therefore some of the matters already described will not be described again.

[0067] The method for producing a coated positive electrode active material for a lithium secondary battery according to this embodiment can include, for example, a coating layer forming solution preparation step, a mixing step, and a drying step.

[0068] In the coating layer forming solution preparation step, a coating layer forming solution for forming a coating layer can be prepared.

[0069] In the mixing step, the positive electrode active material as the base material and the coating layer forming solution can be mixed.

[0070] In the drying step, the mixture obtained in the mixing step can be dried.

[0071] The film thickness, uniformity, crystallinity, and other aspects of the coating layer on the surface of the positive electrode active material can be controlled by the conditions of the coating layer-forming solution, mixing and drying conditions, and the like.

[0072] Each step will be described below: (1) Step of Preparing a Solution for Forming a Coating Layer In the step of preparing a solution for forming a coating layer, a solution for forming the coating layer 12 can be prepared.

[0073] Therefore, the coating layer forming solution preparing step can include a first coating layer forming solution preparing step and a second coating layer forming solution preparing step.

[0074] In the first coating layer forming solution preparation step, for example, a first coating layer forming solution for forming the first coating layer 121 can be prepared.

[0075] In the second coating layer forming solution preparation step, a second coating layer forming solution for forming the second coating layer 122 can be prepared.

[0076] The first coating layer forming solution may contain niobium and lithium, for example, in a solvent (dispersion medium) selected from water and one or more alcohols such as ethanol. When ethanol is used as the solvent, the ethanol may be absolute ethanol.

[0077] The supply sources of niobium and lithium are not particularly limited, and include various compounds containing niobium and lithium. The supply sources of niobium and lithium may be compounds containing niobium and lithium, or may be mixtures of compounds containing niobium and compounds containing lithium. As compounds containing niobium or lithium, various alkoxides such as ethoxides can be used.

[0078] The solution for forming the second coating layer may contain, as a solvent (dispersion medium), one or more selected from water and alcohols such as ethanol, and a precursor of a lithium-containing carboxylate or various lithium-containing alkoxides, etc. When ethanol is used as the solvent, the ethanol may be absolute ethanol.

[0079] When a first coating layer 121 or a second coating layer 122 having different compositions is provided on the surface of the positive electrode active material 11, a coating layer forming solution having a different composition can be prepared for the first coating layer forming solution or the second coating layer forming solution. Furthermore, when a layer other than the first coating layer 121 or the second coating layer 122 is provided on the surface of the positive electrode active material 11, a coating layer forming solution matching the intended composition of the layer to be provided can also be prepared in the coating layer forming solution preparation step. (2) Mixing Step In the mixing step, the positive electrode active material, which is the base material, and the coating layer forming solution can be mixed.

[0080] The mixing method used to mix the positive electrode active material and the coating layer forming solution in the mixing step is not particularly limited as long as it is a method that can coat the base material with the coating layer forming solution. For example, a method of spraying the coating layer forming solution while stirring and fluidizing the base material can be used as the mixing method.

[0081] When the first coating layer 121 and the second coating layer 122 are alternately laminated on the surface of the positive electrode active material 11, the coating layer forming solution to be sprayed can be alternately switched between the first coating layer forming solution and the second coating layer forming solution. Furthermore, when a layer other than the first coating layer 121 and the second coating layer 122 is to be provided on the surface of the positive electrode active material 11, a coating layer forming solution corresponding to the layer to be provided can be sprayed onto the positive electrode active material, which is the base material.

[0082] The method for stirring and fluidizing the base material is not particularly limited, but a method that minimizes the crushing of the particles of the positive electrode active material as the base material or damage to them by impact can be suitably used, for example, a tumbling fluidizer. Examples of devices that can be used to mix the positive electrode active material as the base material and the coating layer forming solution in the mixing step include a tumbling fluidized granulation coating device (MP-micro, manufactured by Powrex Corporation) and a fine particle surface continuous coating device (JD-01, manufactured by Kawata Corporation).

[0083] In addition, the mixture can be dried while being mixed by externally heating the mixing device used in the mixing step or by adjusting the temperature of the gas such as air introduced into the device. That is, the mixing step and at least a part of the drying step can be carried out simultaneously. (3) Drying Step In the drying step, the mixture obtained in the mixing step can be dried.

