Positive electrode active material for lithium-ion secondary battery, and lithium-ion secondary battery

Abstract

This positive electrode active material for a lithium-ion secondary battery comprises a lithium-nickel composite oxide having a layered structure with a space group of R-3m, wherein: the lithium-nickel composite oxide contains, other than oxygen, Li, Ni, and an element M at an amount-of-substance ratio of Li:Ni:M = a:b:c (0.90≤a<1.0, 0.5≤b≤1.0, 0≤c≤0.5, and b+c=1); the particles of the lithium-nickel composite oxide that are composed of single primary particles or secondary particles obtained by the aggregation of single primary particles and multiple primary particles; the number ratio of the single primary particles is at least 30% with respect to the total number of particles; and, when immersed in water, the amount of lithium eluted from the inside of the particles of the lithium-nickel composite oxide is 32% or less with respect to the total amount of lithium eluted into the water.

Description

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

[0001] This invention relates to a positive electrode active material for lithium-ion secondary batteries and to lithium-ion secondary batteries.

[0002] In recent years, with the widespread use of portable electronic devices such as mobile phones and notebook computers, there has been a strong demand for the development of small, lightweight, non-aqueous electrolyte secondary batteries with high energy density and durability. Furthermore, there is a strong demand for the development of high-output secondary batteries for use in power tools and electric vehicles, including hybrid cars. Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, meet these demands. As a positive electrode active material for lithium-ion secondary batteries, lithium cobalt composite oxide (LiCoO) with a layered crystalline structure is used. ２ ) and lithium nickel composite oxide (LiNiO ２ ), Lithium nickel cobalt manganese composite oxide (LiNi １／３ Co １／３ Mn １／３ O ２ Lithium metal composite oxides such as ) are used.

[0003] Among lithium metal composite oxides used as positive electrode active materials in lithium-ion secondary batteries, lithium metal composite oxides with a high Ni ratio (the molar ratio of Ni to elements excluding lithium and oxygen) are known to have high battery capacity. For this reason, lithium metal composite oxides with a high Ni ratio have recently attracted attention as high-energy-density materials that can realize secondary batteries with high battery capacity.

[0004] However, lithium metal composite oxides have a problem where, as the Ni ratio increases, the battery capacity decreases with repeated charging and discharging compared to batteries with a low Ni ratio; in other words, they have inferior cycle characteristics. Furthermore, lithium metal composite oxides have a problem where, as the Ni ratio increases, the battery expands more due to gas generation during repeated charging and discharging compared to batteries with a low Ni ratio.

[0005] Lithium metal composite oxides are typically composed of secondary particles formed by the aggregation of multiple primary particles. In lithium-ion secondary batteries, one of the causes of deterioration in cycle characteristics due to repeated charging and discharging is the cracking, deformation, and fracture of secondary particles due to expansion and contraction during charging and discharging (see Patent Documents 1 and 2).

[0006] Secondary particle cracking is thought to occur at the interfaces (grain boundaries) between primary particles. Therefore, it is expected that reducing the number of interfaces between primary particles in lithium metal composite oxides will improve their cycle characteristics. A lithium nickel composite oxide with reduced interfaces between primary particles can also be described as being composed of, for example, single primary particles or secondary particles formed by the aggregation of a small number of primary particles.

[0007] As an active material composed of single primary particles or secondary particles formed by the aggregation of a small number of primary particles, for example, Patent Document 3 proposes a positive electrode active material for lithium secondary batteries, which is a nickel-lithium metal oxide containing single crystal particles, the size of the single crystal particles being 1 to 8 μm, and the particle size distribution shown as (D90-D10) / D50 being 1.4 or less. Patent Document 3 also discloses that the peak intensity ratio (I(003) / I(104)) measured by X-ray diffraction analysis related to the positive electrode active material is 1.2 to 4.0, and that it further includes a cobalt compound-containing coating layer disposed on the surface of the nickel-lithium metal oxide, the cobalt compound-containing coating layer further containing one or more selected from boron, manganese, phosphorus, aluminum, zinc, zirconium, and titanium.

[0008] Furthermore, Patent Document 4 discloses a group of lithium transition metal composite oxide particles in which the average particle size DSEM based on electron microscopy observation is 1 μm or more and 7 μm or less, the ratio D50 / DSEM of the 50% particle size D50 to the average particle size based on electron microscopy observation in the cumulative particle size distribution based on volume is 1 or more and 4 or less, and the ratio D90 / D10 of the 90% particle size D90 to the 10% particle size D10 in the cumulative particle size distribution based on volume is 4 or less, and the lithium transition metal composite oxide contains nickel in its composition and has a layered structure, and is disclosed as a positive electrode active material for a non-aqueous electrolyte secondary battery.

[0009] Japanese Patent Publication No. 2001-243949, Japanese Patent Publication No. 2004-355824, Japanese Patent Publication No. 2023-036062, Japanese Patent Publication No. 2017-188445

[0010] Patent documents 3 and 4 described above describe improving cycle characteristics by controlling the number, particle size, and particle size distribution of primary particles constituting a lithium metal composite compound. However, there is a need for new methods to improve cycle characteristics in lithium-ion secondary batteries using lithium nickel composite oxide as the positive electrode.

[0011] In view of the problems of the above-mentioned prior art, one aspect of the present invention aims to provide a positive electrode active material for lithium-ion secondary batteries that, when used as the positive electrode of a lithium-ion secondary battery, exhibits excellent charge-discharge efficiency, high capacity, and excellent cycle characteristics.

[0012] To solve the above problems, according to one aspect of the present invention, a lithium nickel composite oxide having a layered structure of space group R-3m is provided, wherein, in addition to oxygen, the lithium nickel composite oxide contains lithium (Li), nickel (Ni), and element M (M) in a molar ratio of Li:Ni:M = a:b:c (where 0.90 ≤ a < 1.0, 0.5 ≤ b ≤ 1.0, 0 ≤ c ≤ 0.5, b + c = 1, and element M is at least one selected from the group consisting of Co, Mn, Al, V, Mg, Mo, Ca, Cr, Zr, Ti, Nb, Na, K, W, Fe, Zn, B, Si, P, and Ta), and the particles of the lithium nickel composite oxide consist of single primary particles or secondary particles formed by the aggregation of single primary particles and multiple primary particles, wherein the number ratio of single primary particles is 30% or more of the total number of particles. The present invention provides a positive electrode active material for lithium-ion secondary batteries, wherein the amount of lithium that dissolves from within the lithium nickel composite oxide particles when immersed in water is 32% or less of the total amount of lithium that dissolves in water.

[0013] According to one aspect of the present invention, it is possible to provide a positive electrode active material for lithium-ion secondary batteries that, when used as the positive electrode of a lithium-ion secondary battery, exhibits excellent charge-discharge efficiency, high capacity, and superior cycle characteristics.

[0014] Figure 1 is a schematic diagram of the coin-type battery used for battery evaluation. Figure 2 is a schematic diagram of the laminate-type battery used for battery evaluation.

[0015] The embodiments for carrying out the present invention will be described in detail below. However, the present invention is not limited to the embodiments described below and can be modified as appropriate without changing the gist of the invention. Also, in the following description, "A to B" means "A or more and B or less". [1] Positive electrode active material for lithium-ion secondary battery The configuration of the positive electrode active material according to this embodiment will be described in detail below.

[0016] The positive electrode active material for a lithium-ion secondary battery according to this embodiment (hereinafter sometimes simply referred to as "positive electrode active material") includes a lithium nickel composite oxide having a layered structure of space group R-3m. Furthermore, the lithium nickel composite oxide particles contained in the positive electrode active material of this embodiment may consist of single primary particles, or secondary particles formed by the aggregation of single primary particles and multiple primary particles (hereinafter sometimes simply referred to as "secondary particles").

[0017] (1) Particle morphology The lithium nickel composite oxide (hereinafter sometimes simply referred to as "lithium composite oxide") contained in the positive electrode active material according to this embodiment may consist only of single primary particles (hereinafter sometimes simply referred to as "single particles"), or it may consist of both single primary particles and secondary particles.

[0018] The lithium composite oxide particles may have a single particle ratio of 30% or more, preferably 50% or more, and more preferably 70% or more, relative to the total number of particles. The lithium composite oxide particles may also have a single particle ratio of 100%. In other words, the lithium composite oxide particles may consist only of single particles. Note that the total number of lithium composite oxide particles refers to both the single particles and secondary particles contained within the lithium composite oxide particles. When the lithium composite oxide particles contain only single particles, the total number of lithium composite oxide particles refers to the single particles.

[0019] Since individual particles are less likely to break due to expansion and contraction during charging and discharging, increasing the ratio of individual particles in lithium composite oxide particles can particularly improve the cycle characteristics of lithium-ion secondary batteries (hereinafter also simply referred to as "secondary batteries"). Furthermore, by increasing the ratio of individual particles in lithium composite oxide particles, the particle surface in contact with the electrolyte can be made wider than in a form where primary particles are aggregated as secondary particles, and an improvement in the output characteristics of the secondary battery can also be expected. By setting the ratio of individual particles contained in lithium composite oxide particles within the above range, the cycle characteristics of the secondary battery are improved, and the surface area in contact with the electrolyte of the lithium composite oxide particles increases, reducing the reaction resistance of the secondary battery and improving its output characteristics.

