Coated positive electrode active material for lithium secondary battery and lithium secondary battery

Abstract

To provide a coated positive electrode active material for a lithium secondary battery, which can be applied to an all-solid-state battery and can suppress degradation of battery capacity even when being charged at high potentials.SOLUTION: There is provided a coated positive electrode active material for a lithium secondary battery, which is used for a positive electrode of an all-solid-state battery, an electrolyte of the coated positive electrode active material being a solid electrolyte. The coated positive electrode active material includes a positive electrode active material and a coating layer disposed on a surface of the positive electrode active material. 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 carboxylates containing lithium.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[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, in the case of a lithium secondary battery using the electrode assembly disclosed in Patent Document 1, when the battery is charged at a high potential, the battery capacity may decrease. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-193940 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

[0009] In order to solve the above problem, according to one aspect of the present invention, A coated positive electrode active material for a lithium secondary battery used in a positive electrode of an all-solid-state battery, wherein the electrolyte is a solid electrolyte, a positive electrode active material and a coating layer disposed on a surface of the positive electrode active material; 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; The second coating layer provides a coated positive electrode active material for a lithium secondary battery, which contains a carboxylate containing lithium. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0011] [Figure 1]FIG. 1 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. [Figure 2] FIG. 2 is a cross-sectional view of a lithium secondary battery. [Figure 3] FIG. 3 is a cross-sectional TEM image of the coated positive electrode active material for a lithium secondary battery obtained in Example 1. [Figure 4] FIG. 4 is a cross-sectional TEM image of the coated positive electrode active material for a lithium secondary battery obtained in Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. However, 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. [Coated cathode active material for lithium secondary batteries] The coated positive electrode active material for a 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] Schematic cross-sectional views of the coated positive electrode active material of this embodiment are shown in FIGS. 1(A) to 1(C). Note that FIGS. 1(A) to 1(C) are merely schematic views. Therefore, the cross-sectional shapes of particles of the coated positive electrode active material 10, the coated positive electrode active material 100, the coated positive electrode active material 110, and the positive electrode active material 11 are not limited to circular shapes, and may have any shape. Furthermore, the coating layer 12 does not need to have a constant thickness. The coated positive electrode active material 100 and the coated positive electrode active material 110 shown in FIGS. 1(B) and 1(C) are modified examples of the configuration of the coating layer 12, and therefore will be described primarily using FIG. 1(A), with FIGS. 1(B) and 1(C) used as needed.

[0014] As shown in FIG. 1(A), 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 of the components 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) through an electrochemical reaction.

[0016] Positive electrode active material 11 can include, 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 x+y+z=1, 0.8≦a≦1.2, and 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 element M is an optional additive element, 0≦z<1 can be satisfied.

[0018] The above 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 may have a layered rock salt structure (α-NaFeO2 structure) or the like.

[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 emission spectroscopy, etc.

[0023] The shape of the positive electrode active material is not particularly limited, and may be, for example, positive electrode active material particles having an average particle diameter of several nm to several tens of μm and in the form of primary particles or secondary particles formed by aggregation of primary particles, or may be a thin film positive electrode film. An example of a thin film positive electrode film is a positive electrode film formed by a PLD (pulsed laser deposition) method. (2) Covering layer The coating layer 12 can 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 more preferable it is.

[0025] The first coating layer 121 and the second coating layer 122 may have a laminated structure in which they are stacked 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 stacked 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 and second coating layers (1st coating layer) The first coating layer 121 can 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 contain one or more compounds selected from, for example, lithium niobate (lithium niobium oxide), lithium oxide (Li2O), niobium oxide (NbO), etc. Examples of lithium niobate include Li3NbO4, LiNbO3, LiNb3O8, and Li8Nb2O9, but the content ratios of lithium, niobium, and oxygen contained in the lithium niobate are 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. Specifically, for example, the following procedure can be used to evaluate whether the substance 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 (FIB) processing. The cross-sectional observation sample is then observed using a transmission electron microscope (TEM), and, if necessary, EELS or EDX measurements are 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, it can be determined that the substance contained in the first coating layer 121 is 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. (2nd coating layer) The second coating layer 122 may contain a carboxylate containing lithium. The elements contained in the second coating layer 122 may be analyzed and identified by EELS, EDX, or the like.

