Positive electrode active material for a coated lithium secondary battery, lithium secondary battery

Abstract

A coated positive electrode active material for lithium secondary batteries is disclosed for use as the positive electrode of an all-solid-state battery with a solid electrolyte. The coated positive electrode active material comprises a positive electrode active material and a coating layer disposed on the 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 stacked structure comprising 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 carboxylate.

Description

Technical Field

[0001] This invention relates to a coated positive electrode active material for lithium secondary batteries and a lithium secondary battery. Background Technology

[0002] In recent years, with the widespread adoption of portable electronic devices such as mobile phones and laptops, there has been a strong demand for the development of small and lightweight lithium-ion batteries with high energy density. Furthermore, there is a strong demand for high-energy-density lithium-ion batteries for use in electric vehicles.

[0003] As a lithium-ion rechargeable battery that meets these requirements, all-solid-state batteries have attracted attention in recent years. All-solid-state batteries consist of a positive electrode layer, a solid electrolyte layer, and a negative electrode layer. Compared with previous batteries that used electrolytes such as organic solvents, they are highly anticipated for practical application due to their high energy density, high output, high voltage, and high stability.

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

[0005] For example, Patent Document 1 discloses an invention that addresses the problem of providing an electrode body capable of reducing interfacial resistance. Patent Document 1 discloses an electrode body characterized by comprising a positive electrode active material and a second solid electrolyte, wherein the positive electrode active material has an active material and a first solid electrolyte covering more than 70% of the surface of the active material. Patent Document 1 also discloses that the first solid electrolyte is lithium niobate and the second solid electrolyte is a sulfide.

[0006] However, for the lithium secondary battery using an electrode body disclosed in Patent Document 1, the battery capacity sometimes decreases when charged at a high potential.

[0007] <Prior art documents> <Patent Documents> Patent Document 1: Japanese Patent Application Publication No. 2009-193940 Summary of the Invention <Problem to be solved by this invention> In view of the problems of the prior art, the present invention aims to provide a coated positive electrode active material for lithium secondary batteries that is applicable to all-solid-state batteries and can suppress battery capacity reduction even when charged at a high potential.

[0008] <Methods for solving problems> To solve the above problems, according to one aspect of the present invention, a coated positive electrode active material for lithium secondary batteries can be provided as follows: It is used as the positive electrode in all-solid-state batteries where the electrolyte is a solid electrolyte. The coated positive electrode active material for lithium secondary batteries comprises: a positive electrode active material and a coating layer disposed on the 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 comprising a first coating layer and a second coating layer. The first coating layer comprises: niobium, lithium, and oxygen. The second coating layer comprises: a lithium-containing carboxylate.

[0009] <The Effects of the Invention> According to one aspect of the present invention, a coated positive electrode active material for lithium secondary batteries can be provided that is suitable for all-solid-state batteries and can suppress battery capacity reduction even when charged at a high potential. Attached Figure Description

[0010] Figure 1A This is a schematic cross-sectional view of a coated positive electrode active material for a lithium secondary battery according to one embodiment of the present disclosure.

[0011] Figure 1B This is a schematic cross-sectional view of a coated positive electrode active material for a lithium secondary battery according to one embodiment of the present disclosure.

[0012] Figure 1C This is a schematic cross-sectional view of a coated positive electrode active material for a lithium secondary battery according to one embodiment of the present disclosure.

[0013] Figure 2 This is a cross-sectional schematic diagram of a lithium secondary battery.

[0014] Figure 3 This is a cross-sectional TEM image of the coated positive electrode active material for lithium secondary batteries obtained in Example 1.

[0015] Figure 4 This is a cross-sectional TEM image of the coated positive electrode active material for lithium secondary batteries obtained in Example 2. Detailed Implementation

[0016] Hereinafter, the embodiments for carrying out the present invention will be described with reference to the accompanying drawings. The present invention is not limited to the embodiments described below, and various modifications and substitutions can be applied to the embodiments described below without departing from the scope of the present invention.

[0017] [Cathode active material for coated lithium secondary batteries] The coated positive electrode active material for lithium secondary batteries in this embodiment (hereinafter also referred to as "coated positive electrode active material") can be used as the positive electrode (positive electrode layer) of an all-solid-state battery with a solid electrolyte.

[0018] exist Figures 1A to 1C The diagram shows a cross-sectional schematic of the coated positive electrode active material according to this embodiment. It should be noted that... Figures 1A to 1C This is for illustrative purposes only. Therefore, the cross-sectional shape of the particles of the coated positive electrode active material 10, coated positive electrode active material 100, coated positive electrode active material 110, or positive electrode active material 11 is not limited to a circle and can have any shape. In addition, the coating layer 12 does not need to be of a fixed thickness. Figure 1B , Figure 1C The coated positive electrode active material 100 and coated positive electrode active material 110 shown are variations of the structure of the coating layer 12, and therefore mainly utilize... Figure 1A Provide explanations, and utilize them when necessary. Figure 1B , Figure 1C Please provide an explanation.

[0019] like Figure 1A As shown, the coated positive electrode active material 10 of this embodiment may have: a positive electrode active material 11 and a coating layer 12 disposed on the surface of the positive electrode active material 11.

[0020] The components contained in the coated positive electrode active material 10 of this embodiment will be described below.

[0021] (1) Positive electrode active material In this embodiment, the positive electrode active material 11 contained in the coated positive electrode active material can be any positive electrode active material that can insert / detach lithium (Li) through an electrochemical reaction.

[0022] The positive electrode active material 11 may include, for example, nickel and cobalt.

[0023] As the positive electrode active material 11, for example, a composite oxide containing lithium, nickel and cobalt can be mentioned. As the positive electrode active material 11, for example, a composite oxide containing lithium (Li), nickel (Ni), cobalt (Co), and element M (M) in a molar ratio of Li:Ni:Co:M = a:x:y:z can be mentioned. It should be noted that a, x, y, and z satisfy x+y+z=1 and 0.8≤a≤1.2. x and y can be selected such that the sum of x, y and z is 1, and can satisfy 0<x<1 and 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 set.

[0024] The above composite oxide can be represented by the general formula: Li a Ni x Co y M z O 2+α It should be noted that α in the general formula preferably satisfies -0.2≤α≤0.2. Since x, y, z, a, and the element M in the general formula have been described, their descriptions are omitted.