[0020] The determination of whether lithium composite oxide particles consist of single particles or secondary particles can be performed by scanning electron microscope (SEM) images of the particles (hereinafter sometimes referred to as "SEM images"). Primary particles are particles that do not have grain boundaries within the particle in SEM observation. Single particles are primary particles that exist individually without agglomerating with other primary particles.

[0021] On the other hand, secondary particles are particles composed of aggregated primary particles, and can be identified as secondary particles if they have grain boundaries within them in an SEM image. In an SEM image, the number of primary particles constituting a secondary particle can be measured by counting the number of primary particles separated by grain boundaries within the secondary particle.

[0022] When determining the number ratio of single particles in lithium composite oxide particles, it is possible to select 50 to 200 lithium composite oxide particles from the SEM image and calculate the number ratio (number proportion) of single particles among the selected lithium composite oxide particles.

[0023] Secondary particles are composed of aggregated primary particles, but it is preferable that the number of primary particles constituting the secondary particles be small. Specifically, it is preferable that the number of primary particles constituting the secondary particles be 20 or less, more preferably 15 or less, and even more preferably 10 or less. There are no particular limitations on how to determine the number of primary particles constituting the secondary particles. For example, 50 to 200 lithium composite oxide particles are selected from an SEM image, and single particles are excluded from the selected lithium composite oxide particles, and the particles that constitute secondary particles are further selected. Then, the number of primary particles contained in the selected secondary particles is counted. Next, the average value (arithmetic mean) of the number of primary particles contained in the evaluated secondary particles can be taken as the number of primary particles constituting the secondary particles.

[0024] However, 10% of the secondary particles that make up the secondary particles can be excluded as outliers, and the number of primary particles that make up the secondary particles can be calculated.

[0025] By keeping the number of primary particles constituting secondary particles within the above range, the number of interfaces between primary particles within the secondary particles can be reduced, thereby suppressing cracking of secondary particles due to expansion and contraction during charging and discharging, and further improving the cycle characteristics of the secondary battery.

[0026] (2) The composition of the lithium composite oxide may contain, in addition to oxygen, lithium (Li), nickel (Ni), and element M (M) as elements in a molar ratio of Li:Ni:M = a:b:c.

[0027] However, it is preferable that a, b, and c in the above formula satisfy 0.90 ≤ a < 1.0, 0.5 ≤ b ≤ 1.0, 0 ≤ c ≤ 0.5, and b + c = 1.

[0028] The element M can be at least one selected from the group consisting of Co (cobalt), Mn (manganese), Al (aluminum), V (vanadium), Mg (magnesium), Mo (molybdenum), Ca (calcium), Cr (chromium), Zr (zirconium), Ti (titanium), Nb (niobium), Na (sodium), K (potassium), W (tungsten), Fe (iron), Zn (zinc), B (boron), Si (silicon), P (phosphorus), and Ta (tantalum).

[0029] The lithium composite oxide is, for example, represented by the general formula Li ａ Ni ｂ M ｃ O ２＋α In the above general formula, α can satisfy -0.2 ≦ α ≦ 0.2. Since a, b, c and the element M have been described above, their explanations are omitted here.

[0030] The positive electrode active material of the present embodiment includes the above-mentioned lithium composite oxide. The positive electrode active material of the present embodiment can also be composed of the above-mentioned lithium composite oxide, but even in this case, this does not exclude that it contains unavoidable impurities that are unintentionally mixed in during the manufacturing process or the like.

[0031] In the following description, lithium is denoted as Li, and elements other than lithium and oxygen contained in the lithium composite oxide, that is, nickel and the element M, may be collectively referred to as the element Me in some cases.

[0032] In the above molar ratio, the value a, which represents the molar ratio of Li, corresponds to the molar ratio of Li to the element Me (Li / Me ratio). In the above molar ratio, the range of a can satisfy 0.90 ≦ a < 1.0. When the range of a falls within the above range, the reaction resistance of the positive electrode decreases, and the output characteristics and charge-discharge efficiency of the secondary battery are improved. When the value of a does not fall within the above range, the reaction resistance increases, and the output characteristics and charge-discharge efficiency of the battery may decrease. Further, the range of a may satisfy 0.92 ≦ a < 0.98.

[0033] In the above ratio of amounts of substance, the range of c, which represents the ratio of the amount of substance of element M, is 0 ≤ c ≤ 0.5, preferably 0.02 ≤ c ≤ 0.40, and more preferably 0.10 ≤ c ≤ 0.30. The type of element M can be appropriately selected according to the required battery characteristics.

[0034] For example, element M may contain at least Co. If c1 is the molar ratio of Co contained in the above molar ratio c, the range of c1 is preferably 0 ≤ c1 ≤ 0.30, more preferably 0.02 ≤ c1 ≤ 0.25, and even more preferably 0.05 ≤ c1 ≤ 0.20. When the value of c1 is within the above range, high thermal stability and power characteristics can be obtained.

[0035] Furthermore, element M may contain at least Mn. When c2 is the molar ratio of Mn contained in c of the above molar ratio, the range of c2 is preferably 0 ≤ c2 ≤ 0.30, more preferably 0.02 ≤ c2 ≤ 0.25, and even more preferably 0.05 ≤ c2 ≤ 0.20. When the range of c2 is within the above range, thermal stability can be improved.

[0036] Furthermore, element M may contain at least Al. When c3 is the molar ratio of Al contained in c in the above molar ratio, the range of c3 is preferably 0 ≤ c3 ≤ 0.10, more preferably 0.01 ≤ c3 ≤ 0.08, and even more preferably 0.01 ≤ c3 ≤ 0.06. When the range of c3 is within the above range, thermal stability can be improved.

[0037] The composition of lithium composite oxides can be measured quantitatively by inductively coupled plasma (ICP) emission spectrometry.

[0038] (3) Ratio of dissolved lithium to total lithium When the lithium composite oxide is immersed in water, the amount of lithium dissolved from within the lithium nickel composite oxide particles (hereinafter also simply referred to as "dissolved lithium") may be 32% or less of the total amount of lithium dissolved in water (hereinafter also simply referred to as "total lithium"). The ratio of dissolved lithium to total lithium may be 31% or less.

[0039] The amount of eluted lithium, the total amount of lithium, and the ratio of the amount of eluted lithium to the total amount of lithium can be determined by the following evaluation method.

[0040] To evaluate the total amount of lithium, the lithium complex oxide is immersed in water to a concentration of 0.2 g / mL, stirred for 5 minutes, filtered, and the resulting filtrate is titrated with an aqueous HCl solution. The titration is evaluated using the Warder method. The titration determines the amount of lithium hydroxide (LiOH) and lithium carbonate (LiOH) in the filtrate. ２ CO ３ The amount of lithium can be calculated, and the sum of the amounts of lithium contained in the calculated lithium hydroxide and lithium carbonate can be used to determine the total amount of lithium.

[0041] To evaluate the amount of eluted lithium, the lithium composite oxide is held in carbon dioxide at 30°C for 12 hours before titration to carbonize any excess lithium hydroxide remaining on the surface of the lithium composite oxide. Then, using the lithium composite oxide after carbonation, the amount of lithium hydroxide (LiOH) in the filtrate is calculated by titration under the same procedure and conditions as for evaluating the total amount of lithium described above. From the calculated amount of lithium hydroxide, the amount of lithium contained in the lithium hydroxide can be further calculated and determined as the amount of eluted lithium.

[0042] Using the total amount of lithium and the amount of dissolved lithium calculated using the above procedure, calculate the ratio (%) of the amount of dissolved lithium to the total amount of lithium.

[0043] When a secondary battery is repeatedly charged and discharged, lithium is repeatedly inserted into and removed from the interlayers of the lithium composite oxide. However, during repeated charging and discharging of a secondary battery, the interlayers expand and contract due to the insertion and removal of lithium. As a result, the crystal structure of conventional lithium composite oxides changes, and lithium that does not contribute to charging and discharging is produced. Therefore, it is thought that the cycle performance deteriorates.

[0044] The lithium composite oxide contained in the positive electrode active material of this embodiment has a ratio of eluted lithium amount to total lithium amount of 32% or less. Therefore, the crystallinity of the lithium composite oxide is high, and it is thought that the crystallinity of the lithium composite oxide can be stabilized even when the secondary battery is repeatedly charged and discharged. Accordingly, even when the secondary battery is repeatedly charged and discharged, the change in the crystal structure of the lithium composite oxide is reduced, and the generation of lithium that does not contribute to charging and discharging is reduced. Therefore, in the positive electrode active material of this embodiment, by reducing the ratio of eluted lithium amount to total lithium amount, it is possible to obtain excellent charge and discharge efficiency, high capacity, and high cycle characteristics when used in a secondary battery. Furthermore, by setting the ratio of eluted lithium amount to total lithium amount to 31% or less, it is possible to further increase crystallinity and further improve cycle characteristics.

[0045] (4) Diffraction Peak Intensity Ratio The positive electrode active material of this embodiment may have a diffraction intensity ratio I(003) / I(104) of 1.8 or more, which is the ratio of the diffraction peak intensity I(003) of the (003) plane to the diffraction peak intensity I(104) of the (104) plane obtained from the X-ray diffraction pattern. The X-ray diffraction pattern used to calculate the above intensity ratio can be measured using Cu-kα rays as the X-ray source and a flat plate sample holder of a Bragg-Brentano optical system.

