Positive electrode active material and lithium-ion secondary battery

Abstract

The present invention aims to provide a positive electrode active material that can produce a lithium-ion secondary battery with a higher discharge capacity, and a lithium-ion secondary battery using said positive electrode active material. Ultimately, this contributes to improved energy efficiency. [Solution] A cobalt-free positive electrode active material using lithium nickel manganese composite oxide as a support, A positive electrode active material comprising particulate silicon dioxide supported on the surface of the lithium nickel manganese composite oxide.

Description

[Technical Field]

[0001] This invention relates to a positive electrode active material and a lithium-ion secondary battery. [Background technology]

[0002] In recent years, research and development has been conducted on secondary batteries that contribute to 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 vehicles.

[0003] The positive electrode active material is attracting attention as an important component that determines the capacity of lithium-ion secondary batteries, and development is progressing. As a positive electrode active material for use in lithium-ion secondary batteries, for example, cobalt-free lithium nickel manganese composite oxide, which has a low resource risk, has been reported (for example, Patent Documents 1 and 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 7446486 [Patent Document 2] Japanese Patent Publication No. 2024-58607 [Overview of the project] [Problems that the invention aims to solve]

[0005] Lithium-ion secondary batteries using cobalt-free lithium nickel manganese composite oxide as the positive electrode active material have a lower discharge capacity compared to conventionally used nickel or cobalt-based materials, indicating room for improvement.

[0006] This invention was made to solve the above-mentioned problems, and aims to provide a positive electrode active material that can produce a lithium-ion secondary battery with a higher discharge capacity, and a lithium-ion secondary battery using said positive electrode active material. Ultimately, this contributes to energy efficiency. [Means for solving the problem]

[0007] To achieve the above objective, the present invention provides the following means. [1] A cobalt-free positive electrode active material supported by a lithium nickel manganese composite oxide, A positive electrode active material comprising particulate silicon dioxide supported on the surface of the lithium nickel manganese composite oxide.

[0008] The positive electrode active material described in [1] 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-resistance layer, and reduces the resistance of the lithium nickel manganese composite oxide. As a result, the discharge capacity of lithium-ion secondary batteries using this positive electrode active material can be increased. Consequently, the number of batteries required can be reduced, contributing to cost reduction. In other words, it can contribute to energy efficiency.

[0009] [2] The positive electrode active material according to [1], wherein the mass ratio of the mass of particulate silicon dioxide (M2) to the sum of the mass of the lithium nickel manganese composite oxide (M1) and the mass of the particulate silicon dioxide (M2) ((M2 / (M1+M2)) × 100)) is greater than 0 mass% and 2.0 mass% or less.

[0010] The positive electrode active material according to [2] satisfies a specific numerical range for the mass ratio ((M2 / (M1 + M2))×100)) of the mass (M2) of particulate silicon dioxide to the total (M1 + M2) of the mass (M1) of lithium nickel manganese composite oxide and the mass (M2) of particulate silicon dioxide. For this reason, the resistance of the lithium nickel manganese composite oxide is further reduced, and the discharge capacity of the lithium ion secondary battery using the positive electrode active material can be further increased. Therefore, the cycle characteristics can be further enhanced and contribute to further energy efficiency improvement.

[0011] The positive electrode active material according to [3] or [1], wherein the particle diameter of the particulate silicon dioxide is 5 nm or more and 300 nm or less.

[0012] The positive electrode active material according to [3] satisfies a specific numerical range for the particle diameter of the particulate silicon dioxide. For this reason, the resistance of the lithium nickel manganese composite oxide is further reduced, and the discharge capacity of the lithium ion secondary battery using the positive electrode active material can be further increased. Therefore, the cycle characteristics can be further enhanced and contribute to further energy efficiency improvement.

[0013] The positive electrode active material according to any one of [1] to [3], wherein the average particle diameter of the lithium nickel manganese composite oxide is 0.25 to 10 μm.

[0014] The positive electrode active material according to [4] satisfies a specific numerical range for the average particle diameter of the lithium nickel manganese composite oxide. For this reason, the productivity of the positive electrode active material can be further increased, and the electrochemical characteristics of the lithium ion secondary battery using the positive electrode active material can be made better. Therefore, the cycle characteristics can be further enhanced and contribute to further energy efficiency improvement.

[0015] A lithium ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains the positive electrode active material according to any one of [1] to [4].

[0016] The lithium-ion secondary battery described in [5] has a positive electrode containing the positive electrode active material. Therefore, the discharge capacity can be increased, the number of batteries required can be reduced, and costs can be lowered. In other words, it can contribute to energy efficiency. [Effects of the Invention]

[0017] According to the positive electrode active material and lithium-ion secondary battery of the present invention, the discharge capacity can be further increased. [Brief explanation of the drawing]

