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

Abstract

A positive electrode active material for a lithium-ion secondary battery comprising lithium nickel composite oxide particles having a hexagonal layered structure, and a coating containing boron, wherein the lithium nickel composite oxide particles include one or more selected from particles composed of primary particles and secondary particles, and have a peak in a region of 190-194 eV in a photoelectron spectrum obtained by photoelectron spectroscopic analysis of the surface of particles in the positive electrode active material; the substance content ratio of the boron on the particle surface to elements other than Li and O that the positive electrode active material contains is 0.25 or greater; and the ratio of the diffraction peak intensity I(003) of a (003) plane to the diffraction peak intensity I(104) of a (104) plane obtained from an X-ray diffraction pattern related to the lithium nickel composite oxide particles is 1.8 or greater.

Description

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

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

[0002] In recent years, with the widespread use of portable electronic devices such as mobile phones and laptop computers, there has been a strong demand for the development of small, lightweight rechargeable batteries with high energy density and durability. Furthermore, there is a strong demand for high-output rechargeable batteries for use in power tools and electric vehicles, including hybrid cars. In addition to the above-mentioned required characteristics, there is also a growing need for rechargeable batteries with high durability that do not degrade easily even after repeated use.

[0003] Lithium-ion batteries are a type of secondary battery that meets these requirements. A lithium-ion battery consists of a negative electrode, a positive electrode, and an electrolyte, and the active materials of the negative and positive electrodes are materials that can detach and insert lithium. As mentioned above, lithium-ion batteries have high energy density, power output characteristics, and durability.

[0004] Various studies have been conducted on the active material of the positive electrode to improve the performance of lithium-ion secondary batteries.

[0005] For example, Patent Document 1 discloses a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, which includes: mixing composite oxide particles containing nickel having predetermined particle size characteristics with a lithium compound to obtain a first mixture; heat-treating the first mixture at a first temperature and a second temperature higher than the first temperature to obtain a first heat-treated product; and dispersing the first heat-treated product. Patent Document 1 also discloses that the first temperature is 850°C or higher and 950°C or lower, and the second temperature is 980°C or higher and 1100°C or lower.

[0006] According to the method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery disclosed in Patent Document 1, it is possible to obtain a positive electrode active material that consists of a single particle or contains lithium transition metal oxide particles such that the number of primary particles constituting one secondary particle is reduced.

[0007] Japanese Patent Application Publication No. 2017-188443

[0008] Regarding the positive electrode active material for lithium-ion secondary batteries, there has been conventional research into using single particles composed of primary particles, or positive electrode active materials containing secondary particles with a reduced number of constituent primary particles. This is because using single particles or positive electrode active materials containing secondary particles with a reduced number of constituent primary particles can prevent particle breakage during positive electrode manufacturing and charging / discharging, thereby improving durability during repeated charging and discharging.

[0009] However, there is a strong demand for improved cycle characteristics in lithium-ion secondary batteries, which is the rate at which the battery capacity is maintained after repeated charging and discharging. For this reason, there is a need to further improve the cycle characteristics of positive electrode active materials in lithium-ion secondary batteries, whether they consist of single primary particles or positive electrode active materials containing secondary particles with a reduced number of constituent primary particles.

[0010] Therefore, in view of the problems of the above-mentioned conventional technology, one aspect of the present invention aims to provide a positive electrode active material for lithium-ion secondary batteries that exhibits excellent cycle characteristics when used in lithium-ion secondary batteries.

[0011] To solve the above problems, according to one aspect of the present invention, a positive electrode active material for a lithium-ion secondary battery comprises lithium nickel composite oxide particles having a hexagonal layered structure, and a coating containing boron disposed on at least a part of the surface of the lithium nickel composite oxide particles, wherein the lithium nickel composite oxide particles include one or more types selected from particles consisting of primary particles and secondary particles formed by the aggregation of multiple primary particles, the photoelectron spectrum obtained by photoelectron spectroscopy analysis of the particle surface of the positive electrode active material for a lithium-ion secondary battery has a peak in the region of 190 eV to 194 eV, and the ratio of the amount of boron on the particle surface to the amount of elements other than lithium and oxygen contained in the positive electrode active material for a lithium-ion secondary battery on the particle surface, obtained by photoelectron spectroscopy analysis of the particle surface of the positive electrode active material for a lithium-ion secondary battery, is 0.25 or more. The present invention provides a positive electrode active material for lithium-ion secondary batteries, 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, as determined from the X-ray diffraction pattern of the lithium nickel composite oxide particles 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 greater.

[0012] According to one aspect of the present invention, it is possible to provide a positive electrode active material for lithium-ion secondary batteries that exhibits excellent cycle characteristics when used in lithium-ion secondary batteries.

[0013] Figure 1 is an explanatory diagram of the configuration of coin-type batteries fabricated in the examples and comparative examples. Figure 2 is an explanatory diagram of the configuration of laminate-type batteries fabricated in the examples and comparative examples. Figure 3 is a schematic cross-sectional view of a positive electrode active material for a lithium-ion secondary battery according to one aspect of the present disclosure.

[0014] The following describes embodiments for carrying out the present invention, but the present invention is not limited to the embodiments described below, and various modifications and substitutions can be made to the embodiments described below without departing from the scope of the present invention. [Positive electrode active material for 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") may have a structure such as that shown in Figure 3. The positive electrode active material 30 of this embodiment may include lithium nickel composite oxide particles 31 having a hexagonal layered structure and a coating 32 disposed on at least a part of the surface of the lithium nickel composite oxide particles 31. Figure 3 is a schematic cross-sectional view of the positive electrode active material 30 of this embodiment, in which the lithium nickel composite oxide particles 31 are shown as circles and an example in which the coating 32 is disposed on the entire surface of the lithium nickel composite oxide particles 31 is shown, but the present invention is not limited to this embodiment. The positive electrode active material of this embodiment may also consist only of lithium nickel composite oxide having a hexagonal layered structure and a coating, but even in this case, the inclusion of unavoidable impurities is not excluded. (1) Regarding lithium nickel composite oxide particles, the lithium nickel composite oxide particles in the positive electrode active material of this embodiment may include one or more types selected from particles consisting of primary particles and secondary particles formed by the aggregation of multiple primary particles.

[0015] A particle composed of primary particles is a single particle made up of only one primary particle; it means that a single particle consists of only one primary particle.

[0016] Furthermore, a secondary particle formed by the aggregation of multiple primary particles means that two or more primary particles have aggregated to form a single secondary particle.

[0017] Lithium nickel composite oxide particles may contain secondary particles, but it is preferable that the number of primary particles constituting the secondary particles be small. The average number of primary particles constituting the secondary particles may be, for example, 20 or less, 10 or less, or 5 or less. The lower limit of the average number of primary particles constituting the secondary particles is not particularly limited and may be, for example, 2 or more.

[0018] The method for measuring the average number of primary particles constituting secondary particles in lithium nickel composite oxide particles is not particularly limited, but can be carried out, for example, by the following procedure.

[0019] First, the positive electrode active material of this embodiment is embedded in a resin, and a cross-section polishing process is performed to prepare a sample for cross-sectional observation. The sample for cross-sectional observation can be prepared by performing a cross-section polishing process so that the cross-section of the lithium nickel composite oxide particles contained in the positive electrode active material of this embodiment, embedded in the resin, is exposed.

[0020] Next, the prepared cross-sectional observation sample is performed using a Scanning Electron Microscope (SEM) to measure the number of primary particles constituting the secondary particles of the lithium nickel composite oxide particles. In the following explanation, the secondary and primary particles of the lithium nickel composite oxide particles will also be referred to simply as "secondary particles" and "primary particles."

[0021] When measuring and calculating the number of primary particles that constitute secondary particles, five to twenty secondary particles of average size are selected from the secondary particles to be evaluated within the SEM observation field of view of the cross-sectional observation sample. If the number of secondary particles in the field of view is less than five, all secondary particles can be included in the evaluation.

[0022] The SEM observation field of view (hereinafter also referred to as "observation field of view") for a sample used for cross-sectional observation is the field of view when the SEM magnification is selected so that a total of 100 to 200 lithium nickel composite oxide particles can be observed. The number of lithium nickel composite oxide particles when selecting the SEM magnification is the total number of particles consisting of primary particles and secondary particles.

[0023] An average-sized secondary particle refers to a secondary particle whose secondary particle diameter is within ±10% of the average secondary particle diameter of all secondary particles present within the observation field, using the average secondary particle diameter of the secondary particles within the observation field as the reference value.

[0024] In calculating the average secondary particle diameter, for 5 to 10 secondary particles present within the observation field, the diameter of the smallest inclusion circle for each secondary particle can be used as the secondary particle diameter. Then, the average value of the secondary particle diameters of the evaluated 5 to 10 secondary particles can be used as the average secondary particle diameter of the secondary particles present within the observation field.

[0025] Then, for a selected secondary particle of average size, the number of constituent primary particles can be measured.

[0026] Using the same procedure, the number of primary particles constituting secondary particles in a total of three fields of view, selected so as not to overlap, is determined. The average value (arithmetic mean) of the number of primary particles constituting secondary particles measured in the three fields of view can then be taken as the average value of the number of primary particles constituting secondary particles in the lithium nickel composite oxide particles.

[0027] Furthermore, when measuring the number of primary particles constituting secondary particles in a sample for cross-sectional observation, if a grain boundary is observed within a single particle, it can be evaluated as a secondary particle, and the number of primary particles separated by the grain boundary can be considered as the number of primary particles constituting that secondary particle.

[0028] Lithium nickel composite oxide particles can also have a higher proportion of primary particles among their constituent particles. By increasing the proportion of primary particles among the particles contained in the lithium nickel composite oxide particles, it is possible to prevent particle breakage during electrode fabrication and repeated charging and discharging, thereby improving durability. Durability here can also be referred to as cycle characteristics, and improving durability and cycle characteristics means that when the positive electrode active material of this embodiment is applied to a lithium-ion secondary battery and repeated charging and discharging is performed, the rate of decrease in discharge capacity can be reduced.

[0029] In lithium nickel composite oxide particles, the number ratio of particles consisting of primary particles may be 60% or more, or 70% or more. By setting the number ratio of particles consisting of primary particles to 60% or more for lithium nickel composite oxide particles, durability can be particularly enhanced.

[0030] Lithium nickel composite oxide particles can also be composed solely of primary particles, so the number ratio of primary particles may be 100% or less. The number ratio of primary particles can be adjusted by selecting conditions when crushing the first raw material mixture after calcination in the lithium nickel composite oxide particle manufacturing method described later. However, it is difficult to completely prevent the inclusion of secondary particles, and setting the number ratio of secondary particles to 0 may lead to a decrease in productivity. For this reason, the number ratio of primary particles in lithium nickel composite oxide particles may be 99% or less, or 90% or less. (Cobalt Coat Layer) Lithium nickel composite oxide particles may also have a cobalt coat layer 33 (see Figure 3) containing cobalt on one or more selected locations on the surface of the primary particles and the surface of the secondary particles.

