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

Abstract

This positive electrode active material for lithium ion secondary batteries contains secondary particles in which a plurality of primary particles are aggregated with each other. The positive electrode active material for lithium ion secondary batteries contains a lithium transition metal composite oxide having a layered rock salt type structure. In the crystal structure of the lithium transition metal composite oxide, the proportion of elements other than Li in the Li site (3b site) is 2%-8%. The composition of the lithium transition metal composite oxide is represented by general formula (1).　General formula (1): Li1-xNiO2+α (where 0.02≤x≤0.08, -0.1≤α≤0.1)

Description

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

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery, a method for producing a positive electrode active material for a lithium ion secondary battery, and a lithium ion secondary battery.

[0002] BACKGROUND ART In recent years, with the widespread use of portable electronic devices such as mobile phone terminals and notebook 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.

[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 can improve the cycle characteristics when used in the lithium ion secondary battery.

[0010] In order to solve the above-mentioned problems, one aspect of the present invention provides a positive electrode active material for a lithium ion secondary battery, comprising secondary particles in which a plurality of primary particles are aggregated together, the positive electrode active material for a lithium ion secondary battery comprising a lithium transition metal composite oxide having a layered rock salt structure, in which in the crystal structure of the lithium transition metal composite oxide, a proportion of elements other than Li in the Li site (3b site) is 2% or more and 8% or less, and the composition of the lithium transition metal composite oxide is represented by the following general formula (1):

[0011] General formula (1): Li １－ｘ NiO ２＋α (where 0.02≦x≦0.08, -0.1≦α≦0.1)

[0012] 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 can improve cycle characteristics when used in the lithium ion secondary battery.

[0013] FIG. 1 shows a flow chart of a method for producing a positive electrode active material for a lithium ion secondary battery.

[0014] 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 the Positive Electrode Active Material The positive electrode active material of this embodiment contains a lithium transition metal composite oxide. The positive electrode active material of this embodiment may be composed only of a lithium transition metal 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.

[0015] The lithium transition metal composite oxide may have a layered rock salt structure. (1-1) Particle Morphology The particle morphology of the positive electrode active material of this embodiment is not particularly limited, but the positive electrode active material of this embodiment may contain secondary particles formed by agglomeration of a plurality of primary particles. Therefore, the positive electrode active material of this embodiment may be composed only of secondary particles, or may contain, in addition to the secondary particles, primary particles that exist independently without agglomeration.

[0016] The particles of the positive electrode active material of this embodiment can contain particles of a lithium transition metal composite oxide, or may be made up of particles of a lithium transition metal composite oxide.

[0017] The particle morphology of the positive electrode active material of this embodiment can be roughly confirmed by a scanning electron microscope (SEM) image of the particle (hereinafter also referred to as an "SEM image"), but this may lack accuracy. In the positive electrode active material of this embodiment, it is preferable to process the cross section of the particle and use the SEM image to determine the presence or absence of primary particles and secondary particles.

[0018] Specifically, for example, it is preferable to embed particles of the positive electrode active material 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. 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 less particles.

[0019] Furthermore, the band contrast of the electron backscatter diffraction (hereinafter also referred to as "EBSD") method can be used, 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) Composition of Lithium Transition Metal Composite Oxide Lithium transition metal composite oxide can be composed of lithium (Li), nickel (Ni), and oxygen (O). Therefore, the composition of the lithium transition metal composite oxide is represented by the following general formula (1):

[0020] General formula (1): Li １－ｘ NiO ２＋α In the above general formula, x and y preferably satisfy 0.02≦x≦0.08 and −0.1≦α≦0.1.

[0021] LiNiO ２ In lithium nickelate represented by the formula (1), the stoichiometric ratio of lithium to nickel is 1.00. In contrast, in the general formula (1) of the lithium transition metal composite oxide contained in the positive electrode active material of this embodiment, the ratio of lithium to nickel by mass is expressed as 1-x. In the general formula of the lithium transition metal composite oxide, 1-x represents the ratio of lithium to nickel by mass, and the value of x corresponds to the amount of lithium missing from the stoichiometric ratio. In the lithium transition metal composite oxide contained in the positive electrode active material of this embodiment, x preferably satisfies the condition 0.02≦x≦0.08. That is, the lithium transition metal composite oxide contained in the positive electrode active material of this embodiment preferably has a ratio of lithium to nickel of 0.98 or less, and has a deficiency.

