Positive electrode active material and lithium ion secondary battery

Abstract

The purpose of the present invention is to provide: a positive electrode active material with which it is possible to obtain a lithium ion secondary battery having further improved discharge capacity; and a lithium ion secondary battery using the positive electrode active material. In addition, efficiency of energy can be improved. The positive electrode active material of the present invention is a cobalt-free positive electrode active material having a lithium nickel manganese composite oxide as a carrier, wherein particulate silica is supported on the surface of the lithium nickel manganese composite oxide.

Description

Technical Field

[0001] This invention relates to positive electrode active materials and lithium-ion secondary batteries. Background Technology

[0002] In recent years, research and development have been conducted on secondary batteries that contribute to higher energy efficiency. In particular, lithium-ion secondary batteries are becoming increasingly important as a power source for electric vehicles (EVs), hybrid electric vehicles (HEVs), and other similar applications.

[0003] Positive electrode active materials have attracted attention as a crucial component determining the capacity of lithium-ion secondary batteries, and their development is progressing. For example, cobalt-free lithium nickel manganese composite oxides with low resource risk have been reported as positive electrode active materials used in lithium-ion secondary batteries (e.g., Patent Document 1 and Patent Document 2).

[0004] Existing technical documents Patent documents Patent Document 1: Japanese Patent No. 7446486 Patent Document 2: Japanese Patent Application Publication No. 2024-58607 Summary of the Invention

[0005] The technical problem that the invention aims to solve Lithium-ion secondary batteries using cobalt-free lithium nickel manganese composite oxides as the positive electrode active material have lower discharge capacity compared to existing nickel- and cobalt-based materials, indicating room for improvement.

[0006] This invention was made to solve the aforementioned technical problems, and its object is to provide a positive electrode active material for a lithium-ion secondary battery that can further improve the discharge capacity, and a lithium-ion secondary battery using the positive electrode active material. Furthermore, it also contributes to higher energy efficiency.

[0007] Technical solutions for solving technical problems To achieve the above objectives, the present invention provides the following technical solutions.

[0008] [1] A positive electrode active material, which is a cobalt-free positive electrode active material with lithium nickel manganese composite oxide as a carrier. The surface of the lithium nickel manganese composite oxide is supported with particulate silicon dioxide.

[0009] [1] The positive electrode active material involved has particulate silicon dioxide supported on the surface of a lithium nickel manganese composite oxide. This promotes the decomposition of lithium carbonate, which is a high-resistivity layer, and reduces the resistance of the lithium nickel manganese composite oxide. Therefore, the discharge capacity of lithium-ion secondary batteries using this positive electrode active material can be further improved. Therefore, the number of batteries required can be reduced, which helps to reduce costs. That is, it can help to improve energy efficiency.

[0010] [2] According to the positive electrode active material described in [1], the mass ratio of the mass of the particulate silicon dioxide (M2) to the total mass of the lithium nickel manganese composite oxide (M1) and the mass of the particulate silicon dioxide (M2) (M1+M2) ((M2 / (M1+M2))×100) is greater than 0% and less than 2.0% by mass.

[0011] [2] In the positive electrode active material involved, the mass ratio of the particulate silicon dioxide (M2) to the total mass of the lithium nickel manganese composite oxide (M1) and the particulate silicon dioxide (M2) (M1+M2) ((M2 / (M1+M2))×100) meets a specific numerical range. Therefore, the resistance of the lithium nickel manganese composite oxide is further reduced, which can further improve the discharge capacity of the lithium-ion secondary battery using this positive electrode active material. Therefore, the cycle characteristics can be further improved, which further contributes to the high efficiency of energy.

[0012] [3] According to the positive electrode active material described in [1] or [2], the particle size of the particulate silicon dioxide is 5 nm or more and 300 nm or less.

[0013] [3] In the positive electrode active material involved, the particle size of the particulate silicon dioxide meets a specific numerical range. Therefore, the resistance of the lithium nickel manganese composite oxide is further reduced, which can further improve the discharge capacity of the lithium-ion secondary battery using this positive electrode active material. Therefore, the cycle characteristics can be further improved, which can further contribute to the high efficiency of energy.

[0014] [4] The positive electrode active material according to any one of [1] to [3], wherein the average particle size of the lithium nickel manganese composite oxide is 0.25 to 10 mm. μ m.

[0015] [4] The average particle size of the lithium nickel manganese composite oxide of the positive electrode active material involved meets a specific numerical range. Therefore, the productivity of the positive electrode active material can be further improved, and the electrochemical characteristics of the lithium-ion secondary battery using this positive electrode active material can be improved. Therefore, the cycle characteristics can be further improved, and energy efficiency can be further improved.

[0016] [5] A lithium-ion secondary battery having a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode contains any one of [1] to [4] positive electrode active material.

[0017] [5] The positive electrode of the lithium-ion secondary battery involved contains the above-mentioned positive electrode active material. Therefore, it is possible to further improve the discharge capacity, reduce the number of batteries required, and contribute to cost reduction. That is, it can contribute to energy efficiency.

[0018] Invention Effects The positive electrode active material and lithium-ion secondary battery according to the present invention can further improve the discharge capacity. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating a method for manufacturing a positive electrode active material according to one embodiment of the present invention.

[0020] Figure 2 This is a schematic cross-sectional view of a lithium-ion secondary battery according to one embodiment of the present invention.

[0021] Figure 3 This is a scanning electron microscope (SEM) image of the positive electrode active material according to one embodiment of the present invention.