[0046] The lithium composite oxide contained in the positive electrode active material of this embodiment may have a hexagonal layered structure. Furthermore, the lithium composite oxide may have diffraction peaks attributed to the (003) plane or the (104) plane in the X-ray diffraction pattern measured by powder X-ray diffraction (hereinafter sometimes referred to as "XRD").

[0047] In this embodiment, the positive electrode active material preferably has a diffraction intensity ratio I(003) / I(104) of 1.8 or more, as described above. I(003) and I(104) represent the diffraction peak intensities corresponding to the heights of the diffraction peaks attributed to the (003) plane and the (104) plane in the XRD pattern, respectively.

[0048] Diffraction peak intensity is affected not only by crystal growth perpendicular to the target surface but also by the orientation of the sample packed into the sample holder. In particular, flat plate sample holders are considered to be susceptible to the influence of the selective orientation of the packed sample. Here, there is a positive correlation between the packing efficiency of lithium composite oxide particles into the flat plate sample holder and the packing efficiency of the electrode (positive electrode) formed by coating it onto a planar current collector. Therefore, it is thought that improving the packing efficiency of the sample in the flat plate sample holder can improve the packing efficiency of the positive electrode active material in the positive electrode.

[0049] The positive electrode active material according to this embodiment contains the lithium composite oxide described above, and the lithium composite oxide is composed of single particles and secondary particles formed by the aggregation of primary particles, with a high number ratio of single particles of 30% or more. As a result, the packing efficiency is increased and the selective orientation effect is strengthened. Therefore, in the case of a positive electrode active material containing lithium composite oxide, the diffraction intensity ratio I(003) / I(104) value measured using a flat plate sample holder has an aspect as an indicator of the packing efficiency of the positive electrode active material, including during positive electrode formation. That is, a positive electrode active material with high packing efficiency exhibits a high diffraction intensity ratio I(003) / I(104) value. In other words, a positive electrode active material with a high number ratio of single particles and a high diffraction intensity ratio I(003) / I(104) value has high packing efficiency and can obtain a high energy density when used in a secondary battery.

[0050] The positive electrode active material of this embodiment has a diffraction intensity ratio I(003) / I(104) of 1.8 or higher, and exhibits high packing properties. Therefore, when used as the positive electrode active material of this embodiment in a secondary battery, it can achieve high capacity and high energy density.

[0051] The upper limit of the diffraction intensity ratio I(003) / I(104) is not particularly limited, but by setting it to 3.0 or less, for example, excessive crystallization and aggregation during firing can be prevented. This reduces damage to the particle surface during crushing. Therefore, the diffraction intensity ratio I(003) / I(104) may be 3.0 or less.

[0052] Therefore, the diffraction intensity ratio I(003) / I(104) can preferably be 1.8 or more and 3.0 or less, more preferably 2.0 or more and 3.0 or less, and even more preferably 2.1 or more and 3.0 or less. By setting the diffraction intensity ratio I(003) / I(104) within the above range, damage to the particle surface can be reduced while obtaining higher capacity and higher energy density when used in a secondary battery.

[0053] (5) Particle size characteristics (5-1) Median diameter The positive electrode active material according to this embodiment can preferably have a volume-based median diameter (D50) of 0.5 μm or more and 20 μm or less, more preferably 0.8 μm or more and 18 μm or less, and even more preferably 1.0 μm or more and 8.0 μm or less.

[0054] By setting the median diameter of the positive electrode active material in this embodiment to 20 μm or less, the contact area with the electrolyte can be sufficiently increased when applied to a secondary battery, thereby increasing the battery capacity. Furthermore, by setting the median diameter of the positive electrode active material in this embodiment to 0.5 μm or more, handling during electrode fabrication can be improved.

[0055] The median diameter (D50) can be determined, for example, from the volume integrated value of the particle size distribution measured by a laser diffraction scattering particle size distribution analyzer, and is the particle size at which the integrated value from the smallest particle size side reaches 50%.

[0056] (5-2) Average diameter of primary particles The average diameter of primary particles contained in the lithium composite oxide particles (hereinafter sometimes simply referred to as "average primary particle diameter") is preferably 0.3 μm or more, more preferably 1.0 μm or more. The upper limit of the average primary particle diameter is preferably 6.0 μm or less, more preferably 5.0 μm or less.

[0057] In other words, the average primary particle diameter of the lithium composite oxide particles can preferably be 0.3 μm or more and 6.0 μm or less, and more preferably 1.0 μm or more and 5.0 μm or less.

[0058] By setting the average primary particle diameter within the above range, a positive electrode active material can be obtained that has sufficiently large primary particles and has improved crystallinity of the lithium composite oxide, thereby reducing grain boundaries. Therefore, when the positive electrode active material of this embodiment, which contains the lithium nickel composite oxide, is applied to a secondary battery, the charge-discharge efficiency and battery capacity can be increased, resulting in a secondary battery with excellent battery capacity and cycle characteristics.

[0059] The average primary particle diameter is determined by selecting primary particles of lithium composite oxide that can be observed entirely using a scanning electron microscope (SEM), measuring their major axis lengths, and averaging the results. While there are no particular limitations on the number of primary particles to be evaluated when calculating the average primary particle diameter, for example, 20 or more primary particles can be selected for evaluation.

[0060] For example, if the lithium composite oxide contains secondary particles, it is preferable to select 20 to 30 primary particles from each of the 10 to 20 secondary particles, and then measure the major axis length of the selected primary particles.

[0061] Furthermore, if the lithium composite oxide contains individual particles that do not constitute secondary particles, it is preferable to evaluate 20 or more of these individual particles. There is no particular upper limit to the number of primary particles to be evaluated, but it is preferable to evaluate 30 or fewer, for example.

[0062] If the lithium composite oxide contains only secondary particles or only single particles, the above number of primary particles can be selected for the secondary particles or single particles. Then, the length of the major axis of the selected primary particles can be measured, and the average value can be taken as the average primary particle diameter of the lithium composite oxide.

[0063] When the lithium composite oxide contains both secondary particles and individual particles, the above number of primary particles can be selected for each of the secondary particles and individual particles. The length of the long axis of all selected primary particles can then be measured, and the average value can be taken as the average primary particle diameter of the lithium composite oxide. [2] Method for manufacturing a positive electrode active material for lithium-ion secondary batteries The method for manufacturing a positive electrode active material for lithium-ion secondary batteries according to this embodiment is not particularly limited as long as a positive electrode active material having the above-described characteristics can be obtained. An example of a method for manufacturing a positive electrode active material according to this embodiment will be described below.

[0064] The method for producing the positive electrode active material according to this embodiment (hereinafter sometimes simply referred to as "production method") may include, for example, a mixing step, a calcination step, a pulverization step, a washing step, and a drying step.

[0065] In the mixing process, a nickel composite compound and a lithium compound are mixed to obtain a mixture.

[0066] In the firing process, the mixture obtained in the mixing process is fired to produce a fired product.

[0067] In the grinding process, the calcined material obtained in the calcination process is ground to produce a pulverized product.

[0068] In the washing process, the pulverized material obtained in the grinding process can be washed with water.

[0069] In the drying process, the pulverized material after washing is dried to obtain lithium composite oxide.

[0070] (1) Mixing Process In the mixing process, the nickel composite compound and the lithium compound are mixed to obtain a mixture. The nickel composite compound, the lithium compound, and a compound containing element M as needed (hereinafter sometimes referred to as "M compound") can be added and mixed, for example, in powder (solid phase). The mixing process will be described below.

[0071] (Nickel composite compound) The nickel composite compound used in the mixing step (S10) can be obtained by known methods. Since the composition of Ni and element M in the nickel composite compound is almost maintained in the resulting lithium composite oxide, the content of Ni and element M can be within the same range as the content in the lithium composite oxide described above.

[0072] The nickel composite compound may be a hydroxide or an oxide. Furthermore, the nickel composite compound may be a mixture of hydroxide and oxide. A method for producing nickel composite hydroxides includes, for example, a neutralization crystallization method using an aqueous solution of a metal salt and an alkaline solution. Alternatively, the nickel composite hydroxide may be heat-treated to remove water, resulting in a nickel composite oxide being produced as part or all of the nickel composite hydroxide.

[0073] (Lithium Compound) The lithium compound is not particularly limited, and any known compound containing lithium can be used. For example, one or more selected from lithium carbonate, lithium hydroxide, and lithium nitrate can be used. The lithium compound may also be a mixture of multiple lithium compounds, such as lithium carbonate and lithium hydroxide. Among the above lithium compounds, one or more selected from lithium carbonate and lithium hydroxide are preferred from the viewpoint of having less influence from residual impurities and dissolving at the calcination temperature. Furthermore, from the viewpoint of obtaining a lithium nickel composite oxide with high crystallinity, lithium hydroxide is more preferable as the lithium compound.

[0074] (M compound) Element M can be added in a form contained in the nickel composite compound, or it can be added by mixing the M compound into the mixture. The M compound is not particularly limited, and for example, oxides, hydroxides, carbonates, chlorides, sulfates, nitrates, etc. containing element M can be used.

[0075] (Mixing method) The method of mixing the nickel composite compound, the lithium compound, and the compound containing element M, which may be added as needed, is not particularly limited, and it is sufficient that they are mixed sufficiently so as not to destroy the individual particles.