[0084] The drying method and drying conditions used in the drying step are not particularly limited, and can be selected so as to remove the solvent contained in the coating layer-forming solution. The drying step can also be performed, for example, by introducing heated air into a mixing device when mixing the positive electrode active material 11 and the coating layer-forming solution in the mixing step. That is, the drying step can also be performed simultaneously with the mixing step. Alternatively, the drying step can be performed by placing the mixture obtained in the mixing step in a dryer, electric furnace, or the like, and drying the mixture.

[0085] The drying temperature is not particularly limited and can be selected so that the desired coating layer can be obtained, for example, so that the solvent of the coating layer-forming solution can be removed. The lower limit of the drying temperature is preferably 80° C. or higher, and more preferably 120° C. or higher. By setting the drying temperature to 80° C. or higher, the time required to remove the solvent can be reduced, and productivity can be increased.

[0086] The upper limit of the drying temperature is preferably 350°C or less, and more preferably 250°C or less. By setting the drying temperature to 350°C or less, it is possible to suppress the reaction between the coating layer and the base material. [Lithium Secondary Battery] The lithium secondary battery of this embodiment can include a positive electrode, a negative electrode, and a solid electrolyte layer. The lithium secondary battery of this embodiment can also be, for example, a lithium secondary battery consisting of a positive electrode, a negative electrode, and a solid electrolyte layer.

[0087] Specifically, the lithium secondary battery of this embodiment can have a positive electrode 21, a solid electrolyte layer 22, and a negative electrode 23, for example, as in the lithium secondary battery 20 shown in Fig. 2. As shown in Fig. 2, the solid electrolyte layer 22 can be disposed between the positive electrode 21 and the negative electrode 23, and these components can be sealed in a container 24. The positive electrode 21 and the negative electrode 23 can be provided with a positive electrode terminal 211 and a negative electrode terminal 231, respectively, and can be configured to be connectable to components outside the container 24.

[0088] The positive electrode may be any electrode that contains at least the coated positive electrode active material according to one embodiment of the present disclosure, and may be composed solely of the coated positive electrode active material according to one embodiment of the present disclosure, or may contain the coated positive electrode active material described above in addition to other positive electrode active materials and a solid electrolyte.

[0089] The solid electrolyte may be, for example, one or more selected from a sulfide-based solid electrolyte, an oxide-based solid electrolyte, and a polymer electrolyte. The positive electrode may contain, for example, the coated positive electrode active material described above and a sulfide-based solid electrolyte. In addition to the positive electrode active material and the solid electrolyte, the positive electrode may also contain materials such as a conductive aid, a binder, an ionic liquid, and other additives.

[0090] When the positive electrode contains a sulfide-based solid electrolyte, the materials described later in the solid electrolyte section can be suitably used as the sulfide-based solid electrolyte. ２ S-P ２ S ５ The negative electrode may contain at least one negative electrode active material, and may be composed of only the negative electrode active material, or may contain both the negative electrode active material and a solid electrolyte.

[0091] The negative electrode active material may be, for example, a lithium-containing material such as metallic lithium or a lithium alloy, or an occlusion material capable of occluding and desorbing lithium ions. The occlusion material is not particularly limited, but examples include natural graphite, artificial graphite, sintered organic compounds such as phenolic resins, and carbonaceous materials such as coke. The solid electrolyte may be, for example, one or more selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes. In addition to the negative electrode active material and solid electrolyte, the negative electrode may also contain materials such as conductive additives, binders, ionic liquids, and other additives. (3) Solid Electrolyte Layer The solid electrolyte layer may contain a lithium-ion conductive solid electrolyte. The solid electrolyte layer may consist solely of the solid electrolyte, or may also contain materials such as a binder.

[0092] The solid electrolyte used in the lithium secondary battery of this embodiment is not particularly limited as long as it is a solid electrolyte having lithium ion conductivity. For example, one or more solid electrolytes selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes can be used.