[0022] Figure 4 This is an SEM image of the positive electrode active material used in SEM-energy dispersive X-ray spectroscopy (EDX).

[0023] Figure 5 It means Figure 4 A graph showing the results of SEM-EDX-based elemental analysis of the particulate matter on the surface of the positive electrode active material.

[0024] Figure 6 It means Figure 4 A graph showing the results of SEM-EDX-based elemental analysis of the cross-section of the positive electrode active material in particulate matter.

[0025] Figure 7 It means Figure 4 The image shows the results of SEM-EDX-based elemental analysis of the surface of the positive electrode active material.

[0026] Figure 8 It means Figure 4 A graph showing the results of SEM-EDX-based elemental analysis of the cross-section of the positive electrode active material.

[0027] Figure 9 This is a graph showing the discharge capacity of lithium-ion secondary batteries using the positive electrode active materials of Example 1 and Comparative Example 1 at a rate of 0.1C.

[0028] Figure 10 This is a graph showing the discharge capacity of lithium-ion secondary batteries using the positive electrode active materials of Example 1 and Comparative Example 1 at a rate of 0.33C.

[0029] Figure 11 This is a graph showing the discharge capacity of lithium-ion secondary batteries using the positive electrode active materials of Example 2 and Comparative Example 2 at a rate of 0.1C.

[0030] Figure 12 This is a graph showing the discharge capacity of lithium-ion secondary batteries using the positive electrode active materials of Example 2 and Comparative Example 2 at a rate of 0.33C.

[0031] Explanation of reference numerals in the attached figures 1. Lithium-ion secondary battery 2 Positive electrode 3 Negative electrode 4. Isolation components 5. Insulating seals (gaskets) 10 Positive electrode container 20 Negative electrode container (negative electrode terminal). Detailed Implementation

[0032] The preferred embodiments of the present invention will now be described in detail.

[0033] [Positive electrode active material] The positive electrode active material in this embodiment is a cobalt-free positive electrode active material with lithium nickel manganese composite oxide as the carrier. Here, "cobalt-free" means that the lithium nickel manganese composite oxide contains no cobalt at all, or contains an unavoidable small amount of cobalt.

[0034] In this embodiment, the positive electrode active material has particulate silicon dioxide supported on the surface of a lithium nickel manganese composite oxide.

[0035] Provided that the function of the present invention is not impaired, the positive electrode active material may also contain components other than lithium nickel manganese composite oxide and silicon dioxide.

[0036] The positive electrode active material in this embodiment may contain only one type of lithium nickel manganese composite oxide, or it may contain two or more types.

[0037] When manufacturing lithium nickel manganese composite oxide as the positive electrode active material, the overall composition ratio (Li:Ni:Mn) of the lithium nickel manganese composite oxide is maintained in the obtained positive electrode active material. Furthermore, the composition ratio of the lithium nickel manganese composite oxide is adjusted to be the same as the required composition ratio for the desired positive electrode active material.

[0038] (Lithium-nickel-manganese composite oxide) The lithium-nickel-manganese composite oxide in this embodiment is a layered rock salt type oxide, which is in the form of particles with an outer layer on the surface.

[0039] In this specification, the average particle size (hereinafter also referred to as "average particle size") of the lithium nickel manganese composite oxide particles is not particularly limited, but is preferably 0.25 to 10 mm. μ m, more preferably 0.25 to 5.0 μ m, more preferably 0.50–4.0 μ m. If the average particle size is above the lower limit mentioned above, the productivity of the positive electrode active material can be further improved. If the average particle size is below the upper limit mentioned above, the electrochemical characteristics of the lithium-ion secondary battery can be improved.

[0040] Average particle size refers to D50, which is measured, for example, by a laser diffraction particle size distribution measuring device.

[0041] <Chemical Composition> In existing lithium nickel manganese composite oxides (e.g., LiNi) 0.5 Mn 0.5 In O2, the presence of lithium carbonate (Li2CO3) with low ionic conductivity on the surface forms a resistive layer, contributing to the low capacity. Conversely, if the Li2CO3 on the surface is washed away with water, the capacity is reduced due to atmospheric and LiNi... 0.5 Mn 0.5 Direct contact with O2 will cause it to react with water, forming a new resistive layer and reducing the capacity. Therefore, in order to improve the electrochemical properties of the positive electrode active material, it is necessary to remove the Li2CO3 on the surface without contact with water.

[0042] This invention is based on the following discovery: through the study of LiNi 0.5 Mn 0.5 To fabricate a battery, particulate silicon dioxide (SiO2) is supported on the surface of O2, thereby creating an air-isolated state. During charging, this promotes the decomposition of lithium carbonate (Li2CO3), which serves as a high-resistivity layer, and enables the formation of a low-resistivity interface. Therefore, in this invention, the amount of Ni used can be reduced while maintaining the electrochemical properties of the positive electrode active material.

[0043] The lithium nickel manganese composite oxide of this embodiment is shown by the following formula (1).

[0044] Li m Ni x Mn y O2 (1) In equation (1), m is in the range of 1.0≤m≤1.06, x is in the range of 0.47≤x≤0.5, y is in the range of 0.47≤y≤0.5, and m+x+y=2.

[0045] In this embodiment, the lithium nickel manganese composite oxide is more preferably in the range of 1.02≤m≤1.04, x≤0.48≤x≤0.49, and y≤0.48≤y≤0.49 in formula (1). Specifically, the chemical composition of the lithium nickel manganese composite oxide used in this invention is in the range of Li 1.02 Ni 0.49 Mn 0.49 O2 to Li 1.04 Ni 0.48 Mn 0.48 The range of O2.