[0018] [Figure 1] This is a flowchart showing a method for manufacturing a positive electrode active material according to one embodiment of the present invention. [Figure 2] This is a schematic cross-sectional view showing a lithium-ion secondary battery according to one embodiment of the present invention. [Figure 3] This is a scanning electron microscope (SEM) image of a positive electrode active material according to one embodiment of the present invention. [Figure 4] This is a SEM image of the cathode active material used in SEM-energy-dispersive X-ray spectroscopy (EDX). [Figure 5] This figure shows the spectral results of elemental analysis of granular material on the surface of the positive electrode active material using SEM-EDX. [Figure 6] This figure shows the spectral results of elemental analysis of granular material in a cross-section of the positive electrode active material, obtained by SEM-EDX. [Figure 7] Figure 4 shows the spectral results of elemental analysis of the surface of the positive electrode active material by SEM-EDX. [Figure 8] This figure shows the spectral results of elemental analysis of the cross-section of the positive electrode active material using SEM-EDX. [Figure 9] This graph shows the discharge capacity at a rate of 0.1C for lithium-ion secondary batteries using the positive electrode active materials of Example 1 and Comparative Example 1. [Figure 10] This graph shows the discharge capacity at a rate of 0.33C for lithium-ion secondary batteries using the positive electrode active materials of Example 1 and Comparative Example 1. [Figure 11]This graph shows the discharge capacity at a rate of 0.1C for lithium-ion secondary batteries using the positive electrode active materials of Example 2 and Comparative Example 2. [Figure 12] This graph shows the discharge capacity at a rate of 0.33C for lithium-ion secondary batteries using the positive electrode active materials of Example 2 and Comparative Example 2. [Modes for carrying out the invention]

[0019] Preferred embodiments of the present invention will be described in detail below.

[0020] [Cathode active material] The positive electrode active material of this embodiment is a cobalt-free positive electrode active material supported by a lithium nickel manganese composite oxide. Here, "cobalt-free" means that the lithium nickel manganese composite oxide contains no cobalt at all, or inevitably contains a small amount of cobalt. In this embodiment, the positive electrode active material has particulate silicon dioxide supported on the surface of a lithium nickel manganese composite oxide. The positive electrode active material may contain components other than lithium nickel manganese composite oxide and silicon dioxide, as long as it does not impair the function of the present invention.

[0021] The positive electrode active material of this embodiment may contain only one type of lithium nickel manganese composite oxide, or it may contain two or more types.

[0022] When lithium nickel manganese composite oxide is manufactured as a positive electrode active material, the overall composition ratio (Li:Ni:Mn) of the lithium nickel manganese composite oxide is maintained in the resulting positive electrode active material. Furthermore, the composition ratio of the lithium nickel manganese composite oxide is adjusted to be the same as the composition ratio required for the desired positive electrode active material.

[0023] (Lithium nickel manganese composite oxide) The lithium nickel manganese composite oxide of this embodiment is a layered rock salt type oxide and is in the form of particles having an outer layer on its surface.

[0024] In this specification, the average particle diameter of the particles of the lithium nickel manganese composite oxide (hereinafter, also simply referred to as "average particle diameter") is not particularly limited. For example, 0.25 to 10 μm is preferable, 0.25 to 5.0 μm is more preferable, and 0.50 to 4.0 μm is even more preferable. When the average particle diameter is not less than the above lower limit value, the productivity of the positive electrode active material can be further increased. When the average particle diameter is not more than the above upper limit value, the electrochemical characteristics of the lithium ion secondary battery can be made better. The average particle diameter means, for example, D50 measured by a laser diffraction particle size distribution measuring device or the like.

[0025] <Chemical composition> In a conventional lithium nickel manganese composite oxide (for example, LiNi 0.5 Mn 0.5 O2), lithium carbonate (Li2CO3) with low ionic conductivity exists on the surface and becomes a resistance layer, which is one of the causes of low capacity. On the other hand, when the Li2CO3 on the surface is washed with water, LiNi 0.5 Mn 0.5 O2 directly contacts with air, reacts with water, and a new resistance layer is formed, reducing the capacity. Therefore, in order to improve the electrochemical characteristics as a positive electrode active material, it is necessary to remove the Li2CO3 on the surface without touching water. The present invention is based on the finding that by producing a battery by supporting particulate silicon dioxide (SiO2) on the surface of LiNi 0.5 Mn 0.5 O2, a state of being blocked from air is created, the decomposition of lithium carbonate (Li2CO3), which is a high resistance layer, is promoted during charging, and an interface of a low resistance layer can be formed. Thereby, in the present invention, the amount of Ni used can be reduced while maintaining good electrochemical characteristics of the positive electrode active material.

[0026] The lithium nickel manganese composite oxide of this embodiment is represented by the following formula (1). Li m Ni x Mn y O2(1) In equation (1), m is within the range of 1.0 ≤ m ≤ 1.06, x is within the range of 0.47 ≤ x ≤ 0.5, y is within the range of 0.47 ≤ y ≤ 0.5, and m + x + y = 2.

[0027] The lithium nickel manganese composite oxide of this embodiment is more preferably such that m in formula (1) is in the range of 1.02 ≤ m ≤ 1.04, x is in the range of 0.48 ≤ x ≤ 0.49, and y is in the range of 0.48 ≤ y ≤ 0.49. Specifically, the lithium nickel manganese composite oxide used in the present invention has a chemical composition of Li 1.02 Ni 0.49 Mn 0.49 O2 to Li 1.04 Ni 0.48 Mn 0.48 It is within the O2 range.

[0028] The chemical composition of the lithium nickel manganese composite oxide in this embodiment can be determined by inductively coupled plasma (ICP) emission spectroscopy.