[0025] It should be noted that the positive electrode active material 11 contained in the coated positive electrode active material 10 of the present embodiment may also be a mixture of a plurality of positive electrode active materials with different compositions.

[0026] The positive electrode active material preferably has a layered structure, that is, a layered crystal structure. This is because in the case of a positive electrode active material having a layered crystal structure, especially when applied to a lithium secondary battery, the output characteristics can be improved. When the positive electrode active material has a layered crystal structure, the positive electrode active material may have a layered rock salt structure (α-NaFeO₂ type structure) or the like.

[0027] As described above, the positive electrode active material 11 preferably contains at least cobalt (Co). When the positive electrode active material 11 contains cobalt, the structure of the positive electrode active material 11 is easily stabilized during charge and discharge, so that deterioration of the positive electrode active material 11 when the battery is used at high voltage can be suppressed.

[0028] The structure of the above positive electrode active material can be identified by, for example, analytical methods such as X-ray diffraction and electron diffraction. The molar ratio of elements contained in the positive electrode active material can be determined by, for example, analytical methods such as fluorescent X-ray analysis and ICP (Inductively Coupled Plasma) emission spectrometry.

[0029] There are no particular restrictions on the shape of the positive electrode active material. For example, it can be a positive electrode active material particle with an average particle size of several nm to tens of μm and the form of primary particles or secondary particles formed by the aggregation of primary particles, or it can be a thin film positive electrode film. As a thin film positive electrode film, for example, a positive electrode film formed by the PLD (Pulsed Laser Deposition) method can be cited.

[0030] (2) Covering layer The coating layer 12 may have a stacked structure comprising a first coating layer 121 and a second coating layer 122. It should be noted that the stacking order of the first coating layer 121 and the second coating layer 122 is not particularly limited. The layers may be stacked starting from the position near the positive electrode active material 11, in the order of the first coating layer 121 and the second coating layer 122, or they may be stacked starting from the position near the positive electrode active material 11, in the order of the second coating layer 122 and the first coating layer 121. The coating layer 12 may contain layers other than the first coating layer 121 and the second coating layer 122, or it may consist only of the first coating layer 121 and the second coating layer 122.

[0031] The coating layer 12 only needs to cover at least a portion of the surface of the positive electrode active material 11. However, the higher the proportion of the area of ​​the positive electrode active material 11 covered by the coating layer 12, the better.

[0032] The first coating layer 121 and the second coating layer 122 may have a stacked structure that is stacked over the entire 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 on at least a portion of the periphery of the positive electrode active material 11 to have a stacked structure.

[0033] (2-1) Regarding the composition of the first and second covering layers (First covering 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 determined using EELS (Electron Energy Loss Spectroscopy), EDX (Energy Dispersive X-ray Spectroscopy), and other methods.

[0034] The elements contained in the first coating layer 121 may form compounds or exist as monomers without forming compounds.

[0035] When the niobium, lithium, and oxygen contained in the first coating layer 121 form a compound, the first coating layer 121 may also contain one or more compounds selected from, for example, lithium niobate (lithium niobate oxide), lithium oxide (Li₂O), and niobium oxide (NbO). It should be noted that examples of lithium niobate include, for example, Li₃NbO₄, LiNbO₃, LiNb₃O₈, and Li₈Nb₂O₉, but the proportions of lithium, niobium, and oxygen contained in lithium niobate are not limited to the composition ratios of the above chemical formulas.

[0036] By including the first coating layer 121 in the coating layer 12, when applied to a lithium secondary battery, it is possible to prevent the formation of a high-resistivity layer between the positive electrode active material 11 and the solid electrolyte, and to suppress the interface resistance.

[0037] The material contained in the first coating layer 121 can also be amorphous. By making the contained material amorphous (non-crystalline) for the first coating layer 121, lithium-ion conductivity can be improved, and in particular, the positive electrode resistance in lithium secondary batteries can be reduced.

[0038] For example, if no crystal diffraction spots are observed when electron diffraction is performed on the first coating layer 121, it can be confirmed that the material contained in the first coating layer 121 is amorphous.

[0039] Specifically, the following steps 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 resin or the like, and then subjected to focused ion beam (FIB) processing to prepare a cross-sectional observation sample. Then, the cross-sectional observation sample is observed using TEM (transmission electron microscopy), and EELS or EDX measurements are performed as needed to determine the position of the first coating layer 121. Next, electron diffraction measurements are performed on the determined first coating layer 121. If no crystalline diffraction spots are observed in the obtained diffraction pattern, it can be evaluated that the material contained in the first coating layer 121 is amorphous. In this case, a halo pattern can be observed in the diffraction pattern. Alternatively, if crystalline diffraction spots are observed in the obtained diffraction pattern, it can be confirmed that the material contained in the first coating layer 121 is crystalline, i.e., it is crystalline.

[0040] (Second covering layer) The second coating layer 122 may contain lithium-containing carboxylates. The elements contained in the second coating layer 122 can be analyzed and determined by EELS, EDX, etc.

[0041] The lithium-containing carboxylate contained in the second coating layer 122 can also be crystalline. Therefore, the lithium-containing carboxylate contained in the second coating layer 122 can also be determined by the diffraction pattern of electron diffraction. 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 for evaluation, except that the position of the second coating layer 122 is determined by TEM observation instead of determining the position of the first coating layer 121, and these points are measured by electron diffraction of the second coating layer 122. When crystalline diffraction spots are observed in the diffraction pattern of the second coating layer 122 and are consistent with the pattern of the lithium-containing carboxylate, it can be determined that the second coating layer 122 contains a lithium-containing carboxylate. Furthermore, it can be determined that the lithium-containing carboxylate contained in the second coating layer 122 is crystalline.

[0042] The lithium-containing carboxylate contained in the second coating layer 122 may be selected from one or more of lithium oxalate, lithium malonate, lithium succinate, lithium glutarate, lithium adipate, etc.

[0043] The valence of the carboxylic acid in lithium-containing carboxylates is not particularly limited; for example, it can be set to 2.