[0046] The chemical composition of the lithium nickel manganese composite oxide in this embodiment can be determined by inductively coupled plasma (ICP) emission spectroscopy.

[0047] The particles of lithium nickel manganese composite oxide have an outer layer on their surface.

[0048] In this specification, "outer layer" refers to the region extending 25 nm from the surface of the particle towards its interior. It should be noted that when the particle size is less than 50 nm, the particle is considered a single-layer structure consisting only of the outer layer.

[0049] The lithium nickel manganese composite oxide particles in this embodiment can be primary particles or secondary particles. From the perspective of obtaining relatively dense particles, the lithium nickel manganese composite oxide particles are preferably secondary particles formed by the aggregation of multiple primary particles.

[0050] The lithium-nickel-manganese composite oxide of this embodiment has particulate silicon dioxide supported on its surface. That is, the lithium-nickel-manganese composite oxide of this embodiment functions as a carrier.

[0051] (Silicon dioxide) In this embodiment, silica is supported as particles on the surface of a lithium nickel manganese composite oxide.

[0052] The particle size of silica is preferably 5 nm or more and 300 nm or less, more preferably 10 nm or more and 100 nm or less, and even more preferably 10 nm or more and 50 nm or less. If the particle size of silica is above or below the aforementioned lower limit, particle aggregation can be suppressed, making it easier to support on the surface of lithium nickel manganese composite oxide. If the particle size of silica is below or below the aforementioned upper limit, it becomes easier to support on the surface of lithium nickel manganese composite oxide.

[0053] The particle size of silica can be determined, for example, by observation using a transmission electron microscope.

[0054] Let the mass of lithium nickel manganese composite oxide be M1, and the mass of silicon dioxide be M2.

[0055] At this point, the mass ratio of silicon dioxide to the total mass of the lithium nickel manganese composite oxide and silicon dioxide (M1+M2) ((M2 / (M1+M2))×100) is preferably greater than 0% and less than 2.0%, more preferably greater than 0.05% and less than 1.0%, and even more preferably greater than 0.1% and less than 0.9%. If the mass ratio of silicon dioxide is greater than or equal to the lower limit mentioned above, the resistance of the lithium nickel manganese composite oxide can be further reduced. If the mass ratio of silicon dioxide is less than or equal to the upper limit mentioned above, the situation where silicon dioxide covers the surface of the lithium nickel manganese composite oxide in a layered manner can be suppressed, and the resistance of the lithium nickel manganese composite oxide can be further reduced.

[0056] In this embodiment, the positive electrode active material, by loading particulate silicon dioxide onto the surface of the lithium nickel manganese composite oxide, can further reduce the resistance of the lithium nickel manganese composite oxide and further improve the discharge capacity of the lithium-ion secondary battery using this positive electrode active material.

[0057] It should be noted that if silicon dioxide covers the entire surface of the lithium nickel manganese composite oxide in a layered manner, it would actually hinder the charge movement of lithium, making it impossible to achieve low resistivity in the lithium nickel manganese composite oxide. Therefore, it is considered important that silicon dioxide is supported on the surface of the lithium nickel manganese composite oxide in a particulate state.

[0058] Whether silica is supported as particles on the surface of lithium nickel manganese composite oxide can be adjusted by the average particle size of silica, its mass ratio, the heat treatment temperature in the firing process described later, and combinations thereof.

[0059] The state of particulate silica supported on the surface of lithium nickel manganese composite oxide can be confirmed by energy dispersive X-ray spectroscopy (EDX) using scanning electron microscopy (SEM).

[0060] When the positive electrode active material of this embodiment is applied to a lithium-ion secondary battery having a liquid electrolyte, it is believed that silica functions as an adsorbent for adsorbing decomposition products in the electrolyte. Therefore, it is believed that an interface capable of forming a low-resistance layer can be formed, thereby reducing the resistance of the lithium nickel manganese composite oxide. As a result, the discharge capacity of the lithium-ion secondary battery using the positive electrode active material of this embodiment can be further improved.

[0061] Furthermore, it is believed that in lithium-ion secondary batteries using the positive electrode active material of this embodiment, the activation energy of the electrochemical reaction is reduced, making it easier for the electrochemical reaction to occur. This is believed to be because the particulate silicon dioxide supported on the lithium nickel manganese composite oxide functions as a catalyst for the electrochemical reaction in the lithium-ion secondary battery.

[0062] [Manufacturing method of positive electrode active material] The positive electrode active material of this embodiment contains the aforementioned lithium-nickel-manganese composite oxide. As the lithium source for the lithium-nickel-manganese composite oxide, known compounds such as hydroxides like lithium hydroxide monohydrate (LiOH·H2O), carbonates like lithium carbonate (Li2CO3), and acetates like lithium acetate (CH3COOLi) and lithium acetate dihydrate (CH3COOLi·2H2O) can be used, without particular limitation. For the compounds used as nickel and manganese sources of transition metals, known oxides, hydroxides, or metal salts of nickel and manganese can also be widely used, without particular limitation.

[0063] For example, nickel hydroxide (Ni(OH)2), nickel(II) chloride (NiCl2), nickel(II) chloride hexahydrate (NiCl2·6H2O), nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O), etc., can be used as nickel compounds, but are not limited to these.