[0031] 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. Whether the lithium-containing carboxylate contained in the second coating layer 122 is crystalline 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.

[0032] 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.

[0033] The valence of the carboxylic acid in the lithium-containing carboxylate is not particularly limited, but may be, for example, divalent.

[0034] The second coating layer 122 has lithium ion conductivity and 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. Furthermore, the first coating layer 121, the cathode active material 11, and the solid electrolyte are prevented from being decomposed by the application of a voltage, and a lithium secondary battery including the coated cathode active material 10 of this embodiment can be prevented from losing battery capacity even when charging at a high potential. In this specification, charging at a high potential means charging by applying a voltage of, for example, 4.4 V or more based on the Li—In alloy potential.

[0035] As described above, the lithium-containing carboxylate contained in the second coating layer 122 may be crystalline. 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.

[0036] 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.

[0037] The second coating layer 122 may also contain a substance other than a lithium-containing carboxylate. The second coating layer 122 may also contain, for example, lithium carbonate (Li2CO3). The lithium carbonate may be intentionally added to the second coating layer 122. The lithium carbonate may also be contained in the second coating layer 122 due to decomposition of the lithium-containing carboxylate contained in the second coating layer 122 or due to impurities contained in the positive electrode active material 11. 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.

[0038] 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 positive electrode active material 11, and the solid electrolyte from being decomposed by the application of voltage, and particularly prevents a decrease in the battery capacity of a lithium secondary battery including the coated positive electrode active material 10 of this embodiment, even when the battery is charged at a high potential. (2-2) Thickness of the first and second coating layers (Thickness of the first coating layer) The thickness T121 of the first coating layer 121 is not particularly limited, but may be, for example, 0.5 nm or more and 200 nm or less, and may be 1 nm or more and 20 nm or less.

[0039] By setting the thickness T121 of the first coating layer 121 to 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 to suppress the interfacial resistance.

[0040] Furthermore, by setting the thickness T121 of the first coating layer 121 to 200 nm or less, when applied to a lithium ion secondary battery, the proportion of layers that do not contribute to charging and discharging can be reduced, and the battery capacity can be particularly increased. (Thickness of the second coating layer) The thickness T122 of the second coating layer 122 is not particularly limited, but may be, for example, 0.5 nm or more and 200 nm or less, and may be 1 nm or more and 20 nm or less.

[0041] By setting the thickness T122 of the second coating layer 122 to 0.5 nm or more, even when the battery is applied to a lithium secondary battery and charging and discharging are performed at a high potential, the voltage applied to the first coating layer 121, the positive electrode 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 positive electrode 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 positive electrode active material 10 of this embodiment, even when the battery is charged at a high potential.

[0042] 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.

[0043] 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 it 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 using, for example, EELS or EDX. Then, the thicknesses of the first coating layer 121 and the second coating layer 122 can be measured by measuring the distance between the boundary lines of each layer at any position in the TEM image of the observed cross section. (2-3) Number of layers of the first and second coating layers The number and arrangement of the first coating layer 121 and the second coating layer 122 included in the coating layer 12 are not particularly limited.

[0044] 1(B) and 1(C), the coating layer 12 may include a structure in which a first coating layer 121 and a second coating layer 122 are alternately laminated. The coating layer 12 may be configured only from a structure in which the first coating layer 121 and the second coating layer 122 are alternately laminated, 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 laminated. The first coating layer 121 and the second coating layer 122 are not limited to the stacking order shown in Figures 1(A), 1(B), and 1(C), 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.

[0045] Furthermore, like the coated positive electrode active material 100 and the coated positive electrode active material 110 shown in Figures 1(B) and 1(C), 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.

[0046] 1(B) and 1(C) 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.

[0047] 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. When the coating layer 12 contains the first coating layer 121 and the second coating layer 122 in the unit structure, it preferably contains 1 to 10 units of the unit structure, and more preferably 1 to 3 units.

