Positive electrode active material for lithium-ion secondary batteries and lithium-ion secondary batteries

Abstract

To provide a Co-free positive electrode active material for lithium-ion secondary batteries, and a lithium-ion secondary battery using the positive electrode active material, which can be used to obtain lithium-ion secondary batteries with higher discharge capacity. [Solution] Li transition metal composite oxide is in the form of particles having an outer layer on its surface, and is represented by formula (1), Li x Ni y Al z Fe w O2(1) (wherein x is in the range of 0.95≦x≦1.05, y is in the range of 0.78≦y≦0.95, z is in the range of 0.01≦z≦0.15, and w is in the range of 0.01≦w≦0.15, and x+y+z+w=2.) A positive electrode active material for a Li-ion secondary battery, wherein the ratio of the number of Fe atoms to the number of Ni atoms in the outer layer to the ratio of the number of Fe atoms to the number of Ni atoms in the entire particle is 0.7 or more and 1.7 or less, and the ratio (I1 / I2) of the integral intensity (I1) of the diffraction peak of the 003 plane to the integral intensity (I2) of the diffraction peak of the 104 plane in space group R-3m measured by X-ray diffraction is 1.15 or more and 1.35 or less.

Description

[Technical Field]

[0001] This invention relates to a positive electrode active material for lithium-ion secondary batteries 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 a crucial component that determines the capacity of lithium-ion secondary batteries, and development is progressing. Conventionally, cobalt-based materials have been used as positive electrode active materials for lithium-ion secondary batteries. However, cobalt-based materials are expensive and pose resource risks. Therefore, there is a need to develop materials based on elements that can replace cobalt. For example, Non-Patent Literature 1 reports a cathode active material made by co-precipitation of nickel, iron, and aluminum hydroxides as a cobalt-free material. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Nitin Muralidharan, et al., “LiNixFeyAlzO2, A New Cobalt-Free Layered Cathode Material For Advanced Li-ion Batteries” Journal of Power Sources. Volume 471, 228389, 30 September 2020 [Overview of the project] [Problems that the invention aims to solve]

[0005] Non-patent document 1 reports that a lithium-ion secondary battery fabricated using a positive electrode active material achieved a high capacity of 190 mAh / g at a rate of 0.1C. However, there is still room for improvement in terms of lithium-ion rechargeable battery capacity.

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

[0007] To achieve the above objective, the present invention provides the following means. [1] A positive electrode active material for lithium-ion secondary batteries, comprising a lithium transition metal composite oxide as the main component, The lithium transition metal composite oxide is in the form of particles having an outer layer on its surface. The aforementioned lithium transition metal composite oxide is represented by the following formula (1): Li x Ni y Al z Fe w O2(1) (In equation (1), x is in the range of 0.95 ≤ x ≤ 1.05, y is in the range of 0.78 ≤ y ≤ 0.95, z is in the range of 0.01 ≤ z ≤ 0.15, and w is in the range of 0.01 ≤ w ≤ 0.15, and x + y + z + w = ​​2.) The ratio of the number of Fe atoms to the number of Ni atoms in the outer layer to the ratio of the number of Fe atoms to the number of Ni atoms in the entire particle is 0.7 or more and 1.7 or less. A positive electrode active material for lithium-ion secondary batteries, wherein the ratio (I1 / I2) of the integral intensity (I1) of the diffraction peak of the 003 plane to the integral intensity (I2) of the diffraction peak of the 104 plane in space group R-3m, as measured by X-ray diffraction, is between 1.15 and 1.35.

[0008] The positive electrode active material for lithium-ion secondary batteries (hereinafter also simply referred to as "positive electrode active material") according to [1] has a ratio of the number of Fe atoms to the number of Ni atoms in the outer layer (Fe / Ni ratio) and the ratio of the number of Fe atoms to the number of Ni atoms in the entire particle (Fe / Ni ratio) (hereinafter also referred to as "Fe / Ni ratio in the 'outer layer / entire particle'") within a specific range. Therefore, the Fe / Ni ratio in the 'outer layer / entire particle' is uniform, and the inhibition of lithium ion movement is suppressed. In addition, because the integrated intensity ratio (I1 / I2) of the positive electrode active material according to [1] is within a specific range, there is less mixing between the lithium metal layer and the transition metal layer (less cation mixing), and lithium ions diffuse easily. Therefore, the discharge capacity of the lithium-ion secondary battery (hereinafter also simply referred to as "secondary battery") can be increased. Consequently, secondary batteries using this positive electrode active material can reduce the number of batteries required, contributing to cost reduction. In other words, it can contribute to energy efficiency.

[0009] [2] The positive electrode active material for lithium-ion secondary batteries according to [1], wherein the lattice constants in the space group R-3m of the lithium transition metal composite oxide are such that the a-axis length is 2.860 Å to 2.890 Å and the c-axis length is 14.18 Å to 14.28 Å.

[0010] The positive electrode active material in [2] has lattice constants such that the a-axis length and c-axis length are within a specific range. Therefore, lithium ions diffuse easily within the primary particles, reducing resistance. Thus, the discharge capacity of the secondary battery can be further increased. Consequently, it can contribute to further energy efficiency.