[0064] As manganese compounds, manganese(II) chloride (MnCl2), manganese(II) chloride tetrahydrate (MnCl2·4H2O), manganese carbonate hexahydrate (MnCO3·6H2O), manganese(II) nitrate hexahydrate (Mn(NO3)2·6H2O), etc., can be used, but are not limited to these.

[0065] In addition to being used individually, the aforementioned transition metal compounds can also be used as composite hydroxides (such as nickel-manganese composite hydroxides) by methods such as co-precipitation.

[0066] Preparation of lithium-nickel-manganese composite oxides In the preparation of lithium-nickel-manganese composite oxides, firstly, a specified amount of lithium compound is added to a nickel-manganese compound as an intermediate, and the mixture is dispersed in a solvent such as ethanol. It should be noted that the specified amount of the intermediate compound and the specified amount of lithium compound can be mixed not only by wet mixing with a solvent, but also by dry mixing without a solvent. For example, when lithium carbonate (Li₂CO₃) is used as the lithium compound to synthesize Li… 1.04 Ni 0.48 Mn 0.48 In the case of O2, it is preferable to weigh 1% to 5% more Li2CO3 than the stoichiometric ratio, for example, weigh 2% more Li2CO3.

[0067] The aforementioned nickel-manganese compounds can be synthesized using known methods. When the nickel-manganese compound is a hydroxide, for example, by weighing nickel sulfate hexahydrate (NiSO4·6H2O) and manganese sulfate pentahydrate (MnSO4·5H2O) such that the molar ratio of Ni:Mn is 1:1, dissolving them in pure water, and then adding an alkaline aqueous solution dropwise to this sulfate aqueous solution, a nickel-manganese complex hydroxide can be co-precipitated.

[0068] The lithium-nickel-manganese composite oxide of this embodiment can be synthesized using known methods. For example, it can be synthesized by: preparing a composite hydroxide or composite oxide of a nickel compound and a manganese compound as an intermediate compound, mixing the intermediate compound with a lithium compound to form a raw material mixture, and subjecting the raw material mixture to heat treatment (e.g., calcination) for a specified time at a specified temperature in a specified atmosphere.

[0069] As a precursor, a mixture of lithium compound and nickel-manganese compound is pulverized to a preferred size, mixed, and then filled into a crucible or the like for heat treatment. Alumina crucibles, alumina crucibles, platinum crucibles, gold crucibles, etc., are used as crucibles. The heat treatment of the mixture is performed, for example, using a firing furnace or roller kiln.

[0070] The mixture described above, placed in a sagger or crucible, is heated at a heating rate of 5°C / min to 25°C / min, preferably 10°C / min to 25°C / min, to reach the heat treatment temperature. The heat treatment atmosphere is not particularly limited and can include the atmosphere (air atmosphere), an oxygen stream, etc. An oxygen stream is preferred. The heat treatment time can be appropriately set according to the heat treatment temperature. It should be noted that the heat treatment time refers to the time spent holding the heat treatment temperature.

[0071] When heat-treating a mixture of lithium compounds (e.g., Li₂CO₃) and nickel-manganese compounds, the heat treatment temperature is preferably 1025°C or higher and 1150°C or lower, more preferably 1050°C or higher and 1125°C or lower. The heat treatment time is preferably 1 minute or higher and 7 hours or lower, more preferably 2 minutes or higher and 6 hours or lower, further preferably 3 minutes or higher and 5 hours or lower, and particularly preferably 5 minutes or higher and 3 hours or lower.

[0072] The lithium-nickel-manganese composite oxide was obtained using the above method.

[0073] Hereinafter, the method for manufacturing the positive electrode active material of this embodiment will be described in detail with reference to the accompanying drawings.

[0074] Figure 1 A flowchart illustrating the method for manufacturing the positive electrode active material according to this embodiment.

[0075] like Figure 1As shown, the method for manufacturing the positive electrode active material in this embodiment includes a dispersion step (S1), a mixing step (S2), a drying step (S3), and a calcination step (S4).

[0076] <Dispersed Process (S1)> The dispersion process (S1) is the process of dispersing and mixing silica in a solvent.

[0077] Silica is preferably nano-oxide particles with an average particle size of 5 nm or more and 300 nm or less.

[0078] Commercially available nano-oxide particles can be used. Examples of commercially available nano-oxide particles include silicon dioxide nanoparticles (5-20 nm particle size, manufactured by Aldrich) and silica nanoparticles (300 nm particle size, also manufactured by Aldrich).

[0079] Examples of solvents used in the dispersion process include alcohols such as methanol and ethanol; ketones such as acetone and methyl ethyl ketone; ethers such as dimethyl ether and diethyl ether; and esters such as methyl acetate and ethyl acetate. Among these solvents, alcohols are preferred from the viewpoint of superior dispersibility and cost, and ethanol is more preferred. Ethanol can also be an aqueous solution.

[0080] In the dispersion process, granular silica is added to a container containing solvent and stirred, thereby dispersing the silica in the solvent.

[0081] There are no particular limitations on the method of stirring; for example, stirring can be done using a stir bar.

[0082] There is no particular limitation on the processing time (stirring time) in the dispersion process, but it is preferably 10 to 60 minutes.

[0083] The processing temperature in the dispersion process is not particularly limited, but is preferably 5 to 30°C.

[0084] <Mixing Process (S2)> The mixing process (S2) is a process of mixing lithium nickel manganese composite oxide with a dispersion of silica and solvent obtained in the dispersion process.

[0085] The preferred lithium-nickel-manganese composite oxide has an average particle size of 0.25–10 mm. μ m particles.