[0048] By including one or more first coating layers 121 and one or more second coating layers 122 in coating layer 12, it is possible to prevent decomposition of first coating layer 121, positive electrode active material 11, and solid electrolyte due to application of voltage. Therefore, for a lithium secondary battery including coated positive electrode active material 10 of this embodiment, it is possible to prevent a decrease in battery capacity even when charged at a high potential. Furthermore, when the coated positive electrode 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 positive electrode active material 11 and the solid electrolyte, thereby suppressing interfacial resistance.

[0049] When the coating layer 12 includes 10 or less first coating layers 121 and 10 or less second coating layers 122, productivity can be improved when producing the coated positive electrode active material of this embodiment.

[0050] 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, as in the coated positive electrode active material 110 shown in Figure 1(C), 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.

[0051] When the coating layer 12 includes multiple layers, the thickness of the first coating layer 121 and the composition of the substances contained therein may be the same or different, and the same is true for the second coating layer 122. (2-4) Titration characteristics The content ratio of the first coating layer 121 and the second coating layer 122 in the coated positive electrode active material 10 of this embodiment can also be determined by titration with hydrochloric acid, for example.

[0052] 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.

[0053] 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. In the stirring and standing step, the mixture obtained in the mixture preparation step can be stirred for 5 minutes by rotating a stirring bar (stirrer) at 400 rpm in the mixture, and then allowed to stand for 5 minutes. 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.

[0054] Then, 60 g of the filtrate obtained through the above process can be titrated potentiometrically by adding a 0.1 mol / L hydrochloric acid standard solution dropwise.

[0055] 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.

[0056] Furthermore, it is preferable that the amount of the hydrochloric acid standard solution added dropwise 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.

[0057] The hydrochloric acid standard solution added until the pH value of the filtrate reaches 8.3 is primarily used to react with substances contained in the second coating layer 122, such as lithium-containing carboxylates. 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.

[0058] 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, thereby increasing the productivity of the coated positive electrode active material of this embodiment.

[0059] 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 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 used in 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.

[0060] 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 or more and 8.3 or less, the proportion of the first coating layer 121 contained in the coated positive electrode active material 10 is within an appropriate range, thereby improving productivity. [Method of manufacturing a positive electrode active material for a coated lithium secondary battery] The method for producing a coated positive electrode active material for a lithium secondary battery according to the present embodiment is not particularly limited. Since the method for producing a coated positive electrode active material according to one aspect of the present disclosure can be used to produce the coated positive electrode active material, some of the matters already described will not be described again. 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.

[0061] In the coating layer forming solution preparation step, a coating layer forming solution for forming a coating layer can be prepared.

[0062] In the mixing step, the positive electrode active material as the base material and the coating layer forming solution can be mixed.

[0063] In the drying step, the mixture obtained in the mixing step can be dried.

[0064] 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.

[0065] Each step will be described below. (1) Preparation process of solution for coating layer formation In the coating layer forming solution preparation step, a coating layer forming solution for forming the coating layer 12 can be prepared.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] The first coating layer forming solution may contain niobium and lithium in a solvent (dispersion medium) selected from, for example, water and one or more alcohols such as ethanol. When ethanol is used as the solvent, it may be absolute ethanol.

[0070] 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.

[0071] 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 precursors such as lithium-containing carboxylates and various lithium-containing alkoxides. When ethanol is used as the solvent, the ethanol may be absolute ethanol. When the first coating layer 121 or the second coating layer 122, which have different compositions, etc., are to be 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 to be 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 process In the mixing step, the positive electrode active material as the base material and the coating layer forming solution can be mixed.

[0072] 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.

[0073] 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 formed on the surface of the positive electrode active material 11, the coating layer forming solution corresponding to the layer to be formed can be sprayed onto the positive electrode active material as the base material.