[0022] By setting the value of x, which corresponds to the amount of lithium element deficiency from the stoichiometric ratio, to 0.02 or more and 0.08 or less to form a lithium transition metal composite oxide having a lithium element deficiency, the amount of lithium compound, i.e., the amount of alkali, attached to the particle surfaces of the positive electrode active material of this embodiment can be reduced. Therefore, when the positive electrode active material of this embodiment is applied to a lithium ion secondary battery (hereinafter also referred to as a "secondary battery"), gas generation during the battery reaction can be reduced.

[0023] Furthermore, by setting the value of x, which corresponds to the amount of deficiency from the stoichiometric ratio of lithium element, to 0.02 or more and 0.08 or less, when the positive electrode active material of this embodiment is applied to a secondary battery, the cycle characteristics can be improved.

[0024] The composition of the lithium nickel composite oxide can be measured by quantitative analysis using inductively coupled plasma (ICP) emission spectrometry. (1-3) Proportion of Elements Other Than Li at the Li Sites (3b Sites) In the lithium transition metal composite oxide contained in the positive electrode active material according to this embodiment, the proportion of elements other than Li at the Li sites (3b sites) in the crystal structure of the lithium transition metal composite oxide is preferably 2% or more and 8% or less, more preferably 2% or more and 6% or less.

[0025] The ratio of elements other than Li to the Li site (3b site) refers to the ratio of nickel to the Li site (3b site). The ratio of elements other than Li to the Li site (3b site) can be obtained by analyzing a powder X-ray diffraction pattern measured by powder X-ray diffraction using the Rietveld method. When analyzing using the Rietveld method, a layered rock salt structure, which is the crystal structure of a lithium transition metal composite oxide, can be used as the crystal structure.

[0026] When a secondary battery is repeatedly charged and discharged, lithium is repeatedly inserted and extracted between the layers of the lithium transition metal composite oxide. However, when a secondary battery using a conventional positive electrode active material is repeatedly charged and discharged, nickel may migrate and occupy some of the voids created by the lithium extraction. It is thought that lithium is no longer inserted into the sites occupied by nickel, resulting in a deterioration in cycle performance.

[0027] In contrast, in the lithium transition metal composite oxide contained in the positive electrode active material of this embodiment, the proportion of elements other than Li in the Li site (3b site) is 2% or more. It is believed that by inserting nickel into the Li site in advance in this way, it is possible to prevent nickel from being inserted into the site created by lithium desorption during charging. Therefore, it is believed that the decrease in battery capacity when charging and discharging is repeated can be suppressed and the cycle characteristics can be improved.

[0028] Furthermore, by setting the proportion of elements other than Li in the Li site (3b site) to 8% or less, a decrease in initial capacity can be suppressed. This ensures a sufficient amount of lithium contributing to charge and discharge, suppresses reaction resistance, and sufficiently increases battery capacity and output. (1-4) Average Particle Diameter of Primary Particles The average particle diameter of the primary particles in the positive electrode active material of this embodiment is preferably 20 nm or more and 250 nm or less, and more preferably 20 nm or more and 150 nm or less.

[0029] By setting the average particle diameter of the primary particles of the positive electrode active material of this embodiment to 20 nm or more, it is possible to sufficiently increase the size of the primary particles contained in the positive electrode active material of this embodiment and the crystallites constituting the primary particles. Therefore, the crystallinity of the lithium transition metal composite oxide contained in the positive electrode active material can be increased, and when the positive electrode active material of this embodiment is applied to a secondary battery, the battery capacity can be increased, resulting in a secondary battery with excellent battery capacity and cycle characteristics.

[0030] However, even if the average particle size of the primary particles of the positive electrode active material of this embodiment 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 250 nm or less.