[0031] By having a cobalt coating layer on the lithium nickel composite oxide particles, the resistance on the surface of the lithium nickel composite oxide particles can be reduced, thereby improving the output characteristics. Therefore, by having a cobalt coating layer on the lithium nickel composite oxide particles contained in the positive electrode active material of this embodiment, it is possible to provide a positive electrode active material for lithium-ion secondary batteries that has a low specific surface area while reducing resistance and exhibiting excellent output.

[0032] The cobalt coating layer 33 refers to a region in the cross-section of the lithium nickel composite oxide particle 31 where, when line analysis is performed along a straight line L from the center O of the lithium nickel composite oxide particle toward the outer surface 31A, the cobalt concentration is locally higher than in other parts. Therefore, the cobalt coating layer may be formed from cobalt alone, or it may further contain one or more elements other than cobalt, such as nickel contained in the lithium nickel composite oxide, or one or more selected from boron, element M, and oxygen. The cobalt coating layer 33 may be a region where the cobalt content is 2% or more higher in terms of molar ratio compared to the parts other than the cobalt coating layer 33. For line analysis, for example, SEM-EDS may be used. EDS stands for Energy Dispersive Spectroscopy. When performing line analysis, the center O of the lithium nickel composite oxide particle is the center of the minimum inclusion circle of the lithium nickel composite oxide particle, and line analysis can be performed along a straight line L corresponding to the radius of the minimum inclusion circle.

[0033] The cobalt coating layer may be one or more selected from a layer formed by the uneven distribution of cobalt within lithium nickel composite oxide particles, and a coating layer covering the surface of lithium nickel composite oxide particles.

[0034] When the positive electrode active material of this embodiment has a cobalt coating layer, as shown in Figure 3, the cobalt coating layer 33 and the coating 32 may be arranged in order from a position close to the lithium nickel composite oxide particles 31. The position close to the lithium nickel composite oxide particles 31 also includes the case where it is located within the lithium nickel composite oxide particles 31. (2) Regarding the coating The positive electrode active material of this embodiment may also have a coating (coating) on ​​the surface of the lithium nickel composite oxide particles. The coating may contain boron, for example. For the coating, for example, an oxide can be used, and examples of oxides include boron oxide and a composite oxide containing lithium and boron. The coating may contain multiple types of oxides, such as boron oxide and a composite oxide containing lithium and boron, or it may contain only one type of oxide, such as boron oxide and a composite oxide containing lithium and boron. In this embodiment, the positive electrode active material has a coating on the surface of the lithium nickel composite oxide particles, which reduces the specific surface area of ​​the positive electrode active material and the contact area with the electrolyte. Therefore, when used in a lithium-ion secondary battery, gas generation can be reduced. Furthermore, the positive electrode active material of this embodiment has a coating on the surface of the lithium nickel composite oxide particles, which reduces the specific surface area of ​​the positive electrode active material and the performance degradation due to reaction with the electrolyte. Therefore, when used in a lithium-ion secondary battery, the cycle characteristics can be improved. Moreover, when the positive electrode active material of this embodiment is used in a lithium-ion secondary battery, the battery capacity can be improved.

[0035] The coating may cover the entire surface of the lithium nickel composite oxide particles, but it is sufficient if it is positioned on at least a portion of the surface. Therefore, the coating may be scattered across the surface of the lithium nickel composite oxide particles. The thickness of the coating does not need to be uniform and may vary from place to place.

[0036] The coating can be produced, for example, by adding and mixing a coating material to lithium nickel composite oxide particles. The coating material can be selected according to the desired composition of the coating, but a boron-containing boron source such as boron oxide or boric acid can be used. The boron source may be boron oxide or boric acid, or it may be elemental boron or a compound other than boron oxide or boric acid that contains boron. When producing the coating, the raw materials can be further heat-treated after mixing. At least a portion of the added boron source, such as boron oxide, may not form a film and may maintain its particle shape. Also, for example, a lithium compound attached to the surface of the lithium nickel composite oxide particles may react with the boron source, such as boron oxide, which is the coating material, to form a composite oxide containing lithium and boron, which may be included in the coating. The coating may also contain elements other than boron. (3) Composition The positive electrode active material of this embodiment may contain lithium (Li), nickel (Ni), boron (B), and element M (M) in a molar ratio of Li:Ni:B:M = a:b:c:d. The positive electrode active material of this embodiment may further contain oxygen in addition to the above elements.

[0037] The positive electrode active material of this embodiment is, for example, a material with the general formula: Li ａ Ni ｂ B ｃ M ｄ O ２＋α It can be expressed as follows. Note that the above general formula applies to the entire positive electrode active material, including the lithium nickel composite oxide and the coating. (3-1) For lithium, a, which is the lithium content, can be, for example, 0.92 ≤ a ≤ 1.20. (3-2) For nickel, in lithium nickel composite oxide, the higher the nickel content, the higher the capacity that can be achieved when used as a positive electrode material for lithium-ion secondary batteries.

[0038] The nickel content b can be, for example, 0.50 ≤ b < 1.00, 0.50 ≤ b ≤ 0.99, 0.80 ≤ b ≤ 0.99, or 0.90 ≤ b ≤ 0.98. By increasing the nickel content, the battery capacity can be increased when the positive electrode active material of this embodiment is used in a lithium-ion secondary battery. Furthermore, by keeping the nickel content within the above range, the capacity retention rate can also be increased.

[0039] The nickel content can be selected such that the total content of nickel, boron, and element M equals 1. That is, the content can be selected such that b + c + d = 1. (3-3) Regarding boron, the positive electrode active material of this embodiment may have a coating on at least a part of the particle surface of the lithium nickel composite oxide, and this coating may contain an oxide, such as boron oxide or a composite oxide containing lithium and boron. The effects of the positive electrode active material of this embodiment having a coating have already been explained, so the explanation will be omitted.

[0040] In the positive electrode active material of this embodiment, the lithium nickel composite oxide particles may also contain boron. Therefore, boron may be contained in either the lithium nickel composite oxide particles or the coating.

[0041] The boron content c of the positive electrode active material in this embodiment can be, for example, 0.00 < c ≤ 0.05. (3-4) Regarding element M, the positive electrode active material of this embodiment may also contain element M. Element M can be at least one element selected from, for example, cobalt (Co), manganese (Mn), aluminum (Al), tungsten (W), molybdenum (Mo), magnesium (Mg), calcium (Ca), chromium (Cr), tantalum (Ta), niobium (Nb), silicon (Si), titanium (Ti), zirconium (Zr), and fluorine (F). In particular, from the viewpoint of improving the thermal stability of the positive electrode active material and reducing, for example, the thermal decomposition of the positive electrode active material, element M may contain at least one or more elements selected from cobalt (Co) and manganese (Mn).

[0042] The value d, which indicates the content of element M, can be set to, for example, 0.00 ≤ d ≤ 0.50.

[0043] Furthermore, if the positive electrode active material contains multiple types of element M, it is preferable that the sum of the content ratios of these multiple types of element M satisfies the above range.

[0044] Element M may be dispersed within the lithium nickel composite oxide particles, or it may be contained in the coating and unevenly distributed on the surface of the lithium nickel composite oxide particles. If the lithium nickel composite oxide particles contain multiple types of element M, their distribution may differ depending on the type of element M. For example, some elements M may be dispersed within the lithium nickel composite oxide particles, while other elements M may be unevenly distributed on the surface of the lithium nickel composite oxide particles. Alternatively, all of the multiple types of element M may be dispersed within the lithium nickel composite oxide particles or unevenly distributed on the surface of the lithium nickel composite oxide particles.

[0045] (Regarding Titanium and Zirconium) Furthermore, element M may contain titanium (Ti) and zirconium (Zr). According to the inventors' research, the inclusion of zirconium (Zr) in the positive electrode active material can promote the growth of primary particles contained in the particles of the positive electrode active material. However, in order to improve battery characteristics such as battery capacity while sufficiently lowering the specific surface area when the particles of the positive electrode active material are crushed to include, for example, particles consisting of primary particles, a further primary particle growth promoting effect is required.

[0046] Therefore, the inventors of the present invention conducted further investigations. As a result, they found that titanium (Ti), which was thought to inhibit the growth of primary particles when included alone as an additive element, can promote the growth of primary particles when included together with zirconium, compared to when zirconium is included alone.

[0047] Therefore, as described above, element M may contain titanium and zirconium.

[0048] As described above, in the positive electrode active material of this embodiment, element M may contain titanium and zirconium. When element M contains titanium and zirconium, if the titanium content is d1 and the zirconium content is d2, then the following conditions may be satisfied: 0.0003 ≤ d1 < 0.02 and 0.0003 ≤ d2 < 0.02, respectively.

[0049] By setting d1, which indicates the titanium content, and d2, which indicates the zirconium content, to 0.0003 or higher, the growth of primary particles can be promoted.

[0050] Conventionally, in order to increase the size of the primary particles contained in the positive electrode active material, the temperature at which the raw material mixture is calcined during the manufacturing process of the positive electrode active material has been raised to a high temperature of 980°C or higher. However, raising the heat treatment temperature during the manufacturing of the positive electrode active material causes problems such as cation mixing, which reduces the battery capacity.

[0051] In contrast, with the positive electrode active material of this embodiment, in the method for producing the positive electrode active material described later, it is possible to produce sufficiently large primary particles without raising the calcination temperature of the first raw material mixture to a high temperature of 980°C or higher, and the specific surface area of ​​the pulverized material obtained by pulverizing after calcination can be sufficiently reduced. For this reason, with the positive electrode active material of this embodiment, it is possible to reduce the specific surface area while also increasing the battery capacity when used in a lithium-ion secondary battery.

[0052] However, since the effect of promoting primary particle growth will saturate even if titanium and zirconium are added excessively, the titanium content (d1) and the zirconium content (d2) can each be set to less than 0.02.

[0053] The ratio of titanium to zirconium content is not particularly limited, but for example, d1, which represents the titanium content, and d2, which represents the zirconium content, may satisfy the condition 0.35 ≤ d1 / d2 ≤ 5.0. By setting d1 / d2 within the above range, the primary particles of the positive electrode active material can be made particularly large, and the variation in particle size of the positive electrode active material can be reduced. In addition, the specific surface area of ​​the positive electrode active material can be made particularly small.

[0054] Titanium and zirconium have the effect of promoting the growth of primary particles of lithium nickel composite oxide particles, and may be contained inside the lithium nickel composite oxide particles in order to particularly promote the growth of primary particles. For this reason, it is preferable that titanium and zirconium be detected when, for example, a sample for cross-sectional observation of lithium nickel composite oxide particles is prepared and observed, and compositional analysis is performed at the center of the primary particles of the lithium nickel composite oxide particles using EDS (Energy Dispersive X-ray Spectroscopy) or the like.