[0076] The mixing device can be a general-purpose mixer, such as a shaker mixer, Redigge mixer, Julia mixer, or V-blender. Thorough mixing reduces variations in the ratio of lithium to elemental Me (Li / Me) between individual particles of the positive electrode active material, thereby improving battery characteristics.

[0077] The lithium compound may be mixed such that the Li / Me ratio in the mixture is 0.90 or higher and less than 1.0, similar to the Li / Me ratio of the lithium composite oxide produced. Since the Li / Me ratio and the ratio of the amount of each element remain almost unchanged before and after the subsequent calcination step, the Li / Me ratio of the mixture in the mixing step becomes the Li / Me ratio of the calcined product (secondary calcined product) obtained in the calcination step.

[0078] On the other hand, the manufacturing method of this embodiment includes a water washing step, which will be described later. The water washing step removes impurities such as unreacted excess lithium present on the particle surface in the calcined product, but a small amount of lithium in the crystal near the particle surface may be extracted. Therefore, in the mixing step, the amount of excess lithium and other impurities removed in the water washing step may be taken into consideration, and the Li / Me ratio in the mixture may be adjusted to be higher than the Li / Me ratio of the lithium composite oxide described above.

[0079] The amount of adjustment for Li / Me can be easily confirmed by small-scale preliminary tests, and the Li / Me of the manufactured lithium composite oxide can be set to the above range by the amount of adjustment confirmed by the preliminary tests. The amount of adjustment for Li / Me increases with excessive washing, but when washing with water under normal conditions, the amount of adjustment can preferably be in the range of 0.01 to 0.06, and more preferably in the range of 0.02 to 0.05. The amount of adjustment for Li / Me is a value added to the Li / Me of the target lithium composite oxide, and the value obtained by adding the amount of adjustment for Li / Me can be used as the Li / Me of the mixture.

[0080] By setting the upper limit of Li / Me in the mixture to less than 1.0, the ratio of eluted lithium to the total amount of lithium can be easily brought within the desired range.

[0081] (2) Firing process In the firing process, the mixture obtained in the mixing process is fired to obtain a fired product.

[0082] The firing process is carried out in two stages. In the first stage of firing (hereinafter sometimes referred to as "first firing"), lithium from the lithium compound is sufficiently diffused into the nickel composite compound to form a fired product containing the reaction product.

[0083] In the second stage of calcination following the first calcination (hereinafter sometimes referred to as "second calcination"), the reaction products can be grown into crystals, increasing their crystallinity and reducing the amount of lithium leached out.

[0084] The atmosphere during firing (hereinafter sometimes referred to as the "firing atmosphere") can be any atmosphere capable of forming reaction products, and can be an oxidizing atmosphere. For example, a mixed gas atmosphere of oxygen gas with an oxygen concentration of 80% by volume or more, preferably 85% by volume or more, and an inert gas, such as nitrogen gas, can be used.

[0085] In the first firing, the holding temperature during firing (hereinafter sometimes referred to as "firing temperature") can preferably be set to 500°C or higher and 750°C or lower, more preferably 550°C or higher and 700°C or lower. By setting the firing temperature within the above range, the nickel composite compound and the lithium in the lithium compound can react sufficiently, forming a reaction product with reduced unreacted material.

[0086] In the first firing, the time for holding at the firing temperature (hereinafter sometimes referred to as "firing time") can preferably be 8 hours or more and 48 hours or less, and more preferably 12 hours or more and 24 hours or less. By setting the firing time within the above range, Li can be sufficiently diffused before the crystal growth of the lithium composite oxide progresses, and even in a state where Li / Me is less than 1 and Li is deficient, a lithium composite oxide with high crystallinity can be obtained after the second firing stage.

[0087] Furthermore, in the second firing, the firing temperature can preferably be set to 700°C to 1100°C, more preferably to 750°C to 1050°C. By setting the firing temperature in the second firing within the above range, the crystal growth of the reaction product can be promoted while reducing the amount of lithium dissolved.

[0088] The firing time in the second firing should be set to allow sufficient crystal growth to proceed. For example, it is preferably 3 hours or more and 24 hours or less, and more preferably 5 hours or more and 18 hours or less.

[0089] The first and second firings may be carried out intermittently, or the second firing may be carried out immediately after the first firing in the same furnace.

[0090] The furnace used in the firing process is not particularly limited, but a batch-type furnace or a continuous furnace that allows for atmosphere control can be used. For example, roller hearth kilns and tunnel furnaces can be preferably used.

[0091] (3) Grinding Process In the grinding process, the calcined material can be ground. In the calcined material obtained in the calcining process, sintering between primary particles is suppressed, but coarse particles may be formed due to weak sintering or aggregation. Therefore, by grinding, the above-mentioned sintering and aggregation can be eliminated, and a ground material can be obtained that contains single particles and secondary particles formed by aggregation of multiple primary particles, preferably 20 or fewer, more preferably 15 or fewer. In addition, the surfaces of the single particles and secondary particles can be exposed, making it easier to remove impurities.

[0092] Furthermore, if necessary, the proportion of individual particles can be increased by crushing after grinding. The crushing method is not particularly limited, but the proportion of individual particles can be easily increased by using grinding equipment such as a jet mill, ball mill, or wet ball mill.

[0093] The crushing described above may be performed immediately after grinding, but it can also be performed after the subsequent washing process. The washing process removes impurities that remain on the particle surface and cause adhesion between particles, making crushing easier and further suppressing damage to the surface of the primary particles. Alternatively, the crushing after the calcination process can be omitted, and only crushing can be performed after the washing process.

[0094] The grinding process can be carried out under selected conditions so that the number ratio of individual particles falls within a desired range. Alternatively, the grinding process may be omitted, and only crushing may be performed.

[0095] (4) Washing Process In the washing process, the pulverized material obtained in the grinding process can be washed with water. By performing the washing process, impurities can be removed from the surface of the particles in the pulverized material. The calcined material obtained in the calcination process has impurities such as unreacted residual lithium remaining on the surface of the primary and secondary particles, so washing with water removes these impurities and reduces their level. In addition, by pre-dissolving lithium, which is easily dissolved from crystals, the crystallinity can be improved, resulting in better capacity and cycle characteristics when used as the positive electrode of a secondary battery.

[0096] During the washing process, lithium from the crystals other than residual lithium may slightly dissolve from the lithium composite oxide. However, by adjusting the Li / Me ratio during the mixing process, the effect of this lithium dissolution can be suppressed.

[0097] Washing can be carried out by mixing the pulverized material with water, preferably pure water, to form a slurry. The washing conditions can be selected, for example, so that the ratio of the amount of eluted lithium to the total amount of lithium is within a desired range. For example, 50 to 200 parts by mass of water can be added to 100 parts by mass of the pulverized material obtained in the pulverization process, mixed to form a slurry, and then stirred for washing.

[0098] By setting the mixing ratio of the pulverized material to water within the above range, impurities can be sufficiently removed, and excessive lithium extraction from the lithium composite oxide can be prevented, thereby further suppressing the decrease in battery capacity and the increase in reaction resistance.

[0099] Other than the mixing ratio mentioned above, the washing conditions are not particularly limited. For example, the washing time, which is the stirring time during washing, can preferably be 1 minute or more and 2 hours or less, and more preferably 5 minutes or more and 50 minutes or less. The temperature of the slurry during washing is preferably 20°C or more and less than 50°C, and more preferably 25°C or more and 45°C or less. By using these conditions, the removal of impurities and the suppression of excessive lithium extraction can be further enhanced.

[0100] Washing with water should be carried out in a decarbonized atmosphere, such as a decarbonized air atmosphere or a nitrogen atmosphere, to suppress the reaction with carbon dioxide in the atmospheric atmosphere and reduce the amount of carbonate in the resulting cathode active material. After washing with water, it is preferable to avoid contact with an atmosphere containing carbon dioxide until drying is complete.

[0101] After washing with water, solid-liquid separation is performed to obtain a precipitate containing lithium complex oxides. The solid-liquid separation method is not particularly limited, and known methods can be used. For example, solid-liquid separation can be performed using a suction filter such as a Nütsch (Buchner funnel), a filter press, a centrifuge, or the like.

[0102] (5) Drying process In the drying process, the pulverized material after washing with water, i.e., the sediment described above, is dried to obtain lithium composite oxide.

[0103] The drying conditions are not particularly limited, but for example, the washed material can be dried by heating it at a temperature of 100°C to 250°C in an oxidizing atmosphere or a vacuum atmosphere. This drying method allows for sufficient evaporation of water from the sediment while suppressing the degradation of the lithium nickel composite oxide.

[0104] Furthermore, the drying atmosphere is preferably one that does not contain water vapor or carbon dioxide in order to avoid the reaction between moisture or carbon dioxide in the atmosphere and the lithium composite oxide. Specifically, the drying atmosphere may be, for example, an oxygen atmosphere with a carbon-containing compound component content of 0.01% by volume or less, a mixed gas of oxygen gas and an inert gas, an oxidizing atmosphere such as a decarbonized atmosphere, or a vacuum atmosphere.

[0105] The drying time is not particularly limited, but it can be set to 0.5 hours or more at the maximum temperature reached during drying in order to sufficiently evaporate the moisture from the sediment. Furthermore, from the standpoint of productivity, the upper limit of the drying time can be set to 48 hours or less at the maximum temperature reached during drying.