[0086] In the mixing process, granular lithium nickel manganese composite oxide is added to a container containing solvent, followed by a dispersion of silica and solvent obtained in the dispersion process. The mixture is stirred to mix the lithium nickel manganese composite oxide with silica.

[0087] The solvent used in the mixing process can be the same as that used in the dispersion process, and preferably the same type of solvent. Specifically, alcohols are preferred as the solvent used in the process, and ethanol is more preferred. Ethanol may also be an aqueous solution.

[0088] There are no particular limitations on the method of stirring; for example, stirring can be done using a stir bar.

[0089] There is no particular limitation on the processing time (stirring time) in the mixing process, but it is preferably 10 to 60 minutes.

[0090] The processing temperature in the mixing process is not particularly limited, but is preferably 5 to 30°C.

[0091] <Drying process (S3)> The drying process (S3) is a process of drying the mixture of lithium nickel manganese composite oxide and silicon dioxide obtained in the mixing process.

[0092] In the drying process, the above mixture is dried using a drying device such as an oven to obtain a mixture with particulate silica adhering to the surface of the lithium nickel manganese composite oxide.

[0093] There is no particular limitation on the processing time (drying time) in the drying process, but it is preferably 2 to 8 hours.

[0094] It should be noted that the drying time refers to the time from the start of heating to the stop of heating, and until the temperature in the drying device returns to normal (e.g., 5–30°C), after the mixture has been placed in the drying device.

[0095] The processing temperature in the drying process is preferably 80 to 120°C.

[0096] <Firing process (S4)> The firing process (S4) is a process of heating and firing the mixture obtained in the drying process.

[0097] In the calcination process, the above mixture is placed in a calcination furnace or other calcination apparatus and heated to obtain a positive electrode active material with particulate silicon dioxide supported on the surface of a lithium nickel manganese composite oxide.

[0098] There is no particular limitation on the processing time (the time for heat treatment) in the firing process, but it is preferably 1 to 8 hours.

[0099] It should be noted that the heating time refers to the time from the start of heating, the stop of heating, to the time when the mixture is placed in the firing apparatus and the temperature in the firing apparatus returns to room temperature (e.g., 5-30°C).

[0100] The processing temperature in the firing process is preferably 200°C or higher and 500°C or lower, more preferably 300°C or higher and 400°C or lower. If the processing temperature in the firing process is above or above the lower limit mentioned above, the catalytic activity of silica can be further improved. If the processing temperature in the firing process is below or above the upper limit mentioned above, the silica loading on the surface of the lithium nickel manganese composite oxide can be sufficiently maintained.

[0101] In the firing process, the mixture described above, which is loaded into the firing apparatus, is heated at a heating rate of 5°C / min to 25°C / min, preferably 10°C / min to 25°C / min, to reach the processing temperature. The heat treatment atmosphere is not particularly limited, and examples include the atmosphere (air atmosphere) and an oxygen stream. An oxygen stream is preferred as the heat treatment atmosphere.

[0102] After the heat treatment, it is preferably cooled at a cooling rate of 5°C / min to 25°C / min, preferably 10°C / min to 25°C / min, to reach room temperature (e.g., 5 to 30°C).

[0103] Through the above processes, the positive electrode active material of this embodiment is obtained.

[0104] [Lithium-ion rechargeable battery] The lithium-ion secondary battery of this embodiment includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode contains the aforementioned positive electrode active material in which particulate silicon dioxide is supported on the surface of a lithium nickel manganese composite oxide. The lithium-ion secondary battery of this embodiment may include other battery elements as needed.

[0105] The lithium-ion secondary battery of this embodiment, except that the positive electrode contains the aforementioned positive electrode active material in which particulate silicon dioxide is supported on the surface of a lithium nickel manganese composite oxide, can directly utilize known lithium-ion secondary battery elements. The lithium-ion secondary battery of this embodiment can have any structure selected from coin-shaped, button-shaped, cylindrical, square, and laminated types. Furthermore, the lithium-ion secondary battery of this embodiment can be applied to a wide range of applications, including mobile devices such as mobile phones and laptops, and automotive applications.

[0106] The following description refers to a lithium-ion secondary battery (coin-type lithium-ion secondary battery) that uses an electrolyte, based on the lithium-ion secondary battery of this embodiment.

[0107] Figure 2 This is a schematic cross-sectional view illustrating the lithium-ion secondary battery according to this embodiment. Figure 2 The image shows an example of a coin-type lithium-ion secondary battery in this embodiment. Figure 2As shown, the lithium-ion secondary battery 1 of this embodiment includes a negative electrode can (negative terminal) 20, a negative electrode 3, an electrolyte-impregnated separator 4, an insulating sealant (gasket) 5, a positive electrode 2, and a positive electrode can 10.

[0108] The positive electrode container 10 is disposed on the lower side of the separator 4, and the negative electrode container 20 is disposed on the upper side of the separator 4, forming the shape of the lithium-ion secondary battery 1. A positive electrode 2 and a negative electrode 3 are disposed between the positive electrode container 10 and the negative electrode container 20, separated by the separator 4 which is impregnated with electrolyte. The positive electrode 2 and the negative electrode 3 are electrically insulated from each other by an insulating seal 5.

[0109] In the lithium-ion secondary battery 1, the positive electrode active material of this embodiment can be combined with conductive agents, binders, etc. to prepare a positive electrode mixture as needed, and then pressed onto the current collector (not shown) to produce the positive electrode 2.