[0031] The average particle size of the primary particles of the positive electrode active material is determined by selecting primary particles of the positive electrode active material that can be observed in their entirety with a scanning electron microscope (SEM), measuring their major axis lengths, and averaging them. When calculating the average particle size of the primary particles, the number of primary particles to be evaluated is not particularly limited.

[0032] For example, when the positive electrode active material contains secondary particles, it is preferable to select 10 to 20 secondary particles, select primary particles from each of the selected secondary particles, and then select a total of 20 to 30 primary particles, and measure the major axis lengths of the selected primary particles.

[0033] Furthermore, when the positive electrode active material contains single primary particles that do not constitute secondary particles, it is preferable to evaluate five or more primary particles of the positive electrode active material. 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 less.

[0034] When the positive electrode active material contains only secondary particles, the above number of secondary particles can be selected, and the above number of primary particles can be selected in total from each secondary particle. 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 particle size of the primary particles of the positive electrode active material.

[0035] When the positive electrode active material contains both secondary particles and single primary particles, the above-mentioned number of primary particles can be selected for each of the selected number of secondary particles and single primary particles. Then, the major axis lengths of all the selected primary particles can be measured, and the average value can be used as the average particle size of the primary particles of the positive electrode active material.

[0036] By setting the average particle size of the primary particles of the positive electrode active material within the above range, when the positive electrode active material is applied to a secondary battery, it is possible to particularly increase both the battery capacity and the cycle characteristics, and to achieve both. [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 it can produce a positive electrode active material having the above characteristics. Hereinafter, an example of the method for producing a positive electrode active material according to this embodiment will be described.

[0037] 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.

[0038] 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).

[0039] In the mixing step (S1), a nickel compound, which is at least one selected from nickel hydroxides and nickel oxides, is mixed with a lithium compound to obtain a lithium mixture. In this case, it is preferable to mix the components so that the atomic ratio of lithium atoms to nickel atoms in the resulting lithium mixture is 0.92 to 0.98.

[0040] In the calcination step (S2), the lithium mixture is calcined to produce a positive electrode active material. In the calcination step, the temperature is preferably increased at a rate of 2°C / min or less and calcined at a temperature of 600°C or more and 700°C or less.

[0041] An example of the configuration of each step will be described below. (1) Mixing Step (S1) In the mixing step (S1), a nickel compound, which is one or more types selected from nickel hydroxide and nickel oxide as described above, is mixed with a lithium compound to obtain a lithium mixture.

[0042] The nickel compound and the lithium compound can be added and mixed, for example, in the form of powder (solid phase). Each of the raw materials used in the mixing step will be described below. (1-1) Raw Materials (Nickel Compound) The nickel compound used in the mixing step (S1) can be obtained by a known method.

[0043] The nickel compound may be nickel hydroxide or nickel oxide. The nickel compound may also be a mixture of nickel hydroxide and nickel oxide. Examples of methods for producing nickel hydroxide include a method of neutralization crystallization using an aqueous solution of a nickel salt and an alkaline solution. Alternatively, nickel hydroxide may be heat-treated to remove moisture from the nickel hydroxide, thereby converting part or all of the nickel hydroxide into nickel oxide. (Lithium Compound) The lithium compound is not particularly limited, and known lithium-containing compounds can be used, including, for example, 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.

[0044] The lithium compound is preferably one or more selected from lithium carbonate and lithium hydroxide from the viewpoints of being less affected by residual impurities and dissolving at the firing temperature. Furthermore, from the viewpoint of obtaining a lithium transition metal composite oxide having particularly high crystallinity, the lithium compound is more preferably lithium hydroxide. (1-2) Mixing Method The method for mixing the nickel compound and the lithium compound is not particularly limited. It is preferable to mix these particles sufficiently to the extent that the particles are not destroyed.

