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

Abstract

Disclosed is a positive electrode active material for lithium ion secondary batteries, the positive electrode active material containing a lithium nickel composite oxide that has a hexagonal layered structure. The lithium nickel composite oxide contains Li, Ni, and element M at an amount-of-substance ratio Li:Ni:M = a:b:c (wherein 0.90 ≤ a < 1.00, 0.80 ≤ b < 1.00, 0.00 < c ≤ 0.20, b + c = 1, and the element M is at least one element that is selected from the group consisting of Mn, Co, Al, Ti, Zr, W, Fe, Si, Nb, Mg, Ca, B, Na, K, Mo, Cu, V, P, and Ba). The nickel occupancy rate in the lithium site is 2.5% to 10.0% inclusive as obtained from the powder neutron diffraction pattern, and the half-value width of the diffraction peak of the (003) plane in the powder X-ray diffraction pattern is not less than 0.054° but less than 0.074°.

Description

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

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery 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 non-aqueous electrolyte secondary batteries that have high energy density and durability. There is also a strong demand for the development of high-power secondary batteries for use in power tools and electric vehicles, including hybrid vehicles.

[0003] Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are available as secondary batteries that meet these requirements. Lithium ion secondary batteries that use a lithium metal composite oxide having a layered or spinel crystal structure as a positive electrode active material can obtain a high voltage of 4 V or so, and are therefore being put into practical use as batteries with high energy density.

[0004] As the lithium metal composite oxide, lithium cobalt composite oxide (LiCoO ２ ), and lithium nickel composite oxide (LiNiO ２ ), lithium nickel cobalt manganese composite oxide (LiNi １／３ Co １／３ Mn １／３ O ２ ), lithium manganese composite oxide (LiMn ２ O ４ ), lithium nickel manganese composite oxide (LiNi ０．５ Mn ０．５ O ２ ) have been proposed.

[0005] In recent years, further improvements in the battery characteristics of lithium-ion secondary batteries have been demanded, and various studies have been conducted.

[0006] For example, Patent Documents 1 to 5 disclose that by controlling the particle characteristics of a lithium composite oxide, it is possible to improve the cycle characteristics, which is the capacity retention rate when repeatedly charged and discharged, of a lithium ion secondary battery using the lithium composite oxide.

[0007] Japanese Patent Publication No. 2001-243949 Japanese Patent Publication No. 2004-355824 Japanese Patent Publication No. 2017-188444 Japanese Patent Publication No. 2017-188445 International Publication No. 2017 / 169129

[0008] However, as lithium ion secondary batteries are used in various applications as described above, further improvements in performance are also being demanded, and therefore, there is a demand for positive electrode active materials for lithium ion secondary batteries to have improved performance, such as cycle characteristics, when used in lithium ion secondary batteries, by methods other than those conventionally used.

[0009] In view of the problems of the above-described conventional techniques, an object of one aspect of the present invention is to provide a positive electrode active material for a lithium ion secondary battery that has excellent cycle characteristics when used in the lithium ion secondary battery.

[0010] According to one aspect of the present invention for achieving the above object, there is provided a positive electrode active material for a lithium ion secondary battery, comprising a lithium nickel composite oxide having a hexagonal layered structure, wherein the lithium nickel composite oxide contains lithium (Li), nickel (Ni), and an element M (M) in a ratio of Li:Ni:M=a:b:c (where 0.90≦a<1.00, 0.80≦b<1.00, 0.00<c≦0.20, b+c=1, and the element M is at least one element selected from the group consisting of Mn, Co, Al, Ti, Zr, W, Fe, Si, Nb, Mg, Ca, B, Na, K, Mo, Cu, V, P, and Ba), and wherein the occupancy rate of nickel present in lithium sites (3b sites) is 2.5% or more and 10.0% or less, as obtained by analyzing a powder neutron diffraction pattern of the lithium nickel composite oxide by the Rietveld method. The lithium nickel composite oxide has a powder X-ray diffraction pattern measured using Cu-Kα radiation as an X-ray source, and the half-width of the diffraction peak for the (003) plane is 0.054° or more and less than 0.074°.

[0011] According to one embodiment of the present invention, it is possible to provide a positive electrode active material for a lithium ion secondary battery that has excellent cycle characteristics when used in the lithium ion secondary battery.

[0012] Fig. 1 shows a flow chart of a method for producing a positive electrode active material. Fig. 2 is a schematic diagram of a coin-type battery used in battery evaluation. Fig. 3 is a schematic diagram of a laminate-type battery used in battery evaluation. Fig. 4 shows a powder neutron diffraction pattern of the lithium nickel composite oxide obtained in Example 1 and a simulation pattern by Rietveld analysis.

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments, and various modifications and substitutions can be made to the following embodiments without departing from the scope of the present invention. [Positive Electrode Active Material for Lithium-Ion Secondary Batteries] Hereinafter, the positive electrode active material for lithium-ion secondary batteries (hereinafter also simply referred to as "positive electrode active material") of this embodiment will be described. (1) Regarding Lithium Nickel Composite Oxide The positive electrode active material of this embodiment contains lithium nickel composite oxide. The positive electrode active material of this embodiment may be composed only of lithium nickel composite oxide, but even in this case, this does not exclude the inclusion of inevitable impurities that are mixed in during the manufacturing process, etc.

[0014] The lithium nickel composite oxide can have a hexagonal layer structure. (1-1) Particle Morphology The particle morphology of the lithium nickel composite oxide is not particularly limited. For example, it can contain at least one of a single primary particle and a secondary particle formed by aggregation of a plurality of primary particles. Therefore, the lithium nickel composite oxide can contain both a single primary particle and the above-mentioned secondary particles. Furthermore, the lithium nickel composite oxide may be composed of only a single primary particle or only secondary particles.

[0015] The lithium nickel composite oxide preferably contains a single primary particle, and the proportion of the particles in the number thereof is preferably high. This is because, when the lithium nickel composite oxide contains a single primary particle and the proportion of the particles in the number thereof is high, the cycle characteristics, which are the capacity retention rate during repeated charge and discharge, can be particularly improved when the positive electrode active material of this embodiment is used in a secondary battery. This is thought to be because, when the lithium nickel composite oxide contains a single primary particle and the proportion of the particles in the number thereof is high, particle collapse during repeated charge and discharge can be suppressed, and a large number of particles that contribute to charge and discharge can be maintained.

[0016] Preferably, the number ratio of single primary particles to the total particles of the lithium nickel composite oxide is 30% or more, and more preferably 50% or more. The lithium nickel composite oxide may have a number ratio of single primary particles of 100%, i.e., may be composed of only single primary particles.

[0017] Furthermore, when the lithium nickel composite oxide contains secondary particles, it is preferable that the number of primary particles constituting the contained secondary particles is small. Specifically, the number of primary particles constituting the secondary particles is preferably 15 or less, and more preferably 10 or less. By reducing the number of primary particles constituting the secondary particles contained in the lithium nickel composite oxide, when the positive electrode active material of this embodiment is used in a lithium ion secondary battery (hereinafter also simply referred to as "secondary battery"), it is possible to particularly improve the cycle characteristics, which are the capacity retention rate when repeatedly charged and discharged.

[0018] Whether the particles of the lithium nickel composite oxide contain primary particles, secondary particles, or both can be roughly confirmed by a scanning electron microscope (SEM) image of the particles (hereinafter also referred to as an "SEM image"), but this may lack accuracy. In the positive electrode active material according to this embodiment, it is preferable to process the cross section of the particles and use the SEM image to determine whether or not the particles contain primary particles and secondary particles.