[0074] The method for stirring and fluidizing the base material is not particularly limited, but a method that minimizes the crushing of the base material positive electrode active material particles or damage due to impact can be suitably used, such as a tumbling fluidizer. Examples of devices that can be used to mix the base material positive electrode active material and the coating layer forming solution in the mixing step include a tumbling fluidized granulation coating device (Powrex Corporation, MP-micro) and a fine particle surface continuous coating device (Kawata Corporation, JD-01).

[0075] Alternatively, the mixing step can be performed while drying by externally heating the mixing device used in the mixing step or by adjusting the temperature of a gas such as air introduced into the device. In other words, the mixing step and at least a part of the drying step can be performed simultaneously. (3) Drying process In the drying step, the mixture obtained in the mixing step can be dried.

[0076] 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.

[0077] 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.

[0078] 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 may include a positive electrode, a negative electrode, and a solid electrolyte layer. For example, the lithium secondary battery of this embodiment may be a lithium secondary battery composed of a positive electrode, a negative electrode, and a solid electrolyte layer.

[0079] 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.

[0080] Each component will be described below. (1) Positive electrode The positive electrode may be any positive electrode that includes at least the coated positive electrode active material according to one embodiment of the present disclosure, and may be configured to include only the coated positive electrode active material according to one embodiment of the present disclosure, or may be configured to include the coated positive electrode active material described above in addition to other positive electrode active materials and a solid electrolyte.

[0081] 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.

[0082] When the positive electrode contains a sulfide-based solid electrolyte, the sulfide-based solid electrolyte may be one of the solid electrolytes described below. The positive electrode preferably contains, as the sulfide-based solid electrolyte, one or more selected from, for example, an argyrodite-type sulfide-based solid electrolyte and a Li2S-P2S5-based solid electrolyte. (2) Negative electrode The negative electrode may contain at least a negative electrode active material, and may be composed of only a negative electrode active material, or may contain a negative electrode active material and a solid electrolyte.

[0083] Examples of the negative electrode active material include lithium-containing materials such as metallic lithium and lithium alloys, and storage materials capable of absorbing and desorbing lithium ions. The storage 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. Examples of the solid electrolyte include 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, and ionic liquids, as well as other additives. (3) Solid electrolyte layer The solid electrolyte layer may be any layer as long as it contains a lithium ion conductive solid electrolyte, and the solid electrolyte layer may be composed of only a solid electrolyte, or may contain a material such as a binder.

[0084] The solid electrolyte used in the lithium secondary battery of this embodiment is not particularly limited as long as it has lithium ion conductivity. For example, the solid electrolyte may be one or more selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes.

[0085] 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 PS 6-x Cl x Argyrodite-type solid electrolytes such as Li7P3S 11 , Li3PS4, Li8P2S9, Li2S-P2S5-LiI, Li2S-P2S5-LiI-LiBr, and other Li2S-P2S5-GeS2 (Li 13 GeP3S 16 , Li 10 GeP2S 12 LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, etc.; or combinations thereof.

[0086] An example of an oxide-based solid electrolyte is Li7La3Zr2O 12 , Li 7-x La3Zr 1-x Nb x O 12 , Li 7-3x La3Zr2Al x O 12 , Li 3x La 2 / 3-x TiO3, Li 1+x Al x Ti 2-x (PO4)3, Li 1+x Al x Ge 2-x (PO4)3, Li3PO4, or Li 3+x PO 4-x N x (LiPON), etc., but are not limited to these.

[0087] Examples of polymer electrolytes include, but are not limited to, polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof.

[0088] The solid electrolyte may be glass or crystallized glass (glass ceramic). [Example]

[0089] 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. First, the evaluation method for 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 carrying out an evaluation by the following method.

[0090] The obtained coated positive electrode active material and a sulfide-based solid electrolyte powder (Li6PS5Cl, a sulfide-based solid electrolyte with 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.

[0091] 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.

[0092] 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.

[0093] 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. Subsequently, 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. The above charge / discharge operation is hereinafter also referred to as the initial charge / discharge. The capacity of the constant-current discharge at a current density of 0.1 C was defined as the "initial discharge capacity."