[0044] The second coating layer 122 possesses lithium-ion conductivity and also functions as an electrical insulating layer. Therefore, even when the coated positive electrode active material 10 of this embodiment is applied to a lithium secondary battery, the voltage applied to the first coating layer 121, the positive electrode active material 11, and the solid electrolyte can be suppressed during high-potential charging, preventing the application of high voltage and inhibiting the oxidative decomposition of the solid electrolyte. Furthermore, the decomposition of the first coating layer 121, the positive electrode active material 11, and the solid electrolyte due to applied voltage can be prevented. For the lithium secondary battery containing the coated positive electrode active material 10 of this embodiment, even when charging at a high potential, the reduction in battery capacity can be prevented.

[0045] It should be noted that, in this specification, charging at a high potential means, for example, applying a voltage of 4.4V or higher using a Li-In alloy potentiometer.

[0046] As described above, the lithium-containing carboxylate contained in the second coating layer 122 can also be crystalline. By making the lithium-containing carboxylate crystalline, the electrical insulation properties of the second coating layer 122 can be particularly improved, and the decomposition of the first coating layer 121, the positive electrode active material 11, and the solid electrolyte due to the application of voltage can be particularly prevented.

[0047] Furthermore, the lithium-containing carboxylate contained in the second coating layer 122 can also have a layered crystal structure, and lithium can be inserted and extracted. 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 also be improved.

[0048] The second coating layer 122 may also contain substances other than lithium-containing carboxylates. For example, the second coating layer 122 may also contain lithium carbonate (Li₂CO₃). For instance, lithium carbonate may be intentionally added to the second coating layer 122. Alternatively, the lithium carbonate may be derived from the decomposition of the lithium-containing carboxylates contained in the second coating layer 122, or from impurities contained in the positive electrode active material 11. It should be noted 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.

[0049] By including lithium carbonate in the second coating layer 122, the electrical insulation properties of the second coating layer 122 can be improved. Therefore, the decomposition of the first coating layer 121, the positive electrode active material 11, and the solid electrolyte due to the applied voltage is particularly prevented. For the lithium secondary battery containing the coated positive electrode active material 10 of this embodiment, even when charged at a high potential, the reduction in battery capacity can be particularly prevented.

[0050] (2-2) Regarding the 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. For example, it can be set to be 0.5nm or more and 200nm or less, or it can be 1nm or more and 20nm or less.

[0051] By setting the thickness T121 of the first coating layer 121 to 0.5 nm or more, when applied to lithium secondary batteries, a high-resistivity layer can be prevented from forming between the positive electrode active material 11 and the solid electrolyte, and the interface resistance can be particularly suppressed.

[0052] By setting the thickness T121 of the first coating layer 121 to less than 200 nm, when applied to lithium-ion secondary batteries, the proportion of layers that do not contribute to charging and discharging can be suppressed, and the battery capacity can be particularly improved.

[0053] (Thickness of the second coating layer) The thickness T122 of the second coating layer 122 is not particularly limited. For example, it can be set to be greater than 0.5 nm and less than 200 nm, or greater than 1 nm and less than 20 nm.

[0054] By setting the thickness T122 of the second coating layer 122 to 0.5 nm or more, even when it is used in a lithium secondary battery and charged and discharged 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 high voltage can be particularly prevented. Therefore, the decomposition of the first coating layer 121, the positive electrode active material 11, and the solid electrolyte due to the applied voltage can be particularly prevented, and for the lithium secondary battery including the coated positive electrode active material 10 of this embodiment, the reduction of battery capacity can be particularly prevented even when charging at a high potential.

[0055] In addition, by setting the thickness T122 of the second coating layer 122 to 200 nm or less, even when it is suitable for lithium secondary batteries and charged and discharged 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 too low.

[0056] The thicknesses T121 of the first coating layer 121 and 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 that cross-section. Which layer is the first coating layer 121 or the second coating layer 122 can be determined, for example, by analyzing the elemental composition of the layer to be evaluated using EELS, EDX, or similar methods. Furthermore, 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 location in the TEM image of the observed cross-section.

[0057] (2-3) Regarding the number of layers of the first and second covering layers The number and arrangement of the first cover layer 121 and the second cover layer 122 of the cover layer 12 are not particularly limited.

[0058] For example, such as Figure 1B , Figure 1C As shown, 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 consist only of a structure in which a first coating layer 121 and a second coating layer 122 are alternately stacked, or it may further include other layers besides the first coating layer 121 and the second coating layer 122, or portions that are irregularly stacked.

[0059] It should be noted that the first coating layer 121 and the second coating layer 122 are not limited to... Figure 1A , Figure 1B , Figure 1C The layering order shown can also be configured by sequentially placing the second coating layer 122 and the first coating layer 121 from the position close to the positive electrode active material 11.

[0060] In addition, such as Figure 1B , Figure 1C Like the coated positive electrode active material 100 and coated positive electrode active material 110 shown, the coating layer 12 may also include at least two units, including a unit structure containing a first coating layer 121 and a second coating layer 122.

[0061] Although Figure 1B , Figure 1C In the coated positive electrode active material 100 and coated positive electrode active material 110, an example is shown where the coating layer 12 includes two unit structures, a first unit 12A and a second unit 12B, but the configuration is not limited to this. The coating layer 12 may also include a unit structure with three or more units, including a first coating layer 121 and a second coating layer 122.

[0062] The coating layer 12 preferably comprises one or more but less than ten first coating layers 121 and second coating layers 122. More preferably, the coating layer 12 comprises one or more but less than three first coating layers 121 and second coating layers 122.

[0063] When the coating layer 12 includes the first coating layer 121 and the second coating layer 122 in the above-described unit structure, it is preferable to include a unit structure with one or more units and less than ten units, and more preferably to include a unit structure with one or more units and less than three units.

[0064] By including one or more first coating layers 121 and second coating layers 122 in each of the coating layers 12, the decomposition of the first coating layer 121, the positive electrode active material 11, and the solid electrolyte due to the applied voltage can be prevented. Therefore, for a lithium secondary battery including the coated positive electrode active material 10 of this embodiment, the reduction in battery capacity can be prevented even when charging at a high potential. In addition, when the coated positive electrode active material of this embodiment is applied to a lithium secondary battery, the formation of a high-resistivity layer between the positive electrode active material 11 and the solid electrolyte can be prevented, and the interfacial resistance can be suppressed.

[0065] By including 10 or fewer layers of the first coating layer 121 and the second coating layer 122, the productivity in manufacturing the coated positive electrode active material of this embodiment can be improved.