[0055] Furthermore, the sample used for cross-sectional observation of lithium nickel composite oxide particles can be the same sample used to evaluate the number ratio of particles consisting of primary particles. The center of the primary particle refers to the center of the smallest inclusion circle of the primary particle observed in the cross-sectional observation sample. (Regarding magnesium and fluorine) Element M may contain one or more elements selected from magnesium (Mg) and fluorine (F). One or more elements selected from magnesium and fluorine may be added in addition to titanium and zirconium.

[0056] The presence of magnesium in element M can promote the growth of primary particles. Furthermore, the presence of fluorine in element M can also promote the growth of primary particles.

[0057] Element M may contain cobalt (Co). The cobalt may be contained in a cobalt coating layer that can be positioned at one or more selected locations on the surface of the particles consisting of primary particles and on the surface of the secondary particles.

[0058] Furthermore, cobalt may be included in the lithium nickel composite oxide as an element that forms the lithium nickel composite oxide.

[0059] Therefore, cobalt may be dispersed within the lithium nickel composite oxide particles, or it may be present unevenly on the surface of the lithium nickel composite oxide particles, either on the surface portion of the lithium nickel composite oxide particles or contained in the coating.

[0060] As described above, the positive electrode active material of this embodiment may contain one or more elements selected from magnesium and fluorine as element M.

[0061] If the positive electrode active material of this embodiment contains magnesium, the molar ratio of magnesium content, d3, may satisfy the condition 0.0003 ≤ d3 < 0.02. By setting the magnesium content d3 to 0.0003 or higher, the growth of primary particles can be particularly promoted.

[0062] Furthermore, if the positive electrode active material of this embodiment contains fluorine, the molar ratio of fluorine content d4 may satisfy the condition 0.0003 ≤ d4 < 0.02. By setting the fluorine content d4 to 0.0003 or higher, the growth of primary particles can be particularly promoted.

[0063] However, excessive addition of magnesium or fluorine may saturate the effect of promoting primary particle growth and may even inhibit it. Therefore, the molar ratio of magnesium and fluorine may be less than 0.02 each.

[0064] The titanium, zirconium, magnesium, and fluorine described so far are included in element M. Since d, which indicates the content ratio of element M, corresponds to the sum of the content ratios of element M in terms of molar mass, d1, d2, d3, and d4, which indicate the content ratios of titanium, zirconium, magnesium, and fluorine, are included in d, which indicates the content ratio of element M. For example, if element M contains only titanium and zirconium, then d = d1 + d2. If element M contains only magnesium or only zirconium, then d = d3 and d = d4, respectively. (3-5) Regarding oxygen, the positive electrode active material of this embodiment may also contain oxygen, and α in 2 + α, which is the oxygen content ratio in the general formula of the positive electrode active material of this embodiment described above, can be set to -0.20 ≤ α ≤ 0.20. (4) Regarding the concentration ratio of titanium and zirconium in primary particles, the positive electrode active material of this embodiment may contain titanium and zirconium as described above. In this case, the ratio R1 (R1 = Ti2 ÷ Ti1) of the titanium concentration Ti1 inside the primary particles of the positive electrode active material of this embodiment to the titanium concentration Ti2 on the surface of the primary particles may be 0.8 or more and 2.0 or less, i.e., 0.8 ≤ R1 ≤ 2.0. The ratio R1 of the titanium concentration Ti1 inside the primary particles of the positive electrode active material of this embodiment to the titanium concentration Ti2 on the surface of the primary particles may be 1.0 or more and 1.9 or less, or 1.2 or more and 1.8 or less.

[0065] Furthermore, the ratio R2 (R2 = Zr2 ÷ Zr1) of the zirconia concentration Zr1 inside the primary particles of the positive electrode active material of this embodiment to the zirconia concentration Zr2 on the surface of the primary particles may be 1.5 or greater, i.e., 1.5 ≤ R2. The ratio R2 of the zirconia concentration Zr1 inside the primary particles of the positive electrode active material of this embodiment to the zirconia concentration Zr2 on the surface of the primary particles may be 1.5 or greater and 5.0 or less, 2.0 or greater and 5.0 or less, or 2.5 or greater and 4.5 or less.

[0066] To measure the concentrations of titanium and zirconium, a sample for cross-sectional observation of the positive electrode active material is prepared and observed using, for example, a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The magnification of the observation field is not particularly limited, but it is preferably 10,000x or more, and more preferably 30,000x or more, in order to observe the surface and interior of the primary particles to be evaluated. In the obtained observation image of the primary particles, the concentrations of titanium and zirconium can be measured by performing point analysis using EDS (Energy Dispersive X-ray Spectroscopy) at the outermost surface and the center of the primary particles.

[0067] The measurement points on the surface of primary particles can be defined as the intersection points of a straight line along the major axis of the primary particle being evaluated and the contour line of the outermost surface of the primary particle. Since there will be two intersection points between the straight line along the major axis of the primary particle and the contour line of the outermost surface of the primary particle, one point can be selected for evaluation.

[0068] Furthermore, the measurement point inside the primary particle can be any single point located on the major axis of the primary particle, within 25% of the major axis diameter from the center of the major axis.

[0069] For evaluation, ten different primary particles can be selected. For the selected primary particles, the ratio R1 (R1 = Ti2 ÷ Ti1) of the titanium concentration Ti1 inside the primary particle to the titanium concentration Ti2 on the primary particle surface can be calculated. In addition, for the selected primary particles, the ratio R2 (R2 = Zr2 ÷ Zr1) of the zirconia concentration Zr1 inside the primary particle to the zirconia concentration Zr2 on the primary particle surface can be calculated.

[0070] Then, the arithmetic mean of the above ratio R1 for the 10 primary particles that were evaluated can be taken as the ratio R1 of the titanium concentration Ti1 inside the primary particles and the titanium concentration Ti2 on the surface of the primary particles for the primary particles of the positive electrode active material that was subjected to evaluation.

[0071] Furthermore, the arithmetic mean of the above ratio R2 for the 10 primary particles that were evaluated can be used as the ratio R2 of the zirconia concentration Zr1 inside the primary particles and the zirconia concentration Zr2 on the surface of the primary particles for the primary particles of the positive electrode active material that was subjected to evaluation. (5) Evaluation results of photoelectron spectroscopy (5-1) Photoelectron spectrum The photoelectron spectrum obtained by photoelectron spectroscopy analysis of the particle surface of the positive electrode active material of this embodiment may have a peak in the region of 190 eV to 194 eV.

[0072] The photoelectron spectrum obtained by analyzing the particle surface of the positive electrode active material of this embodiment has a peak in the region of 190 eV to 194 eV, which means that boron or boron compounds that have the function of enhancing initial characteristics and improving cycle characteristics are arranged on the particle surface of the positive electrode active material. For this reason, the positive electrode active material of this embodiment having the above peak can improve cycle characteristics when used in a lithium-ion secondary battery. (5-2) Coverage The positive electrode active material of this embodiment may have a ratio of the amount of boron on the particle surface of the positive electrode active material to the amount of elements other than lithium and oxygen contained in the positive electrode active material on the particle surface (hereinafter also referred to as "coverage") of 0.25 or more.

[0073] The coverage rate can be determined by analyzing the particle surface of the positive electrode active material using photoelectron spectroscopy.

[0074] For example, suppose the coating contains boron, and the lithium nickel composite oxide is a composite oxide containing lithium, nickel, and element M.

[0075] Then, let B be the amount of boron on the particle surface of the positive electrode active material, and let Ni and M be the amounts of nickel and element M on the particle surface of the positive electrode active material.

[0076] In this case, the coverage rate can be calculated using the following formula.

[0077] (Coverage) = [(B) / (Ni + B + M)] Setting the above coverage to 0.25 or higher means that the surface of the lithium nickel composite oxide particles is sufficiently covered. For this reason, when the positive electrode active material of this embodiment is applied to a lithium-ion secondary battery, the cycle characteristics can be particularly improved.

[0078] The coverage ratio may be 0.30 or higher.

[0079] The upper limit of the coverage rate is not particularly limited, but for example it may be 0.95 or less, or 0.90 or less. Therefore, the coverage rate may be 0.25 or more and 0.95 or less, or 0.30 or more and 0.90 or less. (6) X-ray diffraction pattern The positive electrode active material of this embodiment may have a ratio (I(003) / I(104)) of the diffraction peak intensity of the (003) plane to the diffraction peak intensity I(104) of the (104) plane, determined from the X-ray diffraction pattern relating to the lithium nickel composite oxide particles, of 1.8 or more and 1.9 or more.

[0080] The above X-ray diffraction pattern can be measured using Cu-Kα rays as the X-ray source and a Bragg-Brentano optical system with a flat sample holder.

[0081] When I(003) / I(104) is 1.8 or higher, it means that the lithium nickel composite oxide particles contain primary particles alone, or secondary particles with a sufficiently reduced number of primary particles, in a particularly suitable proportion. Therefore, when manufacturing a positive electrode using the positive electrode active material of this embodiment, the packing efficiency of the positive electrode can be improved, and the battery characteristics of a lithium-ion secondary battery using the positive electrode can be enhanced.

[0082] The upper limit of I(003) / I(104) is not particularly limited, but may be, for example, 3.0 or less. Therefore, I(003) / I(104) may be 1.8 or more and 3.0 or less, or 1.9 or more and 3.0 or less. (7) Specific surface area The positive electrode active material of this embodiment has a specific surface area of ​​0.50 m² determined by the nitrogen adsorption method. ２ / g or more 1.8m ２ It may be less than or equal to 0.7 m ２ / g or more 1.8m２ / g or less.

[0083] In the positive electrode active material of the present embodiment, the specific surface area is 0.50 m ２ / g or more, whereby an appropriate contact area with the electrolyte can be secured, and thus the battery capacity can be increased.

[0084] In the positive electrode active material of the present embodiment, the specific surface area is 1.8 m ２ / g or less, whereby the contact area with the electrolyte can be reduced, so the reaction with the electrolyte can be reduced, durability can be improved, and gas generation due to the reaction with the electrolyte can also be reduced. (8) Particle size characteristics (8-1) Volume average particle size The volume average particle size of the positive electrode active material of the present embodiment is not particularly limited, but the volume average particle size MV obtained from the volume-based particle size distribution by laser diffraction scattering method may be 0.5 µm or more and 6.0 µm or less, or 1.5 µm or more and 5.0 µm or less.

[0085] By setting the volume average particle size MV of the positive electrode active material of the present embodiment to 0.5 µm or more, the particles of the positive electrode active material of the present embodiment are sufficiently large, and the contact area with the electrolyte can be reduced, so the reaction with the electrolyte can be reduced, durability can be improved, and gas generation due to the reaction with the electrolyte can also be reduced.