[0029] The lithium nickel manganese composite oxide particles have an outer layer on their surface. In this specification, the "outer layer" refers to the region extending from the surface of the particle to the interior of the particle up to 25 nm. If the particle diameter is less than 50 nm, the particle is assumed to have a single-layer structure consisting only of the outer layer.

[0030] The lithium nickel manganese composite oxide particles in this embodiment may be primary particles or secondary particles. It is preferable that the lithium nickel manganese composite oxide particles be secondary particles formed by the aggregation of multiple primary particles, as this yields relatively dense particles.

[0031] The lithium nickel manganese composite oxide of this embodiment has particulate silicon dioxide supported on its surface. In other words, the lithium nickel manganese composite oxide of this embodiment functions as a carrier.

[0032] (Silicon dioxide) In this embodiment, silicon dioxide is supported as particles on the surface of a lithium nickel manganese composite oxide. The particle size of silicon dioxide is preferably 5 nm to 300 nm, more preferably 10 nm to 100 nm, and even more preferably 10 nm to 50 nm. If the particle size of silicon dioxide is above the lower limit, particle aggregation can be suppressed, and it becomes easier to support on the surface of the lithium nickel manganese composite oxide. If the particle size of silicon dioxide is below the upper limit, it becomes easier to support on the surface of the lithium nickel manganese composite oxide. The particle size of silicon dioxide can be determined, for example, by observation using a transmission electron microscope.

[0033] Let M1 be the mass of lithium nickel manganese composite oxide, and M2 be the mass of silicon dioxide. In this case, the mass ratio of silicon dioxide to the total mass of lithium nickel manganese composite oxide (M1+M2) ((M2 / (M1+M2))×100) is preferably greater than 0% by mass and 2.0% by mass or less, more preferably 0.05% by mass or more and 1.0% by mass or less, and even more preferably 0.1% by mass or more and 0.9% by mass or less. If the mass ratio of silicon dioxide is above the lower limit, the resistance of the lithium nickel manganese composite oxide can be further reduced. If the mass ratio of silicon dioxide is below the upper limit, the layering of silicon dioxide on the surface of the lithium nickel manganese composite oxide can be suppressed, and the resistance of the lithium nickel manganese composite oxide can be further reduced.

[0034] In this embodiment, the positive electrode active material has particulate silicon dioxide supported on the surface of a lithium nickel manganese composite oxide, which further reduces the resistance of the lithium nickel manganese composite oxide and increases the discharge capacity of the lithium-ion secondary battery using this positive electrode active material. Furthermore, if silicon dioxide were to completely cover the surface of the lithium nickel manganese composite oxide in a layered manner, it would likely inhibit the charge transfer of lithium, thus preventing the reduction of the lithium nickel manganese composite oxide's resistance. Therefore, it is considered important that the silicon dioxide is supported on the surface of the lithium nickel manganese composite oxide as particles. Whether or not silicon dioxide is supported as particles on the surface of the lithium nickel manganese composite oxide can be controlled by the average particle size and mass ratio of silicon dioxide, the heat treatment temperature in the firing process described later, and combinations thereof.

[0035] The presence of particulate silicon dioxide supported on the surface of lithium nickel manganese composite oxide can be confirmed using energy-dispersive X-ray spectroscopy (EDX) with a scanning electron microscope (SEM).

[0036] When the positive electrode active material of this embodiment is applied to a lithium-ion secondary battery equipped with a liquid electrolyte, silicon dioxide is thought to function as an adsorbent that adsorbs decomposition products in the electrolyte. Therefore, it is thought that a low-resistance layer interface can be formed, and the resistance of the lithium nickel manganese composite oxide can be reduced. As a result, the discharge capacity of the lithium-ion secondary battery using the positive electrode active material of this embodiment can be further increased.

[0037] Furthermore, in the lithium-ion secondary battery using the positive electrode active material of this embodiment, the activation energy of the electrochemical reaction is reduced, making the electrochemical reaction more likely to occur. This suggests that 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.

[0038] [Method for manufacturing positive electrode active material] The positive electrode active material of this embodiment contains the lithium nickel manganese composite oxide described above. As the lithium source of the lithium nickel manganese composite oxide, known compounds such as hydroxides such as lithium hydroxide monohydrate (LiOH·H2O), carbonates such as lithium carbonate (Li2CO3), and acetates such as lithium acetate (CH3COOLi) and lithium acetate dihydrate (CH3COOLi·2H2O) can be used, and there are no particular restrictions. As for the nickel source and manganese source compounds of the transition metals, a wide range of known nickel and manganese oxides, hydroxides, or metal salts can be used, and there are no particular restrictions. For example, nickel compounds that can be used include, but are not limited to, nickel hydroxide (Ni(OH)2), nickel(II) chloride (NiCl2), nickel(II) chloride hexahydrate (NiCl2·6H2O), and nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O). Examples of manganese compounds that can be used include, but are not limited to, manganese(II) chloride (MnCl2), manganese(II) chloride tetrahydrate (MnCl2·4H2O), manganese carbonate hexahydrate (MnCO3·6H2O), and manganese(II) nitrate hexahydrate (Mn(NO3)2·6H2O). In addition to using each of the above transition metal compounds individually, they can also be used as composite hydroxides (for example, nickel-manganese composite hydroxides) by coprecipitation or other methods.