[0066] The number of layers of the first covering layer 121 and the second covering layer 122 of the covering layer 12 need not be the same, for example, Figure 1C As shown in the coated positive electrode active material 110, the coating layer 12 comprises three first coating layers 121 and two second coating layers 122, and the number of layers may be different.

[0067] When the first coating layer 121 in the coating layer 12 contains multiple layers, the thickness of the first coating layer 121 and the composition of the contained material can be the same or different. The same applies to the second coating layer 122.

[0068] (2-4) Regarding titration characteristics The proportions of the first coating layer 121 and the second coating layer 122 in the coated positive electrode active material 10 of this embodiment can be determined, for example, by titration with hydrochloric acid.

[0069] Specifically, for example, the filtrate for titration can be prepared by first going through the following steps: mixing preparation, stirring and settling, and filtration.

[0070] In the preparation of the mixture, 100g of pure water is added to 2g of the coated positive electrode active material to be evaluated to prepare the mixture.

[0071] In the stirring and settling process, for the mixture obtained in the mixing preparation process, the stirrer is rotated at 400 rpm for 5 minutes in the mixture, and then settling for 5 minutes.

[0072] In the filtration process, the supernatant of the mixture after the stirring and settling process can be filtered to obtain the filtrate. For example, a syringe filter can be used in the filtration.

[0073] Then, add 0.1 mol / L hydrochloric acid standard solution dropwise to 60 g of the filtrate obtained through the above process, so that potentiometric titration can be performed.

[0074] In this case, it is preferable that the amount of hydrochloric acid standard solution added before the pH of the filtrate reaches 8.3 is more than 1.5 mL and less than 4.5 mL.

[0075] In addition, the amount of hydrochloric acid standard solution added in the region where the pH of the filtrate is above 4.5 and below 8.3 is preferably 0.5 mL or more and 2.5 mL or less.

[0076] The hydrochloric acid standard solution added dropwise before the pH of the filtrate reaches 8.3 is mainly 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 dropwise before the pH of the filtrate reaches 8.3 is 1.5 mL or more, it means that the coated positive electrode active material 10 contains a sufficient proportion of the second coating layer 122. Therefore, it is possible to prevent the first coating layer 121, the positive electrode active material 11, and the solid electrolyte from decomposing due to the applied voltage. In addition, for the lithium secondary battery containing the coated positive electrode active material 10 of this embodiment, even when charged at a high potential, it is possible to prevent the reduction of battery capacity.

[0077] By setting the amount of hydrochloric acid standard solution added before the pH 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 becomes particularly appropriate, thereby improving the productivity of the coated positive electrode active material in this embodiment.

[0078] The hydrochloric acid standard solution added dropwise in the region where the pH of the filtrate is above 4.5 and below 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 added dropwise in the region where the pH of the filtrate is above 4.5 and below 8.3 is 0.5 mL or more, when the coated positive electrode active material 10 is applied to a lithium secondary battery, it is possible to prevent the formation of a high-resistivity layer between the positive electrode active material 11 and the solid electrolyte. Furthermore, it is possible to suppress the interfacial resistance between the positive electrode active material 11 and the solid electrolyte.

[0079] Furthermore, by setting the amount of hydrochloric acid standard solution added in the region where the pH of the filtrate is above 4.5 and below 8.3 to 2.5 mL or less, the proportion of the first coating layer 121 contained in the coated positive electrode active material 10 is within an appropriate range, which can improve productivity.

[0080] [Manufacturing method of positive electrode active material for coated lithium secondary batteries] The method for manufacturing the coated positive electrode active material for lithium secondary batteries according to this embodiment is not particularly limited. Since the method for manufacturing the coated positive electrode active material according to this embodiment can manufacture a coated positive electrode active material according to one aspect of this disclosure, some of the already described aspects are omitted.

[0081] The method for manufacturing a coated positive electrode active material for a lithium secondary battery according to this embodiment may include, for example, a solution preparation step for coating layer formation, a mixing step, and a drying step.

[0082] In the coating formation solution preparation process, a coating formation solution for forming a coating layer can be prepared.

[0083] In the mixing process, the positive electrode active material, which serves as the base material, and the coating layer forming solution can be mixed.

[0084] In the drying process, it is possible to dry the mixture obtained through the mixing process.

[0085] The thickness, uniformity, and crystallinity of the coating layer on the surface of the positive electrode active material can be controlled by the conditions of the solution used for coating layer formation, mixing, and drying.

[0086] The following is a description of each process.

[0087] (1) Solution preparation process for coating formation In the coating layer forming solution preparation process, a coating layer forming solution for forming coating layer 12 can be prepared.

[0088] Therefore, the solution preparation process for coating formation can include a first solution preparation process for coating formation and a second solution preparation process for coating formation.

[0089] The solution preparation step for forming the first coating layer can, for example, prepare a solution for forming the first coating layer 121.

[0090] The solution preparation step for forming the second coating layer can prepare a solution for forming the second coating layer 122.

[0091] The solution for forming the first coating layer may use one or more solvents (dispersants) selected from water, ethanol, and other alcohols, and may contain niobium and lithium. When ethanol is used as a solvent, the ethanol may be anhydrous ethanol.

[0092] There are no particular limitations on the supply sources of niobium and lithium; various compounds containing niobium and lithium can be listed. The supply sources of niobium and lithium can be compounds containing niobium and lithium, or mixtures of compounds containing niobium and lithium. Various alkoxides, such as ethoxides, can be used as compounds containing niobium and lithium.

[0093] The solution for forming the second coating layer may use one or more solvents (dispersants) selected from water, ethanol, and other alcohols, and may contain precursor substances such as lithium-containing carboxylates and various lithium-containing alkoxides. When ethanol is used as a solvent, the ethanol may be anhydrous ethanol.

[0094] When a first coating layer 121 or a second coating layer 122 with different compositions is provided on the surface of the positive electrode active material 11, coating layer forming solutions with different compositions can also be prepared for the first coating layer forming solution or the second coating layer forming solution. Alternatively, when layers other than the first coating layer 121 and the second coating layer 122 are provided on the surface of the positive electrode active material 11, a coating layer forming solution with a target composition matching the configured layer can be prepared during the coating layer forming solution preparation process.

[0095] (2) Mixing process In the mixing process, the positive electrode active material, which serves as the base material, can be mixed with the coating layer forming solution.