[0086] Furthermore, by setting the volume average particle size MV of the positive electrode active material of the present embodiment to 6.0 µm or less, excessive growth of particles of the positive electrode active material of the present embodiment can be prevented, and an appropriate contact area with the electrolyte can be secured, so that the battery capacity can be increased. (8-2) Particle size distribution width For the positive electrode active material of the present embodiment, the particle size distribution width PDW (Particle size Distribution Width) calculated by the following formula (1), which is obtained from a volume-based particle size distribution measured by a laser diffraction scattering method, may be 0.40 or more and 1.2 or less, or 0.6 or more and 1.0 or less.

[0087] PDW = (D90 - D10) / MV ・・・ (1) In formula (1), D90, D10, and MV refer to 90% cumulative volume particle size (D90), which is the volume-based 90% diameter, 10% cumulative volume particle size (D10), which is the volume-based 10% diameter, and volume average particle size MV, respectively, in a volume-based particle size distribution obtained by a laser diffraction scattering method.

[0088] By keeping the particle size variation index PDW within the above range, it means that the particle size distribution of the positive electrode active material in this embodiment is not excessively narrow or excessively wide, and therefore the energy density and cycle characteristics can be particularly increased. (8-3) Fine particle ratio The positive electrode active material in this embodiment may have a fine particle ratio of 18.0% or less, which is the ratio of the number of particles of 0.50 μm or less to the total number of particles determined by flow image analysis using a wet flow particle size and shape analyzer.

[0089] By keeping the fine powder content at 18.0% or less, the proportion of fine particles can be sufficiently reduced, decreasing the contact area with the electrolyte. This reduces the reaction with the electrolyte, improves durability, and also reduces gas generation due to the reaction with the electrolyte. The fine powder content may be 15.0% or less.

[0090] The lower limit of the fine powder content is not particularly limited, but for example, it can be 1.0% or more. Therefore, the fine powder content may be 1.0% or more and 18.0% or less, or 1.0% or more and 15.0% or less. [Method for manufacturing positive electrode active material for lithium-ion secondary batteries] The method for manufacturing positive electrode active material for lithium-ion secondary batteries of this embodiment will be described. According to the method for manufacturing positive electrode active material of this embodiment, a positive electrode active material according to one aspect of the present disclosure can be manufactured. For this reason, some explanations of matters already described will be omitted. Note that the method for manufacturing positive electrode active material according to one aspect of the present disclosure is not limited to the method for manufacturing positive electrode active material described below.

[0091] The method for producing the positive electrode active material of this embodiment may include a first mixing step, a calcination step, a pulverization step, and a second mixing step.

[0092] In the first mixing step, a nickel composite compound and a lithium compound are mixed to prepare a first raw material mixture.

[0093] In the firing process, the first raw material mixture prepared in the first mixing process is fired to obtain a fired product.

[0094] In the grinding process, the calcined material obtained in the calcination process is ground to obtain a pulverized lithium nickel composite oxide.

[0095] In the second mixing step, a coating material can be added to the lithium nickel composite oxide particles obtained after the calcination or pulverization step and mixed to prepare the second raw material mixture.

[0096] The following describes each step. (1) First mixing step In the first mixing step, as described above, the nickel composite compound and the lithium compound can be mixed to prepare the first raw material mixture. In the first mixing step, in addition to the above raw materials, one or more element M sources, such as a titanium element source, a zirconium element source, a magnesium element source, and a fluorine element source, may be added and mixed to prepare the first raw material mixture. The raw materials used will be described below. (Nickel composite compound) The nickel composite compound used in the first mixing step may contain elements other than lithium and oxygen among the elements contained in the target lithium nickel composite oxide particles, namely nickel and element M.

[0097] While not particularly limited, nickel composite compounds can be used, such as nickel composite hydroxides or roasted nickel composite hydroxides. Examples of roasted nickel composite hydroxides include nickel composite oxides and mixtures of nickel composite oxides and nickel composite hydroxides.

[0098] Furthermore, the nickel composite compound may be a mixture of nickel oxide, a material having a coating layer containing element M on the surface of nickel hydroxide, nickel oxide, nickel hydroxide, etc., with element M alone, a compound of element M, etc. Examples of titanium, zirconium, magnesium, and fluorine sources that can be used as element M sources will be described later.

[0099] The nickel composite compound can contain, for example, nickel (Ni) and element M (M) in a molar ratio of Ni:M = x:y. In the above formula, x and y can satisfy the relationship x:y = b:d with b and d in the general formula of the positive electrode active material of this embodiment. If element M is included in the coating, the content ratio of element M in the nickel composite compound can be selected so that the overall positive electrode active material has a desired composition. Therefore, it is not always necessary to satisfy the above relationship.

[0100] Since element M has already been explained in the section on the positive electrode active material, we will omit its explanation here.

[0101] If the nickel composite compound is a nickel composite oxide, for example, the general formula is: Ni ｘ M ｙ O １＋β It can be expressed as follows.

[0102] If the nickel complex compound is a nickel complex hydroxide, for example, the general formula is: Ni ｘ M ｙ (OH) ２＋γ It can be expressed as follows.

[0103] Note that x, y, and element M have already been explained, so their explanations will be omitted. β and γ can be defined, for example, as -0.2 ≤ β ≤ 0.2 and -0.2 ≤ γ ≤ 0.2.

[0104] When using nickel composite hydroxide as the nickel composite compound, the method for producing the nickel composite hydroxide is not particularly limited. For example, nickel composite hydroxide obtained by crystallization methods such as coprecipitation or homogeneous precipitation can be used.

[0105] In the first mixing step, the nickel composite hydroxide described above can be used as is as the nickel composite compound, or it may be used after being oxidized and roasted to form a roasted product.

[0106] The conditions for oxidative roasting of nickel composite hydroxides are not particularly limited, but nickel composite hydroxides can be oxidative roasted, for example, in an oxidizing atmosphere at a temperature of 500°C to 800°C. The oxidative roasting temperature may be 550°C to 750°C.

[0107] When a roasted nickel complex hydroxide is used as the nickel complex compound, the composition ratio of lithium to elements other than lithium and oxygen in the lithium nickel complex oxide can be made particularly stable when the first raw material mixture, which is mixed with lithium compounds, etc., is calcined to obtain a lithium nickel complex oxide.

[0108] The atmosphere used for oxidative roasting is not particularly limited and can be carried out in an oxidizing atmosphere as described above, or in an air atmosphere (air atmosphere) or in an airflow for easier implementation. (Lithium compound) The lithium compound is not particularly limited and any compound containing lithium can be used. As a lithium compound, for example, one or more selected from lithium carbonate, lithium hydroxide, and lithium nitrate can be used. Among these, from the viewpoint of having less influence from residual impurities and dissolving at the calcination temperature, one or more selected from lithium carbonate and lithium hydroxide can be preferably used. Furthermore, from the viewpoint of obtaining a lithium nickel composite oxide with high crystallinity, lithium hydroxide can be more preferably used. (Element M source) As already explained, element M can also contain titanium, zirconium, magnesium, fluorine, etc. For this reason, for example, a titanium element source, a zirconium element source, a magnesium element source, a fluorine element source, etc. can be added as an element M source. Note that the following description is an example of an element M source, and elements other than titanium element sources, etc., can also be used as an element M source. (Titanium Source) The titanium source is a component containing titanium, and one or more types selected from elemental titanium and titanium-containing compounds can be used as the titanium source.

[0109] The titanium element source is not particularly limited, but from the viewpoint of ease of handling, availability, and prevention of contamination with impurities, one or more types selected from titanium oxide, titanium hydroxide, titanium hydride, titanium chloride, titanium nitrate, etc. can be suitably used, and titanium oxide can be used more suitably. (Zirconium element source) The zirconium element source is a component containing zirconium, and as the zirconium element source, one or more types selected from elemental zirconium and zirconium-containing compounds can be used.

[0110] The zirconium element source is not particularly limited, but from the viewpoint of ease of handling, availability, and prevention of contamination with impurities, one or more selected from zirconium oxide, zirconium hydroxide, zirconium hydride, zirconium nitrate, zirconium chloride, zirconium acetate, etc., can be suitably used, with zirconium oxide being particularly suitably used. (Magnesium element source) The magnesium element source is not particularly limited, but from the viewpoint of ease of handling, availability, and prevention of contamination with impurities, one or more selected from magnesium oxide, magnesium hydroxide, magnesium hydride, magnesium sulfate, magnesium chloride, etc., can be suitably used. (Fluorine element source) The fluorine element source is not particularly limited, but from the viewpoint of ease of handling, availability, and prevention of contamination with impurities, one or more selected from ammonium fluoride, lithium fluoride, sodium fluoride, etc., can be suitably used. (Mixing ratio) The mixing ratio of the nickel composite compound, the lithium compound, and the element M source is not particularly limited. However, the composition ratio of lithium to elements other than lithium and oxygen in the calcined product obtained after firing is almost the same as the composition ratio in the first raw material mixture.

[0111] Therefore, it is preferable to adjust the amount of lithium (Li) in the first raw material mixture so that the amount of substance ratio (Li / Me) is 0.92 or more and 1.20 or less with respect to the total amount of nickel and element M (Me) in the first raw material mixture.

[0112] Furthermore, if a washing process is carried out after the calcination process, the proportion of lithium in the first raw material mixture may be adjusted to be higher than the Li / Me ratio described above, taking into account the amount of excess lithium removed in the washing process.

[0113] The apparatus and method for mixing the nickel composite compound and the lithium compound are not particularly limited, as long as they can uniformly mix the two. For example, dry mixers or mixing and granulation devices such as shaker mixers, Redigge mixers, Julia mixers, and V-blenders can be used. (2) Firing process In the firing process, the first raw material mixture can be fired in an oxidizing atmosphere to obtain a fired product. When the first raw material mixture is fired in the firing process, lithium from the lithium compound diffuses into the nickel composite compound and reacts, so a lithium nickel composite oxide is formed. Furthermore, when the first raw material mixture is fired, it is thought that element M contained in the element M source also diffuses into the nickel composite compound.

[0114] In the firing process, the firing temperature for firing the raw material mixture is not particularly limited, but it can be, for example, 600°C or higher and less than 950°C.

[0115] By setting the firing temperature to 600°C or higher, the diffusion of lithium into the nickel composite compound can be sufficiently promoted.

[0116] Furthermore, by setting the firing temperature below 950°C, it is possible to prevent sintering from progressing between the particles of the generated positive electrode active material. In addition, it is possible to prevent the occurrence of abnormal grain growth and the coarsening of the particles contained in the fired product.

[0117] The holding time at the firing temperature is not particularly limited, but it can be, for example, 3 hours or more, or 5 hours or more and 24 hours or less.