[0110] As a current collector, stainless steel mesh and aluminum foil are preferred. As a conductive agent, acetylene black and Ketjen black are preferred. As a binder, tetrafluoroethylene and polyvinylidene fluoride are preferred.

[0111] The composition of the positive electrode active material, conductive agent, and binder in the positive electrode mixture is not particularly limited. The content of the conductive agent in the positive electrode mixture is preferably 1% to 15% by mass, more preferably 0.1% to 5% by mass. The content of the binder in the positive electrode mixture is preferably 0.1% to 10% by mass, more preferably 0.1% to 5% by mass. The remaining portion of the positive electrode mixture (excluding the positive electrode active material and the conductive agent) is preferably composed of the positive electrode active material, the conductive agent, and the binder to form the positive electrode active material.

[0112] In a lithium-ion secondary battery 1, the negative electrode 3, which is the opposite of the positive electrode 2, can be made of known materials that function as the negative electrode active material and can absorb and release lithium, such as metallic materials such as lithium metal and lithium alloys; carbon-based materials such as graphite and MCMB (mesophase carbon microspheres); and silicon-based materials such as silicon (Si), Si alloys, and silicon oxide.

[0113] The separator 4 and the battery container (positive electrode container 10, negative electrode container 20) can adopt known battery elements.

[0114] As the electrolyte, known electrolyte solutions can be used. For example, electrolyte solutions prepared by dissolving electrolytes such as lithium perchlorate or lithium hexafluorophosphate in solvents such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), and diethyl carbonate (DEC) can be used.

[0115] The lithium-ion secondary battery 1 of this embodiment can achieve high capacity because it contains the above-mentioned positive electrode active material in the positive electrode 2, which has particulate silicon dioxide supported on the surface of the lithium nickel manganese composite oxide.

[0116] Example The following describes embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0117] Preparation of lithium-nickel-manganese composite oxides: LiNi 0.5 Mn 0.5 O2” Li2CO3 (high-purity chemically manufactured) and Ni 0.5 Mn 0.5 (OH)₂ (manufactured by Sigma-Aldrich) was weighed in a molar ratio of Li:Ni:Mn = 1:0.5:0.5. Considering the evaporation of Li, it was weighed based on the stoichiometric ratio, with Li₂CO₃ increasing by 3% by mass. This was done by mixing Li₂CO₃ (high-purity chemically manufactured) and Ni... 0.5 Mn 0.5 The total mass of (OH)₂ (manufactured by Sigma-Aldrich) was 2.1 g. It was dispersed in ethanol using a mortar and mixed. Then, it was filled into a JIS-standard platinum crucible. Using a calcining furnace, the mixture in the platinum crucible was heated in air at a heating rate of 15 °C / min, calcined at 1100 °C for 5 minutes, and then allowed to stand until the temperature of the resulting powder reached room temperature (25 °C), yielding lithium nickel manganese oxide.

[0118] Preparation of positive electrode active material Add 0.03g of granular silica (manufactured by Fujifilm and Koichi Pure Chemical Industries, Ltd., purity 99.5%) to a 100mL beaker containing 50mL of ethanol. Use a stir bar to stir for 30 minutes at 25°C to disperse the silica (dispersion process).

[0119] 9.97 g of lithium nickel manganese oxide obtained in the above "Preparation of Lithium Nickel Manganese Composite Oxide" was added to the silica dispersion obtained in the above process. The mixture was stirred for 10 minutes at 25°C using a stir bar to ensure proper mixing (mixing process). The mass ratio of lithium nickel manganese composite oxide to silica was 99.7:0.3.

[0120] The resulting mixture was placed in an oven and dried at 80°C for 20 hours (drying process).

[0121] 10g of the obtained mixture was filled into a JIS standard platinum crucible. Using a calcining furnace, the mixture in the platinum crucible was heated in air at a heating rate of 15°C / min and calcined at 400°C for 4 hours (calcination process). Then, it was left to stand until the temperature of the obtained powder returned to room temperature (25°C) to obtain the positive electrode active material.

[0122] "SEM photography" The positive electrode active material was obtained by imaging with a SEM (JEOL Ltd., model JSM-IT800) at a magnification of 10,000x. The results are shown below. Figure 3 .

[0123] like Figure 3 As shown, the obtained positive electrode active material has a major diameter of 1 to several... μ An aggregate of primary particles (A) with a diameter of approximately m. Primary particles (A) may also aggregate to form secondary particles. Multiple small white granular objects (B) were observed on the surface of the primary particles (A).

[0124] “SEM-EDX Analysis” SEM images of the positive electrode active material were analyzed using a SEM-EDX (manufactured by NEC Corporation, model JSM-IT800). The SEM images of the positive electrode active material were obtained at 33,000x magnification by photographing a cross-sectional sample prepared from the powder sample using a cross-section polishing instrument (manufactured by NEC Corporation, model IB-19520CP). The SEM images used in the SEM-EDX analysis are shown below. Figure 4 .

[0125] like Figure 4 As shown, the positive electrode active material is a particle with a surface (S) and a cross-section (C). White granular substances (B1) and (B2) are observed at various locations on the surface (S) of the particle.

[0126] The energy dispersive spectroscopy (EDX) results of elemental analysis of white granular materials (B1) and (B2) based on SEM-EDX are shown below. Figure 5 and Figure 6 In addition, the results of the composition analysis are shown in Table 1.