[0096] Regarding the mixing method used when mixing the positive electrode active material and the coating forming solution in the mixing process, there are no particular limitations as long as the method can cover the coating forming solution onto the base material. For example, a method can be used where the base material is stirred and made to flow while the coating forming solution is sprayed.

[0097] When the first coating layer 121 and the second coating layer 122 are alternately stacked on the surface of the positive electrode active material 11, the coating layer forming solution can be alternately switched between the first coating layer forming solution and the second coating layer forming solution for spraying. Furthermore, when layers other than the first coating layer 121 and the second coating layer 122 are also provided on the surface of the positive electrode active material 11, the coating layer forming solution corresponding to the provided layers can be sprayed onto the positive electrode active material, which serves as the base material.

[0098] There are no particular limitations on the method of agitating and flowing the base material. Methods that reduce particle breakage of the positive electrode active material as the base material or reduce damage from impact can be used, such as a rotary flow device. In the mixing process, examples of devices capable of mixing the positive electrode active material as the base material with the coating layer forming solution include rotary flow granulation coating devices (manufactured by Powrex, MP-micro) and continuous particle surface coating devices (manufactured by Kawata, JD-01).

[0099] Alternatively, drying can be performed simultaneously with mixing by heating the mixing device used in the mixing process from the outside or by adjusting the temperature of the gas, such as air, introduced into the device. That is, at least a part of the mixing process and the drying process can be performed at the same time.

[0100] (3) Drying process The drying process allows for the drying of the mixture obtained in the mixing process.

[0101] There are no particular limitations on the drying method and conditions used in the drying process. For example, the drying method and conditions can be selected to remove the solvent contained in the coating forming solution. For the drying process, for example, heated air can be introduced into the mixing apparatus when mixing the positive electrode active material 11 with the coating forming solution in the mixing process. That is, the drying process can also be performed simultaneously with the mixing process. Alternatively, the mixture obtained in the mixing process can be placed in a dryer or electric furnace to dry the mixture, thereby performing the drying process.

[0102] The drying temperature can be selected in a way that allows the target coating to be obtained, such as removing the solvent from the coating-forming solution; there is no particular limitation. As a lower limit for the drying temperature, 80°C or higher is preferred, and 120°C or higher is more preferred. By setting the drying temperature to 80°C or higher, the time required for solvent removal can be suppressed, improving productivity.

[0103] As an upper limit for the drying temperature, a temperature below 350°C is preferred, and a temperature below 250°C is more preferred. By setting the drying temperature below 350°C, the reaction between the coating layer and the base material can be suppressed.

[0104] [Lithium-ion rechargeable battery] The lithium secondary battery of this embodiment may include a positive electrode, a negative electrode, and a solid electrolyte layer. The lithium secondary battery of this embodiment may also be, for example, a lithium secondary battery composed of a positive electrode, a negative electrode, and a solid electrolyte layer.

[0105] Specifically, the lithium secondary battery of this embodiment can be, for example, as follows: Figure 2 The lithium secondary battery 20 shown has a positive electrode 21, a solid electrolyte layer 22, and a negative electrode 23. Figure 2 As shown, a 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 are respectively provided with a positive terminal 211 and a negative terminal 231, so that they can be configured to be connected to components outside the container 24.

[0106] The following is a description of each component.

[0107] (1) Positive electrode The positive electrode only needs to include at least one of the coated positive electrode active materials of the present disclosure. It can be a structure consisting only of one of the coated positive electrode active materials of the present disclosure, or it can be a structure that includes the already described coated positive electrode active material and other positive electrode active materials or solid electrolytes.

[0108] As a solid electrolyte, one or more selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes can be used. The positive electrode may contain, for example, a coated positive electrode active material and a sulfide-based solid electrolyte as described above. In addition, the positive electrode may contain conductive additives, binders, ionic liquids, or other additives, in addition to the positive electrode active material and solid electrolyte.

[0109] When the positive electrode contains a sulfide-based solid electrolyte, the solid electrolyte material described later can be suitably used as the sulfide-based solid electrolyte. As the sulfide-based solid electrolyte, the positive electrode preferably includes one or more of, for example, an Argyrodite-type sulfide solid electrolyte and a Li2S-P2S5-based solid electrolyte.

[0110] (2) Negative electrode The negative electrode only needs to contain at least a negative electrode active material. It can be a structure consisting only of a negative electrode active material, or a structure containing a negative electrode active material and a solid electrolyte.

[0111] As the negative electrode active material, lithium-containing materials such as metallic lithium and lithium alloys, or lithium-absorbing materials capable of absorbing and de-intercalating lithium ions, can be used. The absorbing material is not particularly limited; for example, sintered organic compounds such as natural graphite, artificial graphite, and phenolic resin, as well as carbonaceous materials such as coke, can be used. As the solid electrolyte, one or more selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes can be used. Furthermore, in addition to the negative electrode active material and solid electrolyte, the negative electrode may also include conductive additives, binders, ionic liquids, or other additives.

[0112] (3) Solid electrolyte layer The solid electrolyte layer only needs to contain a solid electrolyte that is conductive to lithium ions. The solid electrolyte layer can be a structure consisting only of a solid electrolyte, or it can contain materials such as adhesives.

[0113] The solid electrolyte used in the lithium secondary battery of this embodiment is not particularly limited as long as it is a solid electrolyte with lithium-ion conductivity. For example, one or more types selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes can be used as the solid electrolyte.

[0114] Examples of sulfide-based solid electrolytes include amorphous sulfide-based solid electrolytes, crystalline sulfide-based solid electrolytes, and sulfide-silver-germanium ore type solid electrolytes, but are not limited to these. Specific examples of sulfide-based solid electrolytes include Li... 7-x PS 6-x Cl x Solid electrolytes with an isothioargylgermanium mineral structure; Li7P3S 11 Li2S-P2S5 series solid electrolytes, such as Li3PS4, Li8P2S9, Li2S-P2S5-LiI, Li2S-P2S5-LiI-LiBr; and Li2S-P2S5-GeS2 (Li 13 GeP3S 16 Li 10GeP2S 12 (e.g.), LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, etc.; or combinations thereof, but not limited to these.

[0115] Examples of oxide-based solid electrolytes include 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 not limited to these.