[0118] During the process of raising the temperature to the firing temperature, the mixture can be held in a temperature range from near the melting point of the lithium compound used to the firing temperature, for example, between 400°C and 550°C, for about 1 to 5 hours. Holding the mixture in this temperature range allows the reaction to proceed particularly uniformly.

[0119] The atmosphere during firing can be, for example, an oxidizing atmosphere. The oxidizing atmosphere is not particularly limited, but an oxygen-containing gas atmosphere can be used, for example, an atmosphere with an oxygen concentration of 18% to 100% by volume.

[0120] By maintaining an oxygen concentration of 18% by volume or higher in the atmosphere during firing, the reaction between components in the first raw material mixture can be promoted, thereby increasing the crystallinity of the positive electrode active material.

[0121] When an oxygen-containing gas atmosphere is used, the gas constituting the atmosphere can be, for example, air, oxygen, or a mixture of oxygen and an inert gas.

[0122] Furthermore, when using a mixed gas of oxygen and an inert gas as the gas constituting the oxygen-containing gas atmosphere, for example, as described above, it is preferable that the oxygen concentration in the mixed gas satisfies the above-mentioned range.

[0123] The firing process may be carried out in an oxygen-containing gas stream, or in air or an oxygen stream. Performing the firing process in an oxygen stream can particularly improve battery characteristics.

[0124] The furnace used for firing is not particularly limited and can be any furnace capable of firing the first raw material mixture in a predetermined atmosphere. However, from the viewpoint of maintaining a uniform atmosphere inside the furnace, an electric furnace that does not generate gas can be preferably used. Either a batch-type or continuous-type furnace can be used for firing. (3) Grinding process The particles contained in the fired product obtained after the firing process have had inter-particle sintering prevented, but coarse particles may be formed due to weak sintering or aggregation. In addition, the fired product obtained after the firing process contains secondary particles formed by the aggregation of primary particles.

[0125] Therefore, in the grinding process, the calcined material obtained after the calcination process is ground to produce particles consisting of primary particles or secondary particles formed by the aggregation of multiple primary particles. The secondary particles can be ground in such a way that the number of primary particles they contain is reduced.

[0126] In the grinding process, the equipment used for grinding and the grinding conditions can be selected to achieve the required degree of aggregation and particle size for the positive electrode active material.

[0127] Grinding can be carried out using grinding equipment such as a jet mill, ball mill, or wet ball mill.

[0128] Furthermore, the grinding process can be carried out in multiple stages using multiple grinding devices.

[0129] After grinding, sieving, classification, etc., can be performed as needed, and particle size characteristics such as particle size distribution can be further adjusted and selected.

[0130] (4) Second mixing step, heat treatment step (Second mixing step) In the second mixing step, a coating material can be added to the lithium nickel composite oxide particles obtained after the firing step or the pulverization step and mixed to prepare the second raw material mixture.

[0131] In the second mixing step, a general-purpose mixer can be used to mix the lithium nickel composite oxide particles with the coating material. For example, one or more types of mixers selected from shaker mixers, Redigge mixers, Julia mixers, V-blenders, etc., can be used. While the mixing conditions in the second mixing step are not particularly limited, 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 the lithium nickel composite oxide particles.

[0132] Alternatively, a second mixing step may be performed before the grinding step, and during the grinding step, the lithium nickel composite oxide particles, which are the calcined product, may be mixed with the coating material while being ground.

[0133] The mixing ratio of lithium nickel composite oxide particles to the coating material is not particularly limited. For example, it is preferable to perform photoelectron spectroscopy analysis on the particle surface of the positive electrode active material and select the mixing ratio such that the resulting coating rate falls within a predetermined range.

[0134] The coating material can be selected according to the desired coating composition, but for example, a boron-containing boron source such as boron oxide can be used. The boron source may be boron oxide, elemental boron, or a compound other than an oxide containing boron. The coating can be formed by adding the coating material to lithium nickel composite oxide particles and mixing them. The boron source, such as boron oxide, may react with lithium compounds attached to the surface of the lithium nickel composite oxide particles to form a composite oxide containing lithium and boron, which may be included in the coating.

[0135] By coating the surface of the lithium nickel composite oxide particles, the specific surface area of ​​the positive electrode active material can be reduced, thereby decreasing the contact area with the electrolyte. This reduces gas generation when used in lithium-ion secondary batteries. Furthermore, by coating the surface of the lithium nickel composite oxide particles, the specific surface area of ​​the positive electrode active material is reduced, minimizing performance degradation due to reactions with the electrolyte. This also improves the cycle characteristics when used in lithium-ion secondary batteries. Moreover, the coating can also improve battery capacity when the positive electrode active material of this embodiment is applied to a lithium-ion secondary battery.

[0136] As will be described later, further heat treatment can be performed after the second mixing step. By performing heat treatment, at least a portion of the coating material, such as boron oxide or other boron sources, will dissolve. It is then believed that the dissolved material can form a film-like coating on the surface of the lithium nickel composite oxide particles.

[0137] The second raw material mixture obtained in the second mixing step can be used as the positive electrode active material, but the method for producing the positive electrode active material in this embodiment may also include the following heat treatment step as needed. (Heat treatment step) The method for producing the positive electrode active material in this embodiment may also include a heat treatment step in which the obtained second raw material mixture is heat-treated after the second mixing step. The heat treatment conditions in the heat treatment step are not particularly limited, but for example, the heat treatment can be performed at a temperature of 150°C to 800°C for 1 hour to 24 hours.

[0138] The atmosphere during heat treatment is not particularly limited, but for example, an oxidizing atmosphere can be used. The oxidizing atmosphere is not particularly limited, but an oxygen-containing gas atmosphere can be used, for example, an atmosphere with an oxygen concentration of 18% by volume or more and 100% by volume or less.

[0139] By maintaining an oxygen concentration of 18% by volume or higher in the atmosphere during heat treatment, the decomposition of lithium nickel composite oxide particles during heat treatment can be prevented.

[0140] When an oxygen-containing gas atmosphere is used, the gas constituting the atmosphere can be, for example, air, oxygen, or a mixture of oxygen and an inert gas.

[0141] Furthermore, when using a mixed gas of oxygen and an inert gas as the gas constituting the oxygen-containing gas atmosphere, for example, as described above, it is preferable that the oxygen concentration in the mixed gas satisfies the above-mentioned range.

[0142] The heat treatment may be carried out in an oxygen-containing gas stream, or in the atmosphere or an oxygen stream.

[0143] The furnace used for heat treatment is not particularly limited; any furnace capable of heat-treating the second raw material mixture in a predetermined atmosphere is acceptable. However, from the viewpoint of maintaining a uniform atmosphere inside the furnace, an electric furnace that does not generate gas is preferably used. Either a batch-type or continuous-type furnace can be used for heat treatment.

[0144] Furthermore, the mixing conditions in the second mixing step and the heat treatment conditions in the heat treatment step can be selected so that peaks are generated in a predetermined region in the photoelectron spectrum.

[0145] The method for producing the positive electrode active material of this embodiment may also include a water washing step as needed. (5) Water washing step The method for producing the positive electrode active material of this embodiment may also include a washing step after the calcination step or the pulverization step in which the calcined product or pulverized product is washed with water as needed. By performing the water washing step, excess lithium, which is a lithium component remaining on the surface of the particles, can be removed.

[0146] The washing process can be carried out by mixing the calcined or pulverized material with water to be used for washing to form a slurry, stirring it for a predetermined time, and then performing solid-liquid separation. After solid-liquid separation, drying can also be performed.

[0147] When forming a cobalt coating layer on one or more selected locations on the surface of primary particles or secondary particles of lithium nickel composite oxide particles, a cobalt-containing compound may be added to the water used for washing. The cobalt coating layer can be formed by washing with water containing the cobalt-containing compound and then drying.

[0148] Any water-soluble salt can be used as the cobalt-containing compound, such as cobalt sulfate or cobalt acetate. The thickness of the cobalt coating layer can be selected by adjusting the amount of cobalt-containing compound added, the washing time, and the drying conditions. [Positive electrode for lithium-ion secondary battery, lithium-ion secondary battery] The positive electrode for lithium-ion secondary battery of this embodiment (hereinafter also referred to as "positive electrode") may include a positive electrode active material according to one aspect of this disclosure.

[0149] Furthermore, the lithium-ion secondary battery of this embodiment (hereinafter also referred to as "secondary battery") may have a positive electrode, a negative electrode, and an electrolyte. The positive electrode may include a positive electrode active material according to one aspect of this disclosure.

[0150] The following describes an example of the configuration of the positive electrode and secondary battery of this embodiment, with each component explained separately. The secondary battery of this embodiment includes, for example, a positive electrode, a negative electrode, and a non-aqueous electrolyte, and is composed of components similar to those of a general lithium-ion secondary battery. Note that the positive electrode for lithium-ion secondary batteries and lithium-ion secondary batteries of the embodiments described below are merely examples. The positive electrode for lithium-ion secondary batteries and lithium-ion secondary batteries of this embodiment can be implemented in various modified and improved forms based on the knowledge of those skilled in the art, including the embodiments described below. Furthermore, the secondary battery does not particularly limit its use. (Positive Electrode) The positive electrode of the secondary battery of this embodiment may include a positive electrode active material according to one aspect of this disclosure.

[0151] An example of a method for manufacturing a positive electrode is described below. First, the positive electrode active material (in powder form), conductive material, and binder are mixed to form a positive electrode mixture. Then, activated carbon and solvents for purposes such as viscosity adjustment are added as needed, and this mixture is kneaded to produce a positive electrode mixture paste.

[0152] The mixing ratio of each material in the positive electrode composite is a factor that determines the performance of the lithium-ion secondary battery, and therefore can be adjusted according to the application. The mixing ratio of the materials can be the same as that of the positive electrode of a known lithium-ion secondary battery. For example, if the total mass of the solid content of the positive electrode composite excluding the solvent is 100% by mass, the positive electrode active material can be contained in a ratio of 60% to 95% by mass, the conductive material in a ratio of 1% to 20% by mass, and the binder in a ratio of 1% to 20% by mass.

[0153] The resulting positive electrode composite paste is applied to the surface of a current collector, for example, made of aluminum foil, and dried to remove the solvent, thereby producing a sheet-like positive electrode. If necessary, it can be pressurized using a roll press or the like to increase the electrode density. The sheet-like positive electrode thus obtained can be cut to an appropriate size according to the intended battery and used in the manufacture of the battery.

[0154] As conductive materials, for example, graphite (natural graphite, artificial graphite, and expanded graphite, etc.) and carbon black-based materials such as acetylene black and Ketjenblack (registered trademark) can be used.

[0155] The binder serves to hold the active material particles together, and one or more of the following can be used: polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose resins, and polyacrylic acid.