[0039] "Preparation of Lithium Nickel Manganese Composite Oxide" In the preparation of lithium nickel manganese composite oxide, first, a predetermined amount of lithium compound is added to an intermediate nickel manganese compound, and the mixture is dispersed in a solvent such as ethanol. Note that in addition to wet mixing using a solvent, dry mixing without a solvent may also be used to mix a predetermined amount of the intermediate compound with a predetermined amount of the lithium compound. For example, lithium carbonate (Li2CO3) is used as the lithium compound. 1.04 Ni 0.48 Mn 0.48 When synthesizing O2, it is preferable to weigh in 1% to 5% by mass, for example, 2% by mass more Li2CO3 than the stoichiometric ratio.

[0040] The above nickel-manganese compounds can be synthesized by known methods. When the nickel-manganese compound is a hydroxide, for example, nickel sulfate hexahydrate (NiSO4·6H2O) and manganese sulfate pentahydrate (MnSO4·5H2O) can be weighed in such a Ni:Mn molar ratio of 1:1, dissolved in pure water, and then an alkaline aqueous solution can be added dropwise to this sulfate aqueous solution to coprecipitate the nickel-manganese composite hydroxide.

[0041] The lithium nickel manganese composite oxide of this embodiment can be synthesized using known methods. For example, a composite hydroxide or composite oxide of a nickel compound and a manganese compound can be prepared as an intermediate compound, this intermediate compound can be mixed with a lithium compound to form a raw material mixture, and this raw material mixture can be heat-treated (e.g., calcined) in a predetermined atmosphere at a predetermined temperature for a predetermined time to synthesize the composite oxide.

[0042] As a precursor, a mixture of a lithium compound and a nickel-manganese compound is crushed to a preferred size, mixed, and then packed into a crucible or the like for heat treatment. Examples of crucibles include alumina saggers, alumina crucibles, platinum crucibles, and gold crucibles. For heat treatment of the mixture, for example, a firing furnace or a roller hearth kiln is used.

[0043] The above mixture, placed in a saggar or crucible, is heated to a heat treatment temperature at a heating rate of 5°C / min to 25°C / min, preferably 10°C / min to 25°C / min. The heat treatment atmosphere is not particularly limited and can be in the atmosphere (air atmosphere), oxygen flow, etc. An oxygen flow is preferred for the heat treatment atmosphere. The heat treatment time can be set appropriately according to the heat treatment temperature. Note that the heat treatment time refers to the time for which the heat treatment temperature is maintained.

[0044] When heat-treating a mixture of a lithium compound (e.g., Li2CO3) and a nickel-manganese compound, the heat treatment temperature is preferably 1025°C to 1150°C, more preferably 1050°C to 1125°C. The heat treatment time is preferably 1 minute to 7 hours, more preferably 2 minutes to 6 hours, even more preferably 3 minutes to 5 hours, and particularly preferably 5 minutes to 3 hours.

[0045] By the above method, lithium nickel manganese composite oxide can be obtained. The method for producing the positive electrode active material of this embodiment will be described in detail below with reference to the drawings.

[0046] Figure 1 shows a flow chart of the method for producing the positive electrode active material in this embodiment. As shown in Figure 1, the method for producing the positive electrode active material of this embodiment includes a dispersion step (S1), a mixing step (S2), a drying step (S3), and a calcination step (S4).

[0047] <Dispersion process (S1)> The dispersion step (S1) is a step in which silicon dioxide is dispersed in a solvent and mixed. The silicon dioxide is preferably in the form of nanooxide particles with an average particle diameter of 5 nm to 300 nm. Commercially available nanooxide particles may be used. Examples of commercially available nanooxide particles include silicon dioxide nanopowder, 5-20 nm particle size, manufactured by Aldrich, and Silica nanoparticles, 300 nm particle size, also manufactured by Aldrich.

[0048] 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. Of these solvents, alcohols are preferred, and ethanol is more preferred, due to their excellent dispersibility and cost-effectiveness. Ethanol may also be used in aqueous solution.

[0049] In the dispersion process, particulate silicon dioxide is placed in a container containing a solvent and stirred to disperse the silicon dioxide in the solvent. The method of stirring is not particularly limited; for example, stirring can be done using a stirring bar.

[0050] The processing time (stirring time) in the dispersion process is not particularly limited, but is preferably, for example, 10 to 60 minutes. The processing temperature in the dispersion process is not particularly limited, but is preferably, for example, 5 to 30°C.

[0051] <Mixing process (S2)> The mixing step (S2) is a step in which the lithium nickel manganese composite oxide is mixed with the dispersion of silicon dioxide and solvent obtained in the dispersion step. The lithium nickel manganese composite oxide is preferably composed of particles with an average particle diameter of 0.25 to 10 μm.