[0127] like Figure 5 and Figure 6 As shown, white granules (B1) and (B2) exhibited Si peaks in the energy range of 1.70–1.80 keV. Therefore, it is concluded that white granules (B1) and (B2) are particles of SiO2.

[0128] [Table 1]

[0129] Next, the energy dispersive spectroscopy results of SEM-EDX elemental analysis of the surface (S) and cross-section (C) are shown in the figure. Figure 7 and Figure 8 In addition, the results of the composition analysis are shown in Table 1.

[0130] like Figure 7 and Figure 8 As shown, no Si peaks were observed on the surface (S) and cross-section (C). This confirms that SiO2 is not present on the surface (S) where there are no white granular substances (B1) and (B2).

[0131] SiO2 is also absent in cross section (C), confirming that SiO2 has not penetrated into the interior of the positive electrode active material particles and is not in a layered form, but rather distributed as granular material in an island-like pattern on the surface (S). This confirms that SiO2 is supported on the surface (S) of the lithium nickel manganese composite oxide in a granular form.

[0132] Furthermore, according to Table 1, the carbon content on the surface (S) of the lithium nickel manganese composite oxide is higher than that on the cross-section (C). Based on the analysis results so far, it is believed that this carbon originates from lithium carbonate. When comparing the surface (S) with the white granular materials (B1) and (B2), there is no significant difference in carbon content regardless of the presence or absence of SiO2. Therefore, it is concluded that Li2O3 has not formed on the surface (S) through the following reaction. x SiO y .

[0133] Li₂CO₃ + SiO₂ → Li₂SiO₃ + CO₂

[0134] In summary, the results above confirm that... Figure 3 The primary particles (A) are lithium nickel manganese composite oxides, and the white granular material (B) is silicon dioxide (SiO2).

[0135] Preparation of positive electrode active materials that change the SiO2 content 1 [Example 1, Comparative Example 1] The lithium nickel manganese composite oxide and silicon dioxide were mixed in a mass ratio of 99.97:0.03, 99.95:0.05, 99.93:0.07, 99.9:0.1, 99.7:0.3, 99.5:0.5, 99:1, 98:2, and 97:3. Otherwise, each positive electrode active material was prepared by the same method as described in "Preparation of Positive Electrode Active Material" (Example 1 above).

[0136] In addition, regarding the preparation of the aforementioned "LiNi nickel manganese composite oxide: LiNi 0.5 Mn 0.5In the O2 process, the sintering conditions of the mixture filled in the platinum crucible were set to 1075°C for 30 minutes. In addition, preparations were made using the above-mentioned "LiNi nickel manganese composite oxide preparation: LiNi" method. 0.5 Mn 0.5 Lithium nickel manganese composite oxide prepared by the same method as described in "O2" (Comparative Example 1).

[0137] It should be noted that the average particle size (D50) of the lithium nickel manganese composite oxide used in Example 1 and Comparative Example 1, as measured using a laser diffraction particle size distribution measuring device, was 4.1. μ m.

[0138] "The fabrication of lithium-ion secondary batteries" The positive electrode active material of Example 1 and the lithium nickel manganese composite oxide of Comparative Example 1 were used as positive electrode active materials, respectively. Acetylene black (AB) as a conductive agent and polyvinylidene fluoride (PVDF) as a binder were mixed with NMP (N-methyl-2-pyrrolidone) in a weight ratio of 8:1:1 to prepare a slurry. Then, it was coated onto a 15mm thick substrate. μ Take aluminum foil of m, dry it, and make 14 The positive electrode. The coating area density is 4.5 mg / cm³. 2 Its bulk density is 2.3 g / cm³. 3 For this positive electrode, a thickness of 200 mm is used. μ m, 16 Lithium metal is used as the counter electrode, with a thickness of 20 mm. μ m, 18 A polyethylene microporous membrane was used as the separator. The electrolyte was a 1.2 mol / L solution prepared by dissolving lithium hexafluorophosphate (LiPF6) in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio 3:4:3). Figure 2 The lithium-ion secondary battery (2032 coin-type battery) with the structure shown is manufactured according to known battery construction and assembly methods.

[0139] "Charge and Discharge Test 1" For each lithium-ion secondary battery fabricated, charge-discharge tests were conducted at a constant current rate of 0.1C (1C: 250mA / g) under 25°C conditions, with a current density of 12.5mA / g and cutoff potentials of 4.7V~2.5V or 4.8V~2.5V, to evaluate their respective discharge capacities. The results are presented below. Figure 9 . Figure 9 The dashed line in the figure represents the discharge capacity of the lithium-ion secondary battery in Comparative Example 1.

[0140] like Figure 9As shown, the lithium-ion secondary battery of Comparative Example 1, which uses a positive electrode active material without silicon dioxide supported on a lithium nickel manganese composite oxide, has a discharge capacity of 154.1 mAh g. -1 .

[0141] In contrast, the discharge capacity of the lithium-ion secondary battery of Example 1, which uses a positive electrode active material with silicon dioxide supported on a lithium nickel manganese composite oxide, exceeds that of Comparative Example 1 if the mass percentage of silicon dioxide is 0.05% by mass or more and 0.9% by mass or less. This confirms that if the mass percentage of silicon dioxide is 0.05% by mass or more and 0.9% by mass or less, the discharge capacity can be further improved.

[0142] "Charge and Discharge Test 2" The charge / discharge test rate was changed to 0.33C. Otherwise, the charge / discharge test was conducted using the same method as in "Charge / Discharge Test 1" above, and the discharge capacity of each test was evaluated. The results are shown below. Figure 10 . Figure 10 The dashed line in the figure represents the discharge capacity of the lithium-ion secondary battery in Comparative Example 1.