[0011] [3] The positive electrode active material for lithium-ion secondary batteries according to [1] or [2], wherein the average particle size of the lithium transition metal composite oxide particles is 2 to 30 μm.

[0012] The positive electrode active material in [3] has a specific average particle size. Therefore, the electrochemical properties of the secondary battery can be improved. Thus, it can contribute to further energy efficiency.

[0013] [4] 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 for a lithium-ion secondary battery according to any one of [1] to [3].

[0014] The secondary battery according to [4] has a positive electrode containing the above positive electrode active material. Therefore, the discharge capacity can be further increased, the number of necessary batteries can be reduced, and it can contribute to cost reduction. That is, it can contribute to energy efficiency improvement.

Advantages of the Invention

[0015] According to the positive electrode active material for a lithium-ion secondary battery and the lithium-ion secondary battery of the present invention, the discharge capacity can be further increased.

Brief Description of the Drawings

[0016] [Figure 1] It is a cross-sectional view schematically showing the lithium-ion secondary battery manufactured in the example. [Figure 2] It is a figure showing the X-ray diffraction (XRD) pattern of the positive electrode active material of Example 1. [Figure 3] It is a figure showing the XRD pattern of the positive electrode active material of Example 2. [Figure 4] It is a figure showing the XRD pattern of the positive electrode active material of Comparative Example 1. [Figure 5] It is a figure showing the XRD pattern of the positive electrode active material of Comparative Example 2. [Figure 6] It is a figure showing the XRD pattern of the positive electrode active material of Example 3. [Figure 7] It is a figure showing the XRD pattern of the positive electrode active material of Comparative Example 3. [Figure 8] It is a figure showing the XRD pattern of the positive electrode active material of Comparative Example 4. [Figure 9] It is a figure showing the XRD pattern of the positive electrode active material of Comparative Example 5. [Figure 10] It is a figure showing the discharge curve of the lithium-ion secondary battery of Example 1. [Figure 11] It is a figure showing the discharge curve of the lithium-ion secondary battery of Example 2. [Figure 12] This figure shows the discharge curve of the lithium-ion secondary battery of Comparative Example 1. [Figure 13] This figure shows the discharge curve of the lithium-ion secondary battery of Comparative Example 2. [Figure 14] This figure shows the discharge curve of the lithium-ion secondary battery of Example 3. [Figure 15] This figure shows the discharge curve of the lithium-ion secondary battery of Comparative Example 3. [Figure 16] This figure shows the discharge curve of the lithium-ion secondary battery of Comparative Example 4. [Figure 17] This figure shows the discharge curve of the lithium-ion secondary battery in Comparative Example 5. [Modes for carrying out the invention]

[0017] Preferred embodiments of the present invention will be described in detail below.

[0018] [Active material for positive electrode in lithium-ion secondary batteries] The positive electrode active material for lithium-ion secondary batteries of this embodiment (hereinafter also simply referred to as "positive electrode active material") mainly consists of a lithium transition metal composite oxide and is used as the positive electrode of a lithium-ion secondary battery (hereinafter also simply referred to as "secondary battery"). "Mainly consisting of a lithium transition metal composite oxide" means that the content of the lithium transition metal composite oxide relative to the total mass of the positive electrode active material is 75% by mass or more, preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, and may be 100% by mass. The positive electrode active material may contain components other than the main component, as long as the function of the present invention is not impaired.

[0019] The positive electrode active material of this embodiment may contain only one type of lithium transition metal composite oxide, or it may contain two or more types, as long as it is mainly composed of lithium transition metal composite oxides.

[0020] When a positive electrode active material is manufactured using lithium transition metal composite oxide as the main component, the overall composition ratio (Li:Ni:Al:Fe) of the lithium transition metal composite oxide is maintained in the resulting positive electrode active material. When a positive electrode active material obtained using lithium transition metal composite oxide with such a composition is used in a secondary battery, high capacity can be achieved. Furthermore, the composition ratio of the lithium transition metal composite oxide is adjusted to be similar to the composition ratio required for the desired positive electrode active material.

[0021] (Lithium transition metal composite oxide) The lithium transition metal 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.

[0022] In this specification, the average particle size of the lithium transition metal composite oxide particles is not particularly limited, but is preferably 2 to 30 μm, more preferably 3 to 20 μm, and even more preferably 5 to 15 μm. If the average particle size of the lithium transition metal composite oxide particles is above the lower limit, the productivity of the positive electrode active material can be further increased. If the average particle size of the lithium transition metal composite oxide particles is below the upper limit, the electrochemical properties of the secondary battery can be further improved. The average particle size of lithium transition metal composite oxide particles refers to the D50 value, which is measured, for example, by a laser diffraction particle size distribution analyzer.