[0052] In the mixing step, particulate lithium nickel manganese composite oxide is placed in a container containing the solvent, and then the dispersion of silicon dioxide and solvent obtained in the dispersion step is added and stirred to mix the lithium nickel manganese composite oxide and silicon dioxide. The solvent used in the mixing step is similar to the solvent used in the dispersion step, and is preferably of the same type as the solvent used in the dispersion step. Specifically, alcohols are preferred as the solvent used in the step, and ethanol is more preferred. Ethanol may be an aqueous solution. The method of stirring is not particularly limited; for example, stirring can be done using a stirring bar.

[0053] The processing time (stirring time) in the mixing step is not particularly limited, but is preferably, for example, 10 to 60 minutes. The processing temperature in the mixing step is not particularly limited, but is preferably, for example, 5 to 30°C.

[0054] <Drying process (S3)> The drying step (S3) is a step in which the mixture of lithium nickel manganese composite oxide and silicon dioxide obtained in the mixing step is dried. In the drying process, the above mixture is dried using a drying device such as an oven to obtain a mixture in which particulate silicon dioxide is attached to the surface of the lithium nickel manganese composite oxide.

[0055] The processing time (drying time) in the drying process is not particularly limited, but for example, 2 to 8 hours is preferred. The drying time refers to the time from when the above mixture is placed in the drying apparatus and heating begins until heating is stopped and the temperature inside the drying apparatus returns to room temperature (for example, 5-30°C). The processing temperature in the drying process is preferably, for example, 80 to 120°C.

[0056] <Baking process (S4)> The firing process (S4) is a process in which the mixture obtained in the drying process is heated and fired. In the firing process, the above mixture is placed in a firing apparatus such as a firing furnace and subjected to heat treatment to obtain a positive electrode active material in which particulate silicon dioxide is supported on the surface of a lithium nickel manganese composite oxide.

[0057] The processing time (heat treatment time) in the firing process is not particularly limited, but for example, 1 to 8 hours is preferred. The heating time refers to the time from when the above mixture is placed in the firing apparatus and heating begins until heating is stopped and the temperature inside the firing apparatus returns to room temperature (for example, 5-30°C).

[0058] The processing temperature in the firing process is preferably, for example, 200°C to 500°C, and more preferably 300°C to 400°C. If the processing temperature in the firing process is above the lower limit, the catalytic activity of silicon dioxide can be further enhanced. If the processing temperature in the firing process is below the upper limit, the amount of silicon dioxide supported on the surface of the lithium nickel manganese composite oxide can be sufficiently maintained.

[0059] In the firing process, the mixture placed in the firing apparatus is heated to reach the processing temperature at a heating rate of 5°C / min to 25°C / min, preferably 10°C / min to 25°C / min. The heat treatment atmosphere is not particularly limited and can be in the atmosphere (air atmosphere), oxygen flow, etc. An oxygen flow is preferred for the heat treatment atmosphere. After the heat treatment, it is preferable to cool the material to room temperature (for example, 5 to 30 degrees Celsius) at a cooling rate of 5°C / min to 25°C / min, preferably 10°C / min to 25°C / min.

[0060] The positive electrode active material of this embodiment is obtained through the above process.

[0061] [Lithium-ion rechargeable battery] The lithium-ion secondary battery of this embodiment comprises a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains a positive electrode active material in which particulate silicon dioxide is supported on the surface of the aforementioned lithium nickel manganese composite oxide. The lithium-ion secondary battery of this embodiment may also include other battery elements as needed.

[0062] The lithium-ion secondary battery of this embodiment can use the same battery elements as known lithium-ion secondary batteries, except that the positive electrode contains a positive electrode active material in which particulate silicon dioxide is supported on the surface of the aforementioned lithium nickel manganese composite oxide. The lithium-ion secondary battery of this embodiment may have any of the following configurations: coin type, button type, cylindrical type, prismatic type, or laminate type. Furthermore, the lithium-ion secondary battery of this embodiment can be applied to a wide range of applications, such as mobile devices like mobile phones and laptop computers, and in-vehicle applications.

[0063] The following describes the lithium-ion secondary battery of this embodiment, specifically a lithium-ion secondary battery using an electrolyte (coin-type lithium-ion secondary battery).

[0064] Figure 2 is a schematic cross-sectional view of a lithium-ion secondary battery according to this embodiment. Figure 2 shows an example in which the lithium-ion secondary battery of this embodiment is a coin-type lithium-ion secondary battery. As shown in Figure 2, the lithium-ion secondary battery 1 of this embodiment comprises a negative electrode can (negative electrode terminal) 20, a negative electrode 3, a separator 4 impregnated with electrolyte, an insulating packing (gasket) 5, a positive electrode 2, and a positive electrode can 10.

[0065] The positive electrode can 10 is positioned below the separator 4, and the negative electrode can 20 is positioned above the separator 4. The positive electrode can 10 and the negative electrode can 20 form the outer shape of the lithium-ion secondary battery 1. Between the positive electrode can 10 and the negative electrode can 20, a positive electrode 2 and a negative electrode 3 are provided via a separator 4 impregnated with electrolyte, with the separator 4 separating the positive electrode 2 and the negative electrode 3. The positive electrode can 10 and the negative electrode can 20 are electrically insulated by an insulating packing 5.