[0093] Examples of sulfide-based solid electrolytes include, but are not limited to, sulfide-based amorphous solid electrolytes, sulfide-based crystalline solid electrolytes, and argyrodite-type solid electrolytes. Specific examples of sulfide-based solid electrolytes include Li ７－ｘ P.S. ６－ｘ Cl ｘ solid electrolytes with an argyrodite structure, such as Li ７ P ３ S １１ , Li ３ P.S. ４ , Li ８ P ２ S ９ , Li ２ S-P ２ S ５ - LiI, Li ２ S-P ２ S ５ Li such as -LiI-LiBr ２ S-P ２ S ５ solid electrolyte, Li２ S-P ２ S ５ -GeS ２ (Li １３ GeP ３ S １６ , Li １０ GeP ２ S １２ etc.), LiI-Li ２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５ etc.; or combinations thereof, but are not limited to these.

[0094] Examples of oxide-based solid electrolytes include Li ７ La ３ Zr ２ O １２ , Li ７－ｘ La ３ Zr １－ｘ Nb ｘ O １２ , Li ７－３ｘ La ３ Zr ２ Al ｘ O １２ , Li ３ｘ La ２／３－ｘ TiO ３ , Li １＋ｘ Al ｘ Ti ２－ｘ (P.O. ４ ) ３ , Li １＋ｘ Al ｘ Ge ２－ｘ (P.O. ４ ) ３ , Li ３ P.O. ４ , or Li ３＋ｘ P.O. ４－ｘ N ｘ (LiPON), but is not limited to these.

[0095] Examples of polymer electrolytes include, but are not limited to, polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof.

[0096] The solid electrolyte may be glass or crystallized glass (glass ceramic).

[0097] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0098] First, the evaluation method of the lithium secondary batteries using the coated positive electrode active materials obtained in the following Examples and Comparative Examples will be described. [Evaluation Method] (1) Floating Test The floating test was carried out by preparing an all-solid-state battery containing a sulfide-based solid electrolyte and evaluating it by the following method.

[0099] The obtained coated positive electrode active material and sulfide-based solid electrolyte powder (Li ６ P.S. ５ Cl, a sulfide-based solid electrolyte having an argyrodite structure) were mixed in a mass ratio of coated positive electrode active material:solid electrolyte=70:30, and this mixture was used as a positive electrode.

[0100] The solid electrolyte layer (separator layer) was made of the same solid electrolyte powder as that used for the positive electrode, and the negative electrode was made of a lithium-indium alloy prepared by pressing a small piece of lithium foil onto an indium foil and diffusing lithium into the indium.

[0101] The positive electrode layer, the solid electrolyte layer, and the negative electrode layer were then laminated in this order and pressure-molded to produce an all-solid-state battery, which is a lithium secondary battery.

[0102] The obtained all-solid-state battery was subjected to constant current charging at a current density of 0.1 C up to a cell voltage of 3.93 V (4.55 V based on the Li—In alloy potential) in an environment of 25° C. Subsequently, constant voltage charging was performed at a cell voltage of 3.93 V until the current density reached 0.01 C.

[0103] Thereafter, constant current discharge was performed at a current density of 0.1 C until the cell voltage reached 2.38 V (3.0 V based on the Li—In alloy potential), and then constant voltage discharge was performed at a cell voltage of 2.38 V until the current density reached 0.01 C. Hereinafter, the above charge / discharge operation will also be referred to as the initial charge / discharge. The capacity of the constant current discharge at a current density of 0.1 C at this time was defined as the "initial discharge capacity."

[0104] Thereafter, the all-solid-state battery was placed in a 25°C environment and subjected to constant current charging at a current density of 0.1 C up to a cell voltage of 3.93 V (4.55 V based on the Li-In alloy potential), followed by constant voltage charging at a cell voltage of 3.93 V until the current density reached 0.01 C. The battery was then moved to a 60°C environment, and constant voltage continuous charging (trickle charging) was performed at a cell voltage of 3.93 V for 120 hours (floating test).

[0105] The all-solid-state battery was then returned to 25°C, and constant current discharge and constant voltage discharge were performed under the same conditions as the initial charge and discharge, followed by charge and discharge under the same conditions as the initial charge and discharge. The capacity of the constant current discharge at this time was designated the "capacity after floating test." The ratio of the capacity after floating test to the initial discharge capacity was calculated and designated the "capacity retention rate after floating." (2) Cycle Test An all-solid-state battery was fabricated under the same conditions as the floating test, except that a graphite electrode was used as the negative electrode.