[0023] <Chemical composition> The lithium transition metal composite oxide of this embodiment is represented by the following formula (1). Li x Ni y Al z Fe w O2(1) In equation (1), x is in the range of 0.95 ≤ x ≤ 1.05, y is in the range of 0.78 ≤ y ≤ 0.95, z is in the range of 0.01 ≤ z ≤ 0.15, and w is in the range of 0.01 ≤ w ≤ 0.15, and x + y + z + w = ​​2.

[0024] The lithium transition metal composite oxide of this embodiment is more preferably in the range of x 0.97 ≤ x ≤ 1.03, y 0.80 ≤ y ≤ 0.92, z 0.04 ≤ z ≤ 0.09, and w 0.04 ≤ w ≤ 0.12 in formula (1).

[0025] The chemical composition of the lithium transition metal composite oxide in this embodiment can be determined by inductively coupled plasma (ICP) emission spectroscopy.

[0026] <Surface composition> The lithium transition metal 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.

[0027] In this embodiment, it is preferable that the lithium transition metal composite oxide particles have a uniform ratio of the number of Fe atoms to the number of Ni atoms in the outer layer (Fe / Ni ratio) and a uniform ratio of the Fe / Ni ratio in the entire particle (hereinafter also referred to as the "Fe / Ni ratio in the outer layer / entire particle"). More specifically, the Fe / Ni ratio in the 'outer layer / whole particle' is 0.7 to 1.7, preferably 0.9 to 1.5, and more preferably 1.1 to 1.3. When the Fe / Ni ratio in the 'outer layer / whole particle' is within the above numerical range, it does not hinder the movement of lithium ions, and when used as a positive electrode active material, it can further increase the discharge capacity of the secondary battery.

[0028] The Fe / Ni ratio can be determined by quantitative analysis using X-ray electron spectroscopy (XPS). XPS allows for the analysis of the transition metal element composition across the entire particle. In other words, the analysis results obtained by XPS represent the composition across the entire surface of the particle, not just a localized composition on the entire surface of the particle.

[0029] The particles of the lithium transition metal composite oxide of the present embodiment may be primary particles or secondary particles. Since relatively dense particles can be obtained, the particles of the lithium transition metal composite oxide are preferably secondary particles in which a plurality of primary particles are aggregated with each other.

[0030] <Lattice constant> The lithium transition metal composite oxide of the present embodiment is a rhombohedral layered compound and has a crystal structure of space group R-3m. The lattice constant of the positive electrode active material of the present embodiment mainly composed of the lithium transition metal composite oxide preferably has an a-axis length of 2.860 Å to 2.890 Å and a c-axis length of 14.18 Å to 14.28 Å. When the lattice constant is within the above range, lithium ions diffuse easily within the primary particles of the positive electrode active material and the resistance is low. The lattice constant of the crystal can be determined by the least squares method using each index and its interplanar spacing by measuring the X-ray diffraction pattern of the positive electrode active material.

[0031] <X-ray diffraction (XRD) pattern> For the positive electrode active material of the present embodiment, the ratio (I1 / I2) of the integrated intensity (I1) of the diffraction peak of the 003 plane to the integrated intensity (I2) of the diffraction peak of the 104 plane in the space group R-3m measured by X-ray diffraction (XRD) is 1.15 or more and 1.35 or less, preferably 1.20 or more and 1.30 or less, and more preferably 1.23 or more and 1.27 or less. When the integrated intensity ratio (I1 / I2) is within the above numerical range, cation mixing is less and lithium ions diffuse easily. Therefore, the discharge capacity of the secondary battery can be increased further. The integrated intensity ratio (I1 / I2) is determined by analyzing the XRD pattern.

[0032] [Manufacturing method of positive electrode active material] The positive electrode active material of this embodiment contains the above-mentioned lithium transition metal composite oxide as its main component. As the lithium source of the lithium transition metal 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, aluminum source, and iron source compounds of the transition metal, a wide range of known nickel, aluminum, and iron 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 aluminum compounds that can be used include aluminum chloride (AlCl3), aluminum carbonate (Al2(CO3)3), and aluminum nitrate notahydrate (Al(NO3)3·9H2O), but are not limited to these. Examples of iron compounds that can be used include iron(II) sulfate (Fe(SO4)) and iron(II) hydroxide (Fe(OH)2), but are not limited to these. In addition to using each of the above transition metal compounds individually, they can also be used as composite hydroxides (for example, nickel-aluminum-iron composite hydroxides) by coprecipitation or other methods.

[0033] The lithium transition metal composite oxide of this embodiment can be synthesized using known methods. For example, a composite hydroxide or composite oxide of a nickel compound and an aluminum compound can be prepared as an intermediate compound, and this intermediate compound can be mixed with an iron compound and a lithium compound to form a raw material mixture (mixing step). This raw material mixture can then be heat-treated (e.g., calcined) in a predetermined atmosphere at a predetermined temperature for a predetermined time to synthesize the composite oxide (heat treatment step). Alternatively, a composite hydroxide or composite oxide of a nickel compound, an aluminum compound and an iron compound can be prepared as an intermediate compound, and this intermediate compound can be mixed with a lithium compound to form a raw material mixture. This raw material mixture can then be heat-treated (e.g., calcined) in a predetermined atmosphere at a predetermined temperature for a predetermined time to synthesize the composite oxide.