[0019] Specifically, for example, it is preferable to embed the lithium nickel composite oxide in a resin, and then perform cross-section polishing (CP) processing or the like to make the particle cross-section observable, and then observe the SEM image to determine the presence or absence of primary particles and secondary particles. For the determination, it is preferable to perform the above evaluation on 50 or more particles. There is no particular upper limit on the number of particles to be evaluated, but considering the productivity of the evaluation, it is preferable to perform the evaluation on 200 or fewer particles. From the above evaluation results, the number ratio of primary particles described above can also be calculated.

[0020] Furthermore, the band contrast of the electron backscatter diffraction (hereinafter also referred to as "EBSD") method can be measured, and when a focused ion beam (FIB) processing device is used for cross-section processing, the particle cross-section can also be confirmed using an attached scanning ion microscope (SIM) observation image. (1-2) Constituent Elements The lithium nickel composite oxide contains lithium (Li), nickel (Ni), and element M (M). The element M can include at least one element selected from the group consisting of Mn, Co, Al, Ti, Zr, W, Fe, Si, Nb, Mg, Ca, B, Na, K, Mo, Cu, V, P, and Ba. Small amounts of elements other than the above elements may also be contained.

[0021] The lithium nickel composite oxide can contain lithium (Li), nickel (Ni), and element M (M), which are elements excluding oxygen, in a ratio of the amounts of substances (molar ratio) of Li:Ni:M=a:b:c.

[0022] The above a, b, and c satisfy 0.90≦a<1.00, 0.80≦b<1.00, 0.00<c≦0.20, and b+c=1.

[0023] The lithium nickel composite oxide is Li ａ Ni ｂ M ｃ O ２＋α It is preferable that α satisfies −0.2≦α≦0.2. Explanation of a, b, c, and element M has already been given, so will be omitted.

[0024] Each element will be described below. (Lithium) In the above substance amount ratios, the value of a, which indicates the substance amount ratio of Li, corresponds to the substance amount ratio (Li / Me ratio) of lithium (Li) to an element other than lithium and oxygen (Me). The element other than lithium and oxygen (Me) corresponds to Ni and element M. In the above substance amount ratios, the range of a is 0.90≦a<1.00. In other words, it is preferable that there is more lithium deficiency than in the case of a=1.00, which is the stoichiometric ratio of lithium nickel composite oxide.

[0025] By setting the value of a to 0.90 or more, the reaction resistance can be suppressed and the battery output can be increased. By setting the value of a to less than 1.00, the amount of lithium compound attached to the particle surface of the positive electrode active material of this embodiment, i.e., the amount of alkali, can be suppressed. Therefore, when the positive electrode active material of this embodiment is applied to a secondary battery, gas generation during the battery reaction can be suppressed.

[0026] The above-mentioned a preferably satisfies 0.92≦a<0.99. (Nickel) In the above substance amount ratio, the range of b, which indicates the substance amount ratio of Ni, is 0.80≦b<1.00. When the value of b is within the above range, high battery capacity can be achieved when the positive electrode active material of this embodiment is applied to a secondary battery. (Element M) In the above substance amount ratio, the range of c, which indicates the substance amount ratio of element M, is 0.00<c≦0.20, preferably 0.02≦c≦0.20, and more preferably 0.05≦c≦0.20. Note that the lithium nickel composite oxide may contain multiple types of element M. In this case, it is preferable that the total substance amount ratio of element M contained in the lithium nickel composite oxide satisfies the above-mentioned range. Candidate elements for element M have already been described, so description thereof will be omitted. The type of element M contained in the lithium nickel composite oxide can be appropriately selected depending on the required battery characteristics, etc.

[0027] For example, element M may include Co. If the substance amount ratio of Co contained in c in the above substance amount ratio is c1, the range of c1 is preferably 0<c1≦0.20, more preferably 0.02≦c1≦0.18, and even more preferably 0.03≦c1≦0.15. When the value of c1 is within the above range, high thermal stability and output characteristics can be exhibited when the positive electrode active material of this embodiment is applied to a secondary battery. Note that when only Co is contained as element M, c=c1. Furthermore, as described above, it is preferable that the total substance amount of element M satisfies the above-mentioned range of c. Therefore, for example, it is preferable that c1+c2+c3, which is the sum of c2 and c3 described below, satisfies the above-mentioned suitable range of c.

[0028] Furthermore, the element M may contain Mn. When the substance amount ratio of Mn contained in c in the substance amount ratio is c2, the range of c2 is preferably 0<c2≦0.20, more preferably 0.02≦c2≦0.20, and even more preferably 0.05≦c2≦0.20. When c2 is within the above range, thermal stability can be improved when the positive electrode active material of this embodiment is applied to a secondary battery. Note that when only Mn is contained as the element M, c=c2.

[0029] The element M may contain Al. When the substance amount ratio of Al contained in c of the substance amount ratio is c3, the range of c3 is preferably 0<c3≦0.10, more preferably 0.01≦c3≦0.08, and even more preferably 0.01≦c3≦0.06. When c3 is within the above range, thermal stability can be improved when the positive electrode active material of this embodiment is applied to a secondary battery. Note that when only Al is contained as the element M, c=c3.

[0030] The composition of the lithium nickel composite oxide can be measured by quantitative analysis using inductively coupled plasma (ICP) atomic emission spectrometry. (1-3) Occupancy of nickel in lithium sites (3b sites) The lithium nickel composite oxide contained in the positive electrode active material according to this embodiment has an occupancy of nickel (Ni) in the lithium sites (3b sites), which is obtained by analyzing a powder neutron diffraction pattern measured by powder neutron diffraction using the Rietveld method, of preferably 2.5% or more and 10.0% or less, more preferably 2.6% or more and 8.0% or less, and even more preferably 2.7% or more and 7.0% or less.

[0031] The occupancy rate of nickel in the lithium sites (3b sites) determined from the powder neutron diffraction pattern will hereinafter also be expressed as "Ni(Li(3b))".

[0032] When a secondary battery is repeatedly charged and discharged, lithium is repeatedly inserted into and extracted from the interlayer spaces of the lithium-nickel composite oxide. However, when the secondary battery is repeatedly charged and discharged, the interlayer spaces repeatedly expand and contract as lithium is inserted and extracted, which changes the crystal structure of the lithium-nickel composite oxide and produces lithium that does not contribute to charge and discharge. This is thought to result in a deterioration of the cycle characteristics.

[0033] In contrast, in the lithium nickel composite oxide contained in the positive electrode active material of this embodiment, nickel occupies some of the lithium sites. Therefore, even when lithium is desorbed from between the layers of the lithium nickel composite oxide, nickel remains between the layers. That is, it is believed that the nickel functions as a pillar between the layers, suppressing changes in the interlayer distance during repeated charge and discharge of the secondary battery. Therefore, even when the secondary battery is repeatedly charged and discharged, changes in the crystalline structure of the lithium nickel composite oxide can be suppressed, and the generation of lithium that does not contribute to charge and discharge can be suppressed. Therefore, the positive electrode active material of this embodiment can improve the cycle characteristics when used in a secondary battery by a method not previously used, namely, controlling the occupancy rate of nickel in the lithium sites.

[0034] Specifically, by setting the Ni(Li(3b)) content to 2.5% or more, it is believed that the nickel can fully function as interlayer pillars, stabilizing the crystal structure during charge and discharge and particularly improving cycle characteristics. Furthermore, by setting the Ni(Li(3b)) content to 10.0% or less, it is possible to suppress a decrease in the perfection of the crystals of the lithium nickel composite oxide. Therefore, it is possible to ensure a sufficient amount of lithium that contributes to charge and discharge, suppress reaction resistance, and sufficiently increase battery capacity and output.