[0094] The all-solid-state battery was then 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 placed in a 60°C environment and subjected to constant voltage continuous charging (trickle charging) at a cell voltage of 3.93 V for 120 hours (floating test).

[0095] The all-solid-state battery was then returned to 25°C, and constant current and constant voltage discharges were performed under the same conditions as the initial charge and discharge, followed by charging and discharging under the same conditions as the initial charge and discharge. The constant current discharge capacity 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 in the floating test, except that a graphite electrode was used for the negative electrode.

[0096] The fabricated all-solid-state battery was charged at a constant current of 0.1 C in a 60°C environment up to a cell voltage of 4.35 V, and then charged at a constant voltage of 4.35 V until the current density reached 0.01 C.

[0097] 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. The above charge / discharge operation is hereinafter also 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.

[0098] 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 an environment of 60° C. The capacity of constant current discharge 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) Manufacturing of coated positive electrode active material (1-1) Solution preparation process for coating layer formation (First coating layer forming solution preparation step) Li(OC2H5) and Nb(OC2H5)5 were 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. (Second coating layer formation solution preparation step) Lithium oxalate was added to an anhydrous ethanol solution to prepare a solution for forming a second coating layer. (1-2) Mixing process, drying process Cathode active material powder (LiNi 0.5 Co 0.2 Mn 0.3 O2) was mixed in a tumbling fluidized bed granulation coating device (Powrex Corporation, MP-micro) while spraying the coating layer solution to form a coating layer on the particle surface of the positive electrode active material. In the first and second mixing steps, the spraying times of the first and second coating layer solutions were selected depending on the target thickness of each layer.

[0099] 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).

[0100] 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).

[0101] Between the first mixing step and the second mixing step, the positive electrode active material powder is heated to 100° C., and part of the drying step is also carried out in parallel.

[0102] After the second mixing step, the mixture was dried at 200° C. for 1 hour in an oxygen atmosphere to remove ethanol, which was the solvent, and water adhering to the positive electrode active material, thereby obtaining a coated positive electrode active material.

[0103] By the above operations, a coated positive electrode active material powder was prepared.

[0104] (2) Evaluation of coated positive electrode active material (2-1) Cross-sectional observation A cross-sectional TEM image of the obtained coated positive electrode active material was observed, and the observed image is 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.

[0105] 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 the observation can be seen outside the second coating layer 122, but it does not constitute the coated positive electrode active material 10.

[0106] It was confirmed that the thickness of the first coating layer 121 obtained from the TEM image 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.

[0107] When the first coating layer 121 and the second coating layer 122 were evaluated by 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.

[0108] When electron beam diffraction was measured for the second coating layer 122, crystalline spots were observed, and the observed electron beam diffraction pattern confirmed that the layer was crystalline and contained lithium oxalate (Li2C2O4).

[0109] 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 substance contained in the first coating layer 121 was amorphous.

[0110] (2-2) Measurement of the amount of hydrochloric acid standard solution added to the filtrate The resulting coated cathode 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 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.

[0111] 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.

[0112] In the stirring and standing step, the mixed liquid was stirred for 5 minutes by rotating a stirrer in the mixed liquid at 400 rpm, and then the mixed liquid was stood still for 5 minutes.

[0113] In the filtration step, the supernatant of the mixed liquid after the stirring and standing step was filtered to obtain a filtrate.

[0114] (2-3) Floating test, 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. Example 2 (1) Manufacturing of coated positive electrode active material In the mixing step, the first mixing step and the second mixing step were alternately performed twice each, and two first coating layers 121 and two second coating layers 122 were alternately laminated on the surface of the positive electrode 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 positive electrode active material was produced under the same conditions as in Example 1.

[0115] (2) Evaluation of coated positive electrode active material (2-1) Cross-sectional observation A cross-sectional TEM image of the obtained coated positive electrode active material was observed, and the observed image is 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.

[0116] From the observation image shown in FIG. 4, it can be seen 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.

[0117] 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.

[0118] 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.

[0119] When the first coating layer 121 and the second coating layer 122 were evaluated by 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.