[0034] The inventors have found that by appropriately selecting the above heat treatment conditions, the integral intensity ratio (I1 / I2) of the positive electrode active material can be controlled to a specific numerical range. When the integral intensity ratio (I1 / I2) of the positive electrode active material is within a specific numerical range, the transition metals of the lithium transition metal composite oxide are uniformly dispersed, cation mixing is reduced, and lithium ions diffuse easily. As a result, the discharge capacity of the secondary battery can be increased. The method for producing the positive electrode active material of this embodiment will be described step by step below.

[0035] <Mixing process> The mixing step involves mixing the lithium compound with the aforementioned intermediate compound to obtain a raw material mixture.

[0036] Lithium compounds are not particularly limited, but examples include 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). These lithium compounds may be used individually or in combination of two or more.

[0037] A known mixer can be used to mix the lithium compound and the intermediate compound. Examples of such mixing machines include shaker mixers, Redigge mixers, Julia mixers, and V-blenders. The mixing conditions in the mixing process are not particularly limited, but it is preferable to select conditions such that the raw material components are sufficiently mixed without destroying the physical structure of the raw material particles, such as intermediate compounds.

[0038] It is preferable to pre-adjust the particle size and other properties of the lithium compound and intermediate compound so that the desired lithium transition metal composite oxide can be obtained after the heat treatment process.

[0039] It is preferable that, after the heat treatment process, the lithium compound and the intermediate compound are weighed and mixed so that the ratio of the amounts of lithium transition metal composite oxide is Li:Ni:Al:Fe=x:y:z:w. More specifically, the lithium compound is weighed in an amount of 1% to 5% by mass, preferably 1% to 3% by mass, more than the stoichiometric ratio. Here, x, y, z, and w can be within the same range as those described for lithium transition metal composite oxides in the positive electrode active material.

[0040] <Heat treatment process> The heat treatment process involves heat-treating the raw material mixture obtained in the mixing process in a predetermined atmosphere, at a predetermined temperature, and for a predetermined time. In the heat treatment process, the raw material mixture is filled into a crucible or similar container and then heat-treated. Examples of crucibles include alumina saggers, alumina crucibles, platinum crucibles, and gold crucibles. For heat treatment of the raw material mixture, for example, a firing furnace or a roller hearth kiln is used.

[0041] The raw material mixture placed in a saggar or crucible is heated to reach the 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. Oxygen flow is preferred as 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.

[0042] The heat treatment temperature is preferably 700°C to 800°C, and more preferably 720°C to 780°C. The heat treatment time is preferably 6 hours to 18 hours, and more preferably 8 hours to 15 hours.

[0043] The method for producing the positive electrode active material in this embodiment may include steps other than the mixing step and the heat treatment step. One example of such a process is a cooling process.

[0044] <Cooling process> The cooling step is a process of cooling the lithium transition metal composite oxide obtained in the heat treatment step to a predetermined temperature at a predetermined rate of cooling. In the cooling process, for example, the powder obtained in the heat treatment process is cooled to room temperature (e.g., 25°C) at a cooling rate of 5°C / min to 25°C / min, preferably 10°C / min to 25°C / min. The atmosphere used to cool the powder is not particularly limited and may include air (air atmosphere), airflow, oxygen flow, etc.

[0045] In this embodiment, the Fe / Ni ratio in the 'outer layer / entire particle' of the lithium transition metal composite oxide can be more easily adjusted to a specific range by appropriately selecting the heat treatment temperature and heat treatment time in the heat treatment process.

[0046] [Lithium-ion rechargeable battery] The lithium-ion secondary battery of this embodiment (hereinafter also simply referred to as "secondary battery") comprises a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains a positive electrode active material mainly composed of the lithium transition metal composite oxide described above. The secondary battery of this embodiment may include other battery elements as needed.

[0047] The 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 mainly composed of the lithium transition metal composite oxide described above. The 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 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.

[0048] The following description of the secondary battery of this embodiment will focus on a secondary battery using an electrolyte (coin-type lithium-ion secondary battery). Each battery element described below can also be applied to an all-solid-state lithium-ion secondary battery that does not use an electrolyte.

[0049] Figure 1 is a schematic cross-sectional view showing a lithium-ion secondary battery according to this embodiment. Figure 1 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 1, 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] The separator 4 and battery container (positive electrode container 10, negative electrode container 20) can use known battery components.

[0055] As the electrolyte, known electrolytes such as liquid electrolytes and solid electrolytes can be used. As an electrolyte, for example, an electrolyte such as lithium perchlorate or lithium hexafluoride phosphate can be dissolved in a solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), or diethyl carbonate (DEC) can be used.

[0056] Furthermore, all-solid-state lithium-ion secondary batteries can have the same structure as known all-solid-state lithium-ion secondary batteries, except that they use a positive electrode active material mainly composed of the lithium transition metal composite oxide described above.