[0116] Examples of polymer electrolytes include polyethylene glycol (PEO), polypropylene glycol (PPO), and their copolymers, but are not limited to these.

[0117] Solid electrolytes can be in a glassy state or in a crystallized glass (glass-ceramic) state.

[0118] [Example] The present invention will be further described in detail below with reference to embodiments, but the present invention is not limited to these embodiments in any way.

[0119] First, an evaluation method for lithium secondary batteries using the coated positive electrode active material obtained in the following examples and comparative examples will be explained.

[0120] [Evaluation Method] (1) Floating test The float charge test is conducted by fabricating and evaluating all-solid-state batteries containing sulfide-based solid electrolytes using the following methods.

[0121] The coated positive electrode active material and the sulfide-based solid electrolyte powder (Li6PS5Cl, sulfide-based solid electrolyte with silver-germanium sulfide structure) were mixed at a mass ratio of coated positive electrode active material: solid electrolyte = 70:30, and the mixture was used as the positive electrode.

[0122] The same solid electrolyte layer powder used in the positive electrode is used in the solid electrolyte layer (membrane layer). Additionally, for the negative electrode, a lithium-indium alloy is used, which is made by pressing small pieces of lithium foil onto indium foil, allowing lithium to diffuse into the indium.

[0123] Then, the positive electrode layer, solid electrolyte layer, and negative electrode layer are stacked in sequence and pressurized to form an all-solid-state battery, which serves as a lithium secondary battery.

[0124] The obtained all-solid-state battery was charged at a constant current density of 0.1C at 25°C until the battery voltage reached 3.93V (4.55V based on the Li-In alloy potential). Then, it was charged at a constant voltage of 3.93V until the current density reached 0.01C.

[0125] Then, constant current discharge is performed at a current density of 0.1C until the battery voltage reaches 2.38V (3.0V based on the Li-In alloy potential). Next, constant voltage discharge is performed at the battery voltage of 2.38V until the current density reaches 0.01C. This charge-discharge operation is also referred to as the initial charge-discharge. The capacity of the constant current discharge at this current density of 0.1C is defined as the "initial discharge capacity".

[0126] Next, the all-solid-state battery was charged at a constant current density of 0.1C at 25°C until the battery voltage reached 3.93V (4.55V based on the Li-In alloy potential). Then, it was charged at a constant voltage of 3.93V until the current density reached 0.01C. Following this, the battery was moved to a 60°C environment and subjected to 120 hours of continuous constant voltage charging (trickle charge) at 3.93V (float charge test).

[0127] Then, the all-solid-state battery was returned to 25°C, and constant current and constant voltage discharges were performed under the same conditions as the initial charge-discharge. Then, charge-discharge was performed under the same conditions as the initial charge-discharge. The capacity of the constant current discharge at this point was set as the "capacity after float charge test". Then, the ratio of the capacity after float charge test to the initial discharge capacity was calculated and used as the "capacity retention rate after float charge".

[0128] (2) Cyclic test In addition to using graphite electrodes as the negative electrode, all-solid-state batteries were prepared under the same conditions as in the float charge test.

[0129] The fabricated all-solid-state battery was charged at a constant current of 0.1C at 60°C until the battery voltage reached 4.35V. Then, it was charged at a constant voltage of 4.35V until the current density reached 0.01C.

[0130] Then, constant current discharge was performed at a current density of 0.1C until the battery voltage reached 3.00V, followed by constant voltage discharge at 3.00V until the current density reached 0.01C. This charge-discharge operation is also referred to as the initial charge-discharge. The capacity of the constant current discharge at this current density of 0.1C is taken as the initial discharge capacity.

[0131] Afterwards, except that the current density during constant current charging and discharging was set to 1C at 60°C, the charging and discharging were repeatedly performed under the same conditions as the initial charging and discharging. Then, the capacity of constant current discharge at a current density of 1C 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 and used as the cycle capacity retention rate.

[0132] (Example 1) (1) Manufacturing of coated positive electrode active material (1-1) Solution preparation process for coating formation (First coating layer formation solution preparation step) Li(OC2H5) and Nb(OC2H5)5 were mixed at a molar ratio of Li:Nb of 1:1 and added to an anhydrous ethanol solution to prepare a solution for forming the first coating layer.

[0133] (Second coating layer formation solution preparation step) Lithium oxalate was added to an anhydrous ethanol solution to prepare a solution for forming the second coating layer.

[0134] (1-2) Mixing process and drying process The positive electrode active material powder (LiNi) 0.5 Co 0.2 Mn 0.3 O2 is mixed in a rotating flow granulation coating apparatus (manufactured by Powrex, MP-micro) while a coating layer forming solution is sprayed, thereby forming a coating layer on the surface of the positive electrode active material particles. In the first mixing step and the second mixing step, the spraying time of the first coating layer forming solution and the second coating layer forming solution is selected according to the target thickness of each layer.

[0135] In the mixing process, the positive electrode active material powder is sprayed with the solution for forming the first coating layer for a certain period of time while it is being stirred. Then the spraying of the solution for forming the first coating layer is stopped. After about 5 minutes without spraying the solution for forming the coating layer, the mixing of the positive electrode active material powder continues (first mixing process).

[0136] Next, the solution for forming the second coating layer is sprayed onto the stirred positive electrode active material powder for a certain period of time. Then, the spraying of the solution for forming the second coating layer is stopped. After about 5 minutes without spraying the solution for forming the coating layer, the mixing of the positive electrode active material powder continues (second mixing step).

[0137] It should be noted that between the first and second mixing steps, the positive electrode active material powder is heated to 100°C, and a partial drying process is also carried out.

[0138] After the second mixing process is completed, the material is dried at 200°C for 1 hour in an oxygen atmosphere to remove the water and ethanol used as a solvent attached to the positive electrode active material, thereby obtaining a coated positive electrode active material.

[0139] Through the above operations, coated positive electrode active material powder was produced.

[0140] (2) Evaluation of coated positive electrode active material (2-1) Cross-sectional observation Cross-sectional TEM images of the obtained coated positive electrode active material were observed. The observed images are shown below. Figure 3 In. Figure 3 In order to distinguish the various coating layers, dashed lines are used to show the approximate boundaries between the coating layers.