[0045] The nickel compound and the lithium compound can be mixed 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 / Ni) of lithium (Li) to nickel (Ni) between individual particles of the positive electrode active material, resulting in problems such as insufficient battery performance. Therefore, thorough mixing is preferred. (1-3) Mixing Ratio The lithium compounds are preferably mixed so that the atomic ratio (Li / Ni) of lithium atoms (Li) to nickel atoms (Ni) in the lithium mixture obtained in the mixing step is 0.92 or more and 0.98 or less. In other words, it is preferred to mix the lithium compounds so that the Li / Ni ratio in the lithium mixture is the same as the Li / Ni ratio in the resulting fired product. This is because the ratio of the amounts of lithium and nickel elements does not change before and after the firing step (S2), so the Li / Ni of the lithium mixture in the mixing step (S1) becomes equal to the Li / Ni of the fired product. The contents (ratios) of the elements in the lithium mixture are also substantially maintained in the lithium transition metal composite oxide. (2) Firing step (S2) The firing step (S2) is a step in which the lithium mixture obtained in the mixing step (S1) is fired to obtain a fired product containing a lithium transition metal composite oxide. When the lithium mixture is fired, the lithium in the lithium compound diffuses into the nickel compound, forming a lithium transition metal composite oxide. The lithium compound melts at the firing temperature and penetrates into the nickel compound, forming a fired product of lithium transition metal composite oxide.

[0046] The firing conditions in the firing step (S2) are not particularly limited, and can be selected so that the lithium transition metal composite oxide has a desired composition and properties such as the proportion of elements other than Li in the Li site (3b site) 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 60% by volume or more, more preferably 80% by volume or more, and even 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.

[0047] In this specification, the firing atmosphere refers to the gas supplied to the firing furnace. (Firing temperature) The firing is preferably carried out at a firing temperature of 600°C or higher and 700°C or lower, more preferably at a firing temperature of 610°C or higher and 685°C or lower, and even more preferably at a firing temperature of 625°C or higher and 675°C or lower.

[0048] The lithium mixture can be fired by raising the temperature from room temperature to the firing temperature and then lowering the temperature to room temperature.

[0049] The rate of temperature rise to the firing temperature is preferably 2° C. / min or less, and more preferably 0.1° C. / min or more and 2° C. / min or less.

[0050] By setting the temperature rising rate to 2° C. / ° C. or less, the reaction in the lithium mixture can proceed uniformly, and the crystal growth of the lithium transition metal composite oxide can be promoted.

[0051] The time for which the firing temperature is maintained is not particularly limited, but is preferably 5 hours or more, and more preferably 10 hours to 48 hours.

[0052] By firing a predetermined lithium mixture according to the above temperature pattern, the proportion of elements other than Li in the Li site (3b site) of the lithium transition metal composite oxide can be easily adjusted to fall within the above-mentioned range.

[0053] In addition, the firing conditions, such as the firing temperature, may change depending on the content of lithium and nickel in the target composition of the lithium transition metal composite oxide in order to set the ratio of elements other than Li in the Li site (3b site) within a desired range. Therefore, it is preferable to select an optimal firing temperature within the above temperature range depending on the target composition, and to select temperature conditions, such as the heating rate and holding time.

[0054] 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 crushed powder after the calcination step (S2) or the crushing step.

[0055] 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.

[0056] By carrying out the water washing step, excess lithium remaining on the particle surfaces of the positive electrode active material can be removed.

[0057] In the water washing step, the fired product is mixed with water to form a slurry, which can be washed with water (slurrying step).

[0058] The water used to form a slurry in the water washing step is not particularly limited, but for example, water having an electrical conductivity of preferably less than 10 μS / cm, more preferably 1 μS / cm or less can be used.

[0059] It is preferable to stir the prepared slurry during the water washing.

[0060] 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.

[0061] 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.

[0062] In addition, by washing the fired product produced under the aforementioned conditions of Li / Ni equal to or greater than 1.00 in the mixing step with water, it is possible to obtain a lithium transition metal composite oxide having a Li / Me ratio of 0.92 to 0.98. 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 with respect to the proportion of elements other than Li in the Li site (3b site), 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.

[0063] 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.

[0064] An example of a method for manufacturing a positive electrode is described below. First, the above-described positive electrode active material (powder), a conductive agent, and a binder are mixed to prepare a positive electrode mixture, to which activated carbon and a solvent for viscosity adjustment and the like are added as needed, and the mixture is kneaded to prepare a positive electrode mixture paste.