[0057] In the case of all-solid-state lithium-ion secondary batteries, the electrolyte can be a polymer-based solid electrolyte such as a polyethylene oxide-based polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain, a sulfide-based solid electrolyte, or an oxide-based solid electrolyte.

[0058] For the positive electrode of an all-solid-state lithium-ion secondary battery, for example, in addition to the positive electrode active material, conductive agent, and binder described above, a positive electrode composite material containing a solid electrolyte can be supported on a positive electrode current collector made of aluminum, nickel, stainless steel, or the like.

[0059] In this embodiment, the lithium-ion secondary battery 1 has a positive electrode 2 that contains a positive electrode active material mainly composed of the above-mentioned lithium transition metal composite oxide, thereby increasing the discharge capacity. [Examples]

[0060] The following describes embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0061] [Example 1] <Preparation of nickel-aluminum coprecipitate hydroxide> A stirrer equipped with a 7cm diameter blade and a 5L sealed, flat-bottomed beaker fitted with an overflow tube with an inner diameter of 5mm were used to hold 3L of water. After heating to 50°C, 150mL of 25% by mass ammonia solution was added, and sulfuric acid solution or sodium hydroxide solution was added to adjust the pH to 11.10 (measured at 50°C). The liquid temperature in the beaker was maintained at 50°C, and while stirring at a speed of 600 rpm, a solution of nickel sulfate and aluminum sulfate, adjusted to a molar ratio (Ni:Al = 95.3:4.7) of 10% by weight in terms of metal, was continuously added at a flow rate of 300 mL / h. Subsequently, 25% by mass aqueous ammonia was added at a flow rate of 30 mL / h. In addition, 25% by mass aqueous sodium hydroxide solution was continuously added to maintain the reaction solution in the beaker at pH 11.10 ± 0.05 (measured at 50°C). The reaction continued while controlling the pH of the reaction solution. The slurry discharged from the overflow pipe between 30 and 50 hours after the start of the reaction was dehydrated and washed with water to obtain nickel-aluminum coprecipitate oxide.

[0062] <Iron hydroxide coating process> 955 g (solid weight) of the obtained nickel-aluminum coprecipitate hydroxide powder was placed in a 5 L beaker and diluted with water to a 20 wt% slurry. This slurry was then heated to 75°C. Furthermore, the slurry was adjusted with sulfuric acid aqueous solution or sodium hydroxide aqueous solution to a pH of 9.00 ± 0.05 (measured at 75°C). In addition, a certain amount of nitrogen gas purging was performed to prevent oxidation of iron. Next, while maintaining the temperature at 75°C and stirring the slurry, an aqueous solution of iron(II) sulfate, adjusted to 10% by weight (metal equivalent), was continuously added dropwise so that iron hydroxide equivalent to 4.5 mol% iron precipitated on the nickel hydroxide surface. A 25% by mass sodium hydroxide aqueous solution was also added dropwise to maintain the pH at 9.00 ± 0.05 (measured at 75°C). This yielded a slurry of nickel-aluminum hydroxide coated with iron hydroxide. The obtained slurry was then dehydrated, washed with alkali and water, dried at 110°C for 10 hours, and then passed through a 100-mesh sieve to obtain 1 kg of an intermediate compound in which the surface of the nickel-aluminum hydroxide was coated with iron hydroxide.

[0063] <Fabrication of positive electrode active material> 1.38 g of the obtained intermediate compound and 0.65 g of lithium compound (LiOH·H2O) were dispersed in ethanol in a mortar and mixed. The mixture was then packed into a JIS standard platinum crucible. Using a firing furnace, the mixture in the platinum crucible was heated in air at a heating rate of 10°C / min, firing at 750°C for 10 hours. The resulting powder was then left to cool to room temperature (25°C) to obtain the positive electrode active material of Example 1, as shown in the chemical composition of Table 1. In Table 1, "-" indicates the absence of that element.

[0064] [Example 2] <Preparation of nickel-aluminum coprecipitate hydroxide> A stirrer equipped with a 7cm diameter blade and a 5L sealed flat-bottom beaker fitted with an overflow tube with an inner diameter of 5mm were used to hold 3L of water. After heating to 50°C, 150mL of 25% ammonia solution was added, and sulfuric acid solution or sodium hydroxide solution was added to adjust the pH to 11.00 (measured at 50°C). The liquid temperature in the beaker was maintained at 50°C, and while stirring at a speed of 600 rpm, a solution of nickel sulfate and aluminum sulfate, adjusted to a molar ratio (Ni:Al = 95.3:4.7) of 10% by weight in terms of metal, was continuously added at a flow rate of 300 mL / h. Subsequently, 25% by mass aqueous ammonia was added at a flow rate of 30 mL / h. In addition, 25% by mass aqueous sodium hydroxide solution was continuously added to maintain the reaction solution in the beaker at pH 11.00 ± 0.05 (measured at 50°C). While controlling the pH of the reaction solution, a slurry concentration device was connected to continuously perform solid-liquid separation, and only the filtrate was discharged from the system to increase the slurry concentration. After the reaction was complete, the slurry was dehydrated and washed with water to obtain nickel-aluminum coprecipitate oxide.