[0156] If necessary, a solvent that dissolves the binder and disperses the positive electrode active material, conductive material, etc., can be added to the positive electrode mixture. Specifically, organic solvents such as N-methyl-2-pyrrolidone can be used as the solvent. In addition, activated carbon can be added to the positive electrode mixture to increase the electrical double layer capacitance.

[0157] The method for manufacturing the positive electrode is not limited to the examples given above, and other methods may be used. For example, the positive electrode composite material can be press-molded and then dried in a vacuum atmosphere. (Negative electrode) Lithium metal, lithium alloy, etc. can be used for the negative electrode. Alternatively, the negative electrode may be formed by mixing a binder with a negative electrode active material that can intercept and deintercept lithium ions, adding a suitable solvent to make a paste, coating the surface of a metal foil current collector such as copper with the negative electrode composite material, drying it, and compressing it to increase the electrode density as needed.

[0158] As the negative electrode active material, for example, natural graphite, artificial graphite, and calcined organic compounds such as phenolic resin, and powdered carbon materials such as coke can be used. In this case, as with the positive electrode, a fluororesin such as PVDF can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent for dispersing these active materials and binders. (Separator) A separator can be placed between the positive electrode and the negative electrode as needed. The separator separates the positive electrode and the negative electrode and holds the electrolyte, and known separators can be used, for example, a thin film of polyethylene or polypropylene having many minute pores can be used. (Non-aqueous electrolyte) As the non-aqueous electrolyte, for example, a non-aqueous electrolyte solution can be used.

[0159] 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.

[0160] 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.

[0161] 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.

[0162] 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.

[0163] Examples of inorganic solid electrolytes include oxide-based solid electrolytes and sulfide-based solid electrolytes.

[0164] 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 One or more types selected from the above can be used.

[0165] The sulfide-based solid electrolyte is not particularly limited, and for example, those containing sulfur (S) and having lithium ion conductivity and electronic insulation can be preferably used. Examples of the sulfide-based solid electrolyte include 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.

[0166] Note that, as the inorganic solid electrolyte, those other than the above may be used, for example, Li ３ N, LiI, Li ３ N-LiI-LiOH or the like may also be used.

[0167] 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) The lithium ion secondary battery of the present embodiment described above 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, a positive electrode and a negative electrode can be stacked with a separator interposed therebetween to form an electrode body, and the obtained electrode body can be impregnated with the non-aqueous electrolyte solution. Then, 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, connection can be made using a current collecting lead or the like, and a structure sealed in a battery case can be obtained.

[0168] As previously described, the secondary battery of 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 changed as necessary.

[0169] The secondary battery of this embodiment can be used for various applications, but because it can be a high-capacity, high-output secondary battery, it is suitable for powering small portable electronic devices (such as notebook computers and mobile phone terminals) that always require high capacity. The secondary battery of this embodiment is also suitable for powering electric vehicles that require high output.

[0170] Furthermore, since the secondary battery of this embodiment can be miniaturized and have a high output, it is suitable as a power source for electric vehicles where mounting space is limited. Moreover, the secondary battery of this embodiment can be used not only as a power source for electric vehicles that are driven purely by electrical energy, but also as a power source for so-called hybrid vehicles that are used in conjunction with combustion engines such as gasoline engines and diesel engines.

[0171] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way by these examples.

[0172] First, we will explain the evaluation methods for the positive electrode active material and secondary battery obtained in the following examples and comparative examples. (1) Evaluation Method (1-1) Evaluation Method for Positive Electrode Active Material The following evaluations were performed on the obtained positive electrode active material.

[0173] (a) Composition The composition of the positive electrode active material was evaluated by analysis using an ICP emission spectrometer (VARIAN, 725ES). The analysis results are shown in the "Monetary Ratio" column of Table 1.

[0174] (b) Photoelectron spectroscopy (XPS) Al-K was monochromatized using a monochromator with an XPS instrument (ULVAC-FI, Versa Probe II). α The beam is used as the irradiation X-ray source, 1.0 × 10 －６Photoelectron spectra were measured on the surface of positive electrode active material particles in a vacuum atmosphere below Pa. XPS stands for X-ray Photoelectron Spectroscopy, and an XPS device is an X-ray photoelectron spectrometer.

[0175] For the region between 190 eV and 194 eV in the obtained photoelectron spectrum, the presence or absence of a peak was checked. If a peak was found, its position was identified and indicated in the "Peak Position" column of Table 2. If no peak was found in the region between 190 eV and 194 eV in the obtained photoelectron spectrum, "-" was indicated in the "Peak Position" column of Table 2.

[0176] From the obtained photoelectron spectra, the ratio of the amount of substance of elements other than oxygen contained in the coating on the particle surface of the positive electrode active material to the amount of lithium and other elements contained in the positive electrode active material on the particle surface of the positive electrode active material was calculated. The evaluation results are shown in the "Coverage Rate" column of Table 2. In the following examples, the element other than oxygen contained in the coating on the particle surface of the positive electrode active material is boron. (c) Particle number ratio consisting of primary particles First, the positive electrode active materials prepared in the following examples and comparative examples were embedded in resin, and cross-section polishing was performed to prepare samples for cross-sectional observation.

[0177] The prepared cross-sectional observation samples were observed using a scanning electron microscope (SEM, Hitachi High-Technologies Corporation, Scanning Electron Microscope S-4700).

[0178] In this process, the magnification of the SEM was selected so that a total of 100 to 200 particles of the positive electrode active material were included within the first field of view of the sample used for cross-sectional observation.

[0179] Then, the number of primary particles among all particles contained within the first observation field was counted, and the ratio of primary particles to all particles contained within the first observation field was calculated and defined as the ratio R1 of primary particles in the first observation field.

[0180] In the obtained observation images, particles in which no grain boundaries were observed within the particle were defined as particles consisting of primary particles. In the obtained observation images, particles in which grain boundaries were observed within the particle were defined as secondary particles formed by the aggregation of multiple primary particles.

[0181] The observation field of view was changed, and the cross-sectional sample was observed under the same conditions in the second and third observation fields, in addition to the first observation field of view described above. For the second and third observation fields, positions were selected that did not overlap with the other observation fields for the same cross-sectional sample, and the SEM magnification was selected under the same conditions as for the first observation field of view.

[0182] Then, the number ratio R2 of particles consisting of primary particles in the second observation field and the number ratio R3 of particles consisting of primary particles in the third observation field were calculated.

[0183] Next, the average value of the number ratio of primary particles in the three fields of view, i.e., (R1 + R2 + R3) / 3, was calculated and used as the number ratio of primary particles for the observed cathode active material. The evaluation results are shown in the "Primary Particle Individual Ratio" column of Table 2. (d) Volume-average particle size MV, particle size variation index (Particle size variation index: PDW) For the cathode active materials prepared in the following examples and comparative examples, the volume-based particle size distribution was obtained using the laser diffraction and scattering method with a laser diffraction scattering particle size analyzer (model: Microtrac HRA, manufactured by Nikkiso Co., Ltd.).

[0184] The volume-based particle size distribution was obtained, and the volume-average particle size (MV) was calculated.

[0185] Furthermore, from the obtained volume-based particle size distribution, the 90% cumulative volume particle size (D90) and the 10% cumulative volume particle size (D10) were determined, and together with the volume-average particle size MV, the particle size variation index PDW was calculated using the previously described formula (1).

[0186] The evaluation results are shown in the "Volume Average Particle Size MV" and "Particle Size Variation Index" columns of Table 2, respectively. (e) For the positive electrode active materials prepared in the examples and comparative examples below, the specific surface area of ​​the positive electrode active material was measured by nitrogen adsorption using a fluidized gas adsorption specific surface area measuring device (Multisorb, manufactured by Yuasa Ionics Co., Ltd.). (f) For the positive electrode active materials prepared in the examples and comparative examples below, the ratio of particles smaller than 0.50 μm to the total number of particles was measured and calculated using a flow-type image analysis method with a wet-type flow-type particle size and shape analyzer (Model: FPIA-3000, manufactured by Sysmex Corporation).

[0187] After adjusting the focus using a standard sample, measurements were performed using a dispersion prepared by dispersing the sample powder in water (the dispersion medium) using sonication. The dispersion was kept at a temperature of 25°C. The concentration of the dispersion was adjusted to allow measurement using a wet flow-type particle size and shape analyzer.

[0188] The evaluation results are shown in the "Fine Powder Content" column of Table 2. (g) For positive electrode active materials prepared in the examples and comparative examples, the ratio of the diffraction peak intensity I(003) of the (003) plane to the diffraction peak intensity I(104) of the (104) plane was less than or equal to the ratio of the diffraction peak intensity I(003) of the (104) plane. An X-ray diffractometer (BRUKER, D8 DISCOVER) was used, and monochromatized CuKα was used as the X-ray source. １ The XRD pattern was measured using the Bragg-Brentano optical system's flat sample holder. From the obtained XRD pattern, the intensity I(003) of the peak on the (003) plane located around 2θ = 18° and the intensity I(104) of the peak on the (104) plane located around 2θ = 44° were determined, which are the XRD patterns for lithium nickel composite oxide particles. Then, I(003) / I(104) was calculated. The evaluation results are shown in the "I(003) / I(104)" column of Table 2.

[0189] From the obtained XRD patterns, it was confirmed that the lithium nickel composite oxide contained in the positive electrode active material manufactured in the following examples has a hexagonal layered structure. (1-2) Method for evaluating battery characteristics (a) Discharge capacity The discharge capacity was evaluated using the coin-type batteries shown in Figure 1, which were manufactured in the following examples and comparative examples.

[0190] After preparing the coin-type batteries described in the following examples and comparative examples, leave them for about 12 hours until the open-circuit voltage (OCV) stabilizes, then set the current density to 0.1 mA / cm² for the positive electrode. ２ The charging capacity was defined as the capacity when the battery was charged to a cutoff voltage of 4.3V. Furthermore, 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.

[0191] The evaluation results are shown in the "Discharge Capacity" column under "Coin Cell" in Table 3. The ratio of discharge capacity to specific surface area of ​​the positive electrode active material is also calculated and shown in the "Discharge Capacity / Specific Surface Area" column. (b) Impedance Measurement The coin cells prepared in the following examples and comparative examples were first conditioned. Conditioning was performed in a constant temperature bath maintained at 25°C with a current density of 0.3 mA / cm². ２ The test was performed by charging the battery to a cutoff voltage of 4.2V, letting it rest for 10 minutes, and then discharging it to a cutoff voltage of 2.5V. This cycle was repeated five times. The initial discharge capacity was defined as the capacity at which the battery was discharged to 2.5V after one cycle under the same conditions following the conditioning test.

[0192] After measuring the initial discharge capacity, the coin-type battery was charged to 50% of its initial discharge capacity, allowed to rest for 10 minutes, and then discharged at a 1C rate for 10 seconds. The change in voltage during this 10-second discharge at the 1C rate was then measured.