[0120] When electron beam diffraction was measured for the second coating layer 122, crystalline spots were observed, and the observed electron beam diffraction pattern confirmed that the layer was crystalline and contained lithium oxalate (Li2C2O4).

[0121] 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 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 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.

[0122] (2-3) Floating test, 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 0.5 Co 0.2 Mn 0.3 O2) 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, that is, a coating layer was not formed.

[0123] 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. In addition, an all-solid-state battery was fabricated using the positive electrode active material and subjected to a floating test. The evaluation results are shown in Table 1. (Comparative Example 2) (1) Manufacturing of coated positive electrode active material In the mixing step, only the first mixing step 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 step, 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 would be about 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.

[0124] (2) Evaluation of coated positive electrode active material (2-1) Cross-sectional observation When a cross-sectional TEM image of the obtained coated positive electrode active material was observed, it was confirmed that one layer of first coating layer 121 was disposed on the surface of positive electrode active material 11 .

[0125] When the first coating layer 121 was evaluated by EELS, it was confirmed that the first coating layer 121 contained niobium, lithium, and oxygen.

[0126] 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 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 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.

[0127] (2-3) Floating test, 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 3) (1) Manufacturing of coated positive electrode 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 positive electrode active material 11. In addition, in the second mixing step, the time for spraying the second coating layer forming solution onto the stirred positive electrode active material powder was adjusted so that the target thickness of the second coating layer 122 would be about 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.

[0128] (2) Evaluation of coated positive electrode active material (2-1) Cross-sectional observation When a cross-sectional TEM image of the obtained coated positive electrode active material was observed, it was confirmed that one layer of second coating layer 122 was disposed on the surface of positive electrode active material 11 .

[0129] When the second coating layer 122 was evaluated by EELS, it was confirmed that the second coating layer 122 contained lithium, carbon, and oxygen.

[0130] When electron beam diffraction was measured for the second coating layer 122, crystalline spots were observed, and the observed electron beam diffraction pattern confirmed that the layer was crystalline and contained lithium oxalate (Li2C2O4). (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.

[0131] (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. [Table 1] 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.

[0132] Furthermore, it was confirmed that the lithium secondary batteries using the coated positive electrode active materials of Examples 1 and 2 also had high capacity retention rates after 300 cycles. 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. [Explanation of symbols]

[0133] 10. Coated cathode active material (coated cathode active material for lithium secondary batteries) 100 Coated cathode active material (coated cathode active material for lithium secondary batteries) 110 Coated cathode active material (coated cathode active material for lithium secondary batteries) 11 Cathode active material 12 Covering layer 12A 1st Unit 12B 2nd Unit 121 1st coating layer T121 Thickness 122 Second coating layer T122 Thickness 20. Lithium secondary battery 21 Positive electrode 211 Positive terminal 22 Solid electrolyte layer 23 Negative electrode 231 Negative terminal 24 Container 31 Carbon 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, wherein the electrolyte is a solid electrolyte, a positive electrode active material and a coating layer disposed on a surface of the positive electrode active material; 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; The second coating layer comprises a carboxylate containing lithium.

2. 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. the first coating layer contains an amorphous material; 3. The positive electrode active material for a coated lithium secondary battery according to claim 1, wherein the lithium-containing carboxylate contained in the second coating layer has crystallinity.

4. a mixed solution preparation step of adding 100 g of pure water to 2 g of the coated positive electrode active material for lithium secondary batteries 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 the mixed solution for 5 minutes; a filtration step of filtering a supernatant of the mixed solution after the stirring and standing step to obtain a filtrate; and when 60 g of the filtrate obtained through this step was potentiometrically titrated by adding a 0.1 mol / L hydrochloric acid standard solution dropwise, 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; 3. The positive electrode active material for a coated lithium secondary battery according to claim 1, wherein the amount of the hydrochloric acid standard solution added dropwise 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.

5. a positive electrode, a negative electrode, and a solid electrolyte layer; 3. A lithium secondary battery, wherein the positive electrode comprises the coated positive electrode active material for lithium secondary batteries 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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