[0065] <Iron hydroxide coating process> 955 g (solid weight) of the obtained nickel-aluminum coprecipitate hydroxide powder was placed in a 5 L beaker and diluted with water to a 20 wt% slurry. This slurry was then heated to 75°C. Furthermore, the slurry was adjusted with sulfuric acid aqueous solution or sodium hydroxide aqueous solution to a pH of 9.00 ± 0.05 (measured at 75°C). In addition, a certain amount of nitrogen gas purging was performed to prevent oxidation of iron. Next, while maintaining the temperature at 75°C and stirring the slurry, an aqueous solution of iron(II) sulfate, adjusted to 10% by weight (metal equivalent), was continuously added dropwise so that iron hydroxide equivalent to 4.5 mol% iron precipitated on the nickel hydroxide surface. A 25% by mass sodium hydroxide aqueous solution was also added dropwise to maintain the pH at 9.00 ± 0.05 (measured at 75°C). This yielded a slurry of nickel-aluminum hydroxide coated with iron hydroxide. The obtained slurry was then dehydrated, washed with alkali and water, dried at 110°C for 10 hours, and then passed through a 100-mesh sieve to obtain 1 kg of an intermediate compound in which the surface of the nickel-aluminum hydroxide was coated with iron hydroxide.

[0066] <Fabrication of positive electrode active material> 1.38 g of the obtained intermediate compound and 0.65 g of lithium compound (LiOH·H2O) were dispersed in ethanol in a mortar and mixed. 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 10°C / min and fired at 750°C for 10 hours. After that, the resulting powder was left to cool to room temperature (25°C) to obtain the positive electrode active material of Example 2, as shown in the chemical composition in Table 1.

[0067] [Comparative Example 1] <Preparation of nickel-aluminum-iron coprecipitate oxide> A stirrer equipped with a 7cm diameter blade and a 5L sealed flat-bottom beaker fitted with an overflow tube with an inner diameter of 5mm were used to hold 3L of water. After heating to 50°C, 150mL of 25% ammonia solution was added, and sulfuric acid solution or sodium hydroxide solution was added to adjust the pH to 11.00 (measured at 50°C). The liquid temperature in the beaker was maintained at 50°C, and while stirring at a speed of 600 rpm, a solution of nickel sulfate, aluminum sulfate, and iron(II) sulfate, adjusted to a molar ratio (Ni:Al:Fe = 91.0:4.5:4.5) of 10% by weight in terms of metal, was continuously added at a flow rate of 300 mL / h. Subsequently, 25% by mass aqueous ammonia was added at a flow rate of 30 mL / h. In addition, 25% by mass aqueous sodium hydroxide solution was continuously added to maintain the reaction solution in the beaker at pH 11.00 ± 0.05 (measured at 50°C). While controlling the pH of the reaction solution, a slurry concentrator was connected to continuously perform solid-liquid separation, and only the filtrate was discharged from the system to increase the slurry concentration. After the reaction, the slurry was dehydrated and washed with water to obtain nickel-aluminum coprecipitated oxide. After the reaction, the slurry was dehydrated, washed with alkali and water, dried at 110°C for 10 hours, and then passed through a 100-mesh sieve to obtain nickel-aluminum-iron coprecipitated oxide (intermediate compound).

[0068] <Fabrication of positive electrode active material> 1.38 g of the obtained intermediate compound and 0.65 g of lithium compound (LiOH·H2O) were dispersed in ethanol in a mortar and mixed. The mixture was then 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 10°C / min and fired at 750°C for 10 hours. After that, the resulting powder was left to cool to room temperature (25°C) to obtain the positive electrode active material of Comparative Example 1, as shown in Table 1 for its chemical composition.

[0069] [Comparative Example 2] <Preparation of nickel-aluminum coprecipitate hydroxide> A stirrer equipped with a 7cm diameter blade and a 5L sealed, flat-bottomed beaker fitted with an overflow tube with an inner diameter of 5mm were used to hold 3L of water. After heating to 50°C, 150mL of 25% by mass ammonia solution was added, and sulfuric acid solution or sodium hydroxide solution was added to adjust the pH to 11.10 (measured at 50°C). The liquid temperature in the beaker was maintained at 50°C, and while stirring at a speed of 600 rpm, a solution of nickel sulfate and aluminum sulfate, adjusted to a molar ratio (Ni:Al = 95.3:4.7) of 10% by weight in terms of metal, was continuously added at a flow rate of 300 mL / h. Subsequently, 25% by mass aqueous ammonia was added at a flow rate of 30 mL / h. In addition, 25% by mass aqueous sodium hydroxide solution was continuously added to maintain the reaction solution in the beaker at pH 11.10 ± 0.05 (measured at 50°C). The reaction continued while controlling the pH of the reaction solution. The slurry discharged from the overflow pipe between 30 and 50 hours after the start of the reaction was dehydrated and washed with water to obtain nickel-aluminum coprecipitate oxide.