[0065] 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 active material can be contained in proportions of 60 mass% to 95 mass%, the conductive agent can be contained in proportions of 1 mass% to 20 mass%, and the binder can be contained in proportions of 1 mass% to 20 mass%.

[0066] The resulting positive electrode mixture 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.

[0067] As the conductive agent, for example, graphite (natural graphite, artificial graphite, expanded graphite, etc.) and carbon black materials such as acetylene black and Ketjen Black (registered trademark) can be used.

[0068] The binder serves to bind the active material particles together, and may be one or more selected from, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose-based resin, polyacrylic acid, etc.

[0069] If necessary, a solvent for dispersing the positive electrode active material, conductive agent, 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.

[0070] The method for producing the positive electrode is not limited to the above-mentioned examples, and other methods may also be used. For example, the positive electrode can be produced by press-molding the positive electrode mixture 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 mixture, 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] Examples of inorganic solid electrolytes include oxide-based solid electrolytes and sulfide-based solid electrolytes.

[0077] 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 P.O. ４ N Ｘ , LiBO ２ N Ｘ , LiNbO ３ , LiTaO ３ , Li ２ SiO ３ , Li４ SiO ４ -Li ３ P.O. ４ , Li ４ SiO ４ -Li ３ VO ４ , Li ２ Alumni ２ O ３ -P ２ O ５ , Li ２ O—SiO ２ , Li ２ Alumni ２ O ３ -ZnO, Li １＋Ｘ Al Ｘ Ti ２－Ｘ (P.O. ４ ) ３ (0≦X≦1), Li １＋Ｘ Al Ｘ Ge ２－Ｘ (P.O. ４ ) ３ (0≦X≦1), LiTi ２ (P.O. ４ ) ３ , Li ３Ｘ La ２／３－Ｘ TiO ３ (0≦X≦2 / 3), Li ５ La ３ Ta ２ O １２ , Li ７ La ３ Zr ２ O １２ , Li ６ BaLa ２ Ta ２ O １２ , Li ３．６ Si ０．６ P ０．４ O ４ One or more selected from the above can be used.

[0078] 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-SiS２ , 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.

[0079] 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.

[0080] 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, and the resulting electrode assembly can be impregnated with the nonaqueous electrolyte solution. The positive electrode current collector and the positive electrode terminal connected to the outside, and the negative electrode current collector and the 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] (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.

[0087] (b) Proportion of Elements Other Than Li in the Li Site (3b Site) For the positive electrode active materials prepared in the following Examples and Comparative Examples, X-ray diffraction patterns were measured using an X'PertPRO X-ray diffractometer (XRD) manufactured by Spectris Corporation. The X-ray diffraction pattern measurement was carried out using CuKα radiation.

[0088] From the obtained X-ray diffraction patterns, it was confirmed that the lithium transition metal composite oxides contained in the positive electrode active materials obtained in the following Examples 1 to 7 and Comparative Examples 1 to 3 have a layered rock salt structure.

[0089] The obtained X-ray diffraction pattern was then subjected to Rietveld analysis using the crystal structure analysis software RIETAN. 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 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 site occupancy of nickel at the obtained lithium site (3b site) was calculated, and the results are shown in the column "Proportion of elements other than Li in the Li site (3b site)" in Table 1.

[0090] (c) Average particle size of primary particles In the SEM observation image of the lithium nickel composite oxide particles, 15 secondary particles that could be observed in their entirety by image analysis were selected, and primary particles were selected from each of the selected secondary particles. In this case, a total of 30 primary particles were selected. The major axis lengths of the selected primary particles were then measured, and the average value was taken as the average particle size of the primary particles.

[0091] The evaluation results are shown in the column "Average particle size of primary particles" in Table 1. (Fabrication and evaluation of lithium ion secondary batteries) (a) Fabrication of coin-type batteries Coin-type batteries were fabricated using the positive electrode active materials fabricated in the following examples and comparative examples.

[0092] Specifically, 0.170 g of the positive electrode active material obtained in the following Examples and Comparative Examples was weighed out in a glove box under an argon atmosphere. The weighed positive electrode active material was then placed in a container together with 0.010 g of acetylene black, 0.250 g of polyvinylidene fluoride (PVDF) solution (8% solution manufactured by Kureha), and 70 μL of N-methyl-2-pyrrolidone (NMP), and mixed using a stirrer (AR-100 manufactured by Thinky).