[0106] The drying apparatus is not particularly limited, but from the viewpoint of quickly discharging the water vapor generated during drying and shortening the drying time, a drying apparatus with an exhaust mechanism can be used.

[0107] After evaporating the water from the sediment, the mixture can be heated to a temperature of 200°C to 300°C in an oxidizing atmosphere from which water vapor and carbon dioxide have been removed, for example, in the aforementioned oxidizing atmosphere.

[0108] The above heating process can repair damage to the crystals near the surface caused by excessive lithium extraction, as described above, thereby improving the battery characteristics when used as the positive electrode of a secondary battery.

[0109] The above heating may be carried out continuously after the water has been evaporated using the same apparatus, or it may be carried out separately using a heating apparatus capable of controlling the atmosphere. [3] Lithium-ion secondary battery The lithium-ion secondary battery of this embodiment may have a positive electrode containing a positive electrode active material according to one aspect of the present disclosure.

[0110] The lithium-ion secondary battery according to this embodiment may comprise at least a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode may include the positive electrode active material described above. The secondary battery according to this embodiment may comprise, for example, a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, or it may comprise a positive electrode, a negative electrode, and a solid electrolyte. Furthermore, the secondary battery may be composed of components similar to those of known lithium-ion secondary batteries.

[0111] Specifically, the secondary battery of this embodiment has a structure comprising a case, a positive electrode, a negative electrode, a non-aqueous electrolyte housed within the case, and a separator as needed. More specifically, for example, when using a non-aqueous electrolyte, the positive electrode and negative electrode are stacked via a separator to form an electrode body, and the resulting electrode body can be impregnated with the non-aqueous electrolyte. The secondary battery of this embodiment is then formed by connecting the positive electrode current collector of the positive electrode to the positive electrode terminal that is open to the outside, and the negative electrode current collector of the negative electrode to the negative electrode terminal that is open to the outside, using current collector leads or the like, and sealing the electrode body in the case. The shape of the lithium-ion secondary battery of this embodiment can be various, such as cylindrical or stacked.

[0112] The embodiments described below are merely illustrative, and secondary batteries can be implemented in various forms based on the embodiments described herein, with modifications and improvements made based on the knowledge of those skilled in the art. Furthermore, the secondary battery according to this embodiment does not particularly limit its applications.

[0113] Below, an example of the configuration of a secondary battery according to this embodiment will be described for each component.

[0114] (1) Positive Electrode First, the positive electrode, which is a characteristic feature of the secondary battery of this embodiment, will be described. The positive electrode is a sheet-like material, and is formed, for example, by applying and drying a positive electrode composite paste containing the positive electrode active material described above as the positive electrode active material to the surface of a current collector made of aluminum foil.

[0115] The positive electrode is processed appropriately according to the battery being used. For example, it may be cut to an appropriate size depending on the intended secondary battery, or compressed using a roll press or similar method to increase electrode density.

[0116] The positive electrode composite paste is formed by adding a solvent to the positive electrode composite and kneading it. The positive electrode composite is formed by mixing the powdered positive electrode active material described above with a conductive material and a binder.

[0117] Conductive materials are added to electrodes to provide them with appropriate conductivity. While not particularly limited, conductive materials such as graphite (natural graphite, artificial graphite, and expanded graphite, etc.) or carbon black-based materials such as acetylene black and Ketjenblack® can be used.

[0118] The binder plays the role of holding the positive electrode active material particles together. The binder used in this positive electrode composite is not particularly limited, but examples of binders that can be used include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose resins, and polyacrylic acid.

[0119] Furthermore, activated carbon may be added to the positive electrode composite material, and by adding activated carbon, the electrical double layer capacity of the positive electrode can be increased.

[0120] The solvent dissolves the binder, dispersing the positive electrode active material, conductive material, activated carbon, etc., within the binder. This solvent is not particularly limited, but for example, an organic solvent such as N-methyl-2-pyrrolidone can be used.

[0121] Furthermore, the mixing ratio of each substance in the positive electrode composite paste is not particularly limited. For example, if the solid content of the positive electrode composite excluding the solvent is 100 parts by mass, the content of the positive electrode active material may be 60 parts by mass or more and 95 parts by mass or less, the content of the conductive material may be 1 part by mass or more and 20 parts by mass or less, and the content of the binder may be 1 part by mass or more and 20 parts by mass or less, similar to the positive electrode of a general lithium-ion secondary battery.

[0122] (2) Negative electrode The negative electrode is a sheet-like component formed by applying a negative electrode composite paste to the surface of a metal foil current collector such as copper and drying it. Although the components and formulation of the negative electrode composite paste and the material of the current collector differ, this negative electrode is formed in substantially the same manner as the positive electrode, and various treatments are performed as necessary, just as with the positive electrode.

[0123] The negative electrode composite paste is made by mixing the negative electrode active material and binder, and then adding a suitable solvent to form a paste.

[0124] The negative electrode active material can be, for example, a lithium-containing material such as metallic lithium or lithium alloy, or a storage material capable of intercalating and deintercalating lithium ions.

[0125] The absorbed material is not particularly limited, but for example, natural graphite, artificial graphite, calcined organic compounds such as phenolic resins, and powdered carbon materials such as coke can be used. When such an absorbed material is used as the negative electrode active material, a fluororesin such as PVDF can be used as a binder, similar to the positive electrode, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent for dispersing the negative electrode active material in the binder.

[0126] (3) Separator A separator is placed between the positive electrode and the negative electrode when a non-aqueous electrolyte is used, and has the function of separating the positive electrode and the negative electrode and holding the electrolyte. As a separator, for example, a thin membrane made of polyethylene or polypropylene with many fine pores can be used, but it is not particularly limited as long as it has the above function.

[0127] (4) Non-aqueous electrolytes As a non-aqueous electrolyte, for example, a non-aqueous electrolyte solution can be used.

[0128] As a non-aqueous electrolyte, for example, a lithium salt dissolved in an organic solvent can be used as a supporting salt. Alternatively, a lithium salt dissolved in an ionic liquid may be used as a non-aqueous electrolyte. An ionic liquid is a salt composed of cations and anions other than lithium ions, and is liquid at room temperature.

[0129] As the organic solvent, one of the following may be used alone or in combination: cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; linear carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethyl methyl sulfone and butanesultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate.

[0130] LiPF is used as a supporting salt. ６ LiBF ４ LiClO ４ LiAsF ６ ,LiN(CF ３ SO ２ ) ２ These, and their combined salts, can be used. Furthermore, the non-aqueous electrolyte may contain radical scavengers, surfactants, and flame retardants.

[0131] Furthermore, solid electrolytes may be used as non-aqueous electrolytes. Solid electrolytes have the property of being able to withstand high voltages. Examples of solid electrolytes include inorganic solid electrolytes and organic solid electrolytes.

[0132] Examples of inorganic solid electrolytes include oxide-based solid electrolytes and sulfide-based solid electrolytes.

[0133] The oxide-based solid electrolyte is not particularly limited, and for example, one containing oxygen (O) and having lithium ion conductivity and electronic insulation properties can be suitably used. For example, lithium phosphate (Li ３ PO ４ ), Li ３ PO ４ N Ｘ LiBO ２ N Ｘ LiNbo ３ , LiTaO ３ Li ２ SiO ３ Li ４SiO ４ -Li ３ PO ４ Li ４ SiO ４ -Li ３ VO ４ Li ２ O-B ２ O ３ -P ２ O ５ Li ２ O-SiO ２ Li ２ O-B ２ O ３ -ZnO, Li １＋Ｘ Al Ｘ Ti ２－Ｘ (PO ４ ) ３ (0≦X≦1), Li １＋Ｘ Al Ｘ Ge ２－Ｘ (PO ４ ) ３ (0≦X≦1), LiTi ２ (PO ４ ) ３ Li ３Ｘ La ２／３－Ｘ TiO ３ (0≦X≦2 / 3), Li ５ La ３ Ta ２ O １２ Li ７ La ３ Zr ２ O １２ Li ６ BaLa ２ Ta ２ O １２ Li ３．６ Si ０．６ P ０．４ O ４ You may use one or more types selected from the above.

[0134] The sulfide-based solid electrolyte is not particularly limited, and for example, one containing sulfur (S) and having lithium ion conductivity and electronic insulation properties can be suitably used. For example, Li ２ S-P ２ S ５ Li ２ S-SiS ２, LiI-Li ２ S-SiS ２ , LiI-Li ２ S-P ２ S ５ , LiI-Li ２ S-B ２ S ３ , Li ３ PO ４ -Li ２ S-Si ２ S, Li ３ PO ４ -Li ２ S-SiS ２ , LiPO ４ -Li ２ S-SiS, LiI-Li ２ S-P ２ O ５ , LiI-Li ３ PO ４ -P ２ S ５ One or more selected from the above can be used.

[0135] Note that inorganic solid electrolytes other than those described above may be used, for example, Li ３ N, LiI, Li ３ N-LiI-LiOH or the like may also be used.