[0070] <Iron hydroxide coating process> 955 g (solid weight) of the obtained nickel-aluminum coprecipitate hydroxide powder was placed in a 5 L beaker and diluted with water to a 20 wt% slurry solution. This solution was then heated to 75°C. Furthermore, the slurry solution was adjusted with sulfuric acid aqueous solution or sodium hydroxide aqueous solution to a pH of 11.00 ± 0.05 (measured at 75°C). Next, while maintaining the temperature at 75°C and stirring the slurry, an aqueous solution of iron(II) sulfate, adjusted to 10% by weight (metal equivalent), was continuously added dropwise so that iron hydroxide equivalent to 4.5 mol% iron precipitated on the nickel hydroxide surface. A 25% by mass aqueous solution of sodium hydroxide was also added dropwise to maintain the pH at 11.00 ± 0.05 (measured at 75°C). This yielded a slurry of nickel-aluminum hydroxide coated with iron hydroxide. The obtained slurry was then dehydrated, washed with alkali and water, dried at 110°C for 10 hours, and then passed through a 100-mesh sieve to obtain 1 kg of an intermediate compound in which the surface of the nickel-aluminum hydroxide was coated with iron hydroxide.

[0071] <Fabrication of positive electrode active material> 1.38 g of the obtained intermediate compound and 0.65 g of lithium compound (LiOH·H2O) were dispersed in ethanol in a mortar and mixed. 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 10°C / min and fired at 750°C for 10 hours. After that, the resulting powder was left to cool to room temperature (25°C) to obtain the positive electrode active material of Comparative Example 2, as shown in the chemical composition in Table 1.

[0072] [Example 3] <Fabrication of positive electrode active material> The positive electrode active material for Example 3, shown in the chemical composition of Table 2, was obtained in the same manner as in Example 1, except that the chemical compositions of nickel, aluminum, and iron were adjusted to the values ​​shown in Table 2. In Table 2, "-" means that the element is not present.

[0073] [Comparative Example 3] <Fabrication of positive electrode active material> The positive electrode active material of Comparative Example 3, with the chemical composition shown in Table 2, was obtained in the same manner as in Example 1, except that the heat treatment conditions were adjusted as shown in Table 2.

[0074] [Comparative Example 4] <Fabrication of positive electrode active material> The positive electrode active material of Comparative Example 4, with the chemical composition shown in Table 2, was obtained in the same manner as in Example 1, except that the heat treatment conditions were adjusted as shown in Table 2.

[0075] [Comparative Example 5] <Preparation of nickel-cobalt-aluminum coprecipitate hydroxide> A stirrer equipped with a 7cm diameter blade and a 5L sealed flat-bottom beaker fitted with an overflow tube with an inner diameter of 5mm were used to hold 3L of water. After heating to 50°C, 150mL of 25% ammonia solution was added, and sulfuric acid solution or sodium hydroxide solution was added to adjust the pH to 11.00 (measured at 50°C). The liquid temperature in the beaker was maintained at 50°C, and while stirring at a speed of 600 rpm, a solution of nickel sulfate, cobalt sulfate, and aluminum sulfate, adjusted to a molar ratio (Ni:Co:Al = 91.0:4.5:4.5) equivalent to 10% by weight in terms of metal, was continuously added at a flow rate of 300 mL / h. Subsequently, 25% by mass aqueous ammonia was added at a flow rate of 30 mL / h. In addition, 25% by mass aqueous sodium hydroxide solution was continuously added to maintain the reaction solution in the beaker at pH 11.00 ± 0.05 (measured at 50°C). While controlling the pH of the reaction solution, a slurry concentrator was connected to continuously perform solid-liquid separation, and only the filtrate was discharged from the system to increase the slurry concentration. After the reaction, the slurry was dehydrated and washed with water to obtain nickel-cobalt-aluminum coprecipitated hydroxide. After the reaction, the slurry was dehydrated, washed with alkali and water, dried at 110°C for 10 hours, and then passed through a 100-mesh sieve to obtain nickel-cobalt-aluminum coprecipitated hydroxide (intermediate compound). <Fabrication of positive electrode active material> 1.36 g of the obtained intermediate compound and 0.64 g of lithium compound (LiOH·H2O) were dispersed in ethanol in a mortar and mixed. 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 10°C / min and fired at 750°C for 10 hours. After that, the resulting powder was left to cool to room temperature (25°C) to obtain the positive electrode active material of Comparative Example 5, as shown in the chemical composition in Table 2.

[0076] (analysis) Tables 1 and 2 show the results of chemical composition analysis of the positive electrode active materials obtained in each example, performed using an ICP emission spectrometer (product name: Agilent 5110 VDV, manufactured by Agilent Technologies).

[0077] The X-ray diffraction (XRD) patterns of the positive electrode active materials obtained in each example were measured using a powder X-ray diffractometer (trade name: SmartLab, manufactured by Rigaku). As the target for irradiating the electron beam, Cu (copper) was used, and as the characteristic X-ray, Kα ray was used. Using each index and the interplanar spacing of the obtained XRD patterns, the lattice constant was determined by the least squares method. Also, by analyzing the obtained XRD patterns, the integrated intensity ratio (I1 / I2) was determined. The XRD patterns are shown in Figs. 2 to 9, and the lattice constant values, integrated intensity ratios (I1 / I2), and the presence or absence of diffraction peaks are shown in Tables 1 to 2.