[0106] The produced all-solid-state battery was subjected to constant current charging at a current density of 0.1 C in a 60° C. environment up to a cell voltage of 4.35 V, and then constant voltage charging at a cell voltage of 4.35 V was performed until the current density reached 0.01 C.

[0107] Thereafter, constant current discharge was performed at a current density of 0.1 C until the cell voltage reached 3.00 V, and then constant voltage discharge was performed at a cell voltage of 3.00 V until the current density reached 0.01 C. Hereinafter, the above charge / discharge operation will also be referred to as the initial charge / discharge. The capacity of the constant current discharge at a current density of 0.1 C was taken as the initial discharge capacity.

[0108] Thereafter, charging and discharging were repeatedly performed under the same conditions as the initial charging and discharging, except that the current density during constant current charging and constant current discharging was 1 C in a 60°C environment. The capacity of constant current discharging at a current density of 1 C after 300 cycles was taken as the post-cycle discharge capacity, and the ratio of the post-cycle discharge capacity to the initial discharge capacity was calculated to obtain the cycle capacity retention rate. (Example 1) (1) Production of coated positive electrode active material (1-1) Coating layer forming solution preparation step (first coating layer forming solution preparation step) Li(OC ２ H ５ ) and Nb(OC ２H ５ ) ５ The positive electrode active material powder (LiNi) was mixed so that the mass ratio of Li:Nb was 1:1, and the mixture was added to an anhydrous ethanol solution to prepare a solution for forming a first coating layer. (Preparation of a solution for forming a second coating layer) Lithium oxalate was added to an anhydrous ethanol solution to prepare a solution for forming a second coating layer. (1-2) Mixing and drying steps ０．５ Co ０．２ Mn ０．３ O ２ The coating layer-forming solution was sprayed onto the particle surfaces of the positive electrode active material while mixing the above-mentioned components in a tumbling fluidized bed granulation coating device (MP-micro, manufactured by Powrex Corporation), thereby forming a coating layer on the particle surfaces of the positive electrode active material. In the first mixing step and the second mixing step, the time for spraying the first coating layer-forming solution and the second coating layer-forming solution was selected depending on the target thickness of each layer.

[0109] In the mixing process, the solution for forming the first coating layer was sprayed onto the stirred positive electrode active material powder for a certain period of time, and then the spraying of the solution for forming the first coating layer was stopped, and the mixing of the positive electrode active material powder was continued for about 5 minutes without spraying the solution for forming the coating layer (first mixing process).

[0110] Next, the solution for forming the second coating layer was sprayed onto the stirred positive electrode active material powder for a certain period of time, and then the spraying of the solution for forming the second coating layer was stopped and the mixing of the positive electrode active material powder was continued for about 5 minutes without spraying the solution for forming the coating layer (second mixing process).

[0111] Between the first mixing step and the second mixing step, the positive electrode active material powder was heated to 100° C., and part of the drying step was also carried out in parallel.

[0112] After the second mixing step, the positive electrode active material was dried at 200° C. for 1 hour in an oxygen atmosphere to remove ethanol as the solvent and water adhering to the positive electrode active material, thereby obtaining a coated positive electrode active material.

[0113] By the above operations, a coated positive electrode active material powder was prepared.

[0114] (2) Evaluation of Coated Positive Electrode Active Material (2-1) Cross-Section Observation Cross-sectional TEM images of the obtained coated positive electrode active material were observed. The observed images are shown in Figure 3. In Figure 3, the approximate boundaries between the coating layers are indicated by dotted lines so that each coating layer can be seen.

[0115] 3, it can be seen that a coating layer 12 is disposed on the surface of the positive electrode active material 11. It can be seen that the coating layer 12 has a layered structure including a first coating layer 121 and a second coating layer 122. A carbon vapor deposition film 31 added during observation can be seen outside the second coating layer 122, but it does not constitute the coated positive electrode active material 10.

[0116] It was confirmed from the TEM image that the thickness of the first coating layer 121 was 8.00 nm or more and 9.00 nm or less, and the thickness of the second coating layer 122 was 6.50 nm or more and 7.00 nm or less.