[0136] The organic solid electrolyte is not particularly limited as long as it is a polymer compound exhibiting ionic conductivity, and for example, polyethylene oxide, polypropylene oxide, copolymers thereof and the like can be used. Further, the organic solid electrolyte may contain a supporting salt (lithium salt). (Shape and Configuration of Secondary Battery) As described above, the secondary battery of the present embodiment can be formed into various shapes such as a cylindrical shape and a stacked shape. Regardless of which shape is adopted, when the secondary battery of the present embodiment uses a non-aqueous electrolyte solution as the non-aqueous electrolyte, the positive electrode and the negative electrode are laminated with a separator interposed therebetween to form an electrode body. The obtained electrode body is impregnated with a non-aqueous electrolyte solution, and connections between the positive electrode current collector and the positive electrode terminal leading to the outside, and between the negative electrode current collector and the negative electrode terminal leading to the outside are made using current collecting leads or the like, whereby a structure sealed in a battery case can be obtained.

[0137] Furthermore, the secondary battery according to this embodiment is not limited to a form using a non-aqueous electrolyte solution as the non-aqueous electrolyte; for example, a secondary battery using a solid non-aqueous electrolyte, i.e., an all-solid-state battery, can also be used. In the case of an all-solid-state battery, the components other than the positive electrode active material can be appropriately changed as needed.

[0138] As described above, the secondary battery according to this embodiment uses the composite oxide particles according to this embodiment as the positive electrode material, and therefore has excellent battery capacity and output characteristics. For this reason, the secondary battery according to this embodiment can be suitably used as a rechargeable battery for portable information terminals such as mobile phones, smartphones, tablets, or notebook computers, as well as portable music players, digital cameras, medical devices, and clean energy vehicles. Examples of clean energy vehicles include HEVs (Hybrid Electric Vehicles), EVs (Electric Vehicles), and PHEVs (Plug-in Hybrid Electric Vehicles).

[0139] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way by these examples. [1] Evaluation Methods The methods for analyzing the metals contained in the positive electrode active material and the various evaluation methods for the positive electrode active material in the examples and comparative examples are as follows.

[0140] (1) Analysis of composition: Measured by ICP emission spectrometry.

[0141] The composition of the precursor nickel composite oxide, excluding oxygen, is shown in the "Ni Composite Oxide Composition" column of Table 1. The composition of the obtained cathode active material, excluding oxygen, is shown in the "Active Material Composition" column of Table 2.

[0142] (2) Median diameter (D50) For measuring the median diameter of the positive electrode active material, the particle size distribution was measured on a volume basis using a laser diffraction scattering particle size analyzer (Microtrac HRA, manufactured by Nikkiso Co., Ltd.). The median diameter (D50) of the positive electrode active material was determined from the measured volume-based particle size distribution. For the measurement, a dispersion was used in which 2 g of the sample was ultrasonically stirred in 100 mL of 0.2 mass% sodium hexametaphosphate aqueous solution. The evaluation results are shown in the "D50" column of Table 2.

[0143] (3) Particle morphology The appearance was observed using an SEM (Hitachi High-Technologies Corporation, S-4700) at an acceleration voltage of 5 kV and a magnification of ×5,000. The number ratio of individual primary particles was evaluated from 100 arbitrarily selected lithium composite oxide particles.

[0144] Furthermore, for the lithium composite oxide particles selected above that were identified as secondary particles, the number of constituent primary particles was evaluated. Then, for the secondary particles that were evaluated, excluding those excluded below, the average number of primary particles contained in the secondary particle was calculated. In evaluating the number of primary particles constituting the secondary particle, 10% of the secondary particle count was excluded from the data used to calculate the average, starting with those with the highest number of constituent primary particles.

[0145] (4) Average diameter of primary particles The average diameter of primary particles of lithium composite oxide was determined by selecting primary particles by SEM observation, measuring their major axis lengths, and averaging them.

[0146] For single particles, 20 single particles in which the entire particle could be observed were selected, and the major axis length of the selected primary particles was measured. For secondary particles, 10 secondary particles were selected, and from each secondary particle, a total of 20 primary particles in which the entire particle could be observed and whose major axis length could be measured were selected, and the major axis length of the selected primary particles was measured. The major axis lengths of all primary particles measured for single and secondary particles were averaged numerically, and this average value was taken as the average diameter of the primary particles (average primary particle diameter). The evaluation results are shown in the "Average Primary Particle Diameter" column of Table 2.

[0147] (5) Ratio of dissolved lithium to total lithium The ratio (%) of dissolved lithium to total lithium was calculated using the evaluation method described in "(3) Ratio of dissolved lithium to total lithium" of "Positive electrode active material for lithium-ion secondary batteries" in "[1] Positive electrode active material for lithium-ion secondary batteries". The evaluation results are shown in the "Dissolved Li amount / Total Li amount" column of Table 2.

[0148] (6) Diffraction Peak Intensity Ratio The X-ray diffraction pattern was measured using CuKα rays with a Bragg-Brentano optical system X-ray diffractometer (Panalytical, X'Pert PRO). The sample was packed into the groove of a circular glass holder with an outer diameter of 32 mmφ (flat sample holder, inner diameter 24 mmφ, groove depth 0.5 mm, Panalytical part number 9200 540 12391). The circular glass holder was then fixed to a designated sample holder (Panalytical part number 9430 018 13321) for measurement. The measurement conditions were a tube voltage of 30 kV, a tube current of 15 mA, and a goniometer scanning speed of 2° / min.

[0149] Then, from the measured X-ray diffraction pattern, the diffraction peak intensity I(003) of the (003) plane and the diffraction peak intensity I(104) of the (104) plane of the hexagonal layered structure, corresponding to the height of the diffraction peaks, were determined, and the diffraction intensity ratio I(003) / I(104) was calculated. The evaluation results are shown in the "Peak Intensity Ratio I(003) / I(104)" column of Table 2.

[0150] Furthermore, from the obtained X-ray diffraction patterns, it was confirmed that the lithium composite oxide particles contained in the positive electrode active materials obtained in the following examples and comparative examples all have a hexagonal layered structure with space group R-3m.

[0151] (7) Evaluation of charging capacity, discharging capacity, and Coulomb efficiency (battery characteristics) A coin-type battery was fabricated as a lithium-ion secondary battery using the following method, and the battery characteristics of the positive electrode active material were evaluated.

[0152] (Fabrication of coin-type batteries) As the coin-type battery, a Type 2032 coin battery (hereinafter sometimes referred to as "coin-type battery") CBA, having the structure shown in Figure 1, was used. The coin-type battery CBA is a lithium-ion secondary battery comprising a positive electrode PE, a negative electrode NE, a separator SE1, a gasket GA, a wave washer WW, a positive electrode can PC, and a negative electrode can NC. The positive electrode PE, negative electrode NE, and separator SE1 were impregnated with electrolyte.

[0153] Inside the coin-type battery CBA, the positive electrode PE, separator SE1, negative electrode NE, and wave washer WW are arranged in this order, stacked from the positive electrode can PC towards the negative electrode can NC. The positive electrode PE is in contact with the inner surface of the positive electrode can PC, and the negative electrode NE is in contact with the inner surface of the negative electrode can NC via the wave washer WW.

[0154] The positive electrode can PC and negative electrode can NC, which make up case CA, each have a hollow structure with one end open, and the negative electrode can NC is placed in the opening of the positive electrode can PC. In the coin-type battery CBA, the positive electrode PE, negative electrode NE, separator SE1, gasket GA, and wave washer WW are housed between the positive electrode can PC and the negative electrode can NC by placing the negative electrode can NC in the opening of the positive electrode can PC.

[0155] Furthermore, the gasket GA is positioned between the positive electrode can PC and the negative electrode can NC, and the gasket GA restricts relative movement between the positive electrode can PC and the negative electrode can NC, maintaining a non-contact state, i.e., an electrically insulated state, thereby fixing them in place. The gasket GA also has the function of sealing the gap between the positive electrode can PC and the negative electrode can NC, thereby creating an airtight and liquid-tight barrier between the inside and outside of the coin-type battery CBA.

[0156] The coin-type battery CBA was manufactured as follows.

[0157] First, 52.5 mg of the prepared positive electrode active material, 15 mg of acetylene black, and 7.5 mg of polytetrafluoroethylene (PTFE) were mixed and press-molded at a pressure of 100 MPa to a diameter of 11 mm and a thickness of 100 μm to produce the positive electrode (evaluation electrode) PE shown in Figure 1. Subsequently, the prepared positive electrode PE was dried in a vacuum dryer at 120°C for 12 hours.

[0158] After impregnating the positive electrode PE, negative electrode NE, and separator SE1 with electrolyte, a coin-type battery CBA was fabricated in a glove box with an Ar atmosphere controlled to a dew point of -80°C. The fabricated positive electrode PE, separator SE1, negative electrode NE, and wave washer WW were stacked in this order on the positive electrode can PC. Next, the coin-type battery CBA was assembled by placing the negative electrode can NC over the opening of the positive electrode can PC so that the negative electrode NE contacts the inner surface of the negative electrode can NC via the wave washer WW.

[0159] The negative electrode NE used a negative electrode sheet made of copper foil coated with graphite powder with an average particle size of approximately 20 μm and polyvinylidene fluoride, punched out in a disc shape with a diameter of 14 mm.

[0160] A polyethylene porous membrane with a thickness of 25 μm was used for separator SE1.

[0161] The electrolyte contains 1M LiPF ６ An equivolute mixture (manufactured by Toyama Pharmaceutical Co., Ltd.) was used, in which the mixing ratio of ethylene carbonate (EC) and diethyl carbonate (DEC), with ethylene carbonate as the supporting electrolyte, was 1:1 by volume.