[0066] The lithium-ion secondary battery 1 can be manufactured by preparing a positive electrode composite material by adding a conductive agent, a binder, etc., as needed to the positive electrode active material of this embodiment, and then pressing this composite material onto a current collector (not shown). Preferably, stainless steel mesh, aluminum foil, etc., can be used as the current collector. Preferably, acetylene black, ketchen black, etc., can be used as the conductive agent. Preferably, tetrafluoroethylene, polyvinylidene fluoride, etc., can be used as the binder.

[0067] The composition of the positive electrode active material, conductive agent, and binder in the positive electrode composite material is not particularly limited. The content of the conductive agent in the positive electrode composite material is preferably 1% to 15% by mass, and more preferably 0.1% to 5% by mass. The content of the binder in the positive electrode composite material is preferably 0.1% to 10% by mass, and more preferably 0.1% to 5% by mass. It is preferable to blend the positive electrode active material, conductive agent, and binder so that the remainder of the positive electrode composite material (the portion other than the positive electrode active material and conductive agent) becomes the positive electrode active material.

[0068] In a lithium-ion secondary battery 1, the negative electrode 3 relative to the positive electrode 2 can be any known material that functions as a negative electrode active material and is capable of intercalating and releasing lithium, such as metallic materials like metallic lithium or lithium alloys, carbon-based materials like graphite or MCMB (mesocarbon microbeads), or silicon-based materials like silicon (Si), Si alloys, or silicon oxide.

[0069] The separator 4 and battery container (positive electrode container 10, negative electrode container 20) can use known battery components.

[0070] As the electrolyte, known electrolytes can be used. For example, electrolytes such as lithium perchlorate and lithium hexafluoride phosphate can be dissolved in solvents such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), and diethyl carbonate (DEC).

[0071] The lithium-ion secondary battery 1 of this embodiment can achieve high capacity because the positive electrode 2 contains a positive electrode active material in which particulate silicon dioxide is supported on the surface of the aforementioned lithium nickel manganese composite oxide. [Examples]

[0072] The following describes embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0073] Preparation of Lithium Nickel Manganese Composite Oxide: LiNi 0.5 Mn 0.5 O2" Li2CO3 (high-purity chemicals) and Ni 0.5 Mn 0.5 (OH)2 (Sigma-Aldrich) was weighed in a molar ratio of Li:Ni:Mn = 1:0.5:0.5, and considering the evaporation of Li, the amount was weighed so that Li2CO3 was 3 mass% more than Li2CO3 based on the stoichiometric ratio. Li2CO3 (High Purity Chemicals) and Ni 0.5 Mn 0.5 The total mass of (OH)2 (manufactured by Sigma-Aldrich) was set to 2.1 g. These were dispersed and mixed in ethanol in a mortar. The mixture was then packed into a platinum crucible conforming to JIS standards. Using a firing furnace, the mixture packed in the platinum crucible was heated in air at a heating rate of 15°C / min and fired at 1100°C for 5 minutes. After that, the resulting powder was left to cool to room temperature (25°C) to obtain lithium nickel manganese oxide.

[0074] "Preparation of positive electrode active material" 50 mL of ethanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 99.5% purity) was placed in a 100 mL beaker, 0.03 g of particulate silicon dioxide (manufactured by Aldrich, particle size 5-20 nm) was added, and the mixture was stirred with a stirring bar at 25°C for 30 minutes to disperse (dispersion step). 9.97 g of lithium nickel manganese oxide obtained in the "Preparation of Lithium Nickel Manganese Composite Oxide" described above was added to the silicon dioxide dispersion obtained in the above step, and mixed using a stirring bar for 10 minutes at 25°C (mixing step). The mass ratio of lithium nickel manganese composite oxide to silicon dioxide was set to 99.7:0.3. The resulting mixture was placed in an oven and dried at 80°C for 20 hours (drying process). Ten g of the obtained mixture was packed into a platinum crucible conforming to JIS standards. Using a firing furnace, the mixture packed into the platinum crucible was heated in air at a heating rate of 15°C / min and fired at 400°C for 4 hours (firing process). After that, the obtained powder was left to stand until its temperature reached room temperature (25°C) to obtain the positive electrode active material.

[0075] "SEM imaging" The obtained positive electrode active material was imaged at a magnification of 10,000x using a scanning electron microscope (JEOL Ltd., model number JSM-IT800). The results are shown in Figure 3. As shown in Figure 3, the obtained positive electrode active material was an aggregate of primary particles (A) with a major axis of approximately 1 to several μm. Some of the primary particles (A) had aggregated to form secondary particles. Multiple small white granular substances (B) were observed on the surface of the primary particles (A).

[0076] "Analysis using SEM-EDX" Cross-sectional samples of powdered material were prepared using a cross-section polisher (JEOL Ltd., model IB-19520CP). SEM images of the cathode active material, captured at a magnification of 33,000x, were analyzed using SEM-EDX (JEOL Ltd., model JSM-IT800). The SEM images used for the SEM-EDX analysis are shown in Figure 4. As shown in Figure 4, the positive electrode active material consisted of particles with a surface (S) and a cross-section (C). White granular material (B1) and (B2) were observed in places on the surface (S) of the particles.