[0141] from Figure 3 The observed images confirm that the coating layer 12 is disposed on the surface of the positive electrode active material 11. It can be confirmed that the coating layer 12 has a stacked structure comprising a first coating layer 121 and a second coating layer 122. Although a carbon vapor deposition film 31, added during observation, is visible on the outer side of the second coating layer 122, it is not a component of the coated positive electrode active material 10.

[0142] It can be confirmed that the thickness of the first coating layer 121 determined from the TEM image is 8.00 nm or more and 9.00 nm or less, and the thickness of the second coating layer 122 is 6.50 nm or more and 7.00 nm or less.

[0143] The first coating 121 and the second coating 122 were evaluated using EELS, and it was confirmed that the first coating 121 contains niobium, lithium and oxygen, and the second coating 122 contains lithium, carbon and oxygen.

[0144] Electron diffraction was performed on the second coating layer 122, and crystalline diffraction spots were observed. Based on the observed electron diffraction pattern, it can be confirmed that it is crystalline and contains lithium oxalate (Li2C2O4).

[0145] Electron diffraction was performed on the first coating layer 121, but no crystal diffraction spots were observed. Instead, halos were observed, confirming that the substance contained in the first coating layer 121 is in an amorphous state.

[0146] (2-2) Determination of the amount of hydrochloric acid standard solution added to the filtrate The obtained coated positive electrode active material underwent a mixture preparation process, a stirring and settling process, and a filtration process to obtain a filtrate. Then, 60g of the filtrate was added dropwise with a 0.1mol / L hydrochloric acid standard solution to perform potentiometric titration. The amount of hydrochloric acid standard solution added was then measured for each pH range of the filtrate.

[0147] In the preparation of the mixed solution, 100g of pure water is added to 2g of the coated positive electrode active material to prepare the mixed solution.

[0148] In the stirring and settling process, the mixture is stirred at 400 rpm for 5 minutes and then left to stand for 5 minutes.

[0149] In the filtration process, the supernatant of the mixture after the stirring and settling process is filtered to obtain the filtrate.

[0150] (2-3) Float test, cyclic test All-solid-state batteries were prepared using the obtained coated positive electrode active material, and float charge and cycle tests were conducted. The evaluation results are shown in Table 1.

[0151] (Example 2) (1) Manufacturing of coated positive electrode active material In the mixing process, the first mixing process and the second mixing process are performed alternately twice, and two layers each of the first coating layer 121 and the second coating layer 122 are alternately deposited on the surface of the positive electrode active material 11. In the first mixing process and the second mixing process, the spraying time of the solution for forming the first coating layer and the solution for forming the second coating layer are selected according to the target thickness of each layer. Except for the above points, the coated positive electrode active material was manufactured under the same conditions as in Example 1.

[0152] (2) Evaluation of coated positive electrode active material (2-1) Cross-sectional observation Cross-sectional TEM images of the obtained coated positive electrode active material were observed. The observed images are shown below. Figure 4 In. Figure 4 In order to distinguish the various coating layers, dashed lines are used to show the approximate boundaries between the coating layers.

[0153] from Figure 4 The observed images confirm that the coating layer 12 is disposed on the surface of the positive electrode active material 11. It can be confirmed that the coating layer 12 has a stacked structure with two alternating layers of a first coating layer 121 and a second coating layer 122 disposed on the surface of the positive electrode active material 11. That is, it can be confirmed that the coating layer 12 comprises two units, a first unit 12A and a second unit 12B, and that the first unit 12A and the second unit 12B are unit structures comprising the first coating layer 121 and the second coating layer 122. Although a carbon vapor deposition film 31, added during observation, is visible on the outer side of the second coating layer 122, it is not a component of the coated positive electrode active material 10.

[0154] It can be confirmed that the thickness of the first coating layer 121 in the first unit 12A determined by the TEM image is 4.00 nm or more and 5.00 nm or less, and the thickness of the second coating layer 122 is 2.25 nm or more and 3.00 nm or less.

[0155] It can be confirmed that the thickness of the first coating layer 121 in the second unit 12B determined by the TEM image is more than 2.70 nm and less than 4.00 nm, and the thickness of the second coating layer 122 is more than 2.31 nm and less than 3.31 nm.

[0156] The first coating 121 and the second coating 122 were evaluated using EELS, and it was confirmed that the first coating 121 contains niobium, lithium and oxygen, and the second coating 122 contains lithium, carbon and oxygen.

[0157] Electron diffraction was performed on the second coating layer 122, and crystalline diffraction spots were observed. Based on the observed electron diffraction pattern, it can be confirmed that it is crystalline and contains lithium oxalate (Li2C2O4).

[0158] Electron diffraction was performed on the first coating layer 121, but no crystal diffraction spots were observed. Instead, halos were observed, confirming that the substance contained in the first coating layer 121 is in an amorphous state.

[0159] (2-2) Determination of the amount of hydrochloric acid standard solution added to the filtrate The obtained coated positive electrode active material was used to prepare a filtrate following the same steps as in Example 1, and the amount of hydrochloric acid standard solution added was evaluated. The evaluation results are shown in Table 1.

[0160] (2-3) Float test, cyclic test All-solid-state batteries were prepared using the obtained coated positive electrode active material, and float charge and cycle tests were conducted. The evaluation results are shown in Table 1.

[0161] (Comparative Example 1) The positive electrode active material powder (LiNi) provided in the mixing process 0.5 Co 0.2 Mn 0.3 O2) was used as the positive electrode active material in Comparative Example 1. The solution preparation process for coating formation was not performed, and therefore no coating was formed.

[0162] Regarding the positive electrode active material, the filtrate was prepared following 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.

[0163] In addition, all-solid-state batteries were prepared using positive electrode active materials, and float charge tests were conducted. The evaluation results are shown in Table 1.

[0164] (Comparative Example 2) (1) Manufacturing of coated positive electrode active material In the mixing process, only the first mixing process is performed, and only one first coating layer 121 is deposited on the surface of the positive electrode active material 11. Furthermore, in the first mixing process, the spraying time of the first coating layer forming solution onto the stirred positive electrode active material powder is adjusted so that the target thickness of the first coating layer 121 is approximately twice that of Example 1. Except for the above points, the coated positive electrode active material is manufactured under the same conditions as in Example 1.

[0165] (2) Evaluation of coated positive electrode active material (2-1) Cross-sectional observation By observing the cross-sectional TEM image of the obtained coated positive electrode active material, it can be confirmed that a first coating layer 121 is disposed on the surface of the positive electrode active material 11.