[0162] (Battery Capacity and Efficiency) Battery capacity was evaluated by measuring the charging capacity, discharging capacity, and efficiency. The charging and discharging capacity was measured after leaving the coin-type battery CBA for about 24 hours after its construction and allowing the open-circuit voltage (OCV) to stabilize. The current density to the positive electrode was 0.1 mA / cm². ２ The charge capacity was defined as the capacity when the battery was charged to a cutoff voltage of 4.3V. The discharge capacity was defined as the capacity when the battery was discharged to a cutoff voltage of 2.5V after a one-hour rest period following charging. Furthermore, the Coulomb efficiency (efficiency), which is the ratio of discharge capacity to charge capacity, was calculated. The evaluation results are shown in the "Initial Battery Characteristics" column of Table 2.

[0163] (8) Cycle characteristics (Fabrication of laminated battery) As shown in Figure 2, the laminated battery LBA has a structure in which an electrolyte is impregnated into a laminate of a positive electrode film PS, a separator SE2, and a negative electrode film NS, and then sealed with laminate LA. A positive electrode tab PT is connected to the positive electrode film PS, and a negative electrode tab NT is connected to the negative electrode film NS, and the positive electrode tab PT and negative electrode tab NT are exposed outside the laminate LA.

[0164] This laminated battery LBA was fabricated as follows: 20.0 g of the obtained positive electrode active material, 2.35 g of acetylene black, and 1.18 g of polyvinylidene fluoride were dispersed in N-methyl-2-pyrrolidone (NMP) slurry and spread over aluminum foil to a depth of 1 cm. ２ The positive electrode active material was coated so that 7.0 mg was present per unit area. Next, the slurry containing the positive electrode active material was coated onto aluminum foil and dried in air at 120°C for 30 minutes to remove NMP. The aluminum foil coated with the positive electrode active material was cut into strips 66 mm wide and roll-pressed with a load of 1.2 t to produce a positive electrode film. The positive electrode film was then cut into a rectangle of 50 mm x 30 mm and dried in a vacuum dryer at 120°C for 12 hours, and used as the positive electrode film PS for a laminate-type battery LBA.

[0165] Furthermore, a negative electrode film NS was prepared by coating a copper foil with a negative electrode mixture paste, which is a mixture of graphite powder with an average particle size of approximately 20 μm and polyvinylidene fluoride. A polyethylene porous membrane with a thickness of 20 μm was used for the separator SE2. 1 M LiPF was used as the electrolyte. ６ A mixed solution (manufactured by Ube Industries, Ltd.) was used, in which ethylene carbonate (EC) and diethyl carbonate (DEC), with EC as the supporting electrolyte, were mixed in a volume ratio of 3:7.

[0166] In a dry room controlled to a dew point of -60°C, an electrolyte was impregnated into a laminate of the positive electrode film PS, separator SE2, and negative electrode film NS, and then sealed with laminate LA to produce a laminate-type battery LBA.

[0167] (Cycle Characteristics) Cycle characteristics were evaluated by measuring the capacity retention rate after 500 charge-discharge cycles. Specifically, the laminated battery LBA was first subjected to a current density of 0.3 mA / cm² in a constant temperature bath maintained at 25°C. ２ The battery was then charged to a cutoff voltage of 4.2V. After a 10-minute rest, it underwent conditioning by repeating a cycle of discharging to a cutoff voltage of 2.5V five times. Subsequently, it was heated in a constant temperature bath maintained at 45°C with a current density of 2.0 mA / cm². ２ The battery was charged to a cutoff voltage of 4.2V, then left for 10 minutes, and discharged to a cutoff voltage of 2.5V. This cycle was repeated 500 times. The ratio of the discharge capacity after 500 cycles after conditioning to the discharge capacity after 1 cycle (initial discharge capacity) was defined as the capacity retention rate. For Example 4 and Comparative Example 2, the cutoff voltage during charging was set to 4.3V. The evaluation results are shown in the "Cycle Characteristics" column of Table 2. [2] Conditions and evaluation results in the examples and comparative examples (Example 1) A nickel composite oxide was obtained using a known crystallization method, in which the molar ratio of nickel, manganese, and cobalt was Ni:Mn:Co = 85:10:5.

[0168] Nickel composite oxides were prepared using the following procedure.

[0169] First, nickel sulfate, manganese sulfate, and cobalt sulfate were dissolved in pure water in a molar ratio of Ni:Mn:Co = 85:10:5 to prepare a 2.0 mol / L raw material aqueous solution. A 20% by mass sodium hydroxide aqueous solution as a pH adjuster and a 25% by mass ammonia aqueous solution as an ammonium ion supplier were added dropwise to the raw material aqueous solution, adjusting the liquid temperature to 50°C and the pH value to 12.0 based on a liquid temperature of 25°C, and nickel composite hydroxide particles were co-precipitated.

[0170] The obtained nickel composite hydroxide particles were heat-treated at 650°C to obtain nickel composite oxide. The D50 of the obtained nickel composite oxide was 13.0 μm. (1) Mixing process Next, the above nickel composite oxide and lithium hydroxide monohydrate were thoroughly mixed using a shaker mixer device (Willi E. Bakkofen (WAB) TURBULA Type T2C) to prepare a mixture. At this time, the mixture was weighed and mixed so that the ratio of the amount of substance of lithium (Li) to elements other than lithium (Me) contained in the mixture, Li / Me, was 0.98. The Li / Me of the mixture obtained in the mixing process is shown in the "Initial Li / Me Ratio" column of Table 1. (2) Firing process The mixture was fired in a firing atmosphere with an oxygen concentration of 90% by volume and the remainder being nitrogen, by first firing by raising the temperature to 600°C and holding it for 15 hours, and then subsequently firing by second firing by raising the temperature to 870°C and holding it for 10 hours to obtain the fired product.

[0171] Table 1 shows the "Casting Conditions" column for the calcination process, including the oxygen concentration of the calcination atmosphere, the calcination conditions in the first calcination (calcination temperature, calcination time), and the calcination conditions in the second calcination (calcination temperature, calcination time). (3) Grinding process The calcined material obtained in the calcination process was ground into a pulverized material using a hammer mill. (4) Washing, drying, and crushing process The pulverized material was mixed with 100 parts by mass of water per 100 parts by mass of the pulverized material in a nitrogen atmosphere to form a slurry. The slurry was washed with water by stirring for 15 minutes while maintaining the slurry temperature at 35°C to remove impurities from the pulverized material, and then solid-liquid separation was performed (washing process).

[0172] The washed material obtained in the washing process was dried in a vacuum atmosphere at 200°C for 10 hours (drying process). Each process from stirring to the start of drying was carried out continuously without any waiting time, thereby minimizing the time the material was in contact with the atmosphere.

[0173] After drying, the material was further crushed using a jet mill (manufactured by Seishin Corporation) to obtain a positive electrode active material for lithium-ion secondary batteries (crushing step).

[0174] SEM observation of the obtained positive electrode active material revealed that the lithium composite oxide was composed of 93% primary particles and the remaining secondary particles, with the number of primary particles constituting the secondary particles being 8 or less. Furthermore, the average primary particle diameter of the lithium composite oxide was 1.6 μm, and the median diameter D50 of the positive electrode active material was 1.8 μm. The manufacturing conditions are shown in Table 1, and the evaluation results of the positive electrode active material are shown in Table 2.

[0175] (Example 2) The firing temperature for the second firing was changed to 890°C. Also, the pulverization process was omitted. Except for the above points, the positive electrode active material was obtained and evaluated in the same manner as in Example 1.

[0176] SEM observation of the obtained positive electrode active material revealed that the lithium composite oxide was composed of 35% primary particles and the remaining secondary particles, with more than 30 primary particles constituting the secondary particles. The average primary particle diameter of the lithium composite oxide was 2.0 μm, and the D50 of the positive electrode active material was 17.0 μm. The manufacturing conditions are shown in Table 1, and the evaluation results of the positive electrode active material are shown in Table 2.

[0177] (Example 3) The positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the firing temperature for the second firing was changed to 880°C and the oxygen concentration of the firing atmosphere was set to 95% by volume.

[0178] SEM observation of the obtained positive electrode active material revealed that the lithium composite oxide was composed of 95% primary particles and the remaining secondary particles, with the number of primary particles constituting the secondary particles being 10 or less. Furthermore, the average primary particle diameter of the lithium composite oxide was 1.6 μm, and the D50 of the positive electrode active material was 1.8 μm. The manufacturing conditions are shown in Table 1, and the evaluation results of the positive electrode active material are shown in Table 2.

[0179] (Example 4) Nickel composite hydroxide particles were coprecipitated in the same manner as in Example 1, except that the raw material aqueous solution was prepared with nickel sulfate, manganese sulfate, and cobalt sulfate in a molar ratio of Ni:Mn:Co = 70:20:10.

[0180] The obtained nickel composite hydroxide particles were heat-treated at 650°C to obtain nickel composite oxide. The D50 of the obtained nickel composite oxide was 13.0 μm.

[0181] (1) Mixing Process Next, the nickel composite oxide and lithium hydroxide monohydrate were thoroughly mixed using a shaker mixer device (Willi e Bakkofen (WAB) TURBULA Type T2C) to prepare a mixture. At this time, the mixture was weighed and mixed so that the ratio of the amount of lithium (Li) to the amount of other elements (Me) contained in the mixture, Li / Me, was 0.98.