[0143] like Figure 10 As shown, the lithium-ion secondary battery of Comparative Example 1, which uses a positive electrode active material without silicon dioxide supported on a lithium nickel manganese composite oxide, has a discharge capacity of 142.9 mAh g. -1 .

[0144] In contrast, the discharge capacity of the lithium-ion secondary battery of Example 1, which uses a positive electrode active material with silicon dioxide supported on a lithium nickel manganese composite oxide, exceeds that of Comparative Example 1 if the mass percentage of silicon dioxide is 0.05% by mass or more and 0.9% by mass or less. This confirms that if the mass percentage of silicon dioxide is 0.05% by mass or more and 0.9% by mass or less, the discharge capacity can be further improved.

[0145] Preparation of positive electrode active materials that change the SiO2 content 2 [Example 2, Comparative Example 2] The lithium nickel manganese composite oxide and silicon dioxide were mixed in a mass ratio of 99.7:0.3, 99.3:0.7, 99:1, 98.5:1.5, 98:2, and 97:3. Otherwise, each positive electrode active material was prepared by the same method as described in the "Preparation of Positive Electrode Active Material" section above (Example 2 above).

[0146] In addition, regarding the preparation of the aforementioned "LiNi nickel manganese composite oxide: LiNi 0.5 Mn 0.5In the O2 process, the sintering conditions of the mixture filled in the platinum crucible were set to 1075°C for 30 minutes. In addition, preparations were made using the above-mentioned "LiNi nickel manganese composite oxide preparation: LiNi" method. 0.5 Mn 0.5 Lithium nickel manganese composite oxide prepared by the same method as described in "O2" (Comparative Example 2).

[0147] It should be noted that the average particle size (D50) of the lithium nickel manganese composite oxide used in Example 2 and Comparative Example 2, as measured using a laser diffraction particle size distribution measuring device, was 3.6. μ m.

[0148] "The fabrication of lithium-ion secondary batteries" The positive electrode active material of Example 2 and the lithium nickel manganese composite oxide of Comparative Example 2 were used as positive electrode active materials, and the lithium-ion secondary batteries were prepared in the same manner as in Example 1 and Comparative Example 1.

[0149] "Charge and Discharge Test 3" The discharge capacity of each component was evaluated by conducting charge-discharge tests using the same method as described in "Charge-Discharge Test 1" above. The results are presented below. Figure 11 . Figure 11 The dashed line in the figure represents the discharge capacity of the lithium-ion secondary battery in Comparative Example 2.

[0150] like Figure 11 As shown, the lithium-ion secondary battery of Comparative Example 2, which uses a positive electrode active material without silicon dioxide supported on a lithium nickel manganese composite oxide, has a discharge capacity of 155.1 mAh g. -1 .

[0151] In contrast, for the lithium-ion secondary battery of Example 2, which uses a positive electrode active material with silicon dioxide supported on a lithium nickel manganese composite oxide, the discharge capacity exceeds that of Comparative Example 2 if the mass percentage of silicon dioxide is greater than 0% by mass and less than 2.0% by mass. This confirms that if the mass percentage of silicon dioxide is greater than 0% by mass and less than 2.0% by mass, the discharge capacity can be further improved.

[0152] "Charge and Discharge Test 4" The charge / discharge test rate was changed to 0.33C. Otherwise, the charge / discharge test was conducted using the same method as in "Charge / Discharge Test 3" above, and the discharge capacity was evaluated. The results are shown below. Figure 12 . Figure 12 The dashed line in the figure represents the discharge capacity of the lithium-ion secondary battery in Comparative Example 2.

[0153] like Figure 12As shown, the lithium-ion secondary battery of Comparative Example 2, which uses a positive electrode active material without silicon dioxide supported on a lithium nickel manganese composite oxide, has a discharge capacity of 142.9 mAh g. -1 .

[0154] In contrast, for the lithium-ion secondary battery of Example 2, which uses a positive electrode active material with silicon dioxide supported on a lithium nickel manganese composite oxide, the discharge capacity exceeds that of Comparative Example 2 if the mass percentage of silicon dioxide is greater than 0% by mass and less than 2.0% by mass. This confirms that if the mass percentage of silicon dioxide is greater than 0% by mass and less than 2.0% by mass, the discharge capacity can be further improved.

[0155] As can be seen from the above results, according to the present invention, a positive electrode active material that can further improve the discharge capacity and a lithium-ion secondary battery containing the positive electrode active material can be provided.

Claims

1. A positive electrode active material, characterized in that, It is a cobalt-free positive electrode active material with lithium nickel manganese composite oxide as the carrier. The surface of the lithium nickel manganese composite oxide is supported with particulate silicon dioxide.

2. The positive electrode active material according to claim 1, wherein, The mass ratio of the granular silicon dioxide M2 ​​to the total mass of the lithium nickel manganese composite oxide M1 and the granular silicon dioxide M2, i.e. (M2 / (M1+M2))×100, is greater than 0% and less than 2.0% by mass.

3. The positive electrode active material according to claim 1, wherein the particle size of the particulate silicon dioxide is 5 nm or more and 300 nm or less.

4. The positive electrode active material according to claim 1, wherein, The average particle size of the lithium-nickel-manganese composite oxide is 0.25–10 mm. μ m.

5. A lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery comprises a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains the positive electrode active material as described in any one of claims 1 to 4.

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