[0166] The first coating 121 was evaluated using EELS, and it was confirmed that the first coating 121 contains niobium, lithium, and oxygen.

[0167] Electron diffraction was performed on the first coating layer 121, but no crystal diffraction spots were observed. Instead, halos were observed, confirming that the substance contained in the first coating layer 121 is in an amorphous state.

[0168] (2-2) Determination of the amount of hydrochloric acid standard solution added to the filtrate The obtained coated positive electrode active material was used to prepare a filtrate following the same steps as in Example 1, and the amount of hydrochloric acid standard solution added was evaluated. The evaluation results are shown in Table 1.

[0169] (2-3) Float test, cyclic test All-solid-state batteries were prepared using the obtained coated positive electrode active material, and float charge and cycle tests were conducted. The evaluation results are shown in Table 1.

[0170] (Comparative Example 3) (1) Manufacturing of coated positive electrode active material In the mixing process, only the second mixing process is performed, and only one second coating layer 122 is deposited on the surface of the positive electrode active material 11. Furthermore, in the second mixing process, the spraying time of the second coating layer forming solution onto the stirred positive electrode active material powder is adjusted so that the thickness of the target second coating layer 122 is approximately twice that of Example 1. Except for the above points, the coated positive electrode active material is manufactured under the same conditions as in Example 1.

[0171] (2) Evaluation of coated positive electrode active material (2-1) Cross-sectional observation By observing the cross-sectional TEM image of the obtained coated positive electrode active material, it can be confirmed that the first second coating layer 122 is disposed on the surface of the positive electrode active material 11.

[0172] The second coating 122 was evaluated using EELS, and it was confirmed that the second coating 122 contains lithium, carbon, and oxygen.

[0173] Electron diffraction was performed on the second coating layer 122, and crystalline diffraction spots were observed. Based on the observed electron diffraction pattern, it can be confirmed that it is crystalline and contains lithium oxalate (Li2C2O4).

[0174] (2-2) Determination of the amount of hydrochloric acid standard solution added to the filtrate The obtained coated positive electrode active material was used to prepare a filtrate following the same steps as in Example 1, and the amount of hydrochloric acid standard solution added was evaluated. The evaluation results are shown in Table 1.

[0175] (2-3) Float test All-solid-state batteries were fabricated using the obtained coated positive electrode active material, and float charge tests were conducted. The evaluation results are shown in Table 1.

[0176] [Table 1] Based on the results shown in Table 1, it can be confirmed that in the lithium secondary batteries using coated positive electrode active materials of Examples 1 and 2, which have a first coating layer and a second coating layer, the capacity retention rate after the float charge test is over 90%. That is, it can be confirmed that this method is applicable to all-solid-state batteries, and can suppress the decrease in battery capacity even when charged at a high potential.

[0177] Furthermore, it can be confirmed that in the lithium secondary batteries using the coated positive electrode active materials of Examples 1 and 2, the capacity retention rate is still very high after 300 cycles. In particular, it can be confirmed that the lithium secondary battery using the coated positive electrode active material of Example 2, which has two layers of the first coating layer and two layers of the second coating layer, has an extremely high capacity retention rate of 90.1% and particularly excellent cycle characteristics.

[0178] This application claims priority based on Japanese Patent Application No. 2024-022361 filed with the Japan Patent Office on February 16, 2024, and the entire contents of Japanese Patent Application No. 2024-022361 are incorporated in this international application.

[0179] Explanation of reference numerals in the attached figures 10. Coated positive electrode active material (coated positive electrode active material for lithium secondary batteries) 100 Coated Positive Electrode Active Materials (Coated Positive Electrode Active Materials for Lithium Secondary Batteries) 110 Coated positive electrode active material (coated positive electrode active material for lithium secondary batteries) 11 Positive Electrode Active Material 12 covering layers Unit 1, 12A Unit 2 of 12B 121 First Covering Layer T121 thickness 122 Second Covering Layer T122 thickness 20 Lithium Secondary Battery 21 positive electrode 211 positive extreme particle 22 Solid Electrolyte Layer 23 Negative electrode 231 Negative Extreme Substance 24 containers 31 carbon vapor deposition film

Claims

1. A coated positive electrode active material for lithium secondary batteries, used as the positive electrode in all-solid-state batteries with a solid electrolyte. The coated positive electrode active material for lithium secondary batteries comprises: a positive electrode active material and a coating layer disposed on the 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 comprising a first coating layer and a second coating layer. The first coating layer comprises: niobium, lithium, and oxygen. The second coating layer comprises: a lithium-containing carboxylate.

2. The coated positive electrode active material for lithium secondary batteries according to claim 1, wherein, The coating layer comprises a structure formed by alternating layers of the first coating layer and the second coating layer, and includes at least two unit structures comprising the first coating layer and the second coating layer.

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

4. The coated positive electrode active material for lithium secondary batteries according to claim 1 or claim 2, wherein, When potentiometric titration is performed by adding 0.1 mol / L hydrochloric acid standard solution dropwise to 60 g of the following filtrate, The amount of hydrochloric acid standard solution added dropwise before the pH of the filtrate reaches 8.3 is more than 1.5 mL and less than 4.5 mL. The amount of hydrochloric acid standard solution added dropwise in the region where the pH of the filtrate is above 4.5 and below 8.3 is between 0.5 mL and 2.5 mL. The filtrate is obtained through the following steps: mixing preparation, stirring and settling, and filtration. In the mixture preparation process, 100g of pure water is added to 2g of the coated positive electrode active material for lithium secondary batteries to prepare the mixture. In the stirring and settling process, the mixture is stirred at 400 rpm for 5 minutes, and then allowed to stand for 5 minutes. In the filtration process, the supernatant of the mixture after the stirring and settling process is filtered to obtain the filtrate.

5. A lithium secondary battery, comprising: a positive electrode, a negative electrode, and a solid electrolyte layer. The positive electrode comprises: the coated positive electrode active material for lithium secondary batteries as described in claim 1 or claim 2, and a sulfide-based solid electrolyte.

6. The lithium secondary battery according to claim 5, wherein, The sulfide-based solid electrolyte includes one or more types selected from sulfide-based solid electrolytes of silver-germanium ore and Li2S-P2S5 solid electrolytes.

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