[0093] The slurry obtained by stirring was coated on an aluminum foil, and the coated aluminum foil was vacuum dried at room temperature for 2 hours, and then vacuum dried at 120° C. for 2 hours to obtain an electrode.

[0094] The obtained electrode was punched out to a diameter of 10 mm, and the weight was measured, and this was used as a positive electrode.

[0095] Metallic lithium was used as the negative electrode, and a porous polyethylene (PE) film was used as the separator. The positive electrode, separator, and negative electrode were stacked in that order. The stack of the positive electrode, separator, and negative electrode was then impregnated with an electrolyte and sealed between the positive electrode can and the negative electrode can. A gasket was placed between the positive electrode can and the negative electrode can to restrict relative movement and secure the two components in a non-contact state, i.e., to maintain an electrically insulated state.

[0096] The electrolyte is 1M LiPF ６ A 3:7 mixture (volume ratio) of ethylene carbonate (EC) and diethyl carbonate (DEC) with ethylene carbonate (EC) as a supporting electrolyte was used.

[0097] The coin-type battery was fabricated in a dry room controlled at a dew point of -60°C.

[0098] (b) Evaluation of Discharge Capacity and Cycle Characteristics The capacity retention rate, which indicates cycle characteristics, was evaluated by measuring the capacity retention rate after 100 charge / discharge cycles for a coin-type battery.

[0099] Specifically, a current of 10 mA / g was applied to the coin battery, and the battery was charged to a cutoff voltage of 4.5 V and then discharged to a cutoff voltage of 2.5 V. This cycle was repeated 100 times. The capacity retention rate, which is the ratio of the discharge capacity at the 100th cycle after conditioning to the discharge capacity at the first cycle, was calculated and evaluated.

[0100] The discharge capacity at the first cycle is shown in the "Initial capacity" column of Table 1, and the capacity retention rate is shown in the "Capacity retention rate" column of Table 1. (Production conditions of positive electrode active material) [Example 1] Nickel hydroxide and lithium hydroxide monohydrate, both of which were obtained by a known method, were mixed to prepare a lithium mixture (mixing step).

[0101] In the mixing step, nickel hydroxide and lithium hydroxide monohydrate were thoroughly mixed using a shaker mixer (model: TURBULA Type T2C, manufactured by Willy & Bachofen (WAB)) to prepare a raw material mixture.

[0102] In the mixing step, the raw materials were weighed and mixed so that the ratio of the number of lithium atoms (Li) to the number of nickel atoms (Ni) in the resulting lithium mixture (Li / Ni) was 0.96. The nickel hydroxide was obtained using a crystallization method.

[0103] During the firing step, oxygen was supplied into the furnace, and the lithium mixture was fired in an oxygen atmosphere (firing step).

[0104] The firing was carried out by increasing the temperature to 650° C. at a rate of 2° C. / min and holding the temperature at 650° C. for 12 hours. After holding the firing temperature, the material was cooled to room temperature.

[0105] The fired lithium transition metal composite oxide was crushed (crushing step) to obtain a positive electrode active material made of lithium transition metal composite oxide particles.

[0106] The obtained positive electrode active material was observed by SEM, and it was confirmed that the positive electrode active material was composed of secondary particles formed by aggregation of a plurality of primary particles. Note that it was also confirmed that the positive electrode active material was composed of secondary particles formed by aggregation of primary particles in the following other examples.

[0107] Other evaluation results are shown in Table 1.