[0117] When the first coating layer 121 and the second coating layer 122 were evaluated using EELS, it was confirmed that the first coating layer 121 contained niobium, lithium, and oxygen, and that the second coating layer 122 contained lithium, carbon, and oxygen.

[0118] When the second coating layer 122 was subjected to electron diffraction measurement, crystalline spots were observed. From the observed electron diffraction pattern, it was found that the second coating layer 122 had crystallinity and contained lithium oxalate (Li ２ C ２ O ４ ) was confirmed to be included.

[0119] Electron diffraction was measured on the first coating layer 121, and no crystal spots were observed, but a halo pattern was observed, confirming that the material contained in the first coating layer 121 was amorphous.

[0120] (2-2) Measurement of the amount of hydrochloric acid standard solution added to the filtrate The coated positive electrode active material was subjected to a mixed solution preparation step, a stirring and standing step, and a filtration step to obtain a filtrate, and then potentiometric titration was performed by adding 0.1 mol / L of hydrochloric acid standard solution dropwise to 60 g of the filtrate. The amount of hydrochloric acid standard solution added to the filtrate for each pH range was then measured.

[0121] In the mixed solution preparation step, 100 g of pure water was added to 2 g of the coated positive electrode active material to prepare a mixed solution.

[0122] In the stirring and standing step, the mixed solution was stirred for 5 minutes by rotating a stirrer in the mixed solution at 400 rpm, and then the mixed solution was stood still for 5 minutes.

[0123] In the filtration step, the supernatant of the mixed liquid after the stirring and standing step was filtered to obtain a filtrate.

[0124] (2-3) Floating Test and Cycle Test An all-solid-state battery was fabricated using the obtained coated cathode active material, and a floating test and a cycle test were performed. The evaluation results are shown in Table 1. (Example 2) (1) Production of Coated Cathode Active Material In the mixing step, the first mixing step and the second mixing step were alternately performed twice each, and two layers of the first coating layer 121 and the second coating layer 122 were alternately laminated on the surface of the cathode active material 11. In the first mixing step and the second mixing step, the time for spraying the first coating layer forming solution and the second coating layer forming solution was selected depending on the target thickness of each layer. Except for the above points, the coated cathode active material was produced under the same conditions as in Example 1.

[0125] (2) Evaluation of Coated Positive Electrode Active Material (2-1) Cross-Section Observation Cross-sectional TEM images of the obtained coated positive electrode active material were observed. The observed images are shown in Figure 4. In Figure 4, the approximate boundaries between the coating layers are indicated by dotted lines so that each coating layer can be seen.

[0126] The observation image shown in Figure 4 confirms that a coating layer 12 is disposed on the surface of the positive electrode active material 11. It was confirmed that the coating layer 12 has a laminated structure in which first coating layers 121 and second coating layers 122 are alternately disposed in pairs on the surface of the positive electrode active material 11. In other words, it was confirmed that the coating layer 12 includes two units, a first unit 12A and a second unit 12B, which are unit structures including the first coating layer 121 and the second coating layer 122. A carbon vapor deposition film 31 added during observation can be seen outside the second coating layer 122, but it does not constitute the coated positive electrode active material 10.

[0127] It was confirmed from the TEM image that the thickness of the first coating layer 121 in the first unit 12A was 4.00 nm or more and 5.00 nm or less, and the thickness of the second coating layer 122 was 2.25 nm or more and 3.00 nm or less.

[0128] It was confirmed from the TEM image that the thickness of the first coating layer 121 in the second unit 12B was 2.70 nm or more and 4.00 nm or less, and the thickness of the second coating layer 122 was 2.31 nm or more and 3.31 nm or less.

[0129] When the first coating layer 121 and the second coating layer 122 were evaluated using EELS, it was confirmed that the first coating layer 121 contained niobium, lithium, and oxygen, and that the second coating layer 122 contained lithium, carbon, and oxygen.

[0130] When the second coating layer 122 was subjected to electron diffraction measurement, crystalline spots were observed. From the observed electron diffraction pattern, it was found that the second coating layer 122 had crystallinity and contained lithium oxalate (Li ２ C ２ O ４ ) was confirmed to be included.