[0077] The spectral results of elemental analysis performed by SEM-EDX on the white granular materials (B1) and (B2) are shown in Figures 5 and 6, respectively. The results of the compositional analysis are shown in Table 1. As shown in Figures 5 and 6, Si peaks were observed in the energy range of 1.70–1.80 keV for the white granular substances (B1) and (B2). From this, it can be concluded that the white granular substances (B1) and (B2) are SiO2 particles.

[0078] [Table 1]

[0079] Next, the spectral results of elemental analysis by SEM-EDX on the surface (S) and cross-section (C) are shown in Figures 7 and 8, respectively. The results of the compositional analysis are shown in Table 1. As shown in Figures 7 and 8, no Si peaks were observed on the surface (S) and cross-section (C). This confirmed that SiO2 was not present on the surface (S) where the white granular material (B1) and (B2) were absent. Since SiO2 was not present in the cross-section (C), it was confirmed that SiO2 did not penetrate into the interior of the positive electrode active material particles and was distributed as granular material in island-like formations on the surface (S) rather than in a layered form. From this, it was confirmed that SiO2 is supported as granular material on the surface (S) of the lithium nickel manganese composite oxide.

[0080] Furthermore, Table 1 shows that the carbon content on the surface (S) of the lithium nickel manganese composite oxide is higher than that on the cross-section (C), and based on previous analysis results, it is considered that this carbon originates from lithium carbonate. 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. From this, it can be inferred that on the surface (S), Li is reacted according to the following reaction equation. x SiO y It is believed that it has not formed. Li2CO3 + SiO2 → Li2SiO3 + CO2↑

[0081] In summary, the results in Figure 3 confirm that the primary particles (A) are lithium nickel manganese composite oxide, and the white granular material (B) is silicon dioxide (SiO2).

[0082] "Preparation of positive electrode active material with varying SiO2 content 1" [Example 1, Comparative Example 1] Each positive electrode active material was prepared in the same manner as described in "Preparation of Positive Electrode Active Material" above, except that the lithium nickel manganese composite oxide and silicon dioxide were mixed in mass ratios 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 (Example 1). Furthermore, the above "Preparation of lithium nickel manganese composite oxide: LiNi 0.5 Mn 0.5 In "O2", the only difference is that the conditions for firing the mixture packed in the platinum crucible were set to 1075°C for 30 minutes, as described above in "Preparation of Lithium Nickel Manganese Composite Oxide: LiNi 0.5 Mn 0.5 A lithium nickel manganese composite oxide was prepared by the same method as described in "O2" (Comparative Example 1). The average particle size (D50) of the lithium nickel manganese composite oxide used in Example 1 and Comparative Example 1, as measured by a laser diffraction particle size distribution analyzer, was 4.1 μm.

[0083] "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 the positive electrode active material. Acetylene black (AB) was used as the conductive agent and polyvinylidene fluoride (PVDF) as the binder, and NMP (N-methyl-2-pyrrolidone) was used as the solvent to mix them in a weight ratio of 8:1:1 to prepare a slurry. This slurry was then coated onto a 15 μm thick aluminum foil and dried to produce a 14φ positive electrode. The coating area density was 4.5 mg / cm². 2 The volume density is 2.3 g / cm³. 3 The positive electrode was constructed using a lithium metal counter electrode with a thickness of 200 μm and a diameter of 16φ, and a polyethylene microporous membrane with a thickness of 20 μm and a diameter of 18φ as a separator. The electrolyte was a 1.2 mol / L solution of lithium hexafluoride phosphate (LiPF6) dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio 3:4:3). A lithium-ion secondary battery (2032 coin-type cell) with the structure shown in Figure 2 was fabricated. The battery was fabricated according to known cell configuration and assembly methods.

[0084] "Charge / Discharge Test 1" Each of the fabricated lithium-ion secondary batteries was subjected to charge-discharge tests at a constant current of 0.1C (1C: 250 mA / g) under a temperature of 25°C, with a current density of 12.5 mA / g and cutoff potentials of 4.7V to 2.5V or 4.8V to 2.5V, and its discharge capacity was evaluated. The results are shown in Figure 9. The dashed line in Figure 9 represents the discharge capacity of the lithium-ion secondary battery of Comparative Example 1.

[0085] As shown in Figure 9, the discharge capacity of the lithium-ion secondary battery of Comparative Example 1, which used a positive electrode active material in which silicon dioxide was not supported on lithium nickel manganese composite oxide, was 154.1 mAhg. -1 That was the case. In contrast, the discharge capacity of the lithium-ion secondary battery in Example 1, which used a lithium nickel manganese composite oxide with silicon dioxide supported as the positive electrode active material, exceeded that of Comparative Example 1 when the mass percentage of silicon dioxide was 0.05% by mass or more and 0.9% by mass or less. From this, it was confirmed that the discharge capacity can be further increased when the mass percentage of silicon dioxide is 0.05% by mass or more and 0.9% by mass or less.

[0086] "Charge / Discharge Test 2" Except for changing the charge / discharge test rate to 0.33C, the charge / discharge test was performed in the same manner as in "Charge / Discharge Test 1" above, and the discharge capacity of each was evaluated. The results are shown in Figure 10. The dashed line in Figure 10 represents the discharge capacity of the lithium-ion secondary battery of Comparative Example 1.