[0182] (2) Firing process The mixture was fired in a firing atmosphere with an oxygen concentration of 95% by volume and the remainder being nitrogen, by first firing, raising the temperature to 600°C and holding it for 15 hours, and then second firing, raising the temperature to 940°C and holding it for 10 hours, to obtain a fired product.

[0183] (3) Grinding process The calcined material obtained was ground using a hammer mill to obtain a pulverized product.

[0184] (4) Washing, drying, and crushing process The pulverized material was mixed with 100 parts by mass of water per 100 parts by mass of pulverized material in a nitrogen atmosphere to form a slurry. The slurry was washed with water by stirring for 15 minutes while maintaining the slurry temperature at 35°C to remove impurities from the calcined material, and then solid-liquid separation was performed (washing process).

[0185] The washed material obtained in the washing process was dried in a vacuum atmosphere at 200°C for 10 hours (drying process). Each process from stirring to the start of drying was carried out continuously without any waiting time, thereby minimizing the time the material was in contact with the atmosphere.

[0186] After drying, the material was further crushed using a jet mill (manufactured by Seishin Corporation) to obtain a positive electrode active material for lithium-ion secondary batteries.

[0187] SEM observation of the obtained positive electrode active material revealed that the lithium composite oxide was composed of 98% primary particles and the remaining secondary particles, with the number of primary particles constituting the secondary particles being 8 or less. The average primary particle diameter of the lithium composite oxide was 2.1 μm, and the D50 of the positive electrode active material was 2.3 μm. Table 1 shows the manufacturing conditions, and Table 2 shows the evaluation results of the positive electrode active material.

[0188] (Comparative Example 1) The mixture was weighed so that the ratio of the amount of lithium (Li) to the amount of other elements (Me), i.e., Li / Me, was 1.02. The firing temperature for the first firing was 780°C, the holding time was 3 hours, and the holding time for the second firing was 2 hours. Except for the above points, the positive electrode active material was obtained and evaluated using the same procedure as in Example 1.

[0189] SEM observation of the obtained positive electrode active material revealed that the lithium composite oxide was composed of 95% primary particles and the remaining secondary particles, with the number of primary particles constituting the secondary particles being 12 or less. The average primary particle diameter of the lithium composite oxide was 2.0 μm, and the D50 of the positive electrode active material was 2.2 μm. Table 1 shows the manufacturing conditions, and Table 2 shows the evaluation results of the positive electrode active material.

[0190] (Comparative Example 2) The mixture was weighed so that the ratio of the amount of substance of elements other than lithium (Me), i.e., Li / Me, was 1.02. The holding time for the first calcination was 3 hours, and the holding time for the second calcination was 2 hours. Except for the above points, the positive electrode active material was obtained and evaluated in the same manner as in Example 4.

[0191] SEM observation of the obtained positive electrode active material revealed that the lithium composite oxide was composed of 95% primary particles and the remaining secondary particles, with the number of primary particles constituting the secondary particles being 8 or less. The average primary particle diameter of the lithium composite oxide was 1.2 μm, and the D50 of the positive electrode active material was 1.5 μm. Table 1 shows the manufacturing conditions, and Table 2 shows the evaluation results of the positive electrode active material.

[0192] (Comparative Example 3) The mixture was weighed so that the ratio of the amount of substance of elements other than lithium (Me), i.e., Li / Me, was 1.02. The holding time for the first calcination was 3 hours, and the holding time for the second calcination was 3 hours. Except for the above points, the positive electrode active material was obtained and evaluated in the same manner as in Example 3.

[0193] SEM observation of the obtained positive electrode active material confirmed that the lithium composite oxide is composed of 95% primary particles and the remaining secondary particles. It was confirmed that there were 10 or fewer primary particles constituting the secondary particles. The average primary particle diameter of the lithium composite oxide was 2.0 μm, and the D50 of the positive electrode active material was 2.3 μm. The manufacturing conditions are shown in Table 1, and the evaluation results of the positive electrode active material are shown in Table 2.

[0194]

[0195] In Examples 1 to 4, the number ratio of individual primary particles was 30% or more, and as shown in Table 2, the ratio of eluted lithium to total lithium was 32% or less.

[0196] The positive electrode active materials of Examples 1 to 4 were found to have superior charge / discharge capacity, efficiency, and capacity retention compared to Comparative Examples 1 to 3 with their corresponding compositions. A corresponding composition means that the proportions of elements other than lithium and oxygen are the same or nearly the same. For example, Examples 1 to 3 correspond to Comparative Examples 1 and 3, and Example 4 corresponds to Comparative Example 2.

[0197] Examples 1, 3, and 4, which had a particularly high number ratio of single primary particles, were also found to have particularly superior cycle characteristics compared to Example 2, which had a relatively high number ratio of secondary particles.

[0198] On the other hand, in Comparative Examples 1 to 3, the ratio of the amount of eluted lithium to the total amount of lithium exceeded 32%, so it was confirmed that the capacity retention rate and other properties were lower compared to the examples with the corresponding compositions.

[0199] The positive electrode active material of this embodiment has been confirmed to provide a positive electrode active material for lithium-ion secondary batteries that exhibits excellent charge-discharge efficiency, high capacity, and superior cycle characteristics. [Note] Examples of embodiments of the present disclosure are as follows.

[0200] <1> The lithium nickel composite oxide comprises a layered structure of space group R-3m, wherein, in addition to oxygen, the lithium nickel composite oxide contains lithium (Li), nickel (Ni), and element M (M) in a molar ratio of Li:Ni:M = a:b:c (where 0.90 ≤ a < 1.0, 0.5 ≤ b ≤ 1.0, 0 ≤ c ≤ 0.5, b + c = 1, and element M is at least one selected from the group consisting of Co, Mn, Al, V, Mg, Mo, Ca, Cr, Zr, Ti, Nb, Na, K, W, Fe, Zn, B, Si, P, and Ta), and the particles of the lithium nickel composite oxide consist of single primary particles or secondary particles formed by the aggregation of single primary particles and multiple primary particles, wherein the number ratio of single primary particles is 30% or more of the total number of particles. A positive electrode active material for a lithium-ion secondary battery, wherein the amount of lithium that dissolves from within the lithium nickel composite oxide particles when immersed in water is 32% or less of the total amount of lithium that dissolves in water.

[0201] <2> The positive electrode active material for a lithium-ion secondary battery according to <1>, wherein the ratio of the diffraction peak intensity I(003) of the (003) plane to the diffraction peak intensity I(104) of the (104) plane, obtained from an X-ray diffraction pattern measured using a flat plate sample holder of a Bragg-Brentano optical system with Cu-kα rays as the X-ray source, is 1.8 or more.

[0202] <3> A positive electrode active material for a lithium-ion secondary battery as described in <1> or <2>, wherein the volume-based median diameter (D50) is 0.5 μm or more and 20 μm or less.

[0203] <4> A lithium-ion secondary battery having a positive electrode containing the positive electrode active material for lithium-ion secondary batteries described in any of <1> to <3>.

[0204] This application claims priority based on Japanese Patent Application No. 2025-052627, filed with the Japan Patent Office on 26 March 2025, and the entire contents of Japanese Patent Application No. 2025-052627 are incorporated herein by reference.

[0205] CBA Coin-type battery: PE positive electrode, NE negative electrode, SE1 separator, GA gasket, WW wave washer, CA case, PC positive electrode can, NC negative electrode can. LBA Laminate-type battery: PS positive electrode film, NS negative electrode film, SE2 separator, LA laminate, PT positive electrode tab, NT negative electrode tab.

Claims

1. The lithium nickel composite oxide comprises a layered structure of space group R-3m, wherein, in addition to oxygen, the lithium nickel composite oxide contains lithium (Li), nickel (Ni), and element M (M) in a molar ratio of Li:Ni:M = a:b:c (where 0.90 ≤ a < 1.0, 0.5 ≤ b ≤ 1.0, 0 ≤ c ≤ 0.5, b + c = 1, and element M is at least one selected from the group consisting of Co, Mn, Al, V, Mg, Mo, Ca, Cr, Zr, Ti, Nb, Na, K, W, Fe, Zn, B, Si, P, and Ta), and the particles of the lithium nickel composite oxide consist of single primary particles or secondary particles formed by the aggregation of single primary particles and multiple primary particles, with the number ratio of single primary particles being 30% or more of the total particles. A positive electrode active material for a lithium-ion secondary battery, wherein the amount of lithium that dissolves from within the lithium nickel composite oxide particles when immersed in water is 32% or less of the total amount of lithium that dissolves in water.

2. The positive electrode active material for a lithium-ion secondary battery according to claim 1, wherein the ratio of the diffraction peak intensity I(003) of the (003) plane to the diffraction peak intensity I(104) of the (104) plane obtained from an X-ray diffraction pattern measured using a flat plate sample holder of a Bragg-Brentano optical system with Cu-kα rays as the X-ray source is 1.8 or more.

3. The positive electrode active material for a lithium-ion secondary battery according to claim 1 or claim 2, wherein the volume-based median diameter (D50) is 0.5 μm or more and 20 μm or less.

4. A lithium-ion secondary battery having a positive electrode containing the positive electrode active material for lithium-ion secondary batteries described in claim 1 or claim 2.

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