[0131] Electron diffraction was measured on the first coating layer 121, and no crystalline spots were observed, but a halo pattern was observed, confirming that the substance contained in the first coating layer 121 was amorphous. (2-2) Measurement of the amount of hydrochloric acid standard solution added to the filtrate A filtrate was prepared for the obtained coated positive electrode active material using the same procedure as in Example 1, and the amount of hydrochloric acid standard solution added was evaluated. The evaluation results are shown in Table 1.

[0132] (2-3) Floating Test and Cycle Test An all-solid-state battery was fabricated using the obtained coated positive electrode active material, and a floating test and a cycle test were carried out. The evaluation results are shown in Table 1. (Comparative Example 1) The positive electrode active material powder (LiNi) subjected to the mixing step ０．５ Co ０．２ Mn ０．３ O ２ ) was used as the positive electrode active material of Comparative Example 1, and the step of preparing a solution for forming a coating layer and the like were not carried out, i.e., a coating layer was not formed.

[0133] For the positive electrode active material, a filtrate was prepared in the same manner as in Example 1, and the amount of hydrochloric acid standard solution added was evaluated. The evaluation results are shown in Table 1.

[0134] In addition, an all-solid-state battery was fabricated using the positive electrode active material, and a floating test was performed. The evaluation results are shown in Table 1. (Comparative Example 2) (1) Production of Coated Positive Electrode Active Material In the mixing process, only the first mixing process was performed, and only one first coating layer 121 was laminated on the surface of the positive electrode active material 11. In addition, in the first mixing process, the time for spraying the first coating layer forming solution onto the stirred positive electrode active material powder was adjusted so that the target thickness of the first coating layer 121 was approximately twice that of Example 1. Except for the above points, a coated positive electrode active material was produced under the same conditions as Example 1.

[0135] (2) Evaluation of Coated Positive Electrode Active Material (2-1) Cross-Section Observation When a cross-sectional TEM image of the obtained coated positive electrode active material was observed, it was confirmed that one layer of the first coating layer 121 was disposed on the surface of the positive electrode active material 11.

[0136] When the first coating layer 121 was evaluated by EELS, it was confirmed that the first coating layer 121 contained niobium, lithium, and oxygen.

[0137] Electron beam diffraction was measured on the first coating layer 121, and no crystalline spots were observed, but a halo pattern was observed, confirming that the substance contained in the first coating layer 121 was amorphous. (2-2) Measurement of the amount of hydrochloric acid standard solution added to the filtrate A filtrate was prepared for the obtained coated positive electrode active material using the same procedure as in Example 1, and the amount of hydrochloric acid standard solution added was evaluated. The evaluation results are shown in Table 1.

[0138] (2-3) Floating Test and Cycle Test An all-solid-state battery was fabricated using the obtained coated cathode active material, and a floating test and a cycle test were performed. The evaluation results are shown in Table 1. (Comparative Example 3) (1) Production of Coated Cathode Active Material In the mixing step, only the second mixing step was performed, and only one second coating layer 122 was laminated on the surface of the cathode active material 11. In addition, in the second mixing step, the time for spraying the second coating layer-forming solution onto the stirred cathode active material powder was adjusted so that the target thickness of the second coating layer 122 was approximately twice that of Example 1. Except for the above points, a coated cathode active material was produced under the same conditions as Example 1.

[0139] (2) Evaluation of Coated Positive Electrode Active Material (2-1) Cross-Section Observation When a cross-sectional TEM image of the obtained coated positive electrode active material was observed, it was confirmed that one layer of the second coating layer 122 was disposed on the surface of the positive electrode active material 11.

[0140] When the second coating layer 122 was evaluated by EELS, it was confirmed that the second coating layer 122 contained lithium, carbon, and oxygen.

[0141] When the second coating layer 122 was subjected to electron diffraction measurement, crystalline spots were observed. From the observed electron diffraction pattern, it was found that the second coating layer 122 had crystallinity and was composed of lithium oxalate (Li ２ C ２ O ４ (2-2) Measurement of the amount of hydrochloric acid standard solution added to the filtrate A filtrate was prepared from the obtained coated positive electrode active material in the same manner as in Example 1, and the amount of hydrochloric acid standard solution added was evaluated. The evaluation results are shown in Table 1.