[0078] For the positive electrode active materials obtained in each example, the average particle diameter (D50) was measured using a laser diffraction particle size distribution analyzer (manufactured by BECKMAN COULTER, LS 13 320). The results are shown in Tables 1 to 2. In the tables, "-" means that the measurement of the average particle diameter was not performed.

[0079] X-ray photoelectron spectroscopy (XPS) analyzer (trade name: K-Alpha + , manufactured by Thermo Fisher Scientific), the results of analyzing the surface layer composition of the positive electrode active materials obtained in each example by quantitative analysis are shown in Tables 1 to 2. The measurement conditions for XPS measurement are shown below. 《XPS Measurement Conditions》 Model used: Manufactured by Thermo Fisher Scientific, K-Alpha + (trade name) Irradiated X-ray: Single crystal spectro AlKα (12 keV, 72 W) X-ray spot diameter: 400 μm Neutral electron gun: Used Reference spectrum: C-C, C-H 284.6 eV Detection depth: 6 - 7 nm

[0080]

Table 1

[0081]

Table 2

[0082] From the XRD patterns shown in Figures 2, 3, and 6, it was confirmed that the positive electrode active materials of Examples 1, 2, and 3 had an integrated intensity ratio (I1 / I2) of 1.15 or more and 1.35 or less.

[0083] <Manufacturing of lithium-ion secondary batteries> In each example, the positive electrode active material was mixed with acetylene black (AB) as a conductive agent and polyvinylidene fluoride (PVDF) as a binder in a weight ratio of 8:1:1 using NMP (N-methyl-2-pyrrolidone) as the solvent to prepare a slurry. This slurry was then coated onto 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 200 μm thick, 16φ lithium metal counter electrode and a 20 μm thick, 18φ polyethylene microporous membrane 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 3 was fabricated. The battery was fabricated according to known cell configuration and assembly methods.

[0084] <Charge / Discharge Test> For each lithium-ion secondary battery fabricated, charge-discharge tests were performed at a constant current of 0.05C or 5C (1C: 200 mA / g) at a temperature of 25°C, with a cutoff potential of 4.3V to 2.5V, to evaluate the initial discharge capacity. The charge-discharge tests started with charging. The results are shown in Tables 1 and 2.

[0085] The discharge curves of lithium-ion secondary batteries for each example at 5C are shown in Figures 10 to 17. From the discharge curves in Figures 10 to 17, it was confirmed that the lithium-ion secondary batteries of Examples 1 and 2 showed improved discharge capacity and average discharge voltage compared to the lithium-ion secondary batteries of Comparative Examples 1 and 2. Furthermore, it is generally known that the higher the nickel content in the positive electrode active material, the higher the discharge capacity. The lithium-ion secondary battery of Example 3 showed improved discharge capacity compared to the lithium-ion secondary battery of Comparative Example 3, despite having a lower nickel content.

[0086] From the above results, it was found that the present invention can provide a lithium-ion secondary battery with a higher discharge capacity. [Explanation of symbols]

[0087] 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 positive electrode active material for lithium-ion secondary batteries, mainly composed of a lithium transition metal composite oxide, The lithium transition metal composite oxide is in the form of particles having an outer layer on its surface. The lithium transition metal composite oxide is represented by the following formula (1): From ｘ Yes ｙ Al ｚ Fe ｗ Oh ２ (1) (In equation (1), x is in the range of 0.95 ≤ x ≤ 1.05, y is in the range of 0.78 ≤ y ≤ 0.95, z is in the range of 0.01 ≤ z ≤ 0.15, and w is in the range of 0.01 ≤ w ≤ 0.15, and x + y + z + w = ​​2.) The ratio of the number of Fe atoms to the number of Ni atoms in the outer layer to the ratio of the number of Fe atoms to the number of Ni atoms in the entire particle is 0.7 or more and 1.7 or less. The integrated intensity (I ２ ), of the diffraction peak of the 104 plane in the space group R-3m measured by X-ray diffraction, relative to the integrated intensity (I １ ) of the diffraction peak of the 003 plane, the ratio (I １ / I ２ ) is 1.15 or more and 1.35 or less, a positive electrode active material for a lithium-ion secondary battery.

2. The positive electrode active material for a lithium-ion secondary battery according to claim 1, wherein the lattice constants in the space group R-3m of the lithium transition metal composite oxide are such that the a-axis length is 2.860 Å to 2.890 Å and the c-axis length is 14.18 Å to 14.28 Å.

3. The positive electrode active material for a lithium-ion secondary battery according to claim 1, wherein the average particle size of the lithium transition metal composite oxide particles is 2 to 30 μm.

4. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains a positive electrode active material for lithium-ion secondary batteries as described in any one of claims 1 to 3.

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