[0087] As shown in Figure 10, the discharge capacity of the lithium-ion secondary battery of Comparative Example 1, which used a positive electrode active material in which silicon dioxide was not supported on lithium nickel manganese composite oxide, was 142.9 mAhg. -1 That was the case. In contrast, the discharge capacity of the lithium-ion secondary battery in Example 1, which used a lithium nickel manganese composite oxide with silicon dioxide supported as the positive electrode active material, exceeded that of Comparative Example 1 when the mass percentage of silicon dioxide was 0.05% by mass or more and 0.9% by mass or less. From this, it was confirmed that the discharge capacity can be further increased when the mass percentage of silicon dioxide is 0.05% by mass or more and 0.9% by mass or less.

[0088] "Preparation of positive electrode active material with varying SiO2 content 2" [Example 2, Comparative Example 2] Each cathode active material was prepared in the same manner as described in "Preparation of Cathode Active Materials" above, except that the lithium nickel manganese composite oxide and silicon dioxide were mixed in mass ratios of 99.7:0.3, 99.3:0.7, 99:1, 98.5:1.5, 98:2, and 97:3 (Example 2). Furthermore, the above "Preparation of lithium nickel manganese composite oxide: LiNi 0.5 Mn 0.5In "O2", the only difference is that the conditions for firing the mixture packed in the platinum crucible were set to 1075°C for 30 minutes, as described above in "Preparation of Lithium Nickel Manganese Composite Oxide: LiNi 0.5 Mn 0.5 A lithium nickel manganese composite oxide was prepared using the same method as described in "O2" (Comparative Example 2). The average particle size (D50) of the lithium nickel manganese composite oxide used in Example 2 and Comparative Example 2, as measured by a laser diffraction particle size distribution analyzer, was 3.6 μm.

[0089] "Fabrication of lithium-ion secondary batteries" A lithium-ion secondary battery was fabricated in the same manner as in Example 1 and Comparative Example 1, except that the positive electrode active material of Example 2 and the lithium nickel manganese composite oxide of Comparative Example 2 were used as the positive electrode active material, respectively.

[0090] "Charge / Discharge Test 3" A charge-discharge test was performed using the same method as in "Charge-Discharge Test 1" described above, and the discharge capacity of each was evaluated. The results are shown in Figure 11. The dashed line in Figure 11 represents the discharge capacity of the lithium-ion secondary battery of Comparative Example 2.

[0091] As shown in Figure 11, the discharge capacity of the lithium-ion secondary battery of Comparative Example 2, which used a positive electrode active material in which silicon dioxide was not supported on lithium nickel manganese composite oxide, was 155.1 mAhg. -1 That was the case. In contrast, the discharge capacity of the lithium-ion secondary battery in Example 2, which used a lithium nickel manganese composite oxide with silicon dioxide supported as the positive electrode active material, exceeded that of Comparative Example 2 when the mass percentage of silicon dioxide was greater than 0% by mass and less than or equal to 2.0% by mass. From this, it was confirmed that the discharge capacity can be further increased when the mass percentage of silicon dioxide is greater than 0% by mass and less than or equal to 2.0% by mass.

[0092] "Charge / Discharge Test 4" Except for changing the charge / discharge test rate to 0.33C, the charge / discharge test was performed in the same manner as in "Charge / Discharge Test 3" above, and the discharge capacity of each was evaluated. The results are shown in Figure 12. The dashed line in Figure 12 represents the discharge capacity of the lithium-ion secondary battery of Comparative Example 2.

[0093] As shown in Figure 12, the discharge capacity of the lithium-ion secondary battery of Comparative Example 2, which used a positive electrode active material in which silicon dioxide was not supported on lithium nickel manganese composite oxide, was 142.9 mAhg. -1 That was the case. In contrast, the discharge capacity of the lithium-ion secondary battery in Example 2, which used a lithium nickel manganese composite oxide with silicon dioxide supported as the positive electrode active material, exceeded that of Comparative Example 2 when the mass percentage of silicon dioxide was greater than 0% by mass and less than or equal to 2.0% by mass. From this, it was confirmed that the discharge capacity can be further increased when the mass percentage of silicon dioxide is greater than 0% by mass and less than or equal to 2.0% by mass.

[0094] From the above results, it was found that the present invention can provide a positive electrode active material that can further increase the discharge capacity, and a lithium-ion secondary battery containing the positive electrode active material. [Explanation of symbols]

[0095] 1…Lithium-ion rechargeable battery 2...Positive electrode 3...Negative electrode 4... Separator 5…Insulating packing (gasket) 10…Positive electrode can 20... Negative electrode can (negative electrode terminal)

Claims

1. A cobalt-free positive electrode active material using lithium nickel manganese composite oxide as a support, A positive electrode active material comprising particulate silicon dioxide supported on the surface of the lithium nickel manganese composite oxide.

2. The mass (M) of the lithium nickel manganese composite oxide １ ) and the mass (M) of the particulate silicon dioxide. ２ ) and the sum (M １ +M ２ The mass of the particulate silicon dioxide (M) relative to the mass of the particulate silicon dioxide. ２ The mass ratio of ((M ２ / (M １ +M ２ The positive electrode active material according to claim 1, wherein the ratio of ()) × 100)) is greater than 0% by mass and less than or equal to 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 to 10 μm.

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

Images