[0142] (2-3) Floating Test An all-solid-state battery was fabricated using the obtained coated positive electrode active material, and a floating test was carried out. The evaluation results are shown in Table 1.

[0143] According to the results shown in Table 1, it was confirmed that the lithium secondary batteries using the coated positive electrode active materials of Examples 1 and 2, in which the coating layer included a first coating layer and a second coating layer, had a capacity retention rate of 90% or more after the floating test. In other words, it was confirmed that this method can be applied to all-solid-state batteries, and that it is possible to suppress a decrease in battery capacity even when charging at a high potential.

[0144] Furthermore, it was confirmed that the capacity retention rate after 300 cycles was also high in the lithium secondary batteries using the coated positive electrode active materials of Examples 1 and 2. In particular, it was confirmed that the lithium secondary battery using the coated positive electrode active material of Example 2, which had two first coating layers and two second coating layers, had an extremely high capacity retention rate of 90.1%, and was particularly excellent in cycle characteristics.

[0145] This application claims priority based on Japanese Patent Application No. 2024-022361, filed with the Japan Patent Office on February 16, 2024, the entire contents of which are incorporated herein by reference.

[0146] REFERENCE SIGNS LIST 10 Coated positive electrode active material (coated positive electrode active material for lithium secondary battery) 100 Coated positive electrode active material (coated positive electrode active material for lithium secondary battery) 110 Coated positive electrode active material (coated positive electrode active material for lithium secondary battery) 11 Positive electrode active material 12 Coating layer 12A First unit 12B Second unit 121 First coating layer T121 Thickness 122 Second coating layer T122 Thickness 20 Lithium secondary battery 21 Positive electrode 211 Positive electrode terminal 22 Solid electrolyte layer 23 Negative electrode 231 Negative electrode terminal 24 Container 31 Carbon vapor deposition film

Claims

1. A coated positive electrode active material for a lithium secondary battery used in a positive electrode of an all-solid-state battery in which the electrolyte is a solid electrolyte, the coated positive electrode active material having a positive electrode active material and a coating layer disposed on a surface of the positive electrode active material, the positive electrode active material containing nickel and cobalt and having a layered crystal structure, the coating layer having a laminated structure including a first coating layer and a second coating layer, the first coating layer containing niobium, lithium, and oxygen, and the second coating layer containing a lithium-containing carboxylate.

2. The positive electrode active material for a coated lithium secondary battery according to claim 1, wherein the coating layer has a structure in which the first coating layer and the second coating layer are alternately stacked, and includes at least two or more unit structures each including the first coating layer and the second coating layer.

3. A positive electrode active material for a coated lithium secondary battery according to claim 1 or claim 2, wherein the substance contained in the first coating layer is amorphous, and the lithium-containing carboxylate contained in the second coating layer is crystalline.

4. The coated positive electrode active material for a lithium secondary battery according to claim 1 or 2, wherein, when 60 g of the filtrate obtained through a mixed solution preparation step of adding 100 g of pure water to 2 g of the coated positive electrode active material for a lithium secondary battery to prepare a mixed solution, a stirring and standing step of stirring the mixed solution for 5 minutes by rotating a stirrer in the mixed solution at 400 rpm and then standing for 5 minutes, and a filtration step of filtering the supernatant of the mixed solution after the stirring and standing step to obtain a filtrate, is potentiometrically titrated by dropping a 0.1 mol / L hydrochloric acid standard solution, the amount of the hydrochloric acid standard solution dropped until the pH value of the filtrate reaches 8.3 is 1.5 mL or more and 4.5 mL or less, and the amount of the hydrochloric acid standard solution dropped when the pH value of the filtrate is in the range of 4.5 to 8.3 is 0.5 mL or more and 2.5 mL or less.

5. A lithium secondary battery having a positive electrode, a negative electrode, and a solid electrolyte layer, wherein the positive electrode comprises the positive electrode active material for a coated lithium secondary battery according to claim 1 or 2, and a sulfide-based solid electrolyte.

6. The sulfide-based solid electrolyte is an argyrodite-type sulfide-based solid electrolyte and Li ２ S-P ２ S ５ 6. The lithium secondary battery according to claim 5, comprising one or more solid electrolytes selected from the group consisting of ammonium nitrate, ...

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