[0108] Furthermore, the above-described coin-type batteries were fabricated using the obtained positive electrode active materials and evaluated. The evaluation results are shown in Table 1. [Example 2] In the mixing step, the mixing ratio of lithium hydroxide monohydrate and nickel hydroxide was changed so that the ratio of the number of lithium atoms (Li) to the number of nickel atoms (Ni) in the resulting lithium mixture (Li / Ni) was 0.98. A positive electrode active material and a coin-type battery were fabricated and evaluated under the same conditions as in Example 1, except for the above. The evaluation results are shown in Table 1. [Comparative Example 1] In the mixing step, the mixing ratio of lithium hydroxide monohydrate and nickel hydroxide was changed so that the ratio of the number of lithium atoms (Li) to the number of nickel atoms (Ni) in the resulting lithium mixture (Li / Ni) was 1.00. A positive electrode active material and a coin-type battery were fabricated and evaluated under the same conditions as in Example 1, except for the above. The evaluation results are shown in Table 1. [Comparative Example 2] In the mixing step, the mixing ratio of lithium hydroxide monohydrate and nickel hydroxide was changed so that the ratio (Li / Ni) of the number of lithium atoms (Li) to the number of nickel atoms (Ni) in the resulting lithium mixture was 0.90. Except for the above, a positive electrode active material and a coin-type battery were produced and evaluated under the same conditions as in Example 1. The evaluation results are shown in Table 1. [Examples 3 and 4] A positive electrode active material and a coin-type battery were produced and evaluated under the same conditions as in Example 1, except that a mixed gas of oxygen and air was used as the gas supplied into the furnace during the firing step.

[0109] In Example 3, a mixed gas of 50% oxygen and 50% air by volume was used, resulting in an oxygen concentration of approximately 61% by volume in the firing atmosphere.

[0110] In Example 4, a mixed gas of 70% oxygen and 30% air by volume was used, resulting in an oxygen concentration of approximately 76% by volume in the firing atmosphere.

[0111] The evaluation results are shown in Table 1. [Comparative Example 3] A positive electrode active material and a coin-type battery were produced and evaluated under the same conditions as in Example 1, except that the atmosphere in the firing step was air and the firing temperature was 710°C.

[0112] Therefore, the oxygen concentration in the firing atmosphere is about 21% by volume.

[0113] The evaluation results are shown in Table 1. [Examples 5 to 7] Positive electrode active materials and coin-type batteries were produced and evaluated under the same conditions as in Example 1, except that the firing temperature in the firing step was changed.

[0114] The firing temperature was 630°C in Example 5, 680°C in Example 6, and 700°C in Example 7.

[0115] The evaluation results are shown in Table 1.

[0116] (Evaluation results)

[0117] According to Table 1, it was confirmed that Examples 1 to 7, in which the proportion of elements other than Li in the Li site (3b site) was 2% or more and 8% or less and the contained lithium transition metal composite oxide was a positive electrode active material represented by the above-mentioned general formula (1), had a higher capacity retention rate than Comparative Examples 1 to 3. In other words, it was confirmed that the positive electrode active materials of Examples 1 to 7, when used in a lithium ion secondary battery, could improve the cycle characteristics compared to the positive electrode active materials of Comparative Examples 1 to 3.

[0118] This application claims priority based on Japanese Patent Application No. 2023-123253, filed with the Japan Patent Office on July 28, 2023, the entire contents of which are incorporated herein by reference.

[0119] 10 Flow S1 Mixing process S2 Firing process

Claims

1. A positive electrode active material for a lithium ion secondary battery, comprising secondary particles in which a plurality of primary particles are aggregated together, the positive electrode active material for a lithium ion secondary battery comprising a lithium transition metal composite oxide having a layered rock salt structure, in which in the crystal structure of the lithium transition metal composite oxide, the proportion of elements other than Li in the Li site (3b site) is 2% or more and 8% or less, and the composition of the lithium transition metal composite oxide is represented by the following general formula (1). General formula (1): Li １－ｘ NiO ２＋α (where 0.02≦x≦0.08, −0.1≦α≦0.1) 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the average particle size of the primary particles is 20 nm or more and 250 nm or less.

3. A method for producing a positive electrode active material for a lithium ion secondary battery, comprising: a mixing step of mixing a nickel compound, which is one or more types selected from nickel hydroxide and nickel oxide, with a lithium compound so that the ratio of the number of lithium atoms to the number of nickel atoms in the resulting lithium mixture is 0.92 to 0.98; and a firing step of heating the lithium mixture at a heating rate of 2°C / min or less and firing it at a temperature of 600°C to 700°C.

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 claim 1 or 2.

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