[0035] When calculating the Ni(Li(3b)), it is also possible to use an X-ray diffraction pattern in Rietveld analysis. However, the lithium nickel composite oxide contained in the positive electrode active material of this embodiment can contain, in addition to nickel, elements such as cobalt and manganese as the element M, which are difficult to identify with X-rays. Furthermore, it is difficult to accurately evaluate light elements such as lithium, hydrogen, and oxygen with X-rays. For this reason, when using an X-ray diffraction pattern, it is generally difficult to calculate an accurate value for Ni(Li(3b)).

[0036] In contrast, the use of neutron beams allows for more accurate evaluation of the elements contained in the lithium nickel composite oxide than when an X-ray diffraction pattern is used. Therefore, when calculating the Ni(Li(3b)), it is necessary to use a powder neutron diffraction pattern. (1-4) Half Width of Peak of (003) Plane The lithium nickel composite oxide contained in the positive electrode active material of this embodiment has a diffraction peak assigned to the (003) plane in a powder X-ray diffraction pattern measured by powder X-ray diffractometry (hereinafter also referred to as "XRD") using Cu-Kα radiation as the X-ray source. The lithium nickel composite oxide contained in the positive electrode active material according to this embodiment preferably has a half width of the diffraction peak of the (003) plane in the X-ray diffraction pattern of 0.054° or more and less than 0.074°. The half width of the diffraction peak of the (003) plane refers to the width of the profile at half the intensity value of the diffraction peak assigned to the (003) plane.

[0037] The half-width of the diffraction peak of the (003) plane is an index for determining the crystallinity of the lithium nickel composite oxide. By setting the half-width of the diffraction peak of the (003) plane to less than 0.074°, a lithium nickel composite oxide with particularly excellent crystallinity can be obtained. Therefore, when the positive electrode active material of this embodiment containing the lithium nickel composite oxide is applied to a secondary battery, the battery capacity can be increased, resulting in a secondary battery with excellent battery capacity and cycle characteristics. However, even if the half-width of the diffraction peak of the (003) plane is excessively small, the effect of increasing the battery capacity saturates, so the half-width of the diffraction peak of the (003) plane is preferably 0.054° or greater.

[0038] The X-ray diffraction pattern is preferably measured using an XRD diffractometer equipped with a Bragg-Brentano optical system and a flat sample holder. (1-5) Median Diameter (D50) The median diameter (D50) of the volume-based particle size distribution of the positive electrode active material according to this embodiment measured by a laser diffraction / scattering method is preferably 0.5 μm or more and 20 μm or less, more preferably 1.0 μm or more and 10 μm or less, even more preferably 2.0 μm or more and 8.0 μm or less, and particularly preferably 2.0 μm or more and 7.0 μm or less.

[0039] By setting the median diameter of the positive electrode active material of this embodiment to 20 μm or less, the contact area with the electrolyte can be sufficiently increased when applied to a secondary battery, thereby increasing the battery capacity. Furthermore, by setting the median diameter of the positive electrode active material of this embodiment to 0.5 μm or more, the handleability during electrode fabrication can be improved.

[0040] The median diameter (D50) can be determined, for example, from the integrated volume value measured using a laser light diffraction / scattering particle size distribution analyzer. (1-6) Average Primary Particle Diameter The average primary particle diameter of the lithium nickel composite oxide is preferably greater than 0.5 μm and not greater than 6.0 μm. Increasing the average primary particle diameter of the lithium nickel composite oxide to greater than 0.5 μm means that the size of the primary particles and the crystallites constituting the primary particles can be sufficiently large, thereby improving the crystallinity of the lithium nickel composite oxide. Therefore, when the positive electrode active material of this embodiment containing the lithium nickel composite oxide is applied to a secondary battery, the battery capacity can be increased, resulting in a secondary battery with excellent battery capacity and cycle characteristics.

[0041] However, even if the average primary particle size of the lithium nickel composite oxide is excessively large, the effect of increasing the battery capacity saturates, and therefore the average primary particle size of the lithium nickel composite oxide is preferably 6.0 μm or less.

[0042] The average primary particle diameter of the lithium nickel composite oxide is determined by selecting primary particles of the lithium nickel composite oxide that can be observed in their entirety using a scanning electron microscope (SEM), measuring their major axis lengths, and averaging them. When calculating the average primary particle diameter, there is no particular limitation on the number of primary particles to be evaluated.

[0043] For example, when the lithium nickel composite oxide contains secondary particles, it is preferable to select a total of 20 to 30 primary particles from each of 10 to 20 secondary particles, and measure the major axis length of the selected primary particles.

[0044] Furthermore, when the lithium nickel composite oxide contains single primary particles that do not constitute secondary particles, it is preferable to evaluate five or more primary particles of the lithium nickel composite oxide. There is no particular upper limit on the number of primary particles to be evaluated, but it is preferable to evaluate, for example, 20 or fewer.

[0045] When the lithium nickel composite oxide contains only secondary particles or only single primary particles, the above number of primary particles can be selected for the secondary particles or single primary particles. The lengths of the major axes of the selected primary particles can then be measured, and the average value can be used as the average primary particle diameter of the lithium nickel composite oxide.

[0046] When the lithium nickel composite oxide contains both secondary particles and single primary particles, the above-mentioned number of primary particles can be selected for each of the secondary particles and the single primary particles. The major axis lengths of all the selected primary particles can then be measured, and the average value can be used as the average primary particle diameter of the lithium nickel composite oxide.

[0047] By setting the median diameter (D50) of the positive electrode active material within the above-mentioned range and the average primary particle diameter of the lithium nickel composite oxide contained therein within the above-mentioned range, when the positive electrode active material is applied to a secondary battery, it is possible to particularly improve both the battery capacity and the cycle characteristics, and to achieve both. (1-7) Amount of Eluted Lithium From the viewpoint of suppressing gas generation in a secondary battery, in the positive electrode active material of this embodiment, the amount of lithium eluted into water when the positive electrode active material is immersed in water (hereinafter also referred to as the "amount of eluted lithium") is preferably 0.15% by mass or less, and more preferably 0.10% by mass or less, based on the total amount of the positive electrode active material.

[0048] By setting the amount of eluted lithium within the above range, gas generation in the secondary battery can be suppressed.

[0049] The lower limit of the amount of eluted lithium from the positive electrode active material of this embodiment is not particularly limited, but is preferably, for example, 0.05 mass % or more.

[0050] When the amount of eluted lithium in the positive electrode active material is particularly suppressed, the lithium nickel composite oxide preferably contains Zr as the element M. Furthermore, the amount of eluted lithium can also be suppressed by subjecting the positive electrode active material to a water washing treatment. [Method for producing a positive electrode active material for a lithium ion secondary battery] The method for producing the positive electrode active material for a lithium ion secondary battery is not particularly limited as long as a positive electrode active material having the above-described properties can be obtained. Hereinafter, an example of the method for producing a positive electrode active material according to this embodiment will be described.

[0051] As described above, according to the method for producing a positive electrode active material of this embodiment, the positive electrode active material already described can be produced, and therefore, some of the matters already described will not be described again.

[0052] The method for producing a positive electrode active material of this embodiment can be carried out, for example, according to flow 10 shown in Fig. 1. That is, the method for producing a positive electrode active material of this embodiment can include a mixing step (S1) and a firing step (S2).

[0053] In the mixing step (S1), a nickel composite compound and a lithium compound are mixed to obtain a raw material mixture.

[0054] In the calcination step (S2), the raw material mixture is calcined to produce a lithium nickel composite oxide.

[0055] In the mixing step (S1), the element M may be added as a compound of the element M separately from the nickel composite compound, or may be added by using a nickel composite compound containing the element M.

[0056] Hereinafter, an example of the configuration of each step will be described. (1) Mixing step (S1) In the mixing step (S1), as described above, a nickel composite compound and a lithium compound are mixed to obtain a raw material mixture. In addition, in the mixing step, a compound of element M can also be added and mixed as necessary.

[0057] The nickel composite compound, the lithium compound, and the compound of element M, which is added as needed, can be added and mixed, for example, as powders (solid phases). Each of the raw materials used in the mixing step will be described below. (1-1) Raw Materials (Nickel Composite Compound) The nickel composite compound used in the mixing step (S1) can be obtained by a known method. The content ratio (composition ratio), which is the ratio of amounts of substance between Ni, an element in the nickel composite compound, and element M, such as Co, is substantially maintained in the lithium nickel composite oxide particles. Therefore, it is preferable that the content ratio of each element be in the same range as the content ratio in the lithium nickel composite oxide described above. Therefore, it is preferable that the nickel composite compound contains nickel (Ni) and element M (M) in a ratio of Ni:M = b:c, in terms of the ratio of amounts of substance. Since b, c, and element M have been described in the lithium nickel composite oxide, their description will be omitted.

[0058] The nickel composite compound used in this embodiment may contain small amounts of elements other than the above-mentioned elements (element M such as Ni or Co) and oxygen, as long as the effects of the present invention are not impaired.

[0059] The nickel composite compound may be a hydroxide or an oxide. It may also be a mixture of a hydroxide and an oxide. Examples of methods for producing a nickel composite hydroxide include a method of neutralization crystallization using an aqueous solution of a metal salt and an alkaline solution. Alternatively, a part or all of the nickel composite hydroxide may be converted into a nickel composite oxide by heat-treating the nickel composite hydroxide to remove moisture from the nickel composite hydroxide.

[0060] In addition, when the nickel composite compound is a nickel composite hydroxide, Ni ｂ M ｃ (OH) ２＋β In addition, when the nickel composite compound is a nickel composite oxide, Ni ｂ M ｃ O １＋γIt is preferable that b, c, and element M are the same as those described for the lithium nickel composite oxide, and therefore their description will be omitted. It is preferable that β and γ satisfy -0.2≦β≦0.2 and -0.2≦γ≦0.2, respectively. (Lithium Compound) The lithium compound is not particularly limited, and known lithium-containing compounds can be used, and examples thereof include one or more compounds selected from lithium carbonate, lithium hydroxide, and lithium nitrate. The lithium compound may also be a mixture of two or more compounds selected from the above compounds.

[0061] The lithium compound is more preferably one or more selected from lithium carbonate and lithium hydroxide, from the viewpoint of being less affected by residual impurities and dissolving at the firing temperature. Furthermore, from the viewpoint of obtaining a lithium nickel composite oxide having particularly high crystallinity, the lithium compound is more preferably lithium hydroxide. (Compound of Element M) As described above, in the mixing step, a compound of element M can also be added and mixed as needed. The type of compound of element M is not particularly limited, but it is preferable to use one or more selected from hydroxides, oxides, chlorides, nitrates, sulfates, carbonates, etc. (1-2) Mixing Method There is no particular limit to the method for mixing the nickel composite compound, the lithium compound, and the compound of element M added as needed. It is preferable to mix these particles sufficiently to the extent that their particle shapes are not destroyed.

[0062] The mixing of raw materials such as nickel composite compounds can be carried out using, for example, a common mixer, such as a shaker mixer, a Lödige mixer, a Julia mixer, or a V-blender. Insufficient mixing can result in variations in the mass ratio (Li / Me) between lithium (Li) and elements other than lithium and oxygen (Me) between individual particles of the positive electrode active material, resulting in problems such as insufficient battery performance. Therefore, thorough mixing is preferred. As mentioned above, elements other than lithium and oxygen (Me) correspond to Ni and element M. (1-3) Mixing Ratio The lithium compounds are preferably mixed so that the Li / Me ratio in the mixture obtained in the mixing step is 0.90 or more and less than 1.00. In other words, it is preferred to mix the raw material mixture so that the Li / Me ratio is the same as the Li / Me ratio in the resulting fired product. This is because the Li / Me ratio and the ratio of the amounts of substances of each element for lithium, nickel, and element M do not change before and after the calcination step (S2), so the Li / Me ratio of the raw material mixture in the mixing step (S1) becomes equal to the Li / Me ratio of the calcined product. The contents (ratios) of the elements in the raw material mixture are almost maintained in the lithium nickel composite oxide. (2) Calcination Step (S2) The calcination step (S2) is a step in which the raw material mixture obtained in the mixing step (S1) is calcined to obtain a calcined product containing lithium nickel composite oxide. When the raw material mixture is calcined, lithium in the lithium compound diffuses into the nickel composite compound, forming a lithium nickel composite oxide. The lithium compound melts at the temperature during calcination and penetrates into the nickel composite compound, forming a calcined product of lithium nickel composite oxide.

[0063] The firing conditions in the firing step (S2) are not particularly limited and can be selected so that the lithium nickel composite oxide has a desired composition and properties such as the nickel occupancy rate of the lithium sites (3b sites) fall within a desired range. Preferred examples of firing conditions are described below. (Atmosphere) The firing atmosphere is preferably an oxidizing atmosphere. The oxidizing atmosphere preferably has an oxygen concentration of 80% by volume or more, more preferably 85% by volume or more. The oxidizing atmosphere can also be an oxygen atmosphere, so the oxygen concentration can be 100% by volume or less. (Firing Temperature) The firing is preferably carried out in two steps. That is, it is preferable to heat the mixture to the first firing temperature and maintain it there (first firing step), then heat it to the second firing temperature and maintain it there (second firing step), and then cool it.

[0064] For example, the first firing temperature, which is the temperature for the first stage of firing, is preferably 400° C. or higher and 600° C. or lower, and the holding time at the first firing temperature is preferably 5 hours or longer.

[0065] The second firing temperature, which is the second stage firing temperature, is preferably higher than 770° C. and not higher than 950° C., and the holding time at the second firing temperature is preferably 10 hours or more.

[0066] When calcined using the above temperature pattern, the crystallinity of the lithium-nickel composite oxide is high even when the lithium deficiency is greater than the stoichiometric ratio, making it possible to obtain a positive electrode active material with a high battery capacity. Furthermore, the occupancy rate of nickel in the lithium sites (3b sites) can be easily controlled within the aforementioned range. Furthermore, the crystallinity of the resulting lithium-nickel composite oxide is particularly enhanced. Note that, depending on the lithium and nickel content in the target composition of the lithium-nickel composite oxide, the calcination conditions, such as the calcination temperature, may vary in order to achieve the desired range of nickel occupancy rate in the lithium sites (3b sites). Therefore, it is preferable to select an optimal calcination temperature within the above temperature range and select temperature conditions, such as the heating rate and holding time, depending on the target composition.

[0067] When a two-stage firing process is performed, in the first firing process, which is a first-stage firing process at a first firing temperature, the lithium compound is melted and sufficiently diffused into the nickel composite compound, so that the lithium and the nickel composite compound can react with each other. Then, in the second firing process, which is a second-stage firing process at a second firing temperature, crystal growth can be promoted.

[0068] The method for producing a positive electrode active material of this embodiment may include any step other than the mixing step and the calcination step. (3) Crushing Step The calcined product obtained in the calcination step (S2) may also be used as the positive electrode active material of this embodiment. However, although sintering between particles is suppressed in the calcined product obtained after the calcination step (S2), coarse particles may be formed due to weak sintering or aggregation. In such cases, the method for producing a positive electrode active material of this embodiment may also include a crushing step of crushing the calcined product. Crushing the calcined product can eliminate the sintering and aggregation and adjust the particle size distribution. (4) Pulverization Step The method for producing a positive electrode active material of this embodiment may also include a crushing step of crushing the calcined product or the crushed powder after the calcination step (S2) or the crushing step.

[0069] The method for producing a positive electrode active material according to this embodiment includes a pulverization step, which allows secondary particles to be converted into primary particles, thereby making it possible to obtain a lithium nickel composite oxide mainly composed of primary particles.

[0070] The primary particles obtained after the pulverization step may be used alone or in combination with agglomerated particles.

[0071] After the crushing step or the pulverizing step, sieving may be performed to select particles of a desired particle size. (5) Water-Washing Step The method for producing a positive electrode active material of the present embodiment may also include a water-washing step in which the fired product obtained in the firing step (S2) is washed with water.

[0072] By carrying out the water washing step, excess lithium remaining on the particle surfaces of the lithium nickel composite oxide can be removed.

[0073] In the water washing step, the fired product is mixed with water to form a slurry, which can be washed with water (slurrying step).

[0074] The water used to form a slurry in the water washing step is not particularly limited, but it is preferable to use water having an electrical conductivity of less than 10 μS / cm, and more preferably 1 μS / cm or less.

[0075] It is preferable to stir the prepared slurry during the water washing.

[0076] After the water-washing step, the slurry is subjected to solid-liquid separation, i.e., filtration and dehydration to obtain a water-washed powder (solid-liquid separation step). The apparatus used for filtration and dehydration is not particularly limited, and for example, a centrifugal separator or a filter press type solid-liquid separator can be used.

[0077] In the water-washing step, the water-washed powder containing water obtained after solid-liquid separation is preferably dried (drying step). The drying conditions are not particularly limited, but it is preferable to perform the drying at a temperature of 100° C. or higher and 350° C. or lower in an oxidizing atmosphere or a vacuum atmosphere, for example.

[0078] In addition, by washing with water the calcined product of the lithium nickel composite oxide produced under the aforementioned conditions of Li / Me of 1.00 or more in the mixing step, it is possible to obtain a lithium nickel composite oxide having a Li / Me of 0.90 or more and less than 1.00. However, according to the investigations of the inventors of the present invention, in the above case, it is not possible to obtain a positive electrode active material that satisfies the requirements of the positive electrode active material of this embodiment in terms of the occupancy rate of nickel in the lithium site (3b site), etc., and the effects of the positive electrode active material of this embodiment cannot be obtained. [Lithium Ion Secondary Battery] The lithium ion secondary battery of this embodiment includes at least a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the positive electrode can contain the aforementioned positive electrode active material for lithium ion secondary batteries.

[0079] Below, one example of the configuration of the secondary battery of this embodiment will be described for each component. The secondary battery of this embodiment includes, for example, a positive electrode, a negative electrode, and a non-aqueous electrolyte, and is composed of the same components as a general lithium-ion secondary battery. Note that the embodiment described below is merely an example, and the lithium-ion secondary battery of this embodiment can be implemented in various forms, including the following embodiment, with various modifications and improvements based on the knowledge of those skilled in the art. Furthermore, the use of the secondary battery is not particularly limited. (Positive Electrode) The positive electrode of the secondary battery of this embodiment can include the positive electrode active material described above.

[0080] An example of a method for manufacturing a positive electrode is described below. First, the above-described positive electrode active material (powder), a conductive material, and a binder are mixed to form a positive electrode mixture, and activated carbon or a solvent for viscosity adjustment or the like is added as needed. The mixture is then kneaded to form a positive electrode mixture paste.

[0081] The mixing ratio of each material in the positive electrode mixture is a factor that determines the performance of the lithium-ion secondary battery, and can be adjusted depending on the application. The mixing ratio of the materials can be the same as that of the positive electrodes of known lithium-ion secondary batteries. For example, when the total mass of the solid content of the positive electrode mixture excluding the solvent is taken as 100 mass%, the positive electrode mixture can contain 60 mass% to 95 mass% of the positive electrode active material, 1 mass% to 20 mass% of the conductive material, and 1 mass% to 20 mass% of the binder.

[0082] The resulting positive electrode composite paste is applied to the surface of, for example, an aluminum foil current collector, and dried to remove the solvent, producing a sheet-like positive electrode. If necessary, pressure can be applied 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 depending on the desired battery and used to produce the battery.

[0083] Examples of the conductive material that can be used include graphite (natural graphite, artificial graphite, expanded graphite, etc.) and carbon black materials such as acetylene black and Ketjen Black (registered trademark).

[0084] The binder serves to bind the active material particles together, and may be, for example, one or more selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose-based resin, polyacrylic acid, etc.

[0085] If necessary, a solvent for dispersing the positive electrode active material, conductive material, etc. and dissolving the binder can be added to the positive electrode mixture. Specifically, an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent. Activated carbon can also be added to the positive electrode mixture to increase the electric double layer capacity.

[0086] The method for producing the positive electrode is not limited to the above-mentioned exemplary method, and other methods may also be used. For example, the positive electrode can be produced by press-molding a positive electrode composite and then drying it under a vacuum atmosphere. (Negative Electrode) The negative electrode can be made of metallic lithium, a lithium alloy, or the like. Alternatively, the negative electrode can be made by mixing a binder with a negative electrode active material capable of absorbing and desorbing lithium ions, adding an appropriate solvent to form a paste of the negative electrode composite, applying it to the surface of a metal foil current collector such as copper, drying it, and compressing it to increase the electrode density as needed.

[0087] Examples of the negative electrode active material include natural graphite, artificial graphite, and sintered organic compounds such as phenolic resins, as well as powdered carbonaceous materials such as coke. In this case, a fluorine-containing resin such as PVDF can be used as the negative electrode binder, as in the positive electrode. 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 sandwiched between the positive electrode and negative electrode as needed. The separator separates the positive electrode and negative electrode and retains the electrolyte. Known separators can be used, such as thin membranes of polyethylene or polypropylene with numerous micropores. (Non-aqueous Electrolyte) The non-aqueous electrolyte can be, for example, a non-aqueous electrolytic solution.

[0088] The non-aqueous electrolyte may be, for example, a solution of a lithium salt as a supporting salt dissolved in an organic solvent. Alternatively, the non-aqueous electrolyte may be a solution of a lithium salt dissolved in an ionic liquid. The ionic liquid is a salt that is liquid even at room temperature and is composed of cations and anions other than lithium ions.

[0089] The organic solvent may be selected from the group consisting of cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain 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 butane sultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate. One of these organic solvents may be used alone, or two or more of these may be used in combination.

[0090] The supporting salt is LiPF ６ , LiBF ４ , LiClO ４ , LiAsF ６ , LiN(CF ３ SO ２ ) ２ and complex salts thereof. The non-aqueous electrolyte solution may further contain a radical scavenger, a surfactant, a flame retardant, and the like.

[0091] Alternatively, the non-aqueous electrolyte may be a solid electrolyte, which has the property of being able to withstand high voltages. Examples of the solid electrolyte include inorganic solid electrolytes and organic solid electrolytes.

[0092] Examples of inorganic solid electrolytes include oxide-based solid electrolytes and sulfide-based solid electrolytes.

[0093] The oxide-based solid electrolyte is not particularly limited, and for example, an oxide-based solid electrolyte containing oxygen (O) and having lithium ion conductivity and electronic insulation can be suitably used. For example, lithium phosphate (Li ３ P.O.４ ) Li ３ PO ４ N Ｘ 、LiBO ２ N Ｘ 、LiNbO ３ 、LiTaO ３ 、Li ２ SiO ３ 、Li ４ SiO 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<00000​​​​​​​​​​​​

[0094] The sulfide-based solid electrolyte is not particularly limited, and for example, a sulfide-based solid electrolyte containing sulfur (S) and having lithium ion conductivity and electronic insulation can be suitably used. 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 P.O. ４ -Li ２ S-Si ２ S., Li. ３ P.O. ４ -Li ２ S-SiS ２ , LiPO ４ -Li ２ S-SiS, LiI-Li ２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５ One or more selected from the above can be used.

[0095] In addition, inorganic solid electrolytes other than those mentioned above may be used. For example, Li ３ N, LiI, Li ３ N-LiI-LiOH or the like may also be used.

[0096] The organic solid electrolyte is not particularly limited as long as it is a polymer compound exhibiting ion conductivity. For example, polyethylene oxide, polypropylene oxide, and copolymers thereof can be used. The organic solid electrolyte may also 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 or a laminated shape. Regardless of the shape, if the secondary battery of the present embodiment uses a nonaqueous electrolyte solution as the nonaqueous electrolyte, the positive electrode and the negative electrode can be laminated with a separator interposed therebetween to form an electrode assembly. The resulting electrode assembly can be impregnated with a nonaqueous electrolyte solution. The positive electrode current collector and a positive electrode terminal connected to the outside, and the negative electrode current collector and a negative electrode terminal connected to the outside can be connected using current collecting leads or the like, and the battery can be sealed in a battery case.

[0097] As described above, the secondary battery of this embodiment is not limited to a battery using a non-aqueous electrolytic solution as the non-aqueous electrolyte, and may be, for example, a secondary battery using a solid non-aqueous electrolyte, i.e., an all-solid-state battery. When an all-solid-state battery is used, the components other than the positive electrode active material may be changed as necessary.

[0098] The secondary battery of this embodiment can be used for various purposes. Since the secondary battery of this embodiment can be a high-capacity, high-output secondary battery, it is suitable, for example, as a power source for small portable electronic devices (such as notebook personal computers and mobile phone terminals) that always require high capacity, and is also suitable as a power source for electric vehicles that require high output.

[0099] Furthermore, since the secondary battery of this embodiment can be made smaller and have higher output, it is suitable as a power source for electric vehicles, which are subject to space restrictions. 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 use the secondary battery in combination with a combustion engine such as a gasoline engine or a diesel engine.

[0100] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0101] First, the evaluation methods for the positive electrode active materials and secondary batteries obtained in the following examples and comparative examples will be described. (Evaluation of Positive Electrode Active Materials) The obtained positive electrode active materials were evaluated as follows.

[0102] (a) Evaluation of Composition The composition of the positive electrode active material was analyzed by ICP atomic emission spectrometry. The evaluation results are shown in the "Composition" column of Table 1.

[0103] (b) Nickel Occupancy Rate of the Lithium Site (3b) Approximately 2 g of the positive electrode active material prepared in the following Examples and Comparative Examples was filled into a vanadium tube and then completely sealed. Neutron diffraction measurements were then performed using BL20 (Ibaraki Prefecture Materials Structure Analysis System, iMATERIA) at the Japan Proton Accelerator Research Complex (J-PARC) under conditions that yielded an intensity of 10,000 cps or more in double-frame mode.

[0104] The neutron diffraction pattern obtained from the backside detector bank was subjected to Rietveld analysis using the analysis software Z-Code. At this time, the space group was set to R-3m, with lithium and mixed nickel (nickel in the lithium site) at the 3b site and nickel and element M at the 3a site as chemical species. After refining the scale factor, lattice constant, and global parameters, the site occupancy and atomic coordinates were refined. The atomic displacement parameters for lithium were 0.9 to 1.0 Å. ２ , nickel and element M: 0.3 to 0.5 Å ２ , oxygen: 0.6 to 0.8 Å ２ The site occupancy and atomic coordinates were refined, and the reliability parameter S-value was considered to have converged when it was 3.0 or less. The nickel site occupancy of the resulting lithium site (3b site) was calculated, and the results are shown in the "Ni occupancy of 3b site" column of Table 1. The S-value, which is the reliability parameter obtained when Rietveld analysis was performed, is also shown in the "S-value" column of Table 1.

[0105] Powder neutron diffraction measurements can also be performed using a High Resolution Powder Diffractometer (HRPD) installed in the Japan Atomic Energy Agency's JRR-3 reactor or a Super HRPD at J-PARC.

[0106] (c) (003) Plane Peak Half-Width: X-ray diffraction patterns were measured using an XRD diffractometer (PANalytical, X'Pert PRO) with Cu-Kα radiation as the X-ray source. From the obtained X-ray diffraction patterns, the half-width, which is the width of the profile at half the maximum intensity, was calculated for the diffraction peak of the (003) plane of the hexagonal layered structure. The sample was filled into the groove of a circular glass holder with an outer diameter of 32 mm (flat sample holder, inner diameter 24 mm, groove depth 0.5 mm, PANalytical product number 9200 540 12391) and fixed in a specified sample holder (PANalytical product number 9430 018 13321) for measurement. The XRD diffractometer used was equipped with a Bragg-Brentano optical system.

[0107] The evaluation results are shown in the column "(003) half width" in Table 1.

[0108] From the obtained X-ray diffraction patterns, it was confirmed that the positive electrode active materials prepared in the following Examples and Comparative Examples were made of lithium nickel composite oxide, and that the lithium nickel composite oxide had a hexagonal layered structure.

[0109] (d) Average primary particle diameter In the SEM observation image of the lithium nickel composite oxide particles, 15 secondary particles that could be observed as a whole were selected by image analysis, and primary particles were selected from each of the selected secondary particles. In this case, the primary particles selected were 30 in total.

[0110] When the lithium nickel composite oxide particles contained single primary particles that did not constitute secondary particles, 10 primary particles that could be observed as a whole were selected from the SEM observation image.

[0111] The major axis lengths of the selected primary particles were then measured, and the average value was taken as the average primary particle diameter. The evaluation results are shown in the "Average primary particle diameter" column of Table 1.

[0112] When the lithium nickel composite oxide particles contained only secondary particles, the major axis lengths of 30 primary particles selected from the secondary particles were measured, and the average value of the major axis lengths of all the measured 30 primary particles was taken as the average primary particle diameter of the lithium nickel composite oxide particles.

[0113] When the lithium nickel composite oxide particles contained single primary particles in addition to secondary particles, the major axis lengths of 30 primary particles selected for the secondary particles and 10 primary particles selected for the single primary particles were measured, and the average of the major axis lengths of all 40 measured primary particles was taken as the average primary particle diameter of the lithium nickel composite oxide particles.

[0114] (e) Median diameter (D50) A volumetric particle size distribution was measured using a laser diffraction / scattering particle size analyzer (Microtrac HRA, manufactured by Nikkiso Co., Ltd.), and the median diameter was determined from the particle size distribution.

[0115] The evaluation results are shown in the "D50" column of Table 1.

[0116] (f) Number Ratio of Single Primary Particles Lithium nickel composite oxide particles were embedded in a thermosetting resin and then polished using a cross-section polisher (SM-09010, manufactured by JEOL Ltd.). The cross sections of the obtained particles were observed using a Schottky field emission scanning electron microscope (Ultra 55, manufactured by Carl Zeiss), and the number ratio of single primary particles per 100 particles was evaluated. (Preparation and Evaluation of Secondary Batteries) (a) Evaluation of Charge Capacity, Discharge Capacity, and Coulomb Efficiency (a-1) Preparation of Coin-Type Batteries Coin-type batteries shown in FIG. 2 were prepared using the positive electrode active materials prepared in the following Examples and Comparative Examples.

[0117] As shown in FIG. 2, the coin battery CBA is composed of a case CA and electrodes housed within the case CA.

[0118] The case CA has a hollow positive electrode can PC with one open end and a negative electrode can NC that is placed in the opening of the positive electrode can PC, and is configured so that when the negative electrode can NC is placed in the opening of the positive electrode can PC, a space for accommodating an electrode is formed between the negative electrode can NC and the positive electrode can PC.

[0119] The electrodes consist of a positive electrode PE, a separator SE1, and a negative electrode NE, which are stacked in this order, and are housed in a case CA so that the positive electrode PE contacts the inner surface of a positive electrode can PC, and the negative electrode NE contacts the inner surface of a negative electrode can NC via a wave washer WW.

[0120] The case CA is provided with a gasket GA, which restricts relative movement between the positive electrode can PC and the negative electrode can NC so as to maintain a non-contact state, i.e., an electrically insulated state, and fixes them in place. The gasket GA also has the function of sealing the gap between the positive electrode can PC and the negative electrode can NC, thereby providing an airtight and liquid-tight barrier between the inside of the case CA and the outside.

[0121] The coin-type battery CBA was fabricated as follows: First, 52.5 mg of the positive electrode active material obtained in each Example and Comparative Example, 15 mg of acetylene black, and 7.5 mg of polytetrafluoroethylene (PTFE) resin were mixed. The resulting mixture was then formed into a thin film with a diameter of 11 mm and a weight of approximately 75 mg to fabricate a positive electrode PE. This was then dried in a vacuum dryer at 100°C for 12 hours.

[0122] Using the prepared positive electrode PE, negative electrode NE, separator SE1, and electrolyte solution, a coin-type battery CBA was produced in a glove box in an Ar atmosphere with a dew point controlled at -60°C.

[0123] For the negative electrode NE, a lithium metal punched into a disk shape with a diameter of 13 mm was used.

[0124] The separator SE1 was a 25 μm thick polyethylene porous film. The electrolyte was 1 M LiClO ４ A mixed solution (manufactured by Toyama Pharmaceutical Co., Ltd.) of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:1, with ethylene carbonate (EC) and diethyl carbonate (DEC) as a supporting electrolyte, was used.

[0125] (a-2) Evaluation of charge capacity, discharge capacity, and efficiency After the coin-type battery CBA was prepared, it was left for about 12 hours, and after the open circuit voltage OCV (open circuit voltage) was stabilized, a current density of 0.1 mA / cm ２ The capacity when the battery was charged to a cutoff voltage of 4.3 V was taken as the charge capacity. After charging, the battery was left resting for 1 hour, and then the capacity when the battery was discharged to a cutoff voltage of 2.5 V was taken as the discharge capacity. Furthermore, the coulomb efficiency, which is the ratio of the discharge capacity to the charge capacity, was calculated.

[0126] (b) Cycle Characteristics (b-1) Fabrication of Laminated Battery Using the positive electrode active materials fabricated in the following Examples and Comparative Examples, laminated batteries shown in FIG. 3 were fabricated.

[0127] 3, the laminated battery LBA has a structure in which a laminate of a positive electrode film PS, a separator SE2, and a negative electrode film NS is impregnated with an electrolyte and sealed with a laminate LA. A positive electrode tab PT is connected to the positive electrode film PS, and a negative electrode tab NT is connected to the negative electrode film NS, and the positive electrode tab PT and the negative electrode tab NT are exposed to the outside of the laminate LA.

[0128] This laminated battery LBA was fabricated as follows: 20.0 g of the obtained positive electrode active material, 2.35 g of acetylene black, and 1.18 g of polyvinylidene fluoride were dispersed in N-methyl-2-pyrrolidone (NMP), and the resulting slurry was spread over an Al foil to a thickness of 1 cm. ２ The positive electrode active material was applied so that 7.0 mg of the positive electrode active material was present per aluminum foil. The slurry containing the positive electrode active material was then applied to the aluminum foil and dried in the air at 120 ° C for 30 minutes to remove the NMP. The aluminum foil coated with the positive electrode active material was cut into strips with a width of 66 mm and roll-pressed under a load of 1.2 t to prepare a positive electrode film. The positive electrode film was then cut into a rectangular shape of 50 mm × 30 mm and dried in a vacuum dryer at 120 ° C for 12 hours to be used as the positive electrode film PS of the laminated battery LBA.

[0129] In addition, a negative electrode film NS was prepared by applying a negative electrode composite paste, which was a mixture of graphite powder with an average particle size of about 20 μm and polyvinylidene fluoride, to a copper foil. The separator SE2 was a 20 μm-thick polyethylene porous film, and the electrolyte was 1 M LiPF ６ A 3:7 mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (manufactured by Ube Industries, Ltd.) was used as a supporting electrolyte.

[0130] In a dry room controlled at a dew point of −60° C., a laminate of the positive electrode film PS, separator SE2, and negative electrode film NS was impregnated with an electrolyte solution and sealed with a laminate LA to prepare a laminated battery LBA.

[0131] (b-2) Cycle Characteristics The cycle characteristics were evaluated by measuring the capacity retention rate after 500 charge / discharge cycles. Specifically, the laminated battery LBA was first charged / discharged at a current density of 0.3 mA / cm in a thermostatic chamber maintained at 25°C. ２ The battery was then conditioned by repeating five cycles of charging to a cutoff voltage of 4.2 V, resting for 10 minutes, and discharging to a cutoff voltage of 2.5 V. Subsequently, the battery was placed in a thermostatic chamber maintained at 45°C, and discharged at a current density of 2.0 mA / cm ２ The battery was charged to a cutoff voltage of 4.2 V at a temperature of 100°C, and then discharged to a cutoff voltage of 2.5 V after a 10-minute rest. This cycle was repeated 500 times. The capacity retention rate, which is the ratio of the discharge capacity at the 500th cycle after conditioning to the discharge capacity at the first cycle, was calculated and evaluated. The discharge capacity at the first cycle and the capacity retention rate are shown in the "Initial Discharge Capacity" and "Capacity Retention Rate" columns under "Cycle Characteristics" in Table 1, respectively. (Production Conditions for Positive Electrode Active Material) [Example 1] A nickel composite oxide (D50 particle size: 5.2 μm) having a substance amount ratio of nickel to cobalt of Ni:Co=95.0:5.0, obtained using a known method, was mixed with lithium hydroxide to prepare a raw material mixture (mixing step).

[0132] In the mixing step, the nickel composite oxide and lithium hydroxide were thoroughly mixed using a shaker mixer (model: TURBULA Type T2C, manufactured by Willy & Bachofen (WAB)) to prepare a raw material mixture.

[0133] In the mixing step, the raw materials were weighed and mixed so that the ratio of the amount of substance of lithium (Li) contained in the raw material mixture to the amount of substance of nickel and cobalt (Me) other than lithium, Li / Me, was 0.92. The nickel composite oxide was obtained by heat-treating a nickel composite hydroxide obtained by a crystallization method.

[0134] This raw material mixture was fired in an oxygen-containing atmosphere with an oxygen concentration of 90% by volume and the remainder being nitrogen (firing step). The oxygen concentration in the oxygen-containing atmosphere in the firing step is shown in the "Oxygen concentration" column in Table 1. In the first firing step, the temperature was raised to 500°C, which was the first firing temperature, and held at 500°C for 7 hours. Following the first firing step, a second firing step was carried out. In the second firing step, the temperature was raised to 780°C, which was the second firing temperature, and held at 780°C for 12 hours, thereby performing firing.

[0135] The fired lithium nickel composite oxide was crushed (crushing step) to obtain a positive electrode active material made of lithium nickel composite oxide particles.

[0136] The obtained positive electrode active material was observed by SEM, and it was confirmed that the lithium nickel composite oxide particles had an average primary particle diameter of 1.2 μm and were composed of secondary particles made of the primary particles. That is, the number ratio of individual primary particles was 0. The number ratio of individual primary particles was also 0 in the following Examples 2 and 4.

[0137] Other evaluation results are shown in Table 1.

[0138] Figure 4 shows the powder neutron diffraction pattern (Observed) of the lithium nickel composite oxide measured in Example 1. Figure 4 also shows the simulation pattern (Calculated) obtained by Rietveld analysis, the difference between the measured powder neutron diffraction pattern and the simulation pattern (Difference), and the Bragg positions (Bragg Positions). In Figure 4, region 40 was excluded from the Rietveld analysis because a peak occurred in that region due to a malfunction of the device, which prevented neutrons of a wavelength that would normally be cut during measurement from being cut.

[0139] Furthermore, the above-described coin-type battery and laminate-type battery were fabricated using the obtained positive electrode active material and evaluated. The evaluation results are shown in Table 1. [Example 2] In the mixing step, the mixing ratio of lithium hydroxide and nickel composite oxide was changed so that the Li / Me ratio of the raw material mixture was the value shown in the "Charged Li / Me Ratio" column in Table 1. Furthermore, the holding time at the first firing temperature in the first firing step was set to the conditions shown in Table 1. Except for the above, positive electrode active materials, coin-type batteries, and laminate-type batteries were fabricated and evaluated under the same conditions as in Example 1. The evaluation results are shown in Table 1. [Example 3] The holding time at the first firing temperature in the first firing step, the holding time at the second firing temperature in the second firing step, and the oxygen concentration in the oxygen-containing atmosphere in the firing step were set to the conditions shown in Table 1. Furthermore, the lithium nickel composite oxide after the crushing step was pulverized using a ball mill (pulverization step) to obtain a positive electrode active material composed of lithium nickel composite oxide particles.

[0140] Except for the above points, a positive electrode active material, a coin-type battery, and a laminate-type battery were produced and evaluated under the same conditions as in Example 2. The evaluation results are shown in Table 1.

[0141] The obtained positive electrode active material was observed with an SEM, and it was confirmed that the particles of the lithium nickel composite oxide included a single primary particle and secondary particles formed by aggregation of a plurality of primary particles.

[0142] The number ratio of a single primary particle was calculated to be 91.3%. [Example 4] In the mixing step, a nickel composite oxide in which the substance ratio of nickel, manganese, and cobalt was Ni:Mn:Co = 85.0:10.0:5.0 was used as the nickel composite oxide. In addition, the mixing ratio of lithium hydroxide and nickel composite oxide was selected so that the Li / Me ratio in the raw material mixture was the value shown in the column "Charged Li / Me ratio" in Table 1.

[0143] In the firing step, the holding time at the first firing temperature in the first firing step and the second firing temperature in the second firing step were set to the conditions shown in Table 1. Except for the above, a positive electrode active material, a coin-type battery, and a laminate-type battery were produced and evaluated under the same conditions as in Example 1. The evaluation results are shown in Table 1. [Example 5] The holding time at the first firing temperature in the first firing step, the holding time at the second firing temperature in the second firing step, and the oxygen concentration in the oxygen-containing atmosphere in the firing step were set to the conditions shown in Table 1. Furthermore, the lithium nickel composite oxide after the crushing step was crushed using a ball mill (crushing step) to obtain a positive electrode active material composed of lithium nickel composite oxide particles.

[0144] Except for the above points, a positive electrode active material, a coin-type battery, and a laminate-type battery were produced and evaluated under the same conditions as in Example 4. The evaluation results are shown in Table 1.

[0145] The obtained positive electrode active material was observed with an SEM, and it was confirmed that the particles of the lithium nickel composite oxide included a single primary particle and secondary particles formed by aggregation of a plurality of primary particles.

[0146] The calculated number ratio of the single primary particles was 97.5%. [Comparative Example 1] In the mixing step, the mixing ratio of lithium hydroxide and nickel composite oxide was changed so that the Li / Me ratio of the raw material mixture was the value shown in the "Charged Li / Me Ratio" column in Table 1. Except for the above, a positive electrode active material, a coin-type battery, and a laminate-type battery were produced and evaluated under the same conditions as in Example 2. The evaluation results are shown in Table 1. [Comparative Example 2] Pure water at 20°C was added to the positive electrode active material obtained in Comparative Example 1 to prepare a slurry containing 1250 g of positive electrode active material per 1 L of water (slurrying step). Next, the slurry was stirred for 20 minutes, passed through a filter press, and dehydrated to prepare a washed cake (solid-liquid separation step). Note that the pure water used had an electrical conductivity of 1 μS / cm or less.

[0147] The resulting washed cake was dried in a vacuum atmosphere at 190° C. for 10 hours to obtain a positive electrode active material of Comparative Example 2 (drying step).

[0148] The obtained positive electrode active material was evaluated as described above. The evaluation results are shown in Table 1. In addition, coin-type batteries and laminate-type batteries were fabricated and evaluated in the same manner as in Example 1, except that the obtained positive electrode active material was used. The evaluation results are shown in Table 1. [Comparative Example 3] In the mixing step, the mixing ratio of lithium hydroxide and nickel composite oxide was changed so that the Li / Me ratio in the raw material mixture was the value shown in the "Feed Li / Me ratio" column in Table 1. Except for the above points, positive electrode active materials, coin-type batteries, and laminate-type batteries were fabricated and evaluated under the same conditions as in Example 4. The evaluation results are shown in Table 1.

[0149] (Evaluation Results) According to the results shown in Table 1, it was confirmed that the positive electrode active materials of Examples 1 to 3, in which the occupancy rate of nickel present in the lithium site (3b site) was 2.5% or more and 10.0% or less, were able to improve the cycle characteristics compared to Comparative Examples 1 and 2, in which the contents of nickel and element M were the same.

[0150] The same was also confirmed by comparing Examples 4 and 5 with Comparative Example 3.

[0151] This application claims priority based on Japanese Patent Application No. 2023-123249, filed with the Japan Patent Office on July 28, 2023, the entire contents of which are incorporated herein by reference.

[0152] CBA Coin cell battery PE Positive electrode NE Negative electrode SE1 Separator GA Gasket WW Wave washer CA Case PC Positive electrode can NC Negative electrode can LBA Laminated battery PS Positive electrode film NS Negative electrode film SE2 Separator LA Laminate PT Positive electrode tab NT Negative electrode tab

Claims

1. A positive electrode active material for a lithium ion secondary battery, comprising a lithium nickel composite oxide having a hexagonal layered structure, the lithium nickel composite oxide containing lithium (Li), nickel (Ni), and an element M (M) in a ratio of substance amounts of Li:Ni:M=a:b:c (wherein 0.90≦a<1.00, 0.80≦b<1.00, 0.00<c≦0.20, b+c=1, and the element M is at least one element selected from the group consisting of Mn, Co, Al, Ti, Zr, W, Fe, Si, Nb, Mg, Ca, B, Na, K, Mo, Cu, V, P, and Ba), and an occupancy rate of nickel present in lithium sites (3b sites) obtained by analyzing a powder neutron diffraction pattern of the lithium nickel composite oxide by the Rietveld method is 2.5% or more and 10.0% or less, The lithium-ion secondary battery positive electrode active material has a half-width of a diffraction peak of a (003) plane in a powder X-ray diffraction pattern of the lithium nickel composite oxide measured using Cu-Kα radiation as an X-ray source of 0.054° or more and less than 0.074°.

2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the average primary particle size of the lithium nickel composite oxide is greater than 0.5 μm and not greater than 6.0 μm.

3. A positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, which has a median diameter in a volume-based particle size distribution measured by a laser diffraction / scattering method of 0.5 μm or more and 20 μm or less.

4. A lithium ion secondary battery comprising at least a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode contains the positive electrode active material for lithium ion secondary batteries according to any one of claims 1 to 3.

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