[0193] According to Ohm's law, the voltage change was divided by the current to calculate the resistance (DC-IR), which was used as the reaction resistance, representing the DC resistance of the battery at SOC 50%, 1C, 10 seconds, and 25°C. In this case, since the coin-type batteries prepared in the following examples and comparative examples use the same components other than the positive electrode active material, the above DC resistance of the battery is considered to be an evaluation of the resistance of the positive electrode active material. The evaluation results are shown in the "Impedance Measurement" column under "Coin-type Battery" in Table 3. (c) Volume Expansion Rate The volume of the laminate-type batteries shown in Figure 2, prepared in the following examples and comparative examples, was measured by the Archimedes method and was used as the pre-cycle volume.

[0194] Next, the device was left in a constant temperature chamber maintained at 25°C for about 12 hours. After the open-circuit voltage (OCV) stabilized, conditioning was performed by repeating charge and discharge cycles five times with a cutoff voltage of 2.5V-4.3V.

[0195] Next, the batteries were charged at 60°C using constant current constant voltage (CCCV) charging up to 4.2V. While maintaining the voltage at 4.2V, they were stored in a constant temperature chamber set to 60°C for 14 days using float charging. After 14 days, they were discharged to 2.5V. After discharge, the volume of the laminated battery was measured using the Archimedes method and recorded as the post-cycle volume.

[0196] The volume expansion coefficient was calculated from the volume before and after the cycle using the following formula.

[0197] (Volume expansion rate) = (Volume after cycle) / (Volume before cycle) × 100 The evaluation results are shown in the "Volume expansion rate" column for "Laminated battery" in Table 3. (d) Cycle characteristics The cycle characteristics were evaluated by measuring and calculating the capacity retention rate when 500 charge-discharge cycles were performed using laminated batteries manufactured in the following examples and comparative examples.

[0198] First, the laminated battery was subjected to a current density of 0.3 mA / cm² in a constant temperature bath maintained at 25°C. ２ The conditioning process involved charging to a cutoff voltage of 4.2V, resting for 10 minutes, and then discharging to a cutoff voltage of 2.5V, repeating this cycle five times. After conditioning, the batteries were kept in a constant temperature bath at 45°C with a current density of 2.0 mA / cm². ２As a result, the battery was charged to a cutoff voltage of 4.2V, rested for 10 minutes, and then discharged to a cutoff voltage of 2.5V. This cycle was repeated 500 times. The capacity retention rate, which is the ratio of the discharge capacity after 500 cycles after conditioning to the discharge capacity after 1 cycle, was calculated and evaluated. The evaluation results are shown in Table 3, in the "Capacity Retention Rate" column for "Laminated Battery". (2) Manufacturing Conditions for Positive Electrode Active Material The manufacturing conditions and evaluation results for positive electrode active materials in the examples and comparative examples are described below. [Example 1] (Manufacturing of Positive Electrode Active Material) (First Mixing Step) A nickel composite oxide with a nickel-to-cobalt molar ratio of Ni:Co = 95:5 and lithium hydroxide were mixed to prepare a first raw material mixture. The nickel composite oxide was obtained by heat treatment of nickel composite hydroxide obtained using the crystallization method.

[0199] Lithium hydroxide was weighed out such that the ratio of the number of lithium (Li) atoms to the number of other metals (Me) contained in the first raw material mixture, i.e., was 1.01.

[0200] A shaker mixer (Willi e Bakkofen (WAB) model: TURBULA Type T2C) was used for mixing. (Castration process) The first raw material mixture obtained in the mixing process was calcined at 770°C under an oxygen atmosphere. (Grinding process) The calcined product obtained in the calcination process was ground using a jet mill (NPK, model: PJM100). The grinding conditions were selected in a prior test to ensure that the number ratio of particles consisting of primary particles was sufficiently high.

[0201] (Second mixing step) Boron oxide particles were added as a coating material to the calcined product (pulverized material) after the grinding step and mixed to prepare the second raw material mixture.

[0202] Boron oxide particles were added so that the ratio of boron to elements other than lithium and oxygen in the calcined material was 0.25 mol% in terms of molar amount. (Heat treatment process) The obtained second raw material mixture was heat-treated at 200°C for 15 hours under an oxygen atmosphere to obtain a positive electrode active material having lithium nickel composite oxide particles of Example 1 and a coating containing boron disposed on the surface of the lithium nickel composite oxide particles. Similarly, in the following other examples, the positive electrode active material has lithium nickel composite oxide particles and a coating containing boron.

[0203] The obtained positive electrode active material was evaluated. (Fabrication of secondary battery) A coin-type battery, which is a lithium-ion secondary battery, was fabricated according to the procedure shown in Figure 1. The charge / discharge capacity and reaction resistance of the obtained coin-type battery were also evaluated. The evaluation results are shown in Table 1. (Coin-type battery) As shown in Figure 1, the coin-type battery 10 has a case 11 and an electrode 12 housed inside the case 11.

[0204] The case 11 includes a hollow positive electrode can 111 with one end open, and a negative electrode can 112 positioned at the opening of the positive electrode can 111. The case 11 is shaped such that when the negative electrode can 112 is positioned at the opening of the positive electrode can 111, a space for housing the electrode 12 is formed between the negative electrode can 112 and the positive electrode can 111.

[0205] The electrode 12 has a positive electrode 121, a separator 122, and a negative electrode 123, which are stacked in this order, and is housed in the case 11 such that the positive electrode 121 is in contact with the inner surface of the positive electrode can 111 and the negative electrode 123 is in contact with the inner surface of the negative electrode can 112.

[0206] The case 11 is equipped with a gasket 113, which restricts relative movement between the positive electrode can 111 and the negative electrode can 112, maintaining a non-contact state, i.e., an electrically insulated state, and thus fixing them in place. The gasket 113 also has the function of sealing the gap between the positive electrode can 111 and the negative electrode can 112, thereby creating an airtight and liquid-tight barrier between the inside and outside of the case 11.

[0207] This coin-type battery 10 was manufactured using the following procedure. First, 52.5 mg of the obtained positive electrode active material, 15 mg of acetylene black, and 7.5 mg of polytetrafluoroethylene (PTFE) resin were mixed and pelletized to a size of approximately 75 mg with a diameter of 11 mm to produce the positive electrode 121. The prepared positive electrode was dried in a vacuum dryer at 100°C for 12 hours.

[0208] Using this positive electrode 121, negative electrode 123, separator 122, and electrolyte, a coin-type battery 10 was fabricated in a glove box with an Ar atmosphere where the dew point was controlled to -60°C.

[0209] The negative electrode 123 used lithium metal punched out in the shape of a 13 mm diameter disc.

[0210] A polyethylene porous membrane with a thickness of 25 μm was used for the separator 122. The electrolyte was 1 M LiClO ４ A mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1:1 (manufactured by Toyama Pharmaceutical Co., Ltd.) was used as the supporting electrolyte. (Laminated Battery) As shown in Figure 2, the laminated battery 20 has a structure in which an electrolyte is impregnated into a laminate of a positive electrode film 21, a separator 22, and a negative electrode film 23, and this laminate is sealed with a laminate 24. A positive electrode tab 25 is connected to the positive electrode film 21, and a negative electrode tab 26 is connected to the negative electrode film 23, and the positive electrode tab 25 and negative electrode tab 26 are exposed outside the laminate 24.

[0211] A slurry was prepared by dispersing 20.0 g of the obtained positive electrode active material, 2.35 g of acetylene black, and 1.18 g of polyvinylidene fluoride in N-methyl-2-pyrrolidone (NMP) and spreading it on aluminum foil to a depth of 1 cm. ２The positive electrode active material was coated so that 7.0 mg was present per aluminum foil. Next, the slurry containing the positive electrode active material was coated onto the 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 21 of the laminate-type battery 20.

[0212] Furthermore, a negative electrode film 23 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 about 20 μm and polyvinylidene fluoride. A polyethylene porous film with a thickness of 20 μm was used for the separator 22. 1 M LiPF was used as the electrolyte. ６ A mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7 (manufactured by Ube Industries, Ltd.) was used as the supporting electrolyte.

[0213] In a dry room controlled to a dew point of -60°C, the laminate of the positive electrode film 21, separator 22, and negative electrode film 23 was impregnated with an electrolyte, and then sealed with a laminate 24 to produce a laminate-type battery 20.

[0214] The evaluation results are shown in Tables 1 to 3. [Example 2] In the second mixing step, boron oxide particles were added so that the ratio of boron to elements other than lithium and oxygen in the calcined material was 0.50 mol% in terms of molar ratio. The positive electrode active material was manufactured under the same conditions and procedure as in Example 1, except for the above points.

[0215] Furthermore, coin-type and laminate-type lithium-ion secondary batteries were manufactured and evaluated under the same conditions as in Example 1, except for the use of the obtained positive electrode active material. The evaluation results are shown in Tables 1 to 3. [Example 3] In the second mixing step, boric acid particles were used instead of boron oxide particles for the coating material. The positive electrode active material was manufactured under the same conditions and procedure as in Example 1, except for the points mentioned above.

[0216] Furthermore, coin-type and laminate-type lithium-ion secondary batteries were manufactured and evaluated under the same conditions as in Example 1, except for the use of the obtained positive electrode active material. The evaluation results are shown in Tables 1 to 3. [Example 4] In the second mixing step, boric acid particles were used instead of boron oxide particles for the coating material. The positive electrode active material was manufactured under the same conditions and procedure as in Example 2, except for the points mentioned above.

[0217] Furthermore, coin-type and laminate-type lithium-ion secondary batteries were fabricated and evaluated under the same conditions as in Example 1, except that the obtained positive electrode active material was used. The evaluation results are shown in Tables 1 to 3. [Example 5] After the firing process, 100 g of the fired product obtained in the firing process was mixed with a 0.1 mol / L cobalt sulfate solution, stirred for 15 minutes, and then filtered and dried to produce a positive electrode active material in the same manner as in Example 2, except that it was washed with water and coated with cobalt.

[0218] Furthermore, coin-type and laminate-type lithium-ion secondary batteries were fabricated and evaluated under the same conditions as in Example 1, except for the use of the obtained positive electrode active material. The evaluation results are shown in Tables 1 to 3.

[0219] In the cross-section of the lithium nickel composite oxide particles contained in the positive electrode active material prepared in this example, line analysis was performed by SEM-EDS from the center toward the outer surface along the radius of the smallest inclusion circle of the lithium nickel composite oxide particles. As a result, it was confirmed that a cobalt coating layer was formed on the surface of the lithium nickel composite oxide particles, with a cobalt content that was 2% or more higher in terms of amount of substance compared to other parts. It was also confirmed that the cobalt coating layer and the boron-containing coating were arranged in order from the position closest to the lithium nickel composite oxide particles. [Example 6] In the first mixing step, titanium oxide and zirconium oxide were added so that the total amount of titanium and zirconium was 0.1 mol% and 0.1 mol%, respectively, respectively, but the positive electrode active material was prepared under the same conditions and procedures as in Example 2.

[0220] Furthermore, coin-type and laminate-type lithium-ion secondary batteries were fabricated and evaluated under the same conditions as in Example 1, except that the obtained positive electrode active material was used. The evaluation results are shown in Tables 1 to 3. [Example 7] In the first mixing step, titanium oxide, zirconium oxide, magnesium oxide, and ammonium fluoride were added so that the amounts of titanium, zirconium, magnesium, and fluorine were 0.1 mol% of the total amount of nickel and cobalt. The positive electrode active material was fabricated in the same manner as in Example 2.

[0221] Furthermore, coin-type and laminate-type lithium-ion secondary batteries were manufactured and evaluated under the same conditions as in Example 1, except that the obtained positive electrode active material was used. The evaluation results are shown in Tables 1 to 3. [Comparative Example 1] The positive electrode active material was manufactured under the same conditions and procedures as in Example 1, except that the second mixing step and heat treatment step were not performed. That is, the pulverized material obtained after the pulverization step was used as the positive electrode active material of Comparative Example 1. Therefore, the positive electrode active material of Comparative Example 1 does not have a coating.

[0222] Furthermore, coin-type and laminate-type lithium-ion secondary batteries were fabricated and evaluated under the same conditions as in Example 1, except for the use of the obtained positive electrode active material. The evaluation results are shown in Tables 1 to 3.

[0223]

[0224]

[0225] As shown in Table 3, Examples 1 to 7, which included the coating, demonstrated superior cycle characteristics when used in lithium-ion secondary batteries compared to Comparative Example 1.

[0226] According to the results shown in Table 2, the specific surface area in Examples 1 to 7 was 1.6 m². ２ The value was less than / g, confirming that the specific surface area had been reduced.

[0227] Furthermore, in Examples 1 to 7, it was confirmed that the volume expansion rate was reduced compared to Comparative Example 1. In other words, it was confirmed that gas generation was reduced in Examples 1 to 7.

[0228] In contrast, in Comparative Example 1, which does not contain a coating, it was confirmed that the specific surface area was larger and the cycle characteristics were lower compared to Examples 1 to 7. [Note] Examples of embodiments of the present disclosure are as follows.

[0229] <1> A positive electrode active material for a lithium-ion secondary battery, comprising: lithium nickel composite oxide particles having a hexagonal layered structure; and a coating containing boron disposed on at least a portion of the surface of the lithium nickel composite oxide particles, wherein the lithium nickel composite oxide particles include one or more types selected from particles consisting of primary particles and secondary particles formed by the aggregation of multiple primary particles; the photoelectron spectrum obtained by photoelectron spectroscopy analysis of the particle surface of the positive electrode active material for a lithium-ion secondary battery has a peak in the region of 190 eV to 194 eV; and the ratio of the amount of boron on the particle surface to the amount of elements other than lithium and oxygen contained in the positive electrode active material for a lithium-ion secondary battery on the particle surface, obtained by photoelectron spectroscopy analysis of the particle surface of the positive electrode active material for a lithium-ion secondary battery, is 0.25 or more. A positive electrode active material for a lithium-ion secondary battery, 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, as determined from the X-ray diffraction pattern of the lithium nickel composite oxide particles 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.

[0230] <2> The positive electrode active material for lithium-ion secondary batteries according to <1>, wherein the number ratio of particles consisting of the primary particles among the lithium nickel composite oxide particles is 60% or more.

[0231] <3> The lithium nickel composite oxide particles have a cobalt-containing cobalt coating layer on one or more locations selected from the surface of the particles made up of the primary particles and the surface of the secondary particles, and the cobalt coating layer and the coating are arranged in order from a position close to the lithium nickel composite oxide particles, as described in <1> or <2>, positive electrode active material for lithium-ion secondary battery.

[0232] <4> A positive electrode active material for a lithium-ion secondary battery according to any one of <1> to <3>, wherein the volume average particle size MV, determined from the volume-based particle size distribution by laser diffraction and scattering method, is 0.5 μm or more and 6.0 μm or less, and the particle size variation index [(D90 - D10) / MV] calculated by D90, which is the 90% diameter based on volume, D10, which is the 10% diameter based on volume, and the volume average particle size MV, is 0.40 or more and 1.2 or less.

[0233] <5> The specific surface area determined by the nitrogen adsorption method is 0.50 m². ２ / g or more 1.8m ２ A positive electrode active material for lithium-ion secondary batteries described in any of <1> to <4>, having a concentration of 1 / g or less.

[0234] <6> A positive electrode active material for a lithium-ion secondary battery according to any one of <1> to <5>, comprising lithium (Li), nickel (Ni), boron (B), and element M (M) in a molar ratio of Li:Ni:B:M = a:b:c:d (wherein element M is at least one element selected from Co, Mn, Al, W, Mo, Mg, Ca, Cr, Ta, Nb, Si, Ti, Zr, F, and a, b, c, d satisfying 0.92 ≤ a ≤ 1.20, 0.50 ≤ b < 1.00, 0.00 < c ≤ 0.05, 0.00 ≤ d ≤ 0.50, and b + c + d = 1).

[0235] <7> The positive electrode active material for lithium-ion secondary batteries according to <6>, wherein the element M contains titanium and zirconium.

[0236] <8> The positive electrode active material for lithium-ion secondary batteries as described in <7>, wherein the ratio of the titanium concentration Ti1 inside the primary particle to the titanium concentration Ti2 on the surface of the primary particle (Ti2 ÷ Ti1) is 0.8 or more and 2.0 or less, and the ratio of the zirconia concentration Zr1 inside the primary particle to the zirconia concentration Zr2 on the surface of the primary particle (Zr2 ÷ Zr1) is 1.5 or more.

[0237] <9> The positive electrode active material for lithium-ion secondary batteries according to any one of <6> to <8>, wherein the element M contains one or more selected from magnesium and fluorine.

[0238] <10> A positive electrode for a lithium-ion secondary battery containing the positive electrode active material for a lithium-ion secondary battery described in any of <1> to <9>.

[0239] <11> A lithium-ion secondary battery having 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 of <1> to <9>.

[0240] This application claims priority based on Japanese Patent Application No. 2025-057199, filed with the Japan Patent Office on 28 March 2025, and the entire contents of Japanese Patent Application No. 2025-057199 are incorporated herein by reference.

[0241] 10 Coin-type battery 11 Case 111 Positive electrode can 112 Negative electrode can 113 Gasket 12 Electrode 121 Positive electrode 122 Separator 123 Negative electrode 20 Laminate-type battery 21 Positive electrode film 22 Separator 23 Negative electrode film 24 Laminate 25 Positive electrode tab 26 Negative electrode tab 30 Positive electrode active material 31 Lithium nickel composite oxide particles 31A Outer surface 32 Coating 33 Cobalt coating layer O Center L Straight line

Claims

1. A positive electrode active material for a lithium-ion secondary battery, comprising: lithium nickel composite oxide particles having a hexagonal layered structure; and a coating containing boron disposed on at least a portion of the surface of the lithium nickel composite oxide particles, wherein the lithium nickel composite oxide particles include one or more types selected from particles consisting of primary particles and secondary particles formed by the aggregation of multiple primary particles; the photoelectron spectrum obtained by photoelectron spectroscopy analysis of the particle surface of the positive electrode active material for a lithium-ion secondary battery has a peak in the region of 190 eV to 194 eV; and the ratio of the amount of boron on the particle surface to the amount of elements other than lithium and oxygen contained in the positive electrode active material for a lithium-ion secondary battery on the particle surface, obtained by photoelectron spectroscopy analysis of the particle surface of the positive electrode active material for a lithium-ion secondary battery, is 0.25 or more. A positive electrode active material for a lithium-ion secondary battery, 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, as determined from the X-ray diffraction pattern of the lithium nickel composite oxide particles 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.

2. The positive electrode active material for a lithium-ion secondary battery according to claim 1, wherein the number ratio of particles consisting of the primary particles among the lithium nickel composite oxide particles is 60% or more.

3. The lithium nickel composite oxide particles have a cobalt-containing cobalt coating layer on one or more selected locations on the surface of the particles made up of the primary particles and on the surface of the secondary particles, and the cobalt coating layer and the coating are arranged in order from a position close to the lithium nickel composite oxide particles, as a positive electrode active material for a lithium-ion secondary battery according to claim 1 or 2.

4. A positive electrode active material for a lithium-ion secondary battery according to any one of claims 1 to 3, wherein the volume-average particle size MV, determined from the volume-based particle size distribution by laser diffraction and scattering method, is 0.5 μm or more and 6.0 μm or less, and the particle size variation index [(D90 - D10) / MV] calculated by D90, which is the 90% diameter based on volume, D10, which is the 10% diameter based on volume, and the volume-average particle size MV, is 0.40 or more and 1.2 or less.

5. The specific surface area determined by the nitrogen adsorption method is 0.50 m². ２ / g or more 1.8m ２ A positive electrode active material for a lithium-ion secondary battery according to any one of claims 1 to 4, wherein the amount is less than or equal to / g.

6. A positive electrode active material for a lithium-ion secondary battery according to any one of claims 1 to 5, comprising lithium (Li), nickel (Ni), boron (B), and element M (M) in a molar ratio of Li:Ni:B:M = a:b:c:d (wherein element M is at least one element selected from Co, Mn, Al, W, Mo, Mg, Ca, Cr, Ta, Nb, Si, Ti, Zr, F, and a, b, c, d satisfying 0.92 ≤ a ≤ 1.20, 0.50 ≤ b < 1.00, 0.00 < c ≤ 0.05, 0.00 ≤ d ≤ 0.50, and b + c + d = 1).

7. The positive electrode active material for a lithium-ion secondary battery according to claim 6, wherein the element M contains titanium and zirconium.

8. The positive electrode active material for a lithium-ion secondary battery according to claim 7, wherein the ratio of the titanium concentration Ti1 inside the primary particle to the titanium concentration Ti2 on the surface of the primary particle (Ti2 ÷ Ti1) is 0.8 or more and 2.0 or less, and the ratio of the zirconia concentration Zr1 inside the primary particle to the zirconia concentration Zr2 on the surface of the primary particle (Zr2 ÷ Zr1) is 1.5 or more.

9. The positive electrode active material for a lithium-ion secondary battery according to any one of claims 6 to 8, wherein the element M contains one or more selected from magnesium and fluorine.

10. A positive electrode for a lithium-ion secondary battery comprising the positive electrode active material for a lithium-ion secondary battery described in any one of claims 1 to 9.

11. A lithium-ion secondary battery having a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains a positive electrode active material for a lithium-ion secondary battery as described in any one of claims 1 to 9.

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