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

Abstract

The present invention relates to a positive electrode active material for a lithium ion secondary battery, which contains a lithium-nickel composite oxide having a hexagonal layered structure, and which is characterized in that the lithium-nickel composite oxide has a mass ratio of Li: Ni: M = a: b: c (0.90 < = a < 1.00, 0.80 < = b < 1.00, 0.00 < c < = 0.20, and b + c = 1), and in that the mass ratio of Li: Ni: M = a: b: c (0.90 < = a < 1.00, 0.80 < = b < 1.00, 0.00 < c < = 0.20, and b + c = 1); 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 the occupancy of nickel present in the lithium site as determined by a powder neutron diffraction pattern is 2.5% to 10.0% inclusive.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a positive electrode active material for a lithium-ion secondary battery, and a lithium-ion secondary battery. BACKGROUND

[0002] In recent years, with the spread of portable electronic devices such as portable telephone terminals and notebook personal computers, there is a strong demand for the development of small and lightweight nonaqueous electrolyte secondary batteries having high energy density and durability. In addition, as a battery for electric vehicles typified by electric power tools and hybrid electric vehicles, there is a strong demand for the development of secondary batteries with high output.

[0003] As a secondary battery that meets such a demand, a nonaqueous electrolyte secondary battery such as a lithium-ion secondary battery is used. As a positive electrode active material, a lithium-ion secondary battery using a lithium metal complex oxide having a crystal structure of a layered or spinel type obtains a high voltage of 4 V class, and thus its practical use is being made as a battery having high energy density.

[0004] As a lithium metal complex oxide, a lithium cobalt complex oxide (LiCoO2) that is relatively easy to synthesize, a lithium nickel complex oxide (LiNiO2) using nickel that is cheaper than cobalt, a lithium nickel cobalt manganese complex oxide (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2), a lithium manganese complex oxide (LiMn2O4) using manganese, a lithium nickel manganese complex oxide (LiNi 0.5 Mn 0.5 O2), and the like are proposed.

[0005] In recent years, further improvement in performance has been required for the battery characteristics of lithium-ion secondary batteries, and various studies have been conducted.

[0006] For example, in Patent Literatures 1 to 5, by controlling the particle characteristics of a lithium complex oxide, for a lithium-ion secondary battery using the lithium complex oxide, improvement in cycle characteristics as a capacity retention rate under repeated charge and discharge is disclosed.

[0007] PRIOR ART DOCUMENTS PATENT LITERATURES Patent Literature 1: Japanese Patent Application Laid-Open No. 2001-243949 Patent Literature 2: Japanese Patent Application Laid-Open No. 2004-355824 Patent Literature 3: Japanese Patent Application Laid-Open No. 2017-188444 Patent Literature 4: Japanese Patent Application Laid-Open No. 2017-188445 Patent Literature 5: International Publication No. 2017 / 169129 SUMMARY Problem to be solved by the Invention However, lithium ion secondary batteries are used for various purposes as described above, and further improvement in performance is required. Therefore, for the positive electrode active material for a lithium ion secondary battery, it is required to improve the performance such as cycle characteristics when used for a lithium ion secondary battery by a method different from the past.

[0008] Therefore, in view of the problems of the conventional techniques described above, an object of one aspect of the present application is to provide a positive electrode active material for a lithium ion secondary battery, which is excellent in cycle characteristics when used for a lithium ion secondary battery.

[0009] Method for solving the problem In order to solve the above problem, according to one embodiment of the present application, Provided is a positive electrode active material for a lithium ion secondary battery, which is a positive electrode active material for a lithium ion secondary battery containing a lithium-nickel composite oxide having a layered structure of hexagonal system, 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 The occupancy of nickel at the lithium site (3b site) obtained by analyzing the powder neutron diffraction pattern of the lithium-nickel composite oxide by the Rietveld method is 2.5% or more and 10.0% or less.

[0010] Effects of the Invention According to one embodiment of the present application, it is possible to provide a positive electrode active material for a lithium ion secondary battery, which is excellent in cycle characteristics when used for a lithium ion secondary battery. BRIEF DESCRIPTION OF DRAWINGS

[0011] Figure 1 Flow of the production method of the positive electrode active material.

[0012] Figure 2 Schematic explanatory view of a coin-type battery used for battery evaluation.

[0013] Figure 3 Schematic explanatory view of a laminated-type battery used for battery evaluation.

[0014] Figure 4A powder neutron diffraction pattern of the lithium-nickel composite oxide obtained in Example 2, and a simulated pattern by Rietveld analysis. DETAILED DESCRIPTION

[0015] Hereinafter, for the present embodiment, the description will be made with reference to the accompanying drawings, the present application is not limited to the following embodiments, various modifications and substitutions can be made to the following embodiments without departing from the scope of the present application.

[0016] [Positive electrode active material for lithium-ion secondary battery] Hereinafter, the positive electrode active material for lithium-ion secondary battery (hereinafter, also described as "positive electrode active material") of the present embodiment will be described.

[0017] (1) For lithium-nickel composite oxide The positive electrode active material of the present embodiment contains a lithium-nickel composite oxide. The positive electrode active material of the present embodiment can be composed of only the lithium-nickel composite oxide, even in this case, it is not excluded that inevitable impurities mixed in the manufacturing process or the like are contained.

[0018] The lithium-nickel composite oxide can have a hexagonal crystal system layered structure.

[0019] (1-1) For the shape of the particles The shape of the particles of the lithium-nickel composite oxide is not particularly limited. For example, at least one of a single primary particle, and a secondary particle in which a plurality of primary particles are aggregated can be contained. Therefore, the lithium-nickel composite oxide can contain both the single primary particle and the secondary particle described above. In addition, the lithium-nickel composite oxide can be composed of only the single primary particle, or can be composed of only the secondary particle.

[0020] In determining either one of the primary particle, the secondary particle, or the particles containing both of the lithium-nickel composite oxide, the approximate situation can be confirmed using a scanning electron microscope (SEM) observation image (hereinafter, also described as "SEM image"), and sometimes lacks accuracy. In the positive electrode active material related to the present embodiment, it is preferable to cross-section process the particles, and determine the presence or absence of the primary particle, the secondary particle using the SEM image.

[0021] Specifically, for example, it is preferable to bury the lithium-nickel composite oxide in a resin, and process it using a cross-section polisher (CP) or the like to be a state in which the particles can be cross-section observed, and then perform observation of the SEM image to determine the presence or absence of the primary particle, the secondary particle. The determination is preferably performed for 50 or more particles, and the evaluation described above. The upper limit of the number of particles for evaluation is not particularly limited, and it is preferable to consider the productivity of the evaluation, and perform the evaluation for 200 or less particles.

[0022] Further, in the case where the spectral band contrast of an electron backscatter diffraction (hereinafter, also referred to as "EBSD") method, or a focused ion beam (FIB) processing device is used at the time of cross-section processing, the above particle cross-section can be confirmed using an attached scanning ion microscope (SIM) observation image.

[0023] (1-2) Constituent elements The lithium-nickel composite oxide contains lithium (Li), nickel (Ni), and an element M (M). Further, as the element M, 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 can be cited. Further, an element other than the above elements can be contained in a small amount.

[0024] The lithium-nickel composite oxide can contain lithium (Li), nickel (Ni), and the element M (M) in a ratio of Li:Ni:M = a:b:c in terms of the amount of substance (molar ratio).

[0025] The above a, b, and c satisfy 0.90 ≤ a < 1.00, 0.80 ≤ b < 1.00, and 0.00 < c ≤ 0.20, and b + c = 1.

[0026] Further, the lithium-nickel composite oxide can contain lithium (Li), nickel (Ni), and the element M (M) in a ratio of Li:Ni:M = a:b:c in terms of the amount of substance (molar ratio). a Ni b M c O 2＋α The expression α preferably satisfies -0.2 ≤ α ≤ 0.2. The a, b, c, and element M are described above, and thus the description thereof is omitted.

[0027] Hereinafter, each element will be described.

[0028] (Lithium) In the above amount-of-substance ratio, the value of a indicating the amount-of-substance ratio of Li corresponds to the amount-of-substance ratio of lithium (Li) to an element (Me) other than lithium and oxygen (Li / Me ratio). The element (Me) other than lithium and oxygen corresponds to Ni and the element M. In the above amount-of-substance ratio, a ranges from 0.90 ≤ a < 1.00. That is, it is preferable that lithium be deficient compared to the case where a = 1.00 in the amount-of-substance ratio of the lithium-nickel composite oxide.

[0029] By making the value of a be 0.90 or more, the reaction resistance is suppressed, and the output of the battery is improved. By making the value of a be less than 1.00, the amount of the lithium compound attached to the surface of the particle of the positive electrode active material of the present embodiment, that is, the alkali amount, can be suppressed. Therefore, in the case where the positive electrode active material of the present embodiment is applied to a lithium-ion secondary battery (hereinafter, also referred to as "secondary battery"), the gas generation at the time of battery reaction can be suppressed.

[0030] The above a is preferably 0.92≤a<0.99.

[0031] (Nickel) The above atomic mass ratio, b, which represents the atomic mass ratio of Ni, is in the range of 0.80≤b<1.00. In the case where the value of b is in the above range, when the positive electrode active material of the present embodiment is applied to a secondary battery, a high battery capacity can be obtained.

[0032] (Element M) The above atomic mass ratio, c, which represents the atomic mass ratio of the element M, is in the range of 0.00 The lithium nickel composite oxide can also contain multiple kinds of element M, in which case the total atomic mass ratio of the element M contained in the lithium nickel composite oxide preferably satisfies the above range. The candidate elements for the element M are as described above, and thus the description thereof is omitted. The kind of the element M contained in the lithium nickel composite oxide can be appropriately selected depending on the required battery characteristics and the like.

[0033] For example, the element M can include Co. In the case where the atomic mass ratio of Co contained in the above atomic mass ratio, c, is cl, the range of cl is preferably 0

[0034] Further, the element M can include Mn. In the case where the atomic mass ratio of Mn contained in the above atomic mass ratio, c, is c2, the range of c2 is preferably 0

[0035] The element M can include Al. In a case where the mass ratio of Al contained in the above-described mass ratio c is c3, the range of c3 is preferably 0 < c3 ≤ 0.10, more preferably 0.01 ≤ c3 ≤ 0.08, and further preferably 0.01 ≤ c3 ≤ 0.06. In a case where the range of c3 is the above-described range, in a case where the positive electrode active material of the present embodiment is applied to a secondary battery, the thermal stability can be improved. In addition, as the element M, in a case where only Al is included, c = c3.

[0036] In addition, the composition of the lithium-nickel composite oxide can be determined by quantitative analysis using an inductively coupled plasma (ICP) emission spectrometry.

[0037] (1-3) Nickel occupancy ratio of lithium site (3b site) The positive electrode active material according to the present embodiment contains a lithium-nickel composite oxide, and the nickel (Ni) occupancy ratio of the lithium site (3b site) obtained by analyzing a powder neutron diffraction pattern determined by a powder neutron diffraction method using a Rietveld method is preferably 2.5% or more and 10.0% or less, more preferably 2.6% or more and 8.0% or less, further preferably 2.7% or more and 7.0% or less, and particularly preferably more than 2.9% and 7.0% or less.

[0038] In addition, the nickel occupancy ratio of the lithium site (3b site) obtained from the above-described powder neutron diffraction pattern is described as "Ni(Li(3b))".

[0039] In a case where the charge and discharge of the secondary battery are repeated, the insertion and extraction of lithium into and from the interlayer of the lithium-nickel composite oxide are repeated. However, in a case where the repeated charge and discharge of the secondary battery is performed, the interlayer repeatedly expands and shrinks with the insertion and extraction of lithium, and thus the crystal structure of the lithium-nickel composite oxide changes, and lithium that does not contribute to the charge and discharge is generated. Therefore, it is considered that the cycle characteristics are reduced.

[0040] On the other hand, in the lithium-nickel composite oxide contained in the positive electrode active material of the present embodiment, nickel occupies a part of the sites of lithium. Therefore, even when lithium is extracted from the interlayer of the lithium-nickel composite oxide, nickel remains in the interlayer. That is, it is considered that the nickel functions as a pillar of the interlayer, and the change in the interlayer distance when the secondary battery is repeatedly charged and discharged can be suppressed. Therefore, even in a case where the repeated charge and discharge of the secondary battery is performed, the change in the crystal structure of the lithium-nickel composite oxide can be suppressed, and the generation of lithium that does not contribute to the charge and discharge can be suppressed. Therefore, according to the positive electrode active material of the present embodiment, by controlling the nickel occupancy ratio of the lithium site that has not been used in the past, the cycle characteristics in the case of being used for a secondary battery can be improved.

[0041] Specifically, it is considered that by making the above Ni(Li(3b)) 2.5% or more, the function of using the above nickel as a pillar between layers can be sufficiently exerted, the crystal structure at the time of charge and discharge is stabilized, and in particular, the cycle characteristics are improved. In addition, by making the above Ni(Li(3b)) 10.0% or less, the decrease in the completeness of the crystal of the lithium-nickel composite oxide can be suppressed. Thus, the amount of lithium that contributes to charge and discharge is sufficiently ensured, the reaction resistance is suppressed, and the battery capacity and output are sufficiently improved.

[0042] In addition, when calculating the above Ni(Li(3b)), the Rietveld analysis uses an X-ray diffraction pattern. However, in the lithium-nickel composite oxide contained in the positive electrode active material of the present embodiment, in addition to nickel, as the element M, for example, cobalt, manganese, and the like, which are elements that are difficult to identify using X-rays, can be contained. In addition, lithium, hydrogen, and oxygen, which are light elements, are difficult to accurately evaluate using X-rays. Therefore, in the case of using an X-ray diffraction pattern, it is generally difficult to calculate an accurate value of Ni(Li(3b)).

[0043] On the contrary, by using a neutron line, the elements contained in the lithium-nickel composite oxide can be accurately evaluated compared to the case of using an X-ray diffraction pattern. Therefore, when calculating the above Ni(Li(3b)), it is necessary to use a powder neutron diffraction pattern.

[0044] (1-4)Half-value width of the peak of the (003) plane The lithium-nickel composite oxide contained in the positive electrode active material of the present embodiment has a diffraction peak belonging to the (003) plane in a powder X-ray diffraction pattern measured using a Cu-Kα ray as an X-ray source by a powder X-ray diffraction method (hereinafter, also referred to as "XRD"). The lithium-nickel composite oxide contained in the positive electrode active material related to the present embodiment preferably has a value of the half-value width of the diffraction peak of the (003) plane in the above X-ray diffraction pattern of 0.074° or more and 0.125° or less. The half-value width of the diffraction peak of the (003) plane refers to the width of the pattern in the intensity value of the half of the intensity of the diffraction peak belonging to the (003) plane.

[0045] The half-value width of the diffraction peak of the above (003) plane is an index for grasping the degree of crystallinity and primary particle size of the lithium-nickel composite oxide. Furthermore, by making the half-value width of the diffraction peak of the above (003) plane 0.125° or less, a lithium-nickel composite oxide having particularly excellent crystallinity can be produced. In addition, the size of the primary particle becomes suitable for improvement of the cycle characteristics. Therefore, in the case where the positive electrode active material of the present embodiment containing this lithium-nickel composite oxide is applied to a secondary battery, the battery capacity and the capacity retention rate can also be particularly improved, and a secondary battery having particularly excellent battery capacity and cycle characteristics can be produced. However, even if the half-value width of the diffraction peak of the above (003) plane is made excessively small, the effect of improving the above battery capacity and / or capacity retention rate saturates, and thus the half-value width of the diffraction peak of the above (003) plane is preferably 0.074° or more.

[0046] In addition, the above X-ray diffraction pattern is preferably measured using an XRD diffraction device having a Bragg Brentano optical system, using a flat sample holder.

[0047] (1-5) Median particle diameter (D50) The median particle diameter (D50) in the particle size distribution on a volume basis of the positive electrode active material involved in the present embodiment is preferably 0.1 μm or more and 20 μm or less, more preferably 1.0 μm or more and 18 μm or less, and further preferably 2.0 μm or more and 17.0 μm or less.

[0048] By making the median particle diameter of the positive electrode active material of the present embodiment 20 μm or less, the contact area with the electrolyte when applied to a secondary battery can be sufficiently increased, and the battery capacity can be improved. In addition, by making the median particle diameter of the positive electrode active material of the present embodiment 0.1 μm or more, the workability and the like at the time of electrode production can be improved.

[0049] The median particle diameter (D50) can be calculated from the volume cumulative value measured by a laser diffraction scattering particle size distribution meter, for example.

[0050] (1-6) Average primary particle diameter The average primary particle diameter of the lithium-nickel composite oxide is preferably 0.05 μm or more and 1.0 μm or less. By making the average primary particle diameter of the lithium-nickel composite oxide 0.05 μm or more, it means that the size of the primary particle and the crystallite constituting the primary particle can be sufficiently increased, and the crystallinity of the lithium-nickel composite oxide can be improved. Therefore, in the case where the positive electrode active material of the present embodiment containing this lithium-nickel composite oxide is applied to a secondary battery, the battery capacity can also be improved, and a secondary battery having excellent battery capacity and cycle characteristics can be produced.

[0051] However, even if the average primary particle diameter of the lithium nickel composite oxide is excessively increased, the effect of improving the battery capacity described above saturates, and thus the average primary particle diameter of the lithium nickel composite oxide is preferably 1.0 μm or less.

[0052] The average primary particle diameter of the lithium nickel composite oxide is selected so that primary particles of the lithium nickel composite oxide as a whole can be observed using a scanning electron microscope (SEM), the length of the long axis thereof is measured, and the average is obtained. The number of primary particles evaluated when calculating the average primary particle diameter is not particularly limited.

[0053] For example, in the case where the lithium nickel composite oxide contains secondary particles, it is preferable that, for 10 or more to 20 or less secondary particles, 20 or more to 30 or less primary particles are selected from each of the secondary particles, and the length of the long axis of the selected primary particles is measured.

[0054] Further, in the case where the lithium nickel composite oxide contains primary particles that do not constitute secondary particles, it is preferable that 5 or more primary particles of the lithium nickel composite oxide are evaluated. The upper limit of the number of primary particles evaluated is not particularly limited, and for example, it is preferable that 20 or less primary particles are evaluated.

[0055] In the case where the lithium nickel composite oxide contains only secondary particles or only monomer primary particles, the above number of primary particles can be selected from the secondary particles or the monomer primary particles. Furthermore, the length of the long axis of the selected primary particles can be measured, and the average thereof can be used as the average primary particle diameter of the lithium nickel composite oxide.

[0056] In the case where the lithium nickel composite oxide contains both secondary particles and monomer primary particles, the above number of primary particles can be selected from each of the secondary particles and the monomer primary particles. Furthermore, the length of the long axis of all the selected primary particles can be measured, and the average thereof can be used as the average primary particle diameter of the lithium nickel composite oxide.

[0057] The median particle diameter (D50) is in the range described above for the positive electrode active material, and the average primary particle diameter is in the range described above for the lithium nickel composite oxide contained therein, and thus in the case where the positive electrode active material is applied to a secondary battery, the battery capacity and the cycle characteristics can be particularly improved, and both can be achieved.

[0058] (1-7) Amount of Dissolved Lithium From the viewpoint of suppressing gas generation in a secondary battery, the amount of lithium dissolved in water when the positive electrode active material is immersed in water (hereinafter, also referred to as "amount of dissolved lithium") is preferably 0.15% by mass or less, and more preferably 0.10% by mass or less, with respect to the entire positive electrode active material.

[0059] By making the amount of eluted lithium be in the above range, generation of gas in the secondary battery can be suppressed.

[0060] The lower limit value of the amount of eluted lithium of the positive electrode active material of the present embodiment is not particularly limited, and is preferably 0.05% by mass or greater, for example.

[0061] In the case where 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. Further, by performing water washing treatment on the positive electrode active material, the amount of eluted lithium can be suppressed.

[0062] [Method for manufacturing positive electrode active material for lithium-ion secondary battery] The method for manufacturing the positive electrode active material for lithium-ion secondary battery described above is not particularly limited if a positive electrode active material having the characteristics described above is obtained. Hereinafter, an example of the method for manufacturing the positive electrode active material according to the present embodiment will be described.

[0063] In addition, according to the method for manufacturing the positive electrode active material according to the present embodiment described above, the positive electrode active material described above can be manufactured, and thus a part of the description of the matters already described will be omitted.

[0064] The method for manufacturing the positive electrode active material according to the present embodiment can be implemented according to the flow 10 shown in FIG. 10, for example. That is, the method for manufacturing the positive electrode active material according to the present embodiment can have a mixing step (S1), and a firing step (S2). Figure 1

[0065] In the mixing step (S1), the nickel composite compound and the lithium compound can be mixed to obtain a raw material mixture.

[0066] In the firing step (S2), the raw material mixture described above can be fired to produce the lithium-nickel composite oxide.

[0067] In addition, the element M can be added as a compound of the element M, or can be added using a nickel composite compound containing the element M, in the mixing step (S1), unlike the nickel composite compound described above.

[0068] Hereinafter, a configuration example of each step will be described.

[0069] (1) Mixing step (S1) In the mixing step (S1), the nickel composite compound and the lithium compound can be mixed to obtain a raw material mixture, as described above. Further, in the mixing step, a compound of the element M can be added as necessary and mixed.

[0070] ​Nickel compounds, lithium compounds, and compounds containing element M as needed can be added and mixed, for example, in powder (solid phase). The following describes the materials supplied to the mixing process.

[0071] (1-1) For raw materials (Nickel complex) The nickel composite compound used in the mixing process (S1) can be obtained by known methods. The content ratio (composition ratio) of element Ni and element M (such as Co) in the nickel composite compound is essentially maintained even in lithium nickel composite oxide particles. Therefore, the content ratio of each element is preferably within the same range as that in the lithium nickel composite oxide described above. Therefore, the nickel composite compound preferably contains nickel (Ni) and element M (M) in a mass ratio of Ni:M = b:c. b, c, and element M have been described in the lithium nickel composite oxide, and their description is omitted here.

[0072] In addition, the nickel composite compound used in this embodiment may contain a small amount of the above-mentioned elements (elements M such as Ni and Co) and elements other than oxygen, without hindering the effects of the present invention.

[0073] Nickel complex compounds can be hydroxides or oxides. Furthermore, they can be mixtures of hydroxides and oxides. Methods for manufacturing nickel complex hydroxides include, for example, neutralization crystallization using an aqueous solution of a metal salt and an alkaline solution. Additionally, by heat treatment of the nickel complex hydroxide, or by removing moisture from the nickel complex hydroxide, some or all of the nickel complex hydroxide can be converted into nickel complex oxides.

[0074] Furthermore, when the nickel complex compound is a nickel complex hydroxide, Ni is preferred. b M c (OH) 2＋β Furthermore, when the nickel complex compound is a nickel complex oxide, Ni is preferred. b M c O 1＋γ For elements b, c, and M, which are described in the lithium-nickel composite oxide, the description is omitted. β and γ preferably satisfy -0.2 ≤ β ≤ 0.2 and -0.2 ≤ γ ≤ 0.2, respectively.

[0075] (Lithium compounds) The lithium compound is not particularly limited, and any known compound containing lithium can be used, such as one or more selected from lithium carbonate, lithium hydroxide, and lithium nitrate. Alternatively, the lithium compound can be a mixture of two or more compounds selected from the above-mentioned compounds.

[0076] The lithium compound is less affected by residual impurities, and from the viewpoint of dissolving at the firing temperature, it is more preferable to be one or more selected from the group consisting of lithium carbonate and lithium hydroxide. Furthermore, from the viewpoint of obtaining a lithium-nickel composite oxide having particularly high crystallinity, the lithium compound is more preferably lithium hydroxide.

[0077] (compound of element M) As described above, in the mixing step, the compound of element M can be added as needed and mixed. The kind of the compound of element M is not particularly limited, and one or more selected from the group consisting of hydroxide, oxide, chloride, nitrate, sulfate, carbonate, and the like is preferably used.

[0078] (1-2) Mixing method The mixing method of the nickel composite compound, the lithium compound, and the compound of element M added as needed is not particularly limited. It is preferable to mix these particles to such an extent that the skeleton of these particles is not destroyed, and to sufficiently mix these particles.

[0079] The mixing of the raw materials of the nickel composite compound and the like can be performed using, for example, a general-purpose mixer, and for example, a vibrator mixer, a laser mixer, a Julia mixer, a V-blender, and the like can be used for the mixing. In the case where the mixing is insufficient, the mass ratio (Li / Me) of lithium (Li) to lithium, oxygen, and element (Me) other than nickel can sometimes deviate between the respective particles of the positive electrode active material, and problems such as insufficient battery characteristics can occur, and thus it is preferable to mix sufficiently. As described above, lithium, oxygen, and element (Me) other than nickel correspond to Ni and element M.

[0080] (1-3) Mixing ratio The lithium compound is preferably mixed so that Li / Me in the mixture obtained in the mixing step is 0.90 or more and less than 1.00. That is, Li / Me in the raw material mixture is preferably mixed in the same manner as Li / Me in the obtained fired product. This is because the mass ratio of each element of Li / Me, lithium, nickel, and element M does not change before and after the firing step (S2), and thus Li / Me of the raw material mixture in the mixing step (S1) is equal to Li / Me of the fired product. In addition, the content (ratio) of the elements in the raw material mixture is substantially maintained even in the lithium-nickel composite oxide.

[0081] (2) Firing step (S2) The firing step (S2) is a step of firing the raw material mixture obtained in the mixing step (S1) to obtain a fired product containing a lithium-nickel composite oxide. If the raw material mixture is fired, lithium in the lithium compound diffuses in the nickel composite compound, and a lithium-nickel composite oxide is formed. The lithium compound is molten at the temperature at which firing is performed, penetrates into the nickel composite compound, and a fired product of the lithium-nickel composite oxide is formed.

[0082] The conditions of the firing in the firing step (S2) are not particularly limited, and for the composition of the lithium-nickel composite oxide targeted, can be selected in a manner such that the characteristics of the occupancy rate of nickel at the lithium site (3b site) and the like become in the desired range. Hereinafter, a suitable example of the firing conditions will be described.

[0083] (Ambience) The firing ambience is preferably an oxidizing ambience. The oxidizing ambience preferably has an oxygen concentration of 80% by volume or more, and more preferably an oxygen concentration of 85% by volume or more. In addition, the oxidizing ambience can also be an oxygen gas ambience, and thus the oxygen concentration can be 100% by volume or less.

[0084] (Firing temperature) The firing is preferably performed in 2 stages. That is, it is preferable to raise the temperature up to the firing temperature of the 1st stage, hold after that (1st firing step), raise the temperature up to the firing temperature of the 2nd stage, hold (2nd firing step), and then perform cooling.

[0085] The 1st firing temperature as the firing temperature of the 1st stage is preferably 400°C or more and 600°C or less, and the holding time at the 1st firing temperature is preferably 5 hours or more.

[0086] The 2nd firing temperature as the firing temperature of the 2nd stage is preferably 600°C or more and 770°C or less, and the holding time at the 2nd firing temperature is preferably 10 hours or more. In addition, the 2nd firing temperature is preferably higher than the temperature of the 1st firing temperature.

[0087] In the case of performing the firing using the above-mentioned temperature pattern, for the lithium-nickel composite oxide, even in a state where Li is deficient compared to the stoichiometric ratio, the crystallinity of the lithium-nickel composite oxide becomes high, and a positive electrode active material having a high battery capacity can be obtained. Furthermore, for the occupancy rate of nickel at the lithium site (3b site), it can be easily set to be within the range described above. In addition, sometimes depending on the containing ratio of lithium and nickel in the target composition of the lithium-nickel composite oxide, the firing conditions such as the firing temperature suitable for making the occupancy rate of nickel at the lithium site (3b site) become in the desired range change. Therefore, it is preferable to select the optimum firing temperature within the above-mentioned temperature range, and select the temperature conditions of the temperature increasing rate, the holding time, and the like, according to the target composition.

[0088] In the case of performing the 2-stage firing step, in the 1st firing step as the firing step at the 1st firing temperature of the 1st stage, the lithium compound can be made to melt and sufficiently diffuse in the nickel composite compound, and the lithium and the nickel composite compound can be made to react. Furthermore, in the 2nd firing step as the firing step at the 2nd firing temperature of the 2nd stage, the crystal growth can be promoted.

[0089] The manufacturing method of the positive electrode active material of the present embodiment can have an arbitrary process other than the mixing process and the firing process.

[0090] (3) Pulverization process The fired product obtained by the firing process (S2) can be the positive electrode active material of the present embodiment. However, the sintering between the particles of the fired product obtained after the firing process (S2) is inhibited, but sometimes coarse particles are formed by weak sintering and agglomeration. In such a case, the manufacturing method of the positive electrode active material of the present embodiment can have a pulverization process of pulverizing the fired product. By pulverizing the fired product, the above-mentioned sintering and agglomeration can be eliminated to adjust the particle size distribution.

[0091] (4) Pulverization process The manufacturing method of the positive electrode active material of the present embodiment can have a pulverization process of pulverizing the fired product or the powder after the firing process (S2) or the above-mentioned pulverization process.

[0092] The manufacturing method of the positive electrode active material of the present embodiment has a pulverization process, so that the secondary particles can be the primary particles, and the primary particles can obtain the main body of the lithium-nickel composite oxide.

[0093] The primary particles obtained after the pulverization process can be used alone or mixed with the agglomerated particles.

[0094] In addition, in the pulverization process or after the pulverization process, sieving can be performed in order to select particles of a desired particle size.

[0095] (5) Water washing process The manufacturing method of the positive electrode active material of the present embodiment can have a water washing process of washing the fired product obtained by the firing process (S2) with water.

[0096] By implementing the water washing process, the remaining lithium remaining on the surface of the particles of the lithium-nickel composite oxide can be removed.

[0097] In the water washing process, the fired product can be mixed with water and washed as a slurry (slurrying process).

[0098] The water used in the slurring process in the water washing process is not particularly limited, and for example, water having an electrical conductivity of less than 10 μS / cm is preferably used, and more preferably water having an electrical conductivity of 1 μS / cm or less can be used.

[0099] In addition, during the water washing, the prepared slurry is preferably stirred.

[0100] In the water washing step, the slurry can be subjected to solid-liquid separation, i.e., filtration and dewatering, after the slurry formation, to obtain a water washed powder (solid-liquid separation step). The device used for the filtration and dewatering is not particularly limited, and, for example, a centrifugal separation type, a filter press type solid-liquid separation device can be used.

[0101] In the water washing step, the water washed powder obtained after the solid-liquid separation, which contains water, is preferably dried (drying step). The drying conditions are not particularly limited, and, for example, the drying is preferably performed at a temperature of 100°C or higher and 350°C or lower in an oxidizing atmosphere or a vacuum atmosphere.

[0102] In addition, the calcined product of the lithium nickel composite oxide produced in the mixing step with Li / Me of 1.00 or higher can be subjected to water washing to obtain a lithium nickel composite oxide having Li / Me of 0.90 or higher and less than 1.00. However, in the above case, according to the research by the inventors of the present application, a positive electrode active material satisfying the prescribed positive electrode active material of the present embodiment and the effects of the positive electrode active material of the present embodiment are not obtained in terms of the occupancy of nickel at the lithium site (3b site).

[0103] [Lithium ion secondary battery] The lithium ion secondary battery of the present embodiment at least has a positive electrode, a negative electrode, and a nonaqueous electrolyte, and the positive electrode can contain the positive electrode active material for a lithium ion secondary battery described above.

[0104] Hereinafter, one configuration example of the secondary battery of the present embodiment will be described with respect to each of the constituent elements. The secondary battery of the present embodiment contains, for example, a positive electrode, a negative electrode, and a nonaqueous electrolyte, and is configured by the same constituent elements as those of a general lithium ion secondary battery. In addition, the embodiments described below are merely examples, and the lithium ion secondary battery of the present embodiment can be implemented in a modified form in which various changes and improvements are applied based on the knowledge of those skilled in the art, taking the embodiments described below as a representative. Furthermore, the secondary battery is not particularly limited in its use.

[0105] (Positive electrode) The positive electrode possessed by the secondary battery of the present embodiment can contain the positive electrode active material described above.

[0106] One example of the method for producing the positive electrode will be described below. First, the positive electrode active material (powder), the conductive material, and the binding agent (binder) described above can be mixed to produce a positive electrode mixture, and, if necessary, activated carbon, a solvent for the purpose of viscosity adjustment, and the like can be added, and the resulting mixture can be kneaded to produce a positive electrode mixture paste.

[0107] The mixing ratio of each material in the positive electrode mixture becomes an element that determines the performance of the lithium ion secondary battery, and thus can be adjusted according to the use. The mixing ratio of the materials can be, for example, such that, in a case where the total mass of the solid components of the positive electrode mixture excluding the solvent is set to 100 mass%, the positive electrode active material is contained at 60 mass% or more and 95 mass% or less, the conductive material is contained at 1 mass% or more and 20 mass% or less, and the binding agent is contained at 1 mass% or more and 20 mass% or less, similarly to the positive electrode of a publicly known lithium ion secondary battery.

[0108] The obtained positive electrode mixture paste is, for example, applied to the surface of a current collector made of an aluminum foil, and dried to volatilize the solvent, thereby producing a sheet-shaped positive electrode. If necessary, in order to increase the electrode density, the positive electrode can also be pressed by roll pressing or the like. The sheet-shaped positive electrode obtained by such an operation can be cut to an appropriate size or the like according to the purpose of the battery, and supplied to the production of the battery.

[0109] As the conductive material, for example, graphite (natural graphite, artificial graphite, and expanded graphite, etc.), acetylene black, carbon black-based materials such as Ketjen black (registered trademark), and the like can be used.

[0110] As the binding agent (binder), which plays a role of holding the active material particles, for example, one or more selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ternary ethylene-propylene-diene rubber, styrene butadiene, cellulose-based resins, and polyacrylic acid, and the like can be used.

[0111] If necessary, the positive electrode active material, the conductive material, and the like can be dispersed, and a solvent in which the binding agent is dissolved can be added to the positive electrode mixture. As the solvent, specifically, an organic solvent such as N-methyl-2-pyrrolidone or the like can be used. In addition, in order to increase the double-layer capacity, the positive electrode mixture can be added with activated carbon.

[0112] The method of producing the positive electrode is not limited to the above-described example, and other methods can be used. For example, the positive electrode mixture can be press-molded, and then dried in a vacuum atmosphere to be produced.

[0113] (Negative Electrode) The negative electrode can use metal lithium, lithium alloy, or the like. In addition, the negative electrode can use a negative electrode mixture in which a binding agent is mixed in a negative electrode active material capable of occluding and releasing lithium ions, and an appropriate solvent is added to be made into a paste, which is applied to the surface of a metal foil current collector such as copper, and dried, and if necessary, compressed in order to increase the electrode density, to be formed.

[0114] As the negative active material, for example, a powdery body of natural graphite, artificial graphite, and a sintered body of an organic compound such as a phenol resin, and a carbon material such as coke can be used. In this case, as the negative binder, similarly to the positive electrode, a fluorine-containing resin such as PVDF can be used, and as a solvent for dispersing these active materials and the binder, an organic solvent such as N-methyl-2-pyrrolidone can be used.

[0115] (Separator) A separator can be interposed as necessary between the positive electrode and the negative electrode. The separator is a separator that separates the positive electrode and the negative electrode and holds the electrolyte, and a known separator such as a film of polyethylene, polypropylene, or the like having a large number of minute pores can be used.

[0116] (Nonaqueous electrolyte) As the nonaqueous electrolyte, for example, a nonaqueous electrolytic solution can be used.

[0117] As the nonaqueous electrolytic solution, for example, a nonaqueous electrolytic solution in which a lithium salt as a supporting salt is dissolved in an organic solvent can be used. Further, as the nonaqueous electrolytic solution, a nonaqueous electrolytic solution in which a lithium salt is dissolved in an ionic liquid can be used. In addition, the ionic liquid refers to a salt composed of a cation other than a lithium ion and an anion, which is in a liquid state at ordinary temperature.

[0118] As the organic solvent, one kind selected from among cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trimethyl propyl carbonate, chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate, ether compounds such as tetrahydrofuran, 2-methyl tetrahydrofuran, and dimethoxy ethane, sulfur compounds such as ethyl methyl sulfone and butane sulfone lactone, phosphorus compounds such as triethyl phosphate and trioctyl phosphate, and the like can be used alone, and two or more kinds can be used in mixture.

[0119] As the supporting salt, LiPF6, LiBF4, LiClO4, LiAsF6, LiN(CF3SO2)2, and a complex salt thereof, and the like can be used. Further, the nonaqueous electrolytic solution can contain a radical scavenger, a surfactant, a flame retardant, and the like.

[0120] Further, as the nonaqueous electrolyte, a solid electrolyte can be used. The solid electrolyte has a property of being able to withstand a high voltage. As the solid electrolyte, an inorganic solid electrolyte, an organic solid electrolyte, and the like can be cited.

[0121] As the inorganic solid electrolyte, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, and the like can be cited.

[0122] As the oxide-based solid electrolyte, there is no particular limitation, and for example, an oxide-based solid electrolyte containing oxygen (O) and having lithium ion conductivity and electronic insulation can be suitably used. As the oxide-based solid electrolyte, for example, one or more selected from the group consisting of lithium phosphate (Li3PO4), Li3PO4N X , LiBO2N X , LiNbO3, LiTaO3, Li2SiO3, Li4SiO4-Li3PO4, Li4SiO4-Li3VO4, Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3-ZnO, Li 1＋X Al X Ti 2-X (PO4)3(0≤X≤1), Li 1＋X Al X Ge 2-X (PO4)3(0≤X≤1), LiTi2(PO4)3, Li 3X La 2 / 3-X TiO3(0≤X≤2 / 3), Li5La3Ta2O 12 , Li7La3Zr2O 12 , Li6BaLa2Ta2O 12 , Li 3.6 Si 0.6 P 0.4 O4, and the like can be used.

[0123] As the sulfide-based solid electrolyte, there is no particular limitation, and for example, a sulfide-based solid electrolyte containing sulfur (S) and having lithium ion conductivity and electronic insulation can be suitably used. As the sulfide-based solid electrolyte, for example, one or more selected from the group consisting of Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-Li2S-B2S3, Li3PO4-Li2S-Si2S, Li3PO4-Li2S-SiS2, LiPO4-Li2S-SiS, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, and the like can be used.

[0124] In addition, as the inorganic solid electrolyte, an inorganic solid electrolyte other than the above can be used, and for example, Li3N, LiI, Li3N-LiI-LiOH, and the like can be used.

[0125] As the organic solid electrolyte, if it is a high molecular compound exhibiting ion conductivity, there is no particular limitation, and for example, polyethylene oxide, polypropylene oxide, copolymers thereof, and the like can be used. Furthermore, the organic solid electrolyte can contain a supporting salt (lithium salt).

[0126] (Shape and configuration of the lithium-ion secondary battery) As described above, the lithium-ion secondary battery of the present embodiment can be formed in various shapes such as a cylindrical shape and a stacked shape. Even in the case of adopting any of the shapes, if a non-aqueous electrolyte solution is used as a non-aqueous electrolyte in the secondary battery of the present embodiment, the positive electrode and the negative electrode can be stacked with the separator interposed therebetween to form an electrode body, and in the resulting electrode body, the non-aqueous electrolyte solution is impregnated, the positive electrode current collector and the positive electrode terminal that penetrates the outside are connected with each other using a current-collecting lead or the like, and the negative electrode current collector and the negative electrode terminal that penetrates the outside are connected with each other using a current-collecting lead or the like, thereby forming a structure in which the electrode body is sealed in the battery case.

[0127] In addition, as described above, the secondary battery of the present embodiment is not limited to the form in which a non-aqueous electrolyte solution is used as a non-aqueous electrolyte, and for example, a secondary battery using a solid non-aqueous electrolyte, i.e., an all-solid battery, can be formed. In the case of forming an all-solid battery, the configuration other than the positive electrode active material can be changed as needed.

[0128] The secondary battery of the present embodiment can be used for various purposes. The secondary battery of the present embodiment can be formed as a secondary battery with high capacity and high output, and thus is generally suitable for a power source for a small portable electronic device (a notebook personal computer, a portable telephone terminal, or the like) that requires high capacity, and for a power source for an electric vehicle that requires high output.

[0129] In addition, the secondary battery of the present embodiment can be miniaturized and high-output, and thus is suitable as a power source for an electric vehicle in which mounting space is limited. In addition, the secondary battery of the present embodiment can be used not only as a power source for an electric vehicle that is purely driven by electric power, but also as a power source for a so-called hybrid vehicle that is used in combination with a combustion mechanism such as a gasoline engine or a diesel engine.

[0130] Example Hereinafter, the present application will be described in further detail by examples, which do not limit the present application in any way.

[0131] Here, first, the evaluation method of the positive electrode active material and the secondary battery obtained in the following examples and comparative examples will be described.

[0132] (Evaluation of the positive electrode active material) The obtained positive electrode active material was evaluated as follows.

[0133] (a) Evaluation of the composition The composition of the positive electrode active material was analyzed by ICP emission spectrometry. The evaluation results are shown in the column of "Composition" in Table 1.

[0134] (b) Nickel occupancy ratio at lithium site (3b) site Approximately 2g of the positive electrode active material prepared according to the following examples and comparative examples was filled into a vanadium tube and then completely sealed. Furthermore, neutron diffraction measurements were performed using the BL20 (iMATERIA) of the Japan Proton Accelerator Research Complex (J-PARC) at an intensity of over 10,000 cps in two-frame mode.

[0135] The neutron diffraction pattern obtained from the back-side detector array was analyzed using the Z-Code software using Rietveld. The space group was set to R-3m, and lithium at site 3b and nickel mixed in (nickel at lithium sites) were seeded with nickel and element M at site 3a. After refining the size factor, lattice constant, and global parameters, the seat occupancy and atomic coordinates were also refined. Furthermore, for atomic displacement parameters, lithium was set to 0.9–1.0 Å. 2 Nickel and element M: 0.3–0.5 Å 2 Oxygen: 0.6–0.8 Å 2 The range was fixed. After refining the seat occupancy and atomic coordinates, it was assumed that the S-value of the reliability parameter converged below 3.0. The nickel seat occupancy of the lithium site (3b site) was calculated, and the results are shown in the "Ni Occupancy of 3b Site" column of Table 1.

[0136] In addition, the S-value, which is used as a reliability parameter during Rietveld analysis, is displayed in the "S-value" column of Table 1.

[0137] In addition, powder neutron diffraction measurements can be performed using HRPD (High Resolution Powder Diffractometer) at the JRR-3 reactor of the Japan Atomic Energy Agency and SuperHRPD at J-PARC.

[0138] (c)(003) Half-width of surface peak The X-ray diffraction pattern was measured by an XRD diffractometer (PANalytical Corporation, X'Pert PRO) using Cu-Kα rays as an X-ray source. From the obtained X-ray diffraction pattern, the half-value width of the pattern in the intensity value that is half of the maximum intensity was calculated for the diffraction peak of the (003) plane of the hexagonal layered structure. In addition, the sample was filled into the groove portion of a circular glass holder (flat sample holder, inner diameter 24 mmφ, groove depth 0.5 mm, PANalytical Corporation No. 9200 540 12391) having an outer diameter of 32 mmφ, and fixed to a predetermined sample holder (PANalytical Corporation No. 9430018 13321) to be supplied to the measurement. Further, the XRD diffractometer used had a Bragg Brentano optical system.

[0139] The evaluation results are shown in the column of "(003) half-value width" of Table 1.

[0140] In addition, it was confirmed from the obtained X-ray diffraction pattern that the positive electrode active material produced in each of the Examples and Comparative Examples contained a lithium-nickel composite oxide having a hexagonal layered structure.

[0141] (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 by image analysis were selected, and primary particles were selected from each of the selected secondary particles. At this time, the primary particles were selected in such a manner that the total number became 30. Further, the major axis length of the selected primary particles was measured, and the average value was set as the average primary particle diameter. The evaluation results are shown in the column of "Average primary particle diameter" of Table 1.

[0142] (e) Median particle diameter (D50) The volume-based particle size distribution was measured by a laser diffraction scattering type particle size analyzer (Nikkiso Co., Ltd., Microtrack HRA), and the median particle diameter was calculated from the particle size distribution.

[0143] The evaluation results are shown in the column of "D50" of Table 1.

[0144] (f) Number ratio of individual primary particles After the lithium-nickel composite oxide particles were embedded with a thermosetting resin, polishing with a cross-section polisher (JEOL Ltd., SM-09010) was performed. The obtained particle cross-section was observed using a Schottky field emission type scanning electron microscope (Carl Zeiss Corporation, Ultra 55), and the number ratio of individual primary particles was evaluated from 100 particles.

[0145] (Production and evaluation of secondary batteries) (a) Evaluation of charge capacity, discharge capacity, and coulombic efficiency (a-1) Production of coin-type battery Using the positive electrode active material produced in each of the examples and comparative examples, a coin-type battery was produced as shown in the following. Figure 2

[0146] Figure 2 As shown in the drawing, the coin-type battery CBA is composed of a case CA and an electrode housed in the case CA.

[0147] The case CA has a positive electrode can PC which is hollow and has one end opened, and a negative electrode can NC disposed at the opening of the positive electrode can PC, and if the negative electrode can NC is disposed at the opening of the positive electrode can PC, it is configured in such a manner that a space in which the electrode is housed between the negative electrode can NC and the positive electrode can PC is formed.

[0148] The electrode contains a positive electrode PE, a separator SE1, and a negative electrode NE, which are arranged in order and stacked, the positive electrode PE is in contact with the inner surface of the positive electrode can PC, and the negative electrode NE is housed in the case CA in such a manner as to be in contact with the inner surface of the negative electrode can NC via a wave washer WW.

[0149] In addition, the case CA is provided with a gasket GA, and by the gasket GA, the relative movement of the positive electrode can PC and the negative electrode can NC is regulated in such a manner that a non-contact state, i.e., an electrically insulating state, is maintained, and thus it is fixed. Furthermore, the gasket GA seals the gap between the positive electrode can PC and the negative electrode can NC, and has a function of blocking the inside of the case CA from the outside in airtight and liquid-tight manner.

[0150] The coin-type battery CBA was produced as follows. First, 52.5 mg of the positive electrode active material obtained in each of the examples and comparative examples, 15 mg of acetylene black, and 7.5 mg of polytetrafluoroethylene (PTFE) resin were mixed. Then, the resulting mixture was filmized to a diameter of 11 mm and a weight of about 75 mg, and a positive electrode PE was produced, which was dried in a vacuum dryer at 100°C for 12 hours.

[0151] Using the produced positive electrode PE, a negative electrode NE, a separator SE1, and an electrolyte, the coin-type battery CBA was produced in a glove box in which the dew point of Ar atmosphere was managed at -60°C.

[0152] The negative electrode NE used a lithium metal punched into a disc shape with a diameter of 13 mm.

[0153] The separator SE1 used a polyethylene porous film with a film thickness of 25 μm. The electrolyte used a mixed solution (manufactured by Hokuriku Pharmaceutical Industry Co., Ltd.) in which the mixing ratio of ethylene carbonate (EC) and diethyl carbonate (DEC) as a supporting electrolyte was 1:1 in volume basis, and 1M of LiClO4 was used as the supporting electrolyte. ​

[0154] (a-2) Evaluation of charge capacity, discharge capacity, efficiency After the coin-type battery CBA was produced, it was left to stand for about 12 hours, and after the open circuit voltage (OCV) became stable, the current density with respect to the positive electrode was set to 0.1 mA / cm 2 The capacity at the time of charging until the cutoff voltage of 4.3 V was set as the charge capacity. Further, after the charging, the capacity at the time of discharging until the cutoff voltage of 2.5 V after 1 hour of interruption was set as the discharge capacity. Further, the coulombic efficiency, which is the ratio of the discharge capacity to the charge capacity, was calculated.

[0155] (b) Cycle characteristics (b-1) Production of the laminated battery A laminated battery shown in FIG. 1 was produced using the positive electrode active material produced in the following example and comparative example. Figure 3

[0156] Figure 3 As shown in FIG. 1, the laminated battery LBA has a structure in which a product obtained by impregnating an electrolyte in a laminate of a positive electrode film PS, a separator SE2, and a negative electrode film NS is sealed with a laminate LA. In addition, the positive electrode film PS is connected to a positive electrode tab PT, and the negative electrode film NS is connected to a negative electrode tab NT, and the positive electrode tab PT and the negative electrode tab NT are exposed to the outside of the laminate LA.

[0157] The laminated battery LBA was produced as follows. A slurry of 20.0 g of the positive electrode active material, 2.35 g of acetylene black, and 1.18 g of polyvinylidene fluoride dispersed in N-methyl-2-pyrrolidone (NMP) was coated on an Al foil at 1 cm 2 The Al foil on which the slurry containing the positive electrode active material was coated was dried at 120°C in the atmosphere for 30 minutes to remove the NMP. The Al foil on which the positive electrode active material was coated was cut into a long strip shape with a width of 66 mm, and roll-pressed with a load of 1.2 t to produce a positive electrode film. Furthermore, the positive electrode film was cut into a rectangular shape of 50 mm x 30 mm, and a product dried at 120°C for 12 hours in a vacuum drier was used as the positive electrode film PS of the laminated battery LBA.

[0158] In addition, a negative electrode film NS in which a negative electrode mixture paste, which is a mixture of a graphite powder with an average particle diameter of about 20 μm and polyvinylidene fluoride, was coated on a copper foil was prepared. A separator SE2 was a polyethylene porous film with a film thickness of 20 μm, and an electrolyte was a 3:7 mixture (manufactured by Ube Industries, Ltd.) of ethylene carbonate (EC) and diethyl carbonate (DEC) in which 1 M of LiPF6 was used as a supporting electrolyte. ​

[0159] In a dry room where the dew point was managed to be -60°C, the electrolyte was impregnated in the laminate of the above positive electrode film PS, the separator SE2, and the negative electrode film NS, and the laminate LA was sealed to produce a laminated battery LBA.

[0160] (b-2) Cycle characteristics The cycle characteristics were evaluated by measuring the capacity retention rate at the time of 500 cycles of charge and discharge. Specifically, first, the laminated battery LBA was kept in a thermostat kept at 25°C, and as the current density 0.3 mA / cm 2 , charged to the cut-off voltage 4.2 V, and after 10 minutes of pause, the cycle of discharging to the cut-off voltage 2.5 V was repeated 5 times. Next, in a thermostat kept at 45°C, as the current density 2.0 mA / cm 2 , charged to the cut-off voltage 4.2 V, and after 10 minutes of pause, the cycle of discharging to the cut-off voltage 2.5 V was repeated 500 times. Further, the capacity retention rate of the ratio of the discharge capacity of the 500th cycle after adjustment to the discharge capacity of the 1st cycle was calculated, and the evaluation was performed. The discharge capacity of the 1st cycle and the capacity retention rate are shown in the column of "initial discharge capacity", "capacity retention rate" of "cycle characteristics" of Table 1.

[0161] (Manufacturing conditions of positive electrode active material) [Example 1] A nickel composite oxide (D50 particle size: 5.2 μm) obtained using a known method, in which the mass ratio of nickel to cobalt was Ni:Co = 95.0:5.0, was mixed with lithium hydroxide to prepare a raw material mixture (mixing step).

[0162] In the mixing step, the nickel composite oxide and the lithium hydroxide were sufficiently mixed using a vibrator mixer device (Willy A Bachofen (WAB) Co., Ltd., Type: TURBULA Type T2C) to prepare the raw material mixture.

[0163] In the mixing step, each raw material was weighed so that the ratio of lithium (Li) to nickel and cobalt (Me) as metals other than lithium as the mass of the raw material mixture, Li / Me, was 0.92, and mixed. In addition, the nickel composite oxide was obtained by heat-treating a nickel composite hydroxide obtained using a crystallization method.

[0164] The raw material mixture was fired in an oxygen-containing atmosphere in which the oxygen concentration was 90 vol% and the remainder was nitrogen (firing step). The oxygen concentration in the oxygen-containing atmosphere in the firing step is shown in the column of "oxygen concentration" in Table 1. In the first firing step, the temperature was increased up to 500°C as the first firing temperature, and the temperature was maintained at 500°C as the first firing temperature for 7 hours. After the first firing step, the second firing step was continued. In the second firing step, the temperature was increased up to 700°C as the second firing temperature, and the temperature was maintained at 700°C as the second firing temperature for 12 hours, and thus the firing was performed.

[0165] The lithium nickel composite oxide after the firing was pulverized (pulverization step), and thus a positive electrode active material including lithium nickel composite oxide particles was obtained.

[0166] The positive electrode active material obtained was observed by SEM, and as a result, it was confirmed that the average primary particle diameter of the lithium nickel composite oxide particles was 0.63 μm, and the secondary particles including the primary particles were formed. That is, the number ratio of the primary particles alone was 0. The number ratio of the primary particles alone was 0 in the following other examples as well.

[0167] The other evaluation results are shown in Table 1.

[0168] Furthermore, using the positive electrode active material obtained, the coin-type battery, the laminated-type battery described above were produced, and evaluation was performed. The evaluation results are shown in Table 1.

[0169] [Example 2] In the mixing step, the mixing ratio of lithium hydroxide to the nickel composite oxide was changed so that the Li / Me of the raw material mixture became the value shown in the column of "added Li / Me ratio" 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 points, the positive electrode active material, the coin-type battery, the laminated-type battery were produced under the same conditions as in Example 1, and evaluation was performed. The evaluation results are shown in Table 1.

[0170] Figure 4 The powder neutron diffraction pattern (Observed) of the lithium nickel composite oxide measured in Example 2 is shown. Figure 4 The simulated pattern (Calculated) obtained by Rietveld analysis, the difference between the measured powder neutron diffraction pattern and the simulated pattern, and the Bragg positions are shown together.

[0171] [Example 3, Example 4] In the mixing step, as the nickel composite oxide, a nickel composite oxide having a mass ratio of nickel to manganese to cobalt of Ni:Mn:Co = 92.0:3.0:5.0 was used. Further, the mixing ratio of lithium hydroxide to the nickel composite oxide was selected so as to make the Li / Me with respect to the raw material mixture the value shown in the column of "added Li / Me ratio" of Table 1.

[0172] 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, respectively. Except for the above points, the positive electrode active material, the coin-type battery, and the laminated-type battery were produced and evaluated under the same conditions as in Example 1. The evaluation results are shown in Table 1.

[0173] [Comparative Example 1] In the mixing step, the mixing ratio of lithium hydroxide to the nickel composite oxide was changed so as to make the Li / Me with respect to the raw material mixture the value shown in the column of "added Li / Me ratio" of Table 1. Except for the above points, the positive electrode active material, the coin-type battery, and the laminated-type battery were produced and evaluated under the same conditions as in Example 2. The evaluation results are shown in Table 1.

[0174] [Comparative Example 2] To the positive electrode active material obtained in Comparative Example 1, 1 L of pure water at 20°C was added to prepare a slurry containing 1250 g of the positive electrode active material with respect to the water (slurry preparation step). Subsequently, after the slurry was stirred for 20 minutes, the liquid was passed to a filter press, and dewatering was performed to produce a washed filter cake (solid-liquid separation step). In addition, as the pure water, water having an electric conductivity of 1 μS / cm or less was used.

[0175] The washed filter cake obtained was dried at 190°C under a vacuum atmosphere for 10 hours to obtain the positive electrode active material of Comparative Example 2 (drying step).

[0176] The obtained positive electrode active material was subjected to the evaluation described above. The evaluation results are shown in Table 1. Further, using the obtained positive electrode active material, the coin-type battery and the laminated-type battery were produced and evaluated in the same manner as in Example 1, except for this. The evaluation results are shown in Table 1.

[0177] [Comparative Example 3] In the mixing step, the mixing ratio of lithium hydroxide to the nickel composite oxide was changed so as to make the Li / Me with respect to the raw material mixture the value shown in the column of "added Li / Me ratio" of Table 1. Except for the above points, the positive electrode active material, the coin-type battery, and the laminated-type battery were produced and evaluated under the same conditions as in Example 4. The evaluation results are shown in Table 1.

[0178] [Table 1] (Evaluation results) According to the results shown in Table 1, it was confirmed that the positive electrode active material of Example 1, in which the occupancy rate of nickel existing at the lithium site (3b site) was 2.5% or more and 10.0% or less, had improved cycle characteristics compared to Comparative Examples 1 and 2, in which the same content ratio of nickel and element M was present.

[0179] Further, the same was confirmed from the comparison between Example 3, 4 and Comparative Example 3.

[0180] This application claims priority based on Japanese Patent Application No. 2023-123250 filed on July 28, 2023, with the Japan Patent Office, the entire contents of which are hereby incorporated by reference into the present international application.

[0181] Explanation of symbols CBA coin-type battery PE positive electrode NE negative electrode SE1 separator GA gasket WW wave-shaped gasket CA case PC positive electrode can NC negative electrode can LBA laminate-type battery PS positive electrode film NS negative electrode film SE2 separator LA laminate PT positive electrode tab NT negative electrode tab

Claims

1. A positive electrode active material for lithium-ion secondary batteries, comprising a lithium-nickel composite oxide with a hexagonal crystal system layered structure. The lithium-nickel composite oxide contains lithium (Li), nickel (Ni), and element M (M) in a mass ratio of Li:Ni:M = a:b:c. 0.90≤a<1.00, 0.80≤b<1.00, 0.00<c≤0.20, b+c=1, wherein 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. The percentage of nickel present at lithium sites (i.e., 3b sites) is 2.5% to 10.0%, obtained by analyzing the powder neutron diffraction pattern of the lithium-nickel composite oxide using the Rietveld method.

2. The positive electrode active material for lithium-ion secondary batteries according to claim 1, wherein the half-width of the diffraction peak of the (003) plane in the powder X-ray diffraction pattern of the lithium nickel composite oxide, measured using Cu-Kα rays as an X-ray source, is 0.074° or more and 0.125° or less.

3. The positive electrode active material for lithium-ion secondary batteries according to claim 1 or 2, wherein the average primary particle size of the lithium-nickel composite oxide is 0.05 μm or more and 1.0 μm or less.

4. The positive electrode active material for lithium-ion secondary batteries according to any one of claims 1 to 3, wherein the median particle size in the volume-based particle size distribution obtained by laser diffraction / scattering method is 0.1 μm or more and 20.0 μm or less.

5. A lithium-ion secondary battery, comprising at least a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode comprises the positive electrode active material for lithium-ion secondary batteries as described in any one of claims